Synthesis and characterization of anhydrous proton conducting inorganic–organic composite membranes for medium temperature proton exchange membrane fuel cells (PEMFCs)

Energy 35 (2010) 5260e5268

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Synthesis and characterization of anhydrous proton conducting inorganiceorganic composite membranes for medium temperature proton exchange membrane fuel cells (PEMFCs) G. Lakshminarayana a, Masayuki Nogami a, *, I.V. Kityk b a b

Department of Materials Science and Engineering, Nagoya Institute of Technology, Showa, Nagoya 466-8555, Japan Electrical Engineering Department, Czestochowa Technological University, Al. Armii Krajowej 17, Czestochowa, Poland

a r t i c l e i n f o

a b s t r a c t

Article history: Received 25 January 2010 Received in revised form 24 July 2010 Accepted 26 July 2010 Available online 15 September 2010

We report on anhydrous proton conducting inorganiceorganic composite membranes synthesized from tetraethoxysilane/poly(-dimethylsiloxane)/trimethylphosphate and 1-ethyl-3 methylimidazolium-bis (trifluoromethanesulfonyl) imide ionic liquid as solegel precursors. The Fourier transform infrared spectroscopy, 31P, 1H, and 13C Nuclear magnetic resonance, thermo gravimetric and differential thermal analysis measurements confirmed that the prepared hybrid membranes possess good chemical stability and are thermally stable up to 350
C. Conductivity of all the fabricated hybrid membranes was measured under anhydrous conditions within the temperature range 20e150
C, and a value of 4.87  103 S/cm at 150
C was achieved for 40 wt% [EMI][TFSI] ionic liquid doped 72TEOSe18PDMSe10PO (OCH3)3 (mol %) hybrid membrane. For 40 wt% ionic liquid doped composite membrane, the measured hydrogen permeability value at 150
C was 4  1012 mol/cm s Pa. Ó 2010 Elsevier Ltd. All rights reserved.

Keywords: Inorganiceorganic composites Conductivity Hydrogen permeability

1. Introduction Recently, proton exchange membrane fuel cells (PEMFCs) are recognized as one of the most promising clean energy power sources due to their high energy conversion efficiency, high energy density, and wide range of applicability, which includes applications to stationary, transport, and portable uses [1e6]. Moreover, the operation of PEMFCs at temperatures above 100
C has been considered to be advantageous because of improved tolerance of platinum for carbon monoxide (CO), faster electrode kinetic, higher energy efficiency, and simplified heat management [7e10]. However, typical polymer electrolyte membranes such as the NafionÒ series exhibit decreased proton conductivities that abruptly occur at temperatures above 100
C, mostly due to a loss of water molecules from the membranes. Therefore, there has been a great deal of research in the development of anhydrous electrolyte membranes with high proton conductivities at higher temperatures [11e15]. Inorganic acids such as H2SO4, H3PO4 or HPAs have shown excellent electrical properties, but their high chemical activity is a serious drawback for practical applications [16]. Recently, phosphoric acid doped polybenzimidazoles (PBIs) were reported as promising electrolyte candidates for high

* Corresponding author. Tel./fax: þ81 52 735 5285. E-mail addresses: [email protected], [email protected] (M. Nogami). 0360-5442/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.energy.2010.07.039

temperature proton exchange membrane fuel cells, with high performance [2,17e20]. These proton exchange membranes have shown high proton conductivity at temperatures up to 200
C without humidification, excellent oxidative and thermal stability, high fuel impurity tolerance, low reactant permeability, and nearly zero water drag coefficient. However, the preparation of phosphoric acid doped PBI membranes was relatively expensive, time-consuming, required multiple chemical reaction steps and not suitable for large scale production [21]. Generally, the inorganiceorganic composite membranes based on organosiloxanes would be a combination of siloxane linkages, organic chains and mixed acid species [7]. The inorganic component of siloxane linkages would increase the thermal and chemical stabilities of the membrane where as the organic species would improve the flexibility [22]. Through the solegel reactions, it is possible to incorporate preformed oligomers or polymers that are often functionalized with trialkoxysilyl groups into the organiceinorganic networks via the co-condensation of functionalized oligomers or polymers with metal alkoxides. The backbone of poly(-dimethylsiloxane) (PDMS) and the hydrolysis or condensation products of tetraethoxysilane (TEOS) possess the same nature of chemical bonds (SieOeSi) which help to make the organic and inorganic components more compatible. The basic three reaction schemes between TEOS and PDMS would be as follows [23]:

