Electrochimica Acta 50 (2005) 4771–4777
Preparation and characterization of composite membranes using blends of SPEEK/PBI with boron phosphate S.M. Javaid Zaidi ∗ Department of Chemical Engineering, King Fahd University of Petroleum and Minerals, KFUPM, Dhahran-31261, Saudi Arabia Received 16 August 2004; received in revised form 18 February 2005; accepted 18 February 2005 Available online 25 March 2005
Abstract In this contribution composite membranes have been prepared from acid–base polymer blend and solid inorganic proton conductive boron phosphate (BPO4 ). The blends are composed of sulfonated polyether-ether ketone (SPEEK) as the acidic component and polybenzimidazole (PBI) as the basic component. The contents of solid BPO4 in the composite membrane varied from 10 to 40 wt%. The conductivity of the composite membranes was measured by impedance spectroscopy at room temperature. The conductivity of the composite membranes was found to increase with the incorporation of boron phosphate particles into blend membranes. The highest conductivity of 6 mS/cm was found for composite membrane at room temperature. The membranes were characterized by X-ray diffraction (XRD), differential scanning calorimetry (DSC), and FTIR which showed acid–base interaction in the blend membranes and also confirmed the presence of solid BPO4 into the composite membranes. These membranes show good perspective in the membrane fuel cell applications. © 2005 Elsevier Ltd. All rights reserved. Keywords: Acid–base blends; SPEEK; Composite membranes; Conductivity; Boron phosphate
1. Introduction The solid polymer electrolyte membranes (PEM) have received considerable attention over the last few years, due to their use in fuel cells as a portable power source and as a replacement for batteries. It has resulted in the development of various types of membranes with diverse mechanical and electrical properties. So far perfluorosulfonate ionomer membranes such as Nafion 115 (Du Pont), Flemion (Asahi Glass Co.), Acipex (Asahi Chemical.), and Dow (Dow Chemical) have been used as polymer electrolyte membranes in these fuel cells [1]. These perfluorosulfonic acid membranes are not suitable due to their high price, the decreasing conductivity at temperatures higher than 100 ◦ C, and methanol crossover especially in DMFC applications [2,3]. Optimized proton and water transport properties of the membrane and
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proper water management are crucial for efficient fuel cell operation. Dehydration of the membrane reduces proton conductivity and excess of water can lead to flooding of the electrodes, both conditions may result in poor cell performance. The proton conductivity of the ionomer membranes relies on the presence of water, but because of high evaporation rate at temperature above 100 ◦ C there is a drastic decrease in conductivity. So, modification of the electrolyte membranes to operate at higher temperatures seems to be an alternative. As a result a number of studies have been reported in the literature to improve the properties of Nafion type ionomer membranes, or develop new ones [4–19]. For this purpose different approaches have been followed in the literature, such as: (1) modifying perfluorinated ionomer membranes or prepare acid–base blends to improve their water retention properties [4–7] at temperature above 100 ◦ C; (2) modifying ionomer membranes to improve conductivity [8–11]; and (3) preparing new electrolyte composite membranes based on proton conducting materials [12–20].
