Pseudo-capacitance of ruthenium oxide/carbon black composites for electrochemical capacitors

Hydrous ruthenium oxide was formed by a new process. The precursor was obtained by mixing the aqueous solutions of RuCl3xH2O and NaHCO3. The addition of NaHCO3 led to the formation of an oxide with extremely fine RuO2 particles forming a porous network structure in the oxide electrode. Polyethylene glycol was added as a controller to partly inhibit the sol-gel reaction. The rate capacitance of 530 F·g^-1 was measured for the powder formed at an optimal annealing temperature of 210℃. Several details concerning this new material, including crystal structure, particle size as a function of temperature, and electrochemical properties, were also reported. In addition, the rate capacitance of the composite electrode reached 800 F·g^-1 after carbon black was added. By using the modified electrode of a RuO2/carbon black composite electrode, the electrochemical capacitor exhibits high energy density and stable power characteristics. The values of specific energy and maximum specific power of 24 Wh·kg^-1 and 4 kW·kg^-1, respectively, are demonstrated for a cell voltage between 0 and 1 V.

Chemically Modified Ordered Mesoporous Carbon/Polyaniline Composites for Electrochemical Capacitors

Chemically modified ordered mesoporous carbon CMK-3 materials were prepared by means of an easy wet-oxidative method in 2 mol/L nitric acid aqueous solution. A large amount of oxygen-containing functional groups were introduced onto the CMK-3 surface. Modified CMK-3（m-CMK-3） and aniline monomer were polymerized via an in situ chemical oxidative polymerization method. Morphological characterizations of m-CMK-3/PANI （polyaniline） composites were carried out via field emission scanning electron microscopy（SEM）. Their electrochemical properties were investigated with cyclic voltammetry, galvanostatic charge-discharge, and electrochemical impedance spectroscopy. The m-CMK-3/PANI composites have excellent properties in capacitance, and the highest specific capacitance（SC） value was up to 489 F/g, suggesting their potential application in the electrode material for electrochemical capacitors.

Biomass-derived activated carbon materials with plentiful heteroatoms for high-performance electrochemical capacitor electrodes

Activated carbons for electrochemical capacitor electrodes are prepared from soyabean using chemical activation with KOH. The pore size is easily controllable by changing the mass ratio between KOH and carbonized product. The as-prepared materials possess a large specific surface area, unique structure, well- developed hierarchical porosity and plentiful heteroatoms（mainly O and N）. Thus resulted in its high specific capacitance,good rate capacity and cycling stability. Moreover, attributing to worldwide availability, renewable nature and low-cost, activated carbon prepared from soyabean has a good potential in energy conversion and storage devices.

Review of Electrochemical Capacitors Based on Carbon Nanotubes and Graphene

Electrochemical capacitors, which can store large amount of electrical energy with the capacitance of thousands of Farads, have recently been attracting enormous interest and attention. Carbon nanostructures such as carbon nanotubes and graphene are considered as the potentially revolutionary energy storage materials due to their excellent properties. This paper is focused on the application of carbon nanostructures in electrochemical capacitors, giving an overview regarding the basic mechanism, design, fabrication and achievement of latest research progresses for electrochemical capacitors based on carbon nanotubes, graphene and their composites. Their current challenges and future prospects are also discussed.

The key challenges and future opportunities of electrochemical capacitors

Electrochemical capacitors(ECs)with unique merits of fast charge/discharge rate and long cyclability are one of the representative electrochemical energy storage systems,possessing wide applications in power electronics and automotive transportation,etc.[1,2].Furthermore.

AC Line Filter Electrochemical Capacitors:Materials,Morphology,and Configuration

Recently,more and more supercapacitors(SCs)have been developed as AC line filter capacitors,which are generally named AC line filter electrochemical capacitors(FECs).Compared to traditional bulky aluminum electrolytic capacitors(AECs),FECs have higher capacity and lower space occupancy,which makes them a strong competitor.However,different from the common SCs for energy storage,it is necessary to consider the frequency response of the SCs for AC line filtering,where the contradiction between frequency response and specific capacitance is a challenge.The researchers have proposed different solutions from the perspective of materials,morphology,and configuration for this challenge.Based on the above background,in this review,we briefly introduce the principle and parameters of AC line filter electrochemical capacitors.We systematically summarize the state-of-the-art progresses of FECs and discuss their possible application and development in the future.The development of FECs can greatly promote the planarization,integration,and miniaturization of filter capacitors,and provide a new solution for the utilization of green and unstable energy.

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.

