Remarkable performance of microstructured ceria foams for thermochemical splitting of H2O and CO2 in a novel high-temperature solar reactor

Thermochemical splitting of H2O and CO2 applying redox materials constitutes a sustainable option for synthetic fuel production and CO2 valorization. It consists of two-step process based on the creation of oxygen vacancies in non-stoichiometric oxides during solar-driven thermal reduction, followed by the material re-oxidation with H2O and/or CO2 to generate syngas (H-2/CO), the building block for a wide variety of synthetic hydrocarbon fuels. In this work, a monolithic solar reactor was designed and tested integrating reticulated porous ceria (open-cell foams) heated by concentrated solar energy. The influence of various operating parameters on the thermochemical reactor performance was investigated. Increasing the temperature or decreasing the pressure in the reduction step was found to enhance the maximum reduction extent reached by the redox material (CeO2-delta), thereby improving the fuel production capacity. In addition, a decrease of the oxidation temperature led to higher fuel production rate, despite an increase of the temperature swing between the reduction and oxidation steps. Increasing the oxidant concentration also sharply enhanced the oxidation rate. Peak CO production rate approaching 10 mL/min/g was achieved with ceria foams (exhibiting micron-sized grains forming an interconnected macroporous network within the struts) during their reoxidation upon free cooling with pure CO2 stream (after reduction at 1400 degrees C), thus strongly outperforming (by a factor of about x8) the previous maximum values reported to date. This result was attributed to the fine and stable granular microstructure of the reticulated ceria foams. The solar reactor reliability and robustness during high temperature two-step redox cycling were demonstrated with an average cycle production of 5.1 mL/g of H-2 and CO, and peak solar-to-fuel efficiencies above 8%. The highly reactive reticulated foams with 10 and 20 ppi (pore per inch) were cycled for about 69 h (51 cycles) of continuous on-sun operation without any decrease in performance, thus evidencing their noteworthy thermochemical and microstructural stability. (C) 2020 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.