A facile preparation of graphene/reduced graphene oxide/Ni(OH)2 two dimension nanocomposites for high performance supercapacitors

Zeolitic Imidazolate Framework-67-Derived NiCoMn-Layered Double Hydroxides Nanosheets Dispersedly Grown on the Conductive Networks of Single-Walled Carbon Nanotubes for High-Performance Hybrid Supercapacitors

A supercapacitor's energy storage capability is greatly dependent on electrode materials. Layered double hydroxides (LDHs) were extensively studied as battery-type electrodes because of their 2D structure and quick intercalation/deintercalation of electrolyte ions. However, the energy storage capability for pristine LDHs is limited by their large aggregation tendency and poor electrical conductivity. Herein, a novel NiCoMn-LDH/SWCNTs (single-walled carbon nanotubes) composite electrode material, with ultrathin NiCoMn-LDH nanosheets dispersedly grown among the highly conductive networks of SWCNTs, was prepared via a facile zeolitic imidazolate framework-67 (ZIF-67)-derived in situ etching and deposition procedure. The NiCoMn-LDH/SWCNTs electrode demonstrates a specific capacitance as large as 1704.3 F g-1 at 1 A g-1, which is ascribed to its exposure of more active sites than NiCoMn-LDH. Moreover, the assembled NiCoMn-LDH/SWCNTs//BGA (boron-doped graphene aerogel) hybrid supercapacitor exhibits a superior capacitance of 167.9 F g-1 at 1.0 A g-1, an excellent energy density of 45.7 Wh kg-1 with a power density of 700 W kg-1, and an outstanding cyclic stability with 82.3% incipient capacitance maintained when subjected to 5000 charge and discharge cycles at the current density of 10 A g-1, suggesting the significant potential of NiCoMn-LDH/SWCNTs as the electrode material applicable in supercapacitors.

Facile Synthesis of Sandwich-Type Porous Structured Ni(OH)2/NCNWs/rGO Composite for High Performance Supercapacitor

Nickel hydroxide has ultra-high energy storage capacity in supercapacitors, but poor electrical conductivity limits their further application. The use of graphene to improve its conductivity is an effective measure, but how to suppress the stacking of graphene and improve the overall performance of composite materials has become a new challenge. In this work, a well-designed substrate of N-doped carbon nanowires with reduced graphene oxide (NCNWs/rGO) was fabricated by growing polypyrrole (PPy) nanowires between GO nanosheets layers and then calcining them at high temperatures. This NCNWs/rGO substrate can effectively avoid the stacking of rGO nanosheets, and provides sufficient sites for the subsequent in situ growth of Ni(OH)2, forming a uniform and stable Ni(OH)2/NCNWs/rGO composite material. Benefiting from the abundant pores, high specific surface area (107.2 m2 g-1), and conductive network throughout the NCNWs/rGO substrate, the deposited Ni(OH)2 can not only realize an ultra-high loading ratio, but also exposes more active surfaces (221.3 m2 g-1). After a comprehensive electrochemical test, it was found that the Ni(OH)2/NCNWs/rGO positive materials have a high specific capacitance of 2016.6 F g-1 at a scan rate of 1 mV s-1, and exhibit significantly better stability. The assembled Ni(OH)2/NCNWs/rGO//AC asymmetric supercapacitor could achieve a high energy density of 85.2 Wh kg-1 at power densities of 381 W kg-1. In addition, the asymmetric supercapacitor has excellent stability and could retain 70.1% of initial capacitance after 10,000 cycles. These results demonstrate the feasibility of using NCNWs/rGO substrate to construct high-performance supercapacitor electrode materials, and it is also expected to be promoted in other active composite materials.

Battery-Type-Behavior-Retention Ni(OH)2-rGO Composite for an Ultrahigh-Specific-Capacity Asymmetric Electrochemical Capacitor Electrode

Nanosized battery-type materials applied in electrochemical capacitors can effectively reduce a series of problems caused by low conductivity and large volume changes. However, this approach will lead to the charging and discharging process being dominated by capacitive behavior, resulting in a serious decline in the specific capacity of the material. By controlling the material particles to an appropriate size and a suitable number of nanosheet layers, the battery-type behavior can be retained to maintain a large capacity. Here, Ni(OH)₂, which is a typical battery-type material, is grown on the surface of reduced graphene oxide to prepare a composite electrode. By controlling the dosage of the nickel source, the composite material with an appropriate Ni(OH)₂ nanosheet size and a suitable number of layers was prepared. The high-capacity electrode material was obtained by retaining the battery-type behavior. The prepared electrode had a specific capacity of 397.22 mA h g^-1 at 2 A g^-1. After the current density was increased to 20 A g^-1, the retention rate was as high as 84%. The prepared asymmetric electrochemical capacitor had an energy density of 30.91 W h kg^-1 at a power density of 1319.86 W kg^-1 and the retention rate could reach 79% after 20,000 cycles. We advocate an optimization strategy that retains the battery-type behavior of electrode materials by increasing the size of nanosheets and the number of layers, which can significantly improve the energy density while combining the advantage of the high rate capability of the electrochemical capacitor.

