Optimisation strategies for the preparation of bipolar membranes with reduced salt ion leakage in acid–base electrodialysis

The salt ion fluxes across commercial bipolar membranes (BPMs) result in the salt contamination of the produced acids or bases especially at increased product concentrations. Often, bipolar membrane electrodialysis can only be applied when these fluxes are reduced. Here, a model is presented to predict the salt impurities using the limiting current density measured for a single bipolar membrane. The model is extended to relate the limiting current density to the experimentally determined properties of the separate membrane layers. A direct dependence has been found for the salt ion fluxes across the bipolar membrane on the square of the solution concentration and the effective salt diffusion coefficient. Further, the salt ion transport is inversely dependent on the fixed charge density and the thickness of the layers. The latter is not trivial — the thickness in general does not play a role in the selectivity of separate anion or cation exchange membranes. The dependence of the salt ion transport on the membrane layer properties has been verified experimentally by characterising membranes prepared from commercially available anion exchange membranes and tailor-made cation-permeable layers. The presented model has proven to be both, simple and accurate enough to guide bipolar membrane development towards increased selectivity.