Solar photoelectrochemical synthesis of electrolyte-free H2O2 aqueous solution without needing electrical bias and H2

Abstract

The conventional synthesis of hydrogen peroxide (H₂O₂) such as heterogeneous catalytic and electrochemical processes requires H₂ and O₂ as reagents, costly noble metals, and organic solvents, which are energy/waste-intensive and hazardous. An alternative method of photoelectrochemical (PEC) synthesis that needs only water and sunlight is environment-friendly but its practical application is limited due to the energy-demanding method for the separation of the synthesized H₂O₂ from the electrolytes. Herein, we demonstrated the direct synthesis of an electrolyte-free aqueous solution of pure H₂O₂ by developing a PEC system with solid polymer electrolyte (SPE) and engineered electrodes. Ruthenium catalyst-decorated TiO₂ nanorods (RuO_x/TNR: photoanode) and anthraquinone-anchored graphite rods (AQ/G: cathode) are placed in an anode compartment and a cathode compartment, respectively, while a middle compartment containing SPE is located between these compartments. Upon solar simulating irradiation (AM 1.5G, 100 mW cm⁻²), the photoanode generates H⁺ ions via water oxidation reaction (WOR) and the cathode generates HO₂⁻ ions via two-electron oxygen reduction reaction (ORR), while the SPE selectively transports H⁺ and HO₂⁻ into the middle compartment to form pure H₂O₂ solution. The combined system enabled continuous H₂O₂ synthesis over 100 h even under bias-free (0.0 V of cell voltage) conditions with the production of ∼80 mM H₂O₂ (electrolyte-free) and a faradaic efficiency of ∼90%, which is the highest concentration of pure H₂O₂ obtained using PEC systems. This study successfully demonstrates the proof-of-concept that might enable the production of a concentrated pure (electrolyte-free) aqueous solution of H₂O₂ using sunlight, water, and dioxygen only.