Electrochemistry Volume 10

A Review of Recent Literature

By D. Pletcher

The Royal Society of Chemistry

Copyright © 1985 The Royal Society of Chemistry
All rights reserved.
ISBN: 978-0-85186-087-9

Contents

Chapter 1 Adsorption at Solid Electrodes By P. J. Mitchell, N. A. Hampson, and A. J. S. McNeil, 1,
Chapter 2 Pitting Corrosion of Ferrous Alloys By C. Westcott, 85,
Chapter 3 The Electrochemistry of Conducting Polymers By G. K. Chandler and D. Pletcher, 117,
Chapter 4 Electron Transfer Reactions Studied Using Pulsed High Energy Radiation By J. Grimshaw, 151,
Chapter 5 Organic Electrochemistry By J. B. Kerr, 171,

CHAPTER 1

Adsorption at Solid Electrodes

BY P. J. MITCHELL, N. A. HAMPSON, AND A. J. S. McNEIL

1 Introduction

It is our intention to write a review of the literature on adsorption at solid electrodes as it affects the technology of electrochemistry. This subject is relatively clear at the smooth solid electrode, and following on the work of the well-known pioneers relatively simple ideas of inner layer and diffuse layer structure, broadened out by concepts of physical adsorption, specific adsorption, and chemisorption, are generally sufficient to describe most of what might be termed the thermodynamic behaviour. Parsons has recently commented upon this area and these comments, written at the time that we began to tackle the emerging literature, formed a foundation on which our scholarship could develop. The Parsons paper, together with a subsequent article written from a slightly more technical view put into perspective the relevant fundamental work on solid metal electrodes of crystallographic uniqueness. The main features of this work have been foreshadowed by other scientifically less satisfactory work carried out in the 1960s and 1970s and it is useful to electrotechnologists, as well as electrochemists, briefly to review these studies.

The first problems to be solved were concerned with the purity of materials. The purity of electrodes has been completely solved by the use of such techniques as zone refining, electrolysis, and various vacuum and melting techniques developed for the semiconductor industry as well as for LEED studies of metal surfaces. The metal surface must be structurally well defined as well as pure, and this has necessitated the preparation of single crystal electrodes with crystallographically defined planes exposed. Moreover, the surface must retain its unique identity under the influence of the electrolyte solution. The primary hydration of the metal electrode surface (with or without specific adsorption) is a spontaneous process and there is clearly a chance that this surface hydration energy may cause some reorganization of the electrode surface. Thus a conflict of desirable properties exists; too low a melting point results in an electrode surface which is liable to reorganization by hydration; high melting point refractory metals are more difficult to process. This has resulted in the modern view of the characteristics of solid metals being almost completely established on the metals Cu, Ag, Au, and Pt.

On the electrolyte solution side pre-polarization techniques removed the ionic impurities from solution but were largely ineffective with the non-ionic ones. Adsorption of impurities on to charcoal 3 or some highly porous active surface (e.g. platinum sponge) was the solution to this problem. The combination of these two methods generally suffices to produce the ultra-pure solution demanded to complement the electrode preparation.

2 Adsorption at the Solid Electrode

It has been well established that at each plane of the single crystal electrode a unique double layer structure exists. This has been quantitatively demonstrated in the case of silver where different low index planes exhibit different potentials characteristic of zero charge. This important difference has been confirmed for copper and gold. Valette and Hamelin have further discussed the important consequence of this difference and demonstrated that a polycrystalline electrode exhibits a minimum capacitance in dilute solution near to the pzc of the lowest charge density plane, which will be the one with the most negative pzc. The system is an extremely complicated one, even for an electrode with the simplest double layer structure. What has been done to analyse the data by the established techniques for silver monocrystals indicates an inner layer capacitance, like that on mercury, which is independent of concentration but with a peak amounting to a maximum value of 120 µF cm-2 positioned close to the pzc (the hump), probably explained by the process of reorientation of water molecules adjacent to the metal surface. This view is confirmed by the close fitting of the extrapolated inner layer capacitance curve with theoretical models. The re-orientation of the surface water at such electrodes has not yet been satisfactorily confirmed.

The effect of specifically adsorbed anions at crystallographically unique plane silver electrodes has been studied in detail and yields interpretable results for the case of chloride ions. Three peaks in the differential capacitance curves occur at low (10%), medium, and almost complete coverage. The middle peak corresponds to the usual adsorption effect. The narrow positive peak is due to the onset of chloride penetration to the inner layer water, and the most negative peak marks the complete discharge in the monolayer adsorbed on silver. Thus the general characteristics of the adsorption of Cl- on low index planes on Ag can be understood and extended to other face centred cubic metals. On higher index planes the behaviour may be successfully approximated to a combination of those of the low index steps and planes which go to make the whole surface.

