Metal Oxide Compounds Research Articles

Fabrication of a polyoxometalate-based metal-organic compound and its derivative for enhanced photocatalytic, electrocatalytic, and electrochemical sensing properties

A novel polyoxometalate-based metal-organic compound (POMOC), namely [Ag(3-pna)2(H2PW12O40)]n (compound 1, where 3-pna = N-(pyridine-3-yl)nicotinamide), had been successfully synthesized and structurally characterized. The photocatalytic performance of compound 1 for the degradation of gentian violet (GV) and methylene blue (MB) was investigated in detail, which exhibited good cyclic stability as a photocatalyst. In addition, cyclic voltammetry technology was used to verify that compound 1 exhibited excellent electrocatalytic performance towards KBrO3 and H2O2. To improve the degradation of organic dyes and electrochemical sensing performance, a negatively charged oxygen vacancy metal oxide compound 1@C was prepared via pyrolysis using compound 1 as a precursor. Compound 1@C was investigated as an adsorbent, and was found to effectively remove MB and GV from wastewater, with adsorption rates of 824.36 mg/g and 957.02 mg/g, respectively. The performance of compound 1@C/glassy carbon electrode as an electrochemical sensor was studied using differential pulse dissolution voltammetry (DPV). This method proved effective in detecting heavy metal ions Cd2+ and Pb2+ in wastewater with the limits of detection as 4.65 and 1.69 μg/L, respectively. Compound 1@C/GCE showed good anti-interference capabilities and stability in detecting heavy metal ions in wastewater.

(Invited) Nanowire Catalysts and Adsorbents

The unique properties of nanowire materials like fast charge transport, short diffusion length, controlled surface sites and single crystal nature make them ideal for designing high performance catalysts, adsorbents and electrocatalysts by design. To realize any of these applications, one would need either large quantities of nanowire powders or nanowire arrays on large areas. In this presentation, we will highlight our group’s efforts with scalable methods for continuous manufacturing of nanowire powders and arrays for oxides1 and III-nitride materials.2 The process for producing metal oxide nanowires has been scaled up to ton-scale. The resulting nanowire powders can be used to formulate unique catalyst and adsorbent products.Zinc oxide nanowires alloyed with catalytically active metals and have been formulated into ultra-deep desulfurization catalytic adsorbents.3,4 These materials exhibited ultra-deep desulfurization with sulfur removal down to 1 ppm or below for liquid hydrocarbons (diesel, gasoline and naphthalene) and to less than 1 ppb for natural gas. The desulfurization mechanism does not involve formation of H2S species unlike that typically observed with hydro-desulfurization process involving sulfided catalysts.Titania nanowires supported with nickel and molybdenum clusters have also shown to exhibit high activity and durability as hydro-desulfurization catalysts for removal of sulfur from hydrocarbons by forming H2S.5 The synergistic activity with MoS2 and titania are expected to contribute to enhanced activity with desulfurization.Alloying of nanowires with other elements is also studied to produce compound and alloyed nanowires. Fundamental studies have shown that alloying process occurs at much lower temperatures than decomposition temperatures for precursors used. Alloying studies have clearly shown that the solubility in nanowires to be much more than that expected for bulk materials systems. This presentation will provide several examples in which nanowires were alloyed with elements at compositions more than that expected for thermodynamic solubility.In the case of CO2 sorbents, alkali metal oxide nanowires have shown high capacity for CO2 sorption as high as 75% by wt. The alkali metal oxide nanowires have been alloyed with alkali hydroxides to produce alkali metal oxide compound nanowires. These solid sorbents showed selective CO2 sorption from a variety of streams including air, exhausts containing CO2 at various concentrations. Nanowire geometry for these nanowires allow for non-sinterability, and smaller length scales for rapid diffusion of alkali ions allow for fast kinetics and regenerability. 6,7 Acknowledgements: Authors also acknowledge contributions from Drs. Juan He, Sivakumar Vasireddy, Vivekanand Kumar, Veerendra Atla and Tu Nguyen. Partial support from National Science Foundation (NSF) and US Department of Energy (DOE) EPSCOR programs and full support from NSF and DOE SBIR programs and Kentucky Cabinet for Economic Development for Advanced Energy Materials, LLC are highly appreciated.

