Bimetallic cobalt‐based spinel is sparking much interest, most notably for its excellent bifunctional performance. However, the effect of Fe3+ doping in Co3O4 spinel remains poorly understood, mainly ...because the surface state of a catalyst is difficult to characterize. Herein, a bifunctional oxygen electrode composed of spinel Co2FeO4/(Co0.72Fe0.28)Td(Co1.28Fe0.72)OctO4 nanoparticles grown on N‐doped carbon nanotubes (NCNTs) is designed, which exhibits superior performance to state‐of‐the‐art noble metal catalysts. Theoretical calculations and magnetic measurements reveal that the introduction of Fe3+ ions into the Co3O4 network causes delocalization of the Co 3d electrons and spin‐state transition. Fe3+ ions can effectively activate adjacent Co3+ ions under the action of both spin and charge effect, resulting in the enhanced intrinsic oxygen catalytic activity of the hybrid spinel Co2FeO4. This work provides not only a promising bifunctional electrode for zinc–air batteries, but also offers a new insight to understand the Co‐Fe spinel oxides for oxygen electrocatalysis.
A bifunctional oxygen electrode composed of hybrid spinel Co2FeO4 nanoparticles grown on N‐doped carbon nanotubes is a promising candidate for zinc–air batteries. Theoretical calculations and magnetic measurements reveal that the introduction of Fe cations into the Co3O4 network causes Co 3d electron delocalization and spin‐state transition, resulting in enhanced catalytic activity of the as‐prepared spinel Co2FeO4.
Cobalt spinel oxides are a class of promising transition metal (TM) oxides for catalyzing oxygen evolution reaction (OER). Their catalytic activity depends on the electronic structure. In a spinel ...oxide lattice, each oxygen anion is shared amongst its four nearest transition metal cations, of which one is located within the tetrahedral interstices and the remaining three cations are in the octahedral interstices. This work uncovered the influence of oxygen anion charge distribution on the electronic structure of the redox‐active building block Co−O. The charge of oxygen anion tends to shift toward the octahedral‐occupied Co instead of tetrahedral‐occupied Co, which hence produces strong orbital interaction between octahedral Co and O. Thus, the OER activity can be promoted by pushing more Co into the octahedral site or shifting the oxygen charge towards the redox‐active metal center in CoO6 octahedra.
The oxygen evolution activity of Co‐based spinel oxides is dominated by the catalytically critical TMO6 octahedra. Pushing more active Co into octahedral sites and shifting the oxygen charge to octahedral Co significantly enhance the activity.
This research investigates the oxidation behavior of nanocrystalline/amorphous Si3N4 compounds in atmospheres with different oxygen pressures. High purity nanocrystalline/amorphous Si3N4 and MgAl2O4 ...mixtures were used to investigate oxidation behavior of Si3N4. Si3N4-xMgAl2O4 (0 < x < 90 wt.%) samples were prepared with mixing in a jar mill, followed by pressing and heating in a high vacuum furnace. Specimens were characterized using FE-SEM, EDS, XRD, FT-IR, and ICP. Thermal analysis (TG/DSC) was also employed to investigate the thermal behavior of samples in different atmospheres. Results showed that phase transformation in pure Si3N4 is controlled by holding time and oxygen pressure in the atmosphere. Results also showed an increase in the weight of pure Si3N4 by 5.3 and 15.2 % for 0.5 and 2 h in a vacuum atmosphere, respectively, inferring that crystallization reaction proceeds with holding time. In the temperature range 1000–1400 °C, a noticeable exothermic peak has appeared for pure Si3N4 in the air atmosphere which was attributed to the formation of cristobalite (SiO2) phase. Besides, thermal analyses of Si3N4-xMgAl2O4 (0 < x < 90 wt.%) samples in the nitrogen atmosphere showed that the maximum weight change was attained in the Si3N4-50% MgAl2O4 sample. Mechanisms and implications of oxidation in all cases are thoroughly discussed.
Spinel zinc cobalt oxide (ZnCo2O4) is not considered as a superior catalyst for the electrochemical oxygen evolution reaction (OER), which is the bottleneck reaction in water‐electrolysis. Herein, ...taking advantage of density functional theory (DFT) calculations, we find that the existence of low‐spin (LS) state cobalt cations hinders the OER activity of spinel zinc cobalt oxide, as the t2g6eg0 configuration gives rise to purely localized electronic structure and exhibits poor binding affinity to the key reaction intermediate. Increasing the spin state of cobalt cations in spinel ZnCo2O4 is found to propagate a spin channel to promote spin‐selected charge transport during OER and generate better active sites for intermediates adsorption. The experiments find increasing the calcination temperature a facile approach to engineer high‐spin (HS) state cobalt cations in ZnCo2O4, while not working for Co3O4. The activity of the best spin‐state‐engineered ZnCo2O4 outperforms other typical Co‐based oxides.
