- Email: [email protected]
PMMA-based polymer gel electrolytes containing NH4PF6: Role of molecular weight of polymer
Materials Science and Engineering B 129 (2006) 104–108
PMMA-based polymer gel electrolytes containing NH4PF6: Role of molecular weight of polymer Jitender Paul Sharma a , S.S. Sekhon a,b,∗ a
b
Department of Applied Physics, Guru Nanak Dev University, Amritsar-143005, India Polymer Electrolyte Fuel Cell Research Department, Korea Institute of Energy Research, Daejeon 305-343, Korea Received 1 September 2005; received in revised form 12 December 2005; accepted 24 December 2005
Abstract The effect of the molecular weight of polymethylmethacrylate (PMMA) on the conductivity and viscosity behavior of proton conducting polymer gel electrolytes containing ammonium hexafluorophosphate (NH4 PF6 ) in propylene carbonate (PC) has been studied. The addition of PMMA having molecular weights: 15,000; 120,000; 350,000; 996,000 results in an increase in conductivity and gels with conductivity higher than the corresponding liquid electrolytes have been obtained. The maxima observed in the variation of conductivity with PMMA concentration shifts towards higher concentrations of PMMA with an increase in the molecular weight of PMMA. The increase in conductivity with PMMA addition also depends upon the molecular weight of PMMA and has been found to be more for gels containing PMMA with lowest molecular weight (15,000). The increase in conductivity at low concentrations of PMMA is due to an increase in free ion concentration with the dissociation of ion aggregates, whereas the decrease in conductivity at higher concentrations of PMMA, is due to the exponential increase in viscosity, which lowers mobility and as a result conductivity decreases. These gels show high value of conductivity (∼10−2 S/cm at 25 ◦ C) which does not vary with time and shows only a small increase over the 20–100 ◦ C temperature range and is desirable for their potential use in applications. © 2006 Elsevier B.V. All rights reserved. Keywords: Molecular weight; Viscosity; Polymer gel electrolytes; Free ion concentration; Conductivity
1. Introduction Polymer gel electrolytes belong to the salt–solvent–polymer hybrid system and are generally prepared by immobilizing the salt solution with a suitable polymer matrix and solvent is retained in these electrolytes, which also helps in the conduction process [1,2]. Although initial work on these electrolytes mainly concentrated on lithium ion conducting gels due to their use in lithium batteries, yet proton conducting polymer gel electrolytes are also receiving attention due to their potential use in various devices [3–6]. Polyacrylonitrile (PAN), polymethylmethacrylate (PMMA), polyvinylidene fluoride (PVdF), polyvinylidene fluoride-co-hexafluoropropylene (PVdF-HFP), polyethylene oxide (PEO), etc. are some of the commonly used polymers in the synthesis of polymer gel electrolytes. The addition of different polymers increases the viscosity (η) of the electrolytes which lowers mobility (µ = q/6πrη, where µ is the
∗
Corresponding author. Tel.: +91 183 2711098; fax: +91 183 2258820. E-mail address: sekhon [email protected] (S.S. Sekhon).
0921-5107/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2005.12.019
carrier mobility, q the carrier charge and r is the radius of the carrier ions) and as a result, a decrease in conductivity (σ = nqµ, where σ is the ionic conductivity and n is the concentration of charge carriers) is generally observed [7,8]. The viscosity of the electrolytes will also depend upon the average molecular weight of the polymer used and polymers with different molecular weights may affect the conductivity behavior of gel electrolytes differently [9]. Although the role of different polymers on the conductivity modification of polymer gel electrolytes has been discussed, yet the effect of different molecular weight of the polymer is not generally taken into account [10–13]. The comparison of conductivity and other results reported by different workers on the same system, sometimes become difficult due to the different molecular weight of the polymer used in their study. Thus the effect of using the same polymer but having different molecular weights on the viscosity and conductivity behavior of polymer gel electrolytes will be an interesting study and is the subject for the present investigation. In the present study, polymethylmethacrylate (PMMA) with average molecular weights: 15,000; 120,000; 350,000; 996,000 has been used in the synthesis of polymer gel electrolytes and the
J.P. Sharma, S.S. Sekhon / Materials Science and Engineering B 129 (2006) 104–108
effect of the molecular weight of polymer on the conductivity and viscosity behavior of proton conducting polymer gel electrolytes containing NH4 PF6 in PC has been studied. The change in viscosity of the electrolytes with the addition of polymer having different molecular weights has been monitored by viscosity measurements. The variation of conductivity with the concentration of salt, concentration and molecular weight of polymer, temperature and time has also been investigated. 2. Experimental procedure PMMA (Aldrich, Mw = 15,000, 120,000, 350,000 and 996,000), NH4 PF6 (Aldrich), propylene carbonate (PC) (Merck) were used as the starting materials. Polymer gel electrolytes containing PMMA with different molecular weights in the 1 M solution of NH4 PF6 in PC were prepared by the method described in earlier publications [8,14]. The conductivity of electrolytes was measured by complex impedance spectroscopy using HP 4284A precision LCR meter with a cell having platinum electrodes. Impedance/admittance plots were drawn and conductivity was calculated by using the relation σ = Gl/A, where G is the conductance to be determined from the impedance/admittance plots, l the distance between the electrodes and A is the area of cross section of each electrode. The viscosity of the electrolytes was measured by Fungi lab rotating viscometer (Visco Basic L) using a small sample adapter assembly. 3. Results and discussion 3.1. Liquid electrolytes The conductivity of liquid electrolytes used in the preparation of polymer gel electrolytes and containing different concentrations of NH4 PF6 in PC was measured as a function of salt concentration and is given in Fig. 1. The conductivity of the solvent (∼10−6 S/cm) increases by four orders of magnitude to 10−2 S/cm with the addition of NH4 PF6 . The increase in con-
Fig. 1. Variation of conductivity (σ) of liquid electrolytes (PC–NH4 PF6 ) with the concentration of NH4 PF6 in PC.
