Volume 2 Modern Electrochemistry: An Introduction to an Interdisciplinary Area

863 Pages·1973·39.5 MB·English

by  John O’M. Bockris

VOLUME 2
MODERN
ELECTROCHEMISTRY
VOLUME 2
MODERN
ELECTROCHEMISTRY
An Introduction to an Interdisciplinary Area
John O'M. Bockris
Professor of Electrochemistry
University of Pennsylvania. Philadelphia. Pennsylvania
and
Amulya K. N. Reddy
Professor of Electrochemistry
Indian Institute of Science. Bangalore. India
A Plenum/Rosetta Edition
Library 01 Congress Cataloging in Publication Data
Boc .... ris. John O'M
Modllrn electrochemistry.
"A PienumfAO$IItt8 !!dition."
1. Electrochemistry. I. A!!dctv. AmulY8 K. N. . joint 8uthor.11. Title.
00553.B63 1973 541'.37 73-13712
ISBN-13: 978-0-306-25002-6 e-ISBN-13: 978-1-4613-4560-2
001: 10.1007/978-1-4613-4560-2
ro 19 18 17 16 15 14 13 12
A PlenumIRosetta Edition
Published by Plenum Publishing Corporation
233 Spring Street, New York, N.v. 10013
C 1970 Plenum Press, New York
A Division of Plenum Publishing Corporation
All rights reserved
No part of this publication may be reproduced in any form
without written permission from the publisher
CONTENTS
VOLUME 2
CHAPTER 7
The Electrified Interface
7.1 Electrification of an Interface............................. 623
7.1.1 The Electrode-Electrolyte Interface: The Basis of Electrodics ....... 623
7.1.2 New Forces at the Boundary of an Electrolyte. .. . .. . .... . ... ... . . 623
7.1.3 The Interphase Region Has New Properties and New Structures. .. . . 626
7.1.4 An Electrode Is Like a Giant Central Ion.. . . ... . .. . . .. . ... . . .. . . 626
7.1.5 The Consequences of Compromise Arrangements: The Electrolyte Side
of the Boundary Acquires a Charge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 627
7.1.6 Both Sides of the Interface Become Electrified: The So-Called "Electrical
Double Layer" ............................................... . 629
7.1.7 Double Layers Are Characteristic of All Phase Boundaries ......... . 630
7.1.8 A Look into an Electrified Interface ............................ . 632
Further Reading ...................................................... . 639
7.2 Some Problems in Understanding an Electrified Interface. . 640
7.2.1 What Knowledge Is Required before an Electrified Interface Can Be Re-
garded as Understood? ........................................ 640
7.2.2 Predicting the Interphase Properties from the Bulk Properties of the
Phases. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 641
7.2.3 Why Bother about Electrified Interfaces? ........................ 642
7.2.4 The Need to Clarify Some Concepts. . .. . .. . . ... . ... . . .... . .... . 643
7.2.5 The Potential Difference across Electrified Interfaces .............. 644
7.2.5a What Happens when One Tries to Measure the Absolute Po-
tential Difference across a Single Electrode-Electrolyte Interface 644
v
vi CONTENTS
7.2.5b The Absolute Potential Difference across a Single Electrified
Interface Cannot Be Measured ........................... 648
7.2.5c Can One Measure Changes in the Metal-Solution Potential
Difference? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 650
7.2.5d The Extreme Cases of Ideally Nonpolarizable and Polarizable
Interfaces .............................................. 653
7.2.5e The Development of a Scale of Relative Potential Differences 655
7.2.5/ Can One Meaningfully Analyze an Electrode-Electrolyte Po-
tential Difference? ...................................... 659
7.2.5g A Thought Experiment Involving a Charged Electrode in
Vacuum............................................... 660
7.2.5h The Test Charge Must Avoid Image Interactions with the
Charged Electrode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 660
7.2.5i The Outer Potential tp of a Material Phase in Vacuum ...... 663
7.2.5j What is the Relevance of the Outer Potential to Double-Layer
Studies? ............................................... 665
7.2.5k Another Thought Experiment Involving an Uncharged, Dipole-
