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Article

High performing Bio-based ionic liquid crystal electrolytes for supercapacitorsRenjith Sasi, Sudha Janardhanan Devaki, and Sujatha Sarojam

ACS Sustainable Chem. Eng., Just Accepted Manuscript DOI: 10.1021/

acssuschemeng.6b00585 Publication Date (Web): 16 May 2016

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High performing Bio-based ionic liquid crystal

electrolytes for supercapacitors

Renjith Sasia, Sujatha Sarojam

b, and Sudha J Devaki

a*

* E-mail:[email protected]

aChemical Sciences and Technology Division, CSIR-National Institute for Interdisciplinary

Science and Technology (CSIR-NIIST), Thiruvananthapuram, 695019, India.

bBattery Division, Vikram Sarabhai Space Centre (VSSC), Thiruvananthapuram, 695022,

India.

ABSTRACT: Production and storage of energy in a highly efficient and environmentally

sustainable way is the demand of current century to meet the growing global energy requirement.

Design and development of novel materials and processes that allow precise control over the

electrochemical behavior and conductivity of electrolytes is necessary for acquiring such targets.

Development of ionic liquid crystals with ordered domains endowed with enhanced ionic

conductivity from renewable resources is receiving much interest in this respect. In this paper,

we report a unique strategy for the preparation and utilization of ionic liquid crystalline

electrolyte derived from a renewable resource: cashew nut shell liquid; an abundantly available

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waste by-product from cashew industry. We have prepared imidazolium-based ionic liquid

crystal (PMIMP) from cardanol and studied its structure and liquid crystalline phase formation

by various techniques. The symmetrical supercapacitor fabricated with mesoporous carbon

electrodes employing PMIMP as electrolyte measured a specific capacitance of 131.43 F/g at a

current density of 0.37 A/g with excellent cycle stability and 80 % capacitance retention after

2000 cycles. All these excellent properties of the prepared ionic liquid crystalline electrolyte

suggest its application as an efficient, environmentally friendly and low-cost electrolyte for

energy storage devices.

KEYWORDS: Bio-based ionic liquid crystals, self-assembly, electrolyte, rheology,

supercapacitors, energy storage

INTRODUCTION

Ionic liquid crystals (ILCs) form a versatile class of compounds combining the intriguing

properties of ionic liquids as well as liquid crystals.1They carry labile ionic centres arranged in a

quasi-ordered manner giving one-dimensional stimuli-responsive conductivity suitable for

conduction, transport and storage of electric charge. ILCs consists of rigid aromatic cores for

long range ordering, ionic centres for charge and alkyl chains for mobility modulations.25The

versatility in design, easy exchangeability of ions, tunable charge transport properties, lower

toxicity and vapour pressure makes them hot cakes in the new age energy research.69Unlike

Ionic liquids, ILCs can form conducting films with suitable binders making them wonderful

candidates to be used as solid electrolytes in energy devices which improve connectivity and

avoids the leakage and pollution. As the global concern for green research rises day by day, the

scientific world is in search of alternate eco-friendly resources for energy production and

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storage.10,11Environmentally benign materials for energy harnessing are hot topics nowadays as

the world suffers severely from environmental pollution, global warming, etc. mainly due to the

improper selection and usage of energy resources. Minimization of waste disposal, re-use of

materials and converting waste materials into useful products are three imperative goals put

forwarded by environmentalists to safeguard our mother nature. Exponential material

consumption of modern society had resulted in the accumulation of huge tonnes of wastes in the

environment whose disposition itself costs a tremendous amount of manpower and energy.

Conversion of troublesome wastes into economic and high-throughput energy currencies seems

an intelligent move to conserve the environment and solve the energy crisis.Steps have alreadytaken to convert industrial wastes and by-products into useful articles and energy resources for

making our planet clean.1214Waste derived carbon materials are widely employedfor various

energy storage devices as efficient and profitable electrode materialsrecently.1519The global

scientific community is in search for alternate, low-cost, eco-friendly technology for the

production of electrical energy.20

Storage of electrical energy is also a crucial technological area which needs intense research.

