Improving the hydrogen storage properties of MgH2 by reversibly forming Mg–Al solid solution alloys

Supersaturated Mg(Al) solid solutions with reduced lattice constants were successfully prepared by ball milling Mg and Al powder mixtures. The microstructure and phase transition were investigated by XRD. The results indicated that disproportionation of supersaturated Mg(Al) solid solution to MgH₂ and Al was caused by hydrogenation, then equilibrium Mg(Al) solid solution formed after dehydrogenation, while the intermetallic compound Mg₁₇Al₁₂ reversibly decomposed to MgH₂ and intermediate phase Al₃Mg₂ which could further decompose to MgH₂ and Al by hydriding. These reversible phase transitions make Mg–Al alloys show an observably lowered de/hydriding enthalpy and activation energy in comparison with pure Mg.     

Crystal structures of MgyTi100−y thin film alloys in the as-deposited and hydrogenated state

In situ X-ray diffraction was used to identify the crystal structures of as-deposited and hydrogenated Mg_yTi_100-y thin film alloys containing 70, 80 and 90 at.% Mg. The preferred crystallographic orientation of the films in both the as-prepared and hydrogenated state made it difficult to unambiguously identify the crystal structure up to now. In this work, identification of the unit cells was achieved by in situ recording diffraction patterns at various tilt angles. The results reveal a hexagonal closed packed structure for all alloys in the as-deposited state. Hydrogenating the layers under 10⁵ Pa H₂ transforms the unit cell into face-centered cubic for the Mg₇₀Ti₃₀ and Mg₈₀Ti₂₀ compounds, whereas the unit cell of hydrogenated Mg₉₀Ti₁₀ has a body-centered tetragonal symmetry. The (de)hydrogenation kinetics changes along with the crystal structure of the metal hydrides from rapid for fcc-structured hydrides to sluggish for hydrides with a bct symmetry and emphasizes the influence of the crystal structure on the hydrogen transport kinetics.     

X-ray photoelectron spectroscopy investigation of magnetron sputtered Mg–Ti–H thin films

Thin film samples of Mg₈₀Ti₂₀ (Mg–Ti) and Mg, both with and without H, were investigated in a series of X-ray photoelectron spectroscopy (XPS) measurements. The samples were covered with a thin protective layer of Pd, which was removed by Ar⁺ sputtering prior to data acquisition. This sputtering was found to reduce both oxides and hydrides. A distinct, previously unknown peak was revealed in the Mg KLL spectrum of the Mg–Ti–H samples, located between the metallic and the MgO component. This peak was attributed to trapping of H in very stable interstitial sites at the interface between Ti nano-clusters and the Mg matrix, based on earlier density functional theory calculations and supported by so-called Bader analysis. The latter was performed in order to study the theoretical charge distribution between Mg, Ti and H, establishing a link between the position of the previously unknown peak and the effect of H on the valence state of Mg. The composition of the samples was studied both by energy dispersive spectroscopy using transmission electron microscopy and by quantitative XPS analysis. Final state Auger parameters (AP) were obtained for metallic Mg, MgO and MgH₂, as well as Mg affected by trapped H. No difference between the AP values from the metallic components was found between the Mg and the Mg–Ti samples. The AP values for MgO and MgH₂ were consistent with previous reports in literature; several eV lower than the metallic value. Mg in the vicinity of trapped hydrogen, on the other hand, showed a more metallic character, with its corresponding AP value less than 1 eV below the AP for pure Mg.     

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.     

