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Highly efficient oxygen reduction electrocatalysts based on winged carbon nanotubes.

Cheng Y, Zhang H, Varanasi CV, Liu J - Sci Rep (2013)

Bottom Line: We found that the outer-walls of a special type of carbon nanotubes/nanofibers, when selectively oxidized, unzipped and exfoliated, form graphene wings strongly attached to the inner tubes.After doping with nitrogen, the winged nanotubes exhibited outstanding activity toward catalyzing the ORR through the four-electron pathway with excellent stability and methanol/carbon monoxide tolerance.While the doped graphene wings with high active site density bring remarkable catalytic activity, the inner tubes remain intact and conductive to facilitate electron transport during electrocatalysis.

View Article: PubMed Central - PubMed

Affiliation: 1] Department of Chemistry, Duke University, Durham, NC 27708 United States [2] Center for the Environmental Implication of NanoTechnology (CEINT), Duke University, Durham, NC 27708 United States.

ABSTRACT
Developing electrocatalysts with both high selectivity and efficiency for the oxygen reduction reaction (ORR) is critical for several applications including fuel cells and metal-air batteries. In this work we developed high performance electrocatalysts based on unique winged carbon nanotubes. We found that the outer-walls of a special type of carbon nanotubes/nanofibers, when selectively oxidized, unzipped and exfoliated, form graphene wings strongly attached to the inner tubes. After doping with nitrogen, the winged nanotubes exhibited outstanding activity toward catalyzing the ORR through the four-electron pathway with excellent stability and methanol/carbon monoxide tolerance. While the doped graphene wings with high active site density bring remarkable catalytic activity, the inner tubes remain intact and conductive to facilitate electron transport during electrocatalysis.

No MeSH data available.


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Structural characterization of pristine SC-CNT and winged nanotubes: (a) SEM and (b), (c) TEM images of pristine SC-CNT; (d) SEM and (e), (f) TEM images of winged nanotubes (The scale bar for the inserted image in (d) is 200 nm).
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f1: Structural characterization of pristine SC-CNT and winged nanotubes: (a) SEM and (b), (c) TEM images of pristine SC-CNT; (d) SEM and (e), (f) TEM images of winged nanotubes (The scale bar for the inserted image in (d) is 200 nm).

Mentions: Stacked-cup carbon nanotubes (SC-CNT, also known as herringbone carbon nanofibers) with heat-treatment at 3000°C were selectively used as the starting material due to their outstanding electrical conductivity, good graphitic structure (Figure S1) and ultra-long length (50 ~ 200 μm, Figure 1a,b) compared with typical multi-walled nanotubes24. More significantly, they are commercially available at lower prices than other types of nanotubes and hence economically more attractive for large-scale applications. A high-resolution transmission electron microscopy (TEM) image shown in Figure 1c revealed that such nanotube has stacked-cup carbon structure as inner core and an assembly of concentric cylinders of graphene as external layer. Because of this unique structure, the external layer was more prone to be oxidized when subjected to the oxidization condition as described in the method section. Consequently, these external layers were heavily oxidized after oxidizing whereas the inner tubular structures remained intact (as shown by the fact that their d-spacing did not increase obviously, Figure S2). The defect-rich nature of the as-oxidized nanotubes (ox-NT) is evident from its high D-band/G-band ratio in Raman spectroscopy (Figure S1). After a sonication step (30 min) that was applied to unzip and exfoliate the heavily oxidized external layers, we found that the average diameter of nanotubes was substantially increased from 100 nm to 180 nm (Figure 1d). More importantly, the ultra-long nature of the initial materials was largely preserved due to, presumably, the intact inner core and they were much longer than few-walled carbon nanotubes subjected to a similar condition (hundreds of nanometers) used in our other research projects19. A closer examination of the products (Figure 1e) revealed that the outer layers of the original SC-CNT were successfully exfoliated through the unzipping process as clearly revealed in the TEM images (Figure 1d, inserted)25, forming graphene wings strongly attached to nanotubes. Figure 1f shows a collection of several wNT and all of them exhibit winged structure and the yield of wNT of this method is high. Additionally, we also found that the graphene wings were strongly attached to nanotubes as the winged structure was preserved even after subjecting a prolonged sonication process (10 hours, Figure S3).


Highly efficient oxygen reduction electrocatalysts based on winged carbon nanotubes.

