Electronic and local structures of Pt-based bimetallic alloy and core-shell systems

This thesis investigates the electronic structure of Pt for catalysis applications. The importance of the Pt 5d band is discussed in terms of the bonding capability of Pt. The oxygen reduction reaction in proton exchange membrane fuel cells is chosen as the catalytic reaction model to illustrate the effect of Pt 5d states on Pt-O interaction. Pt-based bimetallic systems are introduced as a solution for the high price and limited resources of Pt. Despite lower usage of Pt, the tuning capability to optimize the Pt 5d band in bimetallic catalysts is supposed to provide superior catalytic activity. Advanced synchrotron X-ray techniques including normal X-ray absorption fine structure (XAFS), X-ray ptychography, and high energy resolution fluorescence detected (HERFD) X-ray absorption/emission spectroscopy (XAS/XES) are combined with laboratory characterization techniques including transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and X-ray powder diffraction (XRD) to study the behavior of Pt upon alloying or forming core-shell structures with 3d transition metals. Three Pt-based bimetallic systems are studied, including Pt-Ni bulk alloys, Pt-Ni nanoparticles (NPs), and Pt-Cu NPs. Pt-Ni bulk alloys are synthesized as model compounds to study the many-body effect, charge redistribution, and local structure of Pt upon alloying. It is found that Pt gains 5d electrons, resulting in more symmetric Pt 4f XPS peaks when diluted in Ni, while Ni loses 4p and 4d electrons, resulting in an increase of K-edge XAS whiteline (WL) intensity, more symmetric Ni 2p XPS peaks, and stronger shake-up satellites. The downshifting of the Pt valence states and upshifting of Ni valence states are also observed with ultraviolet photoelectron spectroscopy (UPS) and density functional theory (DFT) calculations. Pt-Ni NPs, as the Pt-Ni systems in nanoscale, are used to track the evolution process including the alloying and de-alloying of Pt during the synthesis of the bimetallic systems. It is found that Pt goes through five distinct stages, i.e. core frame, tight network, outer frame, thin skin, and particulate shell. The electronic and local structures at each stage are tracked with XAS. Pt-Cu NPs are in the form of core-shell or alloy NPs, with a very low amount of Pt either on the surface or in the bulk. For the 8-nm Cu@Pt core-shell NPs, several monolayers of Pt are deposited on the Cu core, exhibiting good controllability by the polyol reduction method. The advantages of HERFD-XAS/XES are demonstrated in studying the Pt 5d band of Pt-Ni and Pt-Cu bimetallic systems. In the valence-to-core (VTC) XES experiments, the widths of the VTC emission lines and energy transfers show the shrinking and downshifting of the Pt valence band upon alloying with Ni. For HERFD-XAS, significantly narrowed WLs, enhanced near-edge XAS features, and easily removable background have enabled detailed analysis of the WL peaks with high accuracy. Combining the HERFD-XAS results for Pt-Ni and Pt-Cu bimetallic systems, Pt foil, and Pt NPs, a general linear relationship between the WL areas of Pt L3- and L2-edges is established. Physically, this linear relationship indicates that the unoccupied Pt 5d5/2 and 5d3/2 states also have a linear relationship. Experimentally, this finding suggests that measuring the Pt L3-edge alone will provide enough information to study the unoccupied Pt 5d states of Pt-based metallic systems

Silicon-Incorporated Carbon Spheres As Anode Material for Lithium-ion Batteries

In this study, a porous silicon-incorporated carbon material is studied as anode material in Lithium-ion batteries. This material is synthesized with carbonization, spray-pyrolysis and magnesiothermic reduction, from sucrose and silica as carbon and silicon precursors. Carbonization of sucrose was conducted in 0.125 M sulfuric acid with addition of colloidal silica at 90 ℃ for 48 hours. The C/SiO2 spheres obtained from subsequent spray-pyrolysis were reduced by magnesium at 750 ℃ for 2 hours in a home-made Swagelok-type stainless steel reactor. The carbon was sacrificed to maintain the spherical structure of the composite during magnesiothermic reduction while silicon formed a highly porous sponge-like structure inside the spheres. C/Si (1:8) showed high recovery (85%) of specific capacity at the second cycle. However, the rapid capacity loss of porous silicon spheres was found to be caused by fracture the of thin silicon structure. Without the carbon shell, the debris dissolved into the electrolyte easily, leading to a lower availability of the silicon material

