Efficiently ion-selective polybenzimidazole membrane with precisely controlled channels and hydrogen-bond networks for flow batteries

Polybenzimidazole (PBI) exhibits exceptional chemical stability and mechanical strength, positioning it as a highly promising material for ion conductive membranes (ICMs). However, its relatively low proton conductivity limits its performance in vanadium flow batteries (VFBs). In this study, we construct ultrafast proton-conducting molecularly mixed composite membranes (MMCMs) by strategically introducing organic macrocyclic cavitands (OMCs), especially 4-sulfocalix[6]arene (SCA6), into the PBI matrix. The open macrocyclic cavity of SCA6 modulates the channels of PBI membrane, thus facilitating proton transport, while its intrinsic cavity (diameter of ∼0.6 nm) effectively restricts vanadium permeation. Meanwhile, the –SO₃H groups form an extensive and continuous hydrogen-bonding network within the membrane, which greatly enhances proton conductivity. The synergistic combination of precisely controlled channels and hydrogen-bond networks enables efficient ion-selective transport. Experimental results demonstrate that the MMCMs exhibit an excellent energy efficiency (EE) of 80.56 % at 220 mA cm⁻² and achieve long-term stability of 1700 cycles (>1350 h) in the VFB system at 200 mA cm⁻². Our work provides a facile preparation method for highly ion-selective PBI-based battery membranes.