SPEEK-Based Blended Membrane Enhancing Ion Transport with Hydrophilic Microporous Polymers in Aqueous Organic Flow Batteries

Abstract

The membrane is a critical component of aqueous organic redox flow batteries (AORFB), functioning to separate the two half-cells, sustain redox activity, and facilitate the rapid transport of charge-balancing ions. While the incorporation of microporous polymers into the membrane can significantly enhance its performance, achieving effective interfacial compatibility between the porous fillers and the sulfonated polyether ether ketone (SPEEK) base membrane remains a significant challenge in the development of blend membranes. To address this, two hydrophilic microporous polymers, PIM-COOH and PIM-SO₃H, were incorporated at varying mass ratios to modify sulfonated polyether ether ketone (SPEEK) membranes. The addition improved the compatibility of the polymer blend system, enhancing the interfacial bonding between the porous additives and the base membrane. The introduction of a rigid and twisted skeleton structure significantly improved the tensile strength of the membrane. Specifically, the tensile strength increased by 18.21% to 75.83 MPa with the addition of 25 wt % PIM-COOH (S/C-25). Similarly, the addition of PIM-SO₃H (S/S-25) increased the membrane strength by over 25%. This improvement is attributed to the increased stiffness of the polymer resulting from changes in the polarity of the microporous polymer segments. The incorporation of hydrophilic microporous channels enhanced water mobility within the membrane, facilitated ion transport, and improved the energy efficiency of the mixed membrane (S/S-10) by approximately 5% at a current density of 100 mA cm^–2. In long-term cycling tests, the battery assembled with the S/S-10 membrane exhibited stable performance for over 1200 cycles.