Improved Electrochemical Performance of a ZnFe2O4 Nanoflake-Based Supercapacitor Electrode by Using Thiocyanate-Functionalized Ionic Liquid Electrolytes

Article abstract of DOI:10.1002/ejic.201500870

Thiocyanate-functionalized ionic liquids (ILs) 1-butyl-3-methylimidazolium thiocyanate [BMIM][SCN], 1-ethyl-3-methylimidazolium thiocyanate [EMIM][SCN], and 3-methylimidazolium thiocyanate [MIM][SCN], with varying chain lengths, were synthesized by using metathesis followed by ion exchange under reflux conditions. These ILs were investigated for their application as electrolyte with a ZnFe₂O₄ nanoflake-based supercapacitor electrode; they exhibited significantly better performance than any other previously reported ILs used as electrolytes. [BMIM][SCN] showed higher specific capacitance (781 F g^-1), cycle stability (95 % retention, 3000 cycle), and energy density (156 W h kg^-1) than [EMIM][SCN] (590 F g^-1, 86 % retention, and 78 W h kg^-1) and [MIM][SCN] (250 F g^-1, 80 % retention, and 50 W h kg^-1). The ZnFe₂O₄ nanoflake electrodes were able to deliver an energy density of 156 W h kg^-1 even at an extra high power density of 7.11 kW kg^-1 in [BMIM][SCN] electrolyte, justifying its utility as a potential electrolyte. Furthermore, the effect of how the hydrated ions of the ILs in aqueous media influence the overall performance of the supercapacitor was studied systematically and found to be contrary to the reported trend and use of pure ILs as electrolytes.

Full text of DOI:10.1002/ejic.201500870

FULL PAPER
DOI:10.1002/ejic.201500870
Improved Electrochemical Performance of a ZnFe2O4
Nanoflake-Based Supercapacitor Electrode by Using
Thiocyanate-Functionalized Ionic Liquid Electrolytes
Madagonda M. Vadiyar,[a][‡] Sandip K. Patil,[a][‡] Sagar C. Bhise,[a]
Anil V. Ghule,*[b] Sung-Hwan Han,*[c] and Sanjay S Kolekar*[a]
Keywords: Ionic liquids / Supercapacitors / Nanostructures / Electrolytes / Electrochemistry
Thiocyanate-functionalized ionic liquids (ILs) 1-butyl-3-
methylimidazolium thiocyanate [BMIM][SCN], 1-ethyl-3-
methylimidazolium thiocyanate [EMIM][SCN], and 3-meth-
retention, 3000 cycle), and energy density (156 Whkg–1) than
[EMIM][SCN] (590 Fg–1, 86% retention, and 78 Whkg–1
and [MIM][SCN] (250 Fg–1, 80% retention, and 50 Whkg–1).
)
ylimidazolium thiocyanate [MIM][SCN], with varying chain The ZnFe2O4 nanoflake electrodes were able to deliver an
lengths, were synthesized by using metathesis followed by
ion exchange under reflux conditions. These ILs were inves-
tigated for their application as electrolyte with a ZnFe2O4
nanoflake-based supercapacitor electrode; they exhibited
significantly better performance than any other previously
reported ILs used as electrolytes. [BMIM][SCN] showed
higher specific capacitance (781 Fg–1), cycle stability (95%
energy density of 156 Whkg–1 even at an extra high power
density of 7.11 kWkg–1 in [BMIM][SCN] electrolyte, justify-
ing its utility as a potential electrolyte. Furthermore, the ef-
fect of how the hydrated ions of the ILs in aqueous media
influence the overall performance of the supercapacitor was
studied systematically and found to be contrary to the re-
ported trend and use of pure ILs as electrolytes.
