Hydrogen generation from Al/NaBH4 hydrolysis promoted by Li-NiCl2 additives

On-demand hydrogen generation from solid-state Al/NaBH₄ hydrolysis activated by Li-NiCl₂ additives are elaborated in the present paper. Hydrogen generation amount and rate can be regulated by changing Al/NaBH₄ weight ratio, Li and NiCl₂ amount, hydrolytic temperature, etc. The optimized Al−10 wt.% Li−15 wt.% NiCl₂/NaBH₄ mixture (weight ratio of 1:1) yields 1778 ml hydrogen/1 g mixture with 100% efficiency within 50 min at 323 K. The improved hydrolytic performance comes from the effect of Li-NiCl₂ additives, which decrease aluminum particle size in the milling process and produce the catalytic promoter BNi₂/Al(OH)₃ in the hydrolytic process. Compared with the conventional reaction of Al and NaBH₄ in water, there is an interaction of Al/NaBH₄ hydrolysis which improves the hydrolytic kinetics of Al/NaBH₄ via the catalytic effect of hydrolysis by-products Al(OH)₃, BNi₂, and NaBO₂. The Al/NaBH₄ mixture may be applied as a portable hydrogen generation material. Our experimental data lay a foundation for designing practical hydrogen generators.     

Mechanisms of H2 generation for metal doped Al16M (M = Mg and Bi) clusters in water

We have systematically investigated the hydrolysis mechanism of metal doped Al₁₆M (M = Al, Mg and Bi) clusters with H₂O molecules and proposed a reasonable elucidation for the experimentally observed fast H₂ generation rate and high H₂ yield in the Al–Bi based composite. Mg and Bi showed negative effect on the dissociation process of the first H₂O molecule, but accelerated further H₂ generation process. The investigation of persistent hydrolysis reactions demonstrated that the proton-transfer way makes the aluminum–water reaction a lasting process in the long-term H₂ generation in existence of Bi atom, which explains not only the previously observed fast H₂ generation rate but also high H₂ yields in the Bi added Al powder. Our experimental results of hydrogen generation form Al–Bi (Mg) mixture and water are in good agreement with the theory prediction. The facilitated hydrolysis reaction in Al₁₆Bi cluster is attributed to the weakened hydroxide adsorption with the presence of Bi in the aluminum cluster, which is the key factor to accelerate the proton-transfer process.     

Steam reforming of methanol over Cu/ZnO/ZrO2/Al2O3 catalyst

Steam reforming of methanol was investigated over Cu–ZnO–ZrO₂–Al₂O₃ catalysts at 473 and 573 K. The Cu:Zn:(Al + Zr) molar ratio was 3:3:4; however, the Zr:Al molar ratio was varied and the catalysts were pretreated at different calcination and reduction temperatures. The synthesized catalysts were characterized by N₂ physisorption, temperature-programmed reduction with H₂ (H₂-TPR), X-ray diffraction, oxidized surface TPR, and infrared spectroscopy after carbon monoxide chemisorption. The crystalline size of Cu decreased on increasing the calcination temperatures from 573 to 623 K and increased on increasing the reduction temperatures from 523 to 573 K. Among the tested catalysts, the Cu–ZnO–ZrO₂ catalyst exhibited the highest and lowest hydrogen-formation rates at 473 and 573 K, respectively. After the reaction at 573 K, all the tested catalysts exhibited an increase in the Cu crystalline size, causing the catalyst deactivation. Among the tested catalysts, the Cu–ZnO–ZrO₂–Al₂O₃ catalyst, where the Cu:Zn:Al:Zr molar ratio was 3:3:2:2, showed the highest and most stable catalytic activity at 573 K. Cu dispersion and catalyst composition affected the catalytic performance for steam reforming of methanol.     

Insight into the decomposition pathway of the complex hydride Al3Li4(BH4)13

The decomposition pathway of the complex hydride Al₃Li₄(BH₄)₁₃ is in the focus of this study. Initially the compound attracted great interest due to its high H₂ capacity (17.2 wt.%) and desorption at moderate temperatures (<100 °C). This work sheds light on its decomposition reaction by a unique experimental setup of thermogravimetry combined with spectroscopic gas phase analysis (FT-IR and MS) at ambient conditions. It is observed that the compound itself is metastable and decomposes immediately into its components, solid LiBH₄ and Al(BH₄)₃ which is monitored in the gas phase. Carbon addition decreases the observed mass loss and the spectroscopic gas phase analysis is used to learn about the impact of carbon addition.     

