Title: Cold start mode classification based on the water state for proton exchange membrane fuel cells

Abstract: The cold start of proton exchange membrane (PEM) fuel cells is one of the primary factors limiting their large-scale commercialization in the automotive field. The water state has a significant impact on the start-up results. Identifying the water state inside fuel cells is the basis for optimizing cold start capabilities and strategies. In this study, the experimental parameters such as current/temperature distribution, cathode pressure drop, and high-frequency resistance of the PEM fuel cell are measured to study water states such as local icing and water transport. Based on the above water states, the PEM fuel cell cold-start processes are classified into seven modes, with the thermal equilibrium phenomenon serving as a salient benchmark for the thermal gap. The classification is valuable and general; it can significantly reduce the experimental and time costs of providing targeted optimization directions for the cold start processes of different PEM fuel cells.