Volume 4 fuel cells and hydrogen technology 4 06 – hydrogen storage liquid and chemical

Volume 4 fuel cells and hydrogen technology 4 06 – hydrogen storage liquid and chemical Volume 4 fuel cells and hydrogen technology 4 06 – hydrogen storage liquid and chemical Volume 4 fuel cells and hydrogen technology 4 06 – hydrogen storage liquid and chemical Volume 4 fuel cells and hydrogen technology 4 06 – hydrogen storage liquid and chemical

4.06 Hydrogen Storage: Liquid and Chemical P Chen, Dalian Institute of Chemical Physics, Dalian, China © 2012 Elsevier Ltd All rights reserved 4.06.1 4.06.2 4.06.3 4.06.4 4.06.4.1 4.06.4.1.1 4.06.4.1.2 4.06.4.2 4.06.4.2.1 4.06.4.2.2 4.06.5 4.06.5.1 4.06.5.2 4.06.6 References Introduction Physical Hydrogen Storage Metal Hydrides Chemical Hydrides Ammonia Borane Homogeneous catalytic dehydrogenation of AB Solid-state dehydrogenation of AB Amidoboranes and Derivatives Alkali and alkaline earth amidoboranes Derivatives Complex Hydrides Borohydrides Amide–Hydride Systems Pending Issues 157 157 159 159 160 162 163 164 164 165 167 167 169 173 174 4.06.1 Introduction Hydrogen-based energy systems offer a potential solution to the ever-increasing demand for sustainable energy systems Although over the long term, the ultimate technological challenge is large-scale hydrogen production from renewable sources, a critical practical issue is how to store hydrogen efficiently and safely, particularly for onboard hydrogen fuel cell vehicles [1] Tremendous efforts have been devoted to the research and development of systems that can hold sufficient hydrogen in terms of gravimetric and volumetric densities to allow fuel cell vehicles to achieve a satisfactory driving range and in the meantime exhibit acceptable charging/discharging kinetics, safety, and cost (see Table 1) [2, 3] Hydrogen can be stored by either physical or chemical means For physical storage, the conventional options are compressed hydrogen gas and cryogenic adsorption, that is, liquid hydrogen For chemical storage, the hydrogen molecule can be dissociated either homolytically or heterolytically and bonds with other elements to form hydrides The hydrogen uptake and release can be either reversible or irreversible depending on the thermodynamic parameters of the corresponding starting and final materials During the last decade of exploration and study, the scope of candidate hydrogen storage materials has expanded considerably, from conventional metal hydrides, such as LaNi5 and MgH2, to complex and chemical hydrides [3–7] and from activated carbon to carbon nanotubes and to metal organic frameworks (MOFs) [8–18] The employment of advanced synthetic routes has also allowed the physical state of the storage materials to change from being bulk crystalline to amorphous and to nano structures [16, 19, 20] Advanced theoretical simulations also have an increasing impact not only on the description of the physical properties of known materials but also on the prediction of novel structures and reaction paths [21–23] A variety of promising storage systems are under intensive investigations Systems of high hydrogen content are the focus of ongoing studies because they allow more space for material modification and optimization A comprehensive survey by Thomas, Sandrock, and Bowman, as shown in Figure 1, lists over 40 material candidates that are being actively investigated Among them, high surface area porous materials and nitrogen- and boron-containing hydrides are the most studied systems [24, 25] In this chapter, a short survey on existing hydrogen storage techniques will be presented Emphasis will be given to chemical and complex hydrides that have been under intensive research since 2005 4.06.2 Physical Hydrogen Storage Physical storage of hydrogen is normally achieved either under high pressure or at cryogenic temperatures To store high-pressure hydrogen, a compressed hydrogen gas tank made of aluminum or composite materials wrapped with carbon or glass fiber to ensure light weight and high strength is used The hydrogen energy stored in the compressed gas tank increases with an increase in pressure, but not in a directly proportional manner Compressed hydrogen at 350–700 bar has been well developed and adopted in prototype fuel cell vehicles The compression of H2 will consume ∼10–15% of the energy stored, and the size of the tank holding ∼4 kg of H2 is still too large to directly compare with a gasoline tank There have been considerable efforts in the development of lightweight tanks to store hydrogen up to 1000 bar in recent years Even with considerable progress, the tanks are somewhat too expensive (∼$15 kWh−1) to be practically viable [26] A recent demonstration on storing compressed hydrogen at 77 K suggested an alternative method with improved characteristics especially in gravimetric and volumetric storage densities It is, however, of practical importance to take a step forward in reducing the system cost and increasing the energy efficiency (Table 2) Comprehensive Renewable Energy, Volume doi:10.1016/B978-0-08-087872-0.00414-5 157 158 Hydrogen Storage: Liquid and Chemical The vehicle performance targets Table Storage parameter Units 2010 2015 Ultimate System gravimetric capacity kWh kg−1 (kg H2 kg−1) kWh l−1 (kg H2 l−1) $ kWh−1 ($ kg H2−1) $ gge−1 at pump 1.5 (0.045) 0.9 (0.028) (133) 2–3 1.8 (0.055) 1.3 (0.040) (67) 2–3 2.5 (0.075) 2.3 (0.070) TBD °C °C cycles % of mean at % confidence atm atm −30/50 −30/80 1000 90/90 FC/35 ICE 100 −40/60 −40/85 1500 99/90 FC/35 ICE 100 −40/60 −40/85 1500 99/90 FC/35 ICE 100 kg H2 min−1 (g s−1) kW −1 s s %H2 4.2 3.3 (1.2) (1.5) 0.02 0.02 5 0.75 0.75 99.99 (dry basis) scc h−1 Meets or exceeds applicable standards (g h−1) kg H2−1 0.1 System volumetric capacity Storage system cost Fuel cost Durability/operability Operating ambient temperature Min/max delivery temperature Cycle life (1/4 tank to full) Cycle life variation Min H2 delivery pressure Max delivery pressure Charging/discharging rates System fill time (for kg H2) Minimum full-flow rate Start time to full flow Transient response (10% to 90%) Fuel purity Environmental health and safety Permeation and leakage Toxicity Safety Loss of usable H2 2–3 2.5 (2.0) 0.02 0.75 0.05 0.05 Source: http://www.hydrogen.energy.gov/annual_review11_report.html 16 Open symbols denote new mat’ls for FY2009 14 Material capacity must exceed system targets chemical hydrides CH Regen Required 12 Observed H2 capacity (wt.