How Does Water Dissociation Work in Bipolar Membranes?

Bipolar membranes (BPMs) create counteracting pH and electrostatic-potential gradients in electrochemical systems which make them of utility for pH regulation, electrocatalysis, and separations. At the polarized, interfacial junctions of a BPM, the water dissociation (WD, 2H2O ⇌ H3O⁺ + OH⁻) reaction can be driven, but it remains poorly understood. In this Perspective, we integrate molecular insights from bulk-water autoionization and field effects with continuum descriptions of BPM electrostatics and recent WD kinetics analyses. Pristine BPM junctions highlight both the limits of field-only acceleration and the practical challenges of junction stability at extreme reverse bias. Introducing heterogeneous catalyst layers, most commonly metal oxides and graphene oxides, accelerate WD by orders of magnitude through hypothesized coupled effects: i) surface acidbase functionality and hydroxyl-site density mediate proton-transfer steps, and ii) catalyst electronic/ionic polarizability redistributes the junction potential drop, shaping local electric fields and reactive microenvironments. Kinetic analyses suggest two regimes of heterogeneous WD mechanism, including fielddriven ordering of interfacial water and a Second Wien-type dissociation barrier lowering. We conclude by defining the key unknown variables (local pH, electrostatic potential, catalyst charge state and connection between mechanisms) and outlining experimental and multiscale modeling strategies needed for predictive WD catalysis and related ion-transfer reactions.