LITHIUM BOROHYDRIDE AS A HYDROGEN STORAGE MATERIAL: A REVIEW

International Journal of Energy for a Clean Environment 11 (1-4) Lithium Borohydride as a Hydrogen Storage Material : A Review Joydev Manna1, Manvendra Vashistha2, Pratibha Sharma*1 1 Department of Energy Science and Engineering, Indian Institute of Technology Bombay, Powai, Mumbai - 400076, Maharashtra, India 2 Pillai’s Institute of Information Technology, Engineering, Media Studies and Research, New Panvel, Navi Mumbai-410206, Maharashtra, India ABSTRACT The major obstacle in transition to the hydrogen economy is the problem of onboard hydrogen storage. Solid state hydrogen storage is the safest and most efficient method for hydrogen storage. Most of the metal hydrides exhibit very large volumetric storage density but less than 5wt% gravimetric hydrogen density. Light metals like Al, B binds with four hydrogen atom and form together with an alkali metal an ionic or partially covalent compound called complex hydride. LiBH4 is a complex hydride with 18.5 mass% gravimetric hydrogen density and 121 kg/m3 volumetric hydrogen storage capacity. The desorption temperature of LiBH4 is greater than 4700C thus making difficult to use it for storage applications, further the conditions for reversible reaction are unfavorable. The modification of thermodynamics of the hydrogenation and dehydrogenation reaction is possible by using additives which could destabilize LiBH4 by stabilizing the dehydrogenated state. This could decrease the heat of reaction and reduce the desorption temperature at the same time make the conditions for reversible reaction more optimum. Several additives which could destabilize LiBH4 have been reviewed. Keywords: LiBH4, hydrogen storage, hydrogenation/dehydrogenation. *Corresponding author : Pratibha Sharma Phone: +91-22-25767898 fax : +91-22-2576 4890 e-mail: pratibha@me.iitb.ac.in, pratibha_sharma@iitb.ac.in 1 destabilization, complex hydrides, International Journal of Energy for a Clean Environment 11 (1-4) INTRODUCTION The deficit of fossil fuels, in combination with global environmental problems, stipulated a great interest to use hydrogen as an energy carrier for both stationary as well as mobile applications. Hydrogen is the most abundant element on earth, but less than 1% is present in molecular hydrogen gas form. The chemical energy per mass of hydrogen (142MJkg -1) is at least three times larger than that of other chemical fuels, besides the exhaust on use of hydrogen as a fuel is clean[1]. However, the biggest challenge towards use of hydrogen for mobile applications is its storage and is thus the major obstacle in the transition to hydrogen economy. Safe, light weight, low cost and high storage capacity are the essential requirements for any suitable storage media. Current long-term U.S. Department of Energy guidelines call for a system hydrogen storage density of 9wt% and a volumetric density of 81kgH 2/m3 to enable fuel cell powered vehicles to be able to replace petroleum fueled vehicles on a large scale. In addition to these challenging targets, onboard devices must operate under various other performance constraints. For example their operating conditions should preferably be below 100 oC and pressures below 100 bars. Storage materials that meet these targets will be useful not only in fuel cell vehicles but also in a range of other applications for hydrogen as a fuel[2]. Hydrogen can be stored as (i) pressurized gas (ii) cryogenic liquid or (iii) in solid state as physical or chemical combination with materials (viz. surface adsorption, intermetallic hydride, complex hydride and chemical hydride) [3,4]. Storage of hydrogen in gaseous form has its own disadvantages because of the low density of hydrogen in the gaseous state (0.08988 g/L) at 1 atm. The gravimetric storage capacity of 700 bar tank is 4.5wt% which is close to the DOE target; however the volumetric capacity is only 0.025kgH2l-1 which is equivalent to 0.83kWl-1. Increasing the pressures above 700 bars will help to improve the volumetric capacity but at the same time, energy required for compression, safety and conformability of the pressure vessel are the important issues [5]. 2 International Journal of Energy for a Clean Environment 11 (1-4) Cryogenic storage of hydrogen requires very low temperatures, since the critical point of hydrogen is 33K, above which it is a non-condensable gas. Its boiling point is 20K at 1 atmosphere and density under these conditions is 0.0708kgl-1, which gives a respectable volumetric capacity of 2.35kWl-1. Thus liquid hydrogen storage requires the addition of a refrigeration unit, which adds weight and energy costs, and results in a 40% loss in energy content [6]. Solid state storage of hydrogen either physically or chemically bound with different materials has definite advantages from the safety perspective. It can adsorb onto a surface through weak Van der Waals interaction onto the high surface area materials, for example in case zeolites, metal organic frameworks, carbon based and nano materials. However, since the strength of the interactions is very weak, low temperatures are needed to obtain significant amount of hydrogen adsorption. Hydrogen can be stored as chemically bound hydrogen where it forms a chemical compound with the substrate. A great deal of effort has been made on new hydrogen-storage systems, including metal hydrides, chemical or complex hydrides and carbon nanostructures. However the hydrogen-storage capacity for carbon materials is reported between 0.2 and 10 wt% [7,8]. Unfortunately although carbon nano compound have good reversibility properties, they cannot store the amount of hydrogen required for automotive applications [9]. Metal hydrides shows an appreciable volumetric hydrogen storage capacity which reach maximum of 150 kg H2/m3 (Mg2FeH6) more than double that of density of liquid hydrogen. But due to the higher weight of metals gravimetric hydrogen storage capacity of metal hydride is less than or equals to 8 wt% (MgH2) [10]. The light metals of group one, two or three such as Li, Be, Na, Mg, B, Al forms a large variety of metal-hydrogen complexes. They are especially of interest due to their light weight and number of hydrogen atoms per metal atom. The hydrogen in 3 International Journal of Energy for a Clean Environment 11 (1-4) the complex hydrides is located in the corners of a tetrahedron with boron or aluminum in the center. The negative charge of the anion, [BH4]- is compensated by a cation e.g. Li or Na. The pure alkali metal tetra borohydride was first produced in 1940 by Schlesinger et al by the reaction of ethyl lithium (EtLi) with diborane (B2H6) [11]. Recently the preparation of borohydride by direct reaction of metal, hydrogen and natural abundant borax deposit was reported, thereby making the process and material less expensive [12]. LiBH4 is the one of the complex hydride of great interest due to its high gravimetric (18.5 wt %) and volumetric (121 kg/m3) hydrogen storage capacity [10]. Therefore this complex material could be proposed to be the ideal hydrogen storage material for mobile application. However the thermodynamic and kinetic limitations that are essentially imposed by the strong covalent and ionic bonds greatly restrain the practical application of LiBH4. Due to high stability of the material it has a very high decomposition temperature (275oC) and also it required very high pressure and temperature for rehydrogenation [13]. Zuttel et al found the interaction of lithium borohydride with quartz test tube and concluded that the decomposition temperature can be reduced to half using SiO2 [14]. Orimo et al also studied the Raman spectra of alkali borohydride (i.e.NaBH4, KBH4, LiBH4) and suggested that the stability of LiBH4 may be reduced by partial cation substitution [15]. The moderated LiBH4-based materials desorbing and reabsorbing at moderate conditions would represent a major breakthrough in hydrogen storage technology. There are mainly two approaches that have been used for more than 25 years in conventional technology: (1) element substitution and (2) additive interaction [16]. Researchers are exploring to define both experimentally and theoretically the stability, thermodynamics and reversibility of lithium borohydride. Here in this review we will present the hydrogen storage properties of pure lithium borohydride and further that of modified lithium borohydride. 