105
ductivity is very steep at low concentrations of salt, but does not show much change at higher concentrations of salt and reaches a saturation value. The salt upon dissociation provides free ions for conduction, but at higher concentrations of salt, ion aggregates are also formed due to the presence of a large number of ions, which do not contribute to the conduction process and as a result conductivity does not increase at the same rate as observed at low salt concentrations [15,16]. The addition of salt also results in a small increase in viscosity of the electrolytes, which shall lower mobility and hence conductivity also but in the present case, the increase in viscosity with the addition of salt from 3.82 to 5.15 mPa s for liquid electrolytes containing 1 M NH4 PF6 in PC is very small and this may affect the conductivity only marginally. The saturation in the value of conductivity at higher salt concentrations as observed in Fig. 1 may be due to the formation of ion aggregates which do not contribute to conductivity. Thus ion aggregates are present in these liquid electrolytes at higher concentrations of NH4 PF6 in PC. 3.2. Gel electrolytes The variation of conductivity and viscosity with PMMA concentration for polymer gel electrolytes containing PMMA with average molecular weight 350,000 in the 1 M solution of NH4 PF6 in PC is given in Fig. 2. The conductivity of polymer gel electrolytes has been observed to increase with the addition of PMMA, reaches a maximum value of 4.16 × 10−2 S/cm at 3 wt.% PMMA and then decreases to a value of 2.85 × 10−2 S/cm at 10 wt.% PMMA. The presence of maxima is generally indicative of the simultaneous presence of two competing processes, which in the present case are free ion concentration at low PMMA concentrations and viscosity at high PMMA concentrations [17]. The increase in free ion concentration at low PMMA concentration is due to the dissociation of ion aggregates with the addition of polymer [14]. The viscosity of gel electrolytes is very small at low concentrations of PMMA but increases sharply at higher concentrations of PMMA. The large value of viscosity (η) at higher concentra-
Fig. 2. Dependence of conductivity (+) and viscosity () with PMMA (average molecular weight = 350,000) concentration for polymer gel electrolytes containing 1 M NH4 PF6 in PC.
106
J.P. Sharma, S.S. Sekhon / Materials Science and Engineering B 129 (2006) 104–108
Table 1a Maximum conductivity of polymer gel electrolytes containing PMMA with different molecular weights (g: gel; l: liquid) Composition PC + 1 M NH4 PF6 + PMMA with molecular weight
Maximum conductivity (in 10−2 S/cm)
Concentration of PMMA (in wt.%) at which maximum conductivity obtained
σ g (max)/σ l
15,000 120,000 350,000 996,000
4.34 4.25 4.15 4.12
0.5 1.5 3 3
1.23 1.21 1.18 1.17
tions (above 3.5 wt.%) of PMMA as observed in Fig. 2 will lead to a lower value of mobility, as viscosity and mobility are inversely related to each other and as a result a decrease in conductivity is also observed. However, care must be taken to differentiate between the macro-viscosity, which is measured experimentally and micro-viscosity, which is involved in the relation between mobility and viscosity. The relation between viscosity and mobility as given in the introduction part generally hold good for liquid electrolytes as well as for gel electrolytes at low concentrations of polymer but may not hold good at higher concentrations of polymer, where the difference between microand macro-viscosity becomes large. Similar results has also been observed for some proton conducting polymer gel electrolytes containing different weak carboxylic acids [14,18–20] as well as for some lithium ion conducting polymer gel electrolytes [21–23]. The increase in conductivity with polymer addition is generally ascribed to be due to an increase in free ion concentration, although the exact mechanism is not yet clearly understood. Breathing polymeric chain model [14] has been proposed to explain such results in proton conducting polymer gel electrolytes containing weak acids and the pressure/density fluctuations due to the breathing in/out of polymeric chains has been proposed to result in the dissociation of ion aggregates/undissociated acid present in these electrolytes. Similar mechanism is expected to take place in the present case also. The effect of the molecular weight of PMMA on the conductivity and viscosity behavior of polymer gel electrolytes was studied by using PMMA having molecular weights: 15,000; 120,000; 350,000; 996,000 and the variation of conductivity and viscosity with the concentration of PMMA is given in Fig. 3. The increase in conductivity with PMMA addition has been observed for all the gels, however the magnitude of increase observed with the addition of PMMA has been found to decrease
Fig. 3. Dependence of (a) conductivity and (b) viscosity with PMMA (average molecular weight = 15,000 (+), 120,000 (), 350,000 () and 996,000 ()) concentration for polymer gel electrolytes containing 1 M NH4 PF6 in PC.