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Hydrolysis:

SiðORÞ4 þ4H2 O/SiðOHÞ4 þ4ROH

(1)

(R is C2H5, CH3etc.) Co-condensation with PDMS:

(2)

Self-condensation:

SiðOHÞ4 /SiO2 þ 2H2 O

(3)

PDMS could also be maintained in solution with the reactive solegel components during the reaction. If hydrolysis of the metal alkoxide is sufficiently rapid so as to provide hydrolysis products of the alkoxide for reaction with the silanol-terminated PDMS, a better dispersion of the functionalized PDMS in the final network could be achieved. Less microphase separation and improved molecular uniformity could be achieved with the use of PDMS of lower molecular weight. Depending on the amount and molecular weight of PDMS used, the final materials could be either flexible or brittle and all would show optical transparency clearly indicating that phase (domain) size is smaller than the wavelength of visible light. The flexibility of these organiceinorganic hybrids could be achieved by carefully controlling the solegel reaction processes of tetraethoxysilane (TEOS) and silanol-terminated poly(-dimethylsiloxane) (PDMS) moieties [24]. Previous works of rubbery ormosils by Schmidt [25] and ceramers by Wilkes [26] are examples of flexible and thermally stable organiceinorganic hybrids. In those works [24e26], the inorganic parts are incorporated to crosslink flexible polymer such as PDMS and provide structural stability of the hybrid network at the same time [24e29]. Room temperature ionic liquids (RTILs) are molten salts that are liquid over a wide temperature range including ambient temperatures. These ionic liquids have been paid increasing attention as new solvents for a number of applications due to their excellent characteristics such as high thermal stability, very low vapor pressure, nonflammability and high intrinsic ionic conductivity. They typically consist of a large organic cation such as imidazolium, pyridinium, pyrrolidinium, etc. and an anion that possesses a highly    delocalized charge such as PF 6 , BF4 , CF3SO3 , or N(CF3SO2)2 . Generally, the choice of the cation and the anion constituting an ionic liquid has a profound effect on the physical properties such as viscosity, density, conductivity, and polarity. Ionic liquids offer a great flexibility in their properties since the possible combinations of cations and anions are quit high. These cations and anions weakly coordinated to each other. The bulky and asymmetrical configuration of the ions reduces their electrostatic interaction and prevents the ions from neatly packing together in a lattice and, thus, results in salts with low melting temperatures. The above unique properties of ionic liquids make them suitable candidates as advanced electrolyte materials in lithium ion batteries [30e33] and fuel cells [34e37]. Fig. 1 depicts the chemical structure of [EMI] [TFSI] ionic liquid. The main feature of the [TFSI] anion is the extensive charge delocalization making it very weakly coordinating. In addition, the [TFSI] anion has a large flexibility around the SeNeS bond and exists in two conformations (cis and trans). [TFSI] anion has the molecular structure F3CeS(O2)eNeS(O2)eCF3, and the terminal CF3 group can rotate along the SeN bond to give

Fig. 1. Chemical structure of [EMI][TFSI] ionic liquid.

