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Ionomer-silicates composite membranes: Permeability and conductivity studies
EUROPEAN POLYMER JOURNAL
European Polymer Journal 41 (2005) 1350–1356
www.elsevier.com/locate/europolj
Ionomer-silicates composite membranes: Permeability and conductivity studies C.S. Karthikeyan b
a,*
, S.P. Nunes a, K. Schulte
b
a GKSS Research Centre, Institute of Chemistry, Max-Planck Strasse, D-21502 Geesthacht, Germany Polymer Composites, Denickestrasse 15, Technical University of Hamburg-Harburg, D-21703 Hamburg, Germany
Received 22 June 2004; received in revised form 14 December 2004; accepted 16 December 2004 Available online 28 January 2005
Abstract Polymer composite membranes based on sulphonated polymers, such as sulphonated poly(ether ketone) and sulphonated poly(ether ether ketone), and silicates were prepared and characterized for water/methanol permeabilities and proton conductivity studies. The study showed methanol and water permeability in the composite system decreased, with respect to the plain polymer/ionomer, with the increase in content of silicates. The permeability reduction in the composite membranes is discussed using models and theories. It was also found that the proton conductivity of the ionomer-composite membranes increased with the increase in total flux of the system, emphasising a good correlation between the total flux of the composite membranes and proton conductivity. The work clearly demonstrates that the same transport mechanism governs both methanol–water crossover and proton conductivity in these polymer electrolyte composite membranes. 2005 Elsevier Ltd. All rights reserved. Keywords: Ionomers; Layered silicates; Membranes; Composites; Fuel cells
1. Introduction Polymer electrolyte membranes for direct methanol fuel cells (DMFC) are still under research and development in many industries and academics all over the world [1]. The key issue is synthesizing a membrane with good proton conductivity and low methanol permeability as they form a major barrier for the better performance of DMFC. It is quite established now that one
* Corresponding author. Present address: Chair of Materials Processing, University of Bayreuth, 95447 Bayreuth, Germany. Tel.: +49 921 557 209; fax: +49 921 557 205. E-mail address: [email protected] (C.S. Karthikeyan).
means to decrease the methanol permeability/cross over is by the usage of polymer composite membranes [2,3]. The fillers can be chosen in such way they either aid proton conductivity or decrease the methanol permeability. Previous reports from our group [4–6] were based on sulphonated poly(ether ketone), SPEK containing zirconium oxide, prepared by sol–gel method involving the hydrolysis of zirconium tetrapropylate in a solution of SPEK, with the addition of acetyl acetone to avoid precipitation of the inorganic phase. This was quite successful in reducing the methanol and water permeability but the proton conductivity decreased as well [5,6]. In addition, proton conductive inorganic components such as zirconium phosphate [5] and heteropolyacids [6] were incorporated during the membrane preparation in order to compensate for the conductivity [5]. But problems
0014-3057/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2004.12.011
C.S. Karthikeyan et al. / European Polymer Journal 41 (2005) 1350–1356
such as increase in permeability or bleeding of the proton conducting inorganic components from the membrane were encountered. These aspects were taken in to account in developing polymer composite membranes for DMFC for the present investigation. The present work focuses on the ionomer-composite membranes based on sulphonated poly (etherketone) and sulphonated poly(ether ether ketone) containing silicates (layered at nanoscale). Ionomers are ion-containing polymers in which the bulk properties are governed by ionic interactions in discrete regions of the material [7]. Polymer silicates composite materials have gained considerable attention [8,9]; few reviews have been reported [10,11] on their preparation and properties. These silicates generally have high aspect ratio and are low priced. They improve mechanical and thermal properties and decrease gas/vapour permeability [11]. It is claimed [12] that permeability of polymer composites is largely affected by the aspect ratio of the filler. It is also believed that flakes [13] are much more effective in decreasing the permeability. This was the motivation to explore inorganic filler with high aspect ratio such as layered silicates for preparing polymer composite membranes for DMFC.
2. Experimental 2.1. Materials Sulphonated poly(etherketone), SPEK, with 50% degree of sulphonation, was supplied by Fumatech. Sulphonated poly(ether ether ketone), SPEEK, with 67% degree of sulphonation was prepared in our laboratory, as described elsewhere [5], by functionalising poly(ether ether ketone) supplied by Victrex. Both natural and synthetic silicates were used for the present investigation. Montmorillonite, a natural layered silicate, supplied by Su¨d Chemie was utilised to prepare the composite membrane based on SPEK. Laponite, a synthetic layered silicate, was supplied by Solvay Soda Deutschland GmbH, Rheinberg. Magadiite, a silicic acid, was prepared in the laboratory by a procedure described elsewhere [14]. 2.2. Preparation of polymer composite membranes Prior to the incorporation of the silicates in to the polymer matrix, they were modified using organo-silanes by a procedure which can be found elsewhere [15]. Membranes were prepared by a normal casting process. The preparation of composite membranes based on silicates involved two stages. A good dispersion of the silicates in dimethyl formamide (DMF) or N-methyl pyrollidone (NMP) solvent was obtained first. Then, the polymer was added to the silicate dispersion and allowed to stir until a homogeneous system was obtained. Finally, the entire
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system was cast on a silylated glass plate. The composite membranes were obtained by the evaporation of solvent at 50 C for 24 h in the case of SPEK and 70 C, 24 h for SPEEK membranes. The membranes were post-heated in a vacuum oven at 100 C for about 24 h. Thus, SPEK membranes containing 10 and 20 wt% of montmorillonite were prepared and SPEEK membranes with 10, 20 wt% of laponite or magadiite were prepared. In the case of SPEK, homogeneous membranes were achievable only with montmorillonite whereas for SPEEK, montmorillonite was not effective in producing homogeneous membranes. The membranes thus prepared were characterized for methanol and water permeabilityÕs, and proton conductivity. 2.3. Characterisation methods 2.3.1. Pervaporation Prior to the measurements, the membranes were conditioned by the following procedure. The plain and composite membranes were immersed in 20% methanol solution in water for 3 days followed by immersion in 0.33 M sulphuric acid for 24 h. The water and methanol permeabilitiy across the membranes were measured by pervaporation using the experimental set-up described elsewhere [6]. A feed containing 20% methanol solution in water was kept at 55 C and was circulated on one side of the membrane held in a Millipore cell for 47 mm membrane diameter, maintaining the permeate side evacuated. A glass trap immersed in liquid nitrogen was utilised to collect the permeate solution after every 1 h. The condensed permeate was weighed and the composition was determined by refractive index (20 C). As one can visualise from this experimental set-up, one side of the membrane is in direct contact with the liquid feed and in the permeate side there is only the desorbed permeated vapour. Although in DMFC, a more complex construction of the membrane electrode assemblies with catalytic reaction process is involved, the pervaporation technique shown in this work is a good method to represent the methanol and water transport in membrane in DMFC. From the permeate mass, the total flux was calculated. The methanol flux was calculated from the methanol percentage determined from refractive index or gas chromatography. In order to ensure consistency of the values the experiments were repeated for three times. The values reported herein, are therefore, the average of values obtained from three experiments. In addition, the values and units reported herein are normalised for the different thickness of the membranes. 2.3.2. Proton conductivity The proton conductivity was measured by impedance spectroscopy [5] using a HP 4284A spectrometer working in the frequency range between 101 and 106 Hz.
