General Formation of MxCo3−xS4 (M=Ni,Mn, Zn) Hollow Tubular Structures for Hybrid Supercapacitors

A simple and versatile method for general synthesis of uniform one‐dimensional (1D) M_xCo_3?xS₄ (M=Ni, Mn, Zn) hollow tubular structures (HTSs), using soft polymeric nanofibers as a template, is described. Fibrous core–shell polymer@M‐Co acetate hydroxide precursors with a controllable molar ratio of M/Co are first prepared, followed by a sulfidation process to obtain core–shell polymer@M_xCo_3?xS₄ composite nanofibers. The as‐made M_xCo_3?xS₄ HTSs have a high surface area and exhibit exceptional electrochemical performance as electrode materials for hybrid supercapacitors. For example, the MnCo₂S₄ HTS electrode can deliver specific capacitance of 1094 F g^?1 at 10 A g^?1, and the cycling stability is remarkable, with only about 6 % loss over 20 000 cycles.     

Self‐Template Synthesis of Hybrid Porous Co3O4–CeO2 Hollow Polyhedrons for High‐Performance Supercapacitors

In this work, hybrid porous Co₃O₄–CeO₂ hollow polyhedrons have been successfully obtained via a simple cation‐exchange route followed by heat treatment. In the synthesis process, ZIF‐67 polyhedron frameworks are firstly prepared, which not only serve as a host for the exchanged Ce3⁺ ions but also act as the template for the synthesis of hybrid porous Co₃O₄–CeO₂ hollow polyhedrons. When utilized as electrode materials for supercapacitors, the hybrid porous Co₃O₄–CeO₂ hollow polyhedrons delivered a large specific capacitance of 1288.3 F g^?1 at 2.5 A g^?1 and a remarkable long lifespan cycling stability (<3.3 % loss after 6000 cycles). Furthermore, an asymmetric supercapacitor (ASC) device based on hybrid porous Co₃O₄–CeO₂ hollow polyhedrons was assembled. The ASC device possesses an energy density of 54.9 W h kg^?1, which can be retained to 44.2 W h kg^?1 even at a power density of 5100 W kg^?1, indicating its promising application in electrochemical energy storage. More importantly, we believe that the present route is a simple and versatile strategy for the preparation of other hybrid metal oxides with desired structures, chemical compositions and applications.     

Hermetically Coated and Well‐Separated Co3O4 Nanophase within Porous Graphitic Carbon Nanosheets: Synthesis,Confinement Effect,and Improved Lithium‐Storage Capacity and Durability

Considerable lithium‐driven volume changes and loss of crystallinity on cycling have impeded the sustainable use of transition metal oxides (MOs) as attractive anode materials for advanced lithium‐ion batteries that have almost six times the capacity of carbon per unit volume. Herein, Co₃O₄ was used as a model MO in a facile process involving two pyrolysis steps for in situ encapsulation of nanosized MO in porous two‐dimensional graphitic carbon nanosheets (2D‐GCNs) with high surface areas and abundant active sites to overcome the above‐mentioned problems. The proposed method is inexpensive, industrially scalable, and easy to operate with a high yield. TEM revealed that the encaged Co₃O₄ is well separated and uniformly dispersed with surrounding onionlike graphitic layers. By taking advantage of the high electronic conductivity and confinement effect of the surrounding 2D‐GCNs, a hierarchical GCNs‐coated Co₃O₄ (Co₃O₄@GCNs) anode with 43.5 wt % entrapped active nanoparticles delivered a remarkable initial specific capacity of 1816 mAh g^?1 at a current density of 100 mA g^?1. After 50 cycles, the retained capacity is as high as 987 mAh g^?1. When the current density was increased to 1000 mA g^?1, the anode showed a capacity retention of 416 mAh g^?1. Enhanced reversible rate capability and prolonged cycling stability were found for Co₃O₄@GCN compared to pure GCNs and Co₃O₄. The Co₃O₄@GCNs hybrid holds promise as an efficient candidate material for anodes due to its low cost, environmentally friendly nature, high capacity, and stability.     

