Structural Investigation And Hydrogen Storage Properties Of R-Mg-Ni (R=Ca And La) Alloys With Layered Structure

With the deficiency of fossil fuel resources and the deterioration of environment, hydrogen has attracted great attentions as a kind of clean energy with great development potential. The development of the hydrogen storage materials with high storage capacity and low cost is key break for the application of hydrogen energy. Based on the review of the research and development of hydrogen storage alloys, the novel R-Mg-Ni(R = Ca and La)alloys with layered structure were selected as the object of the study in this paper. The alloys were prepared by conduction melting or green compact laser sintering. The phase structure of the alloys was analyzed with the Rietveld refinement program RIETAN-2000. The structure of the new compound was determined with the EXPO program. The sub-structure of the compounds with layered structure was observed by high resolution transmission electron microscope (HRTEM). The microstructures and phase compositions were examined using a scanning electron microscope (SEM) or electron backscattered diffraction (EBSD) with an energy dispersive X-ray spectrometer (EDS). The pressure-composition-temperature (P-C-T) isotherms were measured using a Sieverts-type apparatus, and the enthalpy (ΔH ) and entropy (ΔS) of hydride formation were calculated according to the curves of van't Hoff. The thermodynamics of hydrogen storage were further studied by by combined differential scanning calorimetry and thermogravimetric analysis (DSC-TGA). For same alloys, moreover,the electrochemical properties were carried out by using a Land battery testing system. The structure and hydrogen storage properties of the novel R-Mg-Ni(R = Ca and La)alloys with layered structure were systematically studied, and the composition-structure-property correlation was understood in this paper.The novel Ca3Mg2Ni13 was found in the Ca-Mg-Ni system. Ca3Mg2Ni13 crystallizes in space group R-3m (No. 166); cell parameters: a = 4.9783(2) ? and c = 36.180(2) ?; Z = 3. The Ca3Mg2Ni13 structure has three blocks stacked along the c axis in one period, and each block is composed of two sub-blocks. One only contains one layer of [CaMgNi4] unit, and the other consists of one layer of [CaMgNi4] unit and one layer of [CaNi5] unit. Theoretically, the solid solubility of Mg in Ca3Mg2Ni13-type phase is as high as 22.22 at.%. Moreover, the maximum solid solubility is close to the theoretic value. Therefore, the hydrogen storage properties of the Ca3-xMg2+xNi13 alloys can be adjusted within a wide range of Mg content. The increase of Mg content leads to the decrease in the lattice parameters of Ca3Mg2Ni13–type compound, which effectively improves the thermodynamics of hydrogen absorption–desorption. The enthalpy changes for the hydrogen absorption and desorption of Ca2.0Mg3.0Ni13 are–28 and 30 kJ/mol H2, respectively. Moreover, Ca2Mg3Ni13 shows good cycling stability because the hydrogen–induced amorphization and decomposition do not occur during hydrogen absorption–desorption cycles. For Ca3?xLaxMg2Ni13 alloys, it was found that the La substitution is unfavorable for the formation of the Ca3Mg2Ni13-type phase. The maximum solid solubility of La in the Ca3Mg2Ni13-type phase is around x = 0.124 in the present study. Among the Ca3?xLaxMg2Ni13 alloys, the Ca1.5La1.5Mg2Ni13 alloy has highest equilibrium pressures of hydrogen absorption-desorption and possesses a highest hydrogen desorption capacity of 1.34 wt.% at 318 K. The electrochemical measurements indicated that the cyclic stability of the alloys is improved, and S30 increases from 13.7 to 67.6% when x increases from 0 to 3.In this paper, the (Ca1.0-xMgx)Ni3 (x = 0.16, 0.33, 0.5, 0.67) alloys were prepared by conduction melting. All alloys contained a PuNi3-type main phase (Ca, Mg)Ni3 and a small amount of impure phases. The lattice parameters and unit cell volume of the PuNi3-type (Ca, Mg)Ni3 phase decreased with increasing Mg content. Mg atoms only occupied the 6c (Ca2) sites of PuNi3-type structure. Moreover, the occupation factor of Mg on the 6c site increased with increasing Mg content. The maximum solid solubility of Mg in the (Ca0.33Mg0.67)Ni3 alloy was slightly smaller than the theoretic solid solubility of 16.67 at.% in the PuNi3-type phase. The enthalpy and entropy of hydride formation for the (Ca0.5Mg0.5)Ni3 and (Ca0.67Mg0.33)Ni3 alloys are close to those of the practically applied LaNi5 alloy. La-Mg-Ni alloys were prepared by green compact laser sintering for the first time in this paper. The structure and electrochemical properties of the laser sintered (La0.67Mg0.33)Ni3 alloys were investigated. Except for small amount of LaMgNi4, the alloys consisted of a main phase LaNi5 and a secondary phase with PuNi3 structure. The novel network microstructure for the laser sintered alloys is quite different to that of the alloys prepared by conduction melting. With increasing of sintering power, ternary La-Mg-Ni phase with PuNi3 structure increased. Moreover, the composition of ternary La-Mg-Ni phase with PuNi3 structure changed from (La0.6Mg0.4)Ni3 