Fuel Cells and Hydrogen: From Fundamentals to Applied Research 0128114592, 9780128114599

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Fuel Cells and Hydrogen

Fuel Cells and Hydrogen From Fundamentals to Applied Research Edited by

Viktor Hacker Shigenori Mitsushima

Elsevier Radarweg 29, PO Box 211, 1000 AE Amsterdam, Netherlands The Boulevard, Langford Lane, Kidlington, Oxford OX5 1GB, United Kingdom 50 Hampshire Street, 5th Floor, Cambridge, MA 02139, United States # 2018 Elsevier Inc. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/permissions. This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein). Notices Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary. Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility. To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein. Library of Congress Cataloging-in-Publication Data A catalog record for this book is available from the Library of Congress British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library ISBN: 978-0-12-811459-9 For information on all Elsevier publications visit our website at https://www.elsevier.com/books-and-journals

Publisher: Susan Dennis Acquisition Editor: Kostas Marinakis Editorial Project Manager: Michelle Fisher Production Project Manager: Maria Bernard Cover Designer: Christian Bilbow Typeset by SPi Global, India

Contributors Takuto Araki Yokohama National University, Graduate School of Engineering Science, Yokohama, Japan Angelo Basile University of Calabria, Institute on Membrane Technology of the Italian National Research Council, Rende, Italy Emile Bere University of Poitiers, Plateforme Image UP, Pole Biologie Sante, Poitiers, France Sebastian Bock Graz University of Technology, Institute of Chemical Engineering and Environmental Technology, Graz, Austria Merit Bodner Graz University of Technology, Institute of Chemical Engineering and Environmental Technology, Graz, Austria Christine Canaff University of Poitiers, Institut de Chimie des Milieux et Mat
eriaux de Poitiers (IC2MP), Poitiers, France Bernd Cermenek Graz University of Technology, Institute of Chemical Engineering and Environmental Technology, Graz, Austria Bernd Eichberger Institute of Electronic Sensor Systems, Graz University of Technology, Graz, Austria Bernhard Gollas Graz University of Technology, Institute for Chemistry and Technology of Materials, Graz, Austria Viktor Hacker Graz University of Technology, Institute of Chemical Engineering and Environmental Technology, Graz, Austria Yaovi Holade Ecole Nationale Sup
erieure de Chimie de Montpellier (ENSCM), Institut Europ
een des Membranes, Montpellier, France Akimitsu Ishihara Yokohama National University, Green Hydrogen Research Center, Yokohama, Japan Adolfo Iulianelli University of Calabria, Institute on Membrane Technology of the Italian National Research Council, Rende, Italy Larisa Karpenko-Jereb Graz University of Technology, Institute of Electronic Sensor Systems, Graz, Austria

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Contributors

Boniface Kokoh University of Poitiers, Institut de Chimie des Milieux et Mat
eriaux de Poitiers (IC2MP), Poitiers, France Werner Lehnert €lich GmbH, Institute of Energy and Climate Research IEK-3: Electrochemical Forchungszentrum Ju €lich, Germany Process Engineering, Ju Karin Malli Graz University of Technology, Institute of Chemical Engineering and Environmental Technology, Graz, Austria Kurt Mayer Graz University of Technology, Institute of Chemical Engineering and Environmental Technology, Graz, Austria Shigenori Mitsushima Yokohama National University, Graduate School of Engineering Science, Institute of Advanced Sciences, Yokohama, Japan T^ eko W. Napporn University of Poitiers, Institut de Chimie des Milieux et Mat
eriaux de Poitiers (IC2MP), Poitiers, France Ken-Ichiro Ota Yokohama National University, Green Hydrogen Research Center, Yokohama, Japan Birgit Pichler Graz University of Technology, Institute of Chemical Engineering and Environmental Technology, Graz, Austria Uwe Reimer €lich GmbH, Institute of Energy and Climate Research IEK-3: Electrochemical Forchungszentrum Ju €lich, Germany Process Engineering, Ju Alexander Schenk Graz University of Technology, Institute of Chemical Engineering and Environmental Technology, Graz, Austria Jan Senn Graz University of Technology, Institute of Chemical Engineering and Environmental Technology, Graz, Austria Gaetano Squadrito Italian National Research Council, Institute for Advanced Energy Technologies “Nicola Giordano” (CNR-ITAE), Messina, Italy Gernot Voitic Graz University of Technology, Institute of Chemical Engineering and Environmental Technology, Graz, Austria

Preface Over many decades, research and development have focused on the direct conversion of chemical energy into electrical energy, and companies have repeatedly announced the upcoming market launch for this new, clean, and efficient technology. Direct electrochemical conversion, a new field of research for many engineers and technology developers, has been constantly setting new challenges. Proofs for fuel cell concepts are widely available. In the 1960s, the on-board power supply for spacecraft was developed with alkaline fuel cells, and in parallel, the first generation of phosphoric acid fuel cells was presented for on-site generation, followed by the market introduction of polymer electrolyte fuel cells and several demonstration projects of small combined heat and power plants, buses, and fuel-cell electric vehicles. The Enefarm fuel cell commercialization project started in Japan in 2009, but it was not until 2014 that the car manufacturer Toyota was able to make fuel cell electric vehicles publicly available. Some 200,000 residential fuel cell units were operational in Japan in 2018. What has made realization of the obvious advantages of fuel cells in a marketable competitive product so difficult? Looking back in history, as early as 1839, William R. Grove (a barrister in Swansea, Great Britain) and (almost simultaneously) Christian F. Sch€ onbein (a German-Swiss chemist) discovered the principle of the fuel cell. Grove observed an experiment in which two platinum strips were immersed in highly diluted sulfuric acid, surrounded by closed tubes containing oxygen and hydrogen, and produced a small current flow through preliminary electrolysis of the electrolyte. Only decades later, engines with a four-stroke work process were developed and demonstrated. By that time, scientists such as Gibbs, Helmholtz, and Ostwald had already thermodynamically confirmed that higher levels of efficiency could be achieved by directly converting chemical energy into electricity, and emphasized the positive effects of this technology on the environment. Since then, thousands of publications have appeared, but the highly challenging, interdisciplinary aspects of this “new” technology (in electrochemistry, material sciences, and engineering) and its hydrogen supply-related challenges have continuously decelerated product development and market introduction. Research institutions and universities have taken up and continue to cover all the aspects involved from basic research to system development, but it is only recently that key international industrial players have pushed the commercialization of electric vehicles and mobile applications. In this context, the International Summer School for Advanced Studies of Polymer Electrolyte Fuel Cells began in 2008 as a cooperative effort between Graz University of Technology, Austria, and Yokohama National University, Japan. The summer school alternates yearly between Graz and Yokohama. The international and intercultural training program was well accepted by the students and grew quickly, both in the number of students participating as also in the number of international lecturers active in the field of fuel cell research. This book is a result of this strongly interdisciplinary training program in research topics in the fields of hydrogen production, basic electrochemistry, thermodynamics, kinetics and catalysis, measurement and characterization techniques, lifetime and degradation of polymer electrolyte fuel cells, as also the design and development of complete fuel cell systems. It provides a short and concise introduction to the field of fuel cell research and development for engineers as well as for natural scientists.

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Preface

We hope this book will enhance the development and implementation of this exciting technology on the path to successfully establishing a clean and sustainable energy economy in the 21st century. Viktor Hacker, Shigenori Mitsushima, Ken-Ichiro Ota

Nomenclature LATIN SYMBOLS A A Ac a a a B Cdl c cp c̅p Cw cmax w Cdrag 1 cTdrag D Dg Dw d E E0 Ea Eeq E e F F Fk f f fcr G, g̅ g̅f g ΔG0 ΔG H H, ħ ħf I, i J Jˆ

area (cm2) coefficient in natural logarithm form of Tafel equation catalyst area coefficient activity coefficient in base 10 logarithm form of Tafel equation sulfonic acid group concentration (mol/m3, mol/kg) coefficient in equation for mass transport voltage loss double-layer capacitance (F) concentration (mol m
3) specific heat capacity at constant pressure (J kg
1 K
1) molar specific heat capacity at constant pressure (J mol
1 K
1) normalized water concentration in the membrane normalized water concentration in the membrane, equilibrated with liquid water electro-osmotic coefficient (drag coefficient) in membrane electro-osmotic coefficient in membrane at Cw ¼ 1 and T1 diffusivity (cm2 s
1) diffusion coefficient of gas in the membrane (m2 s
1) water diffusion coefficient in membrane (m2 s
1) separation of charge layers in a capacitor electrode potential (V) standard electrode potential (V) activation energy (J/mol) equilibrium electrode potential (V) electric field (V cm
1) magnitude of the charge on one electron (1.602 10
19 C) Helmholtz free energy (J, J mol
1) Faraday constant (96.485 C mol
1) generalized force (N) reaction rate constant (Hz, s
1) volume fraction of the conducting phase in the membrane critical volume fraction of the conducting phase Gibbs free energy (J, J mol
1) Gibbs free energy of formation per mole (J mol
1) acceleration due to gravity (m s
2) change of Gibbs free energy at standard temperature and pressure with pure reactants (J, J mol
1) change of Gibbs free energy (J, J mol
1) heat (J) enthalpy (J, J mol
1) enthalpy of formation per mole (J mol
1) current (A) molar flux, molar reaction rate (mol cm
2 s
1) mass flux (g cm
2 s
1, kg m
2 s
1)

xv

xvi

j j0 jlim jw jconv w jm w k Khyd L Lmem M m m˙ N NA n n n n˙ P P, p pl,a pl,c Q Q R R r r S S/C T t t U U0 Ueq UT U V VC Va Vr V v v v

Nomenclature

current density (A cm
2) exchange current density at electrolyte/electrode (A cm
2) limiting current density (A cm
2) water flux (mol/m2s) convective water flux through the membrane (mol/m2s) total water flux through the membrane (mol/m2s) Boltzmann’s constant (1.38 10
23 J K
1) hydraulic permeability of the membrane (mol/(m Pa s)) length (cm) membrane thickness (m) molar mass (g mol
1, kg mol
1) mass (kg) mass flow rate (kg s
1) number of moles Avogadro’s number (6.02 1023 mol
1) number of electrons transferred in the reaction number of cells in fuel cell stack number of moles (mol) number of moles per second (mol s
1) power or power density (W or W cm
2) pressure (bar, atm, Pa) hydraulic pressure of liquid water at anode side (Pa) hydraulic pressure of liquid water at cathode site (Pa) heat (J, J mol
1) charge (C) ideal gas constant (8.314 J mol
1 K
1) resistance (Ω) area specific resistance (Ω cm2) degradation rate (1 h
1) entropy (J K
1) steam-to-carbon ratio temperature (K, °C) time (s, min, h, d, a) exponent of the equation of the percolation theory cell voltage, potential (V) difference of standard electrode potentials for a cell (V) equilibrium cell voltage (theoretical OCV) (V) temperature-dependent thermodynamic voltage at reference concentration (V) internal energy (J, J mol
1) voltage (V) average voltage of a cell in a stack (V) activation overvoltage (V) ohmic voltage loss (V) volume (L, cm3) velocity (cm s
1) hopping rate (s
1, Hz) molar flow rate (mol s
1, mol min
1)

Nomenclature

W 0 W y z Z

work (J, J mol
1) work done under isentropic conditions (J, J mol
1) channel length (cm) normal direction membrane (m) impedance (Ω)

GREEK SYMBOLS α ϕ η ηact ηconc ηohmic ηmem λ λ μ μ μ̅ ρ ρ γ H2O σ σ σ mem

charge transfer coefficient water activity in gas phase, relative humidity overvoltage, potential loss (V) activation overvoltage (V) concentration overvoltage (V) ohmic overvoltage (V) membrane overpotential (V) stoichiometric coefficient water content viscosity (kg m s
1) chemical potential (J, J mol
1) electrochemical potential (T, J mol
1) resistivity (Ω cm) density (kg cm
3, kg m
3) water transfer coefficient (m/s) conductivity (S cm
1, (Ω cm
1)) pre-factor of the equation of the percolation theory (S/m) specific membrane conductivity (S/m)

SUPERSCIPTS 0 a c CL conv curr dif diff drag E, e, elec eff f GDL (HHV) i l (LHV) m; mem

standard or reference state anode cathode related to catalyst layer related to convective flux current operating condition diffusion diffusion related to drag transport of water electrical (e.g., Pe, Welec) effective property quantity of formation (e.g., A Hf) related to gas diffusion layer higher heating value species i related to liquid water lower heating value membrane

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mean P R ref rxn SK SYS w; H2O X 1, 2, 3, … σ

Nomenclature

related to an average value product or parasitic reactant reference value, reference operating condition change in a reaction (e.g., ΔHrxn) stack system water substance different stages of a process (e.g., T1, T2) related to membrane conductivity

ABBREVIATIONS 1D 1NL 3D AFC ASR BOP CCL CD CFD CHP CL CPO CPU DC DIR DMFC EMF FC FCV GDL HHV LH2 LHV MCFC MEA NTP OCV PAFC PEFC PEM PEM ppm

one dimensional normal liter, 1 L at NTP three dimensional alkaline (electrolyte) fuel cell area specific resistance, the resistance of 1 cm2 of fuel cell (N.B. total resistance is ASR divided by area) balance of plant cathode catalyst layer current density computational fluid dynamics combined heat and power catalyst layer catalytic partial oxidation central processing unit direct current direct internal reforming direct methanol fuel cell electromotive force fuel cell fuel cell vehicle gas diffusion layer higher heating value liquid (cryogenic) hydrogen lower heating value molten carbonate (electrolyte) fuel cell membrane electrode assembly normal temperature and pressure (20°C and 1 atm or 1.01325 bar) open circuit voltage phosphoric acid (electrolyte) fuel cell proton exchange (membrane) fuel cell or polymer electrolyte (membrane) fuel cell polymer electrolyte membrane proton exchange membrane or polymer electrolyte membrane—different names for the same thing which fortunately have the same abbreviation parts per million

Nomenclature

RH SOFC SPFC SRS STP TEM

relative humidity solid oxide fuel cell solid polymer fuel cell (¼ PEMFC) standard reference state (25°C and 1 bar) standard temperature and pressure (¼ SRS) transmission electron microscope

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CHAPTER

1

INTRODUCTION

Shigenori Mitsushima*, Bernhard Gollas†, Viktor Hacker‡ Yokohama National University, Graduate School of Engineering Science, Institute of Advanced Sciences, Yokohama, Japan* Graz University of Technology, Institute for Chemistry and Technology of Materials, Graz, Austria† Graz University of Technology, Institute of Chemical Engineering and Environmental Technology, Graz, Austria‡

CHAPTER OUTLINE 1.1 Electrochemical Systems and Fuel Cells ............................................................................................1 1.2 Types of Fuel Cells and Their Applications ........................................................................................4 1.3 Thermodynamics ..............................................................................................................................7 1.3.1 Energy Conversion Efficiency of Fuel Cells ..................................................................... 7 1.3.2 Electrochemical and Thermal Energy Conversion Efficiency ........................................... 10 1.3.3 Ideal Reversible Fuel Cell Efficiencies ......................................................................... 10 1.4 Recapitulation ............................................................................................................................... 12 1.5 Comprehension Questions and Exercises ........................................................................................ 13 References .......................................................................................................................................... 13

1.1 ELECTROCHEMICAL SYSTEMS AND FUEL CELLS Electricity is undoubtedly the most versatile energy carrier. Electrochemical systems, such as fuel cells, have a simple structure, and take fuel as an input and produce electrical energy as the output. Fuel cells consist of two electrodes, an electrolyte, a separator, and an external electrical circuit. Each component of the electrochemical system fulfills specific tasks. The electrodes are electronically conductive and have a large electroactive surface area. The electrolyte is an electrical insulator, but has high ionic conductivity. Such ions are typically positively (cation) or negatively (anion) charged atoms, or molecules, that represent the mobile species in electrochemical reactions. Although the set-up of such an electrochemical system looks rather simple, the electrochemical reactions at the interface between the electrodes and the electrolyte are often complex. One of the two electrodes is called an “anode.” The anode is the negative electrode of the system, where the reactant (fuel) is oxidized. During the electrochemical reaction, electrons are released from the reactant and are utilized in an external load. The oxidized (anode) reactant, i.e. cations, are transported away from the anode through the electrolyte to the second electrode called “cathode” due to Fuel Cells and Hydrogen. https://doi.org/10.1016/B978-0-12-811459-9.00001-3 # 2018 Elsevier Inc. All rights reserved.

1

2

CHAPTER 1 INTRODUCTION

the potential gradient (migration) and the concentration gradient (diffusion). The cathode is the positive electrode in the electrochemical system, where the oxidant is reduced. The electrons required for this reduction are supplied from the external circuit and originate from the charge separation at the anode. During its reduction at the cathode the oxidant reacts with the cations from the anode and forms the respective product. The separator ensures separation of the anode and cathode reactants to avoid direct chemical reaction and further prevent a direct electronic contact between the anode and cathode. Typically, hydrogen is the anode reactant, or fuel; whereas oxygen is the cathode reactant or oxidant. The corresponding product of the reaction of hydrogen and oxygen is water. In order to enable an electrochemical reaction, the electrolyte must be ionically conductive. In Fig. 1.1, a schematic drawing of a hydrogen-oxygen electrochemical system with a proton conductive electrolyte is shown. Such a system is generally known as a fuel cell – more specifically as an acidic fuel cell, because acids are typically used as proton conductive electrolytes. Electrochemical and total reactions are as follows: Anode: H2 ! 2 H + + 2 e

(1.1)

1 Cathode: 2 H + + O2 + 2 e ! H2 O 2

(1.2)

1 Total: H2 + O2 ! H2 O 2

(1.3)

Hydrogen (H2) is supplied to the fuel cell, where it is oxidized, forming cations (H3O+, hydrated protons or hydronium ions), usually called protons (H+) and electrons (e, electricity) on the anode of acidic fuel cells. The electrons are transported through the external circuit and power the device, for example, an electric motor. The protons are transported through the electrolyte and reach the cathode, where they react with the electrons and atmospheric oxygen (O2). Through this process, oxygen is reduced to water (H2O), which is the only reaction product. The most common fuel cell is the polymer electrolyte fuel cell (PEFC), whose electrolyte is a solid (i.e., polymeric) cation exchange membrane.

FIG. 1.1 Schematic drawing of a fuel cell with proton conductive electrolyte.

1.1 ELECTROCHEMICAL SYSTEMS AND FUEL CELLS

3

FIG. 1.2 Functions of electrochemical power systems.

In PEFCs, this electrolyte membrane simultaneously fulfills several important tasks, as serves as the proton conductor, the electronic insulator, between anode and cathode, and as separator for the gases disabling the direct contact of hydrogen and oxygen inside the fuel cell. The different operating principles of electric energy devices presented in Fig. 1.2 determine the fields of applications. Closed systems such as capacitors, electric double-layer capacitors, and batteries are used for energy storage. The electric double layer capacitors possess all components of an electrochemical system; capacitors, however, store energy as an electric charge on two electrodes that are separated by a dielectric without faradaic reaction. Batteries are capable of storing energy with faradaic reactions. Reactants and products of fuel cells are generally gaseous or fluid. The catalysts in the electrodes enhance the electrochemical reactions. A composite of a secondary battery and a fuel cell is the redox flow cell (RFC), or redox flow battery. RFCs are electrochemical cells, using technology identical to a reversible fuel cell, where the energy is provided or stored by two chemical components dissolved in liquids contained within storage tanks. Fig. 1.3 shows the specific power and the energy density of capacitors, electrochemical capacitors, batteries, and fuel cells in a so-called Ragone diagram. The discharge time for the end user discloses an opposing trend to the specific energy density [1].

4

CHAPTER 1 INTRODUCTION

FIG. 1.3 Specific energy and power density of energy devices.

1.2 TYPES OF FUEL CELLS AND THEIR APPLICATIONS The five major types of fuel cells are shown in Fig. 1.4. They operate at different temperature levels, have different tolerances on fuel impurities, and performance characteristics. The names are derived from the electrolytes such as alkaline, polymer, phosphoric acid, molten carbonate, and solid oxide electrolytes. In addition, direct fuel cells, which convert (liquid) fuels such as methanol, ethanol, or hydrazine electrochemically at the anode, are named after the respective fuel; for example, direct methanol fuel cells or direct hydrazine fuel cells.

FIG. 1.4 Materials and characteristics of hydrogen oxygen fuel cells.

1.2 TYPES OF FUEL CELLS AND THEIR APPLICATIONS

5

For low-temperature fuel cells (below 200°C) a high electrocatalytic activity is essential. Acidic electrolyte fuel cells use Pt group electrocatalysts, while alkaline fuel cells (AFCs) can use Ni-based electrocatalysts. High-temperature fuel cells (molten carbonate and solid oxide fuel cells (SOFCs)) operate on nickel-based anodes and electronically conductive oxide cathodes. AFCs were first demonstrated by Francis Thomas Bacon. AFCs have very high energy density because of the low oxygen reduction overpotential and electrolyte resistance. Unfortunately, their performance still suffers from the carbonization of the alkaline electrolyte in the presence of atmospheric CO2. The electrode reactions in AFCs are different from those in acidic electrolyte fuel cells: Anode: H2 + 2 OH ! 2 H2 O + 2 e

(1.4)

1 Cathode: O2 + H2 O + 2 e ! 2 OH 2

(1.5)

1 Total: H2 + O2 ! H2 O 2

(1.6)

AFCs were developed and applied for onboard power generation for spacecraft such as Apollo, and for the space shuttle program, and for undersea and military uses. They showed good performance and reliability. For the extraterrestrial applications, pure hydrogen and oxygen were used as fuel and oxidants. The electrolyte of the AFC is concentrated potassium hydroxide. Higher and less concentrated KOH are designed for high- (260°C) and low-temperature operation (80–90°C), respectively. The AFC for the space shuttle operated at a low-temperature, at 470 mA cm2 and 0.86 V. The electrocatalysts were 10 mg cm2 of 80% Pt-20% Pd for the anodes and 20 mg cm2 of 90% Au-10% Pt for the cathodes, bonded with polytetrafluoroethylene (PTFE) to metal mesh screen, respectively. High surface area Raney nickel anodes and Raney silver-based cathodes, or carbon-based porous electrocatalysts have been developed for underwater, mobile, and stationary applications [2]. Ongoing research and development in the field of polybenzimidazole (PBI) membranes for anion exchange membranes (AEMs) will allow applying non-noble metal electrocatalysts in PEFCs. For practical applications of AEMs, the improvements of stability and ionic conductivity are mandatory. The PEFC (also called polymer electrolyte membrane fuel cell PEMFC, proton exchange membrane fuel cell PEMFC, or solid polymer fuel cell, SPFC) uses an ion exchange membrane as an electrolyte. Developed PEFCs use perfluorosulfonic acid membranes (PFSA membranes) as solid electrolytes because of their high stability. The PFSA membrane was developed by DuPont under the Gemini space program. The PEFCs have been developed as a power supply for vehicles since the late 1980s using Dow Chemical’s perfluorinated membranes because of their low area resistance. The PFSA membranes are strongly acidic; therefore, the choice of conventional electrocatalysts is limited to the platinum group. Carbon material is used as a catalyst carrier for the platinum nanoparticles (Pt/C). The platinum loading (Pt/C) is approx. 50 wt% in order to get high activity. The catalyst layer (CL) consists of Pt/C or PtRu/C and the PFSA ionomer. The CLs are assembled with the PFSA membrane and called membrane electrode assembly (MEA). The electrode reactions for PEFCs are given in Eqs. (1.1) and (1.2). The PEFCs reach high energy density because of the very low ionic resistance of the ultrathin cation exchange membrane electrolytes. They have been commercialized in Japan in stationary applications for 1 kW class residential cogeneration systems “ENE  FARM” in 2009 and in mobile applications for fuel cell electric vehicles (FCEV) in 2014.

