The rotary-type solar reactor has been developed and fabricated for solar hydrogen production by a two-step water-splitting process using the reactive ceramics of CeO2 and Ni,Mn-ferrite (Ni0.5Mn0.5Fe2O4). It has a cylindrical rotor and dual cells for discharging O2 and for the H2O splitting reaction. A detailed specification and the efficiency of the rotary-type solar reactor were examined for the two-step water-splitting process. The maximum temperature of the reactive ceramics mounted on the cylindrical rotor was ca. 1623 K by irradiation with a solar simulator of an infrared imaging lamp. Repetition of the two-step water-splitting process using the rotary-type solar reactor with CeO2 was achieved, and successive evolution of H2 was observed in the H2O-splitting reaction cell at the optimum reaction temperatures of the O2-releasing reaction cell (T = 1623 K) and H2O-splitting reaction cell (T = 1273 K). Also, repetition of the two-step water-splitting process was achieved in the case of using the reactive ceramics of Ni,Mn-ferrite, and its optimum reaction temperatures of the O2-releasing and H2-generation reactions were 1473 and 1173 K, respectively. It was confirmed that the higher O2-releasing reaction temperature of above 1800 K was achieved with the about 10-times scaled-up rotary-type solar reactor.

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The reactive ceramics suitable for the rotary-type solar reactor (proposed by Tokyo Institute of Technology, Tokyo) with two-step water-splitting reaction were developed. It is confirmed that O2 gas is evolved in the two-step water-splitting reaction with the reactive ceramics vigorously by rapid heating (α-O2-releasing reaction). The α-O2-releasing reaction is due to the formation of interstitial defect and the conversion of lattice oxygen into O2 gas at a nonequilibrium state. Reactive ceramics (NiFe2O4 and yttria stabilized zirconia (YSZ)-NiFe2O4 solid solution) can absorb solar thermal energy and convert thermal energy into chemical energy under high O2 partial pressure atmosphere in the α-O2-releasing reaction. Repetitive evolutions of O2 gas were observed in the two-step water-splitting reaction with YSZ-Fe3O4 solid solution and cerium based metal oxides (CeO2–NiO, CeO2–ZrO2, and CeO2–Ta2O5) at high O2 partial pressure. The CeO2–Ta2O5(Ce:Ta=90:10) released a large amount of O2 gas (3.95 cm3/g) in the α-O2 releasing reaction in the flow of air.

Solar H2 production by the two-step water splitting process with thermochemical reaction has been proposed to convert solar energy into chemical energy. We succeeded in repeating the cyclic two-step water splitting process composed of the O2-releasing and H2-generation reactions with metal (Fe, Ni) doped ceria. The metal doped ceria with low content of metal ion (Fe3+, Ni2+) formed a solid solution with fluorite-type structure between ceria (CeO2) and metal oxide (Fe2O3, NiO). The empirical formula of the solid solution was Ce1-αMαO2−δ (M = Fe, Ni), and it was assumed that the high reactivity on the two-step water splitting process was due to an oxygen deficiency in the solid solution. The metal doped ceria with different Ce:M mole ratio (Ce:M = 0.97:0.03–0.7:0.3) was prepared through the combustion method. The two-step water-splitting process with metal doped ceria proceeded at 1673K for the O2-releasing reaction and at 1273K for the H2-generation reaction by irradiation of an infrared imaging lamp for a solar simulator. The amounts of H2 gas evolved in the H2-generation reaction with Fe-doped ceria and Ni-doped ceria with different Ce:M (M = Fe, Ni) mole ratio were 0.97–1.8 and 1.7–2.5 cm3/g, respectively, and the evolved H2/O2 ratios were approximately equaled to 2 of the stoichiometric value. The amounts of H2 and O2 gases evolved in the two-step water splitting process varied with the Ce:M mole ratio in the metal doped ceria. It was suggested that the O2-releasing and H2-generation reactions with metal doped ceria was repeated with the reduction and oxidation of Ce4+-Ce3+ enhanced by the presence of Fe or Ni ions. Furthermore, the O2-releasing reaction with Ni-doped ceria proceeded under a high O2 partial pressure atmosphere (pO2 = 0.05 atm) and at the temperature of 1773K. The progress of the O2-releasing reaction under a high pO2 indicates that metal doped ceria can be applicable for the rotary-type solar reactor developed by Tokyo Tech group for solar H2 production.

The rotary-type solar reactor has been developed for solar hydrogen production with the two-step water splitting process using the reactive ceramic (Ni, Mn-ferrite). The rotary-type reactor has the rotating tubular cylinder covered on a reactive ceramic and dual reaction cells for O2-releasing and H2-generation reactions. The successive evolutions of O2 and H2 gases were observed in the O2 releasing and H2 generation reaction cells, respectively, with the prototype (small) reactor (diameter of cylinder ; 4cm). There is an upper limit for the rate of H2 gas evolution in the case of the prototype reactor because of the slow rotation rate in a small irradiation area. To confirm the practical operation of the rotary-type solar reactor with the two-step water splitting process for the simultaneous production of H2 and O2 gases, a scaled-up rotary-type solar reactor with 400cm2 of the irradiation area was fabricated (diameter; 50cm). The scaled-up reactor made of inner and outer short tubular cylinders (stainless steel) has a quartz glass window for the irradiation of reactive ceramic coated on the inner tubular cylinder (cylindrical rotor) and reaction cells were aligned in the sharing spaces between the inner and outer short tubular cylinders with gas sealing mechanisms. In the reactor, reactive ceramic coated on the inner tubular cylinder was heated up to 1800K by using the infrared imaging lamps (solar simulator) with thermal flux = 600kW/m2. The solid solution between YSZ and Ni-ferrite was used as reactive ceramic for the scaled-up reactor in order to prevent from sintering at a high temperature in the O2-releasing reaction, since the sintering of reactive ceramic resulted in lowering the yield of H2 gas evolution in the H2-generation reaction. The amounts of H2 and O2 gases evolved at the rotation rate of 0.3rpm were evaluated to 64cm3 and 30cm3 for 10min with the scaled-up rotary-type solar reactor, respectively, which were much larger than those with the prototype reactor. The simultaneous evolutions of H2 and O2 gases in the two-step water splitting process were repeated by employing the scaled-up reactor with the solid solution between YSZ and Ni-ferrite.