Variation of the electrochemical transfer coefficient with potential

Abstract

The electrochemical electron-transfer rate constant has been determined as a function of the electrode potential for a series of simple electron-transfer processes to organic molecules in media containing acetonitrile or dimethylformamide and a quaternary ammonium salt as supporting electrolyte and using mercury as the electrode material. The reactions and the experimental conditions were selected so as to deal with outer-sphere processes and to minimize the magnitude of double-layer corrections. Convolution potential-sweep voltammetry and the impedance method were used for obtaining the kinetic data. Under these conditions, the electrochemical transfer coefficient was observed, in all cases, to vary, beyond experimental error, with the electrode potential. The magnitude of the variation is of the same order of magnitude as that predicted by the Marcus theory of outer-sphere electron transfer. A more complex reaction, the reduction of benzaldehyde in ethanol, involving dimerization steps following the initial electron transfer was also investigated. A definite variation of the transfer coefficient was again observed. This behaviour, observed for various solvents and functional groups, appears as a general phenomenon in the reduction of organic molecules in the case where charge transfer is fast and mainly governed by solvent reorganization.The present theories of electron transfer at electrodes,' such as the Hush-Marcus theory,2 -4 predict that the electrochemical transfer coefficient should vary with the electrode potential. Being based on an harmonic approximation they imply a quadratic dependence of the activation energy and therefore a linear dependence of the electrochemical transfer coefficient upon the electrode potential. They also predict the magnitude of the variation of the transfer coefficient as a function of the reorganization factor. The smaller the reorganization factor, i.e. the faster the electron transfer, the larger the variation of the transfer coefficient with potential.Over the last 15 years there have been several attempts to detect such variations experimentally and to compare their magnitude with that anticipated from the Earlier work5-11 in this area has not provided a clear answer to the question thus raised. The same systems that were first regarded as exhibiting such a potential dependence were later, after more accurate analysis, shown not to give rise to a definite variation that was clearly greater than experimental error.7 In order to obtain acceptable evidence that the transfer coefficient does or does not vary with the potential, the system under study must fulfil several requirements. The first of these is that the electrochemical reaction should follow a simple mechanism, preferably involving a single one-electron step giving rise to a chemically stable species, at least in the time range of the experiments. This does not mean that it is impossible to investigate the potential dependence of the transfer coefficient in more complex processes, involving for example follow-up ch...

Mathematical Modeling of Electrode Processes – Potential Dependent Transfer Coefficient in Electrochemical Kinetics

“…The significance of the discussed problem is confirmed by simple calculations, based on relevant literature data (Angell & Dickinson, 1972;Garreau et al, 1979;Savéant & Tessier, 1982;Savéant & Tessier et al, 1977), which leads to the conclusion that ETC variability effect is usually 5-10 times stronger than the double layer correction effect (see also Sanecki & Skitał, 2007a). …”

Beyond the Butler–Volmer equation. Curved Tafel slopes from steady-state current–voltage curves

“…The plot is restricted to data that are one order of magnitude greater than the background capacitive charging current (~1 nA), and one order of magnitude less than the diffusion limited current (~81 μA). 6 ] 3+ at a random assembly of carbon micro-disc electrodes. Error curve for the difference between experimental data and the linear line of best fit.…”

“…Such a phenomenon is implied by the weak dependence of the symmetry factor on overpotential [6][7][8]. However, for more than fifty years, attempts to detect this curvature have been frustrated by the labyrinthine complexity of the experiments required.…”

“…For example, Weaver and Anson measured the electrochemical reduction rates of three complexes of Cr(III) over a wide potential range, using a combination of chronocoulometry and dc polarography [12]. The three complexes studied, [Cr(H 2 O) 5 6 ] 3+ all supposedly followed outer-sphere electron transfer mechanisms, but even after applying diffuse double-layer corrections of various magnitudes the authors still could not observe any significant dependence of the symmetry factor on applied potential. The reason for this negative result remains a mystery.…”

“…The hexamminecobalt(III)/(II) redox couple was also of interest because the speciation of cobalt ammine complexes had recently been determined by Ji et al as a function of the concentration of free ammonia [17]. Their results indicated that essentially pure [Co(NH 3 ) 6 ] 3+ was present when the concentration of free ammonia exceeded 10 −2 M. This was doubly convenient because an excess of ammonia in aqueous solution also strongly suppressed the non-specific adsorption of cobalt ammine complexes on carbon electrodes at negative potentials, due to the presence of excess ammonium ions;…”