Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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The invention relates to an electrlcal storage battery with
positive nickel oxide electrodes and negative iron electrodes.
Under the influence of the limited resources of natural energy
and in furtherance of environmental protection, persistent efforts are being
directed toward the application of known electrochemical current sources to
different fields of use, and particularly to electrical traction.
A promising electrochemical system for vehicle storage batteries
consists of the chargeable system dating back to Edison having a positive
nickel oxide electrode and a negative iron electrode in aqueous potassium
hydroxide solution. Although this is the oldest alkaline storage system, it
has subsequently undergone insignificant improvements in the technical reali-
~;~ zation of the practical cell, as well as in the preparation and electrochemi-
cal yield of the active mass.
Among the drawbacks of the nickel oxide/iron system, there is its
thermodynamic instability which results from the fact that the potentials
wh~ch are required for charging the nickel CII) oxide electrode, or rather
the FeCOH)2 electrode lie outside the limits of the thermodynamic stability
range of the water. This manifests itself in a parasitic gas evolution during
charging, particularly from the iron electrode, and this in turn requires a
- 2~ disproportionately high charging input. That charging of the electrodes is
possible under those conditions is attributable to kinetic impedances.
In contra~t to a theoretical load capacity of 260 Wh/kg, based
upon an open cell potential of 1.33 volts, practical nickel oxide/iron cells
have reached during five hour discharge only values between 20 and 25 ~h/kg.
There are known the classical electrode constructions of the
nickel oxide/iron storage battery, namely the positiye tubular electrodes
and the negative plate electrodes, in which the iron mass which consists of
metallic iron, iron oxide, or mixtures thereof is pressed into a metallic
holder. In order to increase the hydrogen excess potential, it is customary
to provide the iron mass also ~ith a predetermined quantity of mercury oxide.
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Measures intended to raise the load capacity of the nickel oxide/iron storage
battery have been principally addressed to the negative iron electrode, whose
theoretical capacity is utilized only at the 28 per cent level and therefore
would appear to provide wide latitude for improvement.
Thus, German patent publication (DT-AS) 1,696,570 teaches the
combination in a storage battery of a positive sinter electrode with a nega-
tive iron sinter electrode which is doped with small quantities of sulfur
compounds whose presence is intended to prevent passivation of the iron.
According to German patent publication (DT-OS) 2,261,997 an iron
electrode is produced by cathodic deposition of iron from an iron (II) nitrate
solution upon an electrically conductive carrier while being simultaneously
brought into contact with a sulfur salt.
United States patent 3,507,696, teaches the production, upon the
particles of an iron oxide powder, of a thin cover layer of molten elements of
the sulfur group, after which a metal fiber plate serving as carrier is
impregnated with this material which has previously been made into a slurry
with water.
However, these and other measures have not sufficed to raise the
load capacity of the respective commercial cells above the values indicated.
A significant obstacle to doing so is represented by the high weight contri-
bution of those cell components which are not directly involved in the
delivery of current and which correspondingly detract from the load capacity.
- In particular, 20 to 35 per cent of the total cell weight are attributable to
the inactive structure of conventional electrodes (support, armature, frame).
Accordingly, it is a primary object of the invention to provide a
nickel oxide/iron cell which, in addition to a good voltage level particularly
during discharge, conforms to the high capacity requirements of electric
traction, particularly through reduction of dead weight.
According to the present invention, there is provided an electric
storage battery having positive nickel oxide electrodes and negative iron
electrodes, wherein the positive electrodes are of compressed powder, and
the negative iron electrodes are of sintered iron powder.
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In another aspect, the invention provides the method of making an
iron electrode for a storage battery comprising pasting onto an iron expanded
metal support an aqueous dispersion of iron powder, water soluble filler
material, and organic thickener, sintering the above at temperatures from
about 600 to 800C, and thereafter dissolving out the filler material.
Preferably, the positive electrodes of pressed powder electrodes,
and the negative iron electrodes of sintered iron powder are positioned so
that a negative electrode is between every two positive electrodes.
Such an electrode combination makes it possible to match the high
surface capacity of the iron electrode with a counter-electrode of correspond- -
ing capacity.
This requirement cannot be met by the positive pocket or tubular
electrodes heretofore used, because the open spaces provided by the perfora-
tions of these electrode armatures amount to only about 15 per cent of the
geometrical electrode surface so that a substantial fraction of the active
mass is more or less covered. It is therefore reached by the charged carries
of the electricity movement only through detours, i.e. with a voltage drop.
It is therefore particularly advantageous to provide, in accordance
with the invention, a metal fabric armature which encloses the positive mass
which has been pressed into it under high pressure. In this armature, which
is preferably a nickel fabric, the open surface portion, at 36 per cent, is
about 2.4 times greater and the individual openings at 1.44X10 2mm2 are only
one-third as large as with conventional pockets. In this way, the filter
effect, that is the retention of the active mass, is substantially improved.
The fraction of the total weight of the electrode constituted by the
armature amounts to only about 20 per cent instead of the 50% of a conventional
pocket electrode; the surface capacity of such an electrode amounts to about
6.5 Ah/dm2.
In accordance with the invention, the negative counter-electrode
consists of sintered iron powder which contains a supporting structure, for
example expanded iron metal. It is preferably produced by applying to iron
expanded metal iron powder which has been made into a paste by means of an
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alcohol-water-methyl cellulose mixture and subsequent sintering.
