Electrochemical Processes in Fuel Cells

287 Pages·1969·8.161 MB·English

by  Manfred W. Breiter (auth.)

Anorganische und allgemeine Chemie
in Einzeldarstellungen
Herausgegeben von Margot Becke-Goehring
------------- Band IX -------------
Manfred W. Breiter
Electrochemical Processes
in Fuel Cells
With 98 Figures
Springer-Verlag New York Inc. 1969
MANFRED W. BRElTER
General Electric Research and Development Center,
Schenectady, N. Y./USA
ISBN-13: 978-3-642-46157-6 e-ISBN-13: 978-3-642-46155-2
DOl: 10.1007/978-3-642-46155-2
All rights reserved. No part of this hook may be translated or reproduced in any form without written permission
from Springer-Verlag. «> by Springer-Verlag New York Inc. 1969. Library of Congress Catalog Card Number
69-17789.
Softcover reprint of the hardcover 1S I edition 1969
The use of general descriptive names. trade names, trade marks etc. in this publication. even if the former are not
especially identified. is not to be taken as a sign that such names, as understood by the Trade Marks and Merchan-
dise Marks Act, may accordingly be used freely by anyone.
Title No. 4276
Preface
The necessity for a better understanding of the basic processes that
determine the operation of fuel cells became evident during the devel
opment of practical units in the last three decades. The search for
efficient electrocatalysts in low-temperature fuel cells intensified the
general study of the nature and the role of the electrode material. Re
search on the complex mechanisms of the anodic oxidation of different
fuels and of the reduction of molecular oxygen on solid electrodes was
stimulated, and the strong influence of adsorbed species on the electrode
reaction in question was investigated. Suitable electrolytes had to be
found for the high-temperature fuel cells. The use of electrodes with
large internal surface lead to the development of models of porous
electrode. structures and to the mathematical analysis of the operation
of these models under certain conditions.
While the chapters I to III introduce the reader to the general field
offuel cells, the progress made in the understanding of the basic problems
in the electrochemistry of fuel cells since the end of the second world
war is reviewed in chapters IV to XVI of this monograph. In contrast,
the technological aspects necessary for the development of practical
units are not covered here.
The open literature published as books or as papers in scientific
journals has been considered up to the time of the writing of the final
draft of the specific chapter, at least till the end of 1967.
I want to express my gratefulness to some of my colleagues at the
General Electric Research and Development Center in Schenectady,
N. Y., for their helpful discussions and suggestions during the writing
of the book: Dr. W. T. GRUBB, Dr. L. W. NIEDRACH, Dr. D. A. VER
MILYEA, and Dr. F. G. WILL. Dr. NIEDRACH kindly supplied me with
the two micrographs of porous Teflon-bonded platinum black electro
des. Prof. G. J. JANZ, Rensselaer Polytechnic Institut, Troy, N. Y.,
advised me on questions concerning chapter XIII. I am very grateful for
the comments on the monograph from Dr. E. L. SIMONS, and on chapter
XIV from Mr. D. W. WHITE, both of the Rand D Center.
It is a pleasure to acknowledge my indebtedness to the management
at the Research and Development Center for their enlightened under
standing and support of my work.
