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Synthesis and characterization of polybenzimidazole/ [alpha]-Zirconium phosphate composites as proton exchange membrane.

INTRODUCTION

Proton exchange membrane fuel cells (PEMFCs) have attracted considerable attention because of their high energy efficiency, excellent durability and environmentally friendly. Proton exchange membrane (PEM) which acts as the proton conductor and the anode/cathode separator is one of the key components in PEMFCs system [1]. At present, perfluorinated sulfonic acid membranes, such as Nafion, have some defects for large-scale applications due to their complicated production technology, poor proton conductivity at high temperature and low humidity condition [2-4]. These technical challenges can be overcome when the membranes were applied at elevated temperature and anhydrous environment. During high temperature operation, PEMFCs are desirable to have higher tolerance to significant quantities of CO, simplify water and heat management and enhance the reaction kinetics at both electrodes [5], Therefore, it is necessary to explore the novel PEMs that can achieve excellent performance at high temperature.

Non-fluorinated polymeric membrane materials have been reviewed in a detailed manner with the goal of achieving improved performance above 100[degrees]C. Among them, PBI has a glass transition temperature ([T.sub.g]) of 420[degrees]C and a completely amorphous structure [6], The PBI membrane has excellent chemical stability, good thermochemical and mechanical properties [7, 8]. However, the lower proton conductivity and poor solubility of the PBI membrane limit its application [9, 10], Therefore, further improvement in the PBI membrane performance is required. Phosphoric acid (PA) and [H.sub.2]S[O.sub.4] doped PBI have been the effective modification methods to improve the proton conductivity of PEMs especially at high temperature [11-13], But one of the disadvantages of these membranes is that the proton conductivity of the PA doped membranes strongly depends on the PA doping level. In addition, the excessive acid reduced mechanical and chemical stability and PA would flow away along with the water in electrode reaction, especially at elevated temperature [14].

In recent years, the incorporation of hydrophilic materials can assist in improving proton conductivity, mechanical properties and so on. The composite membranes of PBI with nanoparticles such as silica, zirconia and other ion conductor have been reported [15, 16]. However, this type of composite PEMs can hardly retain water and maintain high proton conductivity above 130[degrees]C [17]. A growing body of research suggests that [alpha]-ZrP is an inorganic proton conductor. It exhibits high ion exchange capacity (6.64 meq [g.sup.-1]) and high thermal stability at temperature up to 300[degrees]C [18-20]. Zirconium atoms connected through the oxygen atoms of the phosphate groups form the layers where the -OH groups are uniformly distributed and could be readily reacted with amines. The proton is transferred from the POH group to nitrogen or by hydrogen bonding throughout the layers of [alpha]-ZrP through an acid-base reaction. Hence, [alpha]-ZrP may allow the composites with higher proton conductivity and thermal stability at high temperature. These indicate that [alpha]-ZrP can be used to enhance the performance of inorganic-organic composite membranes [20].

In the present work, PB1 containing a large number of ether linkages was prepared with 3,3'-diaminobenzidine and 4,4'oxybis (benzoic acid) by the direct polycondensation in polyphosphoric acid to enhance the flexibility and improve the solubility. The stability of the imidazolium was acquired through introducing heteroatoms into para-position of the phenyl substituent of 2-phenyl-imidazolium systems. [alpha]-ZrP was selected as the impregnating material into the PBI polymeric matrix. A series of PBI/ZrP proton exchange composite membranes were prepared by changing the ratios of ZrP doping. The effects of ZrP content on the microstructure, thermal stability, mechanical properties and proton conductivity of composite membrane were investigated systematically. The proton conductivity mechanism of the composite membranes was studied.

