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Hydrogen storage by Mg-based nanocomposites
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Hydrogen storage by Mg-based nanocomposites M. Jurczyk a,*, M. Nowak a, A. Szajek b, A. Jezierski b a b
Institute of Materials Science and Engineering, Poznan University of Technology, Sklodowska-Curie 5 Sq., 60-965 Poznan, Poland Institute of Molecular Physics, Polish Academy of Sciences, Smoluchowskiego 17 St., 60-179 Poznan, Poland
article info
abstract
Article history:
Hydrogen storage materials research is entered to a new and exciting period with the
Received 7 February 2011
advance of the nanocrystalline alloys, which show substantially enhanced absorption/
Received in revised form
desorption kinetics, even at room temperatures. In this work, hydrogen storage capacities
28 March 2011
and the electrochemical discharge capacities of the Mg2(Ni, Cu)-, LaNi5-, ZrV2-type nano-
Accepted 2 April 2011
crystalline alloys and Mg2Ni/LaNi5-, Mg2Ni/ZrV2-type nanocomposites have been
Available online 27 April 2011
measured. The electronic properties of the Mg2Ni1-xCux, LaNi5 and ZrV2 alloys were calculated. The nanocomposite structure reduced hydriding temperature and enhanced
Keywords:
hydrogen storage capacity of Mg-based materials. The nanocomposites (Mg,Mn)2Ni (50 wt
Magnesium
%)-La(Ni,Mn,Al,Co)5 (50 wt%) and (Mg,Mn)2Ni (75 wt%)-(Zr,Ti)(V,Cr,Ni)2.4 (25 wt%) materials
Hydrogen storage
releases 1.65 wt% and 1.38 wt% hydrogen at 25 C, respectively. The strong modifications of
Nanocomposites
the electronic structure of the nanocrystalline alloys could significantly influence hydro-
Electronic structure
genation properties of Mg-based nanocomposities. Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Metal hydrides (MH) which reversibly absorb and desorb hydrogen at ambient temperature and pressure are regarded as important materials for solving energy and environmental issues. A large number of experimental investigations on LaNi5-, TiFe-, TiNi-, ZrV2-type compounds have been performed up to now in relation to their exceptional hydrogenation properties [1e5]. Magnesium-based hydrogen storage alloys have been also considered to be possible candidates for hydrogen storage as well as for electrodes in Ni-MHx batteries [6e8]. In recent years many other new developments have occurred in metal hydrides, from the introduction of new, non-conventional methods of fabrication, surface treatment, or by discoveries of new hydride phases [4,6,7]. Conventionally, the microcrystalline hydride materials have been prepared by arc or induction melting and annealing. However, either a low storage capacity by weight or poor absorption-desorption kinetics in addition to a complicated
activation procedure have limited the practical use of metal hydrides. Substantial improvements in the hydridingedehydriding properties of metal hydrides could be possibly achieved by the formation of nanocrystalline structures by non-equilibrium processing technique such as mechanical alloying (MA) [6e13]. This process consists of repeated fracture, mixing and cold welding of a fine blend of elemental particles, resulting in size reduction and chemical reactions [14]. As non-equilibrium processing method MA can be used to produce large quantities of materials at relatively low cost. Recently, amorphous 2Mg þ 3d/x wt% Ni materials were prepared by mechanical alloying (MA) of Mg and 3d elemental powders (3d ¼ Fe, Co, Ni, Cu; 0 x 200) under high purity argon atmosphere [10,15]. For all 2Mg þ 3d synthesized materials, mechanical alloying with nickel effectively reduced the degradation rate of the studied electrodes [15]. This result indicates that the nickel content does help to inhibit the corrosion of the alloy electrode in the KOH solution.
* Corresponding author. Tel.: þ48 61 665 3508; fax: þ48 61 665 3576. E-mail address: [email protected] (M. Jurczyk). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.04.012
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As a continuation of our previous studies, in this paper, hydrogen storage nano-scale Mg2(Ni, Cu)-, LaNi5-, ZrV2-type alloys and Mg2Ni/LaNi5-, Mg2Ni/ZrV2-type nanocomposites have been synthesized using the mechanical alloying process. The structure of the samples has been studied by X-ray diffraction (XRD). Hydrogen storage capacities and the electrochemical discharge capacities of the materials have been measured. The electronic properties of the Mg2Ni1-xCux, LaNi5 and ZrV2 alloys were calculated.
2.
Experimental procedure
Mechanical alloying was performed under an argon atmosphere using an SPEX 8000 Mixer Mill. The high purity elemental powders (Mg: 44 mm; Al: 75 mm; Ti: 44 mm; V: 44 mm; Cr: 1e5 mm; Mn: 45 mm; Co: 2 mm; Ni: 3e7 mm; Cu: 3 mm, Zr: 149 mm; La: 425 mm) were mixed and loaded into the vial in the glove box (Labmaster 130) containing an argon atmosphere (O2-2 ppm and H2O-1 ppm). The as-milled powders were heat treated (see text for details) under high purity argon, respectively, to form ordered phases. Additionally, the MA and annealed Mg2Ni-type hydrogen storage materials were mixed with different contents of LaNi5- or ZrV2-type powders and milled for 1 h in an SPEX Mixer Mill. Microcrystalline LaNi5-, ZrV2- and Mg2Ni-, Mg2Cu-type alloys were prepared by arc melting and by diffusion methods, respectively. The MA process of the synthesized materials has been examined by X-ray diffraction (XRD) and microstructural investigations. The particle sizes were estimated by Scherrer method. Independently, the change in the structure of powdered samples was observed using Atomic Force Microscope (Nanoscope IIIa - Digital Instruments, USA). These measurements were done in ambient atmosphere in contact mode using Si3N4 tips with a tip apex radius of 20e60 nm. The nanocrystalline materials with 10 wt% addition of Ni powder, were subjected to electrochemical measurements as working electrodes after pressing (under 80 kN cm2) to 0.5 g pellet form between nickel nets acting as current collector. The electrochemical properties of electrodes were measured in a three-compartment glass cell, using a much larger NiOOH/ Ni(OH)2 counter electrode and a Hg/HgO/6 M KOH reference electrode. All electrochemical measurements were carried out in deaerated 6 M KOH solution prepared from Analar grade KOH and 18 MU cm1 water, at 21 C. Potentiodynamic and galvanostatic techniques with either short or long-term pulses using a conventional apparatus were applied to study the chargeedischarge kinetics of the electrodes. Additionally, these mechanically alloyed materials, in nanocrystalline forms were subjected to thermodynamic measurements. The hydrogen absorption properties (P-C-T isotherms) were investigated using Sievert-type apparatus at temperatures in the range 293e673 K (Particulate Systems). Sample analysis data collection is fully automated to assure quality data and high reproducibility. The electronic properties of Mg2Ni1-xCux-, LaNi5- and ZrV2type alloys were calculated by the first principle scalar relativistic full-potential local-orbital method in the coherent potential approximation (FPLO-CPA) [16e18]. The calculations were performed for experimental values of lattice constants
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within the local density approximation [17] using the exchange correlation potential in the form of Perdew and Wang [19].
3.
Results
In this work, mechanical alloying has been used to make a nanocrystalline Mg2Ni-, Mg2Cu-, LaNi5- and ZrV2-type alloys (Fig. 1). The behaviour of MA processes has been studied by Xray diffraction, microstructural, thermodynamic and electrochemical investigations.
3.1.
