L J Bannenberg

M Heere

H Benzidi

J Montero

E M Dematteis

S Suwarno

T Jaroń
email: t.jaron@cent.uw.edu.pl

M Winny

P A Orlowski

W Wegner

P A Orłowski

A Starobrat

K J Fijałkowski

W Grochala

Z Qian

J.-P Bonnet

I Nuta

W Lohstroh

C Zlotea

O Mounkachi

F Cuevas

C Chatillon

M Latroche

M Fichtner

M Baricco

B C Hauback

A El Kharbachi
email: kharbachi@kit.edu

Metal (boro-) hydrides for

Keywords: hydrogen storage, metal hydrides, metal borohydrides, hydrogen sensors, electrochemical energy storage

published or not. The documents may come   L'archive ouverte pluridisciplinaire

Introduction

It was already in the nineteenth century when British chemist Thomas Graham discovered that palladium can accommodate vast amounts of hydrogen 'being eight to nine hundred times its volume in hydrogen gas' [START_REF] Graham | On the relation of hydrogen to palladium[END_REF]. In the years following this pioneering study, it was found that metal hydrides may accommodate non-stoichiometric amounts of hydrogen up to a hydrogen-to-metal ratio of H/M = 3 for some transition metals, and even higher at P of hundreds GPa. Consequently, some metal hydrides have remarkably high atomic hydrogen densities (e.g. N H = 14.4 10 28 m -3 for VH 2 ) that well exceed the density of liquid hydrogen (N H = 4.2 10 28 m -3 ) [START_REF] Griessen | Science and technology of hydrogen in metals[END_REF]. The hydrogenation of metals may dramatically alter the electronic, optical and magnetic properties of materials, inducing metal-toinsulator changes or changing paramagnets into superconductors at low temperature [START_REF] Griessen | Science and technology of hydrogen in metals[END_REF][START_REF] Huiberts | Yttrium and lanthanum hydride films with switchable optical properties[END_REF].

Starting from the 1970s, metal hydrides were considered as an ideal candidate for hydrogen storage because of their high atomic hydrogen density. Hydrogena clean and sustainable energy carrier with the highest energy density per mass of fuelwill likely play a paramount role in the transition to a carbon neutral economy. Hydrogen can be particularly attractive for mobile applications, for instance powering heavy-duty vehicles as well as for stationary energy storage where it may be used to match the relatively constant energy demand with the fluctuating output of renewable energy sources (solar, wind, hydropower, etc.). Yet, under ambient conditions, hydrogen gas has a low energy density per volume unit, and viable hydrogen storage is therefore key for its successful implementation. Compared to other methods of hydrogen storage, storage in metal hydrides, apart from the high volumetric density, has the particular advantage that no cryogenic cooling is required (as for liquid hydrogen storage) and that is intrinsically safer than high-pressure storage where potential leaks form a safety hazard. In particular, research focused on lightweight metal hydrides and, so called, "complex metal hydrides" that feature relatively high gravimetric hydrogen densities [START_REF] Ajmt | Materials for hydrogen storage[END_REF][START_REF] Orimo | Complex hydrides for hydrogen storage[END_REF][START_REF] Sakintuna | Metal hydride materials for solid hydrogen storage: a review[END_REF][START_REF] Schlapbach | Hydrogen-storage materials for mobile applications[END_REF][START_REF] Milanese | Complex hydrides for energy storage[END_REF]. The latter term denotes multinary ionic compounds composed of cations and polyatomic anions containing hydrogen, in principle covalently bound to the heavier atom, e.g. NaBH 4 , Li 2 B 12 H 12 or LiAlH 4 . The term "complex hydrides" has even been stretched to the compounds containing hydrogen in non-hydridic but rather protonic form (with partial positive charge), like amides, or imides, e.g. NaNH 2 or Li 2 NH.

Although most metal hydrides exhibit a high energy density, their applicability is in general limited both by thermodynamic and kinetic factors. Hydrogen forms relatively strong bonds with most metals, implying reasonably large negative values for the enthalpy and entropy of formation (e.g. ΔH ≈ -74 kJ mol H2 -1 and ΔS ≈ -135 J K -1 mol H2 -1 for MgH 2 [START_REF] Jain | Hydrogen storage in Mg: a most promising material[END_REF][START_REF] Crivello | Review of magnesium hydridebased materials: development and optimisation[END_REF]). As a consequence, hydrogenation readily occurs exothermically under mild conditions, while the endothermic desorption requires high temperatures and the supply of a certain amount of heat. In addition, although the diffusion of hydrogen in metals is in general much faster than the diffusion of other compounds, slow hydrogen (de)sorption as well as poor cyclability remain a problem in most materials [START_REF] Schlapbach | Hydrogen-storage materials for mobile applications[END_REF][START_REF] Milanese | Complex hydrides for energy storage[END_REF][START_REF] Rusman | A review on the current progress of metal hydrides material for solidstate hydrogen storage applications[END_REF].

Especially in the 1990s and early 2000s, various strategies have been employed to overcome these challenges including alloying and nanostructuring [12,[START_REF] De Jongh | Nanoconfined light metal hydrides for reversible hydrogen storage[END_REF]. Although these strategies considerably improve the storage properties of metal hydrides, none of the investigated materials simultaneously meet at the moment the requirements on desorption temperature and pressure, gravimetric and volumetric energy density, cyclability, (de)sorption kinetics and economic feasibility. In other words, continuous efforts are still required to obtain breakthrough materials with high storage efficiency and cost-effective processing [START_REF] Hirscher | Materials for hydrogen-based energy storage -past, recent progress and future outlook[END_REF]. Although advances have been made in the last decades, the prospective advantages of hydrogen storage in metal hydrides faded partly away owing to the rapid development of alternative technologies as Li-ion batteries and gaseous hydrogen storage in light-weight high pressure vessels.

Meanwhile new applications have emerged such as compressors already commercially available [START_REF] Lototskyy | Metal hydride hydrogen compressors: a review[END_REF][START_REF] Bellosta Von Colbe | Application of hydrides in hydrogen storage and compression: Achievements, outlook and perspectives[END_REF], metal hydrides constitute alternative materials for heat and thermochemical storage to be coupled with next generation of solar power plants [START_REF] Manickam | Future perspectives of thermal energy storage with metal hydrides[END_REF]. Metal hydrides are also present in many commercial aqueous batteries (e.g. NiMH batteries [START_REF] Ovshinsky | A nickel metal hydride battery for electric vehicles[END_REF][START_REF] Zhao | Recent progress in hydrogen storage alloys for nickel/metal hydride secondary batteries[END_REF]) which is not approached here. However, owing to their light-weight, low voltage and small hysteresis compared to oxide anodes, metal hydrides are now intensively studied for their applications as high energy density electrodes for LIBs with carbonate-based liquid electrolytes [START_REF] Oumellal | Metal hydrides for lithium-ion batteries[END_REF][START_REF] Berti | Enhanced reversibility of the electrochemical Li conversion reaction with MgH2-TiH2 nanocomposites[END_REF][START_REF] El Kharbachi | Understanding Capacity Fading of MgH2 Conversion-Type Anodes via Structural Morphology Changes and Electrochemical Impedance[END_REF]. In addition, complex metal hydrides are studied for their application as solid-state electrolytes in next-generation all-solid-state batteries [START_REF] Kim | A complex hydride lithium superionic conductor for high-energy-density all-solid-state lithium metal batteries[END_REF][START_REF] Unemoto | Complex hydrides for electrochemical energy storage[END_REF][START_REF] De Jongh | Complex hydrides as roomtemperature solid electrolytes for rechargeable batteries[END_REF][START_REF] Cheng | Metal hydrides for lithium-ion battery application: A review[END_REF]. In these batteries, the highly flammable liquid electrolyte is replaced by a solid one, making these batteries intrinsically safer and potentially increasing the energy density. Furthermore, metal hydride based hydrogen sensors are now considered as a competitive way to reliably and efficiently sense hydrogen over extensive hydrogen pressure ranges [START_REF] Bannenberg | Metal Hydride Based Optical Hydrogen Sensors[END_REF][START_REF] Hubert | Hydrogen sensors -A review[END_REF][START_REF] Nugroho | Metalpolymer hybrid nanomaterials for plasmonic ultrafast hydrogen detection[END_REF][START_REF] Silva | A Review of Palladium-Based Fiber-Optic Sensors for Molecular Hydrogen Detection[END_REF][START_REF] Zhang | Recent advancements in optical fiber hydrogen sensors[END_REF]. These sensors utilize the profound change of the electronic and/or optical properties of most metal hydrides when (partially) hydrogenated as a result of an exposure to a hydrogen environment.

The purpose of this review is to provide an overview of various metal hydrides with a particular focus on applications in a green economy. In Section 2 we discuss single-metal hydrides, multi-metallic systems and a series of metal borohydrides with focus on synthesis, structural and hydrogen storage properties. While Section 3 deals with the application of metal hydrides in hydrogen sensors, Section 4 discusses the incorporation of metal hydrides in rechargeable batteries, including conversion-type electrodes, together with lithium and magnesium-based solid-state electrolytes. Section 5 describes experimental techniques relevant for the study of metal hydrides. In Section 6 we summarize the main recent results and provide an outlook on the application of metal (boro-)hydrides. For guidance throughout this review report abbreviations and acronyms are listed below (Table 1).

Single-metal hydrides

Considering stable (non-radioactive) elements, all of the main group and many transition metals form binary hydrides, MH n , either under mild (P, T) or harsh high pressure (HP) conditions [32]. Here we will briefly refer to this group of hydrides in the context of hydrogen storage. Among such simple hydrides, only those containing the most lightweight elements are potentially able to fulfil the requirement of a sufficiently high gravimetric capacity for onboard hydrogen storage for light-duty fuel cell vehicles of 6.5 wt% [START_REF]Target Explanation Document: Onboard Hydrogen Storage for Light-Duty Fuel Cell Vehicles[END_REF], while this parameter remains not as severely limiting for the multinary hydrides (Fig. 1). At the same time, significant volumetric density of hydrogen may be achieved even for heavier binary hydrideswhile lightweight LiH of 12.6 wt% H stores 98.3 kg m -3 .

Much heavier BaH 2 containing only 1.45 wt% H has a hydrogen density of 60.5 kg m -3 , which still fulfils the U.S. Department of Energy (DOE) target (updated May 2017) of > kg m -3 [START_REF]Target Explanation Document: Onboard Hydrogen Storage for Light-Duty Fuel Cell Vehicles[END_REF]. Despite limited options for gravimetrically efficient hydrogen storage, single-metal hydrides are still an appealing class of materials for energy storage in potential applications other than light-duty vehicles.

This is related to their simplicity, usually combined with emission of pure hydrogen in a simple, oneor two-step thermal decomposition process, according to eq. ( 1):

MH n → M + n/2H 2 ↑ (1)

Comparison of the nominal gravimetric content of hydrogen in simple MH n and selected multinary (complex) hydrides (M(BH 4 ) n , M(AlH 4 ) n ) and (M(NH 2 BH 3 ) n ) for various metals, M.

Single-metal hydrides are traditionally classified into three groups: ionic, covalent and interstitial. This division works fine mostly for a few typical examples in each category, however, with numerous compounds of intermediate properties [START_REF] Grochala | Thermal Decomposition of the Non-Interstitial Hydrides for the Storage and Production of Hydrogen[END_REF]. The typical ionic hydrides (also known as "saline hydrides") are formed by most of the s-block metals: Li -Cs, Mg -Ba, however, already MgH 2 exhibits a partial covalent character of its metal-hydrogen bonding [START_REF] Noritake | Chemical bonding of hydrogen in MgH2[END_REF]. These non-volatile and insulating compounds share densely-packed crystal structures with metal halides, fulfilling the known hydride-fluoride analogy [START_REF] Gingl | The hydride fluoride crystal structure database, HFD[END_REF]. Accordingly, the NaCl-type is adopted by MH, M = Li -Cs. Similarly to MgF 2 , MgH 2 crystallizes in rutile (TiO 2 ) structure, while MH 2 , M = Ca -Ba, are isostructural with PbCl 2 , common with fluorite (CaF 2 ) above ~10 GPa [START_REF] Kavner | Radial diffraction strength and elastic behavior of CaF2 in low-and high-pressure phases[END_REF][START_REF] Cazorla | High-Pressure, High-Temperature Phase Diagram of Calcium Fluoride from Classical Atomistic Simulations[END_REF]. Also the lanthanide trihydrides, LnH 3 , as well as EuH 2 and YbH 2 can be classified as ionic, unlike metallic LnH 2 and the non-stoichiometric compounds [START_REF] Cotton | Lanthanide and Actinide Chemistry[END_REF]. Beryllium and the p-block metals form hydrides of significantly covalent character, which adopt 0D molecular crystals, such as volatile SnH 4 [START_REF] Maley | Solid-state structures of the covalent hydrides germane and stannane[END_REF], or polymeric 3D networks, e.g. via corner sharing of corresponding tetrahedra or octahedra like BeH 2 and AlH 3 , respectively [START_REF] Smith | The crystal and molecular structure of beryllium hydride[END_REF][START_REF] Turley | Crystal structure of aluminum hydride[END_REF]. The binary hydrides of transition metals, some hydrides of lanthanides of a formula LnH n<3 , and various actinide hydrides are classified as interstitial hydrides [START_REF] Zachariasen | Crystal chemical studies of the 5 f -series of elements. XIX. The crystal structure of the higher thorium hydride, Th4H15[END_REF][START_REF] Weaver | Electronic structure of the thorium hydrides ThH2 and Th4H15[END_REF][START_REF] Gibson | Preparation and X-ray diffraction studies of berkelium hydrides[END_REF]. These compounds are often nonstoichiometric across a broad composition range, they show metallic cluster and usually conduct electricity well; conductivity is strongly affected by the molar content of hydrogen which influences electronic occupancy of the conduction band. Many hydrides belonging to this group form by incorporation of hydrogen atoms into suitable interstices (octahedral O, or tetrahedral T, vacancies) in the metallic lattice. However, this process usually occurs with substantial volume change (often 10-20% per hydrogen atom) and is often accompanied by hydrogen-induced phase transitions [START_REF] Fukai | The Metal-Hydrogen System[END_REF][START_REF] Schober | The systems NbH(D), TaH(D), VH(D) : Structures, phase diagrams, morphologies, methods of preparation[END_REF].

Such hydrides often show significant mobility of hydrogen atoms, mainly at elevated temperatures.

This feature is especially noticeable for palladium and allows for "filtration" of hydrogen through membranes based on Pd, or its alloys, resulting in 99.99999% hydrogen purity [START_REF] Adams | The role of palladium in a hydrogen economy[END_REF][START_REF] Grashoff | The Purification of Hydrogen. A review of the technology emphasising the current status of palladium membrane diffusion[END_REF]. Interestingly, palladium is able to reversibly absorb ~400 times its own volume of hydrogen at ambient conditions, while at lower temperature and elevated pressure this value raises almost to 1000. Remarkably, absorption of H 2 is connected with drop of the molar volume of the metal in this case. Unfortunately, PdH 0.6 , with a mere hydrogen capacity of 0.57 wt% H, is a typical stoichiometry which can be reached for palladium-hydrogen system near ambient conditions, and there is no evidence for H:Pd atomic ratio larger than 1 even up to 100 GPa [START_REF] Guigue | An x-ray study of palladium hydrides up to 100 GPa: Synthesis and isotopic effects[END_REF]. These facts, in addition to the high price of palladium, exclude PdH n from broader applications in hydrogen storage; however, due to its excellent reversibility, palladium tritide PdT x , remains a common tritium reservoir in nuclear facilities [START_REF] Emig | Helium release from 19-year-old palladium tritide[END_REF][START_REF] El-Kharbachi | Tritium absorption/desorption in ITER-like tungsten particles[END_REF].

Besides the systems' hydrogen capacity, the temperature of the thermal decomposition (T dec ) is connected with the release of hydrogen and, therefore, one of the key practical parameters related to hydrogen-storage materials. This parameter has been thoroughly analyzed for a number of hydrides.

For most of the MH n compounds the values of T dec well correlate with their thermodynamic parameters, especially with the free energy (ΔG o ) of the reaction, the standard redox potential (E o ) of the M n+ /M 0 redox couple and with the standard enthalpy of MH n formation (ΔH o f ) [START_REF] Grochala | Thermal Decomposition of the Non-Interstitial Hydrides for the Storage and Production of Hydrogen[END_REF]. It appears that T dec remains high (330 -720 °C) for the highly electropositive elements with E o < -2.3 V. T dec adopts moderate temperature values (0 -250 °C) for moderately electropositive metals and metalloids (E o between -2.0 and -0.6 V), while the electronegative metals and most of the semimetals studied (E o between -0.6 and +0.85 V) are rather unstable ( -125 < T dec < -15 °C). While this relation remains monotonic for most of the binary hydrides studied, there are a few exceptions, which are either more or less stable than the prevailing majority of MH n compounds. Some of these discrepancies can be explained by possible excessive kinetic stability, like for CuH, which should remain in equilibrium with the gaseous hydrogen only above an immense pressure of 8.5 GPa, while it shows a half-life of ca. 30 h at ambient conditions [START_REF] Burtovyy | Heat capacity of copper hydride[END_REF][START_REF] Tkacz | Decomposition of the hexagonal copper hydride at high pressure[END_REF][START_REF] Burtovyy | High-pressure synthesis of a new copper hydride from elements[END_REF].

Due to the above-mentioned restrictions on the gravimetric energy density, LiH, MgH 2 and AlH 3 are the binary systems that have been predominantly studied as potential hydrogen storage materials.

Although some tuning of the conditions under which hydrogen can be desorbed appeared possible, still non-satisfactory parameters have been achieved. The first of these compounds, LiH, is the most thermally stable among these metal hydrides (T dec ≈ 720 o C, decomposition is preceded by melting at ca. 689 o C). This temperature has been significantly decreased by doping with Si (Li : Si = 2.35 : 1), which allows for reversible storage of ca. 5 wt% H, released during heating to 490 o C [START_REF] Vajo | Altering Hydrogen Storage Properties by Hydride Destabilization through Alloy Formation: LiH and MgH2Destabilized with Si[END_REF]. Mg-based materials remains the most studied systems for hydrogen, electrochemical and thermal energy storage [START_REF] Yartys | Magnesium based materials for hydrogen based energy storage: Past, present and future[END_REF]. In the case of MgH 2 slow kinetics of de-/re-hydrogenation and high formation enthalpy (-75.2 kJ mol -1 ) are the main problems [START_REF] Grochala | Thermal Decomposition of the Non-Interstitial Hydrides for the Storage and Production of Hydrogen[END_REF]. Due to such parameters, heating to >400 o C is necessary for reversible hydrogen storage. Several methods for improvement have been tested for this parent system like the decreasing of the grain size and introduction of defects via high-energy ball-milling [START_REF] Zaluska | Nanocrystalline magnesium for hydrogen storage[END_REF],

forming nano-fibers utilizing CVD [START_REF] Saita | Hydriding chemical vapor deposition of metal hydride nano-fibers[END_REF], incorporation of MgH 2 into the carbon aerogel nanoscaffold [START_REF] Zhang | The synthesis and hydrogen storage properties of a MgH2 incorporated carbon aerogel scaffold[END_REF], destabilization via ion irradiation [START_REF] Matović | Structural destabilisation of MgH2 obtained by heavy ion irradiation[END_REF][START_REF] Kurko | Changes of hydrogen storage properties of MgH2 induced by boron ion irradiation[END_REF], or using various catalysts [START_REF] Vajo | Altering Hydrogen Storage Properties by Hydride Destabilization through Alloy Formation: LiH and MgH2Destabilized with Si[END_REF][START_REF] Reule | Hydrogen desorption properties of mechanically alloyed MgH2 composite materials[END_REF][START_REF] Barkhordarian | Fast hydrogen sorption kinetics of nanocrystalline Mg using Nb2O5 as catalyst[END_REF]. While some of them led to improved kinetics and lowered temperature of hydrogen release, these achievements are still far from meeting the targets for on-board hydrogen storage [START_REF] Graetz | Thermodynamics of the α, β and γ polymorphs of AlH3[END_REF]. In contrast to the two previous hydrides, AlH 3 is a metastable compound, strongly stabilized by significant barrier of decomposition and due to easy surface passivation by traces of oxygen. Although its room-temperature equilibrium pressure of H 2 is close to 50 GPa (ΔG o f = +48.5 ± 0.4 kJ mol -1 , ΔH o f = -9.9 ± 0.4 kJ mol -1 ), its thermal decomposition occurs around 150 o C [START_REF] Graetz | Thermodynamics of the α, β and γ polymorphs of AlH3[END_REF], or around 100 o C for the freshly prepared compound [START_REF] Graetz | Kinetics and thermodynamics of the aluminum hydride polymorphs[END_REF][START_REF] Maehlen | Thermal decomposition of AlH3 studied by in situ synchrotron X-ray diffraction and thermal desorption spectroscopy[END_REF]. Therefore, despite fair kinetics of hydrogen release, the reversibility of this system in the acceptable pressure range remains the sole unsolved problem [START_REF] Graetz | Kinetically stabilized hydrogen storage materials[END_REF].

It is worth to mention that preparation of so called "reactive hydride composites" is one of the tuning options tested for the mixtures of simple and complex metal hydrides [START_REF] Barkhordarian | Unexpected kinetic effect of MgB2 in reactive hydride composites containing complex borohydrides[END_REF][START_REF] Chen | Interaction of hydrogen with metal nitrides and imides[END_REF][START_REF] Vajo | Reversible storage of hydrogen in destabilized LiBH4[END_REF][START_REF] Heere | Milling time effect of Reactive Hydride Composites of NaF-NaH-MgB2 investigated by in situ powder diffraction[END_REF], in addition to the presence of suitable catalysts [START_REF] Milanese | Complex hydrides for energy storage[END_REF]. Such systems are selected in order to facilitate thermally-induced chemical reactions between their components to change the original (i.e. that for the parent chemical compounds) hydrogen release path and lower the overall reaction enthalpy. For example the MgH 2 /2LiBH 4 composite [START_REF] Bösenberg | Hydrogen sorption properties of MgH 2 -LiBH 4 composites[END_REF] decomposes according to the following scheme:

MgH 2 + 2LiBH 4 → Mg + 2LiBH 4 + H 2 → MgB 2 + 2LiH + 4H 2 (2)
This system shows dramatic improvement of kinetics of hydrogen absorption, which is observed already at 250 o C under 5 MPa hydrogen, while 600 o C and 35 MPa H 2 is required to reverse decomposition of LiBH 4 [START_REF] Orimo | Dehydriding and rehydriding reactions of LiBH4[END_REF]. Other systems involve the original LiH/LiNH 2 [START_REF] Chen | Interaction of hydrogen with metal nitrides and imides[END_REF], more complex 2LiNH 2 /MgH 2 /LiBH 4 composite, which decomposes in seven stages [START_REF] Yang | A Self-Catalyzing Hydrogen-Storage Material[END_REF], or MgH 2 /Ca(BH 4 ) 2 [START_REF] Vajo | Altering Hydrogen Storage Properties by Hydride Destabilization through Alloy Formation: LiH and MgH2Destabilized with Si[END_REF][START_REF] Barkhordarian | Unexpected kinetic effect of MgB2 in reactive hydride composites containing complex borohydrides[END_REF].

A very interesting system utilizing Kubas hydrogen binding: (η 2 -H 2 )-metal interaction, has been recently reported by Antonelli and co-workers [START_REF] Hoang | Exploiting the Kubas Interaction in the Design of Hydrogen Storage Materials[END_REF][START_REF] Morris | A manganese hydride molecular sieve for practical hydrogen storage under ambient conditions[END_REF]. This system, based on amorphous MnH x molecular sieve can be classified as falling in between metal hydrides and physisorption materials. It can be capable to reversibly store 10.5 wt% H, and almost 200 kg m -3 at 120 bar and room temperature, with no loss of performance after more than 50 cycles. As this system remains close to thermo-neutral, it will certainly become an inspiration in further search for even better performing hydrogen storage materials. However, no detailed information has been given about the purity of H 2 gas used for absorption, and that coming from desorption, which raises questions about possibility of absorption of traces of water and/or O 2 by this compound. Similar findings have been reported for other related systems, like those based on chromium(III), where the capacity of ca. 5.1 wt% H was achieved [START_REF] Morris | Thermodynamically neutral Kubas-type hydrogen storage using amorphous Cr(iii) alkyl hydride gels[END_REF].

Selection of binary and ternary intermetallics

bcc Ti-V alloys

Titanium alloy-based metal hydrides are an interesting class of hydrides because of their unique properties, their abundance and relatively low price. The volumetric hydrogen density of TiH 2 (150 kg m -3 ) is more than two times that of liquid hydrogen. The formation enthalpy of TiH 2 is -123.4 kJ mol -1 H 2 at 298 K [START_REF] San-Martin | The H-Ti (Hydrogen-Titanium) system[END_REF], which means that significant heat is released during hydrogenation. This makes Ti and its alloys promising candidates for both on-board hydrogen storage and heat storage systems. The interaction of hydrogen with vanadium is rather complex in comparison to its reaction with titanium.

The formation of various stable vanadium hydrides occurs below 200 °C. The formation enthalpy of fcc VH 2 is -40 kJ mol -1 H 2 [START_REF] Maeland | Investigation of the Vanadium-Hydrogen System by X-Ray Diffraction Techniques1,2[END_REF][START_REF] Reilly | Higher hydrides of vanadium and niobium[END_REF], hence these vanadium hydrides can be categorized as lowtemperature hydrides.

Ti and V can be alloyed with various elements, e.g. transition metals Mn, Cr, Ni, Fe [START_REF] Maeland | Hydride formation rates of BCC group V metals[END_REF][START_REF] Maeland | Hydride formation rates of titanium-based BCC solid solution alloys[END_REF]. This produces either a solid solution alloy or an intermetallic alloy. The intermetallic alloy can have a stoichiometric ratio of AB and AB 2 , where A is Ti or V, and B is usually a weak or non-hydride forming element. Many investigations have already been done on ternary and quaternary alloys [START_REF] Ono | The reaction of hydrogen with alloys of vanadium and titanium[END_REF][START_REF] Kuriiwa | Effects of V content on hydrogen storage properties of V-Ti-Cr alloys with high desorption pressure[END_REF][START_REF] Volodin | Study of hydrogen storage and electrochemical properties of AB2-type Ti0[END_REF][START_REF] Wijayanti | Hydrides of Laves type Ti-Zr alloys with enhanced H storage capacity as advanced metal hydride battery anodes[END_REF]. Ti and V form continuous solid solution bcc alloys in the range 2.7 at.% to 80 at. % vanadium.

bcc alloys can also be synthesized from Ti and V by the addition of beta stabilizer metals such as Cr and Nb. The addition of beta stabilizers has been known to reduce the thermal stability of the corresponding hydrides. The hydrogen capacity depends very much on the phase's constituent, the structure of the crystals, and the microstructure of the alloys. In this respect, Cr is favorable because Cr has a lower density than Ti and V. Addition of Cr to Ti-V based alloys is known to increase the equilibrium plateau pressure of the hydrides. This is related to the fact that Cr decreases the lattice parameter of the Ti-V-Cr bcc alloys since it has a lower metallic radius than Ti and V [START_REF] Kuriiwa | Effects of V content on hydrogen storage properties of V-Ti-Cr alloys with high desorption pressure[END_REF].

