New mechanism and improved kinetics of hydrogen absorption and desorption of Mg(In) solid solution alloy milling with CeF3

This paper presents improving the hydrogen absorption and desorption of Mg(In) solid solution alloy through doped with CeF₃. A nanocomposite of Mg_0.95In_0.05-5 wt% CeF₃ was prepared by mechanical ball milling. The microstructures were systematically investigated by X-ray diffraction, scanning electron microscopy, scanning transmission electron microscopy. And the hydrogen storage properties were evaluated by isothermal hydrogen absorption and desorption, and pressure-composition-isothermal measurements in a temperature range of 230 °C–320 °C. The mechanism of hydrogen absorption and desorption of Mg_0.95In_0.05 solid solution is changed by the addition of CeF₃. Mg_0.95In_0.05-5 wt% CeF₃ nanocomposite transforms to MgH₂, MgF₂ and intermetallic compounds of MgIn and CeIn₃ by hydrogenation. Upon dehydrogenation, MgH₂ reacts with the intermetallic compounds of MgIn and CeIn₃ forming a pseudo-ternary Mg(In, Ce) solid solution, which is a fully reversible reaction with a reversible hydrogen capacity～4.0 wt%. The symbiotic nanostructured CeIn₃ impedes the agglomeration of MgIn compound, thus improving the dispersibility of element In, and finally improving the reversibility of hydrogen absorption and desorption of Mg(In) solution alloy. For Mg_0.95In_0.05-5 wt% CeF₃ nanocomposite, the dehydriding enthalpy is reduced to about 66.1 ± 3.2 kJ⋅mol⁻¹⋅H₂, and the apparent activation energy of dehydrogenation is significantly lowered to 71.9 ± 10.0 kJ⋅mol⁻¹⋅H₂, a reduction of ～73 kJ⋅mol⁻¹⋅H₂ relative to that for Mg_0.95In_0.05 solid solution. As a result, Mg_0.95In_0.05-5 wt% CeF₃ nanocomposite can release ～57% H₂ in 10 min at 260 °C. The improvements of hydrogen absorption and desorption properties are mainly attributed to the reversible phase transition of Mg(In, Ce) solid solution combing with the multiphase nanostructure.     

Enhanced reversible hydrogen storage properties of a Mg–In–Y ternary solid solution

A Mg(In, Y) ternary solid solution was successfully synthesized by two-step method, namely sintering the elemental powders and subsequent milling. The formation of Mg(In, Y) indicates that the solubility of Y in the Mg lattice is expanded due to the existence of In. The as-synthesized Mg₉₀In₅Y₅ solid solution transformed to MgH₂, YH₃, In₃Y and MgIn compound upon hydrogenation, the hydrogenated products except for the YH₃ recovered to Mg(In, Y) solid solution after dehydrogenation. The Mg₉₀In₅Y₅ solid solution exhibited a decreased reaction enthalpy of 62.9 kJ/(mol H₂), reduction by ca. 5 kJ/(mol H₂) or 12 kJ/(mol H₂) than the Mg₉₅In₅ binary solid solution and pure Mg, respectively. The working temperature as well as the activation energies for the hydriding and dehydriding were also decreased in comparison with those of Mg(In) binary solid solution, which are attributed to the reduced reaction enthalpy and the catalytic role of YH₃. Our work indicates that the thermodynamic and kinetic tuning of MgH₂ are realized in the Mg(In, Y) ternary solid solution.     

