De Novo Design of Aminopropyl Quaternary Ammonium-Functionalized Covalent Organic Frameworks for Enhanced Polybenzimidazole Anion Exchange Membranes

Quaternary ammonium functionalized covalent organic frameworks (COFs) have great potential to enhance hydroxide transport owing to crystalline ordered 1D nanochannels, however, suffer from limited quaternary ammonium functional monomers and poor membrane-forming ability. In this work, a novel aminopropyl quaternary ammonium-functionalized COF (DCOF) is designed and synthesized via a bottom-up strategy. The self-supporting DCOF membrane exhibits high crystallinity with a dense and orderly arrangement of quaternary ammonium groups (IEC, 2.07 mmol g-1), achieving a high hydroxide conductivity of 172.5 mS cm-1 and an extremely low water swelling of 5.3% at 80 degrees C. The exfoliated DCOF colloidal suspension is further incorporated into quaternary ammonium di-cation grafted polybenzimidazoles (DPBI) matrix. Molecular simulations reveal strong electrostatic and van der Waals interfacial interactions between DCOF and DPBI, which enable a high doping content of 20 wt.% and interconnected ionic channels through the surface and nanochannels of the DCOF. The DCOF/DPBI-20% membrane exhibits a tensile strength of 29.7 MPa, a hydroxide conductivity of 135.3 mS cm-1, and a low swelling ratio of 37.2% at 80 degrees C. A H2/O2 single cell assembled with the membrane reaches a peak power density of 323 mW cm-2, surpassing most recently reported COF-based membranes.