ZIF-67/rGO/NiPc composite electrode material for high-performance asymmetric supercapacitors

In this reported study, novel multiple dimensional ZIF-67/rGO/NiPc composite materials were prepared for supercapacitors. The electrochemical test showed that the ZIF-67/rGO/NiPc electrode achieved a remarkable specific capacitance of 860 F g^?1 at a current density of 1 A g^?1, which was superior to that of the rGO/NiPc and ZIF-67/rGO electrodes. An asymmetric supercapacitor based on ZIF-67/rGO/NiPc//activated carbon exhibited a high specific capacitance of 200.67 F g^?1 and an extraordinary energy density of 62.7 Wh kg^?1 at a corresponding power density of 750 W kg^?1. In addition, the device demonstrated 94.6% capacitance retention after 5000 cycles. The assembled asymmetric supercapacitors could easily powered a green light-emitting diode. This work revealed a promising research route for the rational construction of multiple dimensioned high-performance electrodes materials for use in new energy storage devices.

     

One-pot fabrication of pitch-derived soft carbon with hierarchical porous structure and rich sp2 carbon for sodium-ion battery

Soft carbons with porous structure have attracted increasing attention as attracting anode active materials for sodium-ion battery. However, the reported anode active materials are plagued by limited capacity and unsatisfying rate performance. Besides, the improvement of electrical conductivity is often performed by post treatment, which is time-consuming and high cost. Herein we report an one-pot fabrication of pitch-derived soft carbon using zinc acetate as hard templates. Zinc acetate can not only play a vital role in constructing hierarchical porous structure but also providing additional sp² carbon during carbonization, resulting in significantly enhanced sodium-ion storage capacity and improved rate performance. As anode active materials for sodium-ion battery, the as-prepared soft carbon with hierarchical porous structure, rich sp² carbon and doped N, O heteroatoms, exhibits an impressively high reversible capacity of 293 mAh g^?1 at 0.05 A g^?1 in sodium-ion half cells, good rate capability of 53 mAh g^?1 at 5 A g^?1, and high capacity retention of 92.2% after 1000 cycles at 1 A g^?1. Our strategy affords a facile way to prepare high-quality soft carbon materials as anode active materials for sodium-ion batteries.

     

Hierarchical 3D All‐Carbon Composite Structure Modified with N‐Doped Graphene Quantum Dots for High‐Performance Flexible Supercapacitors

Flexible supercapacitors have shown enormous potential for portable electronic devices. Herein, hierarchical 3D all‐carbon electrode materials are prepared by assembling N‐doped graphene quantum dots (N‐GQDs) on carbonized MOF materials (cZIF‐8) interweaved with carbon nanotubes (CNTs) for flexible all‐solid‐state supercapacitors. In this ternary electrode, cZIF‐8 provides a large accessible surface area, CNTs act as the electrical conductive network, and N‐GQDs serve as highly pseudocapactive materials. Due to the synergistic effect and hierarchical assembly of these components, N‐GQD@cZIF‐8/CNT electrodes exhibit a high specific capacitance of 540 F g^?1 at 0.5 A g^?1 in a 1 m H₂SO₄ electrolyte and excellent cycle stability with 90.9% capacity retention over 8000 cycles. The assembled supercapacitor possesses an energy density of 18.75 Wh kg^?1 with a power density of 108.7 W kg^?1. Meanwhile, three supercapacitors connected in series can power light‐emitting diodes for 20 min. All‐solid‐state N‐GQD@cZIF‐8/CNT flexible supercapacitor exhibits an energy density of 14 Wh kg^?1 with a power density of 89.3 W kg^?1, while the capacitance retention after 5000 cycles reaches 82%. This work provides an effective way to construct novel electrode materials with high energy storage density as well as good cycling performance and power density for high‐performance energy storage devices via the rational design.     

