Impact of Graphene Quantum Dot Edge Shapes on High-Performance Energy Storage Devices

Abstract

Graphene quantum dots (GQDs) are a promising electrode material for the advancement of energy storage devices due to their fast-moving charge carriers, high surface area to volume ratio and controllable bandgaps. However, the differences between the electronic properties of zigzag and armchair-edged graphene quantum dots (GQDs) have not yet been well defined and are explored in detail in this work.
Nitrogen-doped GQDs with zigzag (ZZ) and armchair (AC) edges were synthesised hydrothermally and electrochemically, respectively. Surface (XRD, TEM, UV-Vis) characterisation was conducted on the synthesised GQDs. It was found that the bandgap of ZZ GQDs is controllable through pH variation and size variations dictated by synthetic reaction times, while this is not the case for AC GQDs. ZZ and AC GQDs were electrodeposited on the surface of carbon fibre electrodes. Electrochemical characterisation (cyclic voltammetry, electrical impedance spectroscopy) was carried out on these electrodes. The specific capacitance of all carbon fibre electrodes increased upon the deposition of GQDs, with the greatest increase observed to be a 275 % improvement in specific capacitance upon the addition of 5 nm zigzag GQDs. Carbon fibre electrodes with deposited ZZ GQDs display a more significant increase in capacitance values (specific capacitance, electrical double layer capacitance, pseudocapacitance and quantum capacitance) due to a greater affinity for edge site doping and a larger surface area compared to AC GQD carbon fibre electrodes. ZZ GQDs are therefore more beneficial for energy storage devices than AC GQDs.