Establishing ZIF-8 as a reference material for hydrogen cryoadsorption: An interlaboratory study.

Hydrogen storage by cryoadsorption on porous materials has the advantages of low material cost, safety, fast kinetics, and high cyclic stability. The further development of this technology requires reliable data on the H2 uptake of the adsorbents, however, even for activated carbons the values between different laboratories show sometimes large discrepancies. So far no reference material for hydrogen cryoadsorption is available. The metal-organic framework ZIF-8 is an ideal material possessing high thermal, chemical, and mechanical stability that reduces degradation during handling and activation. Here, we distributed ZIF-8 pellets synthesized by extrusion to 9 laboratories equipped with 15 different experimental setups including gravimetric and volumetric analyzers. The gravimetric H2 uptake of the pellets was measured at 77 K and up to 100 bar showing a high reproducibility between the different laboratories, with a small relative standard deviation of 3-4 % between pressures of 10-100 bar. The effect of operating variables like the amount of sample or analysis temperature was evaluated, remarking the calibration of devices and other correction procedures as the most significant deviation sources. Overall, the reproducible hydrogen cryoadsorption measurements indicate the robustness of the ZIF-8 pellets, which we want to propose as a reference material.

Impact of Surface Chemistry of Silicon Nanoparticles on the Structural and Electrochemical Properties of Si/Ni3.4Sn4 Composite Anode for Li-Ion Batteries.

Embedding silicon nanoparticles in an intermetallic matrix is a promising strategy to produce remarkable bulk anode materials for lithium-ion (Li-ion) batteries with low potential, high electrochemical capacity and good cycling stability. These composite materials can be synthetized at a large scale using mechanical milling. However, for Si-Ni3Sn4 composites, milling also induces a chemical reaction between the two components leading to the formation of free Sn and NiSi2, which is detrimental to the performance of the electrode. To prevent this reaction, a modification of the surface chemistry of the silicon has been undertaken. Si nanoparticles coated with a surface layer of either carbon or oxide were used instead of pure silicon. The influence of the coating on the composition, (micro)structure and electrochemical properties of Si-Ni3Sn4 composites is studied and compared with that of pure Si. Si coating strongly reduces the reaction between Si and Ni3Sn4 during milling. Moreover, contrary to pure silicon, Si-coated composites have a plate-like morphology in which the surface-modified silicon particles are surrounded by a nanostructured, Ni3Sn4-based matrix leading to smooth potential profiles during electrochemical cycling. The chemical homogeneity of the matrix is more uniform for carbon-coated than for oxygen-coated silicon. As a consequence, different electrochemical behaviors are obtained depending on the surface chemistry, with better lithiation properties for the carbon-covered silicon able to deliver over 500 mAh/g for at least 400 cycles.

Solid-State Li-Ion Batteries Operating at Room Temperature Using New Borohydride Argyrodite Electrolytes.

Using a new class of (BH4)- substituted argyrodite Li6PS5Z0.83(BH4)0.17, (Z = Cl, I) solid electrolyte, Li-metal solid-state batteries operating at room temperature have been developed. The cells were made by combining the modified argyrodite with an In-Li anode and two types of cathode: an oxide, LixMO2 (M = ⅓ Ni, ⅓ Mn, ⅓ Co; so called NMC) and a titanium disulfide, TiS2. The performance of the cells was evaluated through galvanostatic cycling and Alternating Current AC electrochemical impedance measurements. Reversible capacities were observed for both cathodes for at least tens of cycles. However, the high-voltage oxide cathode cell shows lower reversible capacity and larger fading upon cycling than the sulfide one. The AC impedance measurements revealed an increasing interfacial resistance at the cathode side for the oxide cathode inducing the capacity fading. This resistance was attributed to the intrinsic poor conductivity of NMC and interfacial reactions between the oxide material and the argyrodite electrolyte. On the contrary, the low interfacial resistance of the TiS2 cell during cycling evidences a better chemical compatibility between this active material and substituted argyrodites, allowing full cycling of the cathode material, 240 mAhg-1, for at least 35 cycles with a coulombic efficiency above 97%.

