EFFECT OF ANODISING ON THE KINETICS OF THE HYDROGEN EVOLUTION REACTION ON COBALT SILICIDES IN SULPHURIC ACID SOLUTION
Abstract
The effect of the anodizing of Co2Si and CoSi2 electrodes in 0.5 M H2SO4 with the potentials of oxide formation Ef from 0.4 up to 2.0 V (standard hydrogen electrode) on the kinetics of hydrogen evolution reaction (HER) in 0.5 M H2SO4 at ambient temperature was studied. It has been discovered that the behavior of the anode oxide on cobalt silicides with low and high silicon content significantly differ: the oxide films on Co2Si obtained for all the Ef studied are cathodically reduced, but the oxide films on CoSi2 (close to SiO2) are retained in the cathodic region. Polarization curves were obtained for HER on CoSi2 with the thickness d of the oxide film remaining practically unchanged during the measurements. The rate of HER on CoSi2 decreases by ~ 2.5 orders of magnitude with increasing Ef from 0.5 to 2.0 V. In the intervals 0.5 £ Ef £ 1.5 V and 1.8 £ Ef £ 2.0 V the current density of HER, with a constant cathodic potential, decreases exponentially with increasing Ef (and, therefore, with increasing d), and in the Ef region from 1.5 to 1.8 V there is a delay in the change of current density i. The linear dependence of lni on Ef is explained in the framework of the mechanism of direct tunneling of electrons from the conduction band of cobalt disilicide to the unoccupied states of the redox system in the electrolyte. The delay in the change of the HER current at the potentials of formation of the oxide from 1.5 to 1.8 V, which correspond to the beginning of the transpassive region for CoSi2, is explained by the development of an additional tunneling mechanism through the intermediate states, whose role can be played by the oxygen vacancies generated at the silicide/oxide interface upon transition from the passive to the transpassive state.
Based on impedance measurements on an anodized CoSi2 electrode in 0.5 M H2SO4, it was concluded that the cathodic hydrogen evolution at sufficiently low electrode potentials occurs via the discharge-recombination mechanism with the limiting discharge step (electron transfer to protonated silanol groups ºSi-OH2+ on the surface of the oxide film).
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