- Email: [email protected]
Mechanical property enhancement of ultra-thin PBI membrane by electron beam irradiation for PEM fuel cell
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e7
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Mechanical property enhancement of ultra-thin PBI membrane by electron beam irradiation for PEM fuel cell Shiauwu Lai a, Jiwon Park a, Sungoh Cho a,*, Mingchih Tsai b, Hyungsan Lim a, Kueihsien Chen b a
Department of Nuclear and Quantum Engineering, Korea Advanced Institute of Science and Technology (KAIST), 373-1 Guseong, Yuseong, Daejeon 305-701, Republic of Korea b Center for Condensed Matter Sciences, National Taiwan University, 106 Taipei, Taiwan
article info
abstract
Article history:
The ultra-thin cross-linked PBI membrane containing phosphoric acid (PA) for possible use
Received 28 December 2015
in a fuel cell was prepared by electron beam irradiation method. The preparation proce-
Received in revised form
dure involved two steps: (a) irradiation of PBI membranes (PBI-1, PBI-2 and PBI-3) by an
11 April 2016
electron beam with increased dose energy and (b) doping the irradiated membranes with
Accepted 16 April 2016
PA. These membranes were characterized in terms of mechanical properties, chemical and
Available online xxx
thermal stability were evaluated using Universal Testing machine, Ultravioletevisible spectroscopy and thermogravimetric analysis, respectively. Compared to the pristine PBI
Keywords:
membrane of thickness 20 mm, the cross-linked PBI membranes show much better me-
Polybenzimidazole
chanical properties and improved chemical and thermal stability. In addition, the tensile
Electron beam irradiation
strength of the cross-linked PBI membranes with PA ranges from 19 MPa to 27 MPa, which
Polymer cross-linking
is higher than that of the pristine PBI membrane with PA (14 MPa). Besides these, the fuel
PEM fuel cell
cell performance of the PBI-1 is similar to that of the pristine PBI membrane of thickness
Mechanical property
20 mm, but higher than common used PBI membrane of thickness 40 mm. The overall results suggest that the membrane has a great potential for possible application in high temperature PEM fuel cell. © 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Proton exchange membrane fuel cells (PEMFCs) are extensively researched as an electrical power source in order to preserve oil resources and reduce greenhouse gas emissions [1e4]. As of recent years, acid-doped polybenzimidazole (PBI) polymer is used as the proton conducting membrane of fuel cells due to the polymer's property of having relatively high
proton conductivity at temperatures higher than 100 C. It is well known that the operating advantages of high temperature fuel cell include high carbon monoxide (CO) tolerance, fast electrode kinetics, no humidification, and no necessity in water management [5,6]. However, this system still has some drawbacks for proton conducting membrane of PBI-based high temperature PEMFCs is costly and presents large membrane resistance, as well as the decreased performance after long operation times. Therefore, the main focus direction of
* Corresponding author. Tel.: þ82 42 350 3863; fax: þ82 42 350 3810. E-mail address: [email protected] (S. Cho). http://dx.doi.org/10.1016/j.ijhydene.2016.04.111 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Lai S, et al., Mechanical property enhancement of ultra-thin PBI membrane by electron beam irradiation for PEM fuel cell, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.04.111
2
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e7
further development in the system is towards the use of thinner membrane in order to decrease the ohmic resistance within membrane. Nonetheless, the reduction of membrane thickness has a realistic limit due to the required mechanical strength and gas barrier property. Additionally, thickness reduction does not necessarily lead to actual cost reduction, rather, it can significantly reduce the lifetime of the membranes in fuel cell operation [7]. Furthermore, PBI membrane can be protonated by doping it in phosphoric acid to be used as an ionic conductive membrane. This acid doping process can achieve a result of 5e8 mol phosphoric acid per repeat unit of the PBI polymer while the membrane thickness ranged from 500 to 200 mm [7]. The additive, phosphoric acid, work as proton donors/acceptors and play a significant role in defining the properties of the membrane. Higher content of the additive results in higher proton conductivity. Unfortunately, the improvement in conductivity with this method is at the expense of deterioration in mechanical property of the membrane. In fact, it is a general trend that increases in acid-doped content leads to decrease in mechanical strength of membrane. Thus, ultimately, the mechanical strength, acid doping level, and membrane thickness should all be balanced and optimized to produce a membrane of practical use. Previous reports describe the modification PBI membranes by a number of techniques in a bid to improve their performance in high temperature PEMCs. As of recently, polymer cross-linking method became a potential technique to improve the mechanical property of the membrane. Consequently, many approaches have been employed for the preparation of cross-linked PBI membrane. Among these cross-linking approaches, ionic and covalent cross-linking procedures are relatively easy to handle due to them not requiring special curing equipment. The blends required for ionic cross-linking of PBIs were prepared according to Lewis acidicebasic complexes polymer concept, such as PBI/sulfonated polysulfone [8e10], PBI/Nafion [11], and PBI/sulfonated poly(ether ether ketone) [12] blend membranes. In the case of covalent cross-linking of PBI polymers, chemical reactions amongst the functional groups of PBI and various crosslinkers including p-xylene dichloride [13], p-xylene dibromide [14], dichloromethyl phosphoric acid [15], and p-vinylbenzyl chloride [5,16,17], were used to form covalent crosslinked structures. Moreover, it has been shown that the covalent cross-linking of PBI polymers lead to more thermally stable PBI polymer than with the case of ionic cross-linking [18]. Although both ionic and covalent cross-linking methods offer some benefits, the type of cross-linkers and excess reactants affect the physical and chemical properties of PBI polymers. Furthermore, cross-linking mechanisms are difficult to control, which limit the final network structure and properties. In contrast, cross-linking polymers through irradiation offer many advantages over the former method such as controllable crosslinking density and structural properties [19]. It is well known that irradiation creates free radicals which will often chemically react in various ways. The free radicals can recombine forming the crosslinks. In previous reports, Nho et al. developed a sulfonated poly(ether ether ketone) (SPEEK) membrane crosslinked with various
crosslinkers with electron beam irradiation for improving the thermal and hydration stability of membranes at high temperature [20]. Kudo et al. reported the preparation of graft-type sulfonated polybenzimidazole (SPBI) via radiation-induced graft polymerization and appealing the proton conductivity did not decrease for 600 h at 120 C [21]. However, the effects the electron beam irradiated PBI membrane had on high temperature PEM fuel cells have never been reported. Here, we report the enhancement in the extent of crosslinking of PBI polymers by electron beam irradiation method, which leads to an enhancement in the mechanical properties of PBI membrane. For the purpose of demonstration, commercial pristine PBI membrane with ultra-thin thickness of 20 mm were used. To the best of our knowledge, there only exists few report regarding the application of even thinner membrane for PEM fuel cells. The cross-linking of ultra-thin PBI membrane was done through electron beam irradiation at different electron fluences and the products were characterized by TGA, UVeVis, and mechanical analysis. Preliminary experiments indicated that some advantages in mechanical properties could be gained by using ultra-thin PBI membrane exposed to electron beam irradiation. To investigate the electrochemical properties, the PA doping level, impedance, and fuel cell performance were measured at operating temperature.
