Platinum (Pt) is the state-of-the-art catalyst for oxygen reduction reaction (ORR), but its high cost and scarcity limit its large-scale use. However, if the usage of Pt reduces to a sufficiently low level, this critical barrier may be overcome. Atomically dispersed metal catalysts with high activity and high atom efficiency have the possibility to achieve this goal. Herein, we report a locally distributed atomic Pt-Co nitrogen-carbon-based catalyst (denoted as A-CoPt-NC) with high activity and robust durability for ORR (267 times higher than commercial Pt/C in mass activity). The A-CoPt-NC shows a high selectivity for the 4e pathway in ORR, differing from the reported 2e pathway characteristic of atomic Pt catalysts. Density functional theory calculations suggest that this high activity originates from the synergistic effect of atomic Pt-Co located on a defected C/N graphene surface. The mechanism is thought to arise from asymmetry in the electron distribution around the Pt/Co metal centers, as well as the metal atoms' coordination with local environments on the carbon surface. This coordination results from N8V4 vacancies (where N8 represents the number of nitrogen atoms and V4 indicates the number of vacant carbon atoms) within the carbon shell, which enhances the oxygen reduction reaction via the so-called synergistic effect.

Paired, single-atom catalysts have been shown to demonstrate synergistic effects computationally and experimentally which enable them to outperform the benchmark catalyst, Pt/C, for electrochemical reactions. We explore the limit of these catalysts by screening different transition metal atoms (M = Co, Pt, Fe, Ni) in nitrogen-doped graphene for their ability to catalyze the oxygen reduction reaction (ORR). We employ density functional theory methods to explore the electronic factors affecting catalytic activity in an effort to rationalize trends in the performance of materials which are promising candidates for the next generation of electrocatalysts. It is found that CoPt@N8V4, composed of paired Co and Pt in a nitrogen-doped four-atom vacancy in graphene (N8V4), performs ideally for the ORR with an overpotential (η) of 0.30 V, followed closely by Co and Ni (η = 0.35 V) and paired Co (η = 0.37 V). The origin of activity is suggested to be the changing reduction potential of the active Co atom via the local distortion of the pore by the spectating metal partner. We utilize the ORR scaling relations and plot catalytic activity on a volcano plot, which we correlate with the degree of antibonding interactions with the O atom in the OH intermediate of the ORR. We establish that the local tuning of paired catalysts allows for the reactivity of metal atoms to be specifically modified for desirable reactivity.

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