Hierarchical porous NiCo2O4 nanomaterials with excellent cycling behavior for electrochemical capacitors via a hard-templating route

Hierarchical porous nickel cobaltite (NiCo₂O₄) nanomaterials were synthesized via a hard-templating route. The obtained materials consist of nanostructured cubic NiCo₂O₄ spinels and a spot of cubic NiO nanoparticles, and the materials display a typical hierarchical porous structure. The NiCo₂O₄ electrode displays quasireversible dynamics characteristics, mainly Faradaic capacitance behavior and capacitance relaxation feature. The NiCo₂O₄ electrode exhibits an excellent long cycling behavior with no capacitance decays during 5,000 cycles at a current density of 2?A?g^?1 in 1?M KOH electrolytes, and the NiCo₂O₄ electrode exhibits both high power and energy performances even after 5,000 cycles with respective value of 1,758?W?kg^?1 and 8.3?W?h?kg^?1 in 1?M KOH electrolytes, indicating that the NiCo₂O₄ nanomaterials are promising candidates for electrochemical capacitors.     

Honeycomb-like NiCo2O4@Ni(OH)2 supported on 3D N−doped graphene/carbon nanotubes sponge as an high performance electrode for Supercapacitor

In order to increase the energy density of supercapacitor, a new kind electrode material with excellent structure and outstanding electrochemical performance is highly desired. In this article, a new type of three-dimensional (3D) nitrogen-doped single-wall carbon nanotubes (SWNTs)/graphene elastic sponge (TRGN?CNTs?S) with low density of 0.8 mg cm^?3 has been successfully prepared by pyrolyzing SWNTs and GO coated commercial polyurethane (PU) sponge. In addition, high performance electrode of the honeycomb-like NiCo₂O₄@Ni(OH)₂/TRGN-CNTs-S with core-shell structure has been successfully fabricated through hydrothermal method and then by annealing treatment and electrochemical deposition method, respectively. Benefited from 3D structural feature, the compressed NiCo₂O₄@Ni(OH)₂/TRGN-CNTs-S electrode exhibits high gravimetric and volumetric capacitance of 1810 F g^?1, 847.7 F cm^?3 at 1 A g^?1. The high rate performance and long-term stability was also obtained. Furthermore, an asymmetric supercapacitor using NiCo₂O₄@Ni(OH)₂/TRGN-CNTs-S cathode and NGN/CNTs anode delivered high gravimetric and volumetric energy density of 54 W h kg^?1 at 799.9 W kg^?1 and 37 W h L^?1 at 561.5 W L^?1. In summary, an excellent electrochemical electrode with new elastic 3D SWNTs/graphene supports and binder free pseudocapacitive materials was introduced.     

Fabrication and characterization of monodispersed Mn0.8Ni0.2Co2O4 mesoporous microspheres for supercapacitor application

The monodispersed Ni doped MnCo₂O₄ mesoporous microspheres were synthesized through a simple ammonium bicarbonate-assisted solvothermal route. The spinel-type crystal structure with a lattice parameter of 8.199?Å for Mn_0.8Ni_0.2Co₂O₄ composition was obtained by using X-ray diffraction analysis. The Brunauer?Emmett?Teller (BET) specific surface area of the sample was found to be 75.78?m² g^?1 with an average pore diameter of 9.88?nm. Electron microscopy studies revealed that the stable mesoporous microspheres are constituted by well-connected aggregates of nanoparticles. The influence of Ni doping on the pseudo-capacitance of MnCo₂O₄ electrode was investigated by means of cyclic voltammetry in 6?M KOH electrolyte. We found that the spinel-type Mn_0.8Ni_0.2Co₂O₄ mesoporous microspheres exhibit specific capacitances of 1822 F g^?1 at a scan rate of 5?mV/s. Furthermore, the electrochemical impedance spectroscopy analysis revealed the low resistance and good electrochemical stability of the sample.     

An exploration of 3-methoxypropionitrile chemical sensor based on layered hexagonal NiCo2O4 nanoplates as electrode material

Binary metal oxide, nickel-cobalt oxide (NiCo₂O₄), was hydrothermally synthesized for the successful utilization as electrochemical electrode in chemical sensor. Benefited from the large surface area, binary NiCo₂O₄ stacked hexagonal nanoplates (HNPs) based electrode was utilized for the electrochemical sensing application towards teratogenic chemical i.e. 3-methoxypropionitrile (3-MPN). The sensing results displayed a reproducible sensitivity of ~605 μAμM^?1cm^?2, detection limit of ~11.8 μM with the correlation coefficient (R) of ~0.99502 and good linearity from ~10 μM to ~100 μM. Stability and repeatability of binary NiCo₂O₄ stacked HNPs based electrode was investigated with satisfactory results. Our synthesized binary NiCo₂O₄ stacked HNPs based electrode is promising for sensor applications and thus, enlightens the possibility of synthesizing other binary oxide materials.     

