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  1. \chapter{Theoretical Background}

  2. \label{cap: Theorie}

  3. 

  4. 

  5. In this chapter, the fundamental concepts and components of fuel cells are explored to provide a comprehensive understanding of Proton Exchange Membrane Fuel Cells (PEMFCs). Section 2.1 covers the basic principles of fuel cells, followed by Section 2.2, which delves into the electrochemical fundamentals, including the thermodynamics of the cell (2.2.1). Section 2.3 focuses specifically on PEMFCs, discussing their operation (2.3.1), overpotential (2.3.2), and methods of characterization (2.3.4). Finally, Section 2.4 examines degradation mechanisms in PEMFCs, detailing the degradation of the platinum catalyst (2.4.1), membrane degradation (2.4.2), electrochemical carbon corrosion (2.4.3), and overall corrosion processes (2.4.4). This structure lays the groundwork for understanding the challenges and performance factors of PEMFCs in practical applications.

  6. 

  7. 

  8. \section{Fundamentals of the Fuel Cell}

  9. \label{sec: Revox}

  10. 

  11. As said in the introduction, the clear impact of the GHG emissions and specially of the CO$_2$ emissions on climate change and the environment is undeniable. Therefore, new technologies such as fuel cells with almost no emissions and also no noise pollution are becoming a promising alternative to conventional Internal combustion engines (ICEs). This engines continue to depend on fossil fuels to function, whereas fuel cells run on hydrogen and air which undergo an electrochemical reaction within the fuel cell to generate electrical power, this reaction results in water as a byproduct\citep{01_wilberforce_advances_2016,02_baroutaji2015materials}.

  12. Battery electric vehicles (BEVs) are another alternative to ICEs which could also lower GHG as well as CO$_2$ emissions but only if the electric energy is also produced by renewable sources. However, when comparing fuel cells with BEVs there are some advantages that are worth mentioning. Fuel cells can be recharged almost instantly like ICEs and unlike BEVs. Besides that fuel cells can also run on other fuels and not only on pure Hydrogen depending on the fuel cell type. Fuel cells do not need to be disposed like Batteries and have a much longer operation time. Last but not least fuel cells have a wider range of temperatures in which they can be operated \citep{01_wilberforce_advances_2016, 02_lucia2014overview}.

  13. 

  14. Before continuing into the way of operation and the electrochemistry behind the fuel cell, it is essential to briefly explain the different types of fuel cells. These can be categorized based on the type of electrolyte membrane they use into solid oxide fuel cells (SOFCs), molten carbonate fuel cells (MFCSs), alkaline fuel cells (AFCs) and the most important one for this Thesis: polymer electrolyte membrane fuel cells (PEMFCs)\citep{02_wang2020fundamentals}. Also depending on the fuel cell type, operating temperatures may vary between -40 to almost 1000°C which will be explained in the following\citep{02_Abderezzak2018}.

  15. 

  16. \subsubsection{Solid Oxide Fuel Cells (SOFCs)}

  17. 

  18. The operating temperatures of SOFCs is higher than in the other types of fuel cells ranging from 500 to 1000 °C \citep{SOFC_hauser2021effects}. This high operating temperature allows the fuel cell to run not only on pure hydrogen but with gases which also contain methane (CH$_4$), carbon dioxide (CO$_2$) as well as carbon monoxide (CO), water vapour (H$_2$O) and hydrogen H$_2$ \citep{SOFC_lin_analysis_2024}. If the anode fuel has CH$_4$ and water vapour in it, the methane (CH$_4$) can be reformed in the fuel cell by the process of steam reforming shown in the equation \ref{eq:Steam reforming} which will produce hydrogen (H$_2$)and carbon monoxide (CO) \citep{SOFC_Haberman2004}.

  19. \begin{equation}

  20. 	\mathrm{CH}_4+\mathrm{H}_2 \mathrm{O} \leftrightarrow \mathrm{CO}+3 \mathrm{H}_2, \Delta H_{298}=2.06 \times 10^5 \mathrm{KJ} / \mathrm{Kmol}

  21. 	\label{eq:Steam reforming}

  22. \end{equation}

  23. 

  24. Another internal reaction is the water-gas-shift reaction which can turn carbon monoxide and water vapour into carbon dioxide and hydrogen in the SOFC as shown in the following equation \ref{eq:WGS} \citep{SOFC_lin_analysis_2024,SOFC_WGS_Buttler2016}.

  25. 

  26. \begin{equation}

  27. 	\mathrm{CO}+\mathrm{H}_2 \mathrm{O} \leftrightarrow \mathrm{CO}_2+\mathrm{H}_2, \Delta H_{298}=-4.1 \times 10^4 \mathrm{KJ} / \mathrm{Kmol}

  28. 	\label{eq:WGS}

  29. \end{equation}

  30. 

  31. 

  32. \subsubsection{Molten Carbonate Fuel Cells (MCFCs)}

  33. 

  34. MCFCs are also high temperature cells with an operating temperature of about 600 to 700 °C and a Electrolyte made out of molten carbonate (CO$_3$$^{2-}$). Like SOFCs they can also be operated not only with pure hydrogen (H$_2$) but also with biogas which could also contain CH$_4$ as well as CO$_2$ and CO \citep{02_lucia2014overview, 02_wang2020fundamentals}.

  35. Therefore, SOFCs and MCFCs do not any external reformer to convert other fuel types into H$_2$ as they use the same reforming reactions from the equations \ref{eq:Steam reforming} and \ref{eq:WGS}

  36. \citep{MCFScontreras2021molten}.

  37. It is also possible to combine SOFC and MCFC with working temperatures of 550-700°C which are slighlty higher than a normal MCFC at 650

  38. \citep{MCFS_cui2021review}. Furthermor, the electrical efficiency of the MCFCs can reach up to 60\% \citep{wang_preparation_2018}.

  39. 

  40. 

  41. \subsubsection{Alkaline Fuel Cells (AFCs)}

  42. 