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The composite membrane approach (3) represents one of the ways to improve the properties of the polymer electrolyte membranes as the desired properties of the two components can be combined in one composite. This approach was employed in the previous studies [15,17–19] where in order to improve the proton conductivity of the PEEK, several heteropolyacids and solid boron phosphate (BPO4 ) were used as the second phase. However, some of these membranes showed excessive swelling, which makes the membranes brittle upon drying. It has been shown in [21–24] that swelling can be reduced by blending with polymers which are capable of formation of hydrogen bonds. The formation of hydrogen bonds leads to compatibilization of the blend polymer. Recently, it has been reported [24–26] that acid–base blend membranes containing acidic and basic polymers showed reduced methanol permeability and are thermally and chemically stable. Kerres et al. [24] also reported development of blend membranes of SPEEK with polybenzimidazole celazole (PBI), which showed reduction in swelling and methanol permeation, in addition to their high thermal stability and moderate conductivity at higher temperature. In the present work, SPEEK is selected as the acidic polymer and PBI as the basic polymer to make the blend. The acidic polymers are combined with the basic polymers in different acid/base ratios in order to get acid–base blend membranes with improve properties. These blend show moderate swelling with high thermal stabilities. This blend is then used as the polymer matrix to prepare composite membranes containing boron phosphate as the conductive filler. In the acid–base blend membranes specific interactions between acidic and basic components are present: hydrogen bridges and electrostatic forces by proton transfer from the acid to the basic group [22]. However, their conductivities are lowered due to the reduction in the water uptake (or swelling) as compared to the conductivity of the acidic SPEEK polymers. Their conductivity can be enhanced by the incorporation of an inorganic proton conducting solids. The present work is an attempt to improve the performance of the SPEEK/PBI blend membranes by introducing the inorganic proton conducting material, BPO4 . Boron phosphate is a solid proton conducting material developed in a previous study [16]. It has been shown previously that BPO4 under certain conditions reveals the properties of a proton conductor and have a proton conductivity of 6 × 10−2 S/cm at room temperature under full humidification. Another interesting feature of BPO4 is that it can retain water at high temperature. The composite membranes were prepared from SPEEK/PBI blend suspension and boron phosphate, and their conductivity was monitored by impedance spectroscopy as a function of PBI content for the blend and BPO4 loading for composite membranes respectively. The contents of solid BPO4 in the composite membrane varied from 10 to 40 wt%. The membranes were characterized by X-ray diffraction (XRD), differential scanning calorimetry (DSC), and FTIR techniques. The water uptake of these membranes was also investigated.
2. Experimental 2.1. Membrane preparation and characterization Sulfonated polyether-ether ketone (SPEEK) of ion exchange capacity (IEC) of 1.6 meq/g and polybenzimidazole (AB type) of intrinsic viscosity 1.3 dl/g were procured from FumaTech, Germany. Boron phosphate was synthesized from orthophosphoric and boric acids according the procedure described in [16]. First the membranes were prepared from the blends of SPEEK with PBI and then this blend was used as the polymer matrix for the preparation of composite membranes by incorporation of solid powdered boron phosphate. The blend membranes were prepared by first dissolving the SPEEK and PBI separately in appropriate amounts in dimethylsulfoxide (DMSO), then the two solutions were mixed together and stirred for long periods. The membranes were then cast on the glass plate after evaporation of the solvent. The membranes were first dried at room temperature overnight then at 120 ◦ C for 6–8 h. The composite membranes were prepared from the blend of SPEEK/PBI with powdered solid BPO4 of particle size less than 0.5 m in the same way as for the blend membranes. The membrane samples were removed by putting them in water. Differential scanning calorimetry for the membranes was carried out using METTLER TOLEDO Instruments DSC 2910 equipped with STARe software in N2 environment. Membrane samples of 6–10 mg were sliced and then compressed into aluminum pans for testing. Samples were heated first from room temperature to 110 ◦ C at a heating rate of 10 o C/min and kept at this temperature for 5 min, then heated at the above mentioned heating rate to 640 ◦ C. Thermal transitions were obtained from the heating cycle. Temperature calibration was performed using indium (Tm = 156.60 ◦ C, Hf = 28.5 J/g). The X-ray diffraction analysis was carried out using Philips Advanced D-8 diffractometer model. Cu K␣ irradiation source operated at 35 kV, 25 mA with a step size of 0.1◦ and a count time of 1.1 s per point is used. FTIR spectra were measured in transmittance mode on a Perkin-Elmer FC-16 FTIR spectrometer. The spectrum for each membrane sample was taken with the above-mentioned spectrometer in the range 400– 3500 cm−1 . The water uptake of SPEEK membranes was determined from the difference in weight (W) between the dry and the swollen membranes. The membrane, cast from polymer solution after drying, was weighed and then soaked in water until the weight remained constant. It was then taken out, wiped with blotting paper (soaking paper) and weighed again. The percentage of water absorbed was calculated with reference to the weight of the dry specimen from the following equation: (Wwet − Wdry ) Water uptake(%) = × 100 Wdry
