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Polymers 2021, 13, 1386 6 of 14 To perform the NMR measurements, it was necessary to discriminate between the NMR signals of methanol and water, which, in the case of solvents confined in membranes, due to the linewidth of the 1H-NMR signals, is not possible to distinguish, through their chemical shift [48]. Therefore, the membranes were equilibrated in solutions prepared with deuterated molecules, i.e., mixture of CH3OD/D2O and CD3OD/H2O. The reason to use CH3OD instead of CH3OH is due to the fast rate exchange of hydroxyl groups between water and methanol molecules during the “NMR times”, which could affect the measurements. Hence, we used the only signal coming from the methyl groups to perform the NMR diffusometry measurements of methanol confined inside the electrolyte films swelled with CH3OD/D2O solutions, and water self-diffusion measurements on membranes swelled with CD3OD-H2O solutions. Figure 3 shows a comparison of the diffusion coefficients of water and methanol (1 and 5 M solutions) measured in swollen membranes, in the range of temperature 20–80 ◦C. The most important consideration concerns the higher water diffusion respect to methanol diffusion for all three membranes and for both two solution concentrations. Furthermore, the difference becomes larger for the composites, proving the beneficial methanol blocking effect of the nanoparticles dispersed in the polymer. These good properties regarding the blocking effect of methanol molecules envisage a suitable behavior of these membranes in DMFCs operation with high methanol concen- tration (5 M). Accordingly, MEAs prepared with the composite and filler-free membranes were investigated in DMFC using 5 M as the anode feed. Figure 4 shows a comparison of the polarization curves for these MEAs at 30 ◦C (Figure 4a) and 60 ◦C (Figure 4b), feeding dry air at the cathode side under atmospheric pressure. At 30 ◦C, the best performance, in terms of both power density, current density and open circuit voltage (OCV), was obtained with the MEA based on sPSf-SiO2_sulf followed by sPSf-SiO2 and filler-free SPSf membranes. The OCV for the acidic composite membrane (sPSf-SiO2_sulf) was 0.79 V, slightly better than that recorded for the sPSf-SiO2 (0.77 V) and significantly higher than the value of 0.62 V obtained with the filler-free membrane. A higher OCV is a clear indication of a lower amount of MeOH crossing the membrane and reaching the cathode, since the effect of the mixed potential (oxygen reduction and methanol oxidation) at the latter electrode is limited. The same behavior is also confirmed at higher temperatures (60 ◦C), although a slight decrease of the OCV was observed for all MEAs. In fact, as known from the literature [23,28], methanol cross-over increases with the temperature, producing a decrease of OCV and performance, although this latter is usually compensated by the enhanced kinetics of methanol oxidation and oxygen reduction reactions. Furthermore, also the proton conductivity of the membrane increases with the temperature leading to an increase of fuel cell performance. Unfortunately, in this study the performance enhancement is not so significant due to the high concentration of methanol (5 M), which produces a large methanol cross-over not counteracted by the presence of pure oxygen at the cathode side (the tests were carried out under more reliable conditions, using dry air at atmospheric pressure at the cathode); thus, methanol competes with oxygen creating a mixed potential, which reduces the fuel cell performance. As a result, the maximum power density achieved with the sPSf-SiO2_sulf was 26 mW·cm−2 at 30 ◦C, with an increase to 29 mW·cm−2 at 60 ◦C, probably due to the negative effect of methanol crossing the membrane. However, the performance observed with the composite acid filler-based membrane (sPSf-SiO2_sulf) was significantly higher than that achieved with the other two membranes (Figure 3). To confirm the beneficial effect of the composite membranes in reducing methanol permeation through the membrane, methanol crossover measurements, using linear sweep voltammetry, were carried out, both at 30 and 60 ◦C. The curves are reported in Figure 5.PDF Image | Properties of Methanol Transport for Direct Methanol Fuel Cells
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