Time-Resolved X-Ray Fluorescence Analysis of Cerium Ion Transport in Polymer Electrolyte Membranes Under Humidity Gradient
Authors: Morita Kaoruko , Hanahara Rukiya , Ishiguro Yuta , Takezawa Aika , Kitano Naoki , Kuwaki Akira , Kato Akihiko , Yamaguchi Satoshi , Shinozaki Kazuma , Orikasa Yuki 
Published: December 2024

The durability of more than 50,000 hours is required for the widespread application of proton exchange membrane fuel cells [1]. Chemical degradation of perfluorosulfonic acid (PFSA) membrane electrolytes by radicals is one of the degradation factors [2]. To quench the radicals, cerium ions are incorporated into the membrane electrode assembly (MEA) as radical scavengers. However, cerium ions move within the plane of MEA due to humidity gradients [3]. The chemical degradation by the radicals occurs in the depleted areas of cerium ion. Understanding the in-plane transport mechanism of cerium ions under operational conditions, including quantifying parameters like diffusion coefficients, is crucial for developing MEAs with enhanced durability. This study introduces an operando measurement technique employing a custom-built cell to monitor the spatial distribution changes of cerium ions over time. This technique utilizes X-ray fluorescence spectroscopy (XRF) with synchrotron high-energy X-rays, allowing for the analysis of cerium ion distribution under operational conditions while controlling the in-plane humidity gradient. By analyzing the data, we explore the phenomena governing the in-plane transport of cerium ions, driven by humidity and concentration gradients.
Two gas inlets were connected to each end plate with an X-ray transmission window to control two types of humidity environments on the left and right side with the in-plane direction of the MEA. The humidity at each gas line was monitored by using dew point hygrometers placed on gas outlets. For the catalyst-coated membranes (CCMs), a platinum catalyst layer was applied to a PFSA-based PEM with approximately 20% sulfonic acid substituted with cerium ions. Gas diffusion layers (GDLs) with microporous layer (MPL) were used. XRF measurements were performed at beamline BL37XU on SPring-8. The incident energy was 60 keV, and Ce-Kα fluorescence X-rays were counted. The beam size was 0.5×0.5 mm2. The cell temperature was maintained at 80°C, and different relative humidity (RH) in the in-plane direction were established using N2 gas (80% on left side and 50% on right side), and then changed to the entire RH 50% to observe cerium ion relaxation behavior.
Figure 1 shows the Ce-Kα intensity change of the MEA under the condition of humidity difference with in-plane direction. The x-axis corresponds to the horizontal position of the MEA, with RH 80% on the left side and RH 50% on the right side. The y-axis corresponds to the time under the humidity gradient. The darker blue indicates weaker intensity of the Ce-Kα fluorescence X-ray and the red indicates stronger intensity. This suggests that cerium ions move from the high humidity side to the low humidity side with the humidity gradient as the driving force. The time scale for this transport of a few millimeters is shown to be about tens of minutes. Figure 2 compares the Ce-Kα intensity after the humidity gradient test and the relaxation in homogeneous RH 50% for 10 hours. After the humidity gradient test, cerium ion is concentrated on the right side closed to the center. The concentrated cerium ion is slightly diffused into the left part after the relaxation for 10 hours. However, the initial distribution state does not completely disappear, indicating that the driving force of the humidity gradient exceeds that of the concentration gradient at RH 50%.

Reference

[1] Y. Fujii, ECS Trans., 111(6), 21 (2023).
[2] R. Borup, et al., Chem. Rev., 107(10), 3904 (2007).
[3] Y.-H. Lai, et al., J. Electrochem. Soc., 165(6), F3217 (2018).
[4] K. Morita, et al., ECS Trans., submitted.

Acknowledgements: The authors would like to thank M. Toida and T. Tambo (Toyota Motor Corporation) for providing samples and for in-depth discussions.

Figure 1