Thermochemical CO2 splitting via redox cycling of ceria reticulated foam structures with dual-scale porosities

Abstract

Efficient heat transfer of concentrated solar energy and rapid chemical kinetics are desired characteristics of solar thermochemical redox cycles for splitting CO₂. We have fabricated reticulated porous ceramic (foam-type) structures made of ceria with dual-scale porosity in the millimeter and micrometer ranges. The larger void size range, with d_mean = 2.5 mm and porosity = 0.76–0.82, enables volumetric absorption of concentrated solar radiation for efficient heat transfer to the reaction site during endothermic reduction, while the smaller void size range within the struts, with d_mean = 10 μm and strut porosity = 0–0.44, increases the specific surface area for enhanced reaction kinetics during exothermic oxidation with CO₂. Characterization is performed via mercury intrusion porosimetry, scanning electron microscopy, and thermogravimetric analysis (TGA). Samples are thermally reduced at 1773 K and subsequently oxidized with CO₂ at temperatures in the range 873–1273 K. On average, CO production rates are ten times higher for samples with 0.44 strut porosity than for samples with non-porous struts. The oxidation rate scales with specific surface area and the apparent activation energy ranges from 90 to 135.7 kJ mol⁻¹. Twenty consecutive redox cycles exhibited stable CO production yield per cycle. Testing of the dual-scale RPC in a solar cavity-receiver exposed to high-flux thermal radiation (3.8 kW radiative power at 3015 suns) corroborated the superior performance observed in the TGA, yielding a shorter cycle time and a mean solar-to-fuel energy conversion efficiency of 1.72%.