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.
| Original language | English |
|---|---|
| Pages (from-to) | 415-427 |
| Number of pages | 13 |
| Journal | Carbon |
| Volume | 78 |
| DOIs | |
| State | Published - Nov 2014 |
Funding
This work was supported as part of the nm-confined RTIL compound. The authors also acknowledge T. Kozielewski and M. Ohl of JCNS Outstation at SNS for their assistance in setting up the NSE experiments on BL-15 at the SNS, as well as C. Do of ORNL for assistance during beam time on BL-6B at the SNS. Fluid Interface Reactions, Structures and Transport (FIRST) Center , an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences . The SNS NSE portion and bulk liquid diffraction portion of this research conducted at Oak Ridge National Laboratory’s Spallation Neutron Source was sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy . The NIST NSE portion of this work utilized facilities supported in part by the National Science Foundation under Agreement No. DMR-0944772 . G.F. thanks the Palmetto Cluster at Clemson University for providing computational resources. The authors thank the reviewers for their insightful comments that helped improve the clarity and strengthen certain points in this work. The authors would like to acknowledge M. Monkenbusch of the Jülich Centre for Neutron Science JCNS for his helpful discussions during the SNS NSE experiment, review of the manuscript, and helpful comments. The authors acknowledge C. Liao, formerly of Oak Ridge National Laboratory (ORNL) for performing the anion exchange in the deuterated RTIL, as well as X. Wang, formerly of ORNL, for the synthesis of the 5.8
| Funders | Funder number |
|---|---|
| Office of Basic Energy Sciences | |
| Scientific User Facilities Division | |
| National Science Foundation | |
| U.S. Department of Energy | |
| National Institute of Standards and Technology | |
| Office of Science |