Fiber-based all-solid-state flexible supercapacitors for self-powered systems

A fiber asymmetric supercapacitor based on FeOOH/PPy on carbon fibers as an anode electrode with high volumetric energy density for wearable applications

Fiber supercapacitors are promising energy storage devices for wearable applications. However, the fiber supercapacitors are currently limited by the mediocre capacitance performances due to the use of typical carbon materials as the anode, sacrificing the volumetric energy density of the whole device. In addition, the inability to undergo washable cycles and poor self-discharge rate prevents the fiber-shaped supercapacitors from being a true energy textile and affects their practicability. Hence, the porous anode electrode FeOOH/PPy@CF has been firstly prepared with a high volumetric capacitance of 30.17 F cm^-3, contributing to a high volumetric energy density of 2 mWh cm^-3 (based on the whole encapsulated device) for a fiber asymmetric supercapacitor MnO₂@CF//FeOOH/PPy@CF in PVA/LiCl. Good flexibility could be exhibited when it was woven into a glove. Desired working voltage and capacity output could be easily obtained when connecting devices in series and parallel. The encapsulated device could work stably even after it was dipped for multiple cycles in different solutions and with intensive stirring in water that simulates washing cycles. The self-discharge rate could be mitigated when an ionogel electrolyte ([EMIM][TFSI]/FS) was incorporated and this further enhanced the energy density to 3.7 mWh cm^-3. The outstanding properties of our assembled asymmetric fiber supercapacitor device render it a good candidate for practical wearable energy storage devices.

A General Electrode Design Strategy for Flexible Fiber Micro-Pseudocapacitors Combining Ultrahigh Energy and Power Delivery

Herein, a general strategy is proposed to boost the energy storage capability of pseudocapacitive materials (i.e., MnO2) to their theoretical limits in unconventional 1D fiber configuration by rationally designing bicontinuous porous Ni skeleton@metal wire "sheath-core" metallic scaffold as a versatile host. As a proof of concept, the 1D metallic scaffold supported-MnO2 fiber electrode is demonstrated. The proposed "sheath" design not only affords large electrode surface area with ordered macropores for large electrolyte-ion accessibility and high electroactive material loading, but also renders interconnected porous metallic skeleton for efficient electronic and ionic transport, while the metallic "core" functions as an extra current collector to promote long-distance electron transport and electron collection. Benefiting from all these merits, the optimized fiber electrode yields unprecedented specific areal capacitance of 1303.6 mF cm-2 (1278 F g-1 based on MnO2, approaching the theoretical value of 1370 F g-1) in liquid KOH and 847.22 mF cm-2 in polyvinyl alcohol (PVA)/KOH gel electrolyte, 2-350 times of previously reported fiber electrodes. The solid-state fiber micro-pseudocapacitors simultaneously achieve remarkable areal energy and power densities of 18.83 µWh cm-2 and 16.33 mW cm-2, greatly exceeding the existing symmetric fiber supercapacitors, together with long cycle life and high rate capability.

Graphene-Fiber-Based Supercapacitors Favor N-Methyl-2-pyrrolidone/Ethyl Acetate as the Spinning Solvent/Coagulant Combination

One-dimensional flexible fiber supercapacitors (FSCs) have attracted great interest as promising energy-storage units that can be seamlessly incorporated into textiles via weaving, knitting, or braiding. The major challenges in this field are to develop tougher and more efficient FSCs with a relatively easy and scalable process. Here, we demonstrate a wet-spinning process to produce graphene oxide (GO) fibers from GO dispersions in N-methyl-2-pyrrolidone (NMP), with ethyl acetate as the coagulant. Upon chemical reduction of GO, the resulting NMP-based reduced GO (rGO) fibers (rGO@NMP-Fs) are twice as high in the surface area and toughness but comparable in tensile strength and conductivity as that of the water-based rGO fibers (rGO@H₂O-Fs). When assembled into parallel FSCs, rGO@NMP-F-based supercapacitors (rGO@NMP-FSCs) offered a specific capacitance of 196.7 F cm^-3 (147.5 mF cm^-2), five times higher than that of rGO@H₂O-F-based supercapacitors (rGO@H₂O-FSCs) and also higher than most existing wet-spun rGO-FSCs, as well as those FSCs built with metal wires, graphene/carbon nanotube (CNT) fibers, or even pseudocapacitive materials. In addition, our rGO@NMP-FSCs can provide good bending and cycling stability. The energy density of our rGO@NMP-FSCs reaches ca. 6.8 mWh cm^-3, comparable to that of a Li thin-film battery (4 V/500 μAh).

