Ni Hydrogen Storage Alloys Research Articles

Effect of annealed treatment on microstructure and cyclic stability for La–Mg–Ni hydrogen storage alloys

The influence of annealed treatment on the microstructure and the cyclic stability for La–Mg–Ni alloys are systematically investigated by FESEM-EDX, XRD, PCT and electrochemical measurements. It is demonstrated that Ni and La are almost evenly dispersed, but Mg is nonuniform distribution in these alloys. Rietveld analysis results illustrate that the alloys mainly consist of LaNi5, Pr5Co19-type, Ce2Ni7-type and PuNi3-type phases. The mass fraction of each phase in these alloys varies after annealed. The 3d annealed alloy shows the better hydriding/dehydridng capacity and electrochemical cyclic stability. As annealed time is continually prolonged, the hydrogen storage capacity slightly increases, but the electrochemical discharge capacity decreases. Estimating from the pressure-composition isotheral curves, it is considered that the hydriding of these alloys is governed by two processes: the initial process can be expressed by the hydriding of the phases with stacking of [LaMgNi4] and [LaNi5] slabs and the second is controlled by LaNi5 phase.

An investigation of hydrogen storage kinetics of melt-spun nanocrystalline and amorphous Mg 2Ni-type alloys

The nanocrystalline and amorphous Mg 2Ni-type Mg 2- x La x Ni( x=0, 0.2) hydrogen storage alloys were synthesized by melt-spinning technique. The as-spun alloy ribbons were obtained. The microstructures of the as-spun ribbons were characterized by X-ray diffraction (XRD), high resolution transmission electronic microscopy (HRTEM) and electron diffraction (ED). The hydrogen absorption and desorption kinetics of the alloys were measured using an automatically controlled Sieverts apparatus, and their electrochemical kinetics were tested by an automatic galvanostatic system. The electrochemical impedance spectrums (EIS) were plotted by an electrochemical workstation (PARSTAT 2273). The hydrogen diffusion coefficients in the alloys were calculated by virtue of potential-step method. The obtained results showed that no amorphous phase was detected in the as-spun La-free alloy, but the as-spun alloys substituted by La held a major amorphous phase, confirming that the substitution of La for Mg markedly intensified the glass forming ability of the Mg 2Ni-type alloy. The substitution of La for Mg notably improved the electrochemical hydrogen storage kinetics of the Mg 2Ni-type alloy. Furthermore, the hydrogen storage kinetics of the experimental alloys was evidently ameliorated with the spinning rate growing.

Experimental investigation and thermodynamic modeling of the phase equilibria at the Mg–Ni side in the La–Mg–Ni ternary system

The La–Mg–Ni system is considered as one of the most promising candidates for the hydrogen storage alloys, and most of hydrogen storage phases in the La–Mg–Ni ternary system are mainly located near the Mg–Ni side. The present work focused on clarifying the phase relationships at the Mg–Ni side using the Calphad method. The thermodynamic descriptions of phase relationship were established on the available literature data together with our experimental results. Several key specimens were selected, synthesized and annealed at 773 K, 973 K and 1173 K to confirm the accuracy of the phase diagram calculated from the ternary thermodynamic description. Inductively coupled plasma, X-ray diffraction and scanning electron microscopy with electron dispersive spectrometry were used to analyze the alloys. The acceptable agreement on the equilibrium phases between the experimental and calculated results suggested the reliability of the assessed thermodynamic descriptions of phase relationship. The optimized phase equilibrium and thermodynamic information were successfully used to discuss the synthesized technological parameter of the La–Mg–Ni hydrogen storage alloys.

Electrochemical characteristics of amorphous MgTi x Ni (x = 0, 0.1, and 0.2) hydrogen storage alloys

MgTixNi (x = 0, 0.1, and 0.2) alloys were successfully prepared by mechanical alloying (MA), and the influence of milling time on the electrochemical characteristics of the electrodes was discussed. MgTixNi alloys after 90 h milling displayed the best electrochemical performance. The X-ray diffraction patterns showed that the alloy ball-milled for 90 h was amorphous with a widened diffraction peak. The charge-discharge tests indicated that these alloys had good electrochemical activation properties, and the MgTi0.2Ni alloy electrode exhibited the best cycle performance. The initial discharge capacity of the MgTi0.2Ni alloy reached up to 401.1 mAh·g−1, and the retention rate of capacity was 31.0% after 30 cycles, much higher than that of MgNi (17.3%). The Tafel polarization curves revealed that Ti addition could enhance the anticorrosion performance of these alloys in alkali solution, which was responsible for the ameliorated cyclic stability of these alloy electrodes.

