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.     

Synergistic enhancement of supercapacitance upon integration of nickel (II) octa [(3,5-biscarboxylate)-phenoxy] phthalocyanine with SWCNT-phenylamine

Supercapacitive behaviour of a novel functional material, nickel (II) octa [(3,5-biscarboxylate)-phenoxy] phthalocyanine (NiOBCPPc) upon covalent integration with phenylamine functionalized single-walled carbon nanotubes (SWCNT-phenylamine) is reported for the first time. The supercapacitive behaviour of the hybrid (NiOBCPPc-SWCNT-phenylamine) was investigated using galvanostatic charge-discharge and electrochemical impedance spectroscopy. Using a more reliable galvanostatic charge-discharge method, the NiOBCPPc-SWCNT-phenylamine hybrid exhibited superior geometrical capacitance (∼186 mF cm⁻²) compared to either NiOBCPPc (∼54 mF cm⁻²) or SWCNT-phenylamine (∼74 mF cm⁻²) at a current density of 138 μA cm⁻². The NiOBCPPc-SWCNT-phenylamine gave excellent stability of over 1000 charge-discharge continuous cycling.     

Mesoporous nano-Co3O4: A potential negative electrode material for alkaline secondary battery

A potential negative electrode material (mesoporous nano-Co₃O₄) is synthesized via a simple thermal decomposition of precursor Co(OH)₂ hexagonal nanosheets in the air. The structure and morphology of the samples are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). It is found that the nano-Co₃O₄ is present in mesoporous hexagonal nanoparticles. The average size of holes is about 5-15 nm. The electrochemical performances of mesoporous nano-Co₃O₄ as the active starting negative electrode material for alkaline secondary battery are investigated by galvanostatic charge-discharge and cyclic voltammetry (CV) technique. The results demonstrate that the prepared mesoporous nano-Co₃O₄ electrode displays excellent electrochemical performance. The discharge capacity of the mesoporous nano-Co₃O₄ electrode can reach 436.5 mAh g⁻¹ and retain about 351.5 mAh g⁻¹ after 100 cycles at discharge current of 100 mA g⁻¹. A properly electrochemical reaction mechanism of mesoporous nano-Co₃O₄ electrode is also constructed in detail.     

Anodic deposition of manganese oxide electrodes with rod-like structures for application as electrochemical capacitors

The electrochemical properties of nanocrystalline manganese oxide electrodes with rod-like structures were investigated to determine the effect of morphology, chemistry and crystal structure on the corresponding electrochemical behavior of manganese electrodes. Manganese oxide electrodes of high porosity composed of 1-1.5 μm diameter rods were electrochemically synthesized by anodic deposition from a dilute solution of Mn(CH₃COO)₂ (manganese acetate) onto Au coated Si substrates without any surfactants, catalysts or templates under galvanostatic control. The morphology of the electrodes depended on the deposition current density, which greatly influenced the electrochemical performance of the capacitor. Electrochemical property and microstructure analyses of the manganese oxide electrodes were conducted using cyclic voltammetry and microstructural techniques, such as scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The synthesized rod-like manganese oxide electrodes at low current densities exhibited a high specific capacitance due to their large surface areas. The largest value obtained was 185 F g⁻¹ for deposits produced at .5 mA cm⁻². Specific capacity retention for all deposits, after 250 charge-discharge cycles in an aqueous solution of 0.5 M Na₂SO₄, was about 75% of the initial capacity.     

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.     

Intercalation of PF6 anion into graphitic carbon with nano pore for dual carbon cell with high capacity

Intercalation property of PF₆⁻ into graphitic carbon was studied for a hybrid capacitor with different ratio of cathode and anode amount. Graphene sheet distance increased with increasing PF₆⁻ intercalation amount and it saturated at 0.4 nm at high applied potential, which is corresponded to stage 2 structure. On the other hand, it was found that nano size pore into graphene sheet was introduced at higher applied potential with 20 times larger anode carbon and this nano porous carbon shows a large capacity for intercalation capacity of 147 mAh g⁻¹. The estimated energy density of the hybrid capacitor using carbon with nano bubble structure was ca. 400 Wh kg⁻¹.     

Chemically grown, porous, nickel oxide thin-film for electrochemical supercapacitors

A porous nickel oxide film is successfully synthesized by means of a chemical bath deposition technique from an aqueous nickel nitrate solution. The formation of a rock salt NiO structure is confirmed with XRD measurements. The electrochemical supercapacitor properties of the nickel oxide film are examined using cyclic voltammetery (CV), galvanostatic and impedance measurements in two different electrolytes, namely, NaOH and KOH. A specific capacitance of ∼129.5 F g⁻¹ in the NaOH electrolyte and ∼69.8 F g⁻¹ in the KOH electrolyte is obtained from a cyclic voltammetery study. The electrochemical stability of the NiO electrode is observed for 1500 charge-discharge cycles. The capacitative behaviour of the NiO electrode is confirmed from electrochemical impedance measurements.     

