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electrospun nonwoven polyacrylonitrile composite membrane for proton exchange membrane fuel cells
Journal of Membrane Science 446 (2013) 212–219
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Properties of sulfonated poly(arylene ether sulfone)/electrospun nonwoven polyacrylonitrile composite membrane for proton exchange membrane fuel cells Duk Man Yu, Sangjun Yoon, Tae-Ho Kim, Jang Yong Lee, Jaerock Lee, Young Taik Hong n Energy Materials Research Center, Korea Research Institute of Chemical Technology, P.O. Box 107, Yuseong, Daejeon 305-600, Republic of Korea
art ic l e i nf o
a b s t r a c t
Article history: Received 4 March 2013 Received in revised form 30 May 2013 Accepted 18 June 2013 Available online 24 June 2013
A new composite membrane is fabricated by impregnating the sulfonated poly(arylene ether sulfone) (SPAES) copolymer into the nonwoven, electrospun by polyacrylonitrile (PAN) resin, for proton exchange membrane fuel cells (PEMFC). The SPAES copolymer is synthesized with 50 degrees of sulfonation, and the nonwoven PAN is prepared as a highly porous substrate that is anticipated to enhance the mechanical properties, dimensional stability, and durability of the composite membrane. This study focuses on all of these characteristics in addition to the degree of proton conductivity. Furthermore, a fuel cell operating test is conducted to compare the properties of the proposed composite membrane to those of a pristine membrane and to identify the effects of the nonwoven PAN. The dimensional change of the composite membrane decreases from 91% to 51% in water and the Young's modulus exhibited nearly three times the strength of the pristine membrane. However, the proton conductivity (measured at various temperature and humidity levels) decreases in the composite membrane because the nonwoven PAN obstructs the proton pathways. The proposed composite membrane displays suitable performance and notable durability for PEMFC applications. & 2013 Elsevier B.V. All rights reserved.
Keywords: Polymer electrolyte membrane Composite membrane Nonwoven Polyacrylonitrile Sulfonated poly(arylene ether sulfone)
1. Introduction Proton exchange membrane fuel cells (PEMFC) are among the greatly anticipated and promising energy technologies, because they can directly convert chemical energy into electrical energy for portable electronic devices, automobiles, and large buildings [1–3]. Among the PEMFC components, the polymer electrolyte membrane (PEM) is a fundamental component that separates the anode and cathode and determines the efficiency and durability of PEMFCs. Nafion, a perfluorosulfonated ionomer, is a membranous material that has been successfully commercialized for use in fuel cells because of its high proton conductivity, low gas permeability, and excellent dimensional stability and cell performance. However, Nafion membranes are expensive; moreover, its fabrication process is intricate and emits toxic waste products [4]. Sulfonated poly(arylene ether sulfone) (SPAES), a hydrocarbon-based ionomer, has been reviewed as an alternative PEM to Nafion over the past decade because of its high thermal resistance, good mechanical strength, low hydrogen permeability, and inexpensive production process [5]. However, the dimensions of SPAES expand under a fully hydrated condition, which decreases the durability of fuel
n
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cells. In order to overcome this critical defect, composite membranes that include multi-fibrous manufactured materials such as polyimide (PI) [6,7], poly(paraphenylene terephthalamide) (PPTA) [8], poly(vinyl alcohol) (PVA) [9,10], and polyphenylsulfone (PPS) [11] have been developed by many research groups. For example, Seol et al. [12] prepared composite membranes using electrospun nonwoven PI and SPAES copolymers. Their membranes showed enhanced dimensional stability and mechanical properties, but the proton conductivity was reduced in the composite membrane (compared to a pristine membrane) because of the PI substrate, which could not conduct protons. To improve the proton conductivity of the composite membrane, silicate with sulfonic acid groups (3-trihydroxysilyl propane-1-sulfonic acid, THSPSA) was introduced using a sol–gel method on the nonwoven PI surface. The developed composite membrane using modified nonwoven PI showed proton conductivity comparable to a pristine membrane while maintaining excellent dimensional stability. Lin et al. [9] reported on an electrospun, crosslinked PVA, nanofiber thin film and incorporated it into Nafion to fabricate a direct methanol fuel cell (DMFC). A new composite membrane with crosslinked PVA nanofibers allowed for reduced the thickness and better methanol permeability. Although the composite membrane using the crosslinked PVA nanofibers displayed lower proton conduction capabilities, similar to other fibrous composite membranes, it performed better than Nafion because of the advantages
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associated with DMFCs, as previously noted. Li et al. [13] prepared polybenzimidazole (PBI)-based, nonwoven nanofiber for hightemperature PEMFCs. The crosslinked PBI nanofibers showed enhanced mechanical strength and good solvent resistance. PBIbased composite membranes modified with crosslinked PBI nanofibers showed notably improved dimensional stability and proton conductivity at high temperatures under anhydrous conditions compared to pristine membranes. A single cell employing a composite membrane exhibited a 34% increase in its maximum power density. We had previously studied a composite membrane using nonwoven LCP, fabricated by a melt-blown method for DMFCs [14]. Two types of SPAES copolymers based on 4,4′-biphenol (BP) and 9,9′-bis(4-hydroxyphenyl)fluorine (HPF) structures were used to prepare the composite membranes. These composite membranes showed a reduced dimensional change by approximately 45% and an enhanced tensile strength by nearly twofold. The durability of the composite membranes was investigated by wet and dry cycle tests, in which the membranes were repeatedly immersed in water and dried under a vacuum for 24 h in each cycle. After 10 cycles, the composite membranes containing the HPF structure showed cracks because of its higher rigidity compared to the composite membrane containing the BP structure. In an assessment of the performance of a single cell, a composite membrane containing the BP structure also exhibited performance comparable to that of Nafion. In this study, a new hydrocarbon-based composite membrane for PEMFCs is investigated using SPAES copolymers with the BP structure, of which the degree of sulfonation is approximately 50 mol% (SPAES50), and nonwoven polyacrylonitrile (PAN). Economically produced PAN fibers, which merely require a facile production process and further oxidation, have been used in many industries, including the automotive, aircraft, and electronic heating device industries because of their high heat resistance, good chemical stability, lightness, and tolerance to friction under severe environments [15–17]. For a composite membrane in PEMFCs, nonwoven PAN can help retain the dimensional stability and strengthen the interaction between an ionomer and a porous substrate through hydrogen bonding by cyano groups and sulfonic acid [18]. Moreover, it improves the durability of the membrane, which is currently the most important property of a membrane during the operation of a PEMFC. Therefore, we practically measured the durability of membranes using a membrane electrode assembly (MEA) and a single cell test station by a wet–dry cycling test operated mechanically under an open-circuit voltage (OCV) hold condition. In addition, the characteristics of the composite membranes are discussed in terms of their proton conductivity at various temperature and relative humidity (RH) conditions. The water uptake, dimensional changes, and mechanical properties are also assessed to identify the effects of nonwoven PAN in comparison to that of a pristine membrane for PEMFC applications.
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azobisisobutyronitrile (AIBN, Aldrich) were used as received. Anhydrous potassium carbonate (K2CO3, Aldrich) was dried under vacuum at 200 1C for 48 h prior to use. Acrylonitrile (AN) and methyl methacrylate (MMA) were obtained from Samchun Chemical (South Korea). A stainless steel needle (MN-21G-13, Iwashita Engineering, Japan) with an inner diameter of 0.51 mm, an outer diameter of 0.81 mm, and a length of 13 mm was used during the electrospinning process. 2.2. Preparation of nonwoven PAN The PAN copolymer was easily synthesized using AN and MMA monomers with a weight ratio of 94:6, as shown in Fig. 1(a). First, AN (70.5 g) and MMA (4.5 g) monomers were inserted into a fournecked flask equipped with a mechanical stirrer under a N2 atmosphere. Deionized water (279.0 g) was then added to the reactor. After the reactor was heated to 70 1C, 20 wt% AIBN (0.8 g) solution dissolved in DMF was introduced into the mixture, which was stirred for 30 min. The polymerized sample was filtered and washed thoroughly with methanol several times to eliminate residual reactants and then dried under convection at 50 1C for 48 h. The nonwoven PAN was prepared using a conventional electrospinning method. In response to the tensile force generated by an applied electric field, 10 wt% PAN solution (dissolved in DMSO) was ejected from a syringe. The electrospun fibers accumulated on a cylindrical drum collector. The electrical voltage was 10 kV and the syringe pump (KD Scientific-100, USA) was used to supply a constant throughput of the solution. The distance from the nozzle tip to the collector was fixed at 10 cm. The developed nonwoven PAN was dried to eliminate any residual solvent at 160 1C under vacuum for 2 h. Finally, the nonwoven dried PAN was stabilized in an oxidizer at 240 1C for 1.5 h (Fig. 1(b)) [20]. 2.3. Preparation of membranes Aromatic nucleophilic step-growth polymerization using SDFDPS, DFDPS, and BP was conducted in a four-neck flask equipped with a mechanical stirrer, an argon gas inlet, a condenser, and a Dean-Stark trap. The synthesis of SPAES50 was performed as reported in earlier studies [5,21]. Fig. 2 depicts the scheme used to create the synthetic copolymer, which was used as an ionomer for
2. Experimental 2.1. Materials 4, 4′-Difluorodiphenyl sulfone (DFDPS) was purchased from Solvay Advanced Polymers (USA) and recrystallized from ethanol. 3,3′-Disulfonated-4,4′-difluorodiphenyl sulfone (SDFDPS) was synthesized using the procedure reported in literature [5,19]. 4,4′-Bisphenol (BP) (TCI) was also recrystallized from ethanol for high purity. N-methyl-2-pyrrolidone (NMP, Junsei), anhydrous toluene (Aldrich), 95–97% sulfuric acid (Merck), dimethylsulfoxide (DMSO, Aldrich), dimethylformamide (DMF, Aldrich), and
Fig. 1. Scheme for synthesizing PAN copolymer (a) before oxidation and (b) after oxidation.
