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Polymers 2021, 13, 1386 4 of 14 The number of scans for the 1H PFG-NMR measurements were 4, while the number of repetitions was kept equal to 2. The repetition time was about 5 times the longitudinal relaxation time (T1) and the total acquisition time ranged between 2 and 10 min. Figure 1c illustrates the experimental echo-signal attenuation curves in the diffusion measurements for water and methanol in the three membranes. On the basis of the very high SNR (signal to noise ratio), the attenuation higher than 90%, the low standard deviation of the fitting curve and repeatability of the measurements, the uncertainties in the PFG-NMR self-diffusion measurements were calculated to be ca. 3%. Measurements were collected as a function of methanol concentration (e.g., 1–5 M CH3OH) and temperature [42]. The electrodes used in this investigation were prepared as previous described [37,38]. A catalytic ink, consisting of an unsupported Pt-Ru (1:1 atomic ratio, Johnson-Matthey), with a Pt loading of 2.5 mg cm−2, and a 15 wt% Nafion ionomer (Ion Power, 5 wt% solution), was employed at the anode, while a Pt black (Johnson-Matthey, London, UK), with the same amount of metal and Nafion ionomer as the anode, was used at the cathode. The membrane-electrode assemblies (MEAs) were obtained by hot-pressing the electrodes onto the membrane at 90 ◦C and 30 kg·cm−2 for 5 min. The MEAs were tested in a 5 cm2 single cell using an Autolab PGSTAT 302 Potentiostat/Galvanostat (Metrohm Italia, Origgio (MI), Italy) equipped with an FRA module of impedance. The methanol solution was fed at the anode with a flow rate of 3 mL min−1, whereas dry air was fed at the cathode (100 mL min−1). Area specific resistance and ionic conductivity of the investigated polysulfone membranes were obtained by electrochemical impedance spectroscopy (EIS). The EIS measurements were performed in a frequency range from 100 kHz to 0.1 Hz on the cells kept at 0.3 V. Under these conditions, the fuel cell operates in the ohmic region. The amplitude of the sinusoidal excitation signal was 10 mV. The series resistance was determined from the high frequency intercept on the real axis in the Nyquist plot. The crossover measurements, using a 5 M methanol solution, were carried out by linear sweep voltammetry (LSV) experiments at a scan rate of 2 mV·s−1 and in the voltage range from 0 to 0.95 V. A 5 M MeOH solution (3 mL·min−1) was fed to one side of the cell, used as the counter/reference electrode and He (100 cm3·min−1) was supplied to the other compartment (working electrode). Methanol crossing the membrane is oxidized at the working electrode generating a positive current, which reaches a plateau when all methanol is converted to CO2 under steady state conditions [43–45]. 3. Results and Discussions In this study the 1H-PFG NMR technique was used to investigate the transport properties of water and methanol molecules confined in the porous structure of sPSf and composite membranes, and to check the effect of the inorganic fillers added. Firstly, water self-diffusion coefficients were measured on completely hydrated membranes, and the data are displayed in Figure 2 for the temperature range from 20 to 120 ◦C. As observed, the amount of water absorbed from the three membranes was quite different, and already from these data emerged the hydrophilic effect of the silica nanoparticles dispersed: the sPSf pristine membrane shows a water uptake of about 12 wt.%, while in the composites it significantly increased up to about 19 wt.% for the sPSf-SiO2 composite and up to 24 wt% for the sPSf-SiO2_sulf. Evidently, the different water content affected the diffusion coefficients, especially in the region of low temperatures, i.e., below 80 ◦C, where the water evaporation is negligible. Indeed, both composites promote higher mobility of the water molecules with coefficients 3 times higher than the filler-free membrane. In all cases, the self-diffusion, D, increased as the temperature increased due to the thermal motions, but at 80 ◦C the thermal energy was no more able to compensate the effect caused by the water evaporation: diffusion started to fall because most of the “free water” was lost from the membranes, and remained the hydration water to the hydrophilic groups, whose diffusivity was clearly inhibited.PDF Image | Properties of Methanol Transport for Direct Methanol Fuel Cells
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