4 International Journal of Energy for a Clean Environment 11 (1-4) 1. PROPERTIES OF LiBH4: 1.1 Physical Properties: Lithium tetra hydroboride (LiBH4) is a salt-like, white, crystalline, hygroscopic solid material. Its density is 0.666gm/cm3 at 250C, melting point is 2750C and boiling point is 380 oC. At 0oC, its vapour pressure is much less than 10 -5 mbar, and the salt neither decomposes nor sublimes. At 298.15K the values of thermodynamic functions are as follows: ∆Hf = -194.44 kJmol-1, ∆Ff = 128.96 kJmol-1, S0 = 75.91 Jk-1 mol-1, cp = 82.60 JK-1mol-1 [14]. 1.2 Structure: The structure of LiBH4 is orthorhombic where each lithium ion is surrounded by four [BH 4]ions and one [BH4]- ion is surrounded by four Li+ ions[17]. Further theoretical observation on structure of LiBH4 shows that it has two different structural states, α-LiBH4 and β-LiBH4. At ambient conditions with the help of rotation and oscillation photograph it is observed that, LiBH4 crystallizes as α-LiBH4 which has orthorhombic structure as shown in figure 1(a)(space group Pnma with a= 6.8, b= 4.43 and c= 7.17 Angstrom )[18]. Whereas at 381 K it undergoes structural change to hexagonal β-LiBH4 with space group P63mc (a= 8.70, c= 4.41 A) as shown in figure 1(b) [19-21]. Though calculated parameters for α-LiBH4 give ideal orthorhombic structure, experimental results suggest that structure is strongly distorted, where bond lengths varies 1.04 to 1.28 A0 and bond angles (H-B-H =85-1200)[22]. 1.3 Stability: LiBH4 is a very stable compound due to its high lattice energy of 779 kJmol-1[38]. DSC profile of LiBH4 shows three peaks which correspond to (a) structural transitions at 380 K (b) melting reaction at 550 K and (c) dehydriding reactions at 600-700 K respectively carried out at 0.1 MPa pressure and 10 K min-1 heating rate as shown in figure 1(c)[23]. The Differential thermal analysis of LiBH4 showed three endothermic peaks at 108-1120C, 268-2860C and 483-4920C. 5 International Journal of Energy for a Clean Environment 11 (1-4) The first endothermic peak at 108-1120C is reversible and corresponds to polymorphic transformation of LiBH4. The second peak at 268-2860C corresponds to the fusion of LiBH4. The fusion is accompanied by a slight decomposition, which liberates approximately 2% of the hydrogen in the compound. The main evolution of gas starts at 380 0C and liberates 80% of the hydrogen in LiBH4 [14]. 2. DEHYDROGENATION AND REHYDROGENATION OF LiBH4 2.1 Desorption mechanism and kinetics: Lithium borohydride desorbs hydrogen according to the following equation: LiBH4(s) → LiH(s) + B(s) + (3/2) H2 (g) (1) During the reaction (1) theoretically 3/2 mol of hydrogen per mol of LiBH 4 (i.e 13.8 mass %) is released. But maximum 10.9 wt% desorption is observed in practical cases [24]. Phase transition corresponds to release of 0.1 wt% hydrogen. Fedneva et al suggested same mechanism and said that melting of LiBH4 is accompanied by liberation of approximately 2wt% of hydrogen [25]. The main evolution of gas starts at 380OC and liberates 80% of the hydrogen. Heat of reaction value is the main parameter which will determine the forwardness of the reaction. However there are only few citations on the heat of reaction for reaction (1). Mauron et al done the PCT (pressure, concentration, temperature) measurement of LiBH 4 in temperature range of 400-500oC at a constant hydrogen flow rate of 2, 1 and 0.1 cm3(STP) min-1 [24]. Using Van’t Hoff equation they reported that enthalpy and entropy of the reaction for equation (1), ∆rH= 74 kJ mol-1 of H2 and ∆rS=115 J K-1 mol-1 of H2, respectively [24]. These values are almost equal with the experimental values (∆rH= 68.9 kJ mol-1 of H2 and ∆rS= 100.3 J K-1 mol-1 H2) obtained by Smith et al [26]. Table 1 shows heat and entropy of reaction obtained from different sources: 6 International Journal of Energy for a Clean Environment 11 (1-4) First principle study also proposed for following reaction [17]: LiBH4 (s) → Li (s) + B (s) + 2 H2 (2) Theoretically the hydrogen to be released in reaction (2) is very high i.e. 21.5 wt%. Thermodynamic analysis also shows that both the reactions (1) and (2) of these reactions are endothermic, with heat of reaction 115.4 and 195.1 kJ mol-1 of LiBH4, respectively at standard conditions (298.15 K, 1 atm) [26]. Raman spectroscopy and XRD analysis study shows that Li2B12H12 is one of the intermediate compound formed during the dehydrogenation reaction [2932]. 2.2 Rehydrogenation reaction: After dehydrogenation of lithium borohydride, LiH and B are formed as residue. From these residue, researchers are trying to achieve reversible reaction, because reversible hydrogen storage is one of the main demand for onboard fuel cell vehicle. The reaction will be according the following equation: LiH(s) + B(s) + (3/2) H2 (g) → LiBH4 (3) The reaction enthalpy amounts to ∆H = -33kJ/mol and the reaction free energy ∆G= -19kJ/molH2. The equilibrium temperature calculated for 1 bar of hydrogen is 402oC[23]. LiBH4 reversibility will be determined by its dehydrogenation products. The borohydrides that do not emit boron containing volatile gases such as BH3 and B2H6, which hold boron in intermediate compounds such as MgB2 during dehydrogenation, might be reversible. However the unstable borohydrides which are capable of liberating hydrogen at room temperature are most likely irreversible[13]. Also, higher pressures and longer times for rehydrogenation may increase the yield of LiBH4 from the dehydrogenated products at the same time slow reaction kinetics makes complete rehydrogenation difficult. It was reported that even at 350bar and 6000C there is incomplete rehydrogenation. The reason for slow kinetics could be 7 International Journal of Energy for a Clean Environment 11 (1-4) low mobility and inactivity of amorphous boron, as such LiBH 4 is considered to be partially reversible under these conditions[13]. After rehydrogenation upto 600oC only 8.3 wt% of hydrogen is desorbed compared to 10.9 wt% for the starting material. Mainly two pathways proposed for the rehydrogenation reaction. First one is where hydrogen first reacts with boron and form diborane, then diborane reacts with lithium hydride to from LiBH4 (eqn. 4)[33]. Second one is where hydrogen reacts with both boron and lithium hydride to form an intermediate compound (i.e. LiBH2 and/ or Li2B12H12), which again react with hydrogen to give LiBH4 (eqn. 5a, 5b)[ 25,29]. 2LiH + 2B + 3H2 → 2LiH + B2H6 → 2LiBH4 (4) LiH + B + 3/2 H2 → LiBH2 + H2 → LiBH4 (5a) LiH + B + 3/2 H2 → 1/12 Li2B12H12 + 5/6 LiH+ 13/2 H2 → LiBH4 (5b) LiBH4 can also be synthesized from a mixture of LiH and MgB2. The reaction proceeds as follows: 2LiH + MgB2 + 4H2 →2LiBH4 +MgH2 (6) The reaction enthalpy and free energy calculated are ∆H = -23kJ/molH2 and ∆G = -7kJ/molH2. With these values the equilibrium value of temperature at 1 bar hydrogen pressure was reported to be 1690C. Theoretically the reaction (6) has a lower kinetic barrier compared to reaction (3). The possible reason given was that in case of (3) higher activation energy is required for elemental boron to build the necessary [BH4]- complexes[23]. 2.3 Hydrolysis of LiBH4: Hydrolysis of lithium borohydride in presence of catalyst could release hydrogen on demand according to following reaction [34]: LiBH4 + (2+ n) H2O → LiBO2.nH2O + 4H2 8 (7) International Journal of Energy for a Clean Environment 11 (1-4) From the reaction (7) it is evident that half of the moles of hydrogen released are liberated from the water molecule. Thus the combination of borohydride and water should be considered as fuel. Storage capacity depends on the hydrates formed after hydrolysis and hence on the initial composition of the fuel. Table 2 shows how storage capacity changes with the number of moles of water present in the fuel mixture [35]. XRD of hydrolysis product showed the presence of well crystallized two phase product composed of lithium metaborate (LiBO 2.8H2O) and dihydrate of lithium metaborate (LiBO2.2H2O) [35]. 