Boosting Visible‐Light‐Driven CO 2 Reduction to CH 4 for In Situ Resource Utilization Application with BiVO 4 /WO 3 Heterojunction Photoanodes Developed by Magnetron Sputtering

ABSTRACT Photoelectrochemical reduction of CO 2 into energy‐dense hydrocarbons represents a promising strategy for sustainable fuel production and in situ resource utilization. This is particularly attractive in extraterrestrial environments where CO 2 ‐rich atmospheres and solar energy are readily available, enabling the conversion of locally available resources into fuels and chemical feedstocks while reducing reliance on Earth‐supplied materials. In this work, BiVO 4 and BiVO 4 /WO 3 photoanodes are fabricated by magnetron sputtering and integrated into a photoelectrochemical system with a Cu‐based gas diffusion cathode for selective CO 2 reduction to CH 4 using greywater as electrolyte. The BiVO 4 /WO 3 heterojunction enhances charge separation and electron transport, achieving optimal performance with thicknesses of 100 nm (BiVO 4 ) and 130 nm (WO 3 ), with a CH 4 faradaic efficiency of 47.9%, an energy‐to‐fuel conversion efficiency of 2.7%, and a production rate of 36.7 µmol m − 2 s − 1 under gas‐phase operation. These results highlight the potential of heterojunction engineering and scalable sputtering deposition for high‐performance photoelectrochemical CO 2 conversion toward solar‐driven CH 4 production.