Free‐Standing 3D Nanoporous Duct‐Like and Hierarchical Nanoporous Graphene Films for Micron‐Level Flexible Solid‐State Asymmetric Supercapacitors

High energy density and power density within a limited volume of flexible solid‐state supercapacitors are highly desirable for practical applications. Here, free‐standing high‐quality 3D nanoporous duct‐like graphene (3D‐DG) films are fabricated with high flexibility and robustness as the backbones to deposit flower‐like MnO₂ nanosheets (3D‐DG@MnO₂). The 3D‐DG is the ideal support for the deposition of large amount of active materials because of its large surface area, appropriate pore structure, and negligible volume compared with other kinds of carbon backbones. Moreover, the 3D‐DG preserve the distinctive 2D coherent electronic properties of graphene, in which charge carriers move rapidly with a small resistance through the high‐quality and continuous chemical vapor deposition‐grown graphene building blocks, which results in a high rate performance. Marvelously, ultrathin (≈50 μm) flexible solid‐state asymmetric supercapacitors (ASCs) using 3D‐DG@MnO₂ as the positive electrode and 3D hierarchical nanoporous graphene films as the negative electrode display ultrahigh volumetric energy density (28.2 mW h cm^?3) and power density (55.7 W cm^?3) at 2.0 V. Furthermore, as‐prepared ASCs show high cycle stability clearly demonstrating their broad applications as power supplies in wearable electronic devices.     

Large‐Area,Ultrathin Inorganic Network Coverages–Graphene Hierarchical Electrodes for Flexible,Heat‐Resistant Energy Storage Application

Graphene and quasi‐2D graphene‐like materials with an ultrathin thickness have been investigated as a new class of nanoscale materials due to their distinctive properties. A novel “molecular tools‐assistances” strategy is developed to fabricate two kinds of graphene‐based electrodes, ultrathin Fe‐doped MnO₂ network coverage–graphene composites (G‐MFO) and ultrathin MoS₂ network coverage–graphene composites (G‐MoS₂) with special hierarchical structures. Such structures enable a large contact interface between the active materials and graphene and thus fully exploit the synergistic effect from both the high specific capacitance of MFO or MoS₂ and the superb conductivity of graphene. Benefiting from their unique structural features, G‐MFO and G‐MoS₂ films directly use as free‐standing electrodes for flexible asymmetric supercapacitors with a nonaqueous gel electrolyte. The device achieves a high energy/power density, superior flexibility, good rate capability as well as outstanding performance stability even at a high temperature. This work represents a promising prototype to design new generation of hybrid supercapacitors for future energy storage devices.     

Nanostructured Ternary Electrodes for Energy‐Storage Applications

A three‐component, flexible electrode is developed for supercapacitors over graphitized carbon fabric, utilizing γ‐MnO₂ nanoflowers anchored onto carbon nanotubes (γ‐MnO₂/CNT) as spacers for graphene nanosheets (GNs). The three‐component, composite electrode doubles the specific capacitance with respect to GN‐only electrodes, giving the highest‐reported specific capacitance (308 F g^?1) for symmetric supercapacitors containing MnO₂ and GNs using a two‐electrode configuration, at a scan rate of 20 mV s^?1. A maximum energy density of 43 W h kg^?1 is obtained for our symmetric supercapacitors at a constant discharge‐current density of 2.5 A g^?1 using GN–(γ‐MnO₂/CNT)‐nanocomposite electrodes. The fabricated supercapacitor device exhibits an excellent cycle life by retaining ≈90% of the initial specific capacitance after 5000 cycles.     

