O4 Spinel Oxide Research Articles

Coordination Tuning of FeNi-HMT Frameworks Derived Effective Hybrid Catalysts for Water Oxidation.

FeNi-based hybrid materials are promising oxygen evolution reaction (OER) catalysts for water electrolysis in hydrogen generation. In this work, the coordination tuning of FeNi-HMT frameworks was achieved by simply changing the Fe/Ni ratios using hexamethylenetetramine (HMT) as an organic ligand, and the derived hybrid FeNi catalysts with varied compositions were probed for OER. Incorporating varying amounts of Fe3+ by adjusting the Ni/Fe ratio results in different metal-organic framework (MOF) structures, and higher Fe feed leads to the formation of amorphous structures due to the coordination structure destruction from the weaker coordination capacity of Fe3+ compared to Ni2+ combining with the tertiary amine ligand. Among them, the FeNi-HMT (with the Fe/Ni molar ratio of 1/1) derived catalyst, consisting of Fe0.36Ni0.64 alloy/Ni0.4Fe2.6O4 spinel oxide heterostructures supported by graphitized carbon matrix, exhibits the highest OER performance. The unique structure facilitates significant electron transfer at the alloy/spinel interface due to the large work function difference between each phase. This strong electronic effect downshifts the d-band center of the catalyst and optimizes the binding energies to the crucial oxygenated intermediates, thereby promoting the OER kinetics. This work highlights the importance of the coordination tuning of FeNi-HMT frameworks for highly efficient catalyst development.

Comparative study on the magnetic properties of Fe-substituted Co2Sn1−x Fe x O4 spinel oxides and its exchange bias effect

Herein, we report the Fe-substituted Co2Sn1−x Fe x O4 (0 ⩽ x ⩽ 0.4) inverse spinel’s oxide using the solid-state reaction method. X-ray reveals the single-phase cubic structure with space group Fd3m. With increasing Fe in Co2Sn1−x Fe x O4 spinel oxide, the transition temperature rise. The ac susceptibility at different frequencies also confirms a spin-glassy state at lower temperatures. The strong exchange bias effect appears in the sample having Fe substitution (x = 0.2) under the presence of constant temperature ∼10 K. The high-temperature susceptibility of Curie-Wise fitting shows that the system changes from antiferromagnetic exchange (x < 0.2) to ferromagnetic exchange (x > 0.2).

High-Voltage Spinel Cathode Materials: Navigating the Structural Evolution for Lithium-Ion Batteries.

High-voltage LiNi0.5Mn1.5O4 (LNMO) spinel oxides are highly promising cobalt-free cathode materials to cater to the surging demand for lithium-ion batteries (LIBs). However, commercial application of LNMOs is still challenging despite decades of research. To address the challenge, the understanding of their crystallography and structural evolutions during synthesis and electrochemical operation is critical. This review aims to illustrate and to update the fundamentals of crystallography, phase transition mechanisms, and electrochemical behaviors of LNMOs. First, the research history of LNMO and its development into a LIB cathode material is outlined. Then the structural basics of LNMOs including the classic and updated views of the crystal polymorphism, interconversion between the polymorphs, and structure-composition relationship is reviewed. Afterward, the phase transition mechanisms of LNMOs that connect structural and electrochemical properties are comprehensively discussed from fundamental thermodynamics to operando dynamics at intra- and inter-particle levels. In addition, phase evolutions during overlithiation as well as thermal-/electrochemical-driven phase transformations of LNMOs are also discussed. Finally, recommendations are offered for the further development of LNMOs as well as other complex materials to unlock their full potential for future sustainable and powerful batteries.

Enhanced Electrochemical Performance of MnCo1.5Fe0.5O4Spinel for Oxygen Evolution Reaction through Heat Treatment.

