Achieving over 1,000 mW cm−2 Power Density Based on Locally High‐Density Cross‐Linked Polybenzimidazole Membrane Containing Pillar[5]arene Bearing Multiple Alkyl Bromide as a Cross‐Linker

Abstract Cross‐linking is widely accepted as an effective method to improve the mechanical strength and durability of phosphoric acid (PA) doped polybenzimidazole (PBI) membranes. However, the cross‐linked membranes generally exhibit compromised overall performance since their compact network structures decrease the free volumes of membranes, leading to poor proton conductivity. In this study, a locally high‐density cross‐linked polybenzimidazole network based on pillar[5]arene bearing multiple alkyl bromide is constructed for the first time to achieve high proton conductivity, desired mechanical properties, and excellent fuel cell performance. The pillar[5]arene‐cross‐linked network considerably enhances the mechanical strength of membrane (14.6 MPa), particularly with high PA uptake, and provides loose PBI chain segment packing to retain PA (315.9%). Surprisingly, the pillar[5]arene‐cross‐linked PBI membrane displays a high‐power density of 1,084.1 mW cm −2 at 180 °C and 0.6 mg cm −2 Pt loading without backpressure and humidification, that is the highest value reported in cross‐linked membranes for high‐temperature proton exchange membrane fuel cells.