Chaoqi Shen

Surface coating of LiMn 2 O 4 spinel via in situ hydrolysis route: effect of the solution

Thin SiO2 shell consisting of nano-SiO2 particles can be deposited on to LiMn2O4 through the facile hydrolysis of tetraethoxysilane (TEOS) as a solution in either methanol or ethanol. The structure and surface morphologies of the modified and pristine materials were characterized by means of X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The choice of solvent (methanol vs. ethanol) for TEOS hydrolysis has a profound effect on the physical and electrochemical properties of the resultant SiO2-coated LiMn2O4. Coated LiMn2O4 demonstrated an improved cycling ability compared to the uncoated counterpart. Moreover, samples coated using a TEOS–methanol solution showed better cycling ability in extended potential windows and at an elevated temperature than that coated using ethanol.

Can γ-MgH2 improve the hydrogen storage properties of magnesium?

Upon hydrogen absorption, magnesium (Mg) spontaneously converts to the thermodynamically favoured β-MgH2 phase, but this leads to hydrogen release/uptake far from the desired low temperatures and fast kinetics. Conversion to the metastable γ-MgH2 phase should lead to improved hydrogen sorption properties. However, experimental verification of such a hypothesis remained distant. In this work, we report the electrochemical synthesis of nanosized Mg leading to the formation of a mixed γ/β hydride phase upon hydrogen absorption with a high γ-MgH2 content (29.6%). More remarkably, full release of hydrogen from these phases occurred at 200 °C only and upon hydrogen reabsorption at 100 °C, the γ/β-MgH2 mixture was restored. It was thus possible to determine for the first time the effect of γ-MgH2 on the kinetic and thermodynamic properties of the Mg/H2 reaction. The presence of γ-MgH2 was found to lead to a significant reduction of the apparent activation energy from 106.2 ± 4.0 to 69.1 ± 2.9 kJ mol−1 and a reduction of the Mg/H2 reaction enthalpy from 74.8 ± 1.0 to 57.7 ± 5.3 kJ mol−1 H2. However, this was accompanied by a concomitant decrease in entropy limiting the reduction of hydrogen desorption temperatures. γ-MgH2 can thus lead to improve hydrogen kinetics but to reach ambient hydrogen uptake and release ways to tune the enthalpy/entropy compensation effects have to be devised.

Nanosized Magnesium Electrochemically Deposited on a Carbon Nanotubes Suspension: Synthesis and Hydrogen Storage

Herein, we report on a novel method for deposition of Mg nanoparticles at the surface of carbon materials. Through the suspension of carbon nanotubes (CNTs) in an electrolyte containing di-n-butylmagnesium as a precursor, Mg nanoparticles were effectively deposited at the surface of the CNTs as soon as these touched the working electrode. Through this process CNTs supported Mg particles as small as 1 nm were synthesised and the distribution of the nanoparticles was found to be influenced by the concentration of the CNTs in the electrolyte. Hydrogenation of these nanoparticles at 100C was found to lead to low temperature hydrogen release starting at 150C, owing to shorter diffusion paths and higher hydrogen mobility in small Mg particles. However, these hydrogen properties drastically degraded as soon as the hydrogenation temperature exceeded 200C and this may be related to the low melting temperature of ultrasmall Mg particles.