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2.2. Conductivity measurement by electrical impedance spectroscopy (EIS) A conductivity cell was fabricated which comprises of two flat stainless steel plates (12 mm × 12 mm × 2 mm) glued to perplex holder sheets. Electrical connections were made by brazing copper wires at the back of each stainless steel sheet. The holders were held together by nuts and bolts. The cell was connected to a potentiostat (Model 283, EG&G PARC), which in turn was hooked to a lock-in amplifier (Model 2810, EG&G PARC). The lock-in amplifier created potential sine wave of desired frequency and amplitude. The waveform was communicated to the potentiostat (Model 283). Both of the instruments were connected to a microcomputer via a GPIB card. The GPIB card communicated with the instruments via IEEE-488 protocol. The equipment were driven by a software (Power Sine, EG&G PARC), which allowed to choose a range of frequencies and amplitude of the potential waveform. In the current experiment the range of frequencies move 1 mHz–120 kHz. The amplitude was 10 mV with respect to open circuit potential. Conductivity measurements were carried out on 1 cm2 area piece of membrane specimen cut and soaked in water for 24 h. Subsequently, sample was carefully placed in the conductivity cell and its electrical impedance spectroscopy spectra were recorded. The data were analyzed using Zsimpwin (EG&G PARC). They exhibited an excellent fit in an equivalent circuit with a real resistance (Rs ), leaking capacitor (Q) and internal resistance (Rp ). The conductivity of the membrane is related to resistance Rs of this circuit. This is the real resistance and is measured at high frequencies. The values were obtained from the model parameters and also cross checked with the corresponding values of real resistance at highest frequency (120 kHz). The conductivities were calculated by using formula: conductivity = thickness/(resistance × area).
3. Results and discussion 3.1. Water uptake and conductivity The amount of water absorbed or water uptake in composite membranes was determined as follows: the dry SPEEK membranes were immersed in H2 O at room temperature for 24 h until there was no further weight gain, then wiped with blotting paper and weighed. Before conductivity measurement all the membranes were treated with 1 M phosphoric acid and then washed with deionized water until they became neutral. Boron phosphate is an inorganic solid proton conducting material and its presence is expected to enhance the conductivity, which will also result by the increased water uptake of these membranes. The water uptake results of the blend SPEEK/PBI membranes with the varying compositions of PBI into the SPEEK are displayed in Fig. 1. The water uptake decreased with the ad-
Fig. 1. Water uptake of SPEEK/PBI blend membranes.
dition of PBI into SPEEK, which results in reduction of the swelling of the membranes. These results are in agreement with those of Kerres et al. [23,24], where they have shown that blending SPEEK with PBI involves a reduction in swelling in an aqueous medium and, therefore, brings about an improvement in the mechanical properties of the membranes. Similar approach is used here to reduce excessive swelling, by blending it with polymers which are capable of formation of hydrogen bonds. The formation of hydrogen bonds leads to compatibilization of the blend polymers. This reduction in swelling (decrease in water uptake) of the blend membranes brought about a decrease in conductivity of the blend membranes, as pure PBI is not conductive. The results of the conductivity of blend membranes at room temperature are given in Fig. 2. The conductivity was found to decrease with blending of SPEEK with PBI. It decreased from 0.77 mS/cm for pure SPEEK to 0.47 mS/cm when 10% PBI is present in the SPEEK/PBI blend membrane, and drops further with increasing PBI content. The fact that conductivity decreases with increasing PBI content may be explained as follows: the number of free SO3 H groups decreases by the formation of ionic crosslinkings to the imidazole groups by proton transfer, leading to the increase of protonic resistance and hence a decrease in conductivity [24]. Similar trend was reported in [23] for the SPEEK/PBI blends, where a drop in conductivity was
Fig. 2. Conductivity SPEEK/PBI blend membranes with increasing PBI content.
