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functionalized graphene oxide and polybenzimidazole composite membranes for high temperature proton exchange membrane fuel cells
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 4 ) 1 e9
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Graphite oxide/functionalized graphene oxide and polybenzimidazole composite membranes for high temperature proton exchange membrane fuel cells Chao Xue, Jing Zou, Zhaonan Sun, Fanghui Wang, Kefei Han, Hong Zhu* State Key Laboratory of Chemical Resource Engineering, Institute of Modern Catalysis, Department of Organic Chemistry, School of Science, Beijing University of Chemical Technology, Beijing 100029, China
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abstract
Article history:
Graphite oxide/polybenzimidazole synthesized by 3, 30 -diaminobenzidine and 5-tert-butyl
Received 17 January 2014
isophthalic acid (GO/BuIPBI) and isocyanate modified graphite oxide/BuIPBI (iGO/BuIPBI)
Received in revised form
composite membranes were prepared for high temperature polymer proton exchange
3 March 2014
membrane fuel cells (PEMFCs). All membranes were loaded with different content of
Accepted 10 March 2014
phosphoric acid to provide proton conductivity. The GO/BuIPBI and iGO/BuIPBI membranes
Available online xxx
were characterized by SEM which showed that the filler GO or iGO were well dispersed in the polymer matrix and had a strong interaction with BuIPBI, which can improve the
Keywords:
chemical stability of BuIPBI membrane and support a higher acid content. The proton
Polybenzimidazole
conductivities of the GO/BuIPBI and iGO/BuIPBI with high acid loading were 0.016 and
Graphite oxide
0.027 S/cm, respectively, at 140 C and without humidity.
Modified graphite oxide
Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
High temperature proton exchange membrane fuel cell (HT-PEMFC)
Introduction The search for renewable and sustainable energy has become one of significant challenges in our society, both for the transport sector and power generation [1]. Solid polymer electrolyte membrane fuel cell, which has recently received considerable attention, is a promising technology to meet today’s energy needs [2]. These membrane fuel cells are particularly attractive because the fuel cell is clean energy equipment, eliminating many of the air pollutants from traditional energy [3]. Fuel cells, especially proton exchange membrane fuel cells (PEMFCs) have attracted considerable attention because of potential advantages such as rapid start-
up, high energy efficiency, low environmental impact, excellent durability and wide range of applications [4,5]. Proton exchange membrane (PEM), with a solid polymer electrolyte, is a critical component of PEMFCs. The ideal PEMs must satisfy the requirements of chemical stability, thermal stability, electrical insulation, excellent mechanical properties, and low cost for useful applications [6]. Today, a large number of polymers have been examined for this application, all of which offer both advantages and disadvantages [7]. The most used fuel cell membranes are those based on perflurosulphonic acid (PFSA), such as Nafion [3]. However, these membranes are limited by their high cost, water dependent proton conductivity, and humidification requirement for regular operation [8]. Therefore, many polymers
* Corresponding author. Tel.: þ86 10 64411443; fax: þ86 10 64444919. E-mail address: [email protected] (H. Zhu). http://dx.doi.org/10.1016/j.ijhydene.2014.03.061 0360-3199/Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Xue C, et al., Graphite oxide/functionalized graphene oxide and polybenzimidazole composite membranes for high temperature proton exchange membrane fuel cells, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.03.061
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electrolytes and composite electrolytes with high proton conductivity at high temperatures have been found in the past two decades [9e11]. A few have been even used at above150 C [12]. Phosphoric acid (PA)-doped polybenzimidazole (PBI) and its derivatives are promising candidates for anhydrous PEMs developed for intermediate temperatures [13]. The most successful example of these polymers is poly [2, 20 -(m-phenylene)-5, 50 -bibenzimidazole] (mPBI) [14]. Some studies showed that these polymer electrolyte membranes had high proton conductivity at low humidity and gas permeability, excellent thermal stability at temperatures up to 200 C, and a water resistance coefficient close to zero [13]. Methods used to prepare polybenzimidazoles with high proton conductivities include the introduction of protogenic group in the processing of the polymerization [15,16], or chemical grafting at nitrogen atom of the imidazole ring [17]. However, there are two challenges for mPBI in fuel cell applications. One is that mPBI is a rigid aromatic polymer whose main chains have strong intermolecular hydrogen bonding interactions which result in poor solubility of PBI in organic solvent and hence poor processability of mPBI [18]. There have been studies to modify mPBI in order to improve the solubility of the polymer [19], by introducing sulfone, ketone, ether or aliphatic units [16,19e22]. Kang et al. [14] recently reported a way to introduce ether groups to the structure of mPBI, and the resulting membrane with an acid doping level (ADL) of 18.73 mol of H2SO4 for OPBI per repeat unit showed high proton conductivity (190 mS/cm) at 160 C and anhydrous conditions. The other challenge is that the synthesis of mPBI by the polycondensation in liquid-phase (i.e., solution polycondensation) is usually complex and takes a long time, generally 10 h or more [9,23,24]. The slow reaction [25] does not meet the increasing demand of the compound. This challenge turns out to be a major problem. Various investigators [26e28] reported that the use of microwave radiation does not affect the yield, but accelerates the synthesis process by a factor of ten. Such unconventional energy can reduce the polycondensation time by molecular internal uniform heating, but prevents thermal decomposition and other side reactions which are caused by local overheating [14]. Therefore, in this study we also used the microwave radiation method to synthesis some new PBIs. In addition, the new PBI we prepared in this study was doped with graphene oxide (GO) and its derivatives to improve the performance. Graphite oxide has received a great deal of attention because of its fascinating features [29]. It has different oxygen-containing functional groups, such as carboxyl, hydroxyl and epoxy groups. Because of the oxygencontaining functional groups, GO is easy to hydrate. As GO itself is an electronic insulator with differential conductivity, the high proton conductivity of the composite membrane is attributed to the hydrogen bonds in GO [30]. The acidic functional groups such as carboxylic acid and intermolecular hydrogen bonding can even provide additional proton conducting paths [31]. However, the dispersion of GO prepared by the Hummer method is poor in organic solvents such as DMF [32]. To avoid GO aggregates in organic solvents, one of the most effective ways is to graft active groups on the GO surface. Such functionalization of GO has been shown to be possible by Lerf et al., who prepared and studied a number of
Scheme 1 e Synthesis of BuIPBI under microwave irradiation. chemically modified GO derivatives [33]. Stankovich et al. have functionalized GO with isocyanates [34]. The isocyanate (-NCO) groups were introduced onto the surface of GO layers through the reaction of diisocyanates with eOH and eCOOH groups [34e36]. In this work, we used 3, 30 -diaminobenzidine (DAB) and 5tert-butyl isophthalic acid (TBIA) to prepare a polybenzimidazole(BuIPBI) containing large alkyl groups to improve the solubility in organic solvent such as DMAc by loosening its chain packing and reducing its high rigidity [37]. The process of polymerization can finish within 3 h by the use of microwave radiation. Besides we treated the surface of GO with tert-butyl isocyanate (TBI) to improve the dispersibility of modified GO (iGO) in both water and organic media. We then prepared GO/BuIPBI and iGO/BuIPBI composite membranes with different filler contents (1e15 wt%) and compared them with neat BuIPBI membrane. We found that the incorporation of iGO in the BuIPBI membrane can improve the chemical stability of the membrane so that it can be immersed in phosphoric acid with a higher concentration(85 wt%) without dissolve or swelling, and the higher concentration can increase the acid contents in the membrane and hence the proton conductivity of the membrane.
