Supercapacitors based on high-surface-area graphene

Exfoliated graphite oxide was prepared by an improved Hummers method and was then reduced to graphene with hydrazine in the presence of ammonium hydroxide.N2adsorption–desorption measurement showed that graphene so obtained had a specific surface area as high as 818 m2/g.Galvanostatic charge/discharge curves demonstrated that the as-prepared graphene exhibited a specific capacitance of 186.9 F/g at a current density of 0.1 A/g and that about 96%of the specific capacitance was retained after 2000 cycles at a current density of 5 A/g.     

Effect of Ni-doping on electrochemical capacitance of MnO2 electrode materials

Mn/Ni composite oxides as active electrode materials for supercapacitors were prepared by solid-state reaction through the reduction of KMnO4 with manganese acetate and nickel acetate at low temperature. The products were characterized by X-ray diffractometry(XRD) and transmission electron microscopy(TEM). The electrochemical characterizations were performed by cyclic voltammetry (CV) and constant current charge-discharge in a three-electrode system. The effects of different potential windows, scan rates, and cycle numbers on the capacitance behavior of Mn0.8Ni0.2Ox composite oxide were also investigated. The results show that the composite oxides are of nano-size and amorphous structure. With increasing the molar ratio of Ni, the specific capacitance goes through a maximum at molar fraction of Ni of 20%. The specific capacitance of Mn0.8Ni0.2Ox composite oxide is 194.5 F/g at constant current discharge of 5 mA.     

Performance of a combined capacitor based on ultrafine nickel oxide／carbon nanotubes composite electrodes

A new sol-gel process for the preparation of ultrafine nickel hydroxide electrode materials was developed. The composite electrodes consisting of carbon nanotubes and Ni(OH)2 were developed by mixing the hydroxide and carbon nanotubes together in different mass ratios. In order to enhance energy density, a combined type pseudocapacitor/electric double layer capacitor was considered and its electrochemical properties were characterized by cyclic voltammetry and dc charge/discharge test. The combined capacitor shows excellent capacitor behavior with an operating voltage up to 1.6 V in KOH aqueous electrolyte. Stable charge/discharge behaviors were observed with much higher specific capacitance values of 24 F/g compared with that of EDLC (12 F/g) by introducing 60% Ni(OH)2 in the anode material. By using the modified anode of a Ni(OH)E/Carbon nanotubes composite electrode, the specific capacitance of the cell was less sensitive to discharge current density compared with that of the capacitor employing pure nickel hydroxide as anode. The combined capacitor in this study exhibits high energy density and stable power characteristics.     

Electrochemical performance of nickel oxide／KOH／active carbon super-capacitor

The fabrication and characterization of new type Nickel oxide/KOH/Active carbon super-capacitor have been described. Porous nickel oxide was prepared by hydrolysis of nickel acetate and heated in air at 300℃. The resulting nickel oxide behaved as an electrochemical capacitor electrode with a specific capacitance (50-70 F/g) superior to most active carbon electrodes. This kind of nickel oxide maintained high utilization at high rate of discharge (i.e., high power density) and had excellent cycle life more than 1 000 times,while the capacitance of the cell composed of two identical nickel oxide electrodes was poor at high discharge current density and the maximum operational voltage of this type capacitor was limited to 0.5 V. A new type super-capacitor was designed in which the nickel oxide and the active carbon were applied to the positive and negative electrodes respectively. The breakdown voltage of this type super-capacitor was improved effectively to 0.8 V and excellent characteristic of high power discharge was attained in this way. The Nickel oxide/KOH/Active carbon super-capacitor has promising potentials in portable telecommunications, uninterruptable power supplies and battery load leveling applications.     

