Enhancing Electrochemical Water Oxidation toward H₂O₂ via Carbonaceous Electrolyte Engineering

Electrochemical production of H₂O₂ from water is a promising route toward improving the value and utility of water electrolysis. Though several studies have demonstrated the superior performance of bicarbonate electrolytes (especially concentrated potassium bicarbonate, 2 M KHCO₃), none have focused on systematically engineering the electrolyte to further improve H₂O₂ production. Here, we use chronoamperometry to investigate the impact of the bicarbonate and carbonate mole fractions, total dissolved inorganic carbon (DIC) concentration, and electrolyte cations on the selectivity and current density toward H₂O₂ at varying applied potentials. We identify a novel optimized electrolyte composition of 0.5 M KHCO₃ and 3.5 M K₂CO₃ at a voltage of 3.25 V vs RHE that improves the faradaic efficiency toward H₂O₂ from 5% in 2 M KHCO₃ to 45% on a fluorine-doped tin oxide anode. Correspondingly, the current density toward H₂O₂ production (J_H2O2) increases from 0.38 to 4.7 mA/cm², a 12-fold improvement. Moreover, the optimized electrolyte leads to more stable H₂O₂ production over time. Importantly, we find that the optimized electrolyte also improves the faradaic efficiency and H₂O₂ production rate when using other electrode surfaces (e.g., zinc oxide, bismuth vanadate, and titanium dioxide), demonstrating its general applicability. Our techno-economic analysis reveals that the optimized electrolyte reduces the cost of electricity necessary to produce a kilogram of H₂O₂ by over 85% in comparison to 2 M KHCO₃ and the estimated electrolyzer thermal efficiency reaches ∼42% at 3.25 V vs RHE. Together, these results highlight the promise of electrolyte engineering to bring the production of H₂O₂ via water oxidation into practice.