Free-standing Supercapacitor Research Articles

Polyaniline–polypyrrole nanocomposites using a green and porous wood as support for supercapacitors

Wood is an ideal type of support material whose porous structure and surface functional groups are beneficial for deposition of various guest substances for different applications. In this paper, wood is employed as a porous support, combined with two kinds of conductive polymers (i.e., polyaniline (PANI) and polypyrrole (PPy)) using an easy and fast liquid polymerization method. Scanning electron microscope observations indicate that the PANI–PPy complex consists of nanoparticles with a size of ~20 nm. The interactions between oxygen-containing groups of the wood and the nitrogen composition of PANI–PPy were verified by Fourier transform infrared spectroscopy. The self-supported PANI–PPy/wood composite is capable of acting as a free-standing supercapacitor electrode, which delivers a high gravimetric specific capacitance of 360 F·g⁻¹ at 0.2 A·g⁻¹.

Facile, Scalable, Eco-Friendly Fabrication of High-Performance Flexible All-Solid-State Supercapacitors.

A highly porous freestanding supercapacitor electrode has been fabricated through a simple, inexpensive, bulk-scalable, and environmentally friendly method, without using any extra current collector, binder, or conducting additive. Benefiting from its unique micro-tubular hollow structure with a thin cell wall and large lumen, kapok fiber (KF) was used herein as a low-cost template for the successive growth of polypyrrole (PPy) through in situ chemical polymerization. This PPy-coated KF (KF@PPy) was blended with functionalized carbon nanotubes (f-CNTs) to form freestanding conductive films (KF@PPy/f-CNT) through a simple dispersion and filtration method. The hybrid film featuring the optimal composition exhibited an outstanding areal capacitance of 1289 mF cm−2 at a scan rate of 5 mV s−1. Moreover, an assembled all-solid-state symmetric supercapacitor featuring a PVA/H2SO4 gel electrolyte exhibited not only areal capacitances as high as 258 mF cm−2 (at a scan rate of 5 mV s−1) but also excellent cycling stability (97.4% of the initial capacitance after 2500 cycles). Therefore, this efficient, low-cost, scalable green synthesis strategy appears to be a facile and sustainable way of fabricating high-performance flexible supercapacitors incorporating a renewable cellulose material.

Wood Pulp Fiber Wrapped by Fish-Scale Graphene as Flexible and Free-Standing Supercapacitor Electrode

Poplar wood pulp was adopted as both frame and precursor for the synthesis of pulp fiber (PF)/reduced graphene oxide composite via a simple and low-cost method. In this method, the PF based on graphene (PFG) composite film electrode was prepared by a simple vacuum filtration process with various ratios (PF: reduced graphene oxide (RGO) = 5:1, PF:RGO = 5:2, PF:RGO = 5:3, PF:RGO = 5:4, PF:RGO = 5:5). In terms of special structures, the PFG can be used as electrodes without metal-collector, adhesives, and additives. The optimal ratio (PF:RGO = 5:4) film electrode displayed a high areal-specific capacitance of 683 mF/cm2 at 1 mA/cm2 with a mass of 5.3 mg/cm2 (specific capacitance of 129 F/g) and good cycling stability (87.5% capacitance retention after 10,000 cycles at 5 mA/cm2) as well as excellent rate capability and high flexibility (suitable for any angle, even 180°). Moreover, the device could possess a maximum energy density of 47.71 μWh/cm2 and a maximum power density of 1251 μW/cm2. These results suggest that the composite PGF film is a promising electrode material.

Controlled Air‐Etching Synthesis of Porous‐Carbon Nanotube Aerogels with Ultrafast Charging at 1000 A g−1

Supercapacitors are energy storage systems capable of fast charging and discharging, thus generating superior power density. Porous carbon with high surface area and tunable pore size represents a promising candidate to construct ultrafast supercapacitors; so far, most porous carbon-based electrodes can only be charged to a moderate current density (100-200 A g-1), also with significant capacitance loss at increasing rate. Here, it is shown that a 3D aerogel consisting of interconnected 1D porous-carbon nanotubes (PCNs) can serve as a freestanding supercapacitor electrode with excellent rate performance. As a result, the PCN aerogel electrodes achieve 1) ultrafast charging at current densities up to 1000 A g-1 (corresponding to a charge period of 16 ms), which is the highest value among other porous carbon-based supercapacitors, 2) superior cycling stability at high charging rates (88% capacitance retention after 105 cycles at 1000 A g-1). Mechanism study reveals favorable kinetics including a centralized pore size distribution at 0.8 nm which is a dominant factor to allow high-rate charging, a low and linear IR drop, and a metallic feature of 1D PCNs by theoretical calculation. The results indicate that 1D PCNs with controlled porous structures have potential applications in ultrafast energy conversion and storage.