Engineering high‐spin state cobalt cations in spinel ZnCo2O4 propagates a spin channel for spin‐selected charge transport and enhances active sites for intermediate adsorption. High‐spin cobalt cations can be directly engineered by varying the calcination temperature.
With the increasing demand for light, small and high power rechargeable lithium ion batteries in the application of mobile phones, laptop computers, electric vehicles, electrochemical energy storage, ...and smart grids, the development of electrode materials with high-safety, high-power, long-life, low-cost, and environment benefit is in fast developing recently. The spinel lithium titanate Li4Ti5O12 has attracted more and more attention as electrode materials applied in advanced energy storage devices due to its appealing features such as “zero-strain” structure characteristic, excellent cycle stability, low cost and high safety feature. The review focuses on recent studies on spinel lithium titanate (Li4Ti5O12) for the energy storage devices, especially on the structure the reversibility of electrode redox, as well as the synthesis methods and strategies for improvement in the electrochemical performances.
•LMO with specific facets and different morphologies are controlled synthesized.•LMO-MST delivers superior rate capability up to 124.2 mAh g−1 at 10 C.•LMO-MST demonstrates 84.3 % capacity retention ...after 1000 cycles at 10 C.•LMO-MST cathodes still maintains optimal cycling stability at 55 and -5°C.•The superior performance results from sphere-bridged-tube structure and {111} facets.
Recently, spinel-type LiMn2O4 (LMO) cathode has attracted great attention to mitigate manganese dissolution especially at elevated temperature. In this work, LMO with surface specific facets and different morphologies are successfully controlled synthesized via a facile solvothermal-lithiation method. It suggests that microspheres, micro-tubes and hybrid sphere-interconnected-tube microstructures with different surface lattice orientation using urea, oxalic acid and mixture of urea and oxalic acid as precipitants at solvothermal stage, respectively. Specifically, LMO microspheres (LMO-MS) display (111) facets and the micro-tubular LMO materials (LMO-MT) exhibit the high index lattice (311). It is more interesting is that hybrid sphere-interconnected-tube microstructures (LMO-MST) are clearly observed the densest (111) facets at the micro-spherical surface and a new (111) plane appearance on the surface of microtubes. LMO-MST demonstrates excellent cycling performance (84.3 % capacity retention after 1000 cycles at 10 C) and superior rate capability up to 10 C (124.2 mAh g−1 at 10 C). The electrochemical performances of LMO-MST cathodes are also investigated at elevated (55°C) and lower (-5°C) temperatures, under which LMO-MST still maintains optimal cycling stability. The superior electrochemical performance of LMO-MST cathodes can be attributed to the unique sphere-bridged-tube seamless outer structure and the preferentially exposed stable facets on the crystal surface.
Hybrid micro-nanostructured LMO sphere-bridged-tube with {111} facets demonstrates superior rate capability and cycling performance both under 55 and -5°C. Display omitted
Voltage decay and capacity fading are the main challenges for the commercialization of Li‐rich Mn‐based layered oxides (LLOs). Now, a three‐in‐one surface treatment is designed via the pyrolysis of ...urea to improve the voltage and capacity stability of Li1.2Mn0.6Ni0.2O2 (LMNO), by which oxygen vacancies, spinel phase integration, and N‐doped carbon nanolayers are synchronously built on the surface of LMNO microspheres. Oxygen vacancies and spinel phase integration suppress irreversible O2 release and help lithium ion diffusion, while N‐doped carbon nanolayer mitigates the corrosion of electrolyte with excellent conductivity. The electrochemical performance of LMNO after the treatment improves significantly; the capacity retention rate after 500 cycles at 1 C is still as high as 89.9 % with a very small voltage fading rate of 1.09 mV cycle−1. This three‐in‐one surface treatment strategy can suppress the voltage decay and capacity fading of LLOs.
A three‐in‐one surface modification is presented. It consists of oxygen vacancies, spinel phase integration, and N‐doped carbon nanolayers synchronously built on the surface of Li1.2Mn0.6Ni0.2O2 microspheres via the pyrolysis of urea. Voltage and capacity stability are improved by this method.
