Preparation of a natural deep eutectic solvent mediated self polymerized highly flexible transparent gel having super capacitive behaviour

Abstract

A novel natural deep eutectic solvent (NADES) was prepared by the endothermic complexation of choline chloride (hydrogen bond acceptor) and orcinol (hydrogen bond donor) at room temperature. Among N-isopropyl acrylamide (NIPAM), vinyl acetate (VA) and 2-hydroxyethyl methacrylate (HEMA), only HEMA was found to self-polymerized in the above NADES at room temperature at optimized concentrations, resulting in the formation of a highly stretchable gel (>30 times). Polymerization of HEMA in the DES was established and rheological investigations revealed the highly stretchable behaviour of the gels. The gels obtained with HEMA in the ratio of 0.35, 0.5 and 1% v/v with respect to the NADES demonstrated good capacitive behaviour with metal oxide frame works (>200 F g⁻¹).

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1. Introduction

The use of volatile solvents in electrolytes poses several safety problems upon long term application of electrochemical devices.¹ In order to minimize the use of volatile solvents in the preparation of electrolytes used in electrochemical storage devices, the concept of solid electrolytes and the nonvolatile solvent based electrolytes was proposed. The two-electrode cell, which makes use of a gel-like film as both the electrolyte and separator generally gives a slightly high capacitive performance. This is because a major contribution to the internal resistance of a working cell comes from conductivity of electrolyte and spacer or space between electrode and separator. Internal resistance is even larger in three electrode system because of cell configuration. Therefore, polyelectrolyte plays an important and sensitive role in supercapacitors. Among the solid electrolytes, the ion gels have occupied a substantial volume of applications. Conventional ion gels used so far are either solid or semi-solid or even sometimes quasi solid electrolytes and mostly stabilized by polymer support to impart non-flow behaviour to the matrices.^2–5 Several methods are reported for the preparation of ion gels, which includes methods involving chemical,⁶ thermal,^7,8 microwave induced,⁹ UV-initiated,¹⁰ radical polymerization¹¹ etc.

Unique properties such as non-volatile nature, thermal stability, large electrochemical window, high ionic conductivities, nonflammability etc., make ionic liquids (ILs) as preferable choice for applications as electrolytes in number of applications such as electric double layer capacitor (EDLC), Li-ion battery, redox flow batteries, solar cells and fuel cells.^12–18 Among the various ILs, pyrrolidinium and imidazolium based ILs with the anionic moiety, bis[(trifluoromethyl)sulfonyl]imide (TFSI) are commonly used as a hydrophobic electrolytes for the fabrication of solid electrolytes useful in electrochemical devices.^19,20 The ion gels prepared by sol–gel technique using tetraethoxysilane (TEOS) in 1-butyl-3-methylimidazolium tetrafluoroborate [BMIm][BF₄] was found to be potential candidates for electrochemical devices, where high ionic conductivity in solid state materials is desirable.²¹ Due to the merits such as, ease of production, non-toxicity, inexpensive in nature etc., make deep eutectic solvents (DESs) as preferable alternatives to ILs for number of applications. Among the DESs, the one obtained by the complexation of choline chloride with trifluoroacetamide or malonic acid or glycerol were investigated as non-aqueous liquid electrolyte in electrochemistry.²² Further certain DESs obtained by the complexation between choline chloride and glycerol/urea or malonic acid were found to act as liquid electrolyte for EDLCs.²³

Due to the increase in research endeavours in biocompatible ion-conducting gels, ability of formation of ion gels by various biopolymers alone or in the form of composites are investigated thoroughly. The corn starch and cellulose acetate based polymer electrolytes in presence of ILs and DESs were prepared and affect of the ionic solvents on the ionic conductivity was investigated.^24,25 The application of such gels are envisaged typically as optical sensors, biosensors or in slow drug delivery. Agarose, a seaweed polysaccharide is proposed as biodegradable gel electrolyte for applications as flexible supercapacitors.²⁶ Due to the multiple advantages such as substantial durability upon long-term usage, flexible gel capacitors having very good mechanical properties were also prepared.²⁷ We have prepared several ion gels of polysaccharides using ILs^28–30 and electric conductive composite material based on ι-carrageenan and polymerizable ILs.³¹

