Effect of Sm on hydrogen storage performance of La–Sm–Mg–Ni alloys

Abstract

In this study, Sm was adopted in order to completely replace the expensive Pr/Nd elements in the A2B7 type alloy. The results indicate that Sm is a favourable element for forming Ce2Ni7 type and Ce5Co19 type phases. With the increasing amount of Sm, the discharge capacity of the alloy retains a value of 283·3 mAh g^?1 at the current density of 1200 mA g^?1. The maximum discharge capacity of the alloys increases with the increasing Sm content when Mg content is relatively low. By optimising the composition and processing technology, the cycle life the alloy enhances from 74 cycles to more than 540 cycles, and the maximum discharge capacity also increases from 300 to 355 mAh g^?1.     

Effect of Mo–Ni treatment on electrochemical kinetics of La–Mg–Ni-based hydrogen storage alloys

In order to improve kinetic properties of La–Mg–Ni-based hydrogen storage alloys, Mo–Ni treatment was applied to La_0.88Mg_0.12Ni_2.95Mn_0.10Co_0.55Al_0.10 alloy powders. FESEM results showed that after Mo–Ni treatment some network-shaped substance with nano-size formed on the surface of the alloy particles. The EDS results revealed increase in Ni content and emerge of Mo element. EIS and Linear polarization showed that charge-transfer resistance decreased and exchange current density increased for the treated alloy electrode, and the high rate dischargeability (HRD) was consequently improved. HRD at 1500 mA/g increased from 22.5% to 39.5%. Mo- and Ni-single treatments were performed compared with the Mo–Ni treatment, and the results showed that the single treatment improved HRD slightly, far less than the Mo–Ni treatment.     

Study of the performance of Ti–Zr based hydrogen storage alloys

The P–C–I and charging–discharging properties of three Ti–Zr based alloys have been studied. Ni substitution for Mn and Cr in the alloy was found to increase the plateau pressure of the P–C–I curve. In addition, the partial substitution of Cr by V greatly improved the discharge capacity. However, the six-element alloy, Ti_0.5Zr_0.5V_0.2Mn_0.7Cr_0.5Ni_0.6, degraded rapidly in the gas–solid reaction. Hydrogen contents in the alloy under low pressure were increased during hydrogen absorption–desorption cycling. Annealing at 1050°C for 4 h before the P–C–I experiment helped in releasing the retained hydrogen under low pressure. Only a slightly flattened P–C–I slope was obtained for the annealed alloy. Microstructures of the as-cast and annealed alloys were examined and related to the above results. Alloy powder was poisoned after 2-month storage in air, which resulted in the deterioration of discharge capacity. Surface pretreatment on alloy powders by HCl–HF solution decreased the activation time of charge–discharge reaction.     

An investigation of the borohydride oxidation reaction on La–Ni-based hydrogen storage alloys

In this study, LaNi_4.7Sn_0.2Cu_0.1 metal hydride alloys, with and without surface deposits of Pt, are investigated as electrocatalysts for the borohydride oxidation reaction (BOR) in alkaline media. Results obtained for LaNi_4.78Al_0.22 and LaNi_4.78Mn_0.22 are used for comparison. It is observed that wet exposition to hydrogen or sodium borohydride lead to some hydriding of the metal hydride alloy particles, particularly that with a coating of Pt. In the presence of borohydride ions, the hydrided charged alloys present more negative potentials for the (boro)hydride oxidation process, and these enhancements are significantly larger for the Pt-coated material. In the potential range of interest, the results demonstrate considerable activity for the BOR, but just for the alloy with Pt. In the presence of borohydride ions in the solution there is a continuous hydriding the alloy during the discharge of the metal hydride electrode. Differential electrochemical mass spectrometry (DEMS) measurements showed that there is formation of H₂, either by hydrolysis or by partial oxidation of the borohydride ions, but in the absence of Pt the hydrolysis process is quite slow.     

