Kinetics and Mechanism of Heterogeneous Voltage-Driven Water-Dissociation Catalysis

27 April 2023, Version 1
This content is a preprint and has not undergone peer review at the time of posting.


The water-dissociation reaction (WD, H2O → H+ + OH−) affects the rates of electrocatalytic reactions and the performance of bipolar membranes (BPMs). How catalyzed interfacial WD is driven by voltage, however, is not understood. We designed a BPM electrolyzer with two reference electrodes attached laterally to each layer/side (here, poly(arylpiperidinium) and perfluorosulfonic-acid ionomers) to measure WD current and overpotential (ηwd), without soluble electrolyte and as a function of temperature and catalyst-layer properties. Using TiO2-P25 nanoparticles as a model WD catalyst, Arrhenius-type analysis yields a WD activation energy Ea of 25–30 kJ mol−1, only weakly dependent ηwd. The pre-exponential factor is unexpectedly proportional to ηwd. With D2O, ηwd is ~2 to 4 times larger than in H2O, largely due to a lower pre-exponential factor. Without catalyst, ηwd is ~10-fold larger and Ea decreases from 34 to 24 kJ mol−1 as ηwd goes from 0.1 to 1 V. To explain these data, we propose a new WD mechanism where metal-oxide nanoparticles, polarized by the voltage across the BPM junction, serve as i) proton acceptors (from water) on the negative sides of the particle to generate free OH−, ii) proton donors on the positive sides to generate H3O+, and iii) surface proton conductors that connect spatially separate donor/acceptor sites. Increasing electric-field strength with overpotential orients water for proton-transfer elementary steps comprising WD, increasing the pre-exponential factor and hence rate, but is insufficient to lower Ea. This understanding will accelerate development of electrocatalysis, electrodialysis, carbon-capture, and carbon-utilization technologies that require efficient WD.


water dissociation
bipolar membrane
activation energy analysis
heterogeneous catalysis
second Wien effect
kinetic isotope effect
membrane potential sensing

Supplementary materials

Supplementary Information Kinetic Analysis and Mechanism of Voltage-Driven Water-Dissociation Catalysis in Bipolar Membranes
Temperature-dependent WD polarization curves for BPMs with different loadings of TiO2-P25 as WD catalyst; measured WD overpotential ηwd using electrochemical impedance spectroscopy (EIS); correction of degradation over time for a pristine, uncatalyzed BPM; kinetic isotope effect; Arrhenius analysis of the temperature-dependent polarization curves of BPMs with different mass ratio of ACB and TiO2-P25; photographs and set-by-step procedure to assemble membrane-potential-sensing system.


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