Synthesis and Characterization of Proton Conducting Inorganic

Jul 17, 2009 - The story of a chemistry-based start-up is one of discovery paired with risk. That's a compelling combination,... SCIENCE CONCENTRATES ...
0 downloads 0 Views 4MB Size
Download PDF
14540

J. Phys. Chem. C 2009, 113, 14540–14550

Synthesis and Characterization of Proton Conducting Inorganic-Organic Hybrid Nanocomposite Membranes Based on mixed PWA-PMA-TEOS-GPTMS-H3PO4-APTES for H2/O2 Fuel Cells G. Lakshminarayana and Masayuki Nogami* Department of Materials Science and Engineering, Nagoya Institute of Technology, Showa, Nagoya, 466-8555, Japan ReceiVed: March 20, 2009; ReVised Manuscript ReceiVed: June 24, 2009

A series of novel fast proton conductive inorganic-organic nanocomposite hybrid membranes doped with a mixture of phosphotungstic acid (PWA) and phosphomolybdic acid (PMA) have been prepared by sol-gel process with 3-glycidoxypropyltrimethoxysilane (GPTMS), 3-aminopropyltriethoxysilane (APTES), phosphoric acid (H3PO4), and tetraethoxysilane (TEOS) as precursors. These hybrid membranes were studied with respect to their structural, thermal, elastic moduli, and proton conductivity properties. The X-ray diffraction measurement revealed the amorphous nature of the hybrid membranes. The Fourier transform infrared spectroscopy has shown a good complexation of H3PO4 in the membrane matrix and the both characteristic Keggin anions PW12O403- and PMo12O403- were present in the nanocomposite membranes. Thermal analysis including thermogravimetric and differential thermal analysis confirmed that the membranes were thermally stable up to 300 °C. Thermal stability of the membranes was significantly enhanced by the presence of inorganic SiO2 framework. The effect of mixed heteropolyacid (HPA) concentration on the microstructure of the membranes was studied by scanning electron and transmission electron micrographs and no phase separation at the surfaces of the TEOS-GPTMS-H3PO4-APTES-HPA membranes was observed, indicating that these membranes are homogeneous in nature. High proton conductivity of 3 × 10-2 S/cm with composition of 50TEOS-25GPTMS-20H3PO4-5APTES-3PMA-6PWA was obtained (6.35 × 10-3S/cm at 150 °C, 50% RH) at 120 °C under 90% relative humidity. The high proton conductivity of the nanocomposite membranes is due to the proton-conducting path through the GPTMS-derived “pseudopolyethylene oxide” (pseudo-PEO) network in which the trapped solid acids (PWA and PMA) as proton donors are contained. The molecular water absorbed in the polymer matrix is also presumed to provide high proton mobility, resulting in an increase of proton conductivity with an increasing relative humidity. These results indicate that the inorganic-organic hybrids synthesized in this work are promising electrolytes for proton exchange membrane fuel cells and also for future fuel-cell design and development. 1. Introduction Proton exchange membrane fuel cells (PEMFC) have received a lot of attention in recent years because of their high efficiency, long lifetime, low pollution and simplified system design. Proton exchange membrane (PEM) is the most important part in PEMFC. It is well known that the most widely used membranes in PEMFC are Nafion type membranes, the perfluorosulfonic acid polymer membranes, owing to their outstanding chemical stability and high proton conductivity. However, there are two major drawbacks in the Nafion type membranes. First, they rely on water for proton conductivity, so they cannot be operated at high temperatures (>80 °C). Second, they have a lot of methanol crossover, which causes the poisoning of electrodes.1-5 High cost and environmental inadaptability of the fluorinated materials (Nafion) are also serious drawbacks for the practical fuel cell applications. In recent years, the inorganic-organic hybrid materials via sol-gel process have attracted great attention because of their potential advantages over the conventional materials due to the coexistence of the inorganic and organic characteristic properties.6-9 From a practical point of view as * To whom correspondence should be addressed. Tel: +81- 52 735 5285. Fax: +81- 52 735 5285. E-mail: [email protected] or mnogami@ mtj.biglobe.ne.jp.

the separators for fuel cells and sensors, these solid-state proton conductors have a promising future due to unique advantages such as flexibility, high mechanical strength, thermal stability, and excellent processability. In the present study, the polymer phase was incorporated into the inorganic matrix through the epoxy ring-opening and polycondensation reactions of 3-glycidoxypropyltrimethoxysilane (GPTMS) with tetraethoxysilane (TEOS) as the starting precursors. We expected that the Si-O-Si backbone (provides thermal stability and mechanical strength) will be connected with each other by the pseudo PEO-chain (provides good flexibility and processability) to form the nanocomposite materials. The low cost phosphorus-containing compound H3PO4 was added to endow the membrane with proton conductivity. During the process, 3-aminopropyltriethoxysilane (APTES) was added into the precursor sol to shorten the gelation time.10 Also water affinity is expected due to the presence of nitrogen atoms with the oxygen atoms in PEO phase (atoms with unshared electrons are potential acceptors for hydrogen bonding11). Heteropolyacids (HPAs) belongs to a class of inorganic proton-conducting materials known to possess high proton conductivities at room temperature. The ability of these compounds to retain water even at high temperatures and act as super acids makes them an excellent proton conductors, and