2-Hydroxyethyl methacrylate (HEMA) (Fig. 1a) is a commonly used monomer to produce biocompatible polymers after polymerization. Due to the high end applications of the polymer and to avoid contaminations during polymerization, this monomer is preferred to be self-polymerized in some biocompatible solvent. Winther-Jensen et al., (2009) have observed the self polymerization of this monomer in an ionic liquid, choline-formate.³² Although the researchers have studied the ionic conductivities of the polymerized HEMA but they have not studied the detail capacitive profile of the polymer. Furthermore the presence of high flexible nature as investigated by rheological measurements in the present study was not available from the above study.

Fig. 1 Chemical structures of (a) HEMA, (b) choline chloride : orcinol 1 : 1.5 NADES and (c) MOF–carbon nanocomposite used as electrode material and (d) supercapacitor assembly.

Herewith we have investigated the self-polymerization ability of three monomers namely 2-hydroxyethyl methacrylate (HEMA) (Fig. 1a), N-isopropylacrylamide (NIPAM) and vinyl acetate (VA) in a natural deep eutectic solvent (NADES) obtained by the complexation of choline chloride and orcinol, a natural phenolic compound at room temperature without adding any initiators (Fig. 1b). Among all the three monomers, only HEMA was found to be self-polymerized in the NADES. The polymerized gel thus obtained was investigated for its capacitive behaviour. Use of NADES is considered to be more advantageous over their IL counter parts due to several advantages such as lower cost, superior bio-degradability and compatibility etc., associated with the later.

2. Results and discussion

NADES comprising of the hydrogen bond acceptor (HBA) choline chloride and hydrogen bond donor (HBD) orcinol was prepared using different mole ratio of both the HBA and HBD as shown in ESI Table S1.† DES could be obtained for both 1 : 1 and 1 : 1.5 mole ratios of choline chloride and orcinol (ESI†). As stated above, three different monomers such as HEMA, NIPAM and VA were investigated to assess their self-polymerization ability in both ChoCl–Or 1 : 1 and ChoCl–Or 1 : 1.5. Among these, only HEMA was found to self-polymerize at room temperature in both the DESs. But the time required for the self-polymerization of HEMA in ChoCl–Or 1 : 1 was 24 h and in ChoCl–Or 1 : 5, it took only 1.5 h to self-polymerize and hence the later was chosen for the polymerization. The various reaction parameters were optimized for the formation of the gel structure of poly-HEMA. As shown in Table 1, HEMA was added into 1 mL of ChoCl–Or 1 : 1.5 in different quantities ranging from 0.25 mL (2.06 mmol) to 2 mL (16.50 mmol). No gelation was observed in the DES containing 2.06 mmol of HEMA even after standing at room temperature for seven days (A). However, 2.88 and 4.11 mmol of the monomer yielded soft gel and gel like material respectively in 1.5 h of stirring at room temperature (B and C) as evident from rheological measurements. Furthermore, solutions of 8.23 and 16.50 mmol in the DES appeared as stiff gel (D) and very stiff gel (E) upon 2 h of stirring at room temperature, rheological investigation could not be performed due to the absence of flow in these samples. It was previously shown that, formation of small amount of radical components in presence of choline formate is responsible for the gelation of HEMA in the ionic liquid.³² It was also hypothesised in the report that, formate anion might have decomposed to radicals in the IL environment. In the present study, the inhibitor mono methyl ether hydroquinone (MEHQ) present in HEMA was removed by column chromatography and hence the chances for the quenching of the free radicals are less in this case. However, to understand the role of NADES and its ability for the free radical initiated polymerization, hydroquinone (free radical quencher) was added in the HEMA solution (4.11 and 8.23 mmol) in the NADES (F + HQ and G + HQ in Table 1). But no gel formation was observed, which indicates that, free radical formation took place in the NADES similar to choline-formate.³² Since in the case of NADES, there is only chloride anion which is bound with orcinol (Fig. 1b) by hydrogen bonding and hence how the NADES will form a free radical in this case is not clearly understood.

