Boosting the performance of positive electrolyte for VRFB by employing zwitterion molecule containing sulfonic and pyridine groups as the additive

The electrolyte, one of the key components for the energy storage in the vanadium redox flow battery (VRFB), hinders the further commercial applications of the VRFB due to the poor thermal stability of the positive electrolyte at the elevated temperature and the inferior activity on the interface of electrode. To enhance the performance of the positive electrolyte, a zwitterion-type molecule pyridinium propyl sulphobetaine (PPS) with negatively charged sulfonic group and positively charged pyridine group is adopted as the additive for the positive electrolyte. In this study, we have proved that PPS can be adsorbed on the electrode through sulfonic group and pyridine group under both positive potential and negative potential, which can effectively modify the interface of electrode. Compared to other additives containing –NH2/–SO3H groups, PPS containing pyridine group may effectively eliminate the side effects of protonation (–NH3+) and intermolecular hydrogen bonding (-SO3−---+H3N-) in acidic electrolyte. Moreover, the PPS-containing V(V) electrolyte presents a better stability at 50 °C compared to the blank V(V) electrolyte. As a consequence, with the addition of 1% PPS in positive electrolyte, the VRFB exhibits higher efficiencies (80.78% VE and 78.17% EE compared to 78.95% VE and 76.87% EE of the blank VRFB at 100 mA cm−2) and 76.28% capacity retention after 100 cycles which is 5.22% higher than that of the pristine VRFB. Herein, the study of zwitterion molecules provides a meaningful reference for the further research on organic additives for VRFB.