Experimental Materials 3, 30 -diaminobenzidine (DAB, 99%) was obtained from Acros Organics, New Jersey, USA. 5-tert-butyl isophthalic acid (TBIA) and tert-butyl isocyanate (TBI) were purchased from J&K Scientific Ltd., Beijing, China. Polyphosphoric acid (PPA, >84% P2O5) and graphite oxide (prepared from purified natural graphite (flake, natural, 325 mesh, 99.8%)) were purchased from Alfa Aesar Company, Tianjin, China. Sodium bicarbonate (NaHCO3), N, N0 -dimethylacetamide (DMAc), N,N0 -dimethylformamide (DMF), potassium permanganate, phosphoric acid (PA, 85%) and sulfuric acid (H2SO4, 98%) were
Please cite this article in press as: Xue C, et al., Graphite oxide/functionalized graphene oxide and polybenzimidazole composite membranes for high temperature proton exchange membrane fuel cells, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.03.061
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purchased from Beijing Chemical Reagent Co., Ltd., China. Dichloromethane was purchased from Sinopharm Chemical Reagent Ltd., Beijing, China. Hydrogen peroxide (H2O2) was purchased from Tianjin Guangfu Technology Development Co., Ltd., Tianjin, China.
Synthesis of BuIPBI The polybenzimidazole containing tertiary butyl (BuIPBI) was prepared by a solution polycondensation method under microwave irradiation, according to literature methods with some improvements [38]. DAB, TBIA and PPA were added into a 100-mL three-necked round bottom flask under mechanical stirring in nitrogen atmosphere for 30 min in an Apex microwave synthesis system (Pree-Kem Scientific Instruments Co., Ltd., China). The mixture was further stirred at 50 for 10 min and 120 C for 30 min for thorough mixing. Finally, the mixture was further heated at 170 C for 1 h and 200 C for 2 h to complete the polycondensation. The chemical equation for the synthesis of BuIPBI is presented in Scheme 1. The reaction product was isolated immediately by pouring the polycondensation solution into deionized water. NaHCO3 solution (3 wt%) was then added to neutralize the remaining acid. The BuIPBI was filtered and washed several times with deionized water and alcohol until the pH value was ca. 7.0 and finally dried in a vacuum oven at 80 C for 48 h.
Preparation of GO GO was synthesized from natural graphite by the modified Hummers method [29]. Typically, 3 g of graphite was mixed with 360 mL of H2SO4 (98%) and 40 mL of PA in a 1000-mL flask under vigorous stirring for 24 h, KMnO4 (18 g) was then added into the 1000-mL flask with an ice bath, and the reaction temperature was well controlled at <20 C and stirred the reaction mixture for 3 h. Then the reaction mixture was stirred for 12 h at 50 C, and poured into ice water under vigorous stirring. 13 mL of 30% H2O2 was added into the reaction mixture. The obtained graphene oxide was rinsed and centrifuged with 5% HCl and H2O several times and then dried under vacuum.
Preparation of iGO The iGO was prepared by using the method reported by Stankovich et al. [34]. GO (0.3 g) was added into anhydrous DMF (30 mL) and then sonicated for 30 min to form a suspension. TBI (2.5 g) was added after the suspension was stirred under nitrogen for 1 h, and then stirring under nitrogen was continued for 24 h. The resulting slurry was poured into dichloromethane (200 mL) for coagulation. The product was further separated by centrifugation, washed with dichloromethane and dried at 50 C under vacuum for 24 h. The synthesis of iGO is presented in Scheme 2.
Preparation of composite membranes Preparation of pristine BuIPBI membrane The pristine BuIPBI membrane was prepared by a solvent casting method. The BuIPBI was dissolved in DMAc to give a
Scheme 2 e Synthesis of iGO.
homogeneous and viscous solution, and then the undissolved particles were removed by filtration. The membrane was formed by casting the above solution onto a Petri dish. The majority of the solvent was evaporated in a ventilated oven in the temperature range at 60 C. The thickness of the cast membrane (in the range of 80e100 mm) was measured by a micrometer with an accuracy of 5 mm.
Preparation of GO/BuIPBI (iGO/BuIPBI) membrane GO sheets were dispersed in DMAc by ultrasonic treatment for about 1 h and then mixed with BuIPBI/DMAc solution in various ratios of GO to BuIPBI. After the mixture was vigorously stirred, the GO/BuIPBI membrane was prepared by the same method as that for the BuIPBI membrane; so was the iGO/BuIPBI membrane. We obtained several composite membranes, which were denoted by GO (x%)/BuIPBI or iGO (x %)/BuIPBI, where “x%” stands for the weight ratio of GO or iGO in the composite membranes.
