materials

Review Synthesis and Stability of Hydrogen Storage Material Aluminum Hydride

Wenda Su 1, Fangfang Zhao 1, Lei Ma 1, Ruixian Tang 1, Yanru Dong 1, Guolong Kong 1, Yu Zhang 1, Sulin Niu 1, Gen Tang 2, Yue Wang 2, Aimin Pang 2,*, Wei Li 2,* and Liangming Wei 1,*

1 Key Laboratory for Thin Film and Microfabrication Technology of the Ministry of Education, Department of Microelectronics and Nanoscience, School of Electronics Information and Electrical Engineering, Shanghai Jiao Tong University, Dong Chuan Road No. 800, Shanghai 200240, China; [email protected] (W.S.); [email protected] (F.Z.); [email protected] (L.M.); [email protected] (R.T.); [email protected] (Y.D.); [email protected] (G.K.); [email protected] (Y.Z.); [email protected] (S.N.) 2 Science and Technology on Aerospace Chemical Power Laboratory, Hubei Institute of Aerospace Chemotechnology, Xiangyang 441003, China; [email protected] (G.T.); [email protected] (Y.W.) * Correspondence: [email protected] (A.P.); [email protected] (W.L.); [email protected] (L.W.)

Abstract: Aluminum hydride (AlH3) is a binary metal hydride with a mass hydrogen density 3 of more than 10% and bulk hydrogen density of 148 kg H2/m . Pure aluminum hydride can easily release hydrogen when heated. Due to the high hydrogen density and low decomposition temperature, aluminum hydride has become one of the most promising hydrogen storage media for wide applications, including fuel cell, reducing agents, and rocket fuel additive. Compared


with aluminum powder, AlH has a higher energy density, which can significantly reduce the
 3 ignition temperature and produce H2 fuel in the combustion process, thus reducing the relative Citation: Su, W.; Zhao, F.; Ma, L.; mass of combustion products. In this paper, the research progress about the structure, synthesis, and Tang, R.; Dong, Y.; Kong, G.; Zhang, stability of aluminum hydride in recent decades is reviewed. We also put forward the challenges for Y.; Niu, S.; Tang, G.; Wang, Y.; et al. application of AlH3 and outlook the possible opportunity for AlH3 in the future. Synthesis and Stability of Hydrogen Storage Material Aluminum Hydride. Keywords: AlH3; hydrogen storage; high energy material; stability Materials 2021, 14, 2898. https:// doi.org/10.3390/ma14112898

Academic Editor: Haralampos 1. Introduction N. Miras Aluminum hydride (AlH3) has great potential applications in rocket fuel and fuel Received: 26 March 2021 cell due to its high combustion heat and high hydrogen content [1–3]. The bulk hydrogen 3 Accepted: 18 May 2021 density of AlH3 is 148 kg H2/m (more than twice of liquid hydrogen), and the weight Published: 28 May 2021 hydrogen density is more than 10%, which meets the requirements of the U.S. Department of Energy (DOE) for hydrogen and energy storage materials [4–9]. AlH3 also has the Publisher’s Note: MDPI stays neutral advantages of low reaction heat and rapid hydrogen release rate. However, AlH3 is with regard to jurisdictional claims in unstable and easy decomposition occurs at room temperature, and it can react with water published maps and institutional affil- vapor and oxygen in the environment, resulting in the release of hydrogen. Therefore, how iations. to improve the stability of AlH3 has become the key problem to be solved in the practical application of AlH3 [10]. In recent years, researchers are committed to breaking through the bottleneck problems in the practical application of AlH3, including improving the synthesis method [11–14], Copyright: © 2021 by the authors. coating or doping of AlH3 [15–17], and providing a deep understanding of the thermody- Licensee MDPI, Basel, Switzerland. namics and kinetics of AlH3 [18–20]. This review focuses on the recent studies on AlH3. In This article is an open access article this review, we first introduce the physical and chemical properties of aluminum hydride, distributed under the terms and including crystal structure and thermal decomposition reaction kinetics. Next, the common conditions of the Creative Commons synthesis methods of aluminum hydride are described. Last, we put forward the methods Attribution (CC BY) license (https:// to improve the stability of aluminum hydride and outlook the possible opportunities for creativecommons.org/licenses/by/ application of AlH3 in the future. 4.0/).

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Materials 2021, 14, 2898 the methods to improve the stability of aluminum hydride and outlook the possible2 of op 16‐ portunities for application of AlH3 in the future.

2. Physical and Chemical Properties There are at least seven different crystal structures of AlH3 due to the different syn‐ There are at least seven different crystal structures of AlH3 due to the different syn- thesisthesis routesroutes and reactionreaction conditionsconditions (mainly(mainly referring to thethe reaction timetime and tempera-tempera‐ ture)ture) [[21,22].21,22]. Brower Brower et et al. al. [23 [23]] first first synthesized synthesizedα phase α phase of non-solvated of non‐solvated aluminum aluminum hydride, hy‐ dride, and then successfully synthesized six other AlH3 polymorphs:0 α ,β,γ,δ,ε and ζ. and then successfully synthesized six other AlH3 polymorphs: α , β, γ, δ, ε and ζ. It has beenIt has proved been proved by experiment by experiment and theory and theory that the thatα thecrystal α crystal phase phase is the is most the most stable stable one, followedone, followed by the byβ theand β γandcrystal γ crystal phase phase [24]. Figure[24]. Figure1 shows 1 shows several several common common structures struc‐ tures of AlH3 polymorphs [10]. The α phase belongs to the trigonal space group (R3c), of AlH3 polymorphs [10]. The α phase belongs to the trigonal space group (R3c), with thewith lattice the lattice parameters parameters of a = of 4.449 a = 4.449 Å and Å andc = 11.804 c = 11.804 Å [25 Å]. [25]. The The structure structure is composed is composed of alternatingof alternating planes planes of of H H and and Al Al atoms, atoms, which which are are stacked stacked perpendicularly perpendicularly toto thethe C-axis.‐axis. ThereThere are six six H H atoms atoms around around each each Al Al atom atom to toform form regular regular octahedral octahedral coordination. coordination. Be‐ Becausecause the the Al Alatom atom has has only only three three bonding bonding electrons, electrons, two two Al atoms Al atoms form form the Al the‐H Al-H-Al‐Al elec‐ electrontron deficient deficient bridge bridge bond bond by sharing by sharing one electron one electron of H of atom. H atom. The structure The structure is composed is com- posedof an Al of‐ anH‐Al Al-H-Al bridge bridge bond, bond,which which connects connects the atoms the atoms in the in whole the whole unit cell unit into cell a into rela a‐ relativelytively stable stable whole. whole. Therefore, Therefore, the the α αphasephase has has good good chemical chemical stability stability in macroscopicmacroscopic view.view. The crystal parameters of the α phasephase areare listedlisted inin TableTable1 .1. The The ββ phasephase belongsbelongs toto thethe cubiccubic spacespace groupgroup (Fd(Fd33m),m), withwith thethe latticelattice parametersparameters ofof aa == 9.0049.004 ÅÅ [[26].26]. TheThe γγ phasephase belongs to to the the orthorhombic orthorhombic space space group group (Fd (Fd3m),3m), with with the the lattice lattice parameters parameters of a of = a5.3806(1)= 5.3806(1) Å, b Å =, b7.3555(2)= 7.3555(2) Å, Å,c =c 5.77509(5)= 5.77509(5) Å Å[21].[21 ].β β AlH− AlH and3 and γ γ AlH− AlH will3 will spontane sponta-‐ ◦ neouslyously convert convert to to αα − AlHAlH at3 ataround around 100 100 °CC[ [27],27], and and the the reaction reaction heats ofof polymorphpolymorph transformationstransformations are: are: 1.5 1.5kJ/mol kJ/molfor for the theβ −βAlH AlH3 → →α − αAlH AlH3 transition transition and and 2.8 kJ/mol2.8 kJ/molfor thefor γthe− AlHγ AlH3 → α→− αAlH AlH3 transition transition [24]. [24]. According According to the to reaction the reaction heat of heat the of polymorph the poly‐ transformation,morph transformation,α − AlH α3 is AlH proved is proved to be the to mostbe the stable most phase.stable phase. Therefore, Therefore, only the onlyα phasethe α hasphase a practical has a practical application application value. In value. this review, In this all review, aluminum all aluminum hydride involved hydride is inα‐ phasevolved except is α phase for special except indication. for special indication.

