Hydrogenation behavior of high-energy ball milled amorphous Mg2Ni catalyzed by multi-walled carbon nanotubes

Amorphous Mg₂Ni alloy was successfully synthesized by means of mechanical alloying. Then, the multi-walled carbon nanotubes (MWCNTs) were added by high-energy ball milling to catalyze the amorphous alloy. The X-ray diffraction (XRD) spectroscopy reveal that the as-cast Mg₂Ni alloy has presented a completely amorphous state under specific conditions of high-energy ball milling process. Different process parameters of ball-to-powder ratio (10:1, 20:1, 40:1) and milling time have been attempted for the preparation of amorphous Mg₂Ni alloy. The results show that the milling time and ball-to-powder weight ratio have significantly influence on the amorphization process of crystalline Mg₂Ni alloy. Before and after the milling, phase compositions and microstructures of the prepared materials were characterized by XRD, scanning electron microscope (SEM), electron energy dispersion spectrum (EDS) and transition electron microscope (TEM) approaches. The morphology of composite Mg₂Ni/MWCNTs was investigated, the TEM images show that the MWCNTs imbed on the surface of the particles after milling for 1 h, and the MWCNTs with and without tubular structure have been observed. The hydrogen storage properties of amorphous Mg₂Ni alloys were improved by the catalytic effect of MWCNTs. The catalytic effect and mechanism of MWCNTs on the hydrogen storage properties of amorphous Mg₂Ni alloy are discussed and investigated.     

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.     

Structural characterization and electrochemical hydrogen storage properties of Mg2Ni1−xMnx(x = 0, 0.125, 0.25, 0.375) alloys prepared by mechanical alloying

Mg₂Ni_1−xMn_x(x = 0, 0.125, 0.25, 0.375) electrode alloys are prepared by mechanical alloying (MA) under argon atmosphere at room temperature using a planetary high-energy ball mill. The microstructures are characterized by XRD and SEM. XRD analysis results indicate that the substitution of Mn for Ni could inhibit the formation of MgNi₂ phase with the increases of x from 0 to 0.375. Replacing Ni with Mn can also promote the formation of the amorphous phase when x increases from 0 to 0.25 for the MA alloys milled for 48 h. The new phase Mg₃MnNi₂ is formed only when x = 0.375 after 48 h of milling. This new phase belongs to the face-centered cubic lattice (Fd-3m) with the lattice constant a being 1.1484 nm. Estimated from the peaks broadening, the crystallite size and lattice strain of Mg₃MnNi₂ phase are 15.6 ± 3.6 nm and 1.09 ± 0.34%, respectively. Curve fit of XRD shows that amorphous and nanocrystalline Mg₂Ni coexist in the Mg₂Ni_1−xMn_x (x = 0, 0.125, 0.25) alloys milled for 48 h. The SEM observation reveals that all the MA alloys particles are mainly flaky and show cleavage fracture morphology and these particles are agglomerates of many smaller particles, namely subparticles. Electrochemical measurements indicate that all MA alloys have excellent activation properties. The discharge capacities of MA alloys increase with the prolongation of milling time. For 16 h of milling, with the increase of Mn content, the discharge capacities of Mg₂Ni_1−xMn_x (x = 0, 0.125, 0.25, 0.375) MA alloys monotonously decrease. For 24 h of milling, the discharge capacities of the Mg₂Ni_1−xMn_x (x = 0, 0.125, 0.25, 0.375) alloys also show a rough tendency to decrease with the increase of Mn content except Mg₂Ni_0.875Mn_0.125 MA alloy. On the other hand, for 48 h of milling, as the rise of Mn content from x = 0.125 to 0.375, the discharge capacities increase. Mg(OH)₂ is formed during charge/discharge cycles in the KOH solution for all MA alloys. After 48 h of milling, the substitution of Mn for Ni for x = 0.25 improves the cycle stability at the expense of decreasing the discharge capacity. In contrast, Mg₃MnNi₂ phase is relatively stable during charge/discharge cycles and therefore can significantly enhance the cycle stability under simultaneously maintaining a high discharge capacity.     

