Carbon Black: Production, Properties and Uses : Production, Properties and Uses [1 ed.] 9781620814352, 9781612095356

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Copyright © 2011. Nova Science Publishers, Incorporated. All rights reserved. Carbon Black: Production, Properties and Uses : Production, Properties and Uses, Nova Science Publishers, Incorporated, 2011. ProQuest Ebook

Copyright © 2011. Nova Science Publishers, Incorporated. All rights reserved. Carbon Black: Production, Properties and Uses : Production, Properties and Uses, Nova Science Publishers, Incorporated, 2011. ProQuest Ebook

MATERIALS SCIENCE AND TECHNOLOGIES

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CARBON BLACK: PRODUCTION, PROPERTIES AND USES

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Carbon Black: Production, Properties and Uses : Production, Properties and Uses, Nova Science Publishers, Incorporated, 2011. ProQuest Ebook

CARBON BLACK: PRODUCTION, PROPERTIES AND USES

IAN J. SANDERS AND

Copyright © 2011. Nova Science Publishers, Incorporated. All rights reserved.

THOMAS L. PEETEN EDITORS

Nova Science Publishers, Inc. New York Carbon Black: Production, Properties and Uses : Production, Properties and Uses, Nova Science Publishers, Incorporated, 2011. ProQuest Ebook

Copyright © 2011 by Nova Science Publishers, Inc. All rights reserved. No part of this book may be reproduced, stored in a retrieval system or transmitted in any form or by any means: electronic, electrostatic, magnetic, tape, mechanical photocopying, recording or otherwise without the written permission of the Publisher. For permission to use material from this book please contact us: Telephone 631-231-7269; Fax 631-231-8175 Web Site: http://www.novapublishers.com NOTICE TO THE READER The Publisher has taken reasonable care in the preparation of this book, but makes no expressed or implied warranty of any kind and assumes no responsibility for any errors or omissions. No liability is assumed for incidental or consequential damages in connection with or arising out of information contained in this book. The Publisher shall not be liable for any special, consequential, or exemplary damages resulting, in whole or in part, from the readers‘ use of, or reliance upon, this material. Any parts of this book based on government reports are so indicated and copyright is claimed for those parts to the extent applicable to compilations of such works.

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Independent verification should be sought for any data, advice or recommendations contained in this book. In addition, no responsibility is assumed by the publisher for any injury and/or damage to persons or property arising from any methods, products, instructions, ideas or otherwise contained in this publication. This publication is designed to provide accurate and authoritative information with regard to the subject matter covered herein. It is sold with the clear understanding that the Publisher is not engaged in rendering legal or any other professional services. If legal or any other expert assistance is required, the services of a competent person should be sought. FROM A DECLARATION OF PARTICIPANTS JOINTLY ADOPTED BY A COMMITTEE OF THE AMERICAN BAR ASSOCIATION AND A COMMITTEE OF PUBLISHERS. Additional color graphics may be available in the e-book version of this book.

Library of Congress Cataloging-in-Publication Data Carbon black : production, properties, and uses / editors, Ian J. Sanders and Thomas L. Peeten. p. cm. Includes index. ISBN:  (eBook)

1. Carbon-black. I. Sanders, Ian J., 1970- II. Peeten, Thomas L., 1967TP951.C37 2011 662'.93--dc22 2010051516

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CONTENTS Preface Chapter 1

Chapter 2

Chapter 3

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Chapter 4

Chapter 5

i Carbon Blacks in Electrochemical Energy Conversion Devices: Uses and Applications in Fuel Cells Paloma Ferreira-Aparicio Study and Application of Carbon Black Vulcan XC-72R in Polymeric Electrolyte Fuel Cells M. J. Lázaro, L. Calvillo, V. Celorrio, J. I. Pardo, S. Perathoner and R. Moliner

1

41

A Review of Current Analytical Applications Employing Graphitized Carbon Black Christine M. Karbiwnyk and Keith E. Miller

69

Intermolecular and Intramolecular Interaction at Adsorption on the Surface of Graphitized Thermal Carbon Black V. V. Varfolomeeva and A. V. Terent‟ev

93

Heat Transfer and Growth of Primary Black Carbon Particles in Gas Mixture Y. A. Baranyshyn, S. P. Fisenko and O. G. Penyazkov

115

Chapter 6

What Does Carbon Nanomaterial Cause in Human Health? Masakazu Umezawa and Ken Takeda

Chapter 7

Effect of Carbon Black Fillers on the Tribological Properties of PTFE Yoshinori Takeichi

137

The Role of Carbon Black in the Rheology of Rubber Compounds: Dynamic Properties, Nonlinear Behavior and Modeling Fabio Bacchelli and Salvatore Coppola

157

Chapter 8

Chapter 9

Carbon Blacks for Energy Storage and Conversion Gui-xin Wang and Kang-ping Yan

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183

vi Chapter 10

Chapter 11

Contents Carbon Black Treated in Low Pressure Plasma: A New Class of Granular Adsorbents for Acid and Basic Compounds Nicoletta De Vietro

237

Production, Properties and Applications of Carbon Blacks in Rechargeable Lithium-Ion Batteries B. Jin

263

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Index

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279

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PREFACE Carbon black is a material produced by the incomplete combustion of heavy petroleum products. It is used as a pigment and reinforcement in rubber and plastic products. This new book examines the production, properties and uses of carbon black. Topics discussed include carbon blacks in electrochemical energy conversion devices; application of carbon black vulcan XC-72R in polymeric electrolyte fuel cells; heat transfer and growth of primary black carbon particles in gas mixture; and health risks of carbon nanomaterials. (Imprint: Nova Press). Chapter 1 – Carbon materials, and in particular carbon blacks, are widely used in electrochemical applications because they combine unique properties: good electrical conductivity, acceptable corrosion resistance, high purity, low cost, high thermal conductivity, dimensional and mechanical stability, light weight, ease of handling, diversity of physical structures and versatility in its surface chemistry,including composite fabrication. This chapter describes a number of applications and uses of carbon blacks in electrochemical energy conversion devices, and more concretely, in proton exchange membrane fuel cells (PEMFCs). Fuel cell technology has experienced a huge advance by using carbon blacks as the basic material for increasing the electrochemical area of metal electrocatalysts, synthesizing microporous layers for gas diffusion and water management in electrodes, and fabricating composites for flow field plates. Tailoring of carbon blacks by surface modification is a powerful tool to design high performance components for these systems, which constitutes a promising alternative for clean and efficient power in a large variety of stationary, mobile, and portable applications. Chapter 2 – The most developed fuel cells are those working at low temperature using a polymeric electrolyte, PEMFC and DAFC, the latter making use of different alcohols directly as fuel. At present, the most effective fuel cell catalysts, both in cathode and anode, are highly dispersed platinum-based nanoparticles. These Pt nanoparticles are normally supported on carbon materials in order to increase the active surface area of Pt and improve the catalyst utilization. Among other factors, the performance of metal nanoparticles supported on carbon materials depends strongly on the properties of the support. For fuel cell applications, carbon supports should have several characteristics such as high surface area for dispersing catalytic metal particles, high electrical conductivity for providing electrical pathways, and mesoporous structure for the facile diffusion of reactants and by-products. Moreover, the interaction between the carbon support and the Pt plays an important role in the properties of the Pt/C catalyst. This interaction can be improved through the surface modification of the support in order to form proper functional groups and chemical links at the Pt/C interface.

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ii

Ian J. Sanders and Thomas L. Peeten

These functional groups can significantly affect the manufacture and performance of electrocatalysts, and they are responsible for both the acid-base and the redox properties of the carbon supports. Nowadays, among all kinds of carbon supports, carbon blacks are the most commonly used due to their high mesoporous distribution and their graphite characteristics. Vulcan XC-72(R) is the most frequently used in the preparation of commercial electrocatalysts because of its good compromise between electrical conductivity and high surface area. In this chapter, a detailed analysis about the properties of Vulcan XC72R and the effect that different functionalization treatments have on them will be reported. Subsequently, the influence of the properties of the support on platinum supported catalysts will be established. The results of the use of these catalysts at the anode side of a PEM fuel cell will be presented in order to demonstrate the effective application of this carbon material in low temperature fuel cells and to establish the effect of the functionalization of the support on the fuel cell performance. Chapter 3 – Graphitized carbon black (GCB) is a non-porous form of amorphous carbon. GCB materials are hydrophobic. They are often used to effectively trap organic compounds from water or under high humidity conditions; conditions where the performance of other sorbents is reduced. While activated carbon relies on its high surface-area-to-volume ratio to adsorb organic compounds, the surface interactions of GCB depend solely on dispersion (London) forces. GCB materials have been used to trap a wide range of organic compounds from C4 hydrocarbons to polychlorinated biphenyls (PCBs). Typically, compounds are adsorbed on the GCB surface from large volumes of air or water, and then subsequently released either by solvent desorption or thermal desorption resulting in either a concentration of the analytes, solvent exchange or combination of both. This review presents analytical applications of GCBs over the last decade. It includes air monitoring, solid-phase extraction (SPE), purge traps for purification or low-flow air sampling, and chromatography columns utilizing GCB packing material. Chapter 4 – The display of intermolecular interaction at adsorption on the surface of graphitized thermal carbon black (GTCB) is not enough described even in theoretical researches. Scientific literature almost lacks the data about the influence of the intermolecular H-bond on adsorption of molecules of different classes. Intermolecular H-bond was not examined as one of the types of intermolecular interaction of adsorbate molecules with the surface of GTCB. It was considered that physically and mathematically (energetically) the homogeneous surface of GTCB is not inclined to this specific interaction. However, the intermolecular H-bond is in fact one of the reasons which hardens the examination of the intramolecular interaction at adsorption. At present the only method to calculate the thermodynamic characteristics of adsorption (TCA), based on the information about the structure of molecules and allowing predicting this structure is the semiempirical molecularstatistical theory of adsorbtion for quasi-rigid molecule, proposed by A.V. Kiselev and D.P. Poshkus. This theory is based on the principle of additivity of the parameters of the intermolecular interaction adsorbate-adsorbent. On the TCA basis it is possible to predict the interaction of which conformational isomer with the surface of GTCB will be stronger. However, the semiempirical molecular-statistical theory of adsorbtion does not take into consideration the influence of the intramolecular H-bond, intermolecular H-bond and the sorbent force field on the adsorbate geometry. The solution of this question will be possible if the approaches are found allowing calculating the TCA taking into account the specific interactions (intra- and intermolecular H-bond) under the influence of the sorbent force field.

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Preface

iii

Such calculations will allow explaining on the molecular level the reasons of difference of the TCA of structural conformers of the compounds, stable in the gas phase and in the adsorbed state. Not using the quantum-chemical methods with minimum permissible level of the calculations of the hydrogen-bonded molecular systems (for example, MP2/6-311++G(d,p) and B3LYP/6-311++G(d,p)), it is not possible to obtain the structural-energetic parameters of the conformational isomers to calculate their TCA. The presented chapter shows the possibility of using the ab initio and DFT methods in combination with the semiempirical molecular-statistical theory of adsorbtion for TCA calculations of the conformation isomers of molecules taking as the example the aromatic alcohols, amines and thiols. The detailed examination of adsorption of these compounds on the GTCB taking into account the hydrogen bond, its exact qualitative characteristics and quantative specification is important for clarifying the specific features of the separate class of compounds. It is interesting to show how the situation of lone electron pair, increase of the chain length by the -CH2 group, differentiation in electron-donor, electron-accepting and repulsive possibilities of oxygen, nitrogen and sulfur atoms affect hydrogen bond formation. The difficulty of the examination of the chosen object is determined by the comparability of the values of the barriers of internal rotation with the benefit in the heat of adsorption at the expense of the change of molecule conformation while getting into in the GTCB force field. The results of the optimization of the adsorbate molecules on the model fragment of the GTCB by the DFT method have confirmed the forecast about the possibility to form the intermolecular H-bond by -OH, -NH2, -SH groups together with graphite π electrons, which was of made by us earlier. To calculate the TCA of the molecules the automotized system is proposed in the new version of which the integration of all the stages of the calculation in one and the same program system is the necessary demand to provide the TCA calculation: the initial data input, carrying on the TCA calculations, visualization of the results, saving the final results in the data base. The analysis of the initial and calculating data is possible with the help of the graphic module which allows to visualize the 3D molecule model on the GTCB surface, create the 2D and 3D diagrams of the dependence of the TCA on the torsion angles, draw the contour maps of the potential energy surface. The modules are also worked out with the help of which it is possible to carry on the variation of the atom-atom potential (AAP) parameters and examine all the possible ways of conformational transfers (variation of the torsion angles). The special attention is paid to the preparation of the geometric parameters of the molecule in the gas phase and adsorbed state. The results of the search can form the fundamental basis for qualitative and quantitative examination of the display of intermolecular interaction at adsorption on the surface of GTCB. Chapter 5 – Results of the experimental and theoretical study of the interference between heat transfer and growth of primary black carbon particles are presented. Experimental study was performed in shock tube with length about 7 m and inner diameter 50 mm. Black carbon nanoparticles have been formed during the pyrolysis of ethylene, diluted with argon, behind the reflected shock wave. The pyrolysis temperature was in the range 2000 – 3500 K, total pressure was 7.5 - 11 bars. Home-made pulsed photoemission pyrometer with a temporal resolution of 1 μs was used for measurements of temperature of black carbon particles during their growth. Substantial difference (about 900 K) in temperature of black carbon particles and the temperature of gas mixture was discovered. For simulation of nonisothermal growth of black carbon nanoparticles the mathematical model was developed, which considers the heat transfer and growth of black carbon nanoparticles in the supersaturated carbon vapor in