6

CHAPTER 1 INTRODUCTION

Most of the residential cogeneration systems use methane-based city gas. Liquefied petroleum gas is also used with a special fuel processor. The three step conversion of the fuel processor include steam reforming with alumina-supported nickel, the water gas shift reaction with Cu-Fe or Cu-Zn, and the selective oxidation of carbon monoxide with alumina-supported Ru catalysts. CH4 ðgÞ + H2 OðgÞ ! COðgÞ + 3 H2 ðgÞΔr H298 K ¼ 205:9 kJ mol1

(1.7)

COðgÞ + H2 OðgÞ ! CO2 ðgÞ + H2 ðgÞΔr H298 K ¼ 41:1 kJ mol1

(1.8)

1 COðgÞ + O2 ðgÞ ! CO2 ðgÞΔr H298K ¼ 283:0 kJ mol1 2

(1.9)

The maximum content of carbon monoxide in the fuel for stationary PEFCs using PtRu/C anodes is approximately 10 ppm. With the appropriate choice of a highly selective catalyst, the reduction of CO in the synthesis gas from 1% to 50%, and the overall efficiency of the CHP is approximately 90%. The systems operate according to electrical house load demands with a lower limit of power generation to maintain the operating temperature. As shown herein, fuel cell power generation systems have high energy conversion efficiencies, low emissions, and can be utilized in a number of applications, such as onsite CHP systems or fuel cell electric vehicles. Until now, fuel cell systems have not been widely used, because their system costs are higher than conventional systems. In order to introduce fuel cell systems, we have to decrease their costs by developing new materials and production methods and find suitable applications. The strong points of fuel cells, and the energy conversion efficiency of fuel cells and their systems are discussed in following section.

1.3 THERMODYNAMICS 1.3.1 ENERGY CONVERSION EFFICIENCY OF FUEL CELLS Energy conversion efficiency is one of the key characteristics of energy systems. The definition of the energy conversion efficiency is the useful energy output (benefit) divided by the energy input (cost). Energy can be divided into quantity and quality terms. For electric power, quantity and quality are described by current and voltage, respectively. The electric power efficiency of hydrogen fuel cells can be written as follows.

8

CHAPTER 1 INTRODUCTION

εfc ¼

Output energy Electric energy ¼ ¼ ½current eff:½voltage eff: Input energy Chemical energy

¼

(1.14)

finput  fexhaust ΔGH2 U U ¼ εF  0:945  ¼ 0:798  εF  U finput ΔHH2 Uth 1:184

Here, εfc, finput, fexhaust, Δ HH2, Δ GH2, Uth, U, and εF are fuel cell efficiency, flow rate of input hydrogen, flow rate of exhaust hydrogen, enthalpy change of hydrogen combustion, Gibbs energy change of hydrogen combustion, theoretical cell voltage, actual cell voltage, and hydrogen utilization, respectively. The fuel cell efficiency is proportional to the hydrogen utilization and the cell voltage. The hydrogen utilization of systems using pure hydrogen, such as fuel cell electric vehicles, is approximately 99%, because of the hydrogen recirculation loop. On the other hand, most residential fuel cell systems include a fuel processer to combust the exhaust hydrogen of the fuel cell stack to produce the heat for steam reforming. In addition, the stationary systems use synthesis gas diluted by carbon dioxide, which lowers the hydrogen partial pressure and might lead to fuel starvation under high fuel utilization operation. Hydrogen utilization of the reformate system is usually approximately 80%. The system efficiency of the reformate system is composed of the fuel processer, fuel cell, and system operation efficiencies. The efficiency of the fuel processor can be calculated by the enthalpy of the hydrogen output divided by the enthalpy of the fuel input minus the heat recovered from the fuel cell stack and the heat produced by exhaust hydrogen combustion. The system efficiency of the fuel processor is represented by: εsys ¼ εfp εfc εop ¼

ΔHH2
εfc εop ΔHprocessfuel + ΔHheatingfuel  ΔHexhaust + ΔHheat recovery

(1.15)

Here, εsys, εfp, εop, ΔHprocess-fuel, ΔHheating-fuel, ΔHexhaust, and ΔHheat recovery are system efficiency, fuel processing efficiency, system operation efficiency, combustion enthalpy of fuel to produce hydrogen, combustion enthalpy for fuel of the reformer, combustion enthalpy of exhaust gas from the fuel cell stack, and heat recovery from heat production of the fuel cell stack. When the endothermic heat of the reformer is supplied with the combustion heat of the fuel cell exhaust gas, ΔHheating-fuel is almost equal to ΔHexhaust, and ΔHheat recovery is negligible. εfp 

ΔHH2 ΔHprocessfuel



ΔHH2 241:8 kJ mol1 ¼ 120:5% ¼ ΔHCH4 =4 200:7 kJ mol1

(1.16)

The 120.5% fuel processor efficiency corresponds to the inverse hydrogen utilization of the fuel cell stack. The low-temperature fuel cell has almost no heat recovery for the fuel processor because the operation temperature is too low for steam reforming. High-temperature fuel cells use waste heat of the fuel cell stack for fuel processing to increase the system efficiency. When the hydrogen utilization and the operating fuel cell voltage are 80% and 0.75 V, respectively, the electrical efficiency is 47.9%. If system operation and fuel processing efficiencies are 90 and 120.5%, respectively, the system efficiency is 51.9% without heat recovery. High-temperature fuel cells can use the exhaust heat of the fuel cell stack for fuel processing. Then, the hydrogen utilization can increase further. If the hydrogen utilization is 95%, the fuel cell and the system efficiencies become 56.9 and 61.7%, respectively. Therefore, heat recovery of high-temperature fuel cell systems can improve the energy conversion efficiency considerably. The internal reforming systems are characterized by a highly efficient heat exchange and improved fuel conversion efficiency because the reaction product hydrogen is selectively consumed at the electrode and thereby enhances the hydrogen production by reforming in the anode channel.

1.3 THERMODYNAMICS

9

FIG. 1.5 Electric power generation efficiency as a function of the plant power.

Fig. 1.5 shows the electric power generation efficiency as a function of the plant power for fuel cell systems, internal combustion power generation systems, and high-temperature fuel cell combined cycle systems. Electrochemical systems like fuel cells are two-dimensional reactors. The efficiency is size independent, because the energy conversion occurs only at the electrode/electrolyte interface. Internal combustion systems based on thermal cycles are three-dimensional reactors; therefore, the efficiency increases with the plant size, because the surface area per unit volume decreases with the increase of the plant size. As you can see in Fig. 1.6, the efficiencies of the thermal cycle systems of gas turbines, engines, advanced combined cycles, and integrated combined cycles increase with the plant power. On the other hand, the electric power generation efficiencies of the fuel cell cogeneration systems are

FIG. 1.6 Theoretical energy conversion efficiency as a function of temperature for fuel cells, thermal cycle, and fuel cell combined cycles.

10

CHAPTER 1 INTRODUCTION

almost independent from size. Therefore, small size fuel cell cogeneration systems have a major advantage over thermal cycle cogeneration systems, because they are more efficient in power generation and can also supply heat onsite. High-temperature fuel cell combined cycles achieve even higher efficiencies than both fuel cell cogeneration and thermal cycle systems. Their efficiency increases with power because of the characteristics of the thermal bottoming cycle. Distributed CHP systems demonstrate the greatest benefits in terms of efficiency and costs compared with conventional technologies (see Fig. 1.5) [4].

1.3.2 ELECTROCHEMICAL AND THERMAL ENERGY CONVERSION EFFICIENCY Electrochemical systems convert chemical energy into electrical energy and vice versa. For fuel cells, the usual basic reaction is the formation of water by two separated electrochemical reactions between hydrogen and oxygen. According to the stoichiometry of the reactions, each hydrogen molecule produces two electrons. As the electrical work wel is defined as the product of the total charge produced per mole of hydrogen q and electrical potential difference E, the electrical work associated with moving this charge can be denoted as: wel ¼ q  E

(1.18)

wel ¼ 2F  E

(1.19)

Here, wel, q, and E are electrical work, electrical charge, and potential. For a reversible ideal electrochemical system, this electrical work is the maximum achievable work under isothermal and isobaric conditions and corresponds to the change in the Gibbs free energy. wel, rev ¼ ΔG

(1.20)

Therefore, the reversible ideal potential for a fuel cell reaction (EMF electromotive force of the oxidation) involving n electrons can be deduced from: ΔG nF

(1.21)

Δr G0 nF

(1.22)

Erev ¼ U0 ¼

For hydrogen fuel cells, n equals 2 (two moles of electrons are transferred for every mole of H2 gas reacted) and F, being Faraday’s constant (96,485 C mol1, 26.9 Ah/mol; 26.7 Ah/ghydrogen). Therefore, the EMF is calculated with 1.22 and 1.19 for the reaction products of water and steam, respectively. Due to the irreversible entropy changes, only the Gibbs free energy portion Δ G of the total enthalpy change can be converted to electrical energy.

1.3.3 IDEAL REVERSIBLE FUEL CELL EFFICIENCIES The reversible efficiency is applied to compare the maximum efficiency of fuel cells and heat engines. A thermodynamic process is reversible if the original state of the system can be regained without leaving changes in the environment (the reversible process is quasistatic—there should be no imbalances during the process). A process is irreversible if it cannot be completely undone. Although the system can restore its initial state, changes in the environment remain.

1.3 THERMODYNAMICS

11

Conventional energy conversion technologies (such as internal combustion engines) generate heat via the combustion of (fossil) fuels, which is then converted into work in a cyclic process. The reversible Carnot cycle provides an upper limit for the heat engine. In the Carnot cycle, the greatest possible share of the heat produced by combustion is converted into work. The Carnot process consists of two isothermal and two isentropic steps. The ideal energy conversion efficiency of a heat engine is the Carnot efficiency εTH. εTH ¼ 1 

TL TH

(1.23)

Here, TL and TH are lower and higher temperatures in the thermal cycle, respectively. The lower and higher temperatures are as near as possible to the ambient and the combustion temperature, respectively. Fig. 1.6 shows the theoretical energy conversion efficiency as a function of temperature for fuel cells, thermal engines, and combined cycles of fuel cells and thermal engines, based on the following equations. εfc ðT Þ ¼

U 0 ðT Þ UΔH ð298Þ

εTH ðT Þ ¼ 1 

298 T

εC ðT Þ ¼ εfc ðT Þ + ð1  εfc ðT ÞÞ  εTH ðT Þ

(1.24) (1.25) (1.26)

The reversible efficiency of a heat engine improves as the operating temperature increases. In contrast, the reversible efficiency of a fuel cell tends to decrease as the operating temperature increases. The highest achievable electric work, that is, the reversible work Wmax ¼ Wel of an electrochemical reaction at constant temperature and pressure, can be calculated from the free enthalpy change ΔG of the reaction (Faraday’s law) with n, F, and E as the number of exchange electrons, the Faraday constant, and the reversible cell potential, respectively. For a spontaneous reaction, ΔG is negative. Wel ¼ ΔG ¼ nFE

(1.27)

For an isothermal process, a heat flux ΔQrev dependent on the direction and amount of the enthalpy change will arise (a negative sign of ΔQrev indicates that heat is flowing out of the system). According to the second law of thermodynamics for a reversible process, the heat flux is calculated from the reaction entropy ΔS and and the reaction enthalpy ΔH: ΔQrev ¼ TΔS ¼ ΔH  ΔG

(1.28)

The value of TΔS is the amount of heat that is flowing into the cell from the surroundings when ΔS has a positive value (e.g., for a fuel cell using carbon as fuel 2C + O2 ! 2CO). During operation of the fuel cell, irreversible reactions in the cell overcompensate this effect. The reversible efficiency ηrev of the fuel cell is given by the ratio of the reaction free energy ΔG and the reaction enthalpy ΔH. εfc ¼

ΔG TΔS ¼1 ΔH ΔH

(1.29)

The reversible efficiency for hydrogen fuel cells is shown in Fig. 1.6. The efficiency is calculated based on the combustion heat at ambient temperature (LHV). The reaction product of the fuel cell is liquid water below 100°C, and steam above 100°C operation temperature. The energy conversion efficiency

12

CHAPTER 1 INTRODUCTION

of the combined cycle is the sum of the fuel cell efficiency and the heat recovery efficiency that is calculated with the products of exhaust heat and the Carnot efficiency. The fuel cell efficiency decreases with increasing operation temperature, because of the dependence of the Gibbs energy change on temperature. The Carnot efficiency is higher than the fuel cell efficiency above 1000°C. Thus from a thermodynamic point of view, a low-temperature fuel cell is better than a high temperature one, and the efficiency of the thermal engine increases with the scale of the plant. This means that due to their nature, fuel cells are most suitable for a distributed power supply with low-temperature operation. By contrast, the efficiency of the combined cycle increases with temperature at relatively low temperatures and stronger than both the fuel cell and the Carnot efficiency. Therefore, the high-temperature fuel cells should be a large scale combined cycle from the thermodynamic point of view. The thermoneutral cell voltage Eth is the theoretical voltage in the hypothetical case of all enthalpy of formation was transformed into electrical energy. The thermoneutral ideal cell voltage Eth,HHV of 1.48 V is derived by using the higher heating value (HHV) of hydrogen Δ H0HHV, which equals the standard enthalpy of formation (ΔfH0) of liquid water. For high-temperature fuel cells, the reaction product water is in the gaseous state. The thermoneutral ideal cell voltage Eth,LHV is then calculated with 1.25 V using the LHV of hydrogen Δ H0LHV, (ffi standard enthalpy of formation (ΔfH0) of gaseous water).

1.4 RECAPITULATION Fuels cells, like all galvanic cells, convert chemical energy into electricity. In general, fuel cells consist of two electrodes, the anode (negative electrode) and the cathode (positive electrode), an ionically conductive electrolyte, a separator, and an external electrical circuit. The anode is fed with the reactant, in most cases hydrogen, where it is oxidized; the cathode is supplied with atmospheric oxygen, which is reduced to water. Fuel cells convert chemicals to electrical energy, whereas other types of galvanic cells, such as batteries or capacitors, are used to store electrical energy. There are different types of fuel cells that are differentiated by electrolyte, operating temperature, oxidant, and/or fuel. The names are derived from the electrolytes, or in the case of direct cells from the respective fuel. Low-temperature fuel cells need highly selective and active electrocatalysts. Acidic fuel cells use Pt group metal catalysts because of the corrosive environment. In AFCs, as well as in high-temperature fuel cells, non-noble metals are applied as catalysts. PEFCs use solid membranes (e.g., Nafion) as electrolytes and are applied for fuel cell electric vehicles (FCEV) and CHP systems for residential homes. The efficiency of residential CHP systems depends strongly on the integration of the hydrocarbon reformer, the water gas shift reaction, and the carbon monoxide purification (CO 1. Actually, the measured electroosmotic coefficient was reported to be about Cdrag ¼ 4 during water electrolysis. On the contrary to this vehicle mechanism, the protons can hop from one water molecule to another (Grotthuss mechanism). Protons conducted by Grotthuss hopping exhibit an electroosmotic coefficient of Cdrag ¼ 0. Measured values range from 0.2 to 1.5 (see Fig. 3.8); thus, both the vehicle and Grotthuss mechanism occur at the same time in most situations.

3.3.5 PRACTICAL EXAMPLES With the transport factors, the simulation of water distribution and performance of PEFC is possible. Fig. 3.9 depicts an example of the water concentration profile in a PEFC. The highest concentration is observed in the downstream region of the cathode GDL, as marked by the circle in Fig. 3.9. In this section, some examples of measured and calculated current density distributions will be presented to reveal how the mass transport affects the PEFC performance, and provide insight into how to define the operating conditions.

FIG. 3.8 Electroosmotic coefficients as a function of relative humidity.

3.3 SIMPLIFIED APPROACH TO PREDICT PEFC PERFORMANCE

49

FIG. 3.9 Simulated water concentration distribution in a PEFC.

3.3.6 DRY OXYGEN SUPPLY Fig. 3.10 shows current densities with a gas supply of 100 cm3 min1 H2, 300 cm3 min1 O2, and dew point of 30°C. The cell temperature is kept at 60°C. The activation overvoltage is uniform due to a sufficient gas supply of oxygen, so it does not affect the current density. At co-flow (A), current densities increase from the inlet as the membrane and ionomer water content increases with generated water. In counter-flow mode, the current distribution becomes more uniform. In Fig. 3.11, the flow rate is lower compared with Fig. 3.10. The other parameters are unchanged. The current density distributions behave in a similar manner to those of Fig. 3.10. In Fig. 3.10A, the current density reaches a plateau at the middle-stream region, because the relative humidity reaches 100% at the middle-stream region, and the membrane cannot be humidified any further. The area of current density plateau shifts downstream as the average current decreases and less water is generated.

3.3.7 HUMIDIFIED AIR SUPPLY Fig. 3.12 represents current densities with a humidified air supply of TDPa ¼ TDPc ¼ 59°C H2 ¼ 40 cm3 min1, air ¼ 100 cm3 min1. In co-flow (A), the local current density decreases from the inlet, because the oxygen partial pressure decreases and the activation overvoltage increases along the channel. The accumulated water near the cathode outlet decreases the local current density. A bilaterally symmetric distribution is observed in the counter-flow configuration (B), because the effects of the cathode side, such as oxygen convection and flooding, dominate the local current distribution. The consistency of the numerical and measured results indicates that a simulation model with presented transport factors can provide a useful tool for estimating the PEFC power generation performance.

50

CHAPTER 3 MODELING OF POLYMER ELECTROLYTE FUEL CELLS

Current density (A cm−2)

1.0 0.8

iave = 0.4A cm−2,hO2 = 5.1%

0.6

Calculated Measured

0.4

iave = 0.24A cm−2,hO2 = 3.0%

H2 in

H2 out

O2 in

O2 out

iave = 0.08A cm−2,hO = 1.0%

0.2

2

0.0 0

5

(A)

10 15 20 Channel length (cm)

25

Current density (A cm−2)

1.0 0.8 Measured

0.6

iave = 0.4A cm−2,xO2 = 5.1%

0.4 iave = 0.3A cm−2,xO2 = 3.8%

0.2

iave = 0.1A cm−2,xO2 = 1.3%

0.0 0

(B)

Calculated

5

10 15 20 Channel length (cm)

H2 in

H2 out

O2 out

O2 in

25

FIG. 3.10 Current densities with dry oxygen gas supply: (A) co-flow and (B) counter-flow (Tcell ¼ 60°C, TDPa ¼ TDPc ¼ 30°C; H2 ¼ 100 cm3 min1, O2 ¼ 300 cm3 min1).

3.4 MEMBRANE TRANSPORT MODEL The membrane transport model (PEM model) estimates the membrane overpotential. Fig. 3.13 depicts the most important processes taking place in the membrane coated with a catalyst layer in an operating fuel cell [20]. These are the proton and electroosmotic transports, the electrochemical water generation, the water diffusion, and convection. Water diffusion arises due to the water concentration difference between the cathode side and the anode side. Usually, the cathode water concentration is higher. The electroosmosis (often called electroosmotic flux or water drag) is caused by the conjugated transport of the water molecules with the protons; the water molecules mainly move in the protons’ solvation shells. The convective flux of water is caused by a pressure gradient. The membrane transport model suggested in [20,21] considers the three above mentioned mechanisms of the water transport and takes into account temperature effect on the water sorption isotherm of the membrane. The boundary conditions of water transport utilize the modified equations from

3.4 MEMBRANE TRANSPORT MODEL

51

Current density (A cm−2)

1.0 0.8 iave = 0.5A cm−2,hO = 47.5%

Calculated

0.6

2

Measured iave = 0.3A cm−2,hO = 28.5% 2

0.4

iave = 0.1A cm−2,hO = 9.5%

0.2

H2 in

H2 out

O2 in

O2 out

2

0.0 0

5

(A)

10

15

20

25

Channel length (cm)

Current density (A cm−2)

1.0 0.8 Calculated Measured

iave = 0.5A cm−2,hO = 47.5% 2

0.6

iave = 0.3A cm−2,hO = 28.5%

0.4

2

0.2

iave = 0.1A cm−2,hO = 9.5% 2

H2 in

H2 out

O2 out

O2 in

0.0 0

(B)

5

10 15 20 Channel length (cm)

25

FIG. 3.11 Current densities with dry oxygen gas supply: (A) co-flow and (B) counter-flow (Tcell ¼ 60°C, TDPa ¼ TDPc ¼ 30°C; H2 ¼ 40 cm3 min1, O2 ¼ 40 cm3 min1).

Berg et al.’s model [22], the membrane overpotential is calculated using Ohm’s law, and the water diffusion and electroosmotic coefficient, as well as membrane conductivity, are functions of the membrane water concentration Cw and temperature.

3.4.1 GOVERNING EQUATIONS The water diffusion through the polymer electrolyte membrane is expressed by Fick’s law: Dw

dCw jdif dCw ¼ w , jdif w ¼ a  Dw  dz a dz

(3.10)

The electroosmotic flux is expressed by: jdrag ¼ Cdrag  w

i F

(3.11)

52

CHAPTER 3 MODELING OF POLYMER ELECTROLYTE FUEL CELLS

1.0 Current density (A cm−2)

Measured

0.8 Calculated iave = 0.5A cm−2,hO = 95%

0.6

2

0.4 H2 in

H2 out

O2 in

O2 out

iave = 0.3A cm−2,hO2 = 57%

0.2

iave = 0.1A cm−2,hO2 = 19%

0.0 0

(A)

5

10 15 20 Channel length (cm)

25

Current density (A cm−2)

1.0 iave = 0.5A cm−2 ,hO = 95%

0.8

2

Calculated

0.6 Measured

iave = 0.3A cm−2 ,hO = 57% 2

0.4

H2 in

H2 out

O2 out

O2 in

iave = 0.1A cm−2 ,hO = 19%

0.2

2

0.0 0

(B)

5

10

15

20

25

Channel length (cm)

FIG. 3.12 Current densities with humidified air supply: (A) co-flow and (B) counter-flow (Tcell ¼ 60°C, TDPa ¼ TDPc ¼ 59°C H2 ¼ 40 cm3 min1, Air ¼ 100 cm3 min1).

Anode GDL

Cathode

CL

PEM

CL

Ccw Diffusion

GDL Cc*w

Caw

Ca* w

Electroosmosis

Electroosmosis Water generation

Convection H2 → 2 H+ + 2e−

Convection if pl,c>pl,a

O2 + 4 H+ + 4e− → 2 H2O

FIG. 3.13 Transport and chemical phenomena in proton exchange membrane coated by catalyst in an operating PEFC.

3.4 MEMBRANE TRANSPORT MODEL

53

The convective flux is calculated from: jconv ¼ Khyd  w

pl, a  pl, c Lmem

(3.12)

At ϕ < 1 water vapor operating conditions, the convective flux is negligible, and the total water flux through the membrane is found from the sum of diffusive and electroosmotic fluxes: jm w ¼ a  Dw

dCw i + Cdrag  dz F

(3.13)

At ϕ  1, if liquid water is formed at both boundaries of the CL/PEM, the water concentrations at the anode and cathode sites are equal Caw ¼ Ccw. In this case, the diffusive flux is zero, but the convective flux is taken into account, and consequently the total water flux through the membrane is described by: jm w ¼ Cdrag 

i pl, a  pl, c + Khyd  Lmem F

(3.14)

Water diffusion and electroosmotic coefficient, as well as membrane conductivity, are functions of the membrane water concentration Cw and temperature. The membrane overpotential is calculated based on Ohm’s law and expressed by: ηmem ¼

i  Lmem σ mem

(3.15)

3.4.2 EXPERIMENTAL VALIDATION OF THE MODEL The model was implemented in the CFD Code AVL FIRE [23]. The thorough test and validation of the fuel cell model was carried out on a fully coupled fuel cell with an active area of 25 cm2. The fully coupled fuel cell is a test cell with 13 parallel straight channels. Fig. 3.14A demonstrates an image of the bipolar plate and end plate of the test cell with the gas channels. Fig. 3.14B displays the computational meshes of the full coupled fuel cell, which completely corresponds to the geometry of the experimental test cell. Fig. 3.14C–E presents a comparison of the simulated performance of the test cell with the experimental data. As seen in the Fig. 3.14A–E, the calculated values are in agreement with the experimental data, confirming the validity of the PEFC model. The polarization curves at different pressure levels at the cathode outlet (see Fig. 3.14C) indicate an increase of the fuel cell current density with growing pressure. Higher pressure leads to an elevation of the oxygen concentration, resulting in higher reaction rates and current density. Lower inlet relative humidity (see Fig. 3.14E) leads to a less humid membrane, and consequently, to a lower proton conductivity and current density. This performance decrease occurs both in the experiment and in the simulation.

