Two-dimensional Materials For Use In Advanced Energy Storage

Abstract

The high energy density, rapid charging and discharging and high degree of
cyclability make supercapacitors a promising form of electrochemical energy
storage. The storage of energy through the assembly of charge at electrode
surfaces in these devices make the use of nanostructured materials particularly attractive. In this work a simple, scalable and green approach – liquid
phase high shear exfoliation – is used to produce aqueous suspensions of
two dimensional materials in the form of platelets a few layers (< 10) thick
and with lateral dimensions of order a few hundred nanometres to a few
micrometres.
Suspensions of few layer graphene (FLG) are used to produce free-standing,
binder-free supercapacitor electrodes which are characterised by scanning
electron microscopy (SEM), Raman spectroscopy, cyclic voltammetry (CV)
and galvanostatic charging/discharging. Increasing the shear rate during
exfoliation of graphene is found to produce FLG platelets with a smaller
size and hence increasing number of edge defects. With increasing defect
density the specific capacitance of FLG electrodes is found to increase and
the effective series resistance decrease, both of which can be attributed to
increased electronic doping of the FLG with increasing edge defect density.
The FLG suspensions produced in this study were characterised by Ramanspectroscopy based metrics which suffer from a lack of generality and are indirect. Hence, an initial approach towards the automated characterisation of platelets of two-dimensional materials is developed and applied to MoS2, successfully demonstrating the feasibility and promise of direct imaging-based characterisation techniques.