Experimental Materials A PBI membrane with average thickness of 20 mm was purchased from Danish Power Systems Ltd. (Denmark). Commercial N,N-Dimethylacetamide (DMAc), phosphoric acid (PA, 85%) from SigmaeAldrich were used as received. Deionized water with a specific resistance of 18.3 MU cm was obtained from a water purification system (Human Power Iþ, Human Corporation).
Exposure to electron radiations and acid doping First, samples of pristine PBI membrane were cut into a rectangular shape (20 mm 20 mm) and irradiated with an electron beam. The electron beam was generated from a thermionic electron gun under a vacuum condition lower than 2 105 Torr. The electron energy and current density were 50 keV and 30 mA cm2, respectively. The total electron fluence was controlled from 1.19 1016 to 4.77 1016 electrons cm2 by adjusting the irradiation time. The specifics of the irradiation conditions of the tested PBI membranes are list in Table 1. After the irradiating treatment, the resulting samples were cooled and stored for further use. Subsequently, the irradiated PBI membranes were immersed into phosphoric acid in room temperature for a few days. The weight changes of the samples from before and after the acid doping were measured and used for the acid doping level (PAdop) calculation using the following equation: Doping level ¼
ðW2 W1 Þ MH3 PO4 ðW1 =MPBI Þ
Please cite this article in press as: Lai S, et al., Mechanical property enhancement of ultra-thin PBI membrane by electron beam irradiation for PEM fuel cell, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.04.111
3
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e7
Table 1 e The electron fluences, thickness, and PA doping level of pristine and irradiated PBI membranes prepared under 50 keV, 30 mA of electron beam irradiation. Membrane
Total exposure time (min)
Electron fluence (e cm2)
Thickness (mm)
PA doping level
Pristine PBI PBI-1 PBI-2 PBI-3
0 30 60 120
0 1.19 1016 2.39 1016 4.77 1016
20 19 17 15
6.98 6.87 6.85 6.81
where MH3 PO4 and MPBI are the molecular weights of phosphoric acid and the repeating unit of the polymer, respectively. W1 and W2 are the weights of un-doped and doped membranes, respectively. The PAdop data were obtained from averaging the results from three measurements.
Membrane characterization The cross-sectional image and thickness of the membranes were investigated using field emission scanning electron microscopy (FE-SEM, HITACHI). The samples were coated under a vacuum condition with platinum powder before the SEM observations. Thermal gravimetric analysis (TGA) was conducted on both pristine and irradiated PBI membranes to study the effects of radiations on thermal stability. Experiments were carried out using TG 209 F3 (NETZSCH, Germany) instrument. Specimens were heated from 20 to 600 C under a nitrogenous atmosphere at a heating rate of 10 C min1. The extent of cross-linking was characterized by measuring the solubility of all tested membranes in DMAc solvent. Due to its low molecular weight, the linear PBI used was found to completely dissolve in DMAc in room temperature; the crosslinked PBI membrane samples were immersed in DMAc at room temperature while being stirred. After 24 h, the membrane samples were taken out of the solution, and the optical property of solution were measured by UVeVis analysis. Tensile testing of un-doped and PA-doped membranes was performed with 60 mm 10 mm sample strips using a universal testing machine (INSTRON 5583). Rectangular samples were cut from unexposed and irradiated specimens and the test procedure of ASTM D638 was adapted. Experiments were conducted at 20 C, with an elongation speed of 5 mm min1. At least three specimens were tested for each case (pristine vs. irradiated) for the sake of reproducibility in results. Throughplane proton conductivity (s) was calculated from the measured current resistance (R) using following equation:
s ¼ D/(W T R) where D is the distance between the two electrodes, W is the width of the membrane, T is the membrane thickness, and R is the resistance of the membrane that was obtained using best fit model of the impedance data. A device capable of holding a membrane for R measurement was located between the probes. The testing device with a membrane was kept in a thermo-state at 160 C and anhydrous state. The measurement proceeded for at least 1 h until the impedance spectrum reached an equilibrium value. The mean of the three measurements was collected for the data.
PEMFC unit cell test The Pt loading of electrodes were 0.6 and 0.8 mg cm2 for anode and cathode, respectively. The pristine and irradiated PBI membranes prepared in this work were used to fabricate MEAs. Two carbon clothes coated with catalyst layers were placed on both sides of a membrane and hot-pressed at 140 C with a pressure of 50 N cm2 for 30 s to obtain an MEA. Fuel cell tests were carried out with 5 cm2 graphite cell with a serpentine flow fields. Mica filled PTFE inserts were used to surround the flow fields and provide location for the O-ring seal. The temperature of the cell was controlled by thermostatically controlled cartridge heaters inserted into the cell body. The performances of unit cells were tested at 160 C and 1 atm backpressure using Asia Pacific fuel cell testing system. The dehumidified anode H2 and cathode O2 input flow rates were 0.2 and 0.5 standard cubic centimeters per minute (SCCM), respectively.
Results and discussion Physical properties and mechanical properties It is known that the electron beam radiation has been used successfully to cross-link polymers, which can efficiently enhance their chemical stabilities. For fuel cell applications, the chemical stability of polymer conducting membrane is very important in preventing membrane damage during the operating process. Moreover, Pu et al. reported that the solubility of PBI can reflect the degree of cross-linking within the membrane [5]. Thus, to find extent of cross-linking within the irradiated PBI membrane in the study, the solubility of the membranes in DMAc solvent (PBI/DMAc) is measured. The standard working solutions used for establishing the calibration curve (shown in Fig. S1(a)) were prepared by stepwise dilution of pristine PBI membranes in DMAc solvent. As shown in Fig. 1(a), the pristine PBI membrane fully dissolves in DMAc solvent (left side) in room temperature, which was observed for 24 h for linear PBI polymer structure. However, the other irradiated membranes showed sings of remnants after 24 h of dissolution in DMAc due to the polymers cross-linking via electron beam irradiation enhancing the chemical stability. After removing the remnants from the PBI/DMAc solutions, the optical properties of four prepared solutions were analyzed and summarized in Table S1 as well as in the overlaid spectra shown in Fig. S1(b). The UVeVis curves of the obtained four PBI/DMAc solutions showed maximum absorption peaks at wavelengths around 390e400 nm, which can be attributed to the pep* transition of PBI polymer [22,23]. Furthermore, based on the experimental
Please cite this article in press as: Lai S, et al., Mechanical property enhancement of ultra-thin PBI membrane by electron beam irradiation for PEM fuel cell, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.04.111
4
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e7
extent of cross-linking within the PBI polymer after electron beam irradiation. In addition, the PBI-2 shows higher tensile stress and strain than the other membranes, although PBI-1 has a relatively even stress value while lacking in strain value in comparison to those of PBI-2. Interestingly, the tensile strength of PBI membranes first increases and then decreases with increasing electron fluence. The improved mechanical properties of irradiated PBI membranes can be attributed to the cross-linking behavior of rubber polymers [24]. It is important to note, in general, the elongation property of polymers diminishes with increasing irradiation dose energy. However, in this work, an improvement in elongation property of irradiated PBI membranes was observed; a
(a) 200 Pristine PBI
100
Dissolution (%)
80
Stress (MPa)
160
PBI-1
PBI-2
60
120 80 40
40
PBI-3
0 20
0 0
1
2
3
4
10
5
30
40
50
160
200
Strain (%)
Electron fluence (x 1016 e/cm2)
(a)
(b) 30
Stress (MPa)
Fig. 1 e The solubility test of pristine and irradiated PBI membranes immersed in DMAc solvent (a) samples precipitate and (b) dissolution ratio after 24 h at room temperature.
