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Electrochemical impedance studies of hybrids of perfluorosulfonic acid ionomer and silicon oxide by sol-gel reaction from solution
Journal of Electroanalytical Chemistry 445 (1998) 39 – 45
Electrochemical impedance studies of hybrids of perfluorosulfonic acid ionomer and silicon oxide by sol-gel reaction from solution R.A. Zoppi *, S.P. Nunes Instituto de Quı´mica, Uni6ersidade Estadual de Campinas, C. Postal 6154, CEP 13083 -970, Campinas, SP, Brazil Received 4 April 1997; received in revised form 18 July 1997
Abstract Hybrids of Nafion® and silica were prepared from solution, growing the inorganic phase by hydrolysis/condensation of alkoxy silanes. Using tetraethoxysilane (TEOS) as the inorganic precursor, transparent and rigid films were obtained. Substituting part of the TEOS (20 wt% substitution) by 1,1,3,3 tetramethyl-1,3-diethoxydisiloxane (TMDES) more flexible films were obtained. These films were translucent and showed a phase segregation which was clearly observed by transmission electron microscopy. The ionic conductivity of the hybrids was measured by electrochemical impedance spectroscopy using two stainless steel electrodes, a frequency range of 0.1 to 105 Hz, and temperatures from 25 to 100°C. Samples were also characterized by modulated differential scanning calorimetry. © 1998 Elsevier Science S.A. All rights reserved. Keywords: Ionic conductivity; Nafion®; Hybrid materials; Composites
1. Introduction The preparation and characterization of hybrid materials [1,2] and their application as gas separation membranes [3] and as solid electrolytes [4] are subjects of current work in our laboratory. Hybrid films have been formed from mixed organic polymer + tetraethoxysilane (TEOS) solutions. Poly(methylmethacrylate), silicone rubber, poly(amide-6-b-ethylene oxide) and poly(ethylene oxide-co-epichlorhydrin) have been investigated as organic matrices. Recently, Nafion®/TEOS/ 1,1,3,3 tetramethyl-1,3-diethoxydisiloxane (TMDES) hybrids were prepared and characterized by dynamicmechanical analysis, differential scanning calorimetry, wide angle X-ray diffractometry and transmission electron microscopy [5]. The hybrids were formed by growing the inorganic phase in a 5 wt% propanol+ Nafion® solution [5]. Nafion® is a perfluorosulfonate ionomer with a backbone similar to Teflon® and pendant sul* Corresponding author. Tel.: +55 192 397012; fax: + 55 192 393805; e-mail: [email protected] 0022-0728/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved. PII S 0 0 2 2 - 0 7 2 8 ( 9 7 ) 0 0 5 1 3 - 5
fonic acid groups. This polymer has been very successful in the field of ionic exchange membranes due to its high chemical resistance and excellent chemical inertness. As a result of their favorable properties, intensive studies have been made to understand the transport properties of Nafion® membranes [6–9]. Nafion® has received much attention due also to its use as an electrolyte in the polymer-electrolyte-membrane fuel cell, which is of great interest for electric vehicle propulsion [10]. In the field of solid electrolytes, studies of poly(ethylene oxide)/LiX (X is a generic anion) complexes demonstrate very clearly that ion conductivity is present in the amorphous phase and it depends on the mobility of the polymer chain segments, as characterized by Tg. The ion carrier concentration depends not only on the salt concentration but also on its degree of dissociation, which is related to the dielectric constant of the polymer. Different procedures of modification of the solvating polymers have been described to optimize the Li-conducting polymer electrolyte’s applications at room temperature [11]. Crystallinity and Tg reduction
40
R.A. Zoppi, S.P. Nunes / Journal of Electroanalytical Chemistry 445 (1998) 39–45
can be achieved using chemical methods (for example, copolymerization of ethylene oxide and different sequences of the ethylene oxide) [12]), or using structural (for example, chemical or physical reticulation of poly(ethylene oxide) [13,14]) and physical approaches (for example, the utilization of plasticizers [15,16]). With respect to hybrid materials, a silica network can improve the thermal and mechanical properties and it can also increase the amorphous content. The hybrid formation can be an alternative procedure to modify the polymer properties and to improve the ionic conductivity of polymeric electrolytes. In this study electrochemical impedance spectroscopy was used to investigate the conductive properties as a function of the composition of Nafion®/TEOS/TMDES systems, and indirectly, as a function of the presence of more flexible siloxane segments into the inorganic part of the hybrid material. 2. Experimental
2.3. Transmission electron microscopy Thin films were obtained in a FC4E Ultracut LeitzReichert-Jung ultramicrotome at − 80°C with a diamond knife, and observed through a Zeiss CEM-902 transmission electron microscope.
2.4. X-ray spectroscopy/scanning electron microscopy Samples were fractured in liquid nitrogen and coated with carbon by sputtering. The fracture surfaces were observed through a JSM T-300 Jeol scanning electron microscope equipped with an energy dispersive spectrometer (EDS) which allowed for the chemical analysis of different regions in the sample.
2.5. Thermal analysis Differential scanning calorimetry analysis was carried out in a 2910 MDSC TA Instruments thermal analyzer from −100 to 300°C, at 5°C min − 1, under nitrogen.
