ARTICLES Catalytic influence of Ni-based additives on the dehydrogenation properties of ball milled MgH 2 Placidus B. Amama a) and John T. Grant Air Force Research Laboratory, Materials and Manufacturing Directorate, RXB, Wright-Patterson AFB, Ohio 45433; and University of Dayton Research Institute (UDRI), University of Dayton, Dayton, Ohio 45469 Jonathan E. Spowart, Patrick J. Shamberger, and Andrey A. Voevodin Air Force Research Laboratory, Materials and Manufacturing Directorate, RXB, Wright-Patterson AFB, Ohio 45433 Timothy S. Fisher Air Force Research Laboratory, Materials and Manufacturing Directorate, RXB, Wright-Patterson AFB, Ohio 45433; and School of Mechanical Engineering, Purdue University, West Lafayette, Indiana 47907; and Birck Nanotechnology Center, Purdue University, West Lafayette, Indiana 47907 (Received 1 March 2011; accepted 11 July 2011) The catalytic influence of Ni, Zr 2 Ni 5 , and LaNi 5 on the dehydrogenation properties of milled MgH 2 was investigated. MgH 2 milled in the presence of Ni (5 wt%) and Zr 2 Ni 5 (5 wt%) catalysts for 2 h showed apparent activation energies, E A , of 81 and 79 kJ/mol, respectively, corresponding to ;50% decrease in E A and a moderate decrease (;100 °C) in the decomposition temperature (T dec ). A further 27 °C decrease in T dec was observed after milling with 10 wt%Ni. Based on the E A values, the catalytic activity decreased in the following order: Ni  Zr 2 Ni 5 . LaNi 5 . X-ray photoelectron spectroscopy analysis of the milled and dehydrogenated states of the hydrides modified with Ni catalyst revealed that the observed reduction in E A may be due to the ability of Ni catalyst to decrease the amount of oxygen atoms in defective positions that are capable of blocking catalytically active sites thereby enhancing the dehydrogenation kinetics. In particular, our results reveal a strong correlation between the type of oxygen species adsorbed on Ni-modified MgH 2 and the E A of the dehydrogenation reaction. I. INTRODUCTION The development of new and efficient thermal energy storage (TES) materials remains a major challenge in addressing needs in a variety of areas from intermittent solar energy harvesting to thermal management of transient, high-flux heat loads. Conceivable TES appli- cation temperatures could range from near room temper- ature (for cooling electronics, for example) to moderate temperatures commensurate with “temperature lift” heat pumping cycles and waste heat recovery from power generation cycles. The latter temperatures could fall in the range of several hundred degrees Celsius. A variety of passive materials have been developed and used for TES including paraffin waxes, water tanks, and low-capacity reversible metal hydrides, among others. Paraffin wax has been used as a TES medium for decades. 1–4 The current state-of-the-art packaging technology for containing and conducting heat to paraffin wax reduces its effective heat storage density at the system level appreciably. Other material systems of possible interest are summarized in Table I; notably, paraffins, salts, and liquid metals have impractically low inherent enthalpies of phase change when normalized by mass. In fact, the only two example materials that exceed 1 MJ/kg are water (liquid–vapor) and a metal hydride. Regarding water, the slow kinetics of boiling/ evaporation, limited largely by the nucleation, ebullition, and departure time of ;10 ms for each bubble, 5 and the requirement for handling large quantities of vapor make it less practical. Metal hydrides offer a potential materials solution, en- abled by the uniquely high formation enthalpy of hydrogen gas, as well as other advantages such as on-demand cooling, fast thermal response, and system designs that are compact and lightweight. 6 However, they also offer significant cha- llenges to be overcome. First, we note that not all metal hydrides have high-energy storage capacities (cf., LaNi 5 H 6 as shown in Table I). In fact, some well-known intermetallic metal hydrides have been studied recently for reversible and reasonably fast thermal storage, 6 but the low thermal energy density of such “classical” hydrides renders them poorly suited for high-density thermal storage. However, some metal hydrides do offer exceptionally high TES density. For instance, a reaction of Mg and H 2 can be used to store the- rmal energy, and a theoretical energy density of 2.81 MJ/kg is possible. 7 Magnesium hydride (MgH 2 ) is a well-studied a) Address all correspondence to this author. e-mail: Placidus.Amama@wpafb.af.mil DOI: 10.1557/jmr.2011.230 J. Mater. Res., 2011 Ó Materials Research Society 2011 1