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Chemical modification of polysulfone: Composite anionic exchange membrane with TiO2 nano-particles
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 8 ( 2 0 1 3 ) 5 1 1 5 e5 1 2 1
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Chemical modification of polysulfone: Composite anionic exchange membrane with TiO2 nano-particles Patrick T. Nonjola*, Mkhulu K. Mathe, Remegia M. Modibedi Material Science and Manufacturing, Council for Scientific and Industrial Research, PO Box 395, Pretoria 0001, South Africa
article info
abstract
Article history:
Synthesis of quaternary polysulfone/Titanium dioxide (QPSf/TiO2) nanocomposite mem-
Received 30 November 2012
branes by the recasting procedure as suitable electrolyte in alkaline fuel cells is described.
Received in revised form
The composite membranes were characterized by ionic conductivity measurements, TGA,
24 January 2013
SEM, XRD, and AFM. Thermal analysis results showed that the composite membranes have
Accepted 7 February 2013
good thermal properties. The introduction of the inorganic filler supplies the composite
Available online 9 March 2013
membrane with a good thermal resistance. The physico-chemical properties studied by means of SEM and XRD techniques suggested the uniform and homogeneous distribution
Keywords:
of TiO2 at 2.5 wt.% loading, and negligible agglomeration at 10 wt.% loading, also indicated
Quaternized polysulfone
enhancement of crystalline character of these membranes. The energy dispersive X-ray
Nanocomposite membranes
spectra (EDS) analysis gave proportional percentages that the distribution of Titania
Ionic conductivity
element on the surface of the composite membrane was uniform. Observations from the
Alkaline fuel cells
results suggest that QPSf/TiO2 nanocomposite membranes have good prospects for
Thermal stability
possible use in AFC. Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Proton-exchange membrane fuel cells (PEMFCs) have been recognized as promising energy source systems due to their high energy-conversion efficiency, high power density, and low pollutant emission [1e3]. Over the past few decades, much effort has focused on proton-exchange membrane (PEM) development with significant progress made to date [4,5]. However, the crucial drawbacks of PEMFCs, such as their temperature limited operation and the low durability of their metal catalysts under acidic conditions, are restricting their commercialization [6]. Despite tremendous global effort dedicated to researching for non-platinum (Pt) and nonprecious metal catalysts, there has been no demonstrated PEMFC completely free from noble metal catalysts in both the anode and cathode electrodes [7e11]. Recently, increasing
attention has focused to alkaline anion exchange membrane fuel cells (AAEMFCs) because of their potentially higher durability [3,12e14] and reactivity of the metal catalysts in alkaline media [15e17]. In AEMFCs, much effort has been dedicated to the development of good hydroxide-conductive electrolyte membranes, in which polymers carrying quaternary ammonium groups [15,18,19] and hydroxide-doped polymers [17,20] have been studied. These polymeric membranes are expected to be of a lower cost and comparable superior electrochemical properties to PEMs. Although promising, the alkaline fuel cell performance still needs to be substantially improved before commercialization [21]. A widely quoted concern with anion exchange membranes is membrane stability in alkaline conditions, especially at elevated temperatures [13,22,23]. The chemical degradation of AEMs underpins largely from nucleophilic attack on the
* Corresponding author. Tel.: þ27 (0) 12 841 4182. E-mail address: [email protected] (P.T. Nonjola). 0360-3199/$ e see front matter Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2013.02.028
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cationic exchange sites by the hydroxyl ion. This results in the loss of cationic exchange sites followed by a decrease in the ionic conductivity of the polymer. The recognized degradation mechanisms for ammonium cation groups are (i) Hofmann elimination, (ii) nucleophilic substitution and (iii) ylide formation. Hoffman elimination reaction wherein the hydroxyl ion attacks b-hydrogen relative to the cation, forming a double bond between a and b-carbon, resulting in the cation being released. Direct nucleophilic displacement occurs by attack at the a-carbon atom of the cation, which can either detach the cation groups from the AEM or convert quaternary ammonium groups into tertiary amines. Degradation through the ylide pathway start with hydroxyl ion attack on a methyl group of the cations and produces a water molecule along with an ylide intermediate. Most recently, Fujimoto et al. [24] reported that both functional group and the specific attachment site may be important to the stability of AEMs. Several reviews on the chemical modification of polysulfone (PSf) have been reported using different mechanisms for the introduction of functional groups onto the polymer backbone [25,26]. The constant interest by researchers for PSf, a high performance engineering thermo-plastic material, is due to its excellent