Mg2Ni(Cu)-type system
The magnesium-nickel phase diagram shows two compounds Mg2Ni and MgNi2. The first one reacts with hydrogen slowly at room temperature to form the ternary hydride Mg2NiH4. At higher temperatures at pressure (e.g. 200 C, 1.4 MPa), the reaction is rapid enough for useful absorptionedesorption reactions to occur. Mechanical alloying is one of the approaches to produce MgeNi alloys which have been highly expected to be used as hydrogen storage materials [6]. Varin et al. [6] pointed out that ball milling which gives rise to the creation of fresh surfaces and cracks is highly effective for the kinetic improvement in initial hydriding properties. The
Fig. 1 e XRD spectra of nanocrystalline: (a) Mg2Ni-, (b) Mg1.5Mn0.5Ni, (c) Mg2Cu-, (d) LaNi5- and (e) ZrV2-type alloys produced by mechanical alloying followed by annealing (Mg2Ni, Mg1.5Mn0.5Ni: MA 30 h and heat treated at 450 C for 0.5 h; Mg2Cu: MA 18 h and head treated at 450 C for 0.5 h; LaNi3.75Mn0.75Al0.25Co0.25: MA 30 h and head treated at 700 C for 0.5 h; Zr0.35Ti0.65V0.85Cr0.26Ni1.30: MA for 25 h and heat treated at 800 C for 0.5 h).
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Fig. 2 e The discharge capacity as a function of cycle number for MA and annealed Mg2Cu (a), Mg2Ni (b), Mg1.5Mn0.5Ni (c), LaNi5 (d), LaNi3.75Mn0.75Al0.25Co0.25 (e) and Zr0.35Ti0.65V0.85Cr0.26Ni1.30 (f) (solution, 6 M KOH; T [ 20 C). The charge conditions were 40 mA gL1. The cut-off potential vs. Hg/HgO/6 M KOH was L0.7 V.
nanocrystalline Mg2Ni-type alloys were prepared by mechanical alloying followed by annealing. The powder mixture milled for more than 30 h has transformed directly to a hexagonaltype phase. Finally, the obtained powder was heat treated in high purity argon atmosphere at 450 C for 0.5 h (Fig. 1a). All diffraction peaks were assigned to those of the hexagonal ˚ , c ¼ 13.246 A ˚. crystal structure with cell parameters a ¼ 5.216 A The average size of amorphous 2Mg-Ni powders, according to AFM studies, was of the order of 30 nm. At room temperature, the nanocrystalline Mg2Ni alloy absorbs hydrogen, but almost does not desorb it. At temperatures above 250 C the kinetic of the absorptionedesorption
process improves considerably and for nanocrystalline Mg2Ni alloy the reaction with hydrogen is reversible. The hydrogen content in this material at 300 C is 3.25 wt%. The Mg2Ni electrode, mechanically alloyed and annealed, displayed the maximum discharge capacity (100 mA h g1) at the 1st cycle but degraded strongly with cycling (Fig. 2). The poor cyclic behaviour of Mg2Ni electrodes is attributed to the formation of Mg(OH)2 on the electrodes, which has been considered to arise from the charge-discharge cycles. To avoid the surface oxidation, we have examined the effect of magnesium substitution by Mn in Mg2Ni-type material. This alloying greatly improved the discharge capacities. In nanocrystalline Mg1.5Mn0.5Ni alloy discharge capacities up to 241 mA h g1 were measured. In the case of 2Mg-Cu powder mixture (0.433 wt% Mg þ 0.567 wt% Cu), the powder mixture milled for more than 18 h has transformed directly to an orthorhombic-type phase (Fig. 1b). Table 1 reports the cell parameters of the studied material. According to the Scherrer method for XRD profiles, the average size of 2Mg-Cu mechanically alloyed for 18 h powders was of the order of 30 nm; after heat treatment in high purity argon atmosphere at 450 C for 0.5 h the mean crystallite size of the nanocrystalline alloy estimated from AFM experiment was about 50 nm. At room temperature, the original nanocrystalline Mg2Cu alloy absorbs hydrogen, but almost does not desorb it. At temperatures above 250 C the kinetic of the absorptionedesorption process improves considerably and for nanocrystalline Mg2Cu alloy the reaction with hydrogen is reversible. Upon hydrogenation, Mg2Cu transforms into the hydride MgH2 þ MgCu2 phases. At 300 C the maximum absorption capacity reaches 2.25 wt% for pure nanocrystalline Mg2Cu alloy. This is lower than in the microcrystalline Mg2Cu alloy (2.6 wt%) because of a significant amount of strain, chemical disorder and defects introduced into the material during the mechanical alloying process. The Mg2Cu electrode, mechanically alloyed and annealed, displayed the maximum discharge capacity (26.5 mA h g1) at the 1st cycle but degraded strongly with cycling (see Table 1,
Table 1 e Structure type, lattice constants, hydrogen contents and discharge capacities for studied nanocrystalline and nanocomposite materials. Composition
Mg2Ni Mg2Cu Mg1.75Mn0.25Ni Mg1.5Mn0.5Ni LaNi5 LaNi3.75Mn0.75Al0.25Co0.25 50%Mg2Ni/50%LaNi5 50%Mg1.5Mn0.5Ni/50%LaNi3.75Mn0.75Al0.25Co0.25 ZrV2 Zr0.35Ti0.65V0.85Cr0.26Ni1.30 75% Mg1.5Mn0.5Ni/25% Zr0.35Ti0.65V0.85Cr0.26Ni1.30
Structure type
hexagonal orthorhombic hexagonal cubic hexagonal hexagonal composite composite cubic hexagonal composite
Lattice ˚] constants [A
a ¼ 5.216 c ¼ 13.246 a ¼ 9.119 b ¼ 18.343 c ¼ 5.271 a ¼ 5.185 c ¼ 13.097 a ¼ 3.137 a ¼ 5.010 c ¼ 3.972 a ¼ 5.075 c ¼ 4.039 e e a ¼ 7.501 a ¼ 4.921 c ¼ 8.011 e
Hydrogen content [wt%]
3.25 at 300 C 2.25 at 300 C 1.75 at 300 C 0.68 at 300 C 1.1 at 21 C 1.03 at 21 C 0 at 21 C 1.65 at 21 C 0 at 21 C 1.31 at 21 C 1.38 at 21 C
Discharge capacity at 20 C (mA h g1) 1st cycle
3rd cycle
100 26.5 148 241 84 258 e e 0 154 e
5 4.7 90 192 101 210 e e 0 160 e
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Fig. 2). The poor cyclic behaviour of Mg2Cu electrodes is attributed to the formation of Mg(OH)2 on the electrodes.
3.2.
Electronic structure of Mg2Ni1-xCux
Application of Mg-based alloys focused attention on the electronic structure of Mg2Ni1-xCux-type hydrogen storage materials. The theoretical band calculations were performed for ideal single-crystalline Mg2Ni and Mg2Cu alloys. The experimental X-ray photoelectron valence bands measured for MA nanocrystalline Mg2Cu alloy showed a significant broadening of the valence band measured for the MA nanocrystalline sample compared to that obtained for the microcrystalline Mg2Cu [20]. The reasons responsible for the band broadening of the nanocrystalline Mg2Cu alloy are probably associated with a strong deformation of the nanocrystals in the MA samples. The valence band of Mg2Ni1-xCux alloy is wider than for starting ordered Mg2Ni and Mg2Cu compounds (Fig. 3C). It is caused by different positions of Cu and Ni peaks in relation to the Fermi level (EF) (Fig. 3A and B). Additionally, the valence band is broadened by two kinds of disorder: (i) the chemical disorder because of random occupying Cu atoms in Ni sites and vice versa (it causes broadening of usually sharp peaks of ordered systems); (ii) the structural disorder: Mg2Ni and Mg2Cu compounds crystallize in two different structures and for Mg2Ni1-xCux alloy a structural transition takes place. The
Fig. 4 e Total DOS and site projected DOS plots for LaNi5 compound (upper panel) and for LaNi3.75Mn0.75Al0.25Co0.25 (lower panel) where impurities are distributed homogeneously in both sites La(Ni2.25Mn0.45Al0.15Co0.15)3g(Ni1.5Mn0.3Al0.1Co0.1)2c
Fig. 3 e Total density of states (DOS) and calculated photoemission spectra (total and site projected) for Mg2Cu (A) and Mg2Ni (B) compounds. Total DOS for Mg2Ni1-xCux (x [ 0.10, 0.15) with Cu atoms in the 3d site (C) and for Mg2Ni0.9Pd0.1, where Pd atoms are located in the 3b site. (EF [ 0).
