A Microwave Synthesis of Mesoporous NiCo2O4 Nanosheets as Electrode Materials for Lithium‐Ion Batteries and Supercapacitors

A facile microwave method was employed to synthesize NiCo₂O₄ nanosheets as electrode materials for lithium‐ion batteries and supercapacitors. The structure and morphology of the materials were characterized by X‐ray diffraction, field‐emission scanning electron microscopy, transmission electron microscopy and Brunauer–Emmett–Teller methods. Owing to the porous nanosheet structure, the NiCo₂O₄ electrodes exhibited a high reversible capacity of 891 mA h g^?1 at a current density of 100 mA g^?1, good rate capability and stable cycling performance. When used as electrode materials for supercapacitors, NiCo₂O₄ nanosheets demonstrated a specific capacitance of 400 F g^?1 at a current density of 20 A g^?1 and superior cycling stability over 5000 cycles. The excellent electrochemical performance could be ascribed to the thin porous structure of the nanosheets, which provides a high specific surface area to increase the electrode–electrolyte contact area and facilitate rapid ion transport.     

Hierarchical Porous NiCo2O4 Microboxes Constructed by Low-Dimensional Substructures for Electrochemical Supercapacitor

The design and synthesis of new materials/structures for high-performance electrochemical capacitors (ECs) is an ongoing challenge. Herein, a hierarchical porous NiCo₂O₄ microbox superstructure made of low-dimensional substructures was reported. The as-prepared NiCo₂O₄ microboxes are constructed by 2D nanosheets building units, which are futher woven by 0D nanoparticles and 1D nanowires. Such microbox superstructures combine the merits of all material dimensions in electrochemical capacitors, such as high porosity, sufficient active sites, and fast mass and charge transport. Benefiting from the structural advantages, the resultant NiCo₂O₄ microbox electrode exhibits ultra-high capacitor performance, i.e., the initial capacitance of 1820 F · g^–1 and 96.6 % capacitance retention after 4000 cycles at 5 A · g^–1.     

Three‐Dimensional Macroporous NiCo2O4 Sheets as a Non‐Noble Catalyst for Efficient Oxygen Reduction Reactions

A facile and low‐cost strategy is developed to prepare three‐dimensional (3D) macroporous NiCo₂O₄ sheets, which can be used as a highly efficient non‐noble metal electrocatalyst for the oxygen reduction reaction (ORR) in alkaline conditions. The as‐obtained sheets have a thickness of about 150 nm and feature a typical 3D macroporous structure with pore volumes of up to 0.23 cm³ g^?1, which could decrease the mass transport resistance and allow easier access of the reactants to the active surface sites. The as‐prepared macroporous NiCo₂O₄ sheets exhibit high electrocatalytic activity for ORR with a four‐electron pathway, good long‐term stability and high tolerance against methanol. The unique 3D macroporous structure and intrinsic properties may be responsible for their high performance.     

Facile Preparation of Snowflake-Like MnO2@NiCo2O4 Composites for Highly Efficient Electromagnetic Wave Absorption

Snowflake-like MnO₂@NiCo₂O₄ composites were successfully fabricated by employing crossed snowflake-like MnO₂ nanorods as cores and one-dimensional (1D) NiCo₂O₄ nanoneedles as shells. Impressively, the MnO₂@NiCo₂O₄ composites exhibited a highly efficient electromagnetic wave (EMW) absorbing capability, and the minimum reflection loss (RL) value reached −58.4 dB at 6.8 GHz for a thickness of 4.0 mm. The reasons for the improved EMW absorption capability of snowflake-like MnO₂@NiCo₂O₄ composites were analyzed. The unique core–shell structure, good impedance matching, and high dielectric loss were all found to be important contributors. Moreover, the interfacial polarization mainly stemmed from the heterostructure, a microcurrent generated from the 1D MnO₂ nanorods and NiCo₂O₄ nanoneedles under alternating electromagnetic fields, and the synergistic effect from the different components were all beneficial to improve the EMW absorption performance. These results demonstrated that snowflake-like MnO₂@NiCo₂O₄ composites could be utilized as promising materials for practical EMW absorbing applications.     