workability and high mechanical strength [27]. Fuel crossover through the membrane badly affects the performance of the fuel cell, which requires the modification of existing membrane properties or the development of new membranes. For comparison, Yu and Scott [28e30] studied the electrochemical performance of the alkaline direct methanol fuel cell (ADMFC) with anion exchange membranes. The direct methanol fuel cell (DMFC) performance with maximum power density of about 10 mW cm2 was obtained in a commercial quaternary ammonium anion exchange membrane (Morgane-ADP, Solvay SA, Belgium). In addition, the PVA/PWA based membranes have been prepared and applied on DMFC [31,32]. Varcoe et al. [13,33e35] developed and characterized the quaternary ammonium radiation grafted ETFC, PVDF and FEP alkaline anion exchange membrane (AAEM). In the effort of modifying the existing anion exchange membrane properties, the incorporation of inorganic additives are being investigated with the objective of serving the dual functions of improving water retention as well as reducing the fuel crossover through the membrane [36]. Although many types of membranes have been developed, manufactured and commercialized in the past, there is now a lot of emphasis on the development of mixed-matrix and nanocomposite membranes using compatible and cheap materials as well as facile and environmental friendly synthesis. Organic/inorganic hybrids can offer opportunities in many areas and not last in the field of fuel cell materials. Their main characteristic is the capability to combine the properties of the components. Choosing suitable materials, it is thus possible to reach the right features for different applications. The addition of inorganic filler into polymer matrix is to reduce the glass transition temperature (Tg) and the crystallinity of the polymer, and also allow the increase of the amorphous phases of polymer matrix, which then increase the ionic conductivity [37]. There are various inorganic filler, such as Al2O3, TiO2 [38], and SiO2 [39], that have been extensively studied. These experimental results indicated improvements in the ionic conductivity, thermal and mechanical
properties as the different inorganic fillers were added into the polymer matrix. Yang et al. [37] demonstrated that alkaline DMFCs with cross-linked PVA/TiO2 composite membranes show good electrochemical performances at ambient temperatures and pressure. Recently, Sangeetha et al. [40] prepared the quaternized polysulfone/ZrO2 membranes that can be applied in alkaline fuel cells. In this work, the inorganic component added to the organic matrix is the TiO2 nano-particle. The incorporation of the nano-particles in QPSf membrane has several attributes of interest, which includes reduced alcohol permeability and improved morphological stability without compromising ionic conductivity. Different loadings (2.5 wt.% and 10 wt.%) of TiO2 nano-particle have been used to prepare the QPSf/TiO2 composite membranes. The objective of this work is to determine if simple, inexpensive materials can be used in alkaline anion exchange membranes. The membranes were characterized and compared with pure QPSf. The surface morphology, crystallinity, thermal stability and ion conductivity properties are discussed. More evidence to support this proposition will be reported soon.
2.
Experimental section
2.1.
Materials
All the reagents were purchased from SigmaeAldrich and used without further purification unless otherwise stated. Polysulfone P-3500 LCD MB7 was supplied by Solvay Specialty Polymers. Chloroform (98%) anhydrous, Paraformaldehyde (95%), Chlorotrimethylsilane (98%), SnCl4, Trimethylamine (45 wt.% in H2O) and Titanium dioxide powder (TiO2) were all bought from SigmaeAldrich. Potassium Hydroxide KOH (84%) and Ethanol (99%) were bought from Merk Chemicals.
2.2.
Characterization of QPSf/TiO2
The water uptake was measured in 1 M KOH solution at 21 C for one day by monitoring the dry weight and wet weight of the sample. The ion exchange capacity was found by back titration technique. The dried and weighed polymer membrane was soaked in 1 M KOH solution for 24 h. The ionic conductivity of the samples in 1 M KOH solution was measured at 21 C on the potentiostat (Autolab PGSTAT302) from 100 mHz to 100 kHz with oscillating voltage of 10 mV. Thermogravimetry analyses (TGA) was performed from room temperature to 800 C on a thermal analyser (Q500 V20.1) at the heating rate of 10 C/min under nitrogen. Morphology studies were used to view the surface of the polymer
O O
S
CH3 O
C
O
CH3 CH2 N+(CH3)3HO-
n
Fig. 1 e Chemical structure of the quaternary polysulfone (QPSf).
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Table 1 e Water uptake and ionic conductivity of QPSf, QPSf/2.5% TiO2 and QPSf/10% TiO2 in 1 M KOH solution at 21 C. Compound
Water uptake (%)
Ion conductivity (S cm1)
Membrane thickness (mm)
9 32 39 19
0.0487 0.0847 0.1252 0.07
40 41 52 172
QPSf QPSf/TiO2 2.5% QPSf/TiO2 10% Nafion 117
membrane and also to predict the distribution of the nanoparticles into the polymer matrix by a scanning electron microscopy (SEM) (membranes coated with gold), X-ray diffraction (XRD) and the atomic force microscopy (AFM EC_SPM).
2.3.
