zwitterion-coated polyamidoamine dendrimer composite membranes for direct methanol fuel cell applications

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 3 8 ( 2 0 1 3 ) 1 6 4 1 0 e1 6 4 1 7

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Polybenzimidazole/zwitterion-coated polyamidoamine dendrimer composite membranes for direct methanol fuel cell applications ZongZong Gu a, Jianning Ding a,b,*, Ningyi Yuan a,b,*, Fuqiang Chu a, Bencai Lin a,b a Center for Low-Dimensional Materials, Micro-Nano Devices and Systems, Jiangsu Key Laboratory for Solar Cell Materials and Technology, Changzhou University, Changzhou, 213164 Jiangsu, China b Jiangsu Collaborative Innovation Center of Photovolatic Science and Engineering, Changzhou, 213164 Jiangsu, China

article info

abstract

Article history:

The zwitterion-coated polyamidoamine (ZC-PAMAM) dendrimer with ammonium and

Received 24 June 2013

sulfonic acid groups has been synthesized and used as filler for the preparation of PBI-

Received in revised form

based composite membranes for direct methanol fuel cells. Polybenzimidazole (PBI)/ZC-

27 August 2013

PAMAM dendrimer composite membranes were prepared by casting a solution of PBI and

Accepted 22 September 2013

ZC-PAMAM dendrimer, and then evaporating the solvent. The presence of ZC-PAMAM

Available online 18 October 2013

dendrimer was confirmed by FT-IR and energy-dispersive X-ray spectroscopy (EDS) mapping of sulfur and oxygen elements. The water uptake, swelling degree, proton conduc-

Keywords:

tivity, and methanol permeability of the membranes increased with the ZC-PAMAM

Zwitterion-coated polyamidoamine

dendrimer content. For the PBI/ZC-PAMAM-20 membrane with 20 wt% of ZC-PAMAM, it

dendrimer

shows a proton conductivity of 1.83
102 S/cm at 80  C and a methanol permeability of

Composite membrane

5.23
108 cm2 s1. Consequently, the PBI/ZC-PAMAM-20 demonstrates a maximum power

Methanol permeability

density of 26.64 mW cm2 in a single cell test, which was about 2-fold higher than Nafion-

Direct methanol fuel cell

117 membrane under the same conditions. Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

1.

Introduction

Fuel cells are efficient and clean electrochemical power source that convert the chemical energy directly into electricity via electrochemical reaction of fuels and oxygen [1]. Among the

different types of fuel cells, direct methanol fuel cells (DMFCs) have been received much attention because of their high power density, the ease of handling operating conditions, and possible applications in micro fuel cells [2,3]. Proton exchange membrane (PEM) is a key component in DMFC for transferring

* Corresponding authors. Center for Low-dimensional Materials, Micro-nano Devices and System, Changzhou University, Changzhou 213164, China. Tel./fax: þ86 0519 86450008. E-mail addresses: [email protected] (J. Ding), [email protected], [email protected] (N. Yuan). 0360-3199/$ e see front matter Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2013.09.131

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protons from anode to cathode while providing a barrier to fuel crossover between the electrodes. Nafion membranes (DuPont) have long been the standard polymer for use in DMFCs due to their excellent proton conductivity and high chemical durability [4,5]. However, the high methanol permeability decreases the fuel cell efficiency and causes poisoning of the catalyst at the cathode limits the Nafion membranes further commercial application in DMFCs [6]. Therefore, new types of PEMs with high proton conductivity and low methanol crossover is in great demand to promote the development of DMFCs. In recent years, new types of PEMs based on variety of polymer materials have been synthesized and studied [7e10]. Among the various polymers that have been reported, polybenzimidazole (PBI) has been recognized as a good choice for preparation of PEMs because of its excellent chemical stability, good thermochemical and mechanical stabilities [11,12]. However, the proton conductivity of PBI should be enhanced because it has no proton conducting ability. Sulfonated polymers with the sulfonic acid groups has become an important approach in enhancing proton conductivity of the PEMs by increasing the water uptake and rendering its ion-exchange properties [13]. A high degree of sulfonation improves the proton conductivity, whereas lead to a high swelling ratio which result in the loss of the mechanical strength and the increase of the methanol permeability of PEMs [14]. Replacement of water with nonaqueous proton carriers such as phosphoric acid, is another simple and powerful method to improve the proton conductivity of PEMs especially at high temperature. However, the performance of fuel cells could be declined by the phosphate anion adsorption on the platinum surface [15]. The performance of fuel cells also could be affected by the release of the phosphoric acid from PEMs [16]. Recently, composite membranes with hydrophilic inorganic fillers have attracted much attention due to the low methanol permeability and high thermal stability [17,18]. Generally, a low content of organic groups of the inorganic fillers is disadvantageous for homogenous and conductive PEMs. To overcome these problems, inorganic fillers with chemical modification have been prepared and used as fillers in PEMs [19]. Chu et al. reported a polybenzimidazole/ H3PO4/zwitterion-coated silica nanoparticle hybrid membranes, and the zwitterion-coated silica nanoparticles showed high compatibility with the polymeric matrix [16]. More recently, zwitterionic cross-linking agent with sulfonic acid and amine groups was synthesized for preparation of PEMs by Shahi et al. [20], and the zwitterionic hybrid PEMs showed high proton conductivity and low methanol crossover. These results suggest that addition of zwitterioncoated fillers can be a promising process to obtain the desired PEMs for DMFCs. In the present work, zwitterion-coated polyamidoamine (ZC-PAMAM) dendrimer with high concentration of sulfonic acid and amine groups was synthesized and used as fillers of the PBI-based PEMs. PBI/ZC-PAMAM dendrimer composite membranes were prepared by simple solution casting method. The influence of ZC-PAMAM dendrimer content on the membrane properties was systematically described. The