2.1. Sample preparation The 5 wt% propanol+ Nafion® solution was supplied by Du Pont with the trade name Nafion 1100 EW and it contains 35 wt% iso-propanol, 55 wt% n-propanol and 10 wt% water (the solvent composition was determined by gas chromatography). Different volumes of tetraethoxysilane (Aldrich) were added to the 5 wt% propanol+Nafion® solution with stirring. A 0.15 M HCl aqueous solution was added to promote acid hydrolysis of TEOS. The solution was further stirred for 16 h at room temperature (25°C) then kept for 24 h without stirring and transferred to Teflon® Petri dishes (5 ml of the solution in a 5 cm diameter dish). Glass dishes were not used due to undesirable sample adhesion with some compositions. The dishes were kept inside a desiccator saturated with propanol vapor. The desiccator was kept semi-opened to allow slow solvent evaporation over 1 week. The samples were further dried under vacuum at 30°C for 1 week before characterization. Part of the TEOS was substituted by up to 20 wt% 1,1,3,3 tetramethyl-1,3-diethoxy disiloxane (Hu¨lls).
2.2. Electrochemical impedance spectroscopy Electrochemical impedance measurements were carried out in a dry box under argon. The hybrid films were pressed between two stainless steel electrodes, and packed in a button cell (0.785 cm2 area). The measurements were performed using a 1255HF Schlumberger Solartron frequency response analyzer connected to a 273 PAR potentiostat which was interfaced to a computer. The range of frequencies analyzed was 10 − 1 to 105 Hz. The temperature range was from 25 to 100°C.
3. Results and discussion As described earlier, Nafion® is an ionomer with a polymeric backbone similar to Teflon® with pendant sulfonic acid groups. The ionic groups are known to form hydrophilic clusters in the chemically stable hydrophobic matrix. Using transmission electron microscopy, small (about 3 nm diameter) spherical clusters were observed regularly distributed all over a pure Nafion® film (Fig. 1). Fig. 2 shows impedance plots for pure Nafion® films at different temperatures. The Nyquist diagram for pure Nafion® showed two semicircles when the sample was analyzed at 25 or 45°C. The occurrence of two semicircles suggests the possibility of inter- and intra-cluster ionic migration. A similar statement was proposed by Bruce and West [17] for other granular systems. Considering Nafion® as a heterogeneous electrolyte, where individual grains are in contact with each other at the grain boundaries, the a.c. response may then be decomposed into two parallel RC elements representing the intra- and inter-granular regions. The capacitance associated with the grain boundary regions is always larger than the bulk capacitance because the grain boundary regions are thinner than the grains; usually migration across grain boundaries is greatly restricted and hence the intergrain resistances are larger than the intragrain resistances despite the grain boundary regions being thinner. Then, two semicircles may be observed for granular electrolytes [17], the first one (i.e. that one at high frequencies) being assigned to the bulk capacitance and bulk resistance (intra-cluster ionic migration), and the second one being assigned to the capacitance and resis-
R.A. Zoppi, S.P. Nunes / Journal of Electroanalytical Chemistry 445 (1998) 39–45
41
tance associated with the grain boundary regions (intercluster ionic migration). The Nyquist diagrams obtained at 25 and 45°C were fitted using an R(RQ)(RQ) equivalent circuit. R represents resistances and Q represents constant phase ele-
Fig. 2. Impedance plots for pure Nafion® films at 25 ( × ), 45 () and 60°C ( + ). The full lines represent the fit using the Nonlinear Least Square Fit program developed by B.A. Boukamp [27].
ments, CPE. The data could not be modeled by any equivalent circuit composed entirely of frequency-independent components. The CPE had to be included in order to explain circular arcs whose centers lay below the real axis. The CPE is generally believed to originate from a distribution of the current density along the electrode surface as a result of surface inhomogeneity [18]. Table 1 shows the bulk capacitances (Cb), bulk resistances (Rb), and capacitances and resistances associated with the grain boundary regions (Cgb and Rgb) for pure Nafion®. Both Rb and Rgb decreased when the temperature changed from 25 to 45°C; but the resistance associated with the grain boundary regions (Rgb) decreased much more, almost one order of magnitude, when the temperature increased. This behavior indicates that intercluster ionic migration was facilitated. For temperatures higher than 45°C, the two semicircles disappeared and the separation between both inter- and intra-cluster ionic migration processes could not be identified. When the inorganic phase was introduced into Nafion® matrices, drastic changes in the impedance behavior and morphology were observed, as shown in Figs. 1 and 3. The preparation of hybrid membranes constituted of Nafion® has been intensively investigated by Mauritz and co-workers [19–22]. In these cases, Nafion® is Table 1 Cb, Rb, Cgb and Rgb values for Nafion® at 25 and 45°C
Fig. 1. Transmission electron microscopy of (a) a pure Nafion® film and Nafion®/TEOS/TMDES hybrids with compositions equal to (b) 50/47.5/2.5 or (c) 50/40/10.
Parameter
T= 25°C
T=45°C
Rb/V Cb/F Rgb/V Cgb/F
3.4×104 1.6×10−7 1.2×105 3.7×10−6
2.0×104 1.9×10−7 6.6×104 4.1×10−6
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R.A. Zoppi, S.P. Nunes / Journal of Electroanalytical Chemistry 445 (1998) 39–45
Fig. 3. Impedance plots for Nafion®/TEOS/TMDES hybrids with compositions () 50/40/10, (+ ) 50/47.5/2.5 or (× ) 50/50/0 at 25°C. The full lines represent the fit using the Nonlinear Least Square Fit program developed by B.A. Boukamp [27].