La gestion des risques liés aux situations de co-activité dans la phase de planification des projets

Tout projet comporte des dangers et sa réussite dépendra notamment de la façon dont son responsable arrivera à cemer les risques potentiels et à réduire la gravité de leurs conséquences. Les problèmes majeurs rencontrés dans des projets de bâtiment et de génie civil proviennent de la gestion préventive des accidents autant que de leurs conséquences. En Europe continentale la loi exige que la gestion du risque soit prise en compte de façon préalable à la mise en oeuvre de toute action importante sur le site où doit se réaliser le projet concemé. Dans le futur, il y aura même une obligation légale d'inclure, dès la phase d'étude, la gestion du risque dans le management de projets. Un des risques des plus importants qu'il faudra gérer est celui de co-activité. Ce risque peut apparaître quand au moins deux ressources, comme des soudeurs et des peintres, travaillent dans le même lieu et en même temps. Ce mémoire propose une méthode d'analyse et d'aide à la décision afin d' éviter ces risques liés aux situations de co-activité. L'enjeu de ce mémoire est important car une telle analyse constitue un moyen essentiel de préserver la santé et la sécurité des travailleurs, sous la fonne d'un diagnostic en amont des facteurs de risques auxque ls ils peuvent être exposés

Analysis of Deformation of Gas Diffusion Layers and the Impact on Performance of PEM Fuel Cells

