Charge transfer, bonding conditioning and solvation effect in the activation of the oxygen reduction reaction on unclustered graphitic-nitrogen-doped graphene

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.

Colloidal silica assisted fabrication of N,O,S-tridoped porous carbon nanosheets with excellent oxygen reduction performance

We demonstrate a new approach for the fabrication of N,O,S-tridoped porous carbon nanosheets with excellent oxygen reduction performance using colloidal silica as a template and an O source.

Molecular adsorbates as probes of the local properties of doped graphene

Graphene-based sensors are among the most promising of graphene's applications. The ability to signal the presence of molecular species adsorbed on this atomically thin substrate has been explored from electric measurements to light scattering. Here we show that the adsorbed molecules can be used to sense graphene properties. The interaction of porphyrin molecules with nitrogen-doped graphene has been investigated using scanning tunneling microscopy and ab initio calculations. Molecular manipulation was used to reveal the surface below the adsorbed molecules allowing to achieve an atomic-scale measure of the interaction of molecules with doped graphene. The adsorbate's frontier electronic states are downshifted in energy as the molecule approaches the doping site, with largest effect when the molecule sits over the nitrogen dopant. Theoretical calculations showed that, due to graphene's high polarizability, the adsorption of porphyrin induces a charge rearrangement on the substrate similar to the image charges on a metal. This charge polarization is enhanced around nitrogen site, leading to an increased interaction of molecules with their image charges on graphene. Consequently, the molecular states are stabilized and shift to lower energies. These findings reveal the local variation of polarizability induced by nitrogen dopant opening new routes towards the electronic tuning of graphene.