Effect of N and S co-doping of multiwalled carbon nanotubes for the oxygen reduction

• The number of defects created on CNTs increases with the duration of ballmilling.
• The incorporation of N and S into CNTs is favoured by the presence of defects.
• Using precursors containing S and N results in a higher amount of N incorporated into CNTs.
• The activity for the ORR increases with the increasing amount of surface N into the CNT.
• The promotional role of S for the ORR is magnified in alkaline media.

The co-incorporation of S and N into multiwalled carbon nanotubes (CNTs) and the effects for the catalytic performance for the oxygen reduction reaction (ORR) in acidic and alkaline electrolytes has been studied. Subjecting CNTs to different ballmilling periods results in the formation of defects in their graphitic structure. Heteroatoms such as N and S can be actually incorporated into such defects leading to active catalysts for the ORR. In fact, the ORR activity in acid and alkaline media increases with the increasing amount of heteroatoms, especially N, actually incorporated in the catalysts. The use of precursors containing both N and S into their structure such as thiourea, results in a higher incorporation of surface N atoms than with similar N-containing precursors. As a consequence the ORR activity of the S/N/CNTs based catalysts is higher than that of N/CNT ones. This promotional effect of the presence of S is more significant when the ORR is measured in alkaline media suggesting that S-incorporation into the carbon matrix could actually play a direct role for the ORR.

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