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Solid oxide fuel cell (SOFC) is one of the new energy conversion technologies producing electricity and heat from fuel and oxidant through an electrochemical reaction. The high operating temperature has many advantages including high efficiency, flexible fuel adaptability and low emissions. However, it has also some drawbacks, e.g., thermal expansion mismatches among the involved materials, particularly when the temperature reaches around 1400 oC during the sintering process, which may cause non-uniform distribution of thermal stress and even further deformation and warpage observed in the cell manufacturing steps. A three-dimensional CFD (computational fluid dynamics) simulation model is developed to study the thermal stress distributed in the sintered function layers of the anode-supported planar SOFC unit cells (10×10 cm2) using a finite element method. Five layers are included, i.e., cathode and its separation layer (GDC/LSCF, 70 µm), anode support and active layers (Ni/YSZ, 550µm), and electrolyte layer (YSZ, 30 µm) between them. In terms of the thermal stress and deformation, the predicted results are presented and discussed for the sintered unit cells with different angels located around the four cornels. It is found that the maximum thermal stress occurs at the interface between the electrode and electrolyte; the magnitude and distribution of thermal stress at the interfaces are closely related to the material thermal properties; the maximum deformation about 13 mm in the thickness direction is predicted for the 90-angel shaped cornels at the sintering temperature 1400 oC, which is bigger than that for the case with the circular shaped cornels; the deformation magnitude depends not only on the maximum thermal stress, but also the difference between the maximum and minimum thermal stress. The findings and predicted results may be applied for optimization of design and sintering conditions for SOFC unit cells. Acknowledgements This work is supported by the National Key Research and Development Project of China (2018YFB1502204, 2018YFB1502203, 2018YFB1502205), the Ningbo major special projects of the Plan “Science and Technology Innovation 2025” (2018B10048). Figure 1

Now you see (through) me: A modular vanadium redox flow cell is used to examine electrolyte distributions in carbon electrodes by synchrotron X���ray imaging. The impact of three different flow geometries on the flow dynamics is studied, and electrochemical characterizations are concurrently performed with X���ray imaging to visualize the hydrogen evolution. The unique capabilities of this cell design and experiments are outlined. The wetting behavior and affinity to side reactions of carbon���based electrodes in vanadium redox flow batteries (VRFBs) are highly dependent on the physical and chemical surface structures of the material, as well as on the cell design itself. To investigate these properties, a new cell design was proposed to facilitate synchrotron X���ray imaging. Three different flow geometries were studied to understand the impact on the flow dynamics, and the formation of hydrogen bubbles. By electrolyte injection experiments, it was shown that the maximum saturation of carbon felt was achieved by a flat flow field after the first injection and by a serpentine flow field after continuous flow. Furthermore, the average saturation of the carbon felt was correlated to the cyclic voltammetry current response, and the hydrogen gas evolution was visualized in 3D by X���ray tomography. The capabilities of this cell design and experiments were outlined, which are essential for the evaluation and optimization of cell components of VRFBs., publishedVersion

Typical operating temperature for solid oxide fuel cells (SOFC) is between 700~800°C. A large temperature gradient and thermal stress caused by internal losses and electrochemical reactions may cause SOFC stack performance degradation and even structural damage, which has become a hindrance to its applications. In this study, a three-dimensional multiphysics CFD (computational fluid dynamics) model is developed and applied for a planar SOFC stack to study the temperature and thermal stress distribution, as well as effects of structure and design parameters, including the flow channel arrangement (e.g., co- and count-flow) and thickness of the electrolyte layer. The stack is composed of three-unit cells, metallic interconnect layers, sealing and anode/cathode current collectors. The simulation results reveal that the temperature difference in the counter-flow mode is smaller and the thermal stress is lower than those in the co-flow mode. The overall performance of the stack is better when the electrolyte layer thickness becomes smaller, but the stack temperature and the temperature gradient become higher. In addition, a large temperature gradient due to the thin electrolyte layer leads to a significant increase of the thermal stress in the electrolyte. The findings and research method from this study can be applied to optimize the design of the stack structures, by consideration of the maximum thermal stress and its distribution. Acknowledgements This work is supported by the National Key Research and Development Project of China (2018YFB1502204), the Ningbo major special projects of the Plan “Science and Technology Innovation 2025” (2018B10048). References 1. M. Peksen, Progress in Energy and Combustion Science, 2015; 48: 1-20. 2. K. Eichhorn Colombo, V. Kharton, F. Berto, et al., Computers and Chemical Engineering,2020; 140: 106972. 3. P. Pianko-Oprych, T. Zinko, et al., Journal of Power Sources, 2015; 300: 10-23. 4. J. Robinson, L. Brown, R. Jervis, et al., Journal of Power Sources, 2015; 288: 473-481. 5. L. Chang, H. Liu, Y. Shiu, et. al., Journal of Power Sources, 2010; 195: 1895-1904. 6. A. Selimovic, M. Kemm, T. Torison, et. al., Journal of Power Sources, 2005; 145: 463-469. 7. M. Xu, T. Li, M. Yang, et. al., Science Bulletin, 2016; 61: 1333-1336. 8. C. Lin, L. Huang, L. Chiang, Y. Chyou, Journal of Power Sources, 2009; 192: 515-524. 9. M. Peksen, International Journal of Hydrogen Energy, 2013; 38: 553-561. 10. X. Fang, Z. Lin, Applied Energy, 2018; 229: 63-68. 11. D. Cui, M. Cheng, Journal of Power Sources, 2009; 192: 400-407. 12. Q. Li, Z. Xu, M. Cheng, et al., Modern Physics Letters B, 2020; 34(15): 23. 13. W. Zhang, D. Yan, J. Duan, et al. International Journal of Hydrogen Energy, 2013, 38(35): 15371-15378. 14. Y. Zhang, W. Jiang, S. Tu, et al., International Journal of Hydrogen Energy, 2018, 43(9): 4492-4504. Figure 1

