Poly (fluorenyl ether ketone) ionomers containing separated hydrophilic multiblocks used in fuel cells as proton exchange membranes

international journal of hydrogen energy 35 (2010) 682–689

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Poly (fluorenyl ether ketone) ionomers containing separated hydrophilic multiblocks used in fuel cells as proton exchange membranes H. Hu a,b, M. Xiao a,b, S.J. Wang a,b, Y.Z. Meng a,b,* a

State Key Laboratory of Optoelectronic Materials and Technologies, Sun Yat-Sen University, Guangzhou 510275, PR China The Key Laboratory of Low-carbon Chemistry & Energy Conservation of Guangdong Province, Sun Yat-Sen University, Guangzhou 510275, PR China b

article info

abstract

Article history:

A series of sulfonated poly(fluorenyl ether ketone) with different hydrophilic block

Received 20 July 2009

lengths were synthesized via a two-step one-pot polymerization from 9,9’-bis(4-

Received in revised form

Hydroxypheyl) fluorine, 3,3’-disulfonated-4,4’-difluorobenzophenone, and 4,4’-difluor-

12 October 2009

obenzophenone. The resulting sulfonated block polymers with high inherent viscosity

Accepted 28 October 2009

(0.8–1.37 dL/g) were very soluble in polar organic solvents and can form flexible and

Available online 24 November 2009

transparent membranes by casting from their solutions. Transmission electron microscope (TEM) was used to examine the microstructure of the membranes and the results

Keywords:

revealed that significant hydrophilic/hydrophobic microphase separation was produced.

Sulfonation

The effects of the multiblock structure and/or length were investigated by comparison of

Ionomers

the properties of the multiblock copolymer and the corresponding random structure. The

Multiblocks

multiblock structure can provide enhanced proton transport, especially under partially

Proton conductivity

hydrated conditions. The as-made membranes can also exhibit better oxidative stability

Fuel cell

and single cell performance than random copolymer. The multiblock structure design method provides a useful way to prepare proton exchange membrane used in PEM fuel cells. ª 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.

1.

Introduction

Nowadays the knowledge about the future shortage of fossil fuels and regulatory pressure to reduce air-pollution by the greenhouse gas CO2 have stimulated world-wide research activities in the field of fuel cells for stationary and mobile application. There has been great interests in the development of a new proton exchange membrane to meet the requirement of fuel cells [1,2]. It is generally accepted that among the many available kinds of proton conducting

polyelectrolytes, poly(perfluoroalkysulfonic acid) (Nafion) is the most preferable from the standpoint of chemical, thermal stabilities, and proton-conducting properties [3]. However, these membranes still have numerous drawbacks, such as high cost, high methanol permeability, poor performance at higher temperatures (>80  C), as well as the environment hazards associated with their disposal, which attract increasing research of new membrane materials [4,5]. In this respect, many aromatic polymers have been explored as alternative PEM candidates for their excellent thermal and

* Corresponding author. E-mail address: [email protected] (Y.Z. Meng). 0360-3199/$ – see front matter ª 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2009.10.103

international journal of hydrogen energy 35 (2010) 682–689

chemical stability as well as their good mechanical properties and high continuous service temperatures [6–9]. Unfortunately, most of these membranes still meet challenges. It is difficult to balance the high proton conductivity and good stability. These randomly located sulfonic acid groups will lead to isolated morphological domains especially under partially hydrated conditions. As is well known, Nafion exhibits high proton conductivity although with low IEC due to microphase separation between hydrophilic sulfonic acid groups and hydrophobic domains. Many efforts were devoted to develop various sulfonated aromatic polymers with hydrophilic/hydrophobic separation just as that of Nafion [10]. Two general methods are available for the synthesis of microphase separated aromatic polymers: (1) direct copolymerization of monomers with pendant sulfonic acid groups to afford stereo-controlled polymers; (2) two-step polycondensation to synthesize block copolymers. The former way requires designing and preparing complicated branched monomers with tough organic synthesis [11,12]. Whereas, Mcgrath et al. [13–17] readily prepared multiblock sulfonated aromatic proton exchange membranes with a distinct morphological structure using a two-step polycondensation. The obtained membranes exhibited much higher proton conductivity at low hydration levels than corresponding random copolymer membranes. They contributed these advantages to the microstructure and sequence distribution of polymers.

