Control Synthesis of Nitrogen Doped Carbon Nanotubes and Their Application in Energy Storage
Authors: Liu Hao , Mei Jun 
Published: February 2020

Considerable progress has been made in investigating the carbon nanotubes (CNTs), which possess many desirable properties, including high mechanical strength and flexibility, excellent electrical conductivity and conductivity. Although the properties of CNTs are nothing short of exceptional, there are many areas in nano- and molecular-electronics, optics, electromechanics or chemistry where pristine tubes are not the appropriate.

Substitutional doping of CNTs can lead to chemical activation of the passive surface of the CNTs and adds additional electronic states around Fermi level. This disrupts the smooth, largely chemically inert, p-bond and makes the tubes more chemically active. Correspondingly, the mechanical, thermal and transport properties will be changed by the doping of CNTs. For this reason, extrinsically doped tubes should make excellent starting candidates for a new generation of controlled chemically functionalized nanotube based materials. Nitrogen atoms substituting carbon atoms in the graphite matrix are electron donors and promote n-type conductivity. It was shown by tunneling microscopy that nitrogen–doped carbon nanotubes (CNx-NTs) have metallic properties independent of the structural parameters and are characterized by the presence of a donor state close to the Fermi level. The increase of the localized density of states at the Fermi level results in larger emission currents at lower voltages. Due to the metal-type conductivity and stable emission characteristics at low voltages, CNx-NTs are promising materials for field emitters and conducting coatings and composites. Furthermore, the enhancement of the donor properties and electrical conductivity of CNx-NTs after doping with nitrogen increases the activity of the carbon material in electron-transfer reactions.

In this presentation, we will firstly give a straight forward illustration of the control synthesis of nitrogen doped carbon nanotubes on Ni foam current collector for afterward application, in which CNx-NTs were prepared using a floating catalyst chemical vapor deposition method and the nitrogen content can be controlled by precursor amount. Considering the increase of the surface activity of the tubes’ surface after nitrogen doping, the dispersion and distribution of metal oxides on CNx-NTs might be improved when they were grown on CNx-NTs. Hence, a series of compositing materials was synthesized including MoO3, ZnO, NiCo2O4, etc. were composed with CNx-NTs. Thanks to the following merits, the composites present very good performance in both lithium ion batteries and supercapacitors. (1) the 3D conducting network of CNx-NTs not only offers a strong skeleton for homogeneous deposition of metal oxides, but also provides a good electrical conducting pathway for ion and electron transportation and redox kinetics; (2) the large void space between CNx-NTs and metal oxides allows electrolyte easily penetrate into the inner-space and can endure the volume change during electrochemical measurement; (3) the direct deposition of metal oxides on CNx-NTs could ensure good mechanical adhesion and decrease the contact resistance between the composites and the current collectors; (4) the ultrathin and interconnected nanostructure of metal oxides possess numerous active sites for the redox reaction, the short diffusion path and fast ion transport; (5) the high conductive CNx-NTs directly grown on the Ni foam current collector benefit the fast electron transfer and avoid the use of polymer binder and conductive additives.
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