Growth of polyaniline nanowhiskers on mesoporous carbon for supercapacitor application

Vertically aligned polyaniline nanowhiskers (PANI-NWs) doped with (1R)-(−)-10-Camphorsulfonic acid (L-CSA) have been successfully synthesized on the external surface of ordered mesoporous carbon (CMK-3) by chemical oxidative polymerization. The specific surface area of the PANI-NWs/CMK-3 nanocomposite remains as high as 497 m² g⁻¹ by removing mesoporous silica template after the polymerization of aniline. Structural and morphological characterizations of the nanocomposite were further investigated by XRD, FTIR and FE-SEM measurements. The result shows that the nanocomposite with 40 wt% PANI applying in supercapacitor devices possesses a large specific capacitance of 470 F g⁻¹ and good capacitance retention of 90.4% is achieved after 1000 cycles at a current density of 1.0 A g⁻¹. The synergistic effect of small PANI nanowhisker arrays and well-ordered mesoporous carbon endows the composite with high electrochemical capacitance and good cycling stability.     

Electrochemical capacitance of NiO/Ru0.35V0.65O2 asymmetric electrochemical capacitor

A designed asymmetric hybrid electrochemical capacitor was presented where NiO and Ru_0.35V_0.65O₂ as the positive and negative electrode, respectively, both stored charge through reversible faradic pseudocapacitive reactions of the anions (OH⁻) with electroactive materials. And the two electrodes had been individually tested in 1 M KOH aqueous electrolyte to define the adequate balance of the active materials in the hybrid system as well as the working voltage of the capacitor based on them. The electrochemical tests demonstrated that the maximum specific capacitance and energy density of the asymmetric hybrid electrochemical capacitor were 102.6 F g⁻¹ and 41.2 Wh kg⁻¹, respectively, delivered at a current density of 7.5 A cm⁻². And the specific energy density decreased to 23.0 Wh kg⁻¹ when the specific power density increased up to 1416.7 W kg⁻¹. The hybrid electrochemical capacitor also exhibited a good electrochemical stability with 83.5% of the initial capacitance over consecutive 1500 cycle numbers.     

High energy density capacitor using coal tar pitch derived nanoporous carbon/MnO2 electrodes in aqueous electrolytes

Asymmetric aqueous electrochemical capacitors with energy densities as high as 22 Wh kg⁻¹, power densities of 11 kW kg⁻¹ and a cell voltage of 2 V were fabricated using cost effective, high surface carbon derived from coal tar pitch and manganese dioxide. The narrow pore size distribution of the activated carbon (mean pore size ∼0.8 nm) resulted in strong electroadsorption of protons making them suitable for use as negative electrodes. Amorphous manganese dioxide anodes were synthesized by chemical precipitation method with high specific capacitance (300 F g⁻¹) in aqueous electrolytes containing bivalent cations. The fabricated capacitors demonstrated excellent cyclability with no signs of capacitance fading even after 1000 cycles.     

Electrochemically synthesized nanocrystalline spinel thin film for high performance supercapacitor

Spinels are not known for their supercapacitive nature. Here, we have explored electrochemically synthesized nanostructured NiCo₂O₄ spinel thin-film electrode for electrochemical supercapacitors. The nanostructured NiCo₂O₄ spinel thin film exhibited a high specific capacitance value of 580 F g⁻¹ and an energy density of 32 Wh kg⁻¹ at the power density of 4 kW kg⁻¹, accompanying with good cyclic stability.     

Microstructure and electrochemical double-layer capacitance of carbon electrodes prepared by zinc chloride activation of sugar cane bagasse

Activated carbons for supercapacitor electrodes are prepared from sugar cane bagasse using chemical activation with ZnCl₂. The ZnCl₂ activation of bagasse is studied using thermogravimetric analysis and the carbon pore structures are characterised using N₂ and CO₂ adsorption. In two-electrode, sandwich-type supercapacitor cells containing 1 M H₂SO₄ the sugar cane bagasse carbons (SCCs) exhibit specific energy up to 10 Wh kg⁻¹ and specific capacitance close to 300 F g⁻¹. The electrochemical performance of the SCCs is attributed to their high specific surface area and the development of mesopores with ZnCl₂ impregnation ratios of 1 or greater. By contrast, the pyrolysis of bagasse without ZnCl₂ produces a carbon with low specific capacitance. The SCC prepared with a ZnCl₂ ratio of 3.5 shows the most stable electrochemical performance at fast charge-discharge rates.     

