Electrochemical and in situ neutron diffraction investigations of La-Ni-Al-H alloys Page: 3 of 4
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ELECTROCHEMICAL AND IN SITU NEUTRON
DIFFRACTION INVESTIGATIONS OF La-Ni-Al-H
W.Peng*, L.Redey, D.R.Vissers. K. M. Myles
Electrochemical Technology Program
J.Carpenter, J.Richardson, and G.Burr
Intense Pulse Neutron Source
Argonne National Laboratory
9700 S. Cass Ave., Argonne, IL 60439
The nickel/metal hydride (Ni/MH) battery is a strong
contender to replace the nickel/cadmium battery because of
its superior performance, low impedance, and absence of
toxic cadmium. However. the present Ni/MH battery is
limited by hydrogen management problems associated with
charge/discharge operation and self-discharge of the battery
and by long-term capacity loss due to corrosion of the MH
electrode. Rare earth metal-based alloys such as LaNi5,
preferably with the partial substitution of lanthanum or
nickel by a small amount of other metal elements, seem to
be promising MH materials to improve the stability and
capacity of the MH electrodes. However, the role of
alloying components is not yet clearly understood.
Therefore, a combination of electrochemical and neutron
diffraction techniques has been designed to investigate metal
hydrides. This combination of the surface and bulk
investigation techniques provides a unique tool to study the
properties of the alloy electrodes in situ with respect to
composition, atomic structure, and phase change [1-3].
In this work, several Al-substituted LaNi5 alloys were
investigated with respect to their specific capacity (measured
by mAh/La and symbolized by x in LaNis.yAlyHX),
impedance, and cycling stability. In addition to the
extensive electrochemical investigations, in situ neutron
diffraction measurements were performed to characterize the
electrochemically induced phase transformation and
structure change during charge and discharge of the metal
The following simple cell reaction permits simple cell
MHX + xNiO(OH) = M + xNi(OH)2,
The electrochemical testing was carried out in a three-
electrode cell consisting of a commercial NiO(OH) positive
electrode, a Hg/HgO reference electrode with electrolyte
bridge and Luggin capillary. a metal-hydride electrode, and
15% KOH electrolyte. Teflon mesh separator was used
between the working and counter electrodes. The alloy
powder (Rhone-Poulenc, -400 mesh size) was mixed with
5% carbon (Shawinigan Black) conducting additive and 10
% DuPont Teflon 30 suspension and pressed onto an
expanded nickel mesh to form 1-mm-thick electrodes.
To investigate the electrochemical performance of the
metal hydride electrode, plane-parallel or jelly-roll electrode
combinations were used to ensure uniform current density.
The following alloys in this study were investigated: LaNi5.
LaNi4 87A012. and LaNi44Ao.6. The electrochemical
properties of the metal hydride electrodes, such as capacity,
cell voltage, electrode potential, ASI0 ,, and cycle-life, were
investigated at room temperature and ambient (I atm)
pressure by interrupted galvanostatic cycling.
A different cell design was used for the in situ neutron
diffraction measurements because the neutron diffraction
measurements are adversely affected by the presence of
protons. For these in situ experiments, a fully deuterated
cell was designed:
To obtain a deuterated NiO(OD) electrode, two identical
pieces of NiO(OH) electrodes removed from Ni/Cd cells
were cycled in 15% KOD/DO solution for 16-20 cycles.
The electrolyte was exchanged every 4 cycles and analyzed
by FTIR to determine the H/D ratio in the solution, which
indicated the remaining proton content of the NiO(OD)
electrode. A NiO(OD) electrode with an H/D ratio of less
than 0.02 was accepted for the neutron diffraction
measurements. The cell used in the in situ neutron
diffraction measurements was constructed in a quartz tube
(Fig. 1). The neutron diffraction patterns were recorded
simultaneously with the electrochemical data. Neutron
diffraction data from these operating cells comprise
scattering from four components: the LaNis.yAlyDX
electrode, the Ni mesh support, the electrolyte and the quartz
tube. With modern multi-phase analysis techniques,
scattering from the electrolyte and quartz tube can be
modelled as background, leaving only the crystalline Ni and
LaNi5-yAlyD, phases .to consider. Rietveld profile
refinements provide precise data corresponding to phase
identification, deuterium siting, and phase composition as a
function of charge/discharge state.
Results and Discussion
The discharge capacity indicates the change in the value
of x in the formula of metal hydride. Figure 2 shows plots
of open-circuit cell voltage versus x for alloys of three
different Al content. The open-circuit cell voltage was
measured at the end of 150-second-long current
interruptions. In Fig. 2. the data are shown for the fourth
charge/discharge cycle. There are distinct differences in the
capacities of the MH electrodes. LaNi5H, exhibited low
specific capacity, while the Al-substituted alloys have
dramatically increased cell capacity up to more than 10
times higher than that of the pure LaNi5 parent alloy. It is
evident from Fig. 2 that higher Al content of the alloy
produces higher specific capacity. The low specific
capacity of the LaNi5 electrode at ambient pressure is mainly
due to its inherent nature, which would require a high
hydrogen pressure to achieve a higher x value [4). The
effect of higher Al concentrations on the cell performance
improvement can be partly explained by the lowered
hydrogen equilibrium pressure requirement of the Al-
containing alloys . This behavior can be assessed from
the structure analysis of the neutron diffraction investigation.
Subtle crystallographic differences between electrodes
containing Al and those without may provide insight into the
mechanism. The presence of Al does influence the
disposition of deuterium in the LaNi5sAl.DX phases.
Aluminum preferentially substitutes into one of three
available crystallographic sites. When no aluminum is
present. the maximum amount of deuterium that enters the
alloys (x<0.5) is limited to inclusion in the alpha phase.
When deuterium is placed into an alloy containing
aluminum, the amount of deuterium in the alpha phase
remains constant, while the extra deuterium enters the beta
phase. Results correlate with electrochemical measurements
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Peng, W.; Redey, L.; Vissers, D.R.; Myles, K.M.; Carpenter, J.; Richardson et al. Electrochemical and in situ neutron diffraction investigations of La-Ni-Al-H alloys, article, May 1, 1996; Illinois. (digital.library.unt.edu/ark:/67531/metadc668074/m1/3/: accessed November 14, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.