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.     

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.     

Enhanced electrochemical performance of straw-based porous carbon fibers for supercapacitor

The efficient utilization of natural biomass as renewable raw materials is of importance. We herein prepared porous carbon fibers (PCFs) by activation of the extracted cellulose microfibers from the agriculture byproduct of corn straw. Different from the porous carbons (PCs) by directly activating straw, the obtained PCFs had typical one-dimensional morphology with high surface area (2013 m² g^?1) and large pore volume (1.27 cm³ g^?1). The influence of the ZnCl₂/cellulose mass ratio on the electrochemical performance was studied, and the optimized PCF(1:1) possessed a much higher specific capacitance than the PC(1:1) sample, which was attributed to the improved specific surface area as well as the fiber-like morphology where it had short ion diffusion route and small interfacial resistance in comparison to PCs. PCFs have a high specific capacitance of 230 F g^?1 at 0.5 A g^?1, and 183 F g^?1 was retained at 20 A g^?1 (79.6%), revealing an excellent rate capability. The assembled symmetrical supercapacitor exhibited a wide potential window of 1.8 V, small electrochemical impedance, and superior cycle performance. Moreover, a high energy density of 16.0 Wh kg^?1 was obtained at a power density of 450.4 W kg^?1, which was preserved of 6.9 Wh kg^?1 at a high power density of 14,194.3 W kg^?1.     

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.     

Synthesis of micro- and mesoporous carbon spheres for supercapacitor electrode

Micro- and mesoporous carbon spheres (MMCSs) are synthesized by the polymerization of colloidal silica-entrapped resorcinol/formaldehyde in the presence of ammonia as catalyst, followed by carbonization, sodium hydroxide (NaOH) etching to remove silica template, and potassium hydroxide (KOH) activation. The morphology and microstructure are characterized by scanning electron microscopy, transmission electron microscopy, and nitrogen adsorption–desorption. The results show that a typical sample (denoted as MMCS-3) unites the characteristics of regular spherical shape (uniform diameters of 500 nm), high specific surface area (1,620 m² g^?1), large pore volume (1.037 cm³ g^?1), and combined micropores and mesopores (11.0 nm), which endows MMCS-3 good electrochemical performance. MMCS-3 as supercapacitor electrode shows a specific capacitance of 314 F g^?1 under a current density of 0.5 A g^?1 and low internal resistance of 0.2 Ω in 6 M KOH aqueous solution. The electrochemical capacitance still retains 198 F g^?1 at a high current density of 10 A g^?1. After 500 cycle numbers of galvanostatic charge/discharge at 0.5 A g^?1, MMCS-3 electrode still remains the specific capacitance of 301 F g^?1 with the retention of 96 %. This study highlights the potential of well-designed MMCSs as electrodes for widespread supercapacitor applications.     

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.     

A tunable hierarchical porous carbon from starch pretreated by calcium acetate for high performance supercapacitors

Hierarchical porous carbons (HPCs) with abundant mesopores have been prepared by a facile route from the starch that was pretreated by calcium acetate. The scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and N₂ adsorption–desorption tests show that hierarchical porous carbons with bimodal mesopores have been obtained. Moreover, the pore sizes are tunable by simply adjusting the reactants ratio and carbonization temperature. The as-synthesized hierarchical porous carbon materials (HPCs-2-800) possesses the highest Brunauer-Emmett-Teller (BET)-specific surface area of 464 m² g^?1 and mesoporous volume of 0.663 cm³ g^?1 at the carbonization temperature of 800 °C and starch to calcium acetate mass ratio of 2. Electrochemical measurements also display that the HPCs-2-800 electrodes have a high reversible capacity of 244 F g^?1 at the current density of 0.1 A g^?1 and 182 F g^?1 at the current density of 10 A g^?1. When the current density is elevated from 0.1 to 10 A g^?1, the high capacitance retention of 74.6 % reveals a good rate performance. Long charge–discharge cycling measurements disclose good stabilities over 25,000 cycles at different current densities of 1–10 A g^?1 (5000 cycles at each current density) for HPCs-2-800 electrode. The cycling results indicate a high capacitance retention of 99.6 % over 5000 charge–discharge cycles even at the current density of 10 A g^?1. The excellent supercapacitive performances imply that HPCs-2-800 is a promising candidate for supercapacitors.     