The bcc solid solution alloys are different from the intermetallic types in terms of ordering between the elements in the alloy. In bcc alloys, the alloying element is dissolved in the solvent such as Ti or V so that a disordered type alloy is formed [START_REF] Sandrock | A panoramic overview of hydrogen storage alloys from a gas reaction point of view[END_REF]. On the other hand, the intermetallic alloys form ordered structures of two elements. bcc type alloys have not gained much attention apart from their inherent reasonably high hydrogen capacity. One reason for this is the slow activation process. However, Maeland et al. [START_REF] Maeland | Hydride formation rates of titanium-based BCC solid solution alloys[END_REF] showed that the bcc alloys absorb hydrogen fast if they are alloyed with Mo, Nb or Fe even without or only with slight activation process. The bcc type hydrides therefore became a promising alternative. Of the ternary system, several studies have been done on Ti-V-Cr based alloys in which the content of Cr and V were found to control the thermodynamics, kinetics, and hydrogen capacity [START_REF] Kabutomori | Hydrogen absorption properties of TiCrA (A$\equiv$ V, Mo or other transition metal) BCC solid solution alloys[END_REF][START_REF] Tsukahara | Metal hydride electrodes based on solid solution type alloy TiV3Nix (0≦ x≦ 0.75)[END_REF][START_REF] Itoh | Improvement of cyclic durability of BCC structured Ti-Cr-V alloys[END_REF][START_REF] Nomura | H2 Absorbing-desorbing characterization of the TiVFe alloy system[END_REF][START_REF] Yu | Effect of Cr content on hydrogen storage properties for Ti-Vbased BCC-phase alloys[END_REF]. Even though the crystal structure is bcc in most of those cases, an addition of another element may result in minor amounts of C14 Laves phase (hexagonal MgZn 2 structure) [START_REF] Pan | Effects of Cr on the structural and electrochemical properties of TiV-based two-phase hydrogen storage alloys[END_REF][START_REF] Mouri | Hydrogen-absorbing alloys with a large capacity for a new energy carrier[END_REF].

At low hydrogen concentrations, the binary Ti-V alloys form monohydrides where the structure is preserved and known as -Ti 1-y V y H x (where x ranges from 0 to ~1.4) with a lattice parameter expanding linearly with the H content [START_REF] Hagi | Structure and phase diagram of Ti-V-H system at room temperature[END_REF]. The lattice parameter of Ti 1-y V y has been suggested to be linearly dependent on the amount of V at.%, due to differences in the atomic radius between Ti and V [START_REF] Ono | The reaction of hydrogen with alloys of vanadium and titanium[END_REF][START_REF] Hayashi | X-ray diffraction and 1H and 51V NMR study of the Ti V H system[END_REF][START_REF] Hayashi | Structure of Ti1-yVyHx alloys studied by X-ray diffraction and by 1H and 51V NMR[END_REF]. At high concentrations of hydrogen (H/M=1.45-2.00) hydrides of the CaF 2 type form, i.e.

-Ti 1-y V y H 1.50-A bct structure can also be present when the alloy is rich in vanadium. The maximum hydrogen content of the bcc Ti 1-y V y alloy is 2.00 H/M or 3.5-4 wt.%. In the monohydride -Ti 1-y V y H x , hydrogen occupies both O and T sites, and the population of the sites depends on the chemical composition of Ti 1-y V y ; the fraction of T sites occupied by hydrogen is higher in alloys with a high Ti fraction of [START_REF] Ueda | 1H NMR study of local structure and proton dynamics in β-Ti1-yVyHx[END_REF][START_REF] Ueda | Hydrogen motion and local structure of metals in β-Ti 1-y V y H x as studied by H 1 NMR[END_REF].

In the intermetallic hydride class, AB 2 type alloys have gained a lot of attention because of their high hydrogen capacity compared to that of the AB 5 type alloys along with their easy activation [START_REF] Pourarian | Hydrogen sorption properties of non-stoichiometric ZrMn2based systems[END_REF][START_REF] Schlapbach | Surface properties of hydride-forming AB2 compounds[END_REF][START_REF] Shaltiel | Hydride properties of AB2 Laves-phase compounds[END_REF].

According to Iba et al. [START_REF] Iba | Hydrogen absorption and modulated structure in Ti-V-Mn alloys[END_REF], the composition of the alloy determines the phase-structure of the AB x alloy. Nevertheless, in the A-B system, 3 structural phases exist, namely bcc, C14 and C15 (Cu 2 Mg structure). The hydrogen capacity, thermodynamics, and kinetics of the bulk hydrides are affected by the fraction of each phase [START_REF] Iba | The relation between microstructure and hydrogen absorbing property in Laves phase-solid solution multiphase alloys[END_REF][START_REF] Huot | Crystal structure of multiphase alloys (Zr,Ti)(Mn,V)2[END_REF][START_REF] Huot | Crystal structure, phase abundance and electrode performance of Laves phase compounds (Zr, A) V0. 5Ni1. 1Mn0. 2Fe0. 2 (A$\equiv$ Ti, Nb or Hf)[END_REF]. C14 and C15 typically have lower capacities than the bcc alloy.

However, the C14 or C15 structures are considered beneficial to sustain the cycle ability and improve the activation properties of the alloys. In these regards, Iba and Akiba developed multiphase alloys which are better than individual Laves phases or bcc phase alone [START_REF] Iba | Hydrogen absorption and modulated structure in Ti-V-Mn alloys[END_REF][START_REF] Akiba | Hydrogen absorption by Laves phase related BCC solid solution[END_REF].

Hydrogen desorption from Ti-V hydrides was studied by thermal desorption spectroscopy [START_REF] Suwarno | Nonisothermal kinetics and in situ SR XRD studies of hydrogen desorption from dihydrides of binary Ti-V alloys[END_REF]. The desorption spectra of hydrogen desorption are mainly composed of 3 peaks related to the stages of hydrogenation. The first step of dehydrogenation is low-temperature hydrogen desorption from the fcc γ-hydride. The hydrogen desorption then starts to evolve at about 300-400 °C depending on the amount of vanadium. This second mode of hydrogen desorption has the fastest hydrogen release rate.

The last stage is the hydrogen desorption from bcc phase hydride.

The evolution of the phase-structural composition of the -Ti 0.8 V 0.2 H 2 (3.96 wt. % H) dihydride as a function of its thermal decomposition during a non-isothermal dehydrogenation was studied by in situ synchrotron radiation -powder X-ray diffraction (SR-XRD) and is shown in Fig. 2a. It can be seen as three-dimensional plot of the phase transformations of hydrides during dehydrogenation. The sample was heated from 25 °C to 800 °C with a heating rate of 2 °C min -1 under vacuum. The diffraction patterns collected above 320 °C were fitted with a two-phase structural model, i.e. fcc and bct type hydrides. The reason for choosing this model was the observation of asymmetric profiles of the peaks corresponding to the [START_REF] Marynick | Crystal structure of beryllium borohydride[END_REF], [START_REF] Her | Structure of unsolvated magnesium borohydride Mg(BH4)2[END_REF], and (311) of the fcc. This is in line with a tetragonal distortion of the fcc lattice to a bct lattice. A proof that the sample was composed of the  hydride mixtures can be seen by observing the progression of the overlapped peaks  [START_REF] Suwarno | Influence of Cr on the hydrogen storage properties of Ti-rich Ti-V-Cr alloys[END_REF] and (101) at temperatures from 450 °C to 500 °C, shown as an inset in Fig. 2a. Changes in phase abundances were calculated by the Rietveld method and can be seen in Fig. 2b. Phase transformations during the nonisothermal heating proceed according to the following pathways;

hydride)hydride(hydride)bcc alloy. The addition of Cr to these bcc Ti-V alloys changes the step of dehydrogenation to a multi-step process [START_REF] Suwarno | Influence of Cr on the hydrogen storage properties of Ti-rich Ti-V-Cr alloys[END_REF]. In principle, there are two routes for nano-structuring alloy for hydrogen storage, i.e. bottom-up approach with liquid-based synthesis method and bulk approach using ball milling, rapid solidification or severe deformation such as high-pressure torsion (HPT) method. There have been some studies on the effect of rapid solidification on bcc type alloys. Yu et al. [START_REF] Yu | Improvement of activation performance of the quenched Ti-Vbased BCC phase alloys[END_REF] found that the rapid solidification of Ti-V-Mn-Cr increased the maximum hydrogen capacity. This was caused by a suppression of the C14 phase fraction in the alloy microstructure [START_REF] Yu | Improvement of activation performance of the quenched Ti-Vbased BCC phase alloys[END_REF][START_REF] Pan | A study on the cycling stability of the Ti-V-based hydrogen storage electrode alloys[END_REF]. However, rapid solidification degraded the activation process, i.e. the first hydrogenation was slower as compared to the as-cast alloy, possibly due to oxide formation on the surface of the rapidly solidified samples. However, in another investigation, it was suggested that the existence of the C14 phase is helpful for the activation process because the C14 Laves phase easily cracks during cycling and provides paths for hydrogen penetration [START_REF] Mouri | Hydrogen-absorbing alloys with a large capacity for a new energy carrier[END_REF].

For bcc Ti-V, rapid solidification has improved the kinetics when a nanograin microstructure is formed [START_REF] Suwarno | Microstructure and hydrogen storage properties of as-cast and rapidly solidified Ti-rich Ti-V alloys[END_REF][START_REF] Suwarno | The effects of rapid solidification on microstructure and hydrogen sorption properties of binary BCC Ti-V alloys[END_REF]. The distribution of the alloying element is also affected by the nanostructured processing and enhance the hydrogen desorption properties. As can be seen in Fig. 3, the HPT processing results in an inhomogeneous distribution of Ti-V elements, which enhance the hydrogenation properties. Rapid solidification has been used to synthesize Laves phases in Zr based alloys [START_REF] Shu | Effect of rapid solidification process on the alloy structure and electrode performance of Zr[END_REF][START_REF] Shu | Micro-crystalline C14 Laves phase in melt-spun AB2 type Zr-based alloy[END_REF]. It was observed that rapid solidification introduced a change in the phase fraction of C14 and C15 in the final microstructure. It was suggested that the formation of Laves phase microstructures do not only depend on the chemical composition alone, but also on the solidification rate [START_REF] Lü | Structure study on rapidly solidified hydrogen storage alloy Zr(NiM)2.1[END_REF]. Increasing the cooling rate increases the amount of C14 phase, but it leads to decreasing capacity. In addition, the activation process became much more difficult. One of the advantages was the prolonged cycle life of the alloys, i.e. up to 500 cycles without significant loss in capacity [START_REF] Shu | Effect of rapid solidification process on the alloy structure and electrode performance of Zr[END_REF]. In general, the properties of metal hydrides can be altered upon hydrogenation-dehydrogenation cycling. This manifests itself by an alteration in the reversible capacity or the kinetic rate constant of the pressure-composition isotherm (PCI) profile. This change can be positive or negative for a specific application, however, in most cases the change is negative. The negative change or degradation can be categorized into two types:

 Intrinsic. The performance of the metal hydrides is decreased upon cycling due to the formation of more stable hydrides or compounds. This is the so-called disproportionation reaction.

 Extrinsic. The performance of the metal hydrides is decreased because of impurities in the hydrogen gas. These impurities are gases such as CO 2 that are reactive towards the hydride alloys.

For the case of Ti-V based alloys, cycling leads to pulverization and disproportionation of the alloys, which both decrease the reversible capacity. The bcc Ti 0.32 Cr 0.43 V 0.25 alloy has been shown to get lower particle size upon cycling, decreasing from 240 to 60 µm after 1000 cycles, and the hydrogen capacity was observed to decrease as well [START_REF] Cho | Hydrogen absorption-desorption properties of Ti0. 32Cr0. 43V0. 25 alloy[END_REF]. A rather worse case was observed in a bcc Ti-V-Cr-Mn alloy where the hydrogen reversible capacity was observed to decrease from 3.4 wt.% H in the 1 st cycle to only 2 wt.% after 200 cycles. Bulk analysis using XRD and surface studies using XPS revealed that the lattice volume of the bcc crystal was reduced and that the surface was enriched with oxygen as a consequence of the cycling [START_REF] Wan | A study on crystal structure and chemical state of TiCrVMn hydrogen storage alloys during hydrogen absorption-desorption cycling[END_REF][START_REF] Suwarno | High temperature hydrogenation of Ti-V alloys: The effect of cycling and carbon monoxide on the bulk and surface properties[END_REF]. This was suggested to cause the observed capacity decrease. A similar observation has been reported for V based alloys where pulverization has been suggested as the main reason for decreased capacity upon cycling [START_REF] Hao | Cyclic properties of hydrogen absorption and desorption in V-Ti-Cr-Fe(Al,Si) alloy[END_REF]. In Ti-based alloys, the disproportionation is related to the formation of stable Ti hydrides, which have a higher thermal stability than the alloy.

Observations of cycled samples using transmission electron microscopy (TEM) have clearly indicated that disproportionation occurs in TiMn 2 [START_REF] Semboshi | Effect of structural changes on degradation of hydrogen absorbing capacity in cyclically hydrogenated TiMn2 based alloys[END_REF][START_REF] Nayebossadri | Compositional effects on the hydrogen cycling stability of multicomponent Ti-Mn based alloys[END_REF]. Improvements in the production process have contributed to increase the alloy performance upon cycling of Ti-V-Cr alloys [START_REF] Itoh | Improvement of cyclic durability of BCC structured Ti-Cr-V alloys[END_REF], and composition homogeneity was suggested to be the reason for this increased cycling performance.

Ti-Fe based systems

Ti-Fe is another binary metal-based systems that has been extensively studied for hydrogen storage thanks to its tunable hydrogenation properties that can be tailored by elemental substitution, for its low cost, easy production and recycling [START_REF] Cuevas | Hydrogen Storage Materials[END_REF]. The main investigated compound in the Ti-Fe system is the stoichiometric binary TiFe intermetallic. The first report on TiFe for hydrogen storage was published by Reilly et al. in 1974 [127], reporting a maximum gravimetric hydrogen capacity of 1.86 wt.% with full reversibility close to normal conditions of pressure and temperature. This feature makes TiFe suitable for stationary storage of hydrogen in mild conditions. TiFe can be produced by high temperature melting techniques, mechanical alloying, sputtering, or powder sintering.

The Ti-Fe binary phase diagram shows two intermetallic phases, TiFe and TiFe 2 , which can be defined as AB and AB 2 compounds [START_REF] Gąsior | Enthalpy of Formation of Intermetallic Phases from Fe-Ni-Ti System. Comparative Studies / Entalpia Tworzenia Faz Międzymetalicznych Z Układu Fe-Ni-Ti[END_REF]. TiFe exists in a small range of composition, which extends from 49.7 to 52.5 at% Ti at 1085 °C. Both elemental Ti and Fe can form bcc solid solutions above 600 °C with reciprocal solubility up to approx. 10 at% of Ti in Fe at 1300 °C , and up to 20 at% of Fe in Ti at 1100 °C [START_REF] Kaufman | Coupled phase diagrams and thermochemical data for transition metal binary systems[END_REF][START_REF] Schlapbach | The activation of FeTi for hydrogen absorption[END_REF].

As most of Ti-based AB alloys, TiFe crystallizes in a CsCl-type cubic structure (S.G. Pm-3m). The enthalpy of formation of this cubic phase is -22.5 kJ mol -1 [START_REF] Gąsior | Enthalpy of Formation of Intermetallic Phases from Fe-Ni-Ti System. Comparative Studies / Entalpia Tworzenia Faz Międzymetalicznych Z Układu Fe-Ni-Ti[END_REF]. The lattice parameter can vary because of composition fluctuation related to homogeneity domain of the TiFe phase that may arise from different synthesis methods [START_REF] Berdonosova | Calorimetric study of peculiar hydrogenation behavior of nanocrystalline TiFe[END_REF]. Upon hydrogenation, TiFe forms two hydrides adopting orthorhombic structures: the monohydride TiFeH (β phase), and the dihydride TiFeH 2 (γ phase).

Deuterated TiFe was investigated by neutron diffraction to define the hydrides structure.

Consequently, the PCI curves under deuterium of TiFe present two distinguished plateau pressures that correspond to the β (1 st plateau, Fig. 4) and γ-phase (2 nd plateau, Fig. 4) [START_REF] Schefer | Structural phase transitions of FeTi-deuterides[END_REF]. Usually the two plateaus are better defined upon desorption, located at ca. 0.3 and 0.9 MPa at room temperature for the first and second plateau, respectively [START_REF] Wenzl | Phase diagram and thermodynamic parameters of the quasibinary interstitial alloy Fe0.5Ti0.5Hxin equilibrium with hydrogen gas[END_REF]. For absorption, the formation of a solid solution of H in TiFe is firstly observed (α phase). The solid solution α has a maximum H solubility at H/M = 0.04 (corresponding to TiFeH 0.08 ) [START_REF] Reilly | Lattice expansion as a measure of surface segregation and the solubility of hydrogen in α-FeTiHx[END_REF]. Then, through the first plateau, the β phase forms (P222 1 ) [START_REF] Fischer | Deuterium storage in FeTi. Measurement of desorption isotherms and structural studies by means of neutron diffraction[END_REF],

even if a large discussion concerning the TiFe hydride crystal structures is still present in the literature, together with some controversial results such as the orthorhombic structure (C222) reported in Ref. [START_REF] Mohammedi | Ab-initio structural and electronic properties of the intermetallic compound TiFeH2[END_REF]. The dihydride, (γ phase) has an orthorhombic structure (Cmmm) [START_REF] Fischer | Orthorhombic structure of γ-TiFeD≈2[END_REF]. The enthalpies of absorption ΔH abs have been reported equal to -17.5 kJ mol -1 H and -12.5 kJ mol -1 H for the α-β and β-γ transitions, respectively [START_REF] Berdonosova | Calorimetric study of peculiar hydrogenation behavior of nanocrystalline TiFe[END_REF]. The cycling stability of the monohydride is good, while the dihydride has a sloppy plateau that shifts to higher pressures with increasing cycling. The activation of TiFe towards the first hydrogenation is usually challenging, since high temperature (above 400 °C) and long reaction time (days) are necessary for hydrogen atoms to overcome the native oxide layer at its surface. Moreover, TiFe has limited resistance to gas impurities. In addition, heat management and changes in volume (being approx. 18%) during absorption and desorption cycling must be considered [START_REF] Sujan | An overview on TiFe intermetallic for solid-state hydrogen storage: microstructure, hydrogenation and fabrication processes[END_REF]. In conclusion, TiFe is a low cost intermetallic with reversible hydrogen uptake near ambient conditions of pressure and temperature. However, the compound suffers from activation issues, sensitivity to gas impurities and cycling instabilities of the second plateau. Strategies to overcome these issues will be discussed in detail hereafter.

Frequently, as-synthetized and/or annealed TiFe does not react readily with hydrogen, neither at room temperature, nor under high hydrogen pressure (i.e. 10 MPa) [START_REF] Emami | ScienceDirect Hydrogen storage performance of TiFe after processing by ball milling[END_REF]. The harsh activation of the stoichiometric single-phased TiFe alloy is attributed to the presence of native oxide passivating surface layers, since the intermetallic compound is sensible to contaminants such as water or oxygen. Air exposure creates a passivating surface layer, which prevents the hydrogenation. Precautions, such as using high hydrogen purity, must be taken for using TiFe as hydrogen storage medium. However, chemical substitutions and minor amounts of secondary phases at the TiFe grain boundaries are highly beneficial for the activation process, thus three main approaches can help the activation process in general.

The first one consists in cracking the metal-oxide layer by a high temperature (up to 400 °C) heat treatment in vacuum, in combination with applying high pressures of hydrogen (above 6 MPa), to facilitate as well hydrogen diffusion through the oxide layer and into the metal [START_REF] Reilly | Formation and properties of iron titanium hydride[END_REF]. Furthermore, mechanical milling or grinding processes [START_REF] Emami | ScienceDirect Hydrogen storage performance of TiFe after processing by ball milling[END_REF], or performing the absorption and desorption cycling under hydrogen enhance easy activation. By alternating cycles of absorption and desorption, the metal lattice is expanded during absorption and contracted during dehydrogenation. This results in pulverization of the metal particles, creating fresh metallic surfaces as well as a reduction in particle sizes. In this way, there is also the advantage to create a fine microstructure, which contributes in enhancing the kinetic of hydrogen absorption and desorption. Mechanical milling is also of interest as activation process. Indeed, milling can break the oxide layer, create fresh surfaces and refine the alloy microstructure [START_REF] Huot | Mechanochemical synthesis of hydrogen storage materials[END_REF]. However, extensive milling should be avoided to mitigate TiFe amorphization since it reduces its reversible storage capacity [START_REF] Zaluski | Hydrogen Absorption in Amorphous and Nano-Crystalline FeTi[END_REF].

The second approach is to modify the chemical composition of the surface layer to precipitate either hydride-forming compounds or H-permeable phases allowing hydrogen penetration. As a matter of fact, if a secondary active phase such as Ti-rich precipitates are present at the alloy surface, its volume expansion due to the formation of Ti-rich hydrides (e.g. TiH 2 ) will favour cracking of the TiFe particles and expose fresh surfaces to accelerate hydrogen sorption. Pure Ti is expected to face the same oxidation issues, as TiO 2 also forms a passivation layer [START_REF] Fernandez | Surface activation and hydrogenation kinetics of ti sponge[END_REF]. However, in the Ti-Fe system, the formation of Ti-rich precipitates containing iron (Ti 80 Fe 20 ) instead of pure Ti, may help to activate the material [START_REF] Fokin | Study of the Interaction with Hydrogen and Ammonia of Titanium and Its Alloys with Iron[END_REF]. Some oxide phases have been also reported to be reactive to hydrogen, allowing hydrogen diffusion towards TiFe and facilitating activation [START_REF] Rupp | On the charge in physical properties of Ti4-xFe2+xOy during idrogenation. I: activation behaviour of ternary oxydes Ti4-xFe2+xOy and β-Ti[END_REF].

Finally, the third method to improve activation consists of chemical tailoring of composition. By modifying the binary composition of TiFe through suitable chemical substitutions either at the Ti or Fe sublattices, or for both elements, pseudo-binary intermetallic compounds more resistant to poisoning effect can be obtained requiring less laborious activation process [START_REF] Yu | Surface properties on hydrogen storage material TiFeMm[END_REF][START_REF] Patel | Effect of annealing on microstructure and hydrogenation properties of TiFe + X wt% Zr (X = 4, 8)[END_REF]. It is important to note that, once activated; TiFe remains highly sensitive to atmospheric contaminants. Thus, for filling TiFe materials in hydrogen tanks, handling of activated TiFe powders under inert atmosphere (in purified glove box) should be considered. For large-scale applications, this approach is unpractical and therefore the activation process should be performed directly inside the tank. This implies that the operating conditions of the tank should be adapted to the activation conditions, with higher values of H 2 pressure and operative temperature than for the usual working conditions.

Addition of substitutional elements or secondary phases into TiFe compound allows the modification of its thermodynamic and chemical properties. Influence of metallic substitution on sorption properties of TiFe-based systems have been studied extensively and many different elements have been added to TiFe so far. By chemical substitutions, absorption and desorption pressures as well as the lengths of the plateaus are modified. Thus, chemical modifications allow tailoring performances and working conditions towards the final targets of the application. When the element addition results in the precipitation of some secondary phases, the reversible capacity is expected to decrease, especially when the secondary phases are not reversible or reactive to hydrogen at the operating conditions.

Tailoring TiFe means partial substitution of Fe at site 1a (0,0,0), or of Ti at site 1b (½,½,½) in the CsCl-type structure. According to Pearson's crystal database [START_REF] Villars | Pearson's Crystal Data -Crystal Structure Database for Inorganic Compounds[END_REF], 52 different substituted structures are reported, mostly compounds including one or two substitutions, with variations of the cell parameter in the range of 2.861 Å < a < 3.185 Å. The cell parameter of the pure equi-atomic Ti:Fe ratio is 2.976 Å. Introducing a substituent cause either the contraction or expansion of cell parameter and volume, depending on the size of the substituting element. For monosubstituted compounds, Fe can be replaced at site 1a in a large extent by Al, Co, Cu, Mn, Ni, Pd, whereas Ti can be substituted at site 1b by Al and Ru in a smaller compositional range. As for bi-substituted compounds, both 1a and 1b sites can be occupied by Al, Ga, Mn, Ru, and V. Monosubstituted compounds forming hydrides include α solid solution (Ru), β phase monohydride (Ni) and γ phase dihydride (Mn) [START_REF] Blouin | Neutron and in Situ X-ray Investigation of Hydrogen Intake in Titanium-Based Cubic Alloys[END_REF][START_REF] Mintz | Hydrides of ternary TiFexM1-x(M=Cr, Mn, Co, Ni) intermetallics[END_REF][START_REF] Raj | Nickel substituted FeTi hydrides: A critical study of the β-region[END_REF][START_REF] Endo | Hydrogenation of a TiFe-based alloy at high pressures and temperatures[END_REF][START_REF] Cantrell | Comparison of structures and electronic properties between TiCoHx and TiFeHx[END_REF].

It is worth noting that mixed site occupancies occur within the binary TiFe phase along the homogeneity domain, particularly for the Ti-rich side which extends up to 52.5 at% Ti. It was shown by accurate measurements of the density and the lattice parameter that the excess of titanium may substitute iron up to the homogeneity domain limit Ti 1b (Fe 1-x Ti x ) 1a with x = 0.05 [START_REF] Challet | Hydrogen Storage in TiFe(0.70+x)Mn(0.20-x) (0 <= x <= 0.15) and TiFe[END_REF].

As mentioned before, hydrogenation of TiFe by a solid-gas reaction can only be achieved after severe activation due to its high sensitivity to surface poisoning by gaseous impurities [START_REF] Sandrock | Cyclic life of metal hydrides with impure hydrogen: Overview and engineering considerations[END_REF][START_REF] Schlapbach | Hydrogen in Intermetallic Compunds II Surface and Dynamic Properties[END_REF]. However, partial substitutions of Fe by Mn and Ni are effective to improve the hydrogenation reactivity of the alloy, and allows adapting the plateau pressures for hydrogen storage applications [START_REF] Sandrock | A panoramic overview of hydrogen storage alloys from a gas reaction point of view[END_REF]. Furthermore, the hydrogenation of titanium-rich alloys Ti 1+x Fe (0 < x < 0.05), which contain β-Ti as a secondary phase, has been found to occur without any activation [START_REF] Singh | Investigation on synthesis, characterization and hydrogenation behaviour of hydrogen storage material: Fe1-xZrxTi1.3 (x = 0.2)[END_REF][START_REF] Matsumoto | Hydrogenation of FeTi-based alloys containing b-Ti[END_REF][START_REF] Mizuno | Titanium concentration in FeTix (l ⩽ x ⩽ 2) alloys and its effect on hydrogen storage properties[END_REF][START_REF] Lee | Microstructural correlations with the hydrogenation kinetics of FeTi1+ξ alloys[END_REF]. A previous study has shown that in certain cases where the Ti concentration is greater than 50 at%, the maximum hydrogen capacity exceeds that of the TiFe hydride [START_REF] Reilly | Paper presented. 1st World Hydrogen Energy Conference[END_REF]. This could be related to the presence of secondary phases, such as β-Ti precipitate as introduced before. In fact, TiH 2 has a capacity of 4 wt%, however it is not reversible under mild pressure and temperature conditions and, therefore, the reversible capacity of Tirich compounds above x = 0.05 is expected to be lower than for TiFe.