  43. For AFCs the operating temperature is about 50-200 °C which is much lower than those of the SOFC and MCFC but a lot closer to the temperature in which PEMFCs are operated \citep{02_wang2020fundamentals}. Another main difference between AFCs and SOFCs is, that the electrolyte in a AFC is liquid unlike the solid state of the ceramics used in SOFCs. AFCs uses a potassium hydroxide solution (KOH) as an electrolyte which is embedded in the matrix \citep{02_Abderezzak2018,02_wang2020fundamentals}. Unlike SOFCs and MCFC and due to the lower temperature, AFCs can be poisoned by carbon dioxide (CO$_2$). In that case, the alkaline electrolyte could  react directly with the CO$_2$ which could lead to the the following reaction equations \ref{eq:AFC_Poisoning} \citep{AFC_mclean2002assessment}:

  44. 

  45. \begin{equation}

  46. 	\begin{aligned}

  47. 		&\mathrm{CO}_2+2 \mathrm{OH}^{-} \rightarrow \mathrm{CO}_3^{2-}+\mathrm{H}_2 \mathrm{O} \text { and/or }\\

  48. 		&\mathrm{CO}_2+2 \mathrm{KOH} \rightarrow \mathrm{K}_2 \mathrm{CO}_3+\mathrm{H}_2 \mathrm{O}

  49. 	\end{aligned}

  50. \label{eq:AFC_Poisoning}

  51. \end{equation}

  52. 

  53. This reaction could reduce the ionic conductivity of the electrolyte as well as block the pores in the electrode. The carbonate shown in the equation \ref{eq:AFC_Poisoning} could also block the pores of the catalyst resulting in the aforementioned reduction of ionic conductivity in the electrolyte

  54. \citep{AFC_mclean2002assessment, AFC_AlSaleh1994_CO2, AFC_AlSaleh1994_Ni}.

  55. 

  56. \subsubsection{Polymer Electrolyte Membrane Fuel Cells (PEMFCs)}

  57. 

  58. For Polymer electrolyte membrane fuel cells the operating temperature can be lower than those from the other fuel cell types including the AFCs, PEMFCs can be operated at 40 to 80 °C \citep{02_lucia2014overview}. For high temperature PEMFCs the operating temperature can even go as high as 150-180 °C \citep{02_wang2020fundamentals}. Because of its low operating temperature and its high output power density it is highly suited for mobile applications like the automotive industry. PEMFCs run on pure hydrogen as fuel and cannot use Biogas containing methane (CH$_4$), carbon dioxide (CO$_2$) or carbon monoxide CO as they cannot reform it internally to pure hydrogen H$_2$ \citep{PEM_Atuomotive_arrigoni2022greenhouse}. 

  59. 

  60. Its mobile applications have turned the PEMFCs into one of the biggest research fields in the search for greener alternatives to conventional ICEs. It typically features a solid polymer electrolyte membrane and porous carbon electrodes with platinum functioning as catalyst \citep{01_wilberforce_developments_2017}.

  61. 

  62. Since this thesis focuses its attention on automobile applications, the following sections will provide a detailed explanation of the relevant fuel cell. However, before looking into the way of function the subsequent section will first cover the electrochemical fundamentals. 

  63. 

  64. 

  65. 

  66. \section{Electrochemical  Fundamentals}

  67. 

  68. The function of a fuel cell is to transform the chemical energy stored in the fuel (hydrogen H$_2$) in electrical energy. During this electrochemical reaction the fuel is transformed but the fuel cell is not consumed by the energy production unlike in a battery. In a fuel cell the electrochemical redox reaction is split in two between the cathode and the anode which are separated by an electrolyte \citep{Fundamentals_o2016fuel}.

  69. 

  70. \begin{align}

  71. 	\text{Anode:} \quad & \text{H}_2 \rightarrow 2\text{H}^+ + 2e^- \\

  72. 	\text{Cathode:} \quad & \frac{1}{2}\text{O}_2 + 2\text{H}^+ + 2e^- \rightarrow \text{H}_2\text{O} \\

  73. 	\text{Overall:} \quad & \text{H}_2 + \frac{1}{2}\text{O}_2 \rightarrow \text{H}_2\text{O}

  74. 	\label{eq:PEM}

  75. \end{align}

  76. 

  77. In the anode the oxidation part of the reaction takes place. Electrons are removed from the Hydrogen H$_2$ as shown in the equation (2.4). In the Cathode those liberated electrons are consumed by the reduction reaction and oxygen (O$_2$) and 2H$^+$ form water (H$_2$O) as a product. This is summed up in the overall reaction from te equation (2.6) \citep*{Fundamentals_o2016fuel}. Moreover, the anodic reaction can also be called hydrogen oxidation reaction (HOR) and the cathodic reaction is called oxygen reduction reaction (ORR) \citep{Fundamentals_scherer2012fuel}.

  78. 

  79. \subsection{Thermodynamics of the Cell}

  80. 

  81. In addition, the reaction produces heat (or enthalpy H) since there is a difference $\Delta$H between the enthalpy of the products and the enthalpy of the reactants \citep{Fund_barbir2008fuel}.

  82. \begin{equation}

  83. 	\Delta H=\Delta H_{product}- \Delta H_{reactant}

  84. 	\label{eq:Enthalpy}

  85. \end{equation}

  86. 

  87. The Gibbs free energy corresponds to the enthalpy in the reaction that can be converted to electricity. It is defined by the following equation (\ref{eq:Gibbs}). $\Delta$H is the difference of the enthalpy and T$\Delta$S expresses the losses of entropy ($\Delta$S) which are dependent on the temperature

  88. \citep{Fund_barbir2008fuel}.

  89. 

  90. \begin{equation}

  91. 	\Delta G=\Delta H-T \Delta S

  92. 	\label{eq:Gibbs}

  93. \end{equation}

  94. 

  95. Furthermore, with the help of the Faraday constant F (96,487 C/mol) and n for the number of electrons transferred in the reaction as well as a value for $\Delta$G for the Gibbs free energy the reversible theoretical potential E$_{rev}$ of a cell and in standard conditions can be calculated with the following equation

  96. \citep{Fundamentals_scherer2012fuel}:

  97. 

