New preparation methods for composite membranes for medium temperature fuel cells based on precursor solutions of insoluble inorganic compounds

Current PEMFCs, using perfluorinated membranes, can only operate at temperatures no higher than 70-80 degrees C, since their performance is dramatically reduced at higher temperatures. Encouraging results, for avoiding the reduction in performance, can be obtained by filling Nafion (R) membranes with inorganic nano-particles (composite membranes) and/ or by adding inorganic nano-particles at the membrane/ electrode interfaces. However, the efficient insertion of nano-particles, other than metal oxides, inside a polymeric matrix is often very complicated or not possible. Therefore, it was of interest to find simpler methods for the insertion. In this paper, the preparation of organic precursor solutions of the insoluble compound Zr(O3P-OH)(O3P-C6H4SO3H), a proton conductor exhibiting very high conductivity (10(-1) S cm(-1) at 100 degrees C and 90 %RH) is reported. It was found that these precursor solutions are more stable the higher the Kb of the solvent and the lower the temperature. Precursor solutions, stable for many days at room temperature, were found by using proton acceptor solvents, such as alkanols, N,N-dimethyl formamide (DMF), and N-methyl pyrrolidone (NMP). The particularity of these solutions is that insoluble phosphonate particles are formed when the solvent is evaporated. They are therefore very suitable for filling porous membranes, for the preparation of composite proton conducting membranes, and for the preparation of composite electrodes. Some examples of preparation and use are reported and discussed.