Design of a Prussian Blue Analogue/Carbon Nanotube Thin‐Film Nanocomposite: Tailored Precursor Preparation,Synthesis, Characterization,and Application

Multi‐walled carbon nanotubes (MWCNTs) filled with different species of cobalt (metallic cobalt, cobalt oxide) were synthesized by a chemical vapor deposition method through cobaltocene pyrolysis. A systematic study was performed to correlate different experimental conditions with the structure and characteristics of the obtained material. Thin films of Co‐filled CNTs were deposited over conductive substrates through a liquid–liquid interfacial method and were used for cobalt hexacyanoferrate (CoHCFe) electrodeposition by an innovative route in which the Co species encapsulated in the CNTs were employed as reactants. The CNT/CoHCFe films were characterized by different spectroscopic, microscopic, and electrochemical techniques and presented high electrochemical stability in different media. The nanocomposites were applied as both an electrochemical sensor to H₂O₂ and a cathode for ion batteries and showed limits of detection at approximately 3.7 nmol L ^?1 and a capacity of 130 mAh g^?1 at a current density of 5 A g^?1.     

Porous ZnO/Co3O4/N‐doped carbon nanocages synthesized via pyrolysis of complex metal–organic framework (MOF) hybrids as an advanced lithium‐ion battery anode

Metal oxides have a large storage capacity when employed as anode materials for lithium‐ion batteries (LIBs). However, they often suffer from poor capacity retention due to their low electrical conductivity and huge volume variation during the charge–discharge process. To overcome these limitations, fabrication of metal oxides/carbon hybrids with hollow structures can be expected to further improve their electrochemical properties. Herein, ZnO‐Co₃O₄ nanocomposites embedded in N‐doped carbon (ZnO‐Co₃O₄@N‐C) nanocages with hollow dodecahedral shapes have been prepared successfully by the simple carbonizing and oxidizing of metal–organic frameworks (MOFs). Benefiting from the advantages of the structural features, i.e. the conductive N‐doped carbon coating, the porous structure of the nanocages and the synergistic effects of different components, the as‐prepared ZnO‐Co₃O₄@N‐C not only avoids particle aggregation and nanostructure cracking but also facilitates the transport of ions and electrons. As a result, the resultant ZnO‐Co₃O₄@N‐C shows a discharge capacity of 2373 mAh g^?1 at the first cycle and exhibits a retention capacity of 1305 mAh g^?1 even after 300 cycles at 0.1 A g^?1. In addition, a reversible capacity of 948 mAh g^?1 is obtained at a current density of 2 A g^?1, which delivers an excellent high‐rate cycle ability.     

Electrospun Hierarchical CaCo2O4 Nanofibers with Excellent Lithium Storage Properties

Hierarchical CaCo₂O₄ nanofibers (denoted as CCO‐NFs) with a unique hierarchical structure have been prepared by a facile electrospinning method and subsequent calcination in air. The as‐prepared CCO‐NFs are composed of well‐defined ultrathin nanoplates that arrange themselves in an oriented manner to form one‐dimensional (1D) hierarchical structures. The controllable formation process and possible formation mechanism are also discussed. Moreover, as a demonstration of the functional properties of such hierarchical architecture, the 1D hierarchical CCO‐NFs were investigated as materials for lithium‐ion batteries (LIBs) anode; they not only delivers a high reversible capacity of 650 mAh g^?1 at a current of 100 mA g^?1 and with 99.6 % capacity retention over 60 cycles, but they also show excellent rate capability with respect to counterpart nanoplates‐in‐nanofibers and nanoplates. The high specific surface areas as well as the unique feature of hierarchical structures are probably responsible for the enhanced electrochemical performance. Considering their facile preparation and good lithium storage properties, 1D hierarchical CCO‐NFs will hold promise in practical LIBs.     

