Comprehensive analysis of area effects and spatial non uniformity in a large area PEM fuel cell under different cold start strategies using a three dimensional non isothermal multi physics model

Reliable cold start of large-area proton exchange membrane fuel cells (PEMFCs) remains a key bottleneck for fuel cell vehicles, because most mechanistic studies have focused on small laboratory cells whose behavior cannot be directly extrapolated to large-scale devices. This study develops a transient three-dimensional non-isothermal multi-physics model of a 79.5 cm² PEMFC with an eleven-channel serpentine flow field to elucidate coupled heat, water and ice processes during self-cold start at −30 °C. On this basis, three start strategies are examined. The global responses reveal three characteristic stages: reaction-limited, self-heating dominated, and icing/transport-limited. These stages develop in a strongly non-uniform manner: downstream regions heat up faster than inlet zones, and the multi-channel serpentine geometry induces pronounced variations of temperature and current density. Higher ramps increase cathode catalyst layer (CL) heating rates from 0.88 to 1.86 K min⁻¹, but also amplify temperature and current-density non-uniformity. Ice forms preferentially in the cathode CL, where limited gas-phase transport and low saturation vapor pressure hinder water removal, and then propagates into the gas diffusion layer (GDL). In-plane distribution and a quantitative uniformity index reveal that aggressive ramps promote early nucleation of ice clusters near channel bends and downstream regions, followed by rapid coalescence into extended ice-rich bands. Through-plane analysis shows that high ramps drive deep ice penetration into the GDL, whereas moderate ramps keep most ice confined near the CL. These results clarify how large-area effects and current-ramp design jointly control cold-start failure, and provide mechanistic guidance to improve cold-start strategies for automotive PEMFCs.