High-performance composite membrane based on synergistic main-chain/side-chain proton conduction channels for the vanadium redox flow battery

Abstract

A series of composite proton-exchange membranes composed of main-chain sulfonated poly(ether ether ketone) (SPEEK) and side-chain sulfonated poly(p-phenylene terephthalamide) (SPPTA) were prepared through a solution-blending technique. Physicochemical properties, such as proton conductivity, ion selectivity, and vanadium redox flow battery performance were characterized in depth. The synergistic proton-conduction channels composed of main-chain and side-chain –SO₃H groups from SPEEK and SPPTA, respectively, and the formed hydrophilic/hydrophobic microphase separation yielded a trade-off balance between proton conductivity and vanadium ion permeability for SPEEK/SPPTA composite membranes. Against pristine SPEEK (2.8 × 10³ S min cm⁻³) and Nafion 117 (4.0 × 10³ S min cm⁻³), the SPEEK/SPPTA-25 composite membrane exhibited the highest ion selectivity (22.9 × 10³ S min cm⁻³) in terms of the good restriction of vanadium ion penetration. For the single-cell performance, at a current density of 50 mA cm⁻², the voltage efficiency (VE, 92.0%) and energy efficiency (EE, 89.4%) of SPEEK/SPPTA-25 were much higher than those of SPEEK (VE, 86.2% and EE, 80.2%) and Nafion 117 (VE, 88.8% and EE, 83.5%). Moreover, EE was still above 70.0% at a current density of 210 mA cm⁻², and no significant reductions in efficiency appeared after 500 charge–discharge cycles at 100 mA cm⁻². This result demonstrates that SPEEK/SPPTA composite membranes have excellent structural stability and long-term durability, showing great perspective towards vanadium redox flow battery applications.