Pseudocapacitive-dye-molecule-based high-performance flexible supercapacitors

Currently, the development of novel, cheap and high-performance electrode materials for flexible energy-storage devices is being intensively pursued. Organic dyes are regarded as one of the most common water pollution sources, and their discharge not only damages the ecological balance but also wastes a lot of valuable chemical raw materials. With intrinsic redox functional groups, two types of widely used dyes including anthraquinone derivatives and phenothiazine dyes can be effectively removed from their effluent by a facile galvanostatic polymerization method, and resource utilized as electrochemical energy-storage materials. Based on this, a new kind of flexible supercapacitor based on dye molecules has been successfully fabricated, and exhibits excellent electrochemical performance, demonstrating a wise strategy for simultaneous wastewater treatment and preparation of functional devices.

Ultrahigh-Conductivity Polymer Hydrogels with Arbitrary Structures

A poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) hydrogel is prepared by thermal treatment of a commercial PEDOT:PSS (PH1000) suspension in 0.1 mol L^-1 sulfuric acid followed by partially removing its PSS component with concentrated sulfuric acid. This hydrogel has a low solid content of 4% (by weight) and an extremely high conductivity of 880 S m^-1 . It can be fabricated into different shapes such as films, fibers, and columns with arbitrary sizes for practical applications. A highly conductive and mechanically strong porous fiber is prepared by drying PEDOT:PSS hydrogel fiber to fabricate a current-collector-free solid-state flexible supercapacitor. This fiber supercapacitor delivers a volumetric capacitance as high as 202 F cm^-3 at 0.54 A cm^-3 with an extraordinary high-rate performance. It also shows excellent electrochemical stability and high flexibility, promising for the application as wearable energy-storage devices.

Effects of surfactants on the preparation of MnO2 and its capacitive performance

Amorphous hydrated manganese dioxide (MnO2) was prepared as an electrode material for supercapacitors by liquid co-precipitation in the presence of polyvinylpyrrolidone (PVP), polyethylene glycol (PEG) and sodium dodecylbenzenesulfonate (SDBS) respectively. The obtained samples were characterized by x-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), high-resolution transmission electron microscopy (HRTEM), and electrochemical methods. Physical characterizations confirmed that the addition of surfactants played an important role in the preparation of MnO2. The specific surface areas of MnO2 with the addition of PEG, SDBS and PVP were 169.92 m2/g, 137.40 m2/g and 196.64 m2/g, respectively, and the corresponding capacitances were 207.9 F/g, 187.5 F/g and 238.7 F/g. Compared with the sample without surfactants, the specific surface area and capacitance of the sample with the addition of PVP were improved by 92.2% and 53.1%, respectively. Moreover, the electrode showed good cycle stability at the current density of 120 mA/g, and 91.1% of its specific capacitance still remained after 500 cycles. It was concluded that this performance improvement was attributed to the electrostatic stabilization of the multivariate alkyl residue and cyano group (-NCO) as anchoring group, as well as the steric hindrance effect from lateral polarity groups of pentabasic ring in PVP structure.

Flexible and Wearable Fiber Microsupercapacitors Based on Carbon Nanotube-Agarose Gel Composite Electrodes

Fiber electrodes provide interesting opportunities for energy storage by providing both mechanical flexibility and the opportunity to impart multifunctionality to fabrics. We show here carbon nanotube (CNT)-embedded agarose gel composite fiber electrodes, with a diameter of ∼120 μm, consisting of 60 wt % CNTs that can serve as the basis for flexible and wearable fiber microsupercapacitors (mSCs). Via an extrusion process, CNT bundles are induced to align in an agarose filament matrix. Due to the shear alignment of the CNT bundles, the dehydrated filaments have an electrical conductivity as high as 8.3 S cm^-1. The composite fiber electrodes are mechanically stable, enabling formation of twisted two-ply fiber mSCs integrated with a solid electrolyte. The fiber mSC shows a high capacitance (∼1.2 F cm^-3), good rate retention (∼90%) at discharge current densities ranging from 5.1 to 38 mA cm^-3, long cycle life under repeated charging/discharging (10% fade after 10 000 cycles) and good performance after at least 1000 cycles of deformation, with a radius of curvature of 12.3 mm (90° bend). After being coated with a thin layer of poly(dimethylsiloxane), the fiber mSCs could be cycled over 10 000 times under water. Impedance studies indicate that the superior performance is due to the high electrical conductivity along the aligned CNTs and the large electrode surface area that is accessible through the ion-conducting agarose.

From natural cotton thread to sewable energy dense supercapacitors

Considering cost and flexibility, cotton thread is an ideal material for the fabrication of wearable and portable electronics. However, the capacitance of cotton thread based supercapacitors remains extremely low (below 50 mF cm^-2) due to the insufficient capacitive utilization of active materials. In this work, ordered mesoporous carbon (OMC) membranes are rationally coupled with chemical vapour deposition derived graphene (CVD gr), to form a highly conductive carbon coating around cotton yarn. In this material design, OMC membranes act as hydrophilic nanoporous "ion reservoirs" to accumulate sufficient cations from a gel electrolyte, while CVD gr endows the composite thread low liner resistance (3.7 Ω cm^-1) and high mechanical strength. Using a butyl-3-methylimidazolium chloride modified gel as an ionic conducting electrolyte, the efficiency in capacitive utilization of coated MnO₂ microparticles has been doubled, delivering an areal capacitance of 1.1 F cm² with a volumetric energy of 2.7 mWh cm^-3. Such a supercapacitor thread is lightweight, sewable and durable in bending fatigue tests, and can be fabricated through a facile dip-coating method. Impressively, this device can power a photodetector based on TiO₂ nanowires without applying any external bias voltage, which opens up a new opportunity for development of wearable and self-powered nanodevices in the near future.