Facile ordered ZnCo2O4@MnO2 nanosheet arrays for superior-performance supercapacitor electrode

Constructing ZnCo₂O₄ nanosheet arrays (NSAs)@MnO₂ nanosheets core-shell nanostructures directly on the current collector (Ni foam) was successfully realized via hydrothermal process and heat treatment. The whole surfaces of uniform ZnCo₂O₄ NSAs were covered with well-ordered MnO₂ nanosheets, which make the whole system have a large specific surface area. At a low current density of 2 mA cm⁻², supercapacitor electrode made of ZnCo₂O₄@MnO₂ composite gave rise to a superior specific capacity about 929.2 C g⁻¹. Although at an ultrahigh current density of 40 mA cm⁻², it still kept a satisfactory specific capacity about 751.1 C g⁻¹, and retained ∼95.75% of the capacity even after 5000 cycles. Because of the synergistic effect between ZnCo₂O₄ and MnO₂ and the great surface area of the system with the special core-shell structure, ZnCo₂O₄@MnO₂ composite has the excellent rate performance, considerable capacity, and quite good cycle performance, which make it a candidate for a new generation of superior-performance electrochemical supercapacitors.     

Controllable Synthesis of Ultrathin NiCo2O4 Nanosheets Incorporated onto Composite Nanotubes for Efficient Oxygen Reduction

Exploring non‐precious‐metal‐based oxygen reduction reaction (ORR) electrocatalysts featuring high efficiency, low cost, and environmental friendliness is of great importance for the broad applications of fuel cells and metal–air batteries. In this work, ultrathin NiCo₂O₄ nanosheets deposited on 1D SnO₂ nanotubes (SNT) were successfully fabricated through a productive electrospinning technique followed by a sintering and low‐temperature coprecipitation strategy. This hierarchically engineered architecture has ultrathin NiCo₂O₄ nanosheets uniformly and fully erected on both walls of tubular SNTs, which results in improved electrochemical activity as an ORR catalyst, in terms of positive onset potential and high current density, as well as superior tolerance to crossover effects and long‐term durability with respect to the commercial Pt/C catalyst. The excellent performance of SNT@NiCo₂O₄ composites may originate from their rationally designed hierarchical tubular nanostructure with completely exposed active sites and interconnected 1D networks for efficient electron and electrolyte transfer; this makes these composite nanotubes promising candidates to replace platinum‐based catalysts for practical fuel cell and metal–air battery applications.     

High reversible capacity and rate capability of ZnCo2O4/graphene nanocomposite anode for high performance lithium ion batteries

A facile and straightforward method was adopted to synthesize ZnCo₂O₄/graphene nanocomposite anode. In the first step, pure ZnCo₂O₄ nanoparticles were synthesized using urea-assisted auto-combustion synthesis followed by annealing at a low temperature of 400 °C. In the second step, in order to synthesize ZnCo₂O₄/graphene nanocomposite, the obtained pure ZnCo₂O₄ nanoparticles were milled with 10 wt% reduced graphene nanosheets using high energy spex mill for 30 s. The ZnCo₂O₄ nanoparticles, with particle sizes of 25–50 nm, were uniformly dispersed and anchored on the reduced graphene nanosheets. Compared with pure ZnCo₂O₄ nanoparticles anode, significant improvements in the electrochemical performance of the nanocomposite anode were obtained. The resulting nanocomposite delivered a reversible capacity of 1124.8 mAh g⁻¹ at 0.1 C after 90 cycles with 98% Coulombic efficiency and high rate capability of 515.9 mAh g⁻¹ at 4.5 C, thus exhibiting one of the best lithium storage properties among the reported ZnCo₂O₄ anodes. The significant enhancement of the electrochemical performance of the nanocomposite anode could be credited to the strong synergy between ZnCo₂O₄ and graphene nanosheets, which maintain excellent electronic contact and accommodate the large volume changes during the lithiation/delithiation process.     

