Pore characteristics and electrochemical performance of ordered mesoporous carbons for electric double-layer capacitors

Three types of ordered mesoporous carbon materials with different pore characteristics have been synthesized via different routes. Whatever the synthesis route was, triblock copolymer was employed as both a carbon precursor and a structure-directing agent. The relationship between the capacitances of carbon electrodes and their pore characteristics was elucidated in detail. The material C-P exhibits the lowest resistance and highest specific capacitance value of exceeding 170 F/g among these carbon materials, which can be due to not only high surface area but also its appropriate pore size distribution. In addition, the noteworthy is that the maintenance of specific capacitance with increasing current load for each sample is better than that for general activated carbons, where larger mesopores and high mesopore fraction play important roles in the rate capability.     

Catalytic graphitization of templated mesoporous carbons

Graphitic porous carbons with a wide variety of textural properties were obtained by using a silica xerogel as template and a phenolic resin as carbon precursor. The synthetic procedure used to prepare them was as follows: (a) infiltration of the porosity of silica by a solution containing phenolic resin, (b) carbonization of the silica-resin composite, (c) removal of the silica skeleton, (d) impregnation of the templated porous carbon with a metallic salt and (e) catalytic graphitization of the impregnated carbon by heat treatment at 900 °C. The graphitization of the carbons thus prepared varies as a function of the carbonization temperature used and the type of metal employed as catalyst (Fe, Ni or Mn). The porous characteristics of these materials change greatly with the temperatures used during the carbonization step. These graphitized carbons exhibit high electrical conductivities up to two orders larger than those obtained for the non-graphitized samples.     

Adsorption of vitamin B12 on ordered mesoporous carbons coated with PMMA

Ordered mesoporous carbons CMK-3 and CMK-1 were prepared from SBA-15 and MCM-48 materials with pore diameters 3.9 nm and 2.7 nm, respectively. When both mesoporous carbons were coated with about 10 wt.% poly(methyl methacrylate) (PMMA), the pore diameters decreased from 3.9 nm to 3.4 nm for CMK-3 and from 2.7 nm to 2.5 nm for CMK-1. These mesoporous carbons containing about 10 wt.% PMMA were studied as adsorbents of Vitamin B 12 (VB12) from water solutions, and their performances were compared with that of pristine CMK-3, CMK-1. Compared with CMK-1, CMK-3 showed higher vitamin B12 adsorption due to a larger mesopore volume, a higher BET surface and a larger pore diameter. After coated with PMMA, both mesoporous carbons showed higher adsorption capacity than pristine materials. The adsorption properties were influenced by the pore structure and surface properties of mesoporous carbons.     

Template method synthesis of mesoporous carbon spheres and its applications as supercapacitors

Mesoporous carbon spheres (MCS) have been fabricated from structured mesoporous silica sphere using chemical vapor deposition (CVD) with ethylene as a carbon feedstock. The mesoporous carbon spheres have a high specific surface area of 666.8 m²/g and good electrochemical properties. The mechanism of formation mesoporous carbon spheres (carbon spheres) is investigated. The important thing is a surfactant hexadecyl trimethyl ammonium bromide (CTAB), which accelerates the process of carbon deposition. An additional advantage of this surfactant is an increase the yield of product. These mesoporous carbon spheres, which have good electrochemical properties is suitable for supercapacitors.     

Templated mesoporous carbons for supercapacitor application

Mesoporous carbons prepared by an inverse replica technique have been used as electrodes for electrochemical capacitors. Such well-sized carbons were prepared from mesostructured SBA-16 silica materials that served as templates whereas polyfurfuryl alcohol was the carbon precursor. Two highly mesoporous carbons characterized by 3 and 8 nm average pore diameter were tested in various electrolytic solutions (acidic, alkaline and aprotic).It can be concluded that templated mesoporous carbons with tailored pore size distribution are very promising materials to be used as electrodes in supercapacitors. The design of their pore size allows suiting the dimensions of electrolyte ions and efficient charging of the electrical double layer is achieved especially at high current load. Definitively better capacitance performance has been found for carbon with 3 nm pores range, however, cycling performance depends not only on the pore size.     

