A novel method for production of activated carbon from waste tea by chemical activation with microwave energy

This study presents the production of activated carbon from waste tea. Activated carbons were prepared by phosphoric acid activation with and without microwave treatment and carbonisation of the waste tea under nitrogen atmosphere at various temperatures and different phosphoric acid/precursor impregnation ratios. The surface properties of the activated carbons were investigated by elemental analysis, BET surface area, SEM, FTIR. Prior to heat treatment conducted in a furnace, the mixture of the waste tea and H₃PO₄ was treated with microwave heating. The maximum BET surface area was 1157 m²/g for the sample treated with microwave energy and then carbonised at 350 °C. In case of application of conventional method, the BET surface area of the resultant material was 928.8 m²/g using the same precursor and conditions. According to the Dubinin–Radushkevich (DR) method the micropore surface area for the sample treated with microwave energy was higher than the sample obtained from the conventional method. Results show that microwave heating reasonably influenced the micropore surface area of the samples as well as the BET surface area.The samples activated were also characterised in terms of the cumulative pore and micropore volumes according to the BJH, DR and t-methods, respectively.     

Functionalized carbon onions,detonation nanodiamond and mesoporous carbon as cathodes in Li-ion electrochemical energy storage devices

Functionalization of carbon surface leads to the enhancement of ion storage capacity of carbon cathodes due to the additional pseudocapacitive reactions. In order to gain additional insights on the effects of the carbon specific surface area, porosity and oxygeni-containing functional groups on their electrochemical performance, we have investigated thick (>150 μm) electrodes based on carbon onion nanopowder with and without functional groups present on carbon surface as well as nanodiamond soot and mesoporous activated carbon. Oxidation of carbon onion surface was found to result in a 2.4–2.8-fold increase in their specific capacitance. The larger average pore size and the absence of micropores in carbon nanoparticles based electrodes resulted in a better rate performance compared to that of mesoporous activated carbon. However, significant self-discharge was observed in all the oxidized samples. The low electrode density combined with limited overall charge storage capacity of carbon samples resulted in a volumetric capacity of less than 23 mAh cm⁻³, compared to 450–700 mAh cm⁻³ offered by state of the art high-density cathodes used in commercial Li-ion batteries. Even with further improvements, our estimations suggest that porous carbon cathodes will unlikely be able to offer more than 15% of the energy density of traditional cathodes.     

Utilization of Crofton weed for preparation of activated carbon by microwave induced CO2 activation

Crofton weed was converted into a high-quality activated carbon (CWAC) via microwave-induced CO₂ physical activation. The operational variables including activation temperature, activation duration and CO₂ flow rate on the adsorption capability and activated carbon yield were identified. Additionally the surface characteristics of CWAC were characterized by nitrogen adsorption isotherms, FTIR and SEM. The operating variables were optimized utilizing the response surface methodology and were identified to be an activation temperature of 980 °C, an activation duration of 90 min and a CO₂ flow rate of 300 ml/min with a iodine adsorption capacity of 972 mg/g and yield of 18.03%. The key parameters that characterize quality of the porous carbon such as the BET surface area, total pore volume and average pore diameter were estimated to be 1036 m²/g, 0.71 ml/g and 2.75 nm, respectively. The findings strongly support the feasibility of microwave heating for preparation of high surface area porous carbon from Crofton weed via CO₂ activation.     

Chemically activated fungi-based porous carbons for hydrogen storage

A set of porous carbons has been prepared by chemical activation of various fungi-based chars with KOH. The resulting carbon materials have high surface areas (1600–2500 m²/g) and pore volumes (0.80–1.56 cm³/g), regardless of the char precursors. The porosities mainly derived from micropores in activated carbons strongly depend on the activation parameters (temperature and KOH amount). All activated carbons have uniform micropores with pore size of 0.8–0.9 nm, but some have a second set of micropores (1.3–1.4 nm pore size), further broadened to 1.9–2.1 nm as a result of increasing either the activation temperature to 750 °C or KOH/char mass ratio to 5/1. These fungi-based porous carbons achieve an excellent H₂ uptake of up to 2.4 wt% at 1 bar and −196 °C, being in agreement with results from other porous carbonaceous adsorbents reported in the literature. At high pressure (ca. 35 bar), the saturated H₂ uptake reaches 4.2–4.7 wt% at −196 °C for these fungi-based porous carbons. The results imply a great potential of these fungi-based porous carbons as H₂ on-board storage media.     