%) solid AB (NH3BH3) New DOE system targets AB/cat MD C-foam Ultimate sorbents IRMOF-177 PCN-12 C aerogel carbide-derived C B/C bridged cat./IRMOF-8 MOF-74 MD C-foam bridged cat./AX21 −200 −100 H2 sorption temperature (�C) Mg(BH4)2(NH3)2 Mg(BH4)2 Mg(BH4)2(NH3)2 AB ionic liq AIB4H11 Li−AB 10 metal hydrides AIH3 Ca(BH4)2 LiBH4/MgH2 LiBH4/CA MgH2 Li3AIH6/LiNH2 LiMgN 1,6 naphthyridine Li3AIH6/Mg(NH2)2 2015 Ca(BH4)2/2LiBH4 Mg−Li−B−N−H liq AB/cat LiNH2/MgH2 NaAIH4 LiMn(BH4)3 Mg(BH4)(AIH4) NaMn(BH4)4 PANI Na2Zr(BH4)6 Ti-MOF-16 PANI M-doped CA PANI 100 M−B−N−H 200 300 Temperature for observed H release (�C) Figure A survey of hydrogen storage materials http://www.hydrogen.energy.gov/annual_review09_report.html 400 Hydrogen Storage: Liquid and Chemical Table 159 Parameters of compressed and liquid H2 storage Gravimetric energy content (MJ kg−1) Volumetric energy content (MJ m−3) Storage techniques Storing energy (kJ kg−1) Spent energy/ stored energy Compressed H2 (350 bar) Compressed H2 (700 bar) Liquid H2 12 264 0.10 8.04 2492 14 883 0.12 7.20 3599 42 600 0.36 16.81 3999 With reference to http://www.hydrogen.energy.gov/annual_review11_proceedings.html Liquid hydrogen is another option to store hydrogen onboard Comparatively, the energy density is nearly times higher than that of the 700 bar compressed H2; however, the energy cost to liquefy hydrogen reaches 30% or more of the actual hydrogen energy stored There is also continual hydrogen loss when stored onboard (namely boil-off) due to thermal conduction, convection, and thermal radiation One of the other drawbacks is the use of expensive multilayered vacuum superinsulated vessels Another important practical issue is the ‘cooling-down’ losses during refilling of liquid hydrogen at gas stations The entire transfer line between the liquid H2 source and the vehicle tank system has to be cooled down to about −253.8 °C, and therefore, additional H2 evaporation occurs Clearly, these losses cannot be neglected and remain significant There have been a few comprehensive reviews on compressed and liquid hydrogen published recently [26, 27], and the commercial employment of these techniques is still an open issue However, currently, while most of the chemical storage systems to be discussed later remain at a research stage, compressed and liquid H2 storage remain as choices for demonstration and evaluation purposes Porous materials are prone to adsorb hydrogen physically; however, due to the weak interaction of H2 and sorbents, cryogenic conditions typically have to be applied Materials with large surface areas and proper pore size of 2–3 nm are capable of adsorbing up to wt.% of H2 [12–14, 28, 29] Representative sorbent materials include carbon materials [8–11, 30–32], MOFs [12, 16, 17, 33–35], and conjugated polymers [14] A special note on hydrogen storage on MOFs was triggered by breakthroughs in material design and synthesis [12] The interesting bonding nature of the metal and organic link creates a variety of pore structures and active centers, and the nature of the interaction of H2 and the active centers has been a hot topic of study It is obviously of scientific interest to further the research in this area; however, cryogenic adsorption in general is an energy-consuming process and has, in general, relatively low volumetric hydrogen storage density; hence, the remainder of this chapter will focus on alternative solid-state forms of H2 storage 4.06.3 Metal Hydrides The homolytic dissociation of H2 into atomic H followed by diffusion of the H in the lattice of metals, especially transition metals and alloys, leads to the formation of metal hydrides [2, 36] Table shows a list of conventional metal hydrides that have been extensively studied in the past For example, the H content in terms of the volumetric hydrogen density of LaNi5H6 is 115 kg m−3, which is higher than that of compressed hydrogen and liquid hydrogen However, transition metals and their alloys have relatively low gravimetric hydrogen densities (normally < wt.%) A recent work by Matsunaga et al [37] on hybridizing metal hydrides with a high-pressure tank gives a certain level of promise for the improvement of gravimetric density In addition to the search for new multicomponent metal alloys, current research also focuses on the reduction in size of Mg-based materials to enhance kinetics in dehydrogenation [16, 20] 4.06.4 Chemical Hydrides There are a vast variety of natural and manmade chemical hydrides including H2O, NH3, alcohol, boranes, and hydrocarbons The H–X (where X refers to O, N, B, C, etc.) bond is significantly stronger than that of a typical H–M bond of most of the interstitial metal hydrides [1–3] Chemical hydrides that have been investigated for the purpose of hydrogen storage are mainly those Table Structure and hydrogen storage properties of typical metal hydrides Type of metal hydrides Metal Hydrides Structure Mass% of hydrogen Peq, T AB5 AB3 AB2 AB A2B LaNi5 CaNi3 ZrV2 TiFe Mg2Ni LaNi5H6 CaNi3H4.4 ZrV2H5.5 TiFeH1.8 Mg2NiH4 Hexagonal Hexagonal Hexagonal Cubic Cubic 1.4 1.8 3.0 1.9 3.6 bar, 298 K 0.5 bar, 298 K 10−8 bar, 323 K bar, 303 K bar, 555 K 160 Hydrogen Storage: Liquid and Chemical H H H H H H N N B B H H H H H Li Figure Molecular structures of AB and lithium amidoborane Dihydrogen bonding ~ 1.8 Å Figure Packing of AB molecules in the crystal Yellow, blue, and white balls are N, B, and H, respectively Table Summary of dehydrogenation of AB, alkali and alkaline earth amidoboranes, and their derivatives Reactions Conditions a Temperature ( °C) H (mass %) nNH3BH3 → (NH2BH2)n + nH2 → (NHBH)n + 2nH2 NH3BH3 → NBH6−x + xH2 Solid SBA-15 Ir-based catalyst/THF Ni-NHC catalyst/THF Solid In THF Solid Solid Solid Solid Solid Solid 70–200 50–100 Ambient temperature 60 75–95 40–55 80–90 90–245 80–200 50–250 75–300 50–300 12.9 6.7 6.7 16.5 10.9 LiNH2BH3 → LiNBH + 2H2 NaNH2BH3 → NaH + BN + 2H2 Ca(NH2BH3)2 → Ca(NBH)2 + 4H2 Sr(NH2BH3)2 → Sr(NBH)2 + 4H2 LiNH2BH3NH3BH3 → LiN2B2H + 5H2 Ca(NH2BH3)2·2NH3 → Ca(BN2H)2 + 6H2 Mg(NH2BH3)2·NH3 → MgB2N3H + 6H2 a 7.5 8.0 6.8 14.7 8.9 11.8 For solid-state dehydrogenation, most of the materials are under molten or semi-molten state containing H–B, H–N, and H–O bonds [4, 38, 39] A distinctive feature of the most investigated chemical hydrides is the coexistence of both protonic and hydridic H atoms Representative entities are NH3BH3 (ammonia borane, AB) [7, 24, 40–45], metal amidoboranes (MAB) [46–52], and H2O–borohydride systems Figure shows the molecular structures of AB and lithium amidoborane H bonded with N has positive charge, which is opposite to H bonded with B When a crystal is formed, the shortest distance between these two H atoms is found to be less than twice the van der Waals radius of H (see Figure 3) The exceptionally high chemical potentials for the combination of H− and H+ to molecular H2 and the formation of strong B–N or B–O bond are likely to be the driving forces for the dehydrogenation For most of the chemical hydrides, dehydrogenation is an exothermic process The increase in stability of the dehydrogenated product is the result of the formation of strong B–N or B–O covalent bond In this section, the research activities on the development of AB and amidoboranes for hydrogen storage will be reviewed Table presents the conditions applied in the dehydrogenation of AB, alkali and alkaline earth amidoboranes, and their derivatives 4.06.4.1 Ammonia Borane � mm and AB, first synthesized in 1955 [53], is a plastic crystalline solid adopting a tetragonal crystal structure with space group I4 lattice parameters of a = b = 5.240 Å and c = 5.028 Å at room temperature [54, 55] As shown in Figure 2, this molecular crystal is Hydrogen Storage: Liquid and Chemical 161 stabilized by dihydrogen bonding between H(B) and H(N) The crystal melts at ∼100 °C and decomposes to hydrogen (1 equiv.) between 70 and 112 °C to yield polyaminoborane (PAB, [NH2BH2]n) according to eqn [1] Subsequently, [NH2BH2]n decomposes with an additional equiv hydrogen loss, over a broad temperature range around 150 °C, forming amorphous polyiminoborane (PIB, [NHBH]n) and a small fraction of borazine ([N3B3H6]), according to eqns [2] and [3], respectively The decomposition of [NHBH] to boron nitride (BN) occurs at temperatures in excess of 500 °C This final step is not considered practical for hydrogen storage due to high temperatures needed for hydrogen release Thermodynamic analyses and theoretical calculations show that hydrogen release from either AB or PAB or PIB is an exothermic process, revealing the irreversibility of hydrogen desorption from these materials nNH3 BH3 sị ẵNH2 BH2 n sị ỵ nH2 gị ẵ1 ẵNH2 BH2 n sị ẵNHBH n sị ỵ nH2 gị ẵ2 ẵNH2 BH2 n sị ẵN3 B3 H6 n = lị ỵ nH2 gị ẵ3 Figure presents some of the likely forms of products from releasing the first and second equivalent molecules of H2 from AB The structure and composition of the product vary with the conditions applied during dehydrogenation As an example, on catalytic dehydrogenation of AB by iridium (Ir) catalyst in tetrahydrofuran (THF), crystalline PAB is formed [43] A recent report from He et al [68] demonstrated the formation of crystalline linear PAB in the FeB-catalyzed solid-state dehydrogenation of AB at ∼60 °C However, in most cases, the solid product is essentially amorphous and is a mixture of linear, cyclic, branched, and cross-linked B–N structures Although it has an exceptionally high hydrogen content, AB has to overcome a few drawbacks to be practically viable The first two challenges are the mass production of AB and energy-efficient regeneration of the used fuel [56, 57], while the kinetic-borne dehydrogena­ tion of AB and the coproduction of unwanted gaseous products (such as borazine and NH3) are to be alleviated Moreover, severe material foaming in the dehydrogenation is also problematic Tremendous efforts have been devoted to these issues since the first report on the dehydrogenation of AB for hydrogen storage by Wolf et al [40], among which investigations on catalytic modification of AB or dispersing AB into porous materials attract significant attention [43–45, 58–71] Dehydrogenation of AB in ionic liquids shows improved kinetics in comparison with neat AB [42] Moreover, a number of intermediates and products have been identified by in situ nuclear magnetic resonance (NMR) and the density functional theory–gauge including/invariant atomic orbital (DFT–GIAO) calculations As shown in Figure 5, different states of hybridization (sp2 or sp3) and bonding environments (with H or N) have chemical shifts ranging from −26.9 to +39.3 ppm There are a few comprehensive reviews in this area to which the readers may like to further refer [24, 25, 56, 57] Figure Molecular structures of possible dehydrogenation products [56] 162 Hydrogen Storage: Liquid and Chemical (a) H H H H H B N B H H H B N H N B H H H H H H N N B H H H H H H H −9.5 ppm −11.4 ppm −23.0 ppm −11.8 ppm (b) H H H H H N B N B H B H H N H H H B N H H −13.8 ppm H H B −5.9 ppm −22.4 ppm H N H (c) H H B H N H H H H H H −10.4 ppm N H −26.9 ppm H H B H B N H N H H B H H −8.8 ppm +8.7 ppm +33.2 ppm +30.4 ppm +39.3 ppm Figure DFT–GIAO calculated 11B NMR chemical shift for possible structures arising from the dehydropolymerization of AB [42] 4.06.4.1.1 Homogeneous catalytic dehydrogenation of AB As summarized by Hamilton et al [57] and Smythe and Gordon [56], a few homogeneous catalysts including Ru-, Ir-, and Ni-based complexes and Lewis acid (B(C6F5)3) have been developed, which are effective in removing 1–2.5 equiv H2 from AB under mild conditions Figure shows the time dependence of H2 evolution from a AB/THF solution with different concentrations of (POCOPf)Ir(H)2 With mol.