Electrochemical Properties of Two-Dimensional (2D) Materials: From Fundamental to Applications in Capacitive Energy Storage

Abstract

The electrochemistry, especially the capacitance, of 2D materials is explored from the fundamental perspective with specific application to supercapacitors. Highly ordered pyrolytic graphite (HOPG) has been used as a model electrode material, representing the capacitive behaviour of the carbonaceous materials, with the aim of studying the effect of ion identity in aqueous electrolytes on the basal and edge planes of the material. The basal plane capacitance is related to the hydrated ionic size, the capacitance of which increases as the hydrated ion size falls, while the edge plane capacitance is insensitive to ion size, the capacitance of the edge plane is dominated by the population of the quinone species. The electrochemical investigation of the HOPG was further expanded to a highly concentrated potassium fluoride electrolyte, so-called "water-in-salt" system. This "water-in-salt" electrolyte exhibits potential windows of between 2.6 and 3.0 V. Moreover, the applicability of this electrolyte in the coin cell supercapacitor has been demonstrated using graphene and activated carbon as electrode materials, exhibiting excellent cyclic stability, in addition to the wide operating voltage. The charge storage mechanisms (including physisorption, surface faradaic, and electrochemical intercalation) of 2D materials are also described herein, using less or non-hydrated aqueous electrolyte i.e. tetraalkylammonium chlorides. When the ion size is smaller than the interlayer spacing, the capacitance increases by a factor of two compared to that seen for the ionic sizes which are similar to the layer spacing due to the electrochemical intercalation. The larger ions stored the charge through physisorption and surface redox processes. Furthermore, the capacitance of small pores of controlled size has been investigated by fabricating precise Angstrom-scale graphene channels. The effect of ion confinement is described by the distortion of the hydration shell, which occupied more space in the channel leading to lower capacitance.

Date of Award	1 Aug 2020
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Robert Dryfe (Supervisor) & Mark Bissett (Supervisor)