The thermochemical two-step water splitting cycle was examined by using an iron oxide supported on yttrium-stabilized, cubic zirconia (YSZ) as the working material, for the purpose of directly converting solar high-temperature heat to clean hydrogen energy. In the first step of the cycle, the YSZ-supported Fe3O4 was thermally decomposed to the reduced phase at 1400 °C under an inert atmosphere. The reduced solid phase was oxidized back to the original phase (the YSZ-supported Fe3O4) with steam to generate hydrogen below 1000 °C. A new redox pair examined by others was found to serve as the working solid material on this YSZ-supported Fe3O4, as follows. The Fe3O4 reacted with YSZ to produce an Fe2+-containing ZrO2 phase by releasing oxygen molecules in the first step: the Fe2+ ions entered into cubic YSZ lattice. In the second step, the Fe2+-containing YSZ generated hydrogen via steam splitting to reproduce Fe3O4 on the cubic YSZ support. This cyclic reaction could be repeated with a good repeatability of the reaction below 1400 °C.

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