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Fig. 3. Water uptake of composite SPEEK/PBI blend membranes containing BPO4 .
also found with the addition of PBI content in SPEEK. These blends show good conductivities and moderate swelling combined with high thermal stabilities. The sulfonic acid groups interact with N-base by formation of hydrogen bridges and by protonation of the basic N. The SPEEK/PBI blend containing 10% PBI was used as the composting polymer blend for the preparation of composite membranes containing various amounts of BPO4 . The water absorption results of the composite membranes for SPEEK/PBI blend with BPO4 are given in Fig. 3. As can be seen, the water uptake increased with increasing BPO4 content, reached maximum at 30 wt% BPO4 and then drops down. A similar trend as that of water uptake was also observed for the conductivity of these composite membranes. The conductivity results of the fully hydrated SPEEK/PBI composite membranes with powdered solid BPO4 are showed in Fig. 4. The amount of solid PBO4 varied from 10 to 40%. The conductivity was found to increase with the increase of boron phosphate loading in the composite membrane. It increased from 0.47 mS/cm when no BPO4 is present in the blend to 5.9 mS/cm for composites containing 20% BPO4 . Hence, the increase in conductivity is associated with the presence of conductive BPO4 in the composite membranes.
Fig. 4. Conductivity composite SPEEK/PBI blend membranes containing BPO4 .
From the above results it is believed that proton conduction takes place in a more effective way for composite ionomer membranes. From these results it can be seen that proton conductivity is more dependent on BPO4 than the water content. As the water content for these composite membranes is not as high as for SPEEK/PBI blend membranes (e.g. 30% for SPEEK/PBI blend while only 16% for the composite containing 20% BPO4 ), but they show higher conductivity values. In general, it is known that proton conductivity does not directly correlate with either water uptake or ion exchange capacity across different families of polymers [20]. This suggests that the acidity of the sulfonic acid of the matrix polymer may be enhanced by the incorporation of BPO4 and resulting specific interactions between the polymer blend with BPO4 and proton conduction due to new interfacial polymer-particle properties can be assumed [12]. This is also supported by the DSC results which show single Tg for the composite membranes. 3.2. DSC analysis The thermal behavior of membranes was studied by differential scanning calorimetry technique. From DSC it is possible to obtain information about the thermal stability and about the structure of polymer blends (e.g. crystalline content, glass transition temperature, Tg , etc.). The DSC traces of the SPEEK/PBI blend membranes are given in Fig. 5. As can be seen addition of benzimidazole (PBI) into the SPEEK increased the thermal stability of the blend. The increase of thermal stability of SPEEK/PBI blend membranes is due to the extraordinary thermal stability of PBI and due to the specific interactions between the acidic and basic components in the blend. The glass transition temperature was also found to increase with the increase of PBI content into the SPEEK, which is in agreement with those of Kerres et al. [24]. From Fig. 5 follows the Tg of SPEEK sample is 201 ◦ C, the Tg of PBI is 398 ◦ C, and the Tg of SPEEK/PBI blend is 244 ◦ C, which increased further with the increase of PBI content The glass transition temperature, Tg of the blend membranes with the increase of PBI content are plotted in Fig. 6, which show a continuous increase in Tg with increase of the PBI content.
Fig. 5. DSC curves of SPEEK, PBI and SPEEK/PBI blend membranes.
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Fig. 6. Effect of PBI content on the glass transition temperature of SPEEK membranes. Table 1 Glass transition temperature of various membranes Sample designation
Weight fraction (wt%)
Tg (◦ C)
SPEEK/PBI SPEEK/PBI SPEEK/PBI Pure PBI Pure SPEEK [SPEEK + PBI]/BPO4 [SPEEK + PBI]/BPO4
90/10 80/20 60/40 0/100 100/0 90/10 50/50
244 262 364 398 201 239 224
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Fig. 8. XRD of blend SPEEK/PBI membranes.
