A comparative study on the hydriding kinetics of Zr-based AB2 hydrogen storage alloys

Two kinetic models (Jander model and Chou model) are used to investigate the hydrogen absorption kinetic mechanism of Zr-based AB₂ type Laves phase alloys (Ti_0.1Zr_0.9Mn_0.9V_0.1Fe_0.5Co_0.5, Ti_0.1Zr_0.9(Mn_0.9V_0.1)_1.1Fe_0.5Ni_0.5 and Ti_0.1Zr_0.9Mn_0.9V_0.1Fe_0.55Ni_0.55). The analysis shows that the rate-controlling step is the diffusion process at high temperatures in the range from 673 K to 923 K with a low hydrogen concentration (solid solution phase). Both models can well describe the experimental data but Chou model is preferred. Chou model is simpler and easier to use for analyzing the experimental results. The activation energies calculated using Chou model with the least square method are 29.3 kJ/mol H₂ for Ti_0.1Zr_0.9Mn_0.9V_0.1Fe_0.5Co_0.5, 43.8 kJ/mol H₂ for Ti_0.1Zr_0.9(Mn_0.9V_0.1)_1.1Fe_0.5Ni_0.5 and 48.5 kJ/mol H₂ for Ti_0.1Zr_0.9Mn_0.9V_0.1Fe_0.55Ni_0.55, which are close to the values reported in the literature (28.3 kJ/mol H₂ for Ti_0.1Zr_0.9Mn_0.9V_0.1Fe_0.5Co_0.5 and 40.3 ± 1.5 kJ/mol H₂ for both Ti_0.1Zr_0.9(Mn_0.9V_0.1)_1.1Fe_0.5Ni_0.5 and Ti_0.1Zr_0.9Mn_0.9V_0.1Fe_0.55Ni_0.55).     

Hydrogen storage alloys for high-pressure suprapure hydrogen compressor

In this work, AB₅ type rare earth-based and AB₂ type TiCr₂-based hydrogen storage alloys were studied for the purpose of high-pressure hydrogen compression. A pair of hydrogen storage alloys, Ml_0.55Mm_0.2Ca_0.25Ni₅ (Ml: La-rich mischmetal; Mm: Ce-rich mischmetal) and (Ti_0.97Zr_0.03)_1.1Cr_1.6Mn_0.4, with favorable hydrogen storage properties was developed as the alloys for a double-stage high-pressure metal hydride hydrogen compressor (MHHC). With the developed alloy pair, we designed and built a MHHC prototype with hydrogen capacity of 100 L, which could produce high-pressure ultrapure hydrogen with pressure of 45 MPa and purity of 99.9999% from industrial grade hydrogen (98% purity) at pressure of around 2 MPa. During the compression procedure, only hot water is used as the heating source. The compression characteristics were studied and the thermal efficiency was calculated.     

Electrochemical behaviour assessment of novel Mg-rich Mg–Al–RE alloys (RE = Ce, Er)

The electrochemical behaviour of new Mg–Al–RE (RE = Ce, Er) alloys AE91 was investigated in 0.01 M NaCl electrolyte (pH = 12) and compared with that of the most commonly used Mg alloy in the automotive field, the AZ91D. Scanning electron microscopy and quantitative electron probe microanalysis were used to characterize the samples prior to the electrochemical tests. AE91 alloys showed very similar microstructures characterized by a three-phase appearance: a Mg-based solid solution containing only Al and two intermetallic phases γ(Mg₁₇Al₁₂) and (Al_1 − xMg_x)₃Ce or (Al_1 − xMg_x)₂Er. Free corrosion potential measurements, potentiodynamic polarization curves and electrochemical impedance spectroscopy revealed improved passivity behaviour compared to AZ91D alloy. The apparent presence of trace amounts of rare earth oxides in the passive film is presumed to be the reason for the enhanced corrosion resistance of AE91 alloys in the aggressive environment considered.     

