Solar-driven thermochemical conversion of H2O and CO2 into sustainable fuels

Solar-driven thermochemical conversion of H2O and CO2 into sustainable fuels, based on redox cycle, provides a promising path for alternative energy, as it employs the solar energy as high-temperature heat supply and adopts H2O and CO2 as initial feedstock. This review describes the sustainable fuels production system, including a series of physical and chemical processes for converting solar energy into chemical energy in the form of sustainable fuels. Detailed working principles, redox materials, and key devices are reviewed and discussed to provide systematic and in-depth understanding of thermochemical fuels production with the aid of concentrated solar power technology. In addition, limiting factors affecting the solar-to-fuel efficiency are analyzed; meanwhile, the improvement technologies (heat recovery concepts and designs) are summarized. This study therefore sets a pathway for future research works based on the current status and demand for further development of such technologies on a commercial scale.

Methane Catalytic Combustion under Lean Conditions over Pristine and Ir-Loaded La1-xSrxMnO3 Perovskites: Efficiency, Hysteresis, and Time-on-Stream and Thermal Aging Stabilities

The increasing use of natural gas as an efficient, reliable, affordable, and cleaner energy source, compared with other fossil fuels, has brought the catalytic CH4 complete oxidation reaction into the spotlight as a simple and economic way to control the amount of unconverted methane escaping into the atmosphere. CH4 emissions are a major contributor to the 'greenhouse effect', and therefore, they need to be effectively reduced. Catalytic CH4 oxidation is a promising method that can be used for this purpose. Detailed studies of the activity, oxidative thermal aging, and the time-on-stream (TOS) stability of pristine La1-xSrxMnO3 perovskites (LSXM; X = % substitution of La with Sr = 0, 30, 50 and 70%) and iridium-loaded Ir/La1-xSrxMnO3 (Ir/LSXM) perovskite catalysts were conducted in a temperature range of 400-970 °C to achieve complete methane oxidation under excess oxygen (lean) conditions. The effect of X on the properties of the perovskites, and thus, their catalytic performance during heating/cooling cycles, was studied using samples that were subjected to various pretreatment conditions in order to gain an in-depth understanding of the structure-activity/stability correlations. Large (up to ca. 300 °C in terms of T50) inverted volcano-type differences in catalytic activity were found as a function of X, with the most active catalysts being those where X = 0%, and the least active were those where X = 50%. Inverse hysteresis phenomena (steady-state rate multiplicities) were revealed in heating/cooling cycles under reaction conditions, the occurrence of which was found to depend strongly on the employed catalyst pre-treatment (pre-reduction or pre-oxidation), while their shape and the loop amplitude were found to depend on X and the presence of Ir. All findings were consistently interpreted, which involved a two-term mechanistic model that utilized the synergy of Eley-Rideal and Mars-van Krevelen kinetics.

Activity and Thermal Aging Stability of La1-xSrxMnO3 (x = 0.0, 0.3, 0.5, 0.7) and Ir/La1-xSrxMnO3 Catalysts for CO Oxidation with Excess O2

The catalytic oxidation of CO is probably the most investigated reaction in the literature, for decades, because of its extended environmental and fundamental importance. In this paper, the oxidation of CO on La_1-xSr_xMnO₃ perovskites (LSMx), either unloaded or loaded with dispersed Ir nanoparticles (Ir/LSMx), was studied in the temperature range 100-450 °C under excess O₂ conditions (1% CO + 5% O₂). The perovskites, of the type La_1-xSr_xMnO₃ (x = 0.0, 0.3, 0.5 and 0.7), were prepared by the coprecipitation method. The physicochemical and structural properties of both the LSMx and the homologous Ir/LSMx catalysts were evaluated by various techniques (XRD, N₂ sorption-desorption by BET-BJH, H₂-TPR and H₂-Chem), in order to better understand the structure-activity-stability correlations. The effect of preoxidation/prereduction/aging of the catalysts on their activity and stability was also investigated. Results revealed that both LSMx and Ir/LSMx are effective for CO oxidation, with the latter being superior to the former. In both series of materials, increasing the substitution of La by Sr in the composition of the perovskite resulted to a gradual suppression of their CO oxidation activity when these were prereduced; the opposite was true for preoxidized samples. Inverse hysteresis phenomena in activity were observed during heating/cooling cycles on the prereduced Ir/LSMx catalysts with the loop amplitude narrowing with increasing Sr-content in LSMx. Oxidative thermal sintering experiments at high temperatures revealed excellent antisintering behavior of Ir nanoparticles supported on LSMx, resulting from perovskite's favorable antisintering properties of high oxygen storage capacity and surface oxygen vacancies.

