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

Why It Matters

The breakthrough enables higher efficiency and lower self‑discharge in Zn/Br₂ flow batteries, accelerating their viability for grid‑scale energy storage.

Key Takeaways

• EB-COF added to Nafion improves ion conductivity.
• 0.3 wt% EB-COF yields optimal membrane performance.
• Energy efficiency reaches 89.1% at 40 mA cm⁻².
• Quaternary amine groups suppress polybromide shuttle.
• Balanced Zn²⁺/Br⁻ transport enhances power density.

Pulse Analysis

Zn/Br₂ redox flow batteries have emerged as a promising solution for large‑scale, stationary energy storage due to their intrinsic safety and high theoretical efficiency. However, membrane design has remained a bottleneck: increasing anion pathways improves bromide conductivity but also accelerates the polybromide shuttle, leading to self‑discharge and reduced coulombic efficiency. Conventional Nafion membranes lack the selective interactions needed to block Brₙ⁻ while maintaining fast Zn²⁺ transport, forcing researchers to seek hybrid materials that can reconcile these competing demands.

The introduction of a cationic covalent organic framework (EB‑COF) into the Nafion matrix addresses this dilemma through dual functionality. Surface aldehyde and amine groups form continuous water channels that dramatically enhance ionic mobility, while the quaternary ammonium sites electrostatically attract and immobilize bromine species, effectively suppressing the shuttle effect. At a modest loading of 0.3 wt%, the NF/EB‑COF composite achieves a balanced ion flux, delivering superior conductivity without compromising selectivity. This molecular engineering approach exemplifies how tailored nano‑structures can fine‑tune membrane properties beyond the capabilities of traditional polymers.

Performance testing confirms the practical impact: Zn/Br₂ cells equipped with the NF/EB‑COF(0.3) membrane reach 89.1% energy efficiency at 40 mA cm⁻², surpassing benchmarks for comparable systems. The enhanced efficiency translates to higher round‑trip energy retention and lower operational costs, critical factors for utility‑scale deployment. Moreover, the strategy is scalable, leveraging existing Nafion processing while incorporating a low‑percentage COF additive. As the renewable energy sector pushes for cost‑effective storage, such membrane innovations could accelerate the commercial adoption of Zn/Br₂ flow batteries, positioning them as a competitive alternative to vanadium and emerging organic chemistries.