Molecular modeling of ionic liquids: structure, dynamics and electrochemical performance in supercapacitors
Room temperature ionic liquids (RTILs) consisting solely of cations and anions are attracting rapidly increasing research interest from multidisciplinary areas because of their outstanding physiochemical properties including low vapor pressure, high stability, non-combustibility etc. which facilitate their applications as solvents, catalysts, lubricants and electrolytes. However, there are many unresolved questions on the spatial heterogeneity of bulk RTILs, interfacial dynamics and capacitive performance of RTILs electrolytes in supercapacitors. Therefore, this project concentrates on the exploration of the molecular insight into the above-mentioned phenomena of RTILs in atomic level by molecular dynamics (MD) simulation. In this dissertation, it is revealed that: (1) the spatial heterogeneity of RTILs is enhanced with the increase of the alkyl chain length in cations regardless of dicationic or monocationic ionic liquids (DILs or MILs) and DILs are found to exhibit less spatial heterogeneity due to the distinct organization of DILs in contrast to MILs; (2) weak temperature dependence in the diffusion coefficients of RTILs near silica mesopore surface is revealed in contrast to temperature-dependent diffusion coefficient of RTILs at carbon surface due to the distinctive interfacial structure and silica/carbon-RTILs interaction; (3) the performance of RTILs electrolytes including RTILs binary mixture, DILs, RTILs/organic solvents in carbon electrode-based supercapacitors is investigated: the positive temperature dependence of differential capacitance for binary mixture of RTILs-based supercapacitors is verified to correlate with the thickness of electric double layers (EDLs), which increases with the decrease of EDL thickness; DIL electrolytes exhibit different EDL structures in comparison with MILs and differential capacitance-electric potential (C-V) curves are presented for different anions; organic solvents are found to enhance the conductivity of DILs electrolytes as well as the capacitance of DILs-based supercapacitors, which is in agreement with experimental measurement. Finally, this work help lay the foundation for understanding the behaviors of RTILs in either bulk or under confinement, especially for novel types of ionic liquids, such as pyrrolidinium-based RTILs and DILs, which may inspire further research in these systems and facilitate the rational design and application of RTIL electrolytes in energy storage devices.