Here we used density functional theory to investigate the current-voltage characteristics and the distribution of catalytic active sites towards oxygen reduction of nitrogen-doped and defective graphene. The study highlights the importance of considering not only the interaction energy of reaction intermediates, but also the electrical conductivity of such configurations. The results were published in the journal of Carbon.

Joakim Ekspong, Nicolas Boulanger, Eduardo Gracia-Espino
Carbon (2018). DOI: 10.1016/j.carbon.2018.05.050

Introducing heteroatoms and creating structural defects on graphene is a common and rather successful strategy to transform its inert basal plane into an efficient metal-free electrocatalyst for oxygen reduction reaction (ORR). However, the intricate atomic configuration of defective graphenes difficult their optimization as ORR electrocatalysts, where not only a large density of active sites is desirable, but also excellent electrical conductivity is required. Therefore, we used density functional theory to investigate the current-voltage characteristics and the catalytic active sites towards ORR of nitrogen-doped and defective graphene by using 8 zig-zag graphene nanoribbons as model systems. Detailed ORR catalytic activity maps are created for ten different systems showing the distribution of catalytic hot spots generated by each defect. Subsequently, the use of both current-voltage characteristics and catalytic activity maps allow to exclude inefficient systems that exhibit either low electrical conductivity or have adsorption energies far from optimal. Our study highlights the importance of considering not only the interaction energy of reaction intermediates to design electrocatalysts, but also the electrical conductivity of such configurations. We believe that this work is important for future experimental studies by providing insights on the use of graphene as a catalyst towards the ORR reaction.

In our most recent publication we performed ab initio calculations to construct ORR overpotential maps and describe the availability and spatial distribution of catalytic active sites on phosphorus-nitrogen co-doped graphene. The results are published in the Journal of Physical Chemistry C.

Eduardo Gracia-Espino. J. Phys. Chem. C, 120, 27849–27857 (2016)
DOI: 10.1021/acs.jpcc.6b09425

Ab initio calculations are performed to investigate how the simultaneous introduction of phosphorus and nitrogen into graphene modifies the availability and spatial distribution of catalytic active sites for oxygen reduction reaction (ORR). A phosphoryl group (R3-P=O) is selected as a representative for the phosphorus doping, and the ORR is studied under alkaline conditions where a 4e- mechanism is used to determine the limiting step and overpotential (ηORR) along the entire graphene surface. A scanning procedure is used to construct ηORR maps for pristine-, N-, P-, and diverse PN co-doped graphenes. The results indicate that a single N (P) atom activates up to 17 (3) C atoms, while the simultaneous introduction of P and N activates up to 55 C atoms equivalent to 57% of the surface. Additionally, PN co-doped graphenes reveals that the relative location of both dopants has significant effects on the ORR performance, where a P-N separation distance of at least 4 Å minimize the localization of electronic states on the neighboring C atoms and improves the quantity and distribution of active sites. The results shows the importance of designing synthesis procedures to control the dopant concentration and spatial distribution to maximize the number of active sites. Furthermore, the ηORR maps reveal features that could be obtained by scanning tunneling microscopy allowing to experimentally identify and possibly quantify the catalytic active sites on carbon-based materials.

Here we performed quantum transport studies on doped and functionalized 8- and 11-armchair graphene nanoribbons (aGNRs) by means of density functional theory. We introduced B, N, O, Si, P, and S within the lattice of the aGNRs. Other functional groups such as borane, amine, hydroxyl, thiol, silane, silene, phosphine, and phosphorane are also introduced at the nanoribbon's edge. Our results suggest that wider graphene nanoribbons could be functionalized at the inner sections without significantly compromising their transport characteristics while retaining the chemical reactivity that characterize doped nanocarbons. The results were published in the  journal of RSC advances.

E. Gracia-Espino, F. López-Urías, H. Terrones and M. Terrones.
RSC Adv., 2016,6, 21954-21960
DOI: 10.1039/C5RA25278D

Quantum transport studies are performed on doped and functionalized 8- and 11-armchair graphene nanoribbons (aGNRs) by means of density functional theory. Substitutional doping is performed by introducing boron, nitrogen, oxygen, silicon, phosphorus, and sulfur atoms within the lattice of the aGNRs. Other functional groups such as borane, amine, hydroxyl, thiol, silane, silene, phosphine, and phosphorane groups are also introduced at the nanoribbon's edge. The dopant position and the nanoribbon's width strongly influence the current–voltage characteristics, and generally, the narrow 8-aGNRs and edge-doped 11-aGNRs show deteriorated transport properties, mainly due to the formation of irregular edges that create highly localized states disrupting several conducting bands. On the other hand, the inside-doped 11-aGNRs are barely affected, mainly because these systems preserve the edge's structure, thus edge conduction bands still contribute to the electron transport. Our results suggest that wider graphene nanoribbons could be functionalized at the inner sections without significantly compromising their transport characteristics while retaining the chemical reactivity that characterize doped nanocarbons. Such characteristics are highly desirable in fuel cells where doped graphene is used as a catalyst support or as a metal-free catalyst.

This is our most recent work published in the Journal of American Chemical Society. Here we studied the nanoscale interactions of reduced graphene oxide (rGOx) homogeneously decorated with small palladium nanoclusters (2.3 ± 0.3 nm). The Pd nanoparticles anchored to the rGOx-surface exhibit high crystallinity and are consistent with six-shell cuboctahedral and icosahedral clusters containing ∼600 Pd atoms. We also performed ab initio simulations to understand the electronic properties of the graphene−nanoparticle hybrid system.

This article has been published as an open access, so here you can download the published version of the article.

Studies on noble-metal-decorated carbon nanostructures are reported almost on a daily basis, but detailed studies on the nanoscale interactions for well-defined systems are very rare. Here we report a study of reduced graphene oxide (rGOx) homogeneously decorated with palladium (Pd) nanoclusters with well-defined shape and size (2.3 ± 0.3 nm). The rGOx was modified with benzyl mercaptan (BnSH) to improve the interaction with Pd clusters, and N,N-dimethylformamide was used as solvent and capping agent during the decoration process. The resulting Pd nanoparticles anchored to the rGOx-surface exhibit high crystallinity and are fully consistent with six-shell cuboctahedral and icosahedral clusters containing ∼600 Pd atoms, where 45% of these are located at the surface. According to X-ray photoelectron spectroscopy analysis, the Pd clusters exhibit an oxidized surface forming a PdOx shell. Given the well-defined experimental system, as verified by electron microscopy data and theoretical simulations, we performed ab initio simulations using 10 functionalized graphenes (with vacancies or pyridine, amine, hydroxyl, carboxyl, or epoxy groups) to understand the adsorption process of BnSH, their further role in the Pd cluster formation, and the electronic properties of the graphene−nanoparticle hybrid system. Both the experimental and theoretical results suggest that Pd clusters interact with functionalized graphene by a sulfur bridge while the remaining Pd surface is oxidized. Our study is of significant importance for all work related to anchoring of nanoparticles on nanocarbon-based supports, which are used in a variety of applications.