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A solar cavity-receiver containing a reticulated porous ceramic (RPC) foam made of pure CeO2 has been experimentally investigated for CO2 splitting via thermochemical redox reactions. The RPC was directly exposed to concentrated thermal radiation at mean solar flux concentration ratios of up to 3015 suns. During the endothermic reduction step, solar radiative power inputs in the range 2.8–3.8 kW and nominal reactor temperatures from 1400 to 1600 °C yielded CeO2−δ with oxygen deficiency δ ranging between 0.016 and 0.042. In the subsequent exothermic oxidation step at below about 1000 °C, CeO2−δ was stoichiometrically reoxidized with CO2 to generate CO. The solar-to-fuel energy conversion efficiency, defined as the ratio of the calorific value of CO (fuel) produced to the solar radiative energy input through the reactor’s aperture and the energy penalty for using inert gas, was 1.73% average and 3.53% peak. This is roughly four times greater than the next highest reported values to date for a solar-driven device. The fuel yield per cycle was increased by nearly 17 times compared to that obtained with optically thick ceria felt because of the deeper penetration and volumetric absorption of high-flux solar irradiation.

A thermodynamic and experimental investigation of a new class of solar thermochemical redox intermediates, namely, lanthanum−strontium−manganese perovskites, is presented. A defect model based on low-temperature oxygen nonstoichiometry data is formulated and extrapolated to higher temperatures more relevant to thermochemical redox cycles. Strontium contents of x = 0.3 (LSM30) and x = 0.4 (LSM40) in La 1−x Sr x MnO 3−δ result in favorable reduction extents compared to ceria in the temperature range of 1523−1923 K. Oxidation with CO 2 and H 2 O is not as thermodynamically favorable and largely dependent upon the oxidant concentration. The model is experimentally validated by O 2 non-stoichiometry measurements at high temperatures (>1623 K) and CO 2 reduction cycles with commercially available LSM35. Theoretical solar−fuel energy conversion efficiencies for LSM40 and ceria redox cycles are 16 and 22% at 1800 K and 13 and 7% at 1600 K, respectively.

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The thermodynamics of ceria-based metal oxides M x Ce 1−x O 2 , where M = Gd, Y, Sm, Ca, Sr, have been studied in relation to their applicability as reactive intermediates in solar thermochemical redox cycles for splitting H 2 O and CO 2 . Oxygen nonstoichiometry was modeled and extrapolated to high temperatures and reduction extents by applying an ideal solution model in conjunction with a defect interaction model. Subsequently, relevant thermodynamic parameters were computed and equilibrium H 2 and CO concentrations determined as a function of reduction conditions (T, P O 2 ) and ensuing oxidation temperature. At 1 atm and above 1673 K, the degree of reduction is negatively correlated to dopant concentration, regardless of the type of dopant considered. Consequently, at a given reduction temperature, more H 2 and CO is generated at equilibrium for pure ceria compared to any of the other doped ceria materials considered. Although the reduction enthalpy decreases as the dopant concentration increases, the overall solar-to-fuel energy conversion efficiency is greater for pure ceria (20.2% at δ = 0.1, P O 2 = 10 ppm). Only when considering heat recovery of nearly 100% are theoretical efficiencies higher for the dopants.

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 dmean = 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 dmean = 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%.

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We present results on the thermochemical redox performance and analytical characterization of Hf4+, Zr4+, and Sc3+ doped ceria solutions synthesized via a sol–gel technique, all of which have recently been shown to be promising for splitting CO2. Dopant concentrations ranging from 5 to 15 mol % have been investigated and thermally cycled at reduction temperatures of 1773 K and oxidation temperatures ranging from 873 to 1073 K by thermogravimetry. The degree of reduction of Hf and Zr doped materials is substantially higher than those of pure ceria and Sc doped ceria and increases with dopant concentration. Overall, 10 mol % Hf doped ceria results in the largest CO yields per mole of oxide (∼0.5 mass % versus 0.35 mass % for pure ceria) based on measured mass changes during oxidation. However, these yields were largely influenced by their rate of reoxidation, not necessarily thermodynamic limitations, as equilibrium was not achieved for either Hf or Zr doped samples after 45 min exposure to CO2 at all oxidation temperatures. Additionally, sample preparation and grain size strongly affected the oxidation rates and subsequent yields, resulting in slightly decreasing yields as the samples were cycled up to 10 times. X-ray diffraction, Raman, FT-IR, and UV/vis spectroscopy in combination with SEM-EDX have been applied to characterize the elemental, crystalline, and morphological attributes before and after redox reactions.