Synergistic Dual‐Photoreaction Crosslinking Enables High Phosphoric Acid Retention in High‐Temperature Proton Exchange Membranes

ABSTRACT Phosphoric acid (PA)‐doped high‐temperature proton exchange membranes (HT‐PEMs) enable anhydrous proton conduction but remain limited by PA migration, excessive swelling, and conductivity decay. Here, anthracenylmethyl groups are introduced into PEEK side chains through partial quaternization of tertiary amine groups. The resulting polymer retains pre‐existing benzophenone photoreactive units while incorporating anthracene sites capable of UV‐induced [4 + 4] photocycloaddition. The newly formed quaternary ammonium groups enhance polymer–PA interactions. PA is introduced before UV irradiation, allowing a proton‐conducting environment to develop within the membrane. Subsequent anthracene [4 + 4] photocycloaddition and benzophenone type II photochemistry cooperatively stabilize the PA‐doped structure while generating additional subnanometer free volume and a compact crosslinked surface layer. Among the investigated membranes, PEEK‐M70‐50%‐PA‐UV exhibits the most favorable performance, retaining nearly 98% of its PA and 91% of its initial conductivity after 120 h at 80°C and 40% RH. It achieves a proton conductivity of 240 mS cm − 1 and a peak power density of 1150 mW cm − 2 at 160°C, while sustaining non‐humidified fuel‐cell operation for 210 h. This work establishes synergistic crosslinking through anthracene [4 + 4] photocycloaddition and benzophenone type II photochemistry as an effective strategy for limiting PA loss while preserving rapid proton transport in HT‐PEMs.