Free-standing Supercapacitor Electrode Research Articles

Tannic Acid-Assisted Fabrication of N/B-Codoped Hierarchical Carbon Nanofibers from Electrospun Zeolitic Imidazolate Frameworks as Free-Standing Electrodes for High-Performance Supercapacitors

An effective synthetic route has been reported to prepare N/B co-doped hierarchical carbon nanofibers (NB-HCNFs) as free-standing electrodes for high-performance supercapacitors. Zeolitic imidazolate framework (ZIF-8) nanoparticles were embedded into polyacrylonitrile (PAN) through electrospinning to obtain PAN/ZIF-8 nanofibers. Tannic acid (TA) acted as a coating layer for PAN/ZIF-8 to generate hollow ZIF-8 core structures within the fiber and an intermediate to coordinate with 1,4-benzenediboronicacid (BDBA). After carbonization, the obtained flexible N/B co-doped hierarchical porous carbon nanofibers were used as free-standing electrodes for supercapacitors. Thus, unique nanostructure and the existence of heteroatoms could offer remarkably improved electrochemical properties with a high specific capacitance (288.2 F g−1 at a current density of 1 A g−1) and good cycling stability (96.9% capacitance retention over 8000 cycles at 10 A g−1). In addition, the NB-HCNFs films were assembled into symmetric supercapacitors, which displayed a high energy density and excellent stability (99.7% capacitance retention after 8000 cycles at 10 A g−1). The synthetic method might provide an effective and facile strategy to prepare a variety of hierarchical doped carbon nanomaterials for energy storage.

Swallow‐Nest‐Inspired Strategy towards Ultralight Functional Multiwall‐Carbon‐Nanotube‐Based Aerogels for Supercapacitors

AbstractElectrode materials with superior electrochemical properties are at the core of energy‐conversion and storage devices such as supercapacitors. Here, an ultralight functionalized multiwall‐carbon‐nanotube (FMWCNT) based aerogel with swallow nest structure is fabricated by a simple and facile method, and further used as a freestanding electrode for supercapacitors. Besides, soft X‐ray absorption near‐edge structure measurements were performed to probe the valence bonds of the as‐prepared FMWCNT aerogel, revealing the existence of chemical interactions among the atoms. More importantly, a series of experiments including cyclic voltammetry, galvanostatic charge‐discharge cycling, and rate capability tests were performed to evaluate the properties of the as‐prepared FMWCNT aerogel electrodes. Impressively, the specific capacitance of the electrodes was as high as 319 F g−1 in 0.5 M H2SO4 at a constant current density of 10 A g−1, and no decay was observed after 2000 cycles. These superior properties can be attributed to the unique interconnected nanostructure, large specific surface area, and synergistic effect of the FMWCNT aerogel.

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.

Oxygen vacancy-engineered Fe2O3 nanoarrays as free-standing electrodes for flexible asymmetric supercapacitors.

The charge storage performance of Fe2O3 nanoarrays (NAs) as negative electrodes are limited by their poor conductivity and rate capability. Herein, we have reported the delicate interfacial engineering on carbon cloth (CC) fibers and oxygen vacancy (VO) generation on Fe2O3 nanorod arrays to boost the capacitive performance. Polydopamine-derived nitrogen-doped carbon layers were fabricated on CC fibers to govern the growth of FeOOH NAs. Rich VOs were generated in Fe2O3 NAs to construct a unique heterostructure with a crystalline core and amorphous shell via successive N2 thermal treatment and chemical reduction. Optimized by 2 h chemical reduction, the VO-rich Fe2O3 NA electrode, featuring a charged voltage of -1.10 V, exhibited a high areal specific capacitance of 2.63 F cm-2 at 0.5 mA cm-2 and 0.12 F cm-2 even at 60 mA cm-2. Impressively, 86.7% specific capacitance was retained after 10 000 cycles at 10 mA cm-2. The flexible asymmetric supercapacitor by assembling free-standing CN-Fe2O3-2 h (negative electrode) and MnO2 (positive electrode) showed an energy density of 1.33 mW h cm-3 at 15.4 mW cm-3. To the best of our knowledge, these results are the record performance for Fe2O3-based electrodes. The two-step interfacial engineering reported in this study may open a new door in the design of high energy-density electrodes for advanced energy storage.

