B.E. Conway

Modern Aspects of Electrochemistry

This book focuses on topics at the forefront of electrochemical research. Splitting water by electrolysis; splitting water by visible light; the recent development of lithium batteries; theoretical approaches to intercalation; and fundamental concepts of electrode kinetics, particularly as applied to semiconductors are discussed. It is recommended for electrochemists, physical chemists, corrosion scientists, and those working in the fields of analytical chemistry, surface and colloid science, materials science, electrical engineering, and chemical engineering.

The role and utilization of pseudocapacitance for energy storage by supercapacitors

Abstract The principle of utilizing the non-Faradaic double-layer capacitance of electrode interfaces as a means of storing electrical energy was suggested and utilized in technologies initiated some 37 years ago. However, only over the last ten years has major interest been manifested in commercial development of this possibility in so-called ‘supercapacitors’ or ‘ultracapacitors’ based on the large double-layer capacitance achievable at high-area, carbon powder electrodes. In parallel with the utilization of double-layer capacitance is the possibility of use of the large pseudocapacitance that is associated with e.g. electrosorption of H or metal adatoms (underpotential deposition) and especially some redox processes. Such pseudocapacitance arises when, for thermodynamic reasons, the charge q required for progression of an electrode process, e.g. electrosorption or conversion of an oxidized species to a corresponding reduced species in liquid or solid solution, is a continuous function of potential, V ; then the derivative d q /d V corresponds to a capacitance but one of a Faradaic kind. This behavior is different from that with an ideal battery where, according to the Nernst equation, V is invariant with state-of-charge measured by q Various experimental examples are shown and characterized, especially that for RuO 2 and other transition metal oxides. Additionally, electroactive polymers such as polyaniline exhibit analogous pseudocapacitative behavior.

Interfacial processes involving electrocatalytic evolution and oxidation of H2, and the role of chemisorbed H

States of chemisorbed H that can be involved in the mechanism of the cathodic H2 evolution reaction (HER) at metals, specially Pt, and the reverse oxidation reaction (HOR), are examined. Particular attention is paid to possible differences between underpotential-deposited (UPD) H, electrosorbed at potentials below the hydrogen reversible potential, and so-called overpotential-deposited (OPD) H that is involved, kinetically, as an intermediate in the HER. UPD H is involved in the HOR, following dissociative chemisorption of H2, at potentials positive to the RHE potential. The roles of diffusion-control involving H2, both in the HER and the HOR, are discussed in the light of experimental results for cathodic and anodic polarization. Values of exchange current-density, jo, for the HER/HOR at equilibrium depend widely on the adsorptive and electronic properties of the metal, often representable as a ‘volcano’ relation with respect to metal-to-H bond energy. The origin of such relations, especially at the catalytic noble metals, is examined in terms of UPD and OPD states of H. Mechanistic and kinetic aspects of the HER and HOR are treated and their sensitivity to presence of CO and to surface structure at Pt single-crystal faces is reviewed.

The real condition of electrochemically oxidized platinum surfaces: Part I. Resolution of component processes

Summary The problem that 3 peaks can be resolved in the initial stages of Pt surface oxidation below a monolayer of electrosorbed OH (1 e per Pt atom), even on single crystal faces, is examined. A high degree of solution purity is required for this resolution to be observed. Reversibility with respect to these stages in the formation and reduction of the surface oxide layer has been evaluated. The stages of surface oxidation below the OH monolayer limit are related to the geometries of various sub-lattices of OH on the Pt surface rather than to specific stoichiometries of platinum-oxygen surface compounds. It is shown that the cathodic reduction profile can also be resolved into reversible and irreversible components depending on end potential in the anodic direction of polarization and on temperature. Low temperature measurements enable complete resolution of a fast reversible component from an irreversible one, associated with the normal main cathodic peak in Pt surface oxide reduction. The irreversible component arises from a rearranged form of the surface oxide and is formed already to a significant extent well below a monolayer of “PtOH”. It is reduced at less positive potentials and gives rise to the characteristic hysteresis between oxide formation and reduction processes at Pt. The origin of the shape of the i-V profile at Pt and other noble metals is explained. Optical studies by means of ellipsometry show that in pure solutions and with sensitive instrumentation there is no distinguishability, with respect to potential dependence, between the change of optical properties and the change of the surface coverage determined by surface coulometry down to the lowest coverages that can be detected. The surface oxidation and reduction is shown, however, to be optically irreversible with respect to degree of surface oxidation so that a real change of condition of the surface arises between anodic and cathodic directions of polarization.

