Free-standing Supercapacitor Electrode Research Articles

Novel In Situ Synthesis of Freestanding Carbonized ZIF67/Polymer Nanofiber Electrodes for Supercapacitors via Electrospinning and Pyrolysis Techniques.

Zeolitic imidazolate framework-67 (ZIF67) has been regarded as an effective energy storage material due to its high surface area and electroactive cobalt center. Carbonizing ZIF67 can enhance electrical conductivity by converting 2-methylimidazole (2-melm) to carbon with cobalt doping. In this work, a novel in situ electrospinning is proposed to fabricate carbonized ZIF67 on carbon fiber (C67@PAN-OC) as a freestanding supercapacitor electrode. Polyacrylonitrile solution containing a cobalt precursor is used for electrospinning, and produced fibers are immersed in 2-melm to form ZIF67. Individually grown carbonized ZIF67 on carbon fiber is obtained using the in situ electrospinning method, while the one-body mixed carbon electrode is formed using the ex situ electrospinning method. A highest specific capacitance (CF) of 386.3 F/g at 20 mV/s is obtained for the in situ synthesized C67@PAN-OC electrode due to the largest electrochemical surface area and the smallest resistance, while the ex situ synthesized electrode only shows a CF value of 27.7 F/g. A symmetric supercapacitor (SSC) assembled using the optimized C67@PAN-OC electrodes and gel electrolytes shows a maximum energy density of 9.64 kWh/kg at 0.55 kW/kg and a CF retention of 59.5% after 1000 times charge/discharge process. A CF retention of 75.6% after bending 100 times is also obtained for SSC.

Self-organized hierarchically porous carbon coated on carbon cloth for high-performance freestanding supercapacitor electrodes

In this work, a facile method is proposed for the fabrication of self-organized hierarchically porous carbon coated on carbon cloth. The method is based on the thermal treatment of potassium citrate gel, which is derived from mixture of citric acid and potassium hydroxide. The as-prepared porous carbon derived from citrate gel generally covers carbon cloth with fine morphological properties and the sample can be directly used as an electrode material for supercapacitor. In a three-electrode configuration, the optimized electrode exhibits a remarkable specific capacitance of 379.5F g−1 at a current density of 1 A g−1. In addition, a good cycle stability with capacity retention of 97% is achieved after 10,000 cycles at a current density of 10 A g−1. Moreover, a symmetric supercapacitor is prepared by using the as-prepared electrode. The supercapacitor shows a high energy density of 0.69 mWh cm−3 at a power density of 4.91 mW cm−3 and good cycle durability and mechanical flexibility. The porous carbon material reported in this work shows potential as electrode for flexible all-solid-state supercapacitors.

Facile fabrication of hierarchically porous graphene/poly(1,5-diaminoanthraquinone) nanocomposite fibers as flexible and robust free-standing electrodes for solid-state supercapacitors

BackgroundFiber-shaped supercapacitors have attracted more and more interests for portable/wearable electronics. However, the practical application is suffering from their tedious fabrication and limited electrochemical performance. MethodsHere, hierarchically porous graphene/poly(1,5-diaminoanthraquinone) nanocomposite (ErGO/PDAA) fibers were fabricated as flexible and robust free-standing electrodes for supercapacitors, via facile coagulation of expanded graphene oxide (EGO) and PDAA nanomaterials with different morphologies, such as nanoparticles (PNPs), nanotubes (PNTs), and nanoribbons (PNRs), in CaCl2 bath, followed by reduction with hydroiodic acid (HI). Significant findingsAll the three ErGO/PDAA fibers showed distinctly enhanced electrochemical property than the ErGO fiber. Furthermore, with the same feeding ratio of EGO and PDAA (6:4), the ErGO/PNTs fiber possessed better electrochemical capacitive property than the other two, ErGO/PNPs and ErGO/PNRs fibers, owing to the unique hierarchical pores. The solid-state symmetric supercapacitor (SSC) possessed specific capacitance, energy and power densities of 35.0 mF cm−1, 3.11 mWh cm−1, and 61.4 mW cm−1, respectively, and retained 83.3% of its initial capacitance after 5000 cycles.

