Theoretical Density Functional Theory Study of Electrocatalytic Activity of MN₄-Doped (M = Cu, Ag, and Zn) Single-Walled Carbon Nanotubes in Oxygen Reduction Reactions

The mechanism of oxygen reduction reaction (ORR) on transition metal-doped nitrogen codoped single-walled nanotubes, C114H24MN4 (MN4-CNT where M = Zn, Cu, or Ag; N = pyridinic nitrogen), has been studied with the density functional theory method at the ωB97XD/DGDZVP level of theory. The charge density analysis revealed two active sites of the catalyst toward ORR: the MN4 site and the C═C bond of the N–C═C–N metal-chelating fragment (C2 site). The structure of O-containing adsorbates (O2*, HOO*, O*, HO*, etc.) on the two sites and the corresponding adsorption energies were determined. The analysis of the free energy diagrams allows to conclude that the 4e– mechanism of ORR is thermodynamically preferable for all the studied catalysts. The probability of the 2e– mechanism of ORR with the formation of hydrogen peroxide decreases in the order Cu > Ag > Zn. The most and the least exergonic steps of the conventional 4e– mechanism of ORR on each active site of model catalysts as well as the electrode potentials of deceleration and of maximum catalytic activity in both acidic and alkaline media are determined. The relative catalytic activity toward ORR increases in the order Zn < Ag ≪ Cu and is mainly attributed to the C2 site rather than the MN4 site, while combined catalytic activity of the two sites (AgN4/C2 sites) is predicted for the AgN4-CNT catalyst.