Improvement of Performance and Lifetime of Polybenzimidazole Membranes in High Temperature Fuel Cells by Incorporation of Muscovite

Abstract

Polybenzimidazole (PBI) is a basic aromatic heterocyclic polymer with excellent thermal and chemical stability. The phosphoric acid (PA) doped PBI and its derivative membranes have high proton conductivity, low gas permeability, good fuel impurity tolerance, and excellent oxidative and thermal stability at temperatures up to 200 °C [1], hence, PA doped PBI is a promising candidate for high-temperature proton exchange membrane fuel cells. However, PA doped PBI membranes also has some limitations, including the dehydration of phosphoric acid above 140 °C, PA leaching out of the membranes, and deterioration of membrane mechanical strength especially at high acid doping level.The incorporation of clay minerals into membranes attract a great interest due to their hygroscopicity, high surface area and cheap cost [2]. The hygroscopic properties of clay minerals can increase the humidity inside the membranes, interact with PA molecules and maintain a strong proton conductivity. Here we present work on the incorporation of a cheap muscovite into the PBI membrane. As the muscovite is highly hygroscopic and has hydroxyl groups on the surface, an optimal cell performance was observed when the loading of muscovite is 1 wt.%. The PBI membrane with 1 wt.% muscovite showed the membrane resistance of 189 mohm cm2 and peak power density of 612 mW cm-2 at 150 °C, 26% higher than the pure PBI membrane with the membrane resistance of 235 mohm cm2 and the power density of 485 mW cm-2. Furthermore, the durability of inorganic-organic composite membranes was tested by an accelerated stress test (AST) operating the membrane electrode assembly between 0.6 A cm−2 and 1.0 A cm−2 [3]. After running around 50 h AST, the composite PBI membrane with 1 wt.% muscovite showed much lower degradation (9.3% at 0.6 A cm−2 and 12.8% at 1.0 A cm−2) than the pure PBI membrane (11.2% at 0.6 A cm−2 and 22.6% at 1.0 A cm−2). The improved durability of the composite membranes might be explained by the muscovite forming hydrogen bonding with PA molecules and polymer chains, which alleviates the degradation of polymer chains and PA leaching out of the membranes. The PA distribution and retention properties of the composite membranes were also characterized by energy dispersive X-ray spectroscopy and electron probe micro-analysis.