Synthesis and investigation of imidazolium functionalized poly(arylene ether sulfone)s as anion exchange membranes for all-vanadium redox flow batteries

A serials of imidazolium functionalized poly(arylene ether sulfone) as anion exchange membranes (AEMs) for all-vanadium redox flow battery (VRB) application are synthesized successfully in this study. The AEMs obtained with different ionization levels, Im-PAES-0.5/0.7/1.1 (0.5/0.7/1.1 represents the different degrees of bromination, respectively) membranes are intensively characterized. Compared with commercial Nafion 117 membrane, the vanadium ion permeation values of AEMs are lower while their chemical stability in VO2+ are comparable. The ionic conductivities, vanadium ion permeability, stability of the membranes and their performance in VRB cells are greatly affected by ionic exchange capacity (IEC). In order to balance these important performance factors, the Im-PAES-0.7 membrane with a moderate IEC of 2.01 meq g−1 exhibits good stability, high ionic conductivity and low vanadium ion permeation, resulting in high voltage efficiencies (up to 87.5%), high coulombic efficiencies (up to 96.1%), as well as high energy efficiency. The VRB cell with the Im-PAES-0.7 membrane achieves comparable to or even slightly higher energy efficiency values (up to 81.7%) than those of Nafion 117 membranes (up to 78.9%) in the current density range of 60–120 mA cm−2. During 40 charge–discharge cycles at different current densities, the Im-PAES-0.7 anion exchange membrane exhibits a comparable short-term stability to Nafion 117 membrane.