Sulfonated SBA-15 mesoporous silica-incorporated sulfonated poly(phenylsulfone) composite membranes for low-humidity proton exchange membrane fuel cells: Anomalous behavior of humidity-dependent proton conductivity

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Sulfonated SBA-15 mesoporous silica-incorporated sulfonated poly(phenylsulfone) composite membranes for low-humidity proton exchange membrane fuel cells: Anomalous behavior of humidity-dependent proton conductivity Ji-Hye Won a, Hyeon-Ji Lee a, Kyung-Suk Yoon b, Young Taik Hong b,**, Sang-Young Lee a,* a

Department of Chemical Engineering, Kangwon National University, Chuncheon, Kangwondo 200-701, Republic of Korea Energy Materials Research Center, Korea Research Institute of Chemical Technology, P.O. Box 107, Yuseong, Daejeon 305-600, Republic of Korea b

article info

abstract

Article history:

Sulfonated

Received 10 November 2011

phenylsulfone) (SPPSU) composite membranes are fabricated for potential application in

SBA-15

mesoporous

silica

(SM-SiO2)-incorporated

sulfonated

poly(-

Received in revised form

low-humidity proton exchange membrane fuel cells (PEMFCs). The SM-SiO2 particles are

3 March 2012

synthesized using tetraethoxy silane (TEOS) as a mechanical framework precursor, Plur-

Accepted 8 March 2012

onic 123 triblock copolymer as a mesopore-forming template, and mercaptopropyl trime-

Available online 20 April 2012

thoxysilane (MPTMS) as a sulfonation agent. A distinctive feature of the SM-SiO2 particles is the long-range ordered 1-D skeleton of hexagonally aligned mesoporous cylindrical

Keywords:

channels bearing sulfonic acid groups. Based on a comprehensive characterization of the

Proton exchange membrane fuel

SM-SiO2 particles, the effect of SM-SiO2 (as a functional filler) addition on the proton

cells

conductivity of the SPPSU composite membrane is examined as a function of temperature

Sulfonated SBA-15 mesoporous

and relative humidity. An intriguing finding is that the proton conductivity of the SPPSU

silica

composite membrane exhibits a strong dependence on the relative humidity of

Sulfonated poly(phenylsulfone)

measurement conditions. This anomalous behavior is further discussed with an in-depth

Composite membranes

consideration of the characteristics and dispersion state of SM-SiO2 particles, which

Proton conductivity

affect the tortuous path for proton movement, water uptake, and state of water. Notably, at

Low humidity

low-humidity conditions, the SM-SiO2 particles in the SPPSU composite membrane serve as an effective water reservoir to tightly retain water molecules and also as a supplementary proton conductor, whereas they behave as a barrier to proton transport at fully hydrated conditions. Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

1.

Introduction

Proton exchange membrane fuel cells (PEMFCs) have garnered considerable attention as a promising power source for

various applications including portable devices, automobiles, and residential power supplies [1e4]. Among core components of PEMFCs, the polymer electrolyte membrane is considered as a crucial element in terms of realizing

* Corresponding author. Tel.: þ82 33 250 6338; fax: þ82 33 251 3658. ** Corresponding author. Tel.: þ82 42 860 7292; fax: þ82 42 860 7237. E-mail addresses: [email protected] (Y.T. Hong), [email protected] (S.-Y. Lee). 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2012.03.036

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successful commercialization of PEMFCs [5e8]. The primary functions of the polymer electrolyte membrane are to allow proton conduction from the anode to cathode and to maintain electrical isolation between the electrodes. In general, proton transport of water-swollen polymer electrolyte membranes such as perfluorosulfonic acid copolymers (e.g., Nafion) is known to highly depend on the degree of hydration [4e13]. In particular, when the hydrated membranes are exposed to low-humidity conditions, they tend to be easily dehumidified and pose a formidable challenge related to substantially decreased proton conductivity. In efforts to overcome these drawbacks of water-swollen membranes, numerous approaches have been proposed. One promising attempt is the incorporation of hygroscopic inorganic oxide fillers such as SiO2, TiO2, ZrO2, and montmorillonite into polymer electrolyte membranes, which effectively improve water retention in the resulting composite polymer electrolyte membranes [14e19]. Recently, various types of mesoporous metal oxides, which can present excellent thermal stability and water adsorption capability, have been investigated as high-temperature/lowhumidity proton conductors [19e27]. Notably, homogeneous and ordered mesoporous silicas have been studied extensively due to their unusual particle characteristics such as high surface area and well-defined mesopore size/distribution. These mesopore structures also offer a considerable amount of active sites that allow introduction of protogenic groups (e.g., sulfonic acid groups), which contribute to the proton conductivity of the resulting mesoporous silicas [21e27]. In general, the functionalized mesoporous silicas have been exploited as effective functional fillers in composite polymer electrolyte membranes. Owing to the incorporation of the silicas with the high acid strength and adjustable acid group density, the proton conductivity of the composite membranes can be significantly improved. Ma et al. [23] demonstrated that the addition of sulfonated mesoporous silica into Nafion improved the proton conductivity of composite membranes. Lu et al. [24] reported that the thermal properties, water uptake, and proton conductivity of sulfonated polyimide membranes were significantly influenced by sulfonated mesoporous silica additives. Kim et al. [25] showed that the sulfonated mesoporous silica was effective in improving water uptake and water retention of Nafion composite membranes, particularly at high-temperature/low-humidity conditions. Wu et al. [26] introduced sulfonated organically modified mesoporous silica as a new additive for sulfonated poly(ether ether ketone) (SPEEK) proton exchange composite membranes. Kang et al. [27] synthesized triazole-attached mesoporous silica and incorporated it into Nafion for the purpose of improving hightemperature/low-humidity proton conductivity. Over the past decade, sulfonated poly(phenylsulfone) copolymer (SPPSU) has been investigated as a promising alternative to Nafion due to its high thermal stability, excellent mechanical strength, cost competitiveness, and strong resistance to membrane decomposition in an acidic water medium [8e13]. However, similar to other water-swollen polymer electrolytes, the SPPSU membrane tends to easily lose proton conductivity at dehumidified conditions. In the present study, in an endeavor to resolve these limitations of SPPSU, we fabricate a new SPPSU-based proton