Figure 1. Structures of AlH3 polymorphs of α AlH , α AlH0 ,βAlH , γ AlH . Reprinted with Figure 1. Structures of AlH3 polymorphs of α − AlH 3, α − AlH 3, β − AlH 3, γ −AlH3. Reprinted withpermission permission from from ref. [10]. Ref. Copyright [10]. Copyright 2009 Elsevier. 2009 Elsevier.

Table 1. Crystallographic data of α − AlH3 [28]. Cited from Ref. [28] with permission.

Crystal System Trigonal hexagonal axes Space group R3c(167) Formula per unit cell, [Z] 6

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Materials 2021, 14, 2898 3 of 16 Table 1. Crystallographic data of αAlH [28]. Cited from Ref. [28] with permission. Table 1. Crystallographic data of αAlH [28]. Cited from Ref. [28] with permission. Crystal System Trigonal Crystal System Trigonal Table 1. Cont. hexagonal axes Space group hexagonal axes Space group R3c(167) Crystal SystemR3c(167) Trigonal Formula per unit cell, [Z] 6 Formula per unit cell, [Z] 6 A = Ab = b4.449 = 4.449 Å Å Lattice parameters A = b = 4.449 Å Lattice parameters c = 11.804c = 11.804 Å Å Lattice parameters αc = 11.804β = 90◦ ,Åγ = 120◦ α = β = 90°, γ = 120° Density α = β = 1.47790°, γ g/cm = 120°3 Density 1.477 g/cm3 Density 1.477 g/cm3 MolecularMolecular weight weight 30.01 30.01 g/mol g/mol Molecular weight 30.01 g/mol

α α AlH− AlH is a iswhite a white nonvolatile nonvolatile hexagonal hexagonal crystal crystal with with a density a density of 1.477 of 1.477 g/cmg/cm, the3, the α AlH is a3 white nonvolatile hexagonal crystal with a density of 1.477 g/cm, the theoreticaltheoretical hydrogen hydrogen content content of 10.08%, of 10.08%, insoluble insoluble or slightly or slightly soluble soluble in ether, in ether, soluble soluble in in theoretical hydrogen content of 10.08%, insoluble or slightly soluble in ether, soluble in tetrahydrofuran,tetrahydrofuran, and and inert inert to to hydrazine hydrazine [29,30]. [29,30]. When When exposedexposed toto water, water, the the pure pureα −αAlH tetrahydrofuran, and inert to hydrazine [29,30]. When exposed to water, the pure α 3 AlHhydrolyzes hydrolyzes slowly, slowly, and and takes takes several several hours hours for for complete complete hydrolyses hydrolyses [31 ,[31,32].32]. α − αAlH AlH hydrolyzes slowly, and takes several hours for complete hydrolyses [31,32]. α 3 AlHcan can be be stored stored for for several several months months at at roomroom temperature,temperature, andand it it beginsbegins to decompose at AlH can be stored for several months at room temperature, and it begins to decompose at 100100 °C,◦C ,releasing releasing hydrogen hydrogen and and forming forming aluminum [33]. [33]. The optical and scanning electronelec‐ at 100 °C, releasing hydrogen and forming aluminum [33]. The optical and scanning elec‐ tronmicroscopy microscopy pictures pictures of ofα −α AlH AlH3 are are reported reported (as (as shown shown in in Figures Figures2 and 2 and3). It3). can It can be seenbe tron microscopy pictures of α AlH are reported (as shown in Figures 2 and 3). It can be seenthat thatα −α AlH AlH3 is is a densea dense and and transparent transparent crystal crystal with with a gooda good crystal crystal plane plane and and crystal crystal edge seen that α AlH is a dense and transparent crystal with a good crystal plane and crystal edgeconfiguration configuration [28 [28].]. edge configuration [28].

(a) (b) (a) (b) FigureFigure 2. Optical 2. Optical microscopy microscopy pictures pictures of of αAlHα − AlH. 3(.(a)a and,b) are (b) optical are optical microscope microscope pictures pictures of α − ofAlH 3 Figure 2. Optical microscopy pictures of αAlH. (a) and (b) are optical microscope pictures of αAlHat different at different magnification. magnification. Reprinted Reprinted with permission with permission from Ref. from [28]. Copyrightref. [28]. Copyright 2009 WILEY-VCH 2009 αAlH at different magnification. Reprinted with permission from ref. [28]. Copyright 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim. WILEYVerlag‐VCH GmbH Verlag & Co. GmbH KGaA, & Co. Weinheim. KGaA, Weinheim.

(a) (b) (a) (b) Figure 3. SEM pictures of αAlH. (a) and (b) are SEM pictures pictures of αAlH at different Figure 3. SEM pictures of αAlH . (a) and (b) are SEM pictures pictures of αAlH at different magnification.Figure 3. SEM Reprinted pictures with of αpermission− AlH3.( afrom,b) are ref. SEM [28]. pictures Copyright pictures 2009 ofWILEYα − AlH‐VCH3 at Verlag different magnification. Reprinted with permission from ref. [28]. Copyright 2009 WILEY‐VCH Verlag GmbHmagnification. & Co. KGaA, Reprinted Weinheim. with permission from Ref. [28]. Copyright 2009 WILEY-VCH Verlag GmbH GmbH& Co. & Co. KGaA, KGaA, Weinheim. Weinheim.

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3. Synthesis Methods 3.1. Liquid Phase Synthesis Methods

In 1942, Stecher and Wiberg [34] first prepared AlH3·2N(CH3)3 with low yield and purity by liquid phase synthesis. A few years later, Finholt et al. [35] prepared a solution of AlH3 in ether by the reaction of LiAlH4 and AlCl3. The chemical equation of the reaction is:

ether 3LiAlH4 + AlCl3 → 4AlH3 + 3LiCl (1)

Finholt et al. [35] also proposed another reaction route for the preparation of AlH3: the reaction of LiH and AlCl3 in ether solution. The chemical equation of the reaction is as follows: ether 3LiH + AlCl3 → AlH3 + 3LiCl (2) For these two different reaction pathways, Finholt et al. [35] found that although Reaction (2) was simpler, Reaction (1) was more stable and rapid. At the same time, the target product of Reaction (2) can easily continue to react to form LiAlH4, so Reaction (1) was considered to be a better choice. The final product prepared by Finholt et al. was AlH3·0.3[(C2H5)2O] with a yield of 85%. However, due to the presence of a certain amount of ether in the final product, how to remove the ether without loss of hydrogen becomes the focus of the next step. In 1955, Chizinsky [36] prepared AlH3 without ether solvent for the first time. The AlH3 ether solution prepared by the usual method was filtered into inert liquid (pentane and ligroin), and then dried under a high vacuum for 12 h. However, Chizinsky’s exper- imental results have not been effectively repeated. In 1975, Brower et al. [23] published a summary on the preparation of AlH3. They synthesized seven different polymorphs of AlH3. The synthesis method is similar to that of Stecherand Wiberg [34] and Finholt et al. [35], namely the metathesis reaction between LiAlH4 and AlCl3 (Reaction (3)), and then the ether is removed by heating the mixed solution of ether and toluene with the good solubility of ether in toluene (Reaction (4)).

