A method was developed to deagglomerate commercially available multi-walled carbon nanotube (MWCNT) bundles while maintaining the carbon nanotube aspect ratio. The process utilizes the rapid expansion of a supercritical carbon dioxide/MWCNT mixture to separate large primary carbon nanotube agglomerates. High levels of deagglomeration of Baytubes Ò C 150 P and Nanocyl TM NC-7000 MWCNT bundles were observed on the macroscale and nanoscale, resulting in 30-fold and 50-fold decreases in bulk density, respectively, with median agglomerate sizes \8 lm in diameter. These results were obtained while retaining the aspect ratio of the as-received nanomaterial, irrespective of the MWCNT agglomerate morphology. It was found that a temperature and pressure of 40°C and 7.86 MP resulted in maximum deagglomeration without damage to the MWCNTs. Thermodynamic principles were applied to describe the effect of processing variables on the efficiency of the deagglomeration. These results suggest that combining this process with a composite processing step, such as melt compounding, will result in nanocomposites with enhanced electrical properties.
High temperature direct methanol fuel cells (DMFCs) using polybenzimidazole (PBI) membranes could improve the energy density of portable power sources. This study examines the polarization of vapor phase PBI DMFCs constructed with commercial membranes manufactured by a sol-gel method. The polarization of the high temperature DMFCs is compared to similar low temperature membrane electrode assemblies (MEAs) using Nafion® membranes. The results showed that the cathode of the PBI DMFC had higher kinetic losses that are likely due to phosphate poisoning of the Pt electrocatalyst. At the tested conditions, the membrane conductivity of the PBI MEAs was comparable to the Nafion® MEA even with no humidification. Higher cell temperatures significantly improved PBI DMFC performance for Pt electrocatalyst electrodes. In full cell tests, the PBI DMFC MEAs had higher performance than Nafion® MEAs with similar catalyst loadings. The Pt and PtRu catalysts were tested for methanol oxidation and oxygen reduction activity by a rotating disk electrode (RDE) under 0.5 M H2SO4 and 0.5 M H3PO4. The combination of the polarization and RDE results for the PBI and Nafion® DMFCs suggest that Pt is a more active electrocatalyst for methanol oxidation in PBI than in Nafion®.
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