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Chem. Mater. 2007, 19, 2372-2381
Physical and Electrochemical Characterization of Nanocomposite Membranes of Nafion and Functionalized Silicon Oxide Bradley P. Ladewig,†,‡ Robert B. Knott,§ Anita J. Hill,⊥ James D. Riches,# John W. White,| Darren J. Martin,† Joa˜o C. Diniz da Costa,† and Gao Qing Lu*,† ARC Centre for Functional Nanomaterials, School of Engineering & AIBN, and Centre for Microscopy and Microanalysis, The UniVersity of Queensland, St Lucia QLD 4072, Australian Nuclear Science and Technology Organisation, PriVate Mail Bag 1, Menai, NSW 2234, CSIRO Manufacturing Science and Technology, PriVate Mail Bag 33, Clayton South, MDC, VIC 3169, and Research School of Chemistry, Australian National UniVersity, Canberra ACT 0200, Australia ReceiVed December 4, 2006. ReVised Manuscript ReceiVed January 12, 2007
Nafion nanocomposite membranes were prepared from Nafion 117 and a systematic range of organically functionalized silicon alkoxide precursors using an in situ sol gel synthesis technique. The physical structure of the resulting nanocomposite membranes were characterized using small and wide-angle X-ray scattering, small angle neutron scattering, positron annihilation lifetime spectroscopy, and transmission electron microscopy. A structural model is proposed for three typical nanocomposite membranes (Nafion-TEOS, Nafion-MPTMS and Nafion-MPMDMS). The proton and methanol transport properties of the membranes included in the model were evaluated by impedance spectroscopy and pervaporation experiments, respectively, and correlated to their composite microstructure. In particular, this model explains the increased selectivity for transport over protons for nanocomposite membranes produced using (3mercaptopropyl)methyldimethoxysilane as the silicon alkoxide precursor, which is more than six times higher than that of Nafion 117.
Introduction Direct methanol fuel cells (DMFC) are seen as an attractive potential replacement for lithium-ion batteries in portable devices because of their potentially high-energy density, which may be up to 10 times that of conventional secondary batteries.1 For this reason, they are being considered for applications in laptop computers, mobile phones, and military devices.2 A major drawback is the high methanol permeability of Nafion, which is the most commonly used membrane in DMFCs.3 This crossover of methanol from the anode to the cathode of the DMFC leads to reduced cell potential and efficiency4 and is a major impediment to widespread DMFC commercialization. It is well-known that incorporating filler particles into polymer membranes can dramatically alter their transport properties,5 and one approach to decreasing the methanol permeability of Nafion membranes is to incorporate silicon oxide nanoparticles, which hinder the transport of methanol through the composite material. The production of Nafion* Corresponding author. E-mail:
[email protected]. † ARC Centre for Functional Nanomaterials, The University of Queensland. ‡ Current address: Laboratoire des Sciences du Ge ´ nie Chimique, CNRSENSIC, 1 rue Grandville, BP 20451, 54001 Nancy Cedex, France. § Australian Nuclear Science and Technology Organisation (ANSTO). ⊥ CSIRO Manufacturing Science and Technology. # Centre for Microscopy and Microanalysis, The University of Queensland. || Research School of Chemistry, Australian National University.
(1) Service, R. F. Science 2002, 296, 1222. (2) Cowey, K.; Green, K. J.; Mepsted, G. O.; Reeve, R. Curr. Opin. Solid State Mater. Sci. 2004, 8, 367. (3) Mauritz, K. A.; Moore, R. B. Chem. ReV. 2004, 104, 4535. (4) Deluca, N. W.; Elabd, Y. A. J. Polym. Sci. Polym. Phys. 2006, 44, 2201. (5) Giannelis, E. P. AdV. Mater. 1996, 8, 29.
