Abstract
The molecular-scale dynamic properties of the room temperature ionic liquid (RTIL)
1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, or [C4mim+
][Tf2N
],
confined in hierarchical microporous–mesoporous carbon, were investigated using neutron
spin echo (NSE) and molecular dynamics (MD) simulations. Both NSE and MD reveal pronounced slowing of the overall collective dynamics, including the presence of an immobilized fraction of RTIL at the pore wall, on the time scales of these approaches. A fraction of
the dynamics, corresponding to RTIL inside 0.75 nm micropores located along the mesopore surfaces, are faster than those of RTIL in direct contact with the walls of 5.8 nm and
7.8 nm cylindrical mesopores. This behavior is ascribed to the near-surface confined-ion
density fluctuations resulting from the ion–ion and ion–wall interactions between the
micropores and mesopores as well as their confinement geometries. Strong micropore–
RTIL interactions result in less-coordinated RTIL within the micropores than in the bulk
fluid. Increasing temperature from 296 K to 353 K reduces the immobilized RTIL fraction
and results in nearly an order of magnitude increase in the RTIL dynamics. The observed
interfacial phenomena underscore the importance of tailoring the surface properties of porous carbons to achieve desirable electrolyte dynamic behavior, since this impacts the
performance in applications such as electrical energy storage devices.