supplied in insoluble films, which swell in polar solvents. The films are swollen in water+ alcohol solutions and further immersed in alcohol+ metal alkoxy solutions. By using this procedure, asymmetric silicon oxide or zirconium oxide composition profiles along a direction perpendicular to the plane of Nafion® films were created via in situ sol-gel reactions for one-sided metal alkoxy solution permeation [19,20]. Small-angle X-ray scattering studies of these membranes established that the original morphology of unfilled Nafion® persisted even after its invasion by the sol-gel inorganic phase and showed that chemical compositional variance was affected within clusters [22]. On the other hand, using the procedure described here, the inorganic polymer growth is not limited to the already existent clusters, but the clusters themselves are formed simultaneously with the inorganic polymerization reaction and solvent evaporation. The formability of the ionic domains can be affected and, hybrids with different hydration and conduction properties can be obtained. The morphology is also a function of TEOS and TMDES contents, as shown in Fig. 1. In Fig. 1, when 50 wt% TEOS was added to 50 wt% Nafion®, about 1.2 mm wide stripes could be differentiated. When part of the TEOS was substituted by TMDES, the films which were originally transparent, became turbid with TMDES substitutions higher than 10 wt%. A phase separation could be detected for samples with 10, 15 or 20 wt% substitution, as shown in Fig. 1c. In samples with 5 wt% substitution, no turbid films were obtained and the morphology was very similar to that shown in Fig. 1b for 50/47.5/2.5 Nafion®/TEOS/TMDES hybrids. Using EDS analysis, the presence of Si in the hybrid films was confirmed (Fig. 4). For 50/50/0 or 50/47.5/2.5
Nafion®/TEOS/TMDES hybrids, the EDS analysis carried out across the film thickness showed that the inorganic phase was homogeneously dispersed, with a Si peak area/S peak area ratio in the range 2.9 to 3.0. For hybrids with high TMDES content (Fig. 4b and c), it was verified that the presence of Si in domains was predominant and Nafion® was practically excluded, due to the higher hydrophobicity. Beside the changes in the morphology, different behaviors with respect to the conductive properties were detected, as shown in the impedance plots (Fig. 3). The impedance spectra for Nafion®/TEOS/TMDES systems could be represented by an R(RQ)(R)(Q) equivalent circuit. For 50/40/10 Nafion®/TEOS/TMDES hybrids, a semicircle in the high frequency region followed by a straight line in the medium and low frequency region was clearly observed. In this case, the contribution of a bulk capacitance associated with the dielectric polarization of the polymer chains caused by the alternating field (Cb), could be verified. At high frequencies, the impedance of the bulk resistance (Rb) and capacitance (Cb) were of the same magnitude, both contributing significantly to the overall impedance whereas the impedance of the electrode capacitance, Ce, was insignificant. Therefore, at high frequencies, the equiva-
Fig. 4. EDS analysis of Nafion®/TEOS/TMDES hybrids with composition (a) 50/47.5/2.5 and (b,c) 50/40/10 showing (b) the matrix and (c) the domains in the 50/40/10 Nafion®/TEOS/TMDES hybrid film.
R.A. Zoppi, S.P. Nunes / Journal of Electroanalytical Chemistry 445 (1998) 39–45 Table 2 Cb, Rb, Ce and ionic conductivity values for Nafion®/TEOS/TMDES systems at 25°C Hybrid composi- Rb/V tion 50/40/10 50/47.5/2.5 50/50/0
8.2×103 8.1×104 1.1×105
Cb/F
Ce/F
s/S cm−1
3.5×10−9 1.2×10−6 7.0×10−8
4.5×10−6 1.9×10−7 1.5×10−7
5.1×10−6 3.0×10−7 1.4×10−7
lent circuit reduces to a parallel RbCb combination which gives rise to the semicircle in the complex impedance plane. At these low frequencies Cb makes a negligible contribution to the impedance and a straight line appears. The low-frequency response carries information on the electrode electrolyte interface, and from any point on the straight line Ce =1/Z¦v, where Z¦ is the value of the imaginary part of the impedance at a frequency v. Rb is the effective d.c. resistance of the electrolyte and Ce is the electrode capacitance. The electrolyte ionic conductivity can be easily determined by s= l(RbA), where s is the ionic conductivity, Rb is the electrolyte resistance, A the area of the electrode and l the electrolyte thickness. Compared to 50/47.5/2.5 or 50/50/0, the 50/40/10 Nafion®/TEOS/TMDES systems showed the highest values for Ce and the lowest value for Rb (Table 2). Fletcher [23] affirmed that a necessary condition for the appearance of both the straight line in the low frequency region and the semicircular response at high frequencies is that Ce Cb, as was verified for the 50/40/10 Nafion®/TEOS/TMDES system. Tanguy et al. [24] have studied the capacitive and noncapacitive charge in polypyrrole films. They verified that the highest Ce values were obtained for highly doped polypyrrole samples (conductive-oxidized state). Ce was then related to the total charge in the polymer. Ce and s values in Table 2 show that 50/40/10 Nafion®/TEOS/ TMDES hybrids are the most conductive films. In Nafion®/TEOS/TMDES systems, the Nafion® content is always the same; so the amount of charge carriers would also be the same in these systems. Here, the high conductivity observed for 50/40/10 Nafion®/TEOS/ TMDES hybrids was related to the mobility of the charge carriers. As described in a previous paper, the dynamic-mechanical analysis gave information on the flexibility of Nafion®/TEOS/TMDES samples [5]. With the incorporation of silicon oxide in Nafion®, the hybrids became much more rigid compared to pure Nafion®. Substituting TEOS by TMDES and increasing the TMDES content, both the storage modulus (E%) and the dissipative modulus (E¦) decreased reaching values even lower than for pure Nafion® [5]. The role of TMDES was to increase flexibility and, indirectly, it promoted an increase in the hybrid conductivity.
