Abstract
Phosphoric acid doped polybenzimidazole (PBI) membranes are generally prepared by immersing the PBI membrane in concentrated phosphoric acid, with the uptake of phosphoric acid being a function of temperature and time. Alternatively, in the so-called sol-gel method, monomer precursors to PBI are polymerised in a polyphosphoric acid solvent, and phosphoric acid formed within cast membranes from the PPA by hydrolysis. The phosphoric acid content of H3PO4-PBI membranes prepared by the latter method can be significantly higher than that which can be reached by immersion doping, and the conductivity a factor of 10 higher. We have developed a route to stabilise a polybenzimidazole gel membrane formed in polyphosphoric acid by simultaneous cross-linking. With increasing degrees of cross-linking the mechanical properties of the high acid content PBI membranes so prepared are significantly improved, while the phosphoric acid content and the membrane conductivity are not greatly affected. H 3PO4-PBI membranes are generally integrated into an assembly using gas diffusion electrodes. In the present work, the cross-linked PBI membranes have been used as substrates for catalyst coated membrane (CCM) preparation using a Hispec Pt/C catalyst transferred by decal to the membrane surfaces. The final membrane electrode assemblies have been characterized for their high temperature performance and durability, and the results compared to those obtained using an MEA prepared by spraying catalyst ink containing PBI on both sides of a PBI membrane, which was then doped by soaking in H3PO4 (1). This poster will describe the properties of the cross-linked high phosphoric acid content PBI membranes, and relate them to the degree of cross-linking and the acid content, the methodology for preparation of H3PO4-PBI-based CCMs, and the fuel cell performance of the final MEAs. We acknowledge the financial contribution under QuasiDry, contract 256821 of the European 7th Framework Programme.[no pdf]