Interlayer Hydrogen-Bonded Covalent Organic Frameworks as High-Performance Supercapacitors.

Covalent organic frameworks (COFs) have emerged as promising electrode materials in supercapacitors (SCs). However, their insoluble powder-like nature, poor capacitive performance in pristine form, integrated with inferior electrochemical stability is a primary concern for their long-term use in electrochemical devices. Keeping this in perspective, herein we report a redox active and hydrogen bonded COF with ultrahigh stability in conc. H2SO4 (18 M), conc. HCl (12 M) and NaOH (9 M). The as-synthesized COF fabricated as thin sheets were efficiently employed as a free-standing supercapacitor electrode material using 3 M aq. H2SO4 as an electrolyte. Moreover, the pristine COF sheet showcased outstanding areal capacitance 1600 mF cm-2 (gravimetric 169 F g-1) and excellent cyclic stability (>100 000) without compromising its capacitive performance or Coulombic efficiency. Moreover, as a proof-of-concept, a solid-state supercapacitor device was also assembled and subsequently tested.

Preparation of flexible and free-standing graphene-based current collector via a new and facile self-assembly approach: Leading to a high performance porous graphene/polyaniline supercapacitor

Flexible all-solid-state supercapacitors are promising electronic devices have been developed in the past decade. Recent efforts to develop such systems are as to construction of free standing electrodes with high specific capacitance. Among these developments, construction of carbon-based current collectors, with high surface areas and good mechanical properties via a facile and economic way, has been taken a great deal of attention. In this study, porous graphene were used as current collector for placement of the carbon fibers functionalized with polyaniline, to prepare a free-standing supercapacitor electrode through a new and facile method. This method begins through blending of the materials in acetone, as the solvent, followed by addition of dilute solution of poly(vinyl alcohol) as the binder of all components. This new strategy can be used as a scalable method to prepare flexible graphene-based current collectors without any loss in their conductivity due to their bending. Maximum specific capacitance of the electrode was obtained 1.42 F/cm2 (710 F/g) at current density of 4 mA/cm2 (2 A/g). In addition, all-solid-state supercapacitor was shown areal specific capacitance and energy density of 440 mF/cm2 and 0.613 mWh/cm3, respectively.

Flexible and freestanding supercapacitor based on nanostructured poly(m-aminophenol)/carbon nanofiber hybrid mats with high energy and power densities

Nanostructured poly(m-aminophenol) (PmAP) coated freestanding carbon nanofiber (CNF) mats were fabricated through simple in situ rapid-mixing polymerization of m-aminophenol in the presence of a CNF mat for flexible solid-state supercapacitors. The surface compositions, morphology and pore structure of the hybrid mats were characterized by using various techniques, e.g., FTIR, Raman, XRD, FE-SEM, TEM, and N2 absorption. The results show that the PmAP nanoparticles were homogeneously deposited on CNF surfaces and formed a thin flexible hybrid mat, which were directly used to made electrodes for electrochemical analysis without using any binders or conductive additives. The electrochemical performances of the hybrid mats were easily tailored by varying the PmAP loading on a hybrid electrode. The PmAP/CNF-10 hybrid electrode with a relatively low PmAP loading (> 42 wt%) showed a high specific capacitance of 325.8 F g−1 and a volumetric capacitance of 273.6 F cm−3 at a current density of 0.5 A g−1, together with a specific capacitance retention of 196.2 F g−1 at 20 A g−1. The PmAP/CNF-10 hybrid electrode showed good cycling stability with 88.2% capacitance retention after 5000 cycles. A maximum energy density of 45.2 Wh kg−1 and power density of 20.4 kW kg−1 were achieved for the PmAP/CNF-10 hybrid electrode. This facile and cost-effective synthesis of a flexible binder-free PmAP/CNF hybrid mat with excellent capacitive performances encourages its possible commercial exploitation.

