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Synthesis and Stability of Hydrogen Storage Material Aluminum Hydride

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Synthesis and Stability of Hydrogen Storage Material Aluminum Hydride 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/). Materials 2021, 14, 2898. https://doi.org/10.3390/ma14112898 https://www.mdpi.com/journal/materials Materials 2021, 14, x FOR PEER REVIEW 2 of 17 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, " 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 Materials 2021, 14, x FOR PEER REVIEW 3 of 17 Materials 2021, 14, x FOR PEER REVIEW 3 of 17 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,
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