rotational isomers. The low basicity of the TFSI anion decreases the rate of formation of a hydrogen bond with H(2) of the imidazolium cation, resulting in a random aggregation of the ions in the [EMI] [TFSI] ionic liquid [7]. The conformation of the anion is important for the ioneion interactions in the system and for the properties of the material. Under non-humidified conditions, the proton transport could be due to Grotthuss mechanism, in which only protons will move from site to site without the assistance of water molecules [7,22,38]. In this work, the synthesized composite membranes are characterized with respect to their structural, thermal, anhydrous conductivity, and hydrogen permeability properties. 2. Experimental studies 2.1. Preparation of composite membranes The composite membranes were prepared by using tetraethyl orthosilicate (Si (OC2H5)4, TEOS, 99.9%, Colcote, Japan), Poly(-dimethoxysilane), hydroxy terminated (PDMS, 98%) (400e700 g per mole, Aldrich Chemical Com.), trimethylphosphate (PO (OCH3)3, 98%, Colcote, Japan), and [1-ethyl-3 methyl imidazolium][bis-(trifluoromethanesulfonyl) imide] ([EMI][TFSI], 99%, Tokyo Chemical Com. Ltd.) ionic liquid as precursors. All the initial solvents and materials were used as received. Water purified with a Milli-Q system from Millipore (AQUARIUS/GS-20R, Japan) was used for the preparation of hybrid membranes. Several compositions of TEOS ePDMSePO (OCH3)3e[EMI][TFSI] (i.e., 72TEOSe18PDMSe10PO (OCH3)3ex [EMI][TFSI] (x ¼ 10, 20, 30, and 40 wt%)) were selected for content optimization. It should be emphasized that we added [EMI] [TFSI] ionic liquid content in excess wt% to the host membrane 72TEOSe18PDMSe10PO (OCH3)3 (mol %), respectively. Initially, the calculated amount of TEOS was hydrolyzed with water (as 0.15NeHCl aq) and ethanol under constant magnetic stirring for 90 min at 50
C. The molar ratio of TEOS/EtOH/H2O/HCl was 1/8/4/0.01. After, PDMS which was pre-hydrolyzed with water (as 0.15NeHCl aq) and ethanol was added to TEOS solution under constant stirring for 90 min at 50
C. The amount of PDMS taken was equal to 10 wt% of total PDMS þ TEOS weight. The value of Rw (molar ratio of H2O/alkoxide) was fixed at 4 for the preparation of sols 90 min later, when the above mixed solution became transparent, trimethylphosphate dissolved in C2H5OH with 5 mol of H2O (1:5) was added drop-wise under stirring for 1 h. After this reaction, [EMI]

