From basil seed to flexible supercapacitors: Green synthesis of heteroatom-enriched porous carbon by self-gelation strategy

Basil seed-derived multi-heteroatom–doped porous carbons (BHPCs) are successfully synthesized by a facile gelation, followed by a moderate gel water/KOH coactivation process. The BHPC-700 prepared at a relatively low KOH loading and activation temperature possesses large specific surface area (1178.3 m² g⁻¹), well-defined hierarchical micro/meso porosity, and rich self-doping heteroatom functionalities (13.08 at% of oxygen, nitrogen, phosphorous, and sulfur). Electrochemical tests demonstrate that the BHPC-700–based electrode achieves an ultrahigh specific capacitance (464 F g⁻¹ at 0.5 A g⁻¹), outstanding rate performance (retaining 73.3% capacitance at 50 A g⁻¹), and superior cyclic stability (96.8% capacitance retention over 5000 cycles). Furthermore, the BHPC-700 electrodes are assembled into all-solid-state symmetrical supercapacitors. The as-assembled device gives a high energy density of 15.0 Wh kg⁻¹ at a power density of 500 W kg⁻¹ and remarkable flexibility, demonstrating great application prospects in the area of sustainable portable electronics.     

Highly conducting silver nanowire-polyacrylonitrile hollow fibres for flexible supercapacitors

A new approach for making stable, flexible, and conductive hollow fibres of poly (acrylonitrile) using dry-jet-wet spinning technique is investigated, wherein the inner walls of the poly (acrylonitrile) hollow fibres are deposited with silver nanowires using their dispersion in the bore fluid. The bore fluid plays a crucial role in determining the morphology and flexibility of the hollow fibres and entrapment of long silver nanowires on the inner walls. Fibres with AgNW layer having high conductivity of ~10⁴ Scm⁻¹ are obtained with the use of ~2 wt% of silver nanowires. The conducting fibres are successfully assembled into coaxial configuration to yield highly stable, flexible supercapacitors with capacitance value of 128 Fcm⁻³. The unique morphology of these conductive hollow fibres opens the possibility of making flexible and stable devices for wearable electronics.     

Progress in graphene based supercapacitors

超级电容器是最具应用前景的电化学储能技术之一.目前,超级电容器的研究重点是提高能量密度和功率密度,发展具有高比表面积,电导率和结构稳定性的电极材料是关键.石墨烯因具有比表面积大,电子导电性高,力学性能好的特点而成为理想的电容材料,但石墨烯的理论容量不高,在石墨烯基电极制备过程中容易发生堆叠现象,导致材料比表面积和离子电导率下降.因此,发展合适的制备方法,对石墨烯进行修饰或与其他材料形成复合电极材料是一种有效解决途径.本文对石墨烯基电极及其在双电层电容器,法拉第准电容器和混合型超级电容器中的应用的研究进展进行归纳,重点介绍了石墨烯凝胶薄膜电极的制备过程,以促进石墨烯基电极在超级电容器构筑中应用.     

Composite of manganese dioxide impregnated in porous hollow carbon spheres for flexible asymmetric solid‐state supercapacitors

Compared with symmetric supercapacitors, asymmetric supercapacitors are been widely applied in energy storage devices because of delivering an impressible energy density. Herein, a simple temple strategy was used to fabricate the porous hollow carbon spheres (PHCS) with high specific surface area of 793 m² g^?1, large pore volume of 1.0 cm³ g^?1 and pore size distribution from micropores to mesopores, serving as the capacitive electrodes of asymmetric supercapacitors. Subsequently, manganese dioxide (MnO₂) was impregnated into the PHCS to form a faradic electrode with a promising performance, owing to a synergistic effect between high capacity MnO₂ and conductive PHCS. Furthermore, the flexible asymmetric solid‐state devices were constructed with PHCS anode, PHCS@MnO₂ cathode, and PVA/LiCl electrolyte, extending a voltage window up to 1.8 V. The extensive voltage window would lead to an increased energy density. In our case, the flexible asymmetric sandwich exhibit excellent electrochemical performance in terms of a high energy density capacity of 26.5 W·h kg^?1 (900 W kg^?1) and superior cycling performance (10 000 cycles). Therefore, the developed strategy provides a strategy to achieve the PHCS‐based composites for the application in the asymmetric solid‐state supercapacitors, which will enable a widely field of flexible energy storage devices.     

