High performance ZrO2 modified sulfonated poly (ether ether ketone) ion-exchange membrane for Iron-chromium redox flow battery application

A high-performance ion-exchange membrane (IEM) for an iron-chromium redox flow battery (ICRFB) was fabricated by zirconium dioxide (ZrO2) with sulfonated poly (ether ketone) (SPEEK, with a degree of sulfonation: 57 %). A series of physical and chemical tests such as water uptake (WU), swelling ratio (SR), ion exchange capacity (IEC), proton conductivity, strength characteristics and permeability were performed. Additionally, it was notable that the ZrO2 -SPEEK (Zr/S) IEM with the optimal reference ratio (7.5 %) showed higher coulombic efficiency (CE:97.74 %) and energy efficiency (EE:82.85 %) at 100 mA & sdot;cm- 2 than Nafion-212 IEM (CE:96.7 %, EE:82.55 %) and SPEEK-57 IEM (CE:96.86 %, EE:76.84 %) in the ICRFB single-cell test. In the first cycle, the Zr/ S-7.5 % IEM delivered a higher discharge capacity (708 mAh) than the Nafion-212 IEM (508 mAh) and SPEEK-57 IEM (373 mAh). Moreover, the composite membrane could maintain 1.2 V open-circuit voltage for 56.5 h in the self-discharge experiment, surpassing Nafion-212 IEM (4.617 h) and SPEEK-57 IEM (14.783 h).