10.1021/jp902518c CCC: $40.75  2009 American Chemical Society Published on Web 07/17/2009

Proton Conducting Inorganic-Organic Hybrid Nanocomposites membranes with an enhanced proton conduction equates greater power outputs from the fuel cell. There are three main commercially produced HPAs, 12-phosphotungstic acid (PWA), 12-phosphomolybdic acid (PMA), and 12-silicotungstic acid (SiWA). Certain proton conductors, such as HPAs and protonic acids will be dehydrated or thermally decomposed at mediumrange temperatures and relatively low humidity, whereas the thermal stability of an HPA can be improved by incorporating it into a modified silica matrix under high humidity.12 The high proton conductivity of PWA and the potential interactions between PWA and a host material can be understood by considering its structure (see Figure 2a). The PWA can be described by a Keggin unit (KU)-type13 primary structure, that is, the polyanion [PW12O40]3-, and a secondary structure, that is, the regular three-dimensional assembly of the heteropolyanions with counter cations (protons) and additional molecules (water).14 This KU consists of a central PO4 tetrahedron surrounded by four W3O13 sets linked together through oxygen atoms. Four types of oxygen atoms can be distinguished. The central oxygen atom belonging to the PO4 tetrahedron is shared by the three tungsten atoms of the set. The edge-sharing oxygen atom will bridge two tungsten atoms of the same set. The cornersharing oxygen atom bridge two tungsten atoms of different sets. The terminal oxygen atom is associated with a single tungsten atom. The bridging and terminal oxygen atoms are on the periphery of the structure and therefore are available to associate with protons or water molecules to form hydrates. Phosphotungstic acid in the anhydrous form, H3PW12O40, is the strongest acid of the common Keggin units.15 The strong acidity derives from the delocalization of the negative charge over many atoms of the polyanion. In the hydrated state, the water molecules are loosely bound in the structure and PWA acts as Brønsted acid toward them16,17 resulting in high proton conductivity. Thus the proton conductivity depends on the number of water molecules coordinated to the Keggin unit. It should be mentioned that the exact number of coordinated water molecules, and therefore the proton conductivity, depend upon the temperature and the relative humidity of the environment. Molybdophosphoric acid (PMA) and its analogues are highly conductive as long as they contain large amount of water in the crystal structure and hence are so sensitive to humidity that a difficulty similar to that observed for liquid electrolyte fuel cells arise.18 The PMA structure (Figure 2a) is composed of one heteroatom surrounded by four oxygens to form a tetrahedron. The heteroatom is located centrally and caged by 12 octahedral MO6-units linked to one another by the neighboring oxygen atoms. There are a total of 24 bridging oxygen atoms that link the 12 addenda atoms. The metal centers in the 12 octahedra are arranged on a sphere almost equidistant from each other in four M3O13 units giving the complete structure of an overall tetrahedral symmetry. The bond length between atoms varies depending on the heteroatom (X ) P) and the addenda atoms (M ) Mo). To obtain fast proton-conducting, chemically and thermally stable PEM materials that are simple to fabricate and cheap, weinvestigatednewcandidatesforPEMs,basedoninorganic-organic hybrid nanostructures by mixing PWA and PMA. These inorganic-organic hybrids are one of the most promising materials because they possess both inorganic and organic functionality. These materials have been synthesized by utilizing the low-temperature reaction of the sol-gel process. In this study, we used GPTMS/APTES for preparation of hybrid composites with TEOS/H3PO4/HPA. The composite membranes elastic modulus, thermal stability, proton conductivity and