Hierarchical NiCo2S4 Nanotube@NiCo2S4 Nanosheet Arrays on Ni Foam for High‐Performance Supercapacitors

Hierarchical NiCo₂S₄ nanotube@NiCo₂S₄ nanosheet arrays on Ni foam have been successfully synthesized. Owing to the unique hierarchical structure, enhanced capacitive performance can be attained. A specific capacitance up to 4.38 F cm^?2 is attained at 5 mA cm^?2, which is much higher than the specific capacitance values of NiCo₂O₄ nanosheet arrays, NiCo₂S₄ nanosheet arrays and NiCo₂S₄ nanotube arrays on Ni foam. The hierarchical NiCo₂S₄ nanostructure shows superior cycling stability; after 5000 cycles, the specific capacitance still maintains 3.5 F cm^?2. In addition, through the morphology and crystal structure measurement after cycling stability test, it is found that the NiCo₂S₄ electroactive materials are gradually corroded; however, the NiCo₂S₄ phase can still be well‐maintained. Our results show that hierarchical NiCo₂S₄ nanostructures are suitable electroactive materials for high performance supercapacitors.     

Morphology‐Tuned Synthesis of NiCo2O4‐Coated 3D Graphene Architectures Used as Binder‐Free Electrodes for Lithium‐Ion Batteries

Nanostructured NiCo₂O₄ is directly grown on the surface of three‐dimensional graphene‐coated nickel foam (3D‐GNF) by a facile electrodeposition technique and subsequent annealing. The resulting NiCo₂O₄ possesses a distinct flower or sheet morphology, tuned by potential or current variation electrodeposition, which are used as binder‐free lithium‐ion battery anodes for the first time. Both samples exhibit high lithium storage capacity, profiting from the unique binder‐free electrode structures. The flower‐type NiCo₂O₄ demonstrates high reversible discharge capacity (1459 mAh g^?1 at 200 mA g^?1) and excellent cyclability with around 71 % retention of the reversible capacity after 60 cycles, which are superior to the sheet‐type NiCo₂O₄. Such superb performance can be attributed to high volume utilization efficiency with unique morphological character, a well‐preserved connection between the active materials and the current collector, a short lithium‐ion diffusion path, and fast electrolyte transfer in the binder‐free NiCo₂O₄‐coated 3D graphene structure. The simple preparation process and easily controllable morphology make the binder‐free NiCo₂O₄/3D‐GNF hybrid a potential material for commercial applications.     

Clewlike ZnV2O4 Hollow Spheres: Nonaqueous Sol–Gel Synthesis,Formation Mechanism,and Lithium Storage Properties

Hollow ZnV₂O₄ microspheres with a clewlike feature were synthesized by reacting zinc nitrate hexahydrate and ammonium metavanadate in benzyl alcohol at 180 °C for the first time. GC–MS analysis revealed that the organic reactions that occurred in this study were rather different from those in benzyl alcohol based nonaqueous sol–gel systems with metal alkoxides, acetylacetonates, and acetates as the precursors. Time‐dependent experiments revealed that the growth mechanism of the clewlike ZnV₂O₄ hollow microspheres might involve a unique multistep pathway. First, the generation and self‐assembly of ZnO nanosheets into metastable hierarchical microspheres as well as the generation of VO₂ particles took place quickly. Then, clewlike ZnV₂O₄ hollow spheres were gradually produced by means of a repeating reaction–dissolution (RD) process. In this process, the outside ZnO nanosheets of hierarchical microspheres would first react with neighboring vanadium ions and benzyl alcohol and also serve as the secondary nucleation sites for the subsequently formed ZnV₂O₄ nanocrystals. With the reaction proceeding, the interior ZnO would dissolve and then spontaneously diffuse outwards to nucleate as ZnO nanocrystals on the preformed ZnV₂O₄ nanowires. These renascent ZnO nanocrystals would further react with VO₂ and benzyl alcohol, ultimately resulting in the final formation of a hollow spatial structure. The lithium storage ability of clewlike ZnV₂O₄ hollow microspheres was studied. When cycled at 50 mA g^?1 in the voltage range of 0.01–3 V, this peculiarly structured ZnV₂O₄ electrode delivered an initial reversible capacity of 548 mAh g^?1 and exhibited almost stable cycling performance to maintain a capacity of 524 mAh g^?1 over 50 cycles. This attractive lithium storage performance suggests that the resulting clewlike ZnV₂O₄ hollow spheres are promising for lithium‐ion batteries.     

