Simulation of Bubble Behavior and Energy Efficiency in Proton Exchange Membrane Electrolyzers based on Fluid Phase Field under Microgravity Conditions

Proton Exchange Membrane Electrolyzers (PEMELs) constitute a promising, clean, and renewable technology for hydrogen production and represent a critical component of life-support and environmental control systems aboard space stations. At present, PEMEL operation in space environments exhibits reductions in energy-conversion efficiency attributable to complex bubble dynamics within the cell; accordingly, a systematic investigation of intradevice bubble behavior is essential for optimizing overall system performance. In this study, we develop a multiphysics modeling framework that couples a detailed electrochemical model with a fluid phase-field model to perform a comparative analysis of how fundamental operating and geometric parameters — specifically inlet water velocity, outlet pressure, and channel depth — affect bubble dynamics in PEMELs under both normal-gravity and microgravity conditions, and to quantify the consequent effects on energy efficiency; the coupled model is validated through quantitative comparison with data from high-quality published literature. The simulation results indicate that, under both gravity regimes, increasing the inlet water velocity from 0.1 m·s^–1 to 0.5 m·s^–1 drives a transition of flow regime from wavy flow to dispersed bubble flow, yielding higher bubble coverage and a concomitant decrease in energy efficiency, and therefore high inlet water velocities (e.g., 0.5 m·s^–1) should be avoided. As the outlet pressure increases from 1 atm to 10 atm, the bubble regime progressively shifts from dispersed bubble flow toward wavy or thin-film flow, which increases bubble coverage and degrades energy efficiency; accordingly, high outlet pressures (e.g., 10 atm) should be avoided. Increasing channel depth from 0.8 mm to 1.2 mm promotes a transition from wavy flow to dispersed bubble flow, reduces lower-wall bubble coverage, and improves energy efficiency, suggesting an optimal channel depth range of approximately 1.0～1.2 mm. Variations in gravity direction exert only minor influence on bubble behavior and energy efficiency for millimeter-scale flow fields. These findings provide valuable guidance for bubble-management strategies and for the design and operation of PEMELs to enhance energy-conversion efficiency in both terrestrial and microgravity environments.