Langmuir 2007, 23, 5077-5084
5077
Embedded Hydrophilic Nanogranules with Radiating Proton-Conducting Channels in a Hydrophobic Matrix Haiqin Pei, Liang Hong,* and Jim Yang Lee* Department of Chemical & Biomolecular Engineering, National UniVersity of Singapore, 10 Kent Ridge Crescent, Singapore 119260 ReceiVed August 14, 2006. In Final Form: February 21, 2007
It is a significant challenge to develop proton exchange membranes (PEMs) possessing both desired proton conductivity and low methanol permeability for the direct methanol fuel cells (DMFC). In this work, a composite PEM was fabricated from a predominantly hydrophobic framework of three-component polymer blend (TCPB) of acrylic polymers and a dispersed proton-conducting hydrophilic copolymer network. The hydrophilic copolymer was designed to contain three co-monomer units of complementary functionalities: 2-acrylamido-2-methyl propanesulfonic acid (AMPS), 2-hydroxyethyl methacrylate (HEMA), and 2-hydroxyl-3-(diethanolamino)propylmethacrylate (DEAPMA). The resultant PEMs were macroscopically homogeneous but contained microscopic heterogeneity in the form of dispersed nanosize AMPS domains with radiating (HEMA-DEAPMA) segments in the TCPB matrix, forming an overall amphiphilic matrix. Formation of such a texture was consequential upon the association of AMPS units and hydrogen bonding between the HEMA and DEAPMA short blocks of the hydrophilic copolymer with TCPB. The polymer blend membranes therefore acquired dual functionalities, i.e., effective proton transport between AMPS granules through interconnecting (HEMA-DEAPMA) segments, resulting in proton conductivity of the order of 10-2 S/cm, and low water uptake and inhibited methanol passage in the continuous amphiphilic matrix, resulting in methanol permeability of (1.25-8) × 10-7 cm2/s, which is about 3-8 times smaller than that of Nafion117.
Introduction Proton exchange membrane (PEM) is a critical component in low to moderate temperature fuel cells, supporting proton conduction from anode to cathode while at the same time serving as a physical barrier confining the fuel and oxidant to their respective electrode compartments. Perfluorosulfonic polymers such as Nafion have been the PEM material of choice for hydrogen fuel cells; their high proton conductivity and excellent chemical, thermal, and mechanical stability are highly desirable features.1,2 However, Nafion is methanol-permeable and is therefore unsuitable for use in direct methanol fuel cells (DMFCs). Methanol crossover can result in the loss of fuel and cell performance, and is a major impediment to the commercialization of DMFCs.3 The cost and the environmental impact of Nafion are some of the other concerns.4 It continues to be a significant challenge to develop effective, low-cost non-fluorinated or partially perfluorinated PEMs for fuel cell applications. The membranes produced here by blending an inexpensive protonconducting copolymer with acrylic polymers are able to meet most of these application requirements. A three-component polymer blend (TCPB) consisting of poly(4-vinylphenol-co-methylmethacrylate) P(4-VP-MMA), poly(butylmethacrylate) (PBMA), and Paraloid B-82 acrylic copolymer resins was deployed as the methanol blocking phase in this study. The design was based on the known low solubility of methanol in acrylic polymers, with PBMA and Paraloid B-82 resins providing a processible and structurally stable framework * To whom all correspondence should be addressed. L.H.: fax, (65) 67791936; tel, (65) 6516-5029; e-mail,
[email protected]. J.Y.L.: fax, (65) 6779-1936; tel, (65) 6516-2899; e-mail,
[email protected]. (1) Rhee, C. H.; Kim, H. K.; Chang, H.; Lee, J. S. Chem. Mater. 2005, 17, 1691. (2) Kim, D. W.; Choi, H. S.; Lee, C.; A., B.; Kang, Y. Electrochem. Acta 2004, 50 (2-3), 656. (3) Heinzel, A.; Barragan, V. M. J. Power Sources 1999, 84, 70. (4) Kerres, J. A. J. Membr. Sci. 2001, 185, 3.
for membrane fabrication. The proton source was a specially tailored 2-acrylamido-2-methyl propanesulfonic acid (AMPS) copolymer dispersed in the TCPB matrix. While AMPS-based polymers are more capable than Nafion in maintaining ionic conductivity under low-water conditions,5 their high level of water uptake is the most undesirable property,6,7 as we have experienced in our previous study.8 Such propensity was circumvented here by copolymerizing AMPS with two hydrophilic co-monomers, 2-hydroxyethyl methacrylate (HEMA), and 2-hydroxyl-3-(diethanolamino)-propylmethacrylate (DEAPMA). The resultant AMPS copolymer, when blended with TCPB, would form an amphiphilic matrix with reduced water absorptivity. HEMA was an important component in the formulation, as without it, the copolymer would be easily leached from the TCPB matrix when exposed to water. The (HEMA-DEAPMA) units formed a distributed hydrogel system through hydrogen bonding with the P(4-VP-MMA) segments (18.2 wt %) of TCPB and acrylicacrylic interaction, thereby establishing the proton conduction channels in the membranes. At the same time, the AMPS short blocks in the copolymer associated to form microscopic pools of protons which preserved the bound water in the composite matrix. The chemical structures of the hydrophobic and hydrophilic constituents of the polymer blend are illustrated in Scheme 1. The procedures from AMPS copolymer synthesis to the fabrication of proton exchange membranes, based on the poly(AMPS-HEMA-DEAPMA) -TCPB blends, are described in detail in this report. This is followed by a careful examination of the membrane structure and the measurements of properties paramount to DMFC applications such as ion-exchange capacity, proton conductivity, and methanol permeability. (5) Randin, J. P. J. Electrochem. Soc. 1982, 129, 1215. (6) Walker, J. C. W. J. Power Sources 2002, 110, 144. (7) Walker, J. C. W. J. Electrochem. Soc. 2004, 151 (11), A1797. (8) Pei, H. Q.; Hong, L.; Lee, J. Y. J. Membr. Sci. 2006, 270, 169.
