8/2/2019 Handbook of Fuel Cells

1/6

Handbook of

FundamentalsTechnologyand Applications

(S e:r0,"- )IIc

VOLUME 1Fundamentals andSurvey of Systems

EditorsWolf VielstichIQSC, Sao Car/os, Universidade de SaoPaulo, BrazilArnold LommDaimlerChrysler Research and Technology,Ulm, GermanyHubert A. GosteigerFuel Cell Activities, General MotorsCorporation, Honeoye Falls, NY, USA

GQWILEY

8/2/2019 Handbook of Fuel Cells

2/6

Copyright 2003 John Wiley & Sons Ltd,The Atrium,Southern Gale,Chichester,West SussexPOl9 8SQ, EnglandTelephone (+44 ) 1243779777Email (for orders and customer service enquiries); cs-books@wiley.co.ukVisit our Home Page on www.wileyeurope.com or www.wiley.com

All Rights Reserved. No part of this publication may be reproduced, stored in a retrieval system ortransmitted in any form or by any means, electronic, mechanical, photocopying, recording, scanning orotherwise, except under the terms of the Copyright, Designs and Patents Act 1988 or under the terms ofa licence issued by the Copyright Licensing Agency Ltd, 90 Tottenham Court Road, London WIT 4LP,UK, without the permission in writing of the Publisher. Requests to the Publisher should be addressedto the Permissions Department, John Wiley & Sons Ltd, The Atrium, Southern Gate, Chichester, WestSussex POl9 8SQ, England, or emailed to permreq@wiley.co.uk, or faxed to (+44) 1243770620.This publication is designed to provide accurate and authoritative information in regard to the subjectmatter covered. It is sold on the understanding that the Publisher is not engaged in rendering professionalservices. If professional advice or other expert assistance is required, the services of a competentprofessional should be sought.Where articles in the Handbook of Fuel Cells have been written by government employees in the UnitedStates of America, please contact the publisher for information on the copyright status of such works, ifrequired. Works written by US government employees and classified as US Government Works are in thepublic domain in the United States of America.The Editor(s)-in-Chief, Advisory Board and Contributors have asserted their right under the Copyright,Designs and Patents Act, 1988, to be identified as the Editor(s)-in-Chief and Advisory Board of andContributors to this work.Cover ImagesForeground: Cutaway view of General Electric PEM fuel cell system for Gemini spacecraft, 3 stacks of32 cc1is in parallel. (Reproduced by permission of Schenectady Museum.)Background: Scheme of a H2/02 fuel cell.Other Wiley Editorial OfficesJohn Wiley & Sons Inc., II I River Street,Hoboken, NJ 07030, USAJossey-Bass, 989 Market Street,San Francisco, CA 94103-1741, USAWiley-VCH Verlag GmbH, Boschstr. 12,D-69469 Weinheim, GermanyJohn Wiley & Sons Australia Ltd, 33 Park Road,Milton, Queensland 4064, AustraliaJohn Wiley & Sons (Asia) Pte Ltd, 2 Clementi Loop #02-01,Jin Xing Distripark, Singapore 129809John Wiley & Sons Canada Ltd, 22 Worcester Road,Etobicoke, Ontario, Canada M9W 1L IWiley also publishes its books in a variety of electronic formats. Some content that appears in print maynot be available in electronic books.British Library Cataloguing il l Publication DataA catalogue record for this book is available from the British LibraryISBN: 0-471-49926-9Typeset in 101l2.5pt Times by Laserwords Private Limited, Chennai, IndiaThis book is printed on acid-free paper responsibly manufactured from sustainable forestryin which at least two trees are planted for each one used for paper production.

8/2/2019 Handbook of Fuel Cells

3/6

ContentsVOLUME 1: Fundamentals and Survey of Systems

Contributors to Volume 1 viiFOreWOl"d ixPreface xiiiAbbreviations and Acronyms xvPal"t 1: Thermodynamics and kinetics offuelcell reactions 1

The components of an electrochemical cell 3A. H(lmllell2 The electrode-electrolyte interface 13A. Hamlletl3 Thermodynamics of electrodes and cells 21A. Hamllelt

4 Ideal and effective efficiencies of cell reactions and comparison tocarnal cycles 26W. Vie/stich5 Kinetics of electrochemical reactionsA. HOl1/11etl

6 Introduction to fuel-cell typesA. Hall/neft

Part 2: Mass transfer in fuel cells7 Mass tram;fer at two-phase and three-phase interfaces

A. Weber/R. Darling/f. Meyers/J. NelVman8 Mass transfer in flow fields

K. Scott

Part 3: Heat transfer in fuel cells9 Low temperature fuel cellsJ. Divisek10 High temperature fuel cells