High electroactive material loading on a carbon nanotube/carbon nanofiber as an advanced free-standing electrode for asymmetric supercapacitors.

3D hierarchical nanocomposites always lead to excellent electrochemical properties. In this research, we have successfully designed and synthesized a hierarchical three-dimensional cobalt sulfide encapsulated carbon nanotube/carbon nanofiber (Co9S8@CNT/CNF) composite, resulting in a high-performance supercapacitor with an unrivalled capacitance of ≈1580 F g-1 at a current density of 1 A g-1. An asymmetric supercapacitor (ASC) based on these 3D hierarchical Co9S8@CNT/CNF composites exhibits a maximum energy density of 58 W h kg-1 at a power density of 1000 W kg-1 and maintains it at 38 W h kg-1 at a higher power density of 16.7 kW kg-1. Even after 10 000 charge/discharge cycles, the ASC still retains 93% of the initial capacitance at a current density of 5 A g-1. A large number of aligned CNF nanochannels combined with the synergistic effects of cobalt sulfide nanowires encapsulated in CNTs obviously accelerates the ion transfer speed and shortens the ion transfer distance, thereby boosting both power density and energy density.

Porous multishelled NiO hollow microspheres encapsulated within three-dimensional graphene as flexible free-standing electrodes for high-performance supercapacitors.

Exploration of electrode materials with well-defined nanostructures and good flexibility is an efficient approach for achieving high-performance and flexible energy storage systems. However, it is still challenging to well integrate active materials into flexible electrodes and simultaneously maintain satisfactory electrochemical performance. Herein, we successfully synthesize novel three-dimensional graphene (3DG)-encapsulated porous multishelled NiO hollow microsphere (3DG/pMS-NiO) composite aerogels via a modified self-templating method and a dopamine (DA)-assisted self-assembly route. The well-designed highly interconnected porous 3DG network and the close contact NiO-graphene structure of the 3DG/pMS-NiO composite aerogels offer multiple advantages such as high porosity and accessible area, improved conductivity, enhanced electrolyte diffusion and a simple electrode preparation process. Thus, the as-prepared flexible 3DG/pMS-NiO electrodes showed significantly improved specific capacitance of 710.4 F g-1 at 0.5 A g-1 and excellent rate capability with an ultrahigh capacitance retention of 92.5% at 10 A g-1. In addition, the fabricated asymmetric supercapacitors (3DG/pMS-NiO//AC) showed a high specific capacitance of 34.4 F g-1 at 1 A g-1 with a voltage window of 0-1.6 V, a large energy density of 12.3 W h kg-1 at a power density of 815.3 W kg-1, and a decent cycling stability. This work profoundly enlightens the material design and electrode preparation, and even opens up an avenue for the development of high-performance and flexible energy storage systems.

Flexible all-solid-state supercapacitors of polyaniline nanowire arrays deposited on electrospun carbon nanofibers decorated with MOFs

Porous carbons derived from metal–organic frameworks (MOFs) are promising materials for a number of energy- and environment-related applications. To integrate the powder MOFs-derived carbon into feasible engineered materials, a facile strategy to fabricate integrated flexible film is developed by growing MOFs nanoparticles on polyimide electrospun nanofibers, followed by calcination, to fabricate freestanding carbon nanofiber membranes decorated with porous carbon. Then vertically polyaniline nanowire arrays are uniformly deposited on the hierarchical porous carbon substrates by in situ polymerization. Thanks to the good distribution of MOFs-derived porous carbon on carbon nanofibers and the compact configuration interwoven by conducting polymers, the designed hybrid electrode could be used directly as a freestanding electrode for supercapacitors, which displayed a high specific capacitance of 1268 F g−1. The assembled flexible solid-state supercapacitor based on the integrated electrodes demonstrated a high volumetric capacitance of 1973 mF cm−3 and a good capacitance retention of 84.9% after 10 000 cycles, which could power a commercial light emitting diode. This strategy may shed light on the design of MOFs-based flexible materials for practical applications of supercapacitors and other electrochemical devices.