J. Phys. Chem. C, Vol. 113, No. 32, 2009 14541 hydrogen permeability were measured as a function of composition and temperature. 2. Experimental Studies 2.1. Preparation of Hybrid Membranes. The hybrid membranes were prepared by using tetraethyl orthosilicate (Si(OC2H5)4, TEOS, 99.9%, Colcote, Japan), phosphoric acid (H3PO4, 85% aqueous solution, Colcote, Japan), H3PW12O40 (PWA, 99.995%, Nacalai Tesque), H3PMo12O40 (PMA, 99.995%, Kishida Chemicals), APTES (99%, Aldrich), and GPTMS (98%, Aldrich). PWA and PMA were used as the proton sources and all 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 experiments. These inorganicorganic hybrid membranes were fabricated at room temperature under atmospheric pressure by sol-gel process. Several compositions of TEOS-GPTMS-H3PO4-APTES-PMA and PWA (i.e., 50TEOS-25GPTMS-20H3PO4-5APTES-xPMA-yPWA (x ) 1, y ) 2; x ) 2, y ) 4; x ) 3, y ) 6; x ) 4, y ) 8; and x ) 5, y ) 10 mol %)) were selected for optimization. Here, it is noted that we added HPA (PMA + PWA) content in excess mol % to the host hybrid membrane 50TEOS-25GPTMS20H3PO4-5APTES (mol %). The whole synthetic process is shown in Figure 1. Initially, the calculated amount of TEOS was hydrolyzed with water (as 0.15N-HCl aq) and ethanol under stirring for 90 min. Now, GPTMS, which was prehydrolyzed with water (as 0.15N-HCl aq), and ethanol in the ratio of 3:3 for 1 mol of GPTMS was added to TEOS solution under constant stirring for 1 h at room temperature. One hour later, when the above mixed solution become transparent, phosphoric acid was added dropwise under magnetic stirring at room temperature during a time period of 2 h. Next, APTES dissolved in C2H5OH with five times the moles of H2O was added dropwise under constant stirring at room temperature during a time period of 1 h. The GPTMS-TEOS-H3PO4 prehydrolyzed solution became opaque when APTES was added into the mixture and became transparent with a light brown color as the gelation proceeded. Subsequently, PMA and PWA was dissolved in ethanol with the mole ratio of 1:10, followed by adding drop-by-drop into the hydrolyzed TEOS-GPTMS-H3PO4APTES solution under constant stirring for 2 h. The resulting clear transparent solution was called as sol. The sol was cast onto Teflon dishes and dried at 80 °C for 2 days to yield a gel. The gel was further heated at 120-250 °C for 6 h to yield a bulk membrane. The sample thickness ranged from 1 to 0.5 mm. It is possible to prepare thin membranes with desired thickness just by varying the volume of sol solutions of these studied membranes, in the Teflon molds. Because of the high flexibility and proccessibility to make large area membranes with different thickness, these inorganicorganic membranes show an advantage over inorganic phosphosilicate materials, which takes a long time to get thin film materials through sol-gel process. We observed no deliquescence on the surface of the membranes after they were left in ambient condition for several days. It suggests that H3PO4 was incorporated quite well either in the PEO domain19 or condensed silica domain, while the protons can dissociate from H3PO4 under humid conditions to form conductive hydrated protons, such as H3O+ and H5O2+. 2.2. Characterization of Membranes. The powder X-ray diffraction (XRD) profiles were obtained on a Bruker AXS D-5005 diffractometer with a Ni-filter using Cu KR ()1.542 Å) radiation with an applied voltage of 40 kV and 20 mA anode current, calibrated with Si at the rate of 2 °C/min. Flexural elastic

14542

J. Phys. Chem. C, Vol. 113, No. 32, 2009

Lakshminarayana and Nogami

Figure 1. Synthetic procedure for obtaining the inorganic-organic hybrid nanocomposite membranes.

moduli of the prepared nanocomposites were determined from the stress-strain curves using three-point bending method. Fourier transform infrared (FT-IR) spectra of the composite membranes were obtained with JASCO FTIR-460 Plus spectrometer. The FT-IR spectra were measured within the spectral range of 4000-400 cm-1 using the KBr pellet method. The thermal degradation process and stability of the composites were investigated by thermogravimetric analysis (TGA) and differential thermal analysis (DTA) (Thermoplus 2, TG- 8120, Rigaku). The measurements were carried out under dry air and with a heating rate of 5 °C/min. The surface morphology of the prepared nanocomposite membranes were observed by scanning electron microscopy (SEM) (JEOL-JSM-6301, NORAN instrument). The nanostructure of the hybrid membranes was characterized by transmission electron microscopy (TEM) using TEM CM 20 Twin (Philips, Netherlands) with an accelerating voltage of 160 keV. The proton conductivity of the composite membranes was determined from the Cole-Cole plots by an AC method using Solartron SI-1260 impedance analyzer with frequencies ranging from 1 to 107 Hz and signal amplitude of 10 mV. After evaporating the gold electrode with a 0.3 mm diameter onto the prepared nanocomposite membranes, the samples were kept in a constant humidity chamber. The temperature and humidity in the chamber were controlled from 20 to 150 °C and 50-100% relative humidity, respectively. Before the conductivity measurement, these hybrid membranes were kept in vacuum at 30 °C for 2 h to remove surface adsorbed water. The temperature was raised stepwise and the measurements were carried out after keeping at each temperature for 1 h. The proton conductivity (σ) was calculated from the electrolyte resistance (R) obtained from the intercept of the ColeCole plot with the real axis, the thickness (l) and the electrode area (A) according to the equation σ ) l/AR. Hydrogen permeability of the hybrid composite membranes (0.5 mm thick) was measured by using a forced convection drying oven (DO600FA), 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 gas chromatography. 3. Results and Discussion Figure 2b presents the schematic diagram of the prepared nanocomposite hybrid membranes along with the reaction scheme. When the liquid GPTMS reacts with water, its methoxy groups bound with silicon will hydrolyze to form threedimensional silica network structures. The other epoxide groups forms polyethylene oxides, pseudo-PEO networks, which contributes to its good flexibility and processing suitability of the hydrolyzed GPTMS. Previously, Honma20,21 and co-workers developed a series of hybrid materials via sol-gel process based on the SiO2 and low molecular weight polymers (polyethylene (PE), polyethylene oxide (PEO), polypropylene oxide (PPO), and polytetramethylene oxide (PTMO)). These polymers were end-capped with alkoxysilane moieties. It was reported that continuous pathways for proton conduction could be set up in membrane matrix. However, the procedure is quite complicated and the resultant composites showed low chemical stability and poor thermal stability, especially when PEO was directly incorporated. We thought that if the resulting “polyethylene oxide chain” with inorganic ligand (pseudo-PEO) obtained by the sol-gel reaction of GPTMS has proton-conducting properties similar to the well-known proton conducting polymer polyethylene oxide (PEO), fast proton conducting inorganicorganic materials could be easily fabricated. Figure 3 shows the X-ray diffraction patterns of powder samples with various PMA + PWA concentrations. No sharp diffraction peaks are observed from these spectra, suggesting no crystallization for Si3(PO4)4 or SiHPO4 · 2H2O19 in the prepared phosphosilicate matrices. A broadband observed from 2θ ) 20° to 35° is the characteristic of vitreous SiO2 that is the indicative of noncrystalline nature of the studied materials.20 Thus the hybrid membranes are amorphous without long-range ordering. Figure 4 shows the elastic modulus of the prepared nanocomposite membranes with respect to HPA content, measured at room temperature. The elastic modulus gradually increased with an

