Chemical Speciation of Zinc–Halide Complexes in Zinc/Bromine Flow Battery Electrolytes

Authors:
  • TEThomas K. Ellis
    Thomas K. Ellis
  • AAAlexander P. Adams
    Alexander P. Adams
  • BSBehdad Soltani
    Behdad Soltani
  • RZRenwu Zhou
    Renwu Zhou
  • PCPatrick J. Cullen
    Patrick J. Cullen
  • AVAnthony M. Vassallo
    Anthony M. Vassallo
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Abstract
Zinc/bromine flow batteries are a promising solution for utility-scale electrical energy storage. The behavior of complex Zn–halogen species in the electrolyte during charge and discharge is currently not well-understood, and is an important aspect to be addressed in order to facilitate future electrolyte formulations. The speciation of the primary zinc bromide electrolyte with and without a secondary zinc chloride electrolyte is studied in the present work. Raman spectroscopy was carried out on aqueous solutions of zinc bromide at 5 concentrations (2–4 M) to account for the initial and later stages of charging, with 3 concentrations (1–2 M) of zinc chloride. Mixed solutions containing various combinations of each primary and secondary electrolyte concentrations were also studied. Semi-quantitative analysis of peaks after Gaussian and Lorentzian peak deconvolution showed that the proportion of four-ligand coordinated Zn–halides (i.e. [ZnBr 4 ] 2− and [ZnCl 4 ] 2− ) increases with higher salt concentration, as compared to complexes with lower halide coordination numbers. The presence of a previously unassigned peak was observed at the 220 cm −1 band in the Raman spectra of mixed electrolytes. Results from ab-initio molecular modeling using the GAUSSIAN 16 software package suggests this peak is due to the presence of the hybrid-halide anionic complex [ZnBr 2 Cl(H 2 O)] – . Increasing the Cl:Br ratio in electrolytes promotes hybridization and subsequently decreasing the proportion of single-halide Zn–Br complexes. While this speciation study is focused on Zn/Br batteries, the findings are also potentially applicable to other energy storage and electrochemical systems containing zinc halide electrolytes.

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