Easy synthesis of mesoporous carbon using nano-CaCO3 as template

Mesoporous carbon has been synthesized using nano-CaCO₃ as a template and sucrose as carbon precursor. It is shown that the CaCO₃/sucrose weight ratio has a marked effect on the porosity of the carbon. At a weight ratio of CaCO₃/sucrose of 1:1, the surface area of the carbon reaches 892 m² g⁻¹ and the pore size is around 45 nm. The carbon shows a capacitance of 155 F/g with excellent rate capability in 6 mol L⁻¹ KOH aqueous electrolytes.     

Activated carbon fiber cloths as electrodes for high performance electric double layer capacitors

Activated carbon fiber cloth (ACFC) electrodes with high double layer capacitance and good rate capability were prepared from polyacrylonitrile (PAN) fabrics by optimizing the carbonization temperature prior to CO₂ activation. The carbonization temperature has a marked effect on both the pore structure and the electrochemical performances of the ACFCs. Moderate carbonization at 600 °C results in higher specific surface area and larger pore size, and hence higher capacitance and better rate capability. The specific capacitance of the ACFCs in 6 mol L⁻¹ KOH aqueous solution can be as high as 208 F g⁻¹. It remains 129 F g⁻¹ as the current density increases to 10 000 mA g⁻¹.     

Relationship between the nanoporous texture of activated carbons and their capacitance properties in different electrolytes

A series of activated carbons (ACs) with progressively changing nanotextural characteristics was obtained by heat-treatment of a bituminous coal at temperatures ranging from 520 to 1000 °C, and subsequent activation by KOH at 700 °C or 800 °C. As the pre-treatment temperature increases, the total pore volume V_T decreases from 1.28 to 0.30 cm³ g⁻¹, and the BET specific surface area from 3000 to 800 m² g⁻¹. The specific capacitance determined for each series of ACs using symmetric two electrode cells in 6 mol L⁻¹ KOH varies almost linearly with the BET surface area, suggesting that the charge accumulation is controlled primarily by the surface area development. A further analysis of the electrochemical behaviour in different electrolytic media—aqueous and organic—shows that an adequate pore size is more important than a high surface area in order to obtain high values of capacitance. Theoretical values of volumetric capacitance could be evaluated without considering the size of ions, which is always uncertain in solution, and compared with the experimental data as a function of the pore width. The efficiency of pore filling, i.e., of double layer formation, is optimal when the pore size is around 0.7 nm in aqueous media and 0.8 nm in organic electrolyte. A study of the performance of the positive and negative electrodes during the charge/discharge of the capacitor, reveals an additional pseudo-faradaic contribution due to oxygenated functionalities within the working potential window of the negative electrode. This effect is more pronounced for the ACs series obtained at 700 °C, because of their higher oxygen content.     

High performance of nanoporous carbon in cryogenic hydrogen storage and electrochemical capacitance

Nanoporous carbon materials were synthesized by a two-step casting process using zeolite 13X as template. The nanoporous structures were characterized by X-ray diffraction, high resolution transmission electron microcopy and nitrogen sorption at 77 K, and the results show that pore filling in the zeolite channels could play an important role in the replication of zeolite-like structural order. Better pore filling led to a more ordered structure as well as higher surface area and pore volume. Further potassium hydroxide (KOH) activation improved the microporous texture to the carbon framework and resulted in higher surface area and pore volume. A large hydrogen uptake capacity of 6.30 wt.% has been achieved at 77 K and 20 bar. Besides, a high gravimetric capacitance of up to 160 F g⁻¹ and an energy density of 30 W h kg⁻¹ have been obtained when tested as an electrode for supercapacitors. The high performance in cryogenic hydrogen storage and electrochemical capacitance were closely correlated with the pore structures of the carbon materials.     