Proton Conducting Inorganic-Organic Hybrid Nanocomposites

J. Phys. Chem. C, Vol. 113, No. 32, 2009 14543

Figure 2. (a) Primary structures of phosphotungstic acid and phosphomolybdic acid, the Keggin unit [PW12O40]-3 and [PMo12O40]-3. (b) Proposed structure of hybrid membranes.

increase in PMA + PWA (mol %) content. From 1PMA/2PWA (mol %) composite to 5PMA/10PWA (mol %) composite, the elastic modulus increased from 0.39 to 3.18 GPa. FTIR spectroscopy, as a means of providing the structure of the HPAs, is one of the most important tools in deciding which HPAs will give the best results when used as an electrolyte in fuel cell tests. This method is convenient and widely used for the characterization of heteropolyanions. Additional information about the thermal stability as well as about the structure of the supported HPAs can be obtained by FTIR spectroscopy. IR studies have been mostly concerned with Keggin acids and their salts and have been used for elucidating structural relationships between heteropolyanions. IR spectra of pure HPAs can display specific absorption bands corresponding to the Keggin structure at 1064, 965, 864, and 805 cm-1, and these bands can be

assigned to the stretching vibrations νas(P-O), νas(MOdOt), νas(Mo-Oc-Mo), and νas(Mo-Oe-Mo), respectively.22 The PO stretching band for PW11O397- split into 1085 and 1040 cm-1. IR spectra of hydrated H3PW12O40 include a broad OH stretching band and two OH bending bands at 1610 and 1720 cm-1. The latter two correspond to water and protonated water, respectively.22 Figure 5a shows the FT-IR spectra of (a) pure PMA and (b) pure PWA powder. Spectral peaks at the wave numbers 450, 505, 597, 787, 872, 964, 1063, 1394, 1622, 1929, 2352, 3211, and 3432 cm-1 indicate the existence of PMA (Figure 5a(a)), and peaks at 424, 529, 597, 651, 799, 892, 984, 1086, 1217, 1309, 1616, 1751, 1991, 3421, 3604, and 3801 cm-1 indicate that of PWA (in Figure 5a(b)). These are the main IR bands for pure PMA and PWA that are observed from Figure 5a. Figure 5b shows the FT-IR spectra of the hybrid membranes

14544

J. Phys. Chem. C, Vol. 113, No. 32, 2009

Lakshminarayana and Nogami

Figure 3. XRD profiles of 50SiO2-25GPTMS-20H3PO4-5APTESxPMA/yPWA (x ) 1, y ) 2; x ) 2, y ) 4; x ) 3, y ) 6; x ) 4, y ) 8; and x ) 5, y ) 10 mol %) composites.

Figure 5. (a) FT-IR spectra of (a) pure PMA and (b) pure PWA. (b) FT-IR spectra of 50SiO2-25GPTMS-20H3PO4-5APTES-xPMA/yPWA (x ) 1, y ) 2; x ) 2, y ) 4; x ) 3, y ) 6; x ) 4, y ) 8; and x ) 5, y ) 10 mol %) composites. Figure 4. Elastic modulus of 50SiO2-25GPTMS-20H3PO4-5APTESxPMA/yPWA (x ) 1, y ) 2; x ) 2, y ) 4; x ) 3, y ) 6; x ) 4, y ) 8; and x ) 5, y ) 10 mol %) composites, measured at room temperature by three bending method.

with various PMA + PWA (mol %) concentrations. Generally, in the wavenumber range from 400 to 1200 cm-1 pure silica shows several characteristic peaks. They can be assigned to the Si-O-Si and Si-O- stretching vibrations. In the spectra of the composites, some absorption bands that are originated from the GPTMS and PMA/PWA molecules are observed between 1250-1000 cm-1 and 1100-800 cm-1 wavenumber regions, respectively. The peaks at 486, 591, 665, 811, 892, 978, 1211, 1401, 1629, 1702, 2358, 2881, 2942, 3391, and 3617 cm-1 indicate the incorporation of HPAs into the nanocomposites. It is also observed that the wave numbers of PWA in the hybrid membranes were spectrally shifted away from those of pure PWA by a few inverse centimeters, confirming that the Keggin geometry of PWA was preserved inside the membranes. The bands of W-Ob-W and W-Oc-W simultaneously display blue shifts. Rocchiccioli-Deltcheff et al.23 correlated the wavenumber shifts of [W-Od-W], [W-Oc-W], and [W-Ob-W] bands with the strength of anion-anion interactions, which take place as a result of the electrostatic repulsion between the PW anions in the crystalline compounds. They proposed that with increasing anion-anion interactions as a result of distance increment between the oxygens of the neighboring PW anion the pure stretching [W-Od] band would

