Influence of the textual properties of activated carbon nanofibers on the performance of electric double-layer capacitors

To investigate the relationship between textural properties and electrochemical properties, activated carbon nanofibers were manufactured using an electrospinning process followed by chemical activation using KOH or NaOH. The specific surface area of the KOH-activated carbon nanofibers was higher than that of NaOH-activated carbon nanofibers; however, the total pore volume and mesopore volume of the NaOH-activated carbon nanofibers were greater than those of the KOH-activated carbon nanofibers when the same number of moles of KOH and NaOH were used. The specific capacitances increased as the specific surface area and pore volume of the activated carbon nanofibers were increased. However, the specific capacitance obtained at a high scan rate (50 mV/s) and the retained capacitance of the activated carbon nanofibers increased with increasing total pore and mesopore volume, especially for mesopores with diameters of 2–4 nm.     

Nitrogen/sulfur dual-doped mesoporous carbon with controllable morphology as a catalyst support for the methanol oxidation reaction

Ordered mesoporous carbon (OMC) was synthesized by nano-casting method using novel fluidic precursor – acrylonitrile telomer (ANT). By the penetration of mesoporous silica template with pure ANT, followed by the stabilization, carbonization and removal of the template, we obtained highly ordered mesoporous carbon rods (specific area 408 m² g⁻¹). When an acetone solution of ANT (66 and 33 wt.%) was used instead of pure ANT, carbon materials with mesopore ranging from 2 to 7 nm were obtained (specific area 843 and 1012 m² g⁻¹ respectively). Both nitrogen and sulfur atoms were doped into mesoporous carbon with 4 and 0.6 at.% using nitrogen containing monomer and sulfur containing chain transfer agent, without involving complicated synthetic technique and poisonous gaseous compounds. This method was proved to be a facile way to synthesize nitrogen and sulfur containing OMC with partially controllable pore distribution and morphology. More importantly, due to unique mesopore structure and heteroatom doping, Pt nano-particles deposited on the OMCs showed electrocatalytic activity as high as 508 mA mg⁻¹ Pt in methanol oxidation which is 1.7-fold of activity of Pt deposited on commercial Vulcan carbon black.     

Ordered mesoporous carbide-derived carbons prepared by soft templating

Free-standing films of ordered mesoporous silicon and titanium carbide-derived carbons have been synthesized using a novel soft templating approach without employing hydrofluoric acid. Tetraethyl orthosilicate or titanium citrate, alternatively, and a phenolic resin underwent an evaporation induced self-assembly yielding ordered mesoporous silicon carbide/carbon or titanium carbide/carbon composites. High temperature chlorine treatment transformed these materials conformally into carbide-derived carbons (CDC) while the ordered arrangement of mesopores was maintained. The corresponding hierarchical pore structures consist of narrowly distributed micro- and mesopores (distribution maxima at 1 and 5 nm, respectively) with a high surface area and pore volume of up to 1538 m²/g and 2.53 cm³/g, respectively.     

High rate performance activated carbons prepared from ginkgo shells for electrochemical supercapacitors

Partially graphitized ginkgo-based activated carbon (GGAC) is fabricated from ginkgo shells by pyrolysis, KOH activation and heat treatment using cobalt nitrate as graphitization catalyst. The graphitization temperature is 900 °C. The GGAC has a microporous structure and its specific surface area is 1775 m² g⁻¹. XRD patterns show that the carbon becomes more graphitic after heat treatment. The specific capacitance of the GGAC reaches to 178 F g⁻¹ at a potential scan rate of 500 mV s⁻¹, which is superior to that of commercial activated carbons and ordered mesoporous carbons. The high electrochemical performance of the GGAC is attributed to its good electronic conductivity and high surface area. Partially graphitized activated carbon is a promising electrode material for electrochemical supercapacitors with high rate performance.     

Hydrothermal synthesis and activation of graphene-incorporated nitrogen-rich carbon composite for high-performance supercapacitors

Graphene-incorporated nitrogen-rich carbon composite with nitrogen content of ca. 10 wt.% has been synthesized by an effective yet simple hydrothermal reaction of glucosamine in the presence of graphene oxide (GO). The nitrogen content of carbon composite is nearly twice as high as that of hydrothermal carbon without graphene. GO is favorable for the high nitrogen doping in the carbon composite by the reaction between the glucosamine-released ammonia and GO. The hydrothermal carbon composite is further activated by KOH, and graphene in the activated carbon composite demonstrates a positive effect of increasing specific surface area, pore volume and electrical conductivity, resulting in superior electrochemical performance. The activated carbon composite with higher specific surface area and micropore volume possesses higher specific capacitance with a value of 300 F g⁻¹ at 0.1 A g⁻¹ in 6 M KOH aqueous solution in the two electrode cell. Larger mesopore volume and higher conductivity of the activated carbon composite will provide fast ion and electron transfer, thus leading to higher rate capacity with a capacitance retention of 76% at 8 A g⁻¹ in comparison to the activated hydrothermal carbon without graphene.     