Proton exchange membrane (PEM) fuel cells have been promoted due to significant breakthroughs in various aspects and increasing public interests. The porous features of the gas diffusion layer (GDL) and the necessary assembly processes generate localized pressure forces on the channel/shoulder structure of the bi-polar plates (BPP). As a consequence, the assembly pressure acting on a single cell and a fuel cell stack has important influence on the geometric deformation of the GDL resulting in a change in porosity, permeability, and the resistance for heat and charge transfer in PEM fuel cells. It is expected that the cell performance is also affected by these physical parameters. To optimize the cell performance, it is necessary to consider the assembly effects, which is conducted by a numerical method in this work. The effect of the GDL porosity change caused by various compression ratios is investigated by a three-dimensional (3D) PEM fuel cell model based on the finite volume method (FVM). The model was validated and further applied to predict the transport phenomena including heat, mass and charges, as well as the effects on the cell performance. The simulation results show that a high compression ratio on the GDL leads to lower porosity, which is favorable for the heat removal from the cell. However, the compression has contradictory effects on the mass transfer and finally deteriorates the cell performance. To predict the GDL deformation and associated effects on the geometric parameters as well as porosity, mass transport properties and the cell performance, both the finite element method (FEM) and the FVM are applied, respectively. A non-homogeneous deformation, porosity, oxygen diffusion coefficient and the electric resistance of the GDL have been observed across the fuel cell in the in-plane direction. The obtained non-homogeneous physical parameters of the deformed GDL are applied for further computational fluid dynamics (CFD) analysis. The CFD results reveal that a higher assembly pressure decreases the porosity, GDL thickness, gas flow channel cross-sectional areas, oxygen diffusion coefficient, oxygen concentration and cell performance. It is found that, the reduction of the GDL porosity is a dominating factor that decreases the cell performance compared with the decreased gas channel flow area and GDL thickness in the assembly condition. A sufficient GDL thickness is required to ensure transfer of the fresh gas to the reaction sites far away from the channel. As the entire electric resistance is considered, the optimized cell performance is obtained if the cell is operating below 1 MPa assembly pressure. It is found from a newly developed electric resistance model that both through-plane resistance of a cell and the interfacial resistance between the GDL and BPP for electrons decrease with higher assembly pressures. Comparing with a zero-compressed cell, the cell operating at an assembly pressure above 2 MPa creates a new contact area between the GDL and BPP at the vertical interface. Therefore, the corner of a BPP close to the channel becomes the dominating zones for electron transfer. Finally, it is suggested that the assembly pressure should be considered properly in designing and manufacturing of PEM fuel cells.Popular science summaryProton exchange membrane (PEM) fuel cell is one of the promising fuel cells in conversion of chemical energy to electric energy with a relative high efficiency. It is widely known that the PEM fuel cell has nearly-zero pollutants if it is fueled by hydrogen. People can use the sustainable electric power without any noise in home usage, transportation and commutation facilities and so on. The current interest of this device is to replace combustion engines to release the environmental problems like CO2 emissions. A PEM fuel cell involves several technologies. Many achievements have been reached in the past decades. However, the cost and stability are two main limitations preventing wide use of PEM fuel cells. In various research and development fields, such as materials, design and manufacturing, some breakthroughs have been made in improving the cell performance. Even though large efforts have been paid in experiments, the closed-space and small-scale of the cell device make it hard to investigate. Therefore, numerical methods have become very popular and presented efficient ways to investigate the transport phenomena and optimizing the cell performance.The assembly process of a single cell or a cell stack is a necessary step to prevent gas leakage and decrease the contact resistance between the various layers. The porous carbon fibers in the gas diffusion layer (GDL) are touching the channel/shoulder structure of the bi-polar plates (BPP). As a consequence, the physical properties of the GDL, such as dimensions, porosity, mass transfer resistance, and interfacial resistances for heat and electrons will be changed. These factors may result in unexpected or decreased cell performance.In this work, the commercial software ANSYS and the newly developed open source code OpenFOAM (“Open Source Field Operation and Manipulation”) are applied to study the important assembly processes. The model in ANSYS predicts the GDL deformation behavior. Then the deformed GDL and the corresponding yield properties are implemented in the PEM fuel cell model to study the effects of the assembly pressure on the transport phenomena and cell performance. To optimize the cell performance, the electric resistance in the deformed bulk of a cell and the interfacial resistance between the GDL and BPP are considered. All the parameters are expressed as a function of the assembly pressure. To investigate the porosity effects independently, different porosities of the GDL caused by various assumed compression ratios are applied as initial conditions for the PEM fuel cell model. In the study of porosity effects, the GDL deformation and the electric resistance variations are neglected. Then the model is further extended to include real deformation of the GDL and the electron transfer effects, respectively. By evaluating several topics, the cell performance is optimized in terms of assembly pressures or compression ratios. Guidelines for design and manufacturing of PEM fuel cells can be set up based on this thesis

Cuckoo search algorithm based on cloud model and its application

Abstract Cuckoo search algorithm is an efficient random search method for numerical optimization. However, it is very sensitive to the setting of the step size factor. To address this issue, a new cuckoo search algorithm based on cloud model is developed to dynamically configure the step size factor. More specifically, the idea of giving consideration to both fuzziness and randomness of cloud model is innovatively introduced into cuckoo search algorithm, and the appropriate step size factor can be determined according to the membership degree and an exponential function, so as to realize the adaptive adjustment of the control parameter. After that, simulation experiments are conducted on 25 benchmark functions with different dimensions and two chaotic time series prediction problems to comprehensively evaluate the superiority of the proposed algorithm. Numerical results demonstrate that the developed method is more competitive than the other five CS and several non-CS algorithms

