Progress in Materials Science

Progress in Materials Science 58 (2013) 30 75 Contents lists available at SciVerse ScienceDirect Progress in Materials Science journal homepage: www.elsevier.com/locate/pmatsci Mechanochemical synthesis of hydrogen storage materials J. Huot a, D.B. Ravnsbæk b, J. Zhang c, F. Cuevas c,, M. Latroche c, T.R. Jensen b a Université du Québec à Trois-Rivières, 3351 des Forges, Trois-Rivières, Québec, Canada G9A 5H7 b Center for Materials Crystallography (CMC), Interdisciplinary Nanoscience Center (inano), Department of Chemistry, Aarhus University, Langelandsgade 140, DK-8000 Århus C, Denmark c ICMPE, CNRS, UMR 7182, 2-8 rue Henri Dunant, 94320 Thiais Cedex, France article info abstract Article history: Received 13 April 2012 Accepted 9 July 2012 Available online 27 July 2012 New synthesis methods are of utmost importance for most materials science research fields. The present review focuses on mechanochemical synthesis methods for solid hydrogen storage. We anticipate that the general methods and techniques are valuable with a range of other research fields, e.g. the rapidly expanding fields of energy materials science and green chemistry including solvent free synthesis. This review starts with a short historical reminder on mechanochemistry, followed by a general description of the experimental methods. The use of milling tools for tuning the microstructure of metals to modify their hydrogenation properties is discussed. A section is devoted to the direct synthesis of hydrogen storage materials by solid/gas reactions, i.e. by reactive ball milling of metallic constituents in hydrogen, diborane or ammonia atmosphere. Then, solid/solid mechano-chemical synthesis of hydrogen storage materials with a particular attention to alanates and borohydrides is surveyed. Finally, more specialised techniques such as solid/liquid based methods are mentioned along with the common characteristics of mechanochemistry as a way of synthesizing hydrogen storage materials. Ó 2012 Elsevier Ltd. All rights reserved. Contents 1. Introduction................. 31 2. Tuning of metal microstructures by mechanical milling................. 33 2.1. BCC alloys.... 33 Corresponding author. Tel.: +33 1 49 78 12 25; fax: +33 1 49 78 12 03. E-mail address: cuevas@icmpe.cnrs.fr (F. Cuevas). 0079-6425/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.pmatsci.2012.07.001

J. Huot et al. / Progress in Materials Science 58 (2013) 30 75 31 2.2. Ti-based................. 33 2.3. Mg-based BCC............ 36 2.4. Amorphization............ 38 3. Synthesis of hydrides by mechanically-induced solid/gas reactions................... 39 3.1. Binary hydrides........... 40 3.1.1. Magnesium hydride........... 41 3.1.2. Titanium hydride............. 42 3.1.3. Vanadium hydride............ 42 3.2. Ternary hydrides.......... 43 3.2.1. ZrNi hydride................. 43 3.2.2. TiNi hydride................. 43 3.2.3. TiFe hydride................. 43 3.2.4. LaNi 5 hydride................ 43 3.2.5. TiV hydride.................. 44 3.3. Mg-based complex hydrides................. 45 3.3.1. Mg 2 Fe hydride............... 45 3.3.2. Mg 2 Co hydride............... 48 3.3.3. Mg 2 Ni hydride............... 49 3.4. Alanates................. 51 3.4.1. Lithium alanates.............. 51 3.4.2. Sodium alanates.............. 52 3.4.3. Potassium alanates............ 53 3.4.4. Mixed alkali alanates.......... 53 3.4.5. Alkali-earth alanates........... 55 3.5. Synthesis of borohydrides by mechanical milling in diborane gas..... 56 3.6. Synthesis of metal amides by mechanical milling in ammonia gas..... 57 4. Synthesis of hydrides by mechanically-induced solid/solid and solid/liquid reactions.... 57 4.1. Mechanochemical synthesis of metal borohydrides................. 57 4.2. Synthesis of novel alane and metal alanates.... 61 4.3. Novel quaternary hydrides.................. 64 4.3.1. Metal borohydride amides...... 64 4.3.2. Metal alanate amides.......... 65 4.4. Solid liquid mechanically assisted synthesis... 65 5. Final remarks and conclusions.......... 66 Acknowledgments.................... 67 References....... 67 1. Introduction Synthesis of innovative materials for energy conversion and storage has received increasing focus during the past decades due to the world s increasing energy demands and simultaneous needs for environmentally friendly energy technologies. Hydrogen is recognized as a possible renewable energy carrier, but its large-scale utilization is mainly hampered by unsatisfactory properties of known hydrogen storage materials. Hence, preparation and characterization of novel materials are receiving significant attention as reviewed elsewhere [1 4]. Traditionally, hydrogen storage materials, such as metallic or complex hydrides, were prepared by solvent-based synthesis methods or by direct solid gas hydrogenation reactions. However, during the past decade mechanochemical synthesis has become one of the most utilized preparation methods for this class of materials, and is still expected to hold a significant unexplored potential for development of novel approaches, e.g. for green chemistry including solvent free synthesis methods. In this work, recent progress within the experimental methods for preparation of hydrogen storage materials is surveyed. In the mid-eighties, several research groups initiated the use of mechanical activation methods for the synthesis of hydrides [5,6]. Mechanical milling (MM) of mixtures of elements under inert gas atmosphere was used to synthesize intermetallic compounds, which were subsequently exposed to hydro-

32 J. Huot et al. / Progress in Materials Science 58 (2013) 30 75 gen in an external device to form hydrides. This two-step approach allowed modifying the microstructure of the alloy by milling to study its influence on the hydrogenation thermodynamics and kinetics [6 9]. Mechanical milling was very successful at improving hydrogenation kinetics via the synthesis of nanocrystalline materials and simultaneous incorporation of selective additives during the milling process [10 12]. Recently, it has been shown that Severe Plastic Deformation (SPD) techniques could be used for the synthesis and processing of metal alloys and their hydrides [13 26]. A few specific cases will be discussed in the forthcoming sections. A possible advantage of SPD techniques over conventional milling is easier scaling up to industrial level. However, some specific nanocrystalline structures and nanocomposites may only be synthesized through mechanical milling. In the early nineties, solid gas reaction facilitated by mechanical milling in reactive gases (nitrogen, oxygen and hydrogen) was investigated. This approach was initially designated Reactive Mechanical Milling (RMM) and used for preparation of hydrides in hydrogen atmosphere [27]. The experiments were performed in small-sized milling vials under moderate hydrogen pressures (below 2 MPa), often leading to an incomplete reaction between hydrogen and metals. The extent of the metal-hydrogen reaction was determined by ex situ XRD analysis of samples milled during a given period of time [28,29]. Modern devices for RMM synthesis are now equipped with pressure and temperature sensors that allow monitoring, e.g., hydrogen absorption during milling [30 32]. Hydrogenation reactions can be followed in situ as a function of milling time at working pressures up to ca. 15 MPa. Today, mechanochemical synthesis of metal hydrides using ball milling has grown to become one of the most frequently used methods. Typically, planetary ball mills are used, however other types such as rotational, vibratory or attritor mills are also operated [33]. The different types of mills differ in their milling efficiency and capacity and in some cases additional arrangements for cooling, heating, gas loading etc. can be applied. Typically a few grams of material and balls are placed in the planetary ball mill to give a ball-to-powder weight ratio of 10:1 50:1. This approach offers the advantage that the milling vial can be loaded, sealed and unloaded under inert conditions in a glove box, and, if equipped with valve connections, subsequently filled with reactive gas [30,31]. Thereby, the p,t phase space for mechanochemistry has expanded significantly. In some cases, especially for ductile materials, a process control agent (PCA) could be added to inhibit particle agglomeration [33]. The PCAs can be solids, liquids, or gases. A wide range of PCAs has been used in practice at a level of about 1 5 wt% of the total powder charge. The most common PCAs are stearic acid, hexane, methanol, ethanol, graphite and salts [33]. Several parameters can be varied for the ball-milling synthesis: milling speed, total milling time, vial and ball composition, powder-to-ball weight ratio, vial diameter, ball diameter and density, milling temperature, milling atmosphere and pressure of the selected gas. The latter two parameters require a special high-pressure vial. Most planetary mills only allow controlling the speed of the support disk. The speed of the planets, on which the milling vials are mounted, is usually fixed relatively to the speed of the main disk. However, for special mills, such as the Fritsch Vario-Planetary Mill Pulverisette 4, both the speed of the support disk and the planets can be varied freely [34]. Thereby, the trajectory of the balls within the vial may be controlled at least when the number of balls is low. Ideally the milling can be continuously changed from high-energy mode dominated by high-energy ball vial impacts to a grinding mode where the balls mainly follow the circumference of the vial [35]. The latter is also facilitated by a high number of balls in the vial. High-energy impacts tend to produce high mechanical pressure in the grain boundaries and in some cases make the high-pressure polymorph of the product. The grinding mode tends to produce more heat by friction and may in some cases lead to thermal decomposition of the product upon prolonged milling. Heating of the sample may be suppressed by using short periods of milling intervened by breaks where intrinsic heat produced in the grain boundaries can be dissipated and the sample can thermally equilibrate. Therefore, not only the total milling time is important for obtaining the desired compound, but breaks within the period of milling is in some cases crucial, which possibly also reduce agglomeration of the powder on the vial walls and balls [36 42]. Furthermore, the reactant mixture, balls and vial can be placed in a fridge or freezer prior to milling to lower the temperature further and/or the milling can be intervened by cooling of the vial. Milling at cryogenic conditions, i.e. at liquid nitrogen temperature (77 K), known as cryo-milling, has proven effective for preparation of some unstable metal hydrides [42].

J. Huot et al. / Progress in Materials Science 58 (2013) 30 75 33 Within the past two decades, mechanochemistry has expanded widely both within the experimental methods and techniques but also within the variety of materials that can be prepared, e.g. binary and ternary metallic hydrides [43 47], or complex hydrides such as Mg-based transition metal hydrides [48 52], alanates [30,53 57], borohydrides [58], amides [59,60], and multi-component systems [61 64] including the broadly studied Reactive Hydride Composites (RHC) [65 68]. These topics are the focus for further discussion in this review. 2. Tuning of metal microstructures by mechanical milling The use of mechano-chemical methods for the synthesis and modification of hydrogen storage materials has generated an enormous amount of reports. In the last 10 years about a thousand papers have been published on the use of ball milling and mechanical alloying for this specific application. Therefore, this review focus on general aspects by discussion selected details and this section focus on the use of mechanochemical methods to tune the microstructure of metal hydride systems in order to improve their hydrogenation properties. 2.1. BCC alloys A body-centered cubic (BCC) structure is a coarse packing structure and has more interstitial space than face-centered cubic (FCC) and hexagonal close-packed (HCP) structures [69]. Thus, BCC alloys are more attractive candidates to be explored as possible interstitial hydrogen storage materials. Usually, BCC alloys are synthesized by arc melting or induction melting. However, for some alloys the desired composition is difficult to obtain by using these techniques because the constituting elements may have quite different melting temperatures. With mechanical alloying there is in principle no limitation on the nature and number of the raw elements used. For hydrogen storage applications one could distinguish two broad classes of BCC alloys: Ti-based and Mg-based. Each of these classes is discussed below. 2.2. Ti-based BCC alloys of systems Ti V Mn and Ti V Cr have been intensively studied for hydrogen storage [70 75]. This class of alloys may also catalyse hydrogen release and uptake in magnesium [76]. Fig. 1. X-ray powder diffraction pattern of arc-melted TiV 0.9 Mn 1.1 as a function of milling time (Cu Ka radiation) [77].

34 J. Huot et al. / Progress in Materials Science 58 (2013) 30 75 Moderate hydrogen capacities as high as 3.6 wt% have been reported for Ti 25 V 40 Cr 35 alloy, which also possess prolific kinetic and thermodynamic properties. The effect of Severe Plastic Deformation (SPD) on BCC Ti 22Al 27Nb alloy has been investigated by Zhang et al. [25,26]. They showed that the first hydrogenation (activation) was much faster for the deformed alloy compared to the as-quenched sample. The deformed alloy also had faster absorption/ desorption kinetics. However, the beneficial effect of deformation was lost after a few hydrogenation cycles. In these studies, SPD was obtained by cold rolling or compression. Cold rolling is a process by which a sheet metal or powder is introduced between rollers and then compressed and squeezed. In the case of cold rolling one rolling was performed at 10.5% and 80% thickness reduction. Some of the 80% rolled specimen were further rolled to 10% thickness reduction in a perpendicular direction with respect to the first rolling. Huot et al. have made a systematic study of the effect of milling on TiV 0.9 Mn 1.1 alloy [77]. This composition is interesting to study because the as-cast alloy is a mixture of BCC and C14 phases. Therefore, it is a good system to test the effect of milling on the crystalline change and the interaction between phases. Milling was performed on as-cast alloy as well as on mixtures of elemental powders. Fig. 1 shows the effect of milling on as-cast TiV 0.9 Mn 1.1. The presence of NaCl Bragg peaks is explained by the use of a small amount of this salt as an antisticking PCA. It is clear that, with milling time, the C14 phase vanishes and a FCC phase appears. From Rietveld refinement it was found that, for the sample milled 80 h, the crystal structure is a mixture of cubic (FCC) solid solution phase and a BCC solid solution. The coexistence of FCC and BCC structures was also observed for the system Fe Cu and may be due to an enhanced solubility due to the high dislocation density [78]. When milling was performed on the raw elements (Ti, V, and Mn), an identical result was obtained, i.e. formation of a nanocrystalline alloy composed of BCC and FCC phases [77]. The BCC alloys need activation, e.g. by cycling hydrogen release and uptake between p(h 2 ) = 5 MPa and vacuum at elevated temperature of 523 K. In Fig. 2 the hydrogen absorption and desorption isotherm (296 K) for arc-melted TiV 0.9 Mn 1.1 before and after 80 h of milling is presented. The maximum capacity of the as-melted alloy is 1.9 wt% at 7 MPa which corresponds to an H/M ratio of 0.97. After 80 h of milling, the alloy does not absorb hydrogen up to 7 MPa. Because the as-milled materials present both FCC and BCC phases this means that none of them absorbs hydrogen. In the case of BCC phase the reason may be reduction of lattice parameters. Iron contamination (even at this low level) may also play a role as shown by Santos et al. in the Ti V Cr system [79]. Fig. 2. Pressure composition temperature (PCT) curve, at 313 K, of arc-melted TiV 0.9 Mn 1.1 alloy before and after 80 h of milling [77].

J. Huot et al. / Progress in Materials Science 58 (2013) 30 75 35 Fig. 3. TEM micrographs of TiV 1.6 Mn 0.4 after arc melting (top), after ball milled for 5 h (middle), and after 150 cold rolls (bottom). Micrographs on the left are bright field images and micrographs on the right are dark field images [82]. Singh et al. studied the effect of milling an arc-melted Ti 0.32 Cr 0.43 V 0.25 alloy [80]. As they used tungsten carbide balls, some contamination was observed after long milling time. Ball milling did not affect the crystal structure of the alloy. Increase of ball milling time resulted in the increase in lattice strain and the decrease in crystallite size, which in turn increased sub-grain boundaries. Contamination from milling tools and microstructural changes caused an important decrease in the hydrogen storage capacity [80].