Study on the electrochemical properties of Mg 0.8Ti 0.1Al 0.1 −xPd xNi ( x = 0.02–0.08) hydrogen storage alloys

Abstract. The Mg 0.8 Ti 0.1 Al 0.1 −x Pd x Ni ( x = 0.02–0.08) hydrogen storage alloys were prepared by mechanical alloying (MA) and the electrochemical properties of the alloy electrodes were studied systematically. Charge-discharge measurement shows that the maximum discharge capacity decreases from 383.5 mAh g −1 ( x = 0.02) to 360.3 mAh g −1 ( x = 0.08), however, the cycle stability is improved obviously due to the enhancement of the anti-corrosion performance, the capacity retention ratio 100%× C 100 / C 1 increases from 39.6% ( x = 0.02) to 49.0% ( x = 0.04), 58.6% ( x = 0.06) and 58.4% ( x = 0.08), respectively. At the discharge current density 500 mA g −1 , the high rate dischargeability (HRD) increases from 59.2% ( x = 0.02) to 73.5% ( x = 0.08), indicating that the electrochemical kinetics of the alloy electrodes is improved with the increasing of Pd content. In addition, electrochemical impedance spectroscopy (EIS), linear polarization and potentiostatic discharge measurements were also employed to study the kinetics of the alloy electrodes. The results obtained indicate that the electrochemical kinetics of the alloy electrodes is jointly controlled by the charge-transfer reaction rate on the alloy surface and the hydrogen diffusion rate in the bulk of the alloys, and the latter is the rate-determining step of discharge process.

Local and crystal structure of Mg 1.9Al 0.1Ni hydrogen storage alloys during hydrogen absorption–desorption cycling

The evolution of crystal structure and chemical state of Mg 1.9Al 0.1Ni alloy during hydrogen absorption–desorption cycling was examined by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). We research the hydrogen storage capacity of the Mg 1.9Al 0.1Ni by the H/D kinetic curves. The H/D kinetic curves indicate that the hydrogen storage capacity increased with the increased cycles and the samples were activated after 10 cycles have the maximum hydrogen storage capacity. The local structure of Ni atoms was studied by extended X-ray absorption fine structure (EXAFS). The EXAFS results indicate the Ni–Ni bonds distance has no obviously change with the cycles increasing, whereas the Ni–Mg bond lengths increase, and the Ni–Mg bond lengths are longer obviously than before 10 cycles whereas it has no obviously change after 10 cycles.

Characterization of Hydrogen Sorption Properties and Microstructure of Cast Mg-10wt%Ni Alloys

New Mg-10wt%Ni hydrogen storage alloys were fabricated by casting which is a very simple and cost effective production process. Alloying elements such as Nb and Ti, which have relatively high melting temperatures and very low solubility in solid Mg, were successfully dissolved into the liquid Mg-Ni alloy. The Mg-Mg2Ni alloys contain a well-refined lamellar eutectic microstructure after solidification with a large interfacial area between the Mg and Mg2Ni phases which provides for good hydrogen sorption properties. This is considered to be due to the high diffusivity of hydrogen along the interphase boundaries. Addition of transition metals such as Nb and Ti results in the formation of intermetallic phases with a size about 10-20μm during solidification. Furthermore, Ti was found to be strongly segregated to the eutectic Mg-Mg2Ni interface. In the presence of Nb and Ti, the hydrogen sorption kinetics of the Mg-Mg2Ni alloy is further improved. This suggests that the transition metals act as active catalysts that eases and accelerates the hydrogen diffusion during hydrogenation and dehydrogenation. In this paper, we present the hydrogen storage properties and their relationship to the microstructure of the cast Mg-10wt%Ni alloys. Detailed microstructural analysis was carried out in order to further understand the hydrogen diffusion and storage mechanisms.