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exchange composite membrane by exploiting the aforementioned advantages of functionalized mesoporous silica particles. Herein, sulfonated SBA-15 mesoporous silica (hereinafter, referred to as “SM-SiO2”) particles are chosen as a functional filler for the SPPSU composite membrane. The SMSiO2 particles are synthesized using tetraethoxy silane (TEOS) as a mechanical framework precursor and a Pluronic 123 triblock copolymer as a template for the formation of mesopores. Sulfonation of the SBA-15 mesoporous silica was conducted by co-condensation of TEOS and mercaptopropyl trimethoxysilane (MPTMS, thiol precursor) in the presence of hydrogen peroxide (H2O2) under acidic conditions [19,26]. The present work is, to the best of our knowledge, the first report that addresses the fabrication of SPPSU composite membranes incorporating sulfonated SBA-15 mesoporous silica particles, where the SM-SiO2 particles act as a water reservoir to enhance water retention capability and also as a supplementary proton conductor allowing intermolecular proton transfer between adjacent sulfonic acid groups in well-aligned 1-D mesoporous cylindrical channels. Hence, it can be reasonably expected that the SM-SiO2 particles play a viable role in improving the proton conductivity of the SPPSU composite membrane, particularly at dehumidified conditions. The anomalous behavior of proton conductivity of the SPPSU composite membranes is further discussed with an indepth consideration of the characteristics (specifically, chemical structure, particle morphology, and mesoporous architecture) and dispersion state of SM-SiO2 particles, which are believed to affect the tortuous path for proton movement, water uptake, and state of water. In hydrated membranes, the state of water can be broadly classified into two groups: physically adsorbed water (i.e., free- and weakly-bound water) and chemically adsorbed water (i.e., strongly-bound water) [28e31]. Under low-humidity conditions, the contribution of chemically adsorbed water to proton transport of hydrated membranes becomes more important. Therefore, a thorough investigation of the state of water will help provide a better understanding of the effects of SM-SiO2 addition on the lowhumidity proton conductivity of the SPPSU composite membrane.

2.

Experimental

2.1. Synthesis of sulfonated SBA-15 mesoporous silica (SM-SiO2) Sulfonated SBA-15 mesoporous silica (SM-SiO2) particles were synthesized using TEOS (Aldrich) as a mechanical framework precursor, a Pluronic 123 triblock poly(ethylene oxide-bpropylene oxide-b-ethylene oxide) copolymer (Aldrich) as a mesopore-forming template, and MPTMS (Aldrich) and H2O2 (Daejung Chemical, Korea) as sulfonation agents. The detailed synthesis procedure of SM-SiO2 particles was established based on previous publications [19,26]. First, 4 g of Pluronic 123 was added into 125 g of 2 M HCl at room temperature. The mixture was vigorously stirred and then heated to 40  C, prior to adding TEOS (¼7.6874 g). After the TEOS was prehydrolyzed for 45 min, the MPTMS and aqueous solution of H2O2 were added into the solution simultaneously and the resulting

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mixture was further stirred at 40  C for 20 h and aged at 100  C for an additional 24 h under static condition. The molar composition of each mixture for 4 g of Pluronic 123 was xTEOS:(0.041  x)MPTMS:yH2O2:0.24HCl:6.67H2O, where x ¼ 0.0369 and y ¼ 0.0123. The solid product was recovered by conducting filtration and then air-dried at room temperature overnight. The Pluronic 123 template was carefully removed from the as-synthesized material by washing with ethanol under reflux for 24 h (1.5 g of the as-synthesized material per 400 mL of ethanol). Finally, the material was washed several times with ethanol and vacuum-dried at 60  C overnight, yielding rod-shaped SM-SiO2 particles. A schematic illustration depicting the mesoporous architecture of SM-SiO2 particles, along with chemical structures of TEOS, Pluronic 123 triblock copolymer, and MPTMS, is provided in Fig. 1(a).

