Note: Descriptions are shown in the official language in which they were submitted.
2~
--1--
DUAL POROSITY ELECTRODE FOR ELECTROLYTIC CELL
Gas electrodes, in which a gas is passed in
contact with a suitable electrode conductor in the
pxesence of an electrolyte solution are well known.
In typical utilizations, gas electrodes
function in systems capable of generating electricity
(such as fuel cells) or for electrolysis purposes in
which the electrode performs as a depolarized cathode
(such as in chlor-alkali cells). Gas electrodes imple-
ment electrochemical reactions involving the inter-
action with and between three individual phases of a
gas, a liquid electrolyte and electrons provided directly
from a solid conductor surface, all of which are in
simultaneous mutual contact in order to accomplish
desired results. So that, with and for given unit
geometric volumes of the electrode maximization can
be realized of the available surface area on which the
three-phase contact takes place (to obtain greater
current density with the given units), modern gas
electrodes are made to be porous. Because, the reaction
is believed to take place on the interior interstitial
surfaces of the porous electrode, it is important that
the three phase contact area for the reaction be kept
in a stable and at least relatively precise location.
26,725-F
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--2--
.:
. ~.
The means so far developed for localizing the
site of the three-phase reaction within the passageways
of porous electrodes have included onc of the three
e~b~e ways of so doing, namely:
.,,
(A). To treat the pore interiors on the
gas side of the electrode with a material
such as "Teflon~", a fluorinated ethylene
; polymer, which is not wetted by the electro-
lyte so that the electrolyte is prevented
' 10 from penetrating entirely through the elec-
3' trode.
,;
(B). To maintain the desired regional
three-phase contact by careful balance
between gas pressure exerted and capil-
lary pressure generated by the electrolyte
, which is possible by use of a small sized
electrode having a uniform and consistent
porosity. Thus, the porous metallic
electrode is fabricated so as to have a
:t, 20 narrow distribution of pore sizes.
.,
t: (C). To use a dual porosity structure for
a small size electrode wherein the layer
facing the electrolyte has smaller pores
than those in the adjacent, complementary
layer. With this construction, it is possible
to apply a gas pressure through the larger
'',t ~ pored layer that is greater than the median
electrolyte capillary pressure in the large
pores but smaller than that in the small
pore layer so as to maintain the three-phase
contact sector within the interstitial
, 26,725-F -2
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:
. . .
,
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--3--
.
passageways at least approximately in the vicinity
of the joinder boundary of khe layers. This dual
electrade construction is easier to make than the
procedure described in Paragraph ~B) since a
narrow pore size distribution is di~ficult to
manufacture compared to dual porosity layers in
the electrode.
Various aspects relevant to the use of gas
electrodes in galvanic and electrolysis mode applications,
including oxygen depolarized cathodes in electrolytic cells,
are taught in U.S. Patent Nos. 1,474,594; 2,273,795;
2,680,884; 3,035,998; 3,117,034; 3,117,066; 3,262,868;
3,27~,911; 3,316,167; 3,377,265; 3,507,701; 3,544,378;
3,645,796; 3,660,255; 3,711,388; 3,711,396; 3,767,542;
3,864,236; 3,923,628; 3,926,769; 3,935,027; 3,959,112;
3,965,592; 4,035,254; 4,035,255; and 4,086,155; and Canadian
Patent No. 700,933. A good description of dual porosity
electrodes for fuel cell usage is set forth at pages 53-55
of "Fuel Cells" by G. J. Young (Reinhold Publishing Company,
N.Y., 1960).
Considerable difficulties are involved in the
utilization for large-scale, commercial manufacturing
purposes of dual porosity gas electrodes. A significant
problem is the frequent occurrence of bubbling or leak-
ing of reactant gas under full electrolyte restrainingpressure through at least the upwardly disposed elec-
trolyte facing portions of a vertically emplaced elec-
trode. In many commercial installations, the electrolyte
is often contained in electrolytic cells which are fre-
quently more than 4 feet (1.2 met.) deep. With a liquidhead of such magnitude, the catholyte exerts a substantial
hydraulic pressure of at least 1 psig and often on the
~, ~
26,725-F -3-
.
.