3. DESTABILIZATION OF LITHIUM BOROHYDRIDE The work of Reilly and Wiswall[36,37] showed that it is possible to modify the thermodynamics of hydrogenation/dehydrogenation reactions by using additives showed that it is possible to modify the thermodynamics of hydrogenation / dehydrogenation reactions by using additives to form compounds or alloys in the dehydrogenated state that are energetically favourable with respect to the products of the reaction without additives. This concept is known as destabilization[42] The concept of destabilization thus relies on identifying compounds that can lead to formation of stabilized compounds, thus reducing the enthalpy of the overall reaction and increasing the partial pressure of the hydrides. 3.1 Elemental Substitution: From the first principle analysis it is reported that charge transfer from Mn+ to BH4- is one of the main factor which determines the stability of the complex borohydrides [39-41]. There exist a linear relation between the formation enthalpy ∆H of M(BH4)n and the Pauling electronegativity �p of the metal M. Experimentally determined decomposition temperature Td decreases with increasing value �P of the metal [40] as evident in figure 2. Thus the Pauling electronegativity value of M is an important indicator for estimating thermodynamic stability of M(BH4)n, thereby appropriate selection of M (with known electronegativity) can lead to 9 International Journal of Energy for a Clean Environment 11 (1-4) borohydride with required Td. Highly electronegative metal borohydrides, for example zinc and aluminum borohydride are quite unstable and release diborane (B2H6) on heating with hydrogen. Zirconium borohydride is again unstable with low melting point and as such it evaporates easily. However since the electronegativity value of lithium is low, the lithium borohydride is reported to be very stable. Therefore the normal stability of LiBH4 could be adjusted by using the combination of two cations. There are different possibilities of substitution of Li by another metal to form an alloy complex hydride. Mainly substitution of metal in LiBH4 is done by milling with some another metal, metal oxide or halide. This process of destabilization is known as so called elemental substitution. 3.2 Additive interaction: Instead of substitution of Li with some metal substitute in LiBH4, a constant research is going on to find suitable materials which can interact with LiBH4 and minimize the desorption temperature by stabilizing the product material. Using such materials, minimization of decomposition temperature is known as additive interaction or mutual catalysis. This type of interaction can be depicted by a generalized enthalpy diagram as shown in figure 3. In the first case AH2, react with ‘x’ mole of ‘B’ and produce stabilized compound AB x (enthalpy of reaction = dH1) and in 2nd case ABx decomposes to give ‘A’ (metal) and hydrogen (enthalpy of reaction dH2)[51]. As ABx is a stable compound than A, dH1 should be less than dH2. 4. DESTABILIZATION BY METAL AND METAL HYDRIDE To identify the appropriate metal hydride as an additive, heat of reaction value has very important role. Alapati et al estimated that for a material to have desorption temperature in the range appropriate for fuel cell usage requires that 30 ≤ ∆H ≤ 60 kJ/mol H2 [42]. Reactions with ∆H higher than 60kJ/mol H2 would require temperatures that are unacceptably high. ∆H lower than 30 kJ/mol H2 would indicate that the desorption reaction is not easily reversible, such 10 International Journal of Energy for a Clean Environment 11 (1-4) information is valuable in eliminating compounds that clearly have unfavorable thermodynamics. 4.1 MgH2 addition: Stabilizing the dehydrogenated state reduces the enthalpy of dehydrogenation, thereby increasing the equilibrium hydrogen pressure. Using this approach the thermodynamic properties of reversible hydrogen storage material systems can potentially be tuned to an extent finer than would be possible with individual materials[44]. Thus to tune the thermodynamics and kinetics of hydriding reactions two approaches can be used. One approach is to substitute specific atoms in the structure of hydride by a dopant atom, for e.g., LiBH4 is destabilized by substituting 10at% of Li by Mg, as a result of it desorption temperature lowers by 30K. Second approach is to adjust the reaction thermodynamics by choosing the suitable reactants. Instead of trying to synthesize hydrides from elements, compounds of their elements are used as starting materials. This approach is called “reactive hydride composites”. However due to high kinetic barrier of formation in case of complex borohydrides, high temperatures and high hydrogen pressures are required to provide thermodynamic driving force. Thus as per literature borohydrides can reversibly absorb and desorb hydrogen if suitable reactants are selected, for example addition of MgH2 to LiBH4 results in a hydride composite with a reversible storage capacity of about 11wt%[43, 44,49]. The reaction proceeds according to the following equation [44]: LiBH4+ ½ MgH2 ↔ LiH + ½ MgB2 + 2H2 (8) Maximum hydrogen desorption capacity of this reaction is 11.4 wt%. Formation of MgB2 stabilizes the dehydrogenated state and effectively destabilizes the LiBH 4. Hydrogenationdehydrogenation enthalpy is 50.4 kJ/mol of H2 that mean it reduces by 15.1 kJ/mol of H2 compared to LiBH4 [45]. Mg reacts with lithium borohydride and forms magnesium boride, a 11 International Journal of Energy for a Clean Environment 11 (1-4) stable product, which is the reason behind minimization of the reaction enthalpy. So addition of magnesium hydride with LiBH4 is an example of an additive interaction. The above reaction (8) proceeds in two steps, firstly MgH2 decomposes to give hydrogen and magnesium, and then Mg reacts with LiBH4 and forms MgB2 and releases hydrogen from LiBH4 [43,46]. This reaction (8) is reversible, but it suffers with slow reaction kinetics, as such no hydrogen is released below 3000C. The two step desorption is reported in DSC investigations, with maxima approximately at 350oC and 430oC for a heating rate of 5oC under 100 cm3/min flow rate [46]. The amount of hydrogen released in the reaction is reported to be 10.2 wt% on heating up to 100h with same heating rate and flow rate[46]. During rehydrogenation process, at 350oC and 150 bar hydrogen pressure, the dehydrogenated product was found to be completely converted to lithium borohydride [46]. It is also observed that LiH+ ½ MgB2 mixture absorbed 2.5 wt% hydrogen after 2 h heating at 300oC and absorbed >9 wt% hydrogen in presence of 0.03 mole of TiCl3 catalyst (heating rate 2oC min-1) [44]. Though for initial cycle uptake is slow (2.5 wt% at 3000C after 2 h), in the second and third cycle uptake becomes faster and 9 wt% uptake occurs in 2 h at 300oC. Further desorption from the rehydrogenated LiBH4 at 450oC releases 8 wt% in the second and third cycles. Desorption kinetics for the first two cycles are nearly identical. Under vacuum, the dehydrogenation step begins at 2700C with the formation of Mg instead of MgB2 thereby resulting in loss of subsequent rehydrogenation capacity. In presence of TiCl3 catalyst desorption-absorption isotherm has a sloping plateau from 2-8 wt% with a capacity of approximately 10 wt% [44]. The initial work by Vajo et al. showed that the dehydrogenation conditions affected the reaction path [44], under a hydrogen atmosphere the dehydrogenation followed reaction (8). However, if 12 International Journal of Energy for a Clean Environment 11 (1-4) the dehydrogenation was done under dynamic vacuum, magnesium boride was not formed and the following reaction appeared to proceed 2LiBH4 +MgH2→ 2LiH + 2B + Mg + 4H2 (9) The end products so obtained could not be hydrogenated suggesting that MgB2 plays an important role in the reverse reaction. It was reported that the stoichiometry of the starting materials does not affect the overall decomposition, but it affects the ratio of the end products depending on whether one component is in excess or not. It is reported that Li-Mg alloy is also formed if 2:1 mixture of the reactants LiBH4 and MgH2 are heated under vacuum [48].The decomposition environment does have a big effect on the decomposition pathway. On addition of 10mol% MgB2 to the reactants LiBH4 and MgH2, the