Mn substitution of Fe in TiFe has been reported to promote hydrogen uptake without activation treatment, especially in Ti-rich compounds such as TiFe 0.90 . Both Mn and Ti in excess partially replace iron at the 1a site, causing an enlargement of the cell parameter and decreasing the plateau pressures [START_REF] Challet | Hydrogen Storage in TiFe(0.70+x)Mn(0.20-x) (0 <= x <= 0.15) and TiFe[END_REF]. Challet et al. reported experiments on TiFe 0.70 Mn 0.20 , evidencing chemical homogeneity and the absence of slopped plateaus (thanks to thermal treatment at 1000 °C for one week). Activation of the alloy at room temperature and 1 MPa after 2 hours of incubation time, and a hydrogen capacity of 1.98 wt% at room temperature and 2 MPa were observed. Two distinct plateaus were reported in the PCI curves. Furthermore, the initial capacity was retained over 20 cycles. Increasing the amount of Mn in the system produces a slight increase of the total capacity, better kinetics, overall decrease of both plateau pressures and a diminution in the difference of stability between mono and di-hydrides (first and second plateau desorption pressure) compared to Mn-free TiFe material [START_REF] Challet | Hydrogen Storage in TiFe(0.70+x)Mn(0.20-x) (0 <= x <= 0.15) and TiFe[END_REF]161].

By increasing the Fe/Mn ratio in TiFe 0.70+x Mn 0.20-x , the cell volume shrinks and, consequently, the plateau pressures rise. Thus, the plateau pressures at 65 °C of the materials investigated by Challet et al. are close to the pressure range 0.4-2 MPa. However, either the first plateau in desorption (TiFe 0.80 Mn 0.10 ) or the one of the second hydride in absorption (TiFe 0.85 Mn 0.05 ) are still out of this pressure domain. In addition, the pressure difference between the first and the second hydride increases with temperature, making it difficult to adjust both plateau pressures in a narrow pressure range. From the kinetic point of view, the alloys exhibit very fast reaction rate since 90% of the total capacity is absorbed in less than 3 minutes at room temperature and 2 MPa.

V substitution of Ti in TiFe-systems has also evidenced an improved and easy activation process at room temperature and 2.5 MPa of hydrogen, with an increase of the capacity as a function of V content, and no clear plateau pressure difference between mono and di-hydride. However, PCI curves show sloppy plateaus [START_REF] Massicot | Hydrogenation properties of Fe-Ti-V bcc alloys[END_REF].

Chromium and aluminium can also be considered in monosubstituted compounds. A mechanicalsubstituted alloy, with 4 at% Cr and 5 at% Al respectively, shown a reversible capacity of 0.7 wt% at room temperature and between 0.1-10 MPa. In these systems, Al shows a higher plateau pressure compared to Cr-substituted material, and both remarkably reduce hysteresis with respect to pure TiFe, improving also kinetics and activation [START_REF] Zadorozhnyy | Mechanical alloying of nanocrystalline intermetallic compound TiFe doped by aluminum and chromium[END_REF].

Finally, the partial substitution of Fe with Ni, Mn, V, Cr, Al, Co can decrease the first plateau pressure and facilitate the activation [START_REF] Rusman | A review on the current progress of metal hydrides material for solidstate hydrogen storage applications[END_REF][START_REF] Sandrock | A panoramic overview of hydrogen storage alloys from a gas reaction point of view[END_REF][START_REF] Challet | Hydrogen Storage in TiFe(0.70+x)Mn(0.20-x) (0 <= x <= 0.15) and TiFe[END_REF]161,[START_REF] Massicot | Hydrogenation properties of Fe-Ti-V bcc alloys[END_REF][START_REF] Zadorozhnyy | Mechanical alloying of nanocrystalline intermetallic compound TiFe doped with sulfur and magnesium[END_REF][START_REF] Lee | Correlation of substitutional solid solution with hydrogenation properties of TiFe1-xMx (M=Ni, Co, Al) alloys[END_REF]. The plateau pressure decreases considerably passing from non-substituted alloy to TiFe 0.9 Ni 0.1 . The plateau remains as flat as for TiFe, except for TiFe 0.9 Al 0.1 . In this case, the substitution creates a stiff slope which is attributed to the distortion of octahedral sites due to the large difference of metallic radii between Fe and Al (r Fe = 1.24 Å, r Al = 1.43 Å) [START_REF] Zhang | Microstructures and properties of high-entropy alloys[END_REF]. It is hypothesized that it exists a size distribution of octahedral interstitial sites with different hydrogen occupancies. Such inhomogeneity would enhance the slope. Thus, to avoid this issue, best substituting B-type elements would be those with metallic radius comparable to Fe. This kind of local effects have been suggested in the past for AB 2 compounds as well [START_REF] Ivey | Storing energy in metal hydrides: a review of the physical metallurgy[END_REF][START_REF] Ivey | Storing Hydrogen in AB2 Laves-Type Compounds[END_REF]. However, this effect seems not to affect the slope of the plateau in case of the Ti-rich TiFe compounds, where no sloping plateau is observed and r Ti = 1.47 Å, a radius even higher than that of Al.

The remarkable decrease of the plateau pressure with Ni and Co substitutions is linked to the stronger affinity towards hydrogen of TiCo and TiNi with respect to TiFe, which implies a higher enthalpy of formation; for TiNiH 0.9 equal to -30 kJ mol -1 H 2 and for TiCoH 0.9 equal to -27 kJ mol -1 H 2 [START_REF] Hirscher | Handbook of Hydrogen Storage: New Materials for Future Energy Storage[END_REF],

resulting in more stable monohydrides. Substitutions can also act on decreasing hysteresis phenomenon among cycles; particularly effective are Ni and Mn elements. However, the hysteresis loop is affected by the initial temperature and pressure [170].

In conclusion, the variety of ternary substitutions in TiFe intermetallic compounds demonstrates how flexible this material is and how much it can be tuned towards the final application. In most of the cases, substitution is beneficial and improves properties such as hydrogen capacity, low hysteresis, kinetics and easy activation. Some elements have drastic effects and may also modify the pressure step between the first and second plateau. The plateau pressure can be finely tuned as a function of the quantity of substituent introduced. For this reason, thanks to its low cost and easy processing, TiFe substituted systems are promising for large-scale stationary application, and can be easily tuned and coupled with electrolyzer and fuel cells incrementing the spread-out of hydrogen as efficient energy carrier and supporting the development and integration of renewable energies into smart grids.

High entropy systems

Multi-principal element alloys (MPEAs), also known as high-entropy alloys (HEAs), are a relatively new class of materials constituted by several elements in near equimolar concentrations initially reported independently by two groups in 2004 [START_REF] Yeh | Nanostructured High-Entropy Alloys with Multiple Principal Elements: Novel Alloy Design Concepts and Outcomes[END_REF][START_REF] Cantor | Microstructural development in equiatomic multicomponent alloys[END_REF]. In these works, the authors studied two quinary systems, exploring the central region of the multi-component phase diagram as a new approach for alloy development. Interestingly, these alloys are disordered solid solution which can adopt simple crystalline structures (bcc, fcc and hcp). It has been suggested that the high configurational entropy is the cause of the stabilization of the single-phase solid solutions. Behind the HEA term, it is now agreed that the motivation is to find single-phased solid solutions by controlling the configurational entropy [START_REF] Miracle | A critical review of high entropy alloys and related concepts[END_REF]. On the contrary, the MPEA term is employed to remind the vastness of composition space of the central region of phase diagram regardless the entropy contribution or the number and types of phases present. In this respect, the MPEA includes a broader classification of alloys then HEA, but the latter term has acquired a more widespread recognition. In addition, it appears that there are two commonly accepted definitions for HEA, one based on the magnitude of entropy, and another based on the concentration of principal elements. The entropy-based definition imposes the entropic term of a disordered solid solution to be superior to 1.61R (R is the universal gas constant), whereas the definition based on composition emphasizes that the concentration of elemental constituents should be in the range 5-35 at.%.

This new strategy of alloy design is very exciting due to the vast number of possible configurations and has become a playground of many research activities mainly focused on the structural and mechanical properties. In addition, other interesting functional properties, such as thermoelectric effect, photovoltaic conversion, piezoelectricity have been reported. Hydrogen storage in HEA is particularly interesting since these alloys are composed of elements with different atomic radii and thus a high lattice strain is developed possibly providing large interstitial sites for hydrogen occupation. However, the hydrogen sorption properties of HEA are scarcely reported in literature. The mainstream of reports focus on bcc alloys [START_REF] Kunce | Microstructure and hydrogen storage properties of a TiZrNbMoV high entropy alloy synthesized using Laser Engineered Net Shaping (LENS)[END_REF][START_REF] Sahlberg | Superior hydrogen storage in high entropy alloys[END_REF][START_REF] Karlsson | Structure and Hydrogenation Properties of a HfNbTiVZr High-Entropy Alloy[END_REF][START_REF] Zepon | Hydrogen-induced phase transition of MgZrTiFe0.5Co0.5Ni0.5 high entropy alloy[END_REF][START_REF] Zlotea | Hydrogen sorption in TiZrNbHfTa high entropy alloy[END_REF][START_REF] Nygård | Hydrogen storage in highentropy alloys with varying degree of local lattice strain[END_REF][START_REF] Nygård | Counting electrons -A new approach to tailor the hydrogen sorption properties of high-entropy alloys[END_REF][START_REF] Montero | TiVZrNb Multi-Principal-Element Alloy: Synthesis Optimization, Structural, and Hydrogen Sorption Properties[END_REF][START_REF] Shen | Compositional dependence of hydrogenation performance of Ti-Zr-Hf-Mo-Nb high-entropy alloys for hydrogen/tritium storage[END_REF] and hexagonal C14 Laves phases [START_REF] Kao | Hydrogen storage properties of multi-principal-component CoFeMnTixVyZrz alloys[END_REF][START_REF] Kunce | Structure and hydrogen storage properties of a high entropy ZrTiVCrFeNi alloy synthesized using Laser Engineered Net Shaping (LENS)[END_REF][START_REF] Chen | Hydrogen storage of C14-CruFevMnwTixVyZrz alloys[END_REF][START_REF] Edalati | Reversible room temperature hydrogen storage in high-entropy alloy TiZrCrMnFeNi[END_REF]. The present review highlights hydrogen storage properties of refractory bcc alloys since this class of materials in their conventional form (see § 2.2) is well known for hydrogen storage for many years [START_REF] Akiba | Metallic Hydrides III: Body-Centered-Cubic Solid-Solution Alloys[END_REF].

In 2014, Kunce et al. [START_REF] Kunce | Microstructure and hydrogen storage properties of a TiZrNbMoV high entropy alloy synthesized using Laser Engineered Net Shaping (LENS)[END_REF] investigated the hydrogen sorption properties of TiZrNbMoV synthesized by laser engineered net shaping. The single-phased bcc alloy showed a low hydrogen capacity of 0.6

wt.% at room temperature. Later, Sahlberg et al. [START_REF] Sahlberg | Superior hydrogen storage in high entropy alloys[END_REF] reported the hydrogen sorption properties of a refractory HEA TiVZrNbHf showing an outstanding capacity. Interestingly, this alloy has a hydrogen uptake of 2.5 H/M, which is greater than the capacity of individual elements (2 H/M). Moreover, the hydrogenation of this alloy is a reversible single step reaction (bcc  bct). Such high hydrogen content has never been observed in individual hydrides based on transition metals and this implies that hydrogen occupies not only tetrahedral but also the octahedral interstices in the bct (pseudo-fcc) lattice [START_REF] Karlsson | Structure and Hydrogenation Properties of a HfNbTiVZr High-Entropy Alloy[END_REF].

Zepon et al. [START_REF] Zepon | Hydrogen-induced phase transition of MgZrTiFe0.5Co0.5Ni0.5 high entropy alloy[END_REF] synthesized MgZrTiFe 0.5 Co 0.5 Ni 0.5 using high-energy ball milling, a technique suitable for the preparation of alloys containing high vapor pressure metals, such as Mg. In their work, the alloy synthesized under Ar atmosphere crystallizes in a bcc structure and has a maximum hydrogen absorption of 1.2 wt.% (0.7 H/M) at relatively high temperature. A reversible transformation from (pristine) bcc ↔ (hydride) fcc structure upon hydrogenation was proven by in-situ synchrotron diffraction. Later, Zlotea et al. [START_REF] Zlotea | Hydrogen sorption in TiZrNbHfTa high entropy alloy[END_REF] reported the hydrogen storage properties of the single-phase bcc TiZrNbHfTa alloy synthesized by arc melting. The pressure-composition isotherm shows two plateaus, one at low equilibrium pressure reaching a capacity of 1 H/M, followed by a second plateau at 23 bar with a maximum capacity of 2 H/M (1.7 wt.%). SR-XRD experiments prove that this alloy has a two-step reversible transformation from (pristine) bcc ↔ (monohydride) bct ↔ (dihydride) fcc phase. This behavior contrasts with the previously reported refractory HEA with a single hydrogenation step but in agreement with the conventional bcc alloys [START_REF] Akiba | Metallic Hydrides III: Body-Centered-Cubic Solid-Solution Alloys[END_REF]. Recently, Montero et al.

reported a comparative study on the hydrogen sorption properties for Ti-V-Zr-Nb alloys synthesized by three different methods: arc melting, high-energy ball milling in Ar atmosphere and reactive ball milling under high H 2 atmosphere [START_REF] Montero | TiVZrNb Multi-Principal-Element Alloy: Synthesis Optimization, Structural, and Hydrogen Sorption Properties[END_REF]. The purpose of this study was to determine the most suitable technique for the refractory HEA preparation. The first two methods yielded a single-phase bcc alloy, while the synthesis by reactive ball milling directly forms a hydride phase with pseudo-fcc lattice. In terms of capacity, the three materials showed similar hydrogen uptake, 1.7-1.8 H/M (2.6-2.7 wt.%), however, the ball-milled sample showed the slowest absorption kinetics and the highest temperature of desorption. Additionally, the hydrogen absorption/desorption cycling was reported over 20 cycles, such a property is still lacking in the literature. After an initial fading of the capacity during the first cycles, this alloy was able to reversibly store a stable capacity of 1. ternary, quaternary, and quinary refractory alloys with different valence electron concentration (VEC) [START_REF] Nygård | Counting electrons -A new approach to tailor the hydrogen sorption properties of high-entropy alloys[END_REF]. In this work, they found a linear relationship between the hydride stability and the VEC.

Moreover, the electron concentration is already known to affect the hydrogen storage capacity of classical bcc alloys; the increase of this parameter above a certain limit induces a drastic decrease of the capacity [START_REF] Sakaki | Hydrogen storage properties of Nb-based solid solution alloys with a BCC structure[END_REF]. Hydrogen cycling experiments have proven a drop of capacity to zero absorption for most of the alloys after few cycles, with one exception TiVCrNb, which losses capacity from 3 to 2 wt.% after the first cycle and then stabilizes to this value for further cycling. Very recently, Shen et al.

have studied the single-phased bcc TiZrHfMoNb alloys with varied Mo content [START_REF] Shen | Compositional dependence of hydrogenation performance of Ti-Zr-Hf-Mo-Nb high-entropy alloys for hydrogen/tritium storage[END_REF]. These alloys absorb hydrogen reversibly forming hydrides with fcc lattice. The thermal stability of the hydrides decreases with increasing Mo concentration, mainly attributed to smaller cell volume by Mo addition.

The majority of these refractory bcc alloys rapidly absorb hydrogen within a single step reversible reaction bcc  (pseudo) fcc with an equilibrium pressure below 1 bar (Fig. 5, left). However, the hydrides are very stable and high temperature is needed to desorb the hydrogen, typically 300 °C, under vacuum. Therefore, the main challenge in the future is to destabilize HEA hydrides by increasing the equilibrium pressure above 1 bar at room temperature in order to have access to the whole length of the plateau, i.e. a useable capacity of 2 H/M available by playing with the pressure with the dotted box in Fig. 5 (right).

Figure 5.

Most common Pressure-Composition-Isotherms (PCIs) for hydrogen absorption in bcc HEAs with equilibrium pressure below 1 bar (left) and proposed PCIs with equilibrium pressure above 1 bar to have a useable capacity of 2 H/M (right).

Finally, the study of hydrogen absorption/desorption properties of this new class of alloys is at an early stage but the results in the literature are promising. Substantial research efforts are still needed in order to rationalize the behavior of these alloys towards hydrogen due to the extensive number of possible elemental combinations in terms of chemical composition, elemental concentration, VEC and lattice distortion.

Borohydrides of alkali and alkaline earth metals

Sodium borohydride, produced on a scale of a few thousand metric tons annually, remains the most important industrially useful complex metal hydride [START_REF] Wietelmann | Hydrides[END_REF], while also a few other borohydrides are commercially available. The list originally included only the M(BH 4 ) n salts of alkali metals, M = Li -Cs; n=1, and a few years ago it has been expanded by their analogues containing Mg and Ca, n=2.

Among these compounds mostly NaBH 4 and LiBH 4 became the basis for the impressive extension of the number of known solvent-free borohydrides to ca. 150, counting only those of identified crystal structures [START_REF] Černý | The crystal chemistry of inorganic metal borohydrides and their relation to metal oxides[END_REF][START_REF] Paskevicius | Metal borohydrides and derivatives -synthesis, structure and properties[END_REF]. This remarkable expansion of chemistry of borohydrides observed during the last two decades has been mainly inspired by their potential for hydrogen storage [START_REF] Paskevicius | Metal borohydrides and derivatives -synthesis, structure and properties[END_REF][START_REF] Lai | Hydrogen Storage Materials for Mobile and Stationary Applications: Current State of the Art[END_REF][START_REF] Churchard | A multifaceted approach to hydrogen storage[END_REF]. In this section, we will review some of the recent developments concerning the borohydrides of metals of the first two groups of the periodic table. The borohydrides of alkali metals, MBH 4 , clearly reveal an ionic characterthey all crystalize in disordered CsClO 4 -type structure of tetrahedral coordination of metal cation by the borohydride groups (NaCl structural type, if the heavy atoms are only considered), although for LiBH 4 this type of crystal structure is observed only above 10 GPa [START_REF] Filinchuk | High-Pressure Polymorphism as a Step towards Destabilization of LiBH4[END_REF]. At ambient conditions, the latter compound forms an orthorhombic crystal (o-LiBH 4 ) with tetrahedral arrangement of BH 4 -around Li + , while at ca.

110 o C a phase transition to a hexagonal, wurtzite-like, polymorph occurs (h-LiBH 4 ) [START_REF] Soulié | Lithium borohydride LiBH4: I. Crystal structure[END_REF][START_REF] El Kharbachi | Above room temperature heat capacity and phase transition of lithium tetrahydroborate[END_REF]. This hexagonal phase reveals increased solid-state Li + conductivity, and could be stabilized at room temperature by halide substitution. Thus, the solid solutions like h-Li(BH 4 ) 0.7 Br 0.2 Cl 0.1 or Li(BH 4 ) 0.66 I 0.33 show room-temperature conductivity in the order of 10 -5 S cm -1 [START_REF] Maekawa | Halide-stabilized LiBH4, a room-temperature lithium fast-ion conductor[END_REF][START_REF] Gulino | Phase Stability and Fast Ion Conductivity in the Hexagonal LiBH4-LiBr-LiCl Solid Solution[END_REF]. More details about solid-state electrolytes are given in Section 4.

The solid-state structures of alkaline earth borohydrides, M(BH 4 ) 2 , are influenced by partial covalent interactions observed in these compounds. These manifest most clearly for the lighter elements: The Be(BH 4 ) 2 crystal is built of helical chains propagating along the c direction of the unit cell [START_REF] Marynick | Crystal structure of beryllium borohydride[END_REF] while Mg(BH 4 ) 2 and Ca(BH 4 ) 2 reveal very broad structural diversity, with several metastable polymorphs detected at ambient conditions some of them showing rather loose packing [START_REF] Černý | The crystal chemistry of inorganic metal borohydrides and their relation to metal oxides[END_REF][START_REF] Paskevicius | Metal borohydrides and derivatives -synthesis, structure and properties[END_REF]. Among these, γ-Mg(BH 4 ) 2 with ca. 33% empty volume is the most impressive example of borohydride-based porous material, as it reversibly absorbs H 2 , N 2 or even CH 2 Cl 2 [START_REF] Filinchuk | Porous and dense magnesium borohydride frameworks: synthesis, stability, and reversible absorption of guest species[END_REF].

The most convenient method to prepare alkali and alkaline earth metal borohydrides are various solution-based reactions with the exclusion of small samples for research purposes. However, a few interesting, mechanochemical approaches have been reported. One of them is the preparation of Ca(BH 4 ) 2 from synthetic colemaniteone of the important mineral sources of boron [START_REF] Karabulut | Synthesis of Ca(BH4)2 from Synthetic Colemanite Used in Hydrogen Storage by Mechanochemical Reaction[END_REF]:

Ca 2 B 6 O 11 + 12CaH 2 → 3Ca(BH 4 ) 2 + 11CaO (3)
Utilizing the Mg-Al-based waste in mechanochemically-induced reactions with the respective boranes in hydrogen atmosphere leads to an efficient synthesis of LiBH 4 or NaBH 4 [START_REF] Le | Efficient Synthesis of Alkali Borohydrides from Mechanochemical Reduction of Borates Using Magnesium-Aluminum-Based Waste[END_REF], e.g.:

NaBO 2 + 2Mg + 2H 2 → NaBH 4 + 2MgO (4)
These two reactions described in eq. ( 3) and ( 4) are certainly useful to obtain the desired product in a simplified way, straight from the natural sources of boron.

While NaBH 4 and LiBH 4 are available on a large scale via well-established methods [START_REF] Wietelmann | Hydrides[END_REF], the heavier MBH 4 , M = K -Cs, can be prepared in the ion metathesis reaction performed in cold methanol [START_REF] Hagemann | Raman studies of reorientation motions of [BH4]anionsin alkali borohydrides[END_REF]:

MOH + NaBH 4 → MBH 4 ↓ + NaOH (5)
Although this method seems convenient even on a larger scale, it requires well-established protective measures. As methanol is a protic solvent, partial solvolysis of borohydrides is difficult to avoid and may even lead to the explosion of the reaction vessel (!) due to an abrupt pressure increase if no depressurization devices have been used.

The alkali metal borohydrides can be obtained from solvent-mediated metathetic reactions between the halide of corresponding divalent metal and lithium or sodium borohydride, followed by desolvation [START_REF] Ravnsbaek | Novel alkali earth borohydride Sr(BH4)2 and borohydride-chloride Sr(BH4)Cl[END_REF][START_REF] Soloveichik | Magnesium borohydride as a hydrogen storage material: Synthesis of unsolvated Mg(BH4)2[END_REF]. Contrastingly, toxic Be(BH 4 ) 2 , due to its volatility, forms already during heating of such mixture to ca. 155 o C and can be purified by vacuum sublimation [START_REF] Marynick | Crystal structure of beryllium borohydride[END_REF]. Another approach involves the reaction between a metal hydride and borane complexes such as (C 2 H 5 ) 3 N or (CH 3 ) 2 S, eq. ( 6), which has been utilized for the preparation of M(BH 4 ) 2 , M = Mg -Ba, and other borohydrides [START_REF] Ravnsbaek | Novel alkali earth borohydride Sr(BH4)2 and borohydride-chloride Sr(BH4)Cl[END_REF][START_REF] Chłopek | Synthesis and properties of magnesium tetrahydroborate, Mg(BH4)2[END_REF][START_REF] Sharma | Halide Free M(BH4)2 (M = Sr, Ba, and Eu) Synthesis, Structure, and Decomposition[END_REF].

MgH 2 + 2(C 2 H 5 ) 3 N•BH 3 → Mg(BH 4 ) 2 + 2(C 2 H 5 ) 3 N (6)
As it has been discussed for simple metal hydrides, thermal stability of these compounds can be rationalized on the basis of electron acceptor properties of the metal atom, as probed by the respective standard redox potential [START_REF] Grochala | Thermal Decomposition of the Non-Interstitial Hydrides for the Storage and Production of Hydrogen[END_REF]. A similar correlation has been found between T dec of borohydrides, which raise with decreasing Pauling electronegativity of the metal atom [START_REF] Lai | Hydrogen Storage Materials for Mobile and Stationary Applications: Current State of the Art[END_REF][START_REF] Nakamori | Development of metal borohydrides for hydrogen storage[END_REF]. Consequently, it can be expected that borohydrides of alkali and alkaline earth metals will be the most thermally stable.

Indeed, besides the borohydride of relatively electropositive beryllium (T dec of ca. 123 o C), the borohydrides of other metals considered in this section decompose above 300 o C, which is very far from the temperature required for systems coupled with a PEM fuel cell [START_REF]Target Explanation Document: Onboard Hydrogen Storage for Light-Duty Fuel Cell Vehicles[END_REF]. At the same time, the reversibility of storage (i.e. possibility of hydrogen charging and re-charging) remains another important issue which does not show satisfactory parameters for the aforementioned borohydrides.

Therefore, plenty of tuning options have been tested for these and related systems. These include forming of the reactive hydride composites discussed above, testing various catalytic approaches, e.g.

nucleation by amorphous boron [START_REF] Pendolino | Effect of boron on the activation energy of the decomposition of LiBH4[END_REF], nanoconfinement using broad range of scaffolds and particle sizes [START_REF] Shane | LiBH4 in carbon aerogel nanoscaffolds: An NMR study of atomic motions[END_REF][START_REF] Christian | Core-shell strategy leading to high reversible hydrogen storage capacity for NaBH4[END_REF], or modification of chemical composition via preparation of mixed-cation or mixedanion compounds [START_REF] Nickels | Tuning the Decomposition Temperature in Complex Hydrides: Synthesis of a Mixed Alkali Metal Borohydride[END_REF][START_REF] Rude | Anion Substitution in Ca(BH4)2-CaI2: Synthesis, Structure and Stability of Three New Compounds[END_REF].

During the last two decades, Mg(BH 4 ) 2 was mostly considered for its high hydrogen mass content and its low predicted decomposition temperatures. The mass composition of 14.94% H next to 45.02% Mg and 40.05% B, shows among the highest known hydrogen contents. However, the prediction that reversible hydrogen storage could be achieved at temperatures [START_REF] Ozolins | First-principles prediction of a ground state crystal structure of magnesium borohydride[END_REF][START_REF] Mjv | A density functional study of α-Mg(BH4)2[END_REF] suitable for proton-exchange membrane (PEM) fuel cells has not been fulfilled until today. That would have made this compound one of the most favorable materials for solid-state hydrogen storage. In practice, reversibility was achieved but never in respect to the full hydrogen content.

From a structural point of view Mg(BH 4 ) 2 is very interesting, and a number of different polymorphs have been reported. Some crystal structures that are of interest for the dynamics are shown in Fig. 6.

Currently, there are seven known polymorphs, which is the largest variety of structures among the alkali and alkaline-earth borohydrides. Five of these structures have been identified (α-, β-, γ-, δ-, ζ-Mg(BH 4 ) 2 ) and two experimentally found but structurally unsolved (β'-and ε-Mg(BH 4 ) 2 ).