Porous carbons prepared by using metal-organic framework as the precursor for supercapacitors

Several kinds of porous carbons were easily prepared by using metal-organic framework as both template and carbon precursor. Nanocasting is chosen to adjust the textures and structures with phenolic resin or carbon tetrachloride and ethylenediamine as the additional carbon sources. The carbon materials were further activated by potassium hydroxide (KOH). The electrochemical capacitance behaviors of these carbon materials were investigated in both aqueous and organic electrolytes. Energy densities of 9.4 W h kg⁻¹ in 6 M KOH and 31.2 W h kg⁻¹ in 1.5 M tetraethylammonium tetrafluoroborate acetonitrile solution can be obtained for one of the prepared porous carbon materials (MAC-A) with the surface area of 2222 m² g⁻¹ and the total pore volume of 1.14 cm³ g⁻¹. Due to its high packing density of 0.93 g cm⁻³, the related volumetric specific energy densities of 8.8 and 29.0 W h L⁻¹ can be got.     

Capacitance behavior of activated carbon fibers with oxygen-plasma treatment

Modified activated carbon fibers (ACFs) were used as the electrodes of an electric double-layer capacitor and showed an enhanced capacitance effect after a RF-plasma treatment. The capacitance and the surface functional groups of the ACFs were studied. For the plasma-treated ACFs having a specific surface area of 1500 m² g⁻¹, the capacitance increased by 28% compared to the untreated sample and the highest electric capacitance value of 142 F g⁻¹ was achieved with an oxygen feed concentration of 10 vol.%. The Brunauer-Emmett-Teller (BET) surface area was 2103 m² g⁻¹, which was 34% higher than that of the untreated sample. The pore volume was similarly increased to 483.1 cm³ g⁻¹ STP, and from the pore distribution plot, quantities of mesopores of 10 nm or less and micropores also increased. However, in order to enhance the capacitance, the quinone functional group had a significant influence in addition to the BET surface area. The correlation between the capacitance and the number of quinone functional groups was confirmed because quinone is an electron acceptor.     

Improvement of the structural and chemical properties of a commercial activated carbon for its application in electrochemical capacitors

The present paper shows that the performance of an inexpensive activated carbon used in electrochemical capacitors can be significantly enhanced by a simple treatment with KOH at 850 °C. The changes in the specific surface area, as well as in the surface chemistry, lead to high capacitance values, which provide a noticeable energy density.The KOH-treatment of a commercial activated carbon leads to highly pure carbons with effective surface areas in the range of 1300-1500 m² g⁻¹ and gravimetric capacitances as high as three times that of the raw carbon.For re-activated carbons, one obtains at low current density (50 mA g⁻¹) values of 200 F g⁻¹ in aqueous electrolytes (1M H₂SO₄ and 6M KOH) and around 150 F g⁻¹ in 1M (C₂H₅)₄NBF₄ in acetonitrile. Furthermore, the resulting carbons present an enhanced and stable performance for high charge/discharge load in organic and aqueous media.This work confirms the possibilities offered by immersion calorimetry on its own for the prediction of the specific capacitance of carbons in (C₂H₅)₄NBF₄/acetonitrile. On the other hand, it also shows the limitations of this technique to assess, with a good accuracy, the suitability of a carbon to be used as capacitor electrodes operating in aqueous electrolytes (H₂SO₄ and KOH).     

An activation-free method for preparing microporous carbon by the pyrolysis of poly(vinylidene fluoride)

A simple method for the preparation of microporous carbon was presented by pyrolyzing poly(vinylidene fluoride) (PVDF) at high temperature under N₂ atmosphere without activation or any other additional processes. The yield of PVDF-derived carbon is 35.0%. Its specific surface area reaches 1012 m² g with a pore volume of 0.41 cm³ g⁻¹. The carbon is microporous with unimodal pore size distribution at 0.55 nm.     

