Metal oxides as catalysts for improved hydrogen sorption in nanocrystalline Mg-based materials

Abstract

Nanocrystalline MgH₂/Me_xO_y- and Mg₂NiH₄/Me_xO_y-powders were produced by high energy ball milling (Me_xO_y=Sc₂O₃, TiO₂, V₂O₅, Cr₂O₃, Mn₂O₃, Fe₃O₄, CuO, Al₂O₃, SiO₂). The hydrogen absorption and desorption kinetics of the nanocomposite materials were determined with respect to a technical application. Some of the selected oxides lead to an enormous catalytic acceleration of hydrogen sorption compared to pure nanocrystalline materials. In absorption, the catalytic effect of TiO₂, V₂O₅, Cr₂O₃, Mn₂O₃, Fe₃O₄, and CuO is comparable. Concerning desorption, the composite material containing Fe₃O₄ shows the fastest kinetics followed by V₂O₅, Mn₂O₃, Cr₂O₃ and TiO₂. Only 0.2 mole% of the catalysts is sufficient to provide a fast sorption kinetics.

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.