Highly mesoporous carbonaceous material of activated carbon beads for electric double layer capacitor

The activated carbon beads (ACB) are prepared by a new preparation method, which is proposed by mixing the coal tar pitch and fumed silica powder at a certain weight ratio and activation by KOH at different weight ratios and different temperatures. The BET surface area, pore volume and average pore size are obtained based on the nitrogen adsorption isotherms at 77 K by using ASAP 2010 apparatus. The results show that our samples have much high specific surface area (SSA) of 3537 m² g⁻¹and high pore volume value of 3.05 cm³ g⁻¹. The percentage of mesopore volume increases with the weight ratio of KOH/ACB ranging from 4% to 72%. The electrochemical double layer capacitors (EDLCs) are assembled with resultant carbon electrode and electrolyte of 1 mol L⁻¹ Et₄NBF₄/PC. The specific capacitance of the ACB sample could be as high as 191.7 F g⁻¹ by constant current charge/discharge technique, indicating that the ACB presents good characteristics prepared by the method proposed in this work. The investigation of influence of carbon porosity structure on capacitance indicates that the SSA plays an important role on the capacitance and all the pore sizes of less than 1 nm, from 1 to 2 nm and larger than 2 nm contribute to the capacitance. Mesopore structure is beneficial for the performance at high current density.     

New carbon materials with high porosity in the 1-7 nm range obtained by chemical activation with phosphoric acid of resorcinol-formaldehyde aerogels

Chemical activation of resorcinol-formaldehyde aerogels with phosphoric acid results in materials containing both intra-particle microporosity (pore volume ∼0.18 cm³g⁻¹ and mean-pore-width ∼1 nm) and inter-particle micro/mesoporosity. The latter forms as a result of partial collapse of the mesopore structure of the organic aerogel and can be controlled by varying the phosphoric acid/organic aerogel ratio. Increasing this ratio leads to higher pore volume and size and it was possible to obtain micro/mesopore volumes as high as 1.23 cm³g⁻¹ with pore widths up to ∼7 nm. Over 90% of these pores were accessible even after blocking all of the ultramicroporosity by pre-adsorption of n-nonane.     

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.     

Influence of oxidation process on the adsorption capacity of activated carbons from lignocellulosic precursors

A set of activated carbon materials non-oxidised and oxidised, were successfully prepared from two different lignocellulosic precursors, almond shell and vine shoot, by physical activation with carbon dioxide and posterior oxidation with nitric acid. All samples were characterised in relation to their structural properties and chemical composition, by different techniques, namely nitrogen adsorption at 77 K, elemental analysis (C, H, N, O and S), point of zero charge (PZC) and FTIR. A judicious choice was made to obtain carbon materials with similar structural properties (apparent BET surface area ∼ 850-950 m²g⁻¹, micropore volume ∼ 0.4 cm³g⁻¹, mean pore width ∼ 1.2 nm and external surface area ∼ 14-26 m²g⁻¹). After their characterisation, these microporous activated carbons were also tested for the adsorption of phenolic compounds (p-nitrophenol and phenol) in the liquid phase at room temperature. The performance in liquid phase was correlated with their structural and chemical properties. The oxidation had a major impact at a chemical level but only a moderate modification of the porous structure of the samples. The Langmuir and Freundlich equations were applied to the experimental adsorption isotherms of phenolic compounds with good agreement for the different estimated parameters.     

Using alkali metals to control reactivity and porosity during physical activation of demineralised kraft lignin

Demineralised kraft lignin was impregnated with between 6.2% and 50% NaCl or KCl and physically activated in CO₂ at 750 °C. The results presented show that a considerable reduction in activation time even at a comparatively low activation temperature could be achieved, particularly when using KCl. Considering a fixed level of burn-off, the impregnation did not affect the pore volume and only increased the pore width by about 0.1-0.2 nm, depending on the concentration of NaCl or KCl used. By controlling the conditions it was possible to obtain predominantly ultramicroporous materials with mean pore widths over the range 0.53-0.77 nm. On the other hand, at high levels of burn-off there was evidence for micropore widening into the small mesopore range and also for the formation of a secondary mesopore structure. Under these conditions it was possible to obtain materials with pore volumes as high as 0.82 cm³ g⁻¹.     

