Playing with ionic liquids to uncover novel polymer electrolytes

Solid State Ionics 300 (2017) 46–52

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Solid State Ionics journal homepage: www.elsevier.com/locate/ssi

Playing with ionic liquids to uncover novel polymer electrolytes Rita Leones a,b, Rodrigo C. Sabadini c, José M.S.S. Esperança b,d, Agnieszka Pawlicka c, M. Manuela Silva a,⁎ a

Centro de Química, Universidade do Minho, Campus de Gualtar, 4710 – 057, Braga, Portugal Instituto de Tecnologia Química e Biológica António Xavier, Universidade Nova de Lisboa, 2780 – 157 Oeiras, Portugal c Instituto de Química de São Carlos, Universidade de São Paulo, 13566 – 590 São Carlos, Brazil d LAQV, REQUIMTE, Departamento de Química, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, 2829-516 Caparica, Portugal b

a r t i c l e

i n f o

Article history: Received 15 August 2016 Received in revised form 18 November 2016 Accepted 21 November 2016 Available online xxxx Keywords: Solid polymer electrolyte Ionic liquid Chitosan Ionic conductivity

a b s t r a c t Solid polymer electrolytes (SPEs) based on chitosan and fourteen ionic liquids (ILs) were synthesized by solvent casting method. All ILs had in common a 1-ethyl-3-methylimidazolium cation ([C2mim]+). Therefore, this study focuses on the influence of the anion on the thermal, morphological, and electrochemical properties of the SPEs. The samples were analyzed by thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), complex impedance spectroscopy (ionic conductivity) and cyclic voltammetry. The SPEs displayed an amorphous morphology, a thermal stability higher than the one obtained for the pure chitosan matrix (N 140 °C), a maximum room temperature (T = 25 °C) ionic conductivity of 1.61 × 10−3 S cm−1 and a wide electrochemical window of ~4.0 V. © 2016 Elsevier B.V. All rights reserved.

1. Introduction A solid polymer electrolyte (SPE) is an ionically conductive solid solution resulting from the dissolution of an ionic salt in a high molecular weight polymer host containing cation-coordinating groups [1]. SPE's commercial attractiveness has been recognized for more than forty years, since they may be used in fabrication of solid-state electrochemical devices (ECDs) such as sensors, displays, rechargeable batteries, supercapacitors, and electrochromic devices [2–3]. Poly(ethylene oxide) (PEO) has been the most widely studied SPE polymer host because of its extraordinary solvating ability towards salts. Due to many envisaged applications, PEO-based systems doped with lithium salts have been the most extensively studied [4]. However, despite SPEs enormous technological potential, their application in commercial devices has been delayed. The main reason is the low ionic conductivity of SPEs. Additionally, even though PEO has advantageous thermal, electrochemical, chemical, and physical properties, the ionic conductivity reported for SPEs based on PEO are moderate and frequently considered inadequate for many electrochemical applications [1,3]. To overcome this drawback several strategies have been proposed. In this work, we intended to develop high ionic conductive SPEs, which are more environmentally friendly than the current alternatives, through the dissolution of ionic liquids (ILs) in a biopolymer matrix (chitosan).

⁎ Corresponding author. E-mail address: [email protected] (M.M. Silva).

http://dx.doi.org/10.1016/j.ssi.2016.11.018 0167-2738/© 2016 Elsevier B.V. All rights reserved.

It was already shown that the addition of ILs to polymer matrices can be successfully achieved [5–10]. The resulting SPEs show enhanced electrochemical stability and ionic conductivity, without compromising the safety associated to solid electrolytes comparatively to the liquid ones. Actually, many ILs are known for their almost null volatility [11], null flammability [12], high thermal stability [13], high ionic conductivity, wide electrochemical window and design ability [14]. The possibility of changing the structure of both cation and anion in a very extensive and fashionable way allows tailoring the ILs properties accordingly to the foreseen applications. A new challenge exists to determine the ILs that are the most suitable ones for each SPE matrix host. Natural polymers gained their momentum because they are biodegradable, non-toxic, cheap and can be extracted from renewable sources [23–25]. Consequently, chitosan was used as the SPE matrix host. Chitosan is a polysaccharide derived from chitin, which can be found in fungi, molluscan organs, insects, and crustacean shells. It is already widely used in biomedical, pharmaceutical and food packaging industries, and its properties differ accordingly to its molecular weight and degree of acetylation [15–19]. Its polar functional groups, namely ether, hydroxyl, and primary amine make it a very well-suited SPE matrix host due to chitosan ability to dissolve ionic salts [20–22]. This work presents results of transparent and free-standing chitosan and ILs-based SPEs. In this work, fourteen ILs sharing a common cation (1-ethyl-3-methylimidazolium, [C2mim]+) were used to uncover the anion that originates a SPE with a better thermal, morphological and electrochemical properties that can perform well when applied in an ECD. The thermal properties were evaluated by TGA and DSC, the morphological characteristics by XRD, SEM and AFM, and the

R. Leones et al. / Solid State Ionics 300 (2017) 46–52

electrochemical features were measured by means of complex impedance spectroscopy and cyclic voltammetry. 2. Experimental 2.1. Materials Medium molecular weight and 75–85% deacetylated chitosan (Sigma-Aldrich 448877), acetic acid (Sigma-Aldrich), and glycerol (Himedia, 99.5%) were used as received. Milli-Q water was used in all experiments. The structure, abbreviation and supplier of the ionic liquids used is described in Table 1. Their most relevant thermal, physical and electrochemical properties are shown in Table 2. 2.2. SPE preparation 0.2 g of chitosan was dissolved in 10 mL of 1% acetic acid solution and stirred overnight at room temperature, until a homogeneous and viscous solution was formed. Then, 0.4 g of the IL and 0.2 g of glycerol as plasticizer were added to this solution. The resulting solution was casted onto Petri dishes and dried for 8 h at 25 °C, then overnight at 40 °C, followed by 4 h at 60 °C and then cooled down to 25 °C [41]. The notation adopted to identify the samples was chitosan[C2mim][X], where X is the anion of the added IL.