Influences of Ti substitution on the structure and electrochemical properties of Mg1−x Ti x Ni (x = 0, 0.1, 0.2, and 0.3) hydrogen storage alloys

MgNi-based hydrogen storage alloys Mg1−xTixNi (x = 0, 0.1, 0.2, and 0.3) were prepared by means of mechanical alloying. Mg in the alloy was partially substituted by Ti to improve the cycle stability of the alloys. The effects of the substitution of Ti for Mg on the microstructure and electrochemical performances of the alloys were investigated in detail. The results indicate that the substitution of Ti for Mg obviously decreases the discharge capacity, but it significantly improves their cycle stabilities. The microstructure of the alloys analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM) shows that the alloys have a dominatingly amorphous structure. The substitution of Ti for Mg helps to improve the anti-oxidation/corrosion ability of the MgNi alloy but demolishes the electrochemical kinetics of hydrogenation/dehydrogenation. The Mg0.9Ti0.1Ni alloy electrode milled for 80 h exhibits the best integrative capability, which has the maximal discharge capacity of 331.66 mAh/g and the C30/Cmax of 63.65%.

XPS study on Mg 0.9− xTi 0.1Pd xNi ( x = 0.04, 0.06, 0.08, 0.1) hydrogen storage electrode alloys after charge–discharge cycles

The passive film composition of Mg 0.9− x Ti 0.1Pd x Ni ( x = 0.04, 0.06, 0.08, 0.1) hydrogen storage alloys after 40 charge–discharge cycles has been investigated by means of X-ray photoelectron spectroscopy (XPS) in combination with Ar + sputtering technology. With the XPSPEAK software, high resolution spectra of alloy elements and oxygen were deconvolved into individual peaks. Composites formed by metal elements and their relative contents were also deduced. It was found that the composites originated from Mg and Ni were mainly in the form of their oxides and hydroxides, which existed at the top surface of alloys. With the increase of sputtering depth, the hydroxides of Mg and Ni gradually disappeared while corresponding oxides dominated their passive products. According to the analysis results of oxygen spectra, the elemental segregation of Mg and Ni was influenced by the substitution of Pd because the addition of Pd slightly enhanced the surface energy of the alloys and suppressed the formation of Mg hydroxide and oxide. Ti and Pd presented multiple-oxides from the surface to the inner alloys and metallic Pd appeared in the sub-layers of the alloys’ surface. The possible mechanisms of the formation of passive products were suggested on the basis of the discussion in the paper.

Influences of heat treatment on the structure and electrochemical properties of Mg0.9Ti0.1Ni hydrogen storage alloys

In the present study, the effects of heat treatment on the structures and electrochemical properties of Mg0.9Ti0.1Ni hydrogen storage alloys have been investigated in detail. X-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses showed that the samples exhibited a predominantly amorphous structure. Electrochemical investigations revealed that heat treatment can improve the maximum discharge capacity and cyclic stability of the alloy electrodes. For Mg0.9Ti0.1Ni alloy prepare by ball milling, the value of Cmax was only 229.9 mAh g-1, however, the value reached 331.9 mAh g-1 after heat treatment at 873K for 8 h and then milling. The cycle voltammetry, electrochemical impedance spectroscopy and potentiodynamic polarization indicated that heat treatment not only increased the discharge capacity but also improved the charge/discharge kinetics of Mg0.9Ti0.1Ni alloy.

Development of Mg-oxide–Ni hydrogen-storage alloys by reactive mechanical grinding

Mg-oxide and Mg-oxide–Ni hydrogen storage alloys were prepared by mechanical grinding under H 2 (reactive mechanical grinding). Among these alloys, Mg – ( 7.5 wt % Fe 2 O 3 , 7.5 wt%Ni) prepared by grinding for 4 h with nano-structured Fe 2 O 3 particles and Ni showed the best hydrogen storage properties. The as-milled sample absorbed 4.75 wt% H at 593 K under 12 bar H 2 for 60 min. Its activation was accomplished after two hydriding–dehydriding cycles. The activated sample absorbed 4.36 wt% H at 593 K under 12 bar H 2 for 60 min and desorbed 4.26 wt% H at 593 K under 1.0 bar H 2 for 60 min. After hydriding–dehydriding cycling, Mg 2 Ni , Fe and MgO are formed. The large bar-like hydrided particles have round and expanding surfaces, suggesting that they expanded during the hydriding reaction, while the dehydrided particles exhibit contracted surfaces with wrinkles.

