We present the theoretical model reflecting experimental parameters, such that we can highlight critical parameters that merit the most attention in further studies towards the practical realisation of solar-rechargeable redox flow batteries (SRFBs).

Recent advances in photoelectrochemical redox flow cells, such as solar redox flow batteries, have received much attention as an alternative integrated technology for simultaneous conversion and storage of solar energy. Theoretically, it has been reported that even singlephoton devices can demonstrate unbiased photo-charging with high solar-to-chemical conversion efficiency; however, the poor redox kinetics of photoelectrodes reported thus far severely limit the photo-charging performance. Here, we report a band alignment design and propose surface coverage control to reduce the charge extraction barrier and create a facile carrier pathway from both n-and p-type photoelectrodes to the electrolyte with the respective redox reaction. Based on these observations, we develop a single-photon photocharging device with a solar-to-chemical conversion efficiency over 9.4% for a redox flow cell system. Along with these findings, we provide design principles for simultaneous optimisation, which may lead to enhanced conversion efficiency in the further development of solarrechargeable redox flow cells.

MoO X is commonly considered to be a high work-function semiconductor. From X-ray photoelectron spectroscopy and photo-electrochemical analysis, it is shown that MoO X can be considered as an effective holetransfer layer (HTL) for the GaP-based device. Specifically, in the absence of carbon contamination using an ionbeam cleaning step, the oxygen vacancy derived defect band located inside the band-gap becomes the main charge transfer mechanism. We demonstrate, for the first time, a device with a MoO X /GaP junction that functions as an unbiased photo-charging cell for the redox flow battery system with AQS/AQSH 2 ‖I-/I 3redox couples. This work has important implications toward enabling MoO X applications beyond the conventional solar cells, including electrochemical energy storage and chemical conversion systems. SUPPLEMENTARY MATERIAL See the supplementary material for further details on band alignment calculation method, UV-Vis transmittance, XPS survey and depth profile, LSV for n-GaP/MoO X , Pt/n-GaP, and Pt film, CV using a charged electrolyte, and the charging/discharging measurements with normalized capacity data, and Mott-Schottky analysis on the n-Gap.