The Catalytic Activity of Platinum Supported on Carbon for Electrochemical Oxygen Reduction in Phosphoric Acid

An experimental program was conducted to determine the catalytic activity of high surface area platinum supported on carbon for the electrochemical reduction of oxygen in phosphoric acid. The Tafel slope at 160°C in 96 weight per cent phosphoric acid was found to be approximately 90 mV/decade. The exchange current per unit of real surface area of the supported platinum was found to be approximately the same as that of platinum black or a flat platinum sheet if the effect of electrode prepolarization is considered. Additional tests at other temperatures indicated that the Tafel slope varies as and is similar in sulfuric and phosphoric acid at the same temperature. The prediction that a double Tafel slope exists when oxygen diffusional resistance becomes significant in a Teflon‐bonded porous electrode was confirmed.