Molecular Design of Bipolar Redox-Active Molecules\nwith Extended p–n Fusion for Symmetric Redox Flow Batteries

Most\nnonaqueous redox flow batteries (RFBs) encounter the issue\nof irreversible capacity loss due to the crossover of redox-active\nmaterials. To mitigate this, an innovative approach is to construct\nsymmetric RFBs with bipolar redox-active molecules (BRMs). Merging\nestablished p- and n-type redox-active moieties within a single molecule\nthrough fused conjugation has proven to be a practical and effective\nstrategy for the preparation of BRMs. Nonetheless, this strategy presents\nsignificant challenges, primarily involving the destabilization of\nredox intermediates caused by direct and excessive electronic perturbation\nas well as the inherently deleterious interactions between the two\ntypes of redox-active centers. This study proposes a robust design\nstrategy for BRMs that incorporate additional p- and n-type redox\nmoieties within the same conjugated structure to enhance redox stability.\nAs a demonstrator, we designed and synthesized a novel class of BRMs,\nmerging two sets of pyrrolic nitrogen and ketone redox-active moieties.\nThe potential and efficacy of these molecules as BRMs for RFBs were\nevaluated in static cells as a proof of concept. A symmetric static\ncell based on one of these molecules achieves a cell voltage of 2.6\nV and demonstrates satisfactory Coulombic and energy efficiencies\nas well as capacity retention over long-term charge–discharge\ncycles. Our study provides innovative structures and an effective\nstrategy for developing high-performance BRMs.