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

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.

Automated summary

Understanding the impact of H3PO4 on catalyst activity is crucial for HT-PEMFC catalyst research. This study utilized HT-RDE to investigate the ORR in H3PO4 and found that H3PO4 reduction occurs during cyclic voltammetry, forming H3PO3. A modified protocol was developed to avoid H3PO3 generation and reliable ORR measurements were obtained. The results showed that carbon-supported PtM bimetallic alloy catalysts had higher ORR activity than Pt and that H3PO4 primarily reduces available sites for ORR.