PHOSPHORUS-ACID IMPURITIES IN PHOSPHORIC-ACID FUEL-CELL ELECTROLYTES .2. EFFECTS ON THE OXYGEN REDUCTION REACTION AT PLATINUM-ELECTRODES

The effects of phosphorous acid additions on the oxygen reduction reaction at platinum electrodes in concentrated phosphoric acid were studied. The oxygen reduction currents decreased, and the Tafel slopes became more negative upon the addition of small concentrations of phosphorous acid. In addition, the phosphorous acid oxidation current tended to compete with the oxygen reduction current. These effects became more pronounced at higher phosphorous acid concentrations and at higher temperatures. Upon the addition of phosphorous acid the number of electrons involved in the oxygen reduction reaction decreased from a value close to four to a value approaching two, suggesting promotion of a two-electron reduction to peroxide. Therefore, in studies of the electrochemical reduction of oxygen in hot concentrated phosphoric acid or in fuel cell systems using hot concentrated phosphoric acid as electrolyte, it is recommended that precautions be taken against the inadvertent formation of phosphorous acid. The removal of phosphorous acid from concentrated phosphoric acid by repeated potential cycling at 100 mV/s between +0.5 and +1.50 V (vs. dynamic hydrogen electrode) was demonstrated.