Radiation grafted Bi-functional cation exchange membrane for vanadium redox flow battery

Abstract

Vanadium redox flow battery (VRFB) utilizing cation exchange membrane (CEM) has the potential as a non-degradable large-scale energy storage for sustainable energy. Nafion with PTFE backbone with the acidic sulfonic groups is the most common membrane for VRFB. However, Nafion is expensive, and suffers from vanadium ion crossover, produces low ion selectivity results in severe capacity decay and voltage declination in an open circuit, and these drawbacks lead to self-discharge. Polymer-based radiation grafted membrane with two-functional groups of monomer collaborated cells is considered as one alternative to overcome the weakness of Nafion. This research produced a reduced-crossover vanadium ion membrane that has commercial value through one-step pre-irradiation grafting of two functional groups of monomers onto the ETFE polymer backbone. The modification of ETFE membrane was completed by one-step radiation grafting polymerization process. The manipulation of the two types of monomers, types of solvents used, reaction time and temperature were optimized to obtain the targeted degree of grafting. The targeted grafting yield of <180% poly (ethylene tetrafluoroethylene)-g-styrene sulfonic acid-g-N-Vinyl formamide (ETFE-SSS-VNF) was successfully achieved. Characterization analysis of the membranes using FTIR and XPS analyses showed the new peaks from SSS and NVF monomers marked the attachment of C=O, O-H, N-H stretching and S=O functional groups onto ETFE film. The cross-section of modified membrane through FESEM-EDS and mapping displayed the monomers were well distributed through ETFE membrane. The modified ETFE membrane exhibited extremely low vanadium crossover while sustaining high conductivity. In the single-cell VRFB test, the modified membrane provided higher coulombic efficiencies up to 96%, and energy efficiencies (EE: 81-84%) higher than commercial membrane N117(EE: 59.7-60.8%) at a current density of 40 mA/cm2. Incorporating -SO3 group from SSS monomers and -N-C=O group from NVF monomers provided high proton conductivity and hindered vanadium ion crossover, demonstrating their high performance as a potential ion exchange membrane for VRFB application. The modification of ETFE membrane with low vanadium ion crossover is a promising new CEM for VRFB application.