Structures and properties of Mg–La–Ni ternary hydrogen storage alloys by microwave-assisted activation synthesis

It is a challenge to prepare a material meeting two conflicting criteria – absorbing hydrogen strongly enough to reach a stable thermodynamic state and desorbing hydrogen at moderate temperature with a fast reaction rate. With the guide of the Mg–La–Ni phase diagram, microwave sintering (MS) was successfully applied to preparing Mg–La–Ni ternary hydrogen storage alloys from the powder mixture of Mg, La and Ni. Their phase structures, morphologies and hydrogen absorption and desorption (A/D) properties have been studied by X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), pressure-composition-isotherm (PCI) and differential scanning calorimetry (DSC). The metal hydride of 70 Mg–9.72 La–20.28 Ni (wt pct) has the best comprehensive hydriding and dehydriding (H/D) properties, which can absorb 4.1 wt.% H₂ in 600 s and desorb 3.9 wt.% H₂ in 1500 s at 573 K. The DSC results reveal its onset temperatures of hydrogen A/D are the lowest among all the samples, which are 671.4 and 600.9 K. Its activation energy of dehydriding reaction is 113.5 kJ/mol H₂, which is the smallest among all the samples. Also, Chou model was used to analyze the reaction kinetic mechanism.     

Microstructure characteristics,hydrogen storage thermodynamic and kinetic properties of Mg–Ni–Y based hydrogen storage alloys

In this paper, the Mg_95-X-Ni_x-Y₅ (x = 5, 10, 15) alloy were prepared by vacuum induction melting. The X-ray diffraction was used to analytical phase composition in different states, and the Scanning Electron Microscope and Transmission Electron Microscope were used to characterize the microstructure and crystalline state. Meanwhile, the kinetic properties of isothermal hydrogen adsorption and desorption at different temperatures also were tested by the Sievert isometric volume method. The results indicate that the hydrogenated Mg–Ni–Y samples is a nanocrystalline structure consists of MgH₂, Mg₂NiH₄, and YH₃ phases. And, the in-situ formed YH₃ phase not decompose in the process of dehydrogenation and evenly dispersed in the mother alloy, which plays a paly a positive the catalytic role for the reversible cyclic reaction of Mg and Mg₂Ni phases. In addition, the Ni elements are effectively to improve the thermodynamic properties of the Mg-based hydrogen storage alloy, the desorption enthalpy of the Ni₅, Ni₁₀, and Ni₁₅ samples successively decrease to 84.5, 69.1, and 63.5 kJ/mol H₂. The hydrogen absorption and desorption kinetics of the Mg–Ni–Y alloy are improved obviously with the increase of Ni content, especially for Mg₈₀Ni₁₅Y₅ alloy, which the optimal hydrogenated temperature is reduced to 200 °C, and the 90% of the maximum hydrogen storage capacity can be absorbed within 1 min, about 5.4 wt % H₂. Besides, the dehydrogenated activation energy of the Mg₈₀Ni₁₅Y₅ alloy also is reduced to 67.0 kJ/mol, and it can completely release hydrogen at 320 °C within 5 min, which is almost reached the hydrogen desorption capability of Ni₅ alloy at 360 °C. This means that Ni element is a very positive element to reduce the hydrogen desorption temperature.     

Ageing of Mg–Ni–H hydrogen storage alloys

In this work, ageing of Mg/Mg₂Ni mixtures was investigated. It was observed that hydrogen desorption kinetics from hydrided Mg/Mg₂Ni was improved considerably after ageing at room temperature for several days. The ageing was interpreted in terms of phase changes. Even after almost complete hydridation, besides two main phases – MgH₂ and Mg₂NiH₄ – a certain amount of Mg₂NiH_0.3 was always present. Similar as Mg₂NiH₄ phase, Mg₂NiH_0.3 islands were located on the surface of MgH₂ grains. Mg₂NiH_0.3 transformed into Mg₂NiH₄ at the expense of hydrogen from an adjoining MgH₂ grain. In such a way, a clean double layer (Mg)–Mg₂NiH₄ was formed, acting as a gate for easy hydrogen desorption from MgH₂. It was found that the Mg₂NiH₄ phase was slightly enriched on non-twinned modification LT1 during the ageing. As a result, both the creation of (Mg)–Mg₂NiH₄ desorption bridges and enrichment of Mg₂NiH₄ on LT1 during the ageing facilitated onset of rapid hydrogen desorption.     