Electrolytic capacitors and batteries offer a sustainable way for stable and consistent energy

storage. Batteries involving Li-ion or metal/metal hydride couple provide energy discharge with

high energy density. Complimentary to them electrolytic capacitors are capable of discharging

with high power density. Supercapacitors having properties intermediate to batteries and

electrolytic capacitors displays faster charge- discharge rates, higher cycle lives, simple and

flexible designs which make them prospective candidates in electronic devices, medical

appliances, military instruments and in hybrid transportation systems.21The device performance

can be tuned by improving its energy density through the design of new electrode materials, new

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electrolytes, and new electrochemical concepts.22,23Currently both organic and aqueous

electrolytes were employed for extracting high performance from carbon-based supercapacitors.

Energy and power densities of supercapacitors were limited by the electrochemical window of

the electrolytes used for the charge transport. Aqueous electrolytes have lower electrochemical

windows in comparison with organic electrolytes but they are less toxic in nature making them a

primary choice. Electrolyte depletion is another important hindering factor of conventional

electrolytes. The ILCs have the good electrochemical window and appreciable conductivity to be

applicable in electrochemical energy devices as efficient electrolytes.24,25Ionic liquids and ionic

liquid crystals with combined effects of organic and aqueous electrolytes seem to be a betterreplacement for conventional electrolytes.2629 Bio-based ionic liquid crystals are attracting

attention nowadays, as they provide a healthy alternative for petroleum-based materials by

performing their respective function with less harm to the environment.30

Cashew industry has been one of the prime industries in Kerala for a long time. Export of

cashew kernels is earning a considerable share of Keralas annual income. In Kerala, there are a

vast number of cashew factories processing and packaging cashew nuts in large scale. Earlier the

nutshells have been burnt to ashes without knowing their value. Later it was discovered that the

extract of cashew nutshell, known as cashew nut shell liquid (CNSL) is a rich source of a variety

of long alkylated phenols particularly 3-pentadecenyl phenol (cardanol).31,32 A library of

cardanol derived functional materials have been reported by our group as dopants, templates,

sensors and so on.3335In this paper, we are presenting a new class of ionic liquid crystal which is

derived from cardanol (3-Pentadecenyl phenol) to be used as an electrolyte for energy storage

devices. It possesses unique bent-core design for facilitating both liquid crystalline ordering and

excellent conductivity. The well characterized ionic liquid crystal found to be having a good

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electrochemical window and ionic conductivity (~40 mS/cm at 0.50 M in acetonitrile) for

displaying excellent capacitive performance. Symmetric supercapacitors employing mesoporous

carbon as the electrode material were fabricated with 0.50 M solution of Ionic liquid crystal in

acetonitrile as electrolytesto check the supercapacitor performance. It exhibits a high specific

capacitance of 131.43 F/g with excellent cycle stability at a current density of 0.37 A/g.

RESULTS AND DISCUSSION

Scheme 1.Synthesis of PMIMP.

We have distilled CNSL under vacuum at 200 C to obtain cardanol which was further reduced

to 3-pentadecylphenol (3-PDP). O-alkylation of 3-PDP followed by quaternization with 1-methyl

imidazole to yield a class of bio-based ionic liquid crystal with distinct bent-core geometry

(Scheme. 1). Formation of targeted compounds was confirmed by various spectroscopic

techniques such as FT-IR, 1HNMR and HRMS. Details of synthetic procedures and

characterization methods are given in supporting information.

