Electrolyte Composition Effects on Bipolar Membrane Polarization and Transport for Electrochemical Separations

Bipolar membranes (BPM), consisting of an anion exchange polymer membrane layer, a cation exchange polymer membrane layer, and a catalytic interfacial layer in between, enable electrochemical cells with strong gradients of pH between adjacent chambers. Bipolar membrane electrodialysis (BPMED) is a prominent example of an electrochemical separation process relying on this unique type of membrane material. In bipolar membrane electrodialysis, the BPM operates in “reverse bias,” dissociating water into hydronium and hydroxide at the interfacial layer, generating acidic and alkaline streams, while a third electrolyte chamber is essentially desalinated as it provides a source of counterions (<i>e.g. </i>K⁺ and Cl^-) for the acid and base streams. Recently, this process has seen increased attention for applications in resource recovery and for removing anthropogenic carbon dioxide from the environment. Unlike the traditional application of BPMED, to generate pure acid and base as chemical products, the emerging applications involve the BPM contacting electrolytes that include supporting salt or environmental impurities, which can have significant effects on the polarization behavior of the membrane as well as its transport selectivity of protons/hydronium and hydroxide over their counterions and other salts in the electrolyte.<br/><br/>In this work, we systematically investigate the effects of ion size, charge, and concentration on salt crossover in a widely-used commercial BPM (Fumasep FBM) by combining polarization experiments in a four-electrode setup with a 1D continuum model of water dissociation and multi-ion transport. We find that salt crossover dominates at low current density, before the stronger electric field at larger applied potential drives water dissociation to dominate. Further, we investigate how the total ion concentration and ratio of supporting salt to acid and base concentration influence the overpotential across the membrane and the contributions of salt crossover to total current.<br/><br/>Both high overpotentials associated with water dissociation at high currents and the unwanted salt crossover at lower currents provide significant hurdles for the deployment of BPMs in electrochemical technology. This work emphasizes the importance of electrolyte effects on these phenomena, and may provide guidance both for designing new materials and for choosing electrolytes and operating conditions in electrochemical cells.