Y.-H. ZHANG ET AL.
Copyright © 2012 SciRes. AMPC
The limiting current density (IL), another important electro-
chemical kinetics parameter which is mainly dominated by the
hydrogen diffusion in the bulk of the alloy during anodic pola-
rization , can be obtained by measuring the Tafel polariza-
tion curves of an alloy. The Tafel po larizatio n curves of La0.75-x
ZrxMg0.25Ni3.2Co0.2Al0.1 (x=0-0.2) alloys are depicted in Figure
7, from which a clear inflection point in each anodic polariza-
tion curve can be seen, there being a limiting value of the cur-
rent density which is defined as limiting current density (IL). It
indicates that an oxidation reacti on took place on the sur face of
the alloy electrode, and the generated oxidation product resists
further penetration of hydrogen atoms . The IL va lues of the
alloys as a function of the spinning rate and the Zr content are
also inset in Figure 7. It is visible that th e IL values of the Zr0.1
alloy first rise and then decline, and those of the as-spun (10
m/s) alloys always decrease wit h growing t he Zr co ntent.
Figure 7. Tafel polarization curves of the as-cast and spun alloys: (a)
Zr0.1 alloy, (b) As-spun (10 m/s).
Based on the investigation of the electrochemical kinetics,
some elucidation s can be provided as the reasons why the HRD
of the Zr0.1 alloy has a ma xi mum val u e with the variation of the
spinning rate. Upon the refined microstructure by melt spinning,
a lot of new crystallites and grain boundaries are generated,
which may act as the fast diffusion paths for hydrogen absorp-
tion , enhancing the HRD of the alloy. However, it must be
mentioned that the refinement of the grains resulted from the
melt spinning severely impairs the charge-transfer rate on the
alloy surface due to the fact that the refined grains effectively
prohibit the pulverization of the alloy particles, a lower new
surface o f the al loy electrode being formed, decreasing th e rate
of charge transfer at the alloy-electrolyte interface. It is the
above-mentioned contrary impacts engendered by melt spin-
ning that lead to a max imum HRD of the alloys. Furthermore,
the d ecreased HRD of th e as-spun (10 m/s) alloys by Zr substi-
tution is ascribed to the formation of a amorphous-like structur e
which not only increases the charge-transfer resistance of the
alloy electrodes but also hinders the hydrogen diffusion from
inner of the bulk to the surface, and subsequently results in the
drop of the electroch emical ki netic property.
The La-Mg-Ni system A2B7-type La0.75-xZrxMg0.25Ni3.2Co0.2
Al0.1 (x=0, 0.05, 0.1, 0.15, 0.2) electrode alloys were success-
fully synthesized by melt spinning. The as-spun Zr0 alloy exhibits
an enti re n anocr ystalli ne an d micr o-crystall ine struct ure, where as
the as-spun alloy substituted by Zr exhibits a amorphous-like
structure. Both the melt spinning and the substitution of Zr for
La markedly enhance the electrochemical cycle stability of the
electrode alloys. The electrochemical kinetics, including the
HRD, the hydrogen diffusion coefficients (D) as well as the
limiting current density (IL) of the Zr0.1 alloy first increase and
then decline with growing the spinning rate, and those of the
as-spun (10 m/s) alloys monotonously fall with rising the Zr
content, for which the refinement of the grains and the forma-
tion of the amorphous-like structure are principally responsible.
This work is supported by National Natural Science Founda-
tions of China (51161015 and 50961009), National 863 plans
projects of China (2011AA03A408), Natural Science Founda-
tion of Inner Mongolia, China (2011ZD10 and 2010ZD05).
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