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with perforated lids were sealed with each sample inside a glove box filled with dry argon. The analyses were carried out using a Mettler DSC 821e under a flowing argon atmosphere in the temperature range – 60 to 200 °C and at a heating rate of 5 °C min−1. XRD measurements were carried out at room temperature with X-ray Rigaku Ultima 4 diffractometer, power of 50 kV/50 mA, Cu Kα irradiation, speed of 2° min− 1, angle range (2θ) of 3 to 60°. SEM images were obtained at 10 kV with a LEO 440 microscope. AFM images were obtained with a Bruker AFM System (Dimension icon with ScanAsyst). In all AFM analyses the intermittent-contact mode was employed by using silicon AFM probes with a force constant of 48 N m− 1 and a resonance frequency of 190 kHz. An Autolab PGSTAT-12 (Eco Chemie) was used to obtain bulk ionic conductivities of the samples from room temperature to 100 °C using the complex plane impedance technique on a cell GE/polymer electrolyte/GE (GE – ion-blocking gold electrode of 10 mm diameter; Goodfellow, N99.95%), secured in a suitable constant volume support, and over the frequency range from 65 kHz to 500 mHz. The electrochemical stability window was evaluated using a two-electrode cell configuration: a 25 μm diameter gold microelectrode as working electrode and a lithium disk (Aldrich, 99.9%; 19 mm diameter, 0.75 mm thick) as counter and reference electrodes. The cyclic voltammetry measurements were carried out within a Faraday cage under an argon atmosphere and at room temperature; they were recorded by an Autolab PGSTAT-12 (Eco Chemie) at a scan rate of 20 mV s−1.

2.3. Characterization techniques 3. Results and discussion TGA and DSC were performed in order to study the thermal behavior of the produced membranes. TGA analyses were performed with a Shimadzu TGA-50 equipment. The measurements were conducted between 30 and 900 °C, at a heating rate of 10 °C min−1, and under a nitrogen atmosphere with a 60 mL min− 1 rate flow. Before each analysis and aiming to eliminate the traces of solvent and/or absorbed water, all samples were subject to a first run from 30 to 105 °C, at a heating rate of 20 °C min−1, followed by a second isothermal run at 105 °C during 10 min. For the DSC experiments, 40 μL aluminum cans

3.1. ILs dissolution All the SPEs studied in this work were prepared by solvent casting method; however, two out of the fourteen employed ILs, i.e. [C2mim][NTf2] and [C2mim][OTf], did not dissolve in the polymer matrix. At the time of the addition of [C2mim][NTf2] to the chitosan acetic acid solution, the solution went from transparent to opaque. The nonsolubility of [C2mim][NTf2] can be explained by its hydrophobicity

Table 1 Description of the ILs used in this work. Formal name

Structure

Abbreviation

Supplier and purity

1-Ethyl-3-methylimidazolium bis((trifluoromethyl)sulfonyl)imide

[C2mim][NTf2]

1-Ethyl-3-methylimidazolium trifluoromethanesulfonate

[C2mim][OTf]

1-Ethyl-3-methylimidazolium tricyanomethanide

[C2mim][C(CN)3]

Io-li-tec 99.5% Io-li-tec 99% Io-li-tec 98%

1-Ethyl-3-methylimidazolium dicyanamide

[C2mim][N(CN)2]

1-Ethyl-3-methylimidazolium thiocyanate

[C2mim][SCN]

1-Ethyl-3-methylimidazolium L-(+)-lactate

[C2mim][lactate]

1-Ethyl-3-methylimidazolium diethylphosphate

[C2mim][diC2PO4]

Io-li-tec 98%

1-Ethyl-3-methylimidazolium dimethylphosphate

[C2mim][diC1PO4]

Aldrich 98%

1-Ethyl-3-methylimidazolium butanesulfonate

[C2mim][C4SO3]

Prepared according to [26] 99%

1-Ethyl-3-methylimidazolium ethyl sulfonate

[C2mim][C2SO3]

Prepared according to [26] 99%

1-Ethyl-3-methylimidazolium methanesulfonate

[C2mim][C1SO3]

Prepared according to [26] 99%

1-Ethyl-3-methylimidazolium ethyl sulfate

[C2mim][C2SO4]

Io-li-tec 99%

1-Ethyl-3-methylimidazolium methyl sulfate

[C2mim][C1SO4]

Io-li-tec 99%

1-Ethyl-3-methylimidazolium acetate

[C2mim][OAc]

Io-li-tec 95%

Io-li-tec 98% Io-li-tec 98% Sigma-Aldrich N95%

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R. Leones et al. / Solid State Ionics 300 (2017) 46–52

Table 2 Thermal, physical and electrochemical properties of ILs used in this work.

