A cheap asymmetric supercapacitor with high energy at high power: Activated carbon//K0.27MnO2·0.6H2O

Studies of the electrochemical behavior of K_0.27MnO₂·0.6H₂O in K₂SO₄ show the reversible intercalation/deintercalation of K⁺-ions in the lattice. An asymmetric supercapacitor activated carbon (AC)/0.5 mol l⁻¹ K₂SO₄/K_0.27MnO₂·0.6H₂O was assembled and tested successfully. It shows an energy density of 25.3 Wh kg⁻¹ at a power density of 140 W kg⁻¹; at the same time it keeps a very good rate behavior with an energy density of 17.6 Wh kg⁻¹ at a power density of 2 kW kg⁻¹ based on the total mass of the active electrode materials, which is higher than that of AC/0.5 mol l⁻¹ Li₂SO₄/LiMn₂O₄. In addition, this asymmetric supercapacitor shows excellent cycling behavior without the need to remove oxygen from the electrolyte solution. This can be ascribed in part to the stability of the lamellar structure of K_0.27MnO₂·0.6H₂O. This asymmetric aqueous capacitor has great promise for practical applications due to high energy density at high power density.     

Dual functions of activated carbon in a positive electrode for MnO2-based hybrid supercapacitor

Utilizing the dual functions of activated carbon (AC) both as a conductive agent and an active substance of a positive electrode, a hybrid supercapacitor (AC-MnO₂&AC) with a composite of manganese dioxide (MnO₂) and activated carbon as the positive electrode (MnO₂&AC) and AC as the negative electrode is fabricated, which integrates approximate symmetric and asymmetric behaviors in the distinct parts of 2 V operating windows. MnO₂ in the positive electrode and AC in the negative electrode together form a pure asymmetric structure, which extends the operating voltage to 2 V due to the compensatory effect of opposite over-potentials. In the range of 0-1.1 V, both AC in the positive and negative electrode assemble as a symmetric structure via a parallel connection which offers more capacitance and less internal resistance. The optimal mass proportions of electrodes are calculated though a mathematical process. In a stable operating window of 2 V, the capacitance of AC-MnO₂&AC can reach 33.2 F g⁻¹. After 2500 cycles, maximum energy density is 18.2 Wh kg⁻¹ with a 4% loss compared to the initial cycle. The power density is 10.1 kW kg⁻¹ with an 8% loss.     

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.     

An asymmetric anthraquinone-modified carbon/ruthenium oxide supercapacitor

An asymmetric supercapacitor with improved energy and power density, relative to a symmetric Ru oxide device, has been constructed with anthraquinone-modified carbon fabric (Spectracarb 2225) as the negative electrode and Ru oxide as the positive electrode. The performance of the supercapacitor was characterized by cyclic voltammetry and constant current discharging. Use of the anthraquinone-modified electrode extends the negative potential limit that can be used, relative to Ru oxide, and allows higher cell voltages to be used. The maximum energy density obtained was 26.7 Wh kg⁻¹ and an energy density of 12.7 Wh kg⁻¹ was obtained at a 0.8 A cm⁻² discharge rate and average power density of 17.3 kW kg⁻¹. The C-AQ/Ru oxide supercapacitor requires 64% less Ru relative to a symmetric Ru oxide supercapacitor.     

Characterization of MnFe2O4/LiMn2O4 aqueous asymmetric supercapacitor

A new type of asymmetric supercapacitor containing a MnFe₂O₄ negative electrode and a LiMn₂O₄ positive electrode in aqueous LiNO₃ electrolyte has been synthesized and characterized. The nanocrystalline MnFe₂O₄ anode material has a specific capacitance of 99 F g⁻¹ and the LiMn₂O₄ cathode a specific capacity of 130-100 mAh g⁻¹ under 10-100 C rate. The cell has a maximum operating voltage window of ca. 1.3 V, limited by irreversible reaction of MnFe₂O₄ toward reducing potential. The specific power and specific energy of the full-cell increase with increasing anode-to-cathode mass ratio (A/C) and saturate at A/C ∼4.0, which gives specific cell energies, based on total mass of the two electrodes, of 10 and 5.5 Wh kg⁻¹ at 0.3 and 1.8 kW kg⁻¹, respectively. The cell shows good cycling stability and exhibits significantly slower self-discharge rate than either the MnFe₂O₄ symmetric cell or the other asymmetric cells having the same cathode but different anode materials, including activated carbon fiber and MnO₂.     

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.     

