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ELECTROCHEMICAL HYDRIDING OF MAGNESIUM-BASED ALLOYS

Dalibor Vojtěcha, Alena Michalcováa, Magda Morťanikováa, Borivoj

Šustaršičb

a Department of Metals and Corrosion Engineering, Institute of Chemical Technology, Prague, Technická 5 , 166 28 Prague 6, Czech Republic, e-mail:

Dalibor.Vojtech@vscht.cz b Laboratory for Powder Metallurgy, Institute of metals and technology, Lepi pot 11,

SI-1001 Ljubljana, Slovenia, e-mail: sustasic@imt.si Abstract

Magnesium alloys are suitable materials for the reversible hydrogen storage. Generally, hydriding is performed at high temperatures and pressures of gaseous hydrogen which is associated with several problems. In the paper, a new hydriding method – electrochemical hydriding – is presented. This process does not need either high temperatures or pressures. By using this method we prepared an alloy with a relatively high hydrogen content.

Keywords: hydrogen storage, magnesium, hydride, electrochemistry

1. INTRODUCTION

Hydrogen is considered as a pure fuel for the future, because it does not produce carbon dioxide when reacting with oxygen. Hydrogen is a very light gas, explosive when mixed with air. It is capable of penetrating through various solid materials. For this reason, a great attention has been paid to find safe methods of hydrogen storage. At present, three methods are considered [1]: - liquefying and storage in thermally insulated containers or in pressure containers - compression and storage in pressure containers - storage in a solid phase, either in the form of metallic hydrides or absorption in materials with high specific surface. The first two methods are most widely used at present. However, their disadvantage is a high energy consumption associated with liquefying and compression of hydrogen. It is reported that liquefying consumes up to 30% of the total energy that can be obtained from hydrogen. For this reason, hydrogen storage in a solid phase is extensively studied.

Solid state hydrogen storage includes compounds of metals with hydrogen – hydrides. To achieve the maximum gravimetric density of hydrogen in a hydride, it should be based on light metals. Therefore, hydrides derived from magnesium hydride MgH2 are extensively studied. This hydride contains 7.6 wt. % of hydrogen, which means that volume of stored hydrogen is about 1200 times higher than volume of hydride itself. The main drawback of this compound is a relatively high thermodynamic stability, resulting in high decomposition temperatures above 300°C. This is not suitable for practical use, because hydrogen release would consume excessive energy. To reduce the hydride stability, various approaches have been adopted. These include alloying with suitable additives, nanocrystalline structure, additions of fine oxide particles etc. [2].

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Preparation of hydrides usually includes high-pressure and high-temperature synthesis from metals and gaseous hydrogen. Such method is relatively expensive and energy-consuming. An alternative to the direct synthesis is an electrochemical synthesis of hydrides. During electrolysis of an appropriate water solution, a hydrided alloy is a cathode. On its surface, atomic hydrogen released by an electrochemical reaction diffuses inward and transforms the cathode material into hydrides. This method is simple, inexpensive, because it does not require either high hydrogen pressures or temperatures.

In the present paper, application of electrochemical hydriding of a magnesium alloy is demonstrated. Mg-Ni alloy was selected for hydriding because nickel significantly reduces the hydride stability. The alloy was prepared in the form of a rapidly solidified (RS) ribbon. We assumed that the rapidly solidified alloy consists of a refined nano-crystalline structure which accelerates hydrogen diffusion in the material.

2. EXPERIMENTAL PART

The Mg-14wt.%Ni alloy used in our experiment was prepared by melting of pure metals in a vacuum induction furnace under argon. Afterwards, the alloy was subjected to melt spinning procedure under argon to prepare rapidly solidified ribbons with a thickness of 80 µm and a width of 2 mm. Melt spinning consisted in ejecting the melted alloy through a nozzle onto a fast rotating copper wheel. Structure of the rapidly solidified alloy was investigated by scanning (SEM) and transmission (TEM) electron microscopy.