683

In previous works, the random copolymers of sulfonated poly(fluorenyl ether ketone) (SPFEK) were prepared in largescale by direct copolymerization of sulfonated aromatic dihalides, aromatic dihalides and bisphenols [18]. The copolymers showed good performance at elevated temperatures and higher hydration levels. However, the random architecture resulted in the formation of isolated domains, which dramatically restricts proton transport at low hydration levels [19]. In this paper, the design and synthesis of multiblock copolymers with different hydrophilic segment lengths using a two-step and one-pot polymerization is reported. The length effect of hydrophilic segments in the main chain, and the correlation among the morphology, proton conductivity, water uptake and single cell performance were investigated.

2.

Experimental

2.1.

Materials

9,9’-bis(4-Hydroxypheyl) fluorene (BHF) and 4,4’-difluorobenzophenone (DFBP) were purchased from Aldrich. Reagent-grade Fuming sulfuric acid (50%), concentrate sulfuric acid (95–98%), dimethyl sulfoxide (DMSO), toluene, ethanol, and anhydrous potassium carbonate were obtained from commercial sources. BHF and DFBP were recrystallized from toluene and ethanol, respectively. DMSO was dried over

Scheme 1 – Synthesis of multiblock copolymers SPFEK-B6, SPFEK-B8, and SPFEK-B10.

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4A molecule sieves and toluene was dried over sodium wire prior to use. Anhydrous potassium carbonate was vacuum dried at 150  C for 10 h. Other reagents and solvents were used as received.

2.2.

Synthesis of polymers

The disodium salt of 3,3’-disulfonated-4,4’-difluorobenzophenone (SDFBP) was synthesized according to the procedure described by Wang et al. [20]. The typical synthetic procedure for a series of sulfonated block copolymers (SPFEKB) is described in details as follows (Scheme 1). First, 1.0135 g (2.4 mmol) of SDFBP, 0.9811 g (2.8 mmol) of BHF and 0.448 g (3. 5 mmol) of potassium carbonate were added into a threenecked flask equipped with a Dean–Stark trap, a nitrogen inlet/outlet, a condenser, and a magnetic stirrer. DMSO (7 mL) was introduced to afford a 28% (w/v) solid concentration. Toluene (7 mL, usually DMSO/toluene ¼ 1/1, v/v) was used as an azeotroping agent. Nitrogen was purged through the reaction mixture with stirring for 10 min, and the mixture was refluxed at 140  C for 3 h to dehydrate the system. After the produced water was azoetroped off with toluene, the temperature was slowly elevated to 175  C and kept it for 16 h, then cooled to room temperature, removed the Dean–Stark trap, 0.4205 g (1.2 mmol) of BHF, 0.3491 g (1.6 mmol) of DFBP, 0.207 g (1. 5 mmol) of potassium carbonate, DMSO (3 mL) and toluene (7 mL) were charged into the above mentioned reaction mixture. The mixture was refluxed at 140  C for 3 h to dehydrate the system. After distilling off the excess toluene, the mixture was heated at 175  C. The reaction was allowed to proceed for another 16 h, until high viscous polymer solution was obtained. Before cooling down the reaction mixture, 5 mL DMSO was added to the mixture which was poured into the mixture of ethanol/water (1:1, v:v) to precipitate out the polymers. The precipitate was filtered and washed with deionized water and ethanol for several times. After the fibrous product was collected and dried at 110  C under vacuum for 24 h, block copolymers SPFEK-Bx (x refers to the designed average repeat unit number) were prepared. The polymerization degree is represented by n number as shown in Scheme 1. A total of 1.3475 g polymer SPFEK-B6 was obtained in high yield of 94%. SPFEK-B8, yield: 92%. SPFEK-B10, yield: 94%. The corresponding random copolymer SPFEK-R was synthesized using the procedures described earlier [18].

2.3.

3.

Characterizations

3.1.

