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Poly(ether ketone) composite membranes by electrospinning for fuel cell applications
Journal of Power Sources 434 (2019) 226733
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Poly(ether ketone) composite membranes by electrospinning for fuel cell applications Maryam Oroujzadeh, Mohammad Etesami, Shahram Mehdipour-Ataei * Iran Polymer and Petrochemical Institute, P.O. Box 14965/115, Tehran, Iran
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� Membranes of non-sulfonated PEK fibers in 70% sulfonated PEK matrix were prepared. � Tensile strength of membrane increased due to presence of non-sulfonated PEK fibers. � Hydrophobic fibers in a hydrophilic matrix controlled water absorption of membranes. � Proton conductivity of samples was be tween 0.05 and 0.16 S cm 1
A R T I C L E I N F O
A B S T R A C T
Keywords: Poly(ether ketone) Electrospinning Composite membrane Proton exchange membrane fuel cell
This work is dedicated to preparation of the composite membranes to control the water absorption and dimensional stability of a highly sulfonated poly(ether ketone) matrix. To achieve this goal, the non-sulfonated poly(ether ketone) fibers are prepared by electrospinning process and then fibers are impregnated by highly sulfonated poly(ether ketone). Both highly sulfonated and non-sulfonated polymers are synthesized by condensation polymerization reaction and are characterized by spectroscopic methods. The water absorption measurements indicate that introducing 20 and 30 wt% of non-sulfonated fibers lead to the water absorption of 31 and 25% respectively, in comparison to water absorption of highly sulfonated matrix that is 54%. The presence of fibers also improves the mechanical strength of membranes. Tensile strength reaches from 27 to 81 MPa by addition of 30 wt% electrospun fibers. Moreover, in H2/O2 fuel cell performance test, the composite membranes show current density in the range of 700–1090 mA cm 1 at different cell temperatures and back pressure amounts. Also proton conductivity of the samples is examined by electrochemical impedance spec troscopy and the values are between 0.05 and 0.16 S cm 1. Morphology of the fibers and membranes is studied by scanning electron microscopy as well.
1. Introduction Proton exchange membranes (PEMs) as a heart of a fuel cell, assist proton transportation between the anode and the cathode and also
inhibit electrons and reactant gasses crossover. Although Nafion (a perflourosulfonic acid membrane) is a commercial membrane for PEMs, because of some drawbacks like high cost and decreased efficiency at temperatures higher than 80 � C, attempts to replace Nafion with other
* Corresponding author. E-mail address: [email protected] (S. Mehdipour-Ataei). https://doi.org/10.1016/j.jpowsour.2019.226733 Received 30 March 2019; Received in revised form 17 May 2019; Accepted 4 June 2019 Available online 13 June 2019 0378-7753/© 2019 Elsevier B.V. All rights reserved.
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suitable proton conducting membranes are in process [1,2]. lots of polymer families were studied to substitute perflourosulfonic acid membranes such as: polybenzimidazoles [3,4], polyimides [5], poly sulfones [6–8], poly(ether ketone)s (PEK)s [9–11], and so on. Among the vast variety of suggested alternatives to Nafion, poly (ether ketone)s with different structures are of interest because of low cost, high thermal and chemical stability, and high mechanical strength [12]. In the case of sulfonated aromatic substituents of Nafion, at the same ion exchange capacities, the proton conductivity is lower than Nafion because of different microstructures of Nafion and sulfonated aromatic polymers. It was shown that there are large separated hydro philic/hydrophobic regions in Nafion microstructure which facilitate proton conducting, while in sulfonated aromatic substituents this separated regions observed just in high degrees of sulfonation (more than 50% for poly(ether sulfone)s) [13–15]. So, to obtain an acceptable proton conductivity in aromatic substituent membranes high degrees of sulfonation is necessary that cause high water absorption of the mem brane and also dimensional instability during on/off cycles of the cell or at different humidity levels. This dimensional instability can cause delamination of the catalyst layer from the membrane surface and suppressing the cell performance. However, for controlling the water uptake of sulfonated aromatic polymers three types of strategies were reported: 1) crosslinking of the polymer chains [16–19], 2) using inorganic additives, they could be proton conductive [20–25] or non-conductive [26,27] and 3) making blends [12,28,29]. In composite blend membranes, a non-sulfonated polymer or a polymer with low degree of sulfonation was introduced to the membrane to control the water absorption and dimensional instability. In many of these reports one of the polymers is in fiber form and the other polymer/polymers play the matrix role [30–33]. Fibers, matrix or both can be sulfonated and participate in the proton con ducting. However, the degree of sulfonation, fibers orientation, or fibers content affect the hydrophilicity and dimensional stability of the system [34]. This work was focused on producing blend membranes consist of polymer fibers. Electrospinning technique has been used to make fibers. Electro spinning process to produce proton exchange membranes for fuel cell applications was pioneered by Pintauro’s group and as a facile and simple way of fiber fabrication has attracted much attentions recently [35–37]. In this process, submicron fibers draw from the needle of a syringe containing a polymeric solution with the specific concentration and gather on the collector surface. Different parameters like applied voltage, solution flow rate, solution concentration, solvent type, needle to collector distance, and so on control uniformity and diameter of the fibers. In this work, sulfonated and non-sulfonated poly(ether ketone)s (PEK)s were synthesized. Sulfonated PEK synthesized from the sulfo nated monomer (sulfonation degree of the polymer was 70%) used as a matrix of the composite membranes. On the other hand, electrospun fibers of non-sulfonated PEK were prepared. The fibers were impreg nated with 70% sulfonated polymer solution to make blend membranes with controlled water absorption and increased mechanical strength. So, two composite membranes with different amounts of electrospun fibers were prepared and the performance of these membranes was compared with pristine 70% sulfonated PEK membrane (prepared by solution casting method) and also with Nafion 212 membrane.