Hierarchical urchin-like amorphous carbon with Co-adding anchored on nickel foam: A free-standing electrode for advanced asymmetrical supercapacitors and adsorbed Pb (II)

The booming development of carbon materials is of great value for diverse applications, owing to their superior electron conductivity, unique structures, and excellent cycle lifetime. This study presents two hierarchically structured amorphous carbon materials for asymmetric supercapacitor (ASC) device: i) the MOFs-derived urchin-like amorphous carbon anchored on nickel foam (UAC@NF) as positive electrode; ii) high temperature activated graphite carbon felt (GF500) as negative electrode. This ASC device achieves a higher energy density of 0.036 mWh cm−3 at a power density of 0.984 mW cm−3 and demonstrates better cycling performance with 91.4% capacitance retention after 10,000 cycles, compared with the other carbon-based supercapacitor. In addition, the UAC@NF after 10,000 cycles displays much better adsorption performance for Pb (II) compared with the unused UAC@NF. We have demonstrated the relationship between carbon materials’ structure and performance by combining experiment and theoretical calculation. Predominantly, our work can provide a new direction for the common development of amorphous carbon materials in the field of energy and environment.

N-rich reduced graphene oxide film with cross-coupled porous networks as free-standing electrode for high performance supercapacitors

Graphene films suffer from poor electrochemical performance due to the undesirable ion diffusion inside its deep area and only electrical double layer absorption mechanism. Here, we proposed a freeze casting method for fabricating the N-rich holey reduced graphene oxide films (Freeze-dried N-HRGO) bearing cross-coupled porous ion diffusion networks. The H2O2 etching and modified vacuum filtration process generate abundant pore structures inside films, which act as ion diffusion path for promoting electrochemical performance. In addition, the N sites serve as pseudocapacitance contributor to react with electrolyte ions, which greatly enhance the specific capacitance. As a result, the freeze-dried N-HRGO films show a high specific capacitance of 528 F g−1 at a current density of 1 A g−1. When assembled into symmetrical supercapacitor, it delivers a high energy density of 14.0 Wh kg−1 and excellent cycle life (89.3% capacity retention after 5000 cycles at 2 A g−1).

MXene loaded onto clean wiper by a dot-matrix drop-casting method as a free-standing electrode for stretchable and flexible supercapacitors

MXene-loaded textiles can be used as free-standing electrodes for multi-functional supercapacitors (SCs), due to their good conductivity, high electrochemical activity, excellent flexibility, and stretchability. However, the typically self-restacking phenomenon would emerge with the increase in MXene amount, resulting in a significant reduction of the ion-accessible surface areas and active sites of MXene. Herein, a dot-matrix drop-casting method with the advantages of quickness and high uniformity is developed to load high amount (6.1 mg cm−2) of Ti3C2 (a typical MXene) on a clean wiper (CW, a typical textile), and simultaneously to alleviate the resultantly self-restacking phenomenon of Ti3C2. The results show that the obtained composite (CWM) has a high conductivity of 2.3 S cm−1 and large specific capacitance of 282 mF cm−2 at 1 mA cm−2, whereas it remains a high tensile strength of 20.1 MPa and developed porous structure (33.4 m2 g−1). These attractive properties make CWM an ideal free-standing electrode. Further, CW is used as a separator to fabricate an all-CW-derived SC (CWM//CW//CWM), because its porous structure provides abundant routes for facile ion transport. Such a SC has an excellent rate capability with a high capacitance retention of 78.2% from 1 to 20 mA cm−2, large specific capacitance of 118 mF cm−2 at 1 mA cm−2, and high energy density of 10.1 μWh cm−2 at 389.9 μW cm−2. Moreover, an all-solid-state SC (CWM//CWM) is fabricated, and shows an excellent stability on capacitive behaviors during bending (0–120°) or stretching (10–40% strains), manifesting its great potential on wearable and portable electronics.

