Evaluation of ORR active sites in nitrogen-doped carbon nanofibers by KOH post treatment

[Display omitted] •Nitrogen and iron were removed from N-CNFs by KOH and H2SO4 post treatment.•The ORR activity persisted after removing 90% of the Fe and 91% of the N.•C-Nx active sites play a predominant role for the oxygen reduction on N-CNFs.•A third active site originating from a distinct carbo...

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Bibliographic Details
Published in:Catalysis today 2018-03, Vol.301, p.11-16
Main Authors: Muthuswamy, Navaneethan, Buan, Marthe E.M., Walmsley, John C., Rønning, Magnus
Format: Article
Language:English
Subjects:
Active site
Carbon electrocatalysis
Carbon nanofiber
KOH treatment
Nitrogen-doped carbon
Oxygen reduction reaction
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Summary:[Display omitted] •Nitrogen and iron were removed from N-CNFs by KOH and H2SO4 post treatment.•The ORR activity persisted after removing 90% of the Fe and 91% of the N.•C-Nx active sites play a predominant role for the oxygen reduction on N-CNFs.•A third active site originating from a distinct carbon structure is proposed. Oxygen reduction on N-doped carbon nanomaterials is believed to take place at either N-centered active sites (C-Nx) or Fe-centered active sites (Fe-Nx). In this work the origin of the oxygen reduction on nitrogen-doped carbon nanofibers (N-CNFs) is investigated by removing nitrogen and iron from the N-CNF surface using high temperature KOH treatment. The activities for the oxygen reduction reaction (ORR) in 0.5M H2SO4 are correlated with the XPS results and discussed with respect to the contribution from C-Nx and Fe-Nx active sites. Increasing the time and temperature of the KOH treatment decreased the iron and nitrogen content at the N-CNF surface. The contribution from Fe-Nx active sites was found to be minor compared to the C-Nx active sites as the KOH-treated N-CNFs with no iron in the surface still showed considerable ORR activity. Furthermore, the activity was maintained when the fraction of pyridinic-N was greatly reduced compared to quaternary-N. Finally, even when no iron or nitrogen could be detected by XPS, 50% of the initial oxygen reduction activity of the N-CNFs persisted. It is therefore suggested that there are active sites not originating from iron or nitrogen atoms, but rather from a distinct carbon environment.
ISSN:0920-5861
1873-4308
DOI:10.1016/j.cattod.2017.03.045