Fe3O4 nanoparticles encapsulated in single-atom Fe–N–C towards efficient oxygen reduction reaction: Effect of the micro and macro pores

Abstract Atomically dispersed Fe–N–C catalysts with additional Fe-containing nanoparticles including metal, carbides or oxides have shown great potentials towards oxygen reduction reaction (ORR) catalysis. However, the formation of these synergistically active nanoparticles and the effect of the porous carbon structures remain unclear. In this work, a novel single-atom-involved electrochemical catalyst, i.e., Fe3O4 nanoparticles encapsulated in atomically dispersed Fe–N–C (Fe3O4@FeNC) was reported. The optimized Fe3O4@FeNC exhibits excellent ORR activity with a half-wave potential of 0.890 V and a Tafel slope of 58.8 mV dec−1, comparable with recently reported ORR catalysts and superior to these of commercial Pt/C. More importantly, the porous architectures not only affect the mass transfer and active sites, but casts huge influence on the nucleation of Fe3O4 nanoparticles, the graphitization degree of the carbon support, the chemical environments of the elements, and the ORR catalytic pathways. Density functional theory calculations show stronger O2 adsorption on Fe–N–C when supported on Fe3O4 moieties, which may increase the reactant concentration for ORR and promote the overall activity. These findings will provide important references for the future understanding and design of single-atom-involved catalysts with enhanced electrochemical properties.