Nanostructured Fe ₂ O ₃ /Cu _<i>x</i> O heterojunction for enhanced solar redox flow battery performance

Solar redox flow batteries (SRFB) have received much attention as an alternative integrated technology for simultaneous conversion and storage of solar energy. Yet, the photocatalytic efficiency of semiconductor-based single photoelectrodes, such as hematite, remains low due to the trade-off between fast electron hole recombination and insufficient light utilization, as well as inferior reaction kinetics at the solid/liquid interface. Herein, we present an α-Fe₂O₃/Cu _<i>x</i> O p-n junction, coupled with a readily scalable nanostructure, that increases the electrochemically active sites and improves charge separation. Thanks to light-assisted scanning electrochemical microscopy (photo-SECM), we elucidate the morphology-dependent carrier transfer process involved in the photo-oxidation reaction at an α-Fe₂O₃ photoanode. The optimized nanostructure is then exploited in the α-Fe₂O₃/Cu _<i>x</i> O p-n junction, achieving an outstanding unbiased photocurrent density of 0.46 mA cm^-2, solar-to-chemical (STC) efficiency over 0.35% and a stable photocharge-discharge cycling. The average solar-to-output energy efficiency (SOEE) for this unassisted α-Fe₂O₃-based SRFB system reaches 0.18%, comparable to previously reported DSSC-assisted hematite SRFBs. The use of earth-abundant materials and the compatibility with scalable nanostructuring and heterojunction preparation techniques offer promising opportunities for cost-effective device deployment in real-world applications.