Crown-ether block copolymer based poly(isatin terphenyl) anion exchange membranes for electrochemical energy conversion devices

Abstract

Poly(isatin terphenyl) based polymers are first synthesized from dibenzo-18-crown-6, terphenyl, and isatin via the acid-catalyzed polycondensation reaction. The polymers are afterwards quaternized by grafting (6-bromohexyl)-trimethylammonium bromides via the amide sites of isatin. The hydrophilic crown ethers endow the membrane with suitable water uptake, reasonable swelling, and interconnected ion conducting channels according to the experimental results and relevant morphology analyses. The prepared membrane displays hydroxide conductivity as high as 112 mS cm−1 and bromide ion conductivity of 74 mS cm−1 at 80 °C, respectively. In addition, the retention rate of the initial hydroxide conductivity is 85 % after immersing the membrane in 1 mol/L KOH at 80 °C for 1200 h. The membrane-based single fuel cell demonstrates a peak power density of 487 mW cm−2 at 60 °C by feeding with humidified hydrogen and oxygen, and 71 % of its initial voltage is retained after operating at 150 mA cm−2 for 40 h. Consideration of the superior bromide ion conductivity as well as acceptable stability, the QPIT-CE0.5 polymer is used to fabricate a composite membrane with glass fiber for using as the diaphragm in aqueous zinc bromine batteries. This composite membrane exhibits remarkable restriction to polybromide ions. The static zinc bromine battery with the composite separator achieves high coulombic efficiency of 93.0 % at 6 mA cm−2. The cell exhibits superior stability over 1500 cycles with a capacity retention rate of 99.5 %.