Graphene-Based Dual-Metal Sites for Oxygen Reduction Reaction: A Theoretical Study

Single-atom catalysts have expanded the design paradigm for oxygen reduction reaction (ORR) relying on nonplatinum group metals (non-PGM). Here, density functional theory calculations were performed on a variety of dual-metal active centers, consisting of both PGM (Pt and Pd) and non-PGM (Fe, Co, Ni, and Cu) metals, embedded in a monolayer of graphene and coordinated by six pyridinic nitrogen atoms. The dual-metal site stability, OH ligand effect, and electronic structures relevant to ORR were investigated. The ORR reactivities can be depicted in terms of a volcano diagram divided into multiple potential limiting regimes based on a wide range of ΔGOH* values. In addition to OH removal and free molecular O2 protonation as the potential-limiting steps, the protonation of adsorbed O2 and O also emerge as likely potential-limiting steps due to strong O2 adsorptions at certain dual-metal active sites. Among the systems investigated, Fe–Co­(OH)s exhibits the highest activity. Moreover, other PGM-free dual-metal sites such as Fe–Fe­(OH), Fe–Cu­(OH), and Co–Co­(OH) also appear to be competitive and would encourage further explorations for Pt-free ORR electrocatalyst alternatives.