**6. Conclusions**

The effects of three kinds of additives including carbon dioxide gas, EC/DMC, and Sb3+ ions on the electrochemical performance of nonaqueous DES electrolyte redox flow batteries are explored. The ohmic resistance of the deep eutectic solvent (DES) electrolyte decreases significantly when adding carbon dioxide gas and EC/ DMC, and the percentage of reduction increases with the volume percentage of EC/ DMC in electrolyte, while for these two additives, the reaction kinetics almost keeps unchanged.

With CO2 in DES, the electrochemical reaction resistance increases about 10%. For EC/DMC additive, the electrochemical reaction resistance almost keeps the same no matter the amount of additive in electrolyte. For the additive of Sb3+ ions in DES electrolyte, the electrochemical reaction kinetics of active redox couple is enhanced, the diffusion coefficient of active ions increases, and the charge transfer resistance decreases. The electrodeposited Sb3+ ions on electrode surface contribute a catalytic effect on the electrochemical reaction. However, due to the trade-off between the enhanced kinetics and reduced active surface area, the optimum concentration of Sb3+ ions is found to be 15 mM. In addition, the flow battery assembled with negative electrolyte containing Sb3+ ions exhibits 31.2% higher power density. The results in this chapter provide a simple yet effective approach to promote the cell performance of nonaqueous redox flow batteries.
