Hierarchical porous carbons with controlled micropores and mesopores for supercapacitor electrode materials

Various porous carbons were prepared by CO₂ activation of ordered mesoporous carbons and used as electrode materials for supercapacitor. The structures were characterized by using X-ray diffraction, transmission electron microscopy and nitrogen sorption at 77 K. The effects of CO₂ treatment on their pore structures were discussed. Compared to the pristine mesoporous carbons, the samples subjected to CO₂ treatment exhibited remarkable improvement in textural properties. The electrochemical measurement in 6 M KOH electrolyte showed that CO₂ activation leads to better capacitive performances. The carbon CS15A6, which was obtained after CO₂ treatment for 6 h at 950 °C using CMK-3 as the precursor, showed the best electrochemical behavior with a specific gravimetric capacitance of 223 F/g and volumetric capacitance of 54 F/cm³ at a scan rate of 2 mV/s and 73% retained ratio at 50 mV/s. The good capacitive behavior of CS15A6 may be attributed to the hierarchical pore structure (abundant micropores and interconnected mesopores with the size of 3-4 nm), high surface area (2749 m²/g), large pore volume (2.09 cm³/g), as well as well-balanced microporosity and mesoporosity.     

The role of textural characteristics and oxygen-containing surface groups in the supercapacitor performances of activated carbons

It is suggested that the specific capacitance C₀ of activated carbons at low current densities (d ∼ 1 mA cm⁻²) consists, to a good first approximation, of two contributions. For the H₂SO₄ electrolyte they correspond to approximately 0.080 F m⁻² from the total accessible surface area and an additional pseudo-capacitance of 63 F mmol⁻¹ from the surface species generating CO in thermally programmed desorption (TPD). The new correlation proposed here is an alternative to Shi's earlier approach which considered contributions from the microporous and the external surface areas. Furthermore, it appears that the variation of the specific capacitance C at high current densities d (up to 100-150 mA cm⁻²) depends essentially on the CO₂-generating surface groups and on 1/L₀, the inverse of the average micropore width.     

On the determination of surface areas in activated carbons

The paper examines the validity of two approaches frequently used to determine surface areas in activated carbons, namely the BET method and the use of immersion calorimetry. The study is based on 21 well characterized carbons, whose external and microporous surface areas, S_e and S_mi, have been determined by a variety of independent techniques. It appears clearly that S_BET and the real surface area S_mi + S_e are in agreement only for carbons with average pore widths L_o around 0.8-1.1 nm. Beyond, S_BET increases rapidly and S_BET − S_e is practically the monolayer equivalent of the micropore volume W_o. This confirms that a characterization of surface properties based on S_BET is, a priori, not reliable. The study of the enthalpy of immersion of the carbons into benzene at 293 K, based on Dubinin’s theory, shows that Δ_iH consists of three contributions, namely from the interactions with the micropore walls (−0.136 J m⁻²), the external surface (−0.114 J m⁻²), and from the volume of liquid found between the surface layers in the micropores (−141 J cm⁻³). It appears that for carbons where L_o > 1 nm, the real surface area cannot be determined in a reliable way from the enthalpy of immersion and a specific heat of wetting alone.     

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.     

Synthetic carbons activated with phosphoric acid: II. Porous structure

Synthetic activated carbons were prepared by H₃PO₄ activation of a chloromethylated and sulfonated copolymer of styrene and divinylbenzene, using an impregnation weight ratio of 0.75 and carbonization temperatures in the 400-1000 °C range. Other impregnation ratios (0.93 and 1.11) were also used at a carbonization temperature of 800 °C. The porous texture of the resulting carbons was characterized by N₂ adsorption at −196 °C and CO₂ adsorption at 0 °C. All carbons exhibited a multimodal pore size distribution with maxima in the micropore and meso/macropore regions. Maxima in pore volume were attained at 900 °C for micropores and at 500 and 900 °C for mesopores. The mesopore volume was less sensitive than the micropore volume to changes in the impregnation ratio. It is concluded that the porous texture is not a prime factor in determining the outstanding cation exchange capacities of these carbons.     

