CA1136089A - Air electrode for electrolytic cell - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

An electrolytic cell has a housing, an acolyte and an anode within the housing and a cathode spaced from the anode with a portion of the cathode being adjacent to the anolyte. The cathode includes a chamber, an inlet and outlet to supply and remove air from the chamber, an outlet to discharge an alkali hydroxide solution from the chamber, and a multi-layer wall defining the boundary between the anolyte and the interior of the chamber including a separator material adjacent to the anolyte, a perforated electrically conductive supporting material adjacent to the interior of the chamber, and an electrocatalytically active material between the separator and the support.

Description

~1360~9 BACKGROUND OF THE INVENTION

Energy costs are a significant factor in the feasi-bility of the electrolytic production of chlorine and alkali.
The increasing costs of electrical energy will accentuate these considerations further. Technical developments in the chlor-alkali field therefore have as an objective the reduction of the energy consumption in the electrolytic process. One possibilit~ is to reduce the cell voltage by employing air cathodes so as to eliminate the energy con-10 - suming hydrogen development in the cathode fingers~, The , hydrogen developed in conventional electrolysers seldom finds a meaningful use at the chlor-alkali plants. The use of air cathodes will reduce the cell voltage depending on the current density, the temperature and the activity of the air electrode. This reduction of the cell voltage is of great importance in improving the economics of the chlor-alkali process. Several designs of chloro-alkali cells with air cathodes are described in the literature, ' see e.g., U.S. Patent 3,262,868.
Another more radical approach is to introduce a bi-functional hydrogen electrode at the same time in order to adjust the production of chlorine and alkali to the market demand with a minimum sacrifice of electrical energy.
See, for example, U.S. Patent 3,864,236.
A particularly advantageous design for air cathodes which is quite useful for bi-polar chlor-alkali cells is shown in the German Offenlegungsschrift 2627142.1-41.
Chlorine and alkali are produced on a very large scale in all industrialized countries and the amount of capital which has been invested in these chlor-alkali '~
i ~136089 plants is accordingly very large. The useful life of these plants is also quite long. It is not unusual that such plants last 20-30 years or even longer. However, it is necessary to renovate the cells at frequent intervals, change anodes, replace diaphragms etc. It has also been possible to develop existing cells which exhibit better performance e.~. by the introduction of so-called dimen-sionally stable anodes instead of graphite anodes in mer-cury cells as well as in disphragm cells. These different cell types are described e.g. in Xirk-Othmer "Encyclopedia of Chemical Technology", Second Edition, Volume l, pp.
668-707, J. S. Sconce "Chlorine" ACS monograph No. 154, 1962, and e.g. the U.S. patents 3,124,520, 3,262,868 and other publications and patents. Pertinent information may be found in the Proceedings from the Chlorine Bicenten-nial Symposium, ECS, 1974, and Hardie: Electrolytic Manu-facture of Chemicals from Salt" The Chlorine Institute 1975.
The development and introduction of air cathodes in chlor-alkali electrolysis has been concerned with the task of developing a completely new electrolytic cell e.g.
by means of bi-polar electrode designs. Much would be ! gained, however, if a design could be envisaged which would ; make it possible to introduce air cathodes into existing cells of diaphragm or membrane type with monopolar electrodes.
Such air electrodes could then be used with existing elec-trolytic plants and thus provide immediate conservation of electrical energy. The operating costs for the chlor-alkali process would also be reduced considerably with such a modification. It is fair to say that such an innovation would be even more important than the earlier introduction of dimensionally stable anodes.