Zeolitic imidazolate framework-7 textile-derived nanocomposite fibers as freestanding supercapacitor electrodes

Zeolitic imidazole frameworks (ZIFs) are a class of metal organic frameworks (MOFs) with diverse energy-related applications. High-surface area materials derived from ZIFs can serve as electrodes with good long-term capacity retention. Herein, we demonstrate an electrospun ZIF7/carbon nanofiber (CNF) derived nanocomposite as a freestanding electrode for supercapacitors with excellent capacitance retention. The optimal ZIF7 composite nanofiber carbonized at 950 °C exhibited a specific capacitance of 202 F·g−1 at a current density of 1 A·g−1 and ~98% specific capacitance retention after 5000 charge–discharge cycles. N-doped nanoporous C and the Zn framework of ZIF7 composite fibers delivered an energy density of 42 W·h·kg−1 at a power density of 0.6 kW·kg−1. These scalable ZIF7/CNF composite textiles (30 × 10 cm2) can be used as freestanding supercapacitor electrodes without a separate substrate or current collector.

Wearable woven supercapacitor fabrics with high energy density and load-bearing capability

Flexible power sources with load bearing capability are attractive for modern wearable electronics. Here, free-standing supercapacitor fabrics that can store high electrical energy and sustain large mechanical loads are directly woven to be compatible with flexible systems. The prototype with reduced package weight/volume provides an impressive energy density of 2.58 mWh g−1 or 3.6 mWh cm−3, high tensile strength of over 1000 MPa, and bearable pressure of over 100 MPa. The nanoporous thread electrodes are prepared by the activation of commercial carbon fibers to have three-orders of magnitude increase in the specific surface area and 86% retention of the original strength. The novel device configuration woven by solid electrolyte-coated threads shows excellent flexibility and stability during repeated mechanical bending tests. A supercapacitor watchstrap is used to power a liquid crystal display as an example of load-bearing power sources with various form-factor designs for wearable electronics.

Large Areal Mass, Mechanically Tough and Freestanding Electrode Based on Heteroatom-doped Carbon Nanofibers for Flexible Supercapacitors.

A flexible and freestanding supercapacitor electrode with a N,P-co-doped carbon nanofiber network (N,P-CNFs)/graphene (GN) composite loaded on bacterial cellulose (BC) is first designed and fabricated in a simple, low-cost, and effective approach. The porous structure and excellent mechanical properties make the BC paper an ideal substrate that shows a large areal mass of 8 mg cm-2 . As a result, the flexible N,P-CNFs/GN/BC paper electrode shows appreciable areal capacitance (1990 mF cm-2 in KOH and 2588 mF cm-2 in H2 SO4 electrolytes) without sacrificing gravimetric capacitance (248.8 F g-1 and 323.5 F g-1 ), exhibits excellent cycling ability (without capacity loss after 20 000 cycles), and remarkable tensile strength (42.8 MPa). By direct coupling of two membrane electrodes, the symmetric supercapacitor delivers a prominent areal capacitance of 690 mF cm-2 in KOH and 898 mF cm-2 in H2 SO4 , and remarkable power/energy density (19.98 mW cm-2 /0.096 mW h cm-2 in KOH and 35.01 mW cm-2 /0.244 mW h cm-2 in H2 SO4 ). Additionally, it shows stable behavior in both bent and flat states. These results promote new opportunities for N,P-CNFs/GN/BC paper electrodes as high areal performance, freestanding electrodes for flexible supercapacitors.

Hierarchical three-dimensional FeCo2O4@MnO2 core-shell nanosheet arrays on nickel foam for high-performance supercapacitor

Developing supercapacitors with high energy density, good rate capability, and superior cycle life is crucial to the ever-increasing energy storage devices. However, how to design and develop a new type of faradaic electrode material with large surface area, low resistance, and stable microstructures is still a challenge work. Herein, we have successfully realized a high-energy and long-life supercapacitor by developing nickel foam—hierarchical three-dimensional iron cobaltate nanosheet arrays framework, and covering the surface with thin nanosheets of manganese dioxide (Ni@FeCo2O4@MnO2). The free-standing supercapacitor device based on this structure gives rise to a high energy density of 22.2Whkg−1 at a power density of 978.3Wkg−1, and ∼87.2% of the capacitance can be retained after 5000 cycles of charge-discharge. These performance features are excellent among those reported for iron cobaltate based supercapacitors, making it a promising candidate for high-performance electrochemical energy storage.

A flexible polyaniline/graphene/bacterial cellulose supercapacitor electrode

A polyaniline (PANI)/graphene (GN)/bacterial cellulose (BC) flexible and freestanding supercapacitor electrode is synthesized via a facile chemical polymerization and filtering method.