Construction of hierarchical Co-Ni-S nanosheets as free-standing electrode for superior-performance asymmetric supercapacitors

Co-Ni-S nanoarchitectures with considerable conductivity and superior electrochemical performance have been successfully designed and grown on Ni foam substrates. The collective effects of structure and composition endow Co-Ni-S electrodes better electrochemical performance with ultra-high areal capacitances of 6.3 F cm−2 at 1 mA cm−2 (4.2 F cm−2 retained at 8 mA cm−2) and remarkable rate capability of 65% at 10 mA cm−2. Correspondingly, asymmetric supercapacitors are assembled with Co-Ni-S sample and active carbon as the positive and negative electrodes, which display an excellent energy density of ∼50 Wh kg−1 at a power density of ∼0.783 kW kg−1. Furthermore, the capacitance maintains 71% of the initial value after cycling 6000 times at 1 A g−1. These results have great significance for the optimization of asymmetric supercapacitors as the energy-storage devices for the next generation.

Facile Activation of Commercial Carbon Felt as a Low-Cost Free-Standing Electrode for Flexible Supercapacitors.

High cost, low capacitance, and complicated synthesis process are still the key limitations for carbon-negative materials to meet their industrial production and application in high-energy-density asymmetric supercapacitors (ASCs). In this work, we demonstrate the facile preparation of ultrahigh-surface-area free-standing carbon material from low-cost industrial carbon felt (CF) and its application for flexible supercapacitor electrode with outstanding performance. Through a simple freeze-drying-assisted activation method, the as-prepared activated CF (ACF) was endowed with satisfactory flexibility, ultrahigh specific surface area of 2109 m2 g-1, good electric conductivity (311 S m-1), and excellent wettability to aqueous electrolyte. Owing to these merits, the ACF expressed an ultrahigh areal capacitance of 1441 mF cm-2, a high specific capacitance ( Cs) of 280 F g-1 based on the mass of the whole electrode, and an impressive cycling stability (87% retention after 5000 cycles). When applied as a flexible freestanding electrode for MnO2//ACF ASCs, the ACF-based device provided satisfactory areal energy densities of 0.283 and 0.104 mWh cm-2 in aqueous and quasi-solid electrolytes, respectively. The values outperform many previously reported carbon-based electrochemical devices. The low cost of raw material and the facile fabrication process, together with the high electrochemical performance, make our ACF electrode highly applicable for the mass production of flexible energy-storage devices.

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.

PANI coated microporous graphene fiber capable of subjecting to external mechanical deformation for high performance flexible supercapacitors

Graphene based composites have been recognized as one of the most important classes of free-standing electrodes for supercapacitors. Herein, one-dimensional microporous graphene oxide fiber (MG) was designed with polystyrene sulfonate (PSS) microspheres as hard-template through a simple mass production method. The MG fiber exhibited outstanding flexibility and excellent tensile strength of 183.7 MPa. Then the polyaniline (PANI) coated microporous graphene (MGP) fibers were prepared by in-situ electropolymerization of aniline. Benefiting from the unique microporous structure, the optimized MGP fiber electrode exhibited remarkable electrochemical characteristics, such as highest specific capacitance of 36.95 mF/cm at current density of 0.25 mA/cm (204.2 F/cm3 at current density of 1.38 A/cm3), as well as excellent capacitance retention of 96.31% after 2000 cyclic voltammetry cycles. More than that, the MGP fiber electrode remained high retention rate of its initial specific capacitance even under tensile stress or bending, indicating its good tolerance to external damage.

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.

