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.
The thermodynamics of ceria-based metal oxides M x Ce1–x O2, 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 H2O and CO2. 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 H2 and CO concentrations determined as a function of reduction conditions (T, P O2 ) 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 H2 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 O2 = 10 ppm). Only when considering heat recovery of nearly 100% are theoretical efficiencies higher for the dopants.
•Advancements in redox materials for two-step solar thermochemical fuel production.•Effects of varying enthalpy and entropy changes on redox performance.•Perovskites suffer from reduced oxidation ...favorability compared to ceria.•The use of excess oxidant may lead to unrealistic expectations of performance.
Solar thermochemical (STC) redox cycles have made substantial advances in recent years, in large part due to the utilization of nonstoichiometric ceria over other iron oxide and zinc oxide based materials that undergo crystallographic or solid-to-gas phase changes. These changes render their utilization in a cyclic nature to be challenging because of the ever-changing physical properties over time and difficulty in preventing reverse reactions when cooling, for example Zn(g)+0.5O2(g)→ZnO(s). However, such phase changes also have the distinct benefit of being accompanied by large changes in entropy which is typically favorable from a thermodynamic perspective. As a result, the theoretical solar-to-fuel energy conversion efficiencies of ceria-based cycles are usually predicted to be lower than their volatile and nonvolatile stoichiometric counterparts; however, in actuality their measured performance is superior. For this technology to become commercially viable, there is a need to develop new materials that can outperform ceria in terms of solar-to-fuel energy conversion efficiency and operate at more benign conditions. This has been a large focus in the thermochemical community over the last 5–6years and, to date, most of the effort has been centered on reducing the relatively high operating temperatures that are required while maintaining ceria’s desirable characteristics such as favorable oxidation thermodynamics, rapid reaction kinetics, and crystallographic stability. This effort resulted in many perovskite related materials that operate several hundred degrees lower (e.g. 1473K). Unfortunately, however, their entropy change is usually lower than that of ceria and the results are consistently a compromise in the thermodynamic driving force for oxidation that results in less efficient materials overall. This work focuses on the thermodynamic, experimental, and computational aspects related to the discovery and characterization of new and better performing redox materials and the attributes necessary of them in order to drive the next generation of efficient STC redox materials.
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.
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 non-stoichiometry 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 La1–x Sr x MnO3−δ result in favorable reduction extents compared to ceria in the temperature range of 1523–1923 K. Oxidation with CO2 and H2O is not as thermodynamically favorable and largely dependent upon the oxidant concentration. The model is experimentally validated by O2 non-stoichiometry measurements at high temperatures (>1623 K) and CO2 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.
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 (D 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.
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 ≤ p CO2 ≤ 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.
Thermodynamic data for several LaMnO3-based perovskites indicates that in the high oxygen partial pressure (pO2) range (e.g., 10–7 to 10–3 atm), where isothermal thermochemical redox cycling is ...viable, they can undergo larger changes in oxidation state than ceria for a given change in pO2. This suggests that these materials may be more optimal for isothermal operation than ceria and offers the potential for more efficient H2/CO production via thermochemical splitting of H2O/CO2. To investigate this hypothesis, we developed a thermodynamic process model to predict the solar-to-fuel efficiencies of La1–x (Sr,Ca) x Mn1–y Al y O3 perovskites and compared results to ceria and Zr-doped ceria. The calculations were performed for isothermal or near-isothermal cycling from 1473 to 1773 K. Four methods of lowering the reduction pO2 were considered: inert gas sweeping, mechanical vacuum pumping, electrochemical oxygen pumping, and thermochemical oxygen pumping. Considering a reduction pO2 of 10–6 atm and a gas-phase heat recovery effectiveness of 95%, the calculations showed that the perovskites outperformed ceria and Zr-doped ceria during isothermal operation in terms of fuel production and efficiency regardless of the pO2 reduction method. For example, at 1773 K, the calculated efficiencies were 35.17% for La0.6Sr0.4Mn0.6Al0.4O3 and 28.26% for ceria when implementing thermochemical oxygen pumping. Other methods of lowering the reduction pO2 resulted in lower efficiencies, where electrochemical oxygen pumping > inert gas sweeping > vacuum pumping. Small temperature swings using inert gases to lower the pO2 resulted in the highest efficiencies overall. For example, with a reduction temperature of 1773 K and a temperature swing of 100 K, the efficiency of the ceria-based cycle was 35.18% and with a temperature swing of 300 K, the efficiency of the La0.6Ca0.4MnO3 cycle was 40.75%. Importantly, in the case of inert gas sweeping, the efficiency of the ceria-based cycle exceeds that of the candidate materials when the temperature swing is low. The theoretical calculations within this work show that perovskites have the potential for improved solar-to-fuel efficiencies during isothermal or near-isothermal redox cycling beyond those achievable by ceria.
Chemical looping reforming of methane over ceria-based materials is a promising route for the production of synthetic liquid fuel precursors, H2 and CO. In this work, a comprehensive kinetic model ...was established, based on thermogravimetric experiments, to describe the previously unresolved, surface-mediated mechanism of methane dissociation via ceria oxygen removal, the first heterogeneous, noncatalytic reaction within the aforementioned redox cycle. Prior studies have suggested that either a surface oxygen anion or vacancy is responsible for the activation of methane. However, here, these two theories are combined to unambiguously show that the prominence of each pathway is dependent on the availability of surface oxygen. Thus, at sufficiently high oxygen nonstoichiometries or for low-surface-area samples, as examined in this work, the vacancy-mediated dissociation of methane is predominant. This assertion was elucidated by mathematically describing a series of rate-determining steps based on surface interactions of known reactive intermediates and fitting the postulated reaction mechanism to temperature-dependent measurements obtained with multistage isothermal thermogravimetry. Corresponding Arrhenius parameters were extracted with excellent agreement between the model predictions and experimentally measured rates. Further validation of the hypothesized reaction mechanism is supported by (1) close similarity between activation energies obtained through model fits and separate isoconversional techniques and (2) expected trends observed with acceptor-doped ceria that has a higher concentration of extrinsic oxygen vacancies at the onset of the reaction. The quantitative kinetic insight obtained from the model presented herein allows for the evaluation and optimization of ceria-based materials in larger-scale, chemical looping processes.
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