CA1169026A - Massive dual porosity gas electrodes - Google Patents
Massive dual porosity gas electrodesInfo
- Publication number
- CA1169026A CA1169026A CA000345783A CA345783A CA1169026A CA 1169026 A CA1169026 A CA 1169026A CA 000345783 A CA000345783 A CA 000345783A CA 345783 A CA345783 A CA 345783A CA 1169026 A CA1169026 A CA 1169026A
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- CA
- Canada
- Prior art keywords
- electrode
- gas
- layer
- coarse
- pressure
- Prior art date
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
- Y02E60/30—Hydrogen technology
- Y02E60/50—Fuel cells
Abstract
ABSTRACT
A dual porosity gas electrode adapted for utilization as a vertically-disposed oxygen gas-bearing electrochemically reducing cathode in electrolytic cells. For efficient operation an electrolyte liquid pressure of greater than 1 psi (ca. 0.69 dynes/cm2) is employed. The electrode is comprised of distinct juxta-posed, contiguous yet separate porous electrode body wall members or layer sections, one of which is of a relatively fine porous structure and the other of which is of a relatively coarse porous structure. The electrode has a height of at least 3 feet and a bubble point pressure that is larger than the summation of the hydraulic head pressure and the liquid capillary pressure in the coarse porous layer.
A dual porosity gas electrode adapted for utilization as a vertically-disposed oxygen gas-bearing electrochemically reducing cathode in electrolytic cells. For efficient operation an electrolyte liquid pressure of greater than 1 psi (ca. 0.69 dynes/cm2) is employed. The electrode is comprised of distinct juxta-posed, contiguous yet separate porous electrode body wall members or layer sections, one of which is of a relatively fine porous structure and the other of which is of a relatively coarse porous structure. The electrode has a height of at least 3 feet and a bubble point pressure that is larger than the summation of the hydraulic head pressure and the liquid capillary pressure in the coarse porous layer.
Description
2~
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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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-
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
~. .
- ~69~Z6 `
.:
. ~.
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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.
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
,;
(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 `: ~
;,;
:
. . .
, :~ :
.
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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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
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-:
"` ~ 2~
;,`
;
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~
:``
'`, ., :.
~ . , ., ....
i.:' ,~
;.
~ 3~`~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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., ., 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 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 -~. ~
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 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-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 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 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-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 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 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`
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-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 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-
;,.
~:, 26,725-F -5~
:``
'`, ., :.
~ . , ., ....
i.:' ,~
;.
~ 3~`~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-, ~
~,.. .... ...
'' ~, ., .
i ~16~Z6 , . .
., ., 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 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 -~. ~
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 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-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 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 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-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 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 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`
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-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 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-
Claims (11)
1. A dual porosity electrode adapted for use in an electrolytic cell comprising a composite electro-conductive foraminous body of a wall-like con-figuration having two distinct, and adjoining porous layers wherein a first layer of said electrode has a plurality of fine passageways and, when in use, is positioned to face an area of the cell which contains a liquid electrolyte solution, wherein a second layer of the electrode has a plurality of coarse passage-ways 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 sufficient to at least minimize seepage of the electrolyte through the electrode and into the portion of the cell contain-ing that gas, at least a substantial majority of the passageways in each of said porous layers being in communication with one another so as to provide com-plete passageways traversing through the overall wall thickness of said electrode body, wherein the maximum hydraulic head pressure created in the cell 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 26,725-F
height of the electrode is at least about 3 feet (0.9 met) and the capillary pressure effect for con-straining 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 ele-vation 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 pres-sure in the coarse pores, and wherein the dimension of the average nominal radius of the interstitial passageways 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 pas-sageways in the coarse pore layer is from 4 to 6 microns and the thickness of the coarse layer is from 20 to 90 mils.
height of the electrode is at least about 3 feet (0.9 met) and the capillary pressure effect for con-straining 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 ele-vation 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 pres-sure in the coarse pores, and wherein the dimension of the average nominal radius of the interstitial passageways 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 pas-sageways in the coarse pore layer is from 4 to 6 microns and the thickness of the coarse layer is from 20 to 90 mils.
2. The electrode of Claim 1 wherein the capillary effect of said passageways is of a magnitude such that gas at a pressure of at least about 1 psig is permitted ingress into at least the coarse pores in said second layer but is constrained from complete passage through said composite electrode body wherein such passage would cause the gas to bubble thereout.
3. The electrode of Claim 1 wherein the height of the electrode is at least about 4 feet and the capillary pressure effect constraining gas passage through the electrode is at least about 10 psig.
26,725-F
26,725-F
4. The electrode of Claim 1, 2 or 3, wherein the ratio of the height of the electrode to the total electrode body thickness is between at least 320 to 1600 times the body thickness.
5. The electrode of Claim 1, 2 or 3, wherein the average nominal radius of said passageways in the coarse pore layer is about 5 microns.
6. The electrode of Claim 1, 2 or 3, wherein the bubble point pressure increases gradually per lineal foot of increasing electrode height by about 2-1/2 psig ? 10 percent.
7. The electrode of Claim 1 wherein the dimension of the average nominal radius of the inter-stitial passageways in the fine pore layer is from 0.5 to 1.5 microns and the thickness of the fine pore layer is from 15 to 35 mils, and wherein the dimension of the average nominal radius of the interstitial passageways in the coarse pore layer is from 4.5 to 5.5 microns and the thickness of the coarse layer is from 20 to 60 mils.
8. The electrode of Claim 7 wherein the thickness of the fine pore layer is from 1/9 to 2/3 times the thickness of the coarse pore layer.
9. The electrode of Claim 8 wherein the thickness of the fine pore layer is about 1/4 times the thickness of the coarse pore layer.
26,725-F
26,725-F
10. The electrode of Claim 1 wherein the electrode is comprised of a foraminous, metallic con-struction.
11. The electrode of Claim 1 wherein said porous electrode body is of a sintered metal particle construction.
26,725-F
26,725-F
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000345783A CA1169026A (en) | 1980-02-15 | 1980-02-15 | Massive dual porosity gas electrodes |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000345783A CA1169026A (en) | 1980-02-15 | 1980-02-15 | Massive dual porosity gas electrodes |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1169026A true CA1169026A (en) | 1984-06-12 |
Family
ID=4116253
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000345783A Expired CA1169026A (en) | 1980-02-15 | 1980-02-15 | Massive dual porosity gas electrodes |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1169026A (en) |
-
1980
- 1980-02-15 CA CA000345783A patent/CA1169026A/en not_active Expired
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