Catalytic influence of Ni-based additives on the dehydrogenation properties of ball milled MgH 2
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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, Zr2Ni5, and LaNi5 on the dehydrogenation properties of milled MgH2 was investigated. MgH2 milled in the presence of Ni (5 wt%) and Zr2Ni5 (5 wt%) catalysts for 2 h showed apparent activation energies, EA, of 81 and 79 kJ/mol, respectively, corresponding to ;50% decrease in EA and a moderate decrease (;100 °C) in the decomposition temperature (Tdec). A further 27 °C decrease in Tdec was observed after milling with 10 wt%Ni. Based on the EA values, the catalytic activity decreased in the following order: Ni Zr2Ni5 . LaNi5. X-ray photoelectron spectroscopy analysis of the milled and dehydrogenated states of the hydrides modified with Ni catalyst revealed that the observed reduction in EA 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 MgH2 and the EA 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 application temperatures could range from near room temperature (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 a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2011.230 J. Mater. Res., Vol. 26, No. 21, Nov 14, 2011
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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
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