Solvothermal Synthesis and Electrochemical Properties of Octahedral Cobalt Oxide Decorated with Ag_2O

Octahedral CoO with nanostructures decorated with Ag nanoparticles was prepared via a facile solvothermal approach. After being annealed at 500 ℃ for 1 h, an electrochemical capacitor material of Co_3O_4 decorated with Ag_2O was obtained. The cyclic voltammetry and galvanostatic charge-discharge were used to evaluate the electrochemical properties of the as-prepared products. The results indicated that the as-prepared samples exhibited fine pseudo-capacitive performance, and the surface modifications of Ag_2O can significantly increase the capacitance of the Co_3O_4 material. The specific capacitance of Ag2O/Co_3O_4 composite electrode was up to 217.6 F·g~(-1), which was 3.35 times as high as that of pure Co_3O_4. Moreover, Ag2O/Co_3O_4 composite showed an excellent cycle performance, and 65.3% of specific capacitance was maintained after 200 cycles.     

Structural and Electrochemical Performances of α-MnO_2 Doped with Tin for Supercapacitors

To improve the electrochemical performances of α-MnO_2 as electrode materials for supercapacitors, Sn-doped α-MnO_2 in the presence of the doping amount of 1%-4% was successfully synthesized by hydrothermal method. As-prepared α-MnO_2 presents nanorod shape and no other impurities exist. By ultraviolet-visible absorption spectroscopy, it is convinced that the band gaps of α-MnO_2 decrease with increasing Sn-doping amount. Cyclic voltammetry investigation indicates that undoped and doped α-MnO_2 all have regular capacitive response. As the scan rate enlarged, the profiles of curves gradually deviate from rectangle. Compared with undoped α-MnO_2, doped α-MnO_2 has larger specific capacitance. The specific capacitance of 3% doped α-MnO_2 reaches 241.0 F/g while undoped α-MnO_2 only has 173.0 F/g under 50 m A/g current density in galvanostatical charge-discharge measurement. Enhanced conductivity by Sn-doping is considered to account for doped sample's enhanced electrochemical specific capacitance.     

Nanosized Nickel Oxides Derived from the Citrate Gel Process and Performances for Electrochemical Capacitors

Nanosized nickel oxide powders were prepared by thermal decomposition of the nickel citrate gel precursors. The thermal decomposition and powder materials derived from calcination of these gel precursors with various ratios of citric acid （CA） to nickel at different temperatures and times were characterized by thermal analysis （TG/DTA）, scanning electron microscopy （SEM）, x-ray diffraction （XRD）, and measurement of specific surface area （BET） with porosity analyses. The optimized processing conditions of calcination temperature 400℃ for 1 hour with the CA/Ni ratio of 1.2, were determined to produce the nanosized nickel oxide pow- ders with a high specific surface area of 181 m^2/g, nanometer particle sizes of 15-25 nm, micro-pore diameter distribution between 4-10 nm. The capacitance characteristics of the nanosized nickel oxide electrode in various concentrations of KOH solutions were studied by the cyclic voltammetry （CV） exhibiting both a double-layer capacitance and a faradaic pseudocapacitance. The nanosized nickel oxide electrode shows a high cyclic stability and is promising for high performance electrochemical capacitors.     

Development of supercapacitors based on carbon nanotubes

Block-type electrodes made of carbon nanotubes were fabricated by different processes. The volumetric specific capacitance based on such electrodes reached 107 F/cm3, which proves carbon nanotubes to be ideal candidate materials for supercapacitors. The composite electrodes consisting of carbon nanotubes and RuO2 ·xH2O were developed by the deposition of RuO2 on the surface of carbon nanotubes. Supercapacitors based on the composite electrodes show much higher specific capacitance than those based on pure carbon nanotube ones. A specific capacitance of 600 F/g can be achieved when the weight percent of RuO2· xH2O in the composite electrodes reaches 75% . In addition , supercapacitors based on the composite electrodes show both high energy density and high power density characteristics.     

