12-Silicotungstic Acid Doped Phosphoric Acid Imbibed Polybenzimidazole for Enhanced Protonic Conductivity for High Temperature Fuel Cell Applications

Abstract

12-Silicotungstic acid, a heteropoly acid (HPA) - was incorporated into phosphoric acid (PA) doped polybenzimidazole (PBI) membrane that exhibited strong mechanical stability, excellent proton conductivity, and can be used for high temperature proton exchange membrane fuel cells (PEMFCs). At 160°C, an electrochemical impedance spectroscopy (EIS) fitting of the fuel cells data showed the membrane electrode assemblies (MEAs) made of PBI/20%HPA/PA had three times lower ohmic resistance (0.057 ± 0.002 Ohm*cm2) as compared to the control reference of PBI/PA (0.160 Ohm*cm2). In addition, the ohmic resistance of the composite MEA remained unchanged while the charge transfer resistance decreased after 313 hours conditioning. Fourier transform infrared spectroscopy (FTIR), magic angle spinning - nuclear magnetic resonance (MAS-NMR), and thermogravimetric analysis (TGA) showed 12-silicotungstic acid inhibits water from escaping the membrane at elevated temperatures and adds more acid sites, providing additional paths for proton transport. Scanning electron microscope (SEM), transmission electron microscopy (TEM), and small angle X-ray scattering (SAXS) were used to confirm the structure and morphology of PBI/20%HPA/PA membrane prior making the MEAs. Fuel cell polarization curves indicated MEAs made of PBI/20%HPA/PA produced shallower IR slopes compared to MEAs made of PBI/PA, especially, when the hydrogen fuel was humidified.