Metal Oxide Redox Research Articles

Extracting metal oxide redox thermodynamics from TGA measurements requires moving beyond the linearized van ‘t Hoff approach

Thermodynamic modeling of metal oxide reduction is crucial for optimizing chemical processes and materials in systems dependent on off-stoichiometric reduction/re-oxidation cycling. Two prevalent methods for extracting reduction thermodynamics from thermogravimetric data are linearized van ‘t Hoff (VH) analysis and the compound energy formalism (CEF). This work evaluates the accuracy of these methods by constructing invertible ground truth thermodynamic models, generating hypothetical thermogravimetric data, and determining the reduction thermodynamic using both VH and CEF methods. Our findings reveal that the VH method produces absolute errors 3–5 times higher than the CEF in kJ/mol O or J/mol O K for enthalpy and entropy of reduction, respectively. In contrast, the CrossFit CEF (CF-CEF) method yields errors often less than 10 kJ/mol O or J/mol O K. Moreover, the CF-CEF method provides models based on mole fraction, temperature, and extent of reduction, while a typical VH analysis provides thermodynamics of only the specific compositions measured. Although simple to implement, the VH method suffers from significant, non-systematic errors due to entropy/enthalpy compensation and defect modeling. Consequently, we recommend the more complex but robust, CF-CEF method for extracting redox thermodynamics from thermogravimetric measurements.

Porous Monolithic Perovskite Structures for High-Temperature Thermochemical Heat Storage in Concentrated Solar Power (CSP) Plants and Renewable Electrification of Industrial Processes

A novel approach towards thermal energy storage of surplus renewable energy (RE) is introduced via a hybrid thermochemical/sensible heat storage concept implemented with the aid of porous structures made of redox metal oxides, capable of reversible reduction/oxidation upon heating/cooling in direct contact with air, accompanied, respectively, by endothermic/ exothermic heat effects and demonstrating fully reversible dimensional changes under cyclic operation. The proposed modular storage units can be heated during the day to a level exceeding the metal oxide’s reduction onset temperature either by hot air streams from air-operated Concentrated Solar Power (CSP) tower plants or via surplus/cheap RE-electricity from photovoltaics, wind, or other renewable sources (“charging”/energy storage step). When this RE sources become non-available or upon demand, the fully charged system can transfer its energy to a controlled airflow that passes through the porous oxide block and initiates the exothermic oxidation of the reduced metal oxide. Thus, a hot air stream is produced which can be used to provide electricity or exploitable heat for industrial processes. The present work elaborates on the operating principles and the potential application of this concept and reports progress in the preparation and shaping of reticulated porous ceramics (RPCs also known as “ceramic foams”) from CaMnO3-based perovskite compositions and their preliminary testing with respect to cyclic reduction-oxidation.

Fabrication and performance assessment of coin cell supercapacitors with multiwalled carbon nanotube-supported mixed metal oxide nanocomposites and redox additive electrolyte

In the realm of supercapacitor applications, the fabrication of coin cell supercapacitors with superior performances played a crucial role. In this work, an asymmetrical type coin supercapacitor was fabricated with multiwalled carbon nanotube supported mixed metal oxide (CuO/CoO@MWCNT - CCM) nanocomposites (NCs) and potassium ferrocyanide incorporated KOH based redox additive electrolyte (RAE). The synergistic effect and the enhanced conductivity by the mixed metal oxides and MWCNT, respectively, were acted as the performance enhancer for electrode performances. On the other hand, RAE with optimized concentration provided additional redox active sites for superior performances. The combined effect of CCM NCs and RAE, were responsible for the superior performances. CCM NCs were prepared by one-pot hydrothermal technique and characterized. Working electrodes with CCM NCs were fabricated by doctor blade method and evaluated in KOH and RAE. It exhibited 1838.55 Fg−1 of specific capacitance (Csp) in RAE. Assembled asymmetrical supercapacitors with CCM NCs modified working electrode and activated carbon based anode, delivered 123.91 Fg−1 of Csp at 2.75 Ag−1 in RAE. The assembled supercapacitors delivered the highest energy density of 27.53 Wh kg−1 and power density of 1875 W kg−1 in RAE with an impressive 82.89 % of cyclic retention after 5000 GCD cycles. Finally, an asymmetric coin cell supercapacitor (CR2302) was fabricated with CCM NCs and RAE. The fabricated coin cell delivered the charge and discharge capacities of 157.72 and 55.99 mAh g−1, respectively in KOH, whereas these values were significantly improved 172.46 and 126.2 mAh g−1 respectively, in RAE. It proved that the fabricated coin cell supercapacitors performed well in terms of maximum charge and discharge performances in RAE compared to conventional KOH.

