Role of Graphitic Edge Plane Exposure in Carbon Nanostructures for Oxygen Reduction Reaction

Abstract

The role that nanostructure plays in carbon-based oxygen reduction reaction (ORR) catalysts is unclear. In this publication, the role that graphitic edge plane exposure in carbon-based ORR catalysts was examined using two types of nanofibers: one with high edge plane exposure and one with minimal edge plane exposure. These nanofibers were grown, acid oxidized, and then treated with ammonia to incorporate nitrogen. They were characterized using electron microscopy, hydrophobicity imaging, X-ray photoelectron spectroscopy, and temperature-programmed oxidation experiments to verify their nanostructure, surface species, and overall composition. Rotating ring disk electrode experiments were performed to study their activity for ORR. It was found that nanostructure on its own is not a factor for improved ORR activity. Rather, nanofibers with high edge plane exposure, like stacked platelets, provide the appropriate locations for nitrogen to incorporate into the graphitic matrix. It was also found that gross nitrogen content did not play a role in ORR activity. Nitrogen had to be incorporated into the graphitic matrix, not attached as part of a surface functional group.

Carbon catalysts for electrochemical hydrogen peroxide production in acidic media

“…This could be due to the fact that different nitrogen containing functionalities can have different influences on the selectivity for the ORR, as the nature of the active sites for ORR in CN-type catalysts is still controversial (compare for instance refs [75,[77][78][79]). This is in line with these findings of Biddinger and Ozkan [80] and Zhao et al [75], that indicate that the overall nitrogen content is a poor indicator of ORR activity, and only certain types of nitrogen, e.g., on the edges of graphitic sheets promotes the 4-electron process [81,82].…”

Topological Defects in Metal‐Free Nanocarbon for Oxygen Electrocatalysis

“…Such doping-derived sites and edge-rich holes are expected to act as active sites in the electrocatalytic reactions. [ 12,17,40,44,49 ] The electrocatalytic performance of NGM was fi rst investigated on a rotating disk electrode (RDE) with a mass loading of ≈0.25 mg cm −2 . All potentials are calibrated to a reversible hydrogen electrode (RHE).…”

“…Additionally, the surface functional groups, edge dangling bonds, and defective sites in nanocarbon materials also contribute to their electrocatalytic activity. [ 12,17,21,38,39 ] Carbon nanostructures with higher edge exposure for the appropriate incorporation of nitrogen, [ 40 ] especially within the distance of edge effect, [ 12,17 ] are likely to be more active.Recently, it has been demonstrated that even dopant-free nanocarbon can deliver excellent ORR activity, which was attributed to the edge sites. [ 41 ] However, up to now, the exact active sites in nanocarbon materials and their specifi c roles during ORR and OER remain elusive and controversial.…”

One-dimensional nitrogen-containing carbon nanostructures

“…Biddinger and Ozkan [676] studied the role of graphitic edge plane exposure in CNSs for ORR. The authors concluded that nanostructure on its own does not improve ORR activity.…”