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[TFSI] ionic liquid was added drop-wise to the hydrolyzed solution under argon atmosphere and the reaction was carried out for 3 h under constant magnetic stirring. Now, a homogeneous solution was obtained. Finally, (N-N-Dimethylformamide) HCON (CH3)2 (1e3 ml) was added in standard solution followed by 60 min stirring. Such obtained clear transparent solution was called as sol. The sol was cast onto Petri dishes and gelled at 50
C for 14 days. Gel films obtained were then dried at 50
C for 6 h, and consecutively at 100
C for 6 h, 150
C for 6 h, and at 250
C for 6 h. Generally, ionic liquids show thermal stability upto 400
C. Therefore, we have selected the gel films heat-treatment temperature range 50
Ce250
C, in steps of 50
C increment, keeping the membranes for 6 h at each temperature. The sample thickness was varied from 0.1 mm to 0.5 mm. The host phosphosilicate membrane in the absence of [EMI][TFSI] ionic liquid was also prepared for comparison with the same procedure described above. In TEOS/PDMS covalently bonded network, TEOS has controllable hydrolysis reaction rate and PDMS possess good thermal stability. The complete synthetic process is shown in Fig. 2 (A). The typical photograph of the 40 wt% [EMI][TFSI] ionic liquid doped hybrid membrane is shown in Fig. 2(B). Following the photograph one can see that the prepared composite membrane is highly transparent and mechanically stable and also still keeps a casting morphology even after drying 14 days at 50
C. The thickness of the membrane was uniform and no macrocrack and fat edge were observed. Usually, in a solegel process, the evaporation of solvent could cause an internal stress gradient in the gel and when the stress is high enough, cracks would appear. 2.2. Characterization of membranes The optical transmission spectra of all the fabricated membranes were recorded using JASCO Ubest 570 UVeviseNIR spectrophotometer with spectral resolution,1 nm. All the spectra were recorded in air at ambient temperature. The BrunauereEmmetteTeller (BET) specific surface areas of the hybrid membranes were determined from N2 adsorptionedesorption isotherms at liquid nitrogen temperature. Hybrid membranes of approximately 0.05 g were degassed at 250
C for a minimum of 6 h under vacuum. The pore size distributions were analyzed with a Quantochrome-NOVA-1000 nitrogen gas sorption analyzer. The Fourier transform infrared (FT-IR) spectra of the hybrid membranes were measured by JASCO FT-IR-460 Plus spectrometer with spectral resolution about 4 cm1. The FT-IR spectra were obtained within the spectral range 4000e400 cm1 using the KBr pellet for reference. The thermal degradation process and stability of the composites were examined by thermo gravimetric analysis (TGA), differential thermal analysis (DTA) and differential scanning calorimeter (DSC) (Thermoplus 2, TG- 8120, Rigaku), respectively. The measurements were carried out under dry air and with a heating rate of 5
C/min. The 31P magicangle-spinning (MAS) nuclear magnetic resonance (NMR) spectra were measured with a Varian Unity Inova 300 spectrometer at 121.42 MHz frequency with a sample spinning rate of about 5000 Hz, and chemical shifts were measured with reference to 85% aqueous phosphoric acid (H3PO4). The solid state high-resolution 1H and 13C magic-angle-spinning (MAS) NMR spectra were measured by using a Varion UNITY-400 plus spectrometer at a resonance frequency of 400 MHz, respectively. The 1H and 13C NMR spectra were obtained by a single-pulse sequence, and the 31P NMR spectra were traced by a single-pulse sequence with high power 1H decoupling. All the NMR measurements were carried out at room temperature, 25
C. The conductivity of the prepared hybrid membranes was measured by an AC method using Solartron SI-1260 impedance analyzer. Gold was evaporated on both sides of the composite membranes as the electrodes, and conductivity was evaluated from the impedance data obtained in the frequency range 1e107 Hz with signal

amplitude of 10 mV. The temperature was raised in steps of 10
C and the measurements were carried out after keeping the membrane at each temperature for 1 h under dry nitrogen. Conductivity was measured within the temperature range 20e150
C. The conductivity (s) was evaluated from the electrolyte resistance (R) obtained from the intercept of the ColeeCole plot with the real axis, the thickness (l) and the electrode area (A) according to the equation s ¼ l/AR. Hydrogen permeability of the composite membranes (0.5 mm thick) was measured by using a forced convection drying oven (DO-600FA), under dry conditions, consisting of two compartments with a capacity of approximately 50 cm3, separated by a vertical membrane with an effective area of 20 cm2. The contents of the compartments were under constant agitation. Gas concentrations were measured by Varian CP-4900 Micro GC gas chromatography with relative standard deviation (RSD) of 0.13%. 3. Results and discussion Fig. 3 shows the ultravioletevisible (UVevis) transmission spectra of all the produced hybrid membranes, including host phoshosilicate matrix. For the host membrane, a transmittance value of 85% was observed. Moreover, with the ionic liquid weight percent ratio increment, the transmittance value decreased from 85% (host membrane) to 79% (40 wt% [EMI][TFSI] doped hybrid membrane), respectively. However, these composite membranes have shown relatively high optical transmittance value (78%) at 800 nm wavelength. Therefore, from Fig. 3 one can conclude that all the prepared membranes are highly transparent. Fig. 4 shows the FT-IR spectra of all the prepared composite membranes doped with 10e40 wt% [EMI][TFSI] ionic liquid including host phosphosilicate membrane. From this figure, for 72TEOSe18PDMSe10PO (OCH3)3 (mol %) host membrane, absorption bands centered at 450, 585, 805, 855, 946,1058,1205,1275,1664, 2972, and 3506 cm1 wavenumbers are identified. The broad spectral band centered at 1058 cm1 wavenumber could be assigned to SieOeSi bond in the prepared phosphosilicate matrix. The absorption band observed at 1664 cm1 wavenumber could be due to the interstitial H2O molecules bending band. The broad absorption band extending from 3000 to 3750 cm1 wavenumber is due to the combination of OH stretching mode, free and hydrogen-bonded H2O molecules that exist in the host phosphosilicate membrane. These observed OH bonding groups can interact with the OH groups that are connected to SiO2 surface to establish SieOeSi bonds and H2O molecules will be released as the condensation product. The several spectral bands observed within the wavenumber range 450e855 cm1 can be assigned to the SieOeSi bending and stretching vibrations, respectively. The absorption bands appeared at 1205 and 1275 cm1 wavenumbers should be assigned to the antisymmetric stretching in SieOeSi bonds, and O]P stretching bond [39], respectively. The observed broad spectral band within the wavenumber range 900e950 cm1 can be attributed to SiþeO stretching vibration of SieOH bonding groups. In phosphosilicate matrices, the IR absorption band at 890 cm1 can be assigned to PeOeP asymmetric stretching mode. Spectral bands centered at 443, 513, 570, 615, 650, 701, 745, 811, 853, 958, 1057, 1137, 1195, 1271, 1359, 1468, 1572, 1628, 2984, 3113, 3163, and 3458 cm1 are identified for 10e40 wt% [EMI][TFSI] ionic liquid doped 72TEOSe18PDMSe10PO(OCH3)3 (mol%) composite membranes. All the observed spectral bands can be assigned similarly to the pure [EMI][TFSI] ionic liquid [7]. With the ionic liquid weight percent ratio increment from 10 to 40 wt%, all the observed absorption bands intensity increased gradually in the prepared hybrids. Also, following Fig. 4 one can conclude that all the phosphosilicate matrices related absorption bands extending from 400 to 1200 cm1 wavenumber range are overlapped well with ionic liquid related absorption bands. These results conclude that in all the

