(585d) Bipolar Membrane Polarization Governed By Interfacial Ionic Species

Bipolar membranes (BPMs), consisting of an anion exchange polymer membrane layer, a cation exchange polymer membrane layer, with a catalytic interfacial layer in between, enable electrochemical cells with large difference of pH between adjacent chambers. This pH gradient is a working principle of a number of devices for energy and sustainability: acid/base batteries store and release energy by building and depleting a pH difference; BPM electrodialysis reactors capture CO₂ in alkaline electrolytes and release CO₂ by acidifying; CO₂ and H₂O electrolysis devices may employ pH-decoupled electrodes to avoid precious metal electrocatalysts while maintaining low electrode overpotentials. For all of these devices the most efficient operation - in terms of extracting maximum voltage from a battery or driving electrolysis with minimum overpotentials - is achieved when the BPM selectively transports H⁺ and OH^- ions in the cation and anion exchange layers respectively. But in practical devices, salt impurities may be present in the electrolytes, or supporting salt may be intentionally added to improve electrolyte conductivity. In this work, we explore how ion exchange processes in bipolar membranes in impure electrolytes affect the ion composition at the bipolar junction - the interfacial layer where the two membrane layers meet. We propose that the ion composition at this interface determines the voltage measured across the membrane at open-circuit, as well as the polarization behavior of the membrane under applied current.

We investigate the proposed mechanism using a model system comprising electrolytes with KOH, HCl, and KCl at various relative concentrations with a commercially available BPM (Fumasep FBM). We combine analysis of polarization experiments in a four-electrode setup with a 1D continuum model of water dissociation and multi-ion transport. We report that increased relative salt concentration in the electrolytes erodes the open-circuit voltage across the membrane, yielding voltages significantly lower than expected based on the measured pH difference, indicating that H⁺ and OH^- ions within the BPM exchange with ions from the added salt to occupy membrane fixed charges at the bipolar junction. This phenomenon penalizes the energy efficiency of BPM acid/base batteries. Additionally, we find that during polarization, impure electrolytes exhibit salt crossover at low current density and that the magnitude of this crossover depends on the relative concentration of the salt versus H⁺ and OH^- ions. We utilize the 1D continuum model to visualize concentration profiles of ions in the BPM at both open-circuit and applied current conditions.

This work illustrates the importance of electrolyte species on interfacial BPM phenomena and provides guidance both for designing new BPM materials and for choosing electrolytes and operating conditions in electrochemical cells.