Hierarchically porous carbon derived from renewable Chingma Abutilon Seeds for high-energy supercapacitors

The cost-effectively biomass-derived porous carbon is highly promising for usage in electrochemical energy storage as the electrode materials. Herein, a series of hierarchically porous carbons with biomass Chingma Abutilon Seeds as the renewable precursor were synthesized via KOH activation and high-temperature carbonization technique. The resulting carbon material possessed an interconnected structure, high specific surface area (120–3566 m² g^?1), hierarchical pores as well as the heteroatom-substituted functional groups. Based on the synergistic effect of the above-mentioned merits, the optimized material exhibited the remarkably electrochemical performance with high specific capacitance (389 F g^?1 at 0.5 A g^?1) and excellent rate stability (72% capacitance retention at 20 A g^?1) in the three-electrode configuration. More significantly, the symmetric two-electrode device assembled in 6 M KOH delivered a high energy density of 39.2 Wh kg^?1 and excellent chemical stability (90% capacitance retention after 10,000 cycles at 5 A g^?1). Such prominent results might provide a new perspective on the value-added application of the renewable biomass resources in the electrochemical field.     

Making a wearable supercapacitor based on highly porous 3D nitrogen-doped graphene

In this paper, based on the hydrothermal method and using a non-toxic organic molecule, as a spacer (and nitrogen source), we synthesized a highly conductive and porous 3D graphene. Then, graphene is used as an electrode material to make a supercapacitor on the surface of activated carbon cloth electrode. The graphene is characterized by different methods, such as Fourier-transform infrared spectroscopy, thermogravimetric analysis, Raman spectroscopy, X-ray diffraction, energy-dispersive and transmission electron microscopy, energy-dispersive X-Ray spectroscopy, emission scanning electron microscopy, Barrett–Joyner–Halenda, and Brunauer–Emmett–Teller methods. The supercapacitor (2 and 3 electrodes) is studied by different electrochemical techniques, such as cyclic voltammetry, electrochemical impedance spectroscopy, and galvanostatic charge/discharge. The 3-electrode system shows a specific capacity 101 F g^? 1 at the current density of 1.7 A g^? 1 (or 0.5 mA cm^? 2). The 2-electrode system (symmetric capacitor) has a power density of about 8000 W kg^? 1 and a maximum energy density of 12.85 Wh kg^? 1.

     

Ionic Liquid‐Controlled Growth of NiCo2S4 3D Hierarchical Hollow Nanoarrow Arrays on Ni Foam for Superior Performance Binder Free Hybrid Supercapacitors

A significant development in the design of a NiCo₂S₄ 3D hierarchical hollow nanoarrow arrays (HNA)‐based supercapacitor binder free electrode assembled by 1D hollow nanoneedles and 2D nanosheets on a Ni foam collector through controlling ionic liquid 1‐octyl‐3‐methylimidazolium chloride ([OMIm]Cl) concentration is reported. The unique NiCo₂S₄‐HNA electrode acquires high specific capacity (1297 C g^?1 at 1 A g^?1, 2.59 C cm^?2 at 2 mA cm^?2), excellent rate capability (maintaining 73.0% at 20 A g^?1), and long operational life (maintaining 92.4% after 10 000 cycles at 5 A g^?1), which are superior to those for 1D hollow nanoneedle arrays (HNN) and 2D porous nanoflake arrays (PNF). The outstanding electrochemical performance is attributed to the novel 3D structure with large specific surface, hollow cores, high porosity as well as stable architecture. In addition, a hybrid supercapacitor applying 3D NiCo₂S₄‐HNA as the positive electrode and active carbon as the negative electrode exhibits a high energy density of 42.5 Wh kg^?1 at a power density of 2684.2 W kg^?1 in an operating voltage of 1.6 V. Robust cycling stability is also expressed with 84.9% retention after repeating 10 000 cycles at 5 A g^?1, implying their great potential in superior‐performance supercapacitors.     

Inflating strategy to fabricate highly dispersed Fe,N co-doped hierarchically porous carbon for ORR and supercapacitor

Doped-carbon nanomaterials as effective electrocatalysts have been received widespread attention in oxygen reduction reaction (ORR) and supercapacitors system. Herein, the high-active Fe atoms dispersed on hierarchically porous N-doped carbon (FeNC-X) is synthesized via inflating the Fe-ion-denatured egg-white, followed by activation and pyrolysis. Among them, the as-prepared FeNC-900 for ORR that has an inner-connecting hierarchically porous structure shows a superior performance with a limiting current density of 5.28 mA cm^?2 and half-wave potential (E_1/2) of 0.839 V (vs RHE), and exhibits a 4 e^? ORR pathway in the alkaline medium. FeNC-900 also shows better durability and good methanol tolerance than those of commercial Pt/C. Besides, FeNC-900 exhibits an outstanding specific capacity of 258 F g^?1 at 1 A g^?1 for supercapacitor. The method presented here may provide a cost-efficient approach to fabricate carbon-based materials for ORR and supercapacitors.

