NMR and Theoretical Study of In-Pore Diffusivity of Ionic Liquid–Solvent Mixtures

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Cui, Jinlei
Lin, Xiaobo
Zhao, Wei
Cummings, Peter T.
Pruski, Marek
Kobayashi, Takeshi
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Iowa State University Digital Repository, Ames IA (United States)
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Ames National Laboratory
Despite having a lower energy density than common batteries, electric double-layer capacitors (EDLCs) offer several advantages for high-power applications, including high power density, quick charge and discharge time, and long cycle life. Room-temperature ionic liquids (RTILs) have been intensely studied as promising electrolytes for applications in ELDCs because of their wide potential window, low volatility, as well as thermal and chemical stability. The main deficiency of neat RTILs in such applications is the sluggish diffusivity, which restricts the EDLCs’ power density. To alleviate the slow diffusivity, RTILs can be used in a mixture with organic solvents. In this study, we applied two-dimensional exchange nuclear magnetic resonance spectroscopy (2D EXSY NMR) and molecular dynamics (MD) simulations to investigate the diffusivity of anions of an RTIL, namely, 1-butyl-3-methyl-imidazolium bis(trifluoromethylsulfonyl)imide (BMIM+–TFSI–), dissolved in five different organic solvents, in the micropores of activated carbon. We determined that the relative concentrations of ions in solutions in the micropores were higher than those in the bulk solutions and were also solvent-dependent. The ion diffusivities in the pores were found to be almost 2 orders of magnitude slower than in the bulk solutions, with methanol showing the largest relative disparity. These results suggested that the interactions of solvents with the activated carbon are critical not only to the power density of EDLCs but also to the energy density. The comparisons of ion diffusivities between the experiments and the MD simulations suggest the need to consider also the surface functionalities of activated carbon for the simulation of ion diffusion in the micropores of activated carbon.
This document is the unedited Author’s version of a Submitted Work that was subsequently accepted for publication as Cui, Jinlei, Xiaobo Lin, Wei Zhao, Peter T. Cummings, Marek Pruski, and Takeshi Kobayashi. "NMR and Theoretical Study of In-Pore Diffusivity of Ionic Liquid–Solvent Mixtures." The Journal of Physical Chemistry B 126, no. 26 (2022): 4889-4898. Copyright 2022 American Chemical Society after peer review. To access the final edited and published work see DOI: 10.1021/acs.jpcb.2c00860. Posted with permission. DOE Contract Number(s): AC02-07CH11358.
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