Two-dimensional mesoporous carbon sheet-like framework material for high-rate supercapacitors

Two-dimensional mesoporous carbon sheet-like framework (MCSF) material has been prepared using mesoporous SiO₂ nanosheet as template and coal tar pitch as carbon precursor. MCSF sheets consisting of numerous mesopores have a specific surface area of 582.7 m² g⁻¹. As a result, the MCSF electrode possesses a maximum specific capacitance of 264 F g⁻¹ at 5 mV s⁻¹, excellent rate capability (74% retention ratio at 1000 mV s⁻¹), and impressive cycling stability with 91% initial capacitance retained after 5000 cycles at 200 mV s⁻¹ in 6 mol L⁻¹ KOH. MCSF symmetric supercapacitor exhibits a maximum energy density of 9.6 Wh kg⁻¹ at 5 mV s⁻¹ and a maximum power density of 119.4 kW kg⁻¹ based on the total mass of the two electrodes in 1 mol L⁻¹ Na₂SO₄ electrolyte.     

Functionalized three-dimensional graphene networks for high performance supercapacitors

Three-dimensional (3D) thermal reduced graphene network (TRGN) deposition on Ni foam without any conductive agents and polymer binders was successfully synthesized by dipping Ni foam into graphene oxide (GO) suspension and subsequent thermal reduction process. The direct and close contact between thermal reduced graphene and Ni foam is beneficial to the enhanced conductivity of the electrode, as well as the improvement of ion diffusion/transport into the electrode. Additionally, low-temperature reduction of GO possesses a large amount of stable oxygen-containing groups that can provide high pseudocapacitance. As a result, the TRGN electrode delivers a high specific capacitance of 442.8 F g⁻¹ at 2 mV s⁻¹ in 6 mol L⁻¹ KOH. Moreover, symmetric supercapacitor based on TRGN exhibits a maximum energy density of 30.4 Wh kg⁻¹ based on the total mass of the two electrodes in 1 mol L⁻¹ Na₂SO₄ electrolyte, as well as excellent cycling stability with 118% of its initial capacitance after 5000 cycles.     

A high-performance three-dimensional micro supercapacitor based on ripple-like ruthenium oxide–carbon nanotube composite films

A micro-supercapacitor with a three-dimensional configuration has been fabricated using an inductively coupled plasma etching technique. A ruthenium oxide–carbon nanotube (CNT) composite with a ripple-like morphology is successfully synthesized using a cathodic deposition technique while using silica-based three-dimensional microstructures as a template. The desired network of carbon nanotubes in the composite facilitates electrolyte penetration and proton exchange/diffusion. A single three dimensional microelectrode is studied using cyclic voltammetry, and a specific capacitance of 272 mF·cm⁻² is observed at 5 mV s⁻¹ in a neutral Na₂SO₄ solution. The accelerated cycle life is tested at 80 mV s⁻¹, and a satisfactory cyclability is observed. When placed on a chip, the symmetric cell exhibits good supercapacitor properties, the specific capacitance up to 37.23 mF cm⁻² and specific power density up to 19.04 mW cm⁻² were obtained at 50 mA cm⁻².     

Three-dimensional hierarchical ZnCo2O4 flower-like microspheres assembled from porous nanosheets: Hydrothermal synthesis and electrochemical properties

In this work, three-dimensional hierarchical ZnCo₂O₄ flower-like microspheres have been synthesized on a large scale via a facile and economical citrate-mediated hydrothermal method followed by an annealing process in air. The as-synthesized ZnCo₂O₄ flower-like microspheres are constructed by numerous interweaving porous nanosheets. According to the experimental results, a formation mechanism involving the assembly of the nanosheets from nanoparticles into flower-like microsphere is proposed. As a virtue of their beneficial structural features, the ZnCo₂O₄ flower-like microspheres exhibit a high lithium storage capacity and excellent cycling stability (1136 mA h g⁻¹ at 100 mA g⁻¹ after 50 cycles). This remarkable electrochemical performance can be ascribed to the hierarchical structure and porous structures in the nanosheets, which effectively increases the contact area between the active materials and the electrolyte, shortening the Li⁺ diffusion pathway and buffering the volume variation during cycling.     

