Near Neutral pH Redox Flow Battery with Low Permeability and Long-Lifetime Phosphonated Viologen Active Species
Authors: Jin Martin , Fell Eric M. , Vina-Lopez Lucia , Michalak Peter Winston , Jing Yan , Gordon Roy G. , Aziz Michael J. 
Published: December 2020

Aqueous redox flow batteries are potentially well suited for grid scale energy storage for their uncoupled power and energy, safety, cost-effectiveness and longevity.[1] Although organic aqueous flow batteries with quinone redox active species have demonstrated promising results, including high stability, > 1 V open-circuit potential and high solubility, during operation they tend to be solubility-limited at moderate pH and, due to proton-coupled electron transfer, tend to swing the electrolyte pH to extreme values during cycling.[2-5]Viologens, another class of redox active organic molecules, are soluble regardless of solution pH and their redox reactions do not involve coupled protons or hydroxides, thus enabling stable pH during cycling. However, previously reported viologen-based flow batteries suffer from high capacity fade rates, high active material permeability, or low power density.[6-9] Here we present a highly stable phosphonate-functionalized viologen as the redox-active species in or aqueous redox flow batteries (ARFBs) operating at nearly neutral pH. The solubility is 1.23 M and the reduction potential is the lowest of any substituted viologen utilized in a flow battery, reaching -0.462 V vs. SHE at pH 9. The negative charges in both the oxidized and the reduced states of 1,1'-bis(3-phosphonopropyl)-[4,4'-bipyridine]-1,1'-diium dibromide (BPP
-
Vi) effect low permeability in cation exchange membranes and suppress a bimolecular mechanism of viologen decomposition. A flow battery pairing BPP-Vi with a ferrocyanide-based posolyte across an inexpensive, non-fluorinated cation exchange membrane at pH = 9 exhibits an open-circuit voltage of 0.9 V and an extremely low capacity fade rate of 0.016%/day or 0.00069%/cycle. Overcharging leads to viologen decomposition, causing irreversible capacity fade. This work introduces extremely stable, extremely low-permeating and low reduction potential redox active materials into near neutral ARFBs.

Figure Inserted Here

Figure Caption: Extended cycling at 40 mA cm-2 of a 1 m BPP-Vi | Fe(CN)6 flow cell. Electrolytes comprised 6.2 mL of 1.0 m BPP
-
Vi titrated with 14 m NH4OH to pH = 9 (negolyte) and 40 mL of 0.3 m K4Fe(CN)6 and 0.3 m K3Fe(CN)6 in 2 m NH4Cl at pH = 9 (posolyte). The average coulombic efficiency was >99.95%. The fitted discharge fade rate was 0.016%/day or 0.00069%/cycle.

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