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Ian J. Sanders and Thomas L. Peeten

the free molecular approximation (the Knudsen number Kn > 103). It was established that due to very high supersaturation of carbon vapor and the release of the latent heat of sublimation temperature of the particles can exceed the temperature of the gas phase more than one thousand degree. Heat transfer between the carrier gas and black carbon nanoparticle significantly affects on the value of this temperature difference. For determining parameters of growth of primary black carbon nanoparticles the inverse problem was solved using the data of electron microscopy and gasdynamic measurements. It was obtained that in order to reach to final diameter the growth time of primary black carbon particles been less 10 μs and the temperature difference been above 1000 K. The final diameter of primary black carbon particles is in the range 27 - 55 nm. Chapter 6 – Previous studies have shown that exposure to nano-sized particles through the airway affects both the respiratory and extrapulmonary organs. The potential health risks of inhaling nanomaterials are of great concern because of their high specific activity and their unique property of translocation [1, 2]. Inhaled nanoparticles with a diameter of less than 100 nm are the main particles that reach and are deposited in the alveolar region. Furthermore, they can enter the circulatory system and translocate to extrapulmonary tissue [3]. Chapter 7 – Polymer materials are widely used as the sliding surface materials because of their excellent low frictional properties and chemical inertness. However, the wear amount of polymer material after long period of friction is relatively larger than that of metal or ceramic. Therefore, polymer materials are actually used in the form of composites by adding various kinds of filler materials to enhance their wear resistance. Carbon fiber, glass fiber, graphite, molybdenum disulfide and fine bronze powder are the typical fillers used for reducing wear of the polymer material. Carbon black is well known as a filler of the rubber products such as automobile tires in the field of tribology. On the other hand, in case of polymer material, the general purpose of adding carbon black is giving electrical conductivity to polymer materials or reducing degradation of polymer materials caused by ultraviolet light. There is little report on the wear reducing effect of carbon black filler for the polymer materials. We used carbon black as a filler of polytetrafluoroethylene (PTFE) in order to reduce the wear of it. PTFE is widely used as a tribomaterial because it shows excellently low friction coefficient comparing with other polymer materials. The wear and frictional properties of PTFE filled with twelve kinds of carbon black were studied using ring-on-disk tribometer. The effects of the average particle diameter, the nitrogen surface area and the DBP absorption of carbon black on the friction coefficient and specific wear rate were studied. The friction coefficient of PTFE composites decreased linearly with the decrease of the nitrogen surface area and specific wear rate of PTFE composites decreased with the decrease of the average particle diameter. We discussed wear reducing mechanisms of PTFE composites filled with carbon black by assuming simple model of carbon black. It was considered that the wear reducing action of carbon black is to make it difficult to extract the PTFE fibre from the composite by getting caught in the carbon black. Chapter 8 – Rubber compounds, even when reinforced with approximately spherical particles, represent highly concentrated suspensions characterized by complex interactions between filler particles and between particles and polymer. Unlike a concentrated suspensions with low viscosity matrix, the rheological behavior of a rubber compounds can be described using a single medium approach. In comparison with the rheology of polymer melts and solutions, these filled systems display yield-like behavior and very long relaxation phenomena which reflect the processes of flocculation and aggregation of particles as well as

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Preface

v

disruption of particle aggregates in the medium. Long memory of the deformation history is also observed and related to slow restructuring phenomena. Additionally, rubber compounds display highly nonlinear viscoelastic behavior and viscoelastic parameters depending on concentration and structure of particles. Rheological properties are assessed for polybutadiene based rubber compounds and advanced investigative techniques are used to characterize the filler network as well as the state-of-mix. The role of filler interactions in shear and extension is investigated. It is shown that reinforcement is due to hydrodynamic effects together with the build-up of a secondary particulate structure within the rubber matrix and the consequent amount of immobilized polymer. In terms of compound processability, this study attempts to point out the effect of state-of-mix on the transient elongational response of uncured systems. The K-BKZ model is implemented in order to describe and predict the transient rheological response of the investigated rubber compounds in the linear and nonlinear regime. The behavior of uncured systems is also described by means of the semi-empirical Leonov‘s model starting form the linear viscoelastic response of the polymer matrix. Chapter 9 – The key physicochemical properties related to energy storage and conversion of carbon blacks, such as structure, accessible specific surface area, texture, morphology, and surface chemistry, are summarized. The status of the applications of carbon blacks for energy storage and conversion, including primary batteries, rechargeable batteries, supercapacitors, air batteries, fuel cells, solar cells, hydrogen energy, biomass energy, and nuclear energy, has been reviewed. The applications of carbon blacks as conductive additives, electrode materials, current collectors, catalysts, catalyst supports, high efficient fuel, and reducing agents, are described systematically. The main factors affecting the state-of the-art performance are analyzed. The existed issues and challenges of carbon blacks for energy storage and conversion are discussed. Besides the conventional applications of carbon blacks in batteries and supercapacitors, it is considerably promising and competitive for the following perspectives: green catalysts to decompose hydrocarbon, catalyst supports, electrode materials of solar cells, accelerant of biomass energy, reductive of nuclear fuel, and high efficient fuels for direct carbon fuel cells. Some aspects, including feedstock, manufacturing conditions, post treatments and good dispersion, should be paid attention to further improve the performance. Carbon blacks or carbon black composites with specific function will become more important and more competitive for energy storage and conversion. Chapter 10 – The surface chemical composition and morphology of carbon black play a crucial role in its adsorption properties. To improve the adsorption ability for acid and basic compounds in liquid and in gas phases, carbon black granules can be treated in low pressure 13,56 MHz radio frequency glow discharges, generated in suitable special reactors which allow homogeneous processing of powders and granules. In this chapter it is shown that the surface acid or basic character of the granules can be increased by grafting acid or basic chemical groups utilizing glow discharges fed with different gas mixtures: e.g. oxygen/ammonia, acrylic acid/argon and allylamine/argon. The chemical and morphological characterization of treated carbon black was performed by means of X-ray photoelectron spectroscopy, water contact angle measurements, Brunauer-Hemmett-Teller specific surface area evaluations and scanning electron microscopy observations. The study of the adsorption ability for basic and acid compounds in liquid and gas phases was accomplished with Boehm‘s titrations, in basic and acid water solutions, and vapour adsorption tests with ammonia, hydrochloric, acetic and propanoic acids. The results show that plasma treatments significantly improve the adsorption ability of carbon black granules and this effect is mainly

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due to the surface grafting of acid/basic groups and not to a surface morphological modification. Chapter 11 – Carbon black is a family of small particle size carbon pigments, which are formed in the gas phase by the thermal decomposition of hydrocarbons, and has been used widely in lithium secondary batteries, inks, coatings, rubbers and plastics. This chapter is to provide an introduction as to how the carbon blacks are manufactured and their most important physical and electrochemical properties. Primary emphasis is given to introducing the electrochemical properties and the applications in rechargeable lithium-ion batteries. Here we will also draw the electrochemical properties from examples taken from our own work. This contribution consists of four sections. Section 1 is entitled Introduction. The following section (Section 2) describes the manufacturing method. Section 3 focuses on the physical and electrochemical properties and the applications. Section 4 provides summary and future prospects.

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Chapter 1

CARBON BLACKS IN ELECTROCHEMICAL ENERGY CONVERSION DEVICES: USES AND APPLICATIONS IN FUEL CELLS Paloma Ferreira-Aparicio CIEMAT (Centre for Energetic, Environmental and Technological Research)

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ABSTRACT Carbon materials, and in particular carbon blacks, are widely used in electrochemical applications because they combine unique properties: good electrical conductivity, acceptable corrosion resistance, high purity, low cost, high thermal conductivity, dimensional and mechanical stability, light weight, ease of handling, diversity of physical structures and versatility in its surface chemistry,including composite fabrication. This chapter describes a number of applications and uses of carbon blacks in electrochemical energy conversion devices, and more concretely, in proton exchange membrane fuel cells (PEMFCs). Fuel cell technology has experienced a huge advance by using carbon blacks as the basic material for increasing the electrochemical area of metal electrocatalysts, synthesizing microporous layers for gas diffusion and water management in electrodes, and fabricating composites for flow field plates. Tailoring of carbon blacks by surface modification is a powerful tool to design high performance components for these systems, which constitutes a promising alternative for clean and efficient power in a large variety of stationary, mobile, and portable applications.

INTRODUCTION: APPLICABILITY OF CARBON BLACK IN ELECTROCHEMICALDEVICES Carbonaceous materials have many desirable properties that have attracted their use in electrodes and other components for electrochemical systems: galvanic cells, electrolytic cells, fuel cells, flow cells, batteries, etc. Properties such as their good electrical and thermal conductivity, their dimensional, mechanical and chemical stability, their acceptable corrosion

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resistance, high purity, low cost, light weight and easy handling are some of the main features that explain their widespread acceptance in electrochemistry. The important role of carbon in these devices probably started in 1792 with the discovery by Alessandro Volta of the voltaic pile as the forerunner of the storage battery of today. Perhaps the first practical application of carbonaceous materials in batteries was demonstrated in 1866 by Georges Leclanché in cells that bear his name. By 1876, the wet Leclanché cell was made with a compressed block of manganese dioxide. In 1888, Dr. Carl Gassner built the first "dry" version by using a zinc cup as the anode and making the electrolyte with a paste of plaster of Paris (and later, wheat flour) to gel and immobilize the electrolyte. Dry cells were demonstrated by Gassner in 1900 for portable lighting at the World's Fair in Paris. Continual improvements were made to the stability and capacity of zinc-carbon cells throughout the 20th Century and by the end of the century the capacity of a zinc-carbon cell had increased fourfold over its 1910 equivalent. A modern version of the Leclanché cell is the alkaline-manganese dioxide (MnO2) one. A schematic representation of the cross-section of this dry cell battery is shown in Figure 1. The outer case is usually made of a thin metal sheet. Coarsely ground manganese dioxide is mixed with an equal volume of carbon black to form the cathode (positive electrode).

Figure 1. Schematic representation of a zinc-carbon dry cell.

Carbon powders such as carbon black and graphite are commonly used to enhance the conductivity of the positive electrodes that contain metal oxides in this type of battery. The particle morphology plays a significant role in this application. A suitable carbon for this application should have characteristics that include: a) low resistivity in the presence of the electrolyte and active electrode material, b) absorb and retain a significant volume of electrolyte without reducing its capability of mixing with the active material, c) exhibit compressibility and resiliency in the cell, and d) contain low levels of impurities. Graphite has higher electrical conductivity than carbon black but it is not capable of retaining the same volume of electrolyte or demonstrating the same mechanical properties in the cell. The structure of acetylene black provides the capability to retain over three times as much electrolyte per unit of mass as graphite. As shown in Figure 2, carbon blacks consist of aggregates, defined as the smallest dispersible units, composed of partially fused, reasonably spherical primary isotropic particles [1]. Surface area increases as the primary particle size is reduced. The aggregates are held together by attractive Van der Waals forces to form agglomerates. These forces increase as the size of the primary particle is reduced and agglomerate density is increased [2]. Therefore, higher surface areas are obtained in highly structured carbon blacks. The capacity of Leclanché cells is dependent on the amount and type of carbon black that is used. Generally about 55 vol. % carbon black mixed with

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Carbon Blacks in Electrochemical Energy Conversion Devices

3

manganese dioxide yields the maximum capacity. This composition agrees closely with the minimum in the electrical resistivity of the electrode mixture. The high electronic conductivity, chemical inertness and low cost are beneficial for the use of carbon for electrode materials in these primary batteries.

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Figure 2. Isotropic carbon black structure with crystallinity in the range of nanometers.

In general, carbon black materials are widely used for a variety of components in modern electrochemical energy conversion devices. Three main types of electrochemical energy generation systems can be distinguished depending on the way that energy is stored: batteries store energy within the electrode structure; flow batteries store the energy in the reduced and oxidized species that recirculate through the system; and fuel cells store energy in the reactants externally to the cell. Flow batteries are a special type of rechargeable or secondary battery in which the dissolution of active species in the electrolyte permits external storage of reactants thereby allowing independent scale up of power and energy density specifications. Reactants are circulated through the cell stack as required and their external storage avoids self-discharge, which is observed in primary and secondary battery systems. The first such battery used the couple zinc/chlorine. It was first used in 1884 by Charles Renard to power his airship La France, that contained its own on board chlorine generator [3]. The technology was revived in the mid 1970s, with the redox flow cell of Thaller[4]. Modern redox flow batteries are generally two electrolyte systems in which the two electrolytes, acting as liquid energy carriers, are pumped simultaneously through the two half-cells of the reaction system separated by a membrane [5]. On charging, the electrical energy supplied causes a chemical reduction reaction in one electrolyte and an oxidation reaction in the other. The thin ion exchange membrane between the half-cells prevents the electrolytes from mixing but allows selected ions to pass through to complete the redox reaction. On discharge, the chemical energy contained in the electrolyte is released in the reverse reaction and electrical energy can be drawn from the electrodes. When in use, the electrolytes are continuously pumped in a circuit between reactor and storage tanks. High power batteries are constructed using a multiple stack of cells in a bipolar arrangement. The power rating of the system is fixed and determined by the size and number of electrodes in the cell stacks; however, the great advantage of this system is that it provides almost unlimited electrical storage capacity, the limitation being only the capacity of the electrolyte storage reservoirs. Opportunities for