Formation of Fe4+ based on the Fe-O bond lengths in perovskite Eu1-xSrxFeO3

The physical properties of high oxidation state of transition metal oxide compounds have attracted many research interest for its potential applications. A systematic investigation of Fe4+ in Eu1-xSrxFeO3 with x = 0, 0.1, 0.2, 0.3, and 0.4 has been carried out in polycrystalline form obtained using sol-gel method. The single phase of Eu1-xSrxFeO3 is observed up to x = 0.3. The X-Ray powder pattern for low Sr content of x = 0.1, is best refined by using EuFeO3 crystal structure with Sr occupied the Eu-site. On the other hand, for higher Sr contents, two phases of EuFeO3 and Eu1/3Sr2/3FeO3 are required to obtained the best refinement. The result of the refinements shows that the volume and the average bond lengths of Fe-O decreases as increasing the Sr content. These results are consistent to support the existence of Fe4+ upon Sr doping in Eu1-xSrxFeO3. For the Sr content of x = 0.1, an anomalous change of the volume is observed.

Investigation of compound state of SiO2-TiO2 aerogel synthesized through controlled sol-gel reaction

In this work, a compound state of silica-titania aerogel was investigated through sol-gel reaction for hybridization of silica and titania with increasing reactivity by using high surface area and charge imbalances. By minimizing the sol-gel reaction rate difference and preparing a homogeneous and stable sol, a compound state of silica-titania aerogel could be obtained, without separation onto single metal oxide aerogel. The benefits of using the sol-gel synthesis method for synthesizing silica-titania compound aerogels are over a limit of substitution solubility and get binary metal oxide compound material. Diffraction and spectroscopic analyses showed that the heterogeneous linkage resulted from interaction among silicon, titanium, and oxygen ions. The coordination environment changes by the formation of an Si-O-Ti linkage following interaction among ions without phase separation on SiO2 and TiO2 aerogels. Thus, the atomically controlled sol-gel reaction can be used to design new metal-oxide-based compound aerogels and immiscible oxide materials.

Influence of annealing temperature on microstructure and phase transformations of oxide system Bi2O3/TiO2 formed in aqueous solutions

Bismuth titanate is widely used in various fields of science and technology due to its unique physical and chemical properties. Nanostructured metal oxide compounds of the Bi–Ti–O system, consisting of columnar TiO2 nanostructures obtained by electrochemical anodization of a two-layer Ti/Al composition, and platelet Bi2O3 nanostructures formed by sequential ion-layer deposition were synthesized. Morphological changes and phase transformations in the microstructure of the Bi2O3/TiO2 oxide system, which occur during its thermal annealing at temperatures of 150, 300, 500, and 700 °C, have been studied. Annealing of the oxide system in the range of 150–300 °C degrees leads to inconsequential morphological and structural changes: the mixture of oxides is densified, in addition to anatase, a rutile phase appears in TiO2. The crystal system of the Bi2O3 phase is hexagonal. After annealing at 500 °C, not only morphological changes occurred in the studied composite, but also significant transformations in the microstructure. In the film volume, oxide phases Ti2O3 and Bi2O3 began to transform into three-component compound Bi4Ti3O12, and this process is completed at 700 degrees with the formation of single-phase Bi4Ti3O12 nanocomposite with an orthorhombic lattice with the crystal space group Fmmm.