Synthesis of 2D/2D Structured Mesoporous Co3O4 Nanosheet/N‐Doped Reduced Graphene Oxide Composites as a Highly Stable Negative Electrode for Lithium Battery Applications

Mesoporous Co₃O₄ nanosheets (Co₃O₄‐NS) and nitrogen‐doped reduced graphene oxide (N‐rGO) are synthesized by a facile hydrothermal approach, and the N‐rGO/Co₃O₄‐NS composite is formulated through an infiltration procedure. Eventually, the obtained composites are subjected to various characterization techniques, such as XRD, Raman spectroscopy, surface area analysis, X‐ray photoelectron spectroscopy (XPS), and TEM. The lithium‐storage properties of N‐rGO/Co₃O₄‐NS composites are evaluated in a half‐cell assembly to ascertain their suitability as a negative electrode for lithium‐ion battery applications. The 2D/2D nanostructured mesoporous N‐rGO/Co₃O₄‐NS composite delivered a reversible capacity of about 1305 and 1501 mAh g^?1 at a current density of 80 mA g^?1 for the 1st and 50th cycles, respectively. Furthermore, excellent cyclability, rate capability, and capacity retention characteristics are noted for the N‐rGO/Co₃O₄‐NS composite. This improved performance is mainly related to the existence of mesoporosity and a sheet‐like 2D hierarchical morphology, which translates into extra space for lithium storage and a reduced electron pathway. Also, the presence of N‐rGO and carbon shells in Co₃O₄‐NS should not be excluded from such exceptional performance, which serves as a reliable conductive channel for electrons and act as synergistically to accommodate volume expansion upon redox reactions. Ex‐situ TEM, impedance spectroscopy, and XPS, are also conducted to corroborate the significance of the 2D morphology towards sustained lithium storage.     

General Formation of MxCo3−xS4 (M=Ni,Mn, Zn) Hollow Tubular Structures for Hybrid Supercapacitors

A simple and versatile method for general synthesis of uniform one‐dimensional (1D) M_xCo_3−xS₄ (M=Ni, Mn, Zn) hollow tubular structures (HTSs), using soft polymeric nanofibers as a template, is described. Fibrous core–shell polymer@M‐Co acetate hydroxide precursors with a controllable molar ratio of M/Co are first prepared, followed by a sulfidation process to obtain core–shell polymer@M_xCo_3−xS₄ composite nanofibers. The as‐made M_xCo_3−xS₄ HTSs have a high surface area and exhibit exceptional electrochemical performance as electrode materials for hybrid supercapacitors. For example, the MnCo₂S₄ HTS electrode can deliver specific capacitance of 1094 F g⁻¹ at 10 A g⁻¹, and the cycling stability is remarkable, with only about 6 % loss over 20 000 cycles.     

Superior Electrochemical Properties of Nanofibers Composed of Hollow CoFe2O4 Nanospheres Covered with Onion‐Like Graphitic Carbon

Nanofibers composed of hollow CoFe₂O₄ nanospheres covered with onion‐like carbon are prepared by applying nanoscale Kirkendall diffusion to the electrospinning process. Amorphous carbon nanofibers embedded with CoFe₂@onion‐like carbon nanospheres are prepared by reduction of the electrospun nanofibers. Oxidation of the CoFe₂‐C nanofibers at 300 °C under a normal atmosphere produces porous nanofibers composed of hollow CoFe₂O₄ nanospheres covered with onion‐like carbon. CoFe₂ nanocrystals are transformed into the hollow CoFe₂O₄ nanospheres during oxidation through a well‐known nanoscale Kirkendall diffusion process. The discharge capacities of the carbon‐free CoFe₂O₄ nanofibers composed of hollow nanospheres and the nanofibers composed of hollow CoFe₂O₄ nanospheres covered with onion‐like carbon are 340 and 930 mA h g^?1, respectively, for the 1000th cycle at a current density of 1 A g^?1. The nanofibers composed of hollow CoFe₂O₄ nanospheres covered with onion‐like carbon exhibit an excellent rate performance even in the absence of conductive materials.     