Effect of Titanium additive element on the discharging behavior of MgNi Alloy electrode

Mg_0.90Ti_0.10Ni, Mg_0.85Ti_0.15Ni, Mg_0.80Ti_0.20Ni, Mg_0.90Ti_0.15Ni_0.95, Mg_0.90Ti_0.20Ni_0.90 and Mg_0.95Ti_0.15Ni_0.90 ternary alloys were synthesized by mechanical alloying and their electrochemical hydrogen storage characteristics were investigated. Among the Mg-Ti-Ni ternary alloys Mg_0.90Ti_0.20Ni_0.90 alloy showed the best discharge performance. The initial discharge capacity was observed to depend on Mg/Ni atomic ratio rather than Ti/Mg atomic ratio in alloys. As the Ti/Mg atomic ratio increased the alloy charge transfer resistances decreased probably due to the partial selective dissolution of the surface Ti/Ti-oxides and thus the limited enrichment of the surface by the electro-catalytic Ni. The average hydrogen diffusion coefficients in all the Ti-including alloys were higher than that in MgNi alloy. The increase in Ti/Mg atomic ratio, however, did not cause any further increase in the average hydrogen diffusion coefficient.     

Synthesis and hydrogen storage thermodynamics and kinetics of Mg(AlH4)2 submicron rods

Mg(AlH₄)₂ submicron rods with 96.1% purity have been successfully synthesized in a modified mechanochemical reaction process followed by Soxhlet extraction. ∼9.0 wt% of hydrogen is released from the as-prepared Mg(AlH₄)₂ at 125–440 °C through a stepwise reaction. Upon dehydriding, Mg(AlH₄)₂ decomposes first to generate MgH₂ and Al. Subsequently, the newly produced MgH₂ reacts with Al to form a Al_0.9Mg_0.1 solid solution. Finally, further reaction between the Al_0.9Mg_0.1 solid solution and the remaining MgH₂ gives rise to the formation of Al₃Mg₂. The first step dehydrogenation is a diffusion-controlled reaction with an apparent activation energy of ∼123.0 kJ/mol. Therefore, increasing the mobility of the species involved in Mg(AlH₄)₂ will be very helpful for improving its dehydrogenation kinetics.     

Magnesium–titanium alloy thin-film switchable mirrors

In this paper, we show gasochromic and electrochromic switching properties of Pd top capped magnesium–titanium (Mg–Ti) thin films prepared by DC magnetron sputtering. These films show excellent switchable mirror properties. By exposing to 4% H₂ in Ar, Pd (4 nm)/Mg_0.82Ti_0.18 (40 nm) film changed from the metallic state to the transparent state drastically within 5 s. By exposing to air, it goes back to the metallic state within 60 s. The transmittance spectrum in the hydride state is quite flat in the wavelength range from 400 to 2500 nm. It looks complete color neutral and its chromaticity coordinates are x=0.326 and y=0.340. Simple electrochromic device of Mg–Ti thin film using a liquid electrolyte works very well. It can be switched between the mirror state and the color-neutral transparent state.     

Enhanced low temperature hydrogen desorption properties and mechanism of Mg(BH4)2 composited with 2D MXene

Mg(BH₄)₂ occupies a large hydrogen storage capacity of 14.7 wt%, and has been widely recognized to be one of the potential candidates for hydrogen storage. In this work, 2D MXene Ti₃C₂ was introduced into Mg(BH₄)₂ by a facile ball-milling method in order to improve its dehydrogenation properties. After milling with Ti₃C₂, Mg(BH₄)₂–Ti₃C₂ composites exhibit a novel “layered cake” structure. Mg(BH₄)₂ with greatly reduced particle sizes are found to disperse uniformly on Ti₃C₂ layered structure. The initial dehydrogenation temperature of Mg(BH₄)₂ has been decreased to 124.6 °C with Ti₃C₂ additive and the hydrogen liberation process can be fully accomplished below 400 °C. Besides, more than 10.8 wt% H₂ is able to be liberated from Mg(BH₄)₂–40Ti₃C₂ composite at 330 °C within 15 min, while pristine Mg(BH₄)₂ merely releases 5.3 wt% hydrogen. Moreover, the improved dehydrogenation kinetics can be retained during the subsequent second and third cycles. Detailed investigations reveal that not only Ti₃C₂ keeps Mg(BH₄)₂ particles from aggregation during de/rehydrogenation, but also the metallic Ti formed in-situ serves as the active sites to catalyze the decomposition of Mg(BH₄)₂ by destabilizing the B–H covalent bonds. This synergistic effect of size reduction and catalysis actually contributes to the greatly advanced hydrogen storage characteristics of Mg(BH₄)₂.     

