Model-based Comparative Assessment of Frequency Response Diagnostic Methods for Electrochemical Energy Devices

Abstract

Electrochemical energy devices, including electrolyzers, batteries, and fuel cells, hold significant promise as sustainable solutions for energy storage and conversion. Among fuel cells, polymer electrolyte fuel cells are currently the most widely studied technology and the most promising candidate for sustainable power generation in a wide range of applications. To ensure their reliable operation and optimal performance, accurate diagnostic methods are essential. Frequency response analysis has proven to be a valuable tool for evaluating the dynamic behavior and internal characteristics of electrochemical energy devices. The thesis starts with a comprehensive literature review that focuses on diagnostic methods for frequency response analysis in electrochemical energy devices, especially in polymer electrolyte fuel cells. Various approaches, including electrochemical impedance spectroscopy, electrochemical pressure impedance spectroscopy, concentration-alternating frequency response analysis, and concentration admittance spectroscopy, are explored. The aim of this master thesis is to develop and test a method to characterize and quantitatively compare different frequency response diagnostic methods for polymer electrolyte fuel cells. In order to find the best frequency response analysis method or combination of methods, a framework based on linear system theory is used to evaluate the strength or weakness of a given method. An existing analytical solution of a simple electrochemical impedance spectroscopy model of the cathode catalyst layer of a polymer electrolyte fuel cell is utilized to characterize observability, controllability, and parameter sensitivity. A transmission line model of the cathode catalyst layer is used as well to calculate the impedance response and compared with this analytical solution. Next, a transmission line model that takes into account oxygen transport in the cathode catalyst layer is constructed to calculate the impedance response and compared it with an existing analytical result.