Development of spinel bimetallic oxides as low‐cost and high‐efficiency catalysts for catalytic oxidation is highly desired. However, rational design of spinel oxides with controlled structure and ...components still remains a challenge. A general route for large‐scale preparation of spinel CoFe2O4/C nanocubes transformed from organometal‐encapsulated metal–organic frameworks (MOFs) via exchange–coordination and pyrolysis combined method is reported. Strong confinement effect between organometallics and MOFs realizes reconstruction of crystal phase and composition, but not simple metallic oxides support by Co2+ introduction. Compared with Co3O4‐Fe2O3/C, MOFs‐derived cubic nano‐CoFe2O4/C with higher surface area (115.7 m2 g−1) and favorable surface chemistry exhibits excellent catalytic activity (100% CO conversion at 105 °C) and competitive water‐resisting stability (total conversion at 145 °C for 20 h). Turnover frequency of CoFe2O4/C reaches 4.26 × 10−4 s−1 at 90 °C, two orders of magnitude higher than commercial Co3O4 . Theoretical models show that oxygen vacancies (17.7%) at exposed {112} facet on the carbon interface take superiority in nanocubic spinel phase, which allows reactive species to be strongly adsorbed on nanostructured catalysts' surface and plays key roles in hindering deactivation under moisture rich conditions. The progresses offer a promising way in the development of novel spinel oxides with tailored architecture and properties for vast applications.
Confined transformation of organometal‐encapsulated metal–organic frameworks (MOFs) into novel spinel CoFe2O4/C nanocubes is achieved via an innovative methodology combining exchange–coordination and pyrolysis. Strong interaction between the organometallic guest and MOF host leads CoFe2O4/C nanocubes to exhibit superior activity for low temperature oxidation (100% CO conversion at 105 °C) and very competitive water‐resisting stability.
The Li1.02Ni0.05Mn1.93O4 cathode material exhibits a better rate capability than that of the LiNi0.05Mn1.95O4. Moreover, the Li1.02Ni0.05Mn1.93O4 shows the well-developed crystal structure with the ...(111), (110) and (100) crystal planes. The (111) crystal planes possess the minimum Mn dissolution and the (110) and (100) crystal planes are well consistent with the Li+ diffusion channel.
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Various Li-rich spinel Li1+xNi0.05Mn1.95-xO4 (0 ≤ x ≤ 0.10) cathode materials with a truncated octahedron were synthesized by a solution combustion method. The relationship of crystalline structure, particles morphology and electrochemical properties of the as-prepared samples was investigated via a series of physicochemical characterizations. The Li-Ni co-doping changes the lattice parameters and atomic configuration, whilst resulting in a contraction of unit cell dimension and giving rise to a variation of bond length. In this regard, the shrinkage of octahedral MnO6 provides a robust structure and the expansion of tetrahedral LiO4 facilitates a fast electrochemical process. Additionally, the resulted polyhedral Li1+xNi0.05Mn1.95-xO4 samples present the exposed (110), (100), and (111) crystal planes, which provide the favorable Li+ ions diffusion/transmission channel and alleviate Mn dissolution. Owing to these merits of polyhedral structure and Li-Ni co-doping, the optimized Li1.02Ni0.05Mn1.93O4 exhibits good electrochemical performance with high initial discharge capacity of 119.8, 107.1 and 97.9 mAh·g−1 at 1, 5 and 10 C, respectively. Even at a high current rate of 15 C, an excellent capacity retention of 91.7% is obtained after 1000 cycles, whilst the high temperature performance was also improved.
To develop efficient catalysts is one of the major ways to solve the energy and environmental problems. Spinel ferrites, with the general chemical formula of MFe2O4 (where M = Mg2+, Co2+, Ni2+, Zn2+, ...Fe2+, Mn2+, etc.), have attracted considerable attention in catalytic research. The flexible position and valence variability of metal cations endow spinel ferrites with diverse physicochemical properties, such as abundant surface active sites, high catalytic activity and easy to be modified. Meanwhile, their unique advantages in regenerating and recycling on account of the magnetic performances facilitate their practical application potential. Herein, the conventional as well as green chemistry synthesis of spinel ferrites is reviewed. Most importantly, the critical pathways to improve the catalytic performance are discussed in detail, mainly covering selective doping, site substitution, structure reversal, defect introduction and coupled composites. Furthermore, the catalytic applications of spinel ferrites and their derivative composites are exclusively reviewed, including Fenton-type catalysis, photocatalysis, electrocatalysis and photoelectro-chemical catalysis. In addition, some vital remarks, including toxicity, recovery and reuse, are also covered. Future applications of spinel ferrites are envisioned focusing on environmental and energy issues, which will be pushed by the development of precise synthesis, skilled modification and advanced characterization along with emerging theoretical calculation.
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•This review is started off with a short outlook on crystal structure of spinel ferrites and its synthesis methods.•Five strategies for catalytic performance improvement are innovatively proposed.•The catalytic applications of spinel ferrites in environment and energy field are detailed discussed.•Some vital remarks about toxicity are included.