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Fig. 2. Scheme for synthesizing SPAES copolymer.
the composite membrane. The composite membranes were prepared by impregnating a polymer solution (15 wt%) into the nonwoven PAN using a bar coating method to acquire a flat membrane. The polymer solution with the nonwoven PAN was dried on a glass plate at 80 1C for 6 h in an oven and the prepared membrane was consecutively dried for the complete removal of the residual solvent under vacuum at 120 1C for 12 h. Finally, the membrane was acidified in 1.5 M sulfuric acid (H2SO4) for 24 h and subsequently rinsed with deionized water for 24 h at room temperature. 2.4. Characterization of membranes The molecular weight (Mw) of the synthesized PAN copolymer was confirmed with gel permeation chromatography (Waters GPC, Tosoh), and the pore size distribution of the nonwoven PAN was measured using an advanced capillary flow porometer (ACFP1500AE, wet up/dry up method with Galwick solution). The dimensional change and the amount of water uptake were determined by the difference in the volume and weight between a wet membrane measured in a fully hydrated state and a dry membrane measured after drying under vacuum at 120 1C. The mechanical properties of the membranes were evaluated using a material testing machine (LLOYD instrument LR5K) at 25 1C in a fully hydrated state with a crosshead speed of 50 mm/min. The surface and cross-sectional images of the nonwoven PAN and composite membranes were observed using a scanning electron microscope (SEM, XL-30S FEG, Philips). The specimens were coated with platinum in vacuum for 2 min using a sputter coating machine (Sputter Coater, Q150T ES, Quorumtech, USA) before the SEM imaging process. The equivalent of sulfonic acid groups per gram of the membranes, expressed by the ion exchange capacity (IEC), was measured using an automatic titrator (Metroohm 794 Basic Titrino). The proton conductivity measurements of the membranes were conducted with an AC impedance analyzer (SP-300, Impedance/ gain phase analyzer) at various temperatures (from 25 1C to 80 1C) and at 100% RH along the in-plane direction over a frequency range of 0.1 Hz to 4 MHz using a four-probe conductivity cell. Equilibration was permitted for 1 h in a temperature chamber (ESPEC, SH-241) before each measurement. The proton conductivity was calculated using the following Eq. (1): Proton conductivity ðS=cmÞ ¼ l=R S
ð1Þ
where l is the distance between the electrodes, R is the resistance of the membrane as calculated using the intercept value of the x axis (real value, Z′) in the electrochemical impedance spectroscopy (EIS) curve composed of the real value and the imaginary value (Z″), and S is the cross-sectional surface area of the membrane. The proton conductivity at different conditions (from 20% RH and 80% RH) and 80 1C was also examined by an in-plane conductivity system (BekkTech, BT-512), which can control humidity conditions using the same four-probe.
To evaluate the single cell performance, membrane electrode assemblies (MEA) were fabricated with commercially available gas-diffusion layers coated with Pt/C and Nafion binder (50 wt% Pt/C, 0.4 mg Pt/cm2 from FuelCellPower Inc.). The electrodes were applied as the cathode and anode; the active surface area of the electrode was 25 cm2. The fabricated MEAs were activated for 24 h with hydrogen at the anode and air at the cathode at 70 1C (stoichiometric coefficient ¼1.2/2). After activation, the MEAs were evaluated electrochemically by cycling between 0.5 and 1.0 V with a step change of 50 mV per 25 s at 70 1C, 100% RH, and a backpressure of zero using a test station (FCT-TS300, Fuel Cell Technologies, Inc.). Electrochemical impedance spectroscopy measurements were also performed to observe the MEA resistance of the through-plane at a DC potential of 0.85 V with the AC frequency ranging from 10 mHz to 1 MHz. The degree of hydrogen crossover was measured by a DC power supply (Agilent N5744A) after 30 min of nitrogen purging to the cathode. The working and reference electrodes were connected to the cathode and anode, respectively, and a potential of 0.15–0.3 V was then applied to the single cell. The current produced by the oxidation at the cathode of passed hydrogen from the anode was recorded.
3. Results and discussion Prior to investigating of the composite membrane, the nonwoven electrospun PAN, with Mw of 450 kg/mol and feasible oxidative stability, was characterized by SEM photography for the morphological analysis. Fig. 3(a) and (b) show the surface and cross-section of the nonwoven PAN that indicate the multi-fibrous nonwoven was fabricated successfully (nonwoven thickness¼1872 mm). The diameters of the fibers in the nonwoven PAN were determined to range from 600 to 1200 nm, and the PAN fibers appear to have melted slightly, causing them join together. A quantitative characterization of the pore size and pore size distribution of the nonwoven PAN is shown in Fig. 4(a). The average pore size of the nonwoven PAN was observed to be approximately 1 mm, and it is distributed regularly from 0.5 mm to 1 mm. In addition, the weight and volume of the nonwoven PAN was used to estimate its porosity at 80%. To investigate the resistance of the organic solvent, a solubility test was conducted using NMP, because it was the solvent used to formulate the composite membrane with the SPAES50 copolymer. Fig. 4(b) displays an image of the nonwoven PAN in the NMP solvent after several weeks, which shows that the nonwoven PAN is insoluble in NMP and making it appropriate to incorporate with the SPAES50 solution. These results confirm that a noteworthy nonwoven material with nano-sized fibers and an elaborate pore structure was developed for use in a proton exchange composite membrane. Fig. 5 shows the fabricated nonwoven PAN/SPAES50 composite membrane (denoted by PAN/SP50) and its surface and crosssectional SEM images. The composite membrane is black because
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of the oxidative PAN substrate (Fig. 5(a)). The SPAES50 copolymer was successfully impregnated into the nonwoven PAN (Fig. 5(b)). The pores in the nonwoven PAN do not appear on the surface of the composite membrane. The cross-sectional morphology indicates that the nonwoven PAN is located in the center of the membrane and that it is well incorporated with the SPAES50 copolymer. The thickness of composite membrane is approximately 35 mm (Fig. 5(c)).
Fig. 3. SEM images of fabricated nonwoven PAN (a) surface and (b) cross section.
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Typically, a multi-fibrous substrate contributes by reinforcing the mechanical properties of a polymer composite membrane [22]. To investigate the mechanical changes, tensile tests of the composite membranes are performed at an ambient temperature and in a fully hydrated state. These results are summarized in Table 1. The Young's modulus and yield strength of a membrane using PEMFC are important in terms of durability under operation, because these characteristics express the degree of durability and the limits of restorability when change is induced by an external force [23]. The Young's modulus of PAN/SP50 (692.1 MPa) is significantly higher than that of SPAES50 (244.6 MPa) and Nafion 212 (112.6 MPa) owing to the rigid filler effect of the multi-fibrous materials (704.6 MPa). Thus, the yield strength of the membranes shows a similar tendency as that of the Young's modulus. The yield strength of PAN/SP50 (13.8 MPa) is enhanced by 50% over that of SPAES50 (9.2 MPa) and Nafion 212 (9.0 MPa). These mechanical properties are believed to be crucial to the operation of a PEMFC under extreme conditions. Another property related to the durability is the dimensional change in water. For the realization of optimal proton conduction, a membrane requires water during the operation of a PEMFC. Therefore, the membrane absorbs water via its sulfonic acid groups and then expands in volume because of the absorbed water [24–26]. However, the expanded membrane is weakened with the increase in the absorbed water. Fig. 6 shows the change in the dimensions of the composite and pristine membranes compared to Nafion 212. The high dimensional change of SPAES50 is reduced drastically from 91% to 50% in PAN/SP50, and this value for PAN/SP50 is similar to with that of Nafion 212. In particular, a significant decrease occurs at the area-based variation of PAN/SP50, because the swelling of the membrane is suppressed by the nonwoven PAN as illustrated in Fig. 6. The areabased expansion of PAN/SP50 displays a 19% variation, which is lower than that of Nafion 212 (26%). Thus, water uptake correlates with the dimensional change, and it is included in the summarized result in Table 1. The proton conductivity of the composite membrane was investigated at various temperatures under fully hydrated conditions, as shown in Fig. 7, and was compared to that of the pristine membrane and Nafion 212. As the temperature was increased from 25 1C to 80 1C, the proton conductivity of all the measured membranes increased. The proton conductivity of PAN/SP50 was 0.062 S/cm and 0.164 S/cm at 25 1C and 80 1C/100% RH, respectively, both of which are lower than that of SPAES50 (0.092 S/cm and 0.181 S/cm). The lower proton conductivity of the composite
Fig. 4. Pore size distribution of nonwoven PAN and its solubility in NMP.