Electrochemical hydriding was realized in a water solution of KOH and hydriding duration was 2 hours. The RS ribbons were connected as a cathode, while graphite rods served as anodes.

Phase composition of the hydrided alloy was determined by x-ray diffraction analysis (XRD) and hydrogen gravimetric density was measured by LECO analyzer. This analysis includes heating of a sample in a flow of an inert gas. Evolved gases, except hydrogen, are then absorbed in appropriate sorbents and hydrogen is then analyzed by a thermal conductivity detector.

Beside the total hydrogen content, temperatures at which it releases from hydrided materials are also important in practice. These temperatures were determined by the thermogravimetric analysis (TGA) measuring temperature dependence of sample weight. Heat effects associating chemical reactions at elevated temperatures were monitored by the differential scanning calorimetry (DSC). It can be expected that thermal decomposition of hydride is accompanied by endothermal effects.

3. RESULTS AND DISCUSSION 3.1 Structure of rapidly solidified Mg-14wt.%Ni ribbons

Mg-Ni phase diagram is presented in Fig.1. According to this diagram, the Mg-14wt.%Ni alloy is slightly hypoeutectic and consists of primary α(Mg) solid solution and α(Mg) + Mg2Ni eutectic. In Fig.2a, there is a SEM image of the cross-sectioned ribbon. It is observed that the structure shows a gradient and depends on the distance from the cooling wheel. On the right-hand side, which was in contact with the cooling wheel and where the melt was cooled most rapidly, the structure is significantly finer as compared to the opposite side. TEM image in Fig.2b shows that the RS alloy is composed of primary α(Mg) dendrites (light, dendritic branches of

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about 400 nm in size) and Mg2Ni eutectic phase (dark).

Fig.1. Mg-Ni equilibrium phase diagram [3].

a)

b)

Fig.2. Microstructure of rapidly solidified Mg-14wt.%Ni ribbon: a) cross-section (SEM), b)

TEM.

3.2 Properties of electrochemically hydrided Mg-14wt.%Ni ribbons By using the LECO hydrogen analyzer, a hydrogen gravimetric density of 1.2 %

was determined in the hydrided alloy. This value is surprisingly high and approaches those in transition metal hydrides (usually less than 2 % of hydrogen). Hydrogen is a very light element. Volumetrically, volume of hydrogen absorbed in the hydrided alloy is approx. 300 times higher than the volume of the alloy.

Fig.3 presents XRD patterns of the RS ribbon and of the ribbon after 2 hour electrochemical hydriding in KOH solution. At first sight, both XRD patterns are almost identical. It was shown in Fig.2b that the RS alloy contains two phases - Mg a Mg2Ni. However, in the hydrided alloy, there are additional phases, namely MgH2 a Mg2NiHX hydrides. Therefore, we assume that two chemical reactions occur during electrochemical hydriding:

Mg2Ni + X H → Mg2NiHX (1)

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Mg + 2 H → MgH2 (2)

The Mg2NiHX hydride is an interstitial solid solution of hydrogen in the Mg2Ni phase. Both Mg2Ni and hydride have the same hexagonal crystal structure (space group P6222). X value depends on H content and it can vary between 0 and 0.3. However, it is difficult to exactly determine the X value from XRD patterns in Fig.3.

Mechanism of hydride formation includes diffusion of atomic hydrogen into the cathode. Atomic hydrogen is produced by electrochemical reaction on the cathode surface. In the Mg phase, a hydrogen solubility is negligible. Therefore,

particles of MgH2 hydride rapidly precipitate on structural defects in this phase. Formation of a compact layer of MgH2 would probably significantly retard further hydrogen diffusion into the alloy. Fortunately, a large fraction of hydrogen diffuses along the dendrite boundaries where the eutectic Mg2Ni phase is located (Fig.2b). Hydrogen in part dissolves in this phase to form the Mg2NiHX hydride which may contain up to 0.28 % of hydrogen. Simultaneously, hydrogen reacts with the surrounding Mg grains and produces additional MgH2 particles.

