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Startup from subzero temperatures, referred to as “cold start”, has been one of the technical challenges hindering the commercialization of polymer electrolyte membrane fuel cell (PEMFC). In this study, a mathematical model has been developed to simulate the cold start processes in a PEMFC. The present three-dimensional multiphase model uniquely includes the water freezing in the membrane electrolyte, the non-equilibrium mass transfer between the water in the ionomer and the water (vapour, liquid and ice) in the pore region of the catalyst layer (CL), and the water freezing and melting in the CL and gas diffusion layer (GDL). Both the failed and successful cold start processes have been numerically investigated. Numerical results indicate that increasing the ionomer fraction in the cathode CL has more significant effects than increasing the thickness of the membrane layer in reducing the amount of ice …