Preparation of QPSf/TiO2
Polysulfone (PSf) (Udel P-3500, Amoco) was chloromethylated with paraformaldehyde and trimethylchlorosilane as the chloromethylating agent and stannic chloride as the catalyst, according to the procedure described by Avram and co-workers [41]. Specifically, paraformaldehyde (3.39 g, 11.3 mmol) and trimethylchlorosilane (12.3 g, 11.3 mmol) were added to a solution of PSf (5 g or 1.13 mmol PSf in 250 ml chloroform) in a flask equipped with a reflux condenser and a magnetic stirrer, and then stannic chloride (0.0294 g, 0.113 mmol) was added dropwise. The reaction mixture was stirred at 50 C for 48 h. Subsequently the reaction mixture was poured into ethanol (95%), and white chloromethylated polysulfone (CMPSf) precipitated immediately. The precipitate was filtrated, washed thoroughly with ethanol, and dried in vacuum at room temperature for 12 h. The degree of chloromethylation (DC) of the CMPSf obtained was 100% (determined from the 1H NMR spectrum). Quaternary polysulfone (QPSf) given in Fig. 1 was synthesized by quaternary amination of CMPSf with trimethylamine solution. Specifically, CMPSf (1.02 g, 2.52 mmol eCH2Cl) was dissolved in 10 ml of dimethylacetamide (DMAc), and then trimethylamine solution (1.408 g, 2.52 mmol) was added. The reaction mixture was stirred at 80 C for 12 h, and then poured into a Petri dish; the DMAc was removed by evaporation at 40 C over 2 days to obtain QPSf. QPSf-OH was obtained by treating QPSf-Cl in 1 M KOH at room temperature for 48 h; it was washed thoroughly and immersed in deionized water for 48 h to remove residual KOH. The QPSf-OH/TiO2 composite membranes were prepared by using a recasting method [42]. The QPSf-OH was dissolved in a desired amount of DMAc to form a solution containing 10 wt.% of QPSf-OH and different proportion of TiO2 powder was added to this solution and subjected to ultrasonic bath for 15 min and stirred further for 1 h. The resulting mixture was cast onto Petri dish to obtain the desired thickness. The cast membrane was dried at 70 C for 8 h. All membranes were stored in deionized water before any test was performed/tests were performed.
3.
represented as QPSf/2.5 wt.% TiO2 and QPSf/10 wt.% TiO2, respectively. The membranes appeared white due to the interaction between polymer and TiO2 powder and homogeneous with a good mechanical flexibility. The chemical structures of CMPSf and QPSf were confirmed by FT-IR and 1H NMR as indicated by Corti et al. [43]. In this work, we report on the formation of QPSf/TiO2 composite membranes, which has several attributes of interest, including decreased membrane swelling, reduced permeability towards methanol and improved morphological stability without compromising ion conductivity. The ionic conductivity test was used as an evaluation tool for possible application in AFC. Generally, the ionic conductivity is closely linked with ion exchange capacity (IEC) and water uptake [38,40]. The IEC of membranes is largely determined by the degree of chloromethylation (DOC), and thus there are no absolute limits or range. The mechanical properties of the ultimate membrane depend on the water content, which is mostly determined by the IEC. At very high DOC, the considerable swelling in water decreases the mechanical strength and the membrane is of little practical use in fuel cell. As discussed earlier that TiO2 may change the morphology of the membrane by changing the shape of the ionic cluster and water content [36,37]. The values of the ionic conductivity and water absorption corresponding to the composite membrane are represented in Table 1. The increase in ionic conductivity is because of the increase in ion exchange capacity and water content of the composite membrane, which facilitates the transport of ions. The composite membrane QPSf/10 wt.% TiO2 shows a maximum conductivity of 1.25 102 S cm1, which is higher than QPSf/2.5 wt.% TiO2 and pure QPSf [44], respectively. These results were similar to the results observed by Sangeetha et al. [40].
Results and discussions
Two types of QPSf composite membranes were prepared by varying the loading percentage of the inorganic filler (TiO2) by between 2.5 wt.% and 10 wt.%. These types of membranes are
Fig. 2 e TGA curves for QPSf and composite membranes. Heating from 25 C to 700 C at 10 C/min under N2 atmosphere.
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Fig. 3 e SEM images of surface and cross section QPSU (a,b), QPSf/2.5 wt.% TiO2 (c,d), and QPSf/10 wt.% TiO2 (e,f).
In order to get the insight into the composite membranes thermal stability, the thermogravimetry analyses (TGA) experiments were performed from room temperature to 800 C, as shown in Fig. 2. On comparison of the TGA thermograms of the QPSf/TiO2 composite membranes with pure QPSf, it was observed that all the membranes retain more than 90% of its weight up to a temperature of about 190 C. Beside the weight loss below 190 C due to hydration water, another weight loss between 200 C and 400 C, corresponding to the cleavage of quaternary ammonium groups from QPSU was observed. Upon further heating, the weight loss was attributed to the
decomposition of the polymer backbone. Although the decomposition profiles are not modified by the addition of TiO2, its interaction with QPSf modify the temperature range where the decomposition reaction occurs. The QPSf/TiO2 composite membranes showed not much improvement in thermal degradation temperature as compared to with pure QPSf, but it is well beyond the desirable fuel cell operation temperature (25e80 C). Scanning electron microscopy (SEM) images in Fig. 3 show typical microstructures of pure QPSf and QPSf/TiO2 composites. Fig. 3a shows the pure QPSf membrane micrograph which
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Table 2 e AFM roughness for QPSf, QPSf/2.5 wt.% TiO2, and QPSf/10 wt.% TiO2. Parameter Sq Sp Sz
Fig. 4 e XRD patterns of composite membranes at different TiO2 loadings (a) 0%, (b) 2.5% and (c) 10%. Insert pattern TiO2 powder.
