Proton-conducting membranes based on benzimidazole polymers for high-temperature PEM fuel cells. A chemical quest

The development of high-temperature PEM fuel cells (working at 150-200 °C) is pursued worldwide in order to solve some of the problems of current cells based on Nafion® (CO tolerance, improved kinetics, water management, etc .). Polybenzimidazole membranes nanoimpregnated with phosphoric acid have been studied as electrolytes in PEMFCs for more than a decade. Commercially available polybenzimidazole (PBI) has been the most extensively studied and used for this application in membranes doped with all sorts of strong inorganic acids. In addition to this well-known polymer we also review here studies on ABPBI and other polybenzimidazole type membranes. More recently, several copolymers and related derivatives have attracted many researchers' attention, adding variety to the field. Furthermore, besides phosphoric acid, many other strong inorganic acids, as well as alkaline electrolytes have been used to impregnate benzimidazole membranes and are analyzed here. Finally, we also review different hybrid materials based on polybenzimidazoles and several inorganic proton conductors such as heteropoly acids, as well as sulfonated derivatives of the polymers, all of which contribute to a quickly-developing field with many blooming results and useful potential which are the subject of this critical review (317 references). High-performance membranes with high H + conductivity at 160-200 ºC are key for the development of commercial PEM fuel cells. Polybenzimidazoles, their derivatives, copolymers and hybrids have grown as a very promising family with a wealth of combinations still in the making.