No abstract

Acid-doped polybenzimidazole (PBI) membranes are considered as one of the most attractive alternatives to Nafion in polymer fuel cells (PEMFCs). [1][2][3] At present, however, their use at temperatures below $160 8C is impeded by the leaching of the free phosphoric acid from the membrane in presence of liquid water, which causes a drop in conductivity of many orders of magnitude during fuel cell operation. To overcome this problem, we investigated the suitability of silica derivatized with imidazole-based functionalities as fillers able to improve the acid retention capability of the membrane. We show that even small amounts ($10 wt %) of filler increase the permanent proton conductivity of the PBI composite membranes by a factor >10 3 in comparison with that of unfilled PBI. The preparation of PBI composite membranes with basic functionalities is a promising way to make possible their use in PEMFCs operating around 120 8C, that is, the temperature required for automotive applications. During the last ten years, polybenzimidazole-based separators have been widely investigated for what concerns polymer synthesis and casting process, [4] membrane thermal stability, [5]

Polybenzimidazole (PBI) activated with H3PO4 is one of the membranes of choice to replace Nafion® in PEMFCs in order to allow their use above 100 °C. The limits of PBI in terms of acid leaching and low conductivity below 160 °C can be overcome by a proper monomer tailoring, and by the addition of new fillers. Here, we report on new pyridine‐based PBI membranes with: (i) imidazole‐silica (SiO2‐Im) and (ii) mesostructured silica (SBA‐15) fillers. Both the thermal stability and the permanent conductivity are improved by adding 5 wt.‐% of filler, but SiO2‐Im gives the best results. Permanent conductivity values higher than 10–3 S cm–1 are obtained at 120 °C and 50% R.H. Vibrational spectroscopies (FT‐IR and Raman) are used to investigate the relationships among the polymer, the filler and the activating H3PO4 acid.

In spite of the promising properties as polymer electrolytes for fuel cells, H3PO4‐doped PBI systems need to be further optimised for what concerns the oxidative stability, the mechanical performances and the leaching of acid at temperatures lower than 150 °C. To this aim, the preparation of composite and nanocomposite membranes is of interest. Here, we investigate on the actual role of the filler in determining the physical and chemical properties of the membranes. Three types of SiO2 are compared, namely HiSil™ T700, mesoporous SBA‐15 and an imidazole‐functionalised silica, which differ for morphology, microstructure and chemical nature. Particular attention is devoted to the ability of the filler to reduce the drop of conductivity occurring in consequence of the acid leaching. In the as‐doped systems, the proton transport is remarkably improved by the presence of fillers containing active sites for the proton hopping. In the case of washed membranes the acid retention capability and the permanent proton conductivity are increased by fillers with higher surface area or with higher basicity. In the lights of these results, the imidazole‐derivatised silica seems to be an optimal filler for the preparation of PBI membranes for PEMFCs operating at 120 °C.