All‐Printed MnHCF‐MnOx‐Based High‐Performance Flexible Supercapacitors

Here, a simple active materials synthesis method is presented that boosts electrode performance and utilizes a facile screen‐printing technique to prepare scalable patterned flexible supercapacitors based on manganese hexacyanoferrate‐manganese oxide and electrochemically reduced graphene oxide electrode materials (MnHCF‐MnO_x/ErGO). A very simple in situ self‐reaction method is developed to introduce MnO_x pseudocapacitor material into the MnHCF system by using NH₄F. This MnHCF‐MnO_x electrode materials can deliver excellent capacitance of 467 F g^?1 at a current density of 1 A g^?1, which is a 2.4 times capacitance increase compared to MnHCF. In addition a printed, patterned, flexible MnHCF‐MnO_x/ErGO supercapacitor is fabricated, showing a remarkable areal capacitance of 16.8 mF cm^?2 and considerable energy and power density of 0.5 mWh cm^?2 and 0.0023 mW cm^?2, respectively. Furthermore, the printed patterned flexible supercapacitors also exhibit exceptional flexibility, and the capacitance remains stable, even while bending to various angles (60°, 90°, and 180°) and for 100 cycles. The flexible supercapacitor arrays integrated by multiple prepared single supercapacitors can power various LEDs even in the bent states. This approach offers promising opportunities for the development of printable energy storage materials and devices with high energy density, large scalability, and excellent flexibility.     

High‐Performance Fiber‐Shaped All‐Solid‐State Asymmetric Supercapacitors Based on Ultrathin MnO2 Nanosheet/Carbon Fiber Cathodes for Wearable Electronics

Flexible fiber‐shaped supercapacitors have shown great potential in portable and wearable electronics. However, small specific capacitance and low operating voltage limit the practical application of fiber‐shaped supercapacitors in high energy density devices. Herein, direct growth of ultrathin MnO₂ nanosheet arrays on conductive carbon fibers with robust adhesion is exhibited, which exhibit a high specific capacitance of 634.5 F g^?1 at a current density of 2.5 A g^?1 and possess superior cycle stability. When MnO₂ nanosheet arrays on carbon fibers and graphene on carbon fibers are used as a positive electrode and a negative electrode, respectively, in an all‐solid‐state asymmetric supercapacitor (ASC), the ASC displays a high specific capacitance of 87.1 F g^?1 and an exceptional energy density of 27.2 Wh kg^?1. In addition, its capacitance retention reaches 95.2% over 3000 cycles, representing the excellent cyclic ability. The flexibility and mechanical stability of these ASCs are highlighted by the negligible degradation of their electrochemical performance even under severely bending states. Impressively, as‐prepared fiber‐shaped ASCs could successfully power a photodetector based on CdS nanowires without applying any external bias voltage. The excellent performance of all‐solid‐state ASCs opens up new opportunity for development of wearable and self‐powered nanodevices in near future.     

A Three‐Dimensionally Interconnected Carbon Nanotube–Conducting Polymer Hydrogel Network for High‐Performance Flexible Battery Electrodes

High‐performance flexible energy‐storage devices have great potential as power sources for wearable electronics. One major limitation to the realization of these applications is the lack of flexible electrodes with excellent mechanical and electrochemical properties. Currently employed batteries and supercapacitors are mainly based on electrodes that are not flexible enough for these purposes. Here, a three‐dimensionally interconnected hybrid hydrogel system based on carbon nanotube (CNT)‐conductive polymer network architecture is reported for high‐performance flexible lithium ion battery electrodes. Unlike previously reported conducting polymers (e.g., polyaniline, polypyrrole, polythiophene), which are mechanically fragile and incompatible with aqueous solution processing, this interpenetrating network of the CNT‐conducting polymer hydrogel exibits good mechanical properties, high conductivity, and facile ion transport, leading to facile electrode kinetics and high strain tolerance during electrode volume change. A high‐rate capability for TiO₂ and high cycling stability for SiNP electrodes are reported. Typically, the flexible TiO₂ electrodes achieved a capacity of 76 mAh g^–1 in 40 s of charge/discharge and a high areal capacity of 2.2 mAh cm^–2 can be obtained for flexible SiNP‐based electrodes at 0.1C rate. This simple yet efficient solution process is promising for the fabrication of a variety of high performance flexible electrodes.     

Direct Growth of Flower‐Like δ‐MnO2 on Three‐Dimensional Graphene for High‐Performance Rechargeable Li‐O2 Batteries

A challenge still remains to develop high‐performance and cost‐effective air electrode for Li‐O₂ batteries with high capacity, enhanced rate capability and long cycle life (100 times or above) despite recent advances in this field. In this work, a new design of binder‐free air electrode composed of three‐dimensional (3D) graphene (G) and flower‐like δ‐MnO₂ (3D‐G‐MnO₂) has been proposed. In this design, graphene and δ‐MnO₂ grow directly on the skeleton of Ni foam that inherits the interconnected 3D scaffold of Ni foam. Li‐O₂ batteries with 3D‐G‐MnO₂ electrode can yield a high discharge capacity of 3660 mAh g^?1 at 0.083 mA cm^?2. The battery can sustain 132 cycles at a capacity of 492 mAh g^?1 (1000 mAh g_carbon ^?1) with low overpotentials under a high current density of 0.333 mA cm^?2. A high average energy density of 1350 Wh Kg^?1 is maintained over 110 cycles at this high current density. The excellent catalytic activity of 3D‐G‐MnO₂ makes it an attractive air electrode for high‐performance Li‐O₂ batteries.     

