Effects of Phosphoric Acid Concentration on Oxygen Reduction Kinetics at Platinum

The oxygen reduction reaction was investigated at platinum electrodes in phosphoric acid in the concentration range 0.7M (6.6%) to 17.5M (95%) at 25°C using the rotating ring‐disk electrode technique. As a complement, cyclic voltammograms on platinum and potentials of zero charge of mercury were obtained as a function of phosphoric acid concentration. The mechanism of the oxygen electrode reaction is discussed in terms of the direct four‐electron transfer reduction to water and the formation of hydrogen peroxide as an intermediate in a parallel two‐electron transfer reaction. The rate constants of the intermediate reaction steps were calculated from the ring‐disk data for various potentials and electrolyte concentrations. The characteristics of the reaction were found to be markedly dependent on the concentration of phosphoric acid. These results are interpreted in terms of changes in oxygen solubility, proton activity, and double layer characteristics when passing over from a water to a phosphoric acid solvent structure.