Fig. 5 e The DOS plots for cubic ZrV2 and Zr(V0.75Ni0.25)2 systems and hexagonal Ti0.5Zr0.5(V0.27Mn0.27Cr0.13Ni0.33)2 one (Ni(1) and Ni(2)) mean Ni contributions from two sites.
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differences for micro- and nanocrystalline samples are visible in photoemission spectra (see Fig. 6 in Ref. [20]). The ab-initio calculations allow for calculating site preference of particular atoms, for instance, in Mg2Ni1xCux alloys (where x ¼ 0.10, 0.15) the position of Cu atoms in 3d sites is more stable, although the difference of the total energy between different configurations is small (for details on electronic structure modification by impurities in wider range of concentration see Ref. [21]). In the case of Mg2Ni0.9Pd0.1 the palladium atoms prefer 3b site [22].
In nanocrystalline LaNi3.75Mn0.75Al0.25Co0.25 discharge capacities up to 258 mA h g1 was measured. Cobalt in the alloy, played chemical role in the passivation of the alloy surface by the dissolution and precipitation (DP) process and thereby improved the electrode performance. Generally, alloying improves the alkali-resistance of Co, Mn and Al, controlling the rate of the dissolution in LaNi5-type electrode materials. At room temperature hydrogen content in LaNi3.75Mn0.75Al0.25Co0.25 is 1.03 wt%.
3.4. 3.3.
LaNi5-type system
The properties of hydrogen host LaNi5 materials can be modified substantially by alloying. In the transition metal sublattice of LaNi5-type compounds, substitution by Mn, Al and Co has been found to offer the best compromise between high hydrogen capacity and good resistance to corrosion [3]. The mechanically alloyed LaeNi powder mixture (0.322 wt % La þ 0.678 wt% Ni) milled for more than 30 h has transformed completely to the amorphous phase, without formation of another phase. Formation of the nanocrystalline alloy was achieved by annealing the amorphous material in high purity argon atmosphere at 700 C for 0.5h (Fig. 1c). All diffraction peaks were assigned to those of the hexagonal crystal structure of CaCu5-type (Table 1). The average size of amorphous LaeNi powders, according to AFM studies, was of the order of 25 nm. The discharge capacity of electrode prepared by application of MA and annealed LaNi5 alloy powder is low. It was found that alloying elements such as Al, Mn and Co substituting nickel greatly improved the cycle life of LaNi5type material (Fig. 2). With increasing cobalt content in LaNi4xMn0.75Al0.25Cox, the material shows an increase in discharge capacity which passes through a wide maximum for x ¼ 0.25.
Fig. 6 e XRD spectra of 50 wt% LaNi3,75Mn0,75Al0,25Co0,25/ 50 wt% Mg1,5Mn0,5Ni (a) and 25wt% Zr0.35Ti0.65V0.85Cr0.26Ni1.30/75 wt% Mg1.5Mn0.5Ni (b) nanocomposites after 1 h ball milling.
Electronic structure of LaNi5-type system
Binary LaNi5 crystallises with the CaCu5 structure type (space group P6/mmm) in which La occupies site 1(a) and Ni sites 2(c) and 3( g). The battery electrode material LaNi4Al is a substitutional derivative of LaNi5 in which La occupies site 1(a) and Ni and Al sites 2(c) and 3( g) of space group P6/mmm. Experimental results showed that the La sites do not accommodate Ni and Al atoms [23]. Furthermore, the ab-initio calculations [24] showed that the impurity aluminium atoms prefer the 3g positions in agreement with experimental data [25]. Similar situation takes place in the case of LaNi3AlCo but in more complicated systems LaNi3.75Mn0.75Al0.25Co0.25 3g site is no longer preferred by impurities. For this stoichiometry three configurations were considered: (i) impurities in 2c site La(NiNiNi)3g(Ni0.75Mn0.75Al0.25Co0.25)2c, impurities located in 3g site (ii) La(Ni1.75Mn0.75Al0.25Co0.25)3g(NiNi)2c, and finally impurities distributed homogeneously in both sites (ii) La(Ni2.25Mn0.45Al0.15Co0.15)3g(Ni1.5Mn0.3Al0.1Co0.1)2c (Fig. 4). Comparison of the total energies give the following rank: E(i)>E(ii)>E(iii). It concludes that last configuration is the most stable.
3.5.
ZrV2-type system
Materials obtained when Ti was substituted for Zr and Cr, Ni was substituted for V in ZrV2 lead to greatly improved activation behaviour of the electrodes. The ZreTieVeCreNi (16.3 wt% Zr þ 15.8 wt% Ti þ 22.1 wt% V þ 6.9 wt% Cr þ 38.9 wt % Ni) powder mixture milled for more than 25 h has transformed absolutely to the amorphous phase. It is worth noting that before amorphization no shift of the diffraction lines was observed. This result means that the amorphous phase forms directly from the starting mixture of the elements (Zr, Ti, V, Cr and Ni), without formation of other phases. Using the ZreTieVeCreNi mixture composition as the representative material example, the behaviour of the grain size of the crystallites has been studied during the mechanical alloying process. The Ni (111) diffraction line remains visible up to 15 h of milling. This allows an estimation of the change in the crystalline size of Ni with increase of the milling time. The crystallite size decreases strongly from 50 nm at the beginning of the mechanical alloying process. The final size of the crystallites, about 30 nm, seems to be favourable to the formation of an amorphous phase which develops at the Zr/ Ti/V/Cr/Ni interfaces. Formation of alloy with hexagonal C14 type structure was achieved by annealing the amorphous material in high purity argon atmosphere at 800 C for 0.5 h (Fig. 1d). The final diffraction pattern exhibits broadening of the peaks characteristic of nanocrystalline material.
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The electrode prepared with nanocrystalline Zr0.35 Ti0.65V0.85Cr0.26Ni1.30 material showed better activation and higher discharge capacities. This improvement is due to a wellestablished diffusion path for hydrogen atoms along the numerous grain boundaries. In the annealed nanocrystalline Zr0.35Ti0.65V0.85Cr0.26Ni1.30 powders discharging capacities up to 195 mA h g1 have been measured. The hydrogen content in room temperature in Zr0.35Ti0.65V0.85Cr0.26Ni1.30 is 1.31 wt%.
3.6.
Electronic structure of ZrV2-type system
Basic ZrV2-type alloy crystallises in the cubic C15-type structure but the MA procedure for more complicated system Ti0.5Zr0.5(V0.27Mn0.27Cr0.13Ni0.33)2 leads to hexagonal structure. Earlier electronic structure studies [26] showed that the Ni impurities cause a charge transfer from Zr and V to Ni, the valence band is wider and the density of electronic states at the Fermi level decreases by about 30%. Change of crystallographic structure and growing disorder introduced by additional impurities cause broadening of the valence band and reconstruction of the electronic structure (Fig. 5).