polymerization solvent. Ethanol, toluene, tetrahydrofuran (THF), dimethylformamide (DMF), 2-propanol, fuming sulfuric acid (30%) (All from Merck) were used as received. 2.2. Synthesis of sulfonated monomer, highly sulfonated and nonsulfonated PEKs In a three-necked round-bottom flask equipped with a nitrogen inlet and Dean-stark trap, 1.1941 g of sulfonated difluorobenzophenone (sDFB) (synthesized as reported before [11]) and 0.2645 g of DFB as dihalides, mixed with 1.3998 g of a dihydroxy monomer (PBP) in pres ence of 0.636 g of potassium carbonate. Then 10 mL of DMAc was added as a solvent. The mixture was refluxed for 4 h at 140 � C and then 10 h at 175 � C. After that, the viscose polymer solution was cooled and diluted with 10 mL of DMAc. Then it was poured slowly in an excess amounts of deionized water. The obtained white fibrous precipitate stirred in deionized water overnight at 50 � C to remove any remained salts. The non-sulfonated PEK was synthesized in the same way by mixing stoi chiometric amounts of DFB and PBP as dihalide and dihydroxy com pounds, respectively in presence of potassium carbonate. Sulfonated PEK was acidified by boiling in 0.5 M sulfuric acid solu tion for 2 h. Then for washing the excess acid, it was boiled in the deionized water for couple of hours using fresh deionized water every 1 h. At last, the acidified sulfonated PEK was filtered and dried in a vacuum oven at 80 � C overnight. 2.3. Electrospinning of non-sulfonated PEK Non-sulfonated PEK solution was prepared by dissolving the polymer in DMF: THF mixture (1:1). After optimization of the parameters, a 5 wt % solution of the polymer was electrospun at room temperature for about 10 h on rotating drum covered with an aluminum foil in the applied voltage of 24 kV, feeding rate of 1 mL h 1, needle to the collector distance of 14 cm, and the collector rotating speed of 300 rpm. After finishing the electrospinning time, the foil was separated from the col lector and immersed in water for peeling off the mat. The mat was pressed 10 s under 100 kg.f to make it easier to handle and then dried in a vacuum oven overnight at 80 � C. 2.4. Membrane preparation For preparation of pristine sulfonated PEK membrane, 15 wt% so lution of polymer in DMAc was cast on a glass surface with a doctor blade and the membrane was dried under infrared light. The membrane after drying was peeled off by immersing in deionized water bath and then dried in a vacuum oven at 80 � C for 12 h. The pristine 70%-sulfo nated PEK membrane made by solution casting method was named as SC-70. Afterwards, for preparation of composite membranes, nonsulfonated fiber mats were immersed in 70%-sulfonated PEK solution. Before that, the suitable solvent must be selected that just dissolve the sulfonated PEK and did not dissolve the non-sulfonated fibers. After examination of several solvents at different temperatures, methanol at room temperature was selected for this purpose. In this regards, 5 wt% methanol solution of 70%-sulfonated PEK was prepared and the elec trospun mat was dipped in for several times. After each time the mem brane was dried at room temperature on a glass plate and then immersed in polymer solution again. Finally, to ensure wetting of electrospun fi bers with sulfonated polymer solution, membranes were hot-pressed for 30 s at 160 � C. In that way, two composite membranes with different content of non-sulfonated fibers were prepared that named as CE-101 and CE-102 (containing 30 and 20 wt% electrospun fibers, respectively). Nafion 212 membrane also was activated before preparation of the membrane electrode assembly (MEA) in this manner: the membrane was immersed in H2O2 solution (3%) at 80 � C for 1 h and then after washing with deionized water, it boiled in water for 2 h. Then it remained in
2. Experimental 2.1. Materials 4,40 -Difluorobenzophenone (DFB) as dihalide monomer and 4,40 (1,4-phenylene diisopropylidene) bisphenol (PBP) as dihydroxy mono mer both purchased from Aldrich and used as received. Potassium car bonate (from Merck) was dried at 120 � C in a vacuum oven overnight. Anhydrous dimethylacetamide (DMAc) (from Aldrich) utilized as 2
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sulfuric acid solution (4 M) for 1 h and then washed again with hot water several times to ensure removal of excess acid. After all, the membrane was kept in deionized water until use.
resistance, and membrane cross-sectional area, respectively.
2.5. Characterization
Four MEAs including SC-70, CE-101, CE-102, and Nafion 212 membranes were prepared. Pt/C catalyst was utilized as the anode and cathode catalyst with loading of 0.48 mg cm 2. Catalyst ink includes Pt/ C (60%) dispersed in 2-propanol/deionized water and contained 5% Nafion solution. Catalyst ink was painted on a carbon paper gas diffusion layer to make the anode and cathode gas diffusion electrodes. MEAs were prepared by hot-pressing of two electrodes onto the membranes at 130 � C for 2 min under pressure of 100 kgf. Each MEA was well packed in a sealed bag with some drops of the deionized water until use. Po larization curves were obtained in a single cell with active area of 5 cm2. Two cell temperature of 70 and 80 � C were examined and flow rates of H2 and O2 were fixed on 300 mL. min 1. The performance test was performed under humidity of 100% for aforementioned cell tempera tures and the backpressure was adjusted to 10 and 15 psi. At least 3 h of humidification and pretreatment was performed for each MEA sample at potential of 550 mV.
2.6. Fuel cell performance evaluation
Chemical structure of the sulfonated monomer was confirmed using H NMR (Bruker Avance DPX 400 MHz) and FT-IR (Bruker- IFS48) spectroscopy (reported before [11]). FT-IR spectroscopy was used to study the structure of the synthesized sulfonated and non-sulfonated polymers. Thermal properties of the polymers were studied by thermogravi metric analysis (Mettler TGA/DSC) from room temperature to 700 � C at a heating rate of 10 � C min 1 under air atmosphere. Glass transition temperature of the synthesized polymers was obtained from differential scanning calorimetry analysis (DSC) (Metteler) and performed in air and with heating rate of 5 � C min 1. Microstructure and morphology of the electrospun fibers, composite membranes, and solution cast membrane were studied through scanning electron microscopy (SEM) (VEGA TESCAN) at an accelerating voltage of 20 kV. Ion exchange capacity (IEC) of the membranes was determined by titration method. 0.06 g of each membrane sample was immersed in NaCl (2 M) solution for 48 h to replace Hþ ions of sulfonic acid groups with Naþ. Liberated Hþ ions were titrated with 4 mM solution of NaOH in presence of phenolphthalein as the indicator. Following equation was used to calculate IEC values:
1
3. Results and discussion 3.1. Synthesis and characterization of sulfonated monomer, highly sulfonated PEK and non-sulfonated PEK For direct synthesis of sulfonated PEK from sulfonated monomer, sDFB was synthesized via electrophilic aromatic sulfonation reaction. 1H NMR and FT-IR were used to confirm the structure of the sulfonated monomer that were reported before [11] and were shown in supple mentary materials section (Fig. S1 and Fig. S2). 70%-sulfonated PEK and non-sulfonated PEK were synthesized via condensation polymerization of related monomers as mentioned in section 2.2 in details (Scheme 1). The FT-IR spectra for both polymers shown in supplementary materials section (Fig. S3) confirmed the structure of synthesized polymers. The strong absorption band of Ar-O-Ar was observed for both polymers. However, symmetric and asymmetric stretching bands related to the
VNaOH � MNaOH IEC ¼ Ws where, V NaOH and M NaOH are volume and concentration of NaOH so lution and Ws is the weight of the dry membrane sample. Water absorption of samples were calculated from the weight of the absorbed water after 24 h immersion in water at room temperature. After weighing the samples, they dried in a vacuum oven for at least 24 h to obtain weights of dehydrated samples. The equation was as follows: Water Absorption ð%Þ ¼
Ww
Wd Wd
� 100
where, Ww and Wd are the weights of hydrated and dehydrated mem branes, respectively. Hydration number (λ) or number of water molecules around every sulfonic acid groups was calculated as follows using water absorption and IEC values: � � λ¼
WW Wd Wd
18 � IEC
� 1000
where, Ww and Wd are the weights of hydrated and dehydrated mem branes, respectively and 18 is the molecular weight of water. Mechanical properties of the samples were measured by tensile test using a STM-20 according to ASTM-D 882 at the speed of 5 mm min 1 at room temperature. At least 3 measurements were performed for each membrane sample. Proton conductivity of membranes were measured by the electro chemical impedance spectroscopy (EIS) (IVIUM potentiostat/galvano stat) in AC amplitude of 50 mA ranging from 1 to 106 Hz at room temperature. The in-plane proton conductivity measurement was occurred in a two-point probe home-made cell. The frequency that produced minimum imaginary response was taken as the membrane resistance and the proton conductivity was calculated:
σ¼
L RA
where, L, R, And A are distance between two electrodes, membrane
Scheme 1. Synthesis of polymers. 3
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sulfonic acid groups just appeared in sulfonated PEK spectrum at 1014 and 1079 cm 1, respectively. 3.2. Morphological study of the electrospun fibers, composite and noncomposite membranes In this work, composite membranes using electrospun nonsulfonated PEK fibers and highly sulfonated PEK matrix were pre pared. As stated, in sulfonated aromatic membranes, less proton con ductivity observes in comparison to Nafion due to the different microstructures of Nafion and aromatic sulfonated polymers. To over come the problem, membranes with higher degrees of sulfonation should be used which have very high water uptake and consequently dimensional instability. In this work non-sulfonated PEK fiber mats were used to decrease high water absorption of highly sulfonated matrix. This fiber mat structure also brought higher mechanical strength for com posite membranes that were investigated later. For preparing composite membranes, non-sulfonated fiber mats were electrospun firstly. As demonstrated in the SEM image (Fig. 1a), uniform and bead-free fibers were electrospun successfully. As noted in the experimental section, non-sulfonated PEK was electrospun on an aluminum foil (Fig. 1b) and then water bath was used for separating fiber mat off the foil (Fig. 1c and d). Since working with those electro spun mats was not easy, the electrospun mat was pressed to make a packed and firmed structure (Fig. 1e) To prepare composite membranes, the fiber mats were immersed in 70%-sulfonated PEK solution for several times and finally to ensure penetration of polymer solution into the fiber mats, they were hotpressed at 160 � C for 30s. Fig. 2 shows the final composite membrane compared to the primary fiber mat. SEM analysis also was used to confirm the penetration of polymer solution into the fiber mats and additionally to study the morphology of the composite and non-composite membranes. As depicted in Fig. 3 (a) and (b), in cross-sectional view of the composite sample CE-102, fibers were completely recognizable. Moreover, there was no void or defect in the structure of the composite membrane which is very important in a fuel cell membrane because any defect can cause reactant crossover and deteriorating cell performance. Fig. 3 (c) demonstrates the cross-
Fig. 2. Primary non-sulfonated PEK fiber mats (left) and final composite membrane (right).
