AU665037B2 - Electrochemical cell - Google Patents

Description

.L'CT1TT I ~B 24503/9~ I aLVER! SION revised abstract received by the International Bureau after p4 1completion of the technical preparations for international ]Pr publication INTERNATIONAL APPLICATION PUBLISHED UNDER THE PATENT COOPERATION TREATY (PCT) (51) International Patent Classification 5 (11) International Publication Number: WO 93/11281 9 (21) Internati /00, 1/00 A3 (43) International Publication Date: 10 June 1993 (10.06.93) onal Application Number: PCT/GB92/02157 (22) International Filing Date: 20 November 1992 (20.11.92) Priority data: 9125680.0 3 December 1991 (03.12.91) GB (81) Designated States: AU, CA, FI, JP, KR, NO, European patent (AT, BE, CH, DE, DK, ES, FR, GB, GR, IE, IT, LU, MC, NL, SE).

Published With international search report.

Before the expiration of lte linlr limit for amending the claims and to be republished in the event of the receipt oj amendmentis.

(88) Date of publication of the international search report: 8 July 1993 (08.07.93) 665037 (71) Applicant: IMPERIAL CHEMICAL INDUSTRIES PLC [GB/GB]; Imperial Chemical House, Millbank, London SW1P 3JF (GB).

(72) Inventor: BULLEN, Shaun 7a Beswicks Road, Winnington, Northwich, Cheshire CW8 lAP (GB).

(74)Agents: COLLINGWOOD, Anthony, Robert et al.; ICI Group Patents Services Dept., P.O. Box 6, Shire Park, Welwyn Garden City, Hertfordshire AL7 1HD (GB).

(54)Title: ELECTROCHEMICAL CELL (57) Abstract An electrolytic cell having first (14) and second (10) electrode structures in 30 36 which the first electrode structure has a surface area (28) which is in heat transfer 22 relation with the electrolyte and part only of said surface area (18) is electrolytically active The first electrode structure may be an anode structure suitable for use in the production of ozone and the remaining part (28) of the surface area of the anode structure may he rendered electrolytically inactive by J masking this part from the cathode structure. The masked surface of the anode structure is maiainined in heat transfer relation with the electrolyte.

(ReKfrred to in PCT Gazette No. 1b/193, Scctton It) 16 -raun mr-irm--calcr~ ra -7pEaCI CQE WO 93/11281 PCT/GB92/02157 ELECTROCHEMICAL CELL.

This.invention relates to an electrolytic cell. The invention relates in particular to an electrolytic cell in which ozone is to be produced.

Electrolytic cells are known in which water is dissociated into its respective elemental species, i.e. 02 and H 2 which are liberated at the anode and cathode respectively. Under appropriate conditions 03 is also produced at the anode.

Thus in European Patent Specification No. 0 041 365 there is described an electrolytic process for the production of ozone in which an electrolyte comprising an aqueous solution of a very highly electronegative anion, for example the acids or salts of hexa-fluoro-anions, is electrolysed.

High current efficiencies of up to 35Z for the production of ozone have been reported. Current efficiency 1 (also known as production efficiency of ozone) is a measure I of actual ozone production relative to theoretical ozone 11 production for given inputs of electrical current, i.e. I icurrent efficiency means under the conditions stated, the i i i02-03 gases evolved at the anode comprise 35Z (of the I theoretical 03 production) 03 by weight and that 352 of the supplied current is utilised in the production of ozone.

i IElectrolytic cells are also known for the production of Z5 ozone which comprise air cathodes and in which the cathode reaction which takes place is the reduction of ambient air to water in an acidic electrolyte, the water then being oxidised at the anode to oxygen and ozone. Consequently I: (since hydrogen is not produced at the cathode surface, the 30 cell voltage is substantially reduced. Furthermore the Sconstruction of the cell is less complex than cells which comprise hydrogen evolving cathodes, for example a separator between the anode and cathode is not required since hydrogen is not evolved at the cathode, and control of the overall process is simpler, namely the need for periodic additions of water is reduced.