~6~ 6
-4-
order of 2~3 psig (0.6g to 1.38-2.07 dynes/cm2). In
other words, ~all and massive electrodes introduce a
new and important factor wlth which to contend; this
being the non-inconseguential liquid pressure effect
particularly on the bottom portion of an electrode due
to the high static head of the electrolyte in the cell.
If the gas pressure is reduced to avoid bubbling through
the upper portion of the electrode, the increasingly
pressurized liquid at the lower portion of the elec`trode
overcomes the pressure of the applied gas. The electrolyte
will then invariably leak through the pores in the lower
area of the electrode causing other major problems such
as electrolyte seepage loss into the bottom of the gas
chamber into the gas supply system. Such leakage con-
siderably diminishes the effectiveness and productivityof the cell. Not only does leakage of the electrolyte
materially interfere with the cell efficiency (since
the cell loses the advantageous electrochemical and
reduced voltage by reducing the electrolytic reaction
in a desirable stable interstitial area), but it also
results in the escape of reaction gas which is either
totally lost or, if collected, must be handled through
recovery and reprocessing units for subsequent re-use.
In any event, leakage to an appreciable extent increases
the cost of the operation.
The heretofore known dual porosity electrodes
are of relatively small size of less than about 18 inches
(45.7 centimeters) in height when the electrodes are in
a vertical position. In such short cells, the hydraulic
electrolyte pressure heads are negligible and of no
practical concern insofar as is relevant to gas leakage
and associated problems. Rarely does the hydrostatic
pressure head in such cells approach a 1 psig value.
26,725-F -4-
:
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--5--
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In fact, small size electrodes in use today are not
hampered by bubbling or leakage problems. Thus, there
has been no prior art disclosure addressing itself to
the problem of bubbling and/or leaking in dual porosity
~; 5 electrodes of relatively large size.
. . .
Particularly, the present invention resides
~ in an electrode design that will give unexpected results
.~ when used in an electrolytic cell. More specifically,
; the electrode of the invention is more than a mere
, 10 variation in its physical dimensions.
....
By way of comparison, although the electrode
of U.S. Patent No. 4,086,155 appears similar, it is
quite different, both in construction and in operation,
from the electrode of the present invention.
One of the most significant differences between
the two types of electrodes is the relative sizes of
the pores. U.S. Patent No. 4,086,155 teaches that the
ratio of the pore radius of the coarse pores (R) to the
'~ radius of the fine pores (r~ must be between 10 and 100
20 (R/r = 10 to 100). Conversely, the present invention
teaches a wholly different ratio of pore radii. In
` calculating the R/r ratio of the present invention, the
following results are obtained:
:~ R, microns r, microns R/r
254.0 2.6 1.5
; 4.5 2.9 1.6
5.0 3.0 1.6
~, 5.5 3.2 1.7
6.0 3.4 1.8
,.,j
:
;,.
~:,
26,725-F -5~
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.,
:.
~ . ,
.,
....
i.:'
,~
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-6-
Accordingly, the electrode of the present invention is
~uite different from the electrode of U.S~ Patent No.
4,086,155 which teaches an R/r ratio of 10 to 100,
while the ratio of R/r in the present invention may
generally be of a magnitude of 2:1.
The significant difference in pore ratios
between the two types of electrodes causes the~ to
operate in completely different ways. The electrode
of U.S. Patent No. 4,086,155 operates completely filled
with the electrolyte, while the electrode of the inven-
tion operates where it is only partially filled with
the electrolyte. In the electrode of U.S. Patent No.
4,086,155, the electrolytic reaction occurs at the
electrode surface, while in the electrode of the pres-
ent invention, the reaction occurs within or inside of
the electrode.
r
Another significant difference between the
electrodes of U.S. Patent No. 4,086,155 and the elec-
trode of the present invention is the need for a water
repellent coating. When the electrode of U.S. Patent
No. 4,086,155 is used in an electrolytic cell in which
;! one face of the electrode is adjacent to a liquid elec-
trolyte an~ another face is adjacent to a gas containing
chamber, -the electrode face adjacent to the gas must be
; coated with a layer of a material which is permeable to
the gas, but which is not wettable by the electrolyte.
