Nickel–Cobalt Layered Double Hydroxide Nanosheets for High‐performance Supercapacitor Electrode Materials

A facile and novel one‐step method of growing nickel‐cobalt layered double hydroxide (Ni‐Co LDH) hybrid films with ultrathin nanosheets and porous nanostructures on nickel foam is presented using cetyltrimethylammonium bromide as nanostructure growth assisting agent but without any adscititious alkali sources and oxidants. As pseudocapacitors, the as‐obtained Ni‐Co LDH hybrid film‐based electrodes display a significantly enhanced specific capacitance (2682 F g^?1 at 3 A g^?1, based on active materials) and energy density (77.3 Wh kg^?1 at 623 W kg^?1), compared to most previously reported electrodes based on nickel‐cobalt oxides/hydroxides. Moreover, the asymmetric supercapacitor, with the Ni‐Co LDH hybrid film as the positive electrode material and porous freeze‐dried reduced graphene oxide (RGO) as the negative electrode material, exhibits an ultrahigh energy density (188 Wh kg^?1) at an average power density of 1499 W kg^?1 based on the mass of active material, which greatly exceeds the energy densities of most previously reported nickel or cobalt oxide/hydroxide‐based asymmetric supercapacitors.     

High‐Performance Supercapacitor Applications of NiO‐Nanoparticle‐Decorated Millimeter‐Long Vertically Aligned Carbon Nanotube Arrays via an Effective Supercritical CO2‐Assisted Method

Nickel oxide (NiO) nanoparticles are distributed uniformly in the vertically aligned carbon nanotube arrays (VACNTs) with millimeter thickness by an effective supercritical carbon dioxide‐assisted method. The as‐prepared VACNT/NiO hybrid structures are used as electrodes without binders and conducting additives for supercapacitor applications. Due to the synergetic effects of NiO and VACNTs with nanoporous structures and parallel 1D conductive paths for electrons, the supercapacitors exhibit a high capacitance of 1088.44 F g^?1. Furthermore, an asymmetric supercapacitor is assembled using the as‐synthesized VACNTs/NiO hybrids as the positive electrode and the VACNTs as the negative electrode. Remarkably, the energy density of the asymmetric supercapacitor is as high as 90.9 Wh kg^?1 at 3.2 kW kg^?1 and the maximum power density reaches 25.6 kW kg^?1 at 24.9 Wh kg^?1, which are superior to those of the NiO or VACNTs‐based asymmetric supercapacitors. More importantly, the asymmetric supercapacitors exhibit capacitance retention of 87.1% after 2000 cycles at 5 A g^?1. The work provides a novel approach in decorating highly dense and long VACNTs with active materials, which are promising electrodes for supercapacitors with ultrahigh power density and energy density.     

Direct Ink Writing of Adjustable Electrochemical Energy Storage Device with High Gravimetric Energy Densities

3D printing graphene aerogel with periodic microlattices has great prospects for various practical applications due to their low density, large surface area, high porosity, excellent electrical conductivity, good elasticity, and designed lattice structures. However, the low specific capacitance limits their development in energy storage fields due to the stacking of graphene. Therefore, constructing a graphene‐based 2D materials hybridization aerogel that consists of the pseduocapacitive substance and graphene material is necessary for enhancing electrochemical performance. Herein, 3D printing periodic graphene‐based composite hybrid aerogel microlattices (HAMs) are reported via 3D printing direct ink writing technology. The rich porous structure, high electrical conductivity, and highly interconnected networks of the HAMs aid electron and ion transport, further enabling excellent capacitive performance for supercapacitors. An asymmetric supercapacitor device is assembled by two different 4‐mm‐thick electrodes, which can yield high gravimetric specific capacitance (C_g) of 149.71 F g^?1 at a current density of 0.5 A g^?1 and gravimetric energy density (E_g) of 52.64 Wh kg^?1, and retains a capacitance retention of 95.5% after 10 000 cycles. This work provides a general strategy for designing the graphene‐based mixed‐dimensional hybrid architectures, which can be utilized in energy storage fields.     

