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

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