36 J. Huot et al. / Progress in Materials Science 58 (2013) 30 75 Fig. 4. X-ray powder diffraction patterns of as-cast, milled 5 h and cold rolled 150 times TiV 1.6 Mn 0.4 alloy (Cu Ka radiation) [82]. Amira et al. compared the effect of ball milling and cold rolling for Ti Cr system [81]. Unlike ball milling, cold rolling of TiCr x (x = 2, 1.8, 1.5) did not lead to the formation of metastable BCC phase. However, cold rolling was found to be effective to form nanocrystalline C14 Laves phase. Hydrogen sorption experiments showed that cold-rolled alloys have similar hydrogen sorption properties to ball-milled alloys despite different crystal structures. The alloy TiV 1.6 Mn 0.4 has been recently investigated by Couillaud et al. [82]. The effect of extended cold rolling as well as high energy ball milling was a reduction of crystalline size and lattice parameter but no change in the crystal structure. Fig. 3 shows TEM micrographs of TiV 1.6 Mn 0.4 alloy in the ascast, milled, and cold-rolled states. The dark field image shows that all samples are nanocrystalline. The bright field image of the coldrolled sample clearly shows the pile-up of dislocations. The dark field image shows that, contrary to the as-cast and ball-milled samples, the crystallites tend to be aligned along dislocations. Fig. 4 shows the X-ray diffraction patterns of as-cast, milled 5 h and cold rolled 150 times TiV 1.6 Mn 0.4 alloy. From these X-ray powder diffraction patterns it was determined that the crystallite sizes of as-cast, milled, and cold-rolled samples are respectively 17, 11, and 13 nm [82]. Apart from peak broadening due to the reduction of crystallite size, the pattern of the ball-milled sample has the same relative intensities and lattice parameter as the as-cast sample. For the cold-rolled sample, the lattice parameter is also the same as the as-cast sample but there is a very strong texture along (200), which is a common feature of cold-rolled samples. Neither the ball-milled sample nor the cold-rolled samples absorb hydrogen even after 10 cycles of hydrogen pressurization (10 MPa) and vacuum at 423 K. The reason for this significant loss of hydrogen capacity is still not clear. 2.3. Mg-based BCC Recently, Mg-based BCC alloys have been explored in order to achieve higher gravimetric hydrogen storage capacity. In particular, Akiba s group has made an extensive study of the synthesis of Mg Ti [83,84], Mg Co [85 87], and Mg Ni [88,89] BCC alloys by means of ball milling. In this review we will limit our discussion to the Mg Ti system. Binary Mg Ti alloys are being intensively investigated for various applications such as: negative electrodes for Ni MH batteries [90,91], H 2 sources for fuel cells [84,92], switchable mirrors for smart solar collectors [93,94], and optical hydrogen detectors [95]. In the Mg Ti phase diagram equilibrium solid solubility of each metal in each other is less than 2 at.% and no intermetallic compound is found. Therefore, non-conventional synthesis methods based on melting or sintering can be used. Metastable single-phase Mg Ti thin films have been successfully synthesized over a large compositional range by

J. Huot et al. / Progress in Materials Science 58 (2013) 30 75 37 Fig. 5. The powder X-ray diffraction pattern of a magnesium titanium Mg 50 Ti 50 mixture milled for 150 h (Cu Ka radiation) [84]. means of electron-beam and magnetron co-sputter deposition techniques [90,93,94,96,97]. However, these techniques could not be scaled-up to industrial level and other methods have to be investigated. Mechanical alloying has demonstrated its high efficiency for producing metastable Mg Ti alloys starting from elemental Mg and Ti powders [83,91,98 101]. The synthesis of Mg Ti BCC alloys by mechanical alloying has been extensively studied by Asano et al. [83,84,92,100,101]. Although both Mg and Ti have a hexagonal closed packed (HCP) structure, during milling of a Mg and Ti mixture they react differently. In the case of magnesium, the deformation is mainly by basal plane slip {0001}h 12 10i while for titanium twinning deformation is more important [100]. In one investigation, Asano et al. had the idea of adding lithium to magnesium in order to reduce the yield stress of magnesium and also to decrease its lattice parameter [100]. They found that by adding Li to Mg, the deformation of Mg was easier and the Ti crystallite size was reduced. This led to a decrease of synthesis time for BCC phase formation. In a subsequent study, they first synthesized a BCC Mg 50 Ti 50 alloy by ball milling a mixture of 50Mg + 50Ti in a Fritsch P5 planetary ball mill for 150 h at a rotation speed of 200 rpm. Fig. 5 confirms that a BCC phase was obtained, and from the peaks width a crystallite size of 3 nm was determined [84]. A full hydrogenation at 423 K under 8 MPa of hydrogen and for 122 h resulted in the formation of Mg 42 Ti 58 H 177 FCC hydride phase and some MgH 2. By controlling milling conditions and Mg:Ti ratio, Asano et al. have also shown that BCC, FCC or HCP phase could be obtained in the Mg Ti system [83]. In the case of HCP phase it is formed by solution of Ti into Mg while the BCC phase is produced by solution of Mg into Ti and the FCC phase is stabilized by introduction of stacking faults in Mg and Ti which have a HCP structure [83,84]. If MgH 2 is used instead of Mg as the starting material then, after ball milling a 50MgH 2 + 50Ti mixture, the resulting compound is FCC Mg 33 Ti 50 H 94 plus some MgH 2 [92]. The importance of mechanical effect during milling is discussed in ref [101]. It shows that during ball milling of Mg and Ti powders in molar ratio of 1:1, plate-like particles first stuck on the surface of the milling pot and balls. After these plate-like particles fell off from the surface of the milling pot and balls, spherical particles, in which concentric layers of Mg and Ti are disposed, are formed. These particles have an average diameter of 1 mm. These spherical particles are then crushed into spherical particles with a diameter of around 10 lm by introduction of cracks along the boundaries between Mg and Ti layers. Finally, the Mg 50 Ti 50 BCC phase with a lattice parameter of 0.342(1) nm and a grain size of 3 nm is formed. During milling, Ti acts as an abrasive for Mg which had stuck on the surface of the milling pot and balls [101]. Recently, Çakmak et al. showed that mechanical milling of Mg 10 vol% Ti yields large Mg agglomerate, 90 100 lm, with embedded Ti fragments of about 1 lm uniformly distributed within the agglomerates [102]. These Mg agglomerates are made of coherently diffracting volumes (crystallites) of small size. Crystallite size, as determined with X-ray diffraction analysis, can be as small as 26 nm after 30 h of milling.

38 J. Huot et al. / Progress in Materials Science 58 (2013) 30 75 In an investigation of high-energy milling of 50Mg 50Ti mixture, Maweja et al. observed twinning in Ti-rich crystallites at intermediate milling time [103]. They attributed the twinning to the deformation of Ti particles. But they also pointed out that in the Mg Ti system it might also indicate a straininduced martensitic transformation of the metastable x-fcc into BCC. The crystallite boundaries acted as preferential sites for the heterogeneous nucleation of the twins and for the formation of solid solution by release of the lattice strain energy [103]. For electrochemical applications, mechanical alloyed Mg Ti materials must be activated by adding of few at.% of Pd. Rousselot et al. have shown that if a 50Mg 50Ti mixture is pre-milled before adding Pd then the alloying of Pd with pre-milled Mg 50 Ti 50 occurs very rapidly (few minutes) and is complete after 5 h of milling [104]. They also found that the crystalline structure of the Mg 50 Ti 50 alloy (BCC and HCP Mg Ti phase mixture) does not change significantly with the addition of Pd. 2.4. Amorphization In amorphization under mechanical driving forces, two categories of alloys could be defined [105]. The first category consists of intermetallic compounds induced to undergo polymorphic crystal-toamorphous transformation by deformation. In this process the introduction of defects by milling increases the free energy of the equilibrium alloy such that it goes above the free energy of the amorphous state. Thus, the amorphous phase becomes the lowest free energy state and the alloy becomes amorphous. The second category of amorphous alloys contains those formed by intermixing of individual elements that have a negative heat of mixing. In this case, deformation plays the role of enhancing such energy-lowering reactions through deformation-enhanced interdiffusion [105]. From a systematic study of Mg Ni system, Rojas et al. proposed the sequence of phase transformations during milling leading to amorphization as [106]: c-mg þ c-ni! nc-mg þ c-ni! amorphous þ nc-ni þ nc-mg! amorphous þ nc-ni þ nc-mg 2 Ni! nc-mg 2 Ni where c and nc denotes crystalline and nanocrystalline state, respectively. The first step shows the fact that grain refinement in nickel is slower than in magnesium. It has been demonstrated that the grain size attainable by milling depends on the crystal structure of the material being milled [107]. Usually, BCC materials tend to reach the smallest sizes, HCP materials somewhat larger grain sizes, and FCC materials tend to produce the largest grain sizes. Since the crystal structure of Mg is HCP and that of Ni is FCC, such a difference in the grain size after milling is expected. For Mg Ni system, amorphous phase could be prepared by ball milling in less than 10 h [108]. According to Varin et al., the presence of hard MgNi 2 phase helps to reduce crystallite size of Mg 2 Ni phase and thus facilitates amorphization [109,110]. For some compositions and milling parameters, a crystallization amorphous crystallization phenomenon could appear. One example of this was given by El-Eskandarany et al. for Co 75 Ti 25 [111]. A solid-state reaction took place during milling elemental Co and Ti powders and an amorphous phase of Co 75 Ti 25 was formed after 3 h. They showed that this amorphous phase crystallized into an ordered FCC-Co 3 Ti phase upon heating to 880 K. Further milling to 24 h also leads to crystallization and the formed phase was a metastable BCC-Co 3 Ti nanocrystalline phase. They attributed this transformation taking place in the ball mill to the inability of the formed amorphous phase to withstand the impact and shear forces that are generated by the milling media. When the milling time was further increased to 100 h, the crystalline phase was subjected to several points and lattice defects that raised the free energy from the stable BCC-Co 3 Ti phase to an amorphous less stable phase. In this case, the crystalline amorphous transformation which took place was similar to the mechanical grinding method in which the amorphization occurs by relaxing the short-range order without any compositional changes. Further milling leads to the formation of crystalline and/or amorphous phases depending on the milling time. Contamination from milling tools and temperature effect were ruled out as origin of this phenomenon [111]. Fig. 6 shows a schematic illustration of this crystallization amorphization crystallization process.

J. Huot et al. / Progress in Materials Science 58 (2013) 30 75 39 Fig. 6. Schematic illustration of amorphous crystalline amorphous cyclic phase transformations that took place during ballmilling elemental powders of Co 75 Ti 25, using a rotation speed of 4.2 s 1 [111]. 3. Synthesis of hydrides by mechanically-induced solid/gas reactions Mechanical milling of metal powders under reactive gas, i.e. Reactive Mechanical Milling (RMM), is becoming a mature and powerful technique for the synthesis of metallic and complex hydrides. The mechanical treatment induces a chemical reaction between the solids and the gas. The synthesis of several metallic and complex hydrides by RMM is surveyed here. RMM under hydrogen gas allows for the synthesis of binary and ternary metal hydrides, Mg-based complex hydrides and alanates. More recently, this technique has been extended to other reactive gases such as diborane and ammonia for the synthesis of borohydrides and metal amides, respectively. Some particular phenomena such as ultra-fast hydride synthesis, reactive-milling induced amorphization, and multi-step reactions are reported. The obtained hydrides are typically nanocrystalline materials leading to fast kinetic for hydrogen release and uptake reactions useful for hydrogen storage applications. Significant progress on the understanding of RMM process has been provided by the in situ monitoring of the hydrogenation reaction during milling. In 2000, Dunlap et al. connected a ball-milling device to a large hydrogen reservoir by means of a rubber tube [112]. They could follow the hydrogen uptake as a function of milling time for several early transition metals (Ti, Zr, Hf, V, Nb and Ta). Experiments were conducted near atmospheric pressure (p(h 2 ) 0.1 MPa). A similar method was used by Bellosta et al. to monitor hydrogen release during ball-milling of sodium tetra-alanate with TiCl 3 additive [113]. Hydrogen release occurs due to titanium reduction to the zero-valent state on milling. A further improvement was reached by using telemetric systems instead of mechanical connections to in situ register both hydrogen pressure and vial temperature during milling [30,31]. The sensors were mounted on the lid of a stainless steel vial which was able to withstand high pressure

40 J. Huot et al. / Progress in Materials Science 58 (2013) 30 75 (10 MPa). Under these conditions, the one-step direct synthesis of Ti-doped NaAlH 4 using NaH, Al and TiCl 3 as starting powders could be monitored. RMM is generally accomplished in tight stainless steel vials equipped with a connection valve for vacuuming and hydrogen filling. The vial is then placed in a milling device to promote the mechanochemical reaction leading to the hydride formation. For practical reasons regarding p T gauges attachment to vials, most of current milling devices used for RMM are planetary ball mills. Thus, this preparation technique is widely named as reactive ball milling. Nonetheless, also shaker, attritor, and vibration mills have been used successfully [51,112,114,115]. Today, compound synthesis can be anticipated from the in situ monitoring of the hydrogenation reaction and subsequently verified by ex situ crystallographic and chemical analyses. Furthermore, if thermodynamic parameters such as vial volume and gas pressure and temperature are accurately known, the quantity of absorbed hydrogen as a function of time can be reliably obtained. Zhang et al. have recently shown that hydrogen uptake can be determined with an accuracy of 95% [116]. In situ monitoring of changes in gas pressure is certainly a powerful tool for the study of hydride formation kinetics and reaction mechanisms on reactive milling. 3.1. Binary hydrides Though thermodynamically favourable, the formation of AH x binary hydrides by solid gas reaction between hydrogen gas and a metal (A, a metal with strong affinity for hydrogen, here stands for either alkaline earths (Mg) or early transition metals such as Ti and V) is very often hindered by kinetic barriers related to the presence of native oxide layers at the metal surface. Then, severe treatments at high temperature (typically above 700 K) and high pressure (several MPa) are needed in conventional gas-phase hydrogenation for activation. In the course of these treatments, oxygen at the surface might react with the bulk material leading to additional impurities. Such surface limitation can be overcome by RMM of pure metals in hydrogen atmosphere that allows achieving faster synthesis reactions under more moderate conditions. Synthesis conditions by RMM under hydrogen gas of representative binary hydrides are summarized in Table 1. Table 1 Representative binary and ternary metal hydrides synthesised by RMM under hydrogen gas. The employed device, reactants, initial hydrogen pressure, p(h 2 ), total milling time (tmt), milling speed (ms), ball-to-powder weight ratio (BTPWR) and ball diameter (Bd) are given. Compound Device Reactants p(h 2 ) (MPa) tmt (h) ms (rpm) BTPWR Bd (mm) Ref. MgH 2 Planetary Mg 0.34 25 12 [117] MgH 2 Mg + graphite 0.4 1 10:1 [118] MgH 2 Fritsch P6 a Mg 1 9 8 500 10:1 10 [31] MgH 2 Fritsch P4 a Mg 8 2 400 60:1 12 [64] TiH 1.9 Planetary Ti 0.34 5.5 12 [117] TiH 2 Fritsch P4 a Ti 8 0.16 400 60:1 12 [64] VH x Fritsch P5 V 1 0.17 400 30:1 7 [47] ZrNiH 3 Fritsch P5 ZrNi 2 3 30:1 10 [27] ZrH 2 + Ni Fritsch P5 Zr + Ni 2 3 30:1 10 [27] ZrH 2 + NiZr y H x Fritsch P5 Zr + Ni 2 100 30:1 10 [27] b-zrnih Fritsch P7 ZrNi 0.1 0.08 400 30:1 7 [28] c-zrnih 3 Fritsch P7 ZrNi 1 0.08 400 30:1 7 [28] TiNiH 3 Rod-mill Ti + Ni 0.1 200 30:1 10 [119] TiH 2 + Ni Ti + Ni 1.1 40 250 10:1 10 [120] TiH 2 + Fe Spex 8000 TiFe 0.5 7 8:1 6, 12 [114] a-lani 5 H 0.15 Fritsch P7 LaNi 5 1 0.08 400 30:1 7 [121] amph-lani 5 y H x Fritsch P7 LaNi 5 1 10 400 30:1 7 [121] BCC TiVH 0.9 Fritsch P5 TiV or Ti + V 0.2 100 20:1 10 [43] TiVH 2.8 Fritsch P5 TiV or Ti + V 0.4 100 20:1 10 [43] FCC TiVH 4.7 Fritsch P5 TiV or Ti + V 1 100 20:1 10 [43] Ti 0.20 V 0.78 Fe 0.02 H 2 Fritsch P4 * Ti 0.20 V 0.78 Fe 0.02 8 0.17 400 100:1 12 [122] a Pressure and temperature measured in situ in the Evico-magnetics vial.

