Renewable hydrogen produced by solar water-splitting has the potential to balance the intermittent nature of the sunlight and support grid-scale energy storage. In a solar-driven water-splitting device, the cathode surface and the anode surface involve hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), which are tightly coupled with each other, that is, whenever one oxygen molecule was produced at the cathode surface, two hydrogen molecules were produced at the anode surface at the same time.

In this talk, I will show some recent results on an alternative approach to solar water-splitting, where the electron and proton generated at OER was used to charge an aqueous vanadium solution in a 2.0 M sulfuric acid (pH = -0.16) electrolyte with near unity Faradaic efficiency, rather than being used directly to produce hydrogen at the cathode. The produced V²⁺ species in the cathode chamber was then passed through a MoC_x based catalyst to produce hydrogen and to re-generate V³⁺ for the subsequent reduction, with an average hydrogen generation efficiency of 85% at different depths of charging. Coupled to a solar tracker, the solar-driven vanadium redox cell was charged outdoors under real-world illumination during the day and discharged at night to produce hydrogen with a daily average solar to hydrogen (STH) conversion efficiency of 5.8%.