Flexible Supercapacitor Electrodes Research Articles

1T-WS2/Graphene on activated carbon cloth as a flexible electrode for wearable supercapacitors

Transition metal dichalcogenides (TMD's) have recently attracted much interest as energy storage materials due to having a two-dimensional structure. Indeed, the trigonal phase has shown more beneficial characteristics owing to higher electrical conductivity. In this research, in a facile one-pot solvothermal method, tungsten disulfide (WS2) was synthesized on the activated carbon cloth (ACC) as an electrode for wearable supercapacitors. The active material was composited with graphene to increase the capacitive properties of this electrode. The formation of the trigonal phase (1T) was confirmed by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and Raman Spectroscopy. First principle calculations were employed to study the interface of composite and surface energy of related planes. The synthesized nano-flower powder showed a substantial surface area of 761 m2/g. Electrochemical studies revealed that graphene has a beneficial impact on the electrode's charge storage and rate capabilities. Furthermore, remarkable areal capacitance of 2964 mF/cm2 in Na2SO4 electrolyte and 86% capacitance retention after 1000 cycles were achieved.

Recent Progress in Flexible Graphene-Based Composite Fiber Electrodes for Supercapacitors

Graphene has shown the world its fascinating properties, including high specific surface area, high conductivity, and extraordinary mechanical properties, which enable graphene to be a competent candidate for electrode materials. However, some challenges remain in the real applications of graphene-based electrodes, such as continuous preparation of graphene fibers with highly ordered graphene sheets as well as strong interlayer interactions. The combination of graphene with other materials or functional guests hence appears as a more promising pathway via post-treatment and in situ hybridism to produce composite fibers. This article firstly provides a full account of the classification of graphene-based composite fiber electrodes, including carbon allotropy, conductive polymer, metal oxide and other two-dimensional (2D) materials. The preparation methods of graphene-based composite fibers are then discussed in detail. The context further demonstrates the performance optimization of graphene-based composite fiber electrodes, involving microstructure design and surface modification, followed by the elaboration of the application of graphene-based composite fiber electrodes in supercapacitors. Finally, we present the remaining challenges that exist to date in order to provide meaningful guidelines in the development process and prospects of graphene-based composite fiber electrodes.

Reversible faradaic reactions involving redox mediators and oxygen-containing groups on carbon fiber electrode for high-performance flexible fibrous supercapacitors

Flexible fibrous supercapacitors (FFS) are taking account of as the energy storage devices for wearable electronics owing to their high power density, high safety, long cycle life, and simple manufacturing process. Nevertheless, FFSs have the disadvantage of low specific capacitance that results from the electrochemical characteristics of the electrical double layer on the carbon fiber electrode. In this study, for the first time, an FFS comprising surface-activated carbon fibers as an electrode/current collector and a redox additive gel polymer electrolyte (FFS-SARE) was fabricated for use as a wearable energy storage device. The FFS-SARE showed outstanding electrochemical performance, namely, high specific capacitances of 891 and 399 mF cm−2 at current densities of 70.0 and 400 μA cm−2, respectively, and remarkable ultrafast cycling stability over 5000 cycles with 92% capacitance retention at a current density of 400.0 μA cm−2. Moreover, they exhibited mechanical flexibility and had high feasibility, and they showed good energy storage performance that renders them suitable for use in wearable electronic textiles.

Highly hydrophilic electrodeposited NiS/Ni3S2 interlaced nanosheets with surface-enriched Ni3+ sites as binder-free flexible cathodes for high-rate hybrid supercapacitors