Study of amorphous Ni–S(La) alloy used as HER cathode in alkaline medium

The hydriding combustion synthesis (HCS) of Mg₂FeH₆ and Mg₂Ni_1−xFe_x hydrides was systematically studied by changing the value of x from 0.25 to 1.0. This study aimed at improving the Mg₂FeH₆ yield in production and examining the effect of the addition of nickel to the hydrogen storage properties of the Mg₂FeH₆ hydride. In synthesizing metallic hydrides, the raw materials in metallic powders were mechanically activated by a ball mill before the HCS treatment. As a result, the ball-milled 2Mg + Fe recorded as much as 5 mass% in hydrogen storage capacity during the HCS treatment, and the final product successfully indicated a high purity Mg₂FeH₆. Interestingly, the deformation enthalpies of the Mg₂Ni_1−xFe_x hydrides were larger by 10% or more than Mg₂NiH₄ and Mg₂FeH₆, taken individually. This was collateral evidence to prove that the HCS of Mg₂FeH₆ and Mg₂Ni_1−xFe_x hydrides with ball milling (BM) produced a new structure of the Mg–Ni–Fe–H system due to the synergy effect.     

Phase relations and hydrogenation behavior of Sr(Al1−xMgx)2

The phase relations and hydrogenation behavior of Sr(Al_1−xMg_x)₂ alloys were studied. The pseudobinary C36-type Laves phase Sr(Al,Mg)₂ was found as a structural intermediate between the Zintl phase and the C14 Laves phase. The single-phase regions for the Zintl phase, C36 phase and C14 phase, were determined to be x=0–0.10, 0.45–0.68 and 0.80–1, respectively. The Mg-substituted Zintl phase Sr(Al_0.95Mg_0.05)₂ can be hydrogenated to Sr(Al,Mg)₂H₂ at about 473 K. However, the Sr(Al,Mg)₂H₂ directly decomposes into SrH₂ and Sr(Al,Mg)₄ starting at 513 K. When the temperature is 573 K, the C36 Laves phase Sr(Al_0.5Mg_0.5)₂ can be hydrogenated into SrMgH₄ and Al, while the C14 Laves phase Sr(Al_0.1Mg_0.9)₂ is hydrogenated into SrMgH₄, Mg₁₇Al₁₂ and Mg.     

La–Mg–Ni ternary hydrogen storage alloys with Ce2Ni7-type and Gd2Co7-type structure as negative electrodes for Ni/Mh batteries

Hydrogen strorage alloys with formula La_1.5Mg_0.5Ni₇ were prepared by induction melting followed by different annealing treatments (1073, 1123 and 1173 K) for 24 h. The alloy composition, alloy microstructure and electrochemical properties were investigated, respectively. The results showed that the multi-phase structure of as-cast alloy was converted into a double-phase structure (Gd₂Co₇-type phase and Ce₂Ni₇-type phase) through annealing treatments. Mg atoms were mainly located in Laves unit of Gd₂Co₇-type unit cell and Ce₂Ni₇-type unit cell. The electrochemical capacity of alloy electrodes after annealing treatment could be up to 390 mAh/g. The cyclic stability of alloy electrodes was significantly improved by annealing treatments; After 150 charge/discharge cycles, the capacity retention ratio of alloy annealed at 1173 K was the highest (81.9%). The high rate dischargeability of alloy electrodes was also improved due to annealing treatment.     

Microstructural evolution during controlled ball milling of (Mg2Ni+MgNi2) intermetallic alloy

Nearly dual-phase Mg–Ni alloy fabricated by ingot metallurgy (IM) and comprising 30 vol% Mg₂Ni and 61 vol% MgNi₂ intermetallic compounds (remaining 9 vol% of unreacted Mg) was mechanically (ball) milled under controlled shearing for 10, 30, 70 and 100 h. The majority of the medium- and small-sized powder particles exhibited a relatively homogeneous microstructure of milled Mg₂Ni and MgNi₂. A fraction of large-sized particles developed the ‘core and mantel’ microstructure after milling for 70 and 100 h. The ‘core’ contains poorly milled MgNi₂ particles and the ‘mantel’ is a thoroughly milled mixture of Mg₂Ni, MgNi₂ and, possibly, residual Mg. X-ray diffraction provides evidence of nanostructurization and eventual amorphization of a fraction of a heavily ball milled Mg₂Ni phase. The remnant Mg₂Ni developed a nanocrystalline/submicrocrystalline structure. The co-existing MgNi₂ phase developed a submicrocrystalline structure within the powder particles. The results are rationalized in terms of enthalpy effects by the application of Miedema’s semi-empirical model to the phase changes in ball milled intermetallics.     