Electrodeposition and pseudocapacitive properties of tungsten oxide/polyaniline composite

Composite films of tungsten oxide (WO₃) and polyaniline (PANI) have been electrodeposited by cyclic voltammetry in a mixed solution of aniline and precursor of tungsten oxide. Surface morphology and chemical composition of WO₃/PANI composite are characterized by scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The influence of H₂O₂ on the electrodeposition of WO₃/PANI composite film is also investigated. Cyclic voltammetry (CV), chronopotentiometry (CP) and electrochemical impedance spectroscopy (EIS) results show that WO₃/PANI composite film exhibit good pseudocapacitive performance over a wide potential range of −0.5 to 0.7 V vs. SCE with the specific capacitance of 168 F g⁻¹ at current density of 1.28 mA cm⁻² and energy density of 33.6 Wh kg⁻¹, which is 91% higher than that of similarly prepared PANI (17.6 Wh kg⁻¹). An asymmetric model capacitor using WO₃/PANI as negative and PANI as positive electrodes over voltage range of 1.2 V displays a specific capacitance of 48.6 F g⁻¹ and energy density of 9.72 Wh kg⁻¹ at the power density of 53 W kg⁻¹, which is two times higher than that of a symmetric capacitor modeled by using two PANI films as both positive and negative electrodes.     

Electrochemical properties of graphene nanosheets/polyaniline nanofibers composites as electrode for supercapacitors

Graphene nanosheets/polyaniline nanofibers (GNS/PANI) composites are synthesized via in situ polymerization of aniline monomer in HClO₄ solution. The PANI nanofibers homogeneously coating on the surface of GNS greatly improve the charge transfer reaction. The GNS/PANI composites exhibit better electrochemical performances than the pure individual components. A remarkable specific capacitance of 1130 F g⁻¹ (based on GNS/PANI composites) is obtained at a scan rate of 5 mV s⁻¹ in 1 M H₂SO₄ solution compared to 402 F g⁻¹ for pure PANI and 270 F g⁻¹ for GNS. The excellent performance is not only due to the GNS which can provide good electrical conductivity and high specific surface area, but also associate with a good redox activity of ordered PANI nanofibers. Moreover, the GNS/PANI composites present excellent long cycle life with 87% specific capacitance retained after 1000 charge/discharge processes. The resulting composites are promising electrode materials for high-performance electrical energy storage devices.     

Symmetric redox supercapacitor based on micro-fabrication with three-dimensional polypyrrole electrodes

To achieve higher energy density and power density, we have designed and fabricated a symmetric redox supercapacitor based on microelectromechanical system (MEMS) technologies. The supercapacitor consists of a three-dimensional (3D) microstructure on silicon substrate micromachined by high-aspect-ratio deep reactive ion etching (DRIE) method, two sputtered Ti current collectors and two electrochemical polymerized polypyrrole (PPy) films as electrodes. Electrochemical tests, including cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and galvanostatical charge/discharge methods have been carried out on the single PPy electrodes and the symmetric supercapacitor in different electrolytes. The specific capacitance (capacitance per unit footprint area) and specific power (power per unit footprint area) of the PPy electrodes and symmetric supercapacitor can be calculated from the electrochemical test data. It is found that NaCl solution is a good electrolyte for the polymerized PPy electrodes. In NaCl electrolyte, single PPy electrodes exhibit 0.128 F cm⁻² specific capacitance and 1.28 mW cm⁻² specific power at 20 mV s⁻¹ scan rate. The symmetric supercapacitor presents 0.056 F cm⁻² specific capacitance and 0.56 mW cm⁻² specific power at 20 mV s⁻¹ scan rate.     

