Uniform urchin-like nickel cobaltite microspherical superstructures constructed by one-dimension nanowires and their application for electrochemical capacitors

Synthesis and magnetic properties of NiCo2O4urchin-like nanofibers

Single-phase NiCo2O4(NCO) nanoparticles (NPs) with an average particle size of 12 (±3.5) nm were successfully synthesized as aggregates in urchin-like nanofibers via a hydrothermal route. Magnetization data measured as functions of temperature and magnetic field suggest a superparamagnetic-like behavior at room temperature, a ferrimagnetic transition around a Curie temperatureTC∼ 200 K, and a spin blocking transition at a blocking temperatureTB∼ 90 K, as observed at a field of 100 Oe. The spin blocking nature has been investigated by analyses of the field-dependence ofTBin the static magnetization and its frequency-dependence in the ac susceptibility data measured in zero-field cooling regime, both indicate a low-temperature spin glass-like state. BelowTB, the coercivity increases monotonically up to 1.7 kOe with decreasing temperature down to 5 K. Our results indicate that the magnetic behavior of NCO NPs, which is mainly determined by the cations' ratio, oxidation states, and site-occupancy, can be controlled by a synthesis in appropriate particle size and morphology.

Study of solvent variation on controlled synthesis of different nanostructured NiCo2O4 thin films for supercapacitive application

The present investigation deals with controlled synthesis of nanostructured NiCo₂O₄ thin films directly on stainless steel substrates by facile and economical chemical bath deposition technique, without adding a surfactant or a binder. The consequences of different compositions of solvents on morphological and electrochemical properties have been studied systematically. We used different solvent composition as Double Distilled Water (DDW), DDW:Ethanol (1:1) and DDW: N, N dimethylformamide (1:1). The films have been named as NCO-W for DDW, NCO-WE for DDW: Ethanol (1:1) solvent and NCO-WD for DDW: N, N dimethylformamide (1:1) solvent. The morphologies of NiCo₂O₄ thin films modify substantially with change in a solvent. NCO-W exhibited the spikes of Crossandra infundibuliformis like nanostructures. The NCO-WE favored the formation of uniformly distributed leaf-like nanostructure whereas NCO-WD showed randomly oriented nanoplates all over the surface area. The Electrochemical performance of these NiCo₂O₄ thin films were studied using cyclic voltammetry, chronopotentiometry, and electrochemical impedance spectroscopy techniques. The NCO-W, NCO-WE and NCO-WD electrodes showed specific capacitance values of 271, 553 and 140 F/g respectively at the current density of 0.5 mA/cm² and excellent capacitance retention of 90%, 91% and 80% after 2000 cycles for NCO-W, NCO-WE and NCO-WD samples respectively. This result reveals that NiCo₂O₄ is a prominent electrode material for supercapacitor application.

Morphological Modulation of Conducting Polymer Nanocomposites with Nickel Cobaltite/Reduced Graphene Oxide and Their Subtle Effects on the Capacitive Behaviors

This work reports on the urchin-like architecture-based nickel cobaltite (NiCo₂O₄)/reduced graphene oxide (rGO)/conducting polymer [polyaniline (PANI) or polypyrrole (PPy)] nanocomposites prepared through a hydrothermal synthesis procedure, followed by in situ polymerization techniques. Subsequently, these materials are subjected to electrochemical investigation to search for promising electrode materials for energy storage applications. Interestingly, the morphology of NiCo₂O₄ varies upon the addition of rGO as well as nitrogen-doped rGO (N-rGO). When it is composite with rGO, it forms an urchin-like architecture, and with N-rGO, it forms nanoparticle structures having a diameter of 90 ± 10 nm. Further, these nanostructures are intricately coated by conducting polymers (such as PANI and PPy) as evidenced from the field emission scanning electron microscopy and high-resolution transmission electron microscopy observations, and the overall shape does not alter after the modification. All composites are investigated thoroughly by Fourier transform infrared, Raman, X-ray powder diffraction, and X-ray photoelectron spectroscopies to confer the formation and presence of polymers. The NiCo₂O₄/rGO/PPy nanocomposites exhibit an excellent specific capacitance of 1547 ± 5 F/g at a current density of 0.5 A/g, a better energy density of 34.37 ± 0.11 W h/kg at 0.5 A/g, a notable power density of 99.98 ± 0.31 W/kg at 0.5 A/g, and retains its 94 ± 1% specific capacitance after 5000 charge-discharge cycles. The uniform coating over the urchin-like architecture by conducting polymers constructs a typical morphology, and possibly, it is the key factor behind gaining such superior electrochemical behavior owing to (a) the enhancement of surface area and (b) the combination of double-layer capacitive and pseudocapacitive properties. Furthermore, a symmetric flexible supercapacitor device made of NiCo₂O₄/rGO/PPy nanocomposites provides superior electrochemical behavior.