High-performance hybrid carbon nanotube fibers for wearable energy storage

Wearable energy storage devices are of practical interest, but few have been commercially exploited. Production of electrodes with extended cycle life, as well as high energy and power densities, coupled with flexibility, remains a challenge. Herein, we have demonstrated the development of a high-performance hybrid carbon nanotube (CNT) fiber-based supercapacitor for the first time using conventional wet-spinning processes. Manganese dioxide (MnO₂) nanoflakes were deposited onto the as-prepared CNT fibers by electrodeposition to form highly flexible nanocomposites fibers. As-prepared fibers were characterized by electron microscopy, electrical, mechanical, and electrochemical measurements. It was found that the specific capacitance was over 152 F g^-1 (156 F cm^-3), which is about 500% higher than the multi-walled carbon nanotube/MnO₂ yarn-based supercapacitors. The measured energy density was 14.1 Wh kg^-1 at a power density of 202 W kg^-1. These values are 232% and 32% higher than the energy density and power density of MWNT/MnO₂ yarn-based supercapacitor, respectively. It was found that the cyclic retention ability was more stable, revealing a 16% increase after 10 000 cycles. Such substantial enhancements of key properties of the hybrid material can be associated with the synergy of CNT and MnO₂ nanoparticles in the fiber structure. The use of wet-spun hybrid CNT for fiber-based supercapacitors has been demonstrated.

Grafoil-Scotch tape-derived highly conducting flexible substrate and its application as a supercapacitor electrode

Development of flexible supercapacitors is limited by the availability of flexible and durable conducting substrates; a conducting and cheap substrate for the active material deposition is essential for breakthrough progress in this direction. In this report, a highly flexible, conducting, and cheap substrate is prepared by simple stick and peel-off method involving Scotch tape and Grafoil. A Grafoil-Scotch tape derived flexible substrate exhibits a sheet resistance of 7 Ω □^-1 along with a high degree of flexibility and durability. Moreover, its properties are further enhanced by the anodization in order to increase the hydrophilicity and surface area. The substrate is highly thin with a thickness of just 74 μm. Its practical utility has been demonstrated by electrodepositing MnO₂ as an active material and, thereafter, fabricating a solid-state flexible supercapacitor. The fabricated device exhibits high capacitance retention under bent (99%) and twisted (98%) conditions along with a low ESR of 7 Ω.

Stretchable Fiber Supercapacitors with High Volumetric Performance Based on Buckled MnO2 /Oxidized Carbon Nanotube Fiber Electrodes

A stretchable fiber supercapacitor (SC) based on buckled MnO₂ /oxidized carbon nanotube (CNT) fiber electrode is fabricated by a simple prestraining-then-buckling method. The prepared stretchable fiber SC has a specific volumetric capacitance up to 409.4 F cm^-3 , which is 33 times that of the pristine CNT fiber based SC, and shows the outstanding stability and repeatability in performance as a stretchable SC.

Well-Ordered Oxygen-Deficient CoMoO4 and Fe2O3 Nanoplate Arrays on 3D Graphene Foam: Toward Flexible Asymmetric Supercapacitors with Enhanced Capacitive Properties

In this work, we report the development of well-ordered hydrogenated CoMoO₄ (H-CoMoO₄) and hydrogenated Fe₂O₃ (H-Fe₂O₃) nanoplate arrays on 3D graphene foam (GF) and explore their practice application as binder-free electrodes in assembling flexible all-solid-state asymmetric supercapacitor (ASC) devices. Our results show that the monolithic 3D porous GF prepared by solution casting method using Ni foam template possesses large surface area, superior electrical conductivity, and sufficient surface functional groups, which not only facilitate in situ growth of CoMoO₄ and Fe₂O₃ nanoplates but also contribute the double-layer capacitance of the resultant supercapacitor. The well-ordered pseudocapacitive metal oxide nanoplate arrays standing up on 3D GF scaffold can provide efficient space and shorten the length for electrolyte diffusion from the outer to the inner region of the electrode material for Faradaic energy storage. Furthermore, one of our major findings is that the introduction of oxygen vacancies in CoMoO₄ and Fe₂O₃ nanoplates by hydrogenation treatment can increase their electronic conductivity as well as improve their donor density and surface properties, which gives rise to a substantially improved electrochemical performance. Benefiting from the synergistic contributions of different components in the nanohybrid electrode, the resultant flexible ASC device with GF/H-CoMoO₄ as the positive electrode and GF/H-Fe₂O₃ as the negative electrode achieves a wide operation voltage of 1.5 V and a maximum volumetric specific capacitance of 3.6 F cm^-3, which is two times larger than that of the Ni/GF/CoMoO₄//Ni/GF/Fe₂O₃ device (1.8 F cm^-3), and the rate capability is up to 70% as the current density increases from 2 to 200 mA cm^-3. Moreover, the Ni/GF/H-CoMoO₄//Ni/GF/H-Fe₂O₃ device also exhibits a high energy density of 1.13 mWh cm^-3 and a high power density of 150 mW cm^-3, good mechanical flexibility with the decrease in capacitance of less than 4% after being bent inward to different angles and inward to 90° 200 times, and good cycling stability of 93.1% capacitance retention after 5000 cycles.

