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

TLDR

Redox flow batteries based on tunable, abundant organic molecules are increasingly viewed as promising for large‑scale energy storage. This study introduces a sulfonate‑functionalized viologen, (SPr)₂V, as a neutral aqueous anolyte that operates via a cation charge‑transfer mechanism. Density functional theory shows that electrostatic repulsion and size exclusion of (SPr)₂V and its anionic state enable compatibility with cation‑exchange membranes. The (SPr)₂V/KI battery delivers high‑current operation (40–100 mA cm⁻²) with up to 71 % energy efficiency, maintains 67 % efficiency and 99.99 % capacity retention at 60 mA cm⁻² over extended cycling, and demonstrates a viable design for sustainable, benign renewable energy storage.

Abstract

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, (SPr)2V, as an anolyte in neutral aqueous organic redox flow batteries (AORFBs) functioning through a cation charge-transfer mechanism. Demonstrated (SPr)2V/KI AORFBs manifested high current performance from 40 to 100 mA/cm2 with up to 71% energy efficiency. In extended cycling studies, the (SPr)2V/KI redox flow battery delivered stable cycling performance at 60 mA/cm2, up to 67% energy efficiency, and 99.99% capacity retention per cycle. Density functional theory modeling of the electrostatic charge surface of (SPr)2V and its charged state, [(SPr)2V]–1, 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.