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.     

Synthesis and characterisation of nanoporous carbide-derived carbon by chlorination of vanadium carbide

Nanoporous carbide-derived carbon (CDC) was synthesised from vanadium carbide (VC) powder via gas phase chlorination in the temperature range from 500 to 1100 °C. The XRD analysis of nanoporous carbon powder samples was carried out to investigate the structural changes (graphitisation) of nanoporous carbons synthesised. The first-order Raman spectra showed the absorption peak at ∼1582 cm⁻¹ and the disorder-induced (D) peak at ∼1345 cm⁻¹. The low-temperature N₂ adsorption experiments were performed and a specific surface area up to 1305 m² g⁻¹ and total pore volume up to 0.66 cm³ g⁻¹ were obtained.     

Adsorption of bromo-phenol blue from an aqueous solution onto thermally modified granular charcoal

The adsorption phenomenon of bromo-phenol blue onto pristine and thermally evacuated granular charcoal (GC) was studied via a batch technique at 25 °C. The effect of evacuation temperature on the GC surface and pore structure (e.g. pore volume and diameter) was studied by Fourier transform infrared spectroscopy (FT-IR), point of zero charge (PZC), proximate analysis, Brunauer, Emmett and Teller (BET) method and field emission scanning electron microscopy (FE-SEM). The FT-IR spectra of the samples after evacuation showed considerable decrease in the acidic functional groups. PZC showed that the surface of the evacuated charcoal became basic as the evacuation temperature was increased from 300 to 800 °C. Volatile matter decreased while ash and fixed carbon contents increased during evacuation, which led to an increase in the micro-pore volume from 0.25 to 0.42 cm³ g⁻¹, meso-pore volume from 0.04 to 0.13 cm³ g⁻¹, pore diameter from 5.01 to 6.21 nm, and specific surface from 150.32 to 254.70 m² g⁻¹. Adsorption of the bromo-phenol blue onto charcoal, increased as the evacuation temperature was increased from 300 to 800 °C. The interaction of bromo-phenol blue with charcoal was proposed to have occurred via hydrogen bonding. The adsorption data fitted well with the Langmuir equation, which indicated that the monolayer adsorption has occurred at specific sites within the adsorbent.     

Electrochemical enhancement of adsorption capacity of activated carbon fibers and their surface physicochemical characterizations

The adsorption of activated carbon fibers (ACFs) and their surface characteristics were investigated before and after electrochemical polarization. The adsorption kinetics of m-cresol showed the dependence on polarized potential, and the adsorption rate constant increased by 77.1%, from 6.38 × 10⁻³ min⁻¹ at open-circuit (OC) to 1.13 × 10⁻² min⁻¹ at polarization of 600 mV. The adsorption isotherms at different potentials were in good agreement with Langmuir isotherm model, and the maximum adsorption capacity increased from 2.28 mmol g⁻¹ at OC to 3.67 mmol g⁻¹ at polarized potential of 600 mV. These indicated that electrochemical polarization could effectively improve the adsorption rate and capacity of ACFs. The surface characteristics of ACFs before and after electrochemical polarization were evaluated by N₂ adsorption-desorption isotherms, scanning electron microscope (SEM), zeta potential and Fourier transform infrared spectroscopy (FTIR). The results showed that the BET specific surface area and pore size increased as the potential rose. However, the surface chemical properties of ACFs hardly changed under electrochemical polarization of less than 600 mV. This study was beneficial to understand the mechanism of electrochemically enhanced adsorption.     

Preparation, characterization and Methylene Blue adsorption of phosphoric acid activated carbons from globe artichoke leaves

Activated carbons were prepared by the pyrolysis of artichoke leaves impregnated with phosphoric acid at 500 °C for different impregnation ratios: 100, 200, 300 wt.%. Materials were characterized for their surface chemistry by elemental analysis, “Boehm titrations”, point of zero charge measurements, infrared spectroscopy, as well as for their porous and morphological structure by Scanning Electron Microscopy and nitrogen adsorption at 77 K. The impregnation ratio was found to govern the porous structure of the prepared activated carbons. Low impregnation ratios (~ 100 wt.%) led to essentially microporous and acidic activated carbons whereas high impregnation ratios (> 100 wt.%) gave essentially microporous-mesoporous carbons with specific surface areas as high as 2038 m²·g^− 1, pore volume as large as 2.47 cm³·g^− 1, and a slightly acidic surface. The prepared activated carbons were studied for their adsorption isotherms of Methylene Blue at pH = 3 and pH = 9. The supermicroporous structure of the material produced at 200 wt.% H₃PO₄ ratio was found to be appropriate for an efficient adsorption of this dye controlled by dispersive and electrostatic interactions depending on the amount of oxygen at the surface.     