Renewable-emodin-based wearable supercapacitors

With the increasing dependency of human life on wearable electronics, the development of corresponding energy-storage devices is being insensitively pursued. Considering the special usage locations of wearable energy-storage devices, the safety and non-toxicity of electrode materials adopted should be of concern. In this work, a novel all-solid-state wearable supercapacitor based on the renewable-biomolecule emodin, naturally derivable from traditional Chinese herbal rhubarb or Polygonum cuspidatum, was successfully fabricated. Such supercapacitors exhibited excellent charge storage and rate capability with great flexibility and could be integrated into wearable electronics. As a proof of concept, a strap-shaped supercapacitor was fabricated, and it was capable of powering an electronic watch. Our work will promote the development of safe wearable electronics.

Design of Amorphous Manganese Oxide@Multiwalled Carbon Nanotube Fiber for Robust Solid-State Supercapacitor

Solid-state fiber-based supercapacitors have been considered promising energy storage devices for wearable electronics due to their lightweight and amenability to be woven into textiles. Efforts have been made to fabricate a high performance fiber electrode by depositing pseudocapacitive materials on the outer surface of carbonaceous fiber, for example, crystalline manganese oxide/multiwalled carbon nanotubes (MnO₂/MWCNTs). However, a key challenge remaining is to achieve high specific capacitance and energy density without compromising the high rate capability and cycling stability. In addition, amorphous MnO₂ is actually preferred due to its disordered structure and has been proven to exhibit superior electrochemical performance over the crystalline one. Herein, by incorporating amorphous MnO₂ onto a well-aligned MWCNT sheet followed by twisting, we design an amorphous MnO₂@MWCNT fiber, in which amorphous MnO₂ nanoparticles are distributed in MWCNT fiber uniformly. The proposed structure gives the amorphous MnO₂@MWCNT fiber good mechanical reliability, high electrical conductivity, and fast ion-diffusion. Solid-state supercapacitor based on amorphous MnO₂@MWCNT fibers exhibits improved energy density, superior rate capability, exceptional cycling stability, and excellent flexibility. This study provides a strategy to design a high performance fiber electrode with microstructure control for wearable energy storage devices.

Flexible all-solid-state supercapacitors based on polyaniline orderly nanotubes array

Flexible all-solid-state supercapacitors are crucial to meet the growing needs for portable electronic devices such as foldable phones and wearable electronics. As promising candidates for pseudocapacitor electrode materials, polyaniline (PANI) orderly nanotube arrays are prepared via a simple template electrodeposition method. The structures of the final product were characterized using various characterization techniques, including scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR), and X-ray photoelectron spectroscopy (XPS). The obtained PANI nanotube film could be directly used as a flexible all-solid-state supercapacitor electrode. Electrochemical results show that the areal capacitance of a PANI nanotube-based supercapacitor with the deposition cycle number of 100 can achieve a maximum areal capacitance of 237.5 mF cm^-2 at a scan rate of 10 mV s^-1 and maximum energy density of 24.31 mW h cm^-2 at a power density of 2.74 mW cm^-2. In addition, the prepared supercapacitor exhibits excellent flexibility under different bending conditions. It retains 95.2% of its initial capacitance value after 2000 cycles at a current density of 1.0 mA cm^-1, which displays its superior cycling stability. Moreover, the prepared flexible all-solid-state supercapacitor can power a light-emitting-diode (LED), which meets the practical applications of micropower supplies.

Electrochemical synthesis of MnO2porous nanowires for flexible all-solid-state supercapacitor

MnO₂porous nanowires with excellent electrochemical performance were preparedviaa simple electrodeposition method.

Nitrogen-doped reduced graphene oxide intertwined with V2O3 nanoflakes as self-supported electrodes for flexible all-solid-state supercapacitors

Flexible all-solid-state supercapacitors (SCs) have great potential applications in flexible and wearable electronics because of their safety, high power density, flexibility, and portability.

