Electrochemical Evaluation of Diketopyrrolopyrrole Derivatives for Nonaqueous Redox Flow Batteries.

Redox flow batteries (RFBs) employing nonaqueous electrolytes could potentially operate at much higher cell voltages, and therefore afford higher energy and power densities, than RFBs employing aqueous electrolytes. The development of such high-voltage nonaqueous RFBs requires anolytes that are electrochemically stable, especially in the presence of traces of oxygen and/or moisture. The inherent atmospheric reactivity of anolytes mandates judicious molecular design with high electron affinity and electrochemical stability. In this study, diketopyrrolopyrrole (DPP)-based TDPP-Hex-CN4 is proposed as a stable redox-active molecule for anolytes in nonaqueous organic RFBs. We demonstrate organic RFBs using TDPP-Hex-CN4 as anolyte with unisol blue (UB) 1,4-bis(isopropylamino)anthraquinone and 1,4-di-tert-butyl-2,5-bis(2-methoxyethoxy)benzene (DBBB) as catholytes. Cyclic voltammetry measurements with scans repeated over 200 cycles were performed to establish the electrochemical stability of the redox pairs. Symmetric flow-cell studies show that TDPP-Hex-CN4 exhibits stable capacity up to 700 cycles. Redox flow cells employing TDPP-Hex-CN4 /UB and TDPP-Hex-CN4 /DBBB as redox pairs demonstrate that DPP derivatives are propitious materials for anolytes in all organic nonaqueous RFBs.