Modifying a Proton Conductive Membrane by Embedding a “Barrier

J. Phys. Chem. B 2010, 114, 13121–13127

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Modifying a Proton Conductive Membrane by Embedding a “Barrier” Liang Wu,†,‡ Chuanhui Huang,† Jung-Je Woo,‡ Dan Wu,†,‡ Sung-Hyun Yun,‡ Seok-Jun Seo,‡ Tongwen Xu,*,† and Seung-Hyeon Moon*,‡ CAS Key Laboratory of Soft Matter Chemistry, Laboratory of Functional Membranes, School of Chemistry and Materials Science, UniVersity of Science and Technology of China, Hefei, Anhui 230026, People’s Republic of China, and Department of EnVironmental Science and Engineering, Gwangju Institute of Science and Technology, Gwangju, 500-712, Republic of Korea ReceiVed: May 18, 2010; ReVised Manuscript ReceiVed: July 15, 2010

For development of proton conductive membranes, it is a difficult dilemma to balance proton conductivity and methanol permeability; however, this research proposes a simple strategy to solve this problem, i.e., embedding a proton conductive “barrier” into the perflorosulfonated matrix. The strategy is exemplified by embedding the amphoteric sulfonated poly(phthalazinone ether sulfone kentone) (SPPESK) into a semicrystalline perflorosulfonic acid polymer matrix (FSP). After being annealed, the domain of SPPESK is converted to the barrier. Two acid-base interactions constitute the barrier for both the transfer of protons and the blockage of methanol, respectively. On one hand, poorly hydrophilic ionic acid-base interactions (-SO3-...NH+-) are formed between sulfonic acid group and phthalazinone group through annealing and are useful for methanol blocking. On the other hand, more hydrophilic hydrogen-bonded acid-base interaction (-SO3H...(H2O)n...N-, n e 3) can also be formed under hydrated condition and facilitate proton transport according to the Grotthusstype mechanism. As a result, the final membrane exhibits an extremely low methanol permeability (30% of that of Nafion-112) and an excellent fuel cell performance (as compared with Nafion-112 at 80 °C). Introduction Proton conductive membrane (PEM)-based fuel cells continue to be of interest as a potential renewable energy source.1,2 As one of the key components, the PEM functions as a medium for proton transport, structural support for electrodes, and obstacle to parasitical mass transport (water/fuel electro-osmotic permeation). Correspondingly, increasing the proton conductivity as well as mitigating or possibly eliminating the parasitical mass transport of the PEM to achieve higher fuel cell efficiency are currently areas of intense research.3-6 To date, the majority of conventional fuel cell technology is based on perfluorinated sulfonic acid ionomers (Nafion series), which have been wildly studied due to their high proton conductivity and substantial durability.7,8 Those membranes usually have hydrophobic poly(tetrafluoroethylene) (PTFE) molecular backbones and hydrophilic fluorosulfonic acid side chains. This special architecture often results in a separation between hydrophobic and hydrophilic phases on a nanometer scale.9,10 The tough hydrophobic domains, which are usually semicrystalline in nature, give this membrane hydrated morphological stability and excellent long-term stability in the rigorous fuel cell environment. Additionally, in the hydrophilic domain, proton conductive channels are easy to be established by connecting the swelling ionic clusters, which are composed of fluorosulfonic acid side chains.11,12 Despite the excellent proton conductivity, parasitical mass transport, especially the fuel (e.g., methanol) crossover, is thought to occur unavoidably through those proton conductive channels too. This unavoidable fuel crossover usually leads to loss of fuel and establishes a mixed potential at the cathode, resulting in a poor overall * To whom correspondence should be addressed. E-mail: [email protected] (T.X.); [email protected] (S.-H.M.). † University of Science and Technology of China. ‡ Gwangju Institute of Science and Technology.

