Mechanistic Insight into Dual-Metal-Site Catalysts for the Oxygen Reduction Reaction

Incorporating a second transition metal to iron- nitrogen-carbon single-atom catalysts (Fe-N-C SACs) to design dual-metal-site catalysts (DMSCs) was demonstrated to offer a promising opportunity to enhance the oxygen reduction reaction (ORR). However, due to the many possible structural configurations and the dynamic structure evolution of metal centers under reaction conditions, it is challenging to clearly elucidate the structure-property relationship at the atomic level. Here, we develop a computational workflow integrating configuration generations, phase diagram constructions, and reaction free energy calculations to provide an insightful understanding of the active site structures and catalytic mechanisms of ORR on DMSCs. Using Fe-Cu as an example, we generate 31 configurations by tiling the hexagonal lattice of graphene and investigate their atomic structures under reaction conditions. We find that for a wide range of electrode potentials, the Fe site is covered by an *OH intermediate, while the Cu site is not covered by any intermediate. With the OH-ligated structures, we identify the configurations which possess higher catalytic activity than Fe-N-C and Pt(111). We demonstrate that ORR on Fe-Cu DMSCs proceeds via the associative pathway, and the desorption of *OH is the rate-determining step. Further analysis reveals a linear correlation between the limiting potential and the magnetic moment on Fe and suggests a closer distance between the two metal sites benefits the catalytic activity. These mechanistic insights pave the way for the rational design of efficient platinum group metal-free DMSCs for ORR.