Analyzing the Influence of H₃PO₄ as Catalyst Poison in High Temperature PEM Fuel Cells Using <i>in-operando</i> X-ray Absorption Spectroscopy

Abstract

The effect of H3PO4 as a poison in high temperature polymer electrolyte fuel cells using polybenzimidazole (PBI) membranes was studied as a function of phosphoric acid loading, potential, and temperature. In this work, for the first time, extensive in-operando X-ray absorption spectroscopy investigations were carried out on Pt/C fuel cell cathode catalysts at different temperatures and H3PO4 concentrations at varying fuel cell voltages. Under in-operando conditions, significant H3PO4 anion coverage of the Pt nanoparticles is observed. The Δμ-XANES analysis shows that the O(H)/H adsorption onset potential increases/decreases with temperature and that this is a result of phosphate anions being driven off the surface at high temperatures (170 °C). With initial coadsorption of H and O(H), the phosphate anions move into registry with the Pt, whereas random adsorption is observed when only phosphate anions are present on the Pt surface. By varying the temperature and the fuel cell potential, the adsorption geometry of the phosphoric acid anion changes with coverage, but in all cases, the anions block Pt sites and reduce the oxygen reduction reaction (ORR) rate.