performance.13,14 In view of this, most current strategies for development of alternatives are trying to reach a compromise between high proton conductivity and low parasitical transport. Usually, the general trends observed in Nafion membranes and other sulfonic acid containing PEMs reveal that protons and methanol share the channel of hydrated ionic clusters and their fluxes both decrease with a decrease in the water content of the membrane.15-17 Therefore, one crucial parameter that allows control of this crossover is the hydration degree of a membrane. However, a low hydration degree also leads to poor proton conductive performance. For example, at a low hydration degree (lower than six H2O per SO3H), the strong association of protonic charge carriers (-SO3-) and counterions (H3O+) make it difficult for protons to transfer.12 Consequently, recent attempts have been focused on strengthening the density of the membranes, e.g., by covalent cross-linking18,19 or forming copolymers of sulfonated areylenes and partially fluorinated polymers.20-22 Those membranes show low fuel crossover; however, the low proton conduction and brittleness of highly cross-linked polymers may cause failure in fuel cell applications. Alternatively, the cross-linking via acid-base complexation is a better choice to balance proton conductivity and methanol permeability.23-26 The materials suitable for such cross-linking are polymers bearing basic sites (e.g., imidazole, phthalazinone, and pyridine groups) and those bearing acidic sites (usually -SO3H). The two polymers can react and establish two interactions: ionic and hydrogen-bonded acid-base interactions.27-30 The former (-SO3-...NH+-) is formed after protons transfer from the acid site to the base site. This interaction is stable even in hot acidic solutions and contributes to the enhancement of morphological stability and reduction of methanol permeability.27,28 The latter (-SO3H...N) is formed after protons are immobilized between acid and base via hydrogen bonding. In addition, our previous study has demonstrated that in hydrated condition, a third loose

10.1021/jp104514t  2010 American Chemical Society Published on Web 09/28/2010

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acid-base interaction (-SO3H...(H2O)n...N, n e 3), where proton-donating and -accepting groups are linked by an intermediate water bridge, can be formed via hydrogen bonding between sulfonic acid group and phthalazinone group. Particularly, this loose hydrogen-bonded interaction can facilitate the transport of protons according to the Grotthuss mechanism, by which protons diffuse through the hydrogen bond network by the formation and cleavage of hydrogen bonds.15 For example, Siwick et al. found that protons transfer between photoacid 8-hydroxy-1,3,6-pyrenetrisulfonic acid (HPTS) and acetate in aqueous solutions involves a distribution of solvent-separated acid-base complexes. The proton is transferred inside these complexes via the Grotthuss mechanism through a hydrogenbound chain of water molecules connecting HPTS and acetate. Accordingly, aggregation of ionic and hydrogen-bonded acid-base interactions is capable of methanol blockage and proton transport. In this research, a novel strategy is proposed to balance proton conductivity and methanol crossover by embedding aggregations of ionic and hydrogen-bonded acid-base interactions (“barrier”) within a semicrystalline perfluorosulfonate polymer matrix. The membrane preparation, barrier formation, and enhancement of the fuel cell-related performances as well as the thermal stability of the resultant membranes will be considered and discussed in detail. Experimental Sections Materials. Perflorosulfonic acid polymer (FSP) resins (IEC ) 0.91 mmol g-1) were kindly supplied by Shandong Dongyue Polymer Materials (Zibo, China). Sulfonated poly(phthalazinone ether sulfone kentone) (SPPESK) with a sulfonation degree of 114% was synthesized by following a procedure as reported previously.31 Both of the polymers were transformed to the Na+ form by soaking them in 1 M NaOH solution for 24 h. N,N′Dimethylformamide (DMF) was of reagent grade and used as received. Deionized water was obtained by circulating distilled water through a milli-Q water purification system. All other reagents and chemicals were of analytical grades and used without further purification. Membrane Preparation. A DMF solution of the Na+ form FSP (25 wt %) was prepared by dissolving FSP resins with DMF in an autoclave at 160 °C for 5 h. After being purified by a centrifuge, the solution was cast on a clean glass plate and dried at 80 °C for 12 h to remove the solvent. Then the FSP membrane with a thickness of approximately 60 µm was obtained. These dry membranes were equilibrated in a NaCl/ HCl aqueous solution with a preset Na+/H+ concentration ratio. Naturally, there was partial exchange of protons with the sodium cations on FSP ion-exchange sites, establishing the protonation degree of the FSP membrane.32 After washing and drying, the FSP membranes with controlled protonation degrees were dissolved in an appropriate amount of SPPESK-DMF solution using an autoclave at 160 °C for 5 h. The solution was cast onto a glass plate and dried at 80 °C for 12 h to evaporate the solvent completely. The resulting composite membranes were heated in boiling deionized water for 1 h. For convenience, the obtained membranes were donated as (x%HFSP) - y, where x and y represent the protonation degree of FSP and the weight content of FSP component (here y is equal to 85), respectively. Annealing Procedure. Membranes in different ionic forms were heated at a certain temperature (higher than 100 °C) for 1 h and then cooled down slowly in an oven (about 9 °C/h).

