Microstructure and morphology changes in MgH2/expanded natural graphite pellets upon hydrogen cycling

Compacted mixtures based on ball-milled magnesium hydride (MgH2) have gained significant research interest as suitable materials for hydrogen storage tanks. The issue related to their stability during practical service conditions is one of paramount relevance. In this work, we investigate the microstructure and morphology of pellets obtained by the compaction of ball-milled MgH2/Nb2O5 powders mixed with expanded natural graphite. The pellets are subjected to repeated hydrogen sorption cycles to measure hydrogen storage properties and its stability with cycling. Moreover, the effect of air-exposure on the hydrogen sorption behavior is studied. Electron microscopy observations of as-prepared and cycled pellets point to a dramatic modification of the material’s microstructure upon repeated hydrogen cycling. In particular, the appearance of MgH2 particles depleted of the Nb2O5 catalyst and the formation of hollow MgO shells are highlighted. These findings are discussed by a simple model which takes into account the basic mechanisms intervening during the metal-hydride transformation in the pellet

Hydrogen storage properties of Pd-doped thermally oxidized single wall carbon nanohorns.

Single wall carbon nanohorns as well as their thermally oxidized derivatives were decorated with Pd nanoparticles and their H2 sorption performance was examined at 298 K up to 20 bar. The specific surface area of the nanohorns was increased through air oxidation, while both the thermal treatment and the metal doping led to the enhancement of the H2 uptake. The higher uptake of the hybrid materials could not be attributed only to the additive effect of the carbon support and Pd content suggesting the existence of a cooperative mechanism between the metal particles and the carbon surface. This weak chemisorption process was found to be fully reversible after mild heating; still, its contribution to the overall H2 uptake was not found to be of great significance

Hydrogenation-induced microstructure evolution in as cast and severely deformed Mg-10 wt.% Ni alloy

We determined the kinetics of hydrogen absorption of the hypoeutectic Mg-10 wt.% Ni alloy in the as-cast state and after processing by four passes of equal channel angular pressing(ECAP). While during the first hydrogenation cycle the ECAP-modified alloy exhibited faster absorption than its as-cast counterpart, this advantage was lost after the second hydrogenation cycle; parity was regained after six cycles. We attributed these differences in the hydrogen absorption kinetics to the formation of large (tens of micrometers) faceted Mg crystals observed during the first hydrogenation cycle. These crystals were significantly larger in the ECAP-modified alloy than in its as-cast counterpart. Wediscussed the growth of large Mg crystals during hydrogenation in terms of self-diffusion of Mg atoms driven by the metal-hydride transformation stress. The larger size of these crystals in the ECAP-processed alloy was attributed to the acceleration of diffusion by ECAP. Our metallographic studies revealed a number of microstructural changes in the alloys upon hydrogenation, such as cracking, accumulation of plastic strain in large Mg crystals, and re-distribution of the dispersed particles of Mg2Ni phase in the partly hydrogenated alloys

Self-grafting carbon nanotubes on polymers for stretchable electronics

Elementary bidimensional circuitry made of single-wall carbon-nanotube-based conductors, self-grafted on different polymer films, is accomplished in an attempt to develop a simple technology for flexible and stretchable electronic devices. Unlike in other studies of polymer-carbon nanotube composites, no chemical functionalization of single-wall carbon nanotubes is necessary for stable grafting onto several polymeric surfaces, suggesting viable and cheap fabrication technologies for stretchable microdevices. Electrical characterization of both unstretched and strongly stretched conductors is provided, while an insight on the mechanisms of strong adhesion to the polymer is obtained by scanning electron microscopy of the surface composite. As a first example of technological application, the electrical functionality of a carbon-nanotube-based 6-sensor (electrode) grid was demonstrated by recording of subdural electrocorticograms in freely moving rats over approximately three months. The results are very promising and may serve as a basis for future work targeting clinical applications. © 2018, Società Italiana di Fisica and Springer-Verlag GmbH Germany, part of Springer Nature

Improvement of DMFC electrode kinetics by using nanohorns catalyst support

One of the factors limiting direct methanol fuel cells (DMFC) performance is the slow kinetics of methanol oxidation at the anode. The importance of the catalyst support for fuel cells has been recognized and different forms of carbon have been suggested. Single wall nanohorns (SWNH) are a new class of carbon with a similar graphitic structure of carbon nanotubes. They are self-assembling materials that produce aggregates of about 100 nm. In the present study, the comparison of the performance of a DMFC equipped with electrocatalysts supported on a commercial carbon black and on SWNH was carried out. The SWNH were synthesized by the arc discharge method in air. The deposition of the Pt and Pt/Ru catalysts on the carbon supports was accomplished by using ethylene glycol as reducing agent. The synthesized catalyst nanoparticles have a very small diameter size (ca. 2.5 nm) and they are uniformly distributed on both carbon supports. The supported electrode catalysts were tested in a DMFC and results indicate that employing SWNH is very promising showing catalytic activities 60 % higher. © (2010) Trans Tech Publications

Stretchable conductors made of single wall carbon nanotubes self-grafted on polymer films

Aiming at the accomplishment of stretchable and elastic conductive devices, we report in this work electrical, mechanical and thermal characterization of a composite conductive material obtained by self-grafting of single wall carbon nanotubes bundles on different polymeric films. The dependence of resistance of micrometric composite conductors on the applied strain was measured; the current breakdown threshold was also measured together with the corresponding temperature increase. Finally, the dependence of an AC signal attenuation for a bi-layer single wall carbon nanotubes conductor sandwiching a 25 mu m thick poly-ethylene film was obtained as a function of the signal frequency, and the experimental results were satisfactorily compared to a simple RLC model