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direct methanol fuel cell. It is also clear that the values decrease on increasing the content of montmorillonite. The incorporation of silicates in a polymer matrix creates a tortuous path [16] for the permeant to pass through thereby retarding the flux. Similarly, the silicates in SPEK would have created obstacles for methanol–water to pass through the membrane thus preventing them from transferring across the membrane to the permeate side. When the amount of silicates increased to 20% the barriers were even greater resulting in still lower permeability value. Table 2 gives the permeability values of membranes based on sulphonated poly(ether ether ketone). It is clear that the synthetic silicates namely laponite and magadiite are also effective in reducing the methanol and water permeability in membranes. Here again the values decrease with the increase in content of layered silicate emphasizing that layered silicates either natural or synthetic are useful in decreasing the methanol/water flux in the membranes. It is possible to discuss the decrease in permeability using a simple lamellae model of the whole composite membrane system. We believe the flake-filled membrane as lamellae where each lamella has two layers: one of polymer and another containing polymer and flakes. Diffusion of the permeate first occurs through the layer of only polymer and then occurs through the second layer (polymer and flakes) either by wiggling around the flakes or going through the flakes as occurs in some cases. As a result, each lamella creates three resistances; a resistance due to the first pure polymer layer in series with the parallel resistance due to the flakes and wiggling or tortuous path in the second layer.
The measuring cell consisted of two compartments, filled with 0.33 M H2SO4 solution as liquid electrolyte, separated by the membrane. The platinum electrodes with diameter 2.8 cm were immersed in the electrolyte solution, keeping a distance of about 2 mm between them. Before the measurement, samples were immersed in water at room temperature during at least 3 · 24 h to ensure that leaching of any membrane component would not lead to an error in the measured conductivity value. The membrane was then immersed during 24 h in 0.33 M H2SO4 at 50 C. Measurements were done at 25 C, determining the impedance at null phase shift. It was found that there was no leaching of inorganic components from any of the composite membranes reported in this work. 2.3.3. Scanning electron microscopy (SEM) The membranes were fractured in liquid nitrogen and were observed in a LEO 1530 Gemini field emission scanning electron microscope at 1 kV using secondary electron detector.
3. Results and discussion 3.1. Methanol and water permeability Table 1 displays the results of the permeabilities of the methanol and water through membranes based on SPEK. It is evident from the table that the methanol flux decreases on incorporation of montmorillonite, which is essential and desirable for the efficient functioning of
Table 1 Permeability and conductivity values of membranes based on SPEK Membranes
Thickness (lm)
SPEK SPEK/10% montmorillonite SPEK/20% montmorillonite
30 140 180
Conductivity (mS cm1)
Permeability (1017 m2 s1 Pa1) Methanol
Water
14 9 5
90 101 77
54 56 46
Table 2 Permeability and conductivity values of membranes based on SPEEK Conductivity (mS cm1)
Membranes
Thickness (lm)
Permeability (1017 m2 s1 Pa1) Methanol
Water
SPEEK SPEEK/10% SPEEK/20% SPEEK/10% SPEEK/20%
90 112 111 205 132
18 13 7 8 3
129 91 74 92 18
laponite laponite magadiite magadiite
97 116 89 111 102
C.S. Karthikeyan et al. / European Polymer Journal 41 (2005) 1350–1356
the calculations using Eq. (2), it was found that the experimental and theoretical (calculated from right hand side of Eq. (2) for volume fraction 0.1) permeability reduction values agree for aspect ratios 5, 10, 20 for montmorillonite, laponite and magadiite respectively. Although these two equations predict the decrease in flux quantitatively, they are actually based on some approximations that the flakes are uniformly distributed throughout the membrane and they are aligned. These conditions may be difficult to achieve experimentally. We mentioned earlier under preparation that the membranes were prepared by casting polymer solutions containing the silicates followed by evaporation of the solvent hoping that the suspended filler material would remain homogeneously distributed. But the true picture displays (Fig. 2) the non-uniformly distributed system. In fact, this homogeneity is harder to achieve for high concentration (10 and 20 wt%) of silicates as these layered silicates are believed to be completely exfoliated only at quite lower concentrations (65 wt%). Similar results of permeability studies on such larger particles for the same volume fraction (0.1) can be found in the literature [18] where the occurrence of settling of the flakes at the bottom was reported. This is in accordance with the model shown in Fig. 1. In addition, due to this inhomogeneity the volume fraction of the flakes is not uniform throughout the film. It is higher at the bottom where the flakes are present and nearly zero at the upper layer (Fig. 1) because the flakes have settled out of this region.