Improved cyclability of lithium–sulfur battery cathode using encapsulated sulfur in hollow carbon nanofiber@nitrogen-doped porous carbon core–shell composite

Hollow carbon nanofiber@nitrogen-doped porous carbon (HCNF@NPC) core–shell composite, which was carbonized from HCNF@polyaniline, was prepared as an improved high conductive carbon matrix for encapsulating sulfur as a cathode composite material for lithium–sulfur batteries. The prepared HCNF@NPC-S composite with high sulfur content of 77.5 wt.% showed an obvious core–shell structure with an NPC layer coating on the surface of the HCNFs and sulfur homogeneously distributed in the coating layer. This material exhibited much better electrochemical performance than the HCNF-S composite, delivered initial discharge capacity of 1170 mAh g⁻¹, and maintains 590 mAh g⁻¹ after 200 cycles at the current density of 837.5 mA g⁻¹ (0.5 C). The significantly improved electrochemical performance of the HCNF@NPC-S composite was attributed to the synergetic effect between HCNF cores, which provided electronic conduction pathways and worked as mechanical support, and the NPC shells with relatively high surface area and pore volume, which could trap sulfur/polysulfides and provide Li⁺ conductive pathways.     

Molded micro- and mesoporous carbon/silica composite from rice husk and beet sugar

A molded carbon/silica composite with high micro- and mesoporosity, as well as a high bulk density, was fabricated by activating a disk-molded precursor made from carbonized rice husk (RH) and beet sugar (BS) at 875 °C in CO₂. The pore structure of the RH- and BS-based carbon/silica composite (RBC) was analysed in relation to the bulk density. An activation time of 2.0 h provided the largest BET specific surface area (1027 m²/g) and total pore volume (0.68 cm³/g) and a low bulk density (0.54 g/cm³). An RBC that was first activated for 1 h was immersed again in BS syrup and then activated in CO₂ for 1 h. This two-step activation process provided both a high bulk density (0.69 g/cm³) and a highly textured structure (BET specific surface area, 943 m²/g; total pore volume, 0.56 cm³/g). The immersion in BS syrup was useful for improving the texture without reducing the bulk density, in comparison to one-step activation for 1.0 h. The suspension of the RBCs was basic because of the residual inorganic compounds of potassium and calcium. However, the basicity of the suspension was alleviated by washing the RBCs with water.     

Preparation of carbon adsorbents from lignosulfonate by phosphoric acid activation for the adsorption of metal ions

Activated carbons were prepared from sodium lignosulfonate by phosphoric acid activation at carbonization temperatures of 400–1000 °C. The resulting materials were characterized with regard to their surface area, pore volume, pore size distribution, distribution of surface groups and ability to adsorb copper ions. Activated carbons were characterized by nitrogen adsorption, scanning electron microscopy, Fourier transform infrared spectroscopy and thermal gravimetric analyses. The results indicate that with increasing carbonization temperature, the surface area decreased from 770 m²/g at 400 °C to 180 m²/g at 700 °C and increased at higher temperatures to 1370 m²/g at 1000 °C. The phosphorus content peaked at 11% for carbon obtained by carbonization at 800 °C. Potentiometric titration revealed the acidic character of all the phosphoric acid-activated carbons, which were found to have total concentrations of surface groups of up to 3.3 mmol/g. The carbons showed a high adsorption capacity for copper ions even at pH values as low as 2.     

