Earth-abundant transition metal oxides with extraordinary reversible oxygen exchange capacity for efficient thermochemical synthesis of solar fuels

Efficient storage of solar and wind power is one of the most challenging tasks still limiting the utilization of the prime but intermittent renewable energy sources. The direct storage of concentrated solar power in renewable fuels via thermochemical splitting of water and carbon dioxide on a redox material is a scalable approach with up to 54% solar-to-fuel conversion efficiency. Despite progress, the search for earth-abundant materials that can provide and maintain high H-2 and CO production rates over long period of high-temperature cycles continues. Here, we report a strategy to unlock the use of manganese, the 12th most abundant element in the Earth's crust, for thermochemical synthesis of solar fuels, achieving superior thermochemical stability, oxygen exchange capacity, and up to seven times higher mass-specific H-2 and CO yield than cerium dioxide. We observe that incorporation of a small fraction of cerium ions in the manganese (II,III) oxide crystal lattice drastically increases its oxygen ion mobility, allowing its reduction from oxide to carbide during methane partial oxidation with simultaneous Ce exsolution. High CO2 and H2O splitting rates are achieved by re-oxidation of the carbide to manganese (II) oxide with simultaneous reincorporation of the cerium ions. We demonstrate that the oxide to carbide reaction is highly reversible achieving remarkable CO2 splitting rates over 100 thermochemical cycles of methane partial oxidation and CO2 splitting, and preserving the initial oxygen exchange capacity of 0.65 mol(O) mol(Mn)(-1) and 89% of the fuel production rates. Due to this extraordinarily high reversible oxygen exchange capacity, the 3% Ce-doped manganese oxide achieves an average mass-specific CO yield for CO2 splitting of 17.72 mmol(CO) g(-1), which is significantly higher than that previously achieved in thermochemical redox cycles. More generally, these findings suggest that incorporation of small soluble amounts of cerium in earth-abundant transition metal oxides like manganese oxide is a powerful approach to enable solar thermochemical fuel synthesis.