Electrochemical performance of graphitized carbide-derived-carbon with hierarchical micro- and meso-pores in alkaline electrolyte

Graphitized carbide-derived-carbon (CDC) with hierarchical micro- and meso-pores is synthesized by chlorination of titanium carbide powder at 1000 °C. The produced CDC has many bilayer graphenes and some narrow graphite ribbons, which contributes a large amount of micropores (∼1.35 nm) and some mesopores. Although hierarchical pore is an attractive structure for supercapacitor, the low hydrophilicity of the graphitized CDC leads to poor electrochemical performance in alkaline electrolyte. The specific capacitance of the CDC in KOH aqueous electrolyte is only 5 F g⁻¹. A strategy that adding ethanol to alkaline electrolyte is presented to improve its surface wettability. The specific capacitance of the graphitized CDC in KOH aqueous electrolyte with addition of ethanol increases to 60 F g⁻¹ at a scan rate of 20 mV s⁻¹. The optimal content of ethanol in KOH electrolyte is 10 wt.%. In addition, cyclic voltammogram curve can maintain a quasi-rectangular shape well even at a scan rate of 500 mV s⁻¹ and the retention rate of the specific capacitance is about 70%. The specific capacitance is stable at high current density (e.g. 1 A g⁻¹), and almost no performance degradation is observed after 8000 consecutive cycles.     

Template synthesis of hollow carbon spheres anchored on carbon nanotubes for high rate performance supercapacitors

A carbon material consisting of hollow carbon spheres anchored on the surface of carbon nanotubes (CNT–HCS) has been synthesized by an easy chemical vapor deposition process using a CNT–MnO₂ hybrid as template. An electrode made of this material exhibits a maximum specific capacitance of 201.5 F g⁻¹ at 0.5 A g⁻¹ and excellent rate performance (69% retention ratio at 20 A g⁻¹). It has impressive cycling stability with 90% initial capacitance retained after 5000 cycles at 5 A g⁻¹ in 6 mol L⁻¹ KOH. Symmetric supercapacitors based on CNT–HCS achieve a maximum energy density of 11.3 W h kg⁻¹ and power density of 11.8 kW kg⁻¹ operated within a wide potential range of 0–1.6 V in 1.0 mol L⁻¹ Na₂SO₄ solution.     

Encapsulation of manganese oxides nanocrystals in electrospun carbon nanofibers as free-standing electrode for supercapacitors

Flexible composites with manganese oxides (MnO_x) nanocrystals encapsulated in electropun carbon nanofibers were successfully fabricated via a simple and practical combination of electrospinning and carbonization process. The as-formed MnO_x/carbon nanofibers composites have a rough surface with MnO_x nanoparticles well embedded in the carbon nanofibers backbones. When used as electrodes for supercapacitor, the resulting MnO_x/carbon nanofiber composites exhibit good electrochemical performance with a specific capacitance of 174.8 F g⁻¹ at 2 mV s⁻¹ in 0.5 M Na₂SO₄ electrolyte, a good rate capability at high current density and long-term cycling stability. It is expected that such freestanding composites could be promising electrodes for high-performance supercapacitors.     

Structure and electrochemical performance of highly nanoporous carbons from benzoate–metal complexes by a template carbonization method for supercapacitor application

A series of highly nanoporous carbons have been prepared by converting benzoate–metal complexes, including zinc benzoate, magnesium benzoate and aluminium benzoate through a template carbonization process. The carbonization temperature plays a pivotal role in determining the carbon structures as well as the resultant electrochemical behaviors in supercapacitors. The carbon–Zn-900 sample derived from zinc benzoate complex has a high specific surface area (1466.4 m² g^–1), large pore volume (2.54 cm³ g^–1) and hierarchical pore size distribution. It can also deliver a large specific capacitance of 314.1 F g⁻¹ at a current density of 0.5 A g⁻¹, together with a large energy density of 67.2 Wh kg⁻¹ when measured in a three-electrode system using 6 mol L⁻¹ KOH as electrolyte. Besides, the carbon–Zn-900 sample has been tested in a two-electrode system using [EMIm]BF₄/AN as electrolyte at different operation temperatures of 25/50/80 °C.     

Supercapacitors based on carbons with tuned porosity derived from paper pulp mill sludge biowaste

Hydrothermal carbonization followed by chemical activation is utilized to convert paper pulp mill sludge biowaste into high surface area (up to 2980 m² g⁻¹) carbons. This synthesis process employs an otherwise unusable byproduct of paper manufacturing that is generated in thousands of tons per year. The textural properties of the carbons are tunable by the activation process, yielding controlled levels of micro and mesoporosity. The electrochemical results for the optimized carbon are very promising. An organic electrolyte yields a maximum capacitance of 166 F g⁻¹, and a Ragone curve with 30 W h kg⁻¹ at 57 W kg⁻¹ and 20 W h kg⁻¹ at 5450 W kg⁻¹. Two ionic liquid electrolytes result in maximum capacitances of 180–190 F g⁻¹ with up to 62% retention between 2 and 200 mV s⁻¹. The ionic liquids yielded energy density–power density combinations of 51 W h kg⁻¹ at 375 W kg⁻¹ and 26–31 W h kg⁻¹ at 6760–7000 W kg⁻¹. After 5000 plus charge–discharge cycles the capacitance retention is as high at 91%. The scan rate dependence of the surface area normalized capacitance highlights the rich interplay of the electrolyte ions with pores of various sizes.     

