Electrochemical Quartz Crystal Microbalance Analysis of the Oxygen Reduction Reaction on Pt-Based Electrodes. Part 1: Effect of Adsorbed Anions on the Oxygen Reduction Activities of Pt in HF, HClO4, and H2SO4 Solutions

The effects of anion adsorption on the activities for the oxygen reduction reaction (ORR) at a Pt film electrode in electrolyte solutions (HClO4 and HF at various concentrations) were analyzed using an electrochemical quartz crystal microbalance (EQCM) and a rotating disk electrode (RDE). With an increasing HClO4 concentration [HClO4], the onset potential for the Pt oxide formation in the voltammogram shifted in the positive direction, accompanied by a compression of the hydrogen adsorption/oxidation wave to less positive potentials. This is ascribed to a specific adsorption of the ClO4- anion, because the [HClO4] dependence of the mass change Am detected by EQCM in the double-layer region was found to be fitted well by a Frumkin-Temkin adsorption isotherm. The potential dependencies of Am in both 0.1 and 0.5 M HClO4 solutions accord well with those of the v(Cl-O) intensities observed by in situ Fourier transform infrared (FTIR) spectroscopy in the potential range from 0.3 to 0.6 V. The kinetically controlled current densities j(k) for the ORR at the Pt RDE were found to decrease with increasing [HClO4], because of the blocking of the active sites by specifically adsorbed ClO4-. The values of j(k) in the non-adsorbing 0.1 M HF electrolyte solution, however, were smaller than those in 0.1 M HClO4. It was found that the low ORR activity could be ascribed to the low H+ activity in the weak acid solution of HF. We, for the first time, detected a reversible mass change for one or more adsorbed oxygen species on the Pt-EQCM in O-2-saturated and He-purged HF and HClO4 solutions. The coverages of oxygen species theta(o) on Pt were found to increase in the O-2-saturated solution. High values of theta(o) in O-2-saturated 7 mM HF suggest that the ORR rate was limited by the very low H+ activity in the solution, and the adsorbed oxygen species remained on the surface because of a slow consumption rate.