Electrochemical hydrogen storage characteristics of La0.75-xMxMg0.25Ni3.2Co0.2Al0.1 (M =Zr, Pr; x =0, 0.1) alloys prepared by melt spinning

In order to ameliorate the electrochemical hydrogen storage performances of La-Mg Ni system A2B7-type electrode alloys,the partial substitution ofM (M =Zr,Pr) for La was performed.The melt spinning technology was used to fabricate the La0.75-xMxMg0.25Ni3.2Co0.2Al0.1 (M =Zr,Pr; x =0,0.1) electrode alloys.The influences of the melt spinning and substituting La with M (M =Zr,Pr) on the structures and the electrochemical hydrogen storage characteristics of the alloys were investigated.The analysis of XRD,SEM,and TEM reveals that the as-cast and spun alloys have a multiphase structure composed of two main phases (La,Mg)2Ni7 and LaNi5 as well as a residual phase LaNi2.The as-spun (M =Pr) alloy displays an entire nanocrystalline structure,while an amorphous-like structure is detected in the as-spun (M =Zr) alloy,implying that the substitution of Zr for La facilitates the amorphous formation.The electrochemical measurements exhibit that the substitution of Pr for La clearly increases the discharge capacity of the alloys; however,the Zr substitution brings on an adverse impact.Meanwhile,the M (M =Zr,Pr) substitution significantly enhances its cycle stability.The melt spinning exerts an evident effect on the electrochemical performances of the alloys,whose discharge capacity and high rate discharge ability (HRD) first mount up and then fall with the growing spinning rate,whereas their cycle stabilities monotonously augment as the spinning rate increases.     

Degradation behaviors of La–Mg–Ni-based metal hydride alloys:structural stability and influence on hydrogen storage performances

The present work focuses on the structural stability upon hydrogenation of three typical La–Mg–Ni-based alloys:La_(2-)Mg Ni_9,La_3Mg Ni_(14)and La_4Mg Ni_(19).Structural changes during gaseous and electrochemical cycles were characterized,and the influence of the structure distortion on the hydrogen storage properties was concerned.Hydrogen-induced amorphization(HIA)and disproportionation of the three alloys have occurred during both the gaseous and electrochemical cycles.Structural stability of the phase structures in the La–Mg–Ni system is found to follow the order:La Ni_(5-)[(La,Mg)_5Ni_(19)[(La,Mg)_2Ni_7[(La,Mg)Ni_3[(La,Mg)Ni_2.HIA increases thermal stability of the metal hydrides and difficulty to dehydrogenation and leads to degradation of both the gaseous and electrochemical capacities.Interestingly,La_2Mg Ni_9with poor stability presents elevated discharge capability even at 60°C which can be attributed to increase in the hydrogen desorption capability and inhibition of the self-discharge induced by severe HIA at higher temperatures.In addition,HIA in the electrochemical reactions is obviously weaker than the extent during the gaseous cycles,which is mainly due to the slower hydrogenation speed.The development of HIA in the gaseous and electrochemical process is considered to follow the direct and gradual modes,respectively.     

MmM5/Mg multi-layer hydrogen storage thin films prepared by dc magnetron sputtering

MmNi_3.5(CoAlMn)_1.5/Mg (here Mm denoted for mischmetal) multi-layer thin films were deposited on (0 0 1) Si substrate by direct current (dc) magnetron sputtering with dual-target. X-ray diffraction (XRD) and scanning electron microscopy analysis revealed that the microstructure of the MmNi_3.5(CoAlMn)_1.5 layer is amorphous and/or nanocrystalline and that the microstructure of the Mg layer is fine grained crystalline with preferential orientation. Phase analysis of hydrogenated and dehydrogenated MmNi_3.5(CoAlMn)_1.5/Mg multi-layer thin films proved that an apparent absorption of hydrogen in the Mg layer occurs at temperatures higher than 200 °C and that the hydrogen absorbed can be fully released at 250 °C.     

Synergistic effect between Laves phase and Zr-Ni phases in Zr(MnVNi)2 hydrogen storage alloys

The effect of annealing treatment on the crystal structure and electrochemical properties of Zr(Mn_0.25V_0.20Ni_0.55)₂ and Zr(Mn_0.05V_0.40Ni_0.55)₂ hydrogen storage alloys was investigated by means of XRD analysis and electrochemical tests. The results of XRD analysis showed that the as-cast alloys consist of C15, C14 type Laves phase and Zr₉Ni₁₁ and ZrNi phases. The composition of alloys homogenized after annealing treatment. The C15 type Laves phase is still stable while the Zr₉Ni₁₁ and ZrNi phases decompose and C14 phase disappears partially. The final stable structure of the alloys was a mixture phase of C15 and C14 type Laves phases. The results of the electrochemical tests showed that the discharge capacity and the properties of activation as well as high-rate dischargeability are all decreased after annealing treatment. The exchange current density decreases in some degree too.     