43
Fig. 5 shows the ionic conductivity values obtained from Rb as a function of temperature for Nafion®/ TEOS/TMDES hybrids. For both 50/50/0 and 50/47.5/ 2.5 Nafion®/TEOS/TMDES systems, the conductivity reached a maximum value at 45°C (Fig. 5a). At this point it is interesting to note also the impedance behavior of pure Nafion® films at different temperatures (Fig. 2). For temperatures higher than 45°C no inter- and intra-cluster migration process could be detected for pure Nafion®. For 50/47.5/2.5 and 50/50/0 Nafion®/ TEOS/TMDES systems, maximum values of ionic conductivity were observed at 45°C. This behavior can be explained by considering the modulated differential scanning calorimetry analysis carried out for pure Nafion® (Fig. 6). Near 50°C, a reversible exothermic peak, with a DH value equal to 1.8 kJ mol − 1 can be observed. The order of magnitude of DH is very close to that verified in micellar reorganization processes (DH = 1 to 5 kJ mol − 1). Considering the ionic clusters as micellar structures, the reversible exothermic peak
Fig. 5. (a) Ionic conductivity as a function of temperature for Nafion®/TEOS/TMDES hybrids with compositions () 50/47.5/2.5 or ( ) 50/50/0. (b) The same for 50/40/10 Nafion®/TEOS/TMDES films. In plot (b), the full line represents the fit using the VTF equation.
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R.A. Zoppi, S.P. Nunes / Journal of Electroanalytical Chemistry 445 (1998) 39–45
Fig. 6. Modulated differential scanning calorimetry analysis for pure Nafion®.
could be related to a cluster reorganization, which could be responsible for the changes in the behavior observed in the impedance spectrum of pure Nafion® at temperatures higher than 45°C, and the maximum values of ionic conductivity at 45°C shown by both 50/ 47.5/2.5 and 50/50/0 Nafion®/TEOS/TMDES systems. For 50/40/10 Nafion®/TEOS/TMDES hybrids, the ionic conductivity increased exponentially with the temperature (Fig. 5b). In this particular case, a good agreement between the experimental curve and the fit according to the Vogel-Tamman-Fulcker [25] equation was observed. One of the parameters that can be obtained from the fit is the activation energy necessary for the redistribution of the free volume (DE), which is directly related to the energy barrier for ion hopping from one free volume site to another one. In the case of 50/40/10 Nafion®/TEOS/TMDES hybrids, the DE was 45 kJ mol − 1, which is in the range of DE values previously observed for poly(amide 6-b-ethylene oxide)/LiClO4 (from 20 to 30 wt% of salt) electrolytes [26]. For 50/40/10 Nafion®/TEOS/TMDES samples, the compositional segregation or phase separation gave films with different dielectric or charge transport properties from pure Nafion® or hybrids with TMDES substitution lower than 20 wt%.
4. Conclusions Hybrids of Nafion® and silica were prepared from solution, growing the inorganic phase by hydrolysis/ condensation of alkoxy silanes. Using TEOS as the inorganic precursor, transparent and rigid films were obtained. By substituting part of the TEOS with TMDES more flexible films were obtained. Phase separation occurred when the TMDES substitution was higher than 10 wt%.
For pure Nafion®, the inter- and intra-cluster ionic migration processes were identified in electrochemical impedance experiments carried out at temperatures lower than 50°C. At high temperatures, these two processes were not clearly identified. Maximum conductivity values were obtained for 50/47.5/2.5 and 50/50/0 Nafion®/TEOS/TMDES films when the measurement was carried out at 45°C. This behavior was assigned to a reversible exothermic transition related to cluster reorganization, which was observed in modulated DSC analysis. The highest conductivity values were obtained for 50/40/10 Nafion®/TEOS/TMDES hybrids (s = 3.5× 10 − 5 S cm − 1 at 30°C). In this case, as for polymeric electrolytes, the ionic conductivity temperature dependence followed the VTF equation. Here, the compositional segregation or phase separation allowes one to design a material with different charge transport properties.
Acknowledgements The authors thank FAPESP and CNPq for financial support and Dr M.I. Felisberti, Dr M.-A. De Paoli and C.M.N.P. Fonseca for helping with modulated DSC and electrochemical impedance measurements.
References [1] K.F. Silveira, I.V.P. Yoshida, S.P. Nunes, Polymer 36 (1995) 1425. [2] R.A. Zoppi, C.R. Castro, I.V.P. Yoshida, S.P. Nunes, Polymer 38 (1997) 5705. [3] S.P. Nunes, J. Schultz, K.V. Peinemann, J. Mater. Sci. Lett. 15 (1996) 1139. [4] R.A. Zoppi, C.M.N.P. Fonseca, M.-A. De Paoli, S.P. Nunes, Acta Polym. 48 (1997) 131. [5] R.A. Zoppi, I.V.P. Yoshida, S.P. Nunes, Polymer 39 (1997) 1309. [6] M.D. Osborne, H.H. Girault, J. Electroanal. Chem. 370 (1994) 287. [7] F. Opekar, D. Svozil, J. Electroanal. Chem. 385 (1995) 269. [8] Z. Samec, A. Thoja´nek, E. Samcova´, J. Electroanal. Chem. 389 (1995) 1. [9] J.J. Fontanella, M.C. Wintersgill, R.S. Chen, Y. Wu, S.G. Greenbaum, Electrochim. Acta 40 (1995) 2321. [10] Y. Sone, P. Ekdunge, D. Simonsson, J. Electrochem. Soc. 143 (1996) 1254. [11] M. Gauthier, M. Armand, D. Muller, in: T.A. Skotheim (Ed.), Electroresponsive Molecular and Polymeric Systems, vol. 1, Marcel Dekker, New York, 1988, pp. 41. [12] K. Nagaoka, H. Naruse, I. Shinohara, J. Polym. Sci. 22 (1984) 659. [13] A. Killis, J.F. LeNest, A. Gandini, H. Cheradame, J.P. CohenAddad, Solid State Ionics 14 (1984) 231. [14] M. Watanabe, K. Sanui, N. Ogata, Macromolecules 19 (1986) 815. [15] G. Feuillade, P. Perche, J. Appl. Electrochem. 5 (1975) 63. [16] R. Spindler, D.F. Shriver, Macromolecules 19 (1986) 327.
