Oxygen exchange materials for solar thermochemical splitting of H2O and CO2: A review

This review summarizes state of the art metal oxide materials used in two-step thermochemical redox cycles for the production of H2 and CO from H2O and CO2 using concentrated solar energy. Advantages and disadvantages of both stoichiometric (e.g. iron oxide based cycles) and nonstoichiometric (e.g. ceria based cycles) materials are discussed in the context of thermodynamics, chemical kinetics, and material stability. Finally, a perspective aimed at future materials development and requirements necessary for advances of process efficiencies is discussed.ISSN:1369-7021ISSN:1873-410

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 ≤ T ≤ 1823 K, 3 × 10–4 atm ≤ pO2 ≤ 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.ISSN:1932-7455ISSN:1932-744

Oxygen nonstoichiometry and thermodynamic characterization of Zr doped ceria in the 1573-1773 K temperature range

This work encompasses the thermodynamic characterization and critical evaluation of Zr4+ doped ceria, a promising redox material for the two-step solar thermochemical splitting of H2O and CO2 to H2 and CO. As a case study, we experimentally examine 5 mol% Zr4+ doped ceria and present oxygen nonstoichiometry measurements at elevated temperatures ranging from 1573 K to 1773 K and oxygen partial pressures ranging from 4.50 × 10−3 atm to 2.3 × 10−4 atm, yielding higher reduction extents compared to those of pure ceria under all conditions investigated, especially at the lower temperature range and at higher pO2. In contrast to pure ceria, a simple ideal solution model accounting for the formation of isolated oxygen vacancies and localized electrons accurately describes the defect chemistry. Thermodynamic properties are determined, namely: partial molar enthalpy, entropy, and Gibbs free energy. In general, partial molar enthalpy and entropy values of Zr4+ doped ceria are lower. The equilibrium hydrogen yields are subsequently extracted as a function of the redox conditions for dopant concentrations as high as 20%. Although reduction extents increase greatly with dopant concentration, the oxidation of Zr4+ doped ceria is thermodynamically less favorable compared to pure ceria. This leads to substantially larger temperature swings between reduction and oxidation steps, ultimately resulting in lower theoretical solar energy conversion efficiencies compared to ceria under most conditions. In effect, these results point to the importance of considering oxidation thermodynamics in addition to reduction when screening potential redox materials.ISSN:1463-9084ISSN:1463-907

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

Morphological Characterization and Effective Thermal Conductivity of Dual-Scale Reticulated Porous Structures

Reticulated porous ceramic (RPC) made of ceria are promising structures used in solar thermochemical redox cycles for splitting CO2 and H2O. They feature dual-scale porosity with mm-size pores for effective radiative heat transfer during reduction and µm-size pores within its struts for enhanced kinetics during oxidation. In this work, the detailed 3D digital representation of the complex dual-scale RPC is obtained using synchrotron submicrometer tomography and X-ray microtomography. Total and open porosity, pore size distribution, mean pore diameter, and specific surface area are extracted from the computer tomography (CT) scans. The 3D digital geometry is then applied in direct pore level simulations (DPLS) of Fourier’s law within the solid and the fluid phases for the accurate determination of the effective thermal conductivity at each porosity scale and combined, and for fluid-to-solid thermal conductivity from 10−5 to 1. Results are compared to predictions by analytical models for structures with a wide range of porosities 0.09–0.9 in both the strut’s µm-scale and bulk’s mm-scale. The morphological properties and effective thermal conductivity determined in this work serve as an input to volume-averaged models for the design and optimization of solar chemical reactors.ISSN:1996-194

Morphological Characterization and Effective Thermal Conductivity of Dual-Scale Reticulated Porous Structures

Reticulated porous ceramic (RPC) made of ceria are promising structures used in solar thermochemical redox cycles for splitting CO2 and H2O. They feature dual-scale porosity with mm-size pores for effective radiative heat transfer during reduction and µm-size pores within its struts for enhanced kinetics during oxidation. In this work, the detailed 3D digital representation of the complex dual-scale RPC is obtained using synchrotron submicrometer tomography and X-ray microtomography. Total and open porosity, pore size distribution, mean pore diameter, and specific surface area are extracted from the computer tomography (CT) scans. The 3D digital geometry is then applied in direct pore level simulations (DPLS) of Fourier’s law within the solid and the fluid phases for the accurate determination of the effective thermal conductivity at each porosity scale and combined, and for fluid-to-solid thermal conductivity from 10−5 to 1. Results are compared to predictions by analytical models for structures with a wide range of porosities 0.09–0.9 in both the strut’s µm-scale and bulk’s mm-scale. The morphological properties and effective thermal conductivity determined in this work serve as an input to volume-averaged models for the design and optimization of solar chemical reactors

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