Nanostructured Polymer Membranes, Applications (Volume 2) [Volume 2 ed.] 1118831780, 9781118831786

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Nanostructured Polymer Membranes

Scrivener Publishing 100 Cummings Center, Suite 541J Beverly, MA 01915-6106 Publishers at Scrivener Martin Scrivener ([email protected]) Phillip Carmical ([email protected])

Nanostructured Polymer Membranes Volume 2: Applications

Visakh P.M. and Olga Nazarenko

Copyright © 2016 by Scrivener Publishing LLC. All rights reserved. Co-published by John Wiley & Sons, Inc. Hoboken, New Jersey, and Scrivener Publishing LLC, Beverly, Massachusetts. Published simultaneously in Canada. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording, scanning, or otherwise, except as permitted under Section 107 or 108 of the 1976 United States Copyright Act, without either the prior written permission of the Publisher, or authorization through payment of the appropriate per-copy fee to the Copyright Clearance Center, Inc., 222 Rosewood Drive, Danvers, MA 01923, (978) 750-8400, fax (978) 750-4470, or on the web at www.copyright.com. Requests to the Publisher for permission should be addressed to the Permissions Department, John Wiley & Sons, Inc., 111 River Street, Hoboken, NJ 07030, (201) 748-6011, fax (201) 748-6008, or online at http://www.wiley.com/go/permission. Limit of Liability/Disclaimer of Warranty: While the publisher and author have used their best eforts in preparing this book, they make no representations or warranties with respect to the accuracy or completeness of the contents of this book and speciically disclaim any implied warranties of merchantability or itness for a particular purpose. No warranty may be created or extended by sales representatives or written sales materials. he advice and strategies contained herein may not be suitable for your situation. You should consult with a professional where appropriate. Neither the publisher nor author shall be liable for any loss of proit or any other commercial damages, including but not limited to special, incidental, consequential, or other damages. For general information on our other products and services or for technical support, please contact our Customer Care Department within the United States at (800) 762-2974, outside the United States at (317) 572-3993 or fax (317) 572-4002. Wiley also publishes its books in a variety of electronic formats. Some content that appears in print may not be available in electronic formats. For more information about Wiley products, visit our web site at www.wiley.com. For more information about Scrivener products please visit www.scrivenerpublishing.com. Cover design by Russell Richardson Library of Congress Cataloging-in-Publication Data: ISBN 978-1-118-83178-6 

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Contents Preface 1 Nanostructured Polymer Membranes: Applications, State-of-the-Art, New Challenges and Opportunities Visakh. P. M 1.1 Membranes: Technology and Applications 1.2 Polymer Membranes: Gas and Vapor Separation 1.3 Membranes for Wastewater Treatment 1.4 Polymer Electrolyte Membrane and Methanol Fuel Cell 1.5 Polymer Membranes for Water Desalination and Treatment 1.6 Biopolymer Electrolytes for Energy Devices 1.7 Phosphoric Acid-Doped Polybenzimidazole Membranes 1.8 Natural Nanoibers in Polymer Membranes for Energy Applications 1.9 Potential of Carbon Nanoparticles for Pervaporation Polymeric Membranes 1.10 Mixed Matrix Membranes for Nanoiltration Application 1.11 Fundamentals, Applications and Future Prospects of Nanoiltration Membrane Technique References 2 Membranes: Technology and Applications Yang Liu and Guibin Wang 2.1 Introduction 2.1.1 Membrane Process 2.1.2 Membrane Types and Preparations 2.1.2.1 Isotropic Membranes 2.1.2.2 Anisotropic Membranes 2.1.3 Membrane Modules 2.1.3.1 Plate-and-frame Modules 2.1.3.2 Tubular Modules

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Contents 2.1.3.3 Hollow-iber Modules 2.1.3.4 Spiral-wound Modules 2.2 Reverse Osmosis Process 2.2.1 Introduction 2.2.2 Principle 2.2.3 Membrane Materials 2.2.3.1 Cellulose Acetate Membrane 2.2.3.2 Aromatic Polyamide Membrane 2.2.3.3 Other Polymer Membranes 2.2.3.4 Interfacial Composite Membrane 2.2.4 Applications 2.2.4.1 Brackish Water Desalination 2.2.4.2 Seawater Desalination 2.2.4.3 Ultrapure Water 2.2.4.4 Nanoiltration 2.2.5 Conclusions 2.3 Ultrailtration Process 2.3.1 Introduction 2.3.2 Principle 2.3.3 Membrane Structures and Materials 2.3.4 Membrane Fouling and Control 2.3.4.1 Concentration Polarization 2.3.4.2 Fouling Control 2.3.5 Applications 2.3.6 Conclusions 2.4 Pervaporation Process 2.4.1 Introduction 2.4.2 Principle 2.4.3 Membrane Materials and Applications 2.4.3.1 Hydrophilic Membrane (Dehydration Membrane) 2.4.3.2 Hydrophobic Membrane (Organic-Water Separation Membrane) 2.4.3.3 Organophilic Membrane (OrganicOrganic Separation Membrane) 2.4.4 Conclusions and Future Directions 2.5 Microiltration Process 2.5.1 Introduction 2.5.2 Principle 2.5.3 Membrane Materials 2.5.4 Applications 2.5.5 Conclusions

36 37 37 37 38 41 41 42 42 43 45 46 47 48 49 49 50 50 51 52 54 54 55 57 59 59 59 60 62 62 63 64 64 65 65 66 68 68 69

Contents 2.6 Coupled and Facilitated Transport 2.6.1 Introduction 2.6.2 Coupled Transport 2.6.3 Facilitated Transport 2.6.4 Conclusions 2.7 Membrane Distillation 2.7.1 Introduction 2.7.2 Principle 2.7.3 Applications 2.7.4 Conclusions 2.8 Ultrailtration Zeolite and Ceramic Membranes 2.8.1 Introduction 2.8.2 Applications 2.8.3 Conclusions 2.9 Conclusions References 3 Polymeric Membranes for Gas and Vapor Separations Seyed Saeid Hosseini and Sara Najari 3.1 Introduction 3.2 Signiicance and Prominent Industrial Applications 3.2.1 Gas/Gas Separation Applications 3.2.2 Vapor/Gas Separation Applications 3.2.3 Vapor/Vapor Separation Applications 3.3 Fundamentals and Transport of Gases in Polymeric Membranes 3.3.1 Transport Properties in Polymeric Membranes 3.3.2 Factors Contributing to the Transport in Polymeric Membranes 3.3.2.1 Nature and Properties of Gas Molecules 3.3.2.2 Nature and Properties of Membrane Material 3.3.2.3 Operating Conditions 3.4 Polymeric Membrane Materials for Gas and Vapor Separations 3.4.1 High-Performance Engineering Polymers 3.4.1.1 Polysulfones (PSfs) 3.4.1.2 Polyethersulfones (PESs) 3.4.1.3 Celluloce Acetates (CAs) 3.4.1.4 Polyimides (PIs) 3.4.1.5 Polyetherimides/Polyamideimides (PEIs/PAEs)

vii 69 69 70 72 73 73 73 74 78 79 79 79 81 82 83 84 89 89 91 91 98 99 100 100 103 103 105 108 112 112 112 113 113 114 117

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Contents 3.4.2 Novel Polymers 3.4.2.1 Fluoropolymers and Fluorinated Polymers 3.4.2.2 Siloxane 3.4.2.3 Substituted Polyacetylenes 3.4.3 Emerging Polymers 3.4.3.1 Polymers of Intrinsic Microporosity (PIMs) 3.4.3.2 hermally Rearranged (TR) Polymers 3.5 Strategies for Tuning the Transport in Polymeric Membranes through Molecular Design and Architecture 3.5.1 Solubility-Selective Membranes 3.5.2 Difusivity-Selective Membranes 3.6 Process Modeling and Simulation 3.6.1 Modeling of Membrane Gas Separation Process 3.6.2 Phenomenological Models for Gas and Vapor Sorption and Permeation 3.7 Challenges and Future Directions 3.7.1 Trade-of between Permeability and Selectivity 3.7.2 Plasticization and Physical Aging 3.8 Concluding Remarks References

4 Membranes for Wastewater Treatment Alireza Zirehpour and Ahmad Rahimpour 4.1 Introduction 4.2 Membrane heory 4.2.1 Membrane Deinition and Structure 4.2.2 Membrane Principles 4.2.2.1 Membrane Transport 4.2.2.2 Membrane Selectivity 4.2.2.3 Membrane Separation Mechanism 4.2.2.4 Concentration Polarization 4.2.2.5 Critical Flux 4.2.2.6 Membrane Fouling 4.3 Membrane Separation Techniques in Industry 4.3.1 Reverse Osmosis and Nanoiltration Systems 4.3.1.1 Flux, Pressure, and Feed Recovery Rate 4.3.1.2 RO and NF Applications 4.3.1.3 Fouling in NF and RO Process 4.3.2 Ultrailtration and Microiltration Systems

118 118 120 121 124 124 126 128 128 130 132 132 137 141 141 142 144 144 159 160 161 161 162 162 163 164 164 166 166 168 169 171 172 172 173

Contents 4.3.3 Forward Osmosis Systems 4.3.3.1 Draw Solution and Recovery System 4.3.3.2 Fouling in FO Systems 4.4 Membrane Operations in Wastewater Management 4.4.1 Membrane Bioreactor 4.4.1.1 MBR Conigurations 4.4.1.2 MBR Performance Determination and Afecting Factors 4.4.1.3 Membrane Fouling in MBR System 4.4.2 Integrated Membrane Systems 4.4.2.1 Integration of Membrane Process with Conventional Wastewater Treatment 4.4.2.2 MF/UF as NF/RO Pretreatment 4.4.3 High Retention Membrane Bioreactors 4.4.3.1 Nanoiltration Membrane Bioreactor 4.4.3.2 Osmotic Membrane Bioreactor 4.4.3.3 Membrane Distillation Bioreactor 4.5 Existing Membrane Processes 4.5.1 Food Industries 4.5.1.1 Potato Starch Production 4.5.1.2 Treatment of Wastewater from Fruit Juice Production 4.5.1.3 Wastewater from Seafood Industries 4.5.1.4 Wastewater from the Olive Oil Mill Industry 4.5.2 Pulp and Paper Industries 4.5.3 Textile Industry 4.5.4 Laundry Industries 4.5.5 Landill Leachate 4.6 Industrial Development of Membrane Modules 4.6.1 Conventional Membrane Modules 4.6.2 Developing Membrane Modules in Industry 4.6.2.1 Low-diferential-pressure Spiral-wound Modules 4.6.2.2 Full-it Spiral-wound Modules 4.6.2.3 High-productivity Spiral-wound Modules 4.6.2.4 Vibratory Shear Enhanced Processing (VSEP) Modules 4.6.2.5 Other Module Developments 4.7 Conclusion References

ix 175 177 177 178 178 179 179 180 181 181 182 183 183 184 185 185 186 187 187 188 189 190 191 192 193 194 194 196 196 197 197 197 197 198 198

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5 Polymer Electrolyte Membrane and Methanol Fuel Cell Kilsung Kwon and Daejoong Kim 5.1 Introduction 5.2 Polymer Electrolyte Membrane Fuel Cells (PEMFCs) 5.2.1 Components 5.2.1.1 Flow Field Plate 5.2.1.2 Membrane Electrolyte Assembly (MEA) 5.2.2 HT-PEMFC 5.2.3 Applications 5.3 Direct Methanol Fuel Cells (DMFCs) 5.3.1 Applications 5.4 Principle and Working Process of PEMFCs 5.4.1 Flow Field Design 5.4.2 hermal Management 5.5 Principle and Working Process of DMFCs 5.5.1 Fuel Supply Method 5.5.2 Micro DMFC 5.6 Modeling and heory of Polymer Electrolyte Membrane Fuel Cells 5.7 Conclusion References

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6 Polymer Membranes for Water Desalination and Treatment Tânia L. S. Silva, Sergio Morales-Torres, José L. Figueiredo and Adrián M. T. Silva 6.1 Introduction 6.2 Polymer Membranes Used in Distillation 6.2.1 Fabrication and Modiication of Hydrophobic Membranes 6.2.2 Macro-Geometries of MD Membranes 6.3 Membrane Distillation 6.3.1 heory and Mechanistic Fundamentals 6.3.2 Conigurations of MD Systems 6.3.2.1 Direct Contact Membrane Distillation (DCMD) 6.3.2.2 Air Gap Membrane Distillation (AGMD) 6.3.2.3 Sweeping Gas Membrane Distillation (SGMD)

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Contents 6.3.2.4 Vacuum Membrane Distillation (VMD) 6.3.2.5 Other MD Conigurations 6.3.3 Operating Parameters Inluencing MD Systems 6.3.3.1 Feed Temperature 6.3.3.2 Permeate Temperature 6.3.3.3 Feed and Permeate Flow Rates 6.3.3.4 Feed Inlet Concentration 6.3.3.5 Presence of Non-Condensable Gases 6.3.3.6 Air Gap Width 6.4 Desalination Driven by MD Systems 6.5 MD Hybrid Systems for Water Desalination and Treatment 6.6 Conclusions Acknowledgments References 7 Polymeric Pervaporation Membranes: Organic-Organic Separation Francesco Galiano, Francesco Falbo and Alberto Figoli 7.1 General Introduction on Pervaporation 7.2 Brief History of Pervaporation 7.3 Polymeric Materials for Organic-Organic Separation – General Requirements 7.3.1 Nature of the Membrane 7.3.1.1 Polymeric Material 7.3.1.2 Crystallinity 7.3.1.3 Presence of Fillers or Functional Groups 7.3.2 Degree of Crosslinking 7.3.3 Degree of Swelling 7.3.4 hickness of Selective Nonporous Layer 7.4 Pervaporation Case Studies for Organic-Organic Separation 7.4.1 Separation of Methanol/MTBE 7.4.2 Pervaporation Separation of Ethanol/ETBE 7.4.3 Pervaporation Separation of Ethanol/Cyclohexane 7.5 Conclusions and Future Directions References

xi 261 261 262 262 262 263 263 264 264 265 272 275 275 276 287 287 290 291 292 292 293 294 296 296 297 298 298 299 301 303 303

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8 Biopolymer Electrolytes for Energy Devices Tan Winie1 and A. K. Arof 8.1 Introduction 8.2 Chitosan-based Electrolyte Membranes 8.3 Methyl Cellulose-based Electrolyte Membranes 8.4 Biopolymer Electrolytes in Lithium Polymer Batteries 8.4.1 he Structure and Working Principle of a Lithium Polymer Battery 8.4.2 Basic Properties of a Polymer Electrolyte Membrane 8.4.3 Characterization Techniques for a Lithium Polymer Battery 8.4.4 Lithium Batteries Employing Polymer Electrolytes 8.5 Biopolymer Electrolytes in Supercapacitors 8.5.1 he Structure and Working Principle of an EDLC 8.5.2 Characterization Techniques for EDLCs 8.5.3 EDLC Employing Biopolymer Electrolytes 8.6 Polymer Electrolytes in Fuel Cells 8.6.1 he Structure and Working Principle of a Fuel Cell 8.6.2 Characterization Techniques for Fuel Cells 8.6.3 Biopolymer Electrolytes in PEMFCs 8.7 Biopolymer Electrolytes in Dye-sensitized Solar Cells (DSSCs) 8.7.1 he Structure and Working Principle of a DSSC 8.7.2 Basic Properties of a Dye Sensitizer 8.7.3 Characterization of DSSCs 8.7.4 DSSCs Employing Biopolymer Electrolytes 8.8 Conclusions Acknowledgments References 9 Phosphoric Acid-Doped Polybenzimidazole Membranes: A Promising Electrolyte Membrane for High Temperature PEMFC S. R. Dhanushkodi, M. W.Fowler, M. D. Pritzker and W. Merida 9.1 Introduction 9.2 Synthesis of PBI

311 312 312 315 317 317 318 319 320 322 325 326 327 328 329 331 331 332 333 335 336 343 344 346 346

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Contents 9.3 Characterization of PBI 9.3.1 Molecular Weight Distribution (MWD) 9.3.2 hemogravimetric Analysis (TGA) 9.3.3 Fourier Transform Infrared Spectroscopy (FTIR) 9.3.4 Nuclear Magnetic Resonance Spectroscopy (NMR) 9.3.5 Conductivity 9.3.6 Permeability and Mechanical Testings 9.3.7 Fuel Cell Testings 9.4 Research Needs and Conclusions Table of Abbreviations References 10 Natural Nanoibers in Polymer Membranes for Energy Applications Annalisa Chiappone 10.1 Introduction 10.2 Natural Fibers 10.2.1 Cellulose and Chitin Structures 10.2.2 Nanoibers Production 10.3 Polymer Nanocomposite Membranes Based on Natural Fibers: Production, Properties and General Applications 10.3.1 Production 10.3.2 Properties 10.3.3 General Applications 10.4 Applications of Natural Fibers Nanocomposite Membranes in the Energy Field 10.4.1 Composite Polymer Electrolytes for Lithium Batteries 10.4.2 Composite Electrodes for Supercapacitors 10.4.3 Other Devices 10.5 Conclusions References 11 Potential Interests of Carbon Nanoparticles for Pervaporation Polymeric Membranes Anastasia V. Penkova and Denis Roizard 11.1 Introduction 11.2 Principle of Permeation 11.2.1 Liquid Molecular Separations by Pervaporation

xiii 363 363 364 365 367 368 368 370 370 373 374 379 379 380 381 383 386 386 388 392 393 393 399 401 402 403 413 413 415 416

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Contents 11.2.2 Composite Membranes: A Strategic Route to Develop Membranes with Outstanding Properties 11.2.3 Industrial Membranes 11.2.3.1 Composite Polymer Membrane 11.3 Current Requirements for Pervaporation Membranes 11.4 Performances of Nanocomposite Membranes: From Membrane Preparations to Enhanced Properties with Carbon Nanoparticles 11.5 Impact of the Insertion of Carbon Particles in Pervaporation Membranes 11.5.1 Fullerene 11.5.2 Carbon Nanotubes 11.5.3 Graphene Oxide 11.6 Pervaporation Membranes 11.7 Pervaporation with the Use of MMM Containing Pristine Carbon Particles 11.8 Pervaporation with the Use of MMM Containing Functionalized Carbon Particles 11.9 Conclusion Acknowledgment References

12 Mixed Matrix Membranes for Nanoiltraion Application Vahid Vatanpour, Mahdie Safarpour and Alireza Khataee 12.1 Introduction 12.2 Nanoiltration Process: History and Principles 12.3 Mixed Matrix Nanoiltration Membranes 12.3.1 Asymmetric Mixed Matrix Nanoiltration Membranes Prepared by Phase Inversion 12.3.2 hin-ilm Nanocomposite (TFN) Nanoiltration Membranes Prepared by Interfacial Polymerization 12.3.2 Surface Coating Containing Inorganic Materials 12.4 Applications of Mixed Matrix Nanoiltration Membranes 12.5 Conclusion Acknowledgment List of Abbreviations References

416 417 418 418 420 422 422 422 423 423 424 427 434 435 435 441 442 443 444 444 457 466 468 469 470 470 471

Contents 13 Fundamentals, Applications and Future Prospects of the Nanoiltration Membrane Technique Siddhartha Moulik, Shaik Nazia and S. Sridhar 13.1 Introduction 13.1.1 Nanoiltration (NF) 13.1.2 Principle 13.1.2.1 Solution-Difusion Model 13.1.2.2 Preferential Sorption Surface-Capillary Flow 13.1.2.3 Model for Charged NF Membrane 13.2 Membrane Synthesis 13.2.1 Membrane Substrate Preparation 13.2.2 Preparation of Nanoiltration Membrane 13.3 Membrane Characterization 13.3.1 Scanning Electron Microscopy (SEM) 13.3.2 Pore Size Measurement by Bubble Point Method 13.3.3 Solute-Transport Method 13.3.4 Laboratory NF Experimental Set-up 13.4 Equations for Calculation of Operating Parameters 13.4.1 Permeate Flux (J) 13.4.2 Rejection Eiciency 13.4.3 Water Recovery (%) 13.5 Efect of Feed Pressure on Process Flux 13.6 Optimization of NF Process Using Computation Fluid Dynamics (CFD) 13.6.1 Development of 3D Model for NF Process Using ANSYS Fluent 13.6.1.1 Governing Equations 13.6.1.2 Modeling Procedure and Solution Scheme 13.6.1.3 Hydrodynamic Analysis 13.6.2 Development of 2D Model for NF Process 13.7 Applications of NF in Societal Development and Industrial Progress 13.7.1 Drinking Water Puriication 13.7.2 Application of NF in the Dairy Industry 13.7.3 Application of NF in Fruit Juice Concentration 13.7.4 Industrial Eluent Treatment 13.7.5 Other Applications

xv 477 478 479 480 480 482 483 483 483 484 485 485 486 486 486 487 487 487 488 488 490 490 492 492 493 497 501 501 503 507 507 510

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Economics of NF Process for Groundwater Puriication 13.8.1 Capital Cost for 5000 L/h Capacity NF Plant 13.8.2 NF System Operation and Maintenance Cost 13.9 Conclusions References Index

510 510 513 514 515 519

Preface Many recent research accomplishments in the area of polymer nanocomposite membrane materials are summarized in Nanostructured Polymer Membranes: Applications, including the state of the art and new challenges, membrane technology and applications, polymer membranes for gas and vapor separation, membranes for wastewater treatment, polymer electrolyte membrane and methanol fuel cell, polymer  membranes for water desalination and treatment, polymeric pervaporation membranes for organic-organic separation, biopolymer electrolytes for energy devices, phosphoric acid-doped polybenzimidazole membranes as a promising electrolyte, membrane for high-temperature PEMFC, natural ibers in polymer membranes for energy applications, the potential interest in carbon nanoparticles for pervaporation polymeric membranes, mixed matrix membranes for nanoiltration application, and fundamentals, applications and future prospects of nanoiltration membrane technique. As the title indicates, various aspects of nanostructured polymer membranes and their applications are emphasized in this book. It is intended to serve as a “onestop” reference resource for important research accomplishments in the area of nanostructured polymer membranes and their applications. his book will be a very valuable reference source for university and college faculties, professionals, post-doctoral research fellows, senior graduate students, and R&D laboratory researchers working in the area of polymer nano-based membranes and their applications. he various chapters were contributed by prominent researchers from industry, academia and government/private research laboratories across the globe. he book is an up-to-date record on the major indings and observations in the ield of nanostructured polymer membranes and their applications. Chapter 1, which is an introduction to nanostructured polymer membranes and their applications, gives an overview of the state of the art, new challenges and opportunities of nanostructured polymer membranes and their applications, along with a discussion of future trends in polymer membranes. he following chapter lends structure to the previous introductory chapter on membrane technology and applications. It is devoted to the xvii

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science and applications of all kinds of membrane separation processes, including reverse osmosis, nanoiltration, ultrailtration, pervaporation, microiltration, coupled and facilitated transport, membrane distillation, and zeolite and ceramic membranes. Also, the fundamental knowledge and principle of membrane technology and membrane types and modules are presented and the challenges and potential implications of developments for the future of membrane technology are discussed. Polymer membranes for gas and vapor separation are thoroughly explained in Chapter 3. his chapter provides an overview of gas and vapor separation of polymer-based nanomembranes. he authors discuss various topics such as its signiicance and prominent industrial applications, fundamentals and transport of gases and vapors in polymeric membranes, polymeric membrane materials for gas and vapor separations, strategies for molecular design and architecture of polymeric membranes, process modeling and simulation, and challenges and future directions. he next chapter mainly concentrates on membranes for wastewater treatment. It provides a brief overview of membrane-based processes for water reuse and environmental control in the treatment of industrial wastewaters. Applications involving the use of pressure-driven membrane operations, membrane bioreactor, as well as a combination of membrane operations in hybrid systems in the treatment of waste from diferent industries are analyzed and discussed. Polymer electrolyte membrane and methanol fuel cell technologies are explained in Chapter 5. his chapter summarizes the recent advances in proton exchange membrane fuel cell and direct methanol fuel cell technologies. It introduces two major polymer membrane-based fuel cells, proton exchange membrane fuel cells (PEMFCs) and direct methanol fuel cells (DMFCs), followed by the working principles of these fuel cells and modeling and theory of polymer membrane-based fuel cells. Section 5.1 includes a historical introduction and classiications of fuel cells; Sections 5.2 and 5.3 present the basic principle and components in the PEMFC and DMFC, respectively; Sections 5.4 and 5.5 are about the systematic designs of the PEMFC and DMFC; and Section 5.6 explains the fundamentals of electrochemistry and the theoretical model of the PEMFC. Chapter 6 on polymer membranes for water desalination and treatment summarizes the recent progress in the fabrication and modiication of MD membranes, as well as intrinsic aspects of the MD process such as mechanistic fundamentals, conigurations and operating parameters. he chapter also ofers a comprehensive outlook concerning the advances of this technology in water desalination and treatment. Chapter 7 opens with a general overview on pervaporation and a brief description of its history. hen, the main requirements of polymeric

Preface xix membranes in terms of their hydrophobic/hydrophilic nature, crosslinking, swelling degree and thickness of selective layer are described with particular reference to organic-organic separations. Finally, three different case studies on the application of pervaporation in some of the most important organic-organic separations are reported and discussed. Chapter 8 on biopolymer electrolytes for energy devices explains many subtopics such as chitosan-based electrolyte membranes, methyl cellulosebased electrolyte membranes biopolymer electrolyte in lithium polymer batteries, biopolymer electrolyte in supercapacitors, polymer electrolyte in fuel cells, and biopolymer electrolytes in dye-sensitized solar cells (DSSCs). Chapter 9 discusses the phosphoric acid-doped polybenzimidazole membrane, which is a promising electrolyte membrane for hightemperature PEMFC. he authors of this chapter explain the synthesis of polybenzimidazole membranes and their characterization techniques such as molecular weight distribution, thermogravimetric analysis, Fourier transform infrared spectroscopy, nuclear magnetic resonance spectroscopy, permeability, mechanical testing and fuel cell testing. In Chapter 10, natural nanoibers in polymer membranes for energy applications are explained by various works related to many topics such as natural ibers, polymer nanocomposite membranes based on natural ibers, applications of natural ibers nanocomposite membranes in the energy ield, lithium batteries, dye-sensitized solar cells and other energy devices. his chapter also briely introduces natural nanoibers and their production processes and proposes and overviews polymer nanocomposite membranes based on natural ibers, focusing on their application in the energy ield, with a discussion of fundamental research in this area. Chapter 11 on the potential interest of carbon nanoparticles for pervaporation polymeric membranes is devoted to investigations of the inluence of carbon illers, such as pristine and functionalized carbon nanoparticles (e.g., graphene, graphene oxide, carbon nanotubes, fullerene), on the pervaporation transport properties of diferent polymers whose mechanism transport is known to obey the solution-difusion mechanism. When used as nanoillers in membranes’ networks, these carbon particles can be useful for signiicantly improving pervaporation performance. In Chapter 12 on mixed matrix membranes for nanoiltration application, the authors summarize the recent scientiic and technological advances in the development of mixed matrix nanoiltration membranes. hese membranes are classiied according to their preparation method into: (1) asymmetric mixed matrix nanoiltration membranes prepared by phase inversion, (2) thin-ilm nanocomposite (TFN) nanoiltration membranes prepared by interfacial polymerization, and (3) surface coating containing inorganic materials.

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Applications of mixed matrix nanoiltration membranes are also briely discussed. he inal chapter reports on the fundamentals, applications and future prospects of the nanoiltration membrane technique. he chapter’s aim is to provide insight into several distinctive properties of nanoiltration (NF) from the perspective of pore radius as well as surface charge density, which signiies its uniqueness in several ields of applications. Beginning with core fundamentals and principles, recent advances in the synthesis and characterization procedure of NF membranes are thoroughly described. he fundamental transport mechanism in NF membranes is discussed through diferent models, including solution-difusion, preferential sorption surface-capillary low, Donnan equilibrium and dielectric exclusion theory. he editors would like to express their sincere gratitude to all the contributors of this book, whose excellent support resulted in the successful completion of this venture. We are grateful to them for the commitment and sincerity they have shown towards their contributions. Without their enthusiasm and support, the compilation of this book would not have been possible. We would like to thank all the reviewers who have taken their valuable time to make critical comments on each chapter. We would also like to thank the publisher, John Wiley and Sons Ltd. and Scrivener Publishing, for recognizing the demand for such a book, realizing the increasing importance of the area of nanostructured polymer membrane applications, and starting a project on such a new topic, which has yet to be addressed by many other publishers. Visakh. P. M Olga Nazarenko July 2016

1 Nanostructured Polymer Membranes: Applications, State-of-the-Art, New Challenges and Opportunities Visakh. P. M Research Associate, Tomsk Polytechnic University, Department of Ecology and Basic Safety, Tomsk, Russia

Abstract his chapter is a brief account of the various topics presented in Nanostructured Polymer Membranes: Applications. Diferent topics are discussed such as membrane technology; gas and vapor separation of membranes; membranes for wastewater treatment; polymer electrolyte membrane; methanol fuel cell membrane; polymer membranes for water desalination; polymer membrane (optical, electrochemical and anion/polyanion sensors); phosphoric acid-doped polybenzimidazole membranes; natural nanoibers in polymer membranes for energy applications; potential interest of carbon nanoparticles for pervaporation polymeric membranes; mixed matrix membranes for nanoiltration application; and the fundamentals, applications and future prospects of nanoiltration membrane technique. Keywords: Polymer membranes, nanostructured polymer membranes, polymer membrane applications, vapor separation of membranes, nanoiltration application, mixed matrix membranes

1.1 Membranes: Technology and Applications Membrane technologies have gained an important place and made great progress in numerous industries and are used in a broad range of applications. Commercial markets have been spreading very rapidly and throughout the world the most important industrial applications of membranes are Corresponding author: [email protected] Visakh P.M. and Olga Nazarenko (eds.) Nanostructured Polymer Membranes: Volume 2, (1–26) © 2016 Scrivener Publishing LLC

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Nanostructured Polmer Membranes: Volume 2

pure water production and wastewater treatment. Membrane science and technology is interdisciplinary, involving polymer chemistry to develop new membrane materials and structures. here are many membrane applications other than wastewater treatments such as artiicial kidneys (hemodialysis), artiicial lungs (blood oxygenators), and controlled drug delivery and release and other medical applications [1, 2]. In recent years, membranes and membrane separation techniques have grown from a simple laboratory tool to an industrial process with considerable technical and commercial impact. Reverse osmosis is regarded as the most economical desalination process, which has played a crucial role in water treatment such as ultrapure water makeup, pure boiler water makeup in industrial ields, seawater and brackish water desalination in drinking water production, and wastewater treatment and reuse in industrial, agricultural, and indirect drinking water production [3]. Nanoiltration (NF) membranes are mostly of porous structure. he typically excellent performance of NF membrane, such as high lux, small investment and low cost for operation, brings it wider and larger applications [4]. Ultrailtration (UF) membranes have been widely expanded further, such as in the water treatment ield, food and beverage production, the automobile industry, the pharmaceutical industry, the electronic industry, etc. [5]. Chitosan and polyelectrolyte (Naion) membrane also provide equivalent performance in PV dehydration of organics. One major challenge in the chemical industry is the complex separation problem of organic mixtures forming zaeotrope with water. Polyether-polyamide block copolymers (PEBA), combining permeable hydrophilic and stabilizing hydrophobic domains within one material, are successfully used as the organic-water PV separation membrane. Applications for hydrophobic membranes are numerous, such as wastewater treatment, removal of organic traces from ground and drinking water, removal of alcohol from beer and wine, recovery of aromatic compounds in food industries, and separation of compounds from fermentation broth in biotechnology, etc. Some kind of membrane has been developed for the removal and recovery of metals [6–9], including chromium, copper, zinc, cobalt, nickel, strontium and lanthanides, from wastewaters and industry streams. Ceramic membranes have a lot of advantages, conditions under which polymer membranes fail: they are autoclavable, allow sterilization by superheated water, steam, or oxidizing agents, show high temperature resistance, acid and base resistance, solvent resistance, excellent mechanical resistance, and have a long working life and are environment friendly. Zeolites are crystalline microporous silicalite or aluminosilicate materials with a regular three-dimensional pore structure, charge-balanced by

Nanostructured Polymer Membranes 3 cations, which is relatively stable at high temperatures. hey are currently used as catalysts or catalyst supports for a number of high temperature reactions. Membrane technologies are best suited in this context as their basic aspects well satisfy the requirements of process intensiication for a sustainable industrial production. In fact, they are precise and lexible processing techniques.

1.2 Polymer Membranes: Gas and Vapor Separation Various separation technologies have been developed over the years and used in order to respond to the required demands. Polymeric membranes have also been extensively utilized for separation and puriication of hydrogen and helium as valuable light gases. It should be noted that the recovery of hydrogen in reineries is a key approach to meet the increased demand for hydrogen owing to new environmental regulations. Gas and vapor separation using polymeric membranes is an area of growing interest with a variety of prominent applications, particularly in the chemical and petrochemical sector. Polymeric membranes used for air separation typically provide higher permeation rates for oxygen, and as a result, the permeate stream is comprised of oxygen-enriched gas. he main feature of organic vapors that facilitates their separation from a gas stream is their high solubility. Accordingly, rubbery polymers, such as PDMS or silicone rubber, has been introduced and examined as a viable candidate for this purpose [10–12]. Hydrocarbon recovery is considered the largest market for membranes ater acid gas removal and this position is expected to remain for the foreseeable future. Olein/parain separation is an application with considerable potential opportunity for practical applications. One of the most important applications of olein/parain separation is the recovery of propylene vent gas from propylene reactor [13]. Polysulfones are one of the most widely used commercial membrane materials, particularly for a variety of gas separation applications, including hydrogen and air separation [14]. Polyethersulfone is one of the most important polymeric materials for use in gas separation and iltration applications due to its mechanical, chemical and thermal resistances, introducing it as an ideal candidate for asymmetric membranes [15, 16]. Applicability of PESf membrane-based pilot plants for CO2 recovery from LNG-ired boiler lue gas showed that 90% recovery of CO2 with 99% purity was possible. Polyimides are the largest group of organic polymers used for the synthesis of membranes for gas and vapor separation due to their high thermal

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Nanostructured Polmer Membranes: Volume 2

and chemical stability and mechanical strength [17]. hese materials are composed of aromatic dianhydride and diamine monomers and a wide variety of PIs and therefore exist according to varying both dianhydride and diamine [18]. Visser et al. [19] proposed the concept of mutual plasticization in mixed gas studies as opposed to auto-plasticization in pure gas condition. Duthie et al. [20] and Lin and Chung [21] demonstrated that nitrogen, and to a lesser extent oxygen permeability, increased with temperature. However, carbon dioxide permeability decreased with temperature, showing that in this case the relative magnitude of solubility decline was higher than the difusivity augmentation with temperature increase. Substituted polyacetylenes have large oxygen permeability (>1000 Barrer) due to high free volume and unusual free volume distribution derived from their low cohesive energy structure, stif main chain and bulky substituents [22]. he gas separation performance of polymers depended on the degree of the conversion which was afected by the ilm thickness [23]. It was shown that greater chain mobility occurred in thin ilms due to the proximity of the free surfaces and reduced difusional resistance for removal of the volatile compounds of the rearrangement reaction. Moreover, it was revealed that thin ilms of TR membranes derived from 6FDA-HAB polyimides, experienced a greater extent of aging than thick ilms ater 1000 h exposure to diferent gases such as H2, O2, N2 and CH4.

1.3 Membranes for Wastewater Treatment Synthetic polymer membranes are used mostly in the case of wastewater treatment because it is possible to select a polymer suitable for the speciic separation problem from the existing enormous categories of polymers. In comparison with conventional wastewater treatment processes, membrane technology ofers the advantage of selectively removing contaminants based on their sizes. Membranes with diferent pore-size distributions and physical properties remove a wide range of pollutants. he success of membrane operations in wastewater treatment is attributed to the compatibility between diferent membrane operations in integrated systems. he synergy resulting from this integration is the speciic feature of hybrid systems, enhancing the process efectiveness for a particular scenario of wastewater treatment. he wastewater treatment by integrated systems nowadays suggests reducing environmentally harmful efects, decreasing groundwater consumption and energetic requirements, and recovering valuable compounds as a byproduct. Membrane bioreactor (MBR), combining membrane iltration with biological treatment, is recognized as one

Nanostructured Polymer Membranes 5 of the most successful hybrid membrane systems in wastewater treatment. Another interesting application that is a valid alternative to conventional methods is in the treatment of wastewater from the pulp and paper industries containing various solutes with diferent chemical natures. As conventional processes cannot achieve the requirements of water quality for the process, Zhang and coworkers evaluated the performance of an integrated membrane system (IMS), including MBR, UF and RO (reverse osmosis), to treat and reuse paper mill wastewater on a pilot scale [24]. here are several technical drawbacks to the fast commercialization of these innovative technologies in wastewater treatment, including salinity build-up, low permeate lux and membrane degradation [25]. Membrane processes are widely used for the treatment of industrial wastewater due to the increasing costs for processing water as well as wastewater discharge. Probably the most common reason for reducing the water discharge derives from environmental laws that lead industries to use advanced wastewater treatment such as membrane iltration. Membrane iltration processes suggest interesting perspectives and key advantages over conventional technologies in the treatment of wastewaters.

1.4 Polymer Electrolyte Membrane and Methanol Fuel Cell Polymer electrolyte membrane fuel cells (PEMFCs) have partly attained the commercialization stage by reason of their rapid development. here still remain several major challenges. hese are issues related to hydrogen (generation, storage, and infrastructure), system cost, and various technological limitations [26]. Graphite plate satisies many factors as the low ield plate for PEMFC; its size has to be bulky because of its brittle and porous characteristics. he graphite plate is unit for some applications where lightweight is important. he use of thin metallic sheets in the PEMFC promises is promising for mobile and transportation applications. hey are also cheaper than the graphite plate. Composite materials are another option for the low ield plate in the PEMFC. hey are more beneicial than the graphite and metallic bipolar plates with regard to corrosion resistance, lexibility and low weight. hey can be produced by more economical processes, such as compression, transfer and injection molding processes. Precious metals like platinum and gold are typically considered as the low ield plate, since they have excellent electrical conductivity and high collusive resistance [27]. he high cost of these materials makes their commercialization diicult. Aluminum, titanium, and

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nickel are alternative materials for the low ield plate in the PEMFC. hese materials have many advantages, including low density, low cost, and high strength. Higher grades of stainless steel are also efective. A formation of the passive oxide ilm, which reduces surface conductivity or increases contact resistance, is an important problem. A suitable PEM should fulill the following various requirements: high proton conductivity, good electrical insulation, high mechanical and thermal stability, good oxidative and hydrolytic stability, cost efectiveness, good barrier property, low swelling stresses, and capability for fabrication in the MEA. he major application of the PEMFC focuses on transportation due to its potential impact on the environment [28]. Most automobile companies have developed fuel-cell vehicles (FCVs). Direct liquid fuel cells (DLFCs) have been mainly developed as the portable power source for various electronic devices, including notebooks, cellular phones, and tablets, because the handling and storage of liquids are easier than that of hydrogen [29]. During the past two decades, they have received much attention due to several advantages, including the comparatively high volumetric energy density of methanol. Methanol used as fuel can also be produced from various sources like biomass, natural gas, or coal. Naion membranes have excellent properties for the PEMFC, however, several limitations, such as their high production cost and poor ion conductivity with high temperature, still remain. To overcome these challenges, various methods have been studied. hese include a modiication of conventional PFSA membranes and development of new polymer membranes such as polyarylene sulfone, polyeter ether ketone (PEEK), polyimide, polyphenyloxide (PPO), polybenzimidazole (PBI), and polyvinylidene luoride (PVDF) [30]. A water crossover, which is a water permeation from the anode and the cathode through the membrane, may cause two problems in the DMFC. One is water loss from the anode and the other the water looding at the cathode. he water looding is especially responsible for the reduction in the DMFC performance. To solve this problem, diverse water management strategies at the cathode have been developed.

1.5 Polymer Membranes for Water Desalination and Treatment Desalination technologies can be grouped according to three categories: (i) those involving a phase change (e.g., distillation), (ii) those interacting with selective membranes (e.g., reverse osmosis), and (iii) those employing

Nanostructured Polymer Membranes 7 electric ields (e.g., capacitive deionization) [31]. Membrane distillation (MD) and forward osmosis (FO) are considered potential alternatives to the leading desalination technology (i.e., RO), namely for the desalination of high-salinity brines or industrial wastewaters, where RO presents limitations [32]. Hydrophilic polymers and non-polymeric materials such as metal, glass, carbon materials and ceramics (e.g., zirconia and alumina) were also used in the fabrication of MD membranes [33]. When designing MD modules, the heat transfer coeicient, thermal conductivity and heat low become crucial parameters to achieve a high performance. he heat transfer mechanism varies according to the selected MD coniguration [34]. In order to quantify the magnitude of the efect of both phenomena in the overall performance of MD, the temperature and the concentration polarization coeicients can be determined [35]. Generally, MD systems are applied to the desalination of seawaters, brines and saline waters. his process is able to produce high purity water under near complete rejection of nonvolatile electrolytes (e.g., sodium chloride and potassium chloride) and nonelectrolytes (e.g., organics). Highly competitive module conigurations have been designed and scaled-up to a size where low cost applications become possible [36] and have been tested with commercially available membranes. In addition to solar-powered MD, geothermal energy applications for water desalination have also been considered [37]. Geothermal energy is not suitable for traditional desalination technologies due to its low enthalpy [38]. he increasing need for progress and fulillment of quality and environmental management principles associated with the demanding constraints imposed by the concept of process sustainability, have stimulated the development of MD-integrated systems. he FO-MD hybrid system has also shown potential in the treatment of more demanding solutions such as wastewaters, heavy metal-contaminated solutions, and oily or dyed wastewater. Mozia and Morawski [39] obtained a high quality permeate solution when studying the removal of ibuprofen sodium salt from tap water, with a capillary MD module employing nine polypropylene (PP) membranes.

1.6 Biopolymer Electrolytes for Energy Devices A polymer electrolyte is a membrane consisting of organic or inorganic salts dissolved in a polymer. Among the biopolymers, polysaccharides have good potential as hosts for ionic conduction since they are abundant, cheap, eco-friendly and may replace synthetic polymers for use in energy

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generation and storage devices. Chitosan is an example. Blending chitosan with other polymers or incorporating reinforcement illers can improve the mechanical stability of the chitosan membrane. A lithium-ion-conducting polymer membrane is used as both the electrolyte and separator. As the polymer electrolyte must function as both the electrolyte and separator in a lithium polymer battery, a number of properties are critical for its success. From an electrochemical point of view, Koksbang et al. [40] and Gray [41] have listed some requirements that a polymer electrolyte must satisfy. he cycle life of a battery is basically the number of continuous charge and discharge cycles within a speciied voltage that the battery can operate without reaching the maximum capacity fade allowed for a certain application. A graph of discharge capacity versus cycle numbers can be plotted in order to study the cycling characteristics of a battery. A large number of studies have been carried out on lithium batteries having gel polymer electrolytes in which the liquid electrolyte has been immobilized by incorporation into a polymer matrix. Membrane ilms of hexanoyl chitosan-lithium triluoromethanesulfonate (LiCF3SO3), hexanoyl chitosan-LiCF3SO3-ethylene carbonate (EC) and hexanoyl chitosan-LiCF3SO3-EC-propylene carbonate (PC) were prepared by solution casting technique. he conductivity of the order of 10-4 S cm-1 is suitable for battery application. hus, electrochemical cells based on LiCoO2/MCMB couple were assembled using the EC-PC plasticized membrane ilm. he discharge characteristics of the cell can also be inluenced by the resistance of the electrolyte [42]. Capacitors can be generally categorized into: (a) conventional capacitors (CC), (b) electrolytic capacitors and (c) electrochemical capacitors (EC). For a conventional capacitor, an insulating ilm is sandwiched between two metallic electrodes. Aluminium and tantalum electrolytic capacitors are examples of electrolytic capacitors. he characteristics of the electrolytic capacitors can be found in [43]. Types of materials with a high amount of pseudocapacitance include polyaniline and polypyrrole [44], polythiophene [45] and polyacetylene [46]. Activated carbon, carbon aerogels and carbon nanotubes (CNTs) are used in electric double layer capacitors (EDLCs). Chitosan is receiving a lot of attention as ionic conducting host materials for membrane electrolyte in low to moderate temperature PEMFC. he use of low-cost chitosan-based membranes might reduce the fuel cell cost since it is the membrane that is the most expensive component of a polymer electrolyte-based fuel cell. he intrinsic ionic conductivity of chitosan-based electrolyte membranes needs to be further improved for fuel cell applications. Apart from that, the mechanical strength and shelf life of chitosan also need to be studied.

Nanostructured Polymer Membranes 9

1.7 Phosphoric Acid-Doped Polybenzimidazole Membranes Most of the studies on membrane degradation of the PEMFC have reported mechanical failure such as delamination, swelling of the membrane, hydrogen crossover and occurrence of hotspots during high temperature and low humidity operations [47]. Several eforts have been recently made to mitigate the membrane durability issues by replacing Naion with polybenzimidazole (PBI) [48–50]. Invented by Brinker in 1959, PBI is an aromatic heterocyclic base polymer and is generally synthesized by step-growth polymerization of monomers such as a tetraamine and a dicarboxylic acid. It is produced via condensation polymerization. he solubility of the PBIs is generally limited to strong acids and polar aprotic solvents, which cannot donate hydrogen. hus, PBI can be mixed with strong acids or strong bases [51]. At higher temperatures it reduces or eliminates the need for water molecules which can act as a solvating agent and proton transport medium, commonly required in any fuel cell membrane. he protonation of PBI using phosphoric acid is well studied in the literature [52]. he performance of PBI membranes, developed during the early 1970s, showed superior mechanical stability largely due to their higher molecular weight. Low molecular weight PBI developed by diferent synthesis schemes showed poor mechanical property and tensile strength. he increased interest in PBI/PA-doped PBI membranes in the PEMFC industry necessitates identifying a synthesis, characterization of molecular weight distribution (MWD) and morphological changes of the membrane during accelerated stress testing. By adding toluene to the solvent, any reversible change prevailing in the column can be corrected during the analysis. Data interpretation and calibration curve procedures of MWD are discussed in detail in the literature [53, 54]. he decomposition temperature of PBI reported by Sukumar et al. is 550 °C. Importantly, small weight losses observed at 350 °C on the sample are expected to be contributed by crosslinking reactions, which release VOC when PBI undergoes phase change from a thermoplastic state to a thermoset resin. Several researchers have described molecular stretching of PBI [55]. hese two sites can take part in the reactions, which involves hydrogen bonding with appropriate solvent and polymer. he interactions of hydrogen bonding of PBI with diferent acid groups and chemicals that can form miscible polymeric blends are described in the literature by several researchers. Transesterification of phosphate and polyphosphate systems is reported to have contributed to this efect. Transesterification interchange

10 Nanostructured Polmer Membranes: Volume 2 reaction is assumed to be the rationale for the proton conduction mechanism. However, the details of this mechanism are still not well understood. In order to improve the conductivity of the PA-doped PBI membrane at higher temperature and lower humidity, doping level of PA in the PBI should be optimized [56]. He et al. obtained the doping level and conductivity relations of PBI membranes at diferent humidities and temperatures [57]. horough plane permeability measurement of any polymer can be made using the procedure discussed by Gostick et al. [58] and Dhanushkodi et al. [59]. He et al. employed this method to study the permeability of PBI at diferent temperatures [60]. he mechanical properties of the doped membranes made by sol-gel process showed very high tensile strength and percentage of elongation. Zhang et al. studied the performance of the PA-doped PBI membrane under fuel cell conditions by preparing a catalyst-coated membrane with Pt loading of 1.7  mg  cm−2 at both anode and cathode [61]. Membrane researchers need to examine the oxidative stability, ion-exchange capacity and chemical stability of PBI-based MEAs at higher temperature and lower relative humidity (RH) using accelerated stress testing protocols for PEMFC applications. Analyzing the chemical compositions of water and gas from fuel cell outlet during testing can help improve the design and lifetime of PBI-based MEAs in fuel cell applications. his efort can improve the conductivity and durability of the membrane. From the beginning of fuel cell technology, the thrust of membrane studies has been to develop highly durable and stable membranes.

1.8 Natural Nanoibers in Polymer Membranes for Energy Applications Natural ibers may be classiied by their origin from plants, animals or minerals. he most difused ibers in common applications are cotton, lax and hemp; although sisal, jute, kenaf and coconut are also widely used [62]. Animal ibers generally comprise proteins and a few cases of polysaccharides. Mineral ibers are naturally occurring ibers or slightly modiied ibers procured from minerals. Because of the strong tendency for intra- and intermolecular hydrogen bonding, bundles of cellulose molecules aggregate to ibrils, which form either highly ordered (crystalline) or less ordered (amorphous) regions. hese microibrils pass through several crystalline regions (about 60 nm in length) and as a result of further aggregation of microibrils. his also means that cellulose is

Nanostructured Polymer Membranes 11 relatively inert during chemical treatments and also that it is soluble only in a few solvents. he most common cellulose solvents are cupriethylenediamine (CED) and cadmiumethylenediamine (Cadoxen), whereas less well known but powerful solvents are N-methylmorpholine, N-oxide and lithium chloride/dimethylformamide [63]. Cellulose ibers can undergo diferent treatments and processes according to the use they are designated for. hey can be treated in order to obtain short ibers or pulped to produce the slurry used for paper production [64]. Among the natural polymers, chitin is also deserving of particular interest since it is the second most abundant biopolymer. Some research groups have presented works based on materials containing chitin nanoibers for energy applications; thus a brief overview of its structure and properties will now be presented. Chitin microibrils are composed of alternating crystalline and amorphous domains; thus, also for chitin, submitting the material to a strongly acidic environment causes the longitudinal cutting of these microibrils, allowing dissolution of amorphous domains [65]. Nature produces a variety of nanoibers such as collagen triple helix ibers, ibroin ibrils, keratin ibrils, crystalline cellulose and so on. he extraction of nanoibers from biomass is considered to be a “top-down” approach in contrast to “bottomup” processes, such as electrospinning, in which nanoibers are artiicially produced. Diferent methods for extracting the crystalline regions from the biomass exist. he recovered material occurs in the form of polysaccharide nanocrystals, most commonly rod-like nanocrystals (cellulose whiskers, nanocrystalline cellulose, chitin nanowhiskers) [66]. Diferent sources of cellulose were studied and used for producing nanocrystals: tunicin, valonia, cotton, wood pulp, sugar beet pulp, sisal. he impact of hydrolysis conditions on the size and stability of nanocrystals has been investigated and the optimal process conditions have been determined [67] to be at a sulfuric acid concentration of 63.5% (w/w) and a reaction time of approximately 130 min. Advancement in the ield is necessary considering that the market always requires technological innovation for large-scale production, with shorter duration times, higher yield and milder preparation conditions. By omitting the hydrolysis step and only submitting the ibers to high mechanical shearing forces, disintegration of the ibers occurs, leading to a material called microibrillated cellulose (MFC). hese microibrils consist of individual nanoparticles with a lateral dimension of around 5 nm, which aggregate in structures whose lateral dimensions range between about 10 and 30 nm [68]. Abe et al. isolated cellulose nanoiber bundles of approximately 15 nm in width from

12 Nanostructured Polmer Membranes: Volume 2 wood by a method consisting of the isolation of nanoibers and subsequent grinding [69]. Isogai et al. converted wood cellulose to nanoibers of 3–4 nm width and high aspect ratio by TEMPO oxidation. Such a process formed carboxylate groups on the cellulose surface with electrostatic repulsions arising due to anionic charges at the cellulose surface, which caused complete disintegration into nanoibers [70]. Concerning the preparation of composites, one of the main issues that has to be overcome is the homogeneous dispersion of cellulose nanocrystals within a continuous polymeric matrix. It is known that the nanoparticles tend to strongly aggregate because of the hydroxyl groups present on their surface. Interparticle interactions can cause problems during the nanocomposite preparation processes, since the aggregation results in the loss of the nanoscale and limits the potential of mechanical reinforcement. he irst demonstration of the reinforcing efect of cellulose nanocrystals in a membrane prepared from a latex of poly(styrene-co-butyl acrylate) was reported by Favier et al. [71]. his indicates that the stifness of the nanocomposite materials is due to aggregates of cellulose nanocrystals that, above the percolation threshold, can form a three-dimensional continuous pathway through the nanocomposite ilm. Bras et al. [72] reported a study on cellulose whiskers from diferent sources in which they investigated the impact of the geometrical characteristics on the mechanical properties of the percolating networks. Correlating the behavior of a nanocomposite membrane with that of a pure cellulose ilm, it was proven that it is important to choose high aspect ratio whiskers in order to also have an eicient reinforcing efect in nanocomposites. he literature shows that cellulose whiskers with diferent aspect ratios isolated from diferent sources, like cotton, tunicin, sisal, etc., were proposed as a reinforcing phase for various polymer matrices such as poly(styrene-co-butyl acrylate), PLA (24), PVA and others. In many cases it was seen that the mechanical properties are inluenced by the disintegration process; in fact, ibrils with higher surface area result in stifer nanocomposites with high tensile strength. he dispersion in solvents, monomers or polymers is generally promoted by a decrease of the surface energy of the nanoparticles which, as previously stated, can be achieved by coating their surface with a surfactant or by surface chemical modiication. As discussed by Svagan et al. [73], the porosity in a glycerol nanocomposite ilm due to the presence of microibrillated cellulose caused a reduction in the moisture difusivity of the composite membrane because of three main reasons, which were the geometrical limitation given

Nanostructured Polymer Membranes 13 by cellulose, the swelling constraints due to high-modulus/hydrogenbonded MFC network, and strong molecular interactions between iller and matrix. Mainly considered for paper and packaging products, but also for construction, automotive, furniture, pharmaceutical and cosmetic products, up to now neat nanocellulose membranes were used for several diferent applications, ranging from membrane for high quality sound to electronics or biomedicals. Cellulose in the form of ibers has been largely used in energy devices as separator or as structural material for the development of different components [74]. Cellulose has also been proposed as a component of gel or solid polymer electrolytes in which nanoparticles have been used as a reinforcement of the polymer membranes. Ionic conductivity of these membranes was quite consistent with the speciications of lithium batteries (about 10-6 s/cm at room temperature) and was always lower than in the absence of the iller; nevertheless the authors claimed that the beneit of the electrolyte contribution to the internal resistance of the battery exceeded a factor of 100, since such reinforcement might enable, with the same safety level, the lowering of the electrolyte thickness by roughly a factor of 100. Crosslinking is one of the most common methods used to disrupt polymer crystallinity and to ensure mechanical properties. It is classically performed to provide both low-temperature conductivity and hightemperature mechanical stability. he crosslinking density of the polymeric network was found to decrease with the whisker content, most probably due to an interfacial efect, while the presence of tunicin whiskers did not afect the thermal properties of the matrix. he ilms proposed displayed substantially improved mechanical properties, when compared to the nonreinforced lithium perchlorate/ethylene oxide-epichlorohydrin, with small reductions in conductivities. In 2010, Alloin et al. [75] proposed a detailed study on the behavior of nanocomposite PEO membranes reinforced with whiskers from various origins and sisal microibrils prepared by casting process from aqueous suspensions. Nanocomposite membranes were proposed as a combination of nanocellulose and conductive polymers. Liew et al. proposed diferent cellulose-based conductive membranes for supercapacitor applications. A irst study was based on electrodeposited cellulose nanocrystalpolypyrrole PPY-NC nanocomposites [76]. Such material showed high capacitance values even when relatively thick membranes were used. Jiang et al. [77] proposed nanocomposite Naion membranes based on bacterial nanocellulose for both direct methanol fuel cells and proton exchange membrane fuel cells.

14 Nanostructured Polmer Membranes: Volume 2

1.9

Potential of Carbon Nanoparticles for Pervaporation Polymeric Membranes

he use of pervaporation polymeric membranes for the separation of small molecules has already been known at the lab scale for over a century; the principle of the phenomena involves a so-call “sorption-difusion” model. Membrane separation is based on non-equilibrium thermodynamics; a constant driving force is maintained between the upstream and downstream sides of the membrane. Researchers have started to study and prepare inorganic membranes, especially for dehydration applications. Indeed, such membranes can exhibit a strong water ainity without any swelling. Permeances of these membranes were relatively small in relation to the technological challenge linked to the preparation of thin active layer at an industrial scale. he choice of the membrane material, which is suitable for a speciic task, depends on many factors; the main ones are the composition of a separated mixture and the conditions of the process involving a membrane. he most common commercial membranes are polymeric membranes, due to their low cost, high workability and good mechanical strength. he main drawback of polymeric membranes is the insuicient selectivity obtained with high lux due to coupling efects. On the other hand, for inorganic membranes some disadvantages of polymeric membranes are absent. Indeed, inorganic particles have excellent thermal and chemical stability, good mechanical resistance to abrasion, and good transport properties. To overcome the disadvantages of the polymeric and inorganic membranes, the preparation of so-called mixed matrix membrane (MMM) have been proposed in which inorganic illers are dispersed in the polymer matrix. Among the new inorganic modiiers, a special place is occupied by carbon nanoparticles, due to their unique physical and chemical characteristics. he polymer must have good transport properties, processability in the product (membrane), and mechanical strength. Many models have been published to describe and explain pervaporation results; the most popular are based on Fick’s laws [78] Stefan-Maxwell theory [79] and Maxwell’s electric conduction analogy [80]. he most common fullerene is fullerene C60 because of its good symmetry and stability. Fullerene C60 is the most accessible for practical use. Under the introduction of C60 additives to polymers and their interactions, the π-electron system of the fullerene molecule undergoes minor changes. Carbon nanotubes (CNTs) are essentially graphite cavity, rolled into a cylinder. he CNTs can be multilayered, consisting of two or more

Nanostructured Polymer Membranes 15 concentric cylindrical shells arranged coaxially around the central hole with a hollow interlayer separation of 0.34 nm, as in graphite. It should be noted that graphene oxide (GO) is used for the modiication of polymers because GO has better dispersion in the polymeric matrix. A detailed description of GO, including structure illustrations, models and properties of particles, is presented in the review by Dreyer et al. [81]. Among the presented particles (fullerene, carbon nanotubes, graphene oxide, fullerenol), carbon nanotubes (CNTs) are the particles that are very diicult to introduce into the polymer matrixes without additional functionalization because CNTs are not soluble in solvents and have, as a rule, bad dispersion at high content in polymers. he fullerene concentration in the polymer, the separation factor and permeability increase for membrane containing up to 5% fullerene; further increase of fullerene concentration (to 10%) causes the decrease of separation factor because the fullerene aggregates inside of the polymer ilms and causes the inhomogeneity of the structure. he introduction of pristine fullerene C60 (up to 2%) to polyphenylene oxide was studied in the work. he transport properties were studied during the separation of binary ethanol-water and ethyl acetate-water mixtures and the quaternary system modeling of the reaction mixture of the esteriication process of ethyl acetate. An increase of GO content causes the decrease of the separation factor, which can be due to agglomeration of GO nanosheets. Smaller nanosheet size, more structural defects, and less oxygen-containing groups of GO could construct more water channels, while less negative charges and more non-oxide regions further improve the permselectivity of water channels. Due to hydrophobic interaction between the aromatic ring of the PA and fullerene, the polar groups of the polymer were available for the sorption of methanol molecule that led to the increase of membrane performance. he insigniicant pervaporation properties for CNT-containing membranes can be caused by bad dispersion of nanoparticles without additional functionalization and are due to the chosen mixture for the separation. Inclusion of graphene oxide (GO), as a rule, is carried out without additional treatment, as this particle properly changes the hydrophilicity of the system by itself. In some cases, additional functionalization can be done. In the works of Penkova et al. [82] an MMM based on a composite of polyvinyl alcohol with fullerenol (polyhydroxy fullerene) C60(OH)22–24 and C60(OH)12 crosslinked with maleic acid were developed. Carbon nanotube was dispersed by using b-cyclodextrin, while in the work of Peng et al. [83], membranes were modiied by chitosan-wrapped carbon nanotube. he obtained results demonstrated that under inclusion of chitosan-wrapped carbon nanotube, a higher improvement of transport

16 Nanostructured Polmer Membranes: Volume 2 properties was noticed and a smaller amount of CNT is needed for the increase of membrane performance (2 wt% [CNT-chitosan]), as compared with 6 wt% CNT dispersed by -cyclodextrin. In the work of Qiu et al. [84] the solubility and the difusion coeicient of membranes in water, ethanol, and 90% ethanol/water mixtures were obtained. Compared with the calculated difusion coeicient (D90, the CNTs ater acid treatment were additionally functionalized by diisobutyryl peroxide. he permeability of both polymers was improved ater introduction of functionalized CNT. For modiied membranes, the increase of both transport parameters was noticed. he authors explained the rise of the lux by the creation of additional nanochannels in the membrane matrix at the packaging of hydrophilic chains in the presence of CNTs and by transport through the inner cavity of CNT. In the work of Yeang et al. [85], a more complex method for modifying the nanoparticles was used. PVA-functionalized MWCNT was bulk aligned on the poly(vinylidene luoride) (PVDF) membrane by a simple iltration method and further coated with CS to form a novel threelayer nanocomposite membrane. Composite membranes composed of a separating layer of polyvinylamine-poly(vinylalcohol) incorporated with functionalized CNTs supported on a microporous polysulfone substrate were fabricated for the dehydration of ethylene glycol by pervaporation. he use of GO does not lead to the signiicant improvement of transport characteristics but the introduction of GO into the polymer clearly induces the increase of selectivity. Upon the addition of CNT into diferent polymers, various results can be observed that are connected with diferent CNT treatments and the cylindrical CNT structure that is essentially different from two-dimensional carbon particles such as graphene and graphene oxide. he most signiicant improvement in selectivity and lux was observed in the work for membrane based on sodium alginate modiied up to 2 wt% CNTs functionalized by chitosan.

1.10 Mixed Matrix Membranes for Nanoiltration Application Nanoiltration (NF) is a complicated process which is afected by microhydrodynamic and interfacial events occurring at the membrane surface and within the membrane nanopores. Compared to ultrailtration and reverse osmosis, nanoiltration has not always been clearly deined and described. he membrane charge arises from the dissociation of ionizable groups on the membrane surface and within the membrane pores [86]. Polymeric membranes have relatively low thermal and chemical stability.

Nanostructured Polymer Membranes 17 Ceramic membranes typically present more stable lux performance (not vulnerable to compaction), proper mechanical strength, high tolerance at extreme conditions and are more resistant to organic solvents but sufer the disadvantages of brittleness and diiculty in scaling up [87]. he polymer and inorganic phases may be linked via van der Waals forces and covalent or hydrogen bonds, thus fabricating membranes with diferent chemistries [88]. Over the recent decades of intensive membrane preparation research, diferent techniques have been proposed to generate selective and permeable ilms. Among several approaches to production of mixed matrix membranes, blending nanoparticles in the casting solution of phase inversion technique is one of the most applicable approaches. Carbon nanotube and other carbon-based nanomaterials have been considered to be excellent candidates for fabrication of mixed matrix membranes due to their superior properties such as high lexibility, low mass density, and the efective π-π stacking interaction between carbon nanotube and aromatic compounds [89]. he treated MWCNTs exhibited good compatibility with PES and caused increased hydrophilicity and water lux of the prepared membrane. he MWCNTs inluenced mean pore size and porosity of membrane. In addition, fouling of membrane resulting from bovine serum albumin (BSA) iltration could be reduced by importing MWCNTs to the blend membrane. he ultrathin layer is formed when polymerization occurs at the interface of two immiscible aqueous and organic solvents containing reactants. During the last few years, much research has been focused on improving the performance of TFC membranes in regard to selectivity (solute rejection) without any appreciable change in membrane productivity (lux) by altering the thin ilm layer. Wu et al. [90] used an improved interfacial polymerization process to prepare MWCNTs/polyester TFN membranes. he improved process is facilely done by immersing the support membrane into the organic phase before the conventional process of interfacial polymerization. Polyetherimide (PEI)/amino-functionalized silica nanocomposite membrane was used by Namvar-Mahboub and Pakizeh as support layer to fabricate TFC membrane [91]. In order to obtain the stable support, nanocomposite membranes with diferent amounts of modiied silica (0–20 wt%) were prepared. he commonly used surface modiication technologies include thin-ilm coating, self-assembled monolayers, and polymer grating by chemical treatment (e.g., UV or plasma treatment). he inherent hydrophilicity of poly(vinyl alcohol) (PVA), together with its great chemical, thermal, and mechanical stability, makes it an appropriate polymer for fabricating nanoiltration membranes. Baroña et al. [92]

18 Nanostructured Polmer Membranes: Volume 2 prepared a new type of TFN membrane for nanoiltration by incorporating aluminosilicate single-walled nanotubes (SWNTs) within the PVA matrix.

1.11 Fundamentals, Applications and Future Prospects of Nanoiltration Membrane Technique Membrane technology is being utilized extensively in surface and ground water puriication, sea and brackish water desalination, wastewater reclamation, hazardous industrial waste treatment, food and beverage production, gas and vapor separation, energy conversion and storage, air pollution control, hemodialysis and protein concentration, etc. Transport of the components through membrane can be afected by convection or depends on the applied driving force that is provided either by concentration, pressure, temperature or electric potential diference. he membrane preferentially sorbs the solvent on the membrane surface. hus, a layer of almost pure solvent is formed on the membrane surface and drained of under an applied pressure. Extensive research is going on for the preparation and optimization of the membrane processes based on charged NF membrane. Swamy et al. [93] evaluated the eiciency of polyamide NF membrane for the treatment of bulk drug industrial eluent. Computational luid dynamics (CFD) is one of the major computer-based tools for carrying out the in-depth analysis of the luid low mechanism inside the membrane low channels and transport phenomena of momentum and mass transfer in any membrane separation process. Nanoiltration membrane can be used in diferent conigurations such as lat sheet, spiral wound, hollow iber, tubular, etc. Among all these types of conigurations, spiral wound membranes are widely used for industrial separation processes, eluent treatment, desalination and much more, due to their low cost, high packing density, and maximum lifetime. Several membrane separation processes, such as ultrailtration (UF), nanoiltration (NF), and reverse osmosis (RO), are widely applied for the pretreatment. UF is found to be useful in removal of a certain amount of TDS and the bacteria present in the water. Nanoiltration membranes separate particles of sizes in the range of 0.01–0.001 μ, and molecular weight of 200 and above. he neutral ions present in the water are separated by size exclusion, whereas the negative ions are rejected by the negatively charged membrane due to electrostatic repulsion. In the dairy industry,

Nanostructured Polymer Membranes 19 the nanoiltration process helps to enhance the quality of dairy products and improves process eiciency and proitability. Here it is generally used for whey processing, recovery of lactose, and lactic acid separation. Nanoiltration is used to concentrate and partially demineralize liquid whey. Conventionally, a combined ion exchange and electrodialysis process [94] is used for demineralization of whey, whereas NF can replace both these conventional processes for moderate demineralization of whey. In the irst stage, (UF) protein is concentrated from wastewater and then UF permeate is sent to NF for lactose concentration and reusable water production. his process is beneicial for utilization of dairy wastewater for bioenergy production by algae cultivation. he dense population, especially in urban areas, has brought the adequate availability of usable water from natural resources to a crucial stage where the development of new concepts and technologies to conserve water resources and reuse of the waste eluent water has become one of the prime issues of concern to scientists.

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20 Nanostructured Polmer Membranes: Volume 2 10. P. Bernardo, E. Drioli, G. Golemme, Membrane gas separation: A review/state of the art. Ind. Eng. Chem. Res., 48, 4638–4663, 2009. 11. C.A. Scholes, G.W. Stevens, S.E. Kentish, Membrane gas separation applications in natural gas processing. Fuel, 96, 15–28, 2012. 12. H. Bum Park, E.M.V. Hoek, V.V. Tarabara, Gas separation membranes, in: Encyclopedia of Membrane Science and Technology, John Wiley & Sons, Inc., 2013. 13. R.W. Baker, Future directions of membrane gas separation technology. Ind. Eng. Chem. Res., 41, 1393–1411, 2002. 14. D.F. Sanders, Z.P. Smith, R. Guo, L.M. Robeson, J.E. McGrath, D.R. Paul, B.D.  Freeman, Energy-eicient polymeric gas separation membranes for a sustainable future: A review. Polymer, 54, 4729–4761, 2013. 15. C.Y. Feng, K.C. Khulbe, T. Matsuura, A.F. Ismail, Recent progresses in polymeric hollow iber membrane preparation, characterization and applications. Sep. Purif. Technol., 111, 43–71, 2013. 16. A.L. Ahmad, A.A. Abdulkarim, B.S. Ooi, S. Ismail, Recent development in additives modiications of polyethersulfone membrane for lux enhancement. Chem. Eng. J., 223, 246–267, 2013. 17. S.P. Nunes, K.V. Peinemann, Gas separation with membranes, in: Membrane Technology, pp. 39–67, Wiley-VCH Verlag GmbH, 2001. 18. M.G. Dhara, S. Banerjee, Fluorinated high-performance polymers: Poly(arylene ether)s and aromatic polyimides containing triluoromethyl groups. Prog. Polym. Sci., 35, 1022–1077, 2010. 19. T. Visser, M. Wessling, Auto and mutual plasticization in single and mixed gas C3 transport through Matrimid-based hollow iber membranes. J. Membr. Sci., 312, 84–96, 2008. 20. X. Duthie, S. Kentish, C. Powell, K. Nagai, G. Qiao, G. Stevens, Operating temperature efects on the plasticization of polyimide gas separation membranes. J. Membr. Sci., 294, 40–49, 2007. 21. W.-H. Lin, T.-S. Chung, Gas permeability, difusivity, solubility, and aging characteristics of 6FDA-durene polyimide membranes. J. Membr. Sci., 186, 183–193, 2001. 22. K. Nagai, T. Masuda, T. Nakagawa, B.D. Freeman, I. Pinnau, Poly[1(trimethylsilyl)-1-propyne] and related polymers: Synthesis, properties and functions. Prog. Polym. Sci., 26, 721–798, 2001. 23. H. Wang, T.-S. Chung, he evolution of physicochemical and gas transport properties of thermally rearranged polyhydroxyamide (PHA). J. Membr. Sci., 385–386, 86–95, 2011. 24. Y. Zhang, C. Ma, F. Ye, Y. Kong, H. Li, he treatment of wastewater of paper mill with integrated membrane process. Desalination, 236, 349–356, 2009. 25. W. Luo, F.I. Hai, W.E. Price, W. Guo, H.H. Ngo, K. Yamamoto, L.D. Nghiem, High retention membrane bioreactors: Challenges and opportunities. Bioresource Technol., 167, 539–546, 2014. 26. J.-H. Wee, Applications of proton exchange membrane fuel cell systems. Renew. Sust. Energ. Rev., 11, 1720, 2007.

