from oil of Linum usitatissimum by CVD method.
2.3.4 CNF as Electrocatalysts for Microbial Energy Harvesting
CNFs synthesized from biogenic material is being considered as an alternative catalyst for the oxygen reduction reaction. Zhou et al. (2016) [47] have fabricated heteroatom-doped (N and S) porous CNFs via pyrolysis of natural spider silk as a precursor. The CNFs have exhibited very good oxygen reduction activity (half-wave potential of 0.85 V and on-set potential of 0.98 V vs. RHE), superior to that of the Pt/C and many metal-free carbon catalysts under alkaline conditions. The catalytic proficiency of the SS-derived CNFs is attributed to the highly positive charges on the carbon atoms due to the presence of the electronegative nitrogen and sulfur atoms within the carbon lattice; and their high surface area and large number of active sites due to their nano-fibrillar structure and abundant pores. They suggested that since these CNFs exhibited excellent oxygen reduction activity in neutral solution (pH 7.0), they used it as cathode catalysts in microbial fuel cells (MFCs), which gave a maximum power density of 1800 mW/m2 that is 1.56 times more than with a Pt/C cathode.
2.3.5 CNF as Regenerative Medicine
Studies are being conducted on CNF for its use in bone regeneration, neural regeneration, scaffolds; the role of adhesive protein adsorption in drug and gene delivery, etc. For such applications CNFs have to be specifically functionalized to improve their biocompatibility. In the field of regenerative medicine, these nanofibers are becoming increasingly attractive as they can be modified to be integrated into human bodies for promoting tissue regeneration and treatment of various diseases. Despite their tremendous potential, toxicity remains a big concern.
2.3.6 CNF as Deodorizer
The patented work of Sharon and Sharon (2018) has shown the potential of CNF synthesized from agro-waste precursors. Unpleasant odors, malodor, stench or stink is caused by food being left out for too long, excreta, urine or any rotting material. Presently, ozone and activated carbon is used for removing bad odor. However, CNF synthesized from agro-waste via pyrolysis is more efficient nanomaterial that can deodorize, as it has higher surface area. It has very high deodorizing property and can remove 99% of odor from a room if temperature, humidity and air circulation (fan speed) is met and can absorb CO, paint odors, noxious vapors, toilet smells, etc.
2.3.7 CNF Composites for Strong and Lightweight Material
Composites of CNF have been investigated in both fundamental scientific research and practical applications. The need for strong and lightweight materials is being met by promising materials created by reinforcing CNF in polymers for applications that include electrical devices, electrode materials for batteries and supercapacitors, and as sensors. In these applications, the electrical conductivity is a priority need, which depends on the dispersion and percolation status of CNFs in matrix materials. Other important considerations are the effects of the aspect ratio, percolation backbone structure and fractal characteristics of CNFs and the non-universality of the percolation critical exponents on the electrical properties. Apart from the electrical property, the thermal conductivity and mechanical properties of CNF composites also add to the utility of CNF composites, including the melt mixing and solution process, especially for their applications as sensors and electrode materials. Significant application of composite materials has contributed a lot in the defense, aeronautical and automobile industries because of their specific modulus and high strength characteristics. In composite material, CNF-reinforced mat with polymer epoxy resin composites and CNF/polyvinyl alcohol (PVA) mat have been found to improve the flexural strength of the epoxy resin, with 0.015% CNF in PVA giving a better mechanical strength. Cellulose is another plant metabolite which is also secreted by many bacteria and extracted from various marine filamentous algae. Cellulose nanofibers are considered as efficient replacements for conventional polymers due to their nano size, ease of preparation, low cost, tunable surface and enhanced mechanical properties. CNF obtained from algal sources is less compared to plants and bacterial sources. CNF finds a wide variety of applications such as drug carriers, tissue regenerating scaffolds, water purification, etc.
2.3.8 Biogenic CNF as Supercapacitor
Another important application of CNMs is in the development of supercapacitor. Among the CNF obtained from seeds of various plants, it was found by Khairnar et al. (2008) that jackfruit seeds gave the highest capacitance of 92F/g [45]. It has been observed that though the CNM from rice straw has a higher surface area (140.15 m2/g) than that obtained from jackfruit seeds (114.02 m2/g), the capacitance obtained from the CNM of rice straw was lower (83F/g) than that of CNM from jackfruit seed. This apparent anomaly has been resolved by analyzing the SEM micrographs of both the CNMs, which show that CNMs from rice straw show channeltype structure while that from jackfruit seeds show porous but block-type structure (Figure 2.11). The cotton fiber-like structure of the CNM from jackfruit helps in forming the electrical double layer which is not so facile in the CNMs from rice straw.
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