2 W 93/11281 PCT/GB92/02157 Thus, in US Patent No.4,541,989 there is described an I electrolytic cell for the production of ozone which comprises a tubular anode structure, the outer surface of which functions as an anode within a tubular air cathode ii structure, the inner surface of which functions as a j cathode. As disclosed in the aforementioned US Patent it is desirable in electrolytic cells for the production of ozone that the current density at the anode surface is greater Ii than that at the cathode surface, in particular that the current density at the anode surface is at least about twice that of the cathode surface in order that the power consumption due to polarisation losses at the cathode surface may be reduced, thereby avoiding hydrogen evolutior Ir and increasing air cathode lifetime.

Furthermore, there is a requirement in the electrolytic production of ozone that the anode surface is cooled. Use of a cooled anode surface substantially improves the current i efficiency of the process. It is kn)wn for example that i where the temperature of the anode is reduced from 25 0 C to 0 0 C, a fourfold improvement in the current efficiency may be obtained. Furthermore, use of a cooled anode in electrolytic cells for the production of ozone often increases the lifetime of the anode.

The anode is typically cooled by the flow of a refrigerant fluid within the anode. It is therefore important that the surface area of the anode is as large as possible in order that effective heat exchange with the electrolyte may take place thereby ensuring efficient cooling of the anode. On the other hand a low anode surface 30 area relative to that of the cathode is desirable in order that the anode current density may be high.

The concentric tubular arrangement as described in US Patent 4, 541, 989 provides one way in which the current density may be greater at the anode than at the cathode whilst still allowing efficient heat exchange.

However, this concentric tubular arrangement does suffer from the disadvantage that the inter electrode gap, 3 WO 93/11281 PCT/GB92/02157 that is the distance between the outer surface of the anode and the inner surface of the cathode may be disadvantageously large in order to achieve the required low surface area of the anode relative to that of the cathode.

For example, in a cell comprising an inner tubular anode having an external radius of 2 cm and an outer tubular cathode having an internal surface area twice that of the a anode, the internal radius of the cathode will be 4 cm. i.e.

i the inter-electrode gap will be 2 cm. This large 0 inter-electrode gep leads to a high electrical resistance tc Sthe flow of current .hrough the cell and an undesirable Sincrease in the voltage of the cell.

These aforementioned problems arise in particular with this arrangement where the cathode is an air cathode. In 1 addition to the aforementioned operational problems, ai: Scathodes are difficult to fabricate in a tubular form and they lead to mechanical problems during manufacture of the electrolytic cell, for example sealing within the cell.

According to a first aspect of the present invention there is provided an electrolytic cell comprising first and Ssecond electrode structures, at least the first electrode structure having a surface area in heat exchange relation with the electrolyte and part only of said surface area being electrolytically active.

Typically, each of the first and second electrode structures has a surface area which is in heat transfer relation with the electrolyte, a part only of said surface area of the first electrode structure being electrolytically active and the active surface area of said second electrode structure being greater than that of the first electrode jstructure.

In an electrolytic cell in which ozone is produced, the first electrode structure is an anode structure and the second electrode structure is a cathode structure, the electrolytically active surface area of the cathode structure being greater than that of the anode structure.

4- WO 93/11281 PCT/GB92/02157 Although the invention is described hereafter with reference to an electrolytic cell in which ozone is to be produced, we do not exclude its application to generation of products other than ozone.

The active surface areas of the anode and cathode structures will usually be of such an extent that in operation the current density established at the surface cf the anode structure is at least 202 greater than that established at the surface of the cathode structure.

S0 Preferably the current density established at the surfac. of the anode structure is at least 50Z, especially at least 80Z greater than that established at the surface of the cathode structure.

The anode structure may have high surface area in heat 1i exchange relation with the electrolyte, thus facilitati.g efficient heat exchange, yet only a part of that surface area is electrolytically active thus ensuring relatively high anode current densities, relative to the cathode current density.

By "electrolytically active anode surface" there is meant that part of. the anode surface at which the electrolytic process takes place, that is that part of the anode surface with which the cathode surface has substantial electrical interaction, i.e. current in the form of a flow of ions through the electrolyte takes place between the active anode surface and the active cathode surface.