4LiAlH4 + AlCl3 + Et2O → 4AlH3·nEt2O + 3LiCl ↓ +LiAlH4 + Et2O (3)

65°C 4AlH3·Et2O + LiAlH4 + LiBH4 + Et2O = 4AlH3 + Et2O ↑ + LiAlH4 ↓ + LiBH4 ↓ (4) 5h

Brower et al. [23] found that the pure AlH3 etherate would decompose and result in loss of hydrogen when heated in vacuum, but if there was excessive LiAlH4, the ether in AlH3 etherate could be removed. They also found that excess LiAlH4 can decrease the time and temperature of desolvation. The mixed use of LiAlH4 and LiBH4 can further reduce the temperature and time of desolvation reaction. However, when only LiBH4 exists and LiAlH4 is absent, desolvation cannot be achieved [23]. When the molar ratio of LiAlH4: AlH3: LiBH4 is 1:4:1, γ − AlH3 is prepared [23]. Brower et al. [23] carried out X-ray analysis of the prepared γ—AlH3 and found that it has the same X-ray powder pattern as the sample reported by Chizinsky. Therefore, Chizinsky’s experimental results are considered to be caused by accidental use of excessive LiAlH4. Different polymorphs of AlH3 can be prepared by slightly adjusting Reactions (3) and (4). γ − AlH3 was prepared by reducing the molar ratio of LiBH4 and reacting at low temperature, while ε − AlH3 and δ − AlH3 were prepared in the presence of a small amount of water. These nonsolvated phases are not considered to be converted into α − AlH3, and their thermal stability is much worse than that of α − AlH3 [23]. Brower et al. [23] also designed a process to prepare a large number of microcrystalline α − AlH3: ether solution was prepared according to Reaction (3), and then desolvent in toluene (or benzene) using a crystallization flask with a fractionator. After that, the researchers proposed a variety of liquid-phase synthesis methods of AlH3. The researchers found that etherated or pure AlH3 could be prepared by the reaction of binary hydrides Materials 2021, 14, x FOR PEER REVIEW 5 of 17

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found that etherated or pure AlH3 could be prepared by the reaction of binary hydrides or tetrahydroaluminates (aluminates) of the first and second groups with Bronsted‐Lewis acidsor tetrahydroaluminates such as AlCl3 [37]. (aluminates) of the first and second groups with Bronsted-Lewis acidsIn such recent as AlClyears,3 [ researchers37]. have improved the wet synthesis process. Bulychev et al. [38] synthesizedIn recent years, unsolvated researchers AlH3 haveby the improved reaction of the AlBr wet3 or synthesis sulfuric acid process. with LiAlH Bulychev4 in pureet al. toluene [38] synthesized or in toluene unsolvated containing AlH 5–103 by wt% the diethyl reaction ether. of AlBr However,3 or sulfuric the unsolvated acid with AlHLiAlH3 is4 notin pure a high toluene‐purity or crystal, in toluene and the containing concentrated 5–10 wt%sulfuric diethyl acid ether.used in However, the reaction the processunsolvated is comparatively AlH3 is not a high-puritydangerous. crystal, and the concentrated sulfuric acid used in the reactionLacina process et al. is[39] comparatively carried out vacuum dangerous. distillation for the mixed solution of ether com‐ pounds,Lacina and et successfully al. [39] carried obtained out vacuummicrometer distillation grade α—AlH for the3 by mixed using solution a variable of ethertem‐ peraturecompounds, solvent and crystallization successfully obtained method. micrometer Graetz et al. grade [40] αreported—AlH3 bya low using‐temperature, a variable lowtemperature‐pressure, solvent reversible crystallization reaction using method. titanium Graetz‐doped et al. [aluminum40] reported powder a low-temperature, and triethy‐ low-pressure, reversible reaction using titanium-doped aluminum powder and triethylene- lenediamine (TEDA) to regenerate AlH3. They first prepared AlH3 by a traditional diamine (TEDA) to regenerate AlH . They first prepared AlH by a traditional method, method, then doped it with β‐TiCl3.3 After that, the samples were3 dried and decomposed then doped it with β-TiCl . After that, the samples were dried and decomposed in vacuum in vacuum to produce active3 aluminum powder containing titanium. AlH3‐TEDA can be regeneratedto produce active by the aluminum reaction of powder the active containing aluminum titanium. powder AlH with3-TEDA hydrogen can be and regenerated TEDA in THFby the solution reaction at of a thelower active temperature aluminum and powder pressure. with hydrogen and TEDA in THF solution at a lower temperature and pressure. Although the liquid‐phase synthesis method of AlH3 has been mature and perfect, Although the liquid-phase synthesis method of AlH has been mature and perfect, the the liquid‐phase synthesis method itself has some disadvantages,3 such as being unsafe, liquid-phase synthesis method itself has some disadvantages, such as being unsafe, con- consuming a lot of solvents, involving a complex follow‐up processing, and so on. There‐ suming a lot of solvents, involving a complex follow-up processing, and so on. Therefore, fore, the new synthesis method without solvent has attracted the attention of researchers. the new synthesis method without solvent has attracted the attention of researchers.

3.2. Dry Dry Synthesis Synthesis As mentioned above, the synthesis of AlH3 in solvents requires a lot of solvents, and As mentioned above, the synthesis of AlH3 in solvents requires a lot of solvents, and most of these solvents are expensive and toxic, so the method of synthesizing AlH3 with‐ most of these solvents are expensive and toxic, so the method of synthesizing AlH3 without outliquid liquid phase phase solvents solvents has beenhas been developed. developed. One wayOne ofway dry of synthesis dry synthesis is to make is to aluminummake alu‐ minum react with hydrogen directly under high pressure to produce AlH3. Baranowaski react with hydrogen directly under high pressure to produce AlH3. Baranowaski and andTkacz Tkacz [41] [41] have have systematically systematically measured measured the equilibrium the equilibrium between between gaseous gaseous hydrogen hydrogen and andsolid solid aluminum aluminum hydride hydride in the in the temperature temperature range range of 100–150 of 100–150◦C °C and and pressure pressure range range of 3 of0–12 0–12 kPa. kPa. The The results results show show that that AlH AlH3 can can be be synthesized synthesized onlyonly whenwhen thethe temperature exceeds 140 °C◦C[ [41].41]. Appel and Frankel [42] [42] bombarded pure aluminum target samples with deuterons. The The X X-ray‐ray diffraction patterns of the samples showed that a new phase was formed. By comparison, it was confirmed confirmed that AlH3 was obtained on the surface of thethe aluminum target target [42]. [42]. ◦ Saitoh et al. [43] [43] prepared AlH3 via reacting aluminum with hydrogen at 650 °CC and 10.0 GPa GPa for for 24 24 h. h. They They measured measured the the morphology morphology change change of aluminum, of aluminum, as shown as shown in Fig in‐ ureFigure 4, and4, and found found that that the the surface surface of of aluminum aluminum is is covered covered with with a a layer layer of of white particles, which are colorless and transparent crystals under the transmission light microscope. The white particles were confirmedconfirmed to be AlH3 by powder X X-ray‐ray diffraction.

Figure 4. (a) Polarized micrographmicrograph of of a a longitudinal longitudinal section section of of a recovereda recovered disk disk sample sample hydrogenated hydrogen‐ atedat 650 at◦ 650C and °C 10.0and GPa10.0 forGPa 24 for h. (24b) h. Transmitted (b) Transmitted light micrograph light micrograph of the obtainedof the obtained AlH3 single AlH3 crystal. single crystal.Reprinted Reprinted with permission with permission from Ref. from [43 ].ref. Copyright [43]. Copyright 2008 AIP 2008 Publishing. AIP Publishing.

