Kinetics and mechanism of heterogeneous voltage-driven water-dissociation catalysis

The water-dissociation (WD) reaction (H2O → H+ + OH−) affects the rates of electrocatalytic reactions and the performance of bipolar membranes (BPMs), but how WD is driven by voltage and catalyzed is not understood. We report BPM electrolyzers with two reference electrodes (REs) to measure temperature-dependent WD current and overpotential (ηwd) without soluble electrolyte. Using TiO2-P25-nanoparticle catalyst and Arrhenius-type analysis, we found Ea,wd of 25–30 kJ/mol, independent of ηwd, and a pre-exponential factor proportional to ηwd that decreases ∼10-fold in D2O. We propose a new WD mechanism where metal-oxide nanoparticles, polarized by the BPM-junction voltage, serve as proton (1) acceptors (from water) on the negatively charged side of the particle to generate free OH−, (2) donors on the positively charged side to generate H3O+, and (3) surface proton conductors that connect spatially separate donor/acceptor sites. Increasing electric field with ηwd orients water for proton transfer, increasing the pre-exponential factor, but is insufficient to lower Ea.