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Fig. 5. Images of composite membrane composed of SPAES50 and nonwoven PAN: (a) actual, (b) surface, and (c) cross-sectional images.
Table 1 Mechanical properties of nonwoven PAN and membranes and proton conducing characteristics of membranes. Membrane
IEC (meq/g)
Water uptake (wt%)
Dimensional change (vol%)
Young's modulus (MPa)
Yield strength (MPa)
Activation energy (Ea, kJ/mol)
Nafion 212 SPAES 50 PAN/SP50 Nonwoven PAN
0.90 2.01 1.71 –
24 76 51 –
43 91 50 –
112.6 7 8.4 244.67 37.6 692.17 38.9 704.6 7 37.6
9.0 7 0.1 9.2 7 0.1 13.8 7 0.1 15.8 7 0.3
10.9 70.6 10.7 70.7 15.7 70.3 –
Fig. 6. Dimensional changes of Nafion 212, SPAES50, and composite membrane.
membrane is caused by the nonwoven PAN, which decreases the IEC value because it cannot conduct protons [14]. The measured IEC value of PAN/SP50 was 1.71 meq/g, which is expectedly lower than that of SPAES50 (2.01 meq/g). Thus, the reduction degree of the proton conductivity of the composite membrane is determined by the porosity of the nonwoven PAN. Although PAN/SP50 decreases the proton conductivity compared to that of SPAES50, its value is closer to that of Nafion 212 at 80 1C/100% RH. To understand the dependency of proton conduction on the temperature, the activation energy (Ea) was investigated by calculating the slope of proton conductivity plotted against the inverse of the absolute temperature [27]. Similar activation energy was noted for Nafion 212 (10.9 kJ/mol) and SPAES50 (10.7 kJ/mol), whereas PAN/ SP50 showed an increase in its activation energy to 15.7 kJ/mol, as summarized in Table 1. This finding verifies that the nonwoven PAN in the composite membrane increases the minimum energy
Fig. 7. Proton conductivities of Nafion 212, SPAES50, and PAN/SP50 at different temperatures under a 100% RH condition.
for proton transport and decreases the proton conductivity. The proton conductivity at low RH conditions was also evaluated from 20% RH to 80% RH. As the RH is reduced, there are drastically fewer water molecules around the membrane available to transport protons; thus, the proton conductivities of all membranes decrease with reduced humidity. SPAES50 displayed an especially lower proton conductivity than Nafion 212 at low RH because of the ineffective connectivity of the proton pathways by the sulfonic acid groups of SPAES50 when compared to Nafion 212. PAN/SP50 has the lowest connectivity in the measured membranes because the nonwoven PAN plays a proton barrier role and it therefore shows the lowest the proton conductivity at low RH (Fig. 8). To evaluate the ohmic and interfacial contact resistance (OCR) of MEA surface, the EIS method was used. OCR is an important
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Fig. 8. Proton conductivities of Nafion 212, SPAES50, and PAN/SP50 at different humidity conditions (80 1C, 50 kPa backpressure).
property related to the enhanced performance of MEA and it is calculated by the high frequency intercept value of x axis (ZRe) [3]. While using the same catalyst system in MEAs, OCR is influenced by the resistance of the membrane. Fig. 9 depicts the OCR of SPAES50 and PAN/SP50 at 70 1C and 100% RH, which were 0.0072 Ω and 0.0092 Ω, respectively. This result indicates that SPAES50 has approximately a 20% higher proton transport capability than that of PAN/SP50, which is calculated by the inverse of the OCR values. This gap in performance originates from the porosity of the nonwoven PAN. Subsequently, the single cell performances of SPAES50, PAN/SP50, and Nafion 212 were evaluated to compare the composite membrane with the pristine membranes. The current density of PAN/SP50 at 70 1C and 100% RH was 1012 mA/cm2 at 0.6 V, which is slightly lower compared to that of SPAES50 (1109 mA/cm2 at 0.6 V) (Fig. 10). However, PAN/ SP50 displays performance comparable to that of Nafion 212 (1053 mA/cm2 at 0.6 V), showing that the level of cell performance correlates with the proton conductivity results (Fig. 10(a)). The relative performance improvement in PAN/SP50 is because of the fact that it is thinner than SPAES50 (45 mm) and Nafion 212 (50 mm), which reduces the cell resistance by narrowing the proton pathways [28,29]. When the measurement conditions were 70 1C and 50% RH, the performances of SPAES50 (788 mA/cm2 at 0.6 V) and PAN/SP50 (691 mA/cm2 at 0.6 V) were lower than that of Nafion 212 (884 mA/cm2 at 0.6 V), thus showing the same results as with the proton conductivity at low RH conditions. Fig. 11 shows the results of the durability test of the membranes during the operation of a cell under rigorous conditions. The applied durability protocol involves the use of hydration– dehydration cycling and an OCV hold method to simultaneously evaluate the physical and chemical failure characteristics [30–32]. The hydration–dehydration cycle time is 10 min in total (5 min each). SPAES50 shows a drop below 0.9 V after approximately 500 cycles, but PAN/SP50 proceeds to 1000 cycles because of the reinforcing effect of the nonwoven PAN. This finding indicates that the polymer nanofibers can enhance the durability of the membrane during the PEMFC operation under various conditions. Hydrogen crossover describes the diffusion of hydrogen gas from the anode to the cathode through the membrane. Membrane failure in the PEMFC can be confirmed by determining whether or not this diffusion takes place [33]. Fig. 12 shows the hydrogen crossover of SPAES50 and PAN/SP50 according to the hydration/ dehydration cycle. The initial hydrogen crossover levels of
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Fig. 9. MEA resistances of SPAES50 and PAN/SP50 by electrochemical impedance spectroscopy at a DC potential of 0.85 V (70 1C, 100% RH).
Fig. 10. Single cell performances of Nafion 212, SPAES50, and PAN/SP50 at (a) 70 1C and 100% RH and (b) 70 1C and 50% RH conditions.
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nonwoven PAN was analyzed by determining the fiber thickness, pore size, and porosity. The incorporation of the nonwoven PAN was shown to be an effective means to not only improve the dimensional stability but also the mechanical properties. However, proton conductivity was decreased in the composite membrane compared to the pristine membrane because of the nonwoven PAN, which blocks proton conduction pathways. For single cell operation, the composite membrane shows a similar performance as well as enhanced durability compared to the pristine membrane. This excellent durability was also verified by examining hydrogen crossover. These advantageous features of the composite membrane using the nonwoven PAN can improve membrane technology for PEMFC applications. References
Fig. 11. Durability tests of SPAES50 and PAN/SP50 using wet–dry cycling with an OCV holding method.
Fig. 12. Hydrogen crossover amounts of SPAES50 and PAN/SP50 according to cycle numbers.
SPAES50 and PAN/SP50 are barely noticeable, as can be seen from the low current density (0.001 A/cm2). However, after 500 cycles, the hydrogen crossover of SPAES50 increased significantly to 0.012 A/cm2. For PAN/SP50, the increase to 0.006 A/cm2 after more than 1000 cycles was only half of that of SPAES50's at 500 cycles. In addition to the observation of the OCV drop, hydrogen crossover clearly evidenced the failure of the membrane during the operation of the fuel cell. Through the analysis of the results of the single cell performances and the assessment of the durability of SPAES 50 and PAN/SP50, the composite membranes employing the electrospun nonwoven PAN have demonstrated their adequacy for use in PEMFC applications.