3.4.3 ROLE OF WATER DIFFUSION AND ELECTROOSMOSIS IN THE WATER MANAGEMENT Water diffusion and electroosmotic coefficients play an important role in the water management of the PEFC. They contribute to the total water flux, and influence the relative humidity and formation of liquid water in the GDL. An accumulation of condensed water in the GDL can lead to a dramatic drop in the cell voltage. An increase in the water diffusion coefficient of the membrane leads to the

54

CHAPTER 3 MODELING OF POLYMER ELECTROLYTE FUEL CELLS

(A)

(B)

1 U = 0.813 V simulation

1.2

U = 0.690 V simulation

Simulation

0.9

U = 0.813 V experiment

1

101,325 kPa

U = 0.690 V experiment

U (V)

141,325 kPa

0.8

I (A cm−2)

121,325 kPa

0.8 0.6 0.4

0.7

0.2 0.6

0 0

(C)

0.2

0.4

0.6

0.8

1

1.2

−2

0

2

4

(D)

I (A cm )

6 y (cm)

8

10

12

1 RHc = 0,9 simulation RHc = 0,6 RHc = 0,3

0.9

RHc = 0,9 experiment

U (V)

RHc = 0,6 RHc = 0,3

0.8

0.7

0.6 0

(E)

0.2

0.4

0.6

0.8

1

1.2

I (A cm−2)

FIG. 3.14 Experimental validation of the PEFC model: (A) bipolar and end plate; (B) computational meshes of the cell; (C) polarization curves at different pressures at the cathode outlet (λa/λc ¼ 1.5/2.2; T ¼ 70°C; RHa ¼ RHc ¼ 90%; pa ¼ 101,325 Pa); (D) the distribution of the current density along the channel from inlet (y ¼ 0 cm) to outlet (y ¼ 12 cm); (E) polarization curves at different levels of relative humidity at the cathode. In depictions (D) and (E), the measurements are presented by markers, and the simulation by lines.

3.5 MODELING DEGRADATION IN PEFCs

55

Volume fraction of liquid water 0.03 0.027 0.024 0.021 0.018 0.015 0.012 0.009 0.006 0.003 0

(A)

(B)

(C)

FIG. 3.15 The distribution of the liquid water in the anode GDL at the interface to the bipolar plate/channel at the different values of the electroosmotic coefficient in the PEM, cdrag at 298 K (A) 0.054; (B) 0.108; (C) 0.160. The simulation was performed using the CFD Code AVL FIRE [23] with the membrane transport model [20].

formation of condensed water, whereby the volume fraction of the liquid water grows in locations that are remote from the gas channel [24]. Fig. 3.15 illustrates the distribution of the liquid water in the anode at the interface of the GDL/bipolar plate for the tested values of the electroosmotic coefficient. The increase in electroosmotic coefficients reduces the concentration of condensed water in the anode.

3.5 MODELING DEGRADATION IN PEFCs Experimental investigations have reported [25–33] that, under conventional operating conditions, the polymer electrolyte membrane degrades much faster than the gas diffusion and catalyst layers. Theoretical estimations showed that during the cycling operation, a drop in the fuel cell potential is mainly caused by the membrane degradation, with a degradation rate of approximately 7.34  105 V/h [34]. The degradation processes in polymer electrolyte membranes can be divided into three main groups: (1) mechanical destruction causing pinhole and crack formation in the polymer; (2) chemical degradation dealing with decomposition of the polymer chain via a chemical reaction with hydrogen peroxide free radicals; (3) thermal degradation taking place at high temperature (>150°C) and leading to membrane dry-out and decomposition of sulfonyl functional groups. At relative humidities close to 100% and temperatures below 75°C, the voltage degradation can be lower than 1–2 μV/h [25]. However, the degradation rates can increase by orders of magnitude when conditions include some of the following: (1) load cycling; (2) start-stop cycles; (3) temperature

56

CHAPTER 3 MODELING OF POLYMER ELECTROLYTE FUEL CELLS

Table 3.1 Overview of Degradation Models Considered Domain

Level

Methods

Single component: Polymer electrolyte membrane Catalyst layer Gas diffusion layer

Microscale

Ab initio [35–36] Molecular dynamics [38] Monte Carlo [37] Kinetic models coupled with transport models [38–47] Equivalent circuit model [48] Fault tree analysis [34,49] Neutral network modeling [50] Ab initio + CFD simulation [51–54] Semi-empirical model [55] Fault tree analysis [56] Petri nets [57] Particle filtering framework [58–59]

Single fuel cell

Macroscale Macroscale

Fuel cell stack

Multiscale Macroscale

cycling; (4) low humidification; (5) humidification cycling; (6) a temperature of 90°C or higher; (7) fuel starvation. Several degradation models have been published [34–59]. Most of the models consider degradation phenomena in a single component of the fuel cell. In accordance with utilized theory, and a level on which degradation phenomena are described, the models can be divided into the following categories (Table 3.1): (1) microscale; (2) macroscale; (3) multiscale models. The microscale models describe the degradation processes on atomic and molecular levels using approaches of quantum-chemistry such as density functional theory (DFT), the Hartree-Fock Method (HF), molecular-dynamics (MD), and Monte Carlo methods. The macroscale models are usually based on the transport models by taking into account the kinetics of the chemical reactions of the degradation. These models are able to predict changes in the macroproperties of the fuel cell materials and cell performance. An example of a new model [60] for 3D CFD analysis of the PEFC as a function of time, relative to the degradation phenomena in the membrane, is introduced here. The degradation rates of the membrane thickness are assumed to be 3.71  104 1/h [61], and for the ionic conductivity, are 5.64  10–4 1/h [62]. The chemical destruction of the perfluorinated sulfonated membranes is mainly caused by interaction of the hydroxyl radicals with the membrane polymer chains. The basic reactions (3)–(7) of the radical formation, which could take place in the PEFC, are presented in Fig. 3.16. One possible mechanism of Nafion destruction is the unzipping of the polymer main chains (backbone) proceeding in accordance with reactions (8)–(10) [43,56]. These reactions result in shortening main chains, and consequently, in loss of the equivalent mass of the polymer. Other studies [36,63,64] have proved a degradation of the side chains corresponding to reactions (11) and (12). In this case, hydroxyl radicals attack C―O bonds in the ether groups of the side chains. Reactions (11) and (12) lead to loss of the parts of the side chains containing the functional groups, resulting in a decrease of the polymer equivalent mass and membrane conductivity. Results of the degradation model on a single-channel cell with membrane Nafion 115, Lmem ¼ 124 μm is shown in Fig. 3.17. The simulation indicates a decrease in the cell performance after 1000 h of cell operation. The slope of the polarization curve slightly increases due to the raise in the

3.5 MODELING DEGRADATION IN PEFCs

57

I. CF2

CF

CF2

(CF2

CF2)x

O x = 7–12

CF2 II.

CF3

CF O CF2 CF2

SO3H

(A) PEM

Anode

H2 → 2H+ + 2e− (1)

H+

Cathode

4H+ + O2 + 4e− → 2H2O (2)

H2 H • + O2 → HOO •

(3)

O2

H • + HOO → H2O2 (4) H2O2 ↔ 2OH •

(5)

2H+ + O2 + 2e− → H2O2 H2O2 ↔ 2OH •

(6) (7)

Unzipping of backbone (I): R – CF2COOH + OH · → R – CF2 · + CO2 + H2O

(8)

R – CF2 · + OH · → R – CF2OH → R – COF + HF

(9)

R – COF + H2O → R – COOH + HF

(10)

Splitting of the side chain (II): containing functional group:

(B)

R – OCF2CF(CF3)O(CF2)2SO3− + OH · → R – OH + OCF2CF(CF3)O(CF2)2SO3−

(11)

R – OCF2CF(CF3)O(CF2)2SO3− + OH · → R – OCF2CF(CF3)O + HO (CF2)2SO3−

(12)

R – OCF2CF(CF3)O + H2O + OH · → R – (CF2)nCOOH + HF

(13)

FIG. 3.16 (A) Chemical structure of the perfluorinated membrane of Nafion type: (I) main chain of the polymer, (II) the side chain containing the functional group. (B) Proton and gas transport: (1) oxidation of hydrogen; (2) reduction of oxygen; (3)–(5) formation of hydrogen peroxide radicals at the anode; (6) and (7) formation of hydrogen peroxide radicals at the cathode; (8)–(10) unzipping mechanism of the polymer main chain (backbone) of the perfluorinated membrane; (11)–(13) splitting mechanism of the polymer side chain, containing functional (acid) group.

58

CHAPTER 3 MODELING OF POLYMER ELECTROLYTE FUEL CELLS

0h

500 h

1000 h

(A) Reaction current density (A/m2) 3700 3630 3560 3490

H2

3420 3350 3280 3210 3140 3070 3000

(B)

0h

500 h

1000 h

FIG. 3.17 The evolution of: (A) the membrane thickness and (B) the current density in the membrane at the anode side. The simulation is performed on the single channel cell with Nafion 115 (Lmem ¼ 124 μm), at p ¼ 1.01 bar; T ¼ 70°C; RH ¼ 100% with AVL FIRE.

membrane resistance. Mechanical defects (pinholes and cracks) in the membrane occur at a critical membrane thickness and cause rapidly rising hydrogen crossover through the membrane.

3.6 RECAPITULATION With suitable cell and stack models, it is possible to predict the performance of fuel cells, which depends on cell geometry, material properties, and operating conditions. Generalized models of fuel cells consist of coupled submodels describing physical and chemical processes in several components. The accuracy of PEFC simulation is affected decisively by the correctness of the selected submodels. The transport processes in polymer electrolyte fuel cells (as well as their interactions) are complex. Three-dimensional computational fluid dynamics (CFD) simulation, in combination with electrochemical models, helps us understand the physical behavior of PEFCs and, thus, reduces development time and costs considerably.

REFERENCES

59

The current challenge in CFD modeling of the PEFC is simulating the degradation phenomena of cells, components, and materials and predicting the cell performance over the lifetime of the fuel cell. The discussed degradation model is based on the approach that the membrane degradation rate depends on hydroxyl radical concentration, which is a function of oxygen concentration in the ionomer. The investigation of the operating parameters on the performance is used to identify areas or spots in fuel cells with critical operating conditions. Moreover, these simulation tools enable investigation of these effects on material degradation and, thus, help to find an optimal compromise between material costs and performance.

3.7 COMPREHENSIVE QUESTIONS AND EXERCISES – – – – – – – – – – – – – –

What is the main aim of modeling a fuel cell? How can you estimate the theoretical voltage of a PEFC? What factors affect the voltage of a real PEFC? Depict a general principle and coupling in a CFD model of a PEFC. How are CFD tools applied in the investigation and development of PEFCs? Please name the three main steps of PEFC simulation using CFD models. In the membrane, water can be transported via three mechanisms. Name these mechanisms. What law is applied to calculate the membrane overvoltage? How does membrane thickness affect the membrane overvoltage? How can you decrease the membrane resistance by changing the operating conditions of a PEFC? Why does the current density of a PEMFC increase with higher gas pressure at the cathode side? Degradation processes in PEFCs can be divided into three main groups. Please name the three groups. The current degradation models can be divided into three groups. Please name the three groups. What microscale models are currently used to simulate degradation phenomena in PEFCs? What radicals cause the chemical degradation of Nafion membranes?

REFERENCES [1] A.A. Kulikovsky, Analytical Modelling of Fuel Cells, Elsevier, Amsterdam, 2010. [2] E. Hontanon, M.J. Escudero, C. Bautista, P.L. Garcıa-Ybarra, L. Daza, Optimization of flow-field in polymer electrolyte membrane fuel cells using computational fluid dynamics techniques, J. Power Sources 86 (2000) 363–368. [3] N. Ahmadi, S. Rezazadeh, I. Mirzaee, N. Pourmahmoud, Three-dimensional computational fluid dynamic analysis of the conventional PEM fuel cell and investigation of prominent gas diffusion layers effect, J. Mech. Sci. Technol. 26 (2012) 2247–2257. [4] A. Iranzo, F. Rosa, J. Pino, A simulation tool for geometrical analysis and optimization of fuel cell bipolar plates: development, validation and results, Energies 2 (2009) 582–594. [5] A. Kumar, R.G. Reddy, Effect of channel dimensions and shape in the flow-field distributor on the performance of polymer electrolyte membrane fuel cells, J. Power Sources 113 (2003) 11–18. [6] C.-J. Kim, A numerical study on uniform cooling of large-scale PEMFCs with different coolant flow field designs, Appl. Therm. Eng. 31 (2011) 1427–1434.

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4

T^ eko W. Napporn*, Larisa Karpenko-Jereb†, Birgit Pichler‡, Viktor Hacker‡ University of Poitiers, Institut de Chimie des Milieux et Mat eriaux de Poitiers (IC2MP), Poitiers, France* Graz University of Technology, Institute of Electronic Sensor Systems, Graz, Austria† Graz University of Technology, Institute of Chemical Engineering and Environmental Technology, Graz, Austria‡

CHAPTER OUTLINE 4.1 4.2 4.3 4.4

Introduction ...................................................................................................................................63 Components of H2/O2 PEFC Electrode Materials ................................................................................64 Basic Features of Electrode Materials .............................................................................................64 Methods for Fabricating Electrocatalyst (Anode or Cathode) Materials ..............................................65 4.4.1 Polyol Method ........................................................................................................... 65 4.4.2 Water-in-Oil Microemulsion Method ............................................................................ 66 4.4.3 Impregnation-Reduction Process ................................................................................. 67 4.4.4 Bromide Anion Exchange Method ................................................................................ 68 4.4.5 Electrocatalysts Synthesized by the Instant Method ...................................................... 70 4.5 Polymer Electrolyte Materials .........................................................................................................70 4.5.1 Membrane Properties ................................................................................................. 73 4.5.2 Conducting Channels and Proton Transport .................................................................. 75 4.5.3 Water Transport and Conductivity ................................................................................ 77 4.6 Bipolar Plates ................................................................................................................................80 4.7 Recapitulation ................................................................................................................................83 4.8 Comprehensive Questions and Exercises ..........................................................................................84 References ...........................................................................................................................................84

4.1 INTRODUCTION In polymer electrolyte fuel cell (PEFC) systems, one of the key components is the catalytic layer. This layer is composed of the ionomer of the electrolyte membrane and nanomaterial of the catalyst, on which electrochemical reactions occur. The surface morphology and structure of these nanomaterials determine the nature of the reaction products. Therefore, the nanomaterial synthesis is of great interest, as the various methods discussed in this chapter have a major influence on fuel cell performance. Fuel Cells and Hydrogen. https://doi.org/10.1016/B978-0-12-811459-9.00004-9 # 2018 Elsevier Inc. All rights reserved.

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The polymer electrolyte membrane (PEM) possesses ionic conductivity and presents a barrier for electrons. This chapter offers ideas about the inner structure of modern membranes, and the formation of conducting ways for proton transport. It also covers the main characteristics of the PEMs and their behavior after alteration of relative humidity (RH) in the fuel cell. The chapter concludes by discussing the important role of bipolar plates in the mechanical stability of the PEFC stack, as well as electrical conductivity. By milling flow-fields into the bipolar plates, reactant gas is evenly distributed over the whole electrode surface. Furthermore, selection of the right bipolar plate material in connection with a suitable flow-field design provides product water outtake from the cathode and facilitates heat removal. Requirements for the mechanical and chemical stability of the bipolar plate materials are discussed.

4.2 COMPONENTS OF H2/O2 PEFC ELECTRODE MATERIALS The heart of an H2/O2 PEFC is composed of a membrane electrode assembly (MEA) (see Fig. 4.1). The most popular polymer electrolyte used in fuel cells is a Nafion® membrane from Dupont. The schematic view of the MEA (see Fig. 4.1) shows the catalytic layers, which are sandwiched between the membrane and the gas diffusion layer (GDL). Each catalytic layer is composed of supported catalysts (often with carbon as support) and the ionomer of the electrolyte as the proton conducting media. The surface of the nanometer sized catalysts in the catalytic layer at the anode or at the cathode is the electrochemical reaction site. Therefore, their features are very important for the performance of the cell. The catalytic layer of each electrode (anode and cathode) is in contact with the membrane electrolyte as shown in Fig. 4.1. The main features of nanomaterials used as anode or cathode catalysts will depend on the reactions that occur at their interface. For example, an element that is not stable in the highly oxidizing conditions during oxygen evolution reaction cannot be used as anode material in a water electrolyzer.

4.3 BASIC FEATURES OF ELECTRODE MATERIALS Electrochemical reactions occur at the electrode surface through consecutive processes, including electron transfer. Therefore, the electrode material has to be electronically conductive, and then be able to interact with the molecule. The reactions at the anode and the cathode have to occur at low and at high

FIG. 4.1 An image and a schematic view of a membrane electrode assembly (MEA).

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potentials, respectively, in order to reach a cell voltage close to the theoretical value of 1.23 V. The stability of these two materials in such conditions is very important, particularly in the case of the cathode catalyst, which should exhibit a high level of stability during the oxygen reduction reaction (ORR). This reaction is very sensitive to any impurity. Therefore, any remaining organic molecule from the material synthesis can lead to a drastic decrease of the catalyst’s performance. Consequently, the nanomaterials synthesis has to fit properly with the application.

4.4 METHODS FOR FABRICATING ELECTROCATALYST (ANODE OR CATHODE) MATERIALS 4.4.1 POLYOL METHOD In polyol synthesis, polyalcohol is used as the solvent and the reducing agent. It is a versatile method, which permits use of elaborate, supported, core-shell and porous structures. The utilization of various polyalcohols depends on their boiling temperature: ethylene glycol (197.3°C), propylene glycol (188.2°C), butylene glycol (207.5°C), diethylene glycol (244°C), glycerol (290°C), tetraethylene glycol (327°C), benzyl alcohol (205.3°C). These alcohols promote the formation or nucleation, and the growth and stabilization of seeds. The oxidation potential of each polyol is different. The reduction temperature of the precursor is fixed with the nature of the solvent. Moreover, various precursors such as chlorides, acetates, nitrates, hydroxides, or oxides are used because they can be dissolved within the polyol. Ethylene glycol is the most utilized solvent for synthesizing electrocatalysts. It has a relatively high boiling point and a high dielectric constant. The metal nanoparticles, shown in Fig. 4.2, were obtained in a solution at pH 13 and boiled at 130°C. Fig. 4.2 shows high dispersion of nanoparticles on the carbon substrate. However, the optimization of experimental conditions is required for further control of the particle’s shape, size, and distribution.

FIG. 4.2 TEM image of Pt/CVulcan

XC 72

(metal loading was 20 wt%) obtained by polyol method.

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The mechanism of the polyol method (nucleation and growth) can be understood through the pioneer work of Fievet et al. [1] who first focused on the general mechanism of reduction of two metal hydroxides, Ni(OH)2 and Co(OH)2, in ethylene glycol. In this mechanism, the acetaldehyde was considered the main reducing agent for preparing metal nanoparticles. The precursor must have high enough solubility in polyol solvents to promote the following reactions: 2H2 COHdCH2 OH ! 2H3 CdCHO

(4.1)

2H3 CdCHO + MðOHÞ2 ! M + ðH3 CdCOÞ2 + 2H2 O

(4.2)

According to Fievet et al. [1], diacetyl is the main oxidation product, and metal nucleation and growth occur in the liquid phase. Researchers tried to understand the reduction mechanism of the metal precursor by suggesting various approaches: Bonet et al. [2] considered the redox process involving the reduction potential of the precursor and the oxidation potential of the ethylene glycol. External energy as heat is required because the chemical reduction of noble metal ions or species by ethylene glycol is thermodynamically unfavorable. Furthermore, Bock et al. [3] proposed the reduction of the metal precursors by the oxidation of ethylene glycol to aldehydes, carboxylic acids, and CO2. The oxidation of ethylene glycol to glycolic acid and glycolate is very important in the process because these products stabilize the nanoparticles. The stability of the colloid solution of nanoparticles depends mainly on the pH during synthesis. The size and shape of the nanoparticles can be controlled by the pH of the solution. An additional report by Skrabalak et al. [4] using a spectrophotometric method, mentioned how metal ions are reduced by the ethylene glycol oxidation to glycolaldehyde. According to these authors, the product of the ethylene glycol oxidation depends on the atmosphere of the reaction, and glycolaldehyde is the major product when ethylene glycol is heated between 140°C and 160°C in air. The kinetics and the thermodynamics of the nanoparticles’ synthesis are critically important to control their growth, shape, size, and further physical properties. Therefore, the role of the solvent in this process to control the particles’ size, shape, activity, and durability was investigated [5–13]. Other reducing agents, such as poly-vinylpyrrolidone (PVP) [14,15], sodium borohydride (NaBH4) [16,17], and formaldehyde (HdCHO) [17] can be added to the solvent for controlling the electrocatalysts’ surface morphology, and thus their activity and performance. For a fine control of the nanoparticle shape, a surfactant is added to the solvent [18].

4.4.2 WATER-IN-OIL MICROEMULSION METHOD Before describing this synthesis method, it is important to introduce the term “microemulsion,” which was initiated by J.H. Schulman [19–22]. From the various definitions that are in the literature, a microemulsion is a macroscopically monophasic fluid of transparent compounds made by mixing water and hydrocarbon in the presence of suitable surface active agents (surfactants) [21,23–25]. Even if at macroscale a microemulsion solution looks homogeneous, at the nanoscale, it appears heterogeneous. Indeed, the solution consists either of nanospherical monosized droplets, or a bicontinuous phase (10–40 nm) [22,26]. The control of the nanoparticle size strongly depends on these droplets. The structure of a microemulsion is determined at a given temperature by the ratio of its different constituents in water and oil phases. Two systems can be distinguished: “oil-in-water” (o/w) and “water-in-oil” (w/o). In the first system, high concentrations of water result in a water-rich phase, where the internal structure of the microemulsion is composed of small oil droplets in a continuous water phase. Then,

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surfactant molecules are used to stabilize the system, with their hydrophilic portion oriented toward the aqueous phase (water) and the hydrophobic portion toward the organic phase (oil). In these conditions, where the droplets are in the organic phase, it is difficult to control the particles’ size, since the metal ion is mostly in the aqueous phase [27]. In the second system, named “water-in-oil, w/o,” the microemulsion has a high oil concentration, and the internal structure consists of small water droplets in a continuous oil phase (reversed micelles). In this structure, the hydrophilic portion of the surfactant molecules is oriented toward the aqueous phase (water) and their hydrophobic portion is oriented toward the organic phase (oil). The volume of surfactant and its nature play a key role in the size of the formed micelles. The nature of the surfactant can be ionic or non-ionic [21,26,28]. Therefore, the catalyst’s morphology depends on the size of the droplets, and consequently, on the synthesis method. The use of surfactants can become a challenge in electrocatalysis because it is difficult to completely eliminate them during the cleaning step of the catalysts after their synthesis. Indeed, surfactants are organic compounds that have strong interactions with the seeds and/or nanoparticle surface. Nevertheless, various surfactants were used to fabricate nanocatalysts for electrochemistry; for example, bis(2-ethylhexyl)sulphosuccinate or polyethylene glycol dodecyl (Brij® 30). The cleaning process is a very sensitive step in the electrocatalyst’s elaboration, and students must be patient. Usually, the reducing agents are hydrazine (N2H4), sodium borohydride (NaBH4), ascorbic acid, or formic acid. The organic solvent (oil) can be n-heptane, hexane, cyclohexane, or isooctane [26,29–31]. Typically, for synthesizing nanocatalysts with the w/o microemulsion method, two micelles are prepared: one containing the reducing agent, and the second one is composed of the catalyst precursors. Afterward, the two micelles are mixed. Collisions between droplets of the two micelles occur and lead to the formation of seeds, and then surfactant-stabilized nanoparticles. The surfactant allows control of the growth of the nanoparticles via a steric effect, or strong specific adsorption on the seeds in order to reach a good size distribution. The nucleation process occurs inside the droplet, followed by the aggregation process to obtain the final particles [26]. Using the w/o microemulsion method in the beginning of the 1980s, platinum (Pt), palladium (Pd), rhodium (Rh), and iridium (Ir) nanoparticles with a size of about 3–5 nm were successfully synthesized by Boutonnet et al. [25]. As any synthesis method, several parameters such as temperature, the volume of surfactant and the solvent, the concentration of the precursor salt, and the nature of the reducing agent [29–38] play a crucial role. In fuel cells, supported catalysts are used because of their high surface area. Indeed, due to their high specific surface area, carbon-based or oxide supports are often the substrate used for dispersing metallic or oxide nanoparticles. Once the nanoparticles are synthesized by the w/o microemulsion method, the appropriate amount of support is added to this solution. The distribution of the nanoparticles onto the support (e.g., carbon Vulcan® XC72, for example) occurs, and the supported catalysts can be washed and cleaned carefully after 2 h with organic solvents in order to remove the surfactant, and then several times with ultrapure water through filtration under a vacuum (Buchner). Fig. 4.3 shows the well-dispersed Pd0.7Ni0.3/C obtained by the w/o method.

