<i>Operando</i> X-ray Fluorescence Analysis of Through-plane Cerium Ion Radical Quencher Migration in Polymer Electrolyte Fuel Cells (Supporting Information)

Polymer electrolyte fuel cells (PEFCs) need to achieve long-term durability for widespread commercialization. Chemical degradation of the perfluorosulfonic acid (PFSA) membrane caused by radical species (·OH) can be mitigated by adding cerium ions as radical scavengers; however, cerium ions migrate within the membrane, potentially reducing their effectiveness. In this study, we have developed an operando high-energy microbeam X-ray fluorescence (XRF) system to visualize the distribution of cerium ions in the electrolyte membrane and catalyst layers under fuel cell operating conditions. Scanning across the membrane and catalyst layers with sub-micron spatial resolution directly observes rapid migration of cerium ions from the membrane to the cathode side immediately after current loading. Conversely, when the cell is returned to open-circuit voltage (OCV), the cerium ions diffuse back into the membrane. The amount of migrated cerium ions depends on the current density, suggesting that higher current loads accelerate cerium ion transport toward the cathode.聚合物电解质燃料电池（PEFCs）要实现大规模商业化，需具备长期耐久性。由自由基（·OH）引发的全氟磺酸（PFSA）膜化学降解，可通过添加铈离子作为自由基清除剂加以缓解；但铈离子会在膜内迁移，可能削弱其防护效果。本研究开发了一套原位高能微束X射线荧光（XRF）系统，可在燃料电池运行工况下可视化观测铈离子在电解质膜与催化层中的分布。借助亚微米空间分辨率的扫描成像，研究团队直接观测到：加载电流后，铈离子会快速从膜层向阴极侧迁移；而当电池恢复至开路电压（OCV）时，铈离子会扩散回膜层。迁移的铈离子总量与电流密度相关，这表明更高的电流负载会加速铈离子向阴极的传输。