Hydrolysis of AlLi/NaBH4 system promoted by Co powder with different particle size and amount as synergistic hydrogen generation for portable fuel cell

Hydrogen generation from the hydrolysis of aluminum lithium/sodium borohydride (referred to as AlLi/NaBH₄) system activated by Co powder with different particle size and amount was evaluated in this paper. The designed aluminum–lithium–cobalt (referred to as Al–Li–Co/NaBH₄) systems including Al-5 wt% Li-50 wt% nano Co, Al-7.5 wt% Li-25 wt% nano Co, Al-5 wt% Li-50 wt% micro Co, and Al-7.5 wt% Li-25 wt% micro Co had 100% hydrogen yield at 323 K. The hydrogen generation rates of these systems were regulated by Co species, Co amount, as well as consecutive runs of NaBH₄ hydrolysis. The underlying activation mechanism, including the formation of Al_0.94Co_1.06 alloy and highly active and stable Co-based catalyst has been elaborated in this study. Experimental data present an inexpensive and highly efficient hydrogen source for portable fuel cell.     

Destabilization of LiAlH4 by nanocrystalline MgH2

In this work, we report the synthesis, characterization and destabilization of lithium aluminum hydride by ad-mixing nanocrystalline magnesium hydride (e.g. LiAlH₄ + nanoMgH₂). A new nanoparticulate complex hydride mixture (Li–nMg–Al–H) was obtained by solid-state mechano-chemical milling of the parent compounds at ambient temperature. Nanosized MgH₂ is shown to have greater and improved hydrogen performance in terms of storage capacity, kinetics, and initial temperature of decomposition, over the commercial MgH₂. The pressure–composition isotherms (PCI) reveal that the destabilized LiAlH₄ + nanoMgH₂ possess ∼5.0 wt.% H₂ reversible capacity at T ≤ 350 °C. Van't Hoff calculations demonstrate that the destabilized (LiAlH₄ + nanoMgH₂) complex materials have comparable enthalpy of hydrogen release (∼85 kJ/mole H₂) to their pristine counterparts, LiAlH₄ and MgH₂. However, these new destabilized complex hydrides exhibit reversible hydrogen sorption behavior with fast kinetics.     

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.     

Bare and Ni decorated Al12N12 cage for hydrogen storage: A first-principles study

Using density functional theory we have investigated the feasibility of bare and Ni decorated Al₁₂N₁₂ cages for hydrogen storage. In the bare Al₁₂N₁₂ cage, each Al atom is capable of adsorption one H₂ in molecular form with the average adsorption energy of −0.165 eV. In addition, it is shown that hydrogen prefers to remain inside the Al₁₂N₁₂ cage with molecular form. In the Ni decorated Al₁₂N₁₂ cage, the most stable site for Ni atom is the bridge site over the Al–N bond shared by the six-membered rings (BH site) out of the cage. Ni atom of the NiAl₁₂N₁₂ cage has been found to adsorb up to three hydrogen molecules. It is demonstrated that up to 20 hydrogen molecules can be stored on the exterior surface and inside of the NiAl₁₂N₁₂ cage with total gravimetric density of 6.8 wt%. As the weight percentage hydrogen storage is increasing to 6.5 wt%, the minimum value of the Gibbs free energy becomes positive at 25 K. It indicates that high weight percentage hydrogen storage cannot be achieved in NiAl₁₂N₁₂ cages.     

Destabilization of the LiNH2–LiH hydrogen storage system by aluminum incorporation

The lithium amide–lithium hydride system (LiNH₂–LiH) is one of the most attractive light-weight materials for hydrogen storage. In an effort to improve its hydrogen sorption kinetics, the effect of 1 mol% AlCl₃ addition to LiNH₂–LiH system was systematically investigated by differential scanning calorimetry, X-ray diffraction, Fourier transform infrared analysis and hydrogen volumetric measurements. It is shown that Al³⁺ is incorporated into the LiNH₂ structure by partial substitution of Li⁺ forming a new amide in the Li–Al–N–H system, which is reversible under hydriding/dehydriding cycles. This new substituted amide displays improved hydrogen storage properties with respect to LiNH₂–LiH. In fact, a stable hydrogen storage capacity of about 4.5–5.0 wt% is observed under cycling and is completely desorbed in 30 min at 275 °C for the Li–Al–N–H system. Moreover, the concurrent incorporation of Al³⁺ and the presence of LiH are effective for mitigating the ammonia release. The results reveal a common reaction pathway for LiNH₂–LiH and LiNH₂–LiH plus 1 mol% AlCl₃ systems, but the thermodynamic properties are changed by the inclusion of Al³⁺ in the LiNH₂ structure. These findings have important implications for tailoring the properties of the Li–N–H system.     

