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

Summary

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 (η_wd) without soluble electrolyte. Using TiO₂-P25-nanoparticle catalyst and Arrhenius-type analysis, we found E_a,wd of 25–30 kJ/mol, independent of η_wd, and a pre-exponential factor proportional to η_wd 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 η_wd orients water for proton transfer, increasing the pre-exponential factor, but is insufficient to lower E_a.