sectional view of the non-composite SC-70 sample for comparison. As it is clear from Fig. 3 (a, c) there was a brittle fracture in non-composite membrane but in CE-102 sample presence of fibers caused more ductile fracture pattern and therefore more flexible membrane. 3.3. Thermal behavior Thermal behavior of sulfonated and non-sulfonated PEK polymers were studied using DSC (shown in supplementary material section Fig. S4) and TGA analysis. Table 1 shows the glass transition tempera ture of the polymer samples. Tg values were 130 � C for PEK-0 and 159 � C for SC-70 samples. It means that the presence of sulfonic acid groups in SC-70 caused about 30 � C increasing in Tg compared to PEK-0 sample. Existence of sulfonic acid groups increased strong intramolecular in teractions between polymer chains and then increasing the Tg of the polymer. TGA profiles of PEK-0 and SC-70 were shown in Fig. 4. Moreover, T10% data were listed in Table 1 and show that both non-sulfonated and highly sulfonated PEKs were more stable than Nafion because of more thermal stable nature of PEK polymer due to the presence of carbonyl and aromatic linkages. According to Fig. 4, there was an initial weight
Fig. 1. SEM image of electrospun fibers (a), image of non-sulfonated fiber mats immediately after electrospinning (b), soaking the aluminum foil in water bath (c), detached fiber mats (d), and fiber mats after pressing and packing (e). 4
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Fig. 3. SEM images of cross-section of CE-102 (a), same sample in close view (b), and SC-70 (c).
loss for SC-70 at low temperatures that was related to the absorbed at mospheric water showing very high tendency of SC-70 to the water absorption. Thereafter, three subsequent weight losses were observed for SC-70. The first one was related to the leaving of sulfonic acid groups and the next two were attributed to the polymer chain decompositions. However, PEK-0 just showed two weight losses related to the polymer chain decompositions. Comparison of the char content of both samples confirmed the presence of inorganic groups in SC-70 sample. The presence of sulfur atoms caused increasing char yield from 1.39% in PEK-0 to 16.2% in SC70 at 750 � C.
g 1, respectively. Presence of more sulfonated polymer in CE-102 (80 wt % versus 70 wt% for CE-101) caused its higher IEC value (Table 1). As observed, the IEC of composite membranes CE-101 and CE-102 were less than what was estimated according to the fiber content. The difference might be attributed to the probable cross-linking of some sulfonic acid groups during thermal treatment of membranes after impregnation process (section 2-4). It was proved that in presence of residual amounts of amidic solvents like dimethylacetamide or dimethylformamide, an electrophilic aromatic substitution reaction may form a sulfone bridge via linking two aromatic rings during thermal treatment [38–40]. Water absorption is an important parameter in sulfonated proton exchange membranes because the water molecules play carrier role in proton transportation. However, high water absorption causes some difficulties like, dimensional and mechanical instabilities and also delamination of catalyst layer from the membrane in MEAs during on/ off cycles of the cell or at different humidity levels. Consequently, the water uptake value is critical in sulfonated membranes. Water absorp tion was easily calculated from weight of the wet samples after 24 h immersion in water and reported in Table 1. As presented, SC-70 had the highest water absorption that was 54%. Since, sulfonic acid groups are responsible for absorbed water, increasing the sulfonation content, increased the water absorption of CE-102 compared to CE-101 mem brane. CE-102 and CE-101 contained 80 and 70 wt% sulfonated poly mer, respectively. Presence of hydrophobic fiber mats decreased their affinity to water significantly. Nafion 212 showed high water absorption in spite of its low IEC value which is related to the different micro structure of Nafion and larger hydrophilic/hydrophobic domains compared to aromatic membrane samples [14]. Hydration number is the number of water molecules around every sulfonic acid group in the hydration sphere. In other words, hydration number is a measure of the efficiency of the absorbed water for proton conducting. The best condition for a fuel cell membrane is high proton conductivity in low hydration number [41]. Therefore, there is a trade-off between two parameters of proton conductivity and hydration number. As listed in Table 1, CE-102 showed hydration number of 11 in comparison to 9.7 for CE-101. The parameter was 13.2 for SC-70 which is very lower than Nafion 212. Proton conductivity measurements for all membranes performed at room temperature in water. In Fig. S5 of the supplementary materials the nyquist plots were shown and data were reported in Table 2. As shown, SC-70 presented the highest proton conductivity due to its very high IEC value and sulfonation degree. Predictably, increasing of sul fonated PEK content in composite membrane CE-102 increased proton conductivity in comparison to CE-101.
3.4. IEC, water absorption, hydration number, and proton conductivity
3.5. Mechanical properties
The number of sulfonic acid groups per gram of molecule defines as IEC value. The ion-exchange capacity of samples was determined as mentioned in section 2.5 by titration. The IEC value for SC-70 sample was 2.23 meq g 1 as reported in Table 1. Composite membranes of CE101 and CE-102 showed ion exchange capacity of 1.43 and 1.59 meq
Suitable mechanical strength is crucial for durability of a fuel cell membrane in the cell environment. Fig. 5 shows stress-strain curves of composite samples and cast film of highly sulfonated PEK. Quantities for tensile strength, Young’s modulus, and elongation at break were listed in Table 2, as well. As reported, the highest tensile strength belonged to CE-
Table 1 Properties of composite and non-composite membranes. Sample
Sulfonated PEK content (wt%)
IEC (meq/ g)
Water absorption (%)
Hydration number
Tg (�C)
T10% (�C)
CE-101 CE-102 PEK-0 SC-70 Nafion 212
70 80 – 100 –
1.43 1.59 – 2.23 0.84
25 31 – 54 40
9.7 11.0 – 13.2 26.4
– – 130 159 218
– – 439 414 351
Fig. 4. TGA profile of the synthesized polymers.
