The effect of counter-ion substitution on poly(phthalazinone ether ketone) amphoteric ion exchange membranes for vanadium redox flow battery

The study proposed a novel method to prepare poly(phthalazinone ether ketone) amphoteric ion exchange membranes (Q/S-M) with both increased efficiency and chemical stability for vanadium redox flow battery (VRB) applications. Q/S-M membranes were obtained after the successive amination and acidification process of blend base membranes prepared from brominated poly(phthalazinone ether ketone) (BPPEK) and sulfonated poly(phthalazinone ether ketone) substituted by counter-ions (SPPEK-M), where M was defined as counter-ions: Li+, Na+ and K+. The study analyzed the effect of counter-ion size on properties of Q/S-M membranes, which were compared with those for Q/S membrane obtained from BPPEK and SPPEK in acid form. Q/S and Q/S-M membranes maintained the same composition of ionic groups. The Q/S-M membranes induced by bigger counter-ion size showed the increasing of water content and ion diffusion rate, which was higher than that of Q/S membrane. Related to Q/S membrane, Q/S-K membrane possessed almost half area resistance (0.67 Omega cm(2) vs 1.43 Omega cm(2)), and the value was close to that of Nafion115. Q/S-M membranes exhibited a 96.0-98.8% decrease in VO2+ permeability over Nafion115. Q/S-K membrane displayed a 6.5% increase in VE and correspondingly a 6.2% increase in EE over Q/S membrane, maintaining superior EE than Nafion115 (89.7% vs 86.5%). The ex-situ degradation test in VO2+ solutions indicated that despite close EE, Q/S-K membrane showed superior chemical stability over amphoteric ion exchange membranes (AIEMs) in our previous works. There was no obvious efficiency decay of Q/S-K membrane during 100-cycles test, indicating the good duration in operating VRB.