Elementary steps of electrochemical oxidation of single-crystal planes of Au Part II. A chemical and structural basis of oxidation of the (111) plane☆

A detailed experimental and mechanistic analysis of the elementary surface-chemical steps involved in the beginning stages of electrooxidation of Au is given with respect to the processes that occur on the (111) plane. At Au, specific adsorption of anions, even oxyanions such as ClO−4 and SO2−4 or HSO−4, plays a major role in the initial stages of oxidation of the metal. In the case of the (111) plane investigated here, the symmetry of the surface lattice has the strongest effect on the specific adsorption of the above anions compared with behaviour at other planes. This is connected with the matching symmetry of the tetrahedral ions with the trigonal symmetry of an unreconstructed (111) plane. The results indicate that the hydrated anions, which are found to be partially or fully discharged on the metal surface, form overlay-lattice arrays. This network of hydrated anions blocks the surface, preventing the formation of sublattices of OH discharged on the free surface of the metal but allows partial discharge of water molecules of the hydration shells of the ions in two energy states in a fast upd reaction. This provides the path for the initial stages of 2-dimensional oxidation of the surface. The resulting network of anion-OH(1−γ)− complexes stabilizes the surface, preventing the turn-over process which is possible only after, or coupled with, the desorption of the anions in an anion replacement, MOH turn-over process. This is possible when a potential is reached at which partly discharged OH(1−γ)− in complexes with anions can become fully discharged, forming free MOHs which are no longer part of the complex. In this way, the network is destroyed and the stabilization of the anions by the H-bonds of hydration H2O or partially discharged OH no longer remains, allowing the anions to be desorbed and processes of phase-oxide development to begin.

ac Impedance of Faradaic reactions involving electrosorbed intermediates—I. Kinetic theory

The evaluation of the electrochemical adsorption behaviour of chemisorbed intermediates generated in a multi-step Faradaic reaction at appreciable net currents is important for understanding the reaction mechanism of the overall process involved. Measurement of ac impedance of the reaction at controlled potentials provides an important experimental route to the required information about the “overpotential-deposited” ad-atom species. Interpretation of the measurements however, requires further examination. Based on an extension of Armstrongs treatment, it is shown that interpretation of ac impedance measurements directly in terms of the components of an intuitively assumed equivalent circuit is rarely correct; only in the case of underpotential-deposition of an ad-species, where no continuous Faradaic currents pass, in such an approach satisfactory. Kinetic analysis is given for the behaviour of a multistep process with examples from the cathodic H2 evolution reaction where electrochemical and H-recombination desorption pathways are involved. The kinetic analysis enables the steady-state adsorption pseudocapacitance, Cφ, for H to be evaluated as a function of overpotential. Its behavior is clearly distinguishable from the quantity Cp commonly written as the pseudocapacitance element in the equivalent circuit for this type of reaction.