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.     

High-energy density graphite/AC capacitor in organic electrolyte

A high-energy density hybrid capacitor has been designed in organic electrolyte (1 mol L⁻¹ LiPF₆ in 1:1 ethylene carbonate (EC)/dimethyl carbonate (DMC)) using commercial grades of graphite and activated carbon for negative and positive electrodes, respectively. Different approaches have been explored for assembling the hybrid capacitor in order to achieve an optimum ratio between the energy and power density, while keeping a long cycle-life capability. In the optimized hybrid capacitor, the potential of the positive electrode ranges from 1.5 up to 5 V vs. Li/Li⁺, being extended to the whole stability window of the activated carbon in the organic electrolyte, whereas the potential of the negative electrode remains almost constant at around 0.1 V vs. Li/Li⁺. After balancing carefully the respective masses of the electrodes and appropriately formatting the system, it was found that a voltage of 4.5 V is the optimal value for avoiding a capacitance fading of the hybrid capacitor during cycling. Gravimetric and volumetric energy densities as high as 103.8 Wh kg⁻¹ and 111.8 Wh L⁻¹, respectively, were obtained. The noticeable value of volumetric energy density is 10 times higher than for symmetric or asymmetric capacitors built with the same activated carbon.     

Electrochemical capacitance of Co3O4 nanowire arrays supported on nickel foam

Co₃O₄ nanowire arrays freely standing on nickel foam are prepared via template-free growth followed by thermal treatment at 300 °C in air. Their morphology is examined by scanning and transmission electron microscopy. The electrochemical capacitance behavior of the self-supported binderless nanowire array electrode is investigated by cyclic voltammetry, galvanostatic charge-discharge test and electrochemical impedance spectroscopy. The results show that nanowires are formed by nanoplatelets packed roughly layer by layer. They densely cover the nickel foam substrate and have diameters around 250 nm and the lengths up to around 15 μm. The Co₃O₄ nanowires display a specific capacitance of 746 F g⁻¹ at a current density of 5 mA cm⁻². The capacitance loss is less than 15% after 500 charge-discharge cycles. The columbic efficiency is higher than 93%.     

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.     

Optimization of MnO2/vertically aligned carbon nanotube composite for supercapacitor application

The optimization strategy for producing manganese oxide supercapacitors based on vertically aligned carbon nanotubes (VACNTs) deposited on large area electrodes is presented. A single sequential process of sputtering, annealing and plasma enhanced chemical vapour deposition (PECVD) is applied to produce dense and uniform VACNTs electrodes. As dielectric layer of the supercapacitor, manganese oxide is electrodeposited lining the surface of the VACNTs electrodes. The control of the growing parameters such as catalyst thickness layer, temperature and deposition time for tuning the density, length and diameter of the VACNTs and their structure are found to be key points for the optimization of the MnO₂ electrodeposition process in view to improve the efficiency of the supercapacitor devices.The electrochemical properties of the obtained electrodes are characterized using cyclic voltammetry and galvanostatic charge-discharge techniques. A specific capacitance of 642 Fg⁻¹ is obtained for MnO₂/VACNTs nanocomposite electrode at a scan rate of 10 mV s⁻¹.     

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.     

Synthesis and improved electrochemical performances of porous Li3V2(PO4)3/C spheres as cathode material for lithium-ion batteries

Spherical Li₃V₂(PO₄)₃/C composites are synthesized by a soft chemistry route using hydrazine hydrate as the spheroidizing medium. The electrochemical properties of the materials are investigated by galvanostatic charge-discharge tests, cyclic voltammograms and electrochemical impedance spectrum. The porous Li₃V₂(PO₄)₃/C spheres exhibit better electrochemical performances than the solid ones. The spherical porous Li₃V₂(PO₄)₃/C electrode shows a high discharge capacity of 129.1 and 125.6 mAh g⁻¹ between 3.0 and 4.3 V, and 183.8 and 160.9 mAh g⁻¹ between 3.0 and 4.8 V at 0.2 and 1 C, respectively. Even at a charge-discharge rate of 15 C, this material can still deliver a discharge capacity of 100.5 and 121.5 mAh g⁻¹ in the potential regions of 3.0-4.3 V and 3.0-4.8 V, respectively. The excellent electrochemical performance can be attributed to the porous structure, which can make the lithium ion diffusion and electron transfer more easily across the Li₃V₂(PO₄)₃/electrolyte interfaces, thus resulting in enhanced electrode reaction kinetics and improved electrochemical performance.     