Microstructural evolution and hydrogen storage proprieties of melt-spun eutectic Mg76.87Ni12.78Y10.35 alloy with low hydrides formation/decomposition enthalpy

Ternary eutectic Mg_76.87Ni_12.78Y_10.35 (at. %) ribbons with mixed amorphous and nanocrystalline phases were prepared by melt spinning. The microstructures of the melt-spun, hydrogenated and dehydrogenated samples were examined and compared by X-ray diffraction and transmission electron microscopy. The amorphous structure transforms into a thermally stable nanocrystalline structure with a grain size of about 5 nm during hydrogen ab/desorption cycles. The Mg, Mg₂Ni and phases with Y in the melt-spun state transform into MgH₂, Mg₂NiH₄, Mg₂NiH_0.3, YH₂ and YH₃ after hydrogenation, and transform back to Mg, Mg₂Ni and YH₂ upon subsequent dehydrogenation. The reaction enthalpy (ΔH) and entropy (ΔS) of the higher plateau pressure corresponding to Mg₂Ni hydride formation are −53.25 kJ mol⁻¹ and −107.74 J K⁻¹ mol⁻¹, respectively. The amorphous/nanocrystalline structure effectively reduces the enthalpy and entropy of Mg₂Ni hydride formation, but has little effect on Mg. The activation energy for dehydrogenation of the hydrogenated ribbons is 69 kJ mol⁻¹. This suggests that Mg–Ni–Y with ternary eutectic composition can form an amorphous/nanocrystalline structure by melt spinning, and this nanostructure efficiently improves the thermodynamics and kinetics for hydrogen storage.     

Tuning the de/hydriding thermodynamics and kinetics of Mg by mechanical alloying with Sn and Zn

Mg₉₅Sn₃Zn₂ alloy was prepared by mechanical alloying. The phase constituents and phase transition were analyzed by X-ray diffraction (XRD) method. The microstructure was characterized by scanning electron microscope (SEM). The hydrogen storage properties were evaluated in detail by the measurements of isothermal hydrogen absorption and desorption, and pressure-composition isotherms (PCI) using the Sieverts method. The addition of Zn benefits to extend the solubility of Sn in the Mg lattice, as a result supersaturated Mg(Sn, Zn) ternary solid solution was synthesized by mechanical alloying, which decomposed to MgH₂, Sn and MgZn₂ in the hydrogenating process. The in situ formed nanostructure Mg₂Sn and MgZn₂ have positive effects on the hydrogen absorption and desorption of Mg. Mg₉₅Sn₃Zn₂ alloy showed significantly improved kinetics with lowered hydrogen absorption and desorption activation energies of 38.1 kJ/mol and 86.6 kJ/mol respectively, and exhibited a reduced dehydriding enthalpy of 67.0 ± 1.9 kJ/(mol·H₂).     

In-situ hydrogen-induced evolution and de-/hydrogenation behaviors of the Mg93Cu7-xYx alloys with equalized LPSO and eutectic structure

Nanosizing is efficient as the dual-tuning of thermodynamics and kinetics for Mg-based hydrogen storage materials. The in-situ synthesis of nanocomposites through hydrogen-induced decomposition from long-period stacking ordered phase is proved effective to achieve active nano-sized catalysts with uniform dispersion. In this study, the Mg₉₃Cu_7-xY_x (x = 0.67, 1.33, and 2) alloys with equalized Mg–Mg₂Cu eutectic and 14H long-period stacking ordered phase of Mg₉₂Cu_3.5Y_4.5 are prepared. Its solidification path is determined as α-Mg, 14H–Mg₂Cu pair and Mg–Mg₂Cu eutectic. The increased Y/Cu atomic ratio lowers the first-cycle hydrogenation rate of the alloys due to the increased 14H–Mg₂Cu structure and reduced Mg–Mg₂Cu eutectic interfaces. After the hydrogen-induced decomposition of the long-period stacking ordered phase, MgCu₂ and YH₃ nanoparticles are in-situ formed, and the following activation process significantly accelerates. The YH₃ nanoparticles partly decompose to YH₂ at 300 °C in vacuum and Mg–Mg₂Cu-YH_x nanocomposites are in-situ formed. The nano-sized YH₂ helps catalyze H₂ dissociation and the YH_x/Mg interfaces stimulate H diffusion and the nucleation of MgH₂. Therefore, the Mg₉₃Cu₅Y₂ composite shows the fastest absorption rates. However, due to the positive effect of YH_x/Mg interfaces on H diffusion and the negative effect of YH₃ nanophases on the hydride decomposition, the minimum activation energy of 115.43 kJ mol⁻¹ is obtained for the desorption of the Mg₉₃Cu_5.67Y_1.33 hydride.     