Hierarchical NiCo2O4 Hollow Microcuboids as Bifunctional Electrocatalysts for Overall Water‐Splitting

Bifunctional electrocatalysts for the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) in alkaline electrolyte may improve the efficiency of overall water splitting. Nickel cobaltite (NiCo₂O₄) has been considered a promising electrode material for the OER. However, NiCo₂O₄ that can be used as an electrocatalyst in HER has not been studied yet. Herein, we report self‐assembled hierarchical NiCo₂O₄ hollow microcuboids for overall water splitting including both the HER and OER reactions. The NiCo₂O₄ electrode shows excellent activity toward overall water splitting, with 10 mA cm^?2 water‐splitting current reached by applying just 1.65 V and 20 mA cm^?2 by applying just 1.74 V across the two electrodes. The synthesis of NiCo₂O₄ microflowers confirms the importance of structural features for high‐performance overall water splitting.     

Two‐Dimensional Cobalt‐/Nickel‐Based Oxide Nanosheets for High‐Performance Sodium and Lithium Storage

Two‐dimensional (2D) nanomaterials are one of the most promising types of candidates for energy‐storage applications due to confined thicknesses and high surface areas, which would play an essential role in enhanced reaction kinetics. Herein, a universal process that can be extended for scale up is developed to synthesise ultrathin cobalt‐/nickel‐based hydroxides and oxides. The sodium and lithium storage capabilities of Co₃O₄ nanosheets are evaluated in detail. For sodium storage, the Co₃O₄ nanosheets exhibit excellent rate capability (e.g., 179 mA h g^?1 at 7.0 A g^?1 and 150 mA h g^?1 at 10.0 A g^?1) and promising cycling performance (404 mA h g^?1 after 100 cycles at 0.1 A g^?1). Meanwhile, very impressive lithium storage performance is also achieved, which is maintained at 1029 mA h g^?1 after 100 cycles at 0.2 A g^?1. NiO and NiCo₂O₄ nanosheets are also successfully prepared through the same synthetic approach, and both deliver very encouraging lithium storage performances. In addition to rechargeable batteries, 2D cobalt‐/nickel‐based hydroxides and oxides are also anticipated to have great potential applications in supercapacitors, electrocatalysis and other energy‐storage‐/‐conversion‐related fields.     

Reduced Graphene‐Wrapped MnO2 Nanowires Self‐Inserted with Co3O4 Nanocages: Remarkable Enhanced Performances for Lithium‐Ion Anode Applications

A simple synthetic approach for graphene‐templated nanostructured MnO₂ nanowires self‐inserted with Co₃O₄ nanocages is proposed in this work. The Co₃O₄ nanocages were penetrated in situ by MnO₂ nanowires. As an anode, the as‐obtained MnO₂–Co₃O₄–RGO composite exhibits remarkable enhanced performance compared with the MnO₂–RGO and Co₃O₄–RGO samples. The MnO₂–Co₃O₄–RGO electrode delivers a reversible capacity of up to 577.4 mA h g^?1 after 400 cycles at 500 mA g^?1 and the Coulombic efficiency of MnO₂–Co₃O₄–RGO is about 96 %.     

