CATHODE HYDROGEN EVOLUTION ON COBALT MONOSILICIDE IN SULFURIC ACID SOLUTIONS

DOI: 10.6060/tcct.20165911.5333
Izv. Vyssh. Uchebn. Zaved. Khim. Khim. Tekhnol. 2016. V. 59. N 11. P. 63-69

The kinetics of hydrogen evolution reaction on cobalt monosilicide CoSi in sulfuric acid solutions was studied using methods of polarization and impedance measurements. Electrochemical impedance of CoSi was studied in solutions of 0.5 M H2SO4, and 0.05 M H2SO4 + 0.45 M Na2SO4 at 20 °C in the range of potentials of hydrogen evolution. Solutions were prepared from high-purity reagents and de-ionized water (Milli-Q). The working solu-tions were de-aerated with hydrogen (purity 99.999 %). Cobalt silicide was prepared from sil-icon (99.99 % purity) and electrolytic cobalt (99.98 % purity) by Czochralski method (pulling from the melt at speed of 0.4 mm/min). Electrodes were cut using electric-spark method. The working electrode surface was 0.4 – 0.6 cm2. Before measurements, the working electrode surface was abraded with emery papers of 1000 and 2000, cleaned with ethanol and washed in the working solution. Electrochemical measurements were carried out in three-electrode electrochemical cell with cathode and anode compartments separated with a porous glass di-aphragm. Potentials Е were changed from low to high cathode polarizations. Before record-ing an impedance spectrum at each potential value, the electrode was polarized under poten-tiostatic conditions until constant value of current was reached. The electrode potentials are given with respect to the SHE. Impedance measurements were carried out using FRA So-lartron 1255 and potentiostat Solartron 1287 (Solartron Analytical) at frequency range from 10 kHz to 0.01 Hz (10 points per decade). The alternating signal amplitude was 10 mV. The CorrWare2, ZPlot2 and ZView2 software (Scribner Associates, Inc.) was used for measuring and processing the impedance data. In 0.5 М H2SO4 polarization curve for CoSi-electrode has Tafel plot with the slope b = 0.088 V. In 0.05 M H2SO4 + 0.45 M Na2SO4 the slope of po-larization curve is equal to 0.139 V. The constant a in Tafel equation (E = a + blgi) for CoSi was equal to 0.52 V in 0.5 М H2SO4 and 0.71 V in 0.05 M H2SO4 + 0.45 M Na2SO4. It means that cobalt monosilicide belongs to the group of materials with the average overvoltage. Nyquist diagrams consist of two semicircles in capacitive high-frequency and low-frequency region. Impedance spectra of cobalt silicide can not be accurately described by a simple equivalent circuit consisting of parallel-connected charge transfer resistance and the double layer capacitance. In order to describe the behavior of CoSi - electrode in 0.5 M H2SO4 solu-tion at the range of investigated potentials the equivalent circuit was used which allows taking into consideration the penetration of atomic hydrogen into the cathode material and diffusion processes. In order to analyze impedance data we used the diagnostic criteria for hydrogen evolution reaction mechanisms in acidic solutions based on the dependence of equivalent cir-cuit elements on the overvoltage and H+ ion concentration, which were previously proposed. Determination of the kinetic parameters (rate constants and transfer coefficients) of hydro-gen evolution reaction steps on the basis of the impedance data was considered. Hydrogen evolution reaction on CoSi electrode in acidic solutions is discussed using the proposed crite-ria. Hydrogen evolution reaction on CoSi in sulfuric acid solution was shown to proceed through the discharge – electrochemical desorption route with the Langmuir isotherm for hy-drogen adsorption. The reaction proceeds with the diffusion control because for CoSi the equivalent circuit taking into account the hydrogen absorption works well.
Key words: hydrogen evolution reaction, cobalt silicide CoSi, sulfuric acid solution, impedance

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2016, Т. 59, № 11, Стр. 63-69

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