In this study, nanostructured nickel sulfides (NiS, Ni3S2 and NiS/Ni3S2) were fabricated directly on the surface of flexible carbon fiber cloths by simply modifying the deposition parameters of pulse-reversal (PR) electrodeposition method and utilized as binder-free flexible electrodes for aqueous hybrid supercapacitors (SCs). X-ray photoelectron spectroscopy and contact angle measurement studies verifies that the surface of heterostructure NiS/Ni3S2 electrode has enriched Ni3+ sites and highly hydrophilic nature. Consequently, the heterostructure NiS/Ni3S2 electrode demonstrated superior rate capability than that of both single phase NiS and Ni3S2 electrodes. Additionally, the hybrid SC device based on the flexible NiS/Ni3S2 electrode delivered a capacity of 40.4 mAh g−1 at a current density of 2 A g−1 and representing a maximum energy density of 32.3 Wh kg−1 at an impressive power density of 1.6 kW kg−1. Furthermore, the device provided excellent electrochemical stability with a capacity retention of 86.2%, even after a 120-h floating test. Hence, the heterostructure NiS/Ni3S2 with interlaced nanosheets morphology should be considered as promising binder-free flexible electrode materials for next-generation energy storage applications.

In-situ electrochemical polymerization of aniline on flexible conductive substrates for supercapacitors and non-enzymatic ascorbic acid sensors

Polyaniline, as a kind of conductive polymer with commercial application prospects, is still under researches in its synthesis and applications. In this work, polyaniline was fabricated on flexible substrates including carbon cloths and polyethylene naphthalate by in situ electropolymerization method. The synthesized flexible electrodes were characterized by scanning electron microscopy, High resolution transmission electron microscope, atomic force microscope, Fourier transform infrared, x-ray diffraction, and x-ray photoelectron spectroscopy. Owing to the conductivity and the reversible redox property, the polyaniline/carbon cloth electrodes show excellent properties such as decent supercapacitor performance and good detection capability toward ascorbic acid. As supercapacitors, the electrodes exhibit a specific capacitance as high as 776 F g–1 at a current density of 1 A g–1 and a long cycle life of 20 000 times in the three-electrode system. As ascorbic acid sensors, the flexible electrodes demonstrate stable response to ascorbic acid in the range of 1–3000 μM with an outstanding sensitivity (4228 μA mM–1 cm–2), low detection limit (1 μM), and a fast response time. This work holds promise for high-performance and low-cost flexible electrodes for both supercapacitors and non-enzymatic ascorbic acid sensors, and may inspire inventions of self-powered electrochemical sensor.

Fabrication of High-Performance Flexible Supercapacitor Electrodes with Poly(3,4-ethylenedioxythiophene) (PEDOT) Grown on Carbon-Deposited Polyurethane Sponge

Composite porous supercapacitor electrodes were prepared by growing poly(3,4-ethylenedioxythiophene) (PEDOT) on graphite nanoplatelet- or graphene nanoplatelet-deposited open-cell polyurethane (PU) sponges via a vapor phase polymerization (VPP) method. The resulting composite supercapacitor electrodes exhibited great capacitive performance, with PEDOT acting as both the conductive binder and the active material. The chemical composition was characterized by Raman spectroscopy and the surface morphology was characterized by scanning electron microscopy (SEM). Cyclic voltammetry (CV), charge-discharge (CD) tests and electrochemical impedance spectroscopy were utilized to study the electrical performance of the composite electrodes produced in symmetrically configured supercapacitor cells. The carbon material deposited on PU substrates and the polymerization temperature of PEDOT affected significantly the PEDOT morphology and the electrical properties of the resulting composite sponges. The highest areal specific capacitance 798.2 mF cm−2 was obtained with the composite sponge fabricated by VPP of PEDOT at 110 °C with graphene nanoplatelet-deposited PU sponge substrate. The capacitance retention of this composite electrode was 101.0% after 10,000 charging–discharging cycles. The high flexibility, high areal specific capacitance, excellent long-term cycling stability and low cost make these composite sponges promising electrode materials for supercapacitors.

