| hydrogen storage alloys of La-Mg-Ni system are the type candidate of Ni/MH battery negative electrode which process good properties. So far further investigations on the preparation and microstructure of A2B7-type hydrogen storage alloys have been done, focusing on the improvement of electrochemical performances. In order to explore the feasibility of the alloys application in fuel cell area, it is necessary to investigate the gas-solid properties in hydrogen absorption and desorption, which has been rarely studied. La1-xMgxNi3.5 (x=0.2～0.3) and La0.75Mg0.25Ni3.5-xCox (x=0,0.2,0.5) were adopted as the component of A2B7type hydrogen storage alloys in this paper, and the alloys were prepared by an induction melting method and annealed at 850℃ and 900℃, then methods of ICP, SEM, XRD, PCT, RMC and the technology of electrochemical measurement were utilized to investigate the microstructure, hydrogen storage and electrochemical performances of the alloys.Since there were many other phases [LaNi5 and (LaMg)Ni3] besides (LaMg)2Ni7 phase in the alloys prepared by an induction melting method, the alloys were annealed at 850℃ and 900℃. It was found by XRD examinations that La0.75Mg0.25Ni3.5 alloy consisted of (LaMg)2Ni7、LaNi5 and (LaMg)Ni3 phases, and that (LaMg)2Ni7 phase content in the alloy increased significantly after annealing, and 58.38% and 88.13% were achieved as the annealing temperature was 850℃ and 900℃, respectively. At the same time, the achieved maximum hydrogen absorption capacity was 1.132 H/M and the maximum discharge capacity was 370.6 mAh/g after the alloy was annealed at 850℃.With respect to the La0.75Mg0.25Ni3.5 alloy, the hydrogenation thermodynamic, hydrogenation kinetics and hydrogenation cyclic stability were investigated systematically. The maximum hydrogen absorption capacity of the alloy decreased gradually with increasing temperature, and it reached 1.115 H/M at 25℃. The hydrogen absorption rate of the alloy decreased gradually with increasing temperature, while the hydrogen desorption rate increased gradually with increasing temperature. The hydrogen absorption and desorption capacities in the first minute achieved 96.3% and 67.9% of the maximum, respectively, as the temperature was 25℃. The hydrogenation cyclic stability increased gradually with increasing temperature. The capacity retention rate of hydrogen absorption after five hydrogen absorbing and desorbing cycles reached 91.0% at 50℃.(LaMg)2Ni7 phase content in La1-xMgxNi3.5 (x=0.2-0.3) alloys increased to 88.13% from 73.29% as Mg content increased to 0.25 from 0.2, then decreased to 55.94% when Mg content increased to 0.3. The maximum hydrogen absorption capacity of the alloys decreased with increasing Mg content, and it reached 1.153 H/M in the case of x=0.2. The hydrogenation cyclic stability decreased with increasing Mg content. The capacity retention rate of hydrogen absorption after five hydrogen absorbing and desorbing cycles reached 96.0% when x=0.3 at 50℃. The maximum discharge capacity of the alloys electrodes decreased with increasing Mg content, and it reached 378.2 mAh/g in the case of x=0.2. The cyclic stability of the alloy electrodes decreased with increasing Mg content, because Mg was corroded easily, and corrosion of the alloys became more serious with increasing Mg content. The capacity retention rate of the alloy electrodes La0.80Mg0.20Ni3.5 and La0.75Mg0.25Ni3.5 reached 90.5% and 88.1%, respectively, after 30 charge/discharge cycles.After the addition of Co, the (LaMg)2Ni7 phase content in La0.75Mg0.25Ni3.5-xCox (x=0,0.2, 0.5) alloys decreased to 66.06% from 88.13% as Co content increased to 0.2, then decreased to 57.89% as Co content further increased to 0.5. The maximum hydrogen absorption capacity of the alloys increased to the maximum (1.144 H/M) in the case of x=0.2. The hydrogenation cyclic stability decreased when x=0.2 and increased when x=0.5. The capacity retention rate of hydrogen absorption after five hydrogen absorbing and desorbing cycles reached 95.8% in the case of x=0.5 at 50℃. The capacity retention rate of the alloy electrodes reached 63.7% when x=0.2 and reached 60.0% when x=0.5 after 100 charge/discharge cycles.Compared with LaNis-type alloys which were widely used in fuel cell area, La2Ni7-type alloys of this paper possess more hydrogen absorption capacity and faster hydrogen absorption rate (the maximum hydrogen absorption weight exceeds 1.6 wt.%, which is more than 1.4 wt.% of LaNi5-type alloys), and recrystallization can take place in the alloys after several hydrogen absorbing and desorbing cycles, after the alloys were annealed at 300℃ 3h, and the hydrogen absorption weight can be thus recovered. However, some shortcomings also exist in La2Ni7-type alloys, for example, the plateau pressure of hydrogen desorption is lower, which is detrimental for hydrogen desorption. So it is still necessary to do further investigations to promote the properties of La2Ni7-type alloys for their potentials to be used in fuel cell area. |
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