  98. \begin{equation}

  99. 	E_{rev}=\frac{-\Delta G}{n F}

  100. 	\label{eq:E}

  101. \end{equation}

  102. 

  103. E$_{rev}$ can also be referred to as E$^0$ which is the open circuit voltage (OCV) in standard conditions (1 atm and 25°C or 298K)\citep{Fundamentals_scherer2012fuel, F_omran2021mathematical}. Since this equation (\ref{eq:E}) can be used only in standard conditions the reversible potential of a cell in non-standard conditions cannot by calculated by it. In a non-standard case where the temperature or the pressure is another, the Nernst equation can be used

  104. \citep*{Fundamentals_o2016fuel,Nernst_sahu2014performance}.

  105. 

  106. \begin{equation}

  107. 	E=E^0-\frac{R T}{n F} \ln \frac{\prod a_{\text {products }}^{v_i}}{\prod a_{\text {reactants }}^{v_i}}

  108. 	\label{eq:nernst}

  109. \end{equation}

  110. 

  111. To be able to understand the Nernst equation first the concept of chemical potential $\mu$ has to be explained \citep{Nernst_mardle2021examination}. The chemical potential describes how the number of molecules or atoms $n_i$ of a species $i$ afects the thermodynamic potentials. In this case $a$ is the activity of the species. which for an ideal gas is $a_i$ = $p_i$/$p^0$. If the gas is non ideal it has to be multiplied with $\gamma$ wich describes how far away the gas is from an ideal one $(0<\gamma<1)$ with $\gamma=1$ as an ideal gas \citep{Fundamentals_o2016fuel}. In the Nernst equation $v_i$ refers to the stoichiometric coefficient of the products or of the reactants. R is the gas constant (R = 8,314 J/molK)

  112. 

  113. \begin{equation}

  114. 	\mu=\mu^0+R \operatorname{Tln}(a)

  115. 	\label{eq:mu}

  116. \end{equation}

  117. 

  118. The following equation can also be rewritten to describe how the chemical potential relates to the Gibbs free energy.

  119. 

  120. \begin{equation}

  121. 	\mu_i^\alpha=\left(\frac{\partial G}{\partial n_i}\right)_{T, p, n_{j \neq i}}

  122. 	\label{eq:chem1}

  123. \end{equation}

  124. 

  125. Using the equations (\ref{eq:chem1}) and (\ref{eq:mu}) it is possible to calculate how the Gibbs free energy changes with the $i$ different chemical species resulting in the following equation (\ref{eq:gibbs_2}):

  126. 

  127. \begin{equation}

  128. 	d G=\sum_i \mu_i d n_i=\sum_i\left(\mu_i^0+R T \ln a_i\right) d n_i

  129. 	\label{eq:gibbs_2}

  130. \end{equation}

  131. 

  132. Finally this equation (\ref{eq:gibbs_2}) can be inserted into the equation (\ref{eq:E}) to form the Nernst equation (\ref{eq:nernst}) in its general form\citep{Fundamentals_o2016fuel}.

  133. 

  134. \newpage

  135. 

  136. \section{PEMFC}

  137. \label{sec: PEMFC}

  138. 

  139. Since this Thesis focuses on the automotive applications of fuel cells the focus of the following sections will shift to the Polymer Electrolyte Membrane Fuel Cell (PEMFC) which are the most widely used in automotive contexts due to their low operating temperature as well as its high output power density \citep{PEM_Atuomotive_arrigoni2022greenhouse}.

  140. In the following section \ref{subsec:2_wayoffunct} the PEMFC will be described in more detailed way starting with its main components and its way of function.

  141. 

  142. \subsection{Way of Function PEMFCs}

  143. \label{subsec:2_wayoffunct}

  144. 

  145. To be able to produce more energy PEMFCs use not only one cell but a stack formed by hundreds of cells stacked on top of each other in between two monopolar plates at the ends as shown in the left side of the following figure \ref{fig:PEMFC} \citep{PEMSchem_xu2020towards}.

  146. 

  147. %vielleicht eine selber machen? Was ist hier los mit der quelle? Bug

  148. \begin{figure}[htbp]

  149. 	\centering

  150. 	\includegraphics[width=0.8\textwidth]{Figures/Theorie/PEMFC.pdf}

  151. 	\caption{Components of a PEMFC cell and its position in a fuel cell stack. Retrieved from Xu et al. page 816 [33].}

  152. 	\label{fig:PEMFC}

  153. \end{figure}

  154. 

  155. Every cell is composed by two bipolar plates (BPs) each with its anode and cathode side. Between each BP there is the membrane electrode assembly layer (MEA). The MEA consists of a proton exchange membrane in the middle of two catalyst layers (CL) and two gas diffusion layers (GDL) one on the cathode side and one on the anode side. Since the redox reaction in the fuel cell (equation \ref{eq:PEM}) is an exothermic reaction which generates heat the cell needs cooling, therefore the coolant can flow inside specific flow channels of the BP as illustrated in the figure \ref{fig:PEMFC} to prevent the system from overheating \citep{PEMSchem_xu2020towards}. The key components of the PEMFC and its functions will be explained in the following starting with the BPs. Since the focus of this Thesis is the BP corrosion this key component will be presented in a more detailed form.

  156. 

  157. \newpage

  158. \subsubsection{Bipolar Plate (BP)}

  159. 

  160. The main functions of the BPs is to distribute fuel on the anode side and oxidant on the cathode side to the reactive sites in the catalyst layer (CL). It also collects the generated current and removes the byproducts from the reaction. Heat management is also a very important function, therefore special channels (flow channels) transport the coolant and remove the heat from the cell \citep{PEM_baroutaji2015materials}. 

  161. 

  162. Since the BPs are responsible for 60-80 \% of the weight of as well as 20-30\% of the total cost of the fuel stack the materials used for it have been under investigation for some time and the Department of Energy (DOE) has set up Targets for the components of the fuel cell \citep{doe_pemfc_targets}. This targets evaluate not only the cost of the materials but also durability and performance \citep{doe_pemfc_targets}. Due to their durability, excellent mechanical strength, high power density and electric conductivity, the investigations of new BP materials has been primarily focused on stainless steels, titanium alloys and aluminium alloys \citep{antunes2010}. In the past BPs were made out of graphite. Graphite is highly corrosion resistant, unfortunately it has some drawbacks like its high permeability for gases and production costs \citep{PEM_baroutaji2015materials}. 

