Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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CONTROL OP EDGE EFFECTS OF OXIDANT ELEC-TRODE,
Background of the Invention
-The invention is coDcerned with electrode assemblies
and use of same in electrical energy storage devices (EESD),
especially a rechargeable EESD,
An EESD has utility in electric vehicle markets
or in stationary power systems. Both of these markets may
have a requirement to electrodeposit the reducible metal
in a smooth dense manner and to remove it uniformly during
discharge. In the electric vehicle market, there may be
multiple shallow depth discharges occurring prior to a
complete discharge. During discharge, difficulty has arisen
when an oxidant is passed through a porous electrode. It
may be significantly more electrochemically active than
the counter electrode due to its high surface area. Due
to the increase in current density, the metal of the counter
electrode is removed quic~ly during discharge. Additionally,
chemical corrosion of the reducible met~l of the EESD by
the presence of the oxidant in the electr,olyte has a tendency
to decrease the effectiveness of any EESD. These problems
are collectively referred to as the edge activity of an ,
oxidant electrode. The control of the edge effects of a
porous oxidant electroae is the object of the present invention.
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Summary of Invention
The invention is concerned with an electrode assembly
comprising;
a. a porous electrode having a first and second
exterior face wit~ a cavity formed in the interior between
said exterior faces thereby having first and second , , '!
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, interior faces positioned opposite the first and second .
exterior faces; .
b. a counter electrode positioned facing each of '
the first and second exterior faces of the porous
electrode;
, c. means for passing an oxidant through said porous
electrode; and
d. screening means for blocking the interior face
. of the porous electrode a greater amount than the respective
exterior face of the porous electrode, thereby maintaining
a differential of electrode surface between the interior
- face and the exterior face.
The'invention is also concerned with a method of
discharging an electrical energy storage device comprising . ~'
the steps;
1. providing a first electrode comprised of an
electrochemically reducible substance;
2. providing a porous electrode having a first . , .
and second exterior face with a cavity :Eormed in the interior
between said exterior faces, thereby having first and second
interior faces positioned opposite the first and second
exterior faces;
3. providing a current carrying electrolyte between
said electrodes;
4. passing an oxiaant through said porous electrode;
5. screening the electrochemical activity of the
porous electrode by ,blocking the interior face a greater amount
than the respective exterior face of the porous electrode,
thereby maintaining a differential of electrode surface
between the interior face and the exterior face; and
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6. closing the circuit,between the first electrode
and the porous electrode, thereby oxidizing the substance a~
the first electrode and reducing the oxidant at the porous
electrode.
Brief Description of the Drawings
Fig. 1 is a diagrammatic view of the process of
the present invention;
Fig. 2 is a sectional view of a submodule of
assembled electrolytic cells,
Fig. 3 is a case for supporting a submodule stack
of electrolytic cells useful in the process of the present
invention;
Pig. 4 is a sectional view substantially along
lines 4-4 of Fig. 2;
Fig. 5 is a sectional view taken along lines 5-5 -
of Fig. 4;
Pig. 6 is an exploded view of the electrodes
useful in the process of the present invention;
Pig. 7 is a sectional view of the cell distribution
manifold useful in the process of the present invention.
Fig. 8 is a side sectional view of a portion of the
electrode assembly of the present invention showing the
internal/external masking or screening effect.
Detailed Description of the Invention
When porous electrodes are used in an EESD, their
electrochemical activity must be taken into consideration
during the discharge reaction because the oxidant will be
reduced not oniy at the exterior electrode surface (generally
longitudinal face) of the electrode, but also in the interior -
portion of the porous electrode
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In the most preferred manner the electrochemicalreactions of discharge are:
Zn (metal) -~ ~n~ + 2
C12 ~ 2 Cl- 2 ~
Zn (metal) ~ C12 ~ Zn++ + 2 Cl-
It has been found, therefore, to control the edge effects
of the porous electrode there should be a means for decreasing
or screening or masking the electrochemical activity -~ the
porous electrode by having a differential of a mechanical mask
on the front or exterior portion of the electrode (exterior
face) versus the interior face or internal mask of the electrode.
The positive electrodes of the present invention are
primarily poxous electrodes and may be carbonaceous electrodes,
that is, comprised of carbon, activated carbon, graphite,
activated graphite and mixtures thereof with or without other
fillers that may be present in the carbonaceous electrode.