J. Huot et al. / Progress in Materials Science 58 (2013) 30 75 41 3.1.1. Magnesium hydride Magnesium hydride is classically prepared by reaction with hydrogen gas and Mg powder at temperatures around 700 K and hydrogen pressures in the range 7 8 MPa for several hours. However, even under these conditions, the presence of Mg is often detected by XRD as the reaction is not completed [123]. Indeed, a shell of magnesium hydride is reported to form at the surface of micrometer-sized magnesium grains, blocking further hydrogenation of the remaining metal core [124,125]. Consequently, the hydrogenation rate of bulk magnesium is slow. First attempts to form magnesium hydride by RMM were conducted by Chen and Williams using p(h 2 ) = 0.34 MPa and a vertical planetary mill [117]. Complete formation of MgH 2 hydride is reported to occur after long milling time (25 h). Later, similar experiments under 0.5 1 MPa of hydrogen pressure were conducted by different groups [46,126,127]. Neither of them could attain hydride formation above 50 wt%, which was attributed to kinetic effects. This was finally overcome by performing RMM experiments at high temperature (573 K) with the addition of graphite to obtain complete hydrogenation within 1 h [118]. Graphite could act as a PCA to reduce particle agglomeration by cold-welding [33]. Doppiu et al. have however shown that fast MgH 2 formation can also be achieved near room temperature using high-pressure reactive ball milling [31]. The hydrogenation reaction could be monitored by in situ measurements of both pressure, p, and temperature, T, inside the vial during milling by using on-board sensors and radio transmitted data. Syntheses were done with pure Mg powders ball milled with a ball-to-powder weight ratio of 10:1 at 500 rpm and hydrogen pressures of 1, 4 and 9 MPa. From the data collection, it was first observed that the temperature of the vial increases up to 318 K mainly due to mechanical action. Mg absorbs hydrogen in less than 8 h for pressures larger than 4 MPa. Reaction rate was significantly slower for lower pressure (1 MPa). Moreover, a nucleation time, strongly dependant of the pressure is also reported; almost undetectable at 9 MPa, it reaches more than 2 h at 1 MPa. Further investigations by XRD at different milling times show the formation of the metastable orthorhombic c-phase along with the tetragonal b-mgh 2 one. The c-phase can also be achieved by ball milling of magnesium hydride [128]. With increasing milling time, the crystallite size decreases to finally stabilizing at 10 nm. Same amounts of hydride phases (>95 wt% for c + b) and identical crystallite sizes are obtained after 18 h of milling whatever the initial pressure though the rate of formation and the size reduction were faster for higher pressures. This was interpreted on the basis of two different factors. Higher pressures promote a more rapid formation of the hydride that is in turn known to exhibit higher plastic deformation. Then, for higher amounts of MgH 2, the mechanical action is more effective than for ductile Mg. However, it is worth noting that at long milling times, all samples reach the same chemical and microstructural states. Very similar results are also reported by Doppiu et al. who performed reactive milling of an elemental Mg 87 Ni 10 Al 3 powder mixture under hydrogen atmosphere [129]. Milling induces the synthesis of nanocrystalline MgH 2 at the first stage followed by the formation Mg 2 NiH 4 when a high degree of conversion of Mg in the hydride form was reached. A minimum value for the crystallite size of 8 nm P (MPa) 9.0 8.5 8.0 7.5 7.0 6.5 6.0 5.5 (A) 320 316 312 308 304 5.0 300 0 60 120 180 t (min) T (K) H/Mg 2.0 1.5 1.0 0.5 (B) 0.0 0 60 120 180 t (min) Fig. 7. Evolution of the pressure and temperature (A) during RMM of magnesium in hydrogen gas and the calculated H/Mg ratio in the solid state (B) as a function of time [64].

42 J. Huot et al. / Progress in Materials Science 58 (2013) 30 75 was obtained. Small differences in the hydride stability were observed at different milling times. In spite of the oxidation of the sample, fast absorption desorption kinetics were obtained. A typical example of the evolution of the pressure, the temperature and the H concentration as a function of time during RMM of Mg is shown in Fig. 7. The starting material was coarse magnesium powder to reduce the amount of MgO that may develop at the grain surface of powder material. The milling vial was loaded with p(h 2 ) = 8 MPa and operated at 400 rpm. After initial heating due to ball friction, the pressure drop related to hydride formation is observed and the reaction is completed after 2 h. The final H/M value reaches 1.9, a value 5% smaller than expected for MgH 2. XRD analysis shows that the final product is made of 76 wt% of b-mgh 2, 21 wt% of c-mgh 2, and 3 wt% of MgO. The mean crystallite size for the hydride phases is close to 6 nm, in good agreement with previous results. 3.1.2. Titanium hydride The formation of titanium hydride by RMM with composition TiH l.9 was first reported by Chen and Williams in 1995 using a hydrogen filled container p(h 2 ) = 0.34 MPa for 67 h [117]. The hydrogenation reaction was completed in 5.5 h and the TiH 1.9 compound was stable during prolonged milling, with only a reduction of particle size being observed. Very similar results have been published by different groups [44 46]. Dunlap s group has extended this method to other early transition metals such as Zr, Hf, Ta, Nb and V [112]. Short reaction time was explained in terms of clean surface generation and severe reduction of diffusion path. Titanium powder is initially passivated by the presence of surface oxides. Upon milling, fresh and highly reactive surfaces are created promoting the formation of near-surface hydride precipitates. This causes hydrogen embrittlement of the metal, enhances its pulverisation and results in a shorter diffusion path for hydrogen absorption. The formation of hydride TiH 2 during RMM is shown in Fig. 8 [64]. Contrary to magnesium, no nucleation time is observed and the hydride formation proceeds readily and is completed after 10 min. XRD analysis confirms the formation of TiH 2 though small contamination with iron is observed, most probably due to the stainless steel vial abrasion during milling. The mean particle size determined from the diffraction peak widths is around 7 nm. 3.1.3. Vanadium hydride Orimo et al. reported on the preparation of nanostructured VH x prepared by mechanical milling under H 2 atmosphere [47]. Formation of the b 2 phase was observed after 5 min of milling at room temperature whereas conventional gas-phase hydrogenation would need activation treatments under 600 K and 3 MPa. The grain size of the b 2 phase decreases from 80 nm at 5 min milling time to 10 nm after 60 min. Additional milling time (up to 300 min) does not lead to further decrease of the grain size nor the formation of an amorphous state as the b 2 phase remains in crystalline state. From the relationship between hydrogen concentration and unit cell, the hydrogen concentration 8.0 320 2.0 P (MPa) 7.5 7.0 6.5 (A) 315 310 305 300 295 T (K) 6.0 290 0.0 0 60 120 180 240 0 60 120 180 240 t (min) t (min) Fig. 8. Evolution of the pressure, the temperature (A) and the H concentration (B) as a function of time during RMM of titanium in hydrogen gas. H/Ti 1.5 1.0 0.5 (B)

J. Huot et al. / Progress in Materials Science 58 (2013) 30 75 43 was determined as a function of the grain size. It decreases from 0.82 H/M for 80 nm down to 0.72 H/ M for 10 nm indicating a modification of the b 2 c phase boundary in the V H system for nanometer grain sizes. Lower concentration, nearly independent of the grain size and higher diffusivity of hydrogen are also reported in the intergrain domain. 3.2. Ternary hydrides Ternary metal hydrides of general composition ABH x can be easily synthesized by RMM providing that the hydrides are stable under the pressure and temperature milling conditions. In this formulation, A stands for a metal with strong affinity for hydrogen (an early transition or rare-earth metal) and B stands for a metal with weak hydrogen affinity (a late transition metal). Well-known hydrogen storage intermetallic compounds such as ZrNi, TiFe, TiNi and LaNi 5 have been used in RMM experiments (see Table 1). As a general behaviour, two effects are observed on prolonged milling: formation of amorphous ABH x hydrides and compound disproportionation into AH x + B species. Compound amorphization is driven by the large negative heat of mixing between A and B elements while its disproportionation is favoured by the different affinity between both elements for hydrogen. 3.2.1. ZrNi hydride RMM experiments in the Zr Ni system have been first reported by Aoki et al. [27]. They performed RMM (p(h 2 ) = 2 MPa) of both arc-melted ZrNi alloys and equiatomic Zr and Ni powder mixtures. In the first case, ZrNiH 3 hydride is formed after 3 h of milling. On prolonged milling (over 100 h), the crystallite size of the hydride decreases without apparent amorphization. The lack of amorphous phases is attributed to the difficulty to introduce defects in ZrNiH 3 hydride because of its brittleness. For RMM of elemental powders, a mixture of ZrH 2 and elemental Ni is formed at short milling time (<3 h). Further reaction over 100 h leads to the coexistence of ZrH 2 and amorphous Zr-poor NiZr 1 y H x phase. Orimo et al. have studied the effect of hydrogen pressure during RMM of ZrNi compound within the range 0 1 MPa [28]. b-zrnih and c-zrnih 3 hydrides are formed within 5 min of milling. Phase abundance depends on hydrogen pressure. Formation of the most stable b-hydride is observed at 0.1 MPa, whereas that of the less stable c-hydride occurs at 1 MPa. At intermediate pressures, 0.3 MPa, both phases are detected. As observed by Aoki, prolonged milling over 80 h results in the formation of ZrH 2 and amorphous Zr-poor NiZr 1 y H x phase. Such decomposition reaction seems to be delayed with increasing hydrogen pressure. 3.2.2. TiNi hydride RMM experiments (p(h 2 ) = 0.1 MPa) on equiatomic Ti and Ni powder mixture have been conducted in a rod-milling device [119]. Within the first 3 h of milling metallic Ti transforms to TiH 2 and metallic FCC Ni remains unreacted. Further milling up to 200 h leads to the gradual formation of a nanocrystalline (10 nm) single-phase FCC compound. The compound is described as an FCC TiNiH 3 solid solution, though details on the hydrogen content determination are not provided. This result is rather striking since TiNi compound only absorbs 1.4 H/f.u. under normal conditions of pressure and temperature [130]. In fact, later experiments at higher hydrogen pressure (1.1 MPa) failed to get the solid solution TiNiH 3 phase [120]. Instead, formation of poorly crystallized TiH 2 and Ni phases on milling for 40 h is reported. 3.2.3. TiFe hydride Chiang et al. have performed RMM experiments (p(h 2 ) = 0.5 MPa) in TiFe compound [114]. In situ manometric measurements reveal that a total hydrogen uptake of 1.6 H/f.u. occurs within 7 h of milling forming TiFeH 1.6. Ex situ XRD analysis reveals that the ternary hydride decomposes to TiH 2 and Fe. 3.2.4. LaNi 5 hydride Fujii et al. have performed RMM experiments (p(h 2 ) = 1 MPa) on LaNi 5 alloy and observe formation of solid solution a-lani 5 H 0.15 within the first 5 min of milling [121]. Formation of hydride b-lani 5 H 6 phase is not detected, which may indicate that the absorption plateau pressure of this hydride is above

44 J. Huot et al. / Progress in Materials Science 58 (2013) 30 75 1 MPa at the temperature attained on milling. At longer milling times, 5 min < t < 3 h, the a-phase coexist with an amorphous phase. This phase forms faster in the presence of hydrogen than in ballmilling experiments performed under inert gas. The a-phase decomposes into Ni and amorphous Ni-poor LaNi 5 y H x phase upon prolonged milling, 3 h < t < 10 h. Subsequent thermodynamic measurements show a significant reduction of total and reversible hydrogen storage capacity for long-time milled as compared to pristine LaNi 5 compound. This is attributed to a lower hydrogenation capacity of both poorly crystallized inter-grain and amorphous regions as compared to the microcrystalline state. This concurs with the facts that nanocrystalline systems exhibit lower capacity than microcrystalline ones and that hydrogen binding energies expands over a wide energy range in amorphous systems [131,132]. 3.2.5. TiV hydride One should notice that the Ti V system differs from previous ones as concerns the affinity of constituting elements towards hydrogen. Both elements are A-type and exhibit comparable affinity for hydrogen which, in principle, precludes alloy disproportionation by hydrogenation. Furthermore, this system exhibits small heat of mixing so that alloy amorphization is expected to be difficult. RMM (p(h 2 ) = 0.2,0.4 and 2 MPa) of either equiatomic Ti and V powder mixtures or BCC TiV alloy have been conducted by Aoki et al. [43]. At long milling time (100 h), phase constitution of milled products does not depend on the nature of the initial powder. Hydrogen pressure plays, however, a major role. At low (0.2 MPa) and high (1 MPa) pressures, BCC TiVH 0.9 solid solution and FCC TiVH 4.7 hydride are formed, respectively. The hydrogen content of FCC TiVH 4.7 hydride is probably overestimated since maximum hydrogen uptake of both Ti and V is 2 H/M. Nevertheless, both BCC and FCC hydrogenated phases are crystalline. In contrast, at intermediate pressure (0.4 MPa), amorphous TiVH 2.8 phase is obtained. The formation mechanism of this phase depends on the initial reactants. For Ti + V powder mixture, the amorphous phase is formed by reaction between TiH 2 and V, whereas for the arc-melted alloy it results from gradual amorphization of BCC TiVH 2.8 phase on milling. Strikingly, for both cases, the amorphization reaction occurs without changing the hydrogen content. RMM experiments on a BCC Ti V Fe alloy of composition Ti 0.20 V 0.78 Fe 0.02 have been carried out in a device equipped with p and T sensors at rotation speed of 400 rpm and p(h 2 ) = 8 MPa. The hydrogenation curve is shown in Fig. 9 [122]. The alloy absorbs 2 H/f.u. in only ten minutes indicating the formation of a stoichiometric (Ti,V,Fe)H 2 hydride. Further milling produces hydrogen desorption from the milled sample. Its hydrogen content decreases to 1.55 H/f.u. after 8 h of milling. XRD diffraction analysis (Fig. 10) shows that the RMM alloy consists of a mixture of FCC VH 2 -type hydride and amorphous 2.0 1.5 H/f.u. 1.0 0.5 0.0 0 60 120 180 240 300 360 420 480 t (min) Fig. 9. Time-evolution of the H concentration in BCC Ti 0.20 V 0.78 Fe 0.02 alloy during RMM in hydrogen gas.

J. Huot et al. / Progress in Materials Science 58 (2013) 30 75 45 800 I (counts) 600 400 200 amorphous phase 0 30 40 50 60 70 80 2θ Fig. 10. Rietveld analysis of Ti 0.20 V 0.78 Fe 0.02 alloy after RMM for 480 min. Observed (dots), calculated (top line) and difference curves (bottom line) are shown. Vertical bars ( ) correspond to Bragg positions (Cu K a 1,2 ) for FCC VH 2 -type hydride. Large dots stand for the contribution of an amorphous phase to the calculated pattern. phase. The amorphous phase formation accounts for the spontaneous hydrogen desorption on milling since it stores less hydrogen. 3.3. Mg-based complex hydrides Mg-based Mg 2 TH x ternary hydrides (T = Fe, Co and Ni transition metals) are attractive hydrogen storage materials due to their high specific (5.5, 4.5 and 3.6 wt%) and volumetric (150, 125 and 97 g/l) hydrogen contents for Mg 2 FeH 6,Mg 2 CoH 5 and Mg 2 NiH 4, respectively [138]. The synthesis of these hydrides is problematic due to the great difference in vapour pressure and melting point between Mg and T and the lack of stable Mg 2 Fe and Mg 2 Co intermetallic compounds in their respective binary phase diagrams. From these facts, synthesis of Mg 2 TH x ternary hydrides was classically achieved by sintering methods from elemental powder mixtures. Temperatures and hydrogen pressures as high as 750 K and 9 MPa, respectively, and reaction time of several days are required [139]. Synthesis conditions of Mg 2 TH x ternary hydrides by RMM under hydrogen gas are summarized in Table 2. 3.3.1. Mg 2 Fe hydride The synthesis of Mg 2 FeH 6 by RMM (p(h 2 ) = 1 MPa for 20 h) of Mg and Fe powder was first attempted in 1997 [133]. Mg 2 FeH 6 hydride was not obtained but Mg powder got hydrogenated to form intimate MgH 2 Fe mixture. The failure to form the ternary hydride could be due to milling conditions not being sufficiently efficient (ball-to-powder weight ratio of 4:1). It was later discovered that the desired hydride can be obtained in two different ways. The first simply consists in a sintering treatment of the reactive milled product MgH 2 Fe for one day at 625 K under 5 MPa of hydrogen. The second, more complex, was reported in a subsequent paper [134]. Mechanical milling of MgH 2 and Fe powders in molar ratio 2:1 under argon atmosphere was performed for 60 h in a high-energetic shaker mill with ball-to-powder weight ratio of 10:1. The mechanical energy provided under these milling conditions was high enough to promote Mg 2 FeH 6 formation without subsequent sintering. Much probably, the following solid-state reaction takes place: 3MgH 2 ðsþþfeðsþ!mg 2 FeH 6 ðsþþmgðsþ ð1þ The formation of the ternary compound is driven by the fact that the Mg 2 FeH 6 phase is more stable than MgH 2 [140]. In situ SR-PXD patterns measured for a ball milled sample of MgH 2 Fe (2:1) reveal formation of Mg 2 FeH 6 at 673 K at p(h 2 ) = 10 MPa [141].

