Ni-ferrite $(NiFe2O4)$ is a promising reactive ceramics of the ferrite for the solar hydrogen production by a two-step water splitting process using concentrated solar energy. However, it should be pretreated before $H2$-generation reaction by grinding the Ni-ferrite sintered after the $O2$-releasing reaction to make a fine powder. If the Ni-ferrite and yttria stabilized zirconia (YSZ) form a solid solution between these oxides ($YSZ∕NiFe2O4$ solid $solution=YSZ(Ni,Fe)$), it is expected that the two-step water splitting process with the Ni-ferrite system can proceed without treatment of the reduced product because of the high thermal stability of the $YSZ∕NiFe2O4$ solid solution. The $YSZ∕NiFe2O4$ solid solution was prepared by calcination of the mixture of the YSZ balls and $NiFe2O4$ powder at $T=1823K$ for $1h$, and its reactivity and thermal stability were examined for the two-step water splitting process. During the ten times repetition of the two-step water splitting reaction ($T=1773K$ for $O2$-releasing, and $1473K$ for $H2$-generation) with the $YSZ∕NiFe2O4$ solid solution using infrared imaging furnace, the reactivity for $O2$-releasing and $H2$-generation was kept constant. The molar ratio of the released $O2$ gas volume (the average $O2$ gas, $1.9cm3∕g$) and the generated $H2$ gas volume (the average $H2$ gas, $3.8cm3∕g$) was nearly 1:2, indicating that the water decomposition process via two steps proceeds. The X-ray diffractometry (XRD) measurement showed that the YSZ(Ni,Fe) keeps the YSZ phase structure during the ten times repetition of the two-step water splitting process. The successive $H2$ gas production by the two-step water splitting process was performed (ten times repetition of the two-step water splitting process). From comparative study on the reactivity and the thermal stability for the two-step water splitting reaction among the $YSZ∕NiFe2O4$ solid solution, $NiFe2O4$ and $ZnFe2O4$, it is concluded that the $YSZ∕NiFe2O4$ solid solution is superior to the others.

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