DMFC single-cell performance with the composite membranes was also demonstrated.

2.

Experimental

2.1.

Materials

2,2-Bis (4-carboxy-phenyl) hexafluopropane was purchased from TCI and purified by recrystallization from glacial acetic acid. 3,30 -Diaminobenzidine, 1,2-diaminoethaneanhydrous, 2propenoizcidmethylester, 1,3-propane sultone, poly (phosphoric acid), 1,2-diaminoethane anhydrous, 3,30 -diaminobenzidine, methanol, 2-propenoizcidmethylester and poly (phosphoric acid) (PPA) was used as received. N,Ndimethylaceramide (DMAc) and dimethylsulfoxide (DMSO),were purified by distillation. Distilled deionized water was used for all experiments.

2.2. Synthesis of zwitterion-coated polyamidoamine dendrimer Ethylenediamine core PAMAM G4.0 was synthesized as documented in previous literatures [21]. As shown in Scheme 1B, the ZC-PAMAM dendrimer was synthesized as follows: excess 1,3-propane sultone/DMSO solution was slowly added to PAMAM G4.0/DMSO solution at 0  C, and then the mixture was stirred at room temperature for 48 h. The resultant product was washed with acetone twice and then dried in vacuum at 60  C for 24 h. 1H NMR (500 MHz, D2O), d: 3.30 (t, eNCH2Ce), 2.81 (br t, eCCH2COe), 3.32 (br t, eNCH2Ce), 3.48 (br t, eCONCH2Ce), 3.59 (t, eCCH2Ne), 2.59 (br t, eCCH2Ce). CHNS: calcd (C, 44.20; H, 7.40; N, 15.81; S, 9.48); obsd (C, 43.23; H, 6.58; N, 17.08; S, 10.37).

2.3.

Preparation of PBI

PBI was synthesized via the condensation polymerization of 2bis(4-carboxyphenyl) hexafluoropropane and 3,3diaminobenzidine in PPA at 170  C, as shown in Scheme 1A [16].

(A) H2N H2N

NH2 + HOOC NH2

CF3 C CF3

PPA COOH 170 oC

N

H N

N H

N

CF3 C CF3

n

PBI

(B) +

O S O O

DMSO 50 oC

PAMAM G4.0 ZC-PAMAM

PBI DMAc

Scheme 1 e Reaction scheme for the preparation of PBI and PBI/ZC-PAMAM dendrimer membranes.

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2.4. Preparation of PBI/ZC-PAMAM dendrimer composite membranes

IECe ¼

The composite membranes were prepared as follows: a mixture containing 3 wt% PBI in DMAc solution and ZCPAMAM dendrimer was vigorously stirred for 0.5 h at room temperature. Membranes with a thickness of about 50 mm were obtained by casting the homogeneous solution onto glass plates with a doctor’s knife and then dried at 70  C for 24 h.