to (La0.67Mg0.33)Ni3. The discharge capacities of the alloys prepared by laser sintering at 1000 and 1400 W are 321.8 and 344.8 mAh/g, respectively.In order to determine the crystal structure of the A7B23-type La5Mg2Ni23 compound reported by literature [5], the La5Mg2Ni23 alloy was prepared by conduction melting. It was found that the La5Mg2Ni23 alloy consists of Ce2Ni7-type, Gd2Co7-type, LaNi5, Pr5Co19-type, Ce5Co19-type and LaMgNi4 phases. The Ca5Mg2Ni23 alloy contain PuNi3-type (Ca, Mg)Ni3, Gd2Co7-type (Ca, Mg)2Ni7, CaNi5 and a small amount of Ni. Moreover, the La5?xCaxMg2Ni23 (x = 1, 2, 3) alloys consist of PuNi3-type, Gd2Co7-type and CaCu5-type phases. In a word, A7B23-type compound has not been found in La-Mg-Ni, Ca-Mg-Ni and La-Ca-Mg-Ni system. The La5?xCaxMg2Ni23 (x = 0, 1, 2 and 3) alloys can be activated to their maximum discharge capacities within four cycles. Ecorr of the alloys decreased with increasing of Ca content. The cyclic stabilities of the La5?xCaxMg2Ni23 alloys are related to both phase abundance and corrosion potential. The discharge capacities of the Ca-substituted alloys are higher than that of the La5Mg2Ni23 alloy. Among these alloys, the La3Ca2Mg2Ni23 alloy has a highest discharge capacity (404.2 mAh/g) and a best high-rate dischargeability (HRD600 = 61.6%) due to the optimum Ca content and the highest abundance of the PuNi3-type and Gd2Co7-type phases.Similar to the binary Ca-Ni compound, the A2B7-type Ca-Mg-Ni compound has the structure of Gd2Co7. Ca3MgNi14 compound crystallizes in space group R-3m (No. 166); cell parameters: a = 4.9702(2) ?, c = 35.1111(1) ?; Z = 3. These parameters are similar to those of Ca3MgNi14, but the atomic coordinates are quite different.The Ca3MgNi14 structure has three blocks stacked along the c axis in one period, and each block is composed of one layer of [CaMgNi4] unit and two layers of [CaNi5] unit. The solid solubility of Mg in Ca2Ni7 and hydrogen storage properties of the (Ca2-xMgx)Ni7 alloys were investigated in this paper. It was found that the maximum solid solubility of Mg in the (Ca2-xMgx)Ni7 phase is about x = 0.5. The stacking faults in the (Ca, Mg)2Ni7 phase for the (Ca1.75Mg0.25)Ni7 alloy are of'inter-block-layer'type. However, few stacking faults were observed in the (Ca, Mg)2Ni7 phase for the (Ca1.5Mg0.5)Ni7 alloy. Owing to smaller lattice parameters and lower density of stacking faults, the reversibility of hydrogen absorption-desorption of the (Ca1.5Mg0.5)Ni7 alloy can be improved by increasing Mg content. In order to investigate the solid solubility of Mg in La2Ni7, the (La1.4Mg0.6)Ni7 alloys were prepared by laser sintering. It is found that all alloys consist of multiple phases, which are Ce2Ni7-type phase, LaNi5 and LaMgNi4. The solid solubility of Mg in La2Ni7 is same as that in Ca2Ni7. The amount of the La3MgNi14 phase in the laser sintered alloys is higher than that in the alloys prepared by conduction melting. Among these alloys, the alloy sintered at 1200 W has a highest discharge capacity, best cyclic stability and best high-rate dischargeability due to the highest amount of the main phase La3MgNi14.La4MgNi19 compound have two types of structure: a hexagonal structure of the Pr5Co19-type and a rhombohedral structure of the Ce5Co19-type. Each block of La4MgNi19 compound is composed of one layer of [CaMgNi4] unit and three layers of [CaNi5] unit. The Pr5Co19-type structure has two blocks stacked along the c axis in one period (2H), but the Ce5Co19-type has three blocks stacked along the c axis in one period (3R). The La4MgNi19 phase in the Ca-substituted La4?xCaxMgNi19 alloys decreased with increasing of Ca content. It is interesting that the Ca4MgNi19 alloy consists of CaNi5, Gd2Co7-type and PuNi3-type phases. That is, the Ca4MgNi19 compound with La4MgNi19 structure was not found in this paper. The La4MgNi19 alloys with the Pr5Co19-type and Ce5Co19-type phases were prepared by melted alloy-powder metallurgy sintering at high pressure Ar atmosphere and succedent quenching. P-C-T curves indicated the Pr5Co19-type and Ce5Co19-type phases have same hydrogen storage properties. However, X-ray diffraction patterns for the La4MgNi19 alloy at different absorption and desorption hydrogen stage revealed the hydrogen absorption and desorption plateaus of the Pr5Co19-type phase is lower than those of the Ce5Co19-type phase. It is found that the Pr5Co19-type structure is high temperature phase and the Ce5Co19-type is low temperature phase. Furthermore, the phase transformation between the Pr5Co19-type and Ce5Co19-type was very sluggish. The La4MgNi19 phase can steadily exist at about the temperature range of 840~960 oC. The hydrogen storage capacities of the La4MgNi19 alloys quenched at 930 and 870 oC are 1.53 and 1.51 wt.%, respectively. Therefore, the hydrogen storage content of the Pr5Co19-type is comparatively with that of the Ce5Co19-type La4MgNi19 phase. Owing to larger slope of hydrogen storage plateaus for the Ce5Co19-type La4MgNi19 phase, the dischargeability of this phase reduced. The discharge capacities of the alloys quenched at 930 and 870 oC are 374.9 and 358.2 mAh/g, respectively.