Thermochemical Hydrogen Production by Alkali Metal Oxides

In this work, we have focused a thermo chemical hydrogen production from water by using alkali metal oxides. This system consists of three step reactions. First one is H₂ generation by the reaction of hydroxide and metal 2MOH+2M→2M₂O+H₂ (endothermic). From the oxide as the product of first reaction, metal is reproduced to form peroxide at second step by the reaction 2M₂O→M₂O₂+2M (endothermic). Third reaction is hydrolysis of peroxide M₂O₂+H₂O→2MOH+1/2O₂ (exothermic), resulting that hydroxide and O₂ are generated. In previous work, it was reported that this hydrogen production requires more than 800 °C for potassium-system. However, it was expected that the reaction temperature can be reduced by using nonequilibrium condition. In this work, these three reactions under nonequilibrium state were investigated. The target for operating temperature of the systems is 500 °C, which can be obtained by the concentrating solar energy. It was experimentally confirmed that the first reaction proceeded below 500 °C by removing generated H₂, where the Nb₂O₅ doped Mg was used as H₂ absorbent at room temperature. In order to realize the nonequilibrium condition for the second reaction, the cooling part equipped reactor was especially designed. By condensing the metal vapor generated by the second reaction at the cooling part, the separation of metal from reaction field succeeded, resulting that the reaction proceeded below 500 °C. The hydrolysis as third reaction occurred below 200 °C and the generation of O₂ and MOH as reaction products were confirmed. From the results, it was experimentally demonstrated that the hydrogen production can be controlled below 500 °C by using nonequilibrium conditions. Therefore, the system should be recognized as a potential thermo chemical hydrogen production method.