Pseudo-ternary LiBH4·LiCl·P2S5 system as structurally disordered bulk electrolyte for all-solid-state lithium batteries.

The properties of the mixed system LiBH4-LiCl-P2S5 are studied with respect to all-solid-state batteries. The studied material undergoes an amorphization upon heating above 60 °C, accompanied with increased Li+ conductivity beneficial for battery electrolyte applications. The measured ionic conductivity is ∼10-3 S cm-1 at room temperature with an activation energy of 0.40(2) eV after amorphization. Structural analysis and characterization of the material suggest that BH4 groups and PS4 may belong to the same molecular structure, where Cl ions interplay to accommodate the structural unit. Thanks to its conductivity, ductility and electrochemical stability (up to 5 V, Au vs. Li+/Li), this new electrolyte is successfully tested in battery cells operated with a cathode material (layered TiS2, theo. capacity 239 mA h g-1) and Li anode resulting in 93% capacity retention (10 cycles) and notable cycling stability under the current density ∼12 mA g-1 (0.05C-rate) at 50 °C. Further advanced characterisation by means of operando synchrotron X-ray diffraction in transmission mode contributes explicitly to a better understanding of the (de)lithiation processes of solid-state battery electrodes operated at moderate temperatures.

Mechanochemistry of Metal Hydrides: Recent Advances.

This paper is a collection of selected contributions of the 1st International Workshop on Mechanochemistry of Metal Hydrides that was held in Oslo in May 2018. In this paper, the recent developments in the use of mechanochemistry to synthesize and modify metal hydrides are reviewed. A special emphasis is made on new techniques beside the traditional way of ball milling. High energy milling, ball milling under hydrogen reactive gas, cryomilling and severe plastic deformation techniques such as High-Pressure Torsion (HPT), Surface Mechanical Attrition Treatment (SMAT) and cold rolling are discussed. The new characterization method of in-situ X-ray diffraction during milling is described.

A novel method for the synthesis of solvent-free Mg(B3H8)2.

This communication presents a novel and solvent-free method to synthesise Mg(B3H8)2 via the gas-solid reaction between B2H6 and Mg2NiH4, which overcomes the limitations of wet chemical methods requiring solvent removal.

Synthesis, structural and hydrogenation properties of Mg-rich MgH2-TiH2 nanocomposites prepared by reactive ball milling under hydrogen gas.

MgH(2)-TiH(2) nanocomposites have been obtained by reactive ball milling of elemental powders under 8 MPa of hydrogen pressure. The composites consist of a mixture of ß-rutile MgH(2), γ-orthorhombic high pressure MgH(2) and ε-tetragonal TiH(2) phases with nanosized crystallites ranging from 4 to 12 nm. In situ hydrogen absorption curves on milling reveal that nanocomposite formation occurs in less than 50 min through the consecutive synthesis of the TiH(2) and MgH(2) phases. The abrasive and catalytic properties of TiH(2) speed up the formation of the MgH(2) phase. Thermodynamic, kinetic and cycling hydrogenation properties have been determined for the 0.7MgH(2)-0.3TiH(2) composite and compared to nanometric MgH(2). Only the MgH(2) phase desorbs hydrogen reversibly at moderate temperature (523 to 598 K) and pressure (10(-3) to 1 MPa). The presence of TiH(2) does not modify the thermodynamic properties of the Mg/MgH(2) system. However, the MgH(2)-TiH(2) nanocomposite exhibits outstanding kinetic properties and cycling stability. At 573 K, H-sorption takes place in less than 100 s. This is 20 times faster than for a pure nanometric MgH(2) powder. We demonstrate that the TiH(2) phase inhibits grain coarsening of Mg, which allows extended nucleation of the MgH(2) phase in Mg nanoparticles before a continuous and blocking MgH(2) hydride layer is formed. The low crystallinity of the TiH(2) phase and its hydrogenation properties are also compatible with a gateway mechanism for hydrogen transfer from the gas phase to Mg. Mg-rich MgH(2)-TiH(2) nanocomposites are an excellent media for hydrogen storage at moderate temperatures.