46 J. Huot et al. / Progress in Materials Science 58 (2013) 30 75 Table 2 Mg-based complex hydrides synthesised by RMM under hydrogen gas. The employed device, reactants, initial hydrogen pressure, p(h 2 ), total milling time (tmt), milling speed (ms), ball-to-powder weight ratio (BTPWR), ball diameter (Bd), reaction yield and formed side products are given. Compound Device Reactants p(h 2 ) (MPa) tmt (h) ms (rpm) BTPWR Bd Yield (mm) (wt%) Side products Mg 2 FeH 6 Fritsch P5 2Mg + Fe 1 20 325 4:1 10 0 MgH 2,Fe [133] Mg 2 FeH 6 Spex 8000 2MgH 2 + Fe 0.1 60 10:1 10 0 Mg, MgO, [134] Fe Mg 2 FeH 6 Uni-Ball-Mill II 2Mg + Fe 0.5 60 44:1 28 MgO, Fe [49] Mg 2 FeH 6 Szegvari attritor 2Mg + Fe 1 20 400 20:1 6 63 Fe [115] Mg 2 FeH 6 Retsch 2000 2Mg + Fe 0.3 8 32 s 1 16:1 12 90 b Fe [51] vibrating mill Mg 2 FeH 6 Fritsch P4 a 2Mg + Fe 7.5 12 400 60:1 12 77 MgO, Fe [116] Mg 2 CoH 5 Kurimoto 2MgH 2 + Co 0.1 10 700 8:1 4 100 [48] planetary mill Mg 2 CoH 5 Uni-Ball-Mill II 2Mg + Co 0.5 90 44:1 50 Co [135] Mg 2 CoH 5 Fritsch P4 a 2Mg + Co 7.5 12 400 60:1 12 81 MgO [116] Mg 2 NiH 4 Fritsch P5 2Mg + Ni 0.5 22 325 4:1 10 0 Mg, [29] MgH 2,Ni Mg 2 NiH 1.8 Fritsch P7 Mg 2 Ni 1 80 400 30:1 7 100 [136] Mg 2 NiH 4 Kurimoto Mg 2 Ni 1 10 885 5:1 7 70 b amph- [137] planetary mill MgNi Mg 2 NiH 4 Retsch 2000 2Mg + Ni 0.3 16 32 s 1 16:1 12 100 [51] vibrating mill Mg 2 NiH 4 Fritsch P4 a 2Mg + Ni 7.5 12 400 60:1 12 79 MgO [116] Mg 2 (FeH 6 ) 0.5 (CoH 5 ) 0.5 Fritsch P6 a 4Mg + Fe + Co 5 20 400 40:1 10 95 b FeCo [52] a b Pressure and temperature measured in situ in the Evico-magnetics vial. Estimated values. Ref. In 2002, direct though incomplete synthesis of Mg 2 FeH 6 by RMM of elemental powders was simultaneously reported [49,115]. Gennari et al. used a Uni-Ball-Mill II device under 0.5 MPa with hydrogen refilling every 5 h to maintain constant hydrogen pressure in the vial [49]. Mg 2 FeH 6 formation with a yield of 28 wt% was achieved after 60 h of milling. The synthesis was reported to occur in two steps. MgH 2 is formed during the first 40 h by mechanically activated solid gas reaction followed by the solid-state reaction between MgH 2 and Fe at longer milling times. Raman et al. [115] used a Szegvari attritor device under 1 MPa of hydrogen. The ternary hydride started forming after 14 h of milling and a maximum yield of 63 wt% was achieved at 20 h of milling as determined from XRD analysis. Reaction yield is probably overestimated since a high quantity of Fe (37 wt%) was identified as the unique secondary phase, which is not possible from mass-balance considerations. In fact, significant residuals in the Rietveld analysis likely related to MgO phase can be observed, which explains the presence of unreacted Fe (similar effects have been later observed [50]). Crystallite sizes of 12 and 18 nm are reported for Mg 2 FeH 6 and Fe phases, respectively. Formation on MgH 2 as intermediate phase was not detected. Prolonged milling to 30 h is reported to lead to amorphization of the ternary hydride. Milling under hydrogen of 2MgH 2 + Fe and 2Mg + Fe powder mixtures have also been compared [142]. It was found that a faster reaction and higher yield is achieved for elemental powders as compared to 2MgH 2 + Fe. The differences were attributed to the dissimilar mechanical properties and microstructures of the mixtures. The 2Mg + Fe mixture behaves as a ductile ductile pair that results in a higher contact surface between Mg and Fe, and a better intermixing and size reduction. On the contrary, the 2MgH 2 + Fe mixture performs as a ductile brittle combination, with less contact area between the reactants and hence lower yield and longer synthesis time. In 2008, the direct synthesis of Mg 2 FeH 6 by RMM of elemental powders was monitored in situ by manometric means by Baum et al. [51] RMM experiments were performed in a horizontal vibrating mill operated at 32 s 1 under a hydrogen pressure of 0.3 MPa. In spite of using mild milling conditions (one unique ball and ball-to-powder weight ratio of 16:1), the reaction was completed after only 8 h of milling time. The reaction yield is not specified, but judging from the total hydrogen uptake and XRD

J. Huot et al. / Progress in Materials Science 58 (2013) 30 75 47 data it should be around 90 wt%. This record could be related to the fact that enough hydrogen pressure was kept in the system on milling. The system was refilled with hydrogen when the pressure decreased below 0.27 MPa. Moreover, from in situ hydrogen uptake curves the authors could infer, in agreement with Gennari et al. a two-step process with formation of MgH 2 during the first 2 h of milling followed by the formation of Mg 2 FeH 6 from MgH 2 and Fe at longer milling times [49]. These results have later been confirmed by Deledda and Hauback by in situ measurements during RMM in an Evicomagnetics vial operated at 5 MPa of hydrogen pressure [52]. The latter authors observed, however, that the first step exceeded the hydrogen capacity of MgH 2 and proposed additional hydrogen uptake at Mg/Fe interfaces. Such additional capacity is doubtful since the authors used the ideal gas law, which is not valid at the imposed pressures, to estimate hydrogen absorption. The reaction path during RMM synthesis (p(h 2 ) = 7.5 MPa for 12 h) of Mg 2 FeH 6 has been recently studied in an Evico-magnetics vial [116]. The evolution of the hydrogen uptake as a function of milling time is shown in Fig. 11. The result for a similar experiment using only Mg powder is shown in the same figure for comparison. Hydrogen absorption by 2Mg + Fe powder mixture occurs in two steps. 6 5 2Mg+Fe Hydrogen uptake (H/f.u.) 4 3 2 3.4 H/f.u. 2Mg 2 nd step 1 0 1 st step t = 50 min 0 60 120 180 240 Milling time (min) Fig. 11. Time-evolution of the H concentration in solid-state during RMM of Mg powder and 2Mg + Fe powder mixture. Fig. 12. XRD patterns and phase identification of RMM 2Mg + Fe powder mixture after the first (50 min) and the second (720 min) reaction step (Cu Ka radiation).

48 J. Huot et al. / Progress in Materials Science 58 (2013) 30 75 The first step, with t < 50 min, corresponds to the formation of MgH 2 hydride as demonstrated by both the equivalent amount of absorbed hydrogen in the experiment conducted with elemental Mg and ex situ XRD measurements (Fig. 12). The second step corresponds to the reaction between MgH 2 and Fe to form Mg 2 FeH 6. The XRD pattern for 12 h milled product (Fig. 12) shows the presence of three phases Mg 2 FeH 6 (77 wt%), Fe (12 wt%) and MgO (11 wt%) with crystallite sizes of 8, 12 and 4 nm, respectively. In contrast to previous reports no amorphization is observed on prolonged milling [115,143]. MgO contamination is attributed to undesired surface oxidation of fine Mg powder (36 lm) in glove-box and accounts for unreacted Fe residual [144]. According to these results, the two-step reaction path for Mg 2 FeH 6 synthesis could be described as: 2MgðsÞþFeðsÞþ3H 2 ðgþ!2mgh 2 ðsþþfeðsþþh 2 ðgþ!mg 2 FeH 6 ðsþ It is worth noting that reaction kinetics for the first step, i.e. MgH 2 formation, is faster for the 2Mg + Fe mixture than for pure Mg powder. This striking result may be related either to catalytic effects for hydrogen dissociation at the Fe surface or to nucleation phenomena at Mg/Fe interfaces [145]. The reaction path given by Eq. (2) concurs with recent reports on hydrogen absorption by classical solid gas reaction in nanosized Mg + Fe mixtures [146,147]. Based on DFT calculations, it has been proposed that the reaction between iron and magnesium hydride may occur through the formation of a (MgFe)H 2 solid solution which becomes unstable with increasing Fe content with respect to Mg 2 FeH 6 [148]. As we saw in the previous sections, ball milling has been extensively used to synthesize Mg 2 FeH 6. In the case of Severe Plastic Deformation SPD techniques, investigation has only started recently and the literature is much less abundant. Lima et al. observed substantial improvement in the hydrogen sorption kinetics of a Mg Fe powder mixture processed by high pressure torsion HPT [16]. The authors noted that hydrogenation and dehydrogenation of the processed samples did not change the preferential orientation (002) of the Mg phase, i.e. the material retained the microstructure imposed by HPT. In a subsequent investigation, the same authors used a combination of ball milling and extrusion to synthesize Mg 2 FeH 6 [149]. Their results indicate that the iron in the 2Mg Fe mixture produced a beneficial pinning effect on the Mg grains by hindering grain coarsening even after annealing treatments. The desorption kinetics of samples processed by high pressure torsion (HPT) was faster than that of extruded samples, probably due to bulk diffusion limitations [149]. 3.3.2. Mg 2 Co hydride Similarly to Mg 2 FeH 6, first attempts to produce Mg 2 CoH 5 by RMM (p(h 2 ) = 1 MPa for 20 h) of Mg and Co powder were unsuccessful. Instead, a mixture of MgH 2 and Co phases was obtained [133]. Subsequent sintering treatment allowed synthesizing Mg 2 CoH 5 hydride though with a lower yield (26 wt%) than for Mg 2 FeH 6 (65 wt%). The synthesis of Mg 2 CoH 5 hydride by RMM (p(h 2 ) = 0.1 MPa for 10 h) of MgH 2 and Co powders in molar ration 2:1 under hydrogen pressure was first reported by Chen et al. [48]. Ex situ XRD measurements revealed that Mg 2 CoH 5 phase started forming at 1 h milling and became the major phase after 10 h milling. Later, the synthesis could be also achieved using Mg and Co powders as reactants under 0.5 MPa of hydrogen [135]. The powders were previously milled under Ar atmosphere in the same system for 200 h. It was supposed that intimate contact and homogeneity between Mg and Co phases is essential to reach hydride formation as occurring for sintering methods [150]. The Mg 2 CoH 5 phase starts forming after 40 h of milling and a yield of 50 wt% was achieved at 90 h. The formation of an intermediate MgH 2 phase occurs from 10 h of milling. The two-step reaction has been also observed by Baum et al. by in situ monitoring of hydrogen uptake during RMM experiments [51]. The reaction path during RMM synthesis (p(h 2 ) = 7.5 MPa for 12 h) of Mg 2 CoH 5 from Fe and Co powders in molar ratio 2:1 has been recently studied in detail [116]. The evolution of the hydrogen uptake as a function of milling time is shown in Fig. 13. Hydrogen absorption occurs in two steps which, after the analysis of XRD data (Fig. 14), correspond to the following reactions: 2MgðsÞþCoðsÞþ5=2H 2 ðgþ!2mgh 2 ðsþþcoðsþþ1=2h 2 ðgþ!mg 2 CoH 5 ðsþ MgH 2 hydride is formed as an intermediate phase for t < 50 min. The reaction is again faster as compared to Mg milled alone. The second step corresponds to the reaction between MgH 2 and Co to form ð2þ ð3þ

J. Huot et al. / Progress in Materials Science 58 (2013) 30 75 49 5 2Mg+Co Hydrogen uptake (H/f.u.) 4 3 2 1 0 3.4 H/f.u. 2 nd step 1 st step t = 50 min 2Mg 0 60 120 180 240 Milling time (min) Fig. 13. Time-evolution of the H concentration in solid-state during RMM of Mg powder and 2Mg + Co powder mixture. Fig. 14. XRD patterns and phase identification of RMM 2Mg + Co powder mixture after the first (50 min) and the second (720 min) reaction step (Cu Ka radiation). Mg 2 CoH 5. The hydrogen uptake corresponding to this reaction is lower but faster than for Mg 2 FeH 6 formation (Fig. 11). The XRD pattern for 12 h milled product (Fig. 14) shows the formation of nanocrystalline (8 nm) Mg 2 CoH 5 phase without significant amorphous contribution. 3.3.3. Mg 2 Ni hydride Since Mg 2 Ni compound exists as stable phase in the binary Mg Ni phase diagram, the synthesis of Mg 2 NiH 4 ternary hydride by RMM can be attempted either using 2Mg + Ni elemental mixture or Mg 2 Ni powders as initial reactants. This synthesis was first tried by RMM (p(h 2 ) = 0.5 MPa for 22 h) of 2Mg + Ni powders [29]. Some MgH 2 phase was formed from 2 h of milling but no ternary hydride could be detected. Either milling energy (ball-to-powder weight ratio was 4:1 and milling speed 325 rpm) or hydrogen supply was insufficient to promote hydride formation. Orimo et al. later investigated hydrogen absorption in Mg 2 Ni during reactive milling (p(h 2 ) = 1 MPa for 80 h) using stronger energetic conditions: rotation speed of 400 rpm and ball-to-powder weight

50 J. Huot et al. / Progress in Materials Science 58 (2013) 30 75 ratio 30:1 [136]. Instead of ternary hydride formation, a gradual solid solution of hydrogen in the Mg 2 Ni powder with a maximum hydrogen content of 1.8 H/f.u. after 80 h is reported. The position of XRD diffraction lines does not depend however on the hydrogen content. The milled material has a two-phase microstructure formed by nanocrystalline (15 nm) Mg 2 Ni regions with low H-content (0.3 H/f.u.) and disordered intergrain regions that store high amounts of hydrogen. Such a particular microstructure has not later been confirmed by other research groups. Tessier et al. also performed RMM experiments using Mg 2 Ni powder as starting reactant. RMM experiments (p(h 2 ) = 1.0 MPa for 10 h) were performed in a Kurimoto planetary mill at a high rotation speed of 885 rpm [137]. Contrary to Orimo et al., the formation of disordered intergrain regions with high H-content is not observed. The synthesis of 70 wt% of Mg 2 NiH 4 ternary hydride, as estimated from total hydrogen content in the milled product, is detected. The hydride crystallizes as a mixture of low- and high-temperature phases of Mg 2 NiH 4 hydride [151,152]. The high temperature modification is usually formed above 510 K but it seems to be stabilized by mechanical milling [153]. Baum et al. have monitored the in situ hydrogen uptake during RMM of 2Mg + Ni elemental powder mixture with identical milling parameters as mentioned above for the synthesis of Mg 2 FeH 6 [51]. Successful formation of Mg 2 NiH 4 ternary hydride is reported after 16 h of milling. The hydrogen uptake curve exhibits a unique step. This result differs from experiments on Mg 2 FeH 6 and Mg 2 CoH 5 formation and was tentatively attributed to a simpler process for Mg 2 NiH 4 related to the existence of Mg 2 Ni compound. However, this seems not to be the case. Fig. 15 displays the in situ hydrogenation curves monitored during the synthesis of Mg 2 NiH 4 hydride [116]. Indeed, one-hydrogenation step for t < 50 min is observed. As proved by XRD diffraction studies of the sample milled for this time (top pattern in Fig. 16), the reaction is however not completed since a high amount of the starting Ni powder remains unreacted. Further milling for 12 h leads to complete formation of Mg 2 NiH 4 hydride as shown by the XRD pattern displayed in bottom Fig. 16. This second step on the formation of the ternary hydride is a solid solid state reaction, i.e. it occurs without any hydrogen absorption. The reaction path of Mg 2 NiH 4 formation is then described according to reaction scheme: 2MgðsÞþNiðsÞþ2H 2 ðgþ!2mgh 2 ðsþþniðsþ!mg 2 NiH 4 ðsþ According to this reaction and to the previous experiments reported by Baum et al. [51] and Zhang et al. [116], the synthesis of all Mg-based Mg 2 TH x ternary hydrides (T = Fe, Co and Ni) by RMM of elemental pure powders under hydrogen atmosphere occurs through a common reaction path: ð4þ 2MgðsÞþTðsÞþ x 2 H 2ðgÞ!2MgH 2 ðsþþtðsþþ ðx 4Þ H 2 ðgþ!mg 2 2 TH x ðsþ ð5þ with x = 6, 5 and 4 for T = Fe, Co and Ni, respectively. 4 3.4 H/f.u. 2Mg+Ni Hydrogen uptake (H/f.u.) 3 2 1 0 1 st step t = 50 min 2Mg 0 60 120 180 240 Milling time (min) Fig. 15. Time-evolution of the H concentration in solid-state during RMM of Mg powder and 2Mg + Ni powder mixture.