Wu et al. After being annealed, the obtained membranes were stored in water for later use. Ion Exchange Capacity (IEC). IEC was determined by the Mohr method. The membrane in the H+ form was converted into the Na+ form after immersing in a NaCl aqueous solution (0.5 mol dm-3) for 8 h. The chloride ions released from the membrane were titrated with an aqueous solution of 0.01 mol dm-3 NaOH. The IEC values were calculated from the released H+ ions and expressed as mequiv g-1 of dry membrane (in the Cl- form). Protonation Degree. The protonation degree of FSP was calculated in eq 1

protonation degree (%) ) (100)IECNa/H /IEC100%H

(1) where IECNa/H is the IEC value of the partially protonated FSP and IEC100%H is the IEC value of the fully protonated FSP (i.e., 0.91 mmol g-1). Methanol Permeability. The methanol permeability was measured using a homemade permeation cell with two compartments and magnetic stirrers. Compartments A and B were filled with 150 cm3 of a 20% (v/v) methanol solution and 150 cm3 of deionized water, respectively. The membrane was positioned between these two compartments, and the diameter of the effective area was 1.0 cm. The concentration of methanol in compartment B was monitored using a refractive index detector (RI750F, Younglin Instrument Co., Korea), which was driven by a Masterflex pump through a 1 mm diameter silicon pipe at a constant speed of 1.0 cm3 min-1. The output signal was converted to the digital signal by a data module (Autochro, Younglin Instrument Co., Korea) and transferred to a programmed computer. After the change in methanol concentration during permeation was measured and the slope of the resulting curves was calculated, the methanol permeability (P) was calculated in eq 233

CB(t) )

AP C (t - t0) VB L A

(2)

where CA is the initial methanol concentration of the methanol solution, CB(t) is the methanol concentration in compartment B at time t, VB is the liquid volume in compartment B, L is the thickness of the membrane, A is the effective permeation area, and t0 is the time lag, i.e., the selected starting point for linear regressions. Proton Conductivity. The proton conductivity of FSPSPPESK composite membrane was measured using the normal four-point probe technique.34 An Autolab PGSTAT 30 (Eco Chemie, Netherland) was used to record the impedance data. The operation conditions were as follows: galvanostatic mode; ac current amplitude, 0.1 mA; frequency, from 1 MHz to 50 Hz. The determined frequency region (phase angle ≈ 0) was about 10000-500 Hz. Here, the proton conductivity (κ) was calculated in eq 3

κ)

L RWd

(3)

where R is the membrane resistance, L is the distance between potential-sensing electrodes (here 1 cm), and W and d are the width (here 1 cm) and thickness of the membrane, respectively.

Embedding a Proton Conductive Barrier

J. Phys. Chem. B, Vol. 114, No. 41, 2010 13123 followed by removal of the solvent (DMF) by heating at 80 °C for 5 h in a vacuum oven. Then a thin layer with an estimated thickness of 50 nm was obtained for the TEM measurement. The TGA spectra of composite membranes were obtained on a 60H DTG (Shimadzu, Japan). Measurements were performed in nitrogen with a temperature range of 10-600 °C and a heating rate of 10 °C min-1. Results and Discussion

Figure 1. Physical observation of the casting solutions and their corresponding membranes, which are prepared from (a) 100% protonated FSP and the Na+ form SPPESK and (b) 0% protonated FSP and the Na+ form SPPESK, respectively.

