The Study On La-Mg-Ni-Co-based Hydrogen Storage Alloy By The Coprecipitation-Reduction-Diffusion Method

This paper first discusses and reviews the previous research on La-Mg-Ni-Co systemhydrogen storage alloys extensively. On this basis, it is aimed to developLa-Mg-Ni-Co-based hydrogen storage alloys with low price and high electrochemicalproperties. Coprecipitation-Reduction-Diffusion preparation method was selected as thepreparation method of this study for the following research:At first, the specific condition of Coprecipitation-Reduction-Diffusion preparationmethod for La-Mg-Ni-Co-based hydrogen storage alloys was invesgated. The bestpreparation for the process condition is as follow: at the beginning of mixed nitrate liquor,the total of metallic ions concentrated is40g/L. Each of metallic ions is stoichiometricallyadded, and the number of rare earth is more5%than that of normal, the number ofMagnesium ion is more10%. The precipitator is oxalic acid-ethanol liquor, then theprecipitator is added into mixed nitrate liquor with dripping method, and the number ofprecipitator is more50%than that of normal, and the reaction is undertaken at60-70℃.The number of CaH2is1.5-2.0times as the theoretical value. The temperature is risen to950℃and kept4hours in hydrogen atmosphere, then cools slowly to room temperaturein the airtight condition. The homogeneous multivariate alloys are prepared. The XRD andFE-SEM analysis showed that: the alloys are single-phase and the size of the particles ofthe alloys is distributes about200nm.Secondly, the effect of different stoichiometric ratios in La-Mg-（Ni-Co）_x（x=3.0，3.3，3.5） quaternary alloys on the structure and electrochemical properties was investigated.XRD analyses showed that La-Mg-（Ni-Co）_xconsists of LaNi₅phase and Co5La phase. Theincreasement of stoichiometric ratio of B side elements leads to the appearance of a smallamount of MgNi₂phase, LaNi3phase and simple substance Ni miscellaneous peak. Theelectrochemical study indicated that the three alloys have a good activation performance, but with the increase of stoichiometric ratio of B side elements the cycle stability of thealloys decreases. S100decreases from45.1%（x=3.0） to31.4%（x=3.5）. Meanwhile, thedischarge capacity and HRD of the alloy electrodes decrease.Lastly, the effect of replacing Ni by Co in La_0.75Mg_0.25Ni_3.1-xCo_xMn_0.1Al_0.3（x=0-0.4）on the structure and electrochemical properties was invesgated. The results of the studyshowed that the phase composition of all alloys are relatively complicated, when x≤0.1,the major phases in alloys are the LaMg2Ni9phase, MgNi₂phase, AlLa3phase, with part ofAl_0.42Ni_0.58phase, and a small amount of Ni. The adding of Co content leads to theappearance of Mn3Co7phase, but the phase composition shows no obvious change. Andwith the further increasing of Co content, when x>0.1, there appears Al₁₄Co₃Ni₃phaseand Al50Mg48Ni2phase, and then the major phases in alloys change to LaNi₅phase andCo5La phase. The electrochemical study indicated that when x is0.3, the dischargecapacity and the cycle stability of the alloy electrodes have the maximum value,meanwhile, the high rate dischargeability （HRD） is best.