Polyfurfuryl alcohol derived activated carbons for high power electrical double layer capacitors

Polyfurfuryl alcohol (PFA) derived activated carbons were prepared by the acid catalysed polymerization of furfuryl alcohol, followed by potassium hydroxide activation. Activated carbons with apparent BET surface areas ranging from 1070 to 2600 m² g⁻¹, and corresponding average micropore sizes between 0.6 and 1.6 nm were obtained. The porosity of these carbons can be carefully controlled during activation and their performance as electrode materials in electric double layer capacitors (EDLCs) in a non-aqueous electrolyte (1 M Et₄NBF₄/ACN) is investigated.Carbon materials with a low average pore size (<∼0.6 nm) exhibited electrolyte accessibility issues and an associated decrease in capacitance at high charging rates. PFA carbons with larger average pore sizes exhibited greatly improved performance, with specific electrode capacitances of 150 F g⁻¹ at an operating voltage window of 0-2.5 V; which corresponds to 32 Wh kg⁻¹ and 38 kW kg⁻¹ on an active material basis. These carbons also displayed an outstanding performance at high current densities delivering up to 100 F g⁻¹ at current densities as high as 250 A g⁻¹. The exceptionally high capacitance and power of this electrode material is attributed to its good electronic conductivity and a highly effective combination of micro- and fine mesoporosity.     

Effect of pore size distribution of coal-based activated carbons on double layer capacitance

A series of coal-based activated carbons representing a wide range of mesopore content, from 16.7 to 86.9%, were investigated as an electrode in electric double layer capacitors (EDLCs) in 1 mol l⁻¹ H₂SO₄ and 6 mol l⁻¹ KOH electrolytic solutions. The activated carbons (ACs) used in this study were produced from chemically modified lignite, subbituminous and bituminous coals by carbonization and subsequent activation with steam. The BET surface area of ACs studied ranged from 340 to 1270 m² g⁻¹. The performance of ACs as EDLC electrodes was characterized using voltammetry, galvanostatic charge/discharge and impedance spectroscopy measurements. For the carbons with surface area up to 1000 m² g⁻¹, the higher BET surface area the higher specific capacitance (F g⁻¹) for both electrolytes. The surface capacitance (μF cm⁻²) increases also with the mesopore content. The optimum range of mesopore content in terms of the use of ACs studied for EDLCs was found to be between 20 and 50%. A maximum capacitance exceeding 160 F g⁻¹ and a relatively high surface capacitance about 16 μF cm⁻² measured in H₂SO₄ solution were achieved for the AC prepared from a sulfonated subbituminous coal. This study shows that the ACs produced from coals exhibit a better performance as an electrode material of EDLC in H₂SO₄ than in KOH electrolytic solutions. For KOH, the capacitance per unit mesopore surface is slightly lower than that referred to unit micropore surface (9.1 versus 10.1 μF cm⁻²). However, in the case of H₂SO₄ the former capacitance is double and even higher compared with the latter (23.1 versus 9.8 μF cm⁻²). On the other hand, the capacitance per micropore surface area is the same in both electrolytes used, about 10.0 μF cm⁻².     

Polypyrrole/carbon composite electrode for high-power electrochemical capacitors

Nano-thin polypyrrole (PPy) layers with thickness from ∼5 nm to several 10s nm were deposited on vapor grown carbon fibers (VGCF) by an in situ chemical polymerization. Using different concentrations of the pyrrole could control the thicknesses of deposited PPy layers. Surface morphology and thickness of the deposited PPy layers were confirmed by means of scanning electron microscopy and scanning transmission emission microscopy. Pseudo-capacitive behavior of the deposited PPy layers on VGCF investigated by means of cyclic voltammetry. Then, the PPy/VGCF composites were mixed with activated carbons (AC) at various mixing ratios. For the PPy/VGCF/AC composite electrodes, characteristics of specific capacitance and power capability were examined by half-cell tests. As results of this study, it was investigated that nano-thin PPy layer below ∼10 nm deposited on VGCF had high pseudo-capacitance and fast reversibility. Its specific capacitance per averaged weight of active material (PPy) was obtained as ∼588 F g⁻¹ at 30 mV s⁻¹ and maintained as ∼550 F g⁻¹ at 200 mV s⁻¹ of scan rate. Also, from the mixing 60 wt.% of the PPy/VGCF with 25 wt.% of AC, the PPy/VGCF/AC composite electrode exhibited higher power capability maintaining the specific capacitance per active materials of PPy and AC as ∼300 F g⁻¹ at 200 mV s⁻¹ in 6 M KOH.     