     

Tailoring Hierarchically Porous Nitrogen‐, Sulfur‐Codoped Carbon for High‐Performance Supercapacitors and Oxygen Reduction

Heteroatom‐doped carbon materials are intensively studied in supercapacitors and fuel cells, because of their great potential for sustainably bearing on the energy crisis and environmental pollution. Although enormous efforts are put in material perfection with a hierarchically porous microstructure, the simultaneous optimization of both porous structures and surface functionalities is hard to achieve due to inevitable concurrent dopant leaching effect and structural collapse under required high pyrolysis temperature. In this study, an in situ dehalogenation polymerization and activation protocol is introduced to synthesize nitrogen‐ and sulfur‐codoped carbon materials (NS‐PCMs) with hierarchical pore distribution and abundant surface doping, which endows them with good conductivity, abundant accessible active sites, and efficient mass transport. As a result, the as‐prepared carbon materials (NS‐a‐PCM‐1000) show an excellent mass specific capacitance of 461.5 F g^?1 at a current density of 0.1 A g^?1, long cycle life (>23 k, 10 A g^?1), and high device energy and power density (17.3 Wh kg^?1, 250 W kg^?1). Significantly, NS‐a‐PCM‐1000 also exhibits one of the highest oxygen reduction reaction activities (onset potential of 1.0 V vs reversible hydrogen electrode) in alkaline media among all reported metal‐free catalysts.     

Core–Shell Nitrogen‐Doped Carbon Hollow Spheres/Co3O4 Nanosheets as Advanced Electrode for High‐Performance Supercapacitor

Co₃O₄/nitrogen‐doped carbon hollow spheres (Co₃O₄/NHCSs) with hierarchical structures are synthesized by virtue of a hydrothermal method and subsequent calcination treatment. NHCSs, as a hard template, can aid the generation of Co₃O₄ nanosheets on its surface; while SiO₂ spheres, as a sacrificed‐template, can be dissolved in the process. The prepared Co₃O₄/NHCS composites are investigated as the electrode active material. This composite exhibits an enhanced performance than Co₃O₄ itself. A higher specific capacitance of 581 F g^?1 at 1 A g^?1 and a higher rate performance of 91.6% retention at 20 A g^?1 are achieved, better than Co₃O₄ nanorods (318 F g^?1 at 1 A g^?1 and 67.1% retention at 20 A g^?1). In addition, the composite is employed as a positive electrode to fabricate an asymmetric supercapacitor. The device can deliver a high energy density of 34.5 Wh kg^?1 at the power density of 753 W kg^?1 and display a desirable cycling stability. All of these attractive results make the unique hierarchical Co₃O₄/NHCS core–shell structure a promising electrode material for high‐performance supercapacitors.     

Controlled Air‐Etching Synthesis of Porous‐Carbon Nanotube Aerogels with Ultrafast Charging at 1000 A g−1

Supercapacitors are energy storage systems capable of fast charging and discharging, thus generating superior power density. Porous carbon with high surface area and tunable pore size represents a promising candidate to construct ultrafast supercapacitors; so far, most porous carbon–based electrodes can only be charged to a moderate current density (100–200 A g^?1), also with significant capacitance loss at increasing rate. Here, it is shown that a 3D aerogel consisting of interconnected 1D porous‐carbon nanotubes (PCNs) can serve as a freestanding supercapacitor electrode with excellent rate performance. As a result, the PCN aerogel electrodes achieve 1) ultrafast charging at current densities up to 1000 A g^?1 (corresponding to a charge period of 16 ms), which is the highest value among other porous carbon–based supercapacitors, 2) superior cycling stability at high charging rates (88% capacitance retention after 10⁵ cycles at 1000 A g^?1). Mechanism study reveals favorable kinetics including a centralized pore size distribution at 0.8 nm which is a dominant factor to allow high‐rate charging, a low and linear IR drop, and a metallic feature of 1D PCNs by theoretical calculation. The results indicate that 1D PCNs with controlled porous structures have potential applications in ultrafast energy conversion and storage.     