Structure and hydrogen storage performances of La–Mg–Ni–Cu alloys prepared by melt spinning

The (Mg₂₄Ni₁₀Cu₂)_100-xLa_x(x = 0, 5, 10, 15, 20) alloys were prepared adopting the method of melt spinning technology. Adding La brings on the formation of secondary phases of La₂Mg₁₇ and LaMg₃, while it does not change the major phase of Mg₂Ni. Originally, there already have nanocrystals and amorphous structures in the experimental alloys, and the addition of La is more conducive to the formation of glass. With adding La in as-spun alloys, the gaseous hydrogen absorption capacity was significantly reduced, but it markedly improved their hydriding rates. Adding La and melt spinning considerably enhanced the dehydriding rate, the reason for which is the decrease of activation energy incurred by adding La and melt spinning. In addition, the discharge capacity of the alloys were able to reach a maximum value during La content varying, and it obviously increased with spinning rate rising.     

Effect of carbon coating on the electrochemical properties of La–Y–Ni-based hydrogen storage alloys

The A₂B₇-type (LaSmY) (NiMnAl)_3.5 alloys were prepared by induction melting, and then the alloy samples coated with different contents of nano-carbon were prepared by the mixing and sintering method using pitch as carbon source. The effects of the contents and structure of the coated-carbon on the electrochemical properties of alloy samples were investigated. With the carbon content increase from 0.1 to 1.0 wt%, the cyclic stability is improved and the high-rate dischargeabilitiy (HRD) of the alloy electrodes first increase and then decrease. The kinetic results show that the carbon coating improves the electrocatalytic activity and electrical conductivity of the alloy electrodes. The alloy electrode with 0.5 wt% carbon coating exhibits the best electrochemical properties. The maximum discharge capacity (C_max) is 345.7 mAh·g⁻¹, the HRD₁₂₀₀ is 72.49%, and the capacity retention rate (S₃₀₀) is 79.44%.     

Effect of Al content on the structural and electrochemical properties of A2B7 type La–Y–Ni based hydrogen storage alloy

LaY_1.9Ni_10.2−xAl_xMn_0.5 (x = 0–0.6) hydrogen storage alloys have been prepared using a vacuum induction-quenching furnace and annealed at 1148 K for 16 h. The alloys are composed of Ce₂Ni₇- and Gd₂Co₇-type phases and an extra Pr₅Co₁₉-type phase appears when x = 0.6. Aluminum tends to enter the inner AB₅ slabs of Ce₂Ni₇- and Gd₂Co₇-type phases and promotes the generation of new AB₅ slabs. The maximum discharge capacity of the alloy electrodes is stable at approximate 375 mA h/g as x increases from 0 to 0.4 and then decreases to 364.2 mA h/g (x = 0.6). The cycling capacity retention rate at the 300^th cycle is 59.4%, 62.0%, 62.7% and 58.7% for x = 0, 0.2, 0.4 and 0.6, respectively, indicating that the function of aluminum on improving the cyclic stability of the alloy electrodes is limited. The main reason is that the similar pulverization degrees of the alloys are presented during the charge/discharge cycles.     

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 rare earth elements on electrochemical properties of La–Mg–Ni-based hydrogen storage alloys

La_0.60R_0.20Mg_0.20(NiCoMnAl)_3.5 (R = La, Ce, Pr, Nd) alloys were prepared by inductive melting. Variations in phase structure and electrochemical properties due to partial replacement of La by Ce, Pr and Nd, were investigated. The alloys consist mainly of LaNi₅ phase, La₂Ni₇ phase and LaNi₃ phase as explored by XRD and SEM. The maximum discharge capacity decreases with Ce, Pr and Nd substitution for La. However, the cycling stability is improved by substituting Pr and Nd at La sites, capacity retention rate at the 100th cycle increases by 13.4% for the Nd-substituted alloy. The electrochemical kinetics measurements show that Ce and Pr substitution improves kinetics and thus ameliorates the high rate dischargeability (HRD) and low temperature dischargeability. The HRD at 1200 mA g⁻¹ increases from 22.1% to 61.3% and the capacity at 233 K mounts up from 90 mAh g⁻¹ to 220 mAh g⁻¹ for the Ce-substituted alloy.     

