Abstract A multi-dimensional transient numerical alkaline water electrolyzer (AWE) model is applied to zero-gap circular shaped cells in which either a porous separator (Zirfon) or a dense polybenzimidazole (PBI) membrane is sandwiched between the negative and positive electrodes of nickel foam. In general, not only do the model predictions compare well with the experimental data collected under various wt% KOH electrolyte conditions, but they also reveal key features that favor the use of the PBI membrane over the porous separator. Moreover, a lower ohmic polarization and resulting higher cell performance are achieved by using the PBI membrane owing to its higher ionic conductivity and thinner thickness compared to those of the separator. Although a thin PBI membrane of around 50 μ m is employed, simulation results demonstrate that the crossover of hydrogen and oxygen gases is effectively suppressed with a 10-fold thicker separator (500 μ m ).