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Plasticized pectin-based gel electrolytes
Electrochimica Acta 54 (2009) 6479–6483
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Plasticized pectin-based gel electrolytes Juliana R. Andrade, Ellen Raphael, Agnieszka Pawlicka ∗ IQSC, Universidade de São Paulo, C.P. 780, CEP 13560-970, São Carlos, SP, Brazil
a r t i c l e
i n f o
Article history: Received 17 March 2009 Received in revised form 30 May 2009 Accepted 30 May 2009 Available online 11 June 2009 Keywords: Gel electrolyte Ionic conductivity Pectin
a b s t r a c t Pectin is a natural polymer present in plants and, as all natural polymers has biodegradation properties. Chemically, pectin is a polysaccharide composed of a linear chain of 1→4 linked galacturonic acids, which is esterified with methanol at 80%. The pectin-based gel electrolytes in a transparent film form were obtained by a plasticization process with glycerol and addition of LiClO4 . The films showed good ionic conductivity results, which increased from 10−5 S/cm for the samples with 37 wt.% of glycerol to 4.7 × 10−4 S/cm at room temperature for the sample with 68 wt.% of glycerol. The electrochemical behaviors of the samples were studied by electrochemical impedance spectroscopy (EIS), and Nyquist graphs are showed and discussed. The obtained pectin-based samples also presented good adherence to the glass, flexibility, homogeneity (SEM) and transparency (about 70% in the vis) properties. They are good candidates to be applied as gel electrolytes in electrochromic devices. © 2009 Elsevier Ltd. All rights reserved.
1. Introduction Researchers have focused on the development of new devices, which are small, light, safe and cheap. The demand of the market is intense: the number of mobile phones, laptops, audio players and portable media has grown vigorously in recent years and the prevision is to grow much more due to the fast development of the telecommunication industry. Another intense research related to the devices is the production, storage and distribution of energy preferentially at low cost. Therefore, a development of new materials to replace traditional metals, ceramics and synthetic polymers is essential, as these materials are obtained from finite sources. One of the possibilities is then to invent materials from renewable sources and in this context, natural polymers can become very interesting substitutes for synthetic polymers. Among natural polymers, polysaccharides and proteins are very attractive due to their abundance in environment, and natural chemical structure differences, which give different properties. In recent years, surveys have emphasized the development of solid polymer electrolytes (SPEs) as they offer some advantages under liquid electrolytes, such as higher temperatures of operation, no flowing and corrosion after damage, ease of application to electrochemical devices and, in some cases, low cost. As already described in many papers and books, the SPEs are solid or gel ionconducting membranes consisting of a salt dispersed in a polymer matrix which can lead ions to the coordination of the same groups of the polymer chain [1–3]. The mark of the research in the domain
∗ Corresponding author. Fax: +55 16 3373 9952. E-mail address: [email protected] (A. Pawlicka). 0013-4686/$ – see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2009.05.098
of polymer electrolytes was the Wright’s work published in 1975 about the ionic conductivity in the PEO/Na+ complex [4]. Then Armand [5] observed the potential of these materials and proposed the application of ESPs in lithium batteries. From that, every effort has been in favor of improving the ionic conductivity of ESPs without losing the polymeric materials’ behaviors and consolidating the study of polymer electrolytes in the U.S., Japan and Europe [6]. As described elsewhere, among different polymer electrolytes, principally based on poly(ethylene oxide) matrices, ionic conducting membranes can also be obtained by chemical or physical modification of natural polymers or their derivatives, such as hydroxyethyl cellulose (HEC) [7], starch [8], chitosan [9–11], natural rubber [12,13] or gelatin [14]. This article proposes a new ionic conducting system based on pectin, which is also a natural polysaccharide composed of different entities and varies according to the method of extraction, raw material, location, and other environmental factors. Pectin occurs in terrestrial plants and is abundant in vegetables and fruits; it is usually added in food products and has beneficial effects for health, helping to improve digestion and decreasing the concentration of cholesterol. Most of the pectin is obtained from peels of citrus fruits (lemon, orange, lime, and occasionally grapefruit). The pectin gels easily in the presence of dissolved sugar and low pH. Chemically, pectin is a heterogeneous polymer and structurally, its molecules are composed of a linear chain of (1→4)-␣-d-galacturonic acid. Some of the galacturonic acid units are esterified with methyl groups, as showed in Fig. 1, and form the polygalacturonic acid, which can be esterified to metoxyl or have free acid groups. The commercial pectin usually has an esterification degree (DE) of 50%, which gives the mark for commercial pectin classification as high (HM) and low (LM) methoxilated [15].
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J.R. Andrade et al. / Electrochimica Acta 54 (2009) 6479–6483
Fig. 1. Representative chemical formula of pectin.