Template synthesis of large pore ordered mesoporous carbon

Nanocast carbon (NCC-1) with large pores and ordered structure was synthesized via a nanocasting process using aluminum-containing SBA-15 as template and furfuryl alcohol (FA) as carbon precursor. This carbon has several interesting features, such as two steps with distinguished hystereses in the nitrogen sorption isotherm, high surface area of 2000 m²/g and large pore volumes of 3.0 cm³/g. It was found that the key factors in the synthesis of such carbons are the aging temperature of the SBA-15 template, the concentration of furfuryl alcohol (dissolved in trimethylbenzene), and the carbonization temperature. The optimal conditions for materials with high surface area and pore volumes are SBA-15 starting materials aged at 140 °C, 25 vol% of FA solution and 850–1100 °C carbonization temperatures. Moreover, it has been demonstrated that such nanocast carbon can be synthesized in a more facile way than previously reported. Purely siliceous SBA-15 without the need of Al³⁺-incorporation can be directly used as template. In this case, the polymerization catalyst—oxalic acid and FA were simultaneously introduced into the pore space of SBA-15.     

Control of pore size in mesoporous silica templated by a multiarm hyperbranched copolyether in water and cosolvent

Worm-like mesoporous silica with various pore sizes has been prepared successfully templated by a multiarm hyperbranched copolyether (PEHO-star-PPO). The pore size of the resultant mesoporous materials can be controlled easily through adjusting the ratio of water to cosolvent. As the molar ratio of water to ethanol changes from 0.02 to 0.73, the pore size of the calcined materials can be enlarged from 32 to 94 Å, attributing to the aggregation behavior of the multiarm hyperbranched polymer in different systems. Moreover, the pore size of the obtained mesoporous silica can be also adjusted by selecting different kinds of cosolvent. The polarity and solubility of cosolvents are believed to be two important factors in affecting the pore size. Thus, a facile and efficient method for controlling the pore size of mesoporous silica has been developed by using hyperbranched polymers as templates.     

The influence of carbon source on the wall structure of ordered mesoporous carbons

A series of ordered mesoporous carbons (OMCs) have been synthesized by filling the pores of siliceous SBA-15 hard template with various carbon precursors including sucrose, furfuryl alcohol, naphthalene and anthracene, followed by carbonization and silica dissolution. The carbon replicas have been characterized by powder XRD, TEM and N₂ adsorption techniques. Their electrochemical performance used as electric double-layer capacitors (EDLCs) were also conducted with cyclic voltammetry and charge-discharge cycling tests. The results show that highly ordered 2D hexagonal mesostructures were replicated by using all these four carbon sources under the optimal operation conditions. Physical properties such as mesoscopic ordering, surface areas, pore volumes, graphitic degrees, and functional groups are related to the precursors, but pore sizes are shown minor relationship with them. The sources, which display high yields to carbons, for example, furfuryl alcohol and anthracene are favorable to construct highly ordered mesostructures even at high temperatures (1300 °C). OMCs prepared from non-graphitizable sources such as sucrose and furfuryl alcohol display amorphous pore walls, and large surface areas and pore volumes. The functional groups in the precursors like sucrose and furfuryl alcohol can be preserved on carbon surfaces after the carbonization at low temperatures but would be removed at high temperatures. The graphitizable precursors with nearly parallel blocks and weak cross-linkage between them like anthracene are suitable for deriving the OMCs with graphitic walls. Therefore, the OMCs originated from sucrose and furfuryl alcohol behave the highest capacitances at a carbonization of 700 °C among the four carbons due to the high surface areas and plenty of functional groups, and a declination at high temperatures possibly attribute to the depletion of functional groups. Anthracene derived OMCs has the lowest capacitance carbonized at 700 °C, and a steady enhancement when heated at high temperatures, which is attributed to the graphitization. The OMCs derived from naphthalene have the stable properties such as relatively high surface areas, few electroactive groups and limited graphitizable properties, and in turn medium but almost constant capacitances.     

Pore structure and graphitic surface nature of ordered mesoporous carbons probed by low-pressure nitrogen adsorption

Ordered mesoporous carbons (OMCs) were produced by pyrolysis of sucrose adsorbed in two different silica matrices (MCM-48 and SBA-15), followed by dissolution of the matrix in hydrofluoric acid. Subsequently, some of these OMCs were heat-treated at temperatures of up to 1600 °C. The OMC pore structure was studied by low-pressure nitrogen adsorption. Information on the graphitic order of the surface of the mesopore walls was also obtained from the nitrogen adsorption data. These results were correlated to the order of the graphene layers at the outer surface, which was studied by X-ray photoelectron spectroscopy (XPS).

The OMCs were predominantly mesoporous, but they also contained micropores. For OMCs produced in an SBA-15 matrix, the micropore volume decreased upon heating. After heating to 1600 °C, nearly all micropores had disappeared. Furthermore, upon heating the width of the mesopores increased from 35 to 50 Å. All these changes can be explained by a shrinking of the OMC framework upon heating. A different behavior was found for OMCs derived from MCM-48. Upon heating these materials at increasingly high temperatures, the width of the mesopores first decreased, and for temperatures above 1100 °C it increased again. For all OMCs studied the graphitic order of the mesopores and the order of the graphene layers at the outer surface increased upon heating. For a given temperature, the graphitic surface order of OMCs derived from SBA-15 and MCM-48 was similar.     

