(404b) Modeling Local pH and Ionic Fluxes in Bipolar Membranes

Bipolar ion-exchange membranes (BPMs), which consist of an anion-exchange layer and a cation-exchange layer stacked in the through-plane direction, have recently demonstrated potential to become critical components in devices for energy conversion and storage. Due to the opposite charges of the individual ion-exchange layers, BPMs facilitate the operation of electrochemical devices under a pH gradient, enabling greater control of the environments for catalysis of the electrochemical reactions occurring at each electrode in the device. Nonetheless, a complete understanding of ionic transport within BPMs has been lacking. Additionally, the local pH within BPMs, particularly within the interface of the anion- and cation- exchange layers, is challenging to probe experimentally. This gap in understanding motivates the development of a theoretical framework capable of describing local environments and predicting electrochemical performance of BPMs.

In this presentation, we detail a 1-D continuum model of a BPM that adequately predicts local pH and electrolyte environment for a wide range of experimental conditions. The model can resolve internal ionic fluxes and the impacts of the exchanged electrolyte species—namely, the impacts of the buffering anion—on electrochemical behavior of BPMs and can predict polarization behavior consistent with experiment. Additionally, the model identifies the local pH and surface species concentrations within the catalytic interface of the BPM and underscores the importance of employing an internal water dissociation catalyst to achieve high current density. The insights gained from this work enable an understanding of how various design parameters of the individual ionomers impact BPM performance under applied pH gradients, rationalizing and guiding the design of next-generation BPMs for energy applications.