A sandwiched bipolar membrane for all vanadium redox flow battery with high coulombic efficiency

Due to the good balance between the low vanadium permeability and high proton conductivity, AIEM (amphoteric ion exchange membrane) has attracted vast consideration for all vanadium redox flow battery (VRB) application. Similar to the AIEM, BPMs (bipolar membranes) also have both anion and cation exchange groups in the anion-selective layer and cation-selective layer. In this paper, a series of BPMs based on PTFE (polytetrafluoroethylene) reinforced QAPSF (quaternized polysulfone) and SPEEK (sulfonated poly ether ether ketone) are prepared and investigated for the VRB application. As expected, all the membranes have presented typical sandwiched structure with an obvious intermediate layer observed by SEM. Owing to the “Donnan exclusion effect” of anion groups in QAPSF, all the BPMs have shown very low VO²⁺ permeability. Particularly, the battery with Q/P/S-3:2 membrane has shown average coulombic efficiency of 98.9% at current density of 40–80 mA cm⁻². Considering the simple adjustment and abundant choice of cation or anion resins, BPMs membranes should be a promising candidate for VRB application after further investigation.