Probing the electro-catalytic ORR activity of cobalt-incorporated nitrogen-doped CNTs

Catalysts for oxygen reduction reaction (ORR) are crucial for fuel cells. However, the rarity and poor durability of Pt-based catalysts have prohibited its commercialization. Herein, we report cobalt-incorporated nitrogen-doped CNT (Co-NCNT) as a Pt-free, efficient and low-cost catalyst for ORR, which is synthesized by the pyrolysis of melamine formaldehyde in which cobalt nitrate is homogeneously dispersed. The electro-catalyst exhibits reasonable activity for ORR in both acidic and alkaline media. Moreover, its activity remains almost unchanged in 1000 electrochemical cycles in both acidic and alkaline media due to the protection by dense graphite layers. More specifically, detailed electron energy loss spectroscopy investigations show that the doped nitrogen species tend to co-accumulate with cobalt at the arch areas of carbon nanotubes where catalytically active Co-N_x species are proposed to form. Combined with probe tests upon ligating, Co-N_x species enriched at the arches has found to be mainly responsible for the ORR activity in acidic medium, while in alkaline medium, nitrogen doped carbon species (N-C) of Co-NCNTs are proposed to serve as dominant active sites for ORR. The ratio of Co-N_x site density to N-C site density plays a significant role in determining ORR activity.