Graphene‐based materials for flexible electrochemical energy storage

Sustainable development of renewable energy sources is one of the most important themes that humanity faces in this century. Wide use of renewable energy sources will require a drastically increased ability to store electrical energy. Electrochemical energy storage devices are expected to play a key role. With the increased demand in flexible energy resource for wearable electronic devices, great efforts have been devoted to developing high‐quality flexible electrodes for advanced energy storage and conversion systems. Because of its high specific surface area, good chemical stability, high mechanical flexibility, and outstanding electrical properties, graphene, a special allotrope of carbon with two‐dimensional mono‐layered network of sp² hybridized carbon, have been showing great potential in next‐generation energy conversion and storage devices. This review presents the latest advances on the flexible graphene‐based materials for the most vigorous electrochemical energy storage devices, that is, supercapacitors and lithium‐ion batteries. Copyright © 2014 John Wiley & Sons, Ltd.     

High-performance textile electrode enhanced by surface modifications of fiberglass cloth with polypyrrole tentacles for flexible supercapacitors

Textile-based flexible supercapacitors have various desirable advantages in practical applications due to their excellent flexibility, ease of large-scale production, and low cost. In this study, a flexible supercapacitor was designed and fabricated using a two-step polymerization method based on fiberglass cloth and unique morphology of polypyrrole (PPy). In this extraordinary nanostructure, not only do PPy tentacles provide high-speed channels for the transfer of electron and ion, but they also create a larger specific surface area, thus enhancing the energy storage. The fabricated PPy/CFC supercapacitor possesses an excellent area-specific capacitance of 549.6 mF cm⁻² and a remarkable energy density of 48.85 μWh cm⁻². Besides, it achieves the high capacitance retention of 92.4% after 10 000 charge and discharge cycles and 96.08% after 1000 bending cycles. Furthermore, it is demonstrated that the PPy/CFC supercapacitor is capable of ensuring a stable power supply for practical applications by driving an LCD electronic watch. The fiberglass cloth-based supercapacitors with PPy tentacles provide a new approach to the practical applications of wearable power supplies.     

Graphene/single-walled carbon nanotube hybrids and applications in supercapacitors

双电层超级电容器作为新型清洁能源储能器件,具有安全、高功率密度和长寿命的优点。目前发展新电极材料与提高工作电压窗口是提高电容器能量密度的重要方向。本工作利用化学气相沉积法制备了石墨烯-碳纳米管杂化物,具有导电性优良、孔径可调、化学稳定性高、比表面积大（1200～1800 m2/g）的优点,同时避免了单独石墨烯或者碳纳米管电极制备过程中堆叠的缺点。并且系统研究了石墨烯-碳纳米管杂化物在水系、有机电解液和离子液体中的电容性质,考察了以活性炭为主体电极材料,石墨烯-碳纳米管为添加剂的软包电容器的性质,为开发高能量密度和高功率密度的超级电容器提供了基础。     

Sputtered manganese oxide thin film on carbon nanotubes sheet as a flexible and binder‐free electrode for supercapacitors

In this work, flexible carbon nanotubes (CNTs)/manganese oxide (MnO₂) composite electrode was fabricated by direct deposition of MnO₂ nanoparticles on CNTs sheet by RF magnetron sputtering. The surface morphology and microstructure of the CNTs and CNTs/MnO₂ composite electrodes were characterized by X‐ray diffraction (XRD), scanning electron microscope‐energy dispersive spectroscopy (SEM‐EDS), X‐ray photoelectron spectroscopy (XPS), and Raman spectroscopy. It was found that 1‐μm thick MnO₂ film covered the surface of CNTs sheet with MnO₂ mass loading of 0.125 mg/cm². CNTs/MnO₂ composite was tested as electrode materials for supercapacitors in sulfate media (1‐M H₂SO₄ and Na₂SO₄) by cyclic voltammetry (CV) and galvanostatic charge/discharge (GCD). The results obtained showed that CNTs/MnO₂ composite electrode displayed good electrochemical performance in 1‐M Na₂SO₄, while the chemical stability of MnO₂ film was highly affected due its dissolution in acidic medium. A specific capacitance of 940 F/g was retained (with a capacitance retention of about 80%) after 3000 GCD cycles. CNTs/MnO₂ all‐solid symmetric supercapacitor using PVA/H₃PO₄ gel electrolyte exhibited an initial specific capacitance 80 F/g and decreased by 25% after 3000 cycles.     