Advanced Asymmetric Supercapacitors Based on Ni(OH)2/Graphene and Porous Graphene Electrodes with High Energy Density

Hierarchical flowerlike nickel hydroxide decorated on graphene sheets has been prepared by a facile and cost‐effective microwave‐assisted method. In order to achieve high energy and power densities, a high‐voltage asymmetric supercapacitor is successfully fabricated using Ni(OH)₂/graphene and porous graphene as the positive and negative electrodes, respectively. Because of their unique structure, both of these materials exhibit excellent electrochemical performances. The optimized asymmetric supercapacitor could be cycled reversibly in the high‐voltage region of 0–1.6 V and displays intriguing performances with a maximum specific capacitance of 218.4 F g^?1 and high energy density of 77.8 Wh kg^?1. Furthermore, the Ni(OH)₂/graphene//porous graphene supercapacitor device exhibits an excellent long cycle life along with 94.3% specific capacitance retained after 3000 cycles. These fascinating performances can be attributed to the high capacitance and the positive synergistic effects of the two electrodes. The impressive results presented here may pave the way for promising applications in high energy density storage systems.     

Fabrication of Oxygen‐Vacancy Abundant NiMn‐Layered Double Hydroxides for Ultrahigh Capacity Supercapacitors

The rational design of advanced structures consisting of multiple components with excellent electrochemical capacitive properties is one of the crucial hindrances to be overcome for high‐performance supercapacitors (SCs). Herein, a superfast and facile synthesis of flower‐like NiMn‐layered double hydroxides (NiMn‐LDH) with high SC performance using an electrodeposition process on nickel foam is proposed. Oxygen vacancies are then modulated via mild H₂O₂ treatment for the first time, significantly promoting the electrochemical energy storage performance. The oxygen‐vacancy abundant NiMn‐LDH (Ov‐LDH) reaches a maximum specific capacity of 1183 C g^?1 at the current density of 1 A g^?1 and retains a high capacity retention of 835 C g^?1 even at a current density of up to 10 A g^?1. Furthermore, the assembled asymmetric SC device achieves a high specific energy density of 46.7 Wh kg^?1 at a power density of 1.7 kW kg^?1. Oxygen vacancies are proven to play a vital role in the improvement of electrochemistry performance of LDH based on experimental and theoretical studies. This vacancy engineering strategy provides a new insight into SC active materials and should be beneficial for the design of the next generation of energy storage devices.     

Molecular Design of Mesoporous NiCo2O4 and NiCo2S4 with Sub‐Micrometer‐Polyhedron Architectures for Efficient Pseudocapacitive Energy Storage

Spinel‐type NiCo₂O₄ (NCO) and NiCo₂S₄ (NCS) polyhedron architectures with sizes of 500–600 nm and rich mesopores with diameters of 1–2 nm are prepared facilely by the molecular design of Ni and Co into polyhedron‐shaped zeolitic imidazolate frameworks as solid precursors. Both as‐prepared NCO and NCS nanostructures exhibit excellent pseudocapacitance and stability as electrodes in supercapacitors. In particular, the exchange of O^2? in the lattice of NCO with S^2? obviously improves the electrochemical performance. NCS shows a highly attractive capacitance of 1296 F g^?1 at a current density of 1 A g^?1, ultrahigh rate capability with 93.2% capacitance retention at 10 A g^?1, and excellent cycling stability with a capacitance retention of 94.5% after cycling at 1 A g^?1 for 6000 times. The asymmetric supercapacitor with an NCS negative electrode and an active carbon positive electrode delivers a very attractive energy density of 44.8 Wh kg^?1 at power density 794.5 W kg^?1, and a favorable energy density of 37.7 Wh kg^?1 is still achieved at a high power density of 7981.1 W kg^?1. The specific mesoporous polyhedron architecture contributes significantly to the outstanding electrochemical performances of both NCO and NCS for capacitive energy storage.     

