High temperature-polymer electrolyte membrane fuel cell (HT-PEMFC) utilizing polybenzimidazol (PBI) membrane doped with high-level of phosphoric acid has been considered to be a promising alternative for stationary power supplies. High-level of phosphoric acid doping, however, is known to decrease free active sites for oxygen reduction reaction (ORR) on Pt surfaces, therefore, diminished performance of the PBI based HT-PEMFCs (phosphoric acid poisoning). Thus, understanding and mitigating the phosphoric acid poisoning on catalyst surfaces are crucial to improve the performance of HT-PEMFC. On bulk Pt surfaces, FT-IR has been widely utilized to characterize the phosphoric acid poisoning, but is not suitable for analyzing the phosphoric acid poisoning on carbon supported Pt nanoparticle (Pt/C) catalysts, which are the most common electrocatalyst for the ORR in HT-PEMFC. Here, in order to analyze quantitatively the fraction of surface Pt atoms blocked by phosphoric acid poisoning (θPA) in Pt/C, we combined extended X-ray absorption fine structure (EXAFS) analysis and cyclic voltammetry (CV). From the curve fitting of EXAFS spectra using bulk models, coordination number (CN) was determined. Then, the θPA was determined from the CN and a conversion constant which was estimated from adsorption charge density of phosphoric acid and the CN changes at 0.35 V ~ 0.60 V.

Determination of phosphoric acid coverage on carbon supported Pt nanoparticles using in-situ X-ray absorption spectroscopy

2019 Fall National Meeting

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