Odd random phase multisine electrochemical impedance spectroscopy to quantify a non-stationary behaviour: Theory and validation by calculating an instantaneous impedance value

Abstract

Electrochemical impedance spectroscopy (EIS) is frequently used to study electrochemical systems. However, despite all the benefits of this EIS method, it suffers from an important limitation related to the interpretation of the experimental data. Namely, the system under study must fulfill the conditions of causality, linearity and stationarity to correctly define its transfer function. Since it is clear that electrochemical processes can feature a non-stationary behaviour (e.g. corrosion studies) and since the occurrence of this behaviour is in contradiction with the necessary conditions to obtain reliable EIS results, an estimation of this behaviour is needed. Starting from Odd Random Phase multisine electrochemical impedance measurements (ORP-EIS), a new procedure is set-up in this work to quantify and correct for this time-evolution by means of the calculation of an instantaneous impedance. How this is done, will be elucidated in this study. After intuitively illustrating the method, the first article of this series will explain the mathematical background of the approach. Furthermore, the method will be demonstrated on a well-known electrical equivalent circuit.