Effects of deposition temperature and annealing temperature on the morphology and electrochemical capacitance of Ni(OH)2 thin films

In this paper, we report 3D nickel (II) hydroxide thin films with porous nanostructures prepared on Ni foam by direct current electrodeposition from aqueous solution of Ni(NO₃)₂ through basic chemicals. The effect of deposition temperature on Ni(OH)₂ thin film morphology is examined by field emission scanning electron microscopy, which is found to have significant influence on capacitance performance of Ni(OH)₂ thin films. Moreover, the effect of annealing temperature on electrochemical capacitance and long-time stability of Ni(OH)₂ thin films is investigated. An optimum-specific capacitance value of 2,447?farads?g^?1 is obtained for Ni(OH)₂ thin film deposited at 20?°C and annealed at 100?°C.     

The rods-like manganese dioxide films grown on nickel foam for electrochemical capacitor applications

MnO₂ films grown on nickel foam (NF) with a desirable 3D structure are investigated as electrochemical pseudocapacitor materials for potential energy storage applications. The prepared MnO₂ films are characterized by X-ray diffraction, FT-IR and scanning electron microscopy. Results indicate that the products are typical hexagonal ?-MnO₂ with a uniform nanorod structure. The electrochemical measurements showed that the MnO₂ films with rods-like morphology have excellent electrochemical performances and its specific capacitance value as single electrode is up to 664 F g^?1 at a discharge current density of 5.5 A g^?1, which is higher than that of most reported corresponding materials. The specific capacitance retention ratio is 76.7% at the current density range from 5.5 to 30 A g^?1. Furthermore, we found that the deposition conditions such as deposition potential and deposition mass have a pronounced effect on their electrochemical activities.     

A hierarchical porous MnO2-based electrode for electrochemical capacitor

A hierarchical porous MnO₂-based electrode was prepared and its electrochemical performance for electrochemical capacitors was investigated. In this work, porous MnO₂ film with pore size of 2?C3?nm in diameter was deposited on a three-dimensional porous current collector by cathodic electrodeposition associated with subsequent controlled heat treatment at 200°C for 2?h. Transmission electron microscopy and X-ray photoelectron spectroscopy showed that the heat treatment has a great effect on the formation of the porous structure of MnO₂ layer, and the disordered porous structure was caused by dehydration during the heat treatment. Cyclic voltammetry and galvanostatic charge?Cdischarge tests showed that both energy and power densities are enhanced due to the unique hierarchical porous structure. The electrode delivers a high specific capacitance of 385?F?g^?1 at a high current density of 5?A?g^?1 within a potential window of ?0.05????0.85?V, and also exhibits excellent rate capability and electrochemical stability.     

Nickel oxide/expanded graphite nanocomposite electrodes for supercapacitor application

Nickel oxide/expanded graphite (NiO/EG) nanocomposites with different loading of EG were prepared through chemically depositing Ni(OH)₂ in EG followed by thermal annealing and characterized by scanning electron microscopy (SEM), powder X-ray diffraction (XRD), Brunauer–Emmet–Teller (BET) isotherm and electrochemical measurements. The prepared NiO/EG composites were found to be crystalline and highly porous with high specific surface area and pore volume. SEM analysis reveals uniform porous morphology for NiO in the NiO/EG-60 nanocomposites which shows good specific capacitance (510?F?g^?1) at a current density of 100?mA?g^?1 in 6?mol?L^?1 KOH measured by chronopotentiometry employing a three-electrode system. The specific capacitance retention of the NiO/EG-60 nanocomposites was found to be ca. 95% after 500 continuous galvanostatic charge–discharge cycles, indicating that the NiO/EG nanocomposites can become promising electro-active materials for supercapacitor application.     

Effect of surfactant on the morphology and capacitive performance of porous NiCo2O4

A promising nickel cobaltite oxide (NiCo₂O₄) composite electrode material was successfully synthesized by a sol-gel method and followed by a simple sintering process. The microstructure and surface morphology of NiCo₂O₄ modified by hexadecyltrimethylammonium bromide (CTAB) and polyvinyl alcohol were physically characterized by powder X-ray diffraction and scanning electron microscopy. Meanwhile, electrochemical performance was widely investigated in 2 M KOH aqueous electrolyte using cyclic voltammetry, galvanostatic charge-discharge test, and electrochemical impedance spectroscopy. The results show that evident porous microstructure was successfully fabricated by CTAB. The NiCo₂O₄ controlled by CTAB exhibited highly specific capacitance of 1,440 F?g^?1 at a current density of 5 mA?cm^?2. Remarkably, it still displays desirable cycle retention of 94.1 % over 1,000 cycle numbers at a current density of 20 mA?cm^?2. The excellent electrochemical performance suggests its potential application in electrode material for electrochemical capacitors.     