A simple approach for fabrication of interconnected graphitized macroporous carbon foam with uniform mesopore walls by using hydrothermal method

Three-dimensional (3D) highly interconnected graphitized macroporous carbon foam with uniform mesopore walls has been successfully fabricated by a simple and efficient hydrothermal approach using resorcinol and formaldehyde as carbon precursors. The commercially available cheap polyurethane (PU) foam and Pluronic F127 were used as a sacrificial polymer and mesoporous structure-directing templates, respectively. The graphitic structure of carbon foam was obtained by catalytic graphitization method using iron as catalyst. Three different carbon foams such as graphitized macro-mesoporous carbon (GMMC) foam, amorphous macro-mesoporous carbon (AMMC) foam and graphitized macroporous carbon (GMC) foam were fabricated and their physicochemical and mechanical properties were systematically measured and compared. It was found that GMMC possess well interconnected macroporous structure with uniform mesopores located in the macroporous skeletal walls of continuous framework. Besides, GMMC foam possesses a well-defined graphitic framework with high surface area (445 m²/g), high pore volume (0.35 cm³/g), uniform mesopores (3.87 nm), high open porosity (90%), low density (0.30 g/cm³) with good mechanical strength (1.25 MPa) and high electrical conductivity (11 S/cm) which makes it a promising material for many potential applications.     

Nitrogen-doped ordered mesoporous carbons based on cyanamide as the dopant for supercapacitor

Nitrogen-doped ordered mesoporous carbons (N-doped OMCs) with a high surface area of 1741 m²/g and nitrogen content up to 15 wt.% have been synthesized by nanocasting approach by using SBA-15 as a hard template, phenolic resin (resol) as a carbon source and high nitrogen-containing cyanamide as the nitrogen dopant. The introduction of cyanamide not only incorporates high-content nitrogen into the carbon matrix in the primary forms of pyridinic and quaternary species, but also greatly increases the surface area of materials. The obtained N-doped OMCs have large surface area with mesoporosity up to 92%, uniform and appropriate pore size (3.6–4.1 nm), large pore volume (1.2–1.81 cm³/g). These merits together with high nitrogen enrichment lead to a specific capacitance (230 F/g at 0.5 A/g) and good rate capability (175 F/g at 20 A/g with capacitance retention of 77.4%) in 6 M KOH aqueous electrolytes.     

Large-scale synthesis of ordered mesoporous carbon fiber and its application as cathode material for lithium–sulfur batteries

A novel type of one-dimensional ordered mesoporous carbon fiber has been prepared via the electrospinning technique by using resol as the carbon source and triblock copolymer Pluronic F127 as the template. Sulfur is then encapsulated in this ordered mesoporous carbon fibers by a simple thermal treatment. The interwoven fibrous nanostructure has favorably mechanical stability and can provide an effective conductive network for sulfur and polysulfides during cycling. The ordered mesopores can also restrain the diffusion of long-chain polysulfides. The resulting ordered mesoporous carbon fiber sulfur (OMCF-S) composite with 63% S exhibits high reversible capacity, good capacity retention and enhanced rate capacity when used as cathode in rechargeable lithium–sulfur batteries. The resulting OMCF-S electrode maintains a stable discharge capacity of 690 mAh/g at 0.3 C, even after 300 cycles.     

Siberian anthracite as a precursor material for microporous activated carbons

The technology of obtaining active carbon from anthracite mined in Siberia is described. The effect of the activating agent, anthracite/activator ratio and activation temperature has been tested. The activation either with KOH or NaOH has been found to lead to microporous active carbon samples of well-developed surface area reaching from 588 to 2260 m²/g and pore volume from 0.29 to 1.12 m³/g. The structural properties of the active carbons obtained have been found to depend first of all on the anthracite/activating agent ratio, on the kind of activating agent and finally on the temperature of activation.     

Binderfree synthesis of high-surface-area carbon electrodes via CO2 activation of resorcinol–formaldehyde carbon xerogel disks: Analysis of activation process

High-surface-area carbon xerogels were prepared in the form of disks via carbonization of precursor resorcinol–formaldehyde (RF) polymer disks and subsequent activation of the resultant RF carbon xerogels by CO₂. RF carbon xerogels allow the preparation of a set of pre-activated carbon disks having different mesopore volumes. Analysis of the relationship between the mesopore volume of the samples and their CO₂ activation efficiency showed that the presence of mesopores is crucial for obtaining a high-surface-area carbon with minimal burn-off of carbon atoms. Activation of an RF carbon disk with a mesopore volume of 1.0 cm³ g⁻¹ up to a burn-off of 81% yielded an activated carbon disk with a high BET surface area of ∼3000 m² g⁻¹. Such disks could be readily used as electrode materials for an electric double layer capacitor without filler or binder addition and exhibited competitive EDLC performance against other electrode materials previously reported.     