5
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Table 2 Proton conductivity and mechanical properties of samples. Sample
Proton conductivity (S/ cm)
Tensile strength (MPa)
Young’s modulus (GPa)
Elongation@ break (%)
Membrane thickness (dry in μm)
Membrane thickness (wet in μm)
CE-101 CE-102 SC-70 Nafion 212
0.05 0.08 0.16 0.10
81 60 27 42
2.02 3.35 0.95 3.60
7.00 6.63 7.44 252.00
95 125 50 50
120 160 75 70
101 with higher percentage of electrospun fibers. Although CE-102 showed lower tensile strength compared to CE-101, but both CE-101 and CE-102 showed very better tensile strength than SC-70. This observation obviously revealed the effect of the presence of fiber mat in the composite membrane structure. Tensile strength data of both com posite membranes were greater than Nafion 212 that was because of the presence of electrospun fibers. Elongation at break data for both com posite and non-composite samples were almost the same (between 6.63 and 7.44%) that were way smaller than Nafion 212. However, mem branes in a fuel cell are not subjected to the significant tensile stress, so this amounts of elongation would be acceptable [22,34]. Also Young’s modulus for composite samples of CE-101 and CE-102 were higher than SC-70 and the value for CE-102 was close to the Nafion 212. Therefore mechanical properties of pristine sulfonated PEK membrane increased significantly by introducing electrospun fibers to the membrane struc ture [42]. 3.6. Single cell performance In this section, cell performance of composite and non-composite membranes explored. Fig. 6 shows the I–V curves of MEAs fabricated with CE-101 and Nafion 212 at 70 and 80 � C with water saturated reactant gasses at backpressure of 10 psi. It was seen that increasing the temperature at same humidity levels improved current density of the cell due to increasing the anode and cathode reaction rates. The current density of CE-101 was almost half of the Nafion 212 as it was expected because of lower proton conductivity of CE-101. Fig. 7 shows cell performance of CE-102 at different temperatures and backpressure amounts. As it is clear, increasing the cell temperature from 70 to 80 � C increased the current density about 300 mA cm 2. However, raising the back pressure amount caused no significant effect on current density of the cell. As can be seen from Figs. 6 and 7, current density derived from CE-102 was higher than CE-101 at its best condi tion that was because of more sulfonated PEK in the structure of CE-102
Fig. 6. Cell performance of CE-101 at temperatures of 70 and 80 � C compared to Nafion 212, in backpressure of 10 psi.
Fig. 7. Effect of cell temperature and backpressure on performance of CE-102.
compared to CE-101 composite membrane. To evaluate the effectiveness of producing composite PEK mem branes containing non-sulfonated fibers of PEK, MEA of pristine 70%sulfonated PEK membrane (SC-70) was examined in a single cell (Fig. 8). As illustrated, increasing the temperature caused improving the current density of the cell but surprisingly increasing the backpressure from 10 to 15 psi severely reduced the current density from 1090 to 770 mA cm 2. According to the decreasing of OCV from 983 to 795 mV, it could be concluded that some defects had occurred in the membrane after increasing the backpressure that affecting the performance of the
Fig. 5. Stress-strain curves of samples. 6
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Foundation for partial support of this research. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jpowsour.2019.226733. References [1] L. Akbarian-Feizi, S. Mehdipour-Ataei, H. Yeganeh, Int. J. Hydrogen Energy 35 (2010) 9385–9397. [2] M. Hu, X. He, Y. Chen, D. Chen, J. Polym. Res. 19 (2012) 9977. [3] Q. Li, J.O. Jensen, R.F. Savinell, N. Bjerrum, Prog. Polym. Sci. 34 (2009) 449–477. [4] X. Zhang, Q. Liu, L. Xia, D. Huang, X. Fu, R. Zhang, S. Hu, F. Zhao, X. Li, X. Bao, J. Membr. Sci. 574 (2019) 282–298. [5] J.-H. Fang, Polyimide proton exchange membranes, in: Advanced Polyimide Materials, Elsevier, 2018, pp. 323–383. [6] F. Lufrano, I. Gatto, P. Staiti, V. Antonucci, E. Passalacqua, Solid State Ionics 145 (2001) 47–51. [7] N. Ure~ na, M.T. P� erez-Prior, C. del Río, A. V� arez, J.-Y. Sanchez, C. Iojoiu, B. Levenfeld, Electrochim. Acta 302 (2019) 428–440. [8] M. Oroujzadeh, S. Mehdipour-Ataei, M. Esfandeh, RSC Adv. 5 (2015) 72075–72083. [9] S. Matsumura, A.R. Hlil, C. Lepiller, J. Gaudet, D. Guay, Z. Shi, S. Holdcroft, A. Hay, Macromolecules 41 (2008) 281–284. [10] M.J. Parnian, S. Rowshanzamir, A.K. Prasad, S. Advani, J. Membr. Sci. 565 (2018) 342–357. [11] M. Oroujzadeh, S. Mehdipour-Ataei, Int J Polym Mater Po 65 (2016) 330–336. [12] C. Li, X. Liu, Y. Zhang, J. Dong, J. Wang, Z. Yang, H. Cheng, Energy Technol-Ger 7 (2019) 71–79. [13] F. Wang, M. Hickner, Y.S. Kim, T.A. Zawodzinski, J. McGrath, J. Membr. Sci. 197 (2002) 231–242. [14] K. Kreuer, J. Membr. Sci. 185 (2001) 29–39. [15] G. Gebel, Polymer 41 (2000) 5829–5838. [16] B. Campagne, G. David, B. Am� eduri, D.J. Jones, J. Rozi� ere, I. Roche, Int. J. Hydrogen Energy 40 (2015) 16797–16813. [17] J.-D. Kim, A. Donnadio, M.-S. Jun, M. Di Vona, Int. J. Hydrogen Energy 38 (2013) 1517–1523. [18] H.-L. Lin, Y.-C. Chou, T.L. Yu, S. Lai, Int. J. Hydrogen Energy 37 (2012) 383–392. [19] K.T. Park, J.H. Chun, S.G. Kim, B.-H. Chun, S. Kim, Int. J. Hydrogen Energy 36 (2011) 1813–1819. [20] W.-K. Chao, C.-M. Lee, D.-C. Tsai, C.-C. Chou, K.-L. Hsueh, F.-S. Shieu, J. Power Sources 185 (2008) 136–142. [21] S.-H. Kwak, T.-H. Yang, C.-S. Kim, K. Yoon, Electrochim. Acta 50 (2004) 653–657. [22] T. Chuesutham, A. Sirivat, N. Paradee, S. Changkhamchom, K. Wattanakul, S. Anumart, N. Krathumkhet, J. Khampim, Renew. Energy 138 (2019) 243–249. [23] F. Celso, S.D. Mikhailenko, M.A. Rodrigues, R.S. Mauler, S. Kaliaguine, J. Power Sources 305 (2016) 54–63. [24] X. Liu, Z. Yang, Y. Zhang, C. Li, J. Dong, Y. Liu, H. Cheng, Int. J. Hydrogen Energy 42 (2017) 10275–10284. [25] P. Salarizadeh, M. Javanbakht, S. Pourmahdian, M. Hazer, K. Hooshyari, M. Askari, Int. J. Hydrogen Energy 44 (2019) 3099–3114. [26] L. Cui, Q. Geng, C. Gong, H. Liu, G. Zheng, G. Wang, Q. Liu, S. Wen, Polym. Adv. Technol. 26 (2015) 457–464. [27] J.L. Reyes-Rodriguez, J. Escorihuela, A. García-Bernab� e, E. Gim�enez, O. SolorzaFeria, V. Compa~ n, RSC Adv. 7 (2017) 53481–53491. [28] Y. Jiang, J. Hao, M. Hou, S. Hong, W. Song, B. Yi, Z. Shao, Sustain Energ Fuels 1 (2017) 1405–1413. [29] B. Wang, L. Hong, Y. Li, L. Zhao, C. Zhao, H. Na, ACS Appl. Mater. Interfaces 9 (2017) 32227–32236. [30] C. Klose, M. Breitwieser, S. Vierrath, M. Klingele, H. Cho, A. Büchler, J. Kerres, S. Thiele, J. Power Sources 361 (2017) 237–242. [31] C. Boaretti, L. Pasquini, R. Sood, S. Giancola, A. Donnadio, M. Roso, M. Modesti, S. Cavaliere, J. Membr. Sci. 545 (2018) 66–74. [32] J. Reyes-Rodriguez, O. Solorza-Feria, A. García-Bernab� e, E. Gim�enez, O. Sahuquillo, V. Compan, RSC Adv. 6 (2016) 56986–56999. [33] X. Xu, L. Li, H. Wang, X. Li, XZhuang, RSC Adv 5 (2015) 4934–4940. [34] X. Gong, G. He, Y. Wu, S. Zhang, B. Chen, Y. Dai, X. Wu, J. Power Sources 358 (2017) 134–141. [35] J.B. Ballengee, P.N. Pintauro, Macromolecules 44 (2011) 7307–7314. [36] J. Choi, K.M. Lee, R. Wycisk, P.N. Pintauro, P.T. Mather, Macromolecules 41 (2008) 4569–4572. [37] D. Powers, R. Wycisk, P.N. Pintauro, J. Membr. Sci. 573 (2019) 107–116. [38] F.A. Carey, R.J. Sundberg, Advanced Organic Chemistry: Part A: Structure and Mechanisms, Springer Science & Business Media, 2007. [39] B. Maranesi, H. Hou, R. Polini, E. Sgreccia, G. Alberti, R. Narducci, P. Knauth, M. Di Vona, Fuel Cells 13 (2013) 107–117. [40] C. Subramanian, M. Giotto, R. Weiss, M. Shaw, Macromolecules 45 (2012) 3104–3111. [41] A. Roy, X. Yu, S. Dunn, J. McGrath, J. Membr. Sci. 327 (2009) 118–124. [42] J. Kim, D. Reneker, Polym composites 20 (1999) 124–131.