~ Since both the coarse pore layer and the fine pore layer
'$ of the electrode in U.S. Patent 4,086,155 are completely
filled with electrolyte during operation of the electrode,
it is clear that this prior art system depends upon the
, non-wetting layer to prevent passage of the electrolyte
j~ into the gas containing chamber. Conversely, such a
coating is not needed in the electrode of the present
. ~
26,725-F -6-
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--7
.,
.,
invention which does not depend upon a coating to pre-
} vent electrolyte flow through the electrode. Rather,
the electrode of the instant invention is constructed
in a manner so that the capillary effect of the pores
5 ~r-evé ~ electrolyte from flowing through the electrode.
Another significant difference between the
electrodes is that the face of the electrode of U.S.
Patent No. 4,086,155, which contacts the electrolyte,
is coated with a porous layer of a refractory oxide
10 such as zirconium oxide, magnesium oxide, aluminium
oxide, thorium oxide, titanium oxide or of a mixture
of at least two of these oxides. These oxide coatings
are non conductive, and the '155 patent does not suggest
that other materials can be used to form the fine porous
~; 15 coating of ~ electrode. Conversely, the present inven-
, tion teaches the use of any metal or fine particulate
material to form not only the fine porous coating, but
the entire electrode. Moreover, the present invention is
not restricted to the use of a non-conductive coating.
20 Therefore, the present invention allows for a much
< greater flexibility in selecting materials of con-
struction when preparing the electrode since the
invention is not limited to using only refractory
oxides to form a fine porous coating.
. . .
Accordingly, the present invention particularly
pertains to and resides in the general field of electro-
, ~ chemistry and is more particularly applicable to an
improved gas-bearing, particularly an oxygen gas-bearing,
electrode with a dual porosity body structure of a large
constructional size. The electrode is preferably posi-
tioned vertically in a cell and functions without leak-
; age or bubbling in a relatively deep supply portion of
:::
.,
.,.
- 26,725-F -7-
~9~26
--8--
the contacting electrolyte which exerts a substantial,
downwardly-increasing, hydraulic pressure against the
electrode body.
The contemplated large size dual porosity
electrode is adapted to operate in a large capacity
cell with substantially greater electrochemical and
power efficiencies and with a stable three-phase re-
action region ensured within the electrode. The electrode
of the inve~tion avoids the disadvantage and difficulty
of trying to balance the pressure between gas and liquid
phases within a porous electrode or of wet-proofing
portions of electrode pores to prevent the passage of gas
or liquid therethrough. A large size dual porosity
electrode for large electrolytic cells is one of the
principal aims and objectives of the invention.
An improved, dual porosity gas bearing electrode
has been developed. The electrode comprises a composite
electroconductive multi-layered foraminous body of gen-
erally flat and wall-like configuration having a relatively
tall configuration. Thus, the height of the electrode
of the present invention is substantially greater than
heretofore possible. Two distinct, contiguously juxta-
positioned and adjoining porous layer sections of dif-
fering poroslty distinguish the electrode of the present
invention. Specifically, the first of said layer sections
is intended for electrolyte contact and is provided with
a plurality of relatively fine, micro-sized, pore-like
1uid mass transferring and transmitting interstitial
passageways. The second of said layer sections is in-
tended for gas contact and is provided with a pluralityof relatively coarse (as compared to the passageways
in said first layer) micro-sized, pore-like fluid mass
'
26,725-F -8-
~6~6
g
transferring and transmitting interstitial passageways.
At least a substantial majority of the interstitial pas-
sageways in each of said porous layers is in network
communication with each other so as to provide complete
passageways which traverse through the entire wall thick
ness of said electrode body. The relatively fine and
coarse pores in the electrode body-traversing intercon-
nected interstitial passageway network have a capillary
pressure effect, functionally dependent upon the fluid-
-constricting cross-sectional area of the porous passage-
ways in the network, upon and against a fluid when the
same is being forced thereinto under pressure. The capil-
lary effect of the passageway network is of a magnitude
such that gas under a given pressure of at least about 1
pisg is permitted ingress into at least the coarse pores
in said second layer but is constrained from complete
passage through said composite electrode body when said
electrode is positioned between the electrolyte and a
gas plenum in the electrolytic cell.