% of catalyst, ∼1 equiv H2 can be released within A quantitative yield of crystalline PAB was observed [43] The dehydrogenation of AB by the Ni-N heterocyclic carbene (Ni-NHC) complex in a molar ratio of 10:1 shows unprecedented evolution of 2.5 equiv H2 at 60 °C [45] The formation of Ni-NHC is by the reaction of biscyclooctadiene nickel (Ni(cod)2) with Enders’ NHC Theoretical investigation suggests that the formation of the first transition state (highest energy barrier) is through transferring H(N) of AB to ligand carbene, which is different from the β-H elimination of AB in some other cases One way to activate AB by Lewis or Brønsted acid is by attracting a hydridic H(B) from AB to form the initiative cation [H2BNH3]+ [63] As shown in Figure 7, the overall process after the formation of the initiative cation resembles cationic polymerization and dehydrogeneration One equivalent H2 can be removed from AB at ambient temperature Equivalents of H2 O PtBu2 0.5 catalyst = Ir H H O PtBu2 0 10 20 Time (min) Figure Amount of H2 evolved per mole of AB using 0.25 mol.% (●), 0.5 mol.% (▲), and 1.0 mol.% (■) Ir catalyst [43] Hydrogen Storage: Liquid and Chemical H3N BH3 A H2B NH3 + H3NBH3 + H2B NH3 H BH2 163 NH3 H2 + N H2B BHNH3 H H2 ‡ H + N2 H2B H H H H B NH3 Figure Reaction of NH3BH3 with Lewis or Brønsted acid (A) results in the formation of borenium cation Subsequent reaction with another equivalent of NH3BH3 results in the formation of with subsequent expulsion of H2 and concomitant formation of [63] As shown above, the chemistry involved in the catalytic dehydrogenation of AB in solvent is considerably rich and worthy of detailed experimental and theoretical investigations The use of the solvent allows sufficient mobility of both reactant and catalyst but will decrease the energy density of the system It is a subject of system engineering to minimize the side effect of the solvent but retain the efficiency of homogeneous catalysis 4.06.4.1.2 Solid-state dehydrogenation of AB As mentioned in Section 4.06.4.1, the thermal decomposition of solid-state AB is a stepwise process having considerable kinetic barriers at each step Efforts in alleviating the barrier in solid-state dehydrogenation are through dispersing AB into porous substrates [7] and introducing a catalyst in the material (solid form) [68] An introductory work by Gutowska et al demonstrated that, when dispersing AB onto porous SBA-15 nanoscaffold, hydrogen started to release at temperatures just above 50 °C and peaked at ∼100 °C, which is considerably lower than that of neat AB Moreover, the formation of borazine was largely depressed [7] Further isothermal testing showed that the dispersed AB presented a significantly shortened induction period and reduced kinetic barrier As shown in Figure 8, equiv H2 can be released at 50 °C within 150 However, for pristine AB, it has to go through a ∼100-min induction period and another 400–500 to remove the same amount of H2 A few successful attempts in using carbon cryogel, lithium catalysis and mesoporous carbon (Li-CMK-3), nano-BN, poly(methyl acrylate) (PMA), and so on, to improve the dehydrogenation properties of solid AB have been reported in the past years [70, 72, 73] Another approach in improving dehydrogenation of AB is via solid-state catalysis through the use of transition metals or alloys He et al reported that, upon the introduction of nano-sized Co- or Ni- or Fe-based catalyst to solid AB via the so-called coprecipitation method, ∼1.0 equiv or wt.% of H2 can be released at 59 °C (shown in Figure 9) [68] It was observed that the presence of the nano-sized catalyst largely depressed the sample foaming and the coproduction of borazine In the meantime, crystalline rather than amorphous PAB was formed (Figure 10), which should be derived from the catalyst-oriented growth of aminoborane and is significantly different from the ion-initiated dehydrogenation 1.0 0.8 0.8 AB:SBA-15 50 �C 0.6 0.6 neat AB 80 �C 0.4 0.4 0.2 0.0 Extent of reaction Relative heat released (arb units) 1.0 0.2 100 200 300 400 500 600 0.0 700 t (min) Figure Scaled exotherms (solid lines) from isothermal differential scanning calorimetry (DSC) experiments that show the time-dependent release of H2 from AB and AB:SBA-15 (1:1 w/w) The area under the curve for neat AB corresponds to ΔHrxn = −21 kJ mol−1, and the area under the curve for AB:SBA-15 corresponds to ΔHrxn = −1 kJ mol−1 The release of hydrogen from AB proceeds at a more rapid rate and at lower temperatures in SBA-15 The dashed line (-) is the integrated signal intensity; (•) is the point at which the reaction is 50% complete [7] 164 Hydrogen Storage: Liquid and Chemical 1.0 Equiv H2 0.8 b 0.6 c 0.4 0.2 a 0.0 10 15 20 25 30 Time (h) Figure Volumetric hydrogen release measurements at 59 °C on the pristine (a), mol.% Co-doped (b), and mol.% Ni-doped (c) AB samples [68] Intensity (a.u.) PAB PABFeB d (Å) Figure 10 XRD patterns of the postdehydrogenated neat AB (80 °C) and 2.0 mol.% Fe-doped AB samples (60 °C) ▼, crystalline linear PAB 4.06.4.2 4.06.4.2.1 Amidoboranes and Derivatives Alkali and alkaline earth amidoboranes As shown in the previous section, various methods have been employed to lower the decomposition temperature of AB through the use of additives and catalysts A different approach has been applied recently in the manipulation of the thermodynamic properties of compounds through chemical alteration to AB, that is, through substituting one H in the NH3 group in BH3NH3 with a more electron-donating element [46, 49] The rationale behind this approach is to alter the polarity and intermolecular interactions (specifically the dihydrogen bonding) of AB to produce a substantially improved dehydrogenation profile Lithium amidoborane (LiNH2BH3) [47, 49, 50, 52, 74, 75], sodium amidoborane (NaNH2BH3) [48, 49, 76], calcium amidoborane [46, 50], and strontium amidoborane [77] have been synthesized, which show substantially different dehydrogenation characteristics with respect to AB itself