Oxygen Exchange in Dual-Phase La0.65Sr0.35MnO3-CeO2 Composites for Solar Thermochemical Fuel Production

Advances in perovskite membranes for carbon capture & utilization: A sustainable approach to CO2 emissions reduction - A review

Despite agreements like the Paris Agreement, the world continues to face rising temperatures, extreme weather, and ecosystem disruptions, driven by continued use fossil fuel, agricultural emissions, and industrial activities and leading to greenhouse gas contributing to the serious fuelling climate change. Carbon capture and utilization (CCU), particularly thermochemical carbon dioxide (CO2) splitting powered by thermal energy, offers a promising solution. Perovskite-based inorganic membranes, known for their high selectivity and permeability toward various gases, efficiency, and energy-saving potential, have attracted significant interest in gas separation, production and emerged as a leading technology for carbon capture and hydrogen purification. This review explores advancements in perovskite materials, focusing on H2/CO2 separation, CO2 conversion to CO, and optimal operating conditions. It addresses key questions such as improving material performance through innovations in double and composite perovskites, enhancing oxygen removal via thermochemical or electrochemical pumps, and integrating CO2 splitting with fuel production. These strategies aim to reduce costs, boost efficiency, and provide sustainable pathways for addressing climate change.

Microwave-Driven Reduction Accelerates Oxygen Exchange in Perovskite Oxides

Microwave-assisted oxide reduction has emerged as a promising method to electrify chemical looping processes for renewable hydrogen production. Moreover, these thermochemical cycles can be used for thermochemical air separation, electrifying the O2 generation by applying microwaves in the reduction step. This approach offers an alternative to conventional cryogenic air separation, producing pure streams of O2 and N2. The electrification by microwaves lowers the requirements for titanate perovskites (CaTi1-xMnxO3-δ), which typically demand high temperatures for thermochemical cycles. Microwave activation allows for a drastic reduction in the operation conditions of the reduction reaction, leading to unprecedentedly rapid absorption-desorption cycles (<3 min per cycle). For CaTi0.8Mn0.2O3-δ, we achieved a cycle-averaged O2 production of 2.6 mL g-1 min-1 at 800 °C, surpassing conventional values of materials operating in the high-temperature regime. This method could significantly impact thermochemical air separation by enabling a faster oxygen absorption-desorption cycle at more moderate temperatures than those of conventionally heated processes.

Beneficial effect of cerium excess on in situ grown Sr0.86Ce0.14FeO3-CeO2 thermocatalysts for the degradation of bisphenol A

Ce-doped SrFeO3 perovskite-type compounds are known as good thermocatalysts for the abatement of wastewater contaminants of emerging concern. In this work, Sr0.86Ce0.14FeO3-CeO2 perovskite-oxide systems with increasing amounts of cerium excess (0, 5, 10 and 15 mol% Ce), with respect to its maximum solubility in the perovskite, were prepared in one-pot by solution combustion synthesis and the effects of cerium excess on the chemical physical properties and thermocatalytic activity in the bisphenol A degradation were evaluated. The powders were characterized by powder X-ray diffraction combined with Rietveld refinement, X-ray photoelectron spectroscopy, thermal gravimetry, temperature programmed reduction, nitrogen adsorption, scanning electron microscopy and energy dispersive X-ray spectroscopy techniques. Results highlight that the perovskite structural, redox, surface, and morphological properties are affected by the in situ co-growth of the main perovskite phase and ceria and that a larger cerium excess has a beneficial effect on the thermocatalytic performance of the perovskite oxide-ceria biphasic system, although ceria is not active as a thermocatalyst itself. Perovskite properties and performance are enhanced by the tetragonal distortion induced by the introduction of cerium excess in the synthesis. It is supposed that a larger oxygen mobility and an easier reducibility are among the most relevant features that contribute to superior thermocatalytic properties of these perovskite oxide-based systems. These results also suggest new perspectives in the nanocomposite preparation and their catalytic applications.

Thermochemical Activity of Single- and Dual-Phase Oxide Compounds Based on Ceria, Ferrites, and Perovskites for Two-Step Synthetic Fuel Production

This study focuses on the generation of solar thermochemical fuel (hydrogen, syngas) from CO₂ and H₂O molecules via two-step thermochemical cycles involving intermediate oxygen-carrier redox materials. Different classes of redox-active compounds based on ferrite, fluorite, and perovskite oxide structures are investigated, including their synthesis and characterization associated with experimental performance assessment in two-step redox cycles. Their redox activity is investigated by focusing on their ability to perform the splitting of CO₂ during thermochemical cycles while quantifying fuel yields, production rates, and performance stability. The shaping of materials as reticulated foam structures is then evaluated to highlight the effect of morphology on reactivity. A series of single-phase materials including spinel ferrite, fluorite, and perovskite formulations are first investigated and compared to state-of-the-art materials. NiFe₂O₄ foam exhibits a CO₂-splitting activity similar to its powder analog after reduction at 1400 °C, surpassing the performance of ceria but with much slower oxidation kinetics. On the other hand, although identified as high-performing materials in other studies, Ce_0.9Fe_0.1O₂, Ca_0.5Ce_0.5MnO₃, Ce_0.2Sr_1.8MnO₄, and Sm_0.6Ca_0.4Mn_0.8Al_0.2O₃ are not found to be attractive candidates in this work (compared with La_0.5Sr_0.5Mn_0.9Mg_0.1O₃). In the second part, characterizations and performance evaluation of dual-phase materials (ceria/ferrite and ceria/perovskite composites) are performed and compared to single-phase materials to assess a potential synergistic effect on fuel production. The ceria/ferrite composite does not provide any enhanced redox activity. In contrast, ceria/perovskite dual-phase compounds in the form of powders and foams are found to enhance the CO₂-splitting performance compared to ceria.

