New Polybenzimidazole-Based Membranes for Fuel Cells
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0972-AA08-03
New Polybenzimidazole-Based Membranes for Fuel Cells Eliana Quartarone1, Arianna Carollo1, Piercarlo Mustarelli1, Aldo Magistris1, Corrado Tomasi2, Luigi Garlaschelli3, and Pier Paolo Righetti3 1 Department of Physical Chemistry, University of Pavia, Via Taramelli 16, Pavia, 27100, Italy 2 IENI-CNR, Pavia, 27100, Italy 3 Department of Organic Chemistry, University of Pavia, Via Taramelli 12, Pavia, 27100, Italy ABSTRACT One of current trends of the PEM research is the development of new membranes working at high temperature and low humidity. Acid-doped polybenzimidazoles are particularly appealing because of high proton conductivity without humidification and promising fuel cells performances. PBI contains basic functional groups that can easily interact with strong acids, such as H3PO4, H2SO4, allowing proton migration along the anionic chains via a Grotthuss mechanism. In this work phosphoric acid-doped membranes, synthesised from benzimidazole-based monomers with increased basicity and molecular weight, are described and discussed. The influence of the monomer structure on the transport properties and on the physico-chemical interactions between acid and polymer has been investigated by means of impedance spectroscopy and 31P solid-state NMR. Test of methanol crossover and diffusion have been performed to check the membrane suitability for DMFCs.
INTRODUCTION Acid-doped polybenzimidazoles [1] are particularly appealing because of high proton conductivity with no or low humidification and promising fuel cells performances [2]. PBI, in fact, contains basic functional groups which can easily interacts with strong oxo-acids, such as H3PO4 and H2SO4. The acid partially protonates the polymer and partially is freely dispersed in the polymer backbone, so allowing proton migration via Grotthuss mechanism along the anionic chains [3-4]. In particular, poly-(2,2’-m-phenylene-5,5’-bibenzimidazole) was widely investigated for what concerns synthetic approach and casting process [2], membrane thermal stability [5], methanol crossover [6], acid doping procedure, proton transport [3, 4, 7, 8], and fuel cells performances [6, 913]. High-temperature PBI-based MEAs (Celtec®) for PEMFCs were developed by Celanese [14]. Anyway, a technological limit of these systems for fuel cells applications is related to the possible leaching of the free acid in presence of water, which may cause a drop of proton conductivity so limiting the use of PBI-based membranes at temperatures higher than 150°C. In this paper we propose a possible way to reduce the leaching of the free acid in the polybenzimidazole membranes. In particular we report on the development of new H3PO4-doped membranes, starting from PBI-based polymers with an increased number of N sites, and different basicity, interspacing and monomer length. The conductivity of the membranes have been determined before and after the complete removal of the free acid, in order to investigate the actual role of the matrix structure on the polymer protonation degree and acid reten
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