Reduced graphene/nanostructured cobalt oxide nanocomposite for enhanced electrochemical performance of supercapacitor applications

We demonstrate the preparation of nanostructures cobalt oxide/reduced graphene oxide (Co₃O₄/rGO) nanocomposites by a simple one-step cost-effective hydrothermal technique for possible electrode materials in supercapacitor application. The X-ray diffraction patterns were employed to confirm the nanocomposite crystal system of Co₃O₄/rGO by demonstrating the existence of normal cubic spinel structure of Co₃O₄ in the matrix of Co₃O₄/rGO nanocomposite. FTIR and FT-Raman studies manifested the structural behaviour and quality of prepared Co₃O₄/rGO nanocomposite. The optical properties of the nanocomposite Co₃O₄/rGO have been investigated by UV absorption spectra. The SEM/TEM images showed that the Co₃O₄ nanoparticles in the Co₃O₄/rGO nanocomposites were covered over the surface of the rGO sheets. The electrical properties were analyzed in terms of real and imaginary permittivity, dielectric loss and AC conductivity. The electrocatalytic activities of synthesized Co₃O₄/rGO nanocomposites were determined by cyclic voltammetry and charge-discharge cycle to evaluate the supercapacitive performance. The specific capacitance of 754 Fg^-1 was recorded for Co₃O₄/rGO nanocomposite based electrode in three electrode cell system. The electrode material exhibited an acceptable capability and excellent long-term cyclic stability by maintaining 96% after 1000 continuous cycles. These results showed that the prepared sample could be an ideal candidate for high-energy application as electrode materials. The synthesized Co₃O₄/rGO nanocomposite is a versatile material and can be used in various application such as fuel cells, electrochemical sensors, gas sensors, solar cells, and photocatalysis.

Redox-Active Gel Electrolyte Combined with Branched Polyaniline Nanofibers Doped with Ferrous Ions for Ultra-High-Performance Flexible Supercapacitors

In this work, the effects of utilizing an Fe²⁺/Fe³⁺ redox-active electrolyte and Fe²⁺-doped polyaniline (PANI) electrode material on the performance of an assembled supercapacitor (SC) were studied. The concentration of the redox couple additive in the electrolyte of the SC was optimized to be 0.5 M. With the optimized concentration of 0.4 M Fe²⁺, the doped PANI branched nanofibers electropolymerized onto titanium mesh were much thinner, cleaner, and more branched than normal PANI. A specific capacitance (C_s) of 8468 F g⁻¹ for the 0.4 M Fe²⁺/PANI electrode in the 1 M H₂SO₄ + 0.5 M Fe²⁺/Fe³⁺ gel electrolyte and an energy density of 218.1 Wh kg⁻¹ at a power density of 1854.4 W kg⁻¹ for the resultant SC were achieved, which were much higher than those of the conventional PANI electrode tested in a normal H₂SO₄ electrolyte (404 F g⁻¹ and 24.9 Wh kg⁻¹). These results are among the highest reported for PANI-based SCs in the literature so far and demonstrate the potential effectiveness of this strategy to improve the electrochemical performance of flexible SCs by modifying both the electrode and electrolyte.