The porous electrode may also be comprised of a film forming
metal, such as~ titanium, titanium alloys,, tantalum, tantalum '
alloys, zirconium, zirconium alloys, niobium, niobium alloys,
tungsten, tungsten alloys and mixtures thereof. Any of the
èlectrodes may be further comprised of catalytic materials
well Xnown in the art as noble metals as gold and silver and
the like or Group VIII of the Periodic Table of Elements
tHANDBOOR OF CHEMISTRY AND PHYSICS, 55th ed., 1974-1975, ~ -
published by CRC Press) such as ruthenium, rhodium, palladium,
osmium, nickel, iridium, platinum and the oxides thereof and
mixtures thereof and the like. Generally, when the film
forming metais are used, a catalyst is also used, e.g.,
ruthenizea titanium.
The electrode assemblies may be useful in any EESD or
any electrochemical reaction wXere a porous electrode is used,
~ such as the utilization of hydrogen, oxygen, halogens, such as
chlorine, bromine, iodine, fluorine, halodates, such as
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chlorates, bromates, the primary or secondary fuel cells, such as the
metal hydride type, a metal halogen system and the like. Most pre-
ferred is the EESD of the metal halogen hydrate type such as the
metal halogen devi oe described in U.S. 3,713,888 or 4,049,880.
Opera~ions of a zinc chloride battery system are described
in Electric Pcwer Research Institute (EPRI) EM-249 Peport for Pro-
ject 226-1 Interim Report, September 1976; F~ 1051, Parts 1-4, Pro-
ject 226-3 Interim Report, April 1979; Cost Analysis of 50 KWH Zinc-
Chlorine Batteries for Mobile Applications, U.S. Dept. of Energy
Report C00-2966-1, January 1978 and Safety and Environ~ental Aspects
of Zinc-Chlorine Hydrate Batteries for Electric Applications, U.S. ,
Dept. of Energy Report C00-2966-2, March 1978.
It has been found highly decirable that the electrode assemblies
of the present invention are particularly useful in EESD where a current
carrying electrolyte is employed such as an aqueous electrolyte. Any
of the electrolytes well known in the art for the EESDs as described
above may be employed. Electrolytes may be acidic or alkaline. The
most preferred electrolyte is that useful in the metal halogen hydrate
device described in the aforementioned patents, most preferably, a
2p metal halogen device such a a zinc chlorine EESD.
It is preferred that when carbonaceous electrodes are employed
in the electrode assembly in the present invention that the electrodes
be activated in accordan oe with the teachings of copending Canadian ap-
plication serial no. 357,298, filed July 30, 1980rthe disclosure in
~ournal of the Electrochemical Society, August 1978, vol. 125, ~8, "Ab-
stract of Electrochemical Society Meeting" and Extended Abstracts of
the Electrochemical Society, Vol. 78-2 for Fall Meeting, October 15-20,
1978~ in particular Abstract #73. The electrode assemblies are also pre-
ferably used as bipolar electrodes in accordan oe with U.S. Patent
4,100,332.
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Turning now to a discussion of the drawings, Fig. 1 '
is a schematic diagram of the electrode compartment o~ a ,
preferred EESD such as the zinc chlorine chlorine hydrate
system. In a container 10, sealed in place is an
electrolyte reservoir 12 within a plastic reservoir 14
The electrolyte reservoir 12 functions as a sump from which
electrolyte is pumped Vi2 line l6 by means of pump P into
each of the stacks or submodules 18 via independent conduit
` ~ 20. A valve V is placed in line 16 so that the electrolyte
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10 may be changed or dumped~as desired. While the apparatus
10 is shown as containing a hood 22, it is to be appreciated
, that the design of such equipment may be modified to fit
the desired characteristics of the electric vehicle or the
standing power market. It is further to be appreciated
that the electrolyte that is flowing from the sump 12 via
line 16 into submodules 1~ can be heated or cooled as is
desired by auxillary apparatus (not shown).
Fig. 2 is a cross-section of the electrochemical
apparatus of the present invention showing the electrolyte '
f 20 sump 12 being retained in a tray 22 and a series of electrical
cells arranged in bipolar fashion having current terminals
19 and 21. The current is passed through the current
terminals to conventional bus bars which in turn are
connected to connector studs (not shown), thereby passing
the current to each of the individual cells in each
submodule. Each stac~ of electrodes is retained in a sub-
module tray 22, a sectional view of which is shown in Fig.