their excellent properties such as higher ionic conductivity,
moderate viscosity, non-flammability, lower toxicity, low
volatility, good thermal stability, and safety.[8,9] Among the
various ILs, functionalized ILs such as 1-butyl-3-methylimi-
dazolium tetrafluoroborate [BMIM][BF4],[10] 1-methyl-3-
octylimidazolium tetrafluoroborate [OMIM][BF4],[11] 1-
butyl-3-methylimidazolium hexafluorophosphate [BMIM]-
[PF6],[12] 1-ethyl-3-methylimidazolium bis(trifluoromethyl-
sulfonyl)imide [EMIM][TFSI],[13] etc. are promising and
have been explored with great interest as these exhibit high
potential windows and specific capacitance values.[14] How-
ever, the utility of some of these ILs is limited by their avail-
ability, high cost, and physical and chemical properties,
which remains to be addressed.[15,16] Amongst ILs, the
imidazolium cation and thiocyanate anion based electro-
lytes and their tailored forms are considered to be promis-
ing electrolytes, which motivated us to explore the use of
thiocyanate-functionalized ILs as electrolytes. It is known
that the performance of supercapacitors with ILs as the
electrolyte depends on the IL functional group, and the se-
lection of the cations and anions by varying the alkyl chain
length.[17,18] Various synthetic strategies have been reported
for tailoring of the chemical structures of the IL cation or
anion with different functional groups.[19] Shaikh et al. re-
ported a simple Brønsted acidic ionic liquid as an electro-
lyte with Ru-doped CuO thin-film electrode, which demon-
strated a potential window of 1.0 V, specific capacitance of
406 Fg–1, and retained this with cycle stability up to 2000
cycles.[20] Maiti et al. also reported using [EMIM][BF4] as
Introduction
Increasing energy demand has forced the development of
efficient energy conversion and storage devices,[1,2] of which
solar cells, fuel cells, batteries, supercapacitors, etc.[3] are
considered to be promising to address these concerns.[4,5]
Tremendous effort is being focused on developing storage
devices, in particular, supercapacitors owing to their high
power density, fast charge–discharge, longer cycle life, and
stability.[6] Despite these efforts, the energy density is still
lower than that of batteries; this is dependent on the poten-
tial window, which is influenced by the electrolyte.[7] Aque-
ous electrolytes usually offer lower potential windows
(Ͻ 1 V), and, thus, organic electrolytes, including ionic li-
quid (ILs), are being explored with great interest owing to
[a] Analytical Chemistry and Material Science Research
Laboratory, Department of Chemistry, Shivaji University,
Kolhapur 416004, Maharashtra, India
E-mail: sskolekar@gmail.com
http://www.unishivaji.ac.in/dptchem/
[b] Green Nanotechnology Laboratory, Department of Chemistry,
Shivaji University,
Kolhapur 416004, Maharashtra, India
E-mail: anighule@gmail.com
http://www.unishivaji.ac.in/dptchem/
[c] Inorganic Nano-Material Laboratory, Department of
Chemistry, Hanyang University,
Seoul, Republic of Korea
E-mail: shhan@hanyang.ac.kr
http://inml.hanyang.ac.kr/
[‡] M. M. V. and S. K. P. contributed equally to this work
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201500870.
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an electrolyte with an α-Fe2O3 electrode, which showed a
specific capacitance of 147 Fg–1 and energy density of
163 Whkg–1.[21] The use of several other ILs as electrolytes
and their supercapacitive performances are summarized in
Table 2.
Results and Discussion
The synthesis (Scheme 1) and structures of 1-butyl-3-
methylimidazolium thiocyanate ([BMIM][SCN]), 1-ethyl-3-
methylimidazolium thiocyanate ([EMIM][SCN]), and 3-
methylimidazolium thiocyanate ([MIM][SCN]) were con-
firmed by using 1H NMR spectra obtained in dimethyl sulf-
oxide (DMSO) with a 300 MHz instrument (see the Sup-
porting Information, Figures S1–S3). In general, the super-
capacitive properties of the electrolyte depend on its physi-
cal properties such as density, viscosity, ionic conductivity,
Figure 1. (a) Representative XRD pattern of the ZnFe2O4 nano-
flake thin film and (b) SEM image of the ZnFe2O4 nanoflake thin
film.
The XRD spectra recorded for the ZnFe2O4 nanoflakes
etc. Thus, the ionic conductivities (mScm–1) of the ILs were [Figure 1(a)] confirmed the crystal structure to be spinel-
measured by using a digital conductometer. The physical cubic. In Figure 1(a), the peaks corresponding to bare
properties and conductivities of the ILs are summarized in stainless steel are marked as SS, and the peaks at 2θ = 30.0°
Table 1.
(220), 35.3° (311), 44.7° (400), 53.3° (422), 56.6° (511), 62.3°
The ZnFe2O4 nanoflake thin-film electrode was prepared (440), and 74.7° (622) could be attributed to the cubic spinel
by using a mechanochemical approach reported pre- phase of ZnFe2O4 (JCPDS 11-0325).[23] The phase forma-
viously.[22] The structural and morphological characteriza- tion of the ZnFe2O4 nanoflake thin film was characterized
tion of the ZnFe2O4 nanoflake thin-film electrode was per- by Raman spectra (Figure S4). The Raman spectra of the
formed by using X-ray diffraction and scanning electron ZnFe2O4 nanoflakes showed peaks at 299, 171, 412, 501,
microscopy (SEM) (Figure 1).
and 613 cm–1, which could be assigned to the five Raman-
Scheme 1. Synthesis of thiocyanate-functionalized ionic liquids.
Table 1. Physical properties of [MIM][SCN], [EMIM][SCN], and [BMIM][SCN] ILs.