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thermal management are also facilitated by using the electrolytes as the thermal working fluids as they are pumped through the cells. These facts provide a very seductive argument in favor of flow batteries in preference to conventional secondary (rechargeable) cells. For the same power, flow batteries are typically dimensioned to store five times the energy stored in conventional cells [6]. In redox flow batteries, carbon blacks are mainly used in composite electrodes, which have become one of their key components [7]. The main requirements of suitable materials are stability in the electrolyte, high oxygen and hydrogen overvoltage and good conductivity of the electrochemically active parts of the bipolar electrode [8,9]. Given that the performance of the battery depends strongly on the bulk resistivities of the electrode, the bipolar plate, and the contact resistance between them, much research has been focused on the development of electrode-bipolar plate setups. Carbon-plastic composite electrodes were put forward in the 1980s [10] and have played an important and promising role in the last decades of flow battery research, which has included iron/chromium, bromine/polysulphide, vanadium/ bromine, zinc/bromine, zinc/cerium, and vanadium redox couples. In such structures, graphite felt electrodes are heat bonded to the conductive carbon-plastic composite bipolar plates. Such carbon-plastic composite electrodes are cheap, flexible, and lightweight. Moreover, the penetration of graphite felt fibers into the conductive carbon-plastic composite bipolar plate during the heat bonding process leads to an increase in the contact points of the graphite felt fibers with the bipolar plate. As a result, low contact resistances between graphite felt electrodes and bipolar plates can be achieved for the carbon-plastic composite electrodes even under a small assembly compact force. Because of these benefits, more and more research has been focused on the design and manufacture of carbon-plastic composite electrodes and their related materials. Carbon blacks are also widely used in the development of fuel cell components. In 1842, Grove developed the first fuel cell (which he called the gas voltaic battery) that produced electrical energy by combining hydrogen and oxygen. Grove was interested in reversing the process of electrolysis. In developing the cell and showing that steam could be disassociated into oxygen and hydrogen, and the process reversed, he was the first person to demonstrate the thermal dissociation of molecules into their constituent atoms. The first Grove cell used zinc and platinum electrodes exposed to two acids (sulphuric acid) and separated by a porous ceramic pot. The term "fuel cell" was coined in 1889 by Ludwig Mond and Charles Langer, who attempted to use air and coal gas to generate electricity. In 1932, Francis Bacon improved on the platinum catalysts of Mond and Langer, and soon Harry Karl Ihrig, of AllisChalmers Manufacturing Company demonstrated a 20-horsepower fuel cell powered tractor. NASA began using fuel cells in the late 1950s and continues to do so today. Nowadays, the need to reduce CO2 emissions to the atmosphere from the combustion of fossil fuel is fostering the fast development of H2 fuel cells together with the so-called hydrogen economy. In a typical fuel cell, gaseous fuels are fed continuously to the anode (negative electrode) compartment, and an oxidant (for example, oxygen from air) is fed continuously to the cathode (positive electrode) compartment; the electrochemical reactions take place at the electrodes to produce an electric current. The three major components that constitute the heart of a fuel cell are the fuel electrode (anode), the oxygen electrode (cathode), and the electrolyte. These elements are usually arranged in a stack of single cells connected in series and separated by bipolar plates. The fuel (typically a hydrogen-containing gas) and the oxidant (typically oxygen from air) flow in small channels that are part of the bipolar plate. In

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final cells the end plate also serves for current collection. Because the bipolar plate serves as part of the electron conduction path through the fuel cell stack, it must be electronically conductive, but it must not permit gas permeation. This configuration allows a fuel cell stack to be fabricated with over 400 elements arranged in series. A fuel cell differs from a conventional battery in several respects. The battery is an energy storage device; that is, the maximum energy that is available is determined by the amount of chemical reactant stored in the battery itself. Thus, the battery will discharge and cease to produce electrical energy when the chemical reactants are consumed. In a secondary battery (rechargeable), the reactants are regenerated during charge, which involves putting energy into the battery from an external source. The fuel cell, on the other hand, is an energy conversion device, which theoretically has the capability of producing electrical energy for as long as the fuel and oxidant are fed to the electrodes. In fact, only degradation or malfunction of components limits the practical operating life of fuel cells. Two major types of fuel cells rely heavily on carbon. They operate at relatively low temperature (lower than 200ºC) and are referred to as the phosphoric-acid fuel cell (PAFC) and the polymer-electrolyte-membrane fuel cell (PEMFC). The primary interest in PAFCs is for stationary power generation, whereas PEMFC's have attracted widespread interest for use in transportation applications. Without the availability of carbon as a reasonably stable and electronically conductive component, both the PAFC and PEMFC would be even further from commercialization. The catalysts, the electrodes and the bipolar plates in these fuel cells usually contain highly conductive carbon blacks. The electrochemical reactions to oxidize the fuel (hydrogen) and to reduce the oxidant (oxygen) to generate electricity require costly catalysts usually based on platinum. In order to minimize the amount of expensive metal electrocatalyst that is used, it is of major interest to increase the surface area of platinum available for reaction. This is achieved by preparing highly dispersed metal nanoparticles on a conductive high surface area support, usually carbon black. Figure 3 shows a transmission electron microscopy (TEM) image of a typical fuel cell electrocatalyst. The platinum particles, which are around 3 nm in diameter, are evident as the solid black spots dispersed over the surface of larger and lighter particles of carbon black, resulting in a surface area close to 100 m2·g-1. The surface area of the loaded platinum is inversely proportional to the mean Pt crystallite size. Preparing such high-surfacearea-supported electrocatalysts and maintaining the small particle size in the fuel cell are major technical challenges to increase Pt utilization to values close to 100%. Electrodes in fuel cells also rely on carbon blacks as base material for synthesizing conductive hydrophobic microporous layers able to manage the water flow [11]. In addition to the catalytic layer, the gas diffusion layer (GDL) is a key component of a PEM fuel cell electrode that fulfills several functions: a.

reactant gas permeability: providing access for reactant gases from flow-field channels to catalyst layers; b. liquid permeability: providing paths for product water to be removed from the catalyst layer area to flow field channels; c. electronic conductivity: providing passage for electron transport from bipolar plates to catalyst layers; d. heat conductivity: providing efficient heat conduction between bipolar plates and MEA; and

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mechanical strength: providing mechanical support to the MEA.

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Figure 3. Transmission electron microscopy image of a typical electrocatalyst for proton exchange membrane fuel cells: Pt (50 wt.%) on Vulcan XC72.

These functions, especially the interfacial electrical and thermal conductivities between the bipolar plates and catalyst layers, depend significantly on the GDL compression behavior. GDLs are usually formed by a carbon fiber substrate, such as carbon cloth or carbon paper, and a microporous layer (MPL). The micro-porous layer (MPL) consisting of a mixture of carbon black powder and a hydrophobic agent is utilized on one side or two sides of the GDL. This layer is the crucial part in GDL to improve the fuel cell performance as it acts both as a valve that pushes water away from the GDL to the flowfield to minimize water flooding and also transports the input gas from the flow field into the catalyst layer. So it is one of the bottlenecks in fuel cell developments, and many R&D activities during last decade have accomplished amazing achievements in this area [12]. It is an industrial practice to refer to this MPL as a carbon sub-layer. Acetylene black, Black Pearls 2000 or Vulcan XC72R have been widely used materials for this application [13]. Other important components of the PEMFC stack are the bipolar plates [14]. They perform a number of functions including: a) provide series of electrical connections across adjacent cells in the fuel cell stack; b) support intricate gas flowfield design pattern which help in directing and uniformly distributing the reactant gases to the membrane electrode assembly and at the same time allow byproduct water to leave the cell c) act as separator plates in between different cells, thereby separating the fuel on one side from coming in direct contact with the oxidant on the other side; d) help in thermal control of the cell stack via heat removal from the active area; and e) provide the basic support infrastructure for the MEA in the PEM fuel cell stack.

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A high proportion of the fuel cell weight is due to the bipolar and end plates. Therefore, weight reduction is a very important factor, especially for mobile applications. Composite materials are lightweight and can be molded into any shape and size [15]. Thermoplastic polymer-carbon black composites are an interesting alternative to fulfill the requirements of bipolar plates for PEMFC stacks: i.e., specific electrical conductivity higher than gas diffusion electrode (2.5 S cm−1), hydrogen permeability equal or below the range of the permeability of the ionic conducting membrane, thermal and mechanical stability at working conditions, low density, high corrosion resistance, low thermal expansion coefficient, reproducibility, easy finishing and recyclability. Coatings of metal separators with carbon/resin composite layers have been also proposed as viable alternative to produce high performance lightweight bipolar plates [16]. Carbon blacks are not only used as base material for fuel cell components. Another main use includes its application as fuel in high-operating-temperature fuel cells such as solid oxide fuel cells (SOFCs) and molten carbonate fuel cells (MCFCs). Fuel cells able to operate with carbon as fuel are designed as direct carbon fuel cells (DCFCs). In a DCFC, complete oxidation of carbon to gaseous CO2 is accompanied by almost no entropy change, and means that the thermodynamic efficiency is close to 100%. In conventional fuel cells, such as SOFCs fueled with gaseous hydrogen, the entropy variation is negative and the theoretical efficiency is reduced. Hydrogen fueled SOFCs, for example, have a theoretical efficiency limit of 70% at high temperatures – i.e., about 70% of the chemical energy converts to electricity, with the remaining 30% converting to heat. In contrast, virtually all the chemical energy can convert to electrical energy in a DCFC. The DCFC concept is not new. William W. Jacques, a US electrical engineer and chemist, described a DCFC in 1896 [17]. Jacques used coke electrodes in a molten NaOH electrolyte. There were problems, such as including the cost of making carbon electrodes and electrolyte degradation as a result of ash in the fuel and interactions between the fuel and the electrolyte. In addition, the carbon anode also served as the fuel source, and the NaOH electrolyte was consumed in the reaction when it reacted with the CO2 product to form Na2CO3. Because an operator would need to shut down the entire device periodically to refuel and replace the electrolyte, it ran as a battery and not as a fuel cell. In practical terms, the cell was expensive (in part because of the high cost of NaOH). Furthermore, when the energy required to make NaOH is taken into account, the cell was very inefficient. Researchers had trouble duplicating Jacques‘ results until the 1970s, when a series of studies at SRI International (Menlo Park, California) verified that complete electrochemical oxidation of carbon can generate electricity. Although the SRI group proved the concept and added several innovations to the DCFC approach, the system was still impractical. In the 1990s, several other organizations began working on DCFCs and related areas. The latest advances in materials science, fuel cells and basic electrochemistry have given researchers hope that the technology will be commercially viable in limited small-scale applications within five years, and in large-scale applications within 20 years [18]. As in the rest of the applications, research has revealed that the carbon properties affecting DCFC performance include crystallographic disorder, electrical conductivity, contaminants and number of surface reactive sites [19].

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Figure 4. Main electrochemical energy conversion devices using carbon blacks in their manufacture.

These applications illustrated in Figure 4 show the relevant role of carbon blacks in the development of electrochemical devices such as galvanic cells, redox flow batteries, and fuel cells. The structure, morphology and surface chemistry of this carbon material determine numerous physical properties on which the electrochemical performance relies. This chapter will be focused in the application of carbon blacks in proton exchange membrane (PEM) fuel cell components, especially in catalytic and gas diffusion layers of electrodes and in bipolar plates.

CARBON BLACKS IN GAS DIFFUSION ELECTRODES FOR PEM FUEL CELLS Gas diffusion electrodes are important components in PEMFCs thatrely heavily on carbon blacks. They are integrated by a carbon fibermacroporous substrate (usually carbon cloth or carbon paper) on which a first layer composed of carbon black and polytetrafluoroethylene (PTFE) is deposited (Figure 5). PTFE is usually added to the carbon

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fiber support and to the microporous layer (MPL) to increase the hydrophobicity of the electrode and improve water management and gas accessibility to the catalytic layer.

Figure 5. Cross-sections of a) a gas diffusion substrate of carbon paper with PTFE treatment, and b) the hydrophobic substrate in image a with a microporous layer of carbon black and PTFE.

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In addition to the functions of mechanical support to strengthen the MEA and providing paths for the electrical and thermal conductivity, the GDL acts as a valve to regulate water ejection, avoid flooding and guaranteeing gas accessibility. Although the diffusion layer is seemingly a minor component in a fuel cell, it has been shown that altering the composition of the diffusion layer can lead to substantial improvements in the performance of the cell [20]. These improvements are related to the PTFE content, the effect of sintering the microporous layer, its thickness, and the properties of the carbon black used in its fabrication [21]. The final permeability of the gas diffusion layer, which depends on all those structural parameters, has a large impact on the cell performance. This influence can be appreciated in Figure 6, in which the polarization curves obtained for MEAs with GDLs of different characteristics and air permeability are shown.

Figure 6. a) Polarization curves obtained for PEM single cells with carbon catalyzed membranes using gas diffusion layers in anode and cathode with different characteristics; b) correlation between the cell performance and the permeability of the gas diffusion electrodes used [22].

The surface chemistry of the carbon black will determine its hydrophilic/hydrophobic properties, which are essential for the microcroporous layer synthesis. The addition of low PTFE loadings as hydrophobic agent to the microporous layer will contribute to reduce the GDL resistivity. Carbon black is not only used to synthesize microporous layers but also to disperse the electroactive metals in the catalytic layer. The state-of-the-art PEMFC electrocatalysts typically consist of Pt nanoparticles (2-8 nm) or Pt alloys dispersed on a high surface support material which provides a good combination of electron conductivity, corrosion resistance,

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surface properties, and low cost [23,24]. Pt loadings in these catalysts are usually in the range between 20 and 60 wt %. A large variety of carbon blacks and numerous carbon materials including carbon aerogels, cryogels, high surface area graphites, single and multiwall carbon nanotubes, graphitic carbon nanofibers, carbon nanohorns, carbon nanocoils and ordered uniform porous carbon networks have been used as supports to synthesize them [25−34]. Despite of the numerous attempts at looking for materials providing high performance catalysts with low corrosion rates, the traditionally used Vulcan XC-72 carbon black (CB) (Cabot Corp.) remains as the most popular and extensively used material because of its superior behavior in catalysts for PEMFC electrodes [31,35−39]. Vulcan XC-72 CB and PEMFC catalysts prepared on it have been extensively studied in electrochemical devices [40−54] but just a few studies deal with their surface properties[55,56]. Few papers can be found regarding the particularities of the surface chemistry of this CB or its composites [57−60] dealing with the effects that the surface functional groups confer to the Pt nanoparticles in the surroundings, or analyzing the Ptsupport synergy, especially in catalysts with high metal loadings [24,61−66]. Most of these studies are mainly focused in oxygen containing groups [61,66,67] and little attention is paid to surface species containing other heteroatoms [56,68] and are relevant for some catalysts properties, such as their activity and durability. A number of reviews related to the degradation mechanisms affecting the activity and durability of carbon supported Pt catalysts in PEMFC electrodes have been recently published [24,63,66,69−73]. In general, the strategies proposed for improving catalyst durability involve increasing the graphitization degree of the supports, modifying their surface chemistry by building proper surface functional groups, increasing the density of basic sites on carbon supports to enhance the Pt–C interaction, increasing surface stability of carbon support, improving the hydrophobicity of the carbon support through proper surface treatment and preparing catalysts with high platinum uniformity and low platinum load. As a matter of fact, some new trends in PEMFC catalysts research follow these strategies by applying sulfonation treatments [74−77] or modifying the carbon surface with sulfonated polymers [65,68,78,79]. The introduction of surface functional groups on the carbon surface induces properties and characteristics in the supports and the catalysts that must be carefully analyzed, especially regarding their thermal stability [55,66] and their behavior under different atmospheres [67,80,81]. Carbon functional groups like carboxyls, lactones, anhydrides, carbonyls, phenols, quinones or lactones do not exert, in general, a beneficial effect on PEMFC catalysts [66,82]. Anchoring of the metal precursor is improved in the presence of oxygen containing groups during the catalyst synthesis process, but some drawbacks must be considered. They can both decrease the conductivity of catalysts and weaken the interaction between the support and the catalytic metal nanoparticles resulting in an accelerated metal sintering. Furthermore, they contribute to increased hydrophilicity in the electrodes with the subsequent repercussion on the water and gas transport mechanisms. There are some scarcely explored issues on this type of catalysts such as the interaction of probes with surface groups and the thermal stability of surface species that will be analyzed here on the base of desorption, adsorption, and exchange experiments.