A Multi‐Wall Sn/SnO2@Carbon Hollow Nanofiber Anode Material for High‐Rate and Long‐Life Lithium‐Ion Batteries

Multi‐wall Sn/SnO₂@carbon hollow nanofibers evolved from SnO₂ nanofibers are designed and programable synthesized by electrospinning, polypyrrole coating, and annealing reduction. The synthesized hollow nanofibers have a special wire‐in‐double‐wall‐tube structure with larger specific surface area and abundant inner spaces, which can provide effective contacting area of electrolyte with electrode materials and more active sites for redox reaction. It shows excellent cycling stability by virtue of effectively alleviating pulverization of tin‐based electrode materials caused by volume expansion. Even after 2000 cycles, the wire‐in‐double‐wall‐tube Sn/SnO₂@carbon nanofibers exhibit a high specific capacity of 986.3 mAh g^?1 (1 A g^?1) and still maintains 508.2 mAh g^?1 at high current density of 5 A g^?1. This outstanding electrochemical performance suggests the multi‐wall Sn/SnO₂@ carbon hollow nanofibers are great promising for high performance energy storage systems.     

Construction of CoO/Co‐Cu‐S Hierarchical Tubular Heterostructures for Hybrid Supercapacitors

Hierarchical hollow structures for electrode materials of supercapacitors could enlarge the surface area, accelerate the transport of ions and electrons, and accommodate volume expansion during cycling. Besides, construction of heterostructures would enhance the internal electric fields to regulate the electronic structures. All these features of hierarchical hollow heterostructures are beneficial for promoting the electrochemical properties and stability of electrode materials for high‐performance supercapacitors. Herein, CoO/Co‐Cu‐S hierarchical tubular heterostructures (HTHSs) composed of nanoneedles are prepared by an efficient multi‐step approach. The optimized sample exhibits a high specific capacity of 320 mAh g^?1 (2300 F g^?1) at 2.0 A g^?1 and outstanding cycling stability with 96.5 % of the initial capacity retained after 5000 cycles at 10 A g^?1. Moreover, an all‐solid‐state hybrid supercapacitor (HSC) constructed with the CoO/Co‐Cu‐S and actived carbon shows a stable and high energy density of 90.7 Wh kg^?1 at a power density of 800 W kg^?1.     

Hierarchical Nanoboxes Composed of Co9S8−MoS2 Nanosheets as Efficient Electrocatalysts for the Hydrogen Evolution Reaction

The development of hydrogen evolution catalysts based on nonprecious metals is essential for the practical application of water‐splitting devices. Herein, the synthesis of Co₉S₈?MoS₂ hierarchical nanoboxes (HNBs) as efficient catalysts for the hydrogen evolution reaction (HER) is reported. The surface of the hollow cubic structure was organized by CoMoS₄ nanosheets formed through the reaction of MoS₄^2? and Co²⁺ released from the cobalt zeolite imidazole framework (ZIF‐67) templates under reflux in a mixture of water/ethanol. The formation process for the CoMoS₄ HNB structures was characterized by TEM images recorded at various reaction temperatures. The amorphous CoMoS₄ HNBs were converted through sequential heat treatments into CoS_x?MoS₂ and Co₉S₈?MoS₂ HNBs. Owing to their unique chemical compositions and structural features, Co₉S₈?MoS₂ HNBs have a high specific surface area (124.6 m² g^?1) and superior electrocatalytic performances for the HER. The Co₉S₈?MoS₂ HNBs exhibit a low overpotential (η₁₀) of 106 mV, a low Tafel slope of 51.8 mV dec^?1, and long‐term stability in an acidic medium. The electrocatalytic activity of Co₉S₈?MoS₂ HNBs is superior to that of recently reported values, and these HNBs prove to be promising candidates for the HER.     

Meso-macroporous Co3O4 electrode prepared by polystyrene spheres and carbowax templates for supercapacitors