Ionic liquid
Molar mass
[gL–1]
Density at 25 °C
[gcm–3]
Viscosity at 25 °C
[mPas]
Ionic conductivity at 25 °C
[mScm–1]
Pure IL
1 m IL solution
[BMIM][SCN]
[EMIM][SCN]
[MIM][SCN]
198
169
141
1.04
1.08
1.11
54
22
16
11
22
24
38.2
34.8
27.9
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active modes (Eg + 3T2g + A1g) of the ZnFe2O4 spinel struc- three-electrode cell system was constructed.[30] Figure 2(a)
ture with the space group Fd3m.[24]
shows the cyclic voltammetry (CV) curves for the ZnFe2O4
Figure 1(b) shows an SEM image of the ZnFe2O4 thin nanoflakes obtained with 1 m [BMIM][SCN], [EMIM]-
films, which reveals a well-adhered and uniformly intercon- [SCN], or [MIM][SCN] electrolytes at a scan rate of
nected porous nanoflake-like[25] surface morphology 2 μm 10 mVs–1. Deviated rectangular shapes with small redox
thick and with an average flake size of approximately peaks were observed within the negative potential range
25 nm.[26] These nanoflakes were grown concurrently under –1.2 to 0.0 V. This is due to a low contact resistance of ion
the influence of mechanical rotation of the substrates, migration, which indicates excellent capacitive features.[31]
which hindered attainment of an equilibrium during deposi- The CV curves were also recorded at varying scan rates
tion and, thus, the growth of the thin films formed porous (10, 20, 40, 60, 80, and 100 mVs–1) for the [BMIM][SCN],
structures. The porous nature was further confirmed by [EMIM][SCN], and [MIM][SCN] electrolytes, as shown in
Brunauer–Emmett–Teller (BET) multipoint surface area Figure 3(a)–(c). However, the shape of the CV curves re-
(123 m2 g–1) and Barrett–Joyner–Halenda (BJH) pore size corded at fast scan rates (80–100 mVs–1) showed slightly
(average pore distribution ca. 1.7 nm) distribution analysis deviating profiles compared with those at slower scan rates
(Figure S5) of the ZnFe2O4 nanoflakes.[27]
that showed ideal capacitive behavior.[32,33] The charge stor-
The elemental composition and chemical oxidation states age mechanism in the ZnFe2O4 nanoflake-based electrode
of individual elements in the ZnFe2O4 thin films were con- system is of mixed type, containing both EDLC (electric
firmed by X-ray photoelectron spectroscopy (XPS) (Fig-
ure S6), which showed the presence of Zn, Fe, and O with
adventitious C, confirming the formation of the ZnFe2O4
phase.[28] Figure S6 shows the survey spectrum of the
ZnFe2O4 thin films, which reveal the presence of Zn, Fe,
and O. The inset in Figure S6 shows core-level high-resolu-
tion spectra of Zn2p, wherein peaks corresponding to bind-
ing energies of 1021.6 and 1044.8 eV were observed and
could be attributed to Zn2p3/2 and Zn2p1/2, respectively. The
oxidation state of Zn was noted to be +2 in the prepared
thin films. The inset in Figure S6 (core-level high-resolution
Fe2p spectra) showed peaks at 711.46 eV and 725.15 eV cor-
responding to Fe2p3/2 and Fe2p1/2, respectively. The observed
results were in good agreement with earlier reports, indicat-
ing the oxidation state of Fe was +3. Furthermore, the inset
Figure 2. (a) Cyclic voltammetry (CV) curves at a 10 mVs–1 scan
in Figure S6 shows the high-resolution O1s spectra with
rate. (b) Galvanostatic charge–discharge (GCD) measurements at
binding energy of 530.24 eV resulting from the lattice oxy-
3 mAcm–2 current density. (c) Plot of current density [mAcm–2]
gen (O2–) in the prepared ZnFe2O4 thin film.[29]
versus specific capacitance [Fg–1]. (d) Electrochemical impedance
The supercapacitor properties of the ZnFe2O4 nano-
(EIS) measurements of ZnFe2O4 nanoflakes in 1 m [BMIM][SCN],
flakes were studied by using electrochemistry, for which a [EMIM][SCN], or [MIM][SCN] electrolyte.
Figure 3. (a–c) Cyclic voltammetry (CV) and (d–f) galvanostatic charge/discharge (GCD) measurement for the ZnFe2O4 nanoflakes at
different scan rates (10–100 mVs–1) and current densities (3–10 mAcm–2) in [BMIM][SCN], [EMIM][SCN], and [MIM][SCN] electrolytes.
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double layer capacitor) (non-faradic ion adsorption/desorp- the polarization in polar solvent (water), leading to stronger
tion on the surface of ZnFe2O4) and pseudocapacitive char- ionic solvation (Scheme 2).
acteristics (faradic ion diffusion into the porous ZnFe2O4).
This is also reflected by the appearance of broad redox
peaks between –0.5 and –0.9 V (Figures 2 and 3). The rec-
tangular shape is somewhat distorted – a feature that is
commonly observed for pseudocapacitors. Irregular shapes
in the CV might be due to the differences in the sizes of the
cations and anions of the ILs.