Removal of dibenzothiophenes in kerosene by adsorption on rice husk activated carbon

The capacity of rice husk activated carbon (RHAC) to adsorb refractory sulfur compounds of dibenzothiophenes (DBTs) from commercial kerosene was evaluated in terms of their textural and chemical characteristics. Rice husk activated at 850 °C for 1 h showed an acceptable adsorption capacity for DBTs, despite a much lower specific surface area (473 m²/g) and total pore volume (0.267 cm³/g), when compared to micro-porous activated carbon fiber with a large specific surface area (2336 m²/g) and total pore volume (1.052 cm³/g). The volumes of ultramicropores acting as DBTs adsorption sites, and of mesopores leading DBTs into the ultramicropores were closely related to the DBTs adsorption capacity of the RHACs.     

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.     

The effect of mechanical grinding on the mesoporosity of steam‐activated palm kernel shell activated carbons

BACKGROUND: Palm kernel shell activated carbon (OPSA) produced by steam gasification at high temperatures generally results in high surface areas of 1146 to 1600 m² g^?1, attributed to the high volume of micropores (0.43 to 0.56 cm³ g^?1). The mesoporosity of naturally occurring activated carbons is observed to increase with decreasing particle size. Mechanical grinding was therefore performed to investigate its effect on the mesoporosity and microporosity of OPSA. RESULTS: Mechanical grinding had a strong effect on mesopore volume and average pore diameter, with an increase in mesopore volume from 47 to 66% as particle size decreases. Interestingly, no significant effect on the micropore fraction was observed in ground OPSA particles. CONCLUSIONS: The mechanically ground OPSA particles possessed dual adsorption capabilities due to the high microporosity and moderate mesoporosity contained in the structures. This results in interesting porosity behaviour of palm kernel shell activated carbons and the potential to provide materials of distinct sorption capacities with minimal treatment. Copyright © 2009 Society of Chemical Industry     

The co-micelle/emulsion templating route to tailor nano-engineered hierarchically porous macrospheres

Here we present a new class of nano-engineered hierarchically porous materials in which the entire framework is mesoporous. This material is engineered into macrospheres of controllable size with a highly interconnected macropore network to facilitate molecular diffusion access. To achieve this, a new co-micelle/emulsion templating (co-MET) technique was developed. In this technique a block copolymer plays the dual roles of emulsion stabilization and micelle formation within the aqueous phase of that emulsion to produce the hierarchical structures. The emulsion templating provides the macroporous structure while the mesoporous structure is formed by hydrolyzation of silica around block copolymer micelles. Increasing the copolymer concentration improves the mesoporosity up to a certain concentration where the emulsion phase behavior changes and the macroporosity is affected. Unlike other hierarchically porous materials, the walls of the co-MET macrospheres are entirely mesoporous, which provides high surface areas (>500 m² g⁻¹) and pore volumes (>1 cm³ g⁻¹) and narrow mesopore size distributions (∼10 nm). This interconnected hierarchical meso/macroporous structure combined with the controlled particle size makes this new class of materials promising for applications requiring high diffusion and throughput rates, alleviating the problems of using typical fine particle mesoporous materials.     

