Hydrogen production by water splitting with Mn3-xCoxO4 mixed oxides thermochemical cycles: A thermodynamic analysis

Abstract

The high temperature required for hydrogen production by solar driven thermochemical cycles is a critical factor hindering full-scale applications. The thermochemical cycle based on Mn3O4/MnO redox pair is one of the most studied despite the high operating temperature required for complete de reduction step (1623–1723 K). The combination of Mn3O4 with Co3O4, a metal oxide with lower reduction temperature than Mn3O4 that cannot be used for hydrogen production due to thermodynamic limitations, is presented as a way to decrease significantly the energy demand of the cycle. In this work, a complete thermodynamic study of thermochemical cycles with different Mn/Co mixed oxides (Mn3-xCoxO4, 0.9 < x < 2.7) for hydrogen production has been performed. The study of the variation of Gibbs energy with temperature allowed to determine that the thermal reduction of the metal oxide (Mn3-xCoxO4) takes place at temperatures between 1048 and 1173 K, which can be achieved by conventional solar concentration technologies. Unfortunately, the oxidation of the reduced metal oxide (Mn3-xCoxO3) with water to produce H2 is not feasible from a thermodynamically point of view, so a stronger oxidizing agent, as sodium hydroxide, is required. The optimum temperature for the oxidation with NaOH was found to be 1373 K, meaning that this reaction takes place at higher temperatures than those actually required for the reduction, something uncommon in thermochemical cycles for water splitting. On the other hand, after the study of the variables affecting the equilibrium like the inert gas/solid ratio, it can be concluded that in both reactions, thermal requirements can be reduced by operating at lower temperatures by means of a higher inert gas/Mn3-xCoxO4 ratio. Finally, energy and exergy analysis of the system based on the solar absorption efficiency and the energy requirements predicts a solar-to-fuel efficiency and exergy efficiency of 40% and 23%, respectively. These values are comparable or even higher than those found in literature for other metal oxides thermochemical cycles for water splitting, thus with the advantage of working at a considerable lower temperature (1373 K).