Electrochemical characterization of polymer ion-exchange bipolar membranes

A bipolar membrane (BM) is a layered structure composed of one cation and one anion ion-exchange layers joined together in series. Polymer BMs offer promising applications for many industrial processes (e, g., the use of bipolar electrodialysis for environmentally clean technologies and the treatment of salt-water effluents) because of their unique electrochemical properties. The most important of these properties is the electric field enhanced (EFE) water dissociation which arises when an electric current is forced through the membrane. This phenomenon occurs at the bipolar junction of the BM, and its coupling with ion transport, though still poorly understood, is the basis of most of the potential applications of BMs. In this review, we will focus on recent work concerning the physical chemistry of BMs and give a general overview of their electrochemical properties, emphasizing both theoretical and experimental aspects. We will model first the electric double layer at the bipolar junction between the two ion-exchange layers, and describe then the EFE water dissociation occurring in this junction. Later, we will present in detail a complete theoretical model for the coupling of ion transport and water dissociation in BMs, and show that this model is able to explain the experimental trends observed in the electrochemical characterization of polymer BMs by means of membrane potential, current-voltage curve, and impedance measurements.