Schenectady, N. Y., May 1969 MANFRED W. BREITER
Contents
I. Introduction. . . . . . . . . . . . . .
1. Definition and Description of a Fuel Cell 1
2. Classification of Fuel Cells 2
3. Historical Development. 3
References 4
II. General Aspects 6
1. Thermodynamic Considerations and Definitions 6
2. Efficiency of Galvanic Cells· . . . . . . . 9
3. Basic Requirements for a Practical Fuel Cell 10
4. Electrolytes . . . . . 11
References . . . . . 12
III. Mass Transport Processes 13
1. Concept of the Nernst Diffusion Layer 13
2. Convective Diffusion . . . . . . . 14
3. Migration and Convective Diffusion . 17
References . . . . . . . . . . . . 18
IV. Kinetics of Electrode Reactions on Homogeneous Surfaces
and Influence of Electrode Material . . . . . . . 19
1. Single Step Reaction . . . . . . . . . . . . . .. 19
2. Two Step Reaction with Adsorbed Intermediate . .. 22
3. Successive Electrode Reactions with One Rate-Determin-
ing Step . . . . . . . . . . . . . . . . . . . . . 29
4. Some Features of Mechanisms Involving the Simultaneous
Formation of Different Products . . . . . . . . . . . 32
5. Correlation between Hydrogen Overvoltage and Free
Energy of Hydrogen Adsorption . . . . . . 38
References . . . . . . . . . . . . . . . 40
V. Electrode Reactions on Heterogeneous Surfaces. 42
1. Structure and Composition of Surfaces of Solids 42
2. Current Distribution on Heterogeneous Surfaces 43
3. Approximate Kinetic Expressions for Electrocatalytic
Reactions on Heterogeneous Surfaces . 44
References . . . . . . . . . . . . . . . . . . . . 47
VI Contents
VI. Characterization of the Surface of Platinium Metals and Pla
tinum Metal Alloys by Hydrogen Adsorption and Comparison
of the Results with Other Techniques . . . . . . . . .. 48
1. Electrochemical Determination of Isotherms of Hydrogen
Adsorption. . . . . . . . . . . . . . . . . . . . 48
2. Heat of Hydrogen Adsorption as a Function of Coverage 52
3. Langmuir Approximation of the Isotherms of Hydrogen
Adsorption . . . . . . . . . . . . . . . . . . . . 55
4. Influence of Surface Structure on Hydrogen Adsorption at
Platinum . . . . . . . . . . . . . . . . . . . . . 57
5. Determination of the Electrochemically Active Surface 60
6. Hydrogen Adsorption in the Presence of Chemisorbed Car
bonaceous Species . . . . . . . . . . . . . . . .. 62
7. Effect of Pretreatment on the Reactivity of Platinum Metal
Electrodes . . . . . . . . . . . . . . . . . . . . 66
8. Hydrogen Adsorption on Binary Platinum Metal Alloys. 71
References . . . . . . . . . . . . . . . . . . . . 75
VII. Anodic Oxidation of Molecular Hydrogen at Low Temper-
atures 78
1. Mechanism of the H2 Oxidation on Noble Metals and Noble
Metal Alloys . . . . . . . . . . . . . . . . . . . 78
2. Mechanism of the H2 Oxidation on Different Types of
Nickel Electrodes in Alkaline Electrolytes . . . . . . . 84
3. Mechanism of the H2 Oxidation on Platinum in Contact
with an Ion-Exchange Membrane. . . . . . . . . . . 88
References . . . . . . . . . . . . . . . . .. . . 89
VIII. Oxygen Layers on Different Materials and Inhibition of Fuel
Oxidations . . . . . . . . . . . . . . . . . . . .. 91
1. Formation and Reduction of Oxygen Layers on Platinum
Metals and Some Alloys . . . . . . . . . . . 91
2. Nature of the Oxygen Layers on Platinum Metals. 94
3. Oxygen Layers on Nickel in Alkaline Electrolytes. 97
4. Oxygen Layers on Silver in Alkaline Electrolytes 101
5. Oxygen Layers on Carbon. . . . . . . . . . 103
6. Inhibition of Fuel Oxidations by Oxygen Layers 105
References . . . . . . . . . . . . . . . . 108
IX. Adsorption of Carbonaceous Species on Platinum Metals 112
1. Non-EquilibriumAspects oft he Chemisorption of Strongly
Bonded Carbonaceous Species . . . . . . . . .. 112
2. Adsorption of Weakly Bonded Carbonaceous Species 113
Contents VII
3. Rate of Formation of Strongly Bonded Species at Const-
ant Potential . . . . . . . . . . . 115
4. Coverage from Anodic Pulses. . . . . . . 118
5. Coverage from Hydrogen Deposition 123
6. Radiometric Determination of the Coverage 126
7. Coverage and Capacitance of Electrode Impedance 128
8. Determination of the Number of Electrons in the Oxid-
ation of Chemisorbed Species. . . . 132
9. Effect of pH and Anions on Coverage ........ 134
to. Nature of Chemisorbed Species . . . . . . . . . . . 136
