This study established that for getting higher capacitance, in addition to large surface area CNF should also have a large number of pores in a plane parallel to the plane of the electrode for which the surface of CNM should be fluffy like a cotton ball.
Figure 2.11 CNF synthesized from seeds of Jackfruit and Rice straw by CVD method (Courtesy of Vilas Khairnar).
2.3.9 Plant-Derived CNM for Use in Coatings
A platform for the development of new environmentally friendly coatings is provided by CNM from burnt grass (Desmostachya bipinnata). It is found that a miniscule incorporation of the ash improves the overall property [48] of the composite in manifold ways. Studies of the properties and applications of various CNFs synthesized from plant precursor is still in progress. Hence, it may be concluded that CNF synthesized from plant precursors has potential for significant applications in various fields.
2.4 Comparative Structure of Chemically and Biogenically Synthesized CNF
2.4.1 CNF Synthesized from Chemical Precursors
Two methods are mainly used to prepare CNF from chemical precursors. One is catalytic thermal chemical vapor deposition (CVD) growth, and the other is electrospinning followed by heat treatment; whereas CNF is usually prepared mostly by CVD from biological precursors, it could be either with or without catalyst. To fabricate CNFs using the catalytic CVD growth method, some metals and alloys (Fe, Co, Ni, Cr, Mo and V) are chosen as the catalysts. Catalyst plays a very important role in deciding the morphology of CNF. Shapes of catalytic nano-sized metal particles decide the structures of the CNF. In the electrospinning process, the polymer types and the carbonization process play the most important roles in the type and quality of the CNFs. Vapor-grown carbon fibers (VGCFs) or vapor-grown carbon nanofibers (VGCNFs) are cylindrical nanostructures with graphene layers arranged as stacked cones, cups or plates. Whereas carbon with graphene layers wrapped into perfect cylinders are called carbon nanotubes.
Chemical precursors that have been used for the synthesis of CNF are methane, carbon monoxide, synthesis gas (H2/CO), and liquid organic waste from petrochemical industries, which include acetylene (C2H2), ethylene (C2H4) (Figure 2.12), and pyrene (C16H10); and various carbon- containing gases, such as carbon monoxide (CO), methane (CH4), etc., are also used as the carbon source. Che et al. (1998) have shown that solid carbon nanofibers are formed by increasing the deposition time at 900 °C in a CVD unit when ethylene and pyrene were used as precursor [53]. The morphologies of the CNF synthesized from chemical precursors could be coiled or straight CNFs.
Figure 2.12 CNF synthesized from chemical precursors: (a) Ethanol and (b) Acetylene using CVD method in our lab.
Figure 2.13 Schematic illustration of CNF and CNT.
2.4.2 CNF Synthesized from Plant Parts or Plant Metabolites as Precursors
The CVD method is mostly used for the synthesis of CNF from plant parts (stem, leaf, seeds) or plant metabolites that are rich in carbon or hydrocarbons (oil, resins, latex). CNF synthesized from plant parts usually show unique structures that are similar to the carbon-based anatomy of that particular plant part (Figures 2.5, 2.8 and 2.9), whereas plant metabolites produce CNF having similarities with the structure of CNF produced when chemical precursors are used.
2.5 Concluding Remarks
In this chapter, carbon nanofibers (CNF) synthesized from plant parts and plant metabolites were investigated both scientifically and for practical applications. Apart from petroleum hydrocarbon-based precursors, natural precursors are accepted as an alternative to the synthesis of CNFs. In this chapter, factors for biogenic synthesis using the CVD system and the factors affecting it were touched upon. Yield of different unique morphologies of CNF that varied with precursors were presented. The impact of a wide range of temperatures on the quality and quantity of yield and effect of temperature on the performance of metal catalysts, such as deciding solubility of carbon, is believed to increase at higher temperatures, and the catalyst and catalyst support systems enhance the rate of growth of CNFs. The effect of carrier gas is not yet very conclusive. Hydrogen seems to have a marked influence on superior CNF synthesis. Despite numerous studies on CNFs being synthesized from natural precursors, there is still room for further understanding of the mechanism of CNF growth by CVD method. CNF composites are able to be applied as promising materials in many fields, such as electrical devices, electrode materials for batteries and supercapacitors, etc.
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