Evaluation of the Performance in Fuel Cells. The membrane electrode assembly (MEA) was prepared as follows: Pt/C electrode was prepared by spraying a catalyst, consisting of Pt/C catalyst (30 wt % Pt) and Nafion solution (Aldrich, 5 wt %, dissolved in ethanol), onto carbon cloth (E-TEK, U.S.A.). Then, the Pt/C electrode with a Pt loading of 0.381 mg cm-2 was put on the composite membrane, and the MEA was installed in the electrochemical cell chamber. All the fuel cell measurements were conducted in a flow of humidified H2/O2 at 0.1 MPa (H2, 110 mL min-1 and O2, 110 mL min-1) and 80 °C. Instrumentation. Solid-state nuclear magnetic resonance (NMR) spectra of heated and unheated SPPESK membranes were recorded on a Bruker Avance II+ NMR instrument at 400 MHz. The phase image of the membrane was obtained through atomic force microscopy (AFM, XE-100) under a noncontact mode at a lateral scan frequency of 0.1 Hz. Transmission electron microscopy (TEM) study was carried out at 200 keV on a JEM-2100 instrument. Four microliters of 1 wt % casting solution was dropped on to a carbon-coated copper grid,

Formation of the Membrane: Intermolecular Acid-Base Reaction between SPPESK and FSP. A critical parameter that ensures the success of membrane preparation is the protonation degree of FSP component, i.e., the ratio of initial -SO3H groups to total -SO3Na and -SO3H groups in the FSP membrane. Our preliminary tests showed that the cast solution is homogeneous and the final membrane is translucent when the protonation degree of FSP is higher than 5%; otherwise, the cast solution divides into two layers and the obtained membrane is heterogeneous with the morphology of macrophase separation. As evidence, Figure 1 presents the image taken by a digital camera (Canon, IXUS 60) in the cases of two casting solutions, which were prepared from 100% and 0% protonated FSP, respectively (Figure 1a, b). Obviously, the membrane prepared from 100% protonated FSP is homogeneous in macroscale. However, as shown in Figure 2a, such membrane has a multiphase morphology in microscale: the sphere of SPPESK phase domains are encapsulated in “shells”, and those capsules disperse uniformly in FSP. As discussed in our previous investigation,28 the formation of this special morphology is attributed to the intermolecular interaction (i.e., the ionic acid-base interaction) between the sulfonic acid groups of the H+ form FSP and the phthalazinone groups of the Na+ form SPPESK (Figure 2b). Such intermolecular interaction not only makes the composite system miscible but also ensures the success of embedding SPPESK into the FSP polymer matrix. Formation of the “Barrier”: Intramolecular Acid-Base Reaction within the SPPESK Phase. As discussed above, SPPESK can be embedded into FSP matrix through intermolecular interaction. As will be shown later, the embedding makes it possible to form a barrier to the transport of methanol but simultaneously does not affect the transfer of protons. To reveal the formation mechanism of the barrier, the TEM images of four DMF solutions of the H+ form SPPESK (1 wt %) heated at different temperatures are shown in Figure 3. At 120 °C, the solution is homogeneous. However, as the temperature increases from 150 to 170 °C, phase separation happens,

Figure 2. (a) TEM images of the composite membrane which is prepared from 100% protonated FSP and the Na+ form SPPESK; (b) ionic acid-base interaction between the H+ form FSP and the Na+ form SPPESK.

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Figure 3. TEM images of the DMF solutions of the H+ form SPPESK (1 wt %), which are heated at different temperatures: (a) 120 °C; (b) 150 °C; (c) 160 °C; (d) 170 °C.

