Analysis of the Degradation of a PEM Fuel Cell Subjected to Dynamic Load Cycles

1 Equation Chapter 1 Section 1 Trabajo Fin de Grado Grado en Ingeniería de la Energía Analysis of the Degradation of a PEM Fuel Cell Subjected to Dynamic Load Cycles Autor: Adriana Raya Angulo Tutor: Sergio Jesús Navas Herrera Gracia María Cabello González Dpto. Ingeniería Energética Escuela Técnica Superior de Ingeniería Universidad de Sevilla Sevilla, 2024 2 3 Trabajo Fin de Grado Grado en Ingeniería de la Energía Analysis of the Degradation of a PEM Fuel Cell Subjected to Dynamic Load Cycles Autora: Adriana Raya Angulo Tutor: Sergio Jesús Navas Herrera Gracia María Cabello González Dpto. Ingeniería Energética Escuela Técnica Superior de Ingeniería Universidad de Sevilla Sevilla, 2024 4 5 Trabajo Fin de Grado: Analysis of the Degradation of a PEM Fuel Cell Subjected to Dynamic Load Cycles Autora: Adriana Raya Angulo Tutor: Sergio Jesús Navas Herrera Gracia María Cabello González El tribunal nombrado para juzgar el Proyecto arriba indicado, compuesto por los siguientes miembros: Presidente: Vocales: Secretario: Acuerdan otorgarle la calificación de: Sevilla, 2024 El Secretario del Tribunal 6 7 To my family, friends and teachers. 8 9 Agradecimientos Me gustaría expresar mi más sincero agradecimiento a todas aquellas personas que han sido fundamentales en la realización de este trabajo y a lo largo de mi vida. En primer lugar, quiero agradecer profundamente a mis tutores, Sergio y Gracia, por hacer posible este trabajo. Gracias por su dedicación, y por guiarme durante todo el proceso. Su confianza ha sido indispensable para llegar hasta aquí. A mis amigas Yaiza, Inma, Carmen, Raquel, María y Pablo. Gracias por estar siempre a mi lado, por apoyarme durante todos estos años y animarme a seguir intentándolo, incluso en los momentos más difíciles. Vuestra amistad y vuestro apoyo incondicional han sido fundamentales para no rendirme nunca. A mis amigos de la universidad, gracias por haber hecho de los años de carrera, los mejores de mi vida. Gracias por las risas y por enseñarme tanto dentro como fuera de la facultad. Ha sido un verdadero placer crecer junto a vosotros. Por último y más importante, a mi familia. Gracias, mamá, papá y María, por ser mi pilar fundamental todos estos años. Sin vosotros, nada de esto habría sido posible. Sois el motor de mi vida, mi mayor apoyo. Gracias también a Ana y a mis abuelos, por tenerme siempre en sus pensamientos y desearme lo mejor. Y una mención especial a mi tita. "Sé que estarías orgullosa de mí, al final tu niña grande, sí que se hace mayor. Te quiero." Adriana Raya Angulo Grado en Ingeniería de la Energía Sevilla, 2024 16 17 TABLE OF FIGURES Figure 1. IEA, Global CO2 emissions from energy combustion and industrial processes, 1900-2022. [2] ........ 23 Figure 2. Rhodium Group, Global GHG emissions, 1990-2022 [3] ...................................................................... 24 Figure 3. Classification of application areas of fuel cells [5] ................................................................................. 25 Figure 4. Classification of Fuel Cell [7] ................................................................................................................... 26 Figure 5. Graphical representation of a Hydrogen PEM fuel cell .......................................................................... 28 Figure 6. PEM Fuel Cell Components ..................................................................................................................... 29 Figure 7. Catalytic Layer Layout .............................................................................................................................. 30 Figure 8. Parallel Flows Channel ............................................................................................................................. 31 Figure 9. Serpentine Flows Channel ........................................................................................................................ 31 Figure 10. Pin Type Flows Channel ......................................................................................................................... 31 Figure 11. Spiral Flows Channel .............................................................................................................................. 31 Figure 12. Gas Diffusion Layers .............................................................................................................................. 32 Figure 13. Sealing gasket in fuel cells [15] .............................................................................................................. 32 Figure 14. Schematic of fuel cell. [16] ..................................................................................................................... 33 Figure 15. SEM Picture of a new (A) and aged (B) PEMFC MEA [17] .............................................................. 33 Figure 16. Example of a PEMFC Polarization Curve [20] ..................................................................................... 35 Figure 17. Polarization Curve Zones [21] ................................................................................................................ 35 Figure 18. Load Cycle ............................................................................................................................................... 37 Figure 19. Current plot Test 4 ................................................................................................................................... 38 Figure 20. Example Test 4, Cycle 1 ......................................................................................................................... 38 Figure 21. Example Test 4, Cycle 75 ....................................................................................................................... 39 Figure 22. Example Test 4, Cycle 150 ..................................................................................................................... 39 Figure 23.Current Plot Test 5 ................................................................................................................................... 40 Figure 24. Example Test 5, Cycle 1 ......................................................................................................................... 40 Figure 25. Example Test 5, Cycle 75 ....................................................................................................................... 40 Figure 26. Example Test 5, Cycle 150 ..................................................................................................................... 