For further investigation, galvanostatic charge–discharge
(GCD) measurements of the ZnFe2O4 nanoflake thin film
in 1 m [BMIM][SCN], [EMIM][SCN], or [MIM][SCN] elec-
trolyte [Figure 2(b)] were carried out within the potential
window of –1.2 to 0.0 V versus saturated calomel electrode
(SCE) at a current density of 3 mAcm–2; this gave maxi-
mum specific capacitances of 781, 390, and 250 Fg–1,
Scheme 2. (Left) Schematic shows hydrated ionic liquid ions with
different ionic radii interacting with the ZnFe2O4 nanoflake-based
respectively. These observed values are higher than those
electrode. (Right) Contact angle (CA) measurements of water, 1 m
reported previously and are summarized in Table 2. The ca-
pacitance values were derived from the discharge curve and
Equations (1) and (2):[34]
MIM+, 1 m EMIM+, and 1 m BMIM+ hydrated cations in contact
with the ZnFe2O4 nanoflake-based electrode.
Thus, MIM+ cations, which have smaller ionic radii,
demonstrate stronger polarization. This results in attraction
of surrounding water molecules to form hydrated ions with
increased hydrodynamic size (larger radius), which retards
C = (I·Δt)/ΔV
Cs = C/m
(1)
(2)
where I is the constant discharge current [mA], m is the the electrolyte ion transport rate, thereby reducing the inter-
active mass [g] of the working electrode, ΔV is the voltage calation/deintercalation and, subsequently, the supercapaci-
range [V], Δt is the discharge time [s], and Cs is the specific tive properties of the electrode material.[36,37] Conversely,
capacitance [Fg–1].
cations with longer alkyl chains and larger ionic radii, for
The van der Waals ionic radii of MIM+, EMIM+, and example, EMIM+ Ͻ BMIM+, show relatively smaller
BMIM+ have been reported to be 0.28, 0.3, and 0.33 nm, hydrated ions owing to relatively weaker solvation with in-
respectively.[35] Thus, considering the influence of the ionic creasing chain length. This enhances the electrolyte ion
radii of the IL cations on the supercapacitive behavior, the transport rate, that is, faster intercalation/deintercalation,
specific capacitance trend speculated was [BMIM][SCN] Ͻ thereby improving the electrochemical properties. This justi-
[EMIM][SCN] Ͻ [MIM][SCN], where [MIM][SCN] was ex- fies the influence of the hydrated ions on the supercapaci-
pected to show maximum capacitance. However, the results tive behavior, which is further supported by contact angle
observed were reversed, demonstrating the opposite trend, (CA) measurement experiments as shown in Scheme 2
where the specific capacitance values decreased in the order (right). The CA of water in contact with the ZnFe2O4 nano-
[BMIM][SCN] Ͼ [EMIM][SCN] Ͼ [MIM][SCN] with the flake-based electrode was noted to be 152°, showing the
same concentration (1 m) and charge.[36] This led us to hydrophobic nature of the thin film. Conversely, the 1 m
probe the influence of the varying alkyl chains of the imid- solutions
of
[MIM][SCN],
[EMIM][SCN],
and
azolium cations and their hydrated form in electrolytes on [BMIM][SCN] IL electrolytes in contact with the ZnFe2O4
the supercapacitive behavior of the ZnFe2O4 nanoflake thin nanoflake-based electrode showed CAs of 74°, 55°, and 35°,
film. Generally, it is observed that in the case of ions with respectively. Interestingly, hydrated [MIM][SCN] showed a
the same charge, the smaller the ionic radius, the stronger is higher CA (74°) than [EMIM][SCN] (55°) and
Table 2. Comparison of thiocyanate ionic liquid performance with literature reports.
No.
Ionic liquid electrolyte[a]
Electrode material
Scan rate [mVs–1] or current density [mAcm–2]
Specific capacitance [Fg–1]
Year and ref.
1
2
3
4
5
6
7
[EMIM][BF4]
[PYR14][TFSI]
P-TFA
[CMIM][HSO4]
[EMIM][SCN]
[HPMIM][Cl]
[BMIM][SCN]
[EMIM][SCN]
[MIM][SCN]
graphene
activated carbon
RuO2
Ru-doped CuO
MnO2
1 Ag–1
20 mVs–1
10 mVs–1
10 mVs–1
0.1 mAcm–2
10 mVs–1
3 mAcm–2
3 mAcm–2
3 mAcm–2
154
60
83
406
55
60
781
390
250
2010[41]
2007[42]
2006[43]
2011[20]
2009[44]
CuO
2015[45]
ZnFe2O4 nanoflakes
ZnFe2O4 nanoflakes
ZnFe2O4 nanoflakes
present work
present work
present work
[a] PYR: pyrrolidine; TFS: trifluoromethanesulfonylimide; P-TFA: α-picoline/trifluoroacetic acid; CMIM: 3-carboxymethyl-1-methyl-
imidazolium; HPMIM: 3-(1Ј-hydroxypropyl)-1-methylimidazolium.
