Near Neutral pH Redox Flow Battery with Low Permeability and Long-Lifetime Phosphonated Viologen Active Species

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 NH 4 OH to pH = 9 (negolyte) and 40 mL of 0.3 m K 4 Fe(CN) 6 and 0.3 m K 3 Fe(CN) 6 in 2 m NH 4 Cl at pH = 9 (posolyte). The average coulombic efficiency was >99.95%. The fitted discharge fade rate was 0.016%/day or 0.00069%/cycle. References 1 T. Nguyen and R.F. Savinell, "Flow Batteries", The Electrochemical Society Interface 54 (2010). 2 B. Huskinson, M.P. Marshak, C. Suh, S. Er, M.R. Gerhardt, C.J. Galvin, X. Chen, A. Aspuru-Guzik, R.G. Gordon, and M.J. Aziz, "A Metal-Free Organic-Inorganic Aqueous Flow Battery", Nature 505 , 195 (2014). 3 D.G. Kwabi, K. Lin, Y. Ji, E.F. Kerr, M.-A. Goulet, D. De Porcellinis, D.P. Tabor, D.A. Pollack, A. Aspuru-Guzik, R.G. Gordon, and M.J. Aziz, "Alkaline Quinone Flow Battery with Long Lifetime at pH 12", Joule 2 , 1907 (2018). 4 Y. Ji, M.A. Goulet, D.A. Pollack, D.G. Kwabi, S. Jin, D. Porcellinis, E.F. Kerr, R.G. Gordon, and M.J. Aziz, "A Phosphonate‐Functionalized Quinone Redox Flow Battery at near‐Neutral pH with Record Capacity Retention Rate", Advanced Energy Materials 9 , 1900039 (2019). 5 S. Jin, Y. Jing, D.G. Kwabi, Y.L. Ji, L.C. Tong, D. De Porcellinis, M.A. Goulet, D.A. Pollack, R.G. Gordon, and M.J. Aziz, "A Water-Miscible Quinone Flow Battery with High Volumetric Capacity and Energy Density", ACS Energy Letters 4 , 1342 (2019). 6 E.S. Beh, D. De Porcellinis, R.L. Gracia, K.T. Xia, R.G. Gordon, and M.J. Aziz, "A Neutral pH Aqueous Organic-Organometallic Redox Flow Battery with Extremely High Capacity Retention", ACS Energy Letters 2 , 639 (2017). 7 B.H. Camden DeBruler, Jared Moss, Jian Luo, T. Leo Liu, "A Sulfonate-Functionalized Viologen Enabling Neutral Cation Exchange, Aqueous Organic Redox Flow Batteries toward Renewable Energy Storage", ACS Energy Letters 3 , 663 (2018). 8 J. Luo, B. Hu, C. Debruler, Y.J. Bi, Y. Zhao, B. Yuan, M.W. Hu, W.D. Wu, and T.L. Liu, "Unprecedented Capacity and Stability of Ammonium Ferrocyanide Catholyte in pH Neutral Aqueous Redox Flow Batteries", Joule 3 , 149 (2019). 9 T. Janoschka, N. Martin, M.D. Hager, and U.S. Schubert, "An Aqueous Redox-Flow Battery with High Capacity and Power: The TEMPTMA/MV System", Angew Chem Int Ed Engl 55 , 14427 (2016). Figure 1