Tuning bromine complexing agent for flowless zinc bromine batteries

Abstract

Recently, a flowless zinc bromine battery system was proposed, which enhanced the economic competitiveness with system simplifying strategy. The system suffers from self-discharge reaction arising from bromine cross over and the bromine complexing agent is essential in retarding bromine cross over. In this research, the bromine complex formation behavior is modulated with bromine complexing group, imidazole ionic liquid, by tuning side alkyl chain length. From the DFT-calculation, it was confirmed that the complex binding energy can be modulated with side alkyl chain length. Also, from the UV-vis spectroscopy, restricted bromine cross over with stronger complex formation of longer side alkyl chain length was observed. From the EIS and discharge voltage curve comparison, the influence of side alkyl chain length on bromine complex complexation/dissociation dynamics and resulting electrolyte conductivity was deduced. As a result, with side alkyl chain length modulation, at the intermediate length, coulombic efficiency and voltage efficiency showed maximum and minimum values, respectively, which led to weak dependency of energy efficiency on side alkyl chain length variation.