Effect of in-situ formed Mg2Ni/Mg2NiH4 compounds on hydrogen storage performance of MgH2

Magnesium-based hydrogen storage materials (MgH₂) are promising hydrogen carrier due to the high gravimetric hydrogen density; however, the undesirable thermodynamic stability and slow kinetics restrict its utilization. In this work, we assist the de/hydrogenation of MgH₂ via in situ formed additives from the conversion of an MgNi₂ alloy upon de/hydrogenation. The MgH₂–16.7 wt%MgNi₂ composite was synthesized by ball milling of Mg powder and MgNi₂ alloy followed by a hydrogen combustion synthesis method, where most of the Mg converted to MgH₂, and the others reacted with the MgNi₂ generating Mg₂NiH₄, which produced in situ Mg₂Ni during dehydrogenation. Results showed that the Mg₂Ni and Mg₂NiH₄ could induce hydrogen absorption and desorption of the MgH₂, that it absorbed 2.5 wt% H₂ at 473 K, much higher than that of pure Mg, and the dehydrogenation capacity increased by 2.6 wt% at 573 K. Besides, the initial dehydrogenation temperature of the composite under the promotion of Mg₂NiH₄ decreased greatly by 100 K, whereas it is 623 K for MgH₂. Furthermore, benefiting from the catalyst effect of Mg₂NiH₄ during dehydrogenation, the apparent activation energy of the composite reduced to 73.2 kJ mol⁻¹ H₂ from 129.5 kJ mol⁻¹ H₂.     

Characterization of microstructure,hydrogen storage kinetics and thermodynamics of a melt-spun Mg86Y10Ni4 alloy

Ternary Mg₈₆Y₁₀Ni₄ alloy was successfully prepared by vacuum induction melting and subsequent melt-spinning technique. The phase composition and microstructure of the melt-spun and hydrogenated samples were characterized by X-ray diffraction, scanning electron microscopy, and transmission electron microscopy measurements. The melt-spun alloy had an amorphous structure, and it transformed into nanocrystalline during the first hydrogenation process. The hydrogenated sample was composed of MgH₂, Mg₂NiH₄, YH₂, and a small amount of YH₃. The hydrogen absorption/desorption kinetics and thermodynamics were measured by Sievert's apparatus at various temperatures. It was found that the melt-spun Mg₈₆Y₁₀Ni₄ alloy could be fully activated after five hydrogenation and dehydrogenation cycles at 380 °C, and it exhibited a reversible gravimetric hydrogen storage capacity of about 5.3 wt%. The enhanced hydrogen sorption kinetics during the first few cycles can be attributed to the increased specific surface caused by the pulverization and cracking of the alloy particles. The activation energy for dehydrogenation reaction was determined to be 67 kJ/mol and 71 kJ/mol by using Arrhenius equation and Kissinger equation respectively. The thermodynamics of the sample was also evaluated by pressure–composition–isotherms, and the results shown that the enthalpy and entropy changes of Mg/MgH₂ transformation in the Mg₈₆Y₁₀Ni₄ alloy were slightly higher than that of pure Mg/MgH₂.     

Synergistic catalytic effects of ZIF-67 and transition metals (Ni,Cu, Pd,and Nb) on hydrogen storage properties of magnesium

