The Direct Ammonia Oxidation Reaction Mechanism and Selectivity on Ni-YSZ Anodes in Solid Oxide Fuel Cells

Ammonia (NH3), with its high hydrogen content, is a promising candidate for a hydrogen-based energy economy. Despite advancements in mitigating anode degradation, the application of ammonia in solid oxide fuel cells remains constrained by predominant research prioritization of catalytic decomposition activity optimization, overlooking the role of the direct oxidation pathway. To address the limitation in anode catalyst performance, this study employs Ni-YSZ as a model system to investigate the ammonia oxidation mechanisms under different conditions and to identify the key factors influencing pathway activity and selectivity. Thermodynamic analysis and microkinetic modeling reveal that the direct ammonia oxidation pathway is favored under typical operational conditions, including activation overpotential, temperature, and H* coverage. Importantly, the oxygen vacancy formation energy at the triple-phase boundary was identified as a critical descriptor governing both the activity and selectivity of this pathway. Regulating the oxygen chemical potential at the triple-phase boundary (TPB) accelerates electron transfer from OTPB to Ni, thereby elevating direct oxidation pathway selectivity. Furthermore, anode degradation is linked to the N* accumulation on the surface and occupation of active catalytic sites. These findings provide a comprehensive understanding of the anode oxidation mechanism and establish a descriptor-guided strategy for optimizing the selectivity of reaction pathways, enabling the rational design of advanced anode catalysts that minimize energy loss in direct ammonia solid oxide fuel cells (DA-SOFCs).