Characterization methods Structural characterization The structures of BuIPBI, GO, and iGO were characterized by Fourier transform infrared spectra and 1H NMR patterns. The Fourier transform infrared spectra were recorded on a Thermo Scientific Nicolet 6700 FT-IR spectrometer in the wavenumber range of 4000e500 cm1. 1H NMR spectra were recorded on a Brucker AV400 spectrometer (400 MHz) by using Thermal stability The thermal stability of the BuIPBI, GO/BuIPBI and iGO/ BuIPBI membranes was measured by thermogravimetric analysis (TGA) on a Mettler Toledo TGA/DSC 1/1100 SF apparatus under nitrogen atmosphere at a heating rate of 10 C/ min from 25 C to 800 C.
Acid doping ability and swelling degree of composite membranes A membrane was dried in a ventilated oven at 80 C until its weight did not change any more. Then the membrane was immersed into different concentrations of phosphoric acid solution. After 48 h the phosphoric acid on the membrane surface was removed by a filter paper, and the membrane was put into the ventilated oven again at 80 C until the membrane weight did not change any more. The acid weight ratio in membrane was determined by Eq. (1):
Please cite this article in press as: Xue C, et al., Graphite oxide/functionalized graphene oxide and polybenzimidazole composite membranes for high temperature proton exchange membrane fuel cells, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.03.061
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Acid weight ratio in membrane ¼ ðm1 m0 Þ=m0 100% (1) where m0 is the weight of the dry membrane and m1 is the weight of the acid-doped membrane. The acid swelling ratio of the membrane was calculated by the Eq. (2): Acid swelling ratio ¼ ðV1 V0 Þ=V0 100%
(2)
where V0 is the volume of the dry membrane and V1 is the volume of the acid-doped membrane.
Proton conductivity The proton conductivities of the composite membranes were measured by the two-electrode AC impedance method over the frequency range of 1e1 105 Hz from 80 to 170 C on an electrochemical workstation (Zahner Im6ex, Germany). All measurements were carried out under anhydrous conditions. The proton conductivity s was calculated by Eq. (3) [39]: s ¼ L=ðRAÞ
(3)
where L is the thickness of the membrane, R is the resistance of membrane derived from the low intersection of the high frequency semicircle on a complex impedance plane with the real (Z) axis, and A is the membrane area that contacts the two electrodes [14]. To avoid the situation that the GO and iGO may agglomerate and touch the two electrodes, although the GO and iGO are insulators, we used a spray gun to put BuIPBI/DMAc solution onto the GO/BuIPBI and iGO/BuIPBI composite membranes before putting them into an oven to remove the residual solvent. This method can increase the thickness of a composite membrane by 3e5 mm.
to the stretching vibration of C]N groups. The peaks appearing in the wavenumber range 2800e2900 cm1 are ascribed to the CeH stretching of tert-butyl groups [41]. The absorption peak at 3416 cm1 is due to the stretching vibration of isolated NeH of imidazole groups [42]. Fig. 2 shows the FT-IR spectra of GO and iGO. The GO spectrum shows the characteristic absorption peaks at 1732 and 1620 cm1, corresponding to the stretching modes of C]O and C]C, respectively [43,44]. However, after the GO treated with TBI, the peak at 1732 cm1 for the C]O stretching vibration of GO becomes obscured, and the band at 1645 cm1 is assigned to the amide carbonyl stretching mode. The peak at 1378 cm1 is due to the CH3 deformation vibration, and that at 1041 cm1 is due to the CeOeC stretching vibration. From the 1H NMR spectrum shown in Fig. 3, aromatic protons are observed in the range d ¼ 7.59, and the protons for the tert-butyl group clearly show a sharp peak at d ¼ 1.5. The protons attached to the nitrogen of the imidazole group were observed at d w 13.2, consistent with results from the literature [41].
Dispersibility of GO and iGO The dispersibility of GO and iGO is shown in Fig. 4. Compare with the parent GO, GO treated with tert-butyl isocyanate (iGO) does not disperse at all in H2O. However, the iGO disperses and forms stable colloidal dispersions in polar aprotic solvents such as DMF after a brief ultrasonic treatment. Fig. 4 shows that the vials with GO/DMF and iGO/H2O dispersions have visible precipitates, which indicate poor dispersion. However, the dark brown dispersion of iGO/DMF contains no visible precipitate and is stable for weeks.
Results and discussion
SEM study of composite membranes
Characterization of BuIPBI, GO, and iGO
As can be seen in Fig. 5, the BuIPBI membrane shows a smooth cross section, in agreement with previous reports [45,46]. However, the morphology of the composite membranes is completely different from that of the neat membrane. In the GO/BuIPBI composite membrane with 1 wt% of GO, we can clearly see that the GO particles are evenly
The FT-IR spectrum of BuIPBI is shown in Fig. 1. The absence of the carboxyl peak at 1650e1700 cm1 indicates that the cyclization to form the imidazole ring is complete in BuIPBI [40]. The peaks from 1450 cm1 to 1600 cm1 indicate the existence of the benzene ring. The characteristic peaks at 1618 cm1 is due
Fig. 1 e FT-IR spectrum of BuIPBI.
Fig. 2 e FT-IR spectra of GO and iGO.
Please cite this article in press as: Xue C, et al., Graphite oxide/functionalized graphene oxide and polybenzimidazole composite membranes for high temperature proton exchange membrane fuel cells, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.03.061
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Fig. 3 e 1H NMR spectrum of BuIPBI.
dispersed in the BuIPBI matrix. Meanwhile, the iGO/BuIPBI composite films containing 1e15 wt% of iGO have rough cross sections that could be attributed to the strong interfacial adhesion and good compatibility between the BuIPBI matrix and the iGO sheets [32] (Fig. 6), indicating that a iGO/ BuIPBI membrane is not just a simple mixture of the two components.
Chemical stability Fig. 7 shows the photographs of BuIPBI, GO (1%)/BuIPBI, and iGO (1%)/BuIPBI membranes immersed in 85 wt% PA solution at 80 C for 24 h. Since we wanted to quickly study the chemical stability of these membranes, we did not measure the stability under mild conditions such as room temperature and dilute acid solutions. The 85 wt% PA solution containing the BuIPBI and GO (1%)/BuIPBI membranes become brown and the membranes begin to swell because of their poor chemical stability at high PA concentrations. However, the solution with iGO (1%)/BuIPBI remains colorless and transparent, an indication that iGO can improve the chemical stability of the BuIPBI matrix with the hydrogen bonds in the amide groups. Further studies are still needed to investigate the detailed mechanism.