NiCo₂O₄-Based Supercapacitor Nanomaterials

In recent years, the research on supercapacitors has ushered in an explosive growth, which mainly focuses on seeking nano-/micro-materials with high energy and power densities. Herein, this review will be arranged from three aspects. We will summarize the controllable architectures of spinel NiCo₂O₄ fabricated by various approaches. Then, we introduce their performances as supercapacitors due to their excellent electrochemical performance, including superior electronic conductivity and electrochemical activity, together with the low cost and environmental friendliness. Finally, the review will be concluded with the perspectives on the future development of spinel NiCo₂O₄ utilized as the supercapacitor electrodes.

Time and temperature dependent multiple hierarchical NiCo2O4 for high-performance supercapacitors

The time and temperature dependent multiple hierarchical NiCo₂O₄ constructed of nanosheets covered with nanowires shows excellent electrochemical performance.

Electrodeposition of honeycomb-shaped NiCo2O4 on carbon cloth as binder-free electrode for asymmetric electrochemical capacitor with high energy density

Through the matching of the honeycomb-shaped NiCo₂O₄/CC (HSNC) and reduced graphene oxide/carbon cloth (rGO/CC) to obtain the binder-free asymmetric electrochemical capacitor with high energy density good rate capability and excellent cycle life.

Nickel Cobaltite Nanostructures for Photoelectric and Catalytic Applications

Bimetallic oxide nickel cobaltite (NiCo2 O4 ) shows extensive potential for innovative photoelectronic and energetic materials owing to their distinctive physical and chemical properties. In this review, representative fabrications and applications of NiCo2 O4 nanostructures are outlined for photoelectronic conversion, catalysis, and energy storage, aiming to promote the development of NiCo2 O4 nanomaterials in these fields through an analysis and comparison of their diverse nanostructures. Firstly, a brief introduction of the spinel structures, properties, and morphologies of NiCo2 O4 nanomaterials are presented. Then, the advanced progress of NiCo2 O4 nanomaterials for both photoelectronic conversion and energy fields is summarized including such examples as solar cells, electrocatalysis, and lithium ion batteries. Finally, further prospects and promising developments of NiCo2 O4 nanomaterials in these significant fields are proposed.

Hierarchical Core/Shell NiCo2O4@NiCo2O4 Nanocactus Arrays with Dual-functionalities for High Performance Supercapacitors and Li-ion Batteries

We report the synthesis of three dimensional (3D) NiCo2O4@NiCo2O4 nanocactus arrays grown directly on a Ni current collector using a facile solution method followed by electrodeposition. They possess a unique 3D hierarchical core-shell structure with large surface area and dual-functionalities that can serve as electrodes for both supercapacitors (SCs) and lithium-ion batteries (LIBs). As the SC electrode, they deliver a remarkable specific capacitance of 1264 F g(-1) at a current density of 2 A g(-1) and ~93.4% of capacitance retention after 5000 cycles at 2 A g(-1). When used as the anode for LIBs, a high reversible capacity of 925 mA h g(-1) is achieved at a rate of 120 mA g(-1) with excellent cyclic stability and rate capability. The ameliorating features of the NiCo2O4 core/shell structure grown directly on highly conductive Ni foam, such as hierarchical mesopores, numerous hairy needles and a large surface area, are responsible for the fast electron/ion transfer and large active sites which commonly contribute to the excellent electrochemical performance of both the SC and LIB electrodes.

Three-dimensional cobalt oxide microstructures with brush-like morphology via surfactant-dependent assembly

In this study, three-dimensional cobalt oxide microstructures were developed. Cobalt oxide microdumbbells and microspheres, assembled by nanowires and primary particles, were successfully synthesized by a multistep hydrothermal method. Of all of the structures, the cobalt oxide microdumbbell electrode possesses the largest surface area of 70.8 m(2) g(-1) and the highest specific capacitance of 407.5 F g(-1). The as-prepared electrode also demonstrates excellent electrochemical stability and retains 97.5% of the initial capacitance after 2000 charge-discharge cycles. This performance is attributed to the desirable morphology, uniform microarchitecture stability, and high surface area. The results show that the as-fabricated Co3O4 is a promising electrode material for supercapacitor applications.

