Modelling of Ni Nitridation in NH3-Fueled Solid Oxide Fuel Cells

Abstract

In the past decade NH3 fueled solid oxide fuel cells (SOFCs) are getting increasing attention. As compared to hydrogen, ammonia has the advantages of higher energy density and ease of storage and transportation. Due to the high operating temperature of SOFCs, typically in the range of 550-750C, NH3 can be easily decomposed into nitrogen and hydrogen over Ni-based cermet fuel electrodes. Previous studies have demonstrated NH3-fueled SOFCs with high power density, high efficiency and low emissions. One of the remaining challenges is the nitridation of Ni electrode. Under realistic operating conditions, Ni in the support and active layers of the fuel electrode has a tendency of being nitride when exposed to NH3, forming Ni3N and resulting in performance degradation. In this work, the thermodynamics of the Ni-N system is explored using the CALculation of PHAse Diagrams (CALPHAD) method. The kinetics of Ni3N formation in NH3-fuelled SOFCs is then predicted, taking into account the kinetics of both ammonia decomposition and Ni3N formation. The locations inside NH3-fuled SOFCs with potential risks of Ni nitridation are then predicted and strategies to avoid or reduce nitridation are further proposed.