2.2. Fabrication of SPPSU composite membranes incorporating SM-SiO2 particles Prior to the fabrication of the SPPSU composite membranes incorporating SM-SiO2 particles, SPPSU (inherent viscosity ¼ 2.02 dl g1) was synthesized via nucleophilic aromatic substitution polymerization of 4,40 4,40 -dichlordichlorodiphenylsulfone, 3,30 -disulfonated odiphenylsulfone, and 4,40 -biphenol. The detailed synthesis and characterization of SPPSU were described in previous publications [8e13,32,33]. The chemical structure of SPPSU and its 1H NMR spectrum are presented in Fig. 2. From analysis of the 1H NMR spectrum, the degree of sulfonation was determined to be 49.3%.

The synthesized SPPSU was dissolved in dimethylacetamide (DMAc) as a solvent to prepare a SPPSU solution, where the SPPSU concentration in the solution was 10 wt%. A predetermined amount (¼0.5 wt%) of the previously synthesized SM-SiO2 particles was added to the SPPSU solution. The SPPSU/SM-SiO2 mixture was transferred into a zirconium bead-loaded chamber and in turn was vigorously agitated by a mechanical rotor for 2 h. This bead-milling process is known to allow a good dispersion of nanoparticles [33]. Subsequently, the solution was poured into a glass plate and then dried at 80  C for 4 h. Finally, the SPPSU composite membranes were acidified in a 1 N boiling sulfuric acid solution for 2 h and subsequently rinsed with distilled water for 2 h. The final thickness of the SPPSU composite membranes was measured to be around 60 mm. A schematic representation illustrating the structure of the SPPSU composite membranes is also presented in Fig. 1(b). Meanwhile, a pristine SPPSU membrane was prepared as a control sample by casting a 10 wt% SPPSU solution onto a glass plate. Except for employing the SPPSU solution itself without adding the SM-SiO2 particles, the entire fabrication procedure was identical to that used for the SPPSU composite membrane. The thickness of the SPPSU membrane was found to be approximately 60 mm.

2.3. Characterization of SM-SiO2 particles and SPPSU composite membranes Morphological characterization of the SM-SiO2 particles was carried out using a field emission scanning electron microscope (FE-SEM, S-4800, Hitachi) and a transmission electron

Fig. 1 e Schematic representations depicting structures of: (a) SM-SiO2 particles; (b) SPPSU composite membrane, wherein the conceptual proton transport pathway of the SPPSU composite membrane is also illustrated.

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Fig. 2 e Chemical structure and 1H NMR spectrum of sulfonated poly(phenylsulfone) (SPPSU).

microscope (TEM, LEO-912AB Omega, LEO) equipped with an energy-dispersive spectrometer (EDS). X-ray powder diffraction (XRD) patterns of the SM-SiO2 particles were acquired on a PANalytical diffractometer using CuKa radiation (l ¼ 0.1541 nm), where the data were collected from 0.61 to 3.99 (2q) with a resolution of 0.02 . The BrunauereEmmetteTeller (BET) specific surface area of the SM-SiO2 particles was determined by measuring a nitrogen adsorption/desorption isotherm (ASAP 2020, Micromeritics), where the pore size distribution was calculated using the BarretteJoynereHalenda (BJH) formula [19,25e27]. The chemical structure of the SM-SiO2 particles was identified using a FT-IR spectrometer (FT-3000, Bio-Rad) with a spectral resolution of 2 cm1. The dispersion state of the SM-SiO2 particles in the SPPSU matrix was evaluated using a FE-SEM equipped with an EDS. The proton conductivity of the SPPSU composite membrane as well as that of a pristine SPPSU membrane was measured with an impedance analyzer (VSP classic, Bio-Logic) using the four probe method over a frequency range of 101e2  105 Hz [12e14], where the membranes were equilibrated in a temperature/humidity control chamber (SH-241, ESPEC) under given conditions of temperature and relative humidity (RH). The state of water in the SPPSU composite membranes was analyzed by conducting a thermal characterization using differential scanning calorimetry (DSC, Q2000, TA Instruments), where an aluminum hermetic pan was employed to prevent water loss during the measurement. DSC samples were held at 50  C for 0.5 h and then heated to 50  C at a heating rate of 1  C min1. The fraction of physically adsorbed water was quantified by comparing the experimental melting enthalpy (DHexperimental) of the membranes at around 0  C with the theoretical melting enthalpy (DHtheoretical (J g1) ¼ 334.0) of pure water [13,29e31]. The amount of chemically adsorbed water was calculated by subtracting the physically adsorbed water from the total water uptake. More specifically, the number of water molecules (l(H2O/SO 3 )) per sulfonic acid group, physically adsorbed water molecules per

sulfonic acid group (lphysical(H2O/SO 3 )), and chemically adsorbed water molecules per sulfonic acid group (lchemi cal(H2O/SO3 )) can be estimated using the following equations [13,32,38]:
of water molecules=number of SO l H2 O=SO 3 ¼ number 3


 lphysical H2 O=SO3 ¼ DHexperimental =DHtheoretical  l H2 O=SO 3


  lchemical H2 O=SO 3 ¼ l H2 O=SO3  lphysical H2 O=SO3 , where the number of water molecules is given by the water uptake and the number of SO 3 is obtained from the ion exchange capacity (IEC) values. Here, the IEC values were quantitatively measured using a conventional titration method (ASTM D2187 [13,32]).