data, the solutions of pristine PBI, PBI-1, PBI-2 and PBI-3 have the dissolution ratio (by weight) of 100%, 68.5%, 55.4%, and 27.4% (Fig. 1(b)), respectively. This result indicates that the dissolution ratio decreases with increasing degree of crosslinking within the polymer structure. Therefore, the irradiated PBIs exhibit better chemical stability than the pristine PBI membrane, demonstrating the potential use of the former in the fabrication of long-life operation high temperature PEMFCs. A good mechanical property is an important requirement for the membrane to be used in high temperature PEMFCs. The tensile stressestrain curves of pristine and irradiated PBI membranes before and after doping with PA are illustrated in Fig. 2(a) and (b), respectively. As shown in Fig. 2(a), the mechanical property of all PBI membranes was significantly improved after the irradiation process. This phenomenon is a result of the increase in the
20
20
10
0 0
40
80
120
Strain (%)
(b) Fig. 2 e Stressestrain curves of (a) undoped and (b) PAdoped membranes of (þ) pristine 20 mm PBI; ( ) crosslinked PBI-1, ( ) PBI-2, and ( ) PBI-3.
Please cite this article in press as: Lai S, et al., Mechanical property enhancement of ultra-thin PBI membrane by electron beam irradiation for PEM fuel cell, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.04.111
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e7
phenomenon which was primarily a result of the dangling branched entanglement amongst the increased cross-linked network. Youssef et al. presented a similar result of irradiated HSR/SBR blends when increasing irradiation dose in their study [24]. Fig. 2(b) demonstrates that PBI membranes after PA doping have an increased tensile strain and decreased yield stress and tensile stress at break in comparison to PBI membranes without PA doping. This phenomena is due to the PA molecules, doped in the membranes, function as plasticizers resulting in membrane swelling and toughness improving. Although the tensile strength of PA-doped irradiated PBI membranes decreased, the values were still higher than that of PA-doped pristine 20 mm PBI membrane, while the PAdoped PBI-1 membrane showed an enormous increase of around 100% in tensile strength. These observations suggest that irradiated PBI membranes show better performance when exposed to appropriate electron beam irradiation dose. Furthermore, doping with PA enables higher sustainability of the membrane for use in high temperature operating environment. Additionally the thermal stabilities of pristine and irradiated PBI membranes were investigated with TGA under nitrogenous environment and the results are shown in Fig. 3. All tested PBI membranes show a two-step degradation pattern at temperature ranges of 50e120 C and 300e450 C. It has been reported that the first weight loss (degradation) can be attributed to the evaporation of water content in the membranes, while the case of the second weight loss is closely related to degradation of side chains [25]. The irradiated PBI membranes show a smaller second weight loss than pristine PBI membranes, which could possibly be due to the difference in the extent of cross-linking between the former and the latter. These results indicate that irradiated PBI membranes are thermally more stable than the pristine counterpart and suitable for testing in high temperature PEM fuel cell. Figures S2(a) and S2(b) illustrate the SEM micrographs of the plane surfaces of pristine 20 mm PBI and irradiated PBI
Fig. 3 e TGA curves of pristine and irradiated PBI membranes.
5
membranes, respectively. The micrographs show smooth surfaces of these two membranes. Moreover, after electron beam irradiation, the surface morphology of PBI membrane (Fig. S2(b)) does not change. Furthermore, we have obtained the cross-sectional images and thicknesses of all tested PBI membranes. As shown in Figs. S2(c) and S2(d), the PBI-3 membrane has a thinner thickness of 15 mm than the 20 mm of pristine PBI membrane. The thicknesses of all tested membranes irradiated at different electron fluences are listed in Table 1. According to the data, the thickness of irradiated membranes decreases with increasing electron fluences. It is well known that irradiating at higher electron fluences provide higher radiation-absorbed dose leading to more radical production, which in turn increases the cross-linking density within the polymer. Thus, it can be concluded that the irradiated PBI shows more compact cross-linked structure, which leads it to having a thinner thickness than its pristine counterpart.
Cell performances To evaluate the performance of the fuel cell with the irradiated PBI membrane, cross-linked PBI-1, which demonstrated fine mechanical strength in the study, were prepared and single cell performance was carried out. Pristine PBI membrane with 20 mm thickness was also used for cell performance tests for control testing. Fig. 4 shows the polarization and power plots for pristine 20 mm PBI and cross-linked PBI-1 membranes at an operating temperature of 160 C under 1 atm back pressure. From the IeV curves and experimental data, the pristine 20 mm PBI shows better cell performance than that shown by crosslinked PBI-1 membrane with the maximal power densities (PDmax) of the pristine 20 mm PBI and cross-linked PBI-1 membranes being 988 and 934 mW cm2, respectively. However, despite the superior performance boasted by the pristine 20 mm PBI membrane, it still has some issues in fuel cell applications due to its poor mechanical property and inferior chemical stability. In contrast, the cross-linked PBI-1
Fig. 4 e Single cell performances of the pristine 20 mm PBI and cross-linked PBI-1 membranes at 160 C under hydrogen and oxygen as a fuel gas.
Please cite this article in press as: Lai S, et al., Mechanical property enhancement of ultra-thin PBI membrane by electron beam irradiation for PEM fuel cell, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.04.111
6
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e7
membrane shows a slightly inferior cell performance due to its lower PA doping level (Table 1) [26], but it has a largely improved mechanical property, chemical stability, and thermal stability over the pristine 20 mm PBI membrane. Therefore, it can be concluded that the irradiated PBI membrane excel far more heavily in durability than the pristine PBI membrane. Although the ultra-thin membrane is known for its excellent conductance, membranes with thickness range from 40 to 200 mm are more typically used for PEMFC applications because of their lower gas permeability result in higher open circuit voltage (OCV) [7]. Thus, a pristine PBI membrane with 40 mm thickness was tested for comparison. Fig. 5(a) shows the polarization and power plots of crosslinked PBI-1 and pristine 40 mm PBI membranes. Pristine 40 mm PBI is approximately 2 times thicker than the crosslinked PBI-1 membrane. However, the cross-linked PBI-1 (0.85 V) exhibit similar OCV as compared to that of pristine 40 mm PBI membrane (0.87 V) at 160 C. It appears that the irradiated PBI membrane has thinner thickness but improved
gas impermeability property due to polymer cross-linking. Furthermore, the PDmax were found to be 934 and 527 mW cm2 for the cross-linked PBI-1 and pristine 40 mm PBI membranes, respectively. This difference can be attributed to the ohm resistance of membranes with different thicknesses. Fig. 5(b) illustrates the through plane resistance of crosslinked PBI-1 and pristine 40 mm PBI membranes, confirming the result of cell performance. The cross-linked PBI-1 membrane shows a significantly lower ohm resistance in comparison to the pristine 40 mm PBI at high temperature. In other words, the reduced thickness and resistance of irradiated PBI membrane can overcome the gas permeation issue and perform better than the pristine 40 mm PBI membrane. Therefore, maintaining performance and significantly boosting mechanical property can potentially be accomplished with the benefits offered by the cross-linking PBI via electron beam irradiation.