3.7. Nano-scale Mg2Ni/LaNi5 and Mg2Ni/ZrV2 composites The structural and thermodynamic properties on LaNi3,75Mn0,75Al0,25Co0,25/Mg1,5Mn0,5Ni and Zr0.35Ti0.65V0.85Cr0.26Ni1.30/ Mg1.5Mn0.5Ni hydrides materials was studied (Fig. 6). The results show that nanostructured 50 wt% Mg1.5Mn0.5Ni þ 50 wt% LaNi3.75Mn0.75Al0.25Co0.25 and 75 wt% Mg1.5Mn0.5Ni þ 25wt% Zr0.35Ti0.65V0.85Cr0.26Ni1.30 composite materials releases 1.65 and 1.38 wt% hydrogen at room temperature, respectively (Fig. 7 A, B). This is higher than in nanocrystalline Mg1.5Mn0.5Ni (0.68 wt%), LaNi3.75Mn0.75Al0.25Co0.25 (1.03 wt%), Zr0.35Ti0.65V0.85Cr0.26Ni1.30 (1.31 wt%). It is believed that the dehydriding temperature is largely controlled by the thermodynamic configuration of magnesium hydride.
4.
Discussion
The nanocrystalline metal hydrides offer a breakthrough in prospects for practical applications. Their excellent properties (significantly exceeding traditional hydrides) are a result of the combined engineering of many factors: alloy composition, surface properties, microstructure, grain size, and other. The chemical composition of the metal alloy is one of the most important factors in the metal - hydrogen system. The equilibrium conditions are reflected in the phase diagram, which dictates the respective phase composition under given temperature and pressure. Introducing metastable phases may result in totally different behaviour of the alloy. Present studies of potentially used materials for hydrogen storage alloys are not only focused on simple binary systems but on modified alloys which can improve their electrochemical properties. It was found that, the strong modifications of the electronic structure in the MA nanocrystalline Mg2(Ni, Cu)-, LaNi5-, ZrV2-type alloys could significantly influence hydrogenation properties of Mg-based nanocomposites.
Fig. 7 e A PC isotherms at 21 C for: (a) composite 50 wt% Mg2Ni/50 wt% LaNi5 (no hydrogen desorption) and (b) 50 wt % Mg1,5Mn0,5Ni/50 wt% LaNi3,75Mn0,75Al0,25Co0,25 (samples a and b: after 1 h ball milling). B PC isotherms at 21 C for: (a) microcrystallne Zr0.35Ti0.65V0.85Cr0.26Ni1.30 alloy and (b) nanocomposite 25 wt% Zr0.35Ti0.65V0.85Cr0.26Ni1.30/75 wt% Mg1.5Mn0.5Ni after 1 h ball milling (b).
The experimental and theoretical investigations concluded that the following effects are important for understanding the changes in the electronic structure of the host metal due to the hydrogen absorption [27]: (i) The change of lattice constants (usually expansion but in some cases contraction) often accompanied by a change of crystal structure leads to a modification of the symmetry of the states and the band widths. (ii) The attractive potential of proton influence on those metal wave functions which have a finite density at the hydrogen site and leads to the so-called metal-hydrogen bonding band below the metal d-band. Furthermore, some metal states located below the Fermi level (EF) in the pure metal are pulled below EF. (iii) The additional hydrogenehydrogen interactions in the hydrides which have more than one H atom per unit cell lead to new features in the lower part of the density of states. (iv) The presence of the additional electrons introduced by the hydrogen atoms results in a shift of the Fermi level. The experimental XPS valence bands measured for the Mgbased nanocomposites showed a significant broadening compared to those obtained for the microcrystalline thin films. The main reasons responsible for the band broadening of the nanocrystalline alloys and nanocomposites are probably associated with a strong deformation of the nanocrystals in the samples. The strong modifications of the electronic structure of the nanocomposites could significantly influence on their hydrogenation properties, similarly to the behaviour observed earlier for the nanocrystalline Mg2Ni-, FeTi- and LaNi5-type [11] alloys. Additionally, the nanocomposite structure improved the hydridingedehydriding kinetics, reduced hydriding temperature and enhanced hydrogen storage capacity of Mg-based materials.
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5.
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Conclusion
Nanostructured Mg2(Ni, Cu)-, LaNi5-, ZrV2-type alloys and Mg2Ni/LaNi5-, Mg2Ni/ZrV2-type composites have been synthesized using the mechanical alloying process. It was found that the elemental LaNi5 (ZrV2) phase was distributed on the surface of ball milled Mg2Ni alloy particles homogenously and role of these particles is to catalyse the dissociation of molecular hydrogen on the surface of studied alloy. Hydrogen storage capacities and the hydridingedehydriding kinetics of the nanocomposites have been measured using a Sieverts apparatus. Adding LaNi5 (ZrV2) in magnesiumbased alloy improves the hydrogen storage performance at room temperatures. The results show that the (Mg,Mn)2Ni (50 wt%)-La(Ni,Mn,Al,Co)5 (50 wt%) and (Mg,Mn)2Ni (75 wt%)(Zr,Ti)(V,Cr,Ni)2.4 (25 wt%) nanocomposites releases 1.65 wt% and 1.38 wt% hydrogen at 25 C, respectively. This is higher than in the microcrystalline LaNi5 (1.49 wt%) or ZrV2 (0 wt%) alloys. The nanocrystalline metal hydrides offer a breakthrough in prospects for practical applications.
Acknowledgements This work was supported by the Polish Ministry of Education and Science (Grant No. N N508 590739).
