Straightforward synthesis of hierarchical Co3O4@CoWO4/rGO core–shell arrays on Ni as hybrid electrodes for asymmetric supercapacitors

Hierarchical Co₃O₄@CoWO₄/rGO core/shell nanoneedles arrays are successfully grown on 3D nickel foam using a simple, effective method. By virtue of its unique structure, Co₃O₄@CoWO₄/rGO demonstrates an enhanced specific capacitance of 386 F g⁻¹ at 0.5 A g⁻¹ current density. It can be used as an integrated, additive-free electrode for supercapacitors that boasts excellent performance. As illustration, we assemble an asymmetric supercapacitor (ASC) using the as-prepared Co₃O₄@CoWO₄/rGO as the positive electrode and activated carbon as the negative electrode. The optimized ASC displays a maximum energy density of 19.99 Wh kg⁻¹ at a power density of 321 W kg⁻¹. Furthermore, the ASC also presents a remarkably long cycle life along with 88.8% specific capacitance retention after 5000 cycles.     

Facile synthesis of three dimensional flower-like Co3O4@MnO2 core-shell microspheres as high-performance electrode materials for supercapacitors

In this work, we reported the synthesis of three dimensional flower-like Co₃O₄@MnO₂ core-shell microspheres by a controllable two-step reaction. Flower-like Co₃O₄ microspheres cores were firstly built from the self-assembly of Co₃O₄ nanosheets, on which MnO₂ nanosheets shells were subsequently grown through the hydrothermal decomposition of KMnO₄. The MnO₂ nanosheets shells were found to increase the electrochemical active sites and allow faster redox reaction kinetics. Based on these advantages, when used as an electrode for supercapacitors, the prepared flower-like Co₃O₄@MnO₂ core-shell composite electrode demonstrated a significantly enhanced specific capacitance (671 F g⁻¹ at 1 A g⁻¹) as well as improved rate capability (84% retention at 10 A g⁻¹) compared with the pristine flower-like Co₃O₄ electrode. Moreover, the optimized asymmetric supercapacitor device based on the flower-like Co₃O₄@MnO₂//active carbon exhibited a high energy density of 34.1 W h kg⁻¹ at a power density of 750 W kg⁻¹, meaning its great potential application for energy storage devices.     

Facile synthesis of NiMn2O4 nanosheet arrays grown on nickel foam as novel electrode materials for high-performance supercapacitors

Nanostructured spinel NiMn₂O₄ arrays have been fabricated by a facile hydrothermal approach and further investigated as binder-free electrode for high-performance supercapacitors. Compared with Mn₃O₄, NiMn₂O₄ exhibited higher specific capacitances (662.5 F g⁻¹ and 370.5 F g⁻¹ in different electrolytes at the current density of 1 A g⁻¹) and excellent cycling stability (~96% capacitance retention after 1000 cycles) in a three-electrode system. Such a novel microstructure grown directly on the conductive substrate provided sufficient active sites for redox reaction resulting in their enhanced electrochemical behaviors. Their improved performances suggested that ultrathin sheet-like NiMn₂O₄ arrays on Ni foam substrate were a promising electrode material for supercapacitors.     

3D network-like porous MnCo2O4 by the sucrose-assisted combustion method for high-performance supercapacitors

3D network-like porous MnCo₂O₄ nanostructures have been successfully fabricated through a facile and scalable sucrose-assisted combustion route followed by calcination treatment. Benefiting from its advantages of the unique 3D network-like architectures with large specific surface area (216.15 m² g⁻¹), abundant mesoporosity (2–50 nm) and high electronic conductivity, the as-prepared MnCo₂O₄ electrode displays a high specific capacitance of 647.42 F g⁻¹ at a current density of 1 A g⁻¹, remarkable capacitance retention rate of 70.67% at current density of 10 A g⁻¹ compared with 1 A g⁻¹, and excellent cycle stability (only 6.32% loss after 3000 cycles). The excellent electrochemical performances coupled with facile and cost effective method will render the as-fabricated 3D network-like porous MnCo₂O₄ as a promising electrode material for supercapacitors.     

