S. Trasatti

Work function, electronegativity, and electrochemical behaviour of metals: III. Electrolytic hydrogen evolution in acid solutions

Summary The dependence of the exchange current for the electrolytic evolution of hydrogen on metals ( i 0,H ) on the work function () is analyzed on the basic of a new list offor polycrystalline surfaces. It is shown that log i 0,H is linearly related toirrespective of the detailed nature of the mechanism involved in the rate determining step (rds). i 0,H on sp metals depends on the sign of the surface charge. Results confirm previous suggestions that the main difference in double layer structure between sp and transition metals arises as a result of hindered rotation of water molecules on the latter. If the strength of the M−H bond is taken into consideration, then metals divide into two groups: (a) sp metals, with slow discharge at usual overvoltages, and probably slow hydrogen removal close to equilibrium, and (b) transition metals, with slow hydrogen removal as the rate determining step. Mn is anomalous and cannot be assigned to either class in correlations. Dependence of M−H bond strength onis also shown and discussed.

“Inner” and “outer” active surface of RuO2 electrodes

Abstract The dependence of the voltammetric surface charge q* on solution pH and potential scan rate has been investigated using a set of RuO2 electrodes prepared by thermal decomposition of RuCl3 at temperatures in the range 300–500°C. The systematically higher charge in KOH than in HClO4 in the same potential range (vs rhe) is attributed to the stabilization of higher oxidation states of surface Ru atoms in the alkaline environment. The variation of q* with v, the potential scan rate, is shown to be linearizable as a function of v 1 2 . It is thus possible to extrapolate the values of q* to v=0 and v=∞, respectively. The extrapolation enables an “inner” surface to be discriminated from an “outer” surface. The former is pointed out to be composed by the regions of difficult accessibility for the proton-donating species assisting the surface redox reactions. Reasons why the “screened” surface appears to be higher in alkaline than in acid solutions, are discussed. It is stressed that only working with a set of RuO2 electrodes prepared at different temperatures it is possible to discover meaningful correlations.

Electrocatalysis in the anodic evolution of oxygen and chlorine

Abstract Requisites for electrode materials to be suitable for technological applications are outlined and discussed. Oxides with metallic or quasi-metallic conductivity meet these requirements best. Most of these electrodes are prepared by thermal procedures. It is shown that the temperature of preparation affects the catalytic activity through the surface area and the chemical composition (non-stoichiometry). The co-variation of these parameters is best followed in situ by voltammetric curves and point of zero charge measurements. Examples are given for pure RuO 2 , IrO 2 , Co 3 O 4 and IrO 2 + RuO 2 mixtures. Kinetic and mechanistic details are discussed for O 2 evolution on RuO 2 , IrO 2 and Co 3 O 4 and for Cl 2 evolution on RuO 2 and Co 3 O 4 . Finally, the electrocatalytic properties of different oxides are correlated with the energy change involved in the lower → higher valency state transition. Experimental data for both O 2 and Cl 2 evolution can thus be organized into a “volcano” curve enabling predictive interpolations to be made.

PHYSICAL ELECTROCHEMISTRY OF CERAMIC OXIDES

Abstract After an introductory survey of the industrial needs and demands, the present targets of technological research in the field of ceramic oxide electrodes are outlined. Some fundamental properties of paramount importance are then examined and discussed. They include the mechanism of electrical conduction, the hydration of the oxide layer, its ability to exchange protons with the solution, the electrochemical characterization of the oxide surface, the relationship between degree of intimate mixing and synergistic effects for mixed oxides. The paper ends with an analysis of the factors responsible for the electrocatalytic activity of oxides, providing a clue to the understanding of a generalized mechanism of electrode reaction which can be established for most anodic and cathodic electrocatalytic processes.

Real surface area measurements in electrochemistry

Abstract Electrode reaction rates and most double layer parameters are extensive quantities and have to be referred to the unit area of the interface. Knowledge of the real surface area of electrodes is therefore needed. Comparison of experimental data with theories or of experimental results for different materials and/or from different laboratories to each other is physically groundless without normalization to unit real area of the electrode surface. Different methods have been proposed to normalize experimental data specifically with solid electrodes. Some of them are not sufficiently justified from a physical point of view. A few of them are definitely questionable. The purpose of this document is to scrutinize the basis on which the various methods and approaches rest, in order to assess their relevance to the specific electrochemical situation and, as far as possible, their absolute reliability. Methods and approaches are applicable to (a) liquid electrodes, (b) polycrystalline and single crystal face solids, (c) supported, compressed and disperse powders. The applicability of the various techniques to each specific case is to be verified. After an introductory discussion of the “concept” of real surface area, fifteen methods, eleven applied in situ and four ex situ, are scrutinized. For each of them, after a description of the principles on which it is based, limitations are discussed and recommendations are given.