Supercapacitor performance of activated carbon derived from rotten carrot in aqueous,organic and ionic liquid based electrolytes

In this work, we report the synthesis of porous activated carbon (AC). AC was derived from rotten carrot, at different values of activating temperature under inert atmosphere, employing chemical activation method and ZnCl₂ as activation agent. On the basis of results observed by surface area and pore size analysis, effect of activation temperature on synthesized AC was determined. Other material properties such as morphology, thermal stability, vibrational response, and crystal structure of prepared AC were studied using standard techniques of material characterization. Further, the electrochemical performance of synthesized AC was studied as an electrode, in aqueous, organic and ionic liquid based electrolyte. It was found that the synthesized AC based electrode exhibits highest specific capacitance (135.5?F?g^?1 at 10?mHz) in aqueous electrolyte and highest specific energy (29.1?Wh?kg^?1 at 2.2?A?g^?1) and specific power (142.5?kW?kg^?1 at 2.2?A?g^?1) in ionic liquid based electrolyte. This shows the suitability of synthesized material for use in energy storage applications.     

Synthesis, characterization, and supercapacitive properties of β-Co(OH)2 leaf-like nanostructures

Leaf-like nanostructures of ??-Co(OH)₂ were successfully prepared via galvanostatically cathodic electrodeposition at the current density of 1?mA?cm^?2 from an aqueous 0.005?M Co(NO₃)₂ bath. The bath temperature was fixed at 60?°C. The XRD and FTIR results revealed that the prepared sample has a single phase of the hexagonal brucite-like ??-Co(OH)₂. Morphological characterization by SEM showed that the prepared ??-Co(OH)₂ was composed of discrete leaf-like nanostructures with edge lengths ranging from 250 to 450?nm. The electrochemical performance of the prepared ??-Co(OH)₂ was evaluated using cyclic voltammetry and charge?Cdischarge tests. A maximum specific capacitance of 772.8?F?g^?1 was obtained in aqueous 1?M KOH with the potential range of ?0.3?C0.5?V (vs. Ag/AgCl) at scan rate of 10?mV?s^?1, suggesting the potential application of the prepared nanostructures in electrochemical supercapacitors. Result of this work showed that cathodic electrodeposition can be recognized as a facile method for the preparation of cobalt hydroxide.     

Core‐Shell Structured Cobalt Sulfide/Cobalt Aluminum Hydroxide Nanosheet Arrays for Pseudocapacitor Application

The direct synthesis of nanostructured electrode materials on three‐dimensional substrates is important for their practical application in electrochemical cells without requiring the use of organic additives or binders. In this study, we present a simple two‐step process to synthesize a stable core–shell structured cobalt sulfide/cobalt aluminum hydroxide nanosheet (LDH‐S) for pseudocapacitor electrode application. The cobalt aluminum layered double hydroxide (CoAl‐LDH) nanoplates were synthesized in basic aqueous solution with a kinetically‐controlled thickness. Owing to the facile diffusion of electrolytes through the nanoplates, thin CoAl‐LDH nanoplates have higher specific capacitance values than thick nanoplates. The as‐grown CoAl‐LDH nanoplates were transformed into core–shell structured LDH‐S nanosheets by a surface modification process in Na₂S aqueous solution. The chemically robust cobalt sulfide (CoS) shell increased the electrochemical stability compared to the sulfide‐free CoAl‐LDH electrodes. The LDH‐S electrodes exhibited high electrochemical performance in terms of specific capacitance and rate capability with a galvanostatic discharge of 1503 F g^?1 at a current density of 2 A g^?1 and a specific capacitance of 91 % at 50 A g^?1.     