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22 Nanostructured Polmer Membranes: Volume 2 44. T. Liu, L. Finn, M. Yu, H. Wang, T. Zhai, X. Lu, Y. Tong, Y. Li, Polyaniline and polypyrrole pseudocapacitor electrodes with excellent cycling stability. Nano Letters, 14, 2522–2527, 2014. 45. Q. Lu, Y. Zhou, Synthesis of mesoporous polythiophene/MnO2 nanocomposite and its enhanced pseudocapacitive properties. J. Power Sources, 196, 4088–4094, 2011. 46. Yu. M. Volkovich, A.A. Mikhailin, D.A. Bograchev, V.E. Sosenkin and V.S. Bagotsky, Studies of supercapacitor carbon electrodes with high pseudocapacitance, in: Recent Trend in Electrochemical Science and Technology, Ujjal Kumar Sur (Ed.), InTech, 2012. 47. S. Kundu, L.C. Simon, M.W. Fowler, Comparison of two accelerated Naion degradation experiments. Polym. Degrad. Stabil., 93, 214–224, 2008. 48. H. Pu, G. Liu, Synthesis and solubility of poly (N-methylbenzimidazole) and poly (N-ethylbenzimidazole): Control of degree of alkylation. Polym. Int., 54, 175–179, 2004. 49. C. Hasiotis, V. Deimede, C. Kontoyannis, New polymer electrolytes based on blends of sulfonated polysulfones with polybenzimidazole. Electrochim. Acta, 46, 2401–2406, 2001. 50. J.A. Kerres, Development of ionomer membranes for fuel cells. J. Membr. Sci., 185, 3–27, 2001. 51. Y. Yuan, F. Johnson, I. Cabasso, Polybenzimidazole (PBI) molecular weight and Mark-Houwink equation, J. Appl. Polym. Sci., 112, 3436–3441, 2009. 52. H. Pu, W.H. Meyer, G. Wegner, Proton transport in polybenzimidazole blended with H3PO4 or H2SO4. J. Polym. Sci. B: Polym. Phys., 40, 663–669, 2002. 53. Z. Grubisic, P. Rempp, H. Benoit, A universal calibration for gel permeation chromatography. J. Polym. Sci. B: Polym. Phys., 34, 1707–1713, 2003. 54. D. Wif, M. Gehatia, A. Wereta, Molecular weight distribution and mechanical behavior of PBI ibers. J. Polym. Sci.: Polym. Phys. Ed., 13, 275–284, 1975. 55. P. Musto, F.E. Karasz, W.J. MacKnight, Fourier transform infra-red spectroscopy on the thermo-oxidative degradation of polybenzimidazole and of a polybenzimidazole-polyetherimide blend, Polymer, 34, 2934–2945, 1993. 56. J. Wainright, J.T. Wang, D. Weng, R. Savinell, M. Litt, Acid-doped polybenzimidazoles: A new polymer electrolyte. J. Electrochem. Soc., 142, L121–L123, 1995. 57. R. He, Q. Li, G. Xiao, N.J. Bjerrum, Proton conductivity of phosphoric acid doped polybenzimidazole and its composites with inorganic proton conductors. J. Membr. Sci., 226, 169–184, 2003. 58. J.T. Gostick, M.W. Fowler, M.D. Pritzker, M.A. Ioannidis, L.M. Behra, In-plane and through-plane gas permeability of carbon iber electrode backing layers. J. Power Sources, 162, 228–238, 2006. 59. S. Dhanushkodi, M. Fowler, M. Pritzker, X.-Z. Yuan, H. Wang, Degradation and diagnostic analysis of gas difusion layers under humidity cycling, in: Meeting Abstracts, pp. 349–349, he Electrochemical Society, 2010.

Nanostructured Polymer Membranes 23 60. R. He, Q. Li, A. Bach, J.O. Jensen, N.J. Bjerrum, Physicochemical properties of phosphoric acid doped polybenzimidazole membranes for fuel cells. J. Membr. Sci., 277, 38–45, 2006. 61. J. Zhang, Y. Tang, C. Song, J. Zhang, Polybenzimidazole-membrane-based PEM fuel cell in the temperature range of 120–200 °C. J. Power Sources, 172, 163–171, 2007. 62. J.K. Pandey, S.H. Ahn, C.S. Lee, A.K. Mohanty, M. Misra, Recent advances in the application of natural iber based composites. Macromol. Mater. Eng., 295(11), 975–989, 2010. 63. T. Heinze, A. Koschella, Solvents applied in the ield of cellulose chemistry: A mini review. Polímeros, 15, 84–90, 2005. 64. K.W. Britt, Handbook of Pulp and Paper Technology, Reinhold Pub. Corp., 1964. 65. S. Ifuku, Z. Shervani, H. Saimoto, Chitin Nanoibers: Preparations and Applications, 2013. 66. M. Mariano, N. El Kissi, A. Dufresne, Cellulose nanocrystals and related nanocomposites: Review of some properties and challenges. J. Polym. Sci. B: Polym. Phys., 52(12), 791–806, 2014. 67. D. Bondeson, A. Mathew, K. Oksman, Optimization of the isolation of nanocrystals from microcrystalline cellulose by acid hydrolysis. Cellulose, 13(2), 171–180, 2006. 68. N. Lavoine, I. Desloges, A. Dufresne, J. Bras, Microibrillated cellulose – Its barrier properties and applications in cellulosic materials: A review. Carbohydr. Polym., 90(2), 735–764, 2012. 69. K. Abe, S. Iwamoto, H. Yano, Obtaining cellulose nanoibers with a uniform width of 15 nm from wood. Biomacromolecules, 8(10), 3276–3278, 2007. 70. T. Saito, M. Hirota, N. Tamura, S. Kimura, H. Fukuzumi, L. Heux, A. Isogai, Individualization of nano-sized plant cellulose ibrils by direct surface carboxylation using TEMPO catalyst under neutral conditions. Biomacromolecules, 10(7), 1992–1996, 2009. 71. V. Favier, G.R. Canova, J.Y. Cavaillé, H. Chanzy, A. Dufresne, C. Gauthier, Nanocomposite materials from latex and cellulose whiskers. Polym. Adv. Technol., 6(5), 351–355, 1995. 72. J. Bras, D. Viet, C. Bruzzese, A. Dufresne, Correlation between stifness of sheets prepared from cellulose whiskers and nanoparticles dimensions. Carbohydr. Polym., 84(1), 211–215, 2011. 73. A.J. Svagan, M.S. Hedenqvist, L. Berglund, Reduced water vapour sorption in cellulose nanocomposites with starch matrix. Compos. Sci. Technol., 69(3–4), 500–506, 2009. 74. F. Sharii, S. Ghobadian, F.R. Cavalcanti, N. Hashemi, Paper-based devices for energy applications. Renew. Sust. Energ. Rev., 52, 1453–1472, 2015. 75. M. Schroers, A. Kokil, C. Weder, Solid polymer electrolytes based on nanocomposites of ethylene oxide-epichlorohydrin copolymers and cellulose whiskers. J. Appl. Polym. Sci., 93(6), 2883–2888, 2004.

24 Nanostructured Polmer Membranes: Volume 2 76. S.Y. Liew, W. hielemans, D.A. Walsh, Electrochemical capacitance of nanocomposite polypyrrole/cellulose ilms. J. Phys. Chem. C, 114(41), 17926–17933, 2010. 77. G.-P. Jiang, J. Zhang, J.-L.Qiao, Y.-M. Jiang, H. Zarrin, Z. Chen, F. Hong, Bacterial nanocellulose/Naion composite membranes for low temperature polymer electrolyte fuel cells. J. Power Sources, 273, 697–706, 2015. 78. J.G. Wijmans, R.W. Baker, he solution-difusion model: A review. J. Membr. Sci., 107, 1–21, 1995. 79. P. Izák, L. Bartovská, K. Friess, M. Šípek, P. Uchytil, Description of binary liquid mixtures transport through non-porous membrane by modiied Maxwell-Stefan equations. J. Membr. Sci., 214, 293–309, 2003. 80. J. Maxwell, A Treatise on Electricity and Magnetism, vol II, vol. 1, 333–335, 1873. 81. D.R. Dreyer, S. Park, C.W. Bielawski, R.S. Ruof, he chemistry of graphene oxide, Chem. Soc. Rev., 39, 228–240, 2010. 82. A.V. Penkova, S.F.A. Acquah, M.E. Dmitrenko, B. Chen, K.N. Semenov, H.W. Kroto, Transport properties of cross-linked fullerenol-PVA membranes. Carbon N. Y., 76, 446–450, 2014. 83. F. Peng, F. Pan, H. Sun, L. Lu, Z. Jiang, Novel nanocomposite pervaporation membranes composed of poly(vinyl alcohol) and chitosan-wrapped carbon nanotube. J. Membr. Sci., 300, 13–19, 2007. 84. S. Qiu, L. Wu, G. Shi, L. Zhang, H. Chen, C. Gao, Preparation and pervaporation property of chitosan membrane with functionalized multiwalled carbon nanotubes. Ind. Eng. Chem. Res., 49, 11667–11675, 2010. 85. Q.W. Yeang, S.H.S. Zein, A.B. Sulong, S.H. Tan, Comparison of the pervaporation performance of various types of carbon nanotube-based nanocomposites in the dehydration of acetone. Sep. Purif. Technol., 107, 252–263, 2013. 86. A.W. Mohammad, Y.H. Teow, W.L. Ang, Y.T. Chung, D.L. Oatley-Radclife, N. Hilal, Nanoiltration membranes review: Recent advances and future prospects. Desalination, 356, 226–254, 2015. 87. J. Yin, B. Deng, Polymer-matrix nanocomposite membranes for water treatment. J. Membr. Sci., 479, 256–275, 2015. 88. H. Siddique, E. Rundquist, Y. Bhole, L.G. Peeva, A.G. Livingston, Mixed matrix membranes for organic solvent nanoiltration, J. Membr. Sci., 452, 354–366, 2014. 89. V. Vatanpour, S.S. Madaeni, R. Moradian, S. Zinadini, B. Astinchap, Fabrication and characterization of novel antifouling nanoiltration membrane prepared from oxidized multiwalled carbon nanotube/polyethersulfone nanocomposite. J. Membr. Sci., 375, 284–294, 2011. 90. H. Wu, B. Tang, P. Wu, Optimization, characterization and nanoiltration properties test of MWNTs/polyester thin ilm nanocomposite membrane. J. Membr. Sci., 428, 425–433, 2013.

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2 Membranes: Technology and Applications Yang Liu1* and Guibin Wang2* 1

College of Material Science and Engineering, Northeast Forestry University, Harbin, China 2 College of Chemistry, Engineering Research Center of High Performance Plastics, Ministry of Education, Jilin University, Changchun, China

Abstract Membrane technology has emerged as an important unit operation in many separation processes over the last few decades. Compared with various traditional separation technologies, membrane technology is regarded as the most economical, efective and feasible separation method since it involves a number of attractive features, such as low energy consumption and cost, high separation eiciency, unique selectivity, efective recovery and concentration of valuable materials, and separation and removal of pollutants, etc. his chapter is devoted to the science and applications of all kinds of membrane separation processes, including reverse osmosis, nanoiltration, ultrailtration, pervaporation, microiltration, coupled and facilitated transport, membrane distillation, zeolite and ceramic membranes. Also, the fundamental knowledge and principle of membrane technology and membrane types and modules are presented. And the challenges and potential implications of developments for the future of membrane technology are discussed as well. Keywords: Membrane science and technology, membrane processes, membrane materials, polymer, separation, industrial application

2.1 Introduction In the 21st century, because of the vastly expanding world population and the increase in the average standard of living, increasing water demand is causing a non-sustainable explosion of energy consumption, which is

*Corresponding authors: [email protected]; [email protected] Visakh P.M. and Olga Nazarenko (eds.) Nanostructured Polymer Membranes: Volume 2, (27–88) © 2016 Scrivener Publishing LLC

27

28 Nanostructured Polmer Membranes: Volume 2 related to the deterioration of water resource quality and quantity, the greenhouse efect and the scarcity of fossil hydrocarbons [1]. Water and energy are going to be the most precious resources in our world. hus, the efects of environmental, economic, social, political and technical factors have led to the rapid deployment of various generation methods and technologies to solve these water and energy problems. Every year, 15% of the energy produced worldwide is employed for separation processes, and most of them are ineicient [2]. Fortunately, among the separation technologies available today, membrane technology is regarded as the most economical and efective separation process since it involves a number of attractive features. he science related to the subject of membrane has been expanding rapidly in the last decade. his area of science is important, especially in new areas such as environmental control, wastewater, nanotechnology, pharmacy, biotechnology, and chemical industry. In particular, the applications of water treatment are very signiicant. First, separations with membranes do not require additive, they can be performed isothermally at low temperature without phase change, thus allowing temperature-sensitive solutions to be treated without the constituents being damaged or chemically altered, which means more energetically eicient, lower energy consumption and more competitive operating cost compared to other thermal separation processes [1, 3]. his is important in the food and drug industry and in biotechnology where temperaturesensitive products have to be processed. In addition, the main advantages of membrane technology as compared with other unit operations are related to the unique separation principle and transport selectivity. Also, upscaling and downscaling of membrane processes as well as their integration into other processes are easy [4]. herefore, membrane technologies have gained an important place and made great progress in numerous industries and used in a broad range of applications, and commercial markets have been spreading very rapidly throughout the world. he membrane markets are rather diverse—food, pharmaceutical, chemical industry, medical devices (tissue repair)—and the most important industrial market segments are pure water production and wastewater treatment. Membrane technology seems to be coming to the center of the water treatment and desalination technologies. he combined U.S. market for membranes used in liquid and gas separations applications is estimated at approximately $1.7 billion in 2010, and is forecast to grow by 2015 to reach $2.3 billion. he membrane and module sales of the world market in 1998 were estimated at more than $4.4 billion, approximately $6.3 billion in 2004, $9.5 billion in 2009, and grew by 2012 to reach $11.0 billion. It is forecast to increase to $21.2 billion in 2016 and reach $25.0 billion in 2018 [5].

Membranes: Technology and Applications 29

2.1.1 Membrane Process A membrane is an interphase between two adjacent phases acting as a selective barrier, regulating selective components to pass through when mixtures of diferent kinds of components are driven to its surface [4]. Membrane science and technology is interdisciplinary, involving polymer chemistry to develop new membrane materials and structures; physical chemistry and mathematics to describe the transport properties and mechanisms of different membranes using mathematical models to predict their separation characteristics; and chemical engineers to design separation processes for large-scale industrial utilization [6]. Membrane processes are designed to carry out physical or physicochemical separations. he most important element in a membrane process, however, is the membrane itself. To gain an understanding of the signiicance of the various structures used in diferent separation processes, a brief discussion of the basic properties and functions of membranes, and the driving forces and luxes involved, is essential. he membrane processes are being put to hard work today, their classes are very fragmented and complicated, and the industrial applications are divided into six main subgroups: reverse osmosis (RO), ultrailtration (UF), microiltration (MF), pervaporation (PV), gas separation (GS), and electrodialysis (ED) [7]. With the exception of ED, in which ions are separated under the inluence of an electric ield, each of the above membrane processes are pressure driven. Although the various applications of membrane processes have increased rapidly in recent years, systematic studies of membrane processes can be traced to the eighteenth century philosophers of science, and the use of membranes extended back several decades. Since the 1960s, RO membranes have been used for the desalination of water, with more widespread use of nanoiltration (NF) for sotening and the removal of total organic carbon (TOC) dating to the late 1980s. he irst achievements in practical implementation of membrane-based gas separation processes occurred between 1970 and 1980. However, the commercialization of backwashable MF and UF membrane processes for the removal of particulate matter in the early 1990s has had the most profound impact on the use of all types of membrane processes for drinking water treatment [7, 8]. Most membrane applications are water based, there also exist gas-liquid and gas-gas separation processes, although these are more recent developments and have not yet achieved widespread implementation. Today, concurrent with the development of these industrial applications of membranes are the medical separation processes, in particular, the artiicial kidneys (hemodialysis), artiicial lungs (blood oxygenators), and controlled drug delivery and release. he use of membranes in artiicial organs

30 Nanostructured Polmer Membranes: Volume 2 has become a major life-saving method. More than 800,000 people are now sustained by artiicial kidneys and a further million people undergo openheart surgery each year, a procedure made possible by development of the membrane blood oxygenator [7]. he sales of these devices comfortably exceed the total industrial membrane separation market. In recent years, membranes and membrane separation techniques have grown from a simple laboratory tool to an industrial process with considerable technical and commercial impact. In this chapter, four developed industrial membrane processes are discussed, and the status of all of these processes is summarized in Table 2.1. he developed industrial membrane separation processes are MF, UF, RO, and ED, which are all well established, and the membrane market is served by a number of experienced companies. A typical pore size distributions and rejection characteristics of these membranes used for these separation processes are shown in Figure 2.1. Moreover, two developing industrial membrane processes (GS and PV) are also listed in Table 2.1. Today, membranes are used on a large scale to produce potable water from the sea by RO, to clean industrial eluents and recover valuable constituents by ED, to fractionate macromolecular solutions in the food and drug industry by UF, to remove urea and other toxins from the blood stream by dialysis in an artiicial kidney, and to release drugs such as scopolamine, nitroglycerin, etc., at a predetermined rate in medical treatment [7, 9]. Although membrane processes may be very diferent in their modes of operation, in the structures used as separating barriers, and in the driving forces used for the transport of the diferent chemical components, they have several features in common which make them attractive as a separation tool. In many cases, membrane processes are faster, more eicient and more economical than conventional separation techniques. Membranes can also be “tailor-made” so that their properties can be adjusted to a speciic separation task. Reverse osmosis (RO) is the tightest possible membrane process in liquid-liquid separation. Water is in principle the only material passing through the membrane; essentially all dissolved and suspended material is rejected. he more open types of RO membranes are sometimes confused with NF. True NF rejects only ions with more than one negative charge, such as sulfate or phosphate, while passing singly charged ions. NF also rejects uncharged, dissolved materials and positively charged ions according to the size and shape of the molecule in question. Finally, the rejection of sodium chloride with NF varies from 0–50 percent, depending on the feed concentration. In contrast, “loose RO” is an RO

Drinking water from sea, brackish or groundwater, production of ultrapure water for electronics and pharmaceutical industries Drinking water from brackish water; some industrial applications too

Developed

Ions

Water

Electrodialysis Electrically charged ilms

Voltage diference 1–2 V

Tubular, Spiral would, Plate-andframe

Developed

Dissolved salts, Glucose, Amino acids

Water

Dense Solutiondifusion

Reverse osmosis

Pressure diference 100–1000 psi

Removal of colloidal material from wastewater, food process streams

Developed Pressure Tubular, diference Hollow 20–100 psi iber, Spiral would, Plate-andframe

Finely Water, dissolved Macromolecules, microporous solutes DNA, Viruses, 1–100 μm Albumin, Colloids, Protein

Ultrailtration

(Continued)

Removal of suspended solids, bacteria in pharmaceutical, electronics industries

Developed

Tubular, Hollow iber

Pressure diference 5–50 psi

Typical application

Status

Water, dissolved Suspended solids, Microiltration Finely solutes Bacteria, microporous Starch, Blood 0.1–0.10 μm cells Clay

Material passed Material retained

Membrane module

Type of membrane

Driving force

Membrane process

Table 2.1 Summary of the established membrane separation technologies.

Membranes: Technology and Applications 31

Pressure diference 100–1000 psi

Vapour pressure 1–10 psi

Permeable gases Impermeable and vapour gases and vapors

Permeable Impermeable micro-solutes micro-salutes and solvents and solvents

Gas separation Dense, Solutiondifusion

Pervaporation

Dense, Solutiondifusion

Driving force

Type of Membrane

Material passed Material retained

Membrane Process

Table 2.1 Cont. Membrane module

Typical application

Developing Dehydration of solvents (especially ethanol)

Developing Nitrogen from air, hydrogen from petrochemical/ reinery vents, carbon dioxide from natural gas propylene and VOCs from petrochemical vents

Status

32 Nanostructured Polmer Membranes: Volume 2

Membranes: Technology and Applications 33 Pressure bar

Membrane pore size m

Reverse Osmosis (RO)

30–60

10–4–10–3

Nano Filtration (NF)

20–40

10–3–10–2

Ultra Filtration (UF)

1–10

10–2–10–1

Micro Filtration (MF)

|ΔHs|) and thus permeability usually increases with temperature [39]. his can be observed in strongly size-sieving polymeric membranes, especially those used in air separation and H2 removal from hydrocarbon mixtures. In contrast, ED is higher than the absolute values of ΔHs in PDMS and PTMSP membranes used for the removal of volatile organic compounds from light gases or air [38].

3.4 Polymeric Membrane Materials for Gas and Vapor Separations 3.4.1

High-Performance Engineering Polymers

Numerous polymeric materials have been investigated over the past few decades for the development of high-performance membranes for a variety of gas and vapor separation applications [13, 20]. Polysulfones (PSfs), polyethersulfones (PESs), cellulose acetates (CAs), polyphenylene oxides (PPOs) and polycarbonates (PCs), are by far the most prevalent polymers examined for this purpose. However, polyimides (PIs), polyamide-imides and polyetherimides are among the newer generations. he prominent families of the polymers used for various gas and vapor separation applications are discussed in the subsequent sections.

3.4.1.1

Polysulfones (PSfs)

Polysulfones are one of the most widely used commercial membrane materials, particularly for a variety of applications, including hydrogen and air separation [20, 42]. Monsanto irst developed hollow-iber membranes based on PSf in 1997 [42]. Polysulfones are composed of diphenylene sulfone repeating units providing high backbone rigidity and mechanical strength as well as thermal and chemical durability [20]. Membranes based on PSf possess more resistance toward CO2 plasticization than PPO and Matrimid 5218 at 8 atm due to more compact backbone and lower free volume [38]. Scholes et al. [44] studied the inluence of membrane thickness

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on the plasticization and demonstrated that plasticization potential of CO2 increased with decreasing thickness. It was revealed that H2 permeability in PSf membranes is higher than cellulose acetate and lower than Matrimid 5218. However, due to the trade-of between permeability and selectivity they were in similar distances from the performance upper bound limit. he separation performance of polysulfone in the CO2/N2 separation was investigated by Hwang et al. [45], showing permeance and selectivity of 120 GPU and 26.4 respectively. Accordingly, it was concluded that it could be an excellent choice for multi-stage membrane systems. In addition the efect of minor components such as CO and NO in mixed gas separation of CO2/N2 mixture showed that the permeabilities of both species was reduced as a result of competitive sorption and CO had a stronger competitive sorption efect on CO2 than N2, leading to the reduced selectivity [46].

3.4.1.2 Polyethersulfones (PESs) Polyethersulfone is one of the most important polymeric materials used in gas separation and iltration applications due to its mechanical strength, as well as, chemical and thermal resistances, introducing it as an ideal candidate for asymmetric membranes [47–48]. Although PES membranes were mainly applied for iltration separations rather than gas and vapor separations, they showed good potentials in gas separations as well. It was demonstrated that by the optimization of their dual-layer hollow-iber fabrication condition, efective membranes for oxygen enrichment and natural gas separation could be produced [49–51]. Accordingly, Li et al. [49] fabricated PES hollow ibers that showed impressive CO2/CH4 selectivity of 49.8 in the mixed gas condition, while this value was 40 for dual-layer Matrimid 5218/PES hollow ibers fabricated by Jiang et al. [52]. Recently, Choi et al. [48] investigated the applicability of PES membrane-based pilot plants for CO2 recovery from LNG-ired boiler lue gas and showed that 90% recovery of CO2 with 99% purity was achievable.

3.4.1.3 Cellulose Acetates (CAs) Cellulose acetate-based membranes were among the irst generation of commercial membranes for natural gas separation due to desirable transport properties. hese materials have been used for the removal of CO2 and H2S from natural gas and also for the separation of CO2 from hydrocarbons [6, 20, 54]. It was reported that cellulosic hollow-iber membranes showed superior performance compared to polysulfone, PPO and silicone rubber in propane/propylene separation. However, these materials only performed well under either extremely low or extremely high temperature

114 Nanostructured Polmer Membranes: Volume 2 treatments, with ethylcellulose (EC) being the only exception [1]. Currently Cynara and UOP Separex supply cellulose acetate hollow-iber and spiralwound modules respectively for CO2 removal where space is limited such as on ofshore platforms [6]. CA polymers are semicrystalline materials having varying degrees of acetylation of the hydroxyl groups on cellulose. he degree of substitution (DS) plays an important role in the separation performance of CA membranes [6, 20]. According to Sanders et al. [20], by varying the DS from 1.57 to 2.85 CO2 permeability increased from 1.84 to 6.56 Barrer, which was because of the higher free volume due to the mentioned substitution. One of the most important issues regarding CA is that it is susceptible to plasticization in the presence of condensable gases such as carbon dioxide and light oleins [1, 6, 20, 55]. Plasticization caused CO2/ CH4 selectivity to decrease in mixed gas experiments from 35 to 31 as CO2 partial pressure increased from 4.1 to 12.7. Although this efect reduced the methane recovery, it reduced the required membrane area for a deinite amount of CO2 removal [20]. his was attributed to the competitive sorption in the Langmuir sites under mixed gas condition [6].

3.4.1.4 Polyimides (PIs) Polyimides are the largest group of organic polymers used as membranes for gas and vapor separations due to their high thermal and chemical stability and mechanical strength [13, 56–58]. hese materials are comprised of aromatic dianhydride and diamine monomers and a wide variety of PIs exist according to varying both dianhydride and diamine [6, 57, 59]. Chemical structures of several commercial polyimides are presented in Table 3.7. he irst commercial application of PIs was the separation of helium from natural gas and CO2 from various gas streams, ofered by DuPont. Hollow-iber PI membranes for gas separations (N2, H2, CO2 and dehydration) are commercially available from UBE America Inc. [57]. PIs showed good permeability and selectivity in gas and vapor separations as a result of their rigid molecular chains caused by strong intermolecular bonds [60]. hese characteristics made them more attractive than cellulose acetate as membrane materials [6]. Polyimides and copolyimides displayed remarkable separation performance for both gas, (e.g. CO2/CH4, CO2/N2, N2/CH4 and H2/CO2) [2, 6, 20, 24], and vapor separations, (e.g. olein/parain) [1, 29, 61]. One important concern about the performance of PIs was reported to be the loss in the separation performance from pure to mixed gas experiments, which was attributed to the competitive sorption of penetrants in the mixed gas experiments [6]. his issue was observed by David et al. [62]

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Table 3.7 he chemical structure of commercial polyimides used for gas and vapor separation [57]. Polyimide

Chemical structure

6FDA-durene

O

O H3C

CF3

F3C

N

N O

O

Lenzing P84 (HP polymer Inc., Austria now Evonik)

CH3

O

O

n CH3

H3C

O

N

N n

O

O

O

O

O CH3

N

N O

Matrimid 5218 (Huntsman Switzerland)

O

O

H3C

O

N

n

O

O O

O O

N

N

O N

N

H

H

n

O

O

U1tem Polyetherimide (Plastics International)

CH3

N

O

Torlon (Solvay Advanced Polymer)

n

O

O

O N

N O

O

n

O

Kapton (DuPont Electronics)

O

O O N

N

O n

O

O

when H2/CO2 selectivity reduced from 4.2 in pure gas experiments to 2.7 in mixed gas environment using Matrimid 5218 membranes. However, the presence of other noncondensable gases, such as nitrogen and carbon monoxide, did not afect the hydrogen permeability. his efect was the result of preferential sorption of CO2 in the Langmuir sites of the excess free volume portion of the polymer in accordance with the dual-mode sorption. Staudt-Bickel and Koros [33] also showed that in mixed gas experiments up to 20% and 50% lower selectivities were obtained for ethylene/ethane and propylene/propane separation using 6FDA-6FpDA compared to the ideal selectivity. Dual-mode sorption model can predict sorption behavior of glassy polymers, as mentioned in Section 3.3.2, and is widely used to

116 Nanostructured Polmer Membranes: Volume 2 predict the pressure-dependent difusion and solubility coeicients of polyimide membranes. he decrease in the permeability coeicient of O2, N2, CO2 and CH4 with increasing pressure that can be explained by this model was reported by Lin and Chung in the case of 6FDA-Durene [63]. However, at higher concentrations plasticization will occur due to the swelling of the polyimide membranes in the presence of condensable species such as CO2 [58, 64–65] and C3H6 [30, 66]. It is expected that increasing temperature to suppress plasticization due to reduction in the sorption coeicient. his efect was displayed by Duthie et al. [67], who studied the CO2 plasticization in the 6FDA-TMPDA in the temperature range of 3 to 112 °C. Duthie et al. [67] and Lin and Chung [63] demonstrated that nitrogen, and to a lesser extent oxygen permeability, increased with temperature. However, carbon dioxide permeability decreased with temperature, showing that in this case the relative magnitude of solubility decline was higher than the difusivity augmentation with temperature increase [67]. Nevertheless, Das and Koros [30] showed that by increasing propylene temperature from 35 to 70 °C plasticization still occurred in 6FDA-6FpDA polyimide with the onset of 2.5 atm. Chan et al. [66] also reported that both propane and propylene caused plasticization in 6FDA-1,5-NDA at 5 atm, while olein and parafin permeability increased when temperature increased from 30 to 50 °C. According to Wang et al. [68], also permeability of 6FDA-1,5-NDA towards O2, N2, CO2 and CH4 increased when temperature was raised as a result of increased difusivity, which was more pronounced than decreased solubility at 10 atm. Plasticization also depends on the membrane thickness, as indicated by Horn and Paul [69]. Accordingly, thin ilms (182 nm) of Matrimid 5218 were more sensitive to CO2 plasticization than thick ones (20 μm) and onset of plasticization was 6 atm in thin ilms, while it was 14 atm for thick ilms. Moreover, they observed that permeability decreased in thick ilms instead of an expected increase, due to the competition between physical aging and plasticization. Xia et al. [70] reached the same conclusion regarding the efect of Matrimid 5218 ilm thickness on the CO2-induced plasticization. hey also showed that the efect of CO2 plasticization and competitive sorption/permeation on thin Matrimid 5218 ilms depended on the feed mixture and was more severe for CO2/ CH4 mixture than CO2/N2 mixture. Plasticization in mixed gas experiments was also studied in the case of Matrimid 5218 hollow ibers by Visser et al. [37], who proposed the concept of mutual plasticization in mixed gas studies as opposed to auto-plasticization in pure gas condition. Accordingly, propane permeance increased due to propylene plasticization. he same phenomenon occurred in the separation of H2/CO2

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mixture using Matrimid 5218  hollow iber, where H2 permeation was higher in the presence of CO2. It was also concluded that this efect is more pronounced for hollow-iber modules than lat ilms of Matrimid 5218, since maximum decay in the permeation of H2 was 21% for dense ilm and 45% for hollow-iber membranes [71]. Barsema et al. [72] also observed that the plasticization behavior of asymmetric P84 membranes difered from the dense ones. For CO2/N2 separation, when pressure increased up to 30 bar no plasticization occurred, but when pure CO2 was used plasticization occurred even at low pressures. his was attributed to the competition phenomena in the presence of N2. Nevertheles, polyimides would be more promising in natural gas separation. However, they were reported to be more impacted than CA under ield conditions where trace contaminants are present in the feed gas. his is attributed to the fact that the contribution of difusivity factor in polyimides is relatively higher due to the more ordered structure in polyimides [55].

3.4.1.5 Polyetherimides/Polyamideimides (PEIs/PAIs) Polyetherimides are high-performance engineering plastics consisting of imide groups giving chain rigidity and heat resistance with ether linkage providing processability and melting characteristics [57]. Peng et al. [73] produced dual-layer Extem (polyetherimide) hollowiber membranes with an ultra-thin selective layer for O2/N2 separation and obtained selectivity of 6.15. hey reduced the substructure resistance by replacing the sublayer with the same polymer of low concentration. Simons et al. [74] demonstrated that the extent of swelling of the ODPA PEI ilms was very low compared to Matrimid 5218 and SPEEK at elevated pressures (almost 20 bar) in CO2/CH4 separation. In addition, the separation performances of ODPA PEI with 1, 2 or 3 para-arylether substitutions were all superior to that of a widely used commercially available glassy polymer; Matrimid 5218. he selectivity observed for ODPA PEI was 40 (at 35 bar) while that of Matrimid 5218 was 30 at the same pressure [74]. Moreover, PEI hollow ibers also showed selectivity of 45 for He/ N2 separation. However, volatile non-solvent additives were added to dope solution to prepare highly selective membranes [75]. PEO-containing poly(ether-imide) copolymers were reported to be important materials in CO2 separation due to their mechanical and ilm-forming properties. he PEO domains were reported to be responsible for the sorption and difusion of gases [76]. Polyamide-imide materials are commonly synthesized by the reaction between a diamine and a trimellitic anhydride derivative. he

118 Nanostructured Polmer Membranes: Volume 2 combination of amide and imide bonds in the polyamide-imides structure leads to exceptional chemical, thermal and mechanical resistances. An example of a polyamide-imide is Torlon , commercialized by Solvay [57, 77]. CO2/CH4 and O2/N2 selectivities of 44 and 7.7 were obtained by Kosuri et al. [78] using asymmetric hollow-iber membranes fabricated from Torlon 4000T-LV. hese selectivities were lower than that of dense ilms due to the substructure resistance. Hollow-iber membranes showed CO2 resistance up to 1100 psi, which was higher than that of Ultem (400) and Matrimid 5218 (174) [78]. However, hollow ibers fabricated from Torlon AI-10 showed poor performance for propylene/propane separation. he membranes had such low permeance that no selectivity could be determined, while a selectivity of 16 was reported for Matrimid 5218 hollow ibers [36].

3.4.2 Novel Polymers 3.4.2.1 Fluoropolymers and Fluorinated Polymers Perluoropolymers possess extraordinary chemical and thermal resistance due to the high energy of C-F bonds (485 kJ/mol) in the substituent groups of the polymer backbone and the protective sheath of luorine atoms around the carbon backbone [6, 20, 79]. Fluoropolymers are therefore unafected by most chemicals, including acids, bases, organic solvents, oils and strong oxidizers [79]. Since the mid-1980s, there have been limited studies on gas or vapor transport through dense luoropolymers due to the semicrystalline nature and lack of solubility in common solvent that lead to low permeability and poor processability of the existing materials like tetraluoroethylene (PTFE), hexaluoropropylene (FEP), and polychlorotriluoroethylene (PCTFE) [6, 20, 79]. However, the development of amorphous luoropolymers, such as Telon AF (DuPont), Hylon AD (Solvay) and Cytop (Asahi Glass Company), that possess bulky luorine substituent groups to inhibit chain packing created new opportunities for these materials for gas and vapor separations [6, 20]. Z-TOP is another perluoropolymer membrane that was commercialized by Membrane Technology and Research, Inc. (MTR) [6]. he chemical structure of the mentioned perluoropolymers are presented in Table 3.8. Telon AF-2400 has ultra-high free volume and is permeable to permanent gases similar to PTMSP. Permeabilities of 990 and 489 Barrer were reported for this material to oxygen and nitrogen respectively. However, Telon AF-2400 is more permeable to small gas molecules rather than large condensable gases and the main contribution to selectivity in this polymer

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Table 3.8 he chemical structures of selected perluoropolymers [79].

Polymer

Glass transition Nitrogen Density temperature permeability (g/cm3) ( C) (Barrer)

Chemical structure

PTFE

CF2

CF2

2.1

30

1.3

1.74

240

480

1.92

134

24

2.03

108

n

Telon AF

F3C

CF3

O

Hylon AD

O

F

F

F

F

O

O

O

F

x

x

F

F

C

C

F

F

F

F

C

C

F

F

1–x

1–x

CF3

Cytop CF2

CF O

CF2 x CF C F CF2 CF2 z

5.0

CF2 y

n

is due to difusivity selectivity, while PTMSP behavior is completely different [12]. Both Telon AF-2400 and Hylon AD-80 are reported to resist against CO2 plasticization up to 20 bar. Moreover, they also have poor wetting properties for water and hydrocarbons and can be used where water and/or hydrocarbons coexist in the feed [6]. However, Telon AF-2400 membranes have been reported not to be proper candidates for vapor/gas separations, such as the separation of air from organic vapors. Because this membrane material is sensitive to vapor activity, and therefore gas selectivity varies upon feed pressure [12]. he fabrication method and the nature of the precursor materials are the critical issues regarding perluolropolymers. It was indicated that these materials have strong solvent retention properties leading to signiicant changes in permeation properties [6]. In order to solve the latter problem leading to high fabrication costs, luorination of other commercially available polymers that are cheaper was proposed [6, 80]. Fluorine-treated polymer consists of mainly luorinated layer and unmodiied layer separated by

120 Nanostructured Polmer Membranes: Volume 2 a very thin transition layer separated by a very thin transition layer. Rate of luorine penetration from luorinated layer to untreated layer determine the rate of formation of the latter, which is afected by luorine partial pressure and treatment temperature [80]. Kharitonov et al. [81] studied the efect of luorination conditions on the properties of Matrimid 5218 and showed that hydrogen atoms were replaced by luorine, double bonds were saturated with luorine and at least one C-N bond in the ive-membrane ring was disrupted [81]. Syrtsova et al. [82] studied the efect of luorination on the performance of Matrimid 5218 hollow iber and polyvinylthrimethylsilane (PVTMS) lat sheet asymmetric and composite membranes. It was observed that He/CH4 and He/N2 and CO2/CH4 selectivities increased by factor of 10, 5 and 35 respectively at the expence of negligible decrease in permeability for He and CO2 [80, 82].

3.4.2.2

Siloxane

One of the high-performance polymeric materials that was studied widely due to its versatile properties is silicone-based polymers. hese materials are commonly synthesized via ring-opening polymerization of a trimer or tetramer. Polydimethylsiloxane (PDMS), which is the most permeable rubbery polymer, is an important siloxane polymer for gas and vapor separations due to the chain lexibility, rotational mobility, large free volume and low Tg (−123 °C). One of the most important applications of PDMS is the organic vapor/gas separation [12, 83]. PDMS was more permeable to more condensable molecules, such as C3H8, rather than other gases (i.e., CO2, CH4, O2, N2 and H2) due to higher solubility coeicient of C3H8 [84,  85]. Raharjo et al. [86] and Pinnau and He [87] demonstrated that high concentrations of n-C4H10 in PDMS membranes led to enhanced permeability of both CH4 and n-C4H10 due to the vapor-induced swelling of the polymer. In addition, the relative permeability increase became greater as the size of the penetrant in the mixture increased and in turn vapor/gas selectivity increased by increasing vapor concentration in the feed [87]. It was demonstrated that both solubility and difusivity of C3H8 increased by increasing upstream pressure, those values of other gases remained constant and therefore C3H8/gas selectivity increased. At upstream pressure of 7 atm selectivities of C3H8/CO2, C3H8/CH4, C3H8/H2, C3H8/O2 and C3H8/ N2 were reported to be 4, 13, 18, 20 and 36 respectively for the synthesized PDMS [84]. It was, however, observed that by increasing the feed pressure, permeation properties of lighter gases (H2 and CH4) increased, while C3H8 difusivity decreased due to the competitive difusion in mixed gas experiments [88]. In fact, feed pressure has a dual efect on the selectivity. Higher

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selectivity at higher pressures in the pure gas experiments is because the permeability of more soluble penetrants increases with pressure while the of less soluble penetrants stays constant or even decreases. However, in the mixed gas experiments the efect of increasing feed pressure on difusion coeicient of lighter molecules is more signiicant than that on solubility of the heavier molecules, and therefore vapor/gas selectivity decreases by increasing feed pressure [89]. Besides, difusivity and permeability of lighter gases increased with increasing temperature, while difusivity of more condensable molecules increased slightly and conversely their solubility increased largely. Consequently, increasing temperature has an inverse efect on the vapor/gas selectivity [88–89]. In the separation of oleins from nitrogen using PDMS it was found that the solubility of oleins and their solubility selectivity over nitrogen were afected by the critical temperature of the permeating gases and were enhanced by increasing pressure and decreasing temperature [90–91]. Selectivity values of 6.5, 16.7 and 100 for C2H4, C3H6 and C4H8 over N2 were reported at 1 atm and 25 °C [90]. Jha et al. [92] examined the performance of another siloxane-based material, poly(methylphenyl siloxane) (PMPS), in the CO2 separation from N2 and Ar. his copolymer has slightly more rigid structure by having 10% methylphenyl groups that cause steric hinderance to chain motions. Since it was found that the extent to which the structure of the polymer is afected by the CO2 depended on the rigidity of the polymer, PMPS is considered as a better candidate than PDMS for CO2 separation. Gomes et al. determined the permeation properties of poly(ether siloxane urethane urea) for n-C4H10/CH4 separation. It was found that by changing the polysiloxane content of the membrane, higher mixed gas selectivity than that of ideal gas could be obtained, indicating a possibility of application of these membranes for vapor/gas separation.

3.4.2.3 Substituted Polyacetylenes Polyacetylene-based polymers that have been widely studied since the discovery of poly[1-trimethylsilyl)-1-propyne] (PTMSP) in 1983, have been evaluated for gas and vapor separations due to their high gas permeabilities [12, 83, 93–95]. hese amorphous glassy polymers are also characterized by their high glass transition and low density [12]. Substituted polyacetylenes have large oxygen permeability (>1000 Barrer) due to high free volume and unusual free volume distribution derived from their low cohesive energy structure, stif main chain and bulky substituents [95]. PTMSP containing bulky substituents with

122 Nanostructured Polmer Membranes: Volume 2 extremely high oxygen permeability in the range of 6000–12000 Barrer is one of the most studied polymers for membrane applications due to its unique structure and properties. However, strong trade-of efects and aging are the main features of PTMSP that have hampered the industrial application of this polymer [83]. Some of the substituted polyacetylenes are presented in Figure 3.4. All of these materials have permeabilities more than or similar to the most permeable commercial membrane material, PDMS [93]. Oxygen permeation experiments showed that polymers containing spherical substituents, such as t-Bu, Me3Si and Me3Ge groups, were highly Poly(TMSP) and its analogues C Me (a)

C

C n SiMe3

C n SiMe3

Me

Poly(TMSP)

Poly(TMGP)

C

C n CHMe2

Me

Poly(TMGP)

Ring-substituted polydiphenylacetylenes

C

C

C n

C

C n

t-Bu

GeMe3

SiMe3 (b) Poly(p-Me3Si-DPA)

C n

Poly(p-Me3Ge-DPA)

Poly(p-t-Bu-DPA)

Polydiarylacetylenes C

(c)

C

C n

C n

Poly( -NpPA)

Poly(DPA)

Ring-substituted polyphenylacetylenes CH

C n

C n

Me3Si

CF3 CF3 (d)

CH

CF3

Poly(1,4,5-(CF3)3-PA)

CH

C n

CF3 CF3

Me3Si Poly(1,4-(Me3Si)2-PA)

Poly(1,5-(CF3)2-PA)

Figure 3.4 Representative gas-permeable substituted polyacetylenes [93].

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permeable among other substituted polyacetylenes. On the other hand, polyacetylenes possessing long n-alkyl groups, such as n-C6H13, were less permeable. Moreover, when phenyl groups were the main substituent the gas permeability of the resulting polyacetylene was considerably lower [93, 95]. Substituted polyacetylenes have also been used as membrane materials in vapor/gas separation applications due to the fact that they are more permeable to large condensable gases than small ones, similar to the trend observed for rubbery polymers [12, 83, 93]. It was revealed that the permeation of n-alkanes from methane to n-butane was solubility controlled in the substituted polyacetylenes but not based on size-sieving efects like other glassy polymers. However, the nature of the substituent had a signiicant inluence on the permeability coeicients of the membranes. hus substituted polyacetylenes can be designed to have both small-molecule selective (e.g., poly(PP)) and large-molecule selective (e.g., poly(TMSP)) properties by choosing their substituents. Alkyl-substituted polyacetylenes have relatively low permeability, similar to conventional glassy polymers, while PTMSP and poly(1-trimetylgermyl-1-propyne) (PTMGP) exhibit higher hydrocarbon permeability [83, 93]. Results of single vapor permeation are shown in Table 3.9. Another interesting characteristic observed in substituted polyacetylenes that the selectivity in the mixed vapor/gas experiments was more than that obtained from pure gas measurements [83, 93]. It was reported that for PTMSP n-C4H10/CH4 selectivity in the mixture was more than 10 times larger than the permeability ratio of pure components [96, 97]. Moreover, PTMSP has the highest C3+ permeability and the highest C3+/ methane and C3+/hydrogen selectivity compared to any known polymers [12]. his behavior was attributed to the interconnected pores in high free volume PTMSP and possibility of blocking “bottlenecks” of these pores by more condensable component of the mixture [83]. However, this blocking efect was not observed in the separation of CO2/iso-C4H10 mixture using PTMSP and the copermeation of iso-C4H10 had no efect on the CO2 permeation. Accordingly, it was concluded that the permeability decline in the presence of a vapor was observed only for less polar, less condensable gases such as CH4 [93, 95]. Unlike vapor/gas separation, substituted polyacetylenes have shown no attractive separation properties in vapor/vapor separation applications and require modiications for the selectivity improvement [93]. In addition to the above-mentioned interesting properties, PTMSP exhibits an unusual dependence of permeability to temperature. Permeability of most gases in PTMSP and poly(4-methyl-2-pentene) (PMP) decrease with increasing temperature, which is in contrast to other

124 Nanostructured Polmer Membranes: Volume 2 Table 3.9 Alkane permeabilities of substituted polyacetylenes [83]. Polyacetylenes–(CH3)C=C(R)R SiMe3 GeMe3

i-Pr

Title PTMSP PTMGP

PMP

Selectivity (C4/C1)

Permeability (Barrer) CH4

C2H6

C3H6

C4H10

15000

31000 38000

-

-

15400

26000 30300

78800

5.1

14000

-

-

40000

3.5

12000

-

20000

39000

3.2

2900

3700

4160

-

-

2690

3730

4160

-

-

2000

-

-

5000

2.5

8500

-

-

15000

1.8

CH2(i-Pr)

P5M2H

190

280

1700

-

-

C2H4(i-Pr)

P6M2H

112

160

605

-

-

C3H7

P2H

21

16

35

440

21

C5H11

P20

51

99

420

2100

42

C6H13

P2N

64

138

450

1850

29

c7H15

P2D

84

220

740

4600

55

C8H17

P2U

83

240

840

3900

47

–

PTPSDPA

390

760

1900

7300

19

glassy polymers [12, 98]. According to the results of molecular modeling, the smaller cavity size in PTMSP at higher temperatures may contribute to the lower permeability of gases at higher temperatures [98].

3.4.3 Emerging Polymers 3.4.3.1 Polymers of Intrinsic Microporosity (PIMs) Eforts to produce organic-nonorganic hybrid materials that mimic the structure of zeolites have led to the synthesis of PIMs, a family of nonnetwork polymers [99–103]. hese materials simply form microporous solids because their highly rigid and contorted molecular structures cannot ill the interstatial space eiciently; resulting in free volume coupled with chemical functionality, giving strong intermolecular

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interactions  [12,  99,  102, 104–105]. Unlike conventional microporous materials, PIMs are soluble and can be processed readily using solventbased techniques [24, 99]. Extraordinary high solubility coeicients compared to other polymers coupled with high difusion coeicients lead to the gas separation performance of PIMs falling beyond the 1991 Robeson upper bound and near the 2008 upper bound for CO2/CH4 and O2/N2 separations [83, 102, 104]. Although ilm formation is diicult due to their low molecular weights, PIM-1 and PIM-6, having structures shown in Figure 3.5, have been reported to form suitable ilms for membrane applications [20]. PIMs have no rotational freedom in the backbone and incorporate sites of contortion that produce sharp bends in the chain [102]. PTMSP is more permeable but less selective than these PIMs and exhibits microporous character with larger apparent pore sizes [102]. According to a study by homas et al. [107–108], PIM-1 was reported to be more permeable to hydrocarbon vapors than to hydrogen, a behavior similar to that of microporous PTMSP. PIM-1 had a mixed gas n-butane permeability of 4000 Barrer and n-butane/hydrogen selectivity of 25 similar to that of PTMSP and about 2.5-fold higher than that of PDMS [108]. he permeability of n-butane in PIM-1 increased by increasing its concentration in the feed due to the swelling-induced dilation of the polymer matrix by condensable component. his increase was accompanied by the decrease in the permanent gas permeability, which is similar to the blocking behavior observed in PTMSP [107]. However, in contrast to PTMSP, the transition to mixed gas permeation was accompanied by a decrease in the selectivity of C4H10/CH4 from 58 to 24 [83]. he tendency of PIM-1 toward condensable vapors was attributed to the polar

O

CN O

O

O PIM-1

n

N

O O

CN

O O

N N PIM-7

N n

Figure 3.5 Molecular structures of polymers of intrinsic microporosity PIM-1 and PIM-7 [106].

126 Nanostructured Polmer Membranes: Volume 2 functional groups in its structures and this make it an attractive candidate as membrane material for the separation of higher hydrocarbons [104]. In contrast, PIM-7 is not a suitable candidate since it is more permeable to methane than n-butane with a mixed gas CH4/C4H10 selectivity of 2.3 [83]. Recently, by applying the design principles of polymers of intrinsic microporosity to polyimides, new polymers (PIM-PIs) were obtained with gas separation properties exceeding the most permeable conventional polyimides, depending on the structure [106, 109]. According to Ghanem et al. [106], PIM-PI-1 and PIM-PI-3 lied close to Robeson’s 1991 upper bound whilst PIM-PI-8 exceeded the trade-of. his material exhibited the highest permeabilities so far reported for a polyimide [100]. he combination of high permeability and selectivity, resistance to plasticization and high chemical resistance are considered as the main advantages of these novel microporous membranes [20].

3.4.3.2 hermally Rearranged (TR) Polymers hermally rearranged polymers are aromatic polymers with heterocyclic rings prepared by in-situ thermal conversion of polyimides or polyamides with ortho-functional groups (PIOFG) under inert atmosphere. he ortho-functional groups can be hydroxyl (-OH), thiol (-SH) and amine (-NH2) groups, which will be converted to the polybenzoxazoles (PBO), polybenzothiazoles (PBZ), polypyrrolone (PPL) and polybenzimidazoles (PBI), [110]. hese aromatic, insoluble and infusible polymers can be prepared by irreversible molecular rearrangement at about 350 to 450 °C that do not correspond to partial burning (or carbonization) of the polymer structure [111]. hermal rearrangement results in increased fractional free volume and narrow cavity size distribution, which in turn leads to the fast transport of penetrants [110–112]. Park et al. [111] showed that TR polymers demonstrated excellent CO2/CH4 separation performance, exhibiting CO2 permeability of 2000 Barrer with a selectivity of 40. Later it was concluded that by using diferent thermal treatment protocols (e.g., inal temperature and thermal dwell time) along with varying the structure of the precursor polyimides, microporous structure and size distribution of TR polymers could be easily tuned [113–114]. Accordingly, Park et al. [113] prepared TR polymers that showed permeabilities higher than original polymers by two orders of magnitude, conirming the presence of interconnected free volume elements. In these studies, hydroxyl-containing polyimides and amine-containing polyimides converted to PBO and PPL, respectively.

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he TR-PBI membranes also showed exceptional high permeabilities to small gas molecules despite common PBIs, which have low permeation properties due to their rigid structure. he presented TR-PBI membrane displayed advantageous separation performance for H2/CO2 separation at high temperatures (120 °C) [115]. Based on these reports, Jung et al. [116] hypothesized that the use of copolymers might be desirable to generate the desired polymer properties for the preparation of TR polymers. hey prepared thermally stable copolymer membranes composed of PBO and PI converted domains synthesized by thermal rearrangement reaction of hydroxyl-containing polyimides at about 450 °C. It was shown that the separation performance and permeation of O2/N2 and CO2/CH4 were dependent on the ratio of PBO [111]. Moreover, Choi et al. [117] indicated that PBO homopolymers, which have high permeabilities but relatively low selectivities, were tunable through PBO-co-PPL copolymerization. Accordingly, thermally rearranged copolymerization of stif and selective pyrrolone and high free volume, high permeable benzoxazole moieties led to enhanced gas separation performance [117]. Do et al. [118] showed that copolymers of thermally rearranged poly (bezoxazole–co-amide) membranes derived from poly(o-hydroxyamides) exhibited tuned cavity size suitable for H2 separation from CO2 exhibiting H2 permeability of 26.8 Barrer and H2/CO2 selectivity of 8. Copolymers of TR-PBO with PI segments unable to undergo TR which formed poly(bezoxazole-co-imide) membranes, resulted in diferent gas selectivity properties depending on the PI moiety. herefore, it was concluded that the gas separation properties of the TR membranes could be tailored through the addition of non-TR segments [119]. Soo et al. [120] demonstrated that introduction of non-TR-able diamines containing nonpolar bulky side groups, such as MCDEA and DAM, resulted in higher gas permeabilities as a consequence of higher chain rotational motions. Besides, separation of small gas molecules were improved by using the non-TR-able segments having lat and rigid structures. It was concluded that the gas separation performance of TR polymers depended on the degree of the conversion, which was afected by the ilm thickness [121–122]. It was shown that greater chain mobility occurred in thin ilms due to the proximity of the free surfaces and reduced difusional resistance for removal of the volatile compounds of the rearrangement reaction [123]. Moreover, It was revealed that thin ilms of TR membranes derived from 6FDA-HAB polyimides, experienced a greater extent of aging than thick ilms ater 1000 h exposure to diferent gases such as H2, O2, N2 and CH4 [124].

128 Nanostructured Polmer Membranes: Volume 2

3.5 Strategies for Tuning the Transport in Polymeric Membranes through Molecular Design and Architecture Permeability of a polymeric membrane is the product of difusion coeicient and solubility coeicient and therefore the overall selectivity is determined by both solubility selectivity and difusivity selectivity. Consequently, all the polymeric membranes can be partitioned into either difusivity controlled or with solubility controlled permeation. he membranes in the irst class is capable to separate molecules by size-sieving efect. However, the membranes in the second class can facilitate the transport of molecules with larger solubility coeicient [39, 83, 125–126]. Generally, difusion coeicient decreases as the penetrant size increases, while solubility coeicient increases as the condensability of the penetrant increases. It is worth mentioning that glassy polymers are more size selective than rubbery ones since the dependence of difusion coeicient on penetrant size is much stronger for glassy polymers [104]. he main factors contributing to the solubility selectivity or difusivity selectivity as are: 1) mobility of the polymer chains, as relected by glass transition temperature of the polymer, 2) intersegmental spacing as a measure of mean free volume and 3)  penetrant-polymer interactions, as relected by solubility of the penetrants 4) characteristics of penetrants [126]. In order to develop new membrane materials with high permeability and selectivity the chemical structure of the polymer can be tailored to optimize the permeation properties [12, 39].

3.5.1 Solubility-Selective Membranes Transport in solubility-selective membranes is dependent on the gas condensability, critical temperature and their ability to interact with polymer [104]. Solubility selectivity (sometimes termed reverse selectivity or thermodynamic selectivity) may be increased when one component has a high chemical ainity with the polymer [39, 83, 125]. Solubility selectivity contributes signiicantly to the separations of condensable vapors and polar molecules [12]. It is known that beside PDMS and PTMSP, which were presented in the previous section, various other polymers of diferent chemical classes exhibit solubility controlled separation properties [83]. However, since the selectivity of rubbery polymers is governed by the solubility selectivity, vapor-gas or CO2–permanent gas pairs solubility/ selectivity of rubbery polymers is large enough to lead to a high permselectivity. Polar groups that can cause strong interactions between CO2 and polymer were reported to be ether oxygens, nitriles, carbonyls, acetates and

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amides. However, these polar groups negatively afect difusion coeicient of more soluble molecule due to the strong ainity between the polymer and the penetrant. Accordingly, poly(ethylene oxide)-based rubbery polymer membranes are recommended as the best candidates due to the ether linkage that possesses strong chemical interactions with CO2 [12, 76, 125, 127]. However, PEO is reported to have low separation performance and permeability owing to high crystallinity. Moreover, the weak mechanical strength has hampered its industrial application. Several methods, such as using low molecular weight liquid PEO or poly(ethylene glycol) (PEG), designing phase-separated block copolymers with EO-rich segments and developing highly branched crosslinked networks containing PEO, were proposed in order to overcome these drawbacks [128]. By studying transport properties in various phase-separated block copolymers containing PEO, Lin et al. [128] concluded that the values of CO2/N2 selectivity are almost the same to that of PEO, suggesting that PEO is the continuous phase. However, different permeabilities ascribed to diferent morphologies arose from the composition and length of the hard segment as well as the PEO length. In this regard, block copolymers, such as Pebax containing EO units, like poly(amide-b-ether), showed good separation performance in CO2 separation [104, 128–129]. Pebax is composed of linear chains of rigid polyamide segments and lexible polyether segments that provide mechanical strength and high gas permeability [125]. Crystalline amide block is the impermeable phase, whereas ether block acts as the permeable phase due to its high chain mobility. Pebax copolymer showed CO2/N2 and SO2/N2 selectivity of 61 and 500 respectively [130]. Trong Nguyen et al. [129] analyzed the structure-property relationship of the ilms of diferent Pebax grades and blends of Pebax 1657 and PEG or poly(ethylene oxide-co-epichlorhydrine) (PEGEPI). It was concluded that the amount of polar groups and the Pebax phase structure had signiicant inluence on the membrane permeability. Accordingly, they developed membranes containing 20 wt% of PEG 300 and Pebax that exhibited CO2 permeability of 128 Barrer and CO2/N2 selectivity of 80. On the other hand, highly branched crosslinked PEO due to the branches containing –OCH3 end groups, which increase fractional free volume, possessed higher CO2 permeability and CO2/H2 selectivity than block copolymers or blends [128, 131–132]. Permeability of PEO was reported to improve to 420 from 12 Barrer ater crosslinking, resulting in CO2/H2 selectivity of 18 at 35 °C [125]. Polyvinylamine (PVAm) membranes also were reported to have great CO2 ainity due to the amine groups that act as carriers and reversibly – react with CO2 in the form of HCO3 [53, 125, 133]. Moreover, addition of polyvinyl alcohol (PVA) could improve the mechanical strength of

130 Nanostructured Polmer Membranes: Volume 2 the resultant blend because of the strong interaction between the amine and hydroxyl groups in the PVAm and PVA [134]. A CO2/CH4 selectivity of 45 and CO2 permeance of 0.55 m3(STP)/(m2 h bar) and CO2/ N2 selectivity of 160 with CO2 permeance of 0.83 m3(STP)/(m2 h bar) were reported by Deng et al. [135]. However, a large increase in solubility might induce plasticization, which would decrease membrane permselectivity [39, 132, 136]. Although selectivity is generally governed by difusivity selectivity in polyimides, several strategies were suggested in order to increase the solubility selectivity [2, 32, 83]. Shimazu et al. [32] studied the efect of diferent diamine moieties on the solubility selectivity of various 6FDAbased polyimides and concluded that solubility of propylene could be controlled by changing the number of CH3 substituents in the phenylene linkage and changing the connectivity in the main chain. Accordingly, polyimides with three to four CH3 substituents in the phenylene linkage of the main chain, such as 6FDA-TrMPD and 6FDA-TMMPD, exhibit increased Henry’s and Langmuir’s solubility due to chain stifness [32]. Using PIMs and polyacetylenes, which were considered in the previous section, is also another approach to control solubility selectivity, especially for the separation of hydrocarbons with diferent molecular masses [83].

3.5.2 Difusivity-Selective Membranes Difusive-selective membranes mostly represented by glassy materials, are capable of separating molecules by their size-sieving property [58, 83]. Penetrant difusion in glassy polymers is relatively much slower than in rubbery polymers but the sensitivity of the rate of difusion to the penetrant size is much greater [104]. Polyimides, which can be synthesized from the combination of numerous dianhydrides and diamines, are attractive membrane materials due to their good thermal and chemical stability and ilm formability [126]. Desirable tuning of transport properties can be possible by adjusting dianhydrides and diamines in this family of glassy polymers [137]. he main factors afecting the gas transport properties of polyimides represented as polymer chain rigidity, interchain spacing and chain mobility [2]. However, packing density of polymer chains, which oten can be evaluated by free volume, was reported to strongly afect the difusivity selectivity [41]. It was indicated that the meta-linked polymers showed lower FFV and higher difusivity selectivity in the CO2/CH4 separation due to the restricted rotational freedom compared to the para-linked isomers.

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However, in the cases of 6FDA-4,4 -ODA and 6FDA-3,4 -ODA the diffusivity selectivity remained unchanged due to the presence of other para linkages [2]. he same efect was reported comparing transport properties of 6FDA-TPEQ with para-linkage and 6FDA-TPER with meta-linkage for the O2/N2 separation. It was found that the higher permeability of 6FDATPEQ than 6FDA-TPER was mainly due to the difusivity rather than solubility. 6FDA-TPER possessed tighter packing of molecular chains, showing lower FFV. Besides, meta-bonding between the phenoxy rings inhibited the rotational motion of central phenyl rings in TPER [138]. herefore, it can be concluded that lower FFV values and increased inhibition to the segmental mobility result in lower difusivity and higher diffusivity selectivity. he lower Tg for a meta-linkage polymer is due to the same reason [41]. Moreover, the presence of pendant or side groups in the polymers is of paramount signiicance in determining the efects of packing density and chain rigidity on the separation performance of the polyimides [2, 41, 126]. It is shown that both permeability and selectivity might be enhanced by the incorporation of bulky pendant groups to simultaneously decrease the chain packing density and hinder torsional mobility [126]. It was demonstrated that the local motion of polymer chains and side group play an important role in generation and dissipation of small free volume holes or change in the size of free volume holes in the glassy state [41]. he bulky CF3 groups in 6FDA-based polyimides restrict the intersegmental mobility in contrast to the single bonds or carbonyl groups in BPDA- and BTDA-based polyimides with unrestricted motion. herefore, eicient polymer chain packing was suppressed in 6FDA-based polyimides, leading to much higher difusivity selectivity [41, 126]. However, it was postulated that the average size of the free volume elements is also an important factor in determining diferences in the difusivity selectivity of various polyimides. It was concluded that the restriction of local mobility imposed by pivaloylimino group, together with the tighter chain packing, could justify the smaller average size of the free volume elements in PBTPDA, which seemed to be responsible for the higher difusivity selectivity of PBTPDA than in BTPDA [139]. he higher difusivity selectivity of 6FDA-TAPOB than 6FDA-TPEO in O2/N2 separation was also attributed to the low segmental mobility and unique size and distribution of free volumes of the hyperbranched TAPOBcontaining polyimide [138]. In addition, 6FDA-based polyimides with diamines containing ether linkage in their repeat units, such as BATPHF and BTPHF, showed low difusivity selectivity for CO2/CH4 separation as a result of enhanced local segmental mobility arising from high mobility of ether linkage [41].

132 Nanostructured Polmer Membranes: Volume 2

3.6 Process Modeling and Simulation Membrane-based separation applications have gained much attention owing to their advantages over traditional separation processes. Development of high-performance polymeric membrane materials and the improvement of the performance of existing polymers necessitate a comprehensive knowledge of the factors governing the separation performance of the membranes. he main goal of modeling is to assess the efect of process conditions on the transport of the penetrants through membrane as well as permeability and selectivity. hese parameters are related to the properties of the feed, permeate and retentate streams, the low and module conigurations, as well as the properties of the membrane material and the governing transport mechanisms involved [140–145]. he separation performance is based on the diference in sorption and difusion of the penetrants in the membrane. herefore, the accurate mathematical analysis of the transport in the polymeric membrane is of great signiicance, especially due to the growing technological attention toward these materials [40, 146]. Transport through the membrane occurs according to the solution-difusion mechanism in which sorption is a thermodynamic process while difusion is a kinetic process. In order to describe the permeation of penetrants in polymeric membranes, appropriate models describing permeation should be applied to investigate the efect of important parameters on the separation performance.

3.6.1 Modeling of Membrane Gas Separation Process Single-stage permeation with diferent low patterns, such as concurrent, countercurrent, cross low, perfect mixing and one-side mixing, is the simplest situation considered by numerous researchers in order to understand the overall performance of the module and to realize the basis of cascade coniguration [147–157]. he most practical representation of multicomponent gas separation was carried out by Pan [155] in case of high-lux asymmetric hollow-iber membranes [157–158]. he local molar lux of component A through the membrane and into the low-pressure side of the membrane is expressed as follows using Fick’s law:

y AdL

PA

ph x A

pl y A dA

(3.23)

where x and y represent the local compositions of component A in the high- and low-pressure sides of the membrane, respectively. dA is the

Polymeric Membranes for Gas and Vapor Separations

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membrane surface area of the element and dL is the increment of the local high-pressure stream permeated through this area. Pressure of highpressure and low-pressure sides are denoted by ph and pl respectively. P and δ are membrane permeability and thickness, respectively. Overall material balance and component balance can be written as follows:

Lf L f x1

Lr Lr x A,r

Lp Lp y p

(3.24) (3.25)

L is the molar lux and subscripts f, r and p refer to feed, retentate and permeate streams respectively. Moreover, from a material balance for a differential volume element one can obtain:

d(Lx A ) dL

yA

(3.26)

Combining the above governing equations, each of the luxes, mole fractions or required membrane areas would be obtainable depending on the solution approach. Applying the Pan’s approach, Kaldis et al. [159] developed a model for predicting hydrogen separation from light hydrocarbons in oil reineries using a polyimide membrane. Results revealed that hydrogen purity of up to 99% along with total recovery of 90% was achievable for 0.2 to 0.6 stage cut [159]. However, from a mathematical point of view, assuming identical surface and bulk permeate concentrations does not appear to be appropriate for high-lux asymmetric membranes where the microstructure of substrate prevents local permeate mixing [157]. Marriot and Sørensen [160] presented models for hollow-iber and spiral-wound membrane modules using mass, momentum and energy balances with the assumption of ixed permeabilities predict the experimental data simulated by Pan [155]. he generality of the model led to the capability of predicting pervaporation and reverse osmosis as well as gas separation. However, dispersion and difusion coeicients are required to be available to exploit this approach. Chowdhury et al. [161] developed a new solution approach to the Pan’s model, eliminating the trail-and-error procedure required to solve the boundary value problem in the original approach. Moreover, the model was incorporated into the Aspen Plus as a user model for design and optimization of membrane processes [161]. Ahmad et al. [162] also incorporated a two-dimensional cross-low mathematical model for membrane separation with Aspen HYSYS as a user-deined unit operation in order to optimize and design the membrane system for CO2 capture from natural

134 Nanostructured Polmer Membranes: Volume 2 gas. It was observed that two-stage with permeate recycle system ofered the optimum design coniguration due to minimum process gas cost [162]. Besides developing models to describe the gas and vapor permeation, several studies were devoted to investigate the efect of various process parameters on the separation performance of gas separation membranes. Yang et al. [163] studied the efect of low coniguration for the process of CO2 removal from natural gas and revealed that cross-low model showed little diference with the cocurrent and countercurrent low models. hey assessed the inluence of permeability and operating pressure and concluded that by increasing feed side pressure and decreasing permeate side pressure, the membrane area required decreased and the CH4 recovery increased. It was also depicted that CH4 recovery and product purity of >98% was achievable using the two-stage system with a membrane selectivity of 20, with much less energy required than that of the amine absorption process [163]. he dynamic performance of hollow-iber gas separation was modeled via the tanks-in-series model by Katoh et al. [164]. Simulation results for hydrogen gas separation in the stream from hydrotreater, hydrogen recovery from purge gas of ammonia plant and air separation indicated that efects of mixing degree in the feed side is more signiicant as compared to that in the permeate side; and less mixing in the feed side results in higher performance. In addition, for the two-stage CH4 separation with recycle stream for methane recovery, the residue recycle to feed was found to improve CH4 recovery eiciency [164]. Kundu et al. [158] examined the efect of permeate recycle as well as retentate recycle in single-stage air separation. It was shown that 50% oxygen-enriched air as permeate stream was obtainable in single-stage systems. Moreover, permeate recycle led to 60% enrichment in oxygen, while retentate recycle did not have considerable inluence on the productivity and recovery of nitrogen [158]. Khalilpour et al. [165] showed that detailed analysis of feed gas quality, pressure, area, selectivity and permeance resulted in better understanding of operating and design optima. It was shown that both area and pressure are inluential and interactive parameters and, despite common sense, at a speciied membrane area, pressure increase did not enhance the product purity and selectivity and an optimal condition existed depending on the design/operation objectives [165]. Kundu et al. [157] also investigated another general perception about pressure buildup in iber lumen. hey showed that the efect of permeate pressure buildup on the membrane performance was reduced at higher feed pressures and membrane performance was close to its full potential in this condition. he efect of non-idealialities regarding permeation on the gas separation membrane performance has been studied via diferent approaches.

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One important non-ideal efect regarding the membrane gas separation process is the concentration polarization, which is the accumulation of the less permeable species and a depletion of the more permeable components in the boundary layer adjacent to the membrane as a result of high luxes and selectivity as well as low bulk difusion [166–167]. It was demonstrated by Wang et al. [166] that the efect of concentration polarization was much more severe at high stage cuts and the calculated permeance of CO2 and CH4 in the polyimide hollow ibers was underestimated and overestimated respectively due to disregarding this efect in the model. he same conclusion was obtained by Peer et al. [168] studying the separation of H2 from CO using polyimide hollow ibers. Yeom et al. [169] also investigated the efect of concentration polarization, incorporating the resistance-in-series concept on the VOC/N2 separation using PDMS membrane. he authors reached the conclusion that boundary layer resistance due to concentration polarization of VOCs component in the feed side was more signiicant when VOC condensability was greater, VOC content in the feed mixture was higher as well as operating temperature was lower [169]. Mourgues and Sanchez [170] investigated the inluence of concentration polarization when the feed was introduced inside the ibers. It was demonstrated that the permeation rate was the most important factor afecting the polarization. he increase in operation pressure and the selectivity were also reported as important parameters on the polarization phenomena [170]. Scholz etal. [171] linked non-ideal efects, including concentration polarization, the Joule-homson efect, pressure losses and real gas behavior, to the mathematical representation of hollow-iber membrane module. It was observed that considering the Joule-homson efect and real gas behavior led to a better estimation of CO2/ CH4 separation as presented in Figure 3.6. he Joule-homson efect was also taken into account along with temperature and pressure dependence of membrane permeance in the model presented by Ahmad et al. [172] which was implemented as a user-deined model in an Aspen HYSYS simulator. It was demonstrated that considering the mentioned non-idealities led to higher CO2 retentate composition, lower stage cut and membrane loss in comparison to ideal model for the same iber length, which in turn resulted in lower compressor power requirements and gas processing cost [172]. he inluence of various nonideal efects were aslo investigated in an study done by Hosseini et al. [141] as illustrated in Figure 3.7. he results showed that the O2/N2 separation performance was less impressed by nonideal efects compared to the CO2/CH4 separation [171]. Another non-ideality which can alter permeation properties of the membrane is the dependency of penetrant difusion coeicient on concentration. his efect was taken into account in the model presented by Ebadi et al. [173] investigating the

136 Nanostructured Polmer Membranes: Volume 2 0.9 Accumulated efects

CO2 permeate mole fraction

0.8

0.7

Joule Thomson

0.6 Real gas

Ideal, pressure drop, concentration polarization

0.5

0.4

0.3 0.00

0.05

0.10

0.20

0.15

Module length (m)

0.92

0.42

Concentration polarization Pressure changes Real gas behavior Accumulated efect Ideal condition JT efect

0.4 0.38 0.36 0.34 0.32 0.3

Nitrogen at feed-side(mole fraction)

Oxygen at permeate outlet(mole fraction)

Figure 3.6 Mole fraction of CO2 on the permeate side as a function of module length for various nonideal and the accumulated efects as well as for ideal module operation [171].

