O. A. Petrii

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.

Potentials of zero total and zero free charge of platinum group metals

Some fundamental definitions and relations of the thermodynamic theory of the platinum electrode are considered. The notions of the potential of zero total charge (pztc) of the first and second kind are discussed. It is shown that in the presence of excess of surface-inactive ions, the potential of zero total charge of the second kind can be regarded as the potential of zero free charge (pzfc). The pzfc is an analogue of the pzc of metals not adsorbing hydrogen and oxygen. The methods of determination of the pztc and the pzfc of platinum metals, of their dependence on solution pH and salt concentration are considered, as well as the dependence of the pzfc on surface coverage with adsorbed hydrogen. Tables listing the pztc and the pzfc of platinum metals are given. The conditional nature of the notions pzfc and the charge of the double layer in the presence of strong chemisorption of ions is demonstrated and the difference between the formal and the true coefficients of charge transfer is emphasized. The notion of the charging curves and electrocapillary curves of the first and second kind is introduced and the methods of their finding are discussed. The electrocapillary curves of the first and second kind are given for platinum in solutions of different composition.

Electroreduction of nitrate and nitrite anions on platinum metals: A model process for elucidating the nature of the passivation by hydrogen adsorption

Abstract A study of the dependence of the electroreduction of NO 3 − anions on a platinized platinum electrode on the solution pH, the concentration of NO 3 − ions and the adsorption of Cs+, La3+ and Cd2+ cations and Cu adatoms was performed with the aim of elucidating the mechanism of the process and, in particular, the nature of the extremal character of the rate dependence on potential. For comparison, the stationary and potentiodynamic polarization curves for the electroreduction of NO 2 − anions on platinum and NO 3 − anions on palladium and rhodium electrodeposits were measured. The behavior of NO 3 − and NO 2 − anions on platinum was similar, and an increase in the electroreduction rate of NO 3 − anions during the transition from Pt and Pd to Rh was observed. The latter behavior was explained by assuming that the metal-nitrogen bond was stronger in the case of rhodium. Possible reasons for the passivation of cathodic processes on platinum when complete hydrogen surface coverage is approached are discussed.

Potentials of Zero Charge

The notion of the potential of zero charge (pzc) and the relevant term were introduced 50 years ago.(1) Later, the pzc was proved to be an important electrochemical characteristic of metal and to play a major role in electrocapillary and electrokinetic phenomena, electric double-layer structure, adsorption of ions and neutral organic molecules on the electrode, wetting phenomena, physico-chemical mechanics of solids, photoemission of electrons from metal into solution, and in electrochemical kinetics. The introduction of the notion of pzc led to solution of the Volta problem and to rigorous interpretation of the attempts to measure or calculate the “absolute” electrode potential. All this testifies to the fundamental nature of the notion of pzc.

ELECTROCATALYTIC ACTIVITY PREDICTION FOR HYDROGEN ELECTRODE-REACTION - INTUITION, ART, SCIENCE

Abstract The factors which determine the electrocatalytic activity of metals in the hydrogen reaction are described. The problems are formulated which arise when plotting the volcano relationship between the current density logarithm and the metal—hydrogen bond energy. Experimental data on exchange currents and electron work function W e were selected. It was shown that the scatter in electrochemical data and the absence of reliable results for metals with low W e are the deciding factors for plotting the correlation relationships. It was concluded that, to date, one can only suggest that there is a tendency for the exchange-current to increase with the increase in W e , without giving any quantitative relationships.