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Surface Chemistry of the Vulcan XC-72 Carbon Black By analyzing the thermal desorption profiles of a pristine sample of the Vulcan XC-72 under vacuum, some particular characteristics can be found in this carbon black (CB) with its support surface groups. Figure 7a shows the temperature programmed desorption (TPD) curve obtained from the pressure increase in a vacuum chamber resulting from the decomposition of surface species when heating a Vulcan XC-72 sample up to 773 K. Three main desorption processes can be distinguished between 600 and 753 K. The ion current profiles registered by mass spectrometry for the most significativem/z ratios contributing to the global pressure increase are presented in Figure 7b. At the lowest temperature, ion currents corresponding to species like CFx+ (m/z=19, m/z =31, m/z =50, m/z =69), CHy+ (m/z =15), H2+ (m/z =2), CO+ (m/z =28) and COF+ (m/z =47) increase. The ion current intensity for m/z=19, that can be assigned to fluoride ions, contains an important contribution from H2O (m/z=18) as secondary mass. The differences existing between the profiles for masses 18 (H2O+) and 19 (naturally occurring isotopes of H2O+, H3O+, F+), concur with the increase of ion current intensity for the mass 31, to confirm the presence of fluoride species. The formation of a more intense peak at ca. 660 K (S-II in Figure 7a) corresponds mainly to H2O release (m/z =18), although there is also a contribution of CO (m/z =28) and some sulfurcontaining compounds, such as COS (m/z =60). The third peak (S-III in Figure 7a) is mainly due CO2 and to SO2 with contributions of several ion currents (m/z =64, m/z =48, m/z =50, m/z =32). The spectral IR features obtained for the Vulcan XC-72 CB in the range between 2000500 cm-1 confirm the presence of fluorine- and sulfur-containing species (Figure 8). IR bands corresponding to CFx species and sulfonic groups can be assigned by comparison with those reported in the literature for fluoride carbon materials [83,84,97,98] and fluoropolymers [85−93]. Two main regions with bands between 750-500 cm-1 and 1300-900 cm-1 can be attributed to characteristic IR absorption modes for CFx species and sulfonic groups. Symmetric and asymmetric stretching bands of C-F bonds appear at ca. 1154 and 1230 cm-1, respectively. Several vibration frequencies of oxygen containing groups probably present in the carbon black surface also lay in this region. These species are probably contributing to poorly defined peaks in overlapped bands. The absorption peaks lying in this frequency range can be ascribed to –SO2OH groups with characteristic vibration modes at 970, 1150, 1230, and 1425 cm-1[93]. No clear bands can be assigned to the asymmetric stretching of –SO3- in the 1300-1320 cm-1 region[94,95]. However, their presence cannot be completely ruled out because of the numerous low intensity absorption bands in the spectrum. According to the literature, when a substituent, C=O, is directly conjugated to aromatic rings, a doublet is observed at 1625-1575 cm-1[96]. Based on this consideration, the strong band centered at ca. 1590 cm-1 in Figure 8 can be assigned to quinone groups in the Vulcan XC-72[97,98]. Aromatic C=C stretching vibrations such as naphthalene- or anthracene-like structures also lie in the same region resulting in bands within 1620-1580 and 1550-1505 cm-1. Bands corresponding to the different CHx absorption modes can be also identified at 1465, 1380 and 720 cm-1 together with the C-H stretching vibrations in the low-frequency region between 2950 and 2850 cm-1. The bands at 2926 and 2952 cm-1 correspond to the asymmetrical C-H stretching modes of -CH2 and -CH3 groups respectively, and that centered at 2856 cm-1 to their symmetrical C-H stretching modes. The strong and broad band at ca. 3428 cm-1 in Figure 8 reveals the presence of adsorbed water. In addition to the H2O stretching vibration,

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the contribution due to its bending mode is probably superposed to the bands previously assigned to quinone groups in the region of ca. 1590 cm-1.

Figure 7. Temperature programmed desorption profiles for the pristine Vulcan XC-72 carbon black under dynamic vacuum: a) pressure increase in the analysis chamber; b) ion current profiles corresponding to the mass spectrometric analysis of desorbed species.

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Figure 8. FTIR spectra obtained for the pristine Vulcan XC72 carbon black.

XPS analysis of the CB indicates a surface composition corresponding to C, O, and S in atomic proportions of 99.3, 0.5, and 0.2%. Fluorine has not been evaluated due to a very weak signal partially masked by the noise. Its low concentration in combination with a defluorination process induced by the X-ray radiation can be some of the reasons making its detection difficult. Halogen-containing compounds have been found to be especially sensitive to X-ray exposure, resulting in a decrease in the halogen peak intensity and an increase in the C 1s signal [99−102]. Figure 9 shows the signals in the binding energy regions corresponding to C 1s, O 1s,and S 2p. The main C 1s signal from the Vulcan XC-72 is observed at 287.4 eV (74.1%) probably due to C-C/C-H bonds. A broad contribution at higher binding energy was deconvoluted into four contributions at 286.1(13.9%), 287.4 (4.6%), 289.2 (3.6%), 291.0 (3.8%) eV which can be assigned to –COH, –C=O, –COOH or –CF, and CF2 groups, respectively [100−102]. The O 1s signal could be fitted into four components located at approximately 532, 533, 534.5, and 535.2 eV, which could be attributed to –C=O, –C–O–, –O–C=O, –SO3- type groups, respectively [102]. Although the signal-to-noise ratio was low at the S 2p region, at least two main contributions were identified: the main one at 164.7 eV and other at 170.4 eV. The signal has been fitted to Gaussian line shapes. For the resolution of each S 2p1/2-3/2 doublet it was required the intensity ratio of the two components to be 1:2 and their widths to be the same. Poleunis et al.[56] have attributed the most intense S 2p signal at 164.7 eV to thiolate groups (–SH) on Vulcan XC-72. The contribution of sulfur atoms with double bonds to carbon (S=C=S) with typical binding energies at 164 eV cannot be excluded [103]. It is well known that vulcanization processes in carbon blacks involve reactions with sulfur leading to the formation of S=C=S species.

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Figure 9. XPS spectra of the Vulcan XC-72 CB in the C 1s, O 1s,and S 2p regions.

The signal at 170.4 eV can be assigned to sulfonic groups (–SO2OH) [77,104,105]. For the best fit of the S 2p1/2-3/2 doublets, a third contribution was found at a binding energy of 166.2 eV, which could be associated to a sulfite state. It has been reported that sulfur mutates from the sulfonic acid group configuration to a sulfite state induced by irradiation, by X-ray, or ions [104].

Surface Chemistry of the Catalyst: Pt/Vulcan XC-72 The species resulting from the decomposition of the CB surface groups in the presence of Pt nanoparticles have been analyzed by mass spectrometry in an analogous way. The desorption takes place in at least four well defined processes (peaks C-I – C-IV) between 370 and 740 K as revealed in the pressure variation profile in Figure 10a. The most defined and intense peak is the third one (C-III) centered at ca. 610 K. The most significantcontributions

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to the TPD curve is from the result of the catalyst, as revealed by mass spectrometry, and presented in Figure 10b.

Figure 10. Temperature programmed desorption profile for the Pt/Vulcan XC-72 catalyst under dynamic vacuum: a) pressure increase in the analysis chamber; b) ion current profiles corresponding to the mass spectrometric analysis of desorbed species.

A detailed analysis of a number of m/z ratios and their profiles can determine the nature of the main chemical species released at each temperature: the C-I peak (Figure 10b) is mainly due to the release of H2O (m/z =18); peaks C-II and C-III can be assigned to the simultaneous evolution of CO2 (m/z =44, m/z =28), SO2 (m/z =64, m/z =32) and COS (m/z =60, m/z =28), and finally, the broad C-IV peak corresponds to the formation of CFx (m/z =31) and CHy (m/z =15) species, H2 (m/z =2) and CO (m/z =28). All these species have been previously detected during the temperature programmed desorption of the support (Figure 7b), but their decomposition profiles differ substantially in the presence of Pt nanoparticles. The IR features of the catalyst in its pristine form and after treatment in vacuum at 448 K are presented in Figure 11. The main IR absorption bands present in the support are also found in the IR spectrum of the catalyst. However, there is a general change in its profile,

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which is probably the result of the change in the proportion of sulfur and fluorine containing groups after Pt incorporation. This change is even more evident when treating the catalyst under vacuum at 448 K. According to the thermal desorption curve in Figure 10, the treatment of the catalyst at that temperature just leads to the release of most of the adsorbed water without influencing the decomposition of other surface species. As a matter of fact, the broad absorption band in the region ca. 3400 cm-1 due to the fundamental stretching vibration of water becomes dramatically reduced after heating the catalyst at 448 K (spectrum b in Figure 11). In the region 500-1650 cm-1 numerous absorption bands attributable to vibration frequencies of fluorocarbon species, sulfonic and carbonyl groups can be observed. Absorption modes attributable to –SO2OH groups with characteristic vibrations at ca. 970, 1150, 1230 and 1425 cm-1 are probably overlapped with other IR vibrations corresponding to C-F and C-O-C bonds laying in the same range and leading to a broad absorption band in that region [ 93−95].

Figure 11. FTIR spectra obtained for the pristine Pt/Vulcan XC-72 catalyst as received (a) and after its treatment under vacuum at 448 K (b).

Adsorption-Exchange Properties of the Pt/Vulcan XC-72 Catalysts The adsorption properties of the Pt/Vulcan XC-72 catalyst can be evaluated by means of different procedures using H2 and CO as probe molecules. The influence of the support surface groups on the adsorption processes can be analyzed on catalyst samples subjected to different thermal treatments. Exchange experiments using isotopic labeled molecules allow the discovery fordetermining the surface groups that support the adsorptive properties of the catalyst.

A. H2 Chemisorption on Pt/Vulcan Xc72 The application of a static volumetric method of adsorption to a commercial Pt/Vulcan XC72 catalyst (BASF) leads to variable hydrogen uptakes at 298 K, depending on the thermal treatment of the sample. H2 uptakes obtained on the pristine catalyst after being treated under vacuum at temperatures as low as 423 K correspond to atomic ratios H/Pt close to 2.0, where

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Pt is referred to the total amount of Pt in the catalyst. This large ratio reveals that adsorption phenomena induced by groups on the support surface are taking place simultaneously with the hydrogen chemisorption on the metal surface. Those processes interfere with H2 chemisorption measurements and make the determination of the surface Pt difficult. The contribution of parallel adsorption phenomena masking the authentic H2 chemisorption on the surface Pt has been minimized by applying increasing pretreatment temperatures in pristine samples and/or successive H2/O2 titration cycles to oxidize only Pt-chemisorbed H2. As shown in Figure 12, both procedures are effective to reduce H2 uptakes to a constant value of 0.24 moles of H2 per mol of Pt in the catalyst. The Pt dispersion, defined as the percentage of surface Pt related to the total Pt in the catalyst, is 48%. By assuming that the Pt particles have a spherical geometry and that their surface is formed by equal proportions of the main low index planes [106], an average Pt particle size of 2.4±0.2 nm has been estimated.

Figure 12. H2 uptake per mol of Pt for the Pt/Vulcan XC72 catalyst as a function of the number of cycles of H2 adsorption/O2 adsorption and the temperature of treatment under vacuum before H2 adsorption.

In order to verify the validity of that measurement as the actual H2 chemisorption on Pt, the average Pt particle size estimated from the H2 uptake could be compared with the values obtained from the pristine catalyst using XRD and TEM techniques. Both, the analysis of the broadening of the X-ray diffraction line corresponding to Pt (220) plane (2=65.7) and the analysis of the particle size distribution from TEM micrographs, yield mean crystallite sizes slightly above that estimated from the H2 chemisorbed amount. By analyzing the broadening of the Pt X-ray diffraction lines, the estimated volume average diameter is 2.7±0.3 nm. For the statistical determination of the mean Pt particle size from TEM micrographs; numerous samples representative of the catalyst, accounting for several hundred of particles have been examined. The mean particle size defined as the median (D50) of the distribution is close to 3.5 nm.The somewhat larger average particle size obtained by TEM is probably the consequence of the poorer representation of the smallest particles in micrographs, which usually leads to overestimate the average diameter (Figure 13). By using the XRD technique, the estimated values can be affected by a significant error in catalysts with a large proportion of very small Pt particles or where is ainhomogeneous particle size distribution. However, in

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this case, there is a very good agreement among the three applied techniques for the Pt mean diameter estimations.

Figure 13. Particle size distribution analysis for a commercial Pt/Vulcan XC-72 catalyst estimated for TEM micrographs.