Nitrogen-enriched bituminous coal-based active carbons as materials for supercapacitors

The paper presents the results of a study on obtaining N-enriched active carbons from bituminous coal and on testing its use as an electrode material in supercapacitors. The coal was carbonised, activated with KOH and ammoxidised by a mixture of ammonia and air at the ratio 1:3 at 300 °C or 350 °C, at different stages of the production, that is, at those of precursor, carbonisate, and active carbon. The products were microporous N-enriched active carbon samples of well-developed surface area reaching from 1577 to 2510 m²/g and containing 1.0 to 8.5 wt% of nitrogen. The XPS measurements have shown that in the active carbons enriched in nitrogen at the stage of precursor and at the stage of carbonisate, the dominant nitrogen species are the N-5 groups, while in the samples ammoxidised at the last stage of the treatment the dominant nitrogen species are the surface groups of imines and/or nitriles, probably accompanied by amines and amides. The paper reports the results of a comprehensive study of the effect of the structure and chemical composition of a series of active carbon samples of different properties on their capacity performance in water solutions of H₂SO₄ or KOH, with the behaviour of positive and negative electrodes analysed separately.     

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.     

Enhanced life-cycle supercapacitors by thermal treatment of mesophase-derived activated carbons

This paper studies the effect of thermal treatment on the long-term performance of carbon-based supercapacitors. Two active electrode materials were studied: a mesophase derived activated carbon (AC), rich in oxygen functionalities, and an activated carbon obtained from treating AC at 1000 °C (AC-1000), from which most of the functionalities had been removed. Up to 10,000 cycles were performed in aqueous acidic media at different voltage windows (0.6 and 1 V). Both materials showed an excellent life cycle, although the thermally treated carbon showed a significantly better behavior. The initial capacitance of AC-1000 was reduced by only ∼5% after 10,000 cycles, independently of the operating voltage, demonstrating that the thermal treatment of AC produces a very stable electrode material at both ranges of potential. The better performance of AC-1000 was attributed to the absence of functional groups and the higher degree of crystalline order that renders materials more stable. The use of these two materials in an asymmetric device resulted in a capacitor that merges both high stability and high capacitance values. The initial capacitance was reduced by only 4.5% after 10,000 galvanostatic cycles and by 10% after 20,000.     

Ordered mesoporous carbons: Implication of surface chemistry, pore structure and adsorption of methyl mercaptan

Three ordered mesoporous carbon materials (OMC) were prepared by pyrolysis of sucrose filled in the mesoporous channels of SBA-15 at three different temperatures of 600 °C (OMC-600), 850 °C (OMC-850) and 1100 °C (OMC-1100), and followed by dissolution of the silica matrix in hydrofluoric acid. The pore structures and surface characteristics of the OMC materials were evaluated using XRD, nitrogen adsorption, FTIR spectroscopy, pH of carbon surface, and the amount of acidic surface groups from Boehm titration, respectively. The increase of the pyrolysis temperature resulted in an increase in surface pH, but a decrease in the amount of acidic surface groups. The surface area and micro- and mesopore volumes increased by increasing the pyrolysis temperature from 600 °C to 850 °C, but there were no significant changes in these properties above 850 °C. In this paper, adsorption characteristics of methyl mercaptan on the OMC materials were studied using a dynamic adsorption method in a fixed bed. The results showed that the adsorption of methyl mercaptan was strongly influenced by pore structure and surface chemistry of the OMC.     

Surface properties of porous carbons obtained from polystyrene-based polymers within inorganic templates: role of polymer chemistry and inorganic template pore structure

Three inorganic adsorbents were applied as templates to produce porous carbons from polystyrene-based organic polymers. As matrices, amorphous silica gel, mesoporous alumina and microporous zeolite 13X were used. Organic precursors were polystyrene sulfonic acid (co-maleic acid) sodium salt and polystyrene co-maleic acid isobutyl/methyl mixed ester. The impregnated templates were carbonized at 800 °C. After removal of inorganic matrices porous carbons were obtained. Materials were characterized by adsorption of nitrogen, thermal analysis, potentiometric titration and SEM. Owing to the template carbonization, highly mesoporous carbons were obtained (S_BET up to 1500 m²/g, V_t up to 3 cm³/g) with majority of pores with sizes between 20–200 Å. Although the carbons were not replicas of their matrices, the carbonization within the confined space with utilization of self-released pore formers resulted in unique carbonaceous materials with very acidic surface. That acidity is linked to either exothermic effect of sodium reactivity with moist air or susceptibility for air oxidation of small graphene layers formed in the confined pore space.     