1HNMR spectroscopic analysis gave characteristic spectra showing the chemical shift ()

values confirming the formation of targeted compound. Peaks around the chemical shift values

of 8.44 and 7.25 ppm corresponds to the aromatic protons in the imidazolium cation. Resonance

of aromatic protons in the benzene ring gave rise to characteristic peaks around 7.16, 6.76 and

6.67 ppm. Sharp singlet observed at 3.85 ppm attributed to the equivalent protons present in the

methyl group attached to the imidazolium ring. Methylene groups near to the aromatic rings and

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hetero atoms gave peaks at 4.22, 3.95, 2.55 and 2.05 ppm respectively. Remaining methylene

groups resonated to give signals in between 1.25 and 1.81 ppm. Terminal methyl group of

aliphatic chain gave characteristic triplet at 0.88 ppm. 1H NMR spectrum of PMIMP is given in

the Figure S1. In the FT-IR spectrum of PMIMP, The multiple IR peaks in the range of 2800-

3000 cm-1are due to the symmetric and asymmetric C-H stretching vibrations of the alkyl chains

whereas the multiplets in the range of 3000-3200 cm-1 are attributed to the C-H vibrational

modes of aromatic rings. The peaks in the range of 1600-1585 cm-1 are assigned to C-C

stretching and C-N bending vibrations. The peak at 1455 cm -1 is corresponding to C-H alkyl

deformation while C-N stretching vibrations are observed at a frequency of 1189 cm

-1

. The sharppeak at 1158 cm-1 corresponds to the C-O-C stretching of alkyl aryl ether. P-F stretching

vibrations of PF6 anion produce a characteristic peak around 820 cm-1.36 FT-IR spectra of

BBPDB and PMIMP are given in the Figure S2. Molecular geometry optimization by B3LYP

using 6-31g* basis set showed that PMIMP has unique bent-core structural design containing

long aliphatic chain, aromatic cores and ionic centres enabling self-assembly via layer by layer

inter-digitation, - stacking and electrostatic interactions (Figure S3). Such non-covalent

interactions are prone to re-alignment on introducing suitable external stimuli such as

temperature, solvent, electric field, etc. which gave rise to liquid crystalline phases.

Thermotropic phase transitions

Thermotropic phase transitions were studied by DSC analysis in conjugation with a hot stage

equipped PLM. In the heating ramp, PMIMP displayed a transition from crystalline phase to

columnar phase at 90.00 C and the isotropic phase at 118.00 C with enthalpy changes 7.89 J/g

and 34.82 J/g respectively (Figure 1a). On cooling from the isotropic melt, PMIMP developed

columnar phase around 96.90 C and is converted into Smectic F phase at around 69.50 C on

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further cooling. It generated crystalline phase around 61.00C also, and the corresponding

thermodynamic variations are listed in Table.S1. More mobile charge carriers in the quasi-

ordered liquid crystalline phase improved the conductivity of the system at the onset of

crystalline to columnar transition. PMIMP showed a low conductivity of 6.50 10-8 S/cm at

room temperature, and it increased gradually with temperature and above 90.00 C; there

occurred an exponential increment in conductivity due to the formation of columnar phase. It

further increased to 2010-3 S/cm at 115 C, at the onset of Columnar- Isotropic phase change.

After melting, conductivity remains almost constant with increase in temperature. Conductivity

enhancements observed were fitted extremely well with the thermotropic phase changes underDSC as displayed in Figure 1a. Thermotropic liquid crystalline phase formation was followed by

polarized light microscopic analysis also. Images were taken while cooling the ionic liquid

crystal from isotropic phase at 120 C at a rate of 5 C/min. During cooling, the randomly aligned

ionic liquid crystal molecules were brought to supramolecular organized liquid crystalline phase

by various non-covalent interactions such as ionic, electrostatic layer by layer assembling and

other van der Waals interactions leading to the formation of liquid crystalline phases of different

orders which depends on the extent of the tilt of molecular alignment from the principle director