Pinecone biomass-derived activated carbon: the potential electrode material for the development of symmetric and asymmetric supercapacitors

Activated carbon, from biomass (pinecone), was synthesized by conventional pyrolysis/chemical activation process and utilized for the fabrication of supercapacitor electrodes. The pinecone-activated carbon synthesized with 1:4 ratio of KOH (PAC4) showed an increase in surface area and pore density with a considerable amount of oxygen functionalities on the surface. Moreover, PAC4, as supercapacitor electrode, exhibited excellent electrochemical performances with specific capacitance value ∼185 Fg⁻¹ in 1 M H₂SO₄, which is higher than that of nonactivated pinecone carbon and 1:2 ratio KOH-based activated carbon (PAC2) (∼144 Fg⁻¹). The systematic studies were performed to design various forms of devices (symmetric and asymmetric) to investigate the effect of device architecture and operating voltage on the performance and stability of the supercapacitors. The symmetric supercapacitor, designed utilizing PAC4 in H₂SO₄ electrolyte, exhibited a maximum device-specific capacitance of 43 Fg⁻¹ with comparable specific energy/power and excellent stability (∼96% after 10 000 cycles). Moreover, a symmetric supercapacitor was specially designed using PAC4, as a positive electrode, and PAC2, as a negative electrode, under their electrolytic ion affinity, and which operates in aqueous Na₂SO₄ electrolyte for a wide cell voltage (1.8 V) and showed excellent supercapacitance performances. Also, a device was assembled with poly(3,4-ethylene dioxythiophene) (PEDOT) nanostructure, as positive electrode, and PAC4, as a negative electrode, to evaluate the feasibility of designing a hybrid supercapacitor, using polymeric nanostructure, as an electrode material along with biomass-activated carbon electrode.     

A free-standing electrode based on 2D SnS2 nanoplates@3D carbon foam for high performance supercapacitors

A reasonable formation of an electrode material with three-dimensional (3D) microstructure for supercapacitors was proposed. Two-dimensional (2D) SnS₂ nanoplates were uniformly in situ grown on 3D carbon foam (CF) through a controllable strategy. The composite displayed excellent electrochemical performance due to the synergistic effect of SnS₂ and CF. The SnS₂@CF-2 composite containing 23.92 wt% of SnS₂ has a superior specific capacitance of 283.6 F g⁻¹ at the current density of 1 A g⁻¹. Moreover, a symmetric supercapacitor based on SnS₂@CF-2 composite has a capacitance of 82.5 F g⁻¹ at 1 A g⁻¹ and a high energy density of 13.9 Wh kg⁻¹ at the power density of 551.7 W kg⁻¹.     

All-textile flexible supercapacitors using electrospun poly(3,4-ethylenedioxythiophene) nanofibers

Poly(3,4-ethylenedioxythiophene) (PEDOT) nanofibers were obtained by the combination of electrospinning and vapor-phase polymerization. The fibers had diameters around 350 nm, and were soldered at most intersections, providing a strong dimensional stability to the mats. The nanofiber mats demonstrated very high conductivity (60 ± 10 S cm⁻¹, the highest value reported so far for polymer nanofibers) as well as improved electrochemical properties, due to the ultraporous nature of the electrospun mats. The mats were incorporated into all-textile flexible supercapacitors, using carbon cloths as the current collectors and electrospun polyacrylonitrile (PAN) nanofibrous membranes as the separator. The textile layers were stacked and embedded in a solid electrolyte containing an ionic liquid and PVDF-co-HFP as the host polymer. The resulting supercapacitors were totally flexible and demonstrated interesting and stable performances in ambient conditions.     

Advances in supercapacitors: From electrodes materials to energy storage devices

随着绿色储能器件的快速发展,超级电容器作为兼具高比能量与高比功率的优点,在储能领域具有重要发展潜力的新型储能器件,本综述从超级电容器的电极材料出发,详细概括了超级电容器电极材料的发展,包括双电层电容材料、赝电容材料以及双电层/赝电容复合材料;在此基础上,基于固态电解质,深入讨论了近年来全固态超级电容器的典型构型,针对性地总结了提高储能器件储能容量的关键问题。最后,基于电极材料与电解液的研究焦点,对超级电容器的研究提出了未来发展方向。     

Design of high-performance flexible symmetric supercapacitors energized by redox-mediated hydrogels including metal-doped acidic polyelectrolyte