Conclusions A facile and effective method of preparing cross-linked PBI membrane via electron beam irradiation for PEMFC applications is proposed. The degree of cross-linking increases while the solubility of the membrane in DMAc solvent decreases with increasing electron fluence. As a result, the irradiated PBI membranes show outstanding chemical and thermal stability, improved mechanical property, as well as thinner thickness of membrane compared to those shown by the pristine 20 mm PBI membrane. After doping with PA, the pristine 20 mm PBI membrane exhibits the highest cell performance and PDmax of 988 mW cm2, but simultaneously shows the lowest mechanical property. The cross-linked PBI-1 membrane shows excellent mechanical property all the while showing similar performance to that of pristine PBI membrane and reaches the PDmax value of 934 mW cm2. Additionally, to assess the performance against a standard membrane used in PEMFc, a pristine 40 mm PBI membrane cell performance test was conducted. The cross-linked PBI-1 demonstrates an 80% increase in performance over that of the pristine 40 mm PBI due to the former's lower ohm resistance in membrane at high temperature. Consequently, it can be concluded that irradiated ultrathin PBI membrane can be applied to improve PEMFC performance.
Acknowledgments This work was supported by the basic research program of the Korea Atomic Energy Research Institute (KAERI) Grant funded by the Korea government (N04130036).
Appendix A. Supplementary data Fig. 5 e (a) Single cell performance and (b) through plane resistance of the cross-linked PBI-1 and pristine 40 mm PBI membranes at 160 C.
Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2016.04.111.
Please cite this article in press as: Lai S, et al., Mechanical property enhancement of ultra-thin PBI membrane by electron beam irradiation for PEM fuel cell, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.04.111
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e7
references
[1] Haile SM, Pintauro PN. Proton transport for fuel cells. J Mater Chem 2010;20:6211e3. [2] Xiao LX, Zhang HF, Scanlon E, Ramanathan LS, Choe EW, Rogers D, et al. High-temperature polybenzimidazole fuel cell membranes via a sol-gel process. Chem Mater 2005;17:5328e33. [3] Aili D, Cleemann LN, Li QF, Jensen JO, Christensen E, Bjerrum NJ. Thermal curing of PBI membranes for high temperature PEM fuel cells. J Mater Chem 2012;22:5444e53. [4] Pu HT, Wang L, Pan HY, Wan DC. Synthesis and characterization of fluorine-containing polybenzimidazole for proton conducting membranes in fuel cells. J Polym Sci Polym Chem 2010;48:2115e22. [5] Luo HC, Pu HT, Chang ZH, Wan DC, Pan HY. Crosslinked polybenzimidazole via a Diels-Alder reaction for proton conducting membranes. J Mater Chem 2012;22:20696e705. [6] Yu S, Benicewicz BC. Synthesis and properties of functionalized polybenzimidazoles for high-temperature PEMFCs. Macromolecules 2009;42:8640e8. [7] Lin HL, Chou YC, Yu TL, Lai SW. Poly(benzimidazole)epoxide crosslink membranes for high temperature proton exchange membrane fuel cells. Int J Hydrogen Energy 2012;37:383e92. [8] Kerres J, Ullrich A, Meier F, Haring T. Synthesis and characterization of novel acid-base polymer blends for application in membrane fuel cells. Solid State Ion 1999;125:243e9. [9] Hasiotis C, Deimede V, Kontoyannis C. New polymer electrolytes based on blends of sulfonated polysulfones with polybenzimidazole. Electrochim Acta 2001;46:2401e6. [10] Manea C, Mulder M. New polymeric electrolyte membranes based on proton donor-proton acceptor properties for direct methanol fuel cells. Desalination 2002;147:179e82. [11] Zhai YF, Zhang HM, Zhang Y, Xing DM. A novel H3PO4/ Nafion-PBI composite membrane for enhanced durability of high temperature PEM fuel cells. J Power Sources 2007;169:259e64. [12] Zhang HQ, Li XF, Zhao CJ, Fu TZ, Shi YH, Na H. Composite membranes based on highly sulfonated PEEK and PBI: morphology characteristics and performance. J Membr Sci 2008;308:66e74. [13] Wang KY, Xiao YC, Chung TS. Chemically modified polybenzimidazole nanofiltration membrane for the separation of electrolytes and cephalexin. Chem Eng Sci 2006;61:5807e17.
7
[14] Li QF, Pan C, Jensen JO, Noye P, Bjerrum NJ. Cross-linked polybenzimidazole membranes for fuel cells. Chem Mater 2007;19:350e2. [15] Noy P, Li QF, Pan C, Bjerrurn NJ. Cross-linked polybenzimidazole membranes for high temperature proton exchange membrane fuel cells with dichloromethyl phosphinic acid as a cross-linker. Polym Adv Technol 2008;19:1270e5. [16] Sukumar PR, Wu WC, Markova D, Unsal O, Klapper M, Mullen K. Functionalized poly(benzimidazole)s as membrane materials for fuel cells. Macromol Chem Phys 2007;208:2258e67. [17] Guan YS, Pu HT, Jin M, Chang ZH, Wan DC. Preparation and characterisation of proton exchange membranes based on crosslinked polybenzimidazole and phosphoric acid. Fuel Cells 2010;10:973e82. [18] Li QF, Jensen JO, Savinell RF, Bjerrum NJ. High temperature proton exchange membranes based on polybenzimidazoles for fuel cells. Prog Polym Sci 2009;34:449e77. [19] Li Y, Lee EJ, Cai WP, Kim KY, Cho SO. Unconventional method for morphology-controlled carbonaceous nanoarrays based on electron irradiation of a polystyrene colloidal monolayer. ACS Nano 2008;2:1108e12. [20] Song JM, Shin J, Sohn JY, Nho YC. Preparation and characterization of SPEEK membranes crosslinked by electron beam irradiation. Macromol Res 2011;19:1082e9. [21] Park J, Takayama T, Asano M, Maekawa Y, Kudo K. Graft-type polymer electrolyte membranes for fuel cells prepared through radiation-induced graft polymerization into alicyclic polybenzimidazoles. Polymer 2013;54:4570e7. [22] Osaheni JA, Jenekhe SA. Synthesis and processing of heterocyclic polymers as electronic, optoelectronic, and nonlinear-optical materials .4. New conjugated rigid-rod poly(benzobis(imidazole))s. Macromolecules 1995;28:1172e9. [23] Liang ZM, Jiang XS, Xu HJ, Chen D, Yin J. Polybenzimidazoles (PBIs) derived from non-coplanar dibenzoic acid containing imidazole (IDBA): synthesis, characterization and properties. Macromol Chem Phys 2009;210:1632e9. [24] Youssef HA. Effect of electron beam irradiation on the properties of high styrene rubber/styrene butadiene rubber blends. J Macromol Sci A 2009;46:636e42. [25] Kannan R, Kagalwala HN, Chaudhari HD, Kharul UK, Kurungot S, Pillai VK. Improved performance of phosphonated carbon nanotubeepolybenzimidazole composite membranes in proton exchange membrane fuel cells. J Mater Chem 2011;21:7223e31. [26] Fang JH, Qiao JL, Wilkinson DP, Zhang JJ. Electrochemical polymer electrolyte membranes. Taylor & Francis Group, LLC; 2015.