(the plane perpendicular to the aligned molecular layers).37 DSC displayed an exothermic peak

around 97 C, suggesting that PMIMP molecules self-assembled to give columnar mesophase as

confirmed by observation under PLM. Characteristic focal conic domains of columnar phase

obtained at 97 C are shown in Figure 1c. On further cooling from columnar phase, the entropy

of the system again decreased which was compensated by the increment in molecular ordering to

generate higher degree of order and simultaneous tilt of the molecular chains toform another type

of focal conic domains with stripes. This columnar-Smectic liquid crystalline phase transition

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was accompanied by the enhancement of birefringence with the simultaneous appearance of

circular stripes along the focal conic domains. The typical mosaic texture of Smectic mesophase

was observed at 69.50 C as depicted in Figure 1d. This homeotropic mosaic texture together

with planar circularly striped cone texture is typical for liquid crystalline Smectic F phases.38

Temperature dependent rheological studies revealed the variation in viscoelastic behavior at

the onset of thermotropic phase transition.39 Above 120 C, when PMIMP melt, a shear-

dependent viscoelastic profile showing non-Newtonian fluidic behavior with elastic prominence

in the lower shear regime was observed. On increasing the shear strain, both storage (G) and

loss moduli (G) decreases and G overtook G at a shear strain of 4.23 % to obtain a viscousphase. High G value displayed by PMIMP even in the isotropic phase is attributed to the

combined effect of layer-by-layer stacking of alkyl chains and strong ionic interaction between

the imidazolium cation and counter anion. On cooling from the melt, the values of G and G

increased exponentially and become independent of shear rate. At a shear strain of 20 %, G rose

from 98.20 Pa to 12.01 kPa on cooling from isotropic melt to columnar phase at 90.00 C.

Variation of viscoelastic moduli and viscosity with shear strain at isotropic (130.00 C) and

columnar (90.00 C) phases is given in Figure S4. Temperature ramp at an angular frequency of

10 Hz from 130.00 C where isotropic phase prevails to 40.00 C illustrated the clear variation of

viscoelastic properties on the thermotropic phase transition (Figure 1b). Storage modulus

increased exponentially on cooling from 130.00 C before attaining a plateau showing the

establishment of molecular self-assembly.40 As it cooled further intermolecular interactions

reinforced to generate columnar phase observed as a discontinuity in the viscoelastic profile.

Sudden enhancement of G and complex viscosity around 53.00 C is attributed to the formation

of well-packed crystalline phase. These observations are in good agreement with DSC analysis.

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Figure 1.a) DSC heating and cooling scans of PMIMP showing thermotropic behavior and

corresponding variation in conductivity and b) thermotropic variation in viscoelastic moduli of

PMIMP. PLM images of c) focal conic domains of columnar phase and d) Smectic F phase

formed.

PMIMP displayed highly ordered crystalline structure in the room temperature wide angle X-

ray diffraction (WAXD) due to strong ionic interactions pertained between charged counterparts

and layer by layer inter-digitation of long alkyl chains (Figure 2a). On heating, it is converted

into an amorphous profile without any characteristic peaks at 120C showing the complete

conversion into the isotropic state. When cooled from the melt, it generated a quasi-ordered

profile with the lesser number of reflections at 90.00 C corresponding to the columnar

mesophase. The strong peaks in the WAXD profile of the columnar phase with d-spacings 6.43

and 4.23 correspond to the core stacking of the imidazolium cations. It regains its original

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crystalline pattern when cooled down to room temperature suggesting strong thermo-responsive

behavior of PMIMP. The SAXS pattern of PMIMP at 90 C (Figure 2b) displayed peaks with

reversible d-spacing ratio of 1: 3: 2 revealed the high degree of ordering present in the

columnar phase.41A d-spacing of 70 corresponds to (100) peak corresponds to which is more

than double the molecular length suggests that columnar phase is formed predominantly via the

ionic interaction between PMIMP molecules. Optimization of molecular geometry of PMIMP by

Gaussian 09 method using 6-31g* basis set further confirms that the d-spacings correspond to the

molecular ordering pertaining in the mesophase.

