Cobalt and Iron Oxides Co‐supported on Carbon Nanotubes as an Efficient Bifunctional Catalyst for Enhanced Electrocatalytic Activity in Oxygen Reduction and Oxygen Evolution Reactions

Cobalt nanoparticles/ nitrogen, sulfur-codoped ultrathin carbon nanotubes derived from metal organic frameworks as high-efficiency electrocatalyst for robust rechargeable zinc-air battery

It remains a challenge for efficient and facile synthesis of promising non-noble metal electrocatalysts with outstanding properties. This work reported a simple pyrolysis method to prepare cobalt nanoparticles/nitrogen, sulfur-codoped ultrathin carbon nanotubes (Co NPs/N,S-CNTs) with metal organic frameworks (cobalt 2-methylimidazole, ZIF-67), melamine, polyvinylpyrrolidone (PVP) and thiourea. The prepared catalyst exhibited superior catalytic activity towards oxygen reduction reaction (ORR) such as the more positive onset potential of 0.96 V, half-wave potential of 0.86 V and smaller Tafel slope of 67.9 mV dec^-1, outperforming those of commercial Pt/C. Furthermore, the Co NPs/N,S-CNTs based Zn-air battery not only showed good cycling performance, but also displayed a notable peak power density (153.8 mW cm^-2) and large open-circuit voltage (1.433 V). This study provides some valuable guidelines for synthesizing advanced electrocatalysts in renewable energy techniques.

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.