Layered MoS2–graphene composites for supercapacitor applications with enhanced capacitive performance

Layered molybdenum disulfide (MoS₂)–graphene composite is synthesized by a modified l-cysteine-assisted solution-phase method. The structural characterization of the composites by energy dispersive X-ray analysis, X-ray powder diffraction, Fourier transform infrared spectroscopy, XPS, Raman, and transmission electron microscope indicates that layered MoS₂–graphene coalescing into three-dimensional sphere-like architecture. The electrochemical performances of the composites are evaluated by cyclic voltammogram, galvanostatic charge–discharge and electrochemical impedance spectroscopy. Electrochemical measurements reveal that the maximum specific capacitance of the MoS₂–graphene electrodes reaches up to 243 F g⁻¹ at a discharge current density 1 A g⁻¹. The energy density is 73.5 Wh kg⁻¹ at a power density of 19.8 kW kg⁻¹. The MoS₂–graphene composites electrode shows good long-term cyclic stability (only 7.7% decrease in specific capacitance after 1000 cycles at a current density of 1 A g⁻¹). The enhancement in specific capacitance and cycling stability is believed to be due to the 3D MoS₂–graphene interconnected conductive network which promotes not only efficient charge transport and facilitates the electrolyte diffusion, but also prevents effectively the volume expansion/contraction and aggregation of electroactive materials during charge–discharge process. Taken together, this work indicates MoS₂–graphene composites are promising electrode material for high-performance supercapacitors.     

Exaggerated capacitance using electrochemically active nickel foam as current collector in electrochemical measurement

In the past decades, nickel and cobalt oxide/hydroxide materials have been investigated intensively for supercapacitor applications. Some works report very high specific capacitance values, up to 3152 F g⁻¹, for these materials. By contrast, some other works report quite modest capacitance values, up to 380 F g⁻¹ for the same materials prepared using same strategy. It is found that most works reporting very high capacitance value applied nickel foam as current collector. In this paper, surface chemistry and electrochemical properties of nickel foam are investigated by XPS analysis, cyclic voltammetry and galvanostatic charge-discharge measurement. The results show that using nickel foam as current collector can bring about substantial errors to the specific capacitance values of electrode materials, especially when small amount of electrode active material is used in the measurement. It is suggested that an electrochemically inert current collector such as Ti or Pt film should be used for testing electrochemical properties of nickel and cobalt oxide/hydroxide positive electrode materials.     

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.     

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.     

Practical and theoretical limits for electrochemical double-layer capacitors

Two types of double-layer capacitors, based on carbon materials, were analysed: (1) an imaginary nano-capacitor assembled from single graphene sheets, separated by electrolyte layers (thickness of nanometers) and (2) a capacitor based on porous carbons. It has been shown that the maximum specific surface of a porous carbon material which may be used for the construction of a capacitor is ca. 2600 m² g⁻¹. The maximum energy density of an imaginary double-layer ‘nano-capacitor’, is close to 10 kJ kg⁻¹ at a voltage of U = 1 V (aqueous electrolyte) of ca. 40–45 kJ kg⁻¹ at U ≈ 2.3–2.5 V (organic electrolytes), and at the order of 100 kJ kg⁻¹ at voltages close to 4 V (ionic liquids as electrolytes). The real device consists of porous electrodes and a separator, both soaked with the electrolyte, as well as current collectors. Consequently, the maximum electric capacity expressed versus the mass of the device (ca. 20–30 F g⁻¹), is much smaller than the corresponding value expressed versus the mass of the carbon material (ca. 300 F g⁻¹). In order to obtain the energy density of the device at a level of 100 kJ kg⁻¹ (characteristic for the lead-acid battery), the capacitor with porous carbon electrodes should operate at voltages of ca. 4 V (ionic liquids as electrolytes). However, the specific power density of such a capacitor having an acceptable energy density (ca. 100 kJ kg⁻¹) is relatively low (ca. 1 kW kg⁻¹).     

Nickel foam supported Sn-Co alloy film as anode for lithium ion batteries

Sn-Co alloy films are deposited electrochemically directly onto nickel foam in an aqueous solution. The influence of electrochemical current density and heat treatment on the structure and morphology of the electrodeposited films is studied by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The electrochemical properties of the Sn-Co alloy films are further investigated by galvanostatic charge-discharge tests. As anodes for lithium ion batteries, the Sn-Co alloy-film anodes, after further heat treatment at 200 °C for 30 min, delivers a specific capacity of 663 mAh g⁻¹ after 60 cycles. This high capacity retention is attributed to the unique electrode configuration with an enhanced interface strength between the active material and the current collector formed in the heat-treatment process.