A Two-Step Thermochemical Water Splitting by Iron-Oxide on Stabilized Zirconia

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 with a view toward direct conversion of 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, which is different from the $Fe3O4–FeO$ pair previously examined by others, served as the working solid material on this YSZ-supported $Fe3O4$. Our new redox reaction proceeded 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 the 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 $1400°C$.