Freezing Behaviors of Supercooled Water Affected by the Microscopic Contact Angle in the GDL of PEMFC

The microstructure of the gas diffusion layer (GDL) in proton exchange membrane fuel cell (PEMFC) significantly influences the freezing behavior of supercooled water. Microscopic contact angles of water droplets within various GDLs were measured using an environmental scanning electron microscope (ESEM). These measurements reveal a notable discrepancy between microscopic and macroscopic contact angles: the macroscopic contact angle increases more substantially following hydrophobic treatment than its microscopic counterpart. The incorporation of polytetrafluoroethylene (PTFE) slightly increases the microscopic contact angle. Freezing experiments were conducted on isolated GDL samples to examine the supercooling degrees and freezing times for both commercial and laboratory‐treated GDLs. Subsequently, the freezing process of supercooled water was modeled based on reconstructed three‐dimensional (3D) GDL structures and heterogeneous nucleation theory to predict freezing times and supercooling degrees. The results indicate that the microscopic contact angle is one of the primary factors governing the ice nucleation of supercooled water inside the GDL. Notably, even after hydrophobic treatment, residual weakly hydrophobic regions within the pores remain dominant, thereby critically influencing freezing behavior.