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 ≤ T ≤ 1823 K, 3 × 10–4 atm ≤ p 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 ≤ D̃ ≤ 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.
The kinetics of CO2 reduction over nonstoichimetric ceria, CeO2−δ, a material of high potential for thermochemical conversion of sunlight to fuel, has been investigated for a wide range of nonstoichiometries (0.02 ≤ δ ≤ 0.25), temperatures (693 ≤ T ≤ 1273 K), and CO2 concentrations (0.005 ≤ pCO2 ≤ 0.4 atm). Samples were reduced thermally at 1773 K to probe low nonstoichiometries (δ < 0.05) and chemically at lower temperatures in a H2 atmosphere to prevent particle sintering and probe the effect of higher nonstoichiometries (δ < 0.25). For extents greater than δ = 0.2, oxidation rates at a given nonstoichiometry are hindered for the duration of the reaction, presumably because of near-order changes, such as lattice compression, as confirmed via Raman Spectroscopy. Importantly, this behavior is reversible and oxidation rates are not affected at lower δ. Following thermal reduction at very low δ, however, oxidation rates are an order of magnitude slower than those of chemically reduced samples, and rates monotonically increase with the initial nonstoichiometry (up to δ = 0.05). This dependence may be attributed to the formation of stable defect complexes formed between oxygen vacancies and polarons. When the same experiments are performed with 10 mol % Gd3+ doped ceria, in which defect complexes are less prevalent than in pure ceria, this dependence is not observed.
Thermochemical splitting of CO2 via a ceria‐based redox cycle was performed in a solar‐driven thermogravimetric analyzer. Overall reaction rates, including heat and mass transport, were determined under concentrated irradiation mimicking realistic operation of solar reactors. Reticulated porous ceramic (RPC) structures and fibers made of undoped and Zr4+‐doped CeO2, were endothermally reduced under radiative fluxes of 1280 suns in the temperature range 1200–1950 K and subsequently re‐oxidized with CO2 at 950–1400 K. Rapid and uniform heating was observed for 8 ppi ceria RPC with mm‐sized porosity due to its low optical thickness and volumetric radiative absorption, while ceria fibers with μm‐sized porosity performed poorly due to its opacity to incident irradiation. The 10 ppi RPC exhibited higher fuel yield because of its higher sample density. Zr4+‐doped ceria showed increasing reduction extents with dopant concentration but decreasing specific CO yield due to unfavorable oxidation thermodynamics and slower kinetics. © 2016 American Institute of Chemical Engineers AIChE J, 63: 1263–1271, 2017
Porous structures made of redox active ceria are attractive for high‐temperature concentrating solar applications and particularly for the thermochemical splitting of H2O and CO2 as their enhanced heat and mass transport properties lead to fast reaction rates, especially with regard to the absorption of concentrated solar radiation during the endothermic reduction step. Hierarchically ordered porous structures, fabricated by the Schwartzwald replica method on 3D‐printed polymer scaffolds, are experimentally assessed for their ability to volumetrically absorb high‐flux irradiation of up to 670 suns. Temperature distributions across the porosity‐gradient path are measured (peak 1724 K) and compared with that obtained for a reticulated porous ceramic (RPC) structure with a uniform porosity. To assist the analysis, a Monte Carlo ray‐tracing model is developed for pore‐level numerical simulations of the ordered geometries and applied to analyze the absorbing–emitting–scattering exchange and determine the radiation attenuation and the temperature distribution at a radiative equilibrium. In contrast to the Bouguer's law exponential‐decay attenuation of incident radiation observed for the RPC, the ordered structures with a porosity gradient exhibit a step‐wise radiative attenuation that leads to a more uniform temperature distribution across the structure. This in turn predicts a superior redox performance.
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