In the present paper, the pectin-based gel electrolytes were prepared and characterized by AC impedance and FTIR spectroscopic studies. The influences on both the ionic conductivity of plasticizer concentration and the temperature have been studied. The surface morphology of the films was characterized by SEM, the structure of the films was examined by X-ray diffraction measurements and the transmittance of the films was measured by UV–vis spectroscopy. 2. Experimental 2.1. Gel electrolytes The electrolytes were prepared according to the following formula: 0.6 g of commercial pectin (BRS-Z from CP Kelco Limeira S.A.), different quantities of glycerol (0–2.0 g; 0–70 wt.%) as plasticizer and 0.24 g of LiClO4 salt were dispersed in 20 mL of water and heated at 70 ◦ C under magnetic stirring for a few minutes up to complete dissolution. The solution was then placed on a glass plate and left to dry for 48 h in an oven at 50 ◦ C to obtain 2.2 × 10−2 to 4.2 × 10−2 cm thick membranes, which were kept under vacuum. 2.2. Characterization techniques Impedance spectroscopy measurements were used to determine the electrolyte ionic conductivity and its frequency behavior. A 2 cm round and 0.5 mm thick piece of the electrolyte was pressed against two steel electrodes. The system was installed in a glass cell under vacuum. The measurements were performed with an Autolab 30 instrument equipped with a FRA2 module, applying a voltage of 5 mV rms amplitude in the frequency range 106 Hz to 10 mHz. The structure of the film was examined by means of X-ray diffraction. The data were recorded using a Siemens D-5000 instrument with CuK␣ radiation. The thermal analyses were carried out with SHIMADZU DSC50 at nitrogen flow of 20 mL/min. The first run was performed up to 140 ◦ C at 20 ◦ C/min for the solvent elimination; the second and the third runs were in the temperature range from −100 to 120 ◦ C at 10 ◦ C/min. Thermogravimetry measurements (TGA) were performed with SHIMADZU TGA-50 equipment in the temperature range from 25 to 800 ◦ C in a nitrogen atmosphere (50 mL/min) at a heating rate of 10 ◦ C/min. The UV–vis optical spectra of the electrolytes were recorded with an Agilent Spectrophotometer Instrument between 200 and 1100 nm and the FTIR spectra were obtained with BOMEM-MB 102. The SEM measurements were studied by LEO model 440. 3. Results and discussion Polymer electrolytes are developed aiming at the substitution of liquid electrolytes in several electrochemical devices. For most of these devices a very interesting feature is the transparency, which is easily obtained with liquid electrolytes, but not so easily with solid electrolytes. However, some of polymer electrolytes, principally those based on natural polymers, can be obtained in the thin membrane forms with very good transparency in the visible range of electromagnetic spectrum [14]. Also, as can be observed in Fig. 2,
Fig. 2. UV–vis spectra of pectin-based ionic conducting membranes containing LiClO4 and 37 wt.%, 68 wt.% and 70 wt.% of glycerol.
pectin-based ionic conducting membranes show the optical transmittance in the 200–1100 nm range, which increases in function of the wavelength from zero in the UV region at 280 nm to 85% depending on the sample in the vis–NIR region. As observed in this figure the best transmittance values were obtained for the sample plasticized with 68 wt.% and the worst transmittance values with the sample plasticized with 70 wt.% of glycerol. The reason for such a big difference can be due to the exudation process, already observed in HEC-based samples and containing high quantity of plasticizer [7,16], and not to the thicknesses of the membranes, which were almost similar, i.e. 4.2 × 10−2 and 3.9 × 10−2 cm for the samples with 68 and 70 wt.% of glycerol, respectively. These good transparency results up to 1100 nm are very similar to other SPEs samples based on natural polymers as chitosan [11]. The Nyquist plots used to obtain ionic conductivity values of the samples plasticized with 37 and 70 wt.% of glycerol at room temperature and 80 ◦ C are shown in Fig. 3. As can be seen in Fig. 3a, the region of semicircle at high frequencies in the complex plane corresponding to the electrolyte resistance and from which it is possible to obtain ionic conductivity values is very small and shows very low real resistance. The sample plasticized with 37 wt.% of glycerol showed a decrease in the semicircle with the increase of temperature from 23 to 80 ◦ C (insert, Fig. 3a). It can also be observed in this figure that at high frequencies, the Z values go up to zero, which is not the case of the sample plasticized with 70 wt.% of glycerol (Fig. 3b), as it shows a small resistance at high frequencies (insert, Fig. 3b). Munichandraiah et al. [17] observed a similar impedance response for the Li/PEO8 –LiClO4 /Li cell at 80 ◦ C and from the equivalent circuit figure caption they stated that high-frequency resistance without capacitance at high temperature is due to the SPE film. They also observed two semicircles at room temperature and following Thevenin and Muller [18] they discarded the solid electrolyte resistance (SEI) model. The authors of this paper also presented equivalent circuits of a resistance in series with electrolyte resistance with capacitance in parallel. Contrarily, Strauss et al. [19], in very similar case, when the semicircle does not finish at zero attributed a medium-frequency response to the SEI even with two semicircles present. As can be stated from all these measurements, the capacitance values in a high-frequency region, where the semicircle does not finish at zero, are very low and probably exist; however they can be seen in GHz frequency, which was not possible due to the limitation of the MHz frequency equipment. In our case the impedance measurements for the sample with 37 and 70 wt.% of glycerol show difference at high frequencies, which is probably due to the electrode–membrane contact as a consequence of exudation process and confirming the UV–vis results (Fig. 2). Also in this
J.R. Andrade et al. / Electrochimica Acta 54 (2009) 6479–6483
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Fig. 4. Arrhenius plot of pectin-based electrolytes for the samples with different glycerol concentrations (37–70 wt.%).
Fig. 3. Nyquist diagrams of pectin-based ionic conducting membranes containing LiClO4 and 37 wt.% (a) and 70 wt.% (b) of glycerol.
