Some aspects of the EIS study of the rhodium electrode in the hydrogen evolution region

General expression for the frequency dependence of the faradaic electrode admittance is derived for the case when the rate of the hydrogen evolution reaction (HER) depends on the potential, potential dependent surface fraction of adsorbed hydrogen, and potential dependent surface or subsurface concentration of the molecular or absorbed hydrogen, respectively. The simplified version of the general expression was adjusted to take into account two different specific reaction mechanisms of HER, and applied in the analysis of the experimentally measured impedance spectra of the rhodium/H₂SO₄ electrode in the region of low cathodic overpotentials. The results of the fitting procedure suggested proceedings of HER in parallel with diffusion associated with some absorption of hydrogen within the rhodium metal layer.