Coupling ammonia catalytic decomposition and electrochemical oxidation for solid oxide fuel cells: A model based on elementary reaction kinetics

Ammonia (NH3) is a carbon-free Hydrogen carrier with the advantages of high volumetric energy density, developed infrastructure, as well as easy and safe storage. The coupling of NH3 catalytic decomposition with electrochemical oxidation in an anode of solid oxide fuel cells leads to the field of direct Ammonia-fueled solid oxide fuel cell. However, the related coupling mechanism needs clarification. In the present study, by developing a mechanistic model and a library of elementary reaction kinetics, we attempt to have an overview on the synergism of heterogeneous reactions, charge-transfer reactions, bulk diffusion, and charge-transfer processes of direct Ammonia-fueled solid oxide fuel cell. We describe charge-transfer reactions based on hydrogen spillover mechanism as well as oxygen spillover mechanism, and find that only the hydrogen spillover mechanism fits with experimental data. With this validated model, we identify the rate-determining steps as well as the main surface species in H2-fueled and NH3-fueled solid oxide fuel cells. We define the region of NH3 catalytic decomposition and that of electrochemical oxidation to quantify the coupling within the anode. Based on the quantification results, we study the effects of temperature and content of inlet NH3 on cell performance.