Curving Engineering of Hollow Concave-Shaped Rhombic Dodecahedrons of N-Doped Carbon Encapsulated with Fe-Doped Co/Co3O4 Nanoparticles for an Efficient Oxygen Reduction Reaction and Zn–Air Batteries

For oxygen reduction reaction (ORR) electrocatalysts, hybrids based on transition metal compounds with nitrogen-doped carbonaceous materials show great potential for replacing state-of-the-art noble metal-based catalysts. Although great efforts have been devoted to improving metal loading to enhance electrocatalytic properties, a limited number of active sites are exposed on the reaction interface, contributing to the catalytic reaction. Engineering morphologies is one of the effective strategies to provide an abundant surface area with highly accessible active sites. Herein, Fe-doped Co/Co3O4 nanoparticles encapsulated in a nitrogen-doped carbon matrix (Fe–Co/Co3O4@NC) with a unique hollow concave-shaped rhombic dodecahedron structure were synthesized by an NaCl-assisted strategy. The hollow concave-shaped structure with an increased surface area leads to the adequate utilization of active sites and improved mass diffusion during the ORR. The curving effect caused by a hollow concave-shaped structure accounts for the high-frequency collisions of oxygen molecules at the inner and outer surfaces. Excellent ORR activity (E1/2 = 0.84 V vs reversible hydrogen electrode) is achieved on the Fe–Co/Co3O4@NC catalysts. A home-made Zn–air battery catalyzed by Fe–Co/Co3O4@NC demonstrates a desirable performance for practical applications.