4. Conclusion A novel composite membrane was successfully developed by impregnating the SPAES50 copolymer into the nonwoven PAN resin. This nonwoven PAN was prepared using an electrospinning method for the fabrication of composite membranes that are suitable for long-term operation under various conditions. The
[1] S. Gamburzev, A.J. Appleby, Recent progress in performance improvement of the proton exchange membrane fuel cell (PEMFC), J. Power Sources 107 (2002) 5–12. [2] Y. Wang, K.S. Chen, J. Mishler, S.C. Cho, X.C. Adroher, A review of polymer electrolyte membrane fuel cells: technology, applications, and needs on fundamental research, Appl. Energy 88 (2011) 981–1007. [3] Y.J. Yoon, T.-H. Kim, D.M. Yu, J.-Y. Park, Y.T. Hong, Modification of hydrocarbon structure for polymer electrolyte membrane fuel cell binder application, Int. J. Hydrogen Energy 37 (2012) 13452–13461. [4] V. Neburchilov, J. Martin, H. Wang, J. Zhang, A review of polymer electrolyte membranes for direct methanol fuel cells, J. Power Sources 169 (2007) 221–238. [5] F. Wang, M. Hickner, Y.S. Kim, T.A. Zawodzinski, J.E. McGrath, Direct polymerization of sulfonated poly(arylene ether sulfone) random (statistical) copolymers: candidates for new proton exchange membranes, J. Membr. Sci. 197 (2002) 231–242. [6] J.-M. Lim, J.-H. Won, H.-J. Lee, Y.T. Hong, M.-S. Lee, C.H. Ko, S.-Y. Lee, Polyimide nonwoven fabric-reinforced, flexible phosphosilicate glass composite membranes for high-temperature/low-humidity proton exchange membrane fuel cells, J. Mater. Chem. 22 (2012) 18550–18557. [7] J.-R. Lee, N.-Y. Kim, M.-S. Lee, S.-Y. Lee, SiO2-coated polyimide nonwoven/ Nafion composite membranes for proton exchange membrane fuel cells, J. Membr. Sci. 367 (2011) 265–272. [8] J.-H. Seol, J.-H. Won, K.-S. Yoon, Y.T. Hong, S.-Y. Lee, SiO2 nanoparticles-coated poly(paraphenylene terephthalamide) nonwovens as reinforcing porous substrates for proton-conducting, sulfonated poly(arylene ether sulfone)-impregnated composite membranes, Solid State Ionics 190 (2011) 30–37. [9] H.-L. Lin, S.-H. Wang, C.-K. Chiu, T.L. Yu, L.-C. Chen, C.-C. Huang, T.-H. Cheng, J.-M. Lin, Preparation of Nafion/poly(vinyl alcohol) electro-spun fiber composite membranes for direct methanol fuel cells, J. Membr. Sci. 365 (2010) 114–122. [10] S. Molla, V. Compan, Polyvinyl alcohol nanofiber reinforced Nafion membranes for fuel cell applications, J. Membr. Sci. 372 (2011) 191–200. [11] J.B. Ballengee, P.N. Pintauro, Composite fuel cell membranes from dualnanofiber electrospun mats, Macromolecules 44 (2011) 7307–7314. [12] J.-H. Seol, J.-H. Won, M.-S. Lee, K.-S. Yoon, Y.T. Hong, S.-Y. Lee, A proton conductive silicate-nanoencapsulated polyimide nonwoven as a novel porous substrate for a reinforced sulfonated poly(arylene ether sulfone) composite membrane, J. Mater. Chem. 22 (2012) 1634–1642. [13] H.-Y. Li, Y.-L. Liu, Polyelectrolyte composite membranes of polybenzimidazole and crosslinked polybenzimidazole-polybenzoxazine electrospun nanofibers for proton exchange membrane fuel cells, J. Mater. Chem. A 1 (2013) 1171–1178. [14] D.M. Yu, K. Yoon, Y.J. Yoon, T.-H. Kim, J.Y. Lee, Y.T. Hong, Fabrication and properties of reinforced membranes based on sulfonated poly(arylene ether sulfone) copolymers for proton-exchange membrane fuel cells, Macromol. Chem. Phys. 213 (2012) 839–846. [15] M.S.A. Rahaman, A.F. Ismail, A. Mustafa, A review of heat treatment on polyacrylonitrile fiber, Polym. Degrad. Stab. 92 (2007) 1421–1432. [16] M.C. Yang, D.C. Yu, Influence of precursor structure on the properties of polyacrylonitrile-based activated carbon hollow fiber, J. Appl. Polym. Sci. 59 (1996) 1725–1731. [17] J. Mittal, R.B. Mathur, O.P. Bahl, Post spinning modification of PAN fibers—a review, Carbon 35 (1997) 1713–1722. [18] H. Zhang, J. Pang, D. Wang, A. Li, X. Li, Z. Jiang, Sulfonated poly(arylene ether nitrile ketone) and its composite with phosphotungstic acid as materials for proton exchange membranes, J. Membr. Sci. 264 (2005) 56–64. [19] K.B. Wiles, C.M. de Diego, J. de Abajo, J.E. McGrath, Directly copolymerized partially fluorinated disulfonated poly(arylene ether sulfone) random copolymers for PEM fuel cell systems: synthesis, fabrication and characterization of membranes and membrane-electrode assemblies for fuel cell applications, J. Membr. Sci. 294 (2007) 22–29. [20] N. Yusof, A.F. Ismail, Post spinning and pyrolysis processes of polyacrylonitrile (PAN)-based carbon fiber and activated carbon fiber: a review, J. Anal. Appl. Pyrolysis 93 (2012) 1–13.
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[21] S. Oh, J. Park, D.M. Yu, S.-K. Hong, Y.T. Hong, Preparation and characterization of acid–acid blend membranes for direct methanol fuel cell applications, Macromol. Res. 20 (2012) 121–127. [22] J.-H. Seol, J.-H. Won, K.-S. Yoon, Y.T. Hong, S.-Y. Lee, SiO2 ceramic nanoporous substrate-reinforced sulfonated poly(arylene ether sulfone) composite membranes for proton exchange membrane fuel cells, Int. J. Hydrogen Energy 37 (2012) 6189–6198. [23] J. Wu, X.Z. Yuan, J.J. Martin, H. Wang, J. Zhang, J. Shen, S. Wu, W. Merida, A review of PEM fuel cell durability: degradation mechanisms and mitigation strategies, J. Power Sources 184 (2008) 104–119. [24] S. Bose, T. Kuila, T.X.H. Nguyen, N.H. Kim, K. Lau, J.H. Lee, Polymer membranes for high temperature proton exchange membrane fuel cell: recent advances and challenges, Prog. Polym. Sci. 36 (2011) 813–843. [25] Q. Li, R. He, J.O. Jensen, N.J. Bjerrum, Approaches and recent development of polymer electrolyte membranes for fuel cells operating above 100 1C, Chem. Mater. 15 (2003) 4896–4915. [26] Y.S. Kim, L. Dong, M.A. Hickner, T.E. Glass, V. Webb, J.E. McGrath, State of water in disulfonated poly(arylene ether sulfone) copolymers and a perfluorosulfonic acid copolymer (Nafion) and its effect on physical and electrochemical properties, Macromolecules 36 (2003) 6281–6284. [27] S.Y. So, Y.J. Yoon, T.-H. Kim, K. Yoon, Y.T. Hong, Sulfonated poly(arylene ether sulfone)/functionalized silicate hybrid proton conductors for high-
[28]
[29]
[30] [31]
[32]
[33]
219
temperature proton exchange membrane fuel cells, J. Membr. Sci. 381 (2011) 204–210. X. Zhu, H. Zhang, Y. Liang, Y. Zhang, Q. Luo, C. Bi, B. Yi, Challenging reinforced composite polymer electrolyte membranes based on disulfonated poly(arylene ether sulfone)-impregnated expanded PTFE for fuel cell applications, J. Mater. Chem. 17 (2007) 386–397. D.M. Xing, B.L. Yi, F.Q. Liu, Y.Z. Fu, H.M. Zhang, Characterization of sulfonated poly(ether ether ketone)/polytetrafluoroethylene composite membranes for fuel cell applications, Fuel Cells 5 (2005) 406–411. F.A. de Bruijn, V.A.T. Dam, G.J.M. Janssen, Review: durability and degradation issues of PEM fuel cell components, Fuel Cells 8 (2007) 3–22. D. Liu, S. Case, Durability study of proton exchange membrane fuel cells under dynamic testing conditions with cyclic current profile, J. Power Sources 162 (2006) 521–531. R. Solasi, Y. Zou, X. Huang, K. Reifsnider, D. Condit, On mechanical behavior and in-plane modeling of constrained PEM fuel cell membranes subjected to hydration and temperature cycles, J. Power Sources 167 (2007) 366–377. M. Inaba, T. Kinumoto, M. Kiriake, R. Umebayashi, A. Tasaka, Z. Ogumi, Gas crossover and membrane degradation in polymer electrolyte fuel cells, Electrochim. Acta 51 (2006) 5746–5753.
