N-Doped rGO-Like Carbon Prepared from Coconut Shell:Structure and Specific Capacitance

An rGO−like carbon compound has been synthesized from biomass,i.e.,old coconut shell,by a carbonization process followed by heating at 400°C for 5 h.The nitrogen doping was achieved by adding the urea(CH4N2O)and stirring at 70°C for 14 h.The morphology and structure of the rGO-like carbon were investigated by electron microscopies and Raman spectroscopy.The presence of C-N functional groups was analyzed by Fourier transform infrared and synchrotron X-ray photoemission spectroscopy,while the particle and the specific capacitance were measured by particle sizer and cyclic voltammetry.The highest specific capacitance of 72.78 F/g is achieved by the sample with 20%urea,having the smallest particles size and the largest surface area.The corresponding sample has shown to be constituted by the appropriate amount of C–N pyrrolic and pyridinic defects.

Novel hierarchical yolk-shell α-Ni(OH)_(2)/Mn_(2)O_(3) microspheres as high specific capacitance electrode materials for supercapacitors

For high performance supercapacitors, novel hierarchical yolk-shell α-Ni(OH)_(2)/Mn_(2)O_(3) microspheres were controllably synthesized using a facile two-step method based on the solvothermal treatment. The unique α-Ni(OH)_(2) based yolk-shell microstructures decorated with numerous interconnected nanosheets and the heterocomposition features can synergistically enhance reactive site exposure and electron conduction within the microspheres, facilitate charge transfer between electrolyte and electrode materials, and release structural stress during OH− chemisorption/desorption. Moreover, the Mn2O3 sediments distributed over the α-Ni(OH)_(2) microspheres can serve as an effective protective layer for electrochemical reactions. Consequently, when tested in 1 mol·L^(−1) KOH aqueous electrolyte for supercapacitors, the yolk-shell α-Ni (OH)_(2)/Mn_(2)O_(3) microspheres exhibited a considerably high specific capacitance of 2228.6 F·g^(−1) at 1 A·g^(−1) and an impressive capacitance retention of 77.7% after 3000 cycles at 10 A·g^(−1). The proposed α-Ni(OH)_(2)/Mn_(2)O_(3) microspheres with hetero-composition and unique hierarchical yolk-shell microstructures are highly promising to be used as electrode materials in supercapacitors and other energy storage devices.

Perspective on ultrathin layered Ni-doped MoS_(2) hybrid nanostructures for the enhancement of electrochemical properties in supercapacitors

Over the last two decades,extensive study has been done on two-dimensional Molybdenum Sulphide(MoS_(2))due to its outstanding features in energy storage applications.Although MoS_(2)has a lot of active sulphur edges,the presence of inactive surfaces leads to limit conductivity and efficiency.Hence,in this article,we aimed to promote the additional active sites by doping various weight percentages(2%,4%,6%,8%and 10%)of Nickel(Ni)into the MoS_(2)matrix by simple hydrothermal technique,and their doping effects were investigated with the help of Physio-chemical analyses.X-ray diffraction(XRD)pattern,Raman,and chemical composition(XPS)analyses were used to confirm the Ni incorporation in MoS_(2)nanosheets.Microscopic investigations demonstrated that Ni-doped MoS_(2)nanosheets were vertically aligned with enhanced interlayer spacing.Cyclic voltammetry,Galvanostatic charge-discharge,and electrochemical impedance spectroscopy investigations were used to characterize the electrochemical characteristics.The 6%Ni-doped MoS_(2)electrode material showed better CSPof 528.7 F/g@1 A/g and excellent electrochemical stability(85%of capacitance retention after 10,000 cycles at 5 A/g)compared to other electrode materials.Furthermore,the solid-state asymmetric supercapacitor was assembled using Nidoped MoS_(2)and graphite as anode and cathode materials and analysed the electrochemical properties in the two-electrode system.To determine the impact of the Ni-atom on the MoS_(2)surface,firstprinciples computations were performed.Further,it was examined for electronic band structure,the projected density of states(PDOS)and Bader charge transfer analyses.