Additionally, Mg(BH 4 ) 2 can form an X-ray amorphous material, which can be obtained by different methods. These methods are solvent-free synthesis [START_REF] Pistidda | Synthesis of amorphous Mg(BH(4))(2) from MgB(2) and H(2) at room temperature[END_REF], ball milling of the α-and γ-modifications [START_REF] Filinchuk | Porous and dense magnesium borohydride frameworks: synthesis, stability, and reversible absorption of guest species[END_REF] and a pressure collapse of γ-Mg(BH 4 ) 2 [START_REF] Ban | Pressure-collapsed amorphous Mg(BH4)2: An Ultradense complex hydride showing a reversible transition to the porous framework[END_REF]. The latter method also resulted in a novel phase, δ-Mg(BH 4 ) 2 [START_REF] Filinchuk | Porous and dense magnesium borohydride frameworks: synthesis, stability, and reversible absorption of guest species[END_REF]. The δ-polymorph has received special attention as it has one of the highest volumetric hydrogen capacities of all complex metal hydrides with 147 kg m -3 . The other phases have volumetric hydrogen capacities between 82 -117 kg m -3 as well. Mg(BH 4 ) 2 in SG Fddd [220], c) γ-Mg(BH 4 ) 2 in SG Id-3a [201,221]. This figure is adapted from Refs [START_REF] Filinchuk | Porous and dense magnesium borohydride frameworks: synthesis, stability, and reversible absorption of guest species[END_REF][START_REF] Filinchuk | Insight into Mg(BH4)2 with Synchrotron X-ray Diffraction: Structure Revision, Crystal Chemistry, and Anomalous Thermal Expansion[END_REF][START_REF] Her | Structure of unsolvated magnesium borohydride Mg(BH4)2[END_REF][START_REF] Heere | Dynamics of porous and amorphous magnesium borohydride to understand solid state Mg-ion-conductors (accepted)[END_REF].

From diffraction the highly symmetric γ-Mg(BH 4 ) 2 phase in space group Id-3a was determined by Filinchuk et al. [START_REF] Filinchuk | Porous and dense magnesium borohydride frameworks: synthesis, stability, and reversible absorption of guest species[END_REF]. One Mg atom is coordinated by the edges of four tetrahedral anions [BH 4 ] -. It was reported that γ-Mg(BH 4 ) 2 has a three-dimensional mesh of interpenetrating channels. These have an inner and outer diameter of ~8 and ~12.3 Å, respectively. Therefore, this compound can almost be seen as a MOF-like-structure with a porosity of ~33%, which is attractive to adsorb physically guest species in a similar way as for the known nanoporous MOFs materials [START_REF] Filinchuk | Porous and dense magnesium borohydride frameworks: synthesis, stability, and reversible absorption of guest species[END_REF][START_REF] Broom | Concepts for improving hydrogen storage in nanoporous materials[END_REF][START_REF] El Kassaoui | Modeling hydrogen adsorption in the metal organic framework (MOF-5, connector): Zn4O(C8H4O4)3[END_REF]. Filinchuk et al. reported that 3 wt.% of hydrogen can be adsorbed at -193 °C and 105 bar of hydrogen. Moreover, nitrogen and dichloromethane have been reversibly adsorbed.

Heere et al. recently investigated the structure of the amorphous phase and the results are further discussed in Ref. [START_REF] Heere | Dynamics of porous and amorphous magnesium borohydride to understand solid state Mg-ion-conductors (accepted)[END_REF]. The study used X-ray total scattering and pair distribution function (PDF)

analysis, where even hydrogen bonds can be observed, due to the circumstance that H has an oxidation state of -1 and with that a notable electron density. The complete picture (or hydrogen positions) of the structure might not be given, nevertheless, the PDF analysis of the amorphous structure strongly resembles the one of γ-Mg(BH 4 ) 2 . In detail, the PDF can be divided into the local structure up to 5.1 Å with the building blocks Mg -BH 4 -Mg. These results are in agreement with the findings by Filinchuk et al. whose spectroscopic results pointed to characteristic building blocks in the X-ray amorphous phase and their similarity to the internal structure of the γ-and δ-Mg(BH 4 ) 2 [START_REF] Filinchuk | Porous and dense magnesium borohydride frameworks: synthesis, stability, and reversible absorption of guest species[END_REF]. Above ~5.1 Å, a slight oscillation in the PDF is still recognizable, which Heere et al. connected to the interpenetrating channels, see Fig 6c, thus indicating that the fundamental building blocks still exhibit a certain degree of ordering related to the 3D net of interpenetrating channels, although less porous [START_REF] Filinchuk | Porous and dense magnesium borohydride frameworks: synthesis, stability, and reversible absorption of guest species[END_REF][START_REF] Heere | Effect of additives, ball milling and isotopic exchange in porous magnesium borohydride[END_REF]. Above 12.3 Å, i.e. the outer diameter of these channels, the featureless PDF supports a fully randomized structure.

The thermal behavior prior to decomposition is characterized by DSC measurement. It was reported that there are endothermic and exothermic phase changes, which are all specific to the respective crystal phases. The first reported was the transition from α-Mg(BH 4 ) 2 to β-Mg(BH 4 ) 2 during thermal treatment at ~220 °C. β-Mg(BH 4 ) 2 is the high temperature modification which is metastable at room temperature and therefore not reversible. DSC of the γ-Mg(BH 4 ) 2 compound reveals two distinct phase transitions. The first one at ~150 °C is an endothermic event to ε-Mg(BH 4 ) 2 , while the second event at 180 °C belongs to the transformation to β'-Mg(BH 4 ) 2 [START_REF] Heere | Dynamics of porous and amorphous magnesium borohydride to understand solid state Mg-ion-conductors (accepted)[END_REF][START_REF] Heere | Effect of additives, ball milling and isotopic exchange in porous magnesium borohydride[END_REF]. The amorphous phase synthesized via a ball milling reaction [START_REF] Huot | Mechanochemistry of Metal Hydrides: Recent Advances[END_REF] of γ-Mg(BH 4 ) 2 shows an additional exothermic DSC event at 100 °C, where the material crystallizes to γ-Mg(BH 4 ) 2 . The other two events are endothermic and related to the ones of the original γ-Mg(BH 4 ) 2 , though lower in peak temperatures. At temperatures over 300 °C the decomposition of Mg(BH 4 ) 2 appears with four reported reaction steps [START_REF] Zavorotynska | Recent progress in magnesium borohydride Mg(BH4)(2): Fundamentals and applications for energy storage[END_REF]. At these temperatures, very stable compounds are formed, such as MgB 12 H 12 and MgB 2 , which can only be rehydrogenated at extreme conditions of 400 °C and 900 bar H 2 [START_REF] Severa | Direct hydrogenation of magnesium boride to magnesium borohydride: demonstration of >11 weight percent reversible hydrogen storage[END_REF]. More suitable conditions for on-board hydrogen storage can be achieved via an intermediate step and the formation of Mg(B 3 H 8 ) 2 at T ~200 °C. Hence a rehydrogenation to Mg(BH 4 ) 2 was achieved after 48 h at 250 °C and 120 bar H 2 [START_REF] Chong | Reversible dehydrogenation of magnesium borohydride to magnesium triborane in the solid state under moderate conditions[END_REF]. The authors reported further improvement of the conditions and a rehydrogenation of a complex of Mg(B 3 H 8 ) 2 •2THF/MgH 2 after 2 h at 200 °C and 50 bar H 2 [START_REF] Chong | Selective reversible hydrogenation of Mg(B3H8)2/MgH2 to Mg(BH4)2: pathway to reversible borane-based hydrogen storage?[END_REF], with the reaction products being discussed in Ref. [START_REF] Palumbo | Determination of the molecular structure of amorphous Mg(B3H8)2(THF)2 through infrared spectroscopic and computational studies[END_REF]. As mentioned above, reversibility was achieved due to the intermediate product and the reaction pathway from Mg(B 3 H 8 ) 2 to Mg(BH 4 ) 2 , but never in respect to the full hydrogen content. The hydrogen capacity for this reaction is limited to 2.5 wt.% and the reaction conditions are still too high (~120 bar H 2 , T Absorption = 260 °C for 7-8 h, or 280 °C for 3 h) for on-board hydrogen storage [START_REF] Zavorotynska | Kinetics studies of the reversible partial decomposition reaction in Mg(BH4)2[END_REF].

Borohydrides of transition and rare-earth metals

The most emphasized aspects in the research devoted to borohydrides involve their synthesis, identification and physicochemical characterization, including crystal structures, spectroscopic measurements, magnetism, luminescence and ionic transport, as well as the processes related to hydrogen release. In this section we will briefly discuss some of these topics on a basis of various borohydride systems studied recently, mostly based on transition-and rare-earth metals, as well as some less common systems, like those containing ammonium cations.

Synthesis and basic structural identification

The rapid growth in the field of borohydrides, mentioned in the previous section, would not be possible without the development of a universal synthetic methodology facilitating fast screening of a broad range of compounds to assess their potential use for hydrogen storage. Consequently, the mechanochemical reactions performed in solid state via high-energy milling (using balls or discs) became the "golden standard" for synthesis of novel borohydrides [START_REF] Huot | Mechanochemical synthesis of hydrogen storage materials[END_REF][START_REF] Huot | Mechanochemistry of Metal Hydrides: Recent Advances[END_REF][START_REF] Hagemann | Synthetic approaches to inorganic borohydrides[END_REF][START_REF] Frommen | Hydrogen storage properties of rare earth (RE) borohydrides (RE = La, Er) in composite mixtures with LiBH4 and LiH[END_REF]. This approach allowed for performing either ion metathesis, eq. ( 7), or salt addition, eq. ( 8), resulting in either singleand multiple-cation compounds, as exemplified below [START_REF] Sato | Experimental and computational studies on solvent-free rare-earth metal borohydrides R(BH4)3 (R= Y, Dy, and Gd)[END_REF][START_REF] Jaroń | Y(BH4)3 -An old-new ternary hydrogen store aka learning from a multitude of failures[END_REF][START_REF] Frommen | Crystal structure, polymorphism, and thermal properties of yttrium borohydride Y(BH4)3[END_REF][START_REF] Ravnsbaek | Thermal polymorphism and decomposition of Y(BH4)3[END_REF][START_REF] Jaroń | Y(BD4)3, an efficient store of deuterium, and impact of isotope effects on its thermal decomposition[END_REF][START_REF] Jaroń | Probing Lewis acidity of Y (BH 4) 3 via its reactions with MBH 4 (M= Li, Na, K, NMe 4)[END_REF][START_REF] Wegner | MYb(BH4)4 (M = K, Na) from laboratory X-ray powder data[END_REF][START_REF] Starobrat | New hydrogen-rich ammonium metal borohydrides, NH4[M(BH4)4], M = Y, Sc, Al, as potential H2 sources[END_REF][START_REF] Jaroń | M [Y (BH 4) 4] and M 2 Li [Y (BH 4) 6-x Cl x](M= Rb, Cs): new borohydride derivatives of yttrium and their hydrogen storage properties[END_REF][START_REF] Heere | In situ investigations of bimetallic potassium erbium borohydride[END_REF]:

YCl 3 + 3LiBH 4 → Y(BH 4 ) 3 + 3LiCl (7) Y(BH 4 ) 3 + KBH 4 → K[Y(BH 4 ) 4 ] (8)
Using the mechanochemically-induced solid-state reactions even the compounds of fairly complicated stoichiometries can be obtained, like Li 3 MZn 5 (BH 4 ) 15 , M = Mg, Mn, Li 3 K 3 M 2 (BH 4 ) 12 , M = La, Ce or M 2 LiY(BH 4 ) 6 , M = Rb, Cs [START_REF] Jaroń | M [Y (BH 4) 4] and M 2 Li [Y (BH 4) 6-x Cl x](M= Rb, Cs): new borohydride derivatives of yttrium and their hydrogen storage properties[END_REF][START_REF] Černý | Trimetallic borohydride Li3MZn5(BH4)15 (M = Mg, Mn) containing two weakly interconnected frameworks[END_REF][START_REF] Brighi | Fast ion conduction in garnet-type metal borohydrides Li3K3Ce2(BH4)(12) and Li3K3La2(BH4)(12)[END_REF][START_REF] Sadikin | Alkali metal-yttrium borohydrides: The link between coordination of small and large rare-earth[END_REF][START_REF] Gharibdoust | Synthesis, Structure and Li Ion Conductivity of LiLa(BH4)3X, X= Cl, Br, I[END_REF]. This method is also an obvious choice for introducing doping or preparation of composites due to easily achieved very high degree of dispersion [START_REF] Vajo | Altering Hydrogen Storage Properties by Hydride Destabilization through Alloy Formation: LiH and MgH2Destabilized with Si[END_REF][START_REF] Barkhordarian | Unexpected kinetic effect of MgB2 in reactive hydride composites containing complex borohydrides[END_REF].

Identity of the prepared products may depend on the stoichiometry of the initial mixture of precursors, like for NaZn 2 (BH 4 ) 5 and NaZn(BH 4 ) 3 obtained from the ZnCl 2 -LiBH 4 mixtures of 1:2.5 and 1:3 ratio, respectively or for the borohydrides of the heavy alkali metals and yttrium [START_REF] Jaroń | M [Y (BH 4) 4] and M 2 Li [Y (BH 4) 6-x Cl x](M= Rb, Cs): new borohydride derivatives of yttrium and their hydrogen storage properties[END_REF][START_REF] Ravnsbaek | A Series of Mixed-Metal Borohydrides[END_REF]. Interestingly, the ratio of reagents can also determine which polymorph of the product is formed. This has been observed to some extent in the case of Cd(BH 4 ) 2 [START_REF] Ravnsbaek | Screening of Metal Borohydrides by Mechanochemistry and Diffraction[END_REF], and allowed for selection of the polymorph for the series of rare earth (RE) borohydrides, according to eq. ( 9) and ( 10), enabling to study of structurerelated properties [START_REF] Wegner | Polymorphism and hydrogen discharge from holmium borohydride, Ho(BH4)3, and KHo(BH4)(4)[END_REF][START_REF] Wegner | Preparation of a series of lanthanide borohydrides and their thermal decomposition to refractory lanthanide borides[END_REF][START_REF] Wegner | Borohydride as Magnetic Superexchange Pathway in Late Lanthanide Borohydrides[END_REF]:

RECl 3 + 3LiBH 4 → α-RE(BH 4 ) 3 + 3LiCl (9) RECl 3 + 12LiBH 4 → β-RE(BH 4 ) 3 + 3LiCl + 9LiBH 4 (10)
A systematic screening of the early transition metal borohydrides has recently been performed using a convenient mechanochemical approach [253,254]. The screening included the borohydride systems based on combinations of Ti, V, Cr, Mn, Fe with alkali metals, prepared according to eq. ( 11):

3MBH 4 + M'Cl n + nLiBH 4 → M 3 M'(BH 4 ) 3+n + nLiCl ( 11
)
where M = K -Cs, M' = Ti -Fe, n = 2 or 3. While no signs of a reaction were observed for vanadium, the expected borohydrides have been detected for other early transition metals and Fe, mainly in combination with Rb and Cs. For the borohydrides of lighter alkali metals either different products were formed, as in the case of K 2 Mn(BH 4 ) 4 , or no reactions occurred for other tested systems. It is worth mentioning that no borohydrides of trivalent metals formed, as in these systems either reaction were not observed (as for V(III)), or it occurs with subsequent decomposition of the products (as for Fe(III)), or with reduction to the respective divalent form (as for Ti(III)). A similar synthetic path (i.e.

mechanochemical reactions performed at room temperature or <0 o C) has also been attempted for the monometallic borohydrides of M ' , however, it succeeded only for Mn(BH 4 ) 2 [START_REF] Richter | Manganese borohydride; synthesis and characterization[END_REF]. The identity of compounds prepared has been confirmed by the refinement of their crystal structures and are tabulated in Table 2. These borohydrides adopt Cs 3 CoCl 5 -type structure shared by M 3 Mg(BH 4 ) 5 compounds, M = K (only above 94 o C) [START_REF] Schouwink | Bimetallic borohydrides in the system M(BH4)2-KBH4 (M = Mg, Mn): On the structural diversity[END_REF], Rb and Cs [START_REF] Wegner | Organic derivatives of Mg(BH4)2 as precursors towards MgB2 and novel inorganic mixed-cation borohydrides[END_REF]. Contrastingly, Rb 3 Cr(BH 4 ) 5 crystalizes in the structure related to room-temperature modification of K 3 Mg(BH 4 ) 5 [START_REF] Schouwink | Bimetallic borohydrides in the system M(BH4)2-KBH4 (M = Mg, Mn): On the structural diversity[END_REF]. Thermal decomposition of these compounds occurs via exothermic processes within the range of temperatures as low as 50 -120 o C, with emission of markedly pure hydrogen. For M' = Mn hydrogen contaminated with diborane is released at slightly higher temperatures.

Table 2. The parameters of the crystal unit cells of M 3 M'(BH 4 ) 5 compounds prepared mechanochemically according to eq. [START_REF] Rusman | A review on the current progress of metal hydrides material for solidstate hydrogen storage applications[END_REF]. * -data measured at -43 o C due to limited thermal stability, the other diffraction data obtained at ca. 25 o C. The compounds adopt tetragonal crystal structures related to that ofand usually isostructural with -K 3 Mg(BH 4 ) 5 [START_REF] Schouwink | Bimetallic borohydrides in the system M(BH4)2-KBH4 (M = Mg, Mn): On the structural diversity[END_REF]. Estimated standard deviations in parentheses. Although convenient for diverse systems, less laborious than the solvent-mediated procedures and technically simple on a small scale, which strongly increased its popularity, the mechanochemical method has also some serious drawbacks. The most important is the purity of the borohydride product, which is severely limited by the "dead mass" of the halide by-product contributing to up to 50 wt.% of the post-reaction composite, and thus dramatically decreases the effective hydrogen content. Most borohydrides cannot be purified using typical solvents like ethers and amines or by physical methods, like difference in buoyancy, due to formation of very stable solvates and high dispersion, respectively [START_REF] Jaroń | Hydrogen Storage Materials: Room-Temperature Wet-Chemistry Approach toward Mixed-Metal Borohydrides[END_REF][START_REF] Jaron | Facile Formation of Thermodynamically Unstable Novel Borohydride Materials by a Wet Chemistry Route[END_REF][START_REF] Starobrat | Salts of highly fluorinated weakly coordinating anions as versatile precursors towards hydrogen storage materials[END_REF][START_REF] Heere | The influence of LiH on the rehydrogenation behavior of halide free rare earth (RE) borohydrides (RE = Pr, Er)[END_REF]. Moreover, the side reactions connected with the formation of (BH 4 /Cl) solid solutions may be observed, often influencing the process of thermal decomposition [START_REF] Jaroń | Probing Lewis acidity of Y (BH 4) 3 via its reactions with MBH 4 (M= Li, Na, K, NMe 4)[END_REF][START_REF] Jaroń | M [Y (BH 4) 4] and M 2 Li [Y (BH 4) 6-x Cl x](M= Rb, Cs): new borohydride derivatives of yttrium and their hydrogen storage properties[END_REF][START_REF] Starobrat | Two new derivatives of scandium borohydride, MSc(BH4)4, M = Rb, Cs, prepared: Via a one-pot solvent-mediated method[END_REF][START_REF] Park | Thermal properties of Y(BH4)3 synthesized via two different methods[END_REF] and some of the borohydrides are still inaccessible via mechanochemically-induced solid state reactions [START_REF] Jaron | Facile Formation of Thermodynamically Unstable Novel Borohydride Materials by a Wet Chemistry Route[END_REF].

Formula group a [Å] c [Å] V [Å 3 ] ICSD #
These disadvantages stimulated the search for more general solvent-mediated synthetic approaches, which would extend a few rather specific methods of synthesis known at that time [START_REF] Hagemann | Synthetic approaches to inorganic borohydrides[END_REF]. The use of DMS as a solvent for performing halideborohydride metathesis or addition of Me 2 S•BH 3 complex to metal hydrides, followed by desolvation of the products at ca. 140 o C under vacuum, allowed for the preparation of a whole series of monometallic borohydrides [START_REF] Frommen | Rare Earth Borohydrides-Crystal Structures and Thermal Properties[END_REF]. The range includes M(BH 4 ) n compounds of alkali, alkaline earth, transition and rare earth metals obtained in rather pure form [START_REF] Richter | Manganese borohydride; synthesis and characterization[END_REF][START_REF] Ley | Novel solvates M(BH(4))(3)S(CH(3))(2) and properties of halide-free M(BH(4))(3) (M = Y or Gd)[END_REF][START_REF] Richter | From Metal Hydrides to Metal Borohydrides[END_REF][START_REF] Grinderslev | Trends in Synthesis, Crystal Structure, and Thermal and Magnetic Properties of Rare-Earth Metal Borohydrides[END_REF]. Interestingly, in the case of M = Sc the mixed-cation borohydrides, MSc(BH 4 ) 4 , are prepared in the DMS-mediated process instead of the elusive Sc(BH 4 ) 3 , Malkali metal [START_REF] Starobrat | Two new derivatives of scandium borohydride, MSc(BH4)4, M = Rb, Cs, prepared: Via a one-pot solvent-mediated method[END_REF], e.g.:

ScCl 3 + 2MBH 4 → 0.5MSc(BH 4 ) 4 + 0.5M 3 ScCl 6 (12)
A different approach consists of salt metathesis reactions performed in the solvents of much lower coordination ability as compared to DMS, and utilizes unique properties of weakly coordinating anions (WCA) [START_REF] Krossing | Chemistry with weakly-coordinating fluorinated alkoxyaluminate anions: Gas phase cations in condensed phases?[END_REF][START_REF] Krossing | Noncoordinating Anions-Fact or Fiction? A Survey of Likely Candidates[END_REF][START_REF] Krossing | The facile preparation of weakly coordinating anions: Structure and characterisation of silverpolyfluoroalkoxyaluminates AgAl(ORF)4, calculation of the alkoxide ion affinity[END_REF][START_REF] Przemysław | Weakly coordinating anions: building blocks for chemical syntheses: synthesis and characterization of M[Al(OC(CF3)3)4[END_REF][272], as in ref. [START_REF] Jaroń | Hydrogen Storage Materials: Room-Temperature Wet-Chemistry Approach toward Mixed-Metal Borohydrides[END_REF]:

M[An] + [Cat]Zn 2 (BH 4 ) 5 → MZn 2 (BH 4 ) 5 ↓ + [Cat][An] ( 13
)
where M = Li -Cs, [An] -WCA, like [Al{OC(CF 3 ) 3 }6a or [B{3,5-(CF 3 ) 2 C 6 H 3 } 4 ] -, and [Cat]a bulky cation, e.g. tetrabutylammonium. Application of this novel method allowed for the preparation of a number of mixed-cation borohydrides based on alkali, alkaline earth, transition and rare earth metals [START_REF] Wegner | Organic derivatives of Mg(BH4)2 as precursors towards MgB2 and novel inorganic mixed-cation borohydrides[END_REF][START_REF] Jaroń | Hydrogen Storage Materials: Room-Temperature Wet-Chemistry Approach toward Mixed-Metal Borohydrides[END_REF][START_REF] Jaron | Facile Formation of Thermodynamically Unstable Novel Borohydride Materials by a Wet Chemistry Route[END_REF][START_REF] Starobrat | Salts of highly fluorinated weakly coordinating anions as versatile precursors towards hydrogen storage materials[END_REF], usually without significant content of impurities; for some of them it was not possible to achieve this without the mentioned wet approach (Fig. 7a).

The solvent-mediated method with the use of WCA enabled preparation of the metastable borohydrides, like LiY(BH 4 ) 4 and NaY(BH 4 ) 4 [START_REF] Jaron | Facile Formation of Thermodynamically Unstable Novel Borohydride Materials by a Wet Chemistry Route[END_REF], which are not formed in mechanochemical or solid/gas reactions [START_REF] Jaroń | Probing Lewis acidity of Y (BH 4) 3 via its reactions with MBH 4 (M= Li, Na, K, NMe 4)[END_REF]. These compounds decompose at room temperature to the composite of corresponding MBH 4 and Y(BH 4 ) 3 , however, it appeared later that heating such mixture followed by fast quenching also allows for the preparation of these elusive borohydrides [START_REF] Roedern | Solid state synthesis, structural characterization and ionic conductivity of bimetallic alkali-metal yttrium borohydrides MY(BH4)4 (M = Li and Na)[END_REF]. The crystal structures of MY(BH 4 ) 4 , M = Li, Na, obtained from WCA precursors appeared isostructural with their respective scandium analogues (Fig. 7b).

This method requires certain precursors, which stimulated recent development in organic complex 13) wet, with the sample obtained in the mechanochemical proceduredry. Note significant contamination of the latter by LiCl and ZnCl 2 [START_REF] Jaroń | Hydrogen Storage Materials: Room-Temperature Wet-Chemistry Approach toward Mixed-Metal Borohydrides[END_REF], (b) The crystal structures of the metastable Li and Na yttrium borohydride accessible via the solvent-mediated approach utilizing weakly coordinating anions [START_REF] Jaron | Facile Formation of Thermodynamically Unstable Novel Borohydride Materials by a Wet Chemistry Route[END_REF]. This figure is adapted from Refs [START_REF] Jaroń | Hydrogen Storage Materials: Room-Temperature Wet-Chemistry Approach toward Mixed-Metal Borohydrides[END_REF][START_REF] Jaron | Facile Formation of Thermodynamically Unstable Novel Borohydride Materials by a Wet Chemistry Route[END_REF].

Properties and prospective applicability

Most of the unsolvated inorganic borohydrides of which the identity has been confirmed are crystalline materials. However, only few of them are accessible as single crystals, therefore powder diffraction remains the main technique for their structural characterization. This is often supported by the use of DFT, due to difficulties in localization of hydrogen atoms from the routine XRD data. Interestingly, neutron powder diffraction revealed that the high temperature β-phase of Y(BH 4 ) 3 crystallizes in [START_REF] Frommen | Crystal structure, polymorphism, and thermal properties of yttrium borohydride Y(BH4)3[END_REF], while the earlier powder XRD investigations suggested the space group with an 8 times smaller unit cell and half the lattice parameters [START_REF] Ravnsbaek | Thermal polymorphism and decomposition of Y(BH4)3[END_REF]. Borohydrides reveal an impressive structural diversity, ranging from salt-like ionic structures, to various nets governed by more covalent interactions, which has been recently thoroughly discussed [START_REF] Černý | The crystal chemistry of inorganic metal borohydrides and their relation to metal oxides[END_REF][START_REF] Paskevicius | Metal borohydrides and derivatives -synthesis, structure and properties[END_REF][START_REF] Hadjixenophontos | A Review of the MSCA ITN ECOSTORE-Novel Complex Metal Hydrides for Efficient and Compact Storage of Renewable Energy as Hydrogen and Electricity[END_REF]. Among rare earth borohydrides, praseodymium (Pr) is the only element that forms three different monometallic polymorphs with different symmetries, i.e. , and , with the last one even having large voids and being considered as porous [START_REF] Heere | The influence of LiH on the rehydrogenation behavior of halide free rare earth (RE) borohydrides (RE = Pr, Er)[END_REF][START_REF] Gharibdoust | Synthesis, structure, and polymorphic transitions of praseodymium(iii) and neodymium(iii) borohydride, Pr(BH4)3 and Nd(BH4)3[END_REF]. The other popular analytical techniques include vibrational spectroscopy (FT-IR and Raman), which often helps to deduce the coordination mode of borohydride anion and identify the (organic) impurities [START_REF] Marks | Covalent transition metal, lanthanide, and actinide tetrahydroborate complexes[END_REF][START_REF] D'anna | FT-IR spectra of inorganic borohydrides[END_REF]. While nuclear magnetic resonance (NMR) utilizing various nuclides, besides identification [START_REF] Le | Efficient Synthesis of Alkali Borohydrides from Mechanochemical Reduction of Borates Using Magnesium-Aluminum-Based Waste[END_REF][START_REF] Jaroń | Y(BH4)3 -An old-new ternary hydrogen store aka learning from a multitude of failures[END_REF][START_REF] Starobrat | New hydrogen-rich ammonium metal borohydrides, NH4[M(BH4)4], M = Y, Sc, Al, as potential H2 sources[END_REF][START_REF] Kim | LiSc(BH4)4 as a hydrogen storage material: Multinuclear high-resolution solid-state NMR and first-principles density functional theory studies[END_REF][START_REF] Černý | Structure and Characterization of KSc(BH4)4[END_REF][START_REF] Łodziana | NMR chemical shifts of 11B in metal borohydrides from first-principle calculations[END_REF] allows for evaluation of dynamic phenomena, like reorientation motions [START_REF] Jaroń | Phase transition induced improvement in H2 desorption kinetics: The case of the high-temperature form of Y(BH4)3[END_REF][START_REF] Skripov | Nuclear magnetic resonance study of reorientational motion in α-Mg(BH4)2[END_REF][START_REF] Corey | Atomic motions in LiBH4 by NMR[END_REF].

