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.     

Graphitic carbon nitride nanosheet-assisted preparation of N-enriched mesoporous carbon nanofibers with improved capacitive performance

N-enriched mesoporous carbon nanofibers (NMCNFs) were prepared by an electrospinning technique using graphitic carbon nitride (g-C₃N₄) nanosheets both as sacrificial template and N-doping source. The resultant NMCNF film has a high N-doping level of 8.6 wt% and a high specific surface area of 554 m² g⁻¹. When directly used as the electrode material for supercapacitor, the free-standing NMPCNF film shows a significantly improved capacitive performance including a higher specific capacitance (220 F g⁻¹ at 0.2 A g⁻¹) and a better rate capability (∼70% retention at 20 A g⁻¹) than those of microporous carbon nanofiber film prepared using the same process without using g-C₃N₄ nanosheets (145 F g⁻¹ at 0.2 A g⁻¹ and ∼45% retention at 20 A g⁻¹). Moreover, the NMCNFs show superior stability with only a ∼3% decrease of its initial capacitance after 1000 cycles at a high current density of 10 A g⁻¹. More significantly, the energy density of a symmetrical supercapacitor (SC) based on the NMPCNF film can reach 12.5 Wh kg⁻¹ at a power density of 72 W kg⁻¹.     

Three-dimensional flower-like and hierarchical porous carbon materials as high-rate performance electrodes for supercapacitors

Three-dimensional flower-like and hierarchical porous carbon material (FHPC) has been fabricated through a simple and efficient carbonization method followed by chemical activation with flower-like ZnO as template and pitch as carbon precursor. The hierarchical porous structure is composed of numerous micropores and well-defined mesopores in the interconnected macroporous walls. The FHPC electrode can achieve a relatively high capacitance of 294 F g⁻¹ at a scan rate of 2 mV s⁻¹ and excellent rate capability (71% retention at 500 mV s⁻¹) with superior cycle stability (only 2% loss after 5000 cycles) in 6 mol L⁻¹ KOH electrolyte. The symmetric supercapacitor fabricated with FHPC electrodes delivers a high energy density of 15.9 Wh kg⁻¹ at a power density of 317.5 W kg⁻¹ operated in the voltage range of 0–1.8 V in 1 mol L⁻¹ Na₂SO₄ aqueous electrolyte.     

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.     

Dense carbon monoliths for supercapacitors with outstanding volumetric capacitances

A commercially available dense carbon monolith (CM) and four carbon monoliths obtained from it have been studied as electrochemical capacitor electrodes in a two-electrode cell. CM has: (i) very high density (1.17 g cm⁻³), (ii) high electrical conductivity (9.3 S cm⁻¹), (iii) well-compacted and interconnected carbon spheres, (iv) homogeneous microporous structure and (v) apparent BET surface area of 957 m²g⁻¹. It presents interesting electrochemical behaviors (e.g., excellent gravimetric capacitance and outstanding volumetric capacitance). The textural characteristics of CM (porosity and surface chemistry) have been modified by means of different treatments. The electrochemical performances of the starting and treated monoliths have been analyzed as a function of their porous textures and surface chemistry, both on gravimetric and volumetric basis. The monoliths present high specific and volumetric capacitances (292 F g⁻¹ and 342 F cm⁻³), high energy densities (38 Wh kg⁻¹ and 44 Wh L⁻¹), and high power densities (176 W kg⁻¹ and 183 W L⁻¹). The specific and volumetric capacitances, especially the volumetric capacitance, are the highest ever reported for carbon monoliths. The high values are achieved due to a suitable combination of density, electrical conductivity, porosity and oxygen surface content.     

Superhigh-rate capacitive performance of heteroatoms-doped double shell hollow carbon spheres

The salient practical application feature of an ideal supercapacitor is its ability to deliver high energy density stably even at ultrahigh power density. Therefore, a rational design of electrode materials is essentially required for achieving high current, energy and power densities. In this work, a special “in situ replicating” strategy is employed to fabricate double shell hollow carbon spheres with homogeneously doped heteroatoms. The KOH activation introduces micropores to the thin shells of the hollow carbon spheres. Materials characterizations show that these carbon spheres have such merits as large surface area, easy-accessible micropore surface with faradaic reaction sites, and high conductivity. All these result in ultrafast ion transport from electrolyte to the micropores in the carbon spheres and endow the carbon with outstanding capacitive performance, e.g., an unprecedentedly high specific capacitance of 270 F g⁻¹ at a very high current density of 90 A g⁻¹. Moreover, a high energy density of 11.9 Wh kg⁻¹ at a respectable power density of 30,000 W kg⁻¹ is achieved in 6 M KOH 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.     

