Phosphoric acid-doped polybenzimidazole (PA/PBI) is one of the most promising electrolyte materials for high-temperature proton exchange membrane fuel cells (HT-PEMFCs). However, it suffers from severe phosphoric acid leakage and excessive membrane swelling under high doping levels, which critically impair long-term stability. To address this challenge, this work proposes a synergistic strategy combining “physical confinement” and “chemical anchoring”. An epoxy-functionalized polyhedral oligomeric silsesquioxane (epoxy-POSS) was employed as a rigid crosslinking node to construct a three-dimensional crosslinked network with hydroxyl-containing polybenzimidazole (OHPBI). Meanwhile, a phosphonic acid-containing small molecule, 4-hydroxyphenyl dihydrogen phosphate (HDP), was introduced to achieve stable phosphoric acid anchoring and to build efficient proton transport pathways. The effects of the crosslinked network and phosphonic acid groups on membrane structure and performance were systematically investigated. The results show that the EP10 membrane achieves a phosphoric acid uptake of 261% while exhibiting a swelling ratio as low as 112%, demonstrating an excellent balance between high acid loading and dimensional stability compared to pristine OHPBI. Under anhydrous conditions at 180 °C, the EP10 membrane exhibits a proton conductivity of 101.9 mS cm^-1 with an activation energy as low as 13.3 kJ mol^-1. The single-cell peak power density reaches 1.013 W cm^-2, approximately twice that of pristine OHPBI. Cold-start thermal cycling and accelerated stress tests further confirm the excellent operational stability of the membrane under dynamic conditions. These results demonstrate that the dual synergistic strategy based on epoxy-POSS crosslinking and phosphonic acid immobilization provides a viable route toward high-performance and durable high-temperature proton exchange membranes.

Key words: High-temperature proton exchange membrane, Polybenzimidazole, Polyhedral oligomeric silsesquioxane (POSS), Phosphonic acid immobilization, Crosslinking