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Fig. 2. (a) Synthetic procedure for obtaining the 72TEOSe18PDMSe10PO (OCH3)3ex [EMI][TFSI] (x ¼ 10, 20, 30, and 40 wt%) hybrid membranes (b) Photograph of 40 wt% [EMI]TFSI] ionic liquid doped hybrid membrane.

fabricated hybrid membranes, [EMI][TFSI] ionic liquid is mixed well with phosphosilicate matrices. Fig. 5(aed) presents the nitrogen adsorptionedesorption isotherms of 10e40 wt% ionic liquid doped hybrid membranes. From these curves, it is confirmed that all the fabricated hybrid membranes have no porous structure and

BrunauereEmmetteTeller (BET) surface areas are calculated to be lower than 0.5 m2/g. This result suggests that all the fabricated hybrid membranes were structurally dense. In these hybrid membranes, the doped ionic liquid should be filled in the pores of the phosphosilicate membrane structure due to the strong

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Fig. 3. UVevisible transmission spectra of 72TEOSe18PDMSe10PO (OCH3)3ex [EMI] [TFSI](x ¼ 0, 10, 20, 30, and 40 wt%) membranes.

Fig. 4. FT-IR spectra of 72TEOSe18PDMSe10PO (OCH3)3ex [EMI][TFSI](x ¼ 0, 10, 20, 30, and 40 wt%) membranes.

interaction of ionic liquid with phosphosilicate matrix. Recently, different kinds of proton conducting electrolyte membranes using trimethylphosphate as a proton donor were reported [40e44]. In those works, it was noticed that the change in PeOH bonds concentration would cause to a change in the proton conductivity also, due to the variation of charge carriers (protons) density. Fig. 6 exhibits the 31P MAS NMR spectra of 72TEOS e18PDMSe10PO (OCH3)3ex[EMI][TFSI] (x ¼ 0, 20, and 40 wt%) membranes. Generally, silicate and phosphate matrices can be described in terms of QN and Q1N basic structural units [SiO4] or [PO4]

tetrahedra with N bridging oxygen atoms. Following these spectra, for 72TEOSe18PDMSe10PO (OCH3)3 (mol %) host phosphosilicate membrane, a sharp peak at 2.547 ppm is assigned to isolated phosphorus (Q0 unit). For 20, and 40 wt% ionic liquid doped composite membranes, this peak is slightly shifted and found at 2.362, and 2.180 ppm, respectively. This observed shift in Q0 unit for ionic liquid doped composite membranes suggests the interaction between ionic liquid and trimethylphosphate. Proton (1H) and Carbon-13(13C) Nuclear Magnetic Resonance (NMR) are powerful methods used in the determination of organic compounds structure. It is well known