Acid doping ability and swelling degree of composite membranes
Fig. 4 e Vials containing dispersions (1 mg/mL) of GO in DMF (left), tert-butyl isocyanate treated GO (iGO) in water (middle) and DMF (right). The top image shows the dispersions 24 h after preparation. The bottom image shows the vials turned upside down with the precipitates clearly shown on the bottom of the left and middle vials.
Acid-doped polybenzimidazole has been used as polymer electrolyte in fuel cells [47,48]. Because PA can conduct protons without water and its thermal stability is high, PA has become the most interesting acid. The neat and composite membranes were immersed in a phosphoric acid solution at room temperature, and after 24 h, the weight of the membranes did not increase any more, indicating that equilibrium had been reached between the acid and the membranes [4,14].
Please cite this article in press as: Xue C, et al., Graphite oxide/functionalized graphene oxide and polybenzimidazole composite membranes for high temperature proton exchange membrane fuel cells, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.03.061
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Fig. 5 e SEM images of (aec) BuIPBI membrane and (def) GO/BuIPBI membrane with 1% of GO. Because the neat BuIPBI and GO/BuIPBI membranes can dissolve in 85 wt% PA solutions, we put these membranes into 80 wt% PA solutions while the iGO/BuIPBI membrane was immersed into 85 wt% PA solution. The acid doping levels and swelling degree of BuIPBI, GO/BuIPBI and iGO/BuIPBI composite membranes doped with PA at room temperature are shown in Fig. 8. It can be seen that the acid uptake of the composite membranes decreases with increasing amount of GO or iGO. The factors that affect the acid doping ability of PBI
membranes have been studied before [49]. The concentration of the acid solution and the structure of the PBI are the two main factors affecting the acid doping level. Increasing the acid concentration can increase the acid doping level in the membranes. Chemically, PBI is a basic polymer and can readily be doped with PA. But when GO was introduced to BuIPBI, the alkalinity of BuIPBI would be decreased and therefore affect the acid doping ability. Besides, the GO and iGO as fillers could occupy the interspaces of polymer chains
Fig. 6 e SEM images of (aed) iGO(1%)/BuIPBI, (eeh) iGO(10%)/BuIPBI, and (iel) iGO(15%)/BuIPBI membranes. Please cite this article in press as: Xue C, et al., Graphite oxide/functionalized graphene oxide and polybenzimidazole composite membranes for high temperature proton exchange membrane fuel cells, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.03.061
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Fig. 7 e Photographs showing chemical stability of (a) BuIPBI, (b) GO(1%)/BuIPBI, and(c) iGO(1%)/BuIPBI membranes immersed in 85 wt% PA solution at 80 C for 24 h.
and prevent the PA molecules from penetrating the membranes. The acid swelling ratio of the BuIPBI, GO/BuIPBI and iGO/ BuIPBI membranes were calculated according to Eq. (2). As shown in Fig. 8, the swelling ratios of the iGO/BuIPBI membranes immersed in 85 wt% PA are smaller than those of the GO/BuIPBI membranes, though the acid contents are much higher in the iGO/BuIPBI membranes than in the GO/BuIPBI membranes. This result could be explained by the strong interfacial adhesion between the iGO particles and the BuIPBI matrix.
Proton conductivity of composite membranes The proton conductivities of the PA-doped BuIPBI, GO/BuIPBI and iGO/BuIPBI composite membranes were measured at 80e160 C under anhydrous conditions. Fig. 9 shows the temperature dependence of proton conductivity. It can be seen that the proton conductivity of the PA-doped BuIPBI, GO/ BuIPBI, and iGO/BuIPBI composite membranes increases with increasing the test temperature. The GO/BuIPBI and iGO/ BuIPBI membranes have higher conductivities than the neat BuIPBI membrane. The improvement can be attributed to the introduction of GO or iGO in the BuIPBI matrix. However, the conductivities of the GO/BuIPBI and iGO/BuIPBI composite membranes decrease with increasing temperature as the
Fig. 8 e Acid doping ability and swelling degree of membranes at room temperature.
temperature exceeds 140 C because of the degradation of acidic functional groups like carboxylic acid and epoxy oxygen groups in the GO sheets. The incorporation of GO introduces interconnected proton transfer channels, which can facilitate the transfer of protons through the membrane, as reported by Hwang et al. [50]. Meanwhile, the iGO/BuIPBI composite membranes have higher acid doping levels than those of the neat BuIPBI membrane and GO/BuIPBI composite membranes. A higher acid doping level provides higher proton conductivity. The reason why iGO(10%)/BuIPBI composite membrane was better than the iGO(15%)/BuIPBI composite membrane was that the acid doping level and the iGO concentration in the membrane both influenced the proton conductivity of iGO/BuIPBI composite membranes. When the concentration of iGO in the membrane was too high, the iGO could be agglomerated and blocked the path of proton.
Conclusions A series of composite membranes made from GO or iGO and BuIPBI have been prepared for PEMFC applications. Due to the good dispersibility of iGO in organic solvents, BuIPBI membranes with a highly uniform structure with iGO were
Fig. 9 e Temperature dependence of proton conductivity of the PA-doped BuIPBI, GO/BuIPBI and iGO/BuIPBI composite membranes with different acid doping levels at 80e160 C without humidification.
Please cite this article in press as: Xue C, et al., Graphite oxide/functionalized graphene oxide and polybenzimidazole composite membranes for high temperature proton exchange membrane fuel cells, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.03.061
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prepared. Because of the hydrogen bonding between iGO and BuIPBI, the membranes had high chemical stability. These iGO/BuIPBI membranes with higher stability could absorb more acid but undergo only a small amount of swelling. With a high acid content, the iGO/BuIPBI membranes had high proton conductivities at elevated temperatures. The proton conductivities of BuIPBI, GO/BuIPBI, and iGO/BuIPBI membrane were 0.012, 0.016, and 0.027 S/cm, respectively, at 140 C without humidity.
Acknowledgment The authors gratefully acknowledge the financial supports from the National High Technology Research and Development Program of China (No. 2011AA11A273), the National Natural Science Foundation of China (No. 21176022, 21176023 and 21376022), the International S&T Cooperation Program of China (No. 2009DFA63120 and 2013DFA51860), and the Fundamental Research Funds for the Central Universities (ZY1328), the Program for Changjiang Scholars and Innovative Research Team in University (IRT1205).