Meso-macroporous Co₃O₄ electrode is synthesized by drop coating with a mixed solution containing Co(OH)₂ colloid, polystyrene spheres, and carbowax (namely polyethylene glycol), followed by calcining at 400?°C to remove polystyrene spheres and carbowax. For comparison, nonporous Co₃O₄ and mesoporous Co₃O₄ electrodes are prepared by drop coating with Co(OH)₂ colloid and with a mixed solution containing Co(OH)₂ colloid and carbowax under the same condition, respectively. Capacitive property of these electrodes is measured by cyclic voltammetry, potentiometry and electrochemical impedance spectroscopy. The results show that meso-macroporous Co₃O₄ electrode exhibits larger specific capacitance than those of nonporous Co₃O₄ electrode and mesoporous Co₃O₄ electrode at various current densities. The specific capacitance of meso-macroporous Co₃O₄ electrode at the current density of 0.2?A?g^?1 is 453?F?g^?1. Meanwhile, meso-macroporous Co₃O₄ electrode possesses the highest specific capacitance retention ratio at the current density ranging from 0.2 to 1.0?A?g^?1, indicating that meso-macroporous Co₃O₄ electrode suits to high-rate charge?Cdischarge.     

Flame Spray Pyrolysis for Finding Multicomponent Nanomaterials with Superior Electrochemical Properties in the CoOx‐FeOx System for Use in Lithium‐Ion Batteries

High‐temperature flame spray pyrolysis is employed for finding highly efficient nanomaterials for use in lithium‐ion batteries. CoO_x‐FeO_x nanopowders with various compositions are prepared by one‐pot high‐temperature flame spray pyrolysis. The Co and Fe components are uniformly distributed over the CoO_x‐FeO_x composite powders, irrespective of the Co/Fe mole ratio. The Co‐rich CoO_x‐FeO_x composite powders with Co/Fe mole ratios of 3:1 and 2:1 have mixed crystal structures with CoFe₂O₄ and Co₃O₄ phases. However, Co‐substituted magnetite composite powders prepared from spray solutions with Co and Fe components in mole ratios of 1:3, 1:2, and 1:1 have a single phase. Multicomponent CoO_x‐FeO_x powders with a Co/Fe mole ratio of 2:1 and a mixed crystal structure with Co₃O₄ and CoFe₂O₄ phases show high initial capacities and good cycling performance. The stable reversible discharge capacities of the composite powders with a Co/Fe mole ratio of 2:1 decrease from 1165 to 820 mA h g^?1 as the current density is increased from 500 to 5000 mA g^?1; however, the discharge capacity again increases to 1310 mA h g^?1 as the current density is restored to 500 mA g^?1.     

Coralloid and hierarchical Co<Subscript>3</Subscript>O<Subscript>4</Subscript> nanostructures used as supercapacitors with good cycling stability

Coralloid and hierarchical Co₃O₄ nanostructures were synthesized by a facile two-step approach composed of room temperature solution-phase synthesis without any surfactant and calcination of precursor. Owing to the unique structural features, the capacitance of Co₃O₄ could reach up to 591 F g^?1 at a current density of 0.5 A g^?1. Especially the cycling stability remained about 97 % after 2000 cycles at a current density of 1 A g^?1. These results demonstrated that the coralloid and hierarchical Co₃O₄ were excellent candidates for electrochemical supercapacitor devices.     

Cobalt pyrophosphate nano/microstructures as promising electrode materials of supercapacitor

Cobalt pyrophosphate (Co₂P₂O₇) nano/microstructures (oblong plate, microplate, microflower, and hierarchical architectures) have been successfully synthesized through calcination of NH₄CoPO₄·H₂O nano/microstructures. More importantly, supercapacitive performances of Co₂P₂O₇ nano/microstructures were studied using cyclic voltammetry, galvanostatic charge/discharge measurements, and electrochemical impedance spectroscopy methods in 3.0 M KOH solution. These results show that Co₂P₂O₇ hierarchical architecture electrodes exhibit high specific capacitance of 367 F?g^?1 at current density of 0.625 A?g^?1 in aqueous KOH solution. Co₂P₂O₇ hierarchical architecture electrodes remain 96.2 % of the initial specific capacitance after 3,000 charge/discharge cycles.     