These alkali and alkaline earth amidoboranes (MABs) were synthesized mainly through the interactions of alkali or alkaline earth metal hydrides (LiH, NaH, CaH2, and SrH2) with AB (see eqn [4]), which lead to the replacement of hydrogen atom of AB by alkali or alkaline earth metals ẵ4 MHx ỵ xNH3 BH3 MNH2 BH3 ị x ỵ xH2 where x = when M is an alkali metal and x = when M is an alkaline earth metal The replacement of the H of the NH3 group in AB by alkali or alkaline earth element results in the alteration of the crystal structure and dehydrogenation property As shown in Figure 11, LiNH2BH3 crystallizes in the orthorhombic space group Pbca with the lattice constants a = 7.112 74(6) Å, b = 13.948 77(14) Å, c = 5.150 18(6) Å, and V = 510.970(15) Å3 The Li–N bond is essentially ionic and the B–H bond length is slightly longer than that in neat NH3BH3 NaNH2BH3 is of identical structure to LiNH2BH3 [49, 50] Ca(NH2BH3)2, on the other hand, is a monoclinic structure with a = 9.100(2) Å, b = 4.371(1) Å, c = 6.441(2) Å, and β = 93.19°(see Figure 12) [50] Li or Na is coordinated with four NH2BH3 groups Ca, on the other hand, sits in the center of an octahedron made of NH2BH3 groups The THF adduct of Ca(NH2BH3)2 was also determined [46] It is interesting to note that, unlike Ca and Sr, no report on the formation of Mg(NH2BH3)2 has appeared in the literature so far The experimental results show that more than 10 and wt.% of hydrogen desorbs exothermically from LiNH2BH3 and NaNH2BH3, respectively, at around 91 °C (Figure 13) [49] Ca(NH2BH3)2, on the other hand, releases ∼8 wt.% H2 in the temperature range of 100–300 °C In all the cases, borazine production is beyond the detection limit of mass spectrometry (MS) The induction period that is associated with the dehydrogenation of pristine NH3BH3 is absent from these amidoboranes, indicating that a different dehydrogenation mechanism is occurring There are a few theoretical and experimental investigations on the dehydrogenation mechanism of these amidoboranes, especially of LiNH2BH3 [52, 75] Kim et al reported that the dehydrogenation of LiNH2BH3 is via abstracting H from the BH3 group by Li [52] An isotopic investigation also evidenced the bimolecular dehydrogenation Hydrogen Storage: Liquid and Chemical 165 Figure 11 Crystal structure of LiNH2BH3 Li, B, N, and H are represented by red, orange, green, and white spheres, respectively [49] c a b 2.87 Å 3Å 3.0 2.466 Å Figure 12 Crystal structure of Ca(NH2BH3)2 Ca, B, N, and H atoms are represented by orange, green, blue, and white spheres, respectively [50] 4.06.4.2.2 Derivatives A number of compounds and complexes derived from amidoboranes have been synthesized since 2009 [51, 78, 79] NH3 and THF are prone to adduct to amidoboranes Amidoborane ammoniates can be synthesized either by exposing amidoboranes (such as Ca(NH2BH3)2) to NH3 or by reacting amides or imides with AB [51] In general, amidoborane ammoniates were found to release H2 at mild temperatures Bowden et al reported that when NH3BH3 reacts with LiNH2, H2 rather than NH3 was released in the temperature range ∼25–300 °C [79] Chua et al synthesized Ca(NH2BH3)2·2NH3 and Mg(NH2BH3)2·NH3 through the reaction of NH3BH3 with Ca(NH2)2 or MgNH, respectively [51] In both cases, NH3 adducts to metal cations and forms dihydrogen bonding 166 Hydrogen Storage: Liquid and Chemical H content (wt.%) 12 LiNH2BH3 10 NaNH2BH3 BH3NH3 0 10 Time (h) 15 20 Figure 13 Hydrogen evolution from heating neat NH3BH3, LiNH2BH3, and NaNH2BH3 at 91 °C [49] H1 (a) N3 (b) H3 H2 B1 Mg1 N1 N2 2.157 2.104 2.123 1.993 2.129 2.126 B2 H7 H6 H4 H13 H8 H5 H12 H11 H9 H10 c a Figure 14 b Molecular packing and network of N–H⋯H–B dihydrogen bonding in MgAB·NH3 (a) and close contacts around the Mg2+ center (b) (a) Intensity (a.u.) H2 NH3 Borazine H2 content (wt.%) 12 4 No of equiv H2 (b) TPD-MS 0 100 200 300 Temperature (�C) Figure 15 Temperature-programmed desorption (TPD)-MS spectra (a) and volumetric release (b) measurements on Mg(NH2BH3)2·NH3 at the heating rate of (a) and 0.5 °C min−1 (b) with nearby H(B) The shortest (N)H⋯H(B) distance in Mg(NH2BH3)2·NH3, for example, is around 1.92 Å (Figure 14) The difficulty in forming Mg(NH2BH3)2 and the existence of its ammoniate indicate that the unstable crystal of Mg(NH2BH3)2 (probably due to small but dense charged cation (Mg2+) and big anion) can be stabilized by NH3 through the establishment of coordination of a lone pair of N to Mg2+ The thermal decomposition of Mg(NH2BH3)2·NH3 performed in a closed vessel demonstrated a stoichiometric conversion of NH3 and desorption of ∼11.2 wt.% H2 (shown in Figure 15) Hydrogen Storage: Liquid and Chemical 167 As shown above, chemical hydrides are capable of releasing large amounts of H2 at relatively low temperatures However, in most cases, hydrogen desorption is exothermic in nature, showing that these chemicals are kinetically stable Intensive sunshine, impact, impurities in the material, and so on may trigger self-decomposition leading to auto-accelerated dehydrogenation, which may need serious consideration when applied practically 4.06.5 Complex Hydrides Complex hydrides have attracted considerable attention since 1997 when Ti was successfully introduced to NaAlH4 system [6] In the past 13 years, alanates, borohydrides, and amides have been extensively and intensively investigated The following section will mainly report on the progress on borohydrides and amides There have been significant amounts of review work on alanates in the past 10 years [3, 80–83] Table summarizes the systems developed over this period 4.06.5.1 Borohydrides Borohydrides, having a common formula of M(BH4)n, where M refers to the metal element and n the valence of M, have been extensively studied in the past decade Representative systems are LiBH4, Mg(BH4)2, and Ca(BH4)2 Figure 16 shows the structure of LiBH4, Mg(BH4)2, and Ca(BH4)2, respectively LiBH4 crystalizes in an orthorhombic structure (Pnma) at ambient temperature and transfers to a hexagonal structure (P63mc) at 135 °C [84–86] Mg(BH4)2, on