Rational design of hierarchically sulfide and MXene-reinforced porous carbon nanofibers as advanced electrode for high energy density flexible supercapacitors

In this study, in-situ carbon coating technology was used to embed the sulfide-loaded MXene in the carbon nanofibers through electrospinning to improve the sulfide conductivity and ion transfer rate. Polyacrylonitrile (PAN) with a high carbon conversion rate was used as carbon nanofiber, while polyvinylpyrrolidone (PVP) with a low carbon conversion rate was used as a pore-forming sacrificial agent . PAN–PVP-based porous carbon nanofibers (PCNF) with good meso/macropore structure were prepared via a thermally induced phase separation process. FeCo 2 S 4 nanoparticles and ultra-thin Ti 3 C 2 T x MXene were uniformly fixed in PCNF in situ, and a flexible hybrid film was prepared as the electrode material of supercapacitors (FeCo 2 S 4 /MXene/PCNF). The FeCo 2 S 4 /MXene/PCNF hybrid membrane inherits a three-dimensional pore structure and hierarchical PCNF nanostructure . It can provide continuous channels for the rapid electrolyte diffusion, thereby obtaining electrochemically active FeCo 2 S 4 nanoparticles. Moreover, carbon nanofibers can act as a conductive core for providing effective electron transport for the rapid Faraday redox reaction of the FeCo 2 S 4 sheath or as a buffer matrix for reducing local volume expansion/contraction during long-term cycling. Therefore, the optimized FeCo 2 S 4 /MXene/PCNF hybrid membrane has excellent cyclic stability, which fundamentally solves the problems of poor sulfide conductivity and cyclic stability.

Thorn-like nanostructured NiCo2S4 arrays anchoring graphite paper as self-supported electrodes for ultrahigh rate flexible supercapacitors

Vertically stacked thorn-like nanostructured NiCo2S4 arrays are designed to in-situ grow on the electrochemical oxidized graphite paper by a two-step hydrothermal method where graphite papers act as heterogeneous nuclear sites to rationally construct the unique features of NiCo2S4. Bimetal synergistic effect and the presence of sulfur atoms can improve the ionic and electron conductivity of NiCo2S4. The unique structure of NiCo2S4 fabricated in such way can allow more active sites exposed to reversible redox reactions, as well as provide relatively short ions/electrons transport pathways within the broken crystal structure and rich porous structure. As such, graphite paper supported NiCo2S4 as a self-supported electrode (NiCo2S4@EGP) shows an extraordinarily high rate capability with the specific capacitance of 1276 F g−1 at 1 A g−1, and even 1218 F g−1 at 20 A g−1, as well as the excellent cycle stability with capacitance retention of 86 % after 5000 cycles. When assembling into a flexible asymmetric all-solid-state supercapacitor device, this device reaches a high-power density of 55.3 Wh kg−1 at a power density of 747.6 W kg−1, together with a superior high cycle stability (retained 95 % in 5000 cycles). These results obviously open interesting perspectives of using NiCo2S4@EGP as self-supported electrodes to become an advanced flexible power device.

Dipping fabrication of rHGO@NiO@NF flexible supercapacitor electrode and its potential in bendable electronic devices

A flexible nickel foam (NF) based reduced Holey Graphene Oxide (rHGO) wrapping on NiO@NF hybrid supercapacitor electrode is fabricated using facile hydrothermal reaction followed by HGO dispersion dipping to achieve an efficient, bendable and scalable supercapacitor with high capacitance for wearable smart device usage. The sample electrode reveals a typical wrapping morphology of porous rHGO spreading on uniform nickel oxide crystal layer while exposing crystals through recurrent rHGO slits. Benefiting from this morphology and optimization of fabrication parameters, the sample electrode gains better electrochemical performance from higher electric double layer capacitance incorporation and more efficient electrolyte ion transportation, with less than 0.1 Ω equivalent series resistance (ESR), 752.9 F/g of capacitance, and 90% capacitance retention under 10 A/g current density. The sample electrode is then made into an all-solid-state supercapacitor device possessing 0.267 Wh/m2 energy density at a power density of 8.14 W/m2, and the bendable fact of the device endows it 97% capacitance retention after 500 times of curving, making it especially suitable as flexible energy provider. This simple fabrication method should shed light on flexible hybrid supercapacitor development.

