ABSTRACT
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 1.2â V under 365â nm ultraviolet irradiation. The hydrogen production rate in a large PEM-PEC cell (16â cm2 ) was 10â µmol min-1 .
ABSTRACT
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.