3.

Results and discussion

3.1. Structural characterization of sulfonated SBA-15 mesoporous silica (SM-SiO2) Prior to investigating the SPPSU composite membrane, the basic characteristics of the SM-SiO2 particles were evaluated. The FE-SEM photograph (Fig. 3(a)) shows rod-shaped SM-SiO2 particles with average length of approximately 1 mm. In the present study, the SM-SiO2 particles were synthesized via cocondensation of MPTMS and TEOS in the presence of H2O2 and P123 block copolymer under an acidic condition. The inset of Fig. 3(a) depicts the characteristic EDS peaks, which are attributable to Si (silicon), O (oxygen), and S (sulfur), which verify the presence of SiO2 and sulfonic acid groups in the SMSiO2 particles. The sulfonic acid groups of the SM-SiO2 particles were further characterized by analyzing the FT-IR spectrum (Fig. 3(b)). The characteristic FT-IR peaks assigned to the stretching vibrations of SieOeSi linkage [26,27] are observed at around 799 cm1 and 1067 cm1. The weak absorbance at 1355 cm1 is assigned to the asymmetric stretching of sulfonic

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In addition, the mesoporous structure of the SM-SiO2 particles was quantitatively evaluated by analyzing a nitrogen adsorption/desorption isotherm. Fig. 4(d) shows a clear hysteresis loop associated with capillary condensation in the mesopores (type IV isotherm), which indicates the formation of mesoporous structure in the SM-SiO2 particles. The analysis of this isotherm exhibits that the SM-SiO2 particles has a BET surface area of 722.8 m2 g1, pore volume of 0.90 cm3 g1, and pore size of 5.67 nm. Notably, the specific surface area of the SM-SiO2 is about 24 times higher than that of a conventional nonporous SiO2 (for example, specific surface area of SiO2 with average particle size ¼ 40 nm is observed to be approximately 30 m2 g1 [34]). This enormously large surface area of the SMSiO2 particles, in conjunction with their sulfonated 1-D mesoporous cylindrical channels, is expected to play a crucial role in retaining water molecules in the SPPSU composite membranes, which could be effective in improving proton conductivity at low-humidity conditions.

3.2. Morphological uniqueness and temperature-/RHdependent proton conductivity of SPPSU composite membranes

Fig. 3 e Characterization of SM-SiO2 particles: (a) A FE-SEM photograph (an inset is an EDS spectrum of Si and S elements); (b) A FT-IR spectrum of sulfonic acid groups and SieOeSi linkage (an inset verifies the presence of sulfonic acid groups in SM-SiO2).

acid groups (the inset in the Fig. 3(b)). The FT-IR peaks observed at 1000e1200 cm1 are known to represent the S]O stretching vibrations [22]; however, they cannot be resolved due to their overlap with the aforementioned SieOeSi stretching vibrations [26]. This characterization of the FT-IR peaks demonstrates that the SBA-15 mesoporous silica is successfully functionalized with the sulfonic acid groups. The TEM photographs (Fig. 4(a) and (b)) exhibit the evolution of long-range ordered 1-D skeleton of hexagonally aligned mesoporous cylindrical channels in the SM-SiO2 particles. This unique mesoporous architecture of the SM-SiO2 particles was also confirmed by conducting XRD measurements. Fig. 4(c) shows that the SM-SiO2 particles have three characteristic XRD peaks, corresponding to (100), (110), and (200) planes, which are consistent with the results of typical SBA-15 type particles [19,26,27]. Hence, the XRD spectrum of the SMSiO2 particles verifies the long-range-ordered mesoporous structure with excellent textural uniformity of hexagonal space groups.

The dispersion state of the SM-SiO2 particles in the SPPSU composite membrane was characterized by observing its cross-sectional morphology. Fig. 5(a) shows that the SM-SiO2 particles are not easily detected in the FE-SEM photograph, indicating that the SM-SiO2 particles may be homogeneously dispersed in the SPPSU composite membrane. This is also confirmed by observing the high transparency of the SPPSU composite membrane (Fig. 5(b)). The excellent dispersion of the SM-SiO2 particles in the SPPSU composite membrane may be due to the exploitation of bead-milling process allowing intensive mixing [33] and possible hydrogen bonding of sulfonic acid groups [26] between SM-SiO2 and SPPSU. The presence of SM-SiO2 particles in the SPPSU composite membrane was also verified by analyzing the EDS images (the inset in Fig. 5(a)). The bright dots signifying Si elements of SM-SiO2 are uniformly dispersed in the thickness direction of the SPPSU composite membrane. The incorporation of the SM-SiO2 particles into the SPPSU membrane is expected to significantly influence the proton conductivity of the SPPSU composite membrane. Fig. 6(a) shows that at a measurement condition of 100% RH, the temperature-dependent proton conductivities of both the pristine SPPSU membrane and SPPSU composite membrane increase as the temperature is increased from 30 to 80  C. This is attributed to enhanced mobility of hydrated protons and also SPPSU polymer chains at higher temperatures. Interestingly, over a wide range of temperature, the SPPSU composite membranes present lower proton conductivity than the pristine SPPSU membrane. This lower proton conductivity of the fully hydrated SPPSU composite membrane was discussed further by considering its water uptake and tortuous path for proton movement. Fig. 6(b) compares the water uptake of the fully hydrated SPPSU composite membrane with that of the pristine SPPSU membrane as a function of temperature. The addition of hygroscopic SM-SiO2 particles slightly increases the water uptake of the SPPSU composite membrane (for example, at

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Fig. 4 e Characterization of SM-SiO2 particles: (a) and (b) TEM photographs; (c) A XRD spectrum; (d) A nitrogen adsorption/ desorption isotherm.