silicon oxide composite membranes can be achieved by a number of different techniques. The first one is mixing silicon oxide particles with the polymer in solution and casting the composite membrane.6-11 Second, one can use silicon alkoxide precursors and sol-gel chemistry to produce silicon oxide particles in Nafion solution and casting these to form composite membranes.12-18 The third technique is to form silicon oxide particles in the membrane by in situ sol-gel synthesis.19-27 This technique was chosen in this work to (6) Staiti, P.; Arico, A. S.; Baglio, V.; Lufrano, F.; Passalacqua, E.; Antonucci, V. Solid State Ionics 2001, 145, 101. (7) Antonucci, P. L.; Arico, A. S.; Creti, P.; Ramunni, E.; Antonucci, V. Solid State Ionics 1999, 125, 431. (8) Antonucci, V.; Arico, A. S. European Patent EP 0 926 754 A1, 1999. (9) Dimitrova, P.; Friedrich, K. A.; Stimming, U.; Vogt, B. Solid State Ionics 2002, 150, 115. (10) Dimitrova, P.; Friedrich, K. A.; Vogt, B.; Stimming, U. J. Electroanal. Chem. 2002, 532, 75. (11) Stonehart, P.; Watanabe, M. US Patent 5,523,181. 5,523,181, 1996. (12) Li, C. N.; Sun, G. Q.; Ren, S. Z.; Liu, J.; Wang, Q.; Wu, Z. M.; Sun, H.; Jin, W. J. Membr. Sci. 2006, 272, 50. (13) Arico, A. S.; Baglio, V.; Antonucci, V.; Nicotera, I.; Oliviero, C.; Coppola, L.; Antonucci, P. L. J. Membr. Sci. 2006, 270, 221. (14) Jiang, R. C.; Kunz, H. R.; Fenton, J. M. J. Membr. Sci. 2006, 272, 116. (15) Kim, H.-J.; Shul, Y.-G.; Han, H. J. Power Sources 2006, 158, 137. (16) Kim, Y. J.; Choi, W. C.; Woo, S. I.; Hong, W. H. J. Membr. Sci. 2004, 238, 213. (17) Miyake, N.; Wainright, J. S.; Savinell, R. F. J. Electrochem. Soc. 2001, 148, A905. (18) Ren, S.; Sun, G.; Li, C.; Liang, Z.; Wu, Z.; Jin, W.; Qin, X.; Yang, X. J. Membr. Sci. 2006, 282, 450. (19) Baradie, B.; Dodelet, J. P.; Guay, D. J. Electroanal. Chem. 2000, 489, 101. (20) Deng, Q.; Moore, R. B.; Mauritz, K. A. J. Appl. Polym. Sci. 1998, 68, 747. (21) Klein, L. C.; Daiko, Y.; Aparicio, M.; Damay, F. Polymer 2005, 46, 4504.
10.1021/cm0628698 CCC: $37.00 © 2007 American Chemical Society Published on Web 04/07/2007
Nanocomposite Membranes of Nafion and Silicon Oxides
Chem. Mater., Vol. 19, No. 9, 2007 2373
Table 1. Silicon Content of Nafion, Test SiO2, and Nafion Composite Membranes as Determined by Inductively Coupled Plasma Atomic Emission Spectrophotometry sample
weight (mg)
digest Si (µg mL-1)
Si in sample (wt %)
Si/inorganic
Nafion test SiO2 Nafion-TEOS 6.0 wt % Nafion-TEOS 10.0 wt % Nafion-MPTMS 11.6 wt % Nafion-MPTMS 14.4 wt % Nafion-MPMDMS 7.4 wt % Nafion-MPMDMS 11.3 wt %
115.3 12.0 113.2 96.9 90.2 59.9 120.4 106.5
0.1 112.4 25.2 73.7 50.5 64.8 29.8 38.4
0.00 46.83 1.11 3.80 2.80 5.41 1.24 1.80
0.19 0.38 0.24 0.38 0.17 0.16
