In our endeavor to improve the cyclic stability of La-Mg-Ni-Co type alloys, La0.7Mg0.3Ni2.652xMn0.1Co0.75Alx (x=0-0.5) hydrogen storage alloys were prepared and the structure and electrochemical properties of these alloys were investigated systematically. X-ray diffraction and Rietveld analyses revealed that the major phases are the (La, Mg) Ni3 phase and the LaNi5 phase. Electrochemical studies indicate that as Ni is progressively substituted by Al, the cyclic stability of alloy electrodes is noticeably improved due to the formation of a dense oxide film on the alloy surface. The capacity retention of the alloy electrodes for 100 cycles increases from 32.0% (x=0) to 73.8% (x=0.3). The maximum discharge capacity decreases with increasing Al content, and the high rate dischargeability, electrochemical impedance spectra, linear polarization, Tafel polarization, and potential-step studies all indicate that the electrochemical kinetics of the alloy electrodes is deteriorated. We ascribe this phenomenon to the increase of the charge-transfer resistance of the alloy electrodes and reduction of the diffusion rate of hydrogen from interior of the bulk to the surface due to the presence of the aforesaid Al-containing oxide film. The optimal content of Al in La0.7Mg0.3Ni2.652xMn0.1Co0.75Alx alloys in this study is in the range from 0.2 to 0.3.

A novel single phase R3 (Fe, M) 29 (R=Nd, Sm and Gd) compound series with M=Mo as a new stabilizing element has been synthesized. A systematic study of the formation and magnetic properties of R3 (Fe, Mo) 29 compounds has been performed. The lattice parameters and magnetic properties incIuding the Curie temperature Tc, saturation magnetization Ms and anisotropy field Ba at 300 and 4.2K are presented.

A novel single-phase Y3 (Fe, Mo) 29 compound with Mo as a new stabilizing element has been synthesized with nominal composition Y1o.sFe86.7Mo2.8 via annealing at 1313K for three days, and then water quenching. The results of thermomagnetic analysis (TMA) show a magnetic ordering temperature of 376K. X-ray diffraction gives lattice parameters a=10.570 Å, b=8.508 Å, c=9.673 Å and β=96.98º. The temperature dependence of the magnetization Ms of Y10.5Fe86.7Mo2.8 in an applied magnetic field up to 7 T and the temperature dependence of the anisotropy field Ba are also reported.

The optimum compositions and annealing conditions of the preparing single-phase R3 (Fe, Mo) 29 (R=Tb and Dy) are reported. The structural characterization by X-ray powder diffraction provides evidence for a Nd3 (Fe, Ti) 29-type compound with the space group A2/m. The Curie temperatures Tc are 437 K for Tb3 (Fe, Mo) 29 and 408K for Dy3 (Fe, Mo) 29. The saturation magnetization Ms values are 77.5Am2/kg at 4.2K and 70.4Am2/kg at 300K for Tb3 (Fe, Mo) 29, and 73.0Am2/kg at 4.2K and 65.2Am2/kg at 300K for Dyz (Fe, Mo) 29.

The effective exchange coupling parameters JFeFe in the Fe sublattice and JRFe between the R and Fe sublattices have been calculated based on a mean field approximation according to the experimental values of the Curie temperatures of R3 (Fe, M) 29 (R=rare earth, M=V, Ti and Mo). JFeFe is largest when M=V and smallest when M=Mo, but JRFe is largest when M=Ti and smallest when M=Mo for the same R and different M in the R3 (Fe, M) 29 compounds.