Operando 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.