Effect of Thermotropic phase transition on electrochemical properties:

As the formation of thermotropic columnar mesophase improves the conductivity of the

system, the electrochemical properties which heavily rely on the charge carrier density will also

show significant enhancement. Electrochemical behaviors of the material in response to

thermotropic phase transitions were monitored by using a typical test cell of the configuration

ITO/PMIMP/ITO. Cyclic Voltammetry (CV) traces show rectangular shaped electrochemical

window in the region of 1.00 to 2.50 V suggests the ability of the material to store charge

effectively (FigureS5a). When the CV analysis was done at 90.00 C, an enhancement in current

occurred and the area under the CV corresponding to the charge stored in the system also

increased. Galvanostatic charge-discharge studies of the symmetric test cell confirmed the effect

of thermotropic phase transition on the capacitive behavior of the material as the discharge time

increased considerably in the liquid crystalline phase (Figure S5b). The test cell retained its

capacitive behavior even after 100 charge-discharge cycles as displayed by the symmetric

charge-discharge profiles in Figure S5c. This observation further suggests the improvement of

capacitive behavior. Impedance analysis of the cell at room temperature and 90.00 C further

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displayed the influence of mesophase formation on charge conduction. Nyquist plot of PMIMP

test cell gave typical semi-circular arc due to ionic conduction at the high frequency regime and

an inclined Warburg line at the low frequency regime attributed to the diffusion of charge

carriers (Figure S5d). Charge transfer resistance (Rct) of PMIMP obtained by extrapolating

semicircle arc to real axis decreased from 75.60 k (RT) to 28.30 k at 90.00 C vividly

substantiating the improvement of charge conduction in the mesophase. Good electrochemical

window and charge-discharge stability of the material suggest its applicability in solid state

energy storage devices. Also, thermotropic enhancement of capacitive behavior is interesting as

it will be suitable for high temperature molecular electronics.

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Figure 2.a) WAXD patterns of PMIMP at different thermotropic conditions and b) SAXS

pattern of columnar LC phase of PMIMP. c) Optimized geometry of PMIMP showing molecular

dimensions.

Lyotropic phase formation on electrochemical properties

The most important pre-requisites for material to be used as electrolytes are good ionic

conductivity and extended electrochemical window.42PMIMP exhibited good ionic conductivity

in solutions prepared in acetonitrile, a widely used solvent for electrochemical analyses.

Concentration-dependent ionic conductivity measurements of the PMIMP solutions were carried

out in combination with PLM analyses to study the effect of lyotropic phase formation on ionic

conduction. At concentrations lower than 0.10 M, no definite birefringence was observed, and

the conductivity of the solution was found to be in the range of 3.00-6.00 mS/cm. On increasing

the concentration Nematic phase with higher conductivity of 19.40mS/cm was obtained at a

concentration of 0.30 M. Conductivity further increased with concentration as columnar domains

of Sm A phase formed around 0.40 M (33.10 mS/cm)(Figure 3c). As the concentration exceeds

0.60 M, ionic conductivity began to decrease since the extended molecular association of ionic

liquid crystals resulted in the formation of higher ordered Smectic C phase (Figure 3d) and

finally gel networks. Maximum conductivity of 40.30 mS/cm was observed for a concentration

of 0.50 M. The conductivity of the electrolyte solutions depends not only on the concentration of

charge carriers but also on the viscosity of the solution. Shear dependent viscosity analysis of the

solutions displayed that viscosities of solutions were increased with concentration. On moving to

Nematic to columnar phase there occurs not much variation in viscosity but gel network found to

have higher viscosity. Variation of viscosity, as well as ionic conductivity with concentration is

given in Figure S6.

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Figure 3.Lyotropic phases formed by MIMP: AFM images of a) Nematic batonnet phase, b)

columnar phase, PLM images of c)Smectic A phase, and d) Smectic C phase.