Flexible and Stretchable Energy Storage: Recent Advances and Future Perspectives

Energy-storage technologies such as lithium-ion batteries and supercapacitors have become fundamental building blocks in modern society. Recently, the emerging direction toward the ever-growing market of flexible and wearable electronics has nourished progress in building multifunctional energy-storage systems that can be bent, folded, crumpled, and stretched while maintaining their electrochemical functions under deformation. Here, recent progress and well-developed strategies in research designed to accomplish flexible and stretchable lithium-ion batteries and supercapacitors are reviewed. The challenges of developing novel materials and configurations with tailored features, and in designing simple and large-scaled manufacturing methods that can be widely utilized are considered. Furthermore, the perspectives and opportunities for this emerging field of materials science and engineering are also discussed.

Graphene/polyaniline@carbon cloth composite as a high-performance flexible supercapacitor electrode prepared by a one-step electrochemical co-deposition method

A one-step electrochemical co-deposition method was used to prepare a graphene/polyaniline composite on carbon cloth for high-performance flexible supercapacitors.

Highly flexible all-solid-state cable-type supercapacitors based on Cu/reduced graphene oxide/manganese dioxide fibers

An all-solid-state Cu/RGO/MnO₂ fiber supercapacitor showed excellent capacitance and flexibility, and could serve as electrical cable and as energy storage device.

Wearable energy-smart ribbons for synchronous energy harvest and storage

A promising energy source for many current and future applications is a ribbon-like device that could simultaneously harvest and store energy. Due to the high flexibility and weavable property, a fabric/matrix made using these ribbons could be highly beneficial for powering wearable electronics. Unlike the approach of using two separate devices, here we report a ribbon that integrates a solar cell and a supercapacitor. The electrons generated by the solar cell are directly transferred and stored on the reverse side of its electrode which in turn also functions as an electrode for the supercapacitor. When the flexible solar ribbon is illuminated with simulated solar light, the supercapacitor holds an energy density of 1.15 mWh cm⁻³ and a power density of 243 mW cm⁻³. Moreover, these ribbons are successfully woven into a fabric form. Our all-solid-state ribbon unveils a highly flexible and portable self-sufficient energy system with potential applications in wearables, drones and electric vehicles.

Flexible materials for harvesting and storing energy are desirable for wearable electronics, but efficiency is still an issue. Here, the authors demonstrate a flexible and weavable ribbon which integrates a solar cell and supercapacitor via a shared electrode for efficient energy harvesting and storage.

Flexible Microsupercapacitors Using Silk and Cotton Substrates

Flexible microsupercapacitors (MSCs) are needed to power ultrasmall wearable electronic devices. Silk cocoons comprise microfibers of silk, which is an attractive natural resource to fabricate MSCs. These fibers are insulators; hence, they must be converted to conducting surfaces. Polyphenols from green tea have been used as a protective layer that also acted as a reducing agent for silver ions. The reduction of silver ions resulted in the formation of silver nanoparticles that subsequently reduced gold ions to gold. The gold film imparts conductivity to the silk fiber without affecting the mechanical strength of the silk fiber. The mechanical strength of uncoated silk fiber and gold coated silk fiber were found to be 5.2 and 5 GPa, respectively. A pseudocapacitive polymer, poly(3,4-ethylenedioxythiophene), was used as the active material to fabricate MSCs. The MSCs showed an impressive gravimetric capacitance of 500 F/g and areal capacitance of 62 mF/cm². The power and energy densities were calculated to be 2458 W/kg and 44 Wh/kg, respectively. The device was coiled on a cylinder, and the performance of the device was found to be same as that of the uncoiled device. To demonstrate that the approach is not specific to silk, we also coated gold on cotton fibers using the protocol used to coat gold on silk. Coiled and uncoiled supercapacitors were fabricated using PEDOT coated cotton fibers. The gravimetric capacitance was found to be 250 F/g with energy and power densities of 5.5 Wh/kg and 1118 W/kg, respectively. We have also demonstrated that the devices can be connected in parallel and series to improve the performance of the miniaturized devices.

Transition-Metal-Free Biomolecule-Based Flexible Asymmetric Supercapacitors

A transition-metal-free asymmetric supercapacitor (ASC) is successfully fabricated based on an earth-abundant biomass derived redox-active biomolecule, named lawsone. Such an ASC exhibits comparable or even higher energy densities than most of the recently reported transition-metal-based ASCs, and this green ASC generation from renewable resources is promising for addressing current issues of electronic hazard processing, high cost, and unsustainability.

Three-dimensional nanotube electrode arrays for hierarchical tubular structured high-performance pseudocapacitors

