Journal
JOURNAL OF PHYSICAL CHEMISTRY C
Volume 122, Issue 19, Pages 10476-10481Publisher
AMER CHEMICAL SOC
DOI: 10.1021/acs.jpcc.8b02521
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Funding
- Fluid Interface Reactions, Structures and Transport (FIRST) Center, an Energy Frontier Research Center - U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences
- Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy
- U.S. DOE
- Office of Science of the U.S. Department of Energy [DE-AC02-05CH11231]
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Well-tailored mixtures of distinct ionic liquids can act as optimal electrolytes that extend the operating electrochemical window and improve charge storage density in supercapacitors. Here, we explore two room-temperature ionic liquids, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EmimTFSI) and 1-ethyl-3-methylimidazolium tetrafluoroborate (EmimBF4). We study their electric double-layer behavior in the neat state and as binary mixtures on the external surfaces of onion-like carbon electrodes using quasielastic neutron scattering (QENS) and classical density functional theory techniques. Computational results reveal that a mixture with 4:1 EmimTFSI/EmimBF(4) volume ratio displaces the larger [TESI-] anions with smaller [BF4-] ions, leading to an excess adsorption of [Emim(+)] cations near the electrode surface. These findings are corroborated by the manifestation of nonuniform ion diffusivity change, complementing the description of structural modifications with changing composition, from QENS measurements. Molecular-level understanding of ion packing near electrodes provides insight for design of ionic liquid formulations that enhance the performance of electrochemical energy storage devices.
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