Large Areal Mass and High Scalable and Flexible Cobalt Oxide/Graphene/Bacterial Cellulose Electrode for Supercapacitors

Flexible energy storage devices require a simple, scalable and general strategy for fabricating high electrochemical performance and mechanically tough flexible electrodes. Herein, sustainable and biological bacterial cellulose (BC) is developed as substrate for Co3O4/graphene (GN), which permits high flexibility (suitable for bending angle of 180°), excellent tensile strength of 63 MPa, good wettability, and especially large mass loading of 9.61 mg cm–2 for a flexible and free-standing supercapacitor electrode. The Co3O4/GN/BC hybrid electrode exhibits both appreciable areal capacitance of 12.25 F cm–2 and gravimetric capacitance of 1274.2 F g–1. Moreover, the remarkable cycling stability with 96.4% capacitance retention after 20000 can be achieved. This study provides a facile procedure to improve the electrochemical performance and mechanical property of flexible supercapacitor electrodes, which are promising candidates for the application of a flexible power source.

Flexible and Freestanding Supercapacitor Electrodes Based on Nitrogen-Doped Carbon Networks/Graphene/Bacterial Cellulose with Ultrahigh Areal Capacitance.

Flexible energy-storage devices based on supercapacitors rely largely on the scrupulous design of flexible electrodes with both good electrochemical performance and high mechanical properties. Here, nitrogen-doped carbon nanofiber networks/reduced graphene oxide/bacterial cellulose (N-CNFs/RGO/BC) freestanding paper is first designed as a high-performance, mechanically tough, and bendable electrode for a supercapacitor. The BC is exploited as both a supporting substrate for a large mass loading of 8 mg cm-2 and a biomass precursor for N-CNFs by pyrolysis. The one-step carbonization treatment not only fabricates the nitrogen-doped three-dimensional (3D) nanostructured carbon composite materials but also forms the reduction of the GO sheets at the same time. The fabricated paper electrode exhibits an ultrahigh areal capacitance of 2106 mF cm-2 (263 F g-1) in a KOH electrolyte and 2544 mF cm-2 (318 F g-1) in a H2SO4 electrolyte, exceptional cycling stability (∼100% retention after 20000 cycles), and excellent tensile strength (40.7 MPa). The symmetric supercapacitor shows a high areal capacitance (810 mF cm-2 in KOH and 920 mF cm-2 in H2SO4) and thus delivers a high energy density (0.11 mWh cm-2 in KOH and 0.29 mWh cm-2 in H2SO4) and a maximum power density (27 mW cm-2 in KOH and 37.5 mW cm-2 in H2SO4). This work shows that the new procedure is a powerful and promising way to design flexible and freestanding supercapacitor electrodes.

Synthesis of chitin nanofibers, MWCNTs and MnO2 nanoflakes 3D porous network flexible gel-film for high supercapacitive performance electrodes

As the porous structure and conductivity result in improvement of electrochemical properties, the chitin nanofibers (ChNFs), multi-walled carbon nanotubes (MWCNTs) and MnO2 (manganese dioxide) nanoflakes 3D porous network core-shell structure gel-film was fabricated for flexible free-standing supercapacitor electrodes. The electrodes were characterized by various techniques and the results demonstrate that the as-synthesized ChNFs/MWCNTs/MnO2 gel-film electrodes exhibits excellent supercapacitive behaviours. The ChNFs/MWCNTs/MnO2 gel-film electrode shows a high capacitance of 295.2mF/cm2 at 0.1mA/cm2 in 1M Na2SO4 aqueous electrolyte because of its 3D porous structure. Furthermore, the electrodes also showed surprising cycling stability for 5000cycles with retention rate up to 157.14% at 1mA/cm2. The data presents great promise in the application of high-performance flexible supercapacitors with the low cost, light-weight and excellent cycling ability.

Scalable synthesis and characterization of free-standing supercapacitor electrode using natural wood as a green substrate to support rod-shaped polyaniline

Natural wood slice was used as a green substrate to support rod-shaped polyaniline via a scalable easily-operated immersion–oxidative polymerization–freeze drying pathway. The scanning electron microscopy observations show that the wood surface was densely covered with plentiful polyaniline nanorods with diameters of 31–72 nm and lengths of 240–450 nm. The analysis of Fourier transform infrared spectroscopy provides further evidence of polyaniline coating onto the wood substrate. Moreover, the analysis of X-ray photoelectron spectroscopy indicates a strong hydrogen bonding between the nitrogen lone pairs $$(\ddot{\text{N}})$$ of polyaniline and the −OH groups of wood, which plays an important role in the interface bonding. This core–shell composite can serve as a free-standing supercapacitor electrode, which shows a high specific capacitance of 304 F g−1 at 0.1 A g−1, a high coulombic efficiency of 93–100 %, and a moderate cyclic stability with a capacitance retention of 72.3 % after 5000 cycles. These make the natural wood a good alternative green substrate to develop novel eco-friendly energy storage devices.