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.     

Studies on spinel cobaltites,MCo2O4 (M = Mn,Zn, Fe,Ni and Co) and their functional properties

Optimization of electrodes for charge storage with appropriate processing conditions places significant challenges in the developments for high performance charge storage devices. In this article, metal cobaltite spinels of formula MCo₂O₄ (where M = Mn, Zn, Fe, Ni and Co) are synthesized by oxalate decomposition method followed by calcination at three typical temperatures, viz. 350, 550, and 750 °C and examined their performance variation when used as anodes in lithium ion batteries. Phase and structure of the materials are studied by powder x-ray diffraction (XRD) technique. Single phase MnCo2O4,ZnCo2O4 and Co3O4 are obtained for all different temperatures 350 °C, 550 °C and 750 °C; whereas FeCo₂O₄ and NiCo₂O₄ contained their constituent binary phases even after repeated calcination. Morphologies of the materials are studied via scanning electron microscopy (SEM): needle-shaped particles of MnCo₂O₄ and ZnCo₂O₄, submicron sized particles of FeCo₂O₄ and agglomerated submicron particle of NiCo₂O₄ are observed. Galvanostatic cycling has been conducted in the voltage range 0.005–3.0 V vs. Li at a current density of 60 mA g^?1 up to 50 cycles to study their Li storage capabilities. Highest observed charge capacities are: MnCo₂O₄ – 365 mA h g^?1 (750 °C); ZnCo₂O₄ – 516 mA h g^?1 (550 °C); FeCo₂O₄ – 480 mA h g^?1 (550 °C); NiCo₂O₄ – 384 mA h g^?1 (750 °C); and Co₃O₄ – 675 mA h g^?1 (350 °C). The Co₃O₄ showed the highest reversible capacity of 675 mA h g^?1; the NiO present in NiCo₂O₄ acts as a buffer layer that results in improved cycling stability; the ZnCo₂O₄ with long needle-like shows good cycling stability.     

Nanoparticles constructed mesoporous coral-like Mn2O3 as high performance anode for lithium-ion batteries

As well established, the morphology and architecture of electrode materials greatly contribute to the electrochemical properties. Herein, a novel structure of mesoporous coral-like manganese (III) oxide (Mn₂O₃) is synthesized via a facile solvothermal method coupled with the carbonization under air. When fabricated as anode electrode for lithium-ion batteries (LIBs), the as-prepared Mn₂O₃ exhibits good electrochemical properties, showing a high discharge capacity of 1090.4 mAh g^?1 at 0.1 A g^?1, and excellent rate performance of 410.4 mAh g^?1 at 2 A g^?1. Furthermore, it maintains the reversible discharge capacity of 1045 mAh g^?1 at 0.1 A g^?1 after 380 cycles, and 755 mAh g^?1 at 1 A g^?1 after 450 cycles. The durable cycling stability and outstanding rate performance can be attributed to its unique 3D mesoporous structure, which is favorable for increasing active area and shortening Li⁺ diffusion distance.     

One-pot generation of mesoporous carbon supported nanocrystalline H3PW12O40 heteropoly acid with high performance in microwave esterification of acetic acid and isoamyl alcohol

Ordered mesoporous carbon-supported H₃PW₁₂O₄₀ heteropoly acid materials (HPW/OMCs) have been rationally synthesized for the first time. The method is based on the evaporation-induced triconstituent co-assembly effect using the sol–gel process, wherein soluble resol polymer is used as an organic precursor, and triblock copolymer F127 is used as a template. The ordered mesoporous carbon-supported H₃PW₁₂O₄₀ heteropoly acid materials were analyzed and characterized by X-ray diffraction, N₂ adsorption and desorption (BET), and transmission electron microscope. The mesoporous carbon-supported H₃PW₁₂O₄₀ materials possess an ordered mesostructure, narrow pore size distributions (around 2.8–3.6 nm), high pore volumes (up to 0.49 cm³ g^?1), high specific BET surface areas (up to 590 m² g^?1), tailorable HPW content (up to 30 wt%), and well dispersion of HPW particles. Moreover, the resultant mesoporous ordered mesoporous carbon-supported H₃PW₁₂O₄₀ materials exhibit high catalytic activity in microwave esterification of acetic acid and isoamyl alcohol. The obtained 20 % HPW/OMC catalyst exhibits high catalytic performance with 96.7 % of isoamyl acetate yield at temperature of 120 °C, alcohol/acid molar ratio of 2, catalyst amount of 0.2 g, microwave irradiation power of 800 W, and reaction time of 18 min. It was believed that the concentration of H₃PW₁₂O₄₀ have a crucial effect on the HPW/OMCs’ porosity, mesostructure and catalytic performance.     