41 Figure 27. EIS Cycle 14 ............................................................................................................................................ 41 Figure 28. Equivalent Analogue Electronic Circuit ................................................................................................ 42 Figure 29. ZView 2 Modeling .................................................................................................................................. 43 Figure 30. Tests Cycle 1 ........................................................................................................................................... 45 Figure 31. Tests Cycle 75 ......................................................................................................................................... 45 Figure 32. Tests Cycle 150 ....................................................................................................................................... 46 Figure 33. Voltage response Test 1 .......................................................................................................................... 46 Figure 34. Voltage response Test 4 .......................................................................................................................... 47 Figure 35. Voltage response Test 9 .......................................................................................................................... 47 Figure 36. Voltage response Test 13 ........................................................................................................................ 48 Figure 37. Voltage response Test 15 ........................................................................................................................ 48 Figure 38. EIS Graphic ............................................................................................................................................. 49 Figure 39. Equivalent Analogue Electronic Circuit ................................................................................................ 50 Figure 40. CV profile of a Pt/C electrocatalyst [28] ............................................................................................... 53 Figure 41. General framework for AI optimization. [29] ...................................................................................... 54 Figure 1ª.A. Voltage response Test 2………………………………………………………….……………...59 Figure 2ª.A. Voltage response Test 3………………………………………………………….……………...59 Figure 3ª.A. Voltage response Test 6………………………………………………………….……………...60 Figure 4ª.A. Voltage response Test 7………………………………………………………….……………...60 Figure 5ª.A. Voltage response Test 14…..…………………………………………………….……………...61 18 19 TABLE OF TABLES Table 1. Types and characteristics of Fuel Cells [7] ............................................................................................... 27 Table 2. Flow-field design of the bipolar plates [12] .............................................................................................. 31 Table 3. Voltage data from tests ............................................................................................................................... 45 Table 4. ZView 2 Data .............................................................................................................................................. 50 Table 5. R1 Data from ZView 2 ............................................................................................................................... 50 Table 6. Total Resistance Data from ZView 2 ........................................................................................................ 51 20 21 ABBREVIATIONS IEA International Energy Agency C02 Carbon Dioxide PEM Proton Exchange Membrane / Polymer Electrolyte Membrane PEMFCs Proton Exchange Membrane Fuel Cells H2 Hydrogen H+ Protons e- Electrons H2O Water FCTO Fuel Cell Technologies Office FC-PAD Fuel Cell Performance and Durability Consortium Pd/C Catalysts Carbon Supported Palladium Catalysts MEA Membrane Electrode Assembly GDL Gas Diffusion Layers EU European Union EIS Electrochemical Impedance Spectroscopy AC Alternating Current DC Direct Current CL Catalytic Layer JRC EIS Joint Research Centre Electrochemical Impedance Spectroscopy AI CV Artificial Intelligence Cyclic Voltammetry 22 Analysis of the Degradation of a PEM Fuel Cell Subjected to Dynamic Load Cycles 23 Analysis of the Degradation of a PEM Fuel Cell Subjected to Dynamic Load Cycles 1 INTRODUCTION AND OBJECTIVES 1.1 Introduction Over the past century, Earth's temperature has risen at an unprecedented rate, increasing by about 1.5 Celsius degrees. The 2020 report Global Warming of 1.5 ºC, published by the Intergovernmental Panel on Climate Change [1], attributes this rapid warming primarily to human activities. Since the Industrial Revolution began, the burning of fossil fuels like oil, coal, and natural gas in factories, power plants, and vehicles has significantly increased the concentration of carbon dioxide, methane, and other greenhouse gases in the atmosphere. These gases intensify the greenhouse effect by trapping heat, which leads to the ongoing rise in global temperatures, a phenomenon widely recognized as global warming. In Figure 1, the International Energy Agency, IEA, presents the annual increase of 𝐶𝑂2 emissions from energy combustion and industrial processes from 1900 to 2022. [2] Figure 1. IEA, Global CO2 emissions from energy combustion and industrial processes, 1900-2022. [2] As can be seen, the growth of 𝐶𝑂2 emissions in 60 years has been almost exponential, and there is not even a downward trend; in fact, the only time a decrease in 𝐶𝑂2 emissions was observed was during the year the world suffered from the COVID-19 pandemic. As mentioned above, greenhouse gases are made up of different gases. The most relevant and prevalent gas is carbon dioxide, but there are also others. In Figure 2 provided by the Rhodium Group, the growth in the rest of the greenhouse gases between 1990 and 2022 can be observed. [3] . ” Pour ce qui est de l’avenir, il ne s’agit pas de le prévoir, mais de le rendre possible. “ . - Antoine de Saint Exupéry, Citadelle, 1948- 24 Analysis of the Degradation of a PEM Fuel Cell Subjected to Dynamic Load Cycles Figure 2. Rhodium Group, Global GHG emissions, 1990-2022 [3] This increase in the expulsion of gases into the atmosphere has been intensified, highlighting the global warming and contributing to climate change. Climate change is defined as long-term changes in the average