2.5. Characterization of PBI/ZC-PAMAM dendrimer composite membranes Fourier transform infrared (FT-IR) spectroscopic measurements were performed using a Varian CP-3800 spectrometer in the range of 4000e400 cm1. The thermal stability of composite membrane was evaluated by using a Thermal Analysis (TA) Universal Analysis 2000. The samples were scanned at a heating rate of 10  C/min under a nitrogen flow. Scanning electron microscopy was performed on a field-emission SEM with a Hitachi S-4800 microscope with an accelerating voltage of 20 kV. The tensile properties of membranes were measured using an Instron Model 3365 device at 25  C at a crosshead speed of 5 mm/min.

2.6. Water uptake, swelling ratio and ion-exchange capacity The water uptake (WU) and swelling ratio (SR) of the composite membranes were determined by measuring the changes in the weight and dimension before and after the hydration of membranes. The WU and SR of the membranes were calculated with the following equations: ðWs  Wd Þ
100% WUð%Þ ¼ Wd SRð%Þ ¼

ðLs  Ld Þ
100% Ld

(1)

(2)

where Ws and Wd are the weights of wet and dry membranes, Ls and Ld are the length of wet and dry membranes, respectively. The ion exchange capacity (IEC) was measured by classical titration (IECt). The membrane was soaked in 1.0 M HCl solution to convert the composite membrane into Hþ form. The excess HCl in the membrane was washed with distilled water. Then the membrane was equilibrated in a certain volume of 1.0 M NaCl for 24 h to replace Hþ ions in the membrane with Naþ ions. Then the remaining solution was titrated with a 0.1 M NaOH solution using phenolphthalein as an indicator. IECt were calculated using the following equation [22]: IECt ¼

CNaOH VNaOH Wd

(3)

where VNaOH and CNaOH are the volume and concentration of the NaOH solution, respectively. IEC by the elemental analysis method (IECe) was calculated from the sulfur content obtained from elemental analysis using the equation [22]:

1000S A

(4)

where S is the sulfur content determined by elemental analysis; A is 32 g mol1 (the atomic weight of sulfur).

2.7.

Methanol permeability

The methanol permeability of the composite membranes was determined using a two-chamber glass diffusion cell as described in previous literature [23]. The composite membranes were soaked in deionized water for 24 h before being clamped between the two compartments. One compartment of the cell was filled with 5 M methanol solution (compartments A), and the other one was filled with deionized water (compartments B). In each compartment, a magnetic stirrer was used to ensure solution uniformity throughout the measurements. The change of methanol concentration in the water compartment with time was monitored using a gas chromatograph (Nicolet 6700, Thermo). The methanol permeability was determined from the slope of the plot of methanol concentration in the water compartment versus time. The methanol diffusion coefficient P was determined from the following equation: CB ðtÞ ¼

Ae P CA ðt  t0 Þ VB L

(5)

where Ae is the effective membrane area, L is the thickness of membrane, P is the methanol permeability, t0 is the measurement time lag, VB is the volume of compartment B, CA and CB are the concentration of methanol in compartment A and B, respectively.

2.8.

Proton conductivity

The resistance of the membranes was calculated from the electrochemical impedance spectroscopy (EIS) over a frequency from 1 Hz to 1 MHz. The hydrated membranes were sandwiched between two gold electrodes in a glass cell and equilibrated in water vapor at experimental temperature. The proton conductivity of the composite membranes was calculated from the following equation: s¼

L RAcs

(6)

where s is the proton conductivity in S cm1, R is the ohmic resistance of the membrane, L is the distance between reference electrodes, and Acs is the cross-sectional area of the membrane.

2.9.

Direct methanol fuel cell tests

Catalyst ink was prepared by dispersing the unsupported PteRu (50:50 wt%) and Pt black (Johnson Matthey Co.) with a mixture of deionized water, 5 wt% Nafion (EW 1100, Dupont) ionomer solution and isopropyl alcohol. The catalyst layers of PteRu black (anode) and Pt black (cathode) were applied to the blend membranes. The content of the catalyst loading was 4 and 3 mg cm2 for the anode and cathode, respectively. The membrane electrode assembly (active geometric area of 4 cm2) was prepared by sandwiching PBI-based membrane and Nafion-117 between two Etek ELAT electrodes. The fuel cell test was operated at 45  C with 5 M methanol. Steady-state current

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 3 8 ( 2 0 1 3 ) 1 6 4 1 0 e1 6 4 1 7

Fig. 1 e (A) SEM images of the plane surface of PBI/ZCPAMAM-20, (B) distribution of sulfur and (C) oxygen elements in the PBI/ZC-PAMAM-20 composite membranes, as obtained by using SEM and EDS mapping.

density/voltage data were recorded using a single cell test station (Scribner Series 890B). Before the performance testing, the single DMFC was held at constant conditions for 1e2 h.

