Effect of Zn on Size Control and Oxygen Reduction Reaction Activity of Co Nanoparticles Supported on N-Doped Carbon Nanotubes

Recently, N-doped carbon supported transition metals nanoparticles have received extensive attention as promising catalysts for oxygen reduction reaction (ORR). The size of decorated nanoparticles is of great importance to the electrocatalytic activity, however, it is yet difficult to control the particle size during the synthesis by pyrolysis of the precursors at high temperatures. Moreover, the size effects on electrocatalytic activity for carbon based non-noble metal catalysts have been vaguely understood. Herein, Co nanoparticles decorated N-doped carbon nanotubes were in situ grown via direct pyrolysis of Co(NO3)2, Zn(NO3)2, and melamine mixture. It is found that the adding of Zn in the precursor plays an important role in the size control of the Co nanoparticles and the ORR activities. By tuning of the Zn amount in the precursor, Co particles can be varied from submicrometers to tens of nanometers. Meanwhile, the morphology and size of the in situ grown carbon nanotubes also changes. Roughly, the ORR activity increases with the increase of Zn amount in the precursor. Compared with the onset and half-wave potentials, the effect of Zn amount in the precursor is more important to the limiting current density. Among the studied catalysts, the optimal catalyst CoZn–N–C-6 exhibits superior activity with an onset potential of 0.971 V, a half-wave potential of 0.834 V, and a limiting current density of 5.42 mA cm–2 in 0.1 M KOH electrolyte, close to that of commercial Pt/C catalyst. The formation process of the catalyst was also studied by carefully probing the structures of the intermediate products.