Stable and Highly Conductive Polycation-Polybenzimidazole Membrane Blends for Intermediate Temperature Polymer Electrolyte Membrane Fuel Cells

Intermediate-temperature polymer electrolyte membrane fuel cells (IT-PEMFCs), operating with phosphoric acid (H₃PO₄) doped polybenzimidazole (PBI), are severely limited by H₃PO₄ evaporation at high temperatures and poor resiliency in the presence of water. Polycations (PCs), on the other hand, provide good acid retention due to strong ion-pair interactions but have low conductivity due to lower ion-exchange capacity when compared to PBI. In this work, a class of H₃PO₄ doped PC-PBI membrane blends was prepared, and the optimal blend (50:50 ratio) exhibited remarkably high in-plane proton conductivity, near 0.3 S cm^-1 at 240 °C, while also displaying excellent thermal stability and resiliency to water vapor. Microwave dielectric spectroscopy demonstrated that incorporating PBI into the PCs raised the dielectric constant by 50-70% when compared to the PC by itself. This observation explains, in part, the high proton conductivity of the optimal membrane blend. Finally, an all-polymeric membrane electrode assembly with the new materials gave a competitive IT-PEMFC performance of 680 mW cm^-2 at 220 °C under dry H₂/O₂. Importantly, the cell was stable for up to 30 h at 220 °C and over 84 h at 180 °C. The IT-PEMFC had reasonable performance (450 mW cm^-2) with 25% carbon monoxide in the hydrogen fuel.