Porous polylactic acid/carbon nanotubes/polyaniline composite film as flexible free-standing electrode for supercapacitors

Green polymer was used flexible substrate for sustainable energy storage in the present work. Namely, degradable and flexible free-standing composite film electrodes were designed with porous polymer-based nanocomposite films (polylactic acid (PLA)/carbon nanotubes (PLA/CNTs)) as conductive and degradable substrates for supercapacitors, via facile in-situ chemical oxidation polymerization of aniline with rapid-mixing chemical oxidation polymerization technique. The effect of the synthetic condition on the electrical conductivity and electrochemical properties of the resultant nanocomposite films was investigated. The porous polylactic acid/carbon nanotubes/polyaniline (P-PCP) sample P-PCP-3-H-2, which was prepared with an aniline concentration of 0.1 mol L−1 in presence of the porous polylactic acid/carbon nanotubes (P-PLA/CNTs) composite film after heat-treating at 60 °C for 1 h, possessed the best comprehensive performance, excellent mechanical strength with tensile strength of 18.7 MPa, highest specific capacitance of 510.3 F g−1 in 1.0 mol L−1 sulfuric acid electrolyte, good cycling stability of 111.5% after 2000 cycles and favorable flexible stability of 93.4% after 180º-bending for 700 cycles.

Facile construction of Co(OH)2@Ni(OH)2 core-shell nanosheets on nickel foam as three dimensional free-standing electrode for supercapacitors

Fabricating three dimensional (3D) free-standing electrode is considerable attractive in designing supercapacitors of high energy and power densities. Co(OH)2@Ni(OH)2 core-shell nanosheets have been successfully grown on nickel foam (NF) as 3D free-standing electrode (denoted as NCN) for electrochemical capacitors via a facile stepwise electrodeposition process. It is found that the unique 3D free-standing frameworks combined with the core-shell nanosheet-like heterostructure endow the as-prepared electrode good electrochemical performances. When tested as a supercapacitor electrode, it delivers a capacitance of 1520.6 F g−1 at 1 A g−1 and still keeps 1376.6 F g−1 at 10 A g−1, exhibiting excellent rate performance. Moreover, it retains ∼71.4% initial capacitance after 1500 charging/discharging cycles at 5 A g−1, demonstrating good cycling stability. The good electrochemical performance of the NCN electrode makes it a promising electrode candidate for high performance supercapacitors.

Mixed-valent CoxO–Ag/carbon nanofibers as binder-free and conductive-free electrode materials for high supercapacitor

The composite materials, carbon nanofibers-based cobalt oxide and Ag (CoxO–Ag/CNFs) were fabricated through a facile electrospinning method followed by calcination. They were Co2+–Ag/CNFs, Co2+@Co3+–Ag/CNFs. And the Co2+@Co3+–Ag/CNFs sample was obtained through transform Co2+–Co3+ partially by the catalytic oxidation performance of silver, instead of the traditional high-temperature calcination process. Meanwhile, the high conductive of Ag can promote the transportation of electrons and ions between the electrode and electrolyte. The flexible composite materials as free-standing and additive-free film electrodes for supercapacitors. In virtue of the presence of Ag, the composite materials have better conductivity and supercapacitor performance than Co/CNFs-based electrode. Interestingly, the sample of Co2+–Ag/CNFs has a higher capacitors than Co2+@Co3+–Ag/CNFs sample, that indicates the supercapacitor performance has an important relationship with the valence state of cobalt.

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.

Facile deposition of polyaniline on the multi-walled carbon nanotubes/polyvinyl chloride composite films as flexible and robust electrodes for high performance supercapacitors

