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 E_f 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 E_f from 0.5 to 2.0 V. In the intervals 0.5 £ E_f £ 1.5 V and 1.8 £ E_f £ 2.0 V the current density of HER, with a constant cathodic potential, decreases exponentially with increasing E_f (and, therefore, with increasing d), and in the E_f 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 E_f 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).