Study on the influence of anion receptor and porous polyimide on polybenzimidazole membranes for high temperature polymer fuel cells

Abstract

As high temperature polymer membranes, $H_3PO_4$ (phosphoric acid, PA)-doped poly(2,5-benzimidazole) (ABPBI) membranes have been considered as one of promising materials. However, these systems still have problems that PA can be leached out from the electrolyte by liquid water on the cathode side and the mechanical strength of the membranes is reduced significantly as the acid doping level increases, especially at high temperatures. In this study, anion receptor and porous polyimide have been applied as alternative route to overcome these difficulties associated with the long-term stability of the membrane in the fuel cell. To solve acid leakage problem, anion receptor was introduced into PA-doped ABPBI systems. Boron phosphate $(BPO_4)$ was chosen as anion receptor. Lewis acid-base interaction between $BPO_4$ anion receptors and $PO_4^{3-}$, $HPO_4^{2-}$, $H_2PO_4^-$ anions was verified through FT-IR and thermal analysis. This interaction could lead to prevent PA from leaching out by liquid water. In addition, proton conductivity of PA-doped ABPBI membrane including 10wt.-% $BPO_4$ exhibits very high values about $4.8 \times 10^{-1}$ S/cm at 150℃. To enhance mechanical properties, PA-doped ABPBI membranes were reinforced by porous polyimide, which prepared by lamination method. Consequently, the tensile mechanical behavior of PA-doped ABPBI-laminated porous PI membranes has been highly enhanced about 58MPa. The maximum power density at 150℃ was $243 mW/cm^2$ at 700 $mA/cm^2$.