Magnetism of borohydrides of lanthanides (Ln) has received significant attention only very recently LiYb(BH 4 ) 4 , NaYb(BH 4 ) 4 , KHo(BH 4 ) 4 , RbTm(BH 4 ) 4 have been systematically studied using SQUID magnetometry [START_REF] Wegner | Borohydride as Magnetic Superexchange Pathway in Late Lanthanide Borohydrides[END_REF]. Among these compounds α-and β-Dy(BH 4 ) 3 , as well as NaYb(BH 4 ) 4 reveal weak ferromagnetic interactions, while weak antiferromagnetic interactions were observed for the other materials studied. This indicates that although the borohydride anion may serve as a transmitter of magnetic superexchange (via H and B atoms), the corresponding superexchange constants remain small.

As the thermolysis remains the method of choice for the release of hydrogen stored in these chemical compounds, the thermal decomposition of borohydrides has been thoroughly studied. Although the alternative hydrolytic approach has also been investigated for a few systems, it is not preferred due to high thermodynamic stability of the products of hydrolysis which would make in situ hydrogen recharging an energy expensive and rather difficult process [START_REF] Ley | Development of hydrogen storage tank systems based on complex metal hydrides[END_REF][START_REF]Go/No-Go Recommendation for Sodium Borohydride for On-Board Vehicular Hydrogen Storage[END_REF][START_REF] Mohtadi | Complex metal borohydrides: multifunctional materials for energy storage and conversion[END_REF]. However, it is worth to mention that a very recent study reports significantly improved method of the reformation of NaBH 4 after hydrolysis [START_REF] Zhu | Closing the Loop for Hydrogen Storage: Facile Regeneration of NaBH4 from its Hydrolytic Product[END_REF]. While the temperature of hydrogen release from most of the known borohydrides remains too high, the other emit significant amount of diborane. Therefore, a number of means has been tested to achieve the necessary improvement of these properties. Typical approaches involve modification of chemical composition, or significant lowering of particle size and nanoconfinement [START_REF] Paskevicius | Metal borohydrides and derivatives -synthesis, structure and properties[END_REF][START_REF] Lai | Hydrogen Storage Materials for Mobile and Stationary Applications: Current State of the Art[END_REF]. However, in the case of yttrium borohydride it has been observed that the initial stage of thermal decomposition is facilitated for β-Y(BH 4 ) 3 polymorph in comparison with the more common α-Y(BH 4 ) 3 . This manifests by lowering the temperature of the DSC event related to the first stage of decomposition by ca. 8 o C, and more impressively, by almost 3-fold drop of the corresponding apparent activation energy calculated using the Kissinger method, which indicates significant improvement of kinetics [START_REF] Kissinger | Reaction Kinetics in Differential Thermal Analysis[END_REF]. Although this finding demonstrated one of the possible tuning options for hydrogen release from borohydrides, its utilization would rather be limited due to additional difficulty imposed by the requirement of recreation of a particular polymorph during rehydrogenation of the spent hydrogen store (below 400 o C both polymorphs release 7.0-7.4 wt% of hydrogen, as calculated for pure Y(BH 4 ) 3 ).

Another broadly explored attempt to facilitate hydrogen release is the formation of protonic-hydridic compounds [START_REF] Maj | Theoretical Design of Catalysts for the Heterolytic Splitting of H2[END_REF] via a combination of the moieties containing partially negatively charged H atoms bound to boron with those attached to nitrogen, which are partially positive. Such approach often increases the theoretical gravimetric content of hydrogen and lowers the temperature of decomposition, as compared with similar borohydrides lacking nitrogen [START_REF] Wegner | A low temperature pyrolytic route to amorphous quasihexagonal boron nitride from hydrogen rich (NH4)3Mg(BH4)5[END_REF]. Ammonium borohydride, NH 4 BH 4 , containing 24.5 wt.% of hydrogen and releasing most of it below 160 o C is a striking example here [START_REF] Karkamkar | Thermodynamic and Structural Investigations of Ammonium Borohydride, a Solid with a Highest Content of Thermodynamically and Kinetically Accessible Hydrogen[END_REF]. As the latter compound is unstable at room temperature its stabilization has been attempted i.e. by formation of mixed-cation borohydrides, which should render the partial charges on hydridic and protonic hydrogen atoms less pronounced:

NH 4 BH 4 + M(BH 4 ) x → NH 4 M(BH 4 ) x+1 (14)
Such chemical stabilization of NH 4 BH 4 appeared successful for NH 4 Ca(BH 4 ) 3 , which is stable at room temperature, and releases ca. 13.2 wt% of nearly pure hydrogen in several steps within 65-420 o C interval (re-calculated for the pure compound) [START_REF] Schouwink | Increasing Hydrogen Density with the Cation-Anion Pair BH4--NH4+ in Perovskite-Type NH4Ca(BH4)3[END_REF][START_REF] Schouwink | Structure and properties of complex hydride perovskite materials[END_REF]. Surprisingly, most of decomposition steps are endothermic, with one broad exothermic step around 130 o C, which has been assigned to decomposition of ammonia borane forming in situ [START_REF] Lang | Dehydrogenation and reaction pathway of Perovskite-Type NH4Ca(BH4)3[END_REF]. A different behavior has been observed for the series of related borohydrides of trivalent metals: NH 4 M(BH 4 ) 4 , where M = Al, Sc, Y [START_REF] Starobrat | New hydrogen-rich ammonium metal borohydrides, NH4[M(BH4)4], M = Y, Sc, Al, as potential H2 sources[END_REF][START_REF] Dovgaliuk | Solid Aluminum Borohydrides for Prospective Hydrogen Storage[END_REF].

The latter compounds start to decompose at a temperature range comparable with the parent NH 4 BH 4 (53 o C) with most of the decomposition stages being exothermic, making hydrogen release an irreversible process. The latter is rather common feature for protonic-hydridic hydrogen stores, observed even for the species containing much more stable [B 12 H 12 ] 2-anions [START_REF] Derdziuk | Synthesis, structural characterization and thermal decomposition studies of (N2H5)2B12H12 and its solvates[END_REF].

As it has been mentioned above, the borohydrides decomposing within the favorable temperature range of ca. 100 o C emit significant amount of diborane together with hydrogen [START_REF] Paskevicius | Metal borohydrides and derivatives -synthesis, structure and properties[END_REF]. Therefore, the application of a catalyst, which would rapidly and selectively decompose B 2 H 6 , leaving boron in the sample and purifying the released hydrogen, should greatly improve the performance of such systems.

This should also be expected for much more stable borohydrides.

Recently, metallic vanadium, which forms a divalent hydride of rather moderate stability (T dec. of ca.

35 o C) [START_REF] Grochala | Thermal Decomposition of the Non-Interstitial Hydrides for the Storage and Production of Hydrogen[END_REF], as well as its nanoparticles and oxides: V 2 O 3 , VO 2 and V 2 O 5 , have been tested as borohydride dopants [307]. Among these systems no significant influence has been observed for LiBH 4 , and partial decomposition without further catalytic effect occurred for Mg(BH 4 ) 2 . However, NaBH 4 doped with 25 wt% V 2 O 5 revealed significant improvement in comparison with the parent borohydride. This catalyzed system releases 3.6 -5.0 wt.% of H already around 380 -390 o C, i.e. ca.

130 o C lower than pure NaBH 4 [START_REF] Paskevicius | Metal borohydrides and derivatives -synthesis, structure and properties[END_REF]. At the same time, the system doped with vanadium nanoparticles does not reveal similar effects, probably due to the ease of their surface degradation/oxidation. All the vanadium-based catalysts have also some influence on thermal decomposition of LiZn 2 (BH 4 ) 5 , lowering the temperature of the fastest stage of decomposition (by up to 25 o C) already on 5 wt.% doping level. However, the latter systems need more thorough research for better understanding.

Thermolysis of borohydrides can potentially be useful for the preparation of reagents or solid materials. For example, heating of LiZn 2 (BH 4 ) 5 above 85 o C makes it a convenient and relatively safe source of B 2 H 6 which can be used on-demand for different reaction processes [START_REF] Friedrichs | Core shell structure for solid gas synthesis of LiBD4[END_REF]. As borohydrides decompose at temperatures of up to a few hundred o C, which is significantly lower than the temperature range typically used for the preparation of borides (reaching even 2000 o C), a few borohydrides have been assessed as precursors towards the binary compounds of boron. Simple Mg(BH 4 ) 2 and two of its solvates, Mg(BH 4 ) 2 •3THF and Mg(BH 4 ) 2 •1.5DME, were tested for synthesis of MgB 2 superconductor in mostly amorphous forms [START_REF] Wegner | Organic derivatives of Mg(BH4)2 as precursors towards MgB2 and novel inorganic mixed-cation borohydrides[END_REF][START_REF] Yang | Low-temperature mass production of superconducting MgB2 nanofibers from Mg(BH4)2 decomposition and recombination[END_REF]. Similarly, thermolysis of a series of lanthanide borohydrides led to their refractory borides, like ErB 4 , TmB 4 , TbB 4 , and also TmB 6 and TbB 6 . However the purification of these compounds appeared unsuccessful [START_REF] Wegner | Preparation of a series of lanthanide borohydrides and their thermal decomposition to refractory lanthanide borides[END_REF]. In contrast, pure amorphous quasi-hexagonal boron nitride containing minor amount of quasi-cubic form has been recently obtained via thermal decomposition of (NH 4 ) 3 Mg(BH 4 ) 5 [START_REF] Wegner | A low temperature pyrolytic route to amorphous quasihexagonal boron nitride from hydrogen rich (NH4)3Mg(BH4)5[END_REF]. The latter compound has an exceptionally high content of hydrogen (21 wt.%). It decomposes already in the temperature range of 220 -250 o C, while further heating up to 650 o C is beneficial for the quality and yield of obtained BN. This is significantly lower temperature than 900 -1500 o C required in the previously knownand industrially appliedapproaches towards BN [START_REF] Arenal | Boron-nitride and boron-carbonitride nanotubes: synthesis, characterization and theory[END_REF]. Interestingly, boron nitride is the only chemical compound detected in the sample after rinsing with water, while formation of MgB 2a typical product of Mg(BH 4 ) 2 pyrolysisis not observed in this case.

The decomposition temperatures and pathways of the aforementioned rare-earth borohydrides may depend on their synthesis method: solvent free synthesis often uses an excess ratio of 6:1 of the LiBH 4

to the RE halide to form a composite [START_REF] Gennari | Reversible hydrogen storage from 6LiBH4-MCl3 (M= Ce, Gd) composites by in-situ formation of MH2[END_REF]. In such conditions partial Cl -substitution occurs very often, which leads to the formation of Li(BH 4 ) 1-x Cl x . It appears that the presence of Cl -either decreases the temperature of o-LiBH 4 to h-LiBH 4 phase transition or it slows down its kinetics [START_REF] Matsuo | Stabilization of lithium superionic conduction phase and enhancement of conductivity of LiBH4 by LiCl addition[END_REF][START_REF] Zavorotynska | Theoretical and Experimental Study of LiBH4-LiCl Solid Solution[END_REF]. Moreover, selected compounds melt at 200 °C [START_REF] Frommen | Crystal structure, polymorphism, and thermal properties of yttrium borohydride Y(BH4)3[END_REF], and others form an X-ray amorphous material without showing signs of this aforementioned melting behavior [START_REF] Ley | Novel solvates M(BH(4))(3)S(CH(3))(2) and properties of halide-free M(BH(4))(3) (M = Y or Gd)[END_REF]. All these solvent free composites show decomposition between 200 < T dec < 300 °C [START_REF] Frommen | Crystal structure, polymorphism, and thermal properties of yttrium borohydride Y(BH4)3[END_REF][START_REF] Frommen | Synthesis, crystal structure, and thermal properties of the first mixed-metal and anion-substituted rare earth borohydride LiCe(BH4)3Cl[END_REF][START_REF] Gennari | Mechanochemical synthesis of erbium borohydride: Polymorphism, thermal decomposition and hydrogen storage[END_REF]. Among the solid-state decomposition products, the respective RE hydrides, and RE borides form at relatively low temperatures. LiH is also a reaction product, which seems responsible for the possibility to rehydrogenate and reform the precursor LiBH 4 [START_REF] Vajo | Reversible storage of hydrogen in destabilized LiBH4[END_REF][START_REF] Heere | The influence of LiH on the rehydrogenation behavior of halide free rare earth (RE) borohydrides (RE = Pr, Er)[END_REF][START_REF] Heere | Hydrogen Sorption in Erbium Borohydride Composite Mixtures with LiBH4 and/or LiH[END_REF][START_REF] El Kharbachi | A thermodynamic assessment of LiBH4[END_REF].

Metal hydride thin films for hydrogen sensors

Efficient, reliable, and fast detection of hydrogen is a prerequisite for a successful sustainable hydrogen economy and crucial in many industrial processes and bioanalytical applications. For safety reasons, hydrogen leaks have to be detected immediately, ideally well before its concentration gets close to the safe limit of 4% in air. In addition, the monitoring of the hydrogen concentrations and pressures is vital for the efficient operation of hydrogen fuel cells, CO 2 conversion devices, and in a variety of industrial processes. Although there are many types of hydrogen sensors commercially available, including catalytic resistor detectors, electrochemical devices, and sensors based on changes in the thermal conductivity, all of them have major disadvantages. Apart from practical and economic drawbacks such as their relatively large size, need for regular calibration, narrow sensing range, and high costs, there are some serious safety concerns with these conventional sensors: they often require the presence of oxygen and/or current leads, forming a potential explosion hazard [START_REF] Hubert | Hydrogen sensors -A review[END_REF][START_REF] Buttner | An overview of hydrogen safety sensors and requirements[END_REF].

Metal hydride based hydrogen sensors are considered to be a promising alternative to these conventional hydrogen sensors. These sensors utilize the propensity of metal hydrides to hydrogenate when exposed to a partial hydrogen pressure, which, in turn, results in volumetric expansion of the metal hydride and a change in its optical and electronic properties. By probing one of these properties, the hydrogen pressure in the environment of the metal hydride sensors can thus be determined. As the hydrogenation of a metal at a given pressure typically decreases with temperature, metal-hydride based hydrogen sensors should either operate at a fixed temperature or incorporate a thermometer to account for the temperature-dependent signal. One of the major challenges for (metal hydride) hydrogen sensors is the resistance to chemical species other than hydrogen. Especially the catalytic surface of palladium is prone to poisoning by aggressive chemical species as NO x and CO as well as humidity, rendering the sensor inactive. To tackle this problem, several polymeric coatings (≈30 nm) have been developed. Notable examples include polytetrafluoroethylene (PTFE) that reduces the poisonous effect of humid air as well as poly(methylmethacrylate) (PMMA) that prevents protection against CO [START_REF] Nugroho | Metalpolymer hybrid nanomaterials for plasmonic ultrafast hydrogen detection[END_REF][START_REF] Ngene | Polymer-induced surface modifications of Pd-based thin films leading to improved kinetics in hydrogen sensing and energy storage applications[END_REF]. These coatings have also been shown to reduce the activation energy of hydrogen (de)sorption and as such substantially improve the response times of metal hydride hydrogen sensors.

In addition, thin platinum top layers [START_REF] Strohfeldt | Long-term stability of capped and buffered palladium-nickel thin films and nanostructures for plasmonic hydrogen sensing applications[END_REF] or alloying palladium catalyst with elements as Cu have also been employed to reduce the poisoning effects of e.g. CO [START_REF] Darmadi | Rationally Designed PdAuCu Ternary Alloy Nanoparticles for Intrinsically Deactivation-Resistant Ultrafast Plasmonic Hydrogen Sensing[END_REF].

Sensor design and read-out

The earliest application of metal hydrides in hydrogen sensors was by probing the electrical resistivity of palladium [START_REF] Macintyre | A thin film hydrogen sensor[END_REF]. Like most metal hydrides, the exposure of palladium to a hydrogen environment results in the hydrogenation of palladium and, in turn, to an increase of its electrical resistivity. This change in resistivity can easily be detected by a four-points resistivity measurement of a continuous thin film (< 100 nm). Alternatively, when higher sensitivities are required, the resistivity of nanofabricated textures such as patterned nanosheets, single and multiple nanowires in combination with inter-digitated transducers (IDT) and microelectromechanical-systems (MEMS) can be used.

Another class of hydrogen sensors uses an optical read-out, which have, as compared to resistive hydrogen sensors, an inherent safety benefit as they do not require any electrical currents near the sensing area (see, e.g., Silva et al. [START_REF] Silva | A Review of Palladium-Based Fiber-Optic Sensors for Molecular Hydrogen Detection[END_REF], Yang et al. [START_REF] Yang | Fiber optic hydrogen sensors: a review[END_REF], and Zhang et al. [START_REF] Zhang | Recent advancements in optical fiber hydrogen sensors[END_REF] for recent reviews).

Furthermore, those sensors are typically less prone to electromagnetic radiation disturbing the readout.

Several optical detection principles have been suggested that can roughly be categorized into two groups: intensity modulated and frequency modulated optical hydrogen sensors. Intensity modulated sensors are usually based on measuring the changes in the optical reflectance or transmittance of a metal hydride upon its exposure to a hydrogen environment. For example, in a micro-mirror configuration, light is coupled into an optical fibre and partially reflected by a thin film metal hydride with a thickness of 20 -100 nm located at the tip of the fibre. The intensity of the reflected light, which alters when the metal hydride is exposed to hydrogen, is subsequently monitored by e.g. a photodiode and is directly related to the hydrogen pressure.

Frequency-modulated optical hydrogen sensors have the advantage that the read-out is insensitive to intensity fluctuations of the light source. One of the most attractive options is based on the (localized) surface plasmon resonance ((L)SPR) of a metal-hydride that may occur when it is illuminated by light that matches the frequency of the plasmon resonances. As illustrated in Fig. 8, when exposed to hydrogen, the resonance frequency changes, which may be detected by measuring the resonant frequency itself (or, alternatively, the intensity of light reflected by the sensing layer). While SPR effects occurring at the interface between a metallic surface and another dielectric medium have been considered for a longer time [START_REF] Chadwick | Enhanced Optical-Detection of Hydrogen Using the Excitation of Surface-Plasmons in Palladium[END_REF][START_REF] Perrotton | A reliable, sensitive and fast optical fiber hydrogen sensor based on surface plasmon resonance[END_REF], especially hydrogen sensors based on LSPR in nanoparticles are recently studied intensively. In the latter case, when the dimensions of metal hydride nanoparticles are much smaller than the wavelength of the incident light, strong absorption and scattering of the incident light occurs when its frequency matches the resonance frequency of the collectively oscillating free electrons in the particle. As a consequence, a peak, the LSPR peak, is visible in the extinction spectrum of which the wavelength strongly depends on the size, shape and permittivity of the nanoparticle. When nanoparticles are exposed to hydrogen, they hydrogenate, causing a broadening and a red-shift of the LSPR peak that turns out to be directly correlated to the hydrogen-tometal ratio in the particle and from the position or width the hydrogen pressure can thus be deduced [START_REF] Nugroho | Universal Scaling and Design Rules of Hydrogen-Induced Optical Properties in Pd and Pd-Alloy Nanoparticles[END_REF]. The spectral position of this peak can then be measured by considering either the reflected or transmitted intensity.

Frequency-modulated optical hydrogen sensors have also been developed based on interferometry or Fibre-Bragg gratings [START_REF] Maier | Fibre optics in palladium-based hydrogen sensing[END_REF][START_REF] Dai | Optical Fiber Grating Hydrogen Sensors: A Review[END_REF]. In its most basic form, Fibre-Bragg sensors consist of (i) a fibre in which the core contains a grating of alternating layers of materials with different refractive indices and

(ii) a cladding that is partly covered by a metal hydride. As such, the fibre only reflects light with a specific wavelength. Upon exposure to a partial hydrogen pressure, the metal hydride expands, which in turn changes the pitch of the grating in the core of the fibre and thus the characteristic wavelength that is reflected. As these sensors are based on the volumetric expansion of strained materials, their response times are relatively long, rendering them unsuitable for safety reasons and thus most applications. spectrum is computed by taking the relative difference between the incident and transmitted/reflected intensity. In the extinction spectrum, the so called LSPR peak shows up. Upon exposure to hydrogen, the LSPR peak shifts to the red, and this shift is directly related to the hydrogen pressure. (d) In Fibre-Bragg grating sensors, white light is coupled into an optical fibre in which a grating is present in its core and of which e.g. the cladding is covered with a metal hydride layer. As such, light with only one wavelength will be reflected by the grating. When exposed to hydrogen, the metal hydride layer will expand, causing a change of the periodicity of the grating, and thus of the reflected wavelength.

Suitable sensing materials

While there is a wide variety in the design and read-out mechanisms of metal hydride based hydrogen sensors, the requirements to the actual sensing material are largely overlapping. In order to obtain the ideal hydrogen sensor that features a large sensing range, fast response, high sensitivity, enduring stability and chemical selectivity [START_REF] Buttner | An overview of hydrogen safety sensors and requirements[END_REF], metal hydrides with a large hydrogen solubility window (preferably within one phase), no hysteresis, appropriate enthalpy and entropy change and fast hydrogen diffusion are required. In the literature, two approaches have been used to develop these hydrogen sensors: (i) materials with both a hydrogen sensing and dissociation functionality as e.g.

(alloys) of palladium and (ii) materials in which the sensing and dissociation functionality are separated as e.g. palladium capped transition metals [START_REF] Bannenberg | Metal Hydride Based Optical Hydrogen Sensors[END_REF].

Palladium-based materials

Palladium is unarguably the most frequently considered material for metal hydride based hydrogen sensors. Its attractiveness stems from its capability to dissociate molecular hydrogen at room temperature, its modest room temperature sensing range of at least P H2 = 10 +1 -10 +4 Pa, and a reasonably fast response that can be in the order of seconds. However, palladium also has some serious shortcomings: while it already hydrogenates at pressures well before the safe limit of hydrogen in air, its sensing range is relatively limited and the mechanical stability is typically poor. The poor mechanical stability, which greatly reduces the lifetime of the sensor, has been linked to the relatively large volumetric expansion of palladium upon hydrogenation. Especially palladium thin films have been found prone to delamination from their support. However, this problem is typically easily solved by using intermediate layers of e.g. nickel [START_REF] Butler | Micromirror Optical-Fiber Hydrogen Sensor[END_REF] or titanium [START_REF] Pivak | Effect of the substrate on the thermodynamic properties of PdHx films studied by hydrogenography[END_REF].

The most important limitation of palladium-based hydrogen sensors is the severe hysteresis in the readout: as palladium undergoes a first-order metal-to-metal hydride transition from the dilute α to the high-hydrogen concentration metal hydride β-phase, the readout depends strongly on the sensor's pressure history. Alloying of palladium has widely been employed to obtain more reproducible sensing characteristics at room temperature. Typically, elements including Au [START_REF] Nugroho | Universal Scaling and Design Rules of Hydrogen-Induced Optical Properties in Pd and Pd-Alloy Nanoparticles[END_REF][START_REF] Zhao | All-optical hydrogen sensor based on a high alloy content palladium thin film[END_REF][START_REF] Westerwaal | Nanostructured Pd-Au based fiber optic sensors for probing hydrogen concentrations in gas mixtures[END_REF][START_REF] Wadell | Hysteresis-Free Nanoplasmonic Pd-Au Alloy Hydrogen Sensors[END_REF][START_REF] Nugroho | A fiber-optic nanoplasmonic hydrogen sensor via pattern-transfer of nanofabricated PdAu alloy nanostructures[END_REF], Ag [START_REF] Wang | Palladium-silver thin film for hydrogen sensing[END_REF], Ta [START_REF] Westerwaal | Thin film based sensors for a continuous monitoring of hydrogen concentrations[END_REF], Cu [START_REF] Darmadi | Rationally Designed PdAuCu Ternary Alloy Nanoparticles for Intrinsically Deactivation-Resistant Ultrafast Plasmonic Hydrogen Sensing[END_REF] and Ni [START_REF] Hughes | Thin-Films of Pd/Ni Alloys for Detection of High Hydrogen Concentrations[END_REF][START_REF] Lee | Hydrogen gas sensing performance of Pd-Ni alloy thin films[END_REF] are introduced in palladium, tuning the d-band and shifting the critical temperature of T C ≈ 290 °C down, thereby suppressing the first-order phase transition for sufficiently high concentrations of these dopants [START_REF] Sharma | Pd-based ternary alloys used for gas sensing applications: A review[END_REF]. Unfortunately, a small hysteresis can often still be discerned even though the first-order transition is completely eliminated at high dopant concentrations. For example, in the case of Pd 1-y Au y, thin films display a small hysteresis over a wide pressure range owing to profound clamping of the film to the support: upon hydrogenation, the volumetric expansion of the cubic unit cell has to be completely translated into a thickness increase of the film. Such effects are not seen in nanoparticles as they have the ability to expand more freely. The different appearance of hysteresis as well as their dissimilar hydrogen solubilities emphasize that the confinement of materials at the nanoscale the way materials are nanoconfined can dramatically influence its properties and thus their sensing characteristics [START_REF] Bannenberg | Direct Comparison of PdAu Alloy Thin Films and Nanoparticles upon Hydrogen Exposure[END_REF] Alloying palladium also affects the hydrogen solubility. Introducing elements with a larger unit cell like Au, Ag, and Ta increase the hydrogen solubility at lower pressures, thus extending the sensing range. For example, in the case of thin film and nanoparticles of Pd 0.7 Au 0.3 , an extraordinary sensing range of P H2 = 10 0 -10 +6 Pa has been demonstrated, which likely extends to even higher pressures making it a suitable candidate to probe relatively high hydrogen pressures. However, alloying palladium typically reduces the maximum hydrogenation of the metal hydride and thus greatly reduces the maximum change of the optical and electronic properties of the material, thereby compromising the sensitivity of the hydrogen sensor.