Simultaneous Determination of Dopamine and Uric Acid in Real Samples Using a Voltammetric Nanosensor Based on Co-MOF, Graphene Oxide, and 1-Methyl-3-butylimidazolium Bromide

A chemically modified carbon paste electrode, based on a CoMOF-graphene oxide (GO) and an ionic liquid of 1-methyl-3-butylimidazolium bromide (CoMOF-GO/1-M,3-BB/CPE), was fabricated for the simultaneous determination of dopamine (DA) and uric acid (UA). The prepared CoMOF/GO nanocomposite was characterized by field emission-scanning electron microscopy (FE-SEM), the X-ray diffraction (XRD) method, a N₂ adsorption-desorption isotherm, and an energy dispersive spectrometer (EDS). The electrochemical sensor clearly illustrated catalytic activity towards the redox reaction of dopamine (DA), which can be authenticated by comparing the increased oxidation peak current with the bare carbon paste electrode. The CoMOF-GO/1-M,3-BB/CPE exhibits a wide linear response for DA in the concentration range of 0.1 to 300.0 µM, with a detection limit of 0.04 µM. The oxidation peaks' potential for DA and uric acid (UA) were separated well in the mixture containing the two compounds. This study demonstrated a simple and effective method for detecting DA and UA in real samples.

Template-Free Preparation of α-Ni(OH)2 Nanosphere as High-Performance Electrode Material for Advanced Supercapacitor

Although it is one of the promising candidates for pseudocapacitance materials, Ni(OH)₂ is confronted with poor specific capacitance and inferior cycling stability. The design and construction of three-dimensional (3D) nanosphere structures turns out to be a valid strategy to combat these disadvantages and has attracted tremendous attention. In this paper, a 3D α-Ni(OH)₂ nanosphere is prepared via a facile and template-free dynamic refluxing approach. Significantly, the α-Ni(OH)₂ nanosphere possesses a high specific surface area (119.4 m²/g) and an abundant porous structure. In addition, the as-obtained α-Ni(OH)₂ electrodes are investigated by electrochemical measurements, which exhibit a high specific capacitance of 1243 F/g at 1 A/g in 6 M KOH electrolyte and an acceptable capacitive retention of 40.0% after 1500 charge/discharge cycles at 10 A/g, which can be attributed to the sphere’s unique nanostructure. Furthermore, the as-assembled Ni(OH)₂-36//AC asymmetric supercapacitor (ASC) yields a remarkable energy density of 26.50 Wh/kg, with a power density of 0.82 kW/kg. Notably, two ASCs in series can light a 2.5 V red lamp sustainably for more than 60 min, as well as power an LED band with a rated power of 25 W. Hence, this 3D α-Ni(OH)₂ nanosphere may raise great potential applications for next-generation energy storage devices.

Core-shell Ni1.5Sn@Ni(OH)2 nanoflowers as battery-type supercapacitor electrodes with high rate and capacitance

Poor conductivity and aggregation of two-dimensional Ni(OH)₂ nanosheets hinder their extensive applications in supercapacitors. In the current study, a core-shell nanoflower composite is successfully synthesized using a high conductivity Ni_1.5Sn alloy and Ni(OH)₂ nanosheets via a facile two-step hydrothermal reaction. The alloy material enhances the conductivity of the sample and promotes electron transport for Ni(OH)₂. The as-prepared core-shell structure effectively restrains the clustering of nanosheets and improves the specific surface area of active materials. The optimized NS@NL-3 displays an outstanding specific capacitance (1002.2C g^-1 at 1 A g^-1) and satisfactory capacitance retention rate (80.63% at 20 A g^-1) by adjusting the coating amount of Ni(OH)₂ nanosheets, which is significantly higher compared with the performance of pure Ni(OH)₂ (609.6C g^-1 at 1 A g^-1 and 55.64% at 20 A g^-1). The all-solid-state hybrid supercapacitor (HSC) is fabricated with activated carbon (AC) as the negative electrode and NS@NL-3 as the positive electrode, which shows a high energy density of 57.4 Wh kg^-1 at 803.6 W kg^-1 as well as a superior cycling stability (88.45 % after 10,000 cycles). Experiment shows that 42 LEDs are effortlessly lit by two series-wound solid-state HSC devices, which indicates its high potential for practical applications.

The Evolution in Electrochemical Performance of Honeycomb-Like Ni(OH)2 Derived from MOF Template with Morphology as a High-Performance Electrode Material for Supercapacitors

Ni(OH)2 derived from an MOF template was synthesized as an electrode material for supercapacitors. The electrochemical performance of the electrode was adjusted by effectively regulating the morphology of Ni(OH)2. The evolution of electrochemical performance of the electrode with morphology of Ni(OH)2 was highlighted in detail, based on which honeycomb-like Ni(OH)2 was successfully synthesized, and endowed the electrode with outstanding electrochemical performance. For the three-electrode testing system, honeycomb-like Ni(OH)2 exhibited a very high specific capacitance (1865 F·g-1 at 1 A·g-1, 1550 F·g-1 at 5 mV·s-1). Moreover, it also presented an excellent rate capability and cycling stability, due to 59.46 % of the initial value (1 A·g-1) being retained at 10 A·g-1, and 172% of initial value (first circle at 50 mV·s-1) being retained after 20,000 cycles. With respect to the assembled hybrid supercapacitor, honeycomb-like Ni(OH)2 also displayed superior electrochemical performance, with a high energy density (83.9 Wh·kg-1 at a power density of 374.8 W·kg-1). The outstanding electrochemical performance of Ni(OH)2 should be attributed to its unique honeycomb-like structure, with a very high specific surface area, which greatly accelerates the transformation and diffusion of active ions.