Effects of gravity on the cell performance of proton exchange membrane water electrolyzer

Hydrogen is of great significance for building a low-carbon and safe energy system. The proton exchange membrane water electrolyzer (PEMWE) is considered a highly promising hydrogen production device. However, due to the complex physical and chemical processes within the cell, and the impact of gravity effect on the cell characteristic is unclear. Analyzing the impact of gravity on cell performance is critical. In this study, the threedimensional two-phase flow models are developed for the PEMWE, the models couple many physical fields, including the hydrodynamics, electrochemical reaction kinetics, mass transfer, heat transfer, two-phase flow based on Euler model, and gravity effect. The simulation model is verified through experiment results. The effect of gravity on the cell performance is investigated, the influences of gravity under different placement directions are compared, and the impact of the cell voltage on the transport characteristics is discussed under the gravity effect. Results show that the gas transport and heat transfer are enhanced under the gravity, the active sites in the catalytic layer can be timely released due to the accelerated bubble removal, the polarization performance is accordingly improved. The cell performance with the gravity effect under vertical placement is higher than that with the gravity effect under horizontal placement because of the improved mass transfer, but the mixture velocity at anode flow channel is decreased. Moreover, as the cell voltage increases, the electrochemical reaction rate is enhanced, the heat transfer is increased, and the volume fraction of disperse phase in anode flow channel is accordingly increased. This study provides a theoretical reference for further investigating transport characteristics and optimising the cell performance.