Bimodal, templated mesoporous carbons for capacitor applications

Several high capacitance ordered mesoporous carbon (OMC) materials, containing a bimodal pore distribution, were synthesized directly using hexagonal mesoporous silicas (HMS) as the template material. The HMS templates were formed using amine surfactants (C_nH_2n+1NH₂) with hydrophobic chain lengths containing 8-16 carbons (n = 8-16). These HMS structures were found to have an interconnected wormhole structure, high textural mesoporosity, a surface area ranging from 910 to 1370 m²/g, and a total pore volume of 1.09-1.83 cm³/g. Also, evidence for a change in structure from hexagonally ordered to layered (for surfactants of chain length with n > 12) was found. The resulting OMCs, formed using sucrose as the carbon precursor, contain bimodal pores 1.6-1.8 and 3.3-3.9 nm in diameter and have a very high surface area (980-1650 m²/g). The OMCs were evaluated as electrode materials for electrochemical capacitors using cyclic voltammetry in 0.5 M H₂SO₄ solution, giving a tunable gravimetric capacitance that increased linearly with BET area (and surfactant chain length), up to 260 F/g, among the highest yet reported for ordered carbon formed from an HMS templated precursor. All OMCs studied in this work displayed a specific capacitance of ∼0.15 F/m².     

Ordered mesoporous carbons synthesized by a modified sol-gel process for electrosorptive removal of sodium chloride

Capacitive deionization (CDI) represents an alternative process to remove the ions from the brackish water. In this study two series of ordered mesoporous carbons (OMCs) that demonstrated the potential use for capacitive desalination have been synthesized by a modified sol-gel process involving nickel salts. It was shown that the preferred formation of crown-ether type complexes between nickel ions and triblock copolymers resulted in higher BET surface area and smaller mesopores. As the electrode materials for CDI, OMC obtained by the addition of NiSO₄ · 6H₂O exhibited best electrochemical performance compared with other OMCs and a commercial activated carbon either in 0.1 M NaCl solution or in 0.0008 M NaCl solution, plus the amount of adsorbed ions measured by a flow through apparatus reached 15.9 μmol g⁻¹ and the ions could be fully released into the solution. The excellent electrosorption desalination performance of OMC obtained by the addition of NiSO₄ · 6H₂O was ascribed to its high BET surface area of 1491 m² g⁻¹ and ordered mesopores of 3.7 nm. Based on these results, it is deduced that the modified sol-gel process might be a potential method of obtaining the excellent electrode materials for CDI.     

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).     

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.     

Carbon adsorbents from waste ion-exchange resins

A series of activated carbons was prepared from different waste commercial ion-exchange resins and studied by means of adsorption, SEM and IR methods. Samples were additionally washed or washed/frozen. This resulted in increases in micro- and mesoporosity in comparison with initial activated carbons. For some samples, the latter treatment gives enhancement of mesoporosity but reduction of microporosity and vice versa comparing with only washed carbons due to different localization of water droplets in mesopores or micropores. Changes in the morphology of chars and activated samples depended on resin composition and history. Relatively high values of porosity (V_p ≈ 0.4 cm³/g) and specific surface area (S_BET ≈ 600 m²/g) show that activated carbons prepared from waste ion-exchange resins can be utilized for different purposes, especially after additional treatment (such as washing, impregnation by certain compounds and subsequent thermal activation).     

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.     

Optimisation of supercapacitors using carbons with controlled nanotexture and nitrogen content

In order to optimize the performance of supercapacitors, the capacitance of the carbon materials used as electrodes was strictly related to their pores size and also to their redox properties. Well-sized carbons have been elaborated through a template technique using mesoporous silica. For a series of template carbons, a perfect linear dependence has been found for the capacitance values versus the micropore volume determined by CO₂ adsorption. The redox properties of carbons were enhanced by substituting nitrogen for carbon up to ca. 7 wt.%. For carbons with similar nanotextural characteristics, the electrochemical measurements showed a proportional increase of the specific capacitance with the nitrogen content in acidic electrolyte. For an activated carbon from polyacrylonitrile with a specific surface area of only 800 m² g⁻¹, but with a nitrogen content of 7 wt.%, the capacitance reaches 160 F g⁻¹, with very little fading during cycling.     

Hydrothermal and conventional H3PO4 activation of two natural bio-fibers

Two phosphoric acid activation procedures; Activation after Hydrothermal Impregnation (recently published) and Activation after Incipient Wetness Impregnation instead of conventional impregnation are analyzed in two natural bio-fiber precursors: banana pseudostem and coconut fiber matting. Both procedures are compared analyzing, in both precursors, the influence that variables such as H₃PO₄/precursor ratio, activation temperature and impregnation time have on the resulting activated carbons (ACs) properties. The work also pays special attention to the mesoporosity development and the application of these ACs to adsorb gasoline vapors.Both H₃PO₄ activation procedures develop activated carbons having suitable activation yields and porosity developments, giving the Activation after Incipient Wetness Impregnation method better results than the Activation after Hydrothermal Impregnation. Both natural bio-fibers are good precursors, rendering the coconut fiber matting better results than the banana pseudostem. The variables studied affect the porosity development, being precursor and H₃PO₄/precursor ratio the variables that most affect. By a suitable selection of these variables, activated carbons having high adsorption capacities (BET above 2500 m² g^?1 and micropore volume above 1.00 cm³ g^?1) and well developed mesoporosity (reaching 1.41 cm³ g^?1), can be prepared. Most of the samples prepared perform very well for adsorbing gasoline vapors, showing a linear relationship with their resulting volumes.     

