Sulfonated graphene oxide-decorated block copolymer as a proton-exchange membrane: Improving the ion selectivity for all-vanadium redox flow batteries

We describe the fabrication of a new block copolymer, sulfonated poly(ether ketone sulfone) (SPEKS) decorated with sulfonated graphene oxide (sGO) as a potential proton-exchange composite membrane for all-vanadium redox flow batteries (VRBs). The simple design of the SPEKS/sGO composite membrane offers key advantages in VRBs relative to commercial Nafion® membrane. The SPEKS/sGO composite membrane exhibited significantly lower vanadium ion permeability, which resulted in an excellent ion selectivity (10.2 × 105 S min cm-3) in comparison with the pristine SPEKS (7.9 × 105 S min cm-3) and Nafion-212 (1.8 × 105 S min cm-3) membranes. As a result, the SPEKS/sGO composite membrane exhibited exceptional electrochemical performance in a VRB under a mixed-acid system. Further, the impressive cyclability with negligible capacity decay demonstrated the high chemical stability of the designed composite membrane for long-term operation. In addition, excellent battery performance with a Coulombic efficiency of 99.4% and energy efficiency of 82.5% was obtained for the SPEKS/sGO membrane compared with the Nafion-212 (CE, 89.6% and EE, 75.5%) and the pristine SPEKS (CE, 97.2% and EE, 74.7%) membranes in the VRB at 40 mA cm-2 current density. The VRB assembled with a SPEKS/sGO composite membrane had a lower self-discharge rate, retaining an open circuit voltage of 1.30 V for 395 h in comparison with the Nafion-212 (29 h) and pristine SPEKS (240 h) membranes. Therefore, given its remarkable electrochemical performance and improved chemical stability, the SPEKS/sGO composite membrane has good potential to be explored as a promising alternative for the Nafion® membrane currently extensively used in VRB applications.