Influence of gravity on water management and mass transport losses in polymer electrolyte membrane fuel cells.

Traditionally, gravity has been assumed to have a negligible impact on the multiphase transport behaviour in polymer electrolyte membrane (PEM) fuel cells; however, in this study, we reveal how the impacts of gravity and flow field orientation should not be ignored. Gravity-assisted reactant flow provides up to a 21.2 % higher peak power density compared to gravity-opposed reactant flow, owing to enhanced water removal (which we observed via operando synchrotron radiography). We are the first to combine a distribution of relaxation times (DRT) analysis with operando imaging, and for the first time, we separately correlate the presence of liquid water in the channels and GDLs to distinct mass transport loss contributions via this approach. Liquid water accumulation in the cathode GDL is most typically the focus of water management in the PEM fuel cell; however, in this work, we observed significant water accumulation in the anode GDL and channels at gravity-opposed orientations. Specifically, we observed large droplets and slugs in the anode channels which led to significantly higher anode GDL water saturation (≥ 0.23) compared to gravity-assisted angles (≤ 0.11). The force of gravity overpowers the weaker inertial force of reactant hydrogen flow, thereby hindering the removal of water droplets in the anode flow fields, which results in poor water management, reactant starvation and ultimately cell failure.