Impedance investigation and switching strategy study of constant-electrode unitized regenerative fuel cell from electrolytic cell through purging to fuel cell

The unitized regenerative fuel cell (URFC) is an integration of hydrogen production and power generation through the same device. The efficient purging is crucial for smooth and fast mode switching, especially from electrolytic cell to fuel cell (FC). However, there is not a universal standard of mode switching to guide the optimization of purging strategies. This paper investigates the variation in the dynamic responses under various strategies in terms of durations, gas flow rates and mediums. Results show that altering the gas flow rate is an effective means of optimization compared to the duration. An inadequate flow rate can only drive the URFC to certain water content far from flooding. However, the purging time is wasted, and an effective indicator is urgently required to judge the state. The high-frequency resistance during purging is then shown to be useful as a joint indicator for successful FC startup, while the low-frequency resistance is ineffective. Selecting the resistance of 3000 mΩ cm2 of a 10 cm2 URFC could enable remarkable decrease of purging duration from 30 to 6 min, whereas other strategies will cause failed FC startup. The results will be useful for the closed-loop regulation and faster switching in real URFC application.