marginally from a factor of 1.23 to 1.17 with an increase in the molecular weight of PMMA. The position of the conductivity maxima also shifts from 0.5 to 3.0 wt.% towards higher concentration of PMMA with an increase in the molecular weight of PMMA as given in Table 1a. The fractional change in conductivity and viscosity has also been calculated as the ratios σ g /σ l and ηg /ηl (where σ g and ηg are the conductivity and viscosity of gel electrolytes; σ l and ηl are the conductivity and viscosity of liquid electrolytes, respectively) for gels having different concentrations and molecular weights of PMMA and the values are listed in Table 1b. The increase in conductivity at low concentrations of PMMA has been observed for all the gels and the maximum increase by a factor of 1.23 has been
Table 1b Ratio of conductivity (σ) and viscosity (η) of gel (g) and liquid (l) electrolytes containing PMMA with different molecular weights Composition PC + 1 M NH4 PF6 + x (wt.%) PMMA
2 6 10
σ g /σ l for electrolytes containing PMMA with molecular weight
ηg /ηl for electrolytes containing PMMA with molecular weight
15,000
120,000
350,000
996,000
15,000
120,000
350,000
996,000
– 1.02 0.90
– 1.02 0.87
– 1.03 0.81
– 1.03 0.80
2.8 35.7 161.7
3.7 48.4 253.0
5.1 78.4 431.1
10.0 131.8 812.8
J.P. Sharma, S.S. Sekhon / Materials Science and Engineering B 129 (2006) 104–108
observed for gels containing PMMA having lowest molecular weight (15,000). Gel electrolytes containing PMMA with molecular weights: 120,000; 350,000; 996,000 show an increase in conductivity by a factor of 1.21, 1.18 and 1.17, respectively. The position of the maxima in conductivity also shifts towards relatively higher concentration of PMMA with an increase in the molecular weight of PMMA and has been observed at 0.5, 1.5, 3.0 and 3.0 wt.% for PMMA having molecular weights: 15,000; 120,000; 350,000; 996,000, respectively. However, the conductivity of gels containing PMMA with different molecular weights decreases at higher concentrations of PMMA and at 6 wt.% PMMA, all the gels show nearly same value of conductivity, whereas at 10 wt.% PMMA, gels containing PMMA with higher molecular weight (996,000) show lowest conductivity and vice versa. As the conductivity of polymer gel electrolytes depends upon the free ion concentration and viscosity of the electrolytes as discussed above, so the variation of viscosity with PMMA concentration is also given in Fig. 3. At low concentrations of PMMA, the addition of polymer results in the dissociation of ion aggregates, which increases the free ion concentration, and as a result an increase in conductivity has been observed. However, at higher concentrations of PMMA, free ion concentration does not increase at the same rate, as the viscosity of the electrolyte becomes large and plays a dominant role. On the other hand, viscosity of all the gel electrolytes is quite small at very low concentrations of PMMA, but shows a sharp increase at higher concentrations of PMMA. The increase in viscosity for polymer gel electrolytes containing 6 wt.% PMMA (by a factor of 35–131) and at 10 wt.% PMMA (by a factor of 161–812) is directly related to the molecular weight of the polymer and is maximum for gel electrolytes containing PMMA with higher molecular weight (996,000) as given in Table 1b. This large increase in viscosity at higher concentrations of polymer will lead to a lower mobility and as a result, a decrease in conductivity has also been observed at higher concentrations of polymer as given in Fig. 3a. As expected, the conductivity value at higher concentrations (>6 wt.%) of PMMA is more for gels containing PMMA with lower molecular weight (15,000) than for gels containing PMMA with higher molecular weight (996,000). However, the difference in conductivity of polymer gel electrolytes in comparison to that of the corresponding liquid electrolytes is quite small and polymer gel electrolytes containing 10 wt.% PMMA with different molecular weights still possess conductivity of (2.80–3.18) × 10−2 S/cm at 25 ◦ C, which is of the same order as observed for the starting liquid electrolytes (3.52 × 10−2 S/cm). Thus the change in conductivity and viscosity observed with the addition of PMMA has been found to depend upon the molecular weight of polymer. Although the increase in conductivity observed with the addition of PMMA is quite small and is by a small factor only, yet it is quite significant as we start with liquid electrolytes and add PMMA to immobilize the salt solution by increasing its viscosity and along with a small increase in conductivity has also been obtained. The dependence of conductivity of selected polymer gel electrolytes was also studied as a function of temperature and time and the results are given in Fig. 4. The curved nature of the plot between log conductivity and reciprocal temperature indicates
107
Fig. 4. Change of conductivity with (a) temperature and (b) time for polymer gel electrolytes containing 8 wt.% PMMA (average molecular weight = 15,000 (+); 996,000 ()) in PC + 1 M NH4 PF6 .
