Crystal structures of new silver ion conductors Ag7Fe3(X2O7)(4) (X = P, As)

International audienceThe crystal structures of new Ag7Fe3(X2O7)(4) (X = P, As) compounds, prepared through ion exchange from their sodium analogs, are reported. They adopt the monoclinic crystal system and exhibit an Ag ordering on cooling evidenced by a lowering of the symmetry from C-centered to primitive Bravais lattices. Crystal structures were determined from single-crystal X-ray diffraction at 100 K and 298 K for each composition. The structure consists of FeO6 octahedra sharing their corners with P2O7 dimers to form a three-dimensional framework [Fe-3(P2O7)(4)](7-) into which the silver ions are located. The differences between the four structures lie on the distribution of the silver ions within this framework, at the origin of a strong anisotropy in conductivity. Temperature displacement factors on Ag sites are generally higher in the arsenate than in the phosphate, in good correlation with conductivity data

2LiH + M (M = Mg, Ti): New concept of negative electrode for rechargeable lithium-ion batteries

xLiH + M composites, where M = Mg or Ti, are suggested as new candidates for negative electrode for Li-ion batteries. For this purpose, the xLiH + M electrode is prepared using the mechanochemical reaction: MHx + xLi → xLiH + M or by simply grinding a xLiH + M mixture. The most promising electrochemical behaviour is obtained with the (2LiH + Mg) composite prepared via a mechanochemical reaction between MgH2 and metallic Li leading to a very divided composite in which Mg crystallites of 20 nm size are embedded in a LiH matrix. Reversible capacities of 1064 mAh g-1 (three times as much as the one of graphite) and 600 mAh g-1 are reached for these phase mixtures after 1 and 28 h of grinding in vertical and planetary mill, respectively. The (2LiH + Ti) mixture prepared via the mechanochemical reaction between TiH2 and Li exhibits a reversible capacity of 428 mAh g-1. From X-ray diffraction measurements, the performances of the electrodes are attributed to the electrochemical conversion reaction: M + xLiH ↔ MHx + xLi+ + xe- (M = Mg, Ti) followed for M = Mg by an alloying process where M reacts with lithium ions to form Mg1-xLix alloys. © 2009 Elsevier B.V. All rights reserved

Negative thermal expansion in noble metal-based nanoparticles under gaseous atmosphere

SSCI-VIDE+ECI2D:ING+LPI:FMONational audienceAs compared to bulk state, metal nanoparticles stabilized on supports offer larger surface to volume ratios, faster transport properties, altered physical properties and interesting confinement effects that are consequences of combination of their nanoscale dimensions and interaction with the supports. Due to these exciting properties, these nanomaterials are currently studied in the fields of heterogeneous catalysis and energy storage. [1,2] We report here in situ EXAFS results on three noble metal nanoparticles supported on different scaffolds (porous carbon, oxide). This experiment was carried out on the ROCK beam line by the help of the Lytle-type cell furnace. [3] The thermal behavior of Rh (~1.3 nm), Pd (~4.5 nm) and Ir (~1.5 nm) nanoparticles was studied under hydrogen and inert atmosphere up to 250-300 °C. The Figure below shows the thermal evolution of 1st neighbor distance (R) for Pd, Ir and Rh nanoparticles under different gaseous environments. The R distance is decreasing during heating whereas, the Debye-Waller factor (σ2) is increasing with temperature, irrespective of the environment gas and the nature of element. The differences between nanoparticles below 150 °C can be understood in terms of hydrogen interaction (hydrogen desorption from a hydride phase, formation of solid solution). This unexpected behavior is believe to originate from a nanosize effect increasingly important with decreasing the nanoparticle size. Moreover, this seems to be a more general trend, since similar behavior was already reported for supported Pt nanoparticles. [4] [1]L. Piccolo, in Nanoalloys Synth. Struct. Prop., Springer-Verlag (London, 2012).[2]C. Zlotea and M. Latroche, Colloids Surf. Physicochem. Eng. Asp. 439, 117 (2013).[3]C. La Fontaine, L. Barthe, A. Rochet, and V. Briois, Operando IV 4th Int. Congr. Operando Spectrosc. 205, 148 (2013).[4]J. H. Kang, L. D. Menard, R. G. Nuzzo, and A. I. Frenkel, J. Am. Chem. Soc. 128, 12068 (2006)

Role of hydrogen absorption in supported Pd nanocatalysts during CO-PROX: Insights from operando X-ray absorption spectroscopy

SSCI-VIDE+ECI2D:ING+FMO:LPIInternational audienc

Metal hydrides for lithium-ion batteries

Classical electrodes for Li-ion technology operate via an insertion/de-insertion process. Recently, conversion electrodes have shown the capability of greater capacity, but have so far suffered from a marked hysteresis in voltage between charge and discharge, leading to poor energy efficiency and voltages. Here, we present the electrochemical reactivity of MgH2 with Li that constitutes the first use of a metal-hydride electrode for Li-ion batteries. The MgH2 electrode shows a large, reversible capacity of 1,480 mAh g-1 at an average voltage of 0.5 V versus Li+/Li° which is suitable for the negative electrode. In addition, it shows the lowest polarization for conversion electrodes. The electrochemical reaction results in formation of a composite containing Mg embedded in a LiH matrix, which on charging converts back to MgH2. Furthermore, the reaction is not specific to MgH2, as other metal or intermetallic hydrides show similar reactivity towards Li. Equally promising, the reaction produces nanosized Mg and MgH2, which show enhanced hydrogen sorption/desorption kinetics. We hope that such findings can pave the way for designing nanoscale active metal elements with applications in hydrogen storage and lithium-ion batteries. © 2008 Macmillan Publishers Limited. All rights reserved

