Enabling Two-Electron Redox Systems for Energy-Dense Organic-Based Flow Batteries

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Redox flow batteries (RFBs) have emerged as a promising grid-scale energy storage technology to meet increasing energy demands. In RFBs, the electroactive materials are in solution stored in external tanks. This separation of the electrode from the electroactive material allows them to be scalable for large-scale operation. Current RFBs use Vanadium as the active material (VRFB). Despite its ability to provide long-duration cycling, VRFBs are limited by the high cost of vanadium and low energy density. To address this issue, organic-based redox flow batteries (ORFB) have gained attention to replace Vanadium. Redox-active organic molecules (ROMs) based on abundant organic structures could reduce costs and offer the possibility to access greater energy densities. As the energy density is proportional to the number of electrons transferred per molecule, ROMs that display two redox-active behaviors can double the electron concentration and energy density. However, multi-electron ROMs are underexplored compared to one-electron systems due to their challenging discovery and synthesis. To advance the class of such systems, structurally simple and abundant dicarbonyl-based compounds were investigated as two-electron negative ROMs. N-Methylphthalimide (N-MePh), a 1,3-dicarbonyl compound, has previously been used as a one-electron ROM, but the instability of the dianion has inhibited its use as a two-electron ROM. A molecular engineering approach was taken to stabilize the dianion and enable two-electron capacity cycling of N-MePh derivatives. Additional dicarbonyl-based compounds are under further investigation to expand the library of multi-electron ROMs for ORFBs. Figure 1