Instrumentation

The inherent viscosity was determined in the 0.5 g/dL DMSO solution at 20  C with a calibrated Ubbelonhde viscometer. Thermogravimetric analysis (TGA) was carried out using a Seiko SSC-5200 thermogravimetric analyzer under a nitrogen atmosphere (200 mL/min) at a heating rate of 10  C/ min at a temperature ranging from 70  C to 600  C. Tensile strength of wet membranes was measured at 25  C by a mechanical test machine (CMT-4014, SANS, Shenzhen, China). The samples were prepared by cutting them into a dumbbell shape and immersed in de-ionized water at 80  C for 24 h prior to test. The cross-head speed was set at a constant speed of 2 mm/min. For each testing sample, at least three measurements were taken and average value was calculated. Microstructure of the sulfonated block polymers was investigated by a JEM-2010HR transmission electron microscope (TEM) using an accelerating voltage of 200 kV. To prepare the sample for TEM observation, the membranes were stained with silver ions by exchanging protons of sulfonic acid groups in a large excess of 1 M AgNO3 aqueous solution, rinsed with water, and dried at room temperature overnight. The stained samples were embedded in carbonaceous membrane and performed using a Reighert–Jung microtome Ultracut E to yield a 60–100 nm thick microtomy sample and placed on copper grids. Proton conductivity was determined using a Solartron 1255B Frequency Response Analyzer functioning with an oscillating voltage of 10 mV using two probes over the frequency range of 1–10 Hz. The film samples were cut into a circle with the diameter of 1 cm and soaked in de-ionized water for 24 h prior to each test. The cell assembly processing step was followed as described in the literature [21]. Equilibration time at each temperature was fixed at 0.5 h. The proton conductivity (s) of the specimen in the transverse direction (across the membrane) was calculated from the impedance data according the following Eq. (1): s ¼ d=ðRSÞ

(1)

Where d and S are the thickness and the face area of the specimen, respectively, and R is derived from the low intersect of the high frequency arc on a complex impedance plane with the Re (Z’) axis.

Membrane preparation 3.2.

Membranes in the sodium form were prepared by solution casting from DMAc. The copolymers were dissolved in DMAc (5% w/v), and cast onto a flat glass substrates in a dust-free environment and dried at 60  C for 12 h, and then at 80  C under vacuum for 24 h. The resulting membranes (in sodium form) were then converted to their required acid form by immersing them in 1.0 M sulfuric acid solution for 24 h, followed by immersion in de-ionized water at room temperature for several days. The acidified membranes were dried at 90  C under vacuum for 12 h. The thickness of all membranes was controlled in the range of 120–180 mm.

Water uptake and swelling ratio

The water uptake and swelling ratio experiments were conducted by measuring the weight and the length comparisons between fully hydrated membranes and vacuum dried membranes. First, the membranes were dried at 90  C under vacuum for 24 h and their weights (Wdry) were recorded. The dried membranes were then immersed in de-ionized water at 80  C for 24 h. Then the wet membranes were quickly weighted after removing the surface water with tissue paper, which recorded as Wwet. The water uptake and swelling ratio were calculated according to the following Eqs. (2) and (3):

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international journal of hydrogen energy 35 (2010) 682–689

Table 1 – Synthesis data of sulfonated SPFEKs. Polymer

Molar feeda (mmol) Bisphenol

SPFEK-B6 SPFEK-B8 SPFEK-B10 SPFEK-R

hinhb of oligomer (dL/g)

hinhb of polymer (dL/g)

Yield (%)

SDFBP

2.8 2.7 2.64 –

2.4 2.4 2.4 –

0.26 0.33 0.40 –

1.37 0.8 1.21 1.41

94 92 94 95

IEC (mequiv./g) theor

exptlc

1.93 1.93 1.93 1.93

1.92 1.83 1.78 1.78

a Added to prepare oligomer. b Tested in 0.5 g/dL solution in DMAc at 20  C. c Calculated from titration.


 Wdry  100%

(2)


 Ldry  100%

(3)

Water uptake ð%Þ ¼ Wwet  Wdry Swelling ration ð%Þ ¼ Lwet  Ldry

The MEAs with SPFEK membranes were prepared in the same way except that 5 wt% SPFEK solution was used as agglutinant.

where Lwet and Ldry are the lengths of dry and wet samples, respectively.

3.6. 3.3.

Ion exchange capacity (IEC)

The IEC of the membranes were determined by titration according to the literature [22]. The dried membranes in acid form were weighed and immersed in saturated NaCl solution for 8 h to replace the protons of sulfonic acid groups with sodium ions. The solutions were titrated using 0.05 M NaOH solution, with phenolphthalein as indicator. The moles of the proton were equal to the moles of sulfonic group, and the IEC were calculated from the titration data using the following Eq. (4):

IEC (mequiv/g) ¼ DVNaOH$CNaOH/WS  103

Oxidative and hydrolytic stabilities [23]

Oxidative Stability was tested by immersing a small piece of a membrane sample in Fenton’s reagent (3% H2O2 þ 2 ppm FeSO4) at 80  C for 1 h. The stabilities were evaluated by changes in the mass, molecular weight and IEC value.

