Mechanistic Insights and Electrochemical–Catalytic Trade-Offs in Bromate-Mediated Decoupled Water Electrolysis

Bromate-mediated decoupled water electrolysis separates hydrogen production from oxygen evolution through an electrochemical–catalytic Br–/BrO3– cycle, but its implementation requires compatible conditions for bromide electrolysis and catalytic bromate decomposition. Here, we define the operating window, dominant polarization losses, and electrolyte trade-offs of this process in near-neutral NaBr electrolyte using separate batch measurements of the electrolytic and catalytic sub-processes. Bromide electrolysis was demonstrated over a current–density range of 0.001–10 A cm–2, with iR-corrected cell voltages of 1.9, 2.3, and 3.4 V at 0.1, 1, and 10 A cm–2, respectively, and Faradaic efficiencies above 95% under suitable conditions. Electrode-resolved polarization measurements, supported by electrochemical impedance spectroscopy and distribution-of-relaxation-times analysis, identify cathodic hydrogen evolution as the principal source of polarization loss. H3BO3 lowers cathodic polarization at low current density by stabilizing the local acid–base environment and increasing proton availability near the cathode, whereas Na2Cr2O7 suppresses cathodic back-reduction and improves bromate selectivity. However, both additives inhibit catalytic bromate decomposition, establishing a central electrochemical–catalytic trade-off. A composition containing 0.05 M H3BO3 and 0.05 g L–1 Na2Cr2O7 in 1.5 M NaBr solution provides a practical compromise under the conditions examined. Time-dependent batch measurements further show that electrolyte mixing, residence time, and concentration-gradient control will be critical for future continuous operation.