Polarization and Electrocatalyst Selection for Polybenzimidazole Direct Methanol Fuel Cells

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^®.