Effect of Alloying Elements in Melt Spun Mg-alloys for Hydrogen Storage

aAdvanced Materials Group, Instituto de Tecnologías y Ciencias de la Ingeniería “Hilario Fernández Long” – INTECIN, Faculty of Engineering, University of Buenos Aires, Paseo Colón 850, Ciudad de Buenos Aires, C1063ACV, Argentina bInstitut de recherche sur l’hydrogène, Université du Québec à Trois-Rivières, 3351 des Forges, TroisRivières, Québec, G9A-5H7, Canada cDepartment of Mechanical Engineering and Mathematical Science, Oxford Brookes University, Oxford, Wheatley Campus, OX33 1HX, Oxford, United Kingdom dInstitute of Research in Materials Science and Technology – INTEMA, Faculty of Engineering, University of Mar del Plata, Buenos Aires, Av. Juan B. Justo, 4302, Mar del Plata, 7600, Buenos Aires, Argentina eLeibniz Institute for Solid State and Materials Research Dresden – IFW Dresden, Leibniz Institute, Helmholtzstraße, 20, 01069, Dresden, Germany fErich Schmid Institute of Materials Science, Austrian Academy of Sciences and Department Materials Physics, Montanuniversität Leoben, Jahnstraße 12, A-8700 Leoben, Austria


Introduction
Hydrogen is a promising clean energy carrier with great potential for mobile and stationary applications replacing petroleum fuels 1 .There are many technological and economic problems to be solved to extend industrial applications of hydrogen as fuel: the production, storage and energy conversion systems.Hydrogen storage is clearly one of the key challenges in developing hydrogen economy.Three basic hydrogen storage methods are considered, which at the present are: (i) pressurized gas, (ii) cryogenic liquid, (iii) solid fuel as chemical or physical combination with materials, such as metal hydrides and complex hydrides 2 .Each of these options possesses attractive attributes for hydrogen storage 3 .Mg-base alloys have great potential for solid-state hydrogen storage, because Mg has high hydrogen absorption capacity near to 7.6 wt%H for MgH 2 , more than most other metal hydrides, good reversibility, low specific weight, low cost and a comparatively high availability in the earth's crust 1,4 .First works in this area were made by Reilly and Wiswall 5,6 on Mg-Cu-H and Mg-Ni-H systems.However, the hydrogen absorption-desorption kinetics are low and need high temperature (from 300ºC to 400ºC) due to the large formation enthalpy of the Mg hydride and diffusion properties.Possible ways to improve the hydrogen absorption-desorption kinetics are alloying Mg to modify the crystal structure of the hydrides by addition of transition metals, metal oxides or rare earths [7][8][9][10][11][12][13][14][15] , reducing the grain size by alloying elements, mechanical deformation or rapid solidification.In recent years the absorption-desorption of hydrogen in Mg alloys produced by unconventional methods have been investigated [16][17][18] .It was observed that the reaction kinetics are improved when the Mg based materials have nanometric structures 19,20 .The addition of rare earths increases the kinetics of absorption and/or desorption in Mg-Ni and Mg-Cu-Ni alloys 21,22 .It was also observed that absorption-desorption kinetics are favorable in Mg-Al alloys containing amorphous or nanostructured phases when it is compared to pure Mg 23 .In general, absorption-desorption kinetics of alloys with amorphous phases and/or nanocrystalline microstructure are higher at lower temperature when compared to micro-crystalline structures 24 .
Amorphous and/or nano-structured Mg alloys can be achieved by rapid solidification or mechanical grinding processes.In general, in these alloys the crystallization temperature of the amorphous phase is lower than the activation temperature of the hydrogen absorption-desorption processes with reasonable rate, thus the amorphous phase has the effect of generating nanostructured alloys by crystallization 25 .Alloys with nano-crystalline phases or a microstructure composed of nano-crystalline and amorphous phases exhibit higher rate of absorption-desorption kinetics at lower temperature in comparison with microcrystalline materials of the same composition 22 .
The aim of the present work is to study the effect of alloying elements on Mg on the hydrogen absorptiondesorption behavior in different alloys produced by melt spinning, as described in the Table 1 Hydrogen absorption-desorption tests were performed on the as-spun samples in a home-made Sieverts apparatus at 350ºC and 2000 kPa for the absorption and 150 kPa for desorption.Both temperature and pressures used are the usual working parameters for pure Mg and many of its alloys 26,27 .For the tests, each sample was carefully weighed and then inserted in a sample holder, fixing the holder onto the device and purging the whole system with three cycles of 120 kPa hydrogen/vacuum.After this it was ready to start the tests.Under vacuum, the sample was isolated from the system and heated up to its operating temperature.When the sample reached the required temperature and maintaining the sample isolated, hydrogen was entered automatically in the system.Once at the set pressure, it was open the sample isolation valve and proceeded to record data.