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[BMIM][SCN] (35°), which supports the fact that it has a ure shows the results obtained from cycling performance
higher solvation capacity, thereby increasing the hydrated tests of the ZnFe2O4 nanoflake thin-film electrode based
ion radius, and a relatively higher hydrophobicity. This fur- supercapacitor with the three different IL electrolytes by
ther justifies the decrease in hydrophobicity with the in- GCD measurements at high current density (10 mAcm–1).
crease in alkyl chain length in the respective ILs.
The capacitance retention was noted to be over 95% even
Furthermore, charge/discharge measurements were also after 3000 cycles in 1 m [BMIM][SCN] electrolyte, which
carried out with different current densities of 3, 5, 8, and was higher than that for 1 m [EMIM][SCN] (retention:
10 mAcm–2, and the calculated specific capacitances are 86%) and [MIM][SCN] (retention: 80%). These results indi-
781, 416, 200, 166 Fg–1 , 390, 208, 191, 156 Fg–1, and 250, cate that the hydrated ionic radius as well as the charge
155, 91, 62 Fg–1 for [BMIM][SCN], [EMIM][SCN], and transfer during the experiment play key roles in determining
[MIM][SCN], respectively [Figure 3(d)–(f)]. The obtained the electrochemical performances of the ZnFe2O4 nano-
results revealed that the specific capacitance decreases with flake-based supercapacitors in thiocyanate-functionalized
the increase in current density [Figure 2(c)].[38] Interestingly, ionic liquids. The cycle stability study [Figure 4(a)] involv-
the specific capacitance of the ZnFe2O4 nanoflake-based ing up to 3000 cycles at a higher current density of
electrode in 1 m [BMIM][SCN] was found to be higher than 10 mAcm–2 suggests good rate capability of the system.
those of the other ILs. This implies that the ZnFe2O4 nano-
Furthermore, the ZnFe2O4 nanoflake-based supercapaci-
flake-based electrode shows good rate capability even at tor electrode with [BMIM][SCN], [EMIM][SCN], or
higher scan rates (100 mVs–1) and higher current densities [MIM][SCN] electrolyte showed a decrease energy density
(10 mAcm–2).
from 156 to 33 Whkg–1, from 78 to 31 Whkg–1, and from
To further justify the results, electrochemical impedance 50 to 12.5 Whkg–1 , respectively, with an increase in power
(EIS) spectra of the ZnFe2O4 nanoflakes in 1 m [BMIM]- density from 2.2 to 7.4 kWkg–1, from 2.3 to 7.5 kWkg–1,
[SCN], [EMIM][SCN], or [MIM][SCN] electrolyte were and from 2.25 to 7.5 kWkg–1, respectively, as shown in the
also measured [Figure 2(d)]. The figure shows a semicir- Ragone plot in Figure 4(b).
cle[39] in the high-frequency region and a sloping line in the
low-frequency region, confirming the supercapacitor behav-
ior.[40] In the high-frequency region, the equivalent series
resistance (ESR, intersection on the real axis at high fre-
Conclusion
The supercapacitive properties of ZnFe2O4 nanoflake-
based electrodes (specific capacitance) in thiocyanate-func-
tionalized ionic liquid electrolytes have been studied in de-
tail to investigate the effect of the alkyl chains of the imid-
quencies, 1.35, 1.20, and 3.55 Ω) and the charge-transfer
resistance (Rct, the diameter of the semicircle, 3.50, 7.15,
and 15.60 Ω) were estimated in 1 m [BMIM][SCN],
[EMIM][SCN], and [MIM][SCN] electrolyte, respectively,
azolium cation forming hydrated ions with varying ionic
with the ZnFe2O4 nanoflake electrode. The observed
radii. The specific capacitance of the ZnFe2O4 nanoflake-
smaller ESR and Rct indicate higher electrical conductivity
and faster charge-transfer capability of the ZnFe2O4 nano-
flake electrode in 1 m [BMIM][SCN] compared with that
observed for the [EMIM][SCN] and [MIM][SCN] electro-
lytes. Furthermore, the ZnFe2O4 nanoflake electrode in 1 m
[BMIM][SCN] presented a higher slope and a shorter line
based electrodes showed an increasing trend from 250 to
390 and 781 Fg–1 for [MIM][SCN], [EMIM][SCN], and
[BMIM][SCN] IL electrolytes, which form the correspond-
ing hydrated ions with decreasing radii under similar condi-
tions. The [BMIM][SCN] (1 m) IL electrolyte showed the
highest capacitance retention (95%, 3000 cycles). The
in the low-frequency region, suggesting better supercapaci-
hydrated ionic radii as well as the charge transfer during
tive properties.
the experiment play key roles in determining the electro-
Good cycle stability and capacitance retention is consid-
chemical performances of ZnFe2O4 nanoflake-based super-
ered important for practical applications of supercapaci-
capacitors in thiocyanate-functionalized ionic liquids. Thus,
tors; thus, experiments to investigate cycle stability and ca-
thiocyanate-functionalized ILs as electrolytes in superca-
pacitance retention were performed [Figure 4(a)]. The fig-
pacitors are reported for the first time, which opens new
avenues for potential electrolytes for energy-storage devices.