The effects of partial substitution of Cr for Ni on the electrochemical properties of Mg1.75Al0.25Ni1−xCrx (0 ≤ x ≤ 0.3) electrode alloys

In this paper, the substitution of different amounts of Cr for Ni in the hydrogen storage electrode alloy of Mg_1.75Al_0.25Ni has been carried out to form quaternary Mg_1.75Al_0.25Ni_1−xCr_x (0 ≤ x ≤ 0.3) alloys by means of solid diffusion method (DM). The XRD profiles exhibited that the quaternary alloys still kept the same main phase of Mg₃AlNi₂ (S.G. Fd3m) as that of ternary Mg_1.75Al_0.25Ni alloy. The electrochemical studies found that Cr substituted quaternary alloy reached its maximum discharge capacity (165 mAh g⁻¹) after 2 cycles, which was larger than that of the Mg_1.75Al_0.25Ni alloy (154 mAh g⁻¹). Among these quaternary alloys, the Mg_1.75Al_0.25Ni_0.9Cr_0.1 electrode alloy was found possessing the highest cycling capacity retention rate. Cyclic voltammetry (CV) results and anodic polarization curves demonstrated that appropriate content (x lower than 0.1) of Cr effectively improved the reaction activity of electrode and inhibited the cycling capacity degradation to some degree. Electrochemical impedance spectroscopy (EIS) analyses indicated that the increase of Cr content would raise the polarization resistance R_p on the particle surface of these quaternary alloys.     

Electrochemical hydrogen storage properties of Mg2−xAlxNi (x = 0, 0.3, 0.5, 0.7) prepared by hydriding combustion synthesis and mechanical milling

Mg_2−xAl_xNi (x = 0, 0.3, 0.5, 0.7) hydrogen storage alloys used as the negative electrode in a nickel–metal hydride (Ni–MH) battery were successfully prepared by means of hydriding combustion synthesis (HCS) and the selected alloy Mg_1.5Al_0.5Ni was further modified by mechanical milling (MM). The structural and electrochemical hydrogen storage properties of Mg_2−xAl_xNi alloys have been investigated in detail. XRD results show that a new phase Mg₃AlNi₂ that possesses an excellent cycling stability is observed with the substitution of Al for Mg. A short-time mechanical milling has a significant effect on improving the discharge capacity of the HCS product of Mg_1.5Al_0.5Ni. The maximum discharge capacity of Mg_1.5Al_0.5Ni ascends with increasing mechanical milling time and reaches the maximum 245.5 mAh/g when milled for 10 h. The alloy milled for 5 h shows the best electrochemical kinetics, which is due to its smaller mean particle size and uniform distribution of the particles. Further increasing in mechanical milling time could not bring about better electrochemical kinetics, which might be attributed to the agglomeration of the alloy particles and thus the charge-transfer reaction and hydrogen diffusion are restrained. It is suggested that the novel method of HCS + MM is promising to prepare ternary Mg-based intermetallic compound for electrochemical hydrogen storage.     

Kinetic properties of La2Mg17–x wt.% Ni (x = 0–200) hydrogen storage alloys prepared by ball milling

The as-cast La₂Mg₁₇ with different amount of Ni powders were mixed through ball milling to produce a new type of La₂Mg₁₇–x wt.% Ni (x = 50, 100, 150, 200) alloy. The microstructures of the alloys were characterized by XRD technique, the results show that the crystal structure transfers to amorphous one with the increasing amount of Ni powders. La₂Mg₁₇–50 wt.% Ni alloy reaches the highest hydrogen absorption capacity of 5.13 wt.% at 300 °C under 2 MPa hydrogen pressure due to its amorphous structure. Furthermore, La₂Mg₁₇–50 wt.% Ni alloy expresses fast hydriding kinetics and absorbs 4.99 wt.% hydrogen gas in 200 s. The hydrogen desorption ability described as discharge capacity during electrochemical reaction is fade next to La₂Mg₁₇–200 wt.% Ni alloy, attributed to the less Mg₂NiH₄ with lower enthalpies and easier to release H₂. The maximum discharge capacity of La₂Mg₁₇–200 wt.% Ni alloy reaches to exciting 980.90 mAh/g, while the La₂Mg₁₇ alloy is only 18.10 mAh/g with inconspicuous improvement of cycle stability. These dramatic difference in electrochemical performance reflect the consequence of sluggish dehydriding process of La₂Mg₁₇–50 and 100 wt.% Ni alloys again. Whereas La₂Mg₁₇–200 wt.% Ni alloy has lower resistance both on alloy surface and in the bulk.     