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B. CO Chemisorption on Pt/Vulcan Xc72 CO uptakes can be also used to determine the surface area of fresh catalyst samples after their thermal treatment under vacuum. Using different evacuation temperatures, a constant value of 0.22 mol CO/molPt is obtained independently from the applied pretreatment. According to the obtained values for the monolayer coverage using H2 and CO as chemisorption probes, the resulting CO/H stoichiometry is close to 0.5. This ratio is lower than the typical 0.7 saturation limit usually found in the literature for supported Pt particles with low or no interaction with the supports showing adsorption surface properties of Pt(111) [107]. Although the CO uptakes seem to be unaffected by the presence of the support surface groups, the CO adsorption isotherms reveal the influence of the support surface chemistry on the adsorption of CO on the Pt crystallites dispersed on it. A detailed analysis of the CO adsorption isotherms on catalyst samples evacuated at different temperatures indicates that, as far as the support surface is progressively modified at increasing pretreatment temperature (mainly by dehydration), the CO isotherm profile changes (Figure 14). Although CO adsorption on Pt does not meet the premises of the Langmuir‘s adsorption model (i.e., monolayer coverage, adsorption site equivalence and no interaction among adsorbate molecules), its simplicity and strong theoretical reasoning makes it a good approach to describe the obtained isotherms. By considering that the curves approach the ideal model of a type-I Langmuir isotherm at low equilibrium pressures, the CO adsorption strength related to the adsorption constant in the Langmuir equation [CO= (·peq)/(1+·peq)] increases as far as the support surface groups are dehydrated at increasing pre-treatment temperatures.

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Figure 14. CO adsorption isotherms on the Pt/Vulcan XC-72 catalyst after its evacuation at (■) 423 K, ( ) 648 K.

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C. The Role of Surface Groups in Adsorption on Pt/Vulcan Xc72 Adsorption results shown in Figure 12 indicate that a parallel H2 adsorption process takes place simultaneously with H2 chemisorption on Pt when the catalyst has been evacuated at temperatures as low as 423 K. To enlighten the nature of this process, its mechanism, and the species involved in it, two types of experiments have been carried out: isotopic exchange under steady-state conditions and temperature programmed desorption tests after D2 adsorption, CO adsorption and CO-D2 co-adsorption [108]. The ability of the catalyst thermally treated at 448 K under vacuum to exchange hydrogen species from its surface with molecular hydrogen gas has been studied by mass spectrometry using D2 as probe molecule for adsorption. The extent of the surface hydrogen mobility process has been evaluated by analyzing the composition of the gas phase during the contact time between pure D2 gas and the Pt/Vulcan XC-72 catalyst previously treated under high vacuum at 448 K. The analysis of the gas phase composition reveals how the ion current for D2 (m/z=4) decreases whereas those for HD (m/z=3) and H2 (m/z=2) ratios increase progressively when D2 is circulated over the catalyst at 298 K. The observed hydrogen exchange rate is presented in Figure 15 together with the ratio of exchanged atoms per surface Pt center. The exchange rate has been expressed as a turnover number by referring the amount of D exchanged atoms to the Pt surface centers in the catalyst. Pt surface centers have been estimated from the H2 chemisorption measurements performed on the catalyst. As shown in Figure 15, two processes taking place at different rates can be distinguished during the exchange test. The exchange turnover frequency is very fast during the initial period of contact (about 5 min) and then it reaches a steady-state with a constant value of 4.9 h-1. By analyzing the molar ratio of exchanged D per surface Pt, it can be observed that this ratio approaches the unity during the first step. This indicates that the catalyst exchanges practically the whole monolayer of initially Pt-chemisorbed deuterium with the closest surface groups able to exchange hydrogen species. Therefore, the first process after contacting the catalyst with D2 gas can be attributed to a relatively fast exchange between the firstly chemisorbed deuterium monolayer on nanometricPt particles and the proton acceptor

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species on the support located closely at their periphery. After that first step, the exchange rate is observed to decrease to a value which probably corresponds to the proton diffusion rate on the carbon black surface at 298 K.

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Figure 15. Deuterium-hydrogen exchange frequency (exch) (solid line) and amount of moles of exchanged D atoms per mol of surface Pt (dashed line) in the Pt/CB catalyst as a function of contact time between the gas and the sample at 298 K. The catalyst has been previously treated under dynamic vacuum at 448 K for 1 h.

This deuterium-hydrogen exchange process is often observed on catalysts with proton acceptor groups on the support surface. It has been previously shown that hydrogen uptakes measured on this catalyst after vacuum treatment at soft temperatures largely surpass the Pt monolayer coverage. This enhanced hydrogen adsorption is usually known as a spill-over, and is usually ascribed to surface diffusion phenomena by hydrogen species on the support surface [61]. This effect has already been reported for fuel cell catalysts [61,109,110]. Ramirez-Cuesta et al. have provided evidence of this phenomenon by means of inelastic neutron scattering spectroscopy on Pt/C, Ru/C, and PtRu/C [110,111]. The contribution of spilt-over hydrogen to adsorption measurements can be eliminated by applying successive adsorption cycles of H2 and O2 until reaching steady-state H2 coverage. This procedure allows saturating surface sites promoting spill-over and just evaluating surface Pt centers. The particular adsorption properties of Pt/Vulcan XC72 are induced by surface functional groups in the carbon black promoting hydrophilic and hydrophobic characteristics in its surface [61,112,113]. The adsorption of D2 and CO as probe molecules has been carried out on catalyst aliquots previously evacuated at 448 K in order to analyze their desorption profiles. After treatment in vacuum at that temperature, the D2 adsorption at 298 K includes a large contribution of spiltover species. D2 uptake increases from 1.24 mmol·gPt-1 (when no surface diffusion phenomena are involved in the measurement) to 3.95 mmol·gPt-1 for a pristine sample treated under dynamic vacuum at 448 K for 1 h. On the other hand, the CO uptake for identical conditions is 1.08 mmol·gPt-1, which corresponds to a coverage close to 50% of a monolayer. An additional adsorption test was carried out on a CO-equilibrated sample. After saturating its surface with CO at 298 K, the gas phase was removed and then, D2 was dosed. Under these conditions, the measured D2 uptake corresponded to 2.05 mmol·gPt-1, and indicate that the

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presence of CO does not block the hydrogen diffusion pathways between the metal and the support surface groups. Following each adsorption test, a temperature programmed desorption experiment was performed at 10 K/min while analyzing the desorbed species. The pressure in the system was registered during the experiments. Figure 16 presents the global desorption profiles obtained from the catalyst after D2 adsorption, after CO adsorption and after CO-D2 co-adsorption. The curves resulting from the temperature programmed heating of pristine samples of the catalyst and its CB support under vacuum (without any chemisorbed probe) are also included as references. Several striking features can be appreciated in the resulting profiles. The broad desorption bands obtained for the D2-equilibrated sample evidences that only a part of the adsorbed hydrogen is on the Pt surface. The Pt-chemisorbed hydrogen originates a low temperature desorption peak (below 373 K). Large differences can be also found between the profile obtained from the sample in which CO and D2 are co-adsorbed and the profiles for the sample equilibrated with CO or D2 separately. The main desorption peak in the co-adsorption curve appears at much lower temperature and the peak corresponding to the Pt-chemisorbed hydrogen/deuterium disappears.

Figure 16. Chamber pressure due to desorbed products during temperature programmed desorption tests in: a) a pristine sample of Vulcan XC72, b) a pristine sample of the Pt/Vulcan XC72 catalyst, c) D2equilibrated sample, d) CO- equilibrated sample, e) D2 adsorbed over a CO-equilibrated sample. Adsorption of probes has been always performed on samples previously evacuated at 448 K.

The mass spectrometric analysis of the desorbed species during the TPD tests renders a clearer picture of the surface phenomena taking place in the catalyst after the adsorption of these probes. Figure 17 compiles the ionic current curves obtained in each case for the formation of labeled water and SO2 (Figure 17A), labeled molecular hydrogen (Figure 17B) and carbon oxides (CO and CO2) (Figure 17C). It can be observed that water desorption above 373 K is in all cases associated to the release of SO2, which proceeds from the decomposition of sulfonic groups. It must be noted that, when no adsorbed probe is present on the catalyst, the SO2 release curve presents three contributions above 448 K with a predominant maximum at ca. 600 K (Figure 17). This profile is drastically modified after adsorbing CO, D2 or both. Adsorbed CO causes sulfonic groups to reduce their main

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decomposition maximum to 507 K (Figure 17A-III). This process is also associated to the formation of large amounts of CO2 (Figure 17C-III) instead of the reversible release of the adsorbed probe (CO). This evidence of interaction by the adsorbed CO molecules with the support hydrophilic sulfonic groups, contribute to oxidize them before being desorbed. Just a small amount of H2 resulting from the decomposition of some CHx species in the support is detected at temperatures above 500 K (Figure 17B-III). A similar H2 evolution profile can be observed in the TPD test for the pristine catalyst (Figure 10). The analyses of the labeled species desorbed from the D2- and the D2-CO-equilibrated samples reveal several important details on the interaction of the adsorbates with the catalyst surface. In the D2-equilibrated sample, the release of molecular hydrogen isotopes confirms the spill-over phenomenon (Figure 17B-II): three main desorption bands are observed, being the main contribution that of H2 (m/z=2). Hydrogen desorption taking place at low temperature (below 373 K) is usually associated to dissociated hydrogen species chemisorbed on the Pt surface: the most weakly adsorbed species are released as D2 (m/z=4) and HD(m/z=3), whereas H2 is also formed from Pt-adsorbed species resulting from the exchange with hydrogen from the support surface groups. The desorption peak appearing within the 400-500 K range arises only from H2, showing no contribution of the heavier isotopes. It probably results from the recombination of hydrogen species in direct interaction with sulfonic groups, which are released simultaneously with water and SO2. The third peak appearing above 600 K is typical from hydrogen spilt-over species in interaction with the support surface in supported metals. Although the ionic current for mass 2 (H2) is the main contribution to that peak, the ionic current increase for mass 3 in that temperature range reveals the presence of labeled species remaining on the support, that are recombined at high temperature to be released as HD. The water desorption profiles for the D2-equilibrated sample indicate that the spill-over phenomenon and the presence of water as proton acceptor are intimately related, since most of the adsorbed deuterium atoms are finally released in two main peaks in the form of isotopic labeled water (Figure 17A-II). These peaks concur with the molecular hydrogen desorption processes at low and medium temperatures, being D2 and HD their main contributions. It must be noted that there is some CO and CO2 desorption (Figure 17C-II) resulting from the decomposition of surface groups, as previously seen in Fig. 4 for the TPD test in the pristine catalyst. Although the CO2 release takes place simultaneously with that of SO2 in the absence of adsorbates, these processes are decoupled after adsorbing D2 on the sample. The TPD test on the catalyst where D2 has been adsorbed over the CO-equilibrated sample provides new evidence on the hydrogen spill-over process. The profiles for molecular hydrogen isotopes indicate that the Pt surface is able to catalyze the D2 dissociation and diffusion towards the support, even when the Pt surface sites are blocked for hydrogen adsorption on it (Figure 17B-I). Thus, molecular hydrogen evolution is just observed above 373 K. Only H2 is observed in the intermediate range 373-500 K, whereas H2 and HD appear above 600 K. The absence of any molecular hydrogen formation below 373 K indicates that the Pt surface is occupied by adsorbed CO at those temperatures. Regarding the competitive adsorption of H2 and CO, Meland and Kjelstrup[114] have found similar results in a study about the influence of CO poisoning of the anode in a PEMFC using electrochemical impedance spectroscopy. They observed that although CO hinders hydrogen access to the

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surface, by occupying it, and by making it more polarizable, the proton hydration step to diffuse towards the membrane is relatively unaffected by its presence.

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Figure 17. Ionic currents obtained for different masses during the temperature programmed desorption tests in samples of the Pt/Vulcan XC72 catalyst equilibrated with: I) CO and D2 , II) D2, and III) CO. Curves in A: ionic currents for m/z 20 (D2O (_____)), 19 (HDO (_ _ _ _ )), 18 (H2 (· · · ·)) and 64 (SO2 (·-·-·-··-·)). Curves in B: ionic currents for m/z 4 (D2 (_____)), 3 (HD (_ _ _ _ )) and 2 (H2 (· · · ·)). Curves in C: ionic currents for m/z 28 (CO (_____)) and 44 (CO2 (_ _ _ _ )).

In the TPD obtained after co-adsorption, HDO, D2O, and H2O are formed simultaneously to H2 at intermediate temperatures. This indicates that most of the adsorbed D2 is finally released as water (Figure 17A-I). Adsorbed CO is released mainly as CO2 in a peak with a maximum at 460 K, which concurs with the most intense contribution for water and SO2 formation. At temperatures above 530 K water desorption is practically depleted and CO2 formation decreases in favor of CO [61]. An issue requiring special attention is the lability of the Vulcan XC72 surface sulfur species, which experiences a significant change depending on the adsorbates on the catalyst. It is worthy to note that at least three types of sulfur species with different stability are observed to contribute to the ion current curves for mass 64 in the several TPD tests performed for the pristine sample and those equilibrated with CO, D2, or both. A comparison of the SO2 release curve in the D2-CO-equilibrated sample (Figure 17A-I) with those corresponding to the tests after D2 or CO adsorption (Figure 17A II and III) evidences the modification of the stability of the surface sulfur species formed. The main difference lies in the decrease of the SO2 release temperature. Although its formation remains always associated to water removal, the SO2 appearance begins at temperatures below 373 K when D2 and CO are co-adsorbed.

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It is clear from the results that the adsorbed probes have a major interaction with the sulfur surface groups. They also probably interact with the fluorine containing species attached to the Vulcan XC-72. By analyzing the ion current for mass 31, attributable to the CF+ fragment (see Figure 10), the fluoride species release could be also followed. In the case of the D2/CO co-adsorption, the release temperature of those fluorinated groups is observed to decrease by 245 K yielding a peak for the CF+ fragment at 440 K (not shown). The low decomposition temperature for sulfur surface groups in the presence of hydrogen and CO has several implications in the stability of catalytic layers during fuel cell operation. CO contamination of the electrocatalyst is a potential cause of degradation that can be easily avoidable in most cases. The impact of the presence of CO in the PEMFC anode feeding is fairly well known and studied and is considered to be most likely reversible[115]. However, it can be inferred from the TPD results that the presence of CO in a H2 stream fed to a fuel cell anode may accelerate the degradation of catalysts containing some surface groups on its surface such as hydrophilic sulfur species. As a matter of fact, surface groups in Vulcan XC72 resemble those present in the perfluorosulfonic acid ionomer commonly used in catalytic layers. The effect of CO contamination on the ionomer sulfonic groups close to Pt particles could possibly be very similar to those observed in the Pt/Vulcan XC-72 catalyst surface groups, but verifying this assumption needs further research.