Enhancement of CO2 adsorption on phenolic resin-based mesoporous carbons by KOH activation

Carbons with high surface area and large volume of ultramicropores were synthesized for CO₂ adsorption. First, mesoporous carbons were produced by soft-templating method using triblock copolymer Pluronic F127 as a structure directing agent and formaldehyde and either phloroglucinol or resorcinol as carbon precursors. The resulting carbons were mainly mesoporous with well-developed surface area, large total pore volume, and only moderate CO₂ uptake. To improve CO₂ adsorption, these carbons were subjected to KOH activation to enhance their microporosity. Activated carbons showed 2–3-fold increase in the specific surface area, resulting from substantial development of microporosity (3–5-fold increase in the micropore volume). KOH activation resulted in enhanced CO₂ adsorption at 760 mmHg pressure: 4.4 mmol g⁻¹ at 25 °C, and 7 mmol g⁻¹ at 0 °C. This substantial increase in the CO₂ uptake was achieved due to the development of ultramicroporosity, which was shown to be beneficial for CO₂ physisorption at low pressures. The resulting materials were investigated using low-temperature nitrogen physisorption, CO₂ sorption, and small-angle powder X-ray diffraction. High CO₂ uptake and good cyclability (without noticeable loss in CO₂ uptake after five runs) render ultramicroporous carbons as efficient CO₂ adsorbents at ambient conditions.     

Polystyrene-based carbon spheres as electrode for electrochemical capacitors

A series of carbon spheres with various porous texture parameters were prepared from polystyrene-based macroreticular resin spheres by carbonization and activation. The as-prepared carbon spheres had a maximum specific surface area of 996 m² g^?1, total pore volume of 1.34 cm³ g^?1 and average pore size of 5.39 nm. Moreover, these carbon spheres showed a mesopore size distributed mainly in about 40 nm. A high specific capacitance of 153 F g^?1 for carbon sphere by carbonization, 164 F g^?1 for carbon sphere by activation for 1 h and 182 F g^?1 for carbon sphere by activation for 2 h can be obtained. Moreover, a specific energy between 2.3 and 5.1 Wh kg^?1 for these carbon spheres can be obtained in 6 mol L^?1 KOH electrolyte.     

Reduction of [Fe(III)EDTA]− catalyzed by activated carbon modified with KOH solution

NO and SO₂ can be eliminated simultaneously by [Fe(II)EDTA]^2? solution with a pH range of 5.6–8.0 at 25–80 °C. Activated carbon is used to catalyze the regeneration of [Fe(II)EDTA]^2?. In this paper, KOH solution has been utilized to modify the carbon to improve its catalytic capability. Experimental results show that the optimal modification factors are as follow: KOH concentration 6.0 mol l^?1, impregnation time 9 h, activation temperature 700 °C and activation time 4 h. After KOH modification, the surface area of activated carbon decreases. But its basicity is enhanced, which plays an important role in improving the catalytic characteristics of activated carbon in the reduction of [Fe(III)EDTA]^?. The experimental results demonstrate that the activated carbon modified by concentrated KOH solution can get a higher NO removal efficiency than the original activated carbon.     

Mesoporous carbon prepared from direct coal liquefaction residue for methane decomposition

Mesoporous carbons (MCs) were directly prepared from direct coal liquefaction residue (CLR) by KOH activation, and used as catalysts for methane decomposition. The results indicated that the prepared MCs were of a narrow pore size distribution centered at about 3.5 nm. The mineral matters in the CLR and their salts formed during KOH activation process served as templates for mesopore formation, through washing off the mineral matters and the salts occupied in the inner space of the carbon. The resultant MCs showed higher and more stable activity in methane decomposition reaction than commercial coal-based activated carbon and carbon black catalysts.     

Activated carbon xerogels with a cellular morphology derived from hydrothermally carbonized glucose-graphene oxide hybrids and their performance towards CO2 and dye adsorption

We report the preparation of micro-/mesoporous carbon monolithic xerogels by means of a two-step approach that comprises (1) hydrothermal carbonization of glucose in the presence of graphene oxide (GO) sheets as morphology-directing agents and (2) chemical activation of the resulting hydrothermal carbon (HTC) xerogels with KOH. The as-prepared HTC xerogels were made up of a random assembly of thin (<30 nm) carbon platelets, which were interpreted to arise via dehydration and condensation reactions of glucose at catalytically active (acidic) sites present on the surface of GO. The chemical activation afforded xerogels with large surface areas and pore volumes (up to ∼2000 m² g⁻¹ and 1.15 cm³ g⁻¹, respectively) and a cellular morphology, which could be attributed to the combined effect of the activating agent and the unusual, compliant nature of the HTC xerogel. Additionally, the use of different activation conditions allowed fine-tuning the porous texture of the activated xerogels. Finally, the activated carbon xerogels displayed CO₂ uptake capacities up to 4.9 mmol g⁻¹ at 0 °C and 1 bar, as well as an efficient performance (between 600 and 700 mg g⁻¹) in the adsorption of bulky dyes, thus demonstrating their application potential.     