Fig. 5. N2 adsorption isotherms curves for (a) 10 wt% [EMI][TFSI] (b) 20 wt% [EMI][TFSI] (c) 30 wt% [EMI][TFSI] and (d) 40 wt% [EMI][TFSI] ionic liquid doped 72TEOSe18PDMSe10PO (OCH3) (mol%) hybrid membranes.

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Fig. 6. 31P MAS NMR spectra of 72TEOSe18PDMSe10PO (OCH3)3ex [EMI][TFSI](x ¼ 0, 20, and 40 wt%) membranes.

that the chemical shifts of the imidazolium ring protons are anion- and concentration-dependent. Usually, the 1H NMR signals are always single lines for each assigned nucleus without multiple signals. This suggests that, even if there are paired, dissociated, or aggregated ionic species, the exchange rate for the chemical equilibrium between the dissociated and associated ions in the ionic liquids is faster than the time scale of NMR measurements. Fig. 7(a,b) presents the 1H and 13C MAS NMR spectra of 10e40 wt% [EMI][TFSI] ionic liquid doped composite membranes, respectively. 1H chemical shifts at 8.232, 7.089, 3.478, 1.200, and 0.212 ppm; 8.176, 7.040, 3.485, 2.441, 1.145, 0.224, and 0.243 ppm; 8.146, 7.010, 3.491, 2.404, 1.114, and 0.255 ppm; and 8.127, 6.973, 3.792, 3.478, 2.791, 2.367, 1.096, and 0.274 ppm are identified for 10, 20, 30, and 40 wt% ionic liquid doped 72TEOSe18PDMSe10PO(OCH3)3 hybrid membrane, respectively (Fig. 7(a)). As the ionic liquid weight percent ratio increases from 10 to 40 wt%, all the observed 1H NMR signals intensity also increased in the prepared hybrids. Moreover, in the fabricated composite membranes, several chemical shifts related to [EMI][TFSI] ionic liquid [7] were slightly shifted due to the host phosphosilicate matrix interaction with ionic liquid. Also following Fig. 7(a), for all the composite membranes no chemical shift was identified at 10.2 ppm, which is the characteristic 1H chemical shift for HeN bond in HTFSI anion [7]. These results confirmed the presence of [EMI][TFSI] ionic liquid in the composite membranes, even after their heat-treatment at 250
C/6 h, and its interaction with trimethylphosphate. Following the 13C NMR spectra, (Fig. 7(b)), the chemical shifts at [122.230, 55.388, 36.436, 15.188, and 0.046 ppm]; [136.277, 124.428, 122.649, 55.340, 45.668, 36.387, 15.090, and 0.052 ppm]; [136.228, 124.261, 122.600, 55.291, 45.570, 36.338, 14.993, and 0.052 ppm]; and [136.180, 124.261, 122.552, 55.242, 45.521, 36.290, 14.944, and 0.144 ppm], respectively, are identified for 10, 20, 30, and 40 wt% ionic liquid doped 72TEOSe18PDMSe10PO(OCH3)3 composite membrane. Furthermore, it should be emphasized that in all the prepared composite membranes, with the [EMI][TFSI] ionic liquid weight percent ratio increment the intensity of chemical shifts within the range 120e140 ppm, and 36e45 ppm is increased gradually while the chemical shift intensity at 55 ppm is decreased. Compared with pure ionic liquid 13C NMR signals [7], the shifting of some 13C NMR chemical shifts in the ionic liquid doped hybrid membranes indicate the strong interaction of [EMI][TFSI] ionic liquid with inorganic phosphosilicate matrix. It is important to investigate the thermal stability of proton exchange membranes because a medium temperature operation at