In-situ/operando characterization of FeOx-based chemiresistive sensor of acetone vapours by X-ray absorption spectroscopy

Metal oxide thin films are archetypal active elements employed in chemiresistive gas sensors. Most existing metal oxide-based sensors operate at elevated temperatures; however, the sensing mechanism has generally been deduced considering the room-temperature properties of metal oxide compounds. We report an in-operando X-ray absorption spectroscopy study on a chemiresistive sensor of acetone vapours; namely, the chemical structure of iron-oxide nanoparticles (NPs) exposed to trace concentrations of acetone vapours under varying temperature conditions was examined. Our results show that the iron-oxide NPs, identified as maghemite/magnetite (γ-Fe2O3/Fe3O4) compound, partially reduced with temperature. Likewise, the iron oxide NPs were partially reduced upon exposure to trace concentrations of acetone vapour (at constant temperature). The latter observation suggests that acetone molecules are chemically adsorbed on iron oxide. The chemical changes are discussed in terms of the conductive response of a consubstantial chemiresistive sensor. Acetone chemisorption requires a revision of the commonly accepted sensing mechanism based on the physical adsorption of acetone molecules.

Coupling Sb2WO6 microflowers and conductive polypyrrole for efficient potassium storage by enhanced conductivity and K+ diffusivity

Although metal oxide compounds are considered as desirable anode materials for potassium-ion batteries (PIBs) due to their high theoretical capacity, the large volume variation remains a key issue in realizing metal oxide anodes with long cycle life and excellent rate property. In this study, polypyrrole-encapsulated Sb2WO6 (denoted Sb2WO6@PPy) microflowers are synthesized by a one-step hydrothermal method followed by in-situ polymerization and coating by pyrrole. Leveraging the nanosheet-stacked Sb2WO6 microflower structure, the improved electronic conductivity, and the architectural protection offered by the PPy coating, Sb2WO6@PPy exhibits boosted potassium storage properties, thereby demonstrating an outstanding rate property of 110.3 mA h g−1 at 5 A g−1 and delivering a long-period cycling stability with a reversible capacity of 197.2 mA h g−1 after 500 cycles at 1 A g−1. In addition, the conversion and alloying processes of Sb2WO6@PPy in PIBs with the generation of intermediates, K2WO4 and K3Sb, is determined by X-ray photoelectron spectroscopy, transmission electron microscopy, and ex-situ X-ray diffraction during potassiation/depotassiation. Density functional theory calculations demonstrate that the robust coupling between PPy and Sb2WO6 endues it with a much stronger total density of states and a built-in electric field, thereby increasing the electronic conductivity, and thus effectively reduces the K+ diffusion barrier.