weather that have been observed over a long period on Earth. The consequences of climate change have significant implications, which include increased sea levels, more frequent and intense extreme weather events, changes in precipitation, and impacts on ecosystems and biodiversity. On the other hand, factors such as deforestation and overpopulation do not only fail to provide an improvement but worsen the current situation. Governments all around the world have recognized the urgency of addressing climate change, and many have taken steps to mitigate its impacts. That way, the decarbonization of the planet is one of the main goals set by countries worldwide for the year 2050. To achieve this, participation in international agreements is key way countries commit to reducing their greenhouse gas emissions, promoting the use of renewable energies such as solar, wind, and hydropower, and working towards limiting global temperature increases. In this framework, the Paris Agreement, adopted in 2015, is a notable example. The Paris Agreement is a legally binding international contract on climate change. It was established by 196 Parties at the UN Climate Change Conference, COP21, in Paris, in December 2015, and became effective in November 2016. Its main goal is to keep “the increase in the global average temperature to well below 2°C above pre-industrial levels” and to make efforts to “limit the temperature increase to 1.5°C above preindustrial levels.” In recent years, global leaders have emphasized the urgency of limiting global warming at 1.5°C by the century’s end. To achieve the 1.5°C target, greenhouse gas emissions must peak by 2025 at the latest and decrease by 43% by 2030. The Paris Agreement is a landmark in the multilateral climate change process because, for the first time, a binding agreement brings all nations together to combat climate change and adapt to its effects. [4] Due to the intermittent nature of renewable energy generation, which may not always align with energy demand, the need to store surplus renewable energy on a large scale is an active area of research and development. One promising solution is hydrogen, which serves as a means to store and transport energy. Hydrogen produced through chemical electrolysis using electricity derived from renewable energy sources is referred to as green hydrogen. This form of hydrogen is emerging as a key player because it is the most abundant chemical element in nature and its production and consumption are climatically neutral, generating no greenhouse gases emissions. Also, hydrogen can be stored until is needed and then used as fuel in a fuel 25 Analysis of the Degradation of a PEM Fuel Cell Subjected to Dynamic Load Cycles cell. In this process, hydrogen undergoes a chemical reaction with oxygen, transforming chemical energy into electrical energy, thus providing emissions-free electricity on demand. This is the reason why this thesis will focus on the study of the hydrogen fuel cells. 1.1.1 Hydrogen fuel cells There are multiple technologies for generating electrical energy from hydrogen, such as its direct use in internal combustion engines for motor vehicles or in gas turbines. Additionally, hydrogen can be used in industries as a source of energy to achieve high temperatures or to remove impurities, particularly in the petroleum sector. However, one of the most promising end-uses of hydrogen are fuel cells, owing to their potential, efficiency, low emissions, and diverse applications. Fuel cells can be employed in various sectors: as an intriguing alternative for transportation, reducing reliance on fossil fuels; for electricity or heat generation, thus mitigating environmental impact; and even in military applications due to their lightweight nature, low noise, and heat generation. [5] Figure 3 shows the different applications areas of fuel cells. Figure 3. Classification of application areas of fuel cells [5] Prior to developing into the study of hydrogen fuel cells, it is imperative to thoroughly understand both their advantages and disadvantages and it types. Hydrogen fuel cells offer numerous advantages. Firstly, they represent a clean energy solution, generating only water as a byproduct and producing no greenhouse gas emissions, as previously mentioned. Furthermore, their energy efficiency is higher than that of internal combustion engines due to the absence of moving parts, which eliminates mechanical friction that dissipates energy. Additionally, they operate with significantly reduced noise levels compared to internal combustion engines, as they lack internal explosions or noisy moving components. Nevertheless, hydrogen fuel cells also have disadvantages. Hydrogen fuel cells can be costly to produce and support. The materials used, such as platinum, are expensive, and the manufacturing process can be complex. 32 Analysis of the Degradation of a PEM Fuel Cell Subjected to Dynamic Load Cycles Another function performed by this layer is to allow electrons to complete the electrical circuit and provide structural support to the membrane. Additionally, on the cathode side, the GDL helps to evacuate water generated at the electrode towards the flow channels. To enable the proper performance of the aforementioned, the GDL must be composed of a good thermal and electrical conductivity material. It needs to be porous, to allow gas flow and water evacuation. Also, it should be rigid yet flexible enough to support the MEA while keeping electrical contact. For those reasons, carbon materials are commonly used. Figure 13 represents a carbon GDL. [14] Figure 12. Gas Diffusion Layers 1.1.2.2.5 Seals and Current collectors Seals and gaskets are part of the sealing system of the PEMFC. They are the elements responsible for preventing gas leakage to the outside of the stack or the crossover of gases from one side of the cell to the other. They can be individual elements or components integrated in the electrode itself. The most common materials for their manufacture are thermoplastic polymers capable of withstanding the operating conditions of the cell, such as Teflon, PTFE, or silicones. Figure 13 shows what a sealing gasket in fuel cells looks like. Figure 13. Sealing gasket in fuel cells [15] On the other hand, current collectors have the function of closing the electrical circuit of the cell and allowing the connection with the external application that demands the generated electrical current. They are usually an independent element separate from the terminal plates. They are usually made of a very good conductive material, such as copper, or another metal coated with a thin layer of gold