An H3PO4-doped polybenzimidazole/Sn0.95Al0.05P2O7 composite membrane for high-temperature proton exchange membrane fuel cells

A polybenzimidazole (PBI)/Sn_0.95Al_0.05P₂O₇ (SAPO) composite membrane was synthesized by an in situ reaction of SnO₂ and Al(OH)₃-mixed powders with an H₃PO₄ solution in a PBI membrane. The formation of a single phase of SAPO in the PBI membrane was completed at a temperature of 250 °C. Thermogravimetric analysis showed that the PBI membrane was not subject to a serious damage by the presence of SAPO until 500 °C. Scanning electron microscopy revealed that SAPO particles with a diameter of approximately 300 nm were homogeneously dispersed and separated from each other in the PBI matrix. Proton magic angle spinning nuclear magnetic resonance spectra confirmed the presence of new protons originating from the SAPO particles in the composite membrane. As a consequence of the interaction of protons in the SAPO with those in the free H₃PO₄, the H₃PO₄-doped PBI/SAPO composite membrane exhibited conductivities several times higher than those of an H₃PO₄-doped PBI membrane at room temperature to 300 °C, which could contribute to the improved performance of H₂/O₂ fuel cells.