the other hand, transfers from a hexagonal structure to an orthorhombic structure at ∼180 °C [87–89] Three different polymorphs of Ca(BH4)2 have been reported up to date [90–92] The structures of low-temperature α-Ca(BH4)2 (orthorhombic, space group Fddd) and high-temperature β-Ca(BH4)2 (tetragonal, P42/m) phases have been resolved [90, 91], and the third phase γ-Ca(BH4)2, of ortho­ rhombic structure Pbca, was also reported [92] Ca(BH4)2 undergoes phase transformation prior to its thermal decomposition Considerable attention has been paid to LiBH4 due to its high hydrogen content (∼18.4 mass%) [39, 85, 86] However, hydrogen desorption from this chemical is highly endothermic (∼67 kJ mol−1) and, thus, requires temperatures higher than 300 °C A few approaches have been introduced recently to destabilize LiBH4 Comparatively, reacting LiBH4 with chemicals, such as LiNH2 [93–96], MgH2 [22, 38, 97, 98], and CaH2 [99–101], can considerably change the overall dehydrogenation thermodynamics due to the formation of more stable products As examples, hydrogen desorption from the LiBH4–2LiNH2 mixture is an exothermic reaction [93, 96]; combination of LiBH4 and MgH2 in a molar ratio of 2:1 leads to ∼25 kJ mol−1 H2 decrease in enthalpy change compared with the pristine LiBH4 [38] As shown in Figure 17, hydrogen desorption from the composite starts at ∼300 °C and rehydrogenation occurs at ∼250 °C The formation of the stable product MgB2 alters the thermodynamics of the dehydrogenation Table Dehydrogenation of borohydrides and amide–hydride combinations Reactions Mass% of H2 Temperature (°C) a Borohydrides 2LiBH4 → 2LiH + 2B + 3H2 2LiBH4 + MgH2 = 2LiH + MgB2 + 4H2 Mg(BH4)2 → MgB2 + 4H2 3Mg(BH4)2·2(NH3) → Mg3B2N4 + 2BN + 2B + 21H2 Ca(BH4)2 → CaH2 + 2B + 3H2 Zn(BH4)2 → Zn + B2H6 + H2 13.6 11.5 14.8 15.9 8.6 2.1 200–550 270–440 290–500 100–400 300–500 90–140 Amide/hydride LiNH2 + 2LiH = Li2NH + LiH + H2 = Li3N + 2H2 CaNH + CaH2 = Ca2NH + H2 Mg(NH2)2 + 2LiH = Li2Mg(NH)2 + 2H2 3Mg(NH2)2 + 8LiH = 4Li2NH + Mg3N2 + 8H2 Mg(NH2)2 + 4LiH = Li3N + LiMgN + 4H2 2LiNH2 + LiBH4 → ‘Li3BN2H8’ → Li3BN2 + 4H2 Mg(NH2)2 + 2MgH2 → Mg3N2 + 4H2 2LiNH2 + LiAlH4 → LiNH2 + 2LiH + AlN + 2H2 = Li3AlN2 + 4H2 3Mg(NH2)2 + 3LiAlH4 → Mg3N2 + Li3AlN2 + 2AlN+12H2 Mg(NH2)2 + CaH2 → MgCa(NH)2 + 2H2 NaNH2 + LiAlH4 → NaH + LiAl0.33NH + 0.67Al + 2H2 2LiNH2 + CaH2 = Li2Ca(NH)2 + 2H2 4LiNH2 + 2Li3AlH6 → Li3AlN2 + Al + 2Li2NH + 3LiH + 15/2H2 2Li4BN3H10 + 3MgH2 → 2Li3BN2 + Mg3N2 + 2LiH + 12H2 10.5 2.1 5.6 6.9 9.1 11.9 7.4 5.0 8.5 4.1 5.2 4.5 7.5 9.2 150–450 350–650 100–250 150–300 150–300 150–350 20b 20b–500 20b–350 20b–500 20b 100–330 100–500 100–400 a b Experimental observation Under ball milling condition 168 Hydrogen Storage: Liquid and Chemical c c a b a b c b a Figure 16 Crystal structures of LiBH4, Mg(BH4)2, and Ca(BH4)2, respectively Green, orange, and blue spheres represent Li, Mg, and Ca (a) 350 (d) 300 (c) 250 200 (b) 150 (a) 100 Temperature (�C) Hydrogen uptake (wt.%) 10 50 0 Time (hr) (c) 400 (b) 300 200 (a) 100 Temperature (�C) Desorbed hydrogen (wt.%) (b) 10 Time (hr) Figure 17 Hydrogenation and dehydrogenation of milled LiH + ½MgB2 with mol.% TiCl3 (a) Hydrogen uptake during heating in 100 bar of hydrogen Curve (a) shows the temperature profile Curve (b) shows the initial uptake of hydrogen Curves (c) and (d) show uptake during the second and third cycles, respectively (b) Desorption following hydrogenation into a closed evacuated volume Curve (a) shows the temperature profile Curves (b) and (c), respectively, show the wt.% of desorbed hydrogen following the initial and second hydrogenation cycles that are shown in part (a) [38] Hydrogen Storage: Liquid and Chemical (a) 169 TG (mass%) −3 −9 −12 as-synthesized h milling h milling −15 (Exo DTA (a.u.) ) (b) −6 300 400 500 600 Temperature (K) 700 800 Figure 18 (a) Thermogravimetry and (b) differential thermal analysis curves of Mg(BH4)2 for as-synthesized and after and h milling [102] There has been a discussion on whether the dehydrogenation is via a stepwise manner, that is, if the first step is via self-decomposition of LiBH4 to LiH, B, and H2 followed by MgH2 + B to MgB2 and H2 Having a hydrogen content of ∼15 mass%, Mg(BH4)2 is another attractive hydrogen storage candidate [102–105] Mg(BH4)2 can be synthesized via a metathesis of LiBH4 and MgCl2 [104–106] At temperatures above 340 °C, it decomposes to hydrogen (see Figure 18) [103]; the first step in dehydrogenation has an enthalpy change of 39.3 kJ mol−1 H2 and an entropy change of 91.3 J mol−1 H2 [106] An easy calculation from eqn [5] will give a temperature of ∼150 °C to desorb 1.0 bar hydrogen Obviously, it is ∼70 °C higher than the operation temperature of the proton exchange membrane (PEM) fuel cell RTlnPị ẳ H S  T ½5Š Ca(BH4)2 has a hydrogen capacity of 11.6 wt.% and a lower hydrogen desorption (thermal decomposition) temperature compared with that of LiBH4 as predicted by thermodynamic analysis based on an ab initio calculation [107] A previous study showed that Ca(BH4)2 desorbs 9.0 wt.% hydrogen at a temperature as high as 770 K, and CaH2 is the only crystalline phase in the solid residue [108, 109] Doping it with Ti or Nb species did not show obvious catalytic effect on decreasing the decomposition temperature [110] Other borohydrides, such as Zn(BH4)2 [72], also decompose to hydrogen; however, these either tend to have poor thermo­ dynamics or create unwanted side products (i.e., B2H6) Nakamori et al demonstrated an interesting relationship between the heat of formation of metal borohydrides and the electronegativities of metal in M(BH4)n (M = Li, Na, Ca, Mg, Zn, etc.) Further investigation shows that the dehydriding temperature of M(BH4)n decreases with the increase of the electronegativity of M [111] It is worth noting that the dehydriding temperature obtained by the temperature-programmed desorption (TPD) technique [103] is not only a measure of the thermodynamic stability of a reactant but also reflects the kinetic barrier associated with the decomposition process It is also