Glass formation ability and isothermal crystallization of melt-spun NdNiCuAl ribbons

On the basis of the previous work on (Nd_62.5Ni_37.5)₈₅Al₁₅ alloy, Cu is selected to partially substitute Ni to form (Nd_62.5Ni_37.5−xCu_x)₈₅Al₁₅ (x = 0, 10, 20, 30) melt-spun alloys. The glass-forming ability (GFA) of the as-prepared alloys is evaluated by the isochronal differential scanning calorimeter (DSC) measurement. The results show that GFA increases with Cu content according to several different criterions. The isothermal crystallization behaviors in the corresponding supercooled liquid region is discussed by both Johnson–Mehl–Avrami (JMA) equation and some nucleation and growth models. The fitting shows that it is reasonable to divide the whole crystallization processes into two stages. And the fittings with the nucleation and growth models infers that with increasing Cu content, the nucleation mechanism of the primary stage changes from the quenched-in and steady-state nucleation for (Nd_62.5Ni_37.5)₈₅Al₁₅, to the steady-state nucleation for (Nd_62.5Ni_27.5Cu₁₀)₈₅Al₁₅ and (Nd_62.5Ni_17.5Cu₂₀)₈₅Al₁₅, then to the time-dependent nucleation for (Nd_62.5Ni_7.5Cu₃₀)₈₅Al₁₅. And the dependence of crystallization mechanisms on Cu substitution agrees well with the change of their GFAs.     

Two-phase microstructure in Pr-(Fe, Co, Ni)-B sintered magnets

A search was carried out for a two-phase sintered magnet comprising Pr₂TM₁₄B and PrTM, where TM stands for Fe, Co and Ni. Specimens were prepared by a powder metallurgical method using alloys with compositions Pr₁₇(Fe_0.6Co_0.3Ni_0.1)₈₃−_χB_χ (x = 4, 5 and 6). X-Ray diffraction and scanning electron microscopy—energy-dispersive X-ray studies have revealed that two-phase magnets are realized for Pr₁₇(Fe_0.6Co_0.3Ni_0.1)₇₈B₅. The microstructures of these sintered magnets are characterized by Pr₂TM₁₄B grains surrounded by an intergranular Ni-rich PrTM phase whose thickness is less than about 3 μ.     

Observation of hydrogen absorption/desorption reaction processes in Li–Mg–N–H system by in-situ X-ray diffractmetry

The in-situ XRD measurements on dehydrogenation/rehydrogenation of the Li–Mg–N–H system were performed in this work. The ballmilled mixture of 8LiH and 3Mg(NH₂)₂ as a hydrogenated phase gradually changed into Li₂NH as a dehydrogenated phase during heat-treatment at 200 °C in vacuum for 50 h. Neither Mg-related phases nor other intermediate phases were recognized in the dehydrogenated phase. With respect to the hydrogenation process, the dehydrogenated state gradually returned to the mixed phase of the LiH and Mg(NH₂)₂ without appearance of any intermediate phases during heat treatment at 200 °C under 5 MPa H₂ for 37 h and during slow cooling down to room temperature through 24 h. In the hydrogenation process at 200 °C under 1 MPa H₂, however, the growing up of the LiNH₂ and LiH phase was observed in the XRD profiles before the 3Mg(NH₂)₂ and 8LiH phases were formed as the final hydrogenated state. This indicates that the LiNH₂ and LiH phase essentially appears as an intermediate state in the Li–Mg–N–H system composed of 3Mg(NH₂)₂ and 8LiH.     

The structural and kinetic characteristics of Mg1.9Al0.1Ni alloy synthesized by mechanical alloying

The structural and kinetic characteristics of the mechanically alloyed Mg_1.9Al_0.1Ni were investigated. It was found that Mg_1.9Al_0.1Ni can absorb/desorb about 3.55/3.44 mass% H at a high rate and it has a hexagonal crystal structure as Mg₂Ni. The hydriding/dehydriding (H/D) rates in the two-phase (–β) region of Mg_1.9Al_0.1Ni were measured and studied at temperatures ranging from 553 to 623 K under an approximately isobaric condition. The obtained data of H/D rates indicated that hydrogen diffusion was the rate-controlling step through the hydride phase. A new model was successfully used to calculate the kinetic experimental results. It can be seen that theoretical calculation agrees well with experimental data. The corresponding activation energies are 47 600 and 54 500 J/mol H₂ for H/D processes, respectively.     

