Water can be split into hydrogen and oxygen using multi-step chemical cycles at temperatures that are feasible with solar reactors. One of those cycles is based on manganese(III)oxide (Mn2O3)听 that is decomposed at ~1773 K into oxygen and manganese(II)oxide (MnO). The MnO is then reacted with NaOH at ~1000 K to form hydrogen and a-NaMnO2, which can be hydrolyzed with water to regenerate Mn2O3听for the high temperature reduction and NaOH for the hydrogen formation step. All three steps pose some technical challenges such as incomplete hydrolysis with sodium carryover to the high temperature reaction, and slow kinetics of the hydrogen formation. Several pathways to overcome these hurdles are currently investigated, including changes to the chemical composition of the starting materials and modification of the cycle to bypass the hydrolysis step.
A second cycle that has only two steps is based on the decomposition of zinc oxide at temperatures above 2000 K to form zinc metal particles and oxygen. The hydrolysis of the zinc particles collected is then carried out at 673 K to form hydrogen and regenerate the zinc oxide. The kinetics of the hydrolysis step is limited by the formation of zinc oxide layers on the metal particles that slows access of water to the unreacted zinc metal core. One approach to improve the hydrolysis is to carry out the reaction in a high-pressure fluidized bed to allow for continuous operation, long residence times, and to obtain hydrogen at pressures similar to the requirements for pipelines.