Raphaël Janot

Preparation of LiBH4@carbon micro–macrocellular foams: tuning hydrogen release through varying microporosity

Microporous–macroporous carbononaceous monolith-type materials, prepared through a hard template method using silica as exo-templating matrices, have been impregnated with an etheric solution of LiBH4 to prepare LiBH4@carbon samples. It has been shown that the amorphous character of LiBH4 is largely favoured when developing the carbon microporosity (pores smaller than 2 nm) and that, as a consequence, the LiBH4 dehydrogenation is strongly enhanced at low temperatures. The onset temperature of dehydrogenation can be decreased to 200 °C and hydrogen capacity reaching 4.0 wt% is obtained at 300 °C with the carbon having the largest microporous volume, whereas the hydrogen release for bulk LiBH4 is negligible at the same temperature. It is suggested both from DSC and from pressure values reached upon hydrogen release that the enthalpy of the dehydrogenation reaction is strongly modified. In addition to some irreversible reactions with carbon surface groups (and even with the carbon matrix itself according to 11B MAS NMR spectroscopy, which reveals the formation of B–C bonds), the explanation for such modification could lie in the LiBH4 destabilization through confinement to the nanoscale range and associated amorphization. The above feature, where LiBH4 crystalline character is tuned through the imposed microporosity, can be considered as a highly promising approach to control the hydrogen release temperature of complex hydrides.

One-step synthesis of maghemite nanometric powders by ball-milling

Iron powder was milled within water for different duration using a planetary ball mill equipped with stainless steel vials. The in-situ production of hydrogen hinders the hematite formation during the grinding. X-ray diffraction, chemical analysis, high resolution transmission electron microscopy (HRTEM) and Mossbauer spectroscopy reveal that the obtained nanostructured powders consist of maghemite. Direct synthesis of maghemite nanoparticles from iron powder is so realised. Particles of about 15 nanometers are obtained after 48 h of milling.

Ball-milling: the behavior of graphite as a function of the dispersal media

Abstract The ball-milling in liquid media leads to well organized, thin and highly anisometric graphite (HAG) crystals. The presence in the milling container of a liquid, which acts as a lubricant and decreases the violence of the shocks, is relevant. Two liquids are used: n-dodecane and water. With dodecane, inert towards graphite and the metal of the milling tools, the powder consists of pure graphite whereas with water, the graphite particles are covered with nanocrystallites (15 nm) of a magnetic compound: the maghemite (γ Fe2O3). The electrochemical properties of those powders are interesting. The highly anisometric graphite leads to an irreversible capacity around half of that for the initial graphite powder, in contradiction with previous results claiming that higher the surface area, the higher the irreversible capacity. In fact, milling in the presence of dodecane provokes essentially a cleavage, which increases the global area, but does not drastically change the number of edge carbon atoms, responsible for the increase of the large irreversible capacity. The graphite–maghemite composites present a high capacity, partly reversible by oxidation–reduction between iron and wustite (FeO). This reaction is made possible by the nanometric size of the particles, and therefore their high reactivity.

Preparation, Structure, and Electrochemistry of Layered Polyanionic Hydroxysulfates: LiMSO4OH (M = Fe, Co, Mn) Electrodes for Li-Ion Batteries

The Li-ion rechargeable battery, due to its high energy density, has driven remarkable advances in portable electronics. Moving toward more sustainable electrodes could make this technology even more attractive to large-volume applications. We present here a new family of 3d-metal hydroxysulfates of general formula LiMSO4OH (M = Fe, Co, and Mn) among which (i) LiFeSO4OH reversibly releases 0.7 Li(+) at an average potential of 3.6 V vs Li(+)/Li(0), slightly higher than the potential of currently lauded LiFePO4 (3.45 V) electrode material, and (ii) LiCoSO4OH shows a redox activity at 4.7 V vs Li(+)/Li(0). Besides, these compounds can be easily made at temperatures near 200 °C via a synthesis process that enlists a new intermediate phase of composition M3(SO4)2(OH)2 (M = Fe, Co, Mn, and Ni), related to the mineral caminite. Structurally, we found that LiFeSO4OH is a layered phase unlike the previously reported 3.2 V tavorite LiFeSO4OH. This work should provide an impetus to experimentalists for designing better electrolytes to fully tap the capacity of high-voltage Co-based hydroxysulfates, and to theorists for providing a means to predict the electrochemical redox activity of two polymorphs.

Chemical Reduction of SiCl4 for the Preparation of Silicon-Graphite Composites used as Negative Electrodes in Lithium-Ion Batteries

Silicon-graphite composites were prepared by direct reduction of liquid SiCl 4 by various alkali metal graphite-intercalation compounds (GICs). The reaction occurred in all cases and led to intimate mixtures of nanosized silicon and graphite. A cleaning procedure has been used to solubilize the alkali chlorides formed as by-products. Even if silicon is hardly detected when the synthesis is performed from KC 8 , the experiments conducted with LiC 6 lead to nanocrystallized silicon particles well dispersed in the graphite matrix. The Si-graphite composite (9:91 by weight) issued from LiC 6 displays a first-charge capacity of 610 mAh/g, which is in fairly good agreement with the theoretical expected value (656 mAh/g), assuming the formation of the Li 3.75 Si phase. Interestingly, this Si-C composite exhibits a better capacity retention than similar Si-C composites prepared by ballmilling; the charge capacity is still above 500 mAh/g after 10 cycles. This suggests the existence of a strong interaction between carbon and Si nanoparticles, which may be explained by the formation and growth of silicon when in contact with the graphene sheets. These preliminary electrochemical results point out the use of such an approach of chemical reaction between GICs and SiCl 4 for the preparation of Si-C composites.

