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

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

During the last decades, the demand for environmental friendly renewable power source has grown rapidly to achieve the sustainable society. To efficiently utilize the widespread but intermittent renewable energy source such as solar and wind energy, low-cost and reliable large-scale energy storage technologies were required. Aqueous organic redox flow batteries (AORFBs) with the merits of decoupled energy and power, higher current and high power performance, safety features, synthetic tunability of charge storage molecules, and potentially low-cost, are ideal solution for the intermittent energy storage and electricity grids balancing. Herein, we designed and synthesized sulfonate functionalized viologen compound [1,1’-bis(3-sulfonatopropyl)-4,4’-bipyridinium, (SPr)2V] as a low-cost, highly soluble anode material for pH neutral AORFB application. DFT computational studies suggested charge repulsion and size exclusion enable the compatibility of (SPr)2V with a cation exchange membrane. The superior chemical stability of the (SPr)2V anolyte was confirmed by a 0.5 M half-cell (SPr)2V/(SPr)2V.- battery test. Paired with the inexpensive I3-/I- catholyte, the (SPr)2V/I neutral AORFB delivered 1.0 V battery voltage and up to 71% energy efficiency with 99.99% capacity retention per cycle. When combined with (NH4)4[Fe(CN)6], a 0.9 M or 24.1 Wh/L symmetric (SPr)2V/(NH4)4[Fe(CN)6] delivered ultra-stable cycling performance, nearly no capacity decay for 1000 cycles. To further boost the battery voltage and energy density of the (SPr)2V-based AORFBs, Br3-/Br- catholyte was chosen to combine with (SPr)2V for AORFB demonstration. The (SPr)2V/Br AQRFB delivered 1.51 V battery voltage and 30.4 Wh/L operated energy density due to the high potential of Br3-/Br- redox couple and high solubility of (SPr)2V anode material. Furthermore, the (SPr)2V/Br AORFB maintained excellent energy and power performance under 1.5 M concentration, respectively, up to 78% energy efficiency and 228 mW/cm2 power density, which is the highest power density ever reported in neutral AORFBs.

1. DeBruler C.; Hu, B.; Moss, J.; Luo, J.; Liu, T. L. A Sulfonate Functionalized Viologen Enabling Neutral Cation Exchange Aqueous Organic Redox Flow Batteries towards Renewable Energy Storage ACS Energy Letters 2018, 3, 663-668.
2. Luo, J. (co-1st author); Hu, B. (co-1st author); DeBruler C. Zhao, Y., Yuan B. Hu, M. Wu, W. Liu, T. L.* "Unprecedented Capacity and Stability of Ammonium Ferrocyanide Catholyte in pH Neutral Aqueous Redox Flow Batteries", Joule, 2019, 3, 1-15.