Heat and Mass Transfer Model for a Counter-Flow Moving Packed-Bed Oxidation Reactor/Heat Exchanger

Abstract Particle-based thermochemical energy storage (TCES) through metal oxide redox cycling is advantageous compared to traditional sensible and latent heat storage (SHS and LHS) due to its higher operating temperature and energy density, and the capability for long-duration storage. However, overall system performance also depends on the efficiency of the particle-to-working fluid heat exchangers (HXs). Moving packed-bed particle-to-supercritical CO2 (sCO2) HXs have been extensively studied in SHS systems. Integrating the oxidation reactor (OR) for discharging with a particle-to-sCO2 HX is a natural choice, for which detailed analysis is needed for OR/HX design and operation. In this work, a 2D continuum heat and mass transfer model coupling transport phenomena and reaction kinetics is developed for a shell-and-plate moving-bed OR/HX. For the baseline design, the model predicted ∼75% particle bed extent of oxidation at the channel exit, yielding a total heat transfer rate of 16.71 kW for 1.0 m2 heat transfer area per channel, while the same design with inert particles (SHS only) gives only 4.62 kW. A parametric study was also conducted to evaluate the effects of particle, air, and sCO2 flowrates, channel height and width, and average particle diameters. It is found that the respective heat transfer rate and sCO2 outlet temperature can approach ∼25 kW and >1000 °C for optimized designs for the OR/HX. The present model will be valuable for further OR/HX design, scale-up, and optimization of operating conditions.

Thermochemical energy storage performances of Co3O4-based honeycombs with multi-scale composite pores

Metal oxide redox system characterized open-loop operation, high energy density, and high reversibility, which is one of the most promising thermochemical energy storage technologies for the next-generation concentrated solar power plants. Most of the previous studies focused on the material properties, while the energy storage performance of oxide monolithic bodies, such as energy storage density and heat storage/release rate, has rarely been reported. In this paper, a novel and effective method for the improvement of the energy storage performance of oxide honeycombs with multi-scale composite pores is proposed and presented. Methyl cellulose and four common biomass materials, including bagasse, loofah, juniper leaf, and pine needle, were used as pore-forming agents for oxide honeycombs. The pyrolysis of methyl cellulose forms small-scale pores in the honeycomb bodies and contributes the major pore volume. The biomass templates further optimize the pore characteristics, all biomass-templated Co3O4-based honeycomb bodies have higher total pore volume and permeability than that of biomass-free honeycomb body. The honeycomb with 2.5 wt% pine needle achieves the highest energy storage density, with an average of 694.62 kJ/kg during the second to fifteenth cycles. In addition, the honeycomb containing 7.5 wt% pine needle has the highest energy store/release rate. Biomass-templated Co3O4-based honeycomb shows a high crushing strength, and there are no obvious cracks and structural deformations on its surface after 15 thermal cycles. The findings demonstrate that the methyl cellulose and biomass template method can effectively optimize the pore structure and energy storage properties of the oxide honeycombs, thereby expanding the application prospects of Co3O4-based honeycombs in thermochemical energy storage.

Regulating lattice oxygen mobility of FeOx redox catalyst via W-doping for enhanced chemical looping oxidative dehydrogenation of ethane

The precise regulation of lattice oxygen mobility toward matched surface reaction kinetics is the key issue for rational design of chemical looing oxygen carriers. Herein, we successfully regulated lattice oxygen reactivity of FeOx redox catalyst via a W-doping strategy to realize efficient chemical looping oxidative dehydrogenation (CL-ODH) of ethane to ethylene. Specifically, the addition of W could significantly increase the binding energy of Fe-O bond, inhibiting the over-quick lattice oxygen migration toward over-oxidation. On this basis, the mild reduction of active Fe3O4 and slowly released oxygen ions involving in dehydrogenation at a matched kinetic rate could greatly prolong the active period, leading to a record high ethane processing capacity (120 ml·gcat−1·cycle−1) and productivity of ethylene (selectivity ∼83 %) during continuous dehydrogenation-regeneration cycles. This work provides a highly active CL-ODH catalyst and new insight into metal oxide redox chemistry.