Contents lists available at ScienceDirect
Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Properties of sulfonated poly(arylene ether sulfone)/electrospun nonwoven polyacrylonitrile composite membrane for proton exchange membrane fuel cells Duk Man Yu, Sangjun Yoon, Tae-Ho Kim, Jang Yong Lee, Jaerock Lee, Young Taik Hong n Energy Materials Research Center, Korea Research Institute of Chemical Technology, P.O. Box 107, Yuseong, Daejeon 305-600, Republic of Korea
art ic l e i nf o
a b s t r a c t
Article history: Received 4 March 2013 Received in revised form 30 May 2013 Accepted 18 June 2013 Available online 24 June 2013
A new composite membrane is fabricated by impregnating the sulfonated poly(arylene ether sulfone) (SPAES) copolymer into the nonwoven, electrospun by polyacrylonitrile (PAN) resin, for proton exchange membrane fuel cells (PEMFC). The SPAES copolymer is synthesized with 50 degrees of sulfonation, and the nonwoven PAN is prepared as a highly porous substrate that is anticipated to enhance the mechanical properties, dimensional stability, and durability of the composite membrane. This study focuses on all of these characteristics in addition to the degree of proton conductivity. Furthermore, a fuel cell operating test is conducted to compare the properties of the proposed composite membrane to those of a pristine membrane and to identify the effects of the nonwoven PAN. The dimensional change of the composite membrane decreases from 91% to 51% in water and the Young's modulus exhibited nearly three times the strength of the pristine membrane. However, the proton conductivity (measured at various temperature and humidity levels) decreases in the composite membrane because the nonwoven PAN obstructs the proton pathways. The proposed composite membrane displays suitable performance and notable durability for PEMFC applications. & 2013 Elsevier B.V. All rights reserved.
Keywords: Polymer electrolyte membrane Composite membrane Nonwoven Polyacrylonitrile Sulfonated poly(arylene ether sulfone)
1. Introduction Proton exchange membrane fuel cells (PEMFC) are among the greatly anticipated and promising energy technologies, because they can directly convert chemical energy into electrical energy for portable electronic devices, automobiles, and large buildings [1–3]. Among the PEMFC components, the polymer electrolyte membrane (PEM) is a fundamental component that separates the anode and cathode and determines the efficiency and durability of PEMFCs. Nafion, a perfluorosulfonated ionomer, is a membranous material that has been successfully commercialized for use in fuel cells because of its high proton conductivity, low gas permeability, and excellent dimensional stability and cell performance. However, Nafion membranes are expensive; moreover, its fabrication process is intricate and emits toxic waste products [4]. Sulfonated poly(arylene ether sulfone) (SPAES), a hydrocarbon-based ionomer, has been reviewed as an alternative PEM to Nafion over the past decade because of its high thermal resistance, good mechanical strength, low hydrogen permeability, and inexpensive production process [5]. However, the dimensions of SPAES expand under a fully hydrated condition, which decreases the durability of fuel
n
Corresponding author. Tel.: +82 42 860 7292; fax: +82 428607237. E-mail address: [email protected] (Y.T. Hong).
0376-7388/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.memsci.2013.06.028
cells. In order to overcome this critical defect, composite membranes that include multi-fibrous manufactured materials such as polyimide (PI) [6,7], poly(paraphenylene terephthalamide) (PPTA) [8], poly(vinyl alcohol) (PVA) [9,10], and polyphenylsulfone (PPS) [11] have been developed by many research groups. For example, Seol et al. [12] prepared composite membranes using electrospun nonwoven PI and SPAES copolymers. Their membranes showed enhanced dimensional stability and mechanical properties, but the proton conductivity was reduced in the composite membrane (compared to a pristine membrane) because of the PI substrate, which could not conduct protons. To improve the proton conductivity of the composite membrane, silicate with sulfonic acid groups (3-trihydroxysilyl propane-1-sulfonic acid, THSPSA) was introduced using a sol–gel method on the nonwoven PI surface. The developed composite membrane using modified nonwoven PI showed proton conductivity comparable to a pristine membrane while maintaining excellent dimensional stability. Lin et al. [9] reported on an electrospun, crosslinked PVA, nanofiber thin film and incorporated it into Nafion to fabricate a direct methanol fuel cell (DMFC). A new composite membrane with crosslinked PVA nanofibers allowed for reduced the thickness and better methanol permeability. Although the composite membrane using the crosslinked PVA nanofibers displayed lower proton conduction capabilities, similar to other fibrous composite membranes, it performed better than Nafion because of the advantages
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associated with DMFCs, as previously noted. Li et al. [13] prepared polybenzimidazole (PBI)-based, nonwoven nanofiber for hightemperature PEMFCs. The crosslinked PBI nanofibers showed enhanced mechanical strength and good solvent resistance. PBIbased composite membranes modified with crosslinked PBI nanofibers showed notably improved dimensional stability and proton conductivity at high temperatures under anhydrous conditions compared to pristine membranes. A single cell employing a composite membrane exhibited a 34% increase in its maximum power density. We had previously studied a composite membrane using nonwoven LCP, fabricated by a melt-blown method for DMFCs [14]. Two types of SPAES copolymers based on 4,4′-biphenol (BP) and 9,9′-bis(4-hydroxyphenyl)fluorine (HPF) structures were used to prepare the composite membranes. These composite membranes showed a reduced dimensional change by approximately 45% and an enhanced tensile strength by nearly twofold. The durability of the composite membranes was investigated by wet and dry cycle tests, in which the membranes were repeatedly immersed in water and dried under a vacuum for 24 h in each cycle. After 10 cycles, the composite membranes containing the HPF structure showed cracks because of its higher rigidity compared to the composite membrane containing the BP structure. In an assessment of the performance of a single cell, a composite membrane containing the BP structure also exhibited performance comparable to that of Nafion. In this study, a new hydrocarbon-based composite membrane for PEMFCs is investigated using SPAES copolymers with the BP structure, of which the degree of sulfonation is approximately 50 mol% (SPAES50), and nonwoven polyacrylonitrile (PAN). Economically produced PAN fibers, which merely require a facile production process and further oxidation, have been used in many industries, including the automotive, aircraft, and electronic heating device industries because of their high heat resistance, good chemical stability, lightness, and tolerance to friction under severe environments [15–17]. For a composite membrane in PEMFCs, nonwoven PAN can help retain the dimensional stability and strengthen the interaction between an ionomer and a porous substrate through hydrogen bonding by cyano groups and sulfonic acid [18]. Moreover, it improves the durability of the membrane, which is currently the most important property of a membrane during the operation of a PEMFC. Therefore, we practically measured the durability of membranes using a membrane electrode assembly (MEA) and a single cell test station by a wet–dry cycling test operated mechanically under an open-circuit voltage (OCV) hold condition. In addition, the characteristics of the composite membranes are discussed in terms of their proton conductivity at various temperature and relative humidity (RH) conditions. The water uptake, dimensional changes, and mechanical properties are also assessed to identify the effects of nonwoven PAN in comparison to that of a pristine membrane for PEMFC applications.
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azobisisobutyronitrile (AIBN, Aldrich) were used as received. Anhydrous potassium carbonate (K2CO3, Aldrich) was dried under vacuum at 200 1C for 48 h prior to use. Acrylonitrile (AN) and methyl methacrylate (MMA) were obtained from Samchun Chemical (South Korea). A stainless steel needle (MN-21G-13, Iwashita Engineering, Japan) with an inner diameter of 0.51 mm, an outer diameter of 0.81 mm, and a length of 13 mm was used during the electrospinning process. 2.2. Preparation of nonwoven PAN The PAN copolymer was easily synthesized using AN and MMA monomers with a weight ratio of 94:6, as shown in Fig. 1(a). First, AN (70.5 g) and MMA (4.5 g) monomers were inserted into a fournecked flask equipped with a mechanical stirrer under a N2 atmosphere. Deionized water (279.0 g) was then added to the reactor. After the reactor was heated to 70 1C, 20 wt% AIBN (0.8 g) solution dissolved in DMF was introduced into the mixture, which was stirred for 30 min. The polymerized sample was filtered and washed thoroughly with methanol several times to eliminate residual reactants and then dried under convection at 50 1C for 48 h. The nonwoven PAN was prepared using a conventional electrospinning method. In response to the tensile force generated by an applied electric field, 10 wt% PAN solution (dissolved in DMSO) was ejected from a syringe. The electrospun fibers accumulated on a cylindrical drum collector. The electrical voltage was 10 kV and the syringe pump (KD Scientific-100, USA) was used to supply a constant throughput of the solution. The distance from the nozzle tip to the collector was fixed at 10 cm. The developed nonwoven PAN was dried to eliminate any residual solvent at 160 1C under vacuum for 2 h. Finally, the nonwoven dried PAN was stabilized in an oxidizer at 240 1C for 1.5 h (Fig. 1(b)) [20]. 2.3. Preparation of membranes Aromatic nucleophilic step-growth polymerization using SDFDPS, DFDPS, and BP was conducted in a four-neck flask equipped with a mechanical stirrer, an argon gas inlet, a condenser, and a Dean-Stark trap. The synthesis of SPAES50 was performed as reported in earlier studies [5,21]. Fig. 2 depicts the scheme used to create the synthetic copolymer, which was used as an ionomer for
2. Experimental 2.1. Materials 4, 4′-Difluorodiphenyl sulfone (DFDPS) was purchased from Solvay Advanced Polymers (USA) and recrystallized from ethanol. 3,3′-Disulfonated-4,4′-difluorodiphenyl sulfone (SDFDPS) was synthesized using the procedure reported in literature [5,19]. 4,4′-Bisphenol (BP) (TCI) was also recrystallized from ethanol for high purity. N-methyl-2-pyrrolidone (NMP, Junsei), anhydrous toluene (Aldrich), 95–97% sulfuric acid (Merck), dimethylsulfoxide (DMSO, Aldrich), dimethylformamide (DMF, Aldrich), and
Fig. 1. Scheme for synthesizing PAN copolymer (a) before oxidation and (b) after oxidation.
