metals Review Solid State Hydrogen Storage in Alanates and Alanate-Based Compounds: A Review Chiara Milanese 1,* ID , Sebastiano Garroni 2 ID , Fabiana Gennari 3, Amedeo Marini 1, Thomas Klassen 4,5, Martin Dornheim 4 and Claudio Pistidda 4 ID 1 Pavia Hydrogen Lab, C.S.G.I. & Chemistry Department, University of Pavia, Viale Taramelli, 16, 27100 Pavia, Italy; [email protected] 2 International Research Centre in Critical Raw Materials-ICCRAM, University of Burgos, 09001 Burgos, Spain; [email protected] 3 National Council of Scientific and Technological Research (CONICET), Bariloche Atomic Center (National Commission of Atomic Energy) and Balseiro Institute (University of Cuyo) Av. Bustillo 9500, San Carlos de Bariloche, Río Negro 8400, Argentina; [email protected] 4 Institute of Materials Research, Helmholtz-Zentrum Geesthacht, Max-Planck-Straße 1, D-21502 Geesthacht, Germany; [email protected] or [email protected] (T.K.); [email protected] (M.D.); [email protected] (C.P.) 5 Institute of Materials Technology, Helmut Schmidt University, University of the Federal Armed Forces, Holstenhofweg 85, D-22043 Hamburg, Germany * Correspondence: [email protected]; Tel.: +39-0382-987-670 Received: 20 June 2018; Accepted: 18 July 2018; Published: 24 July 2018 Abstract: The safest way to store hydrogen is in solid form, physically entrapped in molecular form in highly porous materials, or chemically bound in atomic form in hydrides. Among the different families of these compounds, alkaline and alkaline earth metals alumino-hydrides (alanates) have been regarded as promising storing media and have been extensively studied since 1997, when Bogdanovic and Schwickardi reported that Ti-doped sodium alanate could be reversibly dehydrogenated under moderate conditions. In this review, the preparative methods; the crystal structure; the physico-chemical and hydrogen absorption-desorption properties of the alanates of Li, Na, K, Ca, Mg, Y, Eu, and Sr; and of some of the most interesting multi-cation alanates will be summarized and discussed. The most promising alanate-based reactive hydride composite (RHC) systems developed in the last few years will also be described and commented on concerning their hydrogen absorption and desorption performance. Keywords: alanates; metal aluminum hydrides; solid state hydrogen storage; complex hydrides; reactive hydride composite 1. Introduction Hydrogen is regarded as an efficient energy storage medium and a green and environmental friendly fuel able to help improve environmental issues linked to the use of fossil fuels [1,2], thanks to its high gravimetric energy density (more than twice the energy content of the most common fossil fuels) and its clean combustion reaction (leading to the formation of water only). Unfortunately, its volumetric capacity is very low, leading to the well-known storage problems [2]. The safest method to store hydrogen is in solid state form, that is, bound to a solid as molecules through physical interactions (physisorption) or in atomic form by chemical bonds (chemisorption). The most promising materials for hydrogen storage concerning the gravimetric and volumetric capacity belong to this last group, suffering anyway from unfavourable working conditions (i.e., high temperature and hydrogen pressure) and scarce cycling ability [1]. The first system to be considered interesting for Metals 2018, 8, 567; doi:10.3390/met8080567 www.mdpi.com/journal/metals Metals 2018, 8, 567 2 of 15 practical applications, thanks to its reversibility in exchanging hydrogen at moderate temperature and pressure conditions, was based on an alanate, that is, titanium-based doped sodium alanate, studied by Bogdanovic and Schwickardi in 1996 [3]. The term ‘alanate’, synonymous of ‘alumino-hydride’, refers to a family of complex hydrides, in which a metallic cation coordinates, with an ionic bond, an anion with Al in the center, covalently binding four hydrogen atoms. Since Bogdanovic’s discovery, many efforts have been made to investigate the hydrogen absorption and desorption mechanism of Na alanate and of the other alkaline and alkaline earth metal alanates, and to optimize their performance by doping with suitable catalysts and destabilizing agents, by wet chemistry infiltration and by mixing in the so-called reactive hydride composite (RHC) approach. Recently, some of them have also been proposed for electrochemical applications in solid state ion batteries. In this review, the preparative methods; the crystal structure; the physico-chemical characteristics; the hydrogen absorption and desorption properties of the alanates of Li, Na, K, Ca, Mg, Y, Sr, and Eu; and the hydrogen absorption and desorption properties of the most appealing multi-cations alanates and of selected systems based on these compounds will be summarized and discussed. The most promising alanate-based RHC systems developed in the last few years will also be described and commented. 