Electrochemical hydrogen storage characteristics of Mg1.5Al0.5−xZrxNi (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5) alloys synthesized by mechanical alloying

Mg_1.5Al_0.5−xZr_xNi (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5) type alloys were synthesized by mechanical alloying and their electrochemical hydrogen storage characteristics were investigated. X-ray diffraction studies showed that Zr facilitated the amorphization of Mg₂Ni phase, while Al retarded the amorphization of this phase. The increase in the Zr content was observed to bring about significant improvement in the discharge capacities at all the ball milling durations. The stepwise replacement of Al with Zr, however, caused considerable reduction in the initial discharge capacities of the alloys. Despite the adverse effect of Al on the initial discharge capacity, it prevented the rapid degradation of Mg₂Ni phase with the charge/discharge cycles. When the beneficial effects of Zr and Al were combined by designing Mg_1.5Al_0.5−xZr_xNi type alloys, Mg_1.5Al_0.2Zr_0.3Ni alloy was found to have the highest discharge capacity at almost all the charge/discharge cycle steps. Among the obtained capacity retaining rates, Mg_1.5Al_0.4Zr_0.1Ni alloy had the best performance. This alloy has kept at least 50% of its initial discharge capacity at 20th cycle. The analysis by the electrochemical impedance spectroscopy revealed that the charge transfer resistances of Al-rich alloys were low at high depth of discharges. This observation was attributed to the formation of the porous unstable Mg(OH)₂ layer due to the intercalation of Al₂O₃ layers, which have the high rate of solubility in strongly basic solutions, and thus the exposition of the underlying electrocatalytically active Ni sites.     

Effects of partial substitution by Fe and Co for Ni in the Mg1.75Al0.25 Ni electrode alloy on their electrochemical performances

In this paper, Co and Fe were selected as the partial substitution elements for Ni to form Mg_1.75Al_0.25Ni_0.9X_0.1 quaternary electrode alloys prepared by means of solid diffusion method (DM) on the basis of the ternary Mg_1.75Al_0.25Ni alloy. The XRD patterns exhibited that quaternary alloys still possessed the main phase of Mg₃AlNi₂ as that of the ternary Mg_1.75Al_0.25Ni alloy when introducing the substitution elements Co or Fe. The electrochemical studies found that Fe- or Co-substituted quaternary alloy possessed larger discharge capacity and higher cycling stability than the Mg_1.75Al_0.25Ni ternary alloy. Cyclic voltammetry (CV) tests demonstrated that the additional Fe and Co could improve the electrocatalytic oxidation activity on the surface and also the discharge capacity of the Mg_1.75Al_0.25Ni electrode alloy. Anodic polarization curves indicated that the additional Fe and Co led to potentials shifting toward position direction and decrease of the corrosion current. Electrochemical impedance spectroscopy (EIS) revealed that the exchange current density decreases considerably with the augmentation of the cycle number.     

The effects of partial substitution of Fe and Co for Ni in the Mg1.75Al0.25Ni electrode alloy on electrochemical performances

In this paper, Co and Fe were selected as the partial substitution elements for Ni to form Mg_1.75Al_0.25Ni_0.9X_0.1 quaternary electrode alloys prepared by means of the solid diffusion method (DM) on the basis of the ternary Mg_1.75Al_0.25Ni alloy. The XRD patterns exhibited that the quaternary alloys still possessed the main phase of Mg₃AlNi₂ as that of the ternary Mg_1.75Al_0.25Ni alloy by introducing the substitution elements Co or Fe. The electrochemical studies found that Fe- or Co-substituted quaternary alloy possessed larger discharge capacity and higher cycling stability than the Mg_1.75Al_0.25Ni ternary alloy. Cyclic voltammetry (CV) tests demonstrated that the additional Fe and Co could improve the electrocatalytic oxidation activity on the surface and also the discharge capacity of the Mg_1.75Al_0.25Ni electrode alloy. Anodic polarization curves indicated that the additional Fe and Co led the potentials shifting toward position direction and to a decrease of the corrosion current. Electrochemical impedance spectroscopy (EIS) revealed that the exchange current density decreases considerably with the augmentation of the cycle number.     

