KINETICS OF CATHODIC HYDROGEN EVOLUTION ON MANGANESE MONOSILICIDE IN SULFURIC ACID ELECTROLYTE

Abstract

Purpose. The purpose of the work is to investigate the kinetics of the hydrogen evolution reaction (h.e.r.) on the MnSi electrode in solutions x M H2SO4 + (0.5 – x) M Na2SO4 (x = 0.5; 0.35; 0.20;0.05).
Methods and methodology. Methods used in the work are polarization and impedance measurements. Тhe research material is manganese silicide MnSi, which was obtained by the Czochralski method. The range of frequencies f (w / 2p) used in impedance measurements is from
20 kHz to 0.02 Hz, the amplitude of the alternating signal is (5–10) mV.
Results. The following results have been obtained. Cathodic polarization curves of the MnSi electrode have two Tafel plots with slopes of 0.17–0.18 V at lg i ≈ –5.3 ... –4.4 and 0.13–0.14 V at lg i ≈ –4.4 ... –3.8. The constant a in the Tafel equation for MnSi is equal to 1.36–1.47 V for the fi rst plot, and 1.12–1.30 V for the second plot. Silicide in sulfuric acid electrolyte refers to materials with high overpotential of hydrogen evolution. Kinetic parameters of h.e.r. (∂ / ∂lg ) , +E icH
(∂ / ∂lg ) + E c H i , (∂ / ∂lg ) + h c H i , (∂lg / ∂lg ) + i c H E , (∂lg / ∂lg ) + i cH h on MnSi are close to theoretical for the slow stage of charge transfer.
The impedance spectra of the MnSi electrode in the studied solutions at lg i ≈ –5.3 ... -3.8 are capacitive semi-circles with an offset center. The impedance spectra are satisfactory (the c2 criterion is ~ 10–4; the error in determining the values of the circuit parameters does not exceed 5 %) described by an equivalent electrical circuit whose Faraday impedance consists of in seriesconnected resistances of charge transfer R1 and a parallel R2C2 chain corresponding to the adsorption of atomic hydrogen on the electrode surface; the impedance of the double-layer capacitance is modeled by the constant phase element CPE1.
The results of determining the values of the parameters R1, R2, C2 of the equivalent circuit and also iR1, iR2 have been analyzed depending on the potential in semi-logarithmic coordinates. 
Conclusions. Conclusions have been made that in the region lg i ≈ –5.3 … –3.8, the experimental values of the slopes , where X = R1, R2, C2, iR1, iR2, are close to the theoretical values of the slopes for the discharge – electrochemical desorption mechanism, in which both stages are irreversible and the transfer coeffi cients of the stages are not equal. Simultaneously with the hydrogen evolution reaction, the hydrogen absorption reaction proceeds with kinetic control; for adsorbed atomic hydrogen the Langmuir adsorption isotherm is performed.

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