There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

Single-layer metal dichalcogenides are two-dimensional semiconductors that present strong potential for electronic and sensing applications complementary to that of graphene.

/pdf/electronics-and-optoelectronics-of-two-dimensional-2jbqr1unvw.pdf

Electronics and optoelectronics of two-dimensional transition metal dichalcogenides.

Energy harvested directly from sunlight offers a desirable approach toward fulfilling, with minimal environmental impact, the need for clean energy. Solar energy is a decentralized and inexhaustible natural resource, with the magnitude of the available solar power striking the earth’s surface at any one instant equal to 130 million 500 MW power plants.1 However, several important goals need to be met to fully utilize solar energy for the global energy demand. First, the means for solar energy conversion, storage, and distribution should be environmentally benign, i.e. protecting ecosystems instead of steadily weakening them. The next important goal is to provide a stable, constant energy flux. Due to the daily and seasonal variability in renewable energy sources such as sunlight, energy harvested from the sun needs to be efficiently converted into chemical fuel that can be stored, transported, and used upon demand. The biggest challenge is whether or not these goals can be met in a costeffective way on the terawatt scale.2

http://pubs.acs.org/doi/pdf/10.1021/cr1002326

Solar Water Splitting Cells

Ultrathin two-dimensional nanosheets of layered transition metal dichalcogenides (TMDs) are fundamentally and technologically intriguing. In contrast to the graphene sheet, they are chemically versatile. Mono- or few-layered TMDs - obtained either through exfoliation of bulk materials or bottom-up syntheses - are direct-gap semiconductors whose bandgap energy, as well as carrier type (n- or p-type), varies between compounds depending on their composition, structure and dimensionality. In this Review, we describe how the tunable electronic structure of TMDs makes them attractive for a variety of applications. They have been investigated as chemically active electrocatalysts for hydrogen evolution and hydrosulfurization, as well as electrically active materials in opto-electronics. Their morphologies and properties are also useful for energy storage applications such as electrodes for Li-ion batteries and supercapacitors.

http://nanotubes.rutgers.edu/PDFs/nchem_TMDs_2013.pdf

The chemistry of two-dimensional layered transition metal dichalcogenide nanosheets

We present a method for calculating the stability of reaction intermediates of electrochemical processes on the basis of electronic structure calculations. We used that method in combination with detailed density functional calculations to develop a detailed description of the free-energy landscape of the electrochemical oxygen reduction reaction over Pt(111) as a function of applied bias. This allowed us to identify the origin of the overpotential found for this reaction. Adsorbed oxygen and hydroxyl are found to be very stable intermediates at potentials close to equilibrium, and the calculated rate constant for the activated proton/electron transfer to adsorbed oxygen or hydroxyl can account quantitatively for the observed kinetics. On the basis of a database of calculated oxygen and hydroxyl adsorption energies, the trends in the oxygen reduction rate for a large number of different transition and noble metals can be accounted for. Alternative reaction mechanisms involving proton/electron transfer to ...

/pdf/origin-of-the-overpotential-for-oxygen-reduction-at-a-fuel-jdt5gxiyj8.pdf

Origin of the Overpotential for Oxygen Reduction at a Fuel-Cell Cathode

The structural parameters, i.e., coordination numbers, bond distances and disorder obtained from the analysis of EXAFS spectra may sometimes be significantly influenced by errors introduced due to the inadequacy of the analysis method applied. Especially in the case of heterogeneous catalysts it has been realized that often there is a need to use an improved EXAFS data analysis compared to the simple harmonic approach which works well for well‐defined bulk structures. This is due to the fact that catalysts contain highly dispersed or disordered structures with pair distribution functions quite different from well‐crystalline bulk materials. In addition, the increased interest for in situ studies, which typically imply high temperatures, makes the influence from anharmonic vibrations on the shape of the pair distribution a significant factor. In this paper, we discuss the importance of asymmetric pair distribution functions for nano‐sized particles and how they influence the structural parameters obtained from the standard data analysis. An alternative method, which takes into account deviations from the Gaussian pair distribution function typically used in the analysis of EXAFS spectra, will be described. The method is based on an analysis of the pair distribution functions derived from molecular dynamics simulations of small metal particles and its reliability is demonstrated by comparing structural parameters obtained from independent X‐ray diffraction experiments.

Asymmetric pair distribution functions in catalysts

It is shown that nanopores are formed during desorption of NH3 from Mg(NH3)6Cl2, which has been proposed as a hydrogen storage material The system of nanopores facilitates the transport of desorbed ammonia away from the interior of large volumes of compacted storage material DFT calculations show that there exists a continuous path from the initial Mg(NH3)6Cl2 material to MgCl2 that does not involve large-scale material transport Accordingly, ammonia desorption from this system is facile

Generation of nanopores during desorption of NH3 from Mg(NH3)6Cl2.