4.4.3 IMPREGNATION-REDUCTION PROCESS The impregnation-reduction method appears as one of the wide and simple synthesis techniques for supported catalyst material preparation. This method uses aqueous media instead of organic, as for the w/o method. Low to room temperatures are sufficient to perform the synthesis [27,39]. For several

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FIG. 4.3 TEM image of Pd0.7Ni0.3/C prepared by w/o method.

years, Pt-based catalysts have been prepared with this method [40–42]. First, the support is immersed in an aqueous solution containing the metal precursor. Various metal precursors are used (e.g., chloride, sulfite, nitrate, and carbonyl complexes). The second step of this method is the reduction process. Metal anions or complexes are reduced with H2 (pure or diluted in Ar or He), or a solution of a reducing agent such as HCHO, HCOOH, NaBH4, N2H4, Na2S2O3 [39,41,43,44]. The nature of the support and the precursor determines the duration of the impregnation. Using this synthesis method, Pt-Sn/C catalysts were prepared by Knani et al. [43] as shown in Fig. 4.4. These electrocatalysts exhibited high tolerance toward methanol during the oxygen reaction reduction. Nevertheless, the disadvantage of this method is the inhomogeneous catalyst’s dispersion and size distribution. Indeed, it appears difficult to obtain full control of the particles’ size and distribution without the use of any surface active agent (surfactant).

4.4.4 BROMIDE ANION EXCHANGE METHOD This method is one of the simple synthesis methods for electrocatalysts, which is free from organics, because it is performed in an aqueous medium. This synthetic route, called bromide anion exchange (BAE), was first carried out by Tonda-Mikiela et al., and revisited and improved by Holade et al. [45–47]. The chloride anions of the metal salts (precursors) are replaced by bromide anions of KBr salt. Due to their large size, the bromide anions insure the steric hindrance around the metal ions, or around the seeds. The different steps of the synthesis are summarized in Fig. 4.5. Different parameters are essential for the success of this method: the metal salts’ concentration, the concentration of bromide ion, and the temperature and the volume of the synthesis reactor. Indeed, the ratio n(KBr)/n(metal salts) is a critical

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FIG. 4.4 TEM image and size distribution of Pt-Sn/C catalyst treated at 300°C. Reprinted from S. Knani, L. Chirchi, S. Baranton, T. W. Napporn, J.-M. Leger, A. Ghorbel, A methanol—tolerant carbon supported Pt–Sn cathode catalysts, Int. J. Hydrog. Energy 39 (2014) 9070–9079. Copyright 2014, Elsevier.

FIG. 4.5 The experimental BAE route for nanoscale material synthesis.

value. Typically, this ratio was optimized at 1.46, with a total molar concentration of metal salts of 1 mmol L1. The temperatures were 25°C and 40°C. The first temperature is kept in steps 1–3, and the second temperature is used in step 4 (the reduction of the metal ions) [48]. A large volume round-bottom flask with 500 mL minimum is suitable for this synthesis. This method allows synthesis of Pt-based, Au-based, and Pd-based nanocatalysts for various organic molecules’ oxidation and for ORR [45,49], Pd-based electrocatalysts for glycerol oxidation [47,49], and also for complete systems, such as biotic and abiotic glucose fuel cells [50,51]. Fig. 4.6 shows the TEM images of Au, Pd/C, and Au-Pt/C catalysts (20 wt% metal loading) fabricated from this method.

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CHAPTER 4 POLYMER ELECTROLYTE FUEL CELLS

FIG. 4.6 TEM-HRTEM micrographs of the nanostructured Au, Pt, and AuPt (20 wt% metal loading) supported on Vulcan XC 72R.

4.4.5 ELECTROCATALYSTS SYNTHESIZED BY THE INSTANT METHOD The instant method is also one of the versatile approaches for synthesizing nanomaterials for fuel cells and water electrolyzers. It consists of two steps as described by Reetz et al. [52]: the first one is the formation of metal oxide and the second is the reduction of this oxide to metal nanoparticles. Typically, in a round-bottom flask containing a certain volume of water, the support (e.g., carbon) and the metal salt are stirred together. The temperature of the round-bottom flask is a critical parameter. A solution of Li2CO3 is added in order to reach a pH of 9–10. The solution is kept at 60°C under stirring for 10 h. In general, in 2–6 h, the oxides are formed according to the reaction: M ðprecursorÞ + Support

Li2 CO3 , H2 O

!

MOx =Support

(4.3)

The second step is the reduction of the metal oxide in metallic nanoparticles. Various reducing agents can be used (e.g., NaBH4, H2, HCOOH, ascorbic acid). A certain volume of the reducing agent is added dropwise (in case of a liquid). Highly dispersed metal nanoparticles are formed, as shown in Fig. 4.7. MOx =Support

Reducing agent

!

M=Support

(4.4)

4.5 POLYMER ELECTROLYTE MATERIALS In the low-temperature fuel cells such as the PEFC, the polymer electrolytes represent thin membranes manufactured from ion-exchange polymers. In general, a membrane is a thin selective barrier. Selectivity means that some compounds can penetrate the membrane, but certain compounds do not [54]. In the PEFCs, the membranes provide transport of ions from one electrode to another, but also serve as a barrier for electrons, fuel, and oxidizing gases. For the application in the commercial fuel cells, the PEMs have to conform to the following requirements: – high ionic conductivity; – impermeable for gases; – impermeable for electrons;

4.5 POLYMER ELECTROLYTE MATERIALS

71

FIG. 4.7 TEM images of Pt/C nanoparticles synthesized from the instant method [53].

– high chemical stability; and – high mechanical durability. The ion-exchange materials are polymers containing charged groups called functional groups (also called “end groups” or “acid groups”). One of the most important physicochemical characteristics of the PEMs is the ion-exchange capacity (IEC), that is, the concentration of the functional groups. A functional group consists of a fixed group and counter ion. When the membrane is hydrated enough with water, the functional group dissociates, and counter-ions are able to move through the membrane. According to the charge of the counter-ion, the ion-exchange membranes are divided into two main classes: – Anion-exchange membranes have positively charged fixed groups and negatively charged counter ions – Cation-exchange membranes have negatively charged fixed groups and positively charged counter ions Both classes of the ion-exchange membranes are widely applied in the fuel cell technology. The anionexchange membranes are used in alkaline and in direct ethanol fuel cells. The cation-exchange membranes are used in proton exchange membrane fuel cells, as well as in direct methanol and ethanol fuel cells. Currently, commercial proton exchange membranes are based on sulfonated aromatic hydrocarbon or perfluorinated polymers, such as: – Perfluorinated ion-exchange membranes (DuPont, Solvay, GEFC and FuMA-Tech) – Hybrid perfluorinated ion-exchange membranes (GEFT and FuMA-Tech), usually they are perfluorinated membranes modified by SiO2 or ZrP

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CHAPTER 4 POLYMER ELECTROLYTE FUEL CELLS

– Sulfonated hydrocarbon membranes (see Fig. 4.8C,D) – Sulfonated poly(trifluorostyrene) (Ballard Power Systems) (see Fig. 4.8E) The more-often utilized ion-exchange materials are sulfonated perfluorinated polymers, whose chemical structures are shown in Fig. 4.8. The perfluorinated membranes of the Nafion type (see Fig. 4.8A) possess long side chains with two ether groups in the middle. The end of the side chain is terminated by the sulfonyl functional group [55]. The perfluorinated membrane Aquivion has a short side chain containing only one ether group, as shown in Fig. 4.8B [56]. Aquivion’s shorter side chain makes it possible to achieve better mechanical properties at the same IEC as long side chain polymers [57]. The hydrocarbon membranes based on the sulfonated poly(ether ether ketone) have remained an object of active research, but have not been widely applied in the commercial fuel cells yet.

FIG. 4.8 Chemical structure of the perfluorinated sulfonated membranes currently used in the commercial PEMFCs: (A) Nafion, (B) Aquivion, (C) sulfonated poly (ether ether) ketone (SPEEK), (D) sulfonated polysulfone (SPSSU), (E) sulfonated poly(trifluorostyrene) (TFS)).

4.5 POLYMER ELECTROLYTE MATERIALS

73

Synthesis of novel PEMs for application in fuel cells is an important research topic of numerous studies. Following are current research trends in the synthesis and formation of new membranes for PEFCs: 1. Incorporation of the perfluorinated membranes with inorganic compounds (SiO2, ZrP) [58–61], with nanofibers [62] or ionic liquids [63]. 2. Chemical modifications of the surface of the polymer films using plasma and radiation techniques to increase selectivity and conducting properties [58]. 3. Synthesis of novel polymer electrolytes from low-cost biopolymers such as chitin and chitosan [64,65]. 4. Synthesis of polymer electrolyte films from organic compounds using modern techniques such as plasma [66,67], radiation [68], and chemical vapor deposition [69]. 5. Formation of composition materials based on glass porous supports filled by ionic conducting materials such as ionic liquids or high-conducting ionomers [58,70].

4.5.1 MEMBRANE PROPERTIES The ion-exchange membranes are described by physicochemical and transport properties [71]. The physicochemical properties are: (1) thickness, (2) density, (3) tensile strength, (4) the swelling factor, (5) equivalent weight (EW), (6) IEC, (7) water uptake, (8) hydration number (membrane water concentration), and (9) water sorption isotherm. The transport properties of the ion-exchange membranes reflect the ability of the membranes to transport water and counter-ions, and they are: (10) diffusion permeability, (11) electroosmotic permeability, and (12) ionic conductivity. The membrane thickness is an important characteristic for the fuel cell, because the ohmic resistance of the cell is directly proportional to the membrane thickness [72]. The thickness of the commercial membranes varies from 7 to 25 μm. The membrane density depends on the membrane water concentration, and, usually, the density decreases with increasing water concentration. The swelling factor characterizes the relative change in the membrane size after swelling in water. For the Nafion membrane, the swelling factor is approximately 15%. The water uptake (W) describes the water content in the membrane. The water content is calculated as the rate of water mass absorbed by the membrane to the weight of the dry membrane: W¼

Wwet  Wdry Wdry

(4.5)

Wwet is the weight of the membrane equilibrated with liquid water and Wdry the weight of the dry membrane. Very often, the water uptake is given in percent. The EW is the molecular weight of a polymer fragment, containing one functional group, measured in (g mol1). The IEC is the concentration of the functional groups per mass unit of the dry (or wet) membrane 1 (mol g1 dry) (or (mol gwet)).

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CHAPTER 4 POLYMER ELECTROLYTE FUEL CELLS

The EW and IEC are connected by the relation: EW ¼

1 IEC

(4.6)

The hydration number (or membrane water concentration) is the amount of water molecules per one functional group: λ¼

W MH2 O ∗IEC

(4.7)

λ is dimensionless, but in the scientific papers, one expresses it in (mol H2 O=mol SO3  ) or [[H2O]/ [SO3H]] in order to reveal the physical meaning of the magnitude. MH2O is the molecular weight of water (g mol1). The water sorption isotherm describes the dependence of the membrane hydration number on the water vapor humidity. This function plays an important role in water management of PEFCs and it is very often used in the mathematical modeling of their performance. The water sorption isotherm by the PEMs has been extensively studied using different experimental methods [73–78]. Special attention is focused on the difference between the equilibriums of the membrane with the saturated water vapor and with liquid water. For the first time, Schroeder [79] indicated that gelatin, an amino acid related to natural polyelectrolytes, absorbs significantly less water from saturated water vapor than at contact with liquid water. At present, this phenomenon is well known as Schroeder’s paradox, and is currently being extensively discussed in the scientific literature [78,80–85]. The existence of this phenomenon has theoretically been proven [80,81]. However, it is an interesting fact that several experimental investigations [77,78,97] did not confirm the presence of Schroeder’s paradox. Utilizing the thermodynamic model approaches, Eikerling and Berg [81], showed that the transition of water from gas to the liquid phase attains a discontinuous jump in the total water concentration in PEMs. Recently, Roldughin [86] suggested a simple mechanism explaining increased water sorption by ion-exchange polymers immersed in liquid water. He associated this observed increased sorption with the electrostatic repulsion of charged surface groups (or action of Maxwell’s stress) at the polymer/water interface, and demonstrated that the predicted value of the ‘excess’ stress in the polymer electrolyte put in liquid water is consistent with published experimental data [86]. The analysis of the experimental data [73,74,76–78] on the water sorption of the Nafion membrane have shown that: 1. The maximal membrane water concentration grows with increasing temperature. 2. The appearance of liquid water in the system causes a sharp but continuous rise of the membrane water concentration. The second observation contradicts Schroeder’s paradox, but it can be explained by the following considerations: at very high RH, water is present in both vapor and condensed (liquid) states (see Fig. 4.9). The water drops are condensed on the membrane surface. Consequently, both phases contact the membrane simultaneously. Based on the theory proposed by Eikerling and Berg [81], we can assume that water droplets will be more easily absorbed by the membrane compared with water vapor. But a few water droplets will not be enough to increase the membrane water concentration to the maximal value. Thus, at the occurrence of the liquid water, the growth of the water content will transit continuously in the membrane.

4.5 POLYMER ELECTROLYTE MATERIALS

75

λ 20

Two phase region

15 Jeck [78] Pimenov [76]

10

Morris [73]

5

0 0

0.2

0.4

0.6

0.8

1

RH

FIG. 4.9 The water sorption isotherm—the dependence of the hydration number (λ) of Nafion membrane on relative humidity (RH). The experimental data are from [73,76,78].

4.5.2 CONDUCTING CHANNELS AND PROTON TRANSPORT In the fully hydrated perfluorinated membranes, the hydrated functional groups strive to form clusters. Numerous structural investigations have revealed that in the perfluorinated membranes, each cluster consists of around three to four hydrated groups forming the inversed micelles: the charged fixed groups with the water molecules are situated in the middle of the micelles; and the micelle boundaries are surrounded by the noncharged main chains. Those clusters are interconnected with narrow channels also containing the single hydrated functional groups. The cluster-network model of the Nafion membrane was for the first time suggested by Hsu and Girke in 1983 [87]. Through the cluster-network of the conducting domains, the protons are transported by two mechanisms: vehicle and Grotthus. While the vehicle mechanism is convenient for every type of ion, and it is characterized by the motion of ions, the Grotthus mechanism is possible only for protons or hydroxyl ions in aqueous media. In this case, the proton builds a water-hydrogen connection from one side of the oxygen atom of the water molecule. Another proton is disconnected from the opposite side of the molecule, and builds a hydrogen connection to the oxygen of the next water molecule (see Fig. 4.10). As shown in many experimental investigations, the proton mobility is much higher in perfluorinated membranes than in hydrocarbon membranes. Thus, in the Nafion membrane, the mobility of the proton is 1.5 times higher than in non-perfluorinated membranes and in hydrocarbon aromatic membranes [88]. In order to understand the reasons for the higher proton mobility in Nafion membranes, we studied the structural features of the different sulfonated membranes using ab initio methods [88]. Fig. 4.11 presents the calculated structure of these membranes with 10 water molecules: (A) the hydrocarbon aromatic membrane; (B) the perfluorinated membrane Nafion.

O

H

H

115° H +

(A) H O

H

Tunneling H

H

O

H H

Tunneling

+ H

+

(B)

O H H

FIG. 4.10 On the top—H3O+ hydronium ion; on the bottom—proton-hopping Grotthus mechanism. Reprinted from H. Strathmann (ed.) Ion-Exchange Membrane Separation Processes, in: (Membrane Science and Technology Series, vol. 9, Elsevier, Amsterdam, 2004, with permission from Elsevier.

FIG. 4.11 Molecular structure of the side chains of sulfo-cationic membranes: (A) perfluorinated membrane Nafion; (B) hydrocarbon aromatic membrane. The functional group hydrated with seven water molecules. The proton is marked by dotted circle.

4.5 POLYMER ELECTROLYTE MATERIALS

77

The ab initio study showed that in Nafion, the water molecules build a stronger network around the sulfo-group than in aromatic membranes. This is also reflected in the OH⋯O hydrogen bond distances ˚ , while in the between the water molecules. Nafion has an average hydrogen bond distance of 1.85 A ˚ aromatic membrane, the distance is 2.02 A. Also, the perfluorosulfonated membrane has a higher water binding energy than the aromatic membrane, consisting of approximately 2 kcal mol1 at the higher hydration level (number of the water molecules per one sulfo-group). Moreover, it was found that in Nafion, the functional groups dissociate when it is hydrated with three or more water molecules, while no dissociation of the functional group was detected in the aromatic membrane in the whole investigated range of the hydration level (λ from 1 up to 10). Thus, we found two reasons explaining the higher proton mobility in the perfluorinated membrane Nafion: (1) the water molecules in the perfluorinated membrane are better networked than in the aromatic one; (2) the sulfonyl functional group dissociates in the perfluorinated membrane more easily than in the hydrocarbon aromatic membrane. The dissociation of the functional group and better connected water promote the proton transport by Grotthus mechanism. Fig. 4.12 demonstrates the inner structure of the membrane, simulated by using the ab initio method and molecular dynamics [89]: gray atoms represent the polymer chains of the Nafion membrane, white spheres are hydrogen atoms of the absorbed water molecules. The gray part of the polymer does not conduct ions and represents the nonconducting phase of the ion-exchange membrane. The protons can move through the cluster-network built from the hydrated sulfonyl functional groups. The conducting phase of the Nafion membrane is separately shown in Fig. 4.12C.

4.5.3 WATER TRANSPORT AND CONDUCTIVITY In general, the volume fraction of the conducting phase depends on the sulfonyl group concentration of a membrane and the amount of absorbed water. The water amount in the membrane (membrane water

FIG. 4.12 Inner structure of the membrane Nafion: (A) side chain terminated by sulfonyl group hydrated by 10 water molecules, the structure was calculated by ab initio method RHF using ORCA program. (B, C) Images of Nafion inner structure obtained using molecular dynamics [89]: (B) macrostructure of hydrated Nafion fragment, consisting of inert polymer chains and conducting phase; (C) image only of the conducting phase.

78

CHAPTER 4 POLYMER ELECTROLYTE FUEL CELLS

concentration) can be varied by changing the RH of the membrane environment. With increasing RH, the membrane water concentration goes up, and consequently, the volume fraction of the conducting phase in the membrane grows. The volume fraction of the conducting phase affects the proton and water flux through the membrane: with an increasing volume of the conducting domain, the water transport is improved. The water diffusion and electro-osmotic coefficients increase, and proton conductivity rises. Fig. 4.13A,B presents the water diffusion coefficient and electro-osmotic coefficient of Nafion membranes as a function of the membrane water concentration based on the experimental data reported in [77,90–97]. As seen in the figures, the Dw and Cdrag grow directly with the rising hydration number of the membrane. In the fuel cell modeling, these dependencies are usually approximated by linear functions shown in the figures. In the dry state, the polymer electrolytes are dielectric (nonconducting), and their specific conductivity at dry conditions is around σ ¼ 106 S m1. After water sorption, the ion-exchange membranes begin to conduct the current, and the conductivity grows dramatically with increasing water concentration in the membrane. The specific conductivity of the proton form of the Nafion membrane, equilibrated with liquid water, is around σ ¼ 101 S m1. As shown in the investigations [98–100], the dependence of membrane conductivity on the water concentration can be successfully described by Dw ·1010 (m2/s)

Cdrag 3

Cdrag = 0.11 × l

Dw = 2.65 × 10−11 × l

6

2.5 2 4

1.5 1

2

Luo [96] Ise [97] Zawodzinski [77]

0.5 l

l

0 0

0

(A)

5

10

15

20

25

(B)

0 4

8

12

16

20

24

Rivin [90]

Suggested fitting

Suresh [91] Jayakody [92] Zawodzinski [93]

Volino [94] Majsztrik [95]

FIG. 4.13 Water diffusion coefficient Dw (A) and electro-osmotic coefficient Cdrag (drag coefficient) (B) as a function of the membrane water concentration (λ). The symbols on the graphs are experimental data reported in [89–97]. The equations in the figures are suggested fittings describing Dw and Cdrag as the functions of the membrane water content.

4.5 POLYMER ELECTROLYTE MATERIALS

79

the equation of the percolation theory. The basic ideas of this theory were formulated by Broadbent and Hammersley in 1957 [101]. According to the theory, after reaching some critical volume fraction of the conducting phase fcr, the conductivity of a composition consisting of dielectric and conductor dramatically begins to grow with the increasing fraction of this phase. This change from dielectric to conductor has been called the percolation transition, and the critical content of the conducting phase fcr is a threshold of the percolation. Fig. 4.14 illustrates the formation of conducting channels in the ionexchange membrane at various f-to-fcr ratios. At f > fcr the dependence of the conductivity on the conducting phase fraction is described by the equation: σ mem ¼ σ  ðf  fcr Þτ

(4.8)

where σ, pre-factor of percolation equation; τ, percolation exponent; f, volume fraction of the conducting phase; and fcr, critical volume fraction of the conducting phase. The quantities σ, fcr, and τ are the parameters of the percolation equation. Calculations based on the theory of probability, as well as experimental investigations, have shown that the percolation parameters lie in the intervals: fcr ¼ 0.15  0.03, τ ¼ 1.6 0.4. As shown herein, the conducting phase in perfluorinated membranes represents the connected hydrophilic cluster domains consisting of hydrated functional groups and networked absorbed water. The following equation has been suggested [99,102] in order to calculate the volume fraction of the conducting phase in ion-exchange membranes: f ¼ IEC  MSO3 + W

(4.9)

FIG. 4.14 Images of the schematic formation of an infinite percolation cluster at various f-to-fcr ratios. The spheres represent the conducting phase in the membrane.

80

CHAPTER 4 POLYMER ELECTROLYTE FUEL CELLS

smem (S/cm) 0.1 lg(smem) −1.2

0.08

−1.4

0.06

−1.6 −1.4

0.04

y = 1.2049x + 0.0661 R2 = 0.9889

−1.2

−1 lg(f − fcr)

(B)

0.02

0 0

0.05

0.1

0.15

0.2

0.25

f

(A) FIG. 4.15 (A) The dependence of the specific conductivity of Nafion membrane on the volume fraction of the conducting phase and (B) bilogarithmic dependence log(σ mem)  log (f  fcr), used for the determination of the exponent τ of the percolation theory equation. The points are experimental data.

where IEC, ion-exchange capacity, that is, the concentration of acid groups; MSO3, molecular weight of the functional group; and W, water mass fraction in PEM. The derivation of the equation is described in detail in the same references [99,102]. The magnitudes of the parameters, σ and τ, of the percolation theory equation are determined from the bilogarithmic dependences of the membrane ionic conductivity on the volume fraction of the membrane conducting phase as shown in Fig. 4.15.