In one form of the invention the anode structure may be constructed from different materials; for example one part of the anode structure may be constructed from a suitable anode material which presents said electrolytically active anode surface, and a second part may be constructed from a material which is not capable of functioning as an electrolytically active anode surface. In this case, the anode surface includes both the surface which is constructed from a suitable anode material and the surface which is not so constructed, although the surface which functions as an

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S f WO 93/11281 PCT/GB92/02157 active anode surface is only that surface which is constructed from a suitable anode material.

However, for simplicity of construction, we prefer that at least that part of the anode structure which has a surface area exposed for contact with the electrolyte in ji operation of the cell is made completely from a material which is suitable for use as an anode and that a portion of the exposed anode surface is prevented from functioning as I an active anode surface by suppressing the electrolytic interaction between that portion and the cathode structure, for example by providing means for masking that portion of j! the anode surface from the cathode surface.

SAccording to a second aspect of the present inventicn there is provided an electrolytic cell comprising first and second electrode structures, part only of the surface area of at least the first electrode structure being electrolytically active, and means for masking the remaining part of the surface area of the first electrode structure in such a way as to render said remaining part of the surface area substantially electrolytically inactive.

The masking means may comprise for example, a coating, cover, insert or any other suitable part, such that there i substantially no ozone producing electrolytic interaction Sbetween the masked portion(s) of the anode surface and the cathode surface.

The masking means may take any suitable shape and form provided that it suppresses the electrolytic interaction between the masked portion of the anode surface and the cathode surface. The material used for effecting masking is typically therefore an electrically insulating material. The material should also be inert to the electrolyte, which may be highly corrosive. Suitable materials include inert polymeric materials s h as polyvinyl chloride or polyfluorinated polymers, for example polytetrafluoroethylene which are electrical insulators and which have resistance to oxidising gases and excellent resistance to highly acidic and corrosive solutions.

11-1-1 I I f 7 j .i-i i i i liil~p~LilIIU13i~i~ 6 WO93/11281 PCT/GB92/02157 The extent of the masking should be such as to achieve the desired current density difference between the active anode surface and the cathode surface. Thus the unmasked anode surface area which functions as an active anode surface should be less than about 80Z of the area of the cathode surface, preferably less than about 60Z of the cathode surface.

The anode structure may have any suitable form, for example it may be in the form of a tube or or it may be a 1o 10 planar anode. The electrolytically active and inactive surface areas of the anode structure may be distributed around the periphery of the anode structure. The anode structure may have an elongate configuration and the active and inactive surfaces may extend longitudinally of the elongate anode structure.

The electrolytic cell preferably further comprises means for circulating a coolant fluid in heat transfer relation with the anode structure in such a way that heat exchange is secured between the coolant and the electrolyte through the active and inactive surface areas of the anode structure. A particularly preferred form of the anode by which such coolant circulation may be achieved is a tubular anode, the lumen of which provides a path for the flow of a coolant fluid. The coolant fluid may flow through the lumen of the anode or the lumen may be provided with a member extending within the lumen, for example a hollow finger through which the coolant fluid is caused to flow. The cold finger may be constructed, for example, of copper.

The surface area of the tubular anode structure which is exposed for contact with the electrolyte may be the outer surface or periphery of the tubular structure, and the electrolytically active and inactive parts of this surface area may therefore be distributed peripherally around, and extending longitudinally of, the tubular structure.

In order that heat exchange may take place through the inactive surfaces of the anode structure, the electrolytic cell preferably comprises means for routing the electrolyte ,e 1 S- 7 I WO 93/11281 PCT/GB92/02157 j| along a path in which it is in heat exchange relation wit: a Isubstantially electrolytically inactive part of the surface area of the anode structure. Where the anode structure is of elongate configuration, the routing means preferably Sprovides at least one flow path extending longitudinally of the anode structure.

,S The active and inactive surfaces of the anode structure .3 may extend generally co-extensively with one another longitudinally of the anode structure.