A pore-scale simulation approach combining the pore-scale model (PSM) and lattice Boltzmann method (LBM) is developed for a gas diffusion layer (GDL) of a proton exchange membrane fuel cell. The effects of mechanical compression on the transport process of gas species, electric current, heat, and liquid water are studied. A solid mechanics model of the GDL is first numerically reconstructed using a stochastic algorithm. The reconstructed model is then compressed using the explicit dynamics method to generate deformed structures at various compression ratios. PSM simulations are subsequently employed to evaluate the transport properties, and LBM is used to simulate the intrusion process of liquid water and compute the permeability. Simulation results show that electric and thermal conductivities increase with compression ratio, whereas gas diffusivity and water permeability decrease with compression ratio. The in-plane transport properties are found to be greater than the through-plane properties. The anisotropy is evident for electric and thermal conductivities and decreases with increasing compression ratio. The PSM results are substituted into a macroscopic fuel cell model to examine the impact of compression on cell performance. It is found that the local current density becomes more diffusion-limited when the compression ratio is increased.

Phosphoric acid as the electrolyte in high-temperature polymer electrolyte membrane fuel cell plays an essential role in its performance and lifetime. Maldistribution of phosphoric acid in the catalyst layer (CL) may result in performance degradation. In the present study, pore-scale simulations were carried out to investigate phosphoric acid’s multiphase flow in a cathode CL. A reconstructed CL model was built using focused ion beam-SEM images, where distributions of pore, carbon support, binder, and catalyst particles can be identified. The multi-relaxation time lattice Boltzmann method was employed to simulate phosphoric acid invading and leaching from the membrane into the CL during the membrane electrode assembly fabrication process. The predicted redistribution of phosphoric acid indicates that phosphoric acid of low viscosity or low wettability is prone to leaching into the CL. The effective transport properties and the active electrochemical active surface area (ECSA) were computed using a pore-scale model. They were subsequently used in a macroscopic model to evaluate the cell performance. A parametric study shows that cell performance first increases with increasing phosphoric acid content due to the increase of ECSA. However, further increasing phosphoric acid content results in performance degradation due to mass transfer limitation caused by acid flooding.

Porous electrodes are commonly used in electrochemical devices such as fuel cells and vanadium redox flow batteries (VRFBs). The performance of these electrodes depends on their transport properties including diffusivity, permeability and electric/thermal conductivities, and further on their surface properties if two-phase flow occurs at high current conditions. This paper reports a pore-scale investigation on the transport processes involved in a carbon felt for VRFB applications. The microstructure of a carbon felt over a range of compression ratios is first reconstructed from micro-computed tomography. Pore-scale model simulation, which solves the coupled transport of electrolyte in porous materials, is employed to compute the effective transport properties of the reconstructed model. The permeability and diffusivity of the carbon felt are found to decrease with increasing compression ratio. Transport of liquid water within the reconstructed carbon felt is studied based on the multiple-relaxation-time lattice Boltzmann method and the simulation result indicates that compression on the electrode causes a large drop in porosity and flow resistance increases accordingly. Furthermore, the reconstructed model is converted to a finite-element model and then solid mechanics simulations are performed to gain insight to the stress distribution of the microstructure under compression. These effective transport properties are used in a two-dimensional macroscopic model to demonstrate the mesoscopic-macroscopic approach. Compression on a porous electrode is found to contribute notably on the increase of vanadium concentrations and pressure drops, which can be beneficial to the performance of VRFB. The methodology developed from this research is readily applicable to other porous electrodes such as the gas diffusion layers for fuel cells.