Development of Cd-doped Co Nanoparticles Encapsulated in Graphite Shell as Novel Electrode Material for the Capacitive Deionization Technology

Because of the low energy requirement and the environmentally safe byproducts, the capacitive deionization water desalination technology has attracted the attention of many researchers. The important requirements for electrode materials are good electrical conductivity, high surface area, good chemical stability and high specific capacitance. In this study, metallic nanoparticles that are encapsulated in a graphite shell(Cd doped Co/C NPs) are introduced as the new electrode material for the capacitive deionization process because they have higher specific capacitance than the pristine carbonaceous materials. Cd doped Co/C NPs perform better than graphene and the activated carbon. The introduced nanoparticles were synthesized using a simple sol gel technique. A typical sol gel composed of cadmium acetate, cobalt acetate and poly(vinyl alcohol)was prepared based on the polycondensation property of the acetates. The physiochemical characterizations that were used confirmed that the drying, grinding and calcination in an Ar atmosphere of the prepared gel produced the Cd doped Co nanoparticles, which were encapsulated in a thin graphite layer. Overall, the present study suggests a new method to effectively use the encapsulated bimetallic nanostructures in the capacitive deionization technology.

Characterization of porous cobalt hexacyanoferrate and activated carbon electrodes under dynamic polarization conditions in a sodium-ion pseudocapacitor

We report here the activated carbon and cobalt hexacyanoferrate composite,which is applied as the electrode materials in symmetric supercapacitors containing a 1.0 M Na_(2)SO_(4) aqueous electrolyte.This novel material combines high specific surface area and electrochemical stability of activated carbon with the redox properties of cobalt hexacyanoferrate,resulting in maximum specific capacitance of 329 F g^(-1) with large voltage working window of 2.0 V.Electrochemical studies indicated that cobalt hexacyanoferrate introduces important pseudocapacitive properties accounting for the overall charge-storage process,especially when I<0.5 A g^(-1).At lower gravimetric currents(e.g.,0.05 A g^(-1))and up to 1.0 V,the presence of cobalt hexacyanoferrate improves the specific energy for more than 300%.In addition,to better understanding the energy storage process we also provided a careful investigation of the electrode materials under dynamic polarization conditions using the in situ Raman spectroscopy and synchrotron light Xray diffraction techniques.Interesting complementary findings were obtained in these studies.We believe that this novel electrode material is promising for applications regarding the energy-storage process in pseudocapacitors with long lifespan properties.

Characterization of Microporous Activated Carbon Electrodes for Electric Double-layer Capacitors

Activated carbons(ACs) with a wide range of surface areas were made from petroleum coke by means of KOH activation. The electrochemical characterization was carried out for several activated carbons used as polarizable electrodes of electric double-layer capacitors(EDLCs) in an aqueous electrolytic solution. The porous structures and electrochemical double-layer capacitance of the activated carbons were investigated by virtue of nitrogen gas adsorption and constant current cycling(CCC) methods. The relationship among the surface area, pore volume of the activated carbons and specific double-layer capacitance was discussed. It was found that the specific capacitance of ACs increased linearly with the increase of surface area. The presence of mesopores in the activated carbons with very high surface area(>2000 m\+2/g) was not very effective for them to be used as EDLCs. The influence of chemical characteristics of the activated carbons on the double layer formation could be considered to be negligible.

Electrochemical Performance of Core-Sheath Dip-Coated Lignin Derived Carbon/Wet-Spun Graphene Electrodes for Fiber-Shaped Supercapacitors

One-dimensional graphene fibers(GFs)possess excellent properties,including high electrical conductivity,good physical and chemical stability,high thermal conductivity,flexibility,etc.GFs are ideal electrode materials for fiber-shaped supercapacitors.However,due to the lack of an effective method to manufacture GFs with high specific capacitance,their low energy density hinders their practical application.Herein,we decorated wet-spun graphene oxide fibers(GOFs)by dip-coating them with graphene oxide(GO)solutions containing different contents of lignin to obtain a core-sheath lignin/GO composite fibers.After carbonization and activation,we successfully prepared lignin derived carbon/GF electrodes.The assembled fiber-shaped supercapacitors(FSSCs)exhibit a specific capacitance of 9.98 mF/cm^(2)and an energy density of 0.89μW·h/cm^(2),about 6 times of those of pure GFs(1.57 mF/cm^(2)and 0.14μW·h/cm^(2),respectively),long cycling life and cycling stability.This suggests that the introduction of lignin derived carbon into GFs can effectively increase the specific capacitance and the energy density of FSSCs.