Intedrcalation in Two-Dimensional Transition Metal Carbides and Nitrides(MXenes) toward Electrochemical Capacitor and Beyond

Since its discovery in 2011,the emerging family of two-dimensional(2D)transition metal carbides,nitrides,and carbonitrides(denoted as MXenes)has shown tremendous promise in the field of energy storages,especially electrochemical capacitors(ECs).The intercalation of foreign species,including but not limited to proton,cations,organic ions,and solvents,is demonstrated as one of the dominant facts during the energy storage process of MXenes,through which interlayer spacing of MXenes can be reversibly tuned,followed with reversible redox reactions on the surface of MXenes.Such a mechanism provides MXenes extremely high capacitance up to 1500 F cm-3,in aqueous acidic electrolyte.In addition,because of the versatile terminations on their surface,the intercalation of ions into MXenes can simultaneously induce the reversible transformation of their electronic structure to trigger some other phenomenon,for example,electrochromic effect,which has great potential on electrochromic smart window—an extended application of ECs.Accordingly,regulating and facilitating the intercalation in MXenes is of great significance for MXene-based ECs.In this review,we summarize the recent progress on the intercalation in MXenes towards ECs,discussing on the intercalated species,working mechanisms,and methods to promote the intercalation.Furthermore,we prospect the future research directions of intercalation of MXenes in ECs,such as the combination of simulation and experiment on finding the best intercalation species,precisely controlling the interlayer spacing and beyond,to boost the electrochemical performance of MXene toward practical applications and multi-functional devices.

Nanosized Ni-Mn Oxides Prepared by the Citrate Gel Process and Performances for Electrochemical Capacitors

Nanosized Ni-Mn oxide powders have been successfully citrate gel precursors. The powder materials derived from prepared by thermal decomposition of the Ni-Mn calcination of the gel precursors with various molar ratios of nickel and manganese at different temperatures and time were characterized using thermal analysis （TG-DSC）, scanning electron microscopy （SEM）, X-ray diffraction （XRD） and Brunauer-Emmet-Teller （BET）. The optimized processing conditions of calcination at 400℃ for 1 h with Ni/Mn molar ratio 6 were proved to produce the nanosized Ni-Mn oxide powders with a high specific surface area of 109.62 m^2/g and nanometer particle sizes of 15-30 nm. The capacitance characteristics of the nanosized Ni-Mn oxide electrode in various concentrations of KOH solutions were studied by the cyclic voltammetry （CV） and exhibited both a doublelayer capacitance and a Faradaic capacitance which could be attributed to the electrode consisting of Ni-Mn oxides and residual carbons from the organic gel thermal decomposition. A specific capacitance of 194.8 F/g was obtained for the electrode at the sweep rate of 10 mV/s in 4 mol/L KOH electrolyte and the capacitor showed quite high cyclic stability and is promising for advanced electrochemical capacitors.

Chemical Treatment of Carbon Nanotubes as Electrodes in Electrochemical Double-Layer Capacitors

Multi-walled carbon nanombes with homogeneous diameters （40 - 60 nm）, produced by chemical vapor deposition of hydrocarbon gas, are purified by nitric acids. Infrared and Raman studies indicate that oxygen containing surface groups, which are predominately carboxylic, phenolic and lactonic groups, are introduced into purified carbon nanotubes. Then three kinds of block-form porous tablets of carbon nanotubes are fabricated as electrodes in electrochemical double-layer capacitors. Using mounded mixture comprising carbon nanotubes and binder powders provides these tablets. Comparison of the effect of different processing on the structural performance of the capacitors is specifically investigated. Using chemically treated electrodes, electrochemical double-layer capacitors with a specific capacitance of about 33 F/g are obtained with 38 wt % H2SO4 as the electrolyte.

A review of neutral pH polymer electrolytes for electrochemical capacitors:Transitioning from liquid to solid devices

The development of neutral pH polymer electrolytes has enabled high-performance solid-state,thin,and flexible electrochemical capacitors(ECs)to provide power for future consumer electronics and Internet-of-Thing devices.Notwithstanding their promising prospect,there is still some lack of understandings or disconnections from fundamental science to practical applications of these electrolytes.In this review,we provide an overview of stateof-the-art studies on ECs with neutral pH electrolytes in both liquid and solid configurations.Starting from the fundamental studies on the voltage window and ion conduction of salt species in liquid solution to polymer electrolytes,key considerations in developing neutral pH polymer electrolytes are discussed.The performance of the polymer electrolytes along with their enabled solid symmetric and asymmetric EC devices,as well as some enhanced functionalities are presented.The future directions for research on neutral pH polymer electrolytes are proposed,expected to provide reference for further enriching the fundamental knowledge and improving the device performances.

