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 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 (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 ∼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.