KOH-doped polybenzimidazole for alkaline direct glycerol fuel cells

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 4 mol L-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 4 mol L-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 FBI at alkali concentrations of 4 mol L-1 or higher, though reveal some membrane degradation processes. The KOH-doped PBI membranes present thermal stabilities up to 573 K, a coefficient of permeability to glycerol of around 10(-7) cm(2) s(-1), and OH- conductivities above 0.01 S cm(-1) at temperatures between 303 and 348 K. The combination of these parameters configures a suitable alkaline membrane capable of producing a maximum power density of 34.2 mW cm(-2) for 1 mol L-1 glycerol and 4 mol L-1 KOH at 348 K. (C) 2015 Elsevier B.V. All rights reserved.