Pure Water‐Fed Photoelectrochemical Water Splitting Using a Porous WO 3 Electrode Surface‐Modified With Perfluorosulfonic Acid Ionomer

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 (WO 3 ) 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 WO 3 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 WO 3 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 WO 3 . 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.