Mechanistic investigations on significantly improved hydrogen storage performance of the Ca(BH4)2-added 2LiNH2/MgH2 system

The hydrogen storage properties and mechanisms of the Ca(BH₄)₂-added 2LiNH₂–MgH₂ system were systematically investigated. The results showed that the addition of Ca(BH₄)₂ pronouncedly improved hydrogen storage properties of the 2LiNH₂–MgH₂ system. The onset temperature for dehydrogenation of the 2LiNH₂–MgH₂–0.3Ca(BH₄)₂ sample is only 80 °C, a ca. 40 °C decline with respect to the pristine sample. Further hydrogenation examination indicated that the dehydrogenated 2LiNH₂–MgH₂–0.1Ca(BH₄)₂ sample could absorb ca. 4.7 wt% of hydrogen at 160 °C and 100 atm while only 0.8 wt% of hydrogen was recharged into the dehydrogenated pristine sample under the same conditions. Structural analyses revealed that during ball milling, a metathesis reaction between Ca(BH₄)₂ and LiNH₂ firstly occurred to convert to Ca(NH₂)₂ and LiBH₄, and then, the newly developed LiBH₄ reacted with LiNH₂ to form Li₄(BH₄)(NH₂)₃. Upon heating, the in situ formed Ca(NH₂)₂ and Li₄(BH₄)(NH₂)₃ work together to significantly decrease the operating temperatures for hydrogen storage in the Ca(BH₄)₂-added 2LiNH₂–MgH₂ system.     

The structural characterization of (NH4)2B10H10 and thermal decomposition studies of (NH4)2B10H10 and (NH4)2B12H12

The structure of (NH₄)₂B₁₀H₁₀ (1) was determined through powder XRD analysis. The thermal decomposition of 1 and (NH₄)₂B₁₂H₁₂ (2) was examined between 20 and 1000 °C using STMBMS methods. Between 200 and 400 °C a mixture of NH₃ and H₂ evolves from both compounds; above 400 °C only H₂ evolves. The dihydrogen bonding interaction in 1 is much stronger than that in 2. The stronger dihydrogen bond in 1 resulted in a significant reduction by up to 60 °C, but with a corresponding 25% decrease in the yield of H₂ in the lower temperature region and a doubling of the yield of NH₃. The decomposition of 1 follows a lower temperature exothermic reaction pathway that yields substantially more NH₃ than the higher temperature endothermic pathway of 2. Heating of 1 at 250 °C resulted in partial conversion of B₁₀H₁₀²⁻ to B₁₂H₁₂²⁻. Both 1 and 2 form an insoluble polymeric material after decomposition. The elements of the reaction network that control the release of H₂ from the B₁₀H₁₀²⁻ can be altered by conducting the experiment under conditions in which pressures of NH₃ and H₂ are either near, or away from, their equilibrium values.     

Characterizations of composite NaNH2-NaBH4 hydrogen storage materials synthesized via ball milling

Composite NaNH₂-NaBH₄ (molar ratio of 2/1) hydrogen storage materials are prepared by a ball milling method with various ball milling times. The compositions and hydrogen generation characteristics are investigated by means of X-ray diffraction (XRD) and thermo gravimetric-differential thermal analysis (TG-DTA). The structural characteristics imply that ball milling produces a new phase of Na₃(NH₂)₂BH₄, and mechanical energy accumulated in the ball milling process may be responsible for the phase change of Na₃(NH₂)₂BH₄. TG-DTA demonstrates that the phase change temperature of the composite NaNH₂-NaBH₄ (2/1) ball milled for 16 h is 141.8 °C, and the melting point is 197.3 °C; below 400 °C, composite hydrogen storage material is mainly decomposed to give hydrogen and Na₃BN₂; while above 400 °C, the previous by-product Na₃BN₂ continues to decompose so as to give metal Na gradually.     

Thermal decomposition of (NH4)2SO4 in presence of Mn3O4

The main objective of this work is to develop a hybrid water-splitting cycle that employs the photon component of sunlight for production of H₂ and its thermal (i.e. IR) component for generating oxygen. In this paper, (NH₄)₂SO₄ thermal decomposition in the presence of Mn₃O₄, as an oxygen evolving step, was systematically investigated using thermogravimetric/differential thermal analyses (TG/DTA), temperature programmed desorption (TPD) coupled with a mass spectrometer (MS), X-ray Diffraction (XRD), and X-ray Photoelectron Spectroscopy (XPS) techniques. Furthermore, thermolysis of ammonium sulfate, (NH₄)₂SO₄, in the presence of Mn₃O₄ was also investigated by conducting flow reactor experiments. The experimental results obtained indicate that at 200-450 °C, (NH₄)₂SO₄ decomposes forming NH₃ and H₂O and sulfur trioxide that in the presence of manganese oxide react to form manganese sulfate, MnSO₄. At still higher temperatures (800∼900 °C), MnSO₄ further decomposed forming SO₂ and O₂.     