EXPERIMENTAL

Materials

3,3'-diaminobenzidine (DAB) was purchased from Aldrich Co. Ltd and stored at 5[degrees]C. 4,4'-oxybis (benzoic acid) (OBBA) and zirconium oxychloride octahydrate (ZrO[Cl.sub.2] x 8[H.sub.2]O) were also purchased from Aldrich Co. Ltd. Polyphosphoric acid (PPA, 85%), phosphorus pentoxide ([P.sub.2][O.sub.5]), dimethylsulfoxide (DMSO) and phosphoric acid were supplied by Shanghai Ling Feng Chemical Reagent Co. Ltd. Hydrochloric acid and anhydrous [Na.sub.2]C[O.sub.3] were obtained from Sinopharm Chemical Reagent Co. Ltd. Deionized water was used for all experiments.

Synthesis of [alpha]-Zirconium Phosphate

[alpha]-ZrP was prepared by the separate nucleation and aging steps (SNAS) method as the literature described [21]. 5.02 g ZrO[Cl.sub.2 x 8[H.sub.2]O was dissolved in 50 mL 2 M hydrochloric acid solution to gain solution A. Then 4 M phosphoric acid and 2 M hydrochloric acid were dissolved in 250 mL deionized water to form the mixed acid solution B. Solution A and solution B were simultaneously added to a beaker, mechanical mixing at 2000 rpm for 5 min. The resulting solution was removed from the beaker and taken into 500-mL four-necked flask containing 20 mL 12 M phosphoric acid. The whole reaction was re fluxed at 90[degrees]C for 72 hr. The final precipitate was filtered, washed thoroughly with deionized water until to the neutral pH value, and dried at 100[degrees]C for 24 hr.

Synthesis of PBI Polymer

The PBI was prepared by the direct polycondensation method as follows. 45 g of PPA and 4.5 g of [P.sub.2][O.sub.5] were added into a 250-mL four-necked round bottom flask equipped with mechanical stirring under nitrogen atmosphere. The mixture was stirred at 80[degrees]C until it dissolved sufficiently, and then cooled down to room temperature. The air bubbles in the system were eliminated. DAB (7.2056 g) and OBBA (5.1648 g) were added into the flask. The reaction system was stirred at 120[degrees]C for 3 hr, at 140[degrees]C for 10 hr, at 170[degrees]C for 2 hr and then at 200[degrees]C for 4 hr until complete polycondensation. Then the polymer was poured into deionized water to get noodle-like product and washed several times with deionized water, then immersed into 5 wt% [Na.sub.2]C[O.sub.3] for 48 hr and vacuum-dried at 100[degrees]C for 16 hr.

Preparation of PBHZrP Composite Membranes

A series of composite membranes with different ZrP doping content were prepared as follows: ZrP (0.000 g, 0.025 g, 0.050 g, 0.075 g, and 0.100 g) was dispersed in DMSO (5 mL) using ultrasonic technique; PBI (0.500 g, 0.475 g, 0.450 g, 0.425 g, 0.400 g) was added into ZrP sol. The system was heated to 60[degrees]C until no polymer particles. In super-clean bench, the polymer solution was placed horizontally on a dust-free glass plate and dried at 80[degrees]C for 24 hr. Then these membranes were immersed in 70 wt% phosphoric acid solution for 48 hr to transform into the acid form and washed with deionized water several times. The products were dried in vacuum oven at 100[degrees]C for 24 hr. These membranes were denoted as PBI, PBI/ ZrP-5%, PBI/ZrP-10%, PBI/ZrP - 15% and PBI/ZrP - 20%.