the amorphous nature of the electrolytes. The experimental data have been fitted with the Vogel–Tamman–Fulcher (VTF) equation for conductivity for such electrolytes. The VTF equation is σ = σ0 exp
−B T − T0
where the constants σ 0 (S/cm), B (K) and T0 (K) are adjustable parameters. The best-fit parameters are shown in Table 1c. The values of best-fit parameters have been found to be in agreement with the values reported by other workers [24,25]. The conductivity does not increase much with an increase in temperature Table 1c VTF equation parameters of ionic conductivity data for polymer gel electrolytes having composition PC − 1 M NH4 PF6 − 8 wt.% PMMA σ = σ 0 exp[−B/(T − T0 )] Molecular weight of PMMA
σ 0 /10−2 (S/cm)
B (K)
T0 (K)
15, 000 996, 000
36.9 33.9
43.8 32.0
210 224
108
J.P. Sharma, S.S. Sekhon / Materials Science and Engineering B 129 (2006) 104–108
over the 20–100 ◦ C temperature range and remains of the same order (10−2 S/cm) even above 100 ◦ C. In addition, the conductivity of these gel electrolytes has been observed to remain constant with time over a period of 12 days and these results are desirable for their potential applications in devices. 4. Conclusions Proton conducting polymer gel electrolytes containing PMMA having different molecular weights show high value of conductivity of ∼10−2 S/cm at 25 ◦ C. At low PMMA concentrations, the addition of PMMA results in an increase in free ion concentration due to the dissociation of ion aggregates which increases conductivity, whose magnitude depends upon the molecular weight of PMMA used and is maximum for PMMA having lowest molecular weight (15,000). At higher concentrations of PMMA, viscosity of gel electrolytes which also depends upon the molecular weight of polymer increases exponentially and lowers mobility and as a result a decrease in conductivity has been observed, which for gel electrolytes containing PMMA with higher molecular weight (996,000) is more. The position of conductivity maxima also shifts towards higher concentrations of PMMA with an increase in the molecular weight of PMMA. The conductivity of gel electrolytes increases by a small amount with an increase in temperature from 20 to 100 ◦ C and remains constant with time over a period of 12 days which is desirable for their use in devices. Acknowledgements One of the author (SSS) is thankful to The Ministry of Science and Technology and The Ministry of Commerce, Industry and Energy, Republic of Korea for financial support. References [1] F.M. Gray, Polymer Electrolytes, The Royal Society of Chemistry, Cambridge, 1997.
[2] G. Feuillade, P.H. Perche, J. Appl. Electrochem. 5 (1975) 63–69. [3] B.B. Owens, J. Power Sources 90 (2000) 2–8. [4] J.Y. Song, Y.Y. Wang, C.C. Wan, J. Power Sources 77 (1999) 183– 197. [5] P. Colomban (Ed.), Proton Conductors: Solids, Membranes and Gels—Materials and Devices, Cambridge University Press, Cambridge, 1992. [6] K.D. Kreuer, Solid State Ionics 97 (1997) 1–15. [7] O. Bohnke, G. Frand, M. Rezrazi, C. Rousselot, C. Truche, Solid State Ionics 66 (1993) 97–104; 105–112. [8] S.S. Sekhon, Pradeep, S.A. Agnihotry, in: B.V.R. Chowdari, K. Lal, S.A. Agnihotry, N. Khare, S.S. Sekhon, P.C. Srivastava, S. Chandra (Eds.), Solid State Ionics: Science & Technology, World Scientific, Singapore, 1998, p. 217. [9] R. Kumar, S.S. Sekhon, unpublished results. [10] M. Caillon-Caravanier, B. Claude-Montigny, D. Lemordant, G. Bosser, Solid State Ionics 156 (2003) 113–127. [11] R. Koksbang, I.I. Oslen, D. Shackle, Solid State Ionics 69 (1994) 320–335. [12] A. Webber, J. Electrochem. Soc. 138 (1991) 2586–2589. [13] E. Cazzanelli, G. Mariotto, G.B. Appetecchi, F. Croce, B. Scrosati, Electrochim. Acta 40 (1995) 2379–2382. [14] S. Chandra, S.S. Sekhon, N. Arora, Ionics 6 (2000) 112–118. [15] M. Deepa, S.A. Agnihotry, D. Gupta, R. Chandra, Electrochim. Acta 49 (2004) 373–383. [16] D.A. Skoog, J.J. Leary, Principles of Instrumental Analysis, 4th ed., Harcourt Brace College Publishers, USA (Chapter 19). [17] S.S. Sekhon, N. Arora, S. Chandra, Eur. Polym. J. 39 (2003) 915–920. [18] A.M. Grillone, S. Panero, B.A. Retamal, B. Scrosati, J. Electrochem. Soc. 146 (1999) 27–31. [19] W. Wieczorek, G. Zukowska, R. Borkowska, S.H. Chung, S. Greenbaum, Electrochim. Acta 46 (2001) 1427–1438. [20] H. Ericson, C. Svanberg, A. Brodin, A.M. Grillone, S. Panero, B. Scrosati, P. Jacobsson, Electrochim. Acta 45 (2000) 1409–1414. [21] Y. Saito, H. Kataoka, C. Capiglia, H. Yamamoto, J. Phys. Chem. B 104 (2000) 2189–2192. [22] C.S. Kim, S.M. Oh, Electrochim. Acta 45 (2000) 2101–2109. [23] B. Singh, R. Kumar, S.S. Sekhon, Solid State Ionics 176 (2005) 1577–1583. [24] P.E. Stallworth, J.J. Fontanella, M.C. Wintersgill, C.D. Scheidler, J.J. Immel, S.G. Greenbaum, A.S. Gozdz, J. Power Sources 81–82 (1999) 739–747. [25] A. Noda, K. Hayamizu, M. Watanabe, J. Phys. Chem. B 105 (2001) 4603–4610.