MoO3 nanorods/Fe2(MoO4)3 nanoparticles composite anode for solid oxide fuel cells

MoO₃ nanorods/Fe₂(MoO₄)₃ nanoparticles composite has been prepared by a hydrothermal method combined with an in situ diffusion growth process. Single cells based on 300 μm LSGM electrolyte have been fabricated with the MoO₃ nanorods/Fe₂(MoO₄)₃ nanoparticles composite anode and a composite cathode consisting of Sr_0.9Ce_0.1CoO_3−δ and Sm-doped ceria (SDC). The peak power densities reach 225, 50, 75 mW cm⁻² at 900 °C in H₂, CH₄ and C₃H₈, respectively. The cell shows excellent long-term stability at 850 °C. The preliminary results demonstrate that the MoO₃ nanorods/Fe₂(MoO₄)₃ nanoparticles composite is a promising alternative anode for solid oxide fuel cells.     

The reactions in LiBH4-NaNH2 hydrogen storage system

Stepwise reactions were observed in the ball milling and heating process of the LiBH₄-NaNH₂ system by means of X-ray diffraction (XRD) and Fourier transform infrared spectrometry (FT-IR). During the ball milling process, two concurrent reactions take place in the mixture: 3LiBH₄ + 4NaNH₂ → Li₃Na(NH₂)₄ + 3NaBH₄ and LiBH₄ + NaNH₂ → LiNH₂ + NaBH₄. The heating process from 50 °C to 400 °C is mainly the concurrent reactions of Li₃Na(NH₂)₄ + 3LiBH₄ → 2Li₃BN₂ + NaBH₄ + 8H₂ and 2LiNH₂ + LiBH₄ → Li₃BN₂H₈ → Li₃BN₂ + 4H₂, where the intermediate phases Li₃Na(NH₂)₄ and LiNH₂ serve as the reagents to decompose LiBH₄. The merged equations for the mechanochemical and the heating reactions below 400 °C can be denoted as 3LiBH₄ + 2NaNH₂ → Li₃BN₂ + 2NaBH₄ + 4H₂. The maximum dehydrogenation capacity in closed system below 400 °C is 5.1 wt.% H₂, which agrees well with the theoretical capacity (5.5 wt.% H₂) of the merged equation and thus demonstrates the suggested pathway. The subsequent step consists of the decompositions of NaBH₄ and Li₃Na(NH₂)₄ within the temperature range of 400 °C-600 °C. The apparent activation energies of the two steps are 114.8 and 123.5 kJ/mol, respectively. They are all lower than that of our previously obtained bulk LiBH₄.     

Improved hydrogen storage reversibility of LiBH4 destabilized by Y(BH4)3 and YH3

In the present study, the synthesis of two different LiBH₄–Y(BH₄)₃ and LiBH₄–YH₃ composites was performed by mechanochemical processing of the 4LiBH₄–YCl₃ mixture and as-milled 4LiBH₄–YCl₃ plus 3LiH. It was found that Y(BH₄)₃ and YH₃ formed in situ during milling are effective to promote LiBH₄ destabilization but differ substantially from each other in terms of the dehydrogenation pathway. During LiBH₄–Y(BH₄)₃ dehydriding, Y(BH₄)₃ decomposes first generating in situ freshly YH₃ and subsequently, it destabilizes LiBH₄ with the formation of minor amounts of YB₄. About 20% of the theoretical hydrogen storage was obtained via the rehydriding of YB₄–4LiH–3LiCl at 400 °C and 6.5 MPa. As a novel result, a compound containing (B₁₂H₁₂)²⁻ group was identified during dehydriding of Y(BH₄)₃. In the case of 4LiBH₄–YH₃ dehydrogenation, the increase of the hydrogen back pressure favors the formation of crystalline YB_4, whereas a reduction to ≤0.1 MPa induces the formation of minor amounts of Li₂B₁₂H₁₂. Although for hydrogen pressures ≤0.1 MPa direct LiBH₄ decomposition can occur, the main dehydriding pathway of 4LiBH₄–YH₃ composite yields YB₄ and LiH. The nanostructured composite obtained by mechanochemical processing gives good hydrogen storage reversibility (about 80%) regardless of the hydrogen back pressure.     