Measurements

The X-ray diffraction (XRD) pattern was collected on a Rigaku D/max 2500 advance X-ray diffractometer with Cu Ka radiation (40 kV, 50 mA). The particle size distribution was determined using a Malvern Mastersizer 2000 laser particle size analyzer. [sup.1]H-NMR spectra was carried out using an A VANCE III 500MHz instrument and deuterated dimethylsulfoxid (DMSO-d6) as solvent. FT-IR spectra of polymer samples were recorded on a TENSOR27 spectrometer using a KBr disk. The thermal stability of the membranes was measured by thermogravimetric analysis (TGA, Mettler Toledo) performing in nitrogen atmosphere at a heating rate of 20[degrees]C [min.sup.-1] in the temperature range from 40 to 800[degrees]C. Mechanical properties of composite membranes were determined from stress-strain curves using an Instron tensile machine at a strain rate of 5 mm [min.sup.-1] in ambient atmosphere. The size of membrane specimens was 40 mm X 10 mm. All samples were measured three times in experiment period, then took an average. The morphologies of composite membranes were investigated using a Zeiss supra 55 scanning electron microscope (SEM).

The phosphoric acid doping level of PBI/ZrP composite membranes was recorded using dry-wet weight method. The samples were dried in a vacuum oven at 120[degrees]C for 24 hr and measured the weight of dry membranes. The dry membranes immersed in 70 wt% phosphoric acid solution at 80[degrees]C for 24 hr. The acid doped membranes were dried in vacuum at 100[degrees]C for 24 hr and measured their weight. The acid doping level was calculated by Eq. 1:

Acid doping level=([W.sub.acid] - [W.sub.dry])/[W.sub.dry] x 100% (1)

where [W.sub.acid] and [W.sub.dry] are the weights of acid doped membrane and dry membrane.

The swelling ratios of the composite membranes were determined by immersing the dry membranes in deionized water at 25[degrees]C for 24 hr. The changes in dimension before and after the hydration of membranes were measured. The swelling ratio was calculated by Eq. 2:

[DELTA]S = ([S.sub.wet] - [S.sub.dry])/[S.sub.dry] x 100% (2)

where [S.sub.wet] and [S.sub.dry] are the area of wet membrane and dry membrane.

The proton conductivity of composite membranes was measured using a four-probe impedance spectroscopy technique over the frequency range of 1-105 Hz at a voltage of 100 mV from 40 to 160[degrees]C under dry nitrogen flow condition. The membrane was placed in a vacuum oven and dried for 8 h at 150[degrees]C before testing to remove absorbed water and retained at least 30 min at a constant temperature before each of the measurement. The membrane was put on the surface of the two platinum electrodes that was a distance of 1 cm. The equipment was placed into a programmable oven in conjunction with a CHI660D electrochemical workstation and the conductivity was measured dependence of temperature. The proton conductivity was calculated according to Eq. 3 [22]:

[sigma] = L/RWd (3)

where [sigma] is the proton conductivity in S [cm.sup.-1], R is the resistance of the testing sample, W and d denote the width and thickness of the membrane, respectively. L is the distance of two platinum electrodes.

RESULTS AND DISCUSSION

Textural Properties of [alpha]-ZrP Microspheres

Figure 1 illustrates the XRD pattern of [alpha]-ZrP nanoparticles. The diffraction peaks were sharp and narrow which implied the sample had a high degree of crystallinity. Generally, [alpha]-ZrP gives a peak at around 2[theta] = 11.542[degrees], 19.689[degrees], 24.921[degrees] corresponding to d-value of [d.sub.002]= (0.766 nm), [d.sub.110] (0.451 nm) and [d.sub.112] (0.358 nm). No individual peak of zirconium oxychloride and zirconia was observed in the XRD diffraction graphics, and it's similar to the reported refined layered structure of [alpha]-ZrP [23], The particle size distribution of [alpha]-ZrP is presented in Fig. 2. The average particle size of the [alpha]-ZrP was circa 99.5 nm.