(Invited) Solid Oxide Membrane (SOM)-Based Electrolytic Production of Metals

Metal producers, such as primary aluminum, ferroalloy, iron and steel, lead, magnesium, and zinc, employ carbothermic, metallothermic, halide reduction, and electrometallurgical methods, to produce the metals and in the process generate significant amounts of pollutants including greenhouse gases (GHG). In the US alone, these industries produce more than 100 million metric tons of CO2e. Because of its impact on the environment there is an urgent need to decarbonize this industrial sector. Electrolysis offers the opportunity to substitute clean electrical energy for fossil fuel energy sources. But electrolytic oxide reduction typically requires carbon anodes and emits CO2 and perfluorocarbons with very high global warming potential. As of this writing, no oxygen-producing inert anode technology has yet scaled commercially beyond a single pilot electrolysis cell.For metal oxide reduction a cost-effective, energy-efficient and low-emissions approach to the inert anode uses a solid oxide membrane (SOM) as a selective electrolyte between the molten salt and anode for the generic production of metals and alloys. The SOM-based electrolysis technology produces metals (Me) and/or their alloys from their respective oxides (MeOx) through direct electrolysis of the oxides: MeOx → Me (cathode) + x/2 O₂ (anode), where x is the stoichiometric amount of oxygen in the metal oxide.The SOM-based electrolytic process brings various advantages such as simplified design, lower cost, lower energy use, lower use of consumables, higher efficiency, zero emissions, and higher purity of metal and oxygen produced. The SOM process features the utilization of an oxygen-ion-conducting membrane-based anode, typically made of stabilized zirconia such as yttria-stabilized zirconia (YSZ) with an oxygen producing electrode on one side of the membrane, a supporting ionic molten salt electrolyte bath, typically an alkali fluoride salt mixture, and a cathode to collect the reduced metal. The oxygen-ion conducting membrane is placed inside the salt and it separates the cathode, also placed in the salt, from the oxygen electrode. The metal oxide is electrolyzed inside the salt medium by applying an electric potential between the cathode and the anode. The metal ions are reduced at the cathode and the oxygen ions migrate through the salt and the membrane and are oxidized at the oxygen electrode. The oxygen produced is pure due to the membrane selectivity, a valuable byproduct, and the membrane completely isolates the anode from the salt bath and metal produced at the cathode. This isolation simplifies anode materials selection, as the anode does not need to be robust to molten salt corrosion. Also, because of the oxygen-ion conducting membrane, the applied potential does not oxidize the halide ions nor the multivalent cations.Since the late 1960s, researchers have recognized and investigated the use of zirconia solid electrolyte for metals production and refining; however, limited progress was achieved due to the poor stability of zirconia membrane in the salt medium. Our work on YSZ membrane stability during SOM electrolysis using eutectic molten salt composition in CaF2 –MgF2 system demonstrated that membrane stability can be greatly enhanced by matching component activity and optical basicity in the salt with that of the membrane. It was shown that the oxide solubility in the salt can be controlled through tailored additions of more stable alkali oxides and monitoring the heat of mixing; higher heat of mixing promotes oxide solubility and vice-versa. Following our initial work, the SOM process has been applied to reduce various metal oxides or oxide compounds to the respective metals or alloys.This talk will provide generic guidelines with examples for the selection of the cell components, including salt electrolyte, cathode, anode configuration, and current collectors. Examples will include both lab and industrial scale applications.

The evaluation of aluminum and iron metal oxide settling behaviors for Hanford insoluble solids waste preprocessing

AbstractThe Hanford site is currently one of the largest and most expensive cleanup sites for hazardous, radioactive waste. Over 20% of the waste found at the Hanford site is in the form of a high activity sludge. The insoluble solids in the sludge will need to be concentrated prior to vitrification in the high‐level waste (HLW) melter. This will minimize the amount of liquid that will be evaporated during the melting process and expedite the melter processing rate. One proposed option for concentrating the insoluble solids is gravity settling in the storage tanks. Metal oxide compounds containing aluminum and iron make up the majority of the insoluble solids in the sludge, therefore understanding the behavior of these compounds in various tank waste matrices can facilitate sludge pretreatment options. A study of non‐radioactive slurry solutions containing Al(OH)3 (gibbsite), AlO(OH) (boehmite), and Fe2O3 (iron (III) oxide) was conducted to determine the time dependent interface behavior and settling rates of these compounds. Variations in solids loading and sodium concentration were evaluated to represent waste processing conditions and the results of these settling studies were compared with prior tank waste settling tests. Information gathered from these studies can be used to inform future decisions on sludge treatment processes of the insoluble solids processed at the Hanford site.

A simple solution method to prepare VO2:Co2+ precursors for thin film deposition by solution-processing method

Solution-processing is a low-cost solution method to preparea variety of organic or inorganic thin films. For metal oxide compounds, a solution-processing solution of an organometallic compound is frequently used as a precursor to be spin coated, followed by a thermal annealing to form metal oxide. In this work, vanadium oxide powders are obtained from a simple acid-base reaction, and then they are dispersed in isopropyl alcohol to form a solution for spin-coating. Different amount of cobalt salt are also added together with VOx into isopropyl alcohol to form VOx:Co2+ solutions. After thermal annealing at 200 °C, continuous transparent thin films are obtained. Optical, structural, morphological and chemical binding energies of those films are analyzed. It is found that amorphous VO2:Co2+ compound is formed in those films with V:Co atomic ratios between 6.6:1 and 1.6:1. Optical absorption onsets of those films are around 2.3 eV. An interesting interconnected porous morphology is observed when the atomic ratio of V:Co is around 4.9:1. It is concluded that porous amorphous cobalt doped vanadium oxide thin films can be obtained from a spin-coating process at low annealing temperature from a simple solution without any complex agent.