A Core–Shell Fe/Fe2O3 Nanowire as a High‐Performance Anode Material for Lithium‐Ion Batteries

The preparation of novel one‐dimensional core–shell Fe/Fe₂O₃ nanowires as anodes for high‐performance lithium‐ion batteries (LIBs) is reported. The nanowires are prepared in a facile synthetic process in aqueous solution under ambient conditions with subsequent annealing treatment that could tune the capacity for lithium storage. When this hybrid is used as an anode material for LIBs, the outer Fe₂O₃ shell can act as an electrochemically active material to store and release lithium ions, whereas the highly conductive and inactive Fe core functions as nothing more than an efficient electrical conducting pathway and a remarkable buffer to tolerate volume changes of the electrode materials during the insertion and extraction of lithium ions. The core–shell Fe/Fe₂O₃ nanowire maintains an excellent reversible capacity of over 767 mA h g^?1 at 500 mA g^?1 after 200 cycles with a high average Coulombic efficiency of 98.6 %. Even at 2000 mA g^?1, a stable capacity as high as 538 mA h g^?1 could be obtained. The unique composition and nanostructure of this electrode material contribute to this enhanced electrochemical performance. Due to the ease of large‐scale fabrication and superior electrochemical performance, these hybrid nanowires are promising anode materials for the next generation of high‐performance LIBs.     

MOF‐Derived Spinel NiCo2O4 Hollow Nanocages for the Construction of Non‐enzymatic Electrochemical Glucose Sensor

Non‐enzymatic glucose sensor is greatly expected to take over its enzymatic counterpart in the future. In this paper, we reported on a facile strategy to construct a non‐enzymatic glucose sensor by use of NiCo₂O₄ hollow nanocages (NiCo₂O₄ HNCs) as catalyst, which was derived from Co‐based zeolite imidazole frame (ZIF‐67). The NiCo₂O₄ HNCs modified glassy carbon electrode (NiCo₂O₄ HNCs/GCE), the key component of the glucose sensor, showed highly electrochemical catalytic activity towards the oxidation of glucose in alkaline media. As a result, the proposed non‐enzymatic glucose sensor afforded excellent analytical performances assessed with the aid of cyclic voltammetry and amperometry (i–t). A wide linear range spanning from 0.18 μΜ to 5.1 mM was achieved at the NiCo₂O₄ HNCs/GCE with a high sensitivity of 1306 μA mM^?1 cm^?2 and a fast response time of 1 s. The calculated limit of detection (LOD) of the sensor was as low as 27 nM (S/N=3). Furthermore, it was demonstrated that the non‐enzymatic glucose sensor showed considerable anti‐interference ability and excellent stability. The practical application of the sensor was also evaluated by determination of glucose levels in real serum samples.     

Heterostructural NiCo2O4 Nanocomposites for Nonenzymatic Electrochemical Glucose Sensing

Hierarchical nanocomposites consisting of NiCo₂O₄ nanorods and NiCo₂O₄ nanoparticles through a straightforward two-step hydrothermal process was employed as a working electrode to examine the electrochemical behavior of glucose. The NiCo₂O₄@NiCo₂O₄ heterostructures was confirmed by the scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray powder diffractometer (XRD), X-ray photoelectron spectroscopy (XPS) and electrochemistry analysis. Results indicated that glucose is electrochemically oxidized with improved sensitivity at the NiCo₂O₄@NiCo₂O₄ sensor, compared to NiCo₂O₄ sensors. Analytical parameters such as the optimal potential (0.45 V), linear range from 0.4 μM to 5.2 mM, limit of detection (1.1 μΜ) (S/N=3), stability and repeatability (2.7 %) demonstrate the suitability of the prepared sensor for glucose analysis. Moreover, the proposed sensor could be used for actual samples analysis in complex matrices.     