Materials with separated hydrogen dissociation and sensing functionality

Separating the hydrogen dissociation and sensing functionality considerably broadens the scope of metal hydride materials that can be used in a hydrogen sensor. Such sensors with separate functionalities have mainly been developed using an optical readout and consist of at least two layers:

(i) a sensing layer that is responsible for the response to hydrogen, typically transition metals and metal hydrides that exhibit a metal-to-insulator transition upon hydrogenation, and (ii) a capping layer that takes care of the dissociation of hydrogen as e.g. a thin (≈ 10 nm) palladium layer.

Transition metals including hafnium and tantalum have very advantageous sensing properties [START_REF] Bannenberg | Optical hydrogen sensing beyond palladium: Hafnium and tantalum as effective sensing materials[END_REF][START_REF] Boelsma | Hafnium-an optical hydrogen sensor spanning six orders in pressure[END_REF]. In these materials, hysteresis arising from a first-order transition is easily circumvented by considering materials with a large solubility window within one single phase. TaH x is a notable example: it has a large solubility range of 0 < x < 0.7. As a result, it entails an exceptionally broad sensing range of at least P H2 = 10 -2 -10 +4 Pa at T = 120 °C. In addition, as the hydrogen uptake of tantalum is considerable larger than for palladium alloy sensing materials, its (optical) contrast is also substantially larger, allowing the development of hydrogen sensors with a better sensitivity [START_REF] Bannenberg | Optical hydrogen sensing beyond palladium: Hafnium and tantalum as effective sensing materials[END_REF].).

The fact that materials as magnesium and yttrium show a metal-to-insulator transition upon hydrogenation implies a substantial change to the optical and electronic properties when these materials are exposed to hydrogen. This large change in properties, which have e.g. also been exploited in switchable mirrors, potentially allows for the development of hydrogen sensors with high sensitivities. However, the applicability of both materials is greatly reduced by the first-order metal-tometal hydride transition, which implies a profound hysteresis. In addition, magnesium and, to a smaller extend yttrium, only exhibit a limited pressure range around their plateau pressures in which they are sensitive to hydrogen. Alloying as well as using the profound thickness dependence of the plateau-pressure of magnesium based films have been employed to increase the sensing range (see, e.g., [START_REF] Slaman | Fiber optic hydrogen detectors containing Mg-based metal hydrides[END_REF][START_REF] Yoshimura | New hydrogen sensor based on sputtered Mg-Ni alloy thin film[END_REF][START_REF] Palmisano | An optical hydrogen sensor based on a Pd-capped Mg thin film wedge[END_REF][START_REF] Radeva | Highly sensitive and selective visual hydrogen detectors based on YxMg1-x thin films[END_REF][START_REF] Ngene | Metal-hydrogen systems with an exceptionally large and tunable thermodynamic destabilization[END_REF][START_REF] Ngene | Eye readable metal hydride based hydrogen tape sensor for health applications[END_REF][START_REF] Victoria | Amorphous Metal-Hydrides for Optical Hydrogen Sensing: The Effect of Adding Glassy Ni-Zr to Mg-Ni-H[END_REF]) and also facilitated the development of low-cost eye-readable hydrogen sensors [START_REF] Palmisano | An optical hydrogen sensor based on a Pd-capped Mg thin film wedge[END_REF][START_REF] Ngene | Metal-hydrogen systems with an exceptionally large and tunable thermodynamic destabilization[END_REF][START_REF] Ngene | Eye readable metal hydride based hydrogen tape sensor for health applications[END_REF]. Unfortunately, these materials typically form extremely stable hydrides and exhibit therefore very slow desorption kinetics, limiting their practical applicability to a one-time-use indicator of the highest hydrogen pressure that was present in the environment of the sensor.

Application in secondary batteries

Metal hydride electrodes for LIBs

Metal hydrides have been considered as potential anode materials for Li-ion batteries due to their large capacities and low average potential value versus Li + /Li 0 . Since the discovery of the electrochemical activity of metal hydrides towards lithium, MgH 2 (theoretical capacity ~2000 mAh g -1 ) remains the most studied in this category of anodes for LIBs using a carbonate-based liquid electrolyte [START_REF] Oumellal | Metal hydrides for lithium-ion batteries[END_REF][START_REF] Berti | Enhanced reversibility of the electrochemical Li conversion reaction with MgH2-TiH2 nanocomposites[END_REF][START_REF] Zaïdi | Carboxymethylcellulose and carboxymethylcellulose-formate as binders in MgH2-carbon composites negative electrode for lithium-ion batteries[END_REF][START_REF] Brutti | Magnesium hydride as a high capacity negative electrode for lithium ion batteries[END_REF][START_REF] Yang | Improvement in the Electrochemical Lithium Storage Performance of MgH2[END_REF][START_REF] El Kharbachi | Morphology effects in MgH2 anode for lithium ion batteries[END_REF][START_REF] El Kharbachi | Understanding Capacity Fading of MgH2 Conversion-Type Anodes via Structural Morphology Changes and Electrochemical Impedance[END_REF][START_REF] Peng | Electrochemical performances of MgH2 and MgH2-C films for lithium ion battery anode[END_REF][START_REF] Huen | Insight into Poor Cycling Stability of MgH2 Anodes[END_REF][START_REF] Brutti | Magnesium hydride as negative electrode active material in lithium cells: A review[END_REF]. It can deliver a reversible capacity of 1480 mAh g -1 during the total conversion reaction (eq. 15):

MgH 2 + 2Li  Mg + 2LiH (15)
However, though the high theoretical capacity, pristine MgH 2 electrode suffers from large initial irreversibility and capacity fading after few cycles, which is connected to the high volume variation (~85%) and LiH interaction with the liquid electrolyte during the (dis)charge processes [START_REF] Brutti | Magnesium hydride as a high capacity negative electrode for lithium ion batteries[END_REF][START_REF] El Kharbachi | Understanding Capacity Fading of MgH2 Conversion-Type Anodes via Structural Morphology Changes and Electrochemical Impedance[END_REF][START_REF] El Kharbachi | Reversibility of metalhydride anodes in all-solid-state lithium secondary battery operating at room temperature[END_REF].

At low cycling rate (1Li/100h), MgH 2 reacts with Li showing a full discharge (Δx ≈ 2.9 Li) with two plateaus at 0.44 V (Δx ≈ 1.8 Li) and 0.095 V. For the first plateau, XRD shows the appearance of the hcp Mg-type and bcc Li-type solid solutions. The formation of these solid solutions can be avoided by limiting the discharge at Δx ≈ 2 Li or 0.15 V using in-house prepared MgH 2 (Mg milled with 10%

MCMB 2528 then hydrogenated under 20 bar H 2 at 350 °C for 10 h). A reversible capacity of 1500 mAh g -1 (25% loss) is reached for the first step, while a reversible capacity 2700 mAh g -1 (33% loss)

is measured for both steps [START_REF] Oumellal | Metal hydrides for lithium-ion batteries[END_REF].

Electronic transfer is an important parameter during any electrochemical reaction. The poor electric conductivity of hydrides has to be taken in consideration. For instance β-MgH 2 and γ-MgH 2 insulators exhibit band gap energies of 5.6 and 5.3 eV, respectively [START_REF] Araújo | Electronic and optical properties of pressure induced phases of MgH2[END_REF]. In addition, the conductivity is also influenced during cycling by the formation of the conducting metallic Mg and insulating LiH. For example, in thin films the presence of small amounts of Mg can be sufficient to reach the minimum conductivity in the electrode, where the addition of carbon is not necessary. Owing to the metallic behavior of TiH 2 , its addition to bulk MgH 2 has been also considered as composite anode for LIBs [START_REF] Oumellal | Metal hydrides for lithium-ion batteries[END_REF][START_REF] Berti | Enhanced reversibility of the electrochemical Li conversion reaction with MgH2-TiH2 nanocomposites[END_REF].

DFT [START_REF] Hohenberg | Inhomogeneous electron gas[END_REF][START_REF] Kresse | Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set[END_REF] has been regarded as a powerful tool to calculate and predict the structures, energies and electrochemical properties of hydrides materials for conversion-type electrodes applications.

Ramzan et al. [START_REF] Ramzan | Transition metal doped MgH2: A material to potentially combine fuel-cell and battery technologies[END_REF] performed ab initio simulations to investigate the MgH 2 electrode, which was in good agreement with experiments in terms of the average working voltage for Li-ion battery.

However, their calculation of doped-MgH 2 as conversion reaction anode did not show much improvement in terms of diffusion of lithium. Qian et al. [START_REF] Qian | Pure and Li-doped NiTiH: Potential anode materials for Li-ion rechargeable batteries[END_REF][START_REF] Qian | Screening study of light-metal and transition-metal-doped NiTiH hydrides as Li-ion battery anode materials[END_REF] explored pure and various metaldoped NiTiH hydrides as anode materials for Li-ion batteries using DFT, in which the enhanced electrochemical capacity and a minor increase in voltage were found in Li-doped NiTiH. The effects of various light-metals (Mg, Al) and transition metals (V, Cr, Mn, Fe, Co, Cu, Zn) on the electrochemical properties of NiTiH conversion electrodes were also screened in detail by the authors.

The doping of Al, Cr, Mn or Fe was found to have the most remarkable effects on pristine NiTiH hydride.

Through materials design and predictions using DFT, conversion electrodes of Li-ion batteries with other complex hydrides also show excellent properties. Mason et al. [START_REF] Mason | First-Principles Study of Novel Conversion Reactions for High-Capacity Li-Ion Battery Anodes in the Li-Mg-B-N-H System[END_REF] determined the phase diagram in the Li-Mg-B-N-H system in the grand canonical ensemble, as a function of lithium chemical potential. Such calculations have also been conducted to study the capacities regarding several new conversion reactions. Qian et al. [START_REF] Jiang | Exploring pristine and Li-doped Mg2NiH4 compounds with potential lithium-storage properties: Ab initio insight[END_REF] explored the pure and Li-doped Mg 2 NiH 4 as conversion anode materials for Li-ion battery applications using DFT and AIMD technique. The most thermodynamically stable Li-doped Mg 2 NiH 4 possesses a smaller band gap than pure Mg 2 NiH 4 and has a theoretical specific capacity of 975.35 mAh g -1 and an average voltage of 0.437 V (versus Li + /Li 0 ). The diffusion behavior of Li-ions in this electrode material at 300 K is also improved by Lidoping. In particular, the diffusion coefficient of Li-ion is increased evidently after Li-doping.

Moreover, Mg 2 NiH 4 doped with nine different elements (Na, Al, Si, K, Ca, Ti, Mn, Fe, Co) has been predicted as conversion-type electrode materials through computations [START_REF] Qian | Atomistic Modeling of Various Doped Mg2NiH4 as Conversion Electrode Materials for Lithium Storage[END_REF]. The electrochemical properties such as specific capacity, volume change and average voltage as well as the electronic band gap of different doped systems have been calculated as shown in Fig. 9. The Na-doped material shows the highest electrochemical specific capacity. The dopants Si and Ti have the most obvious effects to reduce the electronic band gap of the electrode material. All nine doping elements can help to reduce the average voltage of the negative electrodes and tend to have acceptable volume changes. Pure and various doped Mg(AlH 4 ) 2 have also been reported to exhibit remarkable specific capacities [START_REF] Qian | Ab Initio Screening of Doped Mg(AlH4)(2) Systems for Conversion-Type Lithium Storage[END_REF]. The theoretical specific capacity of the Li-doped material can be 2547.64 mAh g -1 with a small volume change of 3.76% during the electrode conversion reaction. The strong hybridization between Li s-state and H s-state influences the performance of the Li-doped material. It is believed that DFT modeling would further help the design and prediction of better light-metal based complex hydrides for conversion electrode applications. In addition to its use as a solid state electrolyte (see § 4.2.1), LiBH 4 was also theoretically and experimentally investigated as conversion type anode material for Li-ion batteries [START_REF] Farina | Electrochemical activity of lightweight borohydrides in lithium cells[END_REF][START_REF] Meggiolaro | Lightweight Borohydrides Electro-Activity in Lithium Cells[END_REF]. Other single-cation alkaline and alkaline earth metal borohydrides NaBH 4 , KBH 4 , Mg(BH 4 ) 2 and Ca(BH 4 ) 2 were simultaneously investigated. All these borohydrides have indeed low average molar masses and a quite substantial hydrogen content, resulting in outstandingly large theoretical inherent gravimetric capacities in the range of 2000-4000 mAh g -1 [START_REF] Meggiolaro | Lightweight Borohydrides Electro-Activity in Lithium Cells[END_REF]. For LiBH 4 notably, electrochemical capacity of 4992 mAh g -1 can be expected , with the following conversion reaction:

LiBH 4 + 3 Li → B + 4LiH (16)
In addition, a reaction potential of 0.42 V vs. Li + /Li° associated with a favorable reaction energy of -123.3 kJ mol -1 are theoretically predicted. For the other borohydrides, one or two steps conversion mechanisms are conceivable. In the first step, elemental boron and metal (Na, K, Mg, Ca) associated with lithium hydride are directly generated, whereas a metal hydride intermediate (NaH, KH, MgH 2 and CaH 2 ) is produced before the metal itself in the second step. However, from the electrochemical tests vs. Li + /Li° using LiPF 6 /EC:DMC 1:1 (LP30) as liquid electrolyte, it appears unfortunately that LiBH 4 present almost no electrochemical reactivity and reversibility. In the case of other borohydrides, only Mg(BH 4 ) 2 and NaBH 4 show some first discharge capacities, but far lower as compared to the theoretical ones, only 540 and 250 mAh g -1 respectively, and very weak reversibility. Despite these poor results, LiBH 4 and more generally alkaline and alkaline earth metal borohydrides, deserve to be deeply investigated mainly in composites, because of the electrochemical activity for some of them vs.

Li + /Li°. In that respect, with an anode composite formulation optimization going hand in hand with an electrode structure control, performances could be greatly enhanced. As a proof of concept, this enhancement was observed for MgH 2 as a conversion material vs. Li + /Li°, with no electroactivity observed for the pristine hydride without the suitable formulation [START_REF] Oumellal | Metal hydrides for lithium-ion batteries[END_REF][START_REF] Aymard | Metal hydrides: an innovative and challenging conversion reaction anode for lithium-ion batteries[END_REF]. Moreover, in the case of the Mg(BH 4 ) 2 conversion, MgH 2 could be formed during reduction and subsequently reacts in the second step, which is conceptually interesting, even if Mg(BH 4 ) 2 conversion is not reversible.

Borohydride-based electrolytes for ASSBs

LiBH 4 -based electrolytes

The complex hydride LiBH 4 and its composites (e.g. with oxides, halides, sulfides) are of considerable interest for solid-state battery applications [START_REF] Latroche | Full-cell hydride-based solid-state Li batteries for energy storage[END_REF][START_REF] Gulino | Enhancing Li-Ion Conductivity in LiBH4-Based Solid Electrolytes by Adding Various Nanosized Oxides[END_REF]. In particular as solid-state electrolyte, the ionic conductivity of LiBH 4 is greater than 10 -3 S cm -1 at a temperature of 393 K [START_REF] Ikeshoji | Fast-ionic conductivity of Li+ in LiBH4[END_REF]. From modelling point of view, it would be interesting to address whether the high Li ion mobility in LiBH 4 can be extended to room temperature or if other complex hydrides with this property are available. It has been proven that mixing metal borohydrides with halides can stabilize the h-LiBH 4 phase at lower temperatures. This has led several authors, for example Yin et al. [START_REF] Yin | Thermodynamically tuning LiBH4 by fluorine anion doping for hydrogen storage: A density functional study[END_REF] and Corno et al. [START_REF] Corno | A computational study on the effect of fluorine substitution in LiBH4[END_REF] to investigate anion substitution by F in the o-LiBH 4 (space group Pnma) [START_REF] Arnbjerg | Structure and Dynamics for LiBH4-LiCl Solid Solutions[END_REF]. Moreover, Maekawa et al. [START_REF] Maekawa | Halide-stabilized LiBH4, a room-temperature lithium fast-ion conductor[END_REF] proposed a superionic solid solution phase at room temperature by doping with lithium halides and also demonstrated that Li(BH 4 ) 0.667 Br 0.333 conserves the hexagonal structure (space group P6 3 mc) from 293 to 573 K [START_REF] Cascallana-Matias | Phase Behavior in the LiBH4-LiBr System and Structure of the Anion-Stabilized Fast Ionic, High Temperature Phase[END_REF]. Further, Gulino et al. [START_REF] Gulino | Phase Stability and Fast Ion Conductivity in the Hexagonal LiBH4-LiBr-LiCl Solid Solution[END_REF] studied the mixed halide anions (Cl/Br) substitution resulting in additional optimization of the electrochemical properties of the hexagonal phase. The influence of different halide anions on the hexagonal structure is studied using DFT modelling, mainly with respect to the following properties:

 The structural and electronic changes during the substitution in the structure Li (BH 4 ) 1-x M x (with M = F, Cl, I, Br and x = 0.125, 0.25, 0.375)

 The mechanical stability of the h-LiBH 4 phase during substitution by halide anions based on atomic vibrations Atomic relaxation and phonon dispersion have been performed for each phase in this study as preliminary results. In the high temperature phase, the [BH 4 ] -anions are arranged along the z-direction in the hexagonal plane, with two nearly equivalent Li + ion sites (Fig. 10). The obtained equilibrium volume, lattice constant and interatomic distances are summarized in Table 3. It can be noticed that LiB, BH, H-H and LiH distances are almost similar for all systems. The structural data agree well with the reported ones for chlorine substitutions at close compositions [START_REF] Zavorotynska | Theoretical and Experimental Study of LiBH4-LiCl Solid Solution[END_REF]. Furthermore, the interatomic distance between Li + and the nearest added halide anions varied from 1.80 to 2.77 Å. Note that, for the substituted structures, it has been demonstrated that there is a significant reduction in volume, owing to a large contraction along the a and c-axis. The substitution of [BH 4 ] -by halide anions (F -, Cl -, Br -, I -) has also been investigated considering three compositions (molar fraction), x equal to 0.125, 0.25 and 0.375 in the hexagonal framework. As a contribution to this review work, our study aims to identify the microscopic behavior during the replacement of the [BH 4 ] -anion by halide (F -, Cl -, Br -or I -) into the h-LiBH 4 phase.

The effect of halides substituted in h-LiBH 4 can be explained based on the interatomic distance.

Except for the hypothetical substitution of fluorine, as the size of the halide ion is larger, the volume of the unit cell during substitution is increased. According to Fig. 10, a reorientation of [BH 4 ] -is observed when [BH 4 ] -anions are randomly replaced by halide ions (F -, Cl -, Br -or I -) that becomes more pronounced with increasing substituent fraction x. While for low values of x a partial reorientation of the nearest [BH 4 ] -anion to the halide anion occurs, total reorientation of [BH 4 ] -anion to the halide anion occurs for x = 0.25 and 0.375. Differently, for x = 0.50, the orientation [BH 4 ] -is again along the z-axis and the structure becomes unstable.

The phonon dispersion has been studied to analyze the structural stability of LiBH 4 substituted by halides. Fig. 10 (d-g) shows that phonon modes are imaginary in case of the substitution for x = 0.125 and lower than observed for h-LiBH 4 . Furthermore, the intensity of the imaginary modes decreases when the halide anion fraction is increased. As a result, the disordered phase is favored, and the phase is more mechanically stable at low temperature, due to the reorientation disordering of anion which is a common behavior of [B n H n ] -groups as described for different systems based on borohydrides [START_REF] Verdal | Evidence of a transition to reorientational disorder in the cubic alkali-metal dodecahydro-closo-dodecaborates[END_REF][START_REF] Martelli | Rotational Motion in LiBH4/LiI Solid Solutions[END_REF][START_REF] Remhof | Rotational disorder in lithium borohydride[END_REF].

With respect to application in batteries, Li(BH 4 ) 0.75 I 0.25 solid electrolyte has been studied and revealed to be suitable for application in LIBs [START_REF] Kisu | Interfacial stability between LiBH4-based complex hydride solid electrolytes and Li metal anode for all-solid-state Li batteries[END_REF][START_REF] Yoshida | Complex hydride for composite negative electrode-applicable to bulk-type all-solid-state Li-ion battery with wide temperature operation[END_REF]. The remarkable properties of LiBH 4 -based materials reside in their reducing character, ductility and compatibility with Li metal anode, which make them as potential candidates for solid-state electrolytes. Progress has been made to improve the ionic conductivity of LiBH 4 -based systems at RT. Similar to halide substitution, the presence of (PS 4 ) 3- groups embedded in the structural unit of Li(BH 4 ) 0.75 I 0.25 could allow the accommodation of the BH 4 -BH 4 distances and less hindered effect for Li mobility [START_REF] El Kharbachi | Reversibility of metalhydride anodes in all-solid-state lithium secondary battery operating at room temperature[END_REF][START_REF] El Kharbachi | Lithium ionic conduction in composites of Li(BH4)0.75I0.25 and amorphous 0.75Li2S•0.25P2S5 for battery applications[END_REF]. These composite electrolytes exhibit high ionic conductivity, ~ 10 -3 S cm -1 , and good chemical compatibility compared to pure LiBH 4 , as well as an electrochemical window up to 5 V. Unlike the Li(BH 4 ) 0.75 I 0.25 system, Li(BH 4 ) 1-x Cl x reverts back to the initial phases, implying the formation of LiBH 4 and LiCl upon cooling to RT [START_REF] Arnbjerg | Structure and Dynamics for LiBH4-LiCl Solid Solutions[END_REF][START_REF] Kharbachi | Borohydride-based Solid-state Electrolytes for Lithium Batteries[END_REF].

Hauback and co-workers studied further the pseudo-ternary LiBH 4 -LiCl-P 2 S 5 system (Fig. 11a) and showed possible incorporation of Cl -in the structure when small amounts of P 2 S 5 are added.

Using the electrolyte composite (LiBH 4 ) 0.73 •(LiCl) 0.24 •(P 2 S 5 ) 0.03 , a battery-cell is demonstrated in the presence of TiS 2 electrode and Li metal anode (Fig. 11b). Furthermore, to monitor the (de)lithiation processes an operando SR-XRD has been demonstrated. Thanks to the low scattering of the bulk electrolyte (Fig. 11c), the expansion of TiS 2 can be explicitly observed which occurs only in c direction due to the layered structure of the material [START_REF] El Kharbachi | Pseudo-ternary LiBH4•LiCl•P2S5 system as structurally disordered bulk electrolyte for all-solid-state lithium batteries[END_REF], and no changes in the bulk electrolyte itself can be detected. The operando SR-XRD setup in transmission mode points to a mean of fast and high resolution structural characterization of a borohydride-based solid-state battery.

Mg(BH 4 ) 2 : dynamics, structure and ion conduction

Current materials considered for all-solid-state batteries (ASSBs) include Na + , Mg 2+ and Ca 2+ -based compounds, while Al 3+ -based compounds are often marginalized due to the lack of suitable electrode materials. This section aims to review the dynamics, structure and ionic conduction process for novel solid-state electrolytes based on complex metal hydrides containing magnesium (Mg) and boron (B), as well as selected solvated derivatives (e.g. ethylenediamine, diglyme and ammonia). The complex Mg(BH 4 ) 2 in addition to the aforementioned solvated derivatives is known to form stable solids, which are very promising Mg ion conductors. Other chemistries for operation in ASSBs are discussed elsewhere [START_REF] Hadjixenophontos | A Review of the MSCA ITN ECOSTORE-Novel Complex Metal Hydrides for Efficient and Compact Storage of Renewable Energy as Hydrogen and Electricity[END_REF][START_REF] Zhao-Karger | Beyond Intercalation Chemistry for Rechargeable Mg Batteries: A Short Review and Perspective[END_REF][START_REF] El Kharbachi | Metal Hydrides and Related Materials. Energy Carriers for Novel Hydrogen and Electrochemical Storage[END_REF].

Rechargeable magnesium batteries are advantageous in many regards when compared to Li-ion battery technology [START_REF] Payandeh | CHAPTER 3 Solid-state Magnesium-ion Conductors[END_REF]. The low electrochemical potential of -2.4 V (Mg/Mg 2+ ) versus a standard hydrogen electrode (SHE) is slightly higher than the electrochemical potential of -3.0 V of Li/Li + . However, the natural abundancy of Mg in the earth crust is more than 2% as compared to 0.0065% for lithium [START_REF] Elia | An Overview and Future Perspectives of Aluminum Batteries[END_REF] and 0.001% of boron [START_REF] Chaussidon | Boron content and isotopic composition of oceanic basalts: Geochemical and cosmochemical implications[END_REF], making Mg not only beneficial from a cost-perspective but as its mining may involve also less geopolitical challenges that may arise from the water intensive production of Li in South America [START_REF] Vaalma | A cost and resource analysis of sodium-ion batteries[END_REF]. Furthermore, Mg is non-toxic, easily manipulated and can be handled in air [START_REF] Gregory | Nonaqueous Electrochemistry of Magnesium: Applications to Energy Storage[END_REF][START_REF] Aurbach | The study of reversible magnesium deposition by in situ scanning tunneling microscopy[END_REF]. Another advantage for Mg is its non-dendritic formation during plating on the metal [START_REF] Jäckle | Microscopic properties of lithium, sodium, and magnesium battery anode materials related to possible dendrite growth[END_REF].

This is a key advantage over Li, in particular for ASSBs, as dendrite growth is one of the main failure mechanisms in ASSBs based on Li, although a recent study suggests a new coating to avoid dendrite formation in lithium ASSBs [START_REF] Yu | A Dynamic, Electrolyte-Blocking, and Single-Ion-Conductive Network for Stable Lithium-Metal Anodes[END_REF]. Consequently, 'pure' Mg metal, with a decent volumetric capacity of 3833 mA•cm -3 , could be used as safe and reliable anode material, although, the appearance of Mg dendrites was suggested recently [399]. The benefits of bivalent Mg-ion migration is also still debated in literature and it has also hampered developments of suitable cathode materials [START_REF] Zhao-Karger | Beyond Intercalation Chemistry for Rechargeable Mg Batteries: A Short Review and Perspective[END_REF].

