Insight into resistance relaxation characteristics of proton exchange membrane fuel cell in purge operation: Experiment, application, and mechanisms

The application of proton exchange membrane fuel cell (PEMFC) is of great significance for alleviating environmental pollution and energy crisis. Cold start of PEMFC at low temperatures presents a major technical barrier to their widespread application and commercialization, and the crucial prerequisite for a successful cold start is to achieve an optimal level of membrane water content through the purge procedure. The high frequency resistance (HFR) of PEMFC is an important parameter and a common method to characterize the membrane water content. However, the HFR relaxation characteristics make the determination of the appropriate water content boundary of purge operation ambiguous. In this paper, a series of purge conditions are carried out to study the effects of different factors on the water removal during both equilibrium purge and on-load purge, and the evolution of HFR with time after the end of gas purge operation is emphasized. The effects of purge gas conditions, load current, and stack temperature on the HFR relaxation characteristics after purging are studied systematically for the first time, the different degrees of HFR relaxation caused by various factors are quantitatively analyzed. Based on this, an effective optimization strategy considering HFR relaxation is proposed for on-load purge, along with the potential mechanisms behind the HFR relaxation phenomenon is revealed. The results show that different purge conditions influence both water removal efficiency and HFR relaxation characteristics, and the compensated on-load purge method improves the control accuracy of water content. Moreover, the mechanisms of HFR relaxation are elusive and intricate, this phenomenon results from the combination of membrane rehydration, membrane water structure reorganization, and other factors, with the dominant causes of HFR relaxation varying depending on the purge method used. This research provides new insights for an in-depth understanding of HFR relaxation characteristics and for guiding the optimization of purge strategies.