Nanocrystalline materials containing 3d metals for hydrogen storage

P. Roupcová^{1, 2}, O. Schneeweiss¹,

¹Institute of Physics of Materials, Academy of Sciences of Czech Republic v.v.i., Zizkova 22, 616 62 Brno, Czech Republic.

²Institute of Material Science and Engineering, Faculty of Mechanical Engineering, BUT, Technická 2, 616 69 Brno, Czech Republic

roupcova@ipm.cz

The some composites of Fe, Ti, V, and Zr 3d metals are well known for hydrogen storage ability. We prepared material by solid state reaction by mixing and dry milling method. The samples are consequently homogenized by high treatment in various surroundings atmospheres and in the pure vacuum. We were able prepared intermetallic compounds TiFe, Zr₂Fe and the pure hydrides ZrH₂, VH₂ in the other hand. These acceptable phases are impure by Fe, Fe₂Zr, Ti₂Fe and oxides (ZrO₂, FeO, Fe₂O₃, Fe₃O₄, V₂O₃, V₂O, TiO and Ti₂O). The dry milling method was used for preparing VFe, Zr₂Fe and Zr₃Fe intermetallic compounds.

Introduction

Transition metals based composites for hydrogen storage belong still to the most expected candidates for hydrogen batteries. Their practical application, however, are connected with some difficulties due to high temperature of hydrogen desorption and relatively slow kinetics of absorption and desorption. This disadvantage could be overcome if these alloys would be applied in a nanocrystalline form. The nanocrystalline alloys or composites exhibit much faster kinetics of hydrogen absorption and desorption and lower temperature of hydriding/dehydriding conventional crystalline materials with the same composition [1-3]. The MgH₂ famous for this capacity contains approximately 7.6 wt. % hydrogen but its high stability (H=-75kJ mol^-1) and high pressure of 0.18 MPa at 300°C [4].

It was shown that the ability of Zr-rich phases of absorbing of hydrogen led to a disproportionation and reproportionation of FeZr₂ and FeZr₃. Hydrogen absorbed at room temperature formed Zr₂FeH₅ hydride. The disproportionation of Zr₂Fe was unstable and it was very quickly followed by a reproportionation [5-6]. Experimentally the hydrogen atoms were detected in octahedral positions of Zr₃Fe with addition of ZrO₂ [7]. Mechanical alloying of a Ti₄₅Zr₃₈Ni₁₇ powder mixture formed an amorphous phase, but subsequent annealing caused the formation of an icosahedral quasicrystal phase with a small amount of the Ti₂Ni-type crystal phase [8]. After high-pressure hydrogenation at 573 K at a hydrogen pressure of 3.8 MPa, the amorphous phase transformed to a TiH₂-type hydride, while the icosahedral phase was structurally stable even after the hydrogenation. The system Ti–Zr–N–H was studied in [9]. In all samples, the dimensions of grains were nanoscaled. The peculiarities of self-propagating high-temperature synthesis processing were discussed.

In this paper show of phase composition of dry milled composites.

Experimental details

The samples were mixed by dry milling and solid state reaction of commercial pure ferrihydrite (Sigma Aldrich) and V, TiH₂, and ZrH₂ (Alfa Aesar) powders in air. The samples were homogenised after the 1 hour milling during the measurement thermomagnetic curves in vacuum (10^-4 Pa) and in the hydrogen (5N) atmosphere at 800°C.

The X-ray diffraction (XRD) and Mössbauer spectroscopy (MS) were applied for characterization of the structure of the as-prepared powder, and after homogenisation. XRD was carried out using X’Pert diffractometer and CoKα radiation with qualitative analysis by HighScore® software and the JCPDS PDF-4 database. For a quantitative analysis HighScore plus® with Rietveld structural models based on the ICSD database was applied. ⁵⁷Fe Mössbauer spectra were measured using ⁵⁷Co/Rh source in standard transmission geometry with detection of 14.4 keV γ-rays. The velocity scale was calibrated with a standard α-iron foil at room temperature. Isomer shifts δ are given relative to α-Fe at room temperature. The computer processing of the spectra was done using CONFIT package [10] which yielded intensities I of the components (atomic fraction of Fe atoms), their hyperfine inductions B_hf, isomer shifts δ, quadrupole splittings ΔE_Q, and quadrupole shifts ε_Q.

Results

The measurement of magnetic moments shows Curie temperatures of (α-Fe, magnetite, and Fe₂Zr) magnetic phases. The existence of those phases was confirmed by XRD and MS. The materials prepared under the same condition are collected in the table 1 and 2.

Table 1. Material prepared by means solid state reaction.

XRD [wt. %]
TiH₂ + ferrihydrite (vacuum)	1.7 FeTiH_0.06	10.2 Ti₂Fe	41.5 α-Fe	37.4 Ti₂O	9.2 TiO_0.325
TiH₂ + ferrihydrite (hydrogen)	7.4 FeTiH_0.02	10.4 Ti₂Fe	11.3 FeTi	70.9 TiO_0.325
V + ferrihydrite (vacuum)	10.7 V₂H	24.6 VO_0.03	64.7 magnetite
V + ferrihydrite (hydrogen)	20.8 V₂H	69.9 α-Fe	9.3 magnetite
Mössbauer spectroscopy [at. %]
TiH₂ + ferrihydrite (vacuum)	0.39 α-Fe	0.32 Fe-Ti	0.29 Fe(III)
V + ferrihydrite (vacuum)	0.93 Fe₃O₄	0.07 Fe(III)
V + ferrihydrite (hydrogen)	0.97 α-Fe	0.03 Fe(III)

Table 2. Material prepared by dry milling.

XRD [wt. %]
TiH₂ + ferrihydrite (vacuum)	34.5 TiH₂	32.4 magnetite	9 α-Fe	24.1 TiO_0.48
TiH₂ + ferrihydrite (hydrogen)	53.9 m-ZrO₂	14.7 t-ZrO₂	7.6 α-Fe	23.9 α-Zr
V + ferrihydrite (vacuum)	10.7 V₂H	24.6 VO_0.03	64.7 magnetite
V + ferrihydrite (hydrogen)	20.8 V₂H	69.9 Fe	9.3 magnetite
Mössbauer spectroscopy [at. %]
TiH₂ + ferrihydrite (vacuum)	0.30 α-Fe	0.6 magnetite	0.05 Fe(III)
V + ferrihydrite (vacuum)	0.06 Fe(III)	0.14 α-Fe	0.8 Fe₂Zr
V + ferrihydrite (hydrogen)	0.07 Fe(III)	0.59 α-Fe	0.34 Fe-Zr-H

Conclusion

The both of this method are not suitable for preparation of the pure intermetallic compounds which are too sensitive to oxidation. The homogenised sample consist majority of oxides, not transformed precursors, hydrides and pure metals.

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Acknowledgements.

This work was supported by the Czech Ministry of Education, Youth and Sports (1M6198959201), Academy of Sciences of the Czech Republic (AV0Z20410507) and Grant Agency of the Czech Republic 106/09/P556.