Covered Phase ......................................... 667
7.2.5/ The Dipole Potential Difference MLlsX across an Electrode-
Electrolyte Interface .................................... 670
7.2.5m The Sum of the Potential Differences Due to Charges and
Dipoles: The Absolute Electrode-Electrolyte (or Galvani)
Potential Difference .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 670
7.2.5n The Outer, Surface, and Inner Potential Differences ........ 673
7.2.50 An Apparent Contradiction: The Sum of the Ll¢'s across a
System of Interfaces Can and the Ll¢ across One Interface
Cannot Be Measured ................................... 674
7.2.5p What Deeper Understanding Has Been Hitherto Gained Re
garding the Absolute Potential Difference Across an Electrified
Interface? ... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 677
7.2.6 The Accumulation and Depletion of Substances at an Interface. . . . . 679
7.2.6a What Would Represent Complete Structural Information Re-
garding an Electrified Interface? ......................... 679
7.2.6b The Concept of Surface Excess .......................... 680
7.2.6c Does Knowledge of the Surface Excess Contribute to Knowledge
of the Distribution of Species in the Interphase Region? . .. .. 683
7.2.6d Is the Surface Excess Equivalent to the Amount Adsorbed? 684
7.2.6e Is the Surface Excess Measurable? ....................... 685
7.2.6/ The Special Position of Mercury in Double-Layer Studies ... 687
Further Reading .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 687
7.3 The Thermodynamics of Electrified Interfaces... . . . . . . . . . . 688
7.3.1 The Measurement of Interfacial Tension as a Function of the Potential
Difference across the Interface ... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 688
7.3.2 Some Basic Facts about Electrocapillary Curves .................. 690
7.3.3 A Digression on the Electrochemical Potential .................... 693
7.3.3a Definition of Electrochemical Potential ................... 693
7.3.3b Can the Chemical and Electrical Work Be Determined Sepa-
rately? ................................................ 695
7.3.3c A Criterion of Thermodynamic Equilibrium between Two
Phases: Equality of Electrochemical Potentials ............ 696
7.3.3d Nonpolarizable Interfaces and Thermodynamic Equilibrium 697
7.3.4 Some Thermodynamic Thoughts on Electrified Interfaces .......... 698
CONTENTS vii
7.3.5 Interfacial Tension Varies with Applied Potential: Determination of
the Charge Density on the Electrode ........................... . 701
7.3.6 Electrode Charge Varies with Applied Potential: Determination of the
Electrical Capacitance of the Interface .......................... . 703
7.3.7 The Potential at Which an Electrode Has a Zero Charge .......... . 706
7.3.8 Surface Tension Varies with Solution Composition: Determination of
the Surface Excess ........................................... . 707
7.3.9 Reflections on Electrocapillary Thermodynamics ................. . 714
7.3.10 Retrospect and Prospect in the Study of Electrified Interfaces .... . 715
Further Reading ...................................................... . 717
7.4 The Structure of Electrified Interfaces .................... . 718
7.4.1 The Parallel-Plate Condenser Model: The Helmholtz-Perrin Theory 718
7.4.2 The Double Layer in Trouble: Neither Perfect Parabolas nor Constant
Capacities .................................................... . 719
7.4.3 The Ionic Cloud: The Gouy-Chapman Diffuse-Charge Model of the
Double Layer ................................................ . 722
7.4.4 Ions under Thermal and Electric Forces near an Electrode ........ . 724
7.4.5 A Picture of the Potential Drop in the Diffuse Layer ............. . 728
7.4.6 An Experimental Test of the Gouy-Chapman Model: Potential Depend-
ence of the Capacitance, but at What Cost? ..................... . 732
7.4.7 Some Ions Stuck to the Electrode, Others Scattered in Thermal Disarray:
The Stern Model .............................................. . 733
