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.     

Decoration of carbon cloth by manganese oxides for flexible asymmetric supercapacitors

Here we describe the production of carbon cloth coated with MnO₂ nanosheets or MnOOH nanorods through a normal temperature reaction or a hydrothermal approach, respectively. Of note, the electrochemical performance of MnO₂-coated carbon cloth was better (429.2 F g⁻¹) than that of MnOOH-coated carbon cloth. When the MnO₂-coated carbon cloth is introduced as the positive electrode and the Fe₂O₃-coated carbon cloth as the negative electrode, a flexible asymmetric supercapacitor was obtained with an energy density of 22.8 Wh kg⁻¹ and a power density of 159.4 W kg⁻¹. Therefore, such a hierarchical MnO₂-coated carbon cloth nanocomposite is a promising high-performance electrode for flexible supercapacitors.     

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.     

Three-dimensional hierarchical ZnCo2O4 flower-like microspheres assembled from porous nanosheets: Hydrothermal synthesis and electrochemical properties

In this work, three-dimensional hierarchical ZnCo₂O₄ flower-like microspheres have been synthesized on a large scale via a facile and economical citrate-mediated hydrothermal method followed by an annealing process in air. The as-synthesized ZnCo₂O₄ flower-like microspheres are constructed by numerous interweaving porous nanosheets. According to the experimental results, a formation mechanism involving the assembly of the nanosheets from nanoparticles into flower-like microsphere is proposed. As a virtue of their beneficial structural features, the ZnCo₂O₄ flower-like microspheres exhibit a high lithium storage capacity and excellent cycling stability (1136 mA h g⁻¹ at 100 mA g⁻¹ after 50 cycles). This remarkable electrochemical performance can be ascribed to the hierarchical structure and porous structures in the nanosheets, which effectively increases the contact area between the active materials and the electrolyte, shortening the Li⁺ diffusion pathway and buffering the volume variation during cycling.     

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.     

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.     

NiCo2O4 nanosheets and nanocones as additive-free anodes for high-performance Li-ion batteries

Development of novel electrode materials with high energy and power densities for lithium-ion batteries (LIBs) is the key to meet the demands of electric vehicles. Transition metal oxides that can react with large amounts of Li⁺ for electrochemical energy storage are considered promising anode materials for LIBs. In this work, NiCo₂O₄ nanosheets and nanocones on Ni foam have been synthesized via general hydrothermal growth and low-temperature annealing treatment. They exhibit high rate capacities and good cyclic performance as LIB anodes owing to their architecture design, which reduces ion and electron transport distance, expands the electrode–electrolyte contact, increases the structural stability, and buffers volume change during cycles. Notably, NiCo₂O₄ nanosheets deliver an initial capacity of 2239 mAh g⁻¹ and a rate capacity of 964 mAh g⁻¹ at current densities of 100 and 5000 mA g⁻¹, respectively. The corresponding values of nanocones are 1912 and 714 mAh g⁻¹. Hence, the as-synthesized NiCo₂O₄ nanosheets and nanocones, which are carbon-free and binder-free with higher energy densities and stronger connections between active materials and current collectors for better stability, are promising for use in advanced anodes for high-performance LIBs.     

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.     

Graphitic carbon nitride nanosheet-assisted preparation of N-enriched mesoporous carbon nanofibers with improved capacitive performance

N-enriched mesoporous carbon nanofibers (NMCNFs) were prepared by an electrospinning technique using graphitic carbon nitride (g-C₃N₄) nanosheets both as sacrificial template and N-doping source. The resultant NMCNF film has a high N-doping level of 8.6 wt% and a high specific surface area of 554 m² g⁻¹. When directly used as the electrode material for supercapacitor, the free-standing NMPCNF film shows a significantly improved capacitive performance including a higher specific capacitance (220 F g⁻¹ at 0.2 A g⁻¹) and a better rate capability (∼70% retention at 20 A g⁻¹) than those of microporous carbon nanofiber film prepared using the same process without using g-C₃N₄ nanosheets (145 F g⁻¹ at 0.2 A g⁻¹ and ∼45% retention at 20 A g⁻¹). Moreover, the NMCNFs show superior stability with only a ∼3% decrease of its initial capacitance after 1000 cycles at a high current density of 10 A g⁻¹. More significantly, the energy density of a symmetrical supercapacitor (SC) based on the NMPCNF film can reach 12.5 Wh kg⁻¹ at a power density of 72 W kg⁻¹.     