J. Huot et al. / Progress in Materials Science 58 (2013) 30 75 51 Fig. 16. XRD patterns and phase identification of RMM 2Mg + Ni powder mixture after the first (50 min) and after 720 min of milling (Cu Ka radiation). Baum et al. have also reported the possibility of synthesizing quaternary Mg 2 T 0.5 T 0.5 H x hydrides with T, T = Fe, Co and Ni [51]. This result has been confirmed by Deledda and Hauback for T =Fe and T =Co[52]. The later authors obtained a quaternary hydride of composition Mg 2 (FeH 6 ) 0.5 (CoH 5 ) 0.5 in which both [FeH 6 ] 4 and [CoH 5 ] 4 and complex anions coexist in the same compound. The synthesized compound desorbed hydrogen in a one-step reaction at temperatures between 500 and 600 K which is between those of Mg 2 FeH 6 and Mg 2 CoH 5. These results open up the possibility of synthesizing other Mg-based transition metal complex hydrides, in which the hydrogen storage properties can be tailored by varying the transition metals and the relative content of the complex anion. 3.4. Alanates Since the pioneering work of Bogdanović and Schwickardi, who found that Ti-catalyzed NaAlH 4 can release hydrogen under moderate conditions, the potential interest of metal aluminum complex hydrides, also known as alanates, for reversible hydrogen storage remains vivid [154]. Alanates are formed by anionic aluminum hydrogen complexes (typically either [AlH 4 ] or [AlH 6 ] 3 ) stabilized by a cation (typically an alkali or alkaline earth metal) [2,155]. Their gravimetric (up to 10.8 wt% though only 5.6 wt% reversible) and volumetric capacities (90 g/l) make these compounds attractive for hydrogen storage. Conventionally, alanates are prepared by a wet chemical route. For instance, to synthesize sodium alanate, NaH and Al are diluted in THF and hydrogen pressure up to 20 MPa at 423 K is applied for few days. Subsequently, the NaAlH 4 in solution is filtered and dried. Since the solubility of NaAlH 4 is very low, a high amount of solvent is necessary. Dymova et al. have shown that the synthesis of NaAlH 4 can also be obtained from Na, Al and H 2 at high temperature (553 K), where Na is in liquid state, and high hydrogen pressure (17.5 MPa) [156]. In 1999, mechanochemical synthesis methods started being employed to synthesize Na 3 AlH 6 and Na 2 LiAlH 6 complex hydrides using NaAlH 4 and LiAlH 4 as reagents [157,158]. Very recently, it has been shown that NaAlH 4 can be formed by milling NaH and Al under a hydrogen atmosphere of 8.3 MPa, 4 mol% of TiCl 3 was added as a dopant [30]. The alanates synthesized by reactive mechanical milling are surveyed hereafter (Table 3). 3.4.1. Lithium alanates The synthesis of LiAlH 4 and Li 3 AlH 6 has been attempted by Kojima et al. under 1 MPa of hydrogen using a planetary ball mill at 400 rpm [165]. After 24 h of milling with addition of TiCl 3, the XRD pattern shows broad peaks of LiH and Al phases but no clear signature of alanate formation. However, the Raman and 27 Al MAS NMR spectra indicate that a small amount of LiAlH 4 was formed. Actually, lithium alanate LiAlH 4 is metastable and it is considered as a non-reversible hydrogen storage compound

52 J. Huot et al. / Progress in Materials Science 58 (2013) 30 75 Table 3 Alanates synthesised by RMM under hydrogen gas. The employed device, reactants, initial hydrogen pressure, p(h 2 ), total milling time (tmt), milling speed (ms), ball-to-powder weight ratio (BTPWR), ball diameter (Bd), reaction yield and formed side products are given. Compound Device Reactants p(h 2 ) (MPa) tmt (h) ms (rpm) BTPWR Bd (mm) Yield (wt%) Side products + Al 90:1 15 97 NaCl 2KH + NaH + Al 10 2 400 90:1 15 98 NaH [57] NaAlH 4 Fritsch P7 NaH + Al 8.3 4 500 50:1 NaCl, Al [30] NaAlH 4 Fritsch P5 NaH + Al 0.6 120 350 50:1 10 Low Na 3 AlH 6, NaH, [53] Al NaAlH 4 Fritsch P5 NaH + Al 1.2 240 230 10:1 10 High NaCl, Al [53] NaAlH 4 Planetary NaH + Al 3 60 350 30:1 8 86 Na 3 AlH 6,Al [159] Na 3 AlH 6 Fritsch P6 NaH + Al 0.85 20 400 40:1 Al [160] Na 3 AlH 6 Planetary NaH + Al 0.5 30 350 30:1 8 58 Al [159] Na 3 AlH 6 Fritsch 3NaH + Al 10 8 400 90:1 15 92 NaCl, NaH, Al [57] P4 a KAlH 4 Fritsch P7 KH + Al 10 10 400 [161] Na 2 LiAlH 6 Fritsch LiH + 2NaH 10 4 400 [57] P4 a K 2 NaAlH 6 Fritsch P4 a Mg(AlH 4 ) 2 Fritsch P7 2AlH 3 + MgH 2 0.1 4 400 175:1 10 81 AlH 3, MgH 2,Al [162] Ca(AlH 4 ) 2 Fritsch P6 2AlH 3 + CaH 2 0.1 10 500 13 [163] CaAlH 5 Fritsch P7 AlH 3 + CaH 2 0.1 3 400 80:1 10 91 CaH 2,Al [162] Ba 2 AlH 7 Fritsch P5 BaH 2 + Al 0.8 10 300 40:1 84 BaH 2,Al [164] BaAlH 5 Fritsch P5 BaH 2 + 2Al 0.8 10 300 40:1 83 Al [164] a Pressure and temperature measured in situ in the Evico-magnetics vial. Ref under moderate temperature and pressure conditions [166,167]. Indeed, the desorption of hydrogen from LiAlH 4 is reported to be an exothermic process [161]. Lithium alanate does not form directly from LiH, Al and H 2 by solid gas reaction. It is classically produced by using liquid complexing agents followed by desolvatation [168,169]. 3.4.2. Sodium alanates The sodium tetra-alanate NaAlH 4 is the best performing and most studied among all the alanates. Bellosta von Colbe et al. have synthesized NaAlH 4 by RMM (p(h 2 ) = 8.3 MPa for 4 h) of NaH and Al powders using 4 mol% TiCl 3 as a dopant [30]. The obtained alanate exhibits a reversible capacity of 4 wt% and very fast kinetics without activation. In the same year, Wang et al. also tried the synthesis of NaAlH 4 by the same means but at a lower pressure of 0.85 MPa and using TiF 3 as a dopant [160]. Within the first 5 h of milling, alanate formation was not observed. However, further milling up to 20 h led to partial synthesis of the more stable Na 3 AlH 6 phase. Eigen et al. have also tried the synthesis of NaAlH 4 at moderate hydrogen pressures (from 0.6 to 1.2 MPa) [53]. NaAlH 4 could be formed with and without the addition of TiCl 4 as catalyst. However, a long milling time (100 h without TiCl 4, 20 h with TiCl 4 ) is needed to obtain a small fraction of NaAlH 4 under 0.6 MPa of hydrogen. Generated heat due to material plastic deformation and the friction between the milling balls and vial wall during milling could lead to a temperature at which the NaAlH 4 is not stable at the imposed hydrogen pressure. A complete formation of NaAlH 4 could be achieved by reducing the milling energy (ball-to-powder weight ratio and rotational speed were decreased from 50:1 and 350 rpm to 10:1 and 230 rpm) and increasing the hydrogen pressure (1.2 MPa). Xiao et al. have also actively worked on the synthesis of NaAlH 4 by RMM under hydrogen pressure of 0.5 3 MPa [159]. They observed that while Na 3 AlH 6 forms under 0.5 MPa hydrogen pressure, NaAlH 4 only forms above 0.8 MPa [159]. These results are similar to those of Eigen et al. and prove that the formation of NaAlH 4 needs a minimum hydrogen pressure about 1 MPa, which is much higher than the equilibrium pressure of NaAlH 4 at room temperature (about 0.1 MPa) [53]. This is attributed to temperature rising on milling. By increasing the hydrogen pressure to 3 MPa, the synthesis yield of NaAlH 4 reached 87 wt% after milling for 60 h.

J. Huot et al. / Progress in Materials Science 58 (2013) 30 75 53 310 T (K) 305 300 295 1.12 Na 3 AlH 6 P o = 9.6 MPa Na 3 AlH 6 (TiCl 3 doped) P o = 10.2 MPa 1.08 P/P o 1.04 1.00 0.96 0 60 120 180 240 300 360 420 480 Milling time (min) Fig. 17. Time-evolution of the hydrogen pressure (bottom) and temperature (top) during the synthesis of Na 3 AlH 6 by RMM with (empty symbols) and without (full symbols) TiCl 3 dopant [57]. Sodium hexa-alanate is not available commercially. Some research groups have synthesized the hexa-sodium alanate by ball milling a mixture of the tetra alanate and the sodium hydride through the solid-sate reaction 2NaH(s) + NaAlH 4 (s)? Na 3 AlH 6 (s) without any additives [157,158]. By using RMM (p(h 2 ) = 10 MPa for 4 h), high yield is obtained through the following reaction [57]: 3NaHðsÞþAlðsÞþ 3 2 H 2mol%TiCl 3 2ðgÞ ƒƒƒƒƒ! Na3 AlH 6 ðsþ ð6þ This experiment was performed in an Evico-magnetics vial. Fig. 17 shows the temperature and hydrogen pressure inside the milling vial for the formation of Na 3 AlH 6 with and without TiCl 3. In both cases, the temperature first increases for t < 30 min due to the heat generated on milling before reaching a plateau around 310 K due to the balance between internal friction heating and ventilation cooling. The hydrogen pressure increases simultaneously because of the temperature rise in the vial. At longer milling time, t > 30 min, the pressure drops markedly for the doped sample whereas it decreases much more slowly for the undoped one. A further advantage on the synthesis of sodium alanates by RMM is the simultaneous incorporation of dopant. The addition of TiF 3 [159,160], TiCl 4 [170], TiCl 3 [57], ScCl 3 and CeCl 3 [171] dopants during milling not only speeds up the formation of NaAlH 4 or Na 3 AlH 6 compounds, but also promotes the dispersion of the dopant in the alanate. The materials synthesized by one-step reactive mechanical milling displayed superior features compared to doping of pre-synthesized NaAlH 4 with TiCl 3 [30,172,173]. 3.4.3. Potassium alanates Potassium alanate KAlH 4 as well as K 3 AlH 6 are stable alanates under normal conditions of pressure and temperature. They decompose at moderate temperatures (above 550 K) [174]. Up till now no reports have been published of the direct synthesis of K 3 AlH 6 by reactive ball milling. KAlH 4 can be synthesized by RMM of KH + Al powder mixture after 10 h milling under 10 MPa of hydrogen pressure [161]. 3.4.4. Mixed alkali alanates Three mixed alkaline alanates are reported to exist: Na 2 LiAlH 6,K 2 LiAlH 6 and K 2 NaAlH 6 [175 177]. In 1999, Na 2 LiAlH 6 was synthesized by ball milling of NaAlH 4 and LiH powder mixtures [157]. Recently, Na 2 LiAlH 6 and K 2 NaAlH 6 mixed alanates have been synthesized by RMM (p(h 2 ) = 10 MPa for 4 h)[57]. These compounds were prepared according to reaction schemes [56,57]: LiHðsÞþ2NaHðsÞþAlðsÞþ 3 2 H 2 mol%ticl 3 2ðgÞ ƒƒƒƒƒ! Na2 LiAlH 6 ðsþ ð7þ

54 J. Huot et al. / Progress in Materials Science 58 (2013) 30 75 NaHðsÞþ2KHðsÞþAlðsÞþ 3 2 H 2 mol%ticl 3 2ðgÞ ƒƒƒƒƒ! K2 NaAlH 6 ðsþ ð8þ In situ hydrogen uptake curves proved that the above reactions typically take place in 2 h. Ex situ XRD analysis after milling demonstrate the completion of the reaction as can be seen, for instance, for Na 2- LiAlH 6 synthesis in Fig. 18. In addition, a small amount of NaCl is detected as a result from the reaction of TiCl 3 with NaH [113]: 3NaHðsÞþTiCl 3 ðsþ!3naclðsþþtiðsþþ 3 2 H 2ðgÞ ð9þ Jeloaica et al. have attempted to synthesize ternary alkaline hexa-alanates of composition Li 4/3 Na 1/3 K 4/ 3AlH 6 and LiNaKAlH 6 [56]. However these compounds could not be obtained by RMM. The obtained I (a.u) 20 30 40 50 60 70 80 Fig. 18. Rietveld analysis of RMM Na 2 LiAlH 6 : observed (dots), calculated (solid line) and difference curves (bottom) are shown. Vertical bars ( ) correspond to Bragg positions for Na 2 LiAlH 6 (top) and NaCl (bottom) phases (Cu Ka 1,2 ). 2θ I (a.u) 20 30 40 50 60 70 80 2θ Fig. 19. Rietveld analysis RMM LiNaKAlH 6 : observed (dots), calculated (solid line) and difference curves (bottom) are shown. Vertical bars ( ) correspond to Bragg positions for K 2 NaAlH 6, KAlH 4, LiNa 2 AlH 6, NaH, LiH and NaCl phases from top to bottom (Cu Ka 1,2 ) [56].

J. Huot et al. / Progress in Materials Science 58 (2013) 30 75 55 materials contain a mixture of alkaline hydrides (LiH, NaH and KH), the tetra-alanate KAlH 4 and binary hexa-alanate (NaK 2 AlH 6 and Na 2 LiAlH 6 ) phases (Fig. 19). Actually, it has been shown by DFT calculations that the nominal Li 4/3 Na 1/3 K 4/3 AlH 6 and LiNaKAlH 6 phases are not stable with respect to the mixture of LiH, the tetra-alanate KAlH 4 and binary hexa-alanate K 2 NaAlH 6 or Na 2 LiAlH 6. 3.4.5. Alkali-earth alanates Conventionally, Mg(AlH 4 ) 2 and Ca(AlH 4 ) 2 have been synthesized by wet chemistry through the metathesis reaction between MCl 2 (M=Mg and Ca) and NaAlH 4 [178,179]. From the same reagents but by using mechanochemical milling, several research groups have also synthesized the magnesium and calcium alanates [180 182]. But all these approaches produce significant amount of magnesium or calcium chloride as by-products. These salts are inert to hydrogen and thus decrease the H-storage capacity of the final products. Reactive mechanical milling of Mg(AlH 4 ) 2 alanate has been attempted in 2005 by Varin et al. using a magneto-ball mill under 0.8 MPa hydrogen [183]. Three stoichiometric Mg + 2Al mixtures, (a) elemental Mg and Al powders, (b) elemental Al powder and commercial AZ91 alloy (Mg Al Zn alloy) and (c) powder of as-cast Mg + 2Al alloy have been used. No successful synthesis of Mg(AlH 4 ) 2 has been achieved. The hydride formed after 270 h of milling is b-mgh 2. In 2009, several groups tried the synthesis of Mg-containing quaternary alkali alanates by RMM under a much higher pressure of hydrogen (10 MPa). No traces of Mg(AlH 4 ) 2 alanate were however detected [184,185]. The lack of success in the synthesis of Mg(AlH 4 ) 2 alanate by RMM could be related to the lower thermodynamic stability of Mg(AlH 4 ) 2 as compared to MgH 2. Interestingly, it has been demonstrated that Mg(AlH 4 ) 2 can be obtained by RMM (p(h 2 ) = 10 MPa for 4 h) of 2AlH 3 and MgH 2 powder mixtures [162]. The reaction yield attained 81 wt% of Mg(AlH 4 ) 2. Experiments were conducted either in argon or hydrogen atmosphere. Strikingly, the milling atmosphere had minor impact on the yield of Mg(AlH 4 ) 2. In contrast, the most important parameter was the pause used to allow vial cooling (2 min of pause following every 10 min of milling). In fact, it was found that during milling, alane decomposes due to the temperature rise and reduces the yield of Mg(AlH 4 ) 2 formation. This suggests that the alanate formation occurs through an all solid-state process: MgH 2 ðsþþ2alh 3 ðsþ!2mgðalh 4 Þ 2 ðsþ ð10þ This reaction could be driven by lower stability of alane as compared to magnesium alanate. Although this synthesis route is successful, it should be taken into account that the preparation of alane is still a challenge [186 188]. As for the calcium alanate, progress on its synthesis by mechanical means is quite similar to the magnesium one. The synthesis of Ca M Al H (M = Na and Li) quaternary alanates by RMM of binary alkali hydrides and Al at 10 MPa of hydrogen pressure was unsuccessful [184]. In contrast, calcium alanate can be formed by mechanical milling using CaH 2 and alane as reactants [162,163]. Strikingly, either Ca(AlH 4 ) 2 or CaAlH 5 compounds are reported to be formed depending on the molar ratio between reactants: 2AlH 3 + CaH 2 in the former case and AlH 3 + CaH 2 in the latter. Other alkaline earth alanates that have been synthesized by RMM under hydrogen atmosphere are Ba 2 AlH 7 and BaAlH 5 [164]. RMM experiments (p(h 2 ) = 0.8 MPa for 10 h) have been performed using BaH 2 and Al powders as reactants. Three ratios of the reagent powder BaH 2 /Al 2:1, 1:1 and 1:2 have been used. Surprisingly, formation of Ba 2 AlH 7 was favoured for 1:1 ratio as compared to 2:1. 2BaH 2 /Al powder mixture forms 42 wt% of Ba 2 AlH 7 (together with 58 wt% of BaH 2 ) whereas BaH 2 /Al powder mixture forms 84 wt% of Ba 2 AlH 7. Elemental Al is not observed in the former case though it is expected from mass balance considerations. As for the synthesis of BaAlH 5, a high yield (83.2 wt%) of this phase is obtained using the ratio 1:2, i.e. with Al in excess. The reason of these striking results was not discussed. RMM has also been used as intermediary step in the synthesis of Sr 2 AlH 7 [189]. Sr 2 Al compound was milled under hydrogen (0.6 MPa) to get intimate 2SrH 2 + Al mixture. Such a mixture could react with hydrogen at 553 K and 7 MPa in an external device to form Sr 2 AlH 7. A similar approach has been used for the synthesis of mixed (Sr,Ca)AlH 7 compounds [190].

