Leveraging sulfonated poly(ether ether ketone) for superior performance in zinc iodine redox flow batteries

Sulfonated poly (ether ether ketone) SPEEK membranes with a degree of sulfonation of approximately 73 % were synthesized and investigated as the cation exchange membrane (CEM) for the zinc iodine (ZI) redox flow battery (RFB). Specifically, SPEEK was used in ZI RFB with 1 mol L−1 electrolyte and its performance was compared to the current benchmark CEM, Nafion™. Notably, columbic and energy efficiencies of 92.9 % and 75.9 %, respectively, were measured for the ZI battery with SPEEK at 17 mA cm−2, which was a 370 % increase in energy density compared to those with Nafion 212 membranes. RFBs with SPEEK exbibit a peak power density of 122 mW cm−2 at a current density of 166 mA cm−2 (98 mW/cm2 at 133 mA/cm2 with Nafion 212) because of their lower overpotential and higher discharge voltage. These results demonstrate the high cation selectivity of the SPEEK membranes in neutral, salt-based electrolytes, and provide the groundwork to develop a suitable replacement to Nafion for low-cost RFBs.