Saitoh et al. [44] studied the reaction process of aluminum and hydrogen at 10 GPa and 600 ◦C by in situ X-ray diffraction. As shown in Figure5, the X-ray diffraction images of

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Saitoh et al. [44] studied the reaction process of aluminum and hydrogen at 10 GPa and 600 °C by in situ X‐ray diffraction. As shown in Figure 5, the X‐ray diffraction images theof the samples samples are are collected collected every every 10 min10 min during during the the hydrogenation hydrogenation process process [44 ].[44]. The The yellow yel‐ andlow redand particles red particles represent represent Al and Al AlHand 3AlHparticles,3 particles, respectively. respectively. Saitoh Saitoh et al. et found al. found that that the growththe growth process process of AlH of AlH3 single3 single crystal crystal will will go throughgo through three three stages: stages: self-pulverization self‐pulverization of aluminumof aluminum (with (with the the increase increase of reaction of reaction time, time, it is found it is found that the that size the of size aluminum of aluminum Bragg latticeBragg decreases),lattice decreases), hydrogenation hydrogenation of pulverized of pulverized aluminum aluminum (X-ray diffraction(X‐ray diffraction image shows image ashows new Bragg a new lattice), Bragg lattice), and solid-state and solid grain‐state growth grain growth of AlH3 of[44 AlH]. 3 [44].

FigureFigure 5.5.X-ray X‐ray diffraction diffraction images images of of the the sample sample during during the hydrogenation.the hydrogenation. (a) Complete (a) Complete Debye Debye rings fromrings aluminumfrom aluminum at 10 GPa,at 10 27GPa,◦C. 27 (b °C.) Bragg (b) Bragg spots spots from from aggregated aggregated aluminum aluminum (colored (colored in yellow). in yel‐ low). This image was taken immediately after the sample was heated to 600 °C at 10 GPa. (c–f) This image was taken immediately after the sample was heated to 600 ◦C at 10 GPa. (c–f) Diffraction Diffraction images taken after 2 h, 6 h, 12 h, and 24 h heat treatment at 10 GPa, 600 °C. Reprinted images taken after 2 h, 6 h, 12 h, and 24 h heat treatment at 10 GPa, 600 ◦C. Reprinted with permission with permission from ref. [44]. Copyright 2010 Elsevier. from Ref. [44]. Copyright 2010 Elsevier.

The direct reaction of aluminum and hydrogen under high pressure to produce AlH3 The direct reaction of aluminum and hydrogen under high pressure to produce AlH often requires strict reaction conditions and low yield, so this kind of method is not con3‐ often requires strict reaction conditions and low yield, so this kind of method is not consideredsidered as a as good a good choice choice [45]. [45 It ].is It considered is considered that that mechanical mechanical ball ballmilling milling is a issimple a simple and andfeasible feasible method method for forthe the synthesis synthesis of of metal metal compound compound hydrides. hydrides. Mechanical Mechanical ball millingmilling has been used to synthesize Mg(AlH4)2 [46,47], Ca(AlH4)2 [48,49], LiMg(AlH4)2 [50], and has been used to synthesize Mg(AlH4)2 [46,47], Ca(AlH4)2 [48,49], LiMg(AlH4)2 [50], and soso on. Brinks Brinks et et al. al. [22] [22 first] first proposed proposed the the mechanical mechanical ball ballmilling milling method method to prepare to prepare AlD3. AlDThey. prepared They prepared AlD3 by AlD usingby usingthe mixed the mixed ball milling ball milling of 3LiAlD of 3LiAlD4 and AlCland3 AlCl. Brinks. Brinks et al. 3 3 4 3 [22] found that in addition to AlD3 (α and α ), the mixture0 of LiCl and Al was also pro‐ et al. [22] found that in addition to AlD3 (α and α ), the mixture of LiCl and Al was duced in planetary milling at room temperature, while only AlD3 and LiCl were produced also produced in planetary milling at room temperature, while only AlD3 and LiCl were in freezing ball milling at 77 K. Sartori et al. [51] found that AlH3 can be prepared by cry‐ produced in freezing ball milling at 77 K. Sartori et al. [51] found that AlH3 can be prepared omilling aluminium halides and alanate. They also found that the yield of AlH3 can be by cryomilling aluminium halides and alanate. They also found that the yield of AlH3 4 3 4 3 4 canincreased be increased by using by 3LiAlD using 3LiAlD + AlBr4 + and AlBr 3NaLiAlD3 and 3NaLiAlD + AlCl4 compared+ AlCl3 compared with using with 3LiAlD using 3 3LiAlD+ AlCl 4. + AlCl3. PaskeviciusPaskevicius etet al.al. [[52]52] studied thethe structure ofof AlH33 prepared by mechanical grinding, andand foundfound that that some someβ βAlH− AlH3and andγ −γAlHAlH3 were were formed formed in in the the product. product. After After ball ball milling, mill‐ LiCling, LiCl in the in product the product was removed was removed via washing via washing with nitromethane/AlCl with nitromethane/AlCl3 solution,3 solution, and AlH and3 separatedAlH3 separated from the from by-product the by‐product was obtained. was obtained. However, However, the AlH the3 nanoparticles AlH3 nanoparticles adversely ad‐ reactedversely withreacted the with nitromethane/AlCl the nitromethane/AlCl3 solution.3 solution. The hydrogen The hydrogen desolvation desolvation kinetics kinetics of the washedof the washed samples samples were significantly were significantly hindered hindered compared compared to the unwashed to the unwashed samples [ 52samples]. The hydrogen[52]. The hydrogen desorption desorption ability of ability the washed of the sample washed is sample only half is only of that half unwashed of that unwashed sample, assample, shown as in shown Figure in6. Figure 6.

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FigureFigureFigure 6. 6. HydrogenHydrogenHydrogen desorption desorptiondesorption data datadata from fromfrom ( ((AAA))) unwashed unwashed andand ( (BB)) washedwashed samplessamples cryogenicallycryogenically milledmilled for for 1 1 h h with with a a 2:1 2:1 LiCl:AlH LiCl:AlH3 3volume volume ratio. ratio. Reprinted Reprinted with with permission permission from from ref. ref. [52]. [52]. Copyright Copyright milled for 1 h with a 2:1 LiCl:AlH3 volume ratio. Reprinted with permission from Ref. [52]. Copyright 20092009 Elsevier. Elsevier. 2009 Elsevier. DuanDuanDuan et etet al. al. [53] [[53]53] proposed proposed a a cost-effective costcost‐‐effectiveeffective solid-state solidsolid‐‐statestate reaction reaction grinding grinding method method with with cheapcheapcheap metal metalmetal hydride hydridehydride and andand aluminum aluminumaluminum chloride chloridechloride as asas the thethe starting startingstarting reagents reagents to to synthesize synthesize AlH AlH333.. . TheyThey found found that that nano nano nano-sized‐‐sizedsized γγγ − AlH AlHAlH can3cancan be be be synthesized synthesized synthesized by by byreactive reactive reactive milling milling milling the the AlCl the AlCl AlCl33 and and3 MgHandMgH MgH22 nanocrystalline. nanocrystalline.2 nanocrystalline. AccordingAccording According toto XRD,XRD, to NMR, XRD,NMR, NMR, andand TEMTEM and analysis,analysis, TEM analysis, thethe averageaverage the average sizesize ofof thesizethe asas of‐‐synthesized thesynthesized as-synthesized γγ AlH AlHγ − phasephaseAlH3 wasphasewas estimatedestimated was estimated toto bebe aroundaround to be around 8.58.5 nmnm 8.5 [53].[53]. nm DuanDuan [53]. Duan etet al.al. [54]et[54] al. also also [54 found] found also foundthat that MgH MgH that2 2can MgHcan be be2 stably canstably be converted converted stably converted to to γγ AlH AlH toγ in−in the AlHthe process process3 in the of of process grinding grinding of thegrindingthe MgH MgH2 the2/AlCl/AlCl MgH33 mixture. mixture.2/AlCl Further3 Furthermixture. studies studies Further by by studiesTEM, TEM, SEM, SEM, by TEM, and and NMR SEM,NMR showed and showed NMR that that showed the the amor amor that‐‐ phousthephous amorphous intermediate intermediate intermediate (AlH (AlH6)6)nn was was (AlH synthesized synthesized6)n was synthesized preferentially preferentially preferentially and and recrystallized recrystallized and recrystallized to to γγ AlH AlH to atγat −the theAlH end end3 atof of the the endreaction reaction of the [54]. [54]. reaction Surface Surface [54 nanocrystallization nanocrystallization]. Surface nanocrystallization is is considered considered is consideredto to be be a a key key step to step be in in a mechanochemicalkeymechanochemical step in mechanochemical synthesis synthesis of of synthesis AlH AlH33, ,and and of AlHthe the synthesis 3synthesis, and the process synthesisprocess can can process be be shown shown can be in in shown Figure Figure in 7 7 [55].Figure[55]. 7[55].