Concentration polarization Pressure changes Real gas behavior Accumulated efect Ideal condition JT efect

0.9 0.88 0.86 0.84 0.82 0.8 0.78

0

(a)

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Narmalized active iber length

0.8

0.9

1

0

(b)

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Narmalized active iber length

Figure 3.7 O2 mole fraction in permeate side stream b) N2 mole fraction in feed side stream along with the active iber length [141].

permeation of ternary gas mixture of H2, CH4 and C3H8 through PDMS ilms using COMSOL sotware. It was demonstrated that considering this dependency led to less deviations from experimental data. Remarkably high inconsistencies were observed assuming constant difusion coeicient due to not considering the efect of propane concentration on the membrane or C3H8-induced plasticization. he same result was also obtained studying CO2 permeation in ABS/PVAc blend membranes. It was observed that considering the dependency of difusion coeicient on concentration, position and time led to a more precise prediction [174]. Variable permeability coeficient is another important factor whose incorporation into the model via a

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proper approach may lead to a better estimation of the permeation process. Safari et al. [175] proposed two models that included both temperature and pressure dependencies of the permeability coeicient and selectivity. Model parameters were obtained via experimental data and the model was used to investigate the efect of various parameters on the separation performance of CO2/CH4 system using 6FDA-2,6-DAT membrane. he model was utilized to ind the optimal membrane area in a two-stage process. In rubbery polymers, Prabhakar et al. [176] presented a model, having few adjustable parameters, to account for the temperature and pressure dependency of permeability in a PDMS membrane. he model was stated to be able to estimate permeability under conditions that are not in the range to be easily experimented [176].

3.6.2 Phenomenological Models for Gas and Vapor Sorption and Permeation Development of advanced polymeric membranes for gas and vapor separation requires a thorough understanding about the factors afecting the transport of penetrants within the membrane. Accordingly, appropriate models should be employed to investigate the efect of important parameters on the membrane permeation and separation performance. Essentially, in glassy polymers, penetrants sorb into both Langmuir and Henry’s sites within the membrane structure and then difuse across the membrane [177]. his is typically presented by the dual mode sorption model that accounts for sorption in the normally densiied and microvoid regions of the non-equilibrium polymer matrix according to Eq. (3.27) [30, 178–180].

CA

kDA pA

CHAbA pA 1 bA pA

(3.27)

where kDA is the Henry’s law constant and CHA is the Langmuir capacity constant. he ainity constant, bA, is the measure of polymer ainity for the sorbed molecule. According to this model, gas is hypothesized to be completely immobilized. Fick’s law, described according to Eq. (3.28), is the most widespread model that presents a simple relation between the lux and concentration gradients of penetrants[180]. According to this model, gas is hypothesized to be completely immobilized.

JA

DA

dC A dz

(3.28)

138 Nanostructured Polmer Membranes: Volume 2 his is in contrast to the partial immobilization model (Eq. (3.29)) which is based on the independent difusion in the Henry and Langmuir modes [181]. Partial immobilization theory provides a more general form than Fick’s law by speculating a inite mobility for the Langmuir sites along with Henry’s law sites [177, 180]:

NA

DDA

dCDA dz

DHA

dCHA dz

(3.29)

where DDA and DHA are the difusion coeicients of component A in the Henry’s and Langmuir sites, respectively. In the most cases, partial immobilization theory gives an appropriate description of mass transport in membrane [182]. his model describes permeation of binary gas mixture using sorption of pure gases, while in binary systems the lux of each component must be considered afected by each other. However, this approach have failed to predict permeability in some polymers due to not taking into account the bulk or convective low of each component arising from the coupling of luxes. Some researchers tried to take this efect into consideration partly by considering the convective lux via applying the frame of reference/bulk low model [30, 181, 183–185]. Accordingly, using bulk low model the permeation of binary gas mixture could be described as a ternary system comprising two penetrants and a membrane as presented in equations 3.30 to 3.32.

nA

nA nB nm w A

DDA

dw A dz

(3.30)

nB

nA nB nm w A

DDB

dw B dz

(3.31)

nm

nA nB nm wm

DDm

dwm dz

(3.32)

where n is the mass lux of components through the membrane (m). he convective term is required to be considered when studying permeation in the membranes since difusion of each component in a mixed gas environment is afected by the presence of other components. his results in an efective bulk low. Accordingly, the transport process is considered as a combination of both bulk and difusive luxes. Bulk lux is a function of sorption level in permeation of pure components; however in the case of binary gas mixtures, bulk lux is a function of both sorption level and

Fraction of bulk lux contribution

Fraction of bulk lux contribution

Polymeric Membranes for Gas and Vapor Separations 0.12 0.1 0.08 0.06 0.04 0.02 0

20

(c)

40

0.12 0.1 0.08 0.06 0.04 0.02 0

30

20

40

Pressure (psia)

Propylene, Mixed gas

0.12 0.1 0.08 0.06 0.04 0.02 0 10

20

(b) Fraction of bulk lux contribution

Fraction of bulk lux contribution

(a)

30 Pressure (psia)

Propylene, Pure gas

(d)

139

30 Pressure (psia)

40

0.12 0.1 0.08 0.06 0.04 0.02 0

20

30 Pressure (psia)

Propane, Mixed gas

40

Propane, Pure gas

Figure 3.8 Fraction of bulk lux contribution of propane and propylene in binary mixture compared to the pure gas in a) 6FDA-6FpDA at 35 °C b) 6FDA-6FpDA at 70 °C c) 6FDADAM at 75 °C and d) 6FDA-TrMPD at 50 °C [194].

lux of mobile components [181]. he contributions of bulk lux of propylene and propane in diferent polyimides were calculated according to the method described by Kamaruddin and Koros [181] and are presented in Figure 3.8. he governing equations should be solved simultaneously in order to achieve the trans-membrane luxes and thus the permeability. his approach was shown to be able to successfully predict permeation of binary gas mixture through some polyimide membranes based on the Fick’s law. However, it failed in predicting the separation performance in several situations [30, 181, 184]. Das and Koros [30, 35] analyzed the results of propylene/propane separation in a 6FDA-6FpDA membrane using both dual mode and frame of reference/bulk low model. It was found that the calculated selectivity by dual mode model was higher than the experimental selectivity for binary gas mixture. However, the results obtained by using the frame of reference/bulk low model were in good agreement with the experimental data. On the other hands, according to the study by Burns and Koros, the

140 Nanostructured Polmer Membranes: Volume 2 frame of reference /bulk low model was not able to predict the separation performance of 6FDA-DAM membrane for the same binary gas mixture [185]. In overall, it may be hypothesized that there are three main cases in which Fick’s law may not appropriately predict the mixed gas separation performance:(1) difusion coeicient is a function of concentration, (2) the concentration of the sorbed penetrants in the polymer matrix are not small and (3) lux of each penetrant is a function of the concentration gradient of other components. To circumvent these limitations in mixed gas separation studies, Maxwell-Stefan model that is developed to describe difusion in gas mixtures has been applied. Maxwell-Stefan equation is as follows [183, 186]:

dA B

xAxB (v A v B ) D AB A

(3.33)

where x and v are mole fraction and velocity of each component, respectively. DAB is the binary difusion coeicient and d is the driving force. Using mass fraction is more useful for describing difusion in membrane systems. his approach successfully predicted permeation of binary gas mixture through some glassy membranes while failed in some other cases. he vast amount of studies on diferent membrane separation systems utilizing the Maxwell-Stefan model have proved the ability of this approach in overcoming the complications of mass transport in membranes [86, 183, 186–193]. he validity of this speculation was assessed in a recent study by Najari et al. [193, 194] by applying the Maxwell-Stefan model that initially accounted for the co-existence of penetrants. According to the results, Maxwell-Stefan estimations approached to those of frame of reference/ bulk low model when the values of kinetic coupling efects were negligible. However, more reliable predictions were obtained using Maxwell-Stefan model that accounted for the thermodynamic coupling efects (TCEs) as illustrated for 6FDA-DAM and 6FDA-TrMPD polyimide membranes, respectively (Figure 3.9) . It was found that the predictions of the aforementioned model deviated from experimental data reported by Burns and Koros [185] and Tanaka et al. [195] in the case of propane permeability. Maxwell-Stefan model with TCE contribution led to a better estimation of permeability of propane. herefore, the assumption that concentration gradient of each component can afect the lux of other component seems to be valid when studying the transport of propylene/propane and probably other similar binary mixture through polymeric membranes.

Polymeric Membranes for Gas and Vapor Separations

40

30 20 10 10

15

30 25 Pressure (psia)

20

45

40

35

4 3 2 10

15

30 25 Pressure (psia)

20

Bulk low model

45

40

35

Maxwell-stefan model

Propylene permeability (Barrer)

5 b

6FDA-TrMPD

50

Propane permeability (Barrer)

Propylene permeability (Barrer) Propane permeability (Barrer)

6FDA-DAM 60 a

141

60 50

a

40 30 20 10 5 5

10

15

20

25 30 Pressure (psia)

35

40

45

10

15

20

25 30 Pressure (psia)

35

40

45

b

4 3 2

5

Maxwell-stefan model with TCE contribution

Experimentel data

Figure 3.9 Comparison of experimental and calculated permeability of (a) propylene, (b) propane for 6FDA-DAM at 75 °C and (c) propylene, (d) propane for 6FDA-TrMPD at 50 °C (Gas composition 50/50 mol.%) [194].

3.7 Challenges and Future Directions 3.7.1 Trade-of between Permeability and Selectivity Since the commercialization of polymeric membranes, obtaining polymeric gas and vapor separation membranes possessing both high permeability and selectivity has been the subject of numerous studies with the aim of structure/property optimization [32, 41, 61, 76, 83]. Nevertheless, the increase in permeability via structural modiications is generally at the expense of selectivity. his trade-of is usually presented using Robeson log-log plots of selectivity versus permeability of the more permeable penetrant [23, 196]. Accordingly, the term upper-bound, was proposed by Robeson [196] as follows:

Ln

AB

ln

AB

AB

ln PA

(3.34)

where β and λ are the empirical properties which could be correlated to the Lennard-Jones kinetic diameters of gases for each gas pair [12]. Following Robeson, Freeman [168] proposed another correlation for the upperbound in which β depends on λ, gas condensability and one adjustable parameter.

ln

AB

dB dA

2

1 lnPA

S ln A SB

dB dA

2

1 b

f

1 a RT

lnSA (3.35)

142 Nanostructured Polmer Membranes: Volume 2 Since λ depends only on penetrant sizes, the slope of upper bound lines are unlikely to change with further polymer development, while tuning β by polymer structural manipulation would result in improved separation performance. Accordingly, enhancing the solubility coeicient or increasing the chain stifness while simultaneously increasing interchain spacing are proposed as strategies for surpassing the upper-bound [197]. he upper-bound was revisited by Robeson [23] in 2008 with two additional upper-bound relationships for CO2/N2 and N2/CH4. However, only minor shits in the position of the upper-bound were observed, which was due to the perluorinated polymers and involved many of the gas pairs comprising He such as He/CH4, He/CO2 and He/H2. In addition to perluorinated polymers, minor shits were observed primarily due to polymers exhibiting rigid, glassy structures, such as polyimides and polypyrrolones, as well as ladder-type polymers [23].

3.7.2 Plasticization and Physical Aging Plasticization is a pressure-dependent phenomena caused by the dissolution of certain components within the polymer matrix which disrupts chain packing and enhances intersegmental mobility [2]. Condensable gases and vapors oten cause plasticization due to their higher solubility than other gases [1–2, 24, 83, 198]. Several symptoms were presented as general signatures of the plasticization by Sanders et al. [20]. he most common is the increase in difusion coeicient and consequently permeability of components upon increase in upstream partial pressure; resulting in deminished selectivity. Moreover, in addition to a rapid increase in permeability with increasing feed pressure, polymers undergoing plasticization may also display a slow increase in permeability over timescales far exceeding those required to achieve steady state. his behavior was observed in Matrimid 5218 polyimide while despite an expected time lag of seconds or minutes, depending on ilm thickness, CO2 permeability continues to increase with time even ater 2 h [20]. In addition, glassy polymers may also show hysteresis while undergoing plasticization, showing diferent permeabilities at a given pressure depending on the pressurization or depressurization process. Plasticization would be more likely for glassy membranes where highly sorbed amount of penetrants facilitate chain mobility, and promote the permeability of less permeable component [83]. he pressure where the gas permeability starts to increase is called the plasticization pressure in which the chain packing disrupts and polymer matrix swells [1]. Permeability exhibits a minimum at plasticization pressure but, since the plasticization occurs considerably below this pressure, examining the absolute heat of

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sorption as a function of pressure would be a more proper diagnosis of determining the onset of plasticization. Accordingly, plasticization occurs at a pressure where the heat of sorption changes from a negative to a positive value [2]. Since plasticization is deleterious for sustainable membrane performance, many attempts have been made to mitigate this phenomenon. Plasticization suppression was reported to be possible via thermal treatment, blending with a polymer that is resistant to plasticization and crosslinking which frustrate suppression of chain mobility [1]. Hosseini et al. [199–200] demonstrated that hollow-iber membranes comprising blends of Matrimid 5218 with polybenzimidazole (PBI) showed antiplasticization behavior toward CO2 as a result of strong hydrogen bonding interaction between PBI and Matrimid 5218 chains. Visser et al. [37, 201] and Bos et al. [202] also indicated that blends of Matrimid 5218 with co-polyimide P84 was very resistant against CO2-induced plasticization. However, in C3H6/C3H8 mixture the efect of blending was less pronounced on the suppression of plasticization [37]. Askari et al. [182] observed that thermally treated 6FDA-based copolyimide membranes (at 425 °C) did not show plasticization up to 40 atm in separation of CO2/CH4 mixture. Dong et al. [203] also revealed that thermal annealing of Matrimid 5218 hollow ibers at 250 °C led to membranes with high plasticization resistance behavior due to redistribution of Henry’s and Langmuir’s sites. It was proposed that Henry’s sites were the main contributor to the dilation of the polymer chain and plasticization [203]. Das and Koros [30] demonstrated that both annealing 6FDA-6FpDA at 210 °C and performing permeation experiments at elevated temperature (70 °C) could suppress plasticization in C3H6/C3H8 mixture due to more densiied chains of annealed dense membranes. Krol et al. [36] also showed that the propylene plasticization suppression in Matrimid 5218 hollow ibers was due to the charge transfer complexes formed in the polyimide matrix as a result of thermal annealing below the Tg. Physical aging is another challenging feature of polymeric membranes, along with permeability/selectivity trade-of and plasticization phenomena. Non-equilibrium state of glassy polymers, which is mainly due to thermal cooling process across the glass transition, leads to excess free volume. herefore, polymer tends to undergo gradual molecular rearrangement to attain the equilibrium state. Densiication of the polymer matrix and therefore reduction of free volume are the consequences of physical aging that reduce gas permeability [2, 20, 24]. Changes in activation energies, heat of solutions and change in optical properties were also reported as symptoms of physical aging [24]. In high free volume polymers the decline of free

144 Nanostructured Polmer Membranes: Volume 2 volume as a consequence of physical aging is more signiicant compared to low free volume polymers [2]. Also thin ilms are reported to be more prone to physical aging than thick ilms due to the diference in relaxation time distribution between thin and thick ilms [2, 24, 70, 204]. However, in multilayered ilms, physical aging was independent of layer thickness compared to free-standing ilms [205]. Nevertheless, much efort should still be put forth to overcome this challenge since the time-dependent nature of physical aging hampers the commercialization of high free volume glassy membrane materials.

3.8 Concluding Remarks he application of membrane technology for gas and vapor separation is a promising area for present and future demands in various industries. Despite the commercialization of several successful processes for separation and puriication of gases and vapors by using polymeric membranes, there is still considerable potential for overcoming the existing challenges for improving the process eiciency and performance. Tailoring the chemical and physical characteristics of polymers through molecular design and architecture, as well as increasing the ainity of the polymer toward more speciic components are among the promising strategies for development of high-performance membranes that can surpass the upper-bound limits. Recent progresses in the ield of novel materials, such as PIMs and TR polymers, have shown promising results for gas and vapor separations. However, preparing a well-engineered membrane that can exhibit high permeability as well as high selectivity, improved stability and processability and lower cost compared to the existing options, is still a matter for research. In this regard, structure/property investigations including modiication of chemical functionalities and structural manipulations aided with modeling and simulation tools are anticipated to remain among the main progressive and attractive areas of research in years to come for achieving industrially viable polymeric membranes for targeted gas and vapor separations.

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156 Nanostructured Polmer Membranes: Volume 2 167. E. Nagy, R. Nagy, J. Dudas, Separate expression of polarization modulus and enrichment by mass transport parameters for membrane gas separation, Ind. Eng. Chem. Res., 52, 10441–10449, 2013. 168. M. Peer, S. Mehdi Kamali, M. Mahdeyarfar, T. Mohammadi, Separation of hydrogen from carbon monoxide using a hollow iber polyimide membrane: Experimental and simulation, Chem. Eng. Technol., 30, 1418–1425, 2007. 169. C.K. Yeom, S.H. Lee, J.M. Lee, H.Y. Song, Modeling and evaluation of boundary layer resistance at feed in the permeation of VOC/N2 mixtures through PDMS membrane. J. Membr. Sci., 204, 303–322, 2002. 170. A. Mourgues, J. Sanchez, heoretical analysis of concentration polarization in membrane modules for gas separation with feed inside the hollow-ibers. J. Membr. Sci., 252, 133–144, 2005. 171. M. Scholz, T. Harlacher, T. Melin, M. Wessling, Modeling gas permeation by linking nonideal efects, Ind. Eng. Chem. Res., 52, 1079–1088, 2012. 172. F. Ahmad, K.K. Lau, A.M. Sharif, Y. Fong Yeong, Temperature and pressure dependence of membrane permeance and its efect on process economics of hollow iber gas separation system. J. Membr. Sci., 430, 44–55, 2013. 173. A. Ebadi Amooghin, P. Moradi Shehni, A. Ghadimi, M. Sadrzadeh, T.  Mohammadi, Mathematical modeling of mass transfer in multicomponent gas mixture across the synthesized composite polymeric membrane. J. Ind. Eng. Chem., 19, 870–885, 2013. 174. A. Ebadi Amooghin, H. Sanaeepur, A. Kargari, A. Moghadassi, Direct determination of concentration-dependent difusion coeicient in polymeric membranes based on the Frisch method. Sep. Purif. Technol., 82, 102–113, 2011. 175. M. Safari, A. Ghanizadeh, M.M. Montazer-Rahmati, Optimization of membrane-based CO2-removal from natural gas using simple models considering both pressure and temperature efects. Int. J. Greenhouse Gas Control, 3, 3–10, 2009. 176. R.S. Prabhakar, R. Raharjo, L.G. Toy, H. Lin, B.D. Freeman, Self-consistent model of concentration and temperature dependence of permeability in rubbery polymers, Ind. Eng. Chem. Res., 44, 1547–1556, 2005. 177. W.J. Koros, R.T. Chern, V. Stannett, H.B. Hopfenberg, A model for permeation of mixed gases and vapors in glassy polymers, Journal of Polymer Science: Polymer Physics Edition, 19, 1513–1530, 1981. 178. W. Koros, D. Punsalan, Polymer glasses: Difusion in, in: Editors-inChief:, K.H.J. Buschow, W.C. Robert, C.F. Merton, I. Bernard, J.K. Edward, M.  Subhash, V. Patrick (Eds.) Encyclopedia of Materials: Science and Technology (Second Edition), Elsevier, Oxford, pp. 7305–7315, 2001. 179. W.J. Koros, Model for sorption of mixed gases in glassy polymers, Journal of Polymer Science: Polymer Physics Edition, 18, 981–992, 1980. 180. D.R. Paul, Gas Sorption and Transport in Glassy Polymers, Berichte der Bunsengesellschat für physikalische Chemie, 83, 294–302, 1979.

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158 Nanostructured Polmer Membranes: Volume 2 194. S. Najari, S.S. Hosseini, M. Omidkhah, N.R. Tan, Phenomenological modeling and analysis of gas transport in polyimide membranes for propylene/ propane separation, RSC Advances, 5, 47199–47215, 2015. 195. K. Tanaka, A. Taguchi, J. Hao, H. Kita, K. Okamoto, Permeation and separation properties of polyimide membranes to oleins and parains, Journal of Membrane Science, 121, 197–207, 1996. 196. L.M. Robeson, Correlation of separation factor versus permeability for polymeric membranes. J. Membr. Sci., 62, 165–185, 1991. 197. B.D. Freeman, Basis of Permeability/selectivity tradeof relations in polymeric gas separation membranes. Macromolecules, 32, 375–380, 1999. 198. S.R. Reijerkerk, K. Nijmeijer, C.P. Ribeiro Jr., B.D. Freeman, M. Wessling, On the efects of plasticization in CO2/light gas separation using polymeric solubility selective membranes. J. Membr. Sci., 367, 33–44, 2011. 199. S.S. Hosseini, N. Peng, T.S. Chung, Gas separation membranes developed through integration of polymer blending and dual-layer hollow iber spinning process for hydrogen and natural gas enrichments. J. Membr. Sci., 349, 156–166, 2010. 200. S.S. Hosseini, M.M. Teoh, T.S. Chung, Hydrogen separation and puriication in membranes of miscible polymer blends with interpenetration networks. Polymer, 49, 1594–1603, 2008. 201. T. Visser, N. Masetto, M. Wessling, Materials dependence of mixed gas plasticization behavior in asymmetric membranes. J. Membr. Sci., 306, 16–28, 2007. 202. A. Bos, I. Pünt, H. Strathmann, M. Wessling, Suppression of gas separation membrane plasticization by homogeneous polymer blending. AIChE J., 47, 1088–1093, 2001. 203. G. Dong, H. Li, V. Chen, Plasticization mechanisms and efects of thermal annealing of Matrimid hollow iber membranes for CO2 removal. J. Membr. Sci., 369, 206–220, 2011. 204. J. Xia, T.-S. Chung, P. Li, N.R. Horn, D.R. Paul, Aging and carbon dioxide plasticization of thin polyetherimide ilms. Polymer, 53, 2099–2108, 2012. 205. T.M. Murphy, D.S. Langhe, M. Ponting, E. Baer, B.D. Freeman, D.R. Paul, Physical aging of layered glassy polymer ilms via gas permeability tracking. Polymer, 52, 6117–6125, 2011.

4 Membranes for Wastewater Treatment Alireza Zirehpour and Ahmad Rahimpour* Membrane Research Center, School of Chemical Engineering, Babol University of Technology, Babol, Iran

Abstract Membrane technologies have developed as one of the main contributors to the resolution of water-related problems over the past two decades. Increasing water scarcity, followed by severe regulations in industrialized countries, have promoted the use of membranes for water and wastewater treatment. From the perspective of wastewater treatment, microiltration (MF), ultrailtration (UF), nanoiltration (NF), and reverse osmosis (RO) are the most common developed membrane separation techniques applied in industries. Forward osmosis (FO) has also been introduced as a recent advance in membrane techniques for wastewater treatment. he success of membrane operations in wastewater treatment is attributed to compatibility between diferent operations in integrated systems. he wastewater treatment by integrated systems nowadays suggests reducing environmentally harmful efects, decreasing groundwater consumption and energetic requirements, and recovering valuable compounds as a by-product. Membrane bioreactor (MBR), combining membrane iltration with biologic treatment, is recognized as one of the most successful hybrid membrane systems in wastewater treatment. his chapter provides a brief overview of membranebased processes for water reuse and environmental control in the treatment of industrial wastewaters. Applications involving the use of pressure-driven membrane operations, MBRs, as well as a combination of membrane operations in hybrid systems in the treatment of waste from diferent industries are analyzed and discussed. Keywords: Membrane operations, integrated membrane systems, industrial membrane processes, membrane fouling, wastewater treatment

*Corresponding author: [email protected] Visakh P.M. and Olga Nazarenko (eds.) Nanostructured Polymer Membranes: Volume 2, (159–208) © 2016 Scrivener Publishing LLC

159

160 Nanostructured Polmer Membranes: Volume 2

4.1 Introduction he world’s population increased in the last century and it will grow about 45% in the following 50 years. Population growth together with urbanization and industrialization has caused increasingly fresh water demand. Rapid industrialization in or near urban centers has resulted in a very high pressure on the carrying capacity of the environment at speciic locations. At these locations water-bodies, such as rivers, lakes, and coastal waters, have been severely disturbed. Freshwater is a vital natural resource, truly at the core of sustainable development, which is inextricably linked to climate change, agriculture, food security, health, equality, gender and education. here is already international agreement that water and sanitation are essential to the achievement of many sustainable development goals [1]. Some industrial activities produce wastewater stream that must be managed. Industrial wastewaters are eluents produced from human activities which are associated with raw material processing and manufacturing. hese streams result from washing, cooking, cooling, heating, extraction, reaction by-products, separation, and conveyance [2]. Unmanaged wastewater streams have caused pollution of some existing fresh water resources. Problems with water are expected to grow in the coming decades, with water scarcity occurring globally, even in regions currently considered water-rich. Hence, many researchers have focused on proper systems to obtain fresh water by purifying and reusing polluted water. Water puriication is the process of eliminating disagreeable agents, such as chemicals, organic and biological contaminants, from water. In water and wastewater treatment, membrane technology has been recognized as the key technology for the separation of contaminants from polluted sources [3]. Membranes are selective barriers that separate two diferent phases, allowing the passage of certain components and the retention of others. he driving force for transport in membrane processes can be a pressure gradient and chemical or electrical potential across the membrane. Membrane processes depend on a physical separation, usually with no phase change and chemicals addition in the feed stream, thus stand out as an alternative wastewater treatment technique to conventional processes (i.e., distillation, precipitation, coagulation/locculation, adsorption by active carbon, ion exchange, biological treatment, etc.) [3, 4]. he low energy consumption, reduction in number of processing steps, greater separation eiciency and higher inal product quality are the main attractions of these processes [3–5]. However, limited resistance of the membranes in terms of chemical, mechanical, and thermal restricts their application.

Membranes for Wastewater Treatment 161 Wide eforts have been implemented to improve both lux and selectivity of the membranes. Furthermore, some researchers have focused on reduce membrane fouling as the most important problem in application of membranes in wastewater treatment. As a result, the performance and commercial markets of membranes have been greatly increased during the past years. Overall, the market for membranes and membrane systems grew from $4.4 billion in 2000 to more than $10 billion in 2010, and the sales of membrane equipment for water treatment exceeded $10.4 billion in 2014 [6]. In the future, further improvements and innovations are needed, especially in the chemical and morphological design of membrane materials, element and module design of membrane systems, antifouling membranes for wastewater treatment, and so on [3]. his chapter provides an overview of membrane-based processes for water reuse and environmental control in the wastewater treatment.

4.2 Membrane heory Membrane separation processes are characterized by instantaneous retention of species and product low through the semipermeable membrane. Membrane performance is based on its high permeate lux and selectivity, good mechanical, chemical and thermal stability of membrane materials, minimal fouling during operation, and good compatibility with the operating environment.

4.2.1 Membrane Deinition and Structure here are a number of meanings of the word “membrane,” based on the object of the process. A membrane is deined as a permselective barrier between two homogeneous phases. For many processes in wastewater treatment, the membrane acts to reject the pollutants, which may be suspended or dissolved, and allows the “puriied” water through it. Synthetic polymer membranes are used mostly in the case of wastewater treatment because it is possible to select a polymer suitable for the speciic separation problem from the existing enormous categories of polymers. Besides, polymeric membranes are oten cheaper than the other membranes. he structural characteristics of the used polymers, such as thermal, chemical and mechanical stability, are important. Membranes can be characterized as porous and nonporous, based on the mechanism by which separation is actually achieved. Separation by

162 Nanostructured Polmer Membranes: Volume 2 nonporous membranes relies somewhat on physicochemical interactions between the permeating components and its material, which results in the highest selectivity. Porous membranes, instead, separate mechanically by size exclusion such as molecular sieve. Hence, the material rejected by the porous membrane may be either dissolved or suspended depending on its size relative to that of the pore [7]. he membranes are divided into two main categories of symmetric or asymmetric, based on their structure. he symmetric membranes have an almost homogeneous structure all over the membrane thickness, while asymmetric membranes are made up of at least two layers. he active layer on the top of the membrane determines its separation behavior and the porous layer below assists as the top layer. he supporting layer ensures the mechanical stability of the membrane with a low resistance to the permeate low. Keeping the active layer of the membrane as thin as possible helps to minimize its resistance during the iltration process.  As a result, the solution-difusion membranes exhibit 50 to 100 times higher lows than comparable symmetric membranes [8]. Phase inversion or interfacial polymerization (IP) usually manufactures the asymmetric membranes. he ones that are fabricated by IP process are called thin-ilm composite (TFC) membranes. he active layer and supporting layer of the TFC membranes consist of diferent materials, so that both layers can be optimized separately. In the phase inversion process, the selective and supporting layers of the membranes are made from the same material.

4.2.2 Membrane Principles Membrane processes are continuous steady-state operations consisting of three streams: feed, retentate and permeate (product) streams. he membrane, a semipermeable barrier, selectively allows the passage of some components but not others, and allows some components to pass through more rapidly than others. In principle, two operating modes of iltration exist, cross-low and dead-end. In the cross-low mode the feed is pumped parallel to the membrane surface, while in dead-end operation the membrane is fed orthogonally [9].

4.2.2.1

Membrane Transport

In membrane processes, the relative low of media and solute transport rates through the membrane control the quality of the product. he membrane

Membranes for Wastewater Treatment 163 permeability and process driving force determine the transport properties of solutes through the membrane. he driving force causes a preferential passage of certain substances through semipermeable membranes. he driving force usually is pressure diference, concentration diference or electrical potential between the two sides of the membrane. he membrane controls the mass transfer between the feed and permeate streams. If the potential diference across the membrane becomes zero and no external forces are applied the system will reach equilibrium. he permeate lux (J) measures the material passing rate through a unit area of membrane per unit time. Based on existing theory models, there is a linear relationship between lux or permeate low and the driving force across the membrane. Several models, including Fick’s and HagenPoisseuille’s laws, describe the mass transport through the membrane. he transport relationship of the membrane processes operated by pressure gradient is as follows [6]:

J

K

P t

(4.1)

where J is the permeate lux, ΔP is the pressure diference across the membrane, K is the permeability constant of the membrane and t is the membrane thickness. he combination of the lux and the total membrane area determine the recovery or conversion of the process, one of the key performance indicators in liquid separations. he recovery percentage is the amount of the feed stream that is converted to the product stream. he recovery is calculated as follows:

R(%) 4.2.2.2

Permeate flow Feed flow

100

(4.2)

Membrane Selectivity

he chemical and physical nature of the membrane material manages its separation. Diferences in size and shape, chemical properties, or electrical charge cause the substances to be separated. he permselective property of the membrane is normally quantiied as the rejection. Rejection coeicient, R, is an indicator to determine the separating ability of a membrane process:

Rejection %

Cf

Cp Cf

100

(4.3)

164 Nanostructured Polmer Membranes: Volume 2 where Cf and Cp are feed and permeate solute concentration, respectively. For an ideal membrane rejection is 100%. For some processes, the accumulation of solute particles on the membrane surface because of concentration polarization (CP) or gel layer formation afects solute transport through the membrane. Hence, the retention coeicient is a better measure of selectivity, as follows:

Retention %

Cm C p Cm

100

(4.4)

where Cm is the solute concentration on the membrane surface. A trade-of between membrane selectivity and membrane productivity determines the membrane performance. he permeability ratio of components through the membrane deines its selectivity (α = A/B). So in the case of water and wastewater treatment, A is the water permeability coeicient and B is the pollutant permeability coeicient.

4.2.2.3 Membrane Separation Mechanism In separation processes, the membranes’ property is their ability to control the permeation of diferent species. Based on the types of membrane separation mechanism, most membranes fall into one of the two general categories. Microporous membranes separate based on molecular iltration (molecular sieve), and dense membranes separate by diferences in the solubility and mobility of species in the membrane (solution-difusion). In molecular-sieving membranes, feed components are separated by pressure-driven low through small pores with ixed position or size. Separation of diferent permeants occurs because of the diference between their sizes. In contrast, in solution-difusion membranes, the membrane material is a dense polymer layer and contains no pores. Permeants dissolve in the membrane material and then difuse through the membrane because of concentration gradient. Diferent permeants are separated based on their solubility diferences in the membrane material and their difusion rate through the membrane. he diference between the molecular sieve and the solution-difusion mechanisms lies in the relative size and lifetime of pores in the membrane.

4.2.2.4

Concentration Polarization

CP is the term to explain the accumulation of solute and particles on the membrane surface within a concentrated boundary layer or liquid ilm (Figure 4.1).

Membranes for Wastewater Treatment 165 Cw

Cg

Jv Permeate lux JConvection

Solution low

Membrane

JDiiusion

Cb Concentration boundary layer

Gel layer

Figure 4.1 Concentration/gel polarization model schematic.

hese solutes and particles transfer to the membrane surface with the solvent by convection and create a high concentration at the membrane surface compared to the bulk solution. he retained solutes that entered into the boundary layer by convection are removed by a generally slower back difusion toward bulk solution. he balance between convection toward the membrane and back-transport from the membrane because of concentration gradient can determine the CP level. he most common back-transport mechanism, represented by the mass transfer coeicient, is difusive back-transport [10]. Normally, CP is assumed to form rapidly at the beginning of iltration [11]. As a laminar boundary layer exists at the membrane surface, the Film heory Model expresses the steady-state solute balance in that laminar layer, as follows [10, 12]:

J (C f

C p ) Ds

dC dx

0

(4.5)

where J is the solvent lux, Cf is the feed concentration of solute, Cp is the permeate concentration of solute, DS the solute difusivity and x the distance from the membrane surface. Ater integration over the boundary layer (x = 0 to δ and C = Cw to Cb) for similar solute and solvent densities, constant difusion coeicient, and constant concentration along the membrane, the following equation is obtained:

J

k ln

Cw C p Cb C p

, k

Ds

(4.6)

166 Nanostructured Polmer Membranes: Volume 2 where k is the mass transfer coeicient, δ is the boundary-layer thickness, and Cw and Cb are the solute concentrations at the membrane surface and in the bulk solution, respectively. For an ideal situation with 100% rejection of solute by membrane (Cp = 0), the equation is modiied to the basic equation of CP:

Cw Cb

exp

J k

CM

(4.7)

where CM is identiied as the polarization modulus. Increasing J and decreasing k enhance the polarization module. Hence, the level of polarization module depends on the solute difusivity. Consequently, any technique that decreases Cw,such as turbulence or surface shear, would minimize the boundary-layer thickness, promote difusive back low and would result in a higher solvent lux through the membrane. he required transmembrane pressure for operation is increased by CP due to raising the efective osmotic pressure at the membrane-solution interface. CP can also afect the permeate water purity by changing the membrane selectivity and increasing the permeation of the rejected materials through the membrane. However, operation under CP conditions can cause scaling on the membrane surface as a more important limitation.

4.2.2.5 Critical Flux Critical lux is deined as the highest lux of a membrane process under which no or little fouling is observed [13]. he permeate lux above the critical lux causes fouling. he applied hydraulic pressure directly afects the pure water lux of membrane in pressure-driven processes. When permeate lux is plotted versus the transmembrane pressure, it is possible to observe a deviation from linearity at a higher applied pressure, attributed to membrane fouling. he lux at the transition between the linearly pressuredependent lux and the onset of fouling has been termed “secondary critical lux” [14]. Critical lux is deined as the limiting lux below which a lux decline over time does not occur [13]. Hence, membrane systems should be operated in the “critical lux” region in order to reduce the gel layer formation on the membrane surface. he critical lux increases with higher crosslow velocity (higher Reynolds number) and lower solute concentration.

4.2.2.6 Membrane Fouling Fouling is one of the inevitable phenomena of membrane processes in which the type of feed water determines its severity. Table 4.1 shows the materials that foul membranes.

Membranes for Wastewater Treatment 167 Table 4.1 Materials can cause membrane fouling. Material Mineral salts Metal hydroxides (Fe, Mn, Al) Colloids

Microbiological Organic acids

Intensity and mechanism Scaling in exceeded solubility conditions Sever fouling Electrically charged fouling Fouling by hydrophobic and charge interactions Bioilm gel layer formation Sever fouling

Oil and grease

Hydrophobic membrane fouling

Suspended solids

Cause fouling in RO/NF applications (more than 0.5 ppm)

Proteins and Polyelectrolytes

Fouling can result in a higher operating cost because of loss in water lux and purity, reducing eiciency of the process, lost service time and early membrane replacement. Furthermore, reduced efective membrane surface area because of fouling results in higher capital costs. hus, preventing and reducing the rate of fouling and CP must be considered in design and operation of membrane systems. he permeate lux of pure water through a membrane is generally related to the applied pressure but for an eluent stream the permeate low may be as low as 20% of that of pure water due to CP and/or fouling. In this case, the permeate lux stabilizes ater an initial rapid decline. he steady-state lux is a function of the feed and operating conditions such as luid shear rate at the membrane surface and transmembrane pressure. Generally, a rapid lux decline implies the presence of foulants in the feed stream. he reasons for fouling vary depending on the nature of the solute and solute-membrane interactions. Fouling is regularly the result of a strong interaction between the membrane and the components in the feed stream. As a general rule, CP results in a reversible lux decline, while an irreversible lux reduction is due to fouling. Most fouling materials are hydrophobic and carry a surface charge [15–17]. Proteins as hydrophobic materials are more readily adsorbed on the surface of hydrophobic membranes due to hydrophobic interactions compared to the hydrophilic solutes. he adsorbed layer is more dificult to remove from a hydrophobic membrane in comparison with a hydrophilic membrane. Additionally, biofouling of membranes is a severe problem; the slightly negative charge of bacteria and cell hydrophobicity results in the formation of a bioilm gel layer [18]. Bioilm formation on the membrane surface is the result of adhesion and the

168 Nanostructured Polmer Membranes: Volume 2 growth of microorganisms on the membrane surface. Biofouling causes a reduction in the membrane performance by decreasing the speciic membrane low [19].

4.3 Membrane Separation Techniques in Industry Membrane technologies have developed as one of the main contributors to the resolution of water-related problems over the past two decades. Increasing water scarcity, followed by severe regulations in industrialized countries, have promoted the use of membranes for water and wastewater treatment. Water companies, municipalities, and industries now treat about 60 million m3/day using thousands of membrane plants [20]. From the perspective of wastewater treatment, microiltration (MF), ultrailtration (UF), nanoiltration (NF), and reverse osmosis (RO) are the most common developed membrane separation techniques applied in industries. he main diference in these processes is the pore size of the membranes. Table 4.2 summarizes the recognized membrane separation technologies in water and wastewater treatment. he current section briely describes each of these membrane technology areas. Besides, recent advances in membrane separation techniques for wastewater treatment, such as forward osmosis (FO), are introduced. Table 4.2 Summary of the recognized membrane separation technologies. (Adapted from [9, 21]) Process MF Driving force 0.1–3 bar Separation Molecular mechanism sieve Suspended Material solids, retained bacteria Water, Material dissolved passed solutes Membrane type

Symmetric polymer or ceramic membranes

UF 0.5–10 bar

NF 2–40 bar SolutionMolecular sieve difusion Micropollutants, Macromolecules, salt, glucose, colloids lactose

RO 7–75 bar Solutiondifusion Dissolved salts

Water, dissolved salts

Water, monovaWater lent salts

Asymmetric polymer composite or ceramic membrane

Asymmetric polymer or thin-ilm composite membrane

hin-ilm composite membrane

Membranes for Wastewater Treatment 169 In comparison with conventional wastewater treatment processes, membrane technology ofers the advantage of selectively removing contaminants based on their sizes. Membranes with diferent pore-size distributions and physical properties remove a wide range of pollutants. Since MF membranes have the largest pore size among the other membrane technologies, they reject large particles and various microorganisms as well as bacteria. UF membranes have smaller pore size than the MF membranes, thus they can reject viruses and slightly soluble macromolecules such as proteins. Whereas RO membranes are efectively nonporous and, therefore, retain particles and even some low molar mass species such as salt ions. NF membranes are relatively porous, and their performance is between that of RO and UF membranes, which can be useful in some applications. For instance, using NF membranes for production of drinking water can reduce the post-treatment cost such as demineralization, because of limiting salt rejection. Based on the feed stream quality and purpose of the product (process water, drinking water, wastewater, reclaimed water), each of the membranes or a combination of these processes can be used. An individual membrane pretreatment and post-treatment can also minimize fouling efects and attain the required water quality for each purpose.

4.3.1 Reverse Osmosis and Nanoiltration Systems Reverse osmosis, a high pressure-driven membrane process, is a technique of desalting saltwater solutions. he membrane preferentially allows water molecules to pass and rejects dissolved components of the feed stream without a phase change. The first successful RO membrane plants were operated with cellulose acetate-based membrane in the 1960s. In order to improve the mechanical resistance of the membrane, Dupont de Nemours developed a polyamide-based membrane in the 1970s. In a normal osmosis process, a semipermeable membrane between pure water and salt solution allows the passage of water but does not permit salt to pass. So, a small diference in water concentration (salt solution) results in lowing water into the salt side of the membrane, and produces an osmotic pressure diference. his low will continue until osmotic equilibrium occurs between the water and salt solution. In RO systems, an applied pressure to the salt solution side reverses the osmotic water low, so that water moves from the salt solution to the pure water side of the membrane. he applied pressure must be higher than the osmotic pressure diference.

170 Nanostructured Polmer Membranes: Volume 2 Solution-difusion is the dominant transport mechanism through RO membranes [22–24], including preferential solvent dissolution as well as difusion across the membrane. Based on the mechanism, water transport occurs in three separate steps: absorption into the membrane surface, difusion across the membrane, and desorption in the permeate side [25, 26]. he water permeate lux and salt rejection are two important parameters that determine the performance of the RO membranes. he water permeate lux through the membrane generally is deined by the following equation:

J

A( P

)

(4.8)

where A is the water permeability of the membrane, ΔP and Δπ are the hydraulic pressure diference and the osmotic pressure diference across the membrane, respectively. When the applied pressure is higher than the osmotic pressure (ΔP > Δπ), water permeates through the membrane from its concentrated side to diluted side. he RO membranes are usually operated at high pressures to achieve the maximum throughput and thus optimize the capital and energy costs. In the case of sea water desalination, a high hydraulic pressure is required, ranging from 40 to 75 bar, to overcome the feed-side osmotic pressure [27]. Instead, the salt lux through RO membranes, Js, is independent of the applied pressure and is relative to the salt concentration diferences (ΔC) through the membrane. Accordingly:

Js

B

C

(4.9)

where B is the salt permeability of the membrane. he rejection is based on size exclusion, charge exclusion and physiochemical interactions between solute, solvent and membrane [28, 29]. he fraction that appears in the product is usually measured in terms of the rejection coeicient of the membrane. An ideal RO membrane has a salt rejection of more than 99% [30]. hus, a completely selective membrane has a 100% rejection, while a completely nonselective membrane has a 0% rejection. Since NF is closely associated with RO, and is sometimes known as loose RO, the basic principles of the RO process are valid for the NF process. But, the rejection of solutes is diferent and depends both on molecular size and Donnan exclusion efects, which are due to the functional groups on the polymer backbone. Ions smaller than the pore size are rejected because of the Donnan exclusion [31–33].

Membranes for Wastewater Treatment 171 he average pore size of the NF membranes is between RO and UF membranes. Likewise, the range of operating pressures is also between those for RO and UF processes. he charge of the NF membrane determines its salts rejection potential [34]. Hence, the salt rejection by a neutral NF membrane depends only on the molecular size of the salt. In the case of positively and negatively charged NF membrane, the membrane surface charge can afect its selectivity so that there may be a higher rejection of smaller salts. From the perspective of wastewater treatment, some advantages of NF over RO and UF are as follows [16, 32]: t
he NF selectively rejects divalent and low molecular weight organic compounds, resulting in a better alternative to RO and UF for treating wastewater streams, such as for removing heavy metals and separating dyes and color compounds in the textile industry. t
In the pulp and paper industry, wastewater treatment based on NF process is cheaper than RO, due to a higher permeate lux as well as a lower energy cost. On the other hand, NF process is more efective than UF process to remove low molecular weight toxic chlorinated compounds.

4.3.1.1 Flux, Pressure, and Feed Recovery Rate he design lux, operating pressure, and recovery rate of the NF/RO techniques are generally determined by the feed stream water quality (salt concentration, temperature range, natural organic matter content, etc.), as well as the type and speciication of membrane employed. Table 4.3 lists a range of operational parameters for RO and NF membranes. Table 4.3 Typical design lux and operating pressure for NF and RO membranes. (Adapted from [20]) Seawater RO (Salt conc.: 15–50 g/l) Operating pressure (bar) Design lux (L/m2.h) Recovery factor (%)

Brackish water RO (Salt conc.: 0.1–15 g/l)

NF

50–75

10–40

5–15

10–18

15–25

15–25

35–45

65–85

75–85

172 Nanostructured Polmer Membranes: Volume 2

4.3.1.2 RO and NF Applications he NF and RO processes are efective and eco-friendly technologies for decontamination and recycling of all kinds of water, including ground, surface, and waste water generated in many industries [35–39]. hey have the ability to reduce the wastewater organic loading in terms of chemical oxygen demand (COD) and to promote its partial desalination for possible water reuse. Moreover, NF as a standalone process has been shown in many cases to reduce total organic carbon (TOC) to less than 0.5 mg/L [40]. he NF and RO membranes can remove a wide range of organic micropollutants. Depending on the compound being removed and properties of the membrane used, removal eiciencies may vary. Organics removal eiciencies may range from 99% to less than 50%, related to the membrane speciication type and treatment goal. Likewise, it is dependent on the molecular weight, shape, and charge of the organic molecule. In general, NF and RO can be applied for treatment of a wide range of wastewaters involving petrochemicals [41–43], electroplating [44], coal and gasiication [45], pulp and paper [46, 47], electronic wastewaters [48], toxic metals and cyanides [49], municipal leachates [50, 51], landill leachate [52, 53], olive mill wastewater [54, 55], agricultural wastewater [56].

4.3.1.3 Fouling in NF and RO Process Membrane fouling hinders the NF and RO potential in water and wastewater treatment. Hence, possible interactions between the feed solution and the membrane, causing organic fouling, scaling, biofouling, or particulate fouling, must be considered systematically [57]. In NF and RO processes fouling is generally caused by deposition of colloidal particles (suspended particles such as silica), organic (natural organic matters such as humic acid) or inorganic compounds (salt precipitations such as metal hydroxides and carbonates) and microbial substance (such as bacteria and fungi) on the membrane surface [58]. High levels of inorganic compounds, such as calcium, silica, phosphate, and carbonate, cause scaling in the membrane processes, resulting in productivity reduction and deterioration of permeate quality. Scaling can be controlled by antiscalant injection, lowering the pH and reducing the recovery rate. he negative efects of biofouling on RO and NF membranes have been recognized and studied widely [59–63]. An appropriate pretreatment by coagulation, iltration, and adsorption can reduce the soluble organic load and thus membrane fouling. However, there are still enough nutrients at the membrane surface for microorganisms to colonize.

Membranes for Wastewater Treatment 173 In the NF and RO processes, biofouling occurs during the following steps: 1) microbial adsorption by hydrophobic or electrostatic interaction, 2) microbes propagation with nutrition in the feed water and 3) exhausted biological material deposition. Microbial attachment on the membrane surface results in the formation of bioilms, which consist of microbial cells embedded in an extracellular polymeric substances matrix produced by the microbes [64]. Bioilms can grow to around 1000 μm in thickness in turbulent conditions of low [20]. Membrane biofouling can severely afect the lux and rejection of the membrane by providing a stagnant boundary layer near to the membrane surface, which enhances concentration polarization. In order to extend membrane life span and avoid severe fouling, feed pretreatment is essential. Pretreatment usually consists of chemical coagulation, ine iltration, MF or UF treatment, antiscalant addition and acidiication [65]. In the case of RO systems, the silt density index (SDI) as a standard for pretreatment has been previously utilized, relative to predict and reduce the biofouling risk [61, 66]. Moreover, many attempts involving the design of new membrane modules [67] and the development of antifouling membranes, have been done to alleviate the fouling problem. Among these, development of antifouling NF and RO membranes, as a fundamental route, has been paid much attention by many researchers and membrane manufacturers. Numerous studies have focused on the membrane fouling resistance enhancement by the selection of new starting monomers, improvement of IP process, surface modiication of conventional membranes by physical and chemical methods, as well as the hybrid organic/inorganic membrane [68–70].

4.3.2 Ultrailtration and Microiltration Systems Ultrailtration (UF) membranes have pore sizes up to around 0.1 μm in diameter, Whereas membranes with pore diameters in the range of 0.1 to 10 μm are considered to be microiltration (MF) membranes. he separation mechanism is normally the molecular sieve, in which the particles whose sizes are smaller than the membrane pore size low freely through the pore, while the larger particles are retained. However, in many cases the particles to be separated are adsorbed onto the surface of the pore, causing a major diminution in the pore size. Hence, the particles that are rejected by the membrane are oten much smaller than the size of the pores. Both UF and MF membranes are porous in nature and are divided into two general categories: screen membrane and depth membrane ilters [9].

174 Nanostructured Polmer Membranes: Volume 2 Screen ilters always have an asymmetric with a much denser skin layer and small surface pores (about 5–50 nm in diameter) on a more open substructure, and generally are applied in UF applications. he smaller pore size and lower surface porosity in the membrane skin produces higher hydrodynamic resistance. Retained material accumulates on the membrane surface. hese membranes have identical surface pores and there is a sharp cut-of between completely retained material and penetrate material in the membrane. Whereas depth membrane ilters have much wider pore sizes (around 1–10 μm in diameter) and are commonly used in MF applications, though many smaller restrictions occur within the membrane. hese membranes retain very large particulates on their surface. On the contrary, smaller particulates enter the membrane and trap within it. he particles accumulated on the membrane surface usually form a cake-like secondary ilter layer known as reversible fouling, while the internal trapped particles are oten irreversible. hese membranes are characterized by their molecular weight cut-of (MWCO). he MWCO is typically described as the molecular weight of solute at which the membrane rejects over 95%. Darcy’s law of low through porous materials deines pure water lux through a UF/MF membrane as follows:

J

A

P

(4.10)

he A value is a function of membrane thickness, porosity and pore size. UF systems usually operate at around 1–5 bar (20–75 psi) pressure. As osmotic pressure efect is not signiicant in UF systems, high operating pressure is not necessary to produce a high permeate lux. Surface fouling is the most important drawback related to UF and MF membranes, controlled by the sweeping action of the feed solution. he composition of the feed solution and the process operating conditions highly afect the UF membranes lux due to the membrane fouling. In the case of removing trace particulates, the feed solution is already clean, and a high permeate lux is achieved. Whereas in the case of food processing streams, industrial wastewaters, or electrocoat paint wastes, a much lower permeate lux can be achieved because of more concentrated and contaminated solutions. he circulating low of solution continuously removes the formed ilm on the membrane surface. Hence, circulating low decreases the fouling layer thickness on the membrane surface, and results in a higher permeate lux through the membrane. However, all the deposited material layers cannot be removed, resulting in a decreased permeate lux through the membrane with time. In order to restore the lux to almost its original value,

Membranes for Wastewater Treatment 175 UF membrane modules are washed periodically by a cleaning solution. Unfortunately, washing cannot completely restore the lux to the starting value when a severe fouling takes place, known as irreversible fouling. One of the large applications of UF membranes is in the food industry to recover milk proteins and to remove lactose and salts, as well as in the metal inishing industry to concentrate oil emulsions. Traditionally, UF membranes have been applied for removing organic compounds with a high molecular weight such as proteins, colloids and oils [15, 71]. Hence, some MF/UF plants are applied to treat industrial wastewater streams. hese processes are commonly integrated with NF/RO process, so that the MF/UF plants provide excellent pretreatment for NF/RO systems to decrease fouling efectively.

4.3.3 Forward Osmosis Systems FO is deined by water passage from a salt solution or a polluted solution (feed) across a semipermeable membrane to a solution containing dissolved matter of higher osmotic pressure (draw solution) [72]. he process uses osmotic pressure diference as the driving force of water transport. Aterward, it is required to regenerate the draw solute and recover the water transferred by the FO as the product of the process. As an alternative membrane process, FO also has the potential to treat wastewater, producing high quality water. he potential advantages of FO technology for the treatment of complex feeds are as follow: t
In the FO process there is an easier separation of the water from the higher osmotic pressure solution than the separation through RO [73]. t
Because of the lack of high hydraulic pressures, FO will be more energy-eicient than RO in systems where recovery is unnecessary [74]. t
FO exhibits high rejections of salts, particles, and pathogens and emerging substances; hence, unlike normal treatment facilities, it eiciently removes total dissolved solids (TDS) from complex solutions [72, 74]. t
While RO and NF are subject to fouling and require pretreatment to increase lifetime and reduce costs, FO systems are independent of extensive pretreatment stages [75, 76]. t
FO has been recognized as an excellent operation in terms of durability, reliability and water quality in cases of highly polluted feeds.

176 Nanostructured Polmer Membranes: Volume 2 t
FO has revealed lexibility and applicability because of scalability of the membrane system, reduced fouling propensity, and simple cleaning [77–79]. he FO process can be applied to treat complex industrial streams, such as textile industries, oil and gas well fracturing, landill leachate, wastewater eluent from municipal sources and even nuclear wastewaters have been mentioned [74, 80–84]. Two main drawbacks of FO systems are reverse solute leakage and internal concentration polarization (ICP) [84, 85]. he former increases operational costs and decreases the driving force of the process, and the latter is exclusive to FO and generally occurs within the porous support layer of the membrane. hick support layer leads to poor performance of membrane in the FO process, which is mainly related to the ICP. Both ICP and external CP (ECP) exist in the FO process. Although ECP has an insigniicant efect compared to that of ICP in FO processes, ICP is still an issue for FO and the main driver for further membrane development. Similar to NF and RO membranes, FO membranes have an asymmetric structure with two diferent layers; an active layer and a porous support layer. he active layer is recognized as the dense selective layer, in which the porous support layer provides its mechanical support. Hence, the FO membranes can be orientated either with the active layer facing the feed solution (AL-FS or FO-mode) or the draw solution (AL-DS, RO-mode or PRO-mode). he membrane orientation signiicantly controls FO performances in terms of permeate lux and fouling. Extensive research has reported higher water luxes in AL-DS mode due to less severe ICP. While this orientation is also more prone to membrane fouling, attributed to the entrapment of the foulants in the support layer [86, 87]. Considering both ICP and ECP phenomena, FO mass transport models are based on solution-difusion and convection-difusion equations [88, 89]. he models, especially for use in wastewater applications, neglect fouling because of the complexity and variability of wastewaters. A theoretical equation of FO water lux (Jv) has been established for the AL-FS mode [90]:

Jv

K m ln

A A

B

draw

feed

Jv

B

(4.11)

where Km is the mass transfer coeicient, A the pure water permeability and B the solute permeability.

Membranes for Wastewater Treatment 177 In order to improve FO membranes in term of ICP recent eforts have focused on the membrane structure parameter (S), which is inversely related to mass transfer through FO membranes [91]:

Km

D , S S

.l

(4.12)

where D is the solute difusion coeicient, l is the support layer thickness, τ is the membrane tortuosity and ε is its porosity. FO membranes with low S values, i.e., thinner and looser membrane supports, result in improved membrane performance because of reduced ICP efect. A higher Km can signiicantly increase FO permeate lux [92].

4.3.3.1 Draw Solution and Recovery System he draw solution in the FO process is an important factor inluencing mass transport and overall process performance, because it is directly related to osmotic pressure. In the case of FO wastewater treatment, the focus of draw solution is on good performance, with high water luxes and low solute leakage. he draw solution should be nontoxic, low cost and easily recoverable. During the iltration, the draw solution is diluted gradually because of the water entering from the feed side. Hence, a closed-loop is required to replenish the draw solution and separate it from the product water. In this regard, some methods such as RO, membrane distillation (MD) and thermal recovery have been applied. he recovery system is selected based on the type of application and solute, the recovery rate required and the energy consumption of the unit. In addition, for wastewater treatment forward solute difusion needs to be considered [73].

4.3.3.2 Fouling in FO Systems In FO systems, fouling results in an additional resistance lowering the efective osmotic pressure and water permeability; whereas FO systems are commonly at a lower risk of membrane fouling due to the absence of hydraulic pressure. However, organic fouling and biofouling can be the most limiting factors of FO systems when employed to treat wastewater [93]. Wastewaters consist of eluent organic matter, such as NOM and microorganisms, causing a serious fouling in FO systems [77, 94]. he feed water quality, physicochemical properties of the FO membrane and operating conditions inluence the severity of biofouling and organic fouling.

178 Nanostructured Polmer Membranes: Volume 2

4.4 Membrane Operations in Wastewater Management In recent decades, membrane technologies have developed as promising contributors to solving water scarcity by desalinating sea/brackish water and treating and reusing wastewater. In addition to their intrinsic characteristics, success of membrane operations in wastewater treatment is attributed to compatibility between diferent membrane operations in integrated systems. Basically, membrane treatment systems are divided into two categories: standalone membrane processes and hybrid/integrated membrane systems (IMS). A standalone membrane process treats water using only a membrane process such as UF, NF and RO. Whereas the term hybrid or integrated membrane system refers to two or more treatment processes with or without conventional operations, performing better functions as a result of combination [33, 95]. hanks to the advantages of integrating the product quality, plant compactness, environmental impact, and energy consumption can be improved [4, 16, 96]. he synergy resultant from this integration is the speciic feature of hybrid systems, enhancing the process efectiveness for a particular scenario of wastewater treatment. Nowadays, wastewater treatment by integrated systems suggests reducing environmentally harmful efects, decreasing groundwater consumption and energetic requirements, and recovering valuable compounds as a by-product.

4.4.1 Membrane Bioreactor Membrane bioreactor (MBR), combining membrane iltration with biological treatment, is recognized as one of the most successful hybrid membrane systems in wastewater treatment. Typically, low-pressure membranes such as UF and MF can be used in MBR systems. he MBR system ofers various advantages over the conventional biological treatment. In the MBR process the membrane acts as an absolute barrier to suspended matter. herefore, the system is capable of operating at high mixed liquor suspended solids (MLSS) concentration (MLSS up to 15 g/l). he process also ofers the possibility to operate at long sludge age (> 20 days). Leading to better removal of refractory organic matter, it supports the development of slow-growing microorganisms, which is impossible in conventional activated sludge systems. Ultimately, using the membrane makes the process very compact compared to the conventional systems.

Membranes for Wastewater Treatment 179 Eluent

(a)

Membrane module

Inluent

Membrane module

Inluent

Eluent

(b) Air

Air

Figure 4.2 Two kinds of MBR conigurations: (a) side-stream, (b) submerged.

4.4.1.1

MBR Conigurations

In MBR systems, membrane iltration occurs either externally through recirculation (side-stream coniguration) or within the bioreactor (submerged/immersed coniguration), as shown in Figure 4.2. he former coniguration requires a high cross-low velocity across the membrane to perform well. Consequently, this coniguration ofers the advantages of more membrane fouling control, easier membrane replacement and high luxes, but at the expense of high operational costs compared to the latter coniguration [97, 98]. In submerged systems, the separation driving force across the membrane is provided by a vacuum pump at the permeate side [99]. he submerged MBRs provide beneits of more moderate operating conditions, much lower energy consumption and fewer rigorous cleaning procedures [97, 100, 101]. Even though the treatment performances of both conigurations are similar, most MBRs are now designed in submerged coniguration due to its much lower capital and operating costs [102].

4.4.1.2 MBR Performance Determination and Afecting Factors Depending on the level of wastewater, diferent operating parameters exist during an MBR operation, covering the membrane features and sludge characteristics [103]. he following factors afect the membrane performance of an MBR system [21]: t
Intrinsic resistance of the membrane; t
Transmembrane pressure;

180 Nanostructured Polmer Membranes: Volume 2 t
he hydrodynamic regime of the solution at the membrane surface; t
Fouling. Moreover, the most important parameters of sludge behavior are as follows: t
t
t
t


Hydraulic residence time (HRT); MLSS; Sludge retention time (SRT); Organic loading rate (OLR).

HRT is an important operating parameter, indicating the time that the feed stream remains in the MBR before treating by the membrane. he HRT formula is as follow [21]:

HRT h

Hydrolic volume (L) Permeate flow (L / h)

(4.13)

Lower HRT values result in smaller reactor volumes required achieving a speciied removal performance. Instead, higher HRTs usually result in a better removal performance. MLSS is a critical operational parameter for aerobic MBR systems and is the concentration of suspended solids in the mixture of raw or settled wastewater in an aeration tank (mixed liquor) [104]. It is applied to control the suspended growth process in wastewater treatment plants. he age of the sludge (SRT) indicates how oten sludge is taken away from the system. SRT afects the mixed liquor characteristics and induces changes in the physiological state of microorganisms [105, 106]. Typically, SRT is selected based on the treatment requirements, and ranges of around 8–10 days can be applied. he OLR, the ratio of feed to microorganism (F/M), is an important design parameter in the MBR process, which is deined as kilograms of COD divided by kilograms of MLSS times days. Where the MBRs are used for wastewater treatment, increasing OLR results in a decrease of the MBR ilterability [21].

4.4.1.3 Membrane Fouling in MBR System In MBR systems, fouling is commonly attributed to pore blocking by colloidal and soluble foulants or to cake layer formation on the membrane surface by proteins and NOMs. Based on the fouling components, all

Membranes for Wastewater Treatment 181 biofouling, organic fouling, and inorganic fouling takes place in an MBR system. All the parameters related to the design and operation of MBR systems either directly or indirectly inluence membrane fouling [97, 107]. Among them, membrane and module characteristics, feed and biomass parameters, and operating conditions are recognized as the three major parameters.

4.4.2 Integrated Membrane Systems Since any single treatment process cannot achieve all the treatment objectives, an IMS is frequently used for wastewater recycling to attain multiple treatment purposes. Nowadays, a number of IMSs have been developed to alleviate membrane fouling, particularly when the feed water contains high concentrations of organic matter [108]. Typically, in IMS, a pretreatment will be applied prior to the membrane iltration unit. his pretreatment process may involve conventional units such as coagulation, locculation, sedimentation or a membrane pretreatment method [109].

4.4.2.1 Integration of Membrane Process with Conventional Wastewater Treatment Sometimes one or more membrane processes are coupled with other conventional treatment processes such as coagulation, locculation, adsorption and ion exchange [110, 111]. Conventional pretreatments may produce feed water to an NF/RO system with acceptable quality. For instance, by integrating a coagulation process with a membrane unit in wastewater treatment, the fouling problem could be reduced signiicantly. Without a suitable pretreatment, contaminants such as suspended and dissolved solids will block the NF/RO membranes and reduce their performance in terms of permeate lux and rejection [112]. his is conirmed by a study carried out with a hybrid of coagulation-NF process to remove NOM, bromide and bromate [113]. he study reported that the hybrid coagulationNF process removed the contaminants successfully. At the same time, the combination improved NF permeate lux due to a lower fouling problem. Nevertheless, coagulation or NF alone could not remove those contaminants efectively. However, there are still a few defects, resulting in the deterioration of the following membrane systems. For example, the conventional pretreatment system failed to eliminate major contributing parameters of biofouling so that the pretreatment stages never succeeded in signiicantly reducing microbial numbers [61]. Besides, in the case of food industries where the

182 Nanostructured Polmer Membranes: Volume 2 wastewater contains valuable compounds, the conventional pretreatments cannot recover these compounds. 

4.4.2.2 MF/UF as NF/RO Pretreatment he integration of pressure-driven membrane systems (such as MF, UF, NF and RO) provides interesting insights to eliminate a wide variety of components from diferent wastewaters, ranging from suspended solids to small organic compounds and ions [39]. he request for MF/UF systems as pretreatment to NF/RO is accentuated by the increasing scarcity of lowfouling feed water sources and the demand to treat more diicult feeds such as industrial wastewaters. As NF and RO processes are highly susceptible to fouling when used for wastewater treatment, advanced pretreatment processes like UF and MF are required to increase the productivity of the systems. UF membranes were tested at the RO pilot plant in Singapore, and the results showed some advantages of the membrane pretreatment system compared to a conventional pretreatment system [114, 115]. he consistently high quality of MF/UF iltrate in terms of turbidity and SDI will allow for a higher design lux for downstream RO. A typical design lux for RO wastewater treatment systems with MF/UF pretreatment can be considered 50–60% higher compared to the systems with conventional pretreatment. An improved lux rate has resulted in lower capital costs for RO elements, pressure vessels and associated piping, as well as higher RO permeate quality with 30 to 50% less salt passage [116]. Bohdziewicz et al. [117] evaluated three diferent hybrid systems in the treatment of landill leachate from a municipal waste dump. Among them, the integration of biological treatment with UF membrane as pretreatment of RO process was the most helpful one. In that system, UF process removed the suspension from the biological treated stream to protect the following RO membranes against fouling. he RO process eliminated organic compounds and inorganic salts let in the UF-treated stream. hus the puriied leachate can be discharged into natural waters. Introducing NF as a pretreatment process to RO can improve the lux and reliability because of the reduction of some fouling agents including turbidity, microorganisms, and hardness. Depending on the membrane type, the NF process eliminates most multivalent ions and 10–50% of monovalent species from the eluent stream to be fed to the following RO system. As a result, the osmotic pressure in the RO system is reduced, allowing the process to operate at higher recovery factors [118]. In fact, by coupling NF and RO the integrated process is more environmentally friendly, because fewer additives are needed to reduce fouling [119].

Membranes for Wastewater Treatment 183 Another interesting application as a valid alternative to conventional methods is in the treatment of wastewater from the pulp and paper industries containing various solutes with diferent chemical natures. As conventional processes cannot achieve the requirements of water quality for the process, Zhang and coworkers evaluated the performance of an IMS including MBR, UF and RO to treat and reuse paper mill wastewater on a pilot scale [120]. hese MBR and UF processes are considered as pretreatment for the RO membranes. he MBR unit removes main polluting substances, including soluble and suspended organic compounds, yielding a stream with a COD of less than 50 mg L−1. RO permeate provides all the standards of process water for the paper mill. hus, the IMS assured the possibility of recovery of more than 65% water recovery in RO process.

4.4.3 High Retention Membrane Bioreactors High retention membrane bioreactor (HR-MBR) has recently been introduced as a combination of the conventional MBRs with high rejection membrane iltrations (i.e., NF, RO and FO) in a single step [121]. Related to conventional MBR, the HR-MBR systems can operate in either submerged or external coniguration. Prolonging organic contaminants retention time in the bioreactor, their biodegradation can be efectively increased in these HR-MBR systems. Consequently, HR-MBR can ofer a reliable solution to high quality product. However, there are several technical drawbacks for fast commercialization of these innovative technologies in wastewater treatment, including salinity buildup, low permeate lux and membrane degradation [121]. hree HR-MBR conigurations have been considered, including nanoiltration membrane bioreactor (NF-MBR) [122], osmotic membrane bioreactor (OMBR) [123] and membrane distillation bioreactor (MDBR) [124]. hese systems take advantage of NF or RO, MD and FO as the high retention membrane separation process. he basic diferences between these systems and conventional MBRs are summarized in Table 4.4.