Lyotropic ordering vividly reflected in the texture and birefringence of the film cast from

respective solutions. PMIMP film cast from solution with Nematic concentration, displayed

randomly oriented mesogens with batonnet rod-like morphology as observed under AFM (Figure

3a). Uniformly oriented Nematic mesogens possess an average height of 20.00 nm (Figure S7a

and S7b).On increasing the concentration, the mesogens were self-assembled to generate

Smectic phase with clear domain boundaries (Figure 3b).Height profile of the conical domains

showed a significant enhancement to ~150.00 nm confirming the molecular association. (Figure

S7c and S7d)

Electrolyte for Symmetric supercapacitors

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Electrolytes are one of the major components of supercapacitors whose performance will

govern the overall efficiency of the device. Even though the charge carrier density is very high

for the crystalline phase, the negligible mobility of carriers owing to the highly ordered

crystalline packing hinders the charge transport and thereby device performance. In comparison

with randomly oriented charge carriers of liquid electrolytes, ordered array of charge carriers

present in liquid crystalline electrolytes seems to deliver exceptional device performance.

Besides, they are having high carrier density and lower tendency to flow which will redeem the

possibility of environmental pollution by electrolyte leakage. In the liquid crystalline phase self-

assembly of ionic molecules result in the formation of ion conducting channels with highconductivity which will be the suitable one to act as electrolyte for supercapacitors as it will

facilitate better charge storage and transport. So, 0.50 M solution of PMIMP in acetonitrile

having excellent conductivity and low viscosity was selected as the electrolyte for fabricating

electrochemical supercapacitors. Symmetric super capacitors were fabricated using

commercially available mesoporous carbon with a specific surface area of 1732.74 m2/g, as the

electrode material. The electrode material is having an average pore width of 20.16 , which was

estimated by Brunauer-Emmett-Teller (BET) method. Average pore diameter of the electrode

material is estimated by Barrett-Joyner-Halenda (BJH) method as 27.49 . Typical BET linear

isotherm and pore volume distribution of mesoporous carbon electrode material are given in the

supporting information (Figure S8). AFM analysis of the electrode material also confirmed the

existence of fine pores for facilitating adsorption of charge carriers for yielding better charge-

discharge properties (Figure S9). The performances of the devices were evaluated using

Electrochemical Impedance Spectroscopy, Cyclic Voltammetry, and galvanostatic charge-

discharge studies. Nyquist impedance plots of the device gave characteristic semicircle arc in the

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higher frequency regime and steeply inclining Warburg line in the lower frequencies (Figure 4a).

Bulk (Rb) and charge transfer resistances (Rct) of the device were calculated from the intercepts

of the semicircles with real axis as 3.60 (Rb) and 1.90 (Rct). Impedance analysis can also be

used to calculate knee frequency (fknee) and response time of the device, two critical parameters

highlighting the rate capability of supercapacitors.43Enhancement of fknee, the frequency at which

the capacitive behavior rapidly changes to resistive behavior implies the fast switching at the

electrode-electrolyte interface. The prepared supercapacitor has a high fknee of 251.00 Hz

showing tremendous capacitive behavior. Response time () of the supercapacitors which

quantifies their rate performance can be calculated from the Bode impedance plots (Figure 4b). is the reciprocal of the response frequency (fres), the frequency at which the real and imaginary

components of impedance coincides or the components are separated by a phase angle of 45.00 .

As expected, PMIMP based supercapacitor has a low response time of 13.20 s confirming fast

charge transfer at the electrode-electrolyte interface.

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Figure 4a) Nyquist and b) Bode impedance plots of PMIMP based supercapacitors.

