Cation Molecular Structure Affects Mobility and Transport of Electrolytes in Porous Carbons

We examined the electrosorption and ion dynamics of imidazolium-based room temperature ionic liquids (RTILs) having short (3-carbon, C3mim(+)) and long (12-carbon, C12mim(+)) cations, that is, 1-propyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide (C3mimTFSI) and 1-dodecyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide (C12mimTFSI), confined in ordered mesoporous carbon (OMC) and analyzed the influence of the cation alkyl chain length on the ion dynamics and the capacitive behavior using electrochemical measurements together with quasi-elastic neutron scattering (QENS) observations and classical density functional theory (cDFT) computations. Electrochemical tests highlighted the significant influence of specific applied potentials on accumulated charge storage densities and on the limits of saturation of larger electrolytes in the pores. Computational analyses corroborated these findings and predicted a 16% increase in the capacitance of the smaller-cation electrolyte under high applied potentials. However, QENS experiments revealed a behavior of decoupling of alkyl chain dynamics from the ring in electrolytes with larger ions. cDFT calculations identified density spikes for C12mim(+) away from the pore walls to further corroborate this unique behavior. Our insights into chain length-dependent dynamics and electrosorption in complex electrolyte-electrode systems deepen fundamental understanding of confined RTIL electrolyte behavior in the porous carbon electrodes. (c) 2019 The Electrochemical Society.