4.4.3.1 Nanoiltration Membrane Bioreactor he NF-MBR coniguration is like conventional MBR, but uses an NF or low pressure RO membrane instead of UF/MF membrane. As NF and RO membranes can efectively reject low molecular weight organic contaminants, the NF-MBR system increases their retention time in the bioreactor for better biological degradation. In this system, a high hydraulic pressure

184 Nanostructured Polmer Membranes: Volume 2 Table 4.4 Basic diferences between conventional MBR and HR-MBRs. (Adapted from [121])

Membrane type

Driving force

HR-MBRs OMBR MDBR FO Porous hydrophobic MF Hydraulic Osmotic Vapor pressure pressure pressure 40–90 ~100 100

Conventional MBR NF-MBR Hydrophilic MF/ NF/RO UF Hydraulic pressure Negligible

NaCl rejection (%) TOC in permeate 3–10 (mg/l) 10–30 Water lux (L/m2.h)

1–4

1, n = 2, x = 5 – 13.5. y = 1000

Figure 5.10 Basic Naion chemical structure.

performance with NR-211 is enhanced in comparison with other Naion membranes due to its smaller thickness. Although Naion membranes have excellent properties for the PEMFC, several limitations, such as high production cost and poor ion conductivity with high temperature, still remain. To overcome these challenges, various methods have been studied. hese include a modiication of conventional PFSA membranes and development of new polymer membranes such as polyarylene sulfone, polyeter ether ketone (PEEK), polyimide, polyphenyloxide (PPO), polybenzimidazole (PBI), and polyvinylidene luoride (PVDF) [44–47]. Figure  5.11 shows the classiication of polymer membranes for the PEMFC. Platinum (Pt)-based catalysts are currently the only choice of electrocatalysts in practical PEMFCs [48]. Most of the recent eforts have focused on reducing the amount of Pt due to its high cost. Various studies related to design and synthesis for the PEMFC catalysts have been actively performed. hese are Pt-alloy catalysts, non-noble catalysts, core-shell structured Pt-based catalysts, Pt-monolayer catalysts, and hollow Pt-based nanocatalysts. he MEA has two catalytic layers where a hydrogen oxidation reaction (HOR) and oxygen reduction reaction (ORR) occur. he HOR on platinum surface at the anode is intrinsically fast, while the ORR at the cathode is very sluggish. A higher platinum loading is required at the cathode than at the anode. At the anode, catalyst poisoning by carbon monoxide (CO) is an important issue because it causes a dramatic drop in the PEMFC performance. here are four strategies to decrease the CO poisoning at the anode: (1) the supply of pure hydrogen gas with the advanced reformer designing at the anode; (2) the operation of the PEMFC at high temperatures above 150 °C; (3) oxygen feeding in the fuel stream; (4) the utilization of CO-tolerant catalysts. Employing CO-tolerant catalysts is an eicient method without the system complications of the PEMFC. A number of platinum alloys or platinum mixtures have been investigated

222 Nanostructured Polmer Membranes: Volume 2 Membrane materials

Perluorinated

Modiied perluorinated

Partially luorinated

Naion/SiO2 Naion/TiO2

PFSA PFCA PFSI Gore-select

Naion/ZrP Naion/CNT Naion/HPA Naion/Zeolite Naion/Imidazole Naion/Cesium

PTFE-g-TFS PVDF-g PSSA

Non-luorinated

Acid base blends

Others

NPI BAM3G SPEEK SPPBP MBS-PBI

SPEEK/PNI/P4VP SPEEK/PEI SPEEK/PSU(NH2)2 SPSU/PBI/P4VP SPSU/PEI SPSU/PSU (NH2)2 PVA/H3PO4

Poly-AMPS Hybrid membrances

Figure 5.11 Classiication of polymer membranes for the PEMFC.

to solve the CO poisoning issue and these includes Pt-ruthenium (Ru), Pt-iron, Pt-molybdenum, platinum-cobalt, Pt-nickel, and Pt-tungsten. Bifunctional and electronic mechanisms can explain the high CO tolerance with these materials. he improvement of the ORR is one of the major challenges in PEMFC development. In the early 1960s, many researchers already recognized the slow kinetics of the ORR in acid solution. A strong O-O bond (498 kJ/mol) causes the diiculty in the oxygen reduction in the PEMFC. Figure 5.12 shows the intrinsic kinetic equation for the ORR in acid media. his kinetic model is composed of four essential elementary reactions. A variety of noble and non-noble metals has been studied for the ORR. Pt has been commonly used as the oxygen reduction catalyst in the PEMFC. he newly developed ORR catalysts have been evaluated in comparison with Pt as the benchmark catalyst. here are several limitations to

Polymer Electrolyte Membrane and Methanol Fuel Cell 223 Table 5.5 Comparison of Pt-based, modiied Pt, and non-Pt catalysts. Advantages

Disadvantages

Pt based Excellent characteristics of high activities for fuel cell reations High cost Low stability

Modiied Pt Low cost High durability under optimized conditions Uncertainty for long-term applications

+H+ +e– 1/2 O2

Non-Pt Low cost Extensive resources Low activity

+H+ +e– OHad

RA

H 2O RD

–

+

+H

DA

+e

RT

Oad

Figure 5.12 Kinetic model for the ORR [49].

Macro porous layer

Figure 5.13 Schematic of the general difusion layer.

employ Pt catalyst for ORR, such as its high cost, gradual degradation, high sensitivity to contaminants. he usage of Pt alloy catalysts can enhance catalytic activity and the reduction of the Pt loading. herefore, many studies have been actively performed to ind the most suitable materials for the ORR. he DL, which is located between the CL and low ield plate, typically consists of only macroporous layer (MPL), as depicted in Figure  5.13. here are two materials for the MPL: carbon-based and metal-based materials. Figure 5.14 shows the detailed classiication of the MPL materials.

224 Nanostructured Polmer Membranes: Volume 2

Macroporous layer

Carbon-based

Metal-based

Carbon paper

Mesh

Carbon cloth

Form

Micromachiend

Figure 5.14 Classiication of materials for the MPL.

Carbon-based materials are widely used because of their great stability in acid environment, high gas permeability, good electrical conductivity, and lexibility. Carbon paper and carbon cloth are the most usual forms of these carbon-based materials in the PEMFC. he related studies have focused on the thickness, porosity, and geometry for the optimal performance. Metal-based MPL, such as metal mesh, metal form, and micromachined metal substrate, have received increasing attention due to its good mechanical strength and high stability over a wide potential range. At the anode, the polarization characteristics were compared with four diferent DL (carbon cloth, carbon paper, and stainless steel wire cloth) by Oedegaard et al. hey found that stainless steel wire cloth is suitable for the DL at the anode in the DMFC because of higher electronic conductivity and excellent two-phase transport of methanol and carbon dioxide from the catalytic layer. Titanium mesh, nickel mesh, and nickel-chromium ally forms have also been evaluated as the DL for the PEMFC. Hydrophobic treatment in the MPL is required for preventing water looding and facilitating oxygen transport at the cathode. Various hydrophobic agents, including polytetraluoroethylene (PTFE), polyvinylidene luoride (PVDF), and luorinated ethylene propylene (FEP), have been utilized to provide such hydrophobicity. Many studies have been reported about the relation between the properties of the gas difusion layer and the contents of the hydrophobic agent. he optimal PEMFC performance was obtained with hydrophobic agents of between 10 and 15%.

Polymer Electrolyte Membrane and Methanol Fuel Cell 225

Micro porous layer Macro porous layer

Figure 5.15 Schematic of the dual-layer structure for the DL.

Table 5.6 Comparison between LT-PEMFC and HT-PEMFC. LT-PEMFC Below 80 °C

HT-PEMFC 120–200 °C

Advantages

Mature component technology

Co tolerance Fast ORR kinetics Simple water and heat management High CO tolerance

Drawbacks

Sluggish ORR kinetics Low CO tolerance

Durability issue Comparability issue Immature component technology

Operating temperature [°C]

here have been several studies about the DL incorporated with the MPL, as shown in Figure 5.15. he microporous layer, which is sandwiched between the catalytic layer and the MPL, reduces the contact resistance and enhances the water management. Many eforts have been made to fabricate the optimal microporous layer.

5.2.2 HT-PEMFC he HT-PEMFC has many beneits compared to the conventional PEMFC (low-temperature PEMFC) operated at the temperature below 80 °C [50–52]. Table 5.6 shows the comparison between LT and HT PEMFCs. As previously stated, the ORR has the slowest electrochemical kinetics and therefore is the determining factor in the overall reaction rate in the LT-PEMFC. he ORR is signiicantly improved at the high temperature. Accordingly, the PEMFC performance can be enhanced.

226 Nanostructured Polmer Membranes: Volume 2

Membrane conductivity

Conventional membrane

PBI membrane

40

60

80

100

120

140

160

Operating temperature ( C)

Figure 5.16 Membrane conductivity of Naion membranes and PBI membrane with the operating temperature.

he platinum catalyst has a signiicant ainity for CO, which is a byproduct of reformation. As a result, the high purity of hydrogen (99.99999%) is needed for the LT-PEMFC. On the other hand, up to 3% CO can be tolerated at the operating temperature of 160 °C. herefore, the production cost for hydrogen can be reduced. Besides, alternative catalysts can be easily applied due to the fact that the increased electrode kinetics water management system can be simpliied as there is a gaseous phase present, and the conventional cooling technologies can be used in the HT-PEMFC. Many eforts have been put into research of membranes, which are considered the biggest barrier to commercialization. he PBI-based membrane is a promising candidate for the HT-PEMFC [53–56]. It can be reliably operated at the higher temperature (up to 200 °C) compared to conventional PEMFC membranes. Ferng and Hou [56] performed the parametric investigation. As the PBI loading, electrode thickness, and electrode porosity decreases, the performance of the HT-PEMFC increases. here are challenges in terms of material durability and compatibility between membrane and electrodes.

5.2.3 Applications In the market, the shipments have been steadily increased with the fast advancement of PEMFC technology. Figure 5.17 shows the PEMFC shipment from 2009 to 2013 (expected data). he PEMFC can generate power from several mW to hundreds of kW. herefore, it can be used in almost

Polymer Electrolyte Membrane and Methanol Fuel Cell 227 200

103 Low power applications

Shipments (–)

150

100

50

0 2009 PEMFC

200

DMFC

2010 PAFC

2011 SOFC

2012 MCFC

AFC

103 High power applications

Shipments (–)

150

100

50

0 2009 PEMFC

DMFC

2010 PAFC

2011 SOFC

MCFC

2012 AFC

Figure 5.17 Annual unit shipments by FC types between 2009 and 2012 [57].

every application such as cellular phones, passenger cars, buses, and distributed power generators. hese are categorized into three groups: transportation, portable, and stationary sectors. he major application of the PEMFC focuses on transportation due to its potential impact on the environment [11]. Most automobile companies have been developing fuel-cell vehicles (FCVs). hese include GM hydrogen 1, Ford Demo IIA, DaimlerChrysler NeCar4a, Honda FCX-V3, Toyota FCHV, Nissan XTERRA and Hyundai Santa Fe FCV (Figure 5.18). Automakers have a commercialization plan of FCVs by 2015. he overall status and target for the automotive PEMFC is shown in Table 5.7. Distributed PEMFC power system is focused on small power ranging between 50–250 kW for decentralized use and below 10 kW for households. However, the high cost is a major barrier for the widespread

228 Nanostructured Polmer Membranes: Volume 2

Toyota FCHV04

Hyundai Santa Fe FCVE

GM Chevrolet Sequel

Honda FCX

Nissan X-TRAIL FCV

DaimlerChrysler NECAR 4

Figure 5.18 FCVs of various automakers.

Table 5.7 Status and target of the PEMFC for the transportation application [58]. Status

Target

61

30

2000–3000

5000

H2 generation cost [/gge]

3–12

2–3

H2 delivery cost [$/gge]

2.30–3.30

1

H2 storage gravimetric

3.0–6.5

7.5

H2 storage volumetric [g/L]

15–50

70

H2 storage cost [$/kWh]

15–23

2

System cost [$/Kw] System durability [h]

commercialization. he application for backup power of companies having high damages associated with sudden power breakdowns also is receiving increasing interest. Plug Power Genesys and Ballard FCgen 1920 ACS fuel cell systems have been developed [11]. he PEMFC can provide continuous power as portable power supply for electric devices like laptops, cell phones, and military radio/communication devices.

5.3 Direct Methanol Fuel Cells (DMFCs) Direct liquid fuel cells (DLFCs) have been mainly developed as the portable power source for various electronic devices, including notebooks,

Polymer Electrolyte Membrane and Methanol Fuel Cell 229 Table 5.8 Advantage and disadvantage of the PEMFC and DMFC. PEMFC Hydrogen

DLFC Liquids

Advantages

Higher eiciency (50–60%) Higher power density

Compact fuel storage Easy handling Higher energy density

Drawbacks

Diicult fuel storage and handling Lower energy density

Higher catalyst loading Lower eiciency (30–40%) Lower power density

Fuel type

Energy density (MJ/L)

25 20 15 10 5 0 Hydrogen

Metahnol

Ethanol

Figure 5.19 Volumetric energy density of hydrogen, methanol, and ethanol.

cellular phones, and tablets, because the handling and storage of liquids are easier than that of hydrogen [59–61]. Table 5.8 shows the comparison of the PEMFC and DMFC. A number of liquids has been proposed and investigated for the DLFC. hese are methanol, ethanol, formic acid, isopropanol, ethylene glycol, demethoxymethane, and so on [60–68]. Among them, methanol is the most popular liquid for the DLFC. In 1990, the DMFC was invented by researchers at several institutions in the United States, including NASA Jet Propulsion Laboratory [7]. During the past two decades, it has received much attention due to several advantages, including the comparatively high volumetric energy density (approximately 4900 Wh/L) of methanol, the ease of storage and handling due to its liquid phase, low operating temperature below 80 °C, and a compact structure. Methanol used as fuel can also be produced from various sources like biomass, natural gas, or coal.

230 Nanostructured Polmer Membranes: Volume 2 Electric load e

e–

–

CH3OH+H2O

H+

O2 Air

Heat

Heat H 2O

CH3OH+H2O

CH3OH and H2O H+

H2O O2 Air

Figure 5.20 Working principle of the DMFC.

he working principle of the DMFC is presented in Figure 5.20. he typical DMFC operates with the anode being supplied with a liquid mixture of methanol and water, while the cathode is provided with air. When methanol is oxidized, protons, electrons and carbon dioxide are generated at the anode. he carbon dioxide is eliminated from the catalytic layer. he protons and electrons are transported through the polymer membrane electrolyte and external electric circuit, respectively. Water is produced by the oxygen reduction at the cathode. he anode, cathode, and overall reactions of the DMFC can be written as: Anode: Cathode: Overall:

CH3OH + H2O → 6H+ + 6e + CO2 1.5O2 + 6H+ + 6e → 3H2O CH3OH + 1.5O2 → 2H2O + CO2

(5.4) (5.5) (5.6)

Methanol crossover through the membrane is the most challenging issue in the DMFC. It can create a mixing potential at the cathode side and drop the fuel eiciency. With these two efects combined, the open-circuit voltage (OCV) and power output are reduced. he methanol crossover can be described by three mechanisms: (1) difusion by methanol concentration gradient; (2) convection by the hydraulic pressure gradient between the anode and the cathode; (3) electro-osmotic drag by proton transport. Han and Liu. [69] described the equation of the methanol crossover, taking these transport mechanisms into account. It can be expressed as:

Polymer Electrolyte Membrane and Methanol Fuel Cell 231

j

DH C L

C2 K P L

ED Mi F

(5.7)

in which j is the methanol lux through the membrane, D is the difusion coeicient, H is the partition coeicient, ΔC is the diference of methanol concentration, and L is the thickness of the membrane; ΔP is the diference in the liquid pressure across the membrane, μ is the viscosity, ED is the electro-osmotic drag coeicient, M is the methanol molar fraction, I is the current density, and F is Faraday’s constant (96485 C/mol). Based on Equation 5.7, the methanol crossover can be alleviated by decreasing the difusivity, increasing the membrane thickness, and reducing the methanol concentration. It can be achieved by the development of membranes which are less permeable for methanol, along with keeping the ionic conductance and optimized methanol feeding strategy so that low-concentration methanol solution (1–2 mol/L) is supplied at the anode side. To alleviate the methanol crossover through the membrane, gas-phase methanol can be used instead of liquid methanol for the DMFC [70, 71] It is known as the vapor-feed DMFC. he eicient vaporization of the liquid methanol is important for the vapor-feed DMFC. his is mainly performed by an external heater and vaporizer. Guo and Faghri [72] described a novel vapor-feed DMFC with a passive thermal-luids management system. At the copper evaporation pad, liquid methanol was vaporized by using the waste heat generated by the DMFC. Kim [73] used a Naion 112 as the vaporizer. he DMFC was able to run 360 h with a power density of 20–25 mW/cm2 at room temperature. Pan [74] applied Naion 117 membranes as the vaporizer and barrier layers in the vapor-feed PEMFC. he DMFC showed a peak power density of 34 mW/cm2 with pure methanol. Abdelkareem et al. [75] investigated the performance of the vapor-feed DMFC by using a hydrophobic porous carbon plate as the vaporizer, located between the fuel reservoir and the current collector. he result shows the peak power density obtained is 24 mW/cm2 with the high methanol concentration (16 M) at room temperature. Eccarius et al. [76] and Ren et al. applied a polydimethysiloxan (PDMS) and Silicon pervaporation membrane respectively for the vapor-feed DMFC. he methanol oxidation has many reaction paths, as given in Figure 5.21 [77]. During methanol oxidation, several possible intermediates are generated. hese occupy the reaction active site and cause the sluggish reaction. CO is generated as intermediate in the methanol oxidation. As previously mentioned, CO gets adsorbed on the Pt surface. It results in the performance degradation. In the DMFC, Pt-Ru alloy is used as the catalyst instead of Pt. he CO oxidation reaction can be accelerated by the second

232 Nanostructured Polmer Membranes: Volume 2 CH3OH

CH2OH

CHOH

COH

CH2O

CHO

CO

HCOOH

COOH CO2

Figure 5.21 Possible reaction path of methanol oxidation reaction.

metal. Stable Ru(OH)ads is generated by water. When the Ru(OH)ads reacts with an adjacent PtCOads, CO is oxidized to CO2. he methanol can be oxidized by free Pt surface.

Pt + CH3OH

PtCOads + 4H+ + 4e

(5.8)

Ru + H2O

Ru(OH)ads + H + e

(5.9)

PtCOad + Ru(OH)ads

+

CO2 + Pt + Ru + H+ + e

(5.10)

A number of metals have been studied as the second metal for the anode catalyst of the DMFC. Ru presently has the best performance. Liu et al. [63] reviewed the various anode catalysts for the DMFC. A water crossover, which is a water permeation from the anode and the cathode through the membrane, may cause two problems in the DMFC. One is water loss from the anode and the other the water looding at the cathode. he water looding is especially responsible for the reduction in the DMFC performance. To solve this problem, diverse water management strategies at the cathode have been developed.

5.3.1 Applications he DMFCs have been tested as power sources for portable, backup power, and transportation applications. Most demonstrations are for portable electronic devices due to the easy storage of liquid methanol. Table 5.9 presents the companies for the portable sectors.

5.4 Principle and Working Process of PEMFCs his section presents the efective design for uniform distribution of reactant gases and thermal management to eiciently release the waste heat produced in the PEMFC.

Polymer Electrolyte Membrane and Methanol Fuel Cell 233 Table 5.9 Companies for the portable sectors [11]. Location USA

Japan

Company CMR Fuel Cells

Details Disposable fuel cartridges for DMFCs

Jadoo Power Systems

Chemical hydride fuel cells 100 W portable electric power supply for aeromedical evacuation application

MTI Micro

Collaboration with equipment manufacturers about external chargers

Neah Power Systems Sony

DMFC units DMFC-powered recharging devices for laptops and mobile phones

Toshiba

10 W DMFC battery charger

UK

CMR Fuel Cells

25 W hybrid DMFC laptop battery charger

China

Horizon

H-racer series of toys and gadgets hobbyist fuel cell system

Korea

Samsung DSI

Military DMFC battery with up to 800% more durability and 54% more power

Germany

SFC Smart Fuel Cell

APUs for camping and leisure, portable soldier-worn military fuel cell sustem

5.4.1 Flow Field Design he low ield design signiicantly afects the PEMFC performance. he reactant gases have to be uniformly distributed from the low channel to the MEA for preventing the non-uniform current density and localized hot spots which induce material degradation. Many geometrical parameters have to be considered in low ield design. hese include coniguration, shape, channel width and depth (including aspect ratio), the number of channels, and low direction. he low ield design can be classiied into two types: an open channel and interdigitated channel [78–80]. Open channel designs include straightparallel, serpentine, cascade, and pin-type low ields, as shown in Figure 5.22. Multiple paths in the parallel and pin-type designs are useful in efectively distributing the reactants on the MEA with low feed pressure. As a result, the parasitic power loss (PEMFC power generation minus

234 Nanostructured Polmer Membranes: Volume 2

(a) Single serpentine channel

(b) Straight parallel channel

(c) Cascade channel

(d) Pin-type channel

Figure 5.22 Open low ield designs for the PEMFC.

pumping power consumption) is relatively low. However, when the asymmetric low resistance occurs by the formation of water droplets in each channel, the non-uniform distribution can be induced to preferentially low the reactants along the path of least resistance. In the parallel design it is also diicult to eiciently remove the blockage due to insuficient pressure. he single serpentine design, which has a single low channel, can easily remove water droplets due to high pressure, while higher pumping loss is induced. he multiple serpentine channel was developed to have the advantages of both single serpentine and parallel channels. Several researchers have investigated various geometries of the multiple serpentine channel for the purpose of optimization [81]. An interdigitated low ield, which was irst devised by Wood et al. [82], indirectly connects inlet and outlet channels, as shown in Figure 5.23. he  reactants low by a convection under the rip. he characteristics of the DL inluence this convection mechanism. he major advantage of the interdigitated channel is the enhancement of catalyst utilization

Polymer Electrolyte Membrane and Methanol Fuel Cell 235

H2

O2

Figure 5.23 Interdigitated low ield.

by eicient water removal from the DL. he high power consumption is a problem. Nam et al. [83] numerically studied the role of under-rip convection. hey found that the diference between neighboring channels afects the convection and transport. he comparison of the current distribution in standard interdigitated and single serpentine channels was performed by Zhang et al. [84]. he current distributions across the interdigitated low were more uniform. Kloess et al. [85] presented bio-inspired intedigitated designs. he results showed the improvement of pressure distribution, decrease in pressure loss, and increase in power density by up to 30% compared to single serpentine and standard interdigitated designs. here are three low conigurations of the reactants: colow, counterlow, and crosslow. High PEMFC performance can be generally obtained with the counterlow because of better membrane hydration and increasing proton conductivity.

5.4.2 hermal Management Heat, oten comparable to the power output, is generated by four factors in the PEMFC. hese include entropic heat of reactions, irreversible heat of electrochemical reactions, water condensation, and ohmic resistance [86, 87]. he overall heat generation rate (q˝) in a single cell can be expressed as:

q

(Etn

Ecell ) i

(5.11)

Here, Etn, Ecell, and i are thermal voltage (also known as the thermoneutral voltage), which represents the imaginary maximum voltage of a FC assuming all the enthalpy change of reaction, the operating voltage, and the current density, respectively.

236 Nanostructured Polmer Membranes: Volume 2 Table 5.10 Heat conductivity of individual components in the PEMFC [86]. Flow ield plate Chemical composition

Heat conductivity [W/m·K]

Graphite

30

DL

CL

PEM

Composed of Mixture of PFSA microporous platinum membrane layer and nanoparticles, carbon iber carbon substrate nanopowders and ionomer materials 0.22 0.27 0.2

he heat generation rate increases with increasing current density in Equation 5.11. At high current density, the heat generation rate is accordingly high. his imposes a great challenge in the cooling of stacks operated at high current density. he thermal properties of individual components dictate the heat removal rate. Andreaus et al. [88] experimentally measured the heat conductivity of each component, as shown in Table 5.10. Each component shows diferent heat transport mechanisms. Only heat conduction afects the heat transfer of the PEM, while both conduction and convection signiicantly contribute to the heat transfer in the CL and DL. Efective cooling is vital for eicient operation of PEMFC stacks. A number of cooling strategies and techniques have been developed. Some of them are applied in practice for PEMFC stacks. Table 5.11 summarizes the various cooling techniques.

5.5 Principle and Working Process of DMFCs his section presents diferent methods of the fuel delivery to the DMFC anode and micro DMFC technologies.

5.5.1 Fuel Supply Method Fuel delivery is critical for portable DMFCs. A right amount of methanol should be supplied to the DMFC anode and CO2 generated by methanol oxidation should be removed from the anode at the same time. Methods for fuel supply can be divided into active and passive types.

Polymer Electrolyte Membrane and Methanol Fuel Cell 237 Table 5.11 Summary of cooling strategies for the PEMFC [87]. Techniques Heat Using highly spreaders thermal /Edge conductive cooling material as heat spreaders

Advantages t
Simple system t
No internal coolant t
Small parasitic power

Disadvantages t
Limited heat transfer length t
Non-availability of cost-efective material with very high thermal conductivity and mechanical properties

Using heat pipes as heat spreaders

t
Simple system t
Small parasitic power t
Very high thermal conductivity

t
Development of heat pipes with small thickness and low weight t
Integration of heat pipes with low ield plates

Cooling with separate air low Liquid cooling

Separate air channels for cooling

t
Simple system t
Small parasitic power

t
Trade-of between cooling performance and parasitic power

Channels integrated in BPPs

t
Strong cooling capability t
Flexible control of cooling capability

t
Coolant degradation t
Large parasitic power

Phase change cooling

Evaporation cooling

t
Simultaneous cooling and internal humidiication t
Simpliied system

t
Dynamic control of water evaporation rate t
hermal mass of liquid water on cold startup

Cooling through t
- Elimination of boiling coolant pump t
Simpliied system

t
Development of suitable working media t
Two-phase low instability

238 Nanostructured Polmer Membranes: Volume 2 Proton exchange membrane Anode/ catalyst

Fuel supply Cathode/ catalyst

Piezoelectric actuator

Fuel low direction

Micromachined layers

Fuel chamber

Nozzle/difuser

Pump chamber

Figure 5.24 Schematic of the DMFC integrated with the valeveless piezoelectric pump [89].

he active fuel supply utilizes external devices such as a pump and a fan. Both the size and power consumption are considered as important criteria for the active method. Micropump technologies are promising candidates as the fuel supplier for the portable DMFC. Zhang and Wang [89] numerically evaluated the possibility of the DMFC integrated with the valveless piezoelectric micropump, which can convert the reciprocating movement of a diaphragm activated by a piezoelectric actuator into pumping. It is shown schematically in Figure 5.24. In the following study, the valveless piezoelectric pump was developed for the DMFC [90]. he results showed the DMFC power increases with the increasing applied voltage of the valveless piezoelectric micropump at the ixed frequency. An electroosmotic (EO) pump, which uses the bulk motion of an electrolyte caused by Coulombic interaction of external electric force, is another option due to its simple structure without moving parts and high low rate per package volume. Buie et al. [91] proposed the EO pump for methanol pumping and investigated the air-breathing DMFC with three methanol concentrations (2, 4, and 8 M). hey found that the EO pump consumed only 5% of DMFC power to supply fuel to the DMFC. Kwon and Kim [92] demonstrated a fuel management system to enhance the energy density by the use of pure methanol. his system consists of three EO pumps which supply the pure methanol, water recirculated from the DMFC anode, and diluted methanol solution, respectively, as shown in Figure 5.25. he results showed that the DMFC with the fuel management system was stably operated during 1 hour and the net power density of 76 mW was obtained. Lee et al. [93] developed the diaphragm air-liquid micropump, which makes it possible to feed the dilute methanol solution and air simultaneously. hey successfully integrated this pump into the DMFC.

Polymer Electrolyte Membrane and Methanol Fuel Cell 239 Carbon dioxide

Water

Second EO pump

Water and carbon dioxide

Methanol

Third EO pump

Diluted methanol solutions

First EO pump

Air-breathing DMFCs

Figure 5.25 Fuel management systems using EO pumps. a.

Vent valve

Channel necks Vgleft Vgright

Methanol solution

Porous membrane

Bubble generator b. Heating rods

Perheater

H

Hydrophilic Hydrophobic c. Vs

left

Vs

right

Thermocouple Anode low ield (a)

(b)

Figure 5.26 (a) Schematic of the natural-circulation fed DMFC [94] and (b) the three-step concept to pump liquid [97].

he passive fuel delivery is based on natural liquid physics, e.g., capillary force and gravity. It has no concern about power consumption and complex system. Ye and Zhao [94] demonstrated a natural-circulation fed DMFC using a gravity mechanism of diferent heights and CO2 production to create pressure drop and upward force, respectively, as shown in Figure 5.26. Yang and Liang [95] investigated the passive delivery based on a surface tension driving mechanism. he passive DMFC has an issue with

240 Nanostructured Polmer Membranes: Volume 2 SiO2 Si (a) Thermal oxidation Resist

Dry ilm (g) Lithography (Dry ilm) Ti/An

(b) Lithography (double side alignment) (h) Ti/Au E-beam deposition Ti/Au (c) Dioxide etching (i) Lift of

Pt

Pt or Pt Ru

(d) Si etching using D-RIE (channels) (j) Catalyst electroplating Glass PEM

(e) Si etching D-RIE (through holes)

(f) Thermal oxidation

(k) Assembly

Figure 5.27 (Let) Micro-DMFC fabrication process and (Right) photo of a fabricated micro-DMFC [99].

orientation. To overcome this challenge, bubble-actuated pumping principles are attractive because these potentially can combine degassing of CO2 and methanol supply without the need of any external devices when applied to a DMFC [96–98].

5.5.2

Micro DMFC

A signiicant efort has been concentrated on miniaturizing the DMFC, in particular by microelectromechanical systems (MEMS) technology. Most of the DMFC components have been successfully redesigned by MEMS technology. Motokawa et al. [99] described a micro-direct methanol fuel cell of 180 mm2 active area, which was prepared using a series of fabrication steps. hese steps included photolithography, deep reactive ion etching (DRIE), and electron beam deposition, as shown in Figure 5.27. hey achieved the maximum power density of 0.44 mW/cm2 and 0.78 mW/cm2 at Pt electrode and Pt-Ru electrode, respectively. Liu et al. [63] performed a parametric study of a silicon-based, Cr/Cu/ Au-coated micro-DMFC. hey reported a maximum power output of 50  mW/cm2 at 200 mA/cm2 for a 2 M methanol solution at operating temperature of 60 °C. Ito et al. [100] presented a lexible micro DMFC which was fabricated on a polymeric lexible substrate containing a micro-hole array. hey obtained high voltage (5.6  V) with 10 cells in series on the substrate. Figure 5.28 shows the fabrication process and image of the lexible micro-DMFC.

Polymer Electrolyte Membrane and Methanol Fuel Cell 241 (a) Preparing the substrate 5 mm

de Catho

mm

m

5m

15

Polysulfone (b) Release Polysulfone substrate Thickness = 0.3 mm

e Anod

m 10 m

(c) Drilling holes = 0.5 mm

m

10 m

on uctor Cond se side r e v ad

t = 0.3 mm Condector on adverse side

(d) Coating PEM PEM

(e) Coating catalyst

n ro to uc nd ide Co ck s ba

m

40 m

Hot press Spacer

Methanol anode Conductor on backside

Pt/Ru black

Oxidant cathode

Pt black (f) Wiring

Au thin ilm

Figure 5.28 Flexible micro DMFC [100].

Voltage [V]

Open circuit voltage

Region 1

Region 2

Region 3

Current density [A/cm2]

Figure 5.29 Schematic of the polarization curve.

5.6 Modeling and heory of Polymer Electrolyte Membrane Fuel Cells he performance of the fuel cell is typically represented by a so-called polarization curve, as shown in Figure 5.29. he polarization curve shows three distinct loss regimes [101]. Activation losses, which result from the high overpotential of the ORR, become a major factor in lowering the overall cell potential at the low current regime. When the current is increased, the potential drop is dominated by ohmic losses. At even higher current, concentration losses caused by mass-transport limitations become a dominant factor of the dramatic voltage drop.

242 Nanostructured Polmer Membranes: Volume 2 he analytical model is expressed on the basis of three losses. he theoretical potential can be derived by the Nernst equation as follows [102, 103]:

E

pH2O RT ln NF pH2 pO2

E0

(5.12)

where E0 is the standard electrode potential, R is the gas constant (8.314  J/mol·K), T is the temperature, n is the number of electrons per reacting molecule, F is the Faraday’s constant, pH2O is the partial pressure of water, pH2 is the partial pressure of hydrogen, and pO2 is the partial pressure of oxygen. he initial drop in the polarization curve stems from the sluggish ORR. his is expressed by the Tafel equation [6].

Eact

a b log(i)

(5.13)

RT ln(i0 ) mF

a

b 2.303

RT mF

(5.14)

(5.15)

Here, α is the activity coeicient, i0 is the exchange current density, and I is the applied current density. In the ohmic loss region, the voltage linearly decreases with the current density [6].

Eohmic

iRohmic

Eohmic

i

i(Relec

Rionic )

Lm A m

(5.16) (5.17)

he concentration loss is due to the balance of the surface concentrations of the reactant species. his can be expressed as a relationship between the rate of the electrode reaction and the supply of the reactants to the catalyst. It is expressed as [6]:

Econc

RT i ln 1 nF ilim it

(5.18)

Polymer Electrolyte Membrane and Methanol Fuel Cell 243 he output voltage delivered to the load is given by:

Ecell

EREV

Eact

Eohmic

Econc

(5.19)

5.7 Conclusion his chapter reviewed the PEMFC and DMFC technologies. hey have been steadily advancing during the past decade. here still remain some barriers for their worldwide commercialization. To overcome the barriers, various studies, including improvement of components (membrane, catalyst, low ield plate, and gas difusion layer) and development of an eicient system of operation, are being actively studied.

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244 Nanostructured Polmer Membranes: Volume 2 13. J.-H. Wee, Applications of proton exchange membrane fuel cell systems, Renew. Sust. Energ. Rev., 11, 1720, 2007. 14. İ. Dinçer and C. Zamirescu, Hydrogen and fuel cell systems, in: Sustainable Energy Systems and Applications, pp. 519, Springer US, 2012,. 15. J. Scholta, B. Rohland, V. Trapp, and U. Focken, Investigations on novel lowcost graphite composite bipolar plates. J. Power Sources, 84, 231, 1999. 16. D.P. Davies, P.L. Adcock, M. Turpin, and S.J. Rowen, Bipolar plate materials for solid polymer fuel cells. J. Appl. Electrochem., 30, 101, 2000. 17. E. Middelman, W. Kout, B. Vogelaar, J. Lenssen, and E. de Waal, Bipolar plates for PEM fuel cells. J. Power Sources, 118, 44, 2003. 18. H. Wang, M.A. Sweikart, and J.A. Turner, Stainless steel as bipolar plate material for polymer electrolyte membrane fuel cells. J. Power Sources, 115, 243, 2003. 19. H.-C. Kuan, C.-C.M. Ma, K.H. Chen, and S.-M. Chen, Preparation, electrical, mechanical and thermal properties of composite bipolar plate for a fuel cell. J. Power Sources, 134, 7, 2004. 20. M.H. Oh, Y.S. Yoon, and S.G. Park, he electrical and physical properties of alternative material bipolar plate for PEM fuel cell system. Electrochim. Acta, 50, 777, 2004. 21. T. Matsuura, M. Kato, and M. Hori, Study on metallic bipolar plate for proton exchange membrane fuel cell. J. Power Sources, 161, 74, 2006. 22. T. Fukutsuka, T. Yamaguchi, S.-I. Miyano, Y. Matsuo, Y. Sugie, and Z. Ogumi, Carbon-coated stainless steel as PEFC bipolar plate material. J. Power Sources, 174, 199, 2007. 23. J. Chen, T. Matsuura, and M. Hori, Novel gas difusion layer with water management function for PEMFC. J. Power Sources, 131, 155, 2004. 24. G.-G. Park, Y.-J. Sohn, T.-H. Yang, Y.-G. Yoon, W.-Y. Lee, and C.-S. Kim, Efect of PTFE contents in the gas difusion media on the performance of PEMFC. J. Power Sources, 131, 182, 2004. 25. T. Hottinen, O. Himanen, S. Karvonen, and I. Nitta, Inhomogeneous compression of PEMFC gas difusion layer: Part II. Modeling the efect. J. Power Sources, 171, 113, 2007. 26. I. Nitta, T. Hottinen, O. Himanen, and M. Mikkola, Inhomogeneous compression of PEMFC gas difusion layer: Part I. Experimental. J. Power Sources, 171, 26, 2007. 27. L. Cindrella, A.M. Kannan, J.F. Lin, K. Saminathan, Y. Ho, C.W. Lin, and J. Wertz, Gas difusion layer for proton exchange membrane fuel cells–A review. J. Power Sources, 194, 146, 2009. 28. D. Cheddie and N. Munroe, Mathematical model of a PEMFC using a PBI membrane, Energ. Convers. Manage., 47, 1490, 2006. 29. Y.-H. Liu, B. Yi, Z.-G. Shao, L. Wang, D. Xing, and H. Zhang, Pt/CNTsNaion reinforced and self-humidifying composite membrane for PEMFC applications. J. Power Sources, 163, 807, 2007.

Polymer Electrolyte Membrane and Methanol Fuel Cell 245 30. Ş. Erce, H. Erdener, R.G. Akay, H. Yücel, N. Baç, and İ. Eroğlu, Efects of sulfonated polyether-etherketone (SPEEK) and composite membranes on the proton exchange membrane fuel cell (PEMFC) performance. Int. J. Hydrogen Energy, 34, 4645, 2009. 31. B.K. Kakati, V.K. Yamsani, K.S. Dhathathreyan, D. Sathiyamoorthy, and A. Verma, he electrical conductivity of a composite bipolar plate for fuel cell applications. Carbon, 47, 2413, 2009. 32. F.G. Boyaci San and G. Tekin, A review of thermoplastic composites for bipolar plate applications. Int. J. Energ. Res., 37, 283, 2013. 33. R. Taherian and M. Nasr, Performance and material selection of nanocomposite bipolar plate in proton exchange membrane fuel cells. Int. J. Energ. Res., n/a, 2013. 34. K. Jayakumar, S. Pandiyan, N. Rajalakshmi, and K.S. Dhathathreyan, Costbeneit analysis of commercial bipolar plates for PEMFC’s. J. Power Sources, 161, 454, 2006. 35. J. Wind, R. Späh, W. Kaiser, and G. Böhm, Metallic bipolar plates for PEM fuel cells. J. Power Sources, 105, 256, 2002. 36. D.P. Davies, P.L. Adcock, M. Turpin, and S.J. Rowen, Stainless steel as a bipolar plate material for solid polymer fuel cells. J. Power Sources, 86, 237, 2000. 37. S. Joseph, J.C. McClure, R. Chianelli, P. Pich, and P.J. Sebastian, Conducting polymer-coated stainless steel bipolar plates for proton exchange membrane fuel cells (PEMFC). Int. J. Hydrogen Energy, 30, 1339, 2005. 38. B.K. Kakati, D. Sathiyamoorthy, and A. Verma, Electrochemical and mechanical behavior of carbon composite bipolar plate for fuel cell. Int. J. Hydrogen Energy, 35, 4185, 2010. 39. S.S. Kocha, Principles of MEA Preparation, in: Handbook of Fuel Cells, John Wiley & Sons, Ltd, 2010. 40. M. Prasanna, E.A. Cho, T.H. Lim, and I.H. Oh, Efects of MEA fabrication method on durability of polymer electrolyte membrane fuel cells. Electrochim. Acta, 53, 5434, 2008. 41. A.K. Mishra, S. Bose, T. Kuila, N.H. Kim, and J.H. Lee, Silicate-based polymer-nanocomposite membranes for polymer electrolyte membrane fuel cells. Prog. Polym. Sci., 37, 842, 2012. 42. M.N. Tsampas, A. Pikos, S. Brosda, A. Katsaounis, and C.G. Vayenas, he efect of membrane thickness on the conductivity of Naion. Electrochim. Acta, 51, 2743, 2006. 43. J. Peron, A. Mani, X. Zhao, D. Edwards, M. Adachi, T. Soboleva, Z. Shi, Z. Xie, T. Navessin, and S. Holdcrot, Properties of Naion NR-211 membranes for PEMFCs. J. Membr. Sci., 356, 44, 2010. 44. H. Wu, X. Shen, Y. Cao, Z. Li, and Z. Jiang, Composite proton conductive membranes composed of sulfonated poly(ether ether ketone) and phosphotungstic acid-loaded imidazole microcapsules as acid reservoirs. J. Membr. Sci., 451, 74, 2014.

246 Nanostructured Polmer Membranes: Volume 2 45. B. Liu, G.P. Robertson, D.-S. Kim, M.D. Guiver, W. Hu, and Z. Jiang, Aromatic poly(ether ketone)s with pendant sulfonic acid phenyl groups prepared by a mild sulfonation method for proton exchange membranes. Macromolecules, 40, 1934, 2007. 46. S. Chen, Y. Yin, H. Kita, and K.-I. Okamoto, Synthesis and properties of sulfonated polyimides from homologous sulfonated diamines bearing bis(aminophenoxyphenyl)sulfone. J. Polym. Sci. A: Polym. Chem., 45, 2797, 2007. 47. K. Miyatake, T. Yasuda, M. Hirai, M. Nanasawa, and M. Watanabe, Synthesis and properties of a polyimide containing pendant sulfophenoxypropoxy groups. J. Polym. Sci. A: Polym. Chem., 45, 157, 2007. 48. X. Zhou, Y. Gan, J. Du, D. Tian, R. Zhang, C. Yang, and Z. Dai, A review of hollow Pt-based nanocatalysts applied in proton exchange membrane fuel cells. J. Power Sources, 232, 310, 2013. 49. J.X. Wang, J. Zhang, and R.R. Adzic, Double-trap kinetic equation for the oxygen reduction reaction on Pt(111) in acidic media. J. Phys. Chem. A, 111, 12702, 2007. 50. J. Zhang, Z. Xie, J. Zhang, Y. Tang, C. Song, T. Navessin, Z. Shi, D. Song, H. Wang, D.P. Wilkinson, Z.-S. Liu, and S. Holdcrot, High temperature PEM fuel cells. J. Power Sources, 160, 872, 2006. 51. S. Bose, T. Kuila, T.X.H. Nguyen, N.H. Kim, K.-t. Lau, and J.H. Lee, Polymer membranes for high temperature proton exchange membrane fuel cell: Recent advances and challenges. Prog. Polym. Sci., 36, 813, 2011. 52. A. Chandan, M. Hattenberger, A. El-kharouf, S. Du, A. Dhir, V. Self, B.G.  Pollet, A.  Ingram, and W. Bujalski, High temperature (HT) polymer electrolyte membrane fuel cells (PEMFC) – A review. J. Power Sources, 231, 264, 2013. 53. Q. Li, R. He, J.O. Jensen, and N.J. Bjerrum, PBI-based polymer membranes for high temperature fuel cells – Preparation, characterization and fuel cell demonstration. Fuel Cells, 4, 147, 2004. 54. J.A. Asensio and P. Gómez-Romero, Recent Developments on proton conducting poly(2,5-benzimidazole) (ABPBI) membranes for high temperature polymer electrolyte membrane fuel cells. Fuel Cells, 5, 336, 2005. 55. Q. Li, J.O. Jensen, R.F. Savinell, and N.J. Bjerrum, High temperature proton exchange membranes based on polybenzimidazoles for fuel cells. Prog. Polym. Sci., 34, 449, 2009. 56. Y.M. Ferng, A. Su, and J. Hou, Parametric investigation to enhance the performance of a PBI-based high-temperature PEMFC. Energ. Convers. Manage., 78, 431, 2014. 57. S. Kocha, Polymer electrolyte membrane (PEM) fuel cells: Automotive applications, in: Fuel Cells, 473, Springer New York, 2013. 58. K. Cowey, K.J. Green, G.O. Mepsted, and R. Reeve, Portable and military fuel cells. Curr. Opin. Solid St. M., 8, 367, 2004.

Polymer Electrolyte Membrane and Methanol Fuel Cell 247 59. W. Qian, D.P. Wilkinson, J. Shen, H. Wang, and J. Zhang, Architecture for portable direct liquid fuel cells. J. Power Sources, 154, 202, 2006. 60. Z. Qi and A. Kaufman, Electrochemical oxidation of 1-methoxy-2-propanol in direct liquid fuel cells. J. Power Sources, 110, 65, 2002. 61. C. Lamy, S. Rousseau, E.M. Belgsir, C. Coutanceau, and J.M. Léger, Recent progress in the direct ethanol fuel cell: Development of new platinum–tin electrocatalysts. Electrochim. Acta, 49, 3901, 2004. 62. H. Liu, C. Song, L. Zhang, J. Zhang, H. Wang, and D.P. Wilkinson, A review of anode catalysis in the direct methanol fuel cell. J. Power Sources, 155, 95, 2006. 63. C.M. Miesse, W.S. Jung, K.-J. Jeong, J.K. Lee, J. Lee, J. Han, S.P. Yoon, S.W. Nam, T.-H. Lim, and S.-A. Hong, Direct formic acid fuel cell portable power system for the operation of a laptop computer. J. Power Sources, 162, 532, 2006. 64. A. Lam, D.P. Wilkinson, and J. Zhang, Novel approach to membraneless direct methanol fuel cells using advanced 3D anodes. Electrochim. Acta, 53, 6890, 2008. 65. X. Yu and P.G. Pickup, Recent advances in direct formic acid fuel cells (DFAFC). J. Power Sources, 182, 124, 2008. 66. A. Serov and C. Kwak, Progress in development of direct dimethyl ether fuel cells. Appl. Catal. B, 91, 1, 2009. 67. C. Lamy and E.M. Belgsir, Other direct-alcohol fuel cells, in: Handbook of Fuel Cells, John Wiley & Sons, Ltd, 2010. 68. J. Han and H. Liu, Real time measurements of methanol crossover in a DMFC. J. Power Sources, 164, 166, 2007. 69. S.L. Ho, S.K. Kamarudin, W.R.W. Daud, and Z. Yaakub, Performance evaluation of a passive direct methanol fuel cell. J. Appl. Sci., 9, 1324, 2009. 70. F.A. Halim, U.A. Hasran, M.S. Masdar, S.K. Kamarudin, and W.R.W. Daud, Overview on vapor feed direct methanol fuel cell. APCBEE Procedia, 3, 40, 2012. 71. Z. Guo and A. Faghri, Vapor feed direct methanol fuel cells with passive thermal-luids management system. J. Power Sources, 167, 378, 2007. 72. H. Kim, Passive direct methanol fuel cells fed with methanol vapor. J. Power Sources, 162, 1232, 2006. 73. Y.H. Pan, Direct methanol fuel cell with concentrated solutions. Electrochem. Solid State Lett., 9, A349, 2006. 74. M.A. Abdelkareem, N. Morohashi, and N. Nakagawa, Factors afecting methanol transport in a passive DMFC employing a porous carbon plate. J. Power Sources, 172, 659, 2007. 75. S. Eccarius, F. Krause, K. Beard, and C. Agert, Passively operated vapor-fed direct methanol fuel cells for portable applications. J. Power Sources, 182, 565, 2008. 76. V.S. Bagotzky, Y.B. Vassiliev, and O.A. Khazova, Generalized scheme of chemisorption, electrooxidation and electroreduction of simple organic

248 Nanostructured Polmer Membranes: Volume 2

77. 78. 79. 80. 81. 82. 83. 84. 85. 86. 87. 88. 89. 90. 91.

compounds on platinum group metals. J. Electroanal. Chem. Interfacial Electrochem., 81, 229, 1977. A.P. Manso, F.F. Marzo, J. Barranco, X. Garikano, and M. Garmendia Mujika, Inluence of geometric parameters of the low ields on the performance of a PEM fuel cell: A review. Int. J. Hydrogen Energy, 37, 15256, 2012. P.J. Hamilton and B.G. Pollet, Polymer electrolyte membrane fuel cell (PEMFC) low ield plate: Design, materials and characterisation. Fuel Cells, 10, 489, 2010. X. Li and I. Sabir, Review of bipolar plates in PEM fuel cells: Flow-ield designs. Int. J. Hydrogen Energy, 30, 359, 2005. R. Boddu, U.K. Marupakula, B. Summers, and P. Majumdar, Development of bipolar plates with diferent low channel conigurations for fuel cells. J. Power Sources, 189, 1083, 2009. D.L. Wood Iii, J.S. Yi, and T.V. Nguyen, Efect of direct liquid water injection and interdigitated low ield on the performance of proton exchange membrane fuel cells. Electrochim. Acta, 43, 3795, 1998. J.H. Nam, K.-J. Lee, S. Sohn, and C.-J. Kim, Multi-pass serpentine low-ields to enhance under-rib convection in polymer electrolyte membrane fuel cells: Design and geometrical characterization. J. Power Sources, 188, 14, 2009. G. Zhang, L. Guo, B. Ma, and H. Liu, Comparison of current distributions in proton exchange membrane fuel cells with interdigitated and serpentine low ields. J. Power Sources, 188, 213, 2009. J.P. Kloess, X. Wang, J. Liu, Z. Shi, and L. Guessous, Investigation of bioinspired low channel designs for bipolar plates in proton exchange membrane fuel cells. J. Power Sources, 188, 132, 2009. S.G. Kandlikar and Z. Lu, hermal management issues in a PEMFC stack–A brief review of current status. Appl. herm. Eng., 29, 1276, 2009. G. Zhang and S.G. Kandlikar, A critical review of cooling techniques in proton exchange membrane fuel cell stacks. Int. J. Hydrogen Energy, 37, 2412, 2012. B. Andreaus, A.J. McEvoy, and G.G. Scherer, Analysis of performance losses in polymer electrolyte fuel cells at high current densities by impedance spectroscopy. Electrochim. Acta, 47, 2223, 2002. T. Zhang and Q.-M. Wang, Valveless piezoelectric micropump for fuel delivery in direct methanol fuel cell (DMFC) devices. J. Power Sources, 140, 72, 2005. T. Zhang and Q.-M. Wang, Performance of miniaturized direct methanol fuel cell (DMFC) devices using micropump for fuel delivery. J. Power Sources, 158, 169, 2006. C.R. Buie, D. Kim, S. Litster, and J.G. Santiago, An electro-osmotic fuel pump for direct methanol fuel cells. Electrochem. Solid State Lett., 10, B196, 2007. K. Kwon and D. Kim, Eicient water recirculation for portable direct methanol fuel cells using electroosmotic pumps. J. Power Sources, 221, 172, 2013.

Polymer Electrolyte Membrane and Methanol Fuel Cell 249 92. S.-M. Lee, Y.-D. Kuan, and M.-F. Sung, Diaphragm air–liquid micro pump applicable to the direct methanol fuel cell. J. Power Sources, 238, 290, 2013. 93. Q. Ye and T.S. Zhao, A natural-circulation fuel delivery system for direct methanol fuel cells. J. Power Sources, 147, 196, 2005. 94. Y. Yang and Y.C. Liang, A direct methanol fuel cell system with passive fuel delivery based on liquid surface tension. J. Power Sources, 165, 185, 2007. 95. N. Paust, C. Litterst, T. Metz, M. Eck, C. Ziegler, R. Zengerle, and P. Koltay, Capillary-driven pumping for passive degassing and fuel supply in direct methanol fuel cells. Microluid. Nanoluidics, 7, 531, 2009. 96. D.D. Meng and C.J. Kim, Micropumping of liquid by directional growth and selective venting of gas bubbles. Lab Chip, 8, 958, 2008. 97. D.D. Meng and C.J. Kim, An active micro-direct methanol fuel cell with selfcirculation of fuel and built-in removal of CO2 bubbles. J. Power Sources, 194, 445, 2009. 98. S. Motokawa, M. Mohamedi, T. Momma, S. Shoji, and T. Osaka, MEMSbased design and fabrication of a new concept micro direct methanol fuel cell (μ-DMFC). Electrochem. Commun., 6, 562, 2004. 99. T. Ito, K. Kimura, and M. Kunimatsu, Characteristics of micro DMFCs array fabricated on lexible polymeric substrate. Electrochem. Commun., 8, 973, 2006. 100. A. Bıyıkoğlu, Review of proton exchange membrane fuel cell models. Int. J. Hydrogen Energy, 30, 1181, 2005. 101. D. Cheddie and N. Munroe, Review and comparison of approaches to proton exchange membrane fuel cell modeling. J. Power Sources, 147, 72, 2005. 102. A.A. Shah, K.H. Luo, T.R. Ralph, and F.C. Walsh, Recent trends and developments in polymer electrolyte membrane fuel cell modelling. Electrochim. Acta, 56, 3731, 2011.

6 Polymer Membranes for Water Desalination and Treatment Tânia L. S. Silva, Sergio Morales-Torres*, José L. Figueiredo and Adrián M. T. Silva LCM – Laboratory of Catalysis and Materials – Associate Laboratory LSRELCM, Faculty of Engineering, University of Porto, Porto, Portugal

Abstract Among the available desalination technologies, membrane distillation (MD) is an emerging thermally driven process presenting several advantages compared to the current leading desalination technology, i.e., reverse osmosis (RO). Desalination of high-salinity brines and wastewaters are two of the many MD applications. However, high energy demand, mass transport resistance or low thermal eiciency, are still some of the drawbacks hindering its widespread commercialization. Innovative indings intended to fulill industry’s expectations and to overcome MD limitations are already being implemented. Among them is the design of high performance membranes and modules, as well as MD integration with other membrane technologies, separation/oxidation processes, or renewable energy sources. his chapter summarizes the recent progress in the fabrication and modiication of MD membranes, as well as intrinsic aspects of the MD process, such as mechanistic fundamentals, conigurations and operating parameters. he chapter also ofers a comprehensive outlook concerning the advances of this technology in water desalination and treatment. Keywords: Membrane distillation, hydrophobic polymer membranes, mechanistic fundamentals, coniguration, operating parameters, desalination, water treatment, hybrid systems

*Corresponding author: [email protected] Visakh P.M. and Olga Nazarenko (eds.) Nanostructured Polymer Membranes: Volume 2, (251–286) © 2016 Scrivener Publishing LLC

251

252 Nanostructured Polmer Membranes: Volume 2

6.1 Introduction Water scarcity is one of the main global concerns of the 21st century. he need to provide suicient clean water to satisfy increasing agricultural, industrial, and municipal demands, underlines the potential of seawater desalination as an important source [1]. Desalination technologies can be grouped according to three categories: (i) involving a phase change (e.g., distillation), (ii) interacting with selective membranes (e.g., reverse osmosis), and (iii) employing electric ields (e.g., capacitive deionization) [2]. Multi-stage lash or multi-efect distillation (MSF and MED, respectively) and mainly reverse osmosis (RO) are the most widely used processes for clean water production [3, 4]. he speciic energy to desalinate seawater by MED and MSF is ca. 18–30 and 24–37 kW h m 3, respectively, while the energy required by membrane processes (e.g., RO) is lower than 4 kW h m 3 [5, 6]. hus, the use of membranes in the desalination of seawater already accounts for 50% of the worldwide clean water production [7]. Membrane distillation (MD) and forward osmosis (FO) are considered potential alternatives to the leading desalination technology (i.e., RO) [2], namely for the desalination of high-salinity brines or industrial wastewaters, where RO presents limitations [8]. FO has received growing interest due to its low hydraulic pressure and lower propensity for membrane fouling [9], while MD is a prominent membrane technology with about 47 years of research, but still requiring additional development to address some of the issues hindering its widespread industrial application. MD is a thermal process driven by a vapor pressure diference (driving force) between both sides of a hydrophobic membrane, which acts as a physical barrier. All nonvolatile solutes in the feed solution are selectively excluded, conceptually allowing only water vapor molecules to cross the membrane pores and to condensate in the liquid/vapor interface formed in the permeate side [10, 11]. MD ofers numerous advantages compared with other membrane separation processes: (i) 100% (theoretical) rejection of inorganic ions, macromolecules and other nonvolatile compounds, (ii) low sensitivity to increasing solute concentrations, and (iii) a lower operating pressure compared to other pressure-driven processes [12]. his chapter is focused on polymer membranes for water desalination, in particular through MD and MD hybrid systems. he properties and fabrication methods of suitable MD membranes, as well as intrinsic aspects, such as mechanistic fundamentals, conigurations and operating parameters, are addressed. he chapter also ofers a comprehensive perspective of the main advances in water desalination and treatment by this emerging technology.

Polymer Membranes for Water Desalination and Treatment 253

6.2 Polymer Membranes Used in Distillation An MD membrane must be a physical barrier with high hydrophobicity, large porosity, and a homogeneous and relative wide pore size distribution. In addition, it should provide high permeate luxes, low vapor transfer resistance, low thermal conductivity (to reduce heat loss across the pores) and resistance towards high temperatures, fouling and scaling, as well as require an easy cleaning procedure [13].

6.2.1 Fabrication and Modiication of Hydrophobic Membranes Surface hydrophobicity is the most important property of MD membranes, so hydrophobic polymers, showing low surface energy, are involved in their preparation. Polymers such as polytetraluoroethylene (PTFE), polyethylene (PE), polypropylene (PP) and polyvinylidene luoride (PVDF) [14–16], are preferably employed, due to their easy processing, modiication, scale-up and low costs [17]. Since PTFE is a highly crystalline polymer with the lowest surface energy (ca. 9–20 × 10 3 N m 1), it has excellent thermal stability and chemical resistance [13]. However, it is a nonpolar polymer in which processing excludes typical synthesis processes, such as nonsolvent induced phase separation (NIPS) or thermally induced phase separation (TIPS). Instead, PTFE membranes are usually manufactured by sintering or meltextrusion-stretching methods [18, 19] and they are oten used in commercial and pilot MD systems due to their excellent wetting resistance, high permeate lux and stability [13] (Figure 6.1a and b). Another highly crystalline polymer is PP, which presents a higher surface energy (30 × 10 3 N m 1) than PTFE [14]. PP membranes have excellent elastic properties and are normally prepared by melt-extrusion-stretching methods (Figure 6.1c) [20], although TIPS can also be employed [21]. hese membranes appear to be advantageous in terms of material and manufacturing costs, but their low performance, favored by their symmetric structure and moderate thermal stability, limits their application in MD [13]. Among the mentioned hydrophobic polymers, PVDF is the most widely used in the lab-scale preparation of MD membranes, due to its solubility in typical solvents such as n-methyl-2-pyrrolidone (NMP), dimethylacetamide (DMAC) and dimethylformamide (DMF). he preparation of these PVDF membranes may involve diferent methods as the mentioned NIPS, TIPS, TIPS/NIPS or even, electrospinning processes [22–24] (Figure 6.1d–f).

254 Nanostructured Polmer Membranes: Volume 2

(a)

(b)

(c)

(d)

(e)

(f)

(g)

(h)

(i)

Figure 6.1 Representative SEM images of diferent MD membranes: (a, b) PTFE membranes fabricated by the melt-extrusion-stretching method; (c) PP membranes prepared from a melt-extrusion-mono-axially-stretching method; (d, e) PVDF membranes synthetized by NIPS and TIPS methods, respectively; (f) electrospun PVDF membrane; (g, h, i) diferent examples of PVDF hollow ibers. (Figures adapted with permission from [13], Copyright (2015) Elsevier; and from [24], Copyright (2011) ACS Publications).

Hylon AD (Solvay Plastics) and ethylene-chlorotriluoroethylene (ECTFE) are copolymers of tetraluoroethylene (TFE), which are also used to prepare asymmetric membranes with enhanced hydrophobicity (contact angles higher than 120 °) [25, 26]. Other authors reported MD membranes made from poly(vinylidene luoride-hexaluoropropylene) (PVDF-HFP) or aromatic luorinated polyoxadiazoles and polytriazoles by using NIPS and/or electrospinning methods, respectively [27, 28]. Hydrophilic polymers and non-polymeric materials, such as metal, glass, carbon materials and ceramics (e.g., zirconia and alumina), were also used in the fabrication of MD membranes [29–31], although in most cases a post-treatment is needed to increase the hydrophobicity. Plasma

Polymer Membranes for Water Desalination and Treatment 255 treatments have been demonstrated as eicient to luorine-containing monomers (e.g., polyethersufone, PES) [32]. he incorporation of additives or micro- or nanoparticles in the dope solution during membrane preparation by NIPS, TIPS or melt-extrusion methods, is another known method to modify the membrane properties, the permeation performance and antifouling character. Small-molecule pore forming agents such as water, alcohols, ethylene glycerol (EG) and poly-ethylene-glycerol (PEG) have been employed with this purpose [33]. In general, these compounds act as nonsolvents in dope solution (e.g., PVDF), increasing its viscosity due to a lower polymer solubility and, consequently, resulting in membranes with a more developed porosity. An enhancement of permeation motivated by larger pores, higher porosity and larger hydrophobicity was also obtained using inorganic salts such as LiCl or LiClO4 [33, 34]. A well-known pore-forming agent is deinitely polyvinylpyrrolidone (PVP), which generally promotes the formation of macrovoids when the PVP content is low, while a sponge-like structure with high porosity is obtained at high contents. However, the hydrophilicity of PVP still prevents its use in MD membranes. On the other hand, inorganic micro- or nanoparticles can be easily incorporated when preparing mixed-matrix membranes. In this context, not only are changes produced in solution rheology properties but also speciic beneits from particles are transferred to membrane properties, such as hydrophilicity/hydrophobicity, enhanced porosity or anti-fouling properties. Hydrophobic modiied nanoclay particles, PTFE particles and carbon materials, like carbon nanotubes (CNTs) and graphene derivatives, have been studied [35–37].

6.2.2 Macro-Geometries of MD Membranes An ideal MD membrane should present a high porosity and a relative large pore size (0.1–0.3 μm) to enhance mass transfer and to reduce temperature polarization. In contrast, these membrane characteristics lead to a mechanical weakness in both axial and radial directions. MD membranes are commonly fabricated with two main macro-geometries: lat sheets and hollow ibers. Flat-sheet membranes are typically placed into plate-andframe or spiral-wound modules to increase the packing density, which corresponds to the ratio between the membrane area and the given packing volume. In plate-and-frame modules, the membranes, the porous plates and the spacers are stacked and placed in an appropriate housing, a packing density of 100–400 m2 m 3 being obtained depending on the number of membranes used. For spiral-wound modules, the same components (spacers, membranes and supports) are enveloped and rolled around a

256 Nanostructured Polmer Membranes: Volume 2 perforated central collection tube, the packing density being increased until 300–1000  m2  m 3 depending on the channel height. Both modules allow obtainment of high feed low rates and reduction of fouling and polarization phenomena. In fact, spiral-wound MD modules have already been employed for desalination at industrial level [38]. Hollow-iber modules composed of a large number of ibers (outer diameter  = 50 and 100 mm) are arranged in parallel as a bundle in a shell tube, the packing density being in the range of 500–9000  m2 m 3. herefore, hollow-iber membranes allow achieving larger speciic surface areas compared to lat-sheet membranes with plate-and-frame and spiralwound conigurations. However, hollow ibers present weak mechanical properties and are more susceptible to scaling, clogging and damage [13]. To overcome this, multi-bore hollow ibers (MBF) and dual-layer hollow ibers were developed [33, 39, 40] (Figure 6.1g–i). On the other hand, hollow-iber modules depend on the membrane lifetime, since damaged ibers cannot be easily replaced, as in the case of lat-sheet modules. Companies have already demonstrated their interest in the fabrication of MD membranes, lat-sheet PTFE membranes with polyester (PET) or PP supports being produced by PALL, Gore, Membrane Solutions, Millipore and GE [14]. Hollow-iber PTFE membranes have been commercialized by Toyobo and several research groups [39], while lat-sheet and hollow-iber PP membranes are available from Celgard (Pollypore) and ACCUREL (Membrana) [39].

6.3 Membrane Distillation 6.3.1 heory and Mechanistic Fundamentals To predict and model the performance of MD, diferent mass transport mechanisms can be used, depending on the selected MD coniguration (described in the next section). In general, mass transfer in MD is described as a contribution of three mechanisms: Knudsen low, Poiseuille or viscous low, and molecular difusion low [41, 42]. More than one mechanism can occur simultaneously throughout the membrane [42]. A way of predicting the best mechanism describing mass low for a given pore size under certain operating conditions is to determine the Knudsen number (kn). It is deined as the ratio between the mean free path (λ) of the transported molecules and the membrane pore size (dp); λ of a certain i species being deined as follows: kBT i

2 P

2 i

(6.1)

Polymer Membranes for Water Desalination and Treatment 257 where kB represents the Boltzmann constant, σi is the collision diameter (e.g., 2.641 Å for water vapor), P the mean pressure inside membrane pores and T the absolute temperature. If kn values are lower than 0.01 (dp > 100 λ), molecular difusion is the dominant transport mechanism, as additional mass transfer resistances can be established due to the presence of nonidentical molecules appearing in the way of the vapor path (e.g., air trapped inside the membrane pores for its low solubility in water). However, for a kn higher than 10, Knudsen difusion prevails, as mass transfer is limited by the molecules bouncing inside the membrane matrix. In the transition region, i.e., 0.01 < kn < 10 (0.1 λ < dp < 100 λ), mass low is better described by the combination of Knudsen and ordinary difusion mechanisms [43]. For large pore sizes in relation to the mean free path of the transported molecules, Poiseuille low prevails, as molecule-molecule collisions become more signiicant than the molecule-pore wall collisions [10]. he permeate lux is inluenced by the membrane characteristics and by the established driving force. It can be enhanced by increasing the membrane pore size and porosity and by decreasing the tortuosity and the thickness of the selected membrane [43]. he mass lux can be determined as: J

(6.2)

Bi pi

where Bi represents the MD coeicient of the membrane, which varies according to the selected coniguration, and Δpi deines the transmembrane vapor pressure diference [43]. he partial pressure diference of pure water is calculated by the Antoine equation [43]: pi

exp 23.1964

3816.44 T 46.13

(6.3)

he equations used to calculate Bi, according to the mechanism better describing mass transfer, are shown in Table 6.1 [43–45]. In MD, there is also heat transfer across the membrane, which is described by two main steps: (i) latent heat transfer accompanying the vapor lux from the bulk to the boundary layer and across the membrane (convection), and (ii) heat transfer by conduction through the membrane matrix [43]. When designing MD modules, the heat transfer coeicient, thermal conductivity and heat low become crucial parameters to achieve a high performance. he heat transfer mechanism varies according to the selected MD coniguration [43], and the heat lux in the diferent regions of the membrane modules can be calculated by applying the equations presented in Table 6.2 [10, 46]. he development of high performance membranes and the design of hydrodynamically improved membrane modules appear to be the

258 Nanostructured Polmer Membranes: Volume 2 Table 6.1 Equations to determine the Bi coeicient for each mass transfer mechanism. Mass transfer mechanism Knudsen difusion

*Equation

Bik

Molecular difusion Knudsen and molecular difusion (transition region)

Mi RT

BiD

Bic

RT 3 2 r 8 Mi

Poiseuille low

0. 5

2 r 8 Mi RT 3

(6.4)

PD Pa 1 2

1

Pa RT PDM

r4 P 1 8 i RT

Biv

(6.5)

(6.6)

(6.7)

* ε, τ, r, δ, T, Pa, Mi, μ, P, D and P correspond to the porosity, pore tortuosity, pore radius, membrane thickness, temperature, air pressure within the membrane pore, molecular weight of water vapor, viscosity, total pressure, difusion coeicient and average pressure in pores, respectively.

Table 6.2 Equations describing heat transfer in each region of the membrane modules [43–45]. Heat transfer region hermal feed boundary layer Membrane hermal permeate boundary layer (SGMD, DCMD) Air gap (AGMD) Condensate ilm (AGMD)

*Equation

Qf Qm Qp Qg Qd

h f Tf

T f ,m

(6.8)

hm T f , m Tp , m

(6.9)

hp Tp , m Tp

(6.10)

kg b

Tp , m T film

hd T film Tcs

(6.11) (6.12)

*Qf, Qm, Qp, Qg, and Qd are the heat luxes for the feed boundary layer, membrane, permeate boundary layer, air gap, and condensation ilm, respectively; hf, hm, hp, and hd are the heat transfer coeicients of feed boundary layer, membrane material, permeate boundary layer, and condensation ilm, respectively; Tf , Tf,m, Tp,m, Tp, Tilm, and Tcs are the temperatures in bulk feed, at the membrane surface of the feed side, at the membrane surface of the permeate side, in the bulk permeate, at the condensation ilm surface, and at the cooling plate surface, respectively; SGMD, DCMD and AGMD stand for sweeping gas membrane distillation, direct contact membrane distillation and air gap membrane distillation, respectively.

Polymer Membranes for Water Desalination and Treatment 259 major drawbacks limiting MD’s wide commercialization. he poor luid dynamics normally associated with MD systems are considered to be responsible for the aggravation of well-known concentration and temperature polarization phenomena [47]. Temperature polarization occurs even when the feed is just water; as heat is removed for its vaporization, the temperature tends to decrease at the membrane surface, thus reducing the required temperature diference (driven force) across both sides of the membrane [10]. If nonvolatile solutes are present in the feed stream, the concentration polarization efect also should be considered, as their concentration will be increasing over time with the decreasing retentate volume. In the regions close to the membrane surface, the concentration of solutes may become higher than in the bulk, thus, forming additional boundary layers that may represent additional resistances to the mass lux [10, 48]. In order to quantify the magnitude of the efect of  both phenomena in the overall performance of MD, the temperature and the concentration polarization coeicients can be determined [49]; these parameters being used as quality indicators of the MD module design. For instance, well-designed MD modules should exhibit a temperature polarization coeicient near unity [43], meaning the temperature at the membrane surface is approaching the temperature in the bulk solution [10, 44].

6.3.2 Conigurations of MD Systems here are four main MD conigurations, where the driving force is obtained by diferent arrangements of vapor condensation in the permeate side (Figure 6.2).

6.3.2.1

Direct Contact Membrane Distillation (DCMD)

he feed and permeate aqueous solutions are kept in direct contact with the membrane surface in DCMD (Figure 6.2a). Evaporation and condensation occur at the liquid/vapor interface found at the pore entrances on the feed and permeate streams, respectively [10, 43]. he vapor crosses the membrane due to the vapor pressure diference established between both sides of the hydrophobic membrane, allowing the selective exclusion of the nonvolatiles that are retained in the feed stream. he most common MD coniguration used in the literature is DCMD, due to its simplicity and because it allows a better assessment of the membrane properties during lab-scale studies. It provides less resistance to the mass lux than other conigurations, but larger heat losses by conduction are registered.

260 Nanostructured Polmer Membranes: Volume 2

6.3.2.2 Air Gap Membrane Distillation (AGMD) In AGMD, an additional chamber illed with stagnant air is placed between the membrane and the condensation surface (Figure 6.2b). Water vapor molecules are forced to cross both the membrane and the air gap until reaching the cooling surface placed inside the MD module. Heat loss by conduction is reduced in this coniguration, as the latent heat used to evaporate the volatiles can be recovered during condensation of the vapor on the cooling plate. Even so, the additional air resistances used in this coniguration result in lower mass luxes, caused by the low temperature diferences; larger membrane areas are required to overcome this. Despite the lower permeability, AGMD is the foundation for some of the most promising modiied MD systems described in the literature, namely the Memstill (Netherlands Organization for Applied Scientiic Research) [50, 51], Scarab AB (Sweden) [52], and Fraunhofer ISE (Germany) [53–55] apparatus.

6.3.2.3 Sweeping Gas Membrane Distillation (SGMD) Inert gas is used at the permeate side to sweep the vapor out of the membrane module to be condensed in an external condenser in SGMD (Figure 6.2c). An additional gas barrier is found, as in the case of AGMD,

Coolant Permeate

(a)

Membrane

Air gap Membrane

Feed

Feed

DCMD

(b)

AGMD Vacuum pump

Condenser

Condenser

Permeate

Permeate

Sweep gas

(c)

Membrane

Membrane

Feed

Feed

SGMD

(d)

VMD

Figure 6.2 Schematic representations of the main MD conigurations.

Condesating plate Permeate

Polymer Membranes for Water Desalination and Treatment 261 but it is not stationary, which increases the mass transfer coeicient. he main drawback of this arrangement is the low volume of permeate difusing in a large sweep gas volume, thereby demanding a larger condenser. In the literature there are fewer studies about SGMD; it has been suggested as a useful approach for the removal of ammonia from aqueous solutions [56–58] or for water desalination [59].

6.3.2.4 Vacuum Membrane Distillation (VMD) he VMD coniguration includes vacuum, which is applied in the permeate stream to reduce the vapor pressure in this compartment, thereby increasing the driving force of the process (Figure 6.2d). External condensation outside the membrane module is also required in this arrangement. he heat loss by conduction is negligible, which in association with its great ability to provide higher driving forces, makes VMD very attractive. If an eicient condenser is used, higher permeate luxes and better thermal eiciencies are achieved in this coniguration. It has been reported in the literature as being useful for many applications, including water desalination [60, 61], water puriication/treatment [62–64], food industry [65] and ethanol recovery [66], among others.