80  C, water uptake w235% for the pristine SPPSU vs. 252% for the SPPSU composite membrane). The larger water uptake of the SPPSU composite membrane may impart an advantageous effect on the proton conductivity. However, contrary to this positive expectation, the experimental results (Fig. 6(a)) show that the presence of SM-SiO2 deteriorates the proton conductivity of the SPPSU composite membrane. This indicates that at the condition of 100% RH, the contribution of the slightly increased water uptake to the proton conductivity of the fully hydrated SPPSU composite membrane is not appreciable. Meanwhile, the tortuous path for proton movement in the SPPSU composite membrane was investigated by quantitatively estimating the tortuosity factor [35,36], which is commonly used in separator membranes for lithium-ion batteries. The tortuosity factor (¼s) is defined by the equation s2 ¼ NM  3 , where NM is the MacMullin number (NM ¼ so/ sm, so is the ionic conductivity of liquid electrolyte, sm is the ionic conductivity of electrolyte-immersed separator membrane), and 3 is the porosity of the separator membrane. In this study, in order to apply this equation to the SPPSU composite membrane, each factor in the equation is replaced as follows: separator membrane / SPPSU composite membrane, ionic conductivity of liquid electrolyte / proton conductivity of pristine SPPSU membrane, ionic conductivity of electrolyte-immersed separator membrane / proton conductivity of SPPSU composite membrane, and porosity of

separator membrane / volume fraction of SPPSU electrolyte in SPPSU composite membrane. Fig. 6(c) exhibits that the tortuosity factor (¼sp) for proton movement increases from 1.000 (pristine SPPSU membrane) to 1.034 (SPPSU composite membrane). This quantitative estimation of the tortuosity factor demonstrates that the SM-SiO2 particles in the fully hydrated SPPSU composite membrane provoke an increase in the tortuous path and therefore may behave as a barrier that hampers proton transport. In other words, at the condition of 100% RH, proton transfer via 1-D mesoporous cylindrical channels of SM-SiO2 particles may be sluggish, in contrast to facile proton movement through water channels formed in the fully hydrated SPPSU electrolytes. This is similar to the results of previous studies on non-functionalized silicaincorporated proton exchange composite membranes [14e19,33]. A schematic representation illustrating the conceptual proton transport pathway of the fully hydrated pristine SPPSU membrane and SPPSU composite membrane is also provided in Fig. 6(c). On the other hand, at low-humidity conditions, significantly different behavior of proton conductivity was observed in the SPPSU composite membrane. Fig. 7(a) shows that, at a low-humidity condition of 80  C/50% RH, the proton conductivities of the pristine SPPSU membrane and SPPSU composite membrane tend to decrease as the measurement time is increased and finally become equilibrated after a certain amount of time (about 100 min) has passed. This

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composite membrane yields larger water uptake than the pristine SPPSU membrane, which is consistent with the proton conductivity results provided in Fig. 7(b). Interestingly, the RH-dependent water uptake between the pristine and composite membrane becomes more pronounced as the RH is decreased. This larger water uptake of the SPPSU composite membrane is ascribed to the presence of sulfonic acid groups and the mesoporous structure of the SM-SiO2 particles, both of which are known to tightly retain water molecules [22e27].

Fig. 5 e Morphological characterization of SPPSU composite membrane: (a) A FE-SEM photograph (cross-section, an inset is an EDS image of Si elements, wherein the bright dots signify Si elements of SPPSU); (b) A photograph of SPPSU composite membrane.

decline of proton conductivity with elapsed time indicates that the membranes are being dehydrated under the lowhumidity condition. An intriguing finding is that the proton conductivity (w5.9 mS cm1) of the SPPSU composite membrane is higher than that (w3.6 mS cm1) of the pristine SPPSU membrane. This reveals that the incorporation of SMSiO2 particles is beneficial in improving the proton conductivity at the low-humidity condition. Fig. 7(b) compares the proton conductivities of the SPPSU composite membrane with those of the pristine SPPSU membrane as a function of RH at 80  C. Over a wide range of RH, the SPPSU composite membrane presents higher proton conductivity than the pristine SPPSU membrane. Notably, the difference in the proton conductivity between the two membranes becomes more pronounced as the RH is decreased (i.e., s (pristine SPPSU membrane)/s (SPPSU composite membrane) ¼ 0.878 at 80% RH vs. 0.612 at 50% RH). This strong dependence of proton conductivity on the RH is further confirmed by measuring the water uptake of the membranes as a function of RH. Fig. 7(c) shows that the SPPSU

Fig. 6 e Comparison of properties between pristine SPPSU membrane and SPPSU composite membrane at a condition of 100% RH (relative humidity): (a) Temperature-dependent proton conductivity; (b) Water uptake; (c) A schematic representation illustrating conceptual proton transport pathway and tortuosity factor for proton movement.