case the increase of temperature promotes a decrease of the real membrane resistance, hence an increase in the ionic conductivity. The temperature-dependent ionic conductivity measurements were then considered to analyze the possible mechanism of ionic conduction in these systems, as well as the ionic conducting stability as a function of temperature. Fig. 4 shows different mechanism of ionic conduction depending on the plasticizer content. For small quantities of glycerol, i.e. 37 and 54 wt.%, there is a linear increase in the ionic conductivity from room temperature to 80 ◦ C, indicating an Arrhenius-type dependence meaning that there is neither a phase transition in the polymer matrix nor a domain formed by the addition of lithium salt. Such results are very similar to the ones of other polysaccharides-based ionic conducting membranes [8]. The ionic conductivity values for both samples were 2.9 × 10−5 and 1.6 × 10−4 S/cm at 23 ◦ C and 1.6 × 10−4 and 1.5 × 10−3 S/cm at 80 ◦ C, respectively. Due to the increase of more than 60 wt.% in the plasticizer concentration it seems that the mechanism of ionic conduction changes to the VTF model, where the ionic transport
is promoted by the polymeric chain movement. In this case, the presence of high plasticizer contents probably promotes a better separation of polymeric chains and, consequently, its more pronounced movements. Comparing with samples with low plasticizer content, an increase in the ionic conductivity values of up to 5 × 10−4 S/cm at 23 ◦ C and 5.4 × 10−3 S/cm at 80 ◦ C is observed. The ionic conductivity measurements as a function of applied dc potential at room temperature for the sample plasticized with 60 and 70 wt.% of glycerol revealed a stability window of 3 V, i.e. in the range of −1.5 to 1.5 V. In the liquid or solid state, the carboxylic acids exist as dimers, due to strong hydrogen bonds [20]. The spectroscopic analyses in the IR region (Fig. 5) of carboxylic acids groups present in the pectin structure reveal an intense and very large absorption of OH stretching in the region of 3300–2500 cm−1 . In this region glycerol hydroxyl groups can also show a stretching vibration. Therefore, the IR peaks observed in this region may be associated with OH groups of acid, glycerol and adsorbed moisture molecules [21] present in pectin gel electrolyte-based samples, as showed in Fig. 5. The band at 2940 cm−1 can be attributed to the stretching vibration of CH2 [22] and CH3 group of the methyl ester [21]. The band at 1740 cm−1 can be attributed to the stretching of C O of COOCH3 and the bands at 1630–1640 and 1420 cm−1 correspond, respectively, to the symmetric and asymmetric stretching vibration of carboxylate ion (COO− ). In the region of 1200–950 cm−1 , the polysaccharides have a strong absorption, called “fingerprint”, characteristic for each
Fig. 5. FTIR spectra of pectin powder and pectin-based ionic conducting membranes containing LiClO4 and 37 wt.% and 70 wt.% of glycerol.
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J.R. Andrade et al. / Electrochimica Acta 54 (2009) 6479–6483 Table 1 Theoretical and experimental chemical analysis values of pectin.
Fig. 6. X-ray diffraction of pure and pectin-based ionic conducting membranes containing LiClO4 and 37 wt.%, 64 wt.% and 70 wt.% of glycerol.
polymer [22]. Following Gnanasambandam and Proctor [23], these bands are frequently difficult to interpret, however it was stated that this region is independent of pectin source and may be used to the identify galacturonic acid [21]. Similar results were obtained by Kaczmarek et al. [24] for the biodegradable samples of blends of pectin and poly(ethylene oxide). Fig. 6 shows typical X-ray diffraction patterns obtained for the pristine pectin and pectin-based electrolytes containing three different glycerol concentrations. The pristine pectin diffractogram reveals a semi-crystalline polymer structure with two principal diffraction peaks, at 2 of ca. 13◦ and of ca. 21◦ . This result is very similar to the results obtained by Lutz et al. [25] for apple pectin. The diffractograms of pectin-based electrolytes show broad diffuse bands centered at about 2 = 21◦ with a small shoulder at 2 = 9◦ for the sample with 37 wt.% of glycerol. Comparing with other natural polymer-based electrolytes again in this case the addition of plasticizer insures the amorphous character of the samples [7]. The TGA measurements showed in Fig. 7 evidences a 10–15% loss of mass from ambient temperature to 120 ◦ C of pure pectin and pectin-based samples, respectively, which can be attributed to free water loss. At this point it should be stated that both glycerol and lithium perchlorate are hydrophilic substances and their presence promotes an increased water absorption capacity of the film [16]. In the region of 200–260 ◦ C, a pure pectin sample showed a very accentuated mass loss of 40%, which can be attributed to the
Fig. 7. Thermal analysis (TG) of pure pectin and pectin-based ionic conducting membranes containing LiClO4 and 37 wt.% and 70 wt.% of glycerol.
Pectin
N (%)
C (%)
H (%)
S (%)
O (%)
Theoretical Experimental of the sample BRS-Z
0 0.53
43.49 38.14
4.97 6.33
0 0
51.54 55.00 (calculated)
decomposition, and continues slowly with the increase of temperature up to 800 ◦ C. The remaining residue was 14%. In the case of the ionic conducting samples, the loss mass behavior as a function of temperature is quite different. The sample degradation starts at 180 ◦ C and occurs in three stages, which can be due to the interactions of lithium salt and glycerol with polysaccharide chain. The 70% weight loss ends at 250 ◦ C and practically there are no more changes up to 800 ◦ C. The 5–9% remaining residues are probably due to the lithium carbonate formation as in the case of LiCMC and NaCMC and their grafted products [26]. In the same figure (Fig. 7) it can be also noted that at 800 ◦ C the residues of 14% of the pure pectin sample are greater than those of the modified samples, as observed by Machado et al. [26]. It can be also explained by the chemical analysis (Table 1), where carbon quantity is lower and hydrogen quantity is higher than the calculated ones. It means that pure pectin probably contain some impurities, which are removed during the gel sample preparation. From the DSC measurements (not showed here), it was stated that the glass transition temperature (Tg) of the pectin-based ionic conducting samples is −51 ◦ C for the sample plasticized with
Fig. 8. SEM pictures of pectin-based ionic conducting membranes containing LiClO4 and 37 wt.% (1000×) (a) and 70 wt.% (500×) (b) of glycerol.