worth highlighting that the chemical state of B in dehydrogenated products, which can bond with metals (such as MgB2) or be in elemental or in amorphous B–Hx states, will considerably change the dehydrogenation thermodynamic parameters [38, 112] An amine complex of Mg(BH4)2, that is, Mg(BH4)2·2NH3, was identified [113], which releases hydrogen endothermically at temperatures above 150 °C (see Figures 19 and 20) Although with side gaseous product(s) such as NH3, this new chemical shows certain advantages over AB 4.06.5.2 Amide–Hydride Systems Studies on hydrogen storage over amide–hydride systems were initiated when researchers accidentally noticed that a mixture of metallic lithium and carbon nanotubes pretreated in a purified N2 atmosphere absorbed large amounts of hydrogen at elevated temperatures (see Figure 21) [4] Through X-ray diffraction characterizations, the hydrogenated solid-state sample was found to contain LiNH2, LiH, and the unreacted carbon nanotubes Further investigations revealed that the N2-treated Li–C mixture converted to Li3N and carbon nanotubes The reversible hydrogen storage over Li3N follows reaction [6], and ∼10.5 mass% of hydrogen can be stored [4, 114] Li3 N ỵ 2H2 ẳ Li2 NH ỵ LiH ỵ H2 ẳ LiNH2 ỵ 2LiH ½6Š Thermodynamic analyses showed that hydrogen desorption from LiNH2–2LiH and LiNH2–LiH is endothermic with the heat of desorption of 80 and 66 kJ mol−1 H2, respectively [4] The operation temperature at 1.0 bar equilibrium H2 pressure is above 250 °C for the LiNH2–LiH (1:1 molar ratio) system, which is too high for practical applications 170 Hydrogen Storage: Liquid and Chemical H5c H5a B5 H4d H4c 2.209 Å H5d 1.999 Å 2.400 Å H5b B4 Mg1 H2c 2.149 Å 2.156 Å 2.030 Å H3b H4a N3 N2 H3a H2a H4b 2.145 Å 2.887 Å H3c H2b Figure 19 Molecular structure of Mg(BH4)22NH3 [113] 14 12 wt.% H2 10 Mg(BH4)2 2NH3 Mg(BH4)2 0 50 100 150 200 250 300 Temperature (�C) 350 400 Figure 20 Gas evolution from Mg(BH4)2·2NH3 (magenta line) and Mg(BH4)2 (blue line) The vertical axis calibration is wt.% H2 and is based on the assumption that all the evolved gas is hydrogen [113] 10 Abs Wt.% H2 Des 50 100 150 200 250 300 350 400 450 Temperature (�C) Figure 21 Weight variations during hydrogen absorption and desorption processes over Li3N samples [4] Li3N has been regarded as a superior Li ion conductor The structure of Li3N is illustrated in Figure 22, which consists of Li and Li2N layers, in which Li migrates along the Li2N layer having a relatively low barrier When hydrogen is pumped in, one-third of the Li will be removed from the Li3N structure, which combines with H to form LiH H replaces Li and bonds to N to form Li2NH Further hydrogenation results in the additional exchange between Li in Li2NH and H in H2 and the formation of LiNH2 and LiH Hydrogen Storage: Liquid and Chemical 171 Figure 22 Crystal structure of Li3N N is in blue and Li in purple Due to a poorer electronegativity, H bonded with N is positively charged (Hδ+) On the contrary, H in hydrides, especially ionic hydrides, is negatively charged (Hδ−) The abnormally high potential of the combination of Hδ+ and Hδ− to H2 together with the strong electrostatic attraction between the N anion in amide and the metal cation in hydride will likely induce a direct reaction between the amide and hydride and lead to the release of H2 [2, 115] One can expect that such an interaction should exist in other amide–hydride combination systems A variety of metal–N–H systems with different hydrogen capacities and thermodynamic parameters have been developed [23, 114, 116–155] (Table 5) As examples, the reaction between Mg(NH2)2 and MgH2 in a molar ratio of ½ gives more than 7.4 mass% H2 and the solid product of Mg3N2 [134]; more than mass% of hydrogen can be released exothermically from a mixture of NaNH2/LiALH4 (1:1 molar ratio) upon energetic ball milling [147] The reactions of amides and complex hydrides including LiBH4 [94, 96, 133, 154, 156–159], LiAlH4, and Li3AlH6 [136, 138, 142, 144, 149] brought considerable interesting features to the amide–hydride system It was reported that more than 11 wt.% of hydrogen can be desorbed from a mixture of 2LiNH2–LiBH3 exothermically in a temperature range of 250–350 °C (see Figure 23) Li3BN2 is the final product [93] The dehydrogenation feature is significantly different from the highly endothermic self-decomposition of LiBH4 and LiNH2 Due to the exceptionally high hydrogen content, this system can be used for onboard hydrogen production, provided that the operation temperature can be substantially reduced Attempts in catalyzing the dehydro­ genation by introducing nano-sized Pd, Pt, Ni, and Co have successfully brought down the dehydrogenation temperature to ∼150 °C [156, 160, 161] However, further reduction of temperature may depend not only on the catalytic modification but also on the optimization of the physical state of the reactant Experimental results show that the catalyzed dehydrogenation reaches the maximum rate upon the melting of Li3BN2H8, indicating that the mobility of the reacting species (Li3BN2H8) is essential to the effective contact of catalyst [161] It is likely that an additive which can form an eutectic compound with Li3BN2H8 could further enhance the reaction kinetics Clearly, such an attempt has to be based on an in-depth structural understanding [94, 133, 159] Yang et al introduced MgH2 to the LiBH4–LiNH2 system and observed a multiple-step reaction involving the formation of Li4BN3H10, 400 10% 300 10.24 wt.% 8% Temperature 364 �C 200 6% 4% LiNH2 + LiBH4 Ball milled 16 hours 2% 100 Desorption 0% 500 1000 1500 Time (min) Figure 23 Volumetric measurement of thermal desorption from Li3BN2H8 heating at 0.5 °C min−1 to 364 °C [93] Temperature (�C) Hydrogen desorption (wt.