Synthesis of nanosized vanadium pentoxide/carbon composites by spray pyrolysis for electrochemical capacitor application

Nanostructured vanadium pentoxide/carbon (V₂O₅/carbon) composite powders with enhanced specific capacitance were synthesized by the spray pyrolysis technique. Electrochemical properties were examined by the cyclic voltammetry technique. Following analysis of powders sprayed at different temperatures, composite powders obtained at an optimum temperature of 450 °C yielded a maximum specific capacitance of 295 F g⁻¹ in 2 M KCl electrolyte at a 5-mV s⁻¹ scan rate. The weight percentage of carbon-related species was 2.7 wt% in this V₂O₅/carbon composite, as detected by thermogravimetric analysis (TGA) and confirmed by transmission electron microscope energy dispersive spectroscopy (TEM-EDS) analysis. Following initial X-ray diffraction (XRD) characterization, scanning electron microscope (SEM), TEM and high-resolution TEM (HRTEM) imaging revealed a specific morphology of spherical shell agglomerates of V₂O₅ nanorods and nanoribbons, with each shell comprising a network of these one- and two-dimensional nanoparticles in an amorphous carbon matrix. The V₂O₅ network was not fully dense, and the majority of the nanorod sizes were in the range of 50-150 nm, with additional long nanoribbons extending from the outsides of the spherical shells. The specific surface area was 18 m² g⁻¹ for the composite powders, and the pore size distribution revealed that the majority of pores had diameters in the range of 40-50 Å, which was relatively larger than the pore diameters obtained at 500 °C and would be beneficial for electrochemical performance. The enhancement of the specific capacitance in V₂O₅/carbon composites was attributed to the distribution of amorphous carbon throughout the V₂O₅ and the particular open nanostructure.     

Metal-organic framework (MOF) as a template for syntheses of nanoporous carbons as electrode materials for supercapacitor

Five nanoporous carbons (NPCs) were prepared by polymerizing and then carbonizing carbon precursor of furfuryl alcohol accommodated in a porous metal-organic framework (MOF-5, [Zn₄O(bdc)₃], bdc = 1,4-benzenedicarboxylate) template. The Brunauer-Emmett-Teller (BET) surface areas for five NPC samples obtained by carbonizing at the temperatures from 530 to 1000 °C fall into the range from 1140 to 3040 m² g⁻¹ and the dependence of BET surface areas on carbonization temperatures shows a “V” shape. All the five NPC samples have a pore size distribution centered at about 3.9 nm. As electrode materials for supercapacitor, the NPC samples obtained at the temperatures higher than 600 °C display the ideal capacitor behaviors and give rise to almost constant specific capacitance (above 100 F g⁻¹ at 5 mV s⁻¹) at various sweep rates, which is associated with their mesoporous characteristics. However, the NPC sample with the highest BET surface area (3040 m² g⁻¹) obtained by carbonizing at 530 °C gives a unusually low capacitance (12 F g⁻¹ at 5 mV s⁻¹), which may be attributed to the poor conductivity of the carbon material due to the low carbonization temperature.     