to improve conduction. Figure 14 shows the currents collectors and where they are located in the fuel cell. 33 Analysis of the Degradation of a PEM Fuel Cell Subjected to Dynamic Load Cycles Figure 14. Schematic of fuel cell. [16] 1.1.3 Hydrogen Fuel Cell Durability One of the main challenges for hydrogen fuel cells widespread adoption is their durability. An increase in durability, performance and cost reduction is imperative for large-scale commercialization of PEMFCs. First of all, the focus must be on the factors affecting their lifespan and the methods used to assess their durability. The degradation of the proton exchange membrane is one of the primary factors affecting PEM durability. They are exposed to high temperatures, contaminants, and pH gradients that can lead to membrane degradation. This result in losses of ionic conductivity and so decreased cell performance. In Figure 15 is represented the MEA degradation. Furthermore, fluctuations in pressure and temperature during cell operation can cause delamination of cell layers and loss of sealing integrity. This means that the components' ability to keep a hermetic seal to prevent fluid leakage is reduced, compromising long-term durability. Figure 15. SEM Picture of a new (A) and aged (B) PEMFC MEA [17] In 2015, the U.S. DOE Fuel Cell Technologies Office, FCTO, formed the Fuel Cell Performance and Durability Consortium (FC-PAD). FC-PAD is a multidisciplinary team formed by five national laboratories focused on enhancing the performance and durability of PEMFCs. Attempts have been made to improve the interface between the catalyst layer and electrolyte membrane to enhance proton transport, reduce resistance, and improve performance. Accelerated stress tests have been used to evaluate and understand PEMFC degradation in order to enhance performance. [8, 18] 34 Analysis of the Degradation of a PEM Fuel Cell Subjected to Dynamic Load Cycles In order to investigate it, in situ and ex situ accelerated stress tests were carried out via potential cycling. Durability tests of the single cells with Pd/C 1 catalysts were performed through an interval constant current density mode. 1.1.3.1 In situ techniques In situ tests are conducted within the actual operating environment of the fuel cell stack, representing assessments performed while the stack is operational and supplying power. These tests facilitate the evaluation of stack durability and performance under real operating conditions, serving as valuable tools for issue identification and ongoing assessment of stack material wear over time. Some examples of in situ tests include: • Electrochemical Impedance Spectroscopy (EIS): This technique measures the impedance of the fuel cell at different frequencies. This test helps to find losses and degradation mechanisms within the cell. • Polarization Curve Analysis: By measuring the voltage and current at different points. This test provides insights into the overall performance of the fuel cell and identifies potential issues like catalyst degradation. • Durability Testing: Long-term operation under standard and accelerated conditions to assess the fuel cell's longevity and show potential failure modes over time. 1.1.3.2 Ex situ techniques Ex situ tests are conducted outside the operational environment of the fuel cell stack. The stack is removed from the system and subjected to tests in a controlled environment, such as a laboratory. Ex situ tests allow for a more detailed analysis of stack components, as precise measurements can be taken, and test conditions can be varied as needed. They are useful for investigating the performance and durability of the stack under specific conditions, as well as for studying degradation mechanisms and seeking improvements in design and materials. Some examples of ex situ tests include: • X-ray Diffraction (XRD): XRD is used to analyze the crystalline structure of the fuel cell materials. It provides information on phase changes and material stability. • Scanning Electron Microscopy (SEM): This technique is used to examine the microstructure of fuel cell components, such as the membrane and catalyst layers, to identify physical degradation or material defects. [10] • Thermogravimetric Analysis (TGA): This test measures changes in the weight of fuel cell components as they are heated, helping to identify thermal stability and decomposition temperatures of materials. A critical tool for the design, optimization, and control of PEMFC, providing valuable insights into their performance and enhancing their efficiency and reliability, is the polarization curve. 1.1.4 Polarization Curve The polarization curve, also known as the i-V curve, is the plot of current density (i) versus voltage (V). It has been widely applied in the study of proton exchange membrane fuel cells. [19] The primary function of the polarization curve is to offer detailed information on how the cell's performance varies in response to distinct levels of load and operating conditions, such as temperature, pressure and fuel or oxidant flow rates. It helps to find the best operating point of the cell, where maximum energy efficiency and performance are achieved. Also, it evaluates the quality and effectiveness of the PEM and CL. 1 Carbon-supported palladium catalysts 35 Analysis of the Degradation of a PEM Fuel Cell Subjected to Dynamic Load Cycles Figure 16. Example of a PEMFC Polarization Curve [20] However, while the polarization curve provides valuable information about the overall performance of the PEMFC, it may not offer detailed insights into specific degradation mechanisms or transient behavior. Figure 16 illustrates the open circuit potential, which is lower than the ideal voltage. This potential should be close to the ideal voltage since no electric current is generated in an open circuit. This indicates that there are losses in the fuel cell. These losses arise from the dependency on operating conditions, making it difficult to generalize performance across all scenarios. Additionally, generating polarization curves usually involves conducting experiments under controlled conditions, which can be both time-consuming and resource intensive. [20] The following subsections will explain the three distinct zone of losses in the polarization curve. These zones can be seen in Figure 17. Figure 17. Polarization Curve Zones [21] 1.1.4.1 Activation polarization zone Activation polarization refers to the voltage overpotential needed to overcome the activation energy of the electrochemical reaction on the catalytic surface. This type of polarization primarily causes losses at low current densities and indicates the effectiveness of the catalyst at a given temperature. In PEMFCs, the anode exchange current density for hydrogen oxidation is significantly higher than the cathode exchanges current density for oxygen reduction. Therefore, the cathode's contribution to activation polarization is often considered insignificant. Activation polarization should increase linearly with temperature, and the purpose of increasing temperature is to decrease activation polarization. [20] 1.1.4.2 Ohmic polarization zone At moderate current densities, the cell potential decreases linearly with current due to ohmic losses. 