ZC-PAMAM dendrimer was confirmed by 1H NMR spectra and elementary analysis (experimental section). Compared with other PBIs, fluorine-containing PBI shows better processability and oxidative-stability [24,25]. Therefore, fluorine-containing PBI was chosen as a polymer matrix for the preparation of composite membranes (Scheme 1A). Polybenzimidazole (PBI)/ ZC-PAMAM dendrimer composite membranes were prepared by casting a solution of PBI and ZC-PAMAM dendrimer, and then evaporating the solvent, as shown in Scheme 1. The composite membranes were denoted as PBI/ZC-PAMAM-X, where X represented the weight percentage of ZC-PAMAM dendrimer in the membranes. The composite membranes are semitransparent, and no obvious phase separation was observed from the photograph as shown in Scheme 1B, indicating a good compatibility of fluorine-containing PBI and ZCPAMAM dendrimer. SEM and a spectrum obtained by mapping with energy dispersive spectroscopy (EDS) provided information about the surface morphology and the elemental composition of the membrane on a micrometer scale. The SEM micrograph of PBI/ ZC-PAMAM-20 shows that the synthesized films were homogeneous and formed a dense membrane (Fig. 1A). Taking into account that ZC-PAMAM dendrimer contained sulfur and oxygen, it was decided to perform a mapping of these elements (Fig. 1B and C). The EDS mappings of sulfur and oxygen of the membranes indicate that both sulfur and oxygen are uniformly distributed throughout the membrane, confirming a homogeneous dispersion of ZC-PAMAM in the polymer matrix. FT-IR spectroscopy measurement was used to confirm the structures of ZC-PAMAM dendrimer and the composite membranes. Fig. 2 shows the FT-IR spectra of ZC-PAMAM dendrimer, pristine PBI and PBI/ZC-PAMAM-20, respectively. As shown in Fig. 2A, absorption peaks at around 3200 cm1 arise from a vibrational mode of NeH groups, and the absorption band of self-associated hydrogen-bonded NeH/N groups at approximately 3060e3450 cm1 [16,26]. The absorption bands present at 1188 cm1 and 1043 cm1 corresponds to the stretching vibration of sulfonic acid groups. Peaks in the regions of 2938 cm1 and 2849 cm1 indicate the presence of methylene groups in ZC-PAMAM dendrimer. Compared with Fig. 2B, the new bands corresponding to ZCPAMAM dendrimer are observed at 2923, 2851 and 1191 cm1 in Fig. 2C. The comparison between the IR spectrum of PBI/ZCPAMAM-20 membrane and that of pure PBI membranes further confirmed the existence of ZC-PAMAM dendrimer in the composite membrane.

3.2. Properties of PBI/ZC-PAMAM dendrimer composite membranes 3.2.1.

3.

Results and discussion

3.1. Characterizations of PBI/ZC-PAMAM dendrimer composite membranes The ZC-PAMAM dendrimer was synthesized by the reaction of 1,3-propane sultone and PAMAM G4.0, and the structure of

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Mechanical properties

Adequate mechanical integrity is essential for PEMs to withstand the fabrication of the membrane electrode assembly. The mechanical properties of composite membranes, pure PBI and Nafion-117 were measured using a tensile testing instrument at room temperature, and the results are showed in Table 1. The composite membranes in dry state had tensile stress at maximum load of 56.33e72.81 MPa, tensile modulus of 1.35e1.73 MPa and elongation at break of 28.1e16.31%.

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Fig. 2 e FT-IR spectra of (A) ZC-PAMAM dendrimer, (B) pristine PBI and (C) PBI/ZC-PAMAM-20 composite membranes. Compared to Nafion-117, the produced composite membranes show a lower tensile modulus and a higher tensile strength. However, the composite membranes showed the improved tensile strength and tensile modulus compared with the pristine PBI membrane. The elongation at break decreased with the introduction of ZC-PAMAM dendrimer probably due to ZC-PAMAM dendrimer restricted the motion of PBI chain segmental. The mechanical properties of PBI/ZC-PAMAM-X membranes in dry states showed they were strong and flexible enough to be used as PEM materials.

3.2.2.