7.4.8 A Consequence of the Stern Picture: Two Potential Drops across an
Electrified Interface ........................................... . 734
7.4.9 Another Consequence of the Stern Model: An Electrified Interface Is
Equivalent to Two Capacitors in Series .......................... . 735
7.4.10 The Relative Contributions of the Helmholtz-Perrin and Gouy-Chap-
man Capacities ............................................... . 737
7.4.11 Some Questions Regarding the Sticking of Ions to the Electrode .... . 738
7.4.12 An Electrode Is Largely Covered with Adsorbed Water Molecules .. . 739
7.4.13 Metal-Water Interactions ...................................... . 740
7.4.14 The Orientation of Water Molecules on Charged Electrodes ...... . 741
7.4.15 How Close Can Hydrated Ions Come to a Hydrated Electrode? .... . 741
7.4.16 Is It Only Desolvated Ions which Contact-Adsorb on the Electrode? 742
7.4.17 The Free-Energy Change for Contact Adsorption ............... . 742
7.4.18 What Determines the Degree of Contact Adsorption? ............ . 743
7.4.19 How Is Contact Adsorption Measured? ........................ . 745
7.4.20 Contact Adsorption, Specific Adsorption, or Superequivalent Adsorption 748
7.4.21 Contact Adsorption: Its Influence of the Capacity of the Interface .. . 749
7.4.22 Looking Back to Look Forward ............................... . 752
7.4.23 The Complete Capacity-Potential Curve ....................... . 753
7.4.24 The Constant-Capacity Region ................................ . 753
7.4.24a The So-Called "Double Layer" Is a Double Layer ........ . 753
7.4.24b The Dielectric Constant of the Water between the Metal and the
Outer Helmholtz Plane ................................. . 756
7.4.24c The Position of the Outer Helmholtz Plane and an Interpreta-
tion of the Constant Capacity .......................... . 757
7.4.25 The Capacitance Hump ........................................ . 761
7.4.26 How Does the Population of Contact-Adsorbed Ions Change with
Electrode Charge? ............................................ . 762
7.4.27 The Test of the Population Law for Contact-Adsorbed Ions ....... . 769
7.4.28 The Lateral-Repulsion Model for Contact Adsorption ............ . 776
viii CONTENTS
7.4.29 Flip-Flop Water on Electrodes ................................. 779
7.4.30 Calculation of the Potential Difference Due to Water Dipoles. . . . . . . 781
7.4.31 The Excess of Flipped Water Dipoles over Flopped Water Dipoles. . . 782
7.4.32 The Contribution of Adsorbed Water Dipoles to the Capacity of the
Interface ...................................................... 78:'
Further Reading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 790
7.5 The Competition between Water and Organic Molecules at
the Electrified Interfaces. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 791
7.5.1 The Relevance of Organic Adsorption.. . . .. . . . . . . . . . . . . . . . . . . . . . 791
7.5.2 The Forces Involved in Organic Adsorption ...................... 792
7.5.3 Does Organic Adsorption Depend on Electrode Charge? .......... 793
7.5.4 The Examination of the Water Flip-Flop Model for Simple Cases of
Organic Adsorption ........................................... 797
7.5.5 At What Potential Does Maximum Organic Adsorption Occur? .. . . .. 798
Further Reading ...................................................... 801
7.6 Electrified Interfaces at Metals Other than Mercury. . . . . . . . 801
Further Reading ...................................................... 803
7.7 The Structure of the Semiconductor-Electrolyte Interface.. 803
7.7.1 How Is the Charge Distributed inside a Solid Electrode? ........... 803
7.7.2 The Band Theory of Crystalline Solids.. . . . . . . .. . . . . . . . . . . . . . . . . 804
7.7.3 Conductors, Insulators, and Semiconductors ..................... 806
7.7.4 Some Analogies between Semiconductors and Electrolytic Solutions. 8ll
7.7.5 The Diffuse-Charge Region inside an Intrinsic Semiconductor: The
