A green SPEEK/lignin composite membrane with high ion selectivity for vanadium redox flow battery

The commercial Nafion proton exchange membranes are most widely used in vanadium redox flow batteries (VRBs). However, their poor ion selectivity and high cost significantly limits the further development of the VRB. Herein, a green biopolymer, lignin, was firstly employed as the additive in the pristine SPEEK membrane to replace the Nafion membrane for the VRB applications. It is found that the abundant hydroxyl groups in lignin not only facilitate the dispersion of the lignin in the SPEEK matrix, but also significantly improve the proton conductivity and ion selectivity of the membrane. With these merits, the VRB single cell based on the SPEEK/lignin membrane exhibits 95.95% and 71.47% capacity retention after 100 and 300 cycles under 120 mA cm−2, respectively. Moreover, its ultrahigh coulombic efficiency (over 99.5%) and energy efficiency (over 83.5%) indicate the outstanding capability of ion selectivity and the superb protons conductivity of the membrane. The excellent stability, low cost, and remarkable device performance of the composite membrane make it a promising candidate for the next generation larger-scale VRB applications.