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H2 Adsorption and Proton Exchange Rate on Gas Diffusion Electrodes with Pt/Vulcan XC72 Additional tests were carried out with gas diffusion electrodes (GDE) containing Pt/Vulcan XC-72 in the catalytic layer for comparison purposes. Two GDEs were prepared with the standard commercial Pt(20wt%)/Vulcan XC-72 catalyst from BASF. Two different inks containing a Nafionperfluorinated ion exchange resin (Aldrich, 5 wt. % solution) were prepared with distinct solventsfor the deposition of the catalytic layers. One of them was dissolved in isopropanol and the other in a mixture of butylacetate, ethanol, glycerol in proportions (45:50:5). The percentage of resin in the catalytic layer was 30%. The third electrode was a commercial GDE from BASF (LT-140E-W ELAT). The GDEs and a catalyst loading of 0.5 mgPt·cm-2, pretreated under vacuum at 448 K before the adsorption or the exchange at 298 K, in the same way as the catalyst. H2 adsorption isotherms on the GDEs showed at least two distinguishable plateaus at increasing equilibrium pressures as it can be noted in Figure 18. The isotherms profile variation probably depends on the coverage of the Pt surface by the perfluorosulfonic ion exchange resin and the interaction between them. The two plateaus could be ascribed to the different hydrogen adsorption/ diffusion rates over the free Pt particles on the support or over Pt particles on Vulcan XC-72 which are covered by the resin or in close interaction with it. By considering that the two homemade GDEs have been prepared with the previously analyzed Pt/Vulcan XC72 catalyst, it is clear that the presence of the ion exchange resin introduces additional difficulties to the determination of the exposed Pt surface area by means of H2 chemisorption. A previous study has been shown the influence of using catalytic inks with different solvents on the catalytic layer microstructure and the performance of the prepared electrodes [116]. During the catalytic layer deposition process, the Nafion dispersion recasts in different configurations which depend on the solvent composition and its evaporation rate. Thus, the Pt/Nafion

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interface, the size of the Nafion aggregates and the catalyst coverage will also determine the effective perimeter in the Pt particles for hydrogen surface diffusion.

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Figure 18. H2 adsorption isotherms obtained for different gas diffusion electrodes (0.5 mgPt/cm²) containing Pt/Vulcan XC72: (···○····) homemade GDE prepared by manual impregnation of an isopropanol based ink on a woven gas diffusion medium; ( - - □ - -) homemade GDE prepared by manual impregnation of a glycerol containing ink on a woven gas diffusion medium; (__▲__) a commercial LT-140E-W ELAT GDE.

The hydrogen adsorption results in relation to Pt surface area suggest that, in general, widely used data on the electrochemically active surface area and the Pt utilization in catalyst layers should be corrected for the effects of the surface groups of the carbon black support, which may store a significant amount of adsorbed hydrogen, and the ionomer. Paulus et al. reported a fundamental investigation regarding to the catalyst utilization at the electrode/membrane interface [117]. They compared GDEs (Pt black on carbon paper and Pt/CB on a woven gas diffusion medium) with model electrodes designed by covering glassy carbon with continuous or discontinuous Pt films. In agreement with our results, their study reveals that the hydrogen adsorption/desorption characteristics and the catalyst surface utilization depend on the nature of the catalyst in the electroactive layer and the pretreatment applied to the GDEs. In all those cases, the degree of Pt utilization is observed to decrease when increasing the sweep rate during cyclic voltammetry. This phenomena is explained in terms of the diffusion of adsorbed species on the Pt surface and surface conductivity in the electrolyte films. Accounting for this effect, Neyerlinget al. developed an analytical model to describe the effective proton resistance in the porous cathode electrode of a PEMFC based on a quick and accurate evaluation of cathode proton resistance by using in situ perturbation measurements [118]. The model they proposed serves as a guideline for experimental design parameters such that catalyst utilization is kept above 90%, which is a prerequisite for measuring the oxygen reduction reaction kinetics. The deuterium exchange rate has been also analyzed in the commercial GDE after heat treatment under vacuum at 448 K. Figure 19 shows the deuterium exchange rate and the moles of exchanged D atoms in the GDE as a function of time. In this case, the proportion the surface Pt in the catalyst has not been determined. Therefore, the exchange rate is referred to the total amount of Pt in the electrode. Three processes occurring at different rates can be distinguished during the deuterium exchange with the GDE, while only the two slower ones were observed during the exchange

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with the bare catalyst. The occurrence of a very fast exchange at short times can be ascribed to the presence of the proton exchange resin in the vicinity of some Pt crystallites. The second and third exchange processes can be assigned to those previously seen for the catalyst itself with the support surface groups, although their rate referred to the Pt (or surface Pt) in the electrode is probably influenced by the presence of the ion exchange resin.

Figure 19. Deuterium-hydrogen exchange frequency (exch) (solid line) and amount of moles of exchanged D atoms per mol of Pt (dotted line) in the commercial GDE as a function of contact time between the gas and the sample at 298 K. The catalyst has been previously treated under dynamic vacuum at 448 K for 1 h.

The mass spectrometric analysis of the isotopic composition of the gas phase during the exchange process allows calculating the surface diffusion coefficient of deuterium in the

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samples by representing the evolution of the exchanged atoms ( N et ) as a function of t . When the surface migration is the rate determining step of the exchange, the measured rate depends both upon the coefficient of surface diffusion (Ds) and the specific perimeter (Io) of the metal particles, which are the source of dissociated deuterium species. By considering the Pt particles to have a semi-spherical geometry and applying the model proposed by Kramer and Andre[119], which has also been used by Cavanagh and Yates [120], and by Duprez and Miloudi[121], the number of atoms diffused ( N et )at the beginning of exchange reaction (when the concentration of labeled species is constant) can be estimated according to Eq. 1: N et 

2



CD  Io  Ds  t withIo = Np(2 π r)

(1)

whereCD, is the atomic concentration of the labeled species (at·m-2), r is the average radius of the metal particles (m), and Io is the length of the metal-support interface, i.e., the total perimeter of the metal particle per m² of catalyst (m·m-2). By assuming that the adsorption/desorption of D2 is very fast, and that the direct exchange of D2 with the support is negligible at the experiment temperature, the coefficient of surface diffusion could be calculated by the following Eq. (2):

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Carbon Blacks in Electrochemical Energy Conversion Devices dN et     S1  S  Ds      with 1 d t  4   C D  I 0 

27

(2)

whereS1 is the initial slope of the curve representing the variation of N et as a function of 7

t.

-2

The fitting of the experimental data to the model yields values of 7.77·10 m·m for the metal-support interface (Io) in the catalyst and a surface diffusion coefficient (Ds) of 1.63 · 1019 m²·s-1. In the gas diffusion electrode the surface diffusion coefficient increases by three orders of magnitude up to 1.83·10-16 m²·s-1 with a particle perimeter length (Io) of 12.37·107 m·m-2. It must be taken into consideration that the isotopic exchange has been performed in the samples after their treatment at a temperature high enough to remove practically all the adsorbed water without decomposition of the support surface groups. In that way, those groups containing in their structure exchangeable hydrogen can be evaluated without the contribution of the deuterium exchange with water molecules. A rough estimation of the amount of exchanged hydrogen associated to surface groups in the studied catalyst within a period of 0.4 h yields approximately a value of 5·10-6 mol·m-². This amount is reduced to 3.5·10-6 mol·m-² in the case of the commercial GDE. The addition of the proton exchange resin to the catalytic layer increases the proton exchange rate in the vicinity of Pt particles, but blocks some groups of the support which could participate in the proton surface diffusion. A challenging breakthrough for improving the proton transfer pathways between the electrodes and the ion exchange membrane consists in designing tailor-made supports that are chemically modified with adequate proton acceptor groups to provide high hydrogen surface diffusion coefficients [68,122,123].

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CARBON BLACKS IN PEMFCFLOW FIELD PLATES Flow field plates or bipolar plates (BPs) for PEMFC applications are usually designed to accomplish many functions, such as distribute reactants uniformly over the active areas, remove heat from the active areas, carry current from cell to cell and prevent leakage of reactants and coolant. They have been traditionallymade of graphite.Both natural as well as synthetic forms of graphite have very low resistivity, resulting in high electrochemical power output, and excellent chemical stability to survive the fuel cell environment. However, the machining process ofgraphite sheets, which were usually impregnated with polymer resin to reduce their porosity, is very expensive and the separator plates obtained in that way comprise nearly 80% of the mass and 45% of the cost in conventional PEMFC stacks [124]. The US Department of Energy has established the following target values for composite bipolar plates[125]: • • • • • •

low weight 25 MPa high flexibility: 3–5% deflection at mid-span high electrical conductivity >100 S cm-1 high thermal conductivity >10 W (m K) -1 low gas permeability 99.9 %) were passed through humidifiers and fed at the anode and cathode sides, respectively. In order to carry out the electrochemical measurements, several MEAs were prepared using different electrodes for the anode, and a commercial 20 % Pt/C on GDM (Gas Diffusion Membrane) electrode (E-TEK Inc.) for the cathode (Pt, 0.4 - 0.6 mg/cm2). The anode electrodes were prepared by depositing a suspension of Nafion solution (10% wt.) and the synthesised electrocatalyst (Nafion content in the suspension = 32 % wt) on pieces of E-TEK ELAT carbon cloth. The final amount of metal active phase in all the prepared electrodes was 0.5 mg/cm2, for their comparison with the commercial electrode E-TEK. The commercial electrode from E-TEK was based on Pt deposited on Vulcan XC-72, which was then deposited on the carbon cloth gas diffusion membrane (Pt, 0.4-0.6 mg cm-2). The final assembly of the electrodes and the Nafion membrane was hot pressed between two metallic plates and heated at 120 ºC, with a pressure of 20 Kg/cm2 for 90 s. 1.0

E-TEK Pt/Vulcan

0.9 0.8

Cell potential (V)

0.6 0.5 0.4 0.3 0.2

A

0.1

30 0.0 0

20

40

60

80

100 120 140 160 180

-2

)

25

Power density (mW cm

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0.7

20 15 10 5

B 0 0

20

40

60

80

100 120 140 160 180 -2

Current density (mA cm )

Figure 15. Polarization (A) and power density (B) curves for Pt/Vulcan based electrode and the commercial one at the anode side in a PEM fuel cell working at room temperature and atmospheric pressure.

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M. J. Lázaro, L. Calvillo, V. Celorrio et al.

64

Figure 15 shows the polarization (cell potential versus current density) and power density curves obtained for the Pt/Vulcan based electrode and the commercial gas diffusion electrode (E-TEK Inc.). Comparing the obtained results, it was observed a better fuel cell performance when the Pt/Vulcan based electrode was used, since less polarization losses were obtained than those obtained with the commercial electrode. Both electrodes were based on Pt supported on Vulcan XC-72 (E-TEK) and Vulcan XC-72R (our catalyst). The only difference between both materials is their morphology: Vulcan XC-72 is formed by pellets, whereas Vulcan XC-72R is a powder. However, even this morphological difference could considerably influences the performance of the fuel cell. Carmo et al. published a comparative study about both supports [Carmo 2007]. They obtained a higher catalytic activity using Vulcan XC-72 than Vulcan XC-72R due to the higher electrical conductivity of the first one. On the contrary, we obtained better results with the catalyst supported on Vulcan XC-72R. Taking into account that both electrodes contained the same Pt loading, this result can be attributed to the preparation of the electrodes. According to the literature, there must be an optimum Nafion content in the electrode in order to obtain a satisfactory cell performance [Sasikumar 2004, Litster 2004]. This result demonstrates that it is necessary not only to optimize the properties of the catalyst but also the properties of the electrode in order to obtain a good fuel cell performance. 1.0

E-TEK Pt/Vulcan Pt/Vulcan NSTa0.5 Pt/Vulcan NcTb0.5 Pt/Vulcan NcTb2

0.9 0.8

Cell potential (V)

0.6 0.5 0.4 0.3 0.2

A

0.1 30 0.0

0

20

40

60

80 100 120 140 160 180 200

B

-2

)

25

Power density (mW cm

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0.7

20 15 10 5 0 0

20

40

60

80 100 120 140 160 180 200 -2

Current density (mA cm )

Figure 16. Polarization (A) and power density (B) curves for Pt electrocatalysts supported on Vulcan with different surface chemistry at the anode side in a PEM fuel cell working at room temperature and atmospheric pressure. Carbon Black: Production, Properties and Uses : Production, Properties and Uses, Nova Science Publishers, Incorporated, 2011. ProQuest Ebook

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Catalysts supported on different oxidized Vulcan carbons were also tested in the fuel cell in order to study the effect of the surface chemistry on their performance. The characteristic curves obtained for these catalysts are showed in Figure 16. For all catalysts, the open circuit potential (OCV) was lower than the reversible potential (1.23 V), which is attributed to irreversible losses during the fuel cell operation. These losses result from three types of polarization effects: activation, ohmic and concentration polarizations. The OCV was about 0.9 V for all catalysts except for Pt/Vulcan NcTb2, which is attributed to the higher agglomeration of platinum nanoparticles. At lower current densities (0 – 10 mA), a sharp drop in the cell potential was observed, associated to the activation polarization. Losses obtained for Pt/Vulcan based electrodes were around 17-36 %, whereas those obtained for the commercial electrode were about 43 %. These losses depend both on the electrode composition and the interaction between hydrogen and the surface of the electrode. Therefore, the lower losses obtained for Pt/Vulcan electrodes are associated to the preparation of the electrode, as was mentioned above. Losses in the second region of the curve are related to the ohmic resistance of the cell components, such as the electric and contact resistances of electrodes, membrane and current collecting components. Taking into account that all experiments were carried out in the same experimental system and all electrodes were prepared by the same method and using the same membrane, differences in the performance of electrodes can be attributed to the electron transport through electrodes during the electrochemical reactions. The electron transport depends on the electrical conductivity of the support and the Pt-support interaction, since an electron transfer from Pt nanoparticles to the conductive gas diffusion layer through the carbon support takes place during the electrochemical reactions. According to the literature, this transfer takes place through the oxygen atoms of the surface of the support [Antolini 2009, Yu and Ye 2007a], therefore, the metal-support interaction can be improved through the surface modification of the carbon support to form proper functional groups and chemical links at the Pt/C interface. Probably, oxygen groups share Pt electron density favouring the electron transfer from catalytic sites to the conductive carbon electrode. However, the functionalization of the support decreases its electrical conductivity. Therefore, it is necessary to establish a compromise between the two effects in order to optimize the cell performance. In Figure 16 can be observed that the functionalization of the support did not improve the fuel cell performance. However, similar ohmic losses were obtained for all Pt/Vulcan based electrodes, despite the differences in the Pt particle size. Finally, the voltage drop in the third region of the polarization curve (high current densities) is associated with mass transport resistance of protons through the membrane and the gas phase through the porous structure of the electrodes. Due to the same membrane was used for preparing all MEAs, differences in the performance of electrodes can be attributed to the diffusion of hydrogen through the electrodes. As can be seen in Figure 16, lower mass transport losses were obtained with electrodes based on oxidized Vulcan. This can be attributed to the higher pore volume of oxidized supports than the original Vulcan, which results in a more effective diffusion of hydrogen through their porous structure.