Graphene–Cellulose Paper Flexible Supercapacitors

A simple and scalable method to fabricate graphene‐cellulose paper (GCP) membranes is reported; these membranes exhibit great advantages as freestanding and binder‐free electrodes for flexible supercapacitors. The GCP electrode consists of a unique three‐dimensional interwoven structure of graphene nanosheets and cellulose fibers and has excellent mechanical flexibility, good specific capacitance and power performance, and excellent cyclic stability. The electrical conductivity of the GCP membrane shows high stability with a decrease of only 6% after being bent 1000 times. This flexible GCP electrode has a high capacitance per geometric area of 81 mF cm^?2, which is equivalent to a gravimetric capacitance of 120 F g^?1 of graphene, and retains >99% capacitance over 5000 cycles. Several types of flexible GCP‐based polymer supercapacitors with various architectures are assembled to meet the power‐energy requirements of typical flexible or printable electronics. Under highly flexible conditions, the supercapacitors show a high capacitance per geometric area of 46 mF cm^?2 for the complete devices. All the results demonstrate that polymer supercapacitors made using GCP membranes are versatile and may be used for flexible and portable micropower devices.     

Structure‐Controlled,Vertical Graphene‐Based,Binder‐Free Electrodes from Plasma‐Reformed Butter Enhance Supercapacitor Performance

Vertical graphene nanosheets (VGNS) hold great promise for high‐performance supercapacitors owing to their excellent electrical transport property, large surface area and in particular, an inherent three‐dimensional, open network structure. However, it remains challenging to materialise the VGNS‐based supercapacitors due to their poor specific capacitance, high temperature processing, poor binding to electrode support materials, uncontrollable microstructure, and non‐cost effective way of fabrication. Here we use a single‐step, fast, scalable, and environmentally‐benign plasma‐enabled method to fabricate VGNS using cheap and spreadable natural fatty precursor butter, and demonstrate the controllability over the degree of graphitization and the density of VGNS edge planes. Our VGNS employed as binder‐free supercapacitor electrodes exhibit high specific capacitance up to 230 F g^?1 at a scan rate of 10 mV s^?1 and >99% capacitance retention after 1,500 charge‐discharge cycles at a high current density, when the optimum combination of graphitic structure and edge plane effects is utilised. The energy storage performance can be further enhanced by forming stable hybrid MnO₂/VGNS nano‐architectures which synergistically combine the advantages from both VGNS and MnO₂. This deterministic and plasma‐unique way of fabricating VGNS may open a new avenue for producing functional nanomaterials for advanced energy storage devices.     

Vanadium Oxide Nanowire–Carbon Nanotube Binder‐Free Flexible Electrodes for Supercapacitors

Vanadium pentoxide (V₂O₅) layered nanostructures are known to have very stable crystal structures and high faradaic activity. The low electronic conductivity of V₂O₅ greatly limits the application of vanadium oxide as electrode materials and requires combining with conducting materials using binders. It is well known that the organic binders can degrade the overall performance of electrode materials and need carefully controlled compositions. In this study, we develop a simple method for preparing freestanding carbon nanotube (CNT)‐V₂O₅ nanowire (VNW) composite paper electrodes without using binders. Coin cell type (CR2032) supercapacitors are assembled using the nanocomposite paper electrode as the anode and high surface area carbon fiber electrode (Spectracarb 2225) as the cathode. The supercapacitor with CNT‐VNW composite paper electrode exhibits a power density of 5.26 kW Kg^?1 and an energy density of 46.3 Wh Kg^?1. (Li)VNWs and CNT composite paper electrodes can be fabricated in similar manner and show improved overall performance with a power density of 8.32 kW Kg^?1 and an energy density of 65.9 Wh Kg^?1. The power and energy density values suggest that such flexible hybrid nanocomposite paper electrodes may be useful for high performance electrochemical supercapacitors.     

