A green and cost-effective zinc-biphenol hybrid flow battery with eutectic electrolyte

Eutectic electrolytes have been widely used in redox flow batteries (RFBs) due to their unique features such as abundant availability, biodegradability, and low cost, as well as their excellent dissolution ability for redox-active substances. Despite these advantages, eutectic electrolytes still have some limitations in important aspects such as reaction kinetics, mass transport and ionic conductivity due to their relatively high intrinsic viscosity. In this work, we explore a green and cost-effective zinc-based eutectic electrolyte containing small molecules additives, which not only has elevated ionic conductivity, but also can dissolve organic cathode materials 3,3′,5,5′-tetramethylaminemethylene-4,4′-biphenol (TABP) at high concentrations. In this zinc-based eutectic electrolyte, TABP exhibits a highly positive redox potential (1.50 V versus Zn/Zn2+), quasi-reversible redox reaction kinetics and good redox stability. When operated in a practical hybrid flow battery, the Zn-TABP cell based on this eutectic electrolyte exhibits excellent rate performance, high capacity utilization, and low capacity decay rate (0.119 % per cycle). Post-analysis for the TABP catholyte suggests that the Michael side reaction rate of TABP in electrochemical cycling appears to depend upon its concentration.