IL

Degradation temperature (°C)

Viscosity (mPa·s at 25 °C)

Conductivity (S cm−1 at 25 °C)

[C2mim][NTf2]

417

33.0

8.30 × 10−3

[C2mim][OTf]

440

42.7

9.30 × 10−3 −2

[C2mim][C(CN)3] [C2mim][N(CN)2]

240

19.6 21.0

2.00 × 10 2.20 × 10−2

[C2mim][SCN]

306

24.5

2.01 × 10−2

340 342

321.0 270.0 531.4 296.1 295.1 95.9

[C2mim][lactate] [C2mim][diC2PO4] [C2mim][diC1PO4] [C2mim][C4SO3] [C2mim][C2SO3] [C2mim][C1SO3] [C2mim][C2SO4] [C2mim][C1SO4] [C2mim][OAc]

408 390 252

78.8 128.4

[42]. For the [C2mim][OTf], a phase separation only occurred after the drying process. Probably, the IL dissolved in the solvent but not in the polymer. The phase separation is confirmed when comparing the opaque chitosan[C2mim][OTf] membrane to the transparent membrane of chitosan[C2mim][OAc] (Fig. 1). 3.2. Thermal behavior The thermal stability of an IL depends mainly on the anion, while the cation influence is relatively insignificant. Generally, as the anion size increases so does the thermal stability. On the other hand, it decreases with the anion hydrophilicity [43]. The TGA curves of chitosan[C2mim][X] SPEs shown in Fig. 2 present a stable plateau up to at least a temperature of about 130–140 °C. Above this temperature, an abrupt mass drop, representing the main degradation process, can be observed. Beyond about 350 °C a plateau is attained, and it persists up to the maximum analyzed temperature of 900 °C. Between 35 and 10% of the samples initial mass remained after this analysis. The remaining residues probably correspond to the residual carbon of the chitosan chains [44] and the ionic liquid. Moreover, the TGA thermograms can be divided into two types: those which exhibit a multi-step decomposition and those which exhibit a typical decomposition curve. The latter case, displayed by chitosan[C2mim][OAc] and chitosan[C2mim][lactate] samples, is consistent with a decomposition via an SN2 reaction mechanism [35]. From Fig. 2, it is also evident that the addition of the ILs increased the SPEs thermal stability, demonstrating that the ILs stabilized the host matrix. The same was verified for other biopolymers and ILs-based SPEs [5,41]. The highest thermal stability of 232 °C was registered for

Fig. 1. Physical appearance of the membranes chitosan[C2mim][OAc] (left) and chitosan[C2mim][OTf] (right).

3.80 × 10−3

Electrochemical window (V)

Ref.

4.6 (−2.5 to 2.1) 3.9 (−2.1 to 1.8)

[27]

3 (−1.6 to 1.4) 3.2 (−2.0 to 1.2)

4.2 (−2.5 to 1.7)

−3

6.02 × 10 2.95 × 10−3

3.2 (−2.3 to 0.9)

[28–30] [31] [30] [30,32–34]

[35] [36] [26] [26] [26,30] [37,38] [38–40] [30,34,37]

chitosan[C2mim][N(CN)2] membrane. However, when comparing the degradation temperatures in Fig. 2 and the values in Table 2, one verifies that the pure ILs have higher thermal stability. Nonetheless, the values obtained here are considered well-suited for solid-state ECDs operating under normal conditions. The DSC thermograms (Fig. 3) indicate that all the SPEs studied here are predominantly amorphous. This conclusion can be drawn from the absence of thermal events in the studied temperature range. Only the samples of chitosan matrix, chitosan[C2mim][N(CN)2] and chitosan[C2mim][diC1PO4] show a thermal event. For the first two, one can observe small peaks above 190 °C that are representative of the materials thermal degradation. Chitosan[C2mim][diC1PO4] DSC curve shows a broad peak around 100 °C that can be associated with solvent evaporation. Beside these few thermal events no others were detected indicating an amorphous morphology of all studied SPEs, which is an obvious benefit in terms of optical and electrochemical properties comparatively to semi-crystalline SPEs [45]. 3.3. Morphology and structure The predominantly amorphous morphology is also confirmed by XRD. The diffractograms in Fig. 4 show a broad band, Gaussian in shape, centered near 20.5°. This indication that the polymer chains are essentially in a disordered organization was already exhibited by chitosan- [41,46,47] and other natural polymers-based SPEs [5–7,10]. Furthermore, a large peak at 2θ = 20.5° can probably be assigned to chitosan crystal form type II [17,48,49]. SEM micrographs of chitosan[C2mim][X] samples (Fig. 5) show homogeneity with no obvious phase separation and good surface uniformity with uniformly distributed light lines that can be due to the membrane formation process. The depicted micro-vesicles are probably undissolved polymer or glycerol [41]. The surface morphology of the chitosan[C2mim][X] samples was evaluated by AFM. The image of the chitosan matrix was acquired with a scanning area of 10.0 μm × 10.0 μm, while for the remaining samples the scanning area was 2.0 μm × 2.0 μm. The roughness mean square (RMS) values (Table 3) showed that the addition of ILs can either increase or decrease the SPEs surface roughness depending on the IL anion. The SPEs surface may influence their ionic conductivity as it is probable to get a better contact between the SPE and electrode for smoother samples [50,51]. The smallest RMS value was found for the chitosan[C2mim][SCN] sample, in agreement with the ionic conductivity results.