R.A. Zoppi, S.P. Nunes / Journal of Electroanalytical Chemistry 445 (1998) 39–45 [17] P.G. Bruce, A.R. West, J. Electrochem. Soc. 130 (1983) 662. [18] G.J. Brug, A.L.G. Van Den Eeden, M. Sluyters-Rehbach, J.H. Sluyters, J. Electroanal. Chem. 176 (1984) 275. [19] R.V. Gummaraju, R.B. Moore, K.A. Mauritz, J. Polym, Sci., B: Polym. Phys. 34 (1996) 2383. [20] W. Apichatachutapan, R.B. Moore, K.A. Mauritz, J. Appl. Polym. Sci. 62 (1996) 417. [21] Q. Deng, Y. Hu, R.B. Moore, C.L. McCormick, K.A. Mauritz, Chem. Mater. 9 (1997) 36.
.
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[22] Q. Deng, K.M. Cable, R.B. Moore, K.A. Mauritz, J. Polym, Sci., B: Polym. Phys. 34 (1996) 1917. [23] S. Fletcher, J. Chem. Soc. Faraday Trans. 89 (1993) 311. [24] J. Tanguy, N. Mermilliod, M. Hoclet, J. Electrochem. Soc. 134 (1987) 795. [25] C.A. Vincent, Prog. Solid State Chem. 17 (1987) 145. [26] R.A. Zoppi, C.M.N.P. Fonseca, M.-A. De Paoli, S.P. Nunes, Solid State Ionics 91 (1996) 123. [27] B.A. Boukamp, Solid State Ionics 20 (1986) 31.
Electrochemical impedance studies of hybrids of perfluorosulfonic acid ionomer and silicon oxide by sol-gel reaction from solution R.A. Zoppi *, S.P. Nunes Instituto de Quı´mica, Uni6ersidade Estadual de Campinas, C. Postal 6154, CEP 13083 -970, Campinas, SP, Brazil Received 4 April 1997; received in revised form 18 July 1997
Abstract Hybrids of Nafion® and silica were prepared from solution, growing the inorganic phase by hydrolysis/condensation of alkoxy silanes. Using tetraethoxysilane (TEOS) as the inorganic precursor, transparent and rigid films were obtained. Substituting part of the TEOS (20 wt% substitution) by 1,1,3,3 tetramethyl-1,3-diethoxydisiloxane (TMDES) more flexible films were obtained. These films were translucent and showed a phase segregation which was clearly observed by transmission electron microscopy. The ionic conductivity of the hybrids was measured by electrochemical impedance spectroscopy using two stainless steel electrodes, a frequency range of 0.1 to 105 Hz, and temperatures from 25 to 100°C. Samples were also characterized by modulated differential scanning calorimetry. © 1998 Elsevier Science S.A. All rights reserved. Keywords: Ionic conductivity; Nafion®; Hybrid materials; Composites
1. Introduction The preparation and characterization of hybrid materials [1,2] and their application as gas separation membranes [3] and as solid electrolytes [4] are subjects of current work in our laboratory. Hybrid films have been formed from mixed organic polymer + tetraethoxysilane (TEOS) solutions. Poly(methylmethacrylate), silicone rubber, poly(amide-6-b-ethylene oxide) and poly(ethylene oxide-co-epichlorhydrin) have been investigated as organic matrices. Recently, Nafion®/TEOS/ 1,1,3,3 tetramethyl-1,3-diethoxydisiloxane (TMDES) hybrids were prepared and characterized by dynamicmechanical analysis, differential scanning calorimetry, wide angle X-ray diffractometry and transmission electron microscopy [5]. The hybrids were formed by growing the inorganic phase in a 5 wt% propanol+ Nafion® solution [5]. Nafion® is a perfluorosulfonate ionomer with a backbone similar to Teflon® and pendant sul* Corresponding author. Tel.: +55 192 397012; fax: + 55 192 393805; e-mail: [email protected] 0022-0728/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved. PII S 0 0 2 2 - 0 7 2 8 ( 9 7 ) 0 0 5 1 3 - 5
fonic acid groups. This polymer has been very successful in the field of ionic exchange membranes due to its high chemical resistance and excellent chemical inertness. As a result of their favorable properties, intensive studies have been made to understand the transport properties of Nafion® membranes [6–9]. Nafion® has received much attention due also to its use as an electrolyte in the polymer-electrolyte-membrane fuel cell, which is of great interest for electric vehicle propulsion [10]. In the field of solid electrolytes, studies of poly(ethylene oxide)/LiX (X is a generic anion) complexes demonstrate very clearly that ion conductivity is present in the amorphous phase and it depends on the mobility of the polymer chain segments, as characterized by Tg. The ion carrier concentration depends not only on the salt concentration but also on its degree of dissociation, which is related to the dielectric constant of the polymer. Different procedures of modification of the solvating polymers have been described to optimize the Li-conducting polymer electrolyte’s applications at room temperature [11]. Crystallinity and Tg reduction
40
R.A. Zoppi, S.P. Nunes / Journal of Electroanalytical Chemistry 445 (1998) 39–45
can be achieved using chemical methods (for example, copolymerization of ethylene oxide and different sequences of the ethylene oxide) [12]), or using structural (for example, chemical or physical reticulation of poly(ethylene oxide) [13,14]) and physical approaches (for example, the utilization of plasticizers [15,16]). With respect to hybrid materials, a silica network can improve the thermal and mechanical properties and it can also increase the amorphous content. The hybrid formation can be an alternative procedure to modify the polymer properties and to improve the ionic conductivity of polymeric electrolytes. In this study electrochemical impedance spectroscopy was used to investigate the conductive properties as a function of the composition of Nafion®/TEOS/TMDES systems, and indirectly, as a function of the presence of more flexible siloxane segments into the inorganic part of the hybrid material. 2. Experimental
2.3. Transmission electron microscopy Thin films were obtained in a FC4E Ultracut LeitzReichert-Jung ultramicrotome at − 80°C with a diamond knife, and observed through a Zeiss CEM-902 transmission electron microscope.
