Thermal stability analysis of cold start processes in PEM fuel cells

[Display omitted] •Thermal stability analysis method for cold-start of PEM fuel cell is developed.•Multiplicity features behind ice formation and thermal behavior are revealed.•Two startup modes and criteria for a successful one are proposed.•Temperature and concentration effects are clarified to improve operability. Cold-start issue of proton exchange membrane (PEM) fuel cells is one of the major factors hindering its commercialization. The key element involved is the competition between ice formation and melting, behind which is the thermal behavior accompanied by nonlinear effects. In this work, the steady-state multiplicity feature during cold-start has been discovered based on the thermal stability analysis approach. The nonlinear heat generation term Qg and linear removal term Qr are separated from conservation equations and constructed against ice fraction. The plot thus obtained could identify the existence of multiplicity which is the intersection of the above two functions. Results indicate that one or two steady states arise for different operating conditions. The startup with only one steady state, namely the “extinguished” state is the worst condition and bound to fail. The startup with two steady states also incorporates an unstable one which denotes not only the critical transition from ice formation to melting but also the limit of the operational domain. Based on the theoretical analysis, simple and explicit criteria are quantitatively developed for the prediction of startup feasibility. Moreover, in order to ensure a reasonable and efficient operation, the impacts of key parameters are further summarized and discussed on the cold-start operability. The proposed approach that develops analytical expressions for the steady-state points provides a systematic yet simple way to reveal the essential physics of cold start.