(637c) Low-Temperature, Manganese Oxide-Based, Thermochemical Water Splitting Cycle

Thermochemical cycles that split water into stoichiometric amounts of hydrogen and oxygen below 1000^oC are highly desirable, because they can convert the waste heat from nuclear power plants into chemical energy in the form of hydrogen. We report a manganese-based thermochemical cycle with the highest operating temperature of 850^oC that is completely recyclable and does not involve toxic or corrosive components. The thermochemical cycle includes the following 4 steps: (1) thermal treatment of a physical mixture of Na₂CO₃ and Mn₃O₄ to produce MnO, CO₂, and alpha-NaMnO₂ at 850^oC, (2) oxidation of MnO in the presence of Na₂CO₃ by water to produce H₂, CO₂ and alpha-NaMnO₂ at 850^oC, (3) Na⁺ extraction from alpha-NaMnO₂ by its suspension in aqueous solutions in the presence of bubbling CO₂ at 80^oC, and (4) recovery of Mn₃O₄ by thermally reducing the sodium ion extracted solid produced in step (3) at 850^oC. The shuttling of Na⁺ into and out of the manganese oxides in the hydrogen and oxygen evolution steps, respectively, provides the key thermodynamic driving forces, and allows for the cycle to be closed at temperatures below 1000^oC. The production of hydrogen and oxygen is fully reproducible for at least 5 cycles.