Hydrogen for electromobility : a promising energy carrier

Electromobility has received important attention in the last few years, but its perception by the public and decision makers is often limited to battery powered vehicles. Alternatives such as hydrogen fuel cells should however be taken into account, as their specific advantages (in particular short refueling times) make electro-mobility as a whole acceptable by a much broader public. Within the SCCER Mobility, PSI and ZHAW work on a novel fuel cell concept aiming at reducing the major limitation to the deployment of fuel cells: their cost

Modeling Oxygen Concentration Oscillation in the Gas channel of Polymer Electrolyte Fuel Cells: a Comparison between Numerical and Analytical Approaches

Considering the complexity of the electrical behavior (polarization curve and impedance spectrum) of PEFC and the lack of relevant models that could be used to identify the main transport parameters in Membrane Electrode Assemblies (MEA), a semi-analytical pseudo-bidimensionnal model is currently developed. Its main characteristics are first recalled. Then the results of a full numerical two-dimensional model are compared to those of the pseudo-bidimensionnal approach and analyzed. It is shown that oxygen diffusion inside the GDL can be considered as a 1D diffusion without any appreciable discrepancy and that the electrode can be modeled as a plane without major error either on impedance spectrum or polarization curve. Oxygen diffusion through the gas channel reveals an important discrepancy between volumetric and plane descriptions, leading to the necessity of a more complex model of the fluid flow in the gas channel

Étude des dégradations dans les piles à combustible PEMFC pendant les phases de démarrage/arrêt

This works contributes to the identification of the various degradation mechanisms in Polymer Electrolyte Membrane Fuel Cell during start-up and shut-down operations. Single start-ups and shut-downs are first analysed using a cell with segmented cathode current collectors. Thus, internal currents which occur during these operations can be measured. Carbon dioxide measured in the cathode exhaust gas reveals that they result partially from carbon oxidation. Another contribution is the reversible or non reversible redox reactions involving platinum. The heterogeneity of the non reversible platinum oxidation between the inlet and outlet of the cathode is evidenced by the in-situ monitoring of the Electrochemical Surface Area during long-term start-up and shut-down aging protocols. Post-mortem analysis reveals another level of heterogeneity, which concerns also carbon oxidation, between land and channel. From these experiments, it appears also that degradations are more important when gases are injected with a low velocity in the anode compartment and when air is used instead of nitrogen to flush the anode compartment during shut-down. The influence of the MEA characteristics on the extent of the degradation observed during these aging protocols is also analyzed. High platinum loading in the anode and high surface carbon electrodes accelerate the drop of the electrical performances, while increasing the cathode platinum loading limits their decay. Finally, numerical simulations of start-ups complete the experimental results. Reversible platinum oxidation was found to be one of the main contribution to the internal currentsCette thèse contribue à l'identification des mécanismes de dégradation qui ont lieu durant les phases de démarrage et d'arrêt des Piles à Combustible à Membrane Échangeuse de Proton. Dans un premier temps, des démarrages et arrêts individuels sont étudiés au moyen d'une cellule équipée de collecteurs de courants segmentés. Les courants internes qui sont produits durant ces opérations peuvent ainsi être mesurés. La mesure du dioxide de carbone dans les gaz d'échappement de la cathode révèle qu'une partie des courants internes correspond à de l'oxydation du carbone. Une autre part provient des réactions (réversibles ou non) d'oxydoréduction impliquant du platine. L'hétérogénéité des dégradations subies par la pile entre l'entrée et la sortie de la cathode est mise en évidence lors de protocoles de vieillissement répétant des démarrages et arrêts. Des analyses post-mortem révèlent un autre niveau d'hétérogénéité, qui concerne également le carbone, entre les dents et les canaux. De ces expériences, il ressort également que les dégradations sont plus importantes lorsque les gaz sont injectés à faible vitesse dans le compartiment anodique mais aussi quand de l'air est utilisé à la place de l'azote pour arrêter la pile. L'influence des caractéristiques de la MEA sur l'intensité des dégradations est aussi étudié. Un chargement en platine élevé à l'anode ou des électrodes avec des surfaces de carbone actif élevées accélèrent la chute des performances électriques. Au contraire accroitre le chargement en platine à la cathode limite ces pertes. Enfin, des simulations numériques des phases de démarrage complètent les résultats expérimentaux. L'oxydation réversible du platine est notamment identifiée comme étant responsable d'une part importante des courants interne