J.R. Andrade et al. / Electrochimica Acta 54 (2009) 6479–6483
37 wt.% of glycerol, and decreases to −65 ◦ C for the sample plasticized with 68 and 70 wt.% of glycerol. These results compared to the ones of hydroxyethyl cellulose plasticized samples [7] reveal that for the similar plasticizer quantities the pectin-based samples show higher ionic conductivity one order of magnitude and Tg values of almost 30◦ . An example is the sample with 36 wt.% of glycerol, where for the HEC-based sample ([O]/[Li] = 6), the ionic conductivity value at room temperature was 8.7 × 01−6 S/cm and Tg = −80 ◦ C and for the pectin-based sample plasticized with 37 wt.% of glycerol the value was = 2.9 × 10−5 S/cm and Tg = −51 ◦ C. SEM pictures of pectin-based electrolytes with 37 wt.% of glycerol reveal good homogeneity without any phase separation (Fig. 8a). SEM micrograph of the sample plasticized with 70 wt.% of glycerol (Fig. 8b) show also good homogeneity with few dark lines, which can be due to the exudation process occurring in the samples, as observed by X-ray diffractograms, and a decrease in the transparency in the vis region. Also these analyzed samples, as other natural polymer-based electrolytes samples, were translucent and showed very good adhesion properties to glass and steel. 4. Conclusions Gel electrolytes based on plasticized pectin and LiClO4 were prepared and analyzed by spectroscopic, thermal, structural and microscopic analyses. The ionic conductivity results as a function of temperature obey an Arrhenius or VTF relationship, depending on the plasticizer contents. The best ionic conductivity values of 4.7 × 10−4 S/cm at 30 ◦ C and 6.3 × 10−3 S/cm at 80 ◦ C were obtained for the samples plasticized with 68 wt.% of glycerol, which showed the transmittance values of 80% in the vis range. Thermal analyses revealed that all analyzed samples are stable up to 150 ◦ C and 10% of weight loss is due to the adsorbed water. The samples were predominantly amorphous and SEM surface visualization evidenced a uniform surface morphology of the samples with lower than 70 wt.% of glycerol concentration. All the samples showed good adhesion to the glass and steel and are very promising materials to be used as solid electrolytes in electrochromic devices.
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Acknowledgments The authors are indebted to FAPESP, CNPq, CAPES, for the financial support given to this research. References [1] F.M. Gray, Solid Polymer Electrolytes Fundamentals and Technological Applications, VCH Publishers Inc., 1991. [2] J.R. Mac Callum, C.A. Vincent, Polymer Electrolyte Reviews-1, Elsevier Applied Science Publishers Ltd., London and New York, 1987. [3] P.G. Bruce, C.A. Vincent, J. Chem. Soc. Faraday Trans. 89 (1993) 3187. [4] P.V. Wright, Br. Polym. J. 7 (1975) 319. [5] M.B. Armand, J.M. Chabagno, M. Duclot, Second International Meeting on Solid Electrolytes, St. Andrews, Scotland, 1978, p. 20 (extended abstracts); Poly-ethers as solid electrolytes, in: P. Vashishta, J.N. Mundy, G.K. Shenoy (Eds.), Fast Ion Transport in Solids, Elsevier North Holland Inc., 1979, p. 131. [6] K. Murata, S. IzuchI, Y. Yoshihisa, Electrochim. Acta 45 (2000) 1501. [7] G.O. Machado, H. Ferreira, A. Pawlicka, Electrochim. Acta 50 (2005) 3827. [8] A. Pawlicka, A.C. Sabadini, E. Raphael, D.C. Dragunski, Mol. Cryst. Liq. Cryst. 485 (2008) 56 [804]. [9] S. Fuentes, P.J. Retuert, G. Gonzalez, Electrochim. Acta 53 (2007) 1417. [10] S.R. Majid, A.K. Arof, Physica B 390 (2007) 209. [11] A. Pawlicka, M. Danczuk, W. Wieczorek, E. Zygadlo-Monikowska, J. Phys. Chem. A 112 (2008) 8888. [12] S.N. Mohamed, N.A. Johari, A.M.M. Ali, M.K. Harun, M.Z.A. Yahya, J. Power Sources 183 (2008) 351. [13] A.M.M. Ali, R.H.Y. Subban, H. Bahron, T. Winie, F. Latif, M.Z.A. Yahya, Ionics 14 (2008) 491. [14] D.F. Vieira, C.O. Avellaneda, A. Pawlicka, Electrochim. Acta 53 (2007) 1404. [15] Genu Pectin, www.cpkelco.com, access on February, 2009. [16] G.O. Machado, R.E. Prud’homme, A. Pawlicka, e-Polymers 115 (2007). [17] N. Munichandraiah, L.G. Scanlon, R.A. Marsh, J. Power Sources 72 (1998) 203. [18] J.G. Thevenin, R.H. Muller, J. Electrochem. Soc. 134 (1987) 273. [19] E. Strauss, D. Golodnitsky, G. Ardel, E. Peled, Electrochim. Acta 43 (1998) 1315. [20] R.M. Silverstein, G.C. Bassler, T.C. Morrill, Identificac¸ão espectrométrica de compostos orgânicos, 5◦ ed., LTC, Rio de Janeiro, 1994. [21] M.A. Monsoor, U. Kalapathyl, A. Proctor, Food Chem. 74 (2001) 233. [22] G.D. Manrique, F.M. Lajolo, Postharvest Biol. Technol. 25 (2002) 99. [23] R. Gnanasambandam, A. Proctor, Food Chem. 68 (2000) 327. [24] H. Kaczmarek, K. Bajer, P. Galka, B. Kotnowska, Polym. Degrad. Stab. 92 (2007) 2058. [25] R. Lutz, A. Aserin, L. Wicker, N. Garti, Food Hydrocolloids 23 (2009) 786. [26] G.O. Machado, A.M. Regiani, A. Pawlicka, Polimery 48 (2003) 273.
