Flow-field-dependent Spatial Freezing Characteristics and Amplification of Flow-Field Effects by Start-Up Conditions During Self-Cold Start of a Large-Area PEM Fuel Cell

Proton exchange membrane fuel cell (PEMFC) self-cold start is strongly affected by flow-field design, especially in large-area cells where local transport non-uniformity can be readily amplified into regional freezing and performance degradation. In this work, a three-dimensional transient non-isothermal model was established to explore the self-cold start behavior of 112 cm2 large-area PEMFCs under different cathode flow-field configurations. Particular attention was given to the amplification of flow-field effects by start-up conditions, as well as to the flow-field-dependent spatial freezing characteristics. The results show that under the baseline self-cold start condition, the multi-channel serpentine flow field increased the peak current density by approximately 5.6%, and extended the failure time from 81 to 91 s. More importantly, its superiority was not simply associated with a lower average ice fraction, but with a more favorable freezing pattern. The multi-channel serpentine flow field suppressed early localized icing at the electrochemical reaction interface and delayed the coalescence of isolated ice spots into connected clusters. It also alleviated the direct freezing burden in the cathode catalyst layer, while part of the freezing tendency shifted toward the cathode gas diffusion layer. Lower start-up temperature intensified the consequence of local ice coalescence, whereas the effect of start-up voltage was non-monotonic, with 0.4 V providing the most favorable compromise between heat accumulation and freezing progression. These findings indicate that, for large-area PEMFCs, temperature uniformity and freezing topology are more critical than average thermal response alone, and that multi-channel serpentine cathode flow fields are more suitable for low-temperature start-up.