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.

Water dissociation (WD, H2O → H+ + OH-) is the core process in bipolar membranes (BPMs) that limits energy efficiency. Both electric-field and catalytic effects have been invoked to describe WD, but the interplay of the two and the underlying design principles for WD catalysts remain unclear. Using precise layers of metal-oxide nanoparticles, membrane-electrolyzer platforms, materials characterization, and impedance analysis, we illustrate the role of electronic conductivity in modulating the performance of WD catalysts in the BPM junction through screening and focusing the interfacial electric field and thus electrochemical potential gradients. In contrast, the ionic conductivity of the same layer is not a significant factor in limiting performance. BPM water electrolyzers, optimized via these findings, use ~30-nm-diameter anatase TiO2 as an earth-abundant WD catalyst, and generate O2 and H2 at 500 mA cm-2 with a record-low total cell voltage below 2 V. These advanced BPMs might accelerate deployment of new electrodialysis, carbon-capture, and carbon-utilization technology.

In alkaline and neutral MEA CO2 electrolyzers, CO2 rapidly converts to (bi)carbonate, imposing a significant energy penalty arising from separating CO2 from the anode gas outlets. Here we report a CO2 electrolyzer uses a bipolar membrane (BPM) to convert (bi)carbonate back to CO2, preventing crossover; and that surpasses the single-pass utilization (SPU) limit (25% for multi-carbon products, C2+) suffered by previous neutral-media electrolyzers. We employ a stationary unbuffered catholyte layer between BPM and cathode to promote C2+ products while ensuring that (bi)carbonate is converted back, in situ, to CO2 near the cathode. We develop a model that enables the design of the catholyte layer, finding that limiting the diffusion path length of reverted CO2 to ~10 μm balances the CO2 diffusion flux with the regeneration rate. We report a single-pass CO2 utilization of 78%, which lowers the energy associated with downstream separation of CO2 by 10× compared with past systems.