FigureFigureFigure 7. 7.7. ((a(a)a )) TheThe The pathwaypathway pathway ofof of thethe the mechanicallymechanically mechanically activatedactivated activated reactionreaction reaction betweenbetween between MgHMgH MgH2 2 andand2 and AlClAlCl AlCl3.3 . ((bb3).) γAlH Mechanism(Mechanismb) Mechanism of of the theof solid thesolid solid state state statesynthesis synthesis synthesis of of nano nano of‐‐sized nano-sizedsized γAlHγ−. .Reprinted ReprintedAlH3. Reprinted with with permission permission with permission from from ref. ref. [56].from[56]. Copyright Copyright Ref. [56]. Copyright 2021 2021 Elsevier. Elsevier. 2021 Elsevier.

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Hlova et al. [57] studied the mechanical grinding process for the synthesis of AlH3 Hlova et al. [57] studied the mechanical grinding process for the synthesis of AlH3 from LiH and AlCl3 at room temperature. The intermediates and final products were char‐ from LiH and AlCl at room temperature. The intermediates and final products were acterized by powder3 X‐ray diffraction and solid‐state 27Al NMR spectroscopy. The exper‐ characterized by powder X-ray diffraction and solid-state 27Al NMR spectroscopy. The imental results show that adding AlCl3 slowly during ball milling can prevent the for‐ experimental results show that adding AlCl slowly during ball milling can prevent the mation of unnecessary intermediate products.3 At the same time, if the reaction is carried formation of unnecessary intermediate products. At the same time, if the reaction is carried outout at at 3 3 MPa MPa hydrogen hydrogen pressure, pressure, the the final final products are not easy to decompose. AlthoughAlthough the mechanical ballball millingmilling method method has has its its unique unique advantages advantages to to synthesize synthe‐ size AlH3, it is difficult to be used in large‐scale production due to its complex preparation AlH3, it is difficult to be used in large-scale production due to its complex preparation process,process, high high reaction reaction conditions conditions (usually (usually at at a a low low temperature temperature or or liquid liquid nitrogen), nitrogen), mixed mixed crystalcrystal phase phase of of reaction reaction products, products, and and difficult difficult purification. purification.

3.3.3.3. Other Other Synthetic Synthetic Methods Methods

InIn the processprocess of of synthesizing synthesizing AlH AlH3 by3 by mechanical mechanical ball milling,ball milling, the reactants the reactants are heated are heatedunevenly, unevenly, and other and complex other complex crystal phases crystal are phases often are produced. often produced. Therefore, Therefore, other synthetic other syntheticmethods methods of AlH3 haveof AlH been3 have proposed been proposed in recent in years. recent Supercritical years. Supercritical technology technology is a new isexperimental a new experimental method method and plays and an plays increasingly an increasingly important important role in role chemical in chemical separation sepa‐ rationand synthesis. and synthesis. McGrady McGrady [58] prepared[58] prepared AlH 3AlHby3 supercritical by supercritical technology. technology. In a In pressure a pres‐ surevessel vessel without without water water and oxygen, and oxygen, aluminum aluminum powder powder and 2% and catalyst 2% catalyst (TiCl3 )(TiCl were3) mixed were mixedevenly, evenly, and liquid and liquid carbon carbon dioxide dioxide (89 MPa) (89 MPa) and hydrogen and hydrogen (50 MPa) (50 MPa) were were introduced, intro‐ duced,heated heated to 60 ◦ toC 60to °C reach to reach supercritical supercritical state, state, stirred stirred at a at rate a rate of of 150 150 r/min r/min for for 11 h, and and thenthen cooled cooled to to room room temperature temperature to toobtain obtain gray gray powder powder AlH AlH3. However,3. However, due to due the to limi the‐ tationlimitation of experimental of experimental conditions, conditions, supercritical supercritical technology technology cannot cannot be applied be applied to the actual to the productionactual production of AlH3 of. AlH3. ZidanetZidanet al. al. [59] [59 synthesized] synthesized α α AlH− AlH by3 byan electrochemical an electrochemical method method for the for first the time: first AlHtime:3∙nTHF AlH3 ·adductnTHF adductwas obtained was obtained by electrolysis by electrolysis with THF with as THFsolvent, as solvent, NaAlH4 NaAlH as electro4 as‐ lyte,electrolyte, aluminum aluminum as anode, as anode, and platinum and platinum as cathode. as cathode. Due Dueto the to strong the strong binding binding force force of AlHof AlH3∙nTHF,3·nTHF, AlH AlH3∙TEA3·TEA was was synthesized synthesized by by introducing introducing triethylamine triethylamine (TEA). (TEA). Then, Then, pure pure α · α—AlH—AlH3 3waswas obtained obtained by by heating heating the the AlH AlH3∙TEA3 TEA solution solution in vacuum in vacuum [59]. [59 This]. This method method can realizecan realize the reversible the reversible cycle cycle of material of material regeneration, regeneration, as shown as shown in Figure in Figure 8. 8.

FigureFigure 8. 8. ProposedProposed reversible reversible fuel fuel cycle cycle for for alane. alane. Reprinted Reprinted with with permission permission from from ref. Ref. [56]. [56 ].Copy Copy-‐ rightright 2021 2021 Elsevier. Elsevier.

4.4. Stability Stability α − ThereThere are are at at least seven different crystal structures inin AlHAlH33, among which α AlH AlH3 α − isis the the most most stable. stable. α AlHAlH will3 will decompose decompose to form to form aluminum aluminum and hydrogen, and hydrogen, as in reac as in‐ tionReaction 5 [60]. (5) [60]. 3 α − AlH → Al + H ↑ (5) 3 23 2 α AlH →Al H ↑ (5) 0 2 α − AlH3/β − AlH3/γ − AlH3 → α − AlH3 (6)

Materials 2021, 14, x FOR PEER REVIEW 9 of 17

Materials 2021, 14, x FOR PEER REVIEW 9 of 17 Materials 2021, 14, 2898 9 of 16 α′ AlH/β AlH/γ AlH →αAlH (6)

3 α′ AlH /γ AlH →Al H ↑ (7) α′ AlH /β AlH /γ AlH →αAlH (6) 0 2 3 α − AlH3/γ − AlH3 → Al + H2 ↑ (7) Sinke et al. [9] investigated the thermodynamic properties of α 3 AlH , and2 they α′ AlH/γ AlH →Al H ↑ (7) found that the enthalpy of formationSinke et of al. α [9 ] AlHinvestigated at room thetemperature thermodynamic was2 −11.4 properties kJ/mol. of α − AlH3, and they α′ AlH, β AlH, andfound γ AlH that are the enthalpyunstable and of formation will transform of α − intoAlH 3αat room AlH temperatureduring was −11.4 kJ/mol. Sinke et al. [9] investigated the thermodynamic properties of α AlH , and they heating (reaction 6) [27].α During0 − AlH the, βheating− AlH process,, and γ the− AlH directare decomposition unstable and of will γ transform AlH into α − AlH dur- found that the3 enthalpy 3of formation of α3 AlH at room temperature was −11.4 kJ/mol. 3 and α′ AlH into Al andα′ing H AlH heating2 was, β also AlH (Reaction observed, and γ (6)) (reaction AlH [27 are]. During7)unstable [61]. Gao theand et heatingwill al. transform[62] process, found into that the α direct AlH during decomposition 0 the outer part of γ AlHheatingof particlesγ − (reactionAlH 3inclinesand 6) [27].α −to During AlHdecompose3 theinto heating Al directly, and process, H 2butwas the the direct also inner observed decomposition part will (Reaction of γ (7))AlH [61]. Gao first convert to α AlHandet, and al.α′ [ then62 AlH] found decompose into that Al and the Hto outer2 Alwas and also part H observed of2, asγ − shownAlH (reaction3 particlesin Figure 7) [61]. inclines 9. Gao et al. to decompose[62] found that directly, but thethe outer inner part part of willγ AlH first particles convert inclines to α − AlHto decompose3, and then directly, decompose but the toinner Al andpart will H2, as shown firstin Figureconvert9 .to α AlH, and then decompose to Al and H2, as shown in Figure 9.