Nanostructured Mg–Mm–Ni hydrogen storage alloy: Structure–properties relationship

Kinetics of H uptake/release in Mg can be improved by alloying with Ni and RE, as well as by reducing the grain size of the Mg alloy. Both these approaches were applied in the present work for the alloy 72 wt.% Mg–20 wt.% Ni–8 wt.% Mm (ternary eutectic Mg–Mg 2Ni–MmMg 12). The alloy was processed by the equal channel angular pressing (ECAP) technique. In ECAP the sample is subjected to heavy plastic strain by pressing it through a die with an angle of 90°. ECAP treatment resulted in a fine microstructure compared to the rather heterogeneous as-cast material. Hydrogenated and non-hydrogenated samples were investigated using SEM and XRD. Hydrogenation properties were studied by TDS and PCT. Hydrogenated samples consist of MgH 2, Mg 2NiH 4 and MmH 2+ x and exhibit a maximum H-storage capacity of ∼5.5 wt.%. To initiate the first hydrogenation, the alloy needs to be activated at ∼300 °C. However, already after one hydrogenation cycle its H-absorption becomes quite fast: 4.5 wt.% H is absorbed in just 15 min. Vacuum TDS (heating rate 0.5 °C/min) shows that desorption starts at low temperature, ∼135 °C, with a peak at ∼210 °C. The alloy was also subjected to high energy ball milling (HEBM) in Ar or H 2 to yield further refinement of the microstructure. ∼1 wt.% Nb 2O 5 was added to facilitate H exchange. The H sorption characteristics of the alloy treated by ECAP and HEBM have been compared with the ones for the as-cast material.

Mg1.8La0.2Ni−xNi Nanocomposites for Electrochemical Hydrogen Storage

Mg1.8La0.2Ni hydrogen storage alloy was ball-milled with Ni powder, leading to the formation of a nanocrystalline and amorphous microstructure with particle sizes less than 50 nm in diameter. Each sample was examined by transmission electron microscopy (TEM), energy-dispersive spectroscopy (EDS), and X-ray diffraction (XRD). This structure was beneficial for the reduction of electrochemical impedance, as well as significant improvement of its discharge capacity, cycle life, and rate capability for electrochemical hydrogen storage in an alkaline solution. When the molar ratio (x) of Ni over Mg1.8La0.2Ni was equal to 2, the dehydriding capacity reached 2.55 wt % from electrochemical pressure-temperature isotherms (P-C-T). It was in good agreement with its initial discharge capacity, 716 mA*h/[g of (Mg1.8La0.2Ni)], observed from the electrochemical charge and discharge process. After 50 cycles, its discharge capacity still reached 381 mA*h/[g of (Mg1.8La0.2Ni)]. Further results showed that this composite had a promising high rate capability. At the current density of 1200 mA/g its discharge capacity reached 48% of its initial capacity.

Hydrogenation and degradation of Mg–10 wt% Ni alloy after cyclic hydriding–dehydriding

Hydrogenation and degradation properties of Mg–10 wt% Ni hydrogen storage alloys were investigated by cyclic hydriding–dehydriding tests. Mg–10 wt% Ni alloy was synthesized by rotation-cylinder method (RCM) under 0.3% HFC-134a/air atmosphere and their hydrogenation and degradation properties were evaluated by pressure-composition-isotherm (PCI) measurement. Hydrogen storage capacities gradually increased following 160 hydriding–dehydriding cycles and thereafter started to decrease. Measured maximum hydrogen capacity of Mg–10 wt% Ni alloy is 6.97 wt% at 623 K. Hydriding and dehydriding plateau pressure were kept constant for whole cycles. Reversible hydrogen capacity started to descend after 280 hydriding–dehydriding cycles. The lamellar eutectic structure of Mg–Ni alloy consists of Mg-rich α -phase and β - Mg 2 Ni . It is assumed that the lamellar eutectic structure enhances hydrogenation properties.