Design of Hierarchical Ni?Co@Ni?Co Layered Double Hydroxide Core–Shell Structured Nanotube Array for High‐Performance Flexible All‐Solid‐State Battery‐Type Supercapacitors

A novel hierarchical nanotube array (NTA) with a massive layered top and discretely separated nanotubes in a core–shell structure, that is, nickel–cobalt metallic core and nickel–cobalt layered double hydroxide shell (Ni? Co@Ni? Co LDH), is grown on carbon fiber cloth (CFC) by template‐assisted electrodeposition for high‐performance supercapacitor application. The synthesized Ni? Co@Ni? Co LDH NTAs/CFC shows high capacitance of 2200 F g^?1 at a current density of 5 A g^?1, while 98.8% of its initial capacitance is retained after 5000 cycles. When the current density is increased from 1 to 20 A g^?1, the capacitance loss is less than 20%, demonstrating excellent rate capability. A highly flexible all‐solid‐state battery‐type supercapacitor is successfully fabricated with Ni? Co LDH NTAs/CFC as the positive electrode and electrospun carbon fibers/CFC as the negative electrode, showing a maximum specific capacitance of 319 F g^?1, a high energy density of 100 W h kg^?1 at 1.5 kW kg^?1, and good cycling stability (98.6% after 3000 cycles). These fascinating electrochemical properties are resulted from the novel structure of electrode materials and synergistic contributions from the two electrodes, showing great potential for energy storage applications.     

Design of Hierarchical NiCo@NiCo Layered Double Hydroxide Core–Shell Structured Nanotube Array for High‐Performance Flexible All‐Solid‐State Battery‐Type Supercapacitors

A novel hierarchical nanotube array (NTA) with a massive layered top and discretely separated nanotubes in a core–shell structure, that is, nickel–cobalt metallic core and nickel–cobalt layered double hydroxide shell (Ni?Co@Ni?Co LDH), is grown on carbon fiber cloth (CFC) by template‐assisted electrodeposition for high‐performance supercapacitor application. The synthesized Ni?Co@Ni?Co LDH NTAs/CFC shows high capacitance of 2200 F g^?1 at a current density of 5 A g^?1, while 98.8% of its initial capacitance is retained after 5000 cycles. When the current density is increased from 1 to 20 A g^?1, the capacitance loss is less than 20%, demonstrating excellent rate capability. A highly flexible all‐solid‐state battery‐type supercapacitor is successfully fabricated with Ni?Co LDH NTAs/CFC as the positive electrode and electrospun carbon fibers/CFC as the negative electrode, showing a maximum specific capacitance of 319 F g^?1, a high energy density of 100 W h kg^?1 at 1.5 kW kg^?1, and good cycling stability (98.6% after 3000 cycles). These fascinating electrochemical properties are resulted from the novel structure of electrode materials and synergistic contributions from the two electrodes, showing great potential for energy storage applications.     

Multicomponent Hierarchical Cu‐Doped NiCo‐LDH/CuO Double Arrays for Ultralong‐Life Hybrid Fiber Supercapacitor