MnCo1.5Fe0.5O4 spinel oxide was synthesized using the sol-gel technique, followed by heat treatment at various temperatures (400, 600, 800, and 1000 °C). The prepared materials were examined as anode electrocatalysts for water-splitting systems in alkaline environments. Solid-state characterization methods, such as powder X-ray diffraction and X-ray absorption spectroscopy (XAS), were used to analyze the materials' crystallographic structure and surface characteristics. The intrinsic activity of the MnCo1.5Fe0.5O4 was fine-tuned by altering the electronic structure by controlling the calcination temperature, and the highest activity was observed for the sample treated at 800 °C. A shift in the valence state of surface cations under oxidative conditions in an alkaline solution during the oxygen evolution reaction was detected through ex situ XAS measurements. Moreover, the influence of the experimental conditions on the electrocatalytic performance of the material, including the pH of the electrolyte and the temperature, was demonstrated.

Fluorine Induced In Situ Formation of High Valent Nickel Species for Ultra Low Potential Electrooxidation of 5-Hydroxymethylfurfural.

The Nickel-based catalysts have a good catalytic effect on the 5-hydroxymethylfurfural electrooxidation reaction (HMFOR), but limited by the conversion potential of Ni2+ /Ni3+ , 1.35V versus RHE, the HMF electrooxidation potential of nickel-based catalysts is generally greater than 1.35V versus RHE. Considering fluorine has the highest Pauling electronegativity and similar atomic radius of oxygen, the introduction of fluorine into the lattice of metal oxides might promote the adsorption of intermediate species, thus improving the catalytic performance. F is successfully doped into the lattice structure of NiCo2 O4 spinel oxide by the strategy of hydrothermal reaction and low-temperature fluorination. As is confirmed by in situ electrochemical impedance spectroscopy and Raman spectroscopy, the introduction of F weakens the interaction force of metal-oxygen covalent bonds of the asymmetric MT -O-MO backbone and improves the valence of Ni in tetrahedra structure, which makes it easier to be oxidized to higher valence active Ni3+ under the action of electric field and promotes the adsorption of OH- , while the decrease of Co valence enhances the adsorption of HMF with the catalyst. Combining the above reasons, F-NiCo2 O4 shows superb electrocatalytic performance with a potential of only 1.297V versus RHE at a current density of 20mA cm-2 , which is lower than the most catalyst.

A new CoFe1.9Li0.1O4 spinel oxide cathode for proton-conducting solid oxide fuel cells

A new spinel oxide CoFe1.9Li0.1O4 (CFLO) has been synthesized as a cathode material for proton-conducting solid oxide fuel cells. CFLO has good structure stability up to 900 °C. In addition, the material can survive in both CO2 and steam-containing atmospheres. Compared with the Li-free sample, the Li-doping strategy significantly regulates the material's electronic structure. The low valence of Li leads to the increased amount of Fe and Co cations with higher valences. Furthermore, doping Li creates more oxygen vacancies at the material surface, which benefits the oxygen reduction reaction (ORR). The improved cathode performance for CFLO has been demonstrated that a cell using CFLO cathode reaches a peak power density of 1052 mW cm−2 at 700 °C. The cell performance is 60% larger than the Li-free cell, and similar results can be detected at other testing temperatures. In addition, the CFLO cell shows good long-term stability in the working condition. This study proposes the Li-doping strategy to modify spinel oxides and also provides a new spinel oxide as a high-performing cathode for H–SOFCs.

One-step synthesis of nitrogen-doped few-layer graphene structures decorated with Mn1.5Co1.5O4 nanoparticles for highly efficient electrocatalysis of oxygen reduction reaction

A nanocomposite consisting of nitrogen-doped few-layer graphene structures, the surface of which is decorated with nanocrystallites of Mn1.5Co1.5O4 spinel oxide, was prepared by a single-stage method of plasma-assisted electrochemical exfoliation of graphite in a solution of 1 M NaNO3 + 0.005 M MnSO4 + 0.005 M CoSO4 + 0.01 M melamine. The high catalytic activity of the synthesized catalyst in the oxygen reduction reaction is due to pyridine nitrogen atoms and Mn1.5Co1.5O4 spinel nanoparticles.