Intrinsic mechanisms on enhancement of hydrogen desorption from MgH2 by (001) surface doping

A first principle study was carried out to investigate the dehydrogenation properties of metal (001) surface doped MgH₂. Site preference of dopants was identified and dehydrogenation properties of the doped systems were analyzed based on the total energy and electronic structure calculations. It was shown that Al and Ti prefer to substitute for Mg atoms, whereas Mn and Ni prefer to occupy interstitial sites. The mechanisms that dopants improve the dehydrogenation properties of the considered systems were clarified. Al weakens the interactions between the Mg and the H atoms and has high potential to drive a formation of an Al-Mg cluster, and therefore improves the dehydrogenation performance of the Al doped system. Ti strongly interacts with its neighboring H atoms, distorts their positions, and could potentially generate a TiH₂ phase by attracting two H atoms. Mn greatly distorts the surface structure and causes a dramatic reduction on the dehydrogenation energy in the Mn interstitially doped system. A Ni-H tetrahedral cluster is observed, which acts as a seed to form Mg₂NiH₄ phase, in the Ni doped MgH₂ (001) surface. Therefore, the improvement of the dehydrogenation properties of Ni doped system is expectable due to the formation of thermodynamically less stable Mg₂NiH₄ phase.     

Pressure DSC study of the hydrogenation and dehydrogenation of some intermetallic compounds Mg2Ni

Pressure differential scanning calorimetry (DSC) has been applied to a study of the hydrogenation and dehydrogenation of some intermetallic compounds Mg₂Ni. The effects of hydrogen pressure, pulverized compound's sizes as well as chemical composition of the compound, partial substitution of Mg in Mg₂Ni by Al, and the second phase MgNi₂ dispersed in Mg₂Ni on the hydrogenation and dehydrogenation of the intermetallic compound Mg₂Ni are elucidated in some detail by this experimental technique. It is emphasized that a pressure DSC is available as a rapid and convenient experimental means for assessing hydrogen absorption and desorption properties of hydrogen storage materials.     

Study on the dehydrogenation properties and reversibility of Mg(BH4)2AlH3 composite under moderate conditions

Mg(BH₄)₂ has been considered as one of the promising light metal complex hydrides due to its high hydrogen capacity and low cost. But its higher thermal stability (dehydrogenation at above 300 °C) needs to be improved for the practical application. In this study, the aluminum hydride AlH₃ was introduced into complex borohydride Mg(BH₄)₂ to synthesize a new Mg(BH₄)₂AlH₃ composite by ball milling method. It is found that the active Al¹ formed from the self-decomposition of AlH₃ can effectively improve the dehydrogenation properties of Mg(BH₄)₂, the Mg(BH₄)₂AlH₃ composite starts to release hydrogen at 130.8 °C with a total hydrogen capacity of 11.9 wt.%. The dehydrogenated products of the composite is composed of Mg₂Al₃ and B at 350 °C, resulting in the improved hydrogen desorption properties of Mg(BH₄)₂AlH₃ composite. The Mg₂Al₃ and B products would be further transformed into MgAlB₄ and Al at 500 °C. Moreover, the Mg₂Al₃ and B dehydrogenated products show better reversible hydrogen storage property than that of the MgAlB₄ and Al products. This research shows a way to alter hydrogen de/hydrogenation route and reversibility of Mg(BH₄)₂ complex hydride by compositing with AlH₃ and controlling the dehydrogenation temperature.     