PMMA-based polymer gel electrolytes containing NH4PF6: Role of molecular weight of polymer Jitender Paul Sharma a , S.S. Sekhon a,b,∗ a
b
Department of Applied Physics, Guru Nanak Dev University, Amritsar-143005, India Polymer Electrolyte Fuel Cell Research Department, Korea Institute of Energy Research, Daejeon 305-343, Korea Received 1 September 2005; received in revised form 12 December 2005; accepted 24 December 2005
Abstract The effect of the molecular weight of polymethylmethacrylate (PMMA) on the conductivity and viscosity behavior of proton conducting polymer gel electrolytes containing ammonium hexafluorophosphate (NH4 PF6 ) in propylene carbonate (PC) has been studied. The addition of PMMA having molecular weights: 15,000; 120,000; 350,000; 996,000 results in an increase in conductivity and gels with conductivity higher than the corresponding liquid electrolytes have been obtained. The maxima observed in the variation of conductivity with PMMA concentration shifts towards higher concentrations of PMMA with an increase in the molecular weight of PMMA. The increase in conductivity with PMMA addition also depends upon the molecular weight of PMMA and has been found to be more for gels containing PMMA with lowest molecular weight (15,000). The increase in conductivity at low concentrations of PMMA is due to an increase in free ion concentration with the dissociation of ion aggregates, whereas the decrease in conductivity at higher concentrations of PMMA, is due to the exponential increase in viscosity, which lowers mobility and as a result conductivity decreases. These gels show high value of conductivity (∼10−2 S/cm at 25 ◦ C) which does not vary with time and shows only a small increase over the 20–100 ◦ C temperature range and is desirable for their potential use in applications. © 2006 Elsevier B.V. All rights reserved. Keywords: Molecular weight; Viscosity; Polymer gel electrolytes; Free ion concentration; Conductivity
1. Introduction Polymer gel electrolytes belong to the salt–solvent–polymer hybrid system and are generally prepared by immobilizing the salt solution with a suitable polymer matrix and solvent is retained in these electrolytes, which also helps in the conduction process [1,2]. Although initial work on these electrolytes mainly concentrated on lithium ion conducting gels due to their use in lithium batteries, yet proton conducting polymer gel electrolytes are also receiving attention due to their potential use in various devices [3–6]. Polyacrylonitrile (PAN), polymethylmethacrylate (PMMA), polyvinylidene fluoride (PVdF), polyvinylidene fluoride-co-hexafluoropropylene (PVdF-HFP), polyethylene oxide (PEO), etc. are some of the commonly used polymers in the synthesis of polymer gel electrolytes. The addition of different polymers increases the viscosity (η) of the electrolytes which lowers mobility (µ = q/6πrη, where µ is the
∗
Corresponding author. Tel.: +91 183 2711098; fax: +91 183 2258820. E-mail address: sekhon [email protected] (S.S. Sekhon).
0921-5107/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2005.12.019
carrier mobility, q the carrier charge and r is the radius of the carrier ions) and as a result, a decrease in conductivity (σ = nqµ, where σ is the ionic conductivity and n is the concentration of charge carriers) is generally observed [7,8]. The viscosity of the electrolytes will also depend upon the average molecular weight of the polymer used and polymers with different molecular weights may affect the conductivity behavior of gel electrolytes differently [9]. Although the role of different polymers on the conductivity modification of polymer gel electrolytes has been discussed, yet the effect of different molecular weight of the polymer is not generally taken into account [10–13]. The comparison of conductivity and other results reported by different workers on the same system, sometimes become difficult due to the different molecular weight of the polymer used in their study. Thus the effect of using the same polymer but having different molecular weights on the viscosity and conductivity behavior of polymer gel electrolytes will be an interesting study and is the subject for the present investigation. In the present study, polymethylmethacrylate (PMMA) with average molecular weights: 15,000; 120,000; 350,000; 996,000 has been used in the synthesis of polymer gel electrolytes and the
J.P. Sharma, S.S. Sekhon / Materials Science and Engineering B 129 (2006) 104–108
effect of the molecular weight of polymer on the conductivity and viscosity behavior of proton conducting polymer gel electrolytes containing NH4 PF6 in PC has been studied. The change in viscosity of the electrolytes with the addition of polymer having different molecular weights has been monitored by viscosity measurements. The variation of conductivity with the concentration of salt, concentration and molecular weight of polymer, temperature and time has also been investigated. 2. Experimental procedure PMMA (Aldrich, Mw = 15,000, 120,000, 350,000 and 996,000), NH4 PF6 (Aldrich), propylene carbonate (PC) (Merck) were used as the starting materials. Polymer gel electrolytes containing PMMA with different molecular weights in the 1 M solution of NH4 PF6 in PC were prepared by the method described in earlier publications [8,14]. The conductivity of electrolytes was measured by complex impedance spectroscopy using HP 4284A precision LCR meter with a cell having platinum electrodes. Impedance/admittance plots were drawn and conductivity was calculated by using the relation σ = Gl/A, where G is the conductance to be determined from the impedance/admittance plots, l the distance between the electrodes and A is the area of cross section of each electrode. The viscosity of the electrolytes was measured by Fungi lab rotating viscometer (Visco Basic L) using a small sample adapter assembly. 3. Results and discussion 3.1. Liquid electrolytes The conductivity of liquid electrolytes used in the preparation of polymer gel electrolytes and containing different concentrations of NH4 PF6 in PC was measured as a function of salt concentration and is given in Fig. 1. The conductivity of the solvent (∼10−6 S/cm) increases by four orders of magnitude to 10−2 S/cm with the addition of NH4 PF6 . The increase in con-
Fig. 1. Variation of conductivity (σ) of liquid electrolytes (PC–NH4 PF6 ) with the concentration of NH4 PF6 in PC.