Studies on dehydrogenation characteristic of Mg(NH2)2/LiH mixture admixed with vanadium and vanadium based catalysts (V,V2O5 and VCl3)

In the present study, we have investigated the effect of vanadium and its compounds (V, V₂O₅ and VCl₃) on desorption characteristics of 1:2 magnesium amide (Mg(NH₂)₂) and lithium hydride (LiH) mixture. The hydrogen storage characteristics of 1:2 Mg(NH₂)₂/LiH mixture gets enhanced with admixing of V, V₂O₅ and VCl₃ separately. The VCl₃ has been found to be the most effective followed by V and V₂O₅. The activation energy for dehydrogenation process of 1:2 Mg(NH₂)₂/LiH mixture with and without catalyst has been evaluated using a method suggested by Ozawa et al. [25]. Based on the experimental results, schematic reaction scheme for decomposition of Mg(NH₂)₂ in the presence of VCl₃ has also been proposed.     

Destabilization effects of Mg(AlH4)2 on MgH2: Improved desorption performances and its reaction mechanism

Both kinetics and thermodynamics properties of MgH₂ are significantly improved by the addition of Mg(AlH₄)₂. The as-prepared MgH₂–Mg(AlH₄)₂ composite displays superior hydrogen desorption performances, which starts to desorb hydrogen at 90 °C, and a total amount of 7.76 wt% hydrogen is released during its decomposition. The enthalpy of MgH₂-relevant desorption is 32.3 kJ mol⁻¹ H₂ in the MgH₂–Mg(AlH₄)₂ composite, obviously decreased than that of pure MgH₂. The dehydriding mechanism of MgH₂–Mg(AlH₄)₂ composite is systematically investigated by using x-ray diffraction and differential scanning calorimetry. Firstly, Mg(AlH₄)₂ decomposes and produces active Al^∗. Subsequently, the in-situ formed Al^∗ reacts with MgH₂ and forms Mg–Al alloys. For its reversibility, the products are fully re-hydrogenated into MgH₂ and Al^∗, under 3 MPa H₂ pressure at 300 °C for 5 h.     

Novel synthesis of LiFePO4-Li3V2(PO4)3 composite cathode material by aqueous precipitation and lithiation

LiFePO₄-Li₃V₂(PO₄)₃ composite cathode material is synthesized by aqueous precipitation of FeVO₄·xH₂O from Fe(NO₃)₃ and NH₄VO₃, following chemical reduction and lithiation with oxalic acid as the reducer and carbon source. Samples are characterized by XRD, SEM and TEM. XRD pattern of the compound synthesized at 700 °C indicates olivine-type LiFePO₄ and monoclinic Li₃V₂(PO₄)₃ are co-existed. TEM image exhibits that LiFePO₄-Li₃V₂(PO₄)₃ particles are encapsulated with a carbon shell 5-10 nm in thickness. The LiFePO₄-Li₃V₂(PO₄)₃ compound cathode shows good electrochemical performance, and its discharge capacity is about 139.1 at 0.1 C, 135.5 at 1 C and 116 mA h g⁻¹ at 3 C after 30 cycles.     

Hydrogen generation from hydrolysis of NH3BH3/MH (M = Li,Na) binary hydrides

Hydrogen release from hydrolysis of LiNH₂BH₃, NaNH₂BH₃, LiH–NH₃BH₃ and NaH–NH₃BH₃ respectively was investigated in this paper. It is shown experimentally that LiNH₂BH₃ and NaNH₂BH₃ hydrolysis can release 3 equiv. of hydrogen at 25 °C. Hydrolysis of LiNH₂BH₃ or NaNH₂BH₃ exhibits greatly improved kinetics in comparison with neat NH₃BH₃ hydrolysis. The electronic and structural changes from NH₃BH₃ to [NH₂BH₃]⁻ play a crucial role in the improvements. The mechanism of LiNH₂BH₃ and NaNH₂BH₃ hydrolysis is the combination of H⁺ and OH⁻ ions of water with the polar ions of LiNH₂BH₃ and NaNH₂BH₃. The process of LiH–NH₃BH₃ and NaH–NH₃BH₃ hydrolysis comprises two steps: LiH or NaH first reacts with water and then the generated heat initiates thermohydrolysis of NH₃BH₃. LiH or NaH hydrolysis is prior to the reaction of LiH or NaH with NH₃BH₃. Our results show a novel strategy to promote hydrogen release kinetics of LiNH₂BH₃ and NaNH₂BH₃. Furthermore, our results also present a novel noncatalytic method for hydrogen release from NH₃BH₃ by co-hydrolyzing it with other highly exothermic hydrides.     