The permeability results are also discussed using some theories. For instance, Maxwell (Eq. (1)) [17] gave a general equation for the reduction in permeability for a system containing impermeable filler in a periodic manner. P 0 1 þ /=2 ¼ P 1/
ð1Þ
where P = permeability of the composite system, P0 = permeability of the plain system, / = concentration (in volume fraction) of the filler. There are two limitations of this model. Firstly, this model is independent of size of the filler, varies only with the volume fraction. Secondly, this equation is valid only for volume fraction 60.1. Using MaxwellÕs model we calculated the permeability reduction for the volume fractions that were used in this work. The calculated values are compared with the permeability reduction values obtained from the pervaporation experiments and shown in Table 3. It is clear the experimental values are always are higher than the predicted (theoretical) ones. The reasons for such discrepancy could attribute to the following facts. The fillers that were used in the present work are flakes or layered materials having certain aspect ratio. The aspect ratio part was not considered in MaxwellÕs model. Second aspect is that this MaxwellÕs model is applicable only for dilute systems whereas in the present work, the volume fractions are higher than is applicable. These factors were later considered by Cussler [13] and gave a modified MaxwellÕs equation (Eq. (2)) containing the aspect ratio (a) of the flakes or layers. P0 a2 /2 ¼1/þ P 1/
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Methanol-water
ð2Þ
The aspect ratio term merits discussion. Any flake has three dimensions: the smallest dimension namely the thickness; the largest dimension namely the length and an intermediate dimension called width. The aspect ratio in Eq. (2) is defined as the half the width divided by the thickness. The length of the flakes is predicted not to affect the permeability. In other words, the length of the flakes is considered important only for mechanical properties not for the permeability of the composites. From
Polymer Polymer and silicates
Reduction in Methanol-water flux
Fig. 1. Lamellae model for the reduction in permeability in composite membranes.
Table 3 Comparison of experiment and theoretical permeability reduction Volume fraction of silicates a
0.04 0.10b 0.13c a b c
Permeability reduction Theoretical
Laponite
Magadiite
Montmorillonite
1.06 1.18 1.22
1.41 1.82 –
1.47 7 –
– – 1.28
For 10 wt% of laponite and magadiite. For 20 wt% of laponite and magadiite. For 20 wt% of montmorillonite.
C.S. Karthikeyan et al. / European Polymer Journal 41 (2005) 1350–1356 Conductivity / mScm-1
1354
60 56
SPEK
SPEK / 10 MM
SPEK / 20 MM
52 48 44 40 70
80
90
100
110
120
2
Permeability / m Pa-1 s-1
Fig. 3. Plot of total permeability (methanol and water) versus proton conductivity of SPEK and composite membranes (MMmontmorillonite). Fig. 2. SEM picture of SPEK/10 wt% montmorillonite.
Surprisingly, in such studies, it was found that settling make the barrier about 10 times less permeable than when the flakes are homogeneously dispersed. In the present work, we have a similar situation hence the permeability reduction reported in this work gives a true picture where the silicates are more effective in decreasing the methanol–water cross-over. It is clear that this type of inhomogeneity may be responsible for some of the experimental deviations from the theoretical models. 3.2. Proton conductivity 3.2.1. Sulphonated poly(ether ketone) Table 1 also displays the results of proton conductivity for membranes based on SPEK, measured in 0.33 M H2SO4. One can notice that the conductivity slightly increases for a composite membrane with 10% montmorillonite. At higher silicate content (20%), the value decreases and is lower than the pure polymer. In the case of 20%, the higher addition of layered silicates, apart from creating obstacles for the methanol/water to pass through the membrane; have also retarded the mobility of the protons in the membrane by the following mechanism. The addition of more amounts of layered silicates prevents water, which is responsible for proton conduction, from entering the membranes thereby resulting in reduction in conductivity in the composite membrane. In other words, the protons in the membrane system are stationary due to lack of water. Therefore, in the case of SPEK/montmorillonite system, the proton conductivity decreases at higher silicate amounts. However, one can observe that the reduction in conductivity is not that drastic. Another explanation for the present result is based on the permeability/flux. There is a correlation between the diffusion coefficient or flux and the ionic conductivity of the membrane, which is in contact with the electrolyte
solution [19]. Fig. 3 depicts the plot between the total permeability or flux (methanol and water) and the proton conductivity obtained from this work. It is clear that when the total permeability is high, the conductivity is also high, as in the case of SPEK/10% montmorillonite. At the same time, the conductivity is lower when the permeability is also low (SPEK/20% montmorillonite). This correlation follows the trend previously observed for composite membranes based on zirconium oxide and zirconium phosphates [20]. Therefore, the increase in conductivity in the case of SPEK/10 wt% montmorillonite may be attributed to increase in flux. This suggests that the same transport mechanism governs both methanol/water crossover and proton conductivity. It is evident that the incorporation of silicates decreases the methanol permeability and slightly increases proton conductivity in the case of membranes containing 10% montmorillonite. Although 20% displays a slightly lower conductivity value than the pure polymer, the decrease is not drastic and its permeability values are encouraging. This novel approach of using layered silicates into a sulphonated polymeric system thus proves to be a desirable polymer electrolyte membrane for direct methanol fuel cells. 3.2.2. Sulphonated poly(ether ether ketone) The proton conductivity values for composite membranes based on sulphonated poly(ether ether ketone) are given in Table 2.The conductivity of membrane with 10 wt% laponite is higher than the plain polymer. When the laponite content increased to 20 wt%, the value decreased. This behaviour is similar to SPEK/montmorillonite membrane system. Table 2 also depicts the proton conductivity values of SPEEK membranes containing magadiite. In this case too, the proton conductivity of membranes with 10 wt% is higher than the plain followed by a lower value for 20 wt%. However, here the conductivity of membrane containing 20 wt% is also higher than the plain polymer, which is encouraging. In other words, the composite membrane containing
C.S. Karthikeyan et al. / European Polymer Journal 41 (2005) 1350–1356
20 wt% of magadiite has very low methanol/water permeability (lower than 10 wt%) and a better proton conductivity with respect to the SPEEK membrane. The proton conductivity results can be explained by making a plot between the total permeability and conductivity (Fig. 4). From Fig. 4 it is evident that the conductivity of the composite membrane based on laponite increases with total permeability, which emphasizes that there exists a relation between total flux and conductivity similar to the SPEK system. Therefore, increase in proton conductivity in composite membrane is due to increase in total permeability. It demonstrates again that same transport mechanism presides over both methanol/water crossover and proton conductivity. Fig. 5 shows the plot of total permeability versus proton conductivity for the membranes based on SPEEK containing magadiite. Here again the conductivity improves with the total flux emphasizing again the existence of a good relation between total flux and conductivity. This relation unambiguously explains the increase in proton conductivity in membranes containing magadiite, a synthetic silicate. In all these cases from Figs. 3–5, the plain membranes are shown in the plot for reference purpose only.
