Jian Luo1 Bo Hu1 Camden Debruler1 Maowei Hu1 Wenda Wu1 Tianbiao Liu1

1, The Department of Chemistry and Biochemistry, Utah State University, Logan, Utah, United States

To efficiently utilize the intermittent renewable energy source such as wind and solar energy and achieve sustainable society, advanced large-scale energy storage technologies are highly demanded. Among various energy storage devices, aqueous organic redox flow batteries (AORFBs) are one of the most promising battery technologies for large scale storage of intermittent energy because of a number technological merits including decoupled energy and power, higher current and high power performance, safety features, and synthetic tunability of charge storage molecules. Herein, we developed a serial of low-cost, highly water soluble sulfonate functionalized viologens as anolyte for AORFB applications. The negative charged sulfonate pendant side chains and favorable molecular sizes of these viologens enable their compatibility with cation exchange membranes. The newly designed viologens were paired with low-cost I3-/I-, Br3-/Br-, and [Fe(CN)6]3-/[Fe(CN)6]4- catholytes in RFBs using cation exchange mechanism. The 1,1’-bis(3-sulfonatopropyl)-4,4’-bipyridinium, (SPr)2V/I RFB delivered 1.0 V battery voltage and reliable battery performance under a pH neutral condition. When the (SPr)2V was paired with a newly designed organometallic catholyte material (NH4)4[Fe(CN)6], a 0.9 M pH neutral (SPr)2V/[Fe(CN)6]4- system delivered unprecedented storage capacity and cycling stability, specifically, 24.1 Ah/L electrolyte capacity, 62.6% energy efficiency and 100% capacity retention in 1000 cycles (more than 45 days testing duration), which represents the best cycling stability among all reported organic RFBs. To further improve battery voltage and energy density, the (SPr)2V was paired with Br3-/Br- catholyte. The (SPr)2V/Br RFB delivered a battery voltage of 1.51V, 78% energy efficiency, and up to a 30.4 Wh/L operated energy density in a 1.5 M battery. Other asymmetric sulfonate functionalized viologens, such as (SEt)(SPr)V and (SPr)(SBu)V, with even higher chemical stability and water solubility were also prepared for AORFB applications.