Investigation of poly(phthalazinone ether ketone) amphoteric ion exchange membranes in vanadium redox flow batteries

Amphoteric ion exchange membranes (Q-x/S) for vanadium redox flow battery (VRFB) are prepared from brominated poly(phthalazinone ether ketone) with the degree of substitution (DS) in the range of 60–95% and sulfonated poly(phthalazinone ether ketone) through blending and amination reaction, and x refers to the DS. Q-x/S membranes contain quaternary ammonium and sulfonic groups, making it easy to adjust the permeability of different vanadium ions and net water transport. When the DS increases from 60 to 95%, the discrepancy of the permeability for VO2+/VO2+ and V2+/V3+ enhances. The permeability of VO2+, VO2+, V3+ and V2+ for Q-x/S membranes was 98.7–99.9%, 98.6–99.9%, 99.5–99.9% and 99.2–99.9% less than those for Nafion115. Compared to Nafion115, Q-88/S membrane shows 81.3 and 84.8% decrease in static and cycling water transport volume. The energy efficiency (EE) of Q-88/S and Q-95/S membranes reaches 90% at the current density of 40 mA cm−2, higher than that of Nafion115 (86.5%). Compared with Nafion115, Q-x/S membranes exhibit stable efficiencies and higher capacity retention in long-term VRFB operation. Low vanadium permeability and net water transport volume make Q-x/S membranes promising to be used in commercial VRFB.