Conductivity / mScm-1
150 SPEEK
SPEEK / 10 lap
SPEEK / 20 lap
120 90 60 30 0
70
90
110
130
150
170
Permeability / m2 Pa-1 s-1
Conductivity / mScm
-1
Fig. 4. Plot of total permeability (methanol and water) versus proton conductivity of SPEEK membranes containing laponite.
SPEEK / 10 mag
SPEEK / 20 mag
105
85 15
35
55
75
95 2
In Fig. 3, sulphonated poly(ether ketone) has flux and conductivity values in between 10 and 20 wt% following the trend that increase in conductivity with total permeability. However, in Figs. 4 and 5 i.e. for laponite and magadiite, sulphonated poly(ether ether ketone) has higher flux than the composite membranes but it also displays a lower conductivity thus proving that this behaviour of increase in conductivity with flux may be applicable only to the composite system.
4. Conclusions The two main requirements for a membrane to function in a direct methanol fuel cell are low methanol/ water permeability and reasonably good proton conductivity. The present work clearly demonstrated that with the incorporation of layered silicates in to the sulphonated polymeric system, methanol/water permeability decreased. It was also observed that the value decreased with increase in content of the silicates. For membranes containing montmorillonite or laponite, the proton conductivity values were higher than the plain polymer for lower weight percentage whereas they decreased for higher percentage. In the case of membranes containing magadiite, the proton conductivity values were higher for 10 and 20 wt% than the plain, the latter displaying lower value than the former, similar to the trend exhibited by membranes with montmorillonite and laponite. The work also presented that there exists a relation between total permeability and proton conductivity in the composite membranes emphasising that the same transport mechanism presides over both methanol/water crossover and proton conductivity. This relationship physically explains the increase in conductivity in the composite system.
Acknowledgements The work was a part of the HGF-Strategiefonds project ‘‘Membranes and Electrodes for Direct Methanol Fuel Cell’’. The first author thanks Dr. Serge Vetter and Dr. Luis Prado for their assistance in preparing the synthetic materials that were utilized in this work. The authors would also like to thank Dr. B. Ruffmann and Mr. Hugo Silva for providing help during conductivity measurements.
125 SPEEK
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115
135
References
-1 -1
Permeability / m Pa s
Fig. 5. Plot of total permeability (methanol and water) versus proton conductivity of SPEEK membranes containing magadiite.
[1] Dillon R, Srinivasan S, Arico AS, Antonucci V. J Power Sources 2004;127:112. [2] Mikhailenko SD, Zaidi SMJ, Kaliagune S. Catal Today 2001;67:225.
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[3] Miyake N, Wainright JS, Savinell RF. J Electrochem Soc 2001;148:A905. [4] Nunes SP, Rikowski E, Ruffmann B, Richau K. New materials for electrochemical systems IV, Montreal, July 2001. p. 373. [5] Nunes SP, Ruffmann B, Rikowski E, Vetter S, Richau K. J Membr Sci 2002;203:215. [6] Ponce ML, Prado L, Ruffmann B, Richau K, Mohr R, Nunes SP. J Membr Sci 2003;217:5. [7] Schlick S. In: Schlick S, editor. Ionomers: characterization, theory and applications. New York: CRC Press; 1995. p. 2. [8] Wang MS, Pinnavaia TJ. Chem Mater 1994;6:468. [9] Gianellis EP. Appl Organomet Chem 1998;12:675. [10] Giannelis EP, Krishnamoorti R, Manias E. Adv Polym Sci 1999;138:107. [11] Alexandre M, Dubois P. Mater Sci Eng R 2000;28:1.
[12] Yano K, Usuki A, Okada A. J Polym Sci Part A: Polym Chem 1997;35:2289. [13] Cussler EL. J Membr Sci 1990;52:275. [14] Karthikeyan CS, Nunes SP, Prado LASA, Ponce ML, Ruffmann B, Schulte K. J Membr Sci, in press. [15] Prado LASA, Karthikeyan CS, Nunes SP, Schulte K. J Non-Cryst Solids, in press. [16] LeBaron PC, Wang Z, Pinnavaia TJ. Appl Clay Sci 1999;15:11. [17] Maxwell JC. Treatise on electricity and magnetism, vol. 1. London: Clanderon Press; 1881. [18] Lape NK, Yang C, Cussler EL. J Membr Sci 2002;209: 271. [19] Dammak L, Lteif R, Bulvestre G, Pourcelly G, Auclair B. Electrochim Acta 2001;47:451. [20] Ruffmann B, Silva H, Schulte B, Nunes SP. Solid State Ionics 2003;162–163:269.