4.6 BIPOLAR PLATES Bipolar plates are, besides the MEA, one of the most important components in PEFC stacks, as they are essential for many functions inside a fuel cell: they support the MEA, giving mechanical stability to the whole stack, allow even gas distribution over the entire electrode surface area, and provide electrical conductivity. In addition, a well-designed bipolar plate helps with heat management and allows easy product water outtake via the exhaust [103–105]. In a PEFC stack, the bipolar plates are alternately stacked with MEAs, so that each plate provides electrical connection on one side for the anode of

4.6 BIPOLAR PLATES

81

one MEA and on the second side for the cathode of the neigboring MEA, respectively. Using thin and compact bipolar plates is therefore essential for reducing the volume of the stack. Over the past decades, various materials and designs have been developed, as will be shortly reviewed in the following. The bipolar plate materials have to fulfill several requirements, such as high resistivity toward the corrosive environment (corrosion rate of C]NdNH2) in a fuel tank without noxious effects [62]. In order to release the carbon-free fuel N2H4, more precisely hydrazine monohydrate (N2H4H2O), again from hydrazone, the adding of a solvent (e.g., H2O) is necessary (see Fig. 5.14) [62,64]. The hydrazine-oxygen powered fuel cell can be operated in alkaline as well as in acidic media. The overall reaction (see Eq. 5.21) results from the hydrazine oxidation reaction and the ORR, and these electrochemical reactions are shown in both media (see Eqs. 5.22–5.26) [53,62]. The research focuses primarily on the alkaline DHFC (see Eqs. 5.22 and 5.23) due to certain benefits, for example, (i) the use of nonnoble elements (e.g., Ni, Zn, Co, or Fe) for anode and cathode catalyst production, respectively, (ii) the faster kinetics of the electrode reactions, and (iii) the addition of alkaline solution to N2H4 suppresses the N2H4 hydrolysis reaction thus resulting in a concentration decrease of the during the hydrazine oxidation reaction formed hydrazonium cation N2 H5 + , resulting in a reduction of the fuel crossover [62–67]. Alkaline media: Anode : N2 H4 + 4OH
! N2 + 4H2 O + 4e
E0 ¼
1:16 V





Cathode : O2 + 2H2 O + 4e ! 4OH

E0 ¼ 0:40 V

(5.22) (5.23)

The oxidation process of hydrazine under acidic and neutral conditions proceeds over the hydrazonium cation N2 H5 + to nitrogen, protons, and electrons (see Eqs. 5.24 and 5.25) [53]. Noncorrosive noble metals with high stability, such as Pt or Ag, are used as anode catalysts for the acidic hydrazine oxidation reaction [62]. Acidic media: Anode : N2 H4 + H2 O ! N2 H5 + + OH


(5.24)

FIG. 5.14 Releasing and storing process of hydrazine. Adapted from U. Martinez, Multifunctional Oxidation Electrocatalysts for Direct Alkaline Fuel Cells, The University of New Mexico, 2013.

5.5 COMPREHENSION QUESTIONS AND EXERCISES

N2 H5 + ! N2 + 5H + + 5e
E0 ¼
0:33 V


Cathode : O2 + 4H + 4e ! 2H2 O E0 ¼ 1:23 V +

111

(5.25) (5.26)

The alkaline and the acidic ORR (see Eqs. 5.23 and 5.26) of the DHFC take place in the same way as AFCs and HT-PEFCs, as described in Sections 5.1 and 5.2. The DHFC performance is influenced by the membrane type (CEM or AEM), the surface area of anode and cathode catalysts, and the fuel cell operating conditions (e.g., hydrazine feed concentration, oxygen feed concentration, and cell temperature) [62,67]. The DHFCs using AEMs show, with a power density of 84 mW cm–2, a higher performance than DHFCs using CEMs [62,67]. The highest power density of 1.02 W cm–2 was achieved using alkaline N2H4 solution as anode fuel, acidic H2O2 as cathode oxidant, NafionTM 112 as CEM, Au/C-based cathode and composite Ni-Pt/C-based anode [62]. Generally, the use of H2O2 as an oxidizer demonstrates some benefits compared to O2 or air, but H2O2 is highly reactive, corrosive, and expensive [62]. The Japanese automobile enterprise Daihatsu Motor successfully produced two prototypes of non-Pt group metal-based DHFC vehicles using AEMs. The prototype stack consisted of 119 fuel cells and exhibited a power of 12 kW [66]. In conclusion, the DHFCs can be applied as direct fuel cells for mobile applications. The AEM DHFC use cost efficient materials (catalyst, membrane) and exhibit higher performance than CEM DHFC.

5.4 RECAPITULATION The principles, challenges, and benefits of HT-PEFC, AFC, and direct fuel cells such as DMFC, DEFC, DBFC, and DHFC were extensively explained and discussed, respectively. HT-PEFCs improve due to high operating temperatures at which the reaction kinetics reduce the CO adsorption strength to the Pt catalyst’s surface, resulting in a higher poisoning tolerance, enhanced ORR kinetics, and simplified heat and water management. AFC technology exhibits some benefits compared to counterpart low-temperature acidic PEFCs, like the potential use of inexpensive non-noble metal catalysts and materials, better ORR kinetics, simplified water management, and the use of cost-efficient AEMs. CO2 sensitivity resulting in carbonate formation and reduced lifetime is still on the research agenda. Direct fuel cells (DMFC, DEFC, DBFC, and DHFC) convert liquid fuels like methanol, ethanol, borohydride, and hydrazine without pre-treatment steps in electricity and operate under alkaline or acidic conditions using AEMs or CEMs. The complex anode reactions (e.g., incomplete oxidation of ethanol, side reactions from borohydride and hydrazine oxidation, and poisonous byproducts of the methanol oxidation reaction) lead to lower power densities. The high volumetric energy densities, the simple handling and production process, and the easy transportation and storage of the fuels make direct fuel cells predestined for portable applications.

5.5 COMPREHENSION QUESTIONS AND EXERCISES 1. The HT-PEFC is a promising energy converter for CHP systems: a. What are the advantages of HT-PEFC systems? b. Explain and draw the proton conduction mechanism within the H3PO4 doped PBI membrane.

112

CHAPTER 5 OTHER POLYMER ELECTROLYTE FUEL CELLS

2. AFC was developed for non-terrestrial applications. a. Describe some technical drawbacks of AFC technology (e.g., CO2 sensitivity)? b. Describe benefits of alkaline fuel cells. 3. Direct fuel cells using liquid fuels like alcohols (DMFC, DEFC), borohydride (DBFC), and hydrazine (DHFC) have a high market potential for portable applications. a. Compare DEFC with DMFC technology. b. The complete oxidation of the liquid fuels in DMFC, DEFC, and DBFC is challenging. Why? c. Calculate practical faradaic efficiencies of acidic and alkaline DEFC. d. Describe and explain the mixed electrolyte DBFC concept. e. Why is the use of hydrazine critical for fuel cells? Which method do you use to store N2H4 safely?

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CHAPTER

PREPARATION OF MEA

6 Gaetano Squadrito

Italian National Research Council, Institute for Advanced Energy Technologies “Nicola Giordano” (CNR-ITAE), Messina, Italy

CHAPTER OUTLINE 6.1. Introduction ............................................................................................................................. 118 6.2. The MEA and the Test-Holder .................................................................................................... 118 6.3. Electrolyte Membrane ............................................................................................................... 121 6.4. Catalyst Layer .......................................................................................................................... 122 6.5. Ink Deposition Methods ............................................................................................................ 126 6.6. Gas Diffusion Layer ................................................................................................................... 130 6.7. MEA Assembling ....................................................................................................................... 134 6.8. Recapitulation .......................................................................................................................... 134 6.9. Comprehension Questions and Exercises .................................................................................... 135 References ........................................................................................................................................ 136 Further Reading ................................................................................................................................. 138

ABBREVIATIONS CCM CL DMFC GDE GDL MPL n.p.v. PEFC PEM PTFE SL

Catalyst coated membrane Catalyst layer Direct methanol fuel cell Ga diffusion electrode Gas diffusion layer Microporous layer Nonpercolating volume Polymer electrolyte fuel cell Polymer electrolyte membrane Polytetrafluoroethylene, chemical formula (C2F4)n Support layer, also named macroporous layer

Fuel Cells and Hydrogen. https://doi.org/10.1016/B978-0-12-811459-9.00006-2 # 2018 Elsevier Inc. All rights reserved.

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6.1 INTRODUCTION PEFCs are considered a commercially viable product for both stationary and portable power, and largescale commercialization is predicted for the next few years, especially regarding hydrogen fuel cell vehicles [1–5]. However, further efforts are required for progressive cost reduction, increased efficiency, the cell/stack increase in duration, and safety assurance. For this purpose, further research on materials, components, modeling, and new applicative studies and new test approaches are necessary for supplying industries with all the information needed both for developing products according to the market necessities, and for assuring good products that are compliant with quality standards [6]. New concepts, materials, components, and design are first tested in laboratories before their application to large-scale systems. Consequently, the preparation of an appropriate testing set up, including a set of test protocols, is one of the most important activities in our laboratory’s everyday practice. This chapter is focused on preparation of membrane electrode assembly (MEA), the heart of the PEFC, for testing new materials, new components’ architecture, or to supply information for modeling or for system management, and so forth. Hereafter, hydrogen-fueled PEMFCs are presented as the reference, taking into account that PEFC could also be fueled with alcohols and other fuels. Many different approaches and techniques can be used for the preparation of MEA in the laboratory according to the desired final characteristics, and based on available machinery. Indeed, some production methods that are very useful at the laboratory scale are not applicable at the industrial level, because they are not scalable, or they are too expensive in terms of time or costs. At the same time, some mass production approaches cannot be easily reproduced at the laboratory scale because they require costly and/or huge machines. For example, the sedimentation method is simple and relatively fast at the laboratory scale, and can be considered for small lots, but it is not advantageous for large-scale applications. These aspects are outside the scope of this chapter. This chapter introduces general approaches and gives recommendations about MEA preparation without looking at scalability. The discussion will be centered on guidelines for the definition of a laboratory procedure. Attention is paid to ink deposition techniques, giving an overview of the most used and available, such as airbrushing and screen printing. Recipes and notes about the drawbacks, the industrial production processes, and future evolutions (ink-jet printing and 3D printing) are reported. For a wider vision, readers should refer to the reviews [6–9]. In this chapter self-prepared or commercial available materials like, Nafion and Gore-Select membranes are applied. Additional information is provided on sputtering and chemical vapor deposition techniques related to onsite preparation of catalysts. The discussion includes the description of the MEA, the single-cell fixture for MEA testing and the MEA component preparation.

6.2 THE MEA AND THE TEST-HOLDER According to the IEC-62282-1 Terminology [10], the membrane electrodes assembly is a component of a fuel cell, usually PEFC/DMFC, consisting of an electrolyte membrane with gas diffusion electrodes (GDEs) on either side.

6.2 THE MEA AND THE TEST-HOLDER

119

Anode gas diffusion layer Anode macroporous layer Anode microporous layer Anode catalyst layer Membrane Cathode catalyst layer Cathode microporous layer Cathode macroporous layer Cathode gas diffusion layer

FIG. 6.1 Schematic of MEA structure. Membrane and catalyst layers are sandwiched by two gas diffusion layers. Each diffusion layer is usually composed of two layers with different porosity called microporous and macroporous, or support layers, respectively.

Fig. 6.1, shows a schematic of an MEA section. The anode’s gas diffusion layer (GDL) is composed of one or two sublayers, depending on the architecture (see Section 6.5). In the catalyst layer hydrogen is splitted into protons and electrons, the membrane allows the transport of protons to the cathode, in the cathode catalyst layer water formation occurs, and the cathode GDL is again built of one or two sublayers. Anode and cathode diffusion layers are composed of two sheets, these are usually called the support layer (SL), or macroporous layer and microporous layer (MPL), due to their different functions and porosities. In operation, hydrogen and oxygen (or air) are supplied to anodes and cathodes, respectively. The produced water is removed by the gas flow in the anode and cathode channels. The produced electric current is collected and transported to the electric load (lamp, machine, electronic device, battery) by electric conductors. For testing, an appropriate MEA test-holder is necessary, usually a single MEA per time is characterized in a “single cell” testing unit. The MEA test-holder interacts with the MEA, and its characteristics influence the experimental results. In Fig. 6.2, a sketch of the section of this embodiment is reported, and three common configurations are illustrated. The MEA is closed between two planar half-shells constituted of dense electric conductor materials that have channels for reactant gas adduction and removing exhausts; a couple of gaskets, usually made of synthetic elastomers, are used to avoid gas leakage. The two half shells are clamped by tie bolts or tie belts or, rarely, by an additional clamping device based on a hydraulic piston or a piston screw. The two half-shells of the MEA holder are usually made up of three main configurations. In the most used configuration, Fig. 6.2A, each half-shell is composed of two parts: a plate, usually graphite, with channels for gas distribution (flow field) hollowed on the face in contact with the GDL of the MEA, and a metallic compression plate also working as a current collector. Tie bolts with insulating washers to avoid short circuits are used for clamping the cell, and a couple of gaskets are used to avoid gas leakage from cell sides. This configuration allows fast and low-cost replacement of flow field

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Current collector

Flow field plate

Tie bolt

(A)

(B)

Gasket MEA

Electric isolator

(C)

FIG. 6.2 Schematic of single-cell enclosure. The most used configuration (A) foresees separate plates for the flow field and for compression/current collection. A single metal plate can be used (B) for reducing the number of pieces. In the configuration (C) two different plates are used for current collection and compression. In this last case, a compression plastic or other nonconductive materials plate could be used.

plates; the main issue is related to the metallic compression plate under voltage that can allow an accidental short circuit due to contact with other conductors. The configuration reported in Fig. 6.2B consists of a single metallic plate hosting both the flow field on the face in contact with the GDL, the holes for the tie bolt on its border, and the electric contactors for current drain. Due to the environment inside the cell, in this case, surface treatments are needed to avoid metal corrosion and surface passivation, also reducing contact resistance between this plate and the GDL. The very simple mounting/dismounting is the only advantage. The main issues are related to weight and accidental short circuits. Another configuration, Fig. 6.2C foresees the use of three plates: usually the one hosting the flow field is made of graphite a second one usually copper made is for current collection, and a third one is for clamping and it is made of stainless steel, aluminum, or plastic. Coming from the stack, this configuration allows a simple electric insulation of compression plates. The MEA holder interacts with the MEA, mainly by the flow field [11–13]. This means that changing the reference flow field also results in an MEA performance change. Moreover, the overtightening of clamping bolts, and the plates’ deformations, negatively influence cell performance [14–16] by damaging the MEA or increasing electric resistance. After insertion of the MEA in the single-cell fixture, the test cell is connected to a test bench that has all the equipment for controlling gas fluxes, the operative conditions, and data acquisition and storage. For MEA characterization, a reference is represented by the IEC technical specification “Fuel cell technologies – part 7.1: Single-cell test methods for polymer electrolyte fuel cells (PEFC)” [17]. In this document, the minimum instrumentation requirements and reference procedures for testing methods are reported. The IEC documents are written specifically for industries.

6.3 ELECTROLYTE MEMBRANE

121

6.3 ELECTROLYTE MEMBRANE The polymer electrolyte membrane (PEM) defines, to a large extent, the operative limits and performance of the cell, and, consequently, also the characteristics required of the other components for obtaining the best performance and endurance. To reduce the cell resistance, and thereby increase the cell efficiency, it is useful to apply a very thin membrane. But this affects the mechanical resistance and the permeability to reactant gases, according to the properties of ions conducting polymer that is used for its preparation, and the preparation technique itself. Consequently, the selected membranes will be the best compromise between minimum resistivity, good mechanical characteristics, and low gas permeability. The PEM is usually supplied ready for use as a foil or as a rolled tape, and for MEA preparation, it has to be cut with the correct shape and dimensions before use. The supplier gives information about the characteristics of the membrane, such as electrical conductibility, water uptake, swelling, operating temperature, glass transition temperature, and so forth. These data are useful both for MEA preparation, and experimental results analysis. For the membrane, 3–15 mm of extra border is added for sealing (Fig. 6.3A) to the electrode area. The extra borders are in contact with the gaskets, which have a double function: sealing the borders of the catalyst and GDL layers, and compensating for their thicknesses, avoiding porous layers’ damage due to mechanical compression stress [18,19]. In common single cells, plain gaskets or O-ring-like sealings are used. The gaskets are introduced as a separate part, or are stamped onto the flow field plate, or onto the membrane [20]. The PEM’s extra borders are not to be covered by electrodes and not to be exposed to mechanical stress due to gaskets. The common solution for protecting the PEM in this area is the addition of protective polymeric layers named pre-gaskets (Fig. 6.3B). The pre-gaskets are applied to the PEM’s extra border by a thin glue layer, or by hot pressing, or rarely, by printing. The use of a glue, such as for adhesive polyester sheets, requires an accurate choice of this component to avoid catalyst poisoning, Stress point

Plane gasket

(A)

Stress point

O-ring gasket

(B) Plane gasket

Pre-gasket

(C) FIG. 6.3 Schematic view of the section of an MEA and the related gaskets for a standard laboratory cell. With plane gasket (A) and O-ring (B), the electrode border introduces a stress on the membrane. The addition of pre-gaskets (C) reduces the membrane stress.

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or spread of the glue out of the requested area. Hot pressing does not introduce additional chemicals, avoiding any possible catalyst pollution; in this case, attention must be paid to the maximum temperature and pressure that can be applied without damaging the membrane. For perfluorosulfonic membranes, a max of 150–160°C is recommended, while max pressure strongly depends on PEM thickness. In printing, both the introduction of additional chemicals and a thermal treatment are required, but usually the curing temperature does not rise above 120–130°C. These aspects, and the compatibility of pre-gasket material with membrane’s base polymer, in terms of chemical and physics properties, must be taken into account in choosing the material for pre-gasketing. In industrial MEA production, the PEM area out of the electrodes can be reinforced with other approaches, such as injection molding, which is useful for large-scale production, and not easy to apply in a laboratory without the appropriate machinery. For the membranes, often pretreatments according to PEMs’ supplier specifications are required in order to remove possible traces of materials that could damage the cell performance. A procedure reported in literature is given here [21–25]: 1. 2. 3. 4. 5.

30–60 min in 3–5% H2O2 aqueous solution at 80°C 1–2 h in deionized water at 80–90°C Treatment in 0.5–1 M H2SO4 aqueous solution at 80–90°C Washing in deionized hot water Storage in deionized water at room temperature

Sometimes additional or alternative purification approaches have been reported for removing any trace of residual impurities. For example, in [25], the following pretreatment has been applied, both to Nafion, and to composite membranes: 1. Reflux washing in a 50:50 mixture by volume of water and concentrated (95%–70.8%) HNO3 for 6–8 h 2. Reflux washing in a 50:50 mixture by volume of water and concentrated (95%–98%) H2SO4 for 6–8 h 3. Reflux washing in distilled water reaching up to a pH over 6.5 4. Storage in deionized water at room temperature

6.4 CATALYST LAYER The catalyst particles have to be in contact with both electronic and proton-conducting materials, and have to be accessible to the reactants. Moreover, the path for product water removal must be available in the CL [26]. This means that three percolating networks must be present: one for electron conduction (catalyst materials), one for ion conduction (ion conducting polymer), and a pore network for gas transport and water removal (Fig. 6.4). The ionomer creates ion-conducting films in contact with the catalyst grains this film allows the transport of protons from the membrane. Electrons coming from the bipolar plate pass through the GDL, migrate along the carbon grains supporting the metal catalyst reaching the reaction site, while oxygen reaches the reaction site by a percolating path of wet proofed pores, or pores that are not water filled. The reaction of water formation can occur only on catalyst grains where protons, oxygen, and

6.4 CATALYST LAYER

123

FIG. 6.4 Scheme of the catalyst layer structure with reactant path at cathode.

electrons are simultaneously present. These sites are named the “triple phase boundary” (red circle, in Fig. 6.4) and are realized where metal particles of the catalyst are in contact with the ionomer film, and there is access to oxygen. In nonpercolation volume (in Fig. 6.4, “n.p.v”), no reactions can arise, because one of the percolating paths is not present; for example, in Fig. 6.4, the ionomer film is not present. So too, at the anode, the hydrogen oxidation reaction is effective only at the triple phase boundary sites. The “triple phase boundary” name comes from the electrochemical cells using a liquid electrolyte, because in this case, there is a liquid (the electrolyte), a solid (the catalyst), and a gas (the reactant) at the same point. In fuel cells with liquid electrolytes, the maintenance of the electrolyte film network without filling the pores is one of the major issues. With a solid electrolyte, a stable ion conduction network that is not filling pores is possible; however, due to penetration and distribution restrictions of the solid electrolyte, limitations in the utilization of the catalyst located in small pores are present. The issues related to the wettability of materials and structure are common to fuel cells with liquid electrolytes and to PEFC, because they influence the ion conduction and the availability to the pores of gases. Finally, water produced during the reaction can be removed by a percolating path of pores, both in vapor and in liquid form, or can be absorbed by the solid electrolyte. In this last case, it can reach the membrane and the anode side. The described CL structure is the result of extensive research on electrode development that started with the Teflon-bonded catalyst layer applied on the GDL (see the next section), a configuration also used in phosphoric acid fuel cells and other electrochemical devices, which finally arrives at the “catalyst coated membrane” (CCM) configuration [7,8,26]. It has to be noted that the presence of percolating paths, like in the catalyst layer, are also necessary in other components of the MEA; yet only in the catalyst layer, three percolating paths are requested. Therefore, in preparing the catalyst layer, the goal is the realization of a volume with three percolating paths without nonpercolating volumes, because these cause a reduction of the MEA performance and, in some cases, can trigger destructive processes. At the same time, it is needed to maximize the catalyst utilization and to optimize the transport of the reactant and the product removal. For example, pores of 20–40 nm diameter are challenging for Nafion ionomer penetration, and consequently, using supported catalysts (such as Pt on carbon), it is possible that the ionomer cannot reach the Pt nanoparticles located in small or deep pores of the support; therefore, these particles will be not useful for the reactions.

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The selection of catalyst supported on mesoporous (5–50 nm) carbons and low porous supports creating mesopores in the agglomeration process, like carbon nanotubes, increases the catalyst utilization [27–29]. However, PEFC catalysts usually consist of a powder of platinum (Pt)-based nanoparticles deposited on a carbon support [6–9,30,31], because a limited number of applications of unsupported metal particles are used. After the catalyst selection, the catalyst layer preparation can start defining the preparation procedure. The first step is the choice of the route to follow. Starting from the catalyst powder, CL can be made onto the GDL or on the membrane, the first approach gives the so-called GDE and the second gives the CCM. Although direct deposition/formation methods on membranes or GDLs have been proposed for catalyst layer preparation: for example, sputtering and physical-chemical deposition [32,33], these methods are special cases and not common practice in laboratory preparation, because they require dedicated machinery and rooms. For preparing GDE and CCM, the starting point is the preparation of a catalyst ink, usually consisting in a suspension of catalyst particles in a solution containing the solid electrolyte ionomer that acts both as ion conductor and binder. Pore formers and other additives could be added to manage the CL architecture, to ameliorate the water management, or to increase the ink stability, just as examples. The inner structure of the CL is determined mainly by on the ink composition, although the deposition method and related deposition parameters have an influence on the final structure. The ionomer has a polymeric part (hydrophobic) and a polar part for ion conduction; consequently, the choice of the solvent for the ink preparation will change the ink properties [34–36], as shown in Fig. 6.5.

FIG. 6.5 Scheme of the proposed ink structure (left) in solution (up) and colloidal (down) methods application, and related basic structures in the catalyst layer after preparation, respectively.

6.4 CATALYST LAYER

125

Referring to Nafion, as reported by M. Uchida et al. [34], with its ionomer, a solution is obtained if the dielectric constant of the solvent is >10, while undergoing precipitation for a solvent dielectric constant of 70%) and good electrical conductivity. These carbon-based materials are light and stable in the PEFC environment, and allow low interface resistance with the catalyst layer. Carbon cloth has a woven structure, and is obtained by carbonization and graphitization of a polymer tissue (Fig. 6.12A). The base polymer, the thickness of polymeric fibers, and the thermal

FIG. 6.11 Scheme of the structure and interaction of GDL, CL, and membrane (cathode side). Area indicated with “n.p.v.” have do not have electron paths available, this will block/reduce the catalyst layer activity close to the interested area.

