State-of-Charge (SOC) Indicators for Alkaline Zinc/Air Redox Flow Batteries

Flexibility in power grid operation will become of paramount importance as the percentage of power generation by intermittent and difficult-to-predict energy sources, like solar and wind power, grows. Storage is an attractive technology to achieve flexibility. It maximizes generation utilization without affecting when and how consumers use electrical power [1]. Due to the low cost, environment-friendliness and wide availability of the active materials, the zinc/air redox flow battery is a promising candidate for stationary energy storage [2]. The reversible air electrode in one half-cell reaction supersedes storage volume for the positive active material, leading to an increased energy density of the system. In order to predict the available energy of the battery and to prevent side reactions, like hydrogen evolution or the formation of zinc dendrites, the state of charge (SOC) has to be known.
We present a study of SOC-determination for the zinc/air redox flow battery. It is possible to determine the SOC from the zinc concentration of the negative electrolyte. The rest potential of the zinc electrode, electrolyte conductivity, electrolyte density, electrolyte refractive index and the limiting current of zinc electrodeposition were investigated at different zinc concentrations and temperatures in NaOH- and KOH-electrolytes. Electrolyte density, electrolyte conductivity and the zinc concentration as well as rest potential and the logarithm of the zinc concentration showed linear correlations.

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[1] C. J. Barnhart, S. M. Benson, Energy Environ.Sci., 6 (2013) 1083-1092.
[2] C. Ponce de León, A. Frías-Ferrer, J. González-García, D. A. Szánto, F. C. Walsh, Journal of Power Sources, 160 (2006) 716-732.