56 J. Huot et al. / Progress in Materials Science 58 (2013) 30 75 To summarize, mechanical milling under hydrogen gas is a very effective method for the synthesis of hydrides. Most hydrides formed by this method can be also obtained by solid gas reaction: metastable hydrides are rarely formed. However, reactive mechanical milling is much faster than conventional solid gas methods even for milder conditions of pressure and temperature. Main reasons for this are probably clean surface generation and shortening of diffusion paths as result of mechanicallyinduced particle pulverization. In systems with more than one metal-constituent, extended and clean interface formation between reactive powders should also play a major role to speed solid/solid reactions. Hydrides produced by RMM are generally nanocrystalline. Prolonged RMM may however lead to amorphization and compound disproportionation as it occurs in ternary metallic ABH x hydrides. This effect has not been observed so far in complex ternary hydrides which may indicate that ionic-covalent compounds are less prone to amorphisation than metallic compounds. 3.5. Synthesis of borohydrides by mechanical milling in diborane gas A novel technique for reactive ball-milling in diborane atmosphere was developed at EMPA, Switzerland, by Remhof, Friedrichs, Borgschulte, Züttel and co-workers. Solvent-free synthesis of lithium borohydride, LiBH 4 from lithium hydride, LiH in diborane, B 2 H 6 gas at 393 K has recently been demonstrated [191]. Diborane gas is produced by heating LiZn 2 (BH 4 ) 5 prepared by ball milling of a 2LiBH 4 :ZnCl 2 mixture. This is considered a convenient and relatively safe source of diborane as compared to pressurized bottles, since diborane is a poisonous and explosive gas [192]. The formation of LiBD 4 by an addition reaction of LiD and B 2 D 6 was analysed by in situ neutron diffraction, which shows that nucleation of LiBD 4 already starts at temperatures of ca. 375 K, i.e. formation of orthorhombic LiBH 4. However, the reaction is incomplete and the yield is only 50% even at elevated temperatures (460 K) and the product contains small amounts of Li 2 B 12 H 12. A passivation layer of LiBH 4 is suggested to form on the surface of the LiH grains retarding the process [193]. Therefore, a custom-made mill connected to a gas/vacuum supply via a flexible hose made of polyamide was used. The mill was equipped with two stainless steel milling vials performing horizontal movements based on the seesaw principle. One vial was equipped with ceramic balls. This approach allows continuous removal of the borohydride surface layer, see Fig. 20 [194]. This new mechanochemical method allows solvent-free synthesis of borohydrides at room temperature demonstrated by the synthesis of three of the most investigated borohydrides at present: LiBH 4, Mg(BH 4 ) 2 and Ca(BH 4 ) 2. Similarly, Y(BH 4 ) 3 was prepared in a reaction with YH 3 and B 2 H 6 with this new RBM [195]. This new gas solid mechanochemical synthesis method is based on the reaction of metal hydrides with diborane to form the corresponding borohydrides. The synthesis method may facilitate preparation of a wide range of different borohydrides in the future. Furthermore, with this Fig. 20. Schematic presentation of a custom-made mill connected via a flexible hose to a gas/vacuum supply [194]. The gas supply consists of a container that is situated in an oven and is filled with a borane desorbing material, i.e. LiZn 2 (BH 4 ) 5.

J. Huot et al. / Progress in Materials Science 58 (2013) 30 75 57 technique it was demonstrated that Li 2 B 12 H 12 and an amorphous Li 2 B 10 H 10 phase can be prepared by a reaction with LiBH 4 and diborane (B 2 H 6 )att 475 K. This tend to suggest that formation of higher boranes, such as Li 2 B 12 H 12, may occur during thermal decomposition of metal borohydrides [196]. Recently, a metal borohydride was prepared by reactive ball milling of a metal boride in hydrogen atmosphere. The starting materials, MgB 2 and hydrogen gas, p(h 2 ) = 10 MPa, was charged into a hardened steel high pressure vial (ball to powder ratio of 30:1) and milled at a rotation speed of 600 rpm for up to 100 h. The product, unsolvated amorphous magnesium borohydride (yield 50%) appeared to form directly but impurities of higher boranes Mg(B n H m ) y (30%) were also identified, which may have formed in side reactions [197]. Interestingly, a new polymorph c-mg(bh 4 ) 2 with 30% open space in the structure transformed from crystalline to semi-amorphous by mechano chemical treatment [198]. The hydrogen absorption mechanism for ball milled samples of 2NaH MgB 2 was also investigated in detail. At high pressures, p(h 2 ) 5 MPa, NaBH 4 formed after observation of an unknown compound, NaMgH 3 and a NaH NaBH 4 molten salt mixture. In contrast, NaBH 4 apparently formed directly at lower hydrogen pressures, i.e. p(h 2 ) = 0.5 MPa. This indicates that the reaction mechanism may be modified by mechano-chemical treatment of the reactants and by the applied hydrogen pressure [199]. This work reveals that reactive ball milling of metal hydrides in diborane, metal boride or mixtures of metal hydrides and borides in elevated hydrogen pressures has a potential for providing new convenient solvent-free routes for preparation of metal borohydrides. 3.6. Synthesis of metal amides by mechanical milling in ammonia gas During the past decade lithium amide, LiNH 2 has received much interest as a hydrogen storage material [200,201]. Recently, LiNH 2 and a range of other metal amides i.e. NaNH 2, Mg(NH 2 ) 2 and Ca(NH 2 ) 2 have been prepared by mechanochemical methods from metal hydrides, MH x and ammonia gas, NH 3 according to reaction scheme Eq. (11) [202,203]. MH x ðsþþxnh 3 ðgþ!mðnh 2 ÞxðsÞþxH 2 ðgþ ð11þ In general, gas loadings of p(nh 3 ) 0.4 0.5 MPa was used with refilled approximately every second hour. A well-known problem of utilization of the metal amides as hydrogen storage materials is the release of NH 3 gas besides hydrogen during the thermal decomposition [59,204,205]. In order to avoid this, a metal hydride can be added to absorb the NH 3 according to reaction scheme Eq. (11). The efficiency of this approach has been studied by milling LiH in p(nh 3 ) = 0.4 MPa corresponding to LiH NH 3 (1:1). Gas chromatographic analysis revealed that 70% of the initial NH 3 reacted with LiH and transformed to H 2 after 30 min of milling [206]. 4. Synthesis of hydrides by mechanically-induced solid/solid and solid/liquid reactions 4.1. Mechanochemical synthesis of metal borohydrides Metal borohydrides are well known for their properties as reducing agents and neutron absorbers. However, due to their high gravimetric hydrogen density this class of compounds has recently received increasing interest as potential hydrogen storage materials [2 4]. Schlesinger et al. reported in 1953 the first mechanochemical synthesis of a metal borohydride i.e. sodium borohydride from sodium hydride and boric oxide according to reaction scheme [207]. 4NaH þ 2B 2 O 3! NaBH 4 þ 3NaBO 2 ð12þ Similar methods are still investigated for preparation of alkali borohydrides, e.g. NaBH 4 or KBH 4 from MgH 2 and Na 2 B 4 O 7 or KBO 2, respectively [208,209]. Due to the high thermal stability of the alkali borohydrides much attention has been given to synthesis and characterization of novel metal borohydrides e.g. based on alkaline earth and transition metals. In 1989 Mal tseva et al. showed that ball milling of alkali borohydrides, MBH 4 (M = Li, Na or K) with zinc chloride, ZnCl 2 resulted in a metathesis reaction, i.e. formation of MCl was observed by

58 J. Huot et al. / Progress in Materials Science 58 (2013) 30 75 Table 4 Metal borohydrides synthesized by mechanochemical methods, reactants used for the synthesis, optimal reactant ratio, formed side products, total milling time (tmt) and milling speed (rounds per minute, rpm) used in the synthesis. Compound Reactants Opt. ratio Side products tmt (min) rpm Ref. NaBH 4 MgH 2 +Na 2 B 4 O 7 4:1 MgO, B 2 O 3 60 2750 [207,209] KBH 4 MgH 2 + KBO 2 2:1 MgO 120 490 [208] Sr(BH 4 )Cl LiBH 4 + SrCl 2 1:1 LiCl, Sr(BH 4 ) 2 120 400 [39] Sr(BH 4 ) 2 LiBH 4 + SrCl 2 1:1 LiCl, Sr(BH 4 )Cl 120 400 [39] LiSc(BH 4 ) 4 LiBH 4 + ScCl 3 4:1 LiCl 180 500 [211,212] NaSc(BH 4 ) 4 NaBH 4 + ScCl 3 2:1 Na 3 ScCl 6 120 400 [213] KSc(BH 4 ) 4 KBH 4 + ScCl 3 2:1 K 3 ScCl 6 120 400 [214] Y(BH 4 ) 3 LiBH 4 + YCl 3 3:1 LiCl 120 200 [215,216] NaY(BH 4 ) 2 Cl 2 NaBH 4 + YCl 3 2:1 Na 3 YCl 6, Na(BH 4 ) 1 x Cl x 120 200 [38] Mn(BH 4 ) 2 LiBH 4 + MnCl 2 2:1 LiCl 350 600 [40] NaBH 4 + MnCl 2 2:1 NaCl 350 600 [40] LiZn 2 (BH 4 ) 5 LiBH 4 + ZnCl 2 5:2 LiCl 120 200 [217] NaZn 2 (BH 4 ) 5 NaBH 4 + ZnCl 2 5:2 Na 2 ZnCl 4, NaCl 120 200 [217] NaZn(BH 4 ) 3 NaBH 4 + ZnCl 2 3:1 Na 2 ZnCl 4, NaCl 120 200 [217] KZn(BH 4 ) 3 KBH 4 + ZnCl 2 2:1 K 2 Zn(BH 4 ) x Cl 4 x,k 3 Zn(BH 4 ) x Cl 5 x 350 [218] KZn(BH 4 )Cl 2 KBH 4 + ZnCl 2 1:1 120 200 [219] Cd(BH 4 ) 2 LiBH 4 + CdCl 2 2:1 LiCl 30 200 [37] NaBH 4 + CdCl 2 14:9 NaCl, Na 6 CdCl 8 30 200 [37] KCd(BH 4 ) 3 KBH 4 + CdCl 2 1:1 KCdCl 3,K 2 Cd(BH 4 ) 4, Cd(BH 4 ) 2 20 200 [37] K 2 Cd(BH 4 ) 4 KBH 4 + CdCl 2 4:3 KCdCl 3 20 200 [37] Li 4 Al 3 (BH 4 ) 13 LiBH 4 + AlCl 3 13:3 LiCl 300 500 [220] Li(BH 4 ) 0.9 Cl 0.1 LiBH 4 + LiCl 120 200 [221,222] Li(BH 4 ) 0.47 Br 0.53 LiBH 4 + LiBr 120 200 [223] Li(BH 4 ) 0.3 I 0.7 LiBH 4 + LiI 120 200 [224] Na(BH 4 ) 0.9 Cl 0.1 NaBH 4 + NaCl 120 200 [225] Ca(BH 4 ) 1.6 I 0.4 Ca(BH 4 ) 2 + CaI 2 120 250 [226] PXD [210]. Such mechanochemical methods have become one of the most common methods for preparation of novel metal borohydrides during the past decade and a significant increase in the number of new compounds and studies of their structural, physical and chemical properties [3,4]. Table 4 provides an overview of reactants and conditions for synthesis of metal borohydrides along with side products, which are obtained in some cases. During the mechanochemical synthesis a variety of reactions can occur and in some cases several competing reactions are observed simultaneously. Metathesis, or double substitution reaction, is a well-known mechanism for chemical reactions during ball milling, here illustrated by the reaction between lithium borohydride LiBH 4 and yttrium(iii)chloride, YCl 3, which results in formation of Y(BH 4 ) 3 and LiCl according to reaction scheme [215,216]. ð1 : 3Þ YCl 3 þ 3LiBH 4! YðBH 4 Þ 3 þ 3LiCl The polymorph a-y(bh 4 ) 3, previously obtained from diethyl ether solutions of LiBH 4 and YCl 3 at RT and also by MM, can be obtained with varying amounts of a new polymorph, denoted b-y(bh 4 ) 3. a- Y(BH 4 ) 3 can be considered as the stable phase at ambient conditions, whereas b-y(bh 4 ) 3, is a hightemperature polymorph formed by an a to b polymorphic phase transition. The high-temperature b-polymorph can be quenched to ambient conditions [215,216,227,228]. It should be noted that many single-cation metal borohydrides have been reported to be prepared by mechanochemical synthesis e.g. Zn(BH 4 ) 2 [229,230], Sc(BH 4 ) 3 [229], Ti(BH 4 ) 3 [229,231], V(BH 4 ) 2 and Cr(BH 4 ) 2 [232]. However, no structural data have been reported for these compounds and their formations are based on observation of alkali chlorides and assumption of a simple metathesis reaction similar to reaction scheme Eq. (13). Further investigations of some of these systems have shown that more complex reactions often take place [213,214,217,219]. The system ZnCl 2 MBH 4 (M = Li, Na or K) can be used to illustrate the complexity of the mechanochemical synthesis. Ball milling a mixture of ZnCl 2 KBH 4 (1:1) leads to an addition reaction and a phase pure product, of KZn(BH 4 )Cl 2 [219]. ð13þ

ZnCl 2 þ KBH 4! KZnðBH 4 ÞCl 2 J. Huot et al. / Progress in Materials Science 58 (2013) 30 75 59 Interestingly, the unit cell volume of KZn(BH 4 )Cl 2 (V/Z = 149.5 Å 3 ) is nearly equal to the sum of formula volumes for the reactants ZnCl 2 (V/Z = 74.3 Å 3 ) and KBH 4 (V/Z = 76.2 Å 3 ). However, there are significant differences between the structure of reactants and the product. The latter contains the heteroleptic complex ion [Zn(BH 4 )Cl 2 ] where Zn coordinates to two chloride ions and two hydrogens in g 2 BH 4, i.e. CN(Zn) = 4. This clearly demonstrates that ball milling induces a complex chemical reaction involving bond breaking and bond formation. In this case, the structure is fully ordered. The addition reaction may also provide a solid solution, e.g. Li(BH 4 ) 1 x Cl x, which will be further discussed later in this section. The mechanochemical synthesis of other new borohydrides in the system ZnCl 2 MBH 4 proceeds via more complex chemical reactions during ball milling [217] ð14þ 2ZnCl 2 þ 5LiBH 4! LiZn 2 ðbh 4 Þ 5 þ 4LiCl 2ZnCl 2 þ 5NaBH 4! NaZn 2 ðbh 4 Þ 5 þ 4NaCl ZnCl 2 þ 3NaBH 4! NaZnðBH 4 Þ 3 þ 2NaCl ð15þ ð16þ ð17þ Reaction schemes (16) and (17) illustrates that small deviations in the composition of reactants may lead to significantly different reaction products both in terms of the stoichiometry and the structural topology, i.e. the structures of NaZn(BH 4 ) 3 and NaZn 2 (BH 4 ) 5 are significantly different, which may suggest that the synthesis mechanism for these compounds is also different. The compounds LiZn 2 (BH 4 ) 5 and NaZn 2 (BH 4 ) 5 are isostructural and built from two identical interpenetrated three-dimensional frameworks consisting of isolated complex anions, [Zn 2 (BH 4 ) 5 ], whereas NaZn(BH 4 ) 3 consists of a single three-dimensional network, containing polymeric anions with the composition ½ZnðBH 4 Þ 3 Š n n (see Fig. 21) [42,217]. These new compounds were prepared by high-energy MM in short intervals (2 min) separated by pauses (2 min). This procedure suppresses heating of the sample by friction heat and keeps the tem- Fig. 21. Crystal structure of LiZn 2 (BH 4 ) 5 (left) built from two identical interpenetrated three-dimensional frameworks consisting of isolated complex anions, [Zn 2 (BH 4 ) 5 ], and NaZn(BH 4 ) 3 (right) consisting of a single three-dimensional network, containing polymeric anions with the composition ½ZnðBH 4Þ 3 Š [42,217].