11. Oxidation Mechanism of Chemisorbed Carbonaceous
Species . . . . . . . . . . . . . . . . 141
References . . . . . . . . . . . . . . . 144
X. Anodic Oxidation of Fuels at Low Temperatures 147
1. Classification of the Oxidation Mechanisms. 147
2. Oxidation of Carbon Monoxide. . . . . . 148
3. Oxidation of Mixtures of Hydrogen and Carbon Monoxide 156
4. Oxidation of Formic Acid . . . . . . . . 157
5. Methanol Oxidation. . . . . . . . . . . 165
6. Oxidation of Higher Alcohols and Aldehydes 168
7. Oxidation of Hydrocarbons . . . . . . . 171
8. Oxidation of Hydrazine . . . . . . . . . 176
9. Oscillatory Phenomena on Solid Electrodes. 179
References . . . . . . . . . . . . . 181
XI. The Oxygen Electrode at Low Temperatures 185
1. Distinction of Reduction Mechanisms. 185
2. The Role of Hydrogen Peroxide in the Oxygen Reduction
on Platinum Metals 188
3. Mechanism of the O2 Reduction on Active Platinum
Metals in the Absence of the Oxygen Layer 198
4. The O Reduction on Platinum Metals in the Presence of
2
Oxygen Layers 199
5. The 02 Reduction on Silver, Nickel, and Silver Alloys. 200
6. The O Reduction on Carbon 203
2
7. The O Reduction on Intermetallic Compounds 205
2
8. The Reversible Oxygen Electrode. 206
References 208
XII. Corrosion of Electrodes at Low Temperatures 211
1. Predictions from Potential-pH Diagrams 211
2. Dissolution of Platinum Metals 213
VIII Contents
3. Dissolution of Nickel, Silver, and Carbon in Alkaline
Electrolytes . . . . . . . . . . . . . . . . . . . . 214
References . . . . . . . . . . . . . . . . . . . . 215
XIII. Processes in Fuel Cells with Molten Carbonate Electrolytes 217
1. General Considerations. . . . . . . . . . . . . . . 217
2. Properties of Molten Carbonate Electrolytes. . . . . . 219
3. Thermal Stability of Molten Carbonates and Corrosion of
Metals . . . . . . . . . . . 220
4. Formation of Carbon Deposits. . . . . . . 223
5. Processes at the Anode . . . . . . . . . . 224
6. The Oxygen Electrode in Molten Carbonates. 226
References . . . . . . . . . . . . . . 228
XIV. Processes in Fuel Cells with Solid Electrolytes 230
1. General Considerations. . . . . 230
2. Properties of Solid Electrolytes. . 233
3. Current-Voltage Characteristics 234
References . . . . . . . . 237
XV. Properties of Porous Electrodes 238
1. Porosity . . . . . . . . . 238
2. Determination of Different Surface Areas 243
3. Experimental Current-Potential Curves for Porous
Electrodes . . . . . . . 244
4. Structure and Performance 246
References 252
XVI. Models of Porous Electrodes. 254
1. Potential Distribution in the Flooded Single Pore without
Influence of Mass Transport Processes . . . . . . . . 254
2. Concentration Distribution in the Flooded Single Pore
under the Influence of Diffusion . . . . . . . . . . . 258
3. Potential Distribution in the Flooded Single Pore in the
Presence of Mass Transport Processes . . . . .. 260
4. Continuum Models of Flooded Porous Electrodes 261
5. The Thin Film Model of the Gas-Diffusion Electrode 263
6. The Meniscus Model of the Gas-Diffusion Electrode 266
7. Simultaneous Consideration of Thin Film and Meniscus. 267
8. Model for the Two-Layer Electrode. 267
References 268
Subject Index 271
Main Symbols
A apparent surface area (cm2)
BET surface area (m2/g)
effective surface in the one-dimensional continuum model
of porous structures (cm -
1 )
activity of species j (dimensionless)
double layer capacity (~F /cm2)
capacitance of electrode impedance in a series analog circuit
(~F/cm2)
capacitance of electrode impendance in a parallel circuit
(~F/cm2)
concentration of species j (Mol/cm3) or (Mol/I)
concentration of species j in the bulk of the solution (Mol/cm 3)
or (Mol/I)
concentration of species j adjacent to the electrode surface
(Mol/cm3) or (Mol/I)
D diffusion coefficient of species j (cm2/sec)
j
E electromotive force (V)
E electric field vector (V /cm)
e electron charge (coul)
F Faraday (couI/Mol)
jj activity coefficient of species j (cm3/Mol) or (l/Mol)
L1 G free energy change (kcaI/Mol)
L1 Giv+ activation energy of anodic processes with respect to the
I, ref
reference electrode ref (kcal/Mol), v = 1, 2, ., ..
L1 G iv + 2, ref activation energy of cathodic processes with respect to the
. reference electrode ref (kcaI/Mol), v = 1, 2, ....
