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.