A Highly Soluble Two-Electron Storage Viologen Negolyte for Neutral Aqueous Organic Redox Flow Batteries

Aqueous organic redox flow batteries (AORFBs), exploiting the reversible redox properties of aqueous soluble organic species to store energy, have been considered as a favorable large-scale and long-term energy storage technology for the deployment of renewable energy. Viologen-based species have been demonstrated as excellent negolyte candidates for AORFBs by virtue of their high water solubility, good electrochemical stability, and diverse molecular structure tunability. However, most viologen derivatives display one-electron redox during operation, limiting the output voltage, power, and energy density of AORFBs. Only a couple of reported viologen derivatives could take full advantage of two-electron reversible redox processes, which are mainly enabled by extending the conjugation skeleton of bipyridinium within viologen, demanding multistep synthesis that is detrimental to mass production. In this context, we proposed the 3-(triethylammonio) propyl viologen tetrachloride (BTMEP-Vi) as a negolyte for AORFBs, which could be acquired via a two-step synthesis from a cost-effective raw material with an acceptable yield. BTMEP-Vi demonstrates a water solubility of 2.56 M and possesses two electron-reversible redox processes at -0.34 and -0.70 V vs standard hydrogen electrode, respectively. A flow battery assembled with BTMEP-Vi displayed a high voltage of 1.50 V and a power density of 168.68 mW cm^-2. Additionally, we investigated the cycle stability and discussed the possible reasons causing capacity fade of the assembled AORFBs and proposed a possible degradation mechanism of BTMEP-Vi.