Since the discovery of Mg batteries by Aurbach et al. [START_REF] Aurbach | Prototype systems for rechargeable magnesium batteries[END_REF] in 2000, research in this field has expanded enormously [START_REF] Zhao-Karger | Beyond Intercalation Chemistry for Rechargeable Mg Batteries: A Short Review and Perspective[END_REF] and even borohydrides were found to have significant conductivities of the metallic cations. With the discovery of high-ionic conductivity in hexagonal-LiBH 4 at ~115 °C [START_REF] Matsuo | Lithium superionic conduction in lithium borohydride accompanied by structural transition[END_REF], much effort has been placed into the research of borohydrides for solid-state electrolytes [START_REF] Hadjixenophontos | A Review of the MSCA ITN ECOSTORE-Novel Complex Metal Hydrides for Efficient and Compact Storage of Renewable Energy as Hydrogen and Electricity[END_REF]. to have a Mg-ion conductivity of up to σ = 6•10 -5 S•cm -1 at 343 K [START_REF] Roedern | Magnesium Ethylenediamine Borohydride as Solid-State Electrolyte for Magnesium Batteries[END_REF]. The synthesis was merely based on ball milling techniques and thermal treatment, while the structure of Mg(en) 1 (BH 4 ) 2 has not been reported so far. This work was expanded to further solvated derivatives, such as diglyme, with measured conductivities of σ = 2 •10 -5 S•cm -1 at 350 K of Mg(BH 4 ) 2 -diglyme 0.5 [START_REF] Burankova | Dynamics of the Coordination Complexes in a Solid-State Mg Electrolyte[END_REF]. Amides also play its role among these conductors and in 2014, Mg(BH 4 )(NH 2 ) was shown to be a solid-state Mg ion conductor [START_REF] Higashi | A novel inorganic solid state ion conductor for rechargeable Mg batteries[END_REF]. More recently, a conductivity of σ = 3•10 -6 S•cm -1 at 373 K was reported for (Mg-B-N-H)-system (of unknown stoichiometry) based on Mg(BH 4 )(NH 2 ) and a correlation to an amorphous phase was postulated [START_REF] Ruyet | Investigation of Mg(BH4)(NH2)-Based Composite Materials with Enhanced Mg2+ Ionic Conductivity[END_REF]. A very recent study even suggests a conductivity of σ =3.3•10 -4 S cm -1 at T = 353 K for Mg(BH 4 ) 2 •NH 3 as well as a new approach to explain the conductivity in these materials [START_REF] Yan | The Mechanism of Mg2+ Conductivity in Ammine Magnesium Borohydride Promoted by a Neutral Molecule[END_REF]. Most of the aforementioned authors reported that some amorphous phase, possibly amorphous Mg(BH 4 ) 2 , has a beneficial influence on the conductivities. This fact is not unheard of and amorphization has also been mentioned to be advantageous for higher ionic conductivity in material classes based on Li 3 PS 4 , which can be identified as glassy solid-state electrolytes [START_REF] Liu | Anomalous high ionic conductivity of nanoporous β-Li3PS4[END_REF][START_REF] Mizuno | High lithium ion conducting glass-ceramics in the system Li2S-P2S5[END_REF]. Therefore, it seemed helpful to investigate the influence of this amorphous phase on the conduction process in complex borohydrides combined with solvated derivatives.

Recently, amorphous-Mg(BH 4 ) 2 has been investigated [START_REF] Heere | Dynamics of porous and amorphous magnesium borohydride to understand solid state Mg-ion-conductors (accepted)[END_REF] by QENS, EIS and PDF analysis. The QENS data depicted as the scattering function S(Q, ∆E) are shown in Fig. 12. These data were used to explore the differences in internal dynamics between the crystalline -phase (blue circles) and the amorphous Mg(BH 4 ) 2 -phase (red squares) at 310 K. In general, QENS is used to analyse dynamic processes such as diffusion or jump rotationsmainly of hydrogen containing species due to the high incoherent cross section of hydrogen. These motions are visible via a broadening around the elastic line which is defined at an energy transfer ∆E = 0 meV.

The data depicted in Fig. 12 show that the quasi-elastic and the inelastic contribution are strongly reliant on the local structure. The -phase exhibits hardly any stochastic motion or quasi-elastic polymorphs of Mg(BH 4 ) 2 also exhibit distinct vibrational spectra explored by inelastic neutron scattering [START_REF] Dimitrievska | Structure-dependent vibrational dynamics of Mg (BH 4) 2 polymorphs probed with neutron vibrational spectroscopy and first-principles calculations[END_REF]. An overview of structure and dynamic investigations in borohydrides by neutron scattering techniques can be found from [START_REF] Lohstroh | Structure and Dynamics of Borohydrides Studied by Neutron Scattering Techniques: A Review[END_REF].

In comparison to alkali metal borohydrides, two points are noteworthy. First, in contrast to the alkali metal borohydrides, no order-disorder transition has been reported for the alkaline-earth borohydrides.

Especially in LiBH 4 , this transition is strongly connected to the enhanced Li + conductivity and especially the reorientational freedom of the [BH 4 ] units. Second, in agreement to the findings for alkali metal borohydrides, enhanced metal-ion conduction, here of the Mg 2+ cations, has been shown experimentally in Mg(BH 4 ) 2 based compounds, which might also be coupled to the rotational mobility of the [BH 4 ] tetrahedra. These findings encourage further research activities towards solid-states conductors for Mg-ASSBs [START_REF] Mohtadi | Magnesium Borohydride: From Hydrogen Storage to Magnesium Battery[END_REF][START_REF] Roedern | Magnesium Ethylenediamine Borohydride as Solid-State Electrolyte for Magnesium Batteries[END_REF][START_REF] Su | A novel rechargeable battery with a magnesium anode, a titanium dioxide cathode, and a magnesium borohydride/tetraglyme electrolyte[END_REF][START_REF] Su | Magnesium borohydride-based electrolytes containing 1-butyl-1-methylpiperidinium bis (trifluoromethyl sulfonyl) imide ionic liquid for rechargeable magnesium batteries[END_REF].

Dedicated characterization tools

5.1.

Experimental methods for thin films

Neutron Reflectometry

Neutron reflectometry is a non-destructive experimental method that utilizes the way neutrons are reflected by sufficiently large (> 100 mm 2 ) and flat surfaces to obtain structural information about the composition, roughness and thickness of thin films and other layered samples with layer thicknesses of 3 -200 nm and with sub-nanometre resolution [START_REF] Fritzsche | Neutron reflectometry. Neutron Scattering and Other Nuclear Techniques for Hydrogen in Materials[END_REF][START_REF] Dura | Nanolayer analysis by neutron reflectometry[END_REF]. In a neutron reflectometry experiment, the sample is illuminated by a neutron beam under a small angle (θ < 5°). Subsequently, the reflectivity of the sample is measured by determining the relative amount of neutrons reflected by the sample as a function of the momentum transfer of the neutron upon reflection, . As Q depends on both the neutrons wavelength λ and the incident angle , Q can be varied by changing either one of these parameters. While using a monochromatic neutron beam in combination with a varying incident angle is most straightforward, measuring at a fixed angle is typically more convenient for kinetic studies. In this method, the time-of-flight method, a pulsed white neutron beam is used, and the neutrons wavelength is discriminated by utilizing the fact that the neutron's velocity is wavelength dependent.

Subsequently, the measured reflectogram is fitted to a simulated one based on a structural model, yielding information about the composition (e.g. hydrogen) content, thickness and roughness of all the layers of the thin film.

Neutron reflectometry is especially a powerful technique to study thin films of metal hydrides. The reflectivity of neutrons by a material is determined by the scattering length density (SLD),

i.e. a material property that depends on the atomic number density N of isotope i and the isotope-dependent scattering length b (a well-known quantity). Owing to the large and negative scattering length of hydrogen (b H = -3.7 fm), neutrons are, in contrast to X-rays, sensitive to hydrogen.

As neutrons are also highly penetrating, it allows for a complex sample environment and, thus, in-situ studying of the hydrogenation (kinetics) of thin films and is able to determine hydrogen concentration profiles in the direction normal to the film's surface.

Neutron reflectometry has for example widely been used to study magnesium-based thin films.

Fritzsche et al. [START_REF] Fritzsche | Neutron reflectometry study of hydrogen desorption in destabilized MgAl alloy thin films[END_REF] and Kallisvaart et al. [START_REF] Kalisvaart | Effect of alloying magnesium with chromium and vanadium on hydrogenation kinetics studied with neutron reflectometry[END_REF] used neutron reflectometry to study the hydrogenation kinetics and structural response of Mg 1-y Al y and other alloy thin films, determining the doping dependence of the kinetics and amount of hydrogen absorbed during hydrogenation. Dura et al. [START_REF] Dura | Porous Mg formation upon dehydrogenation of MgH2 thin films[END_REF] studied palladium-capped magnesium thin films and discovered that once the film hydrogenated from Mg to MgH 2 , the film retained its 25% increase in thickness upon desorption to Mg by incorporating voids. The formation of a porous film explains the different absorption kinetics after the initial exposure to hydrogen. Bannenberg et al. [START_REF] Bannenberg | Impact of Nanostructuring on the Phase Behavior of Insertion Materials: The Hydrogenation Kinetics of a Magnesium Thin Film[END_REF] studied a thin film of magnesium sandwiched between two titanium layers and capped with palladium. In this model system, the hydrogenation of α-MgD x to the insulating β-MgD 2-x phase is governed by a nucleation-and-growth mechanism in which domains can grow of up to millimetres in size [START_REF] Mooij | Nucleation and growth mechanisms of nano magnesium hydride from the hydrogen sorption kinetics[END_REF][START_REF] Mooij | Hysteresis and the role of nucleation and growth in the hydrogenation of Mg nanolayers[END_REF]. By combining optical transmission measurements with neutron reflectometry, they showed that the hydrogen solubilities of both the α-MgD x and the β-MgD 2-x deviated considerably from the solubility limits in bulk magnesium during the phase transformation. It suggests that the enhanced kinetics of phase transformations in nanostructured systems is not only the result of reduced length scales but also caused by the enlarged solubility in the parent phases.

Nuclear Reaction Analysis

Nuclear Reaction Analysis (NRA) uses a nuclear reaction between hydrogen atoms and an incident ion beam to depth-dependent probe the hydrogen concentration of flat samples [START_REF] Fritzsche | Neutron reflectometry. Neutron Scattering and Other Nuclear Techniques for Hydrogen in Materials[END_REF][START_REF] Wilde | Hydrogen detection near surfaces and shallow interfaces with resonant nuclear reaction[END_REF]. Typically, a flat sample is irradiated with a monochromatic beam of 15 N ion that are accelerated to a specific energy.

Subsequently, the ion beam may react with the 1 H atoms present in the sample, forming an excited 16 O nucleus that subsequently relaxes to the 12 C ground state while emitting an α and γ(4.44 MeV) particle:

15 N + 1 H  16 O  12 C + α + γ(4.44 MeV) (17)
As such, the amount of detected gamma's with this specific energy originating from the sample is thus an indirect measure of the number of hydrogen atoms present in the sample.

The probability (cross-section) that the 1 H( 15 N,αγ) 12 C reaction occurs has a strong dependence on the energy of the 15 N ion, showing a narrow resonance peak around 6.385 MeV with a width of 1.8 keV.

As the chance that a reaction occurs outside this energy window (off-resonance) is 10 4 times lower, one may under most conditions neglect this probability. As such, one can probe the amount of hydrogen present on the surface of a material by accelerating the 15 N beam to 6.385 MeV. In order to obtain depth-dependent concentration profiles of the sub-surface hydrogen in the material, one accelerates the beam to energies slightly exceeding the resonance energy. As the ions penetrate into the sample they experience an energy loss of about 1-4 keV/nm for most materials. Hence, by scanning the incident's beam energy, a depth-dependent hydrogen concentration can be obtained as the energy of the 15 N beam matches the resonance energy at different depths. Typically, a 1-5 nm depth resolution is realised while relatively low (> 100 ppm) hydrogen concentrations can be detected for depths up to 2-4 μm. Similar to neutron reflectometry, NRA requires the samples to be flat in order to obtain a good depth resolution.

As NRA measurements are typically performed under ultra-high vacuum conditions to minimize discharges and interactions of the beam with gaseous atoms, performing measurements in which the sample is exposed in-situ to varying hydrogen pressures is challenging and can typically only be conducted at relatively low hydrogen pressures (< 1 mbar [START_REF] Blässer | Atoms. In-situ hydrogen charging of thin Nb films and depth profiling with the 1H (15N, αγ) 12C nuclear reaction[END_REF], usually much lower pressures). To this end, NRA has mainly been used to study the presence of hydrogen at surfaces, interfaces and absorbed within layers of materials that are stable under (ultra)high vacuum conditions (see Wilde and Fukutani [START_REF] Wilde | Hydrogen detection near surfaces and shallow interfaces with resonant nuclear reaction[END_REF] for a recent comprehensive review of applications).

Hydrogenography

Hydrogenography utilizes the fact that the optical transmission of metal hydrides changes upon hydrogenation to study the properties of thin films or arrays of nanoparticles deposited on an optically transparent substrate [START_REF] Gremaud | Hydrogenography: An optical combinatorial method to find new light-weight hydrogen-storage materials[END_REF]. In a typical hydrogenography measurement, the optical transmission as a function of time is monitored at a set temperature while the (partial) hydrogen pressure can be varied stepwise. As such, pressure-optical transmission-isotherms (PTIs) can be obtained. Similar to measure pressure-composition-isotherms, these optical transmission measurements allow the detection of hysteresis and the determination of the enthalpy and entropy of formation through Van 't Hoff's equation: [START_REF] Ovshinsky | A nickel metal hydride battery for electric vehicles[END_REF] where is the equilibrium pressure, the standard pressure, R the gas constant, T the absolute temperature and and are the enthalpy (kJ mol -1 H 2 ) and entropy (JK -1 mol -1 H 2 ) of the hydrogenation reaction, respectively. Indeed, by measuring the optical transmission of a thin film metal hydride as a function of the hydrogen pressure for different temperatures, the temperature dependence of can be determined at a given optical transmission (i.e. halfway the plateau).

Subsequently, from plotting versus the inverse of temperature, (slope) and -(intercept) can be obtained. In this analysis, it is assumed that the experimental conditions (temperature, pressure) at which the optical transmission is the same, correspond to the same hydrogen-to-metal ratio of the metal hydride layer.

Hydrogenography is not confined to materials showing a metal to insulating transition. Indeed, metals such as titanium, hafnium and palladium can also be studied as long as their layer thickness is sufficiently thin (< 100 nm). In addition, it is important to realize that the thermodynamics of thin films may deviate from bulk materials owing to clamping of the film to the support as well as from the increased surface-to-volume ratios.

Generally speaking, a detailed understanding of the changes induced by the hydrogenation of the material on the optical transmission is required to relate the measured changes in the optical transmission to the difference in the hydrogen-to-metal ratio. As such, additional measurements as e.g.

neutron reflectometry measurements need to be performed to translate the changes in optical transmission to the hydrogen-to-metal ratio of the sample. In practice, however, it turns out that the natural logarithm of the relative transmission with respect to the reference (unloaded) state, i.e.

Ln(T/T ref )

, is often proportional to the hydrogen-to-metal ratio. While it directly follows from Lambert-Beer's Law that such a relation is expected for a two phase system with varying fractions of the two phases [START_REF] Bannenberg | Metal Hydride Based Optical Hydrogen Sensors[END_REF], this relation has also been observed experimentally for solid solutions [START_REF] Bannenberg | Metal Hydride Based Optical Hydrogen Sensors[END_REF][START_REF] Bannenberg | Direct Comparison of PdAu Alloy Thin Films and Nanoparticles upon Hydrogen Exposure[END_REF][START_REF] Bannenberg | Optical hydrogen sensing beyond palladium: Hafnium and tantalum as effective sensing materials[END_REF][START_REF] Boelsma | Hafnium-an optical hydrogen sensor spanning six orders in pressure[END_REF][START_REF] Prinz | Combined light and electron scattering for exploring hydrogen in thin metallic films[END_REF][START_REF] Palm | In Situ Optical and Stress Characterization of Alloyed PdxAu1-x Hydrides[END_REF]. It suggests that the effect of hydrogen sorption on the optical properties of the metal hydride is independent of the hydrogen concentration.

Hydrogenography has some distinct advantages over more conventional methods to study the properties of metal hydrides. The enhanced hydrogen (de)sorption kinetics of thin films over bulk materials allows for much faster measurements. In addition, hydrogenography facilitates the simultaneous measurement of a large number of samples, either by multiple samples with dimensions of typically 10 x 10 mm 2 at the same time, or by examining large area films with compositional gradients. These gradients can for instance be obtained by co-deposition from two, three or more offcentred metal sources in a magnetron sputtering set-up [START_REF] Gremaud | Chemical short-range order and lattice deformations in Mg(y)Ti(1-y)H(x) thin films probed by hydrogenography[END_REF][START_REF] Gremaud | Hydrogenography of PdHx thin films: Influence of H-induced stress relaxation processes[END_REF].

Knudsen effusion method for gaseous phase of hydrides

The development of suitable solid-state hydrogen storage and ion conducting hydride materials must pass through the characterization and thermodynamic evaluation of the specific gas phase, being present above a condensed phase, and taking in consideration the following experimental facts:

-When the quantification of partial pressures is possible, the range of low partial pressures corresponds to an ideal gas solution, i.e. the compositions and the partial pressures are identical; this is often the case up to at least 1 Pa pressure and far from the critical point;

-The quantities of material involved in the evaporation flows required for a gas phase measurement are much smaller than those that can exist in the condensed phase, and the disturbances caused by the measurement can therefore be limited;

-The identification of the composition of a gaseous phase of multi-component systems using a pressure determination method with adjacent analytical analyzer (e.g. MS, gas chromatography, etc.) can give the information of adsorption/desorption mechanisms.

The Knudsen effusion (KE) method [START_REF] Knudsen | Die Gesetze der Molekularströmung und der inneren Reibungsströmung der Gase durch Röhren[END_REF][START_REF] Carlson | The Knudsen effusion method[END_REF][START_REF] Cater | The effusion method at age 69: current state of the art[END_REF], in combination for instance with a mass spectrometer, allows the measurement of the vapor pressure in the gaseous phase between 10 -6 -10 Pa until 600 K.

In the simplest thermodynamic application of the KE method, the vapor, composed of a single species, is in equilibrium with its congruently evaporating condensed phase, and flows from the sample surface to the isothermal container and then through a small orifice into an evacuated space at high vacuum.

Since the vapor has a mean free path that is longer than the orifice diameter, the effusing molecules behave like a well-defined molecular beam of atoms or molecules moving in nearly collision-free trajectories with particles distribution that are easily calculable from gas kinetic theory. The Knudsen equation relates the total vapor pressure p above the sample to the mass effusion rate by:

where Δm is the mass loss during the time Δt at the temperature T of the experiment, R the gas constant, M the mass of a molecule in the vapor, s the orifice area and C a correction coefficient, the Clausing factor, [START_REF] Santeler | New concepts in molecular gas flow[END_REF] that quantitatively predicted the departure from the ideal cosine distribution (thin edge orifices) for beams effusing from non-ideal orifices (usually cylindrical or conical orifices).

Coupling the KE method with MS [START_REF] Drowart | High-temperature mass spectrometry: Instrumental techniques, ionization cross-sections, pressure measurements, and thermodynamic data (IUPAC Technical Report)[END_REF][START_REF] Margrave | The characterization of high-temperature vapors[END_REF][START_REF] Grimley | The Characterization of High-Temperature Vapors[END_REF] has the attractive features of high sensitivity and resolution under high vacuum conditions, that is especially a useful tool for the qualitative and quantitative detection of gaseous species. By heating or by milling a sample (solid, liquid, or gaseous) in the Knudsen cell, a molecular beam is formed. This effused flow is ionized, for example by bombarding it with electrons into an ionization chamber. This may cause some of the sample's molecules to breaks into charged fragments or simply become charged without fragmenting. Then, all ions formed by different ionization processes are accelerated by an electric field and then they are separated by a magnetic and/or an electric field according to their mass-to-charge ratio. A Faraday cup or a secondary electron multiplier is used for ion current measurements. The temperature of the sample in the Knudsen cell is measured either with an optical pyrometer or with a thermocouple (Fig. 13a).

Knudsen cell temperatures and intensities of ion currents related to gaseous species are the quantities measured in the course of an investigation by KE-MS.

Partial pressures of species i, p i , at temperature T are defined by the basic mass spectrometric relation [START_REF] Chatillon | Thermodynamic and physico-chemical behavior of the interactions between Knudsen-effusion-cells and the systems under investigation: analysis by high temperature mass spectrometry[END_REF]:

p i •S i = I (i,j) •T ( 20
)
where I (j,i) is the sum of intensities of the ion currents j originating from molecule i, where S i is the sensitivity of the apparatus and T the temperature of the neutral species (i.e. of the Knudsen cell at the time of its evaporation). Partial pressures are obtained by calibration that is generally performed for each measurement separately in order to determine the sensitivity S i that includes several factors as:

where G is a geometric factor involving the solid angle between the Knudsen orifice and the source aperture defining the useful molecular beam, is the ion transmission factor in the spectrometric analyser, is the total ionisation cross section of the molecule i at E ionization energy, is the efficiency of the detector and is the isotopic abundance known or calculated from the constituent atoms of the ion.

There are only a very limited number of studies on the hydrides gaseous phase owing to complexities of the experimental set-up. From metal hydrides, various studies about the gaseous phase showed some aspects:

-Quantitative determination of gaseous species in the binary systems as H-M (Li,B,…) showed that H 2 (g) is the major one, followed by gaseous hydrides such as MH(g), MH 2 (g), or polymers such as M 2 H 2 (g). The relative importance of these gaseous hydrides depends on the composition of the condensed phase: for hydrogen rich composition side of a metal hydride it is H 2 (g) which predominates and on the M side are the metal gaseous hydrides;

-When varying the composition, especially for the M rich side, some liquid phase may appear (this is the case in Li-H system for Li(l)-LiH(s) side) [START_REF] Veleckis | Lithium-lithium hydride system[END_REF];

-Determination of the composition of the gas phase by MS may present difficulties especially for the measurement of H 2 (g), i.e. a so-called permanent gas that is subject to multiple reflections between the two compartments furnace and source, and even in the ionization chamber. It is then recommended to use a restricted collimation [START_REF] Morland | High-temperature mass spectrometry using the Knudsen effusion cell. I-Optimization of sampling constraints on the molecular beam[END_REF] equipped with an adequate molecular beam shutter [START_REF] Froment | Determination of B and C activities in the non-stoichiometric B4-xC compound by the multiple Knudsen-cell Mass Spectrometric method[END_REF][START_REF] Heyrman | Improvements and new capabilities for the multiple Knudsen cell device used in high-temperature mass spectrometry[END_REF] and an in situ liquid nitrogen cooled ionization chamber [START_REF] Tmar | Refinement of the vapour pressures in equilibrium with InP and InAs by mass spectrometry[END_REF];

-Materials for building the effusion cell must be chemically compatible with the condensed hydride, but must also remain impervious (especially with temperature) to hydrogen (avoid diffusion through the walls) so as not to disturb the measurements;

-Handling the sample and loading into the effusion cell and then the cell into the mass spectrometer must have all adapting facilities to carry the sample under air inert conditions. In 1966, Baylis et al. [START_REF] Baylis | Mass Spectrometric Investigation of the Pyrolysis of Boranes. IV. Diborane1[END_REF] used a mass spectrometer equipped with a Knudsen cell, introducing diborane (B 2 H 6 (g)) at low pressure to observe its pyrolysis. From the spectral analysis, the borane (BH 3 ) spectrum was identified at temperatures above 650 K as well as the production of H 2 (g) molecule associated with a dark deposit (probably Li and B) in the Knudsen cell.

For more complex systems as H-M I (Li,…)-M III (B,…), Züttel et al. [START_REF] Züttel | Hydrogen storage properties of LiBH4[END_REF] observed the desorption peak of H 2 (g) during temperature ramps up to 100 °C in LiBH 4 using a mass spectrometer in an ultra-high vacuum (UHV) chamber that is associated with structural transformation of LiBH 4 . Indeed, temperature programmed desorption method (i.e. very slow T ramp type as 5 °C min -1 ) can be used to monitor the different steps of desorption and to give a mechanism of evaporation kinetics. Particularly, the peaks (which in fact more resemble bumps-like) observed as a function of temperature in the evolution of the H 2 + ion (it could be the same for MH + ) correspond to different desorption energies evaluated from the bumps summit. These energies are H-solid breaking bonds and are not the enthalpies of formation for gaseous species. These studies are different from the gas phase study (aimed to acquire thermodynamic data about gaseous species) performed with the KE method, although the latter method may also be used for a desorption kinetics experiment.

An understanding of the processes that occur upon heating LiBH 4 is important for its practical use.

Zhinzhin et al. [START_REF] Zhizhin | Modern aspects of the chemistry of complex boron and aluminum hydrides[END_REF] studied the influence of mechanical activation on the thermal decomposition of LiBH 4 . They observed that the mechanical activation noticeably enhances the reactivity of lithium tetrahydroborate. Two mass spectrometric studies [453,454] have been reported that monitor the gaseous species during LiBH 4 decomposition/vaporization where H 2 is the most important species before and after the mechanical treatment. Gas evolution from a treated sample starts at lower temperature and strongly increases at the second melting step. However, Zhinzhin et al. [START_REF] Zhizhin | Modern aspects of the chemistry of complex boron and aluminum hydrides[END_REF] notices that while the temperature maxima of gas desorption are the same for both studies, in [453] there is a hydrogen excess, whereas in [454] measurements are performed during continuous pumping. Using an airtight Knudsen cell made of stainless steel with controlled opening [START_REF] Violet | A special reactor coupled with a high-temperature mass spectrometer for the investigation of the vaporization and cracking of organometallic compounds[END_REF] coupled with a mass spectrometer, El Kharbachi [START_REF] Kharbachi | Study of complex solid-phase reactions for hydrogen storage[END_REF] measured the total vapor pressure of LiBH 4 solid phase and reported its variation as a function of the inverse temperature from 373 to 573 K (Fig. 13b). The derived enthalpy of reaction, considering the main equilibrium LiBH 4 ↔ LiH + B + 2H 2 , is in agreement with the compiled value in NIST-databased, the latter being based on the experiments of Davis et al. [START_REF] Davis | The Heats of Formation of Sodium Borohydride, Lithium Borohydride and Lithium Aluminum Hydride[END_REF],

using bomb calorimetry in 1949.

Summary and perspectives

Solid-state hydrogen storage. As hydrogen gas at ambient conditions has very low volumetric energy density, effective storage of hydrogen is crucial for its implementation as an energy carrier for stationary and mobile applications. One of the methods of hydrogen storage being developed for the last few decades utilizes hydrogen-rich chemical compounds as solid-state stores. Both simple and complex hydrides are potentially interesting basic materials for this purpose, however, the latter are able to fulfill the required gravimetric and volumetric hydrogen capacity for much broader range of chemical composition. As thermodynamic and kinetic factors limit the performance of chemical hydrogen stores, continuous efforts are still required to obtain cost-effective materials with high storage efficiency. Although advances have been made in the last decades, the prospective advantages of hydrogen storage in metal hydrides faded partly away owing to the rapid development of alternative technologies as Li-ion batteries and gaseous hydrogen storage in lightweight high-pressure vessels.

However, the potential materials for solid-state hydrogen storage are still under development, and various metal hydrides may be unbeaten in some aspects of hydrogen processing. The palladium-based purifying membranes are a classic example here.

The Ti-V based alloys have been known for decades, and many studies have been conducted. The thermodynamic properties of the Ti-V hydrides can be tuned by changing the atomic ratio of the Ti/V and the addition of other alloying elements such as Fe, Cr, and Mn. The hydrogen sorption kinetics is known to be fast, especially when the alloy is nanostructurally prepared. The hydrogen desorption proceeds at least three steps involving phase-structural changes depending on the alloying elements.

However, desorption temperature and hydrogen capacity are the trade-off variables because the addition of alloying elements, in general, decreases the hydrogen capacity of the Ti-V based hydrides.

Since the Ti-V alloys can be tuned in rather large ranges, the application of the Ti-V alloys ranges from low temperatures such as for stationary hydrogen storage or medium temperature such as in hydrogen separation processing. The challenge for further development will be optimizing the hydrogen storage capacity while meeting the desorption temperature. Intermetallic compounds, as TiFe, present renewed interests owing to their low cost and highly tunable hydrogenation properties as concerned equilibrium pressure and temperature close to room temperature, well adapted to electrolyzer output pressures. The productive literature on TiFe H-sorption properties and their possible adjustment by elemental substitutions make a consolidated scientific base for moving toward real applications and prototype systems, especially for stationary energy storage. However, though excellent volumetric capacity, the moderate gravimetric hydrogen capacity of these metal hydrides make their use for mobile application difficult. Nevertheless, the absence of critical raw material in their formulation, their fast kinetics and good cycling properties make them a perfect choice for integrated systems in the production, storage and use of green hydrogen and for energy balancing in smart grids. Future efforts should be focused on the collection of hydrogenation properties and characterization of these materials in scaled-up systems, to move forward the exploitation of TiFebased compounds in real applications and into the market, implementing and diffusing the hydrogen economy locally and globally.