The fabrication of flexible supercapacitors was achieved by employing the novel redox-activated polymer electrolytes comprising poly(vinylphosphonic acid) (PVPA) and nickel nitrate Ni(NO₃)₂, Ni. The hydrogels, PVPA/NiX, were produced in various contents, in which X denotes the doping fraction of Ni in PVPA. The structure, thermal, and morphology of the materials were characterized, and then they were applied for construction of supercapacitors. The performance evaluations of the fabricated devices were carried out by electrochemical impedance spectroscopy, galvanostatic charge-discharge, and cyclic voltammetry experiments. Flexible supercapacitor devices assembled with activated carbon (AC) electrodes and PVPA/NiX hydrogels produced 793 F g⁻¹ specific capacitance with 30 times enhanced capacitance compared with Ni-free system. The energy density of 103.1 Wh kg⁻¹ was yielded from the device at a power density of 500 W kg⁻¹. The supercapacitor demonstrated an excellent performance during 5.000 charge-discharge cycles, while preserving 84% of its initial capacitance. The supercapacitor constructed of 1 × 5 cm dimension, successfully operates the LED after charging at 3 V.     

Nomex-derived activated carbon fibers as electrode materials in carbon based supercapacitors

Electrochemical characterization has been carried out for electrodes prepared of several activated carbon fiber samples derived from poly (m-phenylene isophthalamide) (Nomex) in an aqueous solution. Depending on the burn-off due to activation the BET surface area of the carbons was in the order of 1300–2800 m² g⁻¹, providing an extensive network of micropores. Their capability as active material for supercapacitors was evaluated by using cyclic voltammetry and impedance spectroscopy. Values for the capacitance of 175 F g⁻¹ in sulfuric acid were obtained. Further on, it was observed that the specific capacitance and the performance of the electrode increase significantly with increasing burn-off degree. We believe that this fact can be attributed to the increase of surface area and porosity with increasing burn-off.     

Physical and electrochemical characteristics of supercapacitors based on carbide derived carbon electrodes in aqueous electrolytes

FIB-SEM, XPS and gas adsorption methods have been used for the characterisation of physical properties of microporous carbide derived carbon electrodes prepared from Mo₂C at 600 °C (noted as CDC-Mo₂C). Cyclic voltammetry, constant current charge/discharge, and electrochemical impedance spectroscopy have been applied to establish the electrochemical characteristics for supercapacitors consisting of the 1 M Na₂SO₄, KOH, tetraethyl ammonium iodide or 6 M KOH aqueous electrolyte and CDC-Mo₂C electrodes. The N₂ sorption values obtained have been correlated with electrochemical characteristics for supercapacitors in various aqueous electrolytes. The maximum gravimetric energy, E_max, and gravimetric power, P_max, for supercapacitors (taking into consideration the active material weight) have been obtained at cell voltage 0.9 V for 6 M KOH aqueous supercapacitor (E_max = 5.7 Wh kg⁻¹ and P_max = 43 kW kg⁻¹). For 1 M TEAI based SC somewhat higher E_max (6.2 Wh kg⁻¹) and comparatively low P_max (7.0 kW kg⁻¹) have been calculated.     

Carbon nanotubes for flexible energy storage devices--A review

碳纳米管具有本征sp²共价结构所带来的优异导电,导热及力学性能,并在锂电池,超级电容器等储能体系中具有广泛的应用前景.本文回顾了碳纳米管柔性储能的背景,介绍了碳纳米管优异的性能,独特的结构.这种一维的管状结构既可以作为柔性电极中的支撑骨架,也可以构建优越的长程导电网络,提供能源存储活性位点,进而提高柔性器件的性能.本文详细评述近年来碳纳米管在柔性超级电容器,锂离子电池,锂硫电池等能源存储元件应用中的一些最新进展和研究热点,其中柔性超级电容器,锂离子电池方面研究比较成熟,而锂硫电池尚在起步阶段,预期会取得快速进步.文章介绍了电极构建途径及性能评估方法,展示了碳纳米管基柔性储能器件的进展,并展望了其未来发展方向.     

Mathematical functions for optimisation of conducting polymer/activated carbon asymmetric supercapacitors

Equations routinely used to describe the properties of conventional symmetric electrochemical double-layer capacitors (EDLCs) are expanded to develop straightforward mathematical functions that can effectively describe the performance characteristics of asymmetric supercapacitors based on electrically conducting polymer and activated carbon (ECP–AC) electrodes. Formulae are developed to describe cell parameters (based on total active material mass) such as maximum specific capacitance (F g⁻¹), maximum specific energy (Wh kg⁻¹), and optimum electrode mass ratios that can be used for maximising the specific energy of asymmetric cells. The electrode mass ratios are found to have a significant impact on the swing voltages across the positive and negative electrodes. Illustrative EDLC and ECP–AC devices are explored and employed to verify the derived equations that serve to predict essential parameters of both symmetric and asymmetric systems, irrespective of electrolyte ion concentration, solvent or species and independent of voltage. The utility of the equations is demonstrated by predicting cell parameters for a number of theoretical asymmetric ECP–AC systems and used to correlate experimentally obtained parameters.     