Fabrication of rGO/CoSx-rGO/rGO hybrid film via coassembly and sulfidation of 2D metal organic framework nanoflakes and graphene oxide as free-standing supercapacitor electrode

Porous cobalt sulfide (CoSx)/reduced graphene oxide (rGO) hybrid films has been fabricated as a flexible freestanding supercapacitor electrode via the co-assembly and sulfidation of 2D metal organic framework (MOF) nanoflakes and graphene oxide (GO). Firstly, zeolitic imidazolate-67 (ZIF-67) nanocubes were added into the aqueous solution of GO to yield a mixed dispersion of ZIF-67 and GO (ZIF-GO). It was found that the morphology of ZIF-67 changed from nanocubes to 2D nanoflakes owing to the concentration change, which favored the formation of hybrid film. Secondly, the sandwich-like GO/ZIF-GO/GO hybrid film was fabricated by the successive vacuum membrane filtration of GO, ZIF-GO, and GO solutions. Finally, the hybrid film was sulfidized via a hydrothermal process using thioacetamide as the sulfur source. This process also led to the reduction of GO to rGO. The resulting rGO/CoSx-rGO/rGO hybrid film was shown to have good electrochemical performance because it combined the good pseudocapacitor property of cobalt sulfide as well as the good conductivity and electric double layer capacitor property of rGO. In addition, an all-solid-state asymmetric supercapacitor (aSC) was assembled using rGO/CoSx-rGO/rGO hybrid film and active carbon as the positive and negative electrodes, respectively. It exhibited an energy density of 10.56 Wh kg−1 and a power density of 2250 W kg−1. Also, it retained 92.8% of initial capacitance after 10,000 cycles. The good electrochemical performance revealed that the resulting aSC has great potential in the practical application of supercapacitors.

Mechanically Strong Double-Layered Aramid Nanofibers/MWCNTs/PANI Film Electrode for Flexible Supercapacitor

As the key material for flexible electronics, free-standing supercapacitor electrodes have attracted great research interests in recent years. However, constructing the novel supercapacitor electrodes with excellent mechanical properties and high areal capacitance remains a challenge. This study demonstrates the fabrication of a highly flexible and mechanically robust supercapacitor electrode based on combination of aramid nanofibers (ANFs), multi-walled carbon nanotubes (MWCNTs), and polyaniline (PANI) in form of nanostructured composite film. The ANFs/MWCNTs/PANI film electrode obtained via vacuum-assisted layer-by-layer (VA-LBL) assembly technique achieves a tensile strength of 158.7 MPa and an areal specific capacitance of 497.3 mF cm−2 at a current density of 0.25 mA cm−2, showing an outstanding compatibility between high electrochemical performance and superior mechanical properties. In addition, the ANFs/MWCNTs/PANI film electrode exhibits an excellent cycle life with 89% capacitance retention after 3000 cycles. Synergistic effect from the strategic assembly of ANFs, MWCNTs and PANI in form of a single nanostructured platform makes this kind of free-standing electrode applicable for flexible and solid-state energy storage systems.

α-MnO2 nanotube@δ-MnO2 nanoflake hierarchical structure on three-dimensional graphene foam as a lightweight and free-standing supercapacitor electrode

Three-dimensional graphene foam (3D GF) is one of the promising candidates due to its fascinating characteristics. In the paper, we reported a simple hydrothermal method to fabricate hierarchical α-MnO2 nanotube@δ-MnO2 nanoflake on GF (GF@MNT@MNF) as a lightweight and free-standing electrode of supercapacitor. The 3D GF could serve as an ideal supporter to grow MnO2 active material, and MnO2 mass percentage affects significantly on the electrochemical properties of the composite electrode. When the mass percentage of MnO2 is 60%, a maximum specific capacitance of 202 F·g−1 with respect to the entire electrode and 336 F·g−1 relative to active material is achieved. The GF@MNT@MNF electrode presents a remarkable cyclic stability (maintaining about 97% of original capacitance over 5000 cycles). The excellent electrochemical performance of the GF@MNT@MNF electrode is resulted from the combination of the lightweight, flexible, conductive GFs and MnO2 hierarchical structure with large contacting interface and proper space distribution. Furthermore, an asymmetric supercapacitor based on GF@MNT@MNF composites was assembled, which showed a great energy density of 23.2 Wh·kg−1 under a power density of 119.9 W·kg −1 in wide potential range of 0–1.8 V. The research may provide a method for lightweight, free-standing and high-performance electrode material for efficient and convenient energy storage device.

Poly(Ether Amide)-Derived, Nitrogen Self-Doped, and Interfused Carbon Nanofibers as Free-Standing Supercapacitor Electrode Materials

For free-standing and self-doped electrode materials of energy storage devices, in this study, we investigate the microstructures and electrochemical properties of aromatic poly(ether amide) (PEA)-...

