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Polymers 2019, 11, 914 2 of 10 The family of sulfated metal oxide (S-MO2) is very attractive, thanks to the intrinsic super-acidity properties. It has been widely investigated by us to form nano-composite membranes with good performances at high temperature and low RH [18–20]. In the present work, we propose the use of mesoporous sulfated titanium oxide (TiO2-SO4), newly synthesized, as an additive in Nafion. With respect to other studies already reported by us, the additive here was prepared by a template-driven procedure, to control the structure of the particles. Taking advantage of the properties of TiO2-SO4, we demonstrate the applicability of the proposed composite membranes in PEMFCs at high temperature and low relative humidity. 2. Materials and Methods A sol-gel one-step synthesis was developed to obtain mesoporous TiO2-SO4, with hydrolysis and sulfation happening in a single step [21], then followed by a hydrothermal treatment. Titanium isopropoxide (Sigma Aldrich, St. Louis, MO, USA) was used as a precursor and Pluronic 123 copolymer (Sigma Aldrich, Mw~5800) was used as a structure-directing agent in order to obtain a mesoporous compound. The TiO2-SO4 was prepared according to the following procedure: Pluronic 123 was added to sulfuric acid (H2SO4, 95%–97%, Sigma Aldrich, St. Louis, MO, USA) in vigorous stirring for 15 min at 35 ◦C. Subsequently, titanium isopropoxide was added to the mixture and after 15 min the solution was transferred in an autoclave and it was treated before at 30 ◦C for 20 h and then at 100 ◦C for 48 h. The solid product was filtered and calcinated for 3 h at 550 ◦C (heat rate 3 ◦C/min). Nafion membranes were prepared by a solvent casting procedure, starting from Nafion wt. 5 % solution (E.W. 1100, Ion Power Inc, München Germany), where solvents (water and alcohols) were gradually replaced with N,N-dimethylacetamide (> 99.5%, Sigma Aldrich, St. Louis, MO, USA) at 80 ◦C [9]. For the composite membranes, the TiO2-SO4 was added to the final Nafion solution. A filler concentration of 2 wt %, 5 wt % and 7 wt %, with respect to the dry Nafion content, was chosen. The mixture obtained was casted on a Petri dish and dried at 80 ◦C. After the heating treatment, dry membranes were extracted and hot-pressed at 50 atm, 175 ◦C for 15 min. This method is needed to improve the thermal stability of the membranes and allows a preferable cohesion between ionic clusters [22]. The membranes were finally activated in boiling 3% w/w hydrogen peroxide (H2O2, 34.5%-36.5%, Sigma Aldrich, St. Louis, MO, USA), H2SO4 (0.5 M) and distilled water. Nano-composite membranes have been compared to plain Nafion systems prepared with the same procedure. All samples were stored in bi-distilled water. X-ray diffraction analysis (XRD) was carried out to study the phases of the prepared TiO2-SO4 additive. The X-ray analysis was performed using a Rigaku D-Max Ultima + diffractometer, provided with a graphite monochromator, in the 2θ range 20–90◦. The radiation used was the Kα of Cu. The crystallite size was obtained using Maud code. Nitrogen-adsorption experiments were used for the determination of pores size distributions and for the evaluation of the specific surface area of the inorganic powder by the Brunauer-Emmett-Teller (BET) equation, using a Micromeritics ASAP 2010. Before of the measurement, each sample needed a pre-treatment at 200 ◦C for 2 h in order to remove physisorbed water. Through the scanning electron microscopy (SEM, Phenom, Eindhoven, The Netherlands) analysis, the morphology and size of the inorganic additive were evaluated. Thermal analysis were conducted by means of differential scanning calorimetry (DSC), using DSC821 instrument (Mettler-Toledo, Zaventem, Belgium), and by thermal gravimetric (TG) analysis, performed with a TGA/SDTA851 (Mettler-Toledo, Zaventem, Belgium). DSC was carried out on the membrane samples under nitrogen (N2) flux (60 mL/min) at a scan rate of 20 ◦C/min. Before DSC measurements, membrane samples were equilibrated at 100% relative humidity (RH) for two weeks. TG analysis was carried out on TiO2-SO4 powder sample under air flux (60 mL/min) at a scan rate of 5 ◦C/min. The ion-exchange capacity (IEC), which expresses the amount of exchangeable protons, was evaluated by a titration method both for the powder and for all the membranes. Dry samples werePDF Image | Polymer Electrolyte Membranes Based on Nafion Fuel Cell
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