A highly selective and high-performance sulfonated poly(ether ether ketone)-based hybrid membrane enabled by complexed UiO-66-NH 2 and sulfonated graphitic carbon nitride for vanadium flow batteries

A series of sulfonated poly(ether ether ketone) (SPEEK) hybrid membranes (SPEEK/NF) with constant 1 wt% complexed nanofillers (NF) comprising UiO-66-NH 2 and sulfonated graphitic carbon nitride (s-g-C 3 N 4 ) are prepared through the solution-blending method. As the ratio of UiO-66-NH 2 /s-g-C 3 N 4 amounts to 1 : 1, the SPEEK/NF-1 : 1 membrane shows the highest ion selectivity (111.3 × 10 3 S min cm −3 ) and the lowest vanadium ion permeability (2.5 × 10 −7 cm 2 min −1 ). In the meantime, it further presents outstanding energy efficiency (85.9–76.1%) and ultra-long self-discharge time (136.1 h) at a current density of 100–200 mA cm −2 . The acid–base interaction originating from the –NH 2 and –SO 3 H groups in the SPEEK/NF hybrid membranes provides a stable protonation network, and the synergistic ion screening ability and physical blocking effect from UiO-66-NH 2 and s-g-C 3 N 4 maintain 79.9% energy efficiency, 0.29% capacity decay rate per cycle and good chemical stability for SPEEK/NF-1 : 1. Thus, the manipulation of acid–base interaction and synergistic ion barriers enabled by two-dimensional nanostructured materials allow us to explore highly selective SPEEK/NF hybrid membranes, showing great prospects in VFB applications.