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REFERENCES G.J.K. Acres, J.C. Frost, G.A. Hards, G.A. Potter, T.R. Ralph, D. Thompsett, G.T. Burstein, G.J. Hutchings, Catal. Today 38 (1997) 393-400. A.E. Aksoylu, M. Madalena, A. Freitas, M.F.R. Pereira, J.L. Figueiredo, Carbon 39 (2001) 175-185. G. Álvarez, F. Alcaide, O. Miguel, L. Calvillo, M.J. Lázaro, J.J. Quintana, J.C. Calderón, E. Pastor, I. Esparbé, J. Solid State Electr. 14 (2010) 1027-1034 E. Antolini, Appl. Catal. B: Environ. 88 (2009)1-24. E. Antolini, J.R.C. Salgado, L.G.R.A. Santos, G. Garcia, E.A. Ticianelli, E. Pastor, E.R. Gonzalez, J. Appl. Electrochem. 36 (2006) 355-362. E. Auer, A. Freund, J. Pietsch, T. Tacke, Appl. Catal. A: Gen. 173 (1998) 259-271. R.C. Bansal, J.-B. Donnet, Carbon black, edited by Jean-Baptiste Donnet, Chand Bansal, Meng-Jiao Wang. 2nd ed, rev. & expanded. Marcel Dekker, INC., 270 Madison Avenue, New York 1993. Cap. 4, 175. C.A. Bessel, K. Laubernds, N.M. Rodríguez, R.T.K. Baker, J. Phys. Chem. B 105 (2001) 1115-1118. C.W.B. Bezerra, L. Zhang, H. Liu, K. Lee, A.L.B. Marques, E.P. Marques, H. Wang, J. Zhang, J. Power Sources. 173 (2007) 891-908. L. Calvillo, M.J. Lázaro, E.G. Bordejé, R. Moliner, P.L. Cabot, I. Esparbé, E. Pastor, J.J. Quintana, J. Power Sources. 169 (2007) 59-64. L. Calvillo, M. Gangeri, S. Perathoner, G. Centi, R. Moliner, M.J. Lázaro, J. Power Sources. 192 (2009) 144-150. L. Calvillo, M.J. Lázaro, E.G. Bordejé, R. Moliner, P.L. Cabot, I. Esparbé, E. Pastor, J.J. Quintana, J. Power Sources. 169 (2007) 59-64. M. Carmo, A.R. dos Santos, J.G.R. Poco, M. Linardi, J. Power Sources 173 (2007) 860-866. F. Coloma, A. Sepúlveda-Escribano, J.L.G. Fierro, Rodríguez-Reinoso, Langmuir. 10 (1994) 750-755. H. Darmstadt, C. Roy, S. Kaliaguine, S.J. Choi, R. Ryoo, Carbon. 40 (2002) 2673-2683. A.L. Dicks, J. Power Sources. 156 (2006) 128-141. J.L. Figueiredo, M.F.R. Pereira, M.M.A. Freitas, J.J.M. Orfao, Carbon 37 (1999) 1379-1389. M.A. Fraga, E. Jordao, M.J. Mendes, M.M.A. Freitas, J.L. Faria, J.L. Figueiredo, J. Catal., 209 (2002) 355-364. M. Gangeri, S. Perathoner, G. Centi, Inorg. Chim. Acta. 359 (2006) 4828-4832. H.A. Gasteiger, N. Markivic, P.N. Ross, E.J. Cairns, J. Electrochem. Soc. 141 (1994) 17951803. C. Grolleau, C. Coutanceau, F. Pierre, J.-M. Lèger, Electrochim. Acta. 53 (2008) 7157-7165. A. Guerrero-Ruiz, P. Badenes, I. Rodríguez-Ramos, Appl. Catal. A: Gen., 173 (1998) 313321. S.C. Hall, V. Subramanian, G. Teeter, B. Rambabu, Solid State Ionics. 175 (2004) 809-813. W.M. Hess, C.R. Herd, Carbon black, edited by Jean-Baptiste Donnet, Chand Bansal, MengJiao Wang. 2nd ed, rev. & expanded. Marcel Dekker, INC., 270 Madison Avenue, New York 1993. Cap. 3, 89. M. Inoue, S. Akamaru, A. Taguchi, T. Abe, Vacuum. 83 (2009) 658-663.

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Z.R. Ismagilov, M.A. Kerzhentsev, N.V. Shikina, A.S. Lisitsyn, L.B. Okhlopkova, Ch.N. Barnakov, M. Sakashita, T. Iijima, K. Tadokoro, Catal. Today. 102-103 (2005) 58-66. J. Jiang, A. Kucernak, J. Electroanal. Chem. 543 (2003) 187-199. a J.B. Joo, P. Kim, W. Kim, J. Kim, J. Yi, Catal. Today. 111 (2006) 171-175. M. Kim, J.-N. Park, H. Kim, S. Song, W.-H. Lee, J. Power Sources. 163 (2006) 93-97. M.J. Lázaro, V. Celorrio, L. Calvillo, E. Pastor, R. Moliner, J. Power Sources. (2010), doi:10.1016/j.jpowsour.2010.10.055. H. Liu, C. Song, L. Zhang, J. Zhang, H. Wang, D.P. Wilkinson, J. Power Sources. 155 (2006) 95-110. B.D. McNicol, D.A.J. Rand, K.R. Williams, J. Power Sources. (1999) 83 5-31. C. Moreno-Castilla, M.A. Ferro-García, J.P. Joly, I. Bautista-Toledo, F. Carrasco-Marín, J. Rivera-Utrilla, Langmuir. 11 (1995) 4386-4392. J.S. Noh, J.A. Schwarz, Carbon. 28 (1990) 675-682. C. Prado-Burguete, A. Linares-Solano, F. Rodríguez-Reinoso, C. Salinas-Martínez de Lecea, J. Catal., 115 (1989) 98-106. M.C. Román-Martínez, D. Cazorla-Amorós, A. Linares-Solano, C. Salinas-Martínez de Lecea, Carbon. 33 (1995) 3-13. J.R.C. Salgado, E. Antolini, E.R. Gonzalez, Appl. Catal. B-Environ. 57, (2005) 283-290. J.R.C. Salgado, J.J. Quintana, L. Calvillo, M.J. Lázaro, P.L. Cabot, I. Esparbé , E. Pastor, Phys. Chem. Chem. Phys. 10 (2008) 6796-6806. J.R.C. Salgado, F. Alcaide, G. Álvarez, L. Calvillo, M.J. Lázaro, E. Pastor, J. Power Sources. 195 (2010) 4022-4029. P.V. Samant, F. Gonçalves, M.M.A. Freitas, M.F.R. Pereira, J.L. Figueiredo, Carbon. 42 (2004) 1321-1325. A. Sepúlveda-Escribano, F. Coloma, F. Rodríguez-Reinoso, Appl. Catal. A: Gen., 173 (1998) 247-257. P. Serp, M. Corrias, P. Kalck, Appl. Catal. A: Gen. 253 (2003) 337-358. Y. Shao, G. Yin, J. Zhang, Y. Gao, Electrochim. Acta. 51 (2006) 5853-5857. P. Sivakumar, V. Tricoli, Electrochim. Acta. 51 (2006) 1235-12343. K. Wikander, H. Ekström, A.E.C. Palmqvist, A. Lundblad, K. Holmberg, G. Lindbergh, Fuel Cells. 6 (2006) 21-25. X. Yu, S. Ye, J. Power Sources. 172 (2007) 133-144. a J.-H. Zhou, J.-P. He, Y.-J. Ji, W.-J. Dang, X.-L. Liu, G.-W. Zhao, C.-X. Zhang, J.-S. Zhao, Q.-B. Fu, H.-P. Hu, Electrochim. Acta. 52 (2007) 4691-4695.

Carbon Black: Production, Properties and Uses : Production, Properties and Uses, Nova Science Publishers, Incorporated, 2011. ProQuest Ebook

Copyright © 2011. Nova Science Publishers, Incorporated. All rights reserved. Carbon Black: Production, Properties and Uses : Production, Properties and Uses, Nova Science Publishers, Incorporated, 2011. ProQuest Ebook

In:Carbon Black: Production, Properties and Uses Editors: I. J. Sanders and T. L. Peeten, pp. 69-91

ISBN: 978-1-61209-535-6 © 2011 Nova Science Publishers, Inc.

Chapter 3

A REVIEW OF CURRENT ANALYTICAL APPLICATIONS EMPLOYING GRAPHITIZED CARBON BLACK Christine M. Karbiwnyk*1 and Keith E. Miller2 1

Animal Drugs Research Center U.S. Food and Drug Administration, Denver, Colorado, USA 2 University of Denver, Department of Chemistry and Biochemistry, Denver, CO, USA

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ABSTRACT Graphitized carbon black (GCB) is a non-porous form of amorphous carbon. GCB materials are hydrophobic. They are often used to effectively trap organic compounds from water or under high humidity conditions; conditions where the performance of other sorbents is reduced. While activated carbon relies on its high surface-area-to-volume ratio to adsorb organic compounds, the surface interactions of GCB depend solely on dispersion (London) forces. GCB materials have been used to trap a wide range of organic compounds from C4 hydrocarbons to polychlorinated biphenyls (PCBs). Typically, compounds are adsorbed on the GCB surface from large volumes of air or water, and then subsequently released either by solvent desorption or thermal desorption resulting in either a concentration of the analytes, solvent exchange or combination of both. This review presents analytical applications of GCBs over the last decade. It includes air monitoring, solid-phase extraction (SPE), purge traps for purification or lowflow air sampling, and chromatography columns utilizing GCB packing material.

Keywords: Review; Graphitized carbon black; Solid-phase extraction; Polar compounds

* Corresponding Author: Animal Drugs Research Center U.S. Food and Drug Administration, P.O. Box 25087, Denver, Colorado 80225-0087, USA Carbon Black: Production, Properties and Uses : Production, Properties and Uses, Nova Science Publishers, Incorporated, 2011. ProQuest Ebook

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1. INTRODUCTION Carbon-based adsorbents have several advantages over silica-based adsorbents, including high chemical inertness and thermal stability. Compared to polymer-based adsorbents, carbon adsorbents tolerate a wider pH range, and they can withstand higher desorption temperatures. The use of graphitized carbon blacks in gas chromatography began 40 years ago in packed column gas chromatography (GC), and is still used today for the analysis of alcohols, ketones and esters. Porous layer open tubular (PLOT) columns are capillary columns where the inner surface is coated with a layer of solid porous material. PLOT columns coated with graphitized carbon black (CarbographTM, Alltech Associates) can be used to separate both polar and nonpolar compounds. One advantage of some of the carbon-based columns over alumina-coated columns is that they are generally less affected by water in the sample, which can cause degradation in separation performance (such as changes in retention, selectivity, efficiency, and peak area). The GCB PLOT stationary phase has been applied to gas chromatography of polar compounds such as alcohols, free fatty acids and phenols, and to environmental analyses. Restek™ produces micro-packed GC columns using 0.75mm ID deactivated stainless steel tubing. The coiled tube can be several centimeters up to several meters in length [1]. This represents a future application for GCB materials with the potential for higher capacity compared to PLOT columns and increased efficiency over traditional packed columns. GCBs are not sufficiently pressure resistant to be used in HPLC columns; instead, porous graphitic carbon columns are used [2]. The use of carbonaceous sorbents for solid-phase extraction (SPE) began in the 1980s. Initial applications included the extraction of non-polar analytes such as organochlorinated insecticides [3,4] and then of moderately polar analytes such as triazines and phenoxy acids [5,6]. Interest in GCBs increased when it was found to solve the difficult extraction problem of isolating polar molecules, which have a low affinity for most reversed-phase sorbents, from aqueous samples [7-11].

