Developing Terpyridine-Based Metal Complexes for Non-Aqueous Redox Flow Batteries

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Redox flow batteries (RFBs) that utilize dissolved electroactive materials have shown great potential in storing intermittent solar and wind energy. The unique technological advantages of decoupled energy and power lead to high flexibility and scalability in RFB design, making it possible to satisfy a wide range of energy and power applications.[1,2] In traditional RFBs, inorganic transition-metal salts like all-vanadium, iron-chromium, and zinc-bromine RFBs are commonly used.[3,4] However, the narrow electrochemical stability window of water limits the attainable cell voltages in these aqueous systems. [5,6] To overcome these limitations, recent research has shifted towards non-aqueous RFBs (NARFBs) that use non-aqueous solvents, which can offer a wider electrochemical window up to 5 V.[7,8] Here, we introduce our research efforts on the design and synthesis of a series of terpyridine-based complexes of the first-row transition metals Cr, Mn, Fe, and Co for non-aqueous redox flow batteries (NARFBs). Electrochemical studies reveal that these complexes can undergo multi-electron transfer redox reactions. Notably, the Mn and Fe-based complexes exhibit both low negative redox potentials and high positive redox potential, enabling them to serve as a bipolar electrolyte for symmetric RFBs with a cell voltage of over 2 V. The assembled iron-based symmetric NARFB demonstrates a high cell voltage of 2.3 V, columbic efficiency of 97%, energy efficiency as high as 88%, and stable charge-discharge capacity retention of 60% after 160 cycles, corresponding to 99.75 % capacity retention per cycle. Figure 1