Please cite this article in press as: Lai S, et al., Mechanical property enhancement of ultra-thin PBI membrane by electron beam irradiation for PEM fuel cell, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.04.111
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Mechanical property enhancement of ultra-thin PBI membrane by electron beam irradiation for PEM fuel cell Shiauwu Lai a, Jiwon Park a, Sungoh Cho a,*, Mingchih Tsai b, Hyungsan Lim a, Kueihsien Chen b a
Department of Nuclear and Quantum Engineering, Korea Advanced Institute of Science and Technology (KAIST), 373-1 Guseong, Yuseong, Daejeon 305-701, Republic of Korea b Center for Condensed Matter Sciences, National Taiwan University, 106 Taipei, Taiwan
article info
abstract
Article history:
The ultra-thin cross-linked PBI membrane containing phosphoric acid (PA) for possible use
Received 28 December 2015
in a fuel cell was prepared by electron beam irradiation method. The preparation proce-
Received in revised form
dure involved two steps: (a) irradiation of PBI membranes (PBI-1, PBI-2 and PBI-3) by an
11 April 2016
electron beam with increased dose energy and (b) doping the irradiated membranes with
Accepted 16 April 2016
PA. These membranes were characterized in terms of mechanical properties, chemical and
Available online xxx
thermal stability were evaluated using Universal Testing machine, Ultravioletevisible spectroscopy and thermogravimetric analysis, respectively. Compared to the pristine PBI
Keywords:
membrane of thickness 20 mm, the cross-linked PBI membranes show much better me-
Polybenzimidazole
chanical properties and improved chemical and thermal stability. In addition, the tensile
Electron beam irradiation
strength of the cross-linked PBI membranes with PA ranges from 19 MPa to 27 MPa, which
Polymer cross-linking
is higher than that of the pristine PBI membrane with PA (14 MPa). Besides these, the fuel
PEM fuel cell
cell performance of the PBI-1 is similar to that of the pristine PBI membrane of thickness
Mechanical property
20 mm, but higher than common used PBI membrane of thickness 40 mm. The overall results suggest that the membrane has a great potential for possible application in high temperature PEM fuel cell. © 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Proton exchange membrane fuel cells (PEMFCs) are extensively researched as an electrical power source in order to preserve oil resources and reduce greenhouse gas emissions [1e4]. As of recent years, acid-doped polybenzimidazole (PBI) polymer is used as the proton conducting membrane of fuel cells due to the polymer's property of having relatively high
proton conductivity at temperatures higher than 100 C. It is well known that the operating advantages of high temperature fuel cell include high carbon monoxide (CO) tolerance, fast electrode kinetics, no humidification, and no necessity in water management [5,6]. However, this system still has some drawbacks for proton conducting membrane of PBI-based high temperature PEMFCs is costly and presents large membrane resistance, as well as the decreased performance after long operation times. Therefore, the main focus direction of
* Corresponding author. Tel.: þ82 42 350 3863; fax: þ82 42 350 3810. E-mail address: [email protected] (S. Cho). http://dx.doi.org/10.1016/j.ijhydene.2016.04.111 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Lai S, et al., Mechanical property enhancement of ultra-thin PBI membrane by electron beam irradiation for PEM fuel cell, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.04.111
2
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e7
further development in the system is towards the use of thinner membrane in order to decrease the ohmic resistance within membrane. Nonetheless, the reduction of membrane thickness has a realistic limit due to the required mechanical strength and gas barrier property. Additionally, thickness reduction does not necessarily lead to actual cost reduction, rather, it can significantly reduce the lifetime of the membranes in fuel cell operation [7]. Furthermore, PBI membrane can be protonated by doping it in phosphoric acid to be used as an ionic conductive membrane. This acid doping process can achieve a result of 5e8 mol phosphoric acid per repeat unit of the PBI polymer while the membrane thickness ranged from 500 to 200 mm [7]. The additive, phosphoric acid, work as proton donors/acceptors and play a significant role in defining the properties of the membrane. Higher content of the additive results in higher proton conductivity. Unfortunately, the improvement in conductivity with this method is at the expense of deterioration in mechanical property of the membrane. In fact, it is a general trend that increases in acid-doped content leads to decrease in mechanical strength of membrane. Thus, ultimately, the mechanical strength, acid doping level, and membrane thickness should all be balanced and optimized to produce a membrane of practical use. Previous reports describe the modification PBI membranes by a number of techniques in a bid to improve their performance in high temperature PEMCs. As of recently, polymer cross-linking method became a potential technique to improve the mechanical property of the membrane. Consequently, many approaches have been employed for the preparation of cross-linked PBI membrane. Among these cross-linking approaches, ionic and covalent cross-linking procedures are relatively easy to handle due to them not requiring special curing equipment. The blends required for ionic cross-linking of PBIs were prepared according to Lewis acidicebasic complexes polymer concept, such as PBI/sulfonated polysulfone [8e10], PBI/Nafion [11], and PBI/sulfonated poly(ether ether ketone) [12] blend membranes. In the case of covalent cross-linking of PBI polymers, chemical reactions amongst the functional groups of PBI and various crosslinkers including p-xylene dichloride [13], p-xylene dibromide [14], dichloromethyl phosphoric acid [15], and p-vinylbenzyl chloride [5,16,17], were used to form covalent crosslinked structures. Moreover, it has been shown that the covalent cross-linking of PBI polymers lead to more thermally stable PBI polymer than with the case of ionic cross-linking [18]. Although both ionic and covalent cross-linking methods offer some benefits, the type of cross-linkers and excess reactants affect the physical and chemical properties of PBI polymers. Furthermore, cross-linking mechanisms are difficult to control, which limit the final network structure and properties. In contrast, cross-linking polymers through irradiation offer many advantages over the former method such as controllable crosslinking density and structural properties [19]. It is well known that irradiation creates free radicals which will often chemically react in various ways. The free radicals can recombine forming the crosslinks. In previous reports, Nho et al. developed a sulfonated poly(ether ether ketone) (SPEEK) membrane crosslinked with various
crosslinkers with electron beam irradiation for improving the thermal and hydration stability of membranes at high temperature [20]. Kudo et al. reported the preparation of graft-type sulfonated polybenzimidazole (SPBI) via radiation-induced graft polymerization and appealing the proton conductivity did not decrease for 600 h at 120 C [21]. However, the effects the electron beam irradiated PBI membrane had on high temperature PEM fuel cells have never been reported. Here, we report the enhancement in the extent of crosslinking of PBI polymers by electron beam irradiation method, which leads to an enhancement in the mechanical properties of PBI membrane. For the purpose of demonstration, commercial pristine PBI membrane with ultra-thin thickness of 20 mm were used. To the best of our knowledge, there only exists few report regarding the application of even thinner membrane for PEM fuel cells. The cross-linking of ultra-thin PBI membrane was done through electron beam irradiation at different electron fluences and the products were characterized by TGA, UVeVis, and mechanical analysis. Preliminary experiments indicated that some advantages in mechanical properties could be gained by using ultra-thin PBI membrane exposed to electron beam irradiation. To investigate the electrochemical properties, the PA doping level, impedance, and fuel cell performance were measured at operating temperature.
Experimental Materials A PBI membrane with average thickness of 20 mm was purchased from Danish Power Systems Ltd. (Denmark). Commercial N,N-Dimethylacetamide (DMAc), phosphoric acid (PA, 85%) from SigmaeAldrich were used as received. Deionized water with a specific resistance of 18.3 MU cm was obtained from a water purification system (Human Power Iþ, Human Corporation).
