(135f) Graphene Tube Electrocatalysts for Oxygen Reduction

Recently, we discovered a new method to prepare N-doped carbon nanotubes 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 work demonstrates size-controlled synthesis of large-diameter graphene tube electrocatalysts by varying the metal used in G-NT generation. 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. The synthesized 20% Pt/N-GT composite catalysts demonstrate significantly enhanced ORR activity and H₂-air fuel cell performance relative to those of 20% Pt/C, which is mainly attributed to the intrinsically active N-GT matrix along with possible synergistic effects between the non-precious metal active sites and the Pt nanoparticles. Unlike traditional Pt/C, the hybrid catalysts exhibit excellent stability during the accelerated durability testing, likely due to the unique highly graphitized graphene tube morphologies, capable of providing strong interaction with Pt nanoparticles and preventing their agglomeration. This work provides new insights into the origins of catalytic activity that can be used to design and synthesize further improved nanocarbon catalyst for electrochemical energy conversion.