Pore Size Engineering in Covalent Organic Frameworks for High-Performance Anion Exchange Membranes

Abstract

Anion exchange membranes (AEMs) play a crucial role as separators in electrolytic cells, facilitating gas isolation and ion exchange during hydrogen production via water electrolysis and in fuel cells. To address longstanding challenges related to swelling stability and to enhance ion conductivity, recent studies have shown that AEMs derived from covalent organic frameworks (COFs) utilize their permanently ordered pore structures as effective platforms. These COF-based membranes demonstrate exceptional resistance to swelling and improved ionic conductivity. In this study, we investigate the influence of pore dimensions within COF-based AEM frameworks on membrane performance. We synthesized COF membranes with varying framework sizes, each incorporating identical flexible side chains of 1-methylpiperidine. Among these, COF-Pip-S2 achieved the highest reported ion exchange capacity (IEC) to date, reaching 4.0 mmol g⁻¹. Meanwhile, COF-Pip-2 demonstrated a notable hydroxide ion (OH⁻) conductivity of 207.1 mS cm⁻¹ at 80°C. Our findings indicate that the vehicle mechanism governing ion transport is primarily influenced by the grafted side chain groups. Furthermore, this research highlights the importance of exploring universal principles to achieve an optimal spatial balance between hydrophilic side chains and hydrophobic frameworks, thereby improving the synergy between the two conduction mechanisms and enhancing overall conductivity.