Electrocatalysis: understanding the success of DSA®

Abstract The ‘legend’ of DSA ® (Dimensionally Stable Anodes), one of the greatest technological breakthrough of the past 50 years of electrochemistry, is reviewed with the aim to emphasise the reasons for their success. In this respect, the industrial success, which came first, was due to factors differing from those responsible for the successive boom of fundamental research on the same materials. This article scrutinises these factors highlighting the aspects which make these materials so much intriguing.

Work function, electronegativity, and electrochemical behaviour of metals

Summary Preferred values of , work function, and E z , potential of zero charge, for metals have been collected through a critical examination of much experimental data. It is shown, through three successive approximations, that the best relationship between the two above quantities is expressed by the equations: E z =−4.61–0.40 α, where α=(2.10− x M )/0.6 is defined as the degree of orientation of water at the interface. x M is the effective electronegativity derived from polots ofa gainst the chemical electronegativity given by Pauling. x M =0.50 −0.29 is the experimental relationship for sp metals (including alkali and alkaline earth metals, with the exception of Ga, Zn and Al). For the last three metals the relationship is x M =0.50 −0.55 x M is to be taken as 1.5 for all transition metals. α=1 for transition metals, and α=0 for gold, on whose surface water molecules do not present any preferential orientation. The value of x M for transition metals must be regarded as a particular interaction parameter between surface and water arising from additional phenomena of chemisorption. The problem of the orientation of water at the free surface of solutions and at the interface with metals is discussed in detail. Complete lists of effective electronegativities, electrochemical work functions (derived from electrochemical considerations), and potentials of zero charge calculated for those metals for which no experimental data are available have been compiled.

Ruthenium dioxide: a new electrode material. I. Behaviour in acid solutions of inert electrolytes

The electrochemical behaviour of RuO2 film electrodes, prepared by the thermal decomposition of Ru Cl3 on metallic supports has been investigated in solutions of inert electrolytes. Steady-state pot entiostatici/E curves, cyclic voltammetry and charging curves are presented. The procedure for the preparation of electrodes is described.RuO2 film electrodes exhibit oxygen overvoltages much lower than, and hydrogen overvoltages almost equal to, those on Pt electrodes. Results of measurements in the range of potential between oxygen and hydrogen evolution suggest that modifications arise which, while occurring within the whole film and not only at the surface, involve only a small fraction of the metal atoms. An explanation of this behaviour is advanced in terms of non-stoichiometry of the oxide layer.

Ruthenium dioxide-based film electrodes

Oxygen evolution from acid solutions has been investigated on two kinds of RuO2-based electrodes; compact and cracked (porous) films. The kinetic study has been carried out by means of potentiostatic curves. A method is suggested to estimate the surface concentration of active sites. The reaction order with respect to active sites and the activation energy have been determined. The mechanism is shown to differ on the two kinds of film in relation to theS(site)-OH bond strength and the surface concentration of intermediates. The behaviour of films on silica glass substrates is reported for the first time.

Electrochemical surface properties of Co3O4 electrodes

The electrochemical behaviour of Co3O4 layers deposited by thermal decomposition of Co(NO3)2 at 200–500°C on titanium supports with and without an interlayer of RuO2 has been studied by cyclic voltammetry, chronopotentiometry and potential step experiments in alkaline solutions. Such variables as the calcination temperature, the solution pH, the potential sweep rate and the oxide loading have been investigated in detail to determine their influence on voltammetric peaks and voltammetric charge. Insight has been gained into the relevance of the latter to surface area determination and to proton diffusion into the oxide layer. The role of the support-active layer interface and especially that of the RuO2 interlayer has been scrutinized. The importance of surface studies for the understanding of the electrocatalytic behaviour of Co3O4 electrodes has been analysed.