High-crystalline tetraaniline nanofibers deposited carbon cloth as flexible electrode for high-performance solid-state supercapacitors

To enhance the electrochemical energy storage property of the carbon cloth (CC)-based supercapacitors, high-crystalline tetraaniline (cTA) nanofibers were deposited onto the oxidized carbon cloth (OCC) here, via a facile post-synthetic solution-based self-assembly method. With the optimized depositing condition, the flexible free-standing cTA/OCC-5 electrode was obtained with much higher specific capacitance of 2119.2 mF/cm2 at 1.0 mA/cm2 in comparison with the OCC of 1501.8 mF/cm2 and the tetraaniline impregnated OCC (TA/OCC) of 1623.1 mF/cm2, which was prepared by the solvent evaporation method with the same TA content as the cTA/OCC-5. To further improve the cyclic life of the cTA/OCC-5 electrode in symmetric solid-state supercapacitors, 0.01 M Na2SO4 was added in the H2SO4/PVA gel electrolyte. With the similar energy density and power density, the H2SO4/Na2SO4/PVA gel-based device possessed higher cyclic stability with retention of 99.97% after 10,000 cycles than the H2SO4/PVA gel-based device of 90.58%.

Construction of copper porphyrin-linked conjugated microporous polymer/carbon nanotube composite as flexible electrodes for supercapacitors

Construction of copper porphyrin-linked conjugated microporous polymer/carbon nanotube composite as flexible electrodes for supercapacitors

Flexible iron oxide supercapacitor electrodes by photonic processing

We report the preparation of flexible and nano-porous iron oxide-reduced graphitic oxide (Fe2O3–rGO) electrodes using a novel photonic processing method. Due to this unique technique, high-temperature thermal processing could be accomplished on inexpensive and low-temperature substrates instantaneously as opposed to longer processing times of conventional thermal processing. The nano-porous morphology of the electrode not only accommodates the volume changes of the electrode but also facilitates the transport of the electrolyte ions into the electrodes. The as-prepared electrode showed excellent electrochemical performance with an initial specific capacitance of 179 F/g at 2 A/g. Moreover, it exhibited excellent specific capacitance retention after 5000 cycles (70%), revealing its superior cyclic stability. Along with having specific capacitance comparable to that of rigid electrodes, the as-prepared electrode is bendable and lightweight, signifying its potential application in foldable and wearable consumer electronic devices which require continuous energy supply while going through physical deformation.Graphic abstract

MnMoO4 nanorods-encapsulated carbon nanofibers hybrid mat as binder-free electrode for flexible asymmetric supercapacitors

In this work, the in-situ formation of MnMoO4 nanoparticles and simultaneous encapsulation within the porous carbon nanofiber (CNF) matrix through electrospinning-carbonization-thermal annealing processes were carried out. The results of various spectroscopic and morphological analyses confirmed the successful formation of rectangular-shaped MnMoO4 nanorods within the CNF matrix. The MnMoO4@CNF hybrid yielded a maximum specific capacitance of 389.7 F/g at 0.1 A/g, which is 3-folds more than that of CNF (123.4 F/g). The MnMoO4@CNF symmetric cell with an aqueous KOH electrolyte possessed a maximum energy density of 54.12 Wh/kg at a power density of 10 W/kg. The excellent capacitive performances of as-prepared MnMoO4@CNF nanofibers could be ascribed to their high specific surface area (229.35 m2/g) with wide mesoporous distribution (3–13 nm) and enhanced reactivity of the nano-sized MnMoO4 nanoparticles. The hybrid electrode demonstrated excellent cyclic stability with capacitance retention of 92.1% after 5000 cycles, suggesting restricted extrication of metal oxide from the CNF matrix during the charge-discharge process. A flexible solid-state asymmetric supercapacitor was assembled by using MnMoO4@CNF hybrid mat as a positive electrode and activated carbon cloth as a negative electrode. The polymer gel electrolyte (PVA + KOH) was used as electrolyte-cum-separator. The as-prepared MnMoO4@CNF//AC cell yielded a highest energy density of 37.46 Wh/kg and an equivalent power density of 278 W/kg. The present investigation could open new horizons in developing next-generation flexible solid-state supercapacitors for wearable electronics.