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Plasticized pectin-based gel electrolytes Juliana R. Andrade, Ellen Raphael, Agnieszka Pawlicka ∗ IQSC, Universidade de São Paulo, C.P. 780, CEP 13560-970, São Carlos, SP, Brazil
a r t i c l e
i n f o
Article history: Received 17 March 2009 Received in revised form 30 May 2009 Accepted 30 May 2009 Available online 11 June 2009 Keywords: Gel electrolyte Ionic conductivity Pectin
a b s t r a c t Pectin is a natural polymer present in plants and, as all natural polymers has biodegradation properties. Chemically, pectin is a polysaccharide composed of a linear chain of 1→4 linked galacturonic acids, which is esterified with methanol at 80%. The pectin-based gel electrolytes in a transparent film form were obtained by a plasticization process with glycerol and addition of LiClO4 . The films showed good ionic conductivity results, which increased from 10−5 S/cm for the samples with 37 wt.% of glycerol to 4.7 × 10−4 S/cm at room temperature for the sample with 68 wt.% of glycerol. The electrochemical behaviors of the samples were studied by electrochemical impedance spectroscopy (EIS), and Nyquist graphs are showed and discussed. The obtained pectin-based samples also presented good adherence to the glass, flexibility, homogeneity (SEM) and transparency (about 70% in the vis) properties. They are good candidates to be applied as gel electrolytes in electrochromic devices. © 2009 Elsevier Ltd. All rights reserved.
1. Introduction Researchers have focused on the development of new devices, which are small, light, safe and cheap. The demand of the market is intense: the number of mobile phones, laptops, audio players and portable media has grown vigorously in recent years and the prevision is to grow much more due to the fast development of the telecommunication industry. Another intense research related to the devices is the production, storage and distribution of energy preferentially at low cost. Therefore, a development of new materials to replace traditional metals, ceramics and synthetic polymers is essential, as these materials are obtained from finite sources. One of the possibilities is then to invent materials from renewable sources and in this context, natural polymers can become very interesting substitutes for synthetic polymers. Among natural polymers, polysaccharides and proteins are very attractive due to their abundance in environment, and natural chemical structure differences, which give different properties. In recent years, surveys have emphasized the development of solid polymer electrolytes (SPEs) as they offer some advantages under liquid electrolytes, such as higher temperatures of operation, no flowing and corrosion after damage, ease of application to electrochemical devices and, in some cases, low cost. As already described in many papers and books, the SPEs are solid or gel ionconducting membranes consisting of a salt dispersed in a polymer matrix which can lead ions to the coordination of the same groups of the polymer chain [1–3]. The mark of the research in the domain
∗ Corresponding author. Fax: +55 16 3373 9952. E-mail address: [email protected] (A. Pawlicka). 0013-4686/$ – see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2009.05.098
of polymer electrolytes was the Wright’s work published in 1975 about the ionic conductivity in the PEO/Na+ complex [4]. Then Armand [5] observed the potential of these materials and proposed the application of ESPs in lithium batteries. From that, every effort has been in favor of improving the ionic conductivity of ESPs without losing the polymeric materials’ behaviors and consolidating the study of polymer electrolytes in the U.S., Japan and Europe [6]. As described elsewhere, among different polymer electrolytes, principally based on poly(ethylene oxide) matrices, ionic conducting membranes can also be obtained by chemical or physical modification of natural polymers or their derivatives, such as hydroxyethyl cellulose (HEC) [7], starch [8], chitosan [9–11], natural rubber [12,13] or gelatin [14]. This article proposes a new ionic conducting system based on pectin, which is also a natural polysaccharide composed of different entities and varies according to the method of extraction, raw material, location, and other environmental factors. Pectin occurs in terrestrial plants and is abundant in vegetables and fruits; it is usually added in food products and has beneficial effects for health, helping to improve digestion and decreasing the concentration of cholesterol. Most of the pectin is obtained from peels of citrus fruits (lemon, orange, lime, and occasionally grapefruit). The pectin gels easily in the presence of dissolved sugar and low pH. Chemically, pectin is a heterogeneous polymer and structurally, its molecules are composed of a linear chain of (1→4)-␣-d-galacturonic acid. Some of the galacturonic acid units are esterified with methyl groups, as showed in Fig. 1, and form the polygalacturonic acid, which can be esterified to metoxyl or have free acid groups. The commercial pectin usually has an esterification degree (DE) of 50%, which gives the mark for commercial pectin classification as high (HM) and low (LM) methoxilated [15].
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J.R. Andrade et al. / Electrochimica Acta 54 (2009) 6479–6483
Fig. 1. Representative chemical formula of pectin.
