Graphene-supported nanoelectrocatalysts for fuel cells: synthesis, properties, and applications.

Heteroatom modification represents one of the largest studied areas of research related to nanostructured carbon materials, with integrated applications stretching from energy production and storage to sustainability and medical uses. While a wide variety of dopants (boron, phosphorus, iodine, fluorine, etc.) have been studied, doping carbon structures with nitrogen ad-atoms has arguably experienced the greatest progress and brought the most attention over the last several years. Research in this field has conclusively demonstrated that nitrogen doping is an effective way to tailor the properties of carbon and tune the material for various applications of interest. This review provides a comprehensive overview of advances in the last half decade on state-of-the-art carbon modification with nitrogen heteroatoms. Improvements in well-established fabrication/modification processes are discussed as well as novel strategies. Additionally, recent theoretical and experimental findings related to the benefits and effects of nitrogen modification for specific applications in the energy and environmental fields are reviewed.

Recent progress on nitrogen/carbon structures designed for use in energy and sustainability applications

In this study, we performed first-principles density functional theory (DFT) calculations to investigate the reaction pathway of oxygen reduction reaction (ORR) on Me–N4 (Me = Fe, Co, or Ni) catalytic clusters formed between pores in graphene supports. Our DFT results indicate that formation of Me–N4 clusters at the edges of graphitic pores is energetically feasible. The catalytic mechanism of the Me–N4 clusters for ORR was elucidated by calculating the binding energies of ORR intermediates O2, O, OH, OOH, HOOH, and H2O on these clusters. It was found that O2 could be chemisorbed on the Co–N4 and Fe–N4 clusters but not on the Ni–N4 clusters. Thus, the Co–N4 and Fe–N4 clusters are predicted to be active for catalyzing ORR. Moreover, we observed that the O–O bond in HOOH was broken in the lowest-energy adsorption configuration of HOOH on the Fe–N4 and Co–N4 clusters. This process of breaking the O–O bond suggests that the Fe–N4 and Co–N4 clusters between graphitic pores could promote 4e− reduction of O2 with a single active site. Here, our theoretical study provides an explanation to the experimental findings of 4e− ORR on nitrogen-derived transition metal/carbon electrocatalysts. Furthermore, we illustrated that the ORR activity of the Me–N4 clusters could be gauged by an electronic descriptor related to the d-electron orbitals of the transition metal atom.

A density functional theory study of oxygen reduction reaction on Me–N4 (Me = Fe, Co, or Ni) clusters between graphitic pores

First-principles density functional theory (DFT) calculations were performed to explain the stability of catalytically active sites in Fe–Nx–C electrocatalysts, their ORR activity and ORR mechanism. The results show that the formation of graphitic in-plane Fe–N4 sites in a carbon matrix is energetically favorable over the formation of Fe–N2 sites. Chemisorption of ORR species O2, O, OH, OOH, and H2O and O–O bond breaking in peroxide occur on both Fe–N2 and Fe–N4 sites. In addition to the favorable interaction of ORR species, the computed free energy diagrams show that elementary ORR reaction steps on Fe–Nx sites are downhill. Thus, a complete ORR is predicted to occur via a single site 4e− mechanism on graphitic Fe–Nx (x = 2, 4) sites. Because of their higher stability and working potential for ORR, Fe–N4 sites are predicted to be prime candidate sites for ORR in pyrolyzed Fe–Nx–C electrocatalysts.

A density functional theory study of oxygen reduction reaction on non-PGM Fe–Nx–C electrocatalysts

Linking structure to function: The search for active sites in non-platinum group metal oxygen reduction reaction catalysts

The reaction selectivity of non-Pt based fuel cell oxygen reduction catalysts is studied in mixed methanol/O2 and formic acid/O2 environments. Technologically relevant fuel cell reaction processes are elucidated by combining experiment and theory. Structure-to-property correlations are established utilizing a mutivariant analysis technique (MVA) for catalyst surface species as determined by XPS and electrochemical performance parameters obtained in mixed reactant systems for a family of non-Pt fuel cell catalysts. Computations at the density-functional-theory (DFT) level support our experimental findings. We used first-principle simulations to explore the interactions of cobalt functionalized graphene with H2O2, O2, CO, and N2. We find that N2 and O2 adsorb as molecular species. In contrast H2O2 decomposes to form a strongly bound Co(OH)2 complex. O2 is intermediate, the oxygen−oxygen bond lengthens by ∼10% as compared to the gas phase and its binding energy (per atom) is less strong as compared to H2O2. ...

Selectivity of Cobalt-Based Non-Platinum Oxygen Reduction Catalysts in the Presence of Methanol and Formic Acid

Mesoporous conductive niobium and niobium-ruthenium particles and methods for forming the same are described. In some cases the particles are suitable for use as a fuel cell catalyst. The described aerosol-based synthesis method allows for single step formation and processing of the particles.

Svitlana Pylypenko

Papers

Selectivity of Cobalt-Based Non-Platinum Oxygen Reduction Catalysts in the Presence of Methanol and Formic Acid

Spray pyrolysis synthesis of mesoporous NbRuyOz as electrocatalyst supports in fuel cells

Nanostructured Mesoporous NbRuyOz as a Catalytic Support for Fuel Cells