The chemisorption of species at electrodes which involves the complete electron transfer to form a bond has special importance in the hydrogen–platinum system. Since Will made the original suggestion that different planes, (110) and (100), each contribute characteristic adsorption peaks in the voltammogram, other workers have confirmed that this indeed is so, but surface and experimental control were so difficult that quantitative agreement between investigations has never been demonstrated satisfactorily until relatively recently. Clavilier et al. have shown specific voltammograms characteristic of each of the (111), (110), and (100) surfaces. The results of the other workers can be discussed in relation to the Clavilier results and a measure of unification can be obtained. An interesting point here is that the (111) electrode clearly showed evidence of surface reorganization if the potential range included the formation and removal of the oxide layer.

It is clear therefore that by 1980 a high level of success had been obtained using very pure systems involving well-characterized electrodes of face centred cubic metals such as Pt and Au. This has served to emphasize the very great complexity of polycrystalline electrodes; indeed, bearing in mind the need for a generalized treatment for monolayers at uniform electrode surfaces, a theoretically based description of adsorption at a polycrystalline electrode appears to be beyond the present state of the subject. In view of this, in our review we intend to concentrate on the technological aspects of adsorption at solid metals although theoretical aspects will be briefly treated inasmuch as papers published since the beginning of 1980 are noted; an in-depth review in this area is too large a task at present.

On the basic theory level Mohilner et al. have discussed the concept of congruence or non-congruence of electrosorption with respect to the electrical variable. They showed in the first contribution that congruence is both a necessary and sufficient condition that the activity coefficients of the adsorbed species in the inner layer are independent of the magnitude of the electric field there. The theory of non-congruent electrosorption of organic compounds proposed by Mohilner was shown to be quite general and an expression for the electrosorption isotherm, expressed as a function of the excess electrochemical free energy of mixing of the inner layer, was derived. Moreover, the general theory of differential capacitance in the case of organic electrosorption was derived on the basis of the non-congruent electrosorption. It was shown that the traditional method of calculating electrosorption isotherms from differential capacitance is incorrect. Tests were proposed for extrapolation to zero frequency. Parsons has calculated the contribution to the capitance of an electrode from a species adsorbed with partial charge transfer. A simple model was proposed in which the degree of charge transfer changed rapidly with potential and as such was likely to account for some of the sharp peaks observed experimentally.

Rangarajan et al. have derived two- and three-state models for the adsorption of organics and it is shown how these can be understood at the molecular level. New isotherms are provided for three molecular descriptions.

There have been two important reviews during our review period. Laviron has reviewed (257 references) the voltammetric methods used for the study of adsorbed species and Rangarajan has reported (304 references) much more generally on the double layer. The latter review gives an up-to-date account of the concepts underlying the solvent structure of the interphase and the various theories of adsorption and the reader is referred to this article for the theoretical background to the present review. The Laviron article is effectively a complementary contribution to that of Rangarajan and emphasizes that adsorption is necessary for electrodic transformation. Again the treatment is physicochemical rather than electrotechnological and in view of this excellent treatment it is intended here to review only the generalities of adsorption.

A brief mention here of some of the more outstanding theoretical papers published during the last four years is justified on the grounds that it is necessary to form a link between the highly developed theory of adsorption and the profound effect of adsorption on electrode kinetics.

Myamlin and Krylov have obtained an expression for the adsorption of charged and neutral species on the surface of an energetically non-uniform metal electrode. In a following contribution, the same authors consider the simultaneous adsorption of two sorts of species. The work provides a method for finding the mechanism of complex formation from the experimental data for the adsorption isotherms. For the dual particle adsorption, two types of adsorption site are assumed present on the electrode, each characterized by a particular value of adsorption energy. Equations for the adsorption isotherms are obtained using statistical combinations. For the case of uniformly inhomogeneous electrode surfaces, adsorption isotherms are established and analysed. Another paper describes procedures for calculating the adsorption parameters for the case of two-dimensional adsorbate condensation at solid electrodes. The target was to refine the calculation procedure for the case of a non-uniform surface. Specifically the adsorption parameters of camphor on bismuth were calculated from differential capacitance curves. Capacity curves were calculated on the basis of a segmented electrode consisting of six equal areas. The results obtained indicated that although this technique was satisfactory for use at a liquid electrode, it did not necessarily apply at a solid electrode.