Characteristics of electrochemical hydrogen storage using Ti–Fe based alloys prepared by ball milling

In this paper, the TiFe-based master alloy Ti_1.04Fe_0.7Ni_0.1Zr_0.1Mn_0.1Pr_0.06 was fabricated by conventional induction melting with high purity helium as the protective gas. After that, the as-cast specimens were mechanically milled with nickel powders to synthesize the as-milled Ti_1.04Fe_0.7Ni_0.1Zr_0.1Mn_0.1Pr_0.06 + 10 wt.% Ni composites with excellent electrochemical characteristics. The master alloy is composed of TiFe, Ti₂Fe and Pr phases, which has a typical crystal structure. Mechanically milling the master alloy with nickel powder leads to the reductions of the grain size and particle size, even forming amorphous structure. The experimental results showed that the specimens after ball-milling treatment can be used to hydriding and dehydriding by electrochemistry, getting the maximal discharge capacity in the first cycle, and no activation was required. The discharge capacity of the as-milled composites declined from 264.2 to 133.6 mAh/g with the milling duration extending from 5 to 30 h. The electrochemical kinetics markedly declined with prolonging milling duration. However, the electrochemical cycling stability of the specimens reduced firstly and then increased with the prolongation of grinding duration.     

Ti–Fe based alloys prepared by ball milling for electrochemical hydrogen storage

In this study, the Ti_1.04Fe_0.6Ni_0.1Zr_0.1Mn_0.2Sm_0.06 composite was prepared by using vacuum induction melting under inert atmosphere. Then, the specimen was milled with 5 wt% Ni powders for 10–40 h to realize the general improvements in hydrogenation performance. The phase component was determined and the morphology and microscopic structure were observed using XRD, SEM and HRTEM, respectively. The electrochemical properties of the alloys were studied. The results showed that the as-milled specimens got the maximal discharge capacity without any activation. It reached 305 mAh/g for the 30 h milling specimen, which was better than the other specimens. Besides, ball milling can enhance the electrochemical cyclic stability of the experimental alloys. The capacity retention rate (S₁₀₀) increased from 57.6 to 70.2% after 100 charging and discharging cycles with increasing milling duration from 10 to 40 h. The high rate discharge ability of the 30 h milling specimen had the maximal value of 92.8%.     

Influence of Cr on the hydrogen storage properties of Ti-rich Ti–V–Cr alloys

In the present work, we studied the effects of Cr on the crystal structures and hydrogen storage properties of ternary alloys, Ti_0.7V_0.3−xCr_x and Ti_0.8V_0.2−xCr_x. Metal–hydrogen interactions were characterised by Thermal Desorption Spectroscopy (TDS) and in situ Synchrotron X-ray diffraction (SR-XRD). All initial alloys crystallise with body-centred cubic (BCC) crystal structures formed as solid solutions of V and Cr in Ti. Upon hydrogenation, the dihydrides (Ti,V,Cr)H₂ with face-centred cubic (FCC) structures are formed. An increase in the Cr content leads to systematic changes in the structure and hydrogenation behaviours. The changes include (a) contraction of the unit cells for the initial alloys and for the corresponding dihydrides; (b) slower hydrogen absorption kinetics and an increase in the incubation period for hydrogenation; (c) a decrease in the thermal stability of the saturated hydrides; and (d) a reduction in the apparent activation energy of hydrogen desorption. In situ SR-XRD and TDS studies of the FCC Ti–V–Cr hydrides indicated that their decomposition consists of five individual desorption events.     