Effects of Mg and Al doping on the electronic structure and dehydrogenation of LiBH4·NH3

The electronic structures and bonding characters, the occupation energies of dopants, as well as the formation energies of Frenkel defects in pure LiBH₄·NH₃ and in Mg- and Al-substituted LiBH₄·NH₃ were investigated by using first-principles calculations. The occupation energies show that the substitutions with Mg and Al destabilize LiBH₄·NH₃ and that Mg substitution is easier than Al substitution. Substitution with Mg or Al partly reduced interactions between B–H and N–H atoms, thus improving the dehydrogenation property of LiBH₄·NH₃. At the same time, substitution with Mg or Al increases the interactions between metal and N atoms, which stabilize the NH₃ group and inhibit the release of NH₃ during dehydrogenation. The formation energy of Frenkel defects indicates that Mg or Al doping facilitates the formation of Frenkel defects. Our theoretical studies show that Mg and Al are good candidates but Al is better than Mg for improving the dehydrogenation property of LiBH₄·NH₃.     

Mg2−xTixNi (x = 0, 0.5) alloys prepared by mechanical alloying for electrochemical hydrogen storage: Experiments and first-principles calculations

Mg_2−xTi_xNi (x = 0, 0.5) electrode alloys have been prepared by mechanical alloying (MA) under argon atmosphere at room temperature using a planetary high-energy ball mill. The microstructures of synthesized alloys are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The effects of substitutional doping of Ti in Mg₂Ni phase have been investigated by first-principles density functional theory calculations. XRD analysis results indicate that Ti substitution for Mg in Mg₂Ni-type alloys results in the formation of TiNi (Pm-3m) and TiNi₃ intermetallics. With the increase of milling time, the TiNi phase captures Ni from Mg₂Ni to further form TiNi₃ phase and the MgO phase increases. The calculated results of enthalpy of formation indicate that the most preferable site of Ti substitution in Mg₂Ni lattice is Mg(6i) position and the stability of phase gradually decreases along the sequence TiNi₃ phase > TiNi phase > Mg₉Ti_3Mg(6i)Ni₆ Ti-doped phase > Mg₂Ni phase. SEM observations show that the average particle sizes of Mg₂Ni and Mg_1.5Ti_0.5Ni milled alloys decrease and increase, respectively with increasing the milling time. The TEM analysis results reveal that TiNi and Mg₂Ni coexist as nanocrystallites in the Mg_1.5Ti_0.5Ni alloy milled for 20 h. Electrochemical measurements indicate that the maximum discharge capacities of Mg₂Ni and Mg_1.5Ti_0.5Ni alloys rise and decline, respectively with the prolongation of milling time. The Mg_1.5Ti_0.5Ni alloy milled for 20 h shows the highest discharge capacity among all milled alloys. The capacity retaining rate of Mg_1.5Ti_0.5Ni milled alloys is better than that of Mg₂Ni milled alloys.     

Investigation on gaseous and electrochemical hydrogen storage performances of as-cast and milled Ti1.1Fe0.9Ni0.1 and Ti1.09Mg0.01Fe0.9Ni0.1 alloys

The AB-type Ti_1.1Fe_0.9Ni_0.1 (Mg₀ for short) and Ti_1.09Mg_0.01Fe_0.9Ni_0.1 (Mg_0.01 for short) alloys were fabricated by vacuum induction melting and mechanical milling. The effects of partly substituting Ti with Mg and/or mechanical milling on the structure, morphology, gaseous thermodynamics and kinetics, and electrochemical performances were studied. The results reveal that the as-cast Mg₀ alloy contains the main phase TiFe and a small number of TiNi₃ and Ti₂Ni phases. Substituting Ti with Mg and/or mechanical milling results in the disappearance of the secondary phases. The discharge capacities of the as-cast Mg₀ and Mg_0.01 alloys are 12.6 and 8.8 mAh g^?1, which increase to 52.6 and 80.4 mAh g^?1 after 5 h of mechanical milling. By milling the as-cast alloy powders with carbonyl nickel powders, they are greatly enhanced to 191.6 mAh g^?1 for the Mg₀+7.5 wt% Ni alloy and 205.9 mAh g^?1 for the Mg_0.01+5 wt% Ni alloy at the current density of 60 mA g^?1, respectively. The values of dehydrogenation enthalpy (ΔH_des) and dehydrogenation activation energy (E^des_(a)) are very small, meaning that the thermal stability and the desorption kinetics of the hydrides are not the key influence factors for the discharge capacity. The reduction of the particle size and the generation of the new surfaces without oxide layers have slight improvements on the discharge capacity, while the enhancement of the charge transfer ability of the surfaces of the alloy particles can significantly promote the electrochemical reaction of the alloy electrodes.     