ll36ass OB~ECTS AND SUMMARY OF T~IE INVENTION
One objective of the present invention is there-fore to enable existing chlor-alkali cells of diaphragm or membrane type with monopolar electrodes to be converted to air electrodes.
A second objective is to considerably reduce the consumption of electrical energy for electrolysis due to the fact that existing plants may be converted to air electrodes.
A third objective is to provide a design which makes possible simple renovation of air electrodes-on the same occasion as exchange of dimensionally stable anodes, membranes or diaphragms.
A fourth objective is to reduce the chloride con-tent in the alkali hydroxide solution.
A fifth objective is to increase the concentration of the product solution particularly with diaphragm cells.
Other objectives will be apparent from the follow-ing description of the invention.
The characteristic feature of the air electrodes for electrolysers according to the invention is that it forms a space which is separated from the surrounding outer anolyte, the surface of said space being covered with a separator consisting of an asbestos diaphragm or a cation permeable material; electrocatalytically active material, which is at least partly hydrophobic, disposed therein whereby the separator and the electrocatalytically active material are mechanically supported by an electri-cally conducting supporting structure which is furnished with means for supply and removal of air to a said space;
means for removal of the alkali hydroxide solution which is li36()~9 being formed at the reduction of oxygen in the presence of electrolyte which is moving into said space from the sur-rounding outer space through the separator; and means to bring air and alkali hydroxide solution in contact with the side of the electrocatalytically active material which is facing the interior of said space. Air as defined herein includes oxygen and oxygen enriched air.

113~089 BRIEF DESCRIPTION OF THE DRAWINGS
Figure la and lb show schematically the arrange-ment of the functional elements of a chlor-alkali cell with an air electrode according to the invention.
Figure 2 shows schematically a corresponding cell wall part for a similar cell with a conventional air electrode.
Figure 3 shows the functional design of a bi-polar electrolytic cell with air electrodes according to the invention.
Figure 4 shows how a conventional cathode with hydrogen development may be modified so as to serve as an air electrode according to the invention for a chlor-alkali cell.
Figure 5 shows an embodiment with a porous elec-trolyte holding body disposed in the interior of the air electrode.
Figure 6 shows another embodiment with an air electrode to be used with membrane cells whereby the electrode is sectioned in elements intended for air and : electrolyte.

113~ii089 DETAILED DESCRIPTION OF THE INVENTION
It should be noted that the principle of the air electrode of this invention differs in a very important respect from the state of art air electrodes for chlor-alkali cells; i.e. the electrolyte is also brought into contact with the back side of the air electrode (front side here means that side of the electrode which is facing the counter electrode, in this case the chlorine anode, back side is the other side of the electrode). Conventional air electrodes have during normal operation no electrolyte in contact with the back side of the electrode, see e.g.
U.S. Patents 3,864,236 and 3,262,868.
This new design principle provides a number of practical and process technical advantages. One such practical advantage is, of course, that the air electrode according to the invention makes possible a functioning ; chlor-alkali cell with air electrodes. These practical advantages should be apparent from the following description.
It has, however, also been found that the invention produces

2~ several very surprising process technical advantages compared to conventional air electrodes with the catholyte disposed on the front side of the air electrode and not on its back side according to the present invention. It is thus possible to produce a more concentrated alkali hydroxide solution with the embodiment for diaphragm cells which reduces steam requirements for the concentration operation. Another advantage of great importance is that the chloride concen-tration will be lower which is of particular importance for the diaphragm cells.
The very low energy consumption, the high alkali concentration and the low chloride concentration are factors of outmost importance for the economics of the chlor-alkali ~13~i089 electrolysis. The present invention has in common with several other inventions in the chlor-alkali field, like dimensionally stable anodes, dimensionally stable dia-phragms and efficient membranes, constructive simplicity combined with very high technical efficiency. The inven-tion meets the objectives which were formulated above in every respect. The invention shall now be described by means of a few examples. The air electrode can be intro-duced along three routes:
(1) Addition to existing diaphragm and membrane cells with a minimum of constructive-modi-fication.
(2) Radical modi~ication of the cathode support-ing parts of the cell including the cathode element at existing diaphragm and membrane cells.

(3) Completely new design of the whole electro-lyser so as to get an optimized embodiment of the invention.
The following description is mainly concerned with the design of the air electrode of the present invention.
The state of the art in chlor-alkali technology is well described in the standard references referred to above.
Present designs of diaphragm and membrane cells are des-cribed in printed material published by the leading com-panies in the field, e.g. Hooker Chemicals and Diamond Shamrock. A detailed flow-sheet for a modern chlor-alkali plant with dlaphragm cells has been produced by Diacell AB in Gavle, Sweden, 1977. The artisan will experience little difficulty in the introduction and use of the electrodes according to this invention for chlor-alkali electrolysis.

