Hydroxyl and hydroperoxyl radicals produced by reacting impurities with hydrogen peroxide is one of the degradation factors of polymer electrolyte [1]. A method to suppress this degradation is the addition of cerium into the polymer electrolyte membrane (PEM), which serves as radical quencher [2,3]. However, cerium ion in PEM migrates from the initial state by fuel cell operation [4]. The lack of cerium ion in PEM causes degradation from radical species. Information on cerium migration in PEM is necessary to design the membrane electrode assembly (MEA) for long-term operation. Although the observation of cerium ion diffusion with in-plane direction of PEM has been reported [5], it is difficult to detect the distribution in through-plane of MEA under fuel cell operating condition due to the quite narrow thickness of PEM. This study reports an operando X-ray fluorescence analysis technique to detect cerium distribution in through-plane direction of MEA using high-energy X-ray nanobeam.

MEAs were prepared by a conventional method using Pt/C catalyst and polymer electrolyte with the thickness of 12 mm. The active electrode area was 1 cm². Single cells were assembled using custom-made end-plates that have an X-ray permeation window. Operando X-ray fluorescence spectroscopy measurements were performed at SPring-8, BL37XU (Japan). The monochromatized X-ray at an energy of 45 keV was focused to less than 200 nm by Kirkpatrick-Baez mirror. To detect the cerium distribution in MEA, Ce-Ka fluorescence intensity was counted using a Ge semiconductor detector with scanning the position of the cell. The cell temperature was 80 ℃. O₂ and H₂ gases with relative humidity of 100% were flew to cathode and anode, respectively.

Figure 1 shows line-scan profile of Ce-Kα fluorescence X-ray intensity from the as-prepared MEA. The x-axis represents the through-plain position, where the positive and negative sides corresponds to cathode and anode sides, respectively. The two peaks are observed at 678 and 685 mm and the peak width is approximately 13 mm. This peak shows PEM containing cerium ion. During the fuel cell operation, the line-scan is repeated and cerium ion distribution can be analyzed.

Acknowledgment

Part of this work is based on results obtained from the project commissioned by the New Energy and Industrial Technology Development Organization (NEDO) of Japan.

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[5] A.M. Baker et al., J. Electrochem. Soc. 164 F1272-F1278 (2017).