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ter 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 dif- Polymers 2021, 13, 1386 fusivity was clearly inhibited. 10-5 10-6 10-7 Figure 2. Water self-diffusion coefficients as a function of temperature for the different polysulfone membranes. The analysis of the temperature dependence of the water diffusivity, using Arrhenius equation in the temperature range in which the diffusion increased, i.e., 20–80 ◦C, gives the corresponding activation energy (Ea) of the diffusion process, which is the energy barrier for carrier transfer from one free site to another one. Both the composites’ membranes have Ea values very similar, 3.82 kcal/mol for sPFS-SiO2 and 3.87 kcal/mol for sPFS-SiO2_sulf, which are typical values usually obtained also for Nafion-based electrolytes [46,47]. Instead, the sPSF pristine membrane shows a much higher value, 4.6 kcal/mol, likely associated with a low hydration level inside the hydrophilic pores of this membrane: the water molecules are strongly interacting with the sulfonic acid groups and the structural diffusion (Grotthuss proton transport mechanism) is the preferential path for proton diffusivity respect to the vehicular mechanism. The hopping mechanism foresees the reorientation of the protons in the solid-like sites through the breaking of such electrostatic interaction bonds, therefore the activation energy associated with this motion increases. Concerning the behavior of such membranes with respect to methanol, they were swelled by immersion in aqueous methanol solutions at two different alcoholic concentration, 1 and 5 M. Table 1 shows the uptake values obtained at 20 ◦C. Compared with the water uptake values seen previously, there are some differences, in particular at higher methanol concentration, reaching for the composites an uptake of about 30 wt%, and 20 wt% for the filler-free sPSf. However, if we compare these values with those usually obtained in Nafion based electrolytes (34 wt% with 4 M methanol) [48], the swelling here is quite lower, confirming a lower affinity of the methanol to polysulfone polymer compared to perfluorosulfonic acid membranes. This parameter is important to overcome the major limitation of using high concentrations of methanol solutions in DMFCs. Table 1. Aqueous methanol solution uptakes (wt%) of the membranes at 20 ◦C. 20 40 60 80 100 120 Temperature (°C) 5 of 14 water self-diffusion sPSF filler-free sPSF/SiO2 sPSF/SiO2-sulf (W.U. 12wt%) (W.U. 19 wt%) (W.U. 24 wt%) Membranes sPSf (filler-free) sPSf-SiO2 sPSf-SiO2 _sulf Uptake (wt%) 1 M MeOH Solution 14 24 27 Uptake (wt%) 5 M MeOH Solution 20 30 30 D (cm2 s-1)PDF Image | Properties of Methanol Transport for Direct Methanol Fuel Cells
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