2.4. X-ray spectroscopy/scanning electron microscopy Samples were fractured in liquid nitrogen and coated with carbon by sputtering. The fracture surfaces were observed through a JSM T-300 Jeol scanning electron microscope equipped with an energy dispersive spectrometer (EDS) which allowed for the chemical analysis of different regions in the sample.
2.5. Thermal analysis Differential scanning calorimetry analysis was carried out in a 2910 MDSC TA Instruments thermal analyzer from −100 to 300°C, at 5°C min − 1, under nitrogen.
2.1. Sample preparation The 5 wt% propanol+ Nafion® solution was supplied by Du Pont with the trade name Nafion 1100 EW and it contains 35 wt% iso-propanol, 55 wt% n-propanol and 10 wt% water (the solvent composition was determined by gas chromatography). Different volumes of tetraethoxysilane (Aldrich) were added to the 5 wt% propanol+Nafion® solution with stirring. A 0.15 M HCl aqueous solution was added to promote acid hydrolysis of TEOS. The solution was further stirred for 16 h at room temperature (25°C) then kept for 24 h without stirring and transferred to Teflon® Petri dishes (5 ml of the solution in a 5 cm diameter dish). Glass dishes were not used due to undesirable sample adhesion with some compositions. The dishes were kept inside a desiccator saturated with propanol vapor. The desiccator was kept semi-opened to allow slow solvent evaporation over 1 week. The samples were further dried under vacuum at 30°C for 1 week before characterization. Part of the TEOS was substituted by up to 20 wt% 1,1,3,3 tetramethyl-1,3-diethoxy disiloxane (Hu¨lls).
2.2. Electrochemical impedance spectroscopy Electrochemical impedance measurements were carried out in a dry box under argon. The hybrid films were pressed between two stainless steel electrodes, and packed in a button cell (0.785 cm2 area). The measurements were performed using a 1255HF Schlumberger Solartron frequency response analyzer connected to a 273 PAR potentiostat which was interfaced to a computer. The range of frequencies analyzed was 10 − 1 to 105 Hz. The temperature range was from 25 to 100°C.
3. Results and discussion As described earlier, Nafion® is an ionomer with a polymeric backbone similar to Teflon® with pendant sulfonic acid groups. The ionic groups are known to form hydrophilic clusters in the chemically stable hydrophobic matrix. Using transmission electron microscopy, small (about 3 nm diameter) spherical clusters were observed regularly distributed all over a pure Nafion® film (Fig. 1). Fig. 2 shows impedance plots for pure Nafion® films at different temperatures. The Nyquist diagram for pure Nafion® showed two semicircles when the sample was analyzed at 25 or 45°C. The occurrence of two semicircles suggests the possibility of inter- and intra-cluster ionic migration. A similar statement was proposed by Bruce and West [17] for other granular systems. Considering Nafion® as a heterogeneous electrolyte, where individual grains are in contact with each other at the grain boundaries, the a.c. response may then be decomposed into two parallel RC elements representing the intra- and inter-granular regions. The capacitance associated with the grain boundary regions is always larger than the bulk capacitance because the grain boundary regions are thinner than the grains; usually migration across grain boundaries is greatly restricted and hence the intergrain resistances are larger than the intragrain resistances despite the grain boundary regions being thinner. Then, two semicircles may be observed for granular electrolytes [17], the first one (i.e. that one at high frequencies) being assigned to the bulk capacitance and bulk resistance (intra-cluster ionic migration), and the second one being assigned to the capacitance and resis-
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41
tance associated with the grain boundary regions (intercluster ionic migration). The Nyquist diagrams obtained at 25 and 45°C were fitted using an R(RQ)(RQ) equivalent circuit. R represents resistances and Q represents constant phase ele-
Fig. 2. Impedance plots for pure Nafion® films at 25 ( × ), 45 () and 60°C ( + ). The full lines represent the fit using the Nonlinear Least Square Fit program developed by B.A. Boukamp [27].
ments, CPE. The data could not be modeled by any equivalent circuit composed entirely of frequency-independent components. The CPE had to be included in order to explain circular arcs whose centers lay below the real axis. The CPE is generally believed to originate from a distribution of the current density along the electrode surface as a result of surface inhomogeneity [18]. Table 1 shows the bulk capacitances (Cb), bulk resistances (Rb), and capacitances and resistances associated with the grain boundary regions (Cgb and Rgb) for pure Nafion®. Both Rb and Rgb decreased when the temperature changed from 25 to 45°C; but the resistance associated with the grain boundary regions (Rgb) decreased much more, almost one order of magnitude, when the temperature increased. This behavior indicates that intercluster ionic migration was facilitated. For temperatures higher than 45°C, the two semicircles disappeared and the separation between both inter- and intra-cluster ionic migration processes could not be identified. When the inorganic phase was introduced into Nafion® matrices, drastic changes in the impedance behavior and morphology were observed, as shown in Figs. 1 and 3. The preparation of hybrid membranes constituted of Nafion® has been intensively investigated by Mauritz and co-workers [19–22]. In these cases, Nafion® is Table 1 Cb, Rb, Cgb and Rgb values for Nafion® at 25 and 45°C
Fig. 1. Transmission electron microscopy of (a) a pure Nafion® film and Nafion®/TEOS/TMDES hybrids with compositions equal to (b) 50/47.5/2.5 or (c) 50/40/10.