Characterization of PBI Polymer

The FT-IR spectra of PBI is shown in Fig. 3 a, there were no absorption bands of the carboxyl group between 1650 and 1700 [cm.sup.-1], indicating a complete reaction of carboxyl functional groups during the polymerization process. The characteristic absorption spectrums at 3415 [cm.sup.-1] and 1632 [cm.sup-1] were due to the stretching vibration of N-H groups in aromatic para ammonia and C=N groups, respectively. The above peaks were identified the existence of benzene ring. The absorption band at 1454 [cm.sup-1] was in-plane bending vibration of the imidazole ring. The bands around 1516 [cm.sup-1] and 1252 [cm.sup-1] were stretching vibration absorption bands of C=C and C-O-C band of aromatic ether. This characterization suggested that PBI polymer was preliminary synthesized.

Figure 3b shows [sup.1]H-NMR spectra of PBI polymer in DMSO-d6, together with the structure and the assignment of the signals. The peaks at 9.0-7.0 ppm (b, c, d, e, f H) were assigned to the proton on the aromatic groups. PBI gave [sup.1]H-NMR chemical shifts of 13.0 (a H) representing the imidazole proton signals, which is in good agreement with the previous reports [24]. It illustrated that PBI was successfully synthesized by the polycondensation.

Morphology of PBI/ZrP Composite Membranes

The microscopic morphology and the elemental composition of composite membranes were provided by SEM and energy dispersive spectroscopy (EDS). Figure 4a shows the SEM morphology of cross-section of PBI/ZrP-10% composite membrane. The cross-section micrograph presented a reticular microstructure. The grain size of ZrP was about 100 nm which was well distributed in the composite membrane. Figure 4b shows the EDS mapping of zirconium element. In the mapping image, the bright dots manifested the high concentration of the zirconium element. The uniform distribution of zirconium element throughout the PBI/ZrP composite membrane was observed in Fig. 4b. Figure 4c presents the EDS spectrum of PBI/ZrP composite membrane. The Zr and P peak at the EDS spectrum confirmed the presence of ZrP particles in the membrane.

FT-IR Of PBI/ZrP Composite Membranes

The FT-IR spectra of PBI/ZrP composite membranes can be seen from Fig. 5. The characteristic peak at 1242 [cm.sup.-1] was P-OH band. The absorption band at 530 [cm.sup.-1] was the deformation vibration of the P[O.sub.2], and the absorption peaks between 1000 and 1200 [cm.sup.-1] was stretching vibration of P04. The above results indicated that ZrP was successfully introduced into the membranes. Compared with PBI, some peaks of PBI/ ZrP composite membranes became weak and even disappeared with the increasing of ZrP content, which demonstrated that the interaction between ZrP and PBI changed the structure of PBI.

Thermal Analysis of PBI/ZrP Composite Membranes

The thermal stability of the composite membranes doped with phosphoric acid was investigated by thermogravimetric analysis (TGA), which was displayed in Fig. 6. The first weight loss around 50-120[degrees]C were ascribed to the desorption of the absorbed free water because of the hygroscopic nature of PBI. A continuous weight loss started at about 200[degrees]C was attributed to the thermal changes in phosphoric acid which was the autopolymerisation of the -P[O.sub.4] units to pyrophosphoric ([P.sub.2][O.sub.7]) and triphosphoric (-[P.sub.3][O.sub.10]) ones [25, 26], The degradation of PBI main chain occurred over 550[degrees]C. It is clear that the membranes weight loss get less with increasing ZrP doping level. At 800[degrees]C, the total weight loss of PBI/ZrP composite membranes was 35% lower than that of PBI membrane, which further demonstrated the introduction of ZrP improved the thermal stability of the membranes. These results indicated that the PBI/ZrP composite membranes with high thermal stability are sufficient for use in HT-PEMFC.

Mechanical Properties of PBI/ZrP Composite Membranes

Appropriate mechanical properties are necessary because PEMs perform three functions, including supporting the catalyst, isolating the oxygen and hydrogen, and transporting protons [27], The mechanical properties of composite membranes are listed in Table 1. The membranes exhibited a tensile strength of 42.7-67.4 MPa and an elongation at break of 30.3%-42.0%, respectively. Increasing ZrP doping level resulted in an increase in the tensile strength. The tensile strength of the PBI/ZrP-20% composite membrane increased by 57.8% compared with that of PBI membrane. However, the elongation at break of the composite membranes decreased slightly which might result from the formation of the network structure and the presence of ZrP nanoparticles limited the motion of the PBI main chain. Accordingly, all the membranes displayed strong mechanical properties which were suitable for fuel cell applications.