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CHAPTER 6 PREPARATION OF MEA

FIG. 6.12 Front (up) and section (down) field emission scanning electron microscope images of commercial (A) carbon cloth, (B) carbon paper, and (C) carbon felt realized at low accelerating voltage (5 kV).

treatments to transform it into carbon fibers define the carbon cloth’s electric, thermal, and mechanical properties. Due to its structure, carbon cloth has two main pores classes [48]: large pores are present at the corners of the bundles of the fibers’ intersection, while small pores are located along the bundles. The arranged fibers provide preferential paths for electron, water, and gas transport. Carbon cloth offers very good adaptability under compression thanks to the fibers’ compaction, but this also changes the carbon cloth porosity according to exerted compression. Carbon paper is obtained by gluing carbon fibers together (Fig. 6.12B); the less-ordered structure results in a more homogeneous distribution of porosity and conductivity, and into a more flat surface in comparison with carbon cloth. Also, for carbon paper, a preferential path for water transport has been observed [49,50]. Carbon felt is composed of carbon fibers such as carbon paper, but in this case, the fibers are longer and more flexible, which allows obtaining a felt-like structure where no binder is necessary to maintain the structure. All these carbon-based materials have hydrophobic characteristics, however, are further wet proofed by adding a polymer-like polytetrafluoroethylene (PTFE), or fluorinated-ethylene-propylene (FEP). Commercial materials of the same type and with the same porosity distribution from different suppliers, or from different production batches, might have hydrophobic characteristics with slight differences because they depend on a number of parameters that are difficult to control, such as small variations of fibers’ thickness and roughness. The hydrophobic properties of the GDL also change during operation, depending on operative conditions. As shown in Fig. 6.12, the surface of carbon cloth is textured, and that of carbon paper and carbon felt include irregular large pores. In both cases, a direct contact between SL and the catalyst layer cannot be really effective.

6.6 GAS DIFFUSION LAYER

133

The addition of an MPL surface layer onto the SL improves both the SL surface flatness, and water/ gas management of the whole GDL. The MPL provides a reduction of the electrical contact resistance between SL and the catalyst layer, and a control of water removing from the cathode catalyst layer, thanks to the microporous structure that increases the contact surface and limits the liquid water transport. Usually, the MPL consists of carbon-based particles (carbons, graphite, carbon nanotubes, carbon nanofibers, carbon felt, carbon foams) mixed with a binder also acting as a hydrophobic agent, usually PTFE. Typical pore size of the MPL is in the order of the carbon agglomerates (100–500 nm), with a layer thickness of 5–50 μm, while SLs have a thickness of 100–300 μm and a pore size of 10–30 μm. For a liquid, the pressure necessary to enter a pore increases as the pore radius decreases [48,50]. Consequently, the MPL structure is more favorable to water vapor diffusion than to liquid water transport. Some studies [50,51] demonstrate that the pore distribution in SL has, as a consequence, the creation of a preferential path for liquid water transport, and that MPL defines the liquid water pressure necessary in the catalyst layer to remove the produced water. A review of the influence of materials on GDL properties can be found in the publication of Cho and Mench [52] and Park et al. [53]. GDLs can easily be purchased from outer sources, or produced in the laboratory, according to necessity. In the case of purchase, the supplier information sheet contains the GDL characteristics, and there is rarely the necessity of a GDL characterization to define, for example, porosity and pore distribution, permeability to gases, and hydrophobic properties. In the case of self-production, the starting point is the selection of the SL, because carbon cloth, carbon paper, and other supports in home production are not simple. In common practice, the SL is acquired with the requested wet proofing. The MPL is obtained by deposition of an ink or paste containing the appropriate balance of carbon and PTFE onto the SL by using the same deposition techniques already illustrated for the CL preparation. Usually, 20–30 wt% PTFE is applied for PEMFC operating at 60–80°C with humidified gases. The ink density must be set according to the deposition method. More dense inks are requested for doctor blade or spatula deposition, while lower densities are requested for serigraphy, brushing, air brushing, and spraying. Usually, ink-jet deposition is not recommended in this case, because the PTFE tends to block the ejectors. After drying, a thermal treatment is carried out to allow the PTFE’s sintering. The MPL porosity is defined mainly by the carbon particles’ size and binder loading; while the deposition technique has less influence on it. Also, for the MPL to manage the porosity pore, the previously discussed materials can be added to the ink. The choice of the deposition techniques influences the minimum thickness that can be uniformly deposited, the penetration of the MPL layer in the SL structure, and the possibility of designing the MPL. For example, spray techniques allow very thin layer deposition and the possibility of creating multilayer MPL, where each layer has a different composition. In many cases, like in the catalyst layer, one or more mechanical compaction steps are applied during the GDL preparation route; these steps influence the porosity of the MPL and the penetration of this in the SL. Consequently, accurate control of these steps is very important to grant preparation repeatability and product quality.

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6.7 MEA ASSEMBLING When all components are prepared, the last step is the MEA preparation. The MEA is obtained by sandwiching the membrane between two GDE, or sandwiching the CCM between two GDL. Before this step, the application of a pre-gasket on the membrane area outside the active area to reinforce and protect the membrane is considered a good practice. Hot pressing is used to combine the components. A reference hot pressing procedure, using a laboratory press, is given: 1. Warm up the press hot plates to 110–140°C 2. Align the cathode GDE, membrane, and anode GDE (or cathode GDL—CCM—anode GDL) with the catalyst layers against the membrane 3. Insert the sandwich in the PTFE bag 4. Close the jaws (but do not clamp) for 5 min for temperature equilibration 5. Apply pressure of 1–4 MPa (10–40 kg/cm2) for 1–3 min 6. Unclamp and let the MEA cool down before storing. In the laboratory, hot pressing is made by a press with heated plates. Presses with digital controls are widely available on the market. The manual press can also be used, but only if equipped with a good pressure gauge and a high quality temperature control. Periodical verification of the planarity and the parallelism of the plates is recommended. The MEA is very thin, and small defects can totally destroy the previous work. The use of a template mold can be useful, both for the correct coupling of the different pieces, and for granting uniform compression. It is important to understand that usually, the hot pressing is carried out at a temperature close to the glass transition of the ion-conducting polymer composing the membrane, and used as an ionomer in the catalyst layer. Using GDE hot pressing allows a better match between the CL and the membrane; while using CCM, it allows a better arrangement of the CL and MPL contact surface. For this reason, when GDE are assembled on a membrane, higher temperatures are recommended, while starting from a CCM at a lower temperature could be used. Moreover, for GDE assembling, in some cases, a temperature ramp is used at step 4 or 5; for example, increasing the temperature gradually from 110°C to 130°C. It is advisable to cool the MEA between two cooling plates and apply light pressure to reduce creasing and warping. After cooling, the MEA is ready for insertion in the single-cell fixture or in the stack; if necessary it may be stored in plastic bags, preferably hermetic. Finally, the assembling is done directly inside the single cell, if requested by the membrane characteristics or by the specific test to be performed, but this is not a standard practice. In fact, hot pressing reduces contact resistance between the layers composing the MEA, and allows handling of the MEA as a single piece, permitting a more practical, precise, and simple mounting of the experimental set up, both in laboratory and in stack assembling for industrial production. The MEA preparation steps are resumed in the form of a flow sheet in Fig. 6.13.

6.8 RECAPITULATION This chapter has addressed the fabrication routes for catalyst layers, GDLs, and MEA. In PEFC, the electrochemical reactions take place at the three-phase boundary of fuel cell catalyst layers, which are the active catalyst sites. Consequently, the proton conductor, electron conductor, and reactants must be

6.9 COMPREHENSION QUESTIONS AND EXERCISES

135

FIG. 6.13 A flow sheet for the MEA preparation process.

present at all active catalyst sites. Conduction networks for proton, electron, gas, and water transport must coexist in a catalyst layer, while in the GDL, proton conduction is not required. Basically, the properties of the gas diffusion and catalyst layers are mainly defined by: • • •

Shape and porosity of the catalyst support, ionomer properties and loading, and ink formulation for the catalyst layer Porosity, base material shape and porosity, and applied wet proofing for the SL of the GDL Porosity, material shape and porosity, and the hydrophobic binder load for the microporous layer of the GDL.

The preparation procedures for the GDL, catalyst layers, and MEAs have a strong effect upon resulting architecture of GDLs and CLs. A wide range of gas diffusion and catalyst layers have been designed, and accordingly, many inks and deposition methods have been developed to generate these layers. Within available preparation methods, those based on the preparation of an ink are largely used for their low cost and flexibility. Some of these techniques, such as ink-jet printing, also allow the preparation of layers with variable characteristics on electrode active areas, such as porosity or catalystloading linear variation. To prepare a high quality MEA, it is essential to define a procedure that is completely mastered for and achieves high reproducibility in production.

6.9 COMPREHENSION QUESTIONS AND EXERCISES 1. A catalyst ink is prepared with a mixture of 6 g of isopropanol and 4 g of deionized water, 30 mg of 10% Pt/C powder and 180 mg of 5% Nafion ionomer solution. What method is followed: colloidal or solution? Why? 2. Referring to the catalyst ink of Question 1, having the target of an active area with 0.1 mg/cm2 Pt loading and using a deposition method with losses of 10%, how much will the catalyzed area be? (27 cm2).

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3. Considering the catalyst ink in Question 1, changing the catalyst powder to 30% Pt/C with the same carbon support (e.g., Vulcan), a significant change in catalyst layer structure is expected. Why? 4. Describe the steps for CCM production by the sedimentation method. 5. Describe the steps for GDE production by a deposition method of your choice. 6. Build a flow chart for MEA production.

REFERENCES [1] G. Squadrito, L. Andaloro, M. Ferraro, V. Antonucci, Chapter 16. Hydrogen fuel cell technology, in: A. Basile, A. Iulianelli (Eds.), Advances in Hydrogen Production, Storage and Distribution, Woodhead Publishing, 2014. ISBN 978-0-85709-768-2. [2] The Fuel Cell Industry Review, E4Tech, 2015. December 2014; free download. http://www. fuelcellindustryreview.com/archive/TheFuelCellIndustryReview2015.pdf. Accessed 30 June 2017. [3] 4th Energy Wave, The fuel cell and hydrogen annual review, Free download from, https://www. 4thenergywave.com/fuelcellannualreview, 2016. Accessed 30 June 2017. [4] DOE, Advancing clean transportation and vehicle systems and technologies—fuel cell electric vehicles, in: Quadrennial Technology Review, 2015. ch.8 free download from, https://energy.gov/under-secretaryscience-and-energy/downloads/chapter-8-advancing-clean-transportation-and-vehicle. Accessed 30 June 2017. [5] Fuel Cell Today, Fuel Cell Electric Vehicles: The Road Ahead, free download from, http://www. fuelcelltoday.com/media/1711108/fuel_cell_electric_vehicles_-_the_road_ahead_v3.pdf. Accessed 30 June 2017. [6] Y. Wang, K.S. Chen, J. Mishler, S.C. Cho, X.C. Adroher, A review of polymer electrolyte membrane fuel cells: technology, applications, and needs on fundamental research, Appl. Energy 88 (2011) 981–1007. [7] S. Lister, G. McLean, PEM fuel cell electrodes, J. Power Sources 130 (2004) 61–76. [8] V. Mehta, J.S. Cooper, Review and analysis of PEM fuel cell design and manufacturing, J. Power Sources 114 (2003) 32–53. [9] D. Wheeler, G. Sverdrup, 2007 Status of Manufacturing: Polymer Electrolyte Membrane (PEM) Fuel Cells, Technical Report NREL/TP-560-41655, http://www.nrel.gov/docs/fy08osti/41655.pdf, March 2008. [10] IEC/TS 62282-1: Fuel cell technologies – Part 1: Terminology”, third ed., International Electrotechnical Commission (CH), 2013, ISBN: 978-2-83221-190-8. [11] G. Squadrito, O. Barbera, I. Gatto, G. Giacoppo, F. Urbani, E. Passalacqua, CFD analysis of the flow-field scale-up influence on the electrodes performance in a PEFC, J. Power Sources 152 (2005) 67–74. [12] J.P. Owejan, T.A. Trabold, D.L. Jacobson, M. Arif, S.G. Kandlikar, Effects of flowfield and diffusion layer properties on water accumulation in a PEM fuel cell, Int. J. Hydrog. Energy 32 (2007) 4489–4502. [13] G. Squadrito, O. Barbera, G. Giacoppo, F. Urbani, E. Passalacqua, Computer aided fuel cell design and scaleup, comparison between model and experimental results, J. Appl. Electrochem. 37 (2007) 87–93. [14] R. Montanini, G. Squadrito, G. Giacoppo, Assessment of fuel cell’s endplate out of plane deformation using digital image correlation, in: G. Neri et al., (Ed.), Sensors and Microsystems, ch. 72. Lecture Notes in Electrical Engineering, vol. 91, Springer-Verlag, Dordrecht (NL), 2011, pp. 443–447. [15] X. Wang, Y. Song, B. Zhang, Experimental study on clamping pressure distribution in PEM fuel cells, J. Power Sources 179 (2008) 305–309. [16] R. Montanini, G. Squadrito, G. Giacoppo, Measurement of the clamping pressure distribution in polymer electrolyte fuel cells using piezoresistive sensor arrays and digital image correlation techniques, J. Power Sources 196 (2011) 8484–8493.

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[17] IEC TS 62282-7-1:2017, Fuel cell technologies – part 7.1: Single cell test methods for polymer electrolyte fuel cell (PEFC). [18] J.H. Lin, Y.J.S. WH Chen, T.H. Ko, Effect of gas diffusion layer compression on the performance in a proton exchange membrane fuel cell, Fuel 87 (2008) 2420–2424. [19] S. Escribano, J.E. JF Blachot, A. Morin, R. Mosdale, Characterization of PEMFCs gas diffusion layers properties, J. Power Sources 156 (2006) 8–13. [20] L. Frisch, PEMFC Stack Sealing Using Silicone Elastomers, SAE Technical Paper 2003-01-0801, 2003. [21] T.A. Zawodzinski Jr., M. Neeman, L.O. Sillerud, S. Gottesfeld, Determination of water diffusion coefficients in perfluorosulfonate ionomeric membranes, J. Phys. Chem. 95 (1991) 6040–6044. [22] F. Lufrano, E. Passalacqua, G. Squadrito, A. Patti, L. Giorgi, Improvement in the diffusion characteristics of low Pt-loaded electrodes for PEFCs, J. Appl. Electrochem. 29 (1999) 445–448. [23] E. Passalacqua, F. Lufrano, G. Squadrito, A. Patti, L. Giorgi, CO tolerance of Pt-W electrocatalysts for polymer electrolyte fuel cells, J. New Mater. Electrochem. Syst. 3 (2000) 131–135. [24] H. Wang, J.A. Turner, The influence of metal ions on the conductivity of Nafion 112 in polymer electrolyte membrane fuel cell, J. Power Sources 183 (2008) 576–580. [25] K.T. Adjemian, S.J. Lee, S. Srinivasan, J. Benziger, A.B. Bocarsly, Silicon oxide Nafion composite membranes for proton-exchange fuel cell operation at 80–140°C, J. Electrochem. Soc. 149 (2002) A256–A261. [26] E. Passalacqua, F. Lufrano, G. Squadrito, A. Patti, L. Giorgi, Nafion content in the catalyst layer of polymer electrolyte fuel cells: effects on structure and performance, Electrochim. Acta 46 (2001) 799–805. [27] E. Antolini, Carbon supports for low-temperature fuel cell catalysts, Appl. Catal. B Environ. 88 (2009) 1–24. [28] D. Banham, F. Feng, T. F€urstenhaupt, K. Pei, S. Ye, V. Birss, Effect of Pt-loaded carbon support nanostructure on oxygen reduction catalysis, J. Power Sources 196 (2011) 5438–5445. [29] M. Uchida, Y. Fukuoka, Y. Sugawara, N. Eda, A. Ohta, Effects of microstructure of carbon support in the catalyst layer on the performance of polymer-electrolyte fuel cells, J. Electrochem. Soc. 143 (1996) 2245–2252. [30] E. Antolini, Formation of carbon-supported PtM alloys for low temperature fuel cells: a review, Mater. Chem. Phys. 78 (2003) 563–573. [31] H.A. Gasteiger, S.S. Kocha, B. Sompalli, F.T. Wagner, Activity benchmarks and requirements for Pt, Pt-alloy, and non-Pt oxygen reduction catalysts for PEMFCs, Appl. Catal. B Environ. 56 (2005) 9–35. [32] A.T. Haug, R.E. White, J.W. Weidner, W. Huang, S. Shi, T. Stoner, N. Rana, Increasing proton exchange membrane fuel cell catalyst effectiveness through sputter deposition, J. Electrochem. Soc. 149 (2002) A280–A287. [33] M.D. Gasda, R. Teki, T.-M. Lu, N. Koratkar, G.A. Eisman, D. Gall, Sputter-deposited Pt PEM fuel cell electrodes: particles vs layers, J. Electrochem. Soc. 156 (2009) B614–B619. [34] M. Uchida, Y. Aoyama, N. Eda, A. Ohta, New preparation method for polymer-electrolyte fuel cells, J. Electrochem. Soc. 142 (1995) 463–468. [35] M. Uchida, Y. Fukuoka, Y. Sugawara, H. Ohara, A. Ohta, Improved preparation process of very-low-platinum-loading electrodes for polymer electrolyte fuel cells, J. Electrochem. Soc. 145 (1998) 3708–3713. [36] S.-J. Shin, J.-K. Lee, H.-Y. Ha, S.-A. Hong, H.-S. Chun, I.-H. Oh, Effect of the catalytic ink preparation method on the performance of polymer electrolyte membrane fuel cells, J. Power Sources 106 (2002) 146–152. [37] Y.-G. Yoon, G.-G. Park, T.-H. Yang, J.-N. Han, W.-Y. Lee, C.-S. Kim, Effect of pore structure of catalyst layer in a PEMFC on its performance, Int. J. Hydrog. Energy 28 (2003) 657–662. [38] H. Tang, S. Wang, M. Pan, R. Yuan, Porosity-graded micro-porous layers for polymer electrolyte membrane fuel cells, J. Power Sources 166 (2007) 41–46. [39] F.A. Uribe, T.A. Zawodzinski Jr., A study of polymer electrolyte fuel cell performance at high voltages. Dependence on cathode catalyst layer composition and on voltage conditioning, Electrochim. Acta 47 (2002) 3799–3806.

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[40] J.-H. Kima, H.-J. Kimb, T.-H. Limb, H.-I. Lee, Dependence of the performance of a high-temperature polymer electrolyte fuel cell on phosphoric acid-doped polybenzimidazole ionomer content in cathode catalyst layer, J. Power Sources 170 (2007) 275–280. [41] S. Hirano, J. Kim, S. Srinivasan, High performance proton exchange membrane fuel cells with sputterdeposited Pt layer electrodes, Electrochim. Acta 42 (1997) 1587–1593. [42] J.S. Zheng, T. Tian, Y. Gao, Q. Wu, J.X. Ma, J.-P. Zheng, Ultra-low Pt loading catalytic layer based on buckypaper for oxygen reduction reaction, Int. J. Hydrog. Energy 39 (2014) 13816–13823. [43] M.M. Waje, X. Wang, W. Li, Y. Yan, Deposition of platinum nanoparticles on organic functionalized carbon nanotubes grown in situ on carbon paper for fuel cells, Nanotechnology 16 (2005) S395. [44] Z.Q. Tian, S.H. Lim, C.K. Poh, Z. Tang, Z. Xia, Z. Luo, P.K. Shen, D. Chua, Y.P. Feng, Z. Shen, J. Lin, A highly order-structured membrane electrode assembly with vertically aligned carbon nanotubes for ultra-low Pt loading PEM fuel cells, Adv. Energy Mater. 1 (2011) 1205–1214. [45] K.L. Choy, Chemical vapour deposition of coatings, Prog. Mater. Sci. 48 (2003) 57–170. [46] D. Bevers, N. Wagner, M. Bradke, Innovative production procedure for low cost PEFC electrodes and electrode membrane structures, Int. J. Hydrog. Energy 23 (1998) 57–63. [47] E. Passalacqua, G. Squadrito, F. Lufrano, A. Patti, L. Giorgi, Influence of the structure in low-Pt loading electrodes for polymer electrolyte fuel cells, Electrochim. Acta 43 (1998) 3665–3673. [48] J. Benziger, J. Nehlsen, D. Blackwell, T. Brennan, J. Itescu, Water flow in the gas diffusion layer of PEM fuel cells, J. Membr. Sci. 261 (2005) 98–106. [49] S. Lister, D. Sinton, N. Djilali, Ex situ visualization of liquid water transport in PEM fuel cell gas diffusion layers, J. Power Sources 154 (2006) 95–105. [50] B. Auvity, G. Giacoppo, G. Squadrito, E. Passalacqua, Visualisation study of water flooding in a model fuel cell, Second International conference on Hydrogen Energy (ICHE’10), Hammamet (Tunisia), May 9–11, 2010. [51] O. Chapuis, M. Prat, M. Quintard, E. Chane-Kane, O. Guillot, N. Mayer, Two-phase flow and evaporation in model fibrous media. Application to the gas diffusion layer of PEM fuel cells, J. Power Sources 178 (2008) 258–268. [52] K.T. Cho, M.M. Mench, Effect of material properties on evaporative water removal from polymer electrolyte fuel cell diffusion media, J. Power Sources 195 (2010) 6748–6757. [53] S. Park, J.-W. Lee, B.N. Popov, A review of gas diffusion layer in PEM fuel cells: Materials and designs, Int. J. Hydrog. Energy 37 (2012) 5850–5865.

FURTHER READING [54] M.S. Wilson, S. Gottesfel, High performance catalyzed membranes of ultra-low Pt loadings for polymer electrolyte fuel cells, J. Electrochem. Soc. 139 (1992) L28. [55] M.S. Wilson, S. Gottesfeld, Thin-film catalyst layers for polymer electrolyte fuel cell electrodes, J. Appl. Electrochem. 22 (1992) 1–7.

CHAPTER

DEGRADATION MECHANISMS AND THEIR LIFETIME

7

Merit Bodner, Jan Senn, Viktor Hacker Graz University of Technology, Institute of Chemical Engineering and Environmental Technology, Graz, Austria

CHAPTER OUTLINE 7.1 Introduction ................................................................................................................................ 139 7.2 Polymer Membrane Lifetime ......................................................................................................... 140 7.2.1 Chemical Degradation ................................................................................................ 140 7.2.2 Mechanical Degradation ............................................................................................ 142 7.3 Carbon Corrosion ........................................................................................................................ 143 7.4 Catalyst ...................................................................................................................................... 144 7.5 Impurities ................................................................................................................................... 145 7.5.1 Anode Gas Stream ..................................................................................................... 146 7.5.2 Cathode Gas Stream .................................................................................................. 147 7.5.3 Contaminations of Components .................................................................................... 148 7.6 Costs Versus Lifetime .................................................................................................................. 148 7.7 Recapitulation ............................................................................................................................. 150 7.8 Comprehensive Questions and Exercises ....................................................................................... 150 References ........................................................................................................................................ 151

7.1 INTRODUCTION Polymer electrolyte fuel cells (PEFCs) are a promising technology for realizing a sustainable power supply due to their high efficiency and flexibility. The major drawbacks of fuel cells are the high costs and the limited lifetime. The costs derive from the applied materials, such as the catalyst system. The catalyst platinum is expensive, but via novel preparation methods, the metal content of the catalyst was decreased significantly without loss of power output [1]. Lifetime limitations are mainly caused by degradation of the electrodes, the membranes, and by impurities. Contaminations can have various sources, and often show vast effects on the fuel cell. Catalyst poisoning causes the decrease of fuel cell performance due to a limited catalytically active surface area; whereas other contaminants bind to the ion conducting moieties of the membrane, thereby increasing the membrane resistance vastly. Fuel Cells and Hydrogen. https://doi.org/10.1016/B978-0-12-811459-9.00007-4 # 2018 Elsevier Inc. All rights reserved.

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Many harmful impurities derive from fuel processing, using renewable or fossil fuels, and from system components; for example, by causing contamination of the gas stream [2]. The fuel cell components most prone to degradation are the membrane and the electrodes. As a result, the performance declines, leading to an instable operation and a limitation of the operational lifetime. The polymer electrolyte membrane (PEM) must be able to withstand harsh conditions, including temperature and humidity fluctuations, strong oxidants, alternating load conditions, and reactive radicals [3]. Although perfluorinated sulfonic acid membranes (PFSAs) have shown highly efficient and stable performance in fuel cell applications, degradation of the polymer electrolyte is evident [4–6]. Membrane degradation is defined by three interrelated mechanisms; chemical, mechanical, and thermal degradation [7]. Chemical membrane degradation is expected to be caused by the formation of radicals from hydrogen peroxide, which is formed in open circuit conditions [8]. Degradation of the electrode is predominantly determined by the loss of active surface area of the electrocatalyst. This is caused by poisoning, loss or agglomeration of platinum, or by detachment of the catalyst due to carbon support material corrosion. However, the electrodes also contain ionomers to provide proton conductivity, and polytetrafluoroethylene (PTFE) to adjust the hydrophobicity. Both compounds can undergo decomposition as well. Research has shown that high temperatures, combined with air atmosphere and humid conditions (which match real fuel cell cathode conditions) do not affect the microporous layer of the fuel cell by altering the thermal conductivity; but rather by lowering the water contact angles. The lower hydrophobicity results from a loss of PTFE in the electrode [9].