European Polymer Journal 41 (2005) 1350–1356
www.elsevier.com/locate/europolj
Ionomer-silicates composite membranes: Permeability and conductivity studies C.S. Karthikeyan b
a,*
, S.P. Nunes a, K. Schulte
b
a GKSS Research Centre, Institute of Chemistry, Max-Planck Strasse, D-21502 Geesthacht, Germany Polymer Composites, Denickestrasse 15, Technical University of Hamburg-Harburg, D-21703 Hamburg, Germany
Received 22 June 2004; received in revised form 14 December 2004; accepted 16 December 2004 Available online 28 January 2005
Abstract Polymer composite membranes based on sulphonated polymers, such as sulphonated poly(ether ketone) and sulphonated poly(ether ether ketone), and silicates were prepared and characterized for water/methanol permeabilities and proton conductivity studies. The study showed methanol and water permeability in the composite system decreased, with respect to the plain polymer/ionomer, with the increase in content of silicates. The permeability reduction in the composite membranes is discussed using models and theories. It was also found that the proton conductivity of the ionomer-composite membranes increased with the increase in total flux of the system, emphasising a good correlation between the total flux of the composite membranes and proton conductivity. The work clearly demonstrates that the same transport mechanism governs both methanol–water crossover and proton conductivity in these polymer electrolyte composite membranes. 2005 Elsevier Ltd. All rights reserved. Keywords: Ionomers; Layered silicates; Membranes; Composites; Fuel cells
1. Introduction Polymer electrolyte membranes for direct methanol fuel cells (DMFC) are still under research and development in many industries and academics all over the world [1]. The key issue is synthesizing a membrane with good proton conductivity and low methanol permeability as they form a major barrier for the better performance of DMFC. It is quite established now that one
* Corresponding author. Present address: Chair of Materials Processing, University of Bayreuth, 95447 Bayreuth, Germany. Tel.: +49 921 557 209; fax: +49 921 557 205. E-mail address: [email protected] (C.S. Karthikeyan).
means to decrease the methanol permeability/cross over is by the usage of polymer composite membranes [2,3]. The fillers can be chosen in such way they either aid proton conductivity or decrease the methanol permeability. Previous reports from our group [4–6] were based on sulphonated poly(ether ketone), SPEK containing zirconium oxide, prepared by sol–gel method involving the hydrolysis of zirconium tetrapropylate in a solution of SPEK, with the addition of acetyl acetone to avoid precipitation of the inorganic phase. This was quite successful in reducing the methanol and water permeability but the proton conductivity decreased as well [5,6]. In addition, proton conductive inorganic components such as zirconium phosphate [5] and heteropolyacids [6] were incorporated during the membrane preparation in order to compensate for the conductivity [5]. But problems
0014-3057/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2004.12.011
C.S. Karthikeyan et al. / European Polymer Journal 41 (2005) 1350–1356
such as increase in permeability or bleeding of the proton conducting inorganic components from the membrane were encountered. These aspects were taken in to account in developing polymer composite membranes for DMFC for the present investigation. The present work focuses on the ionomer-composite membranes based on sulphonated poly (etherketone) and sulphonated poly(ether ether ketone) containing silicates (layered at nanoscale). Ionomers are ion-containing polymers in which the bulk properties are governed by ionic interactions in discrete regions of the material [7]. Polymer silicates composite materials have gained considerable attention [8,9]; few reviews have been reported [10,11] on their preparation and properties. These silicates generally have high aspect ratio and are low priced. They improve mechanical and thermal properties and decrease gas/vapour permeability [11]. It is claimed [12] that permeability of polymer composites is largely affected by the aspect ratio of the filler. It is also believed that flakes [13] are much more effective in decreasing the permeability. This was the motivation to explore inorganic filler with high aspect ratio such as layered silicates for preparing polymer composite membranes for DMFC.
2. Experimental 2.1. Materials Sulphonated poly(etherketone), SPEK, with 50% degree of sulphonation, was supplied by Fumatech. Sulphonated poly(ether ether ketone), SPEEK, with 67% degree of sulphonation was prepared in our laboratory, as described elsewhere [5], by functionalising poly(ether ether ketone) supplied by Victrex. Both natural and synthetic silicates were used for the present investigation. Montmorillonite, a natural layered silicate, supplied by Su¨d Chemie was utilised to prepare the composite membrane based on SPEK. Laponite, a synthetic layered silicate, was supplied by Solvay Soda Deutschland GmbH, Rheinberg. Magadiite, a silicic acid, was prepared in the laboratory by a procedure described elsewhere [14]. 2.2. Preparation of polymer composite membranes Prior to the incorporation of the silicates in to the polymer matrix, they were modified using organo-silanes by a procedure which can be found elsewhere [15]. Membranes were prepared by a normal casting process. The preparation of composite membranes based on silicates involved two stages. A good dispersion of the silicates in dimethyl formamide (DMF) or N-methyl pyrollidone (NMP) solvent was obtained first. Then, the polymer was added to the silicate dispersion and allowed to stir until a homogeneous system was obtained. Finally, the entire
1351
system was cast on a silylated glass plate. The composite membranes were obtained by the evaporation of solvent at 50 C for 24 h in the case of SPEK and 70 C, 24 h for SPEEK membranes. The membranes were post-heated in a vacuum oven at 100 C for about 24 h. Thus, SPEK membranes containing 10 and 20 wt% of montmorillonite were prepared and SPEEK membranes with 10, 20 wt% of laponite or magadiite were prepared. In the case of SPEK, homogeneous membranes were achievable only with montmorillonite whereas for SPEEK, montmorillonite was not effective in producing homogeneous membranes. The membranes thus prepared were characterized for methanol and water permeabilityÕs, and proton conductivity. 2.3. Characterisation methods 2.3.1. Pervaporation Prior to the measurements, the membranes were conditioned by the following procedure. The plain and composite membranes were immersed in 20% methanol solution in water for 3 days followed by immersion in 0.33 M sulphuric acid for 24 h. The water and methanol permeabilitiy across the membranes were measured by pervaporation using the experimental set-up described elsewhere [6]. A feed containing 20% methanol solution in water was kept at 55 C and was circulated on one side of the membrane held in a Millipore cell for 47 mm membrane diameter, maintaining the permeate side evacuated. A glass trap immersed in liquid nitrogen was utilised to collect the permeate solution after every 1 h. The condensed permeate was weighed and the composition was determined by refractive index (20 C). As one can visualise from this experimental set-up, one side of the membrane is in direct contact with the liquid feed and in the permeate side there is only the desorbed permeated vapour. Although in DMFC, a more complex construction of the membrane electrode assemblies with catalytic reaction process is involved, the pervaporation technique shown in this work is a good method to represent the methanol and water transport in membrane in DMFC. From the permeate mass, the total flux was calculated. The methanol flux was calculated from the methanol percentage determined from refractive index or gas chromatography. In order to ensure consistency of the values the experiments were repeated for three times. The values reported herein, are therefore, the average of values obtained from three experiments. In addition, the values and units reported herein are normalised for the different thickness of the membranes. 2.3.2. Proton conductivity The proton conductivity was measured by impedance spectroscopy [5] using a HP 4284A spectrometer working in the frequency range between 101 and 106 Hz.
