Elucidating conductivity and nitriding resiliency in (Al/H)-ZnO coatings for NH3-fueled SOFC separators via SEM-EDX and MLP-DFT protocols

Abstract

NH₃ fuel-based solid oxide fuel cells (SOFCs) can overcome the inefficiencies of other hydrogen sources, but NH₃ deteriorates SOFC separators by nitriding. Here, experimental and computational approaches are used to investigate aluminum-doped zinc oxide (AZO) as a SOFC separator coating candidate for nitriding mitigation. Experimental characterization reveals how AZO coating slows down the nitriding of steel separators and that AZO conductivity improves upon NH₃ exposure. A phase diagram construction framework combines a machine learning potential (MLP), density functional theory (DFT), and grand potential energy calculations to confirm the phase stability of AZO-based coatings under operating conditions. An a priori linear response simulation approach is developed to precisely reproduce the ZnO electronic structure and conductivity. This methodology is adapted to explain why AZO-based coating color changes upon nitriding, the physical contributions to which are determined by AZO conductivity versus Al impurity concentration relationships, and explain how steam incorporation into AZO-based coatings can increase their conductivity. Energy-dispersive X-ray (EDX) spectroscopy and the calculation of NH_x adsorption energetics infer that ZnO surface nitriding susceptibility diverts nitrides away from other coating or separator components. This work demonstrates insights into the design of nitriding-resistant materials for NH₃-based SOFCs.