Foreword. Preface. Preface to the Wiley Series on Electrocatalysis and Electrochemistry. Contributors. 1. Hydrogen Reactions on Nanostructured Surfaces (Holger Wolfschmidt, Odysseas Paschos, and Ulrich Stimming). 2. Comparison of Electrocatalysis and Bioelectrocatalysis of Hydrogen and Oxygen Redox Reactions (Marc T. M. Koper and Hendrik A. Heering). 3. Design of Palladium-Based Alloy Electrocatalysts for Hydrogen Oxidation Reaction in Fuel Cells (Sung Jong Yoo and Yung-Eun Sung). 4. Mechanism of an Enhanced Oxygen Reduction Reaction at Platinum-Based Electrocatalysts: Identification and Quantification of Oxygen Species Adsorbed on Electrodes by X-Ray Photoelectron Spectroscopy (Mitsuru Wakisaka, Hiroyuki Uchida, and Masahiro Watanabe). 5. Biocathodes for Dioxygen Reduction in Biofuel Cells (Renata Bilewicz and Marcin Opallo). 6. Platinum Monolayer Electrocatalysts: Improving Structure and Activity (Kotaro Sasaki, Miomir B. Vukmirovic, Jia X. Wang, and Radoslav R. Adzic). 7. The Importance of Enzymes: Benchmarks for Electrocatalysts (Fraser A. Armstrong). 8. Approach to Microbial Fuel Cells and Their Applications (Juan Pablo Busalmen, Abraham Esteve-Nunez, and Juan Miguel Feliu). 9. Half-Cell Investigations of Cathode Catalysts for PEM Fuel Cells: From Model Systems to High-Surface-Area Catalysts (Matthias Arenz and Nenad M. Markovic). 10. Nanoscale Phenomena in Catalyst Layers for PEM Fuel Cells: From Fundamental Physics to Benign Design (Karen Chan, Ata Roudgar, Liya Wang, and Michael Eikerling). 11. Fuel Cells with Neat Proton-Conducting Salt Electrolytes (Dominic Gervasio). 12. Vibrational Spectroscopy for the Characterization of PEM Fuel Cell Membrane Materials (Carol Korzeniewski). 13. Ab Initio Electrochemical Properties of Electrode Surfaces (Ismaila Dabo, Yanli Li, Nic-ephore Bonnet, and Nicola Marzari). 14. Electronic Structure and Reactivity of Transition Metal Complexes (Heather J. Kulik and Nicola Marzari). 15. Quantitative Description of Electron Transfer Reactions (Patrick H.-L. Sit, Agostino Migliore, Michael L. Klein, and Nicola Marzari). 16. Understanding Electrocatalysts for Low-Temperature Fuel Cells (Peter Ferrin, Manos Mavrikakis, Jan Rossmeisl, and Jens K. Norskov). 17. Operando XAS Techniques: Past, Present, and Future (Christina Roth and David E. Ramaker). 18. Operando X-Ray Absorption Spectroscopy of Polymer Electrolyte Fuel Cells (Eugene S. Smotkin and Carlo U. Segre). 19. New Concepts in the Chemistry and Engineering of Low-Temperature Fuel Cells (Fikile R. Brushett, Paul. J. A Kenis, and Andrzej Wieckowski). Index.

Fuel Cell Science: Theory, Fundamentals, and Biocatalysis

Density Functional Theory (DFT) calculations of the adsorption energy of CO, for a platinum overlayer on Ru(0001), have been performed. For all coverages a significant reduction in the binding energy of up to 0.5 eV has been observed compared to that obtained on Pt(111). In addition, a Steady-State Isotopic Transient Kinetic Analysis (SSITKA) study has been performed to determine the desorption rate dependence on the partial pressure of CO over commercial Pt/Ru electrocatalysts. As expected, no significant difference in the rate of exchange of CO at any given pressure is observed on going from Pt to Pt/Ru electrocatalysts when the diluted gas used was argon since the CO states will be filled to the same desorption energy for the two catalysts. However on changing the diluent gas to hydrogen, a reduction in the exchange rate for CO is observed clearly reflecting the lower CO binding energy and the increased competition for sites at the surface of the catalyst. The reduction efficiency of the Pt/Ru electrocatalyst was also studied and found to be highly dependent on whether CO or hydrogen was used. These results will be discussed with reference to the anode catalysis of the Polymer Electrolyte Membrane Fuel Cell (PEMFC).

The Ligand Effect: CO Desorption from Pt/Ru Catalysts

The enzyme nitrogenase catalyzes the biological nitrogen fixation where ${\mathrm{N}}_{2}$ is reduced to ${\mathrm{NH}}_{3}$. Density functional calculations are presented of the bonding and hydrogenation of ${\mathrm{N}}_{2}$ on a ${\mathrm{MoFe}}_{6}{\mathrm{S}}_{9}$ complex constructed to model aspects of the active site of nitrogenase. ${\mathrm{N}}_{2}$ is found to bind end on to one of the Fe atoms. A complete energy diagram for the addition of hydrogen to the ${\mathrm{MoFe}}_{6}{\mathrm{S}}_{9}$ complex with and without ${\mathrm{N}}_{2}$ is given, and a mechanism for ammonia synthesis is proposed on this basis.

/pdf/nitrogen-adsorption-and-hydrogenation-on-a-mofe6s9-complex-1ot3bwdv4k.pdf

Jens K. Nørskov

Papers

Asymmetric pair distribution functions in catalysts

Generation of nanopores during desorption of NH3 from Mg(NH3)6Cl2.

Fuel Cell Science: Theory, Fundamentals, and Biocatalysis

The Ligand Effect: CO Desorption from Pt/Ru Catalysts

Nitrogen Adsorption and Hydrogenation on a MoFe6S9 Complex