Coupled simulation approaches for PEM fuel cells by OpenFOAM

Proton exchange membrane (PEM) fuel cells are known as environmental friendly energy conservation devices, and have the potential to be suitable alternative power sources. The cost and durability of a PEM fuel cell are strongly affected by the involved transport phenomena and reactions, which are two major challenges to be overcome before commercialization. Modeling and simulation are crucial for the cell design and operation. Various “add-on” fuel cell modules are available in commonly-used commercial CFD codes: FLUENT, STAR-CD and COMSOL Multiphysics. However, the length scale of PEM fuel cell’s main components ranges from the micro over the meso to the macro level. The various transport processes at different scales sometimes cannot be captured simultaneously by these codes. On the other hand, physical properties of functional layers used in MEA (membrane electrolyte assembly, consisting of catalyst layers, gas diffusion layers and membrane) play an important role for the cell performance. Therefore coupling of the multi-scale structural and transport characteristics in the functional layers might be an effective way to understand the electrochemical reactions and transient transport phenomena in PEM fuel cells. OpenFOAM (Open Field Operation and Manipulation) is an open source finite volume code having an object-oriented design written in C++, which allows implementation of own models and numerical algorithms. Furthermore, it is possible to integrate other models, e.g., particle-based models, with the OpenFOAM CFD Toolbox. Thus OpenFOAM has the potential to meet the requirements faced in PEM fuel cell simulations as mentioned above. In this paper, various models and applications of OpenFOAM are outlined and reviewed, focusing on the multi-phase transport processes and reactions in PEM fuel cells. The potential methods and challenges coupling OpenFOAM with other modeling techniques are also discussed and highlighted

Modeling of inhomogeneous compression effects of porous GDL on transport phenomena and performance in PEM fuel cells

A comprehensive, three-dimensional model of a proton exchange membrane (PEM) fuel cell based on a steady state code has been developed. The model is validated and further be applied to investigate the effects of various porosity of the gas diffusion layer (GDL) below channel land areas, on thermal diffusivity, temperature distribution, oxygen diffusion coefficient, oxygen concentration, activation loss and local current density. The porosity variation of the GDL is caused by the clamping force during assembling, in terms of various compression ratios, that is, 0%, 10%, 20%, 30% and 40%. The simulation results show that the higher compression ratio on the GDL leads to lower porosity, and this is helpful for the heat removal from the cell. The compression effects of the GDL below the land areas have a contrary impact on the oxygen diffusion coefficient, oxygen concentration, cathode activation loss, local current density and cell performance. Generally, a lower porosity leads to a smaller oxygen diffusion coefficient, a less uniform oxygen concentration, a higher activation loss, a smaller local current density and worse cell performance. In order to have a better cell performance, the clamping force on the cell should be as low as possible but ensure gas sealing

On electric resistance effects of non-homogeneous GDL deformation in a PEM fuel cell

The electric resistance is very important for the performance of a proton exchange membrane (PEM) fuel cell. However, the performance analysis is more complex as the cell operates under assembly conditions. At such conditions, the mass transfer is deteriorated but the electric conductivity is favored. In this paper, the electric resistance of a cell is evaluated by application of a recently developed method in the through-plane direction of the electrodes, together with consideration of the contact resistance between the gas diffusion layer (GDL) and bi-polar plates (BPP) for various assembly pressures. The predicted electric resistance and deformed GDL were implemented in an existing CFD code for evaluation of the PEM fuel cell performance. It is found that the electric current is distributed in a narrow area in the GDL under the shoulders and then redistributed into the BPP above the channels for all cases. The channel/rib structure promotes a non-homogeneous electric conductivity along the cell in the in-plane direction and a concentrated area of the current flow around the corner of the BPP close to the channels as the cell is subject to an assembly pressure. Additional contact areas are created between the GDL and BPP at the vertical interface when the cell operates at an assembly pressure above 2 . MPa. Therefore, both the corner of a BPP close to the channel and the GDL region become the dominating zones, where the electric current under the middle of the channel must cross over a longer distance due to the intrusion of the GDL into the BPP. In addition, the optimized cell performance is obtained as the cell is operating below 1 . MPa assembly pressure. The findings are useful for proper design of PEM fuel cells