ORP-EIS to study the time evolution of the [Fe(CN)6]3-/[Fe(CN)6]4-[Fe(CN)6]3-/[Fe(CN)6]4- reaction due to adsorption at the electrochemical interface

• The time evolution of the ferri/ferrocyanide ET reaction is studied by ORP-EIS.
• ORP-EIS is used to detect and quantify the non-stationary behavior.
• The calculated instantaneous impedance is analyzed to discuss the time-variance.
• The XPS study confirms the adsorption iron-cyanide substances on the gold electrode.
• The physically and statistically validated EIS model considers the time-variance.

The ferri/ferrocyanide system is widely considered as a one-electron transfer reaction. Yet, the redox couple is known to have a more complex mechanism that involves adsorption of species. The electron transfer of the ferri/ferrocyanide reaction on polycrystalline gold is studied by odd random phase multisine electrochemical impedance spectroscopy (ORP-EIS). This technique ensures reliable experiments and provides information of time-varying characteristics of the system. In this work, emphasis is set on the analysis of the non-stationary behavior of the ferri/ferrocyanide couple. The study brings knowledge on the time-dependent phenomena occurring during the electron transfer reaction. The investigation is completed by X-ray photoelectron spectroscopy (XPS) analysis of species adsorbed on gold. The results of the surface analysis confirm the presence of an iron-cyanide layer. This work provides a statistically founded modeling with an insight into the time-variance of the ferri/ferrocyanide reaction on gold.