A molecular dynamics simulation study of the electric double layer and capacitance of [BMIM][PF6] and [BMIM][BF4] room temperature ionic liquids near charged surfaces

Abstract

A molecular dynamics simulation study of electric double layer (EDL) structure and differential capacitance (DC) of two 1-butyl-3-methylimidazolium (BMIM)-based room temperature ionic liquids, i.e. [BMIM][BF₄] and [BMIM][PF₆], has been conducted on basal and prismatic graphite as well as (001) and (011) gold electrode surfaces. The influence of the electrode surface and electrolyte structure on electrode capacitance and EDL structure are discussed. For a given electrode surface both the [BMIM][BF₄] and [BMIM][PF₆] electrolytes generate very similar DC and EDL structures. The DC for these ionic liquids in contact with atomically flat surfaces (i.e. basal graphite and (001)Au) shows very small variations within the electrolyte chemical stability potential window and fluctuates around an average value of ∼5 μF cm⁻². On atomically more corrugated surfaces (i.e., Au(011) and prismatic graphite) the DC shows more variation with electrode potential and depends on the correspondence between dimensions of the surface roughness and electrolyte ion sizes. The trends and dependencies obtained for DC are used to discriminate between mutually contradictory experimental data reported in the literature for related systems.