Thiourea aldehyde resin-based carbon/graphene composites for high-performance supercapacitors

Thiourea aldehyde resin-based heteroatom doping carbon and graphene composites (RHDC/GN) were prepared by an in situ polymerization and carbonization. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images showed that thiourea aldehyde resin deposited on lamellar GO flakes during the polymerization and RHDC/GN composites had a hierarchical structure. The specific capacitance of the RHDC/GN composites was high up to 355 F g^?1, much higher than that of the pure thiourea aldehyde resin-based heteroatom doping carbon (RHDC) with specific capacitance of 135 F g^?1 at a current density of 1.0 A g^?1 in 6-M KOH electrolyte. And the hetroatoms in RHDC/GN composites increase the specific capacitance, and GN enhances the conductivity of the electrodes which is beneficial to improving electrochemical cycling stability of the electrode significantly. The specific capacitance retains 90.97% after 5000 charge-discharge processes at 10 A g^?1, which provides potential as supercapacitors.     

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).     

Mechanochemically prepared polyaniline and graphene-based nanocomposites as electrodes of supercapacitors

Conductive nanocomposites based on polyaniline and graphene (PAni/Gr) were prepared by cheap and efficient mechanochemical method. The uniform distribution of Gr nanoparticles in the polymer matrix and the ordering of the polymer chains due to the action of mechanical shear stresses, which were established by TEM, stipulated high specific capacitance about 920 F g^?1 in ??0.2–1.0 V vs. Ag/AgCl potential range. PAni/Gr-based electrodes are able to provide the specific capacitance of ~?750 F g^?1 at 2 A g^?1 in symmetric supercapacitors (SSC) and stably cycle at the operating voltage V?=?0.65 V for 10,000 charge-discharge cycles with 96% capacitance retention, whereas the increasing of V leads to the loss of stability as a result of the cathode degradation. PAni/Gr-based SSC possessed improved self-discharge showed high rate capability, and the specific power of such SSC could reach ~?10 kW kg^?1 at the specific energy of ~?18 W h kg^?1.     

Converting eucalyptus leaves into mesoporous carbon for its application in quasi solid-state supercapacitors

A high performance activated carbon having pore diameter of 2.8 nm and specific surface area of 841.8 m² g^?1 is prepared by chemical activation of eucalyptus leaves using KOH as an activating agent. The chemically-treated eucalyptus leaves EL(T) as electrode material has a specific capacitance of 663.5 mF cm^?2 (equivalent to single electrode specific capacitance of 442.3 F g^?1) with solid polymer electrolyte. This active material has excellent rate capability and good cycle performance, over 95 % of the original capacitance is retained after 5,000 cycles. The energy density of 101.6 Wh kg^?1 and power density of 2.85 kW kg^?1 has been observed for EL(T) based quasi solid-state supercapacitors.     

The loading of polyoxometalates compound on a biomass derived N-doped mesoporous carbon matrix,a composite material for electrical energy storage

Abstract

A polyoxometalate (POM)-based composite material (NiPW₁₂NP/NMC) was synthesized, in which the nanoparticle of a POM compound (NiPW₁₂NP) distributes on orange juice derived nitrogen doped mesoporous carbon matrix (NMC) homogenously. When employed as a cathode material, NiPW₁₂NP/NMC exhibits high specific capacitance, remarkable rate capability and long-term stability. When the current density is 4?A·g^?1, a specific capacitance as high as 547 F·g^?1 is achieved by NiPW₁₂NP/NMC. With NiPW₁₂NP/NMC serving as cathode and MnO₂ acting as anode, a high performance asymmetric supercapacitor is assembled, which possesses a high energy density of 10.88?Wh·kg^?1 at 0.64?kW·kg^?1. It also shows a good rate capability, when the current density increases from 4 to 12?A·g^?1, its specific capacitance decreases from 113 to 88 F·g^?1, with 77.9% capacitance retention. After 5000 cycles charge-discharge experiments, 92.8% of its capacitance can be maintained, which exhibits good stability.     

Porous nitrogen-doped graphene for high energy density supercapacitors in an ionic liquid electrolyte

Porous nitrogen-doped graphene (PNG) has been prepared via simple thermal treatment of graphene oxide and urea, and the morphology and structure of the PNG have been characterized by using a range of electron microscopy, X-ray photoelectron spectroscopy, and other techniques. The electrochemical performances of the PNG have been investigated in an ionic liquid electrolyte by cyclic voltammetry and galvanostatic charge-discharge via both three-electrode and two-electrode configurations. The PNG electrode delivers a specific capacitance of 310 F g^?1 at 1 A g^?1 with good cycling stability over 4000 cycles. The high electrochemical performance is ascribed to the porous structure and nitrogen-doping in the PNG. The porous structure enables high specific surface area and rapid ion mobility, contributing to double layer capacitance, while the N-doping enhances electrochemical activity and electric conductivity, contributing to pseudocapacitance. Meanwhile, the ionic liquid electrolyte enables a very wide working voltage of 3 V, leading to a high energy density up to 163.8 W h kg^?1. The fabricated supercapacitor can light up a LED for a long while with low self-discharge, showing good potential for practical application.     

