Hydrogen/anion electrosorption at rhodized electrodes as revealed by electrochemical impedance spectroscopy

Electrosorption properties of rhodized electrodes in H2SO4 and HClO4 electrolyte solutions, with and without small concentration of added chloride and (bi)sulphate anions, are investigated by use of the electrochemical impedance spectroscopy. Potentials between the threshold of the hydrogen evolution and initial stage of (oxy)hydroxide formation are applied. Kinetic contributions of hydrogen under-potential deposition (UPD) are over restricted and adsorption of chloride and (bi)sulphate anions over almost whole potential region is clearly marked by present impedance spectra. Added anions, as well as products formed by decomposition of perchlorate ions from HClO4 electrolyte solution, influenced kinetics of the hydrogen UPD at the rhodized electrode. Adsorption of (bi)sulphate ions from H2SO4 electrolyte solution is not detected directly as a kinetic process, but is indicated through increased apparent double-layer capacitance values. Theoretical equation for the frequency dependence of adsorption impedance is derived on the basis of (partial) charge transfer adsorption of ions, the state variable approach, and concept of two-step adsorption. The derived equation is found to be adequate for fitting impedance spectra in the hydrogen/anion transition region. Values of impedance parameters and their potential dependences are discussed and compared to already published data on similar electrodes, including low-index rhodium and platinum single-crystal electrodes.