Performance of templated mesoporous carbons in supercapacitors

By analogy with other types of carbons, templated mesoporous carbons (TMCs) can be used as supercapacitors. Their contribution arises essentially from the double layer capacity formed on their surface, which corresponds to 0.14 F m⁻² in aqueous electrolytes such as H₂SO₄ and KOH and 0.06 F m⁻² for the aprotic medium (C₂H₅)₄NBF₄ in CH₃CN. In the case of a series of 27 TMCs, it appears that the effective surface area determined by independent techniques can be as high as 1500-1600 m² g⁻¹, and therefore exceeds the value obtained for many activated carbons (typically 900-1300 m² g⁻¹). On the other hand, the relatively low amount of surface oxygen in the present TMCs, as opposed to activated carbons, reduces the contribution of pseudo-capacitance effects and limits the gravimetric capacitance to 200-220 F g⁻¹ for aqueous electrolytes. In the case of non-aqueous electrolyte, it rarely exceeds 100 F g⁻¹.It is also shown that the average mesopore diameter of these TMCs does not improve significantly the ionic mobility compared with typical activated carbons of pore-widths above 1.0-1.3 nm.This study suggests that activated carbons remain the more promising candidates for supercapacitors with high performances.     

Surface chemistry of phosphorus-containing carbons of lignocellulosic origin

Activated carbon adsorbents were prepared by phosphoric acid activation of fruit stones in an argon atmosphere at various temperatures in the 400-1000 °C range and at different acid/precursor impregnation ratios (0.63-1.02). The surface chemistry of the carbons was investigated by elemental analysis, cation exchange capacity (CEC, measured by neutralization of NaOH with acidic surface groups), infrared spectroscopy and potentiometric titration. Porous structure was derived from adsorption isotherms (N₂ at −196 °C and CO₂ at 0 °C). It was demonstrated that all carbons show considerable cation exchange capacity, the maximum (CEC = 2.2 mmol g⁻¹) being attained at 800 °C, which coincides with the maximum contents of phosphorus and oxygen. The cation exchange properties of phosphoric acid activated carbons from fruit stones are chemically stable in very acidic and basic solutions. Proton affinity distributions of all carbons show the presence of three types of surface groups with pK at 2.0-3.3, 4.6-5.9 and 7.6-9.1. These pK ranges were ascribed primarily to: (a) phosphorus-containing and carboxylic groups; (b) lactonic groups, and (c) phenolic groups, respectively. Phosphoric acid activated carbons are microporous with a relatively small contribution of mesopores. A maximum BET surface area of 1740 m² g⁻¹ was attained at 400 °C.     

Energy and power performance of electrochemical double-layer capacitors based on molybdenum carbide derived carbon

Cyclic voltammetry, constant current charge/discharge, and electrochemical impedance spectroscopy have been applied to establish the electrochemical characteristics for electric double-layer capacitor (EDLC) consisting of the 1 M (C₂H₅)₃CH₃NBF₄ electrolyte in acetonitrile and micro/mesoporous carbon electrodes prepared from Mo₂C, noted as C(Mo₂C). The N₂ sorption (total BET specific surface area (S_BET ≤ 1855 m² g⁻¹), micropore area (S_micro ≤ 1823 m² g⁻¹), total pore volume (V_tot ≤ 1.399 m³ g⁻¹) and pore size distribution (average NLDFT pore width d_NLDFT ≥ 0.89 nm) values obtained have been correlated with the electrochemical characteristics for EDLCs (region of ideal polarizability (ΔV = 3.0 V), characteristic time constant (τ_R = 1.05 s), gravimetric capacitance (C_m ≤ 143 F g⁻¹)) dependent strongly on the C(Mo₂C) synthesis temperature. High gravimetric energy (35 Wh kg⁻¹) and gravimetric power (237 kW kg⁻¹) values, normalised to the total active mass of both C(Mo₂C) electrodes, synthesised at T_synt = 800 °C, have been demonstrated at cell voltage 3.0 V and T = 20 °C.     

