MoOX is commonly considered to be a high work-function semiconductor. From x-ray photoelectron spectroscopy and photoelectrochemical analysis, it is shown that MoOX can be considered as an effective hole transfer layer for the GaP-based device. Specifically, in the absence of carbon contamination using an ion beam cleaning step, the oxygen vacancy derived defect band located inside the bandgap becomes the main charge transfer mechanism. We demonstrate, for the first time, a device with a MoOX/GaP junction that functions as an unbiased photo-charging cell for the redox flow battery system with AQS/AQSH₂∥I⁻/I₃⁻ redox couples. This work has important implications toward enabling MoOX applications beyond the conventional solar cells, including electrochemical energy storage and chemical conversion systems.

In recent years, research in solar energy storage with photoelectrochemical cells (i.e., solar redox flow batteries: SRFBs) has resurged. This development is emerging in parallel with the growing field of research into organic redox couples intended for aqueous redox flow batteries (RFBs) in a range of different pH environments. In a solar flow battery, the dissolved electroactive molecules are charged directly from solar radiation by semiconductor photoelectrodes. The charged solution can then at a later stage be converted into electricity, and solar flow batteries are as such an approach to build integrated solar energy generation and storage devices. Research in RFBs and SRFBs has from their beginning been mutually linked by use of the same organic redox molecules in the electrolyte, such as quinones. Despite the long research history (since 1976), metallic-based, acidic SRFBs have shown only incremental development, while research in the use of organic redox pairs appears more promising. This review focuses on the historical development of the use of organic redox pairs in both RFBs and SRFBs and in particular on the mutual exchange of methods and materials between the two fields.