Fracture toughness of reactive bonded Co–Mn and Cu–Mn contact layers after long-term aging

Creating a tough bond for the electrical contact between metallic interconnects and ceramic solid oxide cells (SOC) in a stack is challenging due to restrictions on the assembly temperature. The reactive oxidation bonding in the formation of Co2MnO4 (CoMn) and Cu1.3Mn1.7O4 (CuMn) spinel oxides from metallic precursors could provide a potential solution for achieving tough and well-conducting contact layers. These contact layers are deposited from metallic precursors onto CoCe-coated AISI441 substrates to achieve high toughness even after aging for 3000 h at typical operating temperatures for SOCs. The interface fracture energy of CoMn and CuMn contact layers was measured for as-sintered and aged samples by using a modified four-point bending test. After the fracture test, X-ray diffraction, electron microscopy, and energy-dispersive X-ray spectroscopy were used to determine phase evolution and possible reactions at the contact layer/interconnect interface. The results show that the interface fracture energy of sintered CoMn contact layer (6.1 J/m2) decreased to 2.9 J/m2 after aging at 850 ○C for 3000 h while the fracture energy for CuMn increased from 6.4 J/m2 to 19.7 J/m2.

Synthesis and critical behaviour analysis of multicomponent Ni0.43Zn0.28Fe0.57Cu0.31Mn1.41O4 spinel oxide

This paper presents synthesis and analysis of critical behavior of (Ni,Zn,Fe,Cu,Mn)3O4 spinel-oxide ceramics near magnetic phase transition. In order to investigate the critical point, a modification of the standard analysis of Arrott-Noakes diagrams was proposed. This new approach takes into account the quality of straightness of isotherms and the fact that the critical straight line (at T=Tc) must pass through the origin of the coordinate system. The determined values of critical parameters β= 0.446, γ= 1.179, and δ= 3.637 are close to those of the mean-field model (β= 0.5, γ= 1, and δ= 3) signifying mainly the isotropic, long-range exchange interactions in the studied ceramic oxide. The self-consistency of the obtained results was confirmed taking into account the scaling theory, and the asymptotic relations for the behaviour of magnetization M in the limits as T→Tc and H→0.

Synthesis of medium entropy Mn1.56Co0.96Ni0.48O4 films by solid-state reaction

Five metal layers (Mn/Co/Mn/Ni/Mn) were deposited on Si substrate by electron beam evaporation and then annealed in air to form a Mn1.56Co0.96Ni0.48O4 spinel oxide film. The increase in annealing temperature led to a gradual growth in the grain size. While sample annealed at 650 ​°C exhibits a poor surface morphology and lowest electrical resistivity, the sample at 750 ​°C is with the largest values of thermal constant B (4196 ​K) and activation energy Ea (0.361 ​eV). In sum, the proposed synthesis route of medium entropy Mn-based thermosensitive films was feasible and may be meaningful for preparation of other medium/high entropy materials.

Conversion of Co-Mn-Al hydrotalcites in highly active spinel-type catalysts for peroxide decomposition

The fast decomposition of peroxides (M2O2, where M = H+, Li+) at room temperature has gained increased attention given its relevance, from propulsive systems to metal-O2 conversion batteries. This study describes, by a rational design of experiment, the influence of selected parameters such as: pH, initial concentration of K2CO3, precipitate aging, precipitate washing, and thermal treatment for the hydrotalcite co-precipitation, characterizing it for each synthesis variable. The results show that hydrotalcites can efficiently be converted in Co-Mn-Al spinels and as a result, a new highly active Co2Mn0.5Al0.5O4 spinel oxide was synthesized after thermal treatment. Its activity and mechanical resistance were tested in a micro-thruster reactor where it showed total decomposition (> 98%) of H2O2 and in a Li-O2 battery, with increased cyclability, been able to reduce the charging potential in ~200 mV. These encouraging results enhance the importance not only of this class of material, but also, the importance of robust synthesis methodologies for reproducible results.

Electrochemical Oxidation of Nitrogen towards Direct Nitrate Production on Spinel Oxides.

