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Synthesis and characterisation of the nanostructured magnesium-lanthanum-nickel alloys for Ni-metal hydride battery applications
Norwegian University of Science and Technology, Faculty of Natural Sciences and Technology, Department of Materials Science and Engineering.
2012 (English)MasteroppgaveStudent thesis
Abstract [en]

Affordable price, high abundance of magnesium and high densities of hydrogen in the Mg-based hydrides attract interest to these hydrides tailored for hydrogen and energy storage applications. Ternary La-Mg-Ni hydrogen storage alloys with composition La3-xMgxNi9 (x = 0.8-1.2) form a new class of the materials for the negative electrodes in Ni-Metal Hydride (MH) batteries. The electrochemical discharge capacity of such alloys reaches 400 mAh/g which is 25 % greater than that of the commercial AB5-type based electrodes, 315 mAh/g. The La3-xMgxNi9 alloys crystallize with trigonal PuNi3 type of crystal structure. Magnesium replaces lanthanum to form the hybrid LaNi5 + Laves phase structures and favorably changes the thermodynamics of the metal-hydrogen interactions allowing improved performance of the advanced metal hydride battery electrodes. Differences in melting temperatures of lanthanum, nickel and easily evaporating magnesium and a complexity of the phase equilibria in the La-Mg-Ni system cause difficulties in synthesis of the battery electrode alloys with controlled Mg content and a desired phase-structural composition. In present work a La2MgNi9 alloy was in focus. Its successful synthesis has been achieved from the alloy melts containing 0-30 % of overstoichiometric Mg as compared to La2MgNi9 by use of Rapid Solidification performed at various quenching rates, with a copper wheel rotation speed of 3.1, 10.5 and 20.9 m/s. They were analyzed by synchrotron X-ray diffraction (SR XRD) including in situ studies in hydrogen gas performed at Swiss-Norwegian Beam Lines at ESRF, Grenoble, and by Scanning Electron Microscopy (SEM) with electron probe microanalysis (EPMA). Pressure-Composition-Temperature isotherms, hydrogen absorption-desorption cycling and measurements of the electrochemical chargedischarge performances were employed to characterize hydrogenation behaviors of the studied alloys. These studies showed that the melt spinning of the alloy containing 30 % weight excess of Mg quenched using wheel spin speed of 400 RPM allowed obtaining the most homogeneous sample with the optimal microstructure and phase-structural composition corresponding to the formation of La2MgNi9 with the highest yield. Variations in magnesium content and selection of optimal conditions for the Rapid Solidification process provide complementary possibilities in improving properties of the studied La-Mg-Ni alloys as hydrogen storage and battery electrode materials and provide a possibility to upscale production of the battery alloys. This work was performed at Institute for Energy Technology and at Department of Materials Science and Engineering, NTNU.

Place, publisher, year, edition, pages
Institutt for materialteknologi , 2012. , 74 p.
Keyword [no]
ntnudaim:7442, MTMT Materialteknologi, Materialer og energiteknologi
URN: urn:nbn:no:ntnu:diva-18466Local ID: ntnudaim:7442OAI: diva2:565958
Available from: 2012-11-08 Created: 2012-11-08

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