Resultant Carbon Foams Research Articles

Synthesis and Electrochemical Performance of Mesoporous Carbon Foams by Microemulsion Method

In this paper, we demonstrated the preparation and electrochemical performance of mesoporous carbon foams as electrode materials for ultracapacitors. By using n-octane as oil phase, cetyltrimethylammonium bromide (CTAB) and butanol as emulsifiers, resorcinol and formaldehyde dissolved in water as the aqueous phase, an O/W microemulsion system was obtained. Mesoporous carbon foams (MCFs) were prepared by the polymerization of the O/W microemulsion, followed by drying and carbonization and subsequently activation at 1273 K by KOH under nitrogen atmosphere. The mesoporous carbon foams were characterized by scanning electron microscopy (SEM) and Brunauer-Emmett-Teller (BET) analyzer. The results show that MCFs have specific surface area of 666.7 m2/g, total pore volume of 0.36 cm3/g and the most probable pore size of 4 nm. The electrochemical properties of the resultant mesoporous carbon foams have been investigated by cyclic voltammetry (CV) and galvanostatic charge-discharge with a three-electrode system in electrolyte of 6 mol/L KOH solution. The CV curves show quite rectangular shape under the scan rate of 5-20 mV/s, suggesting a typical nonfaradic adsorption/desorption reaction. The mesoporous carbon foams possess linear galvanostatic discharge curves under the current densities of 10-50 mA/cm2 and corresponding specific capacitance values are 132.6-172.1 F/g. Thus the MCFs have good electrochemical performance and they provide an important candidate for electrode materials used in ultracapacitors.

Preparation and characterization of resin-derived carbon foams reinforced by hollow ceramic microspheres

Carbon foam materials were prepared with phenolic resole resin as precursor and hollow ceramic microspheres as reinforcement. The resultant carbon foams had a bulk density of between 0.22 and 0.27 g/cm 3. Analysis of scanning electron micrographs indicated that the cells were mainly open with incomplete cell membranes, and the hollow ceramic microspheres were located in the junctions. Compression test results showed that the carbon foams with 1 wt% hollow ceramic microspheres exhibited the highest compressive strength (7.76 MPa). Thermal conductivity measurements revealed that the introduction of hollow ceramic microspheres improved the thermal insulating property. The thermal conductivity of carbon foams ranged from 0.11 to 0.25 W m −1 K −1 at room temperature, which makes carbon foams a prime choice for thermal insulation.

Preparation of carbon foams with supercritical toluene

Carbon foams with pore sizes of 10–50 μm were prepared with mesophase pitch and toluene as the carbonaceous precursor and supercritical agent, respectively. Results revealed that the light pitch components and dissolved toluene in pitch significantly affected the pore structures of resultant carbon foams. The amount of toluene dissolved in molten pitch is greatly dependent on the foaming conditions, such as the ratio of toluene to pitch, foaming temperature, foaming pressure and saturation time. Carbon foams with hierarchical porous structures are obtained by controlling the amount of light pitch components.

Preparation of mesophase-pitch-based carbon foams at low pressures

A simple method for preparing the mesophase-pitch-based carbon foams at low pressures through prolonging the soaking time in the preparation process of the mesophase pitch was disclosed. The physical properties, morphologies and the crystal structure of the as-obtained foams were investigated. Bulk density of the resultant carbon foams cover a range 0.514–0.624 g/cm 3, under the preparation pressure range 0.5–2 MPa. The SEM micrographs revealed that the thermal shrinkage of the graphitized foams derived from the higher softening point mesophase pitch was less than that of the foam from the lower softening point; Optical micrographs showed that higher softening point mesophase pitch derived carbon foams exhibited better orientation and less microcracks in both junctions and ligaments; The XRD results revealed that higher softening point pitch derived graphitized foams exhibited smaller interlayer spacing and larger crystallite size. The properties of the carbon foam can be severely affected by the properties of the precursor, so it is critical to tailor the properties of the pitch precursor so as to obtain high performance and low cost products.

Carbon foam derived from various precursors

A series of carbon foams was developed by using low-cost precursors, such as coal, coal tar pitch and petroleum pitch. The properties of the resultant carbon foams cover a wide range, e.g., bulk density, 0.32–0.67 g/cm 3, compressive strength, 2.5–18.7 MPa, isotropic and anisotropic microstructure, etc. The investigation of foaming mechanism and the relationship between properties and structure indicate that the fluidity and dilatation of the foaming precursors significantly affect the foaming performance and foam structure. Raw coal samples were foamed directly without pretreatment in this work. However, for the pitch based foaming precursor, a thermal pretreatment is necessary to adjust its thermoplastic properties to meet the foaming requirement. The mechanical strength of carbon foam is found to be related to not only the foam cell structure, but also the fluidity and anisotropic domain size of the foaming precursors. In addition, the micro and mesopore structure in carbon foam matrix was investigated by gas adsorption and it was found that it also affects the strength of carbon foam and is related to the fluidity of foaming precursor.

Morphological characterization of microcellular carbon foams

Microcellular carbon foams have been prepared by the high-temperature carbonization of polyacrylonitrile (PAN) foams in an inert atmosphere. The PAN precursor foams were first prepared by thermally induced phase separation of PAN solutions followed by gelation/extraction or freezing/freeze-drying processes. The resulting carbon foams have low densities and open-celled morphologies. We have developed two complementary techniques to characterize objectively carbon foam morphologies, BET surface area measurements and mercury porosimetry. These two measurement techniques, when used in conjunction with known stereological rules, allow us to calculate densities and average cell sizes or cell size distributions. As a bonus, values of the bulk modulus for low-density carbon foams also can be determined from mercury porosimetry. Unlike other techniques, our determinations do not require carefully prepared surfaces (such as for microscopy) or an assumed morphological model (required for most other techniques).