Exploring the Stability of Fe–Ni Alloy Nanoparticles Exsolved from Double-Layered Perovskites for Dry Reforming of Methane

Exsolution is emerging as a promising route for the creation of nanoparticles that remain anchored to the oxide support, imparting remarkable stability in high temperature chemical processes such as dry reforming of methane. This process takes place at temperatures around 850 °C, which causes sintering-related issues in catalysts prepared using conventional impregnation methods, which could be overcome by using exsolution functionalized oxides. In this work, FeNi3 alloy nanoparticles exsolved from Sr2FexNi1-xMoO6-δ double-layered perovskites were evaluated as a dry reforming catalyst, paying special attention to structure–activity relationships. Our results indicate that increasing the Ni content favors the nanoparticle dispersion, eventually leading to increased CO2 and CH4 conversions. The exsolved nanoparticles presented remarkable nanoparticle size (ca. 30 nm) stability after the 10 h treatment, although the formation of some phase segregations over the course of the reaction caused a minor decrease in the nanoparticle population. Overall, the results presented here serve as materials processing guidelines that could find further potential use in the design of more efficient (electro)catalysts in other fuel production or energy conversion technologies.

CaCo0.05Mn0.95O3-δ: A Promising Perovskite Solid Solution for Solar Thermochemical Energy Storage

The redox cycle of doped CaMnO_3-δ has emerged as an attractive way for cost-effective thermochemical energy storage (TCES) at high temperatures in concentrating solar power. The role of dopants is mainly to improve the thermal stability of CaMnO_3-δ at high temperatures and the overall TCES density of the material. Herein, Co-doped CaMnO_3-δ (CaCo_xMn_1-xO_3-δ, x = 0-0.5) perovskites have been proposed as a promising candidate for TCES materials for the first time. The phase compositions, redox capacities, TCES densities, reaction rates, and redox chemistry of the samples have been explored via experimental analysis and theoretical calculations. The results demonstrate that CaCo_0.05Mn_0.95O_3-δ showed an enhanced redox capacity (1000 °C at pO₂ = 10^-5 bar) without decomposition and provided the highest TCES density of ∼571 kJ kg^-1 reported so far. The effective Co doping tended to increase the valence states of B-site cations in perovskite and facilitate the diffusion of the lattice oxygen atoms into the surface-active oxygen sites. Furthermore, the high cooling rates deteriorated the microstructure of CaCo_0.05Mn_0.95O_3-δ particles and resulted in incomplete heat release, which is instructive to the design and operation of the TCES systems.

Solar thermochemical CO2 splitting with doped perovskite LaCo0.7Zr0.3O3: thermodynamic performance and solar-to-fuel efficiency

The research of thermochemical CO₂ splitting based on perovskites is a promising approach to green energy development. Performance evaluation was performed towards the doped perovskite LaCo_0.7Zr_0.3O₃ (LCZ-73) based two-step thermochemical CO₂ splitting process thermodynamically based on the experimentally derived parameters for the first time. The impacts of vacuum pump and inert gas purge to reduce oxygen partial pressure and CO₂ heating on the performance parameter η_{solar-to-fuel} have been analyzed. The results showed that at the P_O2 of 10⁻⁵ bar, non-stoichiometric oxygen δ increased by more than 3 times as the reduction temperature varied from 1000 °C to 1300 °C, however, no significant deviation of δ was observed between 1300 °C and 1400 °C. The reaction enthalpy ranged from 60 to 130 kJ mol⁻¹ corresponding to δ = 0.05–0.40. Comparing the abovementioned two ways to reduce the oxygen partial pressure, the η_{solar-to-fuel} of 0.39% and 0.1% can be achieved with 75% and without heat recovery with the CO₂ flow rate of 40 sccm under experimental conditions, respectively. The energy cost for CO₂ heating during the thermodynamic process as the n_CO2/n_LCZ-73 increases was obtained from the perspective of energy analysis. The ratio of n_CO2/n_LCZ-73 at lower temperature required more demanding conditions for the aim of commercialization. Finally, the ability of perovskite to split CO₂ and thermochemical performance were tested under different CO₂ flow rates. The results showed that high CO₂ flow rate was conducive to the production of CO, but at the cost of low η_{solar-to-fuel}. The maximum solar-to-fuel efficiency of 1.36% was achieved experimentally at a CO₂ flow rate of 10 sccm in the oxidation step and 75% heat recovery.

