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

The conventional synthesis of hydrogen peroxide (H2O2) such as heterogeneous catalytic and electrochemical processes requires H-2 and O-2 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 (RuOx/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 similar to 80 mM H2O2 (electrolyte-free) and a faradaic efficiency of similar to 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.