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

Abstract

The introduction of Prussian blue (PB), an inexpensive pigment material, elegantly breaks the solubility limit of the [Fe(CN) 6 ] 4À/3À electrolyte, and substantially boosts the capacity via an off-electrode chemical reaction. In the reversible redoxtargeting reaction cycles, PB acts as the energy reservoir, while [Fe(CN) 6 ] 4À/3À plays a role in mediating the reactions between the electrode and storage tank. The volumetric capacity surpasses other reported [Fe(CN) 6 ] 4À/3À -based and most other organic aqueous redox flow batteries.

Single‐Molecule Redox‐Targeting Reactions for a pH‐Neutral Aqueous Organic Redox Flow Battery

“…Given that the charge transfer is influenced by both the reorganization energy and distance between the redox-active center as depicted by the Marcus theory, the hydrogen bonding mediated reaction mechanism implies that the solvation environment may play an important role in determining the SMRT reaction rate and thus affect the power-to-energy ratio. [30] As for the catholyte counterpart, in view of the compositional versatility and facile SMRT reaction of hexacyanoferrate-based materials as demonstrated recently, [28,36] nickel hexacyanoferrate (NiHCF) was selected among various cathode materials, considering its identical redox potential to NaI (Figure 1 b), superior stability as well as fast kinetics. [37,38] Ex situ FTIR and XPS measurements were conducted to validate the reaction in the NiHCF-NaI/NaI 3 electrolyte system.…”

“…The tank capacity can be feasibly enhanced by increasing the granule loading. [28,30] Taking the achieved 83 % material utilization, the maximum achievable granule loading ratio of about 50 % and active material ratio (52 vol %) into the calculations, 0.2 m 2,7-AQDS anolyte exhibits an achievable volumetric capacity of about 97 Ah L À1 (calculation method can be found in the Supporting information). Paired with the NiHCF/NaI catholyte, the full cell energy density can reach 39 Wh L À1 .…”

“…In conjunction with the ion-insensitive mediators, we envisage that the PI/2,7-AQDS electrolyte system can work as a universal anolyte for the previously studied potassium systems, such as the KBr/PB and TEMPTMA/CuHCF. [28,46] To prove the universality as well as increase the theoretical energy density, a full cell of PI/2,7-AQDS anolyte paired with NH 4 Br/Br 2 catholyte was tested, which provided a higher cell voltage of around 1.3 V and promoted the theoretical tank energy density to 127 Wh L À1 . As shown in the Supporting Information, Figure S18, although the full cell test showed a comparable utilization (62 %) at 5 mA cm À2 as the PI/2,7-AQDS k NiHCF/NaI cell, the pH variation in 2,7-AQDS k NH 4 Br cell was found to be serious due to the acidic nature of bromine, which drove the pH of anolyte to around 1 after cycling, even in the presence of 0.1m NH 4 H 2 PO 4 /(NH 4 ) 2 HPO 4 buffer.…”

“…[21][22][23] The redox-targeting reactions, where solid materials are used as capacity boosters in the energy storage tank (Figure 1 a; Supporting Information, Figure S1), have been demonstrated as an efficient method to considerably improve the effective solubility of redox species in various RFBs such as nonaqueous redox flow lithium or sodium batteries and metal complex-based aqueous RFBs, and so on. [24][25][26][27][28] In this study, the redox targeting reaction of quinone with polyimide was investigated to solve the low-solubility hurdle of 2,7-AQDS in pH-neutral electrolyte system.…”

“…For a minimal potential loss and fast reaction kinetics, we screened solid materials based on potential matching and structure similarity; the former is the basis of the single mediator redox targeting (SMRT) reaction while the latter may facilitate the reaction as suggested by the previous Prussian blue-ferrocyanide system. [28,30] Polyimide (PI), a low-cost polymeric material with tunable potential, was chosen here as the capacity booster for anolyte. The 1,4,5,8-naph-thalenetetracarboxylic dianhydride (NTCDA) derived PI was synthesized, as confirmed by the FTIR spectrum with matched characteristic peaks [31,32] (Supporting Information, Figure S3).…”