Thermal analysis

Though the operating temperature is mild for the DMFCs, the fabrication of the membrane electrode assemblies still needs the membrane material has good thermal property [13]. The thermal stability of the composite membrane was evaluated through the thermogravimetric analysis (TGA) experiments. The TGA results for the pure PBI, ZC-PAMAM dendrimer and the composite membranes are shown in Fig. 3. In ZC-PAMAM dendrimer, an obviously weight loss around 100e150  C assigned to the evaporation of absorbed water, which indicating the hygroscopic nature of the ZC-PAMAM dendrimer. The decomposition temperature occurred at about 250  C was assigned to the decomposition of sulfonate groups. The slight weight loss around 100e200  C for all the membranes was attributed to residual solvent (DMAc) and water release from membranes. A major weight loss at about 510  C is due to the degradation of the PBI main chain. Though the introduction of

the ZC-PAMAM dendrimer had slightly effected on the thermal stability of the composite membranes, the composite membranes had a compatible thermal stability which was suitable for the application in DMFCs.

3.2.3. Water uptake, swelling ratio and ion-exchange capacity The water uptake has a major effect on proton conductivity, mechanical and dimensional properties of PEMs [27]. Presence of water in the membrane favors the proton conductivity of PEMs [28]. However, high water uptake usually resulted in a loss of mechanical properties and a decline of methanol resistance of PEMs [29]. Table 2 shows water uptake and swelling ratio of the composite membranes and Nafion-117 at 25  C and 80  C. The pure PBI membrane shows a lower water uptake at 80  C than that at 25  C, which might be caused by the shrinkage of PBI chains at a high temperature [17]. The water uptake of composite membranes increased with increasing the content of ZC-PAMAM dendrimer. Similar results have been observed for PVA/zwitterionic silica hybrid

Table 1 e Mechanical properties of PBI-based composite membranes (Temp. 25  C and 40e45% RH). Membrane

Tensile strength (MPa)

Tensilemodulus Elongation at (MPa) break (%)

PBI PBI/ZCPAMAM-5 PBI/ZCPAMAM-10 PBI/ZCPAMAM-20 Nafion-117

56.33  0.75 57.21  0.98

1.35  0.23 1.51  0.17

28.31  1.42 21.13  2.33

63.12  0.37

1.57  0.29

19.88  2.05

72.81  0.75

1.73  0.31

16.31  1.97

21.11  1.05

6.60  1.35

370.62  22.03

Fig. 3 e TGA plots of pristine PBI, ZC-PAMAM dendrimer and the PBI-based composite membranes under the nitrogen flow. Heating rate: 10  C/min.

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Table 2 e Water uptake (WU), swelling ratio (SR) and ion-exchange capacity (IEC) of the PBI/ZC-PAMAM dendrimer composite membranes and Nafion-117. Membranes

WU (%) 

PBI PBI/ZC-PAMAM-5 PBI/ZC-PAMAM-10 PBI/ZC-PAMAM-20 Nafion-117

SR (%) 



25 C

80 C

25 C

80 C

16.2 23.3 30.4 37.8 26.0

12.6 30.7 39.7 47.9 32.2

1.34 2.37 3.86 5.87 6.02

1.06 3.52 6.19 8.61 7.91

membranes [20] and PEG/POSS-SO3H composite membranes [30]. The increase of the water uptake of the composite membranes must due to the hygroscopic nature of the chemical functionalized inorganic fillers. Just as expected, the change of swelling ratio of the PBI-based membranes the same as the water uptake with the increment of the ZCPAMAM dendrimer content. It is worth noting that the water uptake of PBI/ZC-PAMAM-20 is 37.8% at 25  C, while the value for Nafion-117 membrane is 26.0% at the same condition. Considering that the high water uptake of the composite membranes, the dimensional change of the composite membranes is relatively low (5.87% for PBI/ZC-PAMAM-20 at 25  C). This is probably due to the introduction of ZC-PAMAM dendrimer which restricted the motion of PBI chain as well as the free volume in the composite membrane. Generally, IEC values are responsible for proton transfer and thus are an indirect and a reliable approximation of the proton conductivity. For the composite membrane in the present work, the ion exchangeable groups were sulfonic acid groups. The IEC values of the composite membranes were are listed in Table 2. As the ZC-PAMAM dendrimer content increased from 5 to 20 wt%, the IECt obtained by titrated method and IECe estimated by elemental analysis were found to be 0.13e0.57 mequiv/g and 0.15e0.59 mmol/g, respectively. In the composite membranes, only accessible acidic groups could be estimated by the titration method, hence the value of IECt was lower than that of IECe [22].

