Ion-pair and solvation engineering for high-potential organic catholytes in aqueous flow batteries

Abstract

The low redox potentials (<1.0 V vs. SHE) of many organic catholytes restrict the operating voltages of aqueous organic redox flow batteries (AORFBs). Herein, ion-pair and solvation engineering of 4-nitrophenethylamine hydrobromide (4-NPEA·HBr), a commercially available pharmaceutical intermediate, is used to realize a high-potential aqueous organic catholyte (1.24 V vs. SHE) with favorable transport characteristics. In 1 M HCl, the nitro to hydroxylamine proton-coupled electron transfer operates at 1.24 V vs. SHE with fast interfacial kinetics and an apparent diffusion coefficient on the order of 10⁻⁵ cm² s⁻¹. Spectroscopy and multi-scale simulations (DFT, MD, COMSOL) show that bromide forms dynamic solvent-separated ion pairs with the protonated amine, producing a more flexible solvation environment that enables faster molecular transport. In contrast, the chloride salt forms tighter interactions and displays a simulated diffusion coefficient nearly an order of magnitude lower. Flow-cell tests with 4-NPEA·HBr achieve a peak power density of 69.5 mW cm⁻² at 140 mA cm⁻², voltage efficiency above 95 percent between 5 and 60 mA cm⁻², and stable cycling for more than 100 cycles at 20 mA cm⁻² with coulombic efficiency above 85%. Coupling 4-NPEA·HBr with a zinc anode yields a full-cell battery with a ca. 1.65 V discharge plateau and energy efficiency around 78%. These results establish ion-pair and solvation engineering of low-cost nitrophenethylamine salts as an effective strategy to create high-potential, transport-favorable organic catholytes for aqueous flow batteries.