NMR investigations of acid doped polybenzimidazole, an electrolyte membrane for fuel cells

Fuel cells (FCs) are devices that convert the chemical energy of their reagents into electrical energy without resorting to a thermal Carnot cycle. Polymer electrolyte membrane FCs (PEMFCs) have raised considerable interest for their high energy conversion efficiency, high power density, ease of assembly, silent operation, and good environmental compatibility. Fuel cells consist mainly of an anode, a cathode and a membrane acting as electrolyte and which allows charges to selectively move between anode and cathode. Acid doped polybenzimidazole (PBI) polymers have become a very promising system for use as membranes in high temperature PEMFCs. The conductivity in FCs is well known to be directly related to the mobility of the ions through the membrane, which might involve a multi-step proton hopping process (Grotthuss mechanism) or the bulk mobility of a proton carrier. We report investigations of poly(2,5-benzimidazole) (AB-PBI) doped with phosphoric acid (PA) with different methods of NMR [1]. We have characterized the molecular interactions between the polymer matrix and the acid dopant. Heteronuclear correlation experiments were used to study hydrogen bonding characteristics. We have further studied the dynamics and order in the system at different doping levels and temperatures using 2H-NMR and 1H-Double-quantum spectroscopy. This information is of fundamental importance in order to elucidate the role of phosphoric acid in the conductivity mechanism of polybenzimidazole [2, 3]. [1] F. Conti, S. Willbold, C. Korte, W. Lehnert, D. Stolten, S. Mammi, Chem. Commun. 2012, submitted. [2] C. E. Hughes, S. Haufe, B. Angerstein, R. Kalim, U. Mähr, A. Reiche, M. Baldus, J. Phys. Chem. B., 2004, 108, 13626-13631. [3] S. Suarez, S. Greenbaum, The Chemical Record, 2010, 10, 377-393.