Merging Flow and Non-Flow Batteries: K4fe(CN)6 Electrolyte – Ni(OH)2 Solid Material As Proof of Concept

Abstract

Ferrocyanide (FCY) based alkaline redox-flow batteries are considered promising alternatives to traditional redox-flow batteries that are based on high cost and strategic metals and corrosive acidic media. FCY has been used as active species in the positive side of a variety of redox-flow batteries, e.g. anthraquinone/FCY[1] and phenazines/FCY[2]. However, in all these cases, the energy density is limited by the low solubility of FCY in alkaline media (0.4 M equivalent to 10 AhL-1). In contrast, electroactive solid species that are frequently used in non-flow batteries are able to store more charge per unit volume than soluble electroactive species, e.g. Ni(OH)2 has a volumetric capacity of 1180 Ah·L-1 that is two orders of magnitude higher than that of a FCY solution. However, solid materials cannot be used in flow batteries as there is no electrical contact between the electrode and the solid located in the external tank. In this talk, we will show that soluble electroactive species can be also used as molecular wiring to transport charges to high-energy solid materials confined in the external reservoir.[3] We demonstrate that by adding solid material in the reservoir, volumetric capacities of >25 AhL-1 are achieved (40% utilization rate of Ni(OH)2). We apply the concept in two types of FCY flow batteries: quinone/FCY and phenazines/FCY. For the latter, energy densities of >15 WhL-1 are demonstrated representing the highest value for an alkaline flow battery. This work open up the field for a new generation of batteries, which benefits from the advatages of flow and non-flow configurations. References Lin, K. et al. Alkaline quinone flow battery. Nat. Energy 349, 1–4 (2015).Hollas, A. et al. A biomimetic high-capacity phenazine-based anolyte for aqueous organic redox flow batteries. Nat. Energy 3, 508–514 (2018). Ventosa, E.; Paez, T.; Palma, J, Redox flow battery for energy storage, European Patent Application, EP18382971 Figure 1. a Scheme of a redox-flow battery based on the use of ferrocyanide-Ni(OH)2 in the positive container. b Reversible stored charge during the first 20 cycles of a 2,6-dihydroxi- anthraquinone // ferrocyanide flow battery in the absence (black circles) and in the presence (red circles) of solid Ni(OH)2 in the external positive container. Figure 1