Fiber supercapacitors have aroused great interest in the field of portable and wearable electronic devices. However, the restrained surface area of fibers and limited reaction kinetics of active materials are unfavorable for performance enhancement. Herein, an efficient multicomponent hierarchical structure is constructed by integrating the Cu‐doped cobalt copper carbonate hydroxide@nickel cobalt layered double hydroxide (CCCH@NiCo‐LDH) core–shell nanowire arrays (NWAs) on Cu fibers with highly conductive Au‐modified CuO nanosheets (CCCH@NiCo‐LDH NWAs@Au–CuO/Cu) via a novel in situ corrosion growth method. This multicomponent hierarchical structure contributes to a large accessible surface area, which results in sufficient permeation of the electrolyte. The Cu dopant could reduce the work function and facilitate fast charge transfer kinetics. Therefore, the effective ion diffusion and electron conduction will facilitate the electrochemical reaction kinetics of the electrode. Benefiting from this unique structure, the electrode delivers a high specific capacitance (1.97 F cm^?2/1237 F g^?1/193.3 mAh g^?1) and cycling stability (90.8% after 30 000 cycles), exhibiting superb performance compared with most oxide‐based fiber electrodes. Furthermore, the hybrid fiber supercapacitor of CCCH@NiCo‐LDH NWAs@Au–CuO/Cu//VN/carbon fibers is fabricated, providing a remarkable maximal energy density of 34.97 Wh kg^?1 and a power density of 13.86 kW kg^?1, exhibiting a great potential in high‐performance fiber‐shape energy‐related systems.     

All‐Solid‐State Asymmetric Supercapacitors with Metal Selenides Electrodes and Ionic Conductive Composites Electrolytes

All‐solid‐state flexible asymmetric supercapacitors (ASCs) are developed by utilization of graphene nanoribbon (GNR)/Co_0.85Se composites as the positive electrode, GNR/Bi₂Se₃ composites as the negative electrode, and polymer‐grafted‐graphene oxide membranes as solid‐state electrolytes. Both GNR/Co_0.85Se and GNR/Bi₂Se₃ composite electrodes are developed by a facile one‐step hydrothermal growth method from graphene oxide nanoribbons as the nucleation framework. The GNR/Co_0.85Se composite electrode exhibits a specific capacity of 76.4 mAh g^?1 at a current density of 1 A g^?1 and the GNR/Bi₂Se₃ composite electrode exhibits a specific capacity of 100.2 mAh g^?1 at a current density of 0.5 A g^?1. Moreover, the stretchable membrane solid‐state electrolytes exhibit superior ionic conductivity of 108.7 mS cm^?1. As a result, the flexible ASCs demonstrate an operating voltage of 1.6 V, an energy density of 30.9 Wh kg^?1 at the power density of 559 W kg^?1, and excellent cycling stability with 89% capacitance retention after 5000 cycles. All these results demonstrate that this study provides a simple, scalable, and efficient approach to fabricate high performance flexible all‐solid‐state ASCs for energy storage.     

Novel Core–Shell FeOF/Ni(OH)2 Hierarchical Nanostructure for All‐Solid‐State Flexible Supercapacitors with Enhanced Performance

Well‐controlled core–shell hierarchical nanostructures based on oxyfluoride and hydroxide are for the first time rationally designed and synthesized via a simple solvothermal and chemical precipitation route, in which FeOF nanorod acts as core and porous Ni(OH)₂ nanosheets as shell. When evaluated as electrodes for supercapacitors, a high specific capacitance of 1452 F g^?1 can be obtained at a current density of 1 A g^?1. Even as the current density increases to 10 A g^?1, the core–shell hybrid still reserves a noticeable capacitance of 1060 F g^?1, showing an excellent rate capacity. Furthermore, all‐solid‐state flexible asymmetric supercapacitor based on the FeOF/Ni(OH)₂ hybrid as a positive electrode and activated carbon as a negative electrode shows high power density, high energy density, and long cycling lifespan. The excellent electrochemical performance of the FeOF/Ni(OH)₂ core–shell hybrid is ascribed to the unique microstructure and synergistic effects. FeOF nanorod from FeF₃ by partial substitution of fluorine with oxygen behaves as a low intrinsic resistance, thus facilitating charge transfer processes. While the hierarchical Ni(OH)₂ nanosheets with large surface area provide enough active sites for redox chemical reactions, leading to greatly enhanced electrochemical activity. The well‐controllable oxyfluoride/hydroxide hybrid is inspiring, opening up a new way to design new electrodes for next‐generation all‐solid‐state supercapacitors.     