Mechanochemically prepared polyaniline and graphene-based nanocomposites as electrodes of supercapacitors

Conductive nanocomposites based on polyaniline and graphene (PAni/Gr) were prepared by cheap and efficient mechanochemical method. The uniform distribution of Gr nanoparticles in the polymer matrix and the ordering of the polymer chains due to the action of mechanical shear stresses, which were established by TEM, stipulated high specific capacitance about 920 F g^?1 in ??0.2–1.0 V vs. Ag/AgCl potential range. PAni/Gr-based electrodes are able to provide the specific capacitance of ~?750 F g^?1 at 2 A g^?1 in symmetric supercapacitors (SSC) and stably cycle at the operating voltage V?=?0.65 V for 10,000 charge-discharge cycles with 96% capacitance retention, whereas the increasing of V leads to the loss of stability as a result of the cathode degradation. PAni/Gr-based SSC possessed improved self-discharge showed high rate capability, and the specific power of such SSC could reach ~?10 kW kg^?1 at the specific energy of ~?18 W h kg^?1.     

One-step synthesis of nitrogen-doped porous carbon for supercapacitors utilizing KNO<Subscript>3</Subscript> as an electrolyte

Nitrogen-doped porous carbons were prepared using a facile method, with low-biotechnology fulvic acid potassium salts as a precursor. The prepared carbons had a high surface area (1623 m² g^?1) and good electrochemical properties, making them suitable electrode materials for supercapacitors. Nitrogen-doped porous carbons were tested as an electrode in both 6 M KOH aqueous solution and different concentrations KNO₃ aqueous solution. The nitrogen-doped porous carbons with unique microstructure and nitrogen functionalities exhibited a capacitance of 235 F g^?1 in a 6 M KOH aqueous solution. Electrochemical investigation showed that the nitrogen-doped porous carbons exhibited a broad potential operational window in a 2.5 M KNO₃ aqueous solution. Furthermore, a high capacitance retention of 88.1 % was achieved even after 5000 cycles at 1.7 V. Potassium nitrate solutions in a wide range of concentrations were also proven to be promising electrolytes for electrochemical capacitors because they are cheap, noncorrosive, electrochemically stable, and compatible to diverse current collectors.     

Facile synthesis of nickel cobalt sulfide nano flowers for high performance supercapacitor applications

A facile synthesis of nickel cobalt sulfide (NCS) nanoflowers have been deposited successfully onto binder free 3D nickel foam electrodes using simple successive ionic layer adsorption and reaction (SILAR) method for supercapacitor applications. The obtained NCS nanoflowers manifest ultrahigh specific capacitance of 1899 F g^?1 at a scan rate of 5 mV s^?1. The NCS nanoflowers exhibit a prominent energy density of 55.16 Wh kg^?1 at power density of 495 W kg^?1 and superior cyclic stability of 94% after 10000 cycles. In addition, the asymmetric supercapacitor (ASC) device is fabricated using NCS nanoflower as positive and reduced graphene oxide (rGO) as negative electrodes, respectively. The ASC (NCS//rGO) delivered good capacity with excellent energy and power densities within 1.6 V wider potential window. Hence, NCS nanoflowers are an outstanding material for energy storage applications in near future.     

Electrospun Porous NiCo2O4 Nanotubes as Advanced Electrodes for Electrochemical Capacitors

Novel, porous NiCo₂O₄ nanotubes (NCO‐NTs) are prepared by a single‐spinneret electrospinning technique followed by calcination in air. The obtained NCO‐NTs display a one‐dimensional architecture with a porous structure and hollow interiors. The effect of precursor concentration on the morphologies of the products is investigated. Due to their unique structure, the prepared NCO‐NT electrode exhibits a high specific capacitance (1647 F g^?1 at 1 A g^?1), excellent rate capability (77.3 % capacity retention at 25 A g^?1), and outstanding cycling stability (6.4 % loss after 3000 cycles), which indicates it has great potential for high‐performance electrochemical capacitors. The desirable enhanced capacitive performance of NCO‐NTs can be attributed to the relatively large specific surface area of these porous and hollow one‐dimensional nanostructures.     