Cheng Y, Zhang H, Varanasi CV, Liu J - Sci Rep (2013)

Structural characterization of pristine SC-CNT and winged nanotubes: (a) SEM and (b), (c) TEM images of pristine SC-CNT; (d) SEM and (e), (f) TEM images of winged nanotubes (The scale bar for the inserted image in (d) is 200 nm).
© Copyright Policy - open-access
Related In: Results  -  Collection

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getmorefigures.php?uid=PMC3824170&req=5

f1: Structural characterization of pristine SC-CNT and winged nanotubes: (a) SEM and (b), (c) TEM images of pristine SC-CNT; (d) SEM and (e), (f) TEM images of winged nanotubes (The scale bar for the inserted image in (d) is 200 nm).
Mentions: Stacked-cup carbon nanotubes (SC-CNT, also known as herringbone carbon nanofibers) with heat-treatment at 3000°C were selectively used as the starting material due to their outstanding electrical conductivity, good graphitic structure (Figure S1) and ultra-long length (50 ~ 200 μm, Figure 1a,b) compared with typical multi-walled nanotubes24. More significantly, they are commercially available at lower prices than other types of nanotubes and hence economically more attractive for large-scale applications. A high-resolution transmission electron microscopy (TEM) image shown in Figure 1c revealed that such nanotube has stacked-cup carbon structure as inner core and an assembly of concentric cylinders of graphene as external layer. Because of this unique structure, the external layer was more prone to be oxidized when subjected to the oxidization condition as described in the method section. Consequently, these external layers were heavily oxidized after oxidizing whereas the inner tubular structures remained intact (as shown by the fact that their d-spacing did not increase obviously, Figure S2). The defect-rich nature of the as-oxidized nanotubes (ox-NT) is evident from its high D-band/G-band ratio in Raman spectroscopy (Figure S1). After a sonication step (30 min) that was applied to unzip and exfoliate the heavily oxidized external layers, we found that the average diameter of nanotubes was substantially increased from 100 nm to 180 nm (Figure 1d). More importantly, the ultra-long nature of the initial materials was largely preserved due to, presumably, the intact inner core and they were much longer than few-walled carbon nanotubes subjected to a similar condition (hundreds of nanometers) used in our other research projects19. A closer examination of the products (Figure 1e) revealed that the outer layers of the original SC-CNT were successfully exfoliated through the unzipping process as clearly revealed in the TEM images (Figure 1d, inserted)25, forming graphene wings strongly attached to nanotubes. Figure 1f shows a collection of several wNT and all of them exhibit winged structure and the yield of wNT of this method is high. Additionally, we also found that the graphene wings were strongly attached to nanotubes as the winged structure was preserved even after subjecting a prolonged sonication process (10 hours, Figure S3).

Bottom Line: We found that the outer-walls of a special type of carbon nanotubes/nanofibers, when selectively oxidized, unzipped and exfoliated, form graphene wings strongly attached to the inner tubes.After doping with nitrogen, the winged nanotubes exhibited outstanding activity toward catalyzing the ORR through the four-electron pathway with excellent stability and methanol/carbon monoxide tolerance.While the doped graphene wings with high active site density bring remarkable catalytic activity, the inner tubes remain intact and conductive to facilitate electron transport during electrocatalysis.

View Article: PubMed Central - PubMed

Affiliation: 1] Department of Chemistry, Duke University, Durham, NC 27708 United States [2] Center for the Environmental Implication of NanoTechnology (CEINT), Duke University, Durham, NC 27708 United States.

ABSTRACT
Developing electrocatalysts with both high selectivity and efficiency for the oxygen reduction reaction (ORR) is critical for several applications including fuel cells and metal-air batteries. In this work we developed high performance electrocatalysts based on unique winged carbon nanotubes. We found that the outer-walls of a special type of carbon nanotubes/nanofibers, when selectively oxidized, unzipped and exfoliated, form graphene wings strongly attached to the inner tubes. After doping with nitrogen, the winged nanotubes exhibited outstanding activity toward catalyzing the ORR through the four-electron pathway with excellent stability and methanol/carbon monoxide tolerance. While the doped graphene wings with high active site density bring remarkable catalytic activity, the inner tubes remain intact and conductive to facilitate electron transport during electrocatalysis.

No MeSH data available.


Related in: MedlinePlus