Fig. 8. Single cell performance of SC-70 at temperatures of 70 and 80 � C and backpressures of 10 and 15 psi.
cell strongly. Referring to Fig. 5, SC-70 had the lowest mechanical strength compared to the composite membranes reinforced with the electrospun fibers so it could not be stable at performance test condi tions. It should be noted that high water absorption of SC-70 could also affect its mechanical strength. On the other hand, SC-70 sample showed the proton conductivity of 0.16 S cm 1 (Table 2) while that parameter for CE-102 was 0.08 S cm 1. So in spite of very high proton conductivity of SC-70, the current density of corresponding MEA was 1090 mA cm 1 which was very close to the value for the best current density of CE-102. So CE-102 membrane with proton conductivity of almost half of the SC70 showed the same current density that reveals successful hydropho bic/hydrophilic separation of the microstructure by introducing nonsulfonated fibers in the sulfonated matrix which led to the better performance. 4. Conclusion In this work, composite membranes containing non-sulfonated PEK fibers in highly sulfonated PEK matrix were prepared and compared with non-composite membrane of highly sulfonated PEK. Thermal properties investigation showed that both sulfonated and nonsulfonated PEKs had acceptable thermal stabilities that were more than Nafion. Water absorption of composite membranes showed that presence of hydrophobic fibers decreased the water absorption consid erably. Besides, mechanical properties of membranes, measured by tensile test, revealed that tensile strength of the composite membranes increased significantly because of the presence of submicron fibers of non-sulfonated PEK. Moreover, fuel cell performance test indicated that the composite membrane of CE-102 with proton conductivity of 0.08 S cm 1 had almost the same current density with pristine 70%sulfonated PEK membrane (with proton conductivity of 0.16 S cm 1) due to induced separation of hydrophilic/hydrophobic domains in the microstructure. Finally it could be concluded that using hydrophobic electrospun fibers in a very hydrophilic matrix controlled the water absorption of the membranes and improved mechanical properties without any negative effects on the current density of the final single cell. Acknowledgements The authors would like to appreciate Iran National Science
7
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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Poly(ether ketone) composite membranes by electrospinning for fuel cell applications Maryam Oroujzadeh, Mohammad Etesami, Shahram Mehdipour-Ataei * Iran Polymer and Petrochemical Institute, P.O. Box 14965/115, Tehran, Iran
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� Membranes of non-sulfonated PEK fibers in 70% sulfonated PEK matrix were prepared. � Tensile strength of membrane increased due to presence of non-sulfonated PEK fibers. � Hydrophobic fibers in a hydrophilic matrix controlled water absorption of membranes. � Proton conductivity of samples was be tween 0.05 and 0.16 S cm 1
A R T I C L E I N F O
A B S T R A C T
Keywords: Poly(ether ketone) Electrospinning Composite membrane Proton exchange membrane fuel cell
This work is dedicated to preparation of the composite membranes to control the water absorption and dimensional stability of a highly sulfonated poly(ether ketone) matrix. To achieve this goal, the non-sulfonated poly(ether ketone) fibers are prepared by electrospinning process and then fibers are impregnated by highly sulfonated poly(ether ketone). Both highly sulfonated and non-sulfonated polymers are synthesized by condensation polymerization reaction and are characterized by spectroscopic methods. The water absorption measurements indicate that introducing 20 and 30 wt% of non-sulfonated fibers lead to the water absorption of 31 and 25% respectively, in comparison to water absorption of highly sulfonated matrix that is 54%. The presence of fibers also improves the mechanical strength of membranes. Tensile strength reaches from 27 to 81 MPa by addition of 30 wt% electrospun fibers. Moreover, in H2/O2 fuel cell performance test, the composite membranes show current density in the range of 700–1090 mA cm 1 at different cell temperatures and back pressure amounts. Also proton conductivity of the samples is examined by electrochemical impedance spec troscopy and the values are between 0.05 and 0.16 S cm 1. Morphology of the fibers and membranes is studied by scanning electron microscopy as well.
1. Introduction Proton exchange membranes (PEMs) as a heart of a fuel cell, assist proton transportation between the anode and the cathode and also
inhibit electrons and reactant gasses crossover. Although Nafion (a perflourosulfonic acid membrane) is a commercial membrane for PEMs, because of some drawbacks like high cost and decreased efficiency at temperatures higher than 80 � C, attempts to replace Nafion with other
* Corresponding author. E-mail address: [email protected] (S. Mehdipour-Ataei). https://doi.org/10.1016/j.jpowsour.2019.226733 Received 30 March 2019; Received in revised form 17 May 2019; Accepted 4 June 2019 Available online 13 June 2019 0378-7753/© 2019 Elsevier B.V. All rights reserved.
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suitable proton conducting membranes are in process [1,2]. lots of polymer families were studied to substitute perflourosulfonic acid membranes such as: polybenzimidazoles [3,4], polyimides [5], poly sulfones [6–8], poly(ether ketone)s (PEK)s [9–11], and so on. Among the vast variety of suggested alternatives to Nafion, poly (ether ketone)s with different structures are of interest because of low cost, high thermal and chemical stability, and high mechanical strength [12]. In the case of sulfonated aromatic substituents of Nafion, at the same ion exchange capacities, the proton conductivity is lower than Nafion because of different microstructures of Nafion and sulfonated aromatic polymers. It was shown that there are large separated hydro philic/hydrophobic regions in Nafion microstructure which facilitate proton conducting, while in sulfonated aromatic substituents this separated regions observed just in high degrees of sulfonation (more than 50% for poly(ether sulfone)s) [13–15]. So, to obtain an acceptable proton conductivity in aromatic substituent membranes high degrees of sulfonation is necessary that cause high water absorption of the mem brane and also dimensional instability during on/off cycles of the cell or at different humidity levels. This dimensional instability can cause delamination of the catalyst layer from the membrane surface and suppressing the cell performance. However, for controlling the water uptake of sulfonated aromatic polymers three types of strategies were reported: 1) crosslinking of the polymer chains [16–19], 2) using inorganic additives, they could be proton conductive [20–25] or non-conductive [26,27] and 3) making blends [12,28,29]. In composite blend membranes, a non-sulfonated polymer or a polymer with low degree of sulfonation was introduced to the membrane to control the water absorption and dimensional instability. In many of these reports one of the polymers is in fiber form and the other polymer/polymers play the matrix role [30–33]. Fibers, matrix or both can be sulfonated and participate in the proton con ducting. However, the degree of sulfonation, fibers orientation, or fibers content affect the hydrophilicity and dimensional stability of the system [34]. This work was focused on producing blend membranes consist of polymer fibers. Electrospinning technique has been used to make fibers. Electro spinning process to produce proton exchange membranes for fuel cell applications was pioneered by Pintauro’s group and as a facile and simple way of fiber fabrication has attracted much attentions recently [35–37]. In this process, submicron fibers draw from the needle of a syringe containing a polymeric solution with the specific concentration and gather on the collector surface. Different parameters like applied voltage, solution flow rate, solution concentration, solvent type, needle to collector distance, and so on control uniformity and diameter of the fibers. In this work, sulfonated and non-sulfonated poly(ether ketone)s (PEK)s were synthesized. Sulfonated PEK synthesized from the sulfo nated monomer (sulfonation degree of the polymer was 70%) used as a matrix of the composite membranes. On the other hand, electrospun fibers of non-sulfonated PEK were prepared. The fibers were impreg nated with 70% sulfonated polymer solution to make blend membranes with controlled water absorption and increased mechanical strength. So, two composite membranes with different amounts of electrospun fibers were prepared and the performance of these membranes was compared with pristine 70% sulfonated PEK membrane (prepared by solution casting method) and also with Nafion 212 membrane.