MgTM/ZIF-67 nanocomposites were prepared by the deposition-reduction method using ZIF-67, MgCl₂, and TMCl_x (TM = Ni, Cu, Pd, Nb) as raw materials. The dehydrogenation activation energies of MgTM/ZIF-67 (TM = Ni, Cu, Pd, Nb) nanocomposites were calculated to be 115.4 kJ mol⁻¹ H₂, 115.7 kJ mol⁻¹ H₂, 113.6 kJ mol⁻¹ H₂, and 75.8 kJ mol⁻¹ H₂, respectively; hence, the MgNb/ZIF-67 nanocomposite manifested the best comprehensive hydrogen storage performance. The hydrogen storage capacity of the MgNb/ZIF-67 nanocomposite was hardly attenuated after the 100th hydrogen absorption-desorption cycle. The dehydrogenated enthalpies of MgH₂ and CoMg₂H₅ in MgNb/ZIF-67 hydride were calculated to be 72.4 kJ mol⁻¹ H₂ and 81.0 kJ mol⁻¹ H₂, respectively, which were lower than those of additive-free MgH₂ and Mg/ZIF-67. The improved hydrogen storage properties of MgNb/ZIF-67 can be ascribed to the CoMg₂–Mg(Nb) core-shell structure and the catalytic effects of NbH and niobium oxide nanocrystals.     

Comparative investigation on the hydrogenation/dehydrogenation characteristics and hydrogen storage properties of Mg3Ag and Mg3Y

The hydrogenation/dehydrogenation characteristics and hydrogen storage properties of nominal Mg₃Ag and Mg₃Y alloys prepared by induction melting were investigated. The as-melted Mg₃Ag alloy was composed of Mg₅₄Ag₁₇ phase, while Mg₃Y consisted of Mg₂₄Y₅ and Mg₂Y phases. Mg₅₄Ag₁₇ transformed into MgAg and MgH₂ during the first hydrogenation, and the phase transition of the following hy/dehydrogenation cycles was Mg₃Ag + 2H₂ ↔ MgAg + 2MgH₂. Both Mg₂₄Y₅ and Mg₂Y undertook disproportion reactions and decomposed into MgH₂ and YH₃. Experimental and calculated results demonstrated that there was no necessary relation between the thermodynamic stabilities and the size interstices in these alloys. The dehydrogenation enthalpy change (ΔH) and entropy change (ΔS) of Mg₃Ag were calculated and compared with that of pure Mg, which indicated that the increase of ΔS could counteract the stabilization effect of ΔH, which offered a method for tuning the thermodynamic properties of Mg-based alloys.     

Hydrogen storage properties of Mg98.5Gd1Zn0.5 and Mg98.5Gd0.5Y0.5Zn0.5 alloys containing LPSO phases

In this work, the as-cast Mg-rich Mg_98.5Gd₁Zn_0.5 and Mg_98.5Gd_0.5Y_0.5Zn_0.5 alloys are prepared by the semi-continuous casting method, and their hydrogen storage performance and catalytic mechanisms are investigated by experimental and first-principles calculations approaches. The results show that the LPSO phases decompose and in-situ form the RE(Gd/Y)H_x(x = 2,3) nano-hydrides upon hydrogenation. These nano-hydrides not only serve as the in-situ catalysts to promote the hydrogen ab/desorption of Mg matrix, but also present the pinning effect to inhibit the growth of Mg/MgH₂ grains during hydrogenation and dehydrogenation. Comparatively, the two alloys exhibit the similar hydrogen absorption kinetics, while the hydrogen desorption kinetics of Mg_98.5Gd₁Zn_0.5 is superior to that of Mg_98.5Gd_0.5Y_0.5Zn_0.5. The first-principles calculations reveal that the GdH₂ and YH₂ hydrides exhibit different catalytic effects on weakening the bond strength of H–H within H₂ and Mg–H within MgH₂, which interprets well the differences in the hydrogen ab/desorption kinetics between Mg_98.5Gd₁Zn_0.5 and Mg_98.5Gd_0.5Y_0.5Zn_0.5 alloys.     