Thermodynamics analysis of two-step thermochemical CO₂ splitting with LaCo_0.7Zr_0.3O₃ with gas–gas, gas–solid phase heat recuperation is performed based on experiment.

Thermochemical CO2 splitting performance of perovskite coated porous ceramics

In this paper, we investigate the redox performance of perovskite coated porous ceramics with various architectures. For this purpose, reticulated porous ceramics (RPCs) in three different pore sizes (5, 12, 75 ppi) were fabricated to represent a broad range of structures and pore sizes. The perovskite material is based on lanthanum manganite and was synthesized and doped with Ca and Al through the Pechini method. Using a deep coating method, the surface of RPC substrates was modified by a thin-film coating with a thickness of ∼15 μm. We evaluated the CO₂ conversion performance of the developed materials in a gold-image IR furnace. X-ray micro-computed tomography along with SEM/EDX were utilized in different steps of the work for a thorough study of the bulk and surface features. Results reveal that the intermediate pore size of 12 ppi delivers the maximum perovskite loading with a high degree of coating homogeneity and connectivity while CO₂ conversion tests showed the highest CO yield for 75 ppi. Our results show that the extreme conditions inside the furnace combined with the flow of gaseous phases cause the RPCs to shrink in length up to 23% resulting in the alteration of the pore phase and elimination of small pores reducing the total specific surface area. Further our results reveal an important mechanism resulting in the inhibition of CO₂ conversion where the perovskite coating layer migrates into the matrix of the RPC frame.

A representative volume of LCMA coated porous SiC showing a maximum of 23% shrinkage when subject to high-temperature CO₂ conversion redox reactions. This results in significant structural changes including a reduction in specific surface area.

Non-Stoichiometric Redox Active Perovskite Materials for Solar Thermochemical Fuel Production: A Review

Due to the requirement to develop carbon-free energy, solar energy conversion into chemical energy carriers is a promising solution. Thermochemical fuel production cycles are particularly interesting because they can convert carbon dioxide or water into CO or H2 with concentrated solar energy as a high-temperature process heat source. This process further valorizes and upgrades carbon dioxide into valuable and storable fuels. Development of redox active catalysts is the key challenge for the success of thermochemical cycles for solar-driven H2O and CO2 splitting. Ultimately, the achievement of economically viable solar fuel production relies on increasing the attainable solar-to-fuel energy conversion efficiency. This necessitates the discovery of novel redox-active and thermally-stable materials able to split H2O and CO2 with both high-fuel productivities and chemical conversion rates. Perovskites have recently emerged as promising reactive materials for this application as they feature high non-stoichiometric oxygen exchange capacities and diffusion rates while maintaining their crystallographic structure during cycling over a wide range of operating conditions and reduction extents. This paper provides an overview of the best performing perovskite formulations considered in recent studies, with special focus on their non-stoichiometry extent, their ability to produce solar fuel with high yield and performance stability, and the different methods developed to study the reaction kinetics.

Controlling cation segregation in perovskite-based electrodes for high electro-catalytic activity and durability

Solid oxide cell (SOC) based energy conversion systems have the potential to become the cleanest and most efficient systems for reversible conversion between electricity and chemical fuels due to their high efficiency, low emission, and excellent fuel flexibility. Broad implementation of this technology is however hindered by the lack of high-performance electrode materials. While many perovskite-based materials have shown remarkable promise as electrodes for SOCs, cation enrichment or segregation near the surface or interfaces is often observed, which greatly impacts not only electrode kinetics but also their durability and operational lifespan. Since the chemical and structural variations associated with surface enrichment or segregation are typically confined to the nanoscale, advanced experimental and computational tools are required to probe the detailed composition, structure, and nanostructure of these near-surface regions in real time with high spatial and temporal resolutions. In this review article, an overview of the recent progress made in this area is presented, highlighting the thermodynamic driving forces, kinetics, and various configurations of surface enrichment and segregation in several widely studied perovskite-based material systems. A profound understanding of the correlation between the surface nanostructure and the electro-catalytic activity and stability of the electrodes is then emphasized, which is vital to achieving the rational design of more efficient SOC electrode materials with excellent durability. Furthermore, the methodology and mechanistic understanding of the surface processes are applicable to other materials systems in a wide range of applications, including thermo-chemical photo-assisted splitting of H₂O/CO₂ and metal-air batteries.