Hierarchical mesoporous nickel cobaltite nanoneedle/carbon cloth arrays as superior flexible electrodes for supercapacitors

Hierarchical mesoporous NiCo₂O₄ nanoneedle arrays on carbon cloth have been fabricated by a simple hydrothermal approach combined with a post-annealing treatment. Such unique array nanoarchitectures exhibit remarkable electrochemical performance with high capacitance and desirable cycle life at high rates. When evaluated as an electrode material for supercapacitors, the NiCo₂O₄ nanoneedle arrays supported on carbon cloth was able to deliver high specific capacitance of 660 F g^-1 at current densities of 2 A g^-1 in 2 M KOH aqueous solution. In addition, the composite electrode shows excellent mechanical behavior and long-term cyclic stability (91.8% capacitance retention after 3,000 cycles). The fabrication method presented here is facile, cost-effective, and scalable, which may open a new pathway for real device applications.     

Synthesis of 3D nanoflower-like mesoporous NiCo2O4 N-doped CNTs nanocomposite for solid-state hybrid supercapacitor; efficient material for the positive electrode

In this research work, we report a novel method for developing ternary NiCo₂O₄ compounds using deep eutectic solvents (DESs) and a strategy for improving their pseudocapacitive performance. NiCo₂O₄ composites with N-doped carbon nanotubes (NCNTs) were fabricated on Ni foam using a hydrothermal method. The electrochemical performance of the NiCo₂O₄ was altered with the change in the reaction temperature. The composite of NiCo₂O₄ and NCNTs demonstrated a maximum value of specific capacity of 303 mAh g⁻¹ at a scan rate of 5 mV s⁻¹. The specific capacity for the composite compound was 1.3-fold greater than that of the pristine NiCo₂O₄ sample. For practical applications, we constructed a flexible solid-state hybrid supercapacitor comprised of NiCo₂O₄/NCNTs//activated carbon (AC) cells with an excellent energy density of 12.31 Wh kg⁻¹, outstanding power density of 8.96 kW kg⁻¹, and tremendous electrode stability. The three-dimensional mesoporous nanoflowers and nanotubes-like nanostructures of NiCo₂O₄ are well-suited for use in hybrid devices as well as convenient for flexible electronic devices.     

NiCo2O4 arrays nanostructures on nickel foam: Morphology control and application for pseudocapacitors

The design of electrode materials with desirable morphology is of great importance and challenge to fabricate high-performance supercapacitors. In this work, NiCo₂O₄ nanopetal, nanosheet, nanoneedle and nanorod arrays on nickel foam have been synthesized through a facile hydrothermal method. The morphologies of NiCo₂O₄ arrays can be easily controlled by adjusting the kinds of alkali source and the addition of NH₄F. The electrochemical results show that the NiCo₂O₄ nanoneedles electrode has the optimal electrochemical performance among four samples, demonstrating its promising application potential for high performance supercapacitors. This investigation about morphology control of NiCo₂O₄ electrode materials and the relationship between the morphologies and corresponding electrochemical performances provides strategies to enhance the performance of supercapacitor electrodes.     

Ag nanowires and its application as electrode materials in electrochemical capacitor

Silver nanowires were synthesized on a large scale by using anodic aluminum oxide (AAO) film as templates and serving ethylene glycol as reductant. Their morphological and structural characterizations were characterized with field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and selected area electron diffraction (SAED). The electrochemical properties of silver nanowires as electrode materials for electrochemical capacitors were investigated by cyclic voltammetry (CV) and galvanostatic charge/discharge technique in 6 M KOH aqueous electrolyte. The Ag₂O/Ag coaxial nanowires were formed by the incomplete electrochemical oxidation during the charge step. The maximum specific capacitance of 987 F g^?1 was obtained at a charge–discharge current density of 5 mA cm^?2.     