10.1021/la0623937 CCC: $37.00 © 2007 American Chemical Society Published on Web 03/23/2007
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Pei et al.
Scheme 1: Chemical Structures of Poly (AMPS-HEMA-DEAPMA) and TCPB
Experimental Section Materials. Poly(4-vinylphenol-co-methylmethacrylate) P(4-VPMMA) (containing 51 mol % of 4-VP units), poly(butylmethacrylate) (PBMA) (M h ) 337,000), 2-acrylamido-2-methyl propanesulfonic acid (AMPS), diethanolamine (DEA) (98%), methyl ethyl ketone (MEK) (99%, A.C.S reagent grade), N,N-dimethylformamide (DMF) (HPLC grade), 1,4-dioxane (anhydrous 99.8%), methanol (HPLCgrade), and benzoyl peroxide (BPO) from Aldrich and sodium chloride, and sodium hydroxide from Merck were used without purification. Paraloid B-82 acrylic copolymer resins (56.1 wt % methyl methacrylate and 43.9 wt % ethyl acrylate) were supplied by Rohm & Haas. 2-Hydroxyethyl methacrylate (HEMA) and glycidyl methacrylate (GMA) from Aldrich were used after the removal of the hydroquinone (HQ) inhibitor. Films of Nafion 117 membranes (equivalent weight of 1100) for comparative studies were purchased from Aldrich. Membrane Preparation. The poly(AMPS-HEMA-DEAPMA)-TCPB blend membranes were produced as follows: Solution A. 2-Hydroxyl-3(diethanolamino)-propylmethacrylate (DEAPMA) was synthesized by heating a solution of 0.4 g of GMA and DEA (in mole ratio of 1:1.1) in 10 mL of 1,4-dioxane and DMF (1:1, v/v) at 80 °C for 3 h. This was followed by the addition of 0.4-0.7 g of AMPS and HEMA (in mole ratio of 1:1.6) and BPO (5 wt % of the monomers total). Polymerization was allowed to proceed for 24 h at 75 °C under flowing nitrogen. A viscous clear yellow solution was obtained at the end of the procedure. The resulting poly(AMPS-HEMA-DEAPMA) is expected to be a random copolymer based on consideration of the monomer structures.9 The molecular weight of poly(AMPS-HEMA-DEAPMA) was determined by gel permeation chromatography (GPC) to be in the range of Mw ) 30 000-40 000. (9) Odian, G. Principles of Polymerization, 4th ed.; John Wiley & Sons: Hoboken, NJ, 2004; Chapter 6.
TCPB. A 0.3 g sample of poly (4-vinylphenol-co-methyl methacrylate), poly (butyl methacrylate) (PBMA), and Paraloid B-82 acrylic copolymer resins (in weight ratio of 1:1.7:0.3) was dissolved in 5 mL MEK. A clear yellow solution was obtained after 6 h of stirring at room temperature. Solution A and TCPB were mixed and stirred for 12 h at room temperature. The resulting transparent solution was cast over a Telfon dish and allowed to dry at room temperature for 24 h to remove most of the solvents. Further drying was carried out at 60 °C in a vacuum oven for 48 h. A free-standing, homogeneous and flexible membrane was obtained which could be easily detached from the Teflon dish. Four membranes were prepared by varying the TCPB to poly(AMPS-HEMA-DEAPMA) ratio (Table 1). Materials Characterizations. Determination of the polymer molecular structures by IR spectroscopy was carried out on a BioRad FTS 135 FTIR spectrometer using the KBr pellet method. Each spectrum was the result of 40 scans in the range 400 to 4000 cm-1 sampled at 8 cm-1 resolution. The cross-sectional morphologies of the membranes were examined by field emission scanning electron microscopy using a JEOL JSM-6700F operating at 5 kV. The macro phases in the poly(AMPS-HEMA-DEAPMA)TCPB blend membranes were characterized by differential scanning calorimetry (DSC) on a DuPont TA 2910 calorimeter. The following temperature program was used; heating from -20 to 150 °C at 5 °C/min, heat soak at 150 °C for 1 min, and cooling to -20 °C at 20 °C/min. The first heating and cooling cycle was used to remove thermal history in sample preparation. Consequently DSC profiles were collected from the second heating cycle at 10 °C/min. Water Sorption and State of Water. Membranes after vacuum drying at 70 °C for 24 h were weighed and placed in deionized water at room temperature for 24 h. The total water content in a given membrane at ambient temperature was obtained by the difference between the dry weight and the wet weight of the membrane.
