Spinel Transition Metal Oxides Research Articles

Nanostructured conversion-type anode materials of metal-organic framework-derived spinel XMn2O4 (X = Zn, Co, Cu, Ni) to boost lithium storage

Bimetallic spinel transition metal oxides play a major part in actualizing eco-friendly electrochemical energy storage systems (ESSs). However, structural precariousness and low electrochemical capacitance restrict their actual implementation in lithium-ion batteries (LIBs). To address these demerits, the sacrificial template approach has been considered as a prospective way to strengthen electrochemical stability and rate performance. Herein, metal-organic frameworks (MOFs) derived XMn2O4-BDC (H2BDC = 1,4-dicarboxybenzene, X = Zn, Co, Cu, Ni) are prepared by a hydrothermal approach in order to discover the effects of various metal cations on the electrochemical performance. Among them, ZnMn2O4-BDC displays best electrochemical properties (1321.5 mAh g−1 at the current density of 0.1 A g−1 after 300 cycles) and high efficiency with accelerated Li+ diffusivity. Density functional theory (DFT) calculations confirm the ZnMn2O4 possesses the weakest adsorption energy on Li+ with a minimized value of −0.92 eV. In comparison with other XMn2O4 through traditional fabrication method, MOF-derived XMn2O4-BDC possesses a higher number of Li+ transport channels and better electric conductivity. This tactic provides a feasible and effective method for preparing bimetallic transition metal oxides and enhances energy storage applications.

Regulating metal active sites of atomically-thin nickel-doped spinel cobalt oxide toward enhanced oxygen electrocatalysis

Two-dimensional (2D) transition metal oxides are promising bifunctional electrocatalysts for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). Herein, atomically-thin nickel-doped spinel cobalt oxide (NCO) was prepared as OER/ORR catalyst through a simple hydrothermal method. Electrochemical test results show that the NCO nanosheets exhibit bifunctional catalytic performances with the overpotential of 275 mV for OER and the onset-potential of 0.92 V for ORR. Moreover, the assembled Zn−air battery based on the material exhibit excellent performance among transition metal oxides based oxygen electrocatalysts. Characterization results demonstrate that the Ni dopants contribute to creating the oxygen vacancies and promoting the electron transfer from nickel to cobalt. More importantly, the ratio of Co2+ and Ni3+ in the octahedral oxygen gap are significantly increased, which serve as the active sites for ORR and OER, respectively. In addition, the 2D nanostructure and Ni dopants help to facilitate the charge transfer during the reaction and the real active species of OER process is more biased towards Ni ions which were proven by in-situ Raman. This work offers a strategy for preparing highly active and durable spinel transition metal oxides materials for bifunctional oxygen catalysts.

Anion-cation co-substitution activation of spinel CoMoO4 for efficient oxygen evolution reaction

Developing an efficient earth-abundant catalyst able to accelerate oxygen evolution reaction (OER) is crucial for realizing the scalable application of electrochemical water splitting. Spinel transition metal oxides represent a promising catalyst for this reaction, but to data, their catalytic performance is still further improved to satisfy the practical application. Being different from previous nanostructure engineering and hybridization strategy, herein, we reported an anion-cation co-substitution activation of CoMoO4 for OER through a facile solvothermal treatment of FeCo-Prussian blue analogue (PBA) nanocube in the presence of thiourea and ammoniummolybdate. The combination of experimental results with theoretical analysis uncovered that anion-cation co-substitution allowed the formation of ultrathin porous nanosheet structure with more accessible active sites, enhanced charge and mass transfer ability and optimized adsorption free energy towards oxygen-containing intermediates. As a result, the Fe and S co-substituted CoMoO4 (Fe0.5Co0.5MoO4-xSx) nanoflowers assembled by nanosheets exhibited a drastically improved OER catalytic activity with an overpotential as low as 263 mV to afford a current density of 10 mA cm−2 and a small Tafel slope of 87 mV dec–1, superior to the state-of-the-art IrO2/C and most reported binary transition metal oxide-based electrocatalysts.

