Metal oxides as catalysts for improved hydrogen sorption in nanocrystalline Mg-based materials
Introduction
Hydrogen is the ideal means of storage, transport and conversion of energy for a comprehensive clean-energy concept. Regarding the use of hydrogen as fuel for the zero emission vehicle, the main problem is the storage of hydrogen. Metal hydrides offer a safe alternative to storage in compressed or liquid form. In addition, metal hydrides have the highest storage capacity by volume. Mg hydride has also a high storage capacity by weight and is therefore favored for mobile applications. However, light metal hydrides have not been considered competitive because of their rather sluggish sorption kinetics. Filling a tank could take several hours. E.g., magnesium hydride needs to be heated up to more than 300°C to obtain relevant sorption properties [1], [2], [3], [4]. Therefore, many attempts have been made to qualify magnesium hydride for application by improving the absorption and desorption behavior of the material. Recently, a breakthrough in hydride technology was achieved by preparing nanocrystalline hydrides using high energy ball milling [5], [6], [7], [8], [9], [10], [11], [12]. Nanocrystalline magnesium hydride shows indeed a fast absorption kinetics with a loading time of few minutes at 300°C. However, absorption and desorption at lower temperatures are still a problem [13]. With respect to absorption, this is due to the bad dissociation ability of metallic Mg for hydrogen molecules, because the probability of the adsorption of a H2-molecule on the Mg-surface is only 10−6 [14]. To overcome this problem, catalysts have to be added to magnesium. As has been shown in the past, Pd, Ni, and Fe can be used for a better H2-dissociation at the surface, e.g. for Mg2Ni, FeTi or LaNi5 [2], [15], [16], [17], [18]. Especially for microcrystalline Magnesium, Tanguy et al. have demonstrated that also the addition of V and Ti cause a catalytic acceleration of the hydrogen absorption [19]. Recently, the effect of transition metals (Ti, V, Mn, Fe, Ni) on nanocrystalline Mg has been investigated by Liang et al. [20], [21]. They show that ball milled MgH2 with additions of 5 at.% of the transition metal absorbs at room temperature (1 MPa) and desorbs at 235°C (0.015 MPa). In the present work, we have investigated the influence of cheap metal oxides (Sc2O3, TiO2, V2O5, Cr2O3, Mn2O3, Fe3O4, CuO, Al2O3 and SiO2) on the sorption behavior of nanocrystalline Mg-based systems.
Section snippets
Experimental
The milling experiments were performed with a Fritsch P5 planetary ball mill using hardened Cr-steel milling tools and an initial ball-to-powder weight ratio of 400:40 g. The initial MgH2 powder (95+%, Goldschmidt GmbH, Germany) was pre-milled for 20 h. The Mg2Ni-based composite material was produced by milling MgH2 and nickel powder in the stoichiometric ratio giving the Mg2NiH4 hydride phase. Afterwards, different metal oxides (99.9+% metal) were added in the desired overall ratio, and milled
Results and discussion
The X-ray spectrum in Fig. 1 mirrors exemplarily the phase composition of the material MgH2/(Cr2O3)0.05 after 120 h of milling. Obviously, the initial phase composition does not change: MgH2 and Cr2O3 are still present. From X-ray analysis the crystallite size of MgH2 was estimated to 20 nm using the Scherrer-formula. Though MgO is more stable than Cr2O3 (per mole oxygen), Cr2O3 is not reduced during the milling process. This is probably due to the fact that MgH2 instead of pure Mg was used for
Conclusions
High energy ball-milled MgH2/MexOy-nanocomposites (MexOy=Sc2O3, TiO2, V2O5, Cr2O3, Mn2O3, Fe3O4, CuO, Al2O3, SiO2) were investigated with respect to hydrogen sorption kinetics. The transition-metal oxides act as catalysts for the magnesium-hydrogen reaction. Cr2O3 yields the fastest hydrogen absorption, whereas V2O5 and Fe3O4 cause the most rapid desorption of hydrogen. It is shown that the catalyst content can be as low as 0.2 mole%, i.e. 0.3 vol% or 1 wt.% in the case of Cr2O3. With respect
Acknowledgements
This work is part of a cooperation between Hydro-Quèbec, Montreal (Canada), GfE Metals and Materials mbH, Nürnberg (Germany), and GKSS, Geesthacht (Germany). Discussions with V. Güther, R. Schulz, J. Huot, S. Boily, G. Liang, A. van Neste, and Z. Dehouche are gratefully acknowledged. This project is supported by the Bavarian Government.
References (29)
- et al.
Int. J. Hydrogen Energy
(1981) - et al.
J. Less-Common Metals
(1983) - et al.
Int. J. Hydrogen Energy
(1993) - et al.
J. Alloys Comp.
(1999) - et al.
J. Alloys Comp.
(1995) - et al.
J. Alloys Comp.
(1998) - et al.
J. Alloys Comp.
(1998) - et al.
J. Alloys Comp.
(1996) - et al.
J. Less-Common Metals
(1987) - et al.
J. Less-Common Metals
(1991)
J. Alloys Comp.
Mat. Res. Bull.
J. Alloys Comp.
J. Alloys Comp.
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