Synthesis and application of oxidised carbon nanotubes in supercapacitors

Abstract

In this thesis, two types of supercapacitors based on the functionalised carbon nanotube (CNT) electrodes that were decorated with different amounts of oxygen-containing groups were assembled. The as-purchased CNTs were functionalised by using improved Hummer’s method, which was adjusted to introduce different amounts of oxygen functional groups. These oxygen functional groups changed the surface structures of the CNTs and hence their electrochemical performances, because the oxygen groups are redox active. On the other side, the quantity of the oxygen groups also reduces the electrical conductivity of CNTs, thereby impairing the electrode performance. Therefore, as the first part of this thesis we studied the correlation of structures with electrochemical performances of those functionalised CNTs with different oxygen contents.

In the second part, an asymmetric zinc ion capacitor (ZIC) was constructed, which was composed of a zinc anode, a low-level oxidised CNT cathode and a zinc sulphate-based liquid or gel electrolyte. The low-level oxidised CNTs were characterised by the abundant surface oxygen-containing groups and the intact graphitic inner tubes that maintained the electrical conductivity. These oxygen functional groups provided a large pseudo-capacitance, due to the electrochemical adsorption/desorption of Zn2+ ions as well as improved the electrode wettability, giving rise to a higher capacitance than that of the ZIC with an as-purchased CNT cathode.

In the last part of this work, a symmetric solid-state supercapacitor was fabricated, which used the mildly oxidised CNT electrodes and sulfuric acid/poly(vinyl alcohol) gel electrolyte. The CNTs oxidised in a mild condition showed a hybrid geometrical structure with both closed tubes and unzipped tubes. The unzipped tubes enriched with oxygen-containing groups could provide electrochemical active sites for improving the capacitance while the closed tubes that preserved mostly the sp2 bonding network could endow the electrode with a good electrical conductivity. This hybrid structure rendered a good electrochemical performance in the solid-state device.