An Investigation on Hydrogen Storage Kinetics of the Nanocrystalline and Amorphous LaMg12-type Alloys Synthesized by Mechanical Milling

Nanocrystalline and amorphous LaMg_（12）-type LaMg_（11）Ni ＋ x wt% Ni（x = 100, 200） alloys were synthesized by mechanical milling. Effects of Ni content and milling time on the gaseous and electrochemical hydrogen storage kinetics of as-milled alloys were investigated systematically. The electrochemical hydrogen storage properties of the as-milled alloys were tested by an automatic galvanostatic system. And the gaseous hydrogen storage properties were investigated by Sievert apparatus and a differential scanning calorimeter（DSC） connected with a H_2 detector. Hydrogen desorption activation energy of alloy hydrides was estimated by using Arrhenius and Kissinger methods. It is found that the increase of Ni content significantly improves the gaseous and electrochemical hydrogen storage kinetic performances of as-milled alloys. Furthermore, as ball milling time changes, the maximum of both high rate discharge ability（HRD） and the gaseous hydriding rate of as-milled alloys can be obtained. But the hydrogen desorption kinetics of alloys always increases with the extending of milling time. Moreover, the improved gaseous hydrogen storage kinetics of alloys are ascribed to a decrease in the hydrogen desorption activation energy caused by increasing Ni content and milling time.

Structure and electrochemical performances of as-milled LaMg12-type alloy-Ni composites

LaMg11Ni+x wt.%Ni(x=100,200)alloys were prepared by ball milling and researched by various methods to study their structure and electrochemical hydrogen storage property.The outcomes reveal that increasing Ni content contributes to improving its electrochemical performance.Particularly,the discharge capacity of as-milled(40 h)alloy will be increased from 157.3 to 1053.5 mAh/g through enhancing Ni percentage from 100 to 200 wt.%when discharging at 60 mAh/g.Ball milling obviously affects the electrochemical performanee of alloys.With milling duration prolonging,the discharge capacity of x=100 alloy keeps increasing,while that of the x=200 alloy has a maximum value.Milling time also affects the cycle stability of experimental alloys.Extendi ng milli ng duration clearly decreases the cycle stability of x=100 alloy but weakens that of the x=200 alloy at first and strengthens it later.In addition,prolonging milling time makes the milling prepared x=100 and 200 alloys get the maximal values of high rate discharge ability of 81.60%and 84.52%,respectively.

Hydrogen storage thermodynamics and dynamics of La-Mg-Ni-based LaMg12-type alloys synthesized by mechanical milling

Nanocrystalline/amorphous LaMg(12)-type alloyNi composites with a nominal composition of LaMg(11)Ni+x wt% Ni(x=100,200) were synthesized by mechanical milling.Effects of Ni content and milling time on the gaseous hydrogen storage thermodynamics and dynamics of alloys were systematically investigated.The hydrogen desorption properties were studied by Sievert apparatus and a differential scanning calorimeter(DSC).Thermodynamic parameters(△H and ΔS) for the hydrogen absorption and desorption of alloys were calculated by Van't Hoff equation.Hydrogen desorption activation energy of alloy hydride was estimated by Arrhenius and Kissinger methods.The increase in Ni content has a slight effect on the thermodynamic properties of alloys,but it significantly enhances the hydrogen absorption and desorption kinetics performance of alloys.Moreover,variation of milling time clearly affects the hydrogen storage properties of alloys.Hydrogen absorption capacity(C(100)~a) and hydrogen absorption saturation ratio(R(10)~a)(a ratio of the hydrogen absorption capacity at 10 min to the saturated hydrogen absorption capacity) have maximum values with milling time varying.But hydrogen desorption ratio(R(20)~d)(a ratio of the hydrogen desorption capacity at 20 min to the saturated hydrogen absorption capacity) always increases with milling time prolonging.Particularly,prolonging milling time from 5 to 60 h makes R(20)~d increase from 10.89% to 16.36% for the x=100 alloy and from 13.93% to 21.68% for the x=200 alloy,respectively.

Y_2O_3对LaMg_(12)型储氢合金电化学性能的影响

采用熔盐覆盖法制备了LaMg12型储氢合金,利用高能球磨对其进行Y2O3的掺杂,采用X射线衍射来分析合金的相结构,利用Land充放电仪测试合金的电化学性能.结果表明:Y2O3掺杂能改善LaMg12储氢合金的电化学性能,其中球磨20 h对首次放电容量改善最明显,球磨40 h对循环性能影响最大.综合而言,球磨40 h对合金性能的改善效果最好.

氢致LaMg_(12)相分解的原位X射线衍射分析

分别在Ar气和H2气气氛下对LaMg12进行热分解实验,采用原位X射线衍射（in-situ XRD）分析LaMg12相分解过程.结果表明,该合金在Ar气中加热到330℃,合金处于稳定状态,而在H2气气氛中加热到大约290℃时,开始分解为LaH3和MgH2.Rietveld分析结果表明,分解前LaMg12的点阵常数没有变化,证明氢原子并未进入合金形成LaMg12Hx固溶体.可以推测出,在H2气气氛中,由于氢致作用产生大量的空位,致使La或Mg易于向表面移动,出现分解现象.

Hydrogen Storage Kinetics of Nanocrystalline and Amorphous LaMg_(12)-Type Alloy–Ni Composites Synthesized by Mechanical Milling

The nanocrystalline and amorphous LaMg11Ni ＋ x wt% Ni （x = 100, 200） composites were synthesized by the mechanical milling, and their gaseous and electrochemical hydrogen storage kinetics performance were systematically investigated, The results indicate that the as-milled composites exhibit excellent hydrogen storage kinetic performances, and increasing Ni content significantly facilitates the improvement of the hydrogen storage kinetics properties of the composites. The gaseous and electrochemical hydrogen storage kinetics of the composites reaches a maximum value with the variation of milling time. Increasing Ni content and milling time both make the hydrogen desorption activation energy lower, which are responsible for the enhancement in the hydrogen storage kinetics properties of the composites. The diffusion coefficient of hydrogen atom and activation enthalpy of charge transfer on the surface of the as-milled composites were also calculated, which are considered to be the dominated factors for the electrochemical high rate discharge ability.