105
ductivity is very steep at low concentrations of salt, but does not show much change at higher concentrations of salt and reaches a saturation value. The salt upon dissociation provides free ions for conduction, but at higher concentrations of salt, ion aggregates are also formed due to the presence of a large number of ions, which do not contribute to the conduction process and as a result conductivity does not increase at the same rate as observed at low salt concentrations [15,16]. The addition of salt also results in a small increase in viscosity of the electrolytes, which shall lower mobility and hence conductivity also but in the present case, the increase in viscosity with the addition of salt from 3.82 to 5.15 mPa s for liquid electrolytes containing 1 M NH4 PF6 in PC is very small and this may affect the conductivity only marginally. The saturation in the value of conductivity at higher salt concentrations as observed in Fig. 1 may be due to the formation of ion aggregates which do not contribute to conductivity. Thus ion aggregates are present in these liquid electrolytes at higher concentrations of NH4 PF6 in PC. 3.2. Gel electrolytes The variation of conductivity and viscosity with PMMA concentration for polymer gel electrolytes containing PMMA with average molecular weight 350,000 in the 1 M solution of NH4 PF6 in PC is given in Fig. 2. The conductivity of polymer gel electrolytes has been observed to increase with the addition of PMMA, reaches a maximum value of 4.16 × 10−2 S/cm at 3 wt.% PMMA and then decreases to a value of 2.85 × 10−2 S/cm at 10 wt.% PMMA. The presence of maxima is generally indicative of the simultaneous presence of two competing processes, which in the present case are free ion concentration at low PMMA concentrations and viscosity at high PMMA concentrations [17]. The increase in free ion concentration at low PMMA concentration is due to the dissociation of ion aggregates with the addition of polymer [14]. The viscosity of gel electrolytes is very small at low concentrations of PMMA but increases sharply at higher concentrations of PMMA. The large value of viscosity (η) at higher concentra-
Fig. 2. Dependence of conductivity (+) and viscosity () with PMMA (average molecular weight = 350,000) concentration for polymer gel electrolytes containing 1 M NH4 PF6 in PC.
106
J.P. Sharma, S.S. Sekhon / Materials Science and Engineering B 129 (2006) 104–108
Table 1a Maximum conductivity of polymer gel electrolytes containing PMMA with different molecular weights (g: gel; l: liquid) Composition PC + 1 M NH4 PF6 + PMMA with molecular weight
Maximum conductivity (in 10−2 S/cm)
Concentration of PMMA (in wt.%) at which maximum conductivity obtained
σ g (max)/σ l
15,000 120,000 350,000 996,000
4.34 4.25 4.15 4.12
0.5 1.5 3 3
1.23 1.21 1.18 1.17
tions (above 3.5 wt.%) of PMMA as observed in Fig. 2 will lead to a lower value of mobility, as viscosity and mobility are inversely related to each other and as a result a decrease in conductivity is also observed. However, care must be taken to differentiate between the macro-viscosity, which is measured experimentally and micro-viscosity, which is involved in the relation between mobility and viscosity. The relation between viscosity and mobility as given in the introduction part generally hold good for liquid electrolytes as well as for gel electrolytes at low concentrations of polymer but may not hold good at higher concentrations of polymer, where the difference between microand macro-viscosity becomes large. Similar results has also been observed for some proton conducting polymer gel electrolytes containing different weak carboxylic acids [14,18–20] as well as for some lithium ion conducting polymer gel electrolytes [21–23]. The increase in conductivity with polymer addition is generally ascribed to be due to an increase in free ion concentration, although the exact mechanism is not yet clearly understood. Breathing polymeric chain model [14] has been proposed to explain such results in proton conducting polymer gel electrolytes containing weak acids and the pressure/density fluctuations due to the breathing in/out of polymeric chains has been proposed to result in the dissociation of ion aggregates/undissociated acid present in these electrolytes. Similar mechanism is expected to take place in the present case also. The effect of the molecular weight of PMMA on the conductivity and viscosity behavior of polymer gel electrolytes was studied by using PMMA having molecular weights: 15,000; 120,000; 350,000; 996,000 and the variation of conductivity and viscosity with the concentration of PMMA is given in Fig. 3. The increase in conductivity with PMMA addition has been observed for all the gels, however the magnitude of increase observed with the addition of PMMA has been found to decrease
Fig. 3. Dependence of (a) conductivity and (b) viscosity with PMMA (average molecular weight = 15,000 (+), 120,000 (), 350,000 () and 996,000 ()) concentration for polymer gel electrolytes containing 1 M NH4 PF6 in PC.