Hydrogen-assisted Carbon Dioxide Thermochemical Reduction on La0.9 Ca0.1 FeO3-δ Membranes: A Kinetics Study

Kinetics data for CO₂ thermochemical reduction in an isothermal membrane reactor is required to identify the rate-limiting steps. A detailed reaction kinetics study on this process supported by an La_0.9 Ca_0.1 FeO_3-δ (LCF-91) membrane is thus reported. The dependence of CO₂ reduction rate on various operating conditions is examined, such as CO₂ concentration on the feed side, fuel concentrations on the sweep side, and temperatures. The CO₂ reduction rate is proportional to the oxygen flux across the membrane, and the measured maximum fluxes are 0.191 and 0.164 μmol cm^-2  s^-1 with 9.5 mol% H₂ and 11.6 mol% CO on the sweep side at 990 °C, respectively. Fuel is used to maintain the chemical potential gradient across the membrane and CO is used to derive the surface reaction kinetics. This membrane also exhibits stable performances for 106 h. A resistance-network model is developed to describe the oxygen transport process and the kinetics data are parameterized using the experimental values. The model shows a transition of the rate limiting step between the surface reactions on the feed side and the sweep side depending on the operating conditions.

Solar thermochemical splitting of water to generate hydrogen

Solar photochemical means of splitting water (artificial photosynthesis) to generate hydrogen is emerging as a viable process. The solar thermochemical route also promises to be an attractive means of achieving this objective. In this paper we present different types of thermochemical cycles that one can use for the purpose. These include the low-temperature multistep process as well as the high-temperature two-step process. It is noteworthy that the multistep process based on the Mn(II)/Mn(III) oxide system can be carried out at 700 °C or 750 °C. The two-step process has been achieved at 1,300 °C/900 °C by using yttrium-based rare earth manganites. It seems possible to render this high-temperature process as an isothermal process. Thermodynamics and kinetics of H₂O splitting are largely controlled by the inherent redox properties of the materials. Interestingly, under the conditions of H₂O splitting in the high-temperature process CO₂ can also be decomposed to CO, providing a feasible method for generating the industrially important syngas (CO+H₂). Although carbonate formation can be addressed as a hurdle during CO₂ splitting, the problem can be avoided by a suitable choice of experimental conditions. The choice of the solar reactor holds the key for the commercialization of thermochemical fuel production.

High Redox Capacity of Al-Doped La1-x Srx MnO3-δ Perovskites for Splitting CO2 and H2 O at Mn-Enriched Surfaces

Perovskites are attractive candidates for the solar-driven thermochemical redox splitting of CO₂ and H₂ O into CO and H₂ (syngas) and O₂ . This work investigates the surface activity of La_1-x Sr_x Mn_1-y Al_y O_3-δ (0≤x≤1, 0≤y≤1) and La_0.6 Ca_0.4 Mn_0.6 Al_0.4 O_3-δ . At 1623 K and 15 mbar O₂ , the oxygen non-stoichiometry of La_0.2 Sr_0.8 Mn_0.8 Al_0.2 O_3-δ increases with the strontium content and reaches a maximum of δ=0.351. X-ray photoelectron spectroscopy analysis indicates that manganese is the only redox-active metal at the surface. All La_1-x Sr_x Mn_1-y Al_y O_3-δ compositions exhibit surfaces enriched in manganese and depleted in strontium. We discuss how these compositional differences of the surface from the bulk lead to the beneficially higher reduction extents and lower strontium carbonate concentrations at the aluminum-doped surfaces. Using first principles calculations, we validate the experimental reduction trends and elucidate the mechanism of the partial electronic charge redistribution upon perovskite reduction.

Tunable thermodynamic activity of La x Sr1-x Mn y Al1-y O3-δ (0 ≤ x ≤ 1, 0 ≤ y ≤ 1) perovskites for solar thermochemical fuel synthesis