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[9] Conte M, Prosini PP, Passerini S. Overview of energy/ hydrogen storages: state-of-the-art of the technologies and prospects for nanomaterials. Mater Sci Eng B 2004;108:2e8. [10] Okonska I, Iwasieczko W, Jarzebski M, Nowak M, Jurczyk M. Hydrogenation properties of amorphous 2Mg þ Fe/x wt% Ni materials prepared by mechanical alloying (x¼0, 100, 200). Int J Hydrogen Energ 2007;32:4186e90. [11] Smardz L, Jurczyk M, Smardz K, Nowak M, Makowiecka M, Okonska I. Electronic structure of nanocrystalline and polycrystalline hydrogen storage materials. Renew Energ 2008;33:201e10. [12] Jankowska E, Makowiecka M, Jurczyk M. Electrochemical performance of sealed Ni-MH batteries using nanocrystalline TiNi-type hydride electrodes. Renew Energ 2008;33:211e5. [13] Huang LW, Elkedim O, Jarzebski M, Hamzaoui R, Jurczyk M. Structural characterization and electrochemical hydrogen storage properties of Mg2Ni1-xMnx (x ¼ 0, 0.125, 0.25, 0.375) alloys prepared by mechanical alloying. Int J Hydrogen Energ 2010;35:6794e803. [14] Beniamin JS. Mechanical alloying. Sci Am 1976;234:40e8. [15] Okonska I, Jurczyk M. Electrochemical properties of an amorphous 2Mg þ 3d alloys doped by nickel atoms (3d ¼ Fe, Co, Ni, Cu). J Alloys Compd 2009;475:289e93. [16] Koepernik K, Eschrig H. Full-potential nonorthogonal localorbital minimum-basis band-structure scheme. Phys Rev B 1999;59:1743e57. [17] Eschrig H. Optimized LCAO method and the electronic structure of extended systems. Berlin: Springer; 1989. [18] Koepernik K, Velicky B, Hayn R, Eschrig H. Self-consistent LCAO-CPA method for disordered alloys. Phys Rev B 1997;55: 5717e29. [19] Perdew JP, Wang Y. Accurate and simple analytic representation of the electron-gas correlation energy. Phys Rev B 1992;45:13244e9. [20] Szajek A, Jurczyk M, Smardz L, Okonska I, Jankowska E, Smardz L. Electrochemical and electronic properties of nanocrystalline Mg-based hydrogen storage materials. J Alloys Compd 2007;436:345e50. [21] Jezierski A, Jurczyk M, Szajek A. Electronic structure of Mg2Ni1-xCux. Acta Phys Pol A 2009;115:223e5. [22] Szajek A, Okonska I, Jurczyk M. Electronic and electrochemical properties of Mg2Ni alloy doped by Pd atoms. Mater Sci Pol 2007;25:1251e7. [23] Gurewitz E, Pinto H, Dariel M, Shaked H. Neutron diffraction study of LaNi4Co and LaNi4CoD4. J Phys F Met Phys 1983;13: 545e54. [24] Szajek A, Jurczyk M, Rajewski W. The electronic and electrochemical properties of the LaNi5, LaNi4Al and LaNi3AlCo systems. J Alloys Compd 2000;307:290e6. [25] Joubert JM, Latroche M, Percheron-Gue´gan A, Boure´eVigueron FB. Thermodynamic and structural comparison between two potential metal hydride battery materials LaNi3. 55Mn0.4Al0.3Co0.75 and CeNi3.55Mn0.4Al0.3Co0.75. J Alloys Compd 1998;275-277:118e22. [26] Szajek A, Jurczyk M, Rajewski W. The electronic and electrochemical properties of the ZrV2 and Zr(V0.75Ni0.25) systems. J Alloys Compd 2000;302:299e303. [27] Gupta M, Schlapbach L. Electronic properties. In: Schlapbach L, editor. Hydrogen in intermetallic compounds. Topics in Applied Physics, vol. 63. Berlin: Springer; 1988.
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Hydrogen storage by Mg-based nanocomposites M. Jurczyk a,*, M. Nowak a, A. Szajek b, A. Jezierski b a b
Institute of Materials Science and Engineering, Poznan University of Technology, Sklodowska-Curie 5 Sq., 60-965 Poznan, Poland Institute of Molecular Physics, Polish Academy of Sciences, Smoluchowskiego 17 St., 60-179 Poznan, Poland
article info
abstract
Article history:
Hydrogen storage materials research is entered to a new and exciting period with the
Received 7 February 2011
advance of the nanocrystalline alloys, which show substantially enhanced absorption/
Received in revised form
desorption kinetics, even at room temperatures. In this work, hydrogen storage capacities
28 March 2011
and the electrochemical discharge capacities of the Mg2(Ni, Cu)-, LaNi5-, ZrV2-type nano-
Accepted 2 April 2011
crystalline alloys and Mg2Ni/LaNi5-, Mg2Ni/ZrV2-type nanocomposites have been
Available online 27 April 2011
measured. The electronic properties of the Mg2Ni1-xCux, LaNi5 and ZrV2 alloys were calculated. The nanocomposite structure reduced hydriding temperature and enhanced
Keywords:
hydrogen storage capacity of Mg-based materials. The nanocomposites (Mg,Mn)2Ni (50 wt
Magnesium
%)-La(Ni,Mn,Al,Co)5 (50 wt%) and (Mg,Mn)2Ni (75 wt%)-(Zr,Ti)(V,Cr,Ni)2.4 (25 wt%) materials
Hydrogen storage
releases 1.65 wt% and 1.38 wt% hydrogen at 25 C, respectively. The strong modifications of
Nanocomposites
the electronic structure of the nanocrystalline alloys could significantly influence hydro-
Electronic structure
genation properties of Mg-based nanocomposities. Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Metal hydrides (MH) which reversibly absorb and desorb hydrogen at ambient temperature and pressure are regarded as important materials for solving energy and environmental issues. A large number of experimental investigations on LaNi5-, TiFe-, TiNi-, ZrV2-type compounds have been performed up to now in relation to their exceptional hydrogenation properties [1e5]. Magnesium-based hydrogen storage alloys have been also considered to be possible candidates for hydrogen storage as well as for electrodes in Ni-MHx batteries [6e8]. In recent years many other new developments have occurred in metal hydrides, from the introduction of new, non-conventional methods of fabrication, surface treatment, or by discoveries of new hydride phases [4,6,7]. Conventionally, the microcrystalline hydride materials have been prepared by arc or induction melting and annealing. However, either a low storage capacity by weight or poor absorption-desorption kinetics in addition to a complicated
activation procedure have limited the practical use of metal hydrides. Substantial improvements in the hydridingedehydriding properties of metal hydrides could be possibly achieved by the formation of nanocrystalline structures by non-equilibrium processing technique such as mechanical alloying (MA) [6e13]. This process consists of repeated fracture, mixing and cold welding of a fine blend of elemental particles, resulting in size reduction and chemical reactions [14]. As non-equilibrium processing method MA can be used to produce large quantities of materials at relatively low cost. Recently, amorphous 2Mg þ 3d/x wt% Ni materials were prepared by mechanical alloying (MA) of Mg and 3d elemental powders (3d ¼ Fe, Co, Ni, Cu; 0 x 200) under high purity argon atmosphere [10,15]. For all 2Mg þ 3d synthesized materials, mechanical alloying with nickel effectively reduced the degradation rate of the studied electrodes [15]. This result indicates that the nickel content does help to inhibit the corrosion of the alloy electrode in the KOH solution.
* Corresponding author. Tel.: þ48 61 665 3508; fax: þ48 61 665 3576. E-mail address: [email protected] (M. Jurczyk). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.04.012
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As a continuation of our previous studies, in this paper, hydrogen storage nano-scale Mg2(Ni, Cu)-, LaNi5-, ZrV2-type alloys and Mg2Ni/LaNi5-, Mg2Ni/ZrV2-type nanocomposites have been synthesized using the mechanical alloying process. The structure of the samples has been studied by X-ray diffraction (XRD). Hydrogen storage capacities and the electrochemical discharge capacities of the materials have been measured. The electronic properties of the Mg2Ni1-xCux, LaNi5 and ZrV2 alloys were calculated.
2.
