Abstract:
Acid doped polybenzimidazole (PBI) has emerged as a
promising candidate for a low-cost and high performance
fuel cell membrane material. It has been shown that this
polymer electrolyte membrane exhibits high ionic
conductivity at temperatures up to 200oC.
However,additional progress is still needed for the large-scale application of PBI in fuel cells. Furthermore, the conventional method to prepare acid doped PBI membranes involves a multi-step process while the mechanical properties of the resulting membranes are largely limited by the low molecular weight of PBI used in previous studies. A novel process, previously reported and termed as the PPA process, has been developed to prepare pyridine-based PBI (PPBI) membranes loaded
with high levels of phosphoric acid by direct casting of
the PPA polymerization solution without isolation or
redissolution of the polymers, followed by a sol-gel
transition induced by the hydrolysis of PPA intophosphoric acid.
In an attempt to understand the ion dynamics in these
membranes, two samples of this material prepared in a different manner have been examined by using nuclear
magnetic resonance (NMR) techniques over a range of
temperatures from 290 to 383 K. The first sample BB1 was prepared by soaking the PBI films in phosphoric acid solutions whereas the second sample BB2 was produced by the new sol-gel process which allows for greater levels of phosphoric acid loading. Using experimental techniques described elsewhere3, 1H (I=�) and 31P (I=�)
NMR linewidths, spin-lattice relaxation times T1, and
self-diffusion coefficients D for these materials are
reported. To obtain consistent and reproducible data, the
samples were dried at 423K for 90 minutes. Significant
differences in the diffusion coefficients and relaxations
times before and after drying are noted. For all samples a
single 31P peak centered close to the reference signal from
85% H3P04 was observed. There were no spectral indications of condensed phosphates. In BB1 the line widths and relaxation times show weak temperature dependence in contrast to the data for BB2 where there were indications of motional narrowing and a welldefined T1 minimum. The diffusion data show that protons diffuse faster than the phosphorus carrying species, which means that the inter-phosphate proton transfer is important in these materials. Proton NMR diffusion and T1 data for BB2 are shown at right.