Comprehensive Degradation Analysis in Proton Exchange Membranes Fuel Cells

https://doi.org/10.5281/zenodo.17476307 B1802=B1808 Page 1/5 B1802=B1808 Comprehensive Degradation Analysis in Proton Exchange Membranes Fuel Cells Isabel Vazquez-Fernandez (1), Jonathan Halter (1), Emmanuelle Boehm-Courjault (2), Jan Van Herle (2), Mardit Matian* (1) (1) EH Group Engineering AG, Nyon; (2) Group of Energy Materials, Ecole Polytechnique Federal de Lausanne (EPFL), Sion; *Contact corresponding authors: www.EFCF.com/ContactRequest Abstract The performance of a PEMFC is the result of many internal and external factors, such as assembly, degradation of materials, operational conditions, and impurities or contaminants. Degradation remains one of the primary limitations in stack durability. In this work, different electrochemical tests were performed to get insight into chemical, mechanical, catalytic, nitrogen crossover, dehydration and thermal degradation of cells. Here, nitrogen crossover is determined under different stack operational conditions. Results showed that the gas crossover increases with increasing current density, which is the result of elevated membrane temperature and increased water content. Nitrogen displaces water in the membrane, leading to dehydration and increased resistance until it reaches a limit where the crossover rate stabilises. Post-mortem analysis of the materials was examined by SEM/EDX, TEM and XPS. https://doi.org/10.5281/zenodo.17476307 B1802=B1808 Page 2/5 Introduction The durability of the catalyst coated membrane (CCM) is of utter importance for the overall lifetime of polymer electrolyte membrane fuel cells (PEMFC). Several degradation mechanisms act on the CCM, such as carbon corrosion in the catalyst layers or wet/dry cycles on the membrane itself. Carbon corrosion is induced by high potentials occurring during fuel cell operation. Although carbon corrosion can occur at potentials of 207 mV vs SHE [1], carbon corrosion occurs at significantly higher rates with elevated potentials. Membrane degradation on the other hand is triggered by changes in humidity induced by wet/dry cycles. The membrane ionomer (i.e. Nafion) can take up large amounts of water in a humid environment and swell up, thereby increasing its thickness significantly. In a drier environment, the overall water content of the membrane is reduced again. These changes induce a strong mechanical stress onto the membrane which can cause the formation of cracks or holes in the membrane [2]. These degradation mechanisms act as a roadblock for extending the lifetime and therefore for a deeper market penetration of PEM fuel cells. 1. Scientific Approach In order to study the effect of these degradation mechanisms, accelerated stress tests (ASTs) are used. The US Department of Energy has developed and defined accelerated stress tests for several PEM fuel cell components [2]. Prior to the AST cycle, a fresh MEA is assembled, conditioned according to EH standards and Begin of life characteristics are recorded. After the AST, the same electrochemical tests are carried out at the end of the life (EoL) of the cells. The cells are then disassembled, and different imaging techniques are used to several of the aged components. The imaging technique is selected to provide sufficient resolution and contrast for the selected component. 2. Experiments Prior to accelerated stress test, the begin of life (BoL) characterization was carried out. This involved several electrochemical tests, such as determining the electrochemical active surface area by means of cyclic voltammetry, polarization curves at selected and representative operational conditions and Tafel slope analysis to derive activation losses within the catalyst layers. An AST focusing on catalyst layer degradation is then carried out in this work, which includes 30’000 square wave cycles between 0.6 V and 0.95 V. Each voltage step is held for 3 seconds. After the completion the same electrochemical diagnostic were applied to study the effect of the degradation. To study the effect of carbon corrosion on the particle size distribution within the cathode catalyst layer, high resolution TEM imaging was carried out on the degraded CCM and on a fresh sample. To understand the effects on the membrane degradation, both nitrogen and hydrogen crossover measurement were carried out. An example of degraded membrane and its effect on hydrogen crossover is presented. The BoL measurements for nitrogen crossover are presented here, to act as a