Water Splitting by MnOx/Na2CO3 Reversible Redox Reactions

Thermal water splitting by redox reactants could contribute to a hydrogen-based energy economy. The authors previously assessed and classified these thermo-chemical water splitting redox reactions. The Mn3O4/MnO/NaMnO2 multi-step redox cycles were demonstrated to have high potential. The present research experimentally investigated the MnOx/Na2CO3 redox water splitting system both in an electric furnace and in a concentrated solar furnace at 775 and 825 degrees C, respectively, using 10 to 250 g of redox reactants. The characteristics of all reactants were determined by particle size distribution, porosity, XRD and SEM. With milled particle and grain sizes below 1 mu m, the reactants offer a large surface area for the heterogeneous gas/solid reaction. Up to 10 complete cycles (oxidation/reduction) were assessed in the electric furnace. After 10 cycles, an equilibrium yield appeared to be reached. The milled Mn3O4/Na2CO3 cycle showed an efficiency of 78% at 825 degrees C. After 10 redox cycles, the efficiency was still close to 60%. At 775 degrees C, the milled MnO/Na2CO3 cycles showed an 80% conversion during cycle 1, which decreased to 77% after cycle 10. Other reactant compounds achieved a significantly lower conversion yield. In the solar furnace, the highest conversion (>95%) was obtained with the Mn3O4/Na2CO3 system at 775 degrees C. A final assessment of the process economics revealed that at least 30 to 40 cycles would be needed to produce H-2 at the price of 4 euro/kg H-2. To meet competitive prices below 2 euro/kg H-2, over 80 cycles should be achieved. The experimental and economic results stress the importance of improving the reverse cycles of the redox system.

Automated summary

Thermal water splitting through redox reactions has the potential to contribute to a hydrogen-based energy economy. The efficiency and characteristics of different redox reactions were investigated, and the optimal conditions for the reactions were determined. The experimental and economic results highlight the importance of improving the reverse cycles of the redox system.