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

Area of Science:

• Electrochemistry
• Materials Science
• Energy Storage

Background:

• Zinc-bromine redox flow batteries (Zn/Br$_{2}$ RFBs) are promising for large-scale energy storage due to safety and efficiency.
• Membrane design is crucial for Zn/Br$_{2}$ RFBs, balancing ion transport (Zn$^{2+}$, Br$^{-}$) with suppressing undesirable side reactions like polybromide (Br$_{n}$$^{-}$) shuttling.
• Existing membranes face a trade-off between high ionic conductivity and effective blocking of Br$_{n}$$^{-}$ migration, limiting battery performance.

Purpose of the Study:

• To develop a novel composite membrane for Zn/Br$_{2}$ RFBs that overcomes the ionic conductivity vs. Br$_{n}$$^{-}$ shuttle trade-off.
• To enhance anion conduction while simultaneously suppressing Br$_{n}$$^{-}$ migration through strategic material incorporation.
• To evaluate the performance of the new membrane in Zn/Br$_{2}$ RFBs.

Main Methods:

• Incorporation of a cationic covalent organic framework (COF) derived from ethidium bromide and triformylphloroglucinol (EB-COF) into a Nafion (NF) matrix.
• Characterization of the composite membrane's structure, water network formation, and functional groups (-CHO, -NH$_{2}$, N$^{+}$).
• Assembly and testing of Zn/Br$_{2}$ RFBs using the NF/EB-COF composite membrane, specifically NF/EB-COF(0.3).

Main Results:

• The EB-COF within the Nafion matrix facilitated abundant water networks, significantly enhancing ion conductivity.
• The quaternary amine groups (N$^{+}$) in EB-COF effectively absorbed Br$_{2}$ and mitigated the Br$_{n}$$^{-}$ shuttle effect.
• The NF/EB-COF(0.3) composite membrane demonstrated superior performance, enabling Zn/Br$_{2}$ RFBs to achieve 89.1% energy efficiency at 40 mA cm$^{-2}$.

Conclusions:

• The proposed membrane design strategy effectively addresses the critical trade-off in Zn/Br$_{2}$ RFB membranes.
• The NF/EB-COF composite membrane offers enhanced ion transport and suppressed polybromide crossover, leading to improved battery performance.
• This work presents a viable pathway for developing advanced membranes for efficient and safe large-scale energy storage systems.