Transition Metal‐Nitrogen‐Carbon Active Site for Oxygen Reduction Electrocatalysis: Beyond the Fascinations of TM‐N4.

Low cost transition metal‐nitrogen‐carbon (TM‐N‐C) catalysts hold excellent electrocatalytic activity and stability for oxygen reduction reaction (ORR), and have been considered as the most promising alternative to commercial Pt/C. TM‐N4, once identified as one potential active site for ORR, are brought to the spotlight by several single atom TM‐N‐C catalysts recently. However, it remains ambiguous whether this active site exists and is active for ORR because of the controversial analysis about similar TM‐N‐C catalysts by different groups. Herein, we elucidated the origin, architecture, characterization methods and possible catalytic mechanism of TM‐N4 active site. Some recent cases about single atom TM‐N‐C catalysts with TM‐N4 active site were comprehensively reviewed. Thus, some derived perspectives were presented. Undoubtedly, TM‐N4 enables being qualified as one possible type of active site for ORR. However, a general acceptance may mislead for clarifying, as confirmation of the metal‐centered coordination environment requires for more advanced technologies and strategies. Questions abound! What is TM‐N4 active site in metal‐nitrogen‐carbon? Where did it originate? Why TM‐N4 is active site? How it works? Whether TM‐N4 exists or is active for ORR seems to be a transparent black box. [ABSTRACT FROM AUTHOR]