Available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/he Microchip power compensated calorimetry applied to metal hydride characterization A. Sepu ´ lveda a , A.F. Lopeandı´a a , R. Dome `nech-Ferrer a , G. Garcia a , F. Pi a , J. Rodrı´guez-Viejo a,Ã , F.J. Mun ˜ oz b a Nanomaterials and Microsystems Group, Physics Department, Universitat Auto `noma de Barcelona, 08193 Bellaterra, Spain b Instituto de Microelectro ´nica de Barcelona, Centro Nacional de Microelectro ´nica, Campus UAB, 08193 Bellaterra, Spain article info Article history: Received 31 January 2008 Received in revised form 19 March 2008 Accepted 20 March 2008 Available online 12 May 2008 Keywords: Thin-film calorimetry Nanocalorimetry Power compensated calorimetry Dehydrogenation Metal hydrides abstract In this work, we show the suitability of the thin film membrane-based calorimetric technique to measure kinetically limited phase transitions such as the dehydrogenation of metallic hydrides. Different compounds such as Mg, Mg/Al and Mg 80 Ti 20 have been deposited over the active area of the microchip by electron beam evaporation. After several hydrogenation treatments at different temperatures to induce the hydride formation, calorimetric measurements on the dehydrogenation process of those thin films, either in vacuum or in air, are performed at a heating rate of 10 1C/min. We observe a significant reduction in the onset of dehydrogenation for Mg 80 Ti 20 compared with pure Mg or Mg/Al layers, which confirms the beneficial effect of Ti on dehydrogenation. We also show the suitability of the membrane-based nanocalorimeters to be used in parallel with optical methods. Quantification of the energy released during hydrogen desorption remains elusive due to the semi-insulating to metallic transition of the film which affects the calorimetric trace. & 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. 1. Introduction Hydrogen storage alloys are important materials for the future development of clean hydrogen energy systems and a number of promising metal hydrides have already been reported in the literature [1,2] and references therein. The advantage of metal hydrides comes from their ability to store hydrogen at low pressure but also from the fact that the hydrogen is released through an endothermic reaction, making hydrogen storage inherently safe. However, in spite of thousands of alloys screened up to date, no single composition meets all the requirements needed for its use in mobile applications [3]. Among the hydrides, Mg or Mg- based alloys are promising due to their relatively high percent storage weight capacity, though the high enthalpy of MgH 2 formation, around 78 kJ=mol H 2 , remains a handicap for its commercial use [4,5]. In addition, high hydrogen pressures and high temperatures, 500 K, are typically needed to reach reasonable fast absorption rates in Mg bulk samples. Never- theless, several authors have shown that the kinetics of hydrogen uptake/release can be greatly improved by reducing the grain size to the nanometer scale [6,7]. Also, to decrease the thermodynamic constraints and still promote the use of Mg-based alloys for hydrogen storage, various strategies have been experimentally tested and discussed [8,9]. Forming stable Mg-based compounds by adding elements that exhibit negative heats of mixing is a possible and suited route to lower the reaction enthalpy, as is the case of Mg 2 NiH 4 , however, at the expense of reducing the storage capacity of the base material. Another possible and better solution would ARTICLE IN PRESS 0360-3199/$ - see front matter & 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2008.03.047 Ã Corresponding author. E-mail address: javirod@vega.uab.es (J. Rodrı´guez-Viejo). INTERNATIONAL JOURNAL OF HYDROGEN ENERGY 33 (2008) 2729– 2737