Supervisor:
Vincenzo Liso

Co-Supervisor:
Na Li and Samuel Simon Araya

Assessment Committee:
Thomas Condra (Chair)
Alessandro Hugo Monteverde Videla, Polytechnic University of Turin
Shuang Ma Andersen, University of Southern Denmark

Moderator:
Simon Lennart Sahlin

Abstract:

High-temperature proton exchange membrane (HT-PEM) fuel cells have garnered significant attention for clean energy applications due to their stability at elevated temperatures (120–200 °C), enhanced tolerance to fuel impurities, and reduced complexity in thermal and water management compared to low-temperature PEM fuel cells. However, their commercialization is still limited by a critical challenge: long-term performance degradation.  This degradation is primarily caused by factors such as membrane thinning, poisoning effect of sulfur dioxide (SO2) contaminated air, catalyst sintering, carbon corrosion, and phosphoric acid (PA) loss in the electrolyte. Under real-world operating conditions—such as load variations, start-stop cycles, temperature fluctuations, and fuel dilution—these degradation mechanisms are further exacerbated, leading to accelerated performance loss and shorter cell lifetimes. This not only increases operational costs but also limits the reliability and scalability of HT-PEM fuel cells, posing a major barrier to their widespread applications.

To address these challenges and enable the commercialization of HT-PEM fuel cells, a comprehensive understanding of the degradation mechanisms is essential to improve cell design and optimize operational strategies. This PhD thesis focuses on systematically investigating these mechanisms through an integrated approach that combines numerical modeling and experimental methods. By simulating real-world operating conditions, the thesis aims to identify the key factors driving performance degradation and validate these findings using experimental data and numerical analysis. 

First of all, a degradation model was developed by MATLAB/SIMULINK to simulate HT-PEM fuel cell performance under nonideal conditions, including load variations, start-stop cycles, temperature fluctuations, and fuel impurities. The model captures key degradation mechanisms such as electrochemical surface area (ECSA) loss and PA evaporation, enabling accurate predictions of long-term performance trends and offering actionable strategies for improvement.

Additionally, modeling and experimental results further show that nitrogen dilution increases charge-transfer and mass-transfer resistances, significantly degrading performance. Based on these findings, load cycle and start-stop cycle tests were performed under pure hydrogen and nitrogen-diluted environments. Start-stop cycling in pure hydrogen causes significant degradation, which is further exacerbated by nitrogen dilution due to amplified mass and charge transfer fluctuations. Scanning electron microscope (SEM) analysis confirmed membrane electrode assembly (MEA) structural deterioration, correlating with the observed electrochemical performance loss.

Thermal cycling experiments further revealed the detrimental impact of repeated heating and cooling cycles on HT-PEM fuel cell performance. The most pronounced voltage degradation occurred during start-stop cycles at open circuit voltage. This was confirmed by electrochemical impedance spectroscopy (EIS). This degradation was characterized by notable increases in PA leaching, ohmic resistance, and charge transfer resistance. The findings underscore the adverse effects of thermal cycling on cell performance and highlight the importance of improved thermal management strategies to mitigate degradation.

In conclusion, this thesis provides a comprehensive understanding of degradation mechanisms in HT-PEM fuel cells under diverse operating conditions. The findings not only deepen our understanding of performance degradation but also offer practical strategies to optimize cell design, improve materials, and refine operational protocols. These contributions are critical for advancing the commercialization of HT-PEM fuel cells in sustainable energy applications.