Results and discussion
The surface of all ribbons (facing wheel and facing air) had typical morphologic features of samples produced by melt spinning.Figure 1(a) shows a secondary electrons (SE) image of facing wheel Mg 90 Cu 10 ribbon where traces of roughness of the copper wheel and from bubbles trapped between the liquid and the copper wheel during the solidification process could be seen.Figure 1(b) shows a backscattered electrons (BSE) image of a cross section of the Mg 90 Cu 10 as-spun sample where a dendritic morphology can be observed.

Experimental
Master alloy ingots with the chemical compositions Mg 65 Cu 25 MM 10 and Mg 65 Cu 10 Al 10 MM 10 , were produced from pure elements by diffusion of filed Cu particles in a Mg-MM or Mg-MM-Al molten alloy in a graphite crucible under Argon atmosphere in an electric furnace at 950 o C. The other master alloys were prepared by melting of pure elements in an induction furnace in a graphite crucible under argon atmosphere.
Melt spun samples were prepared under Argon atmosphere using a BN coated quartz tube.Continuous ribbons with 20-35 µm in thickness and ~1 mm in width were obtained for MM containing alloys.Melt spun samples of the other alloys were obtained with thickness between 80-100 µm and ~4 mm in width.
The characterization of the morphology of the melt spun samples has been carried out using a JEOL 6510 LV scanning electron microscope (SEM).The atomic structure of the as-spun and hydrogenated samples was characterized by X-ray diffraction using Cu-Kalpha radiation.The thermal stability and the crystallization process of the melt-spun samples were studied by differential scanning calorimetry (DSC) at a heating rate of 20 K/min under Argon flow.A complementary heat treatment at 360ºC was performed to   The DSC curves obtained for the as-spun crystalline samples, Mg-Cu, Mg-Si and Mg-Cu-Al alloys, did not showed any peak (solid state transformations), and are not showed in this work.
Figure 5      the desorption kinetics 4,33 .J. Lang et al. 34 studied kinetics of absorption and desorption for Mg and Mg hydrides on cold rolled samples, and they found that the cold rolling process produces a reduction of the crystal size (tens of nm), that improves the H-sorption properties, similarly as the results obtained in this work on samples produced by rapid solidification.The subsequent nano-crystallization from the amorphous phase occurs during the activation cycle at 350 o C.
The rapid solidification produces amorphous and/or nano-crystalline phases that lead to the same beneficial effect that the cold rolling process on the H-sorption behaviour.
A reduction of the H 2 capacities between the second and third cycle is observed for both alloys.

Figure 1 (
c) and (d) show SE images of the Mg 90 Si 10 as-spun sample, where in (c) typical ripples of the facing wheel of a melt spun sample are present; and in (d) an equiaxed grain structure on the surface facing the air can be observed.Figures 2(a), (b), (c) and (d), show the X-ray diffractograms of the alloys Mg 90 Cu 10 , Mg 65 Cu 35 , Mg 90 Si 10 and Mg 65 Cu 25 Al 10 , in the as-spun state and after hydrogen absorption-desorption cycles, respectively.In the binary Mg-Cu alloys only stable equilibrium phases are observed in both states; Mg and Mg 2 Cu for Mg 90 Cu 10 and Mg 2 Cu and MgCu 2 for Mg 65 Cu 35 alloy.In the Mg 90 Si 10 alloy, also the stable phases are observed, Mg and Mg 2 Si; in addition traces of the silicon oxide, SiO 2 can be observed.The diffractograms of the ternary alloy, Mg 65 Cu 25 Al 10 , show the presence of the Mg and (Cu,Al) 2 Mg phases.No peaks related to hydrides and no new phases were observed in the diffractograms after the hydrogen absorption-desorption cycle for Mg 90 Cu 10 , Mg 65 Cu 35 and Mg 65 Cu 25 Al 10 alloys.Figures 3(a) and (b) show the X-ray diffractograms of Mg 65 Cu 25 MM 10 and Mg 65 Cu 10 Al 15 MM 10 alloys in the as-spun state, after three cycles of hydrogen absorption-desorption and of the heat treated sample in a continue heating up to 360ºC.The as-spun Mg 65 Cu 25 MM 10 sample shows a typical X-ray difractogram of an amorphous structure with a broad peak at 2θ~35.7º.The X-ray diffractograms after three cycles of hydrogen absorption-desorption show the Mg 2 Cu, MgH 2 , Mg 4 CuMM phases.However, some peaks in the diffractograms could not be identified.The indexed phases after the heat treatment correspond to Mg 2 Cu and Mg 4 CuMM phases and the un-identified peaks present in the diffractogram after