Synthesis of hierarchical core-shell NiFe2O4@MnO2 composite microspheres decorated graphene nanosheet for enhanced microwave absorption performance

A ternary functional composite NiFe₂O₄@MnO₂@graphene was synthesized successfully via a facile method. The phase constitution, microstructures, morphologies and chemical compositions of the samples were investigated by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), Brunauer-Emmett-Teller (BET) and X-ray photoelectron spectroscopy (XPS). It was observed that the NiFe₂O₄ nanoparticles were coated by hierarchically MnO₂ shells and distributed on the surface of graphene. Investigations of EM wave absorption indicated that NiFe₂O₄@MnO₂@ graphene composite has the strongest reflection loss of −47.4 dB at 7.4 GHz at the matching thickness of 3 mm, compared to NiFe₂O₄ and NiFe₂O₄@MnO₂, and its maximum absorption bandwidth (<−10 dB) is 4.3 GHz (from 5.1 to 9.4 GHz). The enhanced microwave absorption performance can be attributed to the hierarchical structure of MnO₂, void space between MnO₂ and graphene, and better impedance matching of ternary composite. The above results indicate that the novel hierarchical NiFe₂O₄@MnO₂@graphene composite, with intense absorption and wide absorption bandwidth, would be a promising absorber with less EM wave interference.     

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.     

Hierarchical Co3O4@ZnWO4 core/shell nanostructures on nickel foam: Synthesis and electrochemical performance for supercapacitors

To improve the electrochemical properties of Co₃O₄ for supercapacitors application, a hierarchical Co₃O₄@ZnWO₄ core/shell nanowire arrays (NWAs) material is designed and synthesized successfully via a facile two-step hydrothermal method followed by the heat treatment. Co₃O₄@ZnWO₄ NWAs exhibits excellent electrochemical performances with areal capacitance of 4.1 F cm⁻² (1020.1 F g⁻¹) at a current density of 2 mA cm⁻² and extremely good cycling stability (99.7% of the initial capacitance remained even after 3000 cycles). Compared with pure Co₃O₄ electrodes, the results prove that this unique hierarchical hybrid nanostructure and reasonable assembling of two electrochemical pseudocapacitor materials are more advantageous to enhance the electrochemical performance. Considering these remarkable capacitive behaviors, the hierarchical Co₃O₄@ZnWO₄ core/shell NWAs nanostructure electrode can be revealed promising for high-performance supercapacitors.     

Effect of core-shell reversal on the structural,magnetic and adsorptive properties of Fe2O3-GO nanocomposites

Effect of core-shell reversal on the nanocomposites of graphene oxide (GO) and ferric oxide (Fe₂O₃) was studied. Fe₂O₃@GO core-shell nanosheets were synthesized by sonication method, while the GO@Fe₂O₃ core-shell nanospheres by employing N,N′-dicyclohexylcarbodimide as the binding agent for the wrapping of GO sheets on pre-formed Fe₂O₃ nanoparticles (NPs). The phase composition, crystallinity and morphology of the nanocomposites were characterized by FT-IR, TEM, SEM-EDS, VSM, BET surface area study and XRD techniques. The saturation magnetization (M_s) was 1.25 and 0.51 emu g⁻¹ for GO@Fe₂O₃ and Fe₂O₃@GO respectively owing to the dependence of magnetic properties on the reversal of core-shell. BET analysis revealed the surface area of 100.32 m² g⁻¹ and 45.69 m² g⁻¹ for GO@Fe₂O₃ and Fe₂O₃@GO nanocomposites respectively. The fabricated nanocomposites were analyzed as adsorbents for the uptake of Pb (II) ions. The impact of various factors affecting adsorption process such as pH, adsorbent dose, contact time, temperature and metal ion concentration was also investigated. GO@Fe₂O₃ core-shell nanospheres showed a higher adsorption capacity for Pb (II) ions as compared to Fe₂O₃@GO core-shell nanosheet with the maximum adsorption capacities (q_m) of 303.0 and 125.0 mg g⁻¹ respectively. The equilibrium data was estimated by Freundlich, Langmuir, D-R and Temkin isotherm models. Thermodynamic analysis confirmed the spontaneous and exothermic nature of the adsorption process. The adsorption kinetics was significantly fitted to pseudo-second order model. The results confirmed that core-shell reversal can significantly alter the adsorptive properties of Fe₂O₃-GO nanocomposite     

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.     

