Mechanistic Investigation of Ammonia Utilization Pathways on a Locally Oxygen Enriched Ni–YSZ Anode for Direct Ammonia-Fed Solid Oxide Fuel Cells: An Ab Initio Study

High Resolution Image Download MS PowerPoint Slide The limited mechanistic knowledge of the key reactions hinders the design of effective anode materials for direct ammonia-fueled solid oxide fuel cells (DA-SOFCs), a technology offering the potential for the single-step conversion of ammonia into power. For the first time, the mechanism of ammonia utilization pathways, namely, direct NH 3 electro-oxidation and the indirect route via NH 3 decomposition followed by H 2 electro-oxidation are investigated, through density functional theory (DFT) calculations, on the locally oxygen-enriched YSZ substrate of the Ni–YSZ (denoted Ni–YSZ + O) anode, with results compared to stoichiometric Ni–YSZ. The results revealed that the N* adatom recombination and desorption would most probably control the overall reaction because of the highest kinetic barrier on both stoichiometric Ni–YSZ (1.24 eV) and Ni–YSZ + O (1.98 eV) surfaces. This high activation energy for N 2 desorption explains the degradation of DA-SOFCs with Ni–YSZ anodes, as adsorbed N* species tend to form nickel nitride (Ni 3 N) rather than being desorbed, leading to active site poisoning. Considering the H 2 oxidation reaction, the H* spillover from Ni to the interface site overcomes an activation energy of 0.67 eV on Ni–YSZ + O, which is approximately five-fold greater than that of Ni–YSZ (0.16 eV), signaling the stoichiometric Ni–YSZ facilitates the spillover more effectively than the oxygen-enriched YSZ counterpart. Notably, the rate-limiting step is shifted from H 2 O interface * formation on stoichiometric Ni–YSZ to OH interface * hydrogenation on the Ni–YSZ + O surface. The H 2 O interface * formation step has a lower activation barrier on Ni–YSZ + O (1.01 eV vs 1.12 eV on stoichiometric Ni–YSZ), implying that the oxygen enrichment renders the YSZ surface more catalytically active toward this step. Interestingly, the NH 3 electro-oxidation reaction is both kinetically sluggish, owing to high kinetic barriers for N–N coupling steps─and thermodynamically unfavored for most Gerischer–Mauerer elementary steps, making NH 3 cracking followed by H 2 electro-oxidation the most viable route in DA-SOFCs, which corroborates the previous experimental studies. Overall, these mechanistic insights not only enable the rational design of anodes to enhance fuel conversion and system efficiency but also shed light on key degradation mechanisms─i.e., nitrogen poisoning from Ni 3 N formation─thereby guiding the development of more durable N -tolerant anode materials. Therefore, this study provides a foundation for designing next-generation DA-SOFC anodes with an improved performance and stability.