Method—Time-Variance In EIS Measurements: The Simple Case of Dilution

Abstract

Classical analysis of Electrochemical Impedance Spectroscopy (EIS) measurements requires them to be performed on a linear, time-invariant system. Many electrochemical systems are time-variant (battery under charge/discharge, corroding sample). The effect of time-variance on these systems is not easy to study because there is no reference to which the time-variant results could be compared to. In this paper, we perform a simple dilution experiment to illustrate the time-variance on EIS measurements performed in a quiescent solution and at a Rotating Disk Electrode (RDE), in an electrolyte containing the classical K3Fe(CN)6/K4Fe(CN)6 redox couple. Using simulations based on analytical expressions of the Faradaic impedance, experimental parameters are defined. Fitting EIS data using Equivalent Circuit Models (ECM allows us to qualify the errors made when trying to fit time-variant data with time-invariant impedance expressions. Repeated impedance measurements and reconstruction of instantaneous impedance graphs via interpolation allows to reduce the effect of time-variance and improve the fitting quality. Finally, another approach is presented, which consists in fitting the time-variant data by an analytical time-dependent expression of the Faradaic impedance where the concentration evolution and its effect are accounted for.