Enhanced selectivity of SPEEK membrane incorporated covalent organic nanosheet crosslinked graphene oxide for vanadium redox flow battery

It is difficult for sulfonated poly(ether ether ketone) (SPEEK) to possess both high proton conduction and vanadium resistance owing to the degree of sulfonation. Herein, the composite membranes (S/GO-TpTG) with cationic covalent organic nanosheet (TpTG) crosslinked graphene oxide (GO-TpTG) are prepared to enhance selectivity by optimizing the ion transport channels. The GO-TpTG can efficiently transport protons utilizing its cationic porous structure and acid-base pairs' interaction with SPEEK. Meanwhile, it can block vanadium ions through the Donnan exclusion and physical blocking effects. The S/GO-TpTG membrane with 3 wt% GO-TpTG exhibits excellent proton conductivity (82.7 mS cm^-1) and selectivity (77.9×10^-7 cm² min^-1). The VRFB with this membrane exhibits excellent energy efficiency (88.6 - 81.0 % at 100-200 mA cm^-2), cycle durability, and self-discharge time (209.8 h). This study confirms the great potential of GO-COF to balance proton conductivity and vanadium resistance, and provides an effective strategy to optimize proton channels.