Technical advances in grid-level energy storage are of critical importance in enabling the widespread penetration of renewable resources that meet consumer requirements for on-demand, safe, and uninterrupted power provided at a reasonable cost. Redox flow batteries (RFBs) are electrochemical systems well-suited for multi-hour storage at the low system costs needed for economically viable grid storage. While aqueous RFBs utilizing inorganic active species have been traditionally studied^{1, 2}, non-aqueous redox flow batteries (NRFBs) utilizing organic active species offer the advantages of a larger electrolyte voltage window and the ability to tailor the molecular structure of the active species to achieve desired properties.  Recently, Darling et al. outlined materials-level benchmarks for NRFBs which align with aggressive cost targets established by the U.S. Department of Energy.³  Specifically, for economic feasibility, NRFBs require active materials with disparate redox potentials (V_cell > 3 V), high solubility in electrolyte solutions (4-5 M charge carriers), and high gravimetric capacity (< 150 g/mol e^-).³

Overcharge protection redox shuttles for lithium-ion batteries provide a good basis for molecular derivatization as the materials developed over the past 30 years share many of the same desired properties, including a high potential, high solubility, and long-term stability^{4, 5}.  Using a dimethoxybenzene core⁴, we combine molecular engineering, electrochemical analysis, and materials characterization to understand the role of structure in redox performance and to develop design principles.  In this presentation, we will report on key findings based on studies of several different dimethoxybenzene-based derivatives.  Specifically, we will highlight means of improving solubility⁶, increasing intrinsic capacity⁷, and raising redox potential, all of which lead to enhanced energy density.  For example, Figure 1 shows the impact of halidization on the redox behavior of a series of dimethoxybenzene derivatives. Further, unintended consequences of these derivatizations, such as changes in active species stability, will be contemplated with a focus on improving the success of molecular design campaigns.  

Acknowledgments

We gratefully acknowledge the financial support of the Joint Center for Energy Storage Research.

References

1. A.Z. Weber, et al. J. Appl. Electrochem., 41, 1137-1164 (2011).

2. M.L. Perry and A.Z. Weber, J.Electrochem. Soc., 163, A5064-A5067 (2016)

3. R. Darling, et al. Energy Environ. Sci., 7, 3459-3477 (2014).

4. L. Zhang, et al., Energy Environ. Sci., 5, 8204-8207 (2012).

5. F.R. Brushett, et al. Adv. Energy Mater., 2, 1390-1396 (2012).

6. J. Huang, et al., Adv. Energy Mater., 5, 1401782. (2015).

7. J. Huang, et al.  J. Mater. Chem. A, 3, 14971-14976 (2015).

Figure 1: Comparative cyclic voltammograms of halide-substituted dimethoxybenzene-based showing the shift in redox potential as a function of electron-withdrawing groups.