The non-local electrostatic approaches to interphasial structures has been reviewed by Russian authors. In this method electric interactions are described using the methods associated with plasma physics and solid state theory. This review is interesting but does not contribute much to the technology.

There are a number of other papers which warrant a brief mention. Karolczak in two preliminary papers considers generalized adsorption at electrodes. Of the recently published equations for adsorption equilibria at electrodes, two general equations are compared which differ in the physical meanings which are implicit in the respective meaning of the surface coverage, θ, and the ratio of the partial molar areas, ‘n’. The contributions illustrate the deductions that can be obtained from the interdependence of the measurable adsorption characteristics on the adsorbate coverage although, in common with other interphasial problems, other interpretations may exist. In general, results must be compared with calculations from the proposed model of the interphasial structure, and good agreement between the two is generally taken as validation for the correctness of the argument. Damaskin et al. analysed the energetic and geometrical characteristics of the inner part of the electrical double layer in the presence of specific adsorption of ions arising from the change in dielectric properties and dimensions of the inner layer. For the specific adsorption of tetra-alkylammonium cations on Bi in ethanol and in aqueous solution, good agreement between experiment and the appropriate theory involving the Frumkin isotherm and the values of the parameters was obtained.

The differential capacitance curves associated with organic adsorption have recently been discussed in detail by Damaskin and co-workers. Congruence of adsorption isotherms with respect to the charge or the potential implies a linear relationship between θ and either the potential or the charge q. For the latter condition, the characteristics of differential capacitance curves are described for the adsorption of an organic particle. For the adsorption of organics at constant electrode potential, Damaskin and Karpov 46 have analysed the effect of the diffuse layer on the form of the isotherm and the energetics of adsorption of organics. The model approach applied to the adsorption of organics has been explored in detail and formulae for the calculation of the differential capacitance curves have been put forward. The theory predicts flat minima at high negative charges, arising via the diffuse structure of the double layer, and these have been verified on the liquid metal Hg and the low melting Bi. Moreover computer calculations yield good agreement although these calculations demanded the use of unrealistic interaction parameters.

The behaviour of a system of interacting adsorbed organic molecules present on the surface of an electrode in such a way that two orientations are possible has been studied by Kharkats. With attraction constants defined for different orientations the relationship between θ and concentration is established. Depending on the relative values of the isotherm parameters, one or two reorientation transitions can be realized in the system. Qualitative similarities exist for the adsorption of bipyridine isomers with the behaviours predicted by the author.

For the case of the co-adsorption of two organic substances Nesterenko et al. have considered the differential capacitance–potential relationships. The effect of the adsorption coefficients of the individual substances on the form of the differential capacitance–potential curve is analysed in detail and it is clear from the interactions that a wide and differing range of behaviours is possible. It is clear from the theoretical papers concerned with the interphasial structure that this area of understanding is far from complete. From the electrotechnologist’s viewpoint this is not likely to be a deterrent to his endeavours to achieve the desired modification to electrode reactions, usually brought about by what are generally referred to as solution additives.

A major area for the electrotechnologist is the inhibiting effect of organic additives. Guidelli et al. have produced a theoretical treatment of the inhibiting effect of neutral organic surfactants at high surface coverages on simple electrode reactions. The authors assume that the activated complex is specifically adsorbed and use the absolute reaction-rate theory applied to a system in which the surfactant is adsorbed under equilibrium conditions. A statistical treatment of different models leads to an expression for the ratio of the rates of the inhibited to the uninhibited reaction. The inhibitory effect of aliphatic alcohols on the kinetics of the electroreduction of Cd2+ and Cu2+ is examined in order to verify the general relationships which arise from the theory. The suggestion is that the ion-dipole interactions between the charged activated complex and a neighbouring water molecule are changed as we pass from a solvent-covered electrode to a surfactant-covered one, and this is responsible for the inhibition. Damaskin and Safanov have calculated the inhibition parameters at various degrees of electrode coverage for the cadmium amalgam/cadmium(II) reaction using a method of least squares. It was shown that the data treatment did not give an unequivocal choice between a relationship containing three fitting parameters

ln(kθ/k01)=ln(1 — θ)-S1θ – S2θβ (1)

and one containing only two inhibition parameters

ln(kθ/k0) = r ln(1 – θ) – Sθ (2)

The most interesting effect of adsorption to the technologist is that on the kinetics of reaction. A number of important theoretical papers have appeared in this area over the last few years.

(Continues…)Excerpted from Electrochemistry Volume 10 by D. Pletcher. Copyright © 1985 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
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