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

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

Three-stage integration system with solid oxide fuel cell, alkali metal thermal electric converter and organic Rankine cycle for synergistic power generation

During operation, solid oxide fuel cell releases quite a great part of hydrogen energy into waste heat, leading to energy waste and even functional component degradation. In this study, solid oxide fuel cell, alkali metal thermal electric converter and organic Rankine cycle are synergistically integrated as a three-stage integration system to gradually and efficiently utilize the waste heat. Accounting a variety of thermodynamic-electrochemical losses within the system, mathematical expressions of power output, energy efficiency, exergy destruction rate, and exergy efficiency for the integration system are deduced. The basic performance features and competitiveness of the integration system are revealed. The maximum power output density of the proposed system allows to be 12407.0 W m−2, which is approximately improved by 103.8 % compared to that of the stand-alone solid oxide fuel cell (6087.4 W m−2). Parametric studies demonstrate that an increase in operation temperature, operation pressure or radiation loss geometric factor enhances the integration system performance, while an increase in β″-alumina solid electrolyte thickness, proportional coefficient or pinch temperature ratio degrades the integration system performance. The results obtained can provide some theoretical support for designing or running such an actual three-stage integration system for efficient power generation

Investigation of effects of non-homogenous deformation of gas diffusion layer in a PEM fuel cell

Proton exchange membrane fuel cells have been promoted due to improved breakthrough and increased commercialization. The assembly pressure put on a single cell and a fuel cell stack has important influence on the geometric deformation of the gas diffusion layers (GDLs) resulting in a change in porosity, permeability, and the resistance for heat and charge transfer in proton exchange membrane fuel cells. In this paper, both the finite element method and the finite volume method are used, respectively, to predict the GDL deformation and associated effects on the geometric parameters, porosity, mass transport property, and the cell performance. It is found that based on the isotropic Young's modulus and the finite element method, the porosity and thickness under a certain assembly pressure are non-homogeneous across the fuel cell in the in-plane direction. The variations of the porosity change and compression ratio in the cross-section plane are localized by three zones, that is, a linear porosity zone, a constant porosity zone, and a nonlinear porosity zone. The results showed that the GDL porosity and compression ratios maintain linear and nonlinear changes in the zone above the shoulders and the zone under the channel but close to the shoulder, respectively. However, a constant value is kept above the middle of the channel. The obtained non-homogeneous porosity distribution is applied together with the deformed GDL for further computational fluid dynamics analysis, in which the finite volume method is implemented. The computational fluid dynamic 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. The maximum oxygen mole fraction occurs where the maximum porosity exists. A sufficient GDL thickness is required to ensure transfer of fresh gas to the reaction sites far away from the channel. However, the reduction of 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. Therefore, the assembly pressure should be balanced to consider both the cell performance and gas sealing security

Enhancing Hydrogen Peroxide Synthesis through Coordination Engineering of Single-Atom Catalysts in the Oxygen Reduction Reaction: A Review

Hydrogen peroxide (H2O2) is an important chemical with a diverse array of applications. However, the existing scenario of centralized high-concentration production is in contrast with the demand for low-concentration decentralized production. In this context, the on-site green and efficient two-electron oxygen reduction reaction (ORR) for H2O2 production has developed into a promising synthetic approach. The development of low-cost, highly active, and durable advanced catalysts is the core requirement for realizing this approach. In recent years, single-atom catalysts (SACs) have become a research hotspot owing to their maximum atom utilization efficiency, tunable electronic structure, and exceptional catalytic performance. The coordination engineering of SACs is one of the key strategies to unlock their full potential for electrocatalytic H2O2 synthesis and holds significant research value. Despite considerable efforts, precisely controlling the electronic structure of active sites in SACs remains challenging. Therefore, this review summarizes the latest progress in coordination engineering strategies for SACs, aiming to elucidate the relevance between structure and performance. Our goal is to provide valuable guidance and insights to aid in the design and development of high-performance SACs for electrocatalytic H2O2 synthesis