60 J. Huot et al. / Progress in Materials Science 58 (2013) 30 75 perature close to room temperature. This suggests that the synthesis is facilitated by high pressure rather than elevated temperature. In fact, this class of M Zn BH 4 compounds decomposes irreversibly by heating at 373 K or prolonged MM with release of diborane [36]. Furthermore, these compounds are metastable, i.e. they decompose when stored at RT within approximately 1 week. These observations suggest that ball milling may lead to a chemical equilibrium state rather than just a statistical distribution of reactants. Reaction scheme Eqs. 13,15,16, and 17 also illustrate a general drawback of the mechanochemical approach involving metathesis reactions, namely that the products may be contaminated with ionic compounds, most often binary metal halides but sometimes also ternary halides. Formation of LiZn 2 (BH 4 ) 5 (reaction scheme Eq. (15)) proceeds completely, i.e. with formation of LiCl as the only side product, whereas formation of NaZn 2 (BH 4 ) 5 and NaZn(BH 4 ) 3 (reaction schemes Eqs. (16) and (17)) only proceeds partly due to formation of a ternary metal chloride by a simultaneous and competing reaction described in reaction scheme [42]. ð1 : 2Þ ZnX 2 þ 2MX! M 2 ZnX 4 Mechanochemical synthesis of M 2 ZnX 4, M = Li or Na, X = Cl or Br from stoichiometric mixtures of MX and ZnX 2 has previously been reported by Solinas et al., who state that reaction times and activation energy decreases as Li 2 ZnCl 4 >Na 2 ZnCl 4 >Na 2 ZnBr 4 [233]. This is in agreement with the results for the zinc-based borohydride systems ZnX 2 MBH 4 (M = Li, Na, K and X = Cl, Br), where formation of M 2 ZnX 4 has increasing dominance over the heavier elements, i.e. K > Na Li and Br > Cl. However, reaction Eq. (18) is only weakly coupled with the formation of the borohydrides. In other words, reaction Eqs. (16) and (17) are faster than reaction Eq. (18). This contrasts the mechanochemical synthesis of NaSc(BH 4 ) 4 and KSc(BH 4 ) 4 from ScCl 3 and NaBH 4 or KBH 4, respectively, which is suggested to proceed as described by reaction scheme [213,214]. ð1 : 4Þ ScCl 3 þ 4NaBH 4! NaScðBH 4 Þ 4 þ 3NaCl ð18þ ð19þ ð1 : 4Þ ScCl 3 þ 4KBH 4! KScðBH 4 Þ 4 þ 3KCl ð20þ Surprisingly, powder X-ray diffraction data show no presence of NaCl or KCl in any of the ball-milled samples of ScCl 3 MBH 4 (M = Na or K) in molar ratios 1:2, 1:3 or 1:4. This indicates that the mechanochemically induced reactions differ from reaction schemes Eqs. (19) and (20). Two sets of unidentified Bragg peaks were observed in all the reaction products and one was assigned to MSc(BH 4 ) 4 and the other to a new ternary sodium scandium chloride M 3 ScCl 6. Furthermore, the samples with the starting ratios ScCl 3 MBH 4 of 1:3 and 1:4 contain different amounts of MBH 4 whereas no diffraction from ScCl 3 was observed. The samples with the starting ratio of 1:2 show neither ScCl 3 nor MBH 4 peaks and appear to contain the largest fraction of the new compounds, NaSc(BH 4 ) 4 and KSc(BH 4 ) 4 for M =NaorK, respectively [213,214]. This suggests that an addition reaction is responsible for the formation of the ternary salts, Na 3 ScCl 6 and K 3 ScCl 6 according to reaction scheme ð1 : 3Þ ScCl 3 þ 3NaCl! Na 3 ScCl 6 ð21þ ð1 : 3Þ ScCl 3 þ 3KCl! K 3 ScCl 6 ð22þ In conclusion, the optimal ratio of reactants ScCl 3 MBH 4 (M = Na or K) for synthesis of MSc(BH 4 ) 4 turns out to be 1:2. The above-mentioned observations can be explained assuming that the formations of the ternary salts, M 3 ScCl 6 (Eqs. (21) and (22)) are much faster than the formations of the borohydrides, MSc(BH 4 ) 4 (Eqs. (19) and (20)). Hence, the overall reactions for the samples ScCl 3 MBH 4 in 1:2 ratio are described in Eqs. (23) and (24) as a sum of the strongly coupled reactions for formation of MSc(BH 4 ) 4 and M 3 ScCl 6. These mechanochemical syntheses lead to maximum borohydride yields of 22 wt% and 18 wt% for NaSc(BH 4 ) 4 and KSc(BH 4 ) 4, respectively [213,214]. ð1 : 2Þ 2ScCl 3 þ 4NaBH 4! NaScðBH 4 Þ 4 þ Na 3 ScCl 6 ð23þ ð1 : 2Þ 2ScCl 3 þ 4KBH 4! KScðBH 4 Þ 4 þ K 3 ScCl 6 ð24þ

J. Huot et al. / Progress in Materials Science 58 (2013) 30 75 61 In contrast, for the system ScCl 3 LiBH 4, only one chemical reaction occurs during ball milling, which produces LiSc(BH 4 ) 4 and LiCl [211,212]. Ionic-covalent type hydrides, i.e. complex hydrides, may also form solid solutions, e.g. ca. 10 mol% lithium chloride may dissolve in solid lithium borohydride by ball milling as described in reaction scheme [221,222]. xlicl þ 1 xlibh 4! LiðBH 4 Þ 1 x Cl x ð25þ On the other hand, Rietveld refinement of powder X-ray diffraction data reveals that Cl readily substitutes for BH 4 in the structure of hexagonal h-libh 4 upon prolonged heating, e.g. at 498 K for 1 h resulting in formation of h-li(bh 4 ) 1 x Cl x, x 0.42 [221]. This observation indicates that formation of such solid solutions is more efficiently facilitated by elevated temperatures rather than by high pressures generated by mechanochemical methods. These types of anion substitution reactions also take place for a range of other metal borohydride-metal halide systems both as a result of mechanochemical treatment and subsequent heating, e.g. LiBH 4 LiBr [223], LiBH 4 LiI [224], NaBH 4 NaCl [225], Ca(BH 4 ) 2 CaF 2, Ca(BH 4 ) 2 CaCl 2 and Ca(BH 4 ) 2 CaI 2 [226,234]. Furthermore, substitution of BH 4 by Cl can also take place in the novel metal borohydrides formed during the mechanochemical synthesis, e.g. Rietveld refinement suggests incorporation of 42 mol% of Cl on one of the two BH 4 sites in Al 3- Li 4 (BH 4 ) 13 [220]. Generally, the smaller anion tends to dissolve in the compound containing the larger anion, and the structure of the latter tends to be preserved in the obtained solid solution in accordance with the observation of a CaI 2 -type trigonal solid solution, tri-ca((bh 4 ) 1 x I x ) 2, i.e. dissolution of Ca(BH 4 ) 2 in CaI 2. This trend follows the relative size of the anions, I > BH 4 > Br > Cl derived by a comparison of the unit cell volumes for different inorganic salts [235,236]. This trend in anion substitution reactions can be interpreted as an increase in the lattice energy due to the clearly observed decrease in the unit cell volume, i.e. a decrease in the average distance between the ions in the structure. Furthermore, anion substitution can occur due to structural similarities between the two compounds, which may partly explain the observation of two solid solutions of mechanically treated LiBH 4 LiI, i.e. dissolution of LiBH 4 in LiI and dissolution of LiI in LiBH 4. The two solid solutions of LiBH 4 LiI, b LiI and h-libh 4 all adopt isostructural hexagonal structures. Upon heating, the two solid solutions of LiBH 4 LiI merge into one [224]. Similarly, prolonged heating of NaBH 4 NaCl produces two solid solutions while NaBH 4 and NaCl both share the same rock salt structure type [225]. This contrasts the behaviour of LiBH 4 LiCl system where there are no indications of any dissolution of LiBH 4 in the alkali halide salts [221,222]. Recently, cation substitution in a borohydride by ball milling was observed for the first time by formation of a Mg 1 x Mn x (BH 4 ) 2 solid solution. This substitution is most likely related to the close structural similarity of Mn(BH 4 ) 2 to a-mg(bh 4 ) 2 and it is interesting to note that this compound retains the framework structure [237]. Recently, a new series of double-cation double-anion borohydride chlorides based on rare-earths elements, LiM (BH 4 ) 3 Cl, M = La, Gd, and Ce, were discovered using combined mechano-chemical synthesis and heat treatment using M Cl 3 LiBH 4 (1:3) mixtures [238,239]. The novel cubic compounds, LiM(BH 4 ) 3 Cl contains isolated tetranuclear anionic clusters [Ce 4 Cl 4 (BH 4 ) 12 ] 4 with a distorted cubane Ce 4 Cl 4 core charge-balanced by Li + cations. The Li + ions are disordered and occupy 2/3 of the 12d Wyckoff sites and DFT calculations indicates that LiCe(BH 4 ) 3 Cl is stabilized by larger entropy rather than smaller energy. The new compound LiM (BH 4 ) 3 Cl simultaneously carries moderate amounts of hydrogen, which is released at relatively low temperatures, and are fast Li-ion conductors. 4.2. Synthesis of novel alane and metal alanates Among the most attractive materials for reversible hydrogen storage is the light-weight aluminium based hydrides due to their relatively high gravimetric hydrogen capacities and, importantly, hydrogen release and uptake at moderate conditions [2,240]. In particular, Ti-doped NaAlH 4 has received significant attention in the past decade [154]. Hydridoaluminates containing a negatively charged complex anion are denoted alanates in the following whereas polymorphs of aluminium hydride,

62 J. Huot et al. / Progress in Materials Science 58 (2013) 30 75 AlH 3 are denoted alane. Alanates synthesised by RMM in hydrogen gas was discussed in Section 3.4 of this review paper. For the most stable alane polymorph, a-alh 3 the dehydrogenation enthalpy is determined by bomb calorimetry to be 7.6(6) kj/(mol H 2 ) and the total dehydrogenation entropy is found to be 30.0(4) J/ (Kmol a-alh 3 ) [241]. From these values and the entropies of Al and H 2, the hydrogenation pressure of Al at room temperature according to reaction scheme Eq. (26) can be estimated to be above 10 GPa. Al þ 3=2H 2! AlH 3 Therefore, the traditional synthesis of Al-based hydrides is based on wet-chemistry methods where a coordinating solvent forms a kinetically-stabilized hydride under mild conditions that can be isolated from the solution [242,243]. Subsequently, the intermediate complex metal hydride solvent complex can be decomposed to recover the pure hydride. Recently, it was discovered that a reversible hydrogenation reaction using Ti-catalyzed Al powder and triethylenediamine (TEDA) in tetrahydrofuran forms the alane adduct (AlH 3 TEDA) at low pressure (p(h 2 ) = 2 MPa, RT) [244]. In contrast, during the past few years mechanochemical techniques at a variety of different conditions have been used with increasing frequency for synthesis of alanates or for formation of homogeneous alanate-additive samples, e.g. in argon or hydrogen atmosphere or at liquid nitrogen temperatures (cryo-milling) [245]. Interestingly, alane, AlH 3 can be prepared directly by cryo-milling LiAlH 4 and AlCl 3 at 77 K resulting in a metathesis reaction according to reaction scheme Eq. (27) [188,246,247]. 3LiAlH 4 þ AlCl 3! 4AlH 3 þ 3LiCl The reaction also proceeds at RT, however it results in low yields of alane and a high content of Al, due to the high milling energy at RT which causes AlH 3 to decompose to Al and H 2. Furthermore, increased brittleness at low temperatures leads to reduced particle sizes, hence shortened diffusion paths. Recently it was found that the yield of alane might be further increased by using NaAlH 4 instead of LiAlH 4 or AlBr 3 instead of AlCl 3. The same study also revealed that the relative amount of a- and a - AlH 3 formed in reaction scheme Eq. (27) can be somewhat controlled by adding 2.4 mol% FeF 3 to the reactant mixture [248]. The thermodynamically unstable AlH 3, which has a three-dimensional covalent network structure, decomposes slightly above room temperature but may be kinetically stabilized by thin oxide layers [249]. This contrasts the ionic sodium hydride, NaH which decomposes at T 700 K [1]. Mechanochemistry reveals an interesting example, namely that reaction between a stable and an unstable compound may provide a new material with intermediate stability. In this case a two-step addition reaction occurs according to reaction scheme Eqs. (28) and (29) and sodium alanate is formed built from discrete [AlH 4 ] complex anions and sodium counter cations [65,155]. 3NaH þ AlH 3! Na 3 AlH 6 Na 3 AlH 6 þ 2AlH 3! 3NaAlH 4 This approach for tailoring thermodynamic properties is also used for destabilising hydrides by formation of reactive hydride composites, e.g. 2LiBH 4 MgH 2 and LiBH 4 Al [66,250 252]. Sodium alanate, NaAlH 4 has moderate formation enthalpies, however the rehydrogenation of NaAlH 4 is kinetically hampered and hydrogen release and uptake in this material need to be catalyzed by titanium [154]. The materials resulting from mechanochemical doping NaAlH 4 with small amounts of catalytic TiCl 3 (<2 wt%) have kinetic and cycling properties that are closer to those required for a practical hydrogen storage medium [172,253]. Doping through mechanical milling of NaAlH 4 with dopant precursors is an effective mean of charging the hydride with catalyst but also activates the material through reduction of the average particle size [173,253]. Furthermore, mechanochemical treatment can promote decomposition of metal alanates with addition of catalysts [113,254,255]. Balema et al. and Easton et al. investigated the decomposition of LiAlH 4 into Li 3 AlH 6 and Al with release of hydrogen at RT during short-time ball milling with addition of catalytic amounts of TiCl 3 (3 and 2 mol%, respectively) [254,255]. Milling times as short as 5 min proved effective for this transformation and according to PXD data LiAlH 4 was completely trans- ð26þ ð27þ ð28þ ð29þ

J. Huot et al. / Progress in Materials Science 58 (2013) 30 75 63 Fig. 22. Ball collision effects on reaction mixtures as a function of ball impact energy. formed, i.e. only Bragg peaks from Li 3 AlH 6, Al and LiCl were observed [255]. Monitoring of the gas pressure within the milling vial as a function of milling time is shown in Fig. 22. The high catalytic activity of TiCl 3 and other additives may be attributed to the in situ formation of nano/microcrystalline, e.g. Al Ti phases, during ball milling [166,256,257]. Similar results was obtained by mechanochemical treatment of NaAlH 4 with TiCl 3, ZrCl 4, FeCl 2 and FeCl 3 [113]. Also it has been demonstrated that ball milling alone to some extent improves the dehydrogenating kinetics of undoped metal alanates, e.g. LiAlH 4 and NaAlH 4 [258,259]. A study of LiAlH 4 showed that milling between 1.5 and 14 h promoted decomposition at 333 K lower than for as-received LiAlH 4. Furthermore, it was also shown that LiAlH 4 milled for less than 14 h decomposes via a twostep reaction pathway (reaction schemes Eqs. (30) and (31)). 3LiAlH 4! Li 3 AlH 6 þ 2Al þ 3H 2 Li 3 AlH 6! 3LiH þ Al þ 1:5H 2 It has also been demonstrated that mechanochemical synthesis is a convenient method for preparation of bialkali metal hydrides from alkali metal hydrides and alkali metal alanates, e.g. Na 2 LiAlH 6,is prepared by ball milling sodium hydride, lithium hydride, and sodium alanate in a stoichiometric composition which reacts by an addition reaction according to reaction scheme Eq. (32) [157]. NaH þ LiH þ NaAlH 4! Na 2 LiAlH 6 Bialkali alanates such as K 2 LiAlH 6 and K 2 NaAlH 6 have been prepared by similar reactions [176,177]. Magnesium and calcium alanate can be readily prepared from sodium or lithium alanate and magnesium or calcium chloride, respectively via a metathesis reaction according to reaction scheme Eq. (33) [161,176,179,260 262]. M 0 Cl 2 þ 2MAlH 4! M 0 ðalh 4 Þ 2 þ 2MClðM 0 ¼ Mg or Ca; M ¼ Li or NaÞ Interestingly, if the amount of lithium alanate in the reaction mixture is increased so that a composition of MgCl 2 :LiAlH 4 1:3 is used, lithium magnesium alanate, LiMg(AlH 4 ) 3 forms according to reaction scheme Eq. (34) [261]. MgCl 2 þ 3LiAlH 4! LiMgðAlH 4 Þ 3 þ 2LiCl Recently, also Sr(AlH 4 ) 2 and lithium beryllium hydrides Li n Be m H n+2m have been prepared mechanochemically [263,264]. Furthermore, rare earth metal chlorides have been ball milled with sodium alanate and partial decomposition was observed due to a metathesis reaction and release of hydrogen to form the more stable hexahydridoaluminate complex ion, AlH 3 6, according to reaction scheme Eq. (35), which is suggested for R = La, Pr, Ce, Nd [265]. ð30þ ð31þ ð32þ ð33þ ð34þ