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direct methanol fuel cell. It is also clear that the values decrease on increasing the content of montmorillonite. The incorporation of silicates in a polymer matrix creates a tortuous path [16] for the permeant to pass through thereby retarding the flux. Similarly, the silicates in SPEK would have created obstacles for methanol–water to pass through the membrane thus preventing them from transferring across the membrane to the permeate side. When the amount of silicates increased to 20% the barriers were even greater resulting in still lower permeability value. Table 2 gives the permeability values of membranes based on sulphonated poly(ether ether ketone). It is clear that the synthetic silicates namely laponite and magadiite are also effective in reducing the methanol and water permeability in membranes. Here again the values decrease with the increase in content of layered silicate emphasizing that layered silicates either natural or synthetic are useful in decreasing the methanol/water flux in the membranes. It is possible to discuss the decrease in permeability using a simple lamellae model of the whole composite membrane system. We believe the flake-filled membrane as lamellae where each lamella has two layers: one of polymer and another containing polymer and flakes. Diffusion of the permeate first occurs through the layer of only polymer and then occurs through the second layer (polymer and flakes) either by wiggling around the flakes or going through the flakes as occurs in some cases. As a result, each lamella creates three resistances; a resistance due to the first pure polymer layer in series with the parallel resistance due to the flakes and wiggling or tortuous path in the second layer.
The measuring cell consisted of two compartments, filled with 0.33 M H2SO4 solution as liquid electrolyte, separated by the membrane. The platinum electrodes with diameter 2.8 cm were immersed in the electrolyte solution, keeping a distance of about 2 mm between them. Before the measurement, samples were immersed in water at room temperature during at least 3 · 24 h to ensure that leaching of any membrane component would not lead to an error in the measured conductivity value. The membrane was then immersed during 24 h in 0.33 M H2SO4 at 50 C. Measurements were done at 25 C, determining the impedance at null phase shift. It was found that there was no leaching of inorganic components from any of the composite membranes reported in this work. 2.3.3. Scanning electron microscopy (SEM) The membranes were fractured in liquid nitrogen and were observed in a LEO 1530 Gemini field emission scanning electron microscope at 1 kV using secondary electron detector.
3. Results and discussion 3.1. Methanol and water permeability Table 1 displays the results of the permeabilities of the methanol and water through membranes based on SPEK. It is evident from the table that the methanol flux decreases on incorporation of montmorillonite, which is essential and desirable for the efficient functioning of
Table 1 Permeability and conductivity values of membranes based on SPEK Membranes
Thickness (lm)
SPEK SPEK/10% montmorillonite SPEK/20% montmorillonite
30 140 180
Conductivity (mS cm1)
Permeability (1017 m2 s1 Pa1) Methanol
Water
14 9 5
90 101 77
54 56 46
Table 2 Permeability and conductivity values of membranes based on SPEEK Conductivity (mS cm1)
Membranes
Thickness (lm)
Permeability (1017 m2 s1 Pa1) Methanol
Water
SPEEK SPEEK/10% SPEEK/20% SPEEK/10% SPEEK/20%
90 112 111 205 132
18 13 7 8 3
129 91 74 92 18
laponite laponite magadiite magadiite
97 116 89 111 102
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the calculations using Eq. (2), it was found that the experimental and theoretical (calculated from right hand side of Eq. (2) for volume fraction 0.1) permeability reduction values agree for aspect ratios 5, 10, 20 for montmorillonite, laponite and magadiite respectively. Although these two equations predict the decrease in flux quantitatively, they are actually based on some approximations that the flakes are uniformly distributed throughout the membrane and they are aligned. These conditions may be difficult to achieve experimentally. We mentioned earlier under preparation that the membranes were prepared by casting polymer solutions containing the silicates followed by evaporation of the solvent hoping that the suspended filler material would remain homogeneously distributed. But the true picture displays (Fig. 2) the non-uniformly distributed system. In fact, this homogeneity is harder to achieve for high concentration (10 and 20 wt%) of silicates as these layered silicates are believed to be completely exfoliated only at quite lower concentrations (65 wt%). Similar results of permeability studies on such larger particles for the same volume fraction (0.1) can be found in the literature [18] where the occurrence of settling of the flakes at the bottom was reported. This is in accordance with the model shown in Fig. 1. In addition, due to this inhomogeneity the volume fraction of the flakes is not uniform throughout the film. It is higher at the bottom where the flakes are present and nearly zero at the upper layer (Fig. 1) because the flakes have settled out of this region.