marginally from a factor of 1.23 to 1.17 with an increase in the molecular weight of PMMA. The position of the conductivity maxima also shifts from 0.5 to 3.0 wt.% towards higher concentration of PMMA with an increase in the molecular weight of PMMA as given in Table 1a. The fractional change in conductivity and viscosity has also been calculated as the ratios σ g /σ l and ηg /ηl (where σ g and ηg are the conductivity and viscosity of gel electrolytes; σ l and ηl are the conductivity and viscosity of liquid electrolytes, respectively) for gels having different concentrations and molecular weights of PMMA and the values are listed in Table 1b. The increase in conductivity at low concentrations of PMMA has been observed for all the gels and the maximum increase by a factor of 1.23 has been
Table 1b Ratio of conductivity (σ) and viscosity (η) of gel (g) and liquid (l) electrolytes containing PMMA with different molecular weights Composition PC + 1 M NH4 PF6 + x (wt.%) PMMA
2 6 10
σ g /σ l for electrolytes containing PMMA with molecular weight
ηg /ηl for electrolytes containing PMMA with molecular weight
15,000
120,000
350,000
996,000
15,000
120,000
350,000
996,000
– 1.02 0.90
– 1.02 0.87
– 1.03 0.81
– 1.03 0.80
2.8 35.7 161.7
3.7 48.4 253.0
5.1 78.4 431.1
10.0 131.8 812.8
J.P. Sharma, S.S. Sekhon / Materials Science and Engineering B 129 (2006) 104–108
observed for gels containing PMMA having lowest molecular weight (15,000). Gel electrolytes containing PMMA with molecular weights: 120,000; 350,000; 996,000 show an increase in conductivity by a factor of 1.21, 1.18 and 1.17, respectively. The position of the maxima in conductivity also shifts towards relatively higher concentration of PMMA with an increase in the molecular weight of PMMA and has been observed at 0.5, 1.5, 3.0 and 3.0 wt.% for PMMA having molecular weights: 15,000; 120,000; 350,000; 996,000, respectively. However, the conductivity of gels containing PMMA with different molecular weights decreases at higher concentrations of PMMA and at 6 wt.% PMMA, all the gels show nearly same value of conductivity, whereas at 10 wt.% PMMA, gels containing PMMA with higher molecular weight (996,000) show lowest conductivity and vice versa. As the conductivity of polymer gel electrolytes depends upon the free ion concentration and viscosity of the electrolytes as discussed above, so the variation of viscosity with PMMA concentration is also given in Fig. 3. At low concentrations of PMMA, the addition of polymer results in the dissociation of ion aggregates, which increases the free ion concentration, and as a result an increase in conductivity has been observed. However, at higher concentrations of PMMA, free ion concentration does not increase at the same rate, as the viscosity of the electrolyte becomes large and plays a dominant role. On the other hand, viscosity of all the gel electrolytes is quite small at very low concentrations of PMMA, but shows a sharp increase at higher concentrations of PMMA. The increase in viscosity for polymer gel electrolytes containing 6 wt.% PMMA (by a factor of 35–131) and at 10 wt.% PMMA (by a factor of 161–812) is directly related to the molecular weight of the polymer and is maximum for gel electrolytes containing PMMA with higher molecular weight (996,000) as given in Table 1b. This large increase in viscosity at higher concentrations of polymer will lead to a lower mobility and as a result, a decrease in conductivity has also been observed at higher concentrations of polymer as given in Fig. 3a. As expected, the conductivity value at higher concentrations (>6 wt.%) of PMMA is more for gels containing PMMA with lower molecular weight (15,000) than for gels containing PMMA with higher molecular weight (996,000). However, the difference in conductivity of polymer gel electrolytes in comparison to that of the corresponding liquid electrolytes is quite small and polymer gel electrolytes containing 10 wt.% PMMA with different molecular weights still possess conductivity of (2.80–3.18) × 10−2 S/cm at 25 ◦ C, which is of the same order as observed for the starting liquid electrolytes (3.52 × 10−2 S/cm). Thus the change in conductivity and viscosity observed with the addition of PMMA has been found to depend upon the molecular weight of polymer. Although the increase in conductivity observed with the addition of PMMA is quite small and is by a small factor only, yet it is quite significant as we start with liquid electrolytes and add PMMA to immobilize the salt solution by increasing its viscosity and along with a small increase in conductivity has also been obtained. The dependence of conductivity of selected polymer gel electrolytes was also studied as a function of temperature and time and the results are given in Fig. 4. The curved nature of the plot between log conductivity and reciprocal temperature indicates
107
Fig. 4. Change of conductivity with (a) temperature and (b) time for polymer gel electrolytes containing 8 wt.% PMMA (average molecular weight = 15,000 (+); 996,000 ()) in PC + 1 M NH4 PF6 .
