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Aqueous electrolyte for Zinc Bromine Batteries

The operating environment for ZBRBs is relatively challenging: the electrolyte mainly consists of 1-2 M ${\text{ZnBr}}_2$ with $\text{KCl}$ and ${\text{ZnCl}}_2$ as supporting salts which increase ionic conductivity. The pH is typically highly acid, in the range of 2-5. In addition, ${\text{Br}}_2$ and polybromide species (${\text{Br}}_3⁻$, ${\text{Br}}_5⁻$, etc.) are highly corrosive and chemically aggressive, which makes material stability and chemical resistance critical for separator design.

This low pH avoids formation of Zinc oxide/hydroxide species, as predicted by solubility-pH charts, that may passivate the anodic surface and decrease the capacity of the battery.

For bromine, at such acidic pH values, there is an equilibrium predicted between bromine gas and polybromide ${\text{Br}}_3⁻$, of which the latter is the dominant species

Image : Theoretical distribution of the equivalent bromine species as a function of pH for c(Br eq ) = 0.1 M, thin dash lines-for the constant c(Br -) = 0.131 M, thick full lines-for the bromine concentration shown in the inset

ZBRB electrolyte preparation protocol

In Bin Luo's lab the negolyte used for ZBRBs is an aqueous solution of 2.0 M ZnBr₂ + 3.0 M KCl, and the posolyte is the same but with 0.4 M MEP, and whilst their industry partner Redflow also added ZnCl₂ as a supporting salt, they do not.

Safety : zinc bromide is corrosive and toxic, and must be handled with proper PPE

For 100 mL of final posolyte and 100 mL of final negolyte (makes two flow cells):

• Anhydrous ZnBr₂ at 99%: 2 M at 225.2 g/mol → 45.04 g
• KCl: 3 M at 74.55 g/mol → 22.365 g
• MEP bromine-complexing agent: 0.4 M at 194.12 g/mol → 7.765 g for 1-ethyl-1-methylpyrrolidinium bromide (ONLY FOR THE POSOLYTE)

Beware : these numbers are for the anhydrous salts in the lab. If new samples of ZnBr₂ or KCl are hydrated, the gram amounts need to be corrected to the hydrate’s molar mass.

Procedure :

1. Add about 20 mL deionised water to a volumetric flask
2. Add and dissolve ZnBr₂
3. Add KCl and mix corked flask until clear
4. Add the MEP bromine-complexing agent and mix
5. Make up to 100 mL with deionised water, mix thoroughly leaving no bubbles

Notes :

• Add Zn salts gradually; these solutions can warm on dissolution.
• The volume of salts added is non negligible with regards to solution volume : be careful to not add too much water
• Use corrosion-resistant labware and good ventilation.
• Because bromine-complexing agents can change phase behaviour, mix until visually uniform.

Further reading :

• Another 2025 paper by Alghamdi et al. used a ZnBr₂ aqueous electrolyte with DMSO as a co-solvent additive, comparing 0% DMSO (control), 10% DMSO, and 30% DMSO.