Hierarchical 3D Cobalt‐Doped Fe3O4 Nanospheres@NG Hybrid as an Advanced Anode Material for High‐Performance Asymmetric Supercapacitors

Hierarchical nanostructure, high electrical conductivity, extraordinary specific surface area, and unique porous architecture are essential properties in energy storage and conversion studies. A new type of hierarchical 3D cobalt encapsulated Fe₃O₄ nanosphere is successfully developed on N‐graphene sheet (Co?Fe₃O₄ NS@NG) hybrid with unique nanostructure by simple, scalable, and efficient solvothermal technique. When applied as an electrode material for supercapacitors, hierarchical Co?Fe₃O₄ NS@NG hybrid shows an ultrahigh specific capacitance (775 F g^?1 at a current density of 1 A g^?1) with exceptional rate capability (475 F g^?1 at current density of 50 A g^?1), and admirable cycling performance (97.1% capacitance retention after 10 000 cycles). Furthermore, the fabricated Co?Fe₃O₄ NS@NG//CoMnO₃@NG asymmetric supercapacitor (ASC) device exhibits a high energy density of 89.1 Wh kg^?1 at power density of 0.901 kW kg^?1, and outstanding cycling performance (89.3% capacitance retention after 10 000 cycles). Such eminent electrochemical properties of the Co?Fe₃O₄ NS@NG are due to the high electrical conductivity, ultrahigh surface area, and unique porous architecture. This research first proposes hierarchical Co?Fe₃O₄ NS@NG hybrid as an ultrafast charge?discharge anode material for the ASC device, that holds great potential for the development of high‐performance energy storage devices.     

Fabrication of a 3D Hierarchical Sandwich Co9S8/α‐MnS@N–C@MoS2 Nanowire Architectures as Advanced Electrode Material for High Performance Hybrid Supercapacitors

Supercapacitors suffer from lack of energy density and impulse the energy density limit, so a new class of hybrid electrode materials with promising architectures is strongly desirable. Here, the rational design of a 3D hierarchical sandwich Co₉S₈/α‐MnS@N–C@MoS₂ nanowire architecture is achieved during the hydrothermal sulphurization reaction by the conversion of binary mesoporous metal oxide core to corresponding individual metal sulphides core along with the formation of outer metal sulphide shell at the same time. Benefiting from the 3D hierarchical sandwich architecture, Co₉S₈/α‐MnS@N–C@MoS₂ electrode exhibits enhanced electrochemical performance with high specific capacity/capacitance of 306 mA h g^?1/1938 F g^?1 at 1 A g^?1, and excellent cycling stability with a specific capacity retention of 86.9% after 10 000 cycles at 10 A g^?1. Moreover, the fabricated asymmetric supercapacitor device using Co₉S₈/α‐MnS@N–C@MoS₂ as the positive electrode and nitrogen doped graphene as the negative electrode demonstrates high energy density of 64.2 Wh kg^?1 at 729.2 W kg^?1, and a promising energy density of 23.5 Wh kg^?1 is still attained at a high power density of 11 300 W kg^?1. The hybrid electrode with 3D hierarchical sandwich architecture promotes enhanced energy density with excellent cyclic stability for energy storage.     

ZTIFs derived nitrogen-introduced high specific area and hierarchical porous carbon for oxygen reduction reaction

It is of great allure to construct nitrogen-doped hierarchical porous carbon to replace Pt-based catalysts for efficient ORR. Here, nitrogen-doped hierarchical porous carbon (NHPC) was prepared by carbonizing ZTIF-1 and KOH activating. The resultant NHPC4-700 catalyst exhibits a hierarchical porous structure and high specific area (2404 m² g^?1), which promoted the exposure of enough active sites as well as simultaneously enhanced the electron transfer rate, shorten the mass transfer pathway, enhanced ionic conductivity and carbon wetting. The results are capable of remarkably improving the ORR activities of carbon materials. The NHPC4-700 catalyst exhibits a great catalytic performance with onset potential at 0.90 V and limiting current density of ??6.0 mA cm^?2, which is close to commercial Pt/C electrocatalyst. Meanwhile, the NHPC4-700 catalysts had better stability and methanol resistance than that of Pt/C toward ORR. These superior electrochemical properties of the NHPC4-700 catalysts were closely related to their nitrogen-doped hierarchical porous structure and high specific area.