Al–Li3AlH6: A novel composite with high activity for hydrogen generation

A novel material for hydrogen generation with high capacity of H₂ generation has been successfully prepared by ball milling the mixture of Al and home-made fresh Li₃AlH₆ powder. Its theoretical capacity of hydrogen released is higher than that of pure Al. Results obtained have shown conversion efficiency of Al–Li₃AlH₆ composite can be close to 100% by increasing the content of Li₃AlH₆. When the content of Li₃AlH₆ is 20 wt%, the maximum hydrogen generation rate and hydrogen yield are 2737.6 mL g⁻¹ min⁻¹ and 1513.1 mL g⁻¹, respectively, at room temperature. By XRD, SEM analyses and reaction heat measurements, it demonstrates that the additive Li₃AlH₆ can provide an additional source of H₂ and an alkaline environment (LiOH) as well as additional heat to promote the Al/H₂O reaction. Therefore, the Al–Li₃AlH₆ composite has a very high activity and high capacity of hydrogen released.     

Theoretical studies on hydrogen adsorption properties of lithium decorated diborene (B2H4Li2) and diboryne (B2H2Li2)

Using ab initio based quantum chemical calculations, we have studied the structure, stability and hydrogen adsorption properties of different boron hydrides decorated with lithium, examples of the corresponding anions being dihydrodiborate dianion, B₂H₂²⁻ and tetrahydrodiborate dianion, B₂H₄²⁻ which can be considered to be analogues and isoelectronic to acetylene (C₂H₂) and ethelene (C₂H₄) respectively. It is shown that there exists a B-B double bond in B₂H₄Li₂ and a B-B triple bond in B₂H₂Li₂. In both the complexes, the lithium sites are found to be cationic in nature and the calculated lithium ion binding energies are found to be very high. The cationic sites in these complexes are found to interact with molecular hydrogen through ion-quadrupole and ion-induced dipole interactions. In both the complexes, each lithium site is found to bind a maximum of three hydrogen molecules which corresponds to a gravimetric density of ∼23 wt% in B₂H₄Li₂ and ∼24 wt% in B₂H₂Li₂. We have also studied the hydrogen adsorption in a model one-dimensional nanowire with C₆H₄B₂Li₂ as the repeating unit and found that it can adsorb hydrogen to the extent 9.68 wt% and the adsorption energy is found to be −2.34 kcal/mol per molecular hydrogen.     

The dehydrogenation performance and reaction mechanisms of Li3AlH6 with TiF3 additive

For Li₃AlH₆ prepared by mechanical milling method, the dissociation reaction enthalpy and activation energy are calculated to be 22.1 kJ mol⁻¹ H₂ and 133.7 ± 2.7 kJ mol⁻¹, respectively. The dehydrogenation performance of Li₃AlH₆ is greatly enhanced by TiF₃ additive, especially in the kinetic behaviors. For the Li₃AlH₆ + 10 mol% TiF₃ sample, the starting temperature of dehydrogenation is obviously decreased by 60 °C from that of pure Li₃AlH₆ (190 °C), and 3.0 wt.% H₂ may be released within 1000 s at 120 °C under an initial vacuum. With the amount of TiF₃ increasing, the starting temperature decreases and the kinetics improves due to the decrease in the activation energy. The X-ray diffraction (XRD) together with thermogravimetric analysis (TGA) results show that there are three mechanochemical reactions involved during milling: i) Li₃AlH₆ + TiF₃ → 3 LiF + Al + Ti + 3H₂, ii) Ti + H₂ → TiH₂, iii) 3 Al + Ti → Al₃Ti. The in-situ formed Ti species (TiH₂ and Al₃Ti) co-catalyze the thermal dehydrogenation of Li₃AlH₆.     

