Filling the in situ-generated vacancies with metal cations captured by C−N bonds of defect-rich 3D carbon nanosheet for bifunctional oxygen electrocatalysis

Nitrogen-doped carbon materials with vacancies/defects have been developed as highly efficient ORR electrocatalysts but with poor activity for OER, which limits their application in rechargeable metal-air batteries. Filling the vacancies/defects with heteroatoms is expected to be an effective strategy to obtain surprising catalytic activities and improve their stability especially under the strongly oxidizing conditions during the OER process. Herein, we successfully transformed the defect-rich 3D carbon nanosheets (DCN) into a bifunctional ORR/OER electrocatalyst (DCN-M) by utilizing the in-situ generated vacancies to capture metal cations via a modified salt-sealed strategy. By varying the metal (Fe, Ni) content, the captured metal cations in DCN-M existed in different chemical states, i.e., metal atoms were stabilized by C−N bonds at low metal contents, while at high metal contents, bimetal particles were covered by graphene layers, taking responsibility for catalyzing the ORR and OER, respectively. In addition, the in-situ formed graphene layers with an interconnected structure facilitate the electron transport during the reactions. The Janus-feature of DCN-M in structures ensures superior bifunctional activity and good stability towards ORR/OER for the rechargeable Zn-air battery. This work provides an effective strategy to design multifunctional electrocatalysts by heteroatom filling into vacancies of carbon materials.