On the relationship between N content, textural properties and catalytic performance for the oxygen reduction reaction of N/CNT

• The activity of N/CNT for the ORR increases with the amount of N.
• The amount of N incorporated into CNTs increases with the amount of defects.
• By generating micropores, more active sites for the ORR are developed.
• Reduction of H2O2 to H2O is favored in active sites are located in the micropores.

Non-precious metal catalysts for the oxygen reduction reaction (ORR) based upon the incorporation of different amounts of N into the CC network of multiwalled carbon nanotubes (CNTs) have been prepared by using CNTs and urea as the carbon and nitrogen sources, respectively. First, and with the aim of generating different levels of defects in the carbon network, the CNTs have been subjected to ballmilling during different periods of time between 0 and 150 h. Then, urea was mixed with the treated CNTs, subjected to further ballmilling and pyrolized at 800 °C. The number of defects, and as a consequence, the amount of N incorporated into the CNTs, increases with the duration of the ballmilling time. Moreover, the structure of the CNTs obtained after longer ballmilling times collapses leading to a carbon material with a high degree of microporisity. The performance of the N/CNT for the ORR, in terms of both the onset potential and mass current activity, increases with the amount of N actually incorporated into the CNT. Moreover, the H2O2 formation during ORR varies with the morphology of the catalyst. Thus, the formation of H2O2 is favored with the electrocatalysts in which the CNT structure is preserved, whereas the total reduction of O2 to H2O is favored for the electrocatalysts in which micropores are formed.

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