Nitrates are widely used as fertilizer and oxidizing agents. Commercial nitrate production from nitrogen involves high-temperature-high-pressure multi-step processes. Therefore, an alternative nitrate production method under ambient environment is of importance. Herein, an electrochemical nitrogen oxidation reaction (NOR) approach is developed to produce nitrate catalyzed by ZnFex Co2-x O4 spinel oxides. Theoretical and experimental results show Fe aids the formation of the first N-O bond on the *N site, while high oxidation state Co assists in stabilizing the absorbed OH- for the generation of the second and third N-O bonds. Owing to the concerted catalysis, the ZnFe0.4 Co1.6 O4 oxide demonstrates the highest nitrate production rate of 130±12 μmol h-1 gMO -1 at an applied potential of 1.6 V versus the reversible hydrogen electrode (RHE).

Effect of electrode loading on the electrochemical performance of LiAl0.1Mn1.9O4 cathode for lithium-ion batteries

LiAl0.1Mn1.9O4 spinel oxide was synthesized by combustion method and investigated as cathode material for Li-ion batteries, taking into account the effect of (i) the type of current collector and (ii) the electrode mass loading. Two different substrates were employed as current collectors: Al foil and carbon paper. The materials with higher loading showed an outstanding capacity retention in rate capability test. Although the samples with lower loading started with higher capacities, they showed poor capacity retention. From a practical point of view, the electrode with 4 mg loading per cm2 presented attractive properties with respect to elevated specific capacity and high retention in long-term stability tests.

Mastering Surface Reconstruction of Metastable Spinel Oxides for Better Water Oxidation.

Developing highly active electrocatalysts for oxygen evolution reaction (OER) is critical for the effectiveness of water splitting. Low-cost spinel oxides have attracted increasing interest as alternatives to noble metal-based OER catalysts. A rational design of spinel catalysts can be guided by studying the structural/elemental properties that determine the reaction mechanism and activity. Here, using density functional theory (DFT) calculations, it is found that the relative position of O p-band and MOh (Co and Ni in octahedron) d-band center in ZnCo2- x Nix O4 (x = 0-2) correlates with its stability as well as the possibility for lattice oxygen to participate in OER. Therefore, it is testified by synthesizing ZnCo2- x Nix O4 spinel oxides, investigating their OER performance and surface evolution. Stable ZnCo2- x Nix O4 (x = 0-0.4) follows adsorbate evolving mechanism under OER conditions. Lattice oxygen participates in the OER of metastable ZnCo2- x Nix O4 (x = 0.6, 0.8) which gives rise to continuously formed oxyhydroxide as surface-active species and consequently enhances activity. ZnCo1.2 Ni0.8 O4 exhibits performance superior to the benchmarked IrO2 . This work illuminates the design of highly active metastable spinel electrocatalysts through the prediction of the reaction mechanism and OER activity by determining the relative positions of the O p-band and the MOh d-band center.

Effectively enhanced structural stability and electrochemical properties of LiNi0.5Mn1.5O4 cathode materials via poly-(3,4-ethylenedioxythiophene)-in situ coated for high voltage Li-ion batteries.

