Quantitative Observation of Water Phase Transition in Gas Diffusion Layers Utilizing Advanced in Situ Differential Scanning Calorimetry

The study investigates the freezing behavior of supercooled water in gas diffusion layers (GDLs) of polymer electrolyte fuel cells (PEFCs). Using an innovative advanced in situ differential scanning calorimetry (advanced DSC) technique, we analyze phase transitions within GDLs at a single-cell scale, excluding impacts of catalyst layers and membranes. This method overcomes the limitations of traditional DSC, allowing variable water saturation and distribution. We find that freezing-thawing and drying-saturating cycles contribute to noticeable variations in the freezing temperature (Tonset) of supercooled water, ranging from -6 degrees C to -15 degrees C, attributed to the heterogeneous surface of GDL and water redistribution. Repeated drying-saturating cycles remove nucleation seeds, lowering the freezing point of water - a "cleaning effect." The freezing probability reduces at lower saturation levels, influenced by waterproof treatment uniformity on GDLs. Residual ice can be detrimental to the cold-start capability of PEFCs, as it promotes freezing of supercooled water around -1 degrees C. Our findings enhance understanding of water behavior in GDLs and its impact on PEFC cold-start performance. Applying this technique to other cell components, such as the catalyst layer, could further elucidate water freezing dynamics in PEFCs and guide the innovation of more reliable cell components for PEFC applications in broader climates.