Multilayer graphene effects on Fe–N-C catalysts: elucidating atomic aggregation and oxygen reduction reaction activity†

Atomic aggregation is a prevalent phenomenon in heterogeneous catalysis, yet the processes governing atomic cluster formation and their effects on catalytic performance are not fully understood, leading to ongoing debate and hindering the rational design and optimization of oxygen reduction reaction (ORR) catalysts. In this study, we systematically investigate the aggregation of Fe atoms in Fe–N-C (FeN₄) catalysts, comparing both monolayer and bilayer graphene models (MGR vs. BGR), and its impact on ORR performance using density functional theory. Our analysis reveals that Fe atom aggregation significantly influences the ORR activity and catalyst stability. Specifically, in Fe_x@FeN₄-BGR models, Fe atoms tend to aggregate between layers, resulting in either enhanced or reduced activity depending on the number of aggregated Fe atoms. In contrast, Fe_x@FeN₄-MGR models exhibit ORR activity variation trends opposite to that of Fe_x@FeN₄-BGR with the same number of Fe atoms due to the absence of stabilizing interlayer effects. Stability assessments indicate that excessive aggregation, particularly without underlying layers, adversely affects the performance. These findings underscore the importance of precisely controlling atomic aggregation in the design of efficient and durable ORR catalysts and highlight the critical role of model accuracy in predicting catalyst behavior.