Spinel LiNi0.5Mn1.5O4 shows promise as a potential candidate for Li-ion batteries due to its high energy density and high rate performance. However, LiNi0.5Mn1.5O4 (LNMO) spinel oxides usually deliver poor cycle life because of the increasing impedance and gradually dissolving Mn resulting in the destruction of crystal structure. Here, a conductive polymer poly-(3,4-ethylenedioxythiophene) (PEDOT) surface modified strategy is introduced to settle the above challenges. The main purpose is to construct a uniform and dense shell film on the surface of LiNi0.5Mn1.5O4 (Industrial Grade), which is prepared by a simple chemical in situ oxidative polymerization method. The Mn dissolving from the lattice during the long-term cycling is well inhibited as the polymer shell protects LiNi0.5Mn1.5O4 from direct exposure to the highly active electrolyte. As expected, the 3 wt% poly-(3,4-ethylenedioxythiophene) coated sample reveals long cycle life with acceptable capacity of 114.5 mA h g−1 and high capacity retention of 91.6% after 200 cycles, compared to 70.9 mA h g−1 and 56.5%, respectively, for the bare LiNi0.5Mn1.5O4 sample. Furthermore, the coated sample demonstrates a higher capacity of 110 mA h g−1 and 63 mA h g−1 at 5C and 10C rate respectively. The improved performance is believed to be attributed to the formation of high conductivity and stable interface structure between electrolyte and LNMO, which is beneficial to suppress the destruction of crystalline structure due to the Mn dissolution and undesired side-reaction between electrolyte and LiNi0.5Mn1.5O4 in long cycle, and improve simultaneously the conductivity and interface stability of LiNi0.5Mn1.5O4 for high voltage lithium-ion batteries.

Surface microstructural evolution of AISI 304 L stainless steel oxy-nitrocarburized in a cyanide-free salt bath and its potential application in solar collectors

The surface microstructural evolution of AISI 304 L stainless steel oxy-nitrocarburized using a cyanide-free salt bath based on the eutectic composition of the system NaCNO-Na2CO3 was studied. The treatments were carried out at 600 °C from 5 to 60 min. Characterization of the materials was carried out using XRD and SEM/EDS, as well as UV/Vis, FT-IR and XPS spectroscopy techniques. The chemistry of the salt bath played a key role in the surface microstructural evolution observed in the samples. Simultaneous oxidation and nitrocarburizing occurring during the thermochemical treatments led to the formation of an outermost oxidized compound layer under which an extensive nitrided diffusion zone was developed. The compound layer was in turn composed by an outermost sublayer constituted by intermixed crystals of Fe3O4, Fe3C and ε-Fe3N phases, as well as by an innermost sublayer constituted either by FeCr2O4 spinel oxide after 5 min of treatment, or by NiFe0.5Cr1.5O4 spinel oxide for longer treatment times. The latter sublayer was rich in Ni and Cr for all treatment times longer than 5 min, which was attributed to the low diffusivity of both metals through the spinel oxide. Only the diffusion zone obeyed a parabolic growth rate, due to the two-layered structure of the compound region. It was found that the material treated for 5 min could be useful for potential applications in solar collectors, since it showed a solar absorptance (α) of 0.64, a thermal emittance (ε) of 0.07, and a selectivity ratio (α/ε) of 9.14, which were close to those of an ideal spectrally selective solar absorber. Thus, the studied process could be an alternative way to modify the surface optical properties of AISI 304 L stainless steel for this kind of applications, which had not been proposed before.

Superexchange Effects on Oxygen Reduction Activity of Edge-Sharing [Cox Mn1-x O6 ] Octahedra in Spinel Oxide.

Mn-Co containing spinel oxides are promising, low-cost electrocatalysts for the oxygen reduction reaction (ORR). Most studies are devoted to the design of porous Mn-Co spinels or to strongly coupled hybrids (e.g., MnCo2 O4 /N-doped-rmGO) to maximize the mass efficiency. The lack of analyses by metal oxide intrinsic activity (activity normalized to catalysts' surface area) hinders the development of fundamental understanding of the physicochemical principles behind the catalytic activities. A systematic study on the composition dependence of ORR in ZnCox Mn2-x O4 (x = 0.0-2.0) spinel is presented here with special attention to the role of edge sharing [Cox Mn1-x O6 ] octahedra in the spinel structure. The ORR specific activity of ZnCox Mn2-x O4 spans across a potential window of 200 mV, indicating an activity difference of ≈3 orders of magnitude. The curve of composition-dependent ORR specific activity as a function of Co substitution exhibits a volcano shape with an optimum Mn/Co ratio of 0.43. It is revealed that the modulated eg occupancy of active Mn cations (0.3-0.9), as a consequence of the superexchange effect between edge sharing [CoO6 ] and [MnO6 ], reflects the ORR activity of edge sharing [Cox Mn1-x O6 ] octahedra in the ZnCox Mn2-x O4 spinel oxide. These findings offer crucial insights in designing spinel oxide catalysts with fine-tuned eg occupancy for efficient catalysis.

