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.     

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.     

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 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.     

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.     

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 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.     

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.     

Carbide-based fuel system for undersea vehicles

In underwater applications such as unmanned undersea vehicle (UUV) propulsion, mass and volume constraints often dictate system energy density and specific energy, which are targeted to exceed 300 Wh L⁻¹ and 300 Wh kg⁻¹, respectively, in order to compete with state-of-the-art battery technologies. To address this need, a novel carbide-based fuel system (CFS) intended for use with a solid oxide fuel cell (SOFC) is under development that is capable of achieving these energy metrics as well as sequestering carbon dioxide. The proposed CFS uses calcium carbide and calcium hydride that react with water to generate acetylene and hydrogen as the fuel and calcium hydroxide as a carbon dioxide scrubber. The acetylene is hydrogenated to ethane and then reformed to syngas (carbon monoxide and hydrogen) before being utilized by the SOFC. Carbon dioxide effluent from the SOFC is reacted with the calcium hydroxide to produce a storable solid, calcium carbonate, thus eliminating gas evolution from the UUV. A system configuration is proposed and discussion follows concerning energy storage metrics, operational parameters and preliminary safety analysis.     

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.     

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 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.     

Electrochemical micro-capacitors of patterned electrodes loaded with manganese oxide and carbon nanotubes

MnO₂ and carbon nanotubes (CNT) composite electrodes have been built on the interdigital stack layers of Fe-Al/SiO₂ and Fe-Al/Au/Ti/SiO₂ for the electrochemical micro-capacitors, using photolithography and thin-film technologies. The electrode properties and the performance of micro-cells are measured and analyzed with cyclic voltammetry (CV), impedance spectroscopy, and galvanostatic charge/discharge test in 0.1 M Na₂SO₄ electrolyte. The vertically aligned CNT, grown on Fe-Al/SiO₂, is more suitable for supporting the pseudocapacitive MnO₂ than the random CNT on Fe-Al/Au/Ti/SiO₂, but ohmic resistance of the former electrode is higher. We have prepared three cells on each stack layer with different electrode materials. The Ragone plot shows systematic variations in power and energy performance, reflecting their differences in electrode structure and polarization loss. The asymmetric cell of a pseudocapacitive positive electrode, loaded with MnO₂ and CNT, exhibits a small IR drop and a high specific energy during discharge. Built on Fe-Al/SiO₂, this asymmetric cell discharges at specific power 0.96 kW kg⁻¹ with specific energy 10.3 Wh kg⁻¹; while on Fe-Al/Au/Ti/SiO₂, the asymmetric cell discharges at power 1.16 kW kg⁻¹ with energy 5.71 Wh kg⁻¹.     

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.     

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).     

Oxygen-selective immobilized liquid membranes for operation of lithium-air batteries in ambient air

In this work, nonaqueous electrolyte-based Li-air batteries with an O₂-selective membrane have been developed for operation in ambient air of 20-30% relative humidity (RH). The O₂ gas is continuously supplied through a membrane barrier layer at the interface of the cathode and ambient air. The membrane allows O₂ to permeate through while blocking moisture. Such membranes can be prepared by loading O₂-selective silicone oils into porous supports such as porous metal sheets and Teflon (PTFE) films. It was found that the silicone oil of high viscosity shows better performance. The immobilized silicone oil membrane in the porous PTFE film enabled the Li-air batteries with carbon black air electrodes to operate in ambient air (at 20% RH) for 16.3 days with a specific capacity of 789 mAh g⁻¹ carbon and a specific energy of 2182 Wh kg⁻¹ carbon. Its performance is much better than a reference battery assembled with a commercial, porous PTFE diffusion membranes as the moisture barrier layer on the cathode, which only had a discharge time of 5.5 days corresponding to a specific capacity of 267 mAh g⁻¹ carbon and a specific energy of 704 Wh kg⁻¹ carbon. The Li-air battery with the present selective membrane barrier layer even showed better performance in ambient air operation (20% RH) than the reference battery tested in the dry air box (<1% RH).     

Al-substituted α-cobalt hydroxide synthesized by potentiostatic deposition method as an electrode material for redox-supercapacitors

Al-substituted α-cobalt hydroxide was prepared by a potentiostatic deposition process at −1.0 V (vs. Ag/AgCl) onto stainless steel electrodes by using a mixed aqueous solution of cobalt nitrate and aluminum nitrate. Their structure and surface morphology were studied by using X-ray diffraction analysis, energy dispersive X-ray spectroscopy and scanning electron microscopy. The SEM images showed changes in the nanostructure of α-cobalt hydroxide by the addition of Al. Galvanostatic charge–discharge curves showed a drastic improvement in the capacitive characteristics of α-cobalt hydroxide, with a specific energy increase from 11.3 to 18.7 Wh kg⁻¹ by the substitution of just 8 at.% Al, and a specific capacitance of 843 F g⁻¹ between 0 and 0.4 V. The cycle stability data suggest no significant changes in the discharge characteristics of α-cobalt hydroxide by the addition of Al.     

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.     

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.     

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.