Flash Converted Graphene for Ultra‐High Power Supercapacitors

Supercapacitors are known for their rapid energy charge–discharge properties, often ten to a hundred times faster than batteries. However, there is still a demand for supercapacitors with even faster charge–discharge characteristics to fulfill the requirements of emerging technologies. The power and rate capabilities of supercapacitors are highly dependent on the morphology of their electrode materials. An electrically conductive 3D porous structure possessing a high surface area for ions to access is ideal. Using a flash of light, a method to produce highly interconnected 3D graphene architectures with high surface area and good conductivity is developed. The flash converted graphene is synthesized by reducing freeze‐dried graphene oxide using an ordinary camera flash as a photothermal source. The flash converted graphene is used in coin cell supercapacitors to investigate its electrode materials properties. The electrodes are fabricated using either a precoating flash conversion or a postcoating flash conversion of graphene oxide. Both techniques produce supercapacitors possessing ultra‐high power (5–7 × 10⁵ W kg^?1). Furthermore, optimized supercapacitors retain >50% of their capacitance when operated at an ultrahigh current density up to 220 A g^?1.     

All‐Manganese‐Based Binder‐Free Stretchable Lithium‐Ion Batteries

Advanced electrode materials with bendability and stretchability are critical for the rapid development of fully flexible/stretchable lithium‐ion batteries. However, the sufficiently stretchable lithium‐ion battery is still underdeveloped that is one of the biggest challenges preventing from realizing fully deformable power sources. Here, a low‐temperature hydrothermal synthesis of a cathode material for stretchable lithium‐ion battery is reported by the in situ growth of LiMn₂O₄ (LMO) nanocrystals inside 3D carbon nanotube (CNT) film networks. The LMO/CNT film composite has demonstrated the chemical bonding between the LMO active materials and CNT scaffolds, which is the most important characteristic of the stretchable electrodes. When coupled with a wrinkled MnO_x /CNT film anode, a binder‐free, all‐manganese‐based stretchable full battery cell is assembled which delivers a high average specific capacity of ≈97 mA h g^?1 and stabilizes after over 300 cycles with an enormous strain of 100%. Furthermore, combining with other merits such as low cost, natural abundance, and environmentally friendly, the all‐manganese design is expected to accelerate the practical applications of stretchable lithium‐ion batteries for fully flexible and biomedical electronics.     

One‐Step Fabrication of Ultrathin Porous Nickel Hydroxide‐Manganese Dioxide Hybrid Nanosheets for Supercapacitor Electrodes with Excellent Capacitive Performance

A facile one‐step hydrothermal co‐deposition method for growth of ultrathin Ni(OH)₂‐MnO₂ hybrid nanosheet arrays on three dimensional (3D) macroporous nickel foam is presented. Due to the highly hydrophilic and ultrathin nature of hybrid nanosheets, as well as the synergetic effects of Ni(OH)₂ and MnO₂, the as‐fabricated Ni(OH)₂‐MnO₂ hybrid electrode exhibits an ultrahigh specific capacitance of 2628 F g^?1. Moreover, the asymmetric supercapacitor with the as‐obtained Ni(OH)₂‐MnO₂ hybrid film as the positive electrode and the reduced graphene oxide as the negative electrode has a high energy density (186 Wh kg^?1 at 778 W kg^?1), based on the total mass of active materials.     

Hierarchical Zn–Co–S Nanowires as Advanced Electrodes for All Solid State Asymmetric Supercapacitors

A facile two‐step strategy is developed to design the large‐scale synthesis of hierarchical, unique porous architecture of ternary metal hydroxide nanowires grown on porous 3D Ni foam and subsequent effective sulfurization. The hierarchical Zn–Co–S nanowires (NWs) arrays are directly employed as an electrode for supercapacitors application. The as‐synthesized Zn–Co–S NWs deliver an ultrahigh areal capacity of 0.9 mA h cm^?2 (specific capacity of 366.7 mA h g^?1) at a current density of 3 mA cm^?2, with an exceptional rate capability (≈227.6 mA h g^?1 at a very high current density of 40 mA cm^?2) and outstanding cycling stability (≈93.2% of capacity retention after 10 000 cycles). Most significantly, the assembled Zn–Co–S NWs//Fe₂O₃@reduced graphene oxide asymmetric supercapacitors with a wide operating potential window of ≈1.6 V yield an ultrahigh volumetric capacity of ≈1.98 mA h cm^?3 at a current density of 3 mA cm^?2, excellent energy density of ≈81.6 W h kg^?1 at a power density of ≈559.2 W kg^?1, and exceptional cycling performance (≈92.1% of capacity retention after 10 000 cycles). This general strategy provides an alternative to design the other ternary metal sulfides, making it facile, free‐standing, binder‐free, and cost‐effective ternary metal sulfide‐based electrodes for large‐scale applications in modern electronics.     