Facile synthesis of nickel-vanadium bimetallic oxide and its catalytic effects on the hydrogen storage properties of magnesium hydride

To improve the dehydrogenation/hydrogenation performance of magnesium hydride (MgH₂), a nickel-vanadium bimetallic oxide (NiV₂O₆) was prepared by a simple hydrothermal method using ammonium metavanadate and nickel nitrate as raw materials. This oxide was used to improve the hydrogen storage performance of MgH₂. NiV₂O₆ reacted with Mg to form Mg₂Ni and V₂O₅; Mg₂Ni and V₂O₅ played an important role in improving the hydrogen storage properties of MgH₂. The NiV₂O₆-doped MgH₂ had an excellent hydrogen absorption and desorption kinetics performance, and it could absorb 5.59 wt% of hydrogen within 50 min at 150 °C and release about 5.3 wt% of hydrogen within 12 min. The apparent activation energies for the dehydrogenation and hydrogenation of MgH₂-NiV₂O₆ were 92.9 kJ mol^?1 and 24.9 kJ mol^?1, respectively. These were 21.7% and 66.3% lower than those of MgH_2, respectively. The mechanism analysis demonstrated that the improved kinetic properties of MgH₂ resulted from the heterogeneous catalysis of vanadium and nickel.     

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.     

Effects of trimesic acid-Ni based metal organic framework on the hydrogen sorption performances of MgH2

A metal-organic framework based on Ni (II) as metal ion and trimasic acid (TMA) as organic linker was synthesized and introduced into MgH₂ to prepare a Mg-(TMA-Ni MOF)-H composite through ball-milling. The microstructures, phase changes and hydrogen storage behaviors of the composite were systematically studied. It can be found that Ni ion in TMA-Ni MOF is attracted by Mg to form nano-sized Mg₂Ni/Mg₂NiH₄ after de/rehydrogenation. The hydriding and dehydriding enthalpies of the Mg-MOF-H composite are evaluated to be −74.3 and 78.7 kJ mol⁻¹ H₂, respectively, which means that the thermodynamics of Mg remains unchanged. The absorption kinetics of the Mg-MOF-H composite is improved by showing an activation energy of 51.2 kJ mol⁻¹ H₂. The onset desorption temperature of the composite is 167.8 K lower than that of the pure MgH₂ at the heating rate of 10 K/min. Such a significant enhancement on the sorption kinetic properties of the composite is attributed to the catalytic effects of the nanoscale Mg₂Ni/Mg₂NiH₄ derived from TMA-Ni MOF by providing gateways for hydrogen diffusion during re/dehydrogenation processes.     

Effects of KNbO3 catalyst on hydrogen sorption kinetics of MgH2

The influences of Nb-containing oxides and ternary compound in hydrogen sorption performance were investigated. As faster desorption kinetic and lower activation energy were reported by addition of a ternary compound catalyst such as K₂NiF₆, the influence of KNbO₃ on hydrogen storage properties of MgH₂ has been investigated for the first time. The MgH₂ - KNbO₃ composite desorbed 3.9 wt% of hydrogen within 10 min, while MgH₂ and MgH₂-Nb₂O₅ composites desorbed 0.66 wt% and 3.2 wt% respectively under similar condition. For MgH₂ with other Nb-contained catalysts such as Nb, NbO and Nb₂O₃, the desorption rate was almost the same as the one registered for as-milled MgH₂. The analysis of differential scanning calorimetry (DSC) showed that MgH₂-KNbO₃ composite started to release hydrogen at ∼335 °C which is 50 °C lower compared to as-milled MgH₂ without any additives. The activation energy for the hydrogen desorption was estimated to be about 104 ± 6.8 kJ mol⁻¹ for this material, while for the as-milled MgH₂ was about 165 ± 2.0 kJ mol⁻¹. It is believed that Nb-ternary oxide catalyst (KNbO₃) showed a good catalytic effect and enhance the sorption kinetics of MgH₂.     

Integrated Ni/Nb2O5 nanocatalytic agent dose for improving the hydrogenation/dehydrogenation kinetics of reacted ball milled MgH2 powders

