Simulation of High Temperature Fuel Cells for Carbon Capture

The aim of this doctoral thesis is to develop and apply a kinetic model for the simulation of High Temperature Fuel Cells for energy conversion and Carbon Capture applications. In particular, the work will focus on the analysis and the modeling of a newly discovered mechanism in Molten Carbonate Fuel Cells that sees the net migration of H2O from the cathode to the anode side in competition with the usually encountered migration of CO2. This mechanism was never reported in the literature and was named "dual-anion mechanism" to underline the parallel migration of carbonate and hydroxide ions. It is important because it can greatly affect the cell’s performance in terms of both energy conversion and CO2 sequestration. The work was performed in collaboration with ExxonMobil that first observed this phenomenon during a campaign to test the use of molten carbonate fuel cells as Carbon Capture devices. The work was also done in partnership with FuelCell Energy, who through an agreement with ExxonMobil obtained all of the experimental data of this phenomenon. The analysis of the mechanism and the development of a model to simulate cells working at such conditions were conducted in a series of different steps. To start, based on experimental data, the mechanism was studied as a function of the reactant gases to understand the main dependences of the occurring phenomena. Consequently, as more data became available, additional dependences to improve the knowledge of the mechanism and the modeling were studied. In particular, the work was focused on the analysis of the effects that the diffusion resistance has on the extent on which one anionic path evolves over the other. Successively, the operating temperature and the carbonate/hydroxide equilibrium were studied and included in the model. The analysis of the experimental data also allowed to observe the effects that the gas atmosphere can have on the cell ohmic resistance as it was determined that the electrolyte melt can change based on equilibria between melt and gas phase. The developed kinetic formulation was implemented into the SIMFC code, a home-made Fortran program realized by the group PERT of the University of Genoa for the simulation of High Temperature Fuel Cells (Molten Carbonate and Solid Oxide). In this way, the model was successfully tested by simulating the experimental data. Additionally, a formulation to consider the direct internal steam reforming of CH4 on the performance of cells was also included into the SIMFC code. The formulation considers the reaction locally with dependence on catalyst loading. As such, it allows the study of the effect of catalyst distribution and degradation. This part of the thesis was developed on Solid Oxide Fuel Cells instead of Molten Carbonate. This choice was dictated by the fact that I spent a period of 8 months during the first year of the Ph.D. program at the Korea Institute of Science and Technology studying solid oxide fuel cells materials, specifically focused on the use of perovskite (a possible solid oxide fuel cells anode material) as catalysts for the CH4 reforming reaction which will be presented. The overall model developed and implemented into the SIMFC code was demonstrated to be very promising in simulating High Temperature Fuel Cells performance under a great range of operating conditions.