6.3.2.5 Other MD Conigurations Adaptations and new conigurations are being proposed in the literature in order to overcome some of the limiting issues associated with other arrangements. he introduction of such concepts as multi-stage and multiefect MD seem to show great potential. For instance, the already mentioned Scarab AB system, based on the AGMD coniguration, was built under the multi-stage layout to minimize energy consumption [52]. Another successful case is the multi-efect spiral-wound MD membrane module designed by the Fraunhofer Institute for Solar Energy Systems (ISE), which uses the same arrangement as the AGMD coniguration, but the air gap is illed with a stagnant volume instead of permeate, which is then named permeate gap MD (PGMD) [55]. Its major advantages are the separation of the distillate from the coolant, which allows the use of diferent cooling solutions, and the reduction of the sensible heat losses due to the additional heat transfer resistance. In addition, other variants can be found in the literature by illing the air gap with sand or other insulating materials, such as polyurethane and polypropylene, which leads to the material gap MD (MGMD) arrangement [67]. he combination of two or more of the common four MD conigurations have also been reported. An example is the new module named thermostatic SGMD (TSGMD), which was described as being able to minimize

262 Nanostructured Polmer Membranes: Volume 2 the increase of the sweeping gas temperature (found to be problematic in the SGMD coniguration) through the use of a cold wall placed in the cold chamber (similar to AGMD); the inert gas being forced to pass through the gap between the membrane and the cooling plate [68].

6.3.3 Operating Parameters Inluencing MD Systems he performance of MD is seriously afected by a group of fundamental operating parameters extensively mentioned in the literature [11, 41, 44, 69, 70]. To predict and explain some of the tendencies in MD experiments, some general and more speciic mathematical models, describing both mass and heat transfers in MD, are presented in Tables 6.1 and 6.2 or reported elsewhere [33, 71]. Some of the most determinant parameters are: the temperatures in the feed and permeate (or gas for SGMD; coolant for AGMD); feed and permeate low rates; solute concentration in the feed aqueous solution; downstream pressure (VMD); presence of noncondensable gases; air/vapor gap width (AGMD and VMD, respectively); and membrane properties. his section focuses on the efect of these operating conditions on the permeate lux in MD systems.

6.3.3.1

Feed Temperature

he feed temperature is normally adjusted in a range between 50–90 °C, the highest temperature standing below the boiling point of the aqueous solution. It has been widely reported in the literature as the main factor afecting MD performance for all conigurations, since an increase in the feed temperature causes an exponential increase of the vapor pressure in the feed stream. Since the transmembrane vapor pressure diference (driving force) is enhanced, the permeate lux is higher [72]. Higher temperatures mean higher vaporization rates, which are associated with larger evaporation eiciencies (the ratio of heat contributing to evaporation); however, the efect of temperature polarization is also larger at higher feed temperatures [10, 73–75].

6.3.3.2

Permeate Temperature

Following the same principle for the permeate temperature, lower temperatures turn into a higher vapor pressure diference. In general, it varies from 10 °C to 40 °C and shows more signiicance for the DCMD coniguration. However, the efect of increasing temperature of the cooling plate is negligible for AGMD, as the heat transfer coeicient in the

Polymer Membranes for Water Desalination and Treatment 263 air gap dominates the overall heat transfer coeicient. A similar scenario is found for the SGMD coniguration, since the temperature of the sweeping gas in the permeate side increases very rapidly from the inlet to the outlet of the membrane module, but its overall performance is not signiicantly afected. Shorter membrane modules are more sensitive to the gas temperature, while its efect can be considered negligible for larger modules [72]. To overcome this issue, an increase in the feed temperature tends to be more convenient than to decrease the sweeping gas temperature.

6.3.3.3 Feed and Permeate Flow Rates Increasing the feed and permeate circulation velocities enhances the heat transfer coeicient and minimizes the efect of temperature and concentration polarization. he signiicance of this variable depends on the MD coniguration considered; being practically negligible for SGMD, when nonvolatile solutes are present, but determinant for DCMD, AGMD and VMD [10]. Some studies reveal that the permeate lux increases with increasing feed low rate up to an asymptotic value [74, 76, 77], while others report a linear relationship between the feed low rate and the distillate lux [78]. he asymptotic level is normally reached for the highest low rates tested, thus indicating a turbulent low regime [77]. Since higher low rates intensify the mixing conditions inside the membrane module, the temperature and the concentration of nonvolatile solutes at the membrane surface tend to approach the temperature and concentration in the bulk phase. However, low rate variations should be performed with due precaution, as pore wetting of the membrane may occur due to an increase of hydraulic pressure and decrease of pressure drop along the MD module. he hydrostatic pressure applied on the membrane should be lower than its liquid entry pressure (LEP), to prevent loss of membrane selectivity [72].

6.3.3.4 Feed Inlet Concentration It has been suggested that MD is very useful for the treatment and puriication of highly concentrated solutions, not exhibiting a signiicant reduction in the membrane permeability as in the case of the pressure-driven membrane processes. However, the presence of nonvolatile solutes in the feed solution results in a decrease of the partial vapor pressure and, therefore, in the driving force of the MD process. Another important contribution to permeate lux reduction is the occurrence of the concentration polarization phenomenon, which is responsible for the formation

264 Nanostructured Polmer Membranes: Volume 2 of a boundary layer near the membrane surface, generally acting as an additional resistance to mass lux [72]. A diferent situation includes the presence of volatile compounds, where the increasing concentration of volatile substances (e.g., alcohols) in the feed solution may afect the thermodynamic properties of the compound itself and even its interaction with water.

6.3.3.5

Presence of Non-Condensable Gases

he presence of non-condensable gases, mostly air, and dissolved gases, such as carbon dioxide (from thermal degradation of bicarbonates), in the feed solution may represent additional resistance to vapor transport. his parameter is particularly important in the AGMD coniguration, as the type of gas present in the air gap may inluence the process eiciency. hese gases tend to accumulate inside the membrane pores, which may reduce the condensation rate and consequently the permeate lux. To overcome this problem, inert gases such as helium and sulphur hexaluoride, were used in AGMD. he difusion rate of the vapor molecules is afected by variable sizes and collision frequencies. herefore, the use of heavier gases instead of air may cause a reduction in the distillate lux, as opposed to lighter gases, such as helium [79]. Additionally, since the MD apparatus are normally exposed to ambient air, feed de-aeration has also been studied to assess eventual process improvements [80]. Decreasing the partial pressure of air, using vacuum, shits the equilibrium conditions to a lower solubility. his results in an increase in the distillate lux and a reduction in the speciic thermal energy consumption [80].

6.3.3.6

Air Gap Width

An additional adjustable air gap width is included in AGMD that determines the difusion path taken by the vapor molecules to reach the cooling surface and condenser. hese membrane modules are designed to easily vary the length of this additional chamber, which in turn may inluence the mechanism of heat and mass transfer registered in the condensing channel. For a gap width which stands below a critical value, the molecular difusion is the main transfer mechanism, while for larger critical values free convection appears to be dominant [10]. In fact, the stagnant air gap represents a 10 to 100 times thicker resistance to mass transfer than the membrane itself; the membrane properties becoming almost negligible in the AGMD coniguration [47]. According to Chouikh et al. [81], a decrease of the gap length from 6–5 mm to 1 mm turned into a 2 to 3.5-fold higher permeate lux.

Polymer Membranes for Water Desalination and Treatment 265

6.4 Desalination Driven by MD Systems Generally, MD systems are applied to the desalination of seawaters, brines and saline waters. his process is able to produce high purity water under near complete rejection of nonvolatile electrolytes (e.g., sodium chloride and potassium chloride) and nonelectrolytes (e.g., organics). Diferent types of membranes have been tested under several operating conditions and MD conigurations. Most of the testing facilities reported in the literature are still at laboratory scale but some pilot plants are already being operated [82–86]. In addition, there are already a few full-scale apparatus under development [55] and, currently, some commercial MD systems are available in the market [87]. One of the most signiicant advantages of MD over the pressure-driven membrane processes is its ability to handle highly concentrated solutions without sufering from the signiicant lux decline typically caused by the decrease of the osmotic pressure (in the case of reverse osmosis –RO) or by fouling. herefore, MD appears to be a suitable technology to solve the problem of RO brines disposal [88–91] or even to recover salts and crystal valuable products from wastewaters and other aqueous solutions [10, 88]. Most of the MD conigurations have proven to be successful for the production of high purity water, providing permeate conductivities as low as 0.4 μS cm 1 and salt rejections of at least 99.90% [10, 92]. Since direct contact between the membrane and nonvolatile solutes is not expected and there is no dependence on the osmotic pressure, MD is able to operate with high salt concentrations. According to Lee et al. [93], similar permeate luxes were achieved when performing DCMD of both distilled water and Red Sea seawater as feed solutions, which proves the mentioned low sensitivity of MD to the increasing solute concentration. hey registered vapor luxes varying from 14 kg m 2 h 1 to 89 kg m 2 h 1 using a PTFE/PP commercial membrane when increasing the feed temperature from 40 °C to 80 °C and keeping the permeate temperature at 20 °C [93]. In fact, permeate luxes were found to be competitive with those achieved with the leading technology [94, 95], since RO plants performing seawater desalination show typical permeate lux values in a range of 11–15 kg m 2 h 1 [96]. Table 6.3 summarizes results reported in the literature with commercial and modiied MD membranes. he same commercial membrane can provide diferent values of permeate lux even when working under the same operating conditions. hese discrepancies may be partially explained by the diferences in the membrane module designs used. Diferent MD conigurations can be found in the literature, employing several types of membrane (lat sheet, capillary or hollow iber) and adopting new design

266 Nanostructured Polmer Membranes: Volume 2 Table 6.3 DCMD permeate lux obtained for water desalination using commercial and commercial-modiied membranes. Flux Membrane Description (kg m-2 h-1) PTFE (lat sheet from Tf = 80 °C; Tp = 20 °C; 39.2 35 g L-1 NaCl Millipore) 34 PTFE (lat sheet from Tf = 80 °C; Tp = 25 °C; Millipore) 45 g L-1 NaCl 85 PTFE-scrim (lat sheet) Tf = 80 °C; Tp = 20 °C; Red -1 Sea (40.8 g L NaCl) PTFE non-woven (lat sheet) 60 PTFE (lat sheet from Pall Inc.) Tf=75 °C; Tp=5 °C; 8 -1 35 g L NaCl Pall-PTFE coated with 4 CNTs-BP* PTFE/PE (lat sheet from Tf = 77 °C; Tp = 5 °C; 110 Millipore) 35 g L-1 NaCl PTFE-scrim (lat sheet from GE Tf = 60 °C; Tp = 20 °C; 25 Osmonics) 1 wt% NaCl 38 PTFE (lat sheet from Tf = 80 °C; Tp = 20 °C; 35 g L-1 NaCl Millipore) 42 PTFE (lat sheet from Millipore) coated with CNTs-ox-BP PVDF (lat sheet from Tf = 77 °C; Tp = 5 °C; 37 Millipore) 3.5 wt% NaCl PVDF (lat sheet from Tf = 40 °C; Tp = 25 °C; 10 hermoscientiic) 10 g L-1 NaCl 18 PVDF (lat sheet from Tf = 60 °C; Tp = 25 C; 10 g L-1 NaCl Millipore) PVDF (lat sheet from 11 Millipore) coated with TiO2/ PEG* PVDF (lat sheet from 12 Millipore) coated with TiO2/ F127* PVDF (lat sheet from 15 Millipore) coated with TiO2/ CTAB* PVDF (lat sheet from Millipore)

Tf = 80 °C; Tp = 20 °C; 35 g L-1 NaCl

28

Ref. 106 107 108 109

110 111 30

112 113 114

23

Polymer Membranes for Water Desalination and Treatment 267 Table 6.3 Cont. Flux (kg Description m-2 h-1) Tf = 80 °C; Tp = 20 °C; 11.6 Red Sea PP Accurel S6/2 MD020TP2N Tf = 70°C; Tp = 15 °C; 35 g 4 (hollow iber from L-1 NaCl Enka-Microdyn) PP Accurel S6/2 MD020CP2N 13 (hollow iber from Microdyn)

Membrane PTFE (hollow iber)

Ref. 115 116

*CNTs = carbon nanotubes; BP = buckypaper; PEG =polyethylene glycol; F127 = Pluronic ; CTAB = cetyltrimethylammonium bromide

strategies in order to achieve the highest vapor lux possible. For instance, Ho et al. [97] proposed a DCMD module using a roughened-surface low channel to increase heat transfer and, consequently, increase the pure water productivity during saline water desalination (Figure 6.3a); an increment of 37% of the mass lux being achieved when using the rougher surface instead of the smooth surface (from 6.48 to 8.64 kg m 2 h 1) with permeate and feed temperatures of 25 °C and 60 °C, respectively, and a low rate of 0.9 mL min 1 for a feed solution of 3.5 wt% NaCl. Fan and Peng [98], suggested a circular MD module with radial low for the hot side, where the feed solution lowed from the center of the membrane to the edge, which may promote more turbulence inside the membrane module and so increase the heat transfer coeicient. Other strategies to improve MD performance may involve the inclusion of turbulence promoters and low distributors, or the use of higher membrane contact areas. Flow distributors (Figure 6.3b) are used to assure a homogeneous distribution of the feed in such a way that its velocity, temperature and concentration remain evenly distributed along the MD module [11]. In fact, diferent membrane arrangements, such as the spiral-wound modules (Figure 6.3c), were adapted and proposed for MD applications for more than 20 years [10]. Spiral-wound modules consist of wrapping a lat-sheet membrane around a perforated central collection tube, using low channel spacers and supports, which results in higher membrane packing density areas, ranging from 300 m2 m 3 to 1000 m2 m 3, depending on the channel height [10]. he application of spacers and the adoption of diferent membrane arrangements was also described as responsible for lux increments of 20% and 36%, as achieved by Teoh et al. [99] during the study of diferent hollow-iber conigurations (Figure 6.3d).

Twisted

String

Nut

(b)

Rough plate

Spacer

Supporter

Fibers

Wryer

(c)

1

5

4

Heat @ 80 C, 30 min

3

2

Braided

Twisted

6 7

10

9

8

Figure 6.3 Schematic representations of some MD modules: (a) rectangular DCMD modules using an extra roughened-surface low channel for enhancement of heat transfer. (Reprinted with permission [97]; Copyright (2013) Elsevier); (b) lat-plate and pyramidal low distributors. (Reprinted with permission from [11]; Copyright (2015) Elsevier); (c) spiral-wound AGMD modules. (Reprinted with permission from [55]; Copyright (2011) Elsevier); and (d) diferent hollow-iber arrangements and inal module coniguration (adapted with permission from [99]; Copyright (2008) Elsevier).

(d)

Fibers

(a)

Membrane

Acrylic plate

Screw

268 Nanostructured Polmer Membranes: Volume 2

Polymer Membranes for Water Desalination and Treatment 269 Another strategy to improve MD performance, not involving changes in the membrane module design, is the optimization of the membrane properties. Most of the membranes applied to MD are originally made for other processes, such as microiltration and ultrailtration, and thus they are oten inadequate, showing low hydrophobicity and permeability or reduced solute rejection [100]. Previous studies have been generally focused on porous hydrophobic membranes made of PTFE, PVDF and PP in both lat-sheet [100, 101] or hollow-iber [102, 103] shapes. Improved membrane surface hydrophilicity was reported as responsible for decreasing the distance of vapor transport to the permeate side [104]. DCMD luxes ranging from 25–27 kg m 2 h 1 with more than 99.6% salt rejection were observed with a N2/H2 plasma-modiied PTFE membrane under a feed temperature of 70 °C and 1.75 m s 1 low rate [104]. Other studies report the use of surface modifying macromolecules (SMMs) to obtain hydrophobic/hydrophilic membranes. Essalhi and Khayet [105], prepared lat-sheet composite hydrophobic/hydrophilic membranes using the hydrophilic host polymer polyetherimide (PEI) and a luorinated SMM (MDI/DPS/BA-L;MDI for the diisocyanate 4,4 -methylene bis(phenyl isocyanate); DPS for the polyol 4,4 -sulfonyldiphenol; and BA-L for the 2-(perluoroalkyl)ethanol, Zonyl luorotelomer intermediate). he permeate lux reached a value of 14.9 kg m 2 h 1 for AGMD and 2.7–3.3 times higher value for DCMD with a salt rejection of 99.4%. A recent trend in the ield of MD membrane preparation is the incorporation of diferent functional materials, in particular CNTs, which can be blended with the membrane matrix or deposited on top of its surface [37, 117–119]. CNTs can upgrade membrane characteristics, such as mechanical strength, chemical resistance and thermal properties, as well as porosity and hydrophobicity [41]. Silva et al. [23] prepared a multiwalled carbon nanotube (MWCNT) PVDF blended membrane with an optimal content of 0.2 wt% pristine MWCNTs, exhibiting a permeate lux 2-fold higher than the neat PVDF membrane (from 19.8 × 10 3 kg m 2 h 1 to 41.8 × 10 3 kg m 2 h 1, respectively) and 1.5-fold higher than the commercial PVDF membrane used as reference. he same scenario was reported by Roy et al. [120] for a PVDF membrane blended with functionalized CNTs containing carboxylic acid groups and supported on porous PP (CNT-COOH-PP) tested in DCMD. he higher lux obtained for the CNT-COOH-PP membrane with a feed temperature of 70 °C and a salt concentration of 10 mg L 1 was 36.8 kg m 2 h 1, which was 51.5% higher than the unmodiied PP membrane [120]. he encapsulation of CNTs in the PVDF matrix was also reported as efective for SGMD, where desalination proved to be better with carboxylated CNTs than with unfunctionalized

270 Nanostructured Polmer Membranes: Volume 2 ones [118]. Other alternatives include the development of thin sheets of randomly oriented CNTs, known as buckypapers (BPs), also described in the literature as having great potential. Dumée et al. [121] prepared self-supporting BPs capable of achieving salt rejections as high as 99% and luxes of approximately 12 kg m 2 h 1. However, membrane aging by delamination was reported as their main limiting factor. In another study, Morales-Torres et al. [30] supported BPs containing diferent amounts of functionalized MWCNTs on a commercial PTFE membrane and evaluated its permeability in DCMD. he membrane including oxidized MWCNTs was described as the most eicient with a lux of 41.8 × 10 3 kg m 2 h 1, which was higher than commercial PTFE alone (38.2 × 10 3 kg m 2 h 1 using distilled water), with total salt exclusion. In addition, the permeate lux was correlated to the content of oxygen-containing surface functional groups, the highest lux being obtained when using BP membranes prepared with the most acidic MWCNTs. On the other hand, large eforts have been carried out on the heat recovery or waste heat, and even the coupling to renewable energies, in order to decrease the energy consumption rate, which is reported as a drawback for MD. In fact, the existing MD pilot plants powered by solar energy exhibit permeate luxes oscillating between 0.5–27 kg m 2 h 1, depending on the operating conditions (namely solar irradiation) [52, 83, 86]. Song et al. [85] also designed a small pilot unit employing two DCMD modules in series, being able to register an even higher value of 55 kg m 2 h 1. Since MD operates at relatively low temperatures (i.e., 50–90 °C) and tolerates luctuating operating conditions (variable temperatures), MD coupling with solar energy collectors appears to be very promising; the aqueous feed being heated by solar-thermal collectors and the pumps recirculation function being powered by photovoltaic modules. MD has the benefit of modularity and simplicity in operation, which highlights its potential for application in autonomous solar-powered desalination units [116]. Serious improvements in terms of total water production costs can be achieved with a reduction from $1.17/m 3 for MD only to $0.64/m 3 for MD using low-grade heat sources [116]. hese values are comparable to the $0.53/m 3 typically reported for the RO plants [96]. In this context, several theoretical and experimental studies have been described in the literature regarding the development of solar-powered MD systems. Also, several research groups have been developing solar MD compact systems, with the aim to improve daily production rates or to study the use of hybrid energy supplies, such as solar and waste heat [122]. Solar-powered MD desalination plants with heat recovery, employing spiral-wound membrane modules and showing a water production of 10 m3 day 1 were

Polymer Membranes for Water Desalination and Treatment 271

PV module MD module

Membrane

Hot seawater

Cold seawater

Waste heat 79 C

85 C

Water vapour

Solar thermal collector

Permeate 20 C

(a)

Discharge/brine

Feed/seawater

(b)

Seawater

Freshwater

26 C Retentate

Figure 6.4 Schematic representation of (a) a solar powered MD pilot plant. (Adapted with permission from [82]; Copyright (2009) Elsevier); and (b) the Memstill technology. (Reprinted with permission from [51]; Copyright (2006) Elsevier).

developed by Fraunhofer ISE (Figure 6.4a) [53, 55, 82, 123]. In fact, six compact systems were operated in ive diferent countries (Egypt, Jordan, Morocco, Germany and Spain) for ive years, exhibiting stable performances with low maintenance for long time periods [82, 83]. Efective membrane areas between 7–12 m2 were used, and gained output ratios (GOR) of around 3–6 were reported (i.e., GOR = ratio between the latent heat used for evaporation and the energy input supplied by external sources) [82]. he same plant was operated with four MD modules in parallel to treat seawater from the Red Sea, showing permeate luxes from 5 to 27 kg m 2 h 1 with conductivities around 20–250 μS cm 1 and salt rejections of 98% ater 90 days of operation [83]. Highly competitive module conigurations have been designed and scaled-up to a size where low cost applications become possible [124] and tested with commercially available membranes. One example is the Memstill technology developed by the Netherlands Organization for Applied Scientiic Research (TNO) (Figure 6.4b), where the feed water lows into a non-permeable condenser to achieve the desired temperature before being in direct contact with the membrane [125]. hey report an average energy consumption of 73.75 MJ m 3 and a production of 25–50 m3 day 1 module 1 for a recovery rate of 50% [51]. In addition to solar-powered MD, geothermal energy applications for water desalination have also been considered [126, 127]. Geothermal energy is not suitable for traditional desalination technologies due to its low enthalpy [127]. However, geothermal water temperature is quite useful in MD applications, since it may vary from 56 °C to 62 °C for reservoir

272 Nanostructured Polmer Membranes: Volume 2 depths comprising 0–24 cm [126]. According to Sarbatly and Chiam [126], if geothermal water is used as feed water and compared to distilled water, VMD operational costs can decrease from $1.22/m 3 to $0.50/m 3, representing a cost savings of approximately 59% for water luxes larger than 6.6 kg m 2 h 1. Despite its potential, further optimization of the operating parameters and improved economic feasibility are required when compared to RO.

6.5 MD Hybrid Systems for Water Desalination and Treatment he increasing needs of progress, fulilment of quality and environmental management principles, associated with the demanding constraints imposed by the concept of process sustainability, have stimulated the development of MD-integrated systems. In this context, MD has been widely studied as a complementary process to further concentrate RO brines (i.e., RO-MD) and to increase water recovery [88, 91, 128], aiming to address the strong impact in the physicochemical and ecological features of the receiving waters caused by the discharge of their hypersaline brines [129]. Recovery rates achieved for seawater RO vary from 35% to 50%, which represents a limitation of the water production capacity of the desalination plants and a critical problem with the disposal of large volumes of RO brines. Its high salt concentration and pH variation may promote serious detrimental efects, such as eutrophication and accumulation of heavy metals in RO receiving waters [89]. To overcome this problem, MD has been selected as a potential alternative to be integrated with conventional technologies. In fact, MD was considered the best option for the concentration of industrial RO from a list of three diferent membrane processes comprising a second RO process, FO and MD, by achieving 65% water recovery with no severe variations on the permeate lux and 99.90% salt rejection [89]. A combination of RO and VMD at bench-scale was also described as promising, reporting a brine volume reduction by a factor of 5.5, recovery rates as high as 89%, and reaching brine concentrations from 64 g L 1 to 300 g L 1 under high permeate luxes [90]. Better performances were reported by Kesieme et al. [130], who achieved a concentration factor of 100-fold (from 30 g L 1 to 361 g L 1), a water recovery higher than 90% under large permeate luxes (from 20–37 kg m 2 h 1) for a DCMD unit treating the RO brine. Other MD process variants, such as MD crystallization (MDC), were also previously studied. Experimental tests performed with artiicial RO

Polymer Membranes for Water Desalination and Treatment 273 concentrates resulted in the production of 21 kg m 3 of NaCl crystals, with a water recovery of 90%, while analogous investigations carried out on RO brines from seawater reported a 20% reduction of the amount of salt crystallized, and 8% decrease of the transmembrane lux, explained by the presence of dissolved organic matter [131]. Tun and Groth [132] also investigated the MD integration with a crystallizer for the treatment of the RO concentrate (conductivity 15 mS cm 1), showing a lux rage of 3–5 kg m−2 h−1 and an overall feed water recovery of 95%. As an alternative, MD can be coupled to FO (i.e., FO-MD) to concentrate the FO draw solution (Figure 6.5). FO is an osmotically driven membrane process, where water moves from the feed solution (low osmotic pressure) to another more concentrated solution, known as draw (high osmotic pressure). he FO-MD hybrid system was already proposed to concentrate protein solutions as bovine serum albumin (BSA) [133], a hydrophilic polybenzimidazole (PBI) membrane and hydrophobic PVDFPTFE hollow ibers being tested in FO and MD units, respectively. In this study, a concentrated NaCl solution (0.5–1.5 M) was used as draw solution to dehydrate proteins in FO, while producing distilled water from the MD unit. he integrated system was found to be stable in continuous operation; the dehydration rate across the FO membrane being the same as the water vapor rate across the MD membrane. In fact, the FO-MD hybrid system is considered promising in applications for the concentration of pharmaceuticals/protein solutions in the near future [133]. he FO-MD hybrid system has also shown potential in the treatment of more demanding solutions, such as wastewaters, heavy metal contaminated

FO membrane

Wastewater

MD membrane

Valve

Pump

Forward osmosis (FO)

Draw solution

Clean water

Membrane distillation (MD)

Figure 6.5 Schematic representation of a lab-scale FO-MD hybrid system. (Reprinted with permission from [134]; Copyright (2013) John Wiley & Sons, Inc)

274 Nanostructured Polmer Membranes: Volume 2 solutions, oily or dyed wastewater [134–138]. Xie et al. [135] demonstrated the efectiveness of the FO-MD system in the treatment of sewer mining streams, a stable water lux and a water recovery of 80% being achieved when operating continuously with raw sewage as feed. High trace organic contaminants (TrOC) removal rates were also achieved ranging from 91 to 98%, largely attributed to the MD process, where only water vapor is transported to the membrane. he results suggest that TrOC transport through the FO membrane is governed by “solute-membrane” interaction, whereas that through the MD membrane is strongly correlated to TrOC volatility [135]. he presence of some contaminants in the draw solution was suggested to be mitigated by granular activated carbon adsorption or ultraviolet oxidation, which resulted in a near complete rejection (>99.5%) of TrOCs [135]. he FO-MD system was also assessed in the treatment of an oily wastewater (containing petroleum, surfactant, NaCl and acetic acid) at 60 °C in batch mode [136]. High feed water recovery (at least 90%) was achieved with only trace amounts of oil and salts, while the FO draw solution was regenerated for consecutive FO-MD runs. In addition, a signiicant amount of acetic acid was also retained in the permeate for further reuse as a chemical additive during the production of crude oil. FO-MD systems were proven to be capable of recovering not only water but also organic additives found in the wastewater for further reuse or other uses [136]. he combination of DCMD with photocatalysis as an advanced oxidation process (AOP) for photodegradation of organic pollutants found in drinking, surface and ground waters, was also reported in the literature [139–141]. Photocatalytic membrane reactors (PMRs), using a photocatalyst suspended in the reaction solution, were built with DCMD membranes, which retain both the catalyst and the nonvolatile solutes present in the feed solution. When studying the removal of ibuprofen sodium salt from tap water, Mozia and Morawski [142] obtained a high quality permeate solution with a capillary MD module employing nine PP membranes (feed temperature of 60 °C and low rate of 0.18 m s 1). he low MD permeate lux was found to be responsible for a longer residence time of the contaminants, which then promoted higher photodegradation eiciencies, as the feed was lowing in a continuous mode in the PMR-DCMD module. he MD/photocatalysis system was also eicient in the removal of azo dyes from water, exhibiting the highest decolorization of the acid yellow 36 (AY36) solution for the highest photocatalyst loading applied (0.5 g TiO2L 1) and highest temperature tested (from 40 °C to 60 °C) [141]. his hybrid system was also employed to treat eluents from municipal sewage treatment plant (STP), its eiciency (concentration of organic carbon 2000 W kg ) 1

Low power density (< 1000 W kg 1)

Low energy density (0.01–0.05 Wh kg 1) High energy density (9–200 Wh kg 1) Longer cycle life (>105 cycles)

Shorter cycle life (< 103 cycles)

Store energy physically (no major change in the structure of the material between charge and discharge states)

Store energy chemically (physical change in the structure of the material between charge and discharge states)

energy density between type (a) and type (b) capacitors with batteries. here are three classes of supercapacitors, namely (a) electrical double layer capacitors (EDLCs), (b) redox supercapacitors or pseudocapacitors and (c) hybrid capacitors. hese supercapacitors are classiied based on the electrode materials employed and their charge storage mechanism. For pseudocapacitors, electrode materials are the transition metal oxides that include NiO, CoO, RuO2, IrO2, Fe3O4 and MnO2 and sulides, e.g., TiS2 [65]. Other types of material with a high amount of pseudocapacitance include polyaniline and polypyrrole [66], polythiophene [67] and polyacetylene [68]. Activated carbon, carbon aerogels and carbon nanotubes (CNTs) are used in EDLCs [69]. he charge storage mechanism in EDLCs is non-Faradaic, i.e., no transfer of charge between electrode and electrolyte but based on the

Biopolymer Electrolytes for Energy Devices

325

accumulation of charges at the electrode/electrolyte interfaces under the application of an electric potential [63, 70]. An “electrical double layer” is formed between the electrodes and the electrolyte and serves as the dielectric. he charge storage mechanism in redox supercapacitors is Faradaic. Faradaic process involves the transfer of charge between electrode and electrolyte such as oxidation-reduction reactions, electrosorption and intercalation process. An example of an oxidation-reduction Faradaic reaction is: RuO2 + xH+ + xe

RuO2–x(OH)x where 0 < x < 2

(8.7)

he reduction reaction proceeds from let to right and oxidation proceeds in reverse. Hybrid capacitor, on the other hand, has the combination of both EDLC and redox capacitor technologies. It was developed to manipulate the advantages and moderate the disadvantages of EDLC and redox capacitors to improve performance. In this chapter, emphasis is given to EDLCs.

8.5.1 he Structure and Working Principle of an EDLC An EDLC consists of two carbon-based current collector electrodes that sandwich the biopolymer electrolyte. At the beginning of charge, the cations in the electrolyte will difuse towards the negative terminal of the charger and the anions towards the opposite terminal as shown in Figure  8.8(a). On completion of charge, the ions will be adsorbed on the electrodes (Figure 8.8(b)). he discharge situation is as shown in Figure 8.8(c).

e– e–

Polymer electrolyte

Polymer electrolyte

(a)

(b)

(c)

Carbon electrode

Figure 8.8 (a) Before charging, (b) ater complete charging and (c) during discharge.

326 Nanostructured Polmer Membranes: Volume 2

8.5.2 Characterization Techniques for EDLCs Experimental techniques employed for the characterization of a battery are suitable for EDLC as well. hese include determining the chargedischarge characteristics, self-discharge characteristics and cyclic voltammetry. When an EDLC cell is charged by a constant current, I, the voltage, V, developed across the cell increases linearly with time, as shown in Figure 8.9(a). During discharge, the voltage of a good EDLC will also decrease linearly with time and exhibits an inverted V shape. However, an abrupt jump in voltage is oten observed when charging and discharging the EDLC (see Figure 8.9(b)). his is attributed to ohmic loss, a result of internal resistance of the EDLC [65]. he charge/discharge capacitance of the EDLC can be calculated from the linear region of the charge-discharge curve according to the relation: C

I

t V

(8.8)

where I is the current applied to charge/discharge the cell and ΔV is the voltage change in an interval of time, Δt. When the charging and discharging currents are the same, the Coulombic eiciency, η, can be calculated from: td 100 tc

%

(8.9)

where Δtd and Δtc are the time intervals for discharging and charging, respectively. he speciic energy, E, and power density, P, can be calculated from: 1 CV 2 2

(8.10)

V2 4mESR

(8.11)

E P

V

V

Charge

(a)

Discharge

Charge

t

Discharge

(b)

Figure 8.9 Charge-discharge curves for (a) ideal and (b) real EDLC cell.

t

Biopolymer Electrolytes for Energy Devices

327

Pseudocapacitance Current (I)

Current (I)

Resistive capacitor

Voltage (V)

Voltage (V)

Ideal capacitor

Figure 8.10 Ideal voltammogram and voltammogram showing pseudocapacitance.

Here C is the discharge capacitance, V is the working voltage, m is the mass of electrode and ESR is the internal resistance. From the self-discharge characteristics of an EDLC, the period of stored energy can be estimated. he self-discharge can be determined by measuring the open circuit voltage (OCV) of the cell as a function of time. Cyclic voltammetry (CV) is a potentiodynamic electrochemical measurement. In CV the working electrode potential is increased linearly with time according to the scan rate (V s 1). When CV reaches a set potential, the potential of the working electrode is reversed. his forward and reverse ramping can happen many times during a single experiment. The current at the working electrode is plotted versus the applied voltage to give the cyclic voltammogram trace. Ideally, the voltammogram should be rectangular. In practice, however, the voltammogram may show the inluence of pseudocapacitance as shown in Figure 8.10.

8.5.3 EDLC Employing Biopolymer Electrolytes Table 8.5 lists some examples of EDLC employing biopolymer electrolytes. It may be worth noting that the EDLC with the cellulose-chitin hybrid gel electrolyte [72] containing speciic ionic liquids and an aqueous H2SO4  solution has shown a higher capacitance and better capacity retention compared to that with only the aqueous H2SO4  for more than 10000 cycles, even at elevated temperatures. his suggests that the use of hybrid cellulose-chitin polymer has given stability to the electrolyte, resulting in better performance.

328 Nanostructured Polmer Membranes: Volume 2 Table 8.5 Examples of EDLC employing biopolymer electrolytes.

EDLC Cells

Energy Power density density Capacitance (F g 1) (Wh kg 1) (kW kg 1) Ref

AC | Chitosan-H3PO4 | AC

0.088

71

AC | Chitosan-H3PO4NH4NO3 |AC

0.220

71

AC | Chitosan-H3PO4NH4NO3-Al2SiO5 |AC

0.218

71

AC | Chitosan-H3PO4Al2SiO5 |AC

0.205

71

AC |Methyl celluloseNH4NO3-PEG | AC

38

3.9

0.14

27

AC | ChitosaniotacarrageenanH3PO4-PEG | AC

35

15

AC | Chitin-celluloseH2SO4 | AC

175

72

AC | Chitosan-1-ethyl3-methylimidazolium tetraluoroborate (EMImBF4) | AC

130

73

he use of chitosan incorporated with 1-ethyl-3-methylimidazolium tetraluoroborate (EMImBF4) ionic liquid in EDLCs with activated carbon electrodes [73] has shown  enhanced discharge capacitance and better rate performance compared to that using only liquid EMImBF4. his was attributed to the high ainity of chitosan for the activated carbon electrode and the higher ionic conductivity of the gel electrolyte. he cell did not show any sign of degradation even at the end of 5000 cycles. Hence, as shown by the authors, chitosan is able to improve the performance of EMImBF4 in EDLCs and possibly that of other electrochemical devices as well. his again shows the advantage of using biopolymer in electrochemical devices.

8.6 Biopolymer Electrolytes in Fuel Cells Fuel cells are an important technology for a wide variety of applications. hese include applications for micropower, auxiliary, transportation and

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329

stationary powers for buildings. Fuel cell development is accelerating rapidly and eforts in the development of fuel cell technology are being made with the aim to obtain a higher and more stable electrochemical performance as well as to reduce cost. here are several types of fuel cells and their characteristics are shown in Table 8.6. a. b. c. d. e.

Polymer electrolyte membrane fuel cell (PEMFC) Alkaline fuel cell (AFC) Phosphoric acid fuel cell (PAFC) Molten carbonate fuel cell (MCFC) Solid oxide fuel cell (SOFC)

In a fuel cell, oxygen in the air is electrochemically combined with a hydrogenated fuel to produce electricity and water as a by-product. he fuels are continuously supplied to the cell instead of being contained as in batteries. Filling up the fuel supply will recharge the fuel cell [74]. Hydrogen can be compressed and stored in vessels, as a cryogenic liquid at a temperature of 253 °C or as hydrogen atom reversibly adsorbed in metal hydrides. Compared to lead acid batteries this storage contains up to 10 times more energy per unit of weight [75]. Fuel cell production cost is still high and this is mainly attributed to the use of expensive materials such as Naion and the low volume production.

8.6.1 he Structure and Working Principle of a Fuel Cell he primary components of the fuel cell are an ion-conducting electrolyte, a cathode, and an anode, as shown in Figure 8.11. he assembly of these three components is referred to as the membrane-electrode assembly (MEA), or simply a single-cell fuel cell. In fuel cell operation, hydrogen is brought into the anode compartment and an oxidant, typically oxygen, into the cathode compartment. Oxygen and hydrogen react to produce water. Direct chemical combustion is prevented by the electrolyte that separates the fuel (H2) from oxidant (O2). he electrolyte serves as a barrier to gas difusion, but will let hydrogen ions or protons migrate across it since it is a proton-conducting membrane. he protons, ater migrating across the electrolyte, will react at the cathode with O to produce water. he O was produced when O2 in the air that entered the cathode was atomized and reduced by the electron from the hydrogen atoms that were oxidized at the anode. Since the by-product is only water, fuel cells are green energy devices.

Platinum

Evaporative

High power density

Intolerate to CO Intolerate to CO in pure H2 in pure H2

Catalyst

Product water management

Advantages

Disadvantages

High eiciency

Evaporative

Platinum

Carbon-based

Carbon-based

Prime-cell components

H

OH¯

H

MCFC

SOFC

O¯

CO3¯

High eiciency

High eiciency

hermal stress failure

Gaseous product

Perovskites

Gaseous product

Nickel

Stainless-steel based Ceramic-based

60

700–1000

45–50

600–700

Low power density Electrolyte instability

Tolerant to impure H2

Evaporative

Platinum

Carbon-based

+

40

150–200

60

90–100

Charge carrier

+

PAFC

Mobilized of Immobilize liquid Immobilized liquid ceramic immobilized phosphoric acid molten carbonate potassium hydroxide

60

50–100

Ion exchange membranes

AFC

Eiciency (%)

Operating temperature (°C)

Electrolyte

PEMFC

Table 8.6 Characteristics of the fuel cells.

330 Nanostructured Polmer Membranes: Volume 2

Biopolymer Electrolytes for Energy Devices

e–

e– H+

Fuel H2

331

Oxidant

H+ H+

2H+ + 2e–

½O2 + e–

O–

H+ H+

By-product Anode

By-product Cathode

Electrolyte

(a)

Power (W cm–2)

Voltage (V)

Figure 8.11 Schematic of a fuel cell.

Current (A cm–2)

Peak power

(b)

Current (A cm–2)

Figure 8.12 Schematic fuel cell performance (a) voltage and (b) power density vs. current density curves.

8.6.2 Characterization Techniques for Fuel Cells he current and power densities are given by the respective equations: Current density Power density

constant current (mA) Effective area (cm 2 )

(8.12)

Voltage (V ) Current (I ) Effective area (cm 2 )

(8.13)

he performance of a PEMFC is usually presented as a plot of voltage and power density versus current density, as shown in Figure 8.12.

8.6.3 Biopolymer Electrolytes in PEMFCs he PEMFC fuel cell uses polymer electrolyte as a membrane for ion transport. Table 8.7 lists some examples of chitosan-based fuel cells.

332 Nanostructured Polmer Membranes: Volume 2 Table 8.7 List of some examples of biopolymer electrolytes in PEMFC. Electrolyte Chitosan-H3PO4

Current density Power density (mW cm 2) Ref. (mA cm 2) 3.9 1.61 71

Chitosan-H3PO4-NH4NO3

21.2

4.63

71

Chitosan-H3PO4-NH4NO3-Al2SiO5

21.9

4.00

71

Chitosan-H3PO4-Al2SiO5

21.0

5.40

71

Chitosan-Adenosine triphosphate-Naion

616

243

76

Chitosan–hydroxyethyl cellulose– phosphotungstic acid

210

58

77

Chitosan–polyvinyl alcohol– sulfosuccinic acid–stabilized silicotungstic acid

400

156

78

Chitosan–polyvinyl alcohol– glutaraldehyde–stabilized silicotungstic acid

300

88

78

Chitosan–1-naphthalene acetic acid

115

16

79

Chitosan– 4-chlorophenoxy acetic acid Chitosan–3-indole acetic acid

125 150

18 25

79 79

Chitosan is receiving a lot of attention as ionic conducting host materials for membrane electrolyte in low to moderate temperature PEMFC. he use of low-cost chitosan-based membranes might reduce the fuel cell cost since it is the membrane that is the most expensive component of a polymer electrolyte-based fuel cell. However, proton conductivity in chitosan-based polymer electrolyte membranes is not yet comparable to that exhibited by the currently used and expensive Naion-based electrolyte membranes. he low proton conductivity will produce PEMFC with low power density compared to that of Naion-based cells. hus, the intrinsic ionic conductivity of chitosan-based electrolyte membranes needs to be further improved for fuel cell applications. Apart from that, the mechanical strength and shelf life of chitosan also need to be studied.

8.7 Biopolymer Electrolytes in Dye-sensitized Solar Cells (DSSCs) As in fuel cells, there are many kinds of solar cells. his section, however, deals with solar cells that are sensitized by dyes and are also known as dye-sensitized solar cells.

Biopolymer Electrolytes for Energy Devices

333

8.7.1 he Structure and Working Principle of a DSSC A DSSC consists of a photoanode, an electrolyte and a counter electrode as shown in Figure 8.13. he photoanode comprises a porous network of metal oxide (MO) coated with a dye. he widely used MOs are TiO2, SnO2 and ZnO. hese MOs are wide-band semiconductor oxides and the porous network is used to get high surface area for good sensitizer binding and eicient solar harvesting. he dye-coated MO is deposited on a transparent conducting oxide (TCO) glass. Examples of TCO glasses are luoride-tin oxide (FTO) and indium-tin oxide (ITO) with 70% and 80% transparency, respectively. A good performing DSSC can have three distinct layers of MOs, as shown in Figure 8.13. he irst layer is a compact layer that prevents the TCO substrate from touching the electrolyte and also reduces the back transfer of electrons from the TCO surface to the photoanode. In addition, the compact layer also provides a larger contact area between MO and TCO in order to enhance the electron transfer to the external circuit. he second MO layer is a porous layer. he porous layer has a larger surface area in which the dye can adhere. he third layer is a layer of nanosized MO that can scatter photons back into the porous layer to excite the dye molecules. Hence, more light is harvested to excite the dye. he porous layer is thicker than the scattering layer and the size of the porous particles are much smaller (~20 nm) than the size of the scattering particles (>100 nm). Figure 8.14 shows the working principle of a DSSC. When light shines on the photoanode, the dye molecules, D, absorb energy from the photons and are excited, D . D h

(8.14)

D

Compact layer of MO Porous layer of MO Scattering layer of MO Electrolyte TCO glass Counter electrode

Figure 8.13 Structure of a DSSC. he MOs in the dye before assembly.

334 Nanostructured Polmer Membranes: Volume 2 Electrolyte

Dye

MO

D* Counter electrode (cathode)

TCO glass CB

TCO glass Oxidation 3I– I3– Reduction D0/D+ Load

Figure 8.14 he working principle of a DSSC with I‒/I3‒ redox mediator.

Since the excited electrons of the dye are in an energy level above the conduction band (CB) of the MO semiconductor, the electrons are able to be injected into the conduction band of the MO. D

D

e(MO)

(8.15)

where D+ is the oxidized dye. he injected electrons percolate through the MO to the TCO and enter the external circuit. On the other hand, in the redox electrolyte, the iodide ion, I donates an electron to D+ and becomes oxidized to a triiodide. 2D

3I

2D I 3

(8.16)

he generated triiodide ion, I3 difuses into the counter electrode (CE) and recombines with the injected electrons that have reached the counter electrode through the external circuit and reduces to I . I3

2e(CE )

3I

(8.17)

he iodide ion, I , moves into the bulk of the DSSC and releases an electron to the hole in the dye, D+, and gets oxidized into I3 again. he above processes are repeated as long as the DSSC is illuminated. he operation of DSSC converts solar energy into electricity. In order to generate high current, the reactions in Equations 8.16 and 8.17 must compete with the recombination reactions D

e MO

D MO

(8.18)

Biopolymer Electrolytes for Energy Devices

335

10–13 s

10–3 s

10–8 s 10–4 s

h

10–2 s I3–/I–

{TCO

TiO2

Sensitizer}

{Electrolyte}

{Counter electrode}

Figure 8.15 Kinetics of a sensitized MO solar cell using I‒/I3‒ redox mediator.

I3

2e(MO)

3I

MO

(8.19)

he kinetics of a DSSC with I‒/I3‒ redox couple is as shown in Figure 8.15.

8.7.2 Basic Properties of a Dye Sensitizer he role of a dye sensitizer is to absorb light to generate electrons and holes and inject them into the MO network. hus, a dye sensitizer should optimally absorb over a wide wavelength range of the electromagnetic spectrum from visible light to near infrared region (400–900 nm) with high extinction coeficient. he excited state of the sensitizer should have a suiciently long life to ensure electron injection into the conduction band of the MO. he energy levels of a dye sensitizer should be compatible with the CB of the MO and the redox potential of the electrolyte used. It should also adhere well to the MO surface to ensure good rate of electron injection. In order to adhere well to the MO surface, a dye sensitizer must have several =O and –OH groups. In addition, the lifetime of a dye must also be consistent with the device life. Ruthenium-based dyes are widely used in DSSCs. An example of a  ruthenium-based dye is the cis-bis(isothiocyanato)bis(2,2-bipyridyl4,4 -dicarboxylato)-ruthenium (II) or N3 for short. Its molecular structure is shown in Figure 8.16. he molar extinction coeicient for N3 dye is of 104 M−1 cm−1 at ~400 nm. he dye adheres to the MO surface through its carboxyl substituents. However, ruthenium-based dyes are expensive and not environmentally friendly. In order to overcome the limitations of ruthenium-based dyes, ruthenium-free dyes (or organic dyes) are used in DSSCs. hese dyes have

336 Nanostructured Polmer Membranes: Volume 2 COOH

HOOC

N N

N=C=S Ru N=C=S

N HOOC

N

COOH

Figure 8.16 he molecular structure of a N3 dye.

higher extinction coeicients ranging from 0.5 to 2.0 × 106 M 1 cm 1, but have a narrower absorption range than ruthenium. Figure 8.17 shows some examples of organic dyes. On the other hand, pigments extracted from leaves, lowers, roots and fruits of plants can serve as natural dyes in DSSCs. Natural dyes are easily available, abundant in supply, environmentally friendly and low cost, which can help further reduce the cost of DSSCs. However, the natural dyes have poor stability compared to synthetic dyes and the eiciency of DSSCs using natural dyes is low. Figure 8.18 shows some examples of natural dyes and their possible sources. Natural dyes include anthocyanin, chrorophyll and carotenoid. he structure of anthocyanin and chlorophyll are as shown in Figure 8.18. here are also diferent types of anthocyanins. Table 8.8 lists a few types of anthocyanins. hey difer in functional groups. Lists signifying the performance of DSSCs using natural dyes are shown in Tables 8.9 to 8.12 for anthocyanin, carotenoid, betalain and chlorophyll dyes. he electrolytes used in these DSSCs are liquid electrolytes.

8.7.3 Characterization of DSSCs Experimental techniques which are frequently employed for DSSC characterization include incident photon to current conversion eiciency (IPCE), optical absorption, electrochemical impedance spectroscopy (EIS) and current measurement under diferent biases (I-V characteristics). IPCE shows how eiciently light of a speciic wavelength is converted to current. It gives a useful estimation of processes that limit the performance of a DSSC. Optical absorption, on the other hand, gives the absorption

Biopolymer Electrolytes for Energy Devices Structure

Description Phthalocyanine is an intensely blue-green-colored aromatic macrocyclic compound. Phthalocyanines form coordination complexes with most elements of the periodic table.

N N

NH

N

N HN

N

337

N

Phthalocyanine O O S

Chemical name is 5-[[4-[4-(2,2-Diphenylethenyl) phenyl]-1,2,3-3a,4,8b-hexahydrocyclopent[b] indol-7-yl]methylene]-2-(3-ethyl-4-oxo-2-thioxo5-thiazolidinylidene)-4-oxo-3-thiazolidineacetic acid.

OH

N S

O

N

S

Indole

Porphyrins are a group of heterocyclic macrocycle organic compounds, composed of four modiied pyrrole subunits interconnected at their α carbon atoms via methine (=CH−) bridges (which are denoted as *).

t-Bu * N * NH N

* N *

N

t-Bu

HN

t-Bu * N *

* N * t-Bu

Porphyrin H3C N

CH

CH *N

H3C I–

+ N

O

N

O

Squaraine

N

H3C H3C

N

O

O

CH3 alkylpyridinum group

Hemicyanine

–

Hemicyanine dyes are positively charged molecules, having an amino group in one end and alkylpyridinium group on the other end.

N CH3

It is an 1,2,3,3-tetramethyl-3H-indolium salt. This type of organic or ruthenium-free dye shows intense luorescence. Absorption maxima are found between 630 and 670 nm and emission maxima are between 650–700 nm. Chemical name is 3-(2-N-Methylbenzimidazolyl)-7N,N-diethylaminocoumarin. It is a blue luorescent dye (emission range ~410 to 470 nm).

Coumarin

Figure 8.17 Some examples of organic dyes.

behavior and range of a DSSC. EIS is used to study the resistances in a DSSC cell and life time of charge carriers. In this section, we concentrate on I-V characteristics of a DSSC. Figure 8.19 shows a typical plot of current density as a function of voltage for a DSSC. Four parameters can be obtained from the I-V characteristics of a DSSC: open-circuit voltage, VOC, short-circuit current, JSC, ill factor, FF, and energy conversion eiciency, η. he open-circuit voltage, VOC, is the voltage exhibited by a DSSC and occurs when no current lows;

338 Nanostructured Polmer Membranes: Volume 2 Description

Structure R1 OH +

O

HO

R2

OH OH

Anthocyanin R

CH=CH2 H2C

CH2 N

CH3

N Mg

H

Black rice (above) is a possible source

2+

N

Porphyrin ring

N CH3

H3C

CH2 H HC CH2

C

O

COOCH3

C

Chlorophyll a: R = CH3 Chlorophyll b: CHO

O

Phytol

This moss bryophyte is a possible source.

Chlorophyll

Figure 8.18 Some examples of natural dyes and their sources.

Table 8.8 he possible functional groups for R1 and R2. Anthocyanin

R1

R2

Cyanidin

OH

H

Delphinidin

OH

OH

OCH3

OCH3

H

H

Peonidin

OCH3

H

Petunidin

OCH3

OH

Malvidin Pelargonidin

the short-circuit current, JSC, is the current from a DSSC when the voltage across the DSSC is zero (see Figure 8.19). he ill factor, FF, is deined as the ratio of maximum obtainable power to the product of the open-circuit voltage and short-circuit current. he relationship is given by: FF

J m Vm J SC VOC

(8.20)

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339

Table 8.9 Anthocyanin dyes and their performance in DSSCs. Dye

JSC (mA cm 2)

VOC (V)

FF (%)

η%

Ref.

Anthocyanin Dyes Blackberries

2.100 10.60 0.750 5.850 1.490 3.140 1.000 0.960 2.410

0.400 0.330 0.340 0.320 0.350 0.520 0.590 0.392 0.325

– 0.420 0.400 0.570 0.480 0.380 0.610 0.470 –

0.560 1.460 0.100 1.070 0.505 0.638 – 0.170 0.460

80 81 82 83 84 85 86 82 87

Black raspberries

0.880

0.389

0.450

0.150

82

Raspberries

0.700

0.310

0.420

0.090

82

Cranberries Cherries Strawberries Mulberry fruit

4.300 0.770 0.850 0.650 1.89

0.520 0.365 0.370 0.285 0.555

0.650 0.400 0.500 0.370 0.490

– 0.110 0.160 0.070 0.548

88 82 82 82 89

Chaste tree fruit Red grapes

1.720 6.100 2.120 0.670

0.422 0.490 0.390 0.297

0.430 0.520 0.480 0.400

– – – 0.080

90 86 90 82

Giacchè grapes

3.060

0.333

0.560

0.570

83

Red Sicilian orange “Moro”

3.840

0.340

0.550

0.660

91

Red Sicilian orange

4.980

0.325

0.480

0.780

83

Red Sicilian orange extract

5.130

0.329

0.590

1.010

83

Skin of Jaboticaba

2.600

0.660

0.620

–

92

Calafate fruit Dragon fruit

3.900 6.20 0.200

0.450 0.470 0.220

0.560 0.360 0.300

– – 0.220

86 92 93

Pomegranate fruit

5.00

0.300

–

–

94

Mangosteen pericarp

2.920

0.611

0.626

1.120

95

Indian blackberries Blueberries

Blueberry juice

(Continued)

340 Nanostructured Polmer Membranes: Volume 2 Table 8.9 Cont. Dye

JSC (mA cm 2)

VOC (V)

FF (%)

η%

Ref.

Radicchio leaves Red cabbage

5.050 0.680

0.322 0.520

0.550 0.700

0.900 0.500

83 87

Skin of eggplant

0.890 0.350 3.400

0.600 0.540 0.350

0.320 0.650 0.400

0.170 0.130 0.480

90 91 87

Red perilla

3.480 1.142 1.287 1.213 3.020 0.390

0.346 0.551 0.550 0.400 0.360 0.510

0.530 0.520 0.334 0.470 0.340 0.670

0.640 – 0.236 0.230 0.370 0.270

83 96 97 12 98 87

Flame tree lower May lower Red frangipani lowers

0.668 1.330 0.940

0.504 0.300 0.495

0.588 0.390 0.650

0.200 0.317 0.301

99 84 100

Amaranth lowers Coleus lowers Erythrina variegata lower

0.435 0.820 0.776

0.876 0.782 0.484

0.560 0.740 0.550

– – –

101 101 96

Rosa xanthina lower Rosa chinensis lower Hibiscus surattensis lower Hibiscus rosasinensis lower Hibiscus sabdarifa L. lower Rosella lower Blue pea lower

0.637 0.802 5.450 4.040 1.630 2.600 0.290 0.270

0.492 0.543 0.392 0.400 0.404 0.330 0.420 0.490

0.520 0.664 0.535 0.633 0.570 0.680 0.610 0.760

– 0.290 1.140 1.020 0.370 0.580 0.070 0.100

96 102 103 103 104 105 106 106

Clitoria ternatea lower Cosmos sulphureus lower

0.370 1.041

0.372 0.447

0.330 0.610

0.050 0.540

104 107

Mucuna lagellipes lower

6.440

0.00011

0.470

0.330

108

Zeamaize lower Ornamental millet leaves Anthurium lower Henna leaves

5.340 0.400 3.200 1.870

0.00025 0.880 0.480 0.610

0.320 0.550 – 0.580

0.430 – – 0.66

108 101 109 110

Black rice

Biopolymer Electrolytes for Energy Devices

341

Table 8.10 Carotenoid dyes and their performance in DSSCs. VOC (V)

JSC (mA cm 2)

Dye

FF (%)

η%

Ref.

Carotenoid Dyes Achiote seeds

1.100

0.570

0.590

0.370

111

Gardenia yellow

0.860

0.590

0.651

0.350

112

Gardenia Jasminoide Ellis

1.930

0.470

0.650

0.590

113

Gardenia fruit

2.840

0.430

0.460

0.560

114

Ivy gourd fruits

0.240

0.644

0.490

0.076

100

Capsicum

0.225

0.412

0.630

–

96

Kerria japonica lower

0.560

0.584

0.678

0.220

102

Allamanda cathartica lower

0.878

0.405

0.540

0.400

107

Frutus lycii

0.530

0.689

0.466

0.170

95

Table 8.11 Betalain dyes and their performance in DSSCs. VOC (V)

FF (%)

η%

Ref.

2.100

0.300

0.570

0.360

115

0.898

0.359

0.520

0.380

107

Red Bougainvillea spectabilis bracts

2.291

0.280

0.760

0.480

116

Red Bougainvillea glabra bracts

2.344

0.260

0.740

0.450

116

Red beet roots

3.758

0.357

0.300

0.402

117

13.910

0.360

0.560

2.710

118

2.420

0.440

0.630

0.670

119

Beet roots (Beta vulgaris)

2.710

0.576

0.572

0.890

120

Sicilian prickly pear

8.800

0.389

0.600

2.060

83

Wild Sicilian prickly pear

9.400

0.350

0.380

1.260

115

Red turnip

9.500

0.425

0.370

1.700

115

9.500

0.480

–

1.75

121

2.700

0.375

0.540

0.500

115

JSC (mA cm 2)

Dye

Betalain Dyes Bougainvillea bracts

Sicilian Indian ig

342 Nanostructured Polmer Membranes: Volume 2 Table 8.12 Chlorophyll dyes and their performance in DSSCs. JSC (mA cm 2)

Dye

VOC (V)

FF (%)

η%

Ref.

Chlorophyll Dyes Pawpaw leaf

0.649

0.504

0.605

0.200

99

Pomegranate leaves

2.050

0.560

0.520

0.597

89

Leaves of Chinar Shiso leaves Bougainvillea leaves

0.012 3.520 3.230

0.468 0.432 0.500

0.004 0.390 0.410

0.550 0.590 0.618

122 123 85

Kelp

0.433

0.441

0.620

-

96

China loropetal

0.840

0.518

0.626

0.270

95

Perilla Petunia Pandanus amaryllifolius leaves

1.360 0.850 1.610

0.522 0.616 0.360

0.696 0.605 0.410

0.500 0.320 0.240

95 95 98

Papaya leaf

0.240

0.566

0.330

0.045

124

JSC

Current density

Vm, Jm

Voltage

VOC

Figure 8.19 I-V curve of a DSSC.

where Jm and Vm are the maximum obtainable current density and voltage, respectively. he energy conversion eiciency is related to the ill factor through %

J SC VOC FF 100 Pin

where Pin is the incident power density (W m 2).

(8.21)

Biopolymer Electrolytes for Energy Devices

343

8.7.4 DSSCs Employing Biopolymer Electrolytes he irst attempt by our group to use chitosan in solar cells [125] resulted in a JSC of the order of μA and open circuit voltage, VOC, between 0.3 and 0.4 V. In that work, chitosan was blended with PEO and no dye was used. Singh et al. [126] have also used chitosan-based electrolyte in their DSSC. hey have also used agarose as the host for ionic conduction [127]. Agarose is a polysaccharide polymer made up of agarobiose repeating units that can be extracted from seaweed. Agarobiose is a disaccharide made up of D-galactose and 3,6-anhydro-L-galactopyranose. Some examples of DSSCs which employed quasi-solid state polymer electrolytes are listed in Table 8.13. Chitosan can be modiied by the method of phthaloylation as reported by Nishimura et al. [130]. Quasi-solid state phthaloyl chitosan-based polymer electrolytes have been prepared and used to fabricate DSSC. A mixture of EC and dimethylformamide (DMF) has been employed as the plasticizing solvent and tetrapropylammonium iodide (TPAI) was used as the doping salt. Table 8.14 shows the efect of TPAI salt on the room temperature conductivity of phthaloyl chitosan-EC-DMF-based polymer electrolyte. he amounts of phthaloyl chitosan, EC and DMF were kept constant. Table 8.13 DSSC employing quasi-solid state polymer electrolyte. JSC (mA cm 2)

VOC (V)

FF

η%

Ref.

Chitosan-NaI Chitosan-NaI-ionic liquid (EMImSCN)

1.05 2.62

0.349 0.53

0.34 0.52

0.13 0.73

126 126

Agarose-KI

3.27

0.670

0.24

0.54

127

Cyanoethylated hydroxypropyl cellulose-MHII-LiI-4Tertbutylpyridine

14.40

0.760

0.700

7.55

128

Chitosan-NH4I(+I2)-BMII

3.1

0.58

0.41

0.74

12

Phthaloyl chitosan-ethylene carbonate-propylene carbonate-KI- tetra propyl ammonium iodide(+I2)

8.72

0.8

0.63

4.37

129

Biopolymer used

*MHII: 1-hexyl-3-methyl imidazolium iodide, EMImSCN: 1-ethyl 3-methylimidazolium thiocyanate

344 Nanostructured Polmer Membranes: Volume 2 Table 8.14. Efect of TPAI salt on the room temperature conductivity of phthaloyl chitosan-EC-DMF based polymer electrolyte. Content of TPAI (g)

Room temperature conductivity, σRT (S cm 1)

0.04

1.41 × 10

3

0.06

2.45 × 10

3

0.08

2.75 × 10

3

0.10

3.38 × 10

3

0.12

4.59 × 10

3

0.14

2.40 × 10

3

he DSSCs have been fabricated using photoanode containing TiO2 coated with N3 dye and platinum (Pt) counter electrode. he polymeric electrolytes were based on phthaloyl chitosan-EC-DMF-TPAI: Cell A: TiO2 Cell B: TiO2 Cell C: TiO2 Cell D: TiO2 Cell E: TiO2

phthaloylchitosan-EC-DMF-TPAI (0.04 g) phthaloylchitosan-EC-DMF-TPAI (0.06 g) phthaloylchitosan-EC-DMF-TPAI (0.08 g) phthaloylchitosan-EC-DMF-TPAI (0.10 g) phthaloylchitosan-EC-DMF-TPAI (0.12 g)

Pt Pt Pt Pt Pt

he fabricated cells were then illuminated with a light source of 100 mW cm 2 and the performances of the cells were plotted as current density versus voltage as shown in Figure 8.20. he extracted and calculated photoelectrochemical parameters of the cells are listed in Table 8.15. Although not always true, the performance of the DSSCs is in correlation with the conductivity behavior of the polymeric electrolytes. he highest energy conversion eiciency, η, of 5% was achieved with the highest conducting electrolyte. Variation of eiciency and JSC for diferent DSSCs is shown in Table 8.15. From the table, it can be observed that the variation of the short circuit photocurrent density dominates the performance of these solar cells. he same patterns can be observed from a previous report [131] where the authors used PAN-EC-PC-TPAI-I2. Hence, phthaloyl chitosan-based electrolyte seems to exhibit similar characteristics as synthetic polymer-based electrolyte.

8.8 Conclusions Biopolymers have received much attention in studies concerning polymer electrolytes and devices that make use of an electrolyte. his is

Biopolymer Electrolytes for Energy Devices

345

14 Photocurrent density/ mA cm–2

D

12

C B

10

E A

8 6 4 2 0 0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Voltage/ V

Figure 8.20 Performance of DSSC cells: TiO2 phthaloyl chitosan-EC-DMF-TPAI Pt.

Table 8.15 he photoelectrochemical parameters of the DSSC cells: TiO2 phthaloyl chitosan-EC-DMF-TPAI Pt. Jsc (mA cm 2)

Voc (V)

FF (%)

η%

A

8.95

0.65

0.64

3.75

B

10.05

0.63

0.65

4.09

C

11.29

0.58

0.65

4.19

D

12.72

0.60

0.66

5.00

E

9.17

0.57

0.69

3.58

DSSC Cells

because of their functionality, low cost and eco-friendliness. he properties of good complexity with ionic salts, high ionic conductivity, nontoxicity and biodegradability are advantages for use in polymer electrolyte membrane fuel cells, batteries, photovoltaic solar cells and supercapacitors. he modiied biopolymers have been used as electrolytes in electrochemical devices. Gel polymer electrolyte based on biopolymers and their derivatives may be considered as suitable electrolytes for DSSC and other electrochemical devices because they also exhibit liquid-like conductivity. To further improve the electrolytic performance, further studies are needed.

346 Nanostructured Polmer Membranes: Volume 2

Acknowledgments he authors wish to thank the Ministry of Higher Education Malaysia for the inancial support through grant FRGS/1/2014/SG06/UITM/02/1 and RACE grant CR011-2013. A. K. Arof extends his acknowledgement to Dr M. H. Buraidah, Dr Faisal Islam Chowdhury, I. M. Noor and I. A. Fadzallah for help with the manuscript. he authors also thank Prof. M. A. Careem for critically reading the manuscript.

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9 Phosphoric Acid-Doped Polybenzimidazole Membranes: A Promising Electrolyte Membrane for High Temperature PEMFC S. R. Dhanushkodi*1,2,3, M. W.Fowler2, M. D. Pritzker2 and W. Merida3 1

Gandhi Batteries, Dindigul, Tamil Nadu, India Department of Chemical Engineering, University of Waterloo, Waterloo, ON, Canada 3 Clean Energy Research Center, University of British Columbia, Vancouver, BC, Canada 2

Abstract Polymer electrolyte membrane fuel cells are considered as an ideal candidate to replace the internal combustion engines in cars and trucks. he use of Naion membranes in the fuel cell can limit the eiciency performance when the cell temperature is above 85 °C. Here, we review a preparation method and structure-property-performance relationship of a polybenzimidazoles (PBI) membrane. Unlike Naion membranes, PBI-based membrane can be operated at high temperature and low relative humidity. Keywords: Polymer electrolyte membrane fuel cell; polybenzimidazoles; conductivity; permeability; structure property relationship; nuclear magnetic resonance spectroscopy-phosphoric acid; I-V curve

9.1 Introduction Fuel cell industries are currently expediting their research to produce low cost and thermally stable proton conducting membranes. Fourth generation fuel-cell-powered electric vehicle trials have been demonstrated by the *Corresponding author: [email protected] Visakh P.M. and Olga Nazarenko (eds.) Nanostructured Polymer Membranes: Volume 2, (357–378) © 2016 Scrivener Publishing LLC

357

358 Nanostructured Polmer Membranes: Volume 2 automobile industries recently [1]. In a proton exchange membrane fuel cell (PEMFC), chemical energy carried by hydrogen is electrochemically converted to electrical energy. he heart of a PEMFC is the membrane electrode assembly (MEA), consisting of gas difusion layers, catalyst layers and membrane. During automotive applications of PEMFC, the post analysis of the components show severe degradation of the membrane [2]. Such trials use polylurosulfonic acid-based membranes, which are subjected to degradation protocols that involve dehydration, crossover and delamination of the membranes at higher temperature and lower relative humidity operations [3]. he automotive sector requires highly conductive and durable electrolyte membrane that can withstand a wide range of operating conditions, including temperature, hydration and potential [4]. During PEMFC operation, the cell voltage degrades when compositional or morphological characteristics of the electrolyte membrane are afected [5]. Most of the studies on membrane degradation of the PEMFC have reported mechanical failure, such as delamination, swelling of the membrane, hydrogen crossover and occurrence of hotspots, during high temperature and low humidity operations [5–9]. Several eforts have been recently made to mitigate the membrane durability issues by replacing Naion with polybenzimidazole (PBI) [10–19]. heir studies have focused on improving chemistry, mechanical stability and functionalization of the PBI. PBIs are attractive membranes for PEMFC in automotive applications because of their higher thermal, mechanical and chemical stabilities [20]. hey are widely used in ireighting fabrics, protective gloves, race driver suits, and aircrat wall fabrics. PBI is an aromatic heterocyclic base polymer invented by Brinker in 1959 [21] and is generally synthesized by step-growth polymerization (Figure 9.1) of monomers such as a tetraamine and a dicarboxylic acid [22]. Polybenzimidazole (PBI) is produced via condensation polymerization. It possess three benzene rings and side groups (Figure 9.2, inset) [17]. It is an amorphous thermoplastic polymer with large numbers of benzimidazole repeating units. he SEM and TEM images of the PBI membranes obtained from the literature are shown in Figure 9.2 [14, 23]. he glass transition temperature of PBI is 4500 °C, which yields good retention of stifness and toughness of the membrane. Several schemes of PBI synthesis are discussed in the literature in detail [24–26]. Table 9.1 summarizes monomers used in the synthesis of PBI and their thermal stabilities [24, 27, 28]. he solubility of the PBIs is generally limited to strong acids and polar aprotic solvents, which cannot donate hydrogen. hus, PBI can be mixed with strong acids or strong bases [29]. he solubility of the PBI polymer is displayed in Table 9.2. Since PBI possesses a rigid molecular structure, it

Phosphoric Acid-Doped Polybenzimidazole Membranes 359

HOOC

H2N

NH2

H2N

NH2

COOH

H N

H N

N

N

COOH

HOOC

PPA 190–220 C –24 h

n

H N

H N

N

N

n

mata-PBI

para-PBI

Figure 9.1 Reaction scheme of PBI [14, 23]. An original scheme of PPA process in the literature is redrawn using ChemDraw in the igure.

(a)

(c)

(b)

H N

H N

N

N

n

(d)

Figure 9.2 SEM image of PBI membranes (a) PBI powder, (b) PBI–PA gel and (c) TEM micrograph of PBI–PA gel. Inset of Figure 9.2d shows the chemical structure of PBI membrane. SEM and TEM images are directly adopted from the literature [34].

is not initially recommended for ion-exchange applications [30]. However, when the polymer is doped with either H3PO4 or H2SO4, the proton conductivity of PBI is increased to up to 200 °C; such an observation at higher temperatures reduces or eliminates the need for water molecules, which can act as a solvating agent and proton transport medium, commonly required in any fuel cell membrane [22].