Nonstoichiometric metal oxides with variable valence are attractive redox materials for thermochemical and electrochemical fuel processing. To guide the design of advanced redox materials for solar-driven splitting of CO₂ and/or H₂O to produce CO and/or H₂ (syngas), we investigate the equilibrium thermodynamics of the La _x Sr_1-x Mn _y Al_1-y O_3-δ perovskite family (0 ≤ x ≤ 1, 0 ≤ y ≤ 1) and La_0.6Ca_0.4Mn_0.8Al_0.2O_3-δ , and compare them to those of CeO₂ as the baseline. Oxygen nonstoichiometry measurements from 1573 to 1773 K and from 0.206 to 180 mbar O₂ show a tunable reduction extent, increasing with increasing Sr content. Maximal nonstoichiometry of 0.32 is established with La_0.2Sr_0.8Mn_0.8Al_0.2O_3-δ at 1773 K and 2.37 mbar O₂. As a trend, we find that oxygen capacities are most sensitive to the A-cation composition. Partial molar enthalpy, entropy and Gibbs free energy changes for oxide reduction are extracted from the experimental data using defect models for Mn⁴⁺/Mn³⁺ and Mn³⁺/Mn²⁺ redox couples. We find that perovskites exhibit typically decreasing enthalpy changes with increasing nonstoichiometries. This desirable characteristic is most pronounced by La_0.6Sr_0.4Mn_0.4Al_0.6O_3-δ , rendering it attractive for CO₂ and H₂O splitting. Generally, perovskites show lower enthalpy and entropy changes than ceria, resulting in more favorable reduction but less favorable oxidation equilibria. The energy penalties due to larger temperature swings and excess oxidants are discussed in particular. Using electronic structure theory, we conclude with a practical methodology estimating thermodynamic activity to rationally design perovskites with variable stoichiometry and valence.

Solar photochemical and thermochemical splitting of water

Artificial photosynthesis to carry out both the oxidation and the reduction of water has emerged to be an exciting area of research. It has been possible to photochemically generate oxygen by using a scheme similar to the Z-scheme, by using suitable catalysts in place of water-oxidation catalyst in the Z-scheme in natural photosynthesis. The best oxidation catalysts are found to be Co and Mn oxides with the e(1) g configuration. The more important aspects investigated pertain to the visible-light-induced generation of hydrogen by using semiconductor heterostructures of the type ZnO/Pt/Cd1-xZnxS and dye-sensitized semiconductors. In the case of heterostructures, good yields of H2 have been obtained. Modifications of the heterostructures, wherein Pt is replaced by NiO, and the oxide is substituted with different anions are discussed. MoS2 and MoSe2 in the 1T form yield high quantities of H2 when sensitized by Eosin Y. Two-step thermochemical splitting of H2O using metal oxide redox pairs provides a strategy to produce H2 and CO. Performance of the Ln0.5A0.5MnO3 (Ln = rare earth ion, A = Ca, Sr) family of perovskites is found to be promising in this context. The best results to date are found with Y0.5Sr0.5MnO3.

Beneficial effects of substituting trivalent ions in the B-site of La0.5Sr0.5Mn1-xAxO3 (A = Al, Ga, Sc) on the thermochemical generation of CO and H2 from CO2 and H2O

The effect of substitution of Al(3+), Ga(3+) and Sc(3+) ions in the Mn(3+) site of La0.5Sr0.5MnO3 on the thermochemical splitting of CO2 to generate CO has been studied in detail. Both La0.5Sr0.5Mn1-xGaxO3 and La0.5Sr0.5Mn1-xScxO3 give high yields of O2 and generate CO more efficiently than La0.5Sr0.5Mn1-xAlxO3 or the parent La0.5Sr0.5MnO3. Substitution of even 5% Sc(3+) (x = 0.05) results in a remarkable improvement in performance. Thus La0.5Sr0.5Mn0.95Sc0.05O3 produces 417 μmol g(-1) of O2 and 545 μmol g(-1) of CO, respectively, i.e. 2 and 1.7 times more O2 and CO than La0.5Sr0.5MnO3. This manganite also generates H2 satisfactorily by the thermochemical splitting of H2O.

Carbon Dioxide Reforming of Methane using an Isothermal Redox Membrane Reactor

The continuous production of carbon monoxide (CO) and hydrogen (H₂) by dry reforming of methane (CH₄) is demonstrated isothermally using a ceramic redox membrane in absence of additional catalysts. The reactor technology realizes the continuous splitting of CO₂ to CO on the inner side of a tubular membrane and the partial oxidation of CH₄ with the lattice oxygen to form syngas on the outer side. La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ (LSCF) membranes evaluated at 840-1030 °C yielded up to 1.27 μmol _CO   s^-1 from CO₂, 3.77 μmol_H₂ g^-1 s^-1 from CH₄ , and CO from CH₄ at approximately the same rate as CO from CO₂. We compute the free energy of the oxygen vacancy formation for La_0.5Sr_0.5B_0.5B'_0.5O_3-δ (B, B'=Mn, Fe, Co, Cu) using electronic structure theory to understand how CO₂ reduction limits dry reforming of methane using LSCF and to show how the CO₂ conversion can be increased by using advanced redox materials such as La_0.5Sr_0.5MnO_3-δ and La_0.5Sr_0.5Mn_0.5Co_0.5O_3-δ .