\chapter{Summary and Outlook}
\label{cap:SummaryAndOutlook}

The final chapter of this master thesis will present a briev summary of the work as well as the motivation behind the work in the first section \ref{sec:5_Summary}. The second and last part of this chapter \ref{sec:5_Outlook} will offer an outlook on potential future research directions.


\section{Summary}
\label{sec:5_Summary}

Since global temperature is on the rise and projections include a 1,5°C until 2030 it is of utmost importance to implement effective measures to mitigate climate change. GHG and especially CO$_2$ emissions have been identified as the main contributor of global climate accounting for 80,6\% of the emissions. The transportation sector in Germany is responsible for 19,8\% of the emissions in the country therefore it is crucial to transition from ICEs to more sustainable alternatives like BEVs and FCEVs. FC technology is on the rise but manufacturing cost continues to be a limiting factor since the BPs constitute 45\% of the manufacturing cost. The search for more cost effective alternatives has suggested a change from Ti BPs to stainless steel plates. Although the plates are more cost effective the corrosion of BP plates raises questions about their durability and how it may affect the performance of the PEMFC. Therefore this thesis aims to deepen the understanding of the corrosion in stainless steel plates by developing a corrosion reinforcing endurance run and studying the effect of the corrosion on the performance of PEMFCs as well as analysing the corrosion damage in the cells.

Chapter \ref{cap: Theorie} lays the theoretical background of this Thesis. Therefore, The fundamentals of the Fuel cell are explained as well as the components of a PEMFC (PEM, GDL, CL and BP). The membrane is composed of Nafion (PFSA) and the CL has Pt as catalyst for the electrochemical reactions. Before explaining the degradation mechanisms first the overpotentials are explained using the polarization curve. At low current densities the activation losses dominate the polarization curve. At medium current densities the ohmic losses dominate the form of the curve and at high current densities the form of the polarization curve is determined by the concentration polarization or mass transport losses related to the diffusion limitations of the cell. Afterwards the main degradation mechanism are explained starting with Pt catalyst dissolution and agglomeration, electrochemical carbon corrosion, membrane degradation and finally corrosion. In the membrane degradation the fenton reaction is of utmost important and this reaction is catalysed by metal ions like Fe$^{2+}$ which react with H$_2$O$_2$  and form hydroxyl radicals which attack the membrane. In this process F$^-$ from the membrane is released. F$^-$ decreases the pH of the product water which can then attack the BPs or reinforce corrosion. Pitting corrosion and crevice corrosion as well as galvanic corrosion can lead to structural damages to the cell, cause pinholes or even gas crossover which will shorten the lifespan of the cell dramatically.

Chapter \ref{chap:Methode} explains the methods used to analyse the operating conditions which reinforce corrosion and afterwards design a corrosion reinforcing endurance run. In the preliminary investigations the pH and the electrical conductivity of the product water at three different operating points is tested. The effect of the operating temperature is then measured by searching for the lowest pH due to membrane degradation and release of F$^-$ and consequently the highest electrical conductivity as a sign from the metal ions released by the BP during a corrosion mechanism. Afterwards two endurance runs are performed: one with corrosion reinforcing operating parameters and a high temperature endurance run to compare. The cell components of the 4 cell stack are then analysed in the ex-situ investigations using microscopy, LIBS as well as REM and EDX.

