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

Abstract

The effect of H 3 PO 4 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 H 3 PO 4 concentrations at varying fuel cell voltages. Under in-operando conditions, significant H 3 PO 4 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.

On the oxygen reduction reaction in phosphoric acid electrolyte: Evidence of significantly increased inhibition at steady state conditions

“…Often it is associated with the same site blocking mechanism observed in electrochemical half-cells, where the phosphate anions completely block the active sites of Pt. Alternatively -or in addition -the low solubility of oxygen in concentrated phosphoric acid was highlighted to explain the poor performance [30][31][32][33][34][35].…”

Electrochemistry of Phosphorous and Hypophosphorous Acid on a Pt electrode

“…H 3 PO 4 and H 2 PO 4 À adsorption on Pt nanoparticles results in a partial blockage of electroactive sites, cf. [2][3][4]. It also affects the rate of the O 2 reduction (ORR) and H 2 oxidation reactions.…”

Effect of Nanostructure of the Electrocatalyst Based on Carbon Nanotube for the Activation Overpotential for PEFC

“…Although the PEFC based on this composite showed high durability, the PEFC exhibited large overpotential in the area of low current density. We considered the one of the possible reason of the overpotential was the poisoning of Pt surface by PVPA coating similar with the poisoning of Pt by phosphoric acid (11). To keep the exposed surface, we newly designed the catalyst doped by acid 'before' Pt loading, in which the PVPA was coated onto the surface of MWNT/PBI prior to the Pt deposition as shown in Figure 1 (denoted as the post-loading catalyst).…”

“…Although this PEFC showed high durability, this catalyst had large overpotential in the area of low current density. Probably, it is caused by the poisoning of Pt surface by PVPA coating (7) or the lack of electrolyte in catalyst layers (8).…”