Thesis for degree: Licentiate of Engineering

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data on YSZ-supported and Ni catalyst anodes is that they are prone to carbon deposition. Small changes during the process of manufacturing can have an effect on the catalytic characteristics. The structural features, for example the particle size distribution, have a strong influence on the anodes catalytic and electrochemical characteristics [33]. Structural parameters and conditions of the experimental cell are not always clearly stated in the literature, which makes it hard to reproduce results numerically. Another issue is that the original kinetic data is often taken from a variety of different catalysis studies which makes the mechanism thermodynamically inconsistent. Due to this issue, some of the original kinetic parameters are often modified to ensure the overall consistency of the enthalpy and entropy. Averaged structural parameters, such as porosity and tortuosity, may have the same value for many different microstructure topologies but the material structure and path ways may differ significantly [32]. The correlations between the large numbers of operating conditions in combination with the simplified structural parameters make it difficult to verify the diffusion in practical ranges [14]. In summary, at the time of writing, no macroscopic parameters can properly describe the microscopic physical behavior. One of these macroscopic parameters is the tortuosity. The tortuosity factor is used to handle the discrepancy of the pressure loss for the complex path ways in porous media and should include both shear forces and elongation forces of the fluid elements in the flow. But even the meaning of tortuosity goes apart, where in some models it represents the effect of additional pathways and in some models just a numerical parameter to fit the experimental data [28]. Microscale models may enhance the knowledge and bridge over this gap. Another important remark is that the functional electrode structures are known to work in favor of electrochemical reaction, mass and charge transport. Several earlier microscale models have adopted a structural schematic spherical approach to reconstruct the porous media [22]. The approaches have until recently been based upon statistical parameters from the random spherical particles to create the model-connected physical parameters. The structure can be created by a random statistical simulation, by size and location, or by computer tomography X-ray absorption contrast (XCT), or similarly, by scanning electron microscope (SEM) of a real existing microstructure of a SOFC electrode. The SEM requires a great deal of effort in terms of measurement and data processing, and still the effect of microstructures on the electrochemical activity remains unclear [27]. According to Asinari et al. [32], the XCT does not provide good enough reconstruction for the microscale and the only viable resolution for SOFCs is provided by SEM, but still XCT is cheaper, faster and demands less effort. XCT can be improved by statistical regression, which is often used when a 3D structure needs to be obtained by a 2D structure [32, 34]. To visualize in a basic schematic way on the microscale structure, it is schematically described in Figure 2.6 by randomly situated spherical particles and transport of the different species when hydrogen is fed. The electron flow is represented and the ionic and electronic feature is represented by the binary particles. Part of the cell is divided into three microscopic regions which are named as cathode functional layer (CFL), electrolyte and anode functional layer (AFL). The functional layer defines the part of the anode or the cathode closest to the electrolyte where most of the active reactions take place. A TPB location is enlarged to show the different material components of an SOFC. 20
Figure 2.7: Schematic illustration of the microstructure components and the main processes. The electrode performance can be increased if there is sufficient porosity so that gas transfer is not limiting and so that the TPB needs to be maximized. While a fine microstructure and a high surface area are clearly desirable, this can lead to low mechanical strength [35]. The anode is often used as the mechanical support for the cell and a change of the anode structure can be problematic. Because the active region in the anode where the electrochemical reactions takes place, extends less than approximately 10 μm from the anode–electrolyte interface, a graded porosity (like functional layers) is sometimes used to maximize the amount of TPB in the active region while still maintaining a high mechanical strength for the rest of the anode which is used primarily as the cell support [35]. Another effect captured by microstructure considerations is that the cell performance can be permanently affected by the electric field. The pore formation in the material can be affected by oxygen potential gradient at the cathode/electrolyte interface and nickel agglomeration at the TPB. This will cause a degradation of the performance by the electrochemical reactions at the TPB due to the sintering of metal particles, which causes a decrease in specific contact area of Ni particles [34, 36]. 21
Page 1 and 2: SOFC modeling from micro to macrosc
Page 3 and 4: Abstract The purpose of this work i
Page 5 and 6: katalytisk omvandling av naturgas o
Page 7 and 8: List of publications Journal public
Page 9 and 10: 3.2 Governing equations for the mac
Page 11 and 12: Q source term (heat), W/m3 q heat f
Page 13 and 14: 1 Introduction After some time of d
Page 15 and 16: 2 SOFC modeling at smaller scales T
Page 17 and 18: etween 0.3 and 1.5 mm. In this conf
Page 19 and 20: Volume Method (FVM) which adopt the
Page 21 and 22: 2.3 Lattice Boltzmann method LBM ha
Page 23 and 24: streaming [3]. The concentration ρ
Page 25 and 26: ease the correspondence between the
Page 27 and 28: applicable for a mixture of two spe
Page 29 and 30: Figure 2.6: Effective boundary arra
Page 31: Table 2.1: Comparison of previous w
Page 35 and 36: model is presented after the data c
Page 37 and 38: 3.2.1 Mass transport In the electro
Page 39 and 40: where h v is the volume heat transf
Page 41 and 42: Achenbach [43], it was found that t
Page 43 and 44: cathode are assumed to be similar t
Page 45 and 46: (3.48) where, besides the parameter
Page 47 and 48: for an isothermal spherical particl
Page 49 and 50: Velocity carried out to check wheth
Page 51 and 52: Figure 4.4: Velocity field [lu/ts]
Page 53 and 54: Table 4.2: Parameters in the SOFC a
Page 55 and 56: eactions are safely fulfilled. This
Page 57 and 58: Figure 4.8: Mole fraction distribut
Page 59 and 60: Table 4.5: Inlet mole fractions for
Page 61 and 62: Figure 4.12: Mole fraction distribu
Page 63 and 64: 5 Conclusions The physics and the t
Page 65 and 66: 7 References [1] Andersson M., Yuan
Page 67 and 68: [25] Latt J., Hydrodynamic Limit of

Thesis for degree: Licentiate of Engineering

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