Investigation on electrochemical performances of melt-spun nanocrystalline and amorphous Mg2Ni1−xMnx (x = 0–0.4) electrode alloys

In order to enhance the glass forming ability of the Mn₂Ni-type electrode alloy, Ni in the Mg₂Ni compound is partially substituted by Mn. The nanocrystalline and amorphous Mg₂Ni-type alloys with nominal compositions of Mg₂Ni_1−xMn_x (x = 0, 0.1, 0.2, 0.3, 0.4) are fabricated by melt-spinning technique. The spun alloy ribbons with a continuous length, a thickness of about 30 μm and a width of about 25 mm are successfully obtained. The microstructures of the as-spun alloy ribbons are characterized by XRD, SEM and TEM. The electrochemical hydrogen storage characteristics of the as-spun alloy ribbons were tested by an automatic galvanostatic system. The electrochemical impedance spectra (EIS) are plotted by an electrochemical workstation (PARSTAT 2273). The hydrogen diffusion coefficients in the alloys are calculated by virtue of potential-step method. The results show that no amorphous structure is detected in the as-spun Mn-free alloy, whereas the as-spun alloys containing Mn display a nanocrystalline and amorphous structure. The amorphization degree of the alloy increases with rising spinning rate, suggesting that the substitution of Mn for Ni facilitates the glass formation in the Mg₂Ni-type alloy. The substitution of Mn for Ni markedly improves the electrochemical hydrogen storage performances of the Mg₂Ni-type alloy, enhancing both the discharge capacity and the electrochemical cycle stability. Furthermore, the high rate dischargeability (HRD), electrochemical impedance spectrum and potential-step measurements all indicate that the electrochemical kinetics of the alloy electrodes first increases then decreases with increasing amount of Mn substitution.     

Improved hydrogen storage behaviours of nanocrystalline and amorphous Mg2Ni-type alloy by Mn substitution for Ni

The nanocrystalline and amorphous Mg₂Ni-type alloys with nominal compositions of Mg₂Ni_1−xMn_x (x = 0, 0.1, 0.2, 0.3, 0.4) were synthesized by melt spinning technique. The structures of the as-cast and spun alloys were characterized by XRD, SEM and HRTEM. The hydrogen absorption and desorption kinetics of the alloys were measured by an automatically controlled Sieverts apparatus. The electrochemical hydrogen storage performances were tested by an automatic galvanostatic system. The results show that the as-spun (x = 0) alloy holds a typical nanocrystalline structure, whereas the as-spun (x = 0.4) alloy displays a nanocrystalline and amorphous structure, confirming that the substitution of Mn for Ni facilitates the glass formation in the Mg₂Ni-type alloy. The hydrogen absorption capacity of the alloys first increases then decreases with rising Mn content, but the hydrogen desorption capacity of the alloys grows with increasing Mn content. Furthermore, the substitution of Mn for Ni significantly improves the electrochemical hydrogen storage performances of the alloys, involving both the discharge capacity and the electrochemical cycle stability. With an increase in the amount of Mn from 0 to 0.4, the discharge capacity of as-spun (30 m/s) alloy grows from 116.7 to 311.5 mAh/g, and its capacity retaining rate at 20th charging and discharging cycle rises from 36.7 to 78.7%.     

Microstructure and electrochemical hydriding/dehydriding properties of ball-milled TiFe-based alloys