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Fig. 2. TGA curves between 30 and 900 °C, at a heating rate of 10 °C min−1 (left), and the degradation temperatures of the samples (right). The dash line represents the chitosan matrix onset degradation temperature.

3.4. Electrochemical properties When commercial applications in ECDs are envisaged, one of the most important requirements that needs to be fulfilled by a SPE is high ionic conductivity. Armand et al. [3] defined 1 × 10−5 S cm−1 as the minimum threshold. The ionic conductivity (σi) is determined according to Eq. (1). σi ¼

d ; Rb A

ð1Þ

where d is the thickness, A is the electrode contact area, and Rb is the bulk resistance of the sample. Rb was obtained from the intercept of the imaginary part of the impedance (minimum value of Z″) with the slanted line in the real part of the impedance (Z′). The plot of Z″ vs Z′, the Nyquist plot (not shown here), is described by three distinct parts: a semicircle at higher frequencies, a straight line at lower frequencies, and the transition between these two [1]. As the temperature increased, the semicircles disappeared gradually, and at high temperatures only straight lines were detected. Taking into account that the semicircles

Fig. 3. DSC thermograms between −60 and 200 °C at a heating rate of 5 °C min−1 of the chitosan-ILs SPEs.

are associated with charge transfer and the straight lines with the diffusion process, one can conclude that these phenomena are temperature related. Hence, the charge transfer and the diffusion of the ionic species are the main processes at lower and higher temperatures, respectively. Observing Fig. 6, one can easily verify how successfully the addition of ILs to a chitosan host matrix was. Fig. 6 also shows that the addition of all the ILs produced an increase of the ionic conductivity, comparatively to the polymer matrix, mainly ascribed to the high ionic charge content within the SPEs membranes. All SPEs show ionic conductivities higher than 1 × 10−5 S cm−1, even at room temperature (Table 4). The ionic conductivity plot shows a non-linear variation of ionic conductivity with temperature, characteristic of predominantly amorphous SPEs, well described by the Vogel-Tammann-Fulcher (VTF) equation. The VTF model implies that the free volume theory is the main mechanism of ion conduction. Ionic conductivity is a transport property governed by the number and mobility of charge carriers, viscosity, and ionic charge. These factors depend in turn on the effective ion sizes and shapes. Ion diffusion is more related to the size of the cations and anions are more likely to form ion complexes and diffuse together at a slower rate [52]. In general, ionic association (e.g., ion pairs) causes a reduction of the ionic conductivity because it decreases the number of available diffusible ions. Also the presence of the ILs affects the dipolar and ionic component of the polymer due to the ionic character of the ILs and the presence of ion-dipole interactions.

Fig. 4. X-ray diffraction patterns of the chitosan[C2mim][X] membranes.

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R. Leones et al. / Solid State Ionics 300 (2017) 46–52

Fig. 5. SEM pictures of the chitosan[C2mim][X] membranes.

As the temperature increases, the ionic conductivity follows the same tendency due to the increase of inter and intra-chain movements [1]. The differences in the conductivity values, as a function of temperature, of the samples with different ILs are related to the type of ILs, i.e., a different anion type alters the viscosity and the interaction between ILs and host polymer [53]. The combination of a polymer matrix and an ion conducting IL affects the properties of SPEs. Like for other systems, a broad and stable amorphous region is observed and it creates the freevolume, facilitating the ion conduction [54]. The improvement of conductivity requires low viscosity ILs to increase the flexibility of polymer electrolytes and to promote the dissociation of paired ions. Several ILs possess such properties. Table 4 shows that all the membranes exhibit conductivity values of the same order of magnitude, except the sample doped with [C2mim][SCN]. The sample that showed the highest ionic conductivity was chitosan[C2mim][SCN] (1.61 × 10− 3 and 1.28 × 10−2 S cm−1, at 25 and 100 °C, respectively). This is probably due to the small size and well delocalized charge of the [SCN]− anion, leading to a weaker coordination bond [55,56]. This result is also in agreement with the values presented in Table 2, as [C2mim][SCN] has the second highest ionic conductivity. The viscosity of [C2mim][SCN] is 24.5 mPa·s, and although it is not the lowest value, it is a low viscosity value, and this may be related to the decrease in the magnitude of the positive charge of the imidazolium cation. The net force of attraction

between the cation and anion decreases resulting in the diminution of the ionic character of [C2mim][SCN], which decreases viscosity [57]. The decrease in viscosity will lead to an increase in mobility and is accompanied by increased conductivity. Moreover, the highest values listed in Table 4 are higher than those reported for SPEs based on synthetic polymers and ILs [8,9,58–61] and on chitosan and different salts [21,51,62–68], and similar to those reported for biopolymers and ILs [6,7,69] which highlights these materials potentialities. Fig. 7 depicts the microelectrode cyclic voltammogram of the chitosan[C2mim][C2SO3] over the − 2.0 to 6.0 V potential range, obtained at room temperature and at a scan rate of 20 mV s − 1 . The voltammogram shows that in the anodic region the sample is stable up to about 3.0 V vs Li/Li+, whereas in the cathodic region it is stable up to − 1.5 V vs Li/Li+. This means that the overall redox stability of the sample spans about 4.5 V. Similar results were found for

Table 3 RMS values found for the chitosan[C2mim][X] samples . RMS (nm) [SCN] [C4SO3] [C(CN)3] [OAc] Matrix [N(CN)2] [diC1PO4] [lactate] [C1SO4] [C2SO3] [C1SO3] [C2SO4] [diC2PO4]

4.69 5.21 6.23 8.47 11.90 12.0 13.5 13.9 15.7 18.0 23.0 25.9 26.6

Fig. 6. Variation of the ionic conductivity of the chitosan[C2mim][X] SPEs with the inverse of temperature.