Agar-based porous electrode and electrolyte for flexible symmetric supercapacitors with ultrahigh energy density

In this work, a symmetric flexible supercapacitor is successfully fabricated using the electrode and electrolyte, which are both derived from source-abundant agar. Agar-based porous carbon electrode is synthesized by foaming and carbonization-activation processes using KHCO3 and HAc as activators. The unique foam-like gel precursor forms during the agar gelation process with simple metathetical reactions, which facilitates the generation of porous structure. Benefitting from the large specific surface area (2134 m2 g−1) and hierarchically porous conductive network, the as-prepared carbon electrode displays a high gravimetric capacitance of 313.7 F g−1 at 0.5 A g−1 and good rate performance in 6 M KOH. Moreover, agar-contained Na2SO4 hydrogel electrolyte is prepared by a simple solvent casting process, and the prepared hydrogel electrolyte shows a broadening electrochemical potential window of 1.8 V and excellent flexibility when used in supercapacitor. The symmetrical flexible aqueous-based supercapacitor that assembled via agar-based porous carbon electrode and agar-based gel electrolyte exhibits an ultrahigh energy density of 22.1 Wh kg−1 at 450 W kg−1. This work provides an effective strategy for preparing both the supercapacitor electrode and electrolyte using the same source, i.e. agar as the starting material, which uncovers a new possibility for the development of flexible capacitors with high energy density.

Polyaniline nanowire arrays oriented on the functionalized rGO/PEDOT/PP fabric substrate for high performance supercapacitors with mechanical flexibility

Fabrication of high-performance supercapacitors with exceptional mechanical flexibility and durability for wearable electronics still remains an elusive challenge. In this paper, oriented polyaniline (PANI) nanowires were in-situ polymerized on a reduced graphene oxide (rGO)/poly(3,4-ethylenedioxythiophene) (PEDOT)/polypropylene (PP) fabric to produce highly robust flexible electrodes for high-performance supercapacitor. The obtained binder-free PANI/rGO/PEDOT/PP electrode fulfills an outstanding electrochemical performance with a specific capacitance (Cs) of 572.7 mF/cm2 and excellent retention of 97.4% over 5000 cycles. More significantly, the corresponding symmetrical supercapacitors could achieve a high Cs of 432.1 mF/cm2 and a satisfactory energy density of 49.6 μWh/cm2. In addition, this device exhibits very stable capacitance properties with the Cs retention of 95.3% and 92.9% under mechanical bending and folding over 1000 times, respectively. All these results indicate a remarkably enhanced electrochemical performance, which provided a valuable reference for the development of flexible supercapacitor electrodes.

Size-refinement enhanced flexibility and electrochemical performance of MXene electrodes for flexible waterproof supercapacitors

Increasing mechanical flexibility without sacrificing electrochemical performance of the electrode material is highly desired in the design of flexible electrochemical energy storage devices. In metal-related materials science, decreasing the grain size introduces more grain boundaries; this stops dislocations and crack propagation under deformation, and results in increased strength and toughness. However, such a size refinement effect has not been considered in the mechanical properties, particle stacking, wetting, and electrochemical performances of flexible supercapacitor electrodes. In this paper, MXene was used as an electrode material to study the size refinement effect of flexible supercapacitors. Size refinement improved the strength and toughness of the MXene electrodes, and this resulted in increased flexibility. Finite elemental analysis provided a theoretical understanding of size refinement-increased flexibility. Moreover, the size refinement also improved the specific surface area, electric conductance, ion transportation, and water wetting properties of the electrode, and the size refinement provided highly increased energy density and power density of the MXene supercapacitors. A highly flexible, water-proof supercapacitor was fabricated using size-refined MXene. The current study provides a new viewpoint for designing tough and flexible energy storage electrodes. The size refinement effect may also be applicable for metal ion batteries and electronic and photo devices composed of MXene and other nanoparticles.