Electrode materials for ionic liquid-based supercapacitors

The use of ionic liquid (IL) electrolytes is a promising strategy to enhance the performance of supercapacitors above room temperature. In this paper we present the results of a study on optimization of electrode materials for IL-based supercapacitors featuring a hybrid configuration with carbon negative electrode and poly(3-methylthiophene) (pMeT) as positive operating at 60 °C with the ILs N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR₁₄TFSI) and 1-ethyl-3-methyl-imidazolium bis(trifluoromethanesulfonyl)imide (EMITFSI). As it concerns the carbon electrode two routes have been pursued: (i) surface modification of commercial activated carbon and (ii) synthesis of mesoporous cryo- and xerogel carbons. Pore size distribution and electrochemical characterization data are related and suggest that the second route should be the most promising for carbons of high specific capacitance and low time constant in IL. For the polymer electrode the nature of the galvanostatic polymerization bath plays a crucial role to provide pMeT of high specific capacitance and the best results may be obtained when pMeT is electropolymerized in the same IL used for the capacitance tests. The strategy of using the acid additive trifluoromethanesulfonimide in IL-based polymerization baths is also described in some detail. This strategy that provides pMeT featuring more than 200 F g⁻¹ in IL is a clean procedure which prevents consumption of the ionic liquid with great advantage in terms of costs.     

Layered molybdenum disulfide coated carbon hollow spheres synthesized through supramolecular self-assembly applied to supercapacitors

Through supramolecular assembly method, molybdenum disulfide (MoS₂) is uniformly anchored on the mesoporous hollow carbon spheres (HCS), which are obtained by hard template method. The introduction of HCS can prevent the agglomeration of MoS₂ and decrease the electric resistance of the compound material MoS₂@HCS. The composite of MoS₂@HCS-17 also owns a specific surface area of 119.0 m² g⁻¹. For MoS₂@HCS-17, SEM and TEM results exhibit that flaky MoS₂ is uniformly covered on hollow carbon spheres and possesses an expanded layered structure. Electrochemical test results show that MoS₂@HCS-17 can reach 314.5F g⁻¹ at 1A g⁻¹. When tested at a scan speed of 50 mV s⁻¹_, there is still 87% specific capacity retention for MoS₂@HCS-17 after 4000 cycles. Moreover, the assembled MnO₂//MoS₂@HCS asymmetric supercapacitor manifests 34.0 Wh kg⁻¹ at 611.6 W kg⁻¹. Bending by 180°, our assembled device still keeps stable capacitive performance. This asymmetric supercapacitor also keeps almost 93% capacity maintained after 2000 cycles.     

Possible improvements in making carbon electrodes for organic supercapacitors

Properties of an Electrical Double Layer Capacitor depend both on the technique used to prepare the electrode and on the current collector structure. Capacitors can be built in a similar way to prismatic cells, with several electrodes connected in parallel for each polarity. Our Electrical Double Layer Capacitors included several carbon/carbon electrodes and all the components (electrodes and separators) were wetted with an organic liquid solvent containing a quaternary ammonium salt as electrolyte. In the present work, electrodes were prepared by two different ways: the first one consisted in spraying a liquid suspension of the electrode materials on a nickel foil, and the second one which consisted in filtering and pressing the electrode materials on to nickel collectors. The first technique allowed us to build seven capacitors of 600 F–2.5 V, with time constants of 12 s. Two banks with series connected supercapacitor cells, one as a 12 V–100 F bank and the other one as a 15 V–85 F were tested on cycling. In our experiments to test these banks of supercapacitors, we also coupled the 100 F–12 V bank to a 12 V–7 Ah secondary lead–acid battery in order to demonstrate the contribution of the supercapacitors during power peaks. Comparing the two techniques used to make the electrodes in terms of performances obtained on the supercapacitors prepared, we obtained the best results by using the second method of electrode preparation. Moreover, two kinds of nickel collectors were studied: expanded nickel grids—and various grades of nickel foams, nickel foams giving the best results. The Equivalent Series Resistance of the electrodes prepared with nickel foams depends on nickel foam grade; it is observed to be 1.75 Ω for a capacitance of 1.37 F in our experimental set up, leading to a time constant of 2.4 s. The mechanical properties of the electrodes were improved as well.