Preparation of MoFs-Derived Cobalt Oxide/Carbon Nanotubes Composites for High-Performance Asymmetric Supercapacitor

Metal-organic frameworks (MOFs)-derived metallic oxide compounds exhibit a tunable structure and intriguing activity and have received intensive investigation in recent years. Herein, this work reports metal-organic frameworks (MOFs)-derived cobalt oxide/carbon nanotubes (MWCNTx@Co3O4) composites by calcining the MWCNTx@ZIF-67 precursor in one step. The morphology and structure of the composite were investigated by scanning electron microscope (SEM) and transmission electron microscope (TEM) characterization. The compositions and valence states of the compounds were characterized by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). Benefiting from the structurally stable MOFs-derived porous cobalt oxide frameworks and the homogeneous conductive carbon nanotubes, the synthesized MWCNTx@Co3O4 composites display a maximum specific capacitance of 206.89 F·g-1 at 1.0 A·g-1. In addition, the specific capacitance of the MWCNT3@Co3O4//activated carbon (AC) asymmetric capacitor reaches 50 F·g-1, and has an excellent electrochemical performance. These results suggest that the MWCNTx@Co3O4 composites can be a potential candidate for electrochemical energy storage devices.

Diverse carbonous nanocomposites of Ce2Y2O7 for boosting hydrogen storage capacity; Synthesis, characterization and electrochemical studies

Carbon based materials due to high conductivity can provide the best conditions for energy storage through electrochemical process. Especially, nanosized carbon materials with active sites can participate in the hydrogen storage process along with metal oxide compounds. This work focuses on the hydrogen storage ability of sol-gel synthesized Ce2Y2O7 nanoribbons as active materials. Effect of carbonous materials with different structures such as GO, N-GQDs and g-C3N4 were compared on the hydrogen storage capacity of Ce2Y2O7 nanoribbons. The provided carbonous nanocomposites were characterized using different methods in terms of identification of phase purity, shape and porosity. The galvanostatic charge-discharge profiles for all samples performed for achieving ideal capacitance. The results illustrate the maximum discharge capacity of 656, 686 and 757 mAhg−1 for nanocomposites of GO/Ce2Y2O7, N-GQDs/Ce2Y2O7 and g-C3N4/Ce2Y2O7, respectively while the pristine Ce2Y2O7 nanoribbons have discharge capacity of 643 mAhg−1 after 15 cycles.

Comparison of on-chip MIS capacitors based on stacked HfO2/Al2O3 nanolaminates

High- κ dielectric materials are commonly used in microelectronic components due to the technological necessity of increasing the capacitance density of dielectric layers. The thickness of the layer is a crucial parameter of this technology because it has a significant influence on dielectric properties, capacitance density, leakage current density–voltage ( J–V ), breakdown voltage, and capacitance density–voltage ( C–V ). Among metal oxide compounds, HfO 2 and Al 2 O 3 have been widely studied due to their good thermodynamic stability in contact with silicon. Thus, in this study, devices are fabricated by atomic layer deposition (ALD) processes on Si wafer. Properties of HfO 2 /Al 2 O 3 -based stack dielectric as on-chip MIS capacitors are investigated. The capacitance density, C–V , J–V , impedance characteristics, equivalent dielectric constant, breakdown voltage, and leakage current are studied on stacks (HfO 2 /Al 2 O 3 ) with a thickness ratio of 1:1. The experimental results indicate very good leakage current and good breakdown voltage. Oxygen vacancies play a significant role in increasing the conductance and contrarily decreasing the equivalent dielectric constant of the stack.