Design and synthesis of three-dimensional needle-like CoNi2S4/CNT/graphene nanocomposite with improved electrochemical properties

In this paper, we described a simple two–step method for preparing needle-like CoNi₂S₄/CNT/graphene nanocomposite with robust connection among its ternary components. The prepared CoNi₂S₄/CNT/graphene nanocomposite has been thoroughly characterized by spectroscopic (Fourier-transform infrared spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy), X-ray diffraction and thermogravimetric analysis. Microscopy techniques (scanning electron microscopy–energy dispersive spectroscopy and transmission electron microscopy) were employed to probe the morphological structures. The electrochemical properties of the as-prepared 3D architectures were investigated with three and two-electrode systems. In addition to its high specific capacitance (710 F g⁻¹ at 20 A g⁻¹), after charging–discharging for 2000 cycles, the electrode still maintained the capacity retention of about 82%. When used as the active electrode material for supercapacitors, the fabricated CoNi₂S₄–g–CNT nanostructure exhibited excellent specific capacitance and good rate capability, making it a promising candidate for next-generation supercapacitors.     

Fabrication of hollow N-doped carbon supported ultrathin NiO nanosheets for high-performance supercapacitor

The polydopamine-assisted hierarchical composites of ultrathin NiO nanosheets uniformly coating on the surface of hollow nitrogen-doped carbon spheres (HNCS-NiO) were successfully fabricated via a facile synthesis method. The hierarchical HNCS-NiO composites as electrode materials for supercapacitors exhibit high capacitance of 550.4 F g^− 1 (880.6 mF cm^− 2) at the current density of 0.5 A g^− 1 (0.8 mA cm^− 2), and present a good rate capability. The composites display excellent improved electrochemical properties not only because their hierarchical hollow nanostructures can provide enough space to buffer the volume expansion during the reversible intercalation/deintercalation processes, but also because their larger specific surface areas can provide adequate active sites for the redox electrochemical reaction.     

Synthesis of hierarchical carbon sphere@NiMoO4 composite materials for supercapacitor electrodes

In this work, hierarchical Carbon sphere@NiMoO₄ (C@NiMoO₄) composite was successfully synthesized by cost-effective two-step hydrothermal method. The samples were characterized by scanning electron microscopy, X-ray photoelectron spectroscopy, X-ray diffraction analysis and Thermogravimetric analysis. The Electrochemical measurement demonstrated that hierarchical C@NiMoO₄ electrode materials exhibited good specific capacitance (Csp) of 268.8 F g⁻¹ at a current density of 1 A g⁻¹ in 2 M NaOH aqueous electrolyte solution, as well as good cycling stability (88.4% retention after 2000 cycles). Compared to pure NiMoO₄, the excellent capacitive properties and stability suggest that the hierarchical structure C@NiMoO₄ could be promising electroactive material for supercapacitors.     

Construction of hierarchical NiMoO4@MnO2 nanosheet arrays on titanium mesh for supercapacitor electrodes

Hierarchical NiMoO₄@MnO₂ nanosheet arrays supported on titanium mesh are synthesized by cost effective hydrothermal methods for binder-free electrode. High specific area of porous MnO₂nanosheets and exceptionally high pseudocapacitive behavior of NiMoO₄nanosheets lead to a specific capacitance of 976 F g⁻¹at a current density of 1 A g⁻¹ with pleasurable rate characteristic in three electrode configuration. The excellent electrochemical performances of the integrated electrode can be ascribed to the unique core-shell nanostructure and synergic interaction. It is believed that the hierarchical NiMoO₄@MnO₂ nanosheet arrays supported on titanium mesh can provide great prospect for energy storage applications.     

