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.     

2D Metal Zn Nanostructure Electrodes for High‐Performance Zn Ion Supercapacitors

Recent supercapacitors show a high power density with long‐term cycle life time in energy‐powering applications. A supercapacitor based on a single metal electrode accompanying multivalent cations, multiple charging/discharging kinetics, and high electrical conductivity is a promising energy‐storing system that replaces conventionally used oxide and sulfide materials. Here, a hierarchically nanostructured 2D‐Zn metal electrode‐ion supercapacitor (ZIC) is reported which significantly enhances the ion diffusion ability and overall energy storage performance. Those nanostructures can also be successfully plated on various flat‐type and fiber‐type current collectors by a controlled electroplating method. The ZIC exhibits excellent pseudocapacitive performance with a high energy density of 208 W h kg^?1 and a power density from 500 W kg^?1, which are significantly higher than those of previously reported supercapacitors with oxide and sulfide materials. Furthermore, the fiber‐type ZIC also shows high energy‐storing performance, outstanding mechanical flexibility, and waterproof characteristics, without any significant capacitance degradation during bending tests. These results highlight the promising possibility of nanostructured 2D Zn metal electrodes with the controlled electroplating method for future energy storage applications.     

3D Printing Quasi‐Solid‐State Asymmetric Micro‐Supercapacitors with Ultrahigh Areal Energy Density

A 3D printing approach is first developed to fabricate quasi‐solid‐state asymmetric micro‐supercapacitors to simultaneously realize the efficient patterning and ultrahigh areal energy density. Typically, cathode, anode, and electrolyte inks with high viscosities and shear‐thinning rheological behaviors are first prepared and 3D printed individually on the substrates. The 3D printed asymmetric micro‐supercapacitor with interdigitated electrodes exhibits excellent structural integrity, a large areal mass loading of 3.1 mg cm^?2, and a wide electrochemical potential window of 1.6 V. Consequently, this 3D printed asymmetric micro‐supercapacitor displays an ultrahigh areal capacitance of 207.9 mF cm^?2. More importantly, an areal energy density of 73.9 µWh cm^?2 is obtained, superior to most reported interdigitated micro‐supercapacitors. It is believed that the efficient 3D printing strategy can be used to construct various asymmetric micro‐supercapacitors to promote the integration in on‐chip energy storage systems.     

High‐Performance and Breathable Polypyrrole Coated Air‐Laid Paper for Flexible All‐Solid‐State Supercapacitors

High‐performance, breathable, conductive, and flexible polypyrrole (PPy) coated paper electrodes are prepared by an interfacial polymerization method using air‐laid paper as a substrate. Owing to the synergistic effect of superior electrical conductivity, high wettability, and the porous architecture, the prepared electrode not only shows an outstanding specific capacitance and rate abilities (3100 and 2579 mF cm^?2 at 1 and 20 mA cm^?2 for a PPy coated paper electrode), but also exhibits excellent flexibility, wearability, and breathability. Based on these superior features, an all‐solid‐state supercapacitor assembled with the PPy coated paper electrodes shows an outstanding energy density of 62.4 µW h cm^?2, remarkable air permeability and excellent flexibility to sustain various deformations. Furthermore, large‐scale fabrication of conductive flexible paper electrode can be easily achieved through this method. Therefore, this work offers a new vision for flexible energy storage.     

Flexible Asymmetric Micro‐Supercapacitors Based on Bi2O3 and MnO2 Nanoflowers: Larger Areal Mass Promises Higher Energy Density

A flexible asymmetric supercapacitor (ASC) with high energy density is designed and fabricated using flower‐like Bi₂O₃ and MnO₂ grown on carbon nanofiber (CNF) paper as the negative and positive electrodes, respectively. The lightweight (1.6 mg cm^?2), porous, conductive, and flexible features make the CNF paper an ideal support for guest active materials, which permit a large areal mass of 9 mg cm^?2 for Bi₂O₃ (≈85 wt% of the entire electrode). Thus, the optimal device with an operation voltage of 1.8 V can deliver a high energy density of 43.4 μWh cm^?2 (11.3 W h kg^?1, based on the total electrodes) and a maximum power density of 12.9 mW cm^?2 (3370 W kg^?1). This work provides an example of large areal mass and flexible electrode for ASCs with high areal capacitance and high energy density, holding great promise for future flexible electronic devices.     