Influence of La-dopant on the Material Characteristics and Supercapacitive Performance of MnO_2 Electrodes

Manganese dioxide was synthesized by electrodeposition method with Mn （CH3COO）24H2O as a raw material. La（NO3）3？6H2O was doped in electroyte during the preparing process to improve the performance of MnO2 electrodes. The micrographs, crystal structure and element content of electrodes were analyzed by SEM, XRD and atomic absorption spectroscopy, respectively. It is found that the La content ratio in the dioxide can be easily controlled by adjusting the composition of the plating solution. Appropriate amount of doped La can increase the surface area of Mn/La materials, resulting in the supercapacitive behavior enhancement. Electrochemical tests show that the specific capacitance is significantely increased from 198.72 F？g-1 to 276.60 F？g-1 by La-doping.

Optimal Electrochemical Performances of CO_2 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.

MnO_(2)/carbon nanocomposite based on silkworm excrement for high-performance supercapacitors

MnO_(2)/biomass carbon nanocomposite was synthesized by a facile hydrothermal reaction.Silkworm excrement acted as a carbon precursor,which was activated by ZnCl_(2) and FeCl_(3) combining chemical agents under Ar atmosphere.Thin and flower-like MnO_(2) nanowires were in-situ anchored on the surface of the biomass carbon.The biomass carbon not only offered high conductivity and good structural stability but also relieved the large volume expansion during the charge/discharge process.The obtained MnO_(2)/biomass carbon nanocomposite electrode exhibited a high specific capacitance(238 F·g^(-1) at 0.5 A·g^(-1))and a superior cycling stability with only 7% degradation after 2000 cycles.The observed good electrochemical performance is accredited to the materials’high specific surface area,multilevel hierarchical structure,and good conductivity.This study proposes a promising method that utilizes biological waste and broadens MnO_(2)-based electrode material application for next-generation energy storage and conversion devices.

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.

Physical and Electrochemical Characterization of Nsutite

Manganese oxides are of interest as, among a number of other applications, supercapacitor materials for energy storage systems. Nsutite, a naturally occurring Manganese Oxide was studied as a possible high-volume source of materials for supercapacitor applications. X-ray diffraction and thermal analysis (DSC/TGA) measurements were carried out to characterize its physical properties, and Cyclic Voltametry and galvanostatic charge-discharge measurements were carried out to obtain its electrochemical properties. The XRD and thermal results support transitions of nsutite, upon heating which were attributed to conversion to MnO2, and to Mn2O3 and eventually to Mn3O4. The electrochemical results of the as mined material show supercapacitance behaviour, suggesting that nsutite, with some heat processing, is to be a promising high-volume source of manganese oxides for supercapacitor applications.

Graphene-Polymer Nanocomposites and Roll-to-Roll (R2R) Compatible Flexible Solid-State Supercapacitor Based on Graphene Nanoplatelets and Ionic Liquid-Polymer Gel

We present the electrical and supercapacitive performance of graphene nanoplatelets in polymer nanocomposites and flexible solid state electrical double layer capacitors (EDLC) respectively. Graphene-doped poly (3,4-ethylenedioxythiophene) (PEDOT) coated polyethylene terephthalate (PET) and glass exhibited transmittance above 95% and electrical conductivity of 2.70 × 10ˉ1 S·cmˉ1 and 9.01 × 10ˉ1 S·cmˉ1 respectively. Graphene loaded polymethyl methacrylate (PMMA) and polystyrene (PS) nanocomposites showed electrical conductivity as high as 2.11 × 10ˉ1 S·cmˉ1 at low loadings of 2 wt%. The use of graphene was necessitated by the need to increase the EDLC capacitance and energy density since it provides high effective surface area. The polymer gel membrane made from polyvinylidene fluoride-co-hexafluoropropylene (PVDF-co-HFP) and the Ionic Liquid (IL) 1-butyl-3-methylimidazolium hexafluorophosphate exhibited high porosity which made it suitable for use as separator in the EDLC. The highest recorded specific capacitance was 133.82 F/g which can be attributed to the porosity of the IL containing PVDF-co-HFP membrane and the large surface area of the graphene electrodes. At an operating voltage of 3.5 V the energy density was found to be 56.92 Wh·Kgˉ1. All chemicals were research grade and were obtained from Sigma Aldrich.

