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High performing Bio-based ionic liquid crystal electrolytes for supercapacitors Renjith 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 Downloaded from http://pubs.acs.org on May 19, 2016

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ACS Sustainable Chemistry & Engineering

High performing Bio-based ionic liquid crystal electrolytes for supercapacitors Renjith Sasia, Sujatha Sarojamb, and Sudha J Devakia* * E-mail:[email protected] a

Chemical Sciences and Technology Division, CSIR-National Institute for Interdisciplinary Science and Technology (CSIR-NIIST), Thiruvananthapuram, 695019, India. b

Battery 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

1 Environment ACS Paragon Plus


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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.1 They 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.2–5 The 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.6–9 Unlike 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 2 Environment ACS Paragon Plus

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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 already taken to convert industrial wastes and by-products into useful articles and energy resources for making our planet clean.12–14Waste derived carbon materials are widely employedfor various energy storage devices as efficient and profitable electrode materialsrecently.15–19The 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.21 The 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.24,25 Ionic liquids and ionic liquid crystals with combined effects of organic and aqueous electrolytes seem to be a better replacement for conventional electrolytes.26–29 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 Kerala’s 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.33–35In 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 ᵒC 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,

1

HNMR and HRMS. Details of synthetic procedures and

characterization methods are given in supporting information. 1

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

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 28003000 cm-1 are 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 sharp peak 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, π-π 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 ᵒC and the isotropic phase at 118.00 ᵒC 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 ᵒC and is converted into Smectic F phase at around 69.50 ᵒC on


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further cooling. It generated crystalline phase around 61.00ᵒC also, and the corresponding thermodynamic variations are listed in Table.S1. More mobile charge carriers in the quasiordered 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 ᵒC; there occurred an exponential increment in conductivity due to the formation of columnar phase. It further increased to 20×10-3 S/cm at 115 ᵒC, 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 under DSC 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 ᵒC at a rate of 5 ᵒC/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 Waal’s 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 ᵒC, 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 ᵒC 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 ᵒC 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 ᵒC, when PMIMP melt, a sheardependent 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 viscous phase. 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 ᵒC. Variation of viscoelastic moduli and viscosity with shear strain at isotropic (130.00 ᵒC) and columnar (90.00 ᵒC) phases is given in Figure S4. Temperature ramp at an angular frequency of 10 Hz from 130.00 ᵒC where isotropic phase prevails to 40.00 ᵒC illustrated the clear variation of viscoelastic properties on the thermotropic phase transition (Figure 1b). Storage modulus increased exponentially on cooling from 130.00 ᵒC 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 ᵒC 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 Xray 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 120ᵒC 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 ᵒC corresponding to the columnar mesophase. The strong peaks in the WAXD profile of the columnar phase with d-spacings 6.43 and 4.23 Å 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 ᵒC (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 Å 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 ᵒC, 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 ᵒC 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 kΩ (RT) to 28.30 kΩ at 90.00 ᵒC 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 selfassembly of ionic molecules result in the formation of ion conducting channels with high conductivity 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 Å, 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 Å. 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 chargedischarge properties (Figure S9). The performances of the devices were evaluated using Electrochemical Impedance Spectroscopy, Cyclic Voltammetry, and galvanostatic chargedischarge 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 Ω (Rb) and 1.90 Ω (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 (τ) of the supercapacitors which quantifies their rate performance can be calculated from the Bode impedance plots (Figure 4b). τ 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 ᵒ. 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, =

4∆ ∆

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 CMK-3A

44 45

Banana fibre carbon 46 OMC-600 47 Waste particle board carbon 48 OMC 49 50

Tri-OMC

Lignin based OMC-CO2 activated 28 Activated carbon MTI carbon

Condition 1 mHz, single electrode 0.50 A/g

Electrolyte Nafion

Capacitance 132.00 F/g

1.00 M Na2SO4

74.00 F/g 105.00 F/g 119.00 F/g

5.00 mV/s

6.00 M KOH 1.00 M TEMABF4/PC 6.00 M KOH/1 M H2SO4 1.00 M TEMABF4/PC 6 M KOH

0.50 A/g 0.18 A/g

PYR14TFSI in BuCN 0.50 M PMIMP in can

125.00 F/g 134.43 F/g

0.70 A/g 0.05 A/g 5.00 mV/s 5.00 mV/s

36.00 F/g 112.00 F/g 102.00 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 =

2 . 3600

=

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/cm2 was 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 a load 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. Rb and Rct values increased to7.26 and 2.56 Ω 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 Ω) 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,

1

HNMR

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. This material 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 also thankful 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 supercapacitors Renjith Sasia, Sujatha Sarojamb, and Sudha J Devakia* a

Chemical Sciences and Technology Division, CSIR-National Institute for Interdisciplinary Science and Technology (CSIR-NIIST), Thiruvananthapuram, 695019, India. b

Battery Division, Vikram Sarabhai Space Centre (VSSC), Thiruvananthapuram, 695022, India.

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

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Ionic liquid crystal as electrolyte for supercapacitors 166x111mm (150 x 150 DPI)

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