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Fig. 7 e Proton conductivities and water uptake of pristine SPPSU membrane and SPPSU composite membrane: (a) Time evolution of proton conductivity at 80  C/50% RH; (b) RH-dependent proton conductivity at 80  C; (c) RHdependent water uptake at 80  C.

These results of water uptake underline that the advantageous effect of SM-SiO2 particles on the water retention of the SPPSU composite membrane is more distinct at lower RH conditions, which in turn contributes to improved proton conductivity of the SPPSU composite membrane at the dehumidified conditions.

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In order to attain a better understanding of the RH-dependent proton conductivity, an in-depth investigation on the state of water in the membranes was carried out. The state of water in the membranes was analyzed using DSC thermograms (Fig. 8(a)) and interpreted in terms of physically adsorbed water and chemically adsorbed water. Meanwhile, Zawodzinski et al. [37] raised a cautionary note on the quantitative use of thermodynamic data for elucidating the state of water in water-swollen membranes. Thus, in the present study, the characterization of water molecules based on the DSC thermograms has been restricted to making a relative comparison between the samples. Prior to conducting the DSC measurement, the fully hydrated membranes were pre-equilibrated for 2 h at 80  C/ 50% RH, which was chosen as a representative measurement condition reflecting the dehumidified environments. At this low-humidity condition, it is expected that free- and weaklybound water molecules could be easily removed from the membranes, which may lead to a considerable loss in the lphysical(H2O/SO 3 ) during the pre-equilibration. Thus, this indicates that the proton conductivity of dehumidified membranes may be strongly dependent on the lchemical(H2O/  SO 3 ) rather than lphysical(H2O/SO3 ). Fig. 8(a) depicts the DSC thermograms of the membranes pre-equilibrated at 80  C/50% RH. Due to the pre-equilibration, the melting enthalpies of the membranes are found to be considerably small (DHexperimental ¼ 0.37 J g1 for the pristine SPPSU membrane vs. DHexperimental ¼ 0.23 J g1 for the SPPSU composite membrane), indicating that lphysical(H2O/SO 3 ) obtained from the melting enthalpies of the membranes may be extremely insignificant, as compared to lchemical(H2O/SO 3 ). For example, for the dehumidified SPPSU composite membrane, which has an IEC value of 1.96 mmol g1, lphysical(H2O/SO 3 ) and lchemical(H2O/ 3 and 2.80. SO 3 ) were respectively estimated to be 1.962  10 A notable finding is that the addition of SM-SiO2 allows an increase in lchemical(H2O/SO 3 ). Fig. 8(b) exhibits that lchemi cal(H2O/SO3 ) of the sPPSU composite membrane (¼2.80) is higher than that (¼1.75) of the pristine SPPSU membrane. This increase in the number of chemically adsorbed water molecules can be reasonably attributed to the incorporated SM-SiO2 particles bearing sulfonic acid groups (Fig. 3) and the substantially large specific surface area of the highly-ordered mesoporous structure (Fig. 4). It is already known that, at lowhumidity conditions, the proton conductivity of polymer electrolyte membranes is strongly influenced by chemically adsorbed water molecules [14,28,34,38]. Therefore, the larger amount of chemically adsorbed water molecules in the SPPSU composite membranes could be solid evidence explaining the improvement of proton conductivity at low-humidity conditions. In addition, it is reasonably speculated that, at lowhumidity conditions where the proton conductivity of the bulk SPPSU electrolyte becomes substantially decreased, proton transport through the 1-D mesoporous cylindrical channels of the SM-SiO2 particles, which is driven by the Grotthuss mechanism based on intermolecular proton transfer between adjacent sulfonic acid groups [19,26], could be appreciable and therefore may offer an additional route for proton conduction. A more detailed investigation on proton conductivity of the SM-SiO2 particles in the SPPSU matrix will

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be performed in future studies. A schematic illustration depicting the conceptual proton transport pathway (via water channels of bulk SPPSU electrolytes and also mesoporous cylindrical channels of SM-SiO2 particles) of the SPPSU

composite membrane at low RH conditions is presented in Fig. 8(c). The aforementioned proton conductivity results of the SPPSU composite membranes emphasize the unusual function of the incorporated SM-SiO2, which heavily depends on RH conditions.

4.