Ultrahigh‐Speed In‐Memory Electronics Enabled by Proximity‐Oxidation‐Evolved Metal Oxide Redox Transistors (Adv. Mater. 20/2022)

In-Memory Processing To achieve in-memory processing, the development of an ultrafast and reconfigurable multi-terminal device is required. In article number 2200122, Mohit Kumar, Hyungtak Seo, and co-workers develop a proof-of-concept ultrafast (≈42 ns) and programmable redox thin-film transistor, which is successfully utilized to build in-memory electronics, including logic-in-memory processing, on-demand multiterminal addressable memory, learning, pattern recognition, and classification.

Ultrahigh-Speed In-Memory Electronics Enabled by Proximity-Oxidation-Evolved Metal Oxide Redox Transistors.

The pursuit of a universal device that combines nonvolatile multilevel storage, ultrafast writing/erasing speed, nondestructive readout, and embedded processing with low power consumption demands the development of innovative architectures. Although thin-film transistors and redox-based resistive-switching devices have independently been proven to be ideal building blocks for data processing and storage, it is still difficult to achieve both well-controlled multilevel memory and high-precision ultrafast processing in a single unit, even though this is essential for the large-scale hardware implementation of in-memory computing. In this work, an ultrafast (≈42ns) and programable redox thin-film transistor (ReTFT) memory made of a proximity-oxidation-grown TiO2 layer is developed, which has on/off ratio of 105 , nonvolatile multilevel analog storage with a long retention time, strong durability, and high reliability. Utilizing the proof-of-concept ReTFTs, circuits capable of performing fundamental NOT, AND, and OR operations with reconfigurable logic-in-memory processing are developed. Further, on-demand signal memory-processing operations, like multi-terminal addressable memory, learning, pattern recognition, and classification, are explored for prospective application in neuromorphic hardware. This device, which operates on a fundamentally different mechanism, presents an alternate solution to the problems associated with the creation of high-performing in-memory processing technology.

Hydrogen production by solar fluidized bed reactor using ceria: Euler-Lagrange modelling of gas-solid flow to optimize the internally circulating fluidized bed

To perform solar thermochemical conversion, by utilizing high-temperature solar heat as an energy source and redox metal oxide particles as a chemical source, fluidized bed reactor has been developed to produce clean fuels. In this study, an Euler-Lagrange model has been developed for the simulation of particulate and gas flows in fluidized bed reactor for hydrogen production, by two-step water splitting cycles, using solar beam down concentrating system. The solid phase is modelled by Discrete Element Method (DEM) using soft-sphere approach and the gas phase is modelled as continuum by Navier-Stokes equations. The flow behavior of newly developed fine ceria particles has been analyzed for various conditions using the 30 kWth fluidized bed reactor prototype. The effect of particle size on the flow-dynamics at the spout, fountain periphery and annulus of the internally circulating fluidized bed has been examined. The results indicate that the particle size distribution should be minimized as much as possible to avoid the segregation behavior of different size particles.

Fluidization behavior of redox metal oxide and spinel particles to develop high-energy-density thermal energy storage system for concentrated solar power applications

Fluidization behavior of redox metal oxide and spinel particles to develop high-energy-density thermal energy storage system for concentrated solar power applications

Redox cycle of calcium manganite for high temperature solar thermochemical storage systems

Thermochemical energy storage via metal oxide redox cycling is a potential cost-effective approach to store solar energy both chemically and thermally at high temperatures, enabling efficient solar thermal power production round-the-clock and on-demand. Perovskite manganites are a low-cost redox material that undergoes reversible reaction without side reactions up to 1000 °C and offers high storage capacities, which makes them promising energy storage materials in next-generation solar thermal power plants. In this study, the thermal properties of a typical perovskite manganite, CaMnO3, has been studied. CaMnO3 shows 893.2 kJ/kg energy storage capacity from 200 °C to 1000 °C, and 0.133 non-stoichiometry in N2. The material exhibits excellent stability with 150 redox cycles between 500 °C and 1000 °C. The thermodynamic efficiencies of two solar-driven combined cycle power systems with CaMnO3 based thermochemical energy storage system are also investigated. The steady-state mass and energy conservation equations are solved for all system components to calculate the thermodynamic properties and mass flow rates at all state points in the system. Preliminary results for a system with 120 MW nominal solar power input at a solar concentration ratio of 1000 are presented, designed for constant round-the-clock operation with 9 h of on-sun and 15 h of off-sun operation. The CaMnO3 particles store and release energy via the sensible heat and reaction heat, yield a net power block efficiency of approximately 60% and an overall energy conversion efficiency of up to 48.7%. Required storage tank sizes for the solids are estimated to be up to 2–3 smaller than those of state-of-the-art molten salt systems.