64 J. Huot et al. / Progress in Materials Science 58 (2013) 30 75 RCl 3 þ 3NaAlH 4! RðAlH 4 Þ 3 þ 3NaCl! RAlH 6 þ 2Al þ H 2 ð35þ 4.3. Novel quaternary hydrides Recently several novel quaternary hydrides based on the amide anion, NH 2 and alanates, AlH 4 or borohydrides, BH 4 or BH 3 have been prepared. This class of materials is of high interest for hydrogen storage, since it combines both high hydrogen densities and storage properties of two types of storage materials. A range of novel materials can be synthesized by mechanical milling utilizing several different types of reactions. 4.3.1. Metal borohydride amides Formation of a quaternary hydride having the approximate composition Li 3 BH 4 (NH 2 ) 2, has been achieved mechanochemically from LiBH 4 LiNH 2 in molar ratio 2:1 according to the addition reaction shown in reaction scheme Eq. (36) [266,267]. 2LiNH 2 þ LiBH 4! Li 3 BH 4 ðnh 2 Þ 2 ð36þ Pinkerton et al. also reported on the extent of the reaction as a function of the milling time. This showed that a substantial amount of Li 3 BH 4 (NH 2 ) 2 is formed after 40 min of milling and that the reaction is completed after 300 min of milling. Milling for a total of 960 min caused no further changes in the reaction product suggesting that the milling does not produce an amorphous phase. Noritake et al. found that varying the LiNH 2 :LiBH 4 molar ratio i.e. using 1:1, 2:1 and 3:1 yielded products of different compositions according to reaction schemes Eqs. (36) (38) [268]. LiNH 2 þ LiBH 4! Li 2 ðbh 4 ÞNH 2 3LiNH 2 þ LiBH 4! Li 4 ðbh 4 ÞðNH 2 Þ 3 ð37þ ð38þ The BH 4 and NH 2 anions are preserved during the mechanochemical synthesis and in the structure of Li 4 (BH 4 )(NH 2 ) 3 they are positioned on specific crystallographic sites, i.e. an ordered structure is formed during ball milling as opposed to formation of a solid solution. The structures of Li 2 (BH 4 )NH 2 and Li 3 BH 4 (NH 2 ) 2 remain to be solved hence the precise stoichiometry of these compounds is not known. However, the compositions of the ball-milled mixtures and the decomposition reactions suggest the compositions stated in reaction schemes Eqs. (36) and (37). Furthermore, a phase diagram study was carried out by Meisner et al. in which samples of LiNH 2 LiBH 4 in a wide range of molar ratios of x = 0.33 0.80 were prepared by ball milling [269]. The study shows that samples of x = 0.6 0.8 yield the cubic Li 4 BH 4 (NH 2 ) 3, while x < 0.5 yield another cubic compound, presumably Li 3 (BH 4 )(NH 2 ) 3. Furthermore, the study also implies existence of two other new compounds, denoted c and d at x = 0.6 and 0.8, respectively. These compounds might be metastable arising from the highly non-equilibrium mechanochemical process. Several metal amidoboranes, M(NH 2 BH 3 ) x have also been prepared by mechanochemical synthesis from amidoborane, NH 3 BH 3 and metal hydrides, e.g. LiNH 2 BH 3, NaNH 2 BH 3 and Ca(NH 2 BH 3 ) 2 according to reaction scheme Eqs. (39) and (40) [270,271]. NH 3 BH 3 ðsþþmhðsþ!mnh 2 BH 3 ðsþþh 2 ðgþðm ¼ Li; NaÞ 2NH 3 BH 3 ðsþþcah 2 ðsþ!caðnh 2 BH 3 Þ 2 ðsþþ2h 2 ðgþ ð39þ ð40þ Recently, LiNH 2 BH 3 has been used as starting material for synthesis of Y(NH 2 BH 3 ) 3 by a metathesis reaction according to reaction scheme Eq. (41) [272]. YCl 3 þ 3LiNH 2 BH 3! 3LiCl þ YðNH 2 BH 3 Þ 3 ð41þ In the same study similar syntheses were tested utilizing different combinations of MNH 2 BH 3 (M = Li, Na) and YX 3 (X = F, Cl), however the reactions seem either to have a very low efficiency or the desired reaction product forms as an amorphous phase.

J. Huot et al. / Progress in Materials Science 58 (2013) 30 75 65 4.3.2. Metal alanate amides In general, ball milling mixtures of alkali amides, MNH 2 and alkali alanates, MAlH 4 (M = Li or Na) result in the release of hydrogen caused by the occurring multistep reactions [273 276]. Furthermore, the presence of amide might facilitate transformation of the tetrahedral AlH 4 into octahedral AlH3 6 by ball milling at ambient conditions. Dolotko et al. observed formation of Li 3 AlH 6 already after 4 min of milling of a LiAlH 4 LiNH 2 sample in molar ratio 1:1 [274]. Xiong et al. report release of approximate one equivalent H 2 as all LiAlH 4 is transformed to Li 3 AlH 6 according to reaction scheme Eq. (42) [273]. 3LiAlH 4! Li 3 AlH 6 þ 2Al þ 3H 2 In comparison negligible amounts of hydrogen were released from LiAlH 4 ball milled for 36 h [273]. Prolonged milling results in formation of amorphous compounds and from combined MAS NMR and PXD analysis the reactions shown in reaction scheme 43 and 44 are suggested [274]. 2LiAlH 4 þ LiNH 2! Li 3 AlH 6 þ AlN þ 2H 2 ð42þ ð43þ Li 3 AlH 6 þ LiNH 2! 4LiH þ AlN þ 2H 2 ð44þ A similar sample of NaAlH 4 NaNH 2 in molar ratio 1:1 was found to react at a much slower rate and requires 60 min of milling to reach its completion. The occurring reactions are similar to the reactions observed for LiAlH 4 LiNH 2 also yielding Na 3 AlH 6 as an intermediate [274]. Similar studies have been performed for samples of LiAlH 4 NaNH 2 (molar ratio 1:2, 1:1 and 2:1) and NaAlH 4 LiNH 2 (molar ratio 1:1) [274 276]. These studies all showed release of two equivalents H 2 during milling. For samples containing LiAlH 4 NaNH 2 the first reaction step is a cation-exchange reaction according to reaction scheme Eq. (45) starting already after 1 min of milling. 3LiAlH 4 þ 4NaNH 2! 3NaAlH 4 þ Li 3 NaðNH 2 Þ 4 Upon further milling of LiAlH 4 NaNH 2 and NaAlH 4 LiNH 2 in molar ratio 1:1, LiNa 2 AlH 6 is formed as an intermediate decomposition product [274]. This formation is facilitated by the presence of the amide, since neither of the pure alanates nor the mixture of the two could transform to AlH 3 6 containing compounds by ball milling alone [277,278]. The amides and alanates present in the sample at this stage are slowly consumed leading to formation of Al, LiH, NaH and an unknown compound, possibly AlN or LiAl 0.33 NH [274,275]. After only 30 min of milling no further changes are observed. Furthermore, Dolotko et al. reported formation of a solid solution, Na 3 x Li x AlH 6 in the NaAlH 4 LiNH 2 sample according to reaction scheme Eq. (46). NaAlH 4 þð2 xþnah þ xlih! Na 3 x Li x AlH 6 The samples of LiAlH 4 NaNH 2 in molar ratio (1:2) and (2:1) exhibit different reaction pathways during the prolonged milling due to the excess of NaNH 2 or LiAlH 4, respectively [276]. The overall reactions suggested to occur during ball milling for the (1:2) and (2:1) samples are shown in reaction schemes Eqs. (47) and (48), respectively. LiAlH 4 þ 2NaNH 2! 2NaH þ LiAlN 2 H 2 þ 2H 2 ð45þ ð46þ ð47þ 2LiAlH 4 þ NaNH 2! NaAlH 4 þ Li 2 AlNH 2 þ 2H 2 ð48þ Apparently, up to now, a mixed borohydride alanate compound has not been prepared. The system LiBH 4 NaAlH 4 was investigated mechanochemically and by hand-mixing, the latter treatment did not lead to any reactions while a metathesis reaction was observed for the ball milled sample, see Eq. (49) [279,280]. LiBH 4 ðsþþnaalh 4 ðsþ!lialh 4 ðsþþnabh 4 ðsþ ð49þ 4.4. Solid liquid mechanically assisted synthesis In some cases addition of a solvent during mechanochemical synthesis is a necessity to promote the desired reaction, e.g. Ca(AlH 4 ) 2 can be prepared by a so-called mechanically assisted synthesis

66 J. Huot et al. / Progress in Materials Science 58 (2013) 30 75 from NaAlH 4 and CaCl 2 mediated by tetrahydrofuran (THF) according to reaction scheme Eq. (50) [281]. 2NaAlH 4 þ CaCl 2 þ 4THF! CaðAlH 4 Þ 2 4THF þ 2NaCl ð50þ The synthesis reported by Fichtner et al. was performed in a glass ball mill reactor to create fresh particle surfaces. The mixture was heated under reflux and the grey solid precipitate was milled by the glass balls. The high yield of the solvated product (84%) was attributed to the high dissolution capacity for NaAlH 4 of THF and that the adduct might form more easily compared to CaAlH 4. After the synthesis the solvent can be removed almost completely from freshly prepared Ca(AlH 4 ) 2 4THF under vacuum yielding a fine white powder of Ca(AlH 4 ) 2. 5. Final remarks and conclusions Recent progress in the field of hydrogen storage materials has received extensive support from mechanochemistry methods. Mechanical milling has been widely used not only to tune metal microstructures for modifying their hydrogenation properties but also as an efficient tool for the synthesis of hydrogen storage materials. Solid/solid, solid/liquid and solid/gas reactions can be activated by mechanochemistry. Mechanical milling acts as a combination of compression and shear on the powder between two colliding balls and between ball and container wall. Upon impact, the powder particles trapped between them will first experience elastic deformations, which are reversible in the elastic region (Fig. 22). If the load increases, the material enters into plastic region and irreversible deformations occur, which may be followed by breakage of the material. Therefore, collisions result in impulses of compression and shear that generate plastic deformation and fracture of the particles. Most of the energy transferred to the powder is mainly used for creating new surfaces and concomitant particle refining. Very similar mechanical effects are produced by Severe Plastic Deformation (SPD) techniques such as Equal Channel Angular Pressing (ECAP), Cold Rolling (CR), and High Pressure Torsion (HPT). Therefore, it could be expected that SPD will have similar impact on hydrogen storage behaviour than mechanical milling. In the particular case of solid/solid reactions, while comminution is an important result of milling, agglomeration assisted by cold-welding processes and thereby the formation of active interfaces is more relevant. Solid-state reactions likely happen at the interface between solid particles of the different reactants. The increase of the surface-area to bulk-volume ratio due to mechanically induced decreasing particle size increases the interfacial contact area between reactive compounds and therefore also the rate of reaction. In addition, plastic deformations due to mechanical work create defects providing fast atomic diffusion pathways and contributing to the increased reaction rate. The intense mixing action provided by the random motion of milled powders and repeated particle fracture and welding favours the formation of active interfaces and ensures the chemical homogeneity of the final product at long milling time. As concerns solid/gas reactions, particle comminution and related effects such as fresh surface generation and diffusion path reduction are determinant. Thus, Mechanical Milling under hydrogen gas is characterized by fast formation of hydride compounds under moderate pressure and temperature. For instance, several days are required for the synthesis of Mg 2 CoH 5 and Mg 2 FeH 6 hydrides by sintering methods at temperatures as high as 750 K, whereas the reaction takes place in only 3 h by reactive ball milling under the same hydrogen pressure (9 MPa) [116,139]. Embrittlement due to hydrogen absorption favours particle refinement and fast synthesis of binary metal hydrides. From this point of view, milling energy (determined by milling process parameters such as rotation speed and ball-to-powder mass ratio) and mechanical properties of reactants (toughness, fracture limit) must play a key role. For the synthesis of ternary hydrides starting from elemental powders, the above-mentioned mechanisms of cold-welding and interface diffusion of solid reactants should be also considered. The feasibility of hydride formation in solid/gas reactions is governed by thermodynamics (i.e. hydride stability under external pressure and temperature conditions). Thus, the formation of highly sta-

J. Huot et al. / Progress in Materials Science 58 (2013) 30 75 67 ble hydrides such as TiH 2, ZrH 2 and MgH 2 is straightforward. In contrast, the synthesis of hydrides that are reversible near room temperature, such as LaNi 5 and NaAlH 4 requires high pressure for the hydride to be stable at the temperatures reached during the milling process [121]. If the pressure is not high enough, only the a-solid solution LaNi 5 H 0.15 or the more stable Na 3 AlH 6 phase are observed, respectively. In this context, the failure to obtain the LiAlH 4 or ternary alkaline hexa-alanates by reactive ball milling is not surprising since they are not stable at the usual operating pressure and temperature [121]. Therefore, in reactive ball-milling experiments, both macroscopic and local pressures as well as temperatures have to be considered in detail. As concerns pressure, one can distinguish the mechanical pressure in the material trapped between two colliding steel balls (internal mechanical pressure) and the gas vial pressure (external isostatic pressure). The gas pressure is not significantly affected by milling due to the high compressibility of the gas phase. Most modern equipment allows for gas pressures up to 15 MPa. In contrast, the internal pressure of the material at mechanical impact may well reach some GPa and is not isotropic. This would explain for instance the formation of the high pressure c-mgh 2 phase by mechanical milling, which otherwise occurs in anvil cells above 2 GPa [282]. Formation of c-mgh 2 phase can be obtained by mechanical milling of thermodynamically stable b-mgh 2 phase under argon atmosphere, which demonstrates that its formation is related to the internal mechanical pressure. As concerns the temperature, one can also distinguish between the temperature of the material trapped between the milling tools (material temperature at impact) and the macroscopic material temperature. For materials with high thermal conductivity (i.e. metals) or by milling under hydrogen gas (which also offers high thermal conductivity), the macroscopic material temperature is not expected to differ much from the vial temperature. The temperature increase in the vial does not exceed some tens of degrees and temperatures in the range 320 350 K have been monitored by temperature gauges. This temperature increase is however non-negligible for hydrogen storage systems that are reversible near normal conditions of pressure and temperature. Several methods can be followed to circumvent this problem such as working at higher hydrogen pressures and minimizing temperature increase by using short milling times (below 10 min). Another and more elegant alternative approach is working at low temperatures, i.e. cryo-milling, though the temperature of the system has to be kept high enough to allow for hydrogen mobility in the bulk powder material. Mechanical milling of powders can be performed under other reactive gases such as diborane and ammonia or in a liquid medium such as THF for the synthesis of hydrogen storage materials. Once again fresh surface generation induced by mechanical work is likely to be determinant to promote solid/gas and solid/liquid reactions. Contrary to reactive mechanical milling under hydrogen gas, these preparation methods are only recently explored in the literature. Further progress is still needed to understand the involved reaction mechanisms and the feasibility of compound formation by these novel routes. A wide research field remains open for the production of new hydrogen storage systems with undiscovered hydrogenation properties. Acknowledgments We thank V. Balema for fruitful discussions regarding this review. The work was supported in part by the Danish National Research Foundation (Center for Materials Crystallography), the Danish Strategic Research Council (Center for Energy Materials and the HyFillFast project), and by the Danish Research Council for Nature and Universe (Danscatt). We are grateful to the Carlsberg Foundation. J.H. would like to thank the Natural Science and Engineering Council of Canada and also the Research Council of Norway for additional funding that permitted a sabbatical leave at the Institute for Energy Technology (IFE) in Norway. ML, FC and JZ would like to thank CNRS and the French agency ANR for financial supports trough research programs ALHAMO and NANOHYDLI. References [1] Grochala W, Edwards PP. Thermal decomposition of the non-interstitial hydrides for the storage and production of hydrogen. Chem Rev 2004;104:1283 315.

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