Molecular Simulations of Ionic Liquids at Charged Graphite Interfaces

PA Bonnaud and H Shiba, JOURNAL OF PHYSICAL CHEMISTRY C, 127, 22917-22933 (2023).

DOI: 10.1021/acs.jpcc.3c06257

Enhancing the energy density of electric double layer capacitors (EDLCs) stands as a challenge that must be overcome to render them competitive for energy storage. We employed molecular dynamics with the constant- potential technique for simulating molecular models of EDLC, where the electrolyte is an ionic liquid (IL) made of 1-butyl-3-methylimidazolium cations and bis-(trifluoromethylsulfonyl) imide anions confined between graphite electrodes. We focus on how the confinement affects the electrical properties of EDLCs. At a gap size of d approximate to 1.4 nm, total capacitances exhibit a peak agreeing with previous molecular- simulation results from the literature. Beyond d = 2 nm, total capacitances are barely influenced by the applied potential and geometrical confinement. The energy storage exhibits a quadratic behavior with respect to the applied potential and values agree fairly with the experiments. The fluid contribution to the differential capacitance mostly shows broad bell shapes in mild confinements (4-6 nm). The higher the temperature, the higher the differential capacitance, and the narrower the bell shape. For gaps between electrodes in the range 1.5-10 nm, differential capacitances show a trend similar to a Langmuir-like sorption model, which highlights the strong relationship between the formation of the EDL and sorption mechanisms. Whereas EDLs are severely disrupted in strong confinements, they are fully formed and span over roughly 3-4 nm from substrate interfaces for d >= 10 nm. This approach enables the understanding of molecular-scale mechanisms in the vicinity of electrode interfaces when the potential is applied. It ultimately contributes to the optimization of the energy storage of EDLCs.

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