Potassium Silanide (KSiH3): A Reversible Hydrogen Storage Material

KSi silicide can absorb hydrogen to directly form the ternary KSiH(3) hydride. The full structure of α-KSiD(3), which has been solved by using neutron powder diffraction (NPD), shows an unusually short Si-D lengths of 1.47 Å. Through a combination of density functional theory (DFT) calculations and experimental methods, the thermodynamic and structural properties of the KSi/α-KSiH(3) system are determined. This system is able to store 4.3 wt% of hydrogen reversibly within a good P-T window; a 0.1 MPa hydrogen equilibrium pressure can be obtained at around 414 K. The DFT calculations and the measurements of hydrogen equilibrium pressures at different temperatures give similar values for the dehydrogenation enthalpy (≈23 kJ mol(-1) H(2)) and entropy (≈54 J K(-1) mol(-1) H(2)). Owing to its relatively high hydrogen storage capacity and its good thermodynamic values, this KSi/α-KSiH(3) system is a promising candidate for reversible hydrogen storage.

Enhanced hydrogen sorption capacities and kinetics of Mg2Ni alloys by ball-milling with carbon and Pd coating

Solid-state hydrogen storage alloys are becoming a practical method to transport and utilize hydrogen as fuel for various technologies. In this paper, the kinetics and capacity of hydrogen desorption from Mg-based alloys have markedly been enhanced by tuning the surface composition of alloy particles. Mg 2 Ni-C 1 . x composites (where t refers to the pregrinding time and x to the Brunauer-Emmet-Teller specific surface area) were prepared by ball-milling the alloy in the presence of preground graphite, and Pd-coated Mg 2 Ni alloy powders were obtained by controlled chemical deposition of Pd on the alloy surface. We have found that the optimization of the pregrinding step of carbon is a determinant factor in enhancing the hydrogen desorption capacity of the Mg 2 Ni-10 wt.% C 1 0 . 3 2 0 composites to 2.6 wt.% at 150 °C, the maximum performance so far reported on desorption for Mg-based alloys. Such value can even be raised to 2.8 wt.% by applying Pd deposition on the composite.

Modification of the hydrogen storage properties of Li3N by confinement into mesoporous carbons

This study presents an innovative synthetic route to Li3N@carbon composites for the purpose of use as hydrogen storage materials. The synthesis method is provided by wet impregnation of mesoporous carbons (graphitic or non-graphitic) using lithium azide solutions, followed by a thermal treatment allowing the transformation of lithium azide into lithium nitride, the latter being formed into the porosity of the carbon hosts. It has been shown by X-ray diffraction that the high-pressure β-phase of Li3N can be stabilized within the carbon matrix. The resulting Li3N@carbon composites have desirable hydrogen storage properties with fast hydrogen absorption/desorption kinetics at 200 °C (much faster than those measured for non-confined Li3N) as well as a completely reversible hydrogen storage process: a 20 wt% Li3N-loaded composite leads to a reversible hydrogen storage capacity of 1.8 wt% (e.g. about 9 wt% per mass of Li3N).

Hydrogenation properties of KSi and NaSi Zintl phases

The recently reported KSi-KSiH(3) system can store 4.3 wt% of hydrogen reversibly with slow kinetics of several hours for complete absorption at 373 K and complete desorption at 473 K. From the kinetics measured at different temperatures, the Arrhenius plots give activation energies (E(a)) of 56.0 ± 5.7 kJ mol(-1) and 121 ± 17 kJ mol(-1) for the absorption and desorption processes, respectively. Ball-milling with 10 wt% of carbon strongly improves the kinetics of the system, i.e. specifically the initial rate of absorption becomes about one order of magnitude faster than that of pristine KSi. However, this fast absorption causes a disproportionation into KH and K(8)Si(46), instead of forming the KSiH(3) hydride from a slow absorption. This disproportionation, due to the formation of stable KH, leads to a total loss of reversibility. In a similar situation, when the pristine Zintl NaSi phase absorbs hydrogen, it likewise disproportionates into NaH and Na(8)Si(46), indicating a very poorly reversible reaction.

Nano-spots induced break of the chemical inertness of boron: a new route toward reversible hydrogen storage applications

Novel LiBH4–metal-loaded carbonaceous foams have been designed to trigger reversible hydrogen storage properties. The metallic nanoparticles favour preferential wetting of LiBH4 on their surface and subsequent nucleation and growth, a configuration in which borate formation is strongly minimized. A cooperative effect between lower boron oxidation and the presence of metallic particles bearing intrinsic high heat capacity (acting as high temperature nanospots) promotes a strong improvement toward the rehydrogenation process, where the chemical inertness of boron has been overcome in this way for the first time. Hence, the LiBH4–M@C-HIPE(25HF) hybrid macrocellular foams (with M = Pd or Au) facilitate a reversible hydrogen storage process with a remnant capacity of about 7.4 wt% H2 (related to LiBH4) after 5 desorption–absorption ) hybrid…” and throughout the article should “absorption” be changed to “adsorption”??>cycles.