3.5.

The performance of the use membrane–electrode assembly (MEA) was carried out using a 4 cm2 single cell in humidified H2/air gases. The single cell was operated at 85  C under 90% RH conditions with 0 MPa gas back-pressure. In order to provide the adequate oxidant-to-fuel ratio, the flow rate of the oxygen was fixed at twice of hydrogen. The polarization curves were measured by applying a constant current for 1 min at each point using a fuel cell test station (Arbin Instruments, 160269). The power densities were calculated from the steady-state voltages and applied currents.

(4)

Where DVNaOH is the consumed volume of NaOH solution, CNaOH is the concentration of NaOH solution and WS is the weight of the membrane sample.

3.4.

Single cell performance

Preparation of membrane electrode assembly (MEA)

The MEA with Nafion 117 membrane was prepared as follows. Pt/C catalyst (40 wt% Pt from Johnson Mattey Corp.), isopropyl alcohol and 5 wt% Nafion solution were mixed by an ultrasonic stirrer bath for 20 min to form catalyst slurry. Electrodes were prepared by direct spraying the catalyst slurry on 2 cm  2 cm carbon papers (EC-TP1-060, Electrochem. Inc.) and dried at 60  C under vacuum. A membrane was clamped between two electrodes and then hot pressed. The loading of Pt in MEA were 0.5 mg cm2 and the mass ratio of dry Nafion and the Pt/C is 3:7.

4.

Results and discussion

4.1.

Polymer synthesis

High molecular weight sulfonated block copolymer with different hydrophilic segment lengths can be readily synthesized via a two-step one-pot method [24,25], as described in Scheme 1. The molecular weights of the hydroxyl-terminated poly(fluorenyl ether ketone) and the copolymers are given in Table 1. The h value of the sulfonated oligomer can be changed and controlled by varying the molar ratio of bisphenol fluorine and SDFBP. In this oligomer series, initial feed ratio of SDFBP/bisphenol were fixed at 6:7, 8:9, 10:11, respectively, thus the oligomers with different inherent viscosity values were obtained successfully. Although gel permeation chromatography measurements were not available because of the insoluble nature of sulfonated polymers in THF, the inherent viscosity of the synthesized polymers can be measured to be ranged from 0.8 to 1.37 dl g1, which is same as that of a random copolymer. This indicates the formation of high-molecular-weight polymers. The IEC of the sulfonated SPFEK determined by titrating was in good agreement with the theoretical values. Moreover, these polymers showed good solubility in polar aprotic solvents, such as DMAc, DMSO and NMP.

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Fig. 1 – TEM images of (a) SPFEK-R and (b) SPFEK-B6.

4.2.

Morphology of membranes

TEM examination was performed to investigate the morphology of SPFEK-B6 and SPFEK-R membranes that were stained with Agþ by ion exchange. As shown in the TEM images in Fig. 1, the dark areas represent hydrophilic domains, and the brighter areas represent hydrophobic domains. Microphase-separated and larger ionic clusters were observed for SPFEK-B6 and the number of these ionic clusters was significantly lower than that of SPFEK-R. The size of the ionic clusters of SPFEK-R appears to be uniform (6–8 nm), while uneven-sized ionic clusters (4–15 nm) existed in SPFEK-B6 [26,27]. These results imply that the hydrophilic ionic clusters provide large and continuous proton transport channels, which accounts for the relatively high conductivities especially under partially hydrated conditions.

4.4.

Mechanical properties of the block copolymers were assessed by comparison with their corresponding random copolymers and Nafion membrane (Table 2). Nafion 117 membrane showed high strain at break (150%) and tensile strength (20 MPa). The multiblock copolymer membranes exhibited comparable both tensile strength and strain at break to SPFEK-R. With different segment lengths, it should be noted that no distinct difference of tensile strength was observed. The tensile strengths of these SPFEKs (21–31 MPa) were slightly higher than those of Nafion 117, but the strain values of block SPFEKs (10–20%) were much smaller than those of SPFEK-R (55%) and Nafion 117. Based on these results, it may be concluded that mechanical property of the block copolymers can meet the requirements for PEMFC applications.