The results as well as the discussion of the results are then presented in Chapter \ref{chap:Ergebnisse und Diskussion}. Two types of cells where used in this thesis: Type 1 made out of Ti-C and with an active area of 273 cm$^2$ (used for the preliminary investigation) and type 2 cells made out of stainless steel 316L and an active are of 285 cm$^2$ (used in the endurance runs). The preliminary investigations concluded, that the lowest pH and also highest electrical conductivity was found at a lower temperature (60°C) at the cathode and the highest pH and lowest electrical conductivity at a temperature of 90°C after 2 hours of voltage cycling between 10s at 0,6V and 15s at 0,85V. The pH found in the literature of 2 was not reached, this could be caused by the higher corrosion resistance of Ti-C compared to 316L. Therefore less metal ions moved from the BP to the membrane to catalyse the fenton reaction and the degradation was less than it would have been with a stainless steel plate. The corrosion endurance run was performed at a cell temperature of 66°C with a 4 cell stack made out of type 2 cells (316L). After the activation of the cells using four 80°C polarization curves the BoL characterization was performed with a 60° polarization curve and a 80° polarization curve as in-situ characterization. Then the cell was set up for 12500 VC of 10s at 0,6V and 10s at 0,88V followed by the in-situ characterization. Then 25000 VC followed by the in-situ characterization and finally 42500 VC followed by the last insitu characterization. The 60° polarization curve showed a higher degradation than the 80 °C polarization curve. A significant difference in the cell voltages at high current densities was measured in the 60°C polarization curve between cells 1 and 4. Cell 1 had presented a Voltage of 0,6V while cell 4 had a voltage of 0,53V. Due to this difference the two cells where further analysed in the ex-situ part. The 80°C polarization curve showed almost no signs of degradation after 81000 VC which can be attributed to the high stoichiometry of 1,5 at the anode and 2 at the cathode and also to the low HFR measured. The low HFR after 81000 VC indicates an optimal humidity level of the cell and therefore a lower ohmic polarization and consequently a better performance of the cell.

The ex-situ analysis of cells 1 and 4 of the corrosion endurance run showed signs of corrosion in the welding seams of both cells as well as corrosion in the cathode outlet of the cell 4. The CCM of both BP 1 and 4 showed Pt agglomeration as well as cracks and a wave structure. The LIBS analysis performed on the welding seam and the cathode outlet showed an increased oxide layer as well as a decreased carbon percentage in the measurement. The Fe, Ni and Cr percentages could not be analysed correctly due to the high standard deviations. The oxide layer as well as the Pt agglomerations could have caused increased losses in the performance of the cell although the losses can also be attributed to the membrane degradation and carbon corrosion (decrease in carbon percentage of the material). A REM and EDX analysis of the cathode CCM in BP 1 and 4 showed no traces of metals from the BP (Fe, Ni, Cr) and therefore no signs of the migration from the BP to the CCM. This could be attributed to the duration of the endurance run of 400h. Finally the next section will present an outlook on the topic an this thesis.


\section{Outlook}
\label{sec:5_Outlook}

Due to time constraints and problems with the first test bench in which the preliminary investigations where conducted some of the experiments will have to be repeated. After a promising result in the pH and electrical Conductivity measurements the next step would have been to repeat this analyses with a stack made out of the type 2 stainless steel cell. This could lead to lower pH and a higher electrical Conductivity as a sign of corrosion which was not the case with the type 1 plates made out of Ti-C. Furthermore a online pH and electrical conductivity measurement of the endurance runs could lead to a better understanding of the operating conditions and the condition of the cell in the different states of the endurance run. Moreover, a more detailed analysis of the product water including Fe, Cr, Ni, Si, Mo, Magnesium (Mg) and Flour could give a better overview of the corrosion mechanism and consequently the dissolution of the metal ions and trace them in the product water. It would also help to create a correlation between the Flour content and the metal ions which cause the membrane degradation (or degradation of Nafion) catalysed by the metal ions in the Fenton reaction.

The use of other analytical methods like X-ray diffraction analysis (XRD), X-ray photoelectron spectroscopy (XPS) and inductively coupled plasma mass spectrometry (ICP-MS) could also produce a much better result when analysing components of the PEMFC. Metal traces of Fe, Ni or Cr could be detected on the CCM, GDL and MPL. XPS could provide deeper insights into the oxide layer on the BP and therefore offering a more detailed understanding of BP corrosion.

Potentiostatic and potentiodynamic measurements of the stainless steel could also be performed to evaluate the BP material and compare it to the targets set by the DOE for 2025 as stated in the theoretical background in section \ref{subsec:2_DOE}. Lastly, the research could also focus on other types of stainless steals like 304, 904L, 321 or even other coatings for the different BP materials to improve corrosion resistance.




