Nitrogen and Iron-Codoped Carbon Hollow Nanotubules as High-Performance Catalysts toward Oxygen Reduction Reaction: A Combined Experimental and Theoretical Study

Abstract

Heteroatom-doped carbons represent a unique class of low-cost, effective catalysts for the electroreduction of oxygen, with a performance that may rival that of commercial Pt/C catalysts. In the present study, Fe and N codoped porous carbon nanotubules were prepared by controlled pyrolysis of tellurium nanowire-supported melamine formaldehyde polymer core–sheath nanofibers at elevated temperatures. Electron microscopic studies showed the formation of hollow carbon nanotubules with the outer diameter of 35–40 nm, inner diameter of 5–10 nm, and length of several hundred nanometers. Elemental mapping and spectroscopic measurements confirmed the doping of the carbon nanotubules with N and Fe including the formation of FeN4 moieties. Electrochemical studies showed that the resulting Fe,N-codoped carbons exhibited much enhanced electrocatalytic activity toward oxygen reduction in alkaline media as compared to the counterparts doped with nitrogen alone and prepared in a similar fashion, and the one prepared at 800 °C stood out as the best among the series, with an activity even better than that of commercial Pt/C. Such a remarkable performance was ascribed to the FeN4 moieties that facilitated the binding of oxygen species. This is further supported by results from DFT calculations, where relevant atomistic models were built based on experimental results and reaction free energies on various possible active sites were computed by first-principles calculations. The computational results suggested that for N-doped carbons, the active sites were the carbon atoms adjacent to nitrogen dopants, while for Fe,N-codoped carbon, the FeN4 moieties were most likely responsible for the much enhanced electrocatalytic activity, in excellent agreement with experimental results. Significantly, from the electronic structure studies, it was found that the high density of states close to the Fermi level and high spin density played a critical role in determining the electrocatalytic activity.

Engineering Local Coordination Environments of Atomically Dispersed and Heteroatom‐Coordinated Single Metal Site Electrocatalysts for Clean Energy‐Conversion

“…Considering the difference in adsorption free energy (Δ G ads ) of the intermediates, the overpotential for the ORR/OER actually depends on the free energy variation (Δ G ) of the potential‐determining step with the highest energy barrier. The Δ G ads for O* (Δ G O* ) and OH* (Δ G OH* ) have been used as activity descriptors of MN x or MC x based SACs for ORR and OER mainly for the purpose of fast screening and rational design of new SACs. However, they are not related to the intrinsic structural characteristics of the active centers.…”

Recent Progress in Defective Carbon‐Based Oxygen Electrode Materials for Rechargeable Zink‐Air Batteries

“…[73] However, the N types contributing to OER/ORR are in serious disagreement, especially for pyridinic N and quaternary N. Reported by Liu and coworkers, [40] N-doped nanoribbon (N-GRW) with abundant edges exhibited outstand-ing bifunctional performance with 0.92 V of onset potential for ORR and 0.36 V overpotential at 10 mA cm À2 for OER in 1 M KOH. [73] However, the N types contributing to OER/ORR are in serious disagreement, especially for pyridinic N and quaternary N. Reported by Liu and coworkers, [40] N-doped nanoribbon (N-GRW) with abundant edges exhibited outstand-ing bifunctional performance with 0.92 V of onset potential for ORR and 0.36 V overpotential at 10 mA cm À2 for OER in 1 M KOH.…”

Nanowire‐Templated Synthesis of FeNx‐Decorated Carbon Nanotubes as Highly Efficient, Universal‐pH, Oxygen Reduction Reaction Catalysts

“…The onset and half‐wave potentials of FeN x ‐NCNT‐2‐NH 3 shifted positively to 0.95 and 0.84 V, respectively, which was in accordance with the CV results. The increased performance can be directly attributed to FeN x species, because it has been shown that metal nitrides, including M−N x active species, form more easily in an ammonia atmosphere, and M−N x species facilitate the binding of oxygen species . The importance of the FeN x species was also demonstrated by washing FeN x ‐NCNT‐2‐NH 3 with concentrated HCl to remove the FeN x active sites.…”