suggests a smooth and dense morphology. Fig. 3c shows the image QPSf/2.5 wt.% TiO2, the image clearly shows the increase in surface roughness compared to that of pure QPSf due to the presence of TiO2 particles evidenced from the Energy Dispersive X-ray spectroscopy (EDS) profiles. Fig. 3e represents the surface of the QPSf/10 wt.% TiO2, which indicates TiO2 apparently not well dispersed with slight agglomeration on the surface of the composite membrane. With increasing the amount of inorganic filler, the surface morphology of the QPSf/TiO2 composite membranes appears to become more rough and dense. As indicated in literature [38], the higher the content of TiO2 fillers (as methanol permeation barrier) in the polymer matrix may help reduce the alcohol permeation in order to increase the current efficiency. The SEM cross-section images of the composite membranes presented in Fig. 3b, d and f, respectively confirm the dense character of the membranes. The TiO2 filler reveals the chemical nature of the functionalized surface and has a strong influence on the microstructure of the membrane. No defects were detected in the surface structure of the membranes. The typical X-ray diffraction (XRD) patterns of TiO2 powder and the composite membranes at different TiO2 loadings are
QPSf
QPSf/2.5% TiO2
QPSf/10% TiO2
1.82 nm 12.2 nm 31.2 nm
3.15 nm 28.9 nm 45.5 nm
3.89 nm 35.4 nm 74.8 nm
represented in Fig. 4. The base PSf polymer is known to be an amorphous polymer with a rigid structure. As indicated in Fig. 4, the pure QPSf reference membrane is fully amorphous: the broad signal around the reflections of crystalline QPSf is indicative of the lack of crystallinity. The peaks observed for TiO2 and pure QPSf are almost identical to those reported in literature [40]. The patterns of QPSf/TiO2 composite membranes have three crystalline characteristic peaks at 2q of 26.25 , 38.21 , 49.35 that is analogous with the characteristic peaks of TiO2 powders (insert diagram in Fig. 4) in addition to the dispersion peaks of pure QPSf. However, their locations are slightly shifted when compared with the pattern of TiO2 particles. The shift of characteristic peaks of TiO2 in QPSf/TiO2 composite membranes indicate that there are likely interactions between QPSf and TiO2, which is expected to influence the mechanical and chemical properties as mentioned in previous section. To further explore the influence of TiO2 particles, atomic force microscopy (AFM) was also used to investigate the membrane surface morphology. 3D topo-graphic images as indicated in Fig. 5, roughness analysis of mean roughness (Ra), root mean square roughness (Rq), and the mean difference height between the five high peaks and five lowest valleys (Ry) for QPSf and composite membranes were obtained in a scan of 8 mm 8 mm. The figures clearly show that the morphology of the surface changes with an increase in the amount of TiO2 percentage loadings in the composite membranes. All the surface areas are without pores, but as indicated in Table 2, pure QPSf membrane exhibited low roughness parameters creating smoother surface compared to QPSf/TiO2 composite membranes. This indication of low roughness parameter values confirms the hydrophilicity of the pure QPSf as it exhibited the lowest contact angle. The increase in the roughness of the membranes was from 1.82 nm for the pure QPSf membrane to 3.89 nm for QPSf/10 wt.% TiO2 membrane.
Fig. 5 e AFM images of (a) QPSf, (b) QPSf/2.5 wt.% TiO2, and (c) QPSf/10 wt.% TiO2 at scan area 8 mm 3 8 mm.
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Conclusion
In this work, we investigated ionic conducting polymer composite membranes based on QPSf/TiO2. The composite membranes were prepared by using DMAc as the casting solvent and TiO2 as the inorganic filler. The composite membranes were extensively characterized using TGA, SEM, XRD, AFM, water uptake and ionic conductivity. TGA results showed that the composite membranes have good thermal properties and also the introduction of the inorganic filler enhances thermal resistance and improve water uptake. However, it is clear that further fine-tuning of the surface treatment is worthwhile to get an optimal compromise between high conductivity and homogeneous microstructure, these developed nanocomposite membranes are qualified with the request for alkaline fuel cells. However, for the practical application in AAEMFCs, more characterizations, including fuel crossover, membrane electrode assembly (MEA) fabrication and catalyst durability will be pursued. The future work will include performance of the same membrane evaluated using Platinum, non-noble metals like Nickel, Cobalt as well as in-house synthesized catalyst on both anode cathode electrodes. Fuel cell performance studies are still in the initial stage in our laboratory and investigations will be reported in the forthcoming paper.
