La-mg-ni System A <sub> 2 </ Sub> B, <sub> 7 </ Sub>-type Hydrogen Storage Performance

In this thesis, the La-Mg-Ni based A₂B₇-type hydrogen storage alloys were set as the subject of this study. By means of XRD, SEM and EDS analysis and thermodynamics and electrochemical test methods such as gas-solid absorption-desorption and galvanostatic charge-discharge etc., the phase structure, the thermodynamics properties and electrochemical properties of La₀.75Mg₀.25Ni₃.0M₀.5 (M=Ni, Co, Cu, Mn, Cr, Fe) alloy were studied systematically. Furthermore we studied the phase structure and electrochemical properties of the La₁.5Mg₀.5Ni₇-xM_x(x=0.l-1.2) sintering alloys. Finally, we explored the semi-experiential formula between enthalpy and microcosmic bond parameters and formula between storage capacity and microcosmic bond parameters.It is found that the main phases of La₀.75Mg₀.25Ni₃.0M₀.5(M=Ni, Co, Cu, Mn, Cr, Fe) alloy were all A₂B₇ type phases, besides some little AB5 type phases were found in the alloys with Cr and Cu. The unite crystal parameters of both kinds of A2B7 type phases increased with the increasing radii of atoms. The storage capacity decreased when Ni was substituded by Cr, Cu, Co, Fe and Mn. The plateau pressure of them decreased by the order Ni>Cu>Co>Cr>Fe>Mn. The hysteresis effect of all alloys presented same orderliness as storage capacity at 25℃, and the enthalpy of them decreased following the Cr>Co>Mn>Fe=Cu>Ni order. The investigation on electrochemical properties of the MH electrodes indicated that all the alloys except with Cr and Cu element are difficult to be activated. Its can be activated after 5 or 7 cycles. The maximal discharge capacity of alloys is consistent with the results of PCT test. The Co element improves the high rate discharge ability of the La₀.75Mg₀.25Ni₃.0 alloy.By investigation La₁.5Mg₀.5Ni₇-xM_x(x=0.l-1.2) sintering alloys, it was found when x changes from 0.1 to 1.2, Cu element can. dissolve and form La₂Ni₇ type phase with well-distributed structure but a small amount of unknown phase and little LaNi₅ phase has been found in the alloy with x≥0.9. The investigation on electrochemical properties of the MH electrodes indicated that with increasing Cu content in the alloys the discharge capacities decreased. The cycle stability of the alloys with x from 0.3 to 0.9 was improved to some extents in comparison x=0.1. The high rate discharge ability (HRD), the exchange current density Io, and the limiting current density II of the alloy electrodes decreased with increasing x from 0 to 0.6 and then increased with increasing x from 0.6 to 1.2. The electrochemical kinetic properties of the electrodes decreased in following order: Cu0.1>Cu0.3>Cul.2>Cu0.9>Cu0.6,The exploration to the models between enthalpy storage capacity and microcosmic bond parameters indicated that the models reflected the main factors influencing the forecasting parameters. The error of enthalpy AH and storage capacity N between calculated value and experimental value are less than 6.3KJ/molH₂ and 0.2 respectively. Its indicate that the enthalpy storage capacity have relation to microcosmic bond parameters of alloys farther.