Figure 4. Solid state 1H NMR spectra of heated and unheated H+ form SPPESK membranes.

and the sizes of dispersed phases increase accordingly. Such change is due to the formation of intramolecular interaction within SPPESK. As is well-known, one SPPESK molecule bears both acidic and basic groups (sulfonic acid groups and phthalazinone groups), and an ionic acid-base reaction similar to that between the sulfonic acid group of the H+ form FSP and the phthalazinone group of the Na+ form SPPESK will occur within the H+ form SPPESK molecules at high temperature. In order to verify this assumption, one piece of H+ form SPPESK membrane was treated at 170 °C for 1 h. The solid state 1H NMR spectra of treated and untreated SPPESK membranes are shown in Figure 4. Obviously, a new peak at 2.3 ppm, which is attributed to the N-H group, appears in the spectrum of the treated H+ form SPPESK membrane. This peak suggests the occurrence of intramolecular acid-base interaction when the proton in an acid site transfers to a base site between SPPESK molecules (chemical structure is shown in the inset of Figure 4). The ionic acid-base interaction at high temperatures occurs in the case of not only the pure SPPESK membrane but also the SPPESK-FSP composite membrane. Figure 5 shows the pictures of the composite membranes before (a) and after (b, c) annealing at 160 °C (b, Na+ form and c, H+ form). Apparently, the annealed H+ form membrane becomes much more flexible

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Figure 5. Digital images of the SPPESK-FSP composite membranes: (a) unannealed membrane; (b) Na+ form membrane annealed at 160 °C; (c) H+ form membrane annealed in the H+ form at 160 °C.

and translucent than the annealed Na+ form membrane. This gives evidence that ionic acid-base reaction can occur in the SPPESK phase domains in the case of composite membranes. However, comparing Figure 5a with Figure 5b, the annealed Na+ form membrane also become more flexible and translucent than the unannealed one. As we know, the ionic acid-base reaction cannot be formed in the Na+ form SPPESK due to the lack of acid sites.28 The possible reason for the morphological change is the recrystallization of FSP domains due to its semicrystalline nature.35 In order to verify this assumption, the morphological change of pure FSP membrane before and after annealing was investigated using AFM. As shown in Figure 6a, the FSP membrane before annealing is almost not crystalline (the white domains represent crystalline regions); however, after annealing, many crystalline domains appear and enhance the morphological stability of membrane (Figure 6b). Therefore, in the system of FSP/SPPESK composite membranes, the annealing-induced ionic acid-base interactions within SPPESK domains form a barrier while recrystallization of FSP domains enhances the mechanical stability of the composite membranes. Evaluation of Methanol Crossover in the Barrier. Figure 7 shows the methanol permeabilities of the H+ form composite membranes annealed at different temperatures. Obviously, as the annealing temperature increases, methanol permeability decreases. Especially, when the temperature is higher than 140 °C, the permeability decreases remarkably. It indicates that the ionic acid-base interactions in both SPPESK domain and interlayer between FSP and SPPESK contribute to the blockage of methanol. Usually, the methanol permeability increases with an increase in the water content due to the high affinity of methanol to water, so the reduction in methanol permeability suggests the poor hydrophilicity of ionic acid-base interactions in the membranes. This point can be supported by the change of IECs in Figure 8. When the annealing temperature increases from 100 to 140 °C, IECs of both the annealed H+ form and Na+ form composite membranes exhibit the same decreasing trend. However, when temperature increases from 140 to 180 °C, IECs of the annealed H+ form membranes decrease sharply, whereas those of the annealed Na+ form membranes decrease slightly. This distinctive difference indicates that intramolecular reaction begins at about 140 °C. When the annealing temperature is lower than 140 °C, only recrystallization occurs in the FSP phase, and some -SO3H groups are buried in the crystalline domains, leading to a decrease in IEC. However, when

Embedding a Proton Conductive Barrier

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Figure 6. AFM phase images of FSP membranes: (a) unannealed; (b) annealed at 160 °C.

Figure 9. Proton conductivities of the composite membranes annealed at different temperatures. Figure 7. Methanol permeabilities of Nafion-112 and the H+ form composite membranes annealed at different temperatures.

