Porous polybenzimidazole membranes with high ion selectivity for the vanadium redox flow battery

Development of high ion-selective polymer membranes is of great importance for vanadium redox flow batteries (VRFBs). In this work, porous polybenzimidazole (PBI) membranes are successfully prepared for improved efficiencies and stability in VRFB applications. The porous structure is constructed via a hard templating method using monodispersed SiO2 solid spheres and 3 M NaOH as template and etching solution, respectively. The influence of the adding content of SiO2 on the microstructure of the membrane is illustrated. The properties of porous membranes, including acid doping, swellings, area resistance, mechanical properties, vanadium ion permeation, and single cell and cycling performance, are investigated systematically. Results show that asymmetric porous PBI membranes exhibit increased sulfuric acid (SA) uptake (>35 wt%), remarkably low area resistance (<0.58 Ω cm2), ultralow vanadium ion permeability (<10-9 cm2 min-1) and superior mechanical strength (>20 MPa) simultaneously. At high current densities (80–200 mA cm-2), the cell with the porous PBI-40%SiO2 membrane possesses superior coulombic efficiencies (CE: 99.5–100%) and energy efficiencies (EE: 87.9–71.5%), together with excellent cycling stability at 120 mA cm-2 in the vanadium flow battery. Accordingly, this work not only provides a kind of advanced membranes for the VRFB, but also offers a facile and effective method to design, optimize and fabricate membranes incorporating the features according to needs.