Figure 9. Schematic picture showing the outer layers and the inner parts of γAlH particle be‐ FigureFigure 9. Schematic 9. Schematic picture picture showing showing the outer the layers outer and layers the inner and parts the of inner γAlH parts particle of γ −be‐AlH3 particle having with different decomposition mechanisms: (a) as‐synthesized γAlH , (b) after partial havingbehaving with withdifferent different decomposition decomposition mechanisms: mechanisms: (a) as‐synthesized (a) as-synthesized γAlH , (b)γ after− AlH partial,( b) after partial decomposition, (c) after full decomposition. Reprinted with permission from ref. [62]. Copyright 3 decomposition,decomposition, (c) after (c) after full decomposition. full decomposition. Reprinted Reprinted with permission with permission from ref. [62]. from Copyright Ref. [62 ]. Copyright 2017 Elsevier. 2017 Elsevier. 2017 Elsevier.

Graetz et al. [63] studiedGraetz the etisothermal al. [63] studied decomposition the isothermal process decomposition of AlH3, processand found of AlH 3, and found Graetz et al. [63] studied the isothermal decomposition process of AlH3, and found that the isothermal decompositionthat the isothermal curve decomposition of AlH3 was curvesigmoid, of AlH including3 was sigmoid, induction including pe‐ induction pe‐ that the isothermal decomposition curve of AlH3 was sigmoid, including induction period, riod, acceleration period,riod,acceleration and acceleration decay period, period, period, as and and shown decay in period, period,Figure as 10. asshown Graetz shown in Figure et in al. Figure [63]10. Graetz also 10. et Graetz al. [63] et also al. [63] also measured that the activationα AlH energy of the α AlH decomposition reaction with a small measured that the activationmeasured energy that of the activation decomposition energy of the α reaction− AlH3 decompositionwith a small reaction with a small grain size of 102 kJ/mol, which is similar to that of the α AlH decomposition reaction grain size of 102 kJ/molgrain, which size is ofsimilar 102 kJ/mol to that, of which the α is similar AlH decomposition to that of the α reaction− AlH decomposition reaction with a large grain size measured by Gabis et al. [64]. This indicates that the3 grain size has with a large grain size measuredwith a large by Gabis grain et size al. measured[64]. This indicates by Gabis that et al. the [64 grain]. This size indicates has that the grain size no significant effect on the activation energy of the AlH3 decomposition reaction. How‐ no significant effect onever, thehas activationthe no above significant measured energy effect of values the on AlH theare significantly activation3 decomposition energy lower reaction.than of thethose AlH How of the3 ‐decomposition stabilized α reaction. ever, the above measured values are significantly lower than those of the stabilized α AlHHowever, (157 kJ/mol the above) reported measured by Herley values et al. [65] are. significantly This indicates lower that the than formation those ofof theα stabilized . AlH (157 kJ/mol) reportedAlHα − surfacebyAlH Herley3 (157 oxide etkJ/mol canal. [65] improve) reported This its indicates stability. by Herley that The etthe enthalpy al. formation [65]. of This dehydrogenation of indicates α that of theα formation AlH surface oxide canAlH ofimproveα determined− AlH its3 surfacestability. by DSC oxide (DifferentialThe canenthalpy improve Scanning of itsdehydrogenation Calorimetry) stability. The is enthalpy5.7 of‐6.6 α kJ/mol of dehydrogenation of H2 [66]. of AlH determined by DSCTherefore,α −(DifferentialAlH AlH3 determined3 is Scanningeasy to release by Calorimetry) DSC hydrogen (Differential atis elevated5.7‐6.6 Scanning kJ/mol temperatures. Calorimetry)of H2 [66]. is 5.7–6.6 kJ/mol of Therefore, AlH3 is easy toH 2release[66]. Therefore, hydrogen AlH at elevated3 is easy temperatures. to release hydrogen at elevated temperatures.

Figure 10. High temperature isothermal decomposition curves from α and γ-AlH3 starting materials at 138 ◦C showing (I) induction period (II), acceleratory period, and (III) decay period. Reprinted with permission from Ref. [63]. Copyright 2007 Elsevier. Materials 2021, 14, x FOR PEER REVIEW 10 of 17

Figure 10. High temperature isothermal decomposition curves from α and γ‐AlH3 starting mate‐ Materials 2021, 14, 2898 rials at 138 °C showing (I) induction period (II), acceleratory period, and (III) decay period. Re10‐ of 16 printed with permission from ref. [63]. Copyright 2007 Elsevier.

As mentioned above, AlH3 has poor stability and is easy to decompose to produce As mentioned above, AlH has poor stability and is easy to decompose to produce hydrogen when the temperature3 rises, which makes the storage and transportation of hydrogen when the temperature rises, which makes the storage and transportation of AlH3 AlH3 to have high risks. The structure, thermodynamics, kinetics, and synthesis methods to have high risks. The structure, thermodynamics, kinetics, and synthesis methods of of AlH3 have been comprehensively and summarized in detail in the latest several reviews AlH3 have been comprehensively and summarized in detail in the latest several reviews about AlH3 [33,56,67]. However, the content of improving the stability of AlH3 is not much about AlH [33,56,67]. However, the content of improving the stability of AlH is not involved. In3 this chapter, we summarize and discuss the latest research papers on improv3 ‐ much involved. In this chapter, we summarize and discuss the latest research papers on ing the stability of AlH3. The key to inhibit the decomposition of AlH3 is how to inhibit improving the stability of AlH . The key to inhibit the decomposition of AlH is how the appearance of the induction3 period [63]. At present, the main methods to stabilize3 to inhibit the appearance of the induction period [63]. At present, the main methods to AlH3 are surface passivation, doping stabilization, and surface coating. stabilize AlH3 are surface passivation, doping stabilization, and surface coating.

4.1.4.1. Surface Surface Passivation Passivation Method Method NakagawaNakagawa et et al. al. [5] [5] observed observed the the dehydrogenation dehydrogenation and and decomposition decomposition process process of of α AlH α − AlH 3byby in in situ situ transmission transmission electron electron microscopy. microscopy. It It was was found found that that with with the the increase increase ofof storage storage time, time, the the thickness thickness of of the the surface surface Al Al2O23 Olayer3 layer increased, increased, and and Al grew Al grew at the at in the‐ terfaceinterface of two of two phases, phases, as shown as shown in Figure in Figure 11. The11. Theresults results depict depict that the that existence the existence of alu of‐ minaalumina film film can cansignificantly significantly delay delay the dehydrogenation the dehydrogenation of α of α AlH− AlH. At3 the. At same the same time, time, the dehydrogenationthe dehydrogenation temperature temperature is independent is independent of the ofthickness the thickness of the alumina of the alumina film, which film, indicateswhich indicates that the that introduction the introduction of the of alumina the alumina film film on the on the surface surface of ofAlH AlH3 is3 is a afeasible feasible methodmethod to to inhibit inhibit the the decomposition. decomposition. In In the the surface surface passivation passivation method, method, dilute dilute acid, acid, buffer buffer solution,solution, organic organic matter matter immersion, immersion, and and heat heat treatment treatment are are used used to to remove remove the the unstable unstable impurityimpurity phase, phase, treat treat the the surface surface of of αα − AlHAlH,3 ,quench quench the the active active sites, sites, and and form form a a dense dense AlAl2(OH)2(OH)3 3oror Al Al2O2O3 layer3 layer to to achieve achieve the the purpose purpose of of stabilization stabilization [5]. [5 ].

