High power supercapacitors using polyacrylonitrile-based carbon nanofiber paper

Porous carbon nanofiber paper has been obtained by one-step carbonization/activation of PAN-based nanofiber paper at temperatures from 700 to 1000 °C in CO₂ atmosphere. The paper was used as supercapacitor electrode without any binder or percolator. At low temperature, e.g., ?900 °C, nitrogen enriched carbons with a poorly developed specific surface area (S_BET ? 400 m²/g) are obtained. In aqueous electrolytes, these carbons withstand high current loads without a noticeable decrease of capacitance, and the normalized capacitance reaches 67 μF/cm². At 10 s time constant, the values of energy and power densities are 3-4 times higher than for activated carbons (AC) presenting higher specific surface area. By carbonization/activation at 1000 °C, subnanometer pores are developed and S_BET = 705 m²/g. Despite moderate BET specific surface area, the capacitance reaches values higher than 100 F/g in organic electrolyte. At high power densities, the nanofiber paper obtained at 1000 °C outperforms the energy density retention of ACs in organic electrolyte. The high power capability of the carbon nanofiber papers in the two kinds of electrolytes is attributed both to the high intrinsic conductivity of the fibers and to the high diffusion rate of ions in the opened mesopores.     

Carbon spheres prepared from zeolite Beta beads

Porous carbon beads were prepared from macroporous anion-exchange resin beads preliminary converted into resin-zeolite Beta composite or pure zeolite Beta spheres. Two synthesis procedures were used depending on the initial template employed. In a series of experiments, the resin from the resin-zeolite Beta composite was directly carbonized into carbon. In another series of experiments, the resin was removed by oxidation at 600 °C leaving behind self-bonded zeolite Beta beads, which were filled with carbon by chemical vapor deposition (CVD) of propylene. As a final step for both procedures, the zeolite was dissolved in hydrofluoric acid. All the carbons prepared inherited the macroscopic spherical shape of the template spheres as well as the morphology of the primary particles building up the beads. The synthesis procedure and the carbonization temperature or the temperature for CVD of carbon employed influenced the ordering and the pore structure of the produced carbons. The carbons prepared by direct carbonization showed relatively low surface areas, less than 1000 m² g⁻¹, and no zeolite structural regularity. The samples obtained via CVD maintained the zeolite ordering with a periodicity of 11.7 Å and had surface areas of over 2000 m² g⁻¹.     

Synthesis and characterization of nanostructured carbons with controlled porosity prepared from sulfonated divinylbiphenyl copolymers

A series of porous carbons have been prepared by the carbonization of spherical porous sulfonated divinylbiphenyl (DVBPh) copolymers. Carbons in spherical bead form were obtained by the pyrolysis of H⁺, Na⁺, Cs⁺, Cu²⁺, Co²⁺ and Fe³⁺ forms of the sulfonated DVBPh beads. Thermogravimetric analysis (TGA) in an inert nitrogen atmosphere (25-900 °C) was carried out on the DVBPh copolymer precursor, the sulfonated copolymer sample and various ionic forms of the resin. The TGA data provides evidence that the sulfonation process thermally stabilized the polymer resulting in a higher final carbon yield. It was found that the pyrolysis yield was ca. 40% for the sulfonic acid derivative and between 40% and 65% for the various sulfonic acid salts. The highest yield was observed for the monovalent sodium and cesium ionic forms of the sulfonated DVBPh copolymers. Low temperature nitrogen adsorption/desorption isotherms provided information on the porous structure of the polymer precursors and the carbons prepared from them. The pore structure in the carbons was found to a large extent to be similar to the porous structure of the starting sulfonated resin material, however, the metal form was found to impact on the micropore structure of the resulting carbons. The carbon materials prepared were characterized by X-ray photoelectron spectroscopy (XPS) to provide information on the form of the residual sulfur in the carbons. XPS results suggest that the ionic form of the sulfonic resin influences the amount and the form of the sulfur and this may be correlated with the yield of the final carbon.     