Facile Synthesis of Hematite Quantum‐Dot/Functionalized Graphene‐Sheet Composites as Advanced Anode Materials for Asymmetric Supercapacitors

For building high‐energy density asymmetric supercapacitors, developing anode materials with large specific capacitance remains a great challenge. Although Fe₂O₃ has been considered as a promising anode material for asymmetric supercapacitors, the specific capacitance of the Fe₂O₃‐based anodes is still low and cannot match that of cathodes in the full cells. In this work, a composite material with well dispersed Fe₂O₃ quantum dots (QDs, ≈2 nm) decorated on functionalized graphene‐sheets (FGS) is prepared by a facile and scalable method. The Fe₂O₃ QDs/FGS composites exhibit a large specific capacitance up to 347 F g^?1 in 1 m Na₂SO₄ between –1 and 0 V versus Ag/AgCl. An asymmetric supercapacitor operating at 2 V is fabricated using Fe₂O₃/FGS as anode and MnO₂/FGS as cathode in 1 m Na₂SO₄ aqueous electrolyte. The Fe₂O₃/FGS//MnO₂/FGS asymmetric supercapacitor shows a high energy density of 50.7 Wh kg^?1 at a power density of 100 W kg^?1 as well as excellent cycling stability and power capability. The facile synthesis method and superior supercapacitive performance of the Fe₂O₃ QDs/FGS composites make them promising as anode materials for high‐performance asymmetric supercapacitors.     

High‐Performance Supercapacitors Based on a Zwitterionic Network of Covalently Functionalized Graphene with Iron Tetraaminophthalocyanine

Graphene derivatives are promising candidates as electrode materials in supercapacitor cells, therefore, functionalization strategies are pursued to improve their performance. A scalable approach is reported for preparing a covalently and homogenously functionalized graphene with iron tetraaminophthalocyanine (FePc‐NH₂) with a high degree of functionalization. This is achieved by exploiting fluorographene's reactivity with the diethyl bromomalonate, producing graphene‐dicarboxylic acid after hydrolysis, which is conjugated with FePc‐NH₂. The material exhibits an ultrahigh gravimetric specific capacitance of 960 F g^?1 at 1 A g^?1 and zero losses upon charging–discharging cycling. The energy density of 59 Wh kg^?1 is eminent among supercapacitors operating in aqueous electrolytes with graphene‐based electrode materials. This is attributed to the structural and functional synergy of the covalently bound components, giving rise to a zwitterionic surface with extensive π–π stacking, but not graphene restacking, all being very beneficial for charge and ionic transport. The safety of the proposed system, owing to the benign Na₂SO₄ aqueous electrolyte, the high capacitance, energy density, and potential of preparing the electrode material on a large‐scale and at low cost make the reported strategy very attractive for development of supercapacitors based on the covalent attachment of suitable molecules onto graphene toward high‐synergy hybrids.     

Flexible Solid‐State Supercapacitors with Enhanced Performance from Hierarchically Graphene Nanocomposite Electrodes and Ionic Liquid Incorporated Gel Polymer Electrolyte

High energy density, durability, and flexibility of supercapacitors are required urgently for the next generation of wearable and portable electronic devices. Herein, a novel strategy is introduced to boost the energy density of flexible soild‐state supercapacitors via rational design of hierarchically graphene nanocomposite (GNC) electrode material and employing an ionic liquid gel polymer electrolyte. The hierarchical graphene nanocomposite consisting of graphene and polyaniline‐derived carbon is synthesized as an electrode material via a scalable process. The meso/microporous graphene nanocomposites exhibit a high specific capacitance of 176 F g^?1 at 0.5 A g^?1 in the ionic liquid 1‐ethyl‐3‐methylimidazolium tetrafluoroborate (EMIBF₄) with a wide voltage window of 3.5 V, good rate capability of 80.7% in the range of 0.5–10 A g^?1 and excellent stability over 10 000 cycles, which is attributed to the superior conductivity (7246 S m^?1), and quite large specific surface area (2416 m² g^?1) as well as hierarchical meso/micropores distribution of the electrode materials. Furthermore, flexible solid‐state supercapacitor devices based on the GNC electrodes and gel polymer electrolyte film are assembled, which offer high specific capacitance of 180 F g^?1 at 1 A g^?1, large energy density of 75 Wh Kg^?1, and remarkable flexible performance under consecutive bending conditions.     

