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A study of the anodic impurity peak that appears in the cyclic voltammograms of platinum electrodes in highly purified, hot, concentrated phosphoric acid is reported. The results indicate that this anodic impurity peak corresponds to the anodic oxidation of phosphorous acid. This phosphorous acid impurity appears to be formed at cathodic potentials by the reaction of the phosphoric acid electrolyte with adsorbed hydrogen. The electrochemical formation and anodic oxidation of the phosphorous acid impurity are enhanced at the elevated temperatures found in phosphoric acid fuel cells.

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.

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