NMR Studies of Mass Transport in High-Acid-Content Fuel Cell Membranes Based on Phosphoric Acid and Polybenzimidazole

Mass-transport studies of phosphoric acid (PA)-doped meta-polybenzimidazole (PBI) fuel cell membranes are described. In this study, the fundamental differences in transport properties between m-PBI/PA membranes prepared by conventional imbibing procedures and the polyphosphoric acid (PPA) process are explored. The membranes were characterized by proton conductivity and multinuclear (1H and 31P) magnetic resonance measurements. Both short-range and long-range dynamical processes were investigated by spin-lattice and spin-spin relaxation time measurements and by pulsed field gradient diffusion, respectively. Comparative data for pure PA and PPA are included. The high proton conductivity (0.13 S/cm at 160 deg C) of the PPA-processed membranes is correlated with rapid proton self-diffusion (3 X 10-6 cm2/s at 180 deg C). The 31P results reveal the presence of both PA and the dimeric pyrophosphoric acid and indicate strong interaction between the phosphate groups and the m-PBI matrix, with negligible anionic transport for both kinds of membranes. The higher concentration of PA in the PPA-processed membranes and differences in membrane morphology may provide an additional proton-transport mechanism involving rapid exchange between the PA and pyrophosphoric acid species.