Fig. 7. (a) 1H and (b) 13C MAS NMR spectra for 72TEOSe18PDMSe10PO (OCH3)3ex [EMI] [TFSI](x ¼ 10, 20, 30, and 40 wt%) hybrid membranes.

higher than 100
C is eventually being aimed for the PEMFCs using these composite membranes. Fig. 8(a,b) shows the thermogravimetry (TG) and differential thermal analysis (DTA) profiles of all the fabricated inorganiceorganic composite membranes including host membrane. For the phosphosilicate host membrane, an initial weight loss of 2 wt% was observed around at 85
C due to the evaporation of adsorbed water molecules. Further, the 10 wt% loss identified around at 500
C could be due to the condensation of structural hydroxyl groups. Generally, in solegel synthesis many independent parameters would govern the nature and properties of the final compound, such as the pH of the sol, the nature of the precursors, the stoichiometry of the reactants, the temperature of the gelation and the water concentration etc. Following the TG profiles of 10, 20 and 30 wt% ionic liquid doped hybrid membranes 2.0 wt% loss was observed below 100
C. The 40 wt% ionic liquid doped composite membrane has not showed any weight loss below 100
C due to large amount of added ionic liquid. This observed good thermal stability of the studied inorganiceorganic composite membranes could be due to homogeneous microstructure formed by SieOeSi backbones and highly stable [TFSI] anion, respectively. Usually, in solegel processes, a partial hydrolysis of the reactants before mixing, also called prehydrolysis stage, leads to better mixed samples on a molecular scale to form hetero-linkages in inorganiceorganic hybrid materials. For 72TEOSe18PDMSe10PO (OCH3)3 (mol %) membrane, two exothermic peaks at 510, and 549
C are

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453; 459; 462; and 462, 455, 463, 468, and 472
C are identified, respectively. These observed decomposition temperatures are relatively high compared with other CeH bonding-based polymers and confirmed the high thermal stability of [TFSI] anion [7]. Following DTA profiles no exothermic peak due to the decomposition of alkoxides or organic residues was noticed after 600
C, indicating that alkoxides were completely hydrolyzed and organic residues including any volatile species are almost removed below 600
C Fig. 8(c) presents the DSC profile of the 40 wt% ionic liquid doped hybrid membrane within the temperature range 100e100
C. From this profile, the glass transition temperature (Tg) with a heat capacity change of the doped ionic liquid was confirmed at 86
C. Fig. 9 exhibits the conductivity of all the fabricated composite membranes, measured within the temperature range 20e150
C under anhydrous conditions. The conductivity of host phosphosilicate membrane was only 9.22  106 S/cm at 80
C. This measured low conductivity value could be due to the loss of water molecules from the membrane under anhydrous conditions with temperature increment. To increase the anhydrous conductivity of the host membrane, we doped ionic liquid because the ionic liquid is able to act as a charge carrier at medium temperatures higher than 100
C. As expected, within 20e150
C the conductivity of the composite membranes was enhanced with [EMI][TFSI] ionic liquid content increment up to 40 wt%. At 150
C, the measured maximum conductivity of 40 wt% ionic liquid doped composite membrane was 4.87  103 S/cm. The conductivity of 40 wt% IL doped hybrid membrane at 80
C was 2.14  103 S/cm. Usually at temperatures higher than 100
C, ionic liquids are able to provide continuous conducting channels in the membrane electrolytes under non-humidified conditions. This could be of particular interest because a medium temperature operation (150
C) under non-humidified conditions is definitely impossible for PEMFCs using water-swollen acidic membranes. It should be emphasized that for all the fabricated composite membranes, the conductivity (s) versus reciprocal temperature (T1) plot (Fig. 9) is not exactly representing straight lines within the temperature range 20e150
C. These results suggest that the conductivity in hybrid membranes does not obey an Arrhenius-type relation, but also represents the VogeleTammaneFulcher (VTF) behavior [7]. Fig. 10 presents the hydrogen permeability values as a function of reciprocal temperature (T1) for 40 wt% ionic liquid based composite membrane within the temperature range 20e150
C including NafionÒ117 membrane (Inset plot). The H2 permeability

