Solid Polymer Electrolyte‐Coated Macroporous Titania Nanotube Photoelectrode for Gas‐Phase Water Splitting

Photoelectrochemical (PEC) water vapor splitting by using n‐type semiconductor electrodes with a proton exchange membrane (PEM) enabled pure hydrogen production from humidity in ambient air. We proved a design concept that the gas–electrolyte–semiconductor triple‐phase boundary on a nanostructured photoanode is important for the photoinduced gas‐phase reaction. A surface coating of solid‐polymer electrolyte on a macroporous titania‐nanotube array (TNTA) electrode markedly enhanced the incident photon‐to‐current conversion efficiency (IPCE) at the gas–solid interface. This indicates that proton‐coupled electron transfer is the rate‐determining step on the bare TNTA electrode for the gas‐phase PEC reaction. The perfluorosulfonate ionomer‐coated TNTA photoanode exhibited an IPCE of 26 % at an applied voltage of 1.2 V under 365 nm ultraviolet irradiation. The hydrogen production rate in a large PEM‐PEC cell (16 cm2) was 10 μmol min−1.