exhibit a decrease in its wavenumber. The opposite shifts were proposed for the other two vibrations because of their mixed bond stretching character. The bands centered at 1040, 978, 892, and 811 cm-1 are assigned to the stretching modes of P-O, terminal W-O, edge-sharing W-O-W and corner-sharing W-O-W units of PWA molecules, respectively.24 Further, typical peaks for the Keggin structure (i.e., the asymmetric stretching vibration of the central PO4 tetrahedron) at 1060 and 1095 cm-1 were partially overlaid by the Si-O frame vibrations. Even after heating the hybrid membranes from 120 to 250 °C, all of the characteristic peaks that indicate the Keggin structure are preserved in the PWA molecules, though the intensity of the C-O-C band at 1200 cm-1 is decreased. On the other hand, in addition to the absorption bands in the 1250-1100 cm-1 regions, the band at 1200 cm-1 due to the C-O-C group is also observed in the composite samples, whereas no band assigned to the epoxy ring is observed at 1260 cm-1. This result strongly suggests that the epoxy ring of GPTMS molecule is opened by the hydrolysis reaction to form the pseudo-PEO network. With the addition of H3PO4, the absorption peak at 1080 cm-1 is broadened and shifted to lower wavenumber due to the presence of Si-O-PO bond25 at 1020 cm-1. It suggests an existence of a complex between H3PO4 and SiO2 matrix. Absorption bands at 1445, 2881, and 2942 cm-1 are ascribed to Vs(CH2), Vas(CH2) and δ(CH2), respectively.26 With APTES addition, an absorption band at 1629 cm-1 due to deformation vibration of amino group, -NH2, is split into 1548 and 1640

Proton Conducting Inorganic-Organic Hybrid Nanocomposites

Figure 6. (a) TG-DTA curves for (a) pure PMA and (b) pure PWA. (b) TG-DTA curves of 50SiO2-25GPTMS-20H3PO4-5APTES-xPMA/ yPWA (x ) 1, y ) 2; x ) 2, y ) 4; x ) 3, y ) 6; x ) 4, y ) 8; and x ) 5, y ) 10 mol %) composites.

cm-1 bands that are assigned to the deformation of -NH3+ and -NH-, respectively.10 The result obtained suggests an interaction of amino groups with H3PO4 and/or epoxy ring.27 Generally, in phosphosilicate matrices spectral peaks centered around 1250 and 1060 cm-1 can be attributed to the asymmetric and symmetric stretching vibrations of PO4 tetrahedra, respectively, with two nonbridging and two bridging oxygen atoms attached to phosphorus.28 The band at 870 cm-1 corresponds to the asymmetric stretching of P-O-P linkages and the corresponding symmetric stretching would appear as a broad spectral band around 780-720 cm-1. For low P2O5 concentration, these centers are primarily two atoms ones with two OdPO3 tetrahedra bonded together through common bridging oxygen atom, and these centers are designated as OdP-O-PdO. The double phosphorus center in phosphosilicate matrices can be simulated by the cluster containing two OdPO3 tetrahedra bonded by each other by common bridging oxygen atom. Each of the tetrahedral can be bonded to two Si atoms with dangling bonds saturated by H atoms.29 Since charge carrier is proton dissociated from the H3PO4, any factor to change the concentration of P-OH leads to the variation of proton conductivity of the membrane. The real analysis of TGA is used for quantification of decomposition, loss of water hydration, desorption of adsorbed species, reaction productions, and drying studies. DTA is commonly used for qualitative and semiquantitative characterization of phase transformations, including glass transition temperatures, and reactions (i.e., endothermic and exothermic) and transformation kinetics. Figure 6a shows the TG and DTA