Comparison of capacitive behavior of activated carbons with different pore structures in aqueous and nonaqueous systems

The pore structures of two activated carbons from sawdust with KOH activation and coconut-shell with steam activation for supercapacitor were analyzed by N2 adsorption method. The electrochemical properties of both activated carbons in 6 mol/L KOH solution and 1 mol/L EtgNPF4/PC were compared, and the effect of pore structure on the capacitance was investigated by cyclic voltammetry, AC impedance and charge-discharge measurements. The results indicate that the capacitance mainly depends on effective surface area, but the power property mainly depends on mesoporosity. At low specific current （1 A/g）, the maximum specific Capacitances of 276.3 F/g in aqueous system and 123.9 F/g in nonaqueous system can be obtained from sawdust activated carbon with a larger surface area of 1 808 m^2/g, butat a high specific current, the specific capacitance of coconut-shell activated carbon with a higher mesoporosity of 75.1% is more excellent. Activated carbon by KOH activation is fitter for aqueous system and that by steam activation is fitter for nonaqueous system.     

Effect of Graphene Surface Functional Groups on the Mechanical Property of PMMA Microcellular Composite Foams

The functional groups on graphene sheets surface affect their dispersion and interfacial adhesion in polymer matrix. We compared the mechanical property of polymethymethacrylate(PMMA) microcellular foams reinforced with graphene oxide(GO) and reduced graphene oxide(RGO) to investigate this influence of functional groups. RGO sheets were fabricated by solvent thermal reduction in DMF medium. UV-Vis, FT-IR and XPS analyses indicate the difference of oxygen-containing groups on GO and RGO sheets surface. The observation of SEM illustrates that the addition of a smaller number of GO or RGO sheets causes a fine cellular structure of PMMA foams with a higher cell density(about 10~(11) cells/cm~3) and smaller cell sizes(about 1-2 μm) owing to their remarkable heterogeneous nucleation effect. Compared to GO reinforced foams, the RGO/PMMA foams own lower cell density and bigger cell size in their microstructure, and their compressive strength is lower even when the reinforcement contents are the same and the foam bulk density is higher. These results indicate that the oxygen-containing groups on GO sheets' surface are beneficial to adhere CO_2 to realize a larger nucleation rate, and their strong interaction with PMMA matrix improves the mechanical property of PMMA foams.     

Preparation and Characterization of Low Density Poly (Imino Imino Ketone) Foam

Poly(Imino Ketone)(PIK) is a group of novel high performance polymer material with excellent thermal properties and dissolubility. Aiming at the requirements of inertial confined fusion(ICF) studies on low density polymer foams, we firstly synthesized poly(imino imino ketone)(PIIK) by palladium catalyzed C-N cross-coupling reaction, and successfully prepared the PIIK foam material with a density of 80-300 mg/cm3 by using PIIK as the raw material with thermal-induced phase separation and lyophilization technique. Mercury injection method was used to determine the structure of PIIK foams, and the results indicated that the mean pore diameter was lower than 5 μm and it had relatively high voidage and specific surface area.     

Optimal electrochemical performances of CO2 activated carbon aerogels for supercapacitors

Activated carbon aerogels（ACAs） derived from sol-gel polycondensation of resorcinol （R） and formaldehyde （F） were pyrolyzed under Ar flow and activated in CO2 atmosphere. The morphology of ACAs was characterized by scanning electron microscopy （SEM） and the structural properties were determined by N2 adsorption at 77 K. The results show that ACAs have a typical three-dimensional nanonetwork structure composing of cross-linking of carbon nanoparticles. The specific surface area and the total pore volume remarkably increase with increasing activation time while the previous porous structure still remains. The specific capacitance of the 950-10-ACA electrode can reach up to 212.3 F/g in 6 mol/L KOH electrolyte. The results of constant-current charge-discharge testing indicate that the ACAs electrodes present fast charge- discharge rate and long cycle life （about 98% capacitance retained after 3000 charge-discharge cycles at 1.25 mA/cm2）. Lower internal resistances can be achieved for 950-10-ACA electrode in KOH electrolyte. Our investigations are very important to improve the wettability and electrochemical performance of electrode for supercapacitors.     