Electrical conductivity and ion transfer behavior of NiCo2O4 with different micro structures

The commercial graphite anode material in lithium-ion batteries has been limited by the low capacity and safety issue own to its low Li-intercalation potential. Spinel transition metal oxides have attracted tremendous attention because their high specific capacity and relatively high Li-intercalation potential. Herein, we investigated the influence of micro-structure on the ion transport properties of NiCo2O4 and their electronic conductivity. The optimized material with nano-cluster constructed macro-sphere demonstrates weak crystal structure and improved electrical conductivity when surrounded with amorphous carbon. Electrochemical impedance spectroscopy (EIS) combined with CV results give more clear explanation of the improved charge transfer resistance and ion transfer properties, which suggests the readjustment of microstructure is an important route for improving electrical conductivity and ion transfer behavior of NiCo2O4 anode.

NiFe2O4 nanoparticles on reduced graphene oxide for supercapacitor electrodes with improved capacitance

Spinel transition metal oxides are regarded as one of the finest pseudocapacitive materials, while unexpected stability hinders their practical application. For addressing such issue, herein, we use a simple hydrothermal method to grow NiFe2O4 nanoparticles on the surface of reduced graphene oxide (rGO). The rGO nanosheets in the hybrid can be used as a conductive substrate to accelerate the movement of electrons and ions, and they can also inhibit the aggregation of NiFe2O4. As the electrochemically active materials, the NiFe2O4 nanoparticles can reduce the self-stacking of rGO as well as increase the pseudocapacitance. The rGO-NiFe2O4 hybrid delivers improved specific capacitance of 215.7 F g−1 at 0.5 A g−1, and exhibits superior cycle stability with no obvious capacity loss after 10000 cycles. Our work demonstrates a method for effectively improving the electrochemical performance of NiFe2O4, and accelerating the commercialization of transition metal oxides as electrode materials for supercapacitors.

Tailoring rGO-NiFe2O4 hybrids to tune transport of electrons and ions for supercapacitor electrodes

Spinel transition metal oxide is a promising pseudocapacitive materials but the instability gives it narrow practical application. It is shown herein, that an effective remedy for these problems can be achieved by introducing highly conductive two dimensional (2D) reduced graphene oxide (rGO) substrate on which NiFe2O4 nanoparticles are well distributed. The rGO-NiFe2O4 hybrids with different NiFe2O4 contents were tailored to explore the source of the improvement in capacitance performance. It is found that rGO nanosheets in the hybrid can be used as a conductive substrate to accelerate the electron transport. Moreover, NiFe2O4 nanoparticles with a certain volume can simultaneously prevent the restacking of rGO and increase the pseudocapacitance. The G-N3 (30 wt% NiFe2O4) hybrid holds enhanced specific capacitance up to 210.9 F g−1 at 0.5 A g−1 far above that of pristine NiFe2O4 (50 F g−1), and exhibits superior cycle stability with no obvious capacity loss even after 5000 cycles. This research indicates that a tailoring method can promote the electrochemical performance of NiFe2O4, and hopefully accelerate the commercialization of spinel transition metal oxides as electrode materials for supercapacitor.

Engineering Surface Structure of Spinel Oxides via High-Valent Vanadium Doping for Remarkably Enhanced Electrocatalytic Oxygen Evolution Reaction.

Spinel oxides (AB2O4) with unique crystal structures have been widely explored as promising alternative catalysts for efficient oxygen evolution reactions; however, developing novel methods to fabricate robust, cost-effective, and high-performance spinel oxide based electrocatalysts is still a great challenge. Here, utilizing a complementary experimental and theoretical approach, pentavalent vanadium doping in the spinel oxides (i.e., Co3O4 and NiFe2O4) has been thoroughly investigated to engineer their surface structures for the enhanced electrocatalytic oxygen evolution reaction. Specifically, when the optimal concentration of vanadium (ca. 7.7 at. %) is incorporated into Co3O4, the required overpotential to reach a certain jGEOM and jECSA decreases dramatically for oxygen evolution reactions in alkaline media. Even after 30 h of chronopotentiometry, the required potential for V-doped Co3O4 just increases by 16.3 mV, being much lower than that of the undoped one. It is observed that the pentavalent vanadium doping introduces lattice distortions and defects on the surface, which in turn exposes more active sites for reactions. DFT calculations further reveal the rate-determining step changing from the step of *-O to *-OOH to the step of *-OH to *-O, while the corresponding energy barriers decrease from 1.73 to 1.57 eV accordingly after high-valent V doping. Moreover, the oxygen intermediate probing method using methanol as a probing reagent also demonstrates a stronger OH* adsorption on the surface after V doping. When vanadium doping is performed in the inverse spinel matrix of NiFe2O4, impressive performance enhancement in the oxygen evolution reaction is as well witnessed. All these results clearly illustrate that the V doping process can not only efficiently improve the electrochemical properties of spinel transition metal oxides but also provide new insights into the design of high-performance water oxidation electrocatalysts.