Acknowledgements The authors acknowledge CSIR (MSM)/NRF Thuthuka Programme (Grant 80581), for financial support. Also acknowledge the National Center for Nanostructured Materials (CSIR) for using their equipment for characterization (SEM, TGA, XRD).
references
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Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Chemical modification of polysulfone: Composite anionic exchange membrane with TiO2 nano-particles Patrick T. Nonjola*, Mkhulu K. Mathe, Remegia M. Modibedi Material Science and Manufacturing, Council for Scientific and Industrial Research, PO Box 395, Pretoria 0001, South Africa
article info
abstract
Article history:
Synthesis of quaternary polysulfone/Titanium dioxide (QPSf/TiO2) nanocomposite mem-
Received 30 November 2012
branes by the recasting procedure as suitable electrolyte in alkaline fuel cells is described.
Received in revised form
The composite membranes were characterized by ionic conductivity measurements, TGA,
24 January 2013
SEM, XRD, and AFM. Thermal analysis results showed that the composite membranes have
Accepted 7 February 2013
good thermal properties. The introduction of the inorganic filler supplies the composite
Available online 9 March 2013
membrane with a good thermal resistance. The physico-chemical properties studied by means of SEM and XRD techniques suggested the uniform and homogeneous distribution
Keywords:
of TiO2 at 2.5 wt.% loading, and negligible agglomeration at 10 wt.% loading, also indicated
Quaternized polysulfone
enhancement of crystalline character of these membranes. The energy dispersive X-ray
Nanocomposite membranes
spectra (EDS) analysis gave proportional percentages that the distribution of Titania
Ionic conductivity
element on the surface of the composite membrane was uniform. Observations from the
Alkaline fuel cells
results suggest that QPSf/TiO2 nanocomposite membranes have good prospects for
Thermal stability
possible use in AFC. Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Proton-exchange membrane fuel cells (PEMFCs) have been recognized as promising energy source systems due to their high energy-conversion efficiency, high power density, and low pollutant emission [1e3]. Over the past few decades, much effort has focused on proton-exchange membrane (PEM) development with significant progress made to date [4,5]. However, the crucial drawbacks of PEMFCs, such as their temperature limited operation and the low durability of their metal catalysts under acidic conditions, are restricting their commercialization [6]. Despite tremendous global effort dedicated to researching for non-platinum (Pt) and nonprecious metal catalysts, there has been no demonstrated PEMFC completely free from noble metal catalysts in both the anode and cathode electrodes [7e11]. Recently, increasing
attention has focused to alkaline anion exchange membrane fuel cells (AAEMFCs) because of their potentially higher durability [3,12e14] and reactivity of the metal catalysts in alkaline media [15e17]. In AEMFCs, much effort has been dedicated to the development of good hydroxide-conductive electrolyte membranes, in which polymers carrying quaternary ammonium groups [15,18,19] and hydroxide-doped polymers [17,20] have been studied. These polymeric membranes are expected to be of a lower cost and comparable superior electrochemical properties to PEMs. Although promising, the alkaline fuel cell performance still needs to be substantially improved before commercialization [21]. A widely quoted concern with anion exchange membranes is membrane stability in alkaline conditions, especially at elevated temperatures [13,22,23]. The chemical degradation of AEMs underpins largely from nucleophilic attack on the
* Corresponding author. Tel.: þ27 (0) 12 841 4182. E-mail address: [email protected] (P.T. Nonjola). 0360-3199/$ e see front matter Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2013.02.028
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cationic exchange sites by the hydroxyl ion. This results in the loss of cationic exchange sites followed by a decrease in the ionic conductivity of the polymer. The recognized degradation mechanisms for ammonium cation groups are (i) Hofmann elimination, (ii) nucleophilic substitution and (iii) ylide formation. Hoffman elimination reaction wherein the hydroxyl ion attacks b-hydrogen relative to the cation, forming a double bond between a and b-carbon, resulting in the cation being released. Direct nucleophilic displacement occurs by attack at the a-carbon atom of the cation, which can either detach the cation groups from the AEM or convert quaternary ammonium groups into tertiary amines. Degradation through the ylide pathway start with hydroxyl ion attack on a methyl group of the cations and produces a water molecule along with an ylide intermediate. Most recently, Fujimoto et al. [24] reported that both functional group and the specific attachment site may be important to the stability of AEMs. Several reviews on the chemical modification of polysulfone (PSf) have been reported using different mechanisms for the introduction of functional groups onto the polymer backbone [25,26]. The constant interest by researchers for PSf, a high performance engineering thermo-plastic