produce a range of nanocomposite membranes using systematically varied silicon alkoxide precursors. The resulting membranes are characterized extensively to gain a detailed understanding of the resulting composite microstructure and their transport properties.28 A mechanistic model is proposed to describe the structure of three of the Nafion composite systems and relate the structural differences to the measured proton conductivity and methanol permeability of these materials, providing an understanding of the synthesis-structure-property relationship for this class of materials. Experimental Section Materials. Nafion 117 was obtained from CG Processing, Inc., and cleaned by placing it in a 3% H2O2 solution at 60 °C for 60 min, rinsing it with distilled water, placing it in 1 M H2SO4 at 60 °C for 60 min, rinsing it with distilled water, and then placing it in distilled water at 60 °C for 60 min. Cleaned membranes were dried at 100 °C overnight and weighed to determine their initial dry weight before inorganic modification. Tetraethyl orthosilicate (TEOS), (3-mercaptopropyl)trimethoxysilane (MPTMS), (3-mercaptopropyl)methyldimethoxysilane (MPMDMS), dimethyldimethoxysilane (DMDMS), and methyltriethoxysilane (MTES) were obtained from Aldrich and used as received. Nanocomposite Membrane Synthesis. The technique used in this work is based on the method first proposed by Mauritz et al.,26 who described the use of the phase-separated morphology of Nafion as a structure-directing template for the reaction of TEOS with water, catalyzed by the immobilized sulfonic acid groups in the hydrophilic clusters. This technique has been modified with the aim of producing smaller inorganic particles by limiting the quantity of water available for the reaction and confining it to the cluster regions. The membranes were exposed to a constant relative humidity of 27% over a saturated salt solution of KCl29 for 60 min, then placed into a 1:8 (mol/mol) solution of silicon alkoxide/ethanol for 60 s. After removal from the silicon alkoxide solution, the membranes were rinsed thoroughly with ethanol to remove any reactants on the membrane surface and dried in an oven at 100 °C (22) (23) (24) (25) (26)
(27) (28) (29)
Mauritz, K. A. Macromolecules 1989, 22, 4483. Mauritz, K. A.; Payne, J. T. J. Membr. Sci. 2000, 168, 39. Mauritz, K. A.; Stefanithis, I. D. Macromolecules 1990, 23, 1380. Mauritz, K. A.; Stefanithis, I. D.; Davis, S. V.; Scheetz, R. W.; Pope, R. K.; Wilkes, G. L.; Huang, H. H. J. Appl. Polym. Sci. 1995, 55, 181. Mauritz, K. A.; Storey, R. F.; Jones, C. K. In Multiphase Polymers: Blends and Ionomers; Utracki, L. A., Weiss, R. A., Eds.; ACS Symposium Series No. 395; American Chemical Society: Washington, DC, 1989; pp 401-417. Xu, W. L.; Lu, T. H.; Liu, C. P.; Xing, W. Electrochim. Acta 2005, 50, 3280. Ladewig, B. P.; Knott, R. B.; Martin, D. J.; Diniz, da Costa, J. C.; Lu, G. Q. Electrochem. Commun. 2007, 9, 781. Acheson, D. T. In Humidity and Moisture: Measurement and Control in Science and Industry; Wexler, A., Ed.; Reinhold: New York, 1965; Vol. 4.