Enhanced supercapacitive performance of chemically grown cobalt-nickel hydroxides on three-dimensional graphene foam electrodes

Chemical growth of mixed cobalt-nickel hydroxides (CoxNi1-x(OH)2), decorated on graphene foam (GF) with desirable three-dimensional (3D) interconnected porous structure as electrode and its potential energy storage application is discussed. The nanostructured CoxNi1-x(OH)2 films with different Ni:Co (x) compositions on GF are prepared by using the chemical bath deposition (CBD) method. The structural studies (X-ray diffraction and X-ray photoelectron spectroscopy) of electrodes confirm crystalline nature of CoxNi1-x(OH)2/GF and crystal structure consists of Ni(OH)2 and Co(OH)2. The morphological properties reveal that nanorods of Co(OH)2 reduce in size with increases in nickel content and are converted into Ni(OH)2 nanoparticles. The electrochemical performance reveals that the Co0.66Ni0.33(OH)2/GF electrode has maximum specific capacitance of ∼1847 F g(-1) in 1 M KOH within a potential window 0 to 0.5 V vs Ag/AgCl at a discharge current density of 5 A g(-1). The superior pseudoelectrochemical properties of cobalt and nickel are combined and synergistically reinforced with high surface area offered by a conducting, porous 3D graphene framework, which stimulates effective utilization of redox characteristics and communally improves electrochemical performance with charge transport and storage.

Rapid microwave-assisted green synthesis of 3D hierarchical flower-shaped NiCo₂O₄ microsphere for high-performance supercapacitor

Binary metal oxides with three-dimensional (3D) superstructure have been regarded as desirable electrode materials for the supercapacitor due to the combination of the improved electrical conductivity and effective porous structure. 3D hierarchical flower-shaped nickel cobaltite (NiCo2O4) microspheres have been fabricated by a rapid and template-free microwave-assisted heating (MAH) reflux approach followed by pyrolysis of the as-prepared precursors. The flower-shaped NiCo2O4 microspheres, composed of ultrathin nanopetals with thickness of about 15 nm, are endowed with large specific surface area (148.5 m(2) g(-1)) and a narrow pore size distribution (5-10 nm). The as-fabricated porous flower-shaped NiCo2O4 microspheres as electrode materials for supercapacitor exhibited high specific capacitance of 1006 F g(-1) at 1 A g(-1), enhanced rate capability, and excellent electrochemical stability with 93.2% retention after 1000 continuous charge-discharge (CD) cycles even at a high current density of 8 A g(-1). The desirable integrated performance enables it to be a promising electrode material for the electrochemical supercapacitor (EC).

Hierarchical multi-villous nickel–cobalt oxide nanocyclobenzene arrays: morphology control and electrochemical supercapacitive behaviors

Self-assembled multi-villous nickel–cobalt oxide nanocyclobenzene arrays were directly grown on nickel foam by a mild and low-cost method, and they exhibited superior capacitive performance and long-life stability as an advanced integrated electrode for high-performance supercapacitors.

Growing nickel cobaltite nanowires and nanosheets on carbon cloth with different pseudocapacitive performance

Controllable synthesis and high yield of functional nanomaterials on conductive substrates are highly desirable for energy conversion and storage applications. In this work, two different porous NiCo2O4 nanoarchitectures (including nanowires and nanosheets) are directly grown on carbon cloth collectors, which display a structure-dependence in their capacitive behaviors. Our results show that the nanowire morphology exhibits higher specific capacitance and better cycling performance in the three-electrode configuration. The pseudocapacitive difference is related to the surface area and pore structure of NiCo2O4 nanocrystals. This comparison among different morphologies reveals a process-structure-property relationship in electrochemical energy storage.