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.
Tsushiro et al. (Mon,) studied this question.