Nanocrystalline/amorphous AB-type hydrogen storage TiFe_1−xCo_x (x = 0.1 and 0.3) and TiFe_0.7Ni_0.2Co_0.1 alloys have been synthesized by mechanical alloying. The high-energy ball milling process is performed in a planetary type mill under an inert atmosphere, and at various milling times (15, 20 and 30 h). The microstructure and the morphology of the as obtained alloy powders are investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The SEM analysis showed that the alloy powders are consisted of agglomerations (10–15 μm) of smaller particles cold-welded together. The average particle size, in the range of 5–7 μm, is dependent on both, the time of milling and in a less extent on the amount of Co in the TiFe_1−xCo_x alloys. The addition of Ni (TiFe_0.7Ni_0.2Co_0.1) results in slight increase of the mean particle size of the ball milled powder. The XRD analysis showed that the samples have composite microstructure. Both, the diffraction peaks of the pure metals and the peaks of the fcc TiFe compound are identified in the diffractograms of all samples. This result indicates that the alloying process is not fully completed even after 30 h of ball milling. The electrochemical cycle life tests of the electrodes, prepared from as-obtained alloys, showed that the Co containing AB type alloys possessed lower maximum discharge capacity and better corrosion stability, compared to the TiFe electrode. In contrast to the TiFeCo alloys the Ni containing material does not show a big difference in the discharge capacity and cycle life performance between the samples milled for different time in the range of 15–30 h. Moreover, the TiFe_0.7Ni_0.2Co_0.1 alloy reveals very small discharge capacity decay, about 90% of the capacity is retained after 100 charge/discharge cycles.     

Structural characterization and electrochemical hydrogen storage properties of Ti2−xZrxNi (x = 0, 0.1, 0.2) alloys prepared by mechanical alloying

Nominal Ti₂Ni was synthesized under argon atmosphere at room temperature using a planetary high-energy ball mill. The effect of milling time and Zr substitution for Ti on the microstructure was characterized by XRD, SEM and TEM, and the discharge capacities of Ti_2−xZr_xNi (x = 0, 0.1, 0.2) were examined by electrochemical measurements at galvanostatic conditions. XRD analysis shows that amorphous phase of Ti₂Ni can be elaborated by 60 h of milling, whereas Zr substitution hinders amorphization process of the system. The products of ball milling nominal Ti_2−xZr_xNi (x = 0.1, 0.2) were austenitic (Ti, Zr)Ni and partly TiO, despite the fact that the operation was carried out under argon atmosphere. By comparing the SEM micrographs, it is found that the amorphous phase of Ti₂Ni was formed in the stage of cold-welding during milling, while with Zr substitution particles were flaky and finer, inhomogeneous in size distribution with massive agglomeration. TEM analysis was carried out and confirmed the observations via XRD. In the electrochemical tests, amorphous Ti₂Ni shows the best discharge capacity at 102 mAh/g at a current density of 40 mA/g. Without need of activation, it exhibits extraordinary cycling stability under room temperature. On the other hand, the effect of Zr substitution on the electrochemical property of Ti₂Ni is tricky, as superficially the discharge capacity drops drastically with Zr substitution, but with increase of Zr content (from x = 0.1 to x = 0.2), the discharge capacity increases generally, which credits to larger unit-cell-volume provided by ZrNi compared to TiNi. It is also found that the Ti–Ni system becomes significantly susceptible to oxidation when Zr is introduced to the initial powders as mechanical alloying is deployed as a synthesis method.     

Formation of AlNi phase and its influence on hydrogen absorption kinetics of Mg77Ni23-xAlx alloys at intermediate temperatures

In order to reduce the obstacle influence of coarse Mg₂Ni phase on hydrogen absorption kinetics in Mg–Ni alloys, aluminum was doped and Mg₇₇Ni_23-xAl_x (x = 0, 3, 6, 9) alloys were prepared. The results show that AlNi phase was formed when Al was added, the size of primary Mg₂Ni phase decreases with increasing Al content till 6 at.%, while primary Mg₂Ni phase was diminished and primary Mg phase was formed when Al content increased to 9 at.%. The initial hydrogenation rates of Mg₇₇Ni_23-xAl_x alloys were increased, which is resulted from the refined primary Mg₂Ni and the catalytic AlNi phase. More importantly, the hydrogenation rates and capacities were significantly improved at 150 °C, especially for the Mg₇₇Ni₁₇Al₆ alloy. The apparent activation energy of the Mg₇₇Ni₁₇Al₆ alloy for hydrogenation was reduced to 73.68 kJ/mol from 102.27 kJ/mol of the Mg₇₇Ni₂₃ alloy. Its enthalpy changes for hydrogenation at low and high platforms are 72.3 kJ/mol and 53.9 kJ/mol, respectively. The multiple channels and short distance for hydrogen atoms diffusion provided by refined primary Mg₂Ni phase, the solid dissolution of Al in Mg₂Ni lattice, and catalytic effect of AlNi on hydrogenation, leading to the improvement of the hydrogen storage properties.     