Structure and Electrical Properties of Mn-Cu-O Spinels

The study presents the results of structural and electrical conductivity investigations of a Cu1.3Mn1.7O4 spinel obtained using EDTA gel processes. An amorphous gel was synthesized and calcinated for 5 h in air at temperatures of 673, 773, 873, and 973 K. When calcinating the gel at temperatures below 973 K, the obtained powders consisted of two phases—the regular Cu1.5Mn1.5O4 spinel and manganese(III) oxide. At 973 K, Mn2O3 was no longer observed, but a new Mn3O4 phase appeared in addition to the Cu1.5Mn1.5O4 spinel. Green bodies prepared from these powders were sintered for 2 h in air at 1393 K. The obtained sinters had a porosity of around 12% and were composed predominantly of the spinel phase, with minor amounts of Mn3O4 and, in the case of three of four sinters—CuO. Electrical conductivity measurements were taken over the temperature range of 300-1073 K. A change in the character of conductivity of the studied sinters was observed in the range of 400-430 K, and it was associated with an increase in activation energy from 0.20 to 0.56 eV. The electrical conductivity of the studied sinters ranged from 74.8 to 88.4 S cm−1, which makes the Cu1.3Mn1.7O4 material suitable for application as a protective-conducting coating in IT-SOFC ferritic stainless steel interconnects.

Tuning of composite cubic spinel structure in Co1.75Fe1.25O4 spinel oxide by thermal treatment and its effects on modifying the ferrimagnetic properties

We have synthesized the Co1.75Fe1.25O4 spinel ferrite by chemical reaction of Co and Fe nitrates at alkaline medium. We have annealed the as-prepared material up to 1100 °C to study the changes in lattice structure and magnetic properties. We have used synchrotron X-ray diffraction and Raman spectroscopy to identify the lattice structure. The material formed a composite spinel structure of Co rich phase (20–30%) and Fe rich phase (80–70%) for annealing temperature range 200–800 °C, and single phased spinel structure at 900 °C. At annealing temperature 1100 °C, the sample showed minor amount of hematite phase, in addition to two spinel phases. All the samples are ferrimagnetic in nature, but ferrimagnetic properties in mixed-phased samples are distinctly different from single phased sample. The analysis of the temperature and magnetic field dependence of magnetization data revealed a wide distribution of exchange interactions, anisotropy barriers, blocking of magnetic domains, and exchange bias for the samples having multi-phase components. On the other hand, single phased sample has shown the highest magnetization and the smallest coercivity with lowest distribution of magnetic exchange energy and anisotropy.

High performance cobalt-free Cu1.4Mn1.6O4 spinel oxide as an intermediate temperature solid oxide fuel cell cathode

In this work Cu1.4Mn1.6O4 (CMO) spinel oxide is prepared and evaluated as a novel cobalt-free cathode for intermediate temperature solid oxide fuel cells (IT-SOFCs). Single phase CMO powder with cubic structure is identified using XRD. XPS results confirm that mixed Cu+/Cu2+ and Mn3+/Mn4+ couples exist in the CMO sample, and a maximum conductivity of 78 S cm−1 is achieved at 800 °C. Meanwhile, CMO oxide shows good thermal and chemical compatibility with a 10 mol% Sc2O3 stabilized ZrO2 (ScSZ) electrolyte material. Impedance spectroscopy measurements reveals that CMO exhibits a low polarization resistance of 0.143 Ω cm2 at 800 °C. Furthermore, a Ni-ScSZ/ScSZ/CMO single cell demonstrates a maximum power density of 1076 mW cm−2 at 800 °C under H2 (3% H2O) as the fuel and ambient air as the oxidant. These results indicate that Cu1.4Mn1.6O4 is a superior and promising cathode material for IT-SOFCs.