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The influence of different amounts of micro-sized titanium dioxide (anatase crystalline variety) dispersed into a polybenzimidazole membrane has been studied for its use as an electrolyte in high temperature polymer electrolyte membrane fuel cells. All of the membranes were cast from a polymer with the same molecular weight. The membranes were physicochemically and electrochemically characterized. From the membranes studied with different amounts of TiO 2 , the membrane that provided the best properties for its use as an electrolyte was the composite one which contained the lowest amount of filler. The composite membrane with 2 weight percent of the filler absorbed larger (absorbed acid: 2.42 g acid g membrane 21 ) and leached lower (remaining acid: 0.78 g acid g membrane 21 ) amounts of acid than the other composite and standard one. Because it has the largest acid and water absorption capability, the 2% TiO 2 -PBI showed also the highest ionic conductivity (0.043 S cm 21 ), measured out of the fuel cell. Fuel cell measurements and also stability tests (. 140 h) were made for the composite membrane with 2% of the filler and the results were compared with a standard polybenzimidazole fuel cell. The TiO 2 -PBI composite membrane showed the best performance and achieved a power density of 450 mW cm 21 at 175 uC. Moreover, the composite also showed the best stability under our operation conditions. Thus, the slope of the increase in the ohmic resistance of the composite membrane was 0.009 mV cm 2 h 21 and the polarization resistance was 1.17 mV cm 2 h 21 which was two times lower than that of the standard membrane. The benefit of inorganic filler introduction when the fuel cell operates at high temperature has been highlighted.

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