Theoretical and experimental studies of imidazolium based ionic liquids for supercapacitors application
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Date
2020-03
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Abstract
The theoretical study of neutral ion pairs of 1-ethyl-3-methylimidazolium tetrafluoroborate
([EMIM][BF4]) and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide
([EMIM][NTf2]) has been performed. The formation of the neutral ion pairs was considered
through interionic reactions involving the [EMIM]+ cation and two anions, [BF4]
– and
[NTf2]
–
. Computations were performed by the DFT/6-311++G(d,p) method with B3LYP5
and CAM–B3LYP functionals. The enthalpies of the interionic reactions ΔrH°(0) have been
determined theoretically for both ion pairs. In addition, following the available experimental
data for [EMIM][NTf2], the enthalpy of the reaction based on experiment was obtained. The
recommended values of ΔrH°(0) are as follows: –359 ± 15 kJ mol–1 for [EMIM][BF4] and –
323 ± 9 kJ mol–1 for [EMIM][NTf2]. The thermodynamic stability of the ion pairs was
interpreted through analysis of the frontier molecular orbitals as well as simple ionic model.
Experimental test on influence of imidazolium-based ionic liquids in electrochemical system
was investigated, with the potential window of 2.8 V for both ionic liquids. The specific
capacitances were found to be 4.1 F g–1 and 4.3 F g–1 for [EMIM][BF4] and [EMIM][NTf2]
respectively, at the lower scan rate of 5 mV s–1
. The maximum current densities reached were
87 mA g–1 and 88 mA g–1 for [EMIM][BF4] and [EMIM][NTf2], respectively.
Electrochemical measurements by cyclic voltammetry and electrochemical impedance
spectroscopy have been conducted on three electrodes system with the IILs under study. A
supercapacitor prototype has been fabricated; conventional charge-discharge tests were done.
Keywords: Ionic liquids, density functional theory, thermodynamic functions, supercapacitor,
cyclic voltammetry, electrochemical impedance spectroscopy
Sustainable Development Goals
A Dissertation Submitted in Partial Fulfillment of the Requirements for the Degree of
Master’s in Materials Science and Engineering of the Nelson Mandela African Institution
of Science and Technology
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Research Subject Categories::NATURAL SCIENCES