Straightforward synthesis of hierarchical Co3O4@CoWO4/rGO core–shell arrays on Ni as hybrid electrodes for asymmetric supercapacitors

Hierarchical Co₃O₄@CoWO₄/rGO core/shell nanoneedles arrays are successfully grown on 3D nickel foam using a simple, effective method. By virtue of its unique structure, Co₃O₄@CoWO₄/rGO demonstrates an enhanced specific capacitance of 386 F g⁻¹ at 0.5 A g⁻¹ current density. It can be used as an integrated, additive-free electrode for supercapacitors that boasts excellent performance. As illustration, we assemble an asymmetric supercapacitor (ASC) using the as-prepared Co₃O₄@CoWO₄/rGO as the positive electrode and activated carbon as the negative electrode. The optimized ASC displays a maximum energy density of 19.99 Wh kg⁻¹ at a power density of 321 W kg⁻¹. Furthermore, the ASC also presents a remarkably long cycle life along with 88.8% specific capacitance retention after 5000 cycles.     

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.     

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.     

Mesoporous N-containing carbon nanosheets towards high-performance electrochemical capacitors

We developed a direct carbonization strategy to efficiently fabricate mesoporous N-containing carbon nanosheets (N-CNSs) by using polyaniline nanosheets as a carbon precursor. Physicochemical characterizations revealed that the as-synthesized N-CNSs with 5.9 wt.% N species possessed a well-developed mesoporous architecture with large specific surface area of 352 m² g⁻¹, high mesoporous volume of 0.32 cm³ g⁻¹, and average pore size of ∼5.2 nm. When further utilized as an electrode for electrochemical capacitors, the mesoporous N-CNSs delivered a large specific capacitance of 239 F g⁻¹ at 0.5 A g⁻¹, and even 197 F g⁻¹ at a high current load of 8 A g⁻¹, indicating its good rate behavior. Furthermore, the capacitance degradation of ∼4% over continuous 5000 charge–discharge cycles at 6 A g⁻¹ further verified its good electrochemical stability at high rates for long-term electrochemical capacitors application.     

The composite material based on Dawson-type polyoxometalate and activated carbon as the supercapacitor electrode

A supercapacitor electrode assembled from activated carbon (AC) and (NH₄)₆[P₂Mo₁₈O₆₂]·14.2H₂O (P₂Mo₁₈) was fabricated for the first time, and showed remarkable electrochemical performance ascribed to the synergy of the double layer capacitance of AC and the pseudocapacitance of P₂Mo₁₈. The investigations indicate that the AC/P₂Mo₁₈ electrode exhibits a specific capacitance of 275 F g^− 1 at a high current density of 6 A g^− 1, which is substantially larger than the 182 F g^− 1 of the AC electrode. In addition, the AC/P₂Mo₁₈ electrode possesses a remarkable rate capability (89%) when the current density is increased from 2 to 6 A g^− 1.     

Facile synthesis of functionalized porous carbon with three-dimensional interconnected pore structure for high volumetric performance supercapacitors

Functionalized porous carbon with three-dimensional (3D) interconnected pore structure has been successfully synthesized through direct heat-treatment of KOH-soaked soybeans. Benefiting from heteroatoms (N, O) doping, interconnected porous carbon framework with high surface area as well as high packing density (up to 1.1 g cm⁻³), the as-obtained porous carbon material exhibits high volumetric capacitance of 468 F cm⁻³, good rate capability and excellent cycling stability (91% of capacitance retention after 10,000 cycles) in 6 M KOH electolyte. More importantly, the as-assembled symmetric supercapacitor delivers high volumetric energy density of 28.6 Wh L⁻¹ in 1 M Na₂SO₄ aqueous solution.     

Preparation and one-step activation of microporous carbon nanofibers for use as supercapacitor electrodes

Microporous carbon nanofibers were prepared by electrospinning from resole-type phenolic resin, followed by one-step activation. KOH was utilized to tune the fiber diameter and improve porous texture. By adjusting KOH content in the spinning solution, the fiber diameter could be controlled in the range of 252–666 nm and the microporous volume and specific surface area could be greatly improved. The electrochemical measurements in 6 M KOH aqueous solution showed that the microporous carbon nanofibers possessed high specific capacitance, considerable rate performance, and superior specific surface capacitance to conventional microporous carbons. The maximal specific capacitance of 256 F g⁻¹ and high specific surface capacitance of 0.51 F m⁻² were achieved at 0.2 A g⁻¹. Furthermore, the specific capacitance could still remain 170 F g⁻¹ at 20 A g⁻¹ with the retention of 67%. Analysis showed that the high specific surface capacitance of the resultant carbons was mainly attributed to optimized pore size (0.7–1.2 nm) and the excellent rate performance should be principally due to the reduced ion transportation distance derived from the nanometer-scaled fibers.     