The permeability results are also discussed using some theories. For instance, Maxwell (Eq. (1)) [17] gave a general equation for the reduction in permeability for a system containing impermeable filler in a periodic manner. P 0 1 þ /=2 ¼ P 1/
ð1Þ
where P = permeability of the composite system, P0 = permeability of the plain system, / = concentration (in volume fraction) of the filler. There are two limitations of this model. Firstly, this model is independent of size of the filler, varies only with the volume fraction. Secondly, this equation is valid only for volume fraction 60.1. Using MaxwellÕs model we calculated the permeability reduction for the volume fractions that were used in this work. The calculated values are compared with the permeability reduction values obtained from the pervaporation experiments and shown in Table 3. It is clear the experimental values are always are higher than the predicted (theoretical) ones. The reasons for such discrepancy could attribute to the following facts. The fillers that were used in the present work are flakes or layered materials having certain aspect ratio. The aspect ratio part was not considered in MaxwellÕs model. Second aspect is that this MaxwellÕs model is applicable only for dilute systems whereas in the present work, the volume fractions are higher than is applicable. These factors were later considered by Cussler [13] and gave a modified MaxwellÕs equation (Eq. (2)) containing the aspect ratio (a) of the flakes or layers. P0 a2 /2 ¼1/þ P 1/
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Methanol-water
ð2Þ
The aspect ratio term merits discussion. Any flake has three dimensions: the smallest dimension namely the thickness; the largest dimension namely the length and an intermediate dimension called width. The aspect ratio in Eq. (2) is defined as the half the width divided by the thickness. The length of the flakes is predicted not to affect the permeability. In other words, the length of the flakes is considered important only for mechanical properties not for the permeability of the composites. From
Polymer Polymer and silicates
Reduction in Methanol-water flux
Fig. 1. Lamellae model for the reduction in permeability in composite membranes.
Table 3 Comparison of experiment and theoretical permeability reduction Volume fraction of silicates a
0.04 0.10b 0.13c a b c
Permeability reduction Theoretical
Laponite
Magadiite
Montmorillonite
1.06 1.18 1.22
1.41 1.82 –
1.47 7 –
– – 1.28
For 10 wt% of laponite and magadiite. For 20 wt% of laponite and magadiite. For 20 wt% of montmorillonite.
C.S. Karthikeyan et al. / European Polymer Journal 41 (2005) 1350–1356 Conductivity / mScm-1
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60 56
SPEK
SPEK / 10 MM
SPEK / 20 MM
52 48 44 40 70
80
90
100
110
120
2
Permeability / m Pa-1 s-1
Fig. 3. Plot of total permeability (methanol and water) versus proton conductivity of SPEK and composite membranes (MMmontmorillonite). Fig. 2. SEM picture of SPEK/10 wt% montmorillonite.
Surprisingly, in such studies, it was found that settling make the barrier about 10 times less permeable than when the flakes are homogeneously dispersed. In the present work, we have a similar situation hence the permeability reduction reported in this work gives a true picture where the silicates are more effective in decreasing the methanol–water cross-over. It is clear that this type of inhomogeneity may be responsible for some of the experimental deviations from the theoretical models. 3.2. Proton conductivity 3.2.1. Sulphonated poly(ether ketone) Table 1 also displays the results of proton conductivity for membranes based on SPEK, measured in 0.33 M H2SO4. One can notice that the conductivity slightly increases for a composite membrane with 10% montmorillonite. At higher silicate content (20%), the value decreases and is lower than the pure polymer. In the case of 20%, the higher addition of layered silicates, apart from creating obstacles for the methanol/water to pass through the membrane; have also retarded the mobility of the protons in the membrane by the following mechanism. The addition of more amounts of layered silicates prevents water, which is responsible for proton conduction, from entering the membranes thereby resulting in reduction in conductivity in the composite membrane. In other words, the protons in the membrane system are stationary due to lack of water. Therefore, in the case of SPEK/montmorillonite system, the proton conductivity decreases at higher silicate amounts. However, one can observe that the reduction in conductivity is not that drastic. Another explanation for the present result is based on the permeability/flux. There is a correlation between the diffusion coefficient or flux and the ionic conductivity of the membrane, which is in contact with the electrolyte
solution [19]. Fig. 3 depicts the plot between the total permeability or flux (methanol and water) and the proton conductivity obtained from this work. It is clear that when the total permeability is high, the conductivity is also high, as in the case of SPEK/10% montmorillonite. At the same time, the conductivity is lower when the permeability is also low (SPEK/20% montmorillonite). This correlation follows the trend previously observed for composite membranes based on zirconium oxide and zirconium phosphates [20]. Therefore, the increase in conductivity in the case of SPEK/10 wt% montmorillonite may be attributed to increase in flux. This suggests that the same transport mechanism governs both methanol/water crossover and proton conductivity. It is evident that the incorporation of silicates decreases the methanol permeability and slightly increases proton conductivity in the case of membranes containing 10% montmorillonite. Although 20% displays a slightly lower conductivity value than the pure polymer, the decrease is not drastic and its permeability values are encouraging. This novel approach of using layered silicates into a sulphonated polymeric system thus proves to be a desirable polymer electrolyte membrane for direct methanol fuel cells. 3.2.2. Sulphonated poly(ether ether ketone) The proton conductivity values for composite membranes based on sulphonated poly(ether ether ketone) are given in Table 2.The conductivity of membrane with 10 wt% laponite is higher than the plain polymer. When the laponite content increased to 20 wt%, the value decreased. This behaviour is similar to SPEK/montmorillonite membrane system. Table 2 also depicts the proton conductivity values of SPEEK membranes containing magadiite. In this case too, the proton conductivity of membranes with 10 wt% is higher than the plain followed by a lower value for 20 wt%. However, here the conductivity of membrane containing 20 wt% is also higher than the plain polymer, which is encouraging. In other words, the composite membrane containing
C.S. Karthikeyan et al. / European Polymer Journal 41 (2005) 1350–1356
20 wt% of magadiite has very low methanol/water permeability (lower than 10 wt%) and a better proton conductivity with respect to the SPEEK membrane. The proton conductivity results can be explained by making a plot between the total permeability and conductivity (Fig. 4). From Fig. 4 it is evident that the conductivity of the composite membrane based on laponite increases with total permeability, which emphasizes that there exists a relation between total flux and conductivity similar to the SPEK system. Therefore, increase in proton conductivity in composite membrane is due to increase in total permeability. It demonstrates again that same transport mechanism presides over both methanol/water crossover and proton conductivity. Fig. 5 shows the plot of total permeability versus proton conductivity for the membranes based on SPEEK containing magadiite. Here again the conductivity improves with the total flux emphasizing again the existence of a good relation between total flux and conductivity. This relation unambiguously explains the increase in proton conductivity in membranes containing magadiite, a synthetic silicate. In all these cases from Figs. 3–5, the plain membranes are shown in the plot for reference purpose only.