J. Phys. Chem. C, Vol. 113, No. 32, 2009 14545 curves for (a) pure PMA and (b) pure PWA powder. For pure PMA (Figure 6a(a)), endothermic peaks are observed at 67, 96, 124, 260, 416, and 517 °C, and the exothermic peaks are found at 78, 110, 194, 233, 280, 436, 492, and 660 °C. From the DTA curve, a small weight loss region was found at 68 °C and larger ones were discovered between temperatures ranging from 100 to 223 °C, at 432 and at 727 °C. For pure PWA (in Figure 6a(b)), endothermic peaks are observed at 180, 197, and 593 °C, and exothermic peaks are observed at 187, 578, and 602 °C from the DTA curve. Similarly, from the TG curve, three weight loss regions could be observed at 141, 198, and 408 °C for pure PWA.The acid forms of heteropoly acids are usually obtained with large amounts of crystallization water, and most of these water molecules will be released below 100 °C. Decomposition, which takes place at 250-600 °C, is believed to occur according to H3PMo12O40 f (1/2)P2O5 + 12MoO3 + (3/2)H2O (Mo ) W). It is assumed that this decomposition proceed via PW12O38 in the case of H3PW12O40.22 The thermal stability also depends on the environment, and in a reducing atmosphere heteropoly compounds decompose more rapidly. The coexistence of oxygen and water vapor enhances the stability at high temperatures and sometimes causes the reformation of the heteropoly structure from a decomposed mixture. Figure 6b shows the TG-DTA profiles of the prepared hybrid nanocomposite membranes with various PMA + PWA (mol %) concentrations. All the hybrid composite membranes exhibited two hydrate loss steps at temperatures 57°C and 300°C as shown in the figure, and these are the two main weight-loss steps in the TG curves. An observed weight loss less than 20 wt % before 300 °C could be due to the evaporation of physically absorbed water in the membranes. It is noticeable that there is no critical change in the TG curves around 100 °C, the boiling point of water. Thus this evaporation temperature of water higher than 100 °C, suggests a good water affinity of the studied membranes. The sol-gel mixture was able to stabilize the crystalline water (from 100 to 300 °C). In the studied membranes, the 300 °C water loss may be due to dehydration from Si(OH)4 to SiO2.22 However, the water loss from the membranes above 100 °C may be directly confirmed with swelling tests. It was found that the mixed HPA hybrid composite membranes were stable up to 300 °C. The weight loss in the final stage is ascribed to the removal of the adsorbed water and hydroxyl groups via strong hydrogen bonding to the oxygen atom of Si-O-Si in the network structure of hybrid nanocomposite membranes. The observed decomposition temperature is relatively high compared with other C-H bondingbased polymers despite the large amount of incorporated acid, and this high thermal stability is attributed to the inorganicorganic hybrid microstructure based on Si-O-Si backbones. Previously, for pure GPTMS, two weight loss regions were reported at 160 and 270 °C, probably due to combustion of the hydrocarbon groups.24 On the other hand, for 1PMA/2PWA (mol %) composite three exothermic peaks at 300, 438, and 497 °C with a small weigh loss are observed, which should be attributed to the oxidation and/or decomposition of the side groups, side chains, and main chains in the polymer phase, respectively. Another peak at 941 °C corresponds to the SiO2 phase transformation. An endothermic peak at 863 °C was also observed from 1PMA/2PWA (mol %) composite DTA curve. Thermal stability of the polymer phase in the membrane is highly enhanced by the nanolevel trapping confinement of the inorganic SiO2 framework up to 300 °C, suggesting it as a suitable electrolyte for medium temperature fuel cells especially at about 120-150 °C. Two exothermic peaks at 820 and 902

14546

J. Phys. Chem. C, Vol. 113, No. 32, 2009

Lakshminarayana and Nogami

Figure 7. SEM images of (a) 1PMA/2PWA (mol %), (b) 2PMA/4PWA (mol %), (c) 3PMA/6PWA (mol %), (d) 4PMA/ 8PWA (mol %), and (e) 5PMA/10PWA (mol %) composites, and (f) TEM image of 3PMA/6PWA (mol %) doped hybrid membrane.

°C are observed for 2PMA/4PWA (mol %) nanocomposite. For 3PMA/6PWA (mol %) composite, two exothermic peaks at 420 and 818 °C are identified. Similarly, for 4PMA/8PWA (mol %) composite three exothermic peaks at 426, 750, and 822 °C are observed. For 5PMA/10PWA (mol %) composite, two exothermic peaks at 410 and 933 °C are noticed. The slight exothermic increase in the DTA curves between 50 and 160 °C can be attributed to an accelerated polymerization reaction of the remaining reactant alkyl groups of alkoxides and GPTMS forming silicon-oxygen bonds. Figure 7 shows the SEM images of (a) 1PMA/2PWA (mol %), (b) 2PMA/4PWA (mol %), (c) 3PMA/6PWA (mol %), (d) 4PMA/ 8PWA (mol %), and (e) 5PMA/10PWA (mol %) nanocomposites. In order to study the effect of HPA concentration on the microstructure of the membranes, SEM micrographs of the surfaces of TEOSGPTMS-H3PO4-APTES-HPA membranes with different HPA contents were obtained. It became clear that the changes in the SEM surfaces of membranes followed a regular trend with an increase in HPA content. It is clear from Figure 7(a-e) that no phase separation at the surfaces of the TEOS-GPTMS-H3PO4APTES-HPA membranes can be observed, indicating that these membranes are homogeneous in nature, while the surface morphologies of these membranes gradually become smoother

and denser as the HPA content in the membrane matrix is increased. It is well known that nanoparticles tend to aggregate to form larger particles.30 These SEM images were obtained from all of the parts. In addition, the particle size of the HPA is seen to range from several nanometers to around 100 nm, which suggests that the prepared membranes can be considered as TEOS- GPTMS-H3PO4-APTES-HPA nanocomposites. These results may be due to the establishment of higher cross-linking density in the inorganic-organic networks as the formation of one network has a significant effect on the formation of the other. During the synthesis, the hydrolysis reaction builds a structure with a high degree of cross-linking. For such highly cross-linked membranes, molecular water and hydroxyl groups have a better chance of forming hydrogen bonds in the membrane structure with Si-O-Si backbone.31 Figure 7f shows the TEM image of the 3PMA/6PWA (mol %) doped hybrid membrane. This photograph clearly indicates that the hybrid membrane is completely homogeneous at nanoscale because no discernible microstructure has been observed. Also, a segregation of the HPA crystals has not been observed. Figure 8 shows the temperature dependence of conductivity of all the prepared hybrid membranes, measured under 50% relative humidity (RH) and plotted as a function of the reciprocal

