Reconstruction of Highly Dense Cu-N 4 Active Sites in Electrocatalytic Oxygen Reduction Characterized by Operando Synchrotron Radiation.

The emerging star of single atomic site (SAS) catalyst has been regarded as the most promising Pt-substituted electrocatalyst for oxygen reduction reaction (ORR) in anion-exchange membrane fuel cells (AEMFCs). However, the metal loading in SAS directly affects the whole device performance. Herein, we report a dual nitrogen source coordinated strategy to realize high dense Cu-N 4 SAS with a metal loading of 5.61 wt% supported on 3D N-doped carbon nanotubes/graphene structure wherein simultaneously performs superior ORR activity and stability in alkaline media. When applied in H 2 /O 2 AEMFC, it could reach an open-circuit voltage of 0.90 V and a peak power density of 324 mW cm -2 . Operando synchrotron radiation analyses identify the reconstruction from initial Cu-N 4 to Cu-N 4 /Cu-nanoclusters (NC) and the subsequent Cu-N 3 /Cu-NC under working conditions, which gradually regulate the d-band center of central metal and balance the Gibbs free energy of *OOH and *O intermediates, benefiting to ORR activity.