.0 In order that as little as possible electrolytic Sinteraction should take place between the cathode structure |i and the inactive anode surface(s), the cell preferably Scomprises means for preventing the flow of electrolyte peripherally of the anode structure from a region where the 3,S electrolyte is in communication with an active surface of the anode structure to a region where the inactive surface area is located.

The cathode structure may take any suitable form although we generally prefer to use a planar or tubular structure. Where the cathode structure is in the form of a tube, the inner surface of the tube may be the S! electrolytically active cathode surface and the radius of the inner surface of the cathode structure may be reduced compared to that hereinbefore described with reference to US Patent 4,541,989 thus reducing the inter electrode gap but maintaining the differential current density between the electrolytically active anode surface and the cathode surface.

Preferably however, the cathode structure is of a planar configuration since we prefer to employ an air cathode (as described hereafter). Air cathodes of planar configuration are readily manufactured and are easily installed in the electrolytic cell. Furthermore, an electrolytic cell comprising at least one tubular anode and at least one planar cathode allows both simple scaling of the cell to much larger sizes as hereinafter described, and facilitates the achievement of the current density WO 93/11281 PCT/GB92/02157 difference between the active anode surface and the cathode surface defined according to the first aspect of the invention.

Where the cathodic structure is of planar configuration, we prefer that two of said cathode structures of planar configuration are present in the cell and that the anode structure is located between, and in spaced relation with, said cathode structures and that the anode structure has separate electrolytically active surfaces in confrontin2 relation with the cathode structures. The anode structures may then have electrolytically inactive surface areas located between the said separate active surface areas. We particularly prefer to employ a plurality of said anode structures, each having electrolytically active and inactive anode surface areas. The anode structures may be arranged between the two planar cathode structures in a direction parallel to said cathode structures.

According to a third aspect of the present invention there is provided an electrolytic cell for the production of ozone comprising at least one tubular structure the outer surface of which functions as an anode surface and at least one planar cathode structure.

A cell in accordance with this third aspect of the invention may incorporate (severally or collectively, or any combination thereof, as the context admits) those features of said first and second aspects of the invention as discussed hereinbefore.

In a preferred embodiment of this third aspect of the invention, the cell comprises at least two planar cathodes between which are located one or more tubular anodes, preferably at least two tubular anodes. The number of tubular anodes which are employed depends at least to some extent upon the desired rate of production of ozone from the cell, the dimensions, in particular the length and diameter of the tubular anodes and the current density at the electrolytically active anode surfaces during operation of the cell. Thus, the cell may comprise up to about 12 tubular i Ii I WO 93/11281 PCT/GB92/02157 anodes where ozone production in the order of 100 g/hour i Sdesired. The ease with which the cell may be scaled up in order to increase the rate of production of ozone, simply by the provision of further tubular anodes between the air cathodes an(. increasing the length of the anode tubes and area dimensions of the planar air cathode, is a substantial advantage of this third aspect of the invention.

Furthermore, the cell may comprise more than two planar cathode structures, with one or more tubular anode i I structures arranged between each pair of cathode structures such that the cell may comprise a row of tubular anodes between each pair of opposingly faced planar cathodes.

Further, in the electrolytic cell according to this third aspect of the invention, preferably only a portion of the outer surface of the tubular anode which is in heat exchange relation with the electrolyte is electrolytically active, and advantageously the cell comprises means masking the remaining part of the surface area of the anode structure in such a way as to render it substantially electrolytically inactive.

Active and inactive portions of the anode surface area may be defined by providing in the cell a first boundary means for bounding together with the cathode structure and a part only of the surface area of the anode structure a 2 5 chamber for enclosure of an electrolyte whereby said part: only of the anode surface area constitutes an electrolytically active surface area of the anode structure and second boundary means for bounding together with a further part or parts of the surface area of said anode structure at least one channel in fluid communication with said chamber to provide for flow of electrolyte over said further part(s) of the surface area of the anode structure, said further parts constituting a substantially electrolytically inactive surface area of the anode structure. The first and second boundary means may further constitute a masking means as hereinbefore described, and may, for example be provided by an insulating body or bodies i0 WO 93/11281 PCT/GB92/02157 extending between the anode surface and the cathode surface.