Mg/MgH2 hydrogen storage system destabilized by recyclable AlH3–NaBH4 composite

While borohydrides, such as NaBH₄, were often used as supplements to improve hydrogen storage properties of Mg/MgH₂ systems, they have long suffered from high decomposition temperature and irreversible dehydrogenation process. Here, we report that NaBH₄ can reversibly serve as a hydrogen storage host and reactant for Mg/MgH₂ systems under mild reaction conditions with the help of Al/AlH₃. 90 wt%MgH₂–5 wt.%AlH₃–5 wt.%NaBH₄ (M-5AB) has been successfully synthesized using the conventional mechanical alloying technique. The dehydrogenation activation energy and enthalpy are 20% and 9% reduced than those of pure Mg/MgH₂. After 10 hydrogen absorption and desorption cycles, the hydrogen storage capacity of M-5AB can reach 6.35 wt%. The X-ray diffraction (XRD) and the transmission electron microscope (TEM) measurements revealed that the interface of additives and Mg/MgH₂ decompose to Mg₁₇Al₁₂, MgAlB₄ and NaH phases. The Mg₁₇Al₁₂ and MgAlB₄ phases reduces the barrier of free energies of hydrogenated and dehydrogenated states, helping NaBH₄ to recover after rehydrogenation. These discoveries indicate that Al species can boost the decomposition and reformation of NaBH₄, providing a wider degree of freedom for the material design of Mg-based hydrogen storage materials.     

Corrosion of novel Pb–Mg–Al–B nuclear shielding alloy in fluoride medium

Abstract

Five kinds of Pb–Mg–Al–B alloys with B atomic fraction of 0, 1·84, 3·62, 5·36 and 7·06% were prepared by induction melting. The corrosion behaviour of Pb–Mg–Al–B in 3·5 wt-%NaF solution was investigated by immersion tests, neutral salt spray and electrochemical measurements. The surface morphology and corrosion products of alloys were measured using scanning electron microscopy (SEM), X-ray diffraction and X-ray photoelectron spectroscopy. Results show that the Pb–Mg–Al–B alloy with 5·36%B shows the best corrosion resistance; the volume fraction and distribution of Mg₂Pb, Mg₁₇Al₁₂ and other phases affect the corrosion resistance significantly; the major corrosion products are Mg₁₇Al₁₂, Pb, PbO, Al₂O₃, etc., and Mg is apt to form a microgalvanic cell with Mg₂Pb (Mg₂Pb–Mg) in corrosive solution, and a bit of product of MgF₂ film has a role to protect alloys; and the different corrosion resistances are caused by the different microstructure of alloys with different B additions.     

Impact of Zn1-xMgxO:Al transparent electrode for buffer-less Cu(In,Ga)Se2 solar cells

Buffer layers such as CdS and ZnS are used in high efficiency Cu(In,Ga)Se₂ (CIGS) thin film solar cells. Eliminating buffer layer is attractive to realize low-cost thin film solar cells by reducing fabrication process. However, the elimination of the buffer layers leads to shunting due to the interface recombination between transparent conductive oxide (TCO) and CIGS layers. To reduce the interface recombination, the control of conduction band offset (CBO) is effective. In this study, we fabricated Zn_1−xMg_xO:Al (ZMO:Al) as the TCO for the CBO control. ZMO:Al was prepared by co-sputtering of ZnO:Al₂O₃ (ZnO:Al) and MgO:Al₂O₃ targets. ZMO:Al shows high transmittance in visible region and the band gap energy widen with the addition of Mg to ZnO:Al. Buffer-less CIGS solar cells with an Al/NiCr/TCO/CIGS/Mo/soda-lime glass structure using ZMO:Al and ZnO:Al were fabricated. For comparison, ZnO/CdS buffered cell was also fabricated. Current density-voltage characteristics of the devices showed the cell with ZMO:Al film achieved higher efficiency compared to the buffer-less cell with ZnO:Al. This result suggested that the control of CBO is important to reduce interface recombination between TCO layer and CIGS absorber.     