High performance wire-shaped supercapacitive electrodes based onactivated carbon fibers core/manganese dioxide shell structures

Micro/nano hierarchical structures with uniformly patterned nanostructures shell and activated internal core are promising for boosting electrochemical performance. Here we report the fabrication of wire-shaped supercapacitive electrodes with manganese dioxide (MnO₂) nanostructures shell integrated onto activated carbon fiber (ACF) core. The ACF core is doped with nitrogen heteroatom and shows good conductivity and hydrophilicity, which endow fast ion and electron transport and high accessibility of electrolyte. The MnO₂ nanostructures shell integrated on the ACF core by electrodeposition method together provide significant pseudocapacitive contribution associated with fast faradaic reactions. The electrochemical performance of the fabricated electrodes was evaluated by cyclic voltammetry, galvanostatic charging/discharging and electrochemical impedance spectroscopy techniques. The integrated wire-shaped electrodes, which boost the synergetic effect of MnO₂ nanostructures and ACF, have excellent current collecting capabilities thus resulting high electrochemical performance (with the specific capacitance of 26.64 mF cm⁻¹ at the current density of 0.1 mA cm⁻¹ and 96% capacitance retention after 8000 charging/discharging cycles at the current density of 1 mA cm⁻¹).     

Improved activity of a graphene–TiO2 hybrid electrode in an electrochemical supercapacitor

We present a simple and fast approach for the synthesis of a graphene–TiO₂ hybrid nanostructure using a microwave-assisted technique. The microstructure, composition, and morphology were characterized by X-ray diffraction, Fourier-transform infrared spectroscopy, Raman microscopy, X-ray photoelectron spectroscopy, and field-emission scanning electron microscopy. The electrochemical properties were evaluated using cyclic voltammetry, electrochemical impedance spectroscopy, and galvanostatic charge–discharge tests. Structural analysis revealed a homogeneous distribution of nanosized TiO₂ particles on graphene nanosheets. The material exhibited a high specific capacitance of 165 F g⁻¹ at a scan rate of 5 mV s⁻¹ in 1 M Na₂SO₄ electrolyte solution. Theenhanced supercapacitance property of these materials could be ascribed to the increased conductivity of TiO₂ and better utilization of graphene. Moreover, the material exhibited long-term cycle stability, retaining ∼90% specific capacitance after 5000 cycles, which suggests that it has potential as an electrode material for high-performance electrochemical supercapacitors.     

Ni@NiCo2O4 core/shells composite as electrode material for supercapacitor

A novel Ni@NiCo₂O₄ core/shells structure consisting of the Ni microspheres skeletons and nanosheet-like NiCo₂O₄ skins was designed and investigated as the electrochemical electrode for supercapacitor. Due to the unique architecture with Ni microspheres as the highly conductive cores improving the electrical conductivity of electrode and external nanosheet-like NiCo₂O₄ shells as the efficient electrochemical active materials facilitating the contact between the electrode and electrolyte, the as-prepared Ni@NiCo₂O₄ exhibited excellent electrochemical performance with high specific capacity of 597 F g⁻¹ (1 A g⁻¹) as well as remarkable capacitance retention of 96% (3000 cycles). These impressive results pave the way to design high-performance electrode materials for energy storage.     

Amaryllis-like NiCo2S4 nanoflowers for high-performance flexible carbon-fiber-based solid-state supercapacitor

Low-cost dynamic materials for Faradaic redox reactions are needed for high-energy storage supercapacitors. A simple and cost-effective hydrothermal process was employed to synthesize amaryllis-like NiCo₂S₄ nanoflowers. The sample was characterized by X-ray powder diffraction, Brunauer–Emmett–Teller method, scanning electron microscopy, and transmission electron microscopy. NiCo₂S₄ nanoflowers were coated onto carbon fiber fabric and used as a binder-free electrode to fabricate a solid-state supercapacitor compact device. The solid-state supercapacitor exhibited excellent electrochemical performance, including high specific capacitance of 360 F g⁻¹ at scan rate of 5 mV s⁻¹ and high energy density of 25 W h kg⁻¹ at power density of 168 W kg⁻¹. In addition, the supercapacitor possessed high flexibility and good stability by retaining 90% capacitance after 5000 cycles. The high conductivity and Faradic-redox activity of NiCo₂S₄ nanoflowers resulted in high specific energy and power. Thus, NiCo₂S₄ nanoflowers are promising pseudocapacitive materials for low-cost and lightweight solid-state supercapacitors.     