material, is due to its excellent workability and high mechanical strength [27]. Fuel crossover through the membrane badly affects the performance of the fuel cell, which requires the modification of existing membrane properties or the development of new membranes. For comparison, Yu and Scott [28e30] studied the electrochemical performance of the alkaline direct methanol fuel cell (ADMFC) with anion exchange membranes. The direct methanol fuel cell (DMFC) performance with maximum power density of about 10 mW cm2 was obtained in a commercial quaternary ammonium anion exchange membrane (Morgane-ADP, Solvay SA, Belgium). In addition, the PVA/PWA based membranes have been prepared and applied on DMFC [31,32]. Varcoe et al. [13,33e35] developed and characterized the quaternary ammonium radiation grafted ETFC, PVDF and FEP alkaline anion exchange membrane (AAEM). In the effort of modifying the existing anion exchange membrane properties, the incorporation of inorganic additives are being investigated with the objective of serving the dual functions of improving water retention as well as reducing the fuel crossover through the membrane [36]. Although many types of membranes have been developed, manufactured and commercialized in the past, there is now a lot of emphasis on the development of mixed-matrix and nanocomposite membranes using compatible and cheap materials as well as facile and environmental friendly synthesis. Organic/inorganic hybrids can offer opportunities in many areas and not last in the field of fuel cell materials. Their main characteristic is the capability to combine the properties of the components. Choosing suitable materials, it is thus possible to reach the right features for different applications. The addition of inorganic filler into polymer matrix is to reduce the glass transition temperature (Tg) and the crystallinity of the polymer, and also allow the increase of the amorphous phases of polymer matrix, which then increase the ionic conductivity [37]. There are various inorganic filler, such as Al2O3, TiO2 [38], and SiO2 [39], that have been extensively studied. These experimental results indicated improvements in the ionic conductivity, thermal and mechanical
properties as the different inorganic fillers were added into the polymer matrix. Yang et al. [37] demonstrated that alkaline DMFCs with cross-linked PVA/TiO2 composite membranes show good electrochemical performances at ambient temperatures and pressure. Recently, Sangeetha et al. [40] prepared the quaternized polysulfone/ZrO2 membranes that can be applied in alkaline fuel cells. In this work, the inorganic component added to the organic matrix is the TiO2 nano-particle. The incorporation of the nano-particles in QPSf membrane has several attributes of interest, which includes reduced alcohol permeability and improved morphological stability without compromising ionic conductivity. Different loadings (2.5 wt.% and 10 wt.%) of TiO2 nano-particle have been used to prepare the QPSf/TiO2 composite membranes. The objective of this work is to determine if simple, inexpensive materials can be used in alkaline anion exchange membranes. The membranes were characterized and compared with pure QPSf. The surface morphology, crystallinity, thermal stability and ion conductivity properties are discussed. More evidence to support this proposition will be reported soon.
2.
Experimental section
2.1.
Materials
All the reagents were purchased from SigmaeAldrich and used without further purification unless otherwise stated. Polysulfone P-3500 LCD MB7 was supplied by Solvay Specialty Polymers. Chloroform (98%) anhydrous, Paraformaldehyde (95%), Chlorotrimethylsilane (98%), SnCl4, Trimethylamine (45 wt.% in H2O) and Titanium dioxide powder (TiO2) were all bought from SigmaeAldrich. Potassium Hydroxide KOH (84%) and Ethanol (99%) were bought from Merk Chemicals.
2.2.
Characterization of QPSf/TiO2
The water uptake was measured in 1 M KOH solution at 21 C for one day by monitoring the dry weight and wet weight of the sample. The ion exchange capacity was found by back titration technique. The dried and weighed polymer membrane was soaked in 1 M KOH solution for 24 h. The ionic conductivity of the samples in 1 M KOH solution was measured at 21 C on the potentiostat (Autolab PGSTAT302) from 100 mHz to 100 kHz with oscillating voltage of 10 mV. Thermogravimetry analyses (TGA) was performed from room temperature to 800 C on a thermal analyser (Q500 V20.1) at the heating rate of 10 C/min under nitrogen. Morphology studies were used to view the surface of the polymer
O O
S
CH3 O
C
O
CH3 CH2 N+(CH3)3HO-
n
Fig. 1 e Chemical structure of the quaternary polysulfone (QPSf).
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Table 1 e Water uptake and ionic conductivity of QPSf, QPSf/2.5% TiO2 and QPSf/10% TiO2 in 1 M KOH solution at 21 C. Compound
Water uptake (%)
Ion conductivity (S cm1)
Membrane thickness (mm)
9 32 39 19
0.0487 0.0847 0.1252 0.07
40 41 52 172
QPSf QPSf/TiO2 2.5% QPSf/TiO2 10% Nafion 117
membrane and also to predict the distribution of the nanoparticles into the polymer matrix by a scanning electron microscopy (SEM) (membranes coated with gold), X-ray diffraction (XRD) and the atomic force microscopy (AFM EC_SPM).
2.3.