Novel flexible and robust polyaniline/multi-walled carbon nanotubes/polyvinyl chloride (PANI/MWCNTs/PVC) composite films were designed as the free-standing electrodes for high performance supercapacitors, by deposition of polyaniline (PANI) via facile in-situ chemical oxidative polymerization of aniline on the multi-walled carbon nanotubes/polyvinyl chloride (MWCNTs/PVC) composite films, in which MWCNTs were introduced into the PVC matrix in order to improve its mechanical property and electrical conductivity. The MWCNTs/PVC composite film with MWCNTs content of 5% was optimized as the conductive flexible substrate, possessing a tensile strength of 44.1 MPa. The electrochemical tests showed that the PANI/MWCNTs/PVC-2 composite film electrode prepared by the rapid-mixing chemical oxidative polymerization exhibited better electrochemical performance than the PANI/MWCNTs/PVC3-1 composite film electrode prepared by the conventional chemical oxidative polymerization, i.e. specific capacitance of 801.1 F g−1 at a current density of 1 A g−1, rate capacitance retaining 66.6% of its original value at high current density of 10 A g−1 and 54.9% at scan rate of 100 mV s−1, and long-term electrochemical cyclic stability with a retention of 90.6% after 2000 cycles of CV test at a scan rate of 100 mV s−1. Furthermore, the PANI/PVC/MWCNTs-2 film electrode showed excellent tensile and tear-resistance performance and flexibility, indicating its promising application as bendable and robust electrode for energy storage devices.

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.

Extremely low fractions of graphene oxide in carbon foam prepared by a spin-coating method as freestanding supercapacitor electrodes

Three-dimensional carbon foam (CF) has been widely employed as a conductive framework for preparing self-supporting supercapacitor electrodes. Herein, based on the spin-drying mechanism of washing machine, a spin-coating method is developed to prepare carbon foam@graphene oxide (CF@GO) composites, in which a special centrifugation tube containing a macroporous shelf is used to separate the foam block and residual GO suspension during centrifugation. The GO fractions in CF@GO composites are controlled easily by tuning the spin speeds. As freestanding electrodes in two-electrode supercapacitors, the CF@GO-4500 (4500 rpm, 1.2 wt% GO) and CF@RGO-1000 electrodes (1000 rpm, 3.2 wt% GO) exhibit the specific capacitances of 159 Fg−1 (258 mF cm−2) and 180 Fg−1 (297 mF cm−2) at 0.5 Ag−1, respectively, which are much higher than those of CF and CF@GO-4 or CF@RGO-4 sample (15.3 wt% GO) prepared by a conventional soaking–freeze-drying method. The results show that a few GO/RGO sheets loading in CF framework exhibit an enhanced electrochemical performance in supercapacitor, rather than filling much GO/RGO.

Sulfur-Deficient Bismuth Sulfide/Nitrogen-Doped Carbon Nanofibers as Advanced Free-Standing Electrode for Asymmetric Supercapacitors.

The use of free-standing carbon-based hybrids plays a crucial role to help fulfil ever-increasing energy storage demands, but is greatly hindered by the limited number of active sites for fast charge adsorption/desorption processes. Herein, an efficient strategy is demonstrated for making defect-rich bismuth sulfides in combination with surface nitrogen-doped carbon nanofibers (dr-Bi2 S3 /S-NCNF) as flexible free-standing electrodes for asymmetric supercapacitors. The dr-Bi2 S3 /S-NCNF composite exhibits superior electrochemical performances with an enhanced specific capacitance of 466 F g-1 at a discharge current density of 1 A g-1 . The high performance of dr-Bi2 S3 /S-NCNF electrodes originates from its hierarchical structure of nitrogen-doped carbon nanofibers with well-anchored defect-rich bismuth sulfides nanostructures. As modeled by density functional theory calculation, the dr-Bi2 S3 /S-NCNF electrodes exhibit a reduced OH- adsorption energy of -3.15 eV, compared with that (-3.06 eV) of defect-free bismuth sulfides/surface nitrogen-doped carbon nanofiber (df-Bi2 S3 /S-NCNF). An asymmetric supercapacitor is further fabricated by utilizing dr-Bi2 S3 /S-NCNF hybrid as the negative electrode and S-NCNF as the positive electrode. This composite exhibits a high energy density of 22.2 Wh kg-1 at a power density of 677.3 W kg-1 . This work demonstrates a feasible strategy to construct advanced metal sulfide-based free-standing electrodes by incorporating defect-rich structures using surface engineering principles.