Electrochemical Performance of Coaxially Wet-Spun Hierarchically Porous Lignin-Based Carbon/Graphene Fiber Electrodes for Flexible Supercapacitors

Electrochemical Performance of Coaxially Wet-Spun Hierarchically Porous Lignin-Based Carbon/Graphene Fiber Electrodes for Flexible Supercapacitors

Microwave Synthesis of MnO2-Lignin Composite Electrodes for Supercapacitors

The demand for energy storage devices made from biodegradable materials has increased significantly due to sustainability. Currently, such devices possess vital issues, such as high manufacturing costs and toxicity, low reliability, as well as poor electrochemical performance. In this research, microwave synthesis was conducted to fabricate a low-cost, high-performing, plant-based electroactive material. MnO2 microparticles fabricated via microwave irradiation were deposited on two plant-based materials as substrates made of Al/lignin and Al/AC/lignin. The quasi-solid-state supercapacitors were assembled using a polymeric gel electrolyte of PVA/H3PO4. Scanning electron microscopy was performed to examine the polydispersity, morphology, and porosity of the micro-MnO2 deposited materials. FTIR and UV-vis spectroscopy were performed to study the composition and verify deposition of micro-MnO2 on the lignin-based matrixes. Cyclic voltammetry (CV) was employed to study the polarization resistance of the system. The cyclic charge-discharge (CCD) and electrochemical impedance spectroscopy (EIS) were performed to observe cyclic performance and interfacial resistances. Electrochemical tests showed that after 700 cycles of charge-discharge, both the supercapacitors exhibited high capacitance retention above 90%. Compared to the existing technology, this method enables consistent material structurization with tunable properties due to the controlled heating time and exposure to radiation with minimal waste. This work provides an alternative approach to synthesize low-cost and scalable green composite electrodes for flexible supercapacitors.

Effect of the morphology of silver layer on electrical conductivity and electrochemical performance of silver/reduced graphene oxide/cotton fabric composite as a flexible supercapacitor electrode

Three different electroless silver plating (ESP) methods were used to prepare the silver nanoparticles (AgNPs)/reduced graphene oxide (RGO)/cotton fabric (CF) composite electrodes. The Ag/RGO/CF composites were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), sheet resistance, water contact angle, and thermal gravimetric analysis (TGA). The electrochemical performance was investigated by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and galvanostatic charge-discharge (GCD). The effect of the ESP method on morphological, structural, electrical, and electrochemical properties of the composite electrodes was evaluated. It was found that the compromise between the amount of deposited AgNPs, conductivity, and electrochemical performance is required to prepare the electrode with high energy storage ability. The simple and rapid ESP method, which was performed on the surface of RGO/CF at room temperature, achieved the electrode with the interconnected porous morphology providing the highest energy storage performance. This morphology provided easy electron transfer, short ion diffusion pathways, and increased penetration of the ions into the electrode materials, leading to improved electrochemical performance. The produced electrode showed a noticeable specific capacitance of 350±9 Fg−1 at a scan rate of 5 mV s−1 in 0.5 M H2SO4 electrolyte and retained 87 % of initial capacitance after 1000 mechanical bending cycles. The symmetric supercapacitor cell based on this electrode exhibited high specific energy of 43.2 Wh Kg−1 and excellent cycle life (95.8 % retention after 3000 charge-discharge cycles). The supercapacitor with asymmetric configuration exhibited enhanced specific energy of 70.6 Wh Kg−1 due to the extended operating voltage range.

Real-Time Biomonitoring Device Based on 2D Black Phosphorus and Polyaniline Nanocomposite Flexible Supercapacitors.

Flexible energy storage devices are becoming significantly important to power wearable and portable devices that monitor physiological parameters for many biomedical applications. Many hybrid nanomaterials based on 2D materials are used in order to improve the performance of flexible energy storage devices. Here, a hybrid nanocomposite is synthesized through in situ polymerization of aniline in the presence of black phosphorus (BP) nanoflakes. This nanocomposite, polyaniline (PANI)@BP, is employed to fabricate flexible supercapacitor (FSC) electrodes. PANI@BP FSCs can provide a power source for biometric devices. The generated signal can be transmitted to a smartphone in real time via wireless communication. Such a compact and lightweight integrated device has been used to track a human heart beat while powered by PANI@BP FSC. These findings are providing a promising example of a flexible energy storage device that can be integrated with different real-time health monitoring devices.