2, California Institute of Technology, Pasadena, California, United States
3, California Institute of Technology, Pasadena, California, United States
4, California Institute of Technology, Pasadena, California, United States
A solar-driven vanadium redox cell, consisting of a carbon cloth cathode in 2.0 M H2SO4(aq) with 0.36 M V2(SO4)3 (pH = 14.21) for vanadium reduction, a Ni mesh anode in 2.5 M KOH(aq) (pH = -0.16) for oxygen evolution reaction (OER) and a bipolar membrane that sustains the pH differentials between the cathode and anode chamber, was constructed to decouple the hydrogen evolution reaction (HER) from OER in space and time. Highly selective reduction of V3+ with a Faradaic efficiency that exceeded 99.8% was observed at the carbon cloth cathode in the presence of a high concentration of protons for HER at different cathodic potentials and at a range of charging depths. The produced V2+ species in the cathode chamber was then passed through a MoCx-based catalyst to produce hydrogen and to regenerate V3+ for 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 diurnal-averaged solar-to-hydrogen conversion efficiency of 5.8%.