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Corresponding Author

Lei Cheng(chenglei@gxnu.edu.cn)

Abstract

Glassy carbon electrode (GCE) is a common basic electrode for various electrochemical sensors, and the detection properties are determined by its interfacial characteristics. In this paper, we established an equivalent circuit including electrolyte resistance (Rel), charge transport resistance (Rct), diffusion impedance (Rdi, Cdi), electrochemical (oxidation/reduction) reaction impedance (RR, CR), surface adsorption impedance (Rads , Cads), double-layer capacitance (CDL), and derived the mathematical expression for the equivalent circuit. The Rel and CDL are contributed by inactive ions in electrolyte to produce non-faradaic impedance, while the Rct and RR are contributed by the active ions of redox reaction in electrolyte to produce faradaic impedance. The Rct directly corresponds to the electrode potential (E) of the reaction, which represents the difficulty of electrode reaction. When the potential E is the only state variable in the impedance spectrum of electrode reaction, that is, there is only one time constant in the impedance spectrum, the Rct can represent the whole Faraday impedance of the system. However, when the electrode reaction is also affected by other variables such as diffusion, surface film or surface adsorption ion coverage, the Faraday impedance of the system also includes the impedance produced by the diffusion impedance and the changes of the surface film (RR, CR) and the coverage of the surface absorbed ions caused by electrochemical reaction (Rads, Cads). The impedance spectrum of the electrode system in different states were simulated by changing the five parameters of the mathematical expression. The contribution of different factors to the impedance spectrum of GCE was revealed. Finally, the impedance spectra of bare/modified GCE in potassium ferricyanide solution were analyzed by the mathematical model. The fitting results are in good agreement with the experimental data. Based on the parameters obtained by fitting, the changes of the electrode surface characteristic before and after modifications were quantitatively compared and analyzed. The charge transport resistance increases from 5827.8 Ω to 25104.3 Ω, and the diffusion conductance of Fe3+/Fe2+ ions on the electrode surface also increases by an order of magnitude. However, there is no significant difference with the double-layer capacitance and the frequency dispersion coefficient. The surface of the modified electrode remains electrically neutral. The aggregation state and oxidation-reduction mechanism of Fe3+/Fe2+ on the electrode surface are the same as those on the bare GCE surface.

Graphical Abstract

Keywords

glass carbon electrode, impedance spectroscopy, equivalent circuit, interface characteristics, dielectric relaxation

Publication Date

2021-10-28

Online Available Date

2020-11-10

Revised Date

2020-10-16

Received Date

2020-08-24

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