FigureFigure 11. 11. TheThe TEM TEM images images of of Al Al2O2O3 films3 films on on AlH AlH3 particles;3 particles; (a) ( awithout) without exposure exposure to toair, air, (b) ( b1) day 1 day afterafter exposure exposure to to air, air, and and (c ()c )7 7days days after after exposure exposure to to air. air. Reprinted Reprinted with with permission permission from from ref. Ref. [5]. [ 5]. CopyrightCopyright 2013 2013 Elsevier. Elsevier.

ChenChen et et al. al. [68] [68] passivated passivated a a layer layer of of nano nano-Al‐Al22OO3 3filmfilm on on the the surface surface of of αα − AlHAlH 3byby atomicatomic layer layer deposition. deposition. The The hydrogen hydrogen retention retention capacity capacity of of passivated passivated particles particles was was four four timestimes higher higher than than that that of of untreated untreated samples. samples. At At the the same same time, time, the the hydrogen hydrogen release release speed speed ofof the the passivated passivated AlH AlH3 3samplessamples was was almost almost the the same same as as that that of of the the untreated untreated samples, samples, as as shownshown in in Figure Figure 1212.. TheThe dehydrogenationdehydrogenation rate rate of of passivated passivated AlH AlH3 particles3 particles was was almost almost the thesame same as thatas that of of untreated untreated samples, samples, which which indicates indicates that that this this is is a practicablea practicable technology technology to tostabilize stabilize AlH AlH3 without3 without sacrificing sacrificing its its energy energy release release ability ability [68 [68].].

4.2. Doping Stabilization Method The doping stabilization method is different from the surface passivation method. The doping method is to introduce metal ions or stabilizers in the synthesis stage of AlH3, and modify the crystal structure and defects of AlH3 to stabilize AlH3. Bulychev et al. [69] used LiAlH4·Mg(AlH4)2 and LiCI·Mg(AlH4)2 as modifying additives to increase the stability of AlH3. According to the data of element phase and X-ray phase analysis, it was found that the lattice parameters of α − AlH3 increased after modification; Mg atoms entered into the lattice of α − AlH3; and the morphology of the sample changed from cubic to spherical with a smooth edge [69]. The results of the hydrogen evolution rate test at 110 ◦C show

that the thermal stability of modified α − AlH3 is significantly improved. Materials 2021, 14, 2898x FOR PEER REVIEW 1111 of of 17 16

Figure 12. 12. (Color(Color online) online) TG TG curves curves of AlH of3 AlH and3 AlHand3@Al AlH2O3@Al3 samples2O3 samples at the heating at the rate heating of 2 °C/min. rate of Reprinted2 ◦C/min. with Reprinted permission with permissionfrom ref. [68]. from Copyright Ref. [68]. 2017 Copyright American 2017 Vacuum American Society. Vacuum Society.

4.2. DopingArdis andStabilization Natoli [Method70] consider that heat, light, radiation, and other factors would α − causeThe the doping reaction stabilization of AlH 3methodradicals, is whichdifferent would from lead the surface to the self passivation decomposition method. of α − AlH3. Therefore, the researchers tried to improve the stability by adding free radical The doping method is to introduce metal ions or stabilizers in the synthesis stage of AlH3, inhibitors. Ardis et al. [70] added free radical inhibitors such as MBT and PTA into the and modify the crystal structure and defects of AlH3 to stabilize AlH3. Bulychev et al. [69] synthesis process of α − AlH3 (as shown in Figure 13) to obtain the modified samples. used LiAlH4∙Mg(AlH4)2 and LiCI∙Mg(AlH4)2 as modifying additives to increase the stabil‐ The stability test showed that the decomposition rate of the modified samples decreased ity of AlH3. According to the data of element phase and X‐ray phase analysis, it was found from 7.5% to 0.6% after 17 days of storage. However, free radical scavengers need to be that the lattice parameters of αAlH increased after modification; Mg atoms entered decomposed to generate active radicals, and then combine with hydrogen radicals, which into the lattice of αAlH ; and the morphology of the sample changed from cubic to will lead to the hydrogen radicals from AlH that have already combined with themselves spherical with a smooth edge [69]. The results3 of the hydrogen evolution rate test at 110 to generate hydrogen before combining with scavengers. Therefore, the possibility of a non- °C show that the thermal stability of modified αAlH is significantly improved. radical reaction between 2-mercaptobenzothiazole and AlH needs further investigation. Ardis and Natoli [70] consider that heat, light, radiation,3 and other factors would The doping stabilization method can significantly improve the stability of AlH3, but other Materials 2021, 14, x FOR PEER REVIEWcause the reaction of αAlH radicals, which would lead to the self decomposition12 of of 17 elements introduced by doping may reduce the energy and purity of AlH , affecting its αAlH 3 compatibility. Therefore, and safety. the researchers tried to improve the stability by adding free radical inhibitors. Ardis et al. [70] added free radical inhibitors such as MBT and PTA into the synthesis process of αAlH (as shown in Figure 13) to obtain the modified samples. The stability test showed that the decomposition rate of the modified samples decreased from 7.5% to 0.6% after 17 days of storage. However, free radical scavengers need to be decom‐ posed to generate active radicals, and then combine with hydrogen radicals, which will lead to the hydrogen radicals from AlH3 that have already combined with themselves to generate hydrogen before combining with scavengers. Therefore, the possibility of a non‐ radical reaction between 2‐mercaptobenzothiazole and AlH3 needs further investigation. The doping stabilization method can significantly improve the stability of AlH3, but other elements introduced by doping may reduce the energy and purity of AlH3, affecting its compatibility and safety.

α − FigureFigure 13. 13. StabilizationStabilization mechanism mechanism of of 2-Mercaptobenzothiazole 2‐Mercaptobenzothiazole (MBT) (MBT) and and αAlHAlH3.. 4.3. Surface Coating Method 4.3. Surface Coating Method Although the above two methods can significantly improve the stability of AlH3, they Although the above two methods can significantly improve the stability of AlH3, they will affect the hydrogen release efficiency, compatibility, and thermal performance of AlH . will affect the hydrogen release efficiency, compatibility, and thermal performance of3 Researchers tend to use the surface coating method, which has little effect on the physical AlH3. Researchers tend to use the surface coating method, which has little effect on the and chemical properties of AlH3, to stabilize AlH3. physical and chemical properties of AlH3, to stabilize AlH3. Matzek et al. [71] reported a method of coating AlH3 with diphenylacetylene. AlH3 was washed with a benzene solution containing diphenylacetylene and then dried in ni‐

trogen. It is found that the thermal stability of AlH3 increases with the increase of coating amount. It takes 13 days for uncoated AlH3 to decompose 1% at 60 °C, while it takes 29 days for AlH3 containing 2.5% diphenylacetylene. At the same time, magnesium‐doped AlH3 coated with diphenylacetylene showed better thermal stability than conventional AlH3 and magnesium‐doped AlH3, and only 0.84% of AlH3 decomposed at 60 °C for 48 days [71]. Flynn [72] reported a method of coating AlH3 with nitrocellulose. The sample mate‐ rial only decomposed 0.63% in 90 days at 60 °C. By coating with nitrocellulose, the com‐ patibility of AlH3 with other components in some solid propellants is also significantly increased [72]. Cai et al. [73] successfully coated α AlH with fluororubber FE26 as the coating agent, and liquid CO as the anti‐solvent and dispersion medium, using supercriti‐ cal fluid technology. The results of differential scanning calorimetry indicate that the en‐ thalpy of coated α AlH increases, and the thermal stability is improved. In addition, the lower electric spark sensitivity of coated α AlH shows improved applicability and stability [73]. At present, the research of coating stabilized AlH3 mainly focus on the screening of the coating agent, but the mechanism of coating stabilized AlH3 is less known. How to accurately control the coating thickness of AlH3 is expected to be the hinge to the next step.