Garrett-Brattain Space Charge ................................. 813
7.7.6 The Differential Capacity Due to the Space Charge. . . . . . . . . . . .. .. . 816
7.7.7 Impurity Semiconductors, n Type and p Type .................... 818
7.7.8 Surface States: The Semiconductor Analogue of Contact Adsorption. . . . . 821
7.7.9 Semiconductor Electrochemistry: The Beginnings of the Electrochem-
istry of Nonmetallic Materials .................................. 823
Further Reading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 823
7.8 A Bird's-Eye View of the Structure of Charged Interfaces. . 824
7.9 Double Layers between Phases Moving Relative to Each Other 826
7.9.1 The Phenomenology of Mobile Electrified Interfaces: Electrokinetic
Properties .................................................... . 826
7.9.2 The Relative Motion of One of the Phases Constituting an Electrified
Interface Produces a Streaming Current ........................ . 829
7.9.3 A Potential Difference Applied Parallel to an Electrified Interface Pro
duces an Electro-osmotic Motion of One of the Phases Relative to the
Other ........................................................ . 831
7.9.4 Electrophoresis: Moving Solid Particles in a Stationary Electrolyte .. 832
Further Reading ...................................................... . 835
7.10 Colloid Chemistry ...................................... .. 835
7.10.1 Colloids: The Thickness of the Double Layer and the Bulk Dimensions
Are of the Same Order ........................................ . 835
7.10.2 The Interaction of Double Layers and the Stability of Colloids ..... . 836
7.10.3 Sols and Gels ................................................ . 839
Further Reading ...................................................... . 841
Appendix 7.1 Measurement of the Electrode-Solution Volta Po-
tential Difference ................................ . 841
CONTENTS ix
CHAPTER 8
Electrodics
8.1 Introduction 845
8.1.1 The Situation Thus Far ....................................... . 845
8.1.2 Charge Transfer: Its Chemical and Electrical Implications ........ . 846
8.1.3 Can an Isolated Electrode-Solution Interface Be Used as a Device? 849
8.1.4 Electrochemical Systems Can Be Used as Devices ............... . 851
8.1.5 An Electrochemical Device: The Substance Producer ............ . 851
8.1.6 Another Electrochemical Device: The Energy Producer ........... . 855
8.1.7 The Electrochemical Undevice: The Substance Destroyer and Energy
Waster ...................................................... . 859
8.1.8 Some Basic Questions ........................................ . 861
8.2 The Basic Electrodic Equation: The Butler-Volmer Equation 862
8.2.1 The Instant of Immersion of a Metal in an Electrolytic Solution ..... 862
8.2.2 The Rate of Charge-Transfer Reactions under Zero Field: The Chemical
Rate Constant ................................................ . 865
8.2.3 Some Consequences of Electron Transfer at an Interface .......... . 868
8.2.4 What Is the Rate of an Electron-Transfer Reaction under the Influence
of an Electric Field? ......................................... . 869
8.2.5 The Two-Way Electron Traffic across the Interface ............... . 873
8.2.6 The Interface at Equilibrium: The Equilibrium Exchange-Current
Density io ................................................... . 876
8.2.7 The Interface Departs from Equilibrium: The Nonequilibrium Drift-
Current Density i ............................................ . 879
8.2.8 The Current-Producing (or Current-Produced) Potential Difference:
The Overpotential 1) •.•....••.••....•............•..•....•.•..• 880
8.2.9 The Basic Electrodic (Butler-Volmer) Equation: Some General and
Special Cases ................................................. . 883
8.2.10 The High-Field Approximation: The Exponential i versus 1) Law .. . 888
8.2.11 The Low-Field Approximation: The Linear i versus 1) Law ....... . 892
8.2.12 Nonpolarizable and Polarizable Interfaces ...................... . 894