Electrochemical performance of carbon-coated lithium manganese silicate for asymmetric hybrid supercapacitors

Nanoscale carbon-coated Li₂MnSiO₄ powder is prepared using a conventional solid-state method and can be used as the negative electrode in a Li₂MnSiO₄/activated carbon (AC) hybrid supercapacitor. Carbon-coated Li₂MnSiO₄ material presents a well-developed orthorhombic crystal structure with a Pmn2₁ space group, although there is a small impurity of MnO. The maximum specific capacitance of the Li₂MnSiO₄/AC hybrid supercapacitor is 43.2 F g⁻¹ at 1 mA cm⁻² current density. The cell delivers a specific energy as high as 54 Wh kg⁻¹ at a specific power of 150 W kg⁻¹ and also exhibits an excellent cycle performance with more than 99% columbic efficiency and the maintenance of 85% of its initial capacitance after 1000 cycles.     

Boron and nitrogen co-doped porous carbon and its enhanced properties as supercapacitor

Boron and nitrogen co-doped porous carbons (BNCs) were prepared through a facile procedure using citric acid, boric acid and nitrogen as C, B and N precursors, respectively. The BNC samples were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and nitrogen sorption at 77 K. Cyclic voltammetry and galvanostatic charge/discharge experiments were adopted to investigate their electrochemical behaviors. The BNC-9 and BNC-15 samples with high specific surface areas of 894 and 726 m² g⁻¹ showed the large specific capacitance up to 268 and 173 F g⁻¹, respectively, with the current of 0.1 A g⁻¹. When the current was set as 1 A g⁻¹, the energy densities were 3.8 and 3.0 Wh kg⁻¹ and the power densities were 165 and 201 W kg⁻¹ for BNC-9 and BNC-15, respectively. Thus, BNC-15 is more suitable to apply in high-power-demanded occasion, while BNC-9 tends to store more energy.     

Supercapacitive properties of hybrid films of manganese dioxide and polyaniline based on active carbon in organic electrolyte

This is the first report about supercapacitive performance of hybrid film of manganese dioxide (MnO₂) and polyaniline (PANI) in an organic electrolyte (1.0 M LiClO₄ in acetonitrile). In this work, a high surface area and conductivity of active carbon (AC) electrode is used as a substrate for PANI/MnO₂ film electro-codeposition. The redox properties of the coated PANI/MnO₂ thin film exhibit ideal capacitive behaviour in 1 M LiClO₄/AN. The specific capacitance (SC) of PANI/MnO₂ hybrid film is as high as 1292 F g⁻¹ and maintains about 82% of the initial capacitance after 1500 cycles at a current density of 4.0 mA cm⁻², and the coulombic efficiency (η) is higher than 95%. An asymmetric capacitor has been developed with the PANI/MnO₂/AC positive and pure AC negative electrodes, which is able to deliver a specific energy as high as 61 Wh kg⁻¹ at a specific power of 172 W kg⁻¹ in the range of 0-2.0 V. These results indicate that the organic electrolyte is a promising candidate for PANI/MnO₂ material application in supercapacitors.     

High energy density PbO2/activated carbon asymmetric electrochemical capacitor based on lead dioxide electrode with three-dimensional porous titanium substrate

A PbO₂/AC asymmetric electrochemical capacitor (AEC) with energy density as high as 49.4 Wh kg⁻¹, power density of 433.2 W kg⁻¹ and specific capacitance of 135.2 F g⁻¹ was fabricated with PbO₂ electrodeposited on three-dimensional porous titanium (3D-Ti/PbO₂) and activated carbon. The high electrochemical active surface of 3D-Ti/PbO₂ resulted in high specific capacity making it suitable for use as positive electrode in PbO₂/AC AEC. The fabricated AEC demonstrated good power performance with an energy density conservation of 30 Wh kg⁻¹ at power density of 2078 W kg⁻¹. The fabricated AEC also showed excellent cycling stability with capacitance retention of 99.2% after 1000 cycles.     

