Computational characterization of nitrogen-doped carbon nanotube functionalized by Fe adatom and Fe substituent for oxygen reduction reaction
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2019 Herein we investigated how Fe-coordinated, N-doped carbon nanotubes (CNT) enhance oxygen reduction reaction (ORR) activity by comparatively studying CNT with different pyridinic and graphitic nitrogen ratios, Fe-CNT-PA and Fe-CNT-Py in reported experimental results. Density functional theory (DFT) calculations revealed that the catalytic activity of Fe or N-doped CNT sponge for ORR was affected by nature of N dopant (pyridinic N (NP) or graphitic N (NG)) and Fe doping states (adatom or substituent). The N components in the carbon network of CNT changed the catalytic properties of metal-free CNT to that of a semi-metal-like CNT by reducing the band-gap energy. Further incorporation of Fe reduced the band-gap energy to zero, producing metal-like conductor properties. When Fe is coordinated with N components on the CNT structure, coordination of Fe as an adatom than as a substituent causes the coordination sites to exhibit higher spin density distribution and stronger hybridization, enhancing the ORR catalytic properties of CNT. This catalytic activity is more substantial when the Fe adatom is combined with a pyridinic N (NP) site by lowering the adsorption energies of ORR intermediates rather than that of O2. Based on these results, we investigated the structural and electronic properties of Fe-CNT-PA and found that a Fe-CNT-PA structure with Fe adatom adjacent to an NP site had zero band-gap energy, higher binding energy, lower working function, and magnetic moment, and thus exhibited more improved ORR activity than Fe-CNT-Py.