Hybrid materials utilizing polyelectrolyte-derivatized carbon nanotubes and vanadium-mixed addenda heteropolytungstate for efficient electrochemical charging and electrocatalysis

Preparation and electrochemical behavior of new hybrid materials composed of multi-walled carbon nanotubes (CNTs) that were derivatized with poly(diallyldimethylammonium) chloride and modified with vanadium-mixed addenda Dawson-type heteropolytungstate, [P₂W₁₇VO₆₂]^8?, is described here. These nanostructured composite systems exhibited fast dynamics of charge propagation. They were characterized by the transport (effectively diffusional) kinetic parameter of approximately 8?×?10^?8 cm^?2 s^?1/2 and the specific capacitance parameter of 82 F g^?1 (at the charging/discharging current of 200 mA g^?1). The latter parameter for bare CNTs was found to be only 50 F g^?1 under analogous conditions. These observations were based on the results of galvanostatic charging–discharging, cyclic voltammetric, and AC impedance spectroscopic measurements. The improved capacitance properties were attributed to the systems’ pseudocapacitive features originating from the fast redox transitions of the [P₂W₁₇VO₆₂]^8? polyanions. In addition to the fast redox conduction, the proposed organic–inorganic hybrid materials exhibited interesting electrocatalytic activity toward reduction of bromate in the broad concentration range (sensitivity, 0.24 mA cm^?2 mmol^?1 dm³).     

Electrospun porous MnMoO<Subscript>4</Subscript> nanotubes as high-performance electrodes for asymmetric supercapacitors

MnMoO₄ nanotubes of diameter about 120 nm were successfully synthesized by a single-spinneret electrospinning technique followed by calcination in air, and their structural, morphological, and electrochemical properties were studied with the aim to fabricate high-performance supercapacitor devices. The obtained MnMoO₄ nanotubes display a 1D architecture with a porous structure and hollow interiors. Benefiting from intriguing structural features, the unique MnMoO₄ nanotube electrodes exhibit a high specific capacitance, excellent rate capability, and cycling stability. As an example, the tube-like MnMoO₄ delivers a specific capacitance of 620 F g^?1 at a current density of 1 A g^?1, and 460 F g^?1 even at a very high current density of 60 A g^?1. Remarkably, almost no decay in specific capacitance is found after continuous charge/discharge cycling for 10,000 cycles at 1 A g^?1. An asymmetric supercapacitor fabricated from this MnMoO₄ nanotubes and activated carbon displayed a maximum high energy density of 31.7 Wh kg^?1 and a power density of 797 W kg^?1, demonstrating a good prospect for practical applications in energy storage electronics.     

Reed straw derived active carbon/graphene hybrids as sustainable high-performance electrodes for advanced supercapacitors

Reed straw-derived active carbon@graphene (AC@GR) hybrids were prepared by one-step carbonization/activation process using a mixture of reed straw and graphene oxide (GO) as raw materials and ZnCl₂ as activation agent. The as-prepared hybrids exhibit high specific surface area in a range of 1971–2497 m² g^?1, abundant porosity, as well as excellent energy storage capability. The symmetric C//C supercapacitor using the hybrid obtained at 700 °C as electrodes demonstrates superior cycling durability, ca. 90 % retention after 6000 cycles at 2 A g^?1, and a high energy density of 6.12 Wh kg^?1 at a power density of up to 4660 W kg^?1 in 6 M KOH aqueous electrolyte. The excellent capacitive performance is attributed to the synergistic effect of AC and GR.     