3.3.



mequiv/g IECt

mmol/g IECe

e 0.13 0.28 0.57 0.91

e 0.15 0.30 0.59

channels for protons and an increase in the mobility of protons in the water phase inside the composite membranes [30]. Moreover, a high surface charge density and considerable sulfonic acid and amine groups of the ZC-PAMAM dendrimer are beneficial to formation hydrogen bond between water molecules and ZC-PAMAM dendrimer which is in favor of proton transport [31,32]. Compared with the composite membranes, Nafion 117 show a higher proton conductivity probably due to the unique hydrophilicehydrophobic phase segregated structure of Nafion [33]. In addition, the conductivities of composite membranes increased with the increasing of temperatures. For example, the conductivity of PBI/ZC-PAMAM-20 was 1.56
102 S/cm at 25  C, and increased to 1.83
102 S/cm at 80  C.

3.4.

Methanol permeability

An ideal PEM should have low methanol permeability because high methanol permeability could decrease the efficiency of DMFCs. Fig. 5 shows the methanol permeation of the composite membranes and Nafion 117. It has already been reported that the methanol transport behavior of PEMs is dependent on its water uptake, the degree of swelling, and the microstructure of the membrane [23]. As shown in Fig. 5, the methanol permeability of the PBI/ZC-PAMAM dendrimer composite membranes increased with increasing the amounts of ZC-PAMAM dendrimer. The methanol

Proton conductivity

Proton conductivity is an important property of PEMs and it also plays an important role on development of fuel cells with good performance. According to the mechanisms proposed for the conductivity, protons can be transported only in water in the present work, and the conductivity is significantly influenced by the water content of the composite membranes. Here, the proton conductivity of the composite membranes was measured with an alternating current impedance spectroscopy in a closed cell at RH 100%. For comparison, the conductivity of Nafion-117 was also measured under the same conditions. Fig. 4 shows the reproducible plot of proton conductivity of Nafion-117 and the composite membranes with different content of ZC-PAMAM dendrimer. As expected, the conductivity of composite membranes was increased as the content of ZC-PAMAM dendrimer increases from 5 to 20%. The proton conductivities of composite membranes were in the range of 0.91e1.83
102 S/cm. The water uptake of composite membranes was increased with increase the content of ZC-PAMAM dendrimer. Water cluster can offer transport

Fig. 4 e Conductivity Arrhenius plots of the PBI/ZC-PAMAM dendrimer composite membranes and Nafion-117.

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were reported by Zhang et al. [34]. The composite membrane of PBI/ZC-PAMAM-20 showed 26.64 mW cm2 maximum power density, whereas for Nafion-117 membrane showed 12.24 mW cm2. Good performance of PBI/ZC-PAMAM-20 was obtained due to the low methanol permeability in spite of its lower proton conductivity compare with Nafion 117 [20]. These results indicated that the PBI/ZC-PAMAM dendrimer composite membranes were suitable for DMFC applications.

4.

Fig. 5 e Methanol permeability of PBI/ZC-PAMAM dendrimer composite membranes and Nafion-117. permeability of the pure PBI membranes was 1.37
108 cm2 s1, reaching a value of 5.23
108 cm2 s1 under the same experimental condition for PBI/ZC-PAMAM20. This tendency was the same as water uptake due to the more water uptake favors the formation of channels to pass methanol molecules [23]. Under the same experimental conditions, methanol permeation of Nafion-117 membrane was measured for comparison (Fig. 5). Compared to Nafion 117 (1.23
106 cm2 s1), the composite membranes showed strong resistance to the methanol permeability.

3.5.

Single cell testing

The polarization and performance curves of DMFCs were collected at 45  C with a methanol feed concentration of 5 M and a 4 cm2 active membrane area as shown in Fig. 6. The open circuit voltage of the cell was 0.687 V, which was slightly higher than that of Nafion-117 (0.658 V), and a similar results

Conclusion

In summary, ZC-PAMAM dendrimer with sulfonic acid and amine groups was developed as filler for preparing PBI-based composite membranes. The semitransparent composite membranes were found to have good thermal stability up to 200  C by TGA analysis. The proton conductivity of composite membranes increased with increasing the content of ZCPAMAM dendrimer. The composite membranes of PBI/ZCPAMAM-20 showed a proton conductivity of 1.83
102 S cm1. In addition, the composite membranes had an excellent performance on restraining methanol diffusion. PBI/ZC-PAMAM-20 showed a methanol permeability of 5.23
108 cm2 s1 which is more than 2 orders of magnitude lower than that of Nafion-117 (1.23
106 cm2 s1). The maximum power density achieved at 45  C for a single cell with a 4 cm2 active area can reach up to 26.64 mW cm2. These results prove that the PBI/ZC-PAMAM dendrimer composite membrane has a strong potential for application in DMFCs. This work on zwitterion-coated fillers for PEMs will give rise to a new developing field in materials and membrane science and have an impact on further investigations of proton conducting membranes for PEMPCs.

