Chemical Speciation of Zinc–Halide Complexes in Zinc/Bromine Flow Battery Electrolytes. Issue 7 (13th July 2021)

• Record Type: Journal Article
• Authors: Rajarathnam, Gobinath P.; Ellis, Thomas K.; Adams, Alexander P.; Soltani, Behdad; Zhou, Renwu; Cullen, Patrick J.; Vassallo, Anthony M.
• Is Part Of: Journal of the Electrochemical Society. Volume 168:Issue 7(2021)
• Publication Date: 2021-07-13
• Subjects: Electrochemistry -- Periodicals; 541.3705
• DOI: 10.1149/1945-7111/ac1034
• Languages: English
• ISSNs: 0013-4651

Abstract

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. [ZnBr4 ] 2− and [ZnCl4 ] 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 [ZnBr2 Cl(H2 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.