Understanding Proton Exchange Membrane Fuel Cell Degradation

Edward Brown

Understanding Proton Exchange Membrane Fuel Cell Degradation

Proton exchange membrane fuel cells (PEMFCs) have emerged as a promising power source for automotive applications. However, the durability issue associated with performance degradation poses a significant challenge for their widespread commercialization. To ensure their integration into vehicles meets strict durability limitations, it is crucial to address the factors contributing to PEMFC degradation, such as extreme operating conditions and material defects.

PEMFCs consist of various components that can experience degradation over time, leading to a decline in performance. In order to understand and mitigate this degradation, accurate performance degradation models are essential for health assessment and prognostics.

In this article, we provide a comprehensive review of the current status and future development of performance degradation modeling for automotive PEMFC systems. We explore the challenges and perspectives associated with PEMFC degradation, covering topics such as fuel cell stack, membrane electrode assembly, fuel supply system, and air supply system.

By delving into the performance degradation mechanisms and exploring different degradation modeling methods, we aim to enhance the understanding of PEMFC degradation and its impact on fuel cell performance. This knowledge will help develop effective mitigation strategies and extend the useful life of PEMFCs.

PEMFC System in Hydrogen Fuel Cell Vehicle

In a hydrogen fuel cell vehicle, the PEMFC system is responsible for generating electricity. It consists of a fuel cell stack and auxiliary systems such as fuel supply, air supply, and thermal management systems.

The fuel supply system feeds hydrogen to the anode, while the air supply system provides compressed air to the cathode. A cooling system removes heat from the system.

The fuel cell stack is comprised of individual fuel cells connected in series, with the membrane electrode assembly (MEA) at the heart of each fuel cell. The MEA consists of a polymer electrolyte membrane (PEM), catalyst layers, and gas diffusion layers. Gaskets and bipolar plates are used for assembly and electron transfer between cells.

Performance Degradation of the PEMFC System

Fuel cell degradation is an inevitable consequence of long-term operation in proton exchange membrane fuel cells (PEMFCs). It encompasses various factors that contribute to the deterioration of the fuel cell’s performance. These factors can be categorized into mechanical degradation, thermal degradation, and chemical/electrochemical degradation.

Mechanical Degradation

Mechanical degradation occurs due to material defects and non-uniform mechanical stresses within the fuel cell system. Material defects, such as cracks or structural imperfections, can lead to localized stress concentrations, resulting in performance loss. Non-uniform mechanical stresses in the fuel cell stack can also cause deterioration and degradation of the cell components over time, impacting overall fuel cell performance.

Thermal Degradation

Thermal degradation is another significant factor that affects PEMFC performance. It occurs when fuel cells operate outside their designated temperature range. High temperatures can accelerate the degradation of critical components within the fuel cell, such as the catalyst layers or membrane electrode assembly (MEA), leading to decreased performance and potentially even failure of the fuel cell system. Conversely, operating the fuel cell at low temperatures may also cause performance degradation due to limited reactant availability and sluggish electrochemical reactions.

Chemical/Electrochemical Degradation

Chemical/electrochemical degradation is mainly caused by aging and the presence of contaminants. Over time, the chemical reactions that take place within the fuel cell, such as oxidation and reduction reactions at the electrodes, can result in the deterioration of catalyst materials and chemical degradation of the membrane. Additionally, the presence of contaminants, such as trace gases or impurities in the fuel or air supply, can lead to chemical reactions that degrade the fuel cell performance.

Understanding these degradation mechanisms is crucial for evaluating fuel cell performance and extending the useful life of PEMFCs. To achieve this, performance degradation models are developed to predict the deterioration of fuel cells over time and assess their state of health. These models integrate various degradation factors, including mechanical, thermal, and chemical degradation, to provide a comprehensive understanding of how performance degradation occurs in PEMFCs. By accurately predicting performance degradation, these models enable informed decision-making and the implementation of appropriate mitigation strategies to ensure the long-term reliability and efficiency of PEMFCs.

End-of-Life Definition/Criterion

The end-of-life (EoL) criteria for fuel cells can vary, with different definitions found in the literature. The most widely used criterion focuses on fuel cell voltage degradation, specifically targeting a 10% decrease in voltage as defined by the US Department of Energy. However, this criterion may not be suitable for dynamic load changes encountered in real-world applications.

Another commonly used criterion, established by the Fuel Cell Testing and Standards Network, sets a fixed threshold voltage value of 0.3V. When the fuel cell voltage drops below this value during a durability test, the test is halted. This criterion provides a clear indicator of when a fuel cell has reached its end-of-life.

It is important to note that the definition of end-of-life may vary depending on the specific application and the requirements of the system designer. Different industries and applications may have their own unique criteria to determine when a fuel cell has surpassed its useful life.

Classification of Degradation Modeling Methods

In order to accurately predict the performance deterioration of proton exchange membrane fuel cells (PEMFCs) and estimate their remaining useful life, it is crucial to employ appropriate degradation modeling methods. These methods can be classified into three main approaches: model-based, data-driven, and hybrid methods.

Model-Based Methods

Model-based degradation modeling methods utilize mathematical equations to predict the aging process of PEMFCs. These methods take into account various degradation mechanisms and their impact on fuel cell performance. While model-based methods provide a solid foundation for understanding degradation, they can be computationally expensive and require extensive knowledge of the underlying physics and chemistry involved.

Data-Driven Methods

Data-driven degradation modeling methods rely on the analysis of collected data to understand the behavior of PEMFC systems. By utilizing large datasets, these methods can uncover complex patterns and correlations that may not be easily captured by mathematical models. However, data-driven methods require a significant amount of data for accurate predictions and may not be suitable for systems with limited data availability.

Hybrid Methods

Hybrid degradation modeling methods combine the strengths of both model-based and data-driven approaches. These methods leverage the mathematical models to provide a foundation for understanding degradation mechanisms, while also incorporating the insights gained from data analysis. Hybrid methods enhance model learning and prediction accuracy. However, designing hybrid models can be challenging, and they may impose additional computational costs.

Choosing the appropriate degradation modeling method is essential for accurately assessing the performance degradation of PEMFCs and estimating their remaining useful life. It requires considering factors such as the availability of data, computational resources, and the level of accuracy required for the specific application.

Mitigation Strategies for PEMFC Degradation

To mitigate PEMFC degradation, researchers and engineers have developed various strategies that target the core components of the fuel cell system. One crucial area of focus is the development of fuel cell membranes with increased durability. By enhancing the materials used in the membrane electrode assembly (MEA), such as the polymer electrolyte membrane (PEM), manufacturers can improve the overall resilience of the fuel cell.

Another important aspect of mitigation is the identification and mitigation of stressors and scavenging reactions that lead to degradation. Researchers study the impact of oxidative species on the membrane’s integrity and work towards developing effective strategies to counteract these effects. Understanding the mechanisms of membrane degradation is essential for designing targeted mitigation techniques.

Additionally, the enhancement of antioxidant properties in the fuel cell system can aid in mitigating degradation. Antioxidants can help neutralize harmful reactive species and prevent damage to the fuel cell components. By incorporating antioxidant materials or coatings, researchers aim to prolong the lifespan of PEMFCs and reduce performance losses.

To validate the effectiveness of mitigation strategies, accelerated aging tests are conducted to simulate the long-term degradation processes. These tests allow researchers to evaluate the performance and durability of the fuel cell under various operating conditions. Furthermore, the implementation of current by-pass mitigation strategies at the stack/system scale is crucial for addressing performance losses and ensuring the longevity of PEMFCs in real-world applications.