Acknowledgments This work was supported by National High Technology Research and Development Program 863 (2011AA050511), Natural Science Foundation of China (No. 51272033 and 51303017) and Jiangsu “333” Project.

references

Fig. 6 e The iev curves of DMFC with PBI/ZC-PAMAM dendrimer composite membranes and Nafion-117 as electrolyte membranes at 45  C.

[1] Peighambardoust SJ, Rowshanzamir S, Amjadi M. Review of the proton exchange membranes for fuel cell applications. Int J Hydrogen Energy 2010;35:9349e84. [2] Iulianelli A, Basile A. Sulfonated PEEK-based polymers in PEMFC and DMFC applications: a review. Int J Hydrogen Energy 2012;37:15241e55. [3] Binsu VV, Nagarale RK, Shahi VK. Phosphonic acid functionalized aminopropyl triethoxysilaneePVA composite material: organiceinorganic hybrid proton-exchange membranes in aqueous media. J Mater Chem 2005;15:4823e31. [4] Jiang Z, Zhao X, Manthiram A. Sulfonated poly(ether ether ketone) membranes with sulfonated graphene oxide fillers for direct methanol fuel cells. Int J Hydrogen Energy 2013;38:5875e84.

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[5] Wycisk R, Chisholm J, Lee J, Lin J, Pintaur PN. Direct methanol fuel cell membranes from Nafionepolybenzimidazole blends. J Power Sources 2006;163:9e17. [6] Nagarale RK, Gohil GS, Shahi VK. Sulfonated poly(ether ethereketone)/polyaniline composite proton-exchange membrane. J Membr Sci 2006;280:389e96. [7] So SY, Hong YT, Kim SC, Lee SY. Control of water-channel structure and state of water in sulfonated poly(arylene ether sulfone)/diethoxydimethylsilane in situ hybridized proton conductors and its influence on transport properties for DMFC membranes. J Membr Sci 2010;346:131e5. [8] Feng SG, Shang YM, Wang SB, Xie XF, Wang YZ, Wang YW, et al. Novel method for the preparation of ionically crosslinked sulfonated poly(aryleneeether sulfone)/ polybenzimidazole composite membranes via in situ polymerization. J Membr Sci 2010;346:105e12. [9] Han MM, Zhang G, Li MY, Wang S, Liu ZG, Li, et al. Sulfonated poly(ether ether ketone)/polybenzimidazole oligomer/epoxy resin composite membranes in situ polymerization for direct methanol fuel cell usages. J Power Sources 2011;196: 9916e23. [10] Zhang Z, Chalkova E, Fedkin M, Wang C, Lvov SN, Komarneni S, et al. Synthesis and characterization of poly(vinylidene fluoride)-g-sulfonated polystyrene graft copolymers for proton exchange membrane. Macromolecules 2008;41:9130e9. [11] Wang JT, Savinell RF, Wainright J, Litt M, Yu H. A H2/O2 fuel cell using acid doped polybenzimidazole as polymer electrolyte. Electrochim Acta 1996;41:193e7. [12] Suryani, Liu YL. Preparation and properties of nanocomposite membranes of polybenzimidazole/ sulfonated silica nanoparticles for proton exchange membranes. J Membr Sci 2009;332:121e8. [13] Zhu J, Zhang G, Shao K, Zhao C, Li H, Na H, et al. Hybrid proton conducting membranes based on sulfonated crosslinked polysiloxane network for direct methanol fuel cell. J Power Sources 2011;196:5803e10. [14] Roelofs KS, Kampa A, Hirth T, Schieste T. Behavior of sulfonated poly(ether ether ketone) in ethanolewater systems. J Appl Polym Sci 2009;6:2998e3009. [15] Neyerlin KC, Singh A, Chu D. Kinetic characterization of a PteNi/C catalyst with a phosphoric acid doped PBI membrane in a proton exchange membrane fuel cell. J Power Sources 2008;176:112e7. [16] Chu F, Lin B, Qiu B, Si Z, Qiu L, Gu Z, et al. Polybenzimidazole/ zwitterion-coated silica nanoparticle hybrid proton conducting membranes for anhydrous proton exchange membrane application. J Mater Chem 2012;22:18411e7. [17] Chuang SW, Hsu SLC, Hsu CL. Synthesis and properties of fluorine-containing polybenzimidazole/montmorillonite nanocomposite membranes for direct methanol fuel cell applications. J Power Sources 2007;168:172e7. [18] Jheng L, Huang C, Hsu SL. Sulfonated MWNT and imidazole functionalized MWNT/polybenzimidazole composite membranes for high temperature proton exchange membrane fuel cells. Int J Hydrogen Energy 2013;38:1524e34. [19] Lin B, Qiu B, Qiu L, Si Z, Chu F, Chen X, et al. Imidazoliumfunctionalized SiO2 nanoparticle doped proton conducting