Ultrafast All‐Solid‐State Coaxial Asymmetric Fiber Supercapacitors with a High Volumetric Energy Density

Fiber‐supercapacitors (FSCs) are promising energy storage devices that can complement or even replace microbatteries in miniaturized portable and wearable electronics. Currently, a major challenge for FSCs is achieving ultrahigh volumetric energy and power densities simultaneously, especially when the charge/discharge rates exceed 1 V s^?1. Herein, an Au‐nanoparticle‐doped‐MnO_x@CoNi‐alloy@carbon‐nanotube (Au–MnO_x@CoNi@CNT) core/shell nanocomposite fiber electrode is designed, aiming to boost its charge/discharge rate by taking advantage of the superconductive CoNi alloy network and the greatly enhanced conductivity of the Au doped MnO_x active materials. An all‐solid‐state coaxial asymmetric FSC (CAFSC) prototype device made by wrapping this fiber with a holey graphene paper (HGP) exhibits excellent performance at rates up to 10 V s^?1, which is the highest charge rate demonstrated so far for FSCs based on pseudocapacitive materials. Furthermore, our fully packaged CAFSC delivers a volumetric energy density of ≈15.1 mW h cm^?3, while simultaneously maintaining a high power density of 7.28 W cm^?3 as well as a long cycle life (90% retention after 10 000 cycles). This value is the highest among all reported FSCs, even better than that of a typical 4 V/500 µA h thin‐film lithium battery.     

Electromechanical Properties of Polymer Electrolyte‐Based Stretchable Supercapacitors

Aligned carbon nanotube (CNT) forests filled with a dehydrated polymer electrolyte are used to fabricate flexible solid state supercapacitors (SSCs) for multifunctional structural‐electronic applications. Local stiffness measurements on the composite electrodes determined through nano­indentation showed an 80% increase over the neat solid polymer electrolyte matrix. Electrochemical properties are monitored as a function of average tensile strain in the SSCs. Galvanostatic charge‐discharge tests with in situ microtensile testing on SSCs are used to show a 10% increase in the specific capacitance through the elastic region of the composite. The increase in capacitance is partly attributed to the enhanced double layer interaction that results from the partial alignment of the polymer electrolyte chains at the electrode‐electrolyte interface. When soaked in 1 m sulfuric acid, the specific capacitance of the CNT‐polymer electrolyte reached approximately 72 F g^–1 at 60 °C.     

Graphene‐Indanthrone Donor–π–Acceptor Heterojunctions for High‐Performance Flexible Supercapacitors

To overcome the low energy density bottleneck of graphene‐based supercapacitors and to organically endow them with high‐power density, ultralong‐life cycles, etc., one rational strategy that couple graphene sheets with multielectron, redox‐reversible, and structurally‐stable organic compounds. Herein, a graphene‐indanthrone (IDT) donor–π–acceptor heterojunction is conceptualized for efficient and smooth 6H⁺/6e^? transfers from pseudocapacitive IDT molecules to electrochemical double‐layer capacitive graphene scaffolds. To construct this, water‐processable graphene oxide (GO) is employed as a graphene precursor, and to in situ exfoliate IDT industrial dyestuff, followed by a hydrothermally‐induced reduction toward GO and self‐assembly between reduced GO (rGO) donors (D) and IDT acceptors (A), affording rGO‐π‐IDT D–A heterojunctions. Electrochemical tests indicate that rGO‐π‐IDT heterojunctions deliver a gravimetric capacitance of 535.5 F g^?1 and an amplified volumetric capacitance of 685.4 F cm^?3. The assembled flexible all‐solid‐state supercapacitor yields impressive volumetric energy densities of 31.3 and 25.1 W h L^?1, respectively, at low and high power densities of 767 and 38 554 W L^?1, while exhibiting an exceptional rate capability, cycling stability, and enduring mechanically‐challenging bending and distortions. The concept and methodology may open up opportunities for other two‐dimensional materials and other energy‐related devices.     

