Building Stable Radical Cations for Non-Aqueous Redox Flow Batteries

Abstract

E-mail: luzhang@anl.gov, shkrob@anl.gov : In advanced electrical grids of the future, electrochemically rechargeable fluids of high energy density could capture the power generated from intermittent sources like solar and wind. To meet this outstanding technological demand there is a need to understand the fundamental limits and interplay of electrochemical potential, stability, and solubility in low-weight redox-active molecules, among which, stability is the crucial factor for determining the real cycling performance. 1 , 2 The derivatives of 1,4-dimethoxybenzene are thus far the best performing catholyte molecules and have been intensively studied for non-aqueous redox flow batteries (NRFBs). However, most of these molecules have been found by trial and error, and the structural factors that determine durability of their radical cations (and even the nature of reactions controlling the life times of these radical cations) are insufficiently understood. In this report, an exploratory study focusing on the correlation between molecular symmetry and electrochemical stability was conducted. As shown in Fig 1, we examined a large selection of previously reported stable redox molecules3-5seeking for the root causes of the radical cation stability. By comparing their cycling performance, we observed that symmetric molecules are more likely to be stable than asymmetric ones. We rationalize this connection through the effect of symmetry on fragmentation (e.g., deprotonation) of a conformationally frozen radical cation. In asymmetric radical cations, the extra positive charge residing on the functional groups is unevenly distributed between them, causing more rapid fragmentation in the more positively charged groups. Following this observation, we have designed and synthesized several novel structures that are expected to be exceptionally stable redox molecules. Figure 1. Molecular Structures for Redox Active compounds. The Symmetry Group and Estimated Dipole Moments (in Debye) are indicated in the Parentheses. (1) Wei, X.; Xu, W.; Huang, J.; Zhang, L.; Walter, E.; Lawrence , C.; Vijayakumar, M.; Henderson, W. A.; Liu, T.; Cosimbescu, L.; Li, B.; Sprenkle, V.; Wang, W. Angewandte Chemie International Edition 2015, 54, 8684. (2) Wei, X.; Cosimbescu, L.; Xu, W.; Hu, J. Z.; Vijayakumar, M.; Feng, J.; Hu, M. Y.; Deng, X.; Xiao, J.; Liu, J.; Sprenkle, V.; Wang, W. Adv. Energy Mater. 2015, 5, 10.1002/aenm.201570002. (3) Huang, J.; Cheng, L.; Assary, R. S.; Wang, P.; Xue, Z.; Burrell, A. K.; Curtiss, L. A.; Zhang, L. Adv. Energy Mater. 2015, 5, 1401782. (4) Zhang, L.; Zhang, Z.; Redfern, P. C.; Curtiss, L. A.; Amine, K. Energy & Environ. Sci. 2012, 5, 8204. (5) Zhang, Z.; Zhang, L.; Schlueter, J. A.; Redfern, P. C.; Curtiss, L.; Amine, K. J. Power Sources 2010, 195, 4957. Figure 1