Electrochemical profile of an asymmetric supercapacitor using carbon-coated LiTi2(PO4)3 and active carbon electrodes

In this work, we reported an asymmetric supercapacitor in which active carbon (AC) was used as a positive electrode and carbon-coated LiTi₂(PO₄)₃ as a negative electrode in 1 M Li₂SO₄ aqueous electrolyte. The LiTi₂(PO₄)₃/AC hybrid supercapacitor showed a sloping voltage profile from 0.3 to 1.5 V, at an average voltage near 0.9 V, and delivered a capacity of 30 mAh g⁻¹ and an energy density of 27 Wh kg⁻¹ based on the total weight of the active electrode materials. It exhibited a desirable profile and maintained over 85% of its initial energy density after 1000 cycles. The hybrid supercapacitor also exhibited an excellent rate capability, even at a power density of 1000 W kg⁻¹, it had a specific energy 15 Wh kg⁻¹ compared with 24 Wh kg⁻¹ at the power density about 200 W kg⁻¹.     

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.     

Graphene-MnO2 and graphene asymmetrical electrochemical capacitor with a high energy density in aqueous electrolyte

The graphene-manganese oxide hybrid material has been prepared by solution-phase assembly of aqueous dispersions of graphene nanosheets and manganese oxide nanosheets at room temperature. The morphology and structure of the obtained material are examined by scanning electron microscopy, transition electron microscopy, X-ray diffraction and N₂ adsorption-desorption. Electrochemical properties are characterized by cyclic voltammetry, galvanostatic charge-discharge and electrochemical impedance spectroscopy. An asymmetric electrochemical capacitor with high energy and power densities based on the graphene-manganese oxide hybrid material as positive electrode and graphene as negative electrode in a neutral aqueous Na₂SO₄ solution as electrolyte is assembled. The asymmetrical electrochemical capacitor could cycle reversibly in a voltage of 0-1.7 V and give an energy density of 10.03 Wh kg⁻¹ even at an average power density of 2.53 kW kg⁻¹. Moreover, the asymmetrical electrochemical capacitor exhibit excellent cycle stability, and the capacitance retention of the asymmetrical electrochemical capacitor is 69% after repeating the galvanostatic charge-discharge test at the constant current density of 2230 mA g⁻¹ for 10,000 cycles.     

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.     

Asymmetric capacitor based on superior porous Ni-Zn-Co oxide/hydroxide and carbon electrodes

Hierarchical porous multi-phase Ni-Zn-Co oxide/hydroxide is synthesized by using metal-organic framework-5 (MOF-5) as the template. Hierarchical porous carbon is obtained by the facile direct decomposition of the MOF-5 framework with phenolic resin. The structures and textures are characterized by X-ray diffraction, high-resolution transmission electron microscopy, scanning electron microscopy, and nitrogen sorption at 77 K. An asymmetric capacitor incorporating the Ni-Zn-Co oxide/hydroxide as the positive electrode and the porous carbon as the negative electrode is fabricated. A maximum energy density of 41.65 Wh kg⁻¹ is obtained, which outperforms many other available asymmetric capacitors. The asymmetric capacitor also shows a good high-rate performance, possessing an energy density of 16.62 Wh kg⁻¹ at the power density of about 2900 W kg⁻¹.     

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.     

Hybrid supercapacitor based on MnO2 and columned FeOOH using Li2SO4 electrolyte solution

The nano-sized columned β-FeOOH was prepared by the hydrolysis process and its electrochemical capacitance performance was evaluated for the first time in Li₂SO₄ solution. A hybrid supercapacitor based on MnO₂ positive electrode and FeOOH negative electrode in Li₂SO₄ electrolyte solution was designed. The electrochemical tests demonstrated that the hybrid supercapacitor has a energy density of 12 Wh kg⁻¹ and a power density of 3700 W kg⁻¹ based on the total weight of the electrode active materials with a voltage range 0–1.85 V. This hybrid supercapacitor also exhibits a good cycling performance and keeps 85% of initial capacity over 2000 cycles.     

Electrodeposited manganese oxides on three-dimensional carbon nanotube substrate: Supercapacitive behaviour in aqueous and organic electrolytes

Thin amorphous manganese oxide layers with a thickness of 3–5 nm are electrodeposited on a carbon nanotube (CNT) film substrate that has a three-dimensional nanoporous structure (denoted as MnO₂/CNT electrode). For the purpose of comparison, manganese oxide films are also electrodeposited on a flat Pt-coated Si wafer substrate (denoted as MnO₂ film electrode). The pseudocapacitive properties of the MnO₂ film and MnO₂/CNT electrodes are examined in both aqueous electrolyte (1.0 M KCl) and non-aqueous organic electrolyte (1.0 M LiClO₄ in propylene carbonate). While both types of electrode show pseudocapacitive behaviour in the aqueous electrolyte, only the MnO₂/CNT electrode does so in the organic electrolyte, due to its high oxide/electrolyte interfacial area and improved electron conduction through the CNT substrate. Compared with the MnO₂ film electrode, the MnO₂/CNT electrode shows a much higher specific capacitance and better high-rate capability, regardless of the electrolyte used. Use of the organic electrolyte results in a ∼6 times higher specific energy compared with that obtained with the aqueous electrolyte, while maintaining a similar specific power. The construction of a three-dimensional nanoporous network structure consisting of a thin oxide layer on a CNT film substrate at the nm scale and the use of an organic electrolyte are promising approaches to improving the specific energy of supercapacitors.     