Table 1 Optimization of reaction parameters for the self-polymerization of HEMA in ChoCl : Or 1 : 1.5 and conductivity values of the suitable gel matrices^a

Code	ChoCl·Or 1 : 1.5 (mL)	HEMA (mL)	Gelation time (h)	Observation	Conductivity σ (mS cm^−1)
a HQ = hydroquinone (50 mg) added in solution; NA: not applicable; G′ = storage modulus and G′′ = loss modulus.
A	1	0.25	NA	Remains at liquid state up to one week upon standing at room temperature	NA
B	1	0.35	1.5	Soft gel with G′ > G′′ in frequency range of 0.1 to 100 Hz	1.47
C	1	0.5	1.5	True gel with G′ ≫ G′′ in frequency range of 0.1 to 100 Hz. Values of G′ is more than 4 times of G′′	0.24
D	1	1	2	Stiff gel	0.21
E	1	2	2	Very stiff gel	0.05
F + HQ	1	0.5	NA	Remains at liquid state up to one week upon standing at room temperature	NA
G + HQ	1	1	NA	Remains at liquid state up to one week upon standing at room temperature	NA
Pure ChoCl : Or 1 : 1.5	NA	NA	NA	NA	2.67

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The photographs of the polymerized transparent ion gels of poly-HEMA thus obtained in the NADES are shown in ESI (ESI, Fig. S1†). Bands observed in FT-IR spectra of HEMA after the polymerization in the DES were identical to those reported for the polymer indicating polymerization of the monomer.^32,33 For sample A (Table 1), HEMA in ChoCl·Or 1 : 1.5 showed typical absorption bands for HEMA such as bands at 1298 and 1320 cm⁻¹ due to =CH stretching along with at 1640 cm⁻¹ due to C=C stretching vibrations (Fig. 2a). However, in the sample B (Table 1), the bands due to =CH and C=C stretching were either disappeared or shifted (Fig. 2b) along with marginal shifting of the band at 1724 cm⁻¹ due to C=O of HEMA indicating polymerization of the monomer in B.

Fig. 2 FT-IR spectra of (a) HEMA in ChoCl·Or 1 : 1.5 and (b) polymerized HEMA in ChoCl·Or 1 : 1.5.

The ¹H NMR of ChoCl·Or 1 : 1.5 showed δ (ppm, D₂O) values of 3.17 [–N(CH̲₃)₃], 3.48 (–CH̲₂–N), 4.02 (–CH̲₂–OH), 2.20 (CH̲₃-orcinol), 6.32 (Ar-H, orcinol). The peaks due to –OH groups are not visible indicating participation of this group in the DES formation (ESI, Fig. S2†). HEMA in the DES prior to polymerization showed peaks due to the monomer at 2.11 (CH̲₃), 4.11 (–O–CH̲₂), 3.54 (–CH̲₂OH), 6.48 (–CH=C–) and peaks due to orcinol as well as choline chloride is also visible (ESI, Fig. S3†). One additional peak at 9.2 ppm in the mixture indicated formation of hydrogen bonds between HEMA and the DES in the mixture (Scheme 1) i.e., as shown in the scheme both the hydroxyl group of orcinol took part in the polymerization. The shifting of peak positions of –OH of HEMA as well as the appearance of the peak due to –OH recorded in d₆-DMSO supports the mechanism depicted in the scheme. All the peaks due to HEMA and DES were visible in the NMR spectrum of the mixture after polymerization, except appearance of peaks at 0.79 and 1.86 ppm due to polymerization of vinyl group of HEMA. Furthermore disappearance of peak at 6.10 and 5.68 ppm due to vinylic protons of HEMA also confirm the complete polymerization of HEMA resulting formation of gel (ESI, Fig. S4†).

