Oxygen-releasing step of ZnFe2O4/(ZnO + Fe3O4)-system in air using concentrated solar energy for solar hydrogen production

The oxygen-releasing step of the ZnFe₂O₄/(ZnO + Fe₃O₄)-system for solar hydrogen production with two-step water splitting using concentrated solar energy was studied under the air-flow condition by irradiation with concentrated Xe lamp beams from a solar simulator. The spinel-type compound of ZnFe₂O₄ (Zn-ferrite) releases O₂ gas under the air-flow condition at 1800 K and then decomposes into Fe₃O₄ () and ZnO with a nearly 100% yield (ZnFe₂O₄ = ZnO + 2/3Fe₃O₄ + 1/6O₂). The ZnO was deposited as the thin layer on the surface of the reaction cell wall. A thermodynamic study showed that the ZnO was produced by the reaction between the O₂ gas in the air and the metal Zn vapor generated from ZnFe₂O₄. With the combined process of the present study on the O₂-releasing step and the previous one on the H₂ generation step (ZnO + 2/3Fe₃O₄ + 1/3H₂O = ZnFe₂O₄ + 1/3H₂) for the ZnFe₂O₄/(ZnO + Fe₃O₄)-system, solar H₂ production was demonstrated by one cycle of the ZnFe₂O₄/(ZnO + Fe₃O₄)-system, where the O₂-releasing step had been carried out in air at 1800 K and the H₂ generation step at 1100 K.