Improvement in photo-device properties of CuO thin films for opto-electronic applications: effects of (Ni, Co) co-doping

We report co-doping effects of transition metal elements (Ni, Co) primarily on the opto-electronic properties of CuO thin films. CuO, CuO:Ni(1%), CuO:Co(1%) and CuO:Ni(1%):Co(1%) thin films were deposited via the spray pyrolysis route. Structural studies revealed the monoclinic CuO structure for all films. For all the films scanning electron microscope (SEM) images showed a crack-free and homogeneous surface. Photoluminescence (PL) spectra of all the films exhibited four emission peaks at 415, 451, 477, and 521 nm wavelengths. The optical bandgap (E g) values were around 2.12 eV, 2.18 eV, 2.05 eV and 1.84 eV for CuO, CuO:Ni(1%), CuO:Co(1%) and CuO:Ni(1%):Co(1%)thin films, respectively. CuO:Ni(1%):Co(1%) photo-device displayed a large responsivity (R) of 0.43 AW−1, external quantum efficiency (EQE) of 100% and detectivity (D *) of 9.55 × 109 Jones. Hence, co-doping of transition metal elements would be one of the effective approaches for enhancing opto-electronic properties of metal oxide compounds.

Synthesis and Formation Mechanism of Metal Oxide Compounds

In this work, the influence of the technological parameters of the detonation method on the formation of metal oxide compounds on the basis of titanium was investigated. The aim of the work was to investigate the method of the effective control of the phase composition and microstructure of titanium-based coatings during detonation spraying. The main parameters that varied in detonation spraying were the volume of filling the detonation barrel with explosive mixture and the oxygen/fuel ratio. The mechanism of formation of the gradient structure of coatings based on metal oxide compounds depending on the technological parameters of detonation spraying was considered. The structural-phase states and tribological properties of detonation coatings were investigated by the following methods: X-ray phase analysis (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDX-mapping), profilometry, and the test for wear-resistance according to the “ball-disk” scheme. It is shown that the phase composition of coatings may significantly change relative to the initial titanium powder during the detonation spraying due to the interaction of particles of powders with components of the gas atmosphere. Varying the spraying parameters may control the chemical reaction and provide conditions for the synthesis of the desired phases in the coatings (titanium oxide TiO2 and Ti2O3).

