Hydrogen peroxide (H2O2) in water has been proposed as a promising solar fuel instead of gaseous hydrogen because of advantages on easy storage and high energy density, being used as a fuel of a one-compartment H2O2 fuel cell for producing electricity on demand with emitting only dioxygen (O2) and water. It is highly desired to utilize the most earth-abundant seawater instead of precious pure water for the practical use of H2O2 as a solar fuel. Here we have achieved efficient photocatalytic production of H2O2 from the most earth-abundant seawater instead of precious pure water and O2 in a two-compartment photoelectrochemical cell using WO3 as a photocatalyst for water oxidation and a cobalt complex supported on a glassy-carbon substrate for the selective two-electron reduction of O2. The concentration of H2O2 produced in seawater reached 48 mM, which was high enough to operate an H2O2 fuel cell.

Decomposition of H2O2 on an electrode was suppressed in the presence of Lewis acids.

Efficient photocatalytic production of H2O2 as a promising solar fuel from H2O and O2 in water has been achieved by the combination of bismuth vanadate (BiVO4) as a durable photocatalyst with a narrow band gap for the water oxidation and a cobalt chlorin complex (CoII(Ch)) as a selective electrocatalyst for the two-electron reduction of O2 in a two-compartment photoelectrochemical cell separated by a Nafion membrane under simulated solar light illumination. The concentration of H2O2 produced in the reaction solution of the cathode cell reached as high as 61 mM, when surface-modified BiVO4 with iron­(III) oxide­(hydroxide) (FeO­(OH)) and CoII(Ch) were employed as a water oxidation catalyst in the photoanode and as an O2 reduction catalyst in the cathode, respectively. The highest solar energy conversion efficiency was determined to be 6.6% under simulated solar illumination adjusted to 0.05 sun after 1 h of photocatalytic reaction (0.89% under 1 sun illumination). The conversion of chemical energy into electric energy was conducted using H2O2 produced by photocatalytic reaction by an H2O2 fuel cell, where open-circuit potential and maximum power density were recorded as 0.79 V and 2.0 mW cm–2, respectively.