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

Abstract

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 CO 2 and H 2 O 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 CO 2 splitting rates over 100 thermochemical cycles of methane partial oxidation and CO 2 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 CO 2 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.

Concentration-Dependent Solar Thermochemical CO 2 /H 2 O Splitting Performance by Vanadia–Ceria Multiphase Metal Oxide Systems

“…The process results in the production of synthesis gas (syngas), a mixture of H 2 and CO, which can be converted into liquid hydrocarbon fuels via the Fischer-Tropsch (FT) process [9][10][11]. In order to achieve high fuel (syngas) yields and high process efficiency in the redox cycling, the oxygen carriers should have a high oxygen exchange capacity and excellent cyclability [12].…”

“…It shows high and stable fuel production rates via a nonstoichiometric oxygen exchange process. The fast redox kinetics and high fuel selectivity are distinct characteristics of CeO 2 as compared to other redox materials [10,11,15,[21][22][23][24][25][26][27]. Fuel selectivity indicates the percentage of product gases (CO, H 2 ) produced upon splitting of CO 2 , H 2 O, CH 4 , etc.…”

“…Doping CeO 2 with transition metals has been proposed to further improve the material chemical, thermal, mechanical, and optical characteristics. However, transition metal-doped CeO 2 has so far demonstrated inferior redox reaction rates and oxygen ion mobility as compared to pure CeO 2 [11,[26][27][28]. In addition, doped CeO 2 shows extensive sintering at high temperatures, which decreases gas-phase mass transfer and consequently decreases the overall fuel production performance [10].…”

Effect of specific surface area on syngas production performance of pure ceria in high-temperature thermochemical redox cycling coupled to methane partial oxidation

“…The cycle (1)- (2) has been demonstrated with different oxygen exchange materials under temperature-swing and isothermal conditions. [3][4][5][6][7] Typical temperatures for the reduction and oxidation steps are above 1700 K and below 1400 K, respectively. The temperature swing between the reduction and oxidation steps necessitates application of heat recovery strategies to maximize process efficiency, and leads to considerable thermal stresses.…”

“…[9][10][11] Alternatively, the reduction reaction (1) can be carried out under carbothermal conditions, which allows for lowering the reduction temperature and attaining higher non-stoichiometry in the metal oxide. 3,12 For methane as the reducing carbonaceous material, the non-stoichiometric reduction reaction of ceria coupled to methane partial oxidation (MPO) can be written as: 3,12 (1/d)CeO 2 + CH 4 (g) / (1/d)CeO 2Àd + CO (g) + 2H 2 (g) (4) High specic surface area (SSA) in porous structures of oxygen exchange materials is a key requirement to enable high oxidation rates, 1,13 low pressure drop 11,14 and enhanced heat and mass transfer, 11,14,15 which in turn result in increased efficiency of syngas production. The effect of the morphology of pure ceria samples, especially their SSA, to enhance syngas production were investigated for three-dimensionally ordered macroporous (3DOM) ceria structures, 13,16,17 reticulated porous ceramics, 15 wood-templated structures, 18 brous structures 19,20 and nanostructured powders.…”