Conclusion

We have developed sulfonated SBA-15 mesoporous silica (SMSiO2) particle-incorporated SPPSU composite membranes for application in low-humidity PEMFCs. The in-depth structural characterization of SM-SiO2 particles exhibited the evolution of sulfonated 1-D mesoporous cylindrical channels with excellent textural uniformity of hexagonal space groups. Owing to the assistance of the bead-milling process and possible hydrogen bonding of sulfonic acid groups between SM-SiO2 and SPPSU, a homogeneous dispersion of SM-SiO2 particles in the SPPSU composite membrane was obtained. In a fully hydrated condition of 100% RH, the SPPSU composite membrane presented lower proton conductivity than the pristine SPPSU membrane, which indicated that the incorporated SM-SiO2 particles might behave as a barrier to proton migration. This decrease in the proton conductivity was also explained by estimating the longer tortuous path for proton movement. In contrast to the proton conductivity results observed at 100% RH, at low-humidity conditions, the SPPSU composite membrane showed higher proton conductivity than the pristine SPPSU membrane, which underlines the advantageous effects of the SM-SiO2 particles. In the SPPSU composite membrane, the increase in the number of chemically adsorbed water molecules as well as the water uptake, in combination with the hexagonally aligned 1-D mesoporous cylindrical channels of sulfonated silica endowing additional proton conduction, played a crucial role in improving the lowhumidity proton conductivity. This anomalous behavior of RH-dependent proton conductivity of the SPPSU composite membrane demonstrates that the incorporated SM-SiO2 particles behave as a barrier to proton transport at fully hydrated conditions, whereas they serve as an effective water reservoir to tightly retain water molecules and also as a supplementary proton conductor at low-humidity conditions. Subsequent studies will be devoted to further improving the proton conductivity of SPPSU composite membranes by fine-tuning the concentration, functionality, and dispersion of mesoporous silica particles, which will be followed by electrochemical characterization of MEA (membraneeelectrode assemblies) incorporating SPPSU composite membranes.

Fig. 8 e Analysis of state of water for pristine SPPSU membrane and SPPSU composite membrane: (a) DSC thermograms after being pre-equilibrated for 2 h at 80  C/ 50% RH; (b) The number of chemically adsorbed water molecules per sulfonic acid group (lchemical(H2O/SOL 3 )). (c) A conceptual proton transport pathway (via water channels of bulk SPPSU electrolytes and mesoporous cylindrical channels of SM-SiO2 particles) of SPPSU composite membrane at low RH conditions.

Acknowledgments This research was supported by a grant from the Fundamental R&D Program for Core Technology of Materials funded by the Ministry of Knowledge Economy. This research was also supported by a grant from the Fundamental R&D Program for Technology of World Premier Materials funded by the Ministry of Knowledge Economy, Republic of Korea.

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 7 ( 2 0 1 2 ) 9 2 0 2 e9 2 1 1

references

[1] Steel BCH, Heinzel A. Materials for fuel-cell technologies. Nature 2001;414:345e52. [2] Devanathan R. Recent developments in proton exchange membranes for fuel cells. Energy Environ Sci 2008;1:101e19. [3] Buratto SK. Fuel cells: engineering the next generation. Nat Nanotechnology 2010;5:176. [4] Peighambardoust SJ, Rowshanzamir S, Amjadi M. Review of the proton exchange membranes for fuel cell applications. Int J Hydrogen Energy 2010;35:9349e84. [5] Mauritz KA, Moore RB. State of understanding of Nafion. Chem Rev 2004;104:4535e85. [6] Kreuer KD. On the development of proton conducting polymer membranes for hydrogen and methanol fuel cells. J Membr Sci 2001;185:29e39. [7] Thiam H, Daud W, Kamarudin S, Mohammad A, Kadhum A, Loh K, et al. Overview on nanostructured membrane in fuel cell applications. Int J Hydrogen Energy 2011;36:3187e205. [8] Hickner MA, Ghassemi H, Kim YS, Ensla BR, McGrath JE. Alternative polymer systems for proton exchange membranes (PEMs). Chem Rev 2004;104:4587e612. [9] Wang F, Hickner M, Kim YS, Zawodzinski TA, McGrath JE. Direct polymerization of sulfonated poly(arylene ether sulfone) random (statistical) copolymers: candidates for new proton exchange membranes. J Membr Sci 2002;197:231e42. [10] Hill ML, Kim YS, Einsla BR, McGrath JE. Zirconium hydrogen phosphate/disulfonated poly(arylene ether sulfone) copolymer composite membranes for proton exchange membrane fuel cells. J Membr Sci 2006;283:102e8. [11] Neburchilov V, Martin J, Wang H, Zhang J. A review of polymer electrolyte membranes for direct methanol fuel cells. J Power Sources 2007;169:221e38. [12] Kwon YH, Kim SC, Lee SY. Nanoscale phase separation of sulfonated poly(arylene ether sulfone)/poly(ether sulfone) semi-IPNs for DMFC membrane applications. Macromolecules 2009;42:5244e50. [13] 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. J Membr Sci 2010;346:131e5. [14] Lee JR, Kim NY, Lee MS, Lee SY. SiO2/PEI-coated polyimide nonwoven/Nafion composite membranes for proton exchange membrane fuel cells. J Membr Sci 2011;367:265e72. [15] Jie Z, Haolin T, Mu P. Fabrication and characterization of selfassembled Nafion-SiO2-ePTFE composite membrane of PEM fuel cell. J Membr Sci 2008;312:41e7. [16] Chen SY, Han CC, Tsai CH, Huang J, Yang YW. Effect of morphological properties of ionic liquid-templated mesoporous anatase TiO2 on performance of PEMFC with Nafion/TiO2 composite membrane at elevated temperature and low relative humidity. J Power Sources 2007;171:363e72. [17] Park KT, Jung UH, Choi DW, Chun K, Lee HM, Kim SH. ZrO2/ SiO2/Nafion composite membrane for polymer electrolyte membrane fuel cells operation at high temperature and low humidity. J Power Sources 2008;177:247e53. [18] Mohtar SS, Ismail AF, Matsuura T. Preparation and characterization of SPEEK/MMT-STA composite membrane for DMFC application. J Membr Sci 2011;371:10e9. [19] Margolese D, Melero JA, Christiansen SC, Chmelka BF, Stucky GD. Direct syntheses of ordered SBA-15 mesoporous silica containing sulfonic acid groups. Chem Mater 2000;12: 2448e59. [20] Tripathi BP, Shahi VK. Organic-inorganic nanocomposite polymer electrolyte membranes for fuel cell applications. Prog Polym Sci 2011;36:945e79.