Parameter
T= 25°C
T=45°C
Rb/V Cb/F Rgb/V Cgb/F
3.4×104 1.6×10−7 1.2×105 3.7×10−6
2.0×104 1.9×10−7 6.6×104 4.1×10−6
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R.A. Zoppi, S.P. Nunes / Journal of Electroanalytical Chemistry 445 (1998) 39–45
Fig. 3. Impedance plots for Nafion®/TEOS/TMDES hybrids with compositions () 50/40/10, (+ ) 50/47.5/2.5 or (× ) 50/50/0 at 25°C. The full lines represent the fit using the Nonlinear Least Square Fit program developed by B.A. Boukamp [27].
supplied in insoluble films, which swell in polar solvents. The films are swollen in water+ alcohol solutions and further immersed in alcohol+ metal alkoxy solutions. By using this procedure, asymmetric silicon oxide or zirconium oxide composition profiles along a direction perpendicular to the plane of Nafion® films were created via in situ sol-gel reactions for one-sided metal alkoxy solution permeation [19,20]. Small-angle X-ray scattering studies of these membranes established that the original morphology of unfilled Nafion® persisted even after its invasion by the sol-gel inorganic phase and showed that chemical compositional variance was affected within clusters [22]. On the other hand, using the procedure described here, the inorganic polymer growth is not limited to the already existent clusters, but the clusters themselves are formed simultaneously with the inorganic polymerization reaction and solvent evaporation. The formability of the ionic domains can be affected and, hybrids with different hydration and conduction properties can be obtained. The morphology is also a function of TEOS and TMDES contents, as shown in Fig. 1. In Fig. 1, when 50 wt% TEOS was added to 50 wt% Nafion®, about 1.2 mm wide stripes could be differentiated. When part of the TEOS was substituted by TMDES, the films which were originally transparent, became turbid with TMDES substitutions higher than 10 wt%. A phase separation could be detected for samples with 10, 15 or 20 wt% substitution, as shown in Fig. 1c. In samples with 5 wt% substitution, no turbid films were obtained and the morphology was very similar to that shown in Fig. 1b for 50/47.5/2.5 Nafion®/TEOS/TMDES hybrids. Using EDS analysis, the presence of Si in the hybrid films was confirmed (Fig. 4). For 50/50/0 or 50/47.5/2.5
Nafion®/TEOS/TMDES hybrids, the EDS analysis carried out across the film thickness showed that the inorganic phase was homogeneously dispersed, with a Si peak area/S peak area ratio in the range 2.9 to 3.0. For hybrids with high TMDES content (Fig. 4b and c), it was verified that the presence of Si in domains was predominant and Nafion® was practically excluded, due to the higher hydrophobicity. Beside the changes in the morphology, different behaviors with respect to the conductive properties were detected, as shown in the impedance plots (Fig. 3). The impedance spectra for Nafion®/TEOS/TMDES systems could be represented by an R(RQ)(R)(Q) equivalent circuit. For 50/40/10 Nafion®/TEOS/TMDES hybrids, a semicircle in the high frequency region followed by a straight line in the medium and low frequency region was clearly observed. In this case, the contribution of a bulk capacitance associated with the dielectric polarization of the polymer chains caused by the alternating field (Cb), could be verified. At high frequencies, the impedance of the bulk resistance (Rb) and capacitance (Cb) were of the same magnitude, both contributing significantly to the overall impedance whereas the impedance of the electrode capacitance, Ce, was insignificant. Therefore, at high frequencies, the equiva-
Fig. 4. EDS analysis of Nafion®/TEOS/TMDES hybrids with composition (a) 50/47.5/2.5 and (b,c) 50/40/10 showing (b) the matrix and (c) the domains in the 50/40/10 Nafion®/TEOS/TMDES hybrid film.
