Tuning the thermal conductivity of hydrogenated porous magnesium hydride composites with the aid of carbonaceous additives L. Popilevsky a,b , V.M. Skripnyuk a , Y. Amouyal a , E. Rabkin a,* a Department of Materials Science and Engineering, Technion-Israel Institute of Technology, Haifa 32000, Israel b Grand Technion Energy Program (GTEP), Technion-Israel Institute of Technology, Haifa 32000, Israel article info Article history: Received 29 November 2016 Accepted 16 April 2017 Available online xxx Keywords: Hydrogen storage Mg-based composites MWCNTs Microstructural characterization Heat transfer abstract We synthesized solid porous pellets of the Mg-2 wt.% multiwall carbon nanotubes com- posite and determined their thermal conductivity in partially hydrogenated state (80e90% of their maximum hydrogen storage capacity). The thermal conductivities of the composite pellets with carbon nanotubes were up to 20% higher than thermal conductivities of the reference pellets of pure Mg prepared in a similar way. We demonstrated that the high energy ball milling employed in the synthesis of the composites has destroyed the carbon nanotubes and transformed them into chains of carbon nanoparticles of 20e30 nm in diameter. The hydride phase grew preferentially along these chains, resulting in aniso- tropic microstructure, with residual Mg phase forming an interpenetrating, percolating network contributing to the long-range thermal transport. On the contrary, the residual Mg formed isolated pockets in the reference pellets of pure Mg. Our work demonstrates that anisotropic additives to the hydride forming metals can improve their thermal conduc- tivity and hydrogen storage properties by changing the topology of the two-phase metal- hydride microstructure. © 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Introduction Magnesium is considered as an attractive material for renewable energy storage applications [1,2], thanks to its high reversible hydrogen storage capacity (about 7.6 wt.% of H 2 ) and relatively low cost. However, its main drawbacks are slow hydrogenation kinetics, low dissociation rate of hydrogen molecules on the surface of Mg, high formation enthalpy of MgH 2 (~75 kJ/mol H 2 at standard conditions), and high reac- tivity toward oxygen. Hydrogenation of metallic Mg begins from H 2 phys- isorption on a surface followed by chemisorption during which the H 2 molecule has to overcome an activation barrier for its dissociation and formation of the hydrogen-metal bond [3,4]. This is followed by atomic hydrogen penetration into the metal and formation of the metal-hydrogen solid solution. During the next step, nucleation of the magnesium hydride phase (MgH 2 ) starts as isolated sites on the surface of the metal particle [5e7]. The shape of MgH 2 nuclei was reported to be circular, suggesting isotropic growth [6,8]. These hydride nuclei/colonies grow into the particle bulk until they impinge on each other in such a way that the surface of the particles becomes completely covered with hydride. It was shown that MgH 2 nuclei impingement determines the final hydrogen * Corresponding author. Fax: þ972 4 8295677. E-mail address: erabkin@campus.technion.ac.il (E. Rabkin). Available online at www.sciencedirect.com ScienceDirect journal homepage: www.elsevier.com/locate/he international journal of hydrogen energy xxx (2017) 1 e11 http://dx.doi.org/10.1016/j.ijhydene.2017.04.088 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Popilevsky L, et al., Tuning the thermal conductivity of hydrogenated porous magnesium hydride composites with the aid of carbonaceous additives, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.04.088