Polyoxometalate‐coupled Graphene via Polymeric Ionic Liquid Linker for Supercapacitors

The integration of electrical double‐layer capacitive and pseudocapacitive materials into novel hybrid materials is crucial to realize supercapacitors with high energy and power densities. Here, high levels of energy and power densities are demonstrated in supercapacitors based on a new type of nanohybrid electrode consisting of polyoxometalate (POM)‐coupled graphene in which a polymeric ionic liquid (henceforth simply PIL) serves as an interfacial linker. The adoption of PIL in the construction of nanohybrids enables a uniform distribution of discrete POM molecules along with a large surface area of graphene sheets. When testing electrochemical characteristics under a two‐electrode system, as‐prepared supercapacitors exhibit a high specific capacitance (408 F g^?1 at 0.5 A g^?1), rapid rate capability (92% retention at 10 A g^?1), a long cycling life (98% retention during 2000 cycles), and high energy (56 Wh kg^?1) and power (52 kW kg^?1) densities. First‐principles calculations and impedance spectroscopy analysis reveal that the PILs enhance the redox reactions of POMs by providing efficient ion transfer channels and facilitating the charge transfer in the nanohybrids.     

Hierarchical Composite Electrodes of Nickel Oxide Nanoflake 3D Graphene for High‐Performance Pseudocapacitors

NiO nanoflakes are created with a simple hydrothermal method on 3D (three‐dimensional) graphene scaffolds grown on Ni foams by microwave plasma enhanced chemical vapor deposition (MPCVD). Such as‐grown NiO‐3D graphene hierarchical composites are then applied as monolithic electrodes for a pseudo‐supercapacitor application without needing binders or metal‐based current collectors. Electrochemical measurements impart that the hierarchical NiO‐3D graphene composite delivers a high specific capacitance of ≈1829 F g^?1 at a current density of 3 A g^?1 (the theoretical capacitance of NiO is 2584 F g^?1). Furthermore, a full‐cell is realized with an energy density of 138 Wh kg^?1 at a power density of 5.25 kW kg^?1, which is much superior to commercial ones as well as reported devices in asymmetric capacitors of NiO. More attractively, this asymmetric supercapacitor exhibits capacitance retention of 85% after 5000 cycles relative to the initial value of the 1^st cycle.     

Hierarchical NiMn Layered Double Hydroxide/Carbon Nanotubes Architecture with Superb Energy Density for Flexible Supercapacitors

A hierarchical nanostructure composed of NiMn‐layered double hydroxide (NiMn‐LDH) microcrystals grafted on carbon nanotube (CNT) backbone is constructed by an in situ growth route, which exhibits superior supercapacitive performance. The resulting composite material (NiMn‐LDH/CNT) displays a three‐dimensional architecture with tunable Ni/Mn ratio, well‐defined core‐shell configuration, and enlarged surface area. An electrochemical investigation shows that the Ni₃Mn₁‐LDH/CNT electrode is rather active, which delivers a maximum specific capacitance of 2960 F g^–1 (at 1.5 A g^–1), excellent rate capability (79.5% retention at 30 A g^–1), and cyclic stability. Moreover, an all‐solid‐state asymmetric supercapacitor (SC) with good flexibility is fabricated by using the NiMn‐LDH/CNT film and reduced graphene oxide (RGO)/CNT film as the positive and negative electrode, respectively, exhibiting a wide cell voltage of 1.7 V and largely enhanced energy density up to 88.3 Wh kg^–1 (based on the total weight of the device). By virtue of the high‐capacity of pseudocapacitive hydroxides and desirable conductivity of carbon‐based materials, the monolithic design demonstrated in this work provides a promising approach for the development of flexible energy storage systems.     