Ordered three-dimensional (3-D) tubular arrays are highly attractive candidates for high performance pseudocapacitor electrodes. Here, we report 3-D fluorine doped tin oxide (FTO) tubular arrays fabricated by a cost-effective ultrasonic spray pyrolysis (USP) method in anodic aluminum oxide (AAO) channels with high uniformity. The large surface area of such a structure leads to remarkable surface area enhancement up to 51.8 times compared to a planar structure. Combining with electrochemically deposited manganese dioxide (MnO2) nanoflakes on the inner side wall of the FTO nanotubes, the unique hierarchical tubular structured pseudocapacitor electrode demonstrated the highest areal capacitance of 193.8 mF cm(-2) at the scan rate of 5 mV s(-1) and 184 mF cm(-2) at the discharge current density of 0.6 mA cm(-2), which is 18.5 times that of a planar electrode. And it also showed a volumetric capacitance of 112.6 F cm(-3) at the scan rate of 5 mV s(-1) and 108.8 F cm(-3) at the discharge current density of 0.6 mA cm(-2). In addition, the cyclic stability test also indicated that a nanostructured pseudocapacitive electrode has a much larger capacitance retention after 3000 cycles of the charge-discharge process compared with a planar electrode, primarily due to the mechanical stability of the nanostructure. Moreover, pseudocapacitor device fabrication based on such electrodes shows the volumetric capacitance of 17.5 F cm(-3), and the highest specific energy of 1.56 × 10(-3) Wh cm(-3). With the merit of facile fabrication procedures and largely enhanced electrochemical performance, such a 3-D structure has high potency for energy storage systems for a wide range of practical applications.

Downsized Sheath-Core Conducting Fibers for Weavable Superelastic Wires, Biosensors, Supercapacitors, and Strain Sensors

Hair-like-diameter superelastic conducting fibers, comprising a buckled carbon nanotube sheath on a rubber core, are fabricated, characterized, and deployed as weavable wires, biosensors, supercapacitors, and strain sensors. These downsized sheath-core fibers provide the demonstrated basis for glucose sensors, supercapacitors, and electrical interconnects whose performance is undegraded by giant strain, as well as ultrafast strain sensors that exploit strain-dependent capacitance changes.

Perovskite Photovoltachromic Supercapacitor with All-Transparent Electrodes

Photovoltachromic cells (PVCCs) are of great interest for the self-powered smart windows of architectures and vehicles, which require widely tunable transmittance and automatic color change under photostimuli. Organolead halide perovskite possesses high light absorption coefficient and enables thin and semitransparent photovoltaic device. In this work, we demonstrate co-anode and co-cathode photovoltachromic supercapacitors (PVCSs) by vertically integrating a perovskite solar cell (PSC) with MoO3/Au/MoO3 transparent electrode and electrochromic supercapacitor. The PVCSs provide a seamless integration of energy harvesting/storage device, automatic and wide color tunability, and enhanced photostability of PSCs. Compared with conventional PVCC, the counter electrodes of our PVCSs provide sufficient balancing charge, eliminate the necessity of reverse bias voltage for bleaching the device, and realize reasonable in situ energy storage. The color states of PVCSs not only indicate the amount of energy stored and energy consumed in real time, but also enhance the photostability of photovoltaic component by preventing its long-time photoexposure under fully charged state of PVCSs. This work designs PVCS devices for multifunctional smart window applications commonly made of glass.

Bottom-Up Fabrication of Activated Carbon Fiber for All-Solid-State Supercapacitor with Excellent Electrochemical Performance

Activated carbon (AC) is the most extensively used electrode material for commercial electric double layer capacitors (EDLC) given its high specific surface area (SSA) and moderate cost. However, AC is primarily used in the forms of powders, which remains a big challenge in developing AC powders into continuous fibers. If AC powders can be processed into fiber, then they may be scaled up for practical applications to supercapacitors (SCs) and satisfy the rapid development of flexible electronics. Herein, we report a bottom-up method to fabricate AC fiber employing graphene oxide (GO) as both dispersant and binder. After chemical reduction, the fiber has high electrical conductivity (185 S m(-1)), high specific surface area (1476.5 m(2) g(-1)), and good mechanical flexibility. An all solid-state flexible SC was constructed using the prepared fiber as electrode, which is free of binder, conducting additive, and additional current collector. The fiber-shaped SC shows high capacitance (27.6 F cm(-3) or 43.8 F g(-1), normalized to the two-electrode volume), superior cyclability (90.4% retention after 10 000 cycles), and good bendability (96.8% retention after bending 1000 times).

Ultrafast-Charging Supercapacitors Based on Corn-Like Titanium Nitride Nanostructures

Ultrahigh rates realized by ALD-made TiN. The symmetric full-cell supercapacitors deliver a typical capacitance of 20.7 F cm^-3 at a scan rate of 1 V s^-1, and retain 4.3 F cm^-3 at high rate of 100 V s^-1. The devices can be charged and discharged for 20 000 cycles with negligible capacitance loss and with an ultralow self-discharge current (≈1 μA).

Carbon Nanotubes and Graphene for Flexible Electrochemical Energy Storage: from Materials to Devices

Flexible electrochemical energy storage (FEES) devices have received great attention as a promising power source for the emerging field of flexible and wearable electronic devices. Carbon nanotubes (CNTs) and graphene have many excellent properties that make them ideally suited for use in FEES devices. A brief definition of FEES devices is provided, followed by a detailed overview of various structural models for achieving different FEES devices. The latest research developments on the use of CNTs and graphene in FEES devices are summarized. Finally, future prospects and important research directions in the areas of CNT- and graphene-based flexible electrode synthesis and device integration are discussed.