On the preparation and characterization of chars and activated carbons from orange skin

Activated carbons were obtained by carbonization of orange skin waste and partial gasification with CO₂. The orange skin contains a significant amount of inorganic matter mainly potassium, calcium and phosphorus. CO₂ gasification is catalyzed by potassium and calcium, resulting in carbons with a microporous structure. Thermal treatment up to 900 °C applied to orange skin-derived activated carbons yields carbons with a highly developed porous structure, and a significant contribution of mesopores, due to the activation effect of potassium compounds. This porous structure is initially blocked by the inorganic matter that is removed by a subsequent acid wash, opening the porous structure of the final carbon; an activated carbon with a very wide porous structure and a specific surface area of around 1200 m²/g was obtained. The activated carbon with high potassium content shows relatively high NO adsorption capacities in the presence of oxygen at 120 °C, probably due to the catalytic effect of potassium on the oxidation of NO. The breakthrough times of the NO adsorption in the presence of oxygen at 120 °C were predicted by the Bohart and Adams model with a relevant agreement between the calculated and the experimental times.     

HEMP-derived activated carbon fibers by chemical activation with phosphoric acid

Activated carbon fibers were prepared by chemical activation of hemp fibers with phosphoric acid at different carbonization temperatures and impregnation ratios. Surface properties of the activated carbons fibers were significantly influenced by the activation temperature and the impregnation ratio. An increase of either of these parameters produced a high development of the porous structure of the fibers. Activated carbon fibers with apparent surface area of 1350 m²/g and mesopore volume of 1.25 cm³/g were obtained at 550 °C with an impregnation ratio of 3. The activated carbon fibers presented a high oxidation resistance, due to the presence of phosphorus compounds on the carbon surface. The oxidation resistance results suggest that C-O-PO₃ and mainly C-PO₃ and C-P groups act as a physical barrier, blocking the active carbon sites for the oxidation reaction.     

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.     

Preparation of a carbon monolith with hierarchical porous structure by ultrasonic irradiation followed by carbonization, physical and chemical activation

A two-step direct and simple method for the preparation of a hierarchical porous carbon monolith with micropores, mesopores and macropores is described. The two stages give more flexibility in the preparation of a porous carbon monolith. In step I a macroporous interconnected carbon monolith is prepared by ultrasonic irradiation during sol-gel polycondensation. The effects of sol-gel temperature, catalyst concentration and ultrasonic power on the structure of the monolith are investigated. In step II, mesopores are induced in the monolith by Ca(NO₃)₂ impregnation followed by CO₂ activation. The effect of activation temperature is also studied. A hierarchical interconnected carbon monolith with mean pore size diameter of 1.2 μm, BET surface area of 624 m²/g, mesopore volume of 0.38 cm³/g and micropore volume of 0.22 cm³/g has been obtained from Ca(NO₃)₂ impregnation of the macroporous carbon monolith followed by CO₂ activation at 850 °C.     

High saturation capacity of activated carbons prepared from mesophase pitch in the removal of volatile organic compounds

A series of binderless activated carbon monoliths (ACMs) have been prepared from petroleum pitch and using KOH as activating agent. Characterization shows that these activated carbons combine a large “apparent” surface area (up to S_BET ∼ 3000 m²/g) together with a well-developed narrow micropore size distribution. Dynamic column adsorption experiments using different volatile organic compounds (VOCs), ethanol and benzene, show that these activated carbons prepared from mesophase-based materials exhibit a superior saturation capacity compared to conventional carbon materials. The total amount adsorbed reaches values as high as 18 g/100 g AC and 40 g/100 g AC, for ethanol and benzene, respectively. These are the best results reported in the literature. The total amount adsorbed for both molecules correlates with the total volume of narrow micropores, thus confirming the pore size specificity required for the adsorption of VOC molecules. Regeneration studies show that ethanol can be easily desorbed at room temperature by flowing clean air through the adsorbent whereas benzene requires a further heating for complete desorption/regeneration.     

Adsorption capacity and removal efficiency of heavy metal ions by Moso and Ma bamboo activated carbons

In order to understand the adsorption capacity and removal efficiency of heavy metal ions by Moso and Ma bamboo activated carbons, the carbon yield, specific surface area, micropore area, zeta potential, and the effects of pH value, soaking time and dosage of bamboo activated carbon were investigated in this study. In comparison with once-activated bamboo carbons, lower carbon yields, larger specific surface area and micropore volume were found for the twice-activated bamboo carbons. The optimum pH values for adsorption capacity and removal efficiency of heavy metal ions were 5.81–7.86 and 7.10–9.82 by Moso and Ma bamboo activated carbons, respectively. The optimum soaking time was 2–4 h for Pb²⁺, 4–8 h for Cu²⁺ and Cd²⁺, and 4 h for Cr³⁺ by Moso bamboo activated carbons, and 1 h for the tested heavy metal ions by Ma bamboo activated carbons. The adsorption capacity and removal efficiency of heavy metal ions of the various bamboo activated carbons decreased in the order: twice-activated Ma bamboo carbons > once-activated Ma bamboo carbons > twice-activated Moso bamboo carbons > once-activated Moso bamboo carbons. The Ma bamboo activated carbons had a lower zeta potential and effectively attracted positively charged metal ions. The removal efficiency of heavy metal ions by the various bamboo activated carbons decreased in the order: Pb²⁺ > Cu²⁺ > Cr³⁺ > Cd²⁺.     