Hydrogen storage characteristics of the nanocrystalline and amorphous Mg–Nd–Ni–Cu-based alloys prepared by melt spinning

Nanocrystalline and amorphous Mg–Nd–Ni–Cu-based (Mg₂₄Ni₁₀Cu₂)_100−xNd_x (x = 0–20) alloys were prepared by melt spinning and their structures as well as hydrogen storage characteristics were investigated. The analysis of XRD, TEM and SEM linked with EDS reveal that all the as-cast alloys hold a multiphase structure, containing Mg₂Ni-type major phase as well as some secondary phases Mg₆Ni, Nd₅Mg₄₁ and NdNi, whose amounts clearly grow with Nd content rising. Furthermore, the as-spun Nd-free alloy displays an entire nanocrystalline structure whereas the as-spun Nd-added alloys have a mixed structure of nanocrystalline and amorphous, moreover, the amorphization degree of the alloys visibly increases with Nd content rising, implying that the addition of Nd facilitates the glass forming in the Mg₂Ni-type alloy. The addition of Nd results in a slight decrease in the hydrogen absorption capacity of the as-cast and spun alloys, but it significantly enhances their hydrogen storage kinetics and hydriding/dehydriding cycle stability of the alloy. In order to reveal the capacity degradation mechanism of the as-spun alloy, the structure evolution of the nanocrystalline and amorphous alloys during the hydriding–dehydriding cycles was investigated. It is found that the root causes of leading to the capacity degradation of the nanocrystalline and amorphous alloys are nanocrystalline coarsening, crystal defect decreasing and amorphous phase crystallizing.     

Hydrogenation and dehydrogenation behaviours of nanocrystalline Mg20Ni10−xCux (x = 0−4) alloys prepared by melt spinning

In order to improve the hydriding and dehydriding kinetics of the Mg₂Ni-type alloys, Ni in the alloy was partially substituted by element Cu, and the nanocrystalline Mg₂Ni-type Mg₂₀Ni_10−xCu_x (x = 0, 1, 2, 3, 4) alloys were synthesized by melt-spinning technique. The structures of the as-cast and spun alloys were studied by XRD, SEM and HRTEM. The hydrogen absorption and desorption kinetics of the alloys were measured using an automatically controlled Sieverts apparatus. The results show that the substitution of Cu for Ni does not change the major phase Mg₂Ni. The hydrogen absorption capacity of the alloys first increases and then decreases with rising Cu content, but the hydrogen desorption capacity of the alloys grows with increasing Cu content. The melt spinning significantly improves the hydrogenation and dehydrogenation capacity and kinetics of the alloys.     

Theoretical and experimental study of the titanium substituted Mg2-xTixNi alloys and Mg2-xTixNiH4 hydrides

The formation of the Ti substituted Mg₂Ni alloys, a promising hydrogen storage material for various applications is studied in detail. Mg_1.95Ti_0.05Ni alloy and ribbons are successfully prepared by vacuum arc melting and melt spinning methods. The phases, microstructures, and thermal behavior of the alloys and ribbons are characterized by XRD, SEM, TEM, DTA/TG. Sievert-type apparatus is used to study hydrogen sorption properties. Apart from the dominant Mg₂Ni phase, the formation of MgNi₂, Mg, and Ni₃Ti phases is seen in both Mg_1·95Ti_0·05Ni alloy and ribbons. During the initial three cycles, Mg_1·95Ti_0·05Ni ribbons showed 2 wt % hydrogen storage capacity. To explain the atomic-scale influence of Ti dopant in the studied alloys and hydrides, FP(L)APW + lo method based on Density Functional Theory (DFT) is applied to Mg_2-xTi_xNi (x = 0.25 and 0.5) alloys and Mg_2-xTi_xNiH₄ (x = 0.25 and 0.5) hydrides. An increase in the Ti dopant on the Mg site leads to the hydrides destabilization. Bader's charge density topology analysis provides insight into the charge transfer and bonding between the constituent atoms.     