Enhanced energy density of asymmetric supercapacitors via optimizing negative electrode material and mass ratio of negative/positive electrodes

An asymmetric supercapacitor based on manganese dioxide/Au/nickel foam (MANF) electrode as positive electrode and graphene or commercial activated carbons (AC) as negative electrode was fabricated. The effect of different negative electrode materials and mass ratios of negative/positive electrodes on the electrochemical properties of the asymmetric supercapacitor was carefully investigated. The results suggest that the mass ratio of negative/positive electrode has a significant impact on the specific capacitance of the asymmetric supercapacitor. Especially, it is found that the optimal mass ratio of the negative/positive electrode is slightly lower than that calculated according to charge balance. On the other hand, the asymmetric supercapacitor with commercialized AC as negative electrode possesses higher specific capacitance and better rate capability than that of the asymmetric supercapacitor with graphene as negative electrode. The negative material has slight impact on the cycle stability of the asymmetric supercapacitor. In addition, the optimized asymmetric supercapacitor with MANF composite as positive electrode and AC as negative electrode can obtain an energy density as high as 65.9 Wh?kg^?1 at a power density of 180 W?kg^?1 and a cell voltage of 1.8 V in the neutral Na₂SO₄ aqueous solution.     

Highly stable asymmetric supercapacitors based on Ni(OH)<Subscript>2</Subscript> deposited on electro-etched carbon paper

Herein, we introduce the application of nickel hydroxide nanosheets on the electro-etched carbon fiber (ECF) formed via a direct electrodeposition, for fabrication of asymmetric supercapacitor. To confirm the practical applicability of prepared Ni(OH)₂–ECF, an asymmetric device was assembled using Ni(OH)₂–ECF in combination with an activated carbon (AC) electrode. Our results showed a substantial cycling stability (96% capacitance retention after 10000 cycles) and considerable rate capability at large discharge currents (60% capacitance retention at 8 A g^??1) for this asymmetric supercapacitor that may have originated from the good contact between Ni(OH)₂ and ECF. A maximum specific capacitance of 88.1 F g^??1 was achieved for Ni(OH)₂–ECF//AC/CF device and showed considerable rate capability at large discharge currents (60% capacitance retention at 8 A g^??1). The results of this study suggest the Ni(OH)₂–ECF electrode is an excellent material for fabrication of supercapacitor electrodes.     

Facile synthesis of multiwalled carbon nanotube–V2O5 nanocomposites as cathode materials for Li-ion batteries

Multiwalled carbon nanotube (MWCNT)–vanadium pentoxide (V₂O₅) nanocomposites have been fabricated using a facile and environmental friendly hydrothermal method without any pretreatment, surfactants, or chelate agents added. The as-annealed nanocomposites are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM), and the results indicate that V₂O₅ nanoparticles grew on MWCNTs. As a cathode material for lithium batteries, it exhibits superior electrochemical performance compare to the pure V₂O₅ powders. A high specific discharge capacity of 253 mA h g^?1 can be obtained for the 15 % MWCNT–V₂O₅ nanocomposite electrodes, which retains 209 mA h g^?1 after 50 cycles. However, the pure V₂O₅ powder electrodes only possess a specific discharge capacity of 157 mA h g^?1 with a capacity retention of 127 mA h g^?1 after 50 cycles. Moreover, the MWCNT–V₂O₅ nanocomposite electrodes show an excellent rate capability with a specific discharge capacity of 180 mA h g^?1 at the current rate of 4 C. The enhanced electrochemical performance of the nanocomposites is attributed to the formation of conductive networks by MWCNTs, and large surface areas of V₂O₅ nanoparticles grew on MWCNTs which stabilizes these nanoparticles against agglomeration.     

Preparation,characterization, and evaluation of LiNi0.4Co0.6O2 nanofibers for supercapacitor applications

We prepared LiNi_0.4Co_0.6O₂ nanofibers by electrospinning at the calcination temperature of 450 °C for 6 h. The prepared LiNi_0.4Co_0.6O₂ nanofibers was characterized by thermal, X-ray diffraction, and Fourier transform infrared (FTIR) studies. The morphology of LiNi_0.4Co_0.6O₂ nanofibers was characterized by scanning electron microscopy studies. The asymmetric supercapacitor was fabricated using LiNi_0.4Co_0.6O₂ nanofibers as positive electrode and activated carbon (AC) as negative electrode and a porous polypropylene separator in 1 M LiPF₆–ethylene carbonate/dimethyl carbonate (LiPF₆–EC:DMC) (1:1?v/v) as electrolyte. Cyclic voltammetry studies were then carried out in the potential range of 0 to 3.0 V at different scan rates which exhibited the highest specific capacitance of 72.9 F g^?1. The electrochemical impedance measurements were carried out to find the charge transfer resistance and specific capacitance of the cell, and they were found to be 5.05 Ω and 67.4 F g^?1, respectively. Finally, the charge–discharge studies were carried out at a current density of 1 mA cm^?2 to find out the discharge-specific capacitance, energy density, and power density of the capacitor cell, and they were found to be 70.9 F g^?1, 180.2 Wh kg^?1, and 248.0 W kg^?1, respectively.     