When ready for use the paste contains, in addition to traces of
what may be a defoamer, 70-80 per cent by weight iron powder, 15-20 per cent
by weight liquid (alcohol and water, for example in the relatioDship 1:15)
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0.5-1 per cent by weight methyl cellulose as thickener, 3-10 per cent by
weight filler material (water soluble inorganic salts, particularly sodium
chloride, sodium carbonate).
The pure iron powder used has a BET surface of 0.1 to 0.3 m2/g.
By BET surface is meant the surface area per unit weight, as calculated by a
procedure attributed to the physicists Brunauer, Emmet, and Teller (hence the
acronym BET) based on the measured volume of nitrogen absorbed by powders or
porous bodies. The predominant portion, e.g. more than 80 per cent, of the
powder has a grain size of less than 30 microns.
After pasting,the electrode is sintered for about one-half hour
at 600-800C, preferably at 700C, in an atmosphere of protective gas.
The defoamer may be a boundary-layer active or surfactant
material, which displaces foam forming substances from the bou~dary layer,
without itself creating a foam. Alternatively, it may be a material which
raises the surface tension of the water. These may be fats or oils, or long-
chain alcohols, such as 2-ethyl hexanol or acetyl alcohol.
The thickener may also be carboxy methyl cellulose, polyvinyl
alcohol, a poly wax or an alginate.
In this electrode, too, theidead weight portion (expanded metal
and lead-out conductor) amounts to only 20 per cent of the total electrode
weight, the surface capacity to abo~t 15-16 Ah/dm2. Consequently, there
still remains a small capacity excess relative to the adjacent positive
electrodes. These therefore limit the cell capacity.
Worthy of note with regard to the iron electrode according to
the invention is its charging characteristic, which differs from that of a
conventional pocket electrode by a more positive voltage level and a definite
voltage step upon complete charging, resembling that of cadmium! This means
that such an iron electrode acts more like cadmium with respect to hydrogen
evolution, that is the gasing rate remains relatively low until the voltage
step is reached. For full charging of the electrode a charge factor of 1.4
suffices.
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For further details reference is made to the discussion which
follows in the light of the accompanying drawings wherein:
Figure 1 shows the operating characteristics of a cell embodying
the invention, and
Figure 2 shows a physical embodiment of such a cell.
Referring to Figure 1, curve 1 shown therein shows the charging
voltage while curve 2 shows the discharge voltage as a function of time of a
cell in accordance with the invention. The load during discharge equals 0.2
CA, where C is the numerical value of the capacity of the cell. For example,
if the cell has a capacity of 50 Ah, then C is equal to 1501, and the dis-
charge current 0,2 CA will be equal to 0,2 x 50 A ~ 10 A.
The negative electrode is separated from the positive electrode
by separators of synthetic material, preferably in the form of a grate, the
spacing between vertical rods being about 8 millimeters and the rod diameter
about 2 millimeters.
The electrode spacing determined by the rod thickness is advanta-
geous relative to the electrolyte quantity and the heat capacity of the cell.
~ue to the relatively close rod spacing, deformation of the electrode under
the influence of expansion pressure can be better counteracted so that the
2C rod separator prevents contact between the positive and negative electrode
and thereby the formation of short circuit connections.
To reduce the fraction of the cell housing relative to the total
weight, particularly for multi-cell storage batteries, in a battery box, the
enclosure for each individual cell, or rather for each individual electrode
set, is a synthetic plastic tube which exhibits sufficient chemical, thermal
and mechanical stability and which can be welded shut together with a compact
bottom plate and a compact lid which insure pole positioning protected from
disldcation. Polyethylene may, for example, be suitable as this material.
With this synthetic plastic envelope as the cell container the
total weight of a cell embodying the invention divides among its various
components as ollows:
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51.5 perccent for the active mass
13.5 per cent for mass armature and vanes
23.0 per cent for the electrolyte
12.0 per cent for the cell container, separators, pole.
This clearly shows that an arrangement according to the invention,
particularly has a very high proportion by weight of active materials. Corr-
espondingly, the energy density of the nickel oxide/iron cell can be raised
by the technique embodying the invention from the conventional figure of
about 24 Wh/kg by a factor of 2, namely to about 50 Wh/kg for five hour
discharge current.
Figure 2 shows a cell embodying the invention and containing in
this case four positive and three negative electrodes, the three front
electrodes being shown partly broken away. These show the principle of the
arrangement.
Between each two positive electrodes 1 having metal fabric
armatures 4, and separated from these by gratelike separators 3, there is a
negative electrode 2.
The current take-off vanes 5 of the positive electrodes 1, which
are attached by spot welds to the metal fabric armature 4, are riveted to the
2~ positive pole shoe 6, whereas the current take-off vanes 7 of the negative
electrodes are similarly attached to the negative pole shoe 8.
Knobs 9, which engage the frame of the rod separator and extend
over the edges of the electrodes, prevent relative sliding of the plates.
To the compact lid 10, which may consist for example of poly-
ethelene, there are welded, spaced by foil material serving as cell housing
11, the two pole lead-throughs 12 and the filler plug 13, all these elements
being of the same synthetic plastic.
After insertion of the negative and positive pole bolts 14 and 15
through lead-throughs 12, the poleshoes are so attached to the underside of
the lid between ribs 16 that they do not rotate during tightening of
hexagonal nuts 17.
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A rigid bottom plate 18, for example of polyethelene, gives-,,*he
electrode package additional mechanical protection at its lower end.
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