Experimental Study of Thermochemical Two-step Water Splitting Cycle for Hydrogen by Fe-CeO2 Redox Metal Oxide Coated foam Device with 3kWth Sun-simulator

Experimental Study of Thermochemical Two-step Water Splitting Cycle for Hydrogen by Fe-CeO2 Redox Metal Oxide Coated foam Device with 3kWth Sun-simulator

Performance Indicators for Benchmarking Solar Thermochemical Fuel Processes and Reactors

Concentrated solar energy offers a source for renewable high-temperature process heat that can be used to efficiently drive endothermic chemical processes, converting the entire spectrum of solar radiation into chemical energy. In particular, solar-driven thermochemical processes for the production of fuels include reforming of methane and other hydrocarbons, gasification of biomass, coal, and other carbonaceous feedstock, and metal oxide redox cycles for splitting H2O and CO2. A notable issue in the development of these processes and their associated solar reactors is the lack of consistent reporting methods for experimental demonstrations and modelling studies, which complicates the benchmarking of the corresponding technologies. In this work we formulate dimensionless performance indicators based on mass and energy balances of such reacting systems, namely: energy efficiency, conversion extent, selectivity, and yield. Examples are outlined for the generic processes mention above. We then provide guidelines for reporting on such processes and reactors and suggest performance benchmarking on four key criteria: energy efficiency, conversion extent, product selectivity, and performance stability.

Identification of step-by-step oxidation process and its driving mechanism in the peroxymonosulfate catalytically activated with redox metal oxides

This study proposed a step-by-step oxidation process based on the in-depth analysis of the catalytic mechanism of peroxymonosulfate (PMS) activated by the common MnO2-based catalyst-manganese octahedral molecular sieve (OMS-2). In the first stage, OMS-2-mediated electron transfer dominated the oxidation process. When PMS was completely consumed, the reaction entered the second stage, at which singlet oxygen (1O2) gradually turned into primary oxidation source. OMS-2-mediated electron transfer was proved based on the results of electrochemical analysis, phosphate substitution experiments, and Raman tests. Meanwhile, the oxidation process of 1O2 was unveiled by radical scavenging tests, electron paramagnetic resonance (EPR), and solvent-exchange experiment (from H2O to D2O). Moreover, superoxide radical (O2•−) generated from the reaction between PMS and metastable manganese intermediate was identified as the precursor of 1O2. O2•− generated in the first stage could significantly extend its half-life by adsorbing on the (211) plane of OMS-2, and then desorbed in the second stage and contributed to the formation of 1O2. More importantly, through the transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and DFT calculations, the essence of step-by-step oxidation driven by O2•− adsorption–desorption process was uncovered.

Operational Limits of Redox Metal Oxides Performing Thermochemical Water Splitting

Solar thermochemical hydrogen production is an attractive technology that stores intermittent solar energy in the form of chemical bonds. Efficient operation requires the identification of a redox‐active metal oxide (MOx) material that can achieve high conversion of water to hydrogen at minimal energy input. Water splitting occurs by consecutive reduction and reoxidation reactions of MOx. MOx is reduced to MOx−δ and, in the second step, is reoxidized by water recovering the initial MOx and generate H2. The material must reduce at temperatures achievable in concentrated solar receiver/reactors, while maintaining a thermodynamic driving force to split water. At equilibrium, extent of reduction depends on temperature and oxygen partial pressure, and in this analysis, a set of thermodynamic properties, namely, enthalpy and entropy of oxygen vacancy formation, is sufficient to represent MOx. Herein, a method to easily classify materials based on these thermodynamic properties under any condition of oxygen partial pressure and temperature is presented. This method is based on fundamental thermodynamic principles and is applicable for any redox material with known thermodynamic properties. Despite the simplicity of the method, it is believed that this analysis will support future research in targeting thermodynamic properties of redox‐active metal oxides.

