Carbon defect functionalization

Of importance to fuel cell applications is the electron conductivity of carbons, when used as electron pathway in the porous electrode. Moreover, their functional groups with respect to hydrophobicity and water transport phenomena are also important. Also their defect density plays a role, when these defects function as nucleation sites during the synthesis of nanoparticulate catalysts and also when anchoring the particles to hinder Ostwald ripening during degradation. This chapter is further subdivided into three parts corresponding to the carbon s three main functions in polymer electrolyte membrane fuel cell (PEMFC) ... 

Research has shown that the surface functionalization of carbon-based fillers, which can both maximize interfacial adhesion between carbon-based fillers and the polymer matrix and increase the dispersion of CB, CNTs, and graphene in polymer matrix, is one of the best approaches to achieve good dispersion of conducting particles in polymer matrix. At present there are several approaches for functionalization of carbon-based materials including defect functionalization, covalent functionalization, and non-covalent functionalization (Gong et al. 2000 Hirsch 2002). Some functional groups, which can improve the interaction between carbon-based fillers and polymer matrix, are covalently bonded directly to the surface of carbon. These functionalization methods will be discussed in Chap. 25 (Vol. 2). 

Potentiodynamic and potentiostatic methods can also be used to activate nanotubes. Similar to chemical oxidation, electrochemical pretreatment can effectively remove impurities and cause the creation of carbon-oxygen functional groups at the exposed edge plane and defect sites. The same authors reported on an electrochemical pretreatment that involved potentiostating the electrode at +1.7 V vs. Ag/AgCl in pH 7 phosphate buffer for 3 min followed by 3 min at —1.5 V (84). Both the chemical and electrochemical oxidations improved the electrode response (smaller voltammetric A p and larger values) for Fe(CN)g " , serotonin, and caffeic acid. 

Covalent functionalization of CNTs is based on the formation of a covalent bond between functional entities and the carbon backbones of CNTs, conducted at the termini of the CNTs or at their sidewalls. It could also be divided into direct covalent sidewall functionalization and indirect covalent functionalization (defect functionalization). Direct covalent sidewall functionalization is associated with a change of hybridization from sp to sp and a simultaneous loss of conjugation. The latter takes advantage of chemical transformations of the already present defect sites, which can be the open... 

An important route to solubilization of carbon nanotubes is to functionalize their surface to form groups that are more soluble in the desired solvent environment. It has been shown that acid treatment of nanotube bundles, particularly with HC1 or HNO3 at elevated temperatures, opens up the aggregate structure, reduces nanotube length, and facilitates dispersion (An et al., 2004 Kordas et al., 2006). Nitric acid treatment oxidizes the nanotubes at the defect sites of the outer graphene sheet, especially at the open ends (Hirsch, 2002 Alvaro et al., 2004), and creates carbonyl, carboxyl, and hydroxyl groups, which aid in their solubility in polar solvents. 

The acid reflux procedure was first described by Rinzler el al. [28], in which raw nanotube materials are refluxed in nitric acid to oxidize the metals and carbon impurities. Acid-treated CNTs are considered to have carboxylic acid groups at the tube ends and, possibly, at defects on the side walls. The functionalized SWNTs have considerably different properties from those of the pristine tubes. 

In another example [56] SWNT was modified with peroxytrifluoroacetic add (PTFAA). Raman spectrum of the carbon nanotubes after the FIFA A treatment shows a D-line substantially increased indicating the formation of defect sites with sp3-hybridized carbon atoms on the sidewalls due to the addition of the functional groups. The RBM bands in the region of 170-270cm-1 decreased and shifted to higher... 

Although a honeycomb lattice theoretically consists of sp2 atoms, the carbon s ability to represent intermediate states of hybridization leads to another kind of defect to counterbalance the strain energy induced by high curvature. This so-called rehybridization results in a higher n-character of the C-C bonds [24]. Furthermore, local sp3 hybridization can be induced though chemical treatment, such as after thermal elimination of functional groups. 

These novel carbon nanostructures can also be modified by (a) doping, that is the addition of foreign atoms into the carbon nanostructure, (b) by the introduction of structural defects that modify the arrangement of the carbon atoms and (c) by functionalization involving covalent or noncovalent bonding with other molecules. These modifications opened up new perspectives in developing novel composite materials with different matrices (ceramic, polymer and metals). For example, polymer composites containing carbon nanostructures have attracted considerable attention due to... 

In the beginning, functionalization reactions were applied to fullerenes [1], later to CNTs [4,3], and recently to graphene [5]. Although both functionalization approaches have clear differences, they share the same intrinsic objective the creation of defects or doping within the surface of the carbon nanostructures in order to facilitate the interactions between the matrix and the filler. 

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