Three-dimensional mesoporous sandwich-like g-C3N4-interconnected CuCo2O4 nanowires arrays as ultrastable anode for fast lithium storage

Inspite of their impressive high theoretical capacity as Lithium-ion batteries (LIBs) anodes, spinel transition-metal oxides (TMOs) suffer serious volume expansion, aggregation and the pulverization of crystal structures during lithiation/delithiation, and this process severely restrict their industrial application. Multi-dimensional morphological engineering of spinel TMO nanostructures is an effective way to solve this issue. In this work, using facile hydrothermal synthetic methods, spinel CuCo2O4 nanowires arrays are synthesized and supported on g-C3N4 nanosheets, thus forming a unique sandwich-like interconnected three-dimensional mesoporous structure containing high amount of void spaces. Addition of g-C3N4 nanosheets to CuCo2O4 nanowire arrays may shorten the Li+ diffusion distance and electron transfer pathway, and may also provide more active sites for Li+ diffusion into electrolyte and buffer for the volume expansion and aggregation of CuCo2O4. As a LIB anode material, CuCo2O4@g-C3N4 shows initial lithiation capacity of 840.6 mAh g−1, and capacity retention of 641.2 mAh g−1 after 60 cycles at the current density of 0.1 A g−1 and 499.2 mAh g−1 after 40 cycles at high current of 1 A g−1, which is significantly better than value of pure CuCo2O4 nanowires. This work affords a new way to tackle the problem of volume expansion of high capacity spinel TMO anode materials using g-C3N4 nanosheets as buffering agent.

Mesoporous spinel NiFe oxide cubes as advanced electrocatalysts for oxygen evolution

Oxygen evolution reaction (OER) is the rate-controlling step of the electrochemical water splitting. The slow kinetics hinders large-scale H2 production. Herein, the spinel NiFe oxides were prepared by directly pyrolyzing nickel hexacynoferrate precursors in air. The NiFe oxides were presented as mesoporous nanocubes with a specific surface area of 125 m2 g−1. The mesoporous spinel NiFe oxide nanocubes can afford a geometric current of 10 mA cm−2 at a low overpotential of a 0.24 V and a small Tafel slope of 41 mV dec−1 in alkaline solution. The specific activity can reach up to 0.37 mA cm−2 with a turnover frequency of 0.93 s−1. The superior OER activity of the NiFe oxide nanocubes (NiFeO NCs) can outperform those of the state-of-the-art IrO2 catalysts, and compare favorably with other spinel transition metal oxides reported recently under identical condition. NiFeO NCs also show a long-term durability without significant loss of the OER activity. Our works provide a new strategy to develop efficient, robust and earth-abundant spinel NiFe oxides as advanced OER electrocatalysts to replace the expensive commercial IrO2 catalysts for water splitting in the industrial scale.

Size-dependent kinetics during non-equilibrium lithiation of nano-sized zinc ferrite

Spinel transition metal oxides (TMOs) have emerged as promising anode materials for lithium-ion batteries. It has been shown that reducing their particle size to nanoscale dimensions benefits overall electrochemical performance. Here, we use in situ transmission electron microscopy to probe the lithiation behavior of spinel ZnFe2O4 as a function of particle size. We have found that ZnFe2O4 undergoes an intercalation-to-conversion reaction sequence, with the initial intercalation process being size dependent. Larger ZnFe2O4 particles (40 nm) follow a two-phase intercalation reaction. In contrast, a solid-solution transformation dominates the early stages of discharge when the particle size is about 6–9 nm. Using a thermodynamic analysis, we find that the size-dependent kinetics originate from the interfacial energy between the two phases. Furthermore, the conversion reaction in both large and small particles favors {111} planes and follows a core-shell reaction mode. These results elucidate the intrinsic mechanism that permits fast reaction kinetics in smaller nanoparticles.

