Measurement of differential capacitance for faradaic systems under potentiodynamic conditions: Considerations of Fourier transform and phase-selective techniques

The differential capacitance (Cdiff) technique is widely employed in studies of metal–liquid interfaces, and is particularly useful for probing electrochemical effects of surface charge variations under voltage control. For such applications, however, it is generally necessary to measure Cdiff under potentiodynamic conditions, and these measurements are typically performed using single frequency phase-selective AC voltammetry. In the present work, we analyze the constraints and variables of these phase-selective measurements by using multi-frequency time resolved Fourier transform electrochemical impedance spectroscopy (FT-EIS). From a detailed comparison of the working principles of single-frequency and spectroscopic impedance methods, we illustrate how different processing of experimental data in the two approaches could lead to different results. As a model system for this study, we consider faradaic surface reactions of Cu in the presence of specifically adsorbing I− anions in KOH solutions. The kinetic parameters of these reactions (obtained with FT-EIS) provide voltage dependent time resolved data for Cdiff, as well as a framework to illustrate the experimental constraints of the phase-selective approach.