R. Leones et al. / Solid State Ionics 300 (2017) 46–52 Table 4 Ionic conductivity values of the SPEs at 25 and 100 °C. 25 °C (S cm−1) Matrix [C(CN)3] [C4SO3] [diC2PO4] [diC1PO4] [lactate] [C1SO4] [C2SO4] [C2SO3] [OAc] [C1SO3] [N(CN)2] [SCN]

3.56 1.35 1.38 1.44 2.07 2.57 2.60 3.50 3.62 4.77 6.69 7.15 1.61

× × × × × × × × × × × × ×

10−7 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−4 10−3

100 °C (S cm−1) 4.46 2.50 2.88 3.14 4.04 4.44 8.20 3.60 5.06 4.22 6.66 8.96 1.28

× × × × × × × × × × × × ×

10−5 10−3 10−3 10−3 10−3 10−3 10−3 10−3 10−3 10−3 10−3 10−3 10−2

the remaining samples (results not shown). On average the chitosan[C 2 mim][X] SPEs presented an electrochemical window of 4.0 V, an indication that these materials display an acceptable stability window for an application in a solid state ECDs. Similarly, 4.0 V is also the average electrochemical window of the ILs (Table 2), showing that the incorporation of the ILs in the host matrix did not reduce their high electrochemical stability. Generally, the IL electrochemical stability is more influenced by the anion rather than the cation [31]. 4. Conclusions The present work demonstrated that chitosan-ILs-based ionic conducting membranes are very promising materials with very high ionic conductivity (1.61 × 10− 3 and 1.28 × 10−2 S cm− 1, at 25 and 100 °C, respectively, for the chitosan[C2mim][SCN] sample) and high electrochemical (4.0 V) and thermal stabilities (N140 °C). The amorphous nature of the chitosan electrolyte system provides a clear advantage relatively to other semi-crystalline systems. The results obtained are encouraging and sufficient to justify further studies. With a special highlight for the SPEs doped with cyano-based ILs. It is highly probable that these materials may find application in “smart windows” and other ECD-based devices. Acknowledgments This work was supported by FEDER through the COMPETE Program and by the Portuguese Foundation for Science and Technology (FCT) in

Fig. 7. Microelectrode cyclic voltammogram of the chitosan[C2mim][C2SO3] membrane at room temperature. The initial sweep direction is anodic and the sweep rate is 20 mV s−1.