Electrophoretic deposition of graphene nanosheets on nickel foams for electrochemical capacitors

Graphene nanosheets are deposited on nickel foams with 3D porous structure by an electrophoretic deposition method using the colloids of graphene monolayers in ethanol as electrolytes. The high specific capacitance of 164 F g⁻¹ is obtained from cyclic voltammetry measurement at a scan rate of 10 mV s⁻¹. When the current densities are set as 3 and 6 A g⁻¹, the specific capacitance values still reach 139 and 100 F g⁻¹, respectively. The high capacitance is attributed to nitrogen atoms in oxidation product of p-phenylene diamine (OPPD) adsorbed on the surface of the graphene nanosheets. The comparable results suggest potential application to electrochemical capacitors based on the graphene nanosheets.     

Preparation of mesoporous carbons from amphiphilic carbonaceous material for high-performance electric double-layer capacitors

Amphiphilic carbonaceous material (ACM), with nanoscale dispersion in alkaline aqueous solutions, is synthesized from green needle coke. As a special precursor with small particle size, plenty of functional groups and widened d₀₀₂ simultaneously, ACM guarantees subsequent ACM-based activated carbons (AACs) with high specific surface area over 3000 m² g⁻¹ as well as well-developed mesoporous structure after KOH activation. Such pore properties enable AACs’ high performances as electrode materials for electric double-layer capacitors (EDLCs). In particular, surface area up to 3347 m² g⁻¹ together with notable mesopore proportion (26.9%) gives sample AAC814 outstanding EDLC behaviors during a series of electrochemical tests including galvanostatic charge/discharge, CV and electrochemical impedance spectroscopy. The electrode gets satisfactory gravimetric and volumetric specific capacitance at the current density of 50 mA g⁻¹, up to 348 F g⁻¹ and 162 F cm⁻³, respectively. Furthermore, for the mesoporosity, there is only a slight capacitance reduction for AAC814 as the current density reaches 1000 mA g⁻¹, indicating its good rate performance. It is all the ACM's unique characteristics that make AACs a sort of competitive EDLC electrode materials, both in terms of specific capacitance and rate capability.     

Nickel hydroxide/activated carbon composite electrodes for electrochemical capacitors

In this paper, a nickel hydroxide/activated carbon (AC) composite electrode for use in an electrochemical capacitor was prepared by a simple chemical precipitation method. The structure and morphology of nickel hydroxide/AC were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results showed that nano-sized nickel hydroxide was loading on the surface of activated carbon. Electrochemical performance of the composite electrodes with different loading amount was studied by cyclic voltammetry and galvanostatic charge/discharge measurements. It was demonstrated that the introduction of a small amount of nickel hydroxide to activated carbon could promote the specific capacitance of a composite electrode. The composite electrodes have good electrochemical performance and high charge–discharge properties. Moreover, when the loading amount of nickel hydroxide was 6 wt.%, the composite electrode showed a high specific capacitance of 314.5 F g⁻¹, which is 23.3% higher than pure activated carbon (255.1 F g⁻¹). Also, the composite electrochemical capacitor exhibits a stable cyclic life in the potential range of 0–1.0 V.     

Solvent-induced porosity control of carbon nanofiber webs for supercapacitor

A simple and scalable method is reported for fabricating a porosity-controlled carbon nanofibers with a skin-core texture by electrospinning a selected blend of polymer solutions. Simple thermal treatment of the electrospun nanofibers from solution blends of various compositions creates suitable ultramicropores on the surface of carbon nanofibers that can accommodate many ions, removing the need for an activation step. The intrinsic properties of the electrode (e.g., nanometre-size diameter, high specific surface area, narrow pore size distribution, tuneable porosity, shallow pore depth, and good ionic accessibility) enable construction of supercapacitors with large specific capacitance (130.7 F g⁻¹), high power (100 kW kg⁻¹), and energy density (15.0 Wh kg⁻¹).     

Supercapacitive properties of polyaniline/Nafion/hydrous RuO2 composite electrodes

Chemically prepared polyaniline is tested for its supercapacitive behaviour in an aqueous electrolyte of 1.0 M H₂SO₄. In order to improve the cycleability of the polyaniline electrode, it is made into a composite with Nafion. This composite electrode shows improved cycleability and higher specific capacitance compared with a pure polyaniline electrode. It is therefore used as a matrix for the electrochemical deposition of hydrous RuO₂. The resulting ternary composite electrode has a high specific capacitance of 475 F g⁻¹ at 100 mV s⁻¹ and 375 F g⁻¹ at 1000 mV s⁻¹ in the voltage range of −0.2 to 0.8 V versus Ag/AgCl. All three types of electrode are characterized by cyclic voltammetry and impedance anaylsis.     