The, o each insulating body may be in the form of, for example, a wedge-shaped member or truncated cone, with its narrow end in contact with and extending from the cathode surface to its wide end in contact with the outer surface of one or more anode structures whereby to mask that portion of the surface area of the anode structure which is not in confronting relation with the cathode surfaces, from the cathode surfaces.

The first and second boundary means, for example one or more insulating bodies may be provided as separate inserts for provision within the cell. Conveniently however, the or each insulating body is formed as part of a cell body in which the anode and cathode structures are supported. Thus the cell body may be so configured that it masks the surfaces of the anode which are not in confronting zelat-or.

with the cathode surfaces.

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The active anode surface will in this case be the surface of the anode adjacent the cathode surface and the mean distance between the active anode surface and the cathode surface is preferably less than 10mm, more preferably less than 5mm and especially less than 4mm.

As hereinbefore described it is desirable that as great an area of the anode structure as possible is in contact with the electrolyte in order to achieve good heat exchange efficiency between the electrolyte and the anode surface whereby efficient cooling of the active anode surface is obtained. We therefore prefer that the masked portion(s) of the anode surface is nevertheless exposed to the electrolyte.

Where the masking means is in the form of one or more wedge-shaped insulating bodies extending from the cathode surface to the anode surface, the wedge may be shaped such that the wedge only comes into contact with a small area of the anode surface to be masked. For example, the wedge may be cut away to form circulation channels, at the portion of the wedge which is adjacent the anode surface to be masked

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WO 93/11281 11 PCT/GB92/02157 so that electrolyte may circulate over the anode surface between the masking wedge member and the anode surface.

Thus, even though the masked portions of the anode structure play no significant part in the electrolytic interaction, the electrolyte may flow freely over substantially all or a section of the masked and electrolytically inactive portion of the anode structure, providing an increased total surface area for heat exchange and thus cooling of the active anode surface.

A further advantage of the provision of, for example, re-circulation channels adjacent the masked and electrolytically inactive anode portion(s) is that a recirculating flow of electrolyte may be achieved over the active anode surface which serves to remove bubbles of 1 5 gaseous products which may form on the active anode surface and which may, if not removed, lead to an increase in the cell voltage. This flow of electrolyte is achieved because substantially no electrolysis takes place within the recirculation channels so that the electrolyte in the channels tends to be ungasified, whereas the formation of gaseous products of electrolysis takes place in the electrolyte chambers thus producing a gasified electrolyte.

A recirculating flow of electrolyte is thereby generated by the density difference between tha gasified and ungasified electrolyte.

i Fluid-tight seals should be maintained between the electrolyte chambers and the re-circulation channels in order to prevent currert leakage from electrolyte between the active anode and cathode surfaces to the electrolyte flowing within the recirculation channels.

SAccording to a fourth aspect of the present invention there is provided an electrolytic cell for the production of ozone comprising an anode structure, a cathode structure and a chamber for containing electrolyte within which electrolysis occurs and which further comprises at least one re-circulation channel in fluid communication with the electrolyte but within which electrolysis does not occur.

WO 93/11281 12 PCT/GB92/02157 The recirculation channels may be provided within the cell itself or they may be provided externally of the cell.

As hereinbefore described we prefer to provide the recirculation channels within the electrolytic cell and especially adjacent the electrolyticaliy inactive anode surface.

Cell head spaces which may serve as both disentrainment areas for product gases and reservoirs for electrolyte may be provided within the cell, into and from which electrolyte from and into both the electrolyte chambers between the active anode and cathode surfaces, and recirculation channels may flow, and from which product gases may be collected.

Air cathodes, which are commercially available components, are typically composed of polytetrafluoroethylene-bonded-carbon containing small amounts of catalytic materials, for example platinum.