reaction kinetics was reported to reduce by half, whereas on addition of 5 mol% of Sc2O3, the hydrogen release kinetics was improved and the reaction time became one fifth compared to pure composites[47]. In presence of 3% TiCl3 the first and second dehydrogenation steps are reported to be lowered by 110C and 220C respectively. Addition of other Ti sources (e.g. TiCl2, TiF3, CpTiCl3; Cp= cyclopentadienyl) give similar results as shown by TiCl3 [50]. Addition of 3% VCl3 or NiCl2 lowers the reaction temperatures for the first step by 9 or 5◦C and the second step by 17 or 5◦C, respectively. The activity with 3% CrCl3 and 3% NdCl5 was similar to that with of 3% VCl3 [50]. Nanoengineering and mechanical activation was used to obtain ultrafine particle size, large surface area, high defect concentration, and nanosized grain which reported to alter thermodynamic and kinetic properties of the material. High energy ball milling of 2:1 mixture LiH+MgB2 using Szevari attritor (this apparatus is effective in preventing the formation of dead zone and producing uniform milling products within powder charge) for 3hr, 24hr and 120hours 13 International Journal of Energy for a Clean Environment 11 (1-4) under argon atmosphere reported to have better properties. For 120 hours ball milled LiH + MgH2 mixture, the total hydrogen absorbed on holding at 265 0C at 90bar pressure for 5hr is much higher (8.3 wt%) compared to that of 24 hr (5.9 wt%) and 3 hr (3.2 wt%) milled mixtures [51]. Though the amount of hydrogen released from 120 h milled hydrogenation product is very low (2 wt% after holding at 265oC for 5 h). V. Alapati et al thermodynamically (USSP-PAW method) calculated heat of reaction value taking LiBH4 and MgH2 in different ratio (4:1, 7:1) and predicted reaction enthalpy of 66.8 kJ/mol and 70 kJ/mol after release of 12.5 and 13 wt% of H 2, respectively, on completion of the reaction [42]. These hypothetical reaction equations are as follows: 4 LiBH4 + MgH2 → MgB4 + 4 LiH + 7 H2 (10) 7 LiBH4 + MgH2 → MgB7 + 7 LiH + 11.5 H2 (11) The reversible dehydrogenation of LiBH4-MgH2 system occurs via a two step process involving the following reactions: MgH2 ↔ Mg + H2 (12) 2LiBH4 + Mg ↔ 2LiH + MgB2 + 3H2 (13) SWNTs are highly effective for improving the H-exchange kinetics of MgH2 and LiBH4 materials. With 10wt% SWNT addition the dehydrogenation kinetics of both the steps improved significantly. It is reported that the onset dehydrogenation temperature for the second step, i.e. reaction of Mg with LiBH4, shifted to 380oC which is about 50oC lower than that of LiBH4MgH2 mixture without SWNT. The composite milled with 10wt% SWNTs took only 20 minutes to desorb nearly 10 wt% hydrogen at 450oC [52]. This improvement may be due to the net like structure of SWNT, which exert “micro-confinement” effect, resulting in inhibition of particle agglomeration/sintering and diffusion of the product phases upon milling with the host material [52]. 14 International Journal of Energy for a Clean Environment 11 (1-4) LiBH4-MgH2 composite with additive Nb2O5 yields a destabilized, reversible hydrogen storage material system. It possess a maximum capacity of 6-8wt% hydrogen releasing below 4000C and could be hydrogenated to 5-6wt% hydrogen capacity in 4h at 4000C and 1.9MPa. Nb2O5 reacts with LiBH4-MgH2 composite and produces an intermediate compound (NbH2) which can improve the kinetics of the reversible reaction at relatively mild conditions. Also the activation energy is reported to decrease from 156.75 kJ/molH2 for LiBH4-MgH2 composite (without additive) to 139.96 kJ/molH2 for milled LiBH4-MgH2 (mass ratio 1:2) + 16wt% Nb2O5 [53]. 4.2 CaH2 addition: In presence of CaH2, enthalpy of decomposition of LiBH4 is reduced due to the stability of the CaB6. Reaction proceeds through as follows: 6LiBH4 + CaH2 ↔ 6LiH + CaB6+ 10H2 (14) Here the theoretical hydrogen capacity for this reaction is 11.7 wt% and volumetric hydrogen density is 0.09 kg H2/lit with calculated reaction enthalpy 60.3 kJ/mol of H2 [2]. Reversible hydrogen storage of 9.1 wt% has been achieved by this reaction in presence of 0.25 mol of TiCl 3 in reaction mixture (weight of TiCl3 is included to calculate the capacity) [54]. So 95% of the total hydrogen can be obtained from this hydride mixture. 6LiBH4+CaH2+0.25TiCl3 sample started desorption at 150oC under 0.13 MPa. Most of the hydrogen desorption occurred after sample reached 350oC and completed after soaking at 450oC for 30 min. Though Barkhordarian et al [49] were unsuccessful to hydrogenate CaB6 and LiH mixture, Pinkerton et al [54] successfully recycled reaction 8 and after reaction 9.1 wt% of hydrogen absorbed within 2700 min heating at 400oC. The success of rehydrogenation reaction was proposed to be due to starting from dehydrogenated state, lesser milling time of 1hr and presence of TiCl3. With different additives (TiF3, TiCl3, TiO2, V2O5) it was reported that in presence of these additives, 15 International Journal of Energy for a Clean Environment 11 (1-4) desorption of hydrogen occurred in the 400-450oC range and desorption temperature being in the order of TiF3< TiO2 ≤ TiCl3 < V2O5 [55]. Presence of CaNi5H4 has no effects towards the destabilization of LiBH4, however the presence of Nickel as one of the product reduced the overall weight percent of hydrogen released to 5wt% for the system. Further addition of LiNH 2 to the CaH2 + LiBH4 mixture was reported to reduce the desorption temperature by 140 0C, but the reaction was irreversible in this case, due to very stable end products[55]. In presence of 0.2 mole NbF5, dehydrogenation temperature estimated as 309 oC under 1 bar pressure and released 9.1 wt% hydrogen up to 400oC temperature with a reaction enthalpy change of 56.5 kJ/ mol H2 [56]. 4.3 Addition of complex hydride and borohydride: A systematic study by Nakamori et al[40] has shown that the stability of metal borohydride can be roughly estimated by the electronegativity of cation: a less electronegative metal can form a more stable metal borohydride. According to this principle, stability of a mixed borohydride LiBH4-M(BH4)n can be tuned by changing the metal M and the mixing ratio[60]. LiBH4 in presence of NaAlH4 (molar ratio 2:3) started releasing hydrogen at 110oC and released 4.2 wt% above 250oC. It was also observed that in presence of 4 mol% TiCl3 (of LiAlH4) hydrogen release starts at room temperature and 3.6 wt% hydrogen is desorbed below 210oC [57]. The amount of hydrogen in TiCl3 doped sample is low due to the consumption of LiAlH4 by reduction reaction of TiCl3 during the ball milling. It is observed that TiF3 or TiO2 doped LiBH4-LiAlH4 system also have lower decomposition temperature. In the case of 5 mol% TiF3, the doped sample started releasing hydrogen at 54oC, releasing 3wt% hydrogen below 2500C and desorbed 5.6 wt% hydrogen after heating to 600 oC [58]. Doping with 3 mol% TiF3 also showed a great reduction in decomposition temperature; this mixture started to decompose between 177 to 2470C and the weight loss reaches up to 7.2 wt% [59]. 16 International Journal of Energy for a Clean Environment 11 (1-4) LiBH4 doped with Ca(BH4)2 have theoretical hydrogen storage capacity of 12.8 wt% and will follow the eqn. 15 [60]. A mixture of xLiBH4+ (1-x) Ca(BH4)2 (x= 0.4, 0.6, 0.8) ball milled releases 10-11 wt% of hydrogen at 400-4500C. The temperature of the major dissociation reaction becomes higher as x increases and is same as that of LiBH4 decomposition temperature (Td) when x=1. 4LiBH4+ Ca(BH4)2 → 4LiH+ CaB6+ 10H2 (15) When LiBH4 is ball milled with Mg(BH4)2 and then heated, a dual cation borohydride with a composition Li1-xMg1-y(BH4)3-x-y is formed and hydrogen release starts at 240oC, which is about 30oC and 170oC lower than that of Mg(BH4)2 + LiBH4 [61]. As reported in the TGA results the amount of hydrogen released was 9 wt% in the temperature range of 200-370oC. When LiBH4 and KBH4 were milled together a double cation borohydride [LiK(BH4)2] is identified [62]. This compound is found to have an orthorhombic structure in the space group Pnma with nearly ideal tetrahedral shape. After first principle study it is observed that decomposition temperature of LiK(BH4)2 lies between those of LiBH4 and KBH4, which suggests that the hydrogen decomposition temperature of metal borohydrides can be precisely adjusted by the appropriate combination of cations. 