Strain-Induced Modulation of Redox Potential and Intermolecular Interactions in <i>N</i> -Phenothiazines for Aqueous Organic Flow Batteries

Aqueous organic flow batteries (AOFBs) hold great potential for stationary energy storage due to abundant resources, flexible tunability, and high safety. Phenothiazines with a stable π-conjugated structure are representative catholytes for AOFBs. However, the relatively low redox potential and molecular aggregation at concentrated conditions constrain the AOFB’s energy density and energy efficiency. Herein, we propose a strain-induced strategy through bridge-N functionalization, which can not only precisely regulate redox potential via molecular strains but also effectively weaken the intermolecular interaction. We find a near-linear correlation between the redox potential and dihedral angles of radical cations and oxidized species, which offers a new route to predict the redox potential of prescreened candidate molecules. Meanwhile, enabled by the low viscosity and fast diffusion coefficient, the 10-ethyl-3,7-bis(dimethylamino)phenothiazine (EDAP)-based AOFBs deliver a high discharge capacity of 84.7 Ah L –1 and a high energy efficiency of 70.0% at 200 mA cm –2 .