PROPERTIES OF MAGNESIUM ALLOYS FOR HYDROGEN STORAGE

Transcription

1 PROPERTIES OF MAGNESIUM ALLOYS FOR HYDROGEN STORAGE Vítězslav KNOTEK, Václav HOŠEK, Dalibor VOJTĚCH, Pavel NOVÁK, Jan ŠERÁK, Alena MICHALCOVÁ, Filip PRŮŠA, Tomáš POPELA, Michal NOVÁK Department of Metals and Corrosion Engineering, Institute of Chemical Technology, Prague, Technická 5, Prague 6, Czech Republic, Abstract The method of hydrogen storage is a key problem in hydrogen economy. Storing of hydrogen in the form of metal hydrides seems to be very prospective. In the first part of this work, as-cast binary MgMm20 and ternary MgNi10Mm5, MgNi10Cu5 and MgNi23P0.3 alloys were exposed to electrochemical hydriding in a 6 mol/l KOH solution at ambient temperature for 16 hours. The best hydriding efficiency is achieved by the ternary MgNi10P0.3 and MgNi10Mm5 alloys, namely about 0.2 wt. %. The second part of paper deals with an evaluation of hydrogen diffusion coefficients in the pure Mg and alloys which were subjected to electrochemical hydriding. The lowest diffusion coefficient of hydrogen was determined in pure Mg and MgNi10Mm5 alloy. The highest one ( m 2 /s) corresponds to MgNi10Cu5. Microstructure of alloys was described and its influence on both the ability to absorb hydrogen and the diffusion coefficients is discussed. Keywords: hydrogen storage, Mg-based alloys, electrochemical hydriding, diffusion coefficients of hydrogen 1 INTRODUCTION Nowadays, hydrogen is considered to be a promising energy carrier for use in various mobile applications, for example in automotive industry. The main task still limiting the practical use of hydrogen is to find safe, simple and inexpensive method of hydrogen storage. The reversible hydrogen storage in light metals, such as magnesium, seems to be very perspective when considering the requirements mentioned above. The main advantages of magnesium for the storage of hydrogen are its relatively low price and theoretical ability to absorb up to 7.6 wt. % of hydrogen, forming MgH 2 hydride. However, pure magnesium suffers from poor thermodynamics and slow kinetics of both hydrogenation and dehydrogenation. MgH 2 desorbs hydrogen very slowly if the temperature is below 573 K [1]. After the hydride layer is formed, MgH 2 prevents further diffusion of hydrogen into the material. Therefore the formation of MgH 2 is practically impossible in the bulk of pure magnesium [2]. Methods of improving the hydrogenation characteristics of magnesium include alloying with appropriate elements such as Ni, Al, Ti, Cu, La, Ce, Nd [3-5] or addition of transition metals oxides of [6], which act as catalysts for hydrogen absorption and desorption. Another method, which can be combined with the above mentioned ones, is to prepare fine nanocrystalline structure by mechanical alloying [7]. In principle there are two ways how to prepare metallic hydride. The first method frequently mentioned in literature [8,9] is the hydriding of alloys by gaseous hydrogen. However this method often requires elevated temperatures and high pressures of pure hydrogen. The second method uses electrochemical production of atomic hydrogen (electrolysis of water-based solution), which can be directly absorbed by an appropriate alloy. This method seems to be technologically and economically preferable in comparison with the first one. After the saturation of alloy by hydrogen, this one can serve as a carrier of hydrogen. Electrochemical hydriding is actually the charging principle of widely used Ni-MH batteries. Although many works deal with the development of new materials based on magnesium for rechargeable batteries [10,11], the application of electrochemical saturation for hydrogen storage is very rarely mentioned [12]. The electrochemical hydriding 1