Properties of functionalized graphene/room temperature vulcanized silicone rubber composites prepared by an In-situ reduction method

Functionalized graphene oxide (FGO) was prepared by treating graphene oxide with γ-aminopropyl triethoxysilane (KH-550) before the mixture was dispersed into α, ω-dihydroxy polydimethylsiloxane to get room temperature vulcanized (RTV) silicone rubber composites by solution casting. The cured composites were then reduced with hydrazine hydrate to obtain functionalized graphene (FG)/RTV silicone rubber composites. The structures of FGO and the resultant composites were characterized by atomic force microscopy, FT-IR spectra and X-ray diffraction. KH-550 was found to be grafted onto graphene sheets, leading to an increased interlayer spacing. Significant improvements in thermal and mechanical properties were obtained. Both the FGO/RTV silicone rubber composite contain 1.0 wt% of FGO, and its reduced product showed an increase of one-step weight loss temperature with 61 ℃ and 133 ℃ higher than that of pure silicone rubber. Tensile strength and elongation at break of FG/RTV silicone rubber composite (with 0.5 wt% FGO content) increased by 175% and 67%, respectively, compared with those of pure silicone rubber.     

Manufacture of high-dispersed W-Cu composite powder by mechano-thermal process

In order to improve the process of co-reduction of oxide powder, a new mechano-thermal process was designed to produce high-dispersed W-Cu composite powder by high temperature oxidation, short time high-energy milling and reduction at lower temperature. The particle size, oxygen content and their sintering abilities of W-Cu composite powder in different conditions were analyzed. The results show that after a quick milling of the oxide powder for about 3-10 h, the reduction temperature of the W-Cu oxide powder can be lowered to about 650 ℃ from 700-750 ℃ owning to the lowering of particle size of the oxide powder. The average particle size of W-Cu powder after reduction at 650 ℃ is about 0. 5μm smaller than that reduced at 750 ℃. After sintering at 1 200℃ for 1 h in hydrogen atmosphere, the relative density and thermal conductivity of final products (W-20Cu) can attain 99. 5% and 210 W ·m-1· K-1 respectively.     

Convenient Design of Hollow Co_3O_4 Nanostructure on Carbon Cloth for Flexible Supercapacitors

We proposed a new approach to construct zeolitic imidazolate frameworks-67 (ZIF-67) powders directly on carbon cloth without any conductive agent and adhesion agent and the ZIF-67 nanoparticles on carbon cloth were successfully converted into hollow Co_3O_4 nanostructure via a facile calcination process.Compared with original ZIF-67 powders,the Co_3O_4@CC electrode materials had admirable specific capacitance of 1 164.8 F·g~(-1) at an area current density of 2.5 mA·cm~(-2).Furthermore,the rate performance remained 42.4% of initial value when the current density was increased to 30 mA·cm~(-2) and the specific capacitance maintained 93.4% of initial capacity after 5 000 cycles at an area current density of 10 mA·cm~(-2).This strategy may have potential prospect for the application of MOFs in the energy storage and conversion field.     

Investigation of Novel Short Fiber-like Polyaniline/Cerium Nitrate Composite

Polyaniline(PANI)/Ce(NO_3)_3 composite with short fiber-like shape was synthesized successfully in a poly(2-arcylamido-2-methylpropane sulfonic acid) aqueous solution. A comparison of SEM images found that short fiber-like composites can be obtained by controlling the dosage of Ce(NO_3)_3. The length and diameter of short fiber-like PANI/Ce(NO_3)_3 composite was about 630 and 200 nm, respectively. A special conjugated structure had formed via Ce~(3+) ions and –NH– group in the quinonoid ring of PANI, which was characterized by means of Fourier transform infrared(FTIR) spectroscopy, Ultraviolet-visible(UV-Vis) spectroscopy and X-ray photoelectron spectroscopy(XPS). Short fiber-like PANI/Ce(NO_3)_3 composite exhibited a high conductivity, a large capacitance and an enhanced anticorrosion property. Linear four-probe method confirmed that the electrical conductivity of composites was improved with the presence of Ce~(3+) ions. The corrosion potential of PANI/Ce(NO_3)_3 composite increased to-79 mV at 10 wt% of Ce(NO_3)_3. Meanwhile, the minimum density of corrosion current(1.4 μA/cm~2) was also achieved.     