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Graphite oxide/functionalized graphene oxide and polybenzimidazole composite membranes for high temperature proton exchange membrane fuel cells Chao Xue, Jing Zou, Zhaonan Sun, Fanghui Wang, Kefei Han, Hong Zhu* State Key Laboratory of Chemical Resource Engineering, Institute of Modern Catalysis, Department of Organic Chemistry, School of Science, Beijing University of Chemical Technology, Beijing 100029, China
article info
abstract
Article history:
Graphite oxide/polybenzimidazole synthesized by 3, 30 -diaminobenzidine and 5-tert-butyl
Received 17 January 2014
isophthalic acid (GO/BuIPBI) and isocyanate modified graphite oxide/BuIPBI (iGO/BuIPBI)
Received in revised form
composite membranes were prepared for high temperature polymer proton exchange
3 March 2014
membrane fuel cells (PEMFCs). All membranes were loaded with different content of
Accepted 10 March 2014
phosphoric acid to provide proton conductivity. The GO/BuIPBI and iGO/BuIPBI membranes
Available online xxx
were characterized by SEM which showed that the filler GO or iGO were well dispersed in the polymer matrix and had a strong interaction with BuIPBI, which can improve the
Keywords:
chemical stability of BuIPBI membrane and support a higher acid content. The proton
Polybenzimidazole
conductivities of the GO/BuIPBI and iGO/BuIPBI with high acid loading were 0.016 and
Graphite oxide
0.027 S/cm, respectively, at 140 C and without humidity.
Modified graphite oxide
Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
High temperature proton exchange membrane fuel cell (HT-PEMFC)
Introduction The search for renewable and sustainable energy has become one of significant challenges in our society, both for the transport sector and power generation [1]. Solid polymer electrolyte membrane fuel cell, which has recently received considerable attention, is a promising technology to meet today’s energy needs [2]. These membrane fuel cells are particularly attractive because the fuel cell is clean energy equipment, eliminating many of the air pollutants from traditional energy [3]. Fuel cells, especially proton exchange membrane fuel cells (PEMFCs) have attracted considerable attention because of potential advantages such as rapid start-
up, high energy efficiency, low environmental impact, excellent durability and wide range of applications [4,5]. Proton exchange membrane (PEM), with a solid polymer electrolyte, is a critical component of PEMFCs. The ideal PEMs must satisfy the requirements of chemical stability, thermal stability, electrical insulation, excellent mechanical properties, and low cost for useful applications [6]. Today, a large number of polymers have been examined for this application, all of which offer both advantages and disadvantages [7]. The most used fuel cell membranes are those based on perflurosulphonic acid (PFSA), such as Nafion [3]. However, these membranes are limited by their high cost, water dependent proton conductivity, and humidification requirement for regular operation [8]. Therefore, many polymers
* Corresponding author. Tel.: þ86 10 64411443; fax: þ86 10 64444919. E-mail address: [email protected] (H. Zhu). http://dx.doi.org/10.1016/j.ijhydene.2014.03.061 0360-3199/Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Xue C, et al., Graphite oxide/functionalized graphene oxide and polybenzimidazole composite membranes for high temperature proton exchange membrane fuel cells, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.03.061
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electrolytes and composite electrolytes with high proton conductivity at high temperatures have been found in the past two decades [9e11]. A few have been even used at above150 C [12]. Phosphoric acid (PA)-doped polybenzimidazole (PBI) and its derivatives are promising candidates for anhydrous PEMs developed for intermediate temperatures [13]. The most successful example of these polymers is poly [2, 20 -(m-phenylene)-5, 50 -bibenzimidazole] (mPBI) [14]. Some studies showed that these polymer electrolyte membranes had high proton conductivity at low humidity and gas permeability, excellent thermal stability at temperatures up to 200 C, and a water resistance coefficient close to zero [13]. Methods used to prepare polybenzimidazoles with high proton conductivities include the introduction of protogenic group in the processing of the polymerization [15,16], or chemical grafting at nitrogen atom of the imidazole ring [17]. However, there are two challenges for mPBI in fuel cell applications. One is that mPBI is a rigid aromatic polymer whose main chains have strong intermolecular hydrogen bonding interactions which result in poor solubility of PBI in organic solvent and hence poor processability of mPBI [18]. There have been studies to modify mPBI in order to improve the solubility of the polymer [19], by introducing sulfone, ketone, ether or aliphatic units [16,19e22]. Kang et al. [14] recently reported a way to introduce ether groups to the structure of mPBI, and the resulting membrane with an acid doping level (ADL) of 18.73 mol of H2SO4 for OPBI per repeat unit showed high proton conductivity (190 mS/cm) at 160 C and anhydrous conditions. The other challenge is that the synthesis of mPBI by the polycondensation in liquid-phase (i.e., solution polycondensation) is usually complex and takes a long time, generally 10 h or more [9,23,24]. The slow reaction [25] does not meet the increasing demand of the compound. This challenge turns out to be a major problem. Various investigators [26e28] reported that the use of microwave radiation does not affect the yield, but accelerates the synthesis process by a factor of ten. Such unconventional energy can reduce the polycondensation time by molecular internal uniform heating, but prevents thermal decomposition and other side reactions which are caused by local overheating [14]. Therefore, in this study we also used the microwave radiation method to synthesis some new PBIs. In addition, the new PBI we prepared in this study was doped with graphene oxide (GO) and its derivatives to improve the performance. Graphite oxide has received a great deal of attention because of its fascinating features [29]. It has different oxygen-containing functional groups, such as carboxyl, hydroxyl and epoxy groups. Because of the oxygencontaining functional groups, GO is easy to hydrate. As GO itself is an electronic insulator with differential conductivity, the high proton conductivity of the composite membrane is attributed to the hydrogen bonds in GO [30]. The acidic functional groups such as carboxylic acid and intermolecular hydrogen bonding can even provide additional proton conducting paths [31]. However, the dispersion of GO prepared by the Hummer method is poor in organic solvents such as DMF [32]. To avoid GO aggregates in organic solvents, one of the most effective ways is to graft active groups on the GO surface. Such functionalization of GO has been shown to be possible by Lerf et al., who prepared and studied a number of
Scheme 1 e Synthesis of BuIPBI under microwave irradiation. chemically modified GO derivatives [33]. Stankovich et al. have functionalized GO with isocyanates [34]. The isocyanate (-NCO) groups were introduced onto the surface of GO layers through the reaction of diisocyanates with eOH and eCOOH groups [34e36]. In this work, we used 3, 30 -diaminobenzidine (DAB) and 5tert-butyl isophthalic acid (TBIA) to prepare a polybenzimidazole(BuIPBI) containing large alkyl groups to improve the solubility in organic solvent such as DMAc by loosening its chain packing and reducing its high rigidity [37]. The process of polymerization can finish within 3 h by the use of microwave radiation. Besides we treated the surface of GO with tert-butyl isocyanate (TBI) to improve the dispersibility of modified GO (iGO) in both water and organic media. We then prepared GO/BuIPBI and iGO/BuIPBI composite membranes with different filler contents (1e15 wt%) and compared them with neat BuIPBI membrane. We found that the incorporation of iGO in the BuIPBI membrane can improve the chemical stability of the membrane so that it can be immersed in phosphoric acid with a higher concentration(85 wt%) without dissolve or swelling, and the higher concentration can increase the acid contents in the membrane and hence the proton conductivity of the membrane.
