Nitrogen-Doped Hollow Carbon Spheres with Embedded Co Nanoparticles as Active Non-Noble-Metal Electrocatalysts for the Oxygen Reduction Reaction

Transition metal (Fe, Co, Ni) complexes on carbon nanomaterials are promising candidates as electrocatalysts towards the oxygen reduction reaction (ORR). In this paper, nitrogen-doped hollow carbon spheres with embedded Co nanoparticles were successfully prepared via a controllable synthesis strategy. The morphology characterization shows that the hollow carbon spheres possess an average diameter of similar to 150 nm with a narrow size distribution and a shell thickness of similar to 14.5 nm. The content of N doping ranges from 2.1 to 6.6 at.% depending on the calcination temperature from 900 to 1050 degrees C. Compared with commercial Pt/C, the Co-containing nitrogen-doped hollow carbon spheres prepared at 900 degrees C (CoNHCS-900) as an ORR electrocatalyst shows a half-wave potential shift of only E-1/2 = 55 mV, but a superior stability of about 90.2% maintenance after 20,000 s in the O-2-saturated 0.1 M KOH at a rotating speed of 1600 rpm. This could be ascribed to the synergistic effects of N-containing moieties, Co-N-x species, and Co nanoparticles, which significantly increase the density of active sites and promote the charge transfer during the ORR process.