Reversible Phase Transformations in Novel Ce‐Substituted Perovskite Oxide Composites for Solar Thermochemical Redox Splitting of CO2

AbstractThermochemical splitting of CO2 and H2O via two‐step metal oxide redox cycles offers a promising approach to produce solar fuels. Perovskite‐type oxides with the general formula ABO3 have recently gained attention as an attractive redox material alternative to the state‐of‐the‐art ceria, due to their high structural and thermodynamic tunability. A novel Ce‐substituted lanthanum strontium manganite perovskite‐oxide composite, La3+0.48Sr2+0.52(Ce4+0.06Mn3+0.79)O2.55 (LSC25M75) is introduced, aiming to bridge the gap between ceria and perovskite oxide‐based materials by overcoming their individual thermodynamic constraints. Thermochemical CO2 splitting redox cyclability of LSC25M75 evaluated with a thermogravimetric analyzer and an infrared furnace reactor over 100 consecutive redox cycles demonstrates a twofold higher conversion extent to CO than one of the best Mn‐based perovskite oxides, La0.60Sr0.40MnO3. Based on complementary in situ high temperature neutron, synchrotron X‐ray, and electron diffraction experiments, unprecedented structural and mechanistic insight is obtained into thermochemical perovskite oxide materials. A novel CO2 splitting reaction mechanism is presented, involving reversible temperature induced phase transitions from the n = 1 Ruddlesden–Popper phase (Sr1.10La0.64Ce0.26)MnO3.88 (I4/mmm, K2NiF4‐type) at reduction temperature (1350 °C) to the n = 2 Ruddlesden–Popper phase (Sr2.60La0.22Ce0.18)Mn2O6.6 (I4/mmm, Sr3Ti2O7‐type) at re‐oxidation temperature (1000 °C) after the CO2 splitting step.

Modeling of Ceria Reduction in a Solar Thermochemical Reactor via DEM Method

Solar thermochemical reactor provides an attractive approach that utilizes the most common solar radiation as the thermal driving force to motivate the reaction between CO2 and metal oxides, which is also called metal oxide redox pair-based thermochemical cycles. The CeO2/CeO2-δ is widely used in the two-step redox process due to its advantages including fast-redox kinetics, high crystallographic stability of a wide range of reacting oxygen non-stoichiometry, and relatively high oxygen solid-state conductivity. In this work, a three-dimensional transient numerical analysis has been completed to study the performance of a CeO2 reduction reaction in a 1/8th segment region of a novel partition cavity-receiver reactor. The porous CeO2 catalyst was analyzed using the discrete element method (DEM) to capture the heat transfer and reactive performances. The catalyst textural properties (particle size and void fraction) and reaction conditions (gas flow rate and radiative power input) were investigated in the CeO2 reduction reaction. The results indicated that increasing the catalyst specific surface area and the temperature are beneficial to the O2 production and further CO2 conversion.

Thermodynamic comparison of solar methane reforming via catalytic and redox cycle routes

Thermochemical methane reforming to syngas is performed on a massive scale in the chemical industry, providing feedstock for many chemical processes such as hydrogen, ammonia and methanol production. The high temperature process heat required for the endothermic reforming reaction could be supplied by conentrated solar energy, in a hybrid solar-fossil process. This can be achieved by re-designing conventional reforming technologies to utilize solar energy as the heat source. Another possible approach is to use a two-step metal oxide redox cycle. Here we compare the two solar thermochemical reforming routes, namely redox reforming and catalytic reforming using thermodynamic analysis and discuss the prospects for both technologies with a focus on methane conversion extents, syngas composition, and energy conversion efficiencies. Further processing of the syngas to liquid fuels is also discussed, in order to highlight how these processes can fit together with gas-to-liquids technologies. The analysis highlights that the redox cycle approach could produce a higher quality syngas, but at the expense of additional thermodynamic constraints, which are sensitive to carbon formation, and also lead to a greater energy demand relative to catalytic reforming.

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.

Insight into the positive effect of Cu0/Cu+ ratio on the stability of Cu-ZnO-CeO2 catalyst for syngas hydrogenation

Cu-ZnO based catalysts are of industrial importance for methanol synthesis. In this work, Cu-ZnO-CeO2 (CZCe) catalyst has been prepared via conventional co-precipitation technique and applied for syngas hydrogenation to methanol with the purpose to study the effect of reversible redox metal oxide (ceria) on the distribution of valence state of active Cu species. Herein, CZCe catalyst exhibited the outstanding catalytic performance with initial methanol yield as high as 0.665 g·g–1 cat.·h–1 and the STYMeOH could well be maintained with a high value (0.600 g·g–1 cat.·h–1) at 230 °C after 70 h of evaluation, which exceeded the reference Cu-ZnO-Al2O3 (CZAl) catalyst prepared by the same method. The excellent performance could be attributed to ceria doping tended to dynamically tune the distributions and/or concentrations of Cu+ and Cu° species on the catalyst surfaces and retard the conversion of Cuδ+ (δ < 2) to Cu2+ in the process of reaction.