Sulfur-doped mesoporous carbon from surfactant-intercalated layered double hydroxide precursor as high-performance anode nanomaterials for both Li-ion and Na-ion batteries

We describe a preparation of sulfur-doped mesoporous amorphous carbon (SMAC) from a commercially available alkyl surfactant sulfonate anion-intercalated NiAl-layered double hydroxide precursor via thermal decomposition and subsequent acid leaching. The resultant amorphous carbon is endowed with the integrated advantage of featuring high reversible capacity and long cycling stability: intrinsic doping of sulfur, large specific area, and broad mesopore size distribution. Electrochemical evaluation shows that the SMAC electrode exhibits highly enhanced electrochemical performances, compared with the electrode of non-doped mesoporous and amorphous carbon prepared by using a different surfactant (sodium laurate). A high reversible capacity of 958 mA h g⁻¹ is achieved for the SMAC electrode after 110 cycles at 200 mA g⁻¹, and especially a superlong cycle life with a reversible capacity of 579 mA h g⁻¹ after 970 cycles at 500 mA g⁻¹. Moreover, the SMAC electrode can facilitate the reversible insertion/extraction of Na ion, owing to the proper specific area and mesopore size distribution, as well as the improved electronic conductivity resulted from doping of sulfur.     

Exceptional electrochemical performance of nitrogen-doped porous carbon for lithium storage

Crumpled nitrogen-doped porous carbon sheets are successfully fabricated via chemical activation of polypyrrole-functionalized graphene sheets with KOH (APGs). The obtained APGs with nitrogen doping, high surface area, porous and crumpled structure exhibit exceptional electrochemical performances as the electrode material for LIBs, including a superhigh reversible specific capacity of 1516.2 mAh g⁻¹, excellent cycling stability over 10,000 cycles, and good rate capability (133.2 mAh g⁻¹ even at a very high current density of 40 A g⁻¹). The chemical activation synthesis strategy might open new avenues for the design of high-performance carbon-based anode materials.     

Analysis of the microporous texture of a glassy carbon by adsorption measurements and Monte Carlo simulation. Evolution with chemical and physical activation

A mesoporous glassy carbon has been chemically (KOH) and physically (CO₂) activated in order to improve its micropore volume while preserving the well-defined mesopore network. The microporosity of the glassy carbon and the evolution of the micropore texture with activation have been studied by means of Monte Carlo simulation and gas adsorption. Micropore size distributions obtained from simulated adsorption isotherms on slit-shaped pores revealed different accessibilities of methane and nitrogen to the microporous texture of the original sample, indicating the presence of constrictions in the micropore network. Both activating agents are able to increase the micropore volume generating new micropores, although KOH showed to be more effective. While CO₂ treatment preserved some hindrances to the access of nitrogen molecules to the micropores, KOH activation generates a more accessible micropore network. Therefore, chemical activation by KOH is a suitable way to increase the adsorption capacity of glassy carbons while preserving the mesoporous structure. Molecular simulation of adsorption linked to experimental adsorption of different gases, has proven to give very satisfactory results in analysing the evolution of the micropore texture and accessibility of carbon materials by different activation treatments.     

A two-step synthesis of ordered mesoporous resorcinol–formaldehyde polymer and carbon

A new two-step method is developed for the synthesis of resorcinol–formaldehyde polymer and carbon with highly ordered mesoporous structures. For this method, resorcinol and formaldehyde is pre-polymerized in the first step under the presence of a basic catalyst to produce resorcinol–formaldehyde resol. Then, the resorcinol–formaldehyde resol is mixed with Pluronic F127 solution followed by the addition of an acid catalyst to allow the rapid self-assembly and condensation in the second step. Compared with the early reported evaporation-induced self-assembly method as well as the one-step liquid phase self-assembly method, in the present two-step liquid method the self-assembly and condensation process can be carried out rapidly by using low amount of base and acid catalysts at room temperature. After the activation by CO₂, the carbon materials maintained ordered mesostructure, and the BET surface area enlarged to 2660 m²/g and total pore volume increased to 2.01 cm³/g. The CO₂ activation not only creates micropores within the carbon frameworks but also enlarges the mesopores by elimination of the carbon pore walls.     

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.