Cold start of proton exchange membrane fuel cells in high-altitude cold regions: coupled heat-water-oxygen mechanisms, system constraints, and engineering validation

Cold start remains a major barrier to the reliable deployment of proton exchange membrane fuel cells (PEMFCs) in subfreezing environments. High-altitude cold regions impose a compound operating boundary in which low temperature, reduced ambient pressure and oxygen partial pressure, dry air, cold-soak history, compressor demand, coolant thermal inertia, and limited auxiliary energy act simultaneously. This critical review integrates previously separated evidence on generic cold start, high-altitude operation, residual-water control, air supply, heating, and liquid-cooled thermal management. The literature is classified according to whether a study directly tests the coupled high-altitude/subfreezing boundary, tests only one constituent boundary, provides validated simulations or architecture-specific evidence, or supports an engineering synthesis. The combined evidence indicates that reduced oxygen availability may constrain the admissible heat-generating current before water production becomes the sole limiting factor, while minimum cell voltage and coolant response emerge as key stack-scale variables. On this basis, the review proposes an altitude-aware sequence linking purge, loading, air supply, heating, and coolant operation and defines success through thermal, electrochemical, system-energy, and durability criteria. Because direct full-size liquid-cooled validation under simultaneous reduced pressure and subfreezing temperature remains scarce, the immediate priorities are compound-environment experiments, platform-specific weakest-cell safety boundaries, and experimentally verified air-current-coolant switching with unified net-energy accounting.