36 Analysis of the Degradation of a PEM Fuel Cell Subjected to Dynamic Load Cycles Conductors have an intrinsic resistance to charge flow, which results in a loss in cell voltage. This phenomenon is called “ohmic polarization,” and it occurs because of the electrical resistance in the cell components. The cell components that contribute to the electrical resistance are the electrolyte, the catalyst layer, the gas diffusion layer, bipolar plates, interface contacts and terminal connections. The reduction in voltage is dominated by internal ohmic losses through the fuel cell. 1.1.4.3 Concentration polarization zone This zone is characterized by a sharp increase in voltage drop as current density increases, due to the depletion of reactants at the electrode surface. This reduction occurs because the rate at which reactants are consumed by the electrochemical reaction exceeds the rate at which they can be supplied from the bulk solution. 1.2 Objectives The aim of this thesis is to analyze the degradation of PEM hydrogen fuel cells under dynamic load cycles. To achieve this objective, three main methods were employed, all of which were conducted in accordance with the conditions outlined in the Joint Research Centre, JRC, protocol. This protocol was used to simulate dynamic operating conditions and assess the degradation behavior of PEM fuel cells, providing a standardized and systematic approach to evaluating performance loss over time in a controlled environment. 1.2.1 Voltage Measurement Voltage measurements in fuel cells are usually performed using polarization curves. These measurements can also be carried out periodically during different stages of endurance tests to observe the evolution of the cell voltage at a given current and to analyze the specific effects of degradation. Long-term standardized tests allow to study the voltage drop over the operating cycles. 1.2.2 Electrochemical Impedance Spectroscopy Electrochemical Impedance Spectroscopy, EIS, offers valuable insights into the different degradation mechanisms by measuring the impedance response across a range of frequencies, helping to identify specific losses such as charge transfer resistance and membrane degradation. Performing EIS tests allows to observe how the internal behavior of the cell varies with the cycles. 1.2.3 Analogue Electronic Circuit An analogue electronic circuit was developed to emulate the dynamic load conditions experienced by PEM fuel cells in real-world applications. This circuit imitates the electrical behavior of the fuel cell, allowing for precise control and analysis of the fuel cell’s response to these conditions, thus aiding in the understanding of the degradation processes. In this way, it is possible to fit the EIS curves to an equivalent circuit and to study the evolution of the different parameters of that circuit with the cycles. 37 Analysis of the Degradation of a PEM Fuel Cell Subjected to Dynamic Load Cycles “I find out what the world needs. Then I go ahead and try to invent it” - Thomas A. Edison - Inventor y científico 2 METHODOLOGY In order to achieve the objective of this thesis, analyze the degradation of PEM hydrogen fuel cells under dynamic load cycles, three methods were employed. These three methods are explained below. To ensure fair and consistent comparisons of results between different PEM fuel cells, a unified test protocol is fundamental. As mentioned above, the Joint Research Centre, JRC, developed such a protocol, in collaboration with industrial and research organizations, to guide the evaluation of PEMFC under standardized conditions like temperature, pressure, voltage, and current. This protocol supports EU policies with independent, evidence-based analysis, ensuring that all fuel cells are tested consistently. The JRC Protocol includes a series of tests designed to evaluate the impact of various operational factors on the performance of the fuel cells, particularly focusing on voltage and current behavior. One of the key aspects of the protocol is the assessment of fuel cell durability, which is defined as the ability of the fuel cell to maintain performance over time without significant irreversible degradation. This is achieved through endurance testing, where the fuel cell is subjected to repetitive load cycles that simulate real-world operating conditions. A graphic example of what a load cycle would look like is shown in Figure 18. By applying these repetitive load profiles over extended periods, the protocol helps identify how the fuel cell's performance degrades over time. Degradation is typically measured by the decline in cell voltage. This testing is crucial for understanding the long-term durability of fuel cells, as it provides insights into their performance stability under conditions that imitate actual usage. [22] Figure 18. Load Cycle 2.1 Voltage Measurement Fuel cell voltage measurements are typically performed using polarization curves. These measurements can also be performed periodically during various stages of the endurance test to check the evolution of cell voltage at a given current and to understand the specific effects of degradation. 