Designer Two-Electron Storage Viologen Anolyte Materials for Neutral Aqueous Organic Redox Flow Batteries

Aqueous organic redox flow batteries (AORFBs) are highly attractive for large-scale energy storage because redox-active organic molecules are synthetically tunable, sustainable, and potentially low cost. Here, we show that rational molecular engineering yielded a series of two-electron storage viologen molecules as anolyte materials for AORFBs. In neutral NaCl solutions, these viologen anolytes have a theoretical capacity of up to 96.5 Ah/L in H2O and exhibit a reduction potential as low as -0.78 V versus normal hydrogen electrode. The neutral aqueous flow batteries with two two-electron storage viologen molecules delivered a cell voltage of up to 1.38 V and outstanding battery performance, including a power density of up to 130 mW/cm(2), capacity retention of up to 99.99% per cycle, and energy efficiency of up to 65% at 60 mA/cm(2). Density functional theory calculations revealed that the 1e(-) and 2e(-) reduced redox states of these molecules were stabilized by the high charge delocalization of their frontier SOMO or HOMO.