First-principles studies of the structures and properties of Al- and Ag-substituted Mg2Ni alloys and their hydrides

The structures and properties of hydrogen storage alloy Mg₂Ni, of aluminum and silver substituted alloys Mg_2−xM_xNi (M = Al and Ag, x = 0.16667), and of their hydrides Mg₂NiH₄, Mg_2−xM_xNiH₄ (M = Al and Ag, x = 0.125) have been calculated from first-principles. Results show that the primitive cell sizes of the intermetallic alloys and hydrides were reduced by substitution of Mg with Al or Ag. Also, the interaction of Ni–Ni was weakened by the substitution. A strong covalent interaction between H and Ni atoms forms tetrahedral NiH₄ units in Mg₂NiH₄. The NiH₄ unit near the Al/Ag atom became tripod-like NiH₃ in Mg_2−xM_xNiH₄ (M = Al, Ag), indicating that the hydrogen storage capacity was decreased by the substitution. The calculated enthalpies of hydrogenation for Mg₂Ni, Mg_2−xAl_xNi and Mg_2−xAg_xNi are −65.14, −51.56 and −53.63 kJ/mol H₂, respectively, implying that the substitution destabilizes the hydrides. Therefore, the substitution is an effective technique for improving the thermodynamic behavior of hydrogenation/dehydrogenation in magnesium-based hydrogen storage materials.     

Electrochemical hydrogen storage performance of Mg–Ti–Zr–Ni alloys

Mg_1.5Ti_0.5−xZr_xNi (x = 0, 0.1, 0.2, 0.3, 0.4), Mg_1.5Ti_0.3Zr_0.1Pd_0.1Ni and Mg_1.5Ti_0.3Zr_0.1Co_0.1Ni alloys were synthesized by mechanical alloying and their electrochemical hydrogen storage characteristics were investigated. X-ray diffraction studies showed that all the replacement elements (Ti, Zr, Pd and Co) perfectly dissolved in the amorphous phase and Zr facilitated the amorphization of the alloys. When the Zr/Ti ratio was kept at 1/4 (Mg_1.5Ti_0.4Zr_0.1Ni alloy), the initial discharge capacity of the alloy increased slightly at all the ball milling durations. The further increase in the Zr/Ti ratio resulted in reduction in the initial discharge capacity of the alloys. The presence of Zr in the Ti-including Mg-based alloys improved the cyclic stability of the alloys. This action of Zr was attributed to the less stable and more porous characteristics of the barrier hydroxide layer in the presence of Zr due to the selective dissolution of the disseminated Zr-oxides throughout the hydroxide layer on the alloy surface. Unlike Co, the addition of Pd into the Mg–Ti–Zr–Ni type alloy improved the alloy performance significantly. The positive contribution of Pd was assumed to arise from the facilitated hydrogen diffusion on the electrode surface in the presence of Pd. As the Zr/Ti atomic ratio increased, the charge transfer resistance of the alloy decreased at all the depths of discharges. Co and Pd were observed to increase the charge transfer resistance of the Mg–Ti–Zr–Ni alloys slightly.     

Effect of ball milling time on microstructure and electrochemical properties of CeMg12+100%Ni hydrogen storage alloy

Abstract

In order to improve hydrogen storage performances of CeMg₁₂ type alloys, ball milling technology was used for preparing nanocrystalline/amorphous CeMg₁₂+100%Ni composite hydrogen storage alloys. The microstructures and morphologies of alloy samples were characterised by X-ray diffraction, scanning electron microscopy and high resolution transmission electron microscopy. The electrochemical hydrogen storage characteristics of as milled alloys were tested by an automatic galvanostatic system. The electrochemical impedance spectra were plotted by an electrochemical workstation (PARSTAT2273). The hydrogen diffusion coefficients D in the alloys were calculated by virtue of potential step method. The results show that the amount of nanocrystalline/amorphous Mg₂Ni phase and Ni phase within alloy samples increase with prolonging milling time. Prolonging of ball milled duration markedly improves the electrochemical discharge properties and cyclic stability of alloy samples. The amorphisation degree of the milling alloys increases with rising milling duration. Furthermore, the high rate dischargeability, electrochemical impedance spectra and potential step measurement all indicate that electrochemical kinetics of alloy electrodes first increases and then decreases with increasing ball milling.