     

Superior capacitive behavior of porous activated carbon tubes derived from biomass waste-cotonier strobili fibers

As supercapacitor electrode materials, the sustainable biomass-derived activated carbons have attracted a great deal of attentions due to their low-cost, abundant, and unwanted natural wastes. In this work, a facile KOH activation method is adopted to prepare activated carbon tubes from the biomass waste-cotonier strobili fibers for the first time. The resultant PTAC-x materials possess highly accessible surface areas and abundant micro-mesopores, which benefit large ion storage and high-rate ion transfer. The optimized material denoted as PTAC-6 demonstrates a high specific capacity (346.1?F?g^?1 at 1?A?g^?1) and a superior rate performance (214.5?F?g^?1 at 50?A?g^?1) in the three-electrode supercapacitors. In addition, the symmetric supercapacitor exhibits excellent cycling stability with a capacitance retention of 84.21% and a columbic efficiency of nearly 100% after 10,000 cycles. Furthermore, the PTAC-6-based symmetric supercapacitor gives a remarkable specific energy of 33.04?Wh?kg^?1 at 160?W?kg^?1. Meanwhile, our proposed porous activated carbon tubes provide a green and low-cost electrode material for high-performance supercapacitors.     

MoS2/NiS Yolk–Shell Microsphere‐Based Electrodes for Overall Water Splitting and Asymmetric Supercapacitor

Rational designing of the composition and structure of electrode material is of great significance for achieving highly efficient energy storage and conversion in electrochemical energy devices. Herein, MoS₂/NiS yolk–shell microspheres are successfully synthesized via a facile ionic liquid‐assisted one‐step hydrothermal method. With the favorable interface effect and hollow structure, the electrodes assembled with MoS₂/NiS hybrid microspheres present remarkably enhanced electrochemical performance for both overall water splitting and asymmetric supercapacitors. In particular, to deliver a current density of 10 mA cm^?2, the MoS₂/NiS‐based electrolysis cell for overall water splitting only needs an output voltage of 1.64 V in the alkaline medium, lower than that of Pt/C–IrO₂‐based electrolysis cells (1.70 V). As an electrode for supercapacitors, the MoS₂/NiS hybrid microspheres exhibit a specific capacitance of 1493 F g^?1 at current density of 0.2 A g^?1, and remain 1165 F g^?1 even at a large current density of 2 A g^?1, implying outstanding charge storage capacity and excellent rate performance. The MoS₂/NiS‐ and active carbon‐based asymmetric supercapacitor manifests a maximum energy density of 31 Wh kg^?1 at a power density of 155.7 W kg^?1, and remarkable cycling stability with a capacitance retention of approximately 100% after 10 000 cycles.     

Flower-like hierarchical porous nitrogen-doped carbon spheres from a facile one-step carbonization method for supercapacitors

A facile one-step carbonization method was developed to fabricate flower-like hierarchical porous nitrogen-doped carbon sphere (FHPNCS) from polyimide using polyurethane foam as macroporous scaffold. The FHPNCS possessed flower-like spherical morphology, well-developed hierarchical porous structure, high specific surface area and nitrogen-containing functional groups. These advantages led to excellent electrochemical performance. The FHPNCS electrode exhibited a high specific capacitance of 251.6 F g^?1 at 1 A g^?1, a high rate capability of 76% capacitance retention at 5 A g^?1, and an outstanding cycling stability of only 4.4% loss in specific capacitance after 2000 cycles. Compared with previously reported multi-step templating methods, the present method only involves a facile thermal treatment procedure and avoids the use of hard templates and toxic raw materials, thus exhibiting great potential for large scale production of nitrogen-doped carbon materials for practical applications in supercapacitors.     

Efficient Supercapacitor Energy Storage Using Conjugated Microporous Polymer Networks Synthesized from Buchwald–Hartwig Coupling