EDLC performance of carbide-derived carbons in aprotic and acidic electrolytes

This study shows that carbide-derived carbons (CDCs) with average pore size distributions around 0.9-1 nm and effective surface areas of 1300-1400 m² g⁻¹ provide electrochemical double-layer capacitors with high performances in both aqueous (2M H₂SO₄) and aprotic (1M (C₂H₅)₄NBF₄ in acetonitrile) electrolytes.In the acidic electrolytic solution, the gravimetric capacitance at low current density (1 mA cm⁻²) can exceed 200 F g⁻¹, whereas the volumetric capacitance reaches 90 F cm⁻³. In the aprotic electrolyte they reach 150 F g⁻¹ and 60 F cm⁻³.A detailed comparison of the capacitive behaviour of CDCs at high current density (up to 100 mA cm⁻²) with other microporous and mesoporous carbons indicates better rate capabilities for the present materials in both electrolytes. This is due to the high surface area, the accessible porosity and the relatively low oxygen content.It also appears that the surface-related capacitances of the present CDCs in the aprotic electrolyte are in line with other carbons and show no anomalous behaviour.     

Preparation of functionalized graphene sheets by a low-temperature thermal exfoliation approach and their electrochemical supercapacitive behaviors

Two kinds of functionalized graphene sheets were produced by thermal exfoliation of graphite oxide. The first kind of functionalized graphene sheets was obtained by thermal exfoliation of graphite oxide at low temperature in air. The second kind was prepared by carbonization of the first kind of functionalized graphene sheets at higher temperature in N₂. Scanning electron microscopy images show that both two kinds of samples possess nanoporous structures. The results of N₂ adsorption-desorption analysis indicate that both of two kinds of samples have high BET surface areas. Moreover, the second kind of functionalized graphene sheets has a relatively higher BET surface area. The results of electrochemical tests is as follows: the specific capacitance values of the first kind of functionalized graphene sheets in aqueous KOH electrolyte are about 230 F g⁻¹; the specific capacitance values of the second kind of functionalized graphene sheets with higher BET surface areas are only about 100 F g⁻¹; however, compared with the first kind of functionalized graphene sheets, the second kind has a higher capacitance retention at large current density because of its good conductive behaviors; furthermore, in non-aqueous EC/DEC electrolyte, the specific capacitance values of the first kind sample and the second kind sample are about 73 F g⁻¹ and 36 F g⁻¹, respectively.     

High capacitance properties of polyaniline by electrochemical deposition on a porous carbon substrate

Electrochemical deposition of polyaniline (PANI) is carried out on a porous carbon substrate for supercapacitor studies. The effect of substrate is studied by comparing the results obtained using platinum, stainless steel and porous carbon substrates. PANI deposited at 100 mV s⁻¹ sweep rate by potentiodynamic technique on porous carbon substrate is found to possess superior capacitance properties. Experimental variables, namely, concentrations of aniline monomer and H₂SO₄ supporting electrolyte are varied and arrived at the optimum concentrations to obtain a maximum capacitance of PANI. Low concentrations of both aniline and H₂SO₄, which produce PANI at low rates, are desirable. The PANI deposits prepared under these conditions possess network morphology of nanofibrils. Capacitance values as high as 1600 F g⁻¹ are obtained and PANI coated carbon electrodes facilitate charge-discharge current densities as high as 45 mA cm⁻² (19.8 A g⁻¹). Electrodes are found to be fairly stable over a long cycle-life, although there is some capacitance loss during the initial stages of cycling.     

Resorcinol-formaldehyde based porous carbon as an electrode material for supercapacitors

Porous carbon materials were prepared using resorcinol and formaldehyde catalyzed by KOH in a sol-gel process followed by carbonization, during which the KOH serves as an activating agent and generates pores mainly located in the micropore range. With an increase of mass ratio of KOH to resorcinol from 1 to 4, both the specific surface area and the pore volume of the carbons increased, from 522 to 2760 m²/g and 0.304 to 1.347 cm³/g, respectively, but the average pore diameter decreased from 4.4 to 2.5 nm. Samples were investigated as electrode materials in supercapacitors and the relevant electrochemical behavior was characterized by cyclic voltammetry, electrochemical impedance spectroscopy and constant current charge-discharge experiments using 30% KOH aqueous solution as electrolyte. The highest specific capacitance of up to 294 F/g was obtained at a current density 1 mA/cm² for the sample with mass ratio of KOH to resorcinol of 2. Only a slight decrease in capacitance for the same sample, from 294 to 242 F/g, was observed when the current density increased from 1 to 30 mA/cm². The specific capacitance only decayed 3% at a current density 30 mA/cm² after 1000 cycles, which indicates that the sample possesses excellent power property and cycle durability.     

