Understanding Water Dissociation Physics through Controlled Oxide Surface Chemistry

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Bipolar membranes (BPMs) provide a platform for the interconversion between electric and chemical potential gradients and the precise control over local ion concentrations and fluxes, making BPMs attractive materials for many electrochemical processes. In reverse bias charge carriers must be generated via heterolytic water dissociation (WD, 2H2O → OH- + H3O+), and which must be accomplished at high rates and low overpotentials to realize scalable BPM applications. It is hypothesized that WD is driven by a combination of electric field and catalytic effects within the bipolar junction. However, the extent to which each contributes to observed rates, and the possible interplay between the two, is yet to be resolved.Here, we explore the interfacial physics within the BPM to understand the kinetics and mechanisms of water dissociation on TiO2 and graphene oxide (GO) derivative catalysts. Using a membrane-potential-sensing testbed we isolate the WD polarization signature in a BPM water electrolyzer. We find that neat-GO exhibits an anomalous exponential current response as a function of WD overpotential, contrary to the linear polarization response of TiO2 (and other metal oxides, reported previously). Similarly, the rates were independent of GO loading, contrasting the typical U-shaped loading dependence on WD performance using metal oxide catalysts. We hypothesize the unique behavior of GO can be linked to the double layer profile within the bipolar junction, which can be screened and localized to the catalyst/AEL interface via the acidic carboxylate moieties on the GO surface. We validate this framework by controlling the degree of hydroxyl/carboxylation on the graphene-oxide surface, and thus the electric double layer profile, to control the polarization response. Using temperature-dependent measurements at varying potentials and a simple Arrhenius-type equation we decouple the apparent enthalpic and entropic driving forces for WD within a confined and polarized heterojunction. Broadly, this work advances our understanding of the interplay between catalyst surface chemistry and electric double layer formation under confinement, and how this effect can be leveraged in (electro)chemical transformations. Figure 1. A large electric field within the catalyst layer increases the WD rate within the BPM Figure 1