La-Mg-Ni-Mn-based AB_2-type La_(1–x)Ce_xMgNi_(3.5)Mn_(0.5)(x=0–0.4) alloys were prepared by melt spinning technology. The detections of X-ray diffraction(XRD) and scanning electron microscopy(SEM) indicated that the experimental alloys consisted of a major phase LaMgNi_4 and a secondary phase LaNi_5. With spinning rate growing, the abundance of LaMgNi_4 phase increased and that of LaNi_5 phase decreased. Moreover, with the melt spinning rate increasing, both the lattice constants and cell volumes increased, and further accelerated the grains refinement of the alloys. The electrochemical tests showed that the as-spun alloys possessed excellent capability of activation, achieving the maximum discharge capacities just at the first cycling without any activation needed. As for the as-spun alloys, its discharge potential characteristics could be improved obviously by adopting the technology of melt spinning. In addition, the melt spinning raised electrochemical cycle stability of the alloys, the main reason was that the melt spinning enhanced the anti-pulverization ability of the alloys. With spinning rate increasing, the discharge capacity of the alloys presented a tendency to increase firstly then decrease. Moreover, the electrochemical kinetics of the alloys showed the same trend under fixed condition.
The melt spinning(MS) and ball milling(BM) technologies are thought to be efficient to prepare nanostructured Mg and Mg-based alloys for improving their hydrogen storage performances. In this paper, two technologies, viz. melt spinning and ball milling, were employed to fabricate the SmMg_(11)Ni alloy. The structure and hydrogen storage performance of these two kinds of alloys were researched in detail. The results reveal that the as-spun and milled alloys both contain nanocrystalline and amorphous structures. By means of the measurement of PCT curves, the thermodynamic parameters of the alloys prepared by MS and BM are ΔN_(Ms)(des) = 82.51 kJ/mol and ΔH_(BM)(des) = 81.68 kJ/mol, respectively, viz.ΔH_(MS)(des) > ΔH_(BM)(des). The as-milled alloy shows a larger hydrogen absorption capacity as compared with the as-spun one. The as-milled alloy exhibits lower onset hydrogen desorption temperature than the as-spun one. As to the as-milled and spun alloys, the onset hydrogen desorption temperatures are557.6 and 565.3 K, respectively. Additionally, the as-milled alloy shows a superior hydrogen desorption property than the as-spun one. On the basis of time that required by desorbing hydrogen of 3 wt% H_2, the as-milled alloy needs 1488.574,390 and 192 s corresponding to hydrogen desorption temperatures 593,613,633 and 653 K, while the as-spun alloy needs 3600,1020,778 and 306 s corresponding to the same temperatures. The dehydrogenation activation energies of the as-milled and spun alloys are 100.31 and105.56 kJ/mol, respectively, the difference of which is responsible for the much faster dehydriding rate of the as-milled alloy.
Nanocrystalline and amorphous LaMg_(12)-type alloy-Ni composites with a nominal composition of LaMg_(11)Ni+x wt.% Ni(x=100,200)were synthesized via ball milling.The influences of ball milling duration and Ni adding amount xon the gaseous and electrochemical hydrogen storage dynamics of the alloys were systematically studied.Gaseous hydrogen storage performances were studied by a differential scanning calorimeter and a Sievert apparatus.The dehydrogenation activation energy of the alloy hydrides was evaluated by Kissinger method.The electrochemical hydrogen storage dynamics of the alloys was investigated by an automatic galvanostatic system.The H atom diffusion and apparent activation enthalpy of the alloys were calculated.The results demonstrate that a variation in Ni content remarkably enhances the gaseous and electrochemical hydrogen storage dynamics performance of the alloys.The gaseous hydriding rate and high-rate discharge(HRD)ability of the alloys exhibit maximum values with varying milling duration.However,the dehydriding kinetics of the alloys is always accelerated by prolonging milling duration.Specifically,rising milling time from 5to 60 h makes the hydrogen desorption ratio(a ratio of the dehydrogenation amount in 20 min to the saturated hydrogenation amount)increase from 57%to 66%for x=100alloy and from 57%to 70%for x=200.Moreover,the improvement of gaseous hydrogen storage kinetics is attributed to the descending of dehydrogenation activation energy caused by the prolonging of milling duration and growing of Ni content.
In order to examine the effects of structure stability on the degradation behaviors of multiphase La_(0.7)Mg_(0.3)Ni_3 alloy,changes of the crystal structure and hydrogen storage properties after gas-solid cycling were investigated in detail.The structural analysis identifies that(La,Mg)Ni_3(PuNi_3-type) phase transforms to amorphous,i.e.,hydrogen-induced amorphization(HIA) occurs whereas LaNi_5(CaCu_5-type),(La,Mg)_2Ni_7(Ce_2Ni_7-type),and(La,Mg)_5Ni_(19)(Pr_5Co_(19)-type) phases still keep crystalline upon hydriding/dehydriding cycling.Partial amorphization remarkably affects both the gas-solid and electrochemical storage performances.The plateau of PCT curves becomes narrow and steep with cycling.Moreover,the maximum electrochemical capacity decreases notably after gas-solid hydrogenation repeats.The electrochemical capacity reduction could be ascribed to both drop of the maximum storage capacity and the slope of plateau induced by partial amorphization.For direct electrochemical cycling,it is suggested that the capacity decay is mainly attributed to HIA in the initial stage.
