Carbon Nanotubes and Graphite Nanofibers

FIGURE 11.1 Typical representation of carbon in graphite form. 

The amount of hydrogen that can be stored on these tubes has been debated. One report shows that maximum storage capacity for a single-walled carbon nanotube (SWCNT) [7] and multiwalled carbon nanotubes [8] is approximately 8% by weight [9], Multiwalled carbon nanotubes are a collection of concentric single-walled nanotubes [10] versus the one-dimensional single-walled tubes [11], SWCNTs have very high surface-to-volume ratios as well as uniform pores, which allow for capillary action and thus the ability to be filled by condensation. 

Adsorption on the interior surface of the tubes can take place. At high temperatures, hydrogen gas condenses to a liquid deep within the tubes, squeezing between the carbon nanotube plates and thus decreasing their vibration activity this allows for greater compression [1], To get the hydrogen into the fuel cell, a vacuum is pulled and the tube heated to 900°C. When hydrogen is inserted into the tube, a pressure of 12,000 kPa is used [1], A pressure of 4,000 kPa must be maintained to keep the H2 in place in the cell [1], 

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Lee, C.-L., et al., Preparation of Pt nanoparticles on carbon nanotubes and graphite nanofibers via self-regulated reduction of surfactants and their application as electrochemical catalyst. Electrochemistry Communications, 2005. 7(4) p. 453-458. 

Simonyan, V.V. and Johnson, J.K. (2002). Hydrogen storage in carbon nanotubes and graphitic nanofibers.Alloys Compd., 330, 659-65. 

Previously, carbon black, carbon nanotubes, and graphite nanofibers have been explored as supports because of their large surface area and high electrical conductivity [225,226]. 

The past two decades have shown an explosion in the development of new nanoporous materials mesoporous molecular sieves, zeolites, pillared clays, sol-gel-derived metal oxides, and new carbon materials (carbon molecular sieves, super-activated carbon, activated carbon fibers, carbon nanotubes, and graphite nanofibers). The adsorption properties for most of these new materials remain largely unexplored. 

Fig. 11.2 Temperature dependence of the gas pressure in a preliminarily evacuated volume (left vertical scale) and its recalculation into the amount of hydrogen evolved from the sample (right scale) upon heating at a rate of 20 K/min for single-walled carbon nanotubes (SWNTs) and graphite nanofibers (GNFs, two heating cycles) saturated with hydrogen at a pressure of 9 GPa and temperatures up to 450°C...

Porous carbons constitnte a fascinating kind of material. Different types with distinctive physical forms and properties (i.e., activated carbons, high-surface-area graphites, carbon blacks, activated carbon cloths and fibers, nanofibers, nanotubes, etc.) find a wide range of indnstrial applications in adsorption and catalysis processes. The main properties of these materials that make them very useful as catalyst supports, as well as some of their applications, have been described. The use of carbon as a catalyst support relies primarily on the relative inertness of its surface, which facilitates the interaction between active phases or between active phases and promoters, thus enhancing the catalytic behavior. This makes porous carbons an excellent choice as catalyst support in a great number of reactions. 

Carbon nanotubes and nanofibers constitute a new family of support offering a good compromise between the advantages of activated carbon and high-surface-area graphite. The main advantages offered by CNT or CNF supports are ... 

Particulate polymer composites with fibers are a very active area of development, particularly carbon nanotubes and nanofiber composites, and the new graphite and polymer composites [35]. This fact, combined with the continued interest in nancomoposites based in nanometric clays [36], suggests that improvements in mechanical properties of particulate and short-fiber polymer composite materials will continue to be reported. 

The dimensions of the added nanoelements also contribute to the characteristic properties of PNCs. Thus, when the dimensions of the particles approach the fundamental length scale of a physical property, they exhibit unique mechanical, optical and electrical properties, not observed for the macroscopic counterpart. Bulk materials comprising dispersions of these nanoelements thus display properties related to solid-state physics of the nanoscale. A list of potential nanoparticulate components includes metal, layered graphite, layered chalcogenides, metal oxide, nitride, carbide, carbon nanotubes and nanofibers. The performance of PNCs thus depends on three major attributes nanoscopically confined matrix polymer, nanosize inorganic constituents, and nanoscale arrangement of these constituents. The current research is focused on developing tools that would enable optimum combination of these unique characteristics for best performance of PNCs. 

Figure 4.11.28 shows the DTG curves for various carbonaceous materials, carbon nanotubes and carbon fibers, active charcoal, and graphite. Obviously, the nanotubes have the highest reactivity, followed by nanofibers, active charcoal, and graphite. It can also be seen that the nanomaterials are quite pure (ideal shape of the DTG signal), whereas a fraction of the charcoal is less reactive than the fraction that reacts first. 

Diazonium salts have been electrografted on a variety of substrates various forms of carbon such as highly ordered pyrolytic graphite graphene,glassy carbon plates,carbon fibers and nanofibers, carbon felts, carbon blacks,ordered mesoporous carbons, ° carbon nanotubes and/or diamond,silicon,AsGa" ° and InAs/GaAs... 

The carbon-based nanofillers are mainly layered graphite, nanotube, and nanofibers. Graphite is an allotrope of carbon, the stmcture of which consists of graphene layers stacked along the c-axis in a staggered array [1], Figure 4.1 shows the layered structure of graphite flakes. 

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