4.4 Effect of other metal hydride addition: It was reported that titanium in the metal hydride form i.e. TiH2, do not substitute for Li in LiBH4 or react with LiBH4. Thus although thermodynamically reaction (16) is a favorable reaction with reduced enthalpy ∆H = 2.2 kJ/mol of H2, experimentally the addition of TiH2 does not reduce the dehydriding temperature and as well as enthalpy of the reaction [63]. 2LiBH4+ TiH2 → 2LiH +TiB2+ 4H2 17 (16) International Journal of Energy for a Clean Environment 11 (1-4) Desorption on addition of ScH2 to LiBH4 releases 8.91 wt% hydrogen with predicted reaction enthalpy ∆H= 34.1 kJ/ mol of H2. 2LiBH4 + ScH2 → ScB2 + 2LiH+ 4H2 (17) But addition of ScH2 does not show any improvement in dehydrogenation kinetics. At temperatures up to 450oC the amount of hydrogen released is less than 5 wt%, which is only half of the theoretical capacity [64]. On addition of CeH2 to LiBH4 the hydrogen release reaction is as follows: 6LiBH4+ CeH2 → 6LiH + CeB6+ 10H2 (18) (hydrogen capacity 7.39 wt %) reaction have a reaction enthalpy of ∆H= 44.1 kJ/mol of H2 with dissociation temperature 173oC. The first dehydrogenation-hydrogenation cycle of this composite system seemed to be reversible in presence of TiCl3 as catalyst with a hydrogen storage capacity 6 wt% [65]. Under static vacuum and in absence of catalyst this composite released only 3.8 wt% hydrogen after 24 h heating at 350oC. But the application of 3 bar hydrogen back pressure on this composite resulted in decomposition of 6.2 wt% hydrogen within 24 h at same temperature [66]. 4LiBH4 + YH3 composite released less than 2 wt% hydrogen after heating at 350 oC under vacuum for 24 h, which is far less than theoretical value of 8.5 wt%. But under 3 bar hydrogen back pressure this composite released 7.2 wt% hydrogen after heating at same temperature for 24 h [66]. 4.5 Destabilization by different metal: It is observed that when certain metals are doped with lithium borohydride desorption temperature is reduced. Formation of the stable metal boride is thermodynamically preferred for the reduction of the decomposition temperature. It is reported that for metal doped borohydride system following are the thermodynamically preferred reaction pathways [46]: 18 International Journal of Energy for a Clean Environment 11 (1-4) 2 LiBH4 + M → MB2 + 2LiH + 3H2 (19) (When M= Al, Cr) 2 LiBH4 + M → 1.5 LiBH4 + 0.75 MH2 + 0.25 MB2 + 0.5 LiH → MB2+ 2LiH+ 3H2 (20) [When M= Mg, Ti, Sc, V* (V*: V forms V2H)] Siegel et al showed thermodynamically that several reaction have sufficient amount of hydrogen storage capacity with optimum enthalpy of the reaction [Table. 3] [45]. It is observed that among Al, Mg, Ni, Ca and In; magnesium and aluminum has a positive effect on the reduction of decomposition temperature of LiBH 4. The material LiBH4+ 0.2 Mg liberated 1 wt% of hydrogen at very slow rate starting from 60 oC and desorbed 9 wt% hydrogen up to 600oC. LiBH4+ 0.2 Al desorbed slowly 0.2 wt% of hydrogen at 80 oC and rate became faster after 300oC and released 7.8 wt% of hydrogen rapidly from 300 to 600 oC [67]. Whereas Ni, In and Ca have negative effect on the desorption temperature of LiBH4. It was also proved that from LiH and AlB2 it is possible to synthesize LiBH4 at 450oC under 13 bar pressure without using any additive or catalyst [68]. For 2LiBH4+Al a broad multistep desorption process is observed, and the onset desorption temperature is approximately 320oC and maximum at 400oC [46]. For different metal doped lithium borohydride system observed and calculated hydrogen storage capacity are tabulated below in Table 4 [46]: The hydrogen storage kinetics of Al doped LiBH4 is further improved by adding TiF3. Al doped sample with TiF3 observed to release over 7.2 wt% hydrogen in about 3h at 450 oC. For sample LiBH4+ 0.5 Al + 0.04 TiF3, dehydriding as well as rehydriding behaviour is improved [70]. In further examination of the cyclic stability, it was found that the LiBH 4/Al system underwent serious capacity loss with increased number of cycles. The H-capacity decreased from the initial 7.2 to about 3wt% after four cycles, with a pronounced decrease of 2.1wt% over the first two cycles. The two reasons proposed for the observed capacity loss with increased number 19 International Journal of Energy for a Clean Environment 11 (1-4) of cycles are: firstly loss of boron secondly could be due to an incomplete reaction between LiBH4 and Al. It was also possible to synthesize LiBH4 from AlB2 and LiH (stoichiometric ratio 2:1) under hydrogen pressure 150 bar with a fixed heating rate of 1oC min-1 [68]. Significant hydrogen absorption is observed after 300oC and released 7.6 wt% after 20 h. LiBH4 with Al/Ti mixed additives reduced the desorption temperature, such that decomposition starts at 200oC and released total 2-2.4 wt% hydrogen (considering weight of whole sample) after heating up to 400oC. The gradual loss of hydrogen storage capacity with cycling is mainly due to the formation of B2H6 [71]. LiBH4 doped with Ni increases the amount of desorbed hydrogen at around 400oC through the following reaction (theoretical capacity 13.6 wt%) [72]. 2LiBH4 + Ni ↔ ½ BNi2 + 2LiH + 3/2 B + 3H2 (21) But experimentally it is observed that this composite shows two distinct dehydrogenation step, one is in the range 300-500oC with a weight loss 13.1 wt% and second step at 500-600oC range with a weight loss of 4.1 wt%. After 600oC presence of Li3.59Ni7.52B6 as product suggests that at this operating temperature LiH also got decomposed. It is also observed that LiBH4-Ni mixture could be rehydrogenated at 600oC under comparatively low pressure (10 MPa) than that of pure LiBH4 (35 MPa). Ball milling physical mixture of Mg50Ni and 10 mol% LiBH4 showed enhanced sorption kinetics. This composite started releasing hydrogen at 250 oC and reached to equilibrium with desorption of 7.0 wt% of hydrogen at 300 to 350oC. Addition of 0.2 mol% of CeCl3 to this composite minimize desorption temperature drastically to 150oC however due to the presence of the catalyst overall storage capacity decreased [73]. Recently it was observed that carbon supported nano-Pt catalyst reduced the hydrogen desorption temperature of LiBH4 to 280oC and hydrogen storage capacity also increased with 20 International Journal of Energy for a Clean Environment 11 (1-4) increase in the amount of catalyst (C/Pt) [74]. Such type of effect is also observed in case of carbon supported nano-Pd, where the decomposition temperature reduced to 220oC and released maximum 18.4 wt% hydrogen for 50% catalyst [75]. Table 5 indicates the hydrogen storage properties of LiBH4 and catalyst mixed for different mass ratios. Mutual catalysis between hydrogenated 40Ti-15Mn-15Cr-30V alloy (HBCC) and LiBH4 was observed and consequently the decomposition temperature of the system is lowered than that of pure LiBH4 and/or HBCC [76]. It is observed that LiBH4 and HBCC (1:3 mass ratio) has two desorption peaks, one at 250oC (corresponds to HBCC desorption, which is 15oC lower than pure HBCC) and second peak is located at 400oC (corresponds to LiBH4 desorption, which is 40oC lower than pure LiBH4). 