Proton Conducting Inorganic-Organic Hybrid Nanocomposites

J. Phys. Chem. C, Vol. 113, No. 32, 2009 14547

Figure 8. Temperature dependence of conductivity of 50SiO225GPTMS-20H3PO4-5APTES-xPMA/yPWA (x ) 1, y ) 2; x ) 2, y ) 4; x ) 3, y ) 6; x ) 4, y ) 8; and x ) 5, y ) 10 mol %) hybrid membranes. Conductivity was obtained under 50% RH. Inset shows the proton conductivity of host membrane without HPA addition, and Nafion 117 under 50% RH.

temperature. The conductivity was measured in the temperature range from 20 to 150 °C. For comparison, conductivity of host membrane without HPA addition and Nafion117 membrane (inset) was also measured at the same conditions as the fabricated hybrid membranes. HPAs exist in a series of hydrated phases of which the stable forms depend strongly on temperature and RH. The maximum proton conductivity was found to be 1.88 × 10-2 S/cm at 120 °C for the 50SiO2-25GPTMS20H3PO4-5APTES-3PMA-6PWA (mol %) composite. This high conductivity, which is more than that of host membrane without HPA addition (9 × 10-4 S/cm) and Nafion117 (1.7 × 10-3S/ cm) at 120 °C, also suggested a better water affinity in the membrane at elevated temperature. Thus doping of mixed HPA (PMA + PWA) into the host membrane increased the proton conductivity, efficiently. The proton conductivity values are increased with the increasing temperature and reach a maximum at 120 °C. It strongly suggests that hybrid membranes with pseudo-PEO network have proton conductive property. These results confirmed that the proton conduction in the hybrid membranes originates primarily from the dissociation among the H3PO4 molecules doped in the membranes. It is notable that the plot of conductivity (σ) versus reciprocal temperature (T-1) is not exactly linear in the range from 20 to 120 °C, indicating the proton conduction does not just follow an Arrhenius-type behavior, but also shows the feature of Vogel-Tamman-Fulcher (VTF) behavior. It suggests that proton transportation in the H3PO4 system follows the Grotthuss mechanism, and H3PO4 dispersed among the hybrid framework shows the interaction with flexible PEO chains formed among GPTMS, indicating that the segmental motion plays a role in the proton conductivity.32 Also, concerning these hybrid nanocomposite membranes, the conductivity increases with increasing HPA (PMA + PWA) content, suggesting that the hydrolysis and polymerization proceeded slowly under the experimental conditions. However, the conductivity values increment was not significant with respect to the PMA and PWA concentration as can be seen in Figure 8. The conductivities were approximately 1.62 × 10-2 and 1.7 × 10-2 S/cm at 120 °C for PMA/PWA content of 4/8 mol %, and 5/10 mol %, which are lower than the corresponding value for the 3/6 mol % composite. Increasing the PMA/PWA content further rendered systematic increases in conductivity,

Figure 9. (a) Cole-Cole representations of impedance data measured for 50SiO2-25GPTMS-20H3PO4-5APTES-3PMA/6PWA (mol %) composite at 50% RH. (b) Typical impedance plot measured for 3PMA/ 6PWA (mol %) doped membrane at 120 °C and 90% RH. Inset shows the proton conductivities of the 3PMA/6PWA (mol %) doped composite at different relative humidities from 50 to 90% RH at 120 °C.

however, never reaching the value of the 3/6 mol % sample at 120 °C. Although the proton conductivity values among the composites were not with large difference, it is evident that the addition of PMA and PWA was an effective means of increasing the conductivity. The high conductivity values were expected as a result of the high proton loading levels. It is also known that these clusters could be functionalized much more easily than the PMAs or the PWA. It is noteworthy that the conductivity decreases with an increase in temperature above 120 °C, although the mechanical properties improved after further heating at 120 °C for 6 h. Above 120 °C the conductivity decreases with increasing temperature and it reaches to 4.5 × 10-3 S/cm for 1PMA/2PWA (mol %), 6.35 × 10-3 S/cm for 3PMA/6PWA (mol %), and 6.1 × 10-3 S/cm for 5PMA/10PWA (mol %) at 150 °C and 50% RH, respectively. This could be possibly due to the decrement of water molecules in the hybrid membranes above 120 °C. The chemical species that comprised the membrane matrix such as polyethylene oxide, silicate, and phosphoric acid are known as good insulators against electronic and/or hole conduction and their combination is understood to very low possibility for further electron/ hole conduction. The conduction in these inorganic organic hybrid membranes must be solely due to protons. Thus, these hybrid membranes may be regarded as novel proton conductive hybrid materials. Figure 9a shows the Cole-Cole representation of impedance data measured for 50SiO2-25GPTMS-20H3PO4-5APTES-3PMA6PWA (mol %) composite at different temperatures under 50%