Chemical synthesis of Co and Mn co-doped NiO nanocrystalline materials as high-performance electrode materials for potential application in supercapacitors

Co and Mn co-doped with NiO nanostructued materials, such as, Ni_0.95Co_0.01Mn_0.04O_1−δ, Ni_0.95Co_0.04Mn_0.01O_1−δ and Ni_0.95Co_0.025Mn_0.025O_1−δ were synthesized by chemical synthesis route and studied for potential application as electrode materials for supercapacitors. The phase structure of the materials was characterized by X-ray diffraction (XRD) and the crystallographic parameters were found out and reported. FTIR (Fourier Transform Infrared) spectroscopy revealed the presence of M–O bond in the compounds. The particle size of the materials was found to be in the range of 291.5–336.5 nm. The morphological phenomenon of the materials was studied by scanning electron microscopy (SEM) and the particles were found to be in spherical shape with average grain size of 14–28 nm. EDAX analysis confirmed the presence of appropriate levels of elements in the samples. The in-depth morphological characteristics were also studied by HR-TEM (High Resolution Tunneling Electron Microscopy). Cyclic voltammetry, chronopotentiometry and electrochemical impedance measurements were applied in an aqueous electrolyte (6 mol L⁻¹ KOH) to investigate the electrochemical performance of the Co and Mn co-doped NiO nanostructured electrode materials. The results indicate that the doping level of Co and Mn in NiO had a significant role in revealing the capacitive behaviors of the materials. Among the three electrode materials studied, Ni_0.95Co_0.025Mn_0.025O_1−δ electrode material shows a maximum specific capacitance of 673.33 F g⁻¹ at a current density of 0.5 A g⁻¹. The electrochemical characteristics of blank graphite sheet were studied and compared with the performance of Co/Mn co-doped NiO based electrode materials. Also, Ni_0.95Co_0.025Mn_0.025O_1−δ has resulted in a degradation level of 4.76% only after 1000 continuous cycles, which shows its excellent electrochemical performance, indicating a kind of potential candidate for supercapacitors.     

Design and self-assembly of metal-organic framework-derived porous Co3O4 hierarchical structures for lithium-ion batteries

Porous metal oxides hierarchical structures with controlled morphologies have received great attention because of their promising applications in catalysis, energy storage, gas sensing, etc. Porous Co₃O₄ hierarchical structures with controlled morphologies were synthesized on the basis of a pyrolytic conversion of Co-based metal-organic frameworks (Co-MOFs), which were initially grown in solutions containing Co(NO₃)₂·6H₂O, 1,3,5-Benzentricarboxylic acid and pyrazine as solute and N,N-dimethylformamide (DMF) as solvent under a solvothermal condition. Porous Co₃O₄ with twin hemispherical and flower-like structures were obtained with the assistance of PVP by adjusting the amount of pyrazine. The results of nitrogen adsorption–desorption indicate the BET surface area (22.6 m² g⁻¹) of twin hemispherical Co₃O₄ structures is lower than that (33.3 m² g⁻¹) of flower-like Co₃O₄ structures. However, the pore size of twin hemispherical Co₃O₄ structures is smaller, which is centered at about 2.5, 4.0 and 20.0 nm. The Co₃O₄ with twin hemispherical structures exhibit more excellent electrochemical performance as anode materials for lithium ion batteries than that of flower-like Co₃O₄ structures, which may be attributed to the smaller particle size and compact porous structures with suitable pore size.     