Plasma-Chemical Synthesis and Properties of Oxide Compounds of Cobalt

To date, the possibilities of using metal oxide particles in various fields have been developed [1]. At the same time, their characteristics largely depend on the method of preparation, which usually determines their structure, size, and physicochemical properties [2].Among the methods for obtaining nanosized particles, one can single out methods of green synthesis [3], electrochemical [4], electropulse [5], and plasmachemical [6] synthesis based on the reduction or oxidation of metal ions in solutions under favorable conditions, followed by aggregation of nanoparticles.The purpose of this work is to analyze the technology for the synthesis of cobalt oxide compounds using contact nonequilibrium low-temperature plasma and to determine the directions for its further improvement.The object of the study was the process of obtaining particles of cobalt oxide using contact non-equilibrium low-pressure plasma during the processing of a liquid medium.The study was carried out using laboratory equipment and software modules of the HSC Chemistry 5.11 package. To find the preliminary conditions for the formation of interphase boundaries, diagrams were used in the coordinates of the E-pH potential of the aquatic environment (Pourbaix diagrams). It has been established that, depending on the initial state of Co(OH)2 in solution, the mass fraction of oxygen in the precipitate can change. In this case, the addition of hydrogen peroxide leads to a decrease in the proportion of hydroxide compounds in sediments. X-ray phase analysis showed the presence of CoO, CoOOH, Co3O4, β-Co(OH)2, Co in dry sediments. It has been established that the particle sizes lie in the range of 10 – 110 nm. The micrographs confirm the agreement between the calculated values and the experimental data.The phase composition of the deposit is influenced by the following factors: current density and anion concentration, process temperature. Increasing the temperature increases the diffusion coefficient but requires a higher current density to produce a powder. It has been established that a decrease in the liquid layer contributes to an increase in the yield of oxide compounds. The results of the analysis allow us to conclude that when using the technologies of plasma-chemical processing of liquid media to obtain compounds of the ultra- and nanoscale range, it becomes necessary to choose the parameters that are optimal for this process. The results of the analysis allow us to conclude that this technology can be optimized by controlling the initial pH of the solution; use of the spent solution in a container with the initial solution for its acidification; refinement of the reactor block for treating the solution in a film mode or close to it; choice of rational parameters for sludge drying. Laurent S., Boutry S., Muller R. N. Metal oxide particles and their prospects for applications //Iron oxide nanoparticles for biomedical applications. – Elsevier, 2018. –P. 3-42.Wang L. et al. Rational design, synthesis, adsorption principles and applications of metal oxide adsorbents: a review //Nanoscale. – 2020. – Vol. 12. – №. 8. – P. 4790-4815.El Shafey A. M. Green synthesis of metal and metal oxide nanoparticles from plant leaf extracts and their applications: A review //Green Processing and Synthesis. – 2020. – Vol. 9. – №. 1. – P. 304-339.Lawrence M. J., Kolodziej A., Rodriguez P. Controllable synthesis of nanostructured metal oxide and oxyhydroxide materials via electrochemical methods //Current Opinion in Electrochemistry. – 2018. – Vol. 10. – P. 7-15.Gan Z. et al. A laser and electric pulse modulated nonvolatile photoelectric response in nanoscale copper dusted metal‐oxide‐semiconductor structures //Advanced Electronic Materials. – 2018. – Vol. 4. – №. 11. – P. 1800234.Shamanin I. et al. Plasmachemical synthesis and evaluation of the thermal conductivity of metal-oxide compounds for prospective nuclear fuel //Journal of Physics: Conference Series. – IOP Publishing, 2019. – Vol. 1145. – №. 1. – P. 012057.

Redox Cycles, Active Materials, and Reactors Applied to Water and Carbon Dioxide Splitting for Solar Thermochemical Fuel Production: A Review

The solar thermochemical two-step splitting of H2O and CO2 based on metal oxide compounds is a promising path for clean and efficient generation of hydrogen and renewable synthetic fuels. The two-step process is based on the endothermic solar thermal reduction of a metal oxide releasing O2 using a high-temperature concentrated solar heat source, followed by the exothermic oxidation of the reduced oxide with H2O and/or CO2 to generate pure H2 and/or CO. This pathway relates to one of the emerging and most promising processes for solar thermochemical fuel production encompassing green H2 and the recycling/valorization of anthropogenic greenhouse gas emissions. It represents an efficient route for solar energy conversion and storage into renewable and dispatchable fuels, by directly converting the whole solar spectrum using heat delivered by concentrating systems. This eliminates the need for photocatalysts or intermediate electricity production, thus bypassing the main limitations of the low-efficient photochemical and electrochemical routes currently seen as the main green methods for solar fuel production. In this context, among the relevant potential redox materials, thermochemical cycles based on volatile and non-volatile metal oxides are particularly attractive. Most redox pairs in two-step cycles proceed with a phase change (solid-to-gas or solid-to-liquid) during the reduction step, which can be avoided by using non-stoichiometric oxides (chiefly, spinel, fluorite, or perovskite-structured materials) through the creation of oxygen vacancies in the lattice. The oxygen sub-stoichiometry determines the oxygen exchange capacity, thus determining the fuel production output per mass of redox-active material. This paper provides an overview of the most advanced cycles involving ZnO/Zn, SnO2/SnO, Fe3O4/FeO, ferrites, ceria, and perovskites redox systems by focusing on their ability to perform H2O and CO2 splitting during two-step thermochemical cycles with high fuel production yields, rapid reaction rates, and performance stability. Furthermore, the possible routes for redox-active material integration and processing in various solar reactor technologies are also described.

