Bipyridine-based polybenzimidazole membranes with outstanding hydrogen fuel cell performance at high temperature and non-humidifying conditions

Abstract

We describe the synthesis of bipyridine-based polybenzimidazole (Bipy-PBI) polymers with various molecular weights (MWs), and used their membranes for fuel cell (FC) applications to reveal the influence of the polymer MWs on the membrane casting, acid-doping level, thermal stability, mechanical properties, and proton conductivity. Bipy-PBI polymers with different MWs ranging from 48 to 141 kDa were successfully obtained. The mechanical properties clearly illustrated the importance of controlling the polymer MWs to fabricate flexible membranes with a sufficient mechanical strength for FC applications. Notably, a 36% increase in the membrane conductivity was obtained for Bipy-PBI compared to PBI of similar MW, thanks to the Bipy-PBI polymer structure which provided additional nitrogen atoms for the potential loading of acid molecules. The conductivity value of the highest MW Bipy-PBI at 120 °C provided a 0.037 S/cm with a146% increase in conductivity compared to the low MW Bipy-PBI. Notably, a 32% increase in the power density was recorded when using the highest MW-based Bipy-PBI membrane electrode assembly. This ratio is remarkable for the membrane effect on the FC performance at high operating temperatures and non-humidifying conditions.