Spinel metal oxide-alkali carbonate-based, low-temperature thermochemical cycles for water splitting

Abstract

Thermochem. cycles that split water into stoichiometric amts. of hydrogen and oxygen below 1000°C are highly desirable, because they can convert the waste heat from nuclear power plants into chem. energy in the form of hydrogen. We report a manganese-based thermochem. cycle with the highest operating temp. of 850°C that is completely recyclable and does not involve toxic or corrosive components. The thermochem. cycle includes the following 4 steps: (1) thermal treatment of a phys. mixt. of Na_2CO_3 and Mn_3O_4 to produce MnO, CO_2, and alpha-NaMnO_2 at 850°C, (2) oxidn. of MnO in the presence of Na_2CO_3 bywater to produce H_2, CO_2 and alpha-NaMnO_2 at 850°C, (3) Na+ extn. from alpha-NaMnO_2 by its suspension in aq. solns. in the presence of bubbling CO_2 at 80°C, and (4) recovery of Mn_3O_4 by thermally reducing the sodium ion extd. solid produced in step (3) at 850°C. The shuttling of Na+ into and out of the manganese oxides in the hydrogen and oxygen evolution steps, resp., provides the key thermodn. driving forces, and allows for the cycle to be closed at temps. below 1000°C. The prodn. of hydrogen and oxygen is fully reproducible for at least 5 cycles.

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