Experimental procedure
Mechanical alloying was performed under an argon atmosphere using an SPEX 8000 Mixer Mill. The high purity elemental powders (Mg: 44 mm; Al: 75 mm; Ti: 44 mm; V: 44 mm; Cr: 1e5 mm; Mn: 45 mm; Co: 2 mm; Ni: 3e7 mm; Cu: 3 mm, Zr: 149 mm; La: 425 mm) were mixed and loaded into the vial in the glove box (Labmaster 130) containing an argon atmosphere (O2-2 ppm and H2O-1 ppm). The as-milled powders were heat treated (see text for details) under high purity argon, respectively, to form ordered phases. Additionally, the MA and annealed Mg2Ni-type hydrogen storage materials were mixed with different contents of LaNi5- or ZrV2-type powders and milled for 1 h in an SPEX Mixer Mill. Microcrystalline LaNi5-, ZrV2- and Mg2Ni-, Mg2Cu-type alloys were prepared by arc melting and by diffusion methods, respectively. The MA process of the synthesized materials has been examined by X-ray diffraction (XRD) and microstructural investigations. The particle sizes were estimated by Scherrer method. Independently, the change in the structure of powdered samples was observed using Atomic Force Microscope (Nanoscope IIIa - Digital Instruments, USA). These measurements were done in ambient atmosphere in contact mode using Si3N4 tips with a tip apex radius of 20e60 nm. The nanocrystalline materials with 10 wt% addition of Ni powder, were subjected to electrochemical measurements as working electrodes after pressing (under 80 kN cm2) to 0.5 g pellet form between nickel nets acting as current collector. The electrochemical properties of electrodes were measured in a three-compartment glass cell, using a much larger NiOOH/ Ni(OH)2 counter electrode and a Hg/HgO/6 M KOH reference electrode. All electrochemical measurements were carried out in deaerated 6 M KOH solution prepared from Analar grade KOH and 18 MU cm1 water, at 21 C. Potentiodynamic and galvanostatic techniques with either short or long-term pulses using a conventional apparatus were applied to study the chargeedischarge kinetics of the electrodes. Additionally, these mechanically alloyed materials, in nanocrystalline forms were subjected to thermodynamic measurements. The hydrogen absorption properties (P-C-T isotherms) were investigated using Sievert-type apparatus at temperatures in the range 293e673 K (Particulate Systems). Sample analysis data collection is fully automated to assure quality data and high reproducibility. The electronic properties of Mg2Ni1-xCux-, LaNi5- and ZrV2type alloys were calculated by the first principle scalar relativistic full-potential local-orbital method in the coherent potential approximation (FPLO-CPA) [16e18]. The calculations were performed for experimental values of lattice constants
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within the local density approximation [17] using the exchange correlation potential in the form of Perdew and Wang [19].
3.
Results
In this work, mechanical alloying has been used to make a nanocrystalline Mg2Ni-, Mg2Cu-, LaNi5- and ZrV2-type alloys (Fig. 1). The behaviour of MA processes has been studied by Xray diffraction, microstructural, thermodynamic and electrochemical investigations.
3.1.
Mg2Ni(Cu)-type system
The magnesium-nickel phase diagram shows two compounds Mg2Ni and MgNi2. The first one reacts with hydrogen slowly at room temperature to form the ternary hydride Mg2NiH4. At higher temperatures at pressure (e.g. 200 C, 1.4 MPa), the reaction is rapid enough for useful absorptionedesorption reactions to occur. Mechanical alloying is one of the approaches to produce MgeNi alloys which have been highly expected to be used as hydrogen storage materials [6]. Varin et al. [6] pointed out that ball milling which gives rise to the creation of fresh surfaces and cracks is highly effective for the kinetic improvement in initial hydriding properties. The
Fig. 1 e XRD spectra of nanocrystalline: (a) Mg2Ni-, (b) Mg1.5Mn0.5Ni, (c) Mg2Cu-, (d) LaNi5- and (e) ZrV2-type alloys produced by mechanical alloying followed by annealing (Mg2Ni, Mg1.5Mn0.5Ni: MA 30 h and heat treated at 450 C for 0.5 h; Mg2Cu: MA 18 h and head treated at 450 C for 0.5 h; LaNi3.75Mn0.75Al0.25Co0.25: MA 30 h and head treated at 700 C for 0.5 h; Zr0.35Ti0.65V0.85Cr0.26Ni1.30: MA for 25 h and heat treated at 800 C for 0.5 h).
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Fig. 2 e The discharge capacity as a function of cycle number for MA and annealed Mg2Cu (a), Mg2Ni (b), Mg1.5Mn0.5Ni (c), LaNi5 (d), LaNi3.75Mn0.75Al0.25Co0.25 (e) and Zr0.35Ti0.65V0.85Cr0.26Ni1.30 (f) (solution, 6 M KOH; T [ 20 C). The charge conditions were 40 mA gL1. The cut-off potential vs. Hg/HgO/6 M KOH was L0.7 V.
nanocrystalline Mg2Ni-type alloys were prepared by mechanical alloying followed by annealing. The powder mixture milled for more than 30 h has transformed directly to a hexagonaltype phase. Finally, the obtained powder was heat treated in high purity argon atmosphere at 450 C for 0.5 h (Fig. 1a). All diffraction peaks were assigned to those of the hexagonal ˚ , c ¼ 13.246 A ˚. crystal structure with cell parameters a ¼ 5.216 A The average size of amorphous 2Mg-Ni powders, according to AFM studies, was of the order of 30 nm. At room temperature, the nanocrystalline Mg2Ni alloy absorbs hydrogen, but almost does not desorb it. At temperatures above 250 C the kinetic of the absorptionedesorption
process improves considerably and for nanocrystalline Mg2Ni alloy the reaction with hydrogen is reversible. The hydrogen content in this material at 300 C is 3.25 wt%. The Mg2Ni electrode, mechanically alloyed and annealed, displayed the maximum discharge capacity (100 mA h g1) at the 1st cycle but degraded strongly with cycling (Fig. 2). The poor cyclic behaviour of Mg2Ni electrodes is attributed to the formation of Mg(OH)2 on the electrodes, which has been considered to arise from the charge-discharge cycles. To avoid the surface oxidation, we have examined the effect of magnesium substitution by Mn in Mg2Ni-type material. This alloying greatly improved the discharge capacities. In nanocrystalline Mg1.5Mn0.5Ni alloy discharge capacities up to 241 mA h g1 were measured. In the case of 2Mg-Cu powder mixture (0.433 wt% Mg þ 0.567 wt% Cu), the powder mixture milled for more than 18 h has transformed directly to an orthorhombic-type phase (Fig. 1b). Table 1 reports the cell parameters of the studied material. According to the Scherrer method for XRD profiles, the average size of 2Mg-Cu mechanically alloyed for 18 h powders was of the order of 30 nm; after heat treatment in high purity argon atmosphere at 450 C for 0.5 h the mean crystallite size of the nanocrystalline alloy estimated from AFM experiment was about 50 nm. At room temperature, the original nanocrystalline Mg2Cu alloy absorbs hydrogen, but almost does not desorb it. At temperatures above 250 C the kinetic of the absorptionedesorption process improves considerably and for nanocrystalline Mg2Cu alloy the reaction with hydrogen is reversible. Upon hydrogenation, Mg2Cu transforms into the hydride MgH2 þ MgCu2 phases. At 300 C the maximum absorption capacity reaches 2.25 wt% for pure nanocrystalline Mg2Cu alloy. This is lower than in the microcrystalline Mg2Cu alloy (2.6 wt%) because of a significant amount of strain, chemical disorder and defects introduced into the material during the mechanical alloying process. The Mg2Cu electrode, mechanically alloyed and annealed, displayed the maximum discharge capacity (26.5 mA h g1) at the 1st cycle but degraded strongly with cycling (see Table 1,
Table 1 e Structure type, lattice constants, hydrogen contents and discharge capacities for studied nanocrystalline and nanocomposite materials. Composition
Mg2Ni Mg2Cu Mg1.75Mn0.25Ni Mg1.5Mn0.5Ni LaNi5 LaNi3.75Mn0.75Al0.25Co0.25 50%Mg2Ni/50%LaNi5 50%Mg1.5Mn0.5Ni/50%LaNi3.75Mn0.75Al0.25Co0.25 ZrV2 Zr0.35Ti0.65V0.85Cr0.26Ni1.30 75% Mg1.5Mn0.5Ni/25% Zr0.35Ti0.65V0.85Cr0.26Ni1.30
Structure type
hexagonal orthorhombic hexagonal cubic hexagonal hexagonal composite composite cubic hexagonal composite
Lattice ˚] constants [A
a ¼ 5.216 c ¼ 13.246 a ¼ 9.119 b ¼ 18.343 c ¼ 5.271 a ¼ 5.185 c ¼ 13.097 a ¼ 3.137 a ¼ 5.010 c ¼ 3.972 a ¼ 5.075 c ¼ 4.039 e e a ¼ 7.501 a ¼ 4.921 c ¼ 8.011 e
Hydrogen content [wt%]
3.25 at 300 C 2.25 at 300 C 1.75 at 300 C 0.68 at 300 C 1.1 at 21 C 1.03 at 21 C 0 at 21 C 1.65 at 21 C 0 at 21 C 1.31 at 21 C 1.38 at 21 C
Discharge capacity at 20 C (mA h g1) 1st cycle
3rd cycle
100 26.5 148 241 84 258 e e 0 154 e
5 4.7 90 192 101 210 e e 0 160 e
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Fig. 2). The poor cyclic behaviour of Mg2Cu electrodes is attributed to the formation of Mg(OH)2 on the electrodes.