The Acid Doping Level and Swelling Ratio of PBI/ZrP Composite Membranes

The acid doping level of the membranes plays an important role in proton conductivity and mechanical properties [28]. The acid doping level as a function of the ZrP doping level at 80[degrees]C are shown in Fig. 7. The acid doping level of PBI/ZrP composite membranes was observed to be 98.7%-109.1%. The acid doping level decreased with the increase in ZrP content. The reason might arise from the amino groups in PBI were blocked with -OH groups of ZrP which resulted in decreasing the interaction of phosphoric acid and PBI chains. But the low acid doping level led to a small improvement in mechanical properties because of avoiding excessive acid swelling.

The swelling ratio of the composite membrane is a crucial parameter to evaluate its dimension stability. The swelling of membrane in water would lead to the poor mechanical property affecting the long-term performance of membrane. Figure 7 shows the swelling ratios of PBI/ZrP composite membranes as a function of ZrP doping level. It could be seen that the swelling ratio of the composite membrane increased as the ZrP content increased. But the swelling ratios were in the range of 3.1%-5.9%. The results demonstrated that the dimensional change of the composite membrane was relatively low, which confirmed that the composite membrane showed better dimensional stability.

The Proton Conductivity of PBI/ZrP Composite Membranes

The proton conductivity is considered as one of the most important properties in the fuel cell performance. The proton conductivities of the PBI/ZrP proton exchange composite membranes at different temperatures (40-160[degrees]C) under anhydrous condition were displayed in Fig. 8. It could be clearly seen that the proton conductivity for all the membranes increased with the temperature increasing. Among them, PBI/ZrP-10% showed the highest proton conductivity, which increased from 0.108 to 0.192 S [cm.sup.-1] when the temperature varied from 40 to 160[degrees]C under anhydrous condition. The high proton conductivity can be attributed to the ZrP and [H.sub.3]P[O.sub.4] dopant in some degree. The proton transfer was followed by Grotthuss mechanism. The proton transport in the membrane can occur from protonated guest molecules to a non-protonated neighbor molecule or from the neighboring proton to the proton defect site [29, 30]. In our study, the P-OH group of ZrP and nitrogen atoms of PBI imidazole rings can behave as both proton donor and acceptor in which the proton transfers from proton donor to an acceptor. [H.sub.3]P[O.sub.4] molecules can also form the protonic defect site (e.g., [H.sub.2]P[O.sub.4]). The neighboring proton can hop to the protonic defect site along the hydrogen bond.

In these acid-base composite materials, the activation energy of proton transport depend on the distance between the hopping sites [29]. The Arrhenius plots of proton conductivity for PBI, PBI/ZrP-10%, PBI/ZrP-20% composite membranes are shown in Fig. 9. The Arrhenius formula for a hopping-like conduction mechanism was calculated as follows:

ln [sigma] = ln A + {-[E.sub.a])/RT (4)

where [sigma] is proton conductivity, A is the frequency factor, [E.sub.a] is the activation energy, R is the molar gas constant, and T is the absolute temperature.