Cyclic voltametric analysis of the coin cell supercapacitor containing PMIMP electrolyte

displayed nearly rectangular shaped window devoid of any redox peaks within a potential range

of 1.00-2.50 V when cycled at a scan rate of 10.00 mV/s shows exquisite capacitive behavior

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(Figure 5a). On increasing the scan rate, the rectangular shape of the potential window remains

the same suggests that the charge transport is taking place at high rates. High rate performance is

one of the most desirable properties for supercapacitors.

Figure 5a) Rectangular CV curve of the super capacitor containing PMIMP electrolyte at

different scan rates, b) galvanostatic charge-discharge profiles of the device at various current

densities.

Galvanostatic charge-discharge studies at the constant current were performed to calculate the

specific capacitance of the device. Symmetrical charging and discharging profiles displayed a

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high degree of reversibility and better storage capability (Figure 5b). The specific capacitance

(Cs) of the devices were calculated from the discharge profile using the equation22,

Where I is the current applied, m is the active mass of the device and V/t is the slope of the

linear part of the discharge profile excluding the initial IR drop. PMIMP based supercapacitors

drew much higher capacities of 131.43 F/g at a current density of 0.37 A/g due to higher

conduction at the electrode-electrolyte interface. Normally supercapacitors with mesoporous

carbon based electrodes deliver capacities in the vicinity of 100.00 F/g. As per literature reports,

different combinations of electrolytes have explored for extracting better capacitive performance

with mesoporous carbon electrodes. In comparison with them PMIMP based electrolyte gave

better specific capacitance. Low cost and eco-friendly nature are other advantages. Comparison

of capacitive behavior of different electrolyte systems used in combination with mesoporous

carbon electrodes and PMIMP is illustrated as Table 1.

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Carbon Condition Electrolyte CapacitanceCMK-3A 1 mHz, single

electrodeNafion 132.00 F/g

Banana fibrecarbon

0.50 A/g 1.00 M Na2SO4 74.00 F/g

OMC-600 0.70 A/g 6.00 M KOH 105.00 F/gWaste particleboard carbon

0.05 A/g 1.00 M TEMA-BF4/PC

119.00 F/g

OMC 5.00 mV/s 6.00 M KOH/1 MH2SO4

36.00 F/g

Tri-OMC 5.00 mV/s 1.00 M TEMA-BF4/PC

112.00 F/g

Lignin basedOMC-CO2activated

5.00 mV/s 6 M KOH 102.00 F/g

Activated carbon 0.50 A/g PYR14TFSI in BuCN 125.00 F/g

MTI carbon 0.18 A/g 0.50 M PMIMP in can 134.43 F/g

Table 1 Comparison of specific capacitance of PMIMP electrolyte with reported electrolytes.

Energy density (in Wh/kg) and power densities (in W/kg) of the devices were also calculated

from discharge profile using the equations given below22

Where C is capacitance, V is the voltage range excluding IR drop, m is the active mass in kg,

and t is the discharge time in hours. 33.79 Wh/kg and 1032.98 W/kg are the energy and power

densities of the supercapacitor containing bio-based ionic liquid crystal solution as the electrolyte

when charged at a constant load current of 0.37 A/g. High energy and power densities obtained

for mesoporous carbon based supercapacitor powered by bio waste-derived electrolyte hold the

key for future energy storage systems. In addition to gravimetric capacitance, the areal

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capacitance of the supercapacitor was also measured. Comparatively high areal capacitance of

1.02 F/cm2was obtained for a constant current loading of 2.90 mA/cm2.

Effect of constant load current on the capacitive performances of super capacitors was

monitored by varying the current densities from 0.18 A/g to 1.80 A/g (Figure 5b). Specific

capacity dropped from 134.73 F/g to 118.29 F/g on increasing the constant load current density

from 0.18 A/g to 1.80 A/g. Variation of specific capacitance with current density is illustrated in

Figure S10. It was justified by the enhancement of ESR values clearly visible from the IR drops

of discharge profiles on increasing load current which will reduce the power density of the

system significantly at higher current densities. Cycling stability studies were carried out at aload current of 0.37 A/g to check the retention of capacities with time. PMIMP based super

capacitors displayed more than 80.00 % capacity retention even after 2000 cycles. The variation

of gravimetric and areal specific capacitance of the device with cycling is depicted in Figure 6

and typical charge-discharge profiles of first ten cycles are given in inset.