2. CHARACTERISTICS OF GRAPHITIZED CARBON BLACK Carbon black is produced by the incomplete combustion of hydrocarbons. Graphitized carbon black (GCB) materials are made by heating carbon black at > 2600 ○C in a furnace with a reduced oxygen atmosphere. This process results in a material that is hydrophobic and non-porous form of amorphous carbon with a particle size of 80–100 μm, and surface area of approximately 200 m2/g [2]. The Carbotrap and Carbopack Graphitized Carbon Black (GCB) product lines from Supelco are very similar, differing only in whether the particle size is larger or smaller than 40 mesh. The GCB materials 40 mesh and larger (e.g. 20/40) belong in the Carbotrap product line, whereas the GCB materials 40 mesh and smaller (e.g. 40/60, 60/80, 80/100, etc.) belong in the Carbopack product line. The larger 20/40 Carbotrap materials are typically used in 4 mm and larger internal diameter (ID) air monitoring tubes, permitting high flow rates without excessive pressure drops. Carbopack 40/60 materials are typically used in 84% and a limit of detection of 0.5 – 1 ng/L, using ENVI-Carb (Supelco) SPE cartridges for extraction and cleanup before LCMS/MS analysis. Hanselman et al. [130] developed a sample preparation method that permits the quantification of four natural steroidal estrogens (17α-estradiol, 17β-estradiol, estrone, and estriol) in flushed dairy manure wastewater (FDMW) by GC-MS. Solid-phase extraction with graphitized carbon black was used for the bulk extraction of estrogens from FDMW and additional sample purification was accomplished with C-18. The sample preparation method allowed estrogens to be detected accurately by GC-MS with average recovery of > 90% measured by spiked recovery experiments. Yang et al. [131] describe a C-18 MSPD and ENVI-Carb SPE cleanup method followed by very high pressure LC–MS/MS technique that was developed to assay trace levels of unconjugated progestogens in eggs. Using the GCBNH2 SPE sample preparation, Yang et al. [132] validated a simple and sensitive method for the simultaneous determination of 50 anabolic hormones with a wide range of polarity and classification in samples of beef, pork, milk and shrimp. The group found the GCB-NH2 SPE extraction resulted in higher recoveries and excellent matrix cleanup for the majority of

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analytes tested compared to C-18 and HLB cartridges. An LC-MS/MS method for the determination of free and conjugated estrogens in complex aqueous matrixes was developed by Gentili et al. [133]. The group used a single Carbograph cartridge in their extraction procedure that enabled differential elution, without derivatization, of the two fractions of the analytes, for the best sensitivity in LC-MS analysis. This also enabled a reduction in the required volume of environmental water needed for extraction. A sensitive analytical method was developed and validated by Shao et al. [134] for simultaneous determination of 16 glucocorticoid (fluorometholone, flumethasone, triamcinolone, aldosterone, clobetasol propionate, methylprednisolone, fluocinolone acetonide, hydrocortisone, prednisone, dexamethasone, beclomethasone, prednisolone, budesonide, triamcinolone acetonide, fludrocortisone acetate, and cortisone) residues in pig tissues (muscle, liver, and kidney). Samples were hydrolyzed with beta-glucuronidase/arylsulfatase enzyme and passed through a Supelclean ENVI-Carb graphitized carbon black solid-phase extraction cartridge, followed by further purification using aminopropyl cartridges. Analytes were separated on an ultraperformance liquid chromatography BEH C-18 column followed by tandem mass spectrometry (MS) with an electrospray ion source in negative ion mode.

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3.2.4. Antibiotics Anthropogenic and ubiquitous use of antibiotics in agriculture (applications to fish ponds, cattle, hogs, etc) and failure of water treatment plants to breakdown or remove these compounds has resulted in the occurrence of antibiotic compounds in lakes and streams [135]. Concern also continues regarding exposure to antibiotics in the food supply [136,137]. Bruno et al. [138] made use of GCB‘s hydrophobic nature and its ability to adsorb acid compounds in the extraction of trace level penicillins from water samples. Sample treatment methods for the analysis of contaminants in food are usually based on deproteinization/ extraction by various solvents followed by a sequence of cleanup steps, which often involve the use of SPE cartridges filled with a variety of sorbent materials. However, several research groups have found that GCB sorbent materials allow one step clean up and extraction without deproteinating the sample matrix. In some cases, sample dilution ensures the proteins won‘t interfere with analyte recovery and additional sorbent materials may be utilized to reduce fatty acid and phospholipid levels from the sample matrix. Thus, a milk matrix required only a slight modification in sample preparation for Bruno et al. [139] to extract 10 β-lactam antibiotics from bovine milk using Carbograph 4 SPE cartridges. Cavaliere et al. [140] extended the method to the analysis of 14 sulfonamide (SAs) residues in milk and eggs. The method uses a single SPE Carbograph 4 cartridge for simultaneous extraction and purification of SAs in the above matrices. After analyte desorption, an aliquot of the final extract is injected into an LC-MS instrument. Recovery of SAs in milk at the 5 ppb level ranged between 76 and 112% with relative standard deviations (RSDs) of ≤13%. Recovery of SAs in egg at the 50 ppb level ranged between 68 and 106% with RSDs of ≤12%. Estimated limits of quantification (S/N = 10) of the method were 1−6 ppb of SAs in whole milk and 5−13 ppb of SAs in eggs. Tetracycline antibiotics (TCAs) have a broad range of activity against grampositive and gram-negative bacteria and are inexpensive. For these reasons, TCAs are widely used in veterinary medicine to prevent and treat several diseases as well as for promoting growth in cattle and poultry. However, the ability of TCAs to form complexes with Ca2+ and Mg2+ ions, which are species abundantly present in both milk and egg, makes them difficult

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to analyze. Bruno et al. [141] found that dilution of milk and egg samples with water containing respectively 10 and 1 mmol/L Na2EDTA succeeded in giving satisfactory analyte recovery from Carbograph 4 cartridges.

3.2.5. Mercury Due to the health risks associated with elemental mercury, methyl mercury, and dimethyl mercury, quantification of these species in a variety of matrices is necessary to determine total mercury concentrations. Dimethyl mercury ((CH3)2Hg) has been found in municipal waste landfill gas, sub-thermocline waters of the Pacific Ocean, sediment cores, soils, and urban air. The highly volatile nature of (CH3)2Hg makes analysis of this compound problematic. Bloom et al. [142], found Carbotrap to be a superior adsorbent for (CH3)2Hg compared to Tenax, a non-polar synthetic polymer often used for semi-volatile compound collection, and several pure carbon-based molecular sieves (Carbosieve S-III, Carboxen-563, Carboxen-564 and Carboxen-569). Bloom et al. [142], estimated recoveries of (CH3)2Hg from Carbotrap sampling cartridges to be 98.9 ± 5.1%, with potentially a worst case scenario of 121.6 ± 6.2% if all flow measurement errors are included. The researchers concluded that Tenax is unsuitable due to its small breakthrough capacity, and that all of the carbon molecular sieves are unsuitable due to the lack of quantitative recovery of (CH3)2Hg. Bloom et al., maintain the observation of very rapid breakthrough of (CH3)2Hg through Tenax is not surprising, and serves to explain the failure of previous Tenax based analytical methods for determining (CH3)2Hg in air.

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3.2.6. Food Contaminants In addition to pesticides and mycotoxins, food undergoes analysis for other toxins and contaminants. Some compounds are the result of environmental contamination (e.g. dioxins and paralytic shellfish toxins). Food safety analysis sometimes addresses an immediate threat to public health, such as illness and renal failure resulting from the consumption of food adulterated with melamine and cyanuric acid [143,144]. The presence of biogenic amines, like putrucine and cadaverine, may be a sign of food spoilage. Fish, shellfish and other food matrices can present challenges in residue analysis. Several research groups have found success using graphitized carbon black adsorbent materials as part of the isolation and cleanup procedures. Dioxins are by-products of various industrial processes, and are commonly regarded as likely human carcinogens that are persistent organic pollutants in the environment. The tolerable daily intake of dioxin has been set very low, 1-4 pg/Kg body weight per day, to allow for uncertainty in the dose response to different toxicity levels of the dioxin compounds and to ensure public safety as far as possible [145]. Hoh et al. [146] developed a relatively fast and easy analytical screening method to monitor fish oil potentially contaminated with dioxin and dioxin-like PCBs at or above current food safety limits. A 2 g cod liver oil sample required only gel permeation chromatography and SPE cleanup with graphitized carbon black before direct sample introduction coupled to 2D gas chromatography time of flight mass spectrometry analysis. Paralytic shellfish poisoning (PSP) is caused by a group of 26 naturally occurring potent neurotoxins. Symptoms begin anywhere from 15 minutes to 10 hours after eating the contaminated shellfish, although they are usually observed within 2 hours. Symptoms are

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generally mild, and begin with numbness or tingling of the face, arms, and legs. This is followed by headache, dizziness, nausea, and muscular uncoordination. In cases of severe poisoning, muscle paralysis and respiratory failure occur, and in these cases death may occur in 2 to 25 hours [147]. Regulatory bodies are recommending lower limits to protect public health. Sayfritz et al. [148] developed an improved LC–MS/MS method, which can be used for the routine regulatory determinations of PSP toxins in shellfish samples in a single chromatographic analysis. The group combined a novel extraction method, improved the sample clean-up with GCB SPE and used gradient elution to lower the overall LOD of saxitoxin (the most potent PSP) to 313 μg/kg. Cyanuric acid (CYA) is a microbicide and disinfectant in water treatment applications as a chlorine stabilizer, but might also be found in the environment from degradation of striazine pesticides. This compound was implicated in renal failure when melamine and CYA were added to dog and cat food ingredients to boost the apparent protein content of certain food commodities [144]. These same contaminated food commodites were used in the production of animal and fish feeds. Karbiwnyk et al. [149] describes an LC–MS/MS method for the analysis of CYA residues in fish and shrimp muscle. CYA was extracted from ground fish or shrimp with an acetic acid solution, defatted with hexane, and isolated with a graphitic carbon black SPE column. Residues were separated from matrix components using a porous graphitic carbon LC column, and then analyzed with electrospray ionization in negative ion mode on a triple quadrupole mass spectrometer. Biogenic amines in fish, cheese and meat products are the result of microbial decarboxylation and, as such, their presence is indicative of spoilage and compromised food safety. Adsorbent tubes filled with 70 mg of Tenax TA 60/80 mesh and Carbotrap 20/40 mesh (Supelco) were prepared by Awan et al. [150] to sample the headspace of stored chicken meat for putrescine and cadaverine spoilage markers. The adsorbent tubes underwent thermal desorption GC- differential mobility spectrometry analysis.

3.2.7. SPME Solid-phase microextraction (SPME) is a solvent free method of analyte extraction. In general, analytes are extracted into a polymetric phase (e.g. poly dimethylsiloxane) immobilized onto a fused silica fiber. Typical applications include passive sampling ambient air, headspace sampling, or placing the fiber into an environmental water sample. After exposing the SPME fiber to the sample, the fiber is introduced into the GC injector where the analytes are thermally desorbed, separated on the column, and identified by a detector. Potter and Pawlizyn [151] created an SPME fiber with a Carbopack B stationary phase and tested the extraction characteristics of a polycyclic aromatic hydrocarbon (PAH) and polychlorinated biphenyl (PCB) mixed standard. The Carbopack B fibers produced virtually zero bleed when desorbed at 300 ◦C for 10 min. The analytes tested (naphthalene, anthracene, benz [a]anthracene, and benzo-[a]pyrene plus two polychlorinated biphenyls, 2,2',5trichlorobiphenyl (PCB3) and 2,2',3,4,5'-pentachlorobiphenyl (PCB5)) had excellent sensitivity and were linear from 10 pg/mL to 2.5 ng/mL. Detection limits were estimated to range from 7 pg/mL for naphthalene to 1 pg/mL for benzo[a]pyrene. Gierak et al. [152], created carbon fibers for SPME by methylene chloride pyrolysis. Results of testing the fibers with benzene, toluene, xylenes, trichloromethane and tetrachloromethane indicated that the fibers can be used for the analysis of organic substances occurring in trace amounts in air or

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liquid matrices. Maeoka et al. [153] quantified PCDDs/PCDFs and co-planar PCBs in soil samples using an SPME carbon-coated fiber and GC-MS. Best results gave an extraction time of 40 min under at 25-30 °C. A desorption temperature of 300 °C of SPME fibers coated with Carbopack B showed the highest response for co-planar PCBs and PCDDs/PCDFs. Until graphitic carbon black SPME fibers become commercially available, significant method development in this area is not anticipated. However, work is being done with activated carbon fibers [154-160], carbon tape SPME fibers [161], Carboxen (carbon molecular sieve) plus polymer fibers [162, 163], and carbon nanotubes [164-172].

4. NOVEL APPLICATIONS

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4.1. SALDI Surface assisted laser desorption/ionisation (SALDI) is a derivation of matrix-assisted laser desorption/ionization (MALDI). MALDI frequently relies on light-absorbing compounds called matrices that are co-crystallized with the analyte to achieve high ionization and desorption efficiencies. Although this approach offers numerous advantages and is an indispensable tool in macromolecule analysis, the presence of the matrix also produces a high chemical background in the region below m/z 700 in the mass spectrum. SALDI substitutes the chemical matrix of MALDI for an active surface, eliminating matrix interference that is seen in the m/z region below 700. SALDI mass spectrometry has evolved in recent years into a technique with great potential to provide insight into many of the challenges faced in modern research [173]. A great variety of substrates have been reported to work in SALDI including graphitic carbon black materials. Han et al. [174] developed a method to obtain laser desorption/ionization mass spectra of organic compounds by depositing sample solutions onto a carbon substrate surface consisting of a thin layer of activated carbon particles immobilized on an aluminum support. In common with the porous carbon suspension samples used in previous SALDI work, the mass spectra contain only a few ―matrix‖ background ion peaks, minimizing interference with analyte ion peaks. A suspension of glycerol, plus 5% – 10% by weight of sucrose, ensured that the ion signals were stable over hundreds of laser shots. Shariatgorji et al. [175] demonstrated that small GCB 4 particles provide a suitable medium for both the solid-phase extraction of diverse compounds with widely varying polarities from liquid samples and SALDI analysis of the compounds. The GCB particles provide a clean spectral background, with no requirements for additives, high SALDI efficiency, and both reversed-phase and ion-exchange SPE capabilities. The group developed a GCB-μ-trap to extract three types of compounds (sulfonamides, human medicines, and organophosphate esters) from water and urine samples. A few particles were removed from all of the GCB-μ-traps for SALDI screening. Negative samples were discarded, whereas the rest were extracted and quantified using LC-MS/MS. Chen et al. [176] combined the use of GCB as an SPE adsorbent and SALDI matrix. Graphitic carbon black SPE tubes were used to extract trace compounds from aqueous solutions. The GCB particles were then directly analyzed by SALDI-MS. The group applied this technique to trace levels of phenolic compounds [176] and quaternary ammonium surfactants [177] in water achieving detection

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Christine M. Karbiwnyk and Keith E. Miller

limits for these compounds in the ppt range. Chen et al. [178,179] also found that surfactants can be used to enhance the SALDI signal from GCB materials. Amin et al. [180] significantly enhanced the signal intensity of low-molecular weight compounds analyzed using surfaceassisted laser desorption/ionization time-of-flight mass spectrometry (SALDI-TOF-MS) when oxidized graphitized carbon black (GCB) particles were used as the desorption/ionization surface. Using this technique, the group successfully extracted and quantified a common pharmaceutical compound, propranolol, from Baltic Sea blue mussels using deuterated propranolol as the internal standard. The calibration curve showed a linear dynamic range of response (0.1-20 μg/mL) and good reproducibility (RSD