HEA is the new classification for alloys usually containing five or more elements owning interesting mechanical properties. In recent years, these alloys have demonstrated promising properties towards hydrogen absorption as solid-state storing materials. With only a dozen or more of reported articles, some bcc HEAs have demonstrated superior hydrogen uptake as compared to conventional bcc alloys.

Additionally, it has been reported that HEA can rapidly absorb hydrogen at room temperature conditions, reaching full capacity within minutes at low equilibrium pressures. Another interesting feature is the single-step reaction of the hydrogenation process, bcc  (pseudo)fcc. However, the main drawback is the desorption that needs high temperature and vacuum. The main characteristic(s) that grant HEA these properties remains unclear and more research is needed in the future to fully understand these systems and propose a material design strategy able to respond to all criteria for use in a practical storage device.

As metal borohydrides contain relatively the largest amount of hydrogen, a remarkable expansion of their chemistry has been observed recently. This was possible due to significant developments in synthesis of new compounds via mechanochemical and solvent-mediated routes of general applicability. Such methods can be utilized for preparation of broad range of products and allow for fast screening of hydrogen storage properties of numerous borohydrides based on main-group and transition metals. Besides hydrogen storage, borohydrides have been tested as potential luminescent or magnetic materials, and as source of potentially useful decomposition products. The latter includes LiZn 2 (BH 4 ) 5 utilized as a convenient, small-scale source of diborane, or various borohydrides tested as precursors for refractory borides.

Optical sensors. Metal hydride based hydrogen sensors are considered to be an effective way to accurately sense hydrogen. Although many different designs exist, all these sensors have in common that they utilize the propensity of metal hydrides to hydrogenate when exposed to a partial hydrogen pressure, which, in turn, results in volumetric expansion of the metal hydride and a change in its optical and electronic properties. By probing one of these properties, the hydrogen pressure in the environment of the metal hydride sensors can thus be determined. Although resistivity based hydrogen films are already considered for long time, hydrogen sensors with an optical readout, based for example on optical transmission/reflectivity or LSPR, have the particular advantage that no currents are required in the sensing area, thus eliminating a potential safety hazard. Palladium has been the most frequently used material in metal-hydride based sensors owing to its large contrast, its modest sensing range and its propensity to dissociate molecular hydrogen at room temperature. However, the first-order metal-to-metal hydride phase transition in palladium has the distinct disadvantage that the sensor output is highly hysteretic. To this end, various other materials, including palladium alloys and transition metals have been considered to obtain sensing response free of hysteresis. Another challenge is to improve the resistance to chemical species other than hydrogen of the sensors, being key for the large-scale implementation of the metal-hydride based sensors.

Electrochemical energy storage. Conversion-type metal hydride anodes remain as potential candidates for LIBs, owing to their high capacity (MgH 2 ~2000 mAh g -1 ) compared to the commercialized graphite (~372 mAh g -1 ). Despite the efforts made to improve the long-term cyclability of MgH 2 anode, the reversibility of the composite Mg/2LiH is still a challenge. The capacity fading is more pronounced when a carbonate-liquid electrolyte is used. However, this

electrode system shows a real potential in solid-state battery with light-weight LiBH 4 -based electrolytes [START_REF] Zeng | Metal hydride-based materials towards high performance negative electrodes for all-solid-state lithium-ion batteries[END_REF][START_REF] El Kharbachi | MgH2-CoO: a conversion-type composite electrode for LiBH4-based all-solid-state lithium ion batteries[END_REF]. Owing to the high capacity of MgH 2 and the beneficial aspects of ASSBs, it is expected that research would consider further these systems in order to reach high energy density electrochemical storage prototypes [START_REF] El Kharbachi | Exploits, advances and challenges benefiting beyond Li-ion battery technologies[END_REF]. Some computational techniques such as quantum DFT and AIMD are helping the acceleration of research and development of better metal hydrides for electrochemical electrode materials especially combining with the cutting-edge materials genome methods such as high-throughput computing, data mining and so on.

The halide substitution in LiBH 4 has been a breakthrough in suppressing the phase transition and maintaining the properties of the high temperature phase at room temperature, i.e. ionic conductivity. This discovery has been followed by the study of interface issues in lithium cells, so that to ensure cycling stability of the cells; however, these cells need to be operated at high temperatures (50-100 °C), which may speed up the appearance of surface degradation aspects. Working below 50 °C became a necessity and a key factor to target larger application fields. A series of materials based on LiBH 4 -LiX-P 2 S 5 (X = halogen) have promising ionic conductivity (~10 -3 S cm -1 ) and chemical properties for lithium batteries. Their different treatments mostly end up with a dominant amorphous structure due to the presence of P 2 S 5 . Efforts are needed to clarify their structure and phases' stability, and how the change in the structural properties will affect ions conduction and many other relevant properties for batteries (mechanical, chemical, thermal etc). On the other hand, ASSBs based on magnesium are still a very young scientific topic and have exciting times ahead. Many challenges, especially technical challenges that are related to the design of low cost materials with high ionic conductivities and, at the same time, low environmental impact, still have to be considered. Mg(BH 4 ) 2 and solvated derivatives, such as ethylenediamine, diglyme and ammonia, tend to form stable compounds. Conductivities were reported such as σ = 6•10 -5 S•cm -1 at 343 K of Mg(en) 1 (BH 4 ) 2 [START_REF] Roedern | Magnesium Ethylenediamine Borohydride as Solid-State Electrolyte for Magnesium Batteries[END_REF], σ = 2 •10 -5 S•cm -1 at 350 K of Mg(BH 4 ) 2 -diglyme 0.5 [START_REF] Burankova | Dynamics of the Coordination Complexes in a Solid-State Mg Electrolyte[END_REF] and σ =3.3•10 -4 S cm -1 at T = 353 K for Mg(BH 4 ) 2 •NH 3 [START_REF] Yan | The Mechanism of Mg2+ Conductivity in Ammine Magnesium Borohydride Promoted by a Neutral Molecule[END_REF]. The later study even postulated a new approach for the Mg diffusion with dihydrogen bonds being involved [START_REF] Yan | The Mechanism of Mg2+ Conductivity in Ammine Magnesium Borohydride Promoted by a Neutral Molecule[END_REF].

In most of the aforementioned materials amorphous Mg(BH 4 ) 2 was formed as a byproduct [START_REF] Roedern | Magnesium Ethylenediamine Borohydride as Solid-State Electrolyte for Magnesium Batteries[END_REF][START_REF] Burankova | Dynamics of the Coordination Complexes in a Solid-State Mg Electrolyte[END_REF].

Its influence on the conduction properties was reviewed here, as it was postulated that the amorphous phase is aiding to increase Mg-ion conductivity. EIS data stated that the conductivity of the amorphous phase is indeed ~2 orders of magnitude higher than the as-received γ-Mg(BH 4 ) 2 at 353 K. From PDF analysis and the reported similar local building blocks, it was suggested that also similar conduction pathways are at play. QENS data indicated a higher fraction of activated rotations in the amorphous sample. Thus it is assumed that the conduction process in amorphous Mg(BH 4 ) 2 is supported by rotating [BH 4 ] units. Upon crystallization at 373 K, the number of rotations decreases as well as the conductivity does. In general, Mg(BH 4 ) 2 with all its derivatives will make an important contribution to future Mg-based ASSBs, while its amorphous structure should not be neglected in those investigations [START_REF] Heere | Dynamics of porous and amorphous magnesium borohydride to understand solid state Mg-ion-conductors (accepted)[END_REF].

Characterization tools. Several specialized characterizations tools are at hand to researchers studying thin film and bulk metal hydrides. For thin films, although neutron reflectometry and NRA can both quantify the hydrogen concentrations, neutron reflectometry has the particular advantage that such experiments do not have to be performed under high vacuum conditions. As such, while NRA can typically achieve a higher lateral resolution and detect smaller quantities of hydrogen, neutron reflectometry facilitates the simultaneous determination of the out-of-plane expansion and the hydrogen concentration in thin films in combination with complex sample environment, thus i.e. in the presence of a hydrogen atmosphere. In addition, pressure-transmission-isotherms can be measured with the dedicated technique of hydrogenography. In such experiments, the optical transmission of thin film metal hydrides deposited on a transparent substrate are measured, allowing e.g. the detection of hysteresis and the determination of the enthalpy and entropy of formation through Van 't Hoff's equation. Hydrogenography has the particular advantage over other methods that the fast (de)sorption

Figure 2 .

 2 Figure 2. (a) Phase evolution of γ-Ti 0.8 V 0.2 H 2 during an increase in temperature from 30 to 800 °C in vacuum studied by in-situ SR-XRD. The figure inset shows overlapping γ-(111) and δ-(101) peaks. Small fraction (∼0.5 wt.%) of stable γ hydride was observed up to 580 °C. (b) Evolution of phase composition calculated from powder diffraction data of Ti 0.8 V 0.2 H x by Rietveld refinement method with Rwp = 3.76-10.28 % and Rp = 2.61-7.19 %. δ-hydride formed from γ-hydride at 320 °C and existed up to about 500 °C. The β hydride appeared at about 390 °C. This figure is taken from Ref [110].

Figure 3 .

 3 Figure 3. Distribution of alloying elements after bulk nano-structuring in Ti-V alloys processed with highpressure torsion (HPT) method. (a) to (d) the images indicate increased severity of deformation. This figure is taken from Ref [119].

Figure 4 .

 4 Figure 4.

  3 H/M (2.0 wt.%) up to 20 cycles. As an extension of this work, Montero et al. have very recently shown that the addition of only 10 at.% of Ta into the quaternary Ti-V-Zr-Nb alloy reported earlier improves the hydrogen sorption performances [188]. The absorption/desorption in the quinary Ti 0.30 V 0.25 Zr 0.10 Nb 0.25 Ta 0.10 alloy is reversible and occurs within one step (bcc alloy  fcc dihydride). The capacity is only slightly fading during the first 10 cycles and then stabilizes at around 1.7 H/M (2.2 wt.%) for the next 10 cycles. In a different approach towards the understanding of the hydrogen sorption properties of HEA, Nygård et al. studied a series of bcc alloys with a scaling degree of lattice distortion [179]. Several alloys in the form TiVZr (1-z) NbTa (z) and TiVZr (1+z) Nb were synthesized in which the amount of Zr relative to the alloy ([Zr]/[M]) was controlled in order to tune the lattice distortion. Most of the alloys have a maximum uptake of 1.8-2.0 H/M but no clear relationship is observed between the hydrogen capacity and the lattice distortion of the HEA nor the lattice parameter a. The stability of the hydride increased as [Zr]/[M] increases. Interestingly, alloys with [Zr]/[M] < 12.5 at.% have recovered their initial bcc structure after desorption, while those with [Zr]/[M] > 12.5 at.% have suffered phase separation after one cycle of hydrogen absorption/desorption. The same group reported a second study of various

Figure 6 .

 6 Figure 6. The most important structures for this chapter are shown a) α-Mg(BH 4 ) 2 in space group (SG) P6 1 22 [219], b) β-

  borohydrides, [Cat]RE(BH 4 ) 3 , [Cat] = [n-Bu 4 N], [Me 4 N], [Ph 4 N], and related salts containing magnesium and transition metals, as well as the salts of some WCA, like M[Al{OC(CF 3 ) 3 } 4 ], M = Li -Cs, and other compounds to grow the number of accessible combinations [257,258,260,274-277].Although the synthetic approach discussed here has been originally designed for the mixed-cation borohydrides, it can also be useful for other salts, as exemplified by M(BH 3 NH 2 BH 2 NH 2 BH 3 ), M = Li -Cs, or even Ag 2 S 2 O 8[START_REF] Owarzany | Complete Series of Alkali-Metal M(BH3NH2BH2NH2BH3) Hydrogen-Storage Salts Accessed via Metathesis in Organic Solvents[END_REF][START_REF] Gilewski | Ag2S2O8 meets AgSO4: the second example of metal-ligand redox isomerism among inorganic systems[END_REF]. It turns out that these long-chain M(BH 3 NH 2 BH 2 NH 2 BH 3 ) materials decompose thermally at rather low temperatures of 100-180 o C and respective MBH 4 are found in the solid residue[START_REF] Owarzany | Complete Series of Alkali-Metal M(BH3NH2BH2NH2BH3) Hydrogen-Storage Salts Accessed via Metathesis in Organic Solvents[END_REF][START_REF] Fijalkowski | BH3NH2BH2NH2BH3) -the missing link in the mechanism of the thermal decomposition of light alkali metal amidoboranes[END_REF].

Figure 7 .

 7 Figure 7. (a) Comparison of the powder X-ray diffraction patterns of LiZn 2 (BH 4 ) 5 prepared according to eq. (13) wet, with the sample obtained in the mechanochemical proceduredry. Note significant contamination of the latter by LiCl and ZnCl 2[START_REF] Jaroń | Hydrogen Storage Materials: Room-Temperature Wet-Chemistry Approach toward Mixed-Metal Borohydrides[END_REF], (b) The crystal structures of the metastable Li and Na yttrium borohydride accessible via the solvent-mediated approach utilizing weakly coordinating anions[START_REF] Jaron | Facile Formation of Thermodynamically Unstable Novel Borohydride Materials by a Wet Chemistry Route[END_REF]. This figure is adapted from Refs[START_REF] Jaroń | Hydrogen Storage Materials: Room-Temperature Wet-Chemistry Approach toward Mixed-Metal Borohydrides[END_REF][START_REF] Jaron | Facile Formation of Thermodynamically Unstable Novel Borohydride Materials by a Wet Chemistry Route[END_REF].

[ 252 ,

 252 292,293]. The borohydride phases containing Ln•••HBH•••Ln bridges propagating in 3 dimensions of the crystal lattice, i.e. α-and β-Ln(BH 4 ) 3 , where Ln = Gd, Tb, Dy, Ho, Er, Tm, as well as selected mixed-cation borohydrides composed of isolated Ln 3+ ions forming [Ln(BH 4 ) 4 ] -complexes, namely

Figure 8 .

 8 Figure 8. Schematic illustration of different metal hydride based hydrogen sensors. (a) In resistivity based hydrogen sensors, the electric resistivity is measured of e.g. a metal hydride thin film, patterned nanosheet or nanowire to probe the hydrogen pressure. (b) In the micro-mirror sensor, a metal hydride sensing layer is deposited at the end of an optical fibre. Light, which is coupled into the fibre, is partly reflected by this metal hydride sensing layer, and subsequently detected by e.g. a charged-coupled device (CCD). The amount of light reflected is directly related to the hydrogen uptake of the sensing layer, and thus the partial hydrogen pressure in the environment of the sensor. (c) In sensors using the localized surface plasmon resonance (LSPR) peak, an array of nanoparticles is typically illuminated by white light, of which the transmission or reflectivity is measured. Subsequently, the extinction

Figure 9 .

 9 Figure 9. Prediction of properties of various doped Mg 2 NiH 4 using DFT: (a) electrochemical lithium-storage capacity; (b) volume change; (c) average voltage (versus Li + /Li 0 ) ; (d) electronic band gap. This figure is taken from Ref [367].

Figure 10 .

 10 Figure 10. Atomic structures of LiBH 4 (a) at low temperature (o-LiBH 4 ) and (b) high temperature (h-LiBH 4 ); (c)-(g) Phonon density of states of the h-LiBH 4 and o-LiBH 4 phases, and of Li(BH 4 ) 1-x X x (with X= F, Cl, Br, I and x= 0.125, 0.25, 0.375).

Figure 11 .

 11 Figure 11. (a) Preparation procedure of the solid electrolyte (SE); (b) galvanostatic discharge/charge cycling (0.05 C-rate) at 50 °C with TiS 2 electrode; (c) sketch of the experimental set-up (left) and 2D operando SR-PXD patterns (right) of the solid-state battery cell during discharge/charge (partial (de)lithiation of TiS 2 electrode, C/10, 60 °C). Red arrows indicate the reflections with the main changes. This figure is taken from Ref [388].

Figure 13 .

 13 Figure 13. (a) Knudsen cell schema and (b) logarithmic variation of the product I*T (proportional to pressure) for H 2 as a function of the inverse of the temperature for LiBH 4 .

2 I

 2 = 67.4 kJ.mol -1 H 2 NIST-database : ∆ r H = 66.6 kJ.mol -1 H

Table 1 .

 1 The acronyms and abbreviations used across this review.

	AIMD	ab initio molecular dynamics
	ASSB	all-solid-state battery
	bcc	body-centered cubic
	bct	body-centered tetragonal
	CVD	chemical vapor deposition
	DFT	density functional theory
	DMC	dimethyl carbonate
	DME	dimethoxyethane
	DMS	dimethyl sulfide
	DSC	differential scanning calorimetry
	EC	ethylene carbonate
	EIS	electrochemical impedance spectroscopy
	en	ethylenediamine
	fcc	face-centered cubic
	FTIR	Fourier-transform infrared spectroscopy
	hcp	hexagonal close packed
	HEA	high-entropy alloy
	h-LiBH 4	hexagonal LiBH 4
	HP	high-pressure
	HPT	high-pressure torsion
	IDT	inter-digitated transducer
	KE	Knudsen effusion
	LIB	Li-ion battery
	(L)SPR	(localized) surface plasmon resonance
	MEMS	microelectromechanical-system
	MH	metal hydride
	MOF	metal-organic framework
	MPEA	multi-principal element alloy
	MS	mass spectrometry
	NMR	nuclear magnetic resonance
	NRA	nuclear reaction analysis
	o-LiBH 4	orthorhombic LiBH 4
	P	pressure
	PCT	pressure-composition isotherm
	PDF	pair distribution function
	PEM	proton-exchange membrane
	PMMA	poly(methylmethacrylate)
	PTFE	polytetrafluoroethylene
	PTI	pressure-optical transmission isotherm
	QENS	quasi-elastic neutron scattering
	RE	rare earth
	RMB	rechargeable magnesium battery
	SE	solid electrolyte
	SHE	standard hydrogen electrode
	SLD	scattering length density
	SQUID	superconducting quantum interference device
	SR-XRD	synchrotron radiation -powder X-ray diffraction
	T	temperature
	TEM	transmission electron microscopy
	THF	tetrahydrofuran
	TOFTOF	high-resolution time-of-flight spectrometer
	UHV	ultra-high vacuum

Table 3 .

 3 Summary of DFT data results for lattice parameter, interatomic distances, optimized equilibrium cell volume and total energy of o-LiBH 4 , h-LiBH 4 and Li(BH 4 ) 1-x X x , (with X= F, Cl, Br, I and x= 0.125, 0.25, 0.375) in hexagonal phase, respectively.

			Parameters (Å)		Interatomic distances (Å)		Volume	Total energy
	System	a	b	c	Li-H	B-H	H-H	Li-B	Li-X	(Å 3 /unit	(Ry/f.u.)
										cell)
	o-LiBH4	7.21	4.36	6.61	1.98	1.22	1.97	2.47		206.01	-25.98
	h-LiBH4	4.19	4.19	7.30	1.89	1.22	1.99	2.48		106.06	-25.97
				h -LiBH4 (supercell 2x2x1)
	Li(BH4)0.875F0.125	4.08	4.08	6.25	1.90	1.22	1.96	2.46	2.03	90.40	-35.43
	Li(BH4)0.75F0.25	4.08	4.08	6.120	1.92	1.22	1.99	2.48	1.94	84.24	-40.17
	Li(BH4)0.625F0.375	4.01	4.01	5.84	1.93	1.22	1.99	2.50	1.80	79.73	-28.47
	Li(BH4)0.875Cl0.125	4.12	4.12	6.62	1.87	1.22	1.95	2.46	2.47	48.57	-30.97
	Li(BH4)0.75Cl0.25	4.13	4.13	6.47	1.91	1.22	1.97	2.46	2.45	46.65	-33.47
	Li(BH4)0.625Cl0.375	4.10	4.10	6.40	1.92	1.22	1.98	2.46	2.42	45.59	-28.78
	Li(BH4)0.875Br0.125	4.14	4.14	6.66	1.90	1.22	1.96	2.46	2.60	99.11	-30.71
	Li(BH4)0.75Br0.25	4.11	4.11	6.63	1.87	1.22	1.96	2.45	2.60	98.16	-33.08
	Li(BH4)0.625Br0.375	4.17	4.10	6.36	1.91	1.22	1.97	2.45	2.65	98.08	-25.98
	Li(BH4)0.875I0.125	4.17	4.17	6.75	1.88	1.22	1.99	2.48	2.77	101.53	-31.58
	Li(BH4)0.75I0.25	4.23	4.23	6.75	1.89	1.22	1.97	2.48	2.72	104.31	-34.38
	Li(BH4)0.625I0.375	4.28	4.28	6.82	1.90	1.22	1.96	2.48	2.73	107.99	-28.34

  The first study of Mg ion conductivity in β-Mg(BH 4 ) 2 was byMatsuo et al. by first-principles molecular dynamics simulations [402]. They determined that due to the close distance of the [BH 4 ] - anions to Mg, the Mg-ions cannot move freely. The authors suggested a partial substitution of [BH 4 ] - by AlH 4 -to increase distances and open the possibility of Mg 2+ conductivity in β-Mg(BH 4 ) 2 . However, since then, the approaches of forming electrolytes can be roughly divided into liquid and solid-state fractions. The first liquid electrolyte was developed by Mohtadi et al., whose study demonstrated the possibility to employ Mg(BH 4 ) 2 dissolved in THF and DME in a secondary magnesium battery [403]. Subsequently, Mg(BH 4 ) 2 was more and more replaced, by Mg(BR 4 ) 2 in DME (R=-OCH(CF 3 ) 2 ). So far, this complex shows the highest reported electrochemical stability window of 4.3 V and is stable in air with a Mg-ion conductivity of σ = 0.011 S•cm -1 in a solution of 0.3 M DME [404]. The second category of solid-state Mg-ion conductors was introduced by Roedern et al. [405]. They synthesized a new compound from Mg(BH 4 ) 2 and ethylenediamine (C 2 H 8 N 2 , 'en'), which was reported

Acknowledgements

TJ's contribution received support from Polish National Science Centre (UMO-2016/21/D/ST5/01966). MW, WW, AS, KF and WG acknowledge the support from the HYDRA project of the Polish National Science Center, NCN (2014/15/B/ST5/05012). PO's work has been financed from the project "Diamond Grant" of the Polish Ministry of Science and Higher Education (DI2015 008845, contract No. 322268). The research was carried out with the use of CePT infrastructure financed by the European Unionthe European Regional Development Fund within the Operational Programme "Innovative economy" for 2007-2013 (POIG.02.02.00-14-024/08-00). ZQ's computational contribution received support from the Natural Science Foundation of China (51801113). EMD, FC, ML and MB acknowledges support through the HyCARE project. This project has received funding from the Fuel Cells and Hydrogen 2 Joint Undertaking (JU) under grant agreement No 826352. The JU receives support from the European Union's Horizon 2020 research and innovation programme and Hydrogen Europe and Hydrogen Europe Research. This work contributes to the research performed at CELEST (Center for Electrochemical Energy Storage Ulm-Karlsruhe).

contribution around the elastic line at an energy transfer ∆E=0 meV. In contrast, the amorphous phase reveals a significant broadening around this elastic line, which is indicative of higher rotational mobility of the [BH 4 ] moieties [START_REF] Heere | Dynamics of porous and amorphous magnesium borohydride to understand solid state Mg-ion-conductors (accepted)[END_REF]. This figure is taken from Ref [START_REF] Lohstroh | TOFTOF: Cold neutron time-of-flight spectrometer[END_REF].

EIS data show that the conductivity of amorphous Mg(BH 4 ) 2 at 80 °C is almost two orders of magnitude higher compared to as received γ-Mg(BH 4 ) 2 . As the PDF analysis already showed similar structural local building blocks, it was suggested that also the conduction pathway might be similar.

However, analysis of the QENS measurements demonstrated that the number of activated [BH 4 ] rotation is significantly higher in the amorphous phase. Under the assumption of similar conduction pathways, it seems likely that those activated rotations support the movement of Mg-ions. This mechanism is already known as the paddle wheel effect [START_REF] Jansen | Volume Effect or Paddle-Wheel Mechanism-Fast Alkali-Metal Ionic Conduction in Solids with Rotationally Disordered Complex Anions[END_REF], and these findings are supported by the fact that when the paddle wheel mechanism stops at the crystallization temperature (i.e. the rotating [BH 4 ] become inactive) the conductivity drops to the same level as crystalline γ-Mg(BH 4 ) 2 [START_REF] Heere | Dynamics of porous and amorphous magnesium borohydride to understand solid state Mg-ion-conductors (accepted)[END_REF].

All structures of Mg(BH 4 ) 2 mentioned above consist of the same building blocks as shown in Fig. 6 ( § 2.4) and Fig. 12. The structural variety found for Mg(BH 4 ) 2 is caused by the ground state energies, which are almost degenerate [START_REF] Ozolins | First-principles prediction of a ground state crystal structure of magnesium borohydride[END_REF][START_REF] Voss | Structural stability and decomposition of Mg (BH4) 2 isomorphs-an ab initio free energy study[END_REF][START_REF] Zhou | First-principles determination of the structure of magnesium borohydride[END_REF]. Previous investigations of the dynamics in Mg(BH 4 ) 2

were centred on the and -modification [START_REF] Silvi | A quasielastic and inelastic neutron scattering study of the alkaline and alkaline-earth borohydrides LiBH4 and Mg(BH4)2 and the mixture LiBH4+ Mg(BH4)2[END_REF][START_REF] Silvi | Hydrogen dynamics in [small beta]-Mg(BH4)2 on the picosecond timescale[END_REF], while the focus here was on the local environment of the BH 4 tetrahedra in the γ-polymorph and its amorphous counterpart, which both have very similar linear H 2 BH 2 -Mg -H 2 BH 2 chains (compare inset in Fig. 12) [START_REF] Heere | Dynamics of porous and amorphous magnesium borohydride to understand solid state Mg-ion-conductors (accepted)[END_REF]. The different kinetics of thin films over bulk materials allows for much faster measurements and that many different samples can be measured simultaneously. As a method of choice for the gaseous phase of hydridebased materials, the Knudsen effusion method coupled with mass spectrometry is a useful tool for both qualitative and quantitative detection of gaseous species, in addition to the study of the kinetic processes and adsorption/desorption mechanisms of solid state materials and thermodynamic properties (enthalpy of formation, Gibbs free energy, ...), in order to contribute to model phase diagrams of complex systems such as metal borohydrides and their mixtures which are still lacking in literature [START_REF] El Kharbachi | A thermodynamic assessment of LiBH4[END_REF][START_REF] Dematteis | A thermodynamic investigation of the LiBH4-NaBH4 system[END_REF][START_REF] Dematteis | Phase diagrams of the LiBH4-NaBH4-KBH4 system[END_REF][START_REF] Dematteis | Exploring Ternary and Quaternary Mixtures in the LiBH4-NaBH4-KBH4-Mg(BH4)2-Ca(BH4)2 System[END_REF].