Multicore Ferrocene Derivative as a Highly Soluble Cathode Material for Nonaqueous Redox Flow Batteries

Nonaqueous redox flow batteries (NARFBs) are considered promising electrochemical energy storage systems to overcome the limited electrochemical window of aqueous counterparts. However, the limited solubility of redox-active materials in the nonaqueous phase, for instance, the intensively investigated ferrocene derivatives, needs to be addressed prior to their practical implementation. In this study, we demonstrate the design and synthesis of a multicore ferrocene derivative, 4,4-diferrocenyl-1-(2-methoxy-ethoxy)-pentane (DFDE), as a highly soluble cathode-active material for NARFBs. Owing to the tailored ether chain as well as the synergistic effect of multiple ferrocene cores, DFDE exhibits a high solubility of 4.5 M equivalent electron concentration in an ether-based solvent as well as a more positive redox potential compared with pristine ferrocene, which are essential to improve the energy density of NARFBs. Moreover, electrochemical characterizations reveal that DFDE undergoes highly reversible redox reactions with a fast electron-transfer rate constant. When paired with N-butyl-phthalimide (BuPh), the cell exhibits a discharge voltage of ca. 1.8 V and stable cycling performance. The use of multicore ferrocene derivatives might represent an effective yet to be explored strategy to overcome the solubility limit of ferrocene-based cathode materials at affordable environmental and ecological load.