" 113~089 * It should, however, be noted that conversion from hydrogen development to oxygen reduction will involve a few simple changes which can easily be carried out by the artisan. The sensible heat in the hydrogen gas stream is frequently recovered for the evaporation of the alkali - hydroxide solution. The sensible heat in the warm air stream leaving the cathode spaces may be utilized in the same way. Sometimes hydrogen is used as a fuel in boilers for production of process steam. These steam generators must, of course, operate on a different fuel in the future.
Piping which has been installed in an existing plant for the hydrogen system may be kept to be used for the air system after change from hydrogen development to oxygen reduction.
The requirement for air, oxygen enriched air, or oxygen for the oxygen reduction of course depends on the oxygen concentration in the supplied gas. If pure oxygen is used the supply will amount to about half of the earlier hydro-gen flow on a volume basis in view of the stoichiometrics of the reaction. Inert components present in the oxygen may, in this case, be vented off periodically to prevent the concentration of these inert components, such as argon and nitrogen, from increasing in the cathode space.
When operating on air it may frequently be suitable with an excess corresponding to the double oxygen requirement, whereby the supply of air will amount to about five times the corresponding flow of hydrogen in the hydrogen mode of operation. In this case about half of the oxygen supplied will be consumed in the electrode reaction whereas remaining oxygen will leave with the outgoing air. It may sometimes be of advantage with preheat and moistening of the air supply. Sometimes one may also have use for the 113~i0~9 cooling and drying effect of the supply of cool and dry air to the cathode space. It may also sometimes be of some advantage to recirculate air back to the cathode spaces after enrichment with fresh air, oxygen enriched air or oxygen. Such recirculation may also be of advantage to reduce the uptake of carbon dioxide by the alkali hydro-xide solution. All of these questions are to be viewed as practical questions of optimization which can be easily decided by the artisan on a case by case basis depending on the operating conditions in question, desired product quality, etc.
The carbon dioxide content of the air is a separate consideration. The carbon dioxide is taken up by the alkali hydroxide solution and causes an increased content of carbonate in the electrolyte. In certain applications it is desirable to minimize the carbonate concentration and it is then necessary to first remove the carbon dioxide in the air in a special scrubber where the air is scrubbed pre~erably with an alkali hydroxide solution which is then i,, 2~ decarbonized in a known manner, e.g. by means of electro-I dialys or causticising, etc.
, The design of the system for air supply does i not present any problems for the artisan which is apparent from the description above.
The change from hydrogen development to oxygen reduction at an existing plant requires a special procedure for the change-over which has to be determined on a case by case basis depending on the extent of cell modification.
It is frequently desired to carry out the change-over step by step without disturbing the production and further-more it is desired to use the facilities which are available for cell maintenance. It is then useful to utilize mobile 113~089 aggregates for individual air supply to a cell unit. After a cell has been rebuilt it will be put back in its place in the cell hall and connected to the system excluding the pipe for outgoing hydrogen. The air supply is then connected whereafter the cell will run on oxygen reduction with no other interference with the system. In this way one may successfully modify a certain number of cells and then join this group to the common air system. When a sufficient number of cells have been converted the common hydrogen system may be disconnected. Of course one may also follow other policies, e.g. complete stop of production during the change-over period or connection of the coverted cells to the new common air system already from the be-ginning. The much lower cell voltage with chlor cells with air cathodes also requires either modification of the elec-trical supply system or expansion of the cell hall so that the available system voltage will be used fully. In this way improved economy of operation may be combined with capacity increase.
;, 20 The active materials in a chlor-alkali cell have limited life and it is therefore necessary to remove a cell from time to time for renovation or a change of e.g.
diaphragm. The electrocatalytically active material in the air electrode also has a limited life. It is therefore use-ful to choose materials and operating conditions so that exchange or reactivation of the electrocatalytically active material may take place at the same time as other operations of maintenance. Of course, it is of particular advantage to regenerate the electrocatalytically active material simultaneously with regeneration or exchange of the dia-phragm or membrane respectively. It is, of course, also important that the air electrode be designed in such a way 113~i0~}9 that the electrocatalytically active material can be e~sily regenerated or exchanged. It is particularly useful to apply this material on the supporting structure by spraying, painting, dipping, electrophoretic precipitation or in other ways without use of mechanical operations.
