CFD modelling of cathode conditions and membrane crossover flux in PEM water electrolysis

Abstract

Proton Exchange Membrane water electrolysis is a high-relevance technology to accomplish the industrial upscale of green hydrogen generation, addressing the urgency of the ecological transition thanks to its well-known principle and high conversion efficiency. In this study a comprehensive three-dimensional, two-phase Proton Exchange Membrane Electrolysis Cell (PEMEC) model is proposed, with the aim to investigate how different operating conditions influence the cell behaviour. Particular attention is posed on cathode pressure and humidification levels. The model was validated against experimental data showing excellent agreement between simulation results and experimental polarization curves for two working temperatures (333.15 K, 353.15 K), confirming that for an applied electric potential of 2.0 V the current density drops approximately 20 % for the lower temperature. Dry cathode conditions do not affect electrolysis efficiency due to the water electro-osmotic drag flux, which remains largely dominant on the water back-diffusion transport, keeping the polymeric membrane fully hydrated. Pressure increase at the negative electrode leads to slightly higher overpotentials for medium-low current densities ($\Delta E_{OCV}\cong +0.05156V$ for $p_c=30bar$) while ohmic losses are reduced allowing similar electrolysis performance for $i>2.0A/c{m}^2$. This means that PEMECs can operate at high efficiencies at optimal conditions for the direct hydrogen storage in specific metal hydrides (MH). Hydrogen cross-flow is highly dependent on the pressure differential and on the flow field, with an increase as the pressure raises and an accumulation in the corner of the serpentine. Even in the most critical condition ($p_c=30bar$) the maximum hydrogen molar concentration ($2.4936\times {10}^{-4}kmol/m^3$), remains below the 4 %mol limit of potentially explosive conditions, thus providing a virtual safety indication.