KOH direct treatment of kombucha and in situ activation to prepare hierarchical porous carbon for high-performance supercapacitor electrodes

Kombucha, a renewable biomass, has been successfully utilized as an accessible carbon source to fabricate kombucha-derived hierarchical porous carbon (KHPC) by KOH direct treatment and in situ activation. The prepared KHPC shows an interconnected hierarchical porous structure, a pore volume of 0.41 cm³ g^?1, and a specific surface area of 917 m² g^?1. Due to the multiple synergistic effects of these advantages, the KHPC-3 exhibits a high specific capacitance of 326 F g^?1 at a current density of 1 A g^?1 in 6 M KOH, good rate capability of 82% retention from 1 to 20 A g^?1, and cycling performance with 91.3% retention over 5000 cycles. Moreover, the KHPC-3 symmetric supercapacitor reveals a good energy density of 20.97 Wh kg^?1 at a power density of 871.2 W kg^?1 and retains 8.08 Wh kg^?1 at 6330 W kg^?1 in 1 M Na₂SO₄ electrolyte. Therefore, the KHPC obtained via the simple synthesis process shows great promise as an electrode material in energy storage devices.     

Co(OH)2/graphene sheet-on-sheet hybrid as high-performance electrochemical pseudocapacitor electrodes

A Co(OH)₂/graphene sheet-on-sheet hybrid has been fabricated by in situ one-step hydrothermal growth for electrochemical pseudocapacitors application. The hybrid delivers a specific capacitance of 436 F?g^?1 at a current density of 50 A?g^?1. Besides, it can keep a specific capacitance of 651 F?g^?1 after 10,000 cycles at 10 A?g^?1. The excellent performance can be ascribed to the high-quality graphene matrix, regular morphology and high crystallinity of Co(OH)₂, and unique sheet-on-sheet structure of the hybrid, endowing enhanced transportation of electrons and Faradic redox reactions. The results demonstrate that the Co(OH)₂/graphene hybrid with a sheet-on-sheet structure is promising for high-performance energy storage applications.     

Water-in-oil microemulsion method preparation and capacitance performance study of Li4Mn5O12

Spinel Li₄Mn₅O₁₂ nanoparticles are successfully prepared by water-in-oil microemulsion method and characterized by X-ray diffraction and scanning electron microscopy. The Li₄Mn₅O₁₂ nanoparticles have sphere-like morphology with particle size less than 50 nm. The Li₄Mn₅O₁₂ and activated carbon (AC) were used as electrodes of Li₄Mn₅O₁₂/AC supercapacitor, respectively. The electrochemical capacitance performance of the supercapacitor was investigated by cyclic voltammetry, galvanostatic charge/discharge, and electrochemical impedance spectroscopy. The results showed that the single electrode was able to deliver specific capacitance 252 F g^?1 within potential range 0–1.4 V at a scan rate of 5 mV s^?1 in 1 mol L^?1 Li₂SO₄ solution, and it also showed high coulombic efficiency close to 100%. This material exhibited a good cycling performance.     

Facile preparation of nitrogen-doped porous carbon for high performance symmetric supercapacitor

In this work, the micromolecule l-glutamic acid (Glu) is employed as nitrogen-rich precursor to prepare a novel porous carbon, and ZnCl₂ is used as activating agent to improve the surface area and electrochemical performance of the carbon. The nitrogen content of the carbon (Glu-2.5) prepared by Glu and ZnCl₂ with a mass ratio of 1:2.5 retains as high as 7.1 % at an activation temperature of 700 °C. The surface area and pore volume of Glu-2.5 are 1007.4 m² g^?1 and 0.57 cm³ g^?1, respectively. Glu-2.5 exhibits a high specific capacitance of 330.6 F g^?1 in 2 M KOH electrolyte at the current density of 1 A g^?1and good cycling stability (89 % retention of capacitance after 5000 charge/discharge cycles). More importantly, the assembled symmetric supercapacitor using Glu-2.5 as electrodes reveals a high energy density (16.7 Wh kg^?1) under the power density of 404.7 W kg^?1. Owing to its inherent advantages, Glu-2.5 could be a promising and scalable alternative applied to energy storage/conversion.     