Scheme 1 Plausible mechanism for the formation of polymerized [2-hydroxyethyl methacrylate] in ChoCl·Or 1 : 1.5.

Quality of gels in terms of their physical strength and their reusability aspects matters a lot, when such gels are applied for practical applications. Hence, the detail gel behaviour of the polymerized HEMA gel in the NADES was studied by rheological measurements. Magnitude of storage modulus (G′) and loss modulus (G′′) provides important information regarding physical state of a matter in rheological measurements. In the case of the HEMA solution in the DES, the G′ was not significantly higher in comparison to G′′ indicating not a very strong gel like behaviour, however the storage modulus was about 4 times higher in comparison to G′′ in the mixture after polymerization indicating formation of true gel (Fig. 3 and Table 1). Moreover, both the modulus did not crossover with increasing frequency in the case of polymerized HEMA, while the moduli crossover at higher frequencies in the mixture before polymerization indicating liquid like behaviour for the system before polymerization (ESI, Fig. S5†).³⁴ Even though, pure HEMA showed good thermal stability in comparison to ChoCl·Or 1 : 1.15, while the thermal stability of the gels further improved after polymerization (ESI, Fig. S6†).

Fig. 3 Time dependent viscoelastic behaviour of HEMA ion gels. ChoCl–Or : HEMA (1 : 0.35 v/v) [blue colour] and ChoCl–Or : HEMA (1 : 0.5 v/v) [green colour].

In order to investigate the stretching/flexible behaviour of the ion gels, recovery of storage modulus (G′) of the ion gels upon relaxation was studied. Ion gel obtained with 1 : 0.35 HEMA (B, Table 1) was initially subjected to varied strain at 1 Hz frequency for 300 s and the values G′ and G′′ were monitored during the process.

It was observed that, values of both the moduli consistently decreased with applied strain and remained almost constant at 500% of applied strain (ESI, Fig. S7†). This strain was further applied to fracture the ion gel. In the case of self-healing and solvent responsive healed ion gels of guar gum in an ionic liquid, it was observed that the value of G′ was decreased, while magnitude of G′′ increased with time indicating fracture of the gels followed by structure recovery upon relaxation.^29,35 But in this case, value of both the moduli remained almost constant in the entire duration of the experiment (Fig. 4). This indicated that, the ion gels in this case was not fractured after the application of very high strain (500%) and even after 12 such repeated cycles, the ion gels did not lose its flexible nature. This experiment showed the excellent flexibility and durable nature of the ion gel. This behaviour was supported by the very high stretchable nature of the ion gel. Upon manual stretching the gel could be stretched more than 30 times of its original shape without any deformation, however more than 100 times stretching of the gel was observed during rheological measurements (ESI, Fig. S8†).

Fig. 4 Storage (G′) and loss modulus (G′′) of HEMA based ion gel versus the deformation induced followed by relaxation.

After the systematic understanding of viscoelastic properties of HEMA-based ion gels prepared in ChoCl–Or 1 : 1.5 with ratio of monomer (1 : 0.35 v/v), (1 : 0.5 v/v) and (1 : 1 v/v) (represented as 0.35%, 0.5% and 1% ion gel respectively), the as prepared ion gels were tested for their ionic conductivity to select the best composition for electrochemical studies. Table 1 gives the list of ion gel and their respective ionic conductivity which ranges from 0.04 mS cm⁻¹ to 1.47 mS cm⁻¹. The dimension of the gels used to measure the conductivity is shown in supporting Table S2.† It was observed that low polymer concentration in ion gel resulted in high conductivity polyelectrolyte. Therefore, 0.35%, 0.5% and 1% ion gels were further used for electrochemical studies.