Exposure to electron radiations and acid doping First, samples of pristine PBI membrane were cut into a rectangular shape (20 mm 20 mm) and irradiated with an electron beam. The electron beam was generated from a thermionic electron gun under a vacuum condition lower than 2 105 Torr. The electron energy and current density were 50 keV and 30 mA cm2, respectively. The total electron fluence was controlled from 1.19 1016 to 4.77 1016 electrons cm2 by adjusting the irradiation time. The specifics of the irradiation conditions of the tested PBI membranes are list in Table 1. After the irradiating treatment, the resulting samples were cooled and stored for further use. Subsequently, the irradiated PBI membranes were immersed into phosphoric acid in room temperature for a few days. The weight changes of the samples from before and after the acid doping were measured and used for the acid doping level (PAdop) calculation using the following equation: Doping level ¼
ðW2 W1 Þ MH3 PO4 ðW1 =MPBI Þ
Please cite this article in press as: Lai S, et al., Mechanical property enhancement of ultra-thin PBI membrane by electron beam irradiation for PEM fuel cell, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.04.111
3
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e7
Table 1 e The electron fluences, thickness, and PA doping level of pristine and irradiated PBI membranes prepared under 50 keV, 30 mA of electron beam irradiation. Membrane
Total exposure time (min)
Electron fluence (e cm2)
Thickness (mm)
PA doping level
Pristine PBI PBI-1 PBI-2 PBI-3
0 30 60 120
0 1.19 1016 2.39 1016 4.77 1016
20 19 17 15
6.98 6.87 6.85 6.81
where MH3 PO4 and MPBI are the molecular weights of phosphoric acid and the repeating unit of the polymer, respectively. W1 and W2 are the weights of un-doped and doped membranes, respectively. The PAdop data were obtained from averaging the results from three measurements.
Membrane characterization The cross-sectional image and thickness of the membranes were investigated using field emission scanning electron microscopy (FE-SEM, HITACHI). The samples were coated under a vacuum condition with platinum powder before the SEM observations. Thermal gravimetric analysis (TGA) was conducted on both pristine and irradiated PBI membranes to study the effects of radiations on thermal stability. Experiments were carried out using TG 209 F3 (NETZSCH, Germany) instrument. Specimens were heated from 20 to 600 C under a nitrogenous atmosphere at a heating rate of 10 C min1. The extent of cross-linking was characterized by measuring the solubility of all tested membranes in DMAc solvent. Due to its low molecular weight, the linear PBI used was found to completely dissolve in DMAc in room temperature; the crosslinked PBI membrane samples were immersed in DMAc at room temperature while being stirred. After 24 h, the membrane samples were taken out of the solution, and the optical property of solution were measured by UVeVis analysis. Tensile testing of un-doped and PA-doped membranes was performed with 60 mm 10 mm sample strips using a universal testing machine (INSTRON 5583). Rectangular samples were cut from unexposed and irradiated specimens and the test procedure of ASTM D638 was adapted. Experiments were conducted at 20 C, with an elongation speed of 5 mm min1. At least three specimens were tested for each case (pristine vs. irradiated) for the sake of reproducibility in results. Throughplane proton conductivity (s) was calculated from the measured current resistance (R) using following equation:
s ¼ D/(W T R) where D is the distance between the two electrodes, W is the width of the membrane, T is the membrane thickness, and R is the resistance of the membrane that was obtained using best fit model of the impedance data. A device capable of holding a membrane for R measurement was located between the probes. The testing device with a membrane was kept in a thermo-state at 160 C and anhydrous state. The measurement proceeded for at least 1 h until the impedance spectrum reached an equilibrium value. The mean of the three measurements was collected for the data.
PEMFC unit cell test The Pt loading of electrodes were 0.6 and 0.8 mg cm2 for anode and cathode, respectively. The pristine and irradiated PBI membranes prepared in this work were used to fabricate MEAs. Two carbon clothes coated with catalyst layers were placed on both sides of a membrane and hot-pressed at 140 C with a pressure of 50 N cm2 for 30 s to obtain an MEA. Fuel cell tests were carried out with 5 cm2 graphite cell with a serpentine flow fields. Mica filled PTFE inserts were used to surround the flow fields and provide location for the O-ring seal. The temperature of the cell was controlled by thermostatically controlled cartridge heaters inserted into the cell body. The performances of unit cells were tested at 160 C and 1 atm backpressure using Asia Pacific fuel cell testing system. The dehumidified anode H2 and cathode O2 input flow rates were 0.2 and 0.5 standard cubic centimeters per minute (SCCM), respectively.
Results and discussion Physical properties and mechanical properties It is known that the electron beam radiation has been used successfully to cross-link polymers, which can efficiently enhance their chemical stabilities. For fuel cell applications, the chemical stability of polymer conducting membrane is very important in preventing membrane damage during the operating process. Moreover, Pu et al. reported that the solubility of PBI can reflect the degree of cross-linking within the membrane [5]. Thus, to find extent of cross-linking within the irradiated PBI membrane in the study, the solubility of the membranes in DMAc solvent (PBI/DMAc) is measured. The standard working solutions used for establishing the calibration curve (shown in Fig. S1(a)) were prepared by stepwise dilution of pristine PBI membranes in DMAc solvent. As shown in Fig. 1(a), the pristine PBI membrane fully dissolves in DMAc solvent (left side) in room temperature, which was observed for 24 h for linear PBI polymer structure. However, the other irradiated membranes showed sings of remnants after 24 h of dissolution in DMAc due to the polymers cross-linking via electron beam irradiation enhancing the chemical stability. After removing the remnants from the PBI/DMAc solutions, the optical properties of four prepared solutions were analyzed and summarized in Table S1 as well as in the overlaid spectra shown in Fig. S1(b). The UVeVis curves of the obtained four PBI/DMAc solutions showed maximum absorption peaks at wavelengths around 390e400 nm, which can be attributed to the pep* transition of PBI polymer [22,23]. Furthermore, based on the experimental
Please cite this article in press as: Lai S, et al., Mechanical property enhancement of ultra-thin PBI membrane by electron beam irradiation for PEM fuel cell, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.04.111
4
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e7
extent of cross-linking within the PBI polymer after electron beam irradiation. In addition, the PBI-2 shows higher tensile stress and strain than the other membranes, although PBI-1 has a relatively even stress value while lacking in strain value in comparison to those of PBI-2. Interestingly, the tensile strength of PBI membranes first increases and then decreases with increasing electron fluence. The improved mechanical properties of irradiated PBI membranes can be attributed to the cross-linking behavior of rubber polymers [24]. It is important to note, in general, the elongation property of polymers diminishes with increasing irradiation dose energy. However, in this work, an improvement in elongation property of irradiated PBI membranes was observed; a
(a) 200 Pristine PBI
100
Dissolution (%)
80
Stress (MPa)
160
PBI-1
PBI-2
60
120 80 40
40
PBI-3
0 20
0 0
1
2
3
4
10
5
30
40
50
160
200
Strain (%)
Electron fluence (x 1016 e/cm2)
(a)
(b) 30
Stress (MPa)
Fig. 1 e The solubility test of pristine and irradiated PBI membranes immersed in DMAc solvent (a) samples precipitate and (b) dissolution ratio after 24 h at room temperature.