Ru oxide supercapacitors with high loadings and high power and energy densities

Supercapacitors with very high energy and power densities have been constructed with hydrous ruthenium oxide powder prepared by a sol–gel method and annealed at 110 °C. Novel features of the capacitors, which improve their performances, are the use of a carbon fibre paper support, a Nafion separator, and Nafion as a binder. 1 M sulfuric acid was employed as the electrolyte. The performances of the supercapacitors were characterized by cyclic voltammetry, impedance spectroscopy and constant current discharging. The interfacial capacitance increased linearly with increasing ruthenium oxide loading to at least 50 mg cm⁻² on each electrode. The gravimetric capacitance of the Ru oxide measure by impedance reached 742 F g⁻¹ (9.66 F cm⁻²) at a loading of 13.0 mg cm⁻², and an interfacial capacitance of 34.9 F cm⁻² (682 F g⁻¹) was obtained at 51.2 mg cm⁻². The average effective series resistance was 0.55 Ω, the electronic resistance of the electrodes was negligible, and their ionic resistances were <0.42 Ω. The average power density for full discharge at 1 A cm⁻² for supercapacitors with 10 mg cm⁻² Ru oxide increased by 39% when 5% Nafion binder was added. The maximum average power density for full discharge was 31.5 W g⁻¹ while the maximum energy density was 31.2 Wh kg⁻¹. At a 1 mA discharge rate a specific capacitance of 977 F g⁻¹ of Ru oxide was obtained.     

Enhancement of the energy storage properties of supercapacitors using graphene nanosheets dispersed with metal oxide-loaded carbon nanotubes

Graphene nanosheets (GNs) dispersed with SnO₂ nanoparticles loaded multiwalled carbon nanotubes (SnO₂-MWCNTs) were investigated as electrode materials for supercapacitors. SnO₂-MWCNTs were obtained by a chemical method followed by calcination. GNs/SnO₂-MWCNTs nanocomposites were prepared by ultrasonication of the GNs and SnO₂-MWCNTs. Electrochemical double layer capacitors were fabricated using the composite as the electrode material and aqueous KOH as the electrolyte. Electrochemical performance of the composite electrodes were compared to that of pure GNs electrodes and the results are discussed. Electrochemical measurements show that the maximum specific capacitance, power density and energy density obtained for supercapacitor using GNs/SnO₂-MWCNTs nanocomposite electrodes were respectively 224 F g⁻¹, 17.6 kW kg⁻¹ and 31 Wh kg⁻¹. The fabricated supercapacitor device exhibited excellent cycle life with ∼81% of the initial specific capacitance retained after 6000 cycles. The results suggest that the hybrid composite is a promising supercapacitor electrode material.     

Electrochemical properties of manganese oxide coated onto carbon nanotubes for energy-storage applications

Birnessite-type manganese dioxide (MnO₂) is coated uniformly on carbon nanotubes (CNTs) by employing a spontaneous direct redox reaction between the CNTs and permanganate ions (MnO₄⁻). The initial specific capacitance of the MnO₂/CNT nanocomposite in an organic electrolyte at a large current density of 1 A g⁻¹ is 250 F g⁻¹. This is equivalent to 139 mAh g⁻¹ based on the total weight of the electrode material that includes the electroactive material, conducting agent and binder. The specific capacitance of the MnO₂ in the MnO₂/CNT nanocomposite is as high as 580 F g⁻¹ (320 mAh g⁻¹), indicating excellent electrochemical utilization of the MnO₂. The addition of CNTs as a conducting agent improves the high-rate capability of the MnO₂/CNT nanocomposite considerably. The in situ X-ray absorption near-edge structure (XANES) shows improvement in the structural and electrochemical reversibility of the MnO₂/CNT nanocomposite after heat-treatment.