Two major limitations of Vanadium Redox Flow Batteries (VRFBs) are (1) The transport losses of getting the electrolyte into the electrode and to the reaction sites with minimum resistance and (2) Lack of access to all reaction sites due to relatively low saturation levels of the electrolyte.1,2 Although the porous carbon electrodes have high porosity, a large pressure may be required to pump the electrolyte through the electrode during operation. A key factor that influences the saturation and pumping pressure is the design of flow fields.3 Also, the presence of hydrogen bubbles can affect the saturation of the carbon electrode, which is formed as a parasitic side product of the V3+ reduction reaction at the anode in VRFBs.4 This affects the efficiency and stability of the VRFB cells. Besides mixed potentials, the Hydrogen bubbles could also damage the pore structure of the electrode and create an inaccessible surface area where otherwise the reaction would occur. To investigate the formation of gas bubbles inside the small pores of the carbon electrode, as well as the influence of flow geometry, we present a novel vanadium redox flow full-cell (Fig. 1(a)) that facilitates synchrotron X-ray imaging. 5 Three different flow geometries, i.e., serpentine, interdigitated, and flow-through, were tested as shown in Fig. 1(b). During the experiment, the electrolyte was injected into the cell using a peristaltic pump. Radiography was conducted during the injection process to track the flow of the electrolyte through the electrode and to calculate the average saturation for each flow geometry. Further experiments include the potential-dependent changes of the saturation and the 3D-visualization of the hydrogen bubble formation during constant potential measurements via tomography (Fig. 1(c)). This setup offers great flexibility in designing experiments for redox flow batteries, allowing the investigation of various electrode materials with different compression ratios and flow geometries under potential control. In the future, the measurements will help us develop theoretical models for a better understanding of the multiphase and interfacial flow phenomena within the porous electrode. These experiments are essential for the evaluation and optimization of electrode materials and manifolds currently being used in VRFBs. References N. Bevilacqua et al., J. Power Sources, 439, 227071 (2019) https://www.sciencedirect.com/science/article/pii/S037877531931064X. R. Banerjee, N. Bevilacqua, L. Eifert, and R. Zeis, J. Energy Storage, 21, 163–171 (2019) https://www.sciencedirect.com/science/article/pii/S2352152X18305851. R. M. Darling and M. L. Perry, J. Electrochem. Soc., 161, A1381–A1387 (2014) http://jes.ecsdl.org/lookup/doi/10.1149/2.0941409jes. L. Eifert, Z. Jusys, R. J. Behm, and R. Zeis, Carbon N. Y., 158, 580–587 (2020) https://www.sciencedirect.com/science/article/abs/pii/S0008622319311546. L. Eifert et al., ChemSusChem, cssc.202000541 (2020) https://onlinelibrary.wiley.com/doi/abs/10.1002/cssc.202000541. Figure 1

Metallic bipolar plates (BPPs) for proton-exchange membrane fuel cells (PEMFCs) are desirable in automotive applications because they (i) offer good mechanical properties and manufacturability, (ii) reduce costs compared with graphite-based BPPs, and (iii) allow flexible flow-channel designs that increase power density. In this study, the relatively unexplored couplings between the mechanical and electrochemical effects due to stack compression were analyzed using a model that accounts for the transport, electrochemical reaction, heat transfer, and stress mechanics. The present model is aimed to be employed into simulation tools for PEMFC design and application. Both the tilt angle and flow-channel width of the BPPs were found to affect the stress distribution in the gas-diffusion layer (GDL) and BPP, as well as the contact resistance. The coolant pressure affected the stress distribution in the BPP, particularly at the welded joint between two adjacent plates. Stack compression not only increased the mass-transfer resistance of the GDL, particularly under the rib region, but also resulted in improved heat transfer, which reduced the PEMFC temperature and improved the uniform temperature distribution. Although the impacts of compression on the heat and mass transfer became more pronounced at higher current densities, the combined effect with the reduced membrane temperature and contact resistance between the GDL and BPP resulted in improved PEMFC performance. Applying the model to investigate a range of mechano-electrochemical conditions revealed that higher stress–strain concentrations resulted in a more nonuniform current–density distribution at the interface between the microporous layer and catalyst layer.

Metallic bipolar plates are desirable for the automotive applications of PEMFCs because they offer excellent mechanical properties over the graphite-based bipolar plates. The flow channel configuration using metallic bipolar plates also enhances a stack’s power density. Modeling and simulation of the mechanical behavior of metallic bipolar plates under compression and the impact of mechanical stress-strain on the transport/electrochemical reactions in the membrane electrode assembly (MEA) are reported in this paper. A two-dimensional MEA model with metallic bipolar plates is developed. A two-stage approach with one-way coupling is employed to study the effects of mechanical stress-strain on transport/electrochemistry, namely, solid mechanics of the model is first solved, followed by the solution of coupled heat and mass transport over the deformed geometry obtained from the solid mechanics solution. The transport equations solved include the conservation of mass, species/charged species, and energy. Transport properties such as the porosity, permeability and contact resistance of the MEA components are either obtained from published works or expressed as functions of strain in the model, which are derived from numerical reconstruction of the materials. Furthermore, membrane degradation reactions are modeled as a function of stress to gain insight to the mechano-chemical coupling in the MEA. The comprehensive model was solved using Multiphysics COMSOL software v.5.3. Figure 1 shows the model with typical distributions of von Mises stress over the computational domain (note different scales of stress in the bipolar plates and the MEA), which are obtained by solving solid mechanics of the model. The stress and strain information of the solid mechanics solution as well as the deformed geometry are subsequently passed to the coupled heat and mass transport solution procedure to compute the distributions of species, potentials, and temperature. The present model establishes a platform to numerically investigate the interplay between mechanical responses and transport/electrochemistry in the MEA. Figure 1