Oxygen reduction reaction mechanism on nitrogen-doped graphene: A density functional theory study

A density functional theory study on oxygen reduction reaction on nitrogen-doped graphene

Nitrogen (N)-doped carbons reportedly exhibit good electrocatalytic activity for the oxygen reduction reaction (ORR) of fuel cells. This work provides theoretical insights into the ORR mechanism of N-doped graphene by using density functional theory calculations. All possible reaction pathways were investigated, and the transition state of each elementary step was identified. The results showed that OOH reduction was easier than O-OH breaking. OOH reduction followed a direct Eley-Rideal mechanism (the OOH species was in gas phase, but H was chemisorbed on the surface) with a significantly low reaction barrier of 0.09 eV. Pathways for both four-electron and two-electron reductions were possible. The rate-determining step of the two-electron pathway was the reduction of O₂ (formation of OOH), whereas that of the four-electron pathway was the reduction of OH into H₂O. After comparing the barriers of the rate-determining steps of the two pathways, we found that the two-electron pathway was more energetically favored than the four-electron pathway.

Free-standing nitrogen-doped carbon nanofiber films as highly efficient electrocatalysts for oxygen reduction

Free-standing nitrogen-doped carbon nanofiber (NCNF) films based on polyacrylonitrile (PAN) were prepared simply by the combination of electrospinning and thermal treatment. We reused the nitrogen-rich gas generated as the byproduct of PAN at elevated temperature, mainly NH3, for surface etching and nitrogen doping. The as-obtained NCNFs exhibited a rougher surface and smaller diameter than pristine carbon nanofibers. Despite the decreased total N content, a significant increase in the content of pyrrolic-N was observed for the NCNFs. In application to electrochemistry, the free-standing NCNF films showed comparable catalytic activity with a close four-electron pathway to a commercial Pt/C catalyst in alkaline medium toward oxygen reduction reaction (ORR), which can be attributed to the nitrogen doping and high hydrophilicity. More importantly, the ORR current density on the NCNFs only dropped 6.6% after 10,000 s of continuous operation, suggesting an enhanced long-time durability. In addition, the NCNFs also showed better electrocatalytic selectivity than Pt/C. Our work reveals a facile but efficient approach for the synthesis of free-standing NCNF films as a promising alternative to Pt-based electrocatalysts in fuel cells.

The interactions of oxygen with small gold clusters on nitrogen-doped graphene

By means of density functional theory, the adsorption properties of O₂ molecule on both isolated and N-graphene supported gold clusters have been studied. The N-graphene is modeled by a C₆₅NH₂₂ cluster of finite size. The results indicate that the catalytic activity and the O₂ adsorption energies of odd-numbered Au clusters are larger than those of adjacent even-numbered ones. The O₂ molecule is in favor of bonding to the bridge sites of odd-numbered Au clusters, whereas for odd-numbered ones, the end-on adsorption mode is favored. The perpendicular adsorption orientation on N-graphene is preferred than the parallel one for Au₂, Au₃ and Au₄ clusters, while for Au₅, Au₆ and Au₇, the parallel ones are favored. When O₂ is adsorbed on N-graphene supported Au clusters, the adsorption energies are largely increased compared with those on gas-phase ones. The increased adsorption energies would significantly facilitate the electron transfer from Au d-orbital to π* orbital of O₂, which would further weakening the O–O bond and therefore enhancing the catalytic activity. The carbon atoms on N-graphene could anchor the clusters, which could make them more difficult to structural distortion, therefore enhance their stability.

Nanostructured metal-free electrochemical catalysts for highly efficient oxygen reduction

Replacing precious and nondurable Pt catalysts with cheap and commercially available materials to facilitate sluggish cathodic oxygen reduction reaction (ORR) is a key issue in the development of fuel cell technology. The recently developed cost effective and highly stable metal-free catalysts reveal comparable catalytic activity and significantly better fuel tolerance than that of current Pt-based catalysts; therefore, they can serve as feasible Pt alternatives for the next generation of ORR electrocatalysts. Their promising electrocatalytic properties and acceptable costs greatly promote the R&D of fuel cell technology. This review provides an overview of recent advances in state-of-the-art nanostructured metal-free electrocatalysts including nitrogen-doped carbons, graphitic-carbon nitride (g-C(3) N(4) )-based hybrids, and 2D graphene-based materials. A special emphasis is placed on the molecular design of these electrocatalysts, origin of their electrochemical reactivity, and ORR pathways. Finally, some perspectives are highlighted on the development of more efficient ORR electrocatalysts featuring high stability, low cost, and enhanced performance, which are the key factors to accelerate the commercialization of fuel cell technology.

Effect of microstructure of nitrogen-doped graphene on oxygen reduction activity in fuel cells

The development of fuel cells as clean-energy technologies is largely limited by the prohibitive cost of the noble-metal catalysts needed for catalyzing the oxygen reduction reaction (ORR) in fuel cells. A fundamental understanding of catalyst design principle that links material structures to the catalytic activity can accelerate the search for highly active and abundant nonmetal catalysts to replace platinum. Here, we present a first-principles study of ORR on nitrogen-doped graphene in acidic environment. We demonstrate that the ORR activity primarily correlates to charge and spin densities of the graphene. The nitrogen doping and defects introduce high positive spin and/or charge densities that facilitate the ORR on graphene surface. The identified active sites are closely related to doping cluster size and dopant-defect interactions. Generally speaking, a large doping cluster size (number of N atoms >2) reduces the number of catalytic active sites per N atom. In combination with N clustering, Stone-Wales defects can strongly promote ORR. For four-electron transfer, the effective reversible potential ranges from 1.04 to 1.15 V/SHE, depending on the defects and cluster size. The catalytic properties of graphene could be optimized by introducing small N clusters in combination with material defects.

Oxygen molecule dissociation on carbon nanostructures with different types of nitrogen doping

The energy barrier of oxygen molecule dissociation on carbon nanotubes or graphene with different types of nitrogen doping is investigated using density functional theory. The results show that the energy barriers can be reduced efficiently by all types of nitrogen doping in both carbon nanotubes and graphene. Graphite-like nitrogen and Stone-Wales defect nitrogen decrease the energy barrier more efficiently than pyridine-like nitrogen, and a dissociation barrier lower than 0.2 eV can be obtained. Higher nitrogen concentration reduces the energy barrier much more efficiently for graphite-like nitrogen. These observations are closely related to partial occupation of π* orbitals and change of work functions. Our results thus provide useful insights into the oxygen reduction reactions.