A simple model to determine the trends of electric-field-enhanced water dissociation in a bipolar membrane. II. Consideration of water electrotransport and monolayer asymmetry

This work elucidates the mechanism of electric-field-enhanced water dissociation. Particular attention has been given to the influences of water electrotransport and monolayer asymmetry on the water dissociation process. A simple model was proposed with consideration of these two factors and mathematically analyzed in term of thickness ratio, fixed group concentration ratio and water diffusivity ratio of the anion selective layer to the cation selective layer on typical current density curves of bipolar membranes. The results suggest that for practical applications, an asymmetric bipolar membrane with proper ion-exchange capacity and high permeability to water is more effective than a symmetric one. Theoretical simulation values were compared with both the theoretically calculated data by a model without consideration of water electrotransport and the experimental current voltage curves. It is shown that the calculated potential across a bipolar membrane is higher at given current density, which permits a more precise prediction of experimental I-V curves for the case of a bipolar membrane with high water permeability. However, for a bipolar membrane with poorer water permeability, it seems that the calculated value with the model without consideration of water electrotransport is closer to the experimental values, but the model with consideration of water electrotransport can effectively predict the over-limiting current density.