Reactive ball milling (RBM) technique was employed to synthesize ultrafine powders of MgH₂, using high energy ball mill operated at room temperature under 50 bar of a hydrogen gas atmosphere. The MgH₂ powders obtained after 200 h of continuous RBM time composed of β and γ phases. The powders possessed nanocrystalline characteristics with an average grain of about 10 nm in diameter. The time required for complete hydrogen absorption and desorption measured at 250 °C was 500 s and 2500 s, respectively. In order to improve the hydrogenation/dehydrogenation kinetics of as synthesized MgH₂ powders, three different types of nanocatalysts (metallic Ni, Nb₂O₅ and (Ni)_x/(Nb₂O₅)_y) were utilized with different weight percentages and independently ball milled with the MgH₂ powders for 50 h under 50 bar of a hydrogen gas atmosphere. The results showed that the prepared nanocomposite MgH₂/5Ni/5Nb₂O₅ powders possessed superior hydrogenation/dehydrogenation characteristics, indexed by low values of activation energy for β-phase (68 kJ/mol) and γ-phase (74 kJ/mol). This nanocomposite system showed excellent hydrogenation/dehydrogenization behavior, indexed by the short time required to uptake (41 s) and release (121 s) of 5 wt% H₂ at 250 °C. At this temperature the synthesized nanocomposite powders possessed excellent absorption/desorption cyclability of 180 complete cycles. No serious degradation on the hydrogen storage capacity could be detected and the system exhibited nearly constant absorption and desorption values of +5.46 and −5.46 wt% H₂, respectively.     

KINETICS OF CHLOROFORM EXTRACTION OF TAR SAND

The kinetics of chloroform extraction from a Jordan tar sand have been studied. The activation energy of the extraction has been evaluated; for 125–180 μm tar sand it is 6⋅53 kJ mol⁻¹ in the initial stage of extraction and 12⋅18 kJ mol⁻¹ in the later stages, for 355–500 μm 1⋅0 kJ mol⁻¹ and 10⋅61 kJ mol⁻¹, respectively. These values are in agreement with the general activation energy of the dissolution of a solid material. It is concluded that the rate of extraction is controlled by the diffusion of extract. © 1997 by John Wiley & Sons, Ltd.     

Improved hydrogen storage kinetics and thermodynamics of RE-Mg-based alloy by co-doping Ce–Y

Aiming at improving hydrogen storage performance of Mg-base alloy, the Mg₉₀Ce₅Y₅ alloy, which has high capacity and high stability, was prepared by vacuum induction melting. The XRD, SEM, TEM, PCI, and DSC were used to characterize the microstructure and phase transformation of alloy as well as hydrogen storage property. The results indicate that the Mg₉₀Ce₅Y₅ alloy consists of multiphase structure, including the CeMg₁₂, Y₅Mg₂₄, Ce₂Mg₁₇ as well as residual Mg phase, besides, part Y dissolved in both Mg and CeMg₁₂/Ce₂Mg₁₇ phase to form solid solutions. After hydrogen absorption, these phases transform into the MgH₂, CeH_2.73 and YH₂ phase, while after hydrogen desorption, the MgH₂ transforms into the Mg phase, but the rare earth hydride phase was not changed. There is another reversible transformation between the CeH_2.73 and CeO₂ phase, which is beneficial for the cyclic stability of the alloy. The alloy has the reversible hydrogen capacity of about 6.0 wt% H₂ as well as the activation energy of 114.3 kJ/mol, and also exhibits enhanced kinetics compared with the pure MgH₂ sample, as a result of the synergistic effect of rare earth hydride phase. Meanwhile, it is also noted that the Mg₉₀Ce₅Y₅ alloy begins to release hydrogen below 250 °C and the rate of hydrogen desorption is mainly dominated by surface controlled.     

Hydrogen storage properties of nanostructured 2MgH2Co powders: The effect of high-pressure compression