  163. 

  164. Stainless steels on the other hand are more cost effective and versatile, its high mechanical strength and malleability results in the possibility of producing thinner BPs which can lead up to a weight reduction of 40\% of the fuell cell stack

  165. \citep{SSweight_li2005review}. This optimal characteristics have attracted the automotive sector and companies like Hyundai, GM and Honda which have produced fuel cell vehicles (FCV) with this stainless steels \citep{Automotive_leng2020}. Toyota on the other hand

  166. uses a titanium bipolar plate nano composite (NC) as a surface treatment for the BPs used in the Mirai stack. This ensured a reduction in the thickness of the Titanium (Ti) plates so that the Platinum (Pt) used in the stack could be reduced by 58\% from its 2008 model to the second-generation Mirai \citep{toyota_technical_review_2021}. Since the collaboration between BMW and Toyota was announced in September 2024, BMW will also be using BPs manufactured by the Toyota Motor Corporation for its BMW iX5 Hydrogen planned for series production in 2028 \citep{bmw_hydrogen_2024}.

  167. 

  168. Stainles Steels BPs have a lower cost than Ti plates but its durability has been questioned since its corrosion resistance is lower than Ti plates and Aluminium plates. When Stainless steel plates corrode they release metal ions like Fe$^{2+}$ which lead to a accelerated chemical degradation of the membrane by contaminating the MEA. \citep{eom2012}. Even though Aluminium has a higher corrosion restistance than stainless steels it would release Al$^{3+}$ ions during the corrosion process which have an even bigger effect than Fe$^{2+}$ on the fuel cell catalyst \citep{sulek2011}. This form of degradation will be explained in the Section \ref{sec:Degradation}. Although SS316L has proven to have a higher corrosion resistance and is therefore used as reference material for Bipolar plates other more cost effective options have also been studied like stainless steels 310L, 304 and 904L\citep{papadias2015degradation,feng2011}.

  169. 

  170. \subsubsection{Membrane Electrode Assembly (MEA)}

  171. 

  172. As mentioned before the Membrane Electrode Assembly (MEA) is conformed by the Gas Diffusion layer (GDL) on the cathode side as well as the GDL on the anode side followed by the two catalyst layers (CL) which contain platinum (Pt) and Nafion and in the middle the proton exchange membrane (PEM) \citep{PEMSchem_xu2020towards}.

  173. The MEA could be looked at as the most important component of a PEMFC as it is responsible for the chemical reactions and consequently for the performance of the fuel cell \citep{MEA_lim2021comparison}. 

  174. 

  175. Three fabrication methods of MEA stand out because of its perfomance, the first one catalyst-coated membranes (CCMs) and catalyst-coated substrates (CCSs) and catalyst-coated electrode(CCE) \citep{MEA_lapicque2012,MEA_bhosale2020}. 

  176.  A comparative study by Bhosale et al. \citep{MEA_bhosale2020} showed that the most effective method is CCE. Nevertheless, CCM shows high performance and many studies suggest that MEA produced with a CCM method can have many advantages over CCS and CCE, therefore it is the most used method \citep{PEM_MEA_parekh2022recent}. 

  177.  

  178.  CCM can lead to an increase of the total reactions in the MEA as well as reducing the Pt amount in the catalyst \citep{MEA_lim2021comparison}.

  179. Since Pt can be very expensive methods to reduce the weight-loading percentage of the catalyst have also been under investigation and even Pt-free catalysts \citep{Pt_liew2014}. Moreover, Pt catalyst can be lost during dynamic operation of the cell (voltage cycling) either by Pt agglomeration or Pt dissolution which also leads to further degradation of the cell and  higher mass transport losses as well as activation losses \citep{thiele2024realistic}. To be able to understand the MEA better now its components will be explained as well.

  180. 

  181. 

  182. \subsubsection{Gas Diffusion Layer (GDL)}

  183. 

  184. Starting with the first layer of the MEA right between the BPs and the catalyst layers (CL) on both sides is the gas diffusion Layers (GDL) and also the microporous layer (MPL)\citep{PEMSchem_xu2020towards}. Their primary function is to offer mechanical support to the MEAs, ensure the flow of the reactants and also the removal of products. Furthermore, they have to enable the electron conduction between the CLs and the BPs on both sides. 

  185. 

  186. Since its main functions are related to diffusion the GDL is made out of porous materials, typically it is made out of carbon paper\citep{02_wang2020fundamentals}.

  187. To enhance water management and prevent flooding in the electrode the carbon paper GDL hast to be hydrophobic. For that reason Polytetrafluorethylene (PTFE) is often added to it as treatment to achieve this hydrophobicity

  188. \citep{02_wang2020fundamentals,GDL_zamel2011}. Since a high PTFE load can cause an obstruction in the pores of the GDL and consequently cause mass transport limitations it is crucial to add the right amount. Studys have shown that 20 wt.\% is an optimal load percentage for PTFE to turn the carbon paper hydrophobic without blocking the pores \citep{GDL_zamel2011}. Also important are the capillary effects of the MPL, since the MPL is also hydrophobic it provides a great drainage as well as stable gas and electron channels \citep{ijaodola2019}. This helps the overall performance of the cell by reducing the flooding of the cell since this layer is in-between of the GDL and CL \citep{majlan2018}.

  189. 

  190. \subsubsection{Catalyst Layer (CT)}

  191. 

  192. The catalyst layers (CTs) are positioned between the PEM and the MPL on both sides. Electrochemical reactions take place here in the CL therefore it has to provide continuous pathways for the different reactants. More specific it has to provide a route for proton transport, its porous structure has to supply the gaseous reactants to the site as well as remove the water while also being able to form a conductive pathway for electrons between the CL and the current collector \citep{02_wang2020fundamentals}.

  193. 