Figure 1 :
Figure 1: SEM micrographs of as-spun samples.(a) SE image facing wheel of the Mg 90 Cu 10 sample; (b) BSE image of a cross section of the Mg 90 Cu 10 sample; (c) SE facing wheel of the Mg 90 Si 10 sample; (d) SE facing air of the Mg 90 Si 10 sample.

Figure 3 :
Figure 3: X-ray diffractograms of the as-spun, after three cycles of hydrogen absorption-desorption and of the heat treated samples in a continue heating up to 360ºC of the alloys: (a) Mg 65 Cu 25 MM 10 , (b) Mg 65 Cu 10 Al 15 MM 10 .hydrogenabsorption-desorption cycles are not present, which suggests those could correspond to an hydride.In the as-spun Mg 65 Cu 10 Al 15 MM 10 sample the X-ray diffractogram shows a broad peak at 2θ~34,7º and some other peaks that could be indexed as corresponding to the Mg 17 MM 2 phase.The microstructure at the as-spun state of this alloy would be composed of Mg 17 MM 2 nanocrystals embedded in an amorphous matrix.These results suggest that the substitution of 15at% Cu by Al reduces the glass forming ability of the Mg 65 Cu 25 MM 10 alloy.The identified phases in the X-ray diffractogram of the Mg-Cu-Al-MM sample after three cycles of hydrogen absorption-desorption are Mg, Mg 2 Cu, (Cu,Al) 2 Mg, Mg 17 MM 2 and MgH 2 also few peaks could not be identified, which are in the same 2θ position as observed before for the Mg 65 Cu 25 MM 10 alloy.After the heat treatment up to 360 o C of the as-spun sample the indexed phases in X-ray diffractogram are Mg 2 Cu, (Cu,Al) 2 Mg, Mg 17 MM 2 phases, and the un-identified peaks present in the diffractogram after hydrogen absorption-desorption cycles are not present, which again suggests those could correspond to an hydride remained in the cycled sample.Figures 4(a) and (b) show DSC curves of as-spun amorphous Mg 65 Cu 25 MM 10 and partially amorphous Mg 65 Cu 10 Al 15 MM 10 samples.During heating the crystallization process consists of several steps for both alloys.The crystallization process of the Mg 65 Cu 25 MM 10 occurs in three steps.A first sharp exothermic peak at Tp 1 ~170 o C is followed by a second step with overlapped peaks that have an average peak temperature at Tp 2 ~205 o C. Finally, the crystallization process ends with a third step with a sharp exothermic peak at Tp 3 ~261 o C. The temperature of the first exothermal peak is ~13 o C lower than that obtained in the work of Murty et al. 28 , which is reasonable considering they used a higher heating speed (40 o C/min) than in our work (20 o C/min).On the other hand, the crystallization process in the partially amorphous Mg 65 Cu 10 Al 15 MM 10 alloy occurs in two exothermic steps with a first broad asymmetric peak at T p1 ~194 o C and a second sharp peak at T p2 ~305 o C. The remained amorphous phase in the Mg 65 Cu 10 Al 15 MM 10 is stable at higher temperature than the amorphous phase in the alloy without Al, moreover the end of the crystallization process is shifted at higher temperature, few degrees below the test temperature (350 o C) used for the hydrogen absorptiondesorption cycles.Thus, the Al-containing alloy could develop a microstructure with a smaller grain size at 350 o C during hydrogen cycles than the Mg 65 Cu 25 MM 10 .The DSC curves obtained for the as-spun crystalline samples, Mg-Cu, Mg-Si and Mg-Cu-Al alloys, did not showed any peak (solid state transformations), and are not showed in this work.Figure5shows the hydrogen absorption curves of the Mg 90 Cu 10 , Mg 65 Cu 35 , Mg 90 Si 10 and Mg 65 Cu 25 Al 10 alloys.The binary alloys present an inert behaviour under hydrogenated atmosphere, doesn´t show capacity for the absorbing reaction and hydrides formation at 350ºC and 2000 kPa.However, the partial substitution of 10at%Cu by Al (ternary alloy Mg 65 Cu 25 Al 10 ) shows a low hydrogen absorption capacity (~1 wt%H after 2hs 15min).The low absorption rate could correspond to: a low fraction of free Mg in the alloy to form hydride and/or some barrier in the sample surface that prevents the diffusion of hydrogen into the volume of the sample.Krozer et al. and Luz et al. 29,30 explain this phenomenon due to a dense surface layer of hydrides which do not allow the diffusion of hydrogen in the sample.Figures 6 (a), (b) and (c) show the activation, second and third cycle of hydrogen absorption for the Mg 65 Cu 25 MM 10 and Mg 65 Cu 10 Al 15 MM 10 alloys respectively.Figures 7(a), (b) and (c) show the activation, second and third cycle of hydrogen desorption as function of the time for Mg 65 Cu 25 MM 10 and Mg 65 Cu 10 Al 15 MM 10 alloys respectively.These alloys were tested to three consecutive cycles of hydrogen absorptiondesorption.It was found that the rate of the first absorption cycle (activation) is low, which is usual for Mg alloys 31 .The sluggish rate during the activation cycle can be related to the presence of an oxide layer on the surface of the sample, which must be broken to start the hydrogenation through the volume of the material 4 .The Al-containing alloy shows a harder activation process than the Mg 65 Cu 25 MM 10 alloy; however, once the hydrogenation is activated the absorption shows the hydrogen absorption curves of the Mg 90 Cu 10 , Mg 65 Cu 35 , Mg 90 Si 10 and Mg 65 Cu 25 Al 10 alloys.The binary alloys present an inert behaviour under hydrogenated atmosphere, doesn´t show capacity for the absorbing reaction and hydrides formation at 350ºC and 2000 kPa.However, the partial substitution of 10at%Cu by Al (ternary alloy Mg 65 Cu 25 Al 10 ) shows a low hydrogen absorption capacity (~1 wt%H after 2hs 15min).The low absorption rate could correspond to: a low fraction of free Mg in the alloy to form hydride and/or some barrier in the sample surface that prevents the diffusion of hydrogen into the volume of the sample.Krozer et al. and Luz et al. 29,30 explain this phenomenon due to a dense surface layer of hydrides which do not allow the diffusion of hydrogen in the sample.Figures 6 (a), (b) and (c) show the activation, second and third cycle of hydrogen absorption for the Mg 65 Cu 25 MM 10 and Mg 65 Cu 10 Al 15 MM 10 alloys respectively.Figures 7(a), (b) and (c) show the activation, second and third cycle of hydrogen desorption as function of the time for Mg 65 Cu 25 MM 10 and Mg 65 Cu 10 Al 15 MM 10 alloys respectively.These alloys were tested to three consecutive cycles of hydrogen absorptiondesorption.It was found that the rate of the first absorption cycle (activation) is low, which is usual for Mg alloys 31 .The sluggish rate during the activation cycle can be related to the presence of an oxide layer on the surface of the sample, which must be broken to start the hydrogenation through the volume of the material 4 .The Al-containing alloy shows a harder activation process than the Mg 65 Cu 25 MM 10 alloy; however, once the hydrogenation is activated the absorption

