Catholyte Blend: Homoleptic and Heteroleptic Iron Complexes for Flow Batteries

Abstract

Nonaqueous redox flow batteries (RFBs) using terpyridine (tpy) iron complex catholytes are an attractive long-duration energy storage system. However, the low solubility of tpy iron complexes hinders their wide deployment. Here, we present a solubility-enhancing catholyte-blend strategy that combines symmetry-breaking molecular design with multicomponent crystal engineering. The blended catholyte exhibits a total Fe solubility of 0.351(7) mol L–1 in acetonitrile (0.194(9) mol L–1 with 0.5 mol L–1 nBu4NPF6), which is more than twice as high as that of each single-component complex. H-cell cycling tests demonstrate stable operation with a coulombic efficiency of ∼99.9% and a capacity fade rate of ∼0.5% day–1. Furthermore, a nonaqueous Mix/EV(PF6)2 (EV = 1,1′-diethyl-4,4′-bipyridinium) flow cell exhibits an initial discharge capacity equal to the theoretical value (2.68 Ah L–1) with an operating voltage of 1.40 V. The capacity fade rate during stable cycling was 1.32(2)% day–1. This blend-based solubility-engineering approach should be broadly applicable to coordination and organic redox-active materials.