5. DESTABILIZATION BY METAL OXIDE AND HALIDE Different types of metal halides and oxides could be used to destabilize LiBH4 and reduce the decomposition temperature of LiBH4. Metal halides or oxides modified lithium borohydride sample are prepared by ball milling process and also other processes (e.g. MTDP). After ball milling the Li metal is partially substituted by another metal. Substitution of Li follows this reaction steps, here MX is metal halide: m MX + n LiBH4 → MmLin-m (BH4)n + m LiX (22) Dehydrogenation reaction of the substituted borohydride should be: MmLin-m (BH4)n → Lin-m MmH + nB + (2n-1/2) H2 (23) In presence of metal oxide, LiBH4 destabilized by following reaction: 2LiBH4 + MO2 → 2LiOH + MB2 + 3H2 (24) Zuttel et al first observed the interaction of LiBH4 with SiO2 and reported that 1:3 mass% mixture of LiBH4 and SiO2 powder starts desorbing hydrogen at low temperature than pure LiBH4 and desorbs 9 mass% below 400 oC[14,77]. When silica gel is used instead of SiO2 21 International Journal of Energy for a Clean Environment 11 (1-4) powder a mass loss of 5 wt% is observed at 300oC (powder took 400oC) under He-flow conditions and heating rate 5 K/min [78]. This is mainly due to the high surface area of silica gel. With addition of TiCl3, NiCl2, PdCl2 to the LiBH4-silica gel mixture total hydrogen release of 12-13 wt% (normalized to the amount of LiBH4) was reported at 550oC. Whereas in presence of LaCl3 the LiBH4-silica gel mixture showed an additional decomposition step at 185 oC with ~2 wt% weight loss, however a total weight loss of 16wt% was reported at 5500C[78]. The destabilization approach for SiO2-doped LiBH4 hydrogen storage composite is indicated by the following reaction: 4LiBH4 + 2SiO2 → Li4SiO4 + 4B + Si + 8H2 (25) Li4SiO4 with high stability is the major obstacle for reverse hydrogenation of LiBH 4-SiO2 composite. TiF3 could be doped in the composite for avoiding the formation of Li4SiO4 and thus enhancing the reversible hydrogen storage properties. For 5:2:3 mixture of LiBH 4, SiO2 and TiF3 the desorption starts at 70oC and weight loss of 8.3 wt% below 500oC reported[79]. This composite is reported to demonstrate the ability of rehydrogenation at 4.5MPa an amount of 4wt% at5000C within 233 minutes. After investigation of desorption properties of LiBH4 in presence of TiO2, ZrO2, V2O3, SnO2, TiCl3 and MgCl2 and it was found that TiO2, ZrO2, V2O3 and SnO2 effectively reduced the temperature of hydrogen desorption in contrast with commercial pure LiBH 4 [80]. The materials LiBH4 75% + TiO2 25%, LiBH4 75%+V2O3 25%, LiBH4 75% + ZrO2 25%, and LiBH4 75% + SnO2 25%, desorbed 8–9 wt% hydrogen starting from 175◦C. LiBH4 75% + TiO2 25% mixture desorbed about 1.5 wt%, 4.5 wt% and 8.0 wt% hydrogen at 200◦C, 300◦C and 400◦C, respectively. After dehydriding at 6000C for 1 h, the materials LiBH4 75% + TiO2 25% and LiBH4 75%+V2O3 25% were rehydrided at 6000C and 100 bar. The dehydrided materials (for LiBH4 75% + TiO2 25% and LiBH4 75%+V2O3 25%) absorb 7.8 wt% and 7.9 wt% hydrogen 22 International Journal of Energy for a Clean Environment 11 (1-4) within 45 min, respectively. The rehydrogenation capacity was above 8 wt%, if absorption time was prolonged to 1 h. To verify the reversibility of LiBH4+ TiO2 system 5th cycle desorptionabsorption operation was performed and observed that storage properties gradually decrease from 9 wt% to 2 wt%. Since the LiBH4-TiO2 system has significant hydrogen sorption capacity, the system was studied for different stoichiometric ratios of the reactants[81]. In the case of 4:1 LiBH4-TiO2 sample there were three main hydrogen evolution steps at 325, 405, 525oC with total hydrogen release of 11.9 wt%. For 1:1 sample it was found that the onset temperature of dehydrogenation decreased to 150oC and hydrogen being evolved in three steps, having peak temperature 245, 390, 465oC with total weight loss 9.0 wt%. But for 1:4 LiBH4-TiO2 sample have only one peak at 245oC with a shoulder at 180oC and total loss was 3.65 wt%. It was also observed that among V2O5, Nb2O5, Fe2O3 and SiO2 mixed LiBH4 have a destabilization order Fe2O3>V2O5>Nb2O5>TiO2>SiO2 [82]. In case of Fe2O3 destabilized LiBH4 showed two desorption peaks at 250 and 363 oC, with most of the hydrogen released in the first step. The desorption behavior of LiBH4-V2O5 composite showed the presence of two distinct peaks at 253 and 397oC. The LiBH4-Nb2O5 sample showed two main evolution peaks at 273 and 443oC, with other two small peaks located at 340 and 370oC. For these entire samples the onset temperature for dehydrogenation was as low as 300oC with release of around 9 wt% of hydrogen. It was found that LiBH4 + 0.2 MgCl2 + 0.1 TiCl3 mixture started releasing hydrogen at 60oC and desorbs total of about 5 wt% of hydrogen from 60oC to 400oC. Increasing the amount of additive shows a decrease in desorption temperature Td but at the same time hydrogen storage capacity also decreases[16]. At 400◦C, the material desorbed 2.5 wt% of hydrogen rapidly in the first 15 min and 4.9 wt% of hydrogen following an incubation period of 18 h. The initial rapid dehydriding could be attributed to surface decomposition. The second desorption at 500◦C was 23 International Journal of Energy for a Clean Environment 11 (1-4) almost the same as the first with 4.9 wt% of hydrogen released. However the desorption kinetics are enough slow for any practical application. Destabilization of LiBH4 is verified in presence of TiCl3, TiF3, ZnCl2, FeCl3, MgCl2, MgF2, SrCl2, CaCl2, TiH2 and observed that the additives TiCl3, TiF3, and ZnF2 reduced dehydriding temperatures remarkably in the first dehydriding cycle [13]. TiH2 don’t have any effect on decomposition temperature. LiBH4 + 0.1TiCl3 desorbed H2 slowly from 100 to 350°C and rapidly from 350 to 450°C. Above 450°C, dehydriding slowed and after 500°C, 6wt % hydrogen was liberated from the sample. It was observed that after heating LiBH4 with ZnCl2, AlCl3, and ZrCl4, Li ion is successfully substituted by these metals and as a result decomposition temperature reduces [41]. It was observed that on heating ZnLi(BH4)3 and AlLi(BH4)4, these borohydrides disproportionate into Zn(BH4)2- or Al(BH4)3- and LiBH4 based phases, respectively. Desorption profile showed two peaks, first one due to the formation of less stable Zn or Al borohydride (at ~ 130oC) and second one for LiBH4 phases (at ~ 430oC). With increasing the value of “m” for ZnLim−4(BH4)m and AlLim−4(BH4)m desorption temperature is increased by 10oC. However the Zr substituted double cation borohydride, ZrLi(BH4)5 and ZrLi2(BH4)6 , did not disproportionate. Decomposition temperature for ZrLim−4(BH4)m, is 167oC (m= 4), 322oC (m= 5) and 377oC (m= 6) and shifts gradually to higher temperature with increasing composition ratio of Li/Zr and continuously approaches to 470oC which is Td for LiBH4. Double cation borohydride LiSc(BH4)4 is also formed after ball milling of ScCl3 and LiBH4, via ionic route, but on desorption of this product ScB2, B, ScH2 are formed [83]. Furthermore reabsorption of H2 to the Li-Sc-B-H system results in the formation of Li2B12H12 compounds along with LiBH4, as such the system can’t be used as a reversible hydrogen storage media. 24 International Journal of Energy for a Clean Environment 11 (1-4) After ball milling of LiBH4 and MnCl2 a double cation borohydride [LiMn(BH4)3] was formed, on doping with 1.5 mol% nano-Ni, onset decomposition temperature is reduced to 107oC and released maximum hydrogen 8.0 wt% between 135 to 155oC under 170 bar pressure [84]. It was also observed that among nano-Ni, nano-Fe, nano-Cu, nano-Zn, nano-Pd, nano-Co and nano-Ti dopants; the nano-Ni and nano-Co destabilized the system and the order of destabilization is nano-Ni > nano-Co > nano-Fe > nano-Cu > nano-Ti > nano-Zn > nano-Pd > undoped. It is also reported that ball milled [3LiBH4+MnCl2] mixture desorbed 4 wt% H2 at 100oC under atmospheric hydrogen pressure (0.1 MPa) within 6 hr [85]. The addition of nano-Ni (SSA= 60.5 m2/g) allowed desorption of 3.7 wt% H2 within 2.5 hr at 100oC. Partial substitution of Li ion by Cu theoretically leads to a good result. It was calculated that compound (Li1-xCux)BH4, (x= 0.25, 0.3, 0.5, 0.75, 1) with optimum value of x= 0.3 could significantly reduce the decomposition temperature [86]. Recently it was observed that mechanically milled 3LiBH4-TiF3 mixture can rapidly dehydrogenated at moderate temperature 70-90oC to obtain 5 wt% of hydrogen gas without undesired gas impurity [87]. Mechanical milling of the sample formed an in situ intermediate Ti(BH4)3 which was rapidly decomposed to give hydrogen (reaction 26). 3LiBH4 + TiF3 → 3LiF + Ti(BH4)3 → 3LiF + 3B + TiH2 + 5H2 6. (26) DESTABILIZATION BY AMIDE LiBH4 mixed with LiNH2 could be proposed to be a good hydrogen storage media. It has hydrogen storage capacity of 11.9 wt% [4 moles of H2 per 1 mole of LiBH4 and 2 moles of LiNH2] according to reaction as follows: LiBH4 + 2LiNH2 → Li3BN2 + 4H2 (27) 25 International Journal of Energy for a Clean Environment 11 (1-4) Thermodynamically (USSP method) enthalpy of the reaction calculated as 23 kJ/mol H 2 (without zero-point energy consideration) [88]. It was reported that on ball milling or by heating above 95oC, a 2:1 molar ratio of LiNH2 and LiBH4 formed a new quaternary hydride with composition Li3BN2H8; which desorbed 7.8 wt% of H2 at 249oC and total 11.2 wt% above 250oC with approximately 2 wt% NH3 [88-91]. LiBH4 and LiNH2 mixture with 2:1, 1:2 ratio showed that as the ratio of LiNH2 increases desorption temperature decreases. In case of 2LiBH4+LiNH2 mixture two peaks are observed. Actually in this case first peak corresponds to desorption from reaction between part of LiBH4 and LiNH2, then rest LiBH4 desorbs alone to give 7.9-9.5 wt% hydrogen and the reactions proceed through eqn. 28 [92]. 