8.2.13 Zero Net Current and the Classical Law of Nernst ............... . 897
8.2.14 The Nernst Equation ......................................... . 901
8.2.15 The Nernst Equation: Its Sphere of Relevance ................... . 906
8.2.16 Looking Back ................................................ . 908
Further Reading ...................................................... . 909
8.3 The Butler-Volmer Equation: Further Details. . . . . . . . . . . . . 910
8.3.1 The Need for a Careful Look at Some Quantities in the Butler-Volmer
Equation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 910
8.3.2 The Relation between Structure at the Electrified Interface and the Rate
of Charge-transfer Reactions ................................... 91 I
8.3.3 The Interfacial Concentrations May Depend on Ionic Transport in
the Electrolyte..................... . . . . . . . . . . . . . .. . . ... . . . . . . . . 916
8.3.4 What Is the Physical Meaning of the Symmetry factor {3? ........ . . 917
8.3.4a The Factor f3 Is at the Center of Electrode Kinetics ........ 917
8.3.4b A Preliminary to a Second Theory of f3: Potential-Energy-
Distance Relations of Particles Undergoing Charge Transfer 918
8.3.4c A Simple Picture of the Symmetry Factor. . . . . . . . . . . . . . . . . 922
x CONTENTS
8.3.4d Is the (3 in the Butler-Volmer Equation Independent of Over-
potential? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 926
8.3.5 Summing-up of Further Details on the Butler-Volmer Equation ... 928
Further Reading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 929
8.4 The Current-Potential Laws at Other Types of Charged Inter-
faces ..................................................... . 930
8.4.1 Semiconductor n-p Junctions ................................... 930
8.4.2 The Current across Biological Membranes ....................... 937
8.4.3 The Hot Emission of Electrons from a Metal into Vacuum ......... 942
8.4.4 The Cold Emission of Electrons from a Metal into Vacuum. . . . . . . . 944
Further Reading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 946
8.5 The Quantum Aspects of Charge-Transfer Reactions at Elec-
trode-Solution Interfaces ................................. 947
8.5.1 A Few Words on the Mechanics of Electrons ..................... 947
8.5.2 The Penetration of Electrons into Classically Forbidden Regions ... 950
8.5.3 The Probability of Electron Tunneling through Barriers ........... 953
8.5.4 The Distribution of Electrons among the Energy Levels in a Metal ... 956
8.5.5 Under What Conditions Do Electrons Tunnel between the Electrode and
Ions in Solution? .............................................. 959
8.5.6 The Tunneling Condition and the Proton-Transfer Curve. . . . . . . . . . 966
8.5.7 Electron Tunneling and the De-electronation Reaction ............. 973
8.5.8 A Perspective View of Charge-Transfer Reactions at an Electrode. . . 974
8.5.9 The Symmetry Factor (3: A Better View. . . . . . . . . . . . . . . . . .. . ... . . 975
8.5.10 Quantifying the Charge-Transfer Picture......................... 977
8.5.11 Some Desirable Refinements and Generalizations ................. 981
8.5.12 Surveying the Progress ......................................... 983
Further Reading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 986
8.6 Electrodic Reactions and Chemical Reactions ........... . 986
Further Reading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 989
Appendix 8.1 The Number of Electrons Having Energy EF Strik-
ing the Surface of a Metal from the Inside ....... 990
CHAPTER 9
Electrodics: More Fundamentals
9.1 Multistep Reactions ...................................... . 991
9.1.1 The Question of Multistep Reactions ........................... . 991
9.1.2 Some Ideas on Queues, or Waiting Lines ........................ . 992
9.1.3 The Overpotential 17 Is Related to the Electron Queue at an Interface 993
9.1.4 A Near-Equilibrium Relation between the Current Density and Over-
potential for a Multistep Reaction ............................. . 994
