Bifunctional effects of halloysite nanotubes in vanadium flow battery membrane

The search for high-performance, stable, and cost-efficient membranes for vanadium flow battery (VFB) has attracted great interest. Herein, we report a physically cross-linked acid-base membrane composed of sulfonated poly(ether ether ketone) (SPEEK) and dopamine-modified halloysite nanotubes (DHNTs). The hollow structure of DHNTs allows the polymer chains getting into the lumen to form a robust mesh structure inside the composite membrane, resulting in improved mechanical stability. Meanwhile, the interface acid-base interaction between DHNTs and SPEEK facilitates the hopping of protons, leading to enhanced proton conductivity. Due to the bifunctional effects of DHNTs, the VFB with an optimized S/DHNTs 3% membrane exhibits outstanding rate performance at 40-200 mA cm(-2) and excellent durability over 500 cycles at 160 mA cm(-2), including extremely stable coulombic efficiency (similar to 99%) and energy efficiency (similar to 78%) along with very slow capacity decay rate (0.099% per cycle). The above results demonstrate that the S/DHNTs composite membrane has great advantages and broad prospects for VFB applications.