Facile synthesis of porous hollow Co3O4 microfibers derived-from metal-organic frameworks as an advanced anode for lithium ion batteries

Co₃O₄, as a promising anode material for the next generation lithium ion batteries to replace graphite, displays high theoretical capacity (890 mAh g⁻¹) and excellent electrochemical properties. However, the drawbacks of its poor cycle performance caused by large volume changes during charge-discharge process and low initial coulombic efficiency due to large irreversible reaction impede its practical application. Herein, we have developed a porous hollow Co₃O₄ microfiber with 500 nm diameter and 60 nm wall thickness synthesized via a facile chemical precipitation method with subsequent thermal decomposition. As an advanced anode for lithium ion batteries, the porous hollow Co₃O₄ microfibers deliver an obviously enhanced electrochemical property in terms of lithium storage capacity (1177.4 mA h g⁻¹ at 100 mA g⁻¹), initial coulombic efficiency (82.9%) and cycle performance (76.6% capacity retention at 200th cycle). This enhancement could be attributed to the well-designed microstructure of porous hollow Co₃O₄ microfibers, which could increase the contact surface area between electrolyte and active materials and accommodate the volume variations via additional void space during cycling.     

Integration of tailored reduced graphene oxide nanosheets and electrospun polyamide-66 nanofabrics for a flexible supercapacitor with high-volume- and high-area-specific capacitance

Nanofiber fabric is firstly introduced to replace common microfiber fabrics as the platform for flexible supercapacitors. Nanofiber and microfiber electrodes can be simply fabricated using a dipping process that impregnates reduced graphene oxide (RGO) nanosheets into electrospun polyamide-66 (PA66) nanofiber and microfiber fabrics. RGO nanosheets are tailored to various sizes and only RGO with a medium diameter of 250–450 nm (denoted as M-RGO) can effectively penetrate the pores of nanofiber fabrics for constructing smooth conductive paths within PA66 nanofiber fabrics. The synergistic effect between suitable sizes of RGO nanosheets and nanofiber fabrics with a high specific area provides a symmetric supercapacitor composed of M-RGO/PA66 nanofiber fabric electrodes with high-volume and high-area specific capacitance (C_S,V and C_S,A, equal to 38.79 F cm⁻³ and 0.931 F cm⁻² at 0.5 A g⁻¹, respectively), which are much larger than that of a symmetric supercapacitor composed of RGO/PA66 microfiber fabric electrodes (8.52 F cm⁻³ and 0.213 F cm⁻² at 0.5 A g⁻¹). The effect of impregnating nanofiber fabrics with suitably sized RGO to promote C_S,V and C_S,A of flexible supercapacitors has been demonstrated.     

High performance MnO2 nanoflower supercapacitor electrode by electrochemical recycling of spent batteries

MnO₂ nanoflower is prepared by electrochemical conversion of Mn₃O₄ obtained by heat treatment of spent zinc‒carbon batteries cathode powder. The heat treated and converted powders were characterized by TGA, XRD, FTIR, FESEM and TEM techniques. XRD analyses show formation of Mn₃O₄ and MnO₂ phases for the heat treated and converted powders, respectively. FESEM images indicate the formation of porous nanoflower structure of MnO_2, while, condensed aggregated particles are obtained for Mn₃O₄. The energy band gap of MnO₂ is obtained from UV‒Vis spectra to be 2.4 eV. The electrochemical properties are investigated using cyclic voltammetry, galvanostatic charge‒discharge and electrochemical impedance techniques using three-electrode system. The specific capacitance of MnO₂ nanoflower (309 F g⁻¹ at 0.1 A g⁻¹) is around six times higher than those obtained from the heat treated one (54 F g⁻¹ at 0.1 A g⁻¹). Moreover, it has high capacitance retention up to 93% over 1650 cycles. Impedance spectra of MnO₂ nanoflower show very small resistances and high electrochemical active surface area (340 m² g⁻¹). The present work demonstrates a novel electrochemical approach to recycle spent zinc-carbon batteries into high value supercapacitor electrode.     