Influence of titanium and nickel dopants on the dehydrogenation properties of Mg(AlH4)2: Electronic structure mechanisms

The structures and dehydrogenation properties of pure and Ti/Ni-doped Mg(AlH₄)₂ were investigated using the first-principles calculations. The dopants mainly affect the geometric and electronic structures of their vicinal AlH₄ units. Ti and Ni dopants improve the dehydrogenation of Mg(AlH₄)₂ in different mechanisms. In the Ti-doped case, Ti prefers to occupy the 13-hedral interstice (Ti_iA) and substitute for the Al atom (Ti_Al), to form a high-coordination structure TiH_n (n = 6, 7). The Ti 3d electrons hybridize markedly with the H 1s electrons in Ti_Al and with the Al 3p electrons in Ti_iA, which weakens the Al–H bond of adjacent AlH₄ units and facilitates the hydrogen dissociation. A TiAl₃H₁₃ intermediate in Ti_iA is inferred as the precursor of Mg(AlH₄)₂ dehydrogenation. In contrast, Ni tends to occupy the octahedral interstice to form the NiH₄ tetrahedron. The tight bind of the Ni with its surrounding H atoms inhibits their dissociation though the nearby Al–H bond also becomes weak. Therefore, Ti is the better dopant candidate than Ni for improving the dehydrogenation properties of Mg(AlH₄)₂ because of its abundant activated hydrogen atoms and low hydrogen removal energy.     

Thermodynamic and experimental approach of electrochemical reduction of CO2 in molten carbonates

This work is dedicated to the feasibility of CO₂ dissolution and transformation into CO by an electroreduction process. It is first based on a predictive thermodynamic study, through the establishment of potential-oxoacidity diagrams, mainly focused on competing reduction processes (CO₂/CO, CO₂/C, H₂O/H₂, M⁺/M) relative to pure Li, Na and K carbonates, Li₂CO₃–K₂CO₃ (62–38 mol.%) and Li₂CO₃–Na₂CO₃ (52–48 mol.%) carbonate eutectics. Due to their lower melting points, only the Li–K and Li–Na eutectics are investigated experimentally by cyclic voltammetry at a gold flag or planar disk working electrode at temperatures from 575 °C to 650 °C. CO₂ reduction wave gives evidence for both eutectics, showing that CO₂/CO system is relatively slow in the present conditions. Re-oxidation of CO formed at the Au electrode only produces a very low intensity signal, indicating a lower solubility of this species. Microelectrolysis at a potential corresponding to the reduction wave of CO₂ increases the amount of CO and an oxidation wave probably corresponding to adsorbed CO can be observed. The whole electrochemical process is complex, involving both soluble and adsorbed CO₂, mostly adsorbed CO and probably other intermediate species.     

Investigation of the thermochemical transformations in the LiAlH4-LiNH2 system

The thermal transformations in the lithium alanate-amide system consisting of lithium aluminum hydride (LiAlH₄) and lithium amide (LiNH₂), mixed in a 1:1 M ratio, were investigated using the pressure-composition-temperature analysis, solid-state nuclear magnetic resonance, X-ray powder diffraction, and residual gas analysis. Below 250 °C, the alanate decomposes into Al, LiH and H₂, through the formation of Li₃AlH₆, whereas the amide remains largely intact. The release of gaseous hydrogen corresponds to approximately 5 wt%. Above 250 °C, additional ∼4 wt% of hydrogen is produced through solid-state reactions among LiNH₂, LiH and metallic Al, through the formation of intermetallic Li-Al binary alloy and an unidentified intermediate. The overall reaction of the thermochemical transformation of the LiAlH₄-LiNH₂ mixture results in the production of Li₃AlN₂, metallic Al, LiH and the release of 9 wt% of gaseous hydrogen. The reaction mechanism of the thermal decomposition is different from one identified earlier during mechanical treatment of the same system. Rehydrogenation of the thermally-decomposed products of LiAlH₄-LiNH₂ mixture using high hydrogen pressure (180 bar) and heating (275 °C) yields LiNH₂ and amorphous aluminum nitride (AlN).     

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.     