CNT@Fe3O4@C Coaxial Nanocables: One‐Pot,Additive‐Free Synthesis and Remarkable Lithium Storage Behavior

By using carbon nanotubes (CNTs) as a shape template and glucose as a carbon precursor and structure‐directing agent, CNT@Fe₃O₄@C porous core/sheath coaxial nanocables have been synthesized by a simple one‐pot hydrothermal process. Neither a surfactant/ligand nor a CNT pretreatment is needed in the synthetic process. A possible growth mechanism governing the formation of this nanostructure is discussed. When used as an anode material of lithium‐ion batteries, the CNT@Fe₃O₄@C nanocables show significantly enhanced cycling performance, high rate capability, and high Coulombic efficiency compared with pure Fe₂O₃ particles and Fe₃O₄/CNT composites. The CNT@Fe₃O₄@C nanocables deliver a reversible capacity of 1290 mA h g^?1 after 80 cycles at a current density of 200 mA g^?1, and maintain a reversible capacity of 690 mA h g^?1 after 200 cycles at a current density of 2000 mA g^?1. The improved lithium storage behavior can be attributed to the synergistic effect of the high electronic conductivity support and the inner CNT/outer carbon buffering matrix.     

Preparation,characterization, and evaluation of LiNi0.4Co0.6O2 nanofibers for supercapacitor applications

We prepared LiNi_0.4Co_0.6O₂ nanofibers by electrospinning at the calcination temperature of 450 °C for 6 h. The prepared LiNi_0.4Co_0.6O₂ nanofibers was characterized by thermal, X-ray diffraction, and Fourier transform infrared (FTIR) studies. The morphology of LiNi_0.4Co_0.6O₂ nanofibers was characterized by scanning electron microscopy studies. The asymmetric supercapacitor was fabricated using LiNi_0.4Co_0.6O₂ nanofibers as positive electrode and activated carbon (AC) as negative electrode and a porous polypropylene separator in 1 M LiPF₆–ethylene carbonate/dimethyl carbonate (LiPF₆–EC:DMC) (1:1?v/v) as electrolyte. Cyclic voltammetry studies were then carried out in the potential range of 0 to 3.0 V at different scan rates which exhibited the highest specific capacitance of 72.9 F g^?1. The electrochemical impedance measurements were carried out to find the charge transfer resistance and specific capacitance of the cell, and they were found to be 5.05 Ω and 67.4 F g^?1, respectively. Finally, the charge–discharge studies were carried out at a current density of 1 mA cm^?2 to find out the discharge-specific capacitance, energy density, and power density of the capacitor cell, and they were found to be 70.9 F g^?1, 180.2 Wh kg^?1, and 248.0 W kg^?1, respectively.     

Triple‐Shelled Manganese–Cobalt Oxide Hollow Dodecahedra with Highly Enhanced Performance for Rechargeable Alkaline Batteries

Precisely carving of multi‐shelled manganese–cobalt oxide hollow dodecahedra (Co/Mn‐HD) with shell number up to three is achieved by a controlled calcination of the Mn‐doped zeolitic imidazolate framework ZIF‐67 precursor (Co/Mn‐ZIF). The unique multi‐shelled and polycrystalline structure not only provides a very large electrochemically active surface area (EASA), but also enhances the structural stability of the material. The residual C and N in the final structures might aid stability and increase their conductivity. When used in alkaline rechargeable battery, the triple‐shelled Co/Mn‐HD exhibits high electrochemical performance, reversible capacity (331.94 mAh g^?1 at 1 Ag^?1), rate performance (88 % of the capacity can be retained with a 20‐fold increase in current density), and cycling stability (96 % retention over 2000 cycles).     

Supercritical CO2-driven, periodic patterning on one-dimensionals carbon nanomaterials

One-dimensional carbon nano-materials, in particular carbon nanotubes (CNTs) and carbon nanofibers (CNFs), are of scientific and technological interest due to their satisfactory properties and ability to serve as templates for directed assembly. In this work, linear high density polyethylene (PE) was periodically decorated on CNTs and CNFs using a supercritical carbon dioxide (scCO₂)antisolvent-induced polymer epitaxy (SAIPE) method, leading to nano-hybrid shish-kebab (NHSK) structures. The formation mechanism of different morphologies of PE lamellae on CNTs and CNFs has been discussed. Palladium nanoparticles were synthesized and immobilized on the PE/CNF NHSK structure with the assistance of scCO₂. The obtained hierarchical nano-hybrid architecture may find applications in microfabrication and other related fields.