# Electrochemical studies of the zinc/zinc(II) and the iron(II)/iron(III) couples in oligomeric electrolytes

#### Abstract

The electrochemical reactions of the Zn/Zn(II) and Fe(II)/Fe(III) couples dissolved in poly(ethylene glycol), PEG (avg. MW = 400), were examined using cyclic voltammetry and chronopotentiometry at microelectrodes. Three electroactive salts were studied: $\rm Zn(CF\sb3SO\sb3)\sb2,$ ZnBr$\sb2,$ and FeBr$\sb3.$ $\rm LiCF\sb3SO\sb3$ and LiBr were also incorporated as nominal supporting electrolytes. The electrochemical reactions of the Fe(II)/Fe(III) couple involved simple redox reactions controlled by diffusion while those of the Zn/Zn(II) couple involved two reactions, namely underpotential deposition and diffusion limited bulk deposition of zinc. The underpotential deposition of zinc was observed on both platinum and gold electrodes. The underpotential shifts due to adsorption on platinum and gold ranged between 0.7-0.9V and 0.5-0.6V respectively, in good agreement with the difference in the work functions of Pt/Zn and Au/Zn. The theory of Wopschall and Shain on the strong adsorption of product was tested by varying the concentration of electroactive species and scan rates. Voltammograms for electrolytes containing $\rm Zn(CF\sb3SO\sb3)\sb2$ and ZnBr$\sb2$ salts were found to be very different. The diffusion limited bulk zinc deposition observed in the $\rm Zn(CF\sb3SO\sb3)\sb2$-PEG electrolytes was absent in the ZnBr$\sb2$-PEG electrolytes. The different electrochemical behavior was attributed to the formation of relatively stable zinc-bromide complexes such as ZnBr$\sb4\sp{2-},$ ZnBr$\sb3\sp-,$ and ZnBr$\sp+,$ in the presence of strongly complexing Br$\sp-$ anions, which are more difficult to reduce than the uncomplexed species. Diffusion coefficients of Zn(II) and Fe(III) species in electrolytes were obtained using chronoamperometry. The diffusion coefficients of Zn(II) species did not vary much with the concentration of Zn(II) at the temperatures studied (23$\sp\circ$C and 90$\sp\circ$C). The diffusion coefficient of Zn(II) was found to be on the order of 10$\sp{-8}$ cm$\sp2$/s at 23$\sp\circ$C and 10$\sp{-7}$ cm$\sp2$/s at 90$\sp\circ$C. The diffusion coefficient of Fe(III) in FeBr$\sb3$-PEG electrolytes could be obtained from both cyclic voltammetry and chronoamperometry at elevated temperatures. The value from the limiting current of voltammetry was found to be 1.4 $\times$ 10$\sp{-6}$ cm$\sp2$/s and that from chronoamperometry was found to be 9.5 $\times$ 10$\sp{-7}$ cm$\sp2$/s at 120$\sp\circ$C, in good agreement with each other. By plotting log(D) as a function of temperature, the activation energy for the diffusion of Fe(III) in PEG was calculated to be 0.22eV.

#### Subject Area

Materials science|Polymers

#### Recommended Citation

Choe, Hyoun Sook, "Electrochemical studies of the zinc/zinc(II) and the iron(II)/iron(III) couples in oligomeric electrolytes" (1992). Dissertations available from ProQuest. AAI9235123.
https://repository.upenn.edu/dissertations/AAI9235123

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