High-performance supercapacitor based on V2O5/carbon nanotubes-super activated carbon ternary composite

A ternary composite of V₂O₅/carbon nanotubes/super activated carbon (V₂O₅/CNTs–SAC) was prepared by a simple hydrothermal method and used as a supercapacitor electrode material. The electrochemical performance of the electrode was analyzed using cyclic voltammetry, galvanostatic charge/discharge measurements, and electrochemical impedance spectroscopy, which were performed in 2 M NaNO₃ as the electrolyte. The V₂O₅/CNTs–SAC nanocomposite exhibited a specific capacitance as high as 357.5 F g⁻¹ at a current density of 10 A g⁻¹, which is much higher than that of either bare V₂O₅ nanosheets or a V₂O₅/CNTs composite. Furthermore, the capacitance increased to 128.7% of the initial value after 200 cycles, with 99.5% of the maximum value being retained after 1000 cycles. These results demonstrated that the V₂O₅/CNTs–SAC ternary composite is suitable for use as an electrode material for supercapacitors.     

High-performance supercapacitors based on defect-engineered carbon nanotubes

Defect-engineered carbon nanotubes (CNTs) were prepared by KOH activation and subsequent nitrogen doping. Controlled KOH activation of the CNTs enlarged the specific surface area to 988 m² g⁻¹, which is about 4.5 times greater than that of pristine CNTs. In addition, a hierarchical pore structure and a rough surface developed at high degrees of activation, which are advantageous features for fast ion diffusion. The subsequent nitrogen doping changed the band structure of the CNTs, resulting in improved electrical properties. Symmetric supercapacitors fabricated using these nitrogen-doped and activated CNTs (NA-CNTs) successfully worked across a wide potential range (0–3.5 V) and exhibited a high capacitance of 98 F g⁻¹ at a current density of 1 A g⁻¹. Furthermore, a low equivalent series resistance (2.2 Ω) was achieved owing to the tailored nanostructure and electrical properties of the electrode materials. Over the voltage range from 0 to 3.5 V, supercapacitors based on NA-CNTs exhibited a high specific energy of 59 Wh kg⁻¹ and a specific power of 1750 W kg⁻¹. In addition, a specific power of 52,500 W kg⁻¹ with a 3-s charge/discharge rate was achieved with a specific energy of 26 Wh kg⁻¹. Moreover, the supercapacitors showed stable performance over 10,000 charge/discharge cycles.     

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.     

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.     

Preparation and characterization of manganese oxide/CNT composites as supercapacitive materials

Manganese oxide was synthesized and dispersed on carbon nanotube (CNT) matrix by thermally decomposing manganese nitrates. CNTs used in this paper were grown directly on graphite disk by chemical vapor deposition technique. The capacitive behavior of manganese oxide/CNT composites was investigated by cyclic voltammetry and galvanostatic charge–discharge method in 1 M Na₂SO₄ aqueous solutions. When the loading mass of MnO₂ is 36.9 μg cm^− 2, the specific capacitance of manganese oxide/CNT composite (based on MnO₂) at the charge–discharge current density of 1 mA cm^− 2 equals 568 F g^− 1. Additionally, excellent charge–discharge cycle stability (ca. 88% value of specific capacitance remained after 2500 charge–discharge cycles) and power characteristics of the manganese oxide/CNT composite electrode can be observed. The effect of loading mass of MnO₂ on specific capacitance of the electrode has also been investigated.     

Nitrogen-doped porous carbons through KOH activation with superior performance in supercapacitors

A series of nitrogen-doped porous carbons are prepared through KOH activation of a nonporous nitrogen-enriched carbon which is synthesized by pyrolysis of the polymerized ethylenediamine and carbon tetrachloride. The porosity and nitrogen content of the nitrogen-doped porous carbons depend strongly on the weight ratio of KOH/carbon. As the weight ratio of KOH/carbon increases from 0.5 to 2, the specific surface area increases from 521 to 1913 m² g⁻¹, while the nitrogen content decreases from 10.8 to 1.1 wt.%. The nitrogen-doped porous carbon prepared with a moderate KOH/carbon weight ratio of 1, which possesses a balanced specific surface area (1463 m² g⁻¹) and nitrogen content (3.3 wt.%), exhibits the largest specific capacitance of 363 F g⁻¹ at a current density of 0.1 A g⁻¹ in 1 M H₂SO₄ aqueous electrolyte, attributed to the co-contribution of double-layer capacitance and pseudocapacitance. Moreover, it shows excellent rate capability (182 F g⁻¹ remained at 20 A g⁻¹) and good cycling stability (97% capacitance retention over 5000 cycles), making it a promising electrode material for supercapacitors.