Cyclic voltammetry and electrochemical quartz crystal microbalance studies of a rhodized platinum electrode in sulfuric acid solution

The adsorption properties of a rhodized platinum electrode in sulfuric acid were studied through simultaneous cyclic voltammetry (CV) and electrochemical quartz crystal microbalance (EQCM) measurements. The processes occurring at the Rh surface during the potential scan between 0.04 and 1.3 V were analyzed in terms of the changes in mass and charge. The apparent molar masses of the species adsorbed on Rh in different potential regions were determined from the mass–charge correlation. The results obtained suggest that the desorption of hydrogen UPD initially occurs in conjunction with the incorporation of mass due to the adsorption of bisulfate/sulfate ions between 0.04 V and 0.12 V; the apparent molar mass (M ₁) in this potential region is consistent with that of bisulfate ion, assuming that one sulfate species occupies the site originally occupied by five H atoms. Between 0.12 V and 0.20 V, the apparent molar mass, M ₂, is less than M ₁. This result may be associated with a structural rearrangement of the adsorbed sulfate species. The apparent molar mass, M ₃, was determined in the potential region of 0.20–0.56 V; the value obtained for M ₃ suggests that the adsorbed species are hydrated bisulfate ions (HSO₄⁻·4H₂O). Additionally, it was found that Rh oxides formed at the surface between 0.56 V and 1.3 V could not be directly identified from the correlation between charge and mass.