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

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.