360 Nanostructured Polmer Membranes: Volume 2 Table 9.1 Structure and stability of several polybenzimidazoles [24, 27, 28]. Monomer 1 Monomer 2 MP(°C) Biphenyl Tetraamine 3,4-Diaminobenzoic acid Benzene Tetraamine Terephthalic acid Biphenyl Tetraamine Isophthalic acid Biphenyl Tetraamine Phthalic acid Biphenyl Tetraamine Biphenyl-4,4 diacid Biphenyl Tetraamine Biphenyl-2,2 diacid

> 600 > 600 > 600 > 500 > 400 > 430

Table 9.2 Solubility information for polybenzimidazoles. Solvent

Solubility

Reference

N-methyl-2-pyrrolidone

Yes

dimethylacetamide Dimethylformamide Methanesulfonic acid Methanol Tetrahydrofuran Choloroform Acetone Ethanol

Yes Yes Yes No No No No No

23 14 61 62 50 63 10 50 50

A summary of the concentration of doping level and conductivity of PBI is reported in Table 9.3. Hence, acid group modiied PBIs have received a great deal of interest from the fuel cell industry due to their improved ion-exchange capacity, especially at high temperature. he protonation of PBI using phosphoric acid is well studied in the literature [31]. Ma et al. reported a mechanism and calculation for conductivity, based on the reaction step shown below [30]. PBI H 3 PO4

H 2 PO4

PBI .H

(9.1)

he equilibrium constant of the above reaction can be written as: K

[H 2 PO4 ][H ] [PBI .H ]

[H 2 PO4 ][PBI .H ] [PBI ][H 3 PO4 ] K H3 PO4 K PBI

10 10

[H 3 PO4 ]

2.16 5.23

10

3.07

[PBI][H ]

(9.2)

Phosphoric Acid-Doped Polybenzimidazole Membranes 361 Table 9.3 Conductivity of PA-doped PBI membranes and their composition.

KOH-doped PBI 15

Conductivity S-cm–1 9 × 10–2

Doping Level 6 M KOH

Reference 19

H2SO4-doped PBI

6 × 10–2

16 M H2SO4

30

H3PO4-doped PBI

2 × 10–3

15 M H3PO4

30

Phosphotungstic aciddoped PBI Sulfonated PBI Diphenyl PA-doped PBI Doped N-benzylsulfonategrated PBI H3PO4-doped PBI/sulfonated polysulfone

PBIH

1.5 × 10

–3

7.5 × 10–5 5 × 10–3 2 × 10–2

10–1

A mixture of phosphotungstic acid adsorbed on Si-doped PBI H2SO4 200% diphenyl PA – 500% H3PO4 doping level, 160 °C, and 80% RH

64 65 66 64

67, 68

in Equation 9.2 shows the degree of protonation PBI of the N number of atoms in repeat units of PBI ilm. he performance of PBI membranes, developed during the early 1970s, showed superior mechanical stability, largely due to their higher molecular weight [27]. Low molecular weight PBI developed by diferent synthesis schemes showed poor mechanical property and tensile strength. Developing a higher molecular weight PBI [17, 18] and identifying the routes to improve the processibillity of the PBIs have been recognized as two main targets for membrane research companies, such as Dupont, PBI Performance Products, Gore and Ion-Exchange, during the last ive years [19, 32]. he increased interest in PBI/PA-doped PBI membranes in the PEMFC industry necessitates identifying the synthesis, characterization of molecular weight distribution (MWD) and morphological changes of the membrane during accelerated  stress testing. Although diferent PBI structures are identiied in the literature, only PBI synthesized by melt/solid-state polymerization process, sulfonated or phosphorylated PBI, PA-doped PBI and poly(2,5-benzimidazole) are capable of serving as a proton-conducting membrane for PEMFC [33]. In spite of the many research indings on PBIbased membranes since 1959, a brief review of the chemistry, synthesis scheme, structure, performance and durability of PBI membranes will be he ratio of

362 Nanostructured Polmer Membranes: Volume 2 helpful for fuel cell companies, such as AFCC and Ballard, by contributing to the fundamental understanding of PBI membrane and its application towards PEMFC. he current report aims to outline some of the important literature indings on PBI polymers and gives an insight into the properties of PBI that are of help to fuel cell research communities.

9.2 Synthesis of PBI he polyphosphoric acid (PPA) process is identiied as one of the reliable methods to produce higher molecular weight and durable PBI membranes for the fuel cell industry [22]. he PPA process yields stable structure of PBI with higher phosphoric acid (PA)-doping levels [22]. he procedure of the synthesis, and characterization of PBI membranes by the PPA process can be found elsewhere [33]. Preparation steps of PBI and PA-doped PBI membrane are outlined in Figure 9.3 [33]. heir process of polymerization involves one-step and one-pot condensation. Isophthalic acid (IA) and polymer grade Tetraaminobiphenyl (TAB) are mixed inside the reaction vessel in an inert atmosphere (N2 glove box). PPA is added with the monomers and stirred continuously

Addition of precursor

Isophthalic acid (12.460 g, 75 mmol)

High Mwt needed Poly-condensation agent PPA (200 to 600 g) HygroscopicHydrolyzes PPA to PA

Mixing three-neck resin reaction lask Tetraaminobiphenyl TAB (16.074 g, 75 mmol)

N2 atm Constant stirring T (PID control) 220 C 16 to 24 h

Separation (dark mass of from water)

Casting

Hydrolyysis PPA-PA Pulverization (brown mass)

Dimethyl acetamide (DMAc) Air atm glass substrate 90 to 220 C 24 h 25 C 40 % RH

Phosphoric acid doped PBI Neutralization (NH4OH)

Drying 24 h at 100 C

Figure 9.3 Process low diagram of PA-doped PBI membrane as described by Xiao et al. [33].

Phosphoric Acid-Doped Polybenzimidazole Membranes 363 at approximately 200 °C for a day. A PTC10 programmable temperature controller from Stanford Research Systems and an oil bath regulates temperature during the reaction for ramping and soaking steps, respectively. Once the reaction mixture turns dark brown and viscous, a small quantity of the mass was transferred into a water container. he brown mass is then extracted from the water and pulverized. An SEM image of PBI powder is presented in Figure 9.1a [34]. Powdered mass is then neutralized with NH4OH. Repeated washing with water and subsequent drying at 100 °C for 24 hours in an oven is carried out to obtain PBI in aqueous solution state from that of the powdered sample. Inherent viscosities (IVs) of the PBI can be measured using a viscometer, such as an Ostwald or Ubbelohde viscometer, in 95% sulphuric acid at room temperature. Calibration and measurements of IV are described by Wilke et al. [35]. he IV of the PBI prepared by this PPA process is 1.1 dl/g at a concentration of 1.0 g/dl in sulphuric acid [18]. he PBI solution is casted into a PBI ilm in a glass substrate by sol-gel process at atmospheric conditions. he presence of moisture in the atmosphere hydrolyzes PPA to PA and yields a highly stable PA-doped PBI. he stability of the gel phase assists PPA to PA conversion during casting of the PBI solution [33]. SEM and TEM images of PA-doped PBI from the literature are shown in Figure 9.1b,c [34].

9.3 Characterization of PBI 9.3.1 Molecular Weight Distribution (MWD) Li et al. [17] synthesized PBI using the approach of Xiao et al. [33] and described the MWD of PBI (Figure 9.4) [18]. To suppress the efect of intra- and intermolecular interaction in the column during gel permeation chromatography analysis, 2 wt% of PBI in dimethylacetamide (DMAc) solution is added with LiBr before the analysis. By adding toluene to the solvent, any reversible change prevailing in the column can be corrected during the analysis. Data interpretation and calibration curve procedures of MWD are discussed in detail in the literature [36, 37]. Figure 9.4 obtained from the literature [17] displays mono-modal MWD, suggesting that both weight-averaged molecular weight (Mw) and the number-averaged molecular weight (Mn) values of polymer can be directly extracted from the MWD curves. MWD of PBI demonstrates a narrow distribution with a peak at a molar mass of 100,000 Da. he slightly broad MWD in Figure 9.4 represents that PA-doped PBI has high mechanical strength [37].

364 Nanostructured Polmer Membranes: Volume 2 2,00

W(log M)

1,50

1,00

0,50

0,00 1.000

10.000 100.000 Molar mass Mw, Da

1.000.000

Figure 9.4 MWD curves reported by Li et al. for PBI membrane [18]. MWD is obtained using GPC size exclusion chromatography. Static light scattering combined with a refractive index detector is used to obtain the concentration signal. An original image of TGA in the literature is modiied using Plot Digitizer in the igure [18].

9.3.2 hemogravimetric Analysis (TGA) Generally, TGA analysis is used to provide information such as (a) the efect of reactive atmosphere on PBI, (b) amount of moisture and volatile contents present in the sample, (c) decomposition kinetics, (d)  crosslinking temperature of PBI and (e) thermal and oxidative stability of the PBI [38]. However, high performance TGA equipment is needed to capture any small level of changes in the initial sample. One such example of TGA analysis of PBI, obtained from Sukumar et al., is shown in Figure 9.5 [39]. It displays the percentage weight loss of PBI as a function of temperature in a N2 atmosphere. heir results showed a mass loss of 5% from the initial weight when PBI is heated from 100 to 140 °C. Removal of unbound moisture at the sample surface, such as water and DMAc present in the PBI, may have contributed to the weight loss. he decomposition temperature of PBI reported by Sukumar et al. is 550 °C [39]. Importantly, the small weight losses observed at 350 °C in the sample are expected to be contributed by crosslinking reactions, which release VOC when PBI undergoes phase change from a thermoplastic state to a thermoset resin.

Phosphoric Acid-Doped Polybenzimidazole Membranes 365 100 90

PBI

80 Weight loss (%)

70 60 50 40 30 20 10 0 0

100

200

300

400 500 T ( C)

600

700

800

Figure 9.5 TGA curve of PBI. he analysis was done with 10 mg of PBI at a ramp rate of 20 °C min-1. An original image of TGA in the literature is modiied using Plot Digitizer in the igure [39].

9.3.3 Fourier Transform Infrared Spectroscopy (FTIR) Types of chemical bonds or functional groups present in the PBI can be identiied with the help of FTIR. he ingerprints of the chemical bond are captured using the wavelength of light absorbed in the spectrum. Several researchers have described molecular stretching of PBI [40–43]. Sannigrahi  et al. described the interaction of PA-doped PBI in gel state with pure PBI [34]. he ingerprints of FTIR from their study are shown in Figure 9.6: (a) for PBI and (b) for PA-doped PBA [34]. he prominent peaks and their details are provided in Table 9.4. Pure PBI polymer has both hydrogen donor (–NH–) and hydrogen acceptor (-N=) groups in its structure [40]. hese two sites can take part in the reactions, which involve hydrogen bonding with appropriate solvent and polymer [34, 44–46]. he interactions of hydrogen bonding of PBI with diferent acid groups and chemicals that can form miscible polymeric blends are described in the literature by several researchers [34, 44–46]. he presence of hydrogen bonding in the membrane is identiied based on the frequency shits and broadening of the peaks in the FTIR signals. Hydrogen bonding interactions are absent in pure PBI. he diference in stretching of N–H, C=C and C=N groups in the PA-doped PBI are apparent in Figure 9.6a,b. Stretching of N–H is a direct evidence of hydrogen bonding in PA-doped PBI. he diference in C=C and C=N stretch

366 Nanostructured Polmer Membranes: Volume 2

1283

1537 1444

(b)

4000

3500

488

1003

1150

1635

3402

2358

887

801

1620

3063

3161

3420

3620

% transmittance

(a)

3000

2500 2000 1500 Wavenumbers (cm–1)

1000

500

Figure 9.6 FTIR spectra of (a) PBI and (b) PBI gel in PA [34]. Original image is directly adopted from the literature [34].

Table 9.4 FTIR details of PA-doped PBI and pure PBI. Wavelength (cm−1) Figure 9.6a – pure PBI 3500–3000  3420  3161  3063  1620 1550–800  1003 and 887

Peak Information N–H stretching of imidazole Isolated, non-hydrogen-bonded free N–H groups Asymmetric peak, Self-associated, hydrogen-bonded N–H groups Low intensity peak, Stretching of C–H groups C=C, C=N Stretching Various ring vibration No Bands

Fig. 9.6b – PA-doped PBI 3402

N–H stretching, Signiicant displacement from PBI (3420 cm1)

2358

Stretching vibration of NH+  Protonation of PBI on the imino nitrogen group contribute to the stretching

(Continued)

Phosphoric Acid-Doped Polybenzimidazole Membranes 367 Table 9.4 (Cont.) Wavelength (cm−1) 1635 1550–800  1003 and 887 1150 

Peak information C=C, C=N Stretching Ring vibration is absent Bands due to free H3PO4 molecules P=O symmetric stretching of H2PO4− species

between PBI and PA-doped PBI are due to electron structure modiication and delocalization caused by protonation of imino nitrogen. Presence of higher PA can be identiied from the vibration at spectral domains from 1550 to 800 cm−1 in PA-doped PBI. Free H3PO4 molecules present in the PA-doped PBI are inferred in the spectral domains from 1003 to 887 cm−1. In summary, FTIR analysis helps to identify the ionic species (H2PO4−) in the PA-doped PBI which are in equilibrium with PBI molecules [34].

9.3.4 Nuclear Magnetic Resonance Spectroscopy (NMR) he 31P-NMR is one of the techniques used that can provide the structure and chemical shit range of pure PBI and PA-doped PBI. Although sensitivity of the 31P-NMR is less compared to proton NMR, it yields sharp peaks and has a broad chemical shit range. Before obtaining NMR spectra, PBI or PA-doped PBI samples should be treated in a vacuum oven for 120 minutes at 150 °C. Ater the treatment, samples are sealed in NMR tubes. Reference NMR is prepared at dry PPA and 100% PA in a similar manner. he details of the sample preparation methods are reported in the literature [47, 48]. A plot of NMR obtained from the literature for PPA and PA-doped PBI is shown in Figure 9.7a–d [48]. heir synthesis scheme [48] of PBI membrane is similar to the one discussed in Section 9.2 of this chapter. he 31P- NMR is obtained for PPA and a PA-doped PBI at both 20 and 140 °C. hree peaks are identiied for the PPA spectrum, which includes a small reference peak and two others at ~1.7 and ~3.7 kHz. he peaks corresponding to 1.7 and 3.7 kHz are assigned to pyrophosphoric acid and oligomeric species, respectively [49]. At ~3.7 kHz, disappearance of a peak in the PA-doped PBI, as shown in Figure 9.7c,d, describes the hydrolysis of the PPA to PA. At a high temperature (Figure 9.7d), PA and dimer components are gradually combined into one broad line. Transesterification of phosphate and polyphosphate systems is reported to have contributed for this efect [48]. Transesterification interchange reaction is assumed to be the rationale for the proton conduction mechanism. However, the details of this mechanism are still not well understood.

368 Nanostructured Polmer Membranes: Volume 2

1.0

0.5

0.5

0.0

0.0

–1.5

–1.5

–2.0

–2.0

AU

1.0

–2.5 –20

–10 0 10 Spectrum width (kHz)

20

–2.5 –20

–10 0 Spectrum width (kHz)

10

20

Figure 9.7 he 31P NMRFTIR spectra of (a) PPA at 20 °C, (b) PPA at 140 °C, (c) PA-doped PBI at 20 °C and (d) PA-doped PBI at 140 °C. Original images from the literature are directly adopted in the igure [48].

9.3.5 Conductivity In order to improve the conductivity of the PA-doped PBI membrane at higher temperature and lower humidity, doping level of PA in the PBI should be optimized [50, 51]. He et al. obtained the doping level and conductivity relations of PBI membranes at diferent humidities and temperatures [52]. he doping level of PA in the PBI membrane is deined by the molar ratio of number of PA molecules to the number of PBI repeat units [30]. Optimum ranges of 20 to 40 mol of PA per mole of PBI repeat unit are observed for the PA-doped PBI membrane developed by sol-gel process [33]. In contrast, Li et al. found the ranges of PA-doping between 13 and 16 mol of PA per PBI for a commercial membrane [17]. he relationship between conductivity and PA-doped PBI as a function of water vapor activity at doping levels and temperature, as reported by Wainright et al., is shown in Figure 9.8a [50]. hey found conductivity of PA-doped PBI to be higher than perluorosulfonic acid membranes. Any increase in the vapor activity in the membrane surface is expected to contribute to higher water content and lower viscosity within the membrane [30, 50]. An increase in temperature increases the mobility and conductivity of the protons in the membrane.

9.3.6 Permeability and Mechanical Testings horough plane permeability measurement of any polymer can be made  using the procedure discussed by Gostick et al. [53] and

Phosphoric Acid-Doped Polybenzimidazole Membranes 369 3.5

0.040 Tensile strength (MPa)

0.030 0.025 0.020 0.015 0.010 0.005 0.000

450

3

0.0

0.1 0.2 0.3 0.4 Water vapor activity (PH2O/Psat)

(a)

Doping level 338 m/o (illed symbols) and 501 m/o (open symobls). The lines drawn are intended only as guides for the eye. , –130 C; , –150 C; , –190 C.

0.5

400 2.5

350

2

300 250

1.5

200 1

150

0.5 0.5 (B)

Elongation at break (%)

Conductivity S cm–1

0.035

500

100 3.5

1 1.5 2 2.5 3 Inherent viscosity (dL/g)

I.V.’s of the polymer samples are measured at a polymer concentration of 0.1 g/dL in concentrated sulphuric acid (96%) at 30 C, using a cannon ubbelohde viscometer.

Figure 9.8 (a) Conductivity of PA-doped PBI vs. water vapor activity; (b) Tensile strength and elongation at break of PBI as a function of polymer IV for PA-doped PBI. Tensile strength and elongation at break of PBI are presented as circle and square respectively in the igure. An original image from the literature is directly adopted in the igure [33].

Table 9.5 Permeability of hydrogen and oxygen at diferent temperatures [55]. 25 °C H2 O2

80 °C H2

120 °C O2

Pure – PBI

–

1.6 × 10−17

Doped – PBI

–

120 × 10−17 30 × 10−17

H2

0.05 × 10−17 3.0 × 10−17

180 °C O2

H2

0.006 × 10−17 4.3 × 10−17

250 × 10−17 70 × 10−17

O2 0.1 × 10−17

380 × 10−17 90 × 10−17

*Permeability is in mol cm cm−2 s−1 Pa−1

Dhanushkodi et al. [54]. He et al. employed this method to study the permeability of PBI at diferent temperatures [55]. Gas permeation can be determined using the solubility, difusivity (Dmem) of the gas and selectively [56]. Small sized molecules (H2 = 2.89  Å) that have higher Dmem are expected to have lower solubility compared to larger sized molecules (O2 = 3.46 Å). Experimental evidence given by He et al. portrayed higher permeability for hydrogen than for oxygen at 80 °C [52]. he results of He et al. show the permeability coeicient of H2 and O2 for pure and PA-doped PBI to increase with an increase in temperature (Table 9.5) [52]. Recent advances in synthesis schemes of PA-doped PBI membranes have produced PBI membranes with very high proton conductivity [16]. heir membranes displayed very good permeability of gases, higher concentrations of PA and doping levels. However, their study reported that the

370 Nanostructured Polmer Membranes: Volume 2 mechanical strength of the membrane was severely compromised in order to produce such high conductivity membranes [14]. Hence, the strength and processability of such membranes are poor for either designing or fabricating MEAs for PEMFC applications. Interestingly, the mechanical properties of the doped membranes made by sol-gel process showed very high tensile strength and percentage of elongation [33]. hese two properties are directly dependent on molecular weight and IV [33]. he PBIs synthesized with higher molecular weight and IV are expected to possess higher mechanical properties and gel stability.

9.3.7 Fuel Cell Testings Zhang et al. studied the performance of the PA-doped PBI membrane under fuel cell conditions, by preparing a catalyst-coated membrane with Pt loading of 1.7 mg cm−2 at both anode and cathode [57]. A cell polarization curve was obtained for the MEAs in the temperature ranges between 120 and 200 °C under ambient pressure and dry conditions. PA-doped PBI MEAs displayed cell potentials of 0.366 and 0.485 V at 1.0 A cm–2 at 120 and 200 °C, respectively. he rationale for the very high performance at 0.1 A cm-2 by PA-doped PBI membranes compared with any other commercial membranes currently available, includes: (a) higher rate of hydrogen oxidation reaction and oxygen reduction reaction kinetics, (b) enhanced water management at the higher temperature, (c) improved thermal management of PBI-based membranes, (d) excellent contamination tolerance and (e) improved proton transfer at the catalyst layer-ionomer interface. he reported performance of PA-doped PBI-based MEAs in the literature for PEMFC are given in Table 9.6 and Figure 9.9.

9.4 Research Needs and Conclusions Despite extensive research eforts on inducting PA-doped PBI in PEMFC, very limited literature is available on the durability aspects of PBI-based MEAs. Membrane researchers need to examine the oxidative stability, ionexchange capacity and chemical stability of PBI-based MEAs at higher temperature and lower RH using accelerated stress testing protocols for PEMFC applications. A key aspect of such work should include the issues associated with the water management of PBI-based MEAs. he relationship between acid concentration and water uptake on the PBI membrane and its impact on fuel cell performance is still not well understood. A summary of the research needs for PBI membranes are given in Table 9.7.

Phosphoric Acid-Doped Polybenzimidazole Membranes 371 Table 9.6 Single cell PEMFC data of acid-doped PBI membranes. Current Pressure Voltage density Membranes Tcell (°C) Reactants (bar) (V) (mAcm−2) Reference 190, 170, 630, 430, H3PO4/PBI 150 & H2/O2 1 0.6 300 & 69 100 160 H3PO4/PBI

150

H2/O2

1

0.54

250

70

H3PO4/PBI

150

H2/air

1

0.41

250

70

170

H2/O2

1

0.6

350

68

130

H2/O2

1

0.6

180

68

H3PO4SPSF/PBI

200

H2/O2

1

0.6

700

67

H3PO4SPSF/PBI

200

3%CO– H2/O2

1

0.6

570

11

H3PO4SPSF/PBI

190

H2/O2

1

0.6

430

11

H3PO4SPSF/ PBI H3PO4SPSF/ PBI

0.6

0.9 0.8

0.5

Cell voltage (V)

0.4

0.6 0.5

0.3 0.4 120 140 160 180 200

0.3 0.2 0.1 0.0 0.0

0.2

C C C C C

0.4 0.6 0.8 1.0 1.2 1.4 Current density (A/cm2)

0.2

Power density (W/cm2)

0.7

0.1

1.6

1.8

2.0

0.0 2.2

Figure 9.9 Polarization curves of MEAs based on PA-doped PBI obtained at diferent temperatures, ambient pressures, 0% RH. he original image from the literature is directly adopted in the igure [57].

372 Nanostructured Polmer Membranes: Volume 2 Table 9.7 Research needs for PBI membranes. Important research needs in PA-doped PBI system for fuel cell industries 1. Investigation of proton transfer mechanism of PA-doped PBI at diferent higher temperatures and RH in fuel cell conditions. 2. Determination of capillary characteristics of PA-doped membranes in water-air systems. 3. Study of ionic transfer mechanisms on PA-doped PBI at diferent higher temperatures. 4. Comparison of physiochemical properties, such as permeability, FTIR, NMR and mechanical testings, between PA-doped PBI with H2SO4-doped PBI. 5. Standardization of the synthesis protocol of PA-doped PBI for fuel cell industries. 6. Durability testing of PA-doped PBI-based MEAs under diferent RH, temperature and potential cycling. 7. Comparison of durability analysis with Naion membranes. 8. Optimization of the ionomer content present in the catalyst layer of PA-doped PBI-based MEAs. 9. Impact of catalyst dissolution during durability testing in PA-doped PBI system in fuel cell conditions. Important shortcomings and short-term research needs in PA-doped PBI Shortcoming

Research Needs

Crossover of gases in PEMFC conditions

Research ideas are lacking in optimizing thickness of the PBI membranes, which can help to overcome the crossover issues associated with the reactants during MEA testing. PBI-based membranes are prepared with the thickness of 80 μm, which is ten times lower than commercial Naion membrane. Further research may be required to improve the ilm casting method or imbibing method to produce a membrane with minimum or no crossover.

Relation between performance of PBI-based MEA with PA-doping level and drying times in fuel cell conditions

PBI-based MEAs are expected to show higher performance when PA-doping levels and their subsequent drying time increase during casting of PBI ilm. Fundamental understanding of this phenomenon needs a more detailed understanding of synthesis, structure and property relationship.

Phosphoric Acid-Doped Polybenzimidazole Membranes 373 Table 9.7 Cont. Contamination efect of PA-doped PBI-based MEAs in fuel cell conditions

Optimum concentration of contaminants, such as CO and SO2, in fuel stream, which can afect the performance of PA-doping in the PBI membranes in fuel cell environment. Detailed research on contaminant level of PBI membranes is not well explored and requires detailed experimental and sensitivity analysis.

Commercial research needs in composite PBI-based MEAs Shortcoming

Research Needs

Stability and reliability of PEMFCs based on composite PBI membranes is yet to be addressed

Durability protocols involving wet-dry, efect of potential cycling at diferent RHs, study of potential cycling at diferent temperatures, and OCV hold on PBI-based composite membranes need to be evaluated.

Diference in conductivity between PBI and Naion

he conductivity of Naion is ten times higher than PBI membranes. Although PA addition to PBI improves the conductivity of PBI, the mechanical properties and durability of the membrane are severely afected. PA-doped nanostructured PBI membranes should be explored in fuel cell studies.

Recently, eforts were taken to prepare composite PBI membrane with less gas crossover [58]. However, the stability of the composite membranes and its ability to retain dopant during long-time operation are yet to be evaluated. Analyzing the chemical compositions of water and gas from fuel cell outlet during testing can help improve the design and lifetime of PBIbased MEAs in fuel cell applications. Another interesting line of research is to focus on preparing crosslinked and sulphonated PBI-based membranes [59, 60]. his efort can improve the conductivity and durability of the membrane. From the beginning of fuel cell technology, the thrust of membrane studies has been on developing highly durable and stable membranes. hus, adaptation of PA-doped PBI in the fuel cell industry can improve the chances of fuel cell commercialization.

Table of Abbreviations AFCC DMAc FTIR

Automotive fuel cell cooperation dimethylacetamide Fourier transform infrared spectroscopy

374 Nanostructured Polmer Membranes: Volume 2 GmbH IA I.V. MEA MWD NMR OCV PA PBI PEMFC PEM PFSA PPA RH SEM TAB TEM TGA VOC H2SO4 H3PO4

Gesellschat mit beschränkter Hatung Isophthalic acid Inherent viscosity Membrane electrode assemblies Molecular weight distribution Nuclear magnetic resonance Open circuit potential Phosphoric acid Polybenzimidazole Polymer membrane electrolyte membrane fuel cells Polymer electrolyte membrane Polylurosulfonic acid Poly phosphoric acid Relative humidity Scanning electron microscopy 3,3’,4,4’-tetramino- biphenyl Transmission electron microscope hemogravimetric analysis Volatile organic compounds Sulphuric acid Phosphoric acid

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Phosphoric Acid-Doped Polybenzimidazole Membranes 375 8. W. Schmittinger, A. Vahidi, A review of the main parameters inluencing long-term performance and durability of PEM fuel cells. J. Power Sources, 180, 1–14, 2008. 9. T. Tingelöf, J.K. Ihonen, A rapid break-in procedure for PBI fuel cells. Int. J. Hydrogen Energy, 34, 6452–6456, 2009. 10. H. Pu, G. Liu, Synthesis and solubility of poly (N-methylbenzimidazole) and poly (N-ethylbenzimidazole): Control of degree of alkylation. Polymer International, 54, 175–179, 2004. 11. C. Hasiotis, V. Deimede, C. Kontoyannis, New polymer electrolytes based on blends of sulfonated polysulfones with polybenzimidazole. Electrochim. Acta, 46, 2401–2406, 2001. 12. J.A. Kerres, Development of ionomer membranes for fuel cells. J. Membr. Sci., 185, 3–27, 2001. 13. L. Qingfeng, H.A. Hjuler, N. Bjerrum, Phosphoric acid doped polybenzimidazole membranes: Physiochemical characterization and fuel cell applications. J. Appl. Electrochem., 31, 773–779, 2001. 14. Q. Li, C. Pan, J.O. Jensen, P. Noyé, N.J. Bjerrum, Cross-linked polybenzimidazole membranes for fuel cells. Chemistry of Materials, 19, 350–352, 2007. 15. J. Yang, Q. Li, L.N. Cleemann, J.O. Jensen, C. Pan, N.J. Bjerrum, R. He, Crosslinked hexaluoropropylidene polybenzimidazole membranes with chloromethyl polysulfone for fuel cell applications, Adv. Energy Mater., 2013. 16. Z.Y. Zhai, Y.G. Shen, B. Jia, Y. Yin, Surface morphology studies on pbi membrane materials of high temperature for proton exchange membrane fuel cells, Adv. Mater. Res., 625, 239–242, 2013. 17. Q. Li, R. He, J. Jensen, N. Bjerrum, PBI-based polymer membranes for high temperature fuel cells–preparation, characterization and fuel cell demonstration, Fuel Cells, 4, 147–159, 2004. 18. Q. Li, H.C. Rudbeck, A. Chromik, J.O. Jensen, C. Pan, T. Steenberg, M. Calverley, N. Bjerrum, J. Kerres, Properties, degradation and high temperature fuel cell test of diferent types of PBI and PBI blend membranes. J. Membr. Sci., 347, 260–270, 2010. 19. B. Xing, O. Savadogo, Hydrogen/oxygen polymer electrolyte membrane fuel cells (PEMFCs) based on alkaline-doped polybenzimidazole (PBI). Electrochem. Commun., 2, 697–702, 2000. 20. E. Powers, G. Serad, High performance polymers: heir origin and development, in: History and Development of Polybenzimidazoles, 355–373, Springer, 1986. 21. K.C. Brinker and I.M. Robinson, PBI, US Patent 2895948, 1959. 22. D. Seel, B. Benicewicz, L. Xiao, T. Schmidt, High-temperature polybenzimidazole-based membranes, in: Handbook of Fuel Cells: Advances in Electrocatalysis, Materials, Diagnostics, and Durability, 5, 300–312, 2010. 23. Y. Zhai, H. Zhang, Y. Zhang, D. Xing, A novel H3PO4/Naion–PBI composite membrane for enhanced durability of high temperature PEM fuel cells. J.  Power Sources, 169, 259–264, 2007.

376 Nanostructured Polmer Membranes: Volume 2 24. T.-S. Chung, A critical review of polybenzimidazoles. J. Macromol. Sci. C: Polym. Rev., 37, 277–301, 1997. 25. T.S. Chung, Z.L. Xu, W. Lin, Fundamental understanding of the efect of airgap distance on the fabrication of hollow iber membranes. J. Appl. Polym. Sci., 72, 379–395, 1999. 26. F. Model, L. Lee, An investigation of PBI hollow iber reverse osmosis membranes. Org. Coat. Plast. Chem., 32, 383, 1972. 27. H. Vogel, C.S. Marvel, Polybenzimidazoles: New thermally stable polymers, J. Polym. Sci., 50, 511–539, 1961. 28. H. Vogel, C.S. Marvel, Polybenzimidazoles. II. J. Polym. Sci. A: General Papers, 1, 1531–1541, 1963. 29. Y. Yuan, F. Johnson, I. Cabasso, Polybenzimidazole (PBI) molecular weight and Mark-Houwink equation. J. Appl. Polym. Sci., 112, 3436–3441, 2009. 30. Y.-L. Ma, J. Wainright, M. Litt, R. Savinell, Conductivity of PBI membranes for high-temperature polymer electrolyte fuel cells. J. Electrochem. Soc., 151, A8–A16, 2004. 31. H. Pu, W.H. Meyer, G. Wegner, Proton transport in polybenzimidazole blended with H3PO4 or H2SO4. J. Polym. Sci. B: Polym. Phys., 40, 663–669, 2002. 32. A.L. Graham, hermodynamics of hermally Stable Polymers: PBI, 2011. 33. L. Xiao, H. Zhang, E. Scanlon, L. Ramanathan, E.-W. Choe, D. Rogers, T.  Apple, B.C. Benicewicz, High-temperature polybenzimidazole fuel cell membranes via a sol-gel process. Chemistry of Materials, 17, 5328–5333, 2005. 34. A. Sannigrahi, S. Ghosh, S. Maity, T. Jana, Polybenzimidazole gel membrane for the use in fuel cell. Polymer, 52, 4319–4330, 2011. 35. J. Wilke, H. Kryk, J. Hartmann, D. Wagner, heory and Praxis of Capillary  Viscometry: An Introduction, Schott-Geräte GmbH, Hoheim, Germany, 1994. 36. Z. Grubisic, P. Rempp, H. Benoit, A universal calibration for gel permeation chromatography. J. Polym. Sci. B: Polym. Phys., 34, 1707–1713, 2003. 37. D. Wif, M. Gehatia, A. Wereta, Molecular weight distribution and mechanical behavior of PBI ibers. J. Polym. Sci.: Polym. Phys. Ed., 13, 275–284, 1975. 38. W. Sichina, Characterization of Polymers Using TGA, Boston, MA: Perkin Elmer, 2001. 39. P.R. Sukumar, W. Wu, D. Markova, Ö. Ünsal, M. Klapper, K. Müllen, Functionalized poly(benzimidazole)s as membrane materials for fuel cells, Macromol. Chem. Phys., 208, 2258–2267, 2007. 40. P. Musto, F.E. Karasz, W.J. MacKnight, Fourier transform infra-red spectroscopy on the thermo-oxidative degradation of polybenzimidazole and of a polybenzimidazole-polyetherimide blend. Polymer, 34, 2934–2945, 1993. 41. J. Lobato, P. Cañizares, M.A. Rodrigo, J.J. Linares, G. Manjavacas, Synthesis and characterisation of poly[2,2-(m-phenylene)-5,5-bibenzimidazole] as polymer electrolyte membrane for high temperature PEMFCs. J. Membr. Sci., 280, 351–362, 2006.

Phosphoric Acid-Doped Polybenzimidazole Membranes 377 42. S. Qing, W. Huang, D. Yan, Synthesis and characterization of thermally stable sulfonated polybenzimidazoles. Eur. Polym. J., 41, 1589–1595, 2005. 43. X. Glipa, B. Bonnet, B. Mula, D.J. Jones, J. Rozière, Investigation of the conduction properties of phosphoric and sulfuric acid doped polybenzimidazole. J. Mater. Chem., 9, 3045–3049, 1999. 44. D. Arunbabu, A. Sannigrahi, T. Jana, Blends of polybenzimidazole and poly(vinylidene luoride) for use in a fuel cell. J. Phys. Chem. B, 112, 5305–5310, 2008. 45. M. Hazarika, D. Arunbabu, T. Jana, Formation of core (polystyrene)–shell (polybenzimidazole) nanoparticles using sulfonated polystyrene as template. J. Colloid Interface Sci., 351, 374–383, 2010. 46. P. Musto, F.E. Karasz, W.J. MacKnight, Hydrogen bonding in polybenzimidazole/poly(ether imide) blends: A spectroscopic study. Macromolecules, 24, 4762–4769, 1991. 47. F.D. Blum, NMR Studies of Organic hin Films, Annual Reports on NMR Spectroscopy, 28, 1994. 48. J. Jayakody, S. Chung, L. Durantino, H. Zhang, L. Xiao, B. Benicewicz, S. Greenbaum, NMR studies of mass transport in high-acid-content fuel cell membranes based on phosphoric acid and polybenzimidazole. J. Electrochem. Soc., 154, B242–B246, 2007. 49. M. Crutchield, C. Callis, R. Irani, G. Roth, Phosphorus nuclear magnetic resonance studies of ortho and condensed phosphates. Inorganic Chemistry, 1, 813–817, 1962. 50. J. Wainright, J.T. Wang, D. Weng, R. Savinell, M. Litt, Acid-doped polybenzimidazoles: A new polymer electrolyte. J. Electrochem. Soc., 142, L121–L123, 1995. 51. J. Weber, K.-D. Kreuer, J. Maier, A. homas, Proton conductivity enhancement by nanostructural control of poly(benzimidazole)-phosphoric acid adducts. Adv. Mater., 20, 2595–2598, 2008. 52. R. He, Q. Li, G. Xiao, N.J. Bjerrum, Proton conductivity of phosphoric acid doped polybenzimidazole and its composites with inorganic proton conductors. J. Membr. Sci., 226, 169–184, 2003. 53. J.T. Gostick, M.W. Fowler, M.D. Pritzker, M.A. Ioannidis, L.M. Behra, In-plane and through-plane gas permeability of carbon iber electrode backing layers. J. Power Sources, 162, 228–238, 2006. 54. S. Dhanushkodi, M. Fowler, M. Pritzker, X.-Z. Yuan, H. Wang, Degradation and diagnostic analysis of gas difusion layers under humidity cycling, in: Meeting Abstracts, pp. 349–349, Electrochemical Society, 2010. 55. R. He, Q. Li, A. Bach, J.O. Jensen, N.J. Bjerrum, Physicochemical properties of phosphoric acid doped polybenzimidazole membranes for fuel cells. J. Membr. Sci., 277, 38–45, 2006. 56. S. Dhanushkodi, M. Fowler, A. Mazza, M. Pritzker, Membrane electrode assembly contamination, in: Proton Exchange Membrane Fuel Cells: Contamination and Mitigation Strategies, 4, 151, CRC Press, 2010.

378 Nanostructured Polmer Membranes: Volume 2 57. J. Zhang, Y. Tang, C. Song, J. Zhang, Polybenzimidazole-membrane-based PEM fuel cell in the temperature range of 120–200 °C. J. Power Sources, 172, 163–171, 2007. 58. O. Savadogo, Emerging membranes for electrochemical systems: Part II. High temperature composite membranes for polymer electrolyte fuel cell (PEFC) applications. J. Power Sources, 127, 135–161, 2004. 59. S. Wang, C. Zhao, W. Ma, N. Zhang, Y. Zhang, G. Zhang, Z. Liu, H. Na, Silanecross-linked polybenzimidazole with improved conductivity for high temperature proton exchange membrane fuel cells, J. Mater. Chem. A, 1, 621–629, 2013. 60. D.W. Seo, Y.D. Lim, S.H. Lee, H.H. Jang, S.Y. Choi, Y.T. Jeon, H. Ju, W.G. Kim, Preparation and properties of phosphoric acid doped sulfonated poly (tetra phenyl phthalazine ether sulfone) copolymers for high temperature proton exchange membrane application. Int. J. Hydrogen Energy, 2013. 61. B. Rufmann, H. Silva, B. Schulte, S. Nunes, Organic/inorganic composite membranes for application in DMFC. Solid State Ionics, 162, 269–275, 2003. 62. M.B. Gieselman, J.R. Reynolds, Aramid and imidazole based polyelectrolytes: Physical properties and ternary phase behavior with poly (benzobisthiazole) in methanesulfonic acid. Macromolecules, 26, 5633–5642, 1993. 63. J.R. Klaehn, T.A. Luther, C.J. Orme, M.G. Jones, A.K. Wertsching, E.S. Peterson, Soluble N-substituted organosilane polybenzimidazoles. Macromolecules, 40, 7487–7492, 2007. 64. J. Rozière, D.J. Jones, M. Marrony, X. Glipa, B. Mula, On the doping of sulfonated polybenzimidazole with strong bases. Solid State Ionics, 145, 61–68, 2001. 65. P. Staiti, F. Lufrano, A. Arico, E. Passalacqua, V. Antonucci, Sulfonated polybenzimidazole membranes: Preparation and physico-chemical characterization. J. Membr. Sci., 188, 71–78, 2001. 66. H. Akita, M. Ichikawa, K. Nosaki, H. Oyanagi, M. Iguchi, Solid polymer electrolytes, in: Google Patents, 2000. 67. C. Hasiotis, L. Qingfeng, V. Deimede, J. Kallitsis, C. Kontoyannis, N. Bjerrum, Development and characterization of acid-doped polybenzimidazole/sulfonated polysulfone blend polymer electrolytes for fuel cells. J. Electrochem. Soc., 148, A513–A519, 2001. 68. V. Deimede, G. Voyiatzis, J. Kallitsis, L. Qingfeng, N. Bjerrum, Miscibility behavior of polybenzimidazole/sulfonated polysulfone blends for use in fuel cell applications. Macromolecules, 33, 7609–7617, 2000. 69. L. Qingfeng, H.A. Hjuler, C. Hasiotis, J. Kallitsis, C. Kontoyannis, N. Bjerrum, A quasi-direct methanol fuel cell system based on blend polymer membrane electrolytes. Electrochem. Solid State Lett., 5, A125–A128, 2002. 70. J.-T. Wang, R. Savinell, J. Wainright, M. Litt, H. Yu, A H2O2 fuel cell using acid doped polybenzimidazole as polymer electrolyte, Electrochim. Acta, 41, 193–197, 1996.

10 Natural Nanoibers in Polymer Membranes for Energy Applications Annalisa Chiappone Italian Institute of Technology (IIT), Torino, Italy

Abstract Natural ibers from cellulose and chitin are largely used as reinforcing materials thanks to their attracting properties combined with low cost. As a consequence, natural iber-polymer composites are extremely interesting because they are lightweight, economical and available in a variety of shapes. By modifying either the resin system or the origin or dimensions of the ibers, biocomposites can be designed for diferent applications. his chapter briely introduces natural nanoibers and their production processes. It proposes and overviews polymer nanocomposite membranes based on natural ibers, focusing on their application in the energy ield, with a discussion of fundamental research in this area. Keywords: Cellulose, chitin, whiskers, microibrils, composite membranes, electrolyte, electrode, energy devices

10.1 Introduction Natural ibers are nowadays largely used as reinforcing materials thanks to the advantages that they present: they are low weight, biodegradable, cheap, renewable, abundantly available, they have interesting speciic properties with good mechanical properties and, furthermore, they are oten waste biomass [1]. As a consequence, natural iber-polymer composites are extremely interesting, being lightweight, economical and available in a variety of shapes; in fact they are not only environmentally friendly but they also have low densities, competitive material properties and high molding lexibility, making them a conceivable alternative to traditional Corresponding author: [email protected] Visakh P.M. and Olga Nazarenko (eds.) Nanostructured Polymer Membranes: Volume 2, (379–412) © 2016 Scrivener Publishing LLC

379

380 Nanostructured Polmer Membranes: Volume 2 materials like ceramic composites [2]. By modifying either the resin system or the origin or dimensions of the ibers, biocomposites can be designed for diferent applications ranging from commodity products to aerospace, examples including electroactive papers, controlled drug release mechanisms and biosensors [3]. his chapter aims to analyze the advancement in the application of natural nanoibers-based materials, with particular attention to their application in the energy ield, with a discussion of fundamental research in this area.

10.2

Natural Fibers

Natural ibers may be classiied by their origin from plants, animals or minerals. Vegetable ibers are generally composed mainly of cellulose or other organic polymers such as starch or lignin. Cellulosic ibers can be further categorized as follows: t
Seed ibers (from seeds or seed cases); e.g., cotton and kapok. t
Leaf ibers (from leaves); e.g., sisal and agave. t
Bast ibers or skin ibers (from the skin or bast surrounding the stem of the plant). hese ibers have high tensile strength and are used for packaging and paper; e.g., lax, jute, kenaf, industrial hemp, ramie, rattan, soybean ibers, banana ibers. t
Fruit ibers (from the fruit of the plant); e.g. coconut (coir) ibers. t
Stalk ibers (ibers are the stalks of the plant); e.g., straws of wheat, rice, barley, and other crops, including bamboo and grass. Tree wood is also such a iber. he most difused ibers in common applications are cotton, lax and hemp, although sisal, jute, kenaf, and coconut are also widely used [4]. Animal ibers generally consist of proteins and in a few cases polysaccharides (cellulose or chitin); examples of proteins include silk, wool, angora, mohair and alpaca. Animal ibers can be classiied as: t
Animal hairs (wool taken from animals or hairy mammals); e.g., sheep’s wool, goat hair (cashmere, mohair), alpaca hair, horse hair, etc. t
Silk ibers (from dried saliva of bugs or insects during the preparation of cocoons); e.g., silk from silk worms. t
Avian ibers (from birds); e.g., feathers and feather ibers.

Natural Nanofibers in Polymer Membranes 381 Mineral ibers are naturally occurring ibers or slightly modiied ibers procured from minerals. hese can be categorized into the following categories: t
Asbestos: he only naturally occurring mineral iber. t
Ceramic ibers; e.g., glass ibers, aluminum oxide, silicon carbide, and boron carbide. t
Metal ibers; e.g., aluminum ibers [2]. Considering the macro, micro and nano sizes, the applications and advantages of each of the categories mentioned above are abundant; in this chapter we will focus on organic ibers, mainly cellulose (which is the most used natural polymer) and chitin reduced at the nanoscale and used as iller for the production of polymer nanocomposites.

10.2.1 Cellulose and Chitin Structures Among the variety of bioproducts, cellulose is the most abundant of the products of photosynthesis in the plant kingdom and it is produced annually in enormous amounts by natural plant growth. It is represented by the formula (C6H10O5)n, where n is indeinite, varying with diferent sources of cellulose, and with the treatment that has been received. he degree of polymerization (DP) of native wood cellulose is of the order of 10000 and lower than that of cotton cellulose (about 15000). hese DP values correspond to molecular lengths of 5.2 and 7.7 mm, respectively. he polydispersity of cellulose is rather low (< 2), indicating that the weight average molecular mass (Mw) and the number average molecular mass do not deviate much from each other [5]. he structure of cellulose is shown in Figure 10.1, the recurring unit of the molecular chain is composed by two consecutive glucopyranose units forming a cellobiose unit. Elementary ibrils

Crystalline part Amorphous region

Cellulose ibre Microibrillated cellulose Cellulose chemical structure

Figure 10.1 Cellulose structure.

382 Nanostructured Polmer Membranes: Volume 2 Because of the strong tendency for intra- and intermolecular hydrogen bonding, bundles of cellulose molecules aggregate to ibrils, which form either highly ordered (crystalline) or less ordered (amorphous) regions. Elementary ibrils form microibrils which have impressive properties; their Young’s modulus is 138 Gpa [6], their tensile strength is at least 2 GPa, and their thermal-expansion coeicient in the axial direction is as small as 0.1 10–6 K–1 [7, 8]. hese microibrils pass through several crystalline regions (about 60 nm in length) and, as a result of further aggregation of microibrils, “cellulose iber wall” with a high degree of crystallinity (60–75%) is formed [8]. his also means that cellulose is relatively inert during chemical treatments and also that it is soluble only in a few solvents. he most common cellulose solvents are cupriethylenediamine (CED) and cadmiumethylenediamine (Cadoxen), whereas less well known but powerful solvents are N-methylmorpholine, N-oxide and lithium chloride/dimethylformamide [9]. Most cellulose occurs in nature and is intimately associated with other substances composing the plant structure, such as hemicelluloses and lignin, from which it can be separated by long processes. Cellulose ibers can undergo diferent treatments and processes according to the use they are designated for. hey can be treated in order to obtain short ibers or pulped to produce the slurry used for paper production [5]. Among the natural polymers, chitin also deserves particular interest being the second most abundant biopolymer, occurring mainly in the exoskeletons of shellish and insects and the cell walls of mushrooms. It is biosynthesized at a rate of about 1010 tons per year [10]. Despite having good properties, chitin, a semicrystalline biopolymer with nanosized ibrillar morphology and excellent material properties, most chitin is thrown away as industrial waste. Some research groups have presented works based on materials containing chitin nanoibers for energy applications; thus a brief overview of its structure and properties will now be presented. Chitin is known to be a cellulose analogue with a (1,4)-b-N-acetyl glycosaminoglycan-repeating structure and a deacetylated derivative, respectively (Figure 10.2). Because of its linear structure with two hydroxyl groups and an acetamide group, chitin, similarly to cellulose, is highly crystalline with strong hydrogen bonding having high binding energy, and is arranged as nanosize chitin nanoibers. he nanoibers are about 2–5 nm in diameter and about 300 nm in length and are embedded in a protein matrix [11–13]. Chitin microibrils are constituted of alternating crystalline and amorphous domains, thus, also for chitin, submitting the material to a strongly

Natural Nanofibers in Polymer Membranes 383 CH3 O

OH

NH

O

O

O

HO

NH

HO O OH

O CH3

n

Figure 10.2 Chitin structure.

acidic environment causes the longitudinal cutting of these microibrils, allowing dissolution of amorphous domains [14].

10.2.2 Nanoibers Production Recently, there has been growing interest in producing nanoibers from biopolymers because of their interesting properties. Nature produces a variety of nanoibers, such as collagen triple helix ibers, ibroin ibrils, keratin ibrils, crystalline cellulose and so on. hese nanoibers can also form a complex hierarchical organization. he extraction of nanoibers from biomass is then considered to be a “top-down” approach in contrast to “bottom-up” processes, such as electrospinning, in which nanoibers are artiicially produced [10]. As shown in Figure 10.1, natural polysaccharides, such as cellulose but also chitin and starch, consist of both crystalline and amorphous regions. Diferent methods for extracting the crystalline regions from the biomass exist. he recovered material occurs in the form of polysaccharide nanocrystals, most commonly rod-like nanocrystals (cellulose whiskers, nanocrystalline cellulose, chitin nanowhiskers) [15]. Concerning cellulose, the main process in the preparation of whiskers is based on strong acid hydrolysis under strictly controlled conditions of temperature, agitation, and time. According to this process the amorphous regions, which can be considered as structural defects, are attacked under acid hydrolysis, leaving the crystalline regions, the more resistant domain, intact. he resulting suspension is washed by successive centrifugations and dialysis is performed using distilled water to remove any free acid molecules [16–18]. Diferent sources of cellulose were studied and used for producing nanocrystals: tunicin [19], valonia [20], cotton [21], wood

384 Nanostructured Polmer Membranes: Volume 2

Figure 10.3 Transmission electron micrographs from dilute suspension of cellulose nanocrystals from (a) cotton (Reprinted with permission from [27]; Copyright (2008) Elsevier), (b) ramie (reproduced with permission from [28]; he Royal Society of Chemistry), (c) tunicine (Reprinted with permission from [29]; Copyright (2011) Wiley).

pulp [22], sugar-beet pulp [23], sisal [24], etc. (Figure 10.3). he diferent source can inluence the dimensions and structure of cellulose nanocrystals [25]. he impact of hydrolysis conditions on the size and stability of nanocrystal has been investigated and the optimal process conditions have been determined [26] to be at a sulfuric acid concentration of 63.5% (w/w) and a reaction time of approximately 130 min. Nanocrystals can be produced with a length ranging between 200 and 400 nm, a width narrower than 10 nm, and a yield that is 30% of the initial weight. In the last decades, novel extractive techniques or upgrading steps have also been developed such as enzymatic combined with acid hydrolysis extraction of cellulose nanocrystals, enzymatic-mediated production of cellulose nanocrystals from recycled pulp, ionic liquid preparation of nanowhiskers, sonochemical-assisted hydrolysis, and combined mechanical shearing [30]. he advancement in the ield is necessary considering that the market always requires technological innovation for large-scale production, with shorter duration times, higher yield and milder preparation conditions. Unfortunately the process of extracting whiskers from plants is still time-consuming and very costly, despite the abundance and low price of the raw materials. By omitting the hydrolysis step and only submitting the ibers to high mechanical shearing forces, disintegration of the ibers occurs, leading to a material called microibrillated cellulose (MFC). hese microibrils consist of individual nanoparticles with a lateral dimension around 5 nm which aggregate in structures whose lateral dimensions range between about 10 and 30 nm [25]. he production and characterization of microibrillar

Natural Nanofibers in Polymer Membranes 385

(a)

(b)

(c)

(d)

Figure 10.4 (a) SEM image of sisal MFC. (b) Optical microscopy image of sisal MFC (Reprinted with permission from [34]; Copyright (2009) American Chemical Society). (c) Gel-like appearance of a MFC water suspension (1 wt%). (d) Optical microscopy image of wood MFC. Reprinted with kind permission from Springer Science+Business Media [35].

cellulose (MFC) from wood ibers was irst described by Turbak et al. [31] and Herrick et al. [32]. his procedure, which only exploits high shear, resulted in the production of a highly entangled network consisting of nanoscale size elements with a gel-like behavior for water dispersions at 1% or lower concentrations of MFC (Figure 10.4) [25]. he MFC particles showed extremely interesting characteristics, such as a very high speciic surface area [33] or the ability to form highly porous network [17], and, for this reason, extensive research has been done in the last years regarding their production and properties [25]. Other treatments have also been proposed for the preparation of cellulose nanoibers using grinders [33, 35, 36], cryocrushing [37], ultrasonic disintegrators [38], or enzymatic methods [39]; but all the proposed cases had diiculties and gave completely disintegrated cellulose nanoibers without high disintegration load and damaged the cellulose because of the complex structure of the cell walls and the strong interibrillar hydrogen bonding. As an alternative, Abe et al. isolated cellulose nanoiber bundles with approximately 15 nm width from wood by a method consisting of the isolation of nanoibers and subsequent grinding [40]. his method has been applied for diferent cellulose sources such as rice straw, potato tuber pulp, and parenchymal cells of bamboo and fruits [41, 42].

386 Nanostructured Polmer Membranes: Volume 2 Isogai et al. converted wood cellulose to nanoibers of 3–4 nm width and high aspect ratio by a TEMPO oxidation. Such a process formed carboxylate groups on the cellulose surface, with electrostatic repulsions arising due to anionic charges at the cellulose surface, which caused complete disintegration into nanoibers [43, 44]. Analogous works have also been proposed for the production of chitin nanowhiskers and nanoibrils. Since crab and prawn shells have a hierarchical structure made up of nanoibers, the cellulose nanoiber isolation method has been considered as applicable to several biomasses consisting of chitin. Chitin nanowhiskers are present in biological tissues, according to structural hierarchies, jointly with proteins and inorganic compounds; therefore to obtain the nanocrystals a puriication step has to be carried out in order to remove remaining proteins and minerals that are present in the animal raw material [10]. As explained for cellulose, various methods have also been employed for the preparation of chitin nanowhiskers (nanocrystals) or nanoibers, including acid hydrolysis [45–49], TEMPO-mediated oxidation [50], ultrasonication [51], electrospinning [52], mechanical or chemimechanical treatments [53]. Acid hydrolysis was used to dissolve regions of low lateral order so that the water-insoluble, highly crystalline residue could be converted into a stable colloidal suspension by subsequent strong mechanical shearing action [54]. Chitin nanowhiskers have been prepared from diferent sources, e.g., crab shells [54], prawn shells [55], squid pens [56], shrimp shells [57], and other sources (Figure 10.5).

10.3 Polymer Nanocomposite Membranes Based on Natural Fibers: Production, Properties and General Applications 10.3.1

Production

he extraction of nanocellulose and the introduction of natural ibers in nanocomposite materials have been identiied as two of the four biggest discoveries since 2000, according to the book Nanotechnology Research Directions for Societal Needs in 2020 [58]. However, concerning the preparation of composites, one of the main issues that has to be overcome is the homogeneous dispersion of cellulose nanocrystals within a continuous polymeric matrix. It is known that nanoparticles tend to strongly aggregate because of the hydroxyl groups present on their surface. his behavior can be advantageous when the formation of load-bearing percolating architectures within the host polymer matrix is desired or when the application of the ibers in the paper and paperboard ield is envisaged.

Natural Nanofibers in Polymer Membranes 387 TEMPO-oxidized -chitin nanowhisher: TOChN

Partially deacetylated -chitin nanowhisher/nanoiber mixture: DEChN

100 nm Acid-hydrolyzed -chitin nanowhisker: HHChN

Squid pen -chitin nanoiber: SQChN

Figure 10.5 TEM images of chitin nanoibers. Reprinted with permission from [50]; Copyright (2012) Elsevier.

However, these inter-particle interactions can cause problems during the nanocomposites preparation processes, since the aggregation results in the loss of the nanoscale and limits the potential of mechanical reinforcement. his aggregation phenomenon is increased when the speciic surface area increases, and then when the size of the particle decreases, and is clearly inluenced by the nanocomposite production technique [59]. he mixing strategies can be summarized in two main groups [60]: t
Use of aqueous or organic solvents and their evaporation ater casting; t
Extrusion with freeze-dried cellulose nanoparticles.

388 Nanostructured Polmer Membranes: Volume 2 he irst technique is the one most commonly used for the production of composite membranes, three systems can be distinguished depending on the polymer used as matrix, which are: 1) water-soluble or polar polymers, 2) polymer emulsions, and 3) apolar polymers [61] (Figure 10.6). Concerning the last case, two routes can be envisaged in order to obtain non-locculated dispersions of cellulose nanocrystals in an appropriated organic medium, which are the use of surfactants [62] or the grating of hydrophobic chains at the surface of cellulose nanocrystals [63,64]. hese procedures allow the preparation of polymer nanocomposite membranes by mixing the nanoparticle suspensions in organic medium with a solution of polymer. Furthermore, it was demonstrated that nanocomposites prepared by casting presented higher mechanical properties than nanocomposites of the same mixture prepared by freeze-drying and hot-pressing thanks to the formation of a rigid whisker/whisker network, probably linked by hydrogen bonds. he formation of this network seems to be more predominant in the evaporated ilms due to lower processing times [15].

10.3.2 Properties he high stifness of crystalline cellulose, which provides strength to plants, is completely exploited when nanoparticles with nanoscale dimensions and high aspect ratio are obtained. Such particles can give nanocomposite with

Nanocellulose

Hydrosoluble systems

Simple mixing

Emulsion systems

Emulsion

Non-hydrosoluble systems

Surfactants or grafting

Figure 10.6 Schematic representation of the preparation methods for natural ibers-based polymer membranes [61].

Natural Nanofibers in Polymer Membranes 389 outstanding mechanical properties when embedded in a polymer matrix even at low iller loading. he irst demonstration of the reinforcing efect of cellulose nanocrystals in a membrane prepared from a latex of poly(styrene-co-butil acrylate) was reported by Favier et al. [65]. In this work, an impressive improvement of the storage modulus above the glass transition temperature of the thermoplastic matrix was measured by DMA in the presence of 6 wt% of tunicine whiskers (Figure 10.7). he experimental data were compared with theoretical results obtained by diferent models and showed to be in good agreement with the prediction obtained by the series-parallel model of Takayanagi which was modiied to include a percolation approach. his indicates that the stifness of the nanocomposite materials is due to aggregates of cellulose nanocrystals that, above the percolation threshold, can form a three-dimensional continuous pathway through the nanocomposite ilm. 10

9

8 Log (G /Pa)

6%

3%

7

1% 6

5

4 200

0%

250 300 Temperature (K)

350

Figure 10.7 Plot of log (G’) vs. temperature, between 200 and 350 °K for polymer composites reinforced with 0, 1, 3 and 6% of tunicine whiskers. Reprinted with permission from [65]; Copyright (2003) Wiley.

390 Nanostructured Polmer Membranes: Volume 2 According to this study, the formation of the rigid pathway is governed by a percolation mechanism and thus the most important parameters are the aspect ratio of the rod-like nanoparticles, which is the ruler of the percolation threshold, and the modulus of the percolating nanoparticle network. Basically it can be stated that the main parameters inluencing the mechanical properties of cellulose-based membranes are: 1) the morphology and dimensions of the nanoparticles, 2) the processing method, and 3) the microstructure of the matrix and matrix/iller interactions. Bras et al. [66] reported a study on cellulose whiskers from diferent sources in which the authors investigated the impact of the geometrical characteristics on the mechanical properties of the percolating networks. It was shown that the tensile modulus of pure cellulose whiskers membranes increases when increasing the aspect ratio of the nanoparticles (Figure 10.8), and thus correlating the behavior of a nanocomposite membrane to that of a pure cellulose ilm, it was proved that it is important to choose high aspect ratio whiskers to also have an eicient reinforcing efect in nanocomposites. he literature shows that cellulose whiskers with diferent aspect ratios isolated from diferent sources, like cotton [27] tunicin [65], sisal [24], etc., were proposed as a reinforcing phase for various polymer matrices such as poly(styrene-co-butyl acrylate) [65], PLA [24], PVA [27, 29] and

16

y = 0.0027x2 – 0.0022x + 0.7581 R2 = 0.8111

12 10 E (GPa)

Tunicin

y = 0.2037x – 1.9822 R2 = 0.7738

14

Capim dourado

y = 0.4965e0.0501x R2 = 0.6977

8

Sisal

Palm tree

6

Wheat straw

4 Cotton 2 0

Ramie 0

Luffa cylindrica

Sugar Cane Bagasse Hardwood 20

40 L/D

60

80

Figure 10.8 Evolution of the Young’s modulus of the cellulosic whiskers ilm determined from tensile tests as a function of the aspect ratio of the constituting whiskers. Reprinted with permission from [66]; Copyright (2011) Elsevier.

Natural Nanofibers in Polymer Membranes 391 others. Summing up the results presented, it can be said that the mechanical properties can be substantially improved by controlling the amount of iller and the homogeneity of its dispersion. Strength and modulus of polymers, especially above Tg, are always improved; on the contrary, the presence of whiskers tends to decrease the elongation at break of the nanocomposites compared to the neat matrix. he inluence of MFC on the mechanical properties of composite membranes was also investigated; microibrils from needle-leaf bleached krat pulp [67], sulite pulp [35], sugar beet [68], and other sources were studied as reinforcement in diferent matrices such as PLA [67], thermoset polymers [35], PVA [69], and others. In many cases it was seen that the mechanical properties are inluenced by the disintegration process; in fact, ibrils with higher surface area result in stifer nanocomposites with high tensile strength. As for all nanocomposites, the reinforcing efect of MFC is also dependent on the interactions of the ibers with the matrix. Regarding the membranes processing method, it was shown that slow casting/evaporation processes can give the best mechanically performing membranes since, during water or polar solvent evaporation, because of Brownian motions, the rearrangement of nanoparticles is possible and strong interactions between hydroxyl groups of nanoparticles can settle for the formation of a strong percolating network. Unfortunately, to obtain the best interactions, the processing is limited to a medium in which the cellulose ibers can be easily dispersed. For this reason, diferent routes to broaden the range of possible polymeric matrices from homogeneous dispersion of the nanoparticles in any liquid medium have been investigated. he dispersion in solvents, monomers or polymers is generally promoted by a decrease of the surface energy of the nanoparticles which, as previously stated, can be achieved by coating their surface with a surfactant or by surface chemical modiication. However, even if the dispersion of the nanoparticles in the liquid medium is improved, the presence of grated or absorbed molecules on the cellulose surface hinders the interactions between nanocrystals, limiting the strength of the percolating network and then the reinforcing efect [25, 70]. Beyond the improvement of mechanical properties, other aspects of the inluence of nanocellulose on polymer membranes have been investigated; concerning the thermal behavior, several works did not show any signiicant change in the glass transition temperature (Tg) and melting temperature (Tm) of nanocomposites upon addition of both whiskers [71, 72–74] or MFC [35, 69, 75]. Diverse trends have been observed according to the nature/size of the nanoparticles and to the kind of matrix; diferent explanations were

392 Nanostructured Polmer Membranes: Volume 2 given mainly based on the possible plasticizing efect of absorbed humidity or surfactants in the case of a decrease of the Tg value [27, 76, 77] or to the strong cellulose matrix interactions in the case of an increase of Tg [78]. Similar observations were made for the degree of crystallinity that in certain cases was increased by the presence of nanocellulose which acted as nucleating agent [69, 72, 79], while in other cases it was reduced because of the hindering efect of nanoparticles [80]. Regarding the thermal stability, most of the works showed that the presence of nanocellulose induced no or slight variations on the membranes behavior [35, 72, 74]. Another largely exploited quality of the nanocellulose reinforcements in membranes is their barrier ability that is due to the great tortuosity provided by nanoparticle networks. As discussed by Svagan et al. [81], the porosity in a glycerol nanocomposite ilm due to the presence of microibrillated cellulose caused a reduction in the moisture difusivity of the composite membrane because of three main reasons, which were: 1) the geometrical limitation given by cellulose, 2) the swelling constraints due to high-modulus/hydrogen-bonded MFC network, and 3) strong molecular interactions between iller and matrix. Further works also demonstrated the beneicial efect of nanocellulose on the barrier properties and moisture sensitivity on membranes composed of diferent polymers [82–85].

10.3.3 General Applications Nanocellulose is potentially applicable in a wide range of sectors. General applications are mainly considered in paper and packaging products, but also in the construction, automotive, furniture, electronics, pharmaceutical, and cosmetics industries. Up to now neat nanocellulose membranes were used for several diferent applications ranging from membrane for high quality sound to electronics or biomedicals [3]. he high strength and stifness, as well as the small dimensions of nanocellulose, may well impart useful properties to composite materials reinforced with these ibers, which, subsequently, could be applied in diferent ields. Nanocellulose and nanochitin composite membranes were proposed for the production of optically transparent and mechanically resistant sheets to be used as optically functional materials for biomedical applications [86, 87]. Exploiting the good barrier properties of nanocellulose composites, membranes containing gluten, cellulose nanocrystals, and titanium dioxide nanoparticles were proposed as coating for paper packagings with good antimicrobial activity [88].

Natural Nanofibers in Polymer Membranes 393 Also, MFC-based nanocomposite membranes with diferent matrices have been developed and largely applied for their low oxygen permeability and good barrier properties [25]. herefore, many composite membranes were applied in the biomedical ield for tissue engineering [89], for burns and chronic wound treatments [90], blood puriication [91] and also in veterinary and dental iedls [92, 93].

10.4 Applications of Natural Fibers Nanocomposite Membranes in the Energy Field he constantly increasing production of a large variety of electronic devices, the urgent request for replacement of polluting, internal combustion cars with hybrid or electric vehicles, and the need for storing energy from renewable but intermitting sources, require the development of new reliable and safe power sources and energy devices, and a huge work is in progress to fulill these requirements [94]. But, at present, little attention has been paid to the environmental impact of energy devices. Because of the unrelenting requests from markets, one of the goals to be pursued is the accomplishment of the “12 Principles of Green Chemistry,” by designing environmentally “conscious” materials, products and processes in order to reduce the environmental impact of systems and devices throughout all phases of their life cycle [95]. For this reason the development of natural ibers-based components deserves much attention. Cellulose in the form of ibers has been largely used in energy devices as separator or as structural material for the development of diferent components [96]. While nanocellulose-based nanocomposite membranes have been generally proposed as innovative electrolytes in various energy devices or as conductive paper-like electrodes; in the following section an overview of the present literature on membranes mainly reinforced with cellulose and chitin nanoibers for energy device applications will be given.

10.4.1 Composite Polymer Electrolytes for Lithium Batteries Cellulose ibers were irstly applied at the micro- or nanoscale in traditional Li-batteries as separator with liquid electrolytes. he ibers were simply used to produce nonwoven separators for lithium batteries, as explained in many works [97–101]. Cellulose was then proposed as a component of gel or solid polymer electrolytes in which nanoparticles were used as a reinforcement of the polymer membranes. Balancing mechanical properties and good ionic conductivity is one of the main goals for the development

394 Nanostructured Polmer Membranes: Volume 2 of polymer electrolytes but the two aspects are oten in conlict as they rely on opposite facts, i.e., the improvement of the movements of the polymer chains for higher ion mobility and their restriction for a higher mechanical strength. A clear example is given by the case of PEO, which has always been of the most investigated materials due to its relatively high ionic conductivity. Generally, PEO is crystalline at room temperature and this implies poor RT conductivity, but, when amorphicity is successfully extended, improving the conductivity, mechanical resistance is lost [102]. he use of nanoiller can be one of the easiest strategies to obtain a calibrated balance of the electrolyte properties if these can enhance the mechanical properties without afecting the ionc conductivity [102]. Nanocomposite electrolytes using cellulose whisker as iller have been studied by Azizi Samir et al. since 2004 [103]. PEO-whiskers solid polymer electrolytes were prepared by casting technique from aqueous solution. In this case it has been shown that the incorporation up to 10 wt% of tunicate whiskers, which are high aspect ratio rodlike natural cellulosic microcrystals, leads to a large increase in mechanical strength but does not help the reduction of crystallinity, resulting in a slight decrease of conductivity values (Figure 10.9). In a more detailed work it was later demonstrated that in PEO/whisker nanocomposites the presence of the nanoparticles afects the crystallization process. And that the melting temperature and the degree of crystallinity decrease for contents higher than 10 wt% [74]. In further works the performances of high molecular weight poly(oxyethylene) matrix containing LiTFSI reinforced by cellulose 1,E–02 Conductivity (S.cm–1)

1,E–03 1,E–04 1,E–05 1,E–06 1,E–07 1,E–08 2,5

2,75

3 1000/T (K–1)

3,25

3,5

Figure 10.9 Arrhenius plot of the ionic conductivity for composite polymer electrolytes containing 0 (black dots), 6 (white dots) and 10 wt% (black triangles) of tunicin whiskers. Adapted with permission from [103]; Copyright (2004) American Chemical Society.

Natural Nanofibers in Polymer Membranes 395 nanocrystals extracted from tunicate were studied in the presence or absence of a tetra(ethylene) glycol dimethyl ether (TEGDME) as plasticizer [104, 105]. In this case, a N,N-dimethylformamide (DMF)-based tunicin whisker solution was prepared without the use of surfactants or chemical modiications. DMF was selected because the choice of an organic medium has always been preferred since water, if not completely removed, could react with the negative electrode and could reduce the battery cycle life. he works showed that the iller stabilizes the storage modulus above the melting point of the complexes POE/LiTFSI thanks to the formation of a rigid whiskers percolating network. However, also in this case, even if the ionic conductivity of these membranes was quite consistent with the speciications of lithium batteries (about 10-6 s/cm at room temperature) it was always lower than in the absence of the iller; nevertheless, the authors claimed that the beneit of the electrolyte contribution to the internal resistance of the battery exceeded a factor of 100 since such reinforcement might enable, with the same safety level, to lower the electrolyte thickness by roughly a factor of 100. he same author showed the preparation of nanocomposite materials by UV crosslinking in the presence of a thermally stable photoinitiator, using an unsaturated PEO-based monomer [73, 105]. Crosslinking is one of the most common methods used to disrupt polymer crystallinity and to ensure mechanical properties. It is classically performed to provide both low-temperature conductivity and high-temperature mechanical stability. he crosslinking density of the polymeric network was found to decrease with the whiskers content, most probably due to an interfacial efect, while the presence of tunicin whiskers did not afect the thermal properties of the matrix. By comparing the behavior of weakly crosslinked polyether illed with tunicin whiskers and the one of unilled materials exhibiting diferent crosslinking density, it has been shown that the cellulosic nanoiller provides a much higher reinforcing efect at high temperature than the crosslinking process and that nanocomposite electrolytes display a higher ionic conductivity on the whole temperature range. his was due to the high crosslinking density that should be used for unilled electrolytes in order to ensure suicient mechanical properties. herefore, the used crosslinked nanocomposite polymer electrolytes allowed conciliating both higher ionic conductivities and higher mechanical performances. Lithium perchlorate-doped nanocomposites of ethylene oxide-epichlorohydrin copolymers and cellulose whiskers were proposed by Schroers et al. [106]. he polymer electrolytes were produced by solution casting of tetrahydrofuran/whiskers water mixtures and subsequent compression molding of the resulting nanocomposites. he ilms proposed

396 Nanostructured Polmer Membranes: Volume 2 displayed substantially improved mechanical properties, when compared to the non-reinforced lithium perchlorate/ethylene oxide-epichlorohydrin, with small reductions in conductivities. In 2010, Alloin et al. [107] proposed a detailed study on the behavior of nanocomposite PEO membranes reinforced with whiskers from various origins and sisal microibrils prepared by casting process from aqueous suspensions. he homogeneous dispersion of the nanoparticles was veriied by SEM analysis (Figure 10.10). Nanocomposite electrolytes based on PEO reinforced by whiskers and sisal MFC exhibited very high mechanical performance with a storage modulus of 160 MPa at high temperature. A weak decrease of the ionic conductivity was observed with the incorporation of 6 wt% of whiskers. he addition of microibrils involved a larger decrease of the conductivity, which was associated with the more restricted PEO mobility due to the addition of entangled nanoibers. UV-cured PEO-based polymer membranes reinforced with microibrillated cellulose to be used as gel or solid polymer electrolytes were also presented [75]. Gel polymer membranes were obtained by casting a mixture of methacrylic oligomers and MFC in water suspension in the presence of a photoinitiator. Ater the evaporation of water, the membranes were UV-cured and subsequently swelled in a liquid electrolyte (1M LiTFSI in EC:DEC). he presence of the microibrils (up to 5 wt%) allowed obtainment of membranes with a Young’s modulus of 80 MPa and evidently improved the dimensional stability and handability ater swelling (Figure 10.11, let). he presence of the iller slightly reduced the ionic conductivity, allowing, however, competitive performances for gel polymer electrolytes. Ionic conductivity was shown to be approaching 10−3 S cm−1 at 25 °C (Figure 10.11, right) and good overall electrochemical performances were also shown.

