Improved Mass Transfer of Methyl Viologen By the Nitrate As Counter Anion for Aqueous Redox Flow

Among the redox active materials, viologen compounds are the most promising redox species, featuring the stable redox reaction MV2+/MV+ and the tunability of the substituents and counter anion, indicating that its electrochemical properties (i.e., diffusion coefficient and electron transfer rate constant) are significantly influenced by the substituents and counter anions[1-8]. Herein, we present the viologen compound employing nitrate as counter anion (MV2+*2NO3 -) as an aqueous redox active material. The MV2+ 2NO3 - was synthesized and characterized by elemental analysis, nuclear magnetic resonance (NMR) spectroscopy, and Raman spectroscopy, and then, the electrochemical properties were studied using cyclic voltammetry and rotating disk electrode techniques. Compared with MV2+ 2Cl- using chloride as the counter anion, which is commonly used an anolyte, it was demonstrated that MV2+ 2NO3 - featured fast mass transfer properties (D = 5.13 x 10-5 cm2 s-1) and comparable kinetics (Ket = 9.68 x 10-3 cms-1. Reference [1] B. Hu, Y. Tang, J. Luo, G. Grove, Y. Guo, T.L. Liu, Improved radical stability of viologen anolytes in aqueous organic redox flow batteries, Chemical communications 54(50) (2018) 6871-6874. [2] B. Hu, C. DeBruler, Z. Rhodes, T.L. Liu, Long-cycling aqueous organic redox flow battery (AORFB) toward sustainable and safe energy storage, Journal of the American Chemical Society 139(3) (2017) 1207-1214. [3] T. Liu, X. Wei, Z. Nie, V. Sprenkle, W. Wang, A total organic aqueous redox flow battery employing a low cost and sustainable methyl viologen anolyte and 4‐HO‐TEMPO catholyte, Advanced Energy Materials 6(3) (2016) 1501449. [4] T. Janoschka, N. Martin, U. Martin, C. Friebe, S. Morgenstern, H. Hiller, M.D. Hager, U.S. Schubert, An aqueous, polymer-based redox-flow battery using non-corrosive, safe, and low-cost materials, Nature 527(7576) (2015) 78-81. [5] B. Dunn, H. Kamath, J.-M. Tarascon, Electrical energy storage for the grid: a battery of choices, Science 334(6058) (2011) 928-935. [6] P.M. Monk, N.M. Hodgkinson, Charge-transfer complexes of the viologens: effects of complexation and the rate of electron transfer to methyl viologen, Electrochimica acta 43(3-4) (1998) 245-255. [7] P. Monk, viologens, Wiley1998. [8] C. Bird, A. Kuhn, Electrochemistry of the viologens, Chemical Society Reviews 10(1) (1981) 49-82.