data, the solutions of pristine PBI, PBI-1, PBI-2 and PBI-3 have the dissolution ratio (by weight) of 100%, 68.5%, 55.4%, and 27.4% (Fig. 1(b)), respectively. This result indicates that the dissolution ratio decreases with increasing degree of crosslinking within the polymer structure. Therefore, the irradiated PBIs exhibit better chemical stability than the pristine PBI membrane, demonstrating the potential use of the former in the fabrication of long-life operation high temperature PEMFCs. A good mechanical property is an important requirement for the membrane to be used in high temperature PEMFCs. The tensile stressestrain curves of pristine and irradiated PBI membranes before and after doping with PA are illustrated in Fig. 2(a) and (b), respectively. As shown in Fig. 2(a), the mechanical property of all PBI membranes was significantly improved after the irradiation process. This phenomenon is a result of the increase in the
20
20
10
0 0
40
80
120
Strain (%)
(b) Fig. 2 e Stressestrain curves of (a) undoped and (b) PAdoped membranes of (þ) pristine 20 mm PBI; ( ) crosslinked PBI-1, ( ) PBI-2, and ( ) PBI-3.
Please cite this article in press as: Lai S, et al., Mechanical property enhancement of ultra-thin PBI membrane by electron beam irradiation for PEM fuel cell, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.04.111
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e7
phenomenon which was primarily a result of the dangling branched entanglement amongst the increased cross-linked network. Youssef et al. presented a similar result of irradiated HSR/SBR blends when increasing irradiation dose in their study [24]. Fig. 2(b) demonstrates that PBI membranes after PA doping have an increased tensile strain and decreased yield stress and tensile stress at break in comparison to PBI membranes without PA doping. This phenomena is due to the PA molecules, doped in the membranes, function as plasticizers resulting in membrane swelling and toughness improving. Although the tensile strength of PA-doped irradiated PBI membranes decreased, the values were still higher than that of PA-doped pristine 20 mm PBI membrane, while the PAdoped PBI-1 membrane showed an enormous increase of around 100% in tensile strength. These observations suggest that irradiated PBI membranes show better performance when exposed to appropriate electron beam irradiation dose. Furthermore, doping with PA enables higher sustainability of the membrane for use in high temperature operating environment. Additionally the thermal stabilities of pristine and irradiated PBI membranes were investigated with TGA under nitrogenous environment and the results are shown in Fig. 3. All tested PBI membranes show a two-step degradation pattern at temperature ranges of 50e120 C and 300e450 C. It has been reported that the first weight loss (degradation) can be attributed to the evaporation of water content in the membranes, while the case of the second weight loss is closely related to degradation of side chains [25]. The irradiated PBI membranes show a smaller second weight loss than pristine PBI membranes, which could possibly be due to the difference in the extent of cross-linking between the former and the latter. These results indicate that irradiated PBI membranes are thermally more stable than the pristine counterpart and suitable for testing in high temperature PEM fuel cell. Figures S2(a) and S2(b) illustrate the SEM micrographs of the plane surfaces of pristine 20 mm PBI and irradiated PBI
Fig. 3 e TGA curves of pristine and irradiated PBI membranes.
5
membranes, respectively. The micrographs show smooth surfaces of these two membranes. Moreover, after electron beam irradiation, the surface morphology of PBI membrane (Fig. S2(b)) does not change. Furthermore, we have obtained the cross-sectional images and thicknesses of all tested PBI membranes. As shown in Figs. S2(c) and S2(d), the PBI-3 membrane has a thinner thickness of 15 mm than the 20 mm of pristine PBI membrane. The thicknesses of all tested membranes irradiated at different electron fluences are listed in Table 1. According to the data, the thickness of irradiated membranes decreases with increasing electron fluences. It is well known that irradiating at higher electron fluences provide higher radiation-absorbed dose leading to more radical production, which in turn increases the cross-linking density within the polymer. Thus, it can be concluded that the irradiated PBI shows more compact cross-linked structure, which leads it to having a thinner thickness than its pristine counterpart.
Cell performances To evaluate the performance of the fuel cell with the irradiated PBI membrane, cross-linked PBI-1, which demonstrated fine mechanical strength in the study, were prepared and single cell performance was carried out. Pristine PBI membrane with 20 mm thickness was also used for cell performance tests for control testing. Fig. 4 shows the polarization and power plots for pristine 20 mm PBI and cross-linked PBI-1 membranes at an operating temperature of 160 C under 1 atm back pressure. From the IeV curves and experimental data, the pristine 20 mm PBI shows better cell performance than that shown by crosslinked PBI-1 membrane with the maximal power densities (PDmax) of the pristine 20 mm PBI and cross-linked PBI-1 membranes being 988 and 934 mW cm2, respectively. However, despite the superior performance boasted by the pristine 20 mm PBI membrane, it still has some issues in fuel cell applications due to its poor mechanical property and inferior chemical stability. In contrast, the cross-linked PBI-1
Fig. 4 e Single cell performances of the pristine 20 mm PBI and cross-linked PBI-1 membranes at 160 C under hydrogen and oxygen as a fuel gas.
Please cite this article in press as: Lai S, et al., Mechanical property enhancement of ultra-thin PBI membrane by electron beam irradiation for PEM fuel cell, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.04.111
6
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e7
membrane shows a slightly inferior cell performance due to its lower PA doping level (Table 1) [26], but it has a largely improved mechanical property, chemical stability, and thermal stability over the pristine 20 mm PBI membrane. Therefore, it can be concluded that the irradiated PBI membrane excel far more heavily in durability than the pristine PBI membrane. Although the ultra-thin membrane is known for its excellent conductance, membranes with thickness range from 40 to 200 mm are more typically used for PEMFC applications because of their lower gas permeability result in higher open circuit voltage (OCV) [7]. Thus, a pristine PBI membrane with 40 mm thickness was tested for comparison. Fig. 5(a) shows the polarization and power plots of crosslinked PBI-1 and pristine 40 mm PBI membranes. Pristine 40 mm PBI is approximately 2 times thicker than the crosslinked PBI-1 membrane. However, the cross-linked PBI-1 (0.85 V) exhibit similar OCV as compared to that of pristine 40 mm PBI membrane (0.87 V) at 160 C. It appears that the irradiated PBI membrane has thinner thickness but improved
gas impermeability property due to polymer cross-linking. Furthermore, the PDmax were found to be 934 and 527 mW cm2 for the cross-linked PBI-1 and pristine 40 mm PBI membranes, respectively. This difference can be attributed to the ohm resistance of membranes with different thicknesses. Fig. 5(b) illustrates the through plane resistance of crosslinked PBI-1 and pristine 40 mm PBI membranes, confirming the result of cell performance. The cross-linked PBI-1 membrane shows a significantly lower ohm resistance in comparison to the pristine 40 mm PBI at high temperature. In other words, the reduced thickness and resistance of irradiated PBI membrane can overcome the gas permeation issue and perform better than the pristine 40 mm PBI membrane. Therefore, maintaining performance and significantly boosting mechanical property can potentially be accomplished with the benefits offered by the cross-linking PBI via electron beam irradiation.
