Analysis of Thermodynamics of Two-Step Solar Water Splitting

A two-step thermo-chemical cycle for solar production of hydrogen from water has been developed and investigated. It is based on metal oxide redox pair systems, which can split water molecules by abstracting oxygen atoms and reversibly incorporating them into their lattice. After proof-of-principle, successful experimental demonstration of several cycles of alternating hydrogen and oxygen production, and elaboration of process strategies presented in previous contributions, the present work describes a thermodynamic study aiming at the fine tuning of the redox system, at the improvement of process conditions, and at the evaluation of the potential of the process. For the redox material the oxygen uptake capability is an essential characteristic, because it is directly connected to the amount of hydrogen which can be produced. In order to evaluate the maximum oxygen uptake potential of a coating material and to be able to find new redox materials theoretical considerations based on thermodynamic laws and properties are helpful and faster than actual tests. Through thermodynamic calculations it is possible to predict the theoretical maximum output of H2 from a specific redox-material under certain conditions. Calculations were focussed on the two mixed iron oxides nickel-iron oxide and zinc-iron oxide. In the simulation the amount of oxygen in the redox-material is calculated before and after the splitting step on the basis of laws of thermodynamics and available material properties for the mixed-iron oxides used. For the simulation the commercial Software FactSage and available databases for the necessary material properties were used. The analysis showed that a maximum hydrogen yield is achieved if the regeneration temperature is raised to the limits of the operation range, if the temperature for the water splitting is lowered below 800 °C and if the partial pressure of oxygen during regeneration is decreased to the lower limits of the operational range. The increased hydrogen yield at lower splitting temperature of about 800 °C could not be confirmed in experimental results, where a higher splitting temperature led to a higher hydrogen yield. As a consequence it can be stated that kinetics must play an important role especially in the splitting step.