(a)

(b)

Figure 10.10 SEM micrographs of the cryo-fractured surface of PEO nanocomposite membranes containing 6 wt% of cotton whiskers (a) and 6 wt% of sisal MFC (b). Reprinted with permission from Elsevier; Copyright (2010).

Natural Nanofibers in Polymer Membranes 397 Lately, UV-cured MFC-based solid polymer electrolytes were produced following the same water-based procedure but, in this case, the lithium salt was directly added to the casted mixture [35]. he presence of MFC did not reduce the crystallinity of the pendant chains in the cured network and slightly reduced the curing conversion. It was then stated that the unreacted monomers can work as plasticizer to help the ionic movements, indeed in this case, the illed membranes presented ionic conductivity values equal or even slightly higher than the neat one (Figure 10.12, let). Furthermore, the presence of the iller largely increased the mechanical properties of the membranes (Figure 10.12, right). Lalia et al. proposed a nanocrystalline cellulose-reinforced poly(vinylideneluoride-co-hexaluoropropylene) nanocomposite ilm prepared by electrospinning [108,109]. Freeze-dried cellulose whiskers were mixed to PVDF-HFP in the presence of an acetone/DMAc mixture and the solution was then electrospun to form a mat. he membrane was then activated by soaking in a solution of lithium salt in ionic liquid (1M LiTFSI in BMPyrTFSI); the choice of an ionic liquid allows obtainment of a quasi-solid membrane, assuring better electrochemical properties. It has been shown that the addition of 2 wt% NCC in PVdF-HFP can improve the electrolyte retention and storage modulus of the separator. he developed electrolyte demonstrated high value of ionic conductivity viz. 4 × 10−4 S cm−1 at 30 °C with wide electrochemical stability and good cycling performances. Recently, Willgert et al. [110] prepared four diferent composite electrolytes constituted of a PEO-based photocured matrix with a nanocellulose Ionic conductivity, /S cm–1

A MFC–0 MFC–1 MFC–3 MFC–5

0.01

1E–3

2.8

2.9 3.0 3.1 3.2 3.3 3.4 1000/T/K–1

Figure 10.11 Let: Appearance of the UV-cured membrane containing 3 wt% of MFC ater two hours of swelling in the liquid electrolyte. Inset: Membrane without MFC ater swelling. Right: Arrhenius plot of ionic conductivity of the UV-cured membranes containing diferent amounts of MFC. Reprinted with permission from [75]; Copyright (2011) Elsevier.

Young,s modulus (MPa)

SP-MFC0 SP-MFC1 SP-MFC3

0.5

0.1 0.05

0.01 2.8

2.9

3.0 3.1 (1000/T)/K

3.2

3.3

Young’s modulus Tensile rasistance

30

3

Young’s Tensile modulus rasistance s (MPa) (MPa) SP-MFC0 – – SP-MFC1 9±2 0.38±0.05 SP-MFC3 32±4 2.3±0.4

20

2

1

10

0

Tensile rasistance (MPa)

Ionic conductivity, ( )/mS cm–1

398 Nanostructured Polmer Membranes: Volume 2

0 0

1

3 2 MFC content (%)

4

Figure 10.12 Let: Arrhenius plot of the ionic conductivity of the solid polymer electrolytes reinforced with MFC. Right: Improvement of the mechanical properties of the membranes. Reprinted with kind permission from Springer Science+Business Media [35].

paper as reinforcement. he reference samples contained unmodiied nanoibers, while in the other samples the cellulose nanoibers were  chemically modiied at the surface to create covalent bonds between the ibers and the polymer matrix. he presence of acrylic groups which reacted with the polymer matrix caused, ater the swelling in the liquid electrolyte, a reduction in ionic conductivity but a further increase in the mechanical properties, leading to gel composite membranes with an elastic modulus > 100 MPa above 100 °C and an ionic conductivity of around 5 10–5 S cm–1 at 25 °C. As a general conclusion it can be summarized that the use of cellulose whiskers or microibrils as a iller for ionic conductive polymer electrolytes for lithium batteries drastically enhances the mechanical properties of the membranes. his represents an advantage from many points of view since it could mean that thin composite electrolytes could be easily used for lexible devices and, at the same time, it would also imply an enhancement in safety thanks to the presence of more robust separators. Unfortunately, a noticeable improvement of the electrochemical behavior of the electrolytes in the presence of nanocellulose has never been shown, meaning that the latter plays only a structural role. Anyway, many of the previously cited works also demonstrated that the use of cellulose does not inluence the electrochemical stability window of the polymer electrolytes and that composite electrolytes can withstand prolonged cycling in a complete cell. Analogous works have been presented by Stephan’s group using chitin nanowhiskers both in PEO [111–113] and PVDF-HFP [114] matrices. Solid composite electrolytes were prepared by mixing the dried nanoparticles with the polymer and the lithium salt and subsequently hot pressing the mixtures. It was demonstrated that the use of chitin nanoparticles enhances the mechanical properties of the polymer and, furthermore, improves the ionic conductivity and transference number of the electrolyte.

Natural Nanofibers in Polymer Membranes 399 his fact was attributed to an increased salt dissociation due to the formation of ion-chitin complexes by Lewis acid-base interactions between the surface groups of the chitin and salt anions, and to the simultaneous formation of conducting paths for Li ions. he presence of the nanoiller improves the interfacial stability, does not inluence the electrochemical stability window and allows good cycling performances (Figure 10.13).

10.4.2 Composite Electrodes for Supercapacitors Nanocellulose has been proposed as mechanically reinforcing material for the development of composite conductive electrodes that have been mainly applied in supercapacitors. Several syntheses routes are proposed in the literature and many of them give materials in the form of aerogel [115–117] or powders [118]. In 2012 Kang [119] proposed a neat bacterial nanocellulose ilm as a substrate for the coating of conductive materials used as electrodes; while in 2014 Wang [120] prepared a hybrid multilayer thin-ilm electrode with the layer-by-layer technique, starting from a transparent paper of cellulose nanoibers, for lexible and relatively transparent supercapacitor applications. A nanoibrillated cellulose ilm was also used as separator for supercapacitors using liquid electrolyte [121]. In all these cases the nanostructured cellulose ilms, assembled with other materials,

(a)

Interfacial resistance (ohm)

4500 4000

Without iller (PE) With iller (NCPE)

3500 3000 2500 2000 1500 1000 500 0

50

100

150 Time (h)

200

250

(b)

Figure 10.13 Let: SEM images of polymer electrolytes composed of (a) PEO + LiPF6 and (b) PEO + LiPF6 + chitin. Right: Interfacial resistance as a function of time of polymeric membranes. Reprinted with permission from [112]; Copyright (2011) Elsevier.

400 Nanostructured Polmer Membranes: Volume 2 gave the possibility to obtain lexible devices without compromising the electrochemical properties of the proposed devices. Beyond these works, true nanocomposite membranes were proposed as a combination of nanocellulose and conductive polymers. Liew et al. proposed diferent cellulose-based conductive membranes for supercapacitor applications. A irst study was based on electrodeposited cellulose nanocrystal-polypyrrole PPY-NC nanocomposites [122, 123]. Such material showed high capacitance values even when relatively thick membranes were used. he authors also prepared a symmetric supercapacitor based on two PPY-NC nanocomposite electrodes and tested it at a cell voltage of 1 V; stability tests revealed that the supercapacitor had good stability over at least 10000 cycles, and retained about 50% of its original capacitance ater 50000 charge-discharge cycles. More recently, the same authors studied electrodeposited polyanilineand poly(ethylenedioxythiophene)-cellulose nanocomposite electrodes (PANI- and PEDOT-NC) [124]. In each case the incorporation of nanocellulose into the polymer ilm led to the formation of porous polymer/NC nanocomposite structures. he speciic capacitances of the materials were higher than those of the neat polymer ilms thanks to the diferent morphology in the presence of nanocellulose, which facilitated ion and solvent transport throughout the ilm structure. he test performed showed that PEDOT/O-NC ilms (where O-NC are oxidized NC) were capacitive at negative potentials, while pure PEDOT ones were resistive, a phenomenon that was attributed to the negative charge on the immobilized O-NCs. PANI/NC ilms formed at high deposition charges exhibited almost ideal capacitive behavior and were highly responsive to fast charging, while the PANI neat ilm was much more resistive when subjected to the same tests. Recently, Wang et al. also proposed polypyrrole nanocellulose composites using cellulose from Cladophora green algae [125, 126]. Pyrrole was polymerized in the presence of NC and subsequently two types of membranes were made using diferent compression forces. It was shown that the compression treatment removes the macropores in the composite, which are responsible for a large part of the dead weight and dead volume of the electrodes, while the mesopores, required for an excellent electrochemical performance, remain essentially unchanged. he dense “paper” electrodes were shown to be able to retain their porous structure, providing the continuous ion transport network yielding very short charge and discharge times and high areal and volumetric capacity. Lately the same group functionalized the surface of nanowhiskers with carboxylate or quaternary amine groups, resulting in ibers with anionic or cationic surface changes [126]. he modiied cellulose was then used as reinforcement for

Natural Nanofibers in Polymer Membranes 401 the polypyrrole membrane. In this case, the presence of nanocellulose also allowed minimization of the macropore volume of the formed composites, maintaining the volume of the micro- and mesopores at the same level. he symmetric, aqueous electrolyte-based devices comprising these porosity-optimized electrodes exhibited interesting device-speciic volumetric energy and power densities. In 2015 Yang and Li also proposed lexible and foldable composite electrodes from the porous 3D network of cellulose nanoibers, carbon nanotubes (MWCNs) and polyaniline (PANI) [127]. he porous network structure of the nanocellulose-MWCNTs ilms allowed a higher absorption of aniline monomer, which resulted in a higher quantity of conductive polymer (PANI) in the composite structure ater polymerization. Due to the coating with the PANI, MWCNTs connected together more efectively, causing a reduction of the charge transfer with good speciic capacitance.

10.4.3

Other Devices

As happened for the previously described devices, neat nanocellulose ilms were also used as lexible substrate for the development of organic solar cells [128–130]. Concerning the ield of DSSC, in 2014 Chiappone et al. [131] proposed a composite UV-cured membrane containing nanoibrillated cellulose to be used as gel-electrolyte. he addition of cellulose gave mechanically resistant membranes; furthermore, the researchers investigated the efect of the cellulose network on the photovoltaic parameters and performance of the resulting photoelectrochemical cells. Nanoibrillated cellulose resulted in having a positive efect on both the photocurrent by means of optical phenomena and the photovoltage through a shielding efect on the recombination phenomena. In the presence of the highest amount of iller (i.e., 30 wt%), sunlight conversion eiciencies were dramatically increased. he presence of the cellulosic network also positively afected the long-term stability, as the same device demonstrated excellent durability (i.e., > 95% eiciency retention ater 500 h of extreme aging conditions) (see Figure 10.14). Nanocomposite membranes were also proposed for application in fuel cells. he microstructure of perluorosulfonate ionomers (Naion) was modiied by the addition of cellulose whiskers. Hasani-Sadrabadi et al. [132] explained that common Naion membranes for direct methanol fuel cells present some deiciencies and, among them, a high methanol crossover; in order to deal with this, incorporation of nanoparticles and creation of tortuous pathways within polyelectrolyte matrices are emerging approaches but, in order to limit the reduction of the protonic

402 Nanostructured Polmer Membranes: Volume 2 1.1

(a)

(b)

Normalized efficiency

1.0 0.9 0.8 0.7

Liquid cell 0% NMFC 5% NMFC 10% NMFC 20% NMFC 30% NMFC

0.6 0.5 0.4 0

(c)

(d)

100

200

300

400

500

Aaina time/h

Figure 10.14 Let: Optical microscope analysis of the MFC dried network of ibers; (c) appearance of a 30 wt% MFC-based membrane; (d) a 30 wt% MFC-based membrane observed with an optical microscope. Right: Normalized light-to-electricity conversion eiciencies versus conservation time at 60 °C for DSSCs assembled with MFC-based membranes. Curve for the corresponding liquid cell is also reported. Reprinted with permission from [131]; Copyright (2014) Wiley.

conduction, 1D illers are preferred. For this reason cellulose whiskers were tested by the authors, the test performed showed the formation of long-range oriented conduction pathways in the vicinity of 1D cellulosic nanostructures, which resulted in concomitantly improved proton conductivity as well as reduced crossover of liquid fuels for high performance applications. Always trying to improve the membrane performances, Jiang et al. [133] proposed nanocomposite Naion membranes based on bacterial nanocellulose for both direct methanol fuel cells and proton exchange membrane fuel cells. It was shown that composite membranes have excellent mechanical and thermal stability and reduced water uptake plus area and volume swelling ratios compared to neat Naion membranes. Furthermore, ater annealing both membranes presented improved electrochemical performances.

10.5 Conclusions Over the last decade, nanocellulose has been intensively used as structural material for the elaboration of nanocomposite membranes for various applications and, among them, whiskers and microibrils have been largely applied in energy storage devices. Particularly in Li-ion batteries, nanocellulose and nanochitin composite membranes have been successfully used as

Natural Nanofibers in Polymer Membranes 403 solid or gelled electrolytes presenting high mechanical stability. Composite membranes based on cellulose and conductive polymers have been applied as electrodes for supercapacitors and a few other applications of cellulose composite membranes have also been proposed for other energy devices. he present chapter shows that, today, nanocellulose represents a real alternative to other illers for the reinforcement of membranes for energy applications which are expected to lead to a new generation of greener energy devices.

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Natural Nanofibers in Polymer Membranes 411 114. Angulakshmi, N.; homas, S.; Nahm, K. S.; Stephan, A. M.; Elizabeth, R. N., Electrochemical and mechanical properties of nanochitin-incorporated PVDF-HFP-based polymer electrolytes for lithium batteries. Ionics 17(5), 407–414, 2011. 115. Gao, K.; Shao, Z.; Li, J.; Wang, X.; Peng, X.; Wang, W.; Wang, F., Cellulose nanoiber-graphene all solid-state lexible supercapacitors. Journal of Materials Chemistry A 1(1), 63–67, 2013. 116. Gao, K.; Shao, Z.; Wang, X.; Zhang, Y.; Wang, W.; Wang, F., Cellulose nanoibers/multi-walled carbon nanotube nanohybrid aerogel for all-solid-state lexible supercapacitors. RSC Advances 3(35), 15058–15064, 2013. 117. Zhang, X.; Lin, Z.; Chen, B.; Zhang, W.; Sharma, S.; Gu, W.; Deng, Y., Solidstate lexible polyaniline/silver cellulose nanoibrils aerogel supercapacitors. Journal of Power Sources 2014, 246, 283–289. 118. Wu, X.; Chabot, V. L.; Kim, B. K.; Yu, A.; Berry, R. M.; Tam, K. C., Costefective and Scalable Chemical Synthesis of Conductive Cellulose Nanocrystals for High-performance Supercapacitors. Electrochimica Acta 2014, 138, 139–147. 119. Kang, Y. J.; Chun, S.-J.; Lee, S.-S.; Kim, B.-Y.; Kim, J. H.; Chung, H.; Lee, S.-Y.; Kim, W., All-Solid-State Flexible Supercapacitors Fabricated with Bacterial Nanocellulose Papers, Carbon Nanotubes, and Triblock-Copolymer Ion Gels. ACS Nano 6(7), 6400–6406, 2012. 120. Wang, X.; Gao, K.; Shao, Z.; Peng, X.; Wu, X.; Wang, F., Layer-by-Layer assembled hybrid multilayer thin ilm electrodes based on transparent cellulose nanoibers paper for lexible supercapacitors applications. Journal of Power Sources 249, 148–155, 2014. 121. Tuukkanen, S.; Lehtimaki, S.; Jahangir, F.; Eskelinen, A. P.; Lupo, D.; Franssila, S. In Printable and disposable supercapacitor from nanocellulose and carbon nanotubes, Electronics System-Integration Technology Conference (ESTC), 2014, 16–18 Sept. 2014, 1–6, 2014. 122. Liew, S. Y.; hielemans, W.; Walsh, D. A., Electrochemical Capacitance of Nanocomposite Polypyrrole/Cellulose Films. he Journal of Physical Chemistry C 114(41), 17926–17933, 2010. 123. Liew, S. Y.; Walsh, D. A.; hielemans, W., High total-electrode and massspeciic capacitance cellulose nanocrystal-polypyrrole nanocomposites for supercapacitors. RSC Advances 3(24), 9158–9162, 2013. 124. Liew, S.; hielemans, W.; Walsh, D., Polyanilineand poly(ethylenedioxythiophene)-cellulose nanocomposite electrodes for supercapacitors. Journal of Solid State Electrochemistry 18(12), 3307–3315, 2014. 125. Wang, Z.; Tammela, P.; Zhang, P.; Stromme, M.; Nyholm, L., High areal and volumetric capacity sustainable all-polymer paper-based supercapacitors. Journal of Materials Chemistry A 2(39), 16761–16769, 2014. 126. Wang, Z.; Carlsson, D. O.; Tammela, P.; Hua, K.; Zhang, P.; Nyholm, L.; Strømme, M., Surface Modiied Nanocellulose Fibers Yield Conducting

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Polymer-Based Flexible Supercapacitors with Enhanced Capacitances. ACS Nano 9(7), 7563–7571, 2015. Yang, C.; Li, D., Flexible and foldable supercapacitor electrodes from the porous 3D network of cellulose nanoibers, carbon nanotubes and polyaniline. Materials Letters 155, 78–81, 2015. Fang, Z.; Zhu, H.; Yuan, Y.; Ha, D.; Zhu, S.; Preston, C.; Chen, Q.; Li, Y.; Han, X.; Lee, S.; Chen, G.; Li, T.; Munday, J.; Huang, J.; Hu, L., Novel Nanostructured Paper with Ultrahigh Transparency and Ultrahigh Haze for Solar Cells. Nano Letters 14(2), 765–773, 2014. Hu, L.; Zheng, G.; Yao, J.; Liu, N.; Weil, B.; Eskilsson, M.; Karabulut, E.; Ruan, Z.; Fan, S.; Bloking, J. T.; McGehee, M. D.; Wagberg, L.; Cui, Y., Transparent and conductive paper from nanocellulose ibers. Energy & Environmental Science 6(2), 513–518, 2013. Zhou, Y.; Fuentes-Hernandez, C.; Khan, T. M.; Liu, J.-C.; Hsu, J.; Shim, J. W.; Dindar, A.; Youngblood, J. P.; Moon, R. J.; Kippelen, B., Recyclable organic solar cells on cellulose nanocrystal substrates. Scientiic Reports 3, 1536, 2013. Chiappone, A.; Bella, F.; Nair, J. R.; Meligrana, G.; Bongiovanni, R.; Gerbaldi, C., Structure–Performance Correlation of Nanocellulose-Based Polymer Electrolytes for Eicient Quasi-solid DSSCs. ChemElectroChem 1(8), 1350– 1358, 2014. Hasani-Sadrabadi, M. M.; Dashtimoghadam, E.; Nasseri, R.; Karkhaneh, A.; Majedi, F. S.; Mokarram, N.; Renaud, P.; Jacob, K. I., Cellulose nanowhiskers to regulate the microstructure of perluorosulfonate ionomers for high-performance fuel cells. Journal of Materials Chemistry A 2(29), 11334–11340, 2014. Jiang, G.-p.; Zhang, J.; Qiao, J.-l.; Jiang, Y.-m.; Zarrin, H.; Chen, Z.; Hong, F., Bacterial nanocellulose/Naion composite membranes for low temperature polymer electrolyte fuel cells. Journal of Power Sources 273, 697–706, 2015.

11 Potential Interests of Carbon Nanoparticles for Pervaporation Polymeric Membranes Anastasia V. Penkova*1 and Denis Roizard*2 1

St. Petersburg State University, St. Petersburg, Russia Reaction and Process Engineering Laboratory – CNRS, University of Lorraine – ENSIC, Nancy, France

2

Abstract his chapter is devoted to investigations of the inluence of carbons illers, such as pristine and functionalized carbon nanoparticles (e.g., graphene, graphene oxide, carbon nanotubes and fullerene), on the pervaporation transport properties of diferent polymers (PVA, chitosan, polydimethylsiloxane, etc.), whose mechanism transport is known to obey the solution-difusion mechanism. hese carbon particles used as nanoillers in membrane networks can be useful to signiicantly improve pervaporation performance. Moreover, the interest of such composite membranes covers a broad range of separations, going from dehydration, the most tested one, to the separation of organic feed mixtures, which is one of the most diicult separation challenges in pervaporation. It was shown that the use of the functionalized carbon nanoparticles can signiicantly promote the pervaporation separation properties due to enhanced compatibility of the particles with the polymer matrix that increases the available free volume. Keywords: Mixed matrix membranes, molecular separation, pervaporation, nanostructuration, carbon nanoparticles

11.1 Introduction Molecular separation of liquid mixtures is a key step in the majority of industrial processes and there are several conventional technologies that *Corresponding authors: [email protected]; [email protected] Visakh P.M. and Olga Nazarenko (eds.) Nanostructured Polymer Membranes: Volume 2, (413–440) © 2016 Scrivener Publishing LLC

413

414 Nanostructured Polmer Membranes: Volume 2 can be used to properly achieve this goal at reasonable economic cost. We can easily cite settler, liquid-liquid extraction and obviously distillation. All these separation methods rely on liquid-liquid or liquid-vapor thermodynamic equilibrium. When thermodynamic features do not promote the molecular separation, more complex and costly separation schemes must be operated. For instance, with distillation it is the case each time that the liquid and vapor compositions of a given mixture do not have signiicant diferences. For example, this occurs with feeds of close-boiling components (like aromatic-aliphatic mixtures) and with azeotropes (ethanol water or ETBE-EtOH). Apart form the above-mentioned cases, the puriication of temperature-sensitive components is also problematic with conventional atmospheric distillation. In such cases, an alternative method using membranes has been clearly identiied in the 1960s: pervaporation. his method uses membrane, which is chosen to favor the transfer of one component of the feed mixture through the membrane. On the downstream side, the permeate species are recovered as a vapor, which needs to be condensed. Hence, this means that the energy of vaporization needs to be provided to the system. hat is why it is usually of interest to favor the transfer of the minor component. Along with pervaporation dedicated to liquid mixtures, one should also be aware of the vapor permeation method, which can be similarly applied to break azeotrope vapor feeds. Originally, pervaporation used only polymeric dense membranes, the qualiicator “dense” referring to active layers having no continuous micropores between the upstream and the downstream sides. Nowadays, it has been proven that inorganic nanoporous membranes can also induce the same type of liquid molecular separations. In this chapter, only polymeric-based membranes will be considered because the schematization of inorganic membranes, which are quite diicult and expensive to produce, is still under study at the lab scale; whereas polymeric membranes, which are about 10 times less expensive, can be readily produced in a thousand meters range without defects. To summarize, pervaporation of polymeric membranes used for the separation of small molecules has already been known for more than a century at the lab scale; the principle of the phenomena involves a so-call “sorption-difusion” model. he membrane acts as a nonvolatile third component which favors the transfer of one molecule of the feed in connection with the higher ainity for the polymeric network. Currently the challenge is to develop this type of molecular separation with composite membranes on an industrial scale, as is the case for other membrane methods like micro- and ultrailtrations (using porous

CNP for Pervaporation Polymeric Membranes 415 membranes) in the ield of water treatment or like gas permeation (also using dense membranes), which were well established in industry for hydrogen recovery since the 1980s. he discovery of molecular separation with dense membranes was based on the observation that molecules can pass through thin polymeric ilms at diferent rates because their solubility in the membrane and their difusivity through the membrane are diferent; conversely to separation using porous structures, no simple sieving efect can occur. Going from permanent gas to liquid molecule, the relative roles of sorption and difusion phenomena on the overall selectivity strongly vary; whereas difusivity usually controls the membrane selectivity for gases, solubility is mainly responsible for the separation of vapors and liquids, as in pervaporation. Hence the membrane separation is based on non-equilibrium thermodynamics, a constant driving force is maintained between the upstream and downstream sides of the membrane as presented in the scheme in Figure 11.1. Most of the time these conditions are achieved by applying a pressure diference between the two sides of the membrane.

11.2 Principle of Permeation An activity gradient is established across the dense polymer.he mass transfer arises through the ilm with respect to the ainities of the mixture components for the polymer and to their difusion ability. he polymer structures are chosen and tailored to enhance permeability and selectivity. he formalism of Fick’s laws or the Stefan-Maxwell approach can be used to describe the transport.

Downstream desorption

Upstream sorption

Permeate:

Feed

Vapourphase

Mixture

Pressure P2 < P1

Pressure Difusion

P1

Figure 11.1 Simpliied pervaporation mechanism. Due to the fact that the difusion is the limiting step, it is generally recommended to apply pervaporation to extract the minor component of a mixture.

416 Nanostructured Polmer Membranes: Volume 2 According to the molecules to be separated, these methods using thin dense ilms are named: t
Gas permeation [1]; e.g., production of pure N2 from air (Air Liquide-Medal) t
Vapor separation [2]; e.g., recovery of VOC, monomers (MTR, GKSS) t
Pervaporation [3]; e.g., dehydration, azeotrope separations (Sulzer)

11.2.1 Liquid Molecular Separations by Pervaporation he mechanism of mass transfer through the polymeric ilm is basically the same for vapors and liquids. he upstream sorption is the most important phenomena, especially with the biggest molecules that are condensable, whereas the difusion is slow and it limits the mass transfer. Of course, in the case of vapors, there is no change of state. his peculiarity induces a simpler process because no re-heaters are needed between vapor permeation modules, contrary to pervaporation modules. he drawback of pervaporation transfer is the strong plasticizing efect which is linked to the sorption phenomena, because the plasticization can modify the intrinsic polymer properties. Obviously it leads to a much more complex situation where coupling efects between the sorbed species might rapidly modify the mass transfer and its selectivity. he initial development of pervaporation was linked to the problem of EtOH-H2O azeotrope breaking in the 1960s; polyvinyl alcohol membranes were studied (mainly in the US and Europe) and successfully developed by GFT (presently Sulzer Chem Tech) to produce 99.95% of pure EtOH. Nowadays the challenges for pervaporation are separations of fully organic mixtures like alcohol-ether mixtures (application to MTBE, ETBE production as octane enhancers), aromatic-alkane ones (decrease of benzene content in fuels), isomers or thermo- sensitive chemicals.

11.2.2 Composite Membranes: A Strategic Route to Develop Membranes with Outstanding Properties As was explained above, the performance obtained in pervaporation suffers from the fact that the swelling of polymeric network results in the transfer of the other feed component. So, as a rule, the increase of the lux causes the decrease of selectivity. To prevent this phenomenon, in

CNP for Pervaporation Polymeric Membranes 417 the 1990s researchers started to study and prepare inorganic membranes, especially for dehydration applications. Indeed, such membranes can exhibit strong water ainity without any swelling. he selectivity of the best inorganic membranes bypassed the usual selectivity of polymeric membranes by at least one order of magnitude. But unfortunately, the permeances of these membranes were relatively small in relation to the technological challenge linked to the preparation of thin active layer at industrial scale. herefore, to overcome the drawbacks of polymeric and inorganic membrane routes the concept of mixed matrix membranes (MMMs) using various types of inorganic particles (zeolites, clays, carbon particles, metal organic frameworks [MOFs]) was launched all over the world. It was expected that membranes would garner the best properties of the organic and inorganic matrixes. According to the authors, these types of membranes combining inorganic particles embedded in polymeric network are called hybrid membranes, mixed matrix membranes or composite membranes. In this chapter, the more general term of composite membranes that will be used hereater is MMM. It should be noted that among diferent inorganic particles the carbon particles take a special role because of their unique physicochemical characteristics [4, 5]. he discovery of fullerene by Richard Smalley, Robert Curl, and Harold Kroto, who were awarded the 1996 Nobel Prize, and graphene by Andre Geim and Konstantin Novoselov, who won the Nobel Prize in Physics in 2010, opened a new frontier for the development of novel composite materials with improved characteristics.

11.2.3 Industrial Membranes Industrial membranes generally have a complex heterogeneous structure made up of several layers having distinct roles and properties. Schematically, the membranes are made of a very thin nonporous separating layer coated on a nonselective porous membrane that is used for the mechanical strength, as shown in Figure 11.2; fully organic, mixed or inorganic membranes are also available. he membrane selection is directed by the targeted separation as well as by the conditions to be achieved in terms of pressure and temperature. Hence a typical industrial membrane is either a multilayer structure associating two or more materials (sintered inorganic support or microiltration polymeric support + one active thin layer) or an asymmetric structure prepared from a single polymer.

418 Nanostructured Polmer Membranes: Volume 2

10 m

Figure 11.2 Asymmetric structure of an industrial membrane.

11.2.3.1

Composite Polymer Membrane

A typical organic composite membrane is shown in Figure 11.2; the active layer (2–3 μm) is in the upper part of the photo. he support structure is an asymmetric microiltration or ultrailtration support from polymer that is chosen according its mechanical properties and low mass transfer resistance (here polyacrylonitrile).

11.3 Current Requirements for Pervaporation Membranes A key element for a successful membrane process is to properly select the membrane material to get both high selectivity and high permeability. According to the classical deinition, a membrane is a selective barrier that separates two diferent phases, which difer physically and chemically from the membrane nature; at the same time a membrane possesses the properties that allow handling of the processes of mass transfer between separated phases under the inluence of the driving force [6]. Currently, a large number of commercial membranes are produced, which have passed the required series of studies and have found applications in membrane processes, going from pilot installations to full-scale manufacture. However, current industrial conditions demand improved quality of the inal product and of the performance of the already existing processes, and what is most important is to cope with the new challenges of lower energy consumption and lower environmental footprint. In this regard, the development of new membranes is a very urgent task. he choice of the membrane material which is suitable for a speciic task depends on many factors; the main ones are the composition of a

CNP for Pervaporation Polymeric Membranes 419 separated mixture and the conditions of the process involving a membrane. To achieve high eiciency of the membrane process the membrane material must meet a number of requirements and at the same time reveal the highest possible performance: t
Transport properties (permeability and selectivity); t
he mechanical strength, chemical and thermal stability in the speciic process conditions; t
he stability over time in a real process (long lifetime of the membrane); t
Construction of membrane modules; t
Development of a membrane process, as well as its modeling, optimization and integration. he most common commercial membranes are polymeric membranes, due to their low cost, high workability and good mechanical strength. However, polymeric membranes are characterized by a number of drawbacks, such as poor resistance to mechanical contamination, relatively poor chemical stability with solvent and low thermal resistance (< 100 °C), except for some speciic polymers like aromatic polyimides. But as already pointed out in the introduction, the main drawback of polymeric membranes is the insuicient selectivity obtained with high lux due to coupling efects. Typically, polymeric membranes are not able to combine high selectivity and high lux (Figure 11.3). 120 Area of interest for industrial processes

Selectivity = P1/P2

100 Inorganic 80

membranes

60

Ex: perovskite

40 20

Glassy membranes

Polymeric

Ex: matrimid

membranes Ex: PDMS

Super glassy membranes Ex. PTMSP

0 0

100 200 300 P1: permeability of component to be removed through the membrance

400

Figure 11.3 Schematic performance of membranes versus their polymeric or inorganic nature and glassy or rubbery structure.

420 Nanostructured Polmer Membranes: Volume 2 On the other hand, for inorganic membranes some disadvantages of polymeric membranes are absent [7]. Indeed, inorganic particles have excellent thermal and chemical stability, good mechanical resistance to abrasion, and good transport properties. However, inorganic membranes also have some disadvantages such as poor mechanical strength, low reproducibility of the selectivity for large membrane area due to minor defects, and high cost of manufacture. To overcome the disadvantages of the polymeric and inorganic membranes, the preparation of so-called mixed matrix membrane (MMM) have been proposed in which inorganic illers are dispersed in the polymer matrix [8]. MMM is supposed to be able to combine the advantages of the operating and transport properties of both types of membranes [5, 9–14]. As a rule, as inorganic additives usually zeolites [15, 16], carbon nanoparticles, silicate [17, 18], metals [19], and metal oxides [20, 21] are used for the organometallic structure [22]. Among the new inorganic modiiers, a special place is occupied by carbon nanoparticles, due to their unique physical and chemical characteristics. he description of carbon nanoparticles that are used for membrane materials preparation is given below.

11.4 Performances of Nanocomposite Membranes: From Membrane Preparations to Enhanced Properties with Carbon Nanoparticles If the concept of mixed matrix membranes is very clear, the way to produce them is a long and tortuous one, whatever the used particles. he main problems to be solved are very well known but not yet solved despite the thousands of papers which have been published since 1990. he main bottlenecks are the following: t
For nanoparticles, the size distribution must be small, their aggregation should be reduced, their activation, if needed, should be possible at a temperature suitable for the polymer stability; t
For polymers, the procedure to prepare the active layer should favor an easy dispersion of nanoparticles but also avoid settling of the particles; t
And last but not least, the compatibility of the polymeric matrix with the inorganic illers has to be achieved to some extent to avoid a quick and irreversible phase separation.

CNP for Pervaporation Polymeric Membranes 421 In the reviewed papers discussed below, the main methods of introduction of nanoparticles into polymers are: 1. Direct mixing of the polymer and nanoparticles in a discrete phase (single particles) or in solution. 2. In-situ polymerization in the presence of nanoparticles. he most common and the easiest method is the irst way to get MMM [23–29]. he choice of the polymer matrix for the preparation of MMM is a suiciently diicult task. he polymer must have good transport properties, processability in the product (membrane), and mechanical strength [30]. As the polymer matrix for MMM preparation, the glassy polymers are frequently used: cellulose acetate, polysulfone, polyimide, polyamide, polyethersulfone, polypropylene, and polyvinylidene luoride [31, 32]. In some cases the MMM with improved structure is prepared by using the matrix block copolymers [33]: polystyrene-butadiene-styrene, [34] co-polyetherurethane [35], and Pebax [36, 37]. Rubbery polymers for MMM are rarely used because their transport characteristics are worse. In the Robson diagram (the dependence of selectivity on permeability) [38] their indicators are located below the data for glassy polymers. he most commonly used rubber is PDMS. To provide better adhesion, the polymer and iller are mixed prior to the crosslinking process, if the crosslinking process for polymer is required [39]. Transport properties of MMM are strongly linked to the good adhesion between the polymeric matrix and the embedded iller. Many models have been published to describe and explain pervaporation results; the most popular are based on Fick’s laws [40], on Stefan-Maxwell theory [41], and on Maxwell electric conduction analogy [42]. To summarize, one can distinguish between either the modeling or predicting approaches of the membrane performances. In the irst case, it uses the background of irreversible thermodynamics, like the solutiondifusion model, which requires the knowledge of the sorption and difusion coeicients; in the second case, it uses the predictive Maxwell’s model type, which requires the permeability of each pure matrix [43]. For each approach, the accuracy is limited by the extent of the binary interaction polymer-penetrants that can induce strong coupling transport of the penetrants, which are diicult to quantify. he predictive models use the analogy with electric and thermal conductions in composite structures. Several predictive models were irst developed to calculate the gas permeability of composite membranes [44]. hey are now also applied for the prediction of pervaporation performances [45, 46].

422 Nanostructured Polmer Membranes: Volume 2 It is worth noting that these permeability-based models can be easily used because the related equations are quite simple and self-evident:

Pcomposite / Ppolymer

2(1 (2

) (1 2 ) dm ) (1 ) dm

(11.1)

where Ppolymer is the permeability of the polymer matrix, φ is the volume fraction occupied by the iller, and λdm is the permeability ratio Piller/Ppolymer. Some equations can take into account the shape of the particles as well as the created voids. he limiting feature can be the determination of the permeability of the iller and the knowledge of the exact role of the iller on the polymeric matrix. However, Gonzo et al. have shown for some cases that the pervaporation literature data can be well modeled with this approach [44].

11.5 Impact of the Insertion of Carbon Particles in Pervaporation Membranes In this chapter, carbon modiications for MMM will be used and described in this section as fullerene, carbon nanotubes, graphene and graphene oxide.

11.5.1 Fullerene Fullerenes are an allotropic form of carbon which were irst obtained in macroscopic quantities in 1990 by thermal evaporation of graphite in an electric arc [47]. he general formula of fullerene is Cn, where n = 20, 24, 28, 32, 36, 50, 60, 70, 74, 76, 84, 164, 192, 216, etc. he most common fullerene is fullerene C60 because of its good symmetry and stability [48]. Fullerene C60 is the most accessible for practical use. Under introduction of C60 additives to polymers and their interactions, the π-electron system of the fullerene molecule undergoes minor changes. On being included in the polymer matrix, the fullerene retains its unique properties but at the same time, the properties of the polymer are changed [49–51].

11.5.2

Carbon Nanotubes

he appearance of nanotubes is also associated with the discovery of fullerenes [52]. Carbon nanotubes (CNTs) are essentially graphite cavity,

CNP for Pervaporation Polymeric Membranes 423 rolled into a cylinder. CNTs can be multilayer, which consist of two or more concentric cylindrical shells arranged coaxially around the central hole with hollow interlayer separation of 0.34 nm, as in graphite. Singlewalled nanotubes consist of a single layer of graphite, having cylinders of a very narrow size distribution of 1–2 nm. Oten single-walled nanotubes are packed into larger structures: ropes. he structure and properties of CNTs are markedly diferent from those of conventional carbon ibers which were used in the industry in recent decades to harden tennis rackets, lying aids and batteries. CNTs have ideal iber structure by ordering carbon atoms.

11.5.3 Graphene Oxide Graphene is a two-dimensional allotropic form of carbon, formed by a layer of carbon just one atom thick, in a state of sp²-hybridization and connected by σ- and π-bonds in the two-dimensional hexagonal crystal lattice. Graphene exhibits unusual carbon structure characteristics such as large theoretical speciic surface area (2630 m2 g−1)) [53], high internal mobility (200,000 cm2 V−1 s−1) [54], a high Young’s modulus (~1.0 TPa ) [55], thermal conductivity [56], optical transmittance (~97.7%) [57], and good electronic conductivity [58]. Graphene can be prepared by various methods, including chemical vapor deposition [59, 88], micromechanical cleavage of graphite molecule [60], detachment of the graphite [61], and restoration of graphene oxide. It should be noted that graphene oxide (GO) is used for the polymer’s modiication because GO has better dispersion in the polymeric matrix. Graphene oxide may be prepared by oxidation of the graphite, and depending on the method of oxidation, the various structures of GO can be received as described by various models. A detailed description of GO, including structure illustrations, models and properties of particles, is presented in the review by Dreyer et al. [62].

11.6 Pervaporation Membranes Diferent allotropic modiications of carbon nanoparticle possess diferent physicochemical properties, solubility and dispersion. Among the presented particles (fullerene, carbon nanotubes, graphene oxide, fullerenol) carbon nanotubes (CNTs) are the particles that are very diicult to introduce into the polymer matrixes without additional functionalization because CNTs are not soluble in solvents and have, as a rule, bad dispersion

424 Nanostructured Polmer Membranes: Volume 2 at high content in polymers. he pervaporation results for MMM with functionalized carbon nanotubes will be discussed in Section 11.4. From the practical and fundamental points of view it is very important to study the introduction of parent carbon particles rather than functionalized particles because the preparation of such composites is time-saving and ecological, and allows studying the inluence of the fundamental nature of carbon particle on diferent polymer matrixes. herefore, the pervaporation results are described in two parts that are devoted to the MMM with pristine and functionalized carbon particles.

11.7 Pervaporation with the Use of MMM Containing Pristine Carbon Particles he transport properties of the MMM based on poly(phenyleneisophtalamide) (PA) modiied by fullerene and carbon nanotubes were investigated by Penkova et al. in works [27] and [63], correspondingly. In the work [27], PA membrane containing up to 10 wt% C60 was applied for the separation of methanol-cyclohexane mixture. It was shown that with an increase of fullerene concentration in the polymer the separation factor and permeability increase for membrane containing up to 5% fullerene; a further increase of fullerene concentration (up to 10%) causes a decrease of separation factor because the fullerene aggregates inside of the polymer ilms and causes the inhomogeneity of the structure. In the work [63], CNT (up to 5%) was added to PA and used for the separation of methanol-methyl tert-butyl ether (MTBE) mixture. his mixture is interesting for the separation because MTBE is an additive that increases the octane number of gasoline and methanol that appears during MTBE synthesis and it has to be removed from the system. his separation is diicult to carry out by traditional separation methods because this system is characterized by an azeotropic point of 14.3 wt% methanol and 85.7 wt% MTBE at 20 °C, 760 mm Hg. Pervaporation can be a good tool for this separation to break azeotrope. It was shown that the best transport properties (selectivity and permeability) were received for membrane with 2% CNT as compared to pristine PA membrane, while with increase of CNT concentration up to 5%, separation factor of this membrane was lower than for PA membranes. his result occurs due to the aggregates of CNT in the membrane that cause the membrane’s defectiveness. It is necessary to mention that the previously reported methanol-cyclohexane mixture is the model mixture for methanol-methyl tert-butyl ether separation because components of these feed mixtures have similar physicochemical

CNP for Pervaporation Polymeric Membranes 425 parameters. herefore, the comparison of pervaporation results for two different carbon particles can be done. Based on the described results it can be concluded that in the case of fullerene the bigger amount of nanoparticle can be added to the polymer matrix to improve the transport parameters (5%) as compared with CNT (2%). It can be explained by better solubility of fullerene in dimethylacetamide and better dispersion in PA membrane. he introduction of pristine fullerene C60 (up to 2%) to polyphenylene oxide was studied in the work of Polotskaya et al. [24]. he transport properties were studied during the separation of binary ethanol-water and ethyl acetate-water mixtures and quaternary system modeling the reaction mixture of esteriication process of ethyl acetate. Spectroscopic methods demonstrated the presence of complex formation in the PPO-C60 system. It was found that the increase of fullerene content up to 2% promotes the increase of the separation factor and lux. he obtained results can be due to good dispersion of the C60 in polymer and the hydrophobic nature of this carbon particle. It is necessary to mention that under separation of water-ethanol mixtures an unusual efect was observed, as the hydrophobic polyphenylene oxide membrane was selective to water. his fact could be explained by membrane swelling in ethanol that produces additional transport channels for small water molecules due to hydrogen bonds. In the work of Yen et al. [64], poly(ether-block-amide) membranes with dispersed CNT particles were studied. In pervaporation of acetone-butanol-ethanol-acetic acid-butyric acid mixture it was shown that with the rise of CNT concentration in the membrane up to 5 wt%, the lux enriched by butanol 1.6-fold increases but further increase of CNT content (10 wt% CNT) does not change transport parameters. In the work of Xue et al. [65], ispolydimethylsiloxane (PDMS) was another polymer which was used for modiication by CNT for the purpose of membrane eiciency improvement for butanol removal in ABE fermentation. he maximal loading concentration of CNT was found to be 10% because with the further increase (12%) membrane cure failed due to the decreased viscosity of the PDMS solution. he hydrophobic CNTs, as well as their interaction with hydrophobic PDMS polymer, provide the high butanol permeance. CNT illers had good compatibility with the PDMS polymer matrix and were uniformly dispersed in PDMS. CNTs acted as active sorption sites that interacted with the feed components and provided an alternative route for mass transport via difusion through the inner channel tubes or along their smooth and hydrophobic surface, allowing the components to permeate through the membrane more easily. Separation factor and total lux of PDMS/CNT (10%) was improved by 95% and 30%, correspondingly. It should be noted that the improvement of mechanical properties for modiied CNT

426 Nanostructured Polmer Membranes: Volume 2 membranes was found as compared to membrane based on neat polymer. Based on the obtained results for these two polymers it can be concluded that not only the nature of carbon nanoparticles can have a big inluence on membrane transport parameters, but the type of polymer as well, in spite of their hydrophobic nature. Polymer membranes containing the carbon particle graphene oxide (GO) have also attracted our attention. he works on this direction are new; the publications on this type of composite have appeared in the literature just in the last four years. In the work of Cao et al. [66], sodium alginate was modiied by pristine graphene oxide (0.4 ÷ 2.4 wt%) and reduced graphene oxide (0.4 ÷ 2.0 wt%) nanosheets and used as selective layers of the membrane that was cast on the polyacrylonitrile support. he obtained MMM was study for the separation of water–ethanol mixture (10:90 wt%). It was shown that the separation factor increases signiicantly for membrane containing 2 wt% GO because of the increased hydrophilicity that allows highly selective channels for water permeability to be built. Further increase of GO content causes the decrease of the separation factor, which can be due to agglomeration of GO nanosheets. It is necessary to mention that the lux of modiied membranes was not signiicantly diferent, which can be explained by the membranes’ crystallinity. he addition of reduced GO nanoparticles in sodium alginate also helped to improve the transport properties. Optimal concentration of the used particles was found as 2% GO and 1.6% reduced GO. In the hybrid membranes, due to the GO structural defects, nanosheet edge-to-edge slits, and interfacial free volume cavities, water channels were constructed. he oxygen-containing groups, structural defects, edge-to-edge slits, and non-oxide regions of GO nanosheet send owed water channels with high selectivity and transport rate toward water molecules. Smaller nanosheet size, more structural defects, and less oxygen-containing groups of GO could construct more water channels, while less negative charges and more non-oxide regions further improved the permselectivity of water channels. he research of Wang et al. [67] was devoted to the improvement of the stability of the membrane; a “pore-illing” membrane was prepared by dynamic pressure-driven assembly of a poly(vinylalcohol)–grapheneoxide (0.05 ÷ 0.4 g/l) nanohybrid layer onto an asymmetric polyacrylonitrile ultrailtration membrane. he prepared membrane was used for toluenen-heptane (50:50 wt%) separation. It was shown that with the rise of GO content up to 0.1 g/l the separation factor increases while lux decreases. he authors explained the decrease of the lux by adsorption of toluene by GO that decreases the difusion. Moreover, nonporous GO particles are a barrier to the difusion of the solution components.

CNP for Pervaporation Polymeric Membranes 427 he work by Choudhari et al. [68] was devoted to modiication of polyether block amides by diferent 2D nanostructured materials, such as graphene and graphene oxide, and application of prepared membranes for the pervaporation separation of butyric acid from anaerobic digestion solution. It was shown that for this polymer the best particle for modiication was graphene (0.75 wt%), as this membrane showed the best performance with butyric acid lux of 24 g/m2h and separation factor of 21 at 70 °C. he data of Table 11.1 clearly demonstrate the diference in the use of various carbon particles for polymer modiications. he most signiicant change was observed for poly(phenyleneisophtalamide) membrane modiied by fullerene under separation of 10% methanol–90% cyclohexane [27] and for sodium alginate modiied by reduced graphene oxide and pristine graphene oxide during separation of 10% water–90% ethanol [66]. Due to hydrophobic interaction between the aromatic ring of the PA and fullerene, the polar groups of the polymer were available for the sorption of methanol molecule that led to the increase of membrane performance; the increase of the selectivity was caused by the decrease of the membrane free volume due to fullerene interaction with polymer chains. In the case of introduction of GO to sodium alginate the improvement of the transport parameters was caused by constructed water channels because of GO structural defects, nanosheet edge-to-edge slits, interfacial free volume cavities and membrane crystallinity. he insigniicant pervaporation properties for CNT-containing membranes can be caused by bad dispersion of nanoparticles without additional functionalization and are due to the chosen mixture for the separation; in some cases, under separation of water-containing mixtures, small water molecules could penetrate through the inner space of CNT and increase membrane performance. Nevertheless, we can observe that a small amount of carbon additives to the membranes (up to 5% [for PDMS 10%]) leads to the improvement of selectivity and permeability, providing big potential to use pristine carbon nanoparticles for MMM (except for [67]).

11.8 Pervaporation with the Use of MMM Containing Functionalized Carbon Particles As was already mentioned, the functionalization of the carbon nanoparticles allows increasing the nanoparticles solubility and improving the particle dispersion inside of the polymer matrix (Table 11.2).

40

10% water–90% ethanol;

76

40

50% toluene–50% n-heptane

37

ABE fermentation: acetone-butanolethanol-acetic acid-butyric acid ABE fermentation: acetone-butanol-ethanol 10% water–90% ethanol

37

50

14.5% methanol–85.5% MTBE

1.83% ethyl acetate-water

T, °C 50

Separation mixture 10% methanol–90% cyclohexane

Separation Flux, factor G/M2h Reference 205 690 27 634 847 78 480 63 112 617 13 11 24 19 19 442 208 464 242 17 85 64 19 153 ≈8 ≈ 72 65 17 93.7 ≈ 510 ≈ 1400 66 1566 1699 ≈ 1590 ≈ 1580 4 42 67 13 27 – – 68 21 24

Anaerobic digestion solution from grass with acid content (g/L): 0.52 70 acetic acid, 0.23 propionic acid, 5.95 butyric and 0.60 valeric acid NB: the irst component of the given separation mixture is the most permeable one through the membrane

Membrane (selective layer/ Carbon particle, support) Wt% poly(phenyleneisophtalamide) 0% С60 5% С60 poly(phenyleneisophtalamide) 0% CNT 2% CNT polyphenyleneoxide 0% С60 2% С60 0% С60 2% С60 poly(ether-block-amide) 0% CNT 5% CNT Polydimethylsiloxane 0% CNT 10% CNT Sodiumalginate/ 0% GO polyacrylonitrile 1.6% reduced GO 2% GO PVA/hydrolyzed 0% GO polyacrylonitrile 0.2% GO Polyetherblockamides 0% 0.75% graphene

Table 11.1 Transport properties of MMM containing neat carbon particles.

428 Nanostructured Polmer Membranes: Volume 2

Functionalization/ modiication PVA, glutaraldehyde

poly(allylamine hydrochloride) Poly(sodium 4-styrenesulfonate)

Н2SO4, HNO3, diisobutyrylperoxide HNO3

Н2SO4/HNO3

– b-cyclodextrin chitosan

0% fullerenol maleic acid 5% fullerenol

CNT, wt% 0% GO 4% GO

0% CNT 6% CNT PVA 0% CNT 2% CNT PVA 0% CNT 3% CNT chitosan/ 0% CNT polyacrylonitrile 2% CNT PVA 0% CNT 2% CNT PVA/polyacrylonitrile 0% CNT 1% CNT PVA/polyacrylonitrile 0% CNT 3% CNT

PVA

Membrane (selective layer/support) Poly(ethyleneimine)/ hydrolyzedpoly acrylonitrile PVA

50% benzene–50% cyclohexane 50% benzene–50% cyclohexane 10% water–90% ethanol 10% water–90% ethanol10% water– 90% isopropanol 10% water– 90% isopropanol 10% water–90% isopropanol

4% water–96% ethanol

Separation mixture 5% water–95% ethanol

Table 11.2 Transport properties of MMM containing functionalized carbon particles.

30

30

30

–

40

50

50

25

T, °C 50

140 ≈ 24 42 20 66 ≈ 30 ≈ 67 112 337 ≈ 150 ≈ 80 229 207 229 168

2140 ≈ 17 36 9.6 53 ≈ 800 ≈ 710 585 571 119 1794 141 948 141 882

77

76

75

78

74

73

72

70

(Continued)

36

2140

Separation Flux, factor g/m2h Reference 212 229 69 394 268

CNP for Pervaporation Polymeric Membranes 429

0% CNT 2% CNT

CNT, wt% 0% CNT 2% CNT CNT

Н2SO4, HNO3

Н2SO4, H2O2, PVA

Functionalization/ modiication chitosan

1% water–99% ethylene glycol

Separation mixture 10% water–90% isopropanol 5% water–95% acetone70

30

T, °C 30

NB: the irst component of the given separation mixture is the most permeable one through the membrane

Chitosan/ poly(vinylidene luoride) polyvinylaminepoly(vinylalcohol)

Membrane (selective layer/support) Sodiumalginate

Table 11.2 Cont

450 1156

≈ 146 146

81

Separation Flux, factor g/m2h Reference 185 122 79 6420 218 1450 89 80

430 Nanostructured Polmer Membranes: Volume 2

CNP for Pervaporation Polymeric Membranes 431 Inclusion of graphene oxide (GO), as a rule, is carried out without additional treatment as this particle properly changes the hydrophilicity of the system by itself. In some cases additional functionalization can be done. Wang et al. [69] studied the complex process of obtaining a composite membrane with a selective layer from polyelectrolyte complex of polyacrylic acid and polyethylene imine containing GO additives that was cast on a polyacrylonitrile substrate and then placed in the PVA solution with further crosslinking by glutaraldehyde. hese membranes were used for the pervaporation of water-ethanol mixture (95/5 wt%). It was shown that membranes with 4 wt% GO possess better separation factor and lux, which were increased by 17% and 85% respectively. It is well known that fullerene, among all other carbon particles, can be dissolved in various solvents; however, the solubility is still very low. he use of fullerene derivatives helps to improve solubility. In the works of Penkova et al. [70, 71], the MMM based on composite of polyvinyl alcohol with fullerenol (polyhydroxy fullerene) C60(OH)22–24 and C60(OH)12 crosslinked with maleic acid were developed. he role of fullerenol was as a nano-additive and as a crosslinking agent for PVA. Several types of PVA crosslinking were applied: physical crosslinking (membrane thermal treatment at 140 °C for 100 min) and chemical crosslinking (introduction of 35 wt% maleic acid to polymer solution and heating at 110 °C during 120 min). he dehydration of binary (ethanol-water) and multicomponent (n-propanol–n-propyl acetate–water; acetic acid–n-propanol–n-propyl acetate–water) systems was carried out with the prepared membranes. It was shown that for chemically crosslinked membranes with fullerenol a slight decrease in separation factor and a signiicant lux increase was noted as compared with the unmodiied PVA membrane; so, during the separation of water-alcohol mixtures by chemically crosslinked PVA-C60(OH)22–24 membrane the lux increases in 2.6 times as compared with the unmodiied PVA membrane [70], while during the study of hybrid processes for the synthesis of n-propyl acetate by esteriication with pervaporation by using chemically crosslinked PVA-C60(OH)12 membranes [71] the lux was improved in 2.6 times and in 1.7 times during the separation of heteroazeotrope of 73 wt% n-propyl acetate, 10 wt% n-propanol and 17 wt% water. All membranes were highly selective to water. It is necessary to mention that prepared PVA-fullerenol composite is prepared by green method, as components of its synthesis (PVA, fullerenol, maleic acid) are soluble in water. Among hydrophilic polymers, PVA is the most used polymer because of its low cost, good ilm-forming properties and physicochemical characteristics. In the works presented in [72–77] PVA was modiied by functionalized carbon nanotubes.

432 Nanostructured Polmer Membranes: Volume 2 In the works of Peng et al. [72,73], PVA membranes were modiied by functionalized carbon nanotubes for pervaporation of benzene-cyclohexane mixture. In their work [72], carbon nanotube was dispersed by using b-cyclodextrin, while in their other work [73], membranes were modiied by chitosan-wrapped carbon nanotube. he obtained results demonstrated that under inclusion of chitosan-wrapped carbon nanotube a higher improvement of transport properties was noticed and a smaller amount of CNT was needed for the increase of membrane performance (2 wt% [CNT-chitosan]), as compared with 6 wt% CNT dispersed by b-cyclodextrin. Other studies were devoted to membranes based on PVA [74] and chitosan [78] modiied by CNT that were preliminarily treated with sulfuric and nitricacids, that led to the functionalization of the surface by -OH and -COOH groups and were used for the separation of water-ethanol mixture (10:90 wt%). In the work of Qiu et al. [78], the CNT ater acid treatment was additionally functionalized by diisobutyryl peroxide. he permeability of both polymers was improved ater introduction of functionalized CNT. he authors explained it by ethanol and water transport through the internal space of CNTs without signiicant resistance. In other works [75–77,79] the investigation of the separation of 10% water–90% isopropanol mixture was done. In some studies PVA was chosen as the polymer matrix [75–77]. CNT was treated by nitric acid [75], polyelectrolytes poly(allylaminehydrochloride) (PAH) and poly(sodium 4-styrenesulfonate) [76,77]. It was shown that the modiication of PVA by functionalized CNT causes the signiicant increase of the separation factor and the fall of the lux. he PVA membranes modiied by CNT treated by HNO3 possess the highest selectivity, while the electrolytes-treated CNT possess the highest lux. he authors explain the low lux results by the decrease of free volume at low concentrations of modiier in the membrane. he best properties for the pervaporation of 10% water–90% isopropanol mixture were found for membrane based on sodium alginate modiied up to 2 wt% CNTs functionalized by chitosan [79]. For modiied membranes the increase of both transport parameters was noticed. he authors explained the rise of the lux by creation of additional nanochannels in membrane matrix at the packaging of hydrophilic chains in the presence of CNTs and by transport through the inner cavity of CNT. he increase of separation factor was explained by the increase in the hydrophilicity of the membrane because of the strong internal interaction between groups of sodium alginate and chitosan-CNTs, resulting in increased interaction between the membrane and the water which is a component of the feed mixture. In the work of Yeang et al. [80], a more complex method for modifying the nanoparticles was used. PVA-functionalized MWCNT was bulk

CNP for Pervaporation Polymeric Membranes 433 aligned on the poly(vinylidene luoride) (PVDF) membrane by a simple iltration method and further coated with CS to form a novel three-layer nanocomposite membrane. his membrane showed improvement on the performance and selectivity. Composite membranes consisting of a separating layer of polyvinylaminepoly(vinylalcohol) incorporated with functionalized CNTs supported on a microporous polysulfone substrate were fabricated for the dehydration of ethylene glycol by pervaporation [81]. Prior functionalization of CNTs was carried out in a mixture of nitric and sulfuric acids. In the pervaporation of ethylene glycol-water mixture containing 0.5–20 wt% water, the best properties were obtained with 2 wt% CNT, leading to a considerable increase of the separation factor (by 2.6 times) at almost constant lux. he data of Table 11.2 demonstrate the signiicant diference of MMM with functionalized carbon particles from MMM modiied by parent carbon particles (Table 11.1). he improvement of lux, selectivity or both these parameters depends on the functionalization. he use of GO does not lead to the signiicant improvement of transport characteristics but introduction of GO to the polymer clearly induces the increase of selectivity [69]. he addition of fullerenol to PVA signiicantly improves the lux at similar selectivity as compared to unmodiied PVA membrane [70]. In the case of addition of CNT to diferent polymers, various results can be observed that are connected with diferent CNT treatments and cylindrical CNT structure that is essentially diferent from 2-dimensional carbon particles such as graphene and graphene oxide. Addition of CNT to polymers can cause the selectivity increase while lux decrease [75–77], the lux increase while selectivity decrease [74,78], and improvement of both transport parameters [72,73,79,81] as compared with pristine polymers. he most signiicant improvement of selectivity and lux was observed in the work of Sajjan et al. [79] for membrane based on sodium alginate modiied up to 2 wt% CNTs functionalized by chitosan. As was already mentioned above, the increase of the lux can occur due to creation of additional nanochannels in the membrane matrix at the packaging of hydrophilic chains in the presence of CNTs and by transport through the inner cavity of CNT. he increase of separation factor was explained by the increase in the hydrophilicity of the membrane because of the strong internal interaction between groups of sodium alginate and chitosan-CNTs, resulting in increased interaction between the membrane and the water which is a component of the feed mixture. It is necessary to mention that in the case of MMM with functionalized carbon particles the optimal amount of carbon particles that cause the improvement of transport parameters is less (maximum 6%) than for MMM with pristine carbon particles (10%).

434 Nanostructured Polmer Membranes: Volume 2

11.9 Conclusion his chapter has focused on the modiication of polymeric membranes by parent graphene, graphene oxide, and fullerene to investigate the potential improvements of membrane separation properties in pervaporation and to try, as much as possible, to understand the origin of transport characteristics enhancements, to be suppressed. Hence this work has shown that numerous groups are now using nanostructures of carbon particles, either pristine or modiied ones, to create new composite membranes. Even if some particular results are worth conirming, the general trend is that these carbon particles used as illers in membranes can really be useful to signiicantly improve pervaporation performance. Moreover, the interest in such composite membranes covers a broad range of separations, going from dehydration, the most tested one, to the separation of organic feed mixtures, which is one of the most diicult separation challenges in pervaporation. As far as we can compare the results of pristine and functionalized carbon nanoparticles, it is striking that “activated” particles give rise to much better enhancements, both for hydrophilic and hydrophobic feed mixtures. Even if some good results were reported with simple dispersion of carbon particles (Table 11.1), the functionalization ways are much more eicient (Table 11.2). Among the corresponding papers in this second category, we can identify two distinct routes used to valorize the carbon particles, i.e., either the physical modiication of the outer surface of the nanoparticles or real chemical functionalization. However, for these two categories, the enhanced compatibility of the particles with the polymer matrix is certainly the main reason for the observed improvements. Among the four types of carbon particles, functionalized CNTs appear to be most the promising one to get mixed matrix membranes with higher separation properties while keeping the lux at least constant. Of course, CNTs are quite speciic nanoparticles because one of their dimensions is rather big compared to the other two. One can suspect this structural particularity to be at the origin of the higher performances obtained with CNTs.   Finally, this review paper also underlines another interesting trend. Contrary to many examples reported in the literature, where there is a minimum of weight amount to be added to the polymer matrix to get clear performance improvement, it seems that with the above carbon particles even a small weight fraction is suicient. Precisely, it is the volume fraction which should be considered; nevertheless, it seems to indicate that one of the modes of action of the carbon particles is to modify the intrinsic properties of the polymeric matrix and not only bring its own properties.

CNP for Pervaporation Polymeric Membranes 435

Acknowledgment A.V. Penkova would like to thank the Fellowship of the President of Russia СП-1153.2015.1 and grant of RFBR No. 15-58-04034. he authors gratefully acknowledge the inancial support from Région Lorraine (ARCUS 3).

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436 Nanostructured Polmer Membranes: Volume 2 16. M. Jaymand, Conductive polymers/zeolite (nano-)composites: Underexploited materials, RSC Adv. 4, 33935–33954, 2014. 17. A.K. Zulhairun, A.F. Ismail, he role of layered silicate loadings and their dispersion states on the gas separation performance of mixed matrix membrane, J. Memb. Sci. 468, 20–30, 2014. 18. A.K. Mishra, S. Bose, T. Kuila, N.H. Kim, J.H. Lee, Silicate-based polymernanocomposite membranes for polymer electrolyte membrane fuel cells, Prog. Polym. Sci. 37, 842–869, 2012. 19. E. Antolini, Iridium as catalyst and cocatalyst for oxygen evolution/reduction in acidic polymer electrolyte membrane electrolyzers and fuel cells, ACS Catal. 4, 1426–1440, 2014. 20. G. Dudek, M. Gnus, R. Turczyn, A. Strzelewicz, M. Krasowska, Pervaporation with chitosan membranes containing iron oxide nanoparticles, Sep. Purif. Technol. 133, 8–15, 2014. 21. Q. Xu, J. Yang, J. Dai, Y. Yang, X. Chen, Y. Wang, Hydrophilization of porous polypropylene membranes by atomic layer deposition of TiO2 for simultaneously improved permeability and selectivity, J. Memb. Sci. 448, 215–222, 2013. 22. H.B. Tanh Jeazet, C. Staudt, C. Janiak, Metal-organic frameworks in mixedmatrix membranes for gas separation, Dalt. Trans. 41, 14003–14027, 2012. 23. G.A. Polotskaya, A. V Penkova, A.M. Toikka, Fullerene-containing polyphenylene oxide membranes for pervaporation, Desalination. 200, 400–402, 2006. 24. G.A. Polotskaya, A. V Penkova, A.M. Toikka, Z. Pientka, L. Brozova, M. Bleha, Transport of small molecules through polyphenylene oxide membranes modiied by fullerene, Sep. Sci. Technol. 42, 333–347, 2007. 25. A. Penkova, A. Toikka, T. Kostereva, N. Sudareva, G. Polotskaya, Structure and transport properties of fullerene-polyamide membranes, Fullerenes Nanotub. Carbon Nanostructures. 16, 666–669, 2008. 26. G.A. Polochtskaya, A.V. Penkova, N.N. Soudareva, A.E. Polochtsky, A.M. Toïkka, Polyamide Ultrailtration Membranes Modiied with Nanocarbone Additives, Journal of Applied Chemistry. 81, 246–250, 2008. 27. A. V Penkova, G.A. Polotskaya, A.M. Toikka, M. Trchová, M. Šlouf, M. Urbanová, et al., Structure and pervaporation properties of poly(phenyleneIso-phthalamide) membranes modiied by fullerene C60, Macromol. Mater. Eng. 294, 432–440, 2009. 28. A. V Penkova, G.A. Polotskaya, V.A. Gavrilova, A.M. Toikka, J.-C. Liu, M. Trchová, et al., Polyamide membranes modiied by carbon nanotubes: Application for pervaporation, Sep. Sci. Technol. 45, 35–41, 2010. 29. N.N. Sudareva, A. V Penkova, T.A. Kostereva, A.E. Polotskii, G.A. Polotskaya, Properties of casting solutions and ultrailtration membranes based on fullerene-polyamide nanocomposites, Express Polym. Lett. 6, 178–188, 2012. 30. R. Nasir, H. Mukhtar, Z. Man, D.F. Mohshim, Material Advancements in Fabrication of Mixed-Matrix Membranes, Chem. Eng. Technol. 36, 717–727, 2013.

CNP for Pervaporation Polymeric Membranes 437 31. A. Iqbal, No Title, University of Technology, 2009. 32. A. Idris, I. Ahmed, Viscosity behavior of microwave-heated and conventionally heated poly(ether sulfone)/dimethylformamide/lithium bromide polymer solutions, J. Appl. Polym. Sci. 108, 302–307, 2008. 33. Y. Bohbot-Raviv, Z.-G. Wang, Discovering new ordered phases of block copolymers, Phys. Rev. Lett. 85, 3428–3431, 2000. 34. H. Odani, No Title, Bull. Inst. Chem. Res. Kyoto Univ., 1975. 35. P.M. Knight, D.J. Lyman, Gas permeability of various block copolyetherurethanes, J. Memb. Sci. 17, 245–254, 1984. 36. I. Blume, I. Pinnau, Composite membrane, method of preparation and use, (1990). 37. R.S. Murali, S. Sridhar, T. Sankarshana, Y.V.L. Ravikumar, Gas Permeation Behavior of Pebax-1657 Nanocomposite Membrane Incorporated with Multiwalled Carbon Nanotubes, Ind. Eng. Chem. Res. 49, 6530–6538, 2010. 38. L.M. Robeson, he upper bound revisited, J. Memb. Sci. 320, 390–400, 2008. 39. M.G. Buonomenna, W. Yave, G. Golemme, Some approaches for high performance polymer based membranes for gas separation: Block copolymers, carbon molecular sieves and mixed matrix membranes, RSC Adv. 2, 10745–10773, 2012. 40. J.G. Wijmans, R.W. Baker, he solution-difusion model: a review, J. Memb. Sci. 107, 1–21, 1995. 41. P. Izák, L. Bartovská, K. Friess, M. Šípek, P. Uchytil, Description of binary liquid mixtures transport through non-porous membrane by modiied MaxwellStefan equations, J. Memb. Sci. 214, 293–309, 2003. 42. J. Maxwell, A Treatise on Electricity and Magnetism : Vol II, Vol. 1, 333–335, 1873. 43. E.E. Gonzo, M.L. Parentis, J.C. Gottifredi, Estimating models for predicting efective permeability of mixed matrix membranes, J. Memb. Sci. 277, 46–54, 2006. 44. R. Pal, Permeation models for mixed matrix membranes, J. Colloid Interface Sci. 317, 191–198, 2008. 45. T. Wang, D.-Y. Kang, Predictions of efective difusivity of mixed matrix membranes with tubular illers, J. Memb. Sci. 485, 123–131, 2015. 46. J.H. Petropoulos, K.G. Papadokostaki, M. Minelli, F. Doghieri, On the role of difusivity ratio and partition coeicient in difusional molecular transport in binary composite materials, with special reference to the Maxwell equation, J. Memb. Sci. 456, 162–166, 2014. 47. G.S. Hammond and V.J. Kuck, Fullerenes: Synthesis, Properties, and Chemistry of Large Carbon Clusters, Washington: ACS, 1992. 48. A. Hirsch, he Chemistry of the Fullerenes, Stuttgart: Georg hieme Verlag, 1994. 49. I. Sapurina, M. Mokeev, V. Lavrentev, V. Zgonnik, M. Trchová, D. Hlavatá, et al., Polyaniline complex with fullerene C60, Eur. Polym. J. 36, 2321–2326, 2000.