Combining Nature‐Inspired,Graphene‐Wrapped Flexible Electrodes with Nanocomposite Polymer Electrolyte for Asymmetric Capacitive Energy Storage

Two kinds of free‐standing electrodes, reduced graphene oxide (rGO)‐wrapped Fe‐doped MnO₂ composite (G‐MFO) and rGO‐wrapped hierarchical porous carbon microspheres composite (G‐HPC) are fabricated using a frozen lake‐inspired, bubble‐assistance method. This configuration fully enables utilization of the synergistic effects from both components, endowing the materials to be excellent electrodes for flexible and lightweight electrochemical capacitors. Moreover, a nonaqueous HPC‐doped gel polymer electrolyte (GPE‐HPC) is employed to broad voltage window and improve heat resistance. A fabricated asymmetric supercapacitor based on G‐MFO cathode and G‐HPC anode with GPE‐HPC electrolyte achieves superior flexibility and reliability, enhanced energy/power density, and outstanding cycling stability. The ability to power light‐emitting diodes also indicates the feasibility for practical use. Therefore, it is believed that this novel design may hold great promise for future flexible electronic devices.     

Wearable High‐Performance Supercapacitors Based on Silver‐Sputtered Textiles with FeCo2S4–NiCo2S4 Composite Nanotube‐Built Multitripod Architectures as Advanced Flexible Electrodes

To achieve high‐performance wearable supercapacitors (SCs), a new class of flexible electrodes with favorable architectures allowing large porosity, high conductivity, and good mechanical stability is strongly needed. Here, this study reports the rational design and fabrication of a novel flexible electrode with nanotube‐built multitripod architectures of ternary metal sulfides' composites (FeCo₂S₄–NiCo₂S₄) on a silver‐sputtered textile cloth. Silver sputtering is applicable to almost all kinds of textiles, and S^2? concentration is optimized during sulfidation process to achieve such architectures and also a complete sulfidation assuring high conductivity. New insights into concentration‐dependent sulfidation mechanism are proposed. The additive‐free FeCo₂S₄–NiCo₂S₄ electrode shows a high specific capacitance of 1519 F g^?1 at 5 mA cm^?2 and superior rate capability (85.1% capacitance retention at 40 mA cm^?2). All‐solid‐state SCs employing these advanced electrodes deliver high energy density of 46 W h kg^?1 at 1070 W kg^?1 as well as achieve remarkable cycling stability retaining 92% of initial capacitance after 3000 cycles at 10 mA cm^?2, and outstanding reliability with no capacitance degradation under large twisting. These are attributed to the components' synergy assuring rich redox reactions, high conductivity as well as highly porous but robust architectures. An almost linear increase in capacitance with devices' area indicates possibility to meet various energy output requirements. This work provides a general, low‐cost route to wearable power sources.     

Integrated Conductive Hybrid Architecture of Metal–Organic Framework Nanowire Array on Polypyrrole Membrane for All‐Solid‐State Flexible Supercapacitors

Metal–organic frameworks (MOFs) with intrinsically porous structures are promising candidates for energy storage, however, their low electrical conductivity limits their electrochemical energy storage applications. Herein, the hybrid architecture of intrinsically conductive Cu‐MOF nanowire arrays on self‐supported polypyrrole (PPy) membrane is reported for integrated flexible supercapacitor (SC) electrodes without any inactive additives, binders, or substrates involved. The conductive Cu‐MOFs nanowire arrays afford high conductivity and a sufficiently active surface area for the accessibility of electrolyte, whereas the PPy membrane provides decent mechanical flexibility, efficient charge transfer skeleton, and extra capacitance. The all‐solid‐state flexible SC using integrated hybrid electrode demonstrates an exceptional areal capacitance of 252.1 mF cm^?2, an energy density of 22.4 µWh cm^?2, and a power density of 1.1 mW cm^?2, accompanied by an excellent cycle capability and mechanical flexibility over a wide range of working temperatures. This work not only presents a robust and flexible electrode for wide temperature range operating SC but also offers valuable concepts with regards to designing MOF‐based hybrid materials for energy storage and conversion systems.     

