Construction of oxygen-vacancy-rich 2D nitrogen-doped carbon nanosheets with CoFe@CoFe₂O₄ heterogeneous interfacial structure for rechargeable Zn-air batteries

Integration of cations doping by using unpaired high-spin electrons through the valence state substitution is deemed as a prospective strategy for construction of oxygen vacancies, regulation of inner electronic structures and enhancement of surface properties. Herein, an effective strategy is proposed to construct massive oxygen vacancies into 2D nitrogen-doped carbon nanosheets by the design of heterogeneous interfaces between CoFe alloy and CoFe2O4 nanoparticles. The valence state transition for Fe and Co endows electronic transfer capability and charge density around the oxygen atoms. The prepared CoFe-CoFe2O4@NCs-4 catalyst exhibits maximum content of oxygen vacancies (Density of O-vac 50.1 %). It shows excellent ORR activity with a half-wave potential of 0.83 V and OER overpotential of 320 mV. Reversible Zn-air batteries assembled by the CoFe-CoFe2O4@NCs-4 catalysts exhibit a power density of 106.32 mW cm(-2) and durable stability (500 h at the current density of 10 mA cm(-2)), which also are superior to those of the commercial 20 wt% Pt/C + RuO2 catalyst. The excellent bifunctional catalytic performance can be attributed to synergistic effects between massive oxygen vacancies, ultrahigh specific surface area (545.9 m(2) g(-1)) and heterogeneous interfacial structure. This work provides valuable insight into designing efficient bifunctional oxygen electrocatalysts for rechargeable Zn-air batteries.