Vanadium Nitride Nanoparticles as Anode Material for Lithium Ion Hybrid Capacitor Applications

A high production efficiency synthesis method was used to produce a stacked vanadium nitride nanoparticle structure with an inexpensive raw material as an anode material and high surface area polystyrene was used the cathode material for lithium ion hybrid capacitors. The Li-HCs cell displayed an excellent specific capacitance of 64.2 F·g~(~(-1)) at a current density of 0.25 A·g~(-1) and a wide potential window of 0.01 to 3.5 V. Furthermore, the device exhibited a high energy density of 109.3 W·h·kg~(-1) at a power density of 512.3 W·kg~(-1) and retained an energy density of 69.2 W·h·kg~(-1) at a high power density of 3 498.9 W· kg~(-1) at 2 A·g~(-1). Due to the short synthesis time and simple raw materials, this method is suitable for industrial production.     

Super-capacitive Properties of Thick RuO2·xH2O Electrodes:Effect of Thickness,Binder and Carbon Black Addition

Hydrous ruthenium oxide,RuO2·xH2O,was synthesized through a sol-gel reaction of ruthenium chloride aqueous solution.The annealed RuO2·xH2O powders are aggregate of very small particles in nano-size of 70-80 nm and have abundant porous structure as observed by SEM.The effect of electrode thickness,PTFE binder and carbon black addition on the super-capacitive properties of thick RuO2·nH2O electrodes was investigated.At low scan rate of 5 mV/s,the specific capacitance of RuO2·xH2O (SCR,based on the weight of RuO2·xH2O only) for pure RuO2·xH2O and RuO2·xH2O-PTFE-CB electrodes can reach 665 F/g.Increasing electrode thickness and mixing PTFE into RuO2·xH2O lead to a degradation of SCR and rate capacity.By adding carbon black into RuO2·xH2O-PTFE mixture,the rate capacity of the thick electrode (RuO2·xH2O-PTFE-CB) can be improved close to the thick pure RuO2·xH2O electrode.However,mixing with PTFE and carbon black leads to a loss of the overall mass-specific capacitance.The reason for the dependence of SCR and rate capacity on electrode thickness,binder and carbon black addition was discussed based on the relation between the resistance and utilization rate of electrodes.     

Performance of a 60 F carbon nanotubes-based supercapacitor for hybrid power sources

A supercapacitor based on charge storage at the interface between a high surface area carbon nanotube electrode and a Li-ClO4/PC electrolyte was assembled. The performance of the capacitor depends on not only the material used in the cell but also the construction of the cell. From a constant charge-discharge test, the capacitance of 60 F was obtained. The performance of the power capacitor for pulse power sources was described. The specific energy (0.8 W·h·kg-1) and the specific power (0.75 kW·kg-1) of the power supercapacitor were demonstrated with a cell of the maximum operating voltage of 2.5 V. A hybrid power source consisting of a lithium ionic battery and the 60 F supercapacitor was demonstrated to power successfully a simulated power load encountered in GSM portable communication equipment. The addition of the supercapacitor to the power train of a cellular phone results in significantly more energy from the battery being used by the load. The experiments indicate that more than 33.8% energy i     

Preparation of spherical and dense LiNi0.8Co0.2O2 lithium-ion battery particles by spray pyrolysis

With citric acid as a polymeric agent layered LiNi0.8Co0.2O2 materials were synthesized by a spray pyrolysis method. The LiNi0.sCo0.2O2 particles were characterized by means of XRD, SEM and TEM. The electrochemical performances of LiNi0.8Co0.2O2 particles were studied in a voltage window of 3.00-4.35 V and at a current density of 30 mA/g. The results show that in the pilot-scale spray pyrolysis process, the morphology of particles is dependent upon the precursor concentration and flux of carrier gas. The initial discharge capacity of the LiNi0.8Co0.2O2particles at 720 ℃ for 12 h is 187.3 mA.h/g, and the capacity remains 96.8% with excellent cycleability after 30 cycles. The LiNi0.8Co0.2O2 samples synthesized under the optimized conditions by the spray pyrolysis method shows a good electrochemical performance.