CNT Electrodes

For the first time, a-Mn02nH20 based composites have been studied in real two electrode capacitors. The a-Mn02nH20/CNTs electrodes demonstrate an ideal capacitive behavior and high values of capacitance. Compared to the conventional carbon black, multi-walled CNTs are a very promising conductivity additive for capacitor or battery electrodes. 

FIGURE 15.3 Cyclic voltammetric curve of 5 mM dopamine in PBS (pH 7.4) at a CNT electrode at 20mV s (Reprinted with permission from [16]. Copyright (1996) Elsevier.)... 

Figure 15.14 illustrates a typical voltammetric result for the determination of dopamine in the presence of ascorbic acid with a CNT-modified electrode. The selective voltammetric detection of uric acid [82] or norepinephrine [83] in the presence of ascorbic acid has been demonstrated with a (3-cyclodextrin-modified electrodes incorporating CNTs. Ye et al. [84] have studied the electrocatalytic oxidation of uric acid and ascorbic acid at a well-aligned CNT electrode, which can be used for the selective determination of uric acid in the presence of ascorbic acid. The simultaneous determination of dopamine and serotonin on a CNT-modified GC electrode has also been described [85],... 

Among those several different types of transducers based on CNTs, the CNT-composite electrode, which was the first CNT electrode tested in 1996, is still widely used with different composite materials such as conducting polymers, nanoparticles, sol-gel, etc. The usefulness of these electrodes is based on their high sensitivity, quick response, good reproducibility, and particularly long-term stability. We expect to see continued research activities using CNT-composite electrodes. 

In this chapter, we will focus on CNTs as advanced materials for the design of electrochemical devices. The next section vdll be devoted to review the structure, electronic, chemical and electrochemical properties of CNTs. Section 3.3 will comprise an overview of the synthesis, purification and (bio)functionalization of CNT, as well as the modification of substrates with CNT. In Section 3.4, we will address the electrochemical applications of functionalized CNT electrodes... 

It is important to point out that not all redox systems will exhibit electrocatalytic activity when probed on high-density edge CNT electrodes or edge pyrolytic graphite. Such phenomenon depends on the particular mechanism of the redox system ]2]. It is important to recall that among the redox species there are some of them whose... 

To summarize, one can say that the electrochemical performance of CNT electrodes is correlated to the DOS of the CNT electrode with energies close to the redox formal potential of the solution species. The electron transfer and adsorption reactivity of CNT electrodes is remarkably dependent on the density of edge sites/defects that are the more reactive sites for that process, increasing considerably the electron-transfer rate. Additionally, surface oxygen functionalities can exert a big influence on the electrode kinetics. However, not all redox systems respond in the same way to the surface characteristics or can have electrocatalytical activity. This is very dependent on their own redox mechanism. Moreover, the high surface area and the nanometer size are the key factors in the electrochemical performance of the carbon nanotubes. 

Carbon nanotubes inevitably contain defects, whose extent depends on the fabrication method but also on the CNT post-treatments. As already seen, oxidizing treatments, such as acid, plasma or electrochemical, can introduce defects that play an important role in the electrochemical performance of CNT electrodes. For instance, Collins and coworkers have published an interesting way to introduce very controlled functionalization points or defects on individual SWNTs by electrochemical means [96]. Other methodologies to introduce artificial defects comprise argon, hydrogen and electron irradiation. Under this context, a number of recent works have appeared with the goal of tailoring the electrochemical behavior of CNT surfaces by the controlled introduction of defects [97, 98]. 

In order to guarantee an efficient performance of the CNT-based electrochemical devices, attention has to be paid not only to CNT synthesis and purification but also to the way that the CNT electrode is built up. There have been many studies in the literature dealing with CNT dispersions either on conducting substrates or forming composites. In this subsection we will address the different carbon-nanotube deposition techniques and carbon-nanotube arrangements on different electrode surfaces. 

We will discuss, on the one hand, the spaghetti-like CNT electrodes on conductive surfaces or randomly dispersed in a polymer matrix and on the other hand, CNT arrays grown in situ on substrates with large-scale control of location and orientation. Finally, individual CNT electrodes will be briefly discussed. Figure 3.10 illustrates these different CNT electrode arrangements. 

FigureS.IO Illustration of different CNT electrode configurations (a) randomly dispersed on a surface, (b) vertically aligned CNTs, (c) randomly dispersed CNT composites, (d) oriented CNTs embedded in a polymer matrix, (e) individual CNT electrode.

Other nanocomposite CNT electrodes have been reported by mixing CNTs with granular Teflon [48], chistosan [110], polystyrene [111], polysulfone [112] and epoxy [103, 113] or by incorporating them into a silicate gel matrix [13, 114]. 

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