Supercapacitor cells built using thermal treated MOF as electrode and above ion gels as polyelectrolyte were scanned for current response at different scan rate. Further, polyelectrolyte performance was analyzed for its interfacial behaviors using electrochemical impedance spectroscopy. To understand the interfacial behaviour of electrode–polyelectrolyte, increasing order of scan rates were applied. It is evident from Fig. 5a–c that at high scan rates, measured current response was negligible. It is important to determine affect of reversing polarity on increased current levels of the working cells. Therefore, three-electrode cell was used to record current responses at different potential window which indicates zero charge point to avoid polarization during CV measurements. However, between −1.0 to 1.0 potential, CV curves shows similar trend for both three-electrode scans. Further, affect of ion gel concentration on current loading from −1.0 to 1.0 range was measured at low scan rates. Fig. 5d shows cell measurements for low concentration polymer in ion gel, which yield up to double the values in comparison to 1% ion gel at lower scan rate of 1 mV s⁻¹ and 5 mV s⁻¹. It is well known that capacitance values determined from CV curves depend on scan rate and the potential window it is operated. Therefore, before using CV curve data for capacitance calculation, we tested current response and stability on number of cycles for optimized 0.35% ion gel at 100 mV s⁻¹. Interestingly, ion gel in combination with microporous MOF electrode showed increased current response for increased cycle number as shown in Fig. 6a. This was stabilized after several cycles of run and CV curve area was used for calculating specific capacitance. The specific capacitance values at lower scan rate of 0.1 V s⁻¹ has shown exceptionally high >200 F g ⁻¹ for 0.35% and 0.5% ion gels as shown in Fig. 6b. However, increased scan rate showed decreasing trend with stable value of ∼50 F g⁻¹ at 10 mV s⁻¹. For 1% ion gel composition (Table S3†), specific capacitance range was found 110 to ∼5 F g⁻¹ for scan rate of 1 to 50 mV s⁻¹, respectively.

Fig. 5 Cyclic voltammograms of three-electrode cell using 0.5% ion gel used for scanning potential window recorded at different potential widows (a) from −0.5 V to 0.5 V, (b) from −1.0 V to 1.0 V and (c) from −3.5 to 3.5 V. (d) Current response for in two-electrode cell with different ion gel compositions at potential range from −1.0 to 1.0 V.

Fig. 6 (a) Number of cycle dependent current response for 0.35% at 100 mV s⁻¹ scan rate (b) graph showing specific capacitance against scan rate for all three ion gel compositions.

Assessment of power density and energy density is an important criterion to evaluate overall performance of electrode and/or polyelectrolyte performance in device form. In this study, ion gel polyelectrolyte in combination with relatively high surface area MOF-based electrode showed remarkably high energy density and power density. Energy density of 26.45 W h kg⁻¹ and power density of 4761 W kg⁻¹ for 0.35% was recorded, whereas, energy density of 33 W h kg⁻¹ and 5940 W kg⁻¹ power density was recorded for 0.5% HEMA ion gel at 1 A g⁻¹. Therefore, ion gel polyelectrolyte with suitable electrode material can be devised to meet various applications. The present study utilizes green solvent in simple and easily scalable experimental procedure to make it viable and sustainable.

3. Experimental section

3.1. Materials

Analytical grade choline chloride, orcinol, acetone, aluminum oxide and hydroquinone were procured from S.D. Fine Chemicals, Mumbai, India. 2-Hydroxyethylmethacrylate (HEMA), N-isopropylacrylamide (NIPAM) and vinyl acetate (VA) were purchased from Sigma-Aldrich. High surface area metal organic framework (MIL-101) was used as high surface area electrode material in this study. Chromium(III) terephthalate (MIL-101) and phosphotungstic acid (PTA) in aqueous system were used as precursors to prepare MIL-101. As prepared MIL-101 were further heat treated to activate to result in high surface area electrode material. BET surface area was noted to be >3200 m² g⁻¹. Conductivity of ion gel in different compositions were measured using unit cell made up of ITO-coated glass having width of 0.75 mm and length of ion gel = 1.75 mm. Conductivity of ion gel was calculated using the formula σ = t(AR_gel)⁻¹, where, σ is conductivity, t is thickness of the gel, A is cross-sectional gel area and R_gel is resistance of the gel. Ni-Foil was used as current collector in all electrochemical studies. All the chemicals were used as received without further purification. HEMA was passed through aluminum oxide to remove added inhibitor prior to polymerization.

