Three Dimensional Multiphysics Modeling of Reversible Solid Oxide Electrochemical Cells for Degradation Studies

Sammanfattning: Solid oxide electrochemical cells (SOCs) are considered a highly promising technology for providing efficient, sustainable and economic conversion between chemical energy and electrical energy. SOCs can be operated as fuel cells, where electric power is obtained from fossil or non-fossil hydrocarbon fuels depending on the origin or as electrolysis cells, whereby using electrical energy from renewable sources like wind or solar, chemical energy can be stored as fuels. However, despite its promising potential, SOCs are still not commercialized in large scales. A hard competition from well-established technologies in the electricity generation market, e.g., gas turbines, for the fuel cell operating mode, hinders its commercialization for large scale applications while electrolysis operation mode has found its way into niche markets. One of the big challenges for its success is to guarantee a long-term stability (4+ years), which is currently not attainable due to degradation issues.Large gradients in temperature, gas composition and local overpotentials in a single cell or a stack cause degradation. Different variables lead to different degradation mechanisms. These gradients occur within the cell, making it difficult to monitor the degradation process. Physical models can be used for retrieving local quantities and thus assist in failure assessment and provide insights on how to mitigate them.A three dimensional multiphysics model has been developed in this work to simulate the performance of SOCs under fuel cell and electrolysis mode. The main transport phenomena are included and coupled to the electrochemical reactions. This enables the calculation of the local partial gas pressures, potentials and temperature distributions through the electrodes and across the cells as function of the operating cell voltage. The model has been validated successfully by comparison to cell test experiments with H2/H2O and CO/CO2 as feedstocks at different temperatures, flows and gas compositions. The highest deviation found, with respect to the cell voltage, is 6 %.Even though the model can operate under both fuel cell and electrolysis modes, the model has been mainly used in electrolysis mode in this work. Apart from studying the effect of the cell operating voltage on the temperature, partial pressure of steam, current density and the overpotential through the cell, the developed model has been used for analyzing one degradation phenomenon, i.e., carbon deposition in the electrolysis operating mode. A detailed analysis of this degradation phenomenon has been performed showing a good agreement between the experimental data and the modeling approach of the local crossing of the thermodynamic carbon deposition threshold. The effect of two different heat boundary conditions on carbon formation have been evaluated as well as the effect of three different structural parameters for the fuel electrode: porosity, electrode thickness and ionic conductivity with a view to seeking cell improvements that can widen the operating window where carbon deposition is avoided.