Nanoscale fine-tuning of porosity of carbide-derived carbon prepared from molybdenum carbide

Micro- and mesoporous carbide-derived carbon (CDC) was synthesised from molybdenum carbide (Mo₂C) powder by gas phase chlorination in the temperature range from 400 to 1200 °C. Analysis of XRD results show that C(Mo₂C), chlorinated at 1200 °C, consist mainly on graphitic crystallites of mean size, L_a = 9 nm and L_c = 7.5 nm. The first-order Raman spectra showed the graphite-like absorption peak at ∼1587 cm⁻¹ and the disorder-induced (D) peak at ∼1348 cm⁻¹. The low-temperature N₂ adsorption experiments were performed and a specific surface area up to 1855 m² g⁻¹ and total pore volume up to 1.399 cm³ g⁻¹ were obtained. Sorption measurements showed the presence of both micro- and mesopores after chlorination at 400-900 °C and only mesopores after chlorination at 1000°-1200 °C. Stepwise formation of micro- and mesopores was achieved and the peak pore size can be shifted from 0.8 nm up to 4 nm by increasing the chlorination temperature.     

Ultra-high specific capacitance of self-doped 3D hierarchical porous turtle shell-derived activated carbon for high-performance supercapacitors

The turtle shell of biomass waste is used as raw material, and the natural inorganic salt contained in it is used as a salt template in combination with a chemical activation method to successfully prepare a high-performance activated carbon with hierarchical porous structure. The role of hydroxyapatite (HAP) and KOH in different stages of preparation was investigated. The prepared turtle shell-derived activated carbon (TSHC-5) has a well-developed honeycomb pore structure, which gives it a high specific surface area (SSA) of 2828 m² g^?1 with a pore volume of 1.91 cm³ g^?1. The excellent hierarchical porous structure and high heteroatom content (O 6.88%, N 5.64%) allow it to have an ultra-high specific capacitance of 727.9 F g^?1 at 0.5 A g^?1 with 92.27% of capacitance retention even after 10,000 cycles. Excitingly, the symmetric supercapacitor assembled from TSHC-5 activated carbon exhibits excellent energy density and cycling stability in a 1 M Na₂SO₄ aqueous solution. The energy density is 45.1 Wh·kg^?1 at a power density of 450 W kg^?1, with 92.05% capacitance retention after 10,000 cycles. Therefore, turtle shell-derived activated carbon is extremely competitive in sustainable new green supercapacitor electrode materials.     

Electrochemical capacitors based on highly porous carbons prepared by KOH activation

Various coal and pitch-derived carbonaceous materials were activated for 5 h at 800 °C using potassium hydroxide and 1:4 component ratio. Porosity development of the resultant activated carbons (ACs) was assessed by N₂ sorption at 77 K and their capability of the charge accumulation in electric double layer was determined using galvanostatic, voltammetric and impedance spectroscopy techniques. ACs produced from different precursors are all microporous in character but differ in terms of the total pore volume (from 1.05 to 1.61 cm³ g⁻¹), BET surface area (from 1900 to 3200 m² g⁻¹) and pore size distribution. Very promising capacitance values, ranging from 200 to 320 F g⁻¹, have been found for these materials operating in acidic 1 mol l⁻¹ H₂SO₄ electrolytic solution. The variations in the electrochemical behavior (charge propagation, self-discharge, frequency response) are considered in relation to the porous texture characteristics, elemental composition but also possible effect of structural ordering due to various precursor materials used. Cycling investigation of all the capacitors has been also performed to compare ability of the charge accumulation for different carbon materials during subsequent cycles.