Excellent electrochemical performance of nitrogen-enriched hierarchical porous carbon electrodes prepared using nano-CaCO3 as template

Recently, tremendous research efforts have been concentrated on developing high-performance electrode materials to meet the ever-increasing energy and power demands in supercapacitors. Herein, we presented a high-capacity supercapacitor material based on nitrogen-enriched hierarchical porous carbons (NHPCs) synthesized by the carbonization of melamine formaldehyde resins using eco-friendly and inexpensive nano-CaCO₃ as template. The effects of carbonization temperature and template content on the porous structure and electrochemical characteristics were compared and discussed in detail. The prepared NHPCs possessed large surface area up to 834 m² g^?1 and high nitrogen content up to 20.94 wt %. As electrode material for supercapacitors, NHPCs exhibited superior electrochemical performances with high specific capacitance (190 F g^?1 at 20 A g^?1), outstanding rate capability (80 %), and excellent cycling stability (over 2,000 cycles at 5 A g^?1) in 1 M sulfuric acid media. The excellent electrochemical performances are due to the synergic effects of unique hierarchical porous microstructure, abundant nitrogen and oxygen functionalities, as well as high degree of graphitization framework.     

Flower-like NiO microspheres prepared by facile method as supercapacitor electrodes

Hierarchical flower-like NiO microsphere was successfully synthesized by a simple one-step template-free hydrothermal process, using l-lysine as precipitator and nickel sulfate as nickel source. The as-synthesized materials were characterized by X-ray diffraction, field emission scanning electron microscope, high-resolution transmission electron microscope, and the electrochemical workstation. The electrochemical results show that the flower-like NiO microspheres exhibit specific capacitance as large as 324 F?g^?1 at the current density of 2 A?g^?1 and the specific capacitance retention can maintain 83 % after 1,000 cycles at the current density of 20 A?g^?1 in 6 M KOH.     

Mesoporous Nickel Oxide Nanowires: Hydrothermal Synthesis,Characterisation and Applications for Lithium‐Ion Batteries and Supercapacitors with Superior Performance

Mesoporous nickel oxide nanowires were synthesized by a hydrothermal reaction and subsequent annealing at 400 °C. The porous one‐dimensional nanostructures were analysed by field‐emission SEM, high‐resolution TEM and N₂ adsorption/desorption isotherm measurements. When applied as the anode material in lithium‐ion batteries, the as‐prepared mesoporous nickel oxide nanowires demonstrated outstanding electrochemical performance with high lithium storage capacity, satisfactory cyclability and an excellent rate capacity. They also exhibited a high specific capacitance of 348 F g^?1 as electrodes in supercapacitors.     

Comparison of carbon aerogel and carbide-derived carbon as electrode materials for non-aqueous supercapacitors with high performance

Two porous carbon materials, one synthesised by pyrolysis of an organic aerogel prepared using sol–gel method and the other synthesised from molybdenum carbide by high temperature chlorination method, were tested as supercapacitor electrode materials in a non-aqueous tetraalkylammonium salt-based electrolyte. The gravimetric capacitance values calculated for the carbon aerogel (CAG)-based system were almost two times smaller (~55?F?g^?1) compared to carbide-derived carbon (C(Mo₂C))-based system (~125?F?g^?1). However, due to the very wide region of ideal polarizability, 3.6?V for C(Mo₂C) and 3.8?V for CAG-based test cells, very high energy densities up to 63?Wh?kg^?1 (34?Wh?dm^?3) and power densities up to 757?kW?kg^?1 (314?kW?dm^?3) were estimated for these systems, respectively. CAG-based system shows very short characteristic charge/discharge time constant values (0.05?s).     

Meso-macroporous Co3O4 electrode prepared by polystyrene spheres and carbowax templates for supercapacitors

Meso-macroporous Co₃O₄ electrode is synthesized by drop coating with a mixed solution containing Co(OH)₂ colloid, polystyrene spheres, and carbowax (namely polyethylene glycol), followed by calcining at 400?°C to remove polystyrene spheres and carbowax. For comparison, nonporous Co₃O₄ and mesoporous Co₃O₄ electrodes are prepared by drop coating with Co(OH)₂ colloid and with a mixed solution containing Co(OH)₂ colloid and carbowax under the same condition, respectively. Capacitive property of these electrodes is measured by cyclic voltammetry, potentiometry and electrochemical impedance spectroscopy. The results show that meso-macroporous Co₃O₄ electrode exhibits larger specific capacitance than those of nonporous Co₃O₄ electrode and mesoporous Co₃O₄ electrode at various current densities. The specific capacitance of meso-macroporous Co₃O₄ electrode at the current density of 0.2?A?g^?1 is 453?F?g^?1. Meanwhile, meso-macroporous Co₃O₄ electrode possesses the highest specific capacitance retention ratio at the current density ranging from 0.2 to 1.0?A?g^?1, indicating that meso-macroporous Co₃O₄ electrode suits to high-rate charge?Cdischarge.     

