Through-Plane Cerium Ion Migration and Diffusion Analysis on Plymer Electrolyte Membrane by Operando X-Ray Fluorescence Spectroscopy

Abstract

High durability is a critical requirement for expanding the use of polymer electrolyte fuel cells (PEFCs). One degradation factor of PEFCs under typical operating conditions is electrolyte membrane degradation. Radical species formed from the reaction between hydrogen peroxide and impurities during fuel cell operation cause significant degradation of the PEM. To scavenge the radicals, cerium species are added to the PEM, which react with the radicals by redoxing the cerium ion [1]. To maintain the effectiveness of the cerium radical quencher in the PEM, the concentration of cerium in the PEM must be maintained during the long-term operation of the PEFC. However, cerium species easily diffuse and migrate out of the PEM [2]. If the cerium ion is depleted, the PEM will be severely attacked by radicals. To prevent such cerium depletion, it is important to understand the cerium migration phenomena. Experimental detection of cerium distribution phenomena through PEM by operando X-ray fluorescence spectroscopy with resolution of several micrometers has been reported [3]. We have also developed operando XRF using focused micron X-ray with submicrometer resolution [4]. In this study, we further develop the analysis method for cerium diffusion through planar PEM with sub-second time resolution. This method is used to estimate the apparent diffusion coefficient of cerium in PEM.MEAs were prepared by a conventional method using a Pt/C catalyst and a cerium-containing polymer electrolyte with a thickness of 12 μm(GORE-SELECT® MEMBRANEM788.12). The active electrode area was 1.0 cm2. Single cells were assembled using custom end plates with an X-ray permeation window. Operando X-ray fluorescence spectroscopy measurements were performed on BL37XU at SPring-8 (Japan). The monochromated X-ray at an energy of 45 keV was focused to less than 500 nm by a Kirkpatrick-Baez mirror. To detect the cerium distribution in MEA, the fluorescence intensity of Ce-Kα was counted with a Ge semiconductor detector by scanning the position of the cell.In the open circuit condition, most of the cerium is present in the PEM, but during cell loading, the cerium moves to the cathode layer. Immediately after the start of 2.0 A cm-2 loading, the intensity of cerium in the cathode layer increases in a few seconds. Simultaneously, a cerium concentration gradient is observed through the MEA. The cerium concentration gradient shows the current density dependence. When the current load is stopped, the concentrated cerium in the catalyst layer diffuses into the PEM. The diffusion coefficient of cerium ion is estimated from the time dependence of cerium concentration in PEM.Acknowledgments: This work is based on results obtained from the project commissioned by the New Energy and Industrial Technology Development Organization (NEDO) of Japan. E. Endoh et al, ECS Electrochem. Lett., 2, F73 (2013).S. M. Stewart et al, ECS Electrochem. Lett., 3, F19 (2014).A. M. Baker et al, ECS Trans., 92, 107 (2019).Y. Orikasa et al, ECS Trans., 109 109 (2022).