Synthesis of nanostructured carbons by the microwave plasma cracking of methane

One of the attractive methods of producing hydrogen and high value-added carbon is plasma-reforming of hydrocarbons. Here, nanostructured carbon was produced by methane cracking in a relatively low-energy cold plasma reactor designed in-house specifically for such purpose. Carbon samples collected at different positions in the reactor show similar structural morphologies, indicating extensive structural uniformity of the carbon during processing. Surface area and microstructure of the materials were characterized by BET surface area analysis, X-ray diffraction and transmission electron microscopy (TEM). The effects of flow rate, temperature and power were evaluated for the formation of the carbon structures. The results show that the BET surface area and pore volume of the carbon materials vary from 74 to 125 m²/g and from 0.12 to 0.20 cm³/g, respectively. Such variations are closely associated with the magnitude of temperature drop at the sample collection position in the cold-plasma chamber before and after methane loading. The highest BET surface area of 125 m²/g is obtained at a power of 2000 W. TEM shows that the carbon consists of spherical particles of 40.8 ± 8.7 nm in diameter and graphene sheets.     

Heteroatom-doped carbon gels from phenols and heterocyclic aldehydes: Sulfur-doped carbon xerogels

Sulfur-doped carbon xerogels were obtained through carbonization of resorcinol/2-thiophenecarboxaldehyde organic gels. The acid-catalyzed sol–gel polymerization of resorcinol and 2-thiophenecarboxaldehyde leads to organic gels whose morphology and texture is dependent on the amount of catalyst used. As a result, monolithic organic gels with sulfur content of up to 19.6 wt.% and easily tailored properties can be produced. After carbonization, a substantial amount of sulfur is retained and porous carbon xerogels with S-content of up to 10 wt.% are produced (at 800 °C). Depending on the sol–gel synthesis conditions, monolithic S-doped carbon xerogels with controllable and enhanced mesoporosity, surface areas of up to 670 m²/g and enhanced mechanical integrity were obtained. Additional KOH activation of the organic or carbon xerogels enables production of micro–mesoporous carbons with surface areas of up to 2550 m²/g while retaining over 5 wt.% of sulfur. Preliminary CO₂ adsorption measurements were performed. On the basis of resorcinol/2-thiophenecarboxaldehyde gel synthesis a more general approach towards heteroatom-doped carbon gels is proposed: sol–gel polymerization of phenols and heterocyclic aldehydes. Thus a variety of heteroatom-doped porous carbon materials with a tailored pore texture and morphology are available via this procedure.     

Preparation of amino-functionalized silica for copper removal from an aqueous solution

In the present research, amino-functionalized silica materials were synthesized to develop absorbents for removing copper (II) ions from water. Three kinds of silica with various BET surface areas and pore volumes (331.4 m²/g, 460.1 m²/g, 717.7 m²/g and 1.38 cm³/g, 1.06 cm³/g, 0.57 cm³/g, respectively) were used to determine an optimum material. 3-Aminopropyltrimethoxysilane (3-APTMS) and N-[3-(trimethoxysilyl)propyl]-ethylenediamine (MSDA) are two amino-functional moieties grafted onto silica surfaces. A maximum copper absorption of 33.45 mg/g was confirmed using the amino-functionalized material at an initial 3-APTMS concentration of 2.52 mmol/g. Silica with a BET surface of 331 m²/g and a pore volume of 1.38 cm³/g demonstrated a good copper absorption capacity. Interference species such as pH, NH₃ and EDTA were also studied in this work.     

Facile preparation and unique H2 adsorption behavior of three-dimensional novel Pt–Ru hollow sphere assemblies

Three-dimensional long range ordered hollow Pt–Ru sphere assemblies were prepared using a sacrificial three-dimensionally ordered macroporous (3DOM) carbon template. Metallic salts, such as a mixture of RuCl₃ with H₂PtCl₆ were infiltrated into the carbon template, and a reduced Pt–Ru phase was produced on the surface of the 3DOM carbon template by a borohydride reduction reaction. The sacrificial template was then burnt off in air at 650 °C. The diameter of the hollow Pt–Ru spheres could be tailored using a different pore size 3DOM carbon template. Assemblies with an outer diameter of 550 nm showed high BET surface area of 584.3 m²/g. In addition, a high hydrogen adsorption stoichiometry (>0.5 H/M) was obtained on the Pt–Ru sphere assemblies, which indicated that most of the metal atoms on the surface were exposed.     