The activation energy of 5.47-6.00 kJ [mol.sup.-1] was calculated from the slopes of these curves. This value is lower than ZrP whose activation energy is in a range of 30-50 kJ [mol.sup.-1] [31]. It illustrated that the proton in PBI/ZrP composite membranes can rapidly transfer to neighboring molecule with small activation energy. But the proton conductivity of the PBl/ZrP-10% composite membrane was higher than that of 15% and 20% ZrP. The decrease in proton conductivity is probably due to the reduced acid doping level. As presented in Fig. 7, the acid doping level of the membranes decreased with increasing the ZrP content. The PBI/ZrP-20% membrane displayed the lowest acid doping level. This disrupts the hydrogen bond structure of the phosphoric acid, which interrupts the continuity of the proton transfer paths through the membrane and results in decrease in proton conductivity. In addition, the excessive amounts of ZrP will lead to a blocking effect of ZrP particles. It must be noticed that although the PBI/ZrP-20% membrane has more ZrP content, which probably could not participate in the proton conductive process. Therefore, the PBI/ZrP-20% membrane displayed the lowest proton conductivity.

CONCLUSIONS

PBI doped with ZrP (0%, 5%, 10%, 15%, and 20%) proton exchange composite membranes have been successfully prepared. The FT-IR and SEM analyses of the composite membranes confirmed the presence of solid ZrP. The composite membrane presented a reticular microstructure. The thermal stability improved with an increase of ZrP and the thermal degradation temperature was up to 550[degrees]C. They exhibited higher mechanical strength and good elongation properties, which may ensure the stability of the battery. The tensile strength and elongation at break were 54.6 MPa and 38.6% for PBI/ZrP-10% composite membrane. The proton conductivity of composite membranes increased with the temperature increasing. The proton conductivity of PBl/ZrP-10% composite membrane reached 0.192 S [cm.sup.-1] at 160[degrees]C under anhydrous condition. Compared with PBI/ZrP membrane, the proton conductivity of Nafion 112 in deionized water was 0.1 S [cm.sup.-1] at 60[degrees]C and 100% RH. However, at 30% RH, the proton conductivity of Nafion 112 was 0.005, which became rather low. These results indicated the PBI/ZrP composite membranes showed excellent proton conductivity compared with Nafion membrane at high temperature or low humidity condition. The proton conducting mechanism of the PBI/ZrP composite membranes was proposed to explain the proton transport phenomena. These properties enabled PBI/ZrP proton exchange composite membranes to be suitable for use in HT-PEMFC under anhydrous or low humidity conditions.

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Qi Zhang, (1) Hui Liu, (1) Xun Li, (1) Rong Xu, (1) Jing Zhong, (1) Ruoyu Chen, (1) Xuehong Gu (2)

(1) Jiangsu Key Laboratory of Advanced Catalytic Materials and Technology, School of Petrochemical Engineering, Changzhou University, Changzhou, Jiangsu 213164, People's Republic of China

(2) State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing University of Technology, Nanjing 210009, China

Correspondence to: J. Zhong; e-mail: zjwyz@cczu.edu.cn

Contract grant sponsor: National Natural Science Foundation of China; contract grant number: 21276029, 21306012; contract grant sponsor: Natural Science Foundation of Jiangsu; contract grant number: BK20131142; contract grant sponsor: Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), Jiangsu Key Laboratory of Advanced Catalytic Materials and Technology; contract grant number: BM2012110; contract grant sponsor: Advanced Catalysis and Green Manufacturing Collaborative Innovation Center, Changzhou University, Jiangsu Overseas Research & Training Program for University Prominent Young & Middle-aged Teachers and Presidents, the State Key Laboratory of Materials-Oriented Chemical Engineering.

DOI 10.1002/pen.24287

TABLE 1. The mechanical properties of composite membranes.

                 Tensile        Elongation
Membrane      strength (MPa)   at break (%)

PBI                42.7            42.0
PBI/ZrP-5%         51.3            39.0
PBI/ZrP-10%        54.6            38.6
PBI/ZrP-15%        59.1            37.3
PBI/ZrP-20%        67.4            30.3
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Author:Zhang, Qi; Liu, Hui; Li, Xun; Xu, Rong; Zhong, Jing; Chen, Ruoyu; Gu, Xuehong
Publication:Polymer Engineering and Science
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Date:Jun 1, 2016
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