438 Nanostructured Polmer Membranes: Volume 2 50. T.-W. Lee, O.O. Park, J. Kim, Y.C. Kim, Application of a novel fullerenecontaining copolymer to electroluminescent devices, Chem. Mater. 14, 4281–4285, 2002. 51. N.P. Yevlampieva, L.V. Vinogradova, E.I. Ryumtsev, Efect of fullerene C60 as a branching point on molecular and polarization properties of star-shaped polystyrenes, Polym. Sci. 48, 106–113, 2006. 52. P. Harris, Carbon nanotubes and related structures. New materials XXI century, (2003). 53. K.I. Bolotin, K.J. Sikes, Z. Jiang, M. Klima, G. Fudenberg, J. Hone, et al., Ultrahigh electron mobility in suspended graphene, Solid State Commun. 146, 351–355, http://dx.doi.org/10.1016/j.ssc.2008.02.024, 2008. 54. S. V. Morozov, K.S. Novoselov, M.I. Katsnelson, F. Schedin, D.C. Elias, J.A. Jaszczak, et al., Giant Intrinsic Carrier Mobilities in Graphene and Its Bilayer, Phys. Rev. Lett. 100, 016602, 2008. 55. C. Lee, X. Wei, J.W. Kysar, J. Hone, Measurement of the Elastic Properties and Intrinsic Strength of Monolayer Graphene, Science (80), 321, 385–388, 2008. 56. A. a Balandin, S. Ghosh, W. Bao, I. Calizo, D. Teweldebrhan, F. Miao, et al., Superior hermal Conductivity of Single-Layer Graphene 2008, Nano Lett. 8, 902–907, 2008. 57. R.R. Nair, P. Blake, a N. Grigorenko, K.S. Novoselov, T.J. Booth, T. Stauber, et al., Fine structure constant deines visual transparency of graphene., Science. 320, 1308, 2008. 58. A.K. Geim, K.S. Novoselov, he rise of graphene, Nat. Mater. 6, 183–191, 2007. 59. G. Nandamuri, S. Roumimov, R. Solanki, Chemical vapor deposition of graphene ilms, Nanotechnology. 21, 145604, 2010. 60. K.S. Novoselov, D. Jiang, F. Schedin, T.J. Booth, V. V Khotkevich, S. V Morozov, et al., Two-dimensional atomic crystals, 102, 10451–10453, 2005. 61. L.M. Viculis, J.J. Mack, O.M. Mayer, H.T. Hahn, R.B. Kaner, Intercalation and exfoliation routes to graphite nanoplatelets, J. Mater. Chem. 15, 974, 2005. 62. D.R. Dreyer, S. Park, C.W. Bielawski, R.S. Ruof, he chemistry of graphene oxide, Chem. Soc. Rev. 39, 228–240, 2010. 63. A. V Penkova, Z. Pientka, G.A. Polotskaya, MWCNT/poly(phenylene isophtalamide) nanocomposite membranes for pervaporation of organic mixtures, Fullerenes Nanotub. Carbon Nanostructures. 19, 137–140, 2011. 64. H.-W. Yen, Z.-H. Chen, I.-K. Yang, Use of the composite membrane of poly(ether-block-amide) and carbon nanotubes (CNTs) in a pervaporation system incorporated with fermentation for butanol production by Clostridium acetobutylicum, Bioresour. Technol. 109, 105–109, 2012. 65. C. Xue, G. Du, L. Chen, J. Ren, J. Sun, F. Bai, et al., polydimethylsiloxane hybrid membrane, 17–19, 2014. 66. K. Cao, Z. Jiang, J. Zhao, C. Zhao, C. Gao, F. Pan, et al., Enhanced water permeation through sodium alginate membranes by incorporating graphene oxides, J. Memb. Sci. 469, 272–283, 2014.

CNP for Pervaporation Polymeric Membranes 439 67. N. Wang, S. Ji, J. Li, R. Zhang, G. Zhang, Poly(vinyl alcohol)-graphene oxide nanohybrid “pore-illing” membrane for pervaporation of toluene/n-heptane mixtures, J. Memb. Sci. 455, 113–120, 2014. 68. S.K. Choudhari, F. Cerrone, T. Woods, K. Joyce, V.O. Flaherty, K.O. Connor, et al., Journal of Industrial and Engineering Chemistry Pervaporation separation of butyric acid from aqueous and anaerobic digestion ( AD ) solutions using PEBA based composite membranes, J. Ind. Eng. Chem. 23, 163–170, 2015. 69. N. Wang, S. Ji, G. Zhang, J. Li, L. Wang, Self-assembly of graphene oxide and polyelectrolyte complex nanohybrid membranes for nanoiltration and pervaporation, Chem. Eng. J. 213, 318–329, 2012. 70. A. V Penkova, S.F.A. Acquah, M.E. Dmitrenko, B. Chen, K.N. Semenov, H.W. Kroto, Transport properties of cross-linked fullerenol-PVA membranes, Carbon N. Y. 76, 446–450, 2014. 71. A. V Penkova, S.F.A. Acquah, M.P. Sokolova, M.E. Dmitrenko, A.M. Toikka, Polyvinyl alcohol membranes modiied by low-hydroxylated fullerenol C-60(OH)12, J. Memb. Sci. 491, 22–27, 2015. 72. F. Peng, C. Hu, Z. Jiang, Novel ploy(vinyl alcohol)/carbon nanotube hybrid membranes for pervaporation separation of benzene/cyclohexane mixtures, J. Memb. Sci. 297, 236–242, 2007. 73. F. Peng, F. Pan, H. Sun, L. Lu, Z. Jiang, Novel nanocomposite pervaporation membranes composed of poly(vinyl alcohol) and chitosan-wrapped carbon nanotube, J. Memb. Sci. 300, 13–19, 2007. 74. J.-H. Choi, J. Jegal, W.N. Kim, Modiication of performances of various membranes using MWNTs as a modiier, Macromol. Symp. 249–250, 610–617, 2007. 75. Y. Shirazi, M.A. Toighy, T. Mohammadi, Synthesis and characterization of carbon nanotubes/poly vinyl alcohol nanocomposite membranes for dehydration of isopropanol, J. Memb. Sci. 378, 551–561, 2011. 76. M. Amirilargani, A. Ghadimi, M.A. Toighy, T. Mohammadi, Efects of poly (allylamine hydrochloride) as a new functionalization agent for preparation of poly vinyl alcohol/multiwalled carbon nanotubes membranes, J. Memb. Sci. 447, 315–324, 2013. 77. M. Amirilargani, M. Ahmadzadeh, T. Mohammadi, B. Sadatnia, Novel Poly (vinyl alcohol)/Multiwalled Carbon Nanotube Nanocomposite Membranes for Pervaporation Dehydration of Isopropanol : Poly (sodium  4 styrenesulfonate) as a Functionalization Agent, 2014. 78. S. Qiu, L. Wu, G. Shi, L. Zhang, H. Chen, C. Gao, Preparation and pervaporation property of chitosan membrane with functionalized multiwalled carbon nanotubes, Ind. Eng. Chem. Res. 49, 11667–11675, 2010. 79. A.M. Sajjan, B.K. Jeevan Kumar, A.A. Kittur, M.Y. Kariduraganavar, Novel approach for the development of pervaporation membranes using sodium alginate and chitosan-wrapped multiwalled carbon nanotubes for the dehydration of isopropanol, J. Memb. Sci. 425–426, 77–88, 2013.

440 Nanostructured Polmer Membranes: Volume 2 80. Q.W. Yeang, S.H.S. Zein, A.B. Sulong, S.H. Tan, Comparison of the pervaporation performance of various types of carbon nanotube-based nanocomposites in the dehydration of acetone, Sep. Purif. Technol. 107, 252–263, 2013. 81. S.Y. Hu, Y. Zhang, D. Lawless, X. Feng, Composite membranes comprising of polyvinylamine-poly(vinyl alcohol) incorporated with carbon nanotubes for dehydration of ethylene glycol by pervaporation, J. Memb. Sci. 417–418, 34–44, 2012.

12 Mixed Matrix Membranes for Nanoiltration Application Vahid Vatanpour*1, Mahdie Safarpour2 and Alireza Khataee2 1

Faculty of Chemistry, Kharazmi University, Tehran, Iran Research Laboratory of Advanced Water and Wastewater Treatment Processes, Department of Applied Chemistry, Faculty of Chemistry, University of Tabriz, Tabriz, Iran

2

Abstract Nanoiltration (NF) membranes with properties in between those of ultrailtration (UF) and reverse osmosis (RO) have come a long way since they was irst introduced in the late 80s. Existing NF membranes, typically polymeric in nature, are still restricted by several challenges, including the trade-of relationship between permeability and selectivity and high fouling tendency. Mixed matrix membranes, a new class of membranes fabricated by combining polymeric materials with inorganic materials, are emerging as a promising solution to these challenges. he advanced mixed matrix nanoiltration membranes could be designed to meet speciic structural and physicochemical properties (e.g., hydrophilicity, porosity, charge density, and thermal and mechanical stability) and for introducing unique functionalities (e.g., antibacterial, photocatalytic or adsorptive capabilities). his chapter summarizes the recent scientiic and technological advances in the development of mixed matrix nanoiltration membranes. hey are classiied according to their preparation method into: (1) asymmetric mixed matrix nanoiltration membranes prepared by phase inversion, (2) thin ilm nanocomposite (TFN) nanoiltration membranes prepared by interfacial polymerization, and (3) surface coating containing inorganic materials. Applications of mixed matrix nanoiltration membranes are also briely discussed. Keywords: Mixed matrix membrane, nanoiltration, nanoparticles, thin ilm nanocomposite, surface modiication, desalination

*Corresponding author: [email protected]; [email protected] Visakh P.M. and Olga Nazarenko (eds.) Nanostructured Polymer Membranes: Volume 2, (441–476) © 2016 Scrivener Publishing LLC

441

442 Nanostructured Polmer Membranes: Volume 2

12.1 Introduction Nanoiltration (NF) membranes with performance characteristics between those of reverse osmosis (RO) and ultrailtration (UF) membranes have come a long way since they were irst introduced in the late 80s. hese membranes are widely used in the separation of inorganic salts and small organic molecules. he main characteristic properties of these membranes are low rejection of monovalent ions, high rejection of divalent ions and higher lux compared to RO membranes. hese characteristics have made NF a favorable option for applications such as water and wastewater treatment, pharmaceutical and biotechnology, and food engineering. Recently, a signiicant growth in research has shown that NF membranes can be used in many diferent areas. According to a recently published review [1], a total of 1642 papers have been published on NF membranes from 2008 to the present in the Scopus database. he main focus areas of these published papers on NF membranes can be classiied as environmental applications (25%), membrane fabrication (19%), miscellaneous topics (11%), fouling (10.68%), desalination (8.95%), pharmaceutical and biotechnology (7.49%), modeling (7.12%), fundamental studies (4%), non-water applications (3.65%), food applications (2.83%) and economics and design (0.55%). Polymeric membranes are currently the most widely used NF membrane due to their straightforward pore-forming mechanism, higher lexibility, smaller footprints required for installation and relatively low costs compared to inorganic membrane equivalents [2]. Existing polymeric membranes, especially membranes used for water treatment, are still restricted by several challenges, including high hydrophobicity, low mechanical strength, the trade-of relationship between permeability and selectivity, and high tendency to fouling. herefore, the development of membranes with high permeability and rejection, and good antifouling property, is much needed for iltration under the context of energy eiciency and cost efectiveness. he combination of polymeric membrane materials and inorganic materials, which is called mixed matrix membrane, has been introduced and investigated as advanced membranes and a promising solution to the mentioned challenges of polymeric membranes [3]. he purpose of this chapter is to review the attempts in the area of mixed matrix NF membranes research. he chapter will start by looking at the principles and history of the nanoiltration process, followed by introducing mixed matrix membranes and their fabrication, characterization, performance evaluation and diferent applications.

Mixed Matrix Membranes for Nanofiltration Application 443

12.2 Nanoiltration Process: History and Principles he history of nanoiltration dates back to the 1970s when research was begun to develop high lux RO membranes operating at relatively low pressures. he high operating pressures of the RO process led to a considerable energy cost, but, on the other hand, the quality of the obtained permeate was very good, and oten even too good. So, the development of membranes with high permeability at relatively low pressures, but with lower rejections of dissolved components, would be a great improvement for separation technology. Such low-pressure RO membranes became established as nanoiltration membranes. By the second half of the 1980s, nanoiltration slowly started to develop, and the irst applications were reported [4, 5]. Compared with ultrailtration and reverse osmosis, nanoiltration has not always had a clear deinition to describe it. Loose NF membranes can oten be classiied as UF membranes, while tight NF membranes are more similar to RO membranes. he distinguishing characteristics of NF membranes are principally the combination of very high rejection of multivalent ions (99%) with low to moderate rejections of monovalent ions (0–70%), and the high rejection (90%) for organic compounds with a molecular weight above the molecular weight of the membrane, which is commonly in the range of 150–300. Nanoiltration is a complicated process which is afected by microhydrodynamic and interfacial events occurring at the membrane surface and within the membrane nanopores. Rejection by NF membranes depends on a combination of steric, Donnan, dielectric and transport efects. he transport of neutral solutes is via the steric mechanism (size-based exclusion) when the classical Donnan efect explains the equilibrium and membrane potential interactions between a charged species and the interface of the charged membrane. he membrane charge arises from the dissociation of ionizable groups on the membrane surface and within the membrane pores [1]. hese groups may be acidic or basic in nature, or indeed, a combination of both, depending on the speciic materials used during the fabrication process. he dissociation of these surface groups is severely afected by the pH of the contacting media and where the membrane surface chemistry is amphoteric in nature, the membrane may exhibit an isoelectric point at a speciic pH. Besides the ionizable surface groups, NF membranes have a poor ion exchange capacity and in some cases ions from the contacting solution may adsorb into the membrane surface, resulting in a slight variation of the membrane charge. Electrostatic repulsion or attraction takes place

444 Nanostructured Polmer Membranes: Volume 2 according to the ion valence and the ixed charge of the membrane that may vary depending on the localized ionic environment as a result of the aforementioned phenomena [6].

12.3 Mixed Matrix Nanoiltration Membranes Polymeric membranes have gained in popularity and a wide range of applications owing to advantages such as inexpensive fabricating materials and easy scale up. However, some drawbacks include lux decline over time (oten due to membrane compaction) and the need for protecting agents, particularly in integrally skinned asymmetric membranes. Moreover, polymeric membranes have relatively low thermal and chemical stability. Ceramic membranes typically present more stable lux performance (not vulnerable to compaction), proper mechanical strength, high tolerance at extreme conditions and are more resistant to organic solvents, but sufer the disadvantages of brittleness and diiculty in scaling up [7]. As a result, the combination of organic and inorganic membrane materials, known as mixed matrix membranes, may be an attractive and promising further strategy to achieve a more desirable membrane [8]. Using this strategy, the advantages of polymeric and ceramic membranes can be combined, and some of the problems faced by polymeric membranes may be solved. A simple method to make these mixed matrix membranes is the incorporation of inorganic materials in polymeric membranes. he polymer and inorganic phases may be linked via van der Waals forces and covalent or hydrogen bonds, thus fabricating membranes with diferent chemistries [9]. hree main methods are reported in general to prepare mixed matrix NF membranes; one by dispersing the nanoparticles in the casting solution and preparation of membrane via phase inversion method, the second by incorporation of nanoparticles into the polyamide layer during interfacial polymerization, and the third by surface coating containing inorganic nanomaterial on a support membrane to fabricate NF membrane. hese techniques will be discussed below.

12.3.1 Asymmetric Mixed Matrix Nanoiltration Membranes Prepared by Phase Inversion During the recent decades of intensive membrane preparation research, diferent techniques have been proposed to generate selective and permeable ilms. he most used and thus important class of techniques is called phase inversion. In this process, a casting solution consisting of polymer

Mixed Matrix Membranes for Nanofiltration Application 445 and solvent is immersed into a nonsolvent coagulation bath. he phase transition occurs due to the interchange of solvent and nonsolvent and membrane is formed [10]. Among several approaches of production of mixed matrix membranes, blending nanoparticles in the casting solution of phase inversion technique is one of the most applicable approaches. Table 12.1 summarizes some of the recently reported mixed matrix NF membranes prepared by phase inversion technique. Inorganic materials incorporated into polymeric NF membranes can be mainly categorized as metals, metal oxides, and carbon-based materials. Among various metals, the incorporation of silver nanoparticles into polymeric matrices for the production of biofouling resistant membrane materials has recently been investigated. Andrade et al. [11] have prepared polysulfone (PSf) membranes containing silver nanoparticles (AgNP) by the wet phase-inversion process. hey have dispersed AgNP into the polymer matrix using two diferent methodologies. First, the AgNPs were synthesized and further dispersed into the polymer solution (ex-situ process). Secondly, the formation of the AgNP was performed in situ. By applying the ex-situ methodology, 45 nm average size AgNPs were uniformly distributed in the internal pores of the membranes. However, by the in-situ formation of AgNP, the nanoparticles were uniformly and preferentially distributed on the top and bottom surfaces of the membrane. he release of nanoparticles during iltration process is a main issue in the mixed matrix membranes. Andrade et al. have measured the silver ion released from the membrane during water iltration using inductively coupled plasma mass spectrometry, which was a silver leaching of approximately 2 μg/L. he mixed matrix NF membranes prepared by the in-situ method exhibited a better antibacterial activity, in comparison to those prepared by ex-situ, and also a decrease in 90% E. coli adhered cells compared to the pristine PSf membranes. he researchers have concluded that the in-situ procedure can be considered a feasible, simple, and reproducible methodology to prepare anti-biofouling PSf membranes containing AgNP. However, there was not any other evidence conirming the nanoiltration performance of the prepared mixed matrix membranes. he metal oxides as another class of inorganic materials introduced to the polymeric membranes have been widely studied in the literature. he most commonly used metal oxides are TiO2 [12–14], silica [15], ZnO [16], and Fe3O4 [17, 18]. Each of these nanoparticles can be incorporated with most of the polymeric materials available in order to produce NF membranes with speciic characteristics, as a result of the synergistic properties between the polymeric materials and nanoparticles.

CA = 69° Skin thickness =1.16 μm Sa = 10.2 nm Mean pore size = 2.35 nm

CA = 60.9°

200

200

Polymer-wrapped MWCNTs (0.1 wt%)

Silver nanoparticles (AgNP) (2.0 wt%)

DMAc/ distilled water

NMP/ deionized water

15 wt% PSf (35,000)/ 2 wt% PVP (40,000)

Optimized surface characteristics

18 wt% PES (58,000)/ 1 wt% PVP (25,000)

Casting thickness (μm)

Inorganic material (optimized content)

Solvent/ nonsolvent

Polymer (Mw,g/mol)/ polymeric additive (Mw, g/mol)

Table 12.1 Reported mixed matrix NF membranes prepared by phase inversion technique.

27

Ref.

he nanocomposite membranes 11 prepared by in-situ method exhibited a better antibacterial activity, in comparison to those prepared by ex-situ, and also a decrease in 90% E. coli adhered cells compared to the pristine PSf membranes. Water lux of membrane containing 2.0 wt% in situ prepared AgNP was 121± 48 kg/m2h.

Pure water lux of PES and 0.1% polymer-wrapped MWCNT membranes was around 6 and 14 kg/m2h, respectively. he optimized membrane prepared in this work revealed completely comparable dye removal eiciency (around 99%) for a highly concentrated acid orange 7 solution.

Separation performance (operating pressure may be diferent)

446 Nanostructured Polmer Membranes: Volume 2

Acid-oxidized MWCNTs (0.4 wt%)

TiO2 (1.0 wt%)

DMAc/ distilled water

NMP/distilled water

18 wt% PES (58,000)/ 1 wt% PVP (25,000)

20 wt% PSf (35,000)/ Poly (amideimide) (PAI) 200

100

CA = 48.1°

CA = 59.6° Sa = 10.5 nm

14

(Continued)

he pure water lux of the pure PSf membranes was 178.4 L/ m2 h and 174.93 L/m2 h for PSf/ PAI blend membranes at 69/30 composition with 1 wt% TiO2 nanoparticles (17.25 bar). With 1 wt% TiO2, rejection of HA increased to 96.4% compared to 67.2% of virgin PSf membranes. he FRR of the pure PSf membranes was 53.6% and increased to 86.7% for PSf/ PAI blend membranes at 69/30 composition with 1.0 wt% TiO2 nanoparticles.

he pure water lux of the pure PES 19 membranes was around 4 L/m2 h and for 0.04 wt% MWCNT blend membranes it increased to around 8 L/m2 h at 4 bar. he lux recovery value of pristine PES membrane was only 29.7%, while it reached to 87.7% in the 0.04 wt% MWCNT membrane. he salt retention sequence for 0.04 wt% MWCNT was Na2SO4 (75%) > MgSO4 (42%) > NaCl (17%).

Mixed Matrix Membranes for Nanofiltration Application 447

Solvent/ nonsolvent

20 wt% PES (58,000)/ 1 wt% PVP (25,000)

DMAc/distilled water

17 wt% poly(1,4- NMP/water phenylene ether–ethersulfone) (PPEES)/8 wt% PEG (1000 g/mol(

Polymer (Mw,g/mol)/ polymeric additive (Mw, g/mol)

Table 12.1 Cont.

Carboxymethyl chitosan/Fe3O4 nanoparticles (CC-Fe3O4 NPs) (0.5 wt%)

TiO2 (0.05 wt%)

Inorganic material (optimized content)

200

150

Casting thickness (μm)

CA = 52.5° Mean pore radius = 5.5 nm Porosity = 86.3%

CA = 46°

Optimized surface characteristics

Ref.

he pure water lux of the membranes increased from 9.2 to 36 kg/m2 h as the additive quantity increased from 0 to 0.5 wt%. he lux recovery value of the unilled PES membrane was only 38%, while the 0.5 wt% CC–Fe3O4 NPs–PES membrane exhibited the highest FRR value (91.7%). he rejection of Direct Red 16 for the unilled PES membrane was 88%. his was altered to about 99.0, 99.0, and 98.5% for 0.1, 0.5 and 1 wt% of the CC-Fe3O4 NPs embedded membranes, respectively.

17

he increase of TiO2 from 0 to 0.05 12 wt% enhances the water lux value from 15.72 to 133.85 L/ m2 h. For Na2SO4 solution, salt rejection decreases from 62 to 52%.

Separation performance (operating pressure may be diferent)

448 Nanostructured Polmer Membranes: Volume 2

TiO2-coated MWCNTs (1 wt%)

SiO2 grated with poly(sodium-4 styrene sulfonate (SiO2-PSS) (3 wt%)

DMAc/ distilled water

DMAc/ deionized water

21 wt% PES (58,000)/ 1 wt% PVP (25,000)

22 wt% PES (58,000)/ 1 wt% PVP (24,000)

100

150

CA = 57.3° Pore radius = 0.64 nm

Porosity = 81% Mean pore radius = 2.52 nm CA = 56.1°. Sa =11.5 nm

15

13

(Continued)

he membrane water lux increased from 29 to 93 L/m2 h with increasing the content of SiO2-PSS from 0 to 3%. he MWCO values of bare NF and 3% SiO2-PSS, was 842 and 655, respectively. he FRR values of bare NF and 3% SiO2-PSS was 56.5 and 93.1%, respectively. he rejection for Reactive Black 5 was maintained above 90% for all hybrid membranes. For Reactive Red 49 the rejection was in the range of 80–90%.

By increasing the amount of TiO2coated MWCNTs from 0 to 1 wt%, the pure water lux was increased from 3.71 to 5.66 kg/ m2 h and FRR value increased from 53.1 to 78.4%. Na2SO4 rejection was 69.5 and 75.8% for bare PES and 1 wt% TiO2-MWCNTs, respectively.

Mixed Matrix Membranes for Nanofiltration Application 449

200

150

Aminefunctionalized multiwalled carbon nanotubes (NH2-MWCNTs) (0.045 wt%)

DMAc/ distilled water

21 wt% PES/ 1 wt% PVP (25,000)

Casting thickness (μm)

MWCNT and PAA-modiied MWCNT (0.1 wt%)

Inorganic material (optimized content)

DMAc/ deionized water

Solvent/ nonsolvent

20 wt% PES (58,000)/ 1 wt% PVP (25,000)

Polymer (Mw,g/mol)/ polymeric additive (Mw, g/mol)

Table 12.1 Cont.

CA = 53.6° Porosity = 73.9% Sa = 9.6 nm

Porosity = 52% CA = 56.7° Sa = 26.36 nm

Optimized surface characteristics

Ref.

25 Increasing the nanotube dosages from 0 to 0.045 wt% led to increase of PWF from 13.6 to 23.7 L/m2 h. he salt retention sequence for 0.045 wt% NH2MWCNT was Na2SO4 (65%) > MgSO4 (45%) > NaCl (20%). he rejection of Na2SO4 for the bare PES membrane was 52%. he unilled PES membrane possessed the lowest FRR value of 68.6%, while FRR values of the 0.045 wt% NH2-MWCNTs embedded nanocomposite membranes were more than 88%.

26 Water lux of the bare PES membrane increased from 9 to 30 L/m2 h using 0.1 wt% PAAmodiied MWCNT. Na2SO4 rejection of bare PES membrane increased from 30 to 64% using 0.1 wt% PAA-modiied MWCNT. FRR value was 52 and 69% for bare PES and 0.1 wt% PAA-modiied MWCNT, respectively.

Separation performance (operating pressure may be diferent)

450 Nanostructured Polmer Membranes: Volume 2

ZnO (nanoparticle and nanorod) (0.1 wt%)

Graphene oxide (GO) nanoplates (0.5 wt%)

DMAc/distilled water

DMAc/distilled water

18 wt% PES (58,000)/ 2 wt% PVP (25,000)

20 wt% PES (58,000)/ 1 wt% PVP (25,000)

CA = 53.2° Mean pore radius = 4.5 nm Sa = 10.2 nm Overall porosity = 83.1%

200

28

16

(Continued)

Pure water lux of the unilled PES increased from 8.2 L/m2 h to 20.4 L/m2 h using 0.5 wt% GO. FRR value was also increased from 35 to 90.5% by increasing GO content from 0 to 0.5 wt%. Rejection of Direct Red 16 was 90% for unilled PES and 96% for 0.5 wt% GO.

CA Pure water lux of the bare PES was 0.1 wt% ZnO: 31 L/m2 h and it increased to nanorod = 54.0° 50 L/m2 h for 0.1 wt% ZnO Nanoparticle: 60.0° nanorods and 48 L/m2 h Sa: for 0.1 wt% ZnO nanoparticles. he FRR value was around 40% 0.1 wt% ZnO: for bare PES and increased to nanorod = 6.1 nm 70% for 0.1 wt% ZnO nanorods Nanoparticle: 6.9 nm and 65% for 0.1 wt% ZnO nanoparticles.

200

Mixed Matrix Membranes for Nanofiltration Application 451

Inorganic material (optimized content)

Reduced graphene oxide (rGO)/TiO2 (0.1 wt%)

ChitosanMontmorillonite (CS-MMT) nanosheets (1 wt%)

Solvent/ nonsolvent

DMAc/ distilled water

DMAc/ distilled water

21 wt% PES (58,000)/ 1 wt% PVP (29,000)

22 wt% PES (55,000)/ 1 wt% PVP (24,000)

Polymer (Mw,g/mol)/ polymeric additive (Mw, g/mol)

Table 12.1 Cont.

CA = 56.3° Mean pore size = 4.68 nm Sa = 6.7 nm Overall porosity = 83.5%

CA = 63.4°

100

Optimized surface characteristics

150

Casting thickness (μm) Ref.

Pure water lux of the neat PES was 30 76.7 L/m h MPa and increased to 156.09 L/m h MPa for 1 wt% CS-MMT. FRR value of neat PES increased from 56.5 to 91.9% using 1 wt% CS-MMT. With respect to neat PES, the rejection of MgSO4 and MgCl2 was around 30%, while that for Na2SO4 and NaCl was less than 20%. For 1 wt% CS-MMT, the rejection of MgSO4 and Na2SO4 was around 20%, while that for NaCl and MgCl2 was less than 10%.

Pure water lux of the bare PES 29 increased from 23 to 43 kg/m2 h using 0.1 wt% rGO/TiO2. FRR value was also increased from 75.2 to 96.7% using 0.1 wt% rGO/TiO2. Rejection of Direct Yellow 12 was 89.0 and 95.4% for Bare PES and 0.1 wt% rGO/TiO2.

Separation performance (operating pressure may be diferent)

452 Nanostructured Polmer Membranes: Volume 2

18 wt% PES/ 1 wt% PVP (25,000)

DMAc/ distilled water

Carboxylated CNTs

120

CA = 54.95° Sa = 10.8 nm 31 he membranes embedded with CNT1 (20 nm diameter) achieved better NF performances. When CNT2 (40 nm diameter) concentration reaches 0.1 wt%, the PES/ CNT2 membranes obtained the highest water lux (38.91 L/m2 h) and Na2SO4 rejection (87.25%) at 4 bar. he solute rejection was in a sequence of (Na2SO4) > R(MgSO4) > R(NaCl).

Mixed Matrix Membranes for Nanofiltration Application 453

454 Nanostructured Polmer Membranes: Volume 2 Rajesh et al. [14] prepared poly(amide-imide) (PAI) and TiO2 nanoparticles-impregnated PSf NF membranes with integral dense layer. he hydrophilicity of the PSf membranes, evaluated by water contact angle measurement, was improved by the addition of PAI and TiO2 nanoparticles due to the favorable orientation of these additives towards the membrane surface. he separation eiciency of humic substances was signiicantly improved using blend membranes. he authors have concluded that by the incorporation of PAI and TiO2 nanoparticles the morphology, pure water lux, porosity, hydrophilicity, separation eiciency and antifouling property of the prepared PSf membranes improved signiicantly. In another study, asymmetric poly(phenylene ether-ether sulfone) (PPEES)-based NF membrane has been modiied by TiO2 nanoparticles for water desalination [12]. he efect of TiO2 content in casting solution on membrane physicochemical properties was studied. Results showed that adding the TiO2 nanoparticles into the membrane matrix caused a sharp increase of water lux from 15.72 to 133.85 L/m2 h. Results revealed that salt rejection was declined initially by the increase of TiO2 nanoparticle content up to 0.05 wt% in the membrane matrix, but with more additive loading, it began rising. he membrane’s tensile strength was improved by the increase of TiO2 concentration in casting solution. he water content and contact angle experiments showed that the membrane hydrophilicity generally was improved by the addition of TiO2. Recently, carbon nanotube (CNT) and other carbon-based nanomaterials have been considered to be excellent candidates for fabrication of mixed matrix membranes due to the superior properties such as high lexibility, low mass density, and the efective π-π stacking interaction between carbon nanotube and aromatic compounds [19]. Although there are numerous advantages of carbon nanotubes for preparation of mixed matrix membranes, there are still some problems such as CNTs being unsuitable for dispersion and dissolution in most organic solvents and polymers and weak interaction of the CNTs and polymer matrix [20, 21]. hese problems may be somewhat obviated by introducing hydrophilic functional groups into the surface of the CNTs, functionalization by chemical agents and attaching polar groups to CNT sidewalls [22–24]. Vatanpour et al. [19, 25] have applied surface modiication with chemical agents and acid treatment to improve the uniform dispersion of CNTs. hey have prepared nanocomposite membranes composed of polyethersulfone (PES) and acid-treatedfunctionalized and amine-functionalized multiwalled carbon nanotubes for nanoiltration application [19, 25]. he treated MWCNTs exhibited good compatibility with PES and caused an increase in hydrophilicity and water lux of the prepared membrane. he MWCNTs inluenced mean pore

Mixed Matrix Membranes for Nanofiltration Application 455 size and porosity of membrane. In addition, fouling of membrane resulting from bovine serum albumin (BSA) iltration could be reduced by importing MWCNTs to the blend membrane. he results conirmed that the surface roughness of membranes plays an important role in anti-biofouling resistance of MWCNT membranes. he salt retention experiments of the nanocomposite membranes indicated that the mechanism of salt rejection is Donnan exclusion, in which membrane surface was negatively charged. Acid-oxidized MWCNTs were also coated by TiO2 nanoparticles via the precipitation of TiCl4 precursor on the MWCNTs and used in the preparation of nanocomposite PES NF membranes [13]. he schematic of preparation of TiO2-coated MWCNTs is presented in Figure 12.1. Contact angle measurements showed that coating of TiO2 nanoparticles on the surface of oxidized MWCNTs increased the hydrophilicity of the prepared mixed matrix membranes. Comparison of fouling resistances and lux recovery ratio of 1 wt% TiO2 and 1 wt% TiO2-coated MWCNT membranes conirmed that coating of MWCNTs with TiO2 had a synergistic efect and improved the induced efect of photocatalytic activity. he TiO2-MWCNTs presented superior anti-biofouling property due to the synergistic efect of coupling nanoparticles compared to the bare TiO2. he salt rejection performance of the prepared mixed matrix membranes

HOOC HOOC

COOH

HOOC 3:1 H2SO4:HNO3 HO 12 h relux HOOC

OH Monohydrate citric acid COOH COOH

COOH Heated in vaccum for 3 h/150 °C HOOC HOOC

COOH COOH COOH COOH

Polycitric acid grafted MWCNTs

Oxidized MWCNT

MWCNT

COOH COOH COOH

HOOC

1) TiCl4/1 M HCI 2) 5 M NH4/stiring for 3 h

Anatase TiO2

Heated at 500 °C in N2 medium for 100 min

C2H5O HOOC C2H5O Ti C2H5O HOOC (C2H5O)Ti-O C2H5O HOOC C2H5O Ti C2H5O HOOC

OC2H5 O C OH OC2H5 OC2H5 O-Ti(OC2H5) COOH COOH COOH OC2H5 COOH Ti OC2H5 OC2H5

TiO2 coated MWCNTs

Figure 12.1 Schematic of the preparation of TiO2-coated MWCNTs. (Adapted from [13] with permission from Elsevier).

456 Nanostructured Polmer Membranes: Volume 2 was higher than the bare PES membrane. Acid-treated MWCNTs have high negative Zeta potential in pH 7.0, leading to negative surface charge of the prepared membranes and improving the salts rejection. In another attempt to functionalize the CNTs, Daraei et al. [26] investigated the simultaneous efect of MWCNTs and grating of polyacrylic acid (PAA) on PES membrane surface to obtain a nanoiltration membrane with enhanced antifouling properties and higher permeate lux. A graphical illustration of the preparation of PAA-modiied MWCNT (PAAg-MWCNTs)/PES nanocomposite membrane is shown in Figure 12.2. he results showed that the presence of PAA-g-MWCNT in the membrane

O

C

OH

O OH O C OH O C OH C

CC H H H

O OH O C OH C

KPS + EG 90 °C, 4 hours

Raw MWCNT

n OH C n

PAA-g-MWCNT

Mixing with DMAc

Polymer addition 24 hours stirring

Casting on glass plate

O

Ultrasonicating

Nanotubes PAA chains

PACNO.1

PAA-g-MWCNT/PES nanocomposite membrane

500 nm

Figure 12.2 Schematic of the preparation steps for PAA-g-MWCNT/PES nanocomposite membrane. (Adapted from [26] with permission from Elsevier)

Mixed Matrix Membranes for Nanofiltration Application 457 matrix improves water permeability and antifouling property of the prepared NF membrane. he highest Na2SO4 rejection was observed using membranes containing 0.1 wt% of modiied MWCNTs. he grat polymerization of PAA on the mixed matrix membranes led to further hydrophilicity improvement of the prepared membranes. he PAA-grated mixed matrix membrane showed high water lux (40 kg/m2 h at 0.4 MPa), superior fouling resistance and more eicient salt rejection, conirming the success of simultaneous use of diferent modiication methods.

12.3.2 hin-ilm Nanocomposite (TFN) Nanoiltration Membranes Prepared by Interfacial Polymerization hin-ilm composite (TFC) membranes are widely used in NF applications. Interfacial polymerization is the most preferred method to fabricate TFC membranes in which a selective polyamide (PA) ultrathin ilm (irst layer) is formed on an asymmetric microporous support (second layer) casted over or placed directly on a nonwoven polyester fabric (third layer). he ultrathin layer is formed when polymerization occurs at the interface of two immiscible aqueous and organic solvents containing reactants [32]. During the last few years, much research have been focused on improving the performance of the TFC membranes regarding selectivity (solute rejection) without any appreciable change in membrane productivity (lux) by altering the thin-ilm layer [33]. Recently, thin-ilm nanocomposite (TFN) membrane has attracted great attention as a new type of composite membrane. TFN membranes are prepared via interfacial polymerization (IP) process and nanoparticles are incorporated within the thin PA dense layer of the TFC membrane with the aim of improving the characteristics of the interfacially polymerized layer. For example, to increase the hydrophilicity and/or surface charge density, without sacriicing the separation eiciency of the TFC membrane [32]. Table 12.2 summarizes the main reported thin-ilm TFN NF membranes prepared by interfacial polymerization technique. Wu et al. [34] used an improved interfacial polymerization process to prepare MWCNTs/polyester TFN membranes. he improved process is facilely done by immersing the support membrane into the organic phase before the conventional process of interfacial polymerization. Figure 12.3 shows a schematic of the preparation of TFN membrane by the improved process of interfacial polymerization of triethanolamine (TEOA) and trimesoyl chloride (TMC) monomers on the PSf supporting membrane. he procedure before the conventional process has a positive efect on the membrane properties and performance because of the introduction of a

Substrate

PSf

dual-layer (PES/ PVDF) hollow iber

Monomers for IP

PIP/TMC

PIP/TMC

Nanoporous SAPO-34/ organic phase

Mesoporous silica SBA-15/ aqueous phase

Inorganic material/ added phase

CA = 41° Sa = 223 nm

CA = 35°

Optimized surface characteristics

he TFN membranes with nanoparticle loading concentration from 0.05 to 0.2 wt% were found to have 106, 119, 144, 157 and 163% increase in lux over TFC membrane. he Na2SO4 rejection was nearly unchanged (from 99.3 to 98.9%) in the TFN membranes with nanoparticle loading ranging from 0 to 0.1 wt%. he developed TFN membranes exhibit a considerable capability for the rejection of rainose (from 99.4 to 98.6%), saccharose (from 96.8 to 95.3%) and glucose (from 88.3 to 82.1%) with the nanoparticle loading from 0.05 to 0.1 wt%, while the rejections dropped to around 85% with the nanoparticle loading of 0.2 wt%.

he introduction of SBA-15 material increased the permeate lux of the NF membrane from 32.4 to 45.6 L/m2 h, accompanied with slight decrease of MgSO4 rejection from 97.88 to 85.23%. he SBA15 modiied membrane exhibits excellent foulingresistance performance, with the permeate lux still retaining 72.53% of the initial lux value ater the antifouling test running for 270 min.

Separation performance

Table 12.2 Reported TFN NF membranes prepared by interfacial polymerization technique.

37

36

Ref.

458 Nanostructured Polmer Membranes: Volume 2

PES

PSf

PES barrier coating on a porous α-Al2O3 hollow ibre

BHTTM/ TMC

PEI/TPC

MPD/TMC

TiO2 nanoparticles (0.1 wt%)/ aqueous and organic phases

SiO2 (0.1 wt%)/ aqueous phase

Silica (0.1% (w/v)/ aqueous phase

CA = 59° Sa = 72 nm

CA = 46° Sa = 21.71 nm

CA = 45.3° hickness of active layer = 200 nm Sa = 5.2 nm

40

39

38

(Continued)

Water lux of the TFC NF membrane increase from12 to 27 L/m2 h using 0.1% TiO2. While NaCl rejection of the TFC NF membrane decreased from 50 to 35% using 0.1% TiO2.

For aqueous solution, the superior lux of 13.3 L/m2 h and superior rejection of 100% were obtained at 0.1 wt% SiO2 nanoparticles. For organic solution an appropriate rejection of 99% was observed.

he NF membrane prepared under the optimum condition (0.1% (w/v) silica) exhibited Na2SO4 rejection of 85.0% and the water lux of 15.2 L/m2 h. he NF membrane was treated by 5000 ppm chlorine solution for 1 h. he salt rejection and water lux of the treated membrane reached to 94.0% and 105 L/m2 h.

Mixed Matrix Membranes for Nanofiltration Application 459

Substrate

PSf

Polyimide (PI)

Monomers for IP

PIP/TMC

EDA/IPC

Table 12.2 Cont.

Aminated and chlorinated TiO2 (0.05 wt%)/aqueous and organic phases

Silica (0.05% w/v)/ organic phase

Inorganic material/ added phase

CA (Chlorinated TiO2)=63.3° CA (Aminated TiO2)=65.6° Sa (Chlorinated TiO2)=48 nm Sa (Aminated TiO2)=62 nm

hickness of active layer = 60.8 nm CA = 18°

Optimized surface characteristics Separation performance

Bromothymol Blue rejection was 94.7% for TFC NF and around 90% for TFN membranes. Crystal Violet rejection was 76% for TFC NF and 94% for TFN (chlorinated TiO2) and 74% for TFN (aminated TiO2).

he separation performance tests reveal that the addition of silica nanospheres can obviously elevate the salt rejection of the pristine TFC membrane from 87.58 ± 0.15 to 94.81 ± 0.17% under 2000 ppm of MgSO4 solution and 0.5 MPa operating pressure, simultaneously accompanied by the increases of permeate lux from 19.36 ± 0.75 to 22.65 ± 0.68 L/m2 h. Additionally, compared with pristine TFC membrane, the fabricated TFN membrane has relatively low salt rejection (43.20 ± 0.27%) in 0.5 MPa operating pressure for 500 ppm of NaCl aqueous solution.

Ref.

42

41

460 Nanostructured Polmer Membranes: Volume 2

PSf

PES

PES

PSf

TEOA/TMC

MPD/TMC

PIP/TMC

PIP/TMC

PMMA-MWNTs (MWNTs grated by poly(methyl methacrylate)/ organic phase

Silica/aqueous phase

TiO2 (~ 5 w%) / organic phase

MCWNTs (0.5 mg/mL)/ aqueous phase

CA = 72.3°

Active layer thickness ≈ 200 nm

–

CA = 22°

35

44

43

34

(Continued)

he rejection of Na2SO4 was high (99%), and the water lux was about 62% increase compared to the TFC membrane when using 0.67 g/L PMMA-MWNTs in the organic phase.

Ater addition of 0.1% (w/v) silica, the rejection of the resulting membrane changed slightly, but the water lux increased 21.1% compared to TFC NF membrane. According to the rejection of polyethylene glycols (PEGs), the MWCO of the resulting membrane was under 600 Da.

Water Flux of the TFC membrane was 17 L/m2 h and increased to 23 L/m2 h using 1.0 wt% TiO2. It decreased to 9.1 L/m2 h using 5.0 wt% TiO2. MgSO4 rejection was 78 and 95% for TFN membrane containing 1.0 and 5.0 wt% TiO2, respectively.

Water Flux of the TFC and TFN membranes was 10.8 and 21.2 L/m2 h, respectively. Na2SO4 rejection was 70 and 80% for TFC and TFN membranes, respectively.

Mixed Matrix Membranes for Nanofiltration Application 461

Polyimide (PI)

PSf

PEI/TMC

Substrate

MPD/TMC

Monomers for IP

Table 12.2 Cont.

TiO2 (0.9 wt%)/ aqueous phase

Metal-organic framework (MOF) nanoparticles (ZIF-8, MIL-53(Al), NH2-MIL53(Al) and MIL-101(Cr))/ organic phase

Inorganic material/ added phase

–

CA: TFN- NH2MIL53(Al)=49° TFN- MIL53(Al)=54° TFN−ZIF-8=75° TFN−MIL101(Cr)=50°

Optimized surface characteristics Separation performance

Water Flux of the TFC and TFN membranes was 8 and 9.5 L/m2 h, respectively. PEG 1000 rejection was 85% for TFC and 92% for TFN membranes. Na2SO4 rejection of TFC and TFN membranes was 16 and 12%, respectively. Rejection of Acid Red 249 and Reactive Black 5 dyes was more than 90% for all of the prepared membranes.

MeOH and THF permeance increased when MOFs were embedded into the PA layer, whereas the rejection remained higher than 90%. he incorporation of nanosized MIL-101(Cr), with the largest pore size of 3.4 nm, led to an exceptional increase in permeance, from 1.5 to 3.9 and from 1.7 to 11.1 L/m2 h bar for MeOH/ PS and THF/PS, respectively.

Ref.

46

45

462 Nanostructured Polmer Membranes: Volume 2

PSf

PAN

PAMAM/ TMC

PEI/TMC

GO (3.0 wt%)/ aqueous phase

SiO2 (1 wt%)/ aqueous phase

CA = 42.1° hickness of active layer ≈ 482 nm

Sa = 28.1 nm

he membranes displayed enhanced solute rejection and adequate solvent lux. 3.0 wt% GO sheets increase the rejection of polyethylene glycol (Mw 200) from 66.2% to 96.8% with the acetone lux of 15.7 L/m2 h, particularly. he incorporation of GO donates promising longterm operation stability and excellent solvent resistance for practical application.

he permeation for SiO2 embedded membrane increased nearly 50% without loss of salt rejection rate by adding 1.0 wt% nano-SiO2 nanoparticles in aqueous solution. he order of rejection to inorganic salts is Na2SO4 > MgSO4 > MgCl2 > NaCl and revealed that both TFC and TFN membrane were negatively charged. he value of MWCO for TFN membrane was about 1000 g/mol. he TFN membrane had a higher stable lux and could remove nearly 50% salts when treated with oily wastewater in one cycle iltration. 48

47

Mixed Matrix Membranes for Nanofiltration Application 463

464 Nanostructured Polmer Membranes: Volume 2 Surface layer Step-1

Step-2

MWNTs Support membrane

Organic phase

Aqueous phase

Organic phase

TMC/n-hexane

TEOA/MWNTs/H2O

TMC/n-hexane

MWNTs/polyester thin ilm nanocmposite membrane

Figure 12.3 Schematic of the preparation of TFN membrane by an improved process of interfacial polymerization of TEOA and TMC on the PSf supporting membrane. (Adapted from [34] with permission from Elsevier).

large amount of hydrophilic and negatively charged carboxyl groups on the membrane surface, as well as being favorable for the incorporation of MWCNTs in the selective layer. he prepared TFN membrane showed improved permeability and excellent selectivity. Shen et al. [35] reported novel TFN membranes composed of polyamide- and poly(methyl methacrylate) (PMMA)-functionalized multiwalled carbon nanotubes (MWNTs) for nanoiltration application. he MWNTs grated by PMMA were synthesized via a microemulsion polymerization of methyl methacrylate (MMA) in the presence of acidmodiied MWNTs. he new approach can improve the MWNT-PMMA dispersion in the organic phase of interfacial polymerization. Figure 12.4 schematically shows the preparation of TFN membrane embedded with MWNT-PMMA. he prepared TFN membranes had a high Na2SO4 rejection (99%) and enhanced water lux (about 62% improvement over the TFC membrane), which demonstrates that PMMA–MWNTs can signiicantly improve selectivity and permeability. Some other researchers have used mixed matrix membranes as the support of the interfacial polymerization process to fabricate the TFC NF membrane [49–51]. Wang et al. [51] added TiO2 sol to PSf casting solution to prepare PSf/titania hybrid membrane. hen, the TFC (NF) membranes were prepared on PSf/TiO2 hybrid membranes by interfacial polymerization of PIP and TMC monomers. he addition of TiO2 sol changed the salt rejection slightly, but the water lux was three times that of the polyamide membrane on the PSf support membrane. he polyamide NF membrane prepared under the optimum condition exhibited Na2SO4 rejection of 96.94% and water lux of 12.84 L m-2 h-1. According to the intercepting experiments of polyethylene glycols, the molecular weight cut-of (MWCO) of the resulting membrane was under 600 Da.

Mixed Matrix Membranes for Nanofiltration Application 465

(a)

(b)

(c)

COOH Microemulsion

H2SO4/HNO3 OH MWNTs

mma PMMA-MWNTs

MWNTs-COOH

TMC

PIP

Water

Support membrane

Toluene

FIP

Primary polyamide layer

Immersed into

Immersed into

aqueous phase

organic phase

Polyamide layer embedded with PMMA-MWNTs

Heat treatment in oven

PMMA-MWNTs

Figure 12.4 A schematic of the preparation process of TFN membrane embedded with poly(methyl methacrylate) hydrophobic modiied multiwalled carbon nanotubes by interfacial polymerization. (Adapted from [35] with permission from Elsevier).

Mollahosseini and Rahimpour [50] coated TiO2 nanoparticles on the PDF UF support irst and then MPD and TMC monomers were interfacially polymerized on the coated TiO2 layer. A smoother and thicker surface without any defects appeared as the concentration of nanoparticle was increased. NaCl rejection was increased from 70% for neat NF membrane to 84% for 0.5 wt% TiO2-modiied NF membrane. Antifouling and permeability behavior of the prepared membranes were improved in the new approach. Antibacterial property of prepared membranes was improved as a result of the photocatalytic characteristic of TiO2 nanoparticles. Polyetherimide (PEI)/amino-functionalized silica nanocomposite membrane was used by Namvar-Mahboub and Pakizeh as support layer

466 Nanostructured Polmer Membranes: Volume 2 to fabricate TFC membrane [49]. In order to obtain the stable support, nanocomposite membranes with diferent amounts of modiied silica (0–20 wt%) were prepared. Results showed that mechanical and chemical stability attained their maximum when silica content was 5%. he solventresistant TFC membranes were prepared using interfacial polymerization with MPD and TMC on the nanocomposite support. A mixture containing dewaxed oil (Mw ≈ 560 g/mol) and dewaxing solvent (toluene and methyl ethyl ketone) was selected to investigate the separation performance of TFC membranes. Oil rejection of 94.72% and permeation lux of 10.4 L/m2 h (at 15 bar) were achieved.

12.3.2

Surface Coating Containing Inorganic Materials

Surface modiication of a substrate membrane is another potential method to fabricate NF membranes with desirable properties. he commonly used surface modiication technologies include thin-ilm coating, self-assembled monolayers, and polymer grating by chemical treatment (e.g., UV or plasma treatment) [52]. Among which, both thin-ilm coating and selfassembled monolayer aim to form a hydrophilic coating layer on the surface of support membrane, thereby improving the membrane’s properties. he inherent hydrophilicity of poly(vinyl alcohol) (PVA) together with its great chemical, thermal, and mechanical stability makes it an appropriate polymer for fabricating nanoiltration membranes. PVA, cast to a UF support membrane, can be used as a selective layer for rejection of divalent ions with notable resistance to fouling [53]. Baroña et al. [53] prepared a new type of TFN membrane for nanoiltration by incorporating aluminosilicate single-walled nanotubes (SWNTs) within the PVA matrix. he nanocomposite PVA ilm contained well-dispersed synthesized aluminosilicate SWNT up to 20% volume fraction cast on a PSf support. he successful incorporation of aluminosilicate SWNT into the polymer matrix was conirmed by X-ray photoelectron spectroscopy. he hydrophilicity of the prepared TFN membranes improved due to the presence of hydrophilic nanotubes (contact angle decreased from 64.2 to 50.5°) and the surface roughness decreased. he TFN membranes containing aluminosilicate single-walled nanotubes had high water lux, sustaining high rejection of divalent (97%) and monovalent ions (59%). Pourjafar et al. [54,55] also prepared PES ultrailtration membrane via immersion precipitation technique and subsequently used it as support layer to fabricate diferent PVA/PES composite membranes. he PVAcoated membranes were immersed in TiO2 nanoparticles solution as membrane surface and performance modiier. hey used the Taguchi method to

Mixed Matrix Membranes for Nanofiltration Application 467 determine the optimum condition controlling factors for preparing PVA/ PES composite membrane [54]. Silica coatings with the appropriate functional groups are promising options with a proper ainity for metal ions. he silanes used to improve the adhesion and substrate surface modiication are usually alkoxysilanes. A tetra-alkoxysilane precursor is employed in most cases for the formation of nanoporous silica. he precursors of the sol undergo polymerization leading to the growth of clusters and ater a collision they link together into a gel. Bauman et al. [56] introduced the sol-gel technique to made such silica coatings on the NF2 membrane that was treated with diferent silica precursors. hey used tetraethoxysilane (TEOS), 3-(trimethoxysilylpropyl) diethylenetriamine (DETA) and 3-(mercaptopropyl) trimethoxysilane (MPTMS) for the sol-gel preparation. he membrane surface modiication was performed using a dip-coating method. Figure 12.5 shows transmission electron microscopy (TEM) images of the silica sols obtained by using TEOS, DETA and MPTMS as alkoxide precursors. he SEM images of the untreated NF2 membrane and the surfacemodiied membranes are shown in Figure 12.6. A comparison of surface SEM images for NF2 and for three other membranes indicated an increased surface roughness for the modiied membrane. It should be noted that the

100 nm

50 nm

(a)

(b)

200 nm

(c)

Figure 12.5 TEM images of colloidal nanoparticles obtained by (a) TEOS, (b) DETA, and (c) MPTMS precursor (Adapted from [56] with permission from Elsevier).

468 Nanostructured Polmer Membranes: Volume 2

(a)

(b)

500 nm

(c)

(d)

Figure 12.6 SEM images of untreated and modiied NF2 membranes: (a) untreated (NF2), (b) DETA (D-NF2), (c) TEOS (T-NF2), and (d) MPTMS (M-NF2) (Adapted from [56] with permission from Elsevier).

deposition of silica particles on the membrane, as is obvious for membranes T-NF2 (image c) and M-NF2 (image d), was not observed in the SEM image of D-NF2. he resolution of the SEM microscope is too small (minimum particle size 50 nm) to visualize the particles deposited on the membrane surface. he TEM analysis of D-NF2 revealed the deposition of nanoparticles with an average size of 10 nm and, therefore, the increased surface roughness of the membrane D-NF2. he surfaces of the modiied membranes showed an amphoteric behavior.

12.4 Applications of Mixed Matrix Nanoiltration Membranes he main applications of mixed matrix NF membranes reported in the literature include desalination [12, 25, 26, 31, 38, 47, 53, 57], water sotening [58], wastewater treatment such as removing heavy metals [18, 59, 60], small organics [14, 37] and dyes [14, 15, 17, 27, 29, 30, 39, 46] and organic solvent nanoiltration [9, 42, 45, 49, 61].

Mixed Matrix Membranes for Nanofiltration Application 469

12.5 Conclusion Polymeric materials are the ideal choice for applications where low-cost iltration is highly preferred. herefore, it is of great importance to further investigate the performance of polymeric membrane with diferent ways of modiication. his chapter discussed various combinations of polymeric and inorganic materials with a focus on the mixed matrix nanoiltration membranes which were fabricated by other researchers. Progress in the development of mixed matrix membranes for various applications has been tremendous in recent years. Besides tuning the physicochemical properties of the membranes (hydrophilicity, porosity, charge density, thermal, and mechanical stability), the incorporation of inorganic materials can provide membranes with some unique properties of nanomaterials and also possibly induce new characteristics and functions based on their synergetic efects. It provides a new dimension to the design of the next generation of polymeric membranes with high performance and antifouling properties. In this chapter, the mixed matrix nanoiltration membranes were classiied according to their preparation method into (1) asymmetric mixed matrix nanoiltration membranes prepared by phase inversion, (2) thin-ilm nanocomposite nanoiltration membranes prepared by interfacial polymerization, and (3) surface coating containing inorganic materials. Applications of mixed matrix nanoiltration membranes were also briely discussed. Despite the extensive research on the mixed matrix nanoiltration membranes, several challenges still need to be addressed to optimize the design of these membranes for industrial applications at a large scale. First, no basic understanding has been developed to systematically relect the efects of nanomaterials on membrane structures and correlate them to the membrane performance changes. Speciic contributions of surface hydrophilicity, pore size, charge density and membrane porosity to the membrane performance are still unclear. Second, the technique for better dispersion of nanomaterials needs to be further explored. Aggregation is a common issue that may prevent nanomaterials from being homogeneously dispersed inside polymer matrices. Improved dispersion of nanoillers could be achieved by modifying nanoiller surfaces or optimizing the embedding process, and the speciic process will depend on the polymer chemistry involved for membrane fabrications. hird, it is important to ensure the compatibility of nanoillers with polymers. he compatibility will determine both the optimal membrane performance and the stability of nanoillers within the host polymer. hey are critical to optimize the loading concentration and durability of nanocomposite membranes.

470 Nanostructured Polmer Membranes: Volume 2 Considering the potential efects of leached nanomaterials to the environment, nanomaterial leakage and its environmental toxicity also need to be systematically evaluated. Finally, there are many laboratory-scale studies on the application of mixed matrix membranes, but the industrial application of these membranes is still not developed. More studies are needed to investigate the cost-efectiveness of large-scale mixed matrix membrane fabrication, including the supply of nanomaterials, additional procedures for nanomaterial incorporation, and monitoring the long-term stability of membranes under practical application conditions.

Acknowledgment he authors thank Kharazmi University, Tehran, and the University of Tabriz, Iran, for all of the support provided.

List of Abbreviations Abbreviation AgNP BHTTM BSA CA CAP CC-Fe3O4 NPs CNT CS-MMT DETA DMAc DMF EDA FRR GO IP IPC MeOH NF MMA MOF MPD

Deinition Silver nanoparticles 2,2’-bis(1-hydroxyl-1-triluoromethyl-2,2,2-triluoroethyl)-4,4’-methylenedianiline Bovine serum albumin Contact angle Cellulose acetate phthalate Carboxymethyl chitosan/Fe3O4 nanoparticles Carbon nanotube Chitosan-Montmorillonite 3-(trimethoxysilylpropyl) diethylenetriamine Dimethylacetamide N,N-Dimethyl formamide Ethylenediamine Flux recovery ratio Graphene oxide Interfacial polymerization Isophthaloyl chloride Methanol Nanoiltration Methyl methacrylate Metal-organic framework m-Phenylenediamine

Mixed Matrix Membranes for Nanofiltration Application 471 MPTMS MWCN MWCO NMP PA PAA PAI PAMAM PAN PEG PEI PES PIP PMMA PPEES PSf PSS PVA PVDF PVP PWF rGO RO Sa SEM SiO2-PSS SWNTs TEM TEOA TEOS TFC TFN THF TPC TMC UF

3-(mercaptopropyl) trimethoxysilane Multiwalled carbon nanotube Molecular weight cut-of N-methyl-2-pyrrolidone Polyamide Polyacrylic acid Poly(amide-imide) Poly(amidoamine) Polyacrylonitrile Polyethylene glycol Polyethyleneimine Polyethersulfone Piperazine Poly(methyl methacrylate) Poly(phenylene ether-ether sulfone) Polysulfone Poly(sodium-4 styrene sulfonate) Poly(vinyl alcohol) Polyvinylidene luoride Polyvinylpyrrolidone Pure water lux Reduced graphene oxide Reverse osmosis Average roughness of membrane surface Scanning electron microscopy SiO2 grated with poly(sodium-4 styrene sulfonate) Single-walled nanotubes Transmission electron microscopy Triethanolamine Tetraethoxysilane hin-ilm composite hin ilm nanocomposite Tetrahydrofuran  Triphthaloyldechloride Trimesoyl chloride Ultrailtration

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474 Nanostructured Polmer Membranes: Volume 2 29. M. Safarpour, V. Vatanpour, A. R. Khataee, Preparation and characterization of graphene oxide/TiO2 blended PES nanoiltration membrane with improved antifouling and separation performance. Desalination, 393, 65–78, 2016. 30. J. Zhu, M. Tian, Y. Zhang, H. Zhang, J. Liu, Fabrication of a novel “loose” nanoiltration membrane by facile blending with chitosan–montmorillonite nanosheets for dyes puriication. Chem. Eng. J., 265, 184–193, 2015. 31. L. Wang, X. Song, T. Wang, S. Wang, Z. Wang, C. Gao, Fabrication and characterization of polyethersulfone/carbon nanotubes (PES/CNTs) based mixed matrix membranes (MMMs) for nanoiltration application. Appl. Surf. Sci., 330, 118–125, 2015. 32. W.J. Lau, S. Gray, T. Matsuura, D. Emadzadeh, J. Paul Chen, A.F. Ismail, A review on polyamide thin ilm nanocomposite (TFN) membranes: History, applications, challenges and approaches. Water Res., 80, 306–324, 2015. 33. S. Kaur, S. Sundarrajan, D. Rana, T. Matsuura, S. Ramakrishna, Inluence of electrospun iber size on the separation eiciency of thin ilm nanoiltration composite membrane. J. Membr. Sci., 392–393, 101–111, 2012. 34. H. Wu, B. Tang, P. Wu, Optimization, characterization and nanoiltration properties test of MWNTs/polyester thin ilm nanocomposite membrane. J. Membr. Sci., 428, 425–433, 2013. 35. J.N. Shen, C.C. Yu, H.m. Ruan, C.j. Gao, B. Van der Bruggen, Preparation and characterization of thin-ilm nanocomposite membranes embedded with poly(methyl methacrylate) hydrophobic modiied multiwalled carbon nanotubes by interfacial polymerization. J. Membr. Sci., 442, 18–26, 2013. 36. Q. Li, Z. Li, H. Yu, X. Pan, X. Wang, Y. Wang, J. Song, Efects of ordered mesoporous silica on the performances of composite nanoiltration membrane. Desalination, 327, 24–31, 2013. 37. T.-Y. Liu, Z.-H. Liu, R.-X. Zhang, Y. Wang, B.V.d. Bruggen, X.-L. Wang, Fabrication of a thin ilm nanocomposite hollow iber nanoiltration membrane for wastewater treatment. J. Membr. Sci., 488, 92–102, 2015. 38. D. Hu, Z.-L. Xu, Y.-M. Wei, A high performance silica–luoropolyamide nanoiltration membrane prepared by interfacial polymerization. Sep. Purif. Technol., 110, 31–38, 2013. 39. M.R.S. Kebria, M. Jahanshahi, A. Rahimpour, SiO2 modiied polyethyleneimine-based nanoiltration membranes for dye removal from aqueous and organic solutions. Desalination, 367, 255–264, 2015. 40. B. Rajaeian, A. Rahimpour, M.O. Tade, S. Liu, Fabrication and characterization of polyamide thin ilm nanocomposite (TFN) nanoiltration membrane impregnated with TiO2 nanoparticles. Desalination, 313, 176–188, 2013. 41. Q. Li, Y. Wang, J. Song, Y. Guan, H. Yu, X. Pan, F. Wu, M. Zhang, Inluence of silica nanospheres on the separation performance of thin ilm composite poly(piperazine-amide) nanoiltration membranes. Appl. Surf. Sci., 324, 757–764, 2015. 42. M. Peyravi, M. Jahanshahi, A. Rahimpour, A. Javadi, S. Hajavi, Novel thin ilm nanocomposite membranes incorporated with functionalized TiO2

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476 Nanostructured Polmer Membranes: Volume 2 56. M. Bauman, A. Košak, A. Lobnik, I. Petrinić, T. Luxbacher, Nanoiltration membranes modiied with alkoxysilanes: Surface characterization using zetapotential. Colloid. Surface. A, 422, 110–117, 2013. 57. J.N. Shen, C.C. Yu, H.m. Ruan, C.J. Gao, B. Van der Bruggen, Preparation and characterization of thin-ilm nanocomposite membranes embedded with poly(methyl methacrylate) hydrophobic modiied multiwalled carbon nanotubes by interfacial polymerization. J. Membr. Sci., 442, 18–26, 2013. 58. F.-Y. Zhao, Q.-F. An, Y.-L. Ji, C.-J. Gao, A novel type of polyelectrolyte complex/ MWCNT hybrid nanoiltration membranes for water sotening. J. Membr. Sci., 492, 412–421, 2015. 59. S.S. Madaeni, S. Zinadini, V. Vatanpour, Convective low adsorption of nickel ions in PVDF membrane embedded with multi-walled carbon nanotubes and PAA coating. Sep. Purif. Technol., 80, 155–162, 2011. 60. E. Salehi, S.S. Madaeni, L. Rajabi, V. Vatanpour, A.A. Derakhshan, S. Zinadini, S. Ghorabi, H. Ahmadi Monfared, Novel chitosan/poly(vinyl) alcohol thin adsorptive membranes modiied with amino functionalized multi-walled carbon nanotubes for Cu(II) removal from water: Preparation, characterization, adsorption kinetics and thermodynamics. Sep. Purif. Technol., 89, 309–319, 2012. 61. I. Soroko, A. Livingston, Impact of TiO2 nanoparticles on morphology and performance of crosslinked polyimide organic solvent nanoiltration (OSN) membranes. J. Membr. Sci., 343, 189–198, 2009.

13 Fundamentals, Applications and Future Prospects of the Nanoiltration Membrane Technique Siddhartha Moulik1,2, Shaik Nazia2 and S. Sridhar2* 1

Academy of Scientiic and Innovative Research (AcSIR), Council of Scientiic and Industrial Research, Anusandhan Bhawan, New Delhi, India 2 Membrane Separations Group, Chemical Engineering Division, Indian Institute of Chemical Technology, Hyderabad, India

Abstract Nanoiltration (NF) is an advanced pressure-driven membrane separation process that utilizes a nanoporous membrane to remove dissolved contaminants and colloidal matter from liquid medium. According to the range of operation it falls in between reverse osmosis (RO) and ultrailtration (UF). he present chapter provides insight into several distinctive properties of NF from the perspectives of pore radius as well as surface charge density, which signify its uniqueness in several ields of applications. Beginning from core fundamentals and principles, recent advances in synthesis and characterization procedure of NF membranes have been thoroughly described. he fundamental transport mechanism in NF membranes is discussed through diferent models, including solution-difusion, preferential sorption surface-capillary low, Donnan equilibrium and dielectric exclusion theory. An in-depth hydrodynamic analysis of luid low mechanism inside the membrane low channels and transport phenomena concerning momentum and mass transfer has been performed to aid the design of commercial membrane systems using computational luid dynamics (CFD). Commercial viability of NF holds promise in areas of drinking water puriication by satisfying stringent speciications for industrial eluent treatment, processing of food and dairy products, and recovery of pharmaceuticals. Finally, an overall economic estimation including operation and maintenance cost of NF process for treating an eluent feed of capacity 8.3 m3/h is reported. his study will help researchers to bring the

*Corresponding author: [email protected] Visakh P.M. and Olga Nazarenko (eds.) Nanostructured Polymer Membranes: Volume 2, (477–518) © 2016 Scrivener Publishing LLC

477

478 Nanostructured Polmer Membranes: Volume 2 technology from lab to land through comprehensive design and help mitigate several obstructions in eluent treatment and water puriication technologies. Keywords: Nanoiltration; membrane synthesis and characterization; drinking water puriication; food processing; industrial eluent treatment; computational luid dynamics; process design; economic estimation

13.1 Introduction he earth is fortunate enough in having the precise condition essential for water to exist in liquid form. One of our prime concerns is to protect potential water resources as well as reduce the unwanted waste in them. hroughout the last few decades, as the public concern has grown for a clean environment, a tremendous output of technology has attained several remarkable inventions in wastewater treatment. Stringent environmental regulations and norms for cost efectiveness have prompted chemical engineers to develop better and alternative separation methods than the presently available conventional techniques. However, the selection of appropriate processes depends upon major factors like yield, eiciency of the process, economic feasibility, environmental hazards and its durability. he traditional molecular separation processes like distillation, evaporation, adsorption, extraction and crystallization have become uneconomical, particularly when operated in small-scale production units. Approaching the zero discharge concept with the reuse of treated water, membrane separation processes and their applications have been a breakthrough with their excellent performance as purifying processes. Membrane technology is being utilized extensively in surface and groundwater puriication, sea and brackish water desalination, wastewater reclamation, hazardous industrial waste treatment, food and beverage production, gas and vapor separation, energy conversion and storage, air pollution control, hemodialysis, protein concentration, etc. herefore, a comprehensive understanding of the separation phenomenon that can be achieved through membranes is necessary. As a brief introduction, membrane acts as permselective barrier through which selective components pass more readily than others by diferences in one or more properties of the components. Transport of the components through membrane can be afected by convection or depends on the applied driving force that is provided either by concentration, pressure, temperature or electric potential diference. hese membranes can be either homo- or heterogeneous, symmetric or asymmetric in structure, which may carry either positive or negative charges or are neutral or bipolar

Nanofiltration Membrane Technique 479 in nature. In recent years, the scope of membrane separation processes is still widening, stimulated by the developments of novel or improved membrane materials with better chemical, thermal and mechanical properties besides enhanced permeability and selectivity characteristics, resulting in reduction of capital and operating costs for commercial success. Membranes used in the modern devices and systems are almost always compact and modular. Among the diferent classiication methods, membranes can be classiied based upon the applied driving force in order to accomplish the separation. Pressure-driven membrane processes such as reverse osmosis (RO), nanoiltration (NF), ultrailtration (UF) and microiltration (MF) have become efective techniques for the removal of environmentally undesirable and hazardous contaminants from aqueous systems such as sewage, ground and polluted surface waters, etc. On the other hand, integration of membrane separation processes with conventional processes has been explored over the last couple of years for drinking water systems throughout the world. hese integrated processes can achieve higher water recoveries of more than 90%, which brings us closer to the goal of zero liquid discharge (ZLD) speciied by pollution control boards. In this chapter a basic foundation for the broader understanding of the nanoiltration process is provided with a general overview, and successively its area of applications and basic modeling approaches of the processes are discussed.

13.1.1 Nanoiltration (NF) Nanoiltration is a pressure-driven membrane separation process that utilizes semipermeable membranes to remove dissolved contaminants and particulate matters, and the range of operation lies between reverse osmosis (RO) and ultrailtration (UF) membrane processes. Nominal pore size of NF membrane is in the range of 1–10 nanometers. Pressure is applied on the feed side of the membrane in order to force the contaminated water to permeate through the porous structure of the membrane, leaving behind most of the dissolved solids. he basic principle of the NF process is shown in Figure 13.1 where a hypothetical polymeric membrane with –CONH groups attached to the surface of the NF membrane is brought in contact with an aqueous salt solution. Here, the separation occurs by molecular sieving only, wherein monovalent salts such as NaCl pass through and bivalent ones like CaCO3 undergo maximum retention. NF inds wide application for brackish water treatment [1], water sotening [2], industrial wastewater treatment [3], food processing  [4,  5], etc.

480 Nanostructured Polmer Membranes: Volume 2 Bivalent lons rejected – CO32

Membrane pore Monovalent lons

Polyvinyl alcohol Polyamide Polyether sulfone Polyester fabric support

–

+

+

Bivalent tons (SO42 . Ca2 . Cu2 etc) Monovalent lons (Na+, CI , H+ etc)

Figure 13.1 Principle of nanoiltration.

Molecular sieving or steric hindrance is the basic mechanism in NF for colloids and large molecules, while for ions and low molecular weight organics the physicochemical interactions of solute and membrane become increasingly important [5].

13.1.2 Principle Several models have been developed to identify the transport mechanism within NF membrane. Uncharged NF membrane theories based on solution-difusion phenomena [6] and preferential sorption surfacecapillary low approach [7] are used to predict the process performance. In the case of charged NF membrane, the Donnan equilibrium model [8] and dielectric exclusion theory [9] are widely used.

13.1.2.1 Solution-Difusion Model In the NF process an external pressure greater than the natural osmotic pressure of a feed solution is applied to force the water to low from the feed solution side to the pure water side through semipermeable membrane. In this process, water sorption mechanism depends on both physical structure and chemical composition of the polymeric membrane. Solutiondifusion mechanism during NF is shown schematically in Figure 13.2. he transport of water molecules through the membrane at a temperature below its glass transition temperature is a three-stage process in which the water molecules initially get sorbed onto the membrane surface at the feed side, then difuse through the thickness and inally get desorbed on the

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Ji C

Ci P

im

Pm Feed side

Permeate side P i