Direct conversion of methane (CH4) into valuable chemicals with low-energy input is an important goal in the sustainable chemical industry. Herein, we report a photoelectrochemical activation of CH4 in the gas phase under visible light irradiation at room temperature. The proof-of-concept study revealed that homocoupling of CH4 to form ethane (C2H6) with high selectivity of 54% was induced by photogenerated holes over a tungsten trioxide (WO3) gas-diffusion photoanode coated with a proton-conducting ionomer in the presence of water vapor. The gas–electrolyte–solid triple-phase boundary enables the oxidation of the inert carbon–hydrogen bond of CH4, and the formation of carbon oxides and ethane with a carbon–carbon bond. The gas-phase photoelectrochemical system shows incident photon-to-current conversion efficiency of 11% under blue light at an applied voltage of 1.2 V. This work is also the first demonstration of a visible-light-driven hydrogen evolution from CH4. The hydrogen is separated from CH4 and oxidized products by a solid polymer electrolyte membrane.

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.

Hydrogen production from humidity in the ambient air reduces the maintenance costs for sustainable solar-driven water splitting. We report a gas-diffusion porous photoelectrode consisting of tungsten trioxide (WO3) nanoparticles coated with a proton-conducting polymer electrolyte thin film for visible-light-driven photoelectrochemical water vapor splitting. The gas–electrolyte–solid triple phase boundary enhanced not only the incident photon-to-current conversion efficiency (IPCE) of the WO3 photoanode but also the Faraday efficiency (FE) of oxygen evolution in the gas-phase water oxidation process. The IPCE was 7.5% at an applied voltage of 1.2 V under 453 nm blue light irradiation. The FE of hydrogen evolution in the proton exchange membrane photoelectrochemical cell was close to 100%, and the produced hydrogen was separated from the photoanode reaction by the membrane. A comparison of the gas-phase photoelectrochemical reaction with that in liquid-phase aqueous media confirmed the importance of the triple phase boundary for realizing water vapor splitting.

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