4.5. 4.3.

Mechanical properties

Water uptake and proton conductivity

Thermal property

Thermal stability of the membranes in proton form derived from the sulfonated copolymers was investigated by both TGA and DSC technologies. Prior to the test, the samples were preheated to 150  C in nitrogen flow to get rid of absorbed moisture, then cooled down to 70  C and finally the heating history from 70 to 600  C was recorded. The TGA curves are shown in Fig. 2. There were no distinct differences observed in the thermal property among the sulfonated multiblock copolymers with different segment lengths [26]. A two-step degradation profile was observed. The first weight loss peak at about 250  C was attributed to the elimination of sulfonic acid groups and the small amount of absorbed water, while the second weight loss at around 400  C was due to the degradation of the main chain of sulfonated polymers. 5% weight loss temperature was above 310  C. From the result of DSC analysis, there was no glass transition temperature observed before thermal decomposition. The Tgs of these polymers might be higher than the first decomposition temperature [28]. The increase in Tgs is believed to result from the intermolecular reactions of sulfonic acid groups.

As mentioned above, in order to achieve similar proton conductivity, the random sulfonated poly(arylene ether

Fig. 2 – TGA curves of sulfonated copolymers.

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Table 2 – Water uptake, proton conductivity and mechanical properties of SPFEK membranes. polymer

SPFEK-B6 SPFEK-B8 SPFEK-B10 SPFEK-R Nafion 117

Water uptakea (%)

63 79 93 35 28

Swelling ratioa (%)

19 23 26 13 9

Hydration numbera Proton (l) conductivityb (mS/cm)

Mechanical propertiesb

30  C

80  C

Tensile strength (Mpa)

Elongation at break (%)

13.5 13.2 14.1 6.3 12.6

29.7 34.5 37.8 11.5 23.1

21 21 31 23 20

17 10 17 55 150

18 23 27 10 15c

c

IEC of Nafion 117 is 0.95–1.01, from the Dupont material data sheet (http://fuelcell.com/techsheets/Nafion%201135%20115%20117.pdf). a Measured after equalized in de-ionized water at 80  C for 24 h, ‘‘l’’ denotes hydration number per sulfonic acid group. b Measured at 100% relative humidity.

ketone) requires a much higher IEC value than Nafion to compensate their narrower and dead end channels [29]. Unfortunately, high IEC value usually leads to high water uptake for membranes, which further results in the loss of mechanical properties and deterioration under the operational conditions in a fuel cell. As can be seen from Table 2, the water uptakes of multiblock copolymers increased from 62.9 to 93.1% at 80  C, and swelling ratios increased from 19.7 to 25.9% with increasing the length of segments in copolymers. Although the water uptake of multiblock copolymer was higher than the corresponding one of random copolymer, the proton conductivity increased from 13.0 to 14.1 mS/cm at 30  C and from 29.0 to 37.8 mS/cm at 80  C, respectively, with increasing the block segment length, which is much higher than those of SPFEK-R. In other words, both proton conductivity and water uptake of multiblock copolymers depend greatly on the morphology of membranes. Bai et al. and Tian et al. [30,31] reported similar observations for their block copolymers. However, the preparation of sulfonated polymer with proper water uptake is crucial to obtain high performance for PEMFC applications. In order to reduce the swelling degree, the length of hydrophilic segments should be controlled to a limited value to balance the proton conductivity and water uptake. The multiblock

copolymers favor the enhancement of proton conductivity, but increase the water uptake of polymers with same sulfonation degree. Considering both proton conductivity and dimensional stability, SPFEK-B6 seems to be a potential candidate for the membrane materials used in fuel cells. Fig. 3 shows relative humidity dependency on proton transport over the RH range of 30–100% at 80  C for the copolymer SPFEK-B6. The performance of block copolymer under partially hydrated conditions was comparable to that of Nafion. For the random copolymer SPFEK-R, proton conductivity dropped significantly under lower RH conditions. It was due to the lack of efficient connectivity among sulfonic acid groups for proton transport under partially hydrated conditions. The presence of long and orderly blocks improved the proton transport along the sulfonic acid groups and water molecules.

4.6.