3.2. Synthesis of MIL-101(Cr)

The procedure followed as per the reported process.³⁶ In a typical reaction, a mixture of 4.0 g of chromium(III) nitrate (Cr(NO₃)₃·9H₂O), 1.66 g of terephthalic acid – BDC, C₆H₄–1,4-(CO₂H)₂, two drop of hydrofluoric acid, HF (40%), and 50 mL of distilled water, was taken in a 75 mL Teflon lined container and kept inside an steel autoclave. Then the autoclave was heated to 493 K and maintained for 8 h in an oven under static condition. Once the synthesis was completed, most of the un-reacted terephthalic acids (white crystal) were removed manually by spatula. Then the remaining green mass was sequentially subjected to a solvothermal/reflux treatment using ethanol or DMF at 353 K for 8 h. The resulting solid was soaked in 1 M of ammonium fluoride (NH₄F) solution at 343 K for 12 h and was immediately filtered and washed with hot water. The green solid was finally dried overnight at 433 K under air atmosphere. The yield was 1.98 g.

3.3. Synthesis of deep eutectic solvents (DESs)

DES was prepared following method described by Abbott et al. (2004).³⁷ In a typical reaction, both hydrogen bond acceptor (HBA) i.e., choline chloride (ChoCl) and donor (HBD) i.e., orcinol (Or) were stirred at room temperature with optimized molar ratio (1 : 1.5) under inert atmosphere until homogenous liquids were formed (ESI, Table S1†). The viscosity of the prepared NADES was measured to be 195 cP at 25 °C and density was found to be 1.12 g mL⁻¹. It should be noted that unlike other DESs, the temperature of the eutectic mixture was reduced to 13 °C from 25 °C (initial temperature) indicating formation of the solvent by endothermic reaction.

3.4. Characterizations

FT-IR was recorded on a Perkin-Elmer FT-IR machine (Spectrum GX, USA). The ¹H NMR was recorded on a Bruker Avance-II, 500 MHz spectrometer. Thermogravimetric measurements (TGA) were carried out on a Mettler Toledo TGA system, Greifensee, Switzerland, machine with heating rate 10 °C min⁻¹ under nitrogen atmosphere. The viscoelasticity measurements were performed on an Anton Paar, Physica MCR 301 rheometer USA, using parallel plate PP50/P-PTD200 geometry (50 mm diameter; 1 mm gap). Temperature was maintained at 25 °C ± 1 °C by Viscotherm VT2 circulating water bath. All impedance measurements were recorded at room temperature under ambient conditions using a HIOKI IM 3570 IMPEDANCE ANALYZER with a built-in frequency response analyzer. Cyclic voltammetry and charge–discharge measurements were carried out with the help of Princeton Applied Research VersaSTAT-3.

3.5. Preparation of ion gels in NADES

The NADES prepared from choline chloride and orcinol were stirred with HEMA, NIPAM, and vinyl acetate at room temperature. However, only HEMA was found to self-polymerize in certain ratios in ambient conditions upon gentle stirring (400 rpm) in ChoCl–Or 1 : 1.5 in dark under inert atmosphere. Even in exposure to ambient conditions, the polymerization time did not varied much. The optimization of various reaction parameters for the formation of HEMA ion gel are shown in Table 1.