A general fabrication approach on spinel MCo2O4 (M = Co, Mn, Fe, Mg and Zn) submicron prisms as advanced positive materials for supercapacitor

The design and fabrication of advanced electrode materials for supercapacitor have been extensively explored recently. The spinel transition metal oxides have drawn considerable attentions because of their high theoretical capacity to store electrical charge. This work introduced a general hydrothermal assisted co-precipitation approach to fabricate five kinds of spinel MCo2O4 (M = Co, Mn, Fe, Mg and Zn) submicron prisms on nickel foams. Among these spinel MCo2O4 electrodes, the MgCo2O4 exhibited the highest specific capacity of 613.5 C g−1 (0.883 C cm−2) at a current density of 2 mA cm−2. All the specific capacities of bimetallic oxides were higher than single metal oxide Co3O4, indicating the enhanced electrochemical performance of bimetallic oxides. The correlations between peak currents and the square root of the scan rates of all prepared electrode materials showed OH− diffusion-controlled characteristic in their redox reactions. Furthermore, an assembled MgCo2O4//AC hybrid supercapacitor (HSC) achieved a specific capacity and a specific energy of 182.8 C g−1 at 0.5 A g−1 and 39.7 W h kg−1, respectively. More impressively, this MgCo2O4//AC HSC showed a subsequent increase about 21.1% in specific capacity after 5000 cycles, suggesting its promising characteristics for the next generation energy storage device.

Visualization of Phase Evolution of Ternary Spinel Transition Metal Oxides (CuFe2O4) during Lithiation

Visualization of Phase Evolution of Ternary Spinel Transition Metal Oxides (CuFe₂O₄) during Lithiation

Inverse spinel transition metal oxides for lithium-ion storage with different discharge/charge conversion mechanisms

Inverse spinel transition metal oxides (Fe3O4, MnFe2O4, Fe3O4/reduced graphene oxide and MnFe2O4/reduced graphene oxide) are prepared by a facile ethylene-glycol-assisted hydrothermal method. The stability of inverse spinel structure and the high specific surface area of nanoscale provide transition metal oxides with high specific capacity. And the surface modification with reduced graphene oxide improves the poor conductivity of pristine transition metal oxides. Pristine Fe3O4 and MnFe2O4 deliver the high initial discharge capacity of 1137.1 and 1088.9mAhg−1, respectively. Fe3O4/reduced graphene oxide and MnFe2O4/reduced graphene oxide get the reversible capacity of 645.8 and 720mAhg−1, respectively, even after 55 cycles. The different discharge/charge conversion mechanisms make them different capacity stability. The great electrochemical performances of composites offer electrodes with suitable characteristics for high-performance energy storage application.

Iron-Based Spinel Oxide Nanoparticles for Electrocatalysis of Oxygen Evolution Reaction

The design of cost-effective and highly active, durable electrocatalysts for the sluggish oxygen evolution reaction (OER) is critical for promoting various energy conversion process such as water splitting and rechargeable metal-air batteries. Due to a wider selection of earth-abundance and low cost catalyst candidates (e.g., mixed 3d-transition metal oxides), OER in alkaline environment has attracted considerable interests during past years. Besides the recently developed Ni-Fe layered double hydroxides (LDH) catalysts, spinel phase transition metal oxides have also drawn particular interests owing to their well-defined rigid structure and thus potentially high stability during electrocatalysis. While improved OER activities have been demonstrated, currently most spinel transition metal oxides (mainly Co-based spinel oxides) still underperformed the state-of-the-art benchmark OER catalyst IrO2and the Ni-Fe LDH catalysts. Moreover, atomic insights into the surface structure that governs their OER reactivities and stabilities still remain very limited. We report here the exploration of iron-based spinel oxide nanoparticles (MxFe3-xO4, M= Mn, Fe, Co, Ni, Cu) as a new class of alkaline OER electrocatalysts with potentially both high electrocatalytic activity and high stability. Emphasis will be placed on atomistic understanding on the surface cation chemistry and surface atomic structures of the spinel oxide nanoparticles and their correlations with OER activities and stabilities. This will be achieved by comparative microscopic and spectroscopic studies of the nanoparticle surfaces before and after OER electrocatalysis using high-resolution (scanning) transmission electron microscopy, electron energy loss spectroscopy and X-ray photon spectroscopy, which will be further complemented by density functional theory calculations. The results will provide important insights into the structure-activity-stability relations of spinel phase transition metal oxides as well as other mixed metal oxides for OER electrocatalysts.