Oxidative stability

Oxidative stability was tested by immersing membranes into a Fenton’s reagent (a 3% H2O2 aqueous solution containing 2 ppm FeSO4) at 80  C for 1 h. The results determined by the loss of weight, IEC and inherent viscosity after treatment are listed in Table 3. For easier comparison, the membranes of random copolymer and Nafion with the same thickness (180 um) were also subjected to the measurement. The experimental results demonstrated that the membranes derived from SPFEK-B6, SPFEK-B8, SPFEK-B10 copolymers retained more than 90 wt % of their original weight. The SPFEK-R membrane can keep its original shape while subjecting 15% loss of its original weight after the oxidative stability testing. Meanwhile, block copolymers showed smaller loss in the IEC when compared with

Table 3 – Oxidative stability of SPFEK membranes.

Fig. 3 – Comparison of conductivity vs. RH for SPFEK-B6, SPFEK-R, and Nafion 117.

SPFEK-B6 SPFEK-B8 SPFEK-B10 SPFEK-R Nafion 117

IEC loss (%)

Weight loss (%)

Inherent viscosity loss (%)

4.8 5.7 4.3 7.7 2.1

6.7 5.0 3.5 15 1.9

21.2 20.0 19.8 20.6 –

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Fig. 4 – Polarization curves and power density of PEMFC using SPFEK-B6, SPFEK-R, and Nafion 117 as PEM. random polymers. From these results, we can conclude that the multiblock copolymers exhibit better oxidative stability than their corresponding random copolymers.

4.7.

Single PEMFC performance

The membranes derived from SPFEK-B6, SPFEK-R and Nafion 117 were used to fabricate membrane electrode assemblies (MEAs), and the MEA properties were then evaluated in single proton exchange membrane fuel cells (PEMFCs). Fig. 4 shows the polarization and power density curves in an H2/O2 single fuel cell at the cell temperature of 85  C, 100% relative humidity, and the back gas pressure of 0 MPa. The highest current densities of the fuel cells using SPFEK-B6, SPFEK-R and Nafion 117 were 0.85, 0.66 and 0.80 A/cm2, respectively, and the highest power densities of the corresponding fuel cells were 0.18, 0.13 and 0.15 W/cm2 in their turns, respectively. The best single cell performance at 85  C was obtained when using SPFEK-B6 as PEM, which is slightly better than the single fuel cell using Nafion 117 membrane. Presumably, the reason for this may be that the optimal operation temperature for Nafion is 75  C [31]. At the effective work potential of 0.5 V, the highest power densities of the single fuel cell using SPFEK-B6 and Nafion 117 were 0.18 and 0.15 W/cm2, respectively. The effective work potential for fuel cell using SPFEK-R was 0.35 V, and the highest power density was 0.13 W/cm2 accordingly. It should be noted that the copolymer SPFEK-B6 exhibited much better fuel cell performance than copolymer SPFEK-R in the whole range of current density. By further comparing the molecular structures of SPFEK-B6 and SPFEK-R, it can be seen that the longer hydrophilic block segments in the main chain result in better cell performance. The presence of longer hydrophilic segments can then result in the formation of two phase structure, consisting of the hydrophilic segment of sulfonic acid containing part and the hydrophobic segment without sulfonic acid group.

5.

Conclusions

A new multiblock SFPEK copolymers with controlled hydrophilic segment lengths can be readily synthesized by a two-

step and one-pot method. The membrane derived from SPFEK-B6 showed higher water uptake (62.9%), higher proton conductivity (29 mS/cm), reasonable welling ratio (19.7%) and highly oxidative stability, when compared with the random SPFEK copolymer with the same IEC value and under the same conditions. TEM images clearly indicated the formation of a multiblock microstructure of the as-prepared copolymers. The multiblock morphology is believed to increase the extent of connectivity among the hydrophilic domains especially under partially hydrated conditions. These characteristics can in turn significantly improve the PEM properties. Among the studied membranes, SPFEK-B6 membrane exhibited much better single cell performance than both SPFEK-R and Nafion 117 membranes. In conclusion, introduction of hydrophilic segments into polymer appears to be an attractive way to improve the comprehensive properties of proton exchange membrane.

Acknowledgments The authors would like to thank the China High-Tech Development 863 Program (Grant No.: 2007AA03Z217), Guangdong Province Sci & Tech Bureau (Key Strategic Project Grant No.: 2003C105004, 2006A10704004, 2006B12401006), and Guangzhou Sci & Tech Bureau (2005U13D2031) for financial support of this work.

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