High-voltage operation of binder-free CNT supercapacitors using ionic liquid electrolytes
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High-voltage ($4.0 V) operation of supercapacitor devices was demonstrated using carbon nanotubes as active electrode materials combined with room temperature ionic liquids as electrolyte. Pouch cells were assembled with four different ionic liquids, 1-butyl-3-methylimidazolium tetrafluoroborate (BMIM-BF4), diethyl-N-methyl-N-(2-methoxyethyl) ammonium bis(trifluoromethanesulfonyl)imide (DEME-TFSI), diethyl-N-methyl-N-(2-methoxyethyl)ammonium tetrafluoroborate (DEME-BF4), and 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (Pyr14-TFSI). Cyclic voltammetry showed the maximum operational voltage to be 4.5 V for DEME-TFSI and 4.7 V for DEME-BF4. Compared to electric double layer capacitor (EDLC) cells using propylene carbonate electrolyte at 2.7 V, capacitance increased by 20% using BMIM-BF4 at 4.0 V, DEME-TFSI at 4.5 V, DEME-BF4 at 4.7 V, and Pyr14-TFSI at 4.3 V, with tripling of energy density and comparable power density using Pyr14-TFSI-based EDLCs. Long-term cyclability using BMIM-BF4 ionic liquid electrolyte operating at 4.0 V showed retention of .80% of initial capacitance after 65,000 continuous cycles without doubling of initial cell equivalent series resistance.
I. INTRODUCTION
Supercapacitors, also known as electric double layer capacitors (EDLCs) or ultracapacitors, have been widely used in applications such as memory power backup, uninterrupted power supply, wind turbines pitch system, stop-start power supply for automobiles, etc. The two most recognized benefits of supercapacitors are long-cycle life and high power performance, which stem from its physical rather than chemical charge storage/ release mechanism at the electrode/electrolyte interface. Porous carbon materials, such as high surface areaactivated carbon (AC) materials, are normally used as the electrode material, and organic electrolytes, using quaternary ammonium salts such as tetraethylammonium tetrafluoroborate (TEA-BF4) or triethylmethylammonium tetrafluoroborate (TEMA-BF4), dissolved in acetonitrile (ACN) or propylene carbonate (PC), are commonly applied as the electrolyte material. Upon charging, two electric double layers are formed at the interface of positive and negative electrodes with organic electrolyte and then dissipated upon discharging. The process is considered to be a physical process, and therefore, the resulting supercapacitors can be exceptionally durable exhibiting extraordinary cycle life, while delivering remarkably high power.
Contributing Editor: Teng Zhai a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2017.455
The power and energy performance of supercapacitor devices scale with the square of operational voltage. In this context, one of the chief limitations with AC-based supercapacitors using organic electrolytes is the operational voltage maximum around 2.7 V. Several studies have revealed that it may not be practically possible to realize AC-based EDLCs with voltages higher than 3.0 V when conventional solvents (ACN and PC) are used.1–3 Therefore, to boost th
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