Journal
ACS ENERGY LETTERS
Volume 1, Issue 1, Pages 21-26Publisher
AMER CHEMICAL SOC
DOI: 10.1021/acsenergylett.6b00010
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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
- National Natural Science Foundation of China [91334203, 21376074]
- 111 Project of China [B08021]
- Chinese Scholarship Council
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Formulating room-temperature ionic liquid (RTIL) mixed electrolytes was recently proposed as an effective and convenient strategy to increase the capacitive performance of electrochemical capacitors. Here we investigate the electrical double-layer (EDL) structure and the capacitance of two RTILs, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMI-TFSI) and 1-ethyl-3-methylimidazolium tetrafluoroborate (EMI-BF4), and their mixtures with onion-like carbon electrodes using experiment and classical density functional theory. The principal difference between these ionic liquids is the smaller diameter of the BF4- anion relative to the TFSI- anion and the EMI+ cation. A volcano-shaped trend is identified for the capacitance versus the composition of the RTIL mixture. The mixture effect, which makes more counterions pack on and more co-ions leave from the electrode surface, leads to an increase of the counterion density within the EDL and thus a larger capacitance. These theoretical predictions are in good agreement with our experimental observations and offer guidance for designing RTIL mixtures for EDL supercapacitors.
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