38 Analysis of the Degradation of a PEM Fuel Cell Subjected to Dynamic Load Cycles In this study, data has been provided from 15 different tests conducted on a test bench. The provided data, consisting of current intensity, voltage, and time values, was exported to an Excel sheet for later graphical analysis. Initially, the number of cycles executed in each test was assessed. Analyzing the repetition of cycles and selecting their peak values revealed that each test averaged approximately 150 cycles, each lasting around 20 minutes. Figure 19 is a graphical example of the 150 cycles per test performed, in this case test 4º. Figure 19. Current plot Test 4 After finding the number of cycles per test; cycles 1, 75, and 150 from each test were selected for detailed analysis. These cycles were graphed to show the voltage degradation, and four points on the load cycle graph were chosen to compare between the cycles of each test. Figures 20, 21 and 22 are an example of the procedure followed. Figure 20. Example Test 4, Cycle 1 I V HH I V HH I V HH 28.86 0.484 15:46:59 27.58 0.475 16:12:40 26.4 0.464 16:38:01 7.744 0.742 15:52:09 7.744 0.73 16:17:50 7.744 0.722 16:43:41 16.918 0.633 16:02:19 16.917 0.605 16:28:00 16.918 0.595 16:54:01 28.66 0.483 16:06:49 27.28 0.473 16:32:20 26.51 0.465 16:58:11 CYCLE 75 CYCLE 1 CYCLE 150 0 5 10 15 20 25 30 35 0 5000 10000 15000 20000 25000 I CURRENT TIME 4º SIMULATION 0 5 10 15 20 25 30 35 15:44:3815:47:3115:50:2415:53:1715:56:1015:59:0216:01:5516:04:4816:07:4116:10:34 I CURRENT TIME LOAD CYCLE 1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0 5 10 15 20 25 30 35 VOLTAGE I CURRENT CYCLE 1 0 5 10 15 20 25 30 16:10:34 16:13:26 16:16:19 16:19:12 16:22:05 16:24:58 16:27:50 16:30:43 16:33:36 I CURRENT TIME LOAD CYCLE 75 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0 5 10 15 20 25 30 VOLTAGE I CURRENT CYCLE 75 0 0.5 1 010 20 30 40 VOLTAGE I CURRENT CYCLE 1 0 0.5 1 010 20 30 VOLTAGE I CURRENT CYCLE 75 0 5 10 15 20 25 30 16:36:29 16:39:22 16:42:14 16:45:07 16:48:00 16:50:53 16:53:46 16:56:38 16:59:31 I CURRENT TIME LOAD CYCLE 150 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 0 5 10 15 20 25 30 VOLTAGE I CURRENT CYCLE 150 0 0.5 1 0 5 10 15 20 25 30 VOLTAGE I CURRENT CYCLE 150 I V HH 28.86 0.484 15:46:59 7.744 0.742 15:52:09 16.918 0.633 16:02:19 28.66 0.483 16:06:49 CYCLE 1 0 5 10 15 20 25 30 35 15:44:3815:47:3115:50:2415:53:1715:56:1015:59:0216:01:5516:04:4816:07:4116:10:34 I CURRENT TIME LOAD CYCLE 1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0 5 10 15 20 25 30 35 VOLTAGE I CURRENT CYCLE 1 0 0.5 1 010 20 30 40 VOLTAGE I CURRENT CYCLE 1 39 Analysis of the Degradation of a PEM Fuel Cell Subjected to Dynamic Load Cycles Figure 21. Example Test 4, Cycle 75 Figure 22. Example Test 4, Cycle 150 I V HH 27.58 0.475 16:12:40 7.744 0.73 16:17:50 16.917 0.605 16:28:00 27.28 0.473 16:32:20 CYCLE 75 16:10:34 35 0 5 10 15 20 25 30 16:10:34 16:13:26 16:16:19 16:19:12 16:22:05 16:24:58 16:27:50 16:30:43 16:33:36 I CURRENT TIME LOAD CYCLE 75 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0 5 10 15 20 25 30 VOLTAGE I CURRENT CYCLE 75 0 0.5 1 010 20 30 VOLTAGE I CURRENT CYCLE 75 I V HH 26.4 0.464 16:38:01 7.744 0.722 16:43:41 16.918 0.595 16:54:01 26.51 0.465 16:58:11 CYCLE 150 16:33:36 30 0 5 10 15 20 25 30 16:36:29 16:39:22 16:42:14 16:45:07 16:48:00 16:50:53 16:53:46 16:56:38 16:59:31 I CURRENT TIME LOAD CYCLE 150 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 0 5 10 15 20 25 30 VOLTAGE I CURRENT CYCLE 150 0 0.5 1 0 5 10 15 20 25 30 VOLTAGE I CURRENT CYCLE 150 40 Analysis of the Degradation of a PEM Fuel Cell Subjected to Dynamic Load Cycles In order to be able to carry out an accurate study, from the 15 initial tests it is necessary to eliminate all those tests who had failed, as in the case of test 5 represented in Figures 23, 24, 25 and 26. As can be seen, the load cycle of cycle 75 and cycle 150 does not follow the model established by the JRC Protocol. Figure 23.Current Plot Test 5 Figure 24. Example Test 5, Cycle 1 Figure 25. Example Test 5, Cycle 75 I V HH 27.86 0.475 23:20:25 7.744 0.736 23:25:45 16.918 0.619 23:35:55 27.62 0.474 23:40:25 CYCLE 1 0 10 20 30 40 50 60 70 0 5000 10000 15000 20000 5º SIMULATION 0 5 10 15 20 25 30 23:16:4823:19:4123:22:3423:25:2623:28:1923:31:1223:34:0523:36:5823:39:5023:42:43 Current Time LOAD CYCLE 1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 0 5 10 15 20 25 30 VOLTAGE CURRENT CYCLE 1 0 0.2 0.4 0.6 0.8 0 5 10 15 20 25 30 V/I 0 5 10 15 20 25 30 0:00:00 4:48:00 9:36:00 14:24:00 19:12:00 0:00:00 4:48:00 Current Time LOAD CYCLE 75 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 0 5 10 15 20 25 30 VOLTAGE CURRENT CYCLE 75 0 5 10 15 20 25 30 0:31:41 0:33:07 0:34:34 0:36:00 0:37:26 0:38:53 0:40:19 0:41:46 0:43:12 0:44:38 0:46:05 Current Time LOAD CYCLE 150 I V HH 27.86 0.475 23:20:25 7.744 0.736 23:25:45 16.918 0.619 23:35:55 27.62 0.474 23:40:25 CYCLE 1 0 5 10 15 20 25 30 23:16:4823:19:4123:22:3423:25:2623:28:1923:31:1223:34:0523:36:5823:39:5023:42:43 Current Time LOAD CYCLE 1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 0 5 10 15 20 25 30 VOLTAGE CURRENT CYCLE 1 0 0.2 0.4 0.6 0.8 0 5 10 15 20 25 30 V/I 23:42:43 30 0 5 10 15 20 25 30 0:00:00 4:48:00 9:36:00 14:24:00 19:12:00 0:00:00 4:48:00 Current Time LOAD CYCLE 75 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 0 5 10 15 20 25 30 VOLTAGE CURRENT CYCLE 75 41 Analysis of the Degradation of a PEM Fuel Cell Subjected to Dynamic Load Cycles Figure 26. Example Test 5, Cycle 150 Additionally, the annexes at the end of this document include the Excel files for each cycle, which provide detailed graphs, as well as the final Excel file for all cycles. 2.2 EIS Description Electrochemical Impedance Spectroscopy, EIS, is a sophisticated analytical technique utilized to prove and understand the properties and behaviors of electrochemical systems. This technique is particularly valuable because it allows the study of the complex interactions within these systems not destructively and with high precision. The principal main of EIS involves applying a small alternating current, AC, signal to an electrochemical cell and measuring the resulting voltage or current response. This process is repeated over a wide range of frequencies. By analyzing how the impedance, the opposition to current flow, of the cell varies with frequency, one can gain insights into various electrochemical processes occurring within the system. When an AC signal is applied to an electrochemical cell, the cell's response to this signal contains valuable information about its internal processes. The impedance measured at different frequencies reflects different aspects of the electrochemical behavior. At high frequencies, the impedance is often dominated by resistive elements, such as the ohmic resistance of the electrolyte and electrode materials. At lower frequencies, the impedance can provide information about slower processes like charge transfer reactions and the diffusion of species. [23] EIS is an essential tool in the field of PEMFCs development because it provides detailed insights into the performance and operational characteristics of these fuel cells. Figure 27 is an example representation of the EIS. Figure 27. EIS Cycle 14 By analyzing the impedance spectra, it can be separated and measured the contributions of different processes. These include the charge transfer resistance at the electrode interfaces, the mass transport limitations within the 0 5 10 15 20 25 30 0:31:41 0:33:07 0:34:34 