2 of alloys by hydrogen is controlled by diffusion. Nevertheless, hydrogen diffusion coefficients in alloys tested for hydrogen storage are either missing in the literature or having big differences in values [13,14]. It has been shown by the previous research that it is possible to prepare hydrides by electrochemical hydriding [12,15]. In this work, the ability of electrochemical hydriding of Mg-based ternary alloys different from previous research was tested. Diffusion coefficients of hydrogen were determined in each alloy. An influence of microstructure and current densities applied in electrochemical hydriding on the amount of absorbed hydrogen by the alloys was described. 2 EXPERIMENTAL 2.1 Characterization of samples In this work, binary MgMm20 and ternary MgNi10Mm5, MgNi10Cu5 and MgNi23P0,3 (all concentrations are in wt. %) were investigated. The Mm ( Mischmetal ) contains 45 % Ce, 38 % La, 12 % Nd and 4 % Pr. The alloys were prepared by melting in a vacuum induction furnace under argon protective atmosphere. Cylindrical ingots of alloys of 100 mm in length and 20 mm in diameter were prepared by pouring the melt into a brass mould. The ingots were cut to thin samples of 0.5 mm in thickness. Surface of samples was treated by grinding on P180-P2500 abrasive papers before determination of hydrogen diffusion coefficients and electrochemical hydriding tests. Pre-treated samples were kept under ethanol to prevent the oxidation before further treatment. Microstructure of samples was studied by the light microscope (Olympus PME-3) and scanning electron microscope equipped with EDS analyzer (Hitachi S 450) after polishing and etching to reveal the microstructure. Phase composition was determined by X-ray difraction analyzer (XRD, X Pert Pro). 2.2 Electrochemical hydriding Samples of alloys for electrochemical hydriding were grinded to constant thickness of 0.3 mm. These slices were connected to a DC source as a cathode. Two platinum electrodes were placed opposite to each side of sample and used as anodes. Electrochemical hydriding was carried out in 6 mol/l KOH solution at ambient temperature. The current density was maintained at 100 A m -2. Hydriding time was 16 hours. Average quantity of hydrogen in samples after hydriding was determined by LECO RH-404 hydrogen analyzer. Results are discussed in relation to the microstructure. 2.3 Determination of hydrogen diffusion coefficients To determine the hydrogen diffusion coefficient in pure Mg and alloys used for hydriding, the electrochemical permeation method was applied. The method was firstly developed by Devanathan and Stachurski in 1962 on palladium and the principle is described in [16]. The experiments were carried out at temperature of 298 K. Cathodic polarization of the entrance side was achieved in a galvanostatic way. The electrolytic solution was a 6 mol/l KOH on the entrance side and N,N -dimethylformamide on the exit side of the sample. N,N dimethylformamide preserves oxidation of Mg-based alloy and it is also sufficiently conductive. In the experiment, the charging current density was 1 ma cm -2 and the detection potential was -100 mv with respect to the spontaneous corrosion potential of sample. Diffusion data was extracted from the experimental permeation curves showing the hydrogen flux versus time. 2

3 3 RESULTS AND DISCUSSION 3.1 Microstructure Microstructure of the as-cast alloys is displayed in Fig. 1. Microstructures of binary MgMm20 and ternary MgNi10Mm5 and MgNi10Cu5 (Fig. 1a-c) are very similar. There are approximately 50 vol. % of primary Mg dendrites and fine eutectic structure. Results of XRD analysis and EDS chemical microanalysis showed that eutectic structure is composed of Mg and Mg 12 Mm phases in the case of MgMm20 alloy (Fig. 1a).Mg 2 Ni phase is present in the eutectic structure together with Mg 12 Mm (Fig 1b) in the case of MgNi10Mm5 alloy. Eutectic mixture in MgNi10Cu5 is composed of Mg, Mg 2 Ni and Mg 2 Cu phases (Fig. 1c). MgNi23P0.3 alloy have very fine eutectic structure of Mg and Mg 2 Ni and is different from the other three alloys (Fig. 1d). This difference is caused by higher amount of Ni since the Ni content of 23 wt. % corresponds to eutectic alloy, in accordance with the Mg-Ni phase diagram [17]. No phosphorus-rich phases were detected. It indicates that phosphorus is distributed almost uniformly in other phases or in the form of fine phosphide particles. Fig. 1. Microstructure of investigated alloys (light microscope) a) MgMm20, b) MgNi10Mm5, c) MgNi10Cu5, MgNi23P Hydrogen concentrations Fig. 2 shows hydrogen concentrations of investigated alloys. Diagram shows that hydriding of ternary MgNi23P0.3 and MgNi10Mm5 alloys absorbed about 0.2 wt. % of hydrogen. These alloys are more suitable for electrochemical hydriding than MgNi10Cu5 and MgMm20. It seems that the eutectic structure of Mg and Mg 2 Ni is the most suitable for hydrogen storage. Presence of mischmetall has also a positive influence on the ability to absorb hydrogen, in comparison with Cu. Since the binary alloy MgMm20 was able to absorb 3

4 only about 0.02 wt. % of H, presence of Ni seems to be most important for effective electrochemical hydriding. Fig. 2. Hydrogen concentration of alloys after 16 h of electrochemical hydriding (100 A/m 2 ) The achieved concentrations of hydrogen are seemingly small. It is be caused by the low specific surface and possible oxidation during hydriding, which inhibits the hydrogen uptake into the structure. We have to take into account that hydriding was carried out at the ambient temperature. By increasing the hydriding temperature and the specific surface of a cathode (cathode could be made of powder), much higher amount of absorbed hydrogen could be probably obtained. 3.3 Diffusion coefficients of hydrogen Diffusion coefficients of hydrogen were determined in the case of each prepared alloy by electrochemical permeation method described above. Fig. 3 shows a drawing of a typical record of permeation measurement. The point t 0 represents the time at which the cathodic current is switched on, t 1/2 the half rise time of the transient current density, and i A : the steady-state permeation current density. Diffusion coefficients of hydrogen are then calculated from the equation (1) [18], where D is diffusion coefficient (m 2 /s), L is thickness of sample (m) and t 1/2 is the half rise of the transient L D = t 1/ 2 2 (1) Fig. 3. Typical record of permeation measurement Diffusion coefficients of hydrogen depend on several parameters, i. e. temperature, structure and measurement method. Therefore it is hard to compare our investigated data with literature. For example diffusion coefficients of hydrogen in Mg (after regression to the same temperature) range from m 2 /s to m 2 /s [19-21]. 4