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the framework of the Strategic Project PEST-C/QUI/UI0686/2013, research units GREEN-it “Bioresources for Sustainability” (UID/Multi/ 04551/2013) and Associated Laboratory for Green Chemistry - Clean Technologies and Processes (UID/QUI/50006/2013), grant SRFH/BD/ 90366/2012 (R.L.) and contract under Investigador FCT 2012 program (J.M.S.S.E.). M. M. Silva acknowledges CNPq (PVE grant 406617/20139), for the mobility grant provided by this institution. References [1] F.M. Gray, Solid Polymer Electrolytes: Fundamentals and Technological Applications, VCH Publishers Inc., New York, 1991. [2] P.V. Wright, Electrical conductivity in ionic complexes of poly(ethylene oxide), Br. Polym. J. 7 (1975) 319. [3] M. Armand, M.T. Duclot, J.M. Chabagno, Proceedings of the Second International Meeting on Solid State Electrolytes, St. Andrews, Scotland, Sept 20–22, 1978, 1978 (Extended Abstract 6.5). [4] J.M. Tarascon, M. Armand, Issues and challenges facing rechargeable lithium batteries, Nature 414 (2001) 359. [5] R. Leones, L.C. Rodrigues, A. Pawlicka, J.M.S.S. Esperança, M.M. Silva, Characterization of flexible DNA films, Electrochem. Commun. 22 (2012) 189. [6] R. Leones, F. Sentanin, L.C. Rodrigues, I.M. Marrucho, J.M.S.S. Esperança, A. Pawlicka, M.M. Silva, Investigation of polymer electrolytes based on agar and ionic liquids, Express Polym. Lett. 6 (2012) 1007. [7] R. Leones, F. Sentanin, L.C. Rodrigues, R.A.S. Ferreira, I.M. Marrucho, J.M.S.S. Esperança, A. Pawlicka, L.D. Carlos, M.M. Silva, Novel polymer electrolytes based on gelatin and ionic liquids, Opt. Mater. 36 (2012) 187. [8] R. Leones, C.M. Costa, A.V. Machado, J.M.S.S. Esperança, M.M. Silva, S. LancerosMendez, Development of solid polymer electrolytes based on poly(vinylidene fluoride-trifluoroethylene) and the [N1 1 1 2(OH)][NTf2] ionic liquid for energy storage applications, Solid State Ionics 253 (2013) 143. [9] R. Leones, C.M. Costa, A.V. Machado, J.M.S.S. Esperança, M.M. Silva, S. LancerosMendez, Effect of ionic liquid type in the performance of solid polymer electrolytes based on poly(vinylidene fluoride-trifluoroethylene), Electroanal. 27 (2015) 457. [10] R. Leones, J.M.S.S. Esperança, A. Pawlicka, V. de Zea Bermudez, M.M. Silva, Polymer electrolyte based on DNA and N,N,N-trimethyl-N-(2-hydroxyethyl)ammonium bis(trifluoromethylsulfonyl)imide, J. Electroanal. Chem. 748 (2015) 70. [11] M.J. Earle, J.M.S.S. Esperança, M.A. Gilea, J.N. Canongia Lopes, L.P.N. Rebelo, J.W. Magee, K.R. Seddon, J.A. Widegren, The distillation and volatility of ionic liquids, Nature 439 (2006) 831. [12] M. Smiglak, W.M. Reichert, J.D. Holbrey, J.S. Wilkes, L.Y. Sun, J.S. Thrasher, K. Kirichenko, S. Singh, A.R. Katritzky, R.D. Rogers, Combustible ionic liquids by design: is laboratory safety another ionic liquid myth? Chem. Commun. 24 (2006) 2554. [13] K.J. Baranyai, G.B. Deacon, D.R. MacFarlane, J.M. Pringle, J.L. Scott, Thermal degradation of ionic liquids at elevated temperatures, Aust. J. Chem. 57 (2004) 145. [14] J.F.D. Holbrey, K.R. Seddon, Ionic liquids, Clean Prod. Process. 1 (1999) 223. [15] M. Dash, F. Chiellini, R.M. Ottenbrite, E. Chiellini, Chitosan – a versatile semi-synthetic polymer in biomedical applications, Prog. Polym. Sci. 36 (2011) 981. [16] C.K.S. Pillai, W. Paul, C.P. Sharma, Chitin and chitosan polymers: chemistry, solubility and fiber formation, Prog. Polym. Sci. 34 (2009) 641. [17] R. Gaf, Chitin Chemistry, MacMillan Press Ltd., Houndmills, 1992. [18] J. Ma, Y. Sahai, Chitosan biopolymer for fuel cell applications, Carbohydr. Polym. 92 (2013) 955. [19] S. Hirano, H. Seino, Y. Akiyama, I. Nonaka, Chitosan: a biocompatible material for oral and intravenous administrations, Progress in Biomedical Polymers, Springer, New York, 1990. [20] S. Fuentes, P.J. Retuert, G. Gonzalez, Transparent conducting polymer electrolyte by addition of lithium to the molecular complex chitosan-poly(aminopropylsiloxane), Electrochim. Acta 48 (2003) 2015. [21] M.Z.A. Yahya, A.K. Arof, Effect of oleic acid plasticizer on chitosan-lithium acetate solid polymer electrolytes, Eur. Polym. J. 39 (2003) 897. [22] N.M. Morni, A.K. Arof, Chitosan-lithium triflate electrolyte in secondary lithium cells, J. Power Sources 77 (1999) 42. [23] A. Pawlicka, M. Danczuk, W. Wieczorek, E. Zygadlo-Monikowska, Influence of plasticizer type on the properties of polymer electrolytes based on chitosan, J. Phys. Chem. 112 (2008) 8888. [24] C.O. Avellaneda, D.F. Vieira, A. Alkahlout, E.R. Leite, A. Pawlicka, M.A. Aegerter, Solidstate electrochromic devices with Nb2O5:Mo thin film and gelatin-based electrolyte, Electrochim. Acta 53 (2007) 1648. [25] G. Machado, H. Ferreira, A. Pawlicka, Influence of plasticizer contents on the properties of HEC-based polymeric electrolytes, Electrochim. Acta 50 (2005) 3827. [26] M. Blesic, M. Swadzba-Kwasny, T. Belhocine, H.Q.N. Gunaratne, J.N.C. Lopes, M.F.C. Gomes, A.H. Pádua, K.R. Seddon, L.P.N. Rebelo, 1-Alkyl-3-methylimidazolium alkanesulfonate ionic liquids, [CnH2n + 1mim][CkH2k + 1SO3]: synthesis and physicochemical properties, Phys. Chem. Chem. Phys. 11 (2009) 8939. [27] H. Matsumto, H. Sakaebe, K. Tatsumi, M. Kikuta, E. Ishiko, M. Kono, Fast cycling of Li/ LiCoO2 cell with low-viscosity ionic liquids based on bis(fluorosulfonyl)imide [FSI], J. Power Sources 160 (2006) 1308. [28] P. Bonhôte, A.P. Dias, N. Papageorgiou, K. Kalyanasundaram, M. Grätzel, Hydrophobic, highly conductive ambient-temperature molten salts, Inorg. Chem. 35 (1996) 1168. [29] E.I. Cooper, E.J.M. O'Sullivan, in: R.J. Gale, G. Blomgren, H. Kojima (Eds.), Proceedings of the 8th International Symposium on Molten Salts, The Electrochemical Society, New Jersey, 1992.