  194. The oxygen reduction reaction (ORR) takes place at the CL and is the most critical process for a PEMFC at the anode \citep{PEM_baroutaji2015materials}. This reaction heavily relies on platinum (Pt) catalyst, by increasing the platinum load  the ORR rate can be enhanced which leads to a higher power output in the cell \citep{PEM_MEA_parekh2022recent}. Contrary to the GDL and MPL platinum particles are not hydrophobic since they present hydrophilic capabilities \citep{CT_malek2011}. 

  195. 

  196. As stated before, the CCM method is currently the most used in the production of the MEA. Since Pt is the most expensive part of the production and CCM production already has a better electrochemical performance than the CCS method and it showcases a lower Pt load\citep{hnat2019}. In this method the catalyst layer is produced by applying catalyst ink on a PEM \citep{MEA_lim2021comparison}. But a big challenge continues to be the search for a more cost effect alternative to Pt catalyst with the same electrochemical performance \citep{PEM_MEA_parekh2022recent}.

  197. 

  198. The Catalyst layer consists of a catalyst (Pt), carbon support, ionomer and a void space. PTFE was subsituted with recast Nafion ionomer as a binder allowing for a significant reduction in Pt loadings. Additionally Pt supported on carbon (Pt/C) also helps to decreases the metal content\citep{ink_zamel2016catalyst}. The ionomer serves a dual purpose as a binder for Pt/C particles and as a proton conductor. An imbalance in ionomer loading can lead to transport or ohmic losses which will be discussed in the subchapter \ref{subsec:losses}. Insufficient ionomer diminishes proton conductivity and an excessive amount can increase a resistance of  gaseous reactant transport \citep{02_wang2020fundamentals}.

  199. 

  200. 

  201. 

  202. 

  203. \subsubsection{Proton Exchange Membrane (PEM)}

  204. 

  205. The proton exchange membrane is the heart of the PEMFC, it is in the middle between cathode and anode followed by CL, MLP and then GDL in that order from the inside to the outside. The PEM has to primary functions. The first one is serving as a barrier, it prevents the mixing of reactant gases and electrons between the anode and cathode. The second one is to facilitate proton conduction from the CL on the anode to the CL on the cathode side. Furthermore, the PEM is impermeable for gas, it stops the oxygen and hydrogen crossover and it has to be electrically insulating. Another requirement for the membrane is a exceptional chemical and mechanical stability to be able to endure the harsh operating conditions of the PEM fuel cells \citep{ghassemzadeh2010chemical}.

  206. 

  207. The most widely used material for the membrane in a PEMFC is perfluorosulfonic acid (PFSA) also referred to as Nafion which was developed by DuPont

  208. \citep{PEM_MEA_parekh2022recent}. Figure \ref{fig:Nafion} shows the chemical structure of Nafion \citep{okonkwo2021nafion}.

  209. 

  210. 

  211. \begin{figure}[htbp]

  212. 	\centering

  213. 	\includegraphics[width=0.5\textwidth]{Figures/Theorie/Nafion.pdf}

  214. 	\caption{Chemical structure of PFSA also called Nafion. Retrieved from Chen et al., page 1436 (1) [59]}

  215. 	\label{fig:Nafion}

  216. \end{figure}

  217. 

  218.  Since the main chain is Teflon-like it has an hydrophobic side and the sulfonic acid groups on the side chains are hydrophilic. This is a great advantage because it facilitates water adsorption and consequently proton conduction. To maintain an effective proton transport proper hydration of the membrane is vital while avoiding excessive moisture that could lead to flooding in the CL and GDL \citep{zaidi2009polymer}.

  219.  

  220.   However the membrane can degrade when it is exposed to low humidity and high temperatures. While degrading Nafion can release F$^-$, CO$_2$, SO$_4^{2-}$, SO$_2$ as well as fluorocarbons \citep{teranishi2006}. Besides this form of degradation the electrochemical reaction in a PEMFC can also produce hydrogen peroxide (H$_2$O$_2$) when the entry of oxygen in the PEM reacts with the hydrogen in the anode as shown in the following equation (\ref{eq:h2o2}) \citep{ren2020degradation}:

  221.   

  222. \begin{equation}

  223.  	\mathrm{O}_2+2 \mathrm{H}^{+}+2 e^{-} \rightarrow \mathrm{H}_2 \mathrm{O}_2, \mathrm{E}_{\mathrm{O}}=0.695 \mathrm{~V} \text { vs. } \mathrm{SHE}\left(75^{\circ} \mathrm{C}\right)

  224.  	\label{eq:h2o2}

  225.  \end{equation}

  226.  

  227.  

  228.  Furthermore, H$_2$O$_2$ in the presence of ferrous ions like Fe$^{2+}$ which are released by the BP when corroding can trigger the formation of hydroxyl radicals which attack the membrane as well. It is thought that an incomplete reduction of the oxygen by the Pt catalyst can trigger the production of H$_2$O$_2$ as well \citep{elferjani_coupling_2021}.

  229. 

  230. 

  231. 

  232. \subsection{Department of Energy Targets}

  233. 

  234. The Deparment of Energy (DOE) of the United States(U.S.) with the help from the U.S. DRIVE partnership has set targets for the components of PEMFC to help FC developers develop them without the need to test the full system

  235. \citep{doe_pemfc_targets}. U.S DRIVE FC team aims to develop a PEMFC system for transportation able to resist 8000 hours and with a mass production cost of 35\$ per Kilowatt (kW) by 2025 \citep{trabia2016}.

  236. 

  237. Targets of the DOE include specifications for the MEA, PEM, electrocatalysis and bipolar plates. The goal for BPs is to reduce the plate cost from 5,4\$ to 2\$ per kW until 2025. Some other goals include the weight reduction of the BPs and increased corrosion resistance as well as a higher electric conductivity\citep{PEM_MEA_parekh2022recent}. Overall the DOE wants to increase cell performance and at the same time reduce production costs to allow PEMFCs in FCV and fuel cell electrical vehicles (FCEV) becoming an cost effective and green alternative to ICE in the series production.

  238. 

  239. 

  240. \subsection{Overpotentials of the PEMFC}

  241. \label{subsec:losses}

  242. 