Preparation of microporous sorbents from cedar nutshells and hydrolytic lignin

Carbon nutshells and hydrolytic lignin were used as starting materials for the preparation of microporous active carbons. Optimum parameters for cedar nutshell carbonization have been selected (temperature of carbonization 700-800 °C, rate of heating less than 3 °C/min) for the preparation of microporous carbons (average pore width 0.56 nm). The textural characteristics of microporous carbons made from nutshell are similar to those of a ‘Coconut’ carbon molecular sieve, but the latter has both a higher CO₂ adsorption capacity and a higher coefficient of N₂/O₂ separation. The influence of carbonization and steam-activation parameters on the microtexture and molecular-sieve properties of granular carbons made from hydrolytic lignin was also investigated. A low rate of heating (less 3 °C/min) promotes the formation of micropores with average sizes around 0.56-0.58 nm at carbonization temperature 700 °C. At the same carbonization temperature the average sizes of micropores were 0.7-0.78 nm at rates of heating more than 3 °C/min. The activation of lignin-char with steam at 800 °C resulted in the formation of active carbons with more developed micropore volume (0.3-0.35 cm³ g⁻¹) and with micropores of widths around 0.6-0.66 nm which are able to separate He from a He-CH₄ mixture. The size of the micropores was varied as a function of burn off value.     

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.     

Preparation and characteristics of rice-straw-based porous carbons with high adsorption capacity

To prepare porous carbons with high adsorption capacity from rice straws, two different kinds of precursors, i.e. one as the raw rice straws (one-stage process) and the other as pre-carbonized rice straws (two-stage process), were activated with KOH of various impregnation ratios. The two-stage process was found very effective for manufacturing porous carbons with high surface area and adsorption capacities for MB and I₂. For example, the porous carbon that was carbonized at 700°C and subsequently activated at 900°C exhibited the surface area of 2410 m²/g, the adsorption capacities of 800 and 1720 mg/g for MB and I₂, respectively, and the total pore volume of 1.4 ml/g. In the two-stage method, there was a preferential optimum impregnation ratio of KOH to a precursor carbon, i.e. 4:1, with which high surface area of porous carbons could be achieved. The formation of uni- and bidentate carboxylic salt structure, induced by reaction between KOH and oxygen containing carbon, that facilitates the formation of azo group (-NN-) on a subsequent heat treatment was considered as one of the key factors for the presence of optimum impregnation ratio of KOH. In contrast, the porous carbons of only moderate adsorption capacity could be obtained from the one-stage method. The original morphology of rice straw was sustained during the two-stage process, yet not during the one-stage process.     

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.     

One-pot synthesis of carbonaceous monolith with surface sulfonic groups and its carbonization/activation

The carbonaceous monoliths rich in surface sulfonic acid groups were synthesized by one-pot hydrothermal carbonization of the mixture of p-toluenesulfonic acid/glucose/resorcinol at 180 °C. The Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy characterizations confirmed the presence of surface sulfonic groups on these monoliths. The catalytic performance of this kind of carbonaceous material as a solid-acid catalyst was studied in the reaction of acetalization of benzaldehyde and the results showed that it has high activity and reusability. Then, these monoliths were further carbonized and activated to form monolithic carbons with high surface area and large pore volume. The surface area and pore volume per mass increased with prolonging the activation time (0–6 h) and the best results on 6-h activated samples were 2337 m²/g and 2.12 cm³/g. Due to the decrease in bulk density the volumetric surface area increased initially until maximum and then slightly dropped down during the activation. These carbonized and activated samples showed better oxidation resistance than one commercial activated carbon under air. Moreover, the adsorption capacity for dye molecules with different size on these activated samples was significant higher than that on commercial activated carbons and a synthetic ordered mesoporous carbon.     

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.     

