Reaction Mechanisms of the Electrosynthesis of Magnetite Nanoparticles Studied by Electrochemical Impedance Spectroscopy.

This work presents a mechanistic study of the electrochemical synthesis of magnetite nanoparticles (NPs) based on the analysis of the electrochemical impedance spectroscopy (EIS) technique. After a discussion of the mechanisms reported in the literature, three models are devised and a prediction of their EIS spectra is presented. The approach consisted of the simulation of EIS spectra as a tool for assessing model validity, as EIS allows to characterize the relaxation of adsorbed intermediates. The comparison between the simulated impedance spectra and the experimental results shows that the mechanisms proposed to date do not explain all of the experimental results. Thus, a new model is proposed, in which up to three adsorbed intermediate species are involved. This model accounts for the number of loops found in experimental impedance data. The closest approximation of the features found in the experimental spectra by this proposed model suggests a better representation of the reaction mechanism within the evaluated potential range.

Effect of Introducing Nonideal Capacitance in the Estimation of the Solution Resistance for Accurate Electrolytic Conductivity Measurements.

The standardization of secondary electrolytic conductivity cells requires the use of a certified reference material. The accepted certification method involves electrochemical impedance spectroscopy (EIS) to estimate the material's solution resistance. This method normally assumes that the impedance's imaginary component can be neglected; and hence, the measured impedance approximates the real impedance. Thus, a linear extrapolation of the impedance versus the period (inverse frequency) yields solution resistance. However, experimental impedance data usually do not exhibit a linear behavior over the spectra of frequency, which strongly suggest that the ideal capacitive assumption may not strictly apply. To account for the observed nonlinear behavior, we have proposed to introduce the concept of a constant phase element (CPE) to the analysis of impedance. This approach leads to the development of a relationship that improves the fitting of experimental data and improves the accuracy of the estimation, by establishing a critical frequency where extrapolation should be done. Finally, we are presenting simulated results to demonstrate how sizeable capacitive effects can influence the determination of solution resistance, and a final analysis to estimate the impact on constant cell or electrolytic conductivity values.

Effect of electrode geometry on the electrolyte resistance measurement over the surface of a skin phantom in a noninvasive manner.

We analyzed the electrode geometry to obtain the potential (E) and current density (J) distributions at the surface of a skin phantom (SP), in this case a planar surface. Two electrode geometries were tested: a circular electrode (CiE) and a rectangular electrode (ReE). First, by a finite element simulation, we calculated the E and J distributions at the surface of the SP. Second, we determined the resistivity properties as a function of the electrochemical impedance. Three- and four-electrode configurations were used to measure the E versus distance between the reference electrodes (d). For the ReE, the electrolyte resistance (Re) measurements show a linear behavior with respect to "d" if the zone of the linear distribution of E and the homogeneous current density (JH) is considered. In contrast, the CiE shows nonlinear behavior due to the absence of that zone of the linear distribution of E and JH in the entire range. For ReE, we deduced that the behavior of Re versus "d" is related to the material resistivity. Consequently, the ReE geometry improves the Re measurements on the surface and shows us a way to control the behavior of this element in planar samples such as skin.