0:36:00 0:37:26 0:38:53 0:40:19 0:41:46 0:43:12 0:44:38 0:46:05 Current Time LOAD CYCLE 150 -5.00E-04 0.00E+00 5.00E-04 1.00E-03 1.50E-03 2.00E-03 2.50E-03 3.00E-03 3.50E-03 0.00E+00 2.00E-03 4.00E-03 6.00E-03 8.00E-03 1.00E-02 1.20E-02 1.40E-02 1.60E-02 1.80E-02 Imaginary Z" (ohm) Real Z' (Ohm) CYCLE 14 48 Analysis of the Degradation of a PEM Fuel Cell Subjected to Dynamic Load Cycles Figure 36. Voltage response Test 13 I = 28 A* is referred to the last point chosen in the load cycle. Figure 37. Voltage response Test 15 I = 28 A* is referred to the last point chosen in the load cycle. The graphs clearly show that low currents place less stress on the materials, while high currents accelerate degradation mechanisms, such as increased internal resistance or electrode deterioration. As a result, the voltage drop is more gradual for currents of I=7A and I=16A compared to I=28A, where the lines are steeper. Additionally, comparing figures 33 and 34, corresponding to tests 1 and 4, reveals a consistent voltage drop across all points of the three cycles. This behavior aligns with the expected performance of PEMFCs as they age. 49 Analysis of the Degradation of a PEM Fuel Cell Subjected to Dynamic Load Cycles However, this pattern does not hold when comparing figures 34 and 35, corresponding to cycles 4 and 9, respectively. In this case, an increase in voltage is observed, attributed to a break period between the two tests during which the PEMFC was able to partially recover from the stress of previous cycles, as mentioned earlier. Lastly, a comparison between figures 36 and 37, representing cycles 13 and 15, once again shows a drop in voltage at all points. The higher the number of tests, and consequently the number of cycles, the more pronounced the voltage drop, indicating a decline in PEMFC performance and increased degradation of its components. 3.2 EIS Results As mentioned in section 2.2, 18 EIS tests were carried out under a current of 25 A. Of these 18 tests, all failed tests were removed; leaving 10 EIS to be studied. Figure 38 represents these ten EIS. Figure 38. EIS Graphic The EIS graph observed in Figure 38, provides insights into the performance of the PEMFC by illustrating its impedance characteristics. The lower left section of the x-axis in the graph corresponds to the ohmic resistance of the PEMFC, including resistances from the membrane, contacts, and other purely resistive components. As the curves extend to the right, they represent the dynamic behavior of the PEMFC, including the effects of charge transfer resistance and diffusion processes within the fuel cell. These dynamics are reflected in the shape and size of the semicircles or curves observed in the plot. The higher the separation along the x-axis between the starting and ending points of each curve indicates a higher total resistance of the PEMFC. This increased resistance is a sign of degradation, as it typically corresponds to issues such as increased charge transfer resistance, catalyst degradation, or membrane drying. In other words, the intercepts on the x-axis, if extrapolated, represent the total resistance of the PEMFC. A shift to the right of these intercepts implies a higher resistance, highlighting the degradation of the fuel cell's performance. Here, as in the previous sections, it is observed that the separation in EIS 9 is greater than in EIS 13. This would mean that the resistance and therefore the degradation is higher in test 9 than in test 13, which is not an expected outcome. As it was mentioned above, the voltage recovery observed in test 13 may be due to the pre- 50 Analysis of the Degradation of a PEM Fuel Cell Subjected to Dynamic Load Cycles treatment performed on the cells prior to the operation. Therefore, it can be deduced that between tests 9 and 13 there was a break in the test bench. 3.3 Analogue Electronic Circuit Results As explained in section 2.3 above, 5 EIS were performed with the ZView 2 software in order to detect degradation issues. Table 4 provides a detailed overview of how the different electrochemical parameters of the PEMFC change over the cycles. It allows the identification of efficiency patterns and possible areas of improvement or degradation within the system. CYCLE R1 CPE1-T CPE1-P R2 Ws1-R Ws1-T Ws1-P CYCLE 01 0.0031445 0.90405 0.83238 0.003514 0.0075553 0.31225 0.31311 CYCLE 01_REP 0.0030665 0.12863 1.02 1.49*10^-10 0.014872 0.21634 0.29092 CYCLE 09 0.0033278 2.684 0.65888 0.0097994 0.0083583 0.33032 0.42221 CYCLE 13 0.0032723 0.1399 1.01 1.6*10^-9 0.014132 0.21262 0.26268 CYCLE 14 0.0033613 0.14937 0.99779 6.6910^-10 0.014716 0.1673 0.27406 Table 4. ZView 2 Data First of all, it is important to understand the meaning of each of the obtained data. [25] [26] Figure 39. Equivalent Analogue Electronic Circuit 3.3.1 R1. Electrolyte Resistance R1 represents the total ohmic resistance of the system. This includes the resistance from the electrolyte, contacts, and electrodes. It appears as a series resistance in the equivalent circuit. CYCLE 01 01_REP 09 13 14 R1*10-3 3.1445 3.0665 3.3278 3.2723 3.3613 Table 5. R1 Data from ZView 2 The R1 values are almost constant throughout the different cycles. It means that there are no significant changes in electrolyte resistance. This stability suggests that the electrolyte maintains its conductive properties over time. However, it can be seen that the resistance in the last cycle is almost 12% higher than the initial resistance. One way to check that the results obtained are correct is to look at the approximate value of the ohmic resistance in Figure 38. It can be seen that R1 should have a value of approximately 0.003, thus verifying that the results obtained make sense. 