Effect of NH2 and CF3 functionalization on the hydrogen sorption properties of MOFs.

The hydrogen adsorption capacity and heat of adsorption at 77 K have been evaluated for several porous metal terephthalate MOFs (MIL-53(Fe), MIL-125(Ti) and UiO-66(Zr)), as well as in their -NH(2) and -(CF(3))(2) functionalized isoreticular structures. The capacity of hydrogen is basically related to the textural properties of the solids and not to their composition. The heats of adsorption at low coverage are on the whole close to those usually reported for MOFs (6-7 kJ mol(-1)), except for the UiO-66(Zr) and MIL-53(Fe)-(CF(3))(2) analogues, whereas the presence of Lewis acid sites and/or a confinement effect enhances significantly the strength of interaction with hydrogen.

Synthesis of small metallic Mg-based nanoparticles confined in porous carbon materials for hydrogen sorption.

MgH2, Mg-Ni-H and Mg-Fe-H nanoparticles inserted into ordered mesoporous carbon templates have been synthesized by decomposition of organometallic precursors under hydrogen atmosphere and mild temperature conditions. The hydrogen desorption properties of the MgH2 nanoparticles are studied by thermo-desorption spectroscopy. The particle size distribution of MgH2, as determined by TEM, is crucial for understanding the desorption properties. The desorption kinetics are significantly improved by downsizing the particle size below 10 nm. Isothermal absorption/desorption cycling of the MgH2 nanoparticles shows a stable capacity over 13 cycles. The absorption kinetics are unchanged though the desorption kinetics are slower on cycling.

Hydrogen spillover measurements of unbridged and bridged metal-organic frameworks--revisited.

The room-temperature hydrogen uptake of unbridged and bridged composites formed by a physical mixture of either (IRMOF-1)-(AC/Pt catalyst) or (IRMOF-1)-(AC/Pt catalyst) and sucrose, respectively, has been carefully measured and compared with previous results reported by Li and Yang. Contrary to their findings, no enhanced hydrogen uptake in the composite was found as compared to the initial IRMOF-1. If any spillover effect occurred it was below the detection limit.

Size-dependent hydrogen sorption in ultrasmall Pd clusters embedded in a mesoporous carbon template.

Hydrogen sorption properties of ultrasmall Pd nanoparticles (2.5 nm) embedded in a mesoporous carbon template have been determined and compared to those of the bulk system. Downsizing the Pd particle size introduces significant modifications of the hydrogen sorption properties. The total amount of stored hydrogen is decreased compared to bulk Pd. The hydrogenation of Pd nanoparticles induces a phase transformation from fcc to icosahedral structure, as proven by in situ XRD and EXAFS measurements. This phase transition is not encountered in bulk because the 5-fold symmetry is nontranslational. The kinetics of desorption from hydrogenated Pd nanoparticles is faster than that of bulk, as demonstrated by TDS investigations. Moreover, the presence of Pd nanoparticles embedded in CT strongly affects the desorption from physisorbed hydrogen, which occurs at higher temperature in the hybrid material compared to the pristine carbon template.

Pd nanoparticles embedded into a metal-organic framework: synthesis, structural characteristics, and hydrogen sorption properties.

The metal-organic framework MIL-100(Al) has been used as a host to synthesize Pd nanoparticles (around 2.0 nm) embedded within the pores of the MIL, showing one of the highest metal contents (10 wt %) without degradation of the porous host. Textural properties of MIL-100(Al) are strongly modified by Pd insertion, leading to significant changes in gas sorption properties. The loss of excess hydrogen storage at low temperature can be correlated with the decrease of the specific surface area and pore volume after Pd impregnation. At room temperature, the hydrogen uptake in the composite MIL-100(Al)/Pd is almost twice that of the pristine material. This can be only partially accounted by Pd hydride formation, and a "spillover" mechanism is expected to take place promoting the dissociation of molecular hydrogen at the surface of the metal nanoparticles and the diffusion of monatomic hydrogen into the porosity of the host metal-organic framework.