Abstract：

Recently, we discovered a new method to prepare N-doped carbon tubes with large diameters (up to 500 nm) and relatively thin walls (less than 10 layers), which we call N-doped graphene tubes (N-GTs). Although the large-diameter tubes contain multiple graphene layers, their wall thickness and ratio of wall thickness to tube diameter are very small compared to conventional multi-walled carbon nanotubes (MWNTs). As a result, their surface areas are much higher than conventional MWNTs. We also demonstrate an effective strategy for tuning the size of large-diameter nitrogen-doped graphene tubes (N-GT) from 50 to 200 nm by varying the transition metal (M=Fe, Co, Ni or Mn) used to catalyze the graphitization of dicyanamide. Fe yielded the largest tube size, followed by Co and Ni. Rather than generating tubes, Mn produced a clot-like carbon morphology. We correlate the carbon morphology to electrochemical properties to guide the development of high-performance precious metal-free catalysts for the oxygen reduction reaction (ORR). The Fe-derived N-GTs, which had the largest diameter, also exhibited the highest activity for the ORR in alkaline media as well as in a more challenging acidic electrolyte. A clear trend of Fe > Co > Ni > Mn for the ORR catalytic activity was observed. The Fe-derived carbon material also exhibited the highest BET surface area (~870 m²/g) and electrochemically accessible surface area (~450 m²/g). More importantly, the Fe-derived G-NTs had the highest concentration of nitrogen incorporated into the graphene planes. Thus, in addition to the intrinsic high activity of Fe catalysts, the high surface area and nitrogen doping contribute to high ORR activity. This effort demonstrates optimal manipulation of morphology and surface area provides an effective approach to further improving the performance of M-N-C precious metal-free catalysts. Furthermore, aiming to improve the activity and stability of conventional Pt catalysts, the ORR active N-GT is used as a matrix to disperse Pt nanoparticles in order to build a unique hybrid Pt cathode catalyst. This is the first demonstration of the integration of a highly active Fe-N-C catalyst with Pt nanoparticles with much improved activity and stability, relative to traditional Pt/C catalyst. This work provides a new concept to design and synthesis novel cathode catalysts for fuel cells.