polymerization solvent. Ethanol, toluene, tetrahydrofuran (THF), dimethylformamide (DMF), 2-propanol, fuming sulfuric acid (30%) (All from Merck) were used as received. 2.2. Synthesis of sulfonated monomer, highly sulfonated and nonsulfonated PEKs In a three-necked round-bottom flask equipped with a nitrogen inlet and Dean-stark trap, 1.1941 g of sulfonated difluorobenzophenone (sDFB) (synthesized as reported before [11]) and 0.2645 g of DFB as dihalides, mixed with 1.3998 g of a dihydroxy monomer (PBP) in pres ence of 0.636 g of potassium carbonate. Then 10 mL of DMAc was added as a solvent. The mixture was refluxed for 4 h at 140 � C and then 10 h at 175 � C. After that, the viscose polymer solution was cooled and diluted with 10 mL of DMAc. Then it was poured slowly in an excess amounts of deionized water. The obtained white fibrous precipitate stirred in deionized water overnight at 50 � C to remove any remained salts. The non-sulfonated PEK was synthesized in the same way by mixing stoi chiometric amounts of DFB and PBP as dihalide and dihydroxy com pounds, respectively in presence of potassium carbonate. Sulfonated PEK was acidified by boiling in 0.5 M sulfuric acid solu tion for 2 h. Then for washing the excess acid, it was boiled in the deionized water for couple of hours using fresh deionized water every 1 h. At last, the acidified sulfonated PEK was filtered and dried in a vacuum oven at 80 � C overnight. 2.3. Electrospinning of non-sulfonated PEK Non-sulfonated PEK solution was prepared by dissolving the polymer in DMF: THF mixture (1:1). After optimization of the parameters, a 5 wt % solution of the polymer was electrospun at room temperature for about 10 h on rotating drum covered with an aluminum foil in the applied voltage of 24 kV, feeding rate of 1 mL h 1, needle to the collector distance of 14 cm, and the collector rotating speed of 300 rpm. After finishing the electrospinning time, the foil was separated from the col lector and immersed in water for peeling off the mat. The mat was pressed 10 s under 100 kg.f to make it easier to handle and then dried in a vacuum oven overnight at 80 � C. 2.4. Membrane preparation For preparation of pristine sulfonated PEK membrane, 15 wt% so lution of polymer in DMAc was cast on a glass surface with a doctor blade and the membrane was dried under infrared light. The membrane after drying was peeled off by immersing in deionized water bath and then dried in a vacuum oven at 80 � C for 12 h. The pristine 70%-sulfo nated PEK membrane made by solution casting method was named as SC-70. Afterwards, for preparation of composite membranes, nonsulfonated fiber mats were immersed in 70%-sulfonated PEK solution. Before that, the suitable solvent must be selected that just dissolve the sulfonated PEK and did not dissolve the non-sulfonated fibers. After examination of several solvents at different temperatures, methanol at room temperature was selected for this purpose. In this regards, 5 wt% methanol solution of 70%-sulfonated PEK was prepared and the elec trospun mat was dipped in for several times. After each time the mem brane was dried at room temperature on a glass plate and then immersed in polymer solution again. Finally, to ensure wetting of electrospun fi bers with sulfonated polymer solution, membranes were hot-pressed for 30 s at 160 � C. In that way, two composite membranes with different content of non-sulfonated fibers were prepared that named as CE-101 and CE-102 (containing 30 and 20 wt% electrospun fibers, respectively). Nafion 212 membrane also was activated before preparation of the membrane electrode assembly (MEA) in this manner: the membrane was immersed in H2O2 solution (3%) at 80 � C for 1 h and then after washing with deionized water, it boiled in water for 2 h. Then it remained in
2. Experimental 2.1. Materials 4,40 -Difluorobenzophenone (DFB) as dihalide monomer and 4,40 (1,4-phenylene diisopropylidene) bisphenol (PBP) as dihydroxy mono mer both purchased from Aldrich and used as received. Potassium car bonate (from Merck) was dried at 120 � C in a vacuum oven overnight. Anhydrous dimethylacetamide (DMAc) (from Aldrich) utilized as 2
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sulfuric acid solution (4 M) for 1 h and then washed again with hot water several times to ensure removal of excess acid. After all, the membrane was kept in deionized water until use.
resistance, and membrane cross-sectional area, respectively.
2.5. Characterization
Four MEAs including SC-70, CE-101, CE-102, and Nafion 212 membranes were prepared. Pt/C catalyst was utilized as the anode and cathode catalyst with loading of 0.48 mg cm 2. Catalyst ink includes Pt/ C (60%) dispersed in 2-propanol/deionized water and contained 5% Nafion solution. Catalyst ink was painted on a carbon paper gas diffusion layer to make the anode and cathode gas diffusion electrodes. MEAs were prepared by hot-pressing of two electrodes onto the membranes at 130 � C for 2 min under pressure of 100 kgf. Each MEA was well packed in a sealed bag with some drops of the deionized water until use. Po larization curves were obtained in a single cell with active area of 5 cm2. Two cell temperature of 70 and 80 � C were examined and flow rates of H2 and O2 were fixed on 300 mL. min 1. The performance test was performed under humidity of 100% for aforementioned cell tempera tures and the backpressure was adjusted to 10 and 15 psi. At least 3 h of humidification and pretreatment was performed for each MEA sample at potential of 550 mV.