Scaleable ultra-thin and high power density graphene electrochemical capacitor electrodes manufactured by aqueous exfoliation and spray deposition

Graphene electrodes of high power density were manufactured by a surfactant-water based exfoliation method followed by a scaleable spray-deposition process. Cyclic voltammetry and galvanostatic charge–discharge experiments revealed a combination of electric double layer and pseudocapacitive behavior that, unlike the many graphene-oxide derived electrodes, was maintained to unusually high scan rates of 10,000 mV s⁻¹, reaching a maximum capacitance of 543 μF cm⁻² and with a capacitive retention of 57% at 10,000 mV s⁻¹. The performance of graphene electrodes was contrasted with carboxylated single walled carbon nanotubes that showed a sharp decrease in capacitance above 200 mV s⁻¹. Electrochemical impedance spectroscopy analysis showed a fast capacitor response of 17.4 ms for as manufactured electrodes which was further improved to 2.3 ms for surfactant-free 40 nm thick electrodes. A maximum energy density of 75.4 nW h cm⁻² gradually decreased as power density increased up to 2.6 mW cm⁻². Graphene electrodes showed 100% capacitance retention for 5000 cycles at the high power scan rate of 10,000 mV s⁻¹.     

Composites of MnO2 nanocrystals and partially graphitized hierarchically porous carbon spheres with improved rate capability for high-performance supercapacitors

Manganese dioxide/carbon nanocomposites with partially graphitized hierarchical porous structure have been designed and synthesized. A high specific capacitance of 412 F g⁻¹ and excellent rate capability of these composites can be achieved owing to the interconnected meso- and micro-porous structure and the graphitic pore walls facilitating the ion diffusion and electron transportation, respectively, which is highly demanded for high-performance supercapacitor electrodes materials. Even at a high scan rate of 100 mV s⁻¹, a specific capacitance of 251 F g⁻¹ can be obtained, corresponding to 61% capacitance retention. Moreover, a long cycling stability with initial capacitance retention of ∼88% is obtained after over 4000 cycles at a current density of 1.0 A g⁻¹. This work presents an efficient electrode materials design and a novel composite which holds great promise in high-performance supercapacitor applications.     

Hydrothermally treated aminated tannin as precursor of N-doped carbon gels for supercapacitors

Aminated tannin submitted to hydrothermal treatment led to nitrogen-doped gels in the absence of any cross-linker. Such gels were subcritically dried, freeze-dried or supercritically dried to obtain organic xerogels, cryogels and aerogels, respectively, having nitrogen contents between 3.0 and 3.7 wt.%. After pyrolysis at 900 °C, the materials presented nitrogen contents ranging from 1.9 to 3.0 wt.%, and surface areas as high as 860, 754 and 585 m² g⁻¹ for carbon aerogels, cryogels and xerogels, respectively. All of them displayed micropores associated with different mesopore volumes, depending on both the drying method and initial dilution of the precursor. When tested as supercapacitor electrodes, these carbon gels presented outstanding specific and normalised capacitances, up to 387.6 F g⁻¹ and 69.5 μF cm⁻², respectively, at a scan rate of 2 mV s⁻¹ in 4 mol L⁻¹ H₂SO₄ aqueous solution. These performances are higher than those obtained with high apparent surface area-activated carbons, as the measured capacitances are indeed among the highest ever reported. The influence of nitrogen- and oxygen-based moieties was investigated, and optimal N and O contents of 2–3 and 17–18 wt.%, respectively, were observed.     

A high-performance asymmetric supercapacitor based on carbon and carbon–MnO2 nanofiber electrodes

An asymmetric supercapacitor with high energy and power densities has been fabricated using MnO₂/carbon nanofiber composites as positive electrode and activated carbon nanofibers as negative electrode in Na₂SO₄ aqueous electrolyte. Both electrode materials are freestanding in nature without any conductive additives or binders and exhibit outstanding electrochemical performances. The as-assembled asymmetric supercapacitor with optimal mass ratio can be operated reversibly over a wide voltage range of 0–2.0 V, and presents a maximum energy density of 30.6 Wh kg⁻¹, which is much higher than those of symmetric supercapacitors. Moreover, the supercapacitor exhibits excellent rate capability (high power density of 20.8 kW kg⁻¹ at 8.7 Wh kg⁻¹) and long-term cycling stability with only 6% loss of its initial capacitance after 5000 cycles. These attractive results make these freestanding materials promising for applications in aqueous electrolyte-based asymmetric supercapacitors with high energy and power densities delivery.     