The materials which are used in air electrodes according to the invention are presently used in the chlor-alkali technology, the fuel cell technology, metal air batterytechnology, etc. As separator materials there may be used diaphragms or membranes of the type now being used in chlor-alkali cells. Different kinds of diaphragms are described in U.S. patents 3,694,281; 3,723,264 and others.
Also other types of diaphragms or membranes for chlor-alkali cells may be used.
Publications pertaining to so-called Nafion membranes are found e.g. in the Proceedings from the Elec-trochemical Society's meeting in Georgia, October 9-14, 1977, pp. 1135-1150.
The electrocatalytically active material contains catalysts for oxygen reduction of kno~m type on the basis of active carbon, silver basis, metal oxides containing nickel and cobalt, so-called perovskite- and spinel structures and of course noble metal catalysts. These catalysts, con-taining conducting additives in the form of carbon, graphite~, nickel powder and structure stabilizing additives like carbides, nitrides, etc. are bonded together to a porous structure of thin thickness (frequently a few tenths of a -millimeter) preferably by means of sintered particles of Teflon. This will at the same time give the desired hydro-phobic property to improve the air contact. This technology is now established above all by progress that has been made in the fuel cell field. Reference is made to the Sweaish B * Trade Mark , li36C~89 patent application 5742/72, Brighton Power Sources Sym-posium No. 6: Paper No. 36 and 37, siemens ser. 5 (1976) No. 5, 266--271.
The mechanically supporting structure may be designed according to designs which have been developed for cathode fingers, see e.g. U.S. Patent 2,897,463. The supporting structure can be manufactured using nickel-coated carbon steel or other combinations of materials which are resistant in the alkaline environment at the electrode potential for oxygen reduction in question. If the dia-phragm is fabricated in a known manner by dipping the structure in a slurry of asbestos fibre whereafter vacuum is put on the interior of the air electrode, the structure must of course be furnished with an interior support to take up the outside pressure. These interior supports are advanta-geously designed so that they simultaneously serve as baffles to bring supplied air in contact with the electrocatalyti-cally active material disposed on the walls of the inner space.
Figure lA depicts schematically the functional design of a chlor-alkali cell with an air electrode accord-ing to the invention. For the sake of simplicity only a single cell element is shown containing a so-called dimen-sionally stable chlorine anode 1 with a surrounding anolyte room 2 and the air cathode with its inner room 3. A cellbase plate 4 carries the anodes. The cathodes are disposed at the cell wall part 5 with means 6 for discharge of alkali hydroxide solution and means 7 and 8 for supply and discharge of process air respectively. The cell cover 9 contains a pipe for discharged chlorine 10 and a connection for supply of alkali chloride solution 11. Supply of electrical energy takes place by the connectors 12 and 13 113~i089 respectively. The anode is insulated from the cellbase plate by the insulation 14 and the cellbase plate is of course electrically insulated from the cell wall part 5 with the insulating gasket 15.
~ As the artisan wlll easily realize, Figure lA
- depicts a completely conventional chlor-alkali cell with the exCeption ~or the neW electrode. The drawing is, however, constructively misleading since the air electrode is at the same time shown in a section by the surface which is facing the anode and in a section through the cell-wall part. In reality, the air electrode will look from the outside very much the same as a cathode finger in a con-ventional chlor-alkali cell. The air electrode (note Figure lB which depicts in detail a portion of Figure lA) contains the separator material 17 which may be an asbestos diaphragm or a Nafion membrane, the electrocatalytically active material 18 which may be a Teflon-bonded porous Raney silver catalyst or active carbon catalyst, the perforated or foraminous supporting structure 19 which delimits the inner room 3 of the air electrode. The supporting structure 19 is furnished with openings 21 and is preferably Teflon-coated so as to make the whole supporting structure electrolyte repellant and thereby facilitate capture of air bubbles for better contact between air and the electrocatalytically active ~aterial 18. It may furthermore be of advantage to make use of a special supporting material 22 for the electrocatalytically active material. This supporting material could be a nickelwire mesh arranged on the supporting structure, porous graphite or carbon paper, etc.