Preparation of QPSf/TiO2
Polysulfone (PSf) (Udel P-3500, Amoco) was chloromethylated with paraformaldehyde and trimethylchlorosilane as the chloromethylating agent and stannic chloride as the catalyst, according to the procedure described by Avram and co-workers [41]. Specifically, paraformaldehyde (3.39 g, 11.3 mmol) and trimethylchlorosilane (12.3 g, 11.3 mmol) were added to a solution of PSf (5 g or 1.13 mmol PSf in 250 ml chloroform) in a flask equipped with a reflux condenser and a magnetic stirrer, and then stannic chloride (0.0294 g, 0.113 mmol) was added dropwise. The reaction mixture was stirred at 50 C for 48 h. Subsequently the reaction mixture was poured into ethanol (95%), and white chloromethylated polysulfone (CMPSf) precipitated immediately. The precipitate was filtrated, washed thoroughly with ethanol, and dried in vacuum at room temperature for 12 h. The degree of chloromethylation (DC) of the CMPSf obtained was 100% (determined from the 1H NMR spectrum). Quaternary polysulfone (QPSf) given in Fig. 1 was synthesized by quaternary amination of CMPSf with trimethylamine solution. Specifically, CMPSf (1.02 g, 2.52 mmol eCH2Cl) was dissolved in 10 ml of dimethylacetamide (DMAc), and then trimethylamine solution (1.408 g, 2.52 mmol) was added. The reaction mixture was stirred at 80 C for 12 h, and then poured into a Petri dish; the DMAc was removed by evaporation at 40 C over 2 days to obtain QPSf. QPSf-OH was obtained by treating QPSf-Cl in 1 M KOH at room temperature for 48 h; it was washed thoroughly and immersed in deionized water for 48 h to remove residual KOH. The QPSf-OH/TiO2 composite membranes were prepared by using a recasting method [42]. The QPSf-OH was dissolved in a desired amount of DMAc to form a solution containing 10 wt.% of QPSf-OH and different proportion of TiO2 powder was added to this solution and subjected to ultrasonic bath for 15 min and stirred further for 1 h. The resulting mixture was cast onto Petri dish to obtain the desired thickness. The cast membrane was dried at 70 C for 8 h. All membranes were stored in deionized water before any test was performed/tests were performed.
3.
represented as QPSf/2.5 wt.% TiO2 and QPSf/10 wt.% TiO2, respectively. The membranes appeared white due to the interaction between polymer and TiO2 powder and homogeneous with a good mechanical flexibility. The chemical structures of CMPSf and QPSf were confirmed by FT-IR and 1H NMR as indicated by Corti et al. [43]. In this work, we report on the formation of QPSf/TiO2 composite membranes, which has several attributes of interest, including decreased membrane swelling, reduced permeability towards methanol and improved morphological stability without compromising ion conductivity. The ionic conductivity test was used as an evaluation tool for possible application in AFC. Generally, the ionic conductivity is closely linked with ion exchange capacity (IEC) and water uptake [38,40]. The IEC of membranes is largely determined by the degree of chloromethylation (DOC), and thus there are no absolute limits or range. The mechanical properties of the ultimate membrane depend on the water content, which is mostly determined by the IEC. At very high DOC, the considerable swelling in water decreases the mechanical strength and the membrane is of little practical use in fuel cell. As discussed earlier that TiO2 may change the morphology of the membrane by changing the shape of the ionic cluster and water content [36,37]. The values of the ionic conductivity and water absorption corresponding to the composite membrane are represented in Table 1. The increase in ionic conductivity is because of the increase in ion exchange capacity and water content of the composite membrane, which facilitates the transport of ions. The composite membrane QPSf/10 wt.% TiO2 shows a maximum conductivity of 1.25 102 S cm1, which is higher than QPSf/2.5 wt.% TiO2 and pure QPSf [44], respectively. These results were similar to the results observed by Sangeetha et al. [40].
Results and discussions
Two types of QPSf composite membranes were prepared by varying the loading percentage of the inorganic filler (TiO2) by between 2.5 wt.% and 10 wt.%. These types of membranes are
Fig. 2 e TGA curves for QPSf and composite membranes. Heating from 25 C to 700 C at 10 C/min under N2 atmosphere.
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Fig. 3 e SEM images of surface and cross section QPSU (a,b), QPSf/2.5 wt.% TiO2 (c,d), and QPSf/10 wt.% TiO2 (e,f).
In order to get the insight into the composite membranes thermal stability, the thermogravimetry analyses (TGA) experiments were performed from room temperature to 800 C, as shown in Fig. 2. On comparison of the TGA thermograms of the QPSf/TiO2 composite membranes with pure QPSf, it was observed that all the membranes retain more than 90% of its weight up to a temperature of about 190 C. Beside the weight loss below 190 C due to hydration water, another weight loss between 200 C and 400 C, corresponding to the cleavage of quaternary ammonium groups from QPSU was observed. Upon further heating, the weight loss was attributed to the
decomposition of the polymer backbone. Although the decomposition profiles are not modified by the addition of TiO2, its interaction with QPSf modify the temperature range where the decomposition reaction occurs. The QPSf/TiO2 composite membranes showed not much improvement in thermal degradation temperature as compared to with pure QPSf, but it is well beyond the desirable fuel cell operation temperature (25e80 C). Scanning electron microscopy (SEM) images in Fig. 3 show typical microstructures of pure QPSf and QPSf/TiO2 composites. Fig. 3a shows the pure QPSf membrane micrograph which
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Table 2 e AFM roughness for QPSf, QPSf/2.5 wt.% TiO2, and QPSf/10 wt.% TiO2. Parameter Sq Sp Sz
Fig. 4 e XRD patterns of composite membranes at different TiO2 loadings (a) 0%, (b) 2.5% and (c) 10%. Insert pattern TiO2 powder.