More specifically, the present invention resides
in a dual porosity electrode adapted for use in an elec-
trolytic cell comprising a composite electroconductive
foraminous body of a wall-like configuration having two
distinct, and adjoining porous layers wherein a first
layer of said electrode has a plurality of fine passage-
ways and, when in use, is positioned to face an area of
the cell which contains a li~uid electrolyte solution,
wherein a second layer of the electrode has a plurality
of coarse passageways and, when in use, is positioned to
face an area of the cell containing a gas which is under
a pressure insufficient to cause substantial bubbling of
the gas through the electrode but is at a pressure suffi-
cient to at least minimize seepage of the electrolyte
through the electrode and into the portion of the cell
containing that gas,
26,725-F -9-
' ~
~6~6
at least a substantial ma~ority of the passageways in
each of said porous layers being in communication with
one another so as to provide complete passageways
traversing through the overall wall thickness of said
electrode body, wherein the maximum hydraulic head
pressure created in the cel.l by said liquid electrolyte
solution is in excess of about 1 psig, wherein the gas
pressure is greater than the maximum hydraulic head
pressure created in the cell by the liquid electrolyte
solution, and wherein the height of the electrode is
at least about 3 feet (0.9 met) and the capillary
pressure effect for constraining gas passage through
the electrode is at least about 7.6 psig; the pore size
ratios in the electrode being selected to ensure that
the bubble point of the electrode throughout its vertical
elevation is larger at any given point then the sum of
the hydraulic head pressure, if any, of the electrolyte
and the liquid fluid-constraining capillary gas pressure
in the coarse pores, and wherein the dimension of the
average nominal radius o-E the interstitial passagew~ays
in the fine pore layer is from 0.05 to 1.5 microns and
the thickness of the fine pore layer is from 10 to 60 mils,
and wherein the dimension of the average nominal radius
of the interstitial passageways in the coarse pore layer
is from 4 to 6 microns and the thickness of the coarse
layer is from 20 to 90 mils.
The invention also resides in electrolytic
cells constructed with the improved dual porosity
electrode as an integral component thereof, as
well as in the method of operating such a cell.
26,725-F = 10 -
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69~ 6
The dual porosity, multiple layer gas electrode
of the present invention will become more readily apparent
and evident from thP ensuing descriptlon when considered
in conjunction with the accompanying Drawings, wherein
(using like reference numerals for like parts):
FIGURE 1 is a schematic view of a cell utilizing
a relatively tall, large scale electrode pursuant to
the invention; and
FIGURE 2 is an enlarged cross-sectional ele-
vational view o~ a cell in which the electrode is posi-
tioned.
With reference to FIGURE 1, there is shown an
electrolytic cell 3 which may be utilized for the pro-
duction of a halogen (such as chlorine) from a corre-
lS sponding acid (such as hydrogen chloride) or alkalimetal (such as sodium chloride). Preferably the cell
3 is used in the electrolysis of sodium chloride brine
into chlorine and sodium hydroxide.
The cell 3 includes an anode compartment 4
with an anode 5, at which the oxidation reaction occurs,
positioned therein. In spaced juxtaposition to the
anode compartment is a cathode compartment 12 having
a dual porosity cathode 13 at which the reduction re-
action occuFs.
25 The dual porosity elec~rode 13 is positioned
between and partitions the catholyte solution 14 in
the cathode compartment 12 from an oxygen-bearing gas
chamber 17 which is supplied with pressurized oxygen
from supply conduit 37. Cathode 13 has a first layer
26,725-F
6~6
-12-
or wall portion 43 containing a multiplicity of rela-
tively fine pores or small interstitlal passageways
44 which faces and is in contact with the catholyte 14
solution. Cathode 13 also has a contiguous second layer
or wall portion 45 containing a multlplicity of coarse
pores or large interstitial passageways 46 which is in
contact with the pressurized gas in chamber 17. ~t
least a substantial proportion or majority of the fine
pores 44 are in matching electrode body-traversing com-
munication with a substantial proportion or majority ofthe coarse pores 46 so as to provide a multipliclty of
continuous passageways through both contiguously adjoin~
ing electrode wall members 43 and 45. ~ny given coarse
pore 46 may connect with more than a single fine pore
44 in the resultant inte~connected pore network.
A diaphragm or ion-exchanging membrane or
screen mesh separator 10, consistent with well known
technology, is centrally positioned in the cell to
divide or separate anode compartment 4 from cathode
compartment 12.