2. Alanates Based-Systems 2.1. Sodium Alanate NaAlH4 A direct synthesis of NaAlH4 from sodium hydride, aluminium, and hydrogen under pressure in several different solvents was described in the early sixties by Ashby et al. and Clasen [4,5]. Dymova et al. [6] proposed a synthetic procedure from the elements at temperatures lower than 553 K and hydrogen pressure of 175 bar. The alanate can be easily prepared contextually with its doping with metals by ball milling under inert atmosphere, via wet chemistry reaction of a NaH–Al-dopant powder mixture and subsequent hydrogenation [7], or by one-step direct synthesis by high energy reactive ball milling [7,8]. The crystal structure of NaAlH4, determined by single crystal X-ray diffraction, is − tetragonal with space group I41/a, consisting of isolated [AlH4] tetrahedra [9]. The Al–D bond length, evaluated more recently by neutron diffraction on the deuterated compound at 295 K, is 1.6262 Å, while the two Na–D distances are 2.431(2) Å and 2.439(2) Å. The shortest (Al–Al) distance is 3.779(1) Å. ◦ ◦ The angles of the [AlD4] tetrahedron are distorted and equal to 107.32 and 113.86 [10]. NaAlH4, with 5.5. wt % H2 theoretical gravimetric capacity and 5.0 wt % hydrogen release reached so far, is amongst the most advanced and intensely investigated materials for hydrogen storage; even if it seems evident that its performance will be not suitable for on-board applications, it is considered promising for stationary purposes [11]. Its dehydrogenation takes place in two steps [12,13]: −1 3NaAlH4 $ Na3AlH6 + 2Al + 3H2; (3.7 wt % H2; DH = 37 kJ·mol H2), (1) −1 Na3AlH6 + 2Al + 3H2 $ 3NaH + 3Al + 9/2 H2; (1.8 wt % H2; DH = 47 kJ·mol H2). (2) −1 The activation energies for the two steps are 118 and 124 kJ·mol H2 [14], respectively. The further dissociation of NaH, responsible of the release of 1.9 wt % H2, takes place above 723 K, making this last process unsuitable for practical applications. The equilibrium pressure calculated from the enthalpy values is 1 bar at 303 K for the first step and at 373 K for the second step respectively, making only reaction (1) useful to feed a proton exchange fuel cell membrane under full throttle conditions (353 K maximum) from a thermodynamic point of view. Considering the kinetic aspects, both reactions (1) and (2) are very slow for practical applications; Ashby and Kobetz [15,16] reported that the formation of Na3AlH6 from NaAlH4 takes place in 3 h at 483–493 K, while to obtain an appreciable speed for the dissociation of the newly formed compound, temperatures higher than 523 K are required. Concerning re-hydrogenation, complete conversion from NaH and Al to NaAlH4 was achieved by Dymova et al. [6] in 3 h under 175 bar hydrogen pressure at 543 K. In 1997, Bogdanovic and co-workers [3] demonstrated that the kinetics of the system could be strongly improved by metal doping, in particular by adding Metals 2018, 8, 567 3 of 15 a few molar percent of Ti, and that the system could be reversibly operated under practical conditions. −1 In these conditions, the activation energies of the two steps reduced to 80 and 96 kJ·mol H2, and the isotherm of the re-absorption of hydrogen followed the desorption isotherm with almost no hysteresis. Anyway, the hydrogenation and dehydrogenation rates of the catalyzed systems are still insufficient for practical applications. Moreover, most of the catalysts were found to react with the hydrides, decreasing the gravimetric capacity of the systems and their whole storage performance. Starting from the results of this work, many Ti-containing compounds (i.e., TiF3, Ti(OBu)4, TiH2, and TiO2) were tested as catalyst, but by-side reactions with formation of compounds in which Ti combines with Al [11] were always observed, wasting the hydrogen absorption and desorption performance [17]. Considering the kinetics of the reactions, a linear increase of the rate as a function of the added amount of the Ti-compounds was recorded up to 5 mol % of catalyst in the active phase. For highest amounts, no variation in the reaction speed was noticed. Concerning other transition metal salts, the addition of 2 mol % of ScCl3 and CeCl3 to NaAlH4 almost halves the re-hydrogenation times at high pressures and reduces them by a factor of ten at low pressures [14]. Moreover, these systems re-charged 3.5 wt % of hydrogen within 1 h at 50 bar, performance that is more promising than that of the titanium doped system. Zidan et al. [18] found zirconium addition acting in a very positive way towards reaction (2), leading to a reversibility higher than 4 wt %. Wang et al. [19], in their systematic study on the action of individual, binary, and ternary combinations of TiCl3, FeCl3, and ZrCl4 on the dehydrogenation kinetics of the Na alanate, reported that that the effect of the presence of more than one catalyst with respect to single doping was pronounced on reaction (1), but almost negligible on reaction (2).