the amorphous nature of the electrolytes. The experimental data have been fitted with the Vogel–Tamman–Fulcher (VTF) equation for conductivity for such electrolytes. The VTF equation is σ = σ0 exp
−B T − T0
where the constants σ 0 (S/cm), B (K) and T0 (K) are adjustable parameters. The best-fit parameters are shown in Table 1c. The values of best-fit parameters have been found to be in agreement with the values reported by other workers [24,25]. The conductivity does not increase much with an increase in temperature Table 1c VTF equation parameters of ionic conductivity data for polymer gel electrolytes having composition PC − 1 M NH4 PF6 − 8 wt.% PMMA σ = σ 0 exp[−B/(T − T0 )] Molecular weight of PMMA
σ 0 /10−2 (S/cm)
B (K)
T0 (K)
15, 000 996, 000
36.9 33.9
43.8 32.0
210 224
108
J.P. Sharma, S.S. Sekhon / Materials Science and Engineering B 129 (2006) 104–108
over the 20–100 ◦ C temperature range and remains of the same order (10−2 S/cm) even above 100 ◦ C. In addition, the conductivity of these gel electrolytes has been observed to remain constant with time over a period of 12 days and these results are desirable for their potential applications in devices. 4. Conclusions Proton conducting polymer gel electrolytes containing PMMA having different molecular weights show high value of conductivity of ∼10−2 S/cm at 25 ◦ C. At low PMMA concentrations, the addition of PMMA results in an increase in free ion concentration due to the dissociation of ion aggregates which increases conductivity, whose magnitude depends upon the molecular weight of PMMA used and is maximum for PMMA having lowest molecular weight (15,000). At higher concentrations of PMMA, viscosity of gel electrolytes which also depends upon the molecular weight of polymer increases exponentially and lowers mobility and as a result a decrease in conductivity has been observed, which for gel electrolytes containing PMMA with higher molecular weight (996,000) is more. The position of conductivity maxima also shifts towards higher concentrations of PMMA with an increase in the molecular weight of PMMA. The conductivity of gel electrolytes increases by a small amount with an increase in temperature from 20 to 100 ◦ C and remains constant with time over a period of 12 days which is desirable for their use in devices. Acknowledgements One of the author (SSS) is thankful to The Ministry of Science and Technology and The Ministry of Commerce, Industry and Energy, Republic of Korea for financial support. References [1] F.M. Gray, Polymer Electrolytes, The Royal Society of Chemistry, Cambridge, 1997.
[2] G. Feuillade, P.H. Perche, J. Appl. Electrochem. 5 (1975) 63–69. [3] B.B. Owens, J. Power Sources 90 (2000) 2–8. [4] J.Y. Song, Y.Y. Wang, C.C. Wan, J. Power Sources 77 (1999) 183– 197. [5] P. Colomban (Ed.), Proton Conductors: Solids, Membranes and Gels—Materials and Devices, Cambridge University Press, Cambridge, 1992. [6] K.D. Kreuer, Solid State Ionics 97 (1997) 1–15. [7] O. Bohnke, G. Frand, M. Rezrazi, C. Rousselot, C. Truche, Solid State Ionics 66 (1993) 97–104; 105–112. [8] S.S. Sekhon, Pradeep, S.A. Agnihotry, in: B.V.R. Chowdari, K. Lal, S.A. Agnihotry, N. Khare, S.S. Sekhon, P.C. Srivastava, S. Chandra (Eds.), Solid State Ionics: Science & Technology, World Scientific, Singapore, 1998, p. 217. [9] R. Kumar, S.S. Sekhon, unpublished results. [10] M. Caillon-Caravanier, B. Claude-Montigny, D. Lemordant, G. Bosser, Solid State Ionics 156 (2003) 113–127. [11] R. Koksbang, I.I. Oslen, D. Shackle, Solid State Ionics 69 (1994) 320–335. [12] A. Webber, J. Electrochem. Soc. 138 (1991) 2586–2589. [13] E. Cazzanelli, G. Mariotto, G.B. Appetecchi, F. Croce, B. Scrosati, Electrochim. Acta 40 (1995) 2379–2382. [14] S. Chandra, S.S. Sekhon, N. Arora, Ionics 6 (2000) 112–118. [15] M. Deepa, S.A. Agnihotry, D. Gupta, R. Chandra, Electrochim. Acta 49 (2004) 373–383. [16] D.A. Skoog, J.J. Leary, Principles of Instrumental Analysis, 4th ed., Harcourt Brace College Publishers, USA (Chapter 19). [17] S.S. Sekhon, N. Arora, S. Chandra, Eur. Polym. J. 39 (2003) 915–920. [18] A.M. Grillone, S. Panero, B.A. Retamal, B. Scrosati, J. Electrochem. Soc. 146 (1999) 27–31. [19] W. Wieczorek, G. Zukowska, R. Borkowska, S.H. Chung, S. Greenbaum, Electrochim. Acta 46 (2001) 1427–1438. [20] H. Ericson, C. Svanberg, A. Brodin, A.M. Grillone, S. Panero, B. Scrosati, P. Jacobsson, Electrochim. Acta 45 (2000) 1409–1414. [21] Y. Saito, H. Kataoka, C. Capiglia, H. Yamamoto, J. Phys. Chem. B 104 (2000) 2189–2192. [22] C.S. Kim, S.M. Oh, Electrochim. Acta 45 (2000) 2101–2109. [23] B. Singh, R. Kumar, S.S. Sekhon, Solid State Ionics 176 (2005) 1577–1583. [24] P.E. Stallworth, J.J. Fontanella, M.C. Wintersgill, C.D. Scheidler, J.J. Immel, S.G. Greenbaum, A.S. Gozdz, J. Power Sources 81–82 (1999) 739–747. [25] A. Noda, K. Hayamizu, M. Watanabe, J. Phys. Chem. B 105 (2001) 4603–4610.