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Figure 6.Cycling stability of PMIMP based supercapacitor. First ten cycles are given in inset.

Lowering of capacitance with cycling is attributed to the enhancement of electrolyte and

charge transfer resistance on prolonged cycling. This was clearly illustrated by the impedance

analysis of the device after cycling. Rband Rctvalues increased to7.26 and 2.56 respectively

which will hinder the smooth transfer of charge carriers. Cyclic voltametric profile also

displayed lowering of current conduction and slight departure from rectangular profile owing to

the increased charge transfer resistance. CV and impedance profiles of the device before and

after cycling are compared in Figure S11. Variation of energy density of the device with power

density on varying the current density is illustrated by typical Ragone plot as in FigureS12. It

shows the characteristic profile of a supercapacitor with a maximum energy density of

38.29Wh/kg corresponding to a low power density of 617.89 W/kg at a current density of 0.18

A/g. Maximum power density of 3582.53 W/kg was observed at a current density of 1.80 A/g

with a lower energy density of 17.11 Wh/kg.

CONCLUSION

A bio-based ionic liquid crystal was prepared by modifying cardanol derived from CNSL, a

waste bye-product from cashew industry. Thermotropic and lyotropic phase analyses of PMIMP

displayed the existence of typical Columnar and Smectic mesophases. Symmetric super

capacitors were prepared using mesoporous carbon based electrodes and the developed ILCs as

electrolytes. PMIMP based super capacitor showed a maximum specific capacitance of 134.73

F/g, high energy density of 38.29 Wh/kg and an impressive power density of 3582.53 W/kg. It

also gives high rate performance with lower charge transfer resistance (1.90 ) and a very small

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response time (13.20s). This strategy of utilization of an industrial waste derived soft liquid

crystalline material as an efficient electrolyte for energy storage systems may expect to add

another gold coin to the field of energy while avoiding environmental issues for a better

sustainable society.

ASSOCIATED CONTENT

Supporting Information. Experimental procedures, characterization methods, 1HNMR

spectrum, FT-IR spectrum, Optimized geometry, DSC parameters, Rheological profiles,

electrochemical characterizations of solid test cell, AFM images, concentration dependent

viscosity and conductivity variation, BET profiles of electrode material and Ragone plot. Thismaterial is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION

Corresponding Author

* E-mail:[email protected]

Author Contributions

All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT

We thank CSIR and UGC for the financial support. We would like to thank Dr. A.

Ajayaghosh, director, CSIR-NIIST, Trivandrum for his constant encouragement and

support. We are thankful to Mr. Aswin Maheswar for AFM analysis. We are alsothankful for the financial support from CSIR network project MULTIFUN (CSC0101).

.

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For TOC use only

High performing Bio-based ionic liquid crystal

electrolytes for supercapacitorsRenjith Sasi

a, Sujatha Sarojam

b, and Sudha J Devaki

a*

aChemical Sciences and Technology Division, CSIR-National Institute for Interdisciplinary

Science and Technology (CSIR-NIIST), Thiruvananthapuram, 695019, India.

bBattery Division, Vikram Sarabhai Space Centre (VSSC), Thiruvananthapuram, 695022,

India.

Renewable resource based low cost eco-friendly ionic liquid crystals as powerelectrolytes for efficient supercapacitors

.

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Ionic liquid crystal as electrolyte for supercapacitors166x111mm (150 x 150 DPI)

ge 27 of 27 ACS Sustainable Chemistry & Engineering

paper lectura 5 cristal liquido de alto rendimiendo para capacitores

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