5 Tab. 1. Diffusion coefficients of hydrogen investigated by Tab. 1. contains diffusion coefficients of electrochemical permeation method (T=298 K) hydrogen for pure Mg and for the alloys, which Alloy D (m 2 /s) were used for hydriding. Advantage of using Mg MgMm20 MgNi23P MgNi10Cu5 MgNi10Mm electrochemical permeation to evaluate the diffusion coefficient of hydrogen in context of electrochemical hydriding, is the same principle of hydrogen saturation - galvanostatic cathodic polarization. It means that similar processes which can influence diffusion may occur both during electrolytic hydriding and electrolytic permeation. The lowest diffusion coefficient of hydrogen was determined in the case of the pure Mg. This can be related to ease of oxidation of Mg to Mg(OH) 2 and probably to the formation of surface hydride, which blocks further penetration of hydrogen to structure. Similar value of diffusion coefficient to magnesium was measured in MgNi10Mm5 alloy. This phenomenon could be probably caused by high portion of Mg in microstructure (Fig. 1b). The highest diffusion coefficient of hydrogen was measured in the case of MgNi10Cu5 alloy. Presence of Cu probably slows down the surface oxidation. Though the hydrogen diffusion in the case of MgNi10Cu5 is relatively high, this material is not able to trap the hydrogen as effectively as MgNi10Mm5 or MgNi23P. 4 CONCLUSION The results of this work show that electrochemical hydriding of Mg-based alloys is a promising approach to simply store hydrogen. The best hydriding efficiency is achieved for the ternary MgNi10P0.3 and MgNi10Mm5 alloys. However, the achieved concentrations of hydrogen are relatively low due to low specific surface and temperature applied for hydriding. Appropriate adjustment of hydriding parameters (temperature, specific surface) might lead to much higher gravimetric densities of absorbed hydrogen. The results of hydrogen diffusion coefficients determination show that the lowest one corresponds to the pure magnesium. Very similar value was determined in the case of ternary MgNi10Mm5. Although the highest diffusion coefficient of hydrogen corresponds to MgNi10Cu5 alloy, the concentration of hydrogen reached by hydriding of this alloy is low. ACKNOWLEDGEMENTS The research on materials for hydrogen storage is financially supported by the Czech Science Foundation (project no. 104/09/0263), the Ministry of Education, Youth and Sports of the Czech Republic (project no. MSM ) and by internal grant project of ICT Prague (Properties of magnesium alloys for hydrogen storage). REFERENCES [1] Palade P., et. al.: Journal of Alloys and Compounds, 415 (2006), [2] Vijay R, et. al.: International Journal of Hydrogen Energy, 30 (2005), [3] Vegge T., et. al.: Journal of Alloy and Compounds, 386 (2005), 1 7. [4] Sato T., Blomqvist H., Noréus D.: Journal of Alloy and Compounds, (2003), [5] Tran N. E., Imann M. A., Feng C. R.: Journal of Alloys and Compounds, 359 (2003), [6] Song M.Y., et. al.: Journal of Alloys and Compounds, 398 (2005) [7] Au M.: Materials Science and Engineering B, 117 (2005),

6 [8] Saita I., et. al.: Journal of Alloys and Compounds, (2003), [9] Zhu M., et. al.: International Journal of Hydrogen Energy, 31 (2006), [10] Zhu Y., Wang Y., Li L.: International Journal of Hydrogen Energy, 33 (2008), [11] Zhao X., Ma L.: International Journal of Hydrogen Energy, 34 (2009), [12] Vojtěch D., et. al.: International Journal of Hydrogen Energy, 34 (2009), [13] Sakaguchi H., et. al.: Journal of Alloys and Compounds, 354 (2003), [14] Cui N., Luo J. L., Chuang K. T.: Journal of Electroanalytical Chemistry, 503 (2001), [15] Vojtěch D., et. al.: Journal of Physics and Chemistry of Solids, 68 (2007), [16] Devanathan M. A. V., Stachurski Z.: Proceedings of the Royal Society A, 270 (1962), [17] Gale W.F., Totemeier T.C.: Smithells Reference Book, eighth ed., Elsevier, Amsterdam, [18] Subramanyan, P. K.: Comprehensive Treatise of Electrochemistry, 4 (1981), [19] Schimmel H.G., et. al.: Journal of Alloys and Compounds, (2005), [20] Nishimura C., Komaki M., Amano M.: Journal of Alloys and Compounds, (1999), [21] Čermák J., Král L.: Acta Materialia, 56 (2008),