High-Temperature Rotating Disk Electrode Study of Platinum Bimetallic Catalysts in Phosphoric Acid

Abstract

Understanding the H3PO4 effect on the catalyst’s activity under a relevant condition is important for high-temperature polymer electrolyte membrane fuel cell (HT-PEMFC) catalyst research. Here, we report a high-temperature rotating disk electrode (HT-RDE) study of oxygen reduction reaction (ORR) in H3PO4. With the regular electrochemical protocol, we found that H3PO4 reduction could occur during cyclic voltammetry study and form a reductive species─phosphorus acid (H3PO3). To obtain reliable ORR measurement, we optimized the protocol to avoid the H3PO3 generation. The ORR activity of carbon-supported PtM (M = Fe, Co, Ni, Ru, Pd, and Ir) bimetallic alloy catalysts measured with this HT-RDE method showed higher ORR activity than Pt. To understand the alloying effect, we combine experiments in diluted solutions to distinguish the alloying effect on Pt–O binding and Pt–H3PO4 binding. The results indicate that H3PO4 mainly reduces available sites for ORR, with little effect on neighboring site’s Pt–O binding via Pt–H3PO4 interaction, which is also supported by the density functional theory calculation of the Pt–O binding energy with/without H2PO4. Further study in a phosphoric acid-doped quaternary ammonium-biphosphate ion pair coordinated polyphenylene (PA-QAPOH) membrane electrode assembly (MEA) shows that the active alloy catalyst has better performance in both the HT-RDE and MEA. Also, the MEA gives higher ORR activity than the HT-RDE because of the higher pressure and less phosphoric acid content of the MEA. Yet, the gap between the HT-RDE and MEA is significantly smaller than that between the room temperature (RT)-RDE and MEA, suggesting the importance of temperature and H3PO4 concentration in understanding ORR in HT-PEMFCs.