2.6. Fuel cell performance evaluation
Chemical structure of the sulfonated monomer was confirmed using H NMR (Bruker Avance DPX 400 MHz) and FT-IR (Bruker- IFS48) spectroscopy (reported before [11]). FT-IR spectroscopy was used to study the structure of the synthesized sulfonated and non-sulfonated polymers. Thermal properties of the polymers were studied by thermogravi metric analysis (Mettler TGA/DSC) from room temperature to 700 � C at a heating rate of 10 � C min 1 under air atmosphere. Glass transition temperature of the synthesized polymers was obtained from differential scanning calorimetry analysis (DSC) (Metteler) and performed in air and with heating rate of 5 � C min 1. Microstructure and morphology of the electrospun fibers, composite membranes, and solution cast membrane were studied through scanning electron microscopy (SEM) (VEGA TESCAN) at an accelerating voltage of 20 kV. Ion exchange capacity (IEC) of the membranes was determined by titration method. 0.06 g of each membrane sample was immersed in NaCl (2 M) solution for 48 h to replace Hþ ions of sulfonic acid groups with Naþ. Liberated Hþ ions were titrated with 4 mM solution of NaOH in presence of phenolphthalein as the indicator. Following equation was used to calculate IEC values:
1
3. Results and discussion 3.1. Synthesis and characterization of sulfonated monomer, highly sulfonated PEK and non-sulfonated PEK For direct synthesis of sulfonated PEK from sulfonated monomer, sDFB was synthesized via electrophilic aromatic sulfonation reaction. 1H NMR and FT-IR were used to confirm the structure of the sulfonated monomer that were reported before [11] and were shown in supple mentary materials section (Fig. S1 and Fig. S2). 70%-sulfonated PEK and non-sulfonated PEK were synthesized via condensation polymerization of related monomers as mentioned in section 2.2 in details (Scheme 1). The FT-IR spectra for both polymers shown in supplementary materials section (Fig. S3) confirmed the structure of synthesized polymers. The strong absorption band of Ar-O-Ar was observed for both polymers. However, symmetric and asymmetric stretching bands related to the
VNaOH � MNaOH IEC ¼ Ws where, V NaOH and M NaOH are volume and concentration of NaOH so lution and Ws is the weight of the dry membrane sample. Water absorption of samples were calculated from the weight of the absorbed water after 24 h immersion in water at room temperature. After weighing the samples, they dried in a vacuum oven for at least 24 h to obtain weights of dehydrated samples. The equation was as follows: Water Absorption ð%Þ ¼
Ww
Wd Wd
� 100
where, Ww and Wd are the weights of hydrated and dehydrated mem branes, respectively. Hydration number (λ) or number of water molecules around every sulfonic acid groups was calculated as follows using water absorption and IEC values: � � λ¼
WW Wd Wd
18 � IEC
� 1000
where, Ww and Wd are the weights of hydrated and dehydrated mem branes, respectively and 18 is the molecular weight of water. Mechanical properties of the samples were measured by tensile test using a STM-20 according to ASTM-D 882 at the speed of 5 mm min 1 at room temperature. At least 3 measurements were performed for each membrane sample. Proton conductivity of membranes were measured by the electro chemical impedance spectroscopy (EIS) (IVIUM potentiostat/galvano stat) in AC amplitude of 50 mA ranging from 1 to 106 Hz at room temperature. The in-plane proton conductivity measurement was occurred in a two-point probe home-made cell. The frequency that produced minimum imaginary response was taken as the membrane resistance and the proton conductivity was calculated:
σ¼
L RA
where, L, R, And A are distance between two electrodes, membrane
Scheme 1. Synthesis of polymers. 3
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sulfonic acid groups just appeared in sulfonated PEK spectrum at 1014 and 1079 cm 1, respectively. 3.2. Morphological study of the electrospun fibers, composite and noncomposite membranes In this work, composite membranes using electrospun nonsulfonated PEK fibers and highly sulfonated PEK matrix were pre pared. As stated, in sulfonated aromatic membranes, less proton con ductivity observes in comparison to Nafion due to the different microstructures of Nafion and aromatic sulfonated polymers. To over come the problem, membranes with higher degrees of sulfonation should be used which have very high water uptake and consequently dimensional instability. In this work non-sulfonated PEK fiber mats were used to decrease high water absorption of highly sulfonated matrix. This fiber mat structure also brought higher mechanical strength for com posite membranes that were investigated later. For preparing composite membranes, non-sulfonated fiber mats were electrospun firstly. As demonstrated in the SEM image (Fig. 1a), uniform and bead-free fibers were electrospun successfully. As noted in the experimental section, non-sulfonated PEK was electrospun on an aluminum foil (Fig. 1b) and then water bath was used for separating fiber mat off the foil (Fig. 1c and d). Since working with those electro spun mats was not easy, the electrospun mat was pressed to make a packed and firmed structure (Fig. 1e) To prepare composite membranes, the fiber mats were immersed in 70%-sulfonated PEK solution for several times and finally to ensure penetration of polymer solution into the fiber mats, they were hotpressed at 160 � C for 30s. Fig. 2 shows the final composite membrane compared to the primary fiber mat. SEM analysis also was used to confirm the penetration of polymer solution into the fiber mats and additionally to study the morphology of the composite and non-composite membranes. As depicted in Fig. 3 (a) and (b), in cross-sectional view of the composite sample CE-102, fibers were completely recognizable. Moreover, there was no void or defect in the structure of the composite membrane which is very important in a fuel cell membrane because any defect can cause reactant crossover and deteriorating cell performance. Fig. 3 (c) demonstrates the cross-
Fig. 2. Primary non-sulfonated PEK fiber mats (left) and final composite membrane (right).
sectional view of the non-composite SC-70 sample for comparison. As it is clear from Fig. 3 (a, c) there was a brittle fracture in non-composite membrane but in CE-102 sample presence of fibers caused more ductile fracture pattern and therefore more flexible membrane. 3.3. Thermal behavior Thermal behavior of sulfonated and non-sulfonated PEK polymers were studied using DSC (shown in supplementary material section Fig. S4) and TGA analysis. Table 1 shows the glass transition tempera ture of the polymer samples. Tg values were 130 � C for PEK-0 and 159 � C for SC-70 samples. It means that the presence of sulfonic acid groups in SC-70 caused about 30 � C increasing in Tg compared to PEK-0 sample. Existence of sulfonic acid groups increased strong intramolecular in teractions between polymer chains and then increasing the Tg of the polymer. TGA profiles of PEK-0 and SC-70 were shown in Fig. 4. Moreover, T10% data were listed in Table 1 and show that both non-sulfonated and highly sulfonated PEKs were more stable than Nafion because of more thermal stable nature of PEK polymer due to the presence of carbonyl and aromatic linkages. According to Fig. 4, there was an initial weight
Fig. 1. SEM image of electrospun fibers (a), image of non-sulfonated fiber mats immediately after electrospinning (b), soaking the aluminum foil in water bath (c), detached fiber mats (d), and fiber mats after pressing and packing (e). 4
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Fig. 3. SEM images of cross-section of CE-102 (a), same sample in close view (b), and SC-70 (c).
loss for SC-70 at low temperatures that was related to the absorbed at mospheric water showing very high tendency of SC-70 to the water absorption. Thereafter, three subsequent weight losses were observed for SC-70. The first one was related to the leaving of sulfonic acid groups and the next two were attributed to the polymer chain decompositions. However, PEK-0 just showed two weight losses related to the polymer chain decompositions. Comparison of the char content of both samples confirmed the presence of inorganic groups in SC-70 sample. The presence of sulfur atoms caused increasing char yield from 1.39% in PEK-0 to 16.2% in SC70 at 750 � C.