for 60 min. They were then removed and weighed to determine the weight percent inorganic uptake. This process of exposure to a humid atmosphere, immersion in silicon alkoxide solution, and drying was repeated until the desired inorganic content was achieved. Membranes produced from MPTMS and MPMDMS were further treated with 30% H2O2 at 60 °C for 60 min to oxidize surface thiol groups to sulfonic acid groups,30 after which they were again rinsed with distilled water, dried at 100 °C for 60 min, and weighed to determine the final inorganic content. Membrane Characterization. The silicon content of the membranes was determined as follows: 50-100 mg of membrane materials was mixed with 500 mg of Na2O2 and fused in a platinum crucible at 500 °C for 30 min to oxidize the sample. The fusion contents were dissolved in boiling deionized water for 30 min, diluted to 50 mL, and filtered. The silicon content of the solution was analyzed using an inductively coupled plasma atomic emission spectrophotometer (ICP-AES). The fusion process was tested for recovery using a reference SiO2 sample. Proton conductivity was measured using a two electrode transverse impedance technique. The blocking electrodes were made of gold-plated copper, 8 mm in diameter, and pressed together with a force of 22.2 N. Samples were soaked overnight in distilled water before being fully submerged in distilled water at 50 °C for testing. A Solartron 1260 frequency response analyzer applied a sinusoidal voltage of 10 mV amplitude over the frequency range 1-107 Hz, and the proton conductivity, σ, was calculated according to eq 131 σ)
l (R - R0)A
(1)
where l is the membrane thickness, A the electrode contact area, R the measured membrane resistance (taken as the membrane impedance at zero-phase angle), and R0 the cell closed circuit resistance. The proton conductivity of each membrane was measured three times, in different areas of the membrane, and the results were averaged. Methanol permeability was determined using a pervaporation technique, in which a 1:1 (mol/mol) CH3OH/H2O solution was circulated over one side of the membrane using a peristaltic pump and 60 mL of He min-1 was passed over the other side, evaporating any permeate, which was subsequently condensed out in a liquid nitrogen cooled condenser. The pervaporation cell was maintained at 50 °C. Three consecutive samples of condensate, collected over 30 min each, were weighed to determine the bulk flux through the membrane. The composition of the condensate was determined using a gas chromatograph (Shimadzu GC 17A) equipped with a capillary column (HP-Plot U) and a thermal conductivity detector. Small angle neutron scattering (SANS) experiments were conducted on the Australian nuclear science and technology (30) Cano-Serrano, E.; Blanco-Brieva, G.; Campos-Martin, J. M.; Fierro, J. L. G. Langmuir 2003, 19, 7621. (31) Khiterer, M.; Loy, D. A.; Cornelius, C. J.; Fujimoto, C. H.; Small, J. H.; McIntire, T. M.; Shea, K. J. Chem. Mater. 2006, 18, 3665.
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Table 2. Molecular Weights for Unhydrolyzed, Hydrolyzed, and Completely Condensed Inorganic Precursors and the Corresponding Silicon Weight Fractiona precursor TEOS MPTMS
MPMDMS
molecular/repeat unit formula
mol wt
Si wt fraction
Si(OCH2CH3)4 Si(OH)4 SiO2 Si(OCH3)(CH2)3SO3H Si(OH)3(CH2)3SO3H Si(O)3/2(CH2)3SO3H Si(O)3/2(CH2)3SH Si(OCH3)2CH3(CH2)3SO3H Si(OH)2CH3(CH2)3SO3H SiOCH3(CH2)3SO3H SiOCH3(CH2)3SH
208.32 96.11 60.08 244.34 202.26 175.24 127.24 228.34 200.29 182.27 134.27
0.13 0.29 0.47 0.11 0.14 0.16 0.22 0.12 0.14 0.15 0.21
a For the two thiol-functionalized precursors, the molecular weights for the unhydrolyzed, hydrolyzed, and completely condensed states assume complete oxidation of the -SH groups to -SO3H after treatment with H2O2. Also included is the molecular weight of the completely condensed, unoxidized state.
Figure 2. SAXS profile for Nafion-TEOS nanocomposite membranes.
Figure 3. SAXS profile for Nafion-MPTMS nanocomposite membranes.
Figure 1. TEM images of (a) 1.3 wt % Nafion-TEOS and (b) 11.1 wt % Nafion-TEOS nanocomposite membranes.
organization (ANSTO) SANS instrument with a sample aperture of 10 mm diameter, neutron wavelength of 3.5 Å, and sample-todetector distance of 4800 mm (total q range 0.01 Å-1 < q
