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Polymers 2021, 13, 1386 2 of 14 at higher methanol concentrations to achieve higher energy densities. These approaches are based on the use of sulfonated aromatic polymers (SAPs) and their modifications [13–17]. The SAP membranes may exhibit high proton conductivity, high fuel cell performance and suitable durability compared to PFSA membranes. Furthermore, the development of inorganic–organic composite membranes based on SAPs for application in DMFCs can (i) improve the self-humidification of the membrane by dispersing hydrophilic inorganic additives in the polymer [18–22]; (ii) reduce the fuel (methanol) crossover through the membrane [23–28]; and (iii) improve the mechanical strength of the membranes without compromising proton conductivity [29,30]. The SAPs could be prepared with a high degree of sulfonation, which is highly de- sirable to achieve a high conductivity. However, this can be accompanied by undesirable higher swelling (or even solubility in hot water) of the membrane and loss of mechanical strength. The addition of an inorganic component into polymer electrolytes is envisaged to compensate these effects, improving the mechanical and chemical stability features and enhancing the thermal stability, proton conductivity and, likely, electrochemical perfor- mance [31–36]. A further strategy is the use of composite sulfonated aromatic membranes with low degree of sulfonation and low water/methanol swelling, modifying and opti- mizing the characteristics of the inorganic fillers, i.e., sulfonation, acid groups, etc. [37,38]. This approach was successfully adopted, as demonstrated in previous papers [37,38], in which an investigation of bare and acidic silica as fillers for sulfonated polysulfone membrane was carried out. The membrane properties were adjusted according to the added fillers; in particular, the acidic-silica-based composite membrane exhibited the best electrochemical performance compared to that with the untreated silica and unmodified sulfonated polysulfone membranes. Moreover, the study demonstrated that this devel- oped sulfonated aromatic polymer can be adapted, in such a way, to exhibits low water uptake, low methanol swelling, reduced methanol crossover, high proton conductivity and suitable DMFC performance. Instead, the aim of this work is to investigate the water and methanol transport characteristics of these membranes (both pristine and composites), particularly when a high concentration of methanol (5 M) is used. For this scope, 1H-PFG NMR technique was used in this study. Furthermore, an analysis of methanol cross-over and direct methanol fuel cell performance was performed using a high methanol concen- tration (5 M), in order to validate the used approach under conditions closer to practical DMFC applications. 2. Experimental The acidic silica material was prepared starting from CAB-O-SIL EH-5 silica (Cabot Corporation, Boston, MA, USA), according to a procedure reported in detail elsewhere [37–39]. Briefly, silica (20.0 g) was reacted with 4.5 mL of chlorosulfonic acid (concentrate) under stirring over a period of 30 min at room temperature. After complete release of HCl (gas), the obtained white solid material was dried at room temperature and stored in a desiccator. A commercial polysulfone polymer (Lati SpA, Varese, Italy) was sulfonated in chloro- form (Sigma-Aldrich, Milano, Italy) solution (8 wt./v.%) at 50 ◦C per 6 h using trimethylsilyl chlorosulfonate (Sigma-Aldrich) as the sulfonating agent and under reflux to produce a silyl sulfonate polysulfone. Thereafter, it was treated with sodium methoxide/methanol solution (30 wt%) at 50 ◦C for 1 h to obtain a sodium sulfonated polysulfone [40]. At the end, a white fine precipitate was recovered and dried at 70 ◦C, for 24 h under vacuum. The bare membrane was prepared by casting method from sulfonated polysulfone solution in dimethylacetamide (15 wt%); the polymer solution was spread on a glass plate using a manual stainless-steel film applicator. The cast membrane was allowed to evaporate for the solvent removal at 50 ◦C for at least 15 h. Composite membranes were prepared in the same way by adding 10 wt% acidic or bare silica to the polymeric dispersion. Mem- branes of uniform thickness (100 m) were prepared. Ion-exchange capacities (IECs) of the membranes were determined by back titration; the values are 1.37, 1.32 and 1.34 mmol/gPDF Image | Properties of Methanol Transport for Direct Methanol Fuel Cells
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