Impedance Spectra of the Hydrogen Evolution Reaction at Low Cathodic Overpotentials

Impedance spectra of the hydrogen evolution reaction, HER, at low cathodic overpotentials were analyzed on the basis of the theoretical admittance/frequency function, initially derived for the Volmer-Heyrovsky-Tafel reaction mechanism involving the hydrogen insertion (absorption + diffusion) step. General principles of the impedance spectroscopy for HER are discussed using schematically constructed impedance spectra. In order to characterize the influence of each impedance parameter on the shape of the spectrum, some simulations of impedance spectra were performed with different values of impedance parameters. Comparative impedance measurements were carried out on deposited rhodium and ruthenium electrodes at –0.040 V vs. rhe, in 0.5 mol dm H2SO4. Impedance parameter values were estimated by curve fitting between theoretical and experimental impedance spectra. In addition to the electron transfer, formation of adsorbed hydrogen and H2 desorption step, participation of a bulk diffusion step of hydrogen was observed for both electrodes at the lowest frequencies. Comparison between impedance parameter values of the two electrodes suggested faster kinetics of HER at the rhodium electrode.