Corrosion Behaviors of Mg 0.9-xTi 0.1Pd xNi( x=0.04-0.1) Hydrogen Storage Alloy Electrodes

The effects of partial substitution of Pd for Mg on the corrosion behaviors of Mg 0.9- x Ti 0.1Pd x Ni ( x=0.04-0.1) hydrogen storage alloys were studied in this paper. The alloys were prepared by mechanical alloying. The alloy structures became amorphous after ball milling for 120 h, which was confirmed by XRD and TEM analysis. The cyclic charge-discharge tests showed that the Pd addition prolonged the cycle life of the alloys effectively. The corrosion performances of the alloys were investigated by open circuit potential measurements, anodic polarization, electrochemical impedance spectroscopy, and XPS experiments. The increase of Pd content shifted the alloy corrosion potentials towards more positive values. The initial corrosion currents of the alloys and its increment rates along with cycle number decreased with the increase of Pd content in the alloys. The measured electrochemical impedance spectra were fitted well with the proposed equivalent circuit model. The analysis suggested that the thickness and the resistance of passive film on the surface of alloys increased with the increase of Pd content in the alloys. The results of XPS experiments demonstrated that the addition of Pd in the alloys suppressed the oxidation and enhanced the anti-corrosion capabilities of the alloys. When the Pd content reached 0.1, the corrosion resistant performances of Mg 0.9- x Ti 0.1Pd x Ni( x=0.04-0.1) alloys are the best and the discharge capacity retention rate is the highest in the present study.

Effects of Pd substitution on the electrochemical properties of Mg 0.9− xTi 0.1Pd xNi ( x = 0.04–0.1) hydrogen storage alloys

Amorphous Mg 0.9− x Ti 0.1Pd x Ni ( x = 0.04–0.1) hydrogen storage alloys were prepared by mechanical alloying (MA). The effects of Pd substitution on the electrochemical properties of the Mg 0.9− x Ti 0.1Pd x Ni ( x = 0.04–0.1) electrode alloys were studied by cyclic charge–discharge, linear polarization, anodic polarization, electrochemical impedance spectroscopy (EIS), and hydrogen diffusion coefficient experiments. It was found that the cyclic capacity retention rate C 50/ C 1 of the quaternary alloys was greatly improved due to the substitution of Pd for Mg. Mg 0.8Ti 0.1Pd 0.1Ni electrode alloy retained the discharge capacity above 200 mAh g −1 even after 80 charge–discharge cycles, possessing the longest cycle life in the studied quaternary alloys. The improvement of cycle life was ascribed to the formation of passive film on the surface of these electrode alloys. X-ray photoelectron spectroscopy (XPS) analysis proved that the passive film was composed of Mg(OH) 2, TiO 2, NiO, and PdO, which synergistically protected the alloy from further oxidation. The Auger Electron Spectroscopy (AES) study revealed that the thickness of passive film increased with augmentation of the Pd content. The electrochemical impedance study of electrode alloys after different cycles demonstrated that the passive film became thicker during cycles and its thickness also increased with Pd content augmentation. It was also found that the augmentation of Pd content resulted in the decrease of exchange current density I 0 and the increase of the charge-transfer resistance R ct. With increasing the Pd amount in the Mg 0.9− x Ti 0.1Pd x Ni ( x = 0.04–0.1) electrode alloys, hydrogen diffusion coefficient D was gradually enhanced at first. Then, it decreased with augmentation of cycle due to the growth of passive film on the surface of the alloys.

Application of volumetric electric-spark dispersion for the fabrication of Ti–Zr–Ni hydrogen storage alloys

Results of X-ray and electrochemical analyses of properties of Ti–Zr–Ni powders obtained by the volumetric electric-spark dispersion method in water, alcohol, kerosene, liquid nitrogen and liquid argon are presented. The influence of working fluids and parameters of discharge pulses on the chemical composition, phase-structural state, electrochemical characteristics and electrical capacity of the obtained powders is considered. The effect of annealing in various media on the electrochemical properties of powders has been studied.