Elementary steps of electrochemical oxidation of single-crystal planes of Au—I. Chemical basis of processes involving geometry of anions and the electrode surfaces

Single-crystal planes of Au electrodes in the (111), (100) and (110) orientation were studied by the potentiodynamic sweep method in solutions of anions which adsorb weakly (HClO4) and strongly (H2SO4) on Au. It is shown how the specific adsorption of anions which appear to be fully discharged on the (111) and (110) planes, but very little or not at all on the (100) planes, depends on the symmetry of the arrangement of surface atoms in relation to the three-fold geometry of the tetrahedral anions. This in turn determines the type and extent of a coordinative deposition (UPD) of OH in between adsorbed anions, which seems to occur with only partial charge transfer (γ = 0.5). The resolution of the processes of OH deposition in sub-lattices amongst the adsorbed anions is found to be specific for the three low-index planes examined and is interpreted in terms of the states of adsorption of the anions in relation to lattice geometry and charge transfer. The adsorbed anions, by having a stabilizing effect on the MOH(1−γ)-species on the surface, influence also the kinetics of the process of replacement of adsorbed anions by deposited OH on the surface and the concerted M/OH turn-over process which constitutes the beginning stage of formation of bulk-phase oxide material on the electrode.

The role of ion adsorption in surface oxide formation and reduction at noble metals: General features of the surface process

Abstract Some important general features of the processes of electrochemical surface oxidation of noble metals are identified and related to effects of specific adsorption of anions. The first three stages in metal oxidation occur generally in successive overlay lattices which arise on account of repulsion between metal-oxygen dipoles; they are reversible on metals, such as Ru, Ir, having little tendency to adsorb anions or on Pt in alkaline solutions where anions other than OH − are absent. Irreversibility increases with oxide coverage, but the monolayer film always shows hysteresis due to place-exchange. The next stage of oxidation occurs over a single broad region, with hysteresis between its formation and reduction, decreased by anion adsorption. The anion effects arise on account of: (a) adsorption competitive with OH/O deposition which blocks initial stages of oxidation (Pt in acid or at Au); (b) change of inner-layer field which modifies the field-assisted place-exchange process and (c) lateral repulsion with metal-oxygen dipoles, which facilitates place-exchange (Au and Pt in acid). Cations, on the contrary, stabilize the unrearranged metal-oxygen dipoles, as is shown by the behaviour of Au in Ba(OH) 2 solution. Studies on the reduction kinetics at Au distinguish one reduction stage, which is a simple surface charge-transfer process, from a second, which is preceded by a slow chemical step, probably reverse place-exchange.

Behavior of Molybdenum Nitrides as Materials for Electrochemical Capacitors Comparison with Ruthenium Oxide

Ruthenium oxide (RuO{sub 2}), formed as a thin film on a Ru or Ti metal substrate, exhibits a large specific (cm{sup {minus}2}) and almost constant, electrochemical capacitance over a 1.35 V range in aqueous H{sub 2}SO{sub 4}. This behavior has led to its investigation and use as a material for fabrication of supercapacitor devices. However, its cost has encouraged search for other materials exhibiting similar behavior. Work reported in the present paper evaluates two nitrides of Mo, Mo{sub 2}N and MoN, as substitutes for RuO{sub 2}. It is shown that very similar capacitance behavior to that of RuO{sub 2} films arises, e.g., in cyclic voltammetry and dc charging curves; in the former, almost mirror-image anodic and cathodic current-response profiles, characteristic of a capacitor, arise. However, the nitride materials have a substantially smaller voltage operating range of only some 0.7 V due to electrochemical decomposition above ca. 0.7 V vs. RHE. This limits their usefulness as a substitute for RuO{sub 2}. Of interest is that the nitride films exhibit potential-decay and potential-recovery on open circuit after respective charge and forced discharge. The decay and recovery processes are logarithmic in time, indicating the role of internal faradaic charge redistribution processes.

Computer simulation of the kinetic behaviour of surface reactions driven by a linear potential sweep: Part I. Model 1-electron reaction with a single adsorbed species*

Abstract The kinetic behaviour of a simple 1 e surface reaction is treated theoretically, especially with regard to (a) characteristic aspects of its behaviour when significant attractive or repulsive interactions arise in the electrodeposited film and (b) formulation of characteristic features of its kinetic and equilibrium behaviour which can provide a reference case for distinguishing, by comparison, the behaviour of other more complex sequential reactions treated in Part II.