Browsing by Author "Chung, S.H."

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    NMR Studies Of Mass Transport In High Acid Content Fuel Cell Membranes Based On PBI/Phosphoric Acid
    (2004) Chung, S.H.; Durantino, L.; Jayakody, J.R.P.; Zhang, H.; Xiao, L.; Benicewicz, B.; Greenbaum, S.G.
    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.
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    NMR Studies of Mass Transport in High-Acid-Content Fuel Cell Membranes Based on Phosphoric Acid and Polybenzimidazole
    (Journal of Electrochemical Society, 2007) Jayakody, J.R.P.; Chung, S.H.; Durantino, L.; Zhang, H.; Xiao, L.; Benicewicz, B.; Greenbaum, S.G.
    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 ?eld gradient diffusion, respectively. Comparative data for pure PA and PPA are included. The high proton conductivity(0.13 S/cm at 160�C) of the PPA-processed membranes is correlated with rapid proton self-diffusion (3 x 10?6 cm2/s at 180�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.