KOH-doped polybenzimidazole for alkaline direct glycerol fuel cells

Abstract

This study addresses the physico-chemical characterization of KOH-doped polybenzimidazole (PBI, poly[2,2-(m-phenylene)-5,5-bibenzimidazole]) membranes for application in alkaline direct glycerol fuel cells. The KOH-doped membranes present a large affinity for water and gradually replace the acid hydrogens in the imidazole rings with K. Only at KOH concentrations above 4molL−1 are “free” KOH molecules present in the membrane structure, along with a small fraction of glycerol molecules. These behaviours impact on the membrane dimensions. At KOH concentrations below 4molL−1, the membrane dimensions become enlarged. However, at higher concentrations, the cross-sectional area shrinks, whereas the thickness expands significantly. This result is confirmed by x-ray diffraction, where an enlargement in the inter-chain distances is evidenced. The infrared spectra support the interaction of KOH and PBI at alkali concentrations of 4molL−1 or higher, though reveal some membrane degradation processes. The KOH-doped PBI membranes present thermal stabilities up to 573K, a coefficient of permeability to glycerol of around 10−7cm2 s−1, and OH− conductivities above 0.01Scm−1 at temperatures between 303 and 348K. The combination of these parameters configures a suitable alkaline membrane capable of producing a maximum power density of 34.2mWcm−2 for 1molL−1 glycerol and 4molL−1 KOH at 348 K.