Experimental Section
Chemicals: The chemicals and reagents used in this work were of
AR grade. 1-Methylimidazole, chlorobutane, bromoethane, potas-
sium thiocyanate, hydrochloric acid, acetone, and dichloromethane
were used for the synthesis of ionic liquids. Whereas zinc chloride
(ZnCl2), ferrous chloride tetrahydrate (FeCl2·4H2O), monoethanol-
Figure 4. (a) Cycle stability of the ZnFe2O4 nanoflakes in 1 m
[BMIM][SCN], [EMIM][SCN], or [MIM][SCN] electrolyte. (b) Ra-
gone plot showing energy density (Whkg–1) versus power density
(kWkg–1) of the ZnFe2O4 nanoflakes in 1 m [BMIM][SCN],
[EMIM][SCN], or [MIM][SCN] electrolyte.
amine (MEA), and ammonia (Merck chemicals) were used for the
thin film deposition.
Synthesis of the Ionic Liquids: The typical procedure for the synthe-
sis of ionic liquid involved two steps. In the first step, 1-methylimid-
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azole (100 mmol) and 1-chlorobutane (150 mmol) were mixed in a
round-bottomed flask with acetone (25 mL) as solvent. The mix-
ture was heated at reflux at 70 °C for 48 h with constant stirring.
Two layers formed, and the upper solvent layer was decanted. The
obtained ionic liquid was purified by washing with ethyl acetate
(3ϫ 10 mL) followed by n-hexane and dried in vacuo. A pale yel-
low, slightly viscous ionic liquid ([BMIM][Cl]) was obtained. In
the second step, a stoichiometric amount of potassium thiocyanate
(KSCN) was added to the freshly prepared [BMIM][Cl] in the pres-
ence of anhydrous dichloromethane. The mixture was stirred vigor-
ously at room temperature for 24 h. The mixture was then filtered
to remove potassium chloride. The solvent was evaporated to ob-
tain a dark yellow, slightly viscous ionic liquid ([BMIM][SCN]),
which was dried in vacuo. A similar procedure was used for the
synthesis of [EMIM][SCN] and [MIM][SCN] ionic liquids as
shown in Scheme 1 except for the addition of bromoethane and
hydrochloric acid instead of chlorobutane.
3 to 10 mAcm–2), and electrochemical impedance spectroscopy
(EIS) in the frequency range from 1 Hz to 100 kHz were performed
by using a CHI 608E electrochemical analyzer.
Construction of a Supercapacitor Cell: The electrochemical cell with
the three-electrode system was composed of graphite (G) as a cur-
rent collector, a saturated calomel electrode (SCE) as the reference
electrode, and the prepared ZnFe2O4 thin film on a stainless steel
(SS) substrate as the working electrode. IL 1 m [BMIM][SCN],
[EMIM][SCN], [MIM][SCN] solutions were used as the electrolyte
as shown in Equations (3)–(5).
SS/ZnFe2O4/1 m [BMIM][SCN]/SCE/G
SS/ZnFe2O4/1 m [EMIM][SCN]/SCE/G
SS/ZnFe2O4/1 m [MIM][SCN]/SCE/G
(3)
(4)
(5)
Synthesis of ZnFe2O4 Nanoflake Thin Films: The ZnFe2O4 nano-
flake thin films were prepared by heating a mixture of aqueous
alkaline solutions of ZnCl2 (0.1 m, 25 mL) and FeCl2·4H2O (0.2 m,
25 mL) in a water bath maintained at 55 °C. The mixture was first
stirred for 15 min to ensure complete dissolution, resulting in a
clear solution. This was followed by addition of monoethanolamine
(MEA, 3 mL) as a complexing agent and dropwise addition of am-
monia to adjust the pH to 10 (Ϯ0.5), forming a blue solution. Dur-
ing the deposition process, the stainless steel substrate was im-
mersed in the solution bath and rotated by using a gear motor with
speed controller during the deposition of the thin film [R-CBD
(rotational chemical bath deposition)]. The heterogeneous reactions
initiate on the substrate through the adsorption of atoms when the
solution reaches super-saturation. The deposition was carried out
for 3 h to ensure completion of the bath reactions. An increase
in thickness of the ZnFe2O4 thin films was noted with increasing
deposition time. The ZnFe2O4 thin film deposited substrates were
removed from the bath and washed thoroughly with doubly dis-
tilled water and subsequently with ethanol. The thin films were
dried in an oven at 60 °C under air for 5 h and then annealed at
550 °C to ensure decomposition of any organic matter and trans-
formation of the hydroxide phase into a crystalline phase.
