Co3Fe7/CoCx nanoparticles encapsulated in nitrogen-doped carbon nanotubes synergistically promote the oxygen reduction reaction in Zn-air batteries

Efficient and stable non-precious metal catalysts (NPMCs) for the oxygen reduction reaction (ORR) are crucial for the advancement of Zn-air batteries. Herein, we report a supramolecular self-scarifying template and confinement pyrolysis strategy to obtain an efficient ORR catalyst of well-dispersed Co3Fe7/CoCx heterostructure nanoparticles encapsulated by nitrogen-doped carbon nanotubes (Co3Fe7/CoCx@N-CNT). The as-synthesized Co3Fe7/CoCx@N-CNT catalyst exhibited outstanding ORR activity, with a half-wave potential of 0.88 V versus a reversible hydrogen electrode, and good stability. The Zn-air battery based on the Co3Fe7/CoCx@N-CNT cathode achieved a peak power density of 265 mW cm-2 and a durability of over 200 h, which is superior to most reported NPMCs and even the Pt/C counterpart. The physical characterization and electrochemical poisoning experiments revealed that the Co3Fe7/CoCx nanoparticles in the core along with pyridine N and Fe-Nx hosted in the carbon nanotube all acted as active sites for the ORR. Further theoretical calculations showed that the charge redistribution between the Co3Fe7/CoCx nanoparticles and the Fe-Nx carbon overlayers downshifted the d-band center of Fe and optimized the adsorption ability, which boosted the ORR kinetics. This work provides an effective strategy to synthesize non-precious metal ORR catalysts with multiple active sites and highlights the synergistic role of encapsulated nanoparticles and carbon support.