Solid Oxide Fuel Cell Modeling at the Cell Scale - Focusing on Species, Heat, Charge and Momentum Transport as well as the Reaction Kinetics and Effects

Sammanfattning: Fuel cells are electrochemical devices that directly transform chemical energy into electricity. They are promising for future energy systems, since they are energy efficient, able to use renewable fuels and, when hydrogen is used as fuel, there are no direct emissions of greenhouse gases. Various improvements are made during the recent years, however the technology is still in the early phases of commercialisation. Fully coupled computational fluid dynamics (CFD) approaches based on the finite element method (with the software COMSOL Multiphysics) in two-dimensions are developed, in several steps, to describe an intermediate temperature SOFC single cell. Governing equations covering heat, gas-phase species, momentum, ion and electron transport are implemented and coupled to kinetics describing internal reforming and electrochemical reactions. Both ordinary and Knudsen diffusion are considered for the gas-phase species transport. For the heat transport a local temperature equilibrium approach is compared to a local temperature non-equilibrium approach, considering the solid- and gas-phases. The Darcy-Brinkman equation enables continuous pressure and velocity fields over the electrode/gas channel interfaces. The electrochemical reaction model is extended from zero-dimension (with only an average value defined) in the early models, to one-dimension covering the variation in current density along the flow direction. Finally a two-dimensional approach including the current density distribution, both along the flow direction and through the electrolyte-electrodes, is developed. The model relies on experimental data from a standard cell developed at Ningbo Institute of Material Technology & Engineering (NIMTE) in China. The anode microscopic structure and catalytic characteristics have a major impact on the internal reforming reaction rates and also on the cell performance. The large difference between the different activation energies and reaction kinetics found in the open literature may be due to the fact that several parameters probably have a significance influence on the reaction rate. Heat is generated due to ohmic, activation and concentration polarizations within the electrolyte and electrodes as well as change of entropy in the cathodic electrochemical reactions. Heat is consumed due to the change of entropy in the anodic electrochemical reactions and the steam reforming reactions within the anode. The activation polarizations in the electrodes and the ohmic polarization due to ion transport in the YSZ material are found to be the major part of the polarizations. The activation polarization is the most significant and as the electrochemical model is extended from one- to two-dimensions, the activation polarization within the cathode becomes smaller than the one within the anode. This difference might be explained by different current density per (active TPB) area and variable area-to-volume-ratios for the electrochemical reactions within the anode and cathode, respectively. The current density and the activation polarization are the highest at the electrolyte-electrode interface and decreases rapidly within the electrodes as the distance from the interface increases. However, the ohmic polarization by ion transfer increases for the positions away from the interface.