The Influence of Supporting Electrolytes on Zinc Half-Cell Performance in Zinc/Bromine Flow Batteries

Abstract

Five supporting electrolytes were studied for their viability as alternatives in the zinc half-cell of a zinc/bromine (Zn/Br) flow battery. The secondary electrolytes studied included sodium salts of the following anions: Br -, SO 4 2-, H 2 PO 4 -and NO 3 -, which were compared against the conventionally employed Cl -. Cyclic voltammetry and Tafel analysis showed improved electrochemical performance from electrolytes containing NaBr, Na 2 SO 4 and NaH 2 PO 4 . Consequently, these chemicals are proposed as potential alternatives in future Zn/Br design work. Electrochemical impedance spectroscopy revealed that the lowering of charge-transfer resistance and diffusion limitation was the contributing reason toward improved performance from those electrolytes. Scanning electron microscopy and X-ray diffraction of zinc electrodeposits obtained during charging showed the type of supporting electrolyte present alters zinc crystallinity. Generation of smaller crystals was related to observations of good half-cell performance during voltammetry. Mossy deposits were linked with higher nucleation overpotentials between zinc plating/de-plating. The well-performing Na 2 SO 4 supporting electrolyte produced mossy deposits, suggesting that contrary to common assumption, such deposition behavior is possibly unrelated to poor zinc-side performance. While the proposed compounds are intended for Zn/Br flow battery applications, they are possibly adaptable to other types of flow batteries utilizing the Zn 2+ /Zn redox couple. Flow batteries are an attractive option for storing electricity at the utility-scale, offering benefits such as the increased uptake of intermittent renewable power sources such as solar and wind.1-8 Zinc/bromine redox flow batteries (Zn/Br RFBs) are a viable candidate for such applications due to factors such as a high theoretical specific energy (440 Wh kg -1 ) 9,10 and relatively low cost of the materials of construction. Consequently, it is of commercial, environmental and social interest to pursue materials research leading to improved performance of such systems.During the charging phase of the Zn/Br battery, the following reactions occur within the zinc and bromine half-cells, respectively (Eqs. 1-2):The reverse occurs during the discharge phase, whereby the primary electrolyte (i.e. ZnBr 2 ) is regenerated. The Zn/Br electrolyte also contains a bromine sequestration agent which complexes with Br 2 evolved at the positive electrode to form a separate phase which is immiscible with the aqueous electrolyte. Organic quaternary ammonium bromides such as 1-ethyl-1-methylpyrrolidinium bromide are commonly employed as sequestration agents. 11,12Dendrite formation during the charging phase has long been a major observation and source of concern during the design of RFBs which employ zinc electrodeposition, with various strategies employed to minimize this issue. [13][14][15][16] Dendrites increase operational risks by causing problems such as short circuiting of cells. It is also beneficial to promote efficient e...

An Energy‐Dense, Powerful, Robust Bipolar Zinc–Ferrocene Redox‐Flow Battery

“…The bipolar, symmetric Zn−Fc AORFB was examined using 0.5 M Zn[Fc(SPr) 2 ] (13.4 Ah L −1 and 7.6 Wh L −1 ) in 3.0 M NH 4 Br aqueous solution as both an anolyte and a catholyte, as well as a low‐cost Daramic membrane ($1/m 2 ). It should be noted that the use of the NH 4 Br supporting electrolyte instead of NH 4 Cl is because the Br − anion is more beneficial than the Cl − anion for the Zn deposition and stripping, including faster deposition/stripping kinetics and smoother Zn morphology [27] . Because of the divalent nature of the Zn 2+ ion, the volume ratio of the catholyte and anolyte was 2 : 1, giving an energy density of 10.1 Wh L −1 .…”

“…It should be noted that the use of the NH 4 Br supporting electrolyte instead of NH 4 Cl is because the Br À anion is more beneficial than the Cl À anion for the Zn deposition and stripping, including faster deposition/stripping kinetics and smoother Zn morphology. [27] Because of the divalent nature of the Zn 2 + ion, the volume ratio of the catholyte and anolyte was 2 : 1, giving an energy density of 10.1 Wh L À 1 . The 0.5 M ZnÀ Fc AORFB was charged/discharged within a voltage window of 0.2 and 1.35 V. As shown in Figures 4B and C, the 0.5 M Zn[Fc(SPr) 2 ] symmetric AORFB was smoothly cycled under high current densities of up to 200 mA cm À 2 .…”

Zinc–Bromine Batteries: Challenges, Prospective Solutions, and Future

“…Recently, dual halogen electrolytes were reported to increase the working voltage window and provide higher energy density due to the combination of different redox reactions. [131][132][133] For instance, Liu et al reported a Br and Cl-containing molten hydrate electrolyte. [134] The dual ion electrolyte enabled the possibility of ion intercalation in carbon materials used at the cathode side (Figure 18a).…”

The characteristics and performance of hybrid redox flow batteries with zinc negative electrodes for energy storage

“…Rajarathnam et al [86] have studied the effect of 0.5 mol dm -3 Br-, SO4 -, H2PO4and NO3anions on the electrode kinetics of the Br -/Br2 reaction. As shown in Figure 6, these supporting electrolytes resulted in different deposit morphologies.…”