3.6. Fabrication of flexible supercapacitor using ion gel as electrolyte

The carbon film coated on current collector Ni-foils were clamped together in a parallel plate geometry using a rectangular polytetrafluoroethylene (PTFE) sheet with pre-fixed dimension (length 10 mm, width 10 mm and thickness 1.75 mm). The physical mixture of ChoCl–Or 1 : 1.5 and HEMA was stirred for 1 h at room temperature resulting formation of a viscous solution. The viscous solutions thus obtained were injected in spacer using a syringe. These viscous solutions were self-polymerised within 1–2 h at room temperature and the electrochemical analyses were carried out after 24 h. AC impedance spectroscopy measurements were performed on ion gels. The physical dimensions (area 1 cm × 1 cm, thickness 0.175 cm) of all the electrodes were maintained in all these experiments (Fig. 1d).

4. Conclusion

In an attempt to self-polymerize monomers such as N-isopropyl acrylamide (NIPAM), vinyl acetate (VA) and 2-hydroxyethylmathacrylate (HEMA) in a natural deep eutectic solvent obtained by the endothermic complexation between choline chloride and orcinol, only HEMA was found to be self-polymerized in the solvent. After the polymerization, it gave formation of a highly stretchable ion gel suitable to be used as solid polyelectrolyte in supercapacitor application. Rheological investigations confirmed highly stretchable nature of the gel. The ion gels obtained with HEMA at 0.35 and 0.5% ratio with respect to the solvent showed high specific capacity with MOF (>200 F g⁻¹) at low scan rates (1.0 mV s⁻¹ and 0.2 V s⁻¹). Considering the non-toxic nature of both the components used for the preparation of solid electrolyte, such electrolytes may find applications in bio-sensor and electrochemical industries.

Acknowledgements

KP thanks CSIR for the Young Scientist Awardees project and overall financial support. SKN gratefully acknowledges the DST, Government of India for DST-INSPIRE Fellowship and Research Grant (IFA12-CH-84). CM and RG acknowledge UGC for senior research fellowships and AcSIR for PhD registration. Mr Mukesh Sharma is acknowledged for his help during the rheological measurements. Analytical division and centralized instrument facility of the institute is acknowledged for providing overall instrumentation facility. This is CSIR-CSMCRI Communication No. 138/2015.