Figure 6 :
Figure 6: Absorption kinetic at 350ºC and 2000kPa of the Mg 65 Cu 25 MM 10 and Mg 65 Cu 10 Al 15 MM 10 alloy; (a) first cycle (activation); (b) second cycle, and (c) third cycle.rate and capacities in the second and third cycles are higher than in the Mg 65 Cu 25 MM 10 alloy.Thus, the absorption time to reach the hydrogen saturation in the second and third cycle for Mg 65 Cu 10 Al 15 MM 10 alloy is lower than for the Mg 65 Cu 25 MM 10 alloy.The absorption time is observed to decrease from the second to the third cycle for both alloys, which is in agreement with a general behaviour of H-sorption properties in Mg alloys 31,32 .The partial substitution of Cu by Al in the Mg 65 Cu 25 MM 10 appears to enhance the kinetic of the hydrogen absorption at 350ºC and 2000 kPa.The hydrogen desorption behavior of Mg 65 Cu 25 MM 10 and Mg 65 Cu 10 Al 15 MM 10 alloys can be analyzed from the curves shown in Figure 7.No incubation time is observed for hydrogen desorption at 350ºC and 150 kPa.The desorption rate is faster than the absorption rate for both alloys.There is also observed that the desorption time for the Al-containing alloy during the second cycle is ~100 s while this time is ~600 s for the Mg 65 Cu 25 MM 10 alloy, both times are shorter than for the first cycle.Then, the desorption time is further reduced for the third cycle.This behavior is in agreement with what was observed by other authors; the presence of Al destabilizes Mg hydrides and consequently increases

