engleski

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

Electrosorption properties of rhodized electrodes in H_2SO_4 and HClO_4 electrolyte solutions, with and without small concentration of added chloride and (bi)sulfate 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 underpotential deposition (UPD) over restricted, and adsorption of chloride and (bi)sulfate anions over almost the whole potential region are clearly marked by present impedance spectra. Added anions, as well as products formed by decomposition of perchlorate anions from HClO_4 electrolyte solution influenced kinetics of the hydrogen UPD at the rhodized electrode. Adsorption of (bi)sulfate anions from H_2SO_4 electrolyte solution is not detected directly as a kinetic process, but is indicated through increased apparent double-layer capacitance. 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 the 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 comapred to already published data for similar electrodes, including low-index rhodium and platinum single-crystal electrodes.

rhodized electrode ; electrochemical impedance spectroscopy ; electrosorption ; hydrogen UPD ; anions

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