Kinetics of hydrogen underpotential deposition at iridium in sulfuric and perchloric acids

Kinetics of hydrogen underpotential deposition (H UPD) reaction was studied at a polycrystalline iridium electrode in sulfuric acid and perchloric acid using cyclic voltammetry and electrochemical impedance spectroscopy. Cyclic voltammograms in both acids displayed peaks due to adsorption and desorption of hydrogen. The hydrogen adsorption properties of Ir are similar to those of Pt and Rh. Differences in cyclic voltammograms of Ir in H2SO4 and HClO4 were due to ionic adsorption. The kinetics of H UPD at Ir was determined using electrochemical impedance spectroscopy. Double layer and adsorption capacitances and charge transfer resistances of the H UPD were determined as functions of electrode potential. It was found that the kinetics of the H UPD reaction at Ir was slower than that at polycrystalline Pt but faster than that at polycrystalline Rh, Ru and Pd. These results give a new insight into electrosorption properties of the iridium catalyst in acidic solutions.