Effect of inorganic cations on the electroreduction of nitrate anions on Pt|Pt electrodes in sulfuric acid solutions

Abstract It is shown that modification of the surface of Pt|Pt electrodes by cations Ni 2+ , Co 2+ , Cd 2+ , and Ge 4+ accelerates the electroreduction of NO − 3 anions in sulfuric acid solutions in the potential region close to a complete coverage of the surface by H ads in the supporting electrolyte solution. The electrocatalytic effect of these surface modifiers increases in the following sequence: Ge>Ni>Co≈Cd and also depends on the conditions of adsorption of the cations and nitrate anions. It is shown that Ge adatoms promote the NO − 3 electroreduction most efficiently when they occupy only a part of the electrode surface, and the remaining part is occupied by adsorbed hydrogen. In the presence of Ni 2+ , Co 2+ , and Cd 2+ cations, ammonium ions are detected as the reaction product, while, in the presence of Ge adatoms, hydroxylamine is also found among the reaction products. Suppositions are made about the possible causes of the electrocatalytic effects observed.

Electrochemical properties of platinum and palladium electrodes in acetonitrile solutions

Abstract Differential capacity curves were plotted for renewed cross-sections of platinum and palladium electrodes under working solutions of perchlorate of lithium, sodium, tetraethylammonium and tetraborate sodium in acetonitrile. In dilute solutions of lithium perchlorate a minimum capacity on the potential of the platinum and palladium in acetonitrile double electric layer at the potential of zero charge. The low values of capacity and the small dependence of the capcity on the potential of the platinum and palladium in acetonitrile permits the assumption that strong chemisorption of molecules of the solvent takes place on the surface of platinum metals. This conclusion is substantiated by a comparison of the differences between the potentials of the zero charge and work function for different metals. Data were obtained on the adsorption of organic substances of different classes on platinum and palladium in acetonitrile.

On the charge transfer in the process of adsorption at the electrode/solution interface: Remarks on the paper “experimental determination and interpretation of the electrosorption valency γ” by J. Schultze and K. Vetter

A critical analysis shows that the approach to the estimation of the charge transfer in the process of adsorption at the electrode/solution interface used by Vetter and Schultze in the general case is not justified without introduction of non-thermodynamic models. This approach can prove useful in the investigation of the electric double layer within the limits of applicability of Grahames model. A number of cases can however be cited where the approach of Schultze and Vetter is inapplicable. The conclusions of Schultze and Vetter regarding the adsorption of hydrogen at platinum are apparently based on some misunderstanding. In this case the thermodynamic approach6, 23 should be used. It would be more correct to call the quantity γ “the formal coefficient of charge transfer”, rather than “the electrosorption valency”.

Nanoparticles of Pt hydrosol immobilised on Au support: an approach to the study of structural effects in electrocatalysis

The electrochemical properties of platinum hydrosol, obtained by the reduction of H2PtCl6 by sodium citrate (NaH2Cyt), were studied for the first time. Hydrosol particles of mean diameter dc≈2.0 nm, as estimated from UV–vis absorbance spectra, were immobilised on a Au support under anodic polarisation to form stable Ptcol/Au electrodes. High Pt coverages were found from a combination of cyclic voltammetry and scanning tunnelling microscopy data. Immobilised Pt particles (most probably, small uniform aggregates) with dc of about 4.5 nm and a spread of size distribution ca. 1.5 nm demonstrated pronounced adsorption of hydrogen and oxygen similar to pure Pt materials, high electrocatalytic activity with respect to formic acid oxidation as well as a relatively high tolerance towards CO poisoning.

Quantum chemical modelling of the heterogeneous electron transfer: from qualitative analysis to a polarization curve ☆

State-of-the-art in the field of quantum chemical modelling of the heterogeneous electron transfer processes is reviewed. Novel approaches originating from interplay between quantum chemistry and modern theory of charge transfer are discussed and illustrated by recent results on the calculation of relevant kinetic parameters for various electrochemical systems. Emphasis is made on the modelling of the inner-sphere reorganization and the works of approach, as well as on the consideration of reaction layer as an orientational ensemble of reagents. Recent approaches to the estimation of electronic transmission coefficient are analyzed. A possibility to employ traditional phenomenological theory to the analysis of experimental data is re-examined in the framework of microscopic treatment.