  243. 

  244. \begin{figure}[htbp]

  245. 	\centering

  246. 	\includegraphics[width=0.8\textwidth]{Figures/Theorie/Polarization.pdf}

  247. 	\caption{Polarization curve of a fuel cell including the different losses. Retrieved from Jung et al., page 741 (4) [64].}

  248. 	\label{fig:losses}

  249. \end{figure}

  250. 

  251. The Nernst equation \ref{eq:E} calculates the reversible cell potential which is the current that should be drawn by the PEMFC. However the actual measured open-circuit voltage (OCV) is lower than the theoretical one calculated by the equation \citep{Loss_mardle2021examination}. 

  252. This deviation can be observed in the figure \ref{fig:losses} \citep{Loss_jung2010dynamic}. This first deviation is caused by the hydrogen (H$_2$) crossover. This occurs when H$_2$ diffuses through the membrane, leading to a mixed potential that lowers the overall open-current potential (OCP). Internal short circuits also lead to OCV losses in this stage as well \citep{Loss_mazzeo2024assessing}.

  253. 

  254. In addition to the first loss the polarization curve experiences other deviations as the current density starts to grow. Starting with the activation losses then the Ohmic losses and at high current densities the mass transport losses which will all be explained in the following \citep{02_lucia2014overview}.

  255. 

  256. 

  257. \subsubsection{Activation Polarization}

  258. 

  259. The activation loss also called activation polarization loss is driven by the voltage loss caused by the activation energy required for the electrochemical reaction to start as the protons move through the reaction interface. Therefore it is a loss related to the kinetic of the cathode and anode electrodes \citep{Loss_li2022new}. As shown in the image \ref{fig:losses} it takes place at a region with low current densities. It$\eta_{act}$ can be calculated using the following equation (\ref{eq:Loss_N}) \citep{ren2020degradation}.

  260. 

  261. \begin{equation}

  262. 	\eta_{\text {act }}=\frac{R T}{\alpha n F} \ln \left(\frac{i_{\text {loss }}}{i_0}\right)

  263. 	\label{eq:Loss_N}

  264. \end{equation}

  265. 

  266. In this equation the following parameters are taken into account: F for the Faraday constant, $i_0$ as the exchange current density for the active area of the FC, $\alpha$ as charge transfer coefficient, R is the gas constant and n the number of molecules or atoms. Furthermore, $i_{loss}$ is formed as the addition of $i_{short}$ for the current density of short-circuits and $i_{crossover}$ which is the gas-crossover current density \citep{ren2020degradation,jouin2016}.

  267. 

  268. \begin{equation}

  269. 	i_{\text {loss }}=i_{\text {crossover }}+i_{\text {short }}

  270. \end{equation}

  271. 

  272. It is worth mentioning that the exchange current density of the oxygen reduction reaction (ORR) can be perceived as a limiting factor in low temperature PEMFC like the ones used for the automotive sector \citep{Loss_mazzeo2024assessing}. 

  273. 

  274. 

  275. \subsubsection{Ohmic Polarization}

  276. 

  277. As the current density increases, ohmic polarization loss becomes the dominant factor in the polarization curve. Voltage decreases in an almost linear way with increasing current density \citep{Loss_li2022new}. Ohmic losses are associated with the resistance encountered by the flow of the electrons through various components of the FC \citep{Loss_mazzeo2024assessing}. The resistance of the hydrogen ion flow into the electrolyte is a significant factor. This resistance is heavily influenced by membrane's hydration level as well as operating temperatures and current density \citep{springer1991}. Mathematically it can be described with the following equation (\ref{eq:Loss_ohm})\citep{ren2020degradation}.

  278. 

  279. \begin{equation}

  280. 	\eta_{\mathrm{ohm}}=\left(R_{\mathrm{ion}}+R_{\mathrm{ele}}+R_{\mathrm{con}}\right) \cdot i

  281. 	\label{eq:Loss_ohm}

  282. \end{equation}

  283. 

  284. In this equation R$_{ion}$ represents the ionic resistance, R$_{con}$ the contact resistance and R$_{ele}$ the electronic resistance. For this section the polarization behaves linearly since it is multiplied with the current density ($i$) \citep{ren2020degradation}.

  285. 

  286. \subsubsection{Concentration Polarization}

  287. 

  288. At high current densities the concentration polarization or concentration loss occurs. The reactants are consumed very quick during the electrochemical reactions at a high current density. Because of transport and diffusion resistance the availability in of the reactants at the reaction sites decrease which limits the the reactions and thereby the efficiency of the PEMFC \citep{Loss_li2022new}.

  289. The ohmic polarization can be calculated using the  following equation (\ref{eq:Loss_con}) 

  290. \citep{ren2020degradation}:

  291. 

  292. \begin{equation}

  293. 	\eta_{\text {con }}=\left(1+\frac{1}{\alpha}\right) \frac{R T}{n F} \ln \left(\frac{i_{\mathrm{L}}}{i_{\mathrm{L}}-i}\right)

  294. 	\label{eq:Loss_con}

  295. \end{equation}

  296. 

  297. The parameters of the equation (\ref{eq:Loss_con}) are the same as in the others before that with the only new one being $i_L$ which stands for the limiting current density.

  298. 

  299. At such high operating it is important to avoid undersupply of the anode which could damage the PEMFC. Therefore, changing the hydrogen-oxygen stoichiometric ratio from 1:1 to 1.5:2.2 can improve the performance of the PEMFC as well as reduce damage caused by the operation on high current densities \citep{liu2024study}.

  300. 

  301. 

  302. \subsection{Characterization of PEMFC}

  303. 

  304. As this thesis includes an endurance run and preliminary investigations to better understand the operating conditions of the PEMFC and identify the optimal point for triggering cell corrosion, this section will detail the in-situ methods employed to characterize the cells. Parameters like the cell potential and the current density can give an insight into the state of health of the cell. By using predefined characterisation curves in-between a specific number of voltage cycles the cell degradation can be tracked. Figure \ref{fig:PolCurve} shows an example of a the polarization curves after a specific number of voltage cycles \citep{mohsin2020electrochemical}.