1R~ o 6~ ~ R /~

113608~1 Figure 2 depicts an air electrode in a cell of the same type. Inspection of the Figure reveals that there is a special catholyte room 23 arranged between the separator 17 and the alr cathode 16 which is not permeable to electrolyte, and a special gas room for air 24. This cathode is thus functionally constructed in the same way as has been described for gas diffusion electrodes for elec-trolysers in the U.S. Patent 3,864,236.
In the operation of the cell according to Figure lA
10 the air is supplied via the conduit 7 and is then brought -''-into contact with active electrode material being exposed via openings 21 in the supporting structure 19. The inner room lS filled up by a more or less continuous air phase and a more or less continuous electrolyte phase, whereby the distribution between air and electrolyte depends on the constructive design of the innèr room, the hydropho-bization, the baffles, the supporting structure, etc.
Figure 3 shows how the invention may be used with bipolar cells. The Figure shows a repetitive element in a pile of bi-polar electrodes 25 with intermediate insul,ating frame elements 26. The notations are the same as in the preceding Figures. The separator 17 is preferably a cation permeable membrane like Nafion.
Figure 4 shows how the essential design according to Figure 1 can be achieved by rebuilding an existing chlor-alkali cell. For simplicity Figure 4 shows only a section through the supporting structure. The cell-wall part with its cathode fingers has been dismounted in a known way and the asbestos diaphragm has been removed. The structure has been nickel-coated galvanically in the known way. A
thin nickel wire mesh has been disposed in such a way ~ f~/~D~ /~
--1~--11360~39 \

that it covers the perforated or foraminous part of the structure. This nickel wire mesh shall serve as support for the electrocatalytical active material. Furthermore an air distributor 27 with holes 28 for supply of air evenly over the inner section of the cathode finger has been introduced in every cathode finger. This air distributor is connected to a main line not shown for . = =--= .
incoming air which in its turn is connected to the common -~~---~
air system. The electrocatalytically active material is then put on the nickelwire mesh by painting of a thin layer (0.1 mm) of a slurry of Raney silver of so-called Siemens' type (see reference above). A suspension of 100 grams of B silver in 100 grams of Teflon dispersion (DuPont Teflon 30 N) is sufficient for 1 Sq.m. The nickel wire mesh should have a mesh number above 60. Sintering occurs at 350C
for 15 minutes in air.
In order to facilitate transfer of electrolyte through the hydrophobic layer (which is frequently disposed on the air side of state of art air electrodes), the layer is perforated with rollers with needles so as to produce holes in the layer. These holes, frequently 0.2-1 mm in diameter, may cover a minor portion of the electrode surface (frequently in the range of 1-10~). After the electrode material structure has been sintered together, e.g. by heating up to 300C for 20 minutes, the asbestos diaphragm is supplied in known manner. It is also possible to sinter the electrocatalytically active material and the diaphragm in one operation.
The modified cell wall part may now be mounted on its cell base plate in the cell hall with the difference .~ ~