ChemInform Abstract: Synthesis of Two‐Dimensional Materials for Capacitive Energy Storage.

AbstractReview: [186 refs.

Synthesis of Two-Dimensional Materials for Capacitive Energy Storage.

The unique properties and great variety of two-dimensional (2D) nanomaterials make them highly attractive for energy storage applications. Here, an insight into the progress made towards the application of 2D nanomaterials for capacitive energy storage is provided. Synthesis methods, and electrochemical performance of various classes of 2D nanomaterials, particularly based on graphene, transition metal oxides, dichalcogenides, and carbides, are presented. The factors that directly influence capacitive performance are discussed throughout the text and include nanosheet composition, morphology and texture, electrode architecture, and device configuration. Recent progress in the fabrication of 2D-nanomaterials-based microsupercapacitors and flexible and free-standing supercapacitors is presented. The main electrode manufacturing techniques with emphasis on scalability and cost-effectiveness are discussed, and include laser scribing, printing, and roll-to-roll manufacture. Various issues that prevent the use of the full energy-storage potential of 2D nanomaterials and how they have been tackled are discussed, and include nanosheet aggregation and the low electrical conductivity of some 2D nanomaterials. Particularly, the design of hybrid and hierarchical 2D and 3D structures based on 2D nanomaterials is presented. Other challenges and opportunities are discussed and include: control of nanosheets size and thickness, chemical and electrochemical instability, and scale-up of electrode films.

Enhanced electrochemical supercapacitance of binder-free nanoporous ternary metal oxides/metal electrode

Free-standing nanoporous Ni-Cu-Mn mixed metal oxides on metal with a high surface area was fabricated by chemically dealloying a Ni8Cu12Mn80 single-phase precursor, followed by electrochemical oxidation in an alkaline solution. Electrochemical analysis shows that first Cu and Mn-based metal oxides formed by the electrochemical oxidation. Ni-based oxides grow later with the increase of electrochemical CV cycles and mix with the Cu/Mn oxides, forming a relatively stable mixed metal oxides thin film on metal ligament network. Due to the different electrochemical properties of each metal and the synergetic effect between them, the mixed ternary metal oxides formed on metal nano-ligament can operate stably between a wide potential window (1.5V) in 1.0M KOH aqueous solution when tested as a free-standing supercapacitor electrode. Due to the high volumetric surface area, wide operating potential window and excellent conductivity, the nanoporous metal oxides@metal composite exhibits a high volumetric capacitance (∼500Fcm−3), high energy density (∼38mWhcm−3) and good cycling stability.

3D Graphene-Nickel Hydroxide Hydrogel Electrode for High-Performance Supercapacitor

3D graphene-based frameworks with interpenetrating macroporous structures have attracted great interests recently since they can serve as robust matrix for accommodating guest nanoparticles for use in a wide range of applications. Here, an adsorption-hydrothermal strategy is adopted for the in-situ growth of Ni(OH)2 nanoplates using three dimensional (3D) nitrogen-containing graphene hydrogel (NG) as the substrate. The NG/Ni(OH)2 nanocomposite hydrogel thus obtained is explored as the monolithic free-standing supercapacitor electrode without adding any other binders or conductive additives. The 3D hierarchical structure of the NG/Ni(OH)2 nanocomposite can not only provide a large accessible surface area, but also facilitate ion diffusion and charge transport for much improved supercapacitive performance. The gel with Ni(OH)2 loading of ∼40% achieves a high specific capacitance of 782Fg−1at the current density of 0.2Ag−1, which equals to a specific capacitance of 1748Fg−1 based on the mass of Ni(OH)2 alone. Excellent cycling stability of only 10% capacitance loss after 10000 cycles is also achieved due to the robust adhesion between the metal hydroxide and nitrogen containing graphene. Furthermore, high capacitance retention of ∼80% can be achieved when the current density is increased 100 fold from 0.2 to 20Ag−1.