Carbon nanotubes are carbon materials with nano-sized tubular structures. Carbon nanotubes are seamless hollow tubes formed by rolling single or multi-layer graphene sheets with special physical and chemical properties. This enables carbon nanotubes to be widely used in composite reinforcement, nanodevices, field emission, catalysts and other fields. In supercapacitors, the electrode material determines the main performance indicators of the capacitor. The resistance of the electrode material and the internal resistance of the electrolyte ions to migrate in the pores of the electrode material are the keys to determine the power and frequency response characteristics of supercapacitors. Among many electrode materials, carbon nanotubes are commonly used electrode materials. Carbon nanotubes are considered as ideal electrode materials for supercapacitors due to their unique hollow structure, good electrical conductivity, large specific surface area, pores suitable for electrolyte ion migration, and intertwining to form nanometer-sized network structures. In order to further improve the performance of carbon nanotube electrode materials, carbon nanotubes can be activated or composited to prepare high-performance and low-cost electrode materials.
Figure 1. Structures of carbon nanotubes
- Electrode material: Both multi-walled carbon nanotubes and single-walled carbon nanotubes can be directly used as electrode materials for supercapacitors. The specific capacity, impedance, power density and other properties of these electrode materials have certain differences due to different preparation methods. In order to obtain electrode materials with excellent performance, it can be prepared by the following methods: 1) Carbon nanotubes with high specific surface area and high purity can obtain high specific capacity; 2) By activating or oxidizing the surface of carbon nanotubes to produce abundant functional groups, the wettability with the electrolyte can be increased, for example, physical activation of carbon nanotubes with KOH, CO2 or air; 3) By optimizing the electrode fabrication process or by directly growing carbon nanotubes on the conductive substrate to prepare an integrated electrode to reduce internal resistance and improve power characteristics; 4) Carbon nanotubes are compounded with quasi-capacitive materials metal oxides or conductive polymers, resulting in a synergistic effect, making carbon nanotubes have higher conductivity, capacitance and cycle performance.
- Khan Abdul Sammed, Lujun Pan, Muhammad Asif, Muhammad Usman, Tianze Cong, Farid Amjad, Muhammad Asif Imran. Reduced holey graphene oxide film and carbon nanotubes sandwich structure as a binder-free electrode material for supercapcitor [J]. Sci. Rep., 2020, 10, 2315.