Formation of Al-Si Composite Oxide Film by Hydrolysis Precipitation and Anodizing

This paper presents a new technique in the high dielectric constant composite oxide film preparation. On the basis of nano-compsite high dielectric constant aluminum oxide fdm growth technology, a new idea of adulterating Si oxide species into the aluminum composite film was proposed. As a result, the specific capacitance and withstanding voltage of the composite oxide film formed at the anodizing voltage of 20V are enhanced, and the leakage current of the aluminum composite oxide fdm is reduced through incorporation of Si oxide species.

Superhydrophilic nickel hydroxide ultrathin nanosheets enable high-performance asymmetric supercapacitors

Superhydrophilic surfaces have been applied for supercapacitor;however,during energy storage reaction,how the wettability affects the process of electrochemical reaction specifically is still unclear.Herein,we demonstrate superhydrophilic surface for promotion of electrochemical reactions by liquid affinity and further explain the mechanism,where the transition of the wettability state caused by the change in surface free energy is the main reason for the obvious increase in specific capacitance.Through citric acid assistance strategy,an intrinsically hydrophobic Ni(OH)_(2)thick nanosheets(HNHTNs,16 nm)can be transitioned into superhydrophilic Ni(OH)_(2)ultrathin nanosheets(SNHUNs,6.8 nm),where the water contact angle was 0°and the surface free energy increased from 8.6to 65.8 mN·m^(-1),implying superhydrophilicity.Compared with HNHTNs,the specific capacitance of SNHUNs is doubled:from 1230 F·g^(-1)(HNHTNs)to 2350 F·g^(-1)(2A·g^(-1))and,even at 20 A·g^(-1),from 833 F·g^(-1)(HNHTNs)to 1670 F·g^(-1).The asymmetric capacitors assembled by SNHUNs and activated carbon show 52.44 Wh·kg^(-1)at 160W·kg^(-1)and excellent stability with~90%retention after5000 cycles(~80%retention after 9500 cycles).The promotion of electrochemical performances is ascribed to the change of surface wettability caused by surface free energy,which greatly increase affinity of electrode to the surrounding liquid environment to reduce the interface resistance and optimize the electron transport path.

Preparation of vanadium-based electrode materials and their research progress in solid-state flexible supercapacitors

Solid-state flexible supercapacitors(SCs)have many advantages of high specific capacitance,excellent flexibility,fast charging and discharging,high power density,environmental friendliness,high safety,light weight,ductility,and long cycle stability.They are the ideal choice for the development of flexible energy storage technology in the future,and provide a good prospect for energy storage applications.At present,solid-state flexible SCs are widely used for portable electronic equipment and wearable energy storage equipment,the research of them has become the focus of a growing number of researchers.Electrode material is the key part of SCs and always determines the electrochemical performance of SCs.It has been a hotspot and focus of research.Vanadium-based compounds are considered to be a promising electrode material for SCs because of variable valence,open structure,high theoretical capacity,and low price.Therefore,this study first gives an overview of solid-state flexible SCs,then reviews the current research status of vanadium-based electrode materials in solid-state flexible SCs,and proposes some strategies to solve some problems of vanadium-based electrode materials.

