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, 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.