THE KINETICS OF CATHODIC PROCESS OF HYDROGEN EVOLUTION ON COBALT DISILICIDE ANODICALLY OXIDIZED IN 0.5 M H2SO4 AT HIGH POTENTIALS
Abstract
The effect of the anodic oxidation of CoSi2 electrode in 0.5 M H2SO4 at the potentials of oxide formation Effrom 2.0 up to 4.0 V (vs. standard hydrogen electrode) on the kinetics of hydrogen evolution reaction (HER) in 0.5 M H2SO4 at 22oC was studied. The secondary passivation of cobalt silicide occurs at E > 2 V, and the oxygen evolution reaction (OER) on the oxidized electrode surface is possible at E > 2.2 V.
Oxide films were formed by potentiostatic polarization for one hour. The cathodic reaction at oxidized silicide was studied in deaerated sulphuric acid solution using polarization and impedance (100 kHz – 0.01 Hz) measurements.
At low polarizations, the cathodic polarization curves have rather high slope, then the slope decreases down to 0.09–0.10 V/decade (nearly the same slope was obtained for the non-oxidized electrode), and at sufficiently high polarizations the slope increases again. The current densities for anodically oxidized CoSi2 are about 2 orders of magnitude lower than ones for the non-oxidized electrode. As the capacitance measurements evidenced, each cathodic polarization curve corresponds to approximately constant thickness of oxide film, i.e. the oxide on cobalt disilicide (compositionally close to SiO2) is stable in the cathodic region. The following peculiarities of the cathodic polarization curves for the electrodes oxidized at Ef > 2 V were revealed: (i) the tendency towards increasing the rate of cathodic reaction with Ef; (ii) fairly weak dependence of the kinetics of cathodic reaction on Ef value.
The experimental results were explained on the assumption that the cathodic process on oxidized silicide occurs at the oxide/solution interface with the participation of electrons tunneling through the oxide film. Fairly weak dependence of the rate of cathodic reaction on Ef suggests that there are several factors acting in opposite directions. Increase in oxide film thickness with Ef is a factor that slows down the electrochemical reaction. Increase in defect concentration in oxide film with Ef could be a factor that facilitates the reaction. The occurrence of appreciable amount of point defects in oxide appears to be associated with OER proceeding at high potentials on CoSi2 electrode. According to the present views, the oxygen vacancies play an important role in the OER kinetics, and these defects can serve as intermediate states in tunnel transfer of electrons through the oxide film in the cathodic process.
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 ≡Si–OH2+ on the surface of the oxide film). The values of the rate constants of the HER steps were estimated.
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