14548

J. Phys. Chem. C, Vol. 113, No. 32, 2009

RH. In the hydrolyzed TEOS/GPTMS molecules, the silica network structures are terminated to hydroxyl groups, which are bound with the water molecules in an ambient atmosphere. These hydroxyl and water molecules act to the proton conduction. The ability to absorb water is due to the large amount of HPA that includes molecular water and generated free space around HPA that can be filled with glycol and silanol groups by introducing humidity in the nanostructure (Figure 2 b). It is well known that the proton conductivity of HPA is strictly related to the number of water molecules coordinated to the Keggin anions. According to Kreuer, the heteropolyacid acts as a Brønsted acid toward the hydration water, which is loosely bound in the structure, resulting in high proton conductivity.33 A large uptake of water is essential for fast proton conduction. The proton conductivity (σ) was calculated from the electrolyte resistance (R) obtained from the intercept of the Cole-Cole plot with the real axis. By increasing the temperature from 20 to 120 °C under 50% RH, the conductivity increased from 1.65 × 10-3 to 1.88 × 10-2 S/cm for 3PMA/6PWA (mol %) composite. The impedance spectra consist of well-defined parts, one related to the contacts [low-frequency wing (LF)] and another one to the impedance of the hybrid membranes. The impedance of the hybrid membranes gives rise to a depressed semicircle [high-frequency wing]. This semicircle represents a typical equivalent circuit of a resistor and a capacitor connected in parallel and corresponds to the bulk electrical properties. The impedance spectra of the hybrid membranes showed the beginning of a second semicircle, indicating contributions from grain boundaries. This part of the semicircle contains a smaller semicircle at higher frequency. The diameter of the semicircle at a higher frequency, R1, is the grain resistance. Beyond this semicircle, there is another semicircle starting at a higher frequency and intersecting the Z1 at a lower frequency, and its diameter R2 represents the grain-boundary resistance.34 The grain resistance is much smaller than the grain-boundary resistance and the total resistance of the hybrid membranes is thus the sum of R1 and R2. Figure 9b shows typical impedance plot of 3PMA/6PWA (mol %) composite at 120 °C under 90% RH. Inset shows the proton conductivities of the 3PMA/6PWA (mol %) doped composite at different relative humidities from 50 to 90% RH at 120 °C. A high proton conductivity of 3 × 10-2 S/cm for 3PMA/6PWA (mol %) composite has been obtained. Previously, the effect of HPA doping, their stabilization, and particle size reduction on commercially available Nafion membranes at elevated temperatures were investigated by some other researchers.35-37 The conductivities of the stabilized heteropolyacid/Nafion composite membranes at 120 °C and 35% relative humidity were on the order of 1.6 × 10-2 S/cm.36 The high conductivity of the studied nanocomposite membranes that was obtained indicates that the proton-conducting path through the pseudo-PEO network, which also contains the trapped solid acid (PMA/PWA) as a proton donor, is successfully formed. When the relative humidity was elevated from 50 to 90% RH at 120 °C, the conductivities were increased largely, as shown in Figure 9b, inset. It strongly suggests that the membrane is microporous in the matrix and able to absorb molecular water to favor the proton transportation. It is well known that the surface mechanism explains38 water-depended conduction in porous ceramic and glassy materials. According to this mechanism, water molecules are absorbed chemically and physically on the wide surface separating porous oxide and atmosphere. Under the low humidity, water molecules are chemisorbed quickly in the defected sites located on the surface presenting a high local

Lakshminarayana and Nogami

Figure 10. Permeability rate as a function of the inverse temperature under a hydrogen feed for the 3PMA/6PWA (mol %) nanocomposite hybrid membrane.

charge density and a strong electrostatic field on exposure of the membrane to the atmosphere. The amount of these molecules, once absorbed, is not further changed by exposure to the humidity. These active sites promote the water dissociation to provide protons as charge carriers, 2H2O T H3O+ + OHof the hopping transporting mechanism known as Groutthuss chain reaction.39 When the hydronium (H3O+) releases a proton to a neighbor water molecule, which accepts it while releasing another proton, charge transportation occurs. When the humidity increases, the subsequent water layers are physically adsorbed, condensed to form liquid water phase in the pores. The electrolytic conduction takes place along with proton transportation in the liquid like phase. This improved proton mobility by molecular water may be another advantage of proton conducting nanocomposites that are incorporated with hydrate compounds such as HPA. These results suggest that continuous paths suitable for fast proton conduction are formed due to the absorption of water molecules in the membranes, heated at different temperatures. Figure 10 shows the hydrogen permeability of the 3PMA/6PWA (mol %) composite membrane. The permeation tests were carried out in the temperature range of 20-120 °C. The membrane displayed an activated transport of all gases that decreased with temperature as the permeation increased as a function of temperature. These results strongly indicate that the membranes were of a high quality without pin holes or microcracks. However, surface diffusion plays only a minor role (