Abstract
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•Higher startup current density causes larger voltage undershoot.•Increasing cathode humidity is more beneficial to FC startup than the anode.•The magnitudes of voltage undershoot depend on the membrane water content.•The variation mechanism of water content in the membrane is proposed and validated.
The reverse voltage is critical for the performance and durability of a proton exchange membrane fuel cell (PEMFC). The reverse process will cause a complex kinetic transformation from normal to abnormal, leading to degradation of catalyst. There are few experimental studies and some numerical simulations reported on this issue, however, the process of reverse voltage under dehydration conditions is still unclear. This article explores the voltage reversal process and dehydration phenomenon of a PEM fuel cell during the start-up process. The effects of operating parameters on the occurrence of the reverse voltage are examined, including the startup current density, the inlet gas relative humidity (RH) and the gas stoichiometric ratio. To understand the reverse process, the voltage, high frequency resistance (HFR) and the outlet gas relative humidity during startup are monitored online in real time. Results show that the reverse voltage occurs when the fuel cell starts up at the condition of low relative humidity, high current density load and high hydrogen stoichiometry at low humidity. From the beginning of the startup process, the humidity of the anode outlet gas decreased significantly, which is correlated with the increase of HFR and the drop of voltage. By time constant analyzing of the process, it was observed that the dehydration on the anode side of the membrane electrode during the startup period led to an instantaneous increase of HFR, which reveals the process mechanism of voltage reversal. Finally, we suggest a startup strategy to reduce or even avoid the reverse voltage.