Conductivity / mScm-1
150 SPEEK
SPEEK / 10 lap
SPEEK / 20 lap
120 90 60 30 0
70
90
110
130
150
170
Permeability / m2 Pa-1 s-1
Conductivity / mScm
-1
Fig. 4. Plot of total permeability (methanol and water) versus proton conductivity of SPEEK membranes containing laponite.
SPEEK / 10 mag
SPEEK / 20 mag
105
85 15
35
55
75
95 2
In Fig. 3, sulphonated poly(ether ketone) has flux and conductivity values in between 10 and 20 wt% following the trend that increase in conductivity with total permeability. However, in Figs. 4 and 5 i.e. for laponite and magadiite, sulphonated poly(ether ether ketone) has higher flux than the composite membranes but it also displays a lower conductivity thus proving that this behaviour of increase in conductivity with flux may be applicable only to the composite system.
4. Conclusions The two main requirements for a membrane to function in a direct methanol fuel cell are low methanol/ water permeability and reasonably good proton conductivity. The present work clearly demonstrated that with the incorporation of layered silicates in to the sulphonated polymeric system, methanol/water permeability decreased. It was also observed that the value decreased with increase in content of the silicates. For membranes containing montmorillonite or laponite, the proton conductivity values were higher than the plain polymer for lower weight percentage whereas they decreased for higher percentage. In the case of membranes containing magadiite, the proton conductivity values were higher for 10 and 20 wt% than the plain, the latter displaying lower value than the former, similar to the trend exhibited by membranes with montmorillonite and laponite. The work also presented that there exists a relation between total permeability and proton conductivity in the composite membranes emphasising that the same transport mechanism presides over both methanol/water crossover and proton conductivity. This relationship physically explains the increase in conductivity in the composite system.
Acknowledgements The work was a part of the HGF-Strategiefonds project ‘‘Membranes and Electrodes for Direct Methanol Fuel Cell’’. The first author thanks Dr. Serge Vetter and Dr. Luis Prado for their assistance in preparing the synthetic materials that were utilized in this work. The authors would also like to thank Dr. B. Ruffmann and Mr. Hugo Silva for providing help during conductivity measurements.
125 SPEEK
1355
115
135
References
-1 -1
Permeability / m Pa s
Fig. 5. Plot of total permeability (methanol and water) versus proton conductivity of SPEEK membranes containing magadiite.
[1] Dillon R, Srinivasan S, Arico AS, Antonucci V. J Power Sources 2004;127:112. [2] Mikhailenko SD, Zaidi SMJ, Kaliagune S. Catal Today 2001;67:225.
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[3] Miyake N, Wainright JS, Savinell RF. J Electrochem Soc 2001;148:A905. [4] Nunes SP, Rikowski E, Ruffmann B, Richau K. New materials for electrochemical systems IV, Montreal, July 2001. p. 373. [5] Nunes SP, Ruffmann B, Rikowski E, Vetter S, Richau K. J Membr Sci 2002;203:215. [6] Ponce ML, Prado L, Ruffmann B, Richau K, Mohr R, Nunes SP. J Membr Sci 2003;217:5. [7] Schlick S. In: Schlick S, editor. Ionomers: characterization, theory and applications. New York: CRC Press; 1995. p. 2. [8] Wang MS, Pinnavaia TJ. Chem Mater 1994;6:468. [9] Gianellis EP. Appl Organomet Chem 1998;12:675. [10] Giannelis EP, Krishnamoorti R, Manias E. Adv Polym Sci 1999;138:107. [11] Alexandre M, Dubois P. Mater Sci Eng R 2000;28:1.
[12] Yano K, Usuki A, Okada A. J Polym Sci Part A: Polym Chem 1997;35:2289. [13] Cussler EL. J Membr Sci 1990;52:275. [14] Karthikeyan CS, Nunes SP, Prado LASA, Ponce ML, Ruffmann B, Schulte K. J Membr Sci, in press. [15] Prado LASA, Karthikeyan CS, Nunes SP, Schulte K. J Non-Cryst Solids, in press. [16] LeBaron PC, Wang Z, Pinnavaia TJ. Appl Clay Sci 1999;15:11. [17] Maxwell JC. Treatise on electricity and magnetism, vol. 1. London: Clanderon Press; 1881. [18] Lape NK, Yang C, Cussler EL. J Membr Sci 2002;209: 271. [19] Dammak L, Lteif R, Bulvestre G, Pourcelly G, Auclair B. Electrochim Acta 2001;47:451. [20] Ruffmann B, Silva H, Schulte B, Nunes SP. Solid State Ionics 2003;162–163:269.