Amination application over nano-Mg?Ni hydrogen storage alloy catalysts

Some nano-intermetallic alloys such as Mg–Ni were prepared by a chemical reduction method (named polyol process) using ethylene glycol, polyvinylpyrrolidone (PVP 10000) and PdCl2 as reduction reagent, protect reagent, and nuclear reagent, respectively, to reduce the metal salts. The particle size of the synthetic Mg–Ni alloy was affected by the amount of the protect agent and nuclear agent and by the reduction temperature. All synthetic Mg–Ni alloys possess higher surface area about 113–315 m2/g and four kinds of components: Mg2Ni, Ni, Mg and MgO. The contents of Mg2Ni, Ni, Mg and MgO are affected by the Mg/Ni molar ratio. High molar ratio of Ni makes the single phase of Ni easy to reduce and results in the increase of Ni crystalline degree. The results of XRD and TEM and electron diffraction pattern indicate that PVP concentration only changes the particle size, but not the structure and composition of synthetic Mg–Ni alloys. The catalytic studies of PEG amination show that the total amine yield reaches to 58.4% over 1 g of Mg–Ni alloy with Mg:Ni=1:2 due to the formation of more single phase of Ni. The formation of Mg2Ni alloy and MgO prevent the aggregation of Ni, and possesses a higher amination activity.

Amination application over nano-Mg–Ni hydrogen storage alloy catalysts

Some nano-intermetallic alloys such as Mg–Ni were prepared by a chemical reduction method (named polyol process) using ethylene glycol, polyvinylpyrrolidone (PVP 10000) and PdCl2 as reduction reagent, protect reagent, and nuclear reagent, respectively, to reduce the metal salts. The particle size of the synthetic Mg–Ni alloy was affected by the amount of the protect agent and nuclear agent and by the reduction temperature. All synthetic Mg–Ni alloys possess higher surface area about 113–315m2/g and four kinds of components: Mg2Ni, Ni, Mg and MgO. The contents of Mg2Ni, Ni, Mg and MgO are affected by the Mg/Ni molar ratio. High molar ratio of Ni makes the single phase of Ni easy to reduce and results in the increase of Ni crystalline degree. The results of XRD and TEM and electron diffraction pattern indicate that PVP concentration only changes the particle size, but not the structure and composition of synthetic Mg–Ni alloys. The catalytic studies of PEG amination show that the total amine yield reaches to 58.4% over 1g of Mg–Ni alloy with Mg:Ni=1:2 due to the formation of more single phase of Ni. The formation of Mg2Ni alloy and MgO prevent the aggregation of Ni, and possesses a higher amination activity.

Cycle durability of Ca–Mg–Ni alloys and factors which cause degradation of hydrogen storage capacity

The cycle durability of Ca–Mg–Ni hydrogen storage alloys, which are investigated in the Japanese national project (International Clean Energy Network), WE-NET, was examined using pure H 2 (99.99999%). The tested alloys were Ca 0.25Mg 0.66Y 0.09Ni 1.86 (cubic MgCu 2-type structure), Ca 0.55Mg 0.45Ni 3 (rhombohedral PuNi 3-type) and Ca 0.5Mg 0.5Ni 2 (MgCu 2-type). Although initially the ‘dynamic’ hydrogen storage capacities were H/M ∼0.7, they decreased steeply within a few cycles, and then they declined gradually. At around 400th cycle, they decreased to nearly half of the maximum capacities regardless of crystal structures and compositions of the tested alloys. However, their ‘equilibrium’ capacities did not decrease as the ‘dynamic’ ones with progress of hydrogen sorption. As for the bulk, not serious disproportionation but pulverization and lattice expansion were observed by X-ray diffractometry (XRD) and microscopic analyses. The expansion indicated that some amounts of hydrogen remained in the lattice cells. It was considered that pulverization of the alloy particles and amorphous shell formed on the surface prevented the alloys from smooth hydrogen sorption, and which led to the decrease of the ‘dynamic’ hydrogen storage capacities without that of the ‘equilibrium’ ones in the cycle tests.