Abstract
Polymer electrolyte fuel cells (PEFCs) face significant challenges during cold starts, where water phase transitions affect critically fuel cell performance. While previous studies have primarily focused on ice formation and melting behavior, the impact of water condensation after breaking through the freezing point remains insufficiently understood. In this study, we apply operando synchrotron X-ray computed tomography to visualize the transient water behavior in a PEFC under three different relative humidity (RH) conditions during a heating/cooling cycle simulating real-world cold-start conditions. The results revealed localized water flooding and hysteresis during the heating/cooling phase. A quantitative layer-by-layer analysis shows that water accumulation in each component layer strongly depends on both RH and the thermal cycle process. Moderate RH conditions promote efficient vapor-phase transport and minimize water flooding while avoiding membrane dehydration. These findings highlight water condensation as a key factor influencing cold-start performance and provide new insights into water management for more robust cold-start strategies toward next-generation PEFC systems.