In the present paper, the pectin-based gel electrolytes were prepared and characterized by AC impedance and FTIR spectroscopic studies. The influences on both the ionic conductivity of plasticizer concentration and the temperature have been studied. The surface morphology of the films was characterized by SEM, the structure of the films was examined by X-ray diffraction measurements and the transmittance of the films was measured by UV–vis spectroscopy. 2. Experimental 2.1. Gel electrolytes The electrolytes were prepared according to the following formula: 0.6 g of commercial pectin (BRS-Z from CP Kelco Limeira S.A.), different quantities of glycerol (0–2.0 g; 0–70 wt.%) as plasticizer and 0.24 g of LiClO4 salt were dispersed in 20 mL of water and heated at 70 ◦ C under magnetic stirring for a few minutes up to complete dissolution. The solution was then placed on a glass plate and left to dry for 48 h in an oven at 50 ◦ C to obtain 2.2 × 10−2 to 4.2 × 10−2 cm thick membranes, which were kept under vacuum. 2.2. Characterization techniques Impedance spectroscopy measurements were used to determine the electrolyte ionic conductivity and its frequency behavior. A 2 cm round and 0.5 mm thick piece of the electrolyte was pressed against two steel electrodes. The system was installed in a glass cell under vacuum. The measurements were performed with an Autolab 30 instrument equipped with a FRA2 module, applying a voltage of 5 mV rms amplitude in the frequency range 106 Hz to 10 mHz. The structure of the film was examined by means of X-ray diffraction. The data were recorded using a Siemens D-5000 instrument with CuK␣ radiation. The thermal analyses were carried out with SHIMADZU DSC50 at nitrogen flow of 20 mL/min. The first run was performed up to 140 ◦ C at 20 ◦ C/min for the solvent elimination; the second and the third runs were in the temperature range from −100 to 120 ◦ C at 10 ◦ C/min. Thermogravimetry measurements (TGA) were performed with SHIMADZU TGA-50 equipment in the temperature range from 25 to 800 ◦ C in a nitrogen atmosphere (50 mL/min) at a heating rate of 10 ◦ C/min. The UV–vis optical spectra of the electrolytes were recorded with an Agilent Spectrophotometer Instrument between 200 and 1100 nm and the FTIR spectra were obtained with BOMEM-MB 102. The SEM measurements were studied by LEO model 440. 3. Results and discussion Polymer electrolytes are developed aiming at the substitution of liquid electrolytes in several electrochemical devices. For most of these devices a very interesting feature is the transparency, which is easily obtained with liquid electrolytes, but not so easily with solid electrolytes. However, some of polymer electrolytes, principally those based on natural polymers, can be obtained in the thin membrane forms with very good transparency in the visible range of electromagnetic spectrum [14]. Also, as can be observed in Fig. 2,
Fig. 2. UV–vis spectra of pectin-based ionic conducting membranes containing LiClO4 and 37 wt.%, 68 wt.% and 70 wt.% of glycerol.
pectin-based ionic conducting membranes show the optical transmittance in the 200–1100 nm range, which increases in function of the wavelength from zero in the UV region at 280 nm to 85% depending on the sample in the vis–NIR region. As observed in this figure the best transmittance values were obtained for the sample plasticized with 68 wt.% and the worst transmittance values with the sample plasticized with 70 wt.% of glycerol. The reason for such a big difference can be due to the exudation process, already observed in HEC-based samples and containing high quantity of plasticizer [7,16], and not to the thicknesses of the membranes, which were almost similar, i.e. 4.2 × 10−2 and 3.9 × 10−2 cm for the samples with 68 and 70 wt.% of glycerol, respectively. These good transparency results up to 1100 nm are very similar to other SPEs samples based on natural polymers as chitosan [11]. The Nyquist plots used to obtain ionic conductivity values of the samples plasticized with 37 and 70 wt.% of glycerol at room temperature and 80 ◦ C are shown in Fig. 3. As can be seen in Fig. 3a, the region of semicircle at high frequencies in the complex plane corresponding to the electrolyte resistance and from which it is possible to obtain ionic conductivity values is very small and shows very low real resistance. The sample plasticized with 37 wt.% of glycerol showed a decrease in the semicircle with the increase of temperature from 23 to 80 ◦ C (insert, Fig. 3a). It can also be observed in this figure that at high frequencies, the Z values go up to zero, which is not the case of the sample plasticized with 70 wt.% of glycerol (Fig. 3b), as it shows a small resistance at high frequencies (insert, Fig. 3b). Munichandraiah et al. [17] observed a similar impedance response for the Li/PEO8 –LiClO4 /Li cell at 80 ◦ C and from the equivalent circuit figure caption they stated that high-frequency resistance without capacitance at high temperature is due to the SPE film. They also observed two semicircles at room temperature and following Thevenin and Muller [18] they discarded the solid electrolyte resistance (SEI) model. The authors of this paper also presented equivalent circuits of a resistance in series with electrolyte resistance with capacitance in parallel. Contrarily, Strauss et al. [19], in very similar case, when the semicircle does not finish at zero attributed a medium-frequency response to the SEI even with two semicircles present. As can be stated from all these measurements, the capacitance values in a high-frequency region, where the semicircle does not finish at zero, are very low and probably exist; however they can be seen in GHz frequency, which was not possible due to the limitation of the MHz frequency equipment. In our case the impedance measurements for the sample with 37 and 70 wt.% of glycerol show difference at high frequencies, which is probably due to the electrode–membrane contact as a consequence of exudation process and confirming the UV–vis results (Fig. 2). Also in this
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Fig. 4. Arrhenius plot of pectin-based electrolytes for the samples with different glycerol concentrations (37–70 wt.%).
Fig. 3. Nyquist diagrams of pectin-based ionic conducting membranes containing LiClO4 and 37 wt.% (a) and 70 wt.% (b) of glycerol.