Figure 7 :
Figure 7: Desorption kinetic at 350 o C and 150 kPa of the Mg 65 Cu 25 MM 10 and Mg 65 Cu 10 Al 15 MM 10 alloy (a) first cycle (activation); (b) second cycle, and (c) third cycle.

From
the hydrogen absorption and desorption tests carried out at 350 o C and 2000 kPa and at 350 o C and 150 kPa respectively, the following effects were demonstrated: • Cu and Si reduce the hydrogen absorption capacity of Mg in the Mg 90 Si 10 , Mg 90 Cu 10 and Mg 65 Cu 35 alloys; • The partial substitution of 10%Cu by Al in the Mg 65 Cu 35 improves the hydrogen activation; while the partial substitution of 10%Cu by MM has stronger effect on activation; • Finally the alloy containing MM and Al, Mg 65 Cu 10 Al 15 MM 10 , has the best behaviour for hydrogen absorption-desorption process.This alloy has higher absorption-desorption kinetic reactions and better reversibility than both ternary alloys, Mg 65 Cu 25 Al 10 and Mg 65 Cu 25 MM 10 ; • The addition of Rare Earths and Al together in Mg base alloys can be used to design new Mg alloys with improved hydrogen absorption-desorption kinetics and reversibility.
. The effect of Cu and Si were studied in the Mg 65 Cu 35 , Mg 90 Cu 10 and Mg 90 Si 10 , (at%) alloys.The effect of Al and rare earths (using MM: mischmetal) were studied in the Mg 65 Cu 25 Al 10 , Mg 65 Cu 25 MM 10 and Mg 65 Cu 10 Al 15 MM 10 alloys.The Mg 65 Cu 25 MM 10 alloy can be obtained in amorphous state by rapid solidification 24 .It is known that the addition of Al to pure Mg destabilizes the MgH 2 , and forms Mg/Al alloys upon dehydrogenation.The Mg 2 Si phase is also attractive as a hydrogen storage material due to favorable desorption enthalpy (ΔH desorption = 36 kJ/mol H 2 ) for room temperature operation.the Mg 65 Cu 25 MM 10 and Mg 65 Cu 10 Al 15 MM 10 alloys in order to analyse the phases in a complete crystallized state.

Table 1 :
Alloys composition (at%) Mg 65 Cu 10 Al 15 MM 10 This reduction is near 40% for Mg 65 Cu 25 MM 10 and only 20% for Mg 65 Cu 10 Al 15 MM 10 .This shows that the Al addition also improves the reversibility of the Mg 65 Cu 25 MM 10 alloy.This behaviour can be explained taking into account that Al destabilizes the MgH 2 4,33 increasing the amount of reversible hydrides.Considering that an alloy absorbs two hydrogen atoms like MgH 2 per each Mg atom in the alloy, from the atomic mass of each composition it can obtain a maximum theoretical hydrogen storage capacity.For Mg 65 Cu 25 MM 10 and Mg 65 Cu 10 Al 15 MM 10 alloys these values are 4.2 wt%H and 4.6 wt%H respectively.For the studied samples, it is observed from Figure 6 that in the second cycle the capacities are 2.3 wt%H for Mg 65 Cu 25 MM 10 and 2.6 wt%H for Mg 65 Cu 10 Al 15 MM 10 ; while for the third cycle these values are 1.4 wt%H and 2.2 wt%H respectively.The partial substitution of Cu by MM or/and by Al in Mg 65 Cu 35 alloy promotes the formation of hydrides and improve the H-sorption properties of the Mg 65 Cu 35 alloy as shown in Figure 3, 6 and 7.The X-ray diffractograms of both Mg 65 Cu 25 MM 10 and Mg 65 Cu 10 Al 15 MM 10 alloys after the third hydrogenation-dehydrogenation cycle confirm the formation of α-hydride MgH 2 .Finally, it is observed that the simultaneous partial substitution of Cu by both Al and MM in the Mg 65 Cu 35 alloy produced by rapid solidification yields an excellent improvement in the H-sorption properties.