Use of Carbon Nanotubes as Electrode Material

Due to their peeuliar properties, earbon nanotubes have reeeived enormous attention for the preparation of electroehemieal sensors, as it was reviewed by Zhao et al [3], Wang [35] and Li et al. [36], Different procedures for immobilizing the nanotubes onto electrochemical transducers have been described and the most representative are presented in the following sections. 

Gustavo A. Rivas, Maria D. Rubianes, Maria L. Pedano et al. 

Oxidation in air is one of the methods for purifying CNTs. However, at the same time that purified them, this oxidative scheme produces the aetivation of CNTs. Palleschi et al. [38] proposed the oxidation at 400 °C using an air flow of 12 mL/min for 1 hour. Sotiropoulou and Chaniotakis [39] proposed two meehanisms for activating MWCNTs grown by CVD on Pt substrate. One of them was performed by air oxidation at 600 °C for 5 min under air flow. 

Compton et al. [40] proposed the activation of MWCNTs in concentrated nitric acid at 60 °C for 20 hours. Mao et al. [41] have reported the pretreatment of MWCNT by refluxing in 3 M nitrie aeid for 12 h. 

A more drastic scheme proposed the aeidie oxidation using a mixture of concentrated sulfuric (98 %) and nitric acid (65 %) in a ratio 3 1 for 8 hours at 40 °C [39]. Under these conditions the tube eaps are opened and the tubules are shortened in fragments of different length. After that, the CNTs were washed with pure water and dried at 100 °C overnight. 

---

The effectiveness of carbon nanotubes as conductive filler to cathode of lithium ion batteries (Fig. 17) was demonstrated by adding small amounts of both carbon nanotubes and acetylene blacks to LiCoOa-based active materials [77]. The merits of using carbon nanotubes together with acetylene blacks as cathode fillers include not only the enhancement of the electrical and the thermal properties of the electrode, but also the enhancement of the density of the electrode and the shortening of the electrolyte absorption time. We envisage that the use of carbon nanotubes as multi-functional fillers will increase in both cathode and anode materials for lithium ion secondary batteries. 

Several types of NiO or Ni(OH)2 electrode such as flowerUke [3], mesoporous [4], nanotubes [5], and nanorod [6] were investigated as active materials for electrochemical capacitors. These Ni oxide electrodes are combined with carbon negative electrodes to construct the hybrid capacitors with high capacity and power density [2]. However, there still remaining serious problems with life cycle, reducibility, long-term stability, etc. It would need some more innovation for the practical use of these high-performance electrode materials. 

CNTs have been one of the most actively studied electrode materials in the past few years due to their unique electronic and mechanical properties. From a chemistry point of view, CNTs are expected to exhibit inherent electrochemical properties similar to other carbon electrodes widely used in various electrochemical applications. Unlike other carbon-based nanomaterials such as C60 and C70 [31], CNTs show very different electrochemical properties. The subtle electronic properties suggest that carbon nanotubes will have the ability to mediate electron transfer reactions with electroactive species in solution when used as the electrode material. Up to now, carbon nanotube-based electrodes have been widely used in electrochemical sensing [32-35], CNT-modified electrodes show many advantages which are described in the following paragraphs. 

It is well known that catalyst support plays an important role in the performance of the catalyst and the catalyst layer. The use of high surface area carbon materials, such as activated carbon, carbon nanofibres, and carbon nanotubes, as new electrode materials has received significant attention from fuel cell researchers. In particular, single-walled carbon nanotubes (SWCNTs) have unique electrical and electronic properties, wide electrochemical stability windows, and high surface areas. Using SWCNTs as support materials is expected to improve catalyst layer conductivity and charge transfer at the electrode surface for fuel cell oxidation and reduction reactions. Furthermore, these carbon nanotubes (CNTs) could also enhance electrocatalytic properties and reduce the necessary amount of precious metal catalysts, such as platinum. 

---