Investigation on structure and hydrogen storage performance of as-milled and cast Mg90Al10 alloys

The as-milled (20 h) and cast Mg₉₀Al₁₀ alloys were prepared by mechanical milling and vacuum induction melting, respectively. The differences in the phase composition, apparent morphology and microstructure of the alloys were analyzed by X-ray diffraction (XRD), scanning electron microscope (SEM) and high resolution transmission microscope (HRTEM). The activation performance, hydrogen absorption/desorption rate and pressure-composition-isotherm (P-C-T) curves of the pure Mg, as-milled (20 h) and cast Mg₉₀Al₁₀ alloys were tested using a Sieverts apparatus. The results show that the alloys both are nanocrystalline structure and consisted of the main phase of Mg phase and the second phase of Al phase or Mg₁₇Al₁₂ phase. Compared to pure Mg, the thermodynamics and kinetics of the as-milled (20 h) and cast Mg₉₀Al₁₀ alloys are improved in different degree. The hydrogen desorption enthalpy (ΔH_de) of the as-milled (20 h) and cast Mg₉₀Al₁₀ alloys are 75.43 and 72.76 kJ mol^?1 H₂, which are smaller than 100.67 kJ mol^?1 H₂ of pure Mg. And the dehydrogenation activation energy (E^de(a)) decreases from 172.61 kJ mol^?1 H₂ of pure Mg to 163.59 and 157.65 kJ mol^?1 H₂ of the as-milled (20 h) and cast Mg₉₀Al₁₀ alloys, respectively. However, the activation performance and the hydrogen absorption capacity have the varying degree to drop.     

Catalytic effect of Ti2C MXene on the dehydrogenation of MgH2

In the present work, the Ti₂C MXene is prepared by selective etching Al layer from Ti₂AlC. The catalytic effect of Ti₂C MXene on the dehydrogenation of MgH₂ is investigated. Compared with the pure MgH₂, the onset desorption temperature, apparent activation energy (E_a) and the overall enthalpy changes (△H) of dehydrogenation of MgH₂-5 wt%Ti₂C decreased by 37 °C, 36.5%, 11%, respectively. The catalytic mechanism of Ti₂C on the dehydrogenation of MgH₂ is investigated by means of X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and transmission electron microscope (TEM). It is suggested that the surface Ti atoms with multivalence serve as the intermediate for electrons shifting between H⁻ and Mg²⁺, which makes the dehydrogenation of MgH₂ easier. In addition, the good hydrogen adsorption ability and thermal conductivity of Ti₂C MXene could also contribute to the improvement of thermodynamics of MgH₂.     

Pd capped MgxTi1−x films: Promising anode materials for alkaline secondary batteries with superior discharge capacities and cyclic stabilities

Pd capped Mg_xTi_1−x films have been prepared by magnetron sputtering, and their electrochemical hydrogen storage properties have been investigated. Results show that the Mg_0.85Ti_0.15 and Mg_0.72Ti_0.28 samples exhibit the most promising electrochemical properties, including short activation period, large discharge capacities, excellent cyclic stabilities and superior anti-corrosion behaviors. The maximum capacity of Mg_0.85Ti_0.15 film is achieved to ∼1100 mAh g⁻¹ after 30 cycles, 80% of which (∼810 mAh g⁻¹) can be maintained even after 150 cycles. Such excellent properties make Pd capped Mg_xTi_1−x films competitive candidates as anode materials of alkaline secondary batteries.