Origin of Limiting and Overlimiting Currents in Bipolar Membranes

Developmentand application of bipolar membranes is thriving.We present a trackable model to predict the behavior of a commercialbipolar membrane throughout its entire operational window (water dissociationand recombination). Bipolar membranes (BPMs), a special class of ion exchangemembraneswith the unique ability to electrochemically induce either water dissociationor recombination, are of growing interest for environmental applicationsincluding eliminating chemical dosage for pH adjustment, resourcerecovery, valorization of brines, and carbon capture. However, iontransport within BPMs, and particularly at its junction, has remainedpoorly understood. This work aims to theoretically and experimentallyinvestigate ion transport in BPMs under both reverse and forward biasoperation modes, taking into account the production or recombinationof H+ and OH-, as well as the transportof salt ions (e.g., Na+, Cl-) insidethe membrane. We adopt a model based on the Nernst-Planck theory,that requires only three input parameters membrane thickness,its charge density, and pK of proton adsorption topredict the concentration profiles of four ions (H+, OH-, Na+, and Cl-) insidethe membrane and the resulting current-voltage curve. The modelcan predict most of the experimental results measured with a commercialBPM, including the observation of limiting and overlimiting currents,which emerge due to particular concentration profiles that developinside the BPM. This work provides new insights into the physicalphenomena in BPMs and helps identify optimal operating conditionsfor future environmental applications.