Polybenzimidazole/Acid Complexes as High-Temperature Membranes

This chapter reviews the progress towards applying acid-doped polybenzimidazoles (PBIs) as polymer electrolyte membrane (PEM) fuel cell membranes over approximately the last ten years. The major focus of the first part of the chapter is on three main systems: (1) the well-developed meta-PBI (poly(2,2'-m-phenylene-5,5'-bibenzimidazole)); (2) the various derivatives and filled systems based on meta-PBI; and (3) poly(2,5-benzimidazole) (AB-PBI). The polymer membrane properties, such as thermal and chemical stability, ionic conductivity; mechanical properties, and ability to be manufactured into a membrane and electrode assembly (MEA), are discussed in detail. Preliminary fuel cell performance is reported for a number of PBI chemistries. The second section of the chapter highlights recent work on developing a novel process to produce phosphoric acid (PA)-doped PBI membranes for use in high-temperature PEM-FCs. This novel sol-gel process, termed the polyphosphoric acid (PPA) process, allows production of a gel membrane that exhibits properties not observed with the "traditionally" prepared PBIs, such as improved ionic conductivity, mechanical properties, fuel cell performance, and long-term stability. The final section of the chapter focuses on the possible degradation modes of the commercially available products from BASF Fuel Cells.