Study of the degradations induced by start-up/shut-down operations in PEMFC

Cette thèse contribue à l'identification des mécanismes de dégradation qui ont lieu durant les phases de démarrage et d'arrêt des Piles à Combustible à Membrane Échangeuse de Proton. Dans un premier temps, des démarrages et arrêts individuels sont étudiés au moyen d'une cellule équipée de collecteurs de courants segmentés. Les courants internes qui sont produits durant ces opérations peuvent ainsi être mesurés. La mesure du dioxide de carbone dans les gaz d'échappement de la cathode révèle qu'une partie des courants internes correspond à de l'oxydation du carbone. Une autre part provient des réactions (réversibles ou non) d'oxydoréduction impliquant du platine. L'hétérogénéité des dégradations subies par la pile entre l'entrée et la sortie de la cathode est mise en évidence lors de protocoles de vieillissement répétant des démarrages et arrêts. Des analyses post-mortem révèlent un autre niveau d'hétérogénéité, qui concerne également le carbone, entre les dents et les canaux. De ces expériences, il ressort également que les dégradations sont plus importantes lorsque les gaz sont injectés à faible vitesse dans le compartiment anodique mais aussi quand de l'air est utilisé à la place de l'azote pour arrêter la pile. L'influence des caractéristiques de la MEA sur l'intensité des dégradations est aussi étudié. Un chargement en platine élevé à l'anode ou des électrodes avec des surfaces de carbone actif élevées accélèrent la chute des performances électriques. Au contraire accroitre le chargement en platine à la cathode limite ces pertes. Enfin, des simulations numériques des phases de démarrage complètent les résultats expérimentaux. L'oxydation réversible du platine est notamment identifiée comme étant responsable d'une part importante des courants internesThis works contributes to the identification of the various degradation mechanisms in Polymer Electrolyte Membrane Fuel Cell during start-up and shut-down operations. Single start-ups and shut-downs are first analysed using a cell with segmented cathode current collectors. Thus, internal currents which occur during these operations can be measured. Carbon dioxide measured in the cathode exhaust gas reveals that they result partially from carbon oxidation. Another contribution is the reversible or non reversible redox reactions involving platinum. The heterogeneity of the non reversible platinum oxidation between the inlet and outlet of the cathode is evidenced by the in-situ monitoring of the Electrochemical Surface Area during long-term start-up and shut-down aging protocols. Post-mortem analysis reveals another level of heterogeneity, which concerns also carbon oxidation, between land and channel. From these experiments, it appears also that degradations are more important when gases are injected with a low velocity in the anode compartment and when air is used instead of nitrogen to flush the anode compartment during shut-down. The influence of the MEA characteristics on the extent of the degradation observed during these aging protocols is also analyzed. High platinum loading in the anode and high surface carbon electrodes accelerate the drop of the electrical performances, while increasing the cathode platinum loading limits their decay. Finally, numerical simulations of start-ups complete the experimental results. Reversible platinum oxidation was found to be one of the main contribution to the internal current

Ensemble-based study of equilibrium liquid water distribution in PEM gas diffusion layer