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Fig. 2. Scheme for synthesizing SPAES copolymer.
the composite membrane. The composite membranes were prepared by impregnating a polymer solution (15 wt%) into the nonwoven PAN using a bar coating method to acquire a flat membrane. The polymer solution with the nonwoven PAN was dried on a glass plate at 80 1C for 6 h in an oven and the prepared membrane was consecutively dried for the complete removal of the residual solvent under vacuum at 120 1C for 12 h. Finally, the membrane was acidified in 1.5 M sulfuric acid (H2SO4) for 24 h and subsequently rinsed with deionized water for 24 h at room temperature. 2.4. Characterization of membranes The molecular weight (Mw) of the synthesized PAN copolymer was confirmed with gel permeation chromatography (Waters GPC, Tosoh), and the pore size distribution of the nonwoven PAN was measured using an advanced capillary flow porometer (ACFP1500AE, wet up/dry up method with Galwick solution). The dimensional change and the amount of water uptake were determined by the difference in the volume and weight between a wet membrane measured in a fully hydrated state and a dry membrane measured after drying under vacuum at 120 1C. The mechanical properties of the membranes were evaluated using a material testing machine (LLOYD instrument LR5K) at 25 1C in a fully hydrated state with a crosshead speed of 50 mm/min. The surface and cross-sectional images of the nonwoven PAN and composite membranes were observed using a scanning electron microscope (SEM, XL-30S FEG, Philips). The specimens were coated with platinum in vacuum for 2 min using a sputter coating machine (Sputter Coater, Q150T ES, Quorumtech, USA) before the SEM imaging process. The equivalent of sulfonic acid groups per gram of the membranes, expressed by the ion exchange capacity (IEC), was measured using an automatic titrator (Metroohm 794 Basic Titrino). The proton conductivity measurements of the membranes were conducted with an AC impedance analyzer (SP-300, Impedance/ gain phase analyzer) at various temperatures (from 25 1C to 80 1C) and at 100% RH along the in-plane direction over a frequency range of 0.1 Hz to 4 MHz using a four-probe conductivity cell. Equilibration was permitted for 1 h in a temperature chamber (ESPEC, SH-241) before each measurement. The proton conductivity was calculated using the following Eq. (1): Proton conductivity ðS=cmÞ ¼ l=R S
ð1Þ
where l is the distance between the electrodes, R is the resistance of the membrane as calculated using the intercept value of the x axis (real value, Z′) in the electrochemical impedance spectroscopy (EIS) curve composed of the real value and the imaginary value (Z″), and S is the cross-sectional surface area of the membrane. The proton conductivity at different conditions (from 20% RH and 80% RH) and 80 1C was also examined by an in-plane conductivity system (BekkTech, BT-512), which can control humidity conditions using the same four-probe.
To evaluate the single cell performance, membrane electrode assemblies (MEA) were fabricated with commercially available gas-diffusion layers coated with Pt/C and Nafion binder (50 wt% Pt/C, 0.4 mg Pt/cm2 from FuelCellPower Inc.). The electrodes were applied as the cathode and anode; the active surface area of the electrode was 25 cm2. The fabricated MEAs were activated for 24 h with hydrogen at the anode and air at the cathode at 70 1C (stoichiometric coefficient ¼1.2/2). After activation, the MEAs were evaluated electrochemically by cycling between 0.5 and 1.0 V with a step change of 50 mV per 25 s at 70 1C, 100% RH, and a backpressure of zero using a test station (FCT-TS300, Fuel Cell Technologies, Inc.). Electrochemical impedance spectroscopy measurements were also performed to observe the MEA resistance of the through-plane at a DC potential of 0.85 V with the AC frequency ranging from 10 mHz to 1 MHz. The degree of hydrogen crossover was measured by a DC power supply (Agilent N5744A) after 30 min of nitrogen purging to the cathode. The working and reference electrodes were connected to the cathode and anode, respectively, and a potential of 0.15–0.3 V was then applied to the single cell. The current produced by the oxidation at the cathode of passed hydrogen from the anode was recorded.
3. Results and discussion Prior to investigating of the composite membrane, the nonwoven electrospun PAN, with Mw of 450 kg/mol and feasible oxidative stability, was characterized by SEM photography for the morphological analysis. Fig. 3(a) and (b) show the surface and cross-section of the nonwoven PAN that indicate the multi-fibrous nonwoven was fabricated successfully (nonwoven thickness¼1872 mm). The diameters of the fibers in the nonwoven PAN were determined to range from 600 to 1200 nm, and the PAN fibers appear to have melted slightly, causing them join together. A quantitative characterization of the pore size and pore size distribution of the nonwoven PAN is shown in Fig. 4(a). The average pore size of the nonwoven PAN was observed to be approximately 1 mm, and it is distributed regularly from 0.5 mm to 1 mm. In addition, the weight and volume of the nonwoven PAN was used to estimate its porosity at 80%. To investigate the resistance of the organic solvent, a solubility test was conducted using NMP, because it was the solvent used to formulate the composite membrane with the SPAES50 copolymer. Fig. 4(b) displays an image of the nonwoven PAN in the NMP solvent after several weeks, which shows that the nonwoven PAN is insoluble in NMP and making it appropriate to incorporate with the SPAES50 solution. These results confirm that a noteworthy nonwoven material with nano-sized fibers and an elaborate pore structure was developed for use in a proton exchange composite membrane. Fig. 5 shows the fabricated nonwoven PAN/SPAES50 composite membrane (denoted by PAN/SP50) and its surface and crosssectional SEM images. The composite membrane is black because
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of the oxidative PAN substrate (Fig. 5(a)). The SPAES50 copolymer was successfully impregnated into the nonwoven PAN (Fig. 5(b)). The pores in the nonwoven PAN do not appear on the surface of the composite membrane. The cross-sectional morphology indicates that the nonwoven PAN is located in the center of the membrane and that it is well incorporated with the SPAES50 copolymer. The thickness of composite membrane is approximately 35 mm (Fig. 5(c)).
Fig. 3. SEM images of fabricated nonwoven PAN (a) surface and (b) cross section.
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Typically, a multi-fibrous substrate contributes by reinforcing the mechanical properties of a polymer composite membrane [22]. To investigate the mechanical changes, tensile tests of the composite membranes are performed at an ambient temperature and in a fully hydrated state. These results are summarized in Table 1. The Young's modulus and yield strength of a membrane using PEMFC are important in terms of durability under operation, because these characteristics express the degree of durability and the limits of restorability when change is induced by an external force [23]. The Young's modulus of PAN/SP50 (692.1 MPa) is significantly higher than that of SPAES50 (244.6 MPa) and Nafion 212 (112.6 MPa) owing to the rigid filler effect of the multi-fibrous materials (704.6 MPa). Thus, the yield strength of the membranes shows a similar tendency as that of the Young's modulus. The yield strength of PAN/SP50 (13.8 MPa) is enhanced by 50% over that of SPAES50 (9.2 MPa) and Nafion 212 (9.0 MPa). These mechanical properties are believed to be crucial to the operation of a PEMFC under extreme conditions. Another property related to the durability is the dimensional change in water. For the realization of optimal proton conduction, a membrane requires water during the operation of a PEMFC. Therefore, the membrane absorbs water via its sulfonic acid groups and then expands in volume because of the absorbed water [24–26]. However, the expanded membrane is weakened with the increase in the absorbed water. Fig. 6 shows the change in the dimensions of the composite and pristine membranes compared to Nafion 212. The high dimensional change of SPAES50 is reduced drastically from 91% to 50% in PAN/SP50, and this value for PAN/SP50 is similar to with that of Nafion 212. In particular, a significant decrease occurs at the area-based variation of PAN/SP50, because the swelling of the membrane is suppressed by the nonwoven PAN as illustrated in Fig. 6. The areabased expansion of PAN/SP50 displays a 19% variation, which is lower than that of Nafion 212 (26%). Thus, water uptake correlates with the dimensional change, and it is included in the summarized result in Table 1. The proton conductivity of the composite membrane was investigated at various temperatures under fully hydrated conditions, as shown in Fig. 7, and was compared to that of the pristine membrane and Nafion 212. As the temperature was increased from 25 1C to 80 1C, the proton conductivity of all the measured membranes increased. The proton conductivity of PAN/SP50 was 0.062 S/cm and 0.164 S/cm at 25 1C and 80 1C/100% RH, respectively, both of which are lower than that of SPAES50 (0.092 S/cm and 0.181 S/cm). The lower proton conductivity of the composite
Fig. 4. Pore size distribution of nonwoven PAN and its solubility in NMP.