Nitrogen-containing microporous carbons prepared from anionic surfactant-melamine/formaldehyde composites

Nitrogen-containing microporous carbons have been synthesized by the carbonization of anionic surfactant-melamine/formaldehyde (MF) composites, which were themselves formed by an electrostatic organic-organic interaction. The carbons prepared from sodium dioctyl sulfosuccinate-MF and sodium dodecyl sulfate-MF mixtures have high surface area of 464 and 539 m² g⁻¹, respectively. The N/C molar ratios of the carbons are 0.11. The resultant carbons showed capacitances higher than 200 F g⁻¹ in an acidic solution of 1 M H₂SO₄ at a scan rate of 1 mV s⁻¹.     

Hydrogen adsorption on KOH activated carbons from mesophase pitch containing Si, B, Ti or Fe

Different activated carbons with large micropore volume (0.78-0.99 cm³/g) have been prepared by KOH activation of mesophase pitch obtained by co-pyrolysis of a petroleum residue and small amounts of different compounds, triphenylsilane, borane pyridine complex, tetrabutyl orthotitanate or ferrocene. During the preparation, the Ti introduced in the petroleum residue concentrate into the activated carbon, whereas some loss of Si and Fe occurs. The compounds modify the size of mesophase structure formed during the co-pyrolysis process, as well as the apparent height of lamelae stack, L_c, both having an important effect in the development of the porosity of the activated carbon. However, there is a scarce influence of all heteroatoms in the adsorption capacity of H₂ at −196 °C and at 25 °C, which seems to be mainly influenced by the volume and size of micropores of the activated carbon. Only the activated carbon containing Fe adsorbs a higher amount of hydrogen at 25 °C and 10 MPa than the expected one according to its micropore volume.     

Overcoming the barrier to graphitization in a polymer-derived nanoporous carbon

A new pathway to synthesize a carbon with both nanoporosity and pre-graphitic structures has been discovered by annealing at 2000 °C a CO₂ activated, non-graphitizing, nanoporous carbon originally derived from polyfurfuryl alcohol. The activation process with CO₂ overcomes the barrier to graphitization normally present in this carbon even when treated at high temperature. Gas adsorption analysis, skeletal density measurements, X-ray diffraction, and transmission electron microscopy are utilized to probe the structure of both the non-activated and the activated carbons at 800, 1200, 1800, and 2000 °C. The influence of activation time is also examined. Prior to activation the nanopore walls are comprised of several layers of disordered graphenes. Activation eliminates the barrier to graphitization by reducing the number of layers below the limit of detection and by removing carbon material highly susceptible to oxidation. Annealing at 2000 °C of the carbon activated to 84% burnoff induces the formation of pre-graphitic domains amongst the nanoporous carbon. The (0 0 2) bands corresponding to 2θ = 24.3°, 26°, and 26.5° are identified and assigned to amorphous, turbostratic, and graphitic morphologies. A pore volume of 0.50 cm³ g⁻¹ localized in pores below 2 nm in size is preserved after annealing.     

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.     

Importance of activated carbon's oxygen surface functional groups on elemental mercury adsorption

The effect of varying physical and chemical properties of activated carbons on adsorption of elemental mercury (Hg⁰) was studied by treating two activated carbons to modify their surface functional groups and pore structures. Heat treatment (1200 K) in nitrogen (N₂), air oxidation (693 K), and nitric acid (6N HNO₃) treatment of two activated carbons (BPL, WPL) were conducted to vary their surface oxygen functional groups. Adsorption experiments of Hg⁰ by the activated carbons were conducted using a fixed-bed reactor at a temperature of 398 K and under N₂ atmosphere. The pore structures of the samples were characterized by N₂ and carbon dioxide (CO₂) adsorption. Temperature-programmed desorption (TPD) and base-acid titration experiments were conducted to determine the chemical characteristics of the carbon samples. Characterization of the physical and chemical properties of activated carbons in relation to their Hg⁰ adsorption capacity provides important mechanistic information on Hg⁰ adsorption. Results suggest that oxygen surface complexes, possibly lactone and carbonyl groups, are the active sites for Hg⁰ capture. The carbons that have a lower carbon monoxide (CO)/CO₂ ratio and a low phenol group concentration tend to have a higher Hg⁰ adsorption capacity, suggesting that phenol groups may inhibit Hg⁰ adsorption. The high Hg⁰ adsorption capacity of a carbon sample is also found to be associated with a low ratio of the phenol/carbonyl groups. A possible Hg⁰ adsorption mechanism, which is likely to involve an electron transfer process during Hg⁰ adsorption in which the carbon surfaces may act as an electrode for Hg⁰ oxidation, is also discussed.