Supercapacitors have received increasing interest as energy storage devices due to their rapid charge–discharge rates, high power densities, and high durability. In this work, novel conjugated microporous polymer (CMP) networks are presented for supercapacitor energy storage, namely 3D polyaminoanthraquinone (PAQ) networks synthesized via Buchwald–Hartwig coupling between 2,6‐diaminoanthraquinone and aryl bromides. PAQs exhibit surface areas up to 600 m² g^?1, good dispersibility in polar solvents, and can be processed to flexible electrodes. The PAQs exhibit a three‐electrode specific capacitance of 576 F g^?1 in 0.5 m H₂SO₄ at a current of 1 A g^?1 retaining 80–85% capacitances and nearly 100% Coulombic efficiencies (95–98%) upon 6000 cycles at a current density of 2 A g^?1. Asymmetric two‐electrode supercapacitors assembled by PAQs show a capacitance of 168 F g^?1 of total electrode materials, an energy density of 60 Wh kg^?1 at a power density of 1300 W kg^?1, and a wide working potential window (0–1.6 V). The asymmetric supercapacitors show Coulombic efficiencies up to 97% and can retain 95.5% of initial capacitance undergo 2000 cycles. This work thus presents novel promising CMP networks for charge energy storage.     

Ultrathin Hierarchical Porous Carbon Nanosheets for High‐Performance Supercapacitors and Redox Electrolyte Energy Storage

The design of advanced high‐energy‐density supercapacitors requires the design of unique materials that combine hierarchical nanoporous structures with high surface area to facilitate ion transport and excellent electrolyte permeability. Here, shape‐controlled 2D nanoporous carbon sheets (NPSs) with graphitic wall structure through the pyrolysis of metal–organic frameworks (MOFs) are developed. As a proof‐of‐concept application, the obtained NPSs are used as the electrode material for a supercapacitor. The carbon‐sheet‐based symmetric cell shows an ultrahigh Brunauer–Emmett–Teller (BET)‐area‐normalized capacitance of 21.4 µF cm^?2 (233 F g^?1), exceeding other carbon‐based supercapacitors. The addition of potassium iodide as redox‐active species in a sulfuric acid (supporting electrolyte) leads to the ground‐breaking enhancement in the energy density up to 90 Wh kg^?1, which is higher than commercial aqueous rechargeable batteries, maintaining its superior power density. Thus, the new material provides a double profits strategy such as battery‐level energy and capacitor‐level power density.     

A facile synthetic method and electrochemical performances of nickel oxide/carbon fibers composites

The uniform and completed nanofilms of nickel oxide (NiO) were electrodeposited on the carbon fibers (CFs) by a facile method of cyclic voltammetric. The as-prepared NiO/CFs composites can be used as a flexible electrode for electrochemical supercapacitors. Electrochemical measurements showed that 1.0-NiO/CFs had a good redox process and reversibility, and displayed the specific capacitances as high as 929 F g^?1 at a current density of 1 A g^?1. After 5000 cycles of charge and discharge, the 1.0-NiO/CFs composite materials could retain more than 88% of initial capacitance and show an excellent cyclability. Meanwhile, this supercapacitor exhibited a higher energy density of 20.8 Wh kg^?1 at a power density of 200 W kg^?1. The carbon fibers acting as active substrate for the composite electrode are a good conductor and have a larger capacitance of electrical double layer. The nanofilm structure of NiO could facilitate the contact of the electrolyte with the active materials, thus increasing the Faradaic pseudo-capacitance.     

High‐Stacking‐Density,Superior‐Roughness LDH Bridged with Vertically Aligned Graphene for High‐Performance Asymmetric Supercapacitors

The high‐performance electrode materials with tuned surface and interface structure and functionalities are highly demanded for advanced supercapacitors. A novel strategy is presented to conFigure high‐stacking‐density, superior‐roughness nickel manganese layered double hydroxide (LDH) bridged by vertically aligned graphene (VG) with nickel foam (NF) as the conductive collector, yielding the LDH‐NF@VG hybrids for asymmetric supercapacitors. The VG nanosheets provide numerous electron transfer channels for quick redox reactions, and well‐developed open structure for fast mass transport. Moreover, the high‐stacking‐density LDH grown and assembled on VG nanosheets result in a superior hydrophilicity derived from the tuned nano/microstructures, especially microroughness. Such a high stacking density with abundant active sites and superior wettability can be easily accessed by aqueous electrolytes. Benefitting from the above features, the LDH‐NF@VG can deliver a high capacitance of 2920 F g^?1 at a current density of 2 A g^?1, and the asymmetric supercapacitor with the LDH‐NF@VG as positive electrode and activated carbon as negative electrode can deliver a high energy density of 56.8 Wh kg^?1 at a power density of 260 W kg^?1, with a high specific capacitance retention rate of 87% even after 10 000 cycles.