Carbon coated porous tin peroxide/carbon composite electrode for lithium-ion batteries with excellent electrochemical properties

A porous tin peroxide/carbon (SnO₂/C) composite electrode coated with an amorphous carbon layer is prepared using a facile method. In this electrode, spherical graphite particles act as supporter of electrode framework, and the interspace among particles is filled with porous amorphous carbon derived from decomposition of polyvinylidene fluoride and polyacrylonitrile. SnO₂ nanoparticles are uniformly embedded in the porous amorphous carbon matrix. The pores in amorphous carbon matrix are able to buffer the huge volume expansion of SnO₂ during charge/discharge cycling, and the carbon framework can prevent the SnO₂ particles from pulverization and re-aggregation. The carbon coating layer on the outermost surface of electrode can further prevent porous SnO₂/C electrode from contacting with electrolyte directly. As a result, the repeated formation of solid electrolyte interface is avoided and the cycling stability of electrode is improved. The obtained SnO₂/C electrode presents an initial coulombic efficiency of 77.3% and a reversible capacity of 742 mA h g⁻¹ after 130 cycles at a current density of 100 mA g⁻¹. Furthermore, a reversible capacity of 679 mA h g⁻¹ is obtained at 1 A g⁻¹.     

Facile synthesis of three dimensional flower-like Co3O4@MnO2 core-shell microspheres as high-performance electrode materials for supercapacitors

In this work, we reported the synthesis of three dimensional flower-like Co₃O₄@MnO₂ core-shell microspheres by a controllable two-step reaction. Flower-like Co₃O₄ microspheres cores were firstly built from the self-assembly of Co₃O₄ nanosheets, on which MnO₂ nanosheets shells were subsequently grown through the hydrothermal decomposition of KMnO₄. The MnO₂ nanosheets shells were found to increase the electrochemical active sites and allow faster redox reaction kinetics. Based on these advantages, when used as an electrode for supercapacitors, the prepared flower-like Co₃O₄@MnO₂ core-shell composite electrode demonstrated a significantly enhanced specific capacitance (671 F g⁻¹ at 1 A g⁻¹) as well as improved rate capability (84% retention at 10 A g⁻¹) compared with the pristine flower-like Co₃O₄ electrode. Moreover, the optimized asymmetric supercapacitor device based on the flower-like Co₃O₄@MnO₂//active carbon exhibited a high energy density of 34.1 W h kg⁻¹ at a power density of 750 W kg⁻¹, meaning its great potential application for energy storage devices.     

Amaryllis-like NiCo2S4 nanoflowers for high-performance flexible carbon-fiber-based solid-state supercapacitor

Low-cost dynamic materials for Faradaic redox reactions are needed for high-energy storage supercapacitors. A simple and cost-effective hydrothermal process was employed to synthesize amaryllis-like NiCo₂S₄ nanoflowers. The sample was characterized by X-ray powder diffraction, Brunauer–Emmett–Teller method, scanning electron microscopy, and transmission electron microscopy. NiCo₂S₄ nanoflowers were coated onto carbon fiber fabric and used as a binder-free electrode to fabricate a solid-state supercapacitor compact device. The solid-state supercapacitor exhibited excellent electrochemical performance, including high specific capacitance of 360 F g⁻¹ at scan rate of 5 mV s⁻¹ and high energy density of 25 W h kg⁻¹ at power density of 168 W kg⁻¹. In addition, the supercapacitor possessed high flexibility and good stability by retaining 90% capacitance after 5000 cycles. The high conductivity and Faradic-redox activity of NiCo₂S₄ nanoflowers resulted in high specific energy and power. Thus, NiCo₂S₄ nanoflowers are promising pseudocapacitive materials for low-cost and lightweight solid-state supercapacitors.     

