ELECTROCHEMICAL BEHAVIOUR OF PLATINUM-IRIDIUM ANODES BY DONALD ARTHUR ...

181 Pages·2010·10.73 MB·English

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ELECTROCHEMICAL BEHAVIOUR
OF PLATINUM-IRIDIUM ANODES
BY
DONALD ARTHUR WENSLEY
B.A.Sc, University of British Columbia, 1970
A THESIS SUBMITTED IN PARTIAL FULFILMENT OF
THE REQUIREMENTS FOR THE DEGREE OF
MASTER OF APPLIED SCIENCE
in the Department
of
METALLURGY .
We accept this thesis as conforming to the
required standard
THE UNIVERSITY OF BRITISH COLUMBIA
June, 1973
In presenting this thesis in partial fulfilment of the requirements for
an advanced degree at the University of British Columbia, I agree that
the Library shall make it freely available for reference and study.
I further agree that permission for extensive copying of this thesis
for scholarly purposes may be granted by the Head of my Department or
by his representatives. It is understood that copying or publication
of this thesis for financial gain shall not be allowed without my
written permission.
Department of Metallurgy
The University of British Columbia
Vancouver 8, Canada
Date October 4, 1973
ii
ABSTRACT
This thesis considers the electrochemistry of platinum-
iridium electrodes in both sulphate- and chloride-containing electro
lytes at 20 - 25°C. Both wire electrodes of appropriate alloy composi
tions and titanium-substrate electrodes were employed. Polarization
curves were obtained, and a technique for measuring the surface area
of the electrodes was employed in order to determine the effect of
potentiostatic electrolysis on the electrochemically active area.
The wire alloy electrodes showed polarization behaviour in
1M NaCl; pH 2 identical to that of platinum electrodes, indicating that
iridium is not effective in reducing the passivation of these electrodes
even with up to 25% alloy content.
The coated electrodes showed irreversible surface area
losses in both sulphate and chloride electrolytes, with the latter pro
ducing significant reductions after very short polarization times. It
is suggested that oxidation of the substrate leading to electrical iso
lation of coating plates is responsible for the area decay.
i ii
TABLE OF CONTENTS
Page
1. INTRODUCTION 1
2. LITERATURE SURVEY AND THEORETICAL CONSIDERATION 5
2.1 ELECTRODE PRETREATMENT AND ACTIVATION 6
2.1.1 Nature of the Problem 6
2.1.2 Methods of Pretreatment and Activation .... 9
2.1.3 Mechanisms of Activation of Noble Metal
Electrodes 11
2.1.3.1 Surface Area Increases 11
2.1.3.2 Active Surface Structure 14
2.1.3.3 Active Oxidized Surface 18
2.1.3.4 Active Reduced Surface 20
2.1.3.5 Activity Induced by Dermasorption
of Oxygen 24
2.1.3.6 Altered Electronic Properties .... 27
2.1.3.7 Impurity Removal 29
2.1.4 Summary 32
2.2 SURFACE AREA OF NOBLE METAL ELECTRODES 33
2.2.1 Bases for Electrochemical Surface Area
Measurement 33
2.2.1.1 Complications due to Simultaneous
Processes 34
2.2.1.2 Compensation for Double Layer
Charging 36
2.2.1.3 Monolayer Formation 37
2.2.1.4 Absorption 38
iv
Page
2.2.1.5 Surface Atom Density 40
2.2.2 Procedures for Surface Area Measurement ... 42
2.2.2.1 Potential Sweep Techniques 42
2.2.2.2 \Galvanostatic Charge Techniques .. 44
2.2.3 Summary 46
2.3 ELECTROLYSIS OF CHLORIDE SOLUTIONS 48
2.3.1 Polarization of Smooth Noble Metal Anodes . 50
2.3.2 Polarization of Titanium-Substrate Anodes . 58
2.3.2.1 Behaviour of Titanium 59
2.3.2.2 Coupling of Platinum Metals with
Titanium 61
2.3.2.3 Polarization Characteristics 63
2.3.3 Summary 64
2.4 DISSOLUTION OF THE NOBLE METALS 65
2.4.1 The Active Dissolution of the Noble Metals 66
2.4.2 Dissolution with Oxygen Participation 72
2.4.3 Dissolution During Activation 78
2.4.4 Degradation of Noble Metal Coatings 83
2.4.4.1 Degradation as a Result of Coat
ing Undermining 83
2.4.4.2 Other Causes of Coating Loss 86
2.4.5 Summary 87
2.5 RELATION TO AIMS OF PRESENT WORK 88
3. EXPERIMENTAL 93
3.1 ELECTRODES 93
V
Page
3.2 ELECTROLYTES 95
3.3 CELLS 96
3.4 PROCEDURES 100
3.4.1 Anodic Galvanostatic Measurements 100
3.4.2 Anodic Potentiostatic Measurements 101
3.4.3 Surface Area Measurements 102
3.4.4 Observation of Electrode Surfaces 105
4. RESULTS 106
4.1 GALVANOSTATIC POLARIZATION CURVES 106
4.2 POTENTIOSTATIC POLARIZATION CURVES 119
