Poly(phthalazinone ether ketone) Amphoteric Ion Exchange Membranes with Low Water Transport and Vanadium Permeability for Vanadium Redox Flow Battery Application

Compact and homogeneous amphoteric ion exchange membranes (Q/S-AIEMs) without acid–base interaction for vanadium redox flow batteries (VRBs) are first prepared through a low-cost and simple way, in which the amounts of different ion exchange groups are controllable in a wide range. Brominated poly(aryl ether ketone) containing 3,5-dimethylphthalazinone moieties (BPPEK) and sulfonated poly(phthalazinone ether ketone) (SPPEK) are mixed forming into a series of blended base membranes. Q/S-AIEMs are prepared through the quaternary amination of the blended base membranes. Based on different effects of divided quaternary ammonium groups and sulfonic groups on vanadium permeability, the relative magnitude of the permeability of different vanadium species could be changed, and the net water transport volume across Q/S-AIEMs is reduced by varying ion exchange group loading, and these are first discussed. At 30% SPPEK, the static and cycling water transport volume across the Q/S-AIEM is 96.0% and 87.8% less than Nafion 115, respectively. Sulfonic groups have greater influence on water transport than quaternary ammonium groups. Q/S-AIEMs show low V3+ and VO2+ permeability, which are only 1.08–1.61% and 0.29–5.08% of values of Nafion 115, respectively. When SPPEK content increases from 10% to 50%, the ratio of VO2+ permeability to V3+ permeability decreases from 2.97 to 0.26. The VRB with Q/S-AIEM steadily conducts for more than 500 h, exhibiting higher efficiencies and capacity retention than Nafion 115. The chemical stability of Q/S-AIEMs improves with the increasing SPPEK content. The good comprehensive performance makes Q/S-AIEMs have the promise to use in commercial VRBs.