Redox flow batteries (RFBs) are emerging as the preferred choice for Mega-watt scale energy storage for the future grids in large part to the decoupled energy/power outputs from the storage system [1]. However, narrow electrochemical stability window of water limits the operating voltage of aqueous flow batteries to < 1.6 V and poses an obstacle to achieving high capacity aqueous RFBs. A higher capacity (energy density) can be obtained if water is replaced with organic solvents such as acetonitrile processing a broader electrochemical stability window (>5 V). While organic solvents increase the cost per kilowatt-hour of the electrolyte this can be offset by the greater energy storage capacity of the RFBs [2]. This tradeoff can be beneficial if less-expensive metal-ligand redox couples are used in non-aqueous RFBs, especially those based on Fe on both the anolyte and catholyte [3]. The other significant cost factor of RFBs is expensive perfluorinated Nafion^® membranes. Therefore, the combination of less-expensive redox couples and hydrocarbon membranes could lead to the DOE long term target of 100 USD per kWh for RFBs.

To achieve these strict economic targets, we have combined the expertise of inorganic synthesis and fuel cell capabilities at the Los Alamos National Laboratory’s towards non-aqueous flow battery development. We are systematically studying the structure-property relationship of Iron and Nickel-containing inorganic redox-active couples as anolyte and catholyte in RFBs. Design aspects of these redox couples and electrochemical characterization will be discussed in this presentation. Besides, results will be presented on specifically tailored anion exchange membrane with high compatibility with organic solvents and the active redox couples. In addition to the component work, results of flow battery testing using these novel membranes and electrolytes will be presented. A voltage window >3 V has been achieved in preliminary results using the Ni and Fe systems. The reversibility of these redox couples and their potential for long-term operation in RFBs will be discussed.

Acknowledgement

This work is supported by Laboratory Directed Research & Development, Los Alamos National Laboratory.

References:

[1] S.H. Shin, S.H. Yun, S.H. Moon, RSC Advances, 3 (2013) 9095-9116.

[2] R.M. Darling, K.G. Gallagher, J.A. Kowalski, S. Ha, F.R. Brushett, Energy & Environmental Science, 7 (2014) 3459-3477.

[3] J. Friedl, M.A. Lebedeva, K. Porfyrakis, U. Stimming, T.W. Chamberlain, Journal of the American Chemical Society, 140 (2018) 401-405.