Correlation between heats of immersion and limiting capacitances in porous carbons

Based on more than 80 carbons, the paper shows that immersion calorimetry into benzene, water and carbon tetrachloride can be used to assess with a good accuracy the limiting capacitance C_o at low current densities in both acidic (2 M H₂SO₄) and aprotic (1 M tetraethyl ammonium tetrafluoroborate in acetonitrile) electrolytic solutions. The enthalpies of immersion Δ_iH(C₆H₆) and Δ_iH(H₂O) provide information on C_o-acidic, where both the surface area and the oxygen content play a role. On the other hand, in the case of the organic electrolyte the oxygen content has only a small influence and C_o-aprotic is directly related to Δ_iH(C₆H₆) and Δ_iH(CCl₄). Carbon tetrachloride has a critical dimension (0.65 nm), which is close to the size of the (C₂H₅)₄N⁺ ion (0.68 nm) and therefore Δ_iH(CCl₄) provides better information in the case of carbons with small micropores. The advantage of this approach lies in the fact that immersion calorimetry, in itself a useful tool for the structural and the chemical characterization of carbons, can also be used to evaluate directly the gravimetric capacitances of these solids at low current densities.     

Ruthenium oxide/carbon composites with microporous or mesoporous carbon as support and prepared by two procedures. A comparative study as supercapacitor electrodes

Composites are prepared by deposition of nanoparticles of RuO₂·xH₂O (1-4 nm) on two carbons: microporous carbon (1.3 nm of average micropore size) and mesoporous carbon (11 nm of average mesopore size). Two-preparation procedures are used: (i) procedure A consisting of repetitive impregnations of the carbons with RuCl₃·0.5H₂O solutions, and (ii) procedure B based on impregnation of the carbons with Ru(acac)₃ vapour. The procedure B leads to supported RuO₂·xH₂O particles that appear more crystalline than those obtained by the procedure A. Specific capacitance and specific surface area of the composites are discussed as functions of the RuO₂ content, and different dependences for the composites derived from the two carbons are found. Mesoporous carbon is better support than microporous carbon. Procedure A leads to supported RuO₂·xH₂O particles with higher specific capacitance than the particles deposited by procedure B.     

Fabrication of activated carbons with well-defined macropores derived from sulfonated poly(divinylbenzene) networks

Poly(divinylbenzene) (PDVB) monoliths with well-defined macropores that have been sulfonated and carbonized to obtain macroporous carbon monoliths. The original macroporous PDVB networks have been synthesized by living radical polymerization accompanied by spinodal decomposition. Sulfonation prevents polymer networks from large shrinkage and weight loss during carbonization by heat-treatment in an inert atmosphere. In the case of PDVB gels sulfonated at 120 °C using conc. H₂SO₄, mesopores in the original skeletons as well as macropores are retained after carbonization. The obtained carbon monoliths are subsequently activated by CO₂, which resulted in activated carbons. The specific surface area of the obtained activated carbons reaches up to 2360 m² g⁻¹.     

Preparation of porous carbons from thermoplastic precursors and their performance for electric double layer capacitors

Porous carbons with high surface area were successfully prepared from thermoplastic precursors, such as poly(vinyl alcohol) (PVA), hydroxyl propyl cellulose and poly(ethylene terephthalate), by the carbonization of a mixture with MgO at 900 °C in an inert atmosphere. After carbonization the MgO was dissolved out using a diluted sulfuric acid and the carbons formed were isolated. The mixing of the PVA carbon precursor with the MgO precursors (reagent grade MgO, magnesium acetate or citrate) was done either in powder form or in an aqueous solution. The BET surface area of the carbons obtained via solution mixing could reach a very high value, such as 2000 m²/g, without any activation process. The pore structure of the resultant carbons was found to depend strongly on the mixing method; the carbons prepared via solution mixing were rich in mesopores, but those produced via powder mixing were rich in micropores. The size of mesopores was found to be almost the same as that of the MgO particles, suggesting a way of controlling the mesopore size in the resultant carbons. Measurement of capacitance was carried out in 1 mol/L H₂SO₄ electrolyte. The porous carbon with a BET surface area of 1900 m²/g prepared at 900 °C through solution mixing of Mg acetate with PVA showed a fairly high EDLC capacitance, about 250 F/g with a current density of 20 mA/g and 210 F/g with 1000 mA/g. The rate performance was closely related to the mesoporous surface area.