Characterization of CoB–silica nanochains hydrogen storage composite prepared by in-situ reduction

The CoB–silica nanochains hydrogen storage composite was prepared by in-situ reduction of cobalt salt on the surface of amine-modified silica nanospheres. The structure and morphology of the sample were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The valence state of atoms was characterized by X-ray photoelectron spectroscopy (XPS). The electrochemical properties of the sample were also investigated. The results demonstrated that the CoB–silica nanochains hydrogen storage composite possessed amorphous nanochains structure by a series of nanospheres connecting in one-dimension. In addition, the material as electroactive negative electrodes showed high reversible discharge capacity (about 500 mAh/g in the first cycle) and good cycling stability. A properly electrochemical reaction mechanism was constructed primarily.     

Insights into the structure–performance relationship in La–Y–Ni-based hydrogen storage alloys

La–Y–Ni-based alloys with different phase structures possess various physicochemical properties and thereby different hydrogen storage performances. In this work, the hydrogen storage and electrochemical properties of LaY_1·9Ni₁₀Mn_0·5Al_0.2 alloys with different phase structures (2H-A₂B₇, 3R-A₂B₇ and 2H-A₅B₁₉) are investigated. All the investigated phases present two plateaus in pressure-composition-temperature (PCT) curves, which is induced by the different hydrogen location (A₂B₄ or AB₅) during the hydrogen absorption process. All of the LaY_1·9Ni₁₀Mn_0·5Al_0.2 series alloys possess good hydrogen storage capacities and electrochemical properties. The cyclic stability of the alloys is determined by the anti-corrosive properties of the alloys to electrolyte, neither the phase transition nor the previously believed pulverization. This work, by systematically investigating phase transitions during the annealed process and elucidating the key factors of influencing the cyclic stability, is useful for the design of La–Y–Ni-based alloy for the application of hydrogen storage and beyond.     

The effect of particle size on the electrode performance of Ti–V-based hydrogen storage alloys

The effect of particle size ranging from 100 mesh to below 500 mesh on the electrochemical properties of _{Ti0.8Zr0.2V2.7Mn0.5Cr0.8Nix}(x=0.75,1.75)${\mathrm{Ti}}_{0.8}{\mathrm{Zr}}_{0.2}{\mathrm{V}}_{2.7}{\mathrm{Mn}}_{0.5}{\mathrm{Cr}}_{0.8}{\mathrm{Ni}}_x(x=0.75,1.75)$ hydrogen storage alloy electrodes was investigated. SEM observation on the surface of the alloy electrodes after charge/discharge cycles revealed that the abilities of anti-pulverization and anti-corrosion of the x=0.75$x=0.75$ alloy were much lower than those of the x=1.75$x=1.75$ alloy. For both of the two alloys, the electrode performance is affected markedly by the particle size. With the increase of the initial particle size, the initial discharge capacity of the alloy electrode decreases and the pulverization of the alloy particles aggravates, especially for the particles of the x=0.75$x=0.75$ alloy with size larger than 400 mesh, the size of which was minimized obviously compared with their initial size. However, the maximum discharge capacity and the cycling stability almost do not correlate to the initial particle size but relate with their actual particle size. As the actual particle size decreases, the maximum discharge capacity increases and the cycling stability declines.     

The hydriding kinetics of Mg–Ni based hydrogen storage alloys: A comparative study on Chou model and Jander model

Two kinds of kinetic models, which are Jander model and Chou model, were applied to investigate the hydriding kinetic behavior of Mg–Ni based alloys. By comparing the calculated values with experimental data, it can be seen that both models were successfully used in the diffusion-controlled hydrogen absorption process of Mg–Ni system. However, Chou model was not only convenient for use but also gave a set of physical meaningful explicit analytic expressions. Chou model should be preferentially recommended to deal with the calculation at multi-temperatures and multi-pressures without multistep calculation. The application of Chou model to Mg₂₀Ni₈Cu₂ and Mg₂₀Ni₈Co₂ alloys shows that the calculated results agreed well with the experimental data and it is reasonable to expect that this model will also suitable for other Mg–Ni based alloys if the mechanism is similar.