A Review of Oxygen Carrier Materials and Related Thermochemical Redox Processes for Concentrating Solar Thermal Applications

Redox materials have been investigated for various thermochemical processing applications including solar fuel production (hydrogen, syngas), ammonia synthesis, thermochemical energy storage, and air separation/oxygen pumping, while involving concentrated solar energy as the high-temperature process heat source for solid-gas reactions. Accordingly, these materials can be processed in two-step redox cycles for thermochemical fuel production from H₂O and CO₂ splitting. In such cycles, the metal oxide is first thermally reduced when heated under concentrated solar energy. Then, the reduced material is re-oxidized with either H₂O or CO₂ to produce H₂ or CO. The mixture forms syngas that can be used for the synthesis of various hydrocarbon fuels. An alternative process involves redox systems of metal oxides/nitrides for ammonia synthesis from N₂ and H₂O based on chemical looping cycles. A metal nitride reacts with steam to form ammonia and the corresponding metal oxide. The latter is then recycled in a nitridation reaction with N₂ and a reducer. In another process, redox systems can be processed in reversible endothermal/exothermal reactions for solar thermochemical energy storage at high temperature. The reduction corresponds to the heat charge while the reverse oxidation with air leads to the heat discharge for supplying process heat to a downstream process. Similar reversible redox reactions can finally be used for oxygen separation from air, which results in separate flows of O₂ and N₂ that can be both valorized, or thermochemical oxygen pumping to absorb residual oxygen. This review deals with the different redox materials involving stoichiometric or non-stoichiometric materials applied to solar fuel production (H₂, syngas, ammonia), thermochemical energy storage, and thermochemical air separation or gas purification. The most relevant chemical looping reactions and the best performing materials acting as the oxygen carriers are identified and described, as well as the chemical reactors suitable for solar energy absorption, conversion, and storage.

Catalytic Activity of LaFe0.4Ni0.6O3/CeO2 Composites in CO and CH4 Oxidation Depending on Their Preparation Conditions

LaFe_0.4Ni_0.6O₃/CeO₂ (1:1) two-phase composite materials were prepared by mechanochemical (MC) and Pechini routes. The catalytic properties of the composites in methane and CO oxidation reactions strongly depend on their preparation conditions. In low-temperature (<600 °C) catalytic CO oxidation the composites demonstrate a higher activity compared with LaFe_0.4Ni_0.6O₃ perovskite. The highest activity was observed for the composite prepared by mechanical treatment of perovskite and fluorite precursors. There is a correlation between activity and the content of weakly bound surface oxygen species. Catalytic activity in high-temperature (>750 °C) catalytic methane oxidation correlates with the reducibility of samples. The highest activity was observed for the composite prepared by the one-pot Pechini route with higher reducibility of the sample up to 600 °C.

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.

Recent Advances in Thermochemical Energy Storage via Solid–Gas Reversible Reactions at High Temperature

The exploitation of solar energy, an unlimited and renewable energy resource, is of prime interest to support the replacement of fossil fuels by renewable energy alternatives. Solar energy can be used via concentrated solar power (CSP) combined with thermochemical energy storage (TCES) for the conversion and storage of concentrated solar energy via reversible solid–gas reactions, thus enabling round the clock operation and continuous production. Research is on-going on efficient and economically attractive TCES systems at high temperatures with long-term durability and performance stability. Indeed, the cycling stability with reduced or no loss in capacity over many cycles of heat charge and discharge of the material is pursued. The main thermochemical systems currently investigated are encompassing metal oxide redox pairs (MOx/MOx−1), non-stoichiometric perovskites (ABO3/ABO3−δ), alkaline earth metal carbonates and hydroxides (MCO3/MO, M(OH)2/MO with M = Ca, Sr, Ba). The metal oxides/perovskites can operate in open loop with air as the heat transfer fluid, while carbonates and hydroxides generally require closed loop operation with storage of the fluid (H2O or CO2). Alternative sources of natural components are also attracting interest, such as abundant and low-cost ore minerals or recycling waste. For example, limestone and dolomite are being studied to provide for one of the most promising systems, CaCO3/CaO. Systems based on hydroxides are also progressing, although most of the recent works focused on Ca(OH)2/CaO. Mixed metal oxides and perovskites are also largely developed and attractive materials, thanks to the possible tuning of both their operating temperature and energy storage capacity. The shape of the material and its stabilization are critical to adapt the material for their integration in reactors, such as packed bed and fluidized bed reactors, and assure a smooth transition for commercial use and development. The recent advances in TCES systems since 2016 are reviewed, and their integration in solar processes for continuous operation is particularly emphasized.