R.A. Zoppi, S.P. Nunes / Journal of Electroanalytical Chemistry 445 (1998) 39–45 Table 2 Cb, Rb, Ce and ionic conductivity values for Nafion®/TEOS/TMDES systems at 25°C Hybrid composi- Rb/V tion 50/40/10 50/47.5/2.5 50/50/0
8.2×103 8.1×104 1.1×105
Cb/F
Ce/F
s/S cm−1
3.5×10−9 1.2×10−6 7.0×10−8
4.5×10−6 1.9×10−7 1.5×10−7
5.1×10−6 3.0×10−7 1.4×10−7
lent circuit reduces to a parallel RbCb combination which gives rise to the semicircle in the complex impedance plane. At these low frequencies Cb makes a negligible contribution to the impedance and a straight line appears. The low-frequency response carries information on the electrode electrolyte interface, and from any point on the straight line Ce =1/Z¦v, where Z¦ is the value of the imaginary part of the impedance at a frequency v. Rb is the effective d.c. resistance of the electrolyte and Ce is the electrode capacitance. The electrolyte ionic conductivity can be easily determined by s= l(RbA), where s is the ionic conductivity, Rb is the electrolyte resistance, A the area of the electrode and l the electrolyte thickness. Compared to 50/47.5/2.5 or 50/50/0, the 50/40/10 Nafion®/TEOS/TMDES systems showed the highest values for Ce and the lowest value for Rb (Table 2). Fletcher [23] affirmed that a necessary condition for the appearance of both the straight line in the low frequency region and the semicircular response at high frequencies is that Ce Cb, as was verified for the 50/40/10 Nafion®/TEOS/TMDES system. Tanguy et al. [24] have studied the capacitive and noncapacitive charge in polypyrrole films. They verified that the highest Ce values were obtained for highly doped polypyrrole samples (conductive-oxidized state). Ce was then related to the total charge in the polymer. Ce and s values in Table 2 show that 50/40/10 Nafion®/TEOS/ TMDES hybrids are the most conductive films. In Nafion®/TEOS/TMDES systems, the Nafion® content is always the same; so the amount of charge carriers would also be the same in these systems. Here, the high conductivity observed for 50/40/10 Nafion®/TEOS/ TMDES hybrids was related to the mobility of the charge carriers. As described in a previous paper, the dynamic-mechanical analysis gave information on the flexibility of Nafion®/TEOS/TMDES samples [5]. With the incorporation of silicon oxide in Nafion®, the hybrids became much more rigid compared to pure Nafion®. Substituting TEOS by TMDES and increasing the TMDES content, both the storage modulus (E%) and the dissipative modulus (E¦) decreased reaching values even lower than for pure Nafion® [5]. The role of TMDES was to increase flexibility and, indirectly, it promoted an increase in the hybrid conductivity.
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Fig. 5 shows the ionic conductivity values obtained from Rb as a function of temperature for Nafion®/ TEOS/TMDES hybrids. For both 50/50/0 and 50/47.5/ 2.5 Nafion®/TEOS/TMDES systems, the conductivity reached a maximum value at 45°C (Fig. 5a). At this point it is interesting to note also the impedance behavior of pure Nafion® films at different temperatures (Fig. 2). For temperatures higher than 45°C no inter- and intra-cluster migration process could be detected for pure Nafion®. For 50/47.5/2.5 and 50/50/0 Nafion®/ TEOS/TMDES systems, maximum values of ionic conductivity were observed at 45°C. This behavior can be explained by considering the modulated differential scanning calorimetry analysis carried out for pure Nafion® (Fig. 6). Near 50°C, a reversible exothermic peak, with a DH value equal to 1.8 kJ mol − 1 can be observed. The order of magnitude of DH is very close to that verified in micellar reorganization processes (DH = 1 to 5 kJ mol − 1). Considering the ionic clusters as micellar structures, the reversible exothermic peak
Fig. 5. (a) Ionic conductivity as a function of temperature for Nafion®/TEOS/TMDES hybrids with compositions () 50/47.5/2.5 or ( ) 50/50/0. (b) The same for 50/40/10 Nafion®/TEOS/TMDES films. In plot (b), the full line represents the fit using the VTF equation.
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R.A. Zoppi, S.P. Nunes / Journal of Electroanalytical Chemistry 445 (1998) 39–45
Fig. 6. Modulated differential scanning calorimetry analysis for pure Nafion®.
could be related to a cluster reorganization, which could be responsible for the changes in the behavior observed in the impedance spectrum of pure Nafion® at temperatures higher than 45°C, and the maximum values of ionic conductivity at 45°C shown by both 50/ 47.5/2.5 and 50/50/0 Nafion®/TEOS/TMDES systems. For 50/40/10 Nafion®/TEOS/TMDES hybrids, the ionic conductivity increased exponentially with the temperature (Fig. 5b). In this particular case, a good agreement between the experimental curve and the fit according to the Vogel-Tamman-Fulcker [25] equation was observed. One of the parameters that can be obtained from the fit is the activation energy necessary for the redistribution of the free volume (DE), which is directly related to the energy barrier for ion hopping from one free volume site to another one. In the case of 50/40/10 Nafion®/TEOS/TMDES hybrids, the DE was 45 kJ mol − 1, which is in the range of DE values previously observed for poly(amide 6-b-ethylene oxide)/LiClO4 (from 20 to 30 wt% of salt) electrolytes [26]. For 50/40/10 Nafion®/TEOS/TMDES samples, the compositional segregation or phase separation gave films with different dielectric or charge transport properties from pure Nafion® or hybrids with TMDES substitution lower than 20 wt%.
4. Conclusions Hybrids of Nafion® and silica were prepared from solution, growing the inorganic phase by hydrolysis/ condensation of alkoxy silanes. Using TEOS as the inorganic precursor, transparent and rigid films were obtained. By substituting part of the TEOS with TMDES more flexible films were obtained. Phase separation occurred when the TMDES substitution was higher than 10 wt%.
For pure Nafion®, the inter- and intra-cluster ionic migration processes were identified in electrochemical impedance experiments carried out at temperatures lower than 50°C. At high temperatures, these two processes were not clearly identified. Maximum conductivity values were obtained for 50/47.5/2.5 and 50/50/0 Nafion®/TEOS/TMDES films when the measurement was carried out at 45°C. This behavior was assigned to a reversible exothermic transition related to cluster reorganization, which was observed in modulated DSC analysis. The highest conductivity values were obtained for 50/40/10 Nafion®/TEOS/TMDES hybrids (s = 3.5× 10 − 5 S cm − 1 at 30°C). In this case, as for polymeric electrolytes, the ionic conductivity temperature dependence followed the VTF equation. Here, the compositional segregation or phase separation allowes one to design a material with different charge transport properties.
Acknowledgements The authors thank FAPESP and CNPq for financial support and Dr M.I. Felisberti, Dr M.-A. De Paoli and C.M.N.P. Fonseca for helping with modulated DSC and electrochemical impedance measurements.
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