Sintering and electrical properties of Ce0.75Sm0.2Li0.05O1.95

Ceria-based electrolytes have been widely investigated in intermediate-temperature solid oxide fuel cell (SOFC), which might be operated at 500–600 °C. Samarium doped (20 mol%) ceria (20SDC) one of the most promising material in this class of compounds. In this work we report effect of lattice substitution of 5 mol % Li on Sm in (20SDC). It was prepared by citrate–nitrate auto combustion synthesis having a powder of average particle size ∼50 nm. The sintered density of more than 98% of the theoretical density at 950 °C has been achieved. Increased ionic conductivity (lattice) at 500 °C has also been achieved in Ce_0.75Sm_0.2Li_0.05O_1.95 compare to that of Ce_0.8Sm_0.2O_1.95. Corresponding activation energy of conduction ∼0.7 eV has been calculated in the temperature range of 200–600 °C. In reducing atmosphere the electrical conductivity has not been altered much. Thus Ce_0.75Sm_0.2Li_0.05O_1.95 has been found to be quite promising in terms of reducing the processing temperature as well as operating temperature of SOFC.     

Electronic and dehydrogenation properties of TiB2 cluster-doped NaAlH4 (101) surface: A first-principle approach

The crystal structures, electronic and dehydrogenation properties of TiB₂ cluster-doped NaAlH₄ (101) surface have been investigated by the first-principles density functional theory method. In the TiB₂ cluster-doped NaAlH₄ (101) surface, a Ti-centered TiB₂–Al₂H₈–AlH₅–AlH₃ complex is observed, and the AlH₃ and (AlH₅)²⁻ units in the TiB₂–Al₂H₈–AlH₅–AlH₃ favor the first-step decomposition reaction of NaAlH₄. The calculated electronic properties show that B–Ti bonds are stronger than B–Al and Ti–H bonds, which demonstrates that TiB₂ does not change its configuration in catalyzing the decomposition reaction of NaAlH₄. The results of hydrogen desorption energies imply that the import of TiB₂ makes the strength of Al–H bonds decreases. Therefore, the removal of H atoms, especially the removal of H atoms in the Ti–H–Al bonds is easier in the TiB₂ cluster-doped NaAlH₄ than in pure NaAlH₄.     

Synergistic hydrogen generation from AlLi alloy and solid-state NaBH4 activated by CoCl2 in water for portable fuel cell

Solid-state AlLi/NaBH₄ mixture activated by CoCl₂ salt is fabricated for hydrogen generation via a milling process, providing uniform dispersion of AlLi alloy and CoCl₂ salt among pulverized NaBH₄ particles in order to improve NaBH₄ hydrolysis through the contact of NaBH₄ with active catalytic sites. The active catalytic sites come from Co₂B loaded in Al(OH)₃ (Bayerite) or LiAl₂(OH)₇ hydrate, generated from the reaction of CoCl₂, AlLi alloy, and NaBH₄ in water. The results show that the gravimetric hydrogen storage capacity is as high as 6.4 wt.% and an efficiency of above 90% in 30-min hydrolysis at 323 K could be achieved using the limited amount of water. The hydrogen generation amount and rate could be regulated by changing the composition, mixing style, mixture/water weight ratio, and hydrolysis temperature. The relative mechanism is explored.     

An improvement of the hydrogen permeability of C/Al2O3 membranes by palladium deposition into the pores

The development of compact hydrogen separator based on membrane technology is of key importance for hydrogen energy utilization, and the Pd-modified carbon membranes with enhanced hydrogen permeability were investigated in this work. The C/Al₂O₃ membranes were prepared by coating and carbonization of polyfurfuryl alcohol, then the palladium was introduced through impregnation–precipitation and colloid impregnation methods with a PdCl₂/HCl solution and a Pd(OH)₂ colloid as the palladium resources, and the reduction was carried out with a N₂H₄ solution. The resulting Pd/C/Al₂O₃ membranes were characterized by means of SEM, EDX, XRD, XPS and TEM, and their permeation performances were tested with H₂, CO₂, N₂ and CH₄ at 25 °C. Compared with the colloid impregnation method, the impregnation–precipitation is more effective in deposition of palladium clusters inside of the carbon layer, and this kind of Pd/C/Al₂O₃ membranes exhibits excellent hydrogen permeability and permselectivity. Best hydrogen permeance, 1.9 × 10⁻⁷ mol/m² s Pa, is observed at Pd/C = 0.1 wt/wt, and the corresponding H₂/N₂, H₂/CO₂ and H₂/CH₄ permselectivities are 275, 15 and 317, respectively.