One-pot hydrothermal synthesis and supercapacitive performance of nitrogen and MnO co-doped hierarchical porous carbon monoliths

Nitrogen and MnO co-doped hierarchical porous carbon monolith (N-MnO-HPCM) materials were synthesized through a facile one-pot hydrothermal method. The resulting N-MnO-HPCM materials had hierarchical porous structure, high BET surface area (606 m²/g), large pore volume (0.33 cm³/g), and contained evenly dispersed MnO nanoparticles of about 6 nm in the carbon matrix. Their electrochemical performances as electrodes for supercapacitors were investigated. N-MnO-HPCM material exhibited an excellent electrochemical performance with a specific capacitance of 261.7 F/g at a current density of 1 A/g. It also showed a good rate capability with 74% capacity retention at high current density (5 A/g), indicating its potential applications in supercapacitors.     

Microporous activated carbons from ammoxidised anthracite and their capacitance behaviours

The paper presents results of a study on obtaining N-enriched active carbons from anthracite mined in Siberia, and on its use as electrode material in supercapacitors. The anthracite was carbonised, activated with KOH and ammoxidised by a mixture of ammonia and air at the ratio 1:3 at 300 or 350 °C, at each stage of the activate production. The products were microporous N-enriched active carbon samples of well-developed surface area reaching from 1255 m²/g to 2011 m²/g and containing from 0.3 to 5.4 wt% of nitrogen. Capacity curves characteristics of the ammoxidised active carbon samples were determined by the galvanostatic and potentiodynamic methods, and by impedance spectroscopy for acidic and basic electrolyte solutions. The best capacity parameters in an acidic medium were obtained for the coal samples ammoxidised at the precursor stage (191 F/g), containing about 0.4 wt% of nitrogen, while in a basic medium—for the coal samples ammoxidised at the stage of active carbon (200 F/g), containing 4.0 wt% of nitrogen.     

Molecular hydrogen and spiltover hydrogen storage on high surface area carbon sorbents

A series of templated carbons with various high surface areas (2033–3798 m²/g) have been prepared using various microporous zeolites as hard templates. Molecular hydrogen storage and spiltover hydrogen storage on these templated carbons were investigated and compared with superactivated carbon AX-21 and other reported porous carbon sorbents at 298 K and 100 atm. Two relationships between the surface areas of these carbons and their hydrogen capacities were obtained. The relationship between molecular hydrogen capacity and surface area showed a 0.23 wt.% H₂/1000 m²/g of carbon sorbent at 298 K and 100 atm, indicating that merely increasing surface areas of the carbon sorbents cannot achieve a significant molecular hydrogen capacity at ambient temperature. Spiltover hydrogen storage was achieved by doping Pt nanoparticles (as dissociative hydrogen source) on these carbons (spiltover hydrogen receptor). Our first result on the relationship between the spiltover hydrogen capacity and surface area showed 0.4 wt.% H₂/1000 m²/g of carbon sorbent at 298 K and 100 atm, which indicated that storage via spillover can lead to an average of 70% enhancement compared to molecular hydrogen storage.     

Preparation of porous carbon nanofibers derived from graphene oxide/polyacrylonitrile composites as electrochemical electrode materials

Porous carbon nanofibers (CNFs) derived from graphene oxide (GO) were prepared from the carbonization of electrospun polyacrylonitrile nanofibers with up to 15 wt.% GO at 1200 °C, followed by a low-temperature activation. The activated CNFs with reduced GOs (r-GO) revealed a specific surface area and adsorption capacity of 631 m²/g and 191.2 F/g, respectively, which are significantly higher than those of pure CNFs (16 m²/g and 3.1 F/g). It is believed that rough interfaces between r-GO and CNFs introduce oxygen pathways during activation, help to produce large amounts of all types of pores compared to pure activated CNFs.     

