A Computational Study of the Interfacial Structure and Capacitance of Graphene in [BMIM][PF₆] Ionic Liquid

Abstract

A combination of graphene-like electrodes and ionic liquid (IL) electrolytes has emerged as a viable and attractive choice for electrochemical double layer (EDL) capacitors. Based on combined classical molecular dynamics and density functional theory calculations, we present the interfacial capacitance between planar graphene and [BMIM][PF 6 ] IL, with particular attention to the relative contributions of the electric double layer capacitance at the graphene/IL interface and the quantum capacitance of graphene. The microstructure of [BMIM][PF 6 ] near the graphene electrode with varying charge densities are investigated to provide a molecular description of EDLs, including BMIM/PF 6 packing and orientation, cation-anion segregation, and electrode charge screening. Although the IL interfacial structures exhibit an alternative cation/anion layering extending a few nanometers, the calculated potential profiles provide evidence of one-ion thick compact EDL formation. The capacitance-potential curve of the EDL is convex-or bell-shaped, whereas the quantum capacitance of graphene is found to have concave-or U-shaped characteristics with a minimum of nearly zero. Consequently, we find that the total interfacial capacitance exhibits a U-shaped trend, consistent with existing experimental observations at a typical carbon/IL interface. Our work highlights the importance of the quantum capacitance in the overall performance of graphene-based EDL capacitors.

Capacitance of carbon-based electrical double-layer capacitors

“…These effects are not ascribable to adsorption or faradic reaction of ions at the graphene surface as verified from the CV test ( Supplementary Fig. 5 36 has shown that MD simulations can partly account for variations of Helmholtz capacitance C H not explained by commonly adopted assumptions. A complete understanding of the effects we observe will likely require the development of new calculation methods that account for the correlated response of carriers in the electrodes to ionic fluctuations.…”

Catalytic reduction of TFSI-containing ionic liquid in the presence of lithium cations

“…It has been proven both theoretically [14,15] and experimentally [16][17][18] that the IL ions are highly structured and form a layered EDL on electrodes [17,18] which can undergo rearrangement under imposed polarization [16,[19][20][21]. When polarized, the biggest changes within the EDL are observed in the first adjacent layer; the strength of the ion-electrode interactions [16], compactness [18,22], and conformational changes of ions [19] vary greatly within this particular layer.…”

Electrochemical double layer near polar reduced graphene oxide electrode: Insights from molecular dynamic study

“…Understanding the interfacial behavior of aqueous/non-aqueous electrolytes in contact with graphene-based electrodes, which is critical for improving the supercapacitor performance, has attracted extensive research efforts, involving theoretical [11][12][13][14], simulation [15][16][17][18][19][20][21][22], and experimental approaches [10,23]. Simulation study, typically molecular dynamic (MD) study is a wellestablished technique, yields a molecular insight into the given model system.…”