Oxygen Reduction on Platinum in 85% Orthophosphoric Acid

Oxygen reduction on Pt in 85 % HaPO4 has been studied using a rotating disk electrode at various temperatures. At a prereduced Pt electrode the Tafel slope is ~--0.12 V/decade. Kinetic studies of O2 reduction on Pt at room tem-perature using rotating r ing-disk technique have shown that in purified 85% I-IsPO4 the paral lel mechanism is operative. The direct reduction to water is the predominant path, and only about 10 % of the reduction proceeds via the production of H202. H202 oxidation and reduction on Pt have also been studied at room temperature using the rotating disk electrode. The oxidation of H202 is one-half order with respect o H20 ~ while the H202 reduction is first order. As an outgrowth of the interest in acid fuel cells, O2 reduction in hot concentrated phosphoric acid has been studied on both smooth (1, 2) and high surface area (3-5) p lat inum electrodes. There is some ques-tion, however, regarding the Tafel slope for the re