9211

[21] Jin Y, Qiao S, Zhang L, Xu Z, Smart S, Costa J. Novel Nafion composite membranes with mesoporous silica nanospheres as inorganic fillers. J Power Sources 2008;185:664e9. [22] Wang XG, Cheng SF, Chan JCC. Propylsulfonic acidfunctionalized mesoporous silica synthesized by in-situ oxidation of thiol groups under template-free condition. J Phys Chem C 2007;111:2156e64. [23] Lin YF, Yen CY, Ma CCM, Liao SH, Lee CH, Hsiao YH, et al. High proton-conducting Nafion/eSO3H functionalized mesoporous silica composite membranes. J Power Sources 2007;171:388e95. [24] Liu D, Geng L, Fu Y, Dai X, Lu¨ C. Novel nanocomposite membranes based on sulfonated mesoporous silica nanoparticles modified sulfonated polyimides for direct methanol fuel cells. J Membr Sci 2011;366:251e7. [25] Hong LY, Oh SY, Matsuda A, Lee CS, Kim DP. Hydrophilic and mesoporous SiO2eTiO2eSO3H system for fuel cell membrane applications. Electrochim Acta 2011;56:3108e14. [26] Wu J, Cui Z, Zhao C, Li H, Zhang Y, Fu T, et al. High proton conductive advanced hybrid membrane based on sulfonated Si-SBA-15. Int J Hydrogen Energy 2009;34:6740e8. [27] Park SJ, Lee DH, Kang YS. High temperature proton exchange membranes based on triazoles attached onto SBA-15 type mesoporous silica. J Membr Sci 2010;357:1e5. [28] Kreuer KD, Paddison SJ, Spohr E, Schuster M. Transport in proton conductors for fuel-cell applications: simulations, elementary reactions, and phenomenology. Chem Rev 2004; 104:4637e78. [29] Kim YS, Dong L, Hickner MA, Glass TE, Webb V, McGrath JE. State of water in disulfonated poly(arylene ether sulfone) copolymers and a perfluorosulfonic acid copolymer (Nafion) and its effect on physical and electrochemical properties. Macromolecules 2003;36:6281e5. [30] Saito M, Arimura N, Hayamizu K, Okada T. Mechanisms of ion and water transport in perfluorosulfonated ionomer membranes for fuel cells. J Phys Chem B 2004;108:16064e70. [31] Su YH, Liu YL, Sun YM, Lai JY, Liu B, Guiver MD. Proton exchange membranes modified with sulfonated silica nanoparticles for direct methanol fuel cells. J Membr Sci 2007;296:21e8. [32] Yoon SJ, Choi JH, Hong YT, Lee SY. Synthesis and characterization of sulfonated poly(arylene ether sulfone) ionomers incorporating perfluorohexylene units for DMFC membranes. Macromol Res 2010;18:352e7. [33] Yoon KS, Choi JH, Hong YT, Hong SK, Lee SY. Control of nanoparticle dispersion in SPAES/SiO2 composite proton conductors and its influence on DMFC membrane performance. Electrochem Comm 2009;11:1492e5. [34] Lee JR, Won JH, Kim NY, Lee MS, Lee SY. Hydrophilicity/ porous structure-tuned, SiO2/polyetherimide-coated polyimide nonwoven porous substrates for reinforced composite proton exchange membranes. J Colloid Interface Sci 2011;362:607e14. [35] Abraham KM. Directions in secondary lithium battery research and development. Electrochim Acta 1993;38: 1233e48. [36] Doyle M, Newman J, Gozdz A, Schumutz CN, Tarascon JM. Comparison of modeling predictions with experimental data from plastic lithium ion cells. J Electrochem Soc 1996;143: 1890e903. [37] Kalapos TL, Decker B, Every HA, Ghassemi H, Zawodzinski TA. Thermal studies of the state of water in proton conducting fuel cell membranes. J Power Sources 2007;172:14e9. [38] So SY, Kim SC, Lee SY. In-situ hybrid Nafion/SiO2-P2O5 proton conductors for high-temperature and low-humidity proton exchange membrane fuel cells. J Membr Sci 2010;360: 210e6.