PMMA/PU/CB Composite Bipolar Plate for Direct Methanol Fuel Cell

Transcription

1 Available online at ScienceDirect Energy Procedia 52 (2014 ) International Conference on Alternative Energy in Developing Countries and Emerging Economies PMMA/PU/CB Composite Bipolar Plate for Direct Methanol Fuel Cell T. Phuttachart a, N. Kreua-ongarjnukool a*, R. Yeetsorn a* and M. Phongaksorn a a a Department of Industrial Chemistry, Faculty of Applied Science, King Mongkut's University of Technology North Bangkok, Bangkok, (Thailand) Abstract This research fabricates bipolar plates for direct methanol fuel cells (DMFC) to improve compressive strength and creep behaviour. The composites comprise of poly (methyl methacrylate) (PMMA), polyurethane (PU) and carbon black (CB) those were fabricated via bulk polymerization in a casting process. The technique can create semiinterpenetrating polymer networks (IPNs) between PMMA and PU. The composites were prepared in different weight ratios PMMA/PU/CB of 75/25_5phr, 80/20_5phr and 85/15_5phr, and their quantity of IPNs in the composites are 99.27%, 97.77% and 96.71% respectively. In terms of compressive strength values, they achieve the requirement for commercial bipolar plates which are around 60 MPa, and a total creep value is approximately 0.23% at 120 minutes, 1 kpa and 80 ๐ C. The surface and volume electrical conductivity values incorporating 5 wt% of CB are 4.25x10-7 Scm -1 and 2.05x10-6 Scm -1 respectively. The developed composite material has a suitable combination of mechanical properties and processability for DMFC bipolar plate applications Published by Elsevier by Elsevier Ltd. This Ltd. is an Selection open access and/or article under peer-review the CC BY-NC-ND under responsibility license of the Research ( Center in Energy and Environment, Thaksin University. Selection and peer-review under responsibility of the Organizing Committee of 2013 AEDCEE Keyword : Direct Methanol Fuel Cell; Bipolar Plate; PMMA/PU/CB; semi-interpenetrating Polymer Networks; Casting Process 1. Introduction Direct methanol fuel cells (DMFC) are one type of proton exchange membrane fuel cells (PEMFCs) but using methanol as fuel gas [1]. One of critical roles for fuel cell developments is material science and *Corresponding author. Tel.: +66 (0) Ext: 4808 and 4811 ; fax +66 (0) address: nkk@kmutnb.ac.th, rmy@kmutnb.ac.th Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license ( Selection and peer-review under responsibility of the Organizing Committee of 2013 AEDCEE doi: /j.egypro

