Molecular dynamics simulations of ionic liquids confined into MXenes

Two-dimensional (2D) transition metal carbides known as MXenes have been extensively investigated as electrode material of electrochemical devices, including supercapacitors. Here we present a detailed computational study of ionic liquids in contact with Ti3C2F2, both at planar surface and under confinement. The simulations employ an extension of the constant potential model (CPMχ) that can properly deal with materials composed of chemical elements with different electronegativities. The average partial charges of MXene atoms calculated with CPMχ are in excellent agreement with the ones obtained by DFT calculations. The differential capacitance and the charging mechanisms are thoroughly analyzed for two ionic liquids with a common cation, [EMIM][BF4] and [EMIM][NTf2]. By varying the MXene interlayer dimension, the best performance in terms of charge storage is obtained in electrodes with 12.6 Å spacing, with outperformance of [EMIM][BF4]-based cells.