11360~9 t that the connection to the hydrogen system is substituted for connection to the system for discharged air and further-more that the air space is connected to the system for air supply.
An important point is of course the adjustment of the hydraulic resistance for transfer of electrolyte from the anolyte room to the interior of the air electrode.
One has to determine by experimentation suitable hydropho-bization, pore structure and eventual perforation of the 0 active air electrode material. One may also simultaneously reduce the thickness of the diaphragm considering the ! transport resistance offered by the air electrode. It is also possible to make diaphragm and electrode materials in to one unit which may be described as elimination of the separator. There are, in principal, two possibilities.
In one case the electrolyte is allowed to "weep" from the anolyte room into the interior room of the air electrode to be collected in the lower part of the air electrode whereby the air electrode is mainly filled with air. The transfer of electrolyte into the catholyte room depends on com-plicated electro-osmotic and other transport processes in the membrane and depends only to a minor extent on the hydrostatic pressure differential between the two rooms.
In the other case the catholyte space is mainly filled with electrolyte and the driving force for transport between the anolyte room and the interior of the air electrode is mainly the hydrostatic pressure difference. This requires that the air electrode be perforated as has been described above. A good contact is still obtained between the air and the electrocatalytically active material since the air bubbles are collected at the openin~s in the supporting ,--113~()B9 structure. The air bubbles are hereby transported success-ively from level to level in the air electrode.
Figure 5 shows another useful embodiment with a porous electrolyte-holding structure 29 disposed in the interior of the air electrode. This electrolyte-containing structure may be manufactured in position within the cathode finger by sintering of alkali-resistant polymers like polysulfon, penton, polyphenyleneoxide, etc., whereby the open porosity is produced by means of 10 spacers like particles of sodium chloride which are then ~ _ leached away. The electrolyte containing structure also contains channels for addition of air 30, respectively discharge or air 31, and channels 32 to produce contact between the air and the electrocatalytically active materials.
The electrolyte-containing structure furthermore contains ribbons 33 or other contact points for conducting electrolyte from the electrocatalytically active material to the elec-trolyte containing structure. This design gives a completely controlled distribution of air and electrolyte in the air electrode with a controlled contact between electrolyte, air a~d the electrocatalytically active material.
Figure 6 shows another special embodiment with separate air element 34 and electrolyte elements 35 disposed in the interior of the electrode. In the finger are inserted standing perforated elements 34 and 35 with electrode material 18 disposed on the surface of the elements 34 against the separator 17, air is conducted towards the bottom of each lement 34. Alkali flows into element 35 and essentially fills up the element.

1~3~i089 Laboratory experiments with functional models designed as in Figure 3 and furnished with material as described above have shown that the electrode can be run at 150 mA/cm2 at 80C, whereby cell voltage is reduced from 3.25 volt for corresponding conventional cell to 2.40 volt. On the basis of these experiments a complete redesign of an existing chlor-alkali plant for diaphragm cells with a capacity of 70,000 metric tons of chlorine per year has been carried out. Specific energy consumption is reduced 24~, the alkali concentration can be increased 18% and the capacity increased 33% without change of the electrical system.
The principles, preferred embodiments and modes of operation of the present invention have been described in the foregoing specification. The invention which is intended to be protected herein, however, is not to be construed as limited to the particular forms disclosed, since these are to be regarded as illustrative rather than restrictive. Variations and changes may be made by those skilled in the art without departing from the spirit of the invention.

Claims (4)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. An electrolytic cell comprising:
a housing;
an anolyte disposed within said housing;
an anode disposed within said housing;
at least one cathode spaced from said anode with at least a portion of the cathode being adjacent said anolyte, the cathode including a chamber;
means to supply and remove air from the chamber;
means to discharge an alkali hydroxide solution from the chamber; and a multi-layer wall defining the boundary between the anolyte and the interior of the chamber comprising a separator material adjacent said anolyte, a perforated electrically conductive support-ing material adjacent the interior of said chamber, and an electrocatalytically active material disposed between said separator material and said supporting material.

2. The electrolytic cell of claim 1 wherein the separator material is selected from the group con-sisting of an asbestos diaphragm and a cation-permeable membrane.

3. The electrolytic cell of claim 1 wherein the electrocatalytically active material is bonded to a thin porous structure disposed between the separator material and the perforated supporting material.

4. The electrolytic cell of claim 1 wherein at least a portion of the electrocatalytically active material is hydrophobic.