Ordered mesoporous carbon prepared from triblock copolymer/novolac composites

Ordered mesoporous carbon is synthesized by the organic–organic self-assembly method with novolac as carbon precursor and two kinds of triblock copolymers (Pluronic F127 and P123) as template. The hexagonal structure and a worm-hole structure are observed by TEM. The carbonization temperature is determined by TG and FT-IR. Characterization of physical properties of mesoporous carbon is executed by N₂ absorption–desorption isotherms and XRD. The mass ratios of carbon precursor/template affect the textural properties of mesoporous carbon. The mesoporous carbon with F127/PF of 1/1 has lager surface area (670 m² g^?1), pore size (3.2 nm), pore volume (0.40 cm³ g^?1), smaller microporous surface area (368 m² g^?1) and wall thickness (3.7 nm) compare to that with F127/PF of 0.5/1 (576 m² g^?1, 2.7 nm, 0.29 cm³ g^?1, 409 m² g^?1 and 4.3 nm, respectively). The mesoporous carbon prepared by carbonization at high temperature (700 °C) exhibits lager surface area, lower pore size and pore volume than the corresponding one obtained at 500 °C. The structure and order of the resulting materials are notably affected with types of templates. The mesoporous carbon with P123 as template exhibits worm-hole structure compare to that with F127 as template with hexagonal structure. In general, the pore size of mesoporous carbon with novolac as precursor is smaller than that with resorcinol–formaldehyde as precursor.     

In situ synthesis of integrated dodecahedron NiO/NiCo2O4 coupled with N-doped porous hollow carbon capsule for high-performance supercapacitors

NiO and NiCo₂O₄ exhibit excellent synergistic effects and broad application prospects in electrochemical applications. However, the apparent interfacial instability between NiO and NiCo₂O₄ limits ion transport kinetics, charge/ion transfer, and electrochemical stability. In response, we developed and designed an integrated dodecahedron NiO/NiCo₂O₄ by a facile in-situ calcination method. Moreover, by utilizing the porous hollow structure of nitrogen-doped carbon capsules (N-Cc) as a conductive network, the N-Cc_x@NiO/NiCo₂O₄ heterostructures with stable interface structure, excellent electrolyte adsorption, and electron transfer pathways were carefully designed. The N-Cc_1.0@NiO/NiCo₂O₄ heterostructures are found to deliver an outstanding specific capacitance of 658.8 F g⁻¹, and a high energy density of 101.40 Wh kg⁻¹ at a power density of 775.03 W kg⁻¹, along with capacitance retention of more than 93.5% after 8000 cycles. Based on the DFT calculations and electrochemical experimental results, this work provides an effective in situ route for the construction of high-performance metal oxide heterostructure electrode materials for new energy storage devices.     

Enhancing the performance and stability of NiCo2O4 nanoneedle coated on Ni foam electrodes with Ni seed layer for supercapacitor applications

We introduce a facile way to improve the performance of NiCo₂O₄ electrode by including a Ni seed layer. The seed layer deposited on Ni foam electrode (NiCo₂O₄/Ni@NF) shows the superior specific capacity of 1142 C g⁻¹ at 1 A g⁻¹ with the excellent cycle stability of ∼96% even after 5000 cycles at a higher current density of 5 A g⁻¹. These values are about 3.7 times higher than that of the electrode (NiCo₂O₄@NF) without a seed layer, which shows the specific capacity of 305 C g⁻¹@1 A g⁻¹ with cycle stability of 84% even at a lower current density of 1 A g⁻¹. The enhanced performance of the NiCo₂O₄/Ni@NF electrode may be attributed to lower interface resistance, fast redox reversible reaction, and improved surface active sites. Further, the asymmetric solid-state supercapacitor device is fabricated by using the NiCo₂O₄/Ni@NF electrode as a positive and reduced graphene oxide (rGO)-Fe₂O₃ nanograin as a negative electrode with PVA-KOH gel electrolyte, and the NiCo₂O₄/Ni20@NF//rGO-Fe₂O₃@NF asymmetric solid state device delivers an areal capacitance of 446 mF cm⁻² with a low capacitance loss of 18% even after 10000 cycles. Further, the fabricated asymmetric solid state device shows a maximum energy density of 124.3 Wh cm⁻² (at 3.58 kW cm⁻²) and power density of 14.88 kW cm⁻² (at 31.41 Wh cm⁻²).     