Cell 3 comprises a housing having top and
bottom sections 31 and 32; side walls 33 and 34, and
front and back walls (not shown). Cell 3 further in-
cludes a source of sodium chloride brine ~not shown)
and a supply conduit 6 to feed the brine into the anode
compartment 4 and maintain the anolyte solution 7 at a
predetermined sodium chloride concentration. Gaseous
chlorine is removed from anode compartment 4 through
conduit 8.
26,725-F -12-
-13-
The dual porosity, depolarized cathode 13 is
spaced apart from side wall 33 of the cell 3 to form
the gas chamber 17. The oxidizing gas such as air,
oxygen-enriched air, oxygen, ozone i5 forced through
S inlet conduit 18 into, preferably, the upper portion
of the chamber 17 and passes in intimate contact with
an outer face or surface of the coarse pore-containing
wall portion 45 of the cathode 13. The oxidizing gas,
following the general flow pattern through compartment
17 depicted by the directional arrows 39 therein, is
then withdrawn from the gas chamber 17 through outlet
conduit 19 for disposal or recycle.
Depending on the nature of the particular
electrolyte(s) and anode employed in a system the
base material for both of the layers 43 and 45 of the
dual porosity cathode 13 may be either me~allic or non-
-metallic in nature. Carbon or graphite, especially
when provided with a catalytically active surface, is
often a suitable non-metallic base, while metals and
oxides thereof such as tantalum or titanium, copper,
various ferrous alloys and metals of the platinum group
includiny gold, iridium, nickel, osmium, rhodium, ru-
thenium, palladium, platinum, and silver (or compositions,
alloys and platings thereof) are useable. As an illu-
stration, a porous copper substrate that is silverplated is useable. The electrode body material must,
inherently or by treatment or modification (such as with
platings, coatings and so forth), be resistant to chemical
attack - at least during cell operation - from the con-
tacting oxyge~ and electrolyte material that is utillzed.
26,725-F -13-
;9~)~6
-14-
The electrode is most preferably catalytically
active to most effectively produce the desired oxygen
reduction in the presence of water within the three-
-phase regions of reaction inside (within) the dual
porosity interstitial passageways of the electrode.
Theoretically, the catalyst activity need only be on
the interior pore surfaces of the electrode body to
provide the desired effect. This allows for the
utilization o catalytic coatings, on the pore sur-
faces to provide the desired reaction-promoting cap-
ability of an electrode body that is not intrinsically
catalytic. While there are many workable catalyst
substances for various electrochemical reactions, the
mentioned platinum group metals and many of their com-
positions, especially the oxides, are useable. Silver
and gold are also good examples as well as nickel. The
latter, for reasons of availability, economy, desirable
physical characteristics and ready workability, is par-
ticularly desirable with or without a catalytic coating,
for electrode body construction. When a catalyst layer
or coating is utilized, it is preferably applied as a
very thin and substantially continuous deposit.
The porous layers 43 and 45 are in the form
of porous sintered or analogously compressed and inter-
bonded metal. Other powdered, fibrous or finely parti-
culated material is useable in the practice of the pre-
sent invention.
The anode is usually constructed in the form of
either a solid body or a foraminous, grid-like structure,
such as a screen. It usually is undesirable for the
anode to be constructed of a ferrous material, especially
26,725-F -14-
~69~Z6
-15-
where it is used in an acidic media. The anode may, for
example, also be constructed of a dimensionally stable
anode material comprised of a base member of a film-
-forming metal such as tantalum or titanium coated
with at least one metal or metal oxide of the platinum
group metals including the same coating materials
above-identified used for construction of the anode.
It may also be beneficial to utilize cir-
culating means (such as agitators, impellers, recircu-
latory pump installations, aerators or gas bubblers,
or ultrasonic vibrators to continuously circulate the
catholyte solution 14 to avoid stagnation thereof within
the cathode compartment 12. Circulation promotes thorough
cathode contact by substantially all of the catholyte.
The rate of catholyte circulation should be sufficient
to ensure adequate liquid contact o~ the cathode inter-
face without causing any physical injury to the sepa-
rator 10.
During cell operation, the catholyte 14 becomes
increasingly enriched with sodium hydoxide which can be
removed in regulated fashion to keep the caustic content
of the catholyte at a controlled, predetermined strength.
