Surface modification, counter-ion exchange effect on thermally annealed sulfonated poly (ether ether ketone) membranes for vanadium redox flow battery

In this work, we report new approaches to sulfonated poly ether ether ketone (SPEEK) membranes to improve its inhibition to vanadium ions along with polymer reinforcement. First approach includes facile thermal annealing of SPEEK membrane (SPEEK-T) which could reinforce the polymer matrix. Second approach was to exchange the labile proton of SPEEK with benzimidazole to protect the hydrolytic decomposition of sulfonic acid followed by thermal annealing (SPEEK-Benz), and third approach focused on creating barrier layer for vanadium ions by coating polyethyleneimine on SPEEK membrane followed by thermal treatment (SPEEK-PEI). The chemical stability study of the membranes revealed acceptable dimension change and VO2+ electrolyte sorption for all the membranes with least dimension change and weight gain of 5 % and 12 %, respectively, for SPEEK-PEI. Amongst four prepared membranes, SPEEK-PEI membrane displayed least diffusion coefficient of 8.04 x 10-7, 8.94 x 10-7, 4.20 x 10-7 cm2 min-1 for V3+, VO2+, and VO2+ ions respectively. VRFB assembled with SPEEK-PEI resulted highest Coulombic efficiency (CE) of 97 % and energy efficiency (EE) of 66 % over 50 charge/discharge cycles at 100 mA cm-2. Peak power density of 255, 248, 240 and 235 mW cm-2 at current density of 300 mA cm-2 was observed for SPEEK-PEI, SPEEK-Benz, SPEEK-T, and SPEEK-48, respectively; these values were higher than 225 mW cm-2 for Nafion & REG;117 in identical experimental conditions. The excellent VRFB performance suggest that the developed new approaches have pronounced effect on the membrane properties and can be further improved to obtain an ideal VRFB separator. Furthermore, the surface modification approaches adopted in this work is facile, reproducible and scalable and can be further designed to develop high performance ion exchange membranes for separation/purification and energy.

Automated summary

This work reports new approaches to enhance the inhibition of sulfonated poly ether ether ketone (SPEEK) membranes towards vanadium ions in flow batteries. The first approach involves thermal annealing of SPEEK membranes, which strengthens the polymer matrix. The second approach involved replacing the labile proton of SPEEK with benzimidazole, providing protection against hydrolytic decomposition of sulfonic acid, followed by thermal annealing. The third approach focused on creating a barrier layer for vanadium ions through the coating of polyethyleneimine on SPEEK membranes, followed by thermal treatment. The study demonstrated that the SPEEK-PEI membrane exhibited the best performance, showing the least diffusion coefficient and resulting in the highest Coulombic efficiency and energy efficiency in vanadium redox flow batteries.