Diffusion coefficient of hydrogen D in the Mg2Ni phase at room temperature is about 9⋅10-14 m2s-1 [4]. Diffusion distance X may be estimated by using the simplified version of the second Fick´s law which relates the diffusion distance to the diffusion time τ: X2 = 2Dτ. (3) By taking X=120 min, we obtain a diffusion distance of 40 µm, about a half of the total RS ribbon thickness.

In practical applications of hydrides as hydrogen storage materials, it is important to know temperatures at which hydrogen evolves from these materials. Fig.4 shows results of TGA and DSC analyses of the hydrided alloy. It is seen that the alloy weight progressively reduces due to heating. The reduction starts already at 50°C and continues slowly up to about 300°C. Above 300°C the weight reduction becomes faster. On the DSC curve there are various endothermal effects at above 80°C corresponding to the weight changes. It is important to note that the observed weight reduction and thermal effects are attributable not only to hydrogen evolution but also to decomposition of other compounds present in the sample. It can be assumed that

Fig.3. X-ray diffraction patterns of rapidly solidified (RS)

Mg-14wt.%Ni alloy and after 2 h hydriding in KOH

solution.

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hydroxides are formed on the alloy surface during the reaction in the alkaline KOH solution.

It is known, for example, that magnesium hydroxide Mg(OH)2 decomposes to MgO and water vapor at about 300°C, suggesting that the weight reduction below 300°C may be assigned to hydrogen release. Low hydrogen evolution temperatures are positive from the practical point of view. Moreover, the total weight reduction within a temperature range of 50-300°C is about 1 %, being very close to the measured

hydrogen gravimetric density. 4. CONCLUSION

New electrochemical method for preparation of Mg-based hydrides is presented in this work. In this method, atomic hydrogen is formed by an electrochemical reaction on the surface of a cathode, followed by its diffusion into the cathode material. It is demonstrated that electrochemical hydriding is efficient, particularly for alloys with fine structure where hydrogen diffusion proceeds rapidly. By electrochemical hydriding of the Mg-14wt.%Ni alloy, a relatively high hydrogen gravimetric density of above 1 % was obtained. Probably, the major part of hydrogen releases at relatively low temperatures. Further research in this field which is now in progress is aimed to find alloys and electrochemical hydriding parameters which would provide higher hydrogen gravimetric densities and lower decomposition temperatures. Such materials might then serve as portable sources of hydrogen.

ACKNOWLEDGEMENTS

Research on hydrogen storage materials is supported by the Czech Science Foundation (project no. 104/09/0263). Preparation of the rapidly solidified nanocrystalline alloy used in this study was also supported by Czech Academy of Sciences (project no. KAN300100801). Authors would like to thank to the Ministry of Education, Youth and Sports of the Czech Republic for its financial support (project no. MSM6046137302). REFERENCES [1] ROSS, D. K. Hydrogen storage: The major technological barrier to the development of hydrogen fuel cell cars. Vacuum, 2006, 80, s. 1084-1089. [2] DUARTE, G. I., BUSTAMANTE, L. A. C. aj. Hydriding properties of an Mg-Al-Ni-Ng hydrogen storage alloy. Scripta Mater, 2007, 56, s. 789-792. [3] GALE, W. F., TOTEMEIER, T. C. Smithells Metals Reference Book, 8th Edition, Elsevier, 2004. [4] CUI, N., LUO, J. L. Electrochemical study of hydrogen diffusion behavior in Mg2Ni-type hydrogen storage alloy electrodes. Int J Hydrogen Energy, 1999, 24, s. 37-42.

Fig.4. TGA and DSC curves for hydrided Mg-14wt.%Ni

alloy.