Investigation of Physical Processes in Fuel Cells via Electrochemical Impedance Spectroscopy and Distribution of Relaxation Times Analysis

his thesis investigates the application of Electrochemical Impedance Spectroscopy (EIS) and Distribution of Relaxation Times (DRT) analysis in both established and novel fuel cell technologies. Firstly the investigation establishes a methodological framework for utilizing EIS and DRT in Solid Oxide Fuel Cells (SOFCs), correlating DRT plot peaks with specific polarization resistances. This correlation facilitates a nuanced interpretation of EIS data, yielding critical insights into operational parameters that significantly affect SOFC performance under real conditions. Key findings reveal significant reductions in gas diffusion resistance and overall impedance when optimizing parameters such as fuel flow, temperature or gas composition. In addition, the thesis discusses the adaptation of EIS techniques for Enzymatic Fuel Cells (EFCs), specifically focusing on glucose oxidase (GOx)-bilirubin oxidase (BOD)-based fuel cells using ferrocene methanol (FcMeOH) as an anodic and 2,2’-Azinobis (3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) as a cathodic mediator to enhance electron transfer. A preliminary EIS analysis highlights the unique challenges and opportunities within these novel systems. It demonstrates how EIS can effectively assess critical parameters, including enzyme immobilization and substrate concentration. Notably, the findings indicate that electrodes constructed from ordered mesoporous carbon CMK-3 sieves outperform multi-walled carbon nanotubes (MWCNTs) in anodic reactions, while the latter show slightly better performance in cathodic reactions. This insight underscores the potential for optimizing both bioanode and biocathode performance. Building upon these insights, the thesis effectively transfers the systematic pproach via EIS and DRT for EFCs, demonstrating the identification of critical performance parameters specific to GOx-loaded bioanodes. DRT analysis delineates key regions within the impedance spectra: region 𝑅1 correlates with high-frequency ionic conduction and substrate concentration; region 𝑅2 is associated with charge transfer kinetics; and region 𝑅3 pertains to diffusion processes influenced by mediator concentration and temperature. These findings provide insights into optimization potentials for EFCs and establish a foundation for future experimental investigations. In summary, this research advances the understanding and application of EIS and DRT in both established and emerging fuel cell technologies, supporting the development of efficient and sustainable energy solutions.