update

Content/Theoretical Background.tex

@@ -373,37 +373,95 @@ Due to an more hydrophilic behaviour because of the carbon corrosion and loss of
 
 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}.
 
-
 \subsection{Membrane Degradation}
 \label{subsec:membrane degradation}
 
-H2O2 und eisen ionen membran degradation fördern
+As membrane degradation plays a huge roll in the performance of the cell it is important to understand how it works and how the PEMFC can be affected by this mechanisms. Since the polyelectrolyte membrane (PEM) is formed by perfluorsulfonic acid (PFSA) also known as Nafion it is important to look at the degradation mechanism of it \citep{okonkwo2021nafion}.
+
+ The chemical degradation of the PFSA ionomer is linked to the membrane decay and can lead to pinhole formations. It is driven by hydrogen peroxide radicals which are known to be formed at potentials below 0,682V and in acid environment \citep{wallnofer2024main}. Since the membrane transports water (H$_2$O), protons (H$^+$) as well as oxygen (O$_2$) form the anode to the cathode it is possible that the Pt in the CL catalyses the reaction of the oxygen with the protons from the hydrogen (H$^+$) forming hydrogen peroxide (H$_2$O$_2$) as an intermediate product \citep{frensch2019impact}. This can be described using the following reaction equation (\ref{eq: h2o2}) \citep{ruvinskiy2011using} .
+ 
+ \begin{equation}
+ 	\mathrm{O}_2+2 \mathrm{H}^{+}+2 \mathrm{e}^{-} \rightarrow \mathrm{H}_2 \mathrm{O}_2
+ 	\label{eq: h2o2}
+ \end{equation}
+
+The PFSA membrane can also be suffer from the chemical degradation caused by an attack of free radicals. Hydroxyl radicals (OH), hydroperoxyl radicals (OOH) and hydrogen radicals (H) can be the species responsible for this attack to the membrane which can again lead to the formation of pinholes in the membrane and therefore cause its failure \citep{ren2020degradation}. With the presence of metal ions like Fe$^{2+}$ or Cu$^{2+}$ the formations of radicals from hydrogen peroxide can be catalysed. This mechanism is called the Fenton reaction and can be seen in the following equation
+\citep{frensch2019impact, ruvinskiy2011using}.
+
+\begin{equation}
+	\mathrm{H}_2 \mathrm{O}_2+\mathrm{Fe}^{2+} \rightarrow \mathrm{Fe}^{3+}+\mathrm{HO}^{+}+\mathrm{HO}^{-}
+\end{equation}
 
-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}
+It is believed, that metal components like the BPs made out of Stainless Steels can release metal ions which can then travel to the membrane and either stay in it or transported out by one of the outlets \citep{elferjani_coupling_2021}.
+Hydrogen peroxide radicals released by the Fenton reaction cause degradation at the weaker points in the ionomer. This could be for example the functional end groups or C-H bonds in the PTFE chain which sometimes arise from the manufacturing process as well as substituted C-F bonds. The  attack leads to either breakdown of the ionomers main or side chain or to elimination of the end groups consequently accelerating the degradation process of the membrane and the PEMFC \citep{wallnofer2024main}.
 
+Pt dissolution as mentioned before can also have a huge impact on the membrane degradation. Studys have shown, that Pt band formation although it is minimal during open circuit conditions can also degrade the membrane. However, Pt particles which infiltrate in the membrane may also act as a catalyst for direct generation of OH free radicals bypassing the intermediate formation of H$_2$O$_2$ \citep{ohma2008}.
 
+Membrane degradation can have a series of devastating consequences in the PEMFC like the formation of pinholes which lead to a high gas crossover rate and consequently to high voltage losses or even the reversal of the current in specific cells \citep{Weber_2008}. The loss of functional end groups in the ionomer is also known to increase the membrane resistance and to change water management properties of the membrane \citep{wallnofer2024main}.
 
