Identification of Efficient Active Sites in Nitrogen-Doped Carbon Nanotubes for Oxygen Reduction Reaction

Abstract

Nitrogen-doped carbon nanotubes (NCNTs) have demonstrated great promise as an electrocatalyst alternative to platinum for oxygen reduction reaction (ORR). However, there is much disagreement regarding which N doping sites are most desirable for the reaction. Here, we report an experimental and computational study to identify the efficient active sites in NCNTs for ORR in alkaline media. In our experiments, we synthesized NCNTs of similar overall N content (∼4.1%) and carbon matrix but varying percentages of different types of N doping species. Our results reveal a positive correlation between ORR activity and the content of graphitic N doping. Furthermore, we performed density functional theory calculations to predict the free energy evolution of ORR on various N doping sites in NCNTs. Our computational results also indicate that graphitic N doping sites exhibit higher thermodynamic limiting potential for ORR than pyridinic N doping sites. Our combined experimental and theoretical study concludes that graphitic N doping sites are more efficient for ORR than pyridinic N doping in NCNTs.

L–Cysteine Modulated ZIF for Deriving Nitrogen‐Doped Porous Carbon: A Highly Efficient and Stable Electrocatalyst for Oxygen Reduction Reactions

“…Among them, the contents of pyridine N and graphite N in NPC-2 are higher than those in NPC-1 and NPC-2, which are 46.95 % and 37.09 %, respectively. These N species [34][35] are very important to improve the ORR activity of the materials.…”

“…The high conductivity of carbon materials, the well‐developed pores in and out of the plane, and the exposure of a large number of active sites greatly promote the transfer of electrons and mass, ensuring excellent synergistic enhancement of ORR catalytic efficiency. At the same time, different morphologies also lead to different N species, it has been widely accepted that the graphitic and pyridinic N make more contribution to oxygen reduction activity [34–35] …”

Engineering Synergistic Edge‐N Dipole in Metal‐Free Carbon Nanoflakes toward Intensified Oxygen Reduction Electrocatalysis

“…Moreover, for zigzag edges, the overpotentials of Z1 and Z2 sites are similar to each other, while on armchair edges, A2 site gives much smaller overpotential than A1 site, indicating that N g is intrinsically more active for ORR than N p , which is in accordance with previous studies. [22,23,25,[56][57][58] More importantly, compared with single N configurations, both the paired N configurations on armchair edges (A4) and zigzag edges (Z3) exhibit distinctly smaller ORR overpotentials, indicating that more propitious adsorption strengths toward OH* are achieved with A4 as the optimal active site for ORR. This clearly validates the presence of an electronic synergism between N p and N g , in which the introduction of N p further enhances the activity of N g , thus theoretically bracing the aforesaid experimental results.…”

“…[11][12][13][14][15][16][17][18][19][20][21] Despite the striking progresses, the conclusions on the accurate structures of ORR active sites remain controversial and even conflicting, particularly for specific configuration of the pivotal N dopants (pyridinic-N versus graphitic-N). [22][23][24][25][26][27][28] For example, based on a N-doped highly-oriented pyrolytic graphite (HOPG) model catalyst, the ORR activity is connected to the presence of pyridinic-N. [24] While study on a series of controlled N-doped carbons reveals the graphitic-N exclusively catalyzes ORR in both alkaline and acidic media, and pyridinic-N has negligible ORR activity. [29] In contrast, another trade-off view states that pyridinic-N and graphitic-N are respectively responsible for the ORR activity in acidic and alkaline media.…”

Carbon Nanotubes‐Based Electrocatalysts: Structural Regulation, Support Effect, and Synchrotron‐Based Characterization

“…Besides intrinsic defects, the replacement of carbon atoms with heteroatoms and the anchoring of external atoms on the surface of CNTs should be also summarized. [52,[72][73][74][75][76] Compared with the intrinsic defects, the types of introduced heteroatoms are diverse and can be extended from nonmetal to metal atoms, from single to multiple elements. [77][78][79][80][81][82][83][84][85][86] The universality of heteroatoms endows CNTs with abundant structural configurations that can trigger different electrochemical reactions.…”