Cycling properties of Sc- and Ce-doped NaAlH4 hydrogen storage materials prepared by the one-step direct synthesis method

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Abstract

The so-called “one-step direct synthesis method” – simultaneous ball-milling (b.m.) of NaH–Al powder doping agent mixtures under H2 pressure – turned out to be a simple way for the preparation of metal-doped sodium alanate hydrogen storage materials, leading to solids with high storage capacity and excellent kinetics. The method has been published until now only for TiCl3 as a doping agent. In combination with ScCl3 or CeCl3 as doping agents, the one-step direct synthesis method delivers materials with hydrogen storage properties which come close to those required by the car industry for hydrogen supply of PEM fuel cells. With respect to the kinetics of the chemical reaction, hydrogenation rates corresponding to 3–5 min time for refuelling of a hydrogen tank can be realized, although removal of the resulting hydrogenation heat in such a short time posses a severe engineering problem. Release of all stored hydrogen in a time compatible with handling of a car is possible without additional heating devices, if instead of the current fuel cells, advanced designs on the basis of polybenzimidazole membranes are used. These fuel cells work at temperatures of 150–200 °C, so that their waste heat temperature level is sufficiently high for desorption of hydrogen from both dehydrogenation steps of the NaAlH4 system. Additionally, it has been reported that using mischmetal with 42 at.% of Ce as a dopant for NaAlH4, at 150 °C, ∼5 wt.% of hydrogen can be desorbed in ∼3 h. Moreover, in a 95 cycles de- and re-hydrogenation test the present Ce-doped NaAlH4 storage material showed stable storage properties.

Introduction

At present the metal-doped NaAlH4 is the only reversible complex metal hydride with high hydrogen content (5.6 wt.%), a decomposition temperature near the working temperature of a PEM fuel cell and potential low prices for the compounds NaH and Al metal, which are the basic materials for the preparation of NaAlH4 [1]. These features are the reasons, that these hydrogen storage materials nowadays are intensively investigated, especially as a potential material for hydrogen supply of PEM fuel cells. To enhance the kinetics of hydrogen desorption and (re)hydrogenation, the reactions must be catalyzed by incorporation of metal containing dopants in the storage material, selected from the transition metals with titanium being the most frequently employed one.

Metal-doped sodium alanate hydrogen storage materials were up to now prepared mainly by the following methods:

  • by ball-milling (b.m.) or wet chemical reaction of pre-synthesized NaAlH4 with doping agents [2], [3], [4], [5], [6], [7];

  • by “direct synthesis” [8], [9], [10], [11], that is doping by ball-milling or wet chemical reaction of NaH/Al powder mixtures with doping agents and subsequent hydrogenation of doped mixtures under pressure;

  • by the “one-step direct synthesis” [12], i.e. simultaneous ball-milling under hydrogen (preferentially under elevated pressure) of NaH/Al powder/doping agent mixtures. Using a commercial ball mill with a telemetric equipment [12], [13] temperature and pressure during the ball-milling reaction can be monitored, thus enabling a direct control of the progress of the ball-milling reaction.

This last method has only been described for the case of TiCl3 as a doping agent [12] and the resulting hydrogen storage material displayed superior features compared to doping of pre-synthesized NaAlH4 with TiCl3 [2], [3], [4], [5], [6], [7]. For example, in a cycle test using material synthesized by the one-step direct synthesis method [12], at 129 °C and 99 bar, the (re)hydrogenation time could be reduced to ∼14 min (3.2 wt.% H2), which is among the best yet achieved hydrogenation times with TiCl3 as a dopant. The one-step direct method is now extended to ScCl3 and CeCl3 as dopants which already showed improved quality as dopants for NaAlH4 compared to previously known systems [14]. The underlying idea of the present work was thus, the combination of an optimal preparation method for the storage material with optimized dopants. In the following we report the synthesis and the exciting cycling properties of these new storage materials.

Section snippets

Experimental

Materials doped with Sc and Ce in one step were prepared analogously to the corresponding Ti material according to Eq. (1). Hydrogenation of NaH/Al/Sc(Ce)Cl3 mixtures in the molar ratio of 1:1:0.4 were carried out in a commercial planetary ball mill (Fritsch Pulverisette P7) equipped with a telemetric system for the recording of the temperature and pressure during the milling process [12], [13] at 30–65 °C/85–55 bar of H2 pressure. Hydrogen absorption was completed in 5–6 h (Fig. 1). 1–1.5 g

Results and discussion

During the investigations of cycling properties of the present storage materials it was noted that hydrogenation rates are higher than those of materials prepared by doping of pre-synthesized NaAlH4 with the same dopants ScCl3 or CeCl3. So, for example, the above-mentioned Sc-doped material, Eq. (1), in the cycles 2, 3, 5 and 9 of the present cycle test (Table 1) was hydrogenated at 122–123 °C/105–100 bar. The hydrogenations (Fig. 2) were practically completed in 10–12 min. For comparison, a

Conclusions

According to the relatively simple preparation way, superior cycling properties and outstanding kinetics, the presented Ce-doped NaAlH4 storage material appears to be a good candidate to be used for supplying hydrogen in combination with advanced fuel cells [20]. The storage capacities fall short of the long-term targets for use in mobile applications and cannot be increased above 5.5 wt.% due to the nature of the system, but presently this and related systems seem to come closest to the

Acknowledgements

Support of our research in this field by General Motors Fuel Cell Activities, in addition to the basic funding provided by the Max-Planck Gesellschaft, is gratefully acknowledged.

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