We have prepared carbon aerogels (CAs) doped with cobalt or nickel through sol-gel polymerization of formaldehyde with the potassium salt of 2,4-dihydroxybenzoic acid, followed by ion exchange with M(NO3)2 (where M = Co2+ or Ni2+), supercritical drying with liquid CO2, and carbonization at temperatures between 400 and 1050 degrees C under a N2 atmosphere. The nanostructures of these metal-doped carbon aerogels were characterized by elemental analysis, nitrogen adsorption, high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD). Metallic nickel and cobalt nanoparticles are generated during the carbonization process at about 400 and 450 degrees C, respectively, forming nanoparticles that are approximately 4 nm in diameter. The sizes and size dispersion of the metal particles increase with increasing carbonization temperatures for both materials. The carbon frameworks of the Ni- and Co-doped aerogels carbonized below 600 degrees C mainly consist of interconnected carbon particles with a size of 15-30 nm. When the samples are pyrolyzed at 1050 degrees C, the growth of graphitic nanoribbons with different curvatures is observed in the Ni- and Co-doped carbon aerogel materials. The distance of graphite layers in the nanoribbons is approximately 0.38 nm. These metal-doped CAs retain the overall open cell structure of metal-free CAs, exhibiting high surface areas and pore diameters in the micro- and mesoporic region.
"The current study is focused on applying the 3D-GCA material as porous substrate for catalytic CVD growth of CNTs, and understanding catalytic growth of carbon nanostructures as a function of growth temperature. Despite the previous claim  , the current study shows that CNT growth is not influenced by the presence of graphitic shell surrounding iron nanoparticles. In addition, graphitization of carbon aerogel prior to CNT growth plays a significant role in improving electrochemical stability along with methanol tolerance in acidic medium. "
[Show abstract][Hide abstract] ABSTRACT: CNTs were grown on iron-modified mesoporous graphitized carbon aerogel (GCA) at 700 °C, 800 °C and 900 °C using catalytic CVD method. Resultant CNT/GCA materials composition, morphology and structure were studied to understand their electrochemical stability and performance for oxygen reduction reaction (ORR) in acidic medium. CNT growth was increased from 700 °C to 800 °C, dominated by MWCNTs formation. In the temperature range from 800 °C to 900 °C, the growth was reduced by forming nanofiber/nanoribbon structures accompanied by MWCNTs. Mesoporosity of CNT/GCA composites declined at 700 °C and 800 °C due to MWCNT formation. However, CNT/GCA growth at 900 °C improved mesoporosity with substantial increase in pore volume (∼3 times of GCA) due to formation of nanofibers and nanoribbons. The structure of CNT/GCA materials revealed nitrogen doping and dispersion of FeNx phase. A synergistic contribution of CNT/GCA material structure and morphology to ORR activity was noticed. Among CNT/GCA materials, CNT-800 °C/GCA material showed ORR activity at lowest onset potential of 0.5 V. However, CNT-900 °C/GCA exhibits the highest ORR mass activity, with a half-wave onset potential difference of 120 mV with Pt (40 wt.%)/C. Moreover, CNT-900 °C/GCA demonstrates high selectivity (>3.97) to 4 electron ORR path, excellent methanol tolerance and electrochemical durability which makes it a potential DMFC cathode candidate.
"Many groups have been working on metal-doped carbon aerogels for catalysis, adsorption and electrode materials. For example, Baumann's group have prepared carbon aerogels doped with cobalt or nickel through sol–gel polymerization of formaldehyde with the potassium salt of dihygroxybenzoic acid, followed by ion exchange with M(NO 3 ) 2 (where M = Co 2+ or Ni 2+ ), and after the samples pyrolyzed at 1050 °C, the growth of graphitic nanoribbons with different curvatures is observed  . Moreno-Castilla's group prepared graphitized domains with three-dimensional stacking order by heating Cr-, Fe-, Co-, or Ni-doped carbon aerogels higher than 1000 °C . "
[Show abstract][Hide abstract] ABSTRACT: An easy co-gelation route has been developed to synthesize porous graphitic carbons with high surface areas by using teraethylorthosilicate (TEOS), furfuryl alcohol (FA), and metal nitrates as precursors. Using a one-pot co-gelation process, a polyfurfuryl alcohol–silica interpenetrating framework with metal ions uniformly dispersed was formed during the polymerization of FA and the hydrolysis of TEOS within an ethanol solution of the three precursors. This synthesis process is simple and time-saving in comparison with the conventional preparation methods. During the heat treatment, Fe7Co3 alloy nanoparticles were produced by carbothermal reduction and they then catalyzed the graphitization of the amorphous carbon. The graphitic carbons obtained have a high crystallinity as shown by X-ray diffraction, Raman spectroscopy, and high-resolution transmission electron microscopy analysis. The degree of graphitization can be controlled by the varying the loading amount of catalyst. The porous texture of the carbons combines miropores and bimodal mesopores, mainly originating from the silica template formed with different sizes and the loose packing of the graphite sheets. The carbons have large surface areas (up to 909 m2/g) and exhibit excellent electrochemical performance.Graphical abstractResearch highlights► An easy co-gelation route has been developed to synthesize porous graphitic carbons. ► This methods is simple and time-saving in comparison with the conventional ones. ► Fe7Co3 were produced by carbothermal reduction and then catalyzed the graphitization. ► The degree of graphitization can be controlled by varying the amount of catalyst. ► The carbons have large surface areas and exhibit excellent electrochemical performance.
[Show abstract][Hide abstract] ABSTRACT: Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2006. Includes bibliographical references (leaves 219-233). Carbon aerogels offer several unique advantages which make them ideal for evaluating a metal's ability to catalyze nanotube growth, including in situ carbothermic reduction of oxidized nanoparticles to their catalytic metallic phase as they form and production of a bulk quantity of nanoparticles which can be easily characterized. In this work, metal-doped carbon aerogels of seven transition metals were synthesized, characterized, and evaluated for their ability to catalyze growth of carbon nanotubes by thermal chemical vapor deposition (CVD). It was found that carbon aerogels doped with Fe, Rh, Re, Au, and Nb all catalyzed the formation of nanotubes in moderate to high yields, resulting in a direct growth of nanotubes on the exterior surfaces of aerogel monoliths. Ta was found to grow nanotubes only after thorough reduction of its oxides. Growth with W was inconclusive. CVD growth of nanotubes throughout the interior porosity of metal-doped carbon aerogels was also achieved by templating a network of interconnected macropores into the monoliths. Surface-based nanoparticles composed of rhenium, gold, and varying combinations of gold and rhenium were investigated for their ability to catalyze carbon nanotube growth. (cont.) Nanoparticles of these metals were nucleated onto silicon wafers from solutions of anhydrous ReCI5 and AuC13. After deposition, the nanoparticles were reduced under hydrogen for 10 min and then oxidized in air for 4 min. The samples were then processed by CVD employing hydrogen and ethanol-saturated Ar for 10 min. Nanoparticles deposited from metal chloride solutions with a 1:1 molar ratio of gold to rhenium or higher were found to result in high yields of single-walled nanotubes, where nanoparticles deposited from solutions with less than a 1:4 gold-to-rhenium ratio resulted in no nanotube growth. Lastly, a new low-pressure CVD system specialized for nanotube growth was developed. The objectives of the system are to provide a flexible architecture for developing new nanotube growth techniques and to lower the minimum temperature required for nanotube growth. The system features a separate sample heating plate for thermally activating nanoparticles and hot filament for carbon feedstock cracking. The system also features the ability to easily install or remove modules for electric field- and plasma-assisted growths. by Stephen Alan Steiner, III. S.M.
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