A Flexible Quasi‐Solid‐State Asymmetric Electrochemical Capacitor Based on Hierarchical Porous V2O5 Nanosheets on Carbon Nanofibers

The development of 3D nanoarchitectures on flexible current collectors has emerged as an effective strategy for preparing advanced portable and wearable power sources. Herein, a flexible and efficient electrode is demonstrated based on electrospun carbon fibers (ECF) substrate with elaborately designed hierarchical porous V₂O₅ nanosheets (V₂O₅–ECF). The unique configuration of V₂O₅–ECF composite film fully enables utilization of the synergistic effects from both high electrochemical performance of V₂O₅ and excellent conductivity of ECF, endowing the films to be an excellent electrode for flexible and lightweight electrochemical capacitors (ECs). Benefiting from their intriguing structural features, V₂O₅–ECF and ECF films, directly used as electrodes for flexible asymmetric quasi‐solid‐state electrochemical capacitors, achieve superior flexibility and reliability, enhanced energy/power density, and outstanding cycling stability. Moreover, the ability to power light‐emitting diodes (LED) also indicates the feasibility for practical use. Therefore, it is believed that this novel design may find promising application in flexible devices in future.     

Large‐Scale Conductive Yarns Based on Twistable Korean Traditional Paper (Hanji) for Supercapacitor Applications: Toward High‐Performance Paper Supercapacitors

A simple and scalable method to fabricate a yarn‐type supercapacitor with a large specific capacitance without the aid of traditional pseudocapacitive electrode materials such as conducting polymers and metal oxides is reported. The yarn‐type supercapacitors are made from twisting reduced graphene oxide (rGO) or/and single‐walled carbon nanotubes (SWNTs)‐coated Korean traditional paper (KTP). The yarn‐type paper supercapacitor displays surprisingly enhanced electrochemical capacitance values, showing synergistic effect between rGO and SWNTs (500 times larger than performance of yarn‐type rGO‐coated paper supercapacitors). Coating rGO or/and SWNTs on KTP gives good morphology to the composite film, in which porosity increases and mean pore diameter decreases. The yarn‐type rGO/SWNT paper supercapacitor shows good mechanical strength, high flexibility, excellent electrochemical performance, and long‐life operation. The yarn‐type supercapacitor has an excellent electrochemical performance with a specific capacitance of 366 F g^?1 at scan rate of 25 mV s^?1 and high stability without any degradation in electrical performance up to 10 000 charge–discharge cycles. The average capacitance of rGO/SWNT@KTP yarn‐type supercapacitors is seven times higher than that of sheet‐type supercapacitors at scan rate of 500 mV s^?1. The lighting of a red light‐emitting diode (LED) is demonstrated by the yarn‐type paper supercapacitor without connecting supercapacitors in series.     

Carbonized Chicken Eggshell Membranes with 3D Architectures as High‐Performance Electrode Materials for Supercapacitors

Supercapacitor electrode materials are synthesized by carbonizing a common livestock biowaste in the form of chicken eggshell membranes. The carbonized eggshell membrane (CESM) is a three‐dimensional macroporous carbon film composed of interwoven connected carbon fibers containing around 10 wt% oxygen and 8 wt% nitrogen. Despite a relatively low surface area of 221 m² g^?1, exceptional specific capacitances of 297 F g^?1 and 284 F g^?1 are achieved in basic and acidic electrolytes, respectively, in a 3‐electrode system. Furthermore, the electrodes demonstrate excellent cycling stability: only 3% capacitance fading is observed after 10 000 cycles at a current density of 4 A g^?1. These very attractive electrochemical properties are discussed in the context of the unique structure and chemistry of the material.     

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.     

NiCo2S4 Nanosheets Grown on Nitrogen‐Doped Carbon Foams as an Advanced Electrode for Supercapacitors

To push the energy density limit of supercapacitors, a new class of electrode materials with favorable architectures is strongly needed. Binary metal sulfides hold great promise as an electrode material for high‐performance energy storage devices because they offer higher electrochemical activity and higher capacity than mono‐metal sulfides. Here, the rational design and fabrication of NiCo₂S₄ nanosheets supported on nitrogen‐doped carbon foams (NCF) is presented as a novel flexible electrode for supercapacitors. A facile two‐step method is developed for growth of NiCo₂S₄ nanosheets on NCF with robust adhesion, involving the growth of Ni‐Co precursor and subsequent conversion into NiCo₂S₄ nanosheets through sulfidation process. Benefiting from the compositional features and 3D electrode architectures, the NiCo₂S₄/NCF electrode exhibits greatly improved electrochemical performance with ultrahigh capacitance (877 F g^?1 at 20 A g^?1) and excellent cycling stability. Moreover, a binder‐free asymmetric supercapacitor device is also fabricated by using NiCo₂S₄/NCF as the positive electrode and ordered mesoporous carbon (OMC)/NCF as the negative electrode; this demonstrates high energy density (≈45.5 Wh kg^?1 at 512 W kg^?1).     

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.