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 (H-1 and P-31) 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 degrees C) of the PPA-processed membranes is correlated with rapid proton self-diffusion (3 x 10(- 6) cm(2)/s at 180 degrees C). The P-31 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. (c) 2006 The Electrochemical Society. All rights reserved.