Anchoring sea-urchin-like Co(OH)2 microspheres on nickel foam as three-dimensional free-standing electrode for high-performance supercapacitors

Transition metal hydroxides have emerged as promising materials in supercapacitor. In this work, a free-standing supercapacitor electrode was prepared. Sea-urchin-like Co(OH)2 microspheres were hydrothermally deposited on a Ni foam substrate, showing a hierarchical 3D free-standing structure. The 3D framework endows the as-prepared electrode with improved supercapacitive performances. The influence of the hydrothermal time on the structure and electrochemical performances of the resulting products are investigated systematically. It delivers a capacitance of 1916 F/g (15.9 F/cm2) and 699 F/g (5.8 F/cm2) at 10 mA/cm2 and 100 mA/cm2, respectively, exhibiting excellent rate performance. The capacitance retains about 79.3% after 5000 charging-discharging cycles at 80 mA/cm2. Various ill-defined rod-like and stacked plate-like CoOOH materials are obtained from the electrodes after the cycles, which is responsible for the property deterioration of Co(OH)2/Ni foam.

Flexible Supercapacitors: Ultralight Flexible Electrodes of Nitrogen‐Doped Carbon Macrotube Sponges for High‐Performance Supercapacitors (Small 1/2021)

In article number 2004827, Min Fu, Ruitao Lv, Yu Lei, and Mauricio Terrones build ultra-light, flexible and three-dimensional freestanding electrodes for supercapacitors. Homogenous deposition of NiFe2O4 nanoparticles from vapor diffusion-precipitation on N-doped carbon sponges result in a composite material which enables rapid charge and ionic transport.

Boosting the volumetric capacitance of MoO3-x free-standing films with Ti3C2 MXene

The high theoretical capacitance of molybdenum trioxide (MoO3) renders it an attractive supercapacitor electrode material. However, its low electronic conductivity restricts charge transfer and slows its reaction kinetics. Herein, we vacuum filtered porous, free-standing, flexible and highly conductive films comprised of oxygen vacancy-rich MoO3-x nanobelts and delaminated Ti3C2 MXene in a mass ratio of 80:20, respectively. When tested as supercapacitor electrodes, in a 5 M LiCl electrolyte, volumetric capacitances of 631 F cm−3 at 1 A g−1, and 474 F cm−3 at 10 A g−1 were obtained. To increase the energy density, asymmetric supercapacitors, wherein the anodes were MoO3-based and the cathodes were nitrogen-doped activated carbon were assembled and tested. The resulting volumetric energy density was 48.6 Wh L−1. After 20,000 continuous charge/discharge cycles at 20 A g−1, 96.3 % of the initial charge remained. These values are outstanding for free-standing supercapacitor electrodes, especially in aqueous electrolytes.

Fabrication and electrochemical properties of flow-through polypyrrole and polypyrrole/polypyrrole nanoarrays

Flow-through polypyrrole (PPY) nanoarrays are well tailored into nanopore, nanotube-in-nanopore and nanorod-in-nanopore structures to act as free-standing supercapacitor electrodes through anodization, electro-polymerization deposition and chemical corrosion processes. Independent TiO2 nanotube array is prepared to act as well-aligned and ordered temperate by an anodic oxidation of titanium sheet. Polypyrrole (PPY) was selectively coated on the external surface and/or internal surface of TiO2 nanotubes to form PPY/TiO2 and PPY/TiO2/PPY through a controlled pulse voltammetry electrodeposition process. Polypyrrole (PPY)/ Polypyrrole (PPY) nanotube-in-nanopore and nanorod-in-nanopore arrays can be formed by HF corrosion dissolution of TiO2 template from PPY/TiO2/PPY. Alternatively, PPY nanopore array can be formed by removing TiO2 template from PPY/TiO2. Polypyrrole (PPY) nanopore array has the pore diameter of 130–200 nm and wall thickness of 25–40 nm. Polypyrrole (PPY) Polypyrrole (PPY)/Polypyrrole (PPY) nanotube-in-nanopore array has the pore diameter of 135–225 nm, nanopore wall thickness of 25–45 nm and nanotube diameter of 90–135 nm. Polypyrrole (PPY)/ Polypyrrole (PPY) nanorod-in-nanopore array has the pore diameter of 115–215 nm, nanopore wall thickness of 20–35 nm and nanorod diameter of 85–135 nm. All these PPY and PPY/PPY nanoarrays have the total length of 0.9–1.1 μm and keep flow through and ordered alignment structure contributing to feasible electrolyte ion diffusion. Electroactive PY nanopore, PPY/PPY nanotube-in-nanopore and PPY/PPY nanorod-in-nanopore arrays directly become free-standing electrodes to achieve the specific capacitance of 13.2, 16.7 and 18.3mF cm−2 at 0.25 mA cm−2 in 1.0 M H2SO4 electrolyte and the rate capacity retention of 77.3, 77.2 and 77.0% at 1.5 mA cm−2. Well-tailored and flow-through PPY and PPY/PPY nanoarrays can act as the promising polymer electrode materials for electrochemical energy storage.