Figure 8. IECs of the composite membranes annealed at different temperatures.

temperature is higher than it, in the H+ form membrane, an acid-base reaction takes place in the SPPESK phase, so more -SO3H groups are consumed and thus there is a quicker

decrease in IEC as compared with those in Na+ form membranes. Obviously, the -SO3H groups trapped in those interactions cannot adsorb as many water molecules (usually more than six H2O per -SO3H) as other free ones. Hence, those poorly hydrophilic acid-base networks do not attract much methanol, resulting in low methanol permeability (Figure 7). This is also the reason why all the composite membranes exhibit lower methanol permeability when compared with Nafion-112. Evaluation of the Proton Conduction in the Barrier. As mentioned above, SPPESK phase domains are successfully converted to a barrier to methanol after annealing. However, proton conductivities of composite membranes annealed at different temperatures in Figure 9 suggest that the barrier is unfavorable for proton conduction. When the annealing temperature is higher than 140 °C, the proton conductivities of the annealed H+ form membranes decrease as the temperature increases. It implies that the poorly hydrophilic ionic acid-base interactions do not facilitate proton conduction. Additionally, a “contradiction” between Figures 8 and 9 suggests that a special structure, other than ionic acid-base interaction, is unfavorable for proton conduction in this barrier too. This contradiction is that the proton conductivities of the H+ form membranes annealed at 150 and 160 °C (0.060 and 0.056 S cm-1 at 25 °C

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Figure 10. Fuel cell performances of Nafion-112 and the composite membranes annealed in the H+ form at different temperatures.

in Figure 9) are lower than that of Nafion-112 (0.1 S cm-1 at 25 °C15) even though their IECs are comparable to each other (about 0.9 mmol g-1 in Figure 8). As is well-known, proton conductivity is usually measured using a normal four-point probe technique and dependent on the amount of protons which are dissociated from sulfonic acid groups. Therefore, this contradiction suggests that, in the barrier, some protons cannot be dissociated from free sulfonic acid groups during the measurement of proton conductivity. As reported, under hydrated conditions, a loose acid-base interaction, in which acid and base are linked by an intermediate water bridge (the maximum number of water molecules is three), can be formed.28-30 Accordingly, in this barrier, similar loose acid-base interaction (Figure 9), in which protons are trapped within this hydrogenbonded complex, exists and results in the lower proton conductivity. Apparently, in Nafion-112, no formation of the loose acid-base interaction due to the lack of basic site makes its conductivity a little higher. Evaluation of the Fuel Cell Performances of the Annealed Composite Membranes. As discussed above, the ionic and hydrogen-bonded acid-base interactions in the barrier are disadvantageous to proton conduction. However, when comparing the fuel cell performances of the composite membranes with those of Nafion-112 (Figure 10), it is surprising to find that the H+ form composite membranes annealed at 150 and 160 °C exhibit better fuel cell performances than Nafion-112 under identical conditions even though their proton conductivities are lower. This contradiction results from the difference in the methods of evaluating proton transfer. Different from the normal four-point probe technique for proton conductivity measurement, the fuel cell measurement is conducted in an electrochemical cell chamber, in which protons can be supplied by the electrode, so the influence of protons trapped in the hydrogen-bonded acid-base interactions on the whole proton transfer efficiency can be neglected. Notably, this hydrogen-bonded acid-base interaction can facilitate proton transfer via the Grouthuss-type mechanism.27,28 Therefore, protons released from one electrode can transfer through this barrier continuously by this unique structure (c.f. the inset in Figure 10): the proton in H3O+ first interacts with the -SO3H group in this hydrogen-bonded acid-base interaction, then transfers along the hydrogen bonds to arrive at the base, and finally moves out of the acid-base pair by hydrogen bonds forming and breaking between the protons and water molecules. Such special structure makes the composite membranes excellent fuel cell performers, even superior to Nafion-112: 0.93 W cm-2 of the maximum power

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Figure 11. TGA and DrTGA data of annealed FSP/SPPESK composite membranes, FSP and SPPESK: (a) H+ form FSP/SPPESK composite membrane annealed at 150 °C; (b) H+ form FSP/SPPESK membrane annealed at 160 °C; (c) SPPESK membrane; (d) FSP membrane.