2LiBH4+LiNH2{=1/2 (LiBH4+ 2LiNH2)+3/2 LiBH4}→ ½ (Li3BN + 4H2)+3/2(LiH+B+ 3/2H2) (28) On ball milling LiBH4+2LiNH2 mixture, a product phase of Li3BN2H8 was observed in XRD profile. The diffraction peaks for Li3BN2H8 are shifted to a higher angle with an increase in the milling time. But the desorption characteristic were reported to be the same for all cases [92]. Bulk Li3BN2H8 dehydrogenated monotonically with the hydrogen desorption midpoint temperature at 324oC. After the complete dehydrogenation, rehydrogenation is not possible, thus demonstrating irreversibility. Recently it is observed that reversibility of Li3BN2H8 could be improved by incorporating the hydrides (LiBH4+ 2LiNH2) into relatively inert nanoporous carbon aerogels and activated carbon nano materials [93]. After dehydrogenation, all samples were rehydrogenated at 3000C under 50 atm H2 for 10 h. This mixture showed decomposition temperature of 3200C and 11.1 wt%of hydrogen is released on complete dehydrogenation. In contrast to bulk Li3BN2H8, the aerogel scaffolded Li3BN2H8 can be rehydrogenated to 32% of the original 11.1 wt% capacity and then 26 International Journal of Energy for a Clean Environment 11 (1-4) completely dehydrogenated on a 2nd cycle. It was also observed that at higher pressure and extended time (e.g. 80 atm and >20 h) the reversible capacity increases up to 75% [93]. For Li3BN2H8 incorporated into the AX-21 sample, the overall desorption temperatures were decreased compared to the bulk hydride. Some amount of hydrogen could even be released from samples at 1650C, with the rest desorbed at a peak temperature of 3050C. The samples could be 34% rehydrogenated at 3000C under 50 atm over 10 h, and release most of the hydrogen during the subsequent dehydrogenation[93]. Improvement of hydrogen storage capacity of the LiBH4+2 LiNH2 mixtures or Li3BN2H8 was investigated on addition of various catalysts of transition metals. A doping of 5 wt% CoCl2 considerably decreased the dehydrogenation temperature of a mixture of LiNH2 and LiBH4 [94]. More than 8 wt% of hydrogen was reported to be released at 1550C. X-Ray absorption near edge structure (XANES) spectroscopy indicated the formation of metallic Co after ball milling CoCl2 with LiNH2 and LiBH4. Hydrogen desorption temperature of Li3BN2H8 was found to decrease with the addition of noble metals (e.g. Pd, Pt, Fe, Zn, Ni) and their chlorides [95,96]. After addition of these type of additives the temperature when desorption of hydrogen becomes half is denoted by ∆T1/2 ( Temperature Shift, ∆T1/2, of the midpoint of the dehydrogenation weight loss measured by TGA relative to the midpoint for additive free alloy at T 1/2) decreases very highly. Effect of different additives to the LiBH4 system and variation on hydrogen released and T 1/2 is tabulated in Table 6. After heating up to 1800C, 3LiNH2 + LiBH4 mixture under oxygen- and moisture-free conditions resulted in the formation of a new phase of stoichiometry Li4BN3H10. The melting of Li4BH4(NH2)3 occur at 2200C. Hydrogen release starts at 2600C and maximizes at around 3300C 27 International Journal of Energy for a Clean Environment 11 (1-4) with total release of 11.1 wt% hydrogen, along with release of small amount of ammonia (5% of total gas desorbed)[97]. 7.1 Ternary hydride mixture: A new type of concept to destabilize and achieve reversibility of the LiBH4 system was ; milling three different hydrides together. Ball milling of 2LiNH2+LiBH4+MgH2 mixture was reported to have an onset hydrogen desorption at 110oC while heated with heating rate 5oC/min under 100sccm argon flow. This mixture released total 8.2 wt% hydrogen through three hydrogen releasing steps. The first 3.5 wt% hydrogen released very quickly within minute at 2600C. Remaining 4.7 wt% hydrogen took 12h to release at 260 0C. After phase analysis of the reactant and product of the sample following reaction steps were proposed corresponding to hydrogen desorption [98]: 2 Li4BH4(NH2)3 + 3MgH2 → 3Li2Mg(NH)2 + 2LiBH4+ 6H2 (29) Mg(NH2)2+ 2LiH → Li2Mg(NH)2+ 2H2 (30) 3Li2Mg(NH)2 + 2LiBH4 → 2 Li3BN2 + Mg3N2 + 2LiH + 6 H2 (31) Above mixture showed reversibility with only 2.8 wt% hydrogen storage capacity, corresponding to only the second desorption step[98]. With the increase in MgH2 amount in the mixture, it was observed that the first desorption step temperature reduced readily to 160 0C. The amount of NH3 in desorbed gas decreased with increase in MgH2 ranging from 2.1 wt% NH3 for X=0.25 to <0.1 wt% NH3 for X=1 (where X= number of mol of MgH2 in reaction mixture) [99]. 7.2 Reaction with NH3: It was observed that LiBH4 didn’t react with NH3 after heating up to 3000C. However, in presence of MgCl2, the reaction occurred, and as a result hydrogen sorption properties of LiBH4 were improved [100]. Here MgCl2 acted as an ammonia carrier and in presence of this carrier 28 International Journal of Energy for a Clean Environment 11 (1-4) decomposition temperature changed and hydrogen released from both the ligands of NH3 and the BH4 anion at temperature as low as 100oC. For the mixture Mg(NH3)nCl2-nLiBH4, for n=6 desorption started at 750C and released maximum 10.5 equiv. H2 with 1.62 equiv. of NH3. It was also mentioned that amount of LiBH4 plays an important role in the reaction as it influences the total amount of the hydrogen desorbed. When x = 0.67 in mixture [2LiNH2+xLiBH4+MgH2] the amount of hydrogen desorbed was reported to be highest (9.1 wt%) at 3200C [101,102]. Taking different composition of LiNH2, LiBH4 and MgH2 it was observed that 3 : 1 : 1.5 ratio was the optimum composition corresponding to the highest hydrogen storage capacity. Size of MgH2 added also effects the desorption temperature of the LiNH2+LiBH4+MgH2 ternary hydride[103]. It was observed that nano-MgH2 (10 nm) released hydrogen at 150oC as compared to other sample at 175oC, which were synthesized using commercial MgH2 crystalline (35-75 nm). Table 7 indicates the hydrogen release temperature and capacity, amount of ammonia released as well as possible reversibility for different ratios of reactants. 7. DESTABILIZATION BY CARBON ADDITIVES Desorption temperature of LiBH4 could be modified by carbon additives such as graphite, carbon nano-tubes, fullerene, mesoporous carbon etc. 7.1 Graphite: Vajo et al reported that for graphite modified LiBH4 the desorption temperature (453oC) does not changes and a 0.22 wt% hydrogen desorption is observed at 300oC for 1 h heating [104,105]. However, recently it was reported that mechanically milled LiBH4-30% graphite could reduce the decomposition temperature by 40oC and desorbed 9.9 wt% hydrogen (without considering graphite weight) upon heating to 510oC [106]. 