INVESTIGATION OF THE ELECTROCHEMICAL PROPERTIES OF GRAPHENE

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Abstract

In this thesis, the synthesis and characterization of nitrogen-doped graphene (NG) and NG-Co3O4 composites are described. Moreover, the effect of airborne contamination and nitrogen doping on the capacitance of graphene was investigated.Firstly, nitrogen-doped thermally expanded graphene oxide (NtGO) was prepared by a facile thermal expansion and hydrothermal doping process. The thermal expansion process plays a vital role in improving the electrochemical performance of N-doped graphene by preventing its aggregation and improving its conductivity. The specific capacitance of NtGO is 270 F g−1 at a discharge current density of 1 A g−1 and the capacitance retention is 97 % after 2000 cycles at this current density. Secondly, a hierarchical electrode structure, consisting of cobalt oxide and nitrogen-doped graphene foam (NGF), has been fabricated with the aim of achieving enhanced charge storage performance. The Co3O4/NGF electrode shows an enhanced charge-storage performance, attributed to the 3D hierarchical structure and the synergistic effect of Co3O4 and NGF. The present study shows that specific capacitances as high as 451 F g−1 can be obtained, indicating that high-performance electrochemical capacitors can be made using electrode materials with advanced structures. Thirdly, a study of the differences between the capacitance of freshly exfoliated highly ordered pyrolytic graphite (HOPG, sample denoted FEG), HOPG aged in air (denoted AAG) and aged in an inert atmosphere (hereafter IAG) is presented in this work. Electrochemical impedance spectroscopy shows the FEG possesses a higher intrinsic capacitance (6.0 µF cm−2 at the potential of minimum capacitance) than AAG (4.3 µF cm−2) and IAG (4.7 µF cm−2). This change in capacitance is correlated with other physical changes of the sample, and attributed to contamination due to airborne hydrocarbons.Finally, the effect of N-doping of graphene prepared by chemical vapour deposition is investigated. The differential capacitance of PG and NG was measured by a microinjection-micromanipulator system. The quantum capacitance of PG and NG was calculated and discussed. The increase in differential capacitance with nitrogen-doping and the growth of the quantum capacitance of NG suggest that the increased capacitance of many electrodes of electrochemical capacitors is primarily due to the modification of the electronic structure of the graphene by the N dopant.

Keyword(s)

electrochemical properties; graphene; supercapacitor

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