density at a current density of 2.3 A cm-2 and a cell voltage of 0.41 V. To sum up, the barrier has a slight negative effect on the proton conductivity, but it does not affect any of the singlecell performances due to the formation of a special structure therein. When the single-cell performances of the membranes annealed at different temperatures are compared, it is shown that the performances decrease as temperature increases and a remarkable drop in membrane performance will be unacceptable if annealed at a temperature higher than 170 °C (Figure 10). The reason is as follows: as the temperature increases, more -SO3H groups are trapped in the ionic acid-base interactions and the membranes become more hydrophobic, and thus the hydrogen-bonded acid-base interactions are separated and cannot form a continuous pathway for proton transfer. Hence, an annealing temperature range (150-160 °C) is recommended for the success of this strategy. Thermal Stability of Composite Membranes. The thermal stability of the composite membranes is the crucial for their practical applications. It was examined by TGA in a nitrogen atmosphere at a heating rate of 10 °C/min. Figure 11 shows the results of annealed FSP/SPPESK composite membranes along with SPPESK and FSP membranes. In comparison, the thermal stability of annealed composite membranes is superior to that of SPPESK membrane and similar to that of FSP membrane. The derivative curves of TG profiles (Figure 11) indicate that all membranes have three decomposition stages: (1) below 100 °C, loss of bonded water; (2) 300-430 °C, splitting of SO3Na and side chains of polymers; (3) over about 430 °C, decomposition of polymer chains. Interestingly, at the second decomposition stage, the beginning decomposition temperature of -SO3Na groups and side chains of composite membranes are all higher than that of FSP membrane. Particularly, the decompositions of composite membranes start at about 350 °C and reach their maxima at about 400 °C while that of FSP membrane starts at about 300 °C and reaches its maximum at about 340 °C (curves a, b, d in Figure 11). All evidence indicates that thermal stability of the composite membranes can be enhanced by acid-base interactions in SPPESK phase and crystalline domains in FSP phase. Conclusion In summary, a simple strategy to prepare proton conductive membranes of high performance is demonstrated by annealing a H+ form FSP/SPPESK composite membrane at 150-160 °C.

Embedding a Proton Conductive Barrier The annealing induces favorable morphological modifications: (1) an increase in the crystallinity of the FSP phase strengthens the morphological stability; (2) the barrier, which is formed from the embedded amphoteric SPPESK by intramolecular acid-base reaction, can block methanol but does not affect any of the proton transfer. As a result, this membrane exhibits a fuel cell performance superior to Nafion-112: 0.93 W cm-2 of the maximum power density at a current density of 2.3 A cm-2 and a cell voltage of 0.41 V. This barrier-embedding strategy is not limited to the above FSP/SPPESK system. It can be applied to any systems containing semicrystalline and amphoteric polymers. Additionally, both FSP and SPPESK are all industrial products. The price of FSP is around 1/3 that of Nafion, while SPPESK is much less expensive than FSP. Considering the simple preparation procedure, the costs of the composite membranes are much lower than that of Nafion. Thus this research will provide a versatile route to prepare low-cost membranes for fuel cells with tailored properties. Acknowledgment. We are thankful for the financial support from the National Natural Science Foundation of China (nos. 20974106 and 20636050), the National Natural Science Funds for Distinguished Young Scholar, the NSFC-KOSEF Scientific Cooperation Program (nos. F01-2009-000-10171-0, 20911140273.), the National Basic Research Program of China (no. 2009CB623403), the Specialized Research Fund for the Doctoral Program of Higher Education (no. 200803580015), and the China Postdoctoral Science Foundation (no. 20090460060). References and Notes (1) Dresselhaus, M. S.; Thomas, I. L. Nature 2001, 414, 332. (2) Schaeffer, G. J.; Uyterlinde, R. J. Power Sources 1998, 71, 256. (3) Yang, Z. Y.; Rajendran, R. G. Angew. Chem., Int. Ed. 2005, 44, 564. (4) Eikerling, M.; Kharkats, M. Y.; Kornyshev, A. A.; Volfkovich, Y. M. J. Electrochem. Soc. 1998, 145, 2684. (5) Scott, K.; Taama, W. M.; Argyropoulos, P.; Sundmacher, K. J. Power Sources 1999, 83, 204. (6) Shukla, A. K.; Christensen, P. A.; Hamnett, A.; Hogarth, M. P. J. Power Sources 1995, 55, 87. (7) Kreuer, K. D. J. Membr. Sci. 2001, 185, 29.

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