In Situ Preparation of Sandwich MoO3/C Hybrid Nanostructures for High‐Rate and Ultralong‐Life Supercapacitors

This work presents a design of sandwich MoO₃/C hybrid nanostructure via calcination of the dodecylamine‐intercalated layered α‐MoO₃, leading to the in situ production of the interlayered graphene layer. The sample with a high degree of graphitization of graphene layer and more interlayered void region exhibits the most outstanding energy storage performance. The obtained material is capable of delivering a high specific capacitance of 331 F g^?1 at a current density of 1 A g^?1 and retained 71% capacitance at 10 A g^?1. In addition, nearly no discharge capacity decay between 1000 and 10 000 continuous charge–discharge cycles is observed at a high current density of 10 A g^?1, indicating an excellent specific capacitance retention ability. The exceptional rate capability endows the electrode with a high energy density of 41.2 W h kg^?1 and a high power density of 12.0 kW kg^?1 simultaneously. The excellent performance is attributed to the sandwich hybrid nanostructure of MoO₃/C with broad ion diffusion pathway, low charge‐transfer resistance, and robust structure at high current density for long‐time cycling. The present work provides an insight into the fabrication of novel electrode materials with both enhanced rate capability and cyclability for potential use in supercapacitor and other energy storage devices.     

Lithiation‐Induced Vacancy Engineering of Co3O4 with Improved Faradic Reactivity for High‐Performance Supercapacitor

Transition metal oxides are promising electrode candidates for supercapacitor because of their low cost, high theoretical capacity, and good reversibility. However, intrinsically poor electrical conductivity and sluggish reaction kinetics of these oxides normally lead to low specific capacity and slow rate capability of the devices. Herein, a commonly used cobalt oxide is used as an example to demonstrate that lithiation process as a new strategy to enhance its electrochemical performance for supercapacitor application. Detailed characterization reveals that electrochemical lithiation of Co₃O₄ crystal reduces the coordination of the Co? O band, leading to substantially increased oxygen vacancies (octahedral Co²⁺ sites). These vacancies further trigger the formation of a new electronic state in the bandgap, resulting in remarkably improved electrical conductivity and accelerated faradic reactions. The lithiated Co₃O₄ exhibits a noticeably enhanced specific capacity of 260 mAh g^?1 at 1 A g^?1, approximately fourfold enhancement compared to that of pristine Co₃O₄ (66 mAh g^?1). The hybrid supercapacitor assembled with lithiated Co₃O₄//N‐doped activated carbon achieves high energy densities in a broad range of power densities, e.g., 76.7 Wh kg^?1 at 0.29 kW kg^?1, 46.9 Wh kg^?1 at a high power density of 18.7 kW kg^?1, outperforming most of the reported hybrid supercapacitors.     

Asymmetric Supercapacitors Based on Graphene/MnO2 and Activated Carbon Nanofiber Electrodes with High Power and Energy Density

Asymmetric supercapacitor with high energy density has been developed successfully using graphene/MnO₂ composite as positive electrode and activated carbon nanofibers (ACN) as negative electrode in a neutral aqueous Na₂SO₄ electrolyte. Due to the high capacitances and excellent rate performances of graphene/MnO₂ and ACN, as well as the synergistic effects of the two electrodes, such asymmetric cell exhibits superior electrochemical performances. An optimized asymmetric supercapacitor can be cycled reversibly in the voltage range of 0–1.8 V, and exhibits maximum energy density of 51.1 Wh kg^?1, which is much higher than that of MnO₂//DWNT cell (29.1 Wh kg^?1). Additionally, graphene/MnO₂//ACN asymmetric supercapacitor exhibits excellent cycling durability, with 97% specific capacitance retained even after 1000 cycles. These encouraging results show great potential in developing energy storage devices with high energy and power densities for practical applications.