case the increase of temperature promotes a decrease of the real membrane resistance, hence an increase in the ionic conductivity. The temperature-dependent ionic conductivity measurements were then considered to analyze the possible mechanism of ionic conduction in these systems, as well as the ionic conducting stability as a function of temperature. Fig. 4 shows different mechanism of ionic conduction depending on the plasticizer content. For small quantities of glycerol, i.e. 37 and 54 wt.%, there is a linear increase in the ionic conductivity from room temperature to 80 ◦ C, indicating an Arrhenius-type dependence meaning that there is neither a phase transition in the polymer matrix nor a domain formed by the addition of lithium salt. Such results are very similar to the ones of other polysaccharides-based ionic conducting membranes [8]. The ionic conductivity values for both samples were 2.9 × 10−5 and 1.6 × 10−4 S/cm at 23 ◦ C and 1.6 × 10−4 and 1.5 × 10−3 S/cm at 80 ◦ C, respectively. Due to the increase of more than 60 wt.% in the plasticizer concentration it seems that the mechanism of ionic conduction changes to the VTF model, where the ionic transport
is promoted by the polymeric chain movement. In this case, the presence of high plasticizer contents probably promotes a better separation of polymeric chains and, consequently, its more pronounced movements. Comparing with samples with low plasticizer content, an increase in the ionic conductivity values of up to 5 × 10−4 S/cm at 23 ◦ C and 5.4 × 10−3 S/cm at 80 ◦ C is observed. The ionic conductivity measurements as a function of applied dc potential at room temperature for the sample plasticized with 60 and 70 wt.% of glycerol revealed a stability window of 3 V, i.e. in the range of −1.5 to 1.5 V. In the liquid or solid state, the carboxylic acids exist as dimers, due to strong hydrogen bonds [20]. The spectroscopic analyses in the IR region (Fig. 5) of carboxylic acids groups present in the pectin structure reveal an intense and very large absorption of OH stretching in the region of 3300–2500 cm−1 . In this region glycerol hydroxyl groups can also show a stretching vibration. Therefore, the IR peaks observed in this region may be associated with OH groups of acid, glycerol and adsorbed moisture molecules [21] present in pectin gel electrolyte-based samples, as showed in Fig. 5. The band at 2940 cm−1 can be attributed to the stretching vibration of CH2 [22] and CH3 group of the methyl ester [21]. The band at 1740 cm−1 can be attributed to the stretching of C O of COOCH3 and the bands at 1630–1640 and 1420 cm−1 correspond, respectively, to the symmetric and asymmetric stretching vibration of carboxylate ion (COO− ). In the region of 1200–950 cm−1 , the polysaccharides have a strong absorption, called “fingerprint”, characteristic for each
Fig. 5. FTIR spectra of pectin powder and pectin-based ionic conducting membranes containing LiClO4 and 37 wt.% and 70 wt.% of glycerol.
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J.R. Andrade et al. / Electrochimica Acta 54 (2009) 6479–6483 Table 1 Theoretical and experimental chemical analysis values of pectin.
Fig. 6. X-ray diffraction of pure and pectin-based ionic conducting membranes containing LiClO4 and 37 wt.%, 64 wt.% and 70 wt.% of glycerol.
polymer [22]. Following Gnanasambandam and Proctor [23], these bands are frequently difficult to interpret, however it was stated that this region is independent of pectin source and may be used to the identify galacturonic acid [21]. Similar results were obtained by Kaczmarek et al. [24] for the biodegradable samples of blends of pectin and poly(ethylene oxide). Fig. 6 shows typical X-ray diffraction patterns obtained for the pristine pectin and pectin-based electrolytes containing three different glycerol concentrations. The pristine pectin diffractogram reveals a semi-crystalline polymer structure with two principal diffraction peaks, at 2 of ca. 13◦ and of ca. 21◦ . This result is very similar to the results obtained by Lutz et al. [25] for apple pectin. The diffractograms of pectin-based electrolytes show broad diffuse bands centered at about 2 = 21◦ with a small shoulder at 2 = 9◦ for the sample with 37 wt.% of glycerol. Comparing with other natural polymer-based electrolytes again in this case the addition of plasticizer insures the amorphous character of the samples [7]. The TGA measurements showed in Fig. 7 evidences a 10–15% loss of mass from ambient temperature to 120 ◦ C of pure pectin and pectin-based samples, respectively, which can be attributed to free water loss. At this point it should be stated that both glycerol and lithium perchlorate are hydrophilic substances and their presence promotes an increased water absorption capacity of the film [16]. In the region of 200–260 ◦ C, a pure pectin sample showed a very accentuated mass loss of 40%, which can be attributed to the
Fig. 7. Thermal analysis (TG) of pure pectin and pectin-based ionic conducting membranes containing LiClO4 and 37 wt.% and 70 wt.% of glycerol.
Pectin
N (%)
C (%)
H (%)
S (%)
O (%)
Theoretical Experimental of the sample BRS-Z
0 0.53
43.49 38.14
4.97 6.33
0 0
51.54 55.00 (calculated)
decomposition, and continues slowly with the increase of temperature up to 800 ◦ C. The remaining residue was 14%. In the case of the ionic conducting samples, the loss mass behavior as a function of temperature is quite different. The sample degradation starts at 180 ◦ C and occurs in three stages, which can be due to the interactions of lithium salt and glycerol with polysaccharide chain. The 70% weight loss ends at 250 ◦ C and practically there are no more changes up to 800 ◦ C. The 5–9% remaining residues are probably due to the lithium carbonate formation as in the case of LiCMC and NaCMC and their grafted products [26]. In the same figure (Fig. 7) it can be also noted that at 800 ◦ C the residues of 14% of the pure pectin sample are greater than those of the modified samples, as observed by Machado et al. [26]. It can be also explained by the chemical analysis (Table 1), where carbon quantity is lower and hydrogen quantity is higher than the calculated ones. It means that pure pectin probably contain some impurities, which are removed during the gel sample preparation. From the DSC measurements (not showed here), it was stated that the glass transition temperature (Tg) of the pectin-based ionic conducting samples is −51 ◦ C for the sample plasticized with
Fig. 8. SEM pictures of pectin-based ionic conducting membranes containing LiClO4 and 37 wt.% (1000×) (a) and 70 wt.% (500×) (b) of glycerol.