baseline for future ASTs. Two methods were used to determine the nitrogen crossover, both based on analysing the anode exhaust gas - one based on mass spectrometry and one on gas chromatography. The effect of a range of current densities, gas pressures and relative humidity values was investigated. https://doi.org/10.5281/zenodo.17476307 B1802=B1808 Page 3/5 3. Results On the left side in Figure 1, a polarization curve both before (BoL) and after (EoL) the AST is shown. Overall, the performance, especially in the activation area related to catalyst activity, was reduced. Figure 1: Polarisation curve and cyclic voltammogram for Begin of Life (Blue) and End of Life (Orange). In the right side of Figure 1, the cyclic voltammogram is shown for both BoL and EoL. One can observe that the electrochemical active surface area was reduced during the voltage cycles in the AST. This also led to a reduced Tafel slop and a lower current at 0.9 V (s. Figure 2). Figure 2: Tafel Slope from begin of life (blue) and end of life (orange). https://doi.org/10.5281/zenodo.17476307 B1802=B1808 Page 4/5 Figure 3: TEM image of the Pt Particle of the cathode catalyst layer of a fresh CCM (left) and from after the AST (right). After the AST, a TEM image of the cathode catalyst layer (s. Figure 3, right) was recorded and the particle size distribution was derived through image processing. This was compared with a fresh sample (Figure 3, left) and and it was obserced that mean particle size increased about 12 % to 2.8 nm. In a second part, the degradation mechanism of the membrane was analyzed. The motivation for the investigation of the membrane stability was an observed membrane failure where membrane holes up to 40 µm diameter were visualized through SEM (s. Figure 4, left)) after operating at elevated pressure difference between anode and cathode. Figure 4: SEM image of a degraded membrane (left); hydrogen crossover rate of the fresh and degraded membrane (right). The observed holes in the membrane affected the H2 crossover rates as determined by linear sweep voltammetry (s. Figure 4, right) which increase drastically. To assess the membrane health, nitrogen crossover measurements were carried out for a fresh CCM. This will act later as a baseline for measurements of degraded CCMs. Two different methods for the analysis of the anode exhaust gas were carried out, namely mass spectroscopy and gas chromatography. A range of relevant parameters were varied to derive their effect on nitrogen crossover; namely, current density, gas pressure and relative humidity. With current, the overall nitrogen crossover decreased, while increasing gas pressure increased the nitrogen crossover. Higher RH led to a decrease in crossover rates. Overall, the measurements of the two analytical methods were in agreement with each other and resulted in similar results. 4. Conclusion https://doi.org/10.5281/zenodo.17476307 B1802=B1808 Page 5/5 The catalyst layer degraded was analysed with an AST cycle as described by the DoE, BoL and EoL diagnostics were used to assess the effect of the AST on the catalyst layer. Although degradation was observed, it did not excess the level given by the DoE. Regarding the membrane degradation, nitrogen crossover rates were chosen as an indicator of membrane health and the BoL diagnostics includes measurement of the crossover rates as a function of current, relative humidity and gas pressure. Acknowledgements The authors acknowledge financial support from the Clean Hydrogen Partnership and its members under the grant agreement H2Marine 101137965. This work was supported by the Swiss State Secretariat for Education, Research and Innovation (SERI) under contract number 23.00624 –10113796 (EH Group) and 24.00109 – 101137965 (EPFL). References [1] J. Kim, J. Lee und Y. Tak, «Relationship between carbon corrision and positive electrode potential in an proton exchange membrane fuel cell during start/stop operation,» Journal of Power Sources, Bd. 192, pp. 674-678, 2009. [2] A. Kusoglu und A. Weber, «New Insights into Perfluorinated Sulfonic-Acid Ionomers,» Chemical REview, Bd. 117, pp. 987-1104, 2017. [3] P. Sharma, D. Aaron, P. Boillat und L. Cheng, «Correlating Catalyst Growth with Liquid Water Distribution in Polymer Electrolyte Fuel Cells,» Small, Bd. 20, Nr. 52, 2024. Keywords: EFCF2025, H2, Low-Temp. Fuel Cells & Electrolysers, Durability, Nitrogen Crossover, Accelerated Stress Test, Catalyst Layer Remark: This work is licensed under Creative Commons Attribution 4.0 International