Alternatingly stacked thin film electrodes-based compact aqueous hybrid electrochemical capacitors for hundred-volts AC line filtering

Filtering capacitor with compact configuration and a wide range of operating voltage has been attracting increasing attention for the smooth conversion of the electric signal in modern circuits.Lossless integration of capacitor units can be regarded as one of the efficient ways to achieve a wider voltage range,which has not yet been fully conquered due to the lack of rational designs of the electrode structure and integration technology.This study presents an alternatingly stacked assemble technology to conveniently fabricate compact aqueous hybrid integrated filtering capacitors on a large scale,in which a unit consists of rGO/MXene composite film as a negative electrode and PEDOT:PSS based film as a positive electrode.Benefiting from the synergistic effect of rGO and MXene components,and morphological characteristics of PEDOT:PSS,the capacitor unit exhibits outstanding AC line filtering with a large areal specific energy density of 1,015 μF V^(2)cm^(-2)(0.28 μW h cm^(-2)) at 120 Hz.After rational integration,the assembled capacitors present compact/lightweight configuration and lossless frequency response,as reflected by almost constant resistor-capacitor time constant of 0.2 ms and dissipation factor of 15% at120 Hz,identical to those of the single capacitor unit.Apart from standing alone steadily on a flower,a small volume(only 8.1 cm^(3)) of the integrated capacitor with 70 units connected in series achieves hundred-volts alternating current line filtering,which is superior to most reported filtering capacitors with sandwich configuration.This study provides insight into the fabrication and application of compact/ultralight filtering capacitors with lossless frequency response,and a wide range of operating voltage.

Structural and Electrochemical Performances of α-MnO2 Doped with Tin for Supercapacitors

To improve the electrochemical performances of α-MnO2 as electrode materials for supercapacitors, Sn-doped α-MnO2 in the presence of the doping amount of 1%-4% was successfully synthesized by hydrothermal method. As-prepared α-MnO2 presents nanorod shape and no other impurities exist. By ultraviolet-visible absorption spectroscopy, it is convinced that the band gaps of α-MnO2 decrease with increasing Sn-doping amount. Cyclic voltammetry investigation indicates that undoped and doped α-MnO2 all have regular capacitive response. As the scan rate enlarged, the profiles of curves gradually deviate from rectangle. Compared with undoped α-MnO2, doped α-MnO2 has larger specific capacitance. The specific capacitance of 3% doped α-MnO2 reaches 241.0 F/g while undoped α-MnO2 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.

Effect of pretreatment on electrochemical etching behavior of Al foil in HCl-H_2SO_4

The aluminum foil for high voltage aluminum electrolytic capacitor was immersed in 0.5 mol/L H3PO4 or 0.125 mol/L NaOH solution at 40 ℃ for different time and then DC electro-etched in 1 mol/L HC1＋2.5 mol/L H2SO4 electrolyte at 80 ℃. The pitting potential and self corrosion potential of A1 foil were measured with polarization curves （PC）. The potentiostatic current--time curve was recorded and the surface and cross section images of etched A1 foil were observed with SEM. The electrochemical impedance spectroscopy （EIS） of etched A1 foil and potential transient curves （PTC） during initial etching stage were measured. The results show the chemical pretreatments can activate A1 foil surface, facilitate the absorption, diffusion and migration of C1- onto the A1 foil during etching, and improve the initiation rate of meta-stable pits and density of stable pits and tunnels, leading to much increase in the real surface area and special capacitance of etched A1 foil.

Effect of citric acid on microstructure and electrochemical characteristics of high voltage anodized alumina film formed on etched Al Foils

Aluminum capacitor foils with a tunnel etch structure were reacted with boiling water and then anodized at 530 V in boric acid solution or boric acid＋citric acid mixed solution.The microstructure and crystallinity of the resulting anodized film were examined by TEM and XRD.The special capacitance,resistance and withstanding voltage of the film were explored with electrochemical impedance spectroscopy（EIS）,LCR meter and small-current charging.The results show that the high voltage anodized oxide film consists of an inner layer with high crystallinity and an outer layer with low crystallinity.However,the crystallinity of the film formed in boric acid＋citric acid mixed solution is higher than that of the film formed in only boric acid solution,leading to an increase in film＇s field strength and special capacitance.Meanwhile,there are more defects from phase transformation in the out layer of the film formed in boric acid＋citric acid mixed solution than in that of film formed in only boric acid solution,leading to a decrease in film＇s resistance and withstanding voltage.

Excellent Electrochemical Performances of Intrinsic Polyaniline Nanofibers Fabricated by Electrochemical Deposition

A simple route to synthesize the polyaniline(PANI) nanofibers with diameter about 150 nm was reported. In this strategy, the PANI nanofibers were fabricated by electrochemical deposition by using two-electrode configuration in 0.01 M aniline and 0.01 M H_2SO_4 electrolytes. The as-prepared materials were characterized by scanning electron microscopy(SEM), infrared spectroscopy(FTIR), Raman spectroscopy and thermogravimetric analysis(TGA). The electrochemical properties of the PANI nanofibers electrode as supercapacitor materials were investigated. The PANI nanofibers electrode showed high capacitance of 485 F·g^(-1)at 0.1 A·g^(-1), and the decrease in the specific capacitance is about 3.5% in 1 000 cycles. The results indicate that the PANI nanofibers electrode shows high stability and retains its electrochemical capacitance property over 1 000 cycles, suggesting PANI nanofibers have promising applications in high-performance supercapacitors.

