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

Abstract

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/cm2, capacity retention of up to 99.99% per cycle, and energy efficiency of up to 65% at 60 mA/cm2. 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. Renewable energy (e.g., solar and wind) can make a significant contribution to meeting the increasing global energy demands. However, its successful penetration into the existing electrical grids requires effective energy-storage solutions to overcome its intermittence. Redox flow batteries (RFBs) are a suitable option for large-scale energy-storage applications (up to MW/MWh). There is an urgent call to develop low-cost and benign RFB technologies to meet the burgeoning energy-storage demands. A new generation of aqueous organic RFBs utilizing sustainable and tunable redox-active organic molecules has emerged as a game changer for electrochemical energy storage. In the present study, we report a class of rationally designed highly reductive and high-charge capacity redox-active viologen molecules as a class of two-electron storage anolyte materials that promise aqueous organic RFBs with high voltage, high power density, and high energy density. Liu and co-workers reported a series of rationally designed two-electron storage viologen molecules as anolytes for high-voltage and high-power pH-neutral aqueous organic redox flow batteries. The synthetic and computational chemistry presented has opened a new avenue for designing energy-dense redox-active organic molecules for building neutral AORFBs with high power density and high energy density, and it promises economical, benign, and widespread uses of redox flow batteries in large-scale energy storage.