Applications of Electrochemical Impedance Spectroscopy to in-situ Dynamic Characterization of Energy Conversion and Storage Systems

Abstract

Electrochemical impedance spectroscopy (EIS) is considered as a powerful and valid technique for non-destructive in-situ dynamic measurement of electrochemical power systems. Together with equivalent circuit (EC) simulation, EIS is competent to perform impedance simulation, system characterization, mechanism validation, performance evaluation, and system diagnostics. The impedance measurement and simulation of both energy conversion systems and energy storage systems are presented in this dissertation. The performance of a commercial high temperature proton exchange membrane (PEM) fuel cell tack module is studied by measuring its impedance at various current loads and different operating temperatures above 100°C. The high temperature operation is achieved by its novel phosphoric acid (PA) doped polybenzimidazole (PBI) membranes. The performance of a traditional PEM stack module operated at a temperature below 65°C is also studied by impedance measurement. A generalized EC model is proposed and validated to simulate both commercial PEM fuel cell stacks. The performance comparison between two stacks and the performance degradation of HT-PEM fuel cell stack are analyzed qualitatively and quantitatively based on EC simulation and impedance interpretation. The impedance study on PEM stacks at commercial level reveals a more realistic status of current fuel cell development. The single cells of tubular solid oxide fuel cell (T-SOFC) fueled directly with reformate mixture are studied by impedance measurement and EC simulation. Due to the complexity and uncertainty of SOFC mechanisms and the difficulty of SOFC operation at varying conditions, the EC simulation lacks validation for impedance interpretation. However, the impedance spectra measured under limited operation conditions still provide the preliminary validation for the physical interpretation of the proposed EC model. The measurements and simulation performed on the T-SOFC single cell tubes provide experimental data for studies on reformate fueled SOFC systems. Impedance measurement and EC simulation are also applied to two commercial Ni-MH D-size rechargeable cells at different state-of-health (SoH). Their impedance spectra are measured and simulated at varying state-of-charge (SoC) levels. A validated EC model can be utilized to find out the correlation between battery impedance and SoC for power prediction and battery diagnostics. The prediction of battery SoC is useful to further develop a powerful and efficient smart-charging system for portable electronics and even electric vehicle development.