52

R. Leones et al. / Solid State Ionics 300 (2017) 46–52

[30] M. Taige, T.J.S. Schubert, Physico-chemical properties of ionic liquids – part II, Ionic Liquids Today, Iolitec GmbH, Heilbronn, 2011. [31] E.H. Cha, S.A. Lim, J.H. Park, D.W. Kim, D.R. Macfarlane, Ionic conductivity studies of gel polyelectrolyte based on ionic liquid, J. Power Sources 178 (2008) 779. [32] M.G. Freire, A.R.R. Teles, M.A.A. Rocha, B. Schroder, C.M.S.S. Neves, P.J. Carvalho, D.V. Evtuguin, L.M.N.B.F. Santos, J.A.P. Coutinho, Thermophysical characterization of ionic liquids able to dissolve biomass, J. Chem. Eng. Data 56 (2011) 4813. [33] G. Sun, K. Li, C. Sun, Application of 1-ethyl-3-methylimidazolium thiocyanate to the electrolyte of electrochemical double layer capacitors, J. Power Sources 162 (2006) 1444. [34] A.W.T. King, A. Parviainen, P. Karhunen, J. Matikainen, L.K.J. Hauru, H. Sixta, I. Kilpelainen, Relative and inherent reactivities of imidazolium-based ionic liquids, RSC Adv. 2 (2012) 8020. [35] W.M.D.W. Normazlan, N.A. Sairi, Y. Alias, A.F. Udaiyappan, A. Jouyban, M. Khoubnasabjafari, Composition and temperature dependence of density, surface tension, and viscosity of EMIM DEP/MMIM DMP + water + 1-propanol/2-propanol ternary mixtures and their mathematical representation using the Jouyban-Acree model, J. Chem. Eng. Data 59 (2014) 2337. [36] Y. Gong, C. Shen, Y.Z. Lu, H. Meng, C.X. Li, Viscosity and density measurements for six binary mixtures of water (methanol or ethanol) with and ionic liquid ([BMIM][DMP] or [EMIM][DMP]) at atmospheric pressure in the temperature range of (293.15 to 333.15) K, J. Chem. Eng. Data 57 (2012) 33. [37] A.B. Pereiro, J.M.M. Araujo, F.S. Oliveira, C.E.S. Bernardes, J.M.S.S. Esperanca, J.N.C. Lopes, I.M. Marrucho, L.P.N. Rebelo, Inorganic salts in purely ionic liquid media: the development of high iconicity ionic liquids (HIILs), Chem. Commun. 48 (2012) 3656. [38] J.D. Holbrey, W.M. Reichert, R.P. Swatloski, G.A. Broker, W.R. Pitner, K.R. Seddon, R.D. Rogers, Efficient, halide free synthesis of new, low cost ionic liquids: 1,3-dialkylimidazolium salts containing methyl- and ethyl-sulfate anions, Green Chem. 4 (2002) 407. [39] A.J.L. Costa, J.M.S.S. Esperança, I.M. Marrucho, L.P.N. Rebelo, Densities and viscosities of 1-ethyl-3-methylimidazolium n-alkyl sulfates, J. Chem. Eng. Data 56 (2011) 3433. [40] B.E. Mbondo Tsamba, S. Sarraute, M. Traïkia, P. Husson, Transport properties and ionic association in pure imidazolium-based ionic liquids as a function of temperature, J. Chem. Eng. Data 59 (2014) 1747. [41] R. Leones, F. Sentanin, S.C. Nunes, J.M.S.S. Esperança, M.C. Gonçalves, A. Pawlicka, V. de Zea Bermudez, M.M. Silva, Effect of the alkyl chain length of the ionic liquid anion on polymer electrolytes properties, Electrochim. Acta 184 (2015) 171. [42] M.G. Freire, C.M.S.S. Neves, P.J. Carvalho, R.L. Gardas, A.M. Fernandes, I.M. Marrucho, L.M.N.B.F. Santos, J.A.P. Coutinho, Mutual solubilities of water and hydrophobic ionic liquids, J. Phys. Chem. B 111 (2007) 13082. [43] A. Fernández, J.S. Torrecilla, J. García, F. Rodríguez, Thermophysical properties of 1ethyl-3-methylimidazolium ethylsulfate and 1-butyl-3-methylimidazolium methylsulfate ionic liquids, J. Chem. Eng. Data 52 (2007) 1979. [44] P. Hong, S.D. Li, C.Y. Ou, C.P. Li, L. Yang, C.H. Zhang, Thermogravimetric analysis of chitosan, J. Appl. Polym. Sci. 105 (2007) 547. [45] M.J. Smith, C.J. Silva, M.M. Silva, The study of a lanthanum triflate based polymer electrolyte using electrochemical and thermal techniques, Solid State Ionics 60 (1993) 73. [46] M. Kaya, T. Baran, S. Erdogan, A. Mentes, M.A. Ozusaglam, Y.S. Cakmak, Physicochemical comparison of chitin and chitosan obtained from larvae and adult Colorado potato beetle (Leptinotarsa decemlineata), Mater. Sci. Eng., C 45 (2014) 72. [47] N. Subhapradha, P. Ramasamy, V. Shanmugam, P. Madeswaran, A. Srinivasan, A. Shanmugam, Physicochemical characterization of β-chitosan from Sepioteuthis lessoniana gladius, Food Chem. 141 (2013) 907. [48] L. Rami, S. Malaise, S. Delmond, J.C. Fricain, R. Siadous, S. Schlaubitz, E. Laurichesse, J. Amedee, A. Montembault, L. David, L. Bordenave, Physicochemical modulation of chitosan-based hydrogels induces different biological responses: interest for tissue engineering, J. Biomed. Mater. Res. A 102 (2014) 3666. [49] M.N.V.R. Kumar, R.A.A. Muzzarelli, C. Muzzarelli, H. Sashiwa, A.J. Domb, Chitosan chemistry and pharmaceutical perspectives, Chem. Rev. 104 (2004) 6017.