51 Analysis of the Degradation of a PEM Fuel Cell Subjected to Dynamic Load Cycles 3.3.2 Total Resistance The total resistance of the analogue electronic circuit is the sum of all the resistances; ohmic, charge and Warburg resistances. CYCLE 01 01_REP 09 13 14 R1*10-3 3.1445 3.0665 3.3278 3.2723 3.3613 R2 0.003514 1.49*10^-10 0.0097994 1.6*10^-9 6.6910^-10 Ws1-R 0.0075553 0.014872 0.0083583 0.014132 0.014716 Rtotal 3.156 3.081 3.346 3.286 3.376 Table 6. Total Resistance Data from ZView 2 R2 represents the charge transfer resistance in the circuit. This resistance occurs when charge moves across the electrode surfaces during electrochemical reactions. Low R2 values indicate efficient charge transfer, while higher values may indicate problems in this process. On the other hand, the resistance parameter for the Warburg element, represents the resistance related to diffusion in the system. When R2 is practically zero, as observed in cycles 01_rep, 13, and 14, it indicates that the Warburg component carries the entire load. This suggests that even if there was a failure in the adjustment with the EIS fitting, the focus should not be only on the specific type of resistance, but rather on the total resistance of the cycle. The restrictions imposed in the operating conditions have to be fulfilled. In fact, it is observed that when R2 is nearly zero, the values of the Warburg resistor values are significantly high. 52 Analysis of the Degradation of a PEM Fuel Cell Subjected to Dynamic Load Cycles 4 CONCLUSION Throughout this document, the process carried out to study the degradation of PEM, Proton Exchange Membrane Fuel Cells, hydrogen fuel cells subjected to dynamic load cycles has been described. To this end, the cell was subjected to multiple tests to collect and compare the data obtained. Each test included Fuel Cell Load Cycle to analyze the voltage at different cycles, as well as electrochemical impedance spectroscopy, EIS, and analogue electronic circuit tests to evaluate the internal resistances and other critical parameters of the PEM cells. All tests used common operating conditions based on the Joint Research Centre, JRC, protocol. The study reveals that the efficiency and performance of the cells progressively decrease with the number of operating cycles. A significant drop in voltage is particularly evident during continuous testing without breaks, indicating increased internal resistance, reduced cell efficiency, and accelerated degradation of PEMFC components. Data from 15 tests on a PEMFC were analyzed, focusing on cell degradation under varying load conditions. The results show a consistent voltage decrease as test cycles increase, confirming internal component wear. Initially, a uniform voltage decline is observed in early cycles, aligning with the expected aging pattern of PEMFCs. However, a temporary voltage increases between cycles 4 and 9 suggests partial performance recovery due to possible rest periods between tests. In advanced cycles, such as 13 and 15, the degradation intensifies, evidenced by sharper voltage drops at higher currents, indicating increased stress on cell materials. Electrochemical Impedance Spectroscopy results further confirm rising internal resistance linked to mechanisms like increased charge transfer resistance. Overall, the findings demonstrate that PEMFC cells experience significant performance degradation under dynamic loads, leading to reduced efficiency over time. These results highlight the importance of continuing to explore methods to minimize these degradation effects and improve the lifetime of PEM hydrogen cells in practical applications. “We really haven't got any great amount of data on the subject, and without data how can we reach any definite conclusions?” - Thomas Edison - 53 Analysis of the Degradation of a PEM Fuel Cell Subjected to Dynamic Load Cycles 5 FUTURE RESEARCH AND DEVELOPMENTS Currently, proton exchange membrane fuel cells, PEMFC, represent a key technology in the transition to clean and renewable energy. PEM fuel cell is considered as a likely candidate for replacing the internal combustion engine in alternative energy transportation applications among other uses. However, as seen in this study, one of the major challenges facing this technology is the durability of its components over time. The durability of PEMFC is influenced by several factors, among which the degradation of its fundamental components stands out. This is critical to their commercial viability on a large scale. In the face of this challenge, it is essential to focus on anticipation. Improvements and research solutions are needed to find out how, and when, PEMFC will degrade and be able to avoid it. 5.1 Diagnostic and modelling techniques Progress in diagnostic techniques and predictive modelling is essential for improving PEMFCs. One of these advances is the quantum optimization method for parameter identification in fuel cells. This technique allows the precise identification of internal cell parameters, such as resistances and capacitances, both critical for its performance. By optimizing these parameters, the efficiency can be improved and the service life of the PEMFC could be extended. [27] Another important development is the identification and correction of a specific problem during cyclic voltammetry, CV. Voltammetry is an electrochemical technique used to study the reactions on the electrodes of PEMFC. The study is performed at this time because the error occurs when there is hydrogen oxidation during the reduction of platinum oxides at electrodes with low catalyst charge. Figure 40. CV profile of a Pt/C electrocatalyst [28] Figure 40 shows a CV plot of a platinum on carbon catalyst. One problem is highlighted in the graph, the spontaneous oxidation of hydrogen on the catalyst surface is mixed with the reduction of the oxidized platinum ” Look closely at the present you are constructing; it should look like the future you are dreaming. “ . - Alice Walker - 54 Analysis of the Degradation of a PEM Fuel Cell Subjected to Dynamic Load Cycles This error can lead to a misinterpretation of the catalytic activity of platinum, making it appear more active than it actually is. Since catalyst activity is crucial for assessing its performance and durability, this advance focuses on developing methods to minimize these errors. In this way, test reliability and understanding of catalyst behavior is improved. [28] 5.2 Artificial Intelligence (AI) The most current development is the implementation of AI based on systems control for real time optimization of PEMFC operating conditions. These techniques allow predicting and adjusting key parameters in real time, avoiding conditions that could lead to system degradation. This advance is possible because AI allows real time analysis of operational data generated by PEMFCs. Thus, it is possible to identify complex patterns and relationships between different variables that may not be evident with traditional methods. Through predictive models based on this data, AI can anticipate changes in operating conditions such as temperature, pressure and internal humidity. Once these changes are predicted, the AI automatically adjusts system parameters to maintain optimal performance. This ensures more stable and longer operation, significantly improving the durability and efficiency of the PEMFC. [29] Figure 41 shows what the general framework for the artificial intelligence optimization looks like. Figure 41. General framework for AI optimization. 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