3D multiphase simulation of a bipolar membrane water electrolyzer

Bipolar membrane water electrolyzers (BPMWE) combine the advantages of alkaline water electrolysis (AWE) and proton exchange membrane water electrolysis (PEMWE). Owing to their hybrid acidic-alkaline membrane architecture, they permit the use of inexpensive, non-noble catalysts and cost-effective porous transport layers on the alkaline side, while retaining the excellent hydrogen-evolution reaction kinetics of an acidic environment. Although substantial progress has been made in device development and experimental testing, there remains a need for numerical tools that can screen new materials, transport layers, and flow field geometries without the time and cost of extensive experimentation. This work presents a modeling study in which a three-dimensional, multiphase, steady-state CFD model for BPMWEs is developed, validated against literature data, and shown to provide actionable design insights. Two literature-based formulations for the bipolar interface reaction rate were implemented and compared in the CFD model. The results showed that a Butler-Volmer-type formulation, employing fixed values for the reference exchange current density and the symmetry factor, based on a trap-assisted mechanism, exhibited improved numerical robustness and smaller deviations from the experimental data. In addition, the simulations could be aligned with experiment only by introducing an additional loss term, the physical interpretation of which remains unresolved. As an application case, a constructal (fractal) flow field was designed and compared with a conventional serpentine architecture at two operating voltages. Under the operating conditions considered, the constructal design achieved a substantially lower pressure drop without discernible penalty in hydrogen production, indicating potential advantages for overall efficiency and reduced mechanical stress. • 3D full-cell multiphase CFD model for bipolar membrane water electrolyzers. • Explicit BPM interface kinetics implemented and comparison of two approaches. • Constructal flow field vs serpentine assessed at two operating voltages. • Resolves gas-liquid mass transport in channels and porous layers. • Constructal flow field reduces pressure drop at comparable electrochemical performance.