The effect of Nd content on the electrochemical properties of low-Co La–Mg–Ni-based hydrogen storage alloys

The low-Co content La_0.80−xNd_xMg_0.20Ni_3.20Co_0.20Al_0.20 (x = 0.20, 0.30, 0.40, 0.50, 0.60) alloys were prepared by inductive melting and the effect of Nd content on the electrochemical properties was investigated. XRD shows that the alloys consist mainly of LaNi₅ phase, La₂Ni₇ phase and minor LaNi₃ phase. The electrochemical P–C–T test shows hydrogen storage capacity increases first and then decreases with increasing x, which is also testified by the electrochemical measurement that the maximum discharge capacity increases from 290 mAh/g (x = 0.20) to 374 mAh/g (x = 0.30), and then decreases to 338 mAh/g (x = 0.60). The electrochemical kinetics test shows exchange current density I₀ increases with x increasing from 0.20 to 0.50 followed by a decrease for x = 0.60, and hydrogen diffusion coefficient D increases with increasing x. Accordingly high rate dischargeability increases with a slight decrease at x = 0.60 and the low temperature dischargeability increases with increase in Nd content. When x is 0.50, the alloy exhibits a better cycling stability.     

Corrosion Behavior of MgZnCa Bulk Amorphous Alloys Fabricated by Spark Plasma Sintering

Centimeter-sized Mg_(65)Zn_(30)Ca_5 bulk amorphous alloys were fabricated by the spark plasma sintering process from the amorphous powders with a size smaller than 5 l m prepared by ball-milling.The sintered Mg_(65)Zn_(30)Ca_5samples were in an amorphous state when the spark plasma sintering was performed at a temperature of 383 K under a pressure of 600 MPa.The data of polarization curves presented that the sintered Mg_(65)Zn_(30)Ca_5bulk amorphous alloys exhibited higher corrosion resistance than pure Mg and AZ31B alloy owing to high content of Zn and homogeneous structure.A calcium phosphate compound layer was formed on the sintered Mg_(65)Zn_(30)Ca_5bulk amorphous sample after immersion in Hanks’solution,which is effective in improving corrosion resistance and bioactivity.The sintered Mg Zn Ca bulk amorphous alloys with large dimensions broaden the potential application of bulk amorphous alloys in the biomedical fields.     

Effect of Co and Mn on the electrochemical properties of La0.7Mg0.3Ni2(Co+Mn) alloys

The effects of the relative Co and Mn content on the electrochemical performance of La_0.7Mg_0.3Ni₂(Co+Mn) hydrogen storage alloys were investigated. The crystal structure, discharge capacity and cycle life of the alloys were evaluated. For all alloys, we found that the higher the Co content, the larger is discharge capacity. The appropriate amount of Mn in La_0.7Mg_0.3Ni₂(Co+Mn) alloys can extend the cycle life of the hydrogen storage alloys although the alloys have less discharge capacity than those with higher Co content. In addition, the LaNi_3.87Mn_1.13 phase appears and the LaNi₅ phase disappears with replacement of Co by Mn.     

Influence of tin on the electrochemical and gas phase hydrogen sorption in Mg2−xSnxNi (x = 0, 0.1, 0.3)

Mg_2−xSn_xNi (x = 0, 0.1, 0.3) alloys were synthesized by reactive ball milling under protective Ar atmosphere and liquid n-heptane. The microstructure and the morphology of the powders were determined by X-ray diffraction and scanning electron microscopy. The as-milled alloys consist of Mg₂Ni nanocrystals with an average grain size in the range 3–7 nm, depending on the alloy composition. Sn containing phases were not detected even in the Sn-rich alloy. Obviously, Sn is dissolved in the Mg₂Ni intermetallic compound. Gas phase sorption of hydrogen was not observed in the alloys containing Sn (Mg_2−xSn_xNi; x = 0.1, 0.3). It was suggested that Sn impedes the process of hydrogen molecules decomposition. The as-milled alloys absorbed reversibly hydrogen electrochemically. Mg₂Ni alloy showed the highest discharge capacity of 300 mAh/g. The capacity of Mg_1.9Sn_0.1Ni and Mg_1.7Sn_0.3Ni was about 260 mAh/g. It was found that Sn improved the cycle life of the electrode.