Synthesis and electrochemical characterization of duo doped spinels (zinc and praseodymium) for use in lithium rechargeable batteries

LiMn₂O₄ and LiZn_xPr_yMn_2?x?yO₄ (x = 0.10–0.24; y = 0.01–0.10) powders have been synthesized by sol–gel method using palmitic acid as chelating agent. The synthesized samples have been subjected to thermo gravimetric and differential thermal analysis, X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray analysis (EDAX). The sol–gel route bestows low calcination temperature, shorter heating time, high purity, good control over stoichiometry, small particle size, high surface area, good surface morphology and better homogeneity, The XRD patterns reveal high degree of crystallinity and better phase purity. SEM and TEM images exhibit nano-sized nature particles with good agglomeration. EDAX peaks of Zn, Pr, Mn and O have been confirmed in actual compositions of LiMn₂O₄ and LiZn_xPr_yMn_2?x?yO₄. Charge–discharge studies of pristine spinel LiMn₂O₄ heated at 850 °C delivers discharge capacity of 132 mA h g^?1 corresponding to columbic efficiency of 73 % during the first cycle. At the end of 10th cycles, it delivers maximum discharge capacity of 112 mA h g^?1 with columbic efficiency of 70 % and capacity fade of 0.15 mA h g^?1 cycle^?1 over the investigated 10 cycles. Inter alia, all dopants concentrations, LiZn_0.10Pr_0.10Mn_1.80O₄ exhibits the better cycling performance (1st cycle discharge capacity: 130 mA h g^?1 comparing to undoped spinel 132 mA h g^?1) corresponding to columbic efficiency of 73 % with capacity fade of 0.12 mA h g^?1 cycle^?1.     

Preparation of activated carbon from cotton stalk and its application in supercapacitor

High specific capacitance and low cost are the critical requirements for a practical supercapacitor. In this paper, a new activated carbon with high specific capacitance and low cost was prepared, employing cotton stalk as the raw material, by using the phosphoric acid (H₃PO₄) chemical activation method. The optimized conditions were as follows: the cotton stalk and activating agent with a mass ratio of 1:4 at an activation temperature of 800 °C for 2 h. The samples were characterized by nitrogen adsorption isotherms at 77 K. The specific surface area and pore volume of activated carbon were calculated by Brunauer–Emmett–Teller (BET) and t-plot methods. With these experimental conditions, an activated carbon with a BET surface area of 1,481 cm²?g^?1 and micropore volume of 0.0377 cm³?g^?1 was obtained. The capacitance of the prepared activated carbon was as high as 114 F?g^?1.The results indicate that cotton stalk can produce activated carbon electrode materials with low cost and high performance for electric double-layer capacitor.     

Optimizing the preparation conditions of polypyrrole electrodes for enhanced electrochemical capacitive performances

Here, we put forward an optimum proposal to prepare high-performance polypyrrole (PPy) electrodes for supercapacitor applications. A detailed study regarding the effects of different preparation conditions including electropolymerized mode, parameter, and current collector on the electrochemical capacitive performances of PPy electrodes is carried out. Fourier transform infrared spectroscopy and X-ray diffraction tests indicate the above preparation conditions have no effect on the component and crystal structure of PPy prepared. Electrochemical measurements manifest a significant effect of current collector on supercapacitive properties of PPy electrodes. Graphite foil as the current collector with low resistance shows remarkably superior capacitive performances compared with FTO-conducting glass and ITO conductive plastic. SEM characterizations show galvanostatically polymerized PPy particles have obviously smaller aggregation degree than potentiostatically polymerized PPy particles, which results in better electrochemical properties for former. Among different preparation conditions, the PPy deposited on graphite foil under galvanostatic mode with 2 mA cm^?2 shows the best electrochemical capacitive properties. The optimized PPy/graphite electrodes show a high specific capacitance of 173.0 mF cm^?2 at 0.2 mA cm^?2, superior rate capability, and outstanding cycling stability (retaining 90.5% of initial capacitance for 5000 cycles).     