Experimental Materials 3, 30 -diaminobenzidine (DAB, 99%) was obtained from Acros Organics, New Jersey, USA. 5-tert-butyl isophthalic acid (TBIA) and tert-butyl isocyanate (TBI) were purchased from J&K Scientific Ltd., Beijing, China. Polyphosphoric acid (PPA, >84% P2O5) and graphite oxide (prepared from purified natural graphite (flake, natural, 325 mesh, 99.8%)) were purchased from Alfa Aesar Company, Tianjin, China. Sodium bicarbonate (NaHCO3), N, N0 -dimethylacetamide (DMAc), N,N0 -dimethylformamide (DMF), potassium permanganate, phosphoric acid (PA, 85%) and sulfuric acid (H2SO4, 98%) were
Please cite this article in press as: Xue C, et al., Graphite oxide/functionalized graphene oxide and polybenzimidazole composite membranes for high temperature proton exchange membrane fuel cells, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.03.061
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purchased from Beijing Chemical Reagent Co., Ltd., China. Dichloromethane was purchased from Sinopharm Chemical Reagent Ltd., Beijing, China. Hydrogen peroxide (H2O2) was purchased from Tianjin Guangfu Technology Development Co., Ltd., Tianjin, China.
Synthesis of BuIPBI The polybenzimidazole containing tertiary butyl (BuIPBI) was prepared by a solution polycondensation method under microwave irradiation, according to literature methods with some improvements [38]. DAB, TBIA and PPA were added into a 100-mL three-necked round bottom flask under mechanical stirring in nitrogen atmosphere for 30 min in an Apex microwave synthesis system (Pree-Kem Scientific Instruments Co., Ltd., China). The mixture was further stirred at 50 for 10 min and 120 C for 30 min for thorough mixing. Finally, the mixture was further heated at 170 C for 1 h and 200 C for 2 h to complete the polycondensation. The chemical equation for the synthesis of BuIPBI is presented in Scheme 1. The reaction product was isolated immediately by pouring the polycondensation solution into deionized water. NaHCO3 solution (3 wt%) was then added to neutralize the remaining acid. The BuIPBI was filtered and washed several times with deionized water and alcohol until the pH value was ca. 7.0 and finally dried in a vacuum oven at 80 C for 48 h.
Preparation of GO GO was synthesized from natural graphite by the modified Hummers method [29]. Typically, 3 g of graphite was mixed with 360 mL of H2SO4 (98%) and 40 mL of PA in a 1000-mL flask under vigorous stirring for 24 h, KMnO4 (18 g) was then added into the 1000-mL flask with an ice bath, and the reaction temperature was well controlled at <20 C and stirred the reaction mixture for 3 h. Then the reaction mixture was stirred for 12 h at 50 C, and poured into ice water under vigorous stirring. 13 mL of 30% H2O2 was added into the reaction mixture. The obtained graphene oxide was rinsed and centrifuged with 5% HCl and H2O several times and then dried under vacuum.
Preparation of iGO The iGO was prepared by using the method reported by Stankovich et al. [34]. GO (0.3 g) was added into anhydrous DMF (30 mL) and then sonicated for 30 min to form a suspension. TBI (2.5 g) was added after the suspension was stirred under nitrogen for 1 h, and then stirring under nitrogen was continued for 24 h. The resulting slurry was poured into dichloromethane (200 mL) for coagulation. The product was further separated by centrifugation, washed with dichloromethane and dried at 50 C under vacuum for 24 h. The synthesis of iGO is presented in Scheme 2.
Preparation of composite membranes Preparation of pristine BuIPBI membrane The pristine BuIPBI membrane was prepared by a solvent casting method. The BuIPBI was dissolved in DMAc to give a
Scheme 2 e Synthesis of iGO.
homogeneous and viscous solution, and then the undissolved particles were removed by filtration. The membrane was formed by casting the above solution onto a Petri dish. The majority of the solvent was evaporated in a ventilated oven in the temperature range at 60 C. The thickness of the cast membrane (in the range of 80e100 mm) was measured by a micrometer with an accuracy of 5 mm.
Preparation of GO/BuIPBI (iGO/BuIPBI) membrane GO sheets were dispersed in DMAc by ultrasonic treatment for about 1 h and then mixed with BuIPBI/DMAc solution in various ratios of GO to BuIPBI. After the mixture was vigorously stirred, the GO/BuIPBI membrane was prepared by the same method as that for the BuIPBI membrane; so was the iGO/BuIPBI membrane. We obtained several composite membranes, which were denoted by GO (x%)/BuIPBI or iGO (x %)/BuIPBI, where “x%” stands for the weight ratio of GO or iGO in the composite membranes.
Characterization methods Structural characterization The structures of BuIPBI, GO, and iGO were characterized by Fourier transform infrared spectra and 1H NMR patterns. The Fourier transform infrared spectra were recorded on a Thermo Scientific Nicolet 6700 FT-IR spectrometer in the wavenumber range of 4000e500 cm1. 1H NMR spectra were recorded on a Brucker AV400 spectrometer (400 MHz) by using Thermal stability The thermal stability of the BuIPBI, GO/BuIPBI and iGO/ BuIPBI membranes was measured by thermogravimetric analysis (TGA) on a Mettler Toledo TGA/DSC 1/1100 SF apparatus under nitrogen atmosphere at a heating rate of 10 C/ min from 25 C to 800 C.