Green Template‐Free Synthesis of Hierarchical Shuttle‐Shaped Mesoporous ZnFe2O4 Microrods with Enhanced Lithium Storage for Advanced Li‐Ion Batteries

In the work, a facile and green two‐step synthetic strategy was purposefully developed to efficiently fabricate hierarchical shuttle‐shaped mesoporous ZnFe₂O₄ microrods (MRs) with a high tap density of ～0.85 g cm³, which were assembled by 1D nanofiber (NF) subunits, and further utilized as a long‐life anode for advanced Li‐ion batteries. The significant role of the mixed solvent of glycerin and water in the formation of such hierarchical mesoporous MRs was systematically investigated. After 488 cycles at a large current rate of 1000 mA g^?1, the resulting ZnFe₂O₄ MRs with high loading of ～1.4 mg per electrode still preserved a reversible capacity as large as ～542 mAh g^?1. Furthermore, an initial charge capacity of ～1150 mAh g^?1 is delivered by the ZnFe₂O₄ anode at 100 mA g^?1, resulting in a high Coulombic efficiency of ～76 % for the first cycle. The superior Li‐storage properties of the as‐obtained ZnFe₂O₄ were rationally associated with its mesoprous micro‐/nanostructures and 1D nanoscaled building blocks, which accelerated the electron transportation, facilitated Li⁺ transfer rate, buffered the large volume variations during repeated discharge/charge processes, and provided rich electrode–electrolyte sur‐/interfaces for efficient lithium storage, particularly at high rates.     

Hybrid nanonet/nanoflake NiCo2O4 electrodes with an ultrahigh surface area for supercapacitors

An integrated electrode consisting of hybrid nanonet/nanoflake NiCo₂O₄ grown on stainless steel mesh substrates exhibits a high specific capacitance while maintaining high-rate capability and good cycling stability. The specific capacitance reaches a maximum of 911 F g^?1 at a current density of 10 A g^?1, which can still retain 864 F g^?1 (94.8 % retention) after 10,000 cycles. These much-improved electrochemical performances are attributed to the unique architecture of NiCo₂O₄ electrode. The interconnected nanonet NiCo₂O₄ with an ultrahigh surface area significantly facilitates the rapid ion/electron transport and guarantees good mechanical adhesion, while the ultrathin nanoflakes further extend the active sites for fast redox reactions for efficient energy storage. Figure

Hybrid nanonet/nanoflake NiCo₂O₄ grown on stainless steel mesh exhibits superior capacitive performance and long-life stability as an integrated electrode for high-performance supercapacitors.     

钴酸镍纳米花/活性炭纤维复合物的制备和表征及其超级电容器性能

利用简单的水热法以及后续热处理, 将钴酸镍纳米花成功生长在活性炭纤维支架上. 场发射扫描电镜(FESEM)及透射电镜(TEM)结果表明, 纳米花是由纳米针自组装而成, 而纳米针呈多孔结构. 这种三维复合多级结构非常有利于电解质离子的渗透和电子的传输. 将该多孔钴酸镍纳米花/活性炭纤维布作为工作电极, 表现出优良的电容性能. 在1 A·g^-1时, 比电容高达1626 F·g^-1; 在10 A·g^-1时, 电容保持率为65%, 具有超高的电容值和优异的倍率特性.     

Mesoporous MnCo2O4 with a Flake‐Like Structure as Advanced Electrode Materials for Lithium‐Ion Batteries and Supercapacitors

A mesoporous flake‐like manganese‐cobalt composite oxide (MnCo₂O₄) is synthesized successfully through the hydrothermal method. The crystalline phase and morphology of the materials are characterized by X‐ray diffraction, field‐emission scanning electron microscopy, transmission electron microscopy, and Brunauer–Emmett–Teller methods. The flake‐like MnCo₂O₄ is evaluated as the anode material for lithium‐ion batteries. Owing to its mesoporous nature, it exhibits a high reversible capacity of 1066 mA h g^?1, good rate capability, and superior cycling stability. As an electrode material for supercapacitors, the flake‐like MnCo₂O₄ also demonstrates a high supercapacitance of 1487 F g^?1 at a current density of 1 A g^?1, and an exceptional cycling performance over 2000 charge/discharge cycles.