In situ synthesis of crosslinked-polyaniline nano-pillar arrays/reduced graphene oxide nanocomposites for supercapacitors

Crosslinked-polyaniline (CPA) nano-pillar arrays adsorbed on the surface of reduced graphene oxide (RGO) sheets were synthesized by in situ solution polymerization through two steps of reduction. The electrochemical analyses demonstrated that the befittingly reduced CPA/RGO composite exhibited high performance as electrode materials for supercapacitors. The CPA/RGO composite showed very high specific capacitance of 1532 F g^?1 at a scan rate of 10 mV s^?1 or 694 F g^?1 at a current density of 2 A g^?1 in 1 M H₂SO₄ electrolyte, as well as great energy density of 61.4 W h kg^?1 at a current density of 2 A g^?1. The electrode material also had decent power density of 4 kW kg^?1 at a current density of 10 A g^?1, and good cycling stability of 92.5 % capacitance retained after 500 cycles of cyclic voltammetry at 500 mV s^?1. The neat microstructures and super electrochemical properties suggest the potential use of the composites in supercapacitors.     

Preparation and performance of polyvinyl alcohol-based activated carbon as electrode material in both aqueous and organic electrolytes

Porous carbon materials with high surface area and different pore structure have been successfully prepared by phenolic resin combined with polyvinyl alcohol (PVA) and KOH as activation agents. The surface morphology, structure, and specific surface area of the carbon materials were studied by scanning electron microscopy, X-ray diffraction, and nitrogen sorption measurement, respectively. Furthermore, the effects of specific surface area, pore structure, and electrolyte on electrochemical properties were investigated by galvanostatic charge–discharge measurement. The results show that KOH–PVA-activated carbon materials display specific capacitance as high as 218 F?g^?1 in 30 wt.% KOH aqueous electrolyte, 147 F?g^?1 in 1 M LiPF₆/(ethylene carbonate (EC) + dimethyl carbonate) (1:1?v/v), and 115 F?g^?1 in 1 M Et₃MeNBF₄/propylene carbonate organic electrolyte, respectively. In addition, the carbon materials demonstrate long-term cycle stability, especially the AK3P-0.30 in aqueous electrolyte and the AK2P-0.30 with excellent rate capability in organic electrolyte. These reveal that the existence of a micro-mesoporous structure of activated carbon is beneficial to store energy in an aqueous supercapacitor and broad pore size distribution of activated carbon is favorable to energy storage in an organic supercapacitor. The carbon materials with pore size distribution in different ranges improve the electrochemical performance of supercapacitor in different electrolytes. A new pore-expand agent (PVA combining with KOH) was used to obtain porous carbons with enhanced properties for supercapacitor.     

Design of high-performance core-shell hollow carbon nanofiber@nickel-cobalt double hydroxide composites for supercapacitive energy storage

The supercapacitive performances of supercapacitor mainly depend on the physical nanostructure and micro-morphology of electrode materials. Here, we demonstrated the design, synthesis and electrochemical performances of core-shell hollow carbon nanofiber@nickel-cobalt-layered double hydroxide (HCNF@ Ni_0.67Co_0.33-LDH) nanocomposites with an optimized Ni/Co molar ratio of 2:1. The HCNF was used as superiorly conductive core to sustain the nanoporous silky Ni_0.67Co_0.33-LDH shell, which can efficiently provide fast transport pathways for electrons and electrolyte ions. The outstanding specific capacitance of 2486 F g^?1 at 1 A g^?1 based on galvanostatic charge-discharge curves were acquired for the highly electroactive HCNF@Ni_0.67Co_0.33-LDH. Furthermore, the HCNF@Ni_0.67Co_0.33-LDH electrode delivered a distinguished rate capability with a specific capacitance of 1890 F g^?1 even at 15 A g^?1. Notably, an asymmetric supercapacitor with HCNF@Ni_0.67Co_0.33-LDH as cathode and HCNF as anode was devised, which presented a prominent specific capacitance of 228 F g^?1, good energy density of 62.1 Wh kg^?1, and impressive cycling stability (90.6% capacitance retention after 10,000 cycles).     

