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.
The Cover Feature shows solar H2 production by splitting of water vapor in ambient air at the sea using an all‐solid photoelectrochemical cell. The TiO2‐nanotube array electrode promotes photoelectrochemical oxidation of water into O2 and protons. The protons are reduced to H2 at the counter electrode, which is separated from the TiO2 photoanode by a proton‐exchange membrane. The splitting of water vapor into H2 and O2 is enhanced by a coating of a solid polymer electrolyte, perfluorosulfonate ionomer, on the TiO2‐nanotube electrode to fabricate gas–electrolyte–semiconductor triple‐phase boundary. More information can be found in the Full Paper by Amano et al. on page 1925 in Issue 9, 2019 (DOI: 10.1002/cssc.201802178).
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