Enhanced adsorptive removal of fluoride using mesoporous alumina

Two different kinds of mesoporous alumina samples were prepared using aluminum tri-sec-butoxide in the presence of either cetyltrimethylammonium bromide (MA-1) or stearic acid (MA-2) as a structure-directing agent, and tested for adsorptive removal of fluoride in water. Both materials contain a wormhole-like mesopore structure, but exhibited different textural properties: surface area (421 or 650 m²/g) and pore volume (0.96 or 0.59 cm³/g). These mesoporous aluminas demonstrated significantly improved adsorption capacity and faster kinetics to those of the commercial activated aluminas in fluoride removal by adsorption process. MA-2 prepared using stearic acid, in particular, demonstrated an adsorption capacity (14.26 mg/g) and initial adsorption rate (14.6 mg/g min) that were respectively 2.2 and 45 times higher than those of a commercial gamma alumina. The textural features of larger surface area and relatively smaller pore size in MA-2 compared to the activated aluminas are believed to be responsible for this enhancement in adsorption process.     

Binderfree synthesis of high-surface-area carbon electrodes via CO2 activation of resorcinol–formaldehyde carbon xerogel disks: Analysis of activation process

High-surface-area carbon xerogels were prepared in the form of disks via carbonization of precursor resorcinol–formaldehyde (RF) polymer disks and subsequent activation of the resultant RF carbon xerogels by CO₂. RF carbon xerogels allow the preparation of a set of pre-activated carbon disks having different mesopore volumes. Analysis of the relationship between the mesopore volume of the samples and their CO₂ activation efficiency showed that the presence of mesopores is crucial for obtaining a high-surface-area carbon with minimal burn-off of carbon atoms. Activation of an RF carbon disk with a mesopore volume of 1.0 cm³ g⁻¹ up to a burn-off of 81% yielded an activated carbon disk with a high BET surface area of ∼3000 m² g⁻¹. Such disks could be readily used as electrode materials for an electric double layer capacitor without filler or binder addition and exhibited competitive EDLC performance against other electrode materials previously reported.     

Preparation of hollow submicrocapsules with a mesoporous carbon shell

The preparation of carbon submicrocapsules with size up to 800 nm and a mesoporous shell by hard silica templating is reported. Washing and template synthesis conditions were varied to promote porosity and avoid deformation of the microcapsules. The silica template synthesis conditions analyzed were: silica nucleus formation time (0.25–6 h), octadecyltrimethoxysilane/tetraethylorthosilicate volume ratio for silica shell formation (0.2–0.6) and silica shell formation time (1–24 h). The samples were characterized by 77 K nitrogen adsorption/desorption, mercury porosimetry and electron microscopy. Under all the washing conditions tested the carbon submicrocapsules were deformed due to the large size of the hollow core and the thickness of the shell. Changes in the silica template synthesis conditions did not result in substantial improvement of the strength of the microcapsules. The synthesis of a silica template with a double shell allowed us to obtain thick shell carbon submicrocapsules without significant deflation and with higher porosity. The characterization of these microcapsules showed that they have a BET surface area of 1541 m²/g and a pore size distribution with peaks centered at 0.75, 0.86, 1.0 nm in the micropore range and 3.5 nm in the mesopore range. The pore volume in the 2–80 nm range was 1.7 cm³/g.     

Microtubes made of carbon nanotubes

Microtubes made of multi-walled carbon nanotubes were prepared via infiltration of CNT-suspension through a microfiltration hollow fiber membrane. Shrinking of the entangled CNT network during the drying allows withdrawal of CNT-microtubes from the hollow fiber. Currently, microtubes have a length of ∼50 cm, outer diameter of ∼1.7 mm and scalable inner diameter by varying the infiltration time resulting in wall thicknesses of 130–320 μm. The BET surface area is 200 m²/g with a porosity of 48–67% and an electrical conductivity ∼20 S/cm. We propose to use such novel CNT-microtubes for the fabrication of tubular electrochemical cells and membrane filtration processes.