Influence of the rare earth composition on the properties of Ni–MH electrodes

doi:10.1016/S0925-8388(03)00340-2

Journal of Alloys and Compounds

Volume 360, Issues 1–2, 6 October 2003, Pages 290-293

https://doi.org/10.1016/S0925-8388(03)00340-2 Get rights and content

Abstract

Lattice parameters, hydrogen absorption properties and electrochemical cycling properties up to 240 cycles have been measured as a function of the Ce content for alloys of composition La_0.82−xCe_xNd_0.15Pr_0.03Ni_3.55Mn_0.4Al_0.3Co_0.75 (0≤x≤0.82). The results show the strong increase of the plateau pressure correlated to the cell volume decrease as a function of x. On the other hand, the hydrogen capacity measured in solid–gas reaction as well as the electrochemical capacity decreases slightly. The results show that both La and Ce have to be present to achieve a good cycle life, the cycling degradation being almost independent of their relative quantities in a broad range of concentrations.

Introduction

Since the early development of metal–hydride electrodes, La has been replaced by a rare-earth mixture (mischmetal) composed of La, Ce, Nd and Pr. This was mainly for economical reasons because mishmetal is produced directly from the ore, without a costly rare earth separation. However, as the mischmetal composition strongly depends on the origin of the ore, and as the composition can be adapted by an adequate enrichment in any of the constituting elements, the search for an optimal composition is still of high technological interest. We have studied the partial or total replacement of La by Ce in the conventional negative electrode composition La_0.82−xCe_xNd_0.15Pr_0.03Ni_3.55Mn_0.4Al_0.3Co_0.75 and its influence on the metallurgical, hydrogenation and electrochemical properties with a special emphasis on the cycle life of the Ni–MH electrodes.

Section snippets

Experimental

The samples were synthesized by arc (x=0.3 to 0.6) or induction melting (x=0 to 0.2 and x=0.82). The arc melted samples were synthesized from natural mischmetal the composition of which was adjusted by the proper amount of pure rare earth, while the samples synthesized by induction melting were synthesized directly from the pure metals. All the samples were annealed at 1000 °C during 15 h in sealed silica tube under argon atmosphere. The alloys were characterized by X-ray diffraction and

Results

The results of the alloy characterization can be found in Table 1. EPMA measurements reveal that the analyzed compositions are in agreement with nominal ones. The lattice parameters and cell volumes are plotted as a function of the Ce content in Fig. 1. The pressure–composition isotherms at 25 and 80 °C are drawn in Fig. 2, Fig. 3. A clear increase of the plateau pressure as a function of the Ce content can be noticed and evaluated in Fig. 4. In the meantime the capacity measured under 10 bar

Discussion

The cell volumes measured as a function of x fairly well compare with the ones measured for the compounds LaNi_3.55Mn_0.4Al_0.3Co_0.75 (90.50 Å³) and CeNi_3.55Mn_0.4Al_0.3Co_0.75 (85.47 Å³) [1], given the amount of Nd and Pr in our study. This indicates, as in this previous study, that the reduced Ce radius due to the intermediate valence state of this element accounts for the large decrease of the cell volume when La is replaced by Ce. Moreover, the linear decrease observed in this work indicates that

Conclusion

The replacement of La by Ce in alloys of composition La_0.82−xCe_xNd_0.15Pr_0.03Ni_3.55Mn_0.4Al_0.3Co_0.75 was studied as a function of x. Decrease of the cell volume and corresponding increase of the plateau pressure was noticed. On the contrary solid-gas and electrochemical capacities remain nearly constant. The cycle life is deeply improved by the presence of Ce and remains good as long as La is present, in a broad range of concentration (0.2≤x≤0.6), demonstrating a synergistic effect between the

Acknowledgments

The authors wish to thank, E. Leroy for the EPMA analyses and V. Lalanne for the pressure–composition measurements. This work was supported by a grant of the Association Franco-Chinoise pour la Recherche Scientifique et Technique (Programme de Recherche Avancé PRA MX 99-03).

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Citation Excerpt :
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Influence of the rare earth composition on the properties of Ni–MH electrodes

Abstract

Introduction

Section snippets

Experimental

Results

Discussion

Conclusion

Acknowledgments

J. Alloys Comp.

J. Alloys Comp.

J. Less-Common Met.

J. Less-Common Met.

J. Alloys Comp.