Effects of thermal treatment of activated carbon on the electrochemical behaviour in supercapacitors

This paper studies the electrochemical behaviour of activated carbons with different oxygen content and investigates the contribution of pseudocapacitance to the global behaviour of the samples. A mesophase-derived activated carbon was further heat treated to 600 or 1000 °C in nitrogen. The changes in texture and surface chemistry induced by the thermal treatment were deeply studied. The electrochemical behaviour of the samples was studied in two- and three-electrode cells. The contribution of pseudocapacitance was evaluated by cyclic voltammetry and by the differences of specific capacitance obtained from galvanostatic tests performed in acidic (H₂SO₄) and basic (KOH) media. The presence of an extra capacitance due to redox reactions has been proved both in acidic and basic media for the samples with high oxygen content, although its contribution in basic media is significantly lower. The results obtained clearly indicate that the oxygen responsible for CO-evolution participates in redox reactions, whereas the oxygen responsible for the CO₂-evolution is of minor importance.     

Synthesis of mesoporous calcium phosphate using hybrid templates

Mesostructured calcium phosphate was synthesized by means of the combination of a soluble metal salt with an aqueous phenylphosphonic acid solution containing sodium dodecyl sulfate (SDS). Phenylphosphonic acid (PhP) was selected as the template to pattern the materials with pores generated by the formation of a lamellar calcium PhP phase. SDS was introduced to improve the thermal stability of the pore structure. The resulting materials were characterized by means of X-ray diffraction (XRD), small-angle X-ray diffraction (SAXS), electron microscopes and BJH gas absorption method. With the aid of SDS, calcium phosphate materials with the surface area and pore volume as 72 m²/g and 569 cm³/g, respectively, were successfully developed at the SDS:PhP molar ratio of 0.3:1. It was found that the addition of SDS could effectively improve the thermal stability of the pore structure. A possible mechanism was proposed to interpret the formation procedure and the improved thermal stability of the mesoporous structure.     

Nano-CaCO3 templated mesoporous carbon as anode material for Li-ion batteries

Mesoporous hard carbon is obtained by pyrolyzing a mixture of sucrose and nanoscaled calcium carbonate (CaCO₃) particles. The microstructure of the carbon is characterized by N₂ adsorption/desorption, Hg porosimetry, field-emission scanning electron microscopy (FESEM), X-ray diffraction (XRD) and Raman spectroscopy. The electrochemical performances of the carbon as an anode material for lithium ion batteries are evaluated by galvanostatic charge/discharge and cyclic voltammetry tests. It is shown that this mesoporous carbon possesses high capacity, good cycling performance and rate capability, indicating the promising application of nano-CaCO₃ particle as template in massive fabrication of mesoporous carbon anode materials for lithium ion batteries.     

Mechanism of lithium electrosorption by activated carbons

Sorption of Li⁺ ions from diluted aqueous medium has been investigated on activated carbons. The pure chemical sorption is controlled by pH of the solution as well as by the nature and concentration of surface groups, showing cation-exchanger properties of the carbon substrate. Lithium sorption is enhanced by applying a cathodic polarization to the activated carbon, and reversing voltage allows a full desorption of trapped lithium. In the selected conditions of investigation, the electrostatic attraction is not responsible of ions sorption as demonstrated by the low values of capacitance measured at constant potential or current. Our experiments proved that local high pH values at the carbon cathode, due to reduction of water, are the cause for increasing the dissociation of surface groups and consequently the amount of adsorbed lithium ions. On the other hand, with positive polarization, water is oxidized at the carbon surface, and proton exchange favors the release of lithium ions to the solution.     

Liquid-phase synthesized mesoporous electrochemical supercapacitors of nickel hydroxide

Electrochemical supercapacitive (ES) properties of liquid-phase synthesized mesoporous (pore size distribution centered ∼12 nm) and of 120 m²/g surface area nickel hydroxide film electrodes onto tin-doped indium oxide substrate are discussed. The amounts of inner and outer charges are calculated to investigate the contribution of mesoporous structure on charge storage where relatively higher contribution of inner charge infers good ion diffusion into matrix of nickel hydroxide. Effect of different electrolytes, electrolyte concentrations, deposit mass and scan rates on the current-voltage profile in terms of the shape and enclosed area is investigated. Specific capacitance of ∼85 F/g at a constant current density of 0.03 A/g is obtained from the discharge curve.