(Digital Presentation) Operando x-Ray Fluorescence Spectroscopic Study of in-Plane Cerium-Ion Radical Quencher Distribution in Polymer Electrolyte Membranes

Abstract

One of the challenges for the widespread use of polymer electrolyte fuel cells (PEFCs) is the suppression of chemical degradation in polymer electrolyte membrane (PEM). This degradation is caused by radical species generated due to the reaction of hydrogen peroxide with impurities in the PEM during PEFC operation. One way to suppress the chemical degradation is to add cerium ions to the PEM as radical quencher, 1) which absorbs the radicals through a redox reaction, thereby suppressing PEM degradation caused by radicals. However, it is known that cerium ions move in the in-plane direction due to the humidity and potential distribution, where the depletion of cerium ions causes PEM degradation.2) Therefore, it is required to elucidate the mechanism of cerium ion transport phenomena in the in-plane direction under PEFC operating conditions. High-energy X-ray fluorescence spectroscopy enables operando measurements of cerium distribution by taking advantage of penetration into the fuel cell inside.3) In this study, we have developed operando measurement cells that can observe the distribution phenomenon of cerium ions by using X-ray fluorescence spectroscopy while controlling the humidity gradient in the in-plane direction. Using the developed method, we analyze the cerium ion transport phenomena under the humidity gradient in the membrane electrode assembly (MEA). For the operando X-ray fluorescence measurements, end-plates with an X-ray transmission window and two gas inlets were used to control two different humidity conditions in the PEM. X-ray was incident at 90 degrees from the MEA plane, and a germanium solid-state detector was positioned perpendicular to the incident X-rays to acquire Ce-Ka fluorescence X-ray from the MEA. The catalyst coated membranes composed of a perfluorosulfonic acid-based cation exchange membrane with cerium-ion substitution, coated with a platinum catalyst layer were used. Operando X-ray fluorescence measurements were performed at BL37XU at SPring-8 (Japan). X-rays with incident energy ranging from 40 to 65 keV were used to scan the cell position and detect Ce-Ka fluorescence X-ray. The cell temperature was set at 60°C. Cerium ions in the MEA were found to migrate from the high humidity side to the low humidity side within a range of millimeters in approximately one hour. The migrated cerium ions tended to diffuse into homogeneous state within in a few hours under the uniform humidity condition. Acknowledgments The authors would like to thank M. Toida (Toyota Motor Corporation) for providing samples and for in-depth discussions. References 1) Endoh, ECS Trans., 16, 1229 (2008).2) Y-H. Lai, K. Rahmoeller, J. Hurst, R. Kukreja, M. Atwan, A. Maslyn and C. Gittleman, Electrochem. Soc., 165, F3217 (2018).3) Y. Orikasa, A. Takezawa, K. Amemiya, Y. Tsuji, T. Asaoka, M. Ohki, O. Sekizawa, and K. Nitta, ECS Trans., 109, 109 (2022).