  305. 

  306. \begin{figure}[htbp]

  307. 	\centering

  308. 	\includegraphics[width=0.7\textwidth]{Figures/Theorie/PolCurve.pdf}

  309. 	\caption{Example of a polarization curve of a PEMFC after different numbers of voltage cycles (VC)  . Retrieved from Mohsin et al., page 24096 (4) [69].}

  310. 	\label{fig:PolCurve}

  311. \end{figure}

  312. 

  313. 

  314. In this polarization curves the potential of the cell is plotted over the current densities. Degradation of the membrane, corrosion, carbon corrosion or as a consequence of it platinum catalyst dissolution causes the polarization curve to have a higher drop in the potential at much lower current densities after more cycles as shown in the aforementioned figure \ref{fig:PolCurve} \citep{Pol_thiele2024realistic}. This mechanisms will be explained in the following section \ref{sec:Degradation}. After a larger number of voltage cycles the higher current densities can no longer be reached as a consequence of the degradation as well as bigger activation, ohmic and concentration losses in the cell \citep{mohsin2020electrochemical}.

  315. 

  316. 

  317. \section{Degradation Mechanisms}

  318. \label{sec:Degradation}

  319. 

  320. Automotive conditions can be very stressful for the PEMFC. Start-stop procedures, idling conditions, operation at maximum power as well as quick changes from full power to stop can speed up the degradation progress of the cell and therefore shorten its lifetime \citep{pei2008}. Accelerated stress tests (AST) are a way of testing the components of a PEMFC in a controlled environment without them being in the actual vehicle. It can shorten the test duration by accelerating the degradation processes and simulating different conditions and automotive scenarios \citep{Pol_thiele2024realistic}. This section will provide information on a few of the most important degradation mechanisms that can be found in a PEMFC like platinum catalyst dissolution, membrane degradation, carbon corrosion and finally corrosion. It is also important to mention that there are a lot more mechanisms which can contribute to the degradation of the fuel cell and that these mechanism impact one another\citep{Pol_thiele2024realistic}.

  321. 

  322. \newpage

  323. 

  324. 

  325. \subsection{Platinum Catalyst Dissolution and Agglomeration}

  326. \label{subsec: Pt}

  327. 

  328. Carbon-supported platinum nanoparticles in the CL of the PEMFC increase the oxygen reduction reaction (ORR) at the cathode making the cell more efficient. With such an important task it is of utmost importance to understand the degradation mechanism. Studys  have shown, that corrosive acidic environments in the PEMFC under a positive potential can lead to platinum dissolving which consequently causes a reduction in the catalyst performance \citep{cherevko2015}.

  329. Pt loss during PEMFC operation is a major contributor to the degradation of the CL. This is driven by processes such as platinum dissolution, Pt-detachment, Pt-migration and Pt-agglomeration. In a study by Luo et al. a 10 cell stack was operated for 200 hours at a temperature of 60 °C. When analysed, the stack showed a reduction from an initial Pt content of 20\% to 13,5\% \citep{luo2010}.

  330. 

  331.  While Pt can remain stable at potentials below 1,188 V at high cell voltages and OCV direct electrochemical dissolution may occur at the cathode. At normal operation conditions or during load cycling the dissolution of Pt is more likely to occur \citep{wallnofer2024main}. Lower electrode potentials as well as the voltage cycling can cause Pt oxide dissolution which can be described by the following equations \citep{takei2016}:

  332. 

  333. 

  334. \begin{equation}

  335. 	\mathrm{Pt} \rightarrow \mathrm{Pt}^{2+}+2 e-\mathrm{E}_0=1.188 \mathrm{~V}

  336. \end{equation}

  337. \begin{equation}

  338. \mathrm{Pt}+\mathrm{H}_2 \mathrm{O}+2 e^{-} \rightarrow \mathrm{PtO}+2 \mathrm{H}^{+} \quad \mathrm{E}_0=0.98 \mathrm{~V}

  339. \end{equation}

  340. \begin{equation}

  341. 	\mathrm{PtO}+2 \mathrm{H}^{+} \rightarrow \mathrm{Pt}^{2+}+\mathrm{H}_2 \mathrm{O}

  342. \end{equation}

  343. 

  344. Since water is produced in the reaction (2.21)  the higher water content in the ionomer leads to a greater mobilty of the dissolved Pt ions which can facilitate the Ostwald ripening of the particles beneath it \citep{takei2016}.

  345. 

  346. Platinum migration is another Problem which can degrade the PEMFC by loss of CL performance. The Pt particles may diffuse into the ionomer phase and then precipitate within the membrane (PEM). Furthermore, hydrogen migrating from the anode to the cathode can reduce the Pt ions forming Pt$^{2+}$ and Pt$^{4+}$. This can again cause the oxidation of Pt to PtO as shown in the previous reactions and consequently decrease the cell performance due to the lingering oxygen \citep{pavlivsivc2018platinum,okonkwo2021platinum}. Agglomeration process can facilitate the formation of oxygenated functional groups on the carbon surface which then lead to an increased hydrophilicity of the carbon support. This altered hydrophilicity can influence the displacement of oxygen towards the Pt by controlling the flooding in the CL. Flooding can limit oxygen access to the active reaction sites within the CL and hence decrease the efficiency of the PEMFC \citep{okonkwo2021platinum}.

  347. 

  348. Losses of activity in the reaction sites can be categorized into two groups. The first one being the unrecoverable losses and the second one are the re-coverable losses. Pt-dettachment as well as agglomeration, dissolution, carbon corrosion and Pt re-deposition are associated with the first group, the unrecoverable losses. Start/ End scenarios expose the cell to very rapid changes in the parameters. Also operating under extreme conditions can accelerate the degradation of the cell and favor the aforementioned mechanisms. 

  349.  \citep{okonkwo2021platinum}. The recovery loss was linked either to the reduction of platinum oxide or the removal of carbon monoxide which is produced because of the carbon corrosion \citep{okonkwo2021platinum} .

  350. 