A green process for the synthesis of controllable mesoporous silica materials

The synthesis, characterization, and application of mesoporous silicas have attracted a lot of attention for over two decades, which stems from their fascinating structures, formation mechanisms and prospects of their applications. Various methods have been developed for the synthesis of these silicas with a tunable pore diameter and a narrow pore size distribution. In this paper, mesoporous silica materials with controllable pore diameters (3-9 nm), narrow pore size distributions, high surface area (>700 m² g⁻¹) and pore volume (>1 cm³ g⁻¹) were prepared by a green template, amphiphilic dendritic polyamidoamine. The resulting silica materials were characterized by ¹H, ¹³C NMR spectroscopy; thermogravimetic analysis; nitrogen adsorption; transmission electron microscope. It was shown that the template could be completely removed and recycled from the silica in an environmentally friendly way by means of a simple water extraction. Furthermore, it was shown that the pore diameter of these materials could be controlled by dendritic polyamidoamine with different generations and functional groups. Meanwhile, the porous framework showed strong thermal stability. Thus, a new environmentally friendly pathway for the controllable synthesis of this fascinating silicas has been proposed.     

New acrylic monolithic carbon molecular sieves for O2/N2 and CO2/CH4 separations

The modification of activated carbon fibres prepared from a commercial textile acrylic fibre into materials with monolithic shape using phenolic resin as binder was studied. The molecular sieving properties for the gas separations CO₂/CH₄ and O₂/N₂ were evaluated from the gas uptake volume and selectivity at 100 s contact time taken from the kinetic adsorption curves of the individual gases. The pseudo-first order rate constant was also determined by the application of the LDF model. The samples produced show high CO₂ and O₂ rates of adsorption, in the range 3-35 × 10⁻³ s⁻¹, and in most cases null or very low adsorption of CH₄ and N₂ which make them very promising samples to use in PSA systems, or similar. Although the selectivity was very high, the adsorption capacity was low in certain cases. However, the gas uptake in two samples reached 23 cm³ g⁻¹ for CO₂ and 5 cm³ g⁻¹ for O₂, which can be considered very good. The materials were heat-treated using a microwave furnace, which is a novel and more economic method, when compared with conventional furnaces, to improve the molecular sieves properties.     

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.     

Regeneration of a solid waste from an edible oil refinery

This work presents a valorization of a solid waste originating from an edible oil refinery called spent bleaching earth (SBE). The SBE material is first impregnated with an ammonium chloride solution (3 M), then treated directly in furnace at 400 °C during an hour followed by a washing in the cold by HCl 1 M. To elucidate the changes in its crystalline structure, induced by the regeneration method, the obtained material (RSBE) is characterized by several physicochemical methods (X-ray diffraction, FTIR, thermal analysis, BET and SEM). The characterization results show that the heat treatment in furnace and the chemical treatment (decomposition of NH₄Cl) don’t affect the structure of montmorillonite of regenerated material (RSBE). The study of porous texture by the nitrogen adsorption technique at −196 °C shows that the specific surface area S_BET and the pore volume increased in the RSBE material compared to those of virgin bleaching earth VBE (unused) and their values are respectively of 145.68 against 115.5 m² g⁻¹ and of 0.287 against 0.234 cm³ g⁻¹. Calculations by the adsorption equations using BJH method, applied to both materials, show that the treatment generate an increase in the micropores in the RSBE material. We belonged the values of the micropores area of S_mic = 41.98 cm² g⁻¹ and of V_mic = 0.074 cm³ g⁻¹ for the volume.     

Electrochemical regeneration of a carbon-based adsorbent loaded with crystal violet dye

The removal of colour from a crystal violet dye solution using a non-porous, electrically conducting carbon-based adsorbent was systematically investigated under different operating conditions. Whilst the adsorptive process was very quick (up to 88% of equilibrium capacity could be achieved within 2 min), the adsorptive capacity of the adsorbent was very low (2 mg g⁻¹) compared with activated carbons. This was due to its low surface area. The conductivity of the adsorbent/electrolyte mixture within the anodic compartment of the electrochemical cell was found to be over 13 times greater with the new adsorbent compared with powdered activated carbon. One hundred percent could be achieved in a simple divided electrochemical cell using treatment times as low as 10 min by passing a charge of 25 C g⁻¹ at a current density of 20 mA cm⁻². The efficiency of electrochemical regeneration depends on a range of variables including charge passed, current density, treatment time, electrolyte type and concentration and the adsorbent bed thickness. Multiple adsorption and regeneration cycles indicate that there is little or no loss in adsorbent capacity on regeneration.     

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.