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(Ion Power) mixed with a solvent [IPA, ethanol (EtOH), or dimethyl sulfoxide (DMSO)] and TiO2 particles. A variety of different sources and prepa- rations of TiO2 particles have been examined. In this article, we focus on 21-nm TiO2 particles (ana- tase) from Degussa-Huls. To prepare a composite membrane, a colloidal suspension of the TiO2 par- ticles and one of the solvents was created by the sonication of the particles in the solvent for over an hour. A Nafion solution was added to the sus- pension, and it was further sonicated. This suspen- sion was cast onto a flat, glass surface, and the sol- vent was removed by evaporation at $80 8C in a vacuum oven (for DMSO) or at $70 8C without a vacuum (for IPA and EtOH). Once the solvents were completely removed, the membranes were annealed via heating to a temperature of $165 8C for 1 h. The membranes were then cleaned and converted to the Hþ form according to the proce- dure detailed previously. Fuel Cell Tests MEA Preparation Commercial gas-diffusion electrodes (20% Pt on carbon, 0.4 mg of Pt/cm2; purchased from E-TEK) were brushed with 5 wt % solubilized Nafion (Aldrich) to impregnate the active layer (0.6 mg/ cm2) and then dried at 80 8C for 1 h. The geometri- cal area of the electrodes was 5 cm2. A membrane was sandwiched between two electrodes and gas- sealing gaskets, and then the MEA was pressed for 2 min at 135 8C at 2 MPa with a Carver hot press. Single-Cell Test Fixture and Performance Evaluation The MEAs, coupled with gas-sealing gaskets, were placed in a Globe Tech, Inc., single-cell test fixture described elsewhere.36 The H2 and O2 (BOC) gases were fed to the single cell at 100 sccm. The gases were bubbled through water in temperature-con- trolled stainless steel bottles to fully humidify the feeds before entry to the fuel cell. The baseline test was performed at a total pressure of 1 bar, cell temperature of 80 8C, and anode and cathode humidifier bottles at 90 8C. The fuel cell perform- ance was characterized by current–voltage meas- urements (polarization curves). These were re- corded at 80 8C and atmospheric pressure. We obtained current–voltage, iv, measurements by connecting the fuel cell to a load resistance (an electronic Amrel load) and sweeping the voltage from 1 to 0.2 V at 10 mV/s, recording the voltage Journal of Polymer Science: Part B: Polymer Physics DOI 10.1002/polb and current. Because the entire current–potential curve for a given temperature/humidification con- dition was obtained in $2 min, it was assumed that the membranes had a constant water content throughout the measurement. The fuel cell was preconditioned by operation at 0.5 V for 2–3 h before the iv measurement. Physical/Chemical Characterization The ion-exchange capacity (IEC) was determined by an exchange of acidic protons with another cat- ion in solution.37,38 The membranes were dried and weighed and then placed in a 1 M NaCl solu- tion at 80 8C overnight to exchange Naþ ions with Hþ. The large excess of Naþ ions ensured virtually complete exchange. The membranes were removed from the solution, and the solution was titrated to the phenolphthalein end point with a 0.1 M NaOH solution to determine the quantity of exchanged Hþ ions. The IEC and equivalent weight (g of poly- mer/mol of Hþ) were calculated with the dry weight of the polymer and the quantity of ex- changed protons. The membranes ($3 cm 3 cm 127 lm) were vacuum-dried at $80 8C for 3 h and then weighed, and the length was measured. The water uptake was measured for membranes placed both in and above liquid water for 24 h at 25 8C and for mem- branes placed in boiling water for $1 h. The mem- branes were removed from the water, blotted to remove droplets, and then weighed and measured. The linear expansion factor (L%) and H2O sorption (W%) were obtained with eqs 1 and 2: L%1⁄4L1 L0 100 ð1Þ L0 TITANIA/NAFION COMPOSITE MEMBRANES 2329 W% 1⁄4 W 0 W1 W0 100 ð2Þ where L0 and L1 are the lengths of the membranes before and after water sorption, respectively, and W0 and W1 are the masses of the membranes before and after water sorption, respectively. Resistivity measurements were carried out with a membrane sheet 0.5 cm wide and 3 cm long. The membrane was placed between two polycar- bonate plates. The top plate had two flush-fit stainless steel electrodes 2.54 cm apart. The alter- nating-current (ac) impedance across the mem- brane was obtained at several frequencies from 0.1 Hz to 100 kHz. We found no change in the im- pedance from 10 Hz to 20 kHz. To record dynamicPDF Image | Properties of Nafion and Titania Nafion Composite Membranes
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