3. The submodule tray has an electrolyte drain cup 24 to
which is attached a conduit 26 which in turn is connected
30 to a passageway for move-ent of electrolyte away from the
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submodule to the sump via exit line 28. In order ta prevent
parasitic losses during tXe charging of the stack and to
decrease the short circuiting that could possibly occur,
the electrolyte passes down the conduit 26 thrdugh a pair
! of opposed serpentina like channels, best shown in Fig. 3
¦ as channel 30 and 32 respectively with flow in the direction
¦ of the arrows.
The most preferred embodiment is that an electrolyte
is flowing through and past the electrodes during the
electrolytic reaction. To provide for the flowing
~ electrolyte, an electrolyte distribution manifold 34 is
I provided for each submodule. The electrolyte flows from
the sump 12 out exit port 36 and is pumped back to the
submodule.
A sectional view showing a portion of a stack of
electrodes with a porous carbonaceous electrode, which,
in the most preferred embodiment, as the chlorine electrode
- of a ~inc chlorine electrical energy storage device, is shown
in Fig. 4. The submodule, which is a stack 18 of ten cells
! 20 is inserted into the interior 35 of the submodule tray 22
wherein the electrolyte distribution manifold 34 would be
joined with the submodule tray by pOSitioniDg the manifold
into channels 38.
The porous chlorine electrode 40 is arranged such
that a pair of porous carbon plates 40a and 40b are joined
together forming a cavity ~1 to allow passage of eiectrolyte
therethrough as shown by arrows 42. Gas venting holes (not
shown) may be provided at the top of the porous chlorine
electrode. The tops of three chlorine electrodes are shown
in the right side of Fig. 4 while the remaining portion of
Fig. 4 lS a sectional view. To prevent distortion of the !
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porous chlorine electrodes, stub 44 is present in the middle
of the chlorine electrode to give strength thereto. The ` -
porous chlorine electrodes are manufactured to have an indented
portion 46, in which the electrolyte feed tube 48 may be
inserted. The electrolyte feed tube in turn is connected
to the internal electrolyte distribution manifold at point
50. The electrolyte distribution manifold is comprised of
a pair of complementary members 52 and 54 whiCh are fastened
together by nuts 56 and bolts 58.
A bipolar intermediate bus 60 is machined to receive
the chlorine electrodes at points 62 and 64, while adjacent
thereto is the metal or zinc electrode 68 which fits into
the intermediate bipolar bus at point 70. To prevent short
circuiting, to insure tight fit, to control discharge rates
of chlorine electrode, and to control the edge effects
thereof, spacers 72 and 74 join together the chlorine and
zinc electrodes which are arranged in bipolar fashion. ~he i,
masking or screening effect is performed by spacers 72 and 74.
In operation the electrolyte is flowed from the
sump 12 through external manifold 80 into i~terior manifold
82 which is a conduit which is connected to the electrolyte
distribution manifo~d at point 84. From the electrolyte
distribution manifold, the electrolyte is passed through
tubes 48 whereby the electrolyte exits from the tube at the
bottom of the halogen electrode at point 83 and the electrolyte
flows through the porous electrodes up the interceil spacing
84 into drain cup 24 down the exit conduit 26, into channels
30 and 32 as described above and out the exit 28 back to
the sump.
The separation between the porous halogen electrode
and the metal electrode ranges from about 40 to about 250 ,
mils, preferably 80 mils (2 mm).
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The differential masking of the present invention
is graphically shown in Fig. 8. The porous electrode is
comprised of two elements lOOa and lOOb which are normally
both comprised of a porous structure joined together at top
tnot shown) and bottom. Fig. 8 shows a "W" shaped ~lement \~
whereby the elements lOOa and lOOb fit within grooves 102a
and 102b respectively formed from an inert plastic as
Kynar (trademark of Penwalt Company for a fluoroplastic).
The porous electrode of Fig. 8 is similar to the porous
electrode of Fig. ~. Electrolyte distribution inlet 106
functions as electrolyte feed`tube 48 of Fig. 6. ~or ease
of distribution of electrolyte an inlet channel 108 is formed
between members 110 and 112. The electrolyte flows from
the sump 12 down distribution inlet 106 to near the base.
of the porous electrode, out channel 108 and fills cavity
114 and then passes through porous electrodes lOOa and lOOb,
first through internal faces 120a and 120b respectively and
out exterior faces 122a and 122b.