29 International Journal of Energy for a Clean Environment 11 (1-4) 7.2 Mesoporous Carbon Aerogel: Confinement of LiBH4 within the carbon mesopores was found to significantly improve both the hydrogen sorption kinetics and thermodynamics, compared to the bulk hydride. Ball milling of LiBH4 with mesoporous carbon CMK-3 reduces the dehydrogenation enthalpy to 40 kJ/mol H2. Major dehydrogenation peak was observed for this sample at about 415 oC and more than 7 wt% of hydrogen (14% of without weight of CMK-3) was desorbed below 600oC [107]. Possible approach for the dehydrogenation reaction was proposed as: LiBH4 → LiH + B + 2H2 (32) LiBH4 + C → Li2C2 + B + H2 (33) LiH + C → Li2C2 + H2 (34) Mesoporous carbon aerogel impregnation to molten LiBH4 reduced the hydrogen decomposition temperature by about 100oC, which varies inversely with pore size. Decomposition temperature decreases in mesoporous carbon due to the reduced diffusion distance and increased surface area of LiBH4. Aerogel with pore volume 0.8-1.4 cm3/g desorbed ~1 wt% hydrogen [104]. This result is observed because of very low (25-45%) loading of LiBH4 into the sample. Hydrogen capacity should increase on improving the pore volume of carbon nanopores. Reversibility was observed in the sample, but the storage capacity was reported to decrease with the number of cycles; after 3rd cycle storage capacity became 0.6 wt%. Aerogel with 13 nm and 25 nm pore size showed decomposition temperature of 381, 3900C with 12.5 and 7.8 wt% h-1 dehydrogenation rates respectively. LiBH4/mesoporous carbon (volume 1.1 cm3/g ) composites, made by process of liquid infiltration of mesoporous carbon by LiBH4 solution in ethers with a 33:67 weight ratio, showed excellent desorption kinetics with a hydrogen release of 3.4 wt% in 90 min at 3350C [108]. For 30 International Journal of Energy for a Clean Environment 11 (1-4) 50:50 weight ratio of LiBH4/mesoporous carbon a 6 wt% hydrogen release at 5000C was reported. The process of impregnation in this case is solubilization of LiBH4 in ethers. 7.3 Activated Carbon: The LiBH4/AC sample starts to release hydrogen from just 220 oC, which is 150oC lower than the onset dehydrogenation temperature of bulk LiBH4 [109]. Total 11.3 wt% of hydrogen was released from the sample at 360oC. For this mixture, chemical impregnation technique was used to load LiBH4 into AC. 7.4 Carbon Nanotubes: The ball milling of LiBH4 with 30% single wall carbon nanotubes (SWNT), resulted in reduction of hydrogen desorption temperature and the onset of decomposition was at 2800C and released 12.3 wt% hydrogen after heating up to 5500C [110,111]. For LiBH4/MWNT mixtures, the initial temperature for hydrogen desorption decreases to 250°C. For 2:1 mass ratio of LiBH4 and MWNT, respectively, a main desorption peak at 460°C with three shoulders at around 300, 350, and 550°C was reported. For 1:1 and 1:2 mass ratio of LiBH4 and MWNT, shoulders disappear and the main temperature peak shifts to 375 and 360 °C respectively. It was found that the rehydrogenation of 4.5wt% hydrogen was observed for LiBH4/MWNT (1:2) at 10 MPa hydrogen pressure[112] and 4000C for 24 h based on a reaction as follows: Li2C2+H2→2LiH+2C (35) 7.5 Fullerene: Fullerenes are graphitic carbon sheets rolled into sphere. Since fullerene has more electronegativity than that of graphite due to the π- and σ-bond interaction in the cage like 31 International Journal of Energy for a Clean Environment 11 (1-4) cluster. Thus more electronegative C60 interfere with the charge transfer from Li to the BH4 moiety, resulting in a minimized ionic bond between Li+ and BH-4, and a weakened covalent bond between B and H. It was observed that after ball-mill-free preparation of LiBH4-C60 composite the hydrogen desorption temperature is lowered by 800C to 3200C [113]. It was also observed that this composite restored 4.2 wt% hydrogen after heating at 3500C for 12 h under ~1.2 x107 Pa H2 pressure. 8. CONCLUSION LiBH4 is the light weight high hydrogen storage capacity material which can be used as a prosperous hydrogen storage media. It has high gravimetric (18.5 wt %) and volumetric (121 kg/m3) hydrogen storage capacity. However the thermodynamic and kinetic limitations greatly restrains the practical application of LiBH4. Due to high stability of the material it has a very high decomposition temperature (275oC) and also the rehydrogenation conditions like high temperature and pressure. The present review provides an overview of the hydrogen storage properties of Lithium borohydride and how the destabilization can result into improved properties. The destabilization could be achieved by selection of proper additives i.e. (a) elemental substitution or (b) additive interaction to form compound or alloy which could make the dehydrogenated state energetically favourable. The requirement is to identify compounds which could lead to formation of stabilized compounds thus reducing the enthalpy of overall reaction and increasing the partial pressure of the hydrides. 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Phys. Chem. C 2008, 112, 17023–17029 [112] X B Yu, Z Wu, Q R Chen, Z L Li, B C Weng, T S Huang; Improved hydrogen storage properties of LiBH4 destabilized by carbon; Applied Physics Letters 90, 034106 (2007) [113] M SWellons, P A Berseth, R Zidan; Novel catalytic effects of fullerene for LiBH4 hydrogen uptake and release; Nanotech. 20 (2009) 204022 (4pp) 45 International Journal of Energy for a Clean Environment 11 (1-4) Figure 1: (a)Crystal structure of LiBH4 in orthorhombic phase at room temperature [20] (b) Crystal structure of LiBH4 in hexagonal phase at high temperature [20] (c) DSC profile of LiBH4 [22] Figure 2 : Dependence of desorption temperature with Pauling electronegativity. Inset shows corelation between desorption temperature and heat of desorption [40]. 46 International Journal of Energy for a Clean Environment 11 (1-4) A + H2 (Dehydrogenated state, dH large and Td high at const. P) dH dH2 ABx + H2 (Dehydrogenated stable state, dH low and Td low ) dH1 AH2 + xB (Hydrogenated state) Figure 3 : Generalized enthalpy diagram of additive interaction Table 1: Heat and entropy of reaction 1 obtained by different method Heat of reaction Entropy of reaction Method/corresponding data (J K-1 mol-1 H2) (kJ/ mol of H2) 68.9 100.3 By reacting compounds with HCl acid, taking ∆fHLiBH4= -194 kJ/mol ∆fHLiH= -90.7 kJ/mol 74 66.6 115 97.4 PCT observation and Vant Hoff equation Taking ∆fHoLiBH4= -190.46 kJ/mol, ∆fHLiH= -90.63 kJ/mol 71.3 DFT with PW91 approximation 76.1 DFT with LDA approximation 75 (69)a DFT with ultrasoft pseudopotential method a: inside parenthesis considering zero point energy. Referen ce 26 24 27 28 28 21 Table 2: Changes in the hydrogen storage capacity with number of moles of water in fuel[35] number of moles of water in fuel Molar mass of fuel (g/mol) Recoverable hydrogen (mass%) mixture 2 (n=0) 57.8 13.8 4 (n=2) 93.6 8.5 6 (n=4) 129.8 6.2 47 International Journal of Energy for a Clean Environment 11 (1-4) Table 3: Hydrogen wt%, volume density, enthalpy and decomposition temperature of different metal doped LiBH4 [45] S. Reactions wt% vol. ∆H kJ/mol Td, P=1 No of H2 Density of H2 (at T= bar 300K) 1 2LiBH4+Al→AlB2+2LiH+3H2 8.6 80 57.9 277 2 2LiBH4+Mg→MgB2+2LiH+3H2 8.9 76 46.4 3 2LiBH4+2Fe→2FeB+2LiH+3H2 3.9 76 12.8 4 2LiBH4+4Fe→2Fe2B+2LiH+3H 2.3 65 1.2 2 5 2LiBH4+Cr→CrB2+2LiH+3H2 170 −163 - 6.3 84 31.7 25 Table 4 : hydrogen storage capacity , calculated and reported for different metal doped LiBH 4 system Doped metal Calculated capacity (wt%)a Ti Sc V Cr Mg Al reported (wt%)b 6.5 6.7 6.3 6.3 8.8 8.5 2.5 2.9 4.4 4.4 5.6 6.8 Tmax 405 420 430 415 430 400 a: source 69, b: source 46 Table 5: Hydrogen storage properties of the mixture of Pt/C and LiBH4 of various mass ratio [74,75] Sample Catalyst doped Onset Main Total amount (%) dehydrogenation dehydrogenation dehydrogenation temperature (oC) temperature (oC) capacity (wt%) LiBH4 0 420 485;610 10.7 Pt/C-LiBH4 5 280 430,458,605 14.5 Pt/C-LiBH4 10 280 353;430;605 16.3 Pt/C-LiBH4 20 280 310;430;590 16.8 Pt/C-LiBH4 33 280 310;420;580 17.9 Pt/C-LiBH4 50 280 370;525 18.4 Pd/C-LiBH4 5 280 440;610 12.8 Pd/C-LiBH4 10 280 430;610 15.8 Pd/C-LiBH4 20 255 420;575 17.2 Pd/C-LiBH4 33 235 390;540 17.9 Pd/C-LiBH4 50 220 350;540 18.4 48 International Journal of Energy for a Clean Environment 11 (1-4) Table 6: Different Additives with wt% and decrease in T1/2 [95,96] Addtives Pt/Vulcan carbon Weight fraction added (%) ∆T1/2 (OC) 1 2 3 5 10 2.4 -38 -53 -69 -90 -87 +1 5 −11 5 −36 5 −44 5 −63 Zn 5 11 5 −104 -112 -4 ZnCl2 5 -8 Fe 5 -7 FeCl2 5 -36 TiCl3 5 -6 Pd PdCl2 5 10 8.3 -43 -64 -76 Pt 5 -49 PtCl2 6.8 -89 Graphite 2 -3 Vulcan Carbon Ni powder Ni flake Raney Ni 2800 Ni nanoparticles NiCl2 Table 7: For different mole ratios of LiNH2: LiBH4: MgH2 the hydrogen and ammonia release behaviour x:y:za Hydrogen Total hydrogen Ammonia release Reversibility desorption capacity(wt%) (up temperature (oC) to 370oC) 2:1:1 140, 250 (2 8.5 Negligible Reversible with steps) 2.5 wt% 2:0.5:1 140, 250 (2 8.6 More Reversible with steps) 3.5 wt% 2:1:2 155, 200, 250 (3 6.6 Negligible Not reversible steps) 1:1:1 155, 200, 250 (3 5.6 Negligible Not reversible steps) a: x:y:z is ratio of mole of LiNH2: LiBH4: MgH2 49