Pyridyl group design in viologens for anolyte materials in organic redox flow batteries

Abstract

Organic redox compounds represent an emerging class of active materials for organic redox-flow batteries (RFBs), which are highly desirable for sustainable electrical energy storage. The structural diversity of organic redox compounds helps in tuning the electrochemical properties as compared to the case of their inorganic counterparts. However, the structural diversity makes the design and identification of redox-active organic materials difficult because it is challenging to achieve appropriate redox potential, solubility and stability together, which are the major concerns regarding the practical applicability of these materials to RFBs. Herein, we report the design, synthesis, and application of viologen molecules as anolyte materials for organic RFBs that are compatible with Li-ion electrolytes. Structural screening assisted by density functional theory (DFT) calculations suggests that the (CH₂)₅CH₃-substituted viologen molecule exhibits reduction potential as low as 2.74 V vs. Li/Li⁺, good structural stability due to effective charge delocalization within the two pyridinium rings, and a solubility of up to 1.3 M in carbonate-based electrolytes. When paired with a 2,2′:6′,2′′-terpyridine–iron complex catholyte, the cell shows a high discharge voltage of 1.3–1.5 V with coulombic efficiency > 98% and energy efficiency > 84%. Both the anolyte and catholyte materials are built from earth-abundant elements and can be produced with high yields; thus, they may represent a promising choice for sustainable electrical energy storage.