During operation (charge and discharge) of an EESD,
the longitudinal faces 122a and 122b are blocked by an
external mask 124a and 12~b which physically covers the
longitudinal (exterior) electrode face opposite the counter -
electrode68. The lnternal masX 126 also physically blocks
the interior faces of the porous electrode. A differential
in physical screening or masking of the (external)
longitudinal face versus the (internal) interior faces is
maintained such that the height of the external mask
(measured from the base of the porous electrode 128a or
128b to the top of external mask 130a or 130b, respectively~
is much less than the height of the internal mas~ (measured
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from the base of the porous electrode 128a or 128b to the .
top of the internal mask 132). The aifferential between
the interior screen or mask and the exterior mask ranges
from about 0.05" tl.27 mm) to about 0,3" (7.62 mm)~
preferably 0.18" (4.57 mm).
Spacers 72 and 74 perform the same function on the
sides of the electrodes shown in Figs. 4 and 6 as the
internal and external screen or mask at the base of the
porous electrode of Fig. 8.
It is to be appreciated that the cells and
submodule described herein can be combined in series or
parallel relationship as is well known in the art.
Any means for storing and/or charging any oxidant can
be used. The storage compartment 25 is connected to line 16
for operation during charging or during discharge of a
primary or secondary (electrically rechargeable) EESD via
line 23.
~ In the most preferred embodiment, chlorine formed
during charging of a zinc chlorine battery with an agueous
zinc chloride electrolyte is converted to chlorine hydrate.
The hydrate is then stored and is available for discharge
by decomposing the chlorine hydrate to chlorine and water,
The halogen hydrate formation apparatus necessary
for forming and storing the halogen hydrate during the
charging and aischarging of the electrical energy storage
device is assembled to the remaining apparatus of Fig. 1.
Any conventional equipment may ~e used such as that
described in ~'i.S. 3,713,888; 3,823,036; or Electric Power
Research Insti~ute and Department of Energy reports discussed
supra. -
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Having described the invention in general, listedbelow are preferred embodiments where all temperatures are
in degrees Centigrade and all parts are parts by weight
unless otherwise indicated.
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Example l
A Kynar (trademark of Pennwalt Company for a
fluoroplastic material) electrode assembly was machined to
the configuration of Fig. 8 incorporating various degrees
of differential masking in order to evaluate their effective-
.lO ness in controlling the discharge edge activity in a zinc
chlorine chlorine hydrate EESD. The evaluation was perf~rmed
in a test cell consisting of two pairs of mechanically framed
chlorine electrodes (4 in. X 2.65 in. X 0.080 in.) and three
~inc electrodes ~4 in. X 2.745 in X 0.390 in.). The exposed
apparent area for each chlorine electrode after framing
(longitudinal face) is calculated to be 61.3 cm2 (245.2 cm2
per cell). l'he exposed apparent area for each zinc electrode
is calculated to be 65.9 cm2 per face. Two porous graphite
electrodes (Union Carbide PG-60) were inserted into the Kynar I .
frame. The cavity between the longitudinal (exterior) faces
of the chlorine electrode is 0.08 in. The temperature of
the electrolyte was controlled by circulating the electrolyte
through a titanium coil immersed in a constant temperature
water bath and held at a temperature of 30C + O.5~C. The
volume of electrolyte used was approximately 800 milliliters.
In the charge mode, chlorine gas produced electrochemically
was vented from the sump. In the discharge mode, the
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required chlorine gas was fed to the sump via a gas dispersion ``~
tube from a chlorine gas cylinder.
