Mediated alkaline flow batteries: from fundamentals to application

Abstract

Alkaline flow batteries are attracting increasing attention for stationary energy storage. Very promising candidates have been proposed as active species for the negative compartment, while potassium ferrocyanide (K4Fe(CN)6) has been the only choice for the positive one. The energy density of this family of batteries is limited by the low solubility of K4Fe(CN)6 in alkaline media. Herein, we propose a general strategy to increase the energy density of this family of alkaline flow batteries by storing energy in commercial Ni(OH)2 electrodes confined in the positive reservoir. In this way, K4Fe(CN)6 dissolved in the electrolyte acts not only as electroactive species but also as charge carrier between current collector and solid Ni(OH)2 particles located in an external reservoir. A storage capacities of 29 Ah L–1 for the positive compartment is demonstrated. The concept is implemented in three systems, Zn–K4Fe(CN)6, anthraquinone–K4Fe(CN)6, and phenazine–K4Fe(CN)6 alkaline flow battery, showing the versatility of the strategy. Challenges and future directions to exceed the 16 Wh Ltotal–1 demonstrated in this work are discussed.