A comparative study of Nafion and sulfonated poly(ether ether ketone) membrane performance for iron-chromium redox flow battery

A low-cost sulfonated poly(ether ether ketone) (SPEEK) membrane with optimized degree of sulfonation (DS) has been successfully synthesized and applied in iron-chromium redox flow battery (ICRFB) in comparison with Nafion membrane. The SPEEK membrane with an optimum DS of 55% exhibits the cell performance of CE 98.53% and EE 79.13% at 80 mA cm(-2) due to its longer charge retention and lower metallic ion crossover, while Nafion 115 membrane exhibits the cell performance of CE 95.97% and EE 82.33%. In addition, the SPEEK membrane demonstrates stable cell performance with slower discharge capacity decline, lower self-discharge rate, and highly stable efficiency. SPEEK membrane performs superior chemical stability in the highly oxidizing electrolytes as well, which is comparable to Nafion 115. Furthermore, the capital cost of ICRBF with SPEEK membrane occupies 5% of the overall system cost, which is 34% lower than that of ICRFB with Nafion membrane for 1 MW-8 h energy storage system.