Anodic processes on Mn5Si3 electrode in alkaline alectrolyte

Abstract

Purpose. The anodic behaviour of a Mn₅Si₃ electrode in solutions of (0.5–3.0) M NaOH in the region from the E corrosion potential up to the E oxygen evolution potential was studied using the methods of polarization and impedance measurements.

Results. The anodic polarization curves of Mn₅Si₃ have a weakly expressed region of active dissolution, regions of primary and secondary passivation, separated by a peak at a potential of ≈ 0.2 V (relative to the standard hydrogen electrode), and an area of oxygen evolution. In the first passive region, passivation of silicide is achieved due to the formation of Mn(OH)₂ on its surface. In the second passive region it is achieved due to the formation of MnO₂. Oxygen evolution is accompanied by the formation of MnO₄^–-ions. Silicon is selectively (chemically and electrochemically) dissolved from the surface layer of silicide. The composition of the films on Mn₅Si₃ may include small amounts of silicon dioxide and silicates along with the products of metal oxidation.

The impedance spectra of Mn₅Si₃ at the potentials of the first passive region consist of a capacitive semicircle with an offset centre at high frequencies and an almost vertical straight line at low frequencies. Using the capacitance values corresponding to the low-frequency line of the impedance spectra, we can calculate the magnitude of the change in the thickness of the passivating film with the potential dδ/dE depending on the potential and the concentration of NaOH. The derivative dd/dE varies in the range of 2.1–17.5 nm/V and decreases with increasing electrode polarization and increasing electrolyte concentration.

At the potentials of the second passive region, the slope of the low-frequency line on the impedance spectra is ~ 45°, which corresponds to the diffusion impedance. The formation of MnO₂ is accompanied by diffusion control over the mass transfer of the implanted OH_--ions into the surface passivating film Mn(OH)₂ towards the silicide/film interface.

Conclusion. The process of oxygen evolution on the Mn₅Si₃ electrode reveals kinetic features that are characteristic for the Mn electrode. The Tafel slope of the E,lgi- curves for silicide is (0.15–0.17) V, the reaction order for OH^-- ions is 0.76, and the dependence of the electrode potential on the solution concentration is –0.12 V. The process rate depends on the discharge rate of OH^-- ions.

CONFLICT OF INTEREST

The authors declare the absence of obvious and potential conflicts of interest related to the publication of this article.

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