In this study, MgH₂ and Co powders were mechanically milled in the molar ratio 2:1 and compressed to hard-packed cylindrical pellets. The microstructure, phase changes, and hydrogen storage properties of the mechanically milled 2MgH₂Co powder and the 2MgH₂Co compressed pellet were analyzed by X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM) and synchronous thermal (DSC/TG) analyses. Dehydrogenation of the 2MgH₂Co compressed pellet is mainly due to the decomposition of Mg₂CoH₅ while it is the dehydriding of MgH₂ for the milled 2MgH₂Co powder. Pressure composition absorption isotherms of the 2MgH₂Co powder and 2MgH₂Co compressed pellet show two and three plateaus, respectively, corresponding to the formation of Mg₆Co₂H₁₁ and Mg₂CoH₅ hydride phases. For the compressed 2MgH₂Co pellet, enthalpies of formation/decomposition were measured to be −58.11±7.69 kJ/mol H₂/55.70±3.34 kJ/mol H₂ for Mg₂CoH₅ and -81.89±10.39 kJ/mol H₂/74.47±5.27 kJ/mol H₂ for Mg₆Co₂H₁₁. In contrast, hydrogenation/dehydrogenation enthalpies of Mg₂CoH₅ and Mg₆Co₂H₁₁ mechanically milled 2MgH₂Co powder were −73.98±10.1 kJ/mol H₂/71.67±1.38 kJ/mol H₂ and -96.86±8.73 kJ/mol H₂/89.95±10.81 kJ/mol H₂, respectively. Fast hydrogenation was observed in the dehydrided 2MgH₂Co compressed pellet with about 2.75 wt% absorbed in less than 1 min at 300 °C and a maximum hydrogen storage capacity of 4.43 wt% (2.32 wt% for the 2MgH₂Co powder) was achieved. The hydrogen absorption activation energy of the 2MgH₂Co compressed pellet (64.34 kJ-mol⁻¹ H₂) is lower than the mechanically milled 2MgH₂Co powder (73.74 kJ-mol⁻¹ H₂). The results show that mechanical milling followed by high-pressure compression is an efficient method for the synthesis of Mg-based complex hydrides with superior hydrogen sorption properties.     

New ionic diffusion strategy to fabricate proton-conducting solid oxide fuel cells based on a stable La2Ce2O7 electrolyte

A composite of NiO–BaZr_0.1Ce_0.7Y_0.2O_3−δ (NiO-BZCY) was successfully prepared by a simple one-step-combustion process and applied as an anode for solid oxide fuel cells based on stable La₂Ce₂O₇ (LCO) electrolyte. A high open circuit voltage of 1.00 V and a maximum power density of 315 mW cm⁻² were obtained with NiO-BZCY anode and LCO electrolyte when measured at 700 °C using humidified hydrogen fuel. SEM-EDX and Raman results suggested that a thin BaCeO₃-based reaction layer about 5 μm in thickness was formed at the anode/electrolyte interface for Ba cations partially migrated from anode into the electrolyte film. Impedance spectra analysis showed that the activation energy for LCO conductivity differed with the anode materials, about 52.51 kJ mol⁻¹ with NiO-BZCY anode and 95.08 kJ mol⁻¹ with NiO-LCO anode. The great difference in these activation energies might suggest that the formed BaCeO₃ reaction layer could promote the proton transferring numbers of LCO electrolyte.     

Hydrogen absorption and desorption kinetics of MgH2 catalyzed by MoS2 and MoO2

The catalytic effect of MoS₂ and MoO₂ on the hydrogen absorption/desorption kinetics of MgH₂ has been investigated. It is shown that MoS₂ has a superior catalytic effect over MoO₂ on improving the hydrogen kinetic properties of MgH₂. DTA results indicated that the desorption temperature decreased from 662.10 K of the pure MgH₂ to 650.07 K of the MgH₂ with MoO₂ and 640.34 K of that with MoS₂. Based on the Kissinger plot, the activation energy of the hydrogen desorption process is estimated to be 101.34 ± 4.32 kJ mol⁻¹ of the MgH₂ with MoO₂ and 87.19 ± 4.48 kJ mol⁻¹ of that with MoS₂, indicating that the dehydriding process energy barrier of MgH₂ can be reduced. The enhancement of the hydriding/dehydriding kinetics of MgH₂ is attributed to the presence of MgS and Mo or MgO and Mo which catalyze the hydrogen absorption/desorption behavior of MgH₂. The detailed comparisons between MoS₂ and MoO₂ suggest that S anion has superior properties than O anion on catalyzing the hydriding/dehydriding kinetics of MgH₂.