J. Dh'isekI I Air-cooled PEM fuel cellsR. VOl' flelmoltlW. Lehner!

31

36

4547

70

9799115

134

Pal"t 4: Fuel cell principles, systems andapplications12 History of low temperature fuel cells

c. SalldstedelE. J. CairnslV. s. Bago/sky/K. Wieseller13 History of high temperature fuel cell development

H. Yokokawa/N. Sakai14 Hydrogen/oxygen (Air) fuel cells with alkaline electrolytes

M. CiJraill/K. Korde.w:h15 Hydrazine fuel cells

H. Kohllke16 Phosphoric acid electrolyte fuel cells

J. M. Killg/H. R. KUllz17 Aqueous carbonate electrolyte fuel cells

E. J. Cairns18 Direct methanol fuel cells (DMFC)

A. Hal/mel(19 Other direct-alcohol fuel cells

C. Willy/E. M. Belgsir20 Solid oxide fuel cells (SOFe)

P. Holtappe!sIU. StilllllJillg21 Biochemical fuel cells

E. Katz/A. N. Shipway//' Willner22 Metal/air batteries: The zinc/air case

O. Haas/F. Ho/zer/K. Miiller/S. Miiller23 Seawater aluminum/air cells

J. P. ludice de SouzaiW. Vie/stich24 Energy storage via electrolysis/fuel cells

J. Divisek/B. Emollls

Contents for Volumes 2, 3 and 4Subject Index

143145

219267

281

287

301

305323335355382409416

433439

8/2/2019 Handbook of Fuel Cells

4/6

Chapter 2The electrode-electrolyte interfaceA. HamnettUniversity of Strathclyde, Glasgow, UK

1 THE ELECTRIFIED DOUBLE LAYEROnce an electrode, which for our purposes may initiallybe treated as a conducting plane, is introduced into anelectrolyte solution, several things change. There is a substantial loss of symmetry, the potential experienced byan ion will now be not only the screened potential ofthe other ions but will contain a term arising from thefield due to the electrode and a term due to the imagecharge in the electrode. The structure of the solvent isalso perturbed: ncxt to the electrode, the oricntation ofthe molecules of solvent will be affccted by the elec,tric field at the electrode surrace, and the net orientationwill derive from both the interaction with the electrodeand with neighboring molecules and ions. Finally, theremay be a sufficiently strong interaction between ions andthe electrode surface that the ions lose at least some oftheir inner solvation sheath and adsorb on the electrodesurface.

The classical model of the electrified interface is shownin Figure 1,[11 and the following features are apparent:I. There is an ordered layer of solvent dipoles next to

the electrode surface, the extent of whose orientationis expected to depcnd on the charge on the electrode.2. There is, or may be, an inner layer of specificallyadsorbed anions on the surface; these anions havedisplaced one or more solvent molecules and have lostpart of their inner solvation sheath. An imaginary planecan be drawn through the centers of these anions toform the inner Helmholtz plane (lHP).

3. The layer of solvent molecules not directly adja,cent to the metal is the closest distance of approachof solvated catiolls. Since the enthalpy of solvationof cations is usually substantially larger than thatof anions, it is normally expected that there willbe insufficient energy to strip the cations of theirinner solvation sheaths, and a second imaginary planecan be drawn through the centers of the solvatedcations: this second plane is termed the outer Helmholtzplane (OHP).

4. Outside the OHP, there may still be an electric field andhence an imbalance of anions and cations extending inthe form of a diffuse layer into the solution.

5. The potential distribution in this model obviously can,sists of two parts: a quasi-linear potential drop betweenthe metal electrode and the IHP or OHP dependingon the charge on the electrode sUIt"ace and the corresponding planar ionic density, and a second part corresponding to the diffuse layer; as we shall see below,in this part, the potential decays roughly exponen,tially through screening" However, there arc subtletiesabout what can actually be measured that need someattention.

2 THE ELECTRODE POTENTIALAny measurement of' potential must describe a referencepoint, and we will take as this point the potential ofan electron well separated from the metal and at restin vacuo. By reference to Figure 2Yl we can define thefollowing quantities:

Handbook ofFIIel Cells - Fundamentals, TechnoLogy alld Applications, Edited by Wolf Vielslich, Hubert A. Gasteiger, Arnold Lamm.Volume I: Fundamentals and Survey ofSystems. 2003 John Wiley & Sons, Ltd. ISBN: 0-471-49926-9.

8/2/2019 Handbook of Fuel Cells

5/6

14 Part J: Thermodynamics and kinetics of uel cell reactions

Metal

MetalplaneInner Outer

Helmholtz Helmholtzplane

cations

anions

Normalwaterstructure

Primary~ " " , W - r - - waterlayerSecondarywaterlayer

Figure 1. Hypothetical structure of the electrolyte double layer.