[20]

[21]

[22]

[23]

[24]

[25]

[26]

[27]

[28]

[29]

[30]

[31]

[32]

[33]

[34]

16417

membranes for anhydrous proton exchange membrane application. Fuel Cells 2013;1:72e8. Chakrabarty T, Singh AK, Shahi VK. Zwitterionic silica copolymer based crosslinked organiceinorganic hybrid polymer electrolyte membranes for fuel cell applications. RSC Adv 2012;2:1949e61. Pan S, Wang C, Zeng X, Wen YT, Wu HM, Feng M. Short multi-armed polylysine-graft-polyamidoamine copolymer as efficient gene vectors. Int J Pharm 2011;420:206e15. Tripathi BP, Shanhi VK. 3-[[3-(Triethoxysilyl)propyl]amino] propane-1-sulfonic acidepoly(vinyl alcohol) cross-linked zwitterionic polymer electrolyte membranes for direct methanol fuel cell applications. ACS Appl Mater Interface 2009;5:1002e12. Diao H, Yan F, Qiu L, Lu J, Lu X, Lin B, et al. High performance cross-linked poly(2-acrylamido-2-methylpropanesulfonic acid)-based proton exchange membranes for fuel cells. Macromolecules 2010;43:6398e405. Chuang SW, Hsu SLC. Synthesis and properties of a newfluorinecontaining polybenzimidazole for high-temperature fuel-cell applications. J Polym Sci A Polym Chem 2006;44:4508e13. Li Q, Rudbeck HC, Chromik A, Jensen JO, Pan C, Steenberg T, et al. Properties, degradation and high temperature fuel cell test of different types of PBI and PBI blend membrane. J Membr Sci 2010;347:260e70. Musto P, Karasz FE, MacKnight WJ. Hydrogen bonding in polybenzimidazole/polyimide systems: a Fourier-transform infra-red investigation using low-molecular-weight monofunctional probes. Polymer 1989;30:1012e21. Li Q, He R, Berg RW, Hjulers HA, Bjerrum NJ. Water uptake and acid doping of polybenzimidazoles as electrolyte membranes for fuel cells. Solid State Ionics 2004;168:177e85. Park CH, Lee CH, Guiver MD, Lee YM. Sulfonated hydrocarbon membranes for medium-temperature and lowhumidity proton exchange membrane fuel cells (PEMFCs). Prog Polym Sci 2011;36:1443e98. Chen WF, Kuo PL. Covalently cross-linked perfluorosulfonated membranes with polysiloxane framework. Macromolecules 2007;40:1987e94. Chang YW, Shin G. Crosslinked poly(ethylene glycol) (PEG)/ sulfonated polyhedraloligosilsesquioxane (sPOSS) hybrid membranes for direct methanol fuel cell applications. J Ind Eng Chem 2011;17:730e5. Helen M, Viswanathan B, Murthy SS. Fabrication and properties of hybrid membranes based on salts of heteropolyacid, zirconium phosphate and polyvinyl alcohol. J Power Sources 2006;63:433e9. O’Dea JR, Economou NJ, Buratto SK. Surface morphology of nafion at hydrated and dehydrated conditions. Macromolecules 2013;46:2267e74. Wang C, Li N, Shin DW, Lee SY, Kang NR, Lee YM, et al. Fluorene-based poly(arylene ether sulfone)s containing clustered flexible pendant sulfonic acids as proton exchange membranes. Macromolecules 2011;44:7296e306. Jang W, Choi S, Lee S, Shul Y, Han H. Characterization and stability of polyimideephosphotungstic acid composite electrolyte membranes for fuel cell. Polym Degrad Stabil 2007;92:1289e96.