Determination of Phosphoric Acid Coverage on Pt/C for High Temperature-Polymer Electrolyte Membrane Fuel Cell (HT-PEMFC) Using in-Situ X-Ray Absorption Spectroscopy

High temperature-polymer electrolyte membrane fuel cell (HT-PEMFC) utilizing polybenzimidazol (PBI) membrane doped with phosphoric acid has been considered to be a promising alternative for stationary power supplies. However, high-level of phosphoric acid doping is known to significantly decrease active sites for oxygen reduction reaction (ORR) on Pt surfaces and, therefore, diminish performance of the PBI based HT-PEMFCs (phosphoric acid poisoning). Thus, mitigating the phosphoric acid poisoning on Pt/C, which is the most common electrocatalyst for the HT-PEMFC, is crucial to improve the performance of HT-PEMFC. For this, measurement of phosphoric acid coverage at various electrode potentials would be highly required for practical Pt/C electrocatalysts.

FT-IR has been widely utilized to characterize the relative coverage and adsorption site of phosphoric acid on bulk Pt surfaces¹, but could not analyze Pt/C catalysts. The characteristics of the phosphoric acid poisoning on Pt/C can be analyzed by X-ray absorption near edge structure (XANES) analysis, but quantitative evaluation is not available². As an electrochemical method, cyclic voltammetry (CV) has been utilized to measure the amount of electrochemically adsorbed phosphoric acid amount³. However, the CV technique is insensitive to the chemically adsorbed phosphoric acids, and therefore total coverage cannot be measured.

In this study, we utilized the extended X-ray absorption fine structure (EXAFS) and CV together to measure the phosphoric acid coverage on Pt/C. For this, coordination numbers (CN) of Pt was determined from the curve fitting of EXAFS spectra using bulk model structures, such as Pt metal, rock salt structured PtO, and β-PtO₂. Then, a conversion constant was estimated from adsorption charge density of phosphoric acid and the CN changes at 0.35 V ~ 0.60 V. Utilizing those values, the phosphoric acid coverage (θ) could be determined as a function of electrode potentials.

References
1. F. C. Nart, T. Iwasita, Electrochim. Acta, 37, 385 (1992).
2. Q. He, B. Shyam, M. Nishijima, D. Ramaker, S. Mukerjee, J. Phys. Chem. C, 117, 4877 (2013).
3. J. Mostany, P. Martínez, V. Climent, E. Herrero, J. M. Feliu, Electrochim. Acta, 54, 5836 (2009).