Influence of nitrogen hetero-substitution on the electrochemical performance of coal-based activated carbons measured in non-aqueous electrolyte

Nitrogen-containing carbons were prepared by modification of activated carbons.The modified carbons were used as electrode materials with improved electrochemical performance.Precursor anthracite was activated by KOH(KOH:anthracite= 1:1), modified by melamine or urea and then treated at 1173 K to obtain the modified carbons.The porous structure, the chemical composition and the electrochemical characteristics of the carbons were investigated by nitrogen sorption, XPS and electrochemical methods respectively.Electrochemical experiments were performed in an organic electrolytic solution of 1 M(C2H5)4NBF4/PC.The samples modified by the different methods showed differences in chemical composition that introduced varying degrees of electrochemical performance enhancement.The presence of nitrogen enhanced the electron donor properties and the surface wettability of the activated carbons:this ensured a sufficient utilization of the exposed surface for charge storage.

Preparation of nano-PANI@MnO_2 by surface initiated polymerization method using as a nano-tubular electrode material:The amount effect of aniline on the microstructure and electrochemical performance

In this study,nano-polyanline and manganese oxide nanometer tubular composites（nano-PANI@MnO2）were prepared by a surface initiated polymerization method and used as electrochemical capacitor electrode materials; and the effect of aniline amount on the microstructure and electrochemical performance was investigated. The microstructures and surface morphologies of nano-PANI@MnO2 were characterized by X-ray diffraction,scanning electron microscopy and fourier transformation infrared spectroscope. The electrochemical performance of these composite materials was performed with cyclic voltammetry,charge–discharge test and electrochemical impedance spectroscopy,respectively. The results demonstrate that the feed ratio of aniline to MnO2 played a very important role in constructing the hierarchically nano-structure,which would,hence,determine the electrochemical performance of the materials. Using the templateassisted strategy and controlling the feed ratio of aniline to MnO2,the nanometer tubular structure of nanoPANI@MnO2 was obtained. A maximum specific capacitance of 386 F/g was achieved in aqueous 1 mol/L Na NO3 electrolyte with the potential range from 0 to 0.6 V（vs. SCE）.

Conductive metal-organic frameworks: Recent advances in electrochemical energy-related applications and perspectives

Metal-organic frameworks(MOFs),typically constructed with metallic nodes and organic linkers,have influenced the development of modular solid materials.Their adjustable molecular structure provides a remarkable variety of MOF-based solid-state structures towards diverse applications.However,the low conductivity of traditional MOFs extremely hinders their applications in electronic and electrochemical devices.The emerging conductive MOFs,generally possessing twodimensional layered structures,are endowed with both the structural merits of common MOFs and exceptional electronic/ionic conductivities.Besides,the selection and optimization of ligands and metal centers,as well as synthetic methods enormously affects the intrinsic conductivity of conductive MOFs.The distinctive crystal structures and superb conductivity promise their appealing applications in electrochemical energy-related fields.In the review,we mainly summarize representative crystal features,conducting mechanisms and recent advances in rational design and synthesis of conductive MOFs,along with their versatile applications as electrodes for electrochemical capacitors and rechargeable batteries,and as catalysts towards electrocatalysis.Finally,the involved challenges and future trends/prospects of the conductive MOFs for electrochemical energyrelated applications are further proposed.

Fabrication and electrochemical performance of graphene-ZnO nanocomposites

Graphene-ZnO nanocomposites were synthesized successfully through a one-step solvothermal approach. The mor-phology, structure, and composition of the prepared nanocomposites were investigated by scanning electron microscopy （SEM）, transmission electron microscope （TEM）, laser micro Raman spectroscopy, and Fourier transform infra-red spec-troscopy （FT-IR）. The outcomes confirmed that this approach is comparatively steady, practicable, and operable compared with other reported methods. The electrochemical performance of the graphene-ZnO electrodes was analyzed through cyclic voltammetry, altering-current （AC） impedance, and chronopotentiometry tests. The graphene-ZnO electrodes exhib-ited an improved electrode performance with higher specific capacitance （115 F·g^-1 ）, higher electrochemical stability, and higher energy density than the graphene electrodes and most reported graphene-ZnO electrodes. Graphene-ZnO nanocom-posites have a steady reversible charge/discharge behavior, which makes them promising candidates for electrochemical capacitors （ECs）.