Impregnation assisted synthesis of 3D nitrogen-doped porous carbon with high capacitance

Three-dimensional (3D) porous carbons with controlled mesopore and micropore structures were prepared through a simple and low-cost ultrasonic and impregnation assisted method from waste air-laid paper. The ammonia management was used to dope the 3D porous carbons with different types of nitrogen heteroatoms in a way that replaced carbon atoms. The N₂ adsorption–desorption characterization suggested that the nitrogen-doped carbons have a high surface area of 1470 m² g⁻¹ with the average pore diameter of 4.2 nm, which are conducive to form electric double layer under high current density. The resulting 3D carbon exhibited a higher capacitance at 296 F g⁻¹ in comparison with the nitrogen-free one at 252 F g⁻¹ in 6 M KOH electrolyte. Moreover, a high power density ca. 0.313 kW kg⁻¹ and energy density ca. 34.3 Wh kg⁻¹ were achieved in the ionic liquid ([EMIm]BF₄). The findings will open a new avenue to use waste materials for high-performance energy-storage devices.     

Carbons with narrow pore size distribution prepared by simultaneous carbonization and self-activation of tobacco stems and their application to supercapacitors

Highly microporous carbons with narrow pore size distribution have been prepared by simultaneous carbonization and self-activation of tobacco wastes at temperatures ranging from 600 to 1000 °C. The efficiency of porosity development, without pores broadening, is attributed to well-distributed alkalis at the molecular level in the tobacco precursor. With Burley tobacco, the BET specific surface area and average micropore size L₀ increased up to 800 °C (Burley 800), where the values reached maxima of 1749 m² g⁻¹ and 1.2 nm, respectively. At temperatures higher than 800 °C, annealing of the materials dominates and provokes a decrease of S_BET and L₀. Burley carbons were implemented in supercapacitors using 1 mol L⁻¹ aqueous Li₂SO₄ or 1 mol L⁻¹ TEABF₄ in acetonitrile. In both electrolytes, the capacitance of Burley carbons followed the same trend as S_BET and L₀. Burley 800 demonstrated outstanding capacitance values of 167 F g⁻¹ (at 0.8 V limit) and 141 F g⁻¹ (at 2.3 V limit) in 1 mol L⁻¹ aqueous Li₂SO₄ and 1 mol L⁻¹ TEABF₄, respectively. Such values, about 50% higher as compared to commercially available carbons, are attributed to the narrow pore size distribution of this carbon with a maximum of pores around 1.2 nm close to the size of solvated ions in these electrolytes.     

Carbonization temperature dependence of pore structure of silicon carbide spheres and their electrochemical capacitive properties as supercapacitors

Three-dimensional silicon carbide-based frameworks with hierarchical micro- and mesoporous structures (2MSiC) are prepared by employing the template method and carbonization reaction using aerosol-spray drying. The mesopores are generated by the self-assembly of a structure-directing agent, whereas the micropores originate from the partial evaporation of Si atoms during the carbonization process. During the carbonization process, the proportion of micro- and mesopores in 2MSiC can be controlled by the carbonization temperature by controlling the amount of partial evaporation of Si atoms. The 2MSiC electrode prepared using a Brij56 structure-directing agent as the mesopore template and carbonized at 1250 °C exhibits a high charge storage capacity with a specific capacitance of 259.9 F g⁻¹ at a scan rate of 5 mV s⁻¹ with 88.1% rate performance from 5 to 500 mV s⁻¹ in 1 M KCl aqueous electrolyte. This outstanding electrochemical performance can be attributed to the synergistic effect of both the enhanced electric double layer properties caused by micropores and reduced resistant pathways for ion diffusion in the pores as well as a large accessible surface area for ion transport/charge storage caused by mesopores. These encouraging results demonstrate that the 2MSiC electrode prepared with Brij56 and carbonized at 1250 °C is a promising candidate for high-performance electrode materials for supercapacitors.     

Hierarchically porous carbon by activation of shiitake mushroom for capacitive energy storage

We present a facile yet effective two-step activation method to prepare a hierarchically porous carbon with natural shiitake mushroom as the starting materials. The first step involves the activation of shiitake mushroom with H₃PO₄, while the second step is to further activate the product with KOH. The resulting carbon is comprised of abundant micro-, mesopores and interconnected macropores that has a specific surface area up to 2988 m² g⁻¹ and pore volume of 1.76 cm³ g⁻¹. With the unique porous nature, the carbon exhibited a specific capacitance of 306 and 149 F g⁻¹ in aqueous and organic electrolyte, respectively. Moreover, this carbon also shows a high capacitance retention of 77% at large current density of 30 A g⁻¹ and exhibited an outstanding cycling stability with 95.7% capacitance preservation after 15,000 cycles in 6 M KOH electrolyte. The far superior performance as compared with those of the commercially most used activated carbon RP20 in both aqueous and non-aqueous electrolyte demonstrates its great potential as high-performance supercapacitor electrode. The two-step method developed herein also represents a very attractive approach for scalable production of various functional carbon materials using diverse biomasses as starting materials.