Ph.D Thesis


Ph.D StudentShany Hershkovitz
SubjectHarnessing Evolutionary Programming for Impedance
Spectroscopy Analysis
DepartmentDepartment of Chemical Engineering
Supervisors Professor Tsur Yoed
Dr. Sioma Baltianski
Full Thesis textFull thesis text - English Version


Abstract

In this research, a novel analysis technique for impedance spectroscopy (IS) measurements is introduced and applied to the investigation of symmetric cells.  IS is a powerful and non destructive method of characterizing electrical properties of materials.  The analysis program is based on genetic programming (GP) which is an evolutionary-based optimization algorithm.  The GP computing approach allows the evolutions of both the model and the numerical parameters of a certain model based on its fitness to a general mathematical problem. In contrast to the conventional analysis methods used for impedance spectroscopy measurements, e.g. equivalent circuits, our program seeks the distribution of relaxation times, DFRT, that has the form of a peak or a sum of several peaks, assuming the Debye kernel .Using this method one finds a functional (parametric) form of the distribution of relaxation times. By finding a functional form of the DFRT, one may develop a physical model and examine its behavior. 

This analysis technique is used to investigate the oxygen reduction reaction at the cathode side of solid oxide fuel cells (SOFC).  Two symmetric cell configurations (SCC) where chosen : (i) The first configuration is composed of  Pt│GDC│Pt,  where the Pt layer serves as cathode material as well as current collector;  GDC is the electrolyte material. (ii) The Second configuration is composed of Pt│LSCF│GDC│LSCF│Pt, where LSCF serves as the electrode material. IS measurements combined with I-V measurements were employed on several samples at several temperatures and several oxygen partial pressures in order to investigate their influence on the oxygen reduction reaction. 

The resulting IS data was analyzed using the ISGP program and the resulting peaks constructing the DFRTs were assigned for different processes that occur at the cathode side. The activation energies as well as the dependence of the processes on the oxygen partial pressure were also evaluated.  The polarization curves obtained were analyzed using the Butler-Volmer (B-V) relations and a proposed model was suggested for the behavior of the examined cell.