Quantitative determination of chemical species in high concentration ZnX2 (X = Br and I) media by steady state voltammetry on Pt ultramicroelectrode

Abstract

In this article, we present the voltanunetric approach to quantitatively determine zinc-halides (Zn-X-, X = Br and I) complexes in the high concentration ZnX2 solutions by the measurements of the steady state current, i(ss) associated with electrochemical oxidation of X- on Pt ultramicroelectrode (UME). At first, i(ss) from electrooxidation of uncoordinated I- as a function of total concentration of I-, CI-,total in the ZnI2 solutions were deviated from the theoretical 4, under the pure diffusion-controlled condition, i(ss),(diffusion) to the lower values, respectively. The deviation from i(ss),(diffusion) to the lower iss in the ZnI2 solutions cannot be explained by migration but is associated with Zn-I- complexation. From the Raman spectra, both ZnI3- and ZnI42- exist at C-ZnI2 > 0.25 M. Since only free I- can be electrochemically oxidized, CI-, free is decreased as C-ZnI2 becomes higher due to the consumption of free I- to form ZnI3- and ZnI42-. Therefore, CI-, free can be determined by the stability constants of ZnI3- and ZnI42-, beta(3,) (zn -I-) and beta(4,) (zn -I-) which were estimated by the finite element analysis under the migration model. From beta(3), (Zn - I) and beta(4), (Zn - I), the accurate concentration of chemical species (I-, Zn2+, ZnI3-, and ZnI42-) were able to be quantitatively determined. beta(2, Zn - Br), beta(3, Zn - Br), and beta(4, Zn) (- Br) in the ZnBr2 solutions were also estimated by the same analytical approach, and the concentration of the chemical species (Br-, Zn+, ZnBr3-, and ZnBr42-) were plotted as a function of CBr-, total. The concentration of the chemical species in the ZnX2 solutions are further predicted as Cx(-), total becomes 14 M, and the lowest limit of energy density in the suggested ZnX2 redox flow batteries (RFBs) is also discussed based on the estimated Cx(-), free as a function of Cx(-), total.