Capacitance of Basal Plane and Edge-Oriented Highly Ordered Pyrolytic Graphite: Specific Ion Effects

Abstract

Carbon materials are ubiquitous in energy storage; however, many of the fundamental electrochemical properties of carbons are still not fully understood. In this work, we studied the capacitance of highly ordered pyrolytic graphite (HOPG), with the aim of investigating specific ion effects seen in the capacitance of the basal plane and edge-oriented planes of the material. A series of alkali metal cations, from Li+, Na+, K+, Rb+, and Cs+ with chloride as the counterion, were used at a fixed electrolyte concentration. The basal plane capacitance at a fixed potential relative to the potential of zero charge was found to increase from 4.72 to 9.39 μF cm–2 proceeding down Group 1. In contrast, the edge-orientated samples display capacitance ca. 100 times higher than those of the basal plane, attributed to pseudocapacitance processes associated with the presence of oxygen groups and largely independent of cation identity. This work improves understanding of capacitive properties of carbonaceous materials, leading to their continued development for use in energy storage.

Specific ion effects at graphitic interfaces

“…However, it is not consistent with experimental measurements of the graphene basal plane reported in ref. 11 , which show that the capacitance should increase with the ionic radius. This indicates that classical descriptions of the ions based on the point charge approximation—while likely adequate for bulk electrolyte solutions—are insufficient for properly capturing the experimentally measured cation effects at interfaces.…”

“…We begin by discussing the behavior of aqueous LiCl, NaCl, KCl, and CsCl solutions near an ideal graphene electrode, which represents a well-defined model system that can be reasonably compared with the recent experimental investigation on HOPG 11 . We first compute the capacitance of these systems using a purely classical description for the solution environment within the RISM approach (see “Methods”), with the electrode explicitly described by DFT.…”

“…However, electrochemical impedance spectroscopy measurements in ref. 11 show that distinctive difference in the capacitance between the cations remains at a low frequency; for instance, the measured capacitance in KCl and LiCl solutions were 8 cm and 5 cm, respectively, resulting in a / ratio as large as 1.6. This indicates that ion kinetics may not be solely responsible for the difference in the capacitance between different aqueous solutions, motivating further development in theoretical descriptions of the electrical response of solutions at graphitic interfaces.…”

“…In a recent study, highly ordered pyrolytic graphite (HOPG) was employed as a model system for investigating specific ion effects on graphene capacitance 11 . By considering solutions with a series of alkali-metal cations, from Li, Na, K, Rb to Cs, with Cl counterions, the authors showed that the basal-plane capacitance of the system increases with the ionic radius of the cations, with CsCl yielding the highest capacitance among the solutions.…”

Controlling the flake size of bifunctional 2D WSe2 nanosheets as flexible binders and supercapacitor materials

“…13 However, graphene stores the charge through a build-up in the electrical double layer, which can limit the energy density of the energy storage. 4,14,15 There are many 2D materials apart from graphene that can be used, such as transition metal dichalcogenides (TMDs, also called MX 2 ), 16 which provide outstanding electrochemical properties in terms of energy storage. 17 This is due to the charge storage mechanism of these materials based on both faradaic and non-faradaic processes.…”