An effective strategy to enhance dimensional-mechanical stability of phosphoric acid doped polybenzimidazole membranes by introducing in situ grown covalent organic frameworks

Poor dimensional-mechanical stability and severe acid leakage have been the crucial issues restraining the practical applications of phosphoric acid-doped polybenzimidazole (PA−PBI) as high-temperature proton exchange membranes (HT-PEMs). In view of their robust, ordered, and tunable porous structures, chemically stable covalent organic frameworks (COFs) show great promise to enhance the performance of PA-doped PBI membrane. In this work, the in situ synthesized COF sheets are introduced to the PBIs for the first time and form a unique composite structure. The composite structure comprising soft PBIs matrix and rigid COF sheets can obviously improve dimensional-mechanical stability of the membranes under high acid doping level. Importantly, the rigid COF sheets help to construct proton transport channel in the membrane, resulting in high proton conductivity and excellent fuel cell performance. Among them, the 40%–COF–OPBI membrane shows the best overall performance, such as excellent proton conductivity (177.7 mS cm−1), desirable mechanical tensile strength (12.1 MPa), and high power density (774.7 mW cm−2) at 160 °C without humidification in H2/O2 fuel cell tests. These results suggest that introducing COFs to PBI is an effective strategy for fabricating high-performance proton exchange membranes for fuel cell applications.