Devising ultra-robust mixed-matrix membrane separators using functionalized MOF–poly(phenylene oxide) for high-performance vanadium redox flow batteries

Abstract

Vanadium redox flow batteries (VRFBs) are promising candidates for the storage of renewable energy, and their performance and long-term durability depend on a core component: stable ion exchange membrane. We report the novel synthesis of a positively charged metal–organic framework (Cd-MOF, here designated as an ionic metal–organic framework (IMOF))-based hybrid polymer membrane with a functionalized backbone. The prepared structurally designed IMOF with fixed positive charges participates in electrostatic and hydrogen bonding interactions with quaternized poly(2,6-dimethyl phenylene oxide) (QPM) to fabricate an IMOF@QPM membrane. A single-cell VRFB with the IMOF@QPM membrane exhibits outstanding performance, with a coulombic efficiency (CE) of 98.0%, a voltage efficiency (VE) of 91.0%, and an energy efficiency (EE) of 85.3% at 100.0 mA cm⁻². Furthermore, the IMOF@QPM membrane maintains a discharge capacity of ∼96.0% for up to 800 cycles, which surpasses the performance of commercial Nafion 117 (∼76.2% up to 400 cycles) at 100.0 mA cm⁻². These observations are attributed to the rationally designed architecture of the IMOF@QPM membrane as a preeminent vanadium ion barrier, with 2.32 × 10⁻⁷ cm² min⁻¹ permeability, 12.00 × 10⁻² S cm⁻¹ conductivity, and 5.2 × 10⁵ S min cm⁻² ion selectivity along with good stability, showing overwhelming advantages over the Nafion® 117 membrane. The reported ultra-robust and high-performance IMOF-based super-cationic membrane may also find potential applications in water treatment and energy conversion devices.