suggests a smooth and dense morphology. Fig. 3c shows the image QPSf/2.5 wt.% TiO2, the image clearly shows the increase in surface roughness compared to that of pure QPSf due to the presence of TiO2 particles evidenced from the Energy Dispersive X-ray spectroscopy (EDS) profiles. Fig. 3e represents the surface of the QPSf/10 wt.% TiO2, which indicates TiO2 apparently not well dispersed with slight agglomeration on the surface of the composite membrane. With increasing the amount of inorganic filler, the surface morphology of the QPSf/TiO2 composite membranes appears to become more rough and dense. As indicated in literature [38], the higher the content of TiO2 fillers (as methanol permeation barrier) in the polymer matrix may help reduce the alcohol permeation in order to increase the current efficiency. The SEM cross-section images of the composite membranes presented in Fig. 3b, d and f, respectively confirm the dense character of the membranes. The TiO2 filler reveals the chemical nature of the functionalized surface and has a strong influence on the microstructure of the membrane. No defects were detected in the surface structure of the membranes. The typical X-ray diffraction (XRD) patterns of TiO2 powder and the composite membranes at different TiO2 loadings are
QPSf
QPSf/2.5% TiO2
QPSf/10% TiO2
1.82 nm 12.2 nm 31.2 nm
3.15 nm 28.9 nm 45.5 nm
3.89 nm 35.4 nm 74.8 nm
represented in Fig. 4. The base PSf polymer is known to be an amorphous polymer with a rigid structure. As indicated in Fig. 4, the pure QPSf reference membrane is fully amorphous: the broad signal around the reflections of crystalline QPSf is indicative of the lack of crystallinity. The peaks observed for TiO2 and pure QPSf are almost identical to those reported in literature [40]. The patterns of QPSf/TiO2 composite membranes have three crystalline characteristic peaks at 2q of 26.25 , 38.21 , 49.35 that is analogous with the characteristic peaks of TiO2 powders (insert diagram in Fig. 4) in addition to the dispersion peaks of pure QPSf. However, their locations are slightly shifted when compared with the pattern of TiO2 particles. The shift of characteristic peaks of TiO2 in QPSf/TiO2 composite membranes indicate that there are likely interactions between QPSf and TiO2, which is expected to influence the mechanical and chemical properties as mentioned in previous section. To further explore the influence of TiO2 particles, atomic force microscopy (AFM) was also used to investigate the membrane surface morphology. 3D topo-graphic images as indicated in Fig. 5, roughness analysis of mean roughness (Ra), root mean square roughness (Rq), and the mean difference height between the five high peaks and five lowest valleys (Ry) for QPSf and composite membranes were obtained in a scan of 8 mm 8 mm. The figures clearly show that the morphology of the surface changes with an increase in the amount of TiO2 percentage loadings in the composite membranes. All the surface areas are without pores, but as indicated in Table 2, pure QPSf membrane exhibited low roughness parameters creating smoother surface compared to QPSf/TiO2 composite membranes. This indication of low roughness parameter values confirms the hydrophilicity of the pure QPSf as it exhibited the lowest contact angle. The increase in the roughness of the membranes was from 1.82 nm for the pure QPSf membrane to 3.89 nm for QPSf/10 wt.% TiO2 membrane.
Fig. 5 e AFM images of (a) QPSf, (b) QPSf/2.5 wt.% TiO2, and (c) QPSf/10 wt.% TiO2 at scan area 8 mm 3 8 mm.
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Conclusion
In this work, we investigated ionic conducting polymer composite membranes based on QPSf/TiO2. The composite membranes were prepared by using DMAc as the casting solvent and TiO2 as the inorganic filler. The composite membranes were extensively characterized using TGA, SEM, XRD, AFM, water uptake and ionic conductivity. TGA results showed that the composite membranes have good thermal properties and also the introduction of the inorganic filler enhances thermal resistance and improve water uptake. However, it is clear that further fine-tuning of the surface treatment is worthwhile to get an optimal compromise between high conductivity and homogeneous microstructure, these developed nanocomposite membranes are qualified with the request for alkaline fuel cells. However, for the practical application in AAEMFCs, more characterizations, including fuel crossover, membrane electrode assembly (MEA) fabrication and catalyst durability will be pursued. The future work will include performance of the same membrane evaluated using Platinum, non-noble metals like Nickel, Cobalt as well as in-house synthesized catalyst on both anode cathode electrodes. Fuel cell performance studies are still in the initial stage in our laboratory and investigations will be reported in the forthcoming paper.
Acknowledgements The authors acknowledge CSIR (MSM)/NRF Thuthuka Programme (Grant 80581), for financial support. Also acknowledge the National Center for Nanostructured Materials (CSIR) for using their equipment for characterization (SEM, TGA, XRD).
references
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