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Langmuir, Vol. 23, No. 9, 2007 5079
Table 1: Compositions and the States of Water in AMPS Copolymer-TCPB Blend Membranes AMPS copolymer
a
membraneb
AMPS (mg)
HEMA (mg)
DEAPMA (mg)
total
free
bound
[bound]/[total]
B1 B2 B3 B4
75 100 120 150
75 100 120 150
380 380 380 380
14.5 16.0 17.0 18.2
3.3 3.6 3.8 4.4
11.2 12.4 13.2 13.8
77.2 77.5 77.6 75.8
[Bound water%]/[total water%] × 100. b The weight of TCPB in all membranes was kept constant at 300 mg. Table 2: IEC Values of AMPS Copolymer-TCPB Blend Membranes IEC
titration ion theoretical SO3H membranes (mmol/g) -SO3H (mmol/g) (meq/g) exchangeability B1 B2 B3 B4
ratio(%)a
water content (%)
0.436 0.548 0.629 0.738
0.436 0.548 0.629 0.738
0.14 0.20 0.27 0.31
32% 36.5% 42.9% 42%
DSC measurements were used to determine the amount of free (bulk) water that was not bound by hydrogen bonding with the hydrophilic groups of the polymer. The measurements were carried out on a DuPont TA 2910 calorimeter using a weighted sample in a hermetically sealed aluminum pan. Only one endothermic peak showing the property of free water, was detected by DSC. The amount of free water in a membrane was estimated by integrating the water peak in the melt endotherm, using 334J/g as the enthalpy of melting for water at 0 °C.10 The amount of bound water was then calculated as the difference between the total water uptake and the amount of free water. The results are shown in Table 1. Nitrogen gas (99.99%) at a flow rate of 50 mL/min was used as the protective gas during the temperature scans. Ion Exchange Capacity (IEC). Ion exchange capacity (IEC) was determined by the classical titration technique. Each membrane was placed in 15 mL 0.05M sodium chloride aqueous solution for 24 h to exchange the protons with sodium ions. The ion-exchanged solution (hydrogen chloride) was titrated to pH 7.0 with 0.05M sodium hydroxide aqueous solution using a pH meter (Schott) for end-point detection. Each exchanged NaCl solution was titrated thrice and the mean titrate volume was used for the IEC calculation. The IEC results are given in Table 2. Proton Conductivity. Sample membranes were cut into circular discs 1.85 cm in diameter, and fully hydrated with deionized water for 24 h prior to the measurements. A membrane disc was sandwiched between two aluminum electrodes to form a symmetric test cell. Proton conductivities were measured in the temperature range 30 °C to 90 °C with 90 min of equilibration at each temperature, and by electrochemical impedance spectroscopy (EIS) between 1 Hz and 1 MHz on an Eco Chemie PGSTAT 30 poteniostat/galvanostat equipped with a frequency response analyzer module. Methanol Permeability. Methanol permeability was measured in a glass diffusion cell. One compartment of the cell (VA ) 50 mL) was filled with 2 M methanol solution (8 vol %, typical concentration used in current DMFCs). The other compartment (VB ) 50 mL) was filled with deionized water. A membrane (4.90 cm2) after fully hydrated with deionized water for 24 h was clamped between the two compartments with the solutions in the two compartments constantly stirred during the measurements. A methanol flux was set up as a result of the concentration difference of methanol between the two compartments. The concentration of methanol diffused from compartment A to B across the membrane was monitored as a function of time, using a Shimadzu GC2010 gas chromatograph with flame ionization detector, and 1-butanol as the internal standard. A detailed description of the experimental setup and procedure can be found elsewhere.11 (10) Samuel, P. K.; Sano, K.; Sudoh, M.; Kensaka, M. Purif. Technol. 2000, 18, 141. (11) Tricoli, V. J. Electrochem. Soc. 1998, 145, 3798.
Viscosity. Solutions of TCPB (150 mg/mL) in MEK and in mixtures of MEK and methanol were prepared. The viscosity changes with solvent composition and shear rate were measured by a Brrokfield DV-II + Pro viscometer at ambient temperature (ca. 25 °C).
Results and Discussion Structure of the AMPS Copolymer-TCPB Blend Membranes. The FTIR spectrum of the hydrophilic AMPS copolymer, poly(AMPS-HEMA-DEAPMA), is shown in Figure 1. The presence of AMPS in the copolymer was confirmed by characteristic IR absorptions at 1150 and 1200 cm-1 due to νSd -1 due to ν CdO from the amide group. The O, and at 1650 cm FTIR spectrum of poly(AMPS-HEMA-GMA) was also measured for comparison, which showed expectedly the characteristic absorption by epoxy ring stretching at 1250 cm-1. Evidence for DEAPMA formation upon reaction between diethanolamine and the epoxy group of GMA was provided by νC-N stretching from the ternary amine group (-N