 \subsection{Corrosion}
 \label{subsec: BP Corrosion}
 
+The use of stainless steel in metallic bipolar plates (BPs) has become increasingly common in PEMFCs due to its low cost and excellent mechanical and electrical properties. However the implementation of stainless steels as a BP material has raised some questions about how its corrosion may affect the durability of FCs since automotive conditions have been known to accelerate the corrosion rate and surface destruction \citep{Corr_ren2022corrosion}. There different types of corrosion like uniform corrosion, galvanic corrosion, interangular corrosion, crevice corrosion and pitting corrosion \citep{jones1996principles}.
+
+Although before getting into the types of corrosion that may occur on the metallic BPs, it is important to first explore the various metals used for their construction. Understanding this materials provides can provide insights into how specific corrosion mechanisms can impact them and consequently damage the PEMFC. Because of its higher corrosion resistance and great properties stainless steels like 304L, 316L and 904L have been under investigation. The composition of these three stainless steels is very similar since they are made out of iron (Fe), chromium (Cr), and nickel (Ni) which could contaminate the MEA \citep{novalin2023demonstrating}. Furthermore, the iron in the   aforementioned stainless steels can catalyse the Fenton reaction seen in equation (\ref{eq: h2o2}) which leads to chemical degradation and the formation of pinholes in the membrane \citep{ruvinskiy2011using,novalin2023demonstrating}.
+
+\subsubsection{Stainless Steel 316L}
+
+ Stainless steel 316 differentiates itself from 304 because of the added molybdenum (Mo) which reinforces its corrosion resistance and offers a higher protection against mechanisms like pitting and crevice corrosion \citep{novalin2023demonstrating}.
+
+\begin{figure}[htbp]
+	\centering
+	\includegraphics[width=0.6\textwidth]{Figures/Theorie/SS316L.pdf}
+	\caption{Comparison of polarization curves at 70 °C and ambient temperature for stainless steel 316L. Retrieved from Wang et al. page 60 [89].}
+	\label{fig:SS316L}
+\end{figure}
+
+In a study performed by Wang et al the electrochemical behaviour of stainless steel 316L was tested in a potentiodynamic test in 0,5M H$_2$SO$_4$ with a a potential reaching from -0,1V to 1,2V with a scanning rate of 1mV/s at room temperature and 70 °C as shown in figure \ref{fig:SS316L}
+\citep{Corr_Mat_wang2010electrochemical}. This polarization curve can be divided into three different parts\citep{Corr_Mat_wang2010electrochemical}:
+\begin{enumerate}
+	\item Active region: OCP to -0,15V.
+	\item Passive region: -0,15V to 0,9V.
+	\item Transpassive region: 0,9V to 1,2V
+\end{enumerate}
+
+Since the curve at high temperatures shows a higher current density a higher operation temperature of a PEMFC can also be associated to a higher corrosion rate. Furthermore the formation of the passive region shows that the Cr in 316L is able to produce a passive film that inhibits further corrosion until the transpassivation is reached with a higher potential \citep{Corr_Mat_wang2010electrochemical}. Although the corrosion is enhanced at the passive region with the formation of an oxide layer, since this layer is less reactive it can also contribute to the performance degradation of the PEMFC
+\citep{laedre2017materials}.
+
+Startup and shutdown conditions in the PEMFC can lead to an increase in the cathode potential which leads to the potential being at the transpassivation region \citep{Corr_ren2022corrosion}. Cycling between the transpassivation region and the passive or passivation region causes the dissolution of Cr species as well as Fe species leading to an extensive structural damage also causing nonuniformity scratches and defects in the surface of the BP\citep{Corr_ren2022corrosion}. The cathode environment because of the contant with oxygen as well as the produced water at the outlet hosts an environment which can lead to the accumulation of metallic elements \citep{Corr_kumagai2012high}.
+
+
+\subsubsection{Pitting Corrosion}
+
+Pitting corrosion is a mechanism which causes localized depassivation, this usually happens at vulnerable surface sites like defects, grain boundaries or impurities \citep{novalin2023demonstrating}. Additionally, changes in the pH can alter the composition of the passivation layer. In the presence of fluoride (F$^-$) this process is intensified leading more severe pitting corrosion which causes the corrosion current density to increase which can be detected by the density of pits formed on the surface \citep{Corr_ren2022corrosion}. Once one or more pits are initiated, the material undergoes rapid dissolution further compromising the integrity of the material \citep{elferjani_coupling_2021}.
+
+The F$^-$ is most commonly encountered in the FC environment due to the membrane degradation (or PFSA). High concentrations of F$^-$  may come from localized evaporations of water droplets which consequently increase the corrosiveness of the run-off water and all in all lead to a more severe degradation of the PEMFC \citep{talbot2018corrosion}. Stainless steel plates are sensible to changes in temperature, humidity and pH therefore a change of any of these parameters can have a big influence on its corrosion resistance. Furthermore 316L has a known depassivation at pH levels ranging from 1,5 to 2 \citep{elferjani_coupling_2021}. Consequently conditions in a PEMFCs creates an environment that may promote pitting corrosion \citep{novalin2023demonstrating}.
+
+
+
+\subsubsection{Crevice Corrosion}
+
+Crevice corrosion has a similar effect than pitting corrosion but unlike pitting corrosion it is driven by geometric features of the components which lead to the creation of highly corrosive micro-environments \citep{talbot2018corrosion}. This type of corrosion may be found in the flow field of the BPs depending on its design. As mentioned before, the molybden in stainless steel 316L provides a higher resistance against crevice corrosion \citep{novalin2023demonstrating}.
+
+
+\subsubsection{Interangular Corrosion}
+
+Intergranular corrosion typically occurs along the boundaries of the grains of the stainless steel alloys therefore it is often associated with the welding process  \citep{talbot2018corrosion}. In this mechanism the boundary acts as the anode , while the surrounding metal serves as the cathode. Due to the big size difference from this anode to the cathode a rapid and concentrated attack to the metal takes place \citep{pe2009fundamentals}. This process leads to significant localized degradation of the material \citep{pe2009fundamentals}.
+
+\subsubsection{Galvanic Corrosion}
+
+Galvanic corrosion can occur when two different metals are in electrical contact in a conductive corrosive environment \citep{al2016modeling}. In this case the driving force is the potential difference between the two metals and the more active metal will act as the anode and corrode. Meanwhile, the more noble metal will function as cathode and be protected from the degradation \citep{al2016modeling}. Since galvanic corrosion leads to the degradation from the anodic metal it will also trigger pitting corrosion \citep{saeed2013effect}.
+
+\subsubsection{Uniform Corrosion}
 