Acid doping ability and swelling degree of composite membranes A membrane was dried in a ventilated oven at 80 C until its weight did not change any more. Then the membrane was immersed into different concentrations of phosphoric acid solution. After 48 h the phosphoric acid on the membrane surface was removed by a filter paper, and the membrane was put into the ventilated oven again at 80 C until the membrane weight did not change any more. The acid weight ratio in membrane was determined by Eq. (1):
Please cite this article in press as: Xue C, et al., Graphite oxide/functionalized graphene oxide and polybenzimidazole composite membranes for high temperature proton exchange membrane fuel cells, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.03.061
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Acid weight ratio in membrane ¼ ðm1 m0 Þ=m0 100% (1) where m0 is the weight of the dry membrane and m1 is the weight of the acid-doped membrane. The acid swelling ratio of the membrane was calculated by the Eq. (2): Acid swelling ratio ¼ ðV1 V0 Þ=V0 100%
(2)
where V0 is the volume of the dry membrane and V1 is the volume of the acid-doped membrane.
Proton conductivity The proton conductivities of the composite membranes were measured by the two-electrode AC impedance method over the frequency range of 1e1 105 Hz from 80 to 170 C on an electrochemical workstation (Zahner Im6ex, Germany). All measurements were carried out under anhydrous conditions. The proton conductivity s was calculated by Eq. (3) [39]: s ¼ L=ðRAÞ
(3)
where L is the thickness of the membrane, R is the resistance of membrane derived from the low intersection of the high frequency semicircle on a complex impedance plane with the real (Z) axis, and A is the membrane area that contacts the two electrodes [14]. To avoid the situation that the GO and iGO may agglomerate and touch the two electrodes, although the GO and iGO are insulators, we used a spray gun to put BuIPBI/DMAc solution onto the GO/BuIPBI and iGO/BuIPBI composite membranes before putting them into an oven to remove the residual solvent. This method can increase the thickness of a composite membrane by 3e5 mm.
to the stretching vibration of C]N groups. The peaks appearing in the wavenumber range 2800e2900 cm1 are ascribed to the CeH stretching of tert-butyl groups [41]. The absorption peak at 3416 cm1 is due to the stretching vibration of isolated NeH of imidazole groups [42]. Fig. 2 shows the FT-IR spectra of GO and iGO. The GO spectrum shows the characteristic absorption peaks at 1732 and 1620 cm1, corresponding to the stretching modes of C]O and C]C, respectively [43,44]. However, after the GO treated with TBI, the peak at 1732 cm1 for the C]O stretching vibration of GO becomes obscured, and the band at 1645 cm1 is assigned to the amide carbonyl stretching mode. The peak at 1378 cm1 is due to the CH3 deformation vibration, and that at 1041 cm1 is due to the CeOeC stretching vibration. From the 1H NMR spectrum shown in Fig. 3, aromatic protons are observed in the range d ¼ 7.59, and the protons for the tert-butyl group clearly show a sharp peak at d ¼ 1.5. The protons attached to the nitrogen of the imidazole group were observed at d w 13.2, consistent with results from the literature [41].
Dispersibility of GO and iGO The dispersibility of GO and iGO is shown in Fig. 4. Compare with the parent GO, GO treated with tert-butyl isocyanate (iGO) does not disperse at all in H2O. However, the iGO disperses and forms stable colloidal dispersions in polar aprotic solvents such as DMF after a brief ultrasonic treatment. Fig. 4 shows that the vials with GO/DMF and iGO/H2O dispersions have visible precipitates, which indicate poor dispersion. However, the dark brown dispersion of iGO/DMF contains no visible precipitate and is stable for weeks.
Results and discussion
SEM study of composite membranes
Characterization of BuIPBI, GO, and iGO
As can be seen in Fig. 5, the BuIPBI membrane shows a smooth cross section, in agreement with previous reports [45,46]. However, the morphology of the composite membranes is completely different from that of the neat membrane. In the GO/BuIPBI composite membrane with 1 wt% of GO, we can clearly see that the GO particles are evenly
The FT-IR spectrum of BuIPBI is shown in Fig. 1. The absence of the carboxyl peak at 1650e1700 cm1 indicates that the cyclization to form the imidazole ring is complete in BuIPBI [40]. The peaks from 1450 cm1 to 1600 cm1 indicate the existence of the benzene ring. The characteristic peaks at 1618 cm1 is due
Fig. 1 e FT-IR spectrum of BuIPBI.
Fig. 2 e FT-IR spectra of GO and iGO.
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Fig. 3 e 1H NMR spectrum of BuIPBI.
dispersed in the BuIPBI matrix. Meanwhile, the iGO/BuIPBI composite films containing 1e15 wt% of iGO have rough cross sections that could be attributed to the strong interfacial adhesion and good compatibility between the BuIPBI matrix and the iGO sheets [32] (Fig. 6), indicating that a iGO/ BuIPBI membrane is not just a simple mixture of the two components.
Chemical stability Fig. 7 shows the photographs of BuIPBI, GO (1%)/BuIPBI, and iGO (1%)/BuIPBI membranes immersed in 85 wt% PA solution at 80 C for 24 h. Since we wanted to quickly study the chemical stability of these membranes, we did not measure the stability under mild conditions such as room temperature and dilute acid solutions. The 85 wt% PA solution containing the BuIPBI and GO (1%)/BuIPBI membranes become brown and the membranes begin to swell because of their poor chemical stability at high PA concentrations. However, the solution with iGO (1%)/BuIPBI remains colorless and transparent, an indication that iGO can improve the chemical stability of the BuIPBI matrix with the hydrogen bonds in the amide groups. Further studies are still needed to investigate the detailed mechanism.
Acid doping ability and swelling degree of composite membranes
Fig. 4 e Vials containing dispersions (1 mg/mL) of GO in DMF (left), tert-butyl isocyanate treated GO (iGO) in water (middle) and DMF (right). The top image shows the dispersions 24 h after preparation. The bottom image shows the vials turned upside down with the precipitates clearly shown on the bottom of the left and middle vials.