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Fig. 5. Images of composite membrane composed of SPAES50 and nonwoven PAN: (a) actual, (b) surface, and (c) cross-sectional images.
Table 1 Mechanical properties of nonwoven PAN and membranes and proton conducing characteristics of membranes. Membrane
IEC (meq/g)
Water uptake (wt%)
Dimensional change (vol%)
Young's modulus (MPa)
Yield strength (MPa)
Activation energy (Ea, kJ/mol)
Nafion 212 SPAES 50 PAN/SP50 Nonwoven PAN
0.90 2.01 1.71 –
24 76 51 –
43 91 50 –
112.6 7 8.4 244.67 37.6 692.17 38.9 704.6 7 37.6
9.0 7 0.1 9.2 7 0.1 13.8 7 0.1 15.8 7 0.3
10.9 70.6 10.7 70.7 15.7 70.3 –
Fig. 6. Dimensional changes of Nafion 212, SPAES50, and composite membrane.
membrane is caused by the nonwoven PAN, which decreases the IEC value because it cannot conduct protons [14]. The measured IEC value of PAN/SP50 was 1.71 meq/g, which is expectedly lower than that of SPAES50 (2.01 meq/g). Thus, the reduction degree of the proton conductivity of the composite membrane is determined by the porosity of the nonwoven PAN. Although PAN/SP50 decreases the proton conductivity compared to that of SPAES50, its value is closer to that of Nafion 212 at 80 1C/100% RH. To understand the dependency of proton conduction on the temperature, the activation energy (Ea) was investigated by calculating the slope of proton conductivity plotted against the inverse of the absolute temperature [27]. Similar activation energy was noted for Nafion 212 (10.9 kJ/mol) and SPAES50 (10.7 kJ/mol), whereas PAN/ SP50 showed an increase in its activation energy to 15.7 kJ/mol, as summarized in Table 1. This finding verifies that the nonwoven PAN in the composite membrane increases the minimum energy
Fig. 7. Proton conductivities of Nafion 212, SPAES50, and PAN/SP50 at different temperatures under a 100% RH condition.
for proton transport and decreases the proton conductivity. The proton conductivity at low RH conditions was also evaluated from 20% RH to 80% RH. As the RH is reduced, there are drastically fewer water molecules around the membrane available to transport protons; thus, the proton conductivities of all membranes decrease with reduced humidity. SPAES50 displayed an especially lower proton conductivity than Nafion 212 at low RH because of the ineffective connectivity of the proton pathways by the sulfonic acid groups of SPAES50 when compared to Nafion 212. PAN/SP50 has the lowest connectivity in the measured membranes because the nonwoven PAN plays a proton barrier role and it therefore shows the lowest the proton conductivity at low RH (Fig. 8). To evaluate the ohmic and interfacial contact resistance (OCR) of MEA surface, the EIS method was used. OCR is an important
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Fig. 8. Proton conductivities of Nafion 212, SPAES50, and PAN/SP50 at different humidity conditions (80 1C, 50 kPa backpressure).
property related to the enhanced performance of MEA and it is calculated by the high frequency intercept value of x axis (ZRe) [3]. While using the same catalyst system in MEAs, OCR is influenced by the resistance of the membrane. Fig. 9 depicts the OCR of SPAES50 and PAN/SP50 at 70 1C and 100% RH, which were 0.0072 Ω and 0.0092 Ω, respectively. This result indicates that SPAES50 has approximately a 20% higher proton transport capability than that of PAN/SP50, which is calculated by the inverse of the OCR values. This gap in performance originates from the porosity of the nonwoven PAN. Subsequently, the single cell performances of SPAES50, PAN/SP50, and Nafion 212 were evaluated to compare the composite membrane with the pristine membranes. The current density of PAN/SP50 at 70 1C and 100% RH was 1012 mA/cm2 at 0.6 V, which is slightly lower compared to that of SPAES50 (1109 mA/cm2 at 0.6 V) (Fig. 10). However, PAN/ SP50 displays performance comparable to that of Nafion 212 (1053 mA/cm2 at 0.6 V), showing that the level of cell performance correlates with the proton conductivity results (Fig. 10(a)). The relative performance improvement in PAN/SP50 is because of the fact that it is thinner than SPAES50 (45 mm) and Nafion 212 (50 mm), which reduces the cell resistance by narrowing the proton pathways [28,29]. When the measurement conditions were 70 1C and 50% RH, the performances of SPAES50 (788 mA/cm2 at 0.6 V) and PAN/SP50 (691 mA/cm2 at 0.6 V) were lower than that of Nafion 212 (884 mA/cm2 at 0.6 V), thus showing the same results as with the proton conductivity at low RH conditions. Fig. 11 shows the results of the durability test of the membranes during the operation of a cell under rigorous conditions. The applied durability protocol involves the use of hydration– dehydration cycling and an OCV hold method to simultaneously evaluate the physical and chemical failure characteristics [30–32]. The hydration–dehydration cycle time is 10 min in total (5 min each). SPAES50 shows a drop below 0.9 V after approximately 500 cycles, but PAN/SP50 proceeds to 1000 cycles because of the reinforcing effect of the nonwoven PAN. This finding indicates that the polymer nanofibers can enhance the durability of the membrane during the PEMFC operation under various conditions. Hydrogen crossover describes the diffusion of hydrogen gas from the anode to the cathode through the membrane. Membrane failure in the PEMFC can be confirmed by determining whether or not this diffusion takes place [33]. Fig. 12 shows the hydrogen crossover of SPAES50 and PAN/SP50 according to the hydration/ dehydration cycle. The initial hydrogen crossover levels of
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Fig. 9. MEA resistances of SPAES50 and PAN/SP50 by electrochemical impedance spectroscopy at a DC potential of 0.85 V (70 1C, 100% RH).
Fig. 10. Single cell performances of Nafion 212, SPAES50, and PAN/SP50 at (a) 70 1C and 100% RH and (b) 70 1C and 50% RH conditions.
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nonwoven PAN was analyzed by determining the fiber thickness, pore size, and porosity. The incorporation of the nonwoven PAN was shown to be an effective means to not only improve the dimensional stability but also the mechanical properties. However, proton conductivity was decreased in the composite membrane compared to the pristine membrane because of the nonwoven PAN, which blocks proton conduction pathways. For single cell operation, the composite membrane shows a similar performance as well as enhanced durability compared to the pristine membrane. This excellent durability was also verified by examining hydrogen crossover. These advantageous features of the composite membrane using the nonwoven PAN can improve membrane technology for PEMFC applications. References
Fig. 11. Durability tests of SPAES50 and PAN/SP50 using wet–dry cycling with an OCV holding method.
Fig. 12. Hydrogen crossover amounts of SPAES50 and PAN/SP50 according to cycle numbers.
SPAES50 and PAN/SP50 are barely noticeable, as can be seen from the low current density (0.001 A/cm2). However, after 500 cycles, the hydrogen crossover of SPAES50 increased significantly to 0.012 A/cm2. For PAN/SP50, the increase to 0.006 A/cm2 after more than 1000 cycles was only half of that of SPAES50's at 500 cycles. In addition to the observation of the OCV drop, hydrogen crossover clearly evidenced the failure of the membrane during the operation of the fuel cell. Through the analysis of the results of the single cell performances and the assessment of the durability of SPAES 50 and PAN/SP50, the composite membranes employing the electrospun nonwoven PAN have demonstrated their adequacy for use in PEMFC applications.