To this end, caustic-rich catholyte is withdrawn from
catholyte chamber 12 through outlet conduit 15.
If and when an ion exchange member is used
as a separator, make-up water is admitted ~coincident
with catholyte withdrawal for balancing the catholyte)
through inlet conduit 16.
26,725-F -15-
-16-
Cell operation can normally be still further
improved by regulated control of the catholyte head
(i.e., the difference, if any, between the upper liquid
s~faces of the anolyte and the catholyte). When an
S ion exchange membrane is used as the separator 10, it is
advantageous to have the surface of the catholyte at a
higher level than that of the anolyte surface. Prefe-
rably, this differential is between about 1 inch (2.54
cm.) and about 3 feet (O.9 met.). On the other hand,
when a flow-through diaphragm separator is employed,
the anolyte level should be higher than the catholyte
Level to facilitate maintenance of a liquid flow rate
through the diaphragm separator to keep the sodium
hydroxide concentration in the catholyte at a con-
15 stant value.
The electrical energy necessary to conductthe electrolysis in cell 3 is obtained from a D.C. power
source 20 connected by a cakle 22 to provide an
electrical current to the anode 5 and cathode 13.
The cathode 13, more clearly illustrated in
Fig. 2, consists of the distinct, yet contiguous, indi-
vidually apertured layers 43 and 45 which are fabricated
and composed as above explained. Due to the greater
physical streng~h of the fine pore layer 43, the thick-
ness of the layer can be less than that of the coarse
or large pore layer 45.
The fine and coarse pores can be described
as forming passageways which are complex, sinuous or
serpentine, winding, coiled or corkscrew-like in either
relatively regular and/or diversely volute ashion,
thick and thin cross-section, forked, or branch-tunneled
26,725-F -16-
~69~26
-17-
pattern. Thus, the indiv1dual pore lengths are seldom
of the same actual path length as the thickness of the
layer being penetrated and generally tending to be much
longer than the layer thickness itself.
As illustrated by arrow 25 in FIGURE 1 and the
downwardly incxeasingly larger, horizontally-directed
arrows 28, 29, 30, 31 and 32 in FIGURE 2, on the one
hand, the pressure head of the catholyte solution 14
progressively increases with depth. On the other hand, the
pressurized gas in chamber 17 is forced into the large
pores 46 of the cathode layer 45 at a relatively con-
stant pressure. In a relatively deep cell housing, the
gas pressure in the gas chamber 17 at the top of electrode
13 (where the catholyte head pressure is at or approaching
zero) is as great as at the electrode bottom ~where the
li~uid head pressure is greatest or approaching its maxi-
mum). The gas pressure would thus not prevent the
catholyte from seeping or leaking through the cathode 13
near the bottom thereof into the gas chamber 17. Con-
versely, gas bubbling or leaking through the cathode,is more likely to occur adjacent the top of the cathode.
Furthermore, it is disadvantageous to have a situation
where the opposing gas and liquid pressures are in
approximate balance in the central vertical portions of
the electrode, as in the vicinity of arrow 30, while
at the same time having the given constant gas pressure
as depicted by arrows 39 being excessive at the top (as
at arrow 28) so as to cause gas bubbling through upper
portions of the electrode while insufficient gas pres-
sure at the bottom (as at arrow 32) will permit liquidlea~age or seepage through lower portions of the elec-
trode.
26,725-F -17-
- ~69~6
-18-
At the same time, it is desirable to maintain
the three-phase reaction within the passageways of the
electrode in the vicinity of the wall sections stable.
This is illustrated by the somewhat exaggerated positions
of the respective menisci 50, 51, 52, 53 and 54 which,
in a descending order, are formed as the liquid/gas inter-
faces within the interconnected pores 44 and 46 at about
the boundary of layer sections 43 and 45. The loci of
the menisci is believed to proceed from within the pores
on layer 43 towards and into the coarse pores 46 as the
catholyte head pressure increases.
The pore size ratios in the electrode are
selected to insure that the bubble point of the electrode
throughou~ its vertical elevation be larger at any given
point that the sum of the hydraulic head pressure, lf
any, of the catholyte and the liquid fluid-constrain-
ing capillary gas pressure in the coarse pores.
