Performance of vanadium redox flow battery with a novel amphoteric ion exchange membrane synthesized by two-step grafting method

A novel ETFE-based amphoteric ion exchange membrane (AIEM) was prepared through a two-step radiation-induced grafting technique. ETFE film was first grafted with styrene (St) (denoted as ETFE-g-PS), followed with a sulfonation treatment to obtain a cation exchange membrane (ETFE-g-PSSA). The ETFE-g-PSSA membrane was subsequently grafted with dimethylaminoethyl methacrylate (DMAEMA) and then protonated, resulting in an AIEM with both anionic and cationic groups. The grafting yields (GY) in the above two steps increased with absorbed dose and leveled off at about 30 kGy and 20 kGy, respectively. Micro-FTIR and XPS analyses testified that the AIEM had been prepared as designed. The obtained AIEM exhibited high ion exchange capacity (IEC) and conductivity, as well as significantly reduced permeability of vanadium ions. Vanadium redox flow battery (VRFB) assembled with the AIEM maintained an open circuit voltage (OCV) higher than 1.3 V after placed for 300 h, and exhibited higher columbic efficiency and energy efficiency than that with Nafion 117 membrane.