4. Conclusion A novel composite membrane was successfully developed by impregnating the SPAES50 copolymer into the nonwoven PAN resin. This nonwoven PAN was prepared using an electrospinning method for the fabrication of composite membranes that are suitable for long-term operation under various conditions. The
[1] S. Gamburzev, A.J. Appleby, Recent progress in performance improvement of the proton exchange membrane fuel cell (PEMFC), J. Power Sources 107 (2002) 5–12. [2] Y. Wang, K.S. Chen, J. Mishler, S.C. Cho, X.C. Adroher, A review of polymer electrolyte membrane fuel cells: technology, applications, and needs on fundamental research, Appl. Energy 88 (2011) 981–1007. [3] Y.J. Yoon, T.-H. Kim, D.M. Yu, J.-Y. Park, Y.T. Hong, Modification of hydrocarbon structure for polymer electrolyte membrane fuel cell binder application, Int. J. Hydrogen Energy 37 (2012) 13452–13461. [4] V. Neburchilov, J. Martin, H. Wang, J. Zhang, A review of polymer electrolyte membranes for direct methanol fuel cells, J. Power Sources 169 (2007) 221–238. [5] F. Wang, M. Hickner, Y.S. Kim, T.A. Zawodzinski, J.E. McGrath, Direct polymerization of sulfonated poly(arylene ether sulfone) random (statistical) copolymers: candidates for new proton exchange membranes, J. Membr. Sci. 197 (2002) 231–242. [6] J.-M. Lim, J.-H. Won, H.-J. Lee, Y.T. Hong, M.-S. Lee, C.H. Ko, S.-Y. Lee, Polyimide nonwoven fabric-reinforced, flexible phosphosilicate glass composite membranes for high-temperature/low-humidity proton exchange membrane fuel cells, J. Mater. Chem. 22 (2012) 18550–18557. [7] J.-R. Lee, N.-Y. Kim, M.-S. Lee, S.-Y. Lee, SiO2-coated polyimide nonwoven/ Nafion composite membranes for proton exchange membrane fuel cells, J. Membr. Sci. 367 (2011) 265–272. [8] J.-H. Seol, J.-H. Won, K.-S. Yoon, Y.T. Hong, S.-Y. Lee, SiO2 nanoparticles-coated poly(paraphenylene terephthalamide) nonwovens as reinforcing porous substrates for proton-conducting, sulfonated poly(arylene ether sulfone)-impregnated composite membranes, Solid State Ionics 190 (2011) 30–37. [9] H.-L. Lin, S.-H. Wang, C.-K. Chiu, T.L. Yu, L.-C. Chen, C.-C. Huang, T.-H. Cheng, J.-M. Lin, Preparation of Nafion/poly(vinyl alcohol) electro-spun fiber composite membranes for direct methanol fuel cells, J. Membr. Sci. 365 (2010) 114–122. [10] S. Molla, V. Compan, Polyvinyl alcohol nanofiber reinforced Nafion membranes for fuel cell applications, J. Membr. Sci. 372 (2011) 191–200. [11] J.B. Ballengee, P.N. Pintauro, Composite fuel cell membranes from dualnanofiber electrospun mats, Macromolecules 44 (2011) 7307–7314. [12] J.-H. Seol, J.-H. Won, M.-S. Lee, K.-S. Yoon, Y.T. Hong, S.-Y. Lee, A proton conductive silicate-nanoencapsulated polyimide nonwoven as a novel porous substrate for a reinforced sulfonated poly(arylene ether sulfone) composite membrane, J. Mater. Chem. 22 (2012) 1634–1642. [13] H.-Y. Li, Y.-L. Liu, Polyelectrolyte composite membranes of polybenzimidazole and crosslinked polybenzimidazole-polybenzoxazine electrospun nanofibers for proton exchange membrane fuel cells, J. Mater. Chem. A 1 (2013) 1171–1178. [14] D.M. Yu, K. Yoon, Y.J. Yoon, T.-H. Kim, J.Y. Lee, Y.T. Hong, Fabrication and properties of reinforced membranes based on sulfonated poly(arylene ether sulfone) copolymers for proton-exchange membrane fuel cells, Macromol. Chem. Phys. 213 (2012) 839–846. [15] M.S.A. Rahaman, A.F. Ismail, A. Mustafa, A review of heat treatment on polyacrylonitrile fiber, Polym. Degrad. Stab. 92 (2007) 1421–1432. [16] M.C. Yang, D.C. Yu, Influence of precursor structure on the properties of polyacrylonitrile-based activated carbon hollow fiber, J. Appl. Polym. Sci. 59 (1996) 1725–1731. [17] J. Mittal, R.B. Mathur, O.P. Bahl, Post spinning modification of PAN fibers—a review, Carbon 35 (1997) 1713–1722. [18] H. Zhang, J. Pang, D. Wang, A. Li, X. Li, Z. Jiang, Sulfonated poly(arylene ether nitrile ketone) and its composite with phosphotungstic acid as materials for proton exchange membranes, J. Membr. Sci. 264 (2005) 56–64. [19] K.B. Wiles, C.M. de Diego, J. de Abajo, J.E. McGrath, Directly copolymerized partially fluorinated disulfonated poly(arylene ether sulfone) random copolymers for PEM fuel cell systems: synthesis, fabrication and characterization of membranes and membrane-electrode assemblies for fuel cell applications, J. Membr. Sci. 294 (2007) 22–29. [20] N. Yusof, A.F. Ismail, Post spinning and pyrolysis processes of polyacrylonitrile (PAN)-based carbon fiber and activated carbon fiber: a review, J. Anal. Appl. Pyrolysis 93 (2012) 1–13.
D.M. Yu et al. / Journal of Membrane Science 446 (2013) 212–219
[21] S. Oh, J. Park, D.M. Yu, S.-K. Hong, Y.T. Hong, Preparation and characterization of acid–acid blend membranes for direct methanol fuel cell applications, Macromol. Res. 20 (2012) 121–127. [22] J.-H. Seol, J.-H. Won, K.-S. Yoon, Y.T. Hong, S.-Y. Lee, SiO2 ceramic nanoporous substrate-reinforced sulfonated poly(arylene ether sulfone) composite membranes for proton exchange membrane fuel cells, Int. J. Hydrogen Energy 37 (2012) 6189–6198. [23] J. Wu, X.Z. Yuan, J.J. Martin, H. Wang, J. Zhang, J. Shen, S. Wu, W. Merida, A review of PEM fuel cell durability: degradation mechanisms and mitigation strategies, J. Power Sources 184 (2008) 104–119. [24] S. Bose, T. Kuila, T.X.H. Nguyen, N.H. Kim, K. Lau, J.H. Lee, Polymer membranes for high temperature proton exchange membrane fuel cell: recent advances and challenges, Prog. Polym. Sci. 36 (2011) 813–843. [25] Q. Li, R. He, J.O. Jensen, N.J. Bjerrum, Approaches and recent development of polymer electrolyte membranes for fuel cells operating above 100 1C, Chem. Mater. 15 (2003) 4896–4915. [26] Y.S. Kim, L. Dong, M.A. Hickner, T.E. Glass, V. Webb, J.E. McGrath, State of water in disulfonated poly(arylene ether sulfone) copolymers and a perfluorosulfonic acid copolymer (Nafion) and its effect on physical and electrochemical properties, Macromolecules 36 (2003) 6281–6284. [27] S.Y. So, Y.J. Yoon, T.-H. Kim, K. Yoon, Y.T. Hong, Sulfonated poly(arylene ether sulfone)/functionalized silicate hybrid proton conductors for high-
[28]
[29]
[30] [31]
[32]
[33]
219
temperature proton exchange membrane fuel cells, J. Membr. Sci. 381 (2011) 204–210. X. Zhu, H. Zhang, Y. Liang, Y. Zhang, Q. Luo, C. Bi, B. Yi, Challenging reinforced composite polymer electrolyte membranes based on disulfonated poly(arylene ether sulfone)-impregnated expanded PTFE for fuel cell applications, J. Mater. Chem. 17 (2007) 386–397. D.M. Xing, B.L. Yi, F.Q. Liu, Y.Z. Fu, H.M. Zhang, Characterization of sulfonated poly(ether ether ketone)/polytetrafluoroethylene composite membranes for fuel cell applications, Fuel Cells 5 (2005) 406–411. F.A. de Bruijn, V.A.T. Dam, G.J.M. Janssen, Review: durability and degradation issues of PEM fuel cell components, Fuel Cells 8 (2007) 3–22. D. Liu, S. Case, Durability study of proton exchange membrane fuel cells under dynamic testing conditions with cyclic current profile, J. Power Sources 162 (2006) 521–531. R. Solasi, Y. Zou, X. Huang, K. Reifsnider, D. Condit, On mechanical behavior and in-plane modeling of constrained PEM fuel cell membranes subjected to hydration and temperature cycles, J. Power Sources 167 (2007) 366–377. M. Inaba, T. Kinumoto, M. Kiriake, R. Umebayashi, A. Tasaka, Z. Ogumi, Gas crossover and membrane degradation in polymer electrolyte fuel cells, Electrochim. Acta 51 (2006) 5746–5753.