7.2 POLYMER MEMBRANE LIFETIME By far, the most commonly used PEM is Nafion (DuPont). The perfluorosulfonic acid-based ionomer shows a high chemical stability, and, depending on the membrane thickness and the operation conditions, a high operational lifetime [10,11]. During operation, the fuel cell membrane is exposed to a variety of harsh conditions, accelerating the chemical and mechanical degradation of the polymer electrolyte.

7.2.1 CHEMICAL DEGRADATION A correlation between the operating conditions and the pH of the effluent water has been found, with a low pH (potential of hydrogen) being an indicator for membrane decay. This is attributed to the formation of hydrogen peroxide, which is decomposed to radicals that attack the perfluorosulfonic acidbased ionomer of the membrane, and the ionomer within the electrodes. A low relative humidity has been found to accelerate hydrogen peroxide (H2O2) production, and promote its stability. Therefore, lower pH values of around two were found with a load in the range of 0.4–0.8 A cm
2 and a relative humidity of 35%. When the humidity was increased to 60%, the pH value increased to 5 [12]. Hydrogen peroxide formation is favored for open circuit potential. A higher concentration is usually found on the anode, despite the lower mobility of oxygen compared with hydrogen [13]. However, the presence of hydrogen peroxide is not sufficient to induce membrane decomposition. Formation of the product of the homolytic cleavage of the OdO bond, or rather, the OH radical, is enabled by the presence of fenton active metal ions such as Fe3+ and Cu2+ [14].

7.2 POLYMER MEMBRANE LIFETIME

141

Chemical decomposition of the ionomer is caused by active radical species, such as hydroxyl radicals. The OH radical is, in fact, one of the most detrimental species for the membrane, as it is able to detach the polymer side chains from the backbone [15], due to interacting with both the secondary ether bridge of the side chains [16] and the sulfonic acid group [17,18]. Another potential starting point for ionomer decomposition is non-perfluorinated end groups of the backbone [19]. The presence of a platinum catalyst, as well as an incomplete oxygen reduction reaction (ORR), can form both hydrogen peroxide and hydrogen-containing radicals [20]. Small quantities of oxygen at the anode side lead to OH radical formation following the mechanics stated as follows (Eqs. 7.1–7.5) [3]: H2 + 2 Pt ! 2 Pt
H

(7.1)

Pt
H + O2 ! Pt +  OOH

(7.2)

OOH + Pt
H ! Pt + H2 O2

(7.3)

H2 O2 + M2 + ! M3 +  OH + OH


(7.4)

OH + H2 O2 ! H2 O +  OOH

(7.5)

As shown in Eq. (7.6), hydroxyl radicals are responsible for the generation of hydrogen radicals [21]: OH + H2 ! H2 O +  H

(7.6)

Hydrogen radicals exceed other membrane-damaging species in terms of their reactivity toward the CdF bonds of the perfluorinated sulfonic acid membranes (PSFA); hence, playing a major role in the chemical decomposition of Nafion [22,23]. The stronger HdF bond (136 kcal mol
1) is thermodynamically favored over the weaker CdF bond within the PSFA [18]. Therefore, under the influence of hydrogen radicals, abstraction of fluorine atoms takes place. The induced damage is consistent with the acceleration of the fluoride release rate in relation to hydrogen crossover [10]. Due to the fact that no hydrogen is consumed and larger amounts of hydrogen radicals are formed at open cell voltage (OCV), the membrane degradation is accelerated. Another potential reason for enforced membrane degradation is the high boiling point of H2O2. While water boils at 100°C, hydrogen peroxide needs 150°C to do so, thus leading to a higher H2O2 concentration at higher temperatures. There are four important factors for an increase in fluoride emission: lower humidity, higher oxygen partial pressure at the cathode, higher temperature, and most significantly, higher hydrogen pressure. The severe effects of chemical degradation are membrane thinning and consequential pinhole formation, leading to augmented hydrogen crossover, which in turn promotes degradation even more [24]. Therefore, chemical degradation is regarded as one of the gravest among all degradation phenomena. The polymer electrolyte can release SO2 and SO3, because of the weak bond between carbon and sulfur. For instance, SO3 is formed by an H2O2 triggered sulfonyl radical mechanism. As a result of the loss of proton conducting species, the main chain scission proceeds, the membrane resistance increases, and the degradation of the PFSA backbone accelerates further [10,25]. The implications of Nafion membrane degradation are the emission of SO2, CO2, CO, HF, carbonyl fluorides, and fluorocarbons [26]. Although state-of-the-art synthesis pathways are designed to circumvent HF formation, hydrogen-containing end groups can still be targeted by hydroxyl radicals (Eqs. 7.7–7.9) [27]: RdCF2 COOH +  OH ! RdCF2  + CO2 + H2 O

(7.7)

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CHAPTER 7 DEGRADATION MECHANISMS AND THEIR LIFETIME

RdCF2  +  OH ! RdCF2 OH ! RdCOF + HF

(7.8)

RdCOF + H2 O ! RdCOOH + HF

(7.9)

In order to determine the decomposition of the polymer backbone, the fluoride emission rate serves as an indicator [28]. However, not only the backbone is prone to degradation, but different sites of the polymer side chain can be the starting point for decomposition as well. The results from different studies are not completely superimposable; they show, however, that the exact mechanism is strongly dependent on the operating conditions [4,18,29]. By investigating the effluent water, side chain fragments are detectable, which allows the conclusion that the conjunctive CdO bond between the side chain and backbone of the polymer is broken [30]. Also, the CdS bond is a potential starting point for side chain degradation, considering it represents the weakest bond in the ionomer [29,31,32]. On the other hand, there are studies declaring that the ether bond is more prone to breakage than the CdS bond [29,33]. The degradation product is highly dependent on the respective mechanism.

7.2.2 MECHANICAL DEGRADATION The formation of cracks or pinholes is mainly caused by mechanical stressors, such as non-uniform contact pressure, high differential gas pressure, or swelling/shrinkage due to changes in temperature or humidity. At alternating humidity conditions, the membrane undergoes swelling and shrinkage, causing mechanical stress. This leads to the formation of cracks and pinholes, causing reactant crossover, and further degradation, as well as limitation of the operational lifetime [34–36]. The implementation of a reinforcement layer has helped to increase the resistivity of the membrane toward the mechanical defect formation [37]. The local operation conditions in the area around the pinhole diverge significantly from the normal operation mode. With unhindered crossover between the electrodes, hydrogen and oxygen can react directly. This leads to a local increase in temperature and a different local humidity; both due to the increased temperature and the water formed during combustion. The direct consumption of the reactant gases also leads to an undersupply of fuel, and thus starvation conditions along the channels. This results in significant electrode degradation, such as carbon corrosion. In general, various effects overlap during the operation of a defective cell [36]. Defects close to the anode inlet have shown the most severe effect on fuel cell operation. Hydrogen crossover results in a local temperature increase at the defective site. This causes an initial improvement of the operation parameters, which is followed by a sudden drop of performance. As the positive effects from the elevated temperature are exceeded by the detrimental effects from electrode degradation and mixed potentials due to crossover, a sudden performance drop occurs [36]. The dominant factors for the degradation rate of the carbon support are the temperature and the potential. In the event of hydrogen starvation, the anode potential rises, the cell voltage is reversed, and carbon is oxidized. In the presence of pinholes near the cathode inlet, a small effect on the fuel cell behavior is observed. If present near the anode inlet, however, the voltage drops severely, resulting in an increased carbon corrosion rate [36], which will be discussed later.

7.3 CARBON CORROSION

143

Pinholes have been shown to contribute to the loss of the active cathode catalyst surface area. This occurs most likely due to the locally increased temperatures in the defective area, leading to an accelerated platinum agglomeration rate [36].

7.3 CARBON CORROSION Most commonly used platinum electro-catalysts are based on carbon support material and exhibit good conductivity, high surface area, inexpensive production in comparison with most other possible materials, as well as satisfactory stability under normal fuel cell operation conditions. However, during the lifetime of a fuel cell, it will eventually encounter undesirable operation modes, inducing degradation. Among others, carbon corrosion is one of the critical failure modes in fuel cells. The loss of carbon support severely affects the fuel cell, as it results in a loss of conductivity, porosity, and loss of catalyst, and accelerates platinum agglomeration and alters the water management [38,39]. As carbon surface oxides are formed, the electrodes lose hydrophobicity. If carbon is further oxidized, gaseous compounds are released. This reduces the electrodes’ electric conductivity irreversibly. Whether carbon is oxidized to carbon monoxide or carbon dioxide depends on the electrode potential [37]. Carbon is released in the form of CO2 at potentials above 1.0 V, while CO is formed at electrode potentials exceeding 1.2 V [40]. To determine carbon support corrosion, the content of carbon oxides in the off-gas can be analyzed [36]. Depending on the potential conditions, the presence of platinum electrocatalyst catalyzes carbon corrosion, resulting in increased carbon loss rates [41]. The carbon support oxidation to carbon dioxide follows a two-step pathway. Initially, carbon is oxidized (see Eq. 7.10), followed by a water-gas-shift reaction (see Eq. 7.11). In the presence of a platinum catalyst, the second rate determining step follows a catalyzed pathway (see Eqs. 7.12 and 7.13) [41]. C + H2 O ! C
Oad + 2H + + 2e
C
Oad + H2 O ! CO2 + 2H + 2e +

Pt + H2 O ! Pt
OHad + H + e +

(7.10)





C
Oad + Pt
OHad ! Pt + CO2 + H + + e


(7.11) (7.12) (7.13)

The catalytically triggered reaction of oxygen with water is another proposed pathway. This leads to the formation of highly active OH and OOH radicals (see Eqs. 7.14–7.19), which react with carbon to surface oxides. At increased temperatures, they further decompose to gaseous CO and CO2 (see Eq. 7.20) [42]. Pt + H2 O $ Pt
ðH2 OÞad

(7.14)

Pt
ðH2 OÞad + O2 ! Pt
OHad + HOO

(7.15)

Pt
OHad + O2 ! PtO + HOO

(7.16)

Pt
OHad + HOO ! PtO + HOOH

(7.17)

HOO  + HOO ! O2 + 2HO

(7.18)

HOO  + HOO ! O2 + HOOH

(7.19)

HOO  =HO  + Cx ! Surface oxides ! CO=CO2

(7.20)

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CHAPTER 7 DEGRADATION MECHANISMS AND THEIR LIFETIME

Water plays a key role during carbon corrosion, irrespective of the applied pathway. In general, the following factors accelerate carbon corrosion: high humidity, high temperature, low pH, large Pt/carbon interface, high oxygen concentration, and high potentials [43]. Another important factor for accelerated carbon corrosion is the depletion of hydrogen at the anode. In the event of fuel starvation, the entire hydrogen is oxidized near the anode inlet, leaving the rest of the channel without fuel to oxidize. Instead of hydrogen, the carbon support material is oxidized according to Eqs. (7.10), (7.12), and (7.13). Operating a fuel cell without sufficient fuel supply leads to a local change of the current density, the overall potential, as well as the anode potential. This potential gradient causes uneven degradation. If the anode exhibits local potentials above 1.23 V, electrolysis occurs (see Eq. 7.21). If carbon corrosion and/or electrolysis occur, the respective products (CO2 and O2) can be found within the anode off-gas [36,44]. 2 H2 O ! O2 + 4 H + + 4 e


(7.21)

Starvation at both the anode and the cathode is also possible due to flooding, which in turn leads to the depolarization of the fuel cell [45,46]. A fuel cell is flooded when the water transport mechanism is not efficient enough to purge the excess reaction water. Poor water transport often derives from a decrease in hydrophobicity, which correlates with the degradation of the PTFE binder in the electrodes [37]. The degradation rate, which is of course related to the extent of the electrolysis, can be reduced with the proper pre-treatment of the carbon support material [47,48].

7.4 CATALYST The loss of electrochemically active surface area is mostly driven by the dissolution of platinum. This phenomenon is highly dependent on the electrode potential, with especially high rates being reported on the cathode at intermediate potentials [49]. The dissolution rate is also affected by the operation conditions [43]. The reasons for platinum degradation are the dissolution and redistribution of the platinum, particle agglomeration and particle growth, as well as blockage of the active catalyst area by poisoning. Redistribution and particle agglomeration take place after the dissolution of platinum, which occurs under standard conditions, and at intermediate potentials [49]. The dissolution occurs primarily on the cathode side, and is strongly dependent on the operation conditions [43,50]. From 0.85 to >1.1 V versus the reversible hydrogen electrode (RHE), various oxidation reactions of platinum and platinum hydroxide are possible, depending on the potential (see Eqs. 7.22–7.26). Adsorbed hydroxyl on platinum (OHPt) is formed irreversibly via place exchange at potentials above 0.95 V versus RHE (see Eq. 7.25). A lower positive reduction potential generally indicates a more stable species. At higher potentials above 1.1 V versus RHE, platinum is further oxidized to PtO, and a protective layer is deposited (see Eq. 7.22) [51,52]. 4 Pt + H2 O ! Pt4 OH + H + + e
ð0:85 V vs:RHEÞ

(7.22)

Pt4 OH + H2 O ! 2 Pt2 OH + H + + e
ð0:94
0:95 V vs:RHEÞ

(7.23)

Pt2 OH + H2 O ! 2 PtOH + H + + e
ð1:04
1:05 V vs:RHEÞ

(7.24)

7.5 IMPURITIES

145

PtOH ! OHPt ð 0:95 V vs:RHEÞ

(7.25)

PtOH=OHPt ! PtO + H + + e
ð> 1:1 V vs:RHEÞ

(7.26)

The oxide film that is formed on the catalyst surface at a potential above 0.85 V versus RHE and 80°C inhibits further dissolution significantly. This protective layer is also confirmed by X-ray analysis. In contrast to lost platinum, platinum oxides from the protective layer can be reduced again during potential cycling or dead-end operation. Afterward, it can be dissolved once more, leading to platinum loss or redeposition of larger platinum particles, and resulting in a lower active surface area [37]. The catalyst degradation mechanism with the highest overall impact is agglomeration. Agglomeration is the coalescence of many small nanoparticles into fewer, larger particles; thereby decreasing both the surface energy and the electrochemically active surface area. The agglomeration process is called Ostwald ripening, and represents the most dominant mechanism for particle growth. After the dissolution of nanoparticles, metallic platinum is eventually reduced on the surface of larger particles, which continue to grow. Driven by electroosmotic drag and chemical diffusion, dissolved platinum particles can migrate from the cathode into the membrane, where reduction via crossover hydrogen occurs. Hence, the lost catalyst is no longer available for the catalytic reaction [43]. Within the electrolyte, the Pt ions form a platinum band. The location depends on the pressure difference between the anode and the cathode, and thus the position of the hydrogen front within the electrolyte [53]. Both the applied mechanism and deposition location of the dissolved catalyst are dependent on the crossover of hydrogen and oxygen, as well as the gas concentration. Hydrogen is considered to reduce platinum oxides, resulting in fewer dissolved Pt ions, as the local concentration of Pt ions in the catalyst layer of the cathode is higher with less hydrogen crossover. It was shown that decreasing H2 crossover leads to increased Pt particle growth, and relative loss of active catalyst surface area [54]. As already mentioned, platinum can be dissolved in an acidic environment at high potentials, thus resulting in Pt ions, which can migrate through the membrane. Together with halogens, those Pt ions can form anionic complexes. Halogens inside the membrane appear in the form of chlorides (deriving from contaminations) or fluorides (deriving from membrane degradation) [55]. If hydrogen is present in the PEM, metallic platinum agglomerates in the range of 10–100 nm are formed [53]. In the absence of hydrogen in the anode compartment of the fuel cell, platinum ions are reduced onto the catalyst surface of the anode, exhibiting a size between 2 and 5 nm [53]. The location of this platinum deposition is determined by the hydrogen concentration, but the oxygen within the cathode compartment plays an important role too [55].

7.5 IMPURITIES Power output and lifetime of PEFCs can be influenced negatively by a considerable variety of contaminations. An extensive amount of contaminations derives from fuel processing. The oxidant (particularly when using ambient air) contributes to the overall contamination, as do several system compounds, for example, the bipolar plates. Three harmful mechanisms can be distinguished, depending on the contamination. If the catalyst is poisoned (e.g., by carbon monoxide or hydrogen sulfide), the hydrogen oxidation reaction at the anode and the ORR at the cathode are inhibited, respectively.

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In the case of membrane contamination, the membrane resistance increases, and the structure of the membrane changes, resulting in poor water management and decreased proton conductivity. Contaminations may also cause the structure of the active layer to be altered, and thus the mass transport properties change as well.

7.5.1 ANODE GAS STREAM In order to ensure the lifetime of PEFCs and to avoid shortening thereof by degradation due to fuel impurities, very pure hydrogen is required as fuel (ISO 14687-2 and ISO 14687-3). The production of hydrogen is usually realized via reformation of hydrocarbons, such as methane. To a smaller extent, hydrogen is produced via water electrolysis [56], as discussed in the chapter on hydrogen production. The commonly used reforming methods are steam reforming, partial oxidation, and auto thermal reforming [59,60]. The produced hydrogen includes, depending on the necessary gas purification system, various impurities in the form of carbon monoxide, carbon dioxide, traces of inert gas, vaporous nitrogen compounds and water vapor, as well as sulfuric compounds [57,58].

7.5.1.1 Carbon monoxide (CO) Because it has such a significant impact on the performance of a fuel cell, the poisoning of the platinum catalyst by carbon monoxide is one of the most researched and best-documented contamination effects regarding the fuel cell. Beside the content of carbon monoxide in the anode stream, the duration of exposure, the temperature, and the composition of the catalyst are important factors as well. Although the exact mechanism of catalyst poisoning cannot be proven yet, it is evident that even small amounts of CO result in a significant voltage loss (see Eq. 7.27). 2CO + 2ðPt
Hads Þ ! 2ðPt
COads Þ + H2

(7.27)

Carbon monoxide is adsorbed and strongly bound to the platinum surface, blocking the catalyst and thus disabling its catalytic properties [61]. This is due to back bonding of platinum to the CO π*-orbitals and bridge bonding of the CO between two atoms, resulting in a strong chemisorption of carbon monoxide on the catalyst surface [62,63]. Furthermore, CO has the ability to replace already adsorbed hydrogen molecules, rendering even more catalytically active sites useless for the hydrogen oxidation reaction and resulting in a further loss of active surface area [64].

7.5.1.2 Carbon dioxide (CO2) Carbon dioxide shows negative influences on the performance of the fuel cell as well. If hydrogen is produced via steam reforming, it contains high amounts of CO2, which may react to carbon monoxide during operation. Especially at high potentials, the water gas-shift reaction from the steam reforming process is reversed (see Eq. 7.28). Catalyzed by platinum, the contained carbon dioxide is reduced back to carbon monoxide, which then inhibits the catalyst as previously explained [65]. CO2 + 2ðPt
Hads Þ ! Pt
COads + H2 O + Pt

(7.28)

Therefore, a marginal amount of CO2 may lead to a significant drop in the performance of the fuel cell as well.

7.5 IMPURITIES

147

7.5.1.3 Hydrogen sulfide (H2S) The fuel gas used for operation also contains traces of sulfuric compounds, such as H2S. Similar to carbon monoxide, those compounds can adsorb and bind to the platinum surface. Hence, the catalytically active sites are blocked, and are thus unable to catalyze the hydrogen oxidation reaction any longer. Already adsorbed sulfuric compounds are able to further react on the catalyst surface, independent of the potential. Mathieu and Primet [66] proposed a number of different dissociative interactions between hydrogen sulfide and platinum. Lopes et al. [67] confirmed the following mechanism for the interaction between the H2S contaminant and the platinum catalyst in PEFCs (Eqs. 7.29–7.31). H2 S + Pt ! Pt
S + H2

(7.29)

H2 + 2 Pt ! 2 PtH

(7.30)

Pt
H + H2 S ! Pt
S + 3=2 H2

(7.31)

7.5.1.4 Ammonia (NH3) Traces of ammonia can be either found in the reformate gas itself, or formed within the fuel cell, if the anode stream is contaminated with nitrogen containing hydrocarbons [68]. Nitrogen and hydrogen can form traces of ammonia at high temperatures during fuel processing as well [69]. In contrast to other contaminants, ammonia adsorbs at the PEM instead of the catalyst surface. Ammonia reacts with protons and forms the ammonium cation, which then in turn is stabilized by the acidic membrane and blocks proton pathways (see Eqs. 7.32 and 7.33). NH3 ðgÞ ! NH3 ðMembraneÞ

(7.32)

NH3 ðMembraneÞ + H + ! NH4 +

(7.33)

For this reason, the membrane resistance increases and the overall cell performance decreases correspondingly. The effect depends on the concentration of ammonia, as well as the duration of exposure. If the exposure was only marginal or for a short period of time, the negative effect of the contamination can be partially reversed by the supply of ultrapure hydrogen. After a certain period of exposure, however, the negative effect becomes irreversible [69].

7.5.2 CATHODE GAS STREAM For obvious reasons, the oxygen source for most fuel cells is ambient air. Because air quality varies heavily with the location, or rather, the environmental situation of the location, air pollution can be more or less vast. Various contaminants such as several nitrogen, sulfur, and carbon oxides; but also volatile organic compounds deriving from transportation and industry can be found in the air.

7.5.2.1 Nitrogen oxides (NOX) Nitrogen dioxide is formed due to the oxidation of nitrogen oxides in the presence of oxygen. Although NO2 does not poison the catalyst itself, it has a negative influence on the ionomer and the ionomercatalyst boundaries. It does, however, decrease the oxygen reduction rate because nitrogen dioxide can be reduced at the active catalyst surface area [70]. Another issue with NO2 is the disproportionation into nitrous acid (HNO2) and nitric acid (HNO3) in the presence of water. In the presence of oxygen, nitrous acid can be further oxidized to nitric acid.

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CHAPTER 7 DEGRADATION MECHANISMS AND THEIR LIFETIME

Strong inorganic acids such as HNO3 increase the proton concentration, which alters the cathode stoichiometry, resulting in a lack of oxygen and increasing the cathode overpotential [71]. Similar to the degradation behavior at the anode regarding ammonia, the negative effect of NOx contamination in the cathode stream can be partially reversed by the supply of pure air, followed by nitrogen purging, if the exposure period occurs for a short period of time. It becomes, however, partially irreversible after long-term exposure [71].

7.5.2.2 Sulfur oxides (SOX) Not unlike hydrogen sulfide in the anode gas, sulfur oxides such as SO2 and SO3 are common impurities in air, and thus in the cathode gas stream. They are adsorbed and strongly bound to the catalyst surface. A marginal amount of sulfur compounds in the gas is sufficient to decrease the catalyst activity regarding the ORR significantly by blocking the active sites. After a certain amount of catalyst is covered with sulfur oxides, even the ORR kinetics are changed from the four-electron pathway to the twoelectron pathway [72]. Most detrimental effects on the cell performance are induced by SO2 with the voltage loss caused by sulfur dioxide being proportional to its amount within the oxidant stream. The severe impact of SO2 can be reduced by enhanced humidification [73].

7.5.2.3 Carbon oxides (COX) The negative effect of carbon monoxide and carbon dioxide on the platinum catalyst is identical for both the anode gas stream and the cathode gas stream. The cell performance deteriorates heavily, because the active sites of the catalyst are blocked, and hence its catalytically active area diminishes drastically. In addition, the carbon monoxide of the anode gas stream can diffuse through the membrane, or cross through pinholes and damage the cathode catalyst as well [74].

7.5.3 CONTAMINATIONS OF COMPONENTS Due to the harsh conditions in PEM fuel cells, metal ions from various sources can be dissolved and harm the cell performance. The ions can derive from the bipolar plates, the seals, the current collectors, the gas inlets, as well as the humidification components of the fuel cell. Many different ions from alkali metals, alkaline earth metals, transition metals, and rare earth metals can be formed through corrosion. The respective metal cations dissolved into the membrane electrode assembly show no effect on the catalyst, but are rather harmful to the membrane. By blocking the acid groups of the PEM, they inhibit the ionic conductivity of the cell. Moreover, through an augmented radical formation, membrane degradation mechanisms are catalyzed as well [58]. Metal cations decrease the proton transportation, accelerate membrane degradation, and lead to poor water management, as well as an overall decline in fuel cell performance.

