Oxygen-releasing step of ZnFe2O4/(ZnOÂ +Â 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 (=FeIIFe2IIIO4) 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.