A different method for producing a flexible LiMn2O4/MWCNT composite electrode for lithium ion batteries

A flexible lithium manganese oxide (LiMn₂O₄)/multi-wall carbon nanotube (MWCNT) composite electrode was produced by casting a slurry-containing powdered LiMn₂O₄ on a previously prepared MWCNT paper. The structure of this new LiMn₂O₄/MWCNT composite electrode was characterized using scanning electron microscopy and X-ray diffraction patterns. Furthermore, the surfaces of these electrodes were coated with gold–palladium alloy using an RF magnetron sputtering technique to prevent Mn dissolution. To investigate the electrochemical performance of this flexible LiMn₂O₄/MWCNT composite electrode, a bare-LiMn₂O₄ electrode was prepared. The discharge capacity of the produced LiMn₂O₄/MWCNT nanocomposite electrode was cyclically tested, and the charge transfer resistance of the electrodes was studied using electrochemical impedance spectroscopy. Consequently, the Au–Pd-coated LiMn₂O₄/MWCNT had a 120 mAh g^?1 discharge capacity and 90 % capacity retention after 100 cycles.     

Polypyrrole-wrapped NiCo2S4 nanoneedles as an electrode material for supercapacitor applications

In this study, dicobalt tetrasulfide (NiCo₂S₄) nanoneedles were successfully synthesized by a two-step hydrothermal method on nickel foam. A layer of polypyrrole (PPy) was further wrapped on the surface of the NiCo₂S₄ nanoneedles by in-situ polymerization. The obtained NiCo₂S₄@PPy composite was investigated for supercapacitor applications, which exhibited a capacitance of 1842.8 F g^?1 at 1 A g^?1. An asymmetric supercapacitor device fabricated with an activated carbon negative electrode and NiCo₂S₄@PPy positive electrodes exhibited an energy density of 41.2 Wh kg^?1 at 402.2 W kg^?1 with a high charge–discharge cycling stability (92.8% after 5000 cycles). These results demonstrate that NiCo₂S₄@PPy electrodes have broad application prospects as energy storage electrode materials.     

Hydrothermally synthesized microrods and microballs of NiCo2O4 for supercapacitor application

The microrods and microballs of NiCo₂O₄ are successfully synthesized by the hydrothermal method. The effect of ammonium hydroxide and ammonium fluoride on the surface microstructure is observed. The prepared microrods and microballs of NiCo₂O₄ are analyzed by various analytical tools like powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), with energy dispersive X-ray spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS). The electrochemical properties are studied by using cyclic voltammetry (CV), galvanostatic charge discharge (GCD), and electrochemical impedance spectroscopy (EIS), using the workstation Biologic SP-200. The maximum specific capacitance of the NiCo₂O₄ microrods electrode is 1671 F/g. The areal specific capacitance of the NiCo₂O₄ microrods electrode is 284 mF/g. The energy density and power density of microrods of NiCo₂O₄ electrode are 19 Wh/Kg and 282 W/kg, respectively. The equivalent series resistance (Rs) is 0.62 Ω for NiCo₂O₄ microrods.     

Vacuum-assisted hard-templating impregnation fabrication of three-dimensional ordered mesoporous samarium oxide

Three-dimensional (3D) ordered mesoporous samarium oxides with cubic polycrystalline structure were fabricated by means of vacuum-assisted impregnation and mill impregnation route using ordered mesoporous SiO₂ (KIT-6) as the hard template with nitrate samarium as the metal source. The structure and morphology of the as-prepared materials were characterized by XRD, TEM and N₂ adsoption-desorption. It is found that the mesoporous Sm₂O₃ sample obtained by vacuum-assisted method have a higher surface area (184 m² g^?1) than that (142 m² g^?1) of mill impregnation and had a better 3D mesoprosity, which has potential applications in the catalytic reaction.     

Flower-like NiCo2O4/NiCo2S4 electrodes on Ni mesh for higher supercapacitor applications

In this research, we have effectively synthesized a novel NiCo₂O₄/NiCo₂S₄ powder by co-precipitation and thin films prepared using a screen-printing method on Ni mesh for supercapacitor applications. Herein, we report the effect of unique hierarchical nanostructures and the systematic effect of Ni and Co on the structural, morphological and electrical properties of the NiCo₂O₄/NiCo₂S₄ electrodes. The optimized NiCo₂O₄/NiCo₂S₄ electrode shows outstanding performance with a specific capacitance of 1966 F g⁻¹ at 5 mV s⁻¹. The cycling stability reports indicate the NiCo₂O₄/NiCo₂S₄ electrodes have an outstanding cyclic stability with 91% capacity retention. From the supercapacitor performance results, we confirmed that the NiCo₂O₄/NiCo₂S₄ electrode is useful for the fabrication of symmetric supercapacitors. These results reveal that the NiCo₂O₄/NiCo₂S₄ electrodes is a capable electrode material for supercapacitor applications in the future.