Hypergrafted nano-silica modified polymer gel electrolyte for high-performance solid-state supercapacitor

In the developing of wearable electronics and smart textiles, thin, lightweight, and flexible energy storage supercapacitor with high energy density has attracted the attention of many researchers in recent years. In this work, we prepared gel nano-composite electrolyte with the hypergrafted poly (amine-ester) nano-silica (HBPAE-SiO₂) as inclusion. The electrochemical properties of the supercapacitor with the alkaline polymer electrolyte were evaluated by cyclic voltammetry, galvanostatic charge–discharge behavior, and electrochemical impedance spectroscopy. It was found that the incorporated HBPAE-SiO₂ can greatly increase the specific capacitance of the supercapacitor, which was due to the enhanced ionic conductivity of gel electrolyte as well as good electrode–electrolyte contact. It is pointed out that the electroactivity of the inclusion may be also one reason. The best specific capacitance with 30 wt% HBPAE-SiO₂ reached 160 F g^?1, which was increased by 36.5 % compared with that of polyvinyl alcohol (PVA)-KOH system. Moreover, the capacity retention of solid-state supercapacitor can be 88 % after 10,000 cycles. The hypergrafted nano-silica modified polymer gel electrolyte is promising for the application of solid-state supercapacitor.     

Electrochemical exfoliation and in situ carboxylic functionalization of graphite in non-fluoro ionic liquid for supercapacitor application

Simultaneous electrochemical generation and functionalization of nano-sized graphite from graphite had been carried out in a non-fluoroanion-based ionic liquid, namely, triethylmethylammonium methylsulfate (TEMAMS) containing water and acetonitrile (AN) in different weight ratios. The oxygen-based functional groups attached with the exfoliated material had been identified using Fourier transform infrared spectroscopy (FTIR), and morphological changes were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). A symmetrical supercapacitor was fabricated using the exfoliated nano-sized graphite, and the influence of surface functionalities on its performance was investigated using electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), and galvanostatic charge–discharge cycles (CC). The highest specific capacitance (C _sp) value of 140 F g^?1 at 0.25 A g^?1 was obtained in 1.0 M H₂SO₄, followed by aqueous TEMAMS (125 F g^?1), TEMAMS/acetonitrile (115 F g^?1), and TEMAMS (106 F g^?1) at 0.10 A g^?1.     

Activated nitrogen-doped carbons from polyvinyl chloride for high-performance electrochemical capacitors

Activated nitrogen-doped carbons (ANCs) were prepared by carbonization/activation approach using aminated polyvinyl chloride (PVC) as precursor. ANCs exhibit larger porosities and higher specific surface areas than those of their nitrogen-free counterparts for the same KOH/carbon ratio. The specific surface area of ANC-1 is up to 1,398 m² g^?1 even at a low KOH/carbon ratio of 1:1. Fourier transform infrared spectroscopy investigation of the nitrogen-enriched resin precursor indicates the efficient dehydrochlorination of PVC by ethylenediamine at a low temperature. The nitrogen content and the population of nitrogen functionalities strongly depend on the KOH/carbon ratios and decrease drastically after KOH activation as seen from the elemental and X-ray photoelectron spectroscopy analysis. The surface concentration of N-6 and N-Q almost disappears and the dominant nitrogen groups become N-5 after KOH activation. The highest specific capacitance of ANCs is up to 345 F g^?1 at a current density of 50 mA g^?1 in 6 M KOH electrolyte. ANCs also exhibit a good capacitive behavior at a high scan rate of 200 mV s^?1 and an excellent cyclability with a capacitance retention ratio as high as ～93 % at a current density of 2,000 mA g^?1 for 5,000 cycles.