Notes and references

1. G. M. A. Girard, M. Hilder, H. Zhu, D. Nucciarone, K. Whitbread, S. Zavorine, M. Moser, M. Forsyth, D. R. MacFarlane and P. C. Howlett, Phys. Chem. Chem. Phys., 2015, 17, 8706–8713.
2. J. L. Bideau, L. Viau and A. Vioux, Chem. Soc. Rev., 2011, 40, 907–925.
3. L. Nègre, B. Daffos, P. L. Taberna and P. Simon, J. Electrochem. Soc., 2015, 162, A5037–A5040.
4. N. Buchtová, A. Guyomard-Lack and J. L. Bideau, Green Chem., 2014, 16, 1149–1152.
5. A. F. Visentin and M. J. Panzer, ACS Appl. Mater. Interfaces, 2012, 4, 2836–2839.
6. L. Wang, S. Fang, Y. Lin, X. Zhou and M. Li, Chem. Commun., 2005, 5687–5689.
7. M. A. B. H. Susan, T. Kaneko, A. Noda and M. Watanabe, J. Am. Chem. Soc., 2005, 127, 4976–4983.
8. A. Noda and M. Watanabe, Electrochim. Acta, 2000, 45, 1265–1270.
9. A. F. Visentin, T. Dong, J. Poli and M. J. Panzer, J. Mater. Chem. A, 2014, 2, 7723–7726.
10. C. Yang, J. Ju, J. Lee, W. Cho and B. Cho, Electrochim. Acta, 2005, 50, 1813–1819.
11. S. Saricilar, D. Antiohos, K. Shu, P. G. Whitten, K. Wagner, C. Wang and G. G. Wallace, Electrochem. Commun., 2013, 32, 47–50.
12. A. Paiva, R. Craveiro, I. Aroso, M. Martins, R. L. Reis and A. L. C. Duarte, ACS Sustainable Chem. Eng., 2014, 2, 1063–1071.
13. X. Baokou and M. Anouti, J. Phys. Chem. C, 2015, 119, 970–979.
14. A. Boisset, S. Menne, J. Jacquemin, A. Balducci and M. Anouti, Phys. Chem. Chem. Phys., 2013, 15, 20054–20063.
15. M. Díaz, A. Ortiz and I. Ortiz, J. Membr. Sci., 2014, 469, 379–396.
16. P. Wang, S. M. Zakeeruddin, P. Comte, I. Exnar and M. Gratzel, J. Am. Chem. Soc., 2003, 125, 1166–1167.
17. T. Welton, Chem. Rev., 1999, 99, 2071–2084.
18. M. H. Chakrabarti, F. S. Mjalli, I. M. AlNashef, M. A. Hashim, M. A. Hussain, L. Bahadori and C. T. J. Low, Renewable Sustainable Energy Rev., 2014, 30, 254–270.
19. S. Zhang, K. H. Lee, C. D. Frisbie and T. P. Lodge, Macromolecules, 2011, 44, 940–949.
20. S. Jeremias, G. A. Giffin, A. Moretti, S. Jeong and S. Passerini, J. Phys. Chem. C, 2014, 118, 28361–28368.
21. S. A. M. Noor, P. M. Bayleyc, M. Forsythc and D. R. MacFarlane, Electrochim. Acta, 2013, 91, 219–226.
22. H. Jhong, D. S. Wong, C. Wan, Y. Wang and T. A. Wei, Electrochem. Commun., 2009, 11, 209–211.
23. Y. Ju, C. Lien, K. Chang, C. Hu and D. S. Wong, J. Chin. Chem. Soc., 2012, 59, 1280–1287.
24. S. Ramesh, R. Shanti and E. Morris, Carbohydr. Polym., 2013, 91, 14–21.
25. S. Ramesh, R. Shanti and E. Morris, J. Mater. Sci., 2012, 47, 1787–1793.
26. W. G. Moon, G. Kim, M. Lee, H. D. Song and J. Yi, ACS Appl. Mater. Interfaces, 2015, 7, 3503–3511.
27. C. Zhao, C. Wang, Z. Yue, K. Shu and G. G. Wallace, ACS Appl. Mater. Interfaces, 2013, 5, 9008–9014.
28. M. Sharma, D. Mondal, C. Mukesh and K. Prasad, Carbohydr. Polym., 2014, 102, 467–471.
29. M. Sharma, D. Mondal, C. Mukesh and K. Prasad, Carbohydr. Polym., 2013, 98, 1025–1030.
30. S. Mine, K. Prasad, H. Izawa, K. Sonoda and J. Kadokawa, J. Mater. Chem., 2010, 20, 9220–9225.
31. K. Prasad and J. Kadokawa, Polym. Compos., 2010, 31, 799–806.
32. O. Winther-Jensen, R. Vijayaraghavan, J. Jiazeng Sun, B. Winther-Jensen and D. R. MacFarlane, Chem. Commun., 2009, 3041–3043.
33. T. S. Perova, J. K. Vij and H. Xu, Colloid Polym. Sci., 1997, 275, 323–332.
34. K. Prasad and J. Bhatt, Rheology of polysaccharide iongels prepared in ionic liquids in “Rheology: Principles, applications and environmental impacts”, ed. E. Karpushkin, NOVA Science Publishers, USA, 2015, ch. 7, pp. 175–190.
35. M. Sharma, D. Mondal, C. Mukesh and K. Prasad, RSC Adv., 2013, 3, 16509–16515.
36. G. Ferey, C. Mellot-Draznieks, F. Serre, C. Millange, J. Dutour, S. Surble and I. Margiolaki, Science, 2005, 309, 2040–2042.
37. A. P. Abbott, D. Boothby, G. Capper, D. L. Davies and R. K. Rasheed, J. Am. Chem. Soc., 2004, 126, 9142–9147.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra03309a

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