Co Nanoparticles Encapsulated in N-Doped Carbon Nanotubes Grafted CNTs as Electrocatalysts for Enhanced Oxygen Reduction Reaction

The exploration of high-efficiency and inexpensive electrocatalysts for oxygen reduction reaction (ORR) is of critical significance for renewable energy conversion. Herein, an in situ catalytic transformation strategy toward a unique hierarchical nanostructure is reported. In the architecture, Co nanoparticles encapsulated at the tip of bamboo-like N-doped carbon nanotubes (NCNTs) are grafted on N-doped polypyrrole-derived CNTs. Thanks to the smart design of unique 3D architecture, the NPCN@Co-NCNTs catalyst displays an extraordinary ORR activity in 0.1 m KOH solution (the onset and half-wave potentials are 0.96 and 0.90 V vs RHE, respectively), which is similar to commercial Pt/C catalyst (0.99 and 0.88 V vs RHE, respectively). Meanwhile, the catalyst shows the low Tafel slope of 78 mV dec −1 and long-time stability. Experimental and theoretical results verify that the improved ORR performance is mainly related to the existence of Co nanoparticles protected by pyridinic-N-doped carbon, which lowers the theoretical overpotential of ORR. Density functional theory calculations reveal that Pyridinic-NC  Co site is the reactive site with the lowest overpotential for ORR (0.57 V). These results unambiguously indicate that the NPCN@Co-NCNTs represent a low-cost yet high-efficiency electrocatalyst for the electrocatalytic ORR.