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Ionic Liquids as Electrolytes for Electrochemical Double-Layer Capacitors: Structures that Optimize Specific Energy.
Maral P S Mousavi1, Benjamin E Wilson1, Sadra Kashefolgheta2
1Department of Chemistry, University of Minnesota , 207 Pleasant Street S.E., Minneapolis, Minnesota 55455, United States.
ACS Applied Materials & Interfaces
|January 16, 2016
Summary
Ionic liquids enhance electrochemical double-layer capacitors (EDLCs) by optimizing electrolyte properties. The study identifies specific ionic liquids that maximize EDLC specific energy through a balance of conductivity and electrochemical stability.
Area of Science:
- Materials Science
- Electrochemistry
- Chemical Engineering
Background:
- Electrochemical double-layer capacitors (EDLCs) are energy storage devices whose performance is dictated by double-layer capacitance and operating potential.
- The operating potential is constrained by the electrolyte's electrochemical window, the voltage range before solvent or electrolyte decomposition.
- Ionic liquids (ILs) are promising electrolytes for EDLCs due to their inherently wide electrochemical windows.
Purpose of the Study:
- To systematically investigate how the physical properties of ionic liquid electrolytes impact the electrochemical stability and performance of EDLCs.
- To correlate ionic liquid structure with electrochemical double-layer capacitance, operating potential, and specific energy in EDLCs.
- To identify optimal ionic liquid electrolytes for maximizing EDLC specific energy.
Main Methods:
- Utilized a mesoporous carbon electrode with uniform, highly interconnected mesopores (3DOm carbon) for EDLC fabrication.
- Investigated a range of ionic liquids with diverse anions (tetrafluoroborate, trifluoromethanesulfonate, trifluoromethanesulfonimide) and cations (imidazolium, ammonium, pyridinium, piperidinium, pyrrolidinium).
- Analyzed the influence of cation size, alkyl chain length, electrolyte viscosity, conductivity, and electrochemical stability on EDLC performance.
Main Results:
- Cation size significantly affects electrolyte viscosity, conductivity, and EDLC capacitance.
- Imidazolium- and pyridinium-based ILs yielded the highest cell capacitance, while ammonium-based ILs provided wider potential windows.
- Increasing alkyl chain length in specific imidazolium ILs did not expand the potential window. The best performing IL was 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide.
Conclusions:
- Specific energy in EDLCs is maximized by ionic liquids with moderate electrochemical stability, small ionic volumes, and low viscosity for high conductivity.
- Ionic liquid electrolytes combining wide potential windows and high capacitance are crucial for advanced EDLCs.
- The study provides a roadmap for selecting optimal ionic liquids to enhance EDLC energy storage capabilities.
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