[50] P.K. Singh, B. Bhattacharya, R.K. Nagarale, S.P. Pandey, K.W. Kim, H.W. Rhee, Ionic liquid doped poly(N-methyl-4-vinylpyridine iodide) solid polymer electrolyte for dye-sensitized solar cell, Synth. Met. 160 (2010) 950. [51] P.K. Singh, B. Bhattacharya, R.K. Nagarale, K.W. Kim, H.W. Rhee, Synthesis, characterization and application of biopolymer-ionic liquid composite membranes, Synth. Met. 160 (2010) 139. [52] R. Zhang, Y. Chen, R. Montazami, Ionic liquid-doped gel polymer electrolyte for flexible lithium-ion polymer batteries, Materials 8 (2015) 2735. [53] A.S. Shaplov, E.I. Lozinskaya, D.O. Ponkratov, I.A. Malyshkina, F. Vidal, P.-H. Aubert, O.V. Okatova, G.M. Pavlov, L.I. Komarova, C. Wandrey, Y.S. Vygodskii, Bis(trifluoromethylsulfonyl)amide based “polymeric ionic liquids”: synthesis, purification and peculiarities of structure–properties relationships, Electrochim. Acta 57 (2011) 74. [54] G.T. Kim, G.B. Appetecchi, M. Carewska, M. Joost, A. Balducci, M. Winter, S. Passerini, UV cross-linked, lithium-conducting ternary polymer electrolytes containing ionic liquids, J. Power Sources 195 (2010) 6130. [55] D.R. MacFarlane, J. Golding, S. Forsyth, M. Forsyth, G.B. Deacon, Low viscosity ionic liquids based on organic salts of the dicyanamide anion, Chem. Commun. 16 (2001) 1430. [56] D.R. MacFarlane, S.A. Forsyth, J. Golding, G.B. Deacon, Ionic liquids based on imidazolium, ammonium and pyrrolidinium salts of the dicyanamide anion, Green Chem. 4 (2002) 444. [57] S. Tsuzuki, K. Hayamizu, S. Seki, Origin of the low-viscosity of [emim][(FSO2)2N] ionic liquid and its lithium salt mixture: experimental and theoretical study of self-diffusion coefficients, conductivities, and intermolecular interactions, J. Phys. Chem. B 114 (2010) 16329. [58] P.K. Singh, K.W. Kim, H.W. Rhee, Development and characterization of ionic liquid doped solid polymer electrolyte membranes for better efficiency, Synth. Met. 159 (2009) 1538. [59] P.K. Singh, K.W. Kim, H.W. Rhee, Ionic liquid (1-methyl-3-propyl imidazolium iodide) with polymer electrolyte for DSSC application, Polym. Eng. Sci. 49 (2009) 862. [60] Y. Ma, L.B. Li, G.X. Gao, X.Y. Yang, J. You, P.X. Yang, Ionic conductivity enhancement in gel polymer electrolyte membrane with N-methyl-N-butyl-piperidinebis(trifluoromethylsulfonyl) imide ionic liquid for lithium ion battery, Colloids Surf. A Physicochem. Eng. Asp. 502 (2016) 130. [61] P.K. Singh, K.W. Kim, R.K. Nagarale, H.W. Rhee, Preparation, characterization and application of ionic liquid doped solid polymer electrolyte membranes, J. Phys. D. Appl. Phys. 42 (2009) 125101. [62] M.H. Buraidah, L.P. Teo, S.R. Majid, A.K. Arof, Ionic conductivity by correlated barrier hopping in NH4I doped chitosan solid electrolyte, Phys. B Condens. Matter 404 (2009) 1373. [63] N.A. Aziz, S.R. Majid, A.K. Arof, Synthesis and characterizations of phthaloyl chitosan-based polymer electrolytes, J. Non-Cryst. Solids 358 (2012) 1581. [64] Z. Osman, Z.A. Ibrahim, A.K. Arof, Conductivity enhancement due to ion dissociation in plasticized chitosan based polymer electrolytes, Carbohydr. Polym. 44 (2001) 167. [65] L.M. Li, S. Yang, S. Dong Fang, Novel polymeric ionic liquid membranes as solid polymer electrolytes with high ionic conductivity at moderate temperature, J. Membr. Sci. 366 (2011) 245. [66] N. Sudhakar, M. Selvakumar, Lithium perchlorate doped plasticized chitosan and starch blend as biodegradable polymer electrolyte for supercapacitors, Electrochim. Acta 78 (2012) 398. [67] L.S. Ng, A.A. Mohamad, Protonic battery based on a plasticized chitosan-NH4NO3 solid polymer electrolyte, J. Power Sources 163 (2006) 382. [68] A. Pawlicka, R.I. Mattos, C.E. Tambelli, I.D.A. Silva, C.J. Magon, J.P. Donoso, Magnetic resonance study of chitosan bio-membranes with proton conductivity properties, J. Membr. Sci. 429 (2013) 190. [69] M.H. Khanmirzaei, S. Ramesh, K. Ramesh, Polymer electrolyte based dye-sensitized solar cell with rice starch and 1-methyl-3-propylimidazolium iodide ionic liquid, Mater. Des. 85 (2015) 833.