Fig. 8. (a) TG and (b) DTA profiles of 72TEOSe18PDMSe10PO (OCH3)3e x [EMI][TFSI] (x ¼ 0,10, 20, 30, and 40 wt%) membranes (c) DSC profile of 40 wt% [EMI][TFSI] ionic liquid doped hybrid membrane.

identified from the DTA profile. These exothermic peaks could be attributed to the thermal decomposition of the reagent organic compounds that are present in the phosphosilicate matrix [7,45]. Generally, compared with SiOH group, the proton in the POH group is strongly hydrogen-bonded with the water molecules. Therefore, high-temperatures are necessary to remove the water from POH groups. On the other hand, from 10, 20, 30, 40 wt% ionic liquid doped composite membranes DTA profiles, exothermic peaks at

Fig. 9. Conductivity of 72TEOSe18PDMSe10PO (OCH3)3ex [EMI][TFSI] (x ¼ 0, 10, 20, 30, and 40 wt%) hybrid membranes.

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and a value of 4  1012 mol/cm s Pa was found at 150
C. The synthesized composite membranes can be used in medium temperature (above 100
C) H2/O2 fuel cell electrolytes. Acknowledgments The authors are grateful to the New Energy and Industrial Technology Development Organization (NEDO), Japan for the financial support. References

Fig. 10. Hydrogen permeation rate as a function of inverse temperature for 40 wt% [EMI][TFSI] doped 72TEOSe18PDMSe10PO(OCH3)3(mol%) hybrid membrane (thickness ¼ 0.5 mm) including NafionÒ117(Inset picture). The permeability measurements were performed under hydrogen feed in the temperature range 20e150
C.

value was decreased from 2.06  1011 to 4  1012 mol/cm s Pa as the temperature increases from 20 to 150
C. On the other hand, the measured permeability value for NafionÒ 117 at 30
C was 3.01  1013 mol/cm s Pa. The fabricated composite membranes have shown the H2 permeability values on the order of 1011e1012 mol/cm s Pa and these are two orders of magnitude higher than the values of NafionÒ membrane. Recently, it was noted that surface diffusion would play only a minor role (<2%) on the total transport mechanism in phosphosilicate matrices [46,47]. It is well known that, for H2/O2 fuel cell applications the membrane electrolytes which exhibit low-hydrogen permeability are necessary. The decrease in hydrogen permeation rates with an increasing temperature may be influenced by the conductivity of the hybrid membrane. Recently, we stated that the reason for the observed high H2 permeability values of ionic liquid doped hybrid membrane might be due to the Knudsen diffusion process that exists in phosphosilicate matrices when the H2 gas permeation decreases with temperature increment [48]. However, we cannot rule out the possible influence of solubility of H2 gas in the ionic liquid medium as one of the probable causes for the observed transport behavior of hybrid membrane. In this assumption, the decrease of permeability with increasing temperature could be related to a decrease of the H2 solubility. Therefore, the hydrogen permeability property of ionic liquids doped inorganiceorganic hybrid membranes required deeper investigations and will a subject of our future work. Further, the permeability coefficient of any gas through a membrane is directly related to the size and shape of the tracks as well as the membrane thickness and applied gas pressure. 4. Conclusions Inorganiceorganic composite membranes with the combination of phosphosiliacte content (TEOS/PDMS/PO (OCH3)3) and 1-ethyl-3 methylimidazolium-bis (trifluoromethanesulfonyl) imide [EMI][TFSI] ionic liquid were synthesized by solegel process. These hybrid membranes possess good chemical stability and are thermally stable up to 350
C in air due to inorganic SiO2 framework and stable [TFSI] anion. Conductivity of 4.87  103 S/cm was measured for 40 wt% [EMI][TFSI] ionic liquid doped composite membrane at 150
C under non-humidified conditions. Within the temperature range 20
Ce150
C, the hydrogen permeability values were decreased for 40 wt% ionic liquid doped composite membrane

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