In Situ Growth of MnO2 Nanosheets on N‐Doped Carbon Nanotubes Derived from Polypyrrole Tubes for Supercapacitors

Nitrogen‐doped porous carbon nanotubes@MnO₂ (N‐CNTs@MnO₂) nanocomposites are prepared through the in situ growth of MnO₂ nanosheets on N‐CNTs derived from polypyrrole nanotubes (PNTs). Benefiting from the synergistic effects between N‐CNTs (high conductivity and N doping level) and MnO₂ nanosheets (high theoretical capacity), the as‐prepared N‐CNTs@MnO₂‐800 nanocomposites show a specific capacitance of 219 F g^?1 at a current density of 1.0 A g^?1, which is higher than that of pure MnO₂ nanosheets (128 F g^?1) and PNTs (42 F g^?1) in 0.5 m Na₂SO₄ solution. Meanwhile, the capacitance retention of 86.8 % (after 1000 cycles at 10 A g^?1) indicates an excellent electrochemical performance of N‐CNTs@MnO₂ prepared in this work.     

The production of activated carbon from cation exchange resin for high-performance supercapacitor

High-performance activated carbon for electrochemical double-layer capacitors (EDLC) has been prepared from cation exchange resin by carbonization and subsequent activation with KOH. The activation temperature has a key role in the determination of porous carbon possessing high surface areas, and large pore structures. The porous carbon activated at 700 °C (carbon-700-1:4) has high surface area (2236 m²?g^?1) and large total pore volume (1.15 cm³?g^?1), which also displays best capacitive performances due to its well-balanced micro- or mesoporosity distribution. In details, specific capacitances of the carbon-700-1:4 sample are 336.5 F?g^?1 at a current density of 1 A?g^?1 and 331.8 F?g^?1 at 2 A?g^?1. At high current density as 20 A?g^?1, the retention of its specific capacitance is 68.4 %. The carbon-700-1:4 sample also exhibits high performance of energy density (46.7 Wh?kg^?1) and long cycle stability (～8.9 % loss after 3,000 cycles). More importantly, due to the amount of waste ion-exchange resins increasing all over the world, the present synthetic method might be adopted to dispose them, producing high-performance porous carbons for EDLC electrode materials.     

Manganese Oxide/Graphene Aerogel Composites as an Outstanding Supercapacitor Electrode Material

Graphene aerogels (GA), prepared with an organic sol–gel process, possessing a high specific surface area of 793 m² g^?1, a high pore volume of 3 cm³ g^?1, and a large average pore size of 17 nm, were applied as a support for manganese oxide for supercapacitor applications. The manganese oxide was electrochemically deposited into the highly porous GA to form MnO₂/GA composites. The composites, at a high manganese oxide loading of 61 wt. %, exhibited a high specific capacitance of 410 F g^?1 at 2 mV s^?1. More importantly, the high rate specific capacitances measured at 1000 mV s^?1 for these composites were two‐fold higher than those obtained with samples prepared in the absence of the GA support. The specific capacitance retention ratio, based on the specific capacitance obtained at 25 mV s^?1, was maintained high, at 85 %, even at the high scan rate of 1000 mV s^?1, in contrast with the significantly lower value of 67 % for the plain manganese oxide sample. For the cycling stability, the specific capacitance of the composite electrode decayed by only 5 % after 50,000 cycles at 1000 mV s^?1. The success of this MnO₂/GA composite may be attributed to the structural advantages of high specific surface areas, high pore volumes, large pore sizes, and three‐dimensionally well‐connected network of the GA support. These structural advantages made possible the high mass loading of the active material, manganese oxide, large amounts of electroactive surfaces for the superficial redox events, fast mass‐transfer within the porous structure, and well‐connected conductive paths for the involved charge transport.