2 T. Phuttachart et al. / Energy Procedia 52 ( 2014 ) engineering technology for the productions of fuel cell components. This article reports on a novel polymer composite for the bipolar plates of DMFC to improve their compressive strength and creep behavior. Since fuel cell applications such as portable electronic devices or transportations; single fuel cell must be assembled as a fuel cell stack to achieve adequate voltages, the bipolar plate is one of the most important constituents of DMFC. In general, the fuel cell stack must be under high clamping force to prevent the leaking of reactant gases and also to reduce an interfacial contact resistance in the stacks in order to achieve nearly the theoretical voltage (1.21 V) [2]. Under this compression, bipolar plate materials may experience compressive stress and some creep deformation. Many researches are developing a polymer composite bipolar plate possessing high compressive strength (81 MPa) by formulating a carbon fiber network in a specific form as the filler component [3]. There has been extensive research on employing of carbon filled polymers for bipolar plates because introducing conductive fillers to the composites will enhance electrical conductivity of the materials. The electrical conductivity is one of the most properties necessitated for bipolar plates. Most of research to date has been paying attention on optimizing the carbon loading in the polymeric base. Lower carbon concentration leads to poor electrical conductivity, while high concentration is detrimental towards mechanical properties. It is well known that mechanical properties of polymer composites increase initially with the addition of a small amount of fillers such as 5 20 vol% CB, but decline with high filler loading [4]. The reduction in processability of bipolar plate shaping also occurs at high filler concentration. Furthermore the crystalline polymer matrix has influent to disperse of conductive fillers, low crystallinity has better filler dispersion than a high crystallinity matrix [5]. The high crystallinity matrix has strong Van der Waals force hence conductive fillers tend to aggregate into larger clusters and small amount of conductive pathways. Poly (methyl methacrylate) (PMMA) is an amorphous polymer that would have positive effect to disperse conductive fillers. Expressions on bipolar plates manufacturing, the polymer blend between PMMA and polyurethane (PU), which is thermoplastic elastomer, can modify impact property and also reduce brittleness of the material. PMMA/PU was fabricated via bulk polymerization in casting process [6]. Many researchers improve mechanical property of the polymer blend by constructing the interpenetrating polymer networks (IPNs) [6-8]. IPNs are mixtures at the microscopic level of two (or more) immiscible polymers with chain entanglements [9]. Typically, techniques which have been used to produce carbon based polymer composite are variable such as melt mixing, solution mixing, in-situ polymerization and the latex approach [10]. This research was interested in the in-situ polymerization, since it can improve the electrical conductivity of the composite at low percolation threshold. For instance, PMMA/graphite nanoplatelet (GNP), that was fabricated via in-situ polymerization with the aid of sonication to GNP dispersion showed the percolation threshold only about 1 wt% of the filler content. Its electrical conductivity is about 10-4 Scm -1 at 5 wt% [11]. The purpose of this research is to develop thermoplastic composite plates with high compressive strength and creep behavior for DMFC components. These polymer composites consist of PMMA, PU and CB, and the high strength composites were fabricated via bulk polymerization in casting process. This technique can create semi-interpenetrating polymer networks (IPNs) of PMMA and PU enhancing compressive strength and creep behavior of the blend. Incorporation of CB into the polymer blend related to the improvement in electrical conductivity of the composites, which is a necessary character of bipolar plates.

3 518 T. Phuttachart et al. / Energy Procedia 52 ( 2014 ) Experimental 2.1 Materials Methyl methacrylate monomer (MMA monomer): (Thai MMA Co., Ltd), linear glycol adipate polyester polyol : (TPRI Co., Ltd), aliphatic hexamethylene diisocyanate: (Bayer Thai Co., Ltd), azo-bis- 2,4-dimethyl valeronitrile (ABVN): (Thai Special Chemical Co., Ltd), ethylene glycol dimethacrylate (EGDM): (Fujimori Ltd) and dibutyl tin laurate (DBTL) : (Bayer Thai Co., Ltd) were supplied by PAN ASIA Industry Co.,Ltd. carbon black (CB) grade acetylene black (AB 50 P) was obtained from IRPC Public Company Limited Thailand, with the following specifications: 0.02% max coarse particles, gcm -3 pour density, 0.25 max.cm electrical resistivity, dichloromethane: (commercial grade). All of these chemicals were used without further purification. 2.2 Preparation of PMMA/PU/CB composite sheet The radical bulk polymerization of PMMA/PU/CB composite processed with various mass fractions of PMMA, PU and CB. The IPNs of PMMA/PU were synthesized by sequential polymerization technique and ratios of MMA:PU:CB used to create PMMA/PU/CB composite were of 75/25, 80/20 and 85/15 [6] under viscous agitation. The acetylene black was used for the source of conducting filler in different weight fractions and added into MMA monomer with variable amount of CB (acetylene black) 1, 2, 3, 4 and 5 wt%. The reaction was carried out at room temperature with continuous stirring. Subsequent to stirring, the polymerization of polyurethane was started by agitation of mixture the diisocyanate and polyol, and then MMA solution was added into a polyurethane monomer. In this step, the mixture should be stirred to get well homogenous solution at room temperature for 30 minutes. To disperse CB in polymer mixture the homogenous mixture was sonicating in the ultrasonic bath (35 khz) for 45 minutes at room temperature. Prior to composite plaque shaping, the solution was degassed using a vacuum pump, and then poured into polymerization casting mould. During the polymerization the mould was left into water bath for 3 hours, at 60 o C. Finally, the polymerization casting cells were placed in the oven at 110 o C for 2 hours in order to complete free radical polymerization. 2.3 Interpenetrating polymer networks (IPNs) Determination The efficiency of a cross-linking reaction was determined by extracting a known weight of the product with dichloromethane for 72 hours with the purpose of estimation the amount of an unreacted starting materials; grafted MMA monomer and uncross-linked PMMA, in the final product. Following the extraction the sample was dried in a vacuum oven. The percentage of IPNs (%IPNs) [12,13] was calculated followed Eq.1: (%) IPNs = W b Wa x 100 (1) Wb Where W a was weight of sample after extraction. W b was weight of sample before extraction.

