General synthesis and definitive structural identification of MN4C4 single-atom catalysts with tunable electrocatalytic activities

Single-atom catalysts (SACs) have recently attracted broad research interest as they combine the merits of both homogeneous and heterogeneous catalysts. Rational design and synthesis of SACs are of immense significance but have so far been plagued by the lack of a definitive correlation between structure and catalytic properties. Here, we report a general approach to a series of monodispersed atomic transition metals (for example, Fe, Co, Ni) embedded in nitrogen-doped graphene with a common MN4C4 moiety, identified by systematic X-ray absorption fine structure analyses and direct transmission electron microscopy imaging. The unambiguous structure determination allows density functional theoretical prediction of MN4C4 moieties as efficient oxygen evolution catalysts with activities following the trend Ni > Co > Fe, which is confirmed by electrochemical measurements. Determination of atomistic structure and its correlation with catalytic properties represents a critical step towards the rational design and synthesis of precious or nonprecious SACs with exceptional atom utilization efficiency and catalytic activities

Strategies for Enhancing the Electrocatalytic Activity of M-N/C Catalysts for the Oxygen Reduction Reaction

The development of highly active and durable nonprecious metal catalysts that can replace expensive Pt-based catalysts for the oxygen reduction reaction (ORR) is of pivotal importance in polymer electrolyte membrane fuel cells. In this line of research, metal and nitrogen codoped carbon (M-N/C) catalysts have emerged as the most promising alternatives to Pt-based catalysts. This review provides an overview of recently developed synthetic strategies for the preparation of M-N/C catalysts to enhance the catalytic activity of the ORR. We present five major strategies, namely the use of metal-organic frameworks as hosts or precursors, the use of sacrificial templates, the addition of heteroelements, the preferential generation of active sites, and a biomimetic approach. For each strategy, the advantages capable of boosting catalytic activity in the ORR are summarized, and notable examples and their catalytic performances are presented. The ORR activities and measurement conditions of high-performing M-N/C catalysts are also tabulated. Finally, we summarize this review with some suggestions for future studie