-Types of corrosion
-unifomr corrosion
+In the uniform corrosion the material occurs consistently across the entire metal surface and leads to the gradual thinning of the material over time weakening its structure \citep{pe2009fundamentals}. The material has be in contact with the corrosive environment with an equal access to the entire area for it to be degraded evenly \citep{pe2009fundamentals}.
 
-galvanic corrosion
-itergranular corrosion
-crevice corrosion
-pitting corrosion
+\subsubsection{Effects of Corrosion}
 
-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. 
+In addition to the degradation of the BP, it is important to consider that the corrosion reaction releases metal ions into the cell which can lead to the contamination of other FC components in the PEMFC. For example metal ions from Fe or Cr can migrate throughout the cell and potentially poison the MEA which can further contribute to the performance loss and degradation of the cell \citep{low2024understanding}. In a study performed by Mele et al. a high accumulation of Fe was found in the MEA and especially in the GDL \citep{Corr_mele2010localised}.
 
-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}
+Structural changes in the BPs, such as variations in shape and thickness caused by corrosion can affect the gas flow  and disrupt the water management of the cell leading to cell flooding or air starvation which will dramatically shorten the lifespan of the PEMFC \citep{low2024understanding}. Pinholes or cracks in the BPs due to corrosion can also lead to gas crossover as well as liquid leakage which can destroy the cell  \citep{low2024understanding}. 
 
+The cathode CL is susceptible to corrosion as a consequence of Pt disintegration or dissolution which is particularly pronounced during fatigue because of voltage cycling or due to high potentials applied to the anode electrode \citep{matsutani2010}.
 
-„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}
-%Rephrase corrosion
-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.
-\citep{Corr_kumagai2012high}
 
-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)
-\citep{Corr_mele2010localised}