A Sulfonate-Functionalized Viologen Enabling Neutral Cation Exchange, Aqueous Organic Redox Flow Batteries toward Renewable Energy Storage

Redox flow batteries using synthetically tunable and resource abundant organic molecules have gained increasing attention for large-scale energy storage. Herein we report a sulfonate-functionalized viologen molecule, 1,1′-bis­(3-sulfonatopropyl)-4,4′-bipyridinium, <b>(SPr)</b><sub><b>2</b></sub><b>V</b>, as an anolyte in neutral aqueous organic redox flow batteries (AORFBs) functioning through a cation charge-transfer mechanism. Demonstrated <b>(SPr)</b><sub><b>2</b></sub><b>V</b>/KI AORFBs manifested high current performance from 40 to 100 mA/cm<sup>2</sup> with up to 71% energy efficiency. In extended cycling studies, the <b>(SPr)</b><sub><b>2</b></sub><b>V</b>/KI redox flow battery delivered stable cycling performance at 60 mA/cm<sup>2</sup>, up to 67% energy efficiency, and 99.99% capacity retention per cycle. Density functional theory modeling of the electrostatic charge surface of <b>(SPr)</b><sub><b>2</b></sub><b>V</b> and its charged state, <b>[(SPr)</b><sub><b>2</b></sub><b>V]</b><sup><b>–1</b></sup>, suggests charge repulsion and size exclusion enable their compatibility with a cation exchange membrane. The present findings expand the battery design of neutral viologen AORFBs and represent an attractive RFB technology for sustainable and benign renewable energy storage.