5. Applications and Challenges

As an energetic material, AlH3 has extraordinary potential in the application of rocket fuel. As the power source of the rocket, solid propellant can be divided into homogeneous propellant and composite propellant [74,75]. Aluminum powder is usually used as a metal fuel to improve the energy characteristics of composite propellants [76,77]. However, the ignition temperature of aluminum powder is high, and the combustion efficiency is low [78–81]. Compared with aluminum powder, AlH3 has a higher energy density, which can significantly reduce the ignition temperature and produce H2 fuel in the combustion pro‐ cess, thus reducing the relative mass of combustion products [82–84]. In recent years, re‐ searchers have conducted extensive research on the combustion performance and com‐ patibility of AlH3, and discussed the application prospect of AlH3 as rocket fuel [85–87].

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Matzek et al. [71] reported a method of coating AlH3 with diphenylacetylene. AlH3 was washed with a benzene solution containing diphenylacetylene and then dried in nitrogen. It is found that the thermal stability of AlH3 increases with the increase of coating ◦ amount. It takes 13 days for uncoated AlH3 to decompose 1% at 60 C, while it takes 29 days for AlH3 containing 2.5% diphenylacetylene. At the same time, magnesium-doped AlH3 coated with diphenylacetylene showed better thermal stability than conventional ◦ AlH3 and magnesium-doped AlH3, and only 0.84% of AlH3 decomposed at 60 C for 48 days [71]. Flynn [72] reported a method of coating AlH3 with nitrocellulose. The sample ma- terial only decomposed 0.63% in 90 days at 60 ◦C. By coating with nitrocellulose, the compatibility of AlH3 with other components in some solid propellants is also significantly increased [72]. Cai et al. [73] successfully coated α − AlH3 with fluororubber FE26 as the coating agent, and liquid CO as the anti-solvent and dispersion medium, using super- critical fluid technology. The results of differential scanning calorimetry indicate that the enthalpy of coated α − AlH3 increases, and the thermal stability is improved. In addition, the lower electric spark sensitivity of coated α − AlH3 shows improved applicability and stability [73]. At present, the research of coating stabilized AlH3 mainly focus on the screening of the coating agent, but the mechanism of coating stabilized AlH3 is less known. How to accurately control the coating thickness of AlH3 is expected to be the hinge to the next step.

5. Applications and Challenges

As an energetic material, AlH3 has extraordinary potential in the application of rocket fuel. As the power source of the rocket, solid propellant can be divided into homogeneous propellant and composite propellant [74,75]. Aluminum powder is usually used as a metal fuel to improve the energy characteristics of composite propellants [76,77]. However, the ignition temperature of aluminum powder is high, and the combustion efficiency is low [78–81]. Compared with aluminum powder, AlH3 has a higher energy density, which can significantly reduce the ignition temperature and produce H2 fuel in the combustion process, thus reducing the relative mass of combustion products [82–84]. In recent years, researchers have conducted extensive research on the combustion performance and compatibility of AlH3, and discussed the application prospect of AlH3 as rocket fuel [85–87]. Bazyn et al. [3] used shock tube and AlO emission and absorption spectroscopy to compare the combustion characteristics of AlH3 and Nano Al in CO2 and O2. The experimental results show that the hydrogen release time of AlH3 is very short. Once H2 is released, the combustion mode of the remaining Al is very similar to that of pure aluminum in a solid rocket motor (i.e., at a similar rate and temperature). Il’in et al. [88] studied the combustion of AlH3 in the air and found that with the increase of the combustion dose of AlH3, the combustion products mainly changed from Al2O3 to AlN. Il’in et al. [88] also found that the combustion of AlH3 can be divided into three stages: (1) Far below the ignition threshold temperature of Al, the combustion stage of AlH3 decomposes to produce H2, forming a mushroom-like flame, and the flame leaves the sample surface; (2) the results show that the combustion of H2 is exhausted, and the flame contacts the sample, which is the low temperature combustion stage of super dispersed porous aluminum powder; (3) the high temperature combustion stage of aluminum. Bazyn et al. [89] studied the combustion characteristics of AlH3 under the condition of a solid rocket motor (400–500 k). The results show that the decomposition time of AlH3 is exponentially correlated with temperature, which satisfies the Arrhenius type rate equation. At 1000 K, 90% of H2 is desorbed from AlH3 particles in only 38 µs. Bazyn et al. [89] also carried out a two-dimensional simulation of the aluminum particles in the propellant mixture. The results showed that AlH3 particles in the surface extrusion mixture decomposed to produce H2 before spraying from the propellant surface. The generated H2 will react with the binder and oxidant in the flame zone, thus affecting the stoichiometry of the zone. Materials 2021, 14, 2898 13 of 16

DeLuca et al. [83] carried out the experiment and numerical analysis of a laboratory composite solid propellant based on AlH3. The ballistic performance of the AlH3-based propellant was evaluated experimentally and compared with the flame structure of the Al-based propellant. The experimental results show that the AlH3-based propellant has a first-class combustion structure, excellent ballistic and aging properties, and high quality two-phase flow. The AlH3-based propellant has a unique competitive advantage over the Al-based propellant. Although AlH3-based rocket propellant is considered to be a promising rocket fuel, the metastability of AlH3 affects its practical application. AlH3 is easy to decompose to produce H2 during storage, which affects the safe storage and continuous combustion of the propellant system. At present, AlH3-based rocket propellant has not been applied in practice.

6. Conclusions

AlH3 is a binary metal hydride with at least seven crystal structures, among which α − AlH3 is the most stable polymorph. During heating, α − AlH3 will decompose into Al and H2, while other crystal forms of AlH3 (such as β—AlH3, γ—AlH3, etc.) will first transform into α − AlH3 and then decompose into Al and H2. AlH3 is generally prepared by the reaction of LiAlH4 and AlCl3 in ether solution. Other synthetic process such as electrochemical hydrogenation, reactive milling synthesis, and regeneration with AlH3 adduct, have also been studied and developed. AlH3 is considered as a promising hydrogen storage medium due to its high volume and weight energy density, low hydrogen release temperature, and fast reaction kinetics. AlH3 can be used as rocket fuel and fuel cells with high combustion heat, non-toxic, and high hydrogen content. However, the stability of AlH3 has become a key problem limiting its practical application (especially when used as rocket fuel). In recent years, methods to improve the stability of AlH3 have been widely studied, including surface passivation, doping, and surface coating. How to improve the stability of AlH3 without sacrificing the hydrogen release efficiency is the linchpin to the current research.

Author Contributions: Conceptualization, W.S. and F.Z.; methodology, W.S.; software, R.T. and Y.Z.; validation, L.M., W.S. and Y.D.; formal analysis, S.N.; investigation, G.K.; resources, W.S.; data curation, G.T. and Y.Z.; writing—original draft preparation, W.S. and Y.Z.; writing—review and editing, Y.W.; visualization, L.W.; supervision, L.W.; project administration, L.W.; funding acquisition, W.L. and A.P. All authors have read and agreed to the published version of the manuscript. Funding: We acknowledge the financial support provided by the National Natural Science Foun- dation of China (No. 51272155, 21875061, 21975066) and Open Research Fund Program of Science and Technology on Aerospace Chemical Power Laboratory (No. STACPL 120201B05). We thank for Instrumental Analysis Center of Shanghai Jiao Tong University for the analysis support. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Data sharing not applicable. Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

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