Acknowledgments
M. M. V. and S. K. P. are thankful to the University Grants Com-
mission (UGC), the University Grants Commission – Special As-
sistance Programme (UGC-SAP), the Department of Science and
Technology – Funds for Improvement of Science and Technology
(DST-FIST), and the Promotion of University Research and Scien-
tific Excellence (PURSE), New Delhi, for financial support under
the UGC-BSR Meritorious Students fellowship and instrument fa-
cilities at the Department of Chemistry, Shivaji University, Kol-
hapur.
[1] B. Dunn, H. Kamath, J. M. Tarascon, Science 2011, 334, 928–
935.
[2] A. Pramanik, S. Maiti, S. Mahanty, Dalton Trans. 2015, 44,
14604–14612.
[3] L. Li, Z. Wu, S. Yuan, X.-B. Zhang, Energy Environ. Sci. 2014,
7, 2101–2122.
[4] H. Sun, Y. Jiang, L. Qiu, X. You, J. Yang, X. Fu, P. Chen, G.
Guan, Z. Yang, X. Sun, H. Peng, J. Mater. Chem. A 2015, 3,
14977–14984.
[5] R. Indhrajothi, I. Prakash, M. Venkateswarlu, N. Satyanaray-
ana, RSC Adv. 2014, 4, 44089–44099.
Material Characterizations: The synthesized thiocyanate-function-
alized ionic liquid samples were characterized by proton nuclear
magnetic resonance (1H NMR) spectroscopy. The ZnFe2O4 thin
film samples were characterized by determining its structure, com-
position, thermal stability, morphology, and supercapacitive prop-
erties. The crystalline quality and phase identification were charac-
terized by XRD (Bruker, D2-Phaser X-ray diffractometer) with Cu-
Kα (λ = 1.5406 Å) radiation in the range 20–80°. Raman spectra
were obtained by using a Horiba Jobin Yvon with Lab RAM HR
system at room temperature with a 532 nm solid-state laser as the
excitation source. The surface morphology of the ZnFe2O4 thin
film was characterized by scanning electron microscopy (SEM)
using a JSM-6360 JEOL SEM and Hitachi S-4700. BET surface
area analysis and pore size analysis were performed by using a
Quantachrome NOVA1000e, USA. X-ray photoelectron spec-
troscopy (XPS-PHI 5300 PHI USA) was used to study the chemical
composition and oxidation state.
[6] B. Guan, D. Guo, L. Hu, G. Zhang, T. Fu, W. Ren, J. Li, Q.
Li, J. Mater. Chem. A 2014, 2, 16116–16123.
[7] M. S. Kolathodi, M. Palei, T. S. Natarajan, J. Mater. Chem. A
2015, 3, 7513–7522.
[8] D. R. MacFarlane, N. Tachikawa, M. Forsyth, J. M. Pringle,
P. C. Howlett, G. D. Elliott, J. H. Davis, M. Watanabe, P. Si-
mon, C. A. Angell, Energy Environ. Sci. 2014, 7, 232–250.
[9] S. A. Pawar, D. S. Patil, S. K. Patil, D. V. Awale, R. S. Devan,
Y.-R. Ma, S. S. Kolekar, J.-H. Kim, P. S. Patil, Electrochim.
Acta 2014, 148, 310–316.
[10] S. K. Ujjain, V. Sahu, R. K. Sharma, G. Singh, Electrochim.
Acta 2015, 157, 245–251.
[11] M. G. Montalbán, C. L. Bolívar, F. G. Díaz Baños, G. Víllora,
J. Chem. Eng. Data 2015, 60, 1986–1996.
[12] M. Sha, Q. Dou, F. Luo, G. Zhu, G. Wu, ACS Appl. Mater.
Interfaces 2014, 6, 12556–12565.
[13] Y. Li, S. Roy, T. Ben, S. Xu, S. Qiu, Phys. Chem. Chem. Phys.
2014, 16, 12909–12917.
[14] S. Pohlmann, T. Olyschläger, P. Goodrich, J. Alvarez Vicente,
J. Jacquemin, A. Balducci, J. Power Sources 2015, 273, 931–
936.
Electrochemical Characterization: The ZnFe2O4 thin film was ex-
plored as an electrode and the 1 m ILs explored as electrolytes for
supercapacitor applications by constructing cells with three elec-
trodes. The cyclic voltammetry (CV) curves were obtained at dif-
ferent scan rates (10–100 mVs–1). The galvanostatic charge/dis-
charge (GCD) study as function of varying current densities (from
[15] J. Yan, Q. Wang, T. Wei, Z. Fan, Adv. Energy Mater. 2014, 4,
1300816.
[16] W. Chen, R. B. Rakhi, M. N. Hedhili, H. N. Alshareef, J. Ma-
ter. Chem. A 2014, 2, 5236–5243.
[17] K. Kim, S. Kim, J. Ind. Eng. Chem. 2015, 26, 136–142.
Eur. J. Inorg. Chem. 2015, 5832–5838
5837
© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.eurjic.org
FULL PAPER
[18]
[19]
[20]
[32] X. He, J. Lei, Y. Geng, X. Zhang, M. Wu, M. Zheng, J. Phys.