Three-Dimensional Microcavity Array Electrodes for High-Capacitance All-Solid-State Flexible Microsupercapacitors

We report novel three-dimensional (3D) microcavity array electrodes for high-capacitance all-solid-state microsupercapactiors. The microcavity arrays are formed in a polymer substrate via a plasma-assisted reactive ion etching (RIE) process and provide extra sidewall surface areas on which the active materials are grown in the form of nanofibers. This 3D structure leads to an increase in the areal capacitance by a factor of 2.56 for a 15-μm-deep cavity etching, agreeing well with the prediction. The fabricated microsupercapactiors exhibit a maximum areal capacitance of 65.1 mF cm(-2) (a volumetric capacitance of 93.0 F cm(-3)) and an energy density of 0.011 mWh cm(-2) (a volumetric energy density of 16.4 mWh cm(-3)) which substantially surpass previously reported values for all-solid-state flexible microsupercapacitors. The devices show good electrochemical stability under extended voltammetry cycles and bending cycles. It is demonstrated that they can sustain a radio frequency (rf) microsystem in a temporary absence of a power supply. These results suggest the potential utility of our 3D microsupercapactiors as miniaturized power sources in wearable and implantable medical devices.

Three-dimensional hierarchical NiCo2O4 nanowire@Ni3S2 nanosheet core/shell arrays for flexible asymmetric supercapacitors

Three-dimensional (3D) hierarchical NiCo2O4@Ni3S2 core/shell arrays on Ni foam were synthesized by a facile, stepwise synthesis approach. The 3D heterogeneous NiCo2O4 nanostructure forms an interconnected web-like scaffold and serves as the core for the Ni3S2 shell. The as-prepared NiCo2O4@Ni3S2 nanowire array (NWA) electrodes exhibited excellent electrochemical performance, such as high specific areal capacitance and excellent cycling stability. The specific areal capacitance of 3.0 F cm(-2) at a current density of 5 mA cm(-2) is among the highest values and the only 6.7% capacitance decay after 10 000 cycles demonstrates the excellent cycling stability. A flexible asymmetric supercapacitor (ASC) was fabricated with activated carbon (AC) as the anode and the obtained NiCo2O4@Ni3S2 NWAs as the cathode. The ASC device exhibited a high energy density of 1.89 mW h cm(-3) at 5.81 W cm(-3) and a high power density of 56.33 W cm(-3) at 0.94 mW h cm(-3). As a result, the hybrid nanoarchitecture opens a new way to design high performance electrodes for electrochemical energy storage applications.

Solid-state supercapacitors with rationally designed heterogeneous electrodes fabricated by large area spray processing for wearable energy storage applications

Supercapacitors are in demand for short-term electrical charge and discharge applications. Unlike conventional supercapacitors, solid-state versions have no liquid electrolyte and do not require robust, rigid packaging for containment. Consequently they can be thinner, lighter and more flexible. However, solid-state supercapacitors suffer from lower power density and where new materials have been developed to improve performance, there remains a gap between promising laboratory results that usually require nano-structured materials and fine-scale processing approaches, and current manufacturing technology that operates at large scale. We demonstrate a new, scalable capability to produce discrete, multi-layered electrodes with a different material and/or morphology in each layer, and where each layer plays a different, critical role in enhancing the dynamics of charge/discharge. This layered structure allows efficient utilisation of each material and enables conservative use of hard-to-obtain materials. The layered electrode shows amongst the highest combinations of energy and power densities for solid-state supercapacitors. Our functional design and spray manufacturing approach to heterogeneous electrodes provide a new way forward for improved energy storage devices.

Suppressing the Coffee-Ring Effect in Semitransparent MnO2 Film for a High-Performance Solar-Powered Energy Storage Window

We introduce a simple and effective method to deposit a highly uniform and semitransparent MnO2 film without coffee-ring effect (CRE) by adding ethanol into MnO2 ink for transparent capacitive energy storage devices. By carefully controlling the amount of ethanol added in the MnO2 droplet, we could significantly reduce the CRE and thus improve the film uniformity. The electrochemical properties of supercapacitor (SC) devices using semitransparent MnO2 film electrodes with or without CRE were measured and compared. The SC device without CRE shows a superior capacitance, high rate capability, and lower contact resistance. The CRE-free device could achieve a considerable volumetric capacitance of 112.2 F cm(-3), resulting in a high volumetric energy density and power density of 10 mWh cm(-3) and 8.6 W cm(-3), respectively. For practical consideration, both flexible SC and large-area rigid SC devices were fabricated to demonstrate their potential for flexible transparent electronic application and capacitive energy-storage window application. Moreover, a solar-powered energy storage window which consists of a commercial solar cell and our studied semitransparent MnO2-film-based SCs was assembled. These SCs could be charged by the solar cell and light up a light emitting diode (LED), demonstrating their potential for self-powered systems and energy-efficient buildings.

