Evaluation of Platinum‐Based Catalysts for Methanol Electro‐oxidation in Phosphoric Acid Electrolyte

Abstract

Carbon‐supported catalysts of Pt, Pt/Ru, Pt/Ru/W, and Pt/Ru/Pd were evaluated for the electro‐oxidation of methanol in phosphoric acid at 180°C. These catalysts were characterized using cyclic voltammetry and x‐ray diffraction. Addition of Ru to a 0.5 mg/cm2 Pt catalyst (1:1 atomic ratio) caused a large reduction in polarization. The open‐circuit voltage was reduced by 100 mV and polarization at 400 mA/cm2 was reduced by 180 mV. A Pt/Ru (5:2) catalyst with the same Pt content lowered the open‐circuit voltage 70 mV. Additions of W to form Pt/Ru/W (1:1:1, atomic ratio) and Pd to form Pt/Ru/Pd (2:2:1), all with the same platinum loading, gave the same performance as Pt/Ru (1:1) without the additions. All of the catalysts showed two Tafel slopes, 140 mV/dec at lower polarizations and 100 to 120 mV/dec at higher polarizations, indicating that the reaction mechanisms are the same for all of the catalysts. Methanol oxidation is greatly enhanced at 180°C in phosphoric acid compared to the lower operating temperatures of a perfluorosulfonic acid electrolyte. The exchange current density for methanol oxidation is higher than that for reduction. Ru metal dissolves from catalysts at high potentials. Hydrogen oxidation in the presence of 1 mole percent carbon monoxide showed carbon monoxide tolerance in the order: Pt/Ru/Pd > Pt/Ru > Pt.