4 T. Phuttachart et al. / Energy Procedia 52 ( 2014 ) Composite morphology analysis The morphologies of the fracture surfaces were observed with a field emission-scanning electron microscopy (FE-SEM S 4800, Hitachi) were used to observe the surface morphology of PMMA/PU/CB composite sheet. 2.5 Compressive strength observation The compressive strength of the PMMA/PU/CB composite specimens was determined according to ASTM D [14]. Specimen size used for all tests was 10 mm x140 mm x 2 mm and five specimens were tested for each sample. These tests were performed using the universal testing machine (Universal Testing: LLOYD LR 10 K) with gage length 10 mm and head speed of 1.5 mm/min. 2.6 Creep behaviour investigation The creep response of the PMMA/PU/CB composite was measured by Dynamic Mechanical Analyzer (TA Instruments Q800 DMA) using tensile mode. The specimens size were 10 mm x 30 mm x 2 mm. Creep experiments were determined as a function of the time at 80 ๐ C and the effect of stress on creep behaviour was also monitored. 2.7 Electrical property measurement Surface electrical conductivity and volume electrical conductivity values were evaluated according to ASTM D [15] using electrometers (Keithley 6517 and Keithley 8009 resistivity test fixture). The specimen size was 75 mm x 75 mm x 2 mm. Three specimens of each composition were used for the measurements. 3. Results and discussion Since, the fuel cell stack must be under high clamping force to prevent the leak of reactant gases and also to reduce an interfacial contact resistance in the stacks, materials for bipolar plates must have high compressive strength. PMMA has a high compressive strength, however; it is brittle. Characteristically, flow channels of bipolar plates should be effortlessly machined and the flow channel matching is suitable for nonbrittle material. To solve this problem PMMA blend PU was fabricated via bulk polymerization in casting process with difference weight ratio. Figure 1 displays the difference between cutting surface of a PMMA plate and PMMA/PU plate. While the cutting surface of the PMMA/PU plate is smooth, the PMMA plate has cracking defects from machining process. As mention in the previous section, this technique creates semi-interpenetrating polymer networks (IPNs) of PMMA and PU, therefore; the percentage of IPNs was determined by solvent extraction. Figure 2 illustrates the IPNs percentage of PMMA/PU with various ratios. The percentage of IPNs decreases with portion of PMMA increases. The reduction is due to the amount of unreacted starting material in the final product excess causing the efficiency of cross-linking reactions to decrease. According to morphology study of previous research literature [16], PMMA/PU blend morphology shows phase separation exhibited in Figure 3. Figure 3(c) shows heterogeneous morphology in which the minor phase forms spherical particles distributed in the continuous PU matrix. When the blend has IPNs, it would have bi-continuous phase structure because of better entanglement [7]. However, the morphology observation of 80/20 of PMMA/PU ratio and 5 wt% of