Both the charge and discharge processes were operated
under constant current. Cell voltage was measured using two
voltage probes, separate from the current carrying terminal
located at the top of the chlorine and zinc bus bars. The
operating conditions are as follows:
TABLE I
Charge: 5 hrs at 27 mA/cm ~i.e. 6.62 amp)
10 Discharge: to 0 volt at 40 mA~cm2 ~i.e. ~.8 amp)
Chlorine Electrode 2
Area 245.2 cm
Electrolyte: Before charge: 25% ZnC12 12.3M) pH: 0.18
Flow rate: 2 ml/cm2/min
C12 concentration: approxmately 2 g/~
The external shoulder (mas~) size was held constant
at 0.05 in. ~mechanical masking on longitudinal face of the
chlorine electrode) while the size of the internal shoulder
(mask) was varied to obtain the various differential mask
sizes ~interior face). To determine the effectiveness of
varying the internal and external mechanical screening or
mask, the internal mask had an increase in size over the
external mask of 0.05 in~ 11.27 mm), 0.09 in. 12.29 mm),
0.20 in. t5.08 ~m) and 0.45 in. (11.43 mm). All tests we~e
conducted with the same electrodes unaer the same operating
conditions. The effect of differential masking on the
charge profi'e was negligible except to the extent that a
good uniform smooth deposit of zinc was obtained. Most
ignific~..Lly were the losses in zinc area coverage at the
30 various discharge steps as is shown below in Table 2.
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TABLE 2
Effect of Differentail Maskins On
The Area Loss of Zinc Coverage
Area Loss of Zinc Coverage (~)
Mask (Inches) at Discharge Depth of
Differential Internal External 50% 75% g
0 05 0.10 0.05 5 12 46
o.o9 0.14 0.05 3 8.25 --
0.20 0.25 0.05 3 4 13
Observation of the zinc metal during various stages
of discharge is quite signigicant. At 50% depth of discharge,
a patch-type zinc plate had àlready developed. The size and
shape of the zinc patch was similar for all differential
mas~ sizes evaluated. At this stage of discharge, the top
edge plate started baring of zinc, averaging 3% loss of zinc
area.
At 75% depth of discharge, the size and shape change
of the zinc deposits h~d become more significant. The decrease
in area covèrage of zinc was 12% for 0.05 in. differential
mask, 8.25% for the 0.09 in. differential mask and 4% for
the 0.20 in. differential mask. It is 5een that the difference
in shape between 50% and 75% depth of discharge was relatively
small for 0.20 in. differential mask, but significantly large
for 0.05 in. differential masX.
--- At 90~ depth of discharge, a very well defined zinc
patch had developed, the decrease in area coverage of zinc
being 46~ for 0.05 in. differential mask as compared to 13%
for 0.20 in. differential mask. At this stage of discharge,
the area coverage of zinc for 0.20 in. differential mask is
still considered to be satisfactory.
In the case of the 0.45 in. differential mask, the
graphite substrate at about 90% depth of discharge showed
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a reverse'shaped patch. The centex portion was bare o~
zinc implying an over-mask effect.
' While applicant does not wish to be held to any
theory, it is believed that with a porous electrode, i.e., a
flow-through mode of operation, a portion of the chlorine
electrode surface, behind the physical eXternal mask, is
participating in chlorine reduction resulting in localized
increased current along the external mask edges which causes
ar. increase in the rate of anodic dissolution at the edges
of the zinc electrode.
Increasing the size of the differential mask decreases the
usable area behind the masks and compensates for the otherwise
higher edge activity on discharge. This is reflected in all
three of the experimental criteria selected for evaluating
the differential masking approach to controlling edge
activity on discharge. As can be seen from the above
example, although the 0.45 in. differential mask size
displayed a satisfactorily flat discharge profile, its
average discharge voltage and coulombic efi'iciency were
low. An over-mask effect was confirmed by visual inspection
of the zinc deposit near the end of the discharge. The
differential mask size of 0.20 in. was the most effective
for retaining the'shape of the zinc deposit near the end
of the discharge and at the same time giving a satisfactory
discharge profile.
It is to be appreciated that the physical mask can
be manufactured in any practical means, such as injection
molding the fluoroplastic Kyna~ or similar inert materials
as polyvinyl chloride or polyester resins. ''
It is to be appréciated that Fig. 8 shows the
30 masking to have been located at the base of the porous j~
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electrode. It shbuld be appreciated that the physical
masking may be on the side of the oxidant electrode as in
Fig. ~ or at the top of the oxidant electrode, depending upon
how one wishes to insert the oxidant into the porous electrode.
Alternatively, the internal screening or masX may be on all
sides of the porous oxidant electrode depending on the oxidant
employed and the utilization of a flowing electrolyte. The
masXing may also take the form of a coating of an inert
substance onto the interior and longitudinal (exterior) faces
of the porous electrode.