Chem. Solids 2009, 70, 738–744.
[33] Q. Cheng, J. Tang, J. Ma, H. Zhang, N. Shinya, L. C. Qin,
Phys. Chem. Chem. Phys. 2011, 13, 17615–17624.
[34] D. P. Dubal, P. Gomez-Romero, B. R. Sankapal, R. Holze,
Nano Energy 2015, 11, 377–399.
[35] H. Tokuda, K. Hayamizu, K. Ishii, M. A. B. H. Susan, M. Wa-
tanabe, J. Phys. Chem. B 2005, 109, 6103–6110.
[36] R. Wang, Q. Li, L. Cheng, H. Li, B. Wang, X. S. Zhao, P. Guo,
Colloids Surf. A 2014, 457, 94–99.
[37] Q. Qu, P. Zhang, B. Wang, Y. Chen, S. Tian, Y. Wu, R. Holze,
J. Phys. Chem. C 2009, 113, 14020–14027.
[38] H. Nan, L. Yu, W. Ma, B. Geng, X. Zhang, Dalton Trans.
2015, 44, 9581–9587.
[39] Y. Wang, C. X. Guo, J. Liu, T. Chen, H. Yang, C. M. Li, Dalton
Trans. 2011, 40, 6388–6391.
[40] K. Bhattacharya, P. Deb, Dalton Trans. 2015, 44, 9221–9229.
[41] C. Liu, Z. Yu, D. Neff, A. Zhamu, B. Z. Jang, Nano Lett. 2010,
10, 4863–4868.
[42] A. Balducci, R. Dugas, P. L. Taberna, P. Simon, D. Plée, M.
Mastragostino, S. Passerini, J. Power Sources 2007, 165, 922–
927.
C. M. Wu, S. Y. Lin, K. Y. Kao, H. L. Chen, J. Phys. Chem. C
2014, 118, 17764–17772.
V. Ivanisˇtsˇev, M. V. Fedorov, R. M. Lynden-Bell, J. Phys.
Chem. C 2014, 118, 5841–5847.
J. S. Shaikh, R. C. Pawar, R. S. Devan, Y. R. Ma, P. P. Salvi,
S. S. Kolekar, P. S. Patil, Electrochim. Acta 2011, 56, 2127–
2134.
[21]
[22]
S. Maiti, A. Pramanik, S. Mahanty, RSC Adv. 2015, 5, 41617–
41626.
M. M. Vadiyar, S. C. Bhise, S. K. Patil, S. A. Patil, D. K. Pawar,
A. V. Ghule, P. S. Patil, S. S. Kolekar, RSC Adv. 2015, 5, 45935–
45942.
X. Yao, C. Zhao, J. Kong, H. Wu, D. Zhou, X. Lu, Chem.
Commun. 2014, 50, 14597–14600.
[23]
[24]
H. Lv, L. Ma, P. Zeng, D. Ke, T. Peng, J. Mater. Chem. 2010,
20, 3665–3672.
[25] G. Anandha Babu, G. Ravi, T. Mahalingam, M. Kumaresav-
anji, Y. Hayakawa, Dalton Trans. 2015, 44, 4485–4497.
[26] Z. Yu, Z. Cheng, S. R. Majid, Z. Tai, X. Wang, S. Dou, Chem.
Commun. 2015, 51, 1689–1692.
[27] A. Shanmugavani, R. K. Selvan, RSC Adv. 2014, 4, 27022–
27029.
[43] D. Rochefort, A. L. Pont, Electrochem. Commun. 2006, 8,
[28] X. Hou, X. Wang, L. Yao, S. Hu, Y. Wu, X. Liu, New J. Chem.
2015, 39, 1943–1952.
[29] Y. Zhang, Q. Shi, J. Schliesser, B. F. Woodfield, Z. Nan, Inorg.
Chem. 2014, 53, 10463–10470.
[30] Z. Wu, X.-L. Huang, Z.-L. Wang, J.-J. Xu, H.-G. Wang, X.-B.
Zhang, Sci. Rep. 2014, 4, 3669.
[31] A. Paravannoor, S. V. Nair, P. Pattathil, M. Manca, A. Balak-
rishnan, Chem. Commun. 2015, 51, 6092–6095.
1539–1543.
[44] J. K. Chang, M. T. Lee, W. T. Tsai, M. J. Deng, H. F. Cheng,
I. W. Sun, Langmuir 2009, 25, 11955–11960.
[45] G. J. Navathe, D. S. Patil, P. R. Jadhav, D. V. Awale, A. M. Teli,
S. C. Bhise, S. S. Kolekar, M. M. Karanjkar, J. H. Kim, P. S.
Patil, J. Electroanal. Chem. 2015, 738, 170–175.
Received: August 2, 2015
Published Online: November 24, 2015
Eur. J. Inorg. Chem. 2015, 5832–5838
5838
© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim