(556g) Energy Storage in Redox-Active Organic Molecules

Low-cost, scalable energy storage systems are required to improve the energy efficiency of the electrical grid (e.g., loading-level, frequency regulation) and to enable the widespread penetration of intermittent renewable energy sources (e.g., wind, solar) [1].  Redox flow batteries may provide the best combination of cost, efficiency, and scalability to enable these applications.  Present flow battery technologies have low energy densities (< 40 Wh/L), due to the use of aqueous electrolytes with low redox species solubilities (~2 M) and low operating voltages (< 1.5 V, limited by water electrolysis), which, in turn, lead to high system-level costs ($280-450/kWh [2]).  Employing non-aqueous electrolytes offers a wider window of electrochemical stability which enables cell operation at dramatically higher potentials (2-4 V) [3-5].  If appropriate redox couples can be identified, operating at higher cell voltages leads to greater system energy (and power) densities and higher roundtrip efficiencies.  Moreover, to achieve the same output as an aqueous system, fewer stack layers, lower flow velocities, smaller tanks, and fewer ancillaries would be required, significantly reducing hardware costs and enhancing system reliability.

Redox-active organic molecules may be good candidate materials for non-aqueous flow batteries.  This is an attractive approach to energy storage as it enables flexibility to design molecules with favorable properties (i.e., redox potential, activity, solubility) by tailoring either the redox-active moiety or the surrounding molecular structure.  Furthermore, research in overcharge protection materials for lithium (Li)-ion batteries has led to the identification of a number of promising materials and to the establishment of some key design principles.  Here, we will present our research efforts on a non-aqueous Li-ion redox flow battery employing 2,5-di-tert-butyl-1,4-dialkoxybenzene and quinoxaline as the high- and low-potential active species.

[1]  Z. Yang, J. Zhang, M.C.W. Kintner-Meyer, X. Lu, D. Choi, J.P. Lemmon, J. Liu, Chem. Rev., 2011, 111, 3577.

[2]  D. Rastler, Electric Perspectives, 2008, Sept/Oct, 30.

[3]  Y. Matsuda, K. Tanaka, M. Okada, Y. Takasu, M. Morita, T. Matsumura-Inoue, J. Appl. Electrochem., 1988, 18, 909.

[4]  Q. Liu, A.A. Shinkle, Y. Li, C.W. Monroe, L.T. Thompson, A.E.S. Sleightholme, Electrochem. Commun., 2010, 12, 1634.

[5]  J.Y. Mun, M.-J. Lee, J.-W. Park, D.-J. Oh, D.-Y. Lee, S.-G. Doo, Electrochem. Solid-State Lett., 2012, 15, A80.