A Computational Study of the Interfacial Structure and Capacitance of Graphene in BMIMPF6 Ionic Liquid
E Paek and AJ Pak and GS Hwang, JOURNAL OF THE ELECTROCHEMICAL SOCIETY, 160, A1-A10 (2013).
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 BMIMPF6 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 BMIMPF6 near the graphene electrode with varying charge densities are investigated to provide a molecular description of EDLs, including BMIM/PF6 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. (C) 2012 The Electrochemical Society. DOI: 10.1149/2.019301jes All rights reserved.
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