Hydrogen Storage And Electrochemical Properties Of La-Mg-Ni Based Alloys

A series of La-Mg-Ni based hydrogen storage alloys with different element constituents and proportions were prepared by induction melting under argon atmosphere. The effects of Ti, Ni, Co, Al, Mn, Pr and Nd elements on crystal phase, hydrogen storage and electrochemical performances of La-Mg-Ni based alloys were studied systematically by XRD, PCT, charge/discharge cyclic stability, high rate dischargeability, electrochemical impedance spectroscopy, linear polarization, Tafel polarization and constant potential-step methods.The X-ray powder diffraction (XRD) results indicated that all the alloys were mainly composed of the LaNi5, LaMg2Ni9, La2Ni7, LaNi3 and LaNi2.28 phases. It can be found that the crystal cell volumes of LaNi5 and LaNi2.28 phases in the La0.7Mg0.3Ni3.4(Co0.7Al0.3)x (x=0.0-0.6) alloys and the (La,Pr)(Ni,Co)5, LaMg2Ni9, (La,Nd)2Ni7, LaNi3 phases in the La0.55Pr0.05Nd0.15Mg0.25Ni3.5(Co0.5Al0.5)x (x=0.0-0.5) alloys increased slightly with the increasing of Co and Al content. The crystal lattice parameters and cell volumes of certain phases were decreased with the smaller atomic radius elements Ti (1.448 A), Pr (1.828 A) and Nd (1.821 A) partial substituting for La (1.877 A) on the A side. For instance, the cell volumes of LaNi5 and LaMg2Ni9 phases decreased with increasing Ti content in the La0.7-xTixMg0.3Ni3.5 (x=0.00-0.10) alloys.The HRD results show that with increasing x in La0.7Mg0.3Ni3.4(Co0.7Al0.3)x alloys, not only decreased the maximum hydrogen storage capacity from 1.44 wt.%(x=0.0) to 1.04 wt.%(x=0.6), but also reduced the absorption/desorption equilibrium pressure and the hysteresis factor. It can be found that the maximum hydrogen storage capacity of La0.7(Pr0.75Ndo.25)xMgo.3Ni3.3(Co0.7Al0.3)0.3 and La0.55Pr0.05Nd0.15Mgo.25Ni3.5(Co0.5Al0.5)x alloys increased firstly and then decreased with increasing x, and the maximum hydrogen storage capacity was 1.47 wt.%(x=0.3) and 1.42 wt.%(x=0.2), respectively.The charge/discharge cyclic stability investigations revealed that the discharge capacity and cyclic stability of alloy electrodes can be improved by modifying the elements on the A side or B side. For example, the discharge capacity of La0.7Mg0.3Ni3.5-x (x=0.0-0.4) alloy electrodes first increased and then decreased with the increasing x, and the maximum discharge capacity reached up to 392 mAh·g-1(x=0.1). What's more, with increasing x, the cyclic capacity retention S65 of La0.7Mg0.3Ni3.4(Co0.7Al0.3)x alloy electrodes increased from 54% (x=0.0) to 76%(x=0.6), and the S100 of La0.55Pr0.08Nd0.15Mg0.25Ni3.5(Co0.5Al0.5)x alloy electrodes increased from 46.6%(x=0.0) to 67.1%(x=0.5), and the S100 of La0.7Pr0.15Nd0.05Mg0.3Ni3.3-xCo0.2Al0.1(Co0.85Mi0.25)x alloy electrodes increased from 53.2%(x=0.0) to 63.0%(x=0.6). The high rate dischargeability of Lao.7(Pro.75Ndo.25)xMg0.3Ni3.3(Co0.7Al0.3)0.3 and La0.7-rTixMg0.3Ni3.5 alloy electrodes decreased with increasing Pr, Nd or Ti element contents. The high rate dischargeability of the La0.55Pr0.05Nd0.15Mg0.25Ni3.5(Co0.5Al0.5)x and La0.7Mg0.3Ni3.4(Co0.7Al0.3)x electrodes were all first increased and then decreased slightly with increasing x in alloys. The similar phenomenas happened in La0.7Mg0.3Ni3.4-x(Al0.3Co0.7)0.2+x and La0.7Pr0.15Nd0.05Mg0.3Ni3.3-xCo0.2Al0.1(Co0.75Mn0.25)x alloys. It was indicated that the Raney Ni-Co film and Ni-rich film formed on the alloy surface enhanced the high dischargeability of electrodes. However, the Al atoms on alloy surface can be easily corroded in alkaline electrolyte, and formed oxidation film which prevented the hydrogen diffusion on the alloy surface reduced the high rate discharge ability of electrodes slightly.The results of electrochemical impedance spectroscopy, linear polarization, Tafel polarization, cyclic voltammogram and constant potential-step analysis showed that with increasing content of Pr, Nd, Co, Al and Mn elements in alloys, the exchange current density I0, anodic peak density Ip, limiting current density IL and hydrogen diffusion coefficient D all increased first and then decreased.