Conclusions A facile and effective method of preparing cross-linked PBI membrane via electron beam irradiation for PEMFC applications is proposed. The degree of cross-linking increases while the solubility of the membrane in DMAc solvent decreases with increasing electron fluence. As a result, the irradiated PBI membranes show outstanding chemical and thermal stability, improved mechanical property, as well as thinner thickness of membrane compared to those shown by the pristine 20 mm PBI membrane. After doping with PA, the pristine 20 mm PBI membrane exhibits the highest cell performance and PDmax of 988 mW cm2, but simultaneously shows the lowest mechanical property. The cross-linked PBI-1 membrane shows excellent mechanical property all the while showing similar performance to that of pristine PBI membrane and reaches the PDmax value of 934 mW cm2. Additionally, to assess the performance against a standard membrane used in PEMFc, a pristine 40 mm PBI membrane cell performance test was conducted. The cross-linked PBI-1 demonstrates an 80% increase in performance over that of the pristine 40 mm PBI due to the former's lower ohm resistance in membrane at high temperature. Consequently, it can be concluded that irradiated ultrathin PBI membrane can be applied to improve PEMFC performance.
Acknowledgments This work was supported by the basic research program of the Korea Atomic Energy Research Institute (KAERI) Grant funded by the Korea government (N04130036).
Appendix A. Supplementary data Fig. 5 e (a) Single cell performance and (b) through plane resistance of the cross-linked PBI-1 and pristine 40 mm PBI membranes at 160 C.
Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2016.04.111.
Please cite this article in press as: Lai S, et al., Mechanical property enhancement of ultra-thin PBI membrane by electron beam irradiation for PEM fuel cell, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.04.111
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e7
references
[1] Haile SM, Pintauro PN. Proton transport for fuel cells. J Mater Chem 2010;20:6211e3. [2] Xiao LX, Zhang HF, Scanlon E, Ramanathan LS, Choe EW, Rogers D, et al. High-temperature polybenzimidazole fuel cell membranes via a sol-gel process. Chem Mater 2005;17:5328e33. [3] Aili D, Cleemann LN, Li QF, Jensen JO, Christensen E, Bjerrum NJ. Thermal curing of PBI membranes for high temperature PEM fuel cells. J Mater Chem 2012;22:5444e53. [4] Pu HT, Wang L, Pan HY, Wan DC. Synthesis and characterization of fluorine-containing polybenzimidazole for proton conducting membranes in fuel cells. J Polym Sci Polym Chem 2010;48:2115e22. [5] Luo HC, Pu HT, Chang ZH, Wan DC, Pan HY. Crosslinked polybenzimidazole via a Diels-Alder reaction for proton conducting membranes. J Mater Chem 2012;22:20696e705. [6] Yu S, Benicewicz BC. Synthesis and properties of functionalized polybenzimidazoles for high-temperature PEMFCs. Macromolecules 2009;42:8640e8. [7] Lin HL, Chou YC, Yu TL, Lai SW. Poly(benzimidazole)epoxide crosslink membranes for high temperature proton exchange membrane fuel cells. Int J Hydrogen Energy 2012;37:383e92. [8] Kerres J, Ullrich A, Meier F, Haring T. Synthesis and characterization of novel acid-base polymer blends for application in membrane fuel cells. Solid State Ion 1999;125:243e9. [9] Hasiotis C, Deimede V, Kontoyannis C. New polymer electrolytes based on blends of sulfonated polysulfones with polybenzimidazole. Electrochim Acta 2001;46:2401e6. [10] Manea C, Mulder M. New polymeric electrolyte membranes based on proton donor-proton acceptor properties for direct methanol fuel cells. Desalination 2002;147:179e82. [11] Zhai YF, Zhang HM, Zhang Y, Xing DM. A novel H3PO4/ Nafion-PBI composite membrane for enhanced durability of high temperature PEM fuel cells. J Power Sources 2007;169:259e64. [12] Zhang HQ, Li XF, Zhao CJ, Fu TZ, Shi YH, Na H. Composite membranes based on highly sulfonated PEEK and PBI: morphology characteristics and performance. J Membr Sci 2008;308:66e74. [13] Wang KY, Xiao YC, Chung TS. Chemically modified polybenzimidazole nanofiltration membrane for the separation of electrolytes and cephalexin. Chem Eng Sci 2006;61:5807e17.
7
[14] Li QF, Pan C, Jensen JO, Noye P, Bjerrum NJ. Cross-linked polybenzimidazole membranes for fuel cells. Chem Mater 2007;19:350e2. [15] Noy P, Li QF, Pan C, Bjerrurn NJ. Cross-linked polybenzimidazole membranes for high temperature proton exchange membrane fuel cells with dichloromethyl phosphinic acid as a cross-linker. Polym Adv Technol 2008;19:1270e5. [16] Sukumar PR, Wu WC, Markova D, Unsal O, Klapper M, Mullen K. Functionalized poly(benzimidazole)s as membrane materials for fuel cells. Macromol Chem Phys 2007;208:2258e67. [17] Guan YS, Pu HT, Jin M, Chang ZH, Wan DC. Preparation and characterisation of proton exchange membranes based on crosslinked polybenzimidazole and phosphoric acid. Fuel Cells 2010;10:973e82. [18] Li QF, Jensen JO, Savinell RF, Bjerrum NJ. High temperature proton exchange membranes based on polybenzimidazoles for fuel cells. Prog Polym Sci 2009;34:449e77. [19] Li Y, Lee EJ, Cai WP, Kim KY, Cho SO. Unconventional method for morphology-controlled carbonaceous nanoarrays based on electron irradiation of a polystyrene colloidal monolayer. ACS Nano 2008;2:1108e12. [20] Song JM, Shin J, Sohn JY, Nho YC. Preparation and characterization of SPEEK membranes crosslinked by electron beam irradiation. Macromol Res 2011;19:1082e9. [21] Park J, Takayama T, Asano M, Maekawa Y, Kudo K. Graft-type polymer electrolyte membranes for fuel cells prepared through radiation-induced graft polymerization into alicyclic polybenzimidazoles. Polymer 2013;54:4570e7. [22] Osaheni JA, Jenekhe SA. Synthesis and processing of heterocyclic polymers as electronic, optoelectronic, and nonlinear-optical materials .4. New conjugated rigid-rod poly(benzobis(imidazole))s. Macromolecules 1995;28:1172e9. [23] Liang ZM, Jiang XS, Xu HJ, Chen D, Yin J. Polybenzimidazoles (PBIs) derived from non-coplanar dibenzoic acid containing imidazole (IDBA): synthesis, characterization and properties. Macromol Chem Phys 2009;210:1632e9. [24] Youssef HA. Effect of electron beam irradiation on the properties of high styrene rubber/styrene butadiene rubber blends. J Macromol Sci A 2009;46:636e42. [25] Kannan R, Kagalwala HN, Chaudhari HD, Kharul UK, Kurungot S, Pillai VK. Improved performance of phosphonated carbon nanotubeepolybenzimidazole composite membranes in proton exchange membrane fuel cells. J Mater Chem 2011;21:7223e31. [26] Fang JH, Qiao JL, Wilkinson DP, Zhang JJ. Electrochemical polymer electrolyte membranes. Taylor & Francis Group, LLC; 2015.
Please cite this article in press as: Lai S, et al., Mechanical property enhancement of ultra-thin PBI membrane by electron beam irradiation for PEM fuel cell, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.04.111