Hydrogen Production via Thermochemical Water Splitting Process by Alkali Metal Redox Cycle

Abstract

In this review, the reaction cycles based on redox reactions of alkali metals such as lithium, sodium, and potassium are systematically investigated as a potential water splitting technology operated at lower temperature than 500 °C. Assuming that all the cycles are operated in thermodynamic equilibrium condition, more than 1000 °C is theoretically required. Thus, equilibrium shift using phase transition of alkali metals is performed to decrease the reaction temperatures. The operating temperature of the lithium cycle is 800 °C because such high temperature is necessary to obtain high vapor pressure. The potassium cycle is potentially water splitting operated below 500 °C. However, strong corrosion is serious problem to develop it as practical application. Among them, the sodium cycle reveals the lowest operating temperature, which is 400 °C. Therefore, the sodium cycle should be categorized as one of the potential water splitting technology.