Flexible All-Solid-State Supercapacitors based on Liquid-Exfoliated Black-Phosphorus Nanoflakes

Flexible all-solid-state supercapacitors are fabricated with liquid-exfoliated black-phosphorus (BP) nanoflakes as an electrode material. These devices deliver high specific volumetric capacitance, power density, and energy density, up to 13.75 F cm(-3) , 8.83 W cm(-3) , and 2.47 mW h cm(-3) , respectively, and an outstanding long life span of over 30 000 cycles, demonstrating the excellent performance of the BP nanoflakes as a flexible electrode material in electrochemical energy-storage devices.

MnO2 nanomaterials for flexible supercapacitors: performance enhancement via intrinsic and extrinsic modification

Increasing power and energy demands for next-generation portable and flexible electronics have raised critical requirements (flexibility, stretch-ability, environmental friendliness, lightweight, etc.) for the energy storage devices. Flexible supercapacitors (SCs), as one of the most promising next-generation energy storage devices, have stimulated intensive interest owing to their outstanding features including small size, low weight, ease of handling, excellent reliability, and high power density. Manganese oxide (MnO₂), has attracted much interest in the development of flexible SCs with high electrochemical performance. Yet, the poor electronic and ionic transport in MnO₂ electrodes still limits its promotion in practical applications. This review aims to describe the recent progress in the application of MnO₂ materials in the development of flexible SCs and summarizes the intrinsic modification of MnO₂via crystallinity, crystal structure, and oxygen vacancy introduction and the extrinsic modification of MnO₂via non-three-dimensional (3D) and 3D flexible conductive scaffolds for high performance flexible SCs. Moreover, we also discuss briefly on the current challenges, future directions, and opportunities for the development of high-performance MnO₂ based flexible SCs.

Layered-MnO₂ Nanosheet Grown on Nitrogen-Doped Graphene Template as a Composite Cathode for Flexible Solid-State Asymmetric Supercapacitor

Flexible solid-state supercapacitors provide a promising energy-storage alternative for the rapidly growing flexible and wearable electronic industry. Further improving device energy density and developing a cheap flexible current collector are two major challenges in pushing the technology forward. In this work, we synthesize a nitrogen-doped graphene/MnO2 nanosheet (NGMn) composite by a simple hydrothermal method. Nitrogen-doped graphene acts as a template to induce the growth of layered δ-MnO2 and improves the electronic conductivity of the composite. The NGMn composite exhibits a large specific capacitance of about 305 F g(-1) at a scan rate of 5 mV s(-1). We also create a cheap and highly conductive flexible current collector using Scotch tape. Flexible solid-state asymmetric supercapacitors are fabricated with NGMn cathode, activated carbon anode, and PVA-LiCl gel electrolyte. The device can achieve a high operation voltage of 1.8 V and exhibits a maximum energy density of 3.5 mWh cm(-3) at a power density of 0.019 W cm(-3). Moreover, it retains >90% of its initial capacitance after 1500 cycles. Because of its flexibility, high energy density, and good cycle life, NGMn-based flexible solid state asymmetric supercapacitors have great potential for application in next-generation portable and wearable electronics.

Graphene-based fibers for supercapacitor applications

Energy conversion and storage devices play an important role in industry and society with the rapid growth of energy consumption. Supercapacitors are very attractive due to their superior power density, fast charge/discharge rates and long cycle lifetime. Graphene fiber (GF), a fascinating material, has drawn considerable attention and shown great potential as an active material in the field of supercapacitors owing to its unique and tunable nanostructure, high electrical conductivity, excellent mechanical flexibility, light weight, and ease of functionalization. This review focuses on the recent significant advances in the fabrication and application of graphene-based fiber as electrode material in supercapacitors. The synthetic strategies and application in the supercapacitor are presented, accompanied with the summary and outlook for the future development of GFs.

Large Scale Synthesis of NiCo Layered Double Hydroxides for Superior Asymmetric Electrochemical Capacitor

We report a new environmentally-friendly synthetic strategy for large-scale preparation of 16 nm-ultrathin NiCo based layered double hydroxides (LDH). The Ni₅₀Co₅₀-LDH electrode exhibited excellent specific capacitance of 1537 F g⁻¹ at 0.5 A g⁻¹ and 1181 F g⁻¹ even at current density as high as 10 A g⁻¹, which 50% cobalt doped enhances the electrical conductivity and porous and ultrathin structure is helpful with electrolyte diffusion to improve the material utilization. An asymmetric ultracapacitor was assembled with the N-doped graphitic ordered mesoporous carbon as negative electrode and the NiCo LDH as positive electrode. The device achieves a high energy density of 33.7 Wh kg⁻¹ (at power density of 551 W kg⁻¹) with a 1.5 V operating voltage.

Wearable Self-Charging Power Textile Based on Flexible Yarn Supercapacitors and Fabric Nanogenerators

A novel and scalable self-charging power textile is realized by combining yarn supercapacitors and fabric triboelectric nanogenerators as energy-harvesting devices.