Fabrication of hierarchical Zn–Ni–Co–S nanowire arrays and graphitic carbon nitride/graphene for solid-state asymmetric supercapacitors

The rational design and subsequent growth of apposite nanostructured transition metal sulfide on conductive scaffold with auspicious morphology to produce an advanced electrode material for supercapacitors is highly desirable to boost the electrochemical performance. Herein, a simplistic two-step strategy is designed to grow 3D-networked and mesoporous Zn–Ni–Co–S nanowire-like arrays (NWAs) on a Ni-foam scaffold, which includes the hydrothermal growth of Zn–Ni–Co-based hydroxyl precursor, followed by an effective sulfidation process. When employed as a free-standing electrode for supercapacitors, Zn–Ni–Co–S NWAs electrode delivers a superb capacity of 418.2 mA h g−1 at 4.6 mA cm−2, good rate capability (233.3 mA h g−1 at 50 mA cm−2), and long cycle life (~89% of capacity retained after 10,000 cycles). Furthermore, a solid-state asymmetric supercapacitor device was constructed based on Zn–Ni–Co-S NWAs as cathode and graphitic carbon nitride/graphene as anode materials, achieves a cell capacity of 110.7 mA h g−1 at 5 mA cm−2, superior capacity retention (54% at 75 mA cm−2) and superb cyclic performance (93% after 10,000 cycles). The device delivers a superb energy density of 88.6 W h kg−1 at a power density of 419.4 W kg−1, and upholds an energy density of 47.6 W h kg−1 at a high power density of 7048.4 W kg−1.

Mxene (Ti3C2Tx)/cellulose nanofiber/porous carbon film as free-standing electrode for ultrathin and flexible supercapacitors

Traditional porous carbon (PC)-based supercapacitors (SCs) are generally stiff and heavy owing to their rigid components and bulky additives, such as binders, conducting materials, and current collectors. Herein, a porous (574.5 m2 g−1), conductive (83.1 S cm−1), and flexible MXene (Ti3C2Tx)/cellulose nanofiber (CNF)/PC hybrid film is prepared by strong interfacial interactions among its components via a facile vacuum-filtration method. Three-dimensional PC provides abundant micropores for charge storage and considerable amount of meso/macropores for fast diffusion of ions. Two-dimensional Ti3C2Tx offers a significantly high conductivity of 2.6 × 103 S cm−1 and a good film-forming ability. A one-dimensional CNF ensures high mechanical properties with a tensile strength of 38.6 MPa for the hybrid film by binding the adjacent Ti3C2Tx flakes and PC. Additionally, the presence of CNF and PC increases the interlayer distance between Ti3C2Tx flakes, thereby providing more ion-accessible surface areas on Ti3C2Tx and creating open gaps for fast ion transport. Benefiting from these electrochemically attractive properties, this film is further employed as a free-standing electrode to fabricate a high-performance quasi-solid-state SC, which demonstrates an ultrathin thickness of 0.2 mm, a high flexibility, a significantly high areal capacitance of 143 mF cm−2 at 0.1 mA cm−2, and a large areal energy density of 2.4 μWh cm−2 at 17.5 μW cm−2, with a high retention of ~50% even after increasing the power density by ~100 fold. This work will pave the way for developing free-standing MXene-based hybrid films that simultaneously possess excellent conductivity, good mechanical properties, and large charge storage capacity for flexible and high-performance SCs.

