Diffusion of Oxygen in Ceria at Elevated Temperatures and Its Application to H₂O/CO₂ Splitting Thermochemical Redox Cycles

Determination of reaction and oxygen diffusion rates at elevated temperatures is essential for modeling, design, and optimization of high-temperature solar thermochemical fuel production processes, but such data for state-of-the-art redox materials, such as ceria, is sparse. Here, we investigate the solid-state reduction and oxidation of sintered nonstoichiometric ceria at elevated temperatures relevant to solar thermochemical redox cycles for splitting H₂O and CO₂ (1673 K ≤ <i>T</i> ≤ 1823 K, 3 × 10^–4 atm ≤ <i>p</i>_O₂ ≤ 2.5 × 10^–3 atm). Relaxation experiments are performed by subjecting the sintered ceria to rapid oxygen partial pressure changes and measuring the time required to achieve thermodynamic equilibrium state. From such data, we elucidate information regarding ambipolar oxygen diffusion coefficients through comparison of experimental data to a numerical approximation of Fick’s second law based on finite difference methods. In contrast to typically applied analytical approaches, where diffusion coefficients are necessarily concentration independent, such a numerical approach is capable of accounting for more realistic concentration dependent diffusion coefficients and also accounts for transient gas phase boundary conditions pertinent to dispersion and oxygen consumption/evolution. Ambipolar diffusion coefficients are obtained in the range 1.5·10^–5 cm² s^–1 ≤ <i>D̃</i> ≤ 4·10^–4 cm² s^–1 between 1673 and 1823 K. These results highlight the rapid nature of ceria reduction to help guide the design of redox materials for solar reactors, the importance of accounting for transient boundary conditions during relaxation experiments (either mass based or conductivity based), and point to the flexibility of using a numerical analysis in contrast to typical analytical approaches

Thermal Reduction of Ceria within an Aerosol Reactor for H₂O and CO₂ Splitting

An aerosol reactor was tested for the thermal reduction of ceria as part of a solar thermochemical redox cycle for producing H₂ and CO from H₂O and CO₂. The design is based on the downward aerosol flow of ceria particles, counter to an argon sweep gas, which are rapidly heated and thermally reduced within residence times of less than 1 s. When operating in the temperature range of 1723–1873 K and at oxygen partial pressures between 5 × 10^–5 and 1.2 × 10^–4 atm, reduction extents of small particles (<i>D</i>_v50 = 12 μm) approached those predicted by thermodynamics. However, heat- and mass-transfer effects were found to limit their conversion when the ceria mass flow rate was increased above 100 mg s^–1. This reactor concept inherently results in separation of the reduced ceria and evolved O₂(g), operates isothermally throughout the day, and decouples the reduction and oxidation steps in both space and time for potential 24-h syngas generation

Diffusion of Oxygen in Ceria at Elevated Temperatures and Its Application to H₂O/CO₂ Splitting Thermochemical Redox Cycles

Determination of reaction and oxygen diffusion rates at elevated temperatures is essential for modeling, design, and optimization of high-temperature solar thermochemical fuel production processes, but such data for state-of-the-art redox materials, such as ceria, is sparse. Here, we investigate the solid-state reduction and oxidation of sintered nonstoichiometric ceria at elevated temperatures relevant to solar thermochemical redox cycles for splitting H₂O and CO₂ (1673 K ≤ <i>T</i> ≤ 1823 K, 3 × 10^–4 atm ≤ <i>p</i>_O₂ ≤ 2.5 × 10^–3 atm). Relaxation experiments are performed by subjecting the sintered ceria to rapid oxygen partial pressure changes and measuring the time required to achieve thermodynamic equilibrium state. From such data, we elucidate information regarding ambipolar oxygen diffusion coefficients through comparison of experimental data to a numerical approximation of Fick’s second law based on finite difference methods. In contrast to typically applied analytical approaches, where diffusion coefficients are necessarily concentration independent, such a numerical approach is capable of accounting for more realistic concentration dependent diffusion coefficients and also accounts for transient gas phase boundary conditions pertinent to dispersion and oxygen consumption/evolution. Ambipolar diffusion coefficients are obtained in the range 1.5·10^–5 cm² s^–1 ≤ <i>D̃</i> ≤ 4·10^–4 cm² s^–1 between 1673 and 1823 K. These results highlight the rapid nature of ceria reduction to help guide the design of redox materials for solar reactors, the importance of accounting for transient boundary conditions during relaxation experiments (either mass based or conductivity based), and point to the flexibility of using a numerical analysis in contrast to typical analytical approaches

Synthesis, Characterization, and Thermochemical Redox Performance of Hf⁴⁺, Zr⁴⁺, and Sc³⁺ Doped Ceria for Splitting CO₂

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