7.6 COSTS VERSUS LIFETIME Degradation is strongly influenced by the respective operation conditions. Other than material properties, the operation mode determines the type of failure, eventually causing the end of life. Fuel cells for automotive applications are the subject of ever-growing interest, as they are a suitable substitute for

7.6 COSTS VERSUS LIFETIME

149

internal combustion engines, operating at a high efficiency and without the loss of customer convenience. However, fuel cells suffer from degradation due to strongly changing power demands, and therefore, instable operation points. This can be mitigated to a certain degree by the implementation of back-up batteries or capacitors to cover load peaks, but some effect will still remain. Also, fuel cells for automotive applications undergo various start/stop cycles and strongly varying temperatures over their lifetime. All those factors contribute to a shortening of their durability, and solely the avoidance of degradation enables a broad commercialization of fuel cells. Stationary systems, on the other hand, benefit from very stable operation conditions, with only a few start/stop cycles over their lifetime. A change of demand is limited to different residential energy requirements, depending on the respective season and the time of the day. Very high efficiencies are possible, as the thermal energy can be utilized as well. In order to assess degradation in stationary systems, two stacks with a power range of 1.5–2 kW with 55 cells each were operated for 12,860 h at a current density of 0.26 A cm
2 and at 60–65 C by De Moor et al. [75]. During operation, the system underwent approximately 250 start/stop cycles, and had to be shut down due to low cell voltage in some cells. The respective cells displayed a higher gas leak, detected at random locations in the cell, but cells near the hydrogen inlet of the stack were more affected. This is due to the strong gradient in humidification, as hydrogen is fed dry, whereas air is humidified and the cell operated in counter flow. Examination of the membrane shows that the polymer electrolyte degrades at a higher rate on the anode than on the cathode. Degradation is, for both electrodes, more extensive near the anode inlet than further down the channel. Extensive anode polymer electrolyte degradation accelerates the degradation of the cathode polymer electrolyte by enabling gas crossover, and thus radical formation. Membrane reinforcement, however, guaranties electronic isolation and prevents shortening. The two key issues that restrict a large scale commercialization of PEFCs are lifetime and cost [76]. PEFC vehicles (cars, trucks, and buses) have a short service lifetime of 0.95 V vs. RHE). Increasing the electrode potential is enough to increase the rate of the charge transfer. But for E < 0.7 V vs. RHE, the other process that limits ORR is the mass transport, also

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CHAPTER 9 METHODS AND CHARACTERIZATION ON THE CELL LEVEL

FIG. 9.10 (A) Current-potential curves recorded at 5 mV s1 on RRDE in a N2-saturated 0.1 mol L1 HClO4 + 10 mmol L1 K3[Fe(III)(CN)6] aqueous solution; (B) collection efficiency at different electrode potentials and RRDE speeds.

FIG. 9.11 Ring (top) and disk (bottom) ORR polarization curves (iR-drop-uncorrected) recorded at 5 mV s1 scan rate in O2saturated electrolyte solution for different speeds of the RRDE: (A) 20 wt% Pt/C (0.1 mol L1 NaOH) and (B) 20 wt % Pd/C (0.1 mol L1 KOH) catalysts prepared from the BAE method. For all panels, the current densities are obtained with the geometry surface area of the ring (0.11 cm2) and disk (0.196 cm2).

9.7 KINETICS OF REACTIONS: ORR

191

referred as the diffusion-limiting process. The principle of the mass transport consists of bringing the reacting species close to the surface, and removing the species formed at the surface, into the bulk solution [26]. The mass transport is typified by a limited current (Idiff l ) and normalized by the geometrical surface diff is determined by Levich’s law as in either Eq. area into diffusion-limited current density (jdiff l ). jl (9.24) when ω is given in radians per second (rad s1); or Eq. (9.25), when Ω is given in revolutions per minute (rpm). Note: “0.62  (2π/60)1/2 ¼ 0.20” for the coefficient conversion from ω to Ω or vice versa. The validity of Levich’s law requires the two following sine qua none conditions: (i) the existence of a mass transport process that is the rate-determining step (rds), (ii) the reaction is of a first-order reaction with respect to the electroreactive species (O2). 1

2

1

1

1

2

1

2

1

1

1

2

jdiff ¼ 0:62nex Fυ 6 CO2 DO2 3 ω2 ¼ nex Bω2 where B ¼ 0:62Fυ 6 CO2 D3 l jdiff ¼ 0:20nex Fυ 6 CO2 DO2 3 Ω2 ¼ nex BΩ2 where B ¼ 0:20Fυ 6 CO2 D3 l

• • • • • • •

(9.24) (9.25)

2 jdiff l , diffusion limiting current density of O2 in the bulk electrolyte (mA cm ); nex, overall exchange number of electrons (total); DO2, diffusion coefficient of O2 in bulk solution (cm2 s1); CO2, the bulk concentration of O2 in the electrolyte (mol mL1 mol cm3); υ, kinematic viscosity of the electrolyte (cm2 s1); F, Faraday’s constant (F ¼ 96,485 C mol1); ω (rad s1) and Ω (rpm), speed of the RRDE.

Based on the data in Table 9.1, the obtained Levich’s plots are displayed in Fig. 9.12A (Pt/C in 0.1 mol L1 NaOH) and B (Pd/C in 0.1 mol L1 KOH). For Pt/C, nex  3.97–4.00 and for Pd/C, nex  3.95–4.00, which corresponds to a quasi-four-electron process. Furthermore, the diffusion layer thickness δ at the RDE/RRDE is given by Eq. (9.26). From Ω ¼ 400 rpm to 2500 rpm, δ ¼ 31–12 μm in both media. In comparison, values of 100–200 μm by a simple stirring highlight the great advantage of the RDE/RRDE technique. δ¼

1 1 1 1 1 1 nex FCO2 DO2 ¼ 1:61υ6 DO2 3 ω 2 ¼ 5:00υ6 DO2 3 Ω 2 diff jl

(9.26)

9.7.5 KINETIC REGION: FROM THE KOUTECKY-LEVICH EQUATION TO FUNDAMENTAL DATA Assuming a planar electrode or a thin film (RDE/RRDE), the use of the Koutecky-Levich equation (K-L) implicitly requires the two following criteria: (i) the existence of an electron transfer process that is the rate-determining step (rds), (ii) a first-order reaction with respect to the electroreactive species (O2).

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CHAPTER 9 METHODS AND CHARACTERIZATION ON THE CELL LEVEL

FIG. 9.12 The Levich plots from the ORR polarization curves recorded at 5 mV s1 scan rate: (A) 20 wt% Pt/C (0.1 mol L1 NaOH) and (B) 20 wt% Pd/C (0.1 mol L1 KOH) catalysts prepared from the BAE method. For all panels, the current densities are obtained with the geometric surface area of the disk (0.196 cm2).

The preceding specific conditions are met in the mixed control regime. Thus, the current densities of the activation (jk) and mass transport (jdiff l ) processes give the total current density ( j) as the sum of reciprocals, Eq. (9.27) and are known as the Koutecky-Levich equation [20,22,26,27]. It should be noted that all the current densities are normalized with the geometric surface area. 1 1 1 1 1 1 + ¼ diff + film + ads + ¼ j jdiff j j j j k l l l l japp k ¼ j0

• • • • • • • • • •

1 θ η0 j0 eb θeq

(9.27)

1 2 1 θ η’ RT eb ; η ¼ E  Eeq ; b’ ¼ ; jdiff ¼ nex BΩ2 where B ¼ 0:201Fυ 6 CO2 D3 θeq αnF l

j, measured ORR current density at the disk (mA cm2); 2 jdiff l , diffusion-limiting current density of O2 in the bulk electrolyte (mA cm ); film jl , diffusion-limiting current density of O2 inside the film of the catalytic ink (mA cm2); 2 jads l , diffusion-limiting current density associated with O2 adsorption at the active site (mA cm ); 2 jk, kinetic current density, which is free from the mass transport (mA cm ); j0, exchange current density (mA cm2); nex, exchange number of electrons; n, number of electrons transferred during the rds (n  nex); b0 , Tafel slope (V); α, symmetric factor or the charge transfer coefficient (0 < α  1);

9.7 KINETICS OF REACTIONS: ORR

• • • • • • •

193

η ¼ │E  Eeq│, overpotential (V); θ and θeq, degree of coverage of the catalyst surface (active sites) by oxygen at potential E and at the equilibrium potential Eeq, respectively; DO2, diffusion coefficient of O2 in solution (cm2 s1); CO2, the bulk concentration of O2 in the electrolyte (mol cm3); υ, kinematic viscosity of the electrolyte (cm2 s1); F, Faraday’s constant (F ¼ 96,485 C mol1); Ω, RRDE speed (rpm).

9.7.6 FROM THE KOUTECKY-LEVICH PLOTS TO THE KINETIC CURRENT The electrochemical and hydrodynamic properties of RDE/RRDE are correlated with the K-L’s equation that can be expressed by Eq. (9.28). Thereafter, it becomes Eq. (9.29) when Ω ! ∞. This relation enables the kinetic current density (jk) to be obtained experimentally. 1 1 1 1 1 + + ads + app ) lim ¼ Ω!∞ j n BΩ12 jfilm j j l l l ex


 1 1 1 1 ¼ film + ads + app j jk jl jl

(9.28)

1 1 1 1 ¼ + ads + app jk jfilm j j k l l

(9.29)

1 1 1 1 1 1 + film + ads + app ) j1 ¼ ðnex BÞ1 Ω 2 + j1 ¼ k 1 j n BΩ2 jl jk jl ex

(9.30)

αKL ¼ ðnex BÞ1 ) nex ¼

1 αKL B

(9.31)

1

From Eq. (9.30), the curve j1 vs. Ω 2 that is known as the “Koutecky-Levich plot” is a straight line characterized by a slope αK-L ¼ (nexB)1 and an intercept βK-L ¼ j1 k . The slope of the straight line enables extraction of the total number of exchanged electrons nex (see Eq. 9.31), and the intercept with the y-axis at the origin (Ω ! ∞) gives the inverse of the kinetic current jk. The KL plots from ORR polarization curves (see Fig. 9.11) at different electrode potentials are reported in Fig. 9.13 for Pt/C in 0.1 mol L1 NaOH and Pd/C in 0.1 mol L1 KOH. From these plots, the determined jk from the intercepts, referred as “absolute activity” [24,28], is expressed in mA per cm2 of disk (sample). Afterward, as recommended [24,28], jk must be normalized as: 2 (i) the “area-specific activity” As (mA per cm2 metal: mA cm2 metal or mA cmreal) by dividing the absolute activity by the surface-area enhancement factor, (ii) the “mass activity” Am (mA per mg of metal: mA mg1 metal) by dividing the absolute activity by the metal loading.

The surface-area enhancement factor (also referred to as the electrochemical surface roughness or electrode roughness) is the ECSA of the catalyst divided by the planar area of the sample (cm2 metal per planar cm2). If a multimetallic catalyst is considered, the noblest metal is considered in both normalizations. But, for active elements such as those from platinum group metals (PGMs), it is not really correct to consider only the most active (e.g., platinum) during the normalization, as it often happens in literature. For presentation in scientific papers, K-L plots should not be plotted for all potentials, and

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CHAPTER 9 METHODS AND CHARACTERIZATION ON THE CELL LEVEL

FIG. 9.13 The Koutecky-Levich plots from the ORR polarization curves recorded at 5 mV s1 scan rate: (A) 20 wt% Pt/C (0.1 mol L1 NaOH) and (B) 20 wt% Pd/C (0.1 mol L1 KOH) catalysts prepared from the BAE method. For all panels, the current densities are obtained with the geometric surface area of the disk (0.196 cm2).

jk in terms of Am and As should not be given for all potentials either. The K-L plots should be given only in the mixed-control region for roughly five representative potentials. To this end, the half-wave potential (E1/2) at 1600 rpm is first considered, and then two potentials are taken at the left and right of it, meaning at E ¼ E1/2  ΔE (ΔE ¼ 50, 20 mV). For example, if E1/2 is 0.85 V vs. RHE, K-L plots will be reported for 0.90, 0.87, 0.85, 0.83, and 0.80 V vs. RHE. Furthermore, histograms of jk by Am and As are currently reported at 0.95, 0.90, and 0.85 V vs. RHE for catalysts based on PGMs and 0.85 and 0.80 V vs. RHE for non-PGMs. It should be stressed that the fundamental aspect of jk is represented by As (activity per active site) and the economical one by Am (activity per mass). It is important to note that the deposited mass can be handled, which enables estimation of the performance, while the active site expressed in cm2 (ECSA) cannot be “handled” experimentally.

9.7.7 FROM THE KINETIC CURRENT DENSITIES TO THE LIMITING CURRENT DENSITY In the K-L’s equation, especially in Eq. (9.29), it is impossible to separate contributions from jfilm and l because they do not depend on the rotation rate. They are gathered as jL (see Eq. 9.32), which is jads l defined as a limiting current density resulting from a mixed-control by the diffusion of O2 inside the film of the catalytic ink and by the adsorption of O2 at the active sites. Thereafter, the expression of jk becomes Eq. (9.33), a relationship between jL and japp k . 1 1 1 ¼ + ads jL jfilm jl l 1 1 ¼ + jk jL

1 θ η0 j0 eb θeq

(9.32) (9.33)

9.7 KINETICS OF REACTIONS: ORR

195

FIG. 9.14 The “jk 1 vs. E” plots for the determination of the limiting current density jL for (A) 20 wt% Pt/C (0.1 mol L1 NaOH) and (B) 20 wt% Pd/C (0.1 mol L1 KOH) catalysts prepared from BAE method. The obtained value is reported in each panel. Note: jk is the kinetic current density, determined from the Koutecky-Levich plots and normalized with the geometric surface area of the disk (0.196 cm2).

1 1 ¼ + jk jL


 1 1 1 1 1 ¼ ¼ + ) lim EEeq θ η0 jL η!∞ θ j j k L j0 eb j0 e b0 θeq θeq

(9.34)

Afterward, Eq. (9.33) becomes Eq. (9.34) when η ! ∞, meaning E ≪ Eeq because E < Eeq for reduction reactions like ORR. Therefore, jL is determined experimentally after extrapolating of reported j1 k 1 values as a function of the potential E. Fig. 9.14 shows plots of j1 NaOH) k vs. E for Pt/C (0.1 mol L and Pd/C (0.1 mol L1 KOH). Here, j1 L can be extrapolated for E < 0.6 V vs. RHE that is far from Eeq ¼ 1.183 V vs. RHE (0.1 mol L1 NaOH) and 1.185 V vs. RHE (0.1 mol L1 KOH). For the investigated catalysts, jL ¼ 70 mA cm1 (Pt/C) and 122 mA cm1 (Pd/C). This difference may result from the 2 nature of the electrolyte and the metal loading (26 μgPt cm2 disk vs. 33 μgPd cmdisk).

9.7.8 DETERMINATION OF THE EXCHANGE CURRENT DENSITY AND THE TAFEL SLOPE Supposing that the coverage of the catalyst surface (active sites) by O2 at potential E and at the equilibrium potential Eeq is the same (θ θeq), Eq. (9.33) becomes a relationship Eq. (9.35). If there is no competitive reaction involving the catalyst surface in the whole potential range of interest, the experiments will meet the hypothesis by bubbling continuously O2. 1 1 1 1 1 ¼ + ¼ + EEeq jk jL j ebη0 jL  b0 0 j0 e

(9.35)

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CHAPTER 9 METHODS AND CHARACTERIZATION ON THE CELL LEVEL


 
 
   jL jk jL jk ) E  Eeq ¼ b log  b log  E  Eeq ¼ b0 ln  j0 jL  jk j0 jL  jk ∂η RT  ) b ¼ αTafel ¼ b0  ln ð10Þ 2:3 αTafel ¼

jk αnF ∂ log jL  jk
 βTafel jL ) j0 ¼ jL  10 b βTafel ¼ b log j0

Plotting either η or (E Eeq) vs. log by αTafel ¼
∂


log

∂η

jk jL  jk



jk jL jk

(9.36) (9.37)

(9.38)

is known as “Tafel plot” and is a straight line characterized

 ¼ b that enables getting the Tafel slope (expressed in mV dec1) via Eq.

(9.37) and the intercept βTafel ¼ b log

 j0 jL

from which the exchange current density j0 is determined

through Eq. (9.38). Obtained j0 values should be compared with those from the literature [19,20,29–31]. In practice, the parameter “αn,” including the symmetric factor (α) and the number of electrons transferred during the rds (n), can be estimated from Eq. (9.39). b 2:3

RT RT ½59 mV dec1  ) αn 2:3

at 25 °C αnF bF b

(9.39)

Furthermore, the value of b itself is a reaction mechanism indicator, and nex and j0 are indicators of catalytic activity [15]. Basically, b ¼ 60 mV dec1 (αn ¼ 1) corresponds to a pseudo two-electron reaction as the rds, in which b ¼ 120 mV dec1 (αn ¼ 0.5), suggests the first-electron reduction of oxygen as rds. Currently, α is taken as 0.5. Thus, one can easily verify that n ¼ 2 for 60 mV dec1 and n ¼ 1 for 120 mV dec1. But according to the International Union of Pure and Applied Chemistry (IUPAC, Technical Report 2014), the simultaneous transfer of more than one electron is highly improbable [32]. Fig. 9.15 shows the obtained Tafel plots for Pt/C (0.1 mol L1 NaOH) and Pd/C (0.1 mol L1 KOH) with two distinguished slopes. At the same electrode material, a small slope in the range of 60–70 mV dec1 is obtained at low overpotential (high potential) and a large slope in the range of 120–130 mV dec1 is obtained at high overpotential (low potential). This disparity comes from the catalyst surface. Indeed, the theoretical slope of 60 mV dec1 is situated at higher potentials where the catalyst surface is oxidized (attributed to M/MOx, M ¼ Pt, Pd, etc.). Meanwhile, the theoretical slope of 120 mV dec1 is situated at lower potentials, where the catalyst surface is composed of pure metal (M). It should be emphasized that these values correspond to Temkin adsorption isotherms of oxygenated species (low η), and Langmuir for high η (where the surface is free from metal oxides) [33,34]. Thus, the Tafel plot undergoes a slope change around the potential where the corresponding metal oxide is reduced (typically 0.80 V vs. RHE for Pt/C material and 0.65 V vs. RHE for Pd/C one (see Fig. 9.9). Here, this value is 0.82 and 0.70 V vs. RHE for Pt/C and Pd/C, respectively. Furthermore, two values of j0 will derivate from the two slopes according to Eq. (9.24). For Pt/C in 0.1 mol L1 NaOH, j0 ¼ 0.05  103 mA cm2 at high potential (low η: αn ¼ 0.89 ’ 1) and 42.09  103 mA cm2 at low potential (high η: αn ¼ 0.44 ’ 0.5). For Pd/C in 0.1 mol L1 KOH, j0 ¼ 0.03  103 mA cm2 at high potential (low η: αn ¼ 0.91 ’ 1) and 136.39  103 mA cm2 at low potential (high η: αn ¼ 0.47 ’ 0.5),

9.7 KINETICS OF REACTIONS: ORR

−200

−200

y = −(397±1)−(64±1)x

mV

de

64

mV

−500

−500 3 12

−600

−1

−2

(A)

−700

−1

0

1

log[ jk/( jL− jk)]

2

y = −(363±4)−(123±2)x

−800

y = −(425±2)−(132±2)x

−800

−1

c de

−700

c de

V m

V m

−600

de

c −1

−400

c −1

2 13

(E − Eeq) (mV)

−400

65

y = −(426±2)−(64±2)x −300

(E − Eeq) (mV)

−300

197

−2

3

(B)

−1

0

1

2

3

4

log[ jk/( jL− jk)]

FIG. 9.15 Tafel plots from the “Koutecky-Levich’s model” for (A) 20 wt% Pt/C (0.1 mol L1 NaOH) and (B) 20 wt% Pd/C (0.1 mol L1 KOH) catalysts prepared from BAE method. The limiting current density jL was determined from the “jk 1 vs. potential” plots and Eeq ¼ 1.185 V vs. RHE. Note: jk is the kinetic current density, determined from the Koutecky-Levich plots and normalized with the geometric surface area of the disk (0.196 cm2).

comparable to what has been reported in the literature [31,35,36]. It is assumed that the metric of b is related to the catalytic mechanism of the electrode reaction, whereas j0, which is obtained when η is assumed to be zero, describes the intrinsic catalytic activity of the electrode material under equilibrium conditions [37,38]. The metric of j0 is inversely proportional to the charge transfer resistance (Rct, correlated to the number of electrons that are transferred from the catalytic surface to the reactant(s), as well as intermediate(s) formation inside the double layer, and it is desired that it be as small as possible for faster electrochemical kinetics). Then, a catalytic material having a high j0 and a small b is desirable, because a smaller value of b means that increasing the same current density requires smaller overpotential, implying faster charge transfer kinetics. It is important to note that the theoretical metrics of 60 and 120 mV dec1 are free from any potential drop resulting from the iR-drop contribution. By assuming a solution resistance between WE and RE of about 10–20 Ω, a nonnegligible potential drop of 10–20 mV is observed for a kinetic current of 1 mA [4]. Consequently, any Tafel slope obtained from iR-drop-uncorrected polarization curves should not be exactly equal to “60 or 120 mV dec1.”

9.7.9 FROM WATER FORMATION EFFICIENCY TO THE NUMBER OF EXCHANGE ELECTRONS The major benefit of RRDE equipment for ORR is the determination of the reaction intermediate. RRDE apparatus enables determining the reaction product formation efficiency at the disk that is typically H2O (acid medium) or HO– (alkaline medium), in direct proportion to the amount of the reaction

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CHAPTER 9 METHODS AND CHARACTERIZATION ON THE CELL LEVEL

intermediate (H2O2 or HO2  because pKa(H2 O2 =HO2  ) ¼ 11.75). The method was first developed by Jakobs et al. [39] in 1985, and then revised by Vork and Barendrecht in 1990 [40]. During the ORR, O2 is reduced at the disk (current: ID < 0), and the intermediate H2O2 or HO2  is radially swept outward, away from the disk and toward the ring where it is oxidized (current: IR > 0). The set relationship is given by Eq. (9.40) [40]. It is very important to note that even if N is provided in a percentage (e.g., 20.5%), it is used in fraction form, meaning 0.205. The O2 reduction to H2O involves 4e and only 2e if the reaction process leads to H2O2 (or HO2  ). Thus, if both H2O and H2O2 (or HO2  ) are produced, nex is easily accessed through Eq. (9.41) [39,40]. Fig. 9.16 is an example of plots at 1600 rpm for Pt/C (0.1 mol L1 NaOH) and Pd/C (0.1 mol L1 KOH). NI D + IR pðH2 OÞ ¼ 1  pðH2 O2 Þ ¼ ¼ NI D  IR

IR N jID j IR 1+ N jID j

1

nex ¼ nðO2 !H2 OÞ + nðO2 !H2 O2 Þ ¼ 4pðH2 OÞ + 2pðH2 O2 Þ ¼ 2½pðH2 OÞ + 1 ¼

4N jID j ¼ N jID j + IR

(9.40)

4 1+

IR N jID j

4

(9.41)

FIG. 9.16 HO2  (H2O2 in acid media) percentage from the incomplete ORR (left Y-axis) and the number of electrons exchanged nex (right Y-axis) determined from ORR polarization curves in O2-saturated 0.1 mol L1 KOH at 5 mV s1 scan rate and RRDE speed of 1600 rpm. Catalysts 20 wt% Pt/C and 20 wt% Pd/C are prepared from the BAE method.

9.8 ELECTROCHEMICAL IMPEDANCE SPECTROSCOPY (EIS)

199

9.7.10 TIPS FOR A PROPER REPORT OF ORR RESULTS The assessment of the entire fundamental parameters requires the utilization of an RRDE. The different steps are listed as follows, and resumed in Fig. 9.17. Step 1: Determination of the electrochemically active surface area (ECSA) Step 2: RRDE experimental setup: determination of the collection efficiency N (