Dual template method to prepare hierarchical porous carbon nanofibers for high-power supercapacitors

Hierarchical porous carbon nanofibers serving as electrode materials are prepared through carbonization and hydrofluoric acid treatment of polyacrylonitrile-based electrospinning involving dual templates. The hierarchical porous structures are synergistically tailored by varying template contents in the spinning solution. The carbon nanofibers prepared from the electrospinning of polyacrylonitrile containing 15/15 wt.% polymethylmethacrylate/tetraethyl orthosilicate exhibit the largest specific surface area (699 m² g^?1) and microporous volume (0.196 cm³ g^?1). In 6 M KOH electrolyte, a symmetrical supercapacitor equipped with the hierarchical porous carbon nanofibers demonstrates its high-end specific capacitance of 170 F g^?1, superior rate capability, and high-power density output up to 14.7 kW kg^?1. Cycling evolution indicates capacitance fading is only 5.8 % of initial capacitance at a current density of 1 A g^?1 even after 8,000 cycles. The excellent electrochemical performances of the carbon nanofiber are mainly ascribed to the optimized pore size distributions of both micropores and mesopores and the unique hierarchical pore structures possessed by abundant micropores.     

Facile fabrication of mesoporous manganese oxides as advanced electrode materials for supercapacitors

Mesoporous manganese oxides (MnO₂) were synthesized via a facile chemical deposition strategy. Three kinds of basic precipitants including sodium carbonate (Na₂CO₃), sodium bicarbonate (NaHCO₃), and sodium hydroxide (NaOH) were employed to adjust the microstructures and surface morphologies of MnO₂ materials. The obtained MnO₂ materials display different microstructures. Great differences are observed in their specific surface area and porosity properties. The microstructures and surface morphologies characteristics of MnO₂ materials largely determine their pseudocapacitive behavior for supercapacitors. The MnO₂ prepared with Na₂CO₃ precipitant exhibits the optimal microstructures and surface morphologies compared with the other two samples, contributing to their best electrochemical performances for supercapacitors when conducted either in the single electrode tests or in the capacitor measurements. The optimal MnO₂ electrode exhibits a high specific capacitance (173 F g^–1 at 0.25 A g^?1), high-rate capability (123 F g^?1 at 4 A g^?1), and excellent cyclic stability (no capacitance loss after 5,000 cycles at 1 A g^?1). The optimal activated carbon//MnO₂ hybrid capacitor exhibits a wide working voltage (1.8 V), high-power and high-energy densities (1,734 W kg^?1 and 20.9 Wh kg^?1), and excellent cycling behavior (93.8 % capacitance retention after 10,000 cycles at 1 A g^?1), indicating the promising applications of the easily fabricated mesoporous MnO₂ for supercapacitors.     

Pomegranate rind-derived activated carbon as electrode material for high-performance supercapacitors

In this paper, activated carbon materials were synthesized from pomegranate rind through carbonization and alkaline activation processes. The effects of pyrolytic temperature on the textual properties and electrochemical performance were investigated. The surface area of the activated carbon can reach at least 2200 m² g^?1 at different pyrolytic temperatures. It was found that, at the range of 600–900 °C, decreasing the carbonization temperature leads to the increase of t-plot micropore area, t-plot micropore volume, and capacitance. Further decreasing the carbonization temperature to 500 °C also leads to the increase of t-plot micropore area and t-plot micropore volume, but the capacitance is slightly poorer. The activated carbon carbonized at 600 °C and activated at 800 °C possesses very high specific area (2931 m² g^?1) and exhibits very high capacitance (～268 F g^?1 at 0.1 A g^?1 and ～242 F g^?1 at 1 A g^?1). There is no capacitance fading after 2000th cycle.     