  351. There is also a second way of classifying the degradation mechanisms. Since one degradation mechanism can trigger or favour another mechanism they can also be classified as primary or secondary depending on their ability to start or intensify another mechanism. For example carbon corrosion is a primary mechanism since it can be responsible for Pt agglomeration and detachment leading to an increased degradation \citep{okonkwo2021platinum}.

  352. 

  353. \subsection{Electrochemical Carbon Corrosion}

  354. \label{subsec: Carbon corrosion}

  355. 

  356. Since carbon corrosion is a primary degradation mechanism for PEMFCs it is essential to examine this process in greater detail. A deeper understanding of electrochemical carbon corrosion can provide insights into its impact on other mechanisms and especially on the performance and durability of the fuel cell.

  357. 

  358. Studys have shown, that start-stop cycles of fuel cells primarily initiate surface corrosion of the carbon support in the CL. Repeated cycles of start-stop conditions can modify the crystalline carbon which will be transformed into a more corrosion-prone amorphous carbon \citep{park2016effects}. Due to the carbon corrosion the CL experiences a loss of thickness which results in detachment of the Pt particles especially at the cathode. This mechanism is not only intensified by start-stop cycles but also by square wave cycles and triangular wave cycles. The first one triggering the corrosion on its surface and internally while the second one targets surface defects \citep{zhao2021carbon}. It is also worth mentioning, that results of an AST in which the cell was exposed to various conditions like load change cycles and start-stop cycles the decreasing perfomance could be attributed by one third to the shutdown and startup cycles proving how demanding this step can be for a cell and how it can increase the degradation \citep{zhao2021carbon,lin2015investigating}. Dependening on the cell conditions either one of the following three reactions can lead to carbon corrosion \citep{wallnofer2024main}.

  359. 

  360. \begin{equation}

  361. 	\mathrm{C}+2 \mathrm{H}_2 \mathrm{O} \rightarrow \mathrm{CO}_2+4 \mathrm{H}^{+}+4 e^{-} \mathrm{E}_0=0.207 \mathrm{~V}

  362. \end{equation}

  363. \begin{equation}

  364. 	\mathrm{C}+\mathrm{H}_2 \mathrm{O} \rightarrow \mathrm{CO}+2 \mathrm{H}^{+}+2 e^{-} \mathrm{E}_0=0.518 \mathrm{~V}

  365. \end{equation}

  366. \begin{equation}

  367. 	\mathrm{CO}+\mathrm{H}_2 \mathrm{O} \rightarrow \mathrm{CO}_2+2 \mathrm{H}^{+}+2 e^{-} \mathrm{E}_0=-0.103 \mathrm{~V}

  368. \end{equation}

  369. 

  370. Another effect of carbon corrosion is the reduction in the hydrophobicity of the GDL which could be attributed to the loss of PTFE and its hydrophobic properties if the cell is flooded while operating at a high current \citep{pei2008}. As mentioned before carbon corrosion can be increased not only by start-stop conditions but also by high potentials. This can then be observed in the polarization curve causing a bigger activation loss at low current densities \citep{Pol_thiele2024realistic}. 

  371. 

  372. Due to an more hydrophilic behaviour because of the carbon corrosion and loss of PTFE the membrane responds slow to quick changes from high to low load which results in water accumulating on the anode side and therefore a reduced hydrogen supply, furthermore the pressure difference between inlet and outlet on the anode hinders the water from being removed. As a consequence of this when the load is increased again quickly the anode can suffer from partial hydrogen starvation which again intensifies the carbon corrosion because of the high electrode potential forming on the cathode side \citep{Pol_thiele2024realistic}.

  373. 

  374. Lastly not only the activation polarization is affected by the carbon corrosion but also the ohmic loss is increased. This increase in the ohmic loss results from the decrease activity of the Pt catalyst since the loss in thickness from the CL and its consequent detachment of Pt particles at the cathode weaken its activity \citep{ren2020degradation}. To be more precise the degradation of the porous structure in the CL causes extended pathways for electrons which increase the contact resistance of the PEMFC \citep{wallnofer2024main}.

  375. 

  376. 

  377. \subsection{Membrane Degradation}

  378. \label{subsec:membrane degradation}

  379. 

  380. H2O2 und eisen ionen membran degradation fördern

  381. 

  382. EDS analysis was performed on the sample tested at RH ¼ 36\% to study Pt dissolution into the membrane since it has been reported that Pt band formation is responsible for membrane degradation [34]. The Pt concentration inside membrane is very low in Fig. 10, indicating that no significant Pt band formation under open circuit conditions. However, any Pt particles that penetrate into the membrane may act as a catalyst for OH free radical direct generation without the H2O2 intermediate and cause membrane degradation \citep{ohma2008}

  383. 

  384. 

  385. 

  386. \subsection{Corrosion}

  387. \label{subsec: BP Corrosion}

  388. 

  389. 

  390. Types of corrosion

  391. unifomr corrosion

  392. 

  393. galvanic corrosion

  394. itergranular corrosion

  395. crevice corrosion

  396. pitting corrosion

  397. 

  398. the ability of pt to support corrosion particularly at high cathode possibilities is another challenge and can decline under load cycling, poor material combination, and high-temperature activity. 

  399. 

  400. Indeed, the cathode CL can corrode as a result of Pt disintegration, appearing particularly during the fatigue loading and applied high potentials to anode-electrode \citep{matsutani2010}

  401. 

  402. 

  403. „which is much more serious in the cathode side. Admittedly, the corrosion of metallic BP in actual fuel cells is almost inevitable even for one with excellent coatings, ascribed to the nonuniformity, defects, and scratches“ \citep{Corr_ren2022corrosion}

  404. %Rephrase corrosion

  405. corrosion characteristics of metallic BP in the PEM fuel cell, especially in the cathode environment revealed the  accumulation  of metallic elements especially Cr and Fe. Constant testing of MEA 200 h even in fuel cell with carbon coated SS.

  406. \citep{Corr_kumagai2012high}

  407. 

  408. Corrosion phenomena on the cathode side rib surface of SS316 BP and found highest accumulation of Fe element in the MEA especially in the gas diffusion layer (GDL)

  409. \citep{Corr_mele2010localised}