Mitigating Phosphate Anion Poisoning of Cathodic Pt/C Catalysts in Phosphoric Acid Fuel Cells

Abstract

The effect of phosphate anion poisoning of cathodic Pt-based catalysts in phosphoric acid fuel cells was investigated by observing the kinetics of the oxygen reduction reaction (ORR) in the presence of varied concentrations of H3PO4. In an attempt to reduce phosphate anion poisoning, a Pt-based Ni alloy catalyst was synthesized. Detailed electrochemical and X-ray absorption spectroscopy investigations have been carried out on both Pt and PtNi electrocatalysts under in situ conditions. It was found that the phosphate poisoning effect on the ORR is much less severe on PtNi/C than on Pt/C. The Δμ X-ray absorption near-edge spectroscopy analysis suggests that the adsorbed phosphate anions actually remain to higher potential on PtNi than on Pt, and this inhibits OH adsorption from water activation and the conversion from OH to O, thus allowing more sites for the ORR. Due to the presence of phosphate anions on the atop sites of PtNi/C, OH cannot occupy such sites now either due to steric hindrance or just lower availability of atop sites, and thus, it appears to keep the remaining sites (2-fold and 3-fold) free for molecular oxygen to undergo the ORR.