Diffusion controlled porous WO3 thin film photoanodes for efficient solar-driven photoelectrochemical permanganic acid production

Abstract

Photoelectrochemical (PEC) oxidation of the divalent manganese ion to the permanganate ion was achieved by using porous WO₃ thin film photoanodes in sulfuric acid electrolytes under simulated sunlight. In this reaction system, the subsequent reaction of the evolved MnO₄⁻ (Mn⁷⁺) with residual Mn²⁺ caused deterioration of photoanodes by Mn solid deposit formation. However, we realized stable PEC reaction by diffusion control of the evolved Mn⁷⁺ in the pores of the photoanodes. We also confirmed that the PEC reaction for the evolution of Mn⁷⁺ proceeded intrinsically very efficiently, and the faradaic efficiency reached 94.8% in an electrolyte containing 1.0 mM Mn²⁺ and 1.0 M H₂SO₄ at a low bias potential voltage of 1.3 V_RHE. To use the photoelectrochemically evolved Mn⁷⁺ in sulfuric acid solutions effectively, a biphasic reaction involving the oxidation of ethylbenzene was carried out. The ethylbenzene added to the anode cell was converted to acetophenone without any byproducts, and this conversion occurred concurrently with the PEC reaction that produced Mn⁷⁺. At the cathode, evolution of hydrogen occurred with 95–99% faradaic efficiency. Thus, we have successfully demonstrated dual benefits of this reaction system: the production of high-value-added organic reagents mediated by the PEC-derived Mn⁷⁺ and the production of solar hydrogen.