Critical overview of polyanionic frameworks as positive electrodes for Na-ion batteries

Na-ion batteries (NIBs) are an important technological alternative to Li-ion batteries (LIBs) for developing energy storage systems that are cost-effective and less constrained by geographical supply chains, with similar energy densities. Analogous to LIBs, cathodes play a critical role in determining the energy density of NIBs, and layered transition metal oxide compounds are widely used as NIB cathodes. However, the instability-driven irreversible phase transitions in layered frameworks, especially at large degrees of Na removal, has necessitated the exploration of rigid structural frameworks that are resistant to structural changes with Na exchange. Thus, polyanionic frameworks, which primarily consist of transition metal polyhedra interconnected by PO4, SiO4, and/or SO4 units, with or without the presence of fluorine, have been investigated as NIB cathodes. This review provides a critical overview of recent studies using polyanion cathodes, which will be useful in guiding the scientific community towards the development of better NIB cathodes.Graphical abstract

Core-Shell ZIF67@ZIF8 Modified with Phytic Acid as an Effective Flame Retardant for Improving the Fire Safety of Epoxy Resins.

Despite many important industrial applications, epoxy resin (EP) suffers from high flammability and toxicity emission, extremely hampering their applications. To circumvent the problem, core–shell structured ZIF67@ZIF8 is successfully synthesized and further functionalized with phytic acid (PA) to obtain PA-ZIF67@ZIF8 hybrids. Then, it is used as an efficient flame retardant to reduce the fire risk of EP. The fire test results show a significant reduction in heat and smoke production. Compared with EP, incorporating 5.0 wt % PA-ZIF67@ZIF8 into EP, the peak heat release rate, total heat release, and peak carbon monoxide production are dramatically reduced by 42.2, 33.0, and 41.5%, respectively. Moreover, the EP/PA-ZIF67@ZIF8 composites achieve the UL-94 V-0 rating and the limiting oxygen index increases by 29.3%. These superior fire safety properties are mainly attributed to the excellent dispersion and the catalytic effect of metal oxide and phosphorus-containing compounds. This work provides an efficient strategy for preparing a promising ZIF-based flame retardant for enhancing flame retardancy and smoke toxicity suppression of EP.

Efficient removal and selective recovery of phosphorus by calcined La-doped layered double hydroxides

Advanced phosphate removal technologies are necessary to mitigate eutrophication problems. Since lanthanum and phosphate have a strong affinity for each other, lanthanum-doped composite materials are widely applied in phosphate removal. In this study, Mg/Fe/La LDOs (MFL) was synthesized successfully by calcination, which considerably enhanced the adsorption capacity of phosphate by reconstructing and compositing metal oxide compounds. MFL-1/10-350 was characterized by XRD, BET, zeta potential, SEM, TGA, XPS, and FTIR. The MFL-1/10-350 showed a typical BET surface area of 154.61 m2/g and was composed of Mg, Fe, La, and O. MFL-1/10-350 exhibited a high Langmuir maximum adsorption capacity of 123.15 mg P/g at 25 °C and strong selectivity for phosphate. The mechanisms of phosphate adsorption on the samples are mainly electrostatic interactions, surface complexation effects, and ion exchange. When 0.5 g/L of MFL-1/10-350 was added to real wastewater with a phosphate content of 3.6 mg P/L, the phosphate concentration quickly decreased to less than 0.12 mg P/L. Phosphorus-adsorbed LDOs (LDOs-P) can provide phosphorus continuously to promote rice growth. The composite material of MFL-1/10-350 was harmless to the growth of rice during the experiment. The results show that MFL-1/10-350 is an ideal material for the effective adsorption and recovery of phosphorus. This provides a possible way for phosphorus removal and recycling in water.