A Stable and High-Capacity Redox Targeting-Based Electrolyte for Aqueous Flow Batteries

Abstract

© 2019 Elsevier Inc. Aqueous redox flow batteries (ARFBs) have received considerable attention for large-scale energy storage because of their salient feature of decoupled energy storage and power generation; however, their deployment is critically constrained by low energy density and relatively high cost. Here, we report a low-cost, high-capacity ferrocyanide/ferricyanide ([Fe(CN)6]4−/3−)-based electrolyte system via the redox targeting reactions with Prussian blue (Fe4[Fe(CN)6]3, PB). The [Fe(CN)6]4−/3−-PB electrolyte exhibits an excellent capacity retention of 99.991% per cycle and an unprecedented capacity of 61.6 Ah L−1. A Zn/[Fe(CN)6]3−-PB flow cell with energy density of 97.4 Wh L−1 at 20 mA cm−2 and a [Fe(CN)6]4−/3−/Br− flow cell with PB as the sole solid material were demonstrated. The battery chemistry and associated redox targeting reactions were scrutinized with computational, neutron diffraction, and spectroscopic studies. The ultra-stable and capacity-intensive [Fe(CN)6]4−/3−-PB electrolyte system presents an intriguing paradigm for advanced cost-effective large-scale energy storage. The ever-growing penetration of renewable energy necessitates a large-scale energy storage system to buffer the intermittent and wavering solar and wind electricity when incorporating into the power grids. Aqueous redox flow batteries (ARFBs) attract considerable attention for their great safety, scalability and operation flexibility. Ferrocyanide/ferricyanide ([Fe(CN)6]4−/3−), as a robust and low-cost redox-active material for neutral ARFBs, is obstructed by the low solubility leading to a low volumetric capacity. Here we report an ultra-stable and cost-effective [Fe(CN)6]4−/3−-based electrolyte system with Prussian blue (PB) as a low-cost capacity booster. This new electrolyte system presents unprecedentedly high capacity via the reversible redox-targeting reaction of [Fe(CN)6]4−/3− with PB. Such a concept is applicable to other flow battery systems for enhanced energy density. The introduction of Prussian blue (PB), an inexpensive pigment material, elegantly breaks the solubility limit of the [Fe(CN)6]4−/3− electrolyte, and substantially boosts the capacity via an off-electrode chemical reaction. In the reversible redox-targeting reaction cycles, PB acts as the energy reservoir, while [Fe(CN)6]4−/3− plays a role in mediating the reactions between the electrode and storage tank. The volumetric capacity surpasses other reported [Fe(CN)6]4−/3−-based and most other organic aqueous redox flow batteries.