Reversible hydrogen storage in LiBH4/Ca(AlH4)2 systems

To improve the hydrogen storage property of LiBH₄, the LiBH₄/Ca(AlH₄)₂ reactive systems with various ratios were constructed, and their de-/hydrogenation properties as well as the reaction mechanisms were investigated experimentally. It was found that the sample with the LiBH₄ to Ca(AlH₄)₂ molar ratio of 6:1 exhibits the best comprehensive hydrogen storage properties, desorbing hydrogen completely (8.2 wt.%) within 35 min at 450 °C and reversibly absorbing 4.5 wt.% of hydrogen at 450 °C under a hydrogen pressure as low as 4.0 MPa. During the first dehydrogenation process of the LiBH₄/Ca(AlH₄)₂ systems, the CaH₂ and Al particles were in situ precipitated via the self-decomposition of Ca(AlH₄)₂, and then reacted with LiBH₄ to form CaB₆, AlB₂ and LiH. Whereafter, the sample can cycle between LiBH₄ + Ca(BH₄)₂ + Al in the hydrogenated state and CaB₆ + AlB₂ + LiH in the dehydrogenated state.     

Influence of metal oxide on LiBH4/2LiNH2/MgH2 system for hydrogen storage properties

In this study, various nanoscale metal oxide catalysts, such as CeO₂, TiO₂, Fe₂O₃, Co₃O₄, and SiO₂, were added to the LiBH₄/2LiNH₂/MgH₂ system by using high-energy ball milling. Temperature programmed desorption and MS results showed that the Li–Mg–B–N–H/oxide mixtures were able to dehydrogenate at much lower temperatures. The order of the catalytic effect of the studied oxides was Fe₂O₃ > Co₃O₄ > CeO₂ > TiO₂ > SiO₂. The onset dehydrogenation temperature was below 70 °C for the samples doped with Fe₂O₃ and Co₃O₄ with 10 wt.%. More than 5.4 wt.% hydrogen was released at 140 °C. X-ray diffraction indicated that the addition of metal oxides inhibited the formation of Mg(NH₂)₂ during ball milling processes. It is thought that the changing of the ball milling products results from the interaction of oxide ions in metal oxide catalysts with hydrogen atoms in MgH₂. The catalytic effect depends on the activation capability of oxygen species in metal oxides on hydrogen atoms in hydrides.     

Effect of Al on the hydrogen storage properties of Mg(BH4)2

The various Mg–B–Al–H systems composed of Mg(BH₄)₂ and different Al-sources (metallic Al, LiAlH₄ and its decomposition products) have been investigated as potential hydrogen storage materials. The role of Al was studied on the dehydrogenation and the rehydrogenation of the systems. The results indicate that the different Al-sources exhibit a similar improving effect on the dehydrogenation properties of Mg(BH₄)₂. Taking the Mg(BH₄)₂ + LiAlH₄ system as an example, at first LiAlH₄ rapidly decomposes into LiH and Al, then Mg(BH₄)₂ decomposes into MgH₂ and B, finally the MgH₂ reacts with Al, LiH and B, which forms Mg₃Al₂ and MgAlB₄. The system starts to desorb H₂ at 140 °C and desorbs 3.6 wt.% H₂ below 300 °C, while individual Mg(BH₄)₂ starts to desorb H₂ at 250 °C and desorbs only 1.3 wt.% H₂ below 300 °C. The isothermal desorption kinetics of the Mg–B–Al–H systems is about 40% faster than that of Mg(BH₄)₂ at the hydrogen desorption ratio of 90%. In addition, the Mg–B–Al–H systems show partial reversibility at moderate temperature and pressure. For Al-added system, the product of rehydrogenation is MgH₂, while for LiAlH₄-added system the product is composed of LiBH₄ and MgH₂.     