I. The Fermi energy cr which is thc difference in energybetween the bottom of the conduction band and theFermi levcl; it is positive and in the simple Sommerfeldtheory of metals [3] c = h2k2/2m = h2(3rrn )2/3/2m'F Fe'here nc is the number density of electrons.

2. The work function q,M which is the energy rcquiredto remove an electron from the inner Fermi levelto vacuum.

3. The surface potential of the electrode, XM, due tothe presence of surface dipoles. At the metal-vacuuminterface, these dipoles arise from the fact that the

Vacuumlevel-----T--- --

Figure 2. Potential energy profile at the metal-vacuum boundary.Bulk and surface contributions to V are separately shown. (Reproduced from Trassatti (1980)[2] with permission from KluwerAcademic/Plenum Publishers.)

electrons in the metal can relax at the surface to somedegrcc, extending outwards by a distance of the orderof I A, and giving rise to a spatial imbalance of chargeat the surface.

4. The chemical potential of the electrons in the metal,~ ~ , a negative quantity.

5. The electrochemical potential i i ~ o f the electrons inmetal M is defined as - eo1JrM - eoXM, where 1JrMis the mean electrostatic potential just outside the metal,which will tend to zero as the free-charge density, (J ,on the metal tends to zero; as (J --+ 0, then ! I ~ --+

- eoxM== _q,M from Figure 2. Hence, for (J # 0,i i ~ = _q,M - eo1JrM6. The potcntial energy of the elcctrons, V, which is anegative quantity that can be partitioned into bulk andsurface contributions as shown. Clearly, from Figure 2,= EF + Vi r

Of the quantities shown in Figure 2,

8/2/2019 Handbook of Fuel Cells

6/6

densities of the two phases will be unaltered, and only thevalue of the potential V will have changed, If we assumethat, on putting the metals together, the XM vanish, and wedefine the potential inside the metal as , then the equalityof electrochemical potentials also leads to

(1)This internal potential, , is not directly measurable; it istermed the Galvani potential, and is the target of most ofthe modeling discussed below, Clearly, if the electrons aretransferred across the free surfaces and vacuum betweenthe two metals, we have [ ; ~ : = [ ; ~ : + [ ; ~ : > V ,

Once a metal is immersed in a solvent, a second dipolarlayer will form at the metal surface due to the alignment ofthe solvent dipoles, Again, this contribution to the potential is not directly measurable, and, in addition, the mctaldipole contribution itself will change since the distributionof the electron cloud will be modified by the presence ofthe solvent. Finally, there will be a contribution from freecharges both on the metal and in the electrolyte. The overall contribution to the Galvani potential difference bctweenmetal and solution then consists of thesc four quantities, asshown in Figure 3.[21 If the potential due to dipoles at thcmetal-vacuum interface for the metal is XM and for thesolvent-vacuum interface is X , then the Galvani potential difference between metal and solvent can be writteneither as

(2)or as

(3)where OX M, OX s, are the changes in surface dipole for metaland solvent on forming the interface. In equation (2) wepass across the interface, and in equation (3) we pass intothe vacuum from both metal and solvent. As bcfore, thevalue of [ ; ~ > V , the Volta potcntial differcnce, is measurableexperimentally, but it is evident that we cannot associatethis potential difference with that due to free charges atthe interface, since there are changes in dipole contributionon both sides as wcll. Evcn if there are no free chargesat the interface (at the point of zero charge (PZC)), theVolta potential difference is not zero unless oXM = oXs i.e.,the free surfaces of the two phases will still be chargedunless the changes in surface dipole of solvent and metalbalance exactly. In practice, this is not the case: carefulmeasurements[41 show that [ ; ~ ; o >V = -O.26V at the PZC,showing that the dipole changes do not, in fact, compensate.Historically, this discussion is of considerable interest, since

The electrode-electrolyte ilUe1face 15

M

O(e)\---,-,,- - - I+1

1

g ~ ( i o n )

1111()1111o110)

s

Figure 3. Components of the Galvani potential difference at ametal-solution interface. (Reproduced from Trassatti (1980)121with permission from Kluwer Academic/Plenum Publishers.)

a bitter dispute between Galvani and Volta over the originof the electromotive force (EMF) when two different metalsare immersed in the same solution could, in principle, bedue just to the Volta potential difference between the metals.In fact, it is easy to see that if conditions arc such thaLthere are no free charges on either metal, the differencein potential between them, again a measurable quantity, isgiven by

(4)showing that the difference in work functions would onlyaccount for the difference in electrode potentials if the twoVolta terms were actually zero.

3 INTERFACIAL STATISTICALTHERMODYNAMICS OF THEDIFFUSE LAYER

Development of a self-consistent theory for the double layerhas proven extremely difficult, since the presence of anelectrode introduces an essentially non-isotropic elementinto the equations. This manifests itself in the need for