Decoupling H₂(g) and O₂(g) Production in Water Splitting by a Solar-Driven V^3+/2+(aq,H₂SO₄)|KOH(aq) Cell

A solar-driven V3+/2+(aq,H2SO4)|KOH(aq) cell, consisting of a carbon-cloth cathode in 2.0 M H2SO4(aq) with 0.36 M V2(SO4)3 (pH −0.16), a Ni mesh anode in 2.5 M KOH(aq) (pH 14.21) for the oxygen-evolution reaction (OER), and a bipolar membrane that sustained the pH differentials between the catholyte and anolyte, enabled water splitting with spatial and temporal decoupling of the hydrogen evolution reaction (HER) from the OER and produced H2(g) locally under pressure upon demand. Over a range of potentials and charging depths, V3+ was selectively reduced with >99.8% faradic efficiency. The V2+ species produced in the catholyte was then passed subsequently on demand over a MoCx-based HER catalyst to produce H2(g) and regenerate V3+ for subsequent reduction. Under a base hydrogen pressure of 1, 10, and 100 atm, the discharge efficiency of the V3+ to hydrogen was 83%, 65.2%, and 59.8%, respectively. In conjunction with a solar tracker and a photovoltaic device, the V3+/2+(aq,H2SO4)|KOH(aq) cell was charged outdoors under sunlight and discharged at night with a daily averaged diurnal solar-to-hydrogen (STH) energy conversion efficiency of 3.7% and a STH conversion efficiency of 5.8% during daylight operation.