High-load Mg2Ni nanoparticle-carbon nanofiber composites for hydrogen storage.

Herein, a simple synthesis method for Mg2Ni composites with carbon nanofibers capable of hydrogen storage is presented. Specifically, n-butyl-sec-butyl-magnesium solution in hexane (C8H18Mg, 0.7 M) and bis-cyclopentadienyl nickel(II) (nickelocene or NiCp2) were used as precursors for the Mg2Ni nanoparticles. Subsequently, the nanoparticles were composited with carbon nanofibers (CNF) with high loading of Mg2Ni of 50 wt%, 75 wt%, 90 wt%, and 100 wt%. The physicochemical characterization of the materials indicated that the size of the as-prepared Mg2Ni nanoparticles was less than 5 nm and they were highly agglomerated due to a carbon-based binder. The best hydrogen storage values were determined to be 2.6-2.7 wt%. Among the tested materials, the composite with 75 wt% of Mg2Ni in CNF presented the best hydrogen uptake. The pressure-composition-temperature curves indicated changes in the hydriding equilibrium pressures of the Mg2Ni nanoparticles compared to the material with a similar composition produced using ball-milling and thermodynamic calculations. Thus, the results presented herein indicate the beneficial effect of nanosizing on hydriding reactions.

Fast Hydrogen Sorption Kinetics in Mg-VCl3 Produced by Cryogenic Ball-Milling.

Hydrogen storage in Mg/MgH2 materials is still an active research topic. In this work, a mixture of Mg-15wt.% VCl3 was produced by cryogenic ball milling and tested for hydrogen storage. Short milling time (1 h), liquid N2 cooling, and the use of VCl3 as an additive produced micro-flaked particles approximately 2.5-5.0 µm thick. The Mg-15wt.% VCl3 mixture demonstrated hydrogen uptake even at near room-temperature (50 °C). Mg-15wt.% VCl3 achieved ~5 wt.% hydrogen in 1 min at 300 °C/26 bar. The fast hydriding kinetics is attributed to a reduction of the activation energy of the hydriding reaction (Ea hydriding = 63.8 ± 5.6 kJ/mol). The dehydriding reaction occurred at high temperatures (300-350 °C) and 0.8-1 bar hydrogen pressure. The activation energy of the dehydriding reaction is 123.11 ± 0.6 kJ/mol. Cryomilling and VCl3 drastically improved the hydriding/dehydriding of Mg/MgH2.

Metal Borohydrides beyond Groups I and II: A Review.

This review consists of a compilation of synthesis methods and several properties of borohydrides beyond Groups I and II, i.e., transition metals, main group, lanthanides, and actinides. The reported properties include crystal structure, decomposition temperature, ionic conductivity, photoluminescence, etc., when available. The compiled properties reflect the rich chemistry and possible borohydrides' application in areas such as hydrogen storage, electronic devices that require an ionic conductor, catalysis, or photoluminescence. At the end of the review, two short but essential sections are included: a compilation of the decomposition temperature of all reported borohydrides versus the Pauling electronegativity of the cations, and a brief discussion of the possible reactions occurring during diborane emission, including some strategies to reduce this inconvenience, particularly for hydrogen storage purposes.

Resolving the Ultrafast Changes of Chemically Inequivalent Metal-Ligand Bonds in Photoexcited Molecular Complexes with Transient X-ray Absorption Spectroscopy.

Photoactive transition-metal complexes that incorporate heteroleptic ligands present a first coordination shell, which is asymmetric. Although it is generally expected that the metal-ligand bond lengths respond differently to photoexcitation, resolving these fine structural changes remains experimentally challenging, especially for flexible multidentate ligands. In this work, ultrafast X-ray absorption spectroscopy is employed to capture directly the asymmetric elongations of chemically inequivalent metal-ligand bonds in the photoexcited spin-switching FeII complex [FeII(tpen)]2+ solvated in acetonitrile, where tpen denotes N,N,N',N'-tetrakis(2-pyridylmethyl)-1,2-ethylenediamine. The possibility to correlate precisely the nature of the donor/acceptor coordinating atoms to specific photoinduced structural changes within a binding motif will provide advanced diagnostics for optimizing numerous photoactive chemical and biological building blocks.

Alanates, a Comprehensive Review.

Hydrogen storage is widely recognized as one of the biggest not solved problem within hydrogen technologies. The slow development of the materials and systems for hydrogen storage has resulted in a slow spread of hydrogen applications. There are many families of materials that can store hydrogen; among them, the alanate family can be of interest. Basic research papers and reviews have been focused on alanates of group 1 and 2. However, there are many alanates of transition metals, main group, and lanthanides that deserve attention in a review. This work is a comprehensive compilation of all known alanates. The approaches towards tuning the kinetics and thermodynamics of alanates are also covered in this review. These approaches are the formation of reactive composites, double cation alanates, or anion substitution. The crystallographic and X-ray diffraction characteristics of each alanate are presented along with this review. In the final sections, a discussion of the infrared, Raman, and thermodynamics was included.

Visualizing the non-equilibrium dynamics of photoinduced intramolecular electron transfer with femtosecond X-ray pulses.

Ultrafast photoinduced electron transfer preceding energy equilibration still poses many experimental and conceptual challenges to the optimization of photoconversion since an atomic-scale description has so far been beyond reach. Here we combine femtosecond transient optical absorption spectroscopy with ultrafast X-ray emission spectroscopy and diffuse X-ray scattering at the SACLA facility to track the non-equilibrated electronic and structural dynamics within a bimetallic donor-acceptor complex that contains an optically dark centre. Exploiting the 100-fold increase in temporal resolution as compared with storage ring facilities, these measurements constitute the first X-ray-based visualization of a non-equilibrated intramolecular electron transfer process over large interatomic distances. Experimental and theoretical results establish that mediation through electronically excited molecular states is a key mechanistic feature. The present study demonstrates the extensive potential of femtosecond X-ray techniques as diagnostics of non-adiabatic electron transfer processes in synthetic and biological systems, and some directions for future studies, are outlined.

Towards longer-lived metal-to-ligand charge transfer states of iron(II) complexes: an N-heterocyclic carbene approach.

A 9 ps (3)MLCT lifetime was achieved by a Fe(II) complex based on C(NHC)^N(py)^C(NHC) pincer ligands. This is the longest known so far for any kind of complexes of this abundant metal, and increased by almost two orders of magnitude compared to the reference Fe(II) bis-terpyridine complex.

Toward Highlighting the Ultrafast Electron Transfer Dynamics at the Optically Dark Sites of Photocatalysts.

Building a detailed understanding of the structure-function relationship is a crucial step in the optimization of molecular photocatalysts employed in water splitting schemes. The optically dark nature of their active sites usually prevents a complete mapping of the photoinduced dynamics. In this work, transient X-ray absorption spectroscopy highlights the electronic and geometric changes that affect such a center in a bimetallic model complex. Upon selective excitation of the ruthenium chromophore, the cobalt moiety is reduced through intramolecular electron transfer and undergoes a spin flip accompanied by an average bond elongation of 0.20 ± 0.03 Å. The analysis is supported by simulations based on density functional theory structures (B3LYP*/TZVP) and FEFF 9.0 multiple scattering calculations. More generally, these results exemplify the large potential of the technique for tracking elusive intermediates that impart unique functionalities in photochemical devices.