In-situ Pd-Pt nanoalloys growth in confined carbon spaces and their interactions with hydrogen

International audienceOptimized synthesis of Pd–Pt nanoalloys confined in mesoporous carbons by a simple and fast one-pot microwave assisted approach is reported herein. The influence of several synthetic parameters (Pd–Pt composition, cross-linker type, Pd precursor type and its addition time) on the carbon framework and Pd–Pt nanoparticles formation and characteristics was investigated. Small and uniform distributed nanoparticles on tailored mesoporous carbons are obtained in short time compared to the classical approaches. The metallic composition has a great influence on the nanoparticle size and an important effect on the carbon pore size distribution. When Pt content increases (from 10 to 90 at.%), an increase in the particle size (from 6.5 to 18 nm) and in the pore size distribution of the carbon support (from 3 to 13 nm) is observed. Bulk immiscible Pd–Pt alloys were formed in the whole composition range as highlighted by the linear relationship between the lattice parameter and the metal content. The Pd precursor, the cross-linker and the addition time proved to have a significant effect in the final size/shape of the Pd–Pt nanoparticles. The optimized synthetic method can successfully tailor the size and the confinement of nanoparticles into the carbon matrix. Consequently, the hydrogen absorption properties and hydride formation can be tuned by the particle size for the richest Pd-composition nanoalloy (PdView the MathML source90PtView the MathML source10). The smaller Pd–Pt nanoparticles (6 to 20 nm) are confined in the carbon matrix and are surrounded by a graphitic layer preventing the hydrogen absorption while larger particles (50 nm) absorb hydrogen with metal hydride formation

Ultra-Small MgH2 Nanoparticles Embedded Into an Ordered Microporous Carbon Showing Rapid Hydrogen Sorption Kinetics

MgH2 nanoparticles with different average sizes have been prepared into an ordered microporous carbon by tuning the Mg amount from 15 to 50 wt%. Ultra-small particles with mean size of 1.3 and 3.0 nm have been obtained for 15 and 25wt% Mg content, respectively. The hydrogen desorption properties strongly depend on the nanoparticle size, as evidenced by different thermal analysis techniques. The onset temperature of hydrogen desorption for MgH2 nanoparticles below 3 nm occurs at temperature about 245 K lower than microcrystalline material. Two distinct hydrogen desorption peaks are noticed for nanoparticles with mean size of 1.3 and 3.0 nm, as confirmed by TDS and HP-DSC. 1H NMR investigations suggest the presence of two MgH2 populations with enhanced hydrogen mobility, as compared to the microcrystalline hydride. The short hydrogen diffusion path and the enhanced hydrogen mobility may explain the increased desorption kinetics of these ultra-small nanoparticles

One-pot synthesis of tailored Pd-Co nanoalloy particles confined in mesoporous carbon

The synthesis of mesoporous carbon with confined PdCo nanoalloy particles by a one-pot soft-template approach is reported herein. The influence of metal precursor type, metal composition and annealing temperature on the carbon@PdCo material characteristics was systematically investigated. Small and homogeneously distributed PdCo nanoparticles in the carbon matrix were achieved. The PdCo particle size could be tuned from 8 to 25 nm and the metal composition proved to be the main parameter controlling the size. When the amount of Pd is low (Pd50Co50) the particle is small and increases with the increase of temperature. On the contrary, for richer Pd contents (Pd75Co25, Pd90Co10) the particle size is larger but they exhibit extremely high stability with no obvious modification of size between 450 and 800 °C. The tuned experimental parameters had less impact on the materials textural characteristics, they exhibit similar surface area and uniform pore size distribution. Among the materials, only the Pd rich alloys (Pd90Co10) are able to form metallic hydride and consequently, showing the highest hydrogen adsorption capacity

Carboxymethylcellulose and carboxymethylcellulose-formate as binders in MgH₂-carbon composites negative electrode for lithium-ion batteries

Influence of carboxymethylcellulose sodium salt (CMC) and carboxymethylcellulose-formate (CMC-f) binders on the cyclability of a MgH2–33.3% CMC type binder–33.3%Ct,x electrodes has been investigated for the first time. These electrodes show a large reversible capacity of 1800–1900mAhg−1 at an average voltage of 0.5V vs. Li+/Li◦ which is suitable for the negative electrode in lithium-ion batteries. Moreover, addition of CMC or CMC-f binder with Ct,x carbon leads to an improved capacity retention with 240mAhg−1 and 542mAhg−1, respectively, compare to 174mAhg−1 for MgH2–18%Ct,x after 40 cycles