Synthesis and thermal decomposition properties of a novel dual-cation/anion complex hydride Li2Mg(BH4)2(NH2)2

A novel dual-cation/anion complex hydride (Li₂Mg(BH₄)₂(NH₂)₂), which contains a theoretical hydrogen capacity of 12.1 wt%, is successfully synthesized for the first time by ball milling a mixture consisting of MgBH₄NH₂ and Li₂BH₄NH₂. The prepared Li₂Mg(BH₄)₂(NH₂)₂ crystallizes in a triclinic structure, and the [NH₂] and [BH₄] groups remain intact within the structure. Upon heating, the prepared Li₂Mg(BH₄)₂(NH₂)₂ decomposes to release approximately 8.7 wt% hydrogen in a three-step reaction at 100–450 °C. In addition, a small amount of ammonia is evolved during the first and second thermal decomposition steps as a side product. This ammonia is responsible for the lower experimental dehydrogenation amount compared to the theoretical hydrogen capacity. The XRD and FTIR results reveal that Li₂Mg(BH₄)₂(NH₂)₂ first decomposes to LiMgBN₂, LiBH₄, BN, LiH and MgBNH₈ at 100–250 °C, and then, the newly formed MgBNH₈ reacts with LiH to form Mg, LiBH₄ and BN at 250–340 °C. Finally, the decomposition of LiBH₄ releases hydrogen and generates LiH and B at 340–450 °C.     

Evaluation of the hydrogen storage behavior of a LiNH2 + MgH2 system with 1:1 ratio

A one-to-one molar ratio of LiNH₂ to MgH₂ was ball milled and characterized to evaluate the proposed hydrogen storage reaction: LiNH₂ + MgH₂ ⇔ LiMgN + 2H₂. The pressure–composition isotherm shows that less than 3.4 wt.% H₂ is released at a plateau pressure near 20 atm at 210 °C. Furthermore, X-ray diffraction show that the products of the reaction include Li₂Mg₂(NH)₃ rather than LiMgN. Combined thermogravimetric and residual gas analyses reveal that large quantities of ammonia are released from the system.     

Effect of Co3(PO4)2 coating on Li[Co0.1Ni0.15Li0.2Mn0.55]O2 cathode material for lithium rechargeable batteries

To prepare a high-capacity cathode material with improved electrochemical performance for lithium rechargeable batteries, Co₃(PO₄)₂ nanoparticles are coated on the surface of powdered Li[Co_0.1Ni_0.15Li_0.2Mn_0.55]O₂, which is synthesized by a simple combustion method. The coated powder prepared under proper conditions for Co₃(PO₄)₂ content and annealing temperature shows an optimum coating layer that consists of a Li_xCoPO₄ phase formed by reaction with lithium impurities during heat treatment. A sample coated with 3 wt.% Co₃(PO₄)₂ and annealed at 800 °C proves to be the best in terms of specific capacity, cycle performance and rate capability. Thermal stability is also enhanced by the coating, as demonstrated a decrease in the onset temperature and/or the heat generated during thermal runaway.     

Promoted hydrogen release from 3LiBH4/MnF2 composite by doping LiNH2: Elimination of diborane release and reduction of decomposition temperature

The evolution of diborane accompanying H₂ release during the decomposition of transition metal borohydrides reduces the purity of evolved hydrogen and results in capacity loss during cycling. To solve the problem, a small amount of LiNH₂ is doped into a 3LiBH₄/MnF₂ composite and the decomposition properties are investigated. The results show that after doping LiNH₂, the formation of diborane during decomposition is effectively suppressed meanwhile the decomposition temperature is significantly reduced. Around 5 wt.% pure hydrogen can be released at 95–140 °C from 5 wt.% LiNH₂-doped 3LiBH₄/MnF₂ composite. These improvements in the decomposition performance are mainly attributed to the prevention of the formation of B–H–B bonds for B₂H₆ and the destabilization of B–H bonds in borohydrides by the interaction of BH₄⁻ and NH₂⁻.     

NH3BH3/LiBH4·NH3 modified by metal hydrides for advanced dehydrogenation

The significantly enhanced dehydrogenation performance of binary complex system, NH₃BH₃/LiBH₄·NH₃, were achieved through a chemical modification of LiH to form ternary composites of x (LiH–NH₃BH₃)/LiBH₄·NH₃. Among the studied composites, 3LiH–3NH₃BH₃/LiBH₄·NH₃ released ca. 10 wt. % high-pure hydrogen (>99.9 mol%) below 100 °C with fast kinetics, while less than 8 wt. % hydrogen, accompanied with a fair number of volatile byproducts, were released from 3NH₃BH₃/LiBH₄·NH₃ at the same conditions. Further investigations revealed that the hydrogen emission from x (LiH–NH₃BH₃)/LiBH₄·NH₃ composites is based on the combination mechanism of H^δ+ and H^δ− through the interaction between LiH–NH₃BH₃ and NH₃ group in LiBH₄·NH₃, in which the controllable protic hydrogen source from the stabilized NH₃ group played a crucial role in providing optimal stoichiometric ratio of H^δ+ and H^δ−, and thus leading to the significant improvement of dehydrogenation capacity and purity.