Determination of kinetic parameters for the hydrogen evolution reaction on the electrodeposited Ni-MoO2 composite coating in alkaline solution

Abstract

As a result of research on the non-noble metal-based composite electrocatalysts for the hydrogen evolution reaction (HER), the Ni-MoO2 composite coating was produced by electrodeposition from an NH4Cl and NiCl2 containing solution with suspended MoO2 powder particles, exhibiting almost the identical activity for the HER as the commercial cathode in conditions of industrial application and satisfactory mechanical stability. In this work, investigations were extended to the kinetics and mechanism of the HER on the Ni-MoO2 electrode in 8 M NaOH solution at 30 degrees C. The steady-state polarization curve for the HER was characterized by only one Tafel slope of -122 mV/dec. Kinetic parameters for the HER, as well as a contribution of surface roughness to the catalytic activity, were evaluated using an electrochemical impedance spectroscopy (EIS) technique. On the basis of the presented theoretical model for the faradaic impedance of the HER, the rate constants of the individual steps were determined by simulating both polarization and EIS experimental data. It was found that the reaction equally proceeds via the Volmer-Heyrovsky and Volmer-Tafel routes at lower overpotentials, while at higher overpotentials the Volmer-Heyrovsky pathway dominates the process with the Heyrovsky step being the rate determining step (rds). A comparison of intrinsic activities of the Ni-MoO2 composite and a flat Ni electrode proved that the combination of Ni and MoO2 produced a true catalytic effect for the HER.