Ion Exchange Mediates the Voltage of Bipolar Membranes in Impure Electrolytes

Understanding ion transport processes in polymeric ion exchange layers is an important direction for the development of bipolar membranes, particularly as the applications of these composite materials expand into concentrated and compositionally complex electrolytes. This work shows that the presence of salt (e.g. KCl) as an impurity in strong acid/base causes erosion of the transmembrane potential (the open circuit voltage, OCV) beyond what is predicted by the Nernst equation, and that this OCV erosion is related to increased salt crossover current under reverse bias polarization. We interrogate these phenomena through a combined approach of electrochemical experiments in a four-electrode setup and 1D continuum modeling. The results reveal that ion-ion associative interactions, which we represent in the model as reactions between mobile ions and fixed charge sites within the membrane, provide mechanistic pathways for the experimentally observed salt crossover and OCV erosion.