Using nano-materials to catalyze magnesium hydride for hydrogen storage

Babak Shalchi Amirkhiz
2011
We have designed and engineered bi-catalyst magnesium hydride composites with superior sorption performance to that of ball milled magnesium hydride catalyzed with the individual baseline catalysts. We have examined the eect of single-walled carbon nanotube (SWCNT)-metallic nanoparticle additions on the hydrogen desorption behavior of MgH 2 after high-energy co-milling. We showed the synergy between SWCNT's and metallic nanoparticles in catalyzing the sorption of magnesium hydride. The optimum
more » ... dride. The optimum microstructure for sorption, obtained after 1 h of co-milling, consists of highly defective SWCNTs in intimate contact with metallic nanoparticles and with the hydride. This microstructure is optimum, presumably because of the dense and uniform coverage of the defective SWCNTs on the MgH 2 surface. Cryo-stage transmission electron microscopy (TEM) analysis of the hydride powders revealed that they are nanocrystalline and in some cases multiply twinned. Since defects are an integral component of hydride-to-metal phase transformations, such analysis sheds new insight regarding the fundamental microstructural origins of the sorption enhancement due to mechanical milling. The nanocomposite shows markedly improved cycling as well. Activation energy analysis demonstrates that any catalytic eect due to the metallic nanoparticles is lost during cycling. Improved cycling performance is instead achieved as a result of the carbon allotropes preventing MgH 2 particle agglomeration and sintering. The nanocomposite received over 100 sorption cycles with fairly minor kinetic degradation. We investigated the catalytic eect of Fe + Ti bi-metallic catalyst on the desorption kinetics of magnesium hydride. Sub-micron dimensions for MgH 2 particles and excellent nanoscale catalyst dispersion was achieved by high-energy milling. The composites containing Fe shows DSC desorption temperature of 170 o C lower than as-received MgH 2 powder, which makes it suitable to be cycled at relatively low temperature of 250 o C. The low cycling temperature also prevents the formation of Mg 2 FeH 6 . The ternary Mg-Fe-Ti composite shows best performance when compared to baseline ball milled magnesium hydride with only one catalytic addition. With a very high BET surface area it also shows much less degradation during cycling. The synergy between Fe and Ti is demonstrated through use of TEM and by carefully measuring the activation energies of the baseline and the ternary composites.
doi:10.7939/r3fh9g fatcat:j7sug7ryhvfb5oqfiwx4ygl6q4