The kinetics of cathodic hydrogen evolution on titanium disilicide in a sulphuric acid solution

Abstract

The kinetics and mechanism of hydrogen evolution reaction on the TiSi2 electrode in x M H2SO4 + (0.5 – x) M Na2SO4 (x = 0.5; 0.35; 0.20; 0.05) solutions were studied using the methods of polarisation and impedance measurements. The cathodic polarisation curves of TiSi2 were characterised by a Tafel section with a slope of 0.116–0.120 V at E ranging from –0.30 to –0.48 V (SHE). The value of hydrogen evolution overpotential at i = 1 A/cm2 for TiSi2 was 0.90–0.96 V. The kinetic parameters of the hydrogen evolution reaction on silicide were close to the theoretical values for the slow stage of charge transfer. Based on the measurements of the differential capacitance of the TiSi2 electrode (at f = 10 kHz), depending on the cathodic polarisation and acidity of the electrolyte, it was concluded that a thin dielectric film of silicon dioxide (Si + 2H2O →SiO2 + 4H+ + 4e–) was present on the surface of the silicide. The film was not reduced with low cathodic polarisations. The impedance spectra of the TiSi2 electrode at the potentials of the Tafel region were capacitive semicircles with a misplaced centre. The impedance spectra were described by an equivalent electrical circuit, the Faraday impedance of which consisted of a series-connected charge transfer resistance R1 and a parallel R2C2 chain corresponding to the adsorption of atomic hydrogen on the electrode surface. The impedance of the double layer capacitance was modelled using the constant phase element CPE1. The χ2 criterion for the circuit was (1.3-3.7)·10-4 (when using data modulus weighting), the sum of square deviations was (1.5–4.1)·10–2, and the error in the determination of the values of circuit parameters did not exceed 10 %.The experimental values of the slopes of lg R1,E-, lg R2,E-, and lg C2,E-dependences were close to the theoretical values of the slopes for the discharge-electrochemical desorption mechanism, in which both stages were irreversible and the transfer coefficients of the stages were not equal, when the Langmuir isotherm for adsorbed atomic hydrogen was fulfilled. The reaction of absorption of hydrogen with the kinetic control proceeded at the same time as the hydrogen evolution reaction.