The marriage of crystalline/amorphous Co/Co3O4 heterostructures with N-doped hollow carbon spheres: efficient and durable catalysts for oxygen reduction

Abstract

The design of non-precious metal-N-C (M-N-C) materials with efficient oxygen reduction activity are extremely necessary for renewable energy technologies. These M-N-C electrocatalysts are often composed of metal nanoparticles and M-Nx sites simultaneously due to their structural heterogeneity induced by the high-temperature strategy. However, these metal nanoparticles usually suffer from a partial electrochemical/chemical oxidation or dissolution in strong alkaline electrolytes, which may decrease the number of active sites for electrocatalytic oxygen reduction reaction (ORR). Herein, we demonstrated a facile strategy to get this obstacle that fabricate the core-shell crystalline/amorphous Co/Co3O4 heterostructures incorporated in N-doped hollow carbon spheres (Co/Co3O4@NHCS). Specifically, the amorphous Co3O4 shell could effectively prevent the electrochemical oxidation/dissolution of metal Co core in strong alkaline solution. It could also create abundant interface that provide sufficient electron supply and reaction places, and thus make it promising the high efficiency of electron transfer. Furthermore, the N-doped defective graphitic carbon could optimize and facilitate the charge/mass transport that could maximize the catalytic activity. Deliberately coupling the crystalline/amorphous Co/Co3O4 heterostructures with Co-Nx sites are advantageous to achieve the remarkable catalytic activity and superior durability for ORR in alkaline media. As expected, the resulted Co/Co3O4@NHCS catalysts possess the superior ORR activity with a half-wave potential of 0.82V (vs. RHE), similar to that of the commercial Pt/C. Most importantly, it shows excellent durability with almost negligible loss in catalytic performance after 3500 cycles. This work opens an avenue for rational design the efficient ORR electrocatalysts toward practical applications in fuel cells.