Visualizing non-equilibrium lithiation of spinel oxide via in situ transmission electron microscopy.

Spinel transition metal oxides are important electrode materials for lithium-ion batteries, whose lithiation undergoes a two-step reaction, whereby intercalation and conversion occur in a sequential manner. These two reactions are known to have distinct reaction dynamics, but it is unclear how their kinetics affects the overall electrochemical response. Here we explore the lithiation of nanosized magnetite by employing a strain-sensitive, bright-field scanning transmission electron microscopy approach. This method allows direct, real-time, high-resolution visualization of how lithiation proceeds along specific reaction pathways. We find that the initial intercalation process follows a two-phase reaction sequence, whereas further lithiation leads to the coexistence of three distinct phases within single nanoparticles, which has not been previously reported to the best of our knowledge. We use phase-field theory to model and describe these non-equilibrium reaction pathways, and to directly correlate the observed phase evolution with the battery's discharge performance.

Combustion synthesis of Mg–Er ferrite nanoparticles: Cation distribution and structural, optical, and magnetic properties

Rare earth ion (Er3+)-doped magnesium ferrite nanoparticles of basic composition MgFe2−xErxO4 (x=0, 0.02, 0.04, and 0.06) were synthesized for the first time by a combustion method with use of glycine as fuel. The synthesized nanoparticles were characterized by X-ray diffraction, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, UV-diffuse reflectance spectroscopy, and vibrating sample magnetometer analysis in order to study the structural, compositional, morphological, and magnetic changes with addition of dopant. The X-ray diffraction pattern revealed a single phase with cubic spinel structure, and from the Scherrer formula and the Williamson–Hall formula, the average grain sizes ranged from 35 to 56nm and from 31 to 54nm, respectively. The lattice parameter (a) increases with the increase of the Er3+ concentration x in the lattice. The cation distributions among the tetrahedral (A) and octahedral (B) sites of spinel-type Er-doped magnesium ferrites were also investigated. The Fourier transform infrared spectroscopy spectra of synthesized samples illustrate that the higher-frequency bands lying in the range from 550 to 620cm−1 and the lower-frequency bands lying in the range from 410 to 450cm−1 are associated with the asymmetric stretching modes of the AB2O4 type of spinel transition metal oxides. Information about the chemical elements and oxidation states of the samples was obtained from high-resolution core-level X-ray photoelectron spectroscopy spectra of Mg 1s, Fe 2p, Er 4d, and O 1s. Further information about the morphology of the nanoparticles was obtained by scanning electron microscopy. From UV-diffuse reflectance spectroscopy studies, the optical band gaps were found to range from 1.81 to 1.96eV. The magnetic hysteresis curves clearly indicate the soft ferromagnetic nature of the samples. Various magnetic properties such as saturation magnetization, coercivity, and remanent magnetization obtained from M–H loops were observed to increase with Er3+ substitution.

The Valence Electronic Structure of Co3O4: Is It a Charge-Transfer Insulator?

AbstractDespite the relevance to a variety of materials applications, the electronic and bonding properties of spinel transition metal oxides are not well established. We report here the slow oxidation of CoO(100) to Co3O4, studied by photoemission (UPS and XPS), low energy electron diffraction (LEED) and high resolution electron energy loss spectroscopy (HREELS) with the aim of elucidating the valence band electronic structure of the Co3O4 spinel. The original Mott insulator picture of the parent CoO substrate has been revised in recent times, after careful analyses and extensive debate, to the more detailed charge-transfer insulator model which includes some admixture of oxygen 2p levels in the 3d-derived valence band. No equivalent band structure analysis has been performed on the spinel oxides, perhaps in part because of the greater complexity of the 56-atom unit cell with two different cation lattice sites and oxidation states. In this study, we determine the valence band structure of the spinel oxide and address the question of whether Co3O4 can be modeled as a charge-transfer insulator in analogy with its closely related rocksalt substrate.