Preparation and capacitance performance of nitrided lithium titanate nanoarrays

Nitrided lithium titanate (N-Li₄Ti₅O₁₂) nanoarrays with nanowire and nanotube structures were designed as the electrode materials of lithium-ion supercapacitor for electrochemical energy storage. Two types of TiO₂ nanoarrays were used as the precursor which involved TiO₂ nanowire array prepared by hydrothermal process and TiO₂ nanotube array prepared by anodization process. Li₄Ti₅O₁₂ nanoarrays were formed through hydrothermal reaction or sonochemical reaction of TiO₂ nanoarrays with lithium hydroxide and then calcination treatment process. Finally, N-Li₄Ti₅O₁₂ nanoarrays were formed through nitriding treatment of Li₄Ti₅O₁₂ using ammonia as nitrogen source. The electroactive N-Li₄Ti₅O₁₂ nanowire array and nanotube array exhibited the specific capacitance of 607.2 F g⁻¹ and 814.4 F g⁻¹ at a current density of 1 A g⁻¹, respectively. The corresponding capacitance retention was determined to be 92.1% and 94.2% after 1000 cycles at high current density of 5 A g⁻¹. The corresponding capacitance still kept 182.9 and 352.1 F g⁻¹ at much higher current density of 20 A g⁻¹, presenting reasonable rate capability for N-Li₄Ti₅O₁₂ nanoarrays. The improved capacitance performance of N-Li₄Ti₅O₁₂ nanotube array was ascribed to the more amount of TiN and more accessible nanotube surface area, which contributed to the improved conductivity and fast diffusion of electrolyte ions on the surface of electrode. Both N-Li₄Ti₅O₁₂ nanowire array and nanotube array with well-aligned integrative structure exhibited an excellent cycling stability during continuous charge/discharge process. Well-designed N-Li₄Ti₅O₁₂ nanoarrays with high capacitance, good cycling stability and rate capability presented the promising application as feasible electrode materials of lithium-ion supercapacitors for the energy storage.     

Preparation of on chip,flexible supercapacitor with high performance based on electrophoretic deposition of reduced graphene oxide/polypyrrole composites

A new concept is introduced to fabricate flexible, on-chip supercapacitors by electrophoretically depositing highly dispersed reduced graphene oxide/polypyrrole on interdigital-like electrodes. By the unique method, the deposited films could construct on the substrate facilely and uniformly. The prepared all-solid-state device demonstrates high volumetric capacitance (about 147.9 F cm⁻³), high energy density (13.15 mWh cm⁻³ at a power density of 1300 mW cm⁻³) and excellent cycling stability (approximately 71.7% of the initial capacitance retained after 5000 cycles). Compared with other supercapacitor, the device demonstrated here is lightweight, flexible and inexpensive.     

Porous Ca3Co4O9 with enhanced thermoelectric properties derived from Sol–Gel synthesis

Highly porous Ca₃Co₄O₉ thermoelectric oxide ceramics for high-temperature application were fabricated by sol–gel synthesis and subsequent conventional sintering. Growth mechanism of misfit-layered Ca₃Co₄O₉ phase, from sol–gel synthesis educts and upcoming intermediates, was characterized by in-situ X-ray diffraction, scanning electron microscopy and transmission electron microscopy investigations. The Ca₃Co₄O₉ ceramic exhibits a relative density of 67.7%. Thermoelectric properties were measured from 373 K to 1073 K. At 1073 K a power factor of 2.46 μW cm⁻¹ K⁻², a very low heat conductivity of 0.63 W m⁻¹ K⁻¹ and entropy conductivity of 0.61 mW m⁻¹ K⁻² were achieved. The maintained figure of merit ZT of 0.4 from sol–gel synthesized Ca₃Co₄O₉ is the highest obtained from conventional, non-doped Ca₃Co₄O₉. The high porosity and consequently reduced thermal conductivity leads to a high ZT value.