Macro-homogenous models enable the study of the complex interplay between operating conditions, materials response and cell performance, which is needed for the optimisation of the cell performances. However, input parameters to the macro-homogenous models are needed to account for the micro-scale properties of the single components and interfaces. For this purpose the water filled and open pore domains of gas diffusion layers (GDL) are determined depending on the boundary (and operating) conditions using the geometric input of the fiber/pore morphologies provided by tomography. The ultimate aim is to calculate the effective transport properties for different degrees of saturation, based on water distributions that are obtained with pore-scale modeling and from tomography. Considering the liquid water as statistical ensemble, a Monte Carlo-based approach based on the energetic balance between solid, liquid and gaseous phases in the porous medium, has been used to determine water distribution. PSMC (Pore Scale Monte Carlo) code we developed, is shown to be able to simulate the equilibrium distribution of the water (EWD) in the porous structure, minimizing the surface free energy of the water-solid-air system. Such an information will be later used to calculate the effective transport properties (e.g. permeability, conductivity) needed for a macroscopic simulation of the full PEM fuel cell

Validation of pore network simulations of ex-situ water distributions in a gas diffusion layer of proton exchange membrane fuel cells with X-ray tomographic images

International audienceUnderstanding and modeling two-phase flows in the gas diffusion layer (GDL) of proton exchange membrane fuel cells are important in order to improve fuel cells performance. They are scientifically challenging because of the peculiarities of GDLs microstructures. In the present work, simulations on a pore network model are compared to X-ray tomographic images of water distributions during an ex-situ water invasion experiment. A method based on watershed segmentation was developed to extract a pore network from the 3D segmented image of the dry GDL. Pore network modeling and a full morphology model were then used to perform two-phase simulations and compared to the experimental data. The results show good agreement between experimental and simulated microscopic water distributions. Pore network extraction parameters were also benchmarked using the experimental data and results from full morphology simulations

Influence of Operating Conditions and Material Properties on the Mass Transport Losses of Polymer Electrolyte Water Electrolysis

Three different porous transport layer (PTL) structures, based on titanium sintered powders, were characterized using X-ray tomographic microscopy to determine key geometric properties such as porosity, pore and particle size distributions as well as effective transport properties. The mass transport through the PTL contributes to the voltage losses in the polymer electrolyte water electrolysis cell. Therefore, influence of the PTL structure on the mass transport overpotential is investigated as function of current densities (≤ 4 A·cm−2), operating pressures (1–100 bar) and temperatures (40–60°C), respectively. A decrease of transport losses was observed with increasing pressure and temperature for all investigated PTLs. At around 100 bar balanced pressure, the transport losses for all PTLs converge to about 40 mV per applied A·cm−2, suggesting that other parts of the cell such as the catalyst layer or their interface contribute to these remaining losses. The performance loss, induced by the different PTL structures, shows a stronger correlation with geometric parameters such as pore and particle size distributions than transport properties like effective diffusivity and permeability. The finest materials with d50 pore and particle diameters of 40–48 and 68 μm, respectively, are performing better than the coarsest material with diameters roughly twice the sizes.ISSN:0013-4651ISSN:1945-711

Voxel-based modelling of water distribution in PEFC porous media

The macro-homogenous approach for modelling proton exchange membrane fuel cells (PEFCs) strives for optimisation of the fuel cell and its components. Water management in the gas diffusion layer (GDL) plays a fundamental role in the overall predictive power of a PEFC model. In our work the 3D-microstructures of the different cell components are investigated with modern and complementary imaging techniques such as X-ray tomography (GDL), FIB-tomography (MPL) and BIB-SEM (cross-sections of layer compounds). Transport relevant morphological parameters (i.e. phase volume fraction, tortuosity, constrictivity) are analysed for materials in the dry and in wet state. To circumvent the lack of a significant length scale separation between the pore-scale and the GDL thickness, we implemented a stochastic model that treats the water evolution in porous media on the corresponding voxel-data from tomography. The analysis, is based on the minimisation of the surface free energy and leads to a statistical physics characterisation of the water-medium interaction through advanced Monte Carlo simulation techniques. In such way the effective properties (permeability, conductivity) can be linked with the mentioned microstructure parameters and are used as input for modeling of the cell performance on a macro-homogeneous scale

About fuel starvation in PEMFC: characterization of the mechanism of degradation and mitigation strategies

International audienc