The nominal diameter measure of the pores
44 capable of preventing gas bubbling or electrolyte
leakage through the cathode of the present invention,
(under maximum electrolyte head pressures that are
substantially greater that at least 1 psig) is selected
within a range of between about 0.1 and about 3 microns.
The thickness of the layer 43 is between about 10 mils
and about 60 mils (2.54 and 15.2 mm). The nominal coarse
pore 46 diameter in layer 45 is between about 8 and
about 12 microns with a layer thickness of between about
20 and about 90 mils (5.08 and 30.4 mm). More advan-
tageously in electrode bodies having a minimum height
of at least about 4 feet, the associated respective
nominal pore diameters a~d layer thickness are: for
the fine pore layer 43, a nominal pore diameter between
26,725-F -18-
~9~Z6`
--19--
about 1 and 3 microns with a layer thickness of about
15 to 35 mil (3.81 to 8.89 mm) and, for the coarse pore
layer 45, a nominal pore diameter between about 9 and
about 11 microns with a layer thickness of about 20 to
60 mil (5.08 to 15.24 mm).
As is apparent, the ratio of the total electrode
body thickness to the vertical height of the electrode
is usually very small. Accordingly, with a 4 feet tall
electrode, the vertical height of the structure is be-
tween minimums of about 1600 to about 320 - preferably
from about 1370 to about 500 times as high as is the
thickness of the complete, composite electrode body.
The ratio of the thickness between the fine pore layer
and the coarse pore layer may also vary over a wide
range depending upon the particularly structural char-
acteristics and strengths of cathode materials and
particular use applications. Typical ratios for a flne
pore layer thickness are from only about 1/9 of to 2/3
times the coarse pore layer thickness. In a 4 feet
tall electrode, taken as a reference standard, the ratio
is from about 1/4 to 3/4 times the fine pore layer thick-
ness relative to that of the coarse pore layer.
The bubble point pressure of dual porosity
electrodes that are of a height of at least about 4
feet should generally be at least abou~ 10 psig (6.9
dynes/cm2~. From this, it can be seen, for most cases,
the bubble point pressure should gradually increase
by about 2-1/2 psig i 10 percent per lineal added verti-
cal foot of incr~asing electrode height.
26,725-F -19-
26
-20-
By way of illustrat1on, a flat section of a
dual porosity electrode was prepared from a sintered,
powdered, nickel electrode material that was in the form
of a flat, 8 inch to a side (20.32 cm) square. The fine
pore layer was about 40 mils thick with nominal pore
slze diameters of about 1 micron. The coarse pore layer
had pores of nominal diameters of about 10 microns
in an approximate 50 mil thick layer. The electrode
structure, mounted in a"Plexiglas" (A) frame, was placed in
contact with a typical aqueous effluent from a chlor-
-alkali cell of about 100 gms/1 NaOH, about 175 gms/1
NaCl. Under these conditions, the bubble point of the
electrode was about 13 psig which is to say that a 13
psig differential gas pressure had to be applied to the
gas side of the electrode before bubbles would appear
on the liquid catholyte side. ~owever, only about 1-2
psig of gas pressure had to be applied in order to pre-
vent cell effluent from penetrating into the coarse
pore layer.
With the above electrode material contained
in a 72 inch (173 cm) high structure, the hydraulic head
pressure near the bottom of the electrode is about 3
psig. This adds directly to the capillary pressure
so that a gas pressure of about 5 psig would be required
to maintain the same gas/liquid pressure balance as with
the smaller electrode previously tested. At the top
of the 72 in h electrode, the hydraulic head pressure
was substantially zero. Thus, if the bubble point
there were not greater than about 5 psig, gas would
~0 bubble through. But, since the tested dual porosity
electrode material had a bubble point that was on the
order of 13 psig, no gas bubbling problem would be
encountered in a 72 inch high electrode made thereof
(A) a Trademark for thermoplastic poly(methyl methacrylate)
type polymers
26,725-F -20-
~6g~;~6
-21-
when an adequate gas pressure is applied to the coarse
pore layer of the cathode to prevent liquid leakage
through the bottom portion thereof.
Electrodes made in accordance with the teach-
ings of the present invention achieved reductions in
power requirements and cell voltage needs in large-
-scale, hiyh-volume, commercial cell installations in
which they were employed of at least one-third of that
necessary for comparable conventional cells.
26,725-F -21-