9.1.5 The Concept of a Rate-Determining Step ....................... . 997
9.1.6 Rate-Determining Steps and Energy Barriers for Multistep Reactions. 1002
9.1.7 How Many Times Must the Rate-Determining Step Take Place for the
Overall Reaction to Occur Once? The Stoichiometric Number v .... . 1004
9.1.8 The Order of an Electrodic Reaction ........................... . 1008
CONTENTS xi
9.1.9 Blockage of the Electrode Surface during Charge Transfer: The Sur-
face-Coverage Factor .......................................... 1014
Further Reading....................................................... 1017
9.2 The Transient Behavior of Interfaces ..................... 1017
9.2.1 The Interface under Equilibrium, Transient, and Steady-State Conditions 1017
9.2.2 How an Interface Is Stimulated to Show Time Variations ......... . 1019
9.2.3 Some Ideas on the Understanding of Transients ................. . 1020
9.2.4 Intermediates in Electrodic Reactions and Their Effects on Potential-
Time Transients .............................................. . 1026
9.2.5 Experimental Methods for the Determination of Partial Coverage, with
Adsorbed Entities, of the Surface of Electrocatalysts ............. . 1029
9.2.5a Radiotracer Method ................................... . 1030
9.2.5b Galvanostatic Transient Method ........................ . 1030
9.2.5c Potentiostatic Transients ................................ . 1032
9.2.5d The Potential-Sweep, or Potentiodynamic, Method ....... . 1033
Further Reading ...................................................... . 1035
9.3 Transport in the Electr.olyte Effects Charge Transfer at the
Interface ................................................. . 1036
9.3.1 lonics Looks after the Material Needs of the Interface ............ . 1036
9.3.2 How the Transport Flux Is Linked to the Charge-Transfer Flux: The
Flux-Equality Condition '" .................................... . 1038
9.3.3 Appropriations from the Theory of Heat Transfer ................ . 1040
9.3.4 A Qualitative Study of How Diffusion Affects the Response of an
Interface to a Constant Current ................................ . 1041
9.3.5 A Quantitative Treatment of How Diffusion to an Electrode Affects the
Response with Time of an Interface to a Constant Current ........ . 1044
9.3.6 The Concept of Transition Time ............................... . 1047
9.3.7 Convection Can Maintain Steady Interfacial Concentrations ...... . 1050
9.3.8 The Origin of Concentration Overpotential ..................... . 1052
9.3.9 The Diffusion Layer .......................................... . 1055
9.3.10 The Limiting Current Density and Its Practical Importance ....... . 1059
9.3.10a Polarography: The Dropping-Mercury Electrode ......... . 1060
9.3.10b The Rotating-Disc Electrode .......................... . 1070
9.3.11 The Steady-State Current-Potential Relation under Conditions of Trans-
port Control ................................................ . 1072
9.3.12 Transport-Controlled De-electronation Reactions ................ . 1074
9.3.13 What Is the Effect of Electrical Migration on the Limiting Diffusion-
Current Density? 1075
9.3.14 Some Summarizing Remarks on the Transport Aspects of Electrodics 1076
Further Reading ...................................................... . 1079
9.4 Determining the Stepwise Mechanism of an Electrodic Re-
action ................................................... . 1080
9.4.1 How One Tries to Determine the Reaction Mechanism ........... . 1080
9.4.2 Which Is the Rate-Determining Step in the Iron Deposition and Dis-
solution Reaction? ........................................... . 1083
9.4.3 The Transfer Coefficient a and Reaction Mechanisms ............ . 1089
9.4.4 Summarizing Remarks Concerning Mechanistic Studies ........... . 1090
Further Reading ..................................................... . 1093
9.5 More on Mechanism Determination ..................... . 1093
9.5.1 Why Review Mechanism Determination? ........................ 1093