4.3 CHANGE OF SURFACE AREA WITH POTENTIOSTATIC
ANODIZATION 122
4.4 OBSERVATIONS OF ELECTRODE SURFACES 132
5. DISCUSSION 140
5.1 ANODIC GALVANOSTATIC MEASUREMENTS 140
5.2 ANODIC POTENTIOSTATIC MEASUREMENTS 142
5.3 SURFACE AREA CHANGES 143
6. PROPOSALS FOR FUTURE WORK 148
BIBLIOGRAPHY 153
APPENDIXES 163
APPENDIX I Electrode Surface Conditions 163
APPENDIX II Surface Area Measurement 167
APPENDIX III X-ray Diffraction Results 170
vi
LIST OF TABLES
TABLE Page
1. Hydrogen and oxygen monolayer charges for
platinum and iridium electrodes 41
2. Electrode areas measured after determination of the
polarization curves 109
3. Tafel parameters for Pt and Pt/Ir wire electrodes for
the lower Tafel region of the polarization curves in
1M NaCl; pH 2 I ll
4. Tafel parameters for Pt and Pt/Ir wire electrodes for
the ascending and descending upper Tafel regions of
the polarization curve in 1M NaCl; pH 2 I ll
5. Passivation data for Pt and Pt/Ir wire electrodes from
polarization curves in 1M NaCl; pH 2 113
6. Surface area changes as a result of potentiostatic
polarization in chloride electrolytes with Pt/30 Ir-
Ti electrodes 118
7. Effect of potentiostatic polarization in 1M ^SO^ on
the surface area of Pt/30 Ir-Ti electrodes 119
8. Effect of treatment in aqua regia on the surface area
of Pt/30 Ir-Ti electrodes 120
9. Effects of "activation" procedures on the surface area
of Pt/30 Ir-Ti electrodes 126
10. Surface conditions of wire electrodes used in galvano-
static polarization experiments 163
11. Surface conditions of coated electrodes used in potent
iostatic polarization and surface area determinations .. 165
12. Identification of X-ray diffraction peaks for a new
titanium substrate electrode 170
13. Identification of X-ray diffraction peaks for a used
titanium substrate electrode (3 weeks in 1M H„S0, at
.2 A/ft.2 and 40°C) 171
vii
LIST OF FIGURES
FIGURE Page
1. Galvanostatic cell 98
2. Cell for surface area measurement 99
3. Galvanostatic polarization curve for platinum wire
electrode in helium-saturated 1M NaCl; pH 2 109
4. Galvanostatic polarization curve for platinum/5% iridium
wire electrode in helium-saturated 1M NaCl; pH 2 110
5. Galvanostatic polarization curve for platinum/10% iridium
wire electrode in helium-saturated 1M NaCl; pH 2 I ll
6. Galvanostatic polarization curve for platinum/20% iridium
wire electrode in helium-saturated 1M NaCl; pH 2 112
7. Galvanostatic polarization curve for platinum/25% iridium
wire electrode in helium-saturated 1M NaCl; pH 2 113
8. Potentiostatic polarization curve for Pt/30 Ir-Ti in
unstirred 1M l^SO^ 120
9. Potentiostatic polarization curve for Pt/30 Ir-Ti in
unstirred 1M NaCl; pH 2 121
10. Current/time relations for potentiostatic polarization
with Pt/30 Ir-Ti electrodes at 1800 mV (S.C.E.) in
various electrolytes 127
11. Current/time relations for a Pt/30 Ir-Ti electrode for
potentiostatic electrolysis of IM H^SO^ at 1800 mV and
25°C, after various pretreatment times in aqua regia .. 128
12. Current/time relations for a Pt/30 Ir-Ti electrode for
potentiostatic electrolysis of 1M H^SO^ at 2000 mV and
25°C, before and after potentiostatxc electrolysis of
1M NaCl; pH 2 129
13. S.E.M. Observation of Pt/30 Ir-Ti surfaces 134
14. S.E.M. Observation of used Pt/30 Ir-Ti electrodes 135
viii
FIGURE Page
15. S.E.M. Observation of platinum sheet 136
16. S.E.M. Observation of platinum wire electrodes 137
17. S.E.M. Observation of Pt/25 Ir wire electrodes 138
18. E.P. Observation of new Pt/30 Ir-Ti electrode 139
19. Schematic representation of the potential history of a
wire electrode used in galvanostatic polarization
experiments ; 164
20. Schematic representation of the potential hostory of a
coated electrode used in potentiostatic polarization
experiments 166
21. Representation of a typical anodic charge curve in de-
aerated 1M I^SO^ at 20°C, showing constructions for
determining oxygen deposition charge 169
I wish to thank Dr. I.H. Warren for his guidance
throughout the course of this research, the staff of the Science
Division, Main Library, "U.B.C. for their invaluable aid, and my
wife, Darlene, for her enduring patience. Financial support from
the National Research Council and International Nickel Company is
also acknowledged.