title: Size-controlled large-diameter and few-walled carbon nanotube catalysts for oxygen reduction.
abstract: We demonstrate a new strategy for tuning the size of large-diameter and few-walled nitrogen-doped carbon nanotubes (N-CNTs) from 50 to 150 nm by varying the transition metal (TM = Fe, Co, Ni or Mn) used to catalyze graphitization of dicyandiamide. Fe yielded the largest tubes, followed by Co and Ni, while Mn produced a clot-like carbon morphology. We show that morphology is correlated with electrocatalytic activity for the oxygen reduction reaction (ORR). A clear trend of Fe > Co > Ni > Mn for the ORR catalytic activity was observed, in both alkaline media and more demanding acidic media. The Fe-derived N-CNTs exhibited the highest BET (∼870 m(2) g(-1)) and electrochemically accessible (∼450 m(2) g(-1)) surface areas and, more importantly, the highest concentration of nitrogen incorporated into the carbon planes. Thus, in addition to the intrinsic high activity of Fe-derived catalysts, the high surface area and nitrogen doping contribute to high ORR activity. This work, for the first time, demonstrates size-controlled synthesis of large-diameter N-doped carbon tube electrocatalysts by varying the metal used in N-CNT generation. Electrocatalytic activity of the Fe-derived catalyst is already the best among studied metals, due to the high intrinsic activity of possible Fe-N coordination. This work further provides a promising route to advanced Fe-N-C nonprecious metal catalysts by generating favorable morphology with more active sites and improved mass transfer.
authors.affiliation: Department of Chemical and Biological Engineering, University at Buffalo (SUNY), Buffalo, New York 14260, USA. swihart@buffalo.edu gangwu@buffalo.edu.
authors.forename: Xianliang
authors.initials: X
authors.lastname: Wang
authors.forename: Qing
authors.initials: Q
authors.lastname: Li
authors.forename: Hengyu
authors.initials: H
authors.lastname: Pan
authors.forename: Ye
authors.initials: Y
authors.lastname: Lin
authors.forename: Yujie
authors.initials: Y
authors.lastname: Ke
authors.forename: Haiyang
authors.initials: H
authors.lastname: Sheng
authors.forename: Mark T
authors.initials: MT
authors.lastname: Swihart
authors.forename: Gang
authors.initials: G
authors.lastname: Wu