Tandem catalysis of Cu/Ni multi-sites promotes oxygen reduction reaction

The special electronic characteristics and high atom usage efficiency of metal-nitrogen-carbon (M-N-C) materials have made them extremely attractive for oxygen reduction reactions (ORRs). However, it is inevitable that hydrogen peroxide (H₂O₂) will be formed via the two-electron pathway in ORRs. Herein, the Cu nanoparticles (NPs) have been encapsulated into Ni doped hollow mesoporous carbon spheres (Ni-HMCS) to reduce the generation of H₂O₂ in ORR. Electrochemical tests confirm that the introduction of Cu NPs improves the ORR performance greatly. The obtained Cu/Ni-HMCS exhibits a half-wave potential of 0.82 V vs. reversible hydrogen electrode and a limited current density of 5.5 mA cm⁻², which is comparable with the commercial Pt/C. Moreover, Cu/Ni-HMCS has been used in Zn-air battery, demonstrating a high power density of 161 mW cm⁻² and a long-term recharge capability (50 h at 5 mA cm⁻²). The theoretical calculation proposes a tandem catalysis pathway for Cu/Ni multi-sites catalysis, that is, H₂O₂ released from the Ni–N₄ and Cu–N₄ sites migrates to the Cu (111) face, on which the captive H₂O₂ is further reduced to H₂O. This work demonstrates an interesting tandem catalytic pathway of dual-metal multi-sites for ORR, which provides an insight into the development of effective fuel cell electrocatalysts.