%) 12% 172 Hydrogen Storage: Liquid and Chemical Li2Mg(NH)2, Li3BN2, and Mg3N2 [150] Each step has different thermodynamic and kinetic parameters Investigations on the interaction between LiNH2 and LiAlH4 revealed that the transition of [AlH4]− to [AlH6]3− is fairly easy in the presence of LiNH2 [149] From a 2LiNH2–LiAlH4 sample, equiv H was desorbed during a ball milling treatment NMR measurements of samples collected at different intervals of ball milling showed that an Al–N bond was established upon the contact of these two chemicals, revealing a direct interaction between –NH2 and [AlH4]− Complete dehydrogenation at elevated temperatures results in the formation of Li3AlN2, which can only be partially rehydrogenated to LiNH2, LiH, and AlN (Table 5) [149] The LiNH2–Li3AlH6 combination was also investigated [136, 138] Different results were observed by different groups Kojima et al detected the formation of Li3AlN2, Li2NH, Al, and LiH after the dehydrogenation of Li3AlH6–2LiNH2 [136] The Al–N bonding was not observed by Lu et al A fully reversible reaction between Li3AlH6 and LiNH2 (molar ratio 1:3) was reported [138] Large amounts of hydrogen desorption from Mg(NH2)2–LiAlH4 [142] and Mg(NH2)2–Li3AlH6 [144] were also observed, in which 6.2 wt.% of hydrogen can be reversibly stored in a Mg(NH2)2–Li3AlH6 combination at 300 °C Comparatively, the Mg(NH2)2–LiH system has attracted more attention due to its reversible nature and suitable thermodynamic parameters [5, 116, 117, 119, 122, 123, 128, 129, 139, 140, 145, 163–178] A few Mg(NH2)2 and LiH combinations have been investigated thus far, which gave different reaction paths and hydrogen capacity [119, 140, 145, 179] However, Mg(NH2)2–2LiH provides more ‘usable’ hydrogen at lower temperatures [117, 122] The dehydrogenation of Mg(NH2)2 and 2LiH takes place in the temperature range of 150–250 °C, which gives 5.6 mass% hydrogen and a solid product Li2Mg(NH)2, which is a new compound of an orthorhombic structure (see Figure 24) Due to almost identical ion radii, Li+ and Mg2+ are indistinguishable in the lattice, thus bringing particular interest to the crystallographic analyses [180] Pressure–composition–temperature (PCT) measurements show that dehydrogenation of Mg(NH2)2 + 2LiH exhibits a pressure plateau and a slope region [117, 122] The heat of hydrogen desorption within the pressure plateau is ∼38.9 kJ mol−1 H2 The results of a van’t Hoff plot (see Figure 25) indicate that the temperature to desorb hydrogen at 1.0 bar equilibrium pressure is ∼80 °C [162], which is clearly in the range of typical operational temperatures of PEM fuel cells However, there is a severe kinetic barrier that probably originates from interface reactions and mass transport through the product layer, which provides a hurdle to low-temperature dehydrogenation [174] Catalytic modification to the system is challenging partly due to the catalytic additives that have to be involved in the interface reactions and/or mass transport Hu et al introduced LiBH4 to the system and lowered the energy barrier It is observed that complete dehydrogenation and hydrogenation can be achieved at 140 and 100 °C, respectively [174] More recently, Wang et al introduced ∼3 mol.% K in the Mg(NH2)2–2LiH system and enabled a full dehydrogenation and rehydrogenation cycle at a temperature near 100 °C (see Figure 26) [178] Theoretical investigations indicated that K may bond with N and activate the N–H bond It is rather interesting to understand the way K functions in the dehydrogenation and hydrogenation as it may shine a light on catalyst design for complex hydrides As mentioned earlier, the diverse combinations of amides and hydrides enable a series of novel chemical processes for hydrogen storage and production On top of that, the superior bonding capability of N with metal and hydrogen allows the formation of a variety of new compounds, such as Li2Mg(NH)2 [122, 180], Li2Ca(NH)2 [146, 153], MgCa(NH)2 [132], Li4BN3H10 [93, 133], and Li2BNH6 [159] Although a number of promising materials have been identified, the further development of amide–hydride systems for hydrogen storage largely relies on the proper match of N–H and N–M bonds in Figure 24 The crystal structure of Li2Mg(NH)2 Li and Mg are in purple, N in blue, and hydrogen in white Pressure (bar) Hydrogen Storage: Liquid and Chemical 173 ln(P) = −4683.65/T + 13.47 ΔH = 38.9 KJ mol−1 H2 ΔS = 112 J mol−1 K−1 H2 10 0.0020 0.0022 0.0024 0.0026 0.0028 0.0030 1/T (K−1) Pressure (bar) 100 10 ln(Peq /P Θ ) Figure 25 van’t Hoff plot of Mg(NH2)2-2LiH system [162] y = −5052.50x + 14.26 Δ H = 42.0 kJmol−1 H2 2.4 1.8 1.2 0.00225 0.00240 0.00255 1/T (K−1) 130 °C 107 °C Pristine sample at 107 �C 0.1 0.0 0.5 1.0 1.5 2.0 2.5 H content 3.0 3.5 4.0 Figure 26 Pressure–composition–temperature (PCT) desorption isotherms of the K-modified samples at 107 (●) and 130 °C (○) and the post-milled pristine sample at 107 °C (□) For the pristine sample, the dehydrogenation is too slow to reach equilibrium H content refers to the equivalent H atoms desorbed from the sorbent The inset shows the van’t Hoff plot of the K-modified sample [178] amides and M–H bond in hydrides In this regard, theoretical simulation is essential in predicting the potential systems [181] In the meantime, in-depth understanding of the reaction mechanisms will also significantly benefit the kinetic optimization 4.06.6 Pending Issues The comparative H content in complex and chemical hydrides is high More than 10 attracting systems have been developed within the past 10 years [3, 83, 182], and further promising systems are continually emerging In the meantime, considerable new chemistry of B- and N-containing compounds has been accumulated, which will greatly facilitate the ongoing research and development The 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817  ... 12H2 10.5 2.1 5.6 6.9 9.1 11.9 7 .4 5.0 8.5 4. 1 5.2 4. 5 7.5 9.2 150? ?45 0 35 0–6 50 10 0–2 50 15 0–3 00 15 0–3 00 15 0–3 50 20b 20b–500 20b–350 20b–500 20b 10 0–3 30 10 0–5 00 100? ?40 0 a b Experimental observation... + 4H2 Mg(BH4)2 → MgB2 + 4H2 3Mg(BH4)2·2(NH3) → Mg3B2N4 + 2BN + 2B + 21H2 Ca(BH4)2 → CaH2 + 2B + 3H2 Zn(BH4)2 → Zn + B2H6 + H2 13.6 11.5 14. 8 15.9 8.6 2.1 20 0–5 50 270? ?44 0 29 0–5 00 100? ?40 0 30 0–5 00... 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