Complexes Containing Redox Noninnocent Ligands for Symmetric, Multielectron Transfer Nonaqueous Redox Flow Batteries

Redox flow batteries (RFBs) hold promise for use in large-scale energy storage applications, but new electrolyte chemistries are needed to significantly enhance their energy densities and lower their cost. The energy density is governed by the cell voltage, active species concentration, and number of electrons transferred at each electrode. Nonaqueous solvents offer wider voltage windows than water; however, most if not all of the previously reported active species have low solubilities and/or are limited to single electron transfer at each electrode. In this paper we describe the design, synthesis, and characterization of metal coordination complexes containing noninnocent ligands that demonstrate enhanced solubilities at different oxidation states along with multiple electron transfers. In particular, a series of ester-functionalized chromium bipyridine complexes are demonstrated that afford six reversible redox couples over similar to 2 V and solubilities approaching 1 M. These characteristics allow the same complex to be used at the negative and positive electrodes. Using an electrolyte consisting of the tris(4,4'-(bis(2-(2-methoxyethoxy)-ethyl)ester)-2,2'-bipyridine)chromium complex ([Cr(L3)(3)]) in acetonitrile, we demonstrate two reversible electron transfers at each electrode in an unoptimized, symmetric H-cell with efficiencies of similar to 70%. With further enhancements in the electrolyte chemistry and cell design, this approach could lead to the demonstration of highly energy dense RFB chemistries for grid-scale storage applications.