g 1, respectively. Presence of more sulfonated polymer in CE-102 (80 wt % versus 70 wt% for CE-101) caused its higher IEC value (Table 1). As observed, the IEC of composite membranes CE-101 and CE-102 were less than what was estimated according to the fiber content. The difference might be attributed to the probable cross-linking of some sulfonic acid groups during thermal treatment of membranes after impregnation process (section 2-4). It was proved that in presence of residual amounts of amidic solvents like dimethylacetamide or dimethylformamide, an electrophilic aromatic substitution reaction may form a sulfone bridge via linking two aromatic rings during thermal treatment [38–40]. Water absorption is an important parameter in sulfonated proton exchange membranes because the water molecules play carrier role in proton transportation. However, high water absorption causes some difficulties like, dimensional and mechanical instabilities and also delamination of catalyst layer from the membrane in MEAs during on/ off cycles of the cell or at different humidity levels. Consequently, the water uptake value is critical in sulfonated membranes. Water absorp tion was easily calculated from weight of the wet samples after 24 h immersion in water and reported in Table 1. As presented, SC-70 had the highest water absorption that was 54%. Since, sulfonic acid groups are responsible for absorbed water, increasing the sulfonation content, increased the water absorption of CE-102 compared to CE-101 mem brane. CE-102 and CE-101 contained 80 and 70 wt% sulfonated poly mer, respectively. Presence of hydrophobic fiber mats decreased their affinity to water significantly. Nafion 212 showed high water absorption in spite of its low IEC value which is related to the different micro structure of Nafion and larger hydrophilic/hydrophobic domains compared to aromatic membrane samples [14]. Hydration number is the number of water molecules around every sulfonic acid group in the hydration sphere. In other words, hydration number is a measure of the efficiency of the absorbed water for proton conducting. The best condition for a fuel cell membrane is high proton conductivity in low hydration number [41]. Therefore, there is a trade-off between two parameters of proton conductivity and hydration number. As listed in Table 1, CE-102 showed hydration number of 11 in comparison to 9.7 for CE-101. The parameter was 13.2 for SC-70 which is very lower than Nafion 212. Proton conductivity measurements for all membranes performed at room temperature in water. In Fig. S5 of the supplementary materials the nyquist plots were shown and data were reported in Table 2. As shown, SC-70 presented the highest proton conductivity due to its very high IEC value and sulfonation degree. Predictably, increasing of sul fonated PEK content in composite membrane CE-102 increased proton conductivity in comparison to CE-101.
3.4. IEC, water absorption, hydration number, and proton conductivity
3.5. Mechanical properties
The number of sulfonic acid groups per gram of molecule defines as IEC value. The ion-exchange capacity of samples was determined as mentioned in section 2.5 by titration. The IEC value for SC-70 sample was 2.23 meq g 1 as reported in Table 1. Composite membranes of CE101 and CE-102 showed ion exchange capacity of 1.43 and 1.59 meq
Suitable mechanical strength is crucial for durability of a fuel cell membrane in the cell environment. Fig. 5 shows stress-strain curves of composite samples and cast film of highly sulfonated PEK. Quantities for tensile strength, Young’s modulus, and elongation at break were listed in Table 2, as well. As reported, the highest tensile strength belonged to CE-
Table 1 Properties of composite and non-composite membranes. Sample
Sulfonated PEK content (wt%)
IEC (meq/ g)
Water absorption (%)
Hydration number
Tg (�C)
T10% (�C)
CE-101 CE-102 PEK-0 SC-70 Nafion 212
70 80 – 100 –
1.43 1.59 – 2.23 0.84
25 31 – 54 40
9.7 11.0 – 13.2 26.4
– – 130 159 218
– – 439 414 351
Fig. 4. TGA profile of the synthesized polymers.
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Journal of Power Sources 434 (2019) 226733
Table 2 Proton conductivity and mechanical properties of samples. Sample
Proton conductivity (S/ cm)
Tensile strength (MPa)
Young’s modulus (GPa)
Elongation@ break (%)
Membrane thickness (dry in μm)
Membrane thickness (wet in μm)
CE-101 CE-102 SC-70 Nafion 212
0.05 0.08 0.16 0.10
81 60 27 42
2.02 3.35 0.95 3.60
7.00 6.63 7.44 252.00
95 125 50 50
120 160 75 70
101 with higher percentage of electrospun fibers. Although CE-102 showed lower tensile strength compared to CE-101, but both CE-101 and CE-102 showed very better tensile strength than SC-70. This observation obviously revealed the effect of the presence of fiber mat in the composite membrane structure. Tensile strength data of both com posite membranes were greater than Nafion 212 that was because of the presence of electrospun fibers. Elongation at break data for both com posite and non-composite samples were almost the same (between 6.63 and 7.44%) that were way smaller than Nafion 212. However, mem branes in a fuel cell are not subjected to the significant tensile stress, so this amounts of elongation would be acceptable [22,34]. Also Young’s modulus for composite samples of CE-101 and CE-102 were higher than SC-70 and the value for CE-102 was close to the Nafion 212. Therefore mechanical properties of pristine sulfonated PEK membrane increased significantly by introducing electrospun fibers to the membrane struc ture [42]. 3.6. Single cell performance In this section, cell performance of composite and non-composite membranes explored. Fig. 6 shows the I–V curves of MEAs fabricated with CE-101 and Nafion 212 at 70 and 80 � C with water saturated reactant gasses at backpressure of 10 psi. It was seen that increasing the temperature at same humidity levels improved current density of the cell due to increasing the anode and cathode reaction rates. The current density of CE-101 was almost half of the Nafion 212 as it was expected because of lower proton conductivity of CE-101. Fig. 7 shows cell performance of CE-102 at different temperatures and backpressure amounts. As it is clear, increasing the cell temperature from 70 to 80 � C increased the current density about 300 mA cm 2. However, raising the back pressure amount caused no significant effect on current density of the cell. As can be seen from Figs. 6 and 7, current density derived from CE-102 was higher than CE-101 at its best condi tion that was because of more sulfonated PEK in the structure of CE-102
Fig. 6. Cell performance of CE-101 at temperatures of 70 and 80 � C compared to Nafion 212, in backpressure of 10 psi.
Fig. 7. Effect of cell temperature and backpressure on performance of CE-102.
compared to CE-101 composite membrane. To evaluate the effectiveness of producing composite PEK mem branes containing non-sulfonated fibers of PEK, MEA of pristine 70%sulfonated PEK membrane (SC-70) was examined in a single cell (Fig. 8). As illustrated, increasing the temperature caused improving the current density of the cell but surprisingly increasing the backpressure from 10 to 15 psi severely reduced the current density from 1090 to 770 mA cm 2. According to the decreasing of OCV from 983 to 795 mV, it could be concluded that some defects had occurred in the membrane after increasing the backpressure that affecting the performance of the
Fig. 5. Stress-strain curves of samples. 6
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Journal of Power Sources 434 (2019) 226733
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Fig. 8. Single cell performance of SC-70 at temperatures of 70 and 80 � C and backpressures of 10 and 15 psi.
cell strongly. Referring to Fig. 5, SC-70 had the lowest mechanical strength compared to the composite membranes reinforced with the electrospun fibers so it could not be stable at performance test condi tions. It should be noted that high water absorption of SC-70 could also affect its mechanical strength. On the other hand, SC-70 sample showed the proton conductivity of 0.16 S cm 1 (Table 2) while that parameter for CE-102 was 0.08 S cm 1. So in spite of very high proton conductivity of SC-70, the current density of corresponding MEA was 1090 mA cm 1 which was very close to the value for the best current density of CE-102. So CE-102 membrane with proton conductivity of almost half of the SC70 showed the same current density that reveals successful hydropho bic/hydrophilic separation of the microstructure by introducing nonsulfonated fibers in the sulfonated matrix which led to the better performance. 4. Conclusion In this work, composite membranes containing non-sulfonated PEK fibers in highly sulfonated PEK matrix were prepared and compared with non-composite membrane of highly sulfonated PEK. Thermal properties investigation showed that both sulfonated and nonsulfonated PEKs had acceptable thermal stabilities that were more than Nafion. Water absorption of composite membranes showed that presence of hydrophobic fibers decreased the water absorption consid erably. Besides, mechanical properties of membranes, measured by tensile test, revealed that tensile strength of the composite membranes increased significantly because of the presence of submicron fibers of non-sulfonated PEK. Moreover, fuel cell performance test indicated that the composite membrane of CE-102 with proton conductivity of 0.08 S cm 1 had almost the same current density with pristine 70%sulfonated PEK membrane (with proton conductivity of 0.16 S cm 1) due to induced separation of hydrophilic/hydrophobic domains in the microstructure. Finally it could be concluded that using hydrophobic electrospun fibers in a very hydrophilic matrix controlled the water absorption of the membranes and improved mechanical properties without any negative effects on the current density of the final single cell. Acknowledgements The authors would like to appreciate Iran National Science
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