Acid doped polybenzimidazole as a polymer electrolyte for methanol fuel cells

In order to take advantage of the high volumetric energy density of liquid hydrocarbon fuels there has been considerable interest in developing PEM fuel cells based on reformed methanol or direct oxidation of methanol. Recently, direct and indirect methanol fuel cells have been constructed and tested using phosphoric acid doped polybenzimidazole as a polymer electrolyte. Polybenzimidazole (PBI) is a relatively low cost ($70-100/lb), commercially available polymer known to have excellent oxidative and thermal stability. Films of PBI that have been doped with phosphoric or sulfuric acid form a single phase polymer electrolyte with the acid dissolved in the polymer. For direct methanol fuel cells, the doped polybenzimidazole electrolyte membrane provides suitable proton conductivity (>0.05 S.cm(-1)) and low methanol crossover rates (<10 mA.cm(-2)) at temperatures up to 200 degrees C. At temperatures above 150 degrees C, fuel cells based on PBI are tolerant to 1% CO in H-2 with low catalyst loadings(anode potentials of approximate to 10 mV RHE at 500 mA.cm(-2) with 2.5 mgPt.cm(-2)) and can be thermally integrated with a reformer for indirect methanol operation.