3.2.
Electronic structure of Mg2Ni1-xCux
Application of Mg-based alloys focused attention on the electronic structure of Mg2Ni1-xCux-type hydrogen storage materials. The theoretical band calculations were performed for ideal single-crystalline Mg2Ni and Mg2Cu alloys. The experimental X-ray photoelectron valence bands measured for MA nanocrystalline Mg2Cu alloy showed a significant broadening of the valence band measured for the MA nanocrystalline sample compared to that obtained for the microcrystalline Mg2Cu [20]. The reasons responsible for the band broadening of the nanocrystalline Mg2Cu alloy are probably associated with a strong deformation of the nanocrystals in the MA samples. The valence band of Mg2Ni1-xCux alloy is wider than for starting ordered Mg2Ni and Mg2Cu compounds (Fig. 3C). It is caused by different positions of Cu and Ni peaks in relation to the Fermi level (EF) (Fig. 3A and B). Additionally, the valence band is broadened by two kinds of disorder: (i) the chemical disorder because of random occupying Cu atoms in Ni sites and vice versa (it causes broadening of usually sharp peaks of ordered systems); (ii) the structural disorder: Mg2Ni and Mg2Cu compounds crystallize in two different structures and for Mg2Ni1-xCux alloy a structural transition takes place. The
Fig. 4 e Total DOS and site projected DOS plots for LaNi5 compound (upper panel) and for LaNi3.75Mn0.75Al0.25Co0.25 (lower panel) where impurities are distributed homogeneously in both sites La(Ni2.25Mn0.45Al0.15Co0.15)3g(Ni1.5Mn0.3Al0.1Co0.1)2c
Fig. 3 e Total density of states (DOS) and calculated photoemission spectra (total and site projected) for Mg2Cu (A) and Mg2Ni (B) compounds. Total DOS for Mg2Ni1-xCux (x [ 0.10, 0.15) with Cu atoms in the 3d site (C) and for Mg2Ni0.9Pd0.1, where Pd atoms are located in the 3b site. (EF [ 0).
Fig. 5 e The DOS plots for cubic ZrV2 and Zr(V0.75Ni0.25)2 systems and hexagonal Ti0.5Zr0.5(V0.27Mn0.27Cr0.13Ni0.33)2 one (Ni(1) and Ni(2)) mean Ni contributions from two sites.
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differences for micro- and nanocrystalline samples are visible in photoemission spectra (see Fig. 6 in Ref. [20]). The ab-initio calculations allow for calculating site preference of particular atoms, for instance, in Mg2Ni1xCux alloys (where x ¼ 0.10, 0.15) the position of Cu atoms in 3d sites is more stable, although the difference of the total energy between different configurations is small (for details on electronic structure modification by impurities in wider range of concentration see Ref. [21]). In the case of Mg2Ni0.9Pd0.1 the palladium atoms prefer 3b site [22].
In nanocrystalline LaNi3.75Mn0.75Al0.25Co0.25 discharge capacities up to 258 mA h g1 was measured. Cobalt in the alloy, played chemical role in the passivation of the alloy surface by the dissolution and precipitation (DP) process and thereby improved the electrode performance. Generally, alloying improves the alkali-resistance of Co, Mn and Al, controlling the rate of the dissolution in LaNi5-type electrode materials. At room temperature hydrogen content in LaNi3.75Mn0.75Al0.25Co0.25 is 1.03 wt%.
3.4. 3.3.
LaNi5-type system
The properties of hydrogen host LaNi5 materials can be modified substantially by alloying. In the transition metal sublattice of LaNi5-type compounds, substitution by Mn, Al and Co has been found to offer the best compromise between high hydrogen capacity and good resistance to corrosion [3]. The mechanically alloyed LaeNi powder mixture (0.322 wt % La þ 0.678 wt% Ni) milled for more than 30 h has transformed completely to the amorphous phase, without formation of another phase. Formation of the nanocrystalline alloy was achieved by annealing the amorphous material in high purity argon atmosphere at 700 C for 0.5h (Fig. 1c). All diffraction peaks were assigned to those of the hexagonal crystal structure of CaCu5-type (Table 1). The average size of amorphous LaeNi powders, according to AFM studies, was of the order of 25 nm. The discharge capacity of electrode prepared by application of MA and annealed LaNi5 alloy powder is low. It was found that alloying elements such as Al, Mn and Co substituting nickel greatly improved the cycle life of LaNi5type material (Fig. 2). With increasing cobalt content in LaNi4xMn0.75Al0.25Cox, the material shows an increase in discharge capacity which passes through a wide maximum for x ¼ 0.25.
Fig. 6 e XRD spectra of 50 wt% LaNi3,75Mn0,75Al0,25Co0,25/ 50 wt% Mg1,5Mn0,5Ni (a) and 25wt% Zr0.35Ti0.65V0.85Cr0.26Ni1.30/75 wt% Mg1.5Mn0.5Ni (b) nanocomposites after 1 h ball milling.
Electronic structure of LaNi5-type system
Binary LaNi5 crystallises with the CaCu5 structure type (space group P6/mmm) in which La occupies site 1(a) and Ni sites 2(c) and 3( g). The battery electrode material LaNi4Al is a substitutional derivative of LaNi5 in which La occupies site 1(a) and Ni and Al sites 2(c) and 3( g) of space group P6/mmm. Experimental results showed that the La sites do not accommodate Ni and Al atoms [23]. Furthermore, the ab-initio calculations [24] showed that the impurity aluminium atoms prefer the 3g positions in agreement with experimental data [25]. Similar situation takes place in the case of LaNi3AlCo but in more complicated systems LaNi3.75Mn0.75Al0.25Co0.25 3g site is no longer preferred by impurities. For this stoichiometry three configurations were considered: (i) impurities in 2c site La(NiNiNi)3g(Ni0.75Mn0.75Al0.25Co0.25)2c, impurities located in 3g site (ii) La(Ni1.75Mn0.75Al0.25Co0.25)3g(NiNi)2c, and finally impurities distributed homogeneously in both sites (ii) La(Ni2.25Mn0.45Al0.15Co0.15)3g(Ni1.5Mn0.3Al0.1Co0.1)2c (Fig. 4). Comparison of the total energies give the following rank: E(i)>E(ii)>E(iii). It concludes that last configuration is the most stable.
3.5.
