Cationic Covalent Organic Framework-Based Membranes for High-Performance Zn/Br₂ Redox Flow Batteries

Zn/Br₂ redox flow batteries (Zn/Br₂ RFBs) have attracted significant attention for large-scale energy storage applications due to their high safety and efficiency. The membrane is a critical component of Zn/Br₂ RFBs, directly influencing their efficiency and power density. However, designing suitable membranes is challenging due to an intrinsic trade-off between achieving fast bi-ionic species transport (Zn²⁺ and Br^-). Increasing anion transport pathways improves Br^- conductivity but simultaneously accelerates Br_n ^- shuttling and self-discharge, while strong Br_n ^- blocking typically restricts anion mobility and causes ionic imbalance. To address this trade-off, a membrane design strategy is proposed that enhances anion conduction while simultaneously suppressing Br_n ^- migration by incorporating a cationic COF construced from ethidium bromide and triformylphloroglucinol (EB-COF) into a Nafion (NF) matrix. The EB-COF exhibits -CHO and -NH₂ functional groups on its surface. The interaction between water molecules and these functional groups forms continuous, abundant water networks within the composite membrane, significantly enhancing its ion conductivity. In addition, the abundant quaternary amine groups (N⁺) of the EB-COF significantly reduce the polybromide (Br_n ^-) shuttle by absorbing Br₂ while forming Br_n ^-. In particular, NF/EB-COF(0.3), in which 0.3 wt.% EB-COF is introduced into the NF polymer matrix, exhibits the most effective characteristics and has excellent performance for Zn/Br₂ RFBs. Consequently, the Zn/Br₂ RFBs assembled with the NF/EB-COF composite membrane demonstrate outstanding performance, achieving an energy efficiency of 89.1% at a current density of 40 mA cm^{- 2}.