Self-assembled activated carbon sandwiched graphene film for symmetrical supercapacitors

Activated carbon (AC) particles sandwiched reduced graphene oxide sheets (rGO) film has been successfully fabricated via a facile self-assemble approach. The as-formed AC/rGO film is self-standing, flexible and mechanically robust, allowing to be transferred to any substrate on demand without rupture. Since AC particles effectively suppressed the restacking of the rGO sheet, AC/rGO film exhibits loose layer-by-layer stacking structures with various gaps between AC particles and rGO sheets, which is different from compact structures of pure graphene films. The as-formed gaps provide fast diffusion channels for electrolyte ions and enhanced accessible surface area of rGO. Therefore, the AC/rGO electrode delivers improved electrochemical performance over the voltage range of 0.0—3.0 V. This work offers a promising strategy to design free-standing supercapacitor electrodes based on traditional nanocarbon materials.

Architectural Ni3Se2 nanosheet-on-nanoforests as free-standing electrodes for high-performance hybrid supercapacitors

A unique electrode architecture consisting of Ni3Se2 nanosheet-on-Ni3Se2-nanoforests grown on nickel foam was obtained by a simple one-step solvothermal method, which subsequently used as a free-standing electrode for supercapacitor. The Ni3Se2 exhibited a high capacity of up to 600 μAh cm−2 at 3 mA cm−2, excellent rate capability and cycling stability at high cycling rates. Impressively, the optimized hybrid supercapacitor containing Ni3Se2 cathode still released 94.4% of initial capacity for 505 μAh cm−2 after 6000 cycles at 20 mA cm−2. The results suggest that the Ni3Se2 nanoforests developed electrode has great potential for practical applications in hybrid supercapacitors.

Preparation of novel 3D hierarchical porous carbon membrane as flexible free-standing electrode for supercapacitors

A flexible 3D hierarchical porous carbon membrane was simply fabricated for supercapacitor electrode by using polyacrylonitrile (PAN) and polyvinylidene fluoride (PVDF) as raw material and polyvinyl pyrrolidone (PVP) as additive via nonsolvent induced phase separation (NIPS) and carbonization. Experimental results show that the as-prepared carbon membranes display typical spongy skeleton structure and excellent flexibility. The specific surface area, micropore, mesopore, and total pore volume of the carbon membranes are increased with the increase of PVDF content. The prepared carbon membranes show large specific surface area (491 m2 g−1) and 3D hierarchical porous structure. As free-standing electrode for supercapacitors, the carbon membranes exhibit a high specific capacitance of 265 F g−1 in three-electrode system and 212 F g−1 in two-electrode system at 0.05 A g−1 in 6 M KOH aqueous electrolyte. Such outstanding capacitive performance is due to the hierarchical porous structure and ameliorated surface chemical functional groups, offering a favorable pathway for ion penetration, considerable surface for accumulation of electrolyte ions and improving the surface accessibility for electrolyte ions. It is believed that the simple and effective approach to hierarchical porous carbon membranes have good application prospect in production of freestanding electrode of supercapacitors.

Flexible carbon nanofibers for high-performance free-standing supercapacitor electrodes derived from Powder River Basin coal

To maximize the utilization of Wyoming sub-bituminous coal and relieve the negative environmental impacts, green synthesis of supercapacitor materials from Powder River Basin (PRB) coal residual were employed to convert coal residuals to high-value carbon materials. The free-standing and binder-free activated carbon nanofibers (ACNFs) were synthesized eco-friendly in CO2 through the carbonization and activation of polyaromatic hydrocarbon-rich extract, which was extracted from the supercritical PRB coal residual by THF Soxhlet. The flexible ACNFs featured various interconnected porous structures endowing the carbon nanofibers (CNFs) with a high specific surface area (743 m2 g−1) and high total pore volume (0.613 cm3 g−1). Moreover, the high-temperature activation in CO2 enhanced the electrical conductivity and reduced the number of oxygen-containing compounds on the surface, which leads to lower charge-transfer resistance. The supercapacitor electrodes prepared from ACNFs have a specific capacitance of 409 F g−1 with some excellent and stable electrochemical performances. Therefore, the green synthesis of supercapacitor electrodes with the extract from PRB coal residuals can expand our horizons for the utilization of low-rank coal.