Mechanical alloying and electrochemical hydrogen storage of Mg-based systems
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P.H.L. Notten Eindhoven University of Technology, 5600 MB Eindhoven, The Netherlands; and Philips Research Laboratories, 5656 AE Eindhoven, The Netherlands (Received 11 March 2008; accepted 1 May 2008)
Results on mechanical alloying of binary and ternary Mg–Ti-based mixtures are reported. Using fine-powdered reactants and a process-control-agent, a mixture of two face-centered cubic compounds is obtained. Using a coarse Mg precursor without addition of a milling agent results in a hexagonal-solid solution of Ti in Mg due to a lower oxygen content in the Mg starting material. Upon introduction of Ni or Al as a third element, the amount of dissolved Ti decreases to form a nanocrystalline secondary phase. The electrochemical charging capacity of the hexagonal compounds is far superior to that of the cubic ones, whereas the discharge capacity is significantly increased only upon addition of Ni. The secondary TiNi phase acts as a rapid diffusion path for hydrogen, greatly improving the rate capability of the alloys. The reversible hydrogen storage capacity reaches values of up to 3.2 wt% at room temperature for (Mg0.75Ti0.25)0.90Ni0.10.
I. INTRODUCTION
Finding an efficient and safe hydrogen storage medium with a small volume and low weight is the biggest technological challenge that the future hydrogen economy faces. Reversible metal hydrides have the advantage of moderate operating temperatures and pressures, compared to pressurized or liquefied hydrogen, which makes them a serious option for mobile applications such as fuel-cell powered cars.1 A different application area of metal hydrides is in rechargeable nickel–metal hydride (NiMH) batteries. Unfortunately, AB5-type materials used in present-day NiMH batteries have a rather low gravimetric capacity, which have caused them to be almost completely replaced by Li-ion batteries for portable applications over the past decade.2 Therefore, materials that can store more than the relatively low 1.1 wt% H of the present-day AB5-type materials3–5 are actively sought after. MgH2 has a high reversible storage capacity of 7.6 wt% hydrogen. A major disadvantage, however, is its slow (de)sorption rate,6 which makes its practical application difficult. However, it was shown in a series of studies that alloying Mg with cheap transition metals,
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2008.0261 J. Mater. Res., Vol. 23, No. 8, Aug 2008
such as Ti, V, and Cr, vastly improved the (de)sorption kinetics of the material compared to pure Mg,7,8 reaching reversible capacities of 6.5, 6.1, and 4.9 wt% for Mg0.80Ti0.20, Mg0.80V0.20, and Mg0.80Cr0.20 thin films, respectively.8 The improved discharge rate was ascribed to a more favorable crystal structure of the hydride, namely, the fluorite-type structure of the transition metalhydride instead of the rutile structure of MgH2. This change in crystal structure had already been observed for bulk Mg–Sc hydrides.9–12 Quantitative evidence of increased hydrogen motion rates in the fluo
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