L1H free enthalpy change (kcaI/Mol)
L1 Hiv+ 1 free enthalpy change of anodic processes (kcaI/Mol)
L1Hiv+2 free enthalpy change of cathodic processes (kcaI/Mol)
h Planck's constant (erg sec)
J current (A)
partial current due to reaction v (A)
exchange current (A)
limiting current (A)
limiting diffusion current (A)
current density J/A (A/cm2)
current density of reaction v (A/cm2)
x Main Symbols
io exchange current density (A/cm2)
i limiting current density (A/cm2)
l
id limiting diffusion current density (A/cm2)
jd diffusional flux vector (Moljsec cm2)
K, tortuosity factor (dimensionless)
k Boltzmann's constant (ergtC)
k2v+ 1 rate constant of anodic processes, given with respect to a
reference electrode, in A if I ~ aj and in A cm3/Mol if I ~ cj
k2v+2 rate constant of cathodic processes (A) or (A cm 3/Mol)
L characteristic length (cm)
M Moljl
Pe Peelet number (dimensionless)
Pr Prandtl number (dimensionless)
P, total porosity (dimensionless)
P specific porosity concerning pores of type v (dimensionless)
v
Pj pressure of gaseous species j (atm)
pressure of species j in the bulk of the solution (atm)
bPj
pressure of species j adjacent to electrode surface (atm)
sPj
QH charge due to the removal or the formation of adsorbed H
atoms (mcouljcm2)
charge corresponding to a monolayer ofH atoms
(mcouljcm2)
charge corresponding to a monolayer of H atoms when
other species are adsorbed simultaneously (mcouljcm2)
charge due to the formation of an oxygen layer (mcoul/cm2)
charge due to the reduction of an oxygen layer (mcoul/cm2)
charge due to the anodic formation of an adsorbed layer of
carbonaceous species (mcouljcm2)
charge due to the anodic formation ofthe maximum coverage
with carbonaceous species (mcouljcm2)
charge due to the anodic removal of an adsorbed layer of
carbonaceous species (mcoul/cm2) ,
charge due to the anodic removal of the maximum coverage
with carbonaceous species (mcouljcm2)
gas constant (Joule/Mol DC) or (kcaljMol DC)
solution resistance per unit pore length (Q cm -1)
resistance of electrocatalyst layer perunit pore length (Q cm - 1)
electrolytic r~sistance (Q)
ohmic component of electrode impendance in an analog
series circuit (Q cm2)
ohmic component of electrode impedance III a parallel
circuit (Q cm2)
entropy change (kcal/Mol)
Main Symbols XI
T temperature (OK)
8- temperature Cc)
t time (sec) or (min) or (hr)
tj transference number ()f species j (dimensionless)
U electrode-solution potential difference of test electrode less
ref
that of a reference electrode, for instance ref = cal for calomel
electrode, (V)
U electrode-solution potential difference of test electrode less
that of the hydrogen electrode in the same solution (V)
U electrode-solution potential difference of test electrode, at
rev. ref
which the equilibrium of an electrode reaction is established,
less that of a reference electrode. The subscript ref is omitted
if the reference electrode is mentioned in the text (V)
U under standard conditions (with respect to the mentioned
rev
reference electrode) (V)
volume (cm3)
apparent volume for porous structure (cm3)
specific volume concerning pores of type v (cm3/g)
velocity vector (cm/sec)
work (Joule/Mol) or (erg/Mol)
impedance per unit pore length (0 cm)
z number of faradays carried if reaction proceeds so that the
number of moles of each product formed is equal to the
stoichiometric coefficient in the reaction (dimensionless)
rj surface excess of species j (Mol/cm2)
y surface tension (erg/cm)
c5j thickness of diffusion boundary layer of species j (cm)
" overvoltage of an electrode reaction (V)
9 electrode coverage with species j (dimensionless)
j
l/K penetration depth (cm)
l/K effective penetration depth in the continuum model of a
p
porous structure (cm)
Jlj((X) chemical potential of species j in the phase (X (kcal/Mol)
iij ((X) electrochemical potential of species j in the phase (X (kcal/Mol)
p density (g/cm3)
Uj specific conductivity of species j (0 -1 cm -1)
Up effective conductivity in a porous structure (0-1 cm -1)
• transition time (sec)
tP" inner electric potential of phase (X (V)
cp average potential in the continuum model of a porous
structure (V)
OJ angular velocity (sec - 1)