A PEMFC recirculation system without an external humidifier in an air-independent environment

A proton exchange membrane fuel cell (PEMFC) generates electricity through an electrochemical reaction between hydrogen and oxygen, characterized by low operating temperatures and a fast response. In air-independent environments, such as underwater or in space, where an air supply is unavailable, pure oxygen is used. In these systems, recirculation is often applied to increase the rate of reactant utilization. Additionally, the absence of nitrogen allows for sufficient humidification with less water, making it possible to configure a system without an external humidifier. However, while this characteristic simplifies humidification, it also increases the condensation tendency and the potential risk of electrode flooding. To mitigate this condensation tendency, a study of system-level operating conditions is necessary. Hence, the goal of this study is to model a PEMFC recirculation system without an external humidifier. The objective is to investigate the effects of the gas-liquid separator temperature, stack temperature, current load, and electrode-specific recirculation ratios on the performance and water-condensation tendency within the PEMFC. Consequently, while the separator temperature has a minor impact on maximum net power (varying within 5.034–5.064 kW), it substantially changes the onset SR for zero stack-outlet liquid fraction from 3.5 to over 7.0. Lower stack temperatures show a high tendency for liquid water condensation, while 90 °C causes dehydration and about 0.46 kW power loss. Regarding electrode-specific stoichiometric ratios, achieving a zero stack-outlet liquid fraction requires an anode SR of 4.5, whereas the cathode SR must reach 6.5, indicating that the electrode-wise flow distribution significantly influences the condensation tendency. Finally, the onset SR for zero stack-outlet liquid fraction remains at 4.0 regardless of the current density; however, the volumetric flow rate increases.