The as-cast RE-Mg-Ni-b ased AB2-type La1-xPrxMgNi3.6Co0.4(x=0-0.4)alloys were prepared by vacuum induction melting followed by annealing treatment.The phase composition and structure were characterized by X-ray diffraction(XRD)and scanning electron microscope(SEM).The results show that LaMgNi4 and LaNi5 coexist in as-cast alloys,but only LaMgNi4 is detected in the annealed alloys.The morphology of annealed alloys is more homogeneous than that of as-cast alloys.The gaseous hydrogen storage and electrochemical properties were investigated by pressure-composition isotherm(P-C-T)and electrochemical measurements.The P-C-T curves of annealed alloys show flatter and wider pressure plateaus corresponding to absorption/desorption pressure plateaus of LaMgNi4 hydride.But the maximum hydrogen storage content of annealed alloys is lower than that of as-cast alloys.In consideration of the electrochemical properties,the annealed La0.8Pr0.2MgNi3.6Co0.4alloy exhibits a maximum discharge capacity of354.2 mAh·g-1.
The casting and annealing technologies were applied to fabricate the La0.8Mg0.2Ni3.3Co0.2Six(x = 0-0.2) electrode alloys. The effects of Si content and annealing temperature on the structure and electrochemical performances of the alloys were investigated systematically. The analyses of XRD and SEM show that all the alloys possess a multiphase structure, involving two main phases(La, Mg)2Ni7 and La Ni5 as well as a residual phase La Ni3. The addition of Si brings on an evident increase in the La Ni5 phase and a decrease in the(La, Mg)2Ni7 phase, without altering the main phase component of the alloy, which also makes the lattice constants and cell volumes of the alloy enlarged. Likewise, the annealing treatment engenders the same action on the lattice constants and cell volumes as adding Si. Simultaneously, it gives rise to the variation of the phase abundance and the coarsening of the alloy grains. The electrochemical measurements indicate that the addition of Si ameliorates the cycle stability of the as-cast and annealed alloys significantly, but impairs their discharge capacities clearly. Similarly, the annealing treatment makes a positive contribution to the cycle stability of the alloy evidently, and the discharge capacity of the alloy shows a maximum value with annealing temperature rising. Furthermore, the high rate discharge ability(HRD) first augments and then declines with the rising of Si content and annealing temperature.
The nanocrystalline and amorphous Mg2Ni-type electrode alloys with a composition of Mg20-xYxNi10(x=0, 1, 2, 3 and 4) were fabricated by mechanical milling. Effects of Y content on the structures and electrochemical hydrogen storage performances of the alloys were investigated in detail. The inspections of X-ray diffraction(XRD), transmission electron microscopy(TEM) and scanning electron microscopy(SEM) revealed that the substitution of Y for Mg brought on an obvious change in the phase composition of the alloys. The substitution of Y for Mg resulted in the formation of secondary YMgN i4 phases without altering the major phase Mg2 Ni when Y content x≤1. But with the further increase of Y content, the major phase of the alloys changed into YMgN i4 phase. In addition, such substitution facilitated the glass forming of the Mg2Ni-type alloy. The discharge capacities of the as-milled alloys had the maximum values with Y content varying, but Y content with which the alloy yielded the biggest discharge capacity was changeable with milling time varying. The substitution of Y for Mg had an insignificant effect on the activation ability of the alloys, but it dramatically improved the cycle stability of the as-milled alloys. The effect of Y content on the electrochemical kinetics of the alloys was related to milling time. When milling time was 10 h, the high rate discharge ability(HRD), diffusion coefficient of hydrogen atom(D) and charge transfer rate all had the maximum value with Y content increasing, but they always decreased in the same condition when milling time increased to 70 h.
Mg2Ni-type Mg20-xYxNi10(x=0,1,2,3 and4) electrode alloys were fabricated by vacuum induction melting.Subsequently,the as-cast alloys were mechanically milled on a planetary-type ball mill.The effects of milling time and Y content on the microstructures and electrochemical performances of the alloys were investigated in detail.The results show that nanocrystalline and amorphous structure can be successfully obtained through mechanical milling.The substitution of Y for Mg facilitates the glass forming of the Mg2Ni-type alloy and significantly enhances the electrochemical characteristics of the alloy electrodes.Moreover,the discharge capacity of Y-free alloy monotonously grows with the milling time prolonging,while that of the Y-substituted alloys has the maximum values in the same case.The milling time of obtaining the greatest discharge capacity markedly decreases with Y content increasing.The electrochemical kinetics of the alloys,including high rate discharge ability(HRD),diffusion coefficient(D),limiting current density(IL) and charge transfer rate,monotonously increase with milling time extending.
Yang-Huan ZhangXi-Ping SongPei-Long ZhangYong-Guo ZhuHui-Ping RenBao-Wei Li