Carbon nanotube/MnO2 composites synthesized by microwave-assisted method for supercapacitors with high power and energy densities

Carbon nanotube (CNT)/MnO₂ composites are synthesized by reduction of potassium permanganate under microwave irradiation. The morphology and microstructure of samples are examined by scanning electron microscopy (SEM), transition electron microscopy (TEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). Electrochemical properties are characterized by cyclic voltammetry (CV), galvanostatic charge/discharge and electrochemical impedance spectroscopy (EIS). Birnessite-type MnO₂ homogeneously coats on the surfaces of CNTs. For CNT–15%MnO₂ composite, the specific capacitance based on MnO₂ is 944 (85% of the theoretical capacitance) and 522 F g⁻¹ at 1 and 500 mV s⁻¹, respectively. When the content of MnO₂ reaches 57 wt%, the composites have the maximum power density (45.4 kW kg⁻¹, the energy density is 25.2 Wh kg⁻¹). Therefore, CNT/MnO₂ composites prepared by microwave irradiation are promising electrode materials in hybrid vehicle systems.     

Physical and electrochemical characteristics of supercapacitors based on carbide derived carbon electrodes in aqueous electrolytes

FIB-SEM, XPS and gas adsorption methods have been used for the characterisation of physical properties of microporous carbide derived carbon electrodes prepared from Mo₂C at 600 °C (noted as CDC-Mo₂C). Cyclic voltammetry, constant current charge/discharge, and electrochemical impedance spectroscopy have been applied to establish the electrochemical characteristics for supercapacitors consisting of the 1 M Na₂SO₄, KOH, tetraethyl ammonium iodide or 6 M KOH aqueous electrolyte and CDC-Mo₂C electrodes. The N₂ sorption values obtained have been correlated with electrochemical characteristics for supercapacitors in various aqueous electrolytes. The maximum gravimetric energy, E_max, and gravimetric power, P_max, for supercapacitors (taking into consideration the active material weight) have been obtained at cell voltage 0.9 V for 6 M KOH aqueous supercapacitor (E_max = 5.7 Wh kg⁻¹ and P_max = 43 kW kg⁻¹). For 1 M TEAI based SC somewhat higher E_max (6.2 Wh kg⁻¹) and comparatively low P_max (7.0 kW kg⁻¹) have been calculated.     

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.     

Flexible supercapacitor based on polyaniline nanowires/carbon cloth with both high gravimetric and area-normalized capacitance

Flexible supercapacitor is successfully fabricated using polyaniline nanowires/carbon cloth (PANI-NWs/CC) nanocomposite. High gravimetric capacitance of 1079 F g⁻¹ at a specific energy of 100.9 Wh kg⁻¹ and a specific power of 12.1 kW kg⁻¹ is obtained. Moreover, this approach also offers an exceptionally high area-normalized capacitance of 1.8 F cm⁻². The diffusion length of protons within the PANI-NWs is estimated to be about 60 nm by electrochemical impedance analysis, which indicates that the electrochemical performance of the electrode is not limited by the thickness of PANI-NWs. The electrochemical performance of PANI-NWS/CC remains without any deterioration, even when the cell is bent under high curvature. These results clearly present a cost-effective and simple method of fabrication of the nanostructured polymers with enormous potential in flexible energy storage device applications.     

Development of all-solid-state mediator-enhanced supercapacitors with polyvinylidene fluoride/lithium trifluoromethanesulfonate separators

All-solid-state supercapacitors (SCs) were fabricated using a polyvinylidene fluoride (PVDF)/lithium trifluoromethanesulfonate (LiTFS) membrane as the separator and poly(ethylene oxide) (PEO)/lithium perchlorate (LiClO₄) as the polymer electrolyte in the porous carbon electrodes. Two types of mediators, NaI/I₂ and K₃Fe(CN)₆/K₄Fe(CN)₆, were added into the PEO/LiClO₄ polymer electrolyte that was used to fabricate the electrodes. The voltage window in which the SCs operated was 2.5-3 V. The results of electrochemical measurements, including cyclic voltammetry and galvanostatic charge/discharge, indicated that NaI/I₂-containing and K₃Fe(CN)₆/K₄Fe(CN)₆-containing SCs yielded high specific capacitances of 209.0 and 138.8 F g⁻¹, respectively. In addition to high specific capacitances for the two mediator-containing SCs, both SCs delivered high specific energies (49.1 Wh kg⁻¹ at 1.6 kW kg⁻¹ for the NaI/I₂-containing SC and 33.6 Wh kg⁻¹ at 1.3 kW kg⁻¹ for the K₃Fe(CN)₆/K₄Fe(CN)₆-containing SC) due to the wide voltage window and fast redox reactions between mediators.     

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).