ZrV2-type system
Materials obtained when Ti was substituted for Zr and Cr, Ni was substituted for V in ZrV2 lead to greatly improved activation behaviour of the electrodes. The ZreTieVeCreNi (16.3 wt% Zr þ 15.8 wt% Ti þ 22.1 wt% V þ 6.9 wt% Cr þ 38.9 wt % Ni) powder mixture milled for more than 25 h has transformed absolutely to the amorphous phase. It is worth noting that before amorphization no shift of the diffraction lines was observed. This result means that the amorphous phase forms directly from the starting mixture of the elements (Zr, Ti, V, Cr and Ni), without formation of other phases. Using the ZreTieVeCreNi mixture composition as the representative material example, the behaviour of the grain size of the crystallites has been studied during the mechanical alloying process. The Ni (111) diffraction line remains visible up to 15 h of milling. This allows an estimation of the change in the crystalline size of Ni with increase of the milling time. The crystallite size decreases strongly from 50 nm at the beginning of the mechanical alloying process. The final size of the crystallites, about 30 nm, seems to be favourable to the formation of an amorphous phase which develops at the Zr/ Ti/V/Cr/Ni interfaces. Formation of alloy with hexagonal C14 type structure was achieved by annealing the amorphous material in high purity argon atmosphere at 800 C for 0.5 h (Fig. 1d). The final diffraction pattern exhibits broadening of the peaks characteristic of nanocrystalline material.
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The electrode prepared with nanocrystalline Zr0.35 Ti0.65V0.85Cr0.26Ni1.30 material showed better activation and higher discharge capacities. This improvement is due to a wellestablished diffusion path for hydrogen atoms along the numerous grain boundaries. In the annealed nanocrystalline Zr0.35Ti0.65V0.85Cr0.26Ni1.30 powders discharging capacities up to 195 mA h g1 have been measured. The hydrogen content in room temperature in Zr0.35Ti0.65V0.85Cr0.26Ni1.30 is 1.31 wt%.
3.6.
Electronic structure of ZrV2-type system
Basic ZrV2-type alloy crystallises in the cubic C15-type structure but the MA procedure for more complicated system Ti0.5Zr0.5(V0.27Mn0.27Cr0.13Ni0.33)2 leads to hexagonal structure. Earlier electronic structure studies [26] showed that the Ni impurities cause a charge transfer from Zr and V to Ni, the valence band is wider and the density of electronic states at the Fermi level decreases by about 30%. Change of crystallographic structure and growing disorder introduced by additional impurities cause broadening of the valence band and reconstruction of the electronic structure (Fig. 5).
3.7. Nano-scale Mg2Ni/LaNi5 and Mg2Ni/ZrV2 composites The structural and thermodynamic properties on LaNi3,75Mn0,75Al0,25Co0,25/Mg1,5Mn0,5Ni and Zr0.35Ti0.65V0.85Cr0.26Ni1.30/ Mg1.5Mn0.5Ni hydrides materials was studied (Fig. 6). The results show that nanostructured 50 wt% Mg1.5Mn0.5Ni þ 50 wt% LaNi3.75Mn0.75Al0.25Co0.25 and 75 wt% Mg1.5Mn0.5Ni þ 25wt% Zr0.35Ti0.65V0.85Cr0.26Ni1.30 composite materials releases 1.65 and 1.38 wt% hydrogen at room temperature, respectively (Fig. 7 A, B). This is higher than in nanocrystalline Mg1.5Mn0.5Ni (0.68 wt%), LaNi3.75Mn0.75Al0.25Co0.25 (1.03 wt%), Zr0.35Ti0.65V0.85Cr0.26Ni1.30 (1.31 wt%). It is believed that the dehydriding temperature is largely controlled by the thermodynamic configuration of magnesium hydride.
4.
Discussion
The nanocrystalline metal hydrides offer a breakthrough in prospects for practical applications. Their excellent properties (significantly exceeding traditional hydrides) are a result of the combined engineering of many factors: alloy composition, surface properties, microstructure, grain size, and other. The chemical composition of the metal alloy is one of the most important factors in the metal - hydrogen system. The equilibrium conditions are reflected in the phase diagram, which dictates the respective phase composition under given temperature and pressure. Introducing metastable phases may result in totally different behaviour of the alloy. Present studies of potentially used materials for hydrogen storage alloys are not only focused on simple binary systems but on modified alloys which can improve their electrochemical properties. It was found that, the strong modifications of the electronic structure in the MA nanocrystalline Mg2(Ni, Cu)-, LaNi5-, ZrV2-type alloys could significantly influence hydrogenation properties of Mg-based nanocomposites.
Fig. 7 e A PC isotherms at 21 C for: (a) composite 50 wt% Mg2Ni/50 wt% LaNi5 (no hydrogen desorption) and (b) 50 wt % Mg1,5Mn0,5Ni/50 wt% LaNi3,75Mn0,75Al0,25Co0,25 (samples a and b: after 1 h ball milling). B PC isotherms at 21 C for: (a) microcrystallne Zr0.35Ti0.65V0.85Cr0.26Ni1.30 alloy and (b) nanocomposite 25 wt% Zr0.35Ti0.65V0.85Cr0.26Ni1.30/75 wt% Mg1.5Mn0.5Ni after 1 h ball milling (b).
The experimental and theoretical investigations concluded that the following effects are important for understanding the changes in the electronic structure of the host metal due to the hydrogen absorption [27]: (i) The change of lattice constants (usually expansion but in some cases contraction) often accompanied by a change of crystal structure leads to a modification of the symmetry of the states and the band widths. (ii) The attractive potential of proton influence on those metal wave functions which have a finite density at the hydrogen site and leads to the so-called metal-hydrogen bonding band below the metal d-band. Furthermore, some metal states located below the Fermi level (EF) in the pure metal are pulled below EF. (iii) The additional hydrogenehydrogen interactions in the hydrides which have more than one H atom per unit cell lead to new features in the lower part of the density of states. (iv) The presence of the additional electrons introduced by the hydrogen atoms results in a shift of the Fermi level. The experimental XPS valence bands measured for the Mgbased nanocomposites showed a significant broadening compared to those obtained for the microcrystalline thin films. The main reasons responsible for the band broadening of the nanocrystalline alloys and nanocomposites are probably associated with a strong deformation of the nanocrystals in the samples. The strong modifications of the electronic structure of the nanocomposites could significantly influence on their hydrogenation properties, similarly to the behaviour observed earlier for the nanocrystalline Mg2Ni-, FeTi- and LaNi5-type [11] alloys. Additionally, the nanocomposite structure improved the hydridingedehydriding kinetics, reduced hydriding temperature and enhanced hydrogen storage capacity of Mg-based materials.
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5.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 3 6 5 2 e3 6 5 8
Conclusion
Nanostructured Mg2(Ni, Cu)-, LaNi5-, ZrV2-type alloys and Mg2Ni/LaNi5-, Mg2Ni/ZrV2-type composites have been synthesized using the mechanical alloying process. It was found that the elemental LaNi5 (ZrV2) phase was distributed on the surface of ball milled Mg2Ni alloy particles homogenously and role of these particles is to catalyse the dissociation of molecular hydrogen on the surface of studied alloy. Hydrogen storage capacities and the hydridingedehydriding kinetics of the nanocomposites have been measured using a Sieverts apparatus. Adding LaNi5 (ZrV2) in magnesiumbased alloy improves the hydrogen storage performance at room temperatures. The results show that the (Mg,Mn)2Ni (50 wt%)-La(Ni,Mn,Al,Co)5 (50 wt%) and (Mg,Mn)2Ni (75 wt%)(Zr,Ti)(V,Cr,Ni)2.4 (25 wt%) nanocomposites releases 1.65 wt% and 1.38 wt% hydrogen at 25 C, respectively. This is higher than in the microcrystalline LaNi5 (1.49 wt%) or ZrV2 (0 wt%) alloys. The nanocrystalline metal hydrides offer a breakthrough in prospects for practical applications.
Acknowledgements This work was supported by the Polish Ministry of Education and Science (Grant No. N N508 590739).
references
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