N,P-codoped porous carbon derived from chitosan with hierarchical N-enriched structure and ultra-high specific surface Area toward high-performance supercapacitors

In this work,a facile"carbonization-activation"strategy is developed to synthesize N,P-codoped hierarchical porous carbon.Phosphoric acid is innovatively introduced during the hydrothermal process to achieve in-situ P doping as well as create abundant pores,and the employment of sodamide is of vital importance to simultaneously serve as activating agent and N-source to succeed a high-level N doping.Thus,the obtained samples exhibit a unique three-dimensional hierarchical structure with an ultra-high specific surface area(3646 m^(2)g^(-1))and ultra-high N-doping level(9.81 at.%).Computational analyses confirm that N,P co-doping and higher N content can enhance active sites and widen potential differences of carbon materials to improve their capacitance.The as-prepared carbon materials demonstrate superior electrochemical performances,such as an ultra-high capacitance of 586 Fg^(-1)at 1 Ag^(-1),a superior rate capability of 409 Fg^(-1)at 20 Ag^(-1),and excellent long-term stability of 97%capacitance retention after10,000 cycles in 6 M KOH.Moreover,an assembled symmetric supercapacitor delivers a high energy density of 28.1 Wh kg^(-1)at the power density of 450 W kg^(-1)in 1 M Na_(2)SO_(4),demonstrating a great potential for applications in supercapacitors.

Moiré-superlattice MXenes enabled ultra-stable K-ion storage in neutral electrolyte

Ti_(3)C_(2)MXene is an auspicious energy storage material due to its metallic conductivity and layered assembly.However,in the real working condition of electrochemical energy storage with long cycle charging-discharging,a structural collapse is usually caused by the stacking of its layers creating a large attenuation of specific capacitance.Inspired by the superlattice effect of magic angle graphene,we conducted microscopical regulation of rotation mismatch on the Ti_(3)C_(2)lattice;consequently,a hexagonal fewlayered Ti_(3)C_(2)free-standing film constructed with Moiré-superlattices.Such finding not only solves the problem of Ti_(3)C_(2)structural collapse but also dramatically improves the specific capacitance of Ti_(3)C_(2)as a supercapacitor electrode under long cycle charging and discharging.The ultra-stable energy storage of this electrode material in a neutral aqueous electrolyte was realized.Moreover,the formation mechanism of rotating Moirépattern is revealed through microscopy and microanalysis of the produced Moirépattern,assisted with modeling and analyzing the underlying mechanism between the Moirépattern and the rotation angle.Our work provides experimental and theoretical support for future construction of Moiré-superlattice structure for a wide range of MXene phases and is undoubtedly promoting the development of MXene materials in the field of energy storage.

Coral reef-like polyanaline nanotubes prepared by a reactive template of manganese oxide for supercapacitor electrode

Coral reef-like PANI nanotubes composed of nanopaticles were successfully synthesized by a reactive template of manganese oxide.The structure was characterized by using SEM,TEM,and FT-IR,and the supercapacitive behaviors of these nanotubes were investigated with cyclic voltammetry（CV）,and charge-discharge tests,respectively.A maximum specific capacitance of 533 F/g could be achieved in 1mol/L aqueous H_2SO_4 with the potential range of -0.2 to 0.8 V（vs.the saturated calomel electrode） in a half-cell setup configuration for PANI electrode,suggesting its potential application in the electrode material for electrochemical capacitors.

Progress and challenges of ceramics for supercapacitors

Supercapacitors(SCs)are one of the most promising electrical energy storage technologies systems due to their fast storage capability,long cycle stability,high power density,and environmental friendliness.Enormous research has focused on the design of nanomaterials to achieve low cost,highly efficient,and stable electrodes.Ceramic materials provide promising candidates for SCs electrodes.However,the low specific surface area and relatively low surface activity severely hinder the SCs performance of ceramic materials.Therefore,the basic understanding of ceramic materials,the optimization strategy,and the research progress of ceramic electrodes are the key steps to enable good electrical conductivity and excellent electron transport capabilities,and realize economically feasible ceramic electrodes in industry.Herein,we review recent achievements in manufacturing the ceramic electrodes for SCs,including metal oxide ceramics,multi-elemental oxide ceramics,metal hydroxide ceramics,metal sulfide ceramics,carbon-based ceramics,carbide and nitride ceramics,and other special ceramics(MXene).We focus on the unique and key factors in the component and structural design of ceramic electrodes,which correlate them with SCs performance.In addition,the current technical challenges and perspectives of ceramic electrodes for SCs are also discussed.