Ph.D Thesis

Ph.D StudentOz Alon
SubjectNovel Approach to Analyze Impedance Spectroscopy of
Electrochemical Power Sources
DepartmentDepartment of Energy
Supervisor PROF. Yoed Tsur
Full Thesis textFull thesis text - English Version


Electrochemical impedance spectroscopy (EIS) is one of the most celebrated characterization techniques to unfold the nature of complex electrochemical systems such as a supercapacitors, fuel-cells and batteries. It utilizes the fact that the polarization losses of the tested sample differ in their characteristic time constant and frequency response. When an electrochemical system is subjected to a potential difference, polarization of charges takes place. Upon reversing the potential, each interface in the cell will react differently and therefore can be detected at a different characteristic frequency (or reciprocal time). In that manner, the unique response of each interface can be separated and analyzed. Thus, given a proper analysis technique, making EIS a powerful, comprehensive and nondestructive electrical characterization method of electrochemical systems . Section 1.1 details the fundamental aspects of EIS.

In this work, a novel approach to analyze EIS has been developed and implemented on electrochemical power sources. Impedance Spectroscopy Genetic Programing (ISGP), which serves as the cornerstone of our analysis approach and laid out in sections 1.1, utilizes evolutionary programming to find the most suitable distribution function of relaxation times (DFRT). The analysis yields a DFRT model comprised of linear combination of peaks. Each peak in the model has its own characteristic relaxation time and area, and can be assigned to one or more processes in the tested sample. By plotting the DFRT as a function of frequency and monitoring the changes in each peak's relaxation time and area at different conditions, we are able to distinguish how each process resistance influence the overall polarization resistance of the tested sample.

Chapter 3 focuses on implementing our ISGP-based approach on supercapacitors. Transforming the impedance data to complex capacitance representation allowed us to develop a DFRT model that can be correlated to different physical processes. In that manner, we studied the effect of different structural features on supercapacitors, their degradation mechanism and their pseudo capacitance capabilities. Using the new model parameters enables us to predict the upcoming failure of the tested samples in the degradation study, earlier than other common indicators.

In chapter 4 the EIS response of SOFC is analyzed using ISGP. The oxygen reduction reaction of several systems, both oxygen and proton conducting, was studied, in a symmetrical cell configuration. ISGP allows a straightforward comparison of the tested systems, making the decision which of the cell assemblies has the optimal performance much more apparent.

Chapter 5 deals with Aluminum air battery and it is analyzed using ISGP in two configurations, which allows us to follow the activation process of the battery and the state of the battery during its discharge.

Chapter 6 lays out the improvements and changes made to ISGP during this research.