A high mass loading electrode based on ultrathin Co<Subscript>3</Subscript>S<Subscript>4</Subscript> nanosheets for high performance supercapacitor

There is a growing need for the electrode with high mass loading of active materials, where both high energy and high power densities are required, in current and near-future applications of supercapacitor. Here, an ultrathin Co₃S₄ nanosheet decorated electrode (denoted as Co₃S₄/NF) with mass loading of 6 mg cm^?2 is successfully fabricated by using highly dispersive Co₃O₄ nanowires on Ni foam (NF) as template. The nanosheets contained lots of about 3～5 nm micropores benefiting for the electrochemical reaction and assembled into a three-dimensional, honeycomb-like network with 0.5～1 μm mesopore structure for promoting specific surface area of electrode. The improved electrochemical performance was achieved, including an excellent cycliability of 10,000 cycles at 10 A g^?1 and large specific capacitances of 2415 and 1152 F g^?1 at 1 and 20 A g^?1, respectively. Impressively, the asymmetric supercapacitor assembled with the activated carbon (AC) and Co₃S₄/NF electrode exhibits a high energy density of 79 Wh kg^?1 at a power density of 151 W kg^?1, a high power density of 3000 W kg^?1 at energy density of 30 Wh kg^?1 and 73 % retention of the initial capacitance after 10,000 charge-discharge cycles at 2 A g^?1. More importantly, the formation process of the ultrathin Co₃S₄ nanosheets upon reaction time is investigated, which is benefited from the gradual infiltration of sulfide ions and the template function of ultrafine Co₃O₄ nanowires in the anion-exchange reaction.

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Graphical abstract The ultrathin 2D Co₃S₄ nanosheets fabricated on 3D Ni foam and the formation process of the ultrathin Co₃S₄ nanosheets upon reaction times has been investigated. At the same time, the Co₃S₄/NF electrode displays an outstanding specific capacitance of 2420 F g^?1 at 1 A g^?1 with high mass loading of 6 mg cm^?2.

     

Coralloid and hierarchical Co<Subscript>3</Subscript>O<Subscript>4</Subscript> nanostructures used as supercapacitors with good cycling stability

Coralloid and hierarchical Co₃O₄ nanostructures were synthesized by a facile two-step approach composed of room temperature solution-phase synthesis without any surfactant and calcination of precursor. Owing to the unique structural features, the capacitance of Co₃O₄ could reach up to 591 F g^?1 at a current density of 0.5 A g^?1. Especially the cycling stability remained about 97 % after 2000 cycles at a current density of 1 A g^?1. These results demonstrated that the coralloid and hierarchical Co₃O₄ were excellent candidates for electrochemical supercapacitor devices.     

KOH‐Activated Porous Carbons Derived from Chestnut Shell with Superior Capacitive Performance

Porous carbons (PC) were prepared from a waste biomass named chestnut shell via a two‐step method involving carbonization and KOH activation. The morphology, pore structure and surface chemical properties were investigated by scanning electron microscopy (SEM), N₂ sorption, Raman spectroscopy, X‐ray diffraction (XRD) and X‐ray photoelectron spectroscopy (XPS). The carbons have been evaluated as the electrode materials for supercapacitors by a two‐electrode system in 6 mol/L KOH electrolyte. Benefiting from the porous texture, high surface area and high oxygen content, the PCs derived from chestnut shell have exhibited high specific capacitance of 198.2 (PC‐1), 217.2 (PC‐2) and 238.2 F·g^?1 (PC‐3) at a current density of 0.1 A·g^?1, good rate capability of 55.7%, 56.6% and 54.9% in a range of 0.1–20 A·g^?1 and high energy density of 5.6, 6.1 and 6.7 Wh·kg^?1, respectively. This is believed to be due to electric double layer capacitance induced by the abundant micropores and extra pseudo‐capacitance generated by oxygen‐containing groups. At a power density of 9000 Wh·kg^?1, the energy density is 3.1, 3.5 and 3.7 Wh·kg^?1 for PC‐1, PC‐2 and PC‐3, respectively, demonstrating the potential of the carbons derived from chestnut shells in energy storage devices.     

Resorcinol-formaldehyde carbon spheres/polyaniline composite with excellent electrochemical performance for supercapacitors

Well-dispersed resorcinol-formaldehyde-based carbon spheres (RFCs) have been prepared by the polycondensation of resorcinol and formaldehyde with ammonia as catalyst and subsequent carbonization of the obtained polymer. In situ polymerization of the aniline occurred in the suspension of the RFC, and RFC was surrounded by the polyaniline (PANI) wires. The PANI and RFC hybrid network (PRFC) formed gradually. In a three-electrode mode, the specific capacitance (C _sp) of PRFC reaches 315 F g^?1 at a current density of 1 A g^?1 in 2 M H₂SO₄, much higher than that of pure PANI (225 F g^?1) and RFC (121.7 F g^?1). Furthermore, the C _sp of PRFC retains 80.0 % after 1000 charge-discharge processes at a current density of 5 Ag^?1. The enhanced electrochemical performance of the PRFC came from its homogeneous three-dimensional hierarchical network structure, good electric conductivity of the PANI around the RFC, and the synergistic effect between the RFC and PANI.