Acid-doped polybenzimidazole has been used as polymer electrolyte in fuel cells [47,48]. Because PA can conduct protons without water and its thermal stability is high, PA has become the most interesting acid. The neat and composite membranes were immersed in a phosphoric acid solution at room temperature, and after 24 h, the weight of the membranes did not increase any more, indicating that equilibrium had been reached between the acid and the membranes [4,14].
Please cite this article in press as: Xue C, et al., Graphite oxide/functionalized graphene oxide and polybenzimidazole composite membranes for high temperature proton exchange membrane fuel cells, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.03.061
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Fig. 5 e SEM images of (aec) BuIPBI membrane and (def) GO/BuIPBI membrane with 1% of GO. Because the neat BuIPBI and GO/BuIPBI membranes can dissolve in 85 wt% PA solutions, we put these membranes into 80 wt% PA solutions while the iGO/BuIPBI membrane was immersed into 85 wt% PA solution. The acid doping levels and swelling degree of BuIPBI, GO/BuIPBI and iGO/BuIPBI composite membranes doped with PA at room temperature are shown in Fig. 8. It can be seen that the acid uptake of the composite membranes decreases with increasing amount of GO or iGO. The factors that affect the acid doping ability of PBI
membranes have been studied before [49]. The concentration of the acid solution and the structure of the PBI are the two main factors affecting the acid doping level. Increasing the acid concentration can increase the acid doping level in the membranes. Chemically, PBI is a basic polymer and can readily be doped with PA. But when GO was introduced to BuIPBI, the alkalinity of BuIPBI would be decreased and therefore affect the acid doping ability. Besides, the GO and iGO as fillers could occupy the interspaces of polymer chains
Fig. 6 e SEM images of (aed) iGO(1%)/BuIPBI, (eeh) iGO(10%)/BuIPBI, and (iel) iGO(15%)/BuIPBI membranes. Please cite this article in press as: Xue C, et al., Graphite oxide/functionalized graphene oxide and polybenzimidazole composite membranes for high temperature proton exchange membrane fuel cells, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.03.061
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Fig. 7 e Photographs showing chemical stability of (a) BuIPBI, (b) GO(1%)/BuIPBI, and(c) iGO(1%)/BuIPBI membranes immersed in 85 wt% PA solution at 80 C for 24 h.
and prevent the PA molecules from penetrating the membranes. The acid swelling ratio of the BuIPBI, GO/BuIPBI and iGO/ BuIPBI membranes were calculated according to Eq. (2). As shown in Fig. 8, the swelling ratios of the iGO/BuIPBI membranes immersed in 85 wt% PA are smaller than those of the GO/BuIPBI membranes, though the acid contents are much higher in the iGO/BuIPBI membranes than in the GO/BuIPBI membranes. This result could be explained by the strong interfacial adhesion between the iGO particles and the BuIPBI matrix.
Proton conductivity of composite membranes The proton conductivities of the PA-doped BuIPBI, GO/BuIPBI and iGO/BuIPBI composite membranes were measured at 80e160 C under anhydrous conditions. Fig. 9 shows the temperature dependence of proton conductivity. It can be seen that the proton conductivity of the PA-doped BuIPBI, GO/ BuIPBI, and iGO/BuIPBI composite membranes increases with increasing the test temperature. The GO/BuIPBI and iGO/ BuIPBI membranes have higher conductivities than the neat BuIPBI membrane. The improvement can be attributed to the introduction of GO or iGO in the BuIPBI matrix. However, the conductivities of the GO/BuIPBI and iGO/BuIPBI composite membranes decrease with increasing temperature as the
Fig. 8 e Acid doping ability and swelling degree of membranes at room temperature.
temperature exceeds 140 C because of the degradation of acidic functional groups like carboxylic acid and epoxy oxygen groups in the GO sheets. The incorporation of GO introduces interconnected proton transfer channels, which can facilitate the transfer of protons through the membrane, as reported by Hwang et al. [50]. Meanwhile, the iGO/BuIPBI composite membranes have higher acid doping levels than those of the neat BuIPBI membrane and GO/BuIPBI composite membranes. A higher acid doping level provides higher proton conductivity. The reason why iGO(10%)/BuIPBI composite membrane was better than the iGO(15%)/BuIPBI composite membrane was that the acid doping level and the iGO concentration in the membrane both influenced the proton conductivity of iGO/BuIPBI composite membranes. When the concentration of iGO in the membrane was too high, the iGO could be agglomerated and blocked the path of proton.
Conclusions A series of composite membranes made from GO or iGO and BuIPBI have been prepared for PEMFC applications. Due to the good dispersibility of iGO in organic solvents, BuIPBI membranes with a highly uniform structure with iGO were
Fig. 9 e Temperature dependence of proton conductivity of the PA-doped BuIPBI, GO/BuIPBI and iGO/BuIPBI composite membranes with different acid doping levels at 80e160 C without humidification.
Please cite this article in press as: Xue C, et al., Graphite oxide/functionalized graphene oxide and polybenzimidazole composite membranes for high temperature proton exchange membrane fuel cells, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.03.061
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prepared. Because of the hydrogen bonding between iGO and BuIPBI, the membranes had high chemical stability. These iGO/BuIPBI membranes with higher stability could absorb more acid but undergo only a small amount of swelling. With a high acid content, the iGO/BuIPBI membranes had high proton conductivities at elevated temperatures. The proton conductivities of BuIPBI, GO/BuIPBI, and iGO/BuIPBI membrane were 0.012, 0.016, and 0.027 S/cm, respectively, at 140 C without humidity.
Acknowledgment The authors gratefully acknowledge the financial supports from the National High Technology Research and Development Program of China (No. 2011AA11A273), the National Natural Science Foundation of China (No. 21176022, 21176023 and 21376022), the International S&T Cooperation Program of China (No. 2009DFA63120 and 2013DFA51860), and the Fundamental Research Funds for the Central Universities (ZY1328), the Program for Changjiang Scholars and Innovative Research Team in University (IRT1205).
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Please cite this article in press as: Xue C, et al., Graphite oxide/functionalized graphene oxide and polybenzimidazole composite membranes for high temperature proton exchange membrane fuel cells, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.03.061