of BPO4 solid as the composite show no degradation peak while a small degradation was shown on pure SPEEK/PBI polymer at 250 ◦ C. However, the glass transition temperature decreased with the addition of BPO4 . This may indicate that inorganic BPO4 solid is not miscible or compatible with the SPEEK/PBI blend. The reasons are not clear for this decrease and more detailed analyses is needed to investigate this behavior. 3.3. X-ray diffraction
The increase in Tg with the increase of PBI content shows a strong acid–base interactions between the acid and base component of the two polymer blend. Also, only one Tg was observed for these blends, which indicate that the two polymers are miscible. These results are summarized in Table 1. In another set of membranes, composite membranes were prepared by incorporating BPO4 into the SPEEK/PBI polymer matrix. For this purpose SPEEK containing 10 wt% PBI was used. The loading of solid BPO4 into the SPEEK/PBI was varied between 10 and 50 wt%. The DSC results for the composite membrane containing BPO4 into SPEEK/PBI polymer matrix are given in Fig. 7. The thermal behavior of these composite membranes was improved by the presence
The nanostructure of membranes and its blends was investigated using X-ray diffraction scattering. The XRD analysis on the membranes was carried out for the hydrated as well as the dry membranes. The X-ray diffraction analysis for 20 wt% PBI into SPEEK membranes and also for SPEEK/PBI composite membranes containing solid BPO4 showed similar trend. All the blend and composite membranes show peaks at similar diffraction angles 2θ as shown in Figs. 8 and 9. By looking at the results, the dry membranes showed low intensities whereas the intensity of the hydrated membranes is stronger than the dry ones at the same/similar diffractions angles and d-spacing. Comparing XRD patterns
Fig. 7. DSC results for the composite SPEEK/PBI membranes containing BPO4 .
Fig. 9. XRD of composite membranes from blends of SPEEK/PBI and BPO4 .
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Fig. 10. FTIR Spectra of blend and composite membranes.
of these membrane samples with the XRD of solid BPO4 (not shown here), indicate that the sharp peaks in the XRD patterns are attributed to the presence of solid boron phosphate. Some of these peaks match up with the typical peaks found in various forms of crystalline BPO4 . This confirms that BPO4 incorporated into ionomer stays in the polymer matrix. 3.4. FTIR spectra It is known from the literature that the ionic interactions in the blend of polymeric acids and bases can be observed via FTIR spectroscopy. The FTIR analysis of the acid–base blend and composite membranes showed characteristic bands indicating the ionic crosslink formation between the sulfonated and basic polymers occurring by proton transfer. The FTIR spectra of the SPEEK/PBI blend and composite membranes are shown in Fig. 10. The IR acid–base interaction bands found in SPEEK/PBI blends have been reported previously by Kerres et al. [24,26]. In their work the characteristic bands have been observed between 1300 and 1800 cm−1 . In the present work the characteristic bands have also been observed in the range of 1300–1800 cm−1 for the SPEEK/PBI blend membranes. This shows the ionic interaction between the sulfonic acid groups and the benzimidazole. The FTIR spectra of the composite membrane containing BPO4 in the SPEEK/PBI blend, also shown in Fig. 10, show bands similar to the SPEEK/PBI blends except that some bands shifted slightly and small characteristic bands of BPO4 at 998, 1510 and 1996 cm−1 , are found. This confirms the existence of solid boron phosphate in the composite membrane.
4. Conclusions In this study preparation, conductivity and characterization of acid/base polymer blend membranes and their composites containing solid boron phosphate has been presented. The acid–base blend membranes showed very good ther-
mal stability, moderate swelling and good proton conductivity. By DSC analysis a marked increase in glass transition temperature was found, which is an indication of interactions between the acidic and the basic blend components and ionic crosslinking. The proton conductivity of the acid–base blend varied in a broad range. The characteristic bands in the FTIR spectra of the blend membranes also suggest the ionic crosslink formation between the SPEEK and PBI. The composite blend membranes containing boron phosphate showed an enhancement in conductivity with the addition of BPO4 particles into SPEEK/PBI blend. The conductivity reached to 6 mS/cm for composite membrane containing 20 wt% BPO4 . The increase in conductivity of these composite membranes is associated with the presence of BPO4 particles, which forms polymer–BPO4 interface. The glass transition temperature of the composite membranes decreased with the incorporation of BPO4 , which indicate that blends of SPEEK/PBI with BPO4 are not compatible. The FTIR and XRD analyses of the composite membranes confirmed the presence of solid BPO4 into the membranes. These membranes showed conductivity comparable to Nafion membrane (of the order of 10−2 S/cm), have glass transition temperature (Tg ) more than 220 ◦ C, whereas the Tg of Nafion is 80 ◦ C and are mechanically stronger than Nafion membranes. They have a strong potential for use in fuel cells.
Acknowledgement The financial support of the King Fahd University of Petroleum & Minerals is highly acknowledged.
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