Nonstationary Impedance Measurements in Forced and Natural Dynamic Conditions using Dynamic Multi-Frequency Analysis

Abstract

This thesis is concerned with the characterization of electrochemical systems by means of dynamic impedance spectroscopy. Electron transfer reactions across electrified interfaces involve a variety of processes, occurring at different time scales. A detailed understanding of the reaction kinetics requires the investigation of the system over a wide range of frequencies in order to capture all time constants related to the different underlying processes. In electrochemical impedance spectroscopy (EIS) a small-amplitude sinusoidal potential perturbation is imposed on the system by successively changing the frequency of the sinusoidal wave. The system's response to the perturbation signal provides an insight into the mechanism governing the reaction. EIS is a powerful tool that allows for the investigation of a variety of electrode processes in a single measurement. This method requires the stationarity of the system throughout the acquisition time of one full impedance spectrum. However, in many electrochemical systems a stable steady-state cannot be established, since the system undergoes irreversible changes during the measurement. Dynamic multi-frequency analysis (DMFA) uses a multi-sine waveform to apply all frequencies to the system simultaneously, thus allowing for the acquisition of impedance spectra in dynamic conditions. Here, DMFA is employed for the investigation of the nonstationary impedance behavior of different electrochemical systems, exhibiting either forced or natural reaction dynamics. Fundamental principles of data recording and analysis are examined by application of DMFA to a simple redox process. It is demonstrated that the temporal evolution of physical variables can be extracted from data analysis using suitable electrical equivalent circuits (EECs). Based on these results, different stages of the hydrogen evolution reaction (HER) are examined by means of DMFA. To do so, the system is subjected to forced dynamic conditions by combining quasi-$dc$ voltammetry and multi-sine impedance spectroscopy, thus providing a concatenated picture of the overall process. The investigation of the temporal evolution of the kinetic variables gives new insight into the mechanism of the HER. Finally, DMFA is employed for the examination of self-sustained current oscillations during the anodic electrodissolution of p-type silicon in fluoride containing electrolytes. The analysis of the system in dynamic equilibrium conditions provides the foundation for the investigation of the electrodissolution process in the oscillatory regime. DMFA allows for the characterization of different types of current oscillations in terms of their oscillatory reaction dynamics.