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37 wt.% of glycerol, and decreases to −65 ◦ C for the sample plasticized with 68 and 70 wt.% of glycerol. These results compared to the ones of hydroxyethyl cellulose plasticized samples [7] reveal that for the similar plasticizer quantities the pectin-based samples show higher ionic conductivity one order of magnitude and Tg values of almost 30◦ . An example is the sample with 36 wt.% of glycerol, where for the HEC-based sample ([O]/[Li] = 6), the ionic conductivity value at room temperature was 8.7 × 01−6 S/cm and Tg = −80 ◦ C and for the pectin-based sample plasticized with 37 wt.% of glycerol the value was = 2.9 × 10−5 S/cm and Tg = −51 ◦ C. SEM pictures of pectin-based electrolytes with 37 wt.% of glycerol reveal good homogeneity without any phase separation (Fig. 8a). SEM micrograph of the sample plasticized with 70 wt.% of glycerol (Fig. 8b) show also good homogeneity with few dark lines, which can be due to the exudation process occurring in the samples, as observed by X-ray diffractograms, and a decrease in the transparency in the vis region. Also these analyzed samples, as other natural polymer-based electrolytes samples, were translucent and showed very good adhesion properties to glass and steel. 4. Conclusions Gel electrolytes based on plasticized pectin and LiClO4 were prepared and analyzed by spectroscopic, thermal, structural and microscopic analyses. The ionic conductivity results as a function of temperature obey an Arrhenius or VTF relationship, depending on the plasticizer contents. The best ionic conductivity values of 4.7 × 10−4 S/cm at 30 ◦ C and 6.3 × 10−3 S/cm at 80 ◦ C were obtained for the samples plasticized with 68 wt.% of glycerol, which showed the transmittance values of 80% in the vis range. Thermal analyses revealed that all analyzed samples are stable up to 150 ◦ C and 10% of weight loss is due to the adsorbed water. The samples were predominantly amorphous and SEM surface visualization evidenced a uniform surface morphology of the samples with lower than 70 wt.% of glycerol concentration. All the samples showed good adhesion to the glass and steel and are very promising materials to be used as solid electrolytes in electrochromic devices.
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Acknowledgments The authors are indebted to FAPESP, CNPq, CAPES, for the financial support given to this research. References [1] F.M. Gray, Solid Polymer Electrolytes Fundamentals and Technological Applications, VCH Publishers Inc., 1991. [2] J.R. Mac Callum, C.A. Vincent, Polymer Electrolyte Reviews-1, Elsevier Applied Science Publishers Ltd., London and New York, 1987. [3] P.G. Bruce, C.A. Vincent, J. Chem. Soc. Faraday Trans. 89 (1993) 3187. [4] P.V. Wright, Br. Polym. J. 7 (1975) 319. [5] M.B. Armand, J.M. Chabagno, M. Duclot, Second International Meeting on Solid Electrolytes, St. Andrews, Scotland, 1978, p. 20 (extended abstracts); Poly-ethers as solid electrolytes, in: P. Vashishta, J.N. Mundy, G.K. Shenoy (Eds.), Fast Ion Transport in Solids, Elsevier North Holland Inc., 1979, p. 131. [6] K. Murata, S. IzuchI, Y. Yoshihisa, Electrochim. Acta 45 (2000) 1501. [7] G.O. Machado, H. Ferreira, A. Pawlicka, Electrochim. Acta 50 (2005) 3827. [8] A. Pawlicka, A.C. Sabadini, E. Raphael, D.C. Dragunski, Mol. Cryst. Liq. Cryst. 485 (2008) 56 [804]. [9] S. Fuentes, P.J. Retuert, G. Gonzalez, Electrochim. Acta 53 (2007) 1417. [10] S.R. Majid, A.K. Arof, Physica B 390 (2007) 209. [11] A. Pawlicka, M. Danczuk, W. Wieczorek, E. Zygadlo-Monikowska, J. Phys. Chem. A 112 (2008) 8888. [12] S.N. Mohamed, N.A. Johari, A.M.M. Ali, M.K. Harun, M.Z.A. Yahya, J. Power Sources 183 (2008) 351. [13] A.M.M. Ali, R.H.Y. Subban, H. Bahron, T. Winie, F. Latif, M.Z.A. Yahya, Ionics 14 (2008) 491. [14] D.F. Vieira, C.O. Avellaneda, A. Pawlicka, Electrochim. Acta 53 (2007) 1404. [15] Genu Pectin, www.cpkelco.com, access on February, 2009. [16] G.O. Machado, R.E. Prud’homme, A. Pawlicka, e-Polymers 115 (2007). [17] N. Munichandraiah, L.G. Scanlon, R.A. Marsh, J. Power Sources 72 (1998) 203. [18] J.G. Thevenin, R.H. Muller, J. Electrochem. Soc. 134 (1987) 273. [19] E. Strauss, D. Golodnitsky, G. Ardel, E. Peled, Electrochim. Acta 43 (1998) 1315. [20] R.M. Silverstein, G.C. Bassler, T.C. Morrill, Identificac¸ão espectrométrica de compostos orgânicos, 5◦ ed., LTC, Rio de Janeiro, 1994. [21] M.A. Monsoor, U. Kalapathyl, A. Proctor, Food Chem. 74 (2001) 233. [22] G.D. Manrique, F.M. Lajolo, Postharvest Biol. Technol. 25 (2002) 99. [23] R. Gnanasambandam, A. Proctor, Food Chem. 68 (2000) 327. [24] H. Kaczmarek, K. Bajer, P. Galka, B. Kotnowska, Polym. Degrad. Stab. 92 (2007) 2058. [25] R. Lutz, A. Aserin, L. Wicker, N. Garti, Food Hydrocolloids 23 (2009) 786. [26] G.O. Machado, A.M. Regiani, A. Pawlicka, Polimery 48 (2003) 273.