Visibly‐Transparent Organic Solar Cells on Flexible Substrates with All‐Graphene Electrodes

Portable electronic devices have become increasingly widespread. Because these devices cannot always be tethered to a central grid, powering them will require low‐cost energy harvesting technologies. As a response to this anticipated demand, this study demonstrates transparent organic solar cells fabricated on flexible substrates, including plastic and paper, using graphene as both the anode and cathode. Optical transmittance of up to 69% at 550 nm is achieved by combining the highly transparent graphene electrodes with organic polymers that primarily absorb in the near‐IR and near‐UV regimes. To address the challenge of transferring graphene onto organic layers as the top electrode, this study develops a room temperature dry‐transfer technique using ethylene‐vinyl‐acetate as an adhesion‐promoting interlayer. The power conversion efficiency achieved for flexible devices with graphene anode and cathode devices is 2.8%–3.8% at for optical transmittance of 54%–61% across the visible regime. These results demonstrate the versatility of graphene in optoelectronic applications and it is important step toward developing a practical power source for distributed wireless electrical systems.     

AuPd–MnOx/MOF–Graphene: An Efficient Catalyst for Hydrogen Production from Formic Acid at Room Temperature

The safe and efficient storage and release of hydrogen are widely recognized as the main challenges for the establishment of a fuel‐cell‐based hydrogen economy. Formic acid (FA) has great potential as a safe and convenient source of hydrogen for fuel cells. Despite tremendous efforts, the development of heterogeneous catalysts with high activity and relatively low cost remains a major challenge. The synthesis of AuPd–MnO_x nanocomposite immobilized on ZIF‐8–reduced‐graphene‐oxide (ZIF‐8–rGO) bi‐support by a wet‐chemical method is reported here. Interestingly, the resultant AuPd–MnO_x/ZIF‐8–rGO shows excellent catalytic activity for the generation of hydrogen from FA, and the initial turnover frequency (TOF) reaches a highest value of 382.1 mol H₂ mol catalyst^?1 h^?1 without any additive at 298 K. This good performance of AuPd–MnO_x/ZIF‐8–rGO results from the modified electronic structure of Pd in the AuPd–MnO_x/ZIF‐8–rGO composite, the small size and high dispersion of the AuPd–MnO_x nanocomposite, and also the strong metal‐support interaction between the AuPd–MnO_x and ZIF‐8–rGO bi‐support.     

Blood‐Capillary‐Inspired,Free‐Standing,Flexible, and Low‐Cost Super‐Hydrophobic N‐CNTs@SS Cathodes for High‐Capacity,High‐Rate,and Stable Li‐Air Batteries

With the rising demand for flexible and wearable electronic devices, flexible power sources with high energy densities are required to provide a sustainable energy supply. Theoretically, rechargeable, flexible Li‐O₂/air batteries can provide extremely high specific energy densities; however, the high costs, complex synthetic methods, and inferior mechanical properties of the available flexible cathodes severely limit their practical applications. Herein, inspired by the structure of human blood capillary tissue, this study demonstrates for the first time the in situ growth of interpenetrative hierarchical N‐doped carbon nanotubes on the surface of stainless‐steel mesh (N‐CNTs@SS) for the fabrication of a self‐supporting, flexible electrode with excellent physicochemical properties via a facile and scalable one‐step strategy. Benefitting from the synergistic effects of the high electronic conductivity and stable 3D interconnected conductive network structure, the Li‐O₂ batteries obtained with the N‐CNTs@SS cathode exhibit superior electrochemical performance, including a high specific capacity (9299 mA h g^?1 at 500 mA g^?1), an excellent rate capability, and an exceptional cycle stability (up to 232 cycles). Furthermore, as‐fabricated flexible Li‐air batteries containing the as‐prepared flexible super‐hydrophobic cathode show excellent mechanical properties, stable electrochemical performance, and superior H₂O resistibility, which enhance their potential to power flexible and wearable electronic devices.