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.     

Rational design of nickel cobalt sulfide/oxide core-shell nanocolumn arrays for high-performance flexible all-solid-state asymmetric supercapacitors

The development of wearable electronics has created a surge of interest in designing flexible energy storage device with high energy density and long lifespan. In this work, we have successfully fabricated a flexible asymmetric supercapacitor (ASC) based on the NiCo₂S₄@NiCo₂O₄ nanocolumn arrays (NCAs). The nickel cobalt sulfide/oxide core-shell nanostructures were rationally synthesized through a facile stepwise approach. The NiCo₂S₄@NiCo₂O₄ NCAs based electrode delivered a high specific capacitance of 2258.9 F g⁻¹ at a current density of 0.5 A g⁻¹. The as-assembled flexible ASC device exhibited a high energy density of 44.06 Wh kg⁻¹, a high power density of 6.4 kW kg⁻¹, and excellent cycling stability by retaining 92.5% after 6000 cycles. Excitingly, the electrochemical property of the ASC device could be maintained under severe bending, indicating superior flexibility and mechanical stability. The NiCo₂S₄@NiCo₂O₄ core-shell NCAs possess enormous potential for future wearable electronic applications.     

Fabrication of carbon encapsulated Co3O4 nanoparticles embedded in porous graphitic carbon nanosheets for microwave absorber

Carbon-encapsulated Co₃O₄ nanoparticles homogeneously embedded 2D (two-dimensional) porous graphitic carbon (PGC) nanosheets were prepared by a facile and scalable synthesis method. With assistance of sodium chloride, the Co₃O₄ nanoparticles (10–20 nm) with magnetic loss were well encapsulated by onion-like carbon shells homogeneously embedded porous graphitic carbon nanosheets (thickness of less than 50 nm) with dielectric loss. In the architecture, the well impedance matching for microwave absorption can be obtained by the synergetic effect between Co₃O₄ nanoparticles and encapsulated porous carbon nanosheets. The minimum reflection loss value of −32.3 dB was observed at 11.4 GHz with a matching thickness of 2.3 mm for 2D Co₃O₄@C@PGC nanosheets. The 2D Co₃O₄@C@PGC nanosheets can be used as a kind of candidate for microwave absorbing materials.     

Three-dimensional flower-like and hierarchical porous carbon materials as high-rate performance electrodes for supercapacitors

Three-dimensional flower-like and hierarchical porous carbon material (FHPC) has been fabricated through a simple and efficient carbonization method followed by chemical activation with flower-like ZnO as template and pitch as carbon precursor. The hierarchical porous structure is composed of numerous micropores and well-defined mesopores in the interconnected macroporous walls. The FHPC electrode can achieve a relatively high capacitance of 294 F g⁻¹ at a scan rate of 2 mV s⁻¹ and excellent rate capability (71% retention at 500 mV s⁻¹) with superior cycle stability (only 2% loss after 5000 cycles) in 6 mol L⁻¹ KOH electrolyte. The symmetric supercapacitor fabricated with FHPC electrodes delivers a high energy density of 15.9 Wh kg⁻¹ at a power density of 317.5 W kg⁻¹ operated in the voltage range of 0–1.8 V in 1 mol L⁻¹ Na₂SO₄ aqueous electrolyte.     

Template synthesis of hollow carbon spheres anchored on carbon nanotubes for high rate performance supercapacitors

A carbon material consisting of hollow carbon spheres anchored on the surface of carbon nanotubes (CNT–HCS) has been synthesized by an easy chemical vapor deposition process using a CNT–MnO₂ hybrid as template. An electrode made of this material exhibits a maximum specific capacitance of 201.5 F g⁻¹ at 0.5 A g⁻¹ and excellent rate performance (69% retention ratio at 20 A g⁻¹). It has impressive cycling stability with 90% initial capacitance retained after 5000 cycles at 5 A g⁻¹ in 6 mol L⁻¹ KOH. Symmetric supercapacitors based on CNT–HCS achieve a maximum energy density of 11.3 W h kg⁻¹ and power density of 11.8 kW kg⁻¹ operated within a wide potential range of 0–1.6 V in 1.0 mol L⁻¹ Na₂SO₄ solution.     

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.