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.