5 520 T. Phuttachart et al. / Energy Procedia 52 ( 2014 ) CB loading in this work does not show phase separation of these two polymers. Therefore, it is important for further work to continue studying the effect of IPNs on the blend morphology. (a) (b) Fig.1 Cutting surface of (a) PMMA ; ( b) PMMA/PU plaques IPNs (%) /25_0 75/25_5 80/20_5 85/15_5 Ratio of PMMA/PU/CB composite (wt%) Fig. 2 Relation between %IPNs and ratio of PMMA/PU/CB composites. Concerns over electrical conductivity improvement and the casting processability of composites have brought forth the idea of producing a PMMA/PU/CB composite for bipolar plate application. First of all, maximum load of CB that can be incorporated into polymerization were investigated. The result indicates that 5 wt% of CB loading is the maximum load because higher CB loading affects the viscosity of polymer solution to increase. Consequently the high viscosity is obstacle to shape bipolar plates. To measure the influence of IPNs and CB on mechanical properties of PMMA/PU/CB composites, the PMMA/PU/CB with 5 wt% of CB loading were introduced into the blends that have 75, 80 and 85 wt% of PMMA weight proportions. It found that the compressive strength of PMMA decreases with the content of PU increases (Figure 4). It can be explained that concentration of PU, which has high elastic behavior, produces more effect than INPs generated from the polymerization process. Moreover, CB introduced into the system reinforced the blend as the results show an increase in compressive strength. When the applied force is over the elastic deformation stress, the CB has a stress transfer effect which enhances the strength and plasticity of the polymer matrix. The feasible rationale is that CB network structure is created, and it can absorb more mechanical loading from the matrix when it is under stress

6 T. Phuttachart et al. / Energy Procedia 52 ( 2014 ) [17]. The ultimate compressive strength of 5 wt% CB in PMMA/PU (80/20) is higher than 60 MPa which is higher than the compressive strength of commercial bipolar plates (50 MPa). (a) (b) 50 m 50 m (c) (d) PMMA CB PU 100 m S kV 7.4mm x20.0k SE(M) 2 m Fig.3 SEM images of PMMA, PU, PMMA/PU blend membranes [16] and PMMA/PU/CB blend with IPNs (a) PMMA [16]; (b) PU [16]; (c) PMMA/PU ratio 40:60 no IPNs [16]; (d) PMMA/PU/CB IPNs 80/20_5 wt% CB IPNs % (x20, 000) no CB 50 Yield strength at 2% offset Compressive strength (MPa) wt % CB Stress (MPa) Secant modulus 0 75/25 80/20 85/15 PMMA 0 75/25 75/25_5 80/20 80/20_5 85/15 85/15_5 Ratio of PMMA/PU/CB composite (wt%) Ratio of PMMA /PU/CB composite (wt%) Fig. 4 Compressive strength of PMMA/PU/CB composites. Fig. 5 Yield strength and secant modulus of PMMA/PU/CB composite difference weight fraction.

7 522 T. Phuttachart et al. / Energy Procedia 52 ( 2014 ) Strain (%) PMMA/PU/CB 1 kpa PMMA/PU/CB 2 kpa PMMA/PU/CB 3 kpa PMMA/PU 1 kpa PMMA/PU 2 kpa PMMA/PU 3 kpa Time (min) Fig. 6 Creep curve of PMMAPU ratio 75/25 with and without 5 wt% CB loading at 80 ๐ C and difference stress. Electrical conductivity (Scm -1 ) 1.0E-03 volume conductivity 1.0E-05 surface conductivity 1.0E E E E E Carbon black content (wt%) Fig. 7 Electrical conductivity of PMMA/PU ratio 75/25 as a function of CB weigh fraction. Electrical conductivity (Scm -1 ) 1.0E E E E E E-13 volume conductivity surface conductivity Electrical conductivity (Scm -1 ) 1.0E E E E E E-13 volume conductivity surface conductivity 1.0E Carbon black content (wt%) Fig. 8 Electrical conductivity of PMMA/PU ratio 80/20 as a function of CB weight fraction. 1.0E Carbon black content (wt%) Fig. 9 Electrical conductivity of PMMA/PU ratio 85/15 as a function of CB weight fraction. Electrical conductivity (Scm -1 ) 1.0E E E E E E E-15 Volume conductivity Surface conductivity 75/25_0 75/25_5 80/20_5 85/15_5 Ratio of PMMA/PU/CB composite (wt%) Fig. 10 Electrical conductivity of PMMA/PU/CB composite plaques.

