Pure Water-Fed Photoelectrochemical Water Splitting Using a Porous WO₃ Electrode Surface-Modified With...

Hydrogen production via photoelectrochemical (PEC) water splitting is a promising approach for efficient renewable-energy storage and transport. This study investigates a proton exchange membrane (PEM)-PEC system using pure water operated without adding supporting electrolyte. A porous tungsten oxide (WO₃) photoanode was surface-modified with a perfluorosulfonic acid (PFSA) ionomer coating. Under 365-nm UV irradiation and an applied cell voltage of 1.2 V, the PFSA ionomer-modified WO₃ electrode improved the incident photon-to-current conversion efficiency (IPCE) from 15% to 36% compared with the unmodified electrode. The pure water-fed system achieved an IPCE of 35%, comparable to vapor-fed systems, despite requiring less than one-tenth of the ionomer loading (0.06 mg cm^-2), indicating a stronger ionomer effect in aqueous environments. Product analysis confirmed oxygen evolution at the WO₃ electrode and hydrogen at the cathode, with nearly all photocurrent contributing to water splitting. The photocurrent density proportionally increased with light intensity and followed the bandgap absorption of WO₃. These results demonstrate that PFSA ionomer loading effectively eliminates proton transport limitations in electrolyte-free conditions, highlighting the importance of surface protonics of the porous photoanodes in PEM-PEC systems.