Impact of relative humidity on vapor-fed photoelectrochemical water splitting with a perfluorosulfonated ionomer-functionalized photoanode

Photoelectrochemical (PEC) water splitting with semiconductor electrodes is a cutting-edge approach for sustainable hydrogen production. Arid regions hold potential for solar-driven hydrogen generation, but the scarcity of liquid water presents a significant challenge. This study investigates the impact of water vapor pressure and temperature on the performance of a vapor-fed PEC cell utilizing a gas-diffusion photoanode specifically designed to operate with water vapor rather than liquid water. A proton exchange membrane (PEM) was used as the solid electrolyte, while a porous strontium titanate (SrTiO 3 ) electrode, functionalized with a perfluorosulfonic acid (PFSA) ionomer as a surface electrolyte, was employed as the gas-diffusion photoanode for oxygen evolution reaction. The PEM-based PEC system was supplied with humidified argon and irradiated with UV light (365 nm wavelength) on the PFSA ionomer-coated SrTiO 3 photoelectrode, with an applied bias of 1.2 V. Photocurrent measurements revealed a positive correlation with increased water vapor pressure at 27°C. While the photocurrent increased as cell resistance decreased, using a thinner PEM with lower resistance did not enhance the photocurrent density, suggesting that the local proton conductivity of the ionomer layer, which is an indicator of the proton transfer on the photoelectrode surface, rather than the proton transport in the PEM (ohmic voltage loss), is crucial for optimizing PEC performance. This study highlights the feasibility of vapor-fed PEC systems and underscores the pivotal role of interfacial proton transfer in enabling efficient solar hydrogen production in water-limited environments. • Vapor-fed PEC cell for hydrogen production in water-limited conditions • PFSA ionomer enables oxygen evolution on porous SrTiO₃ gas-diffusion photoanode • Higher water vapor pressure improves photocurrent under UV irradiation at 25–75 °C • Photocurrent is limited by interfacial proton transfer kinetics, not PEM bulk resistance • PEC performance is governed by humidity rather than temperature at fixed RH