8 T. Phuttachart et al. / Energy Procedia 52 ( 2014 ) All of PMMA/PU/CB composites at 5 wt% of CB loading have yield strength, the lowest stress which leads to permanent deformation, at 2% offset stress stain compressive curve over than 26 MPa and the secant modulus over than 31 MPa shown in Figure 5. The modulus and yield strength values increase with PMMA content increases but the results of composites with 75:25 ratio of PMMA: PU do not have the same tendency. These unexpected results may occur from the synergistic effect of IPNs and CB particles. The data of creep behavior test of PMMA/PU (75/25 of ratio) and PMMA/PU (75/25 of ratio) with 5 wt% of CB is shown in Figure 6. The figure illustrates the traces of the maximum creep strains as a function of time (120 minutes) at 80 ๐ C applying varies stress at 1, 2 and 3 kpa. The creep strain of PMMA/PU/CB composites increases with higher stress. From the curves, the creep stages have instantaneous deformation primary and decreases a little bit in secondary creeps because polymer relaxation can be clearly observed there is no evidence of tertiary creep. Furthermore, addition of carbon black into the blend reduces total creep of the composite since CB particles would distribute in vacancy spaces between IPNs. For that reason, CB distribution would confine the movement of polymer chains when they were under constant load. The surface and volume electrical conductivity of difference weight fraction PMMA/PU/CB composites is displayed in Figure 7-9. The percolation s-curve of them had appeared the percolation threshold at 3 wt% of CB for PMMA/PU ratio 75:25 and 2 wt% of CB for PMMA/PU ratio 80:20 and 85:15. It implies that IPNs may disconnect the conductive network of CB or high percentage of IPNs would not give advantage for CB dispersion in polymer matrix. The surface and volume electrical conductivity of composites incorporating 5 wt% of CB in PMMA/PU/CB composite shown in Figure 10 are 4.25x10-7 Scm -1 and 2.05x10-6 Scm -1 respectively. Although the electrical conductivity exhibits a dramatic increase with the increasing CB content, it is still falling short of the DOE target (>100 Scm -1 ) which implies that introducing a single conductive filler as CB may not be satisfactory to provide adequate conductive pathways for a desired electrical conductivity. 4. Conclusions This study has explored the potential of using PMMA/PU/CB composites as novel polymeric bipolar plates for direct methanol fuel cells. PMMA/PU/CB composites manufactured by bulk polymerization via casting process were successful in further improving mechanical properties of the composites such as brittleness, compression strength and creep behavior. The compressive strength of composite plates is approximately 60 MPa which achieves DOE requirement. Total creep at 120 minutes, 1 kpa and 80 ๐ C is in the region of 0.2%. Furthermore, the surface and volume electrical conductivity of composites incorporating 5 wt% of CB are 4.25x10-7 Scm -1 and 2.05x10-6 Scm -1 respectively. In terms of mechanical properties, these successes are promising for future opportunities to use PMMA/PU/CB composites as a DMFC component, even though the developed materials are currently inferior to the performance of commercial bipolar plates. Therefore, there are many characterizations of a bipolar plate that must be concerned and discussed to develop composite formulas for achieving the required properties of bipolar plates. To understand more characters of composites as a function of bipolar plate role, in-situ and ex-situ creep behavior tests should be investigated as future work. The morphology study involving the effect of CB dispersion on conductive network also needed to study. Moreover, DMFC performance tests to observe the reliability for using composites in DMFC conditions should be evidencing.

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