The materials used as the anode surface in the electrolytic cell of the present invention may be conventional anode materials as described more fully in, for example, European Patent 0 041 365. The anode surface may be constructed from platinum or lead dioxide, particularly lead dioxide in the beta crystalline form. However, a special form of carbon, specifically vitreous or glassy carbon is particularly preferred for use as the anode surface material since it has a high oxygen overpotential and thus a high efficiency for ozone production, it is stable in strong acid electrolytes and is stable to oxidising conditions generated in the cell. Furthermore, glassy carbon is a material which possesses poor electrical conductivity so that where current is fed to the anode structure through a conducting memeber provided adjacent only the electrolytically active surfaces of.the anode (as described hereafter), current tends not to leak from the electrolytically active anode surface to the electrolytically inactive anode surfaces.

The electrolyte used in the electrolytic cell is typically a known electrolyte, for example, an aqueous I 13 13 WO 93/11281 PCT/GB92/02157 solution of a highly electronegative anion (and associated cation) such as are, for example, described in European Patent 0 041 365. The electronegative anion used is preferably as electronegative as possible, and more preferably is a fluoro-anion. The fluoro-anion may be the fluoro-anion of a Group V-B element of the Periodic Table, for example phosphorous and arsenic which form hexa-fluoro anions. Other related non-metallic elements such as Si and Sb also form hexa-fluoro-anions. Other suitable fluoro-anions may be mentioned, inter alia PO 2 F2-, HTiF 6 NbF 7 2 TaF 7 2 NiF 6 2 ZrF62-, GeF 6 2 FeF6 2 The phosphorous, arsenic, boron and silicon fluoro-anions are the preferred anions for addition to the aqueous electrolyte and in particular polyhalogenated boranes. We especially j prefer to employ the tetrafluoroborate anion.

The fluoro-anions may be added to the aqueous electrolyte solution in the form of their respective acids or as water-soluble salts. Whereas the acid-form of the Sfluoro-anions may be preferred because of their higher solubilities in water, the fluoro-anion salts, for example sodium or potassium, offer the advantage that their aqueous solutions have higher pH's than do the solutions of their respective acid forms, and they therefore are less corrosive towards the cathodes.

For current efficiency, it is desirable to increase the fluoro-anion concentration in the electrolyte to its maximum solubility since increasing the anion concentration increases ozone current efficiency. However, an increase in the anion concentration also increases the corrosivity of the electrolyte towards the cathodes. Suitable anion concentrations may be readily determined by routine experimentation.

The construction of the electrolytic cell may, apart from its construction according to the various aspects of the invention as hereinbefore defined, follow conventional technology taking into consideration the corrosive nature of the fluoro-anion electrolytes and the high oxidising power 14 WO 93/11281 PCT/GB92/02157 of the ozone gases. Thus, the parts of the cell in contact with the corrosive electrolyte and oxidising products of electrolysis are preferably constructed of material- which are inert both to the highly corrosive electrolyte and the oxidising gases. The cell body may therefore be constructed from, or coated with, an inert material, for example an inert polymeric material such as polyvinyl chloride or j polyfluorinated polymers, for example polytetrafluoroethylene which have resistance to oxidising gases and excellent resistance to highly acid and corrosive solutions.

Where the cathodic reaction taking place in the cell i produces hydrogen, the anode and cathode compartments of the ji cell should be separated such that the hydrogen evolved at the cathode is not in fluid-flow contact with the gases evolved at the anode. Such separators are well known ir. the art. Conventionally they are prepared from a perfluorinatc i polymeric cation exchange material, for example "Nafion" (registered Trademark of E.I.Du Pont). Such a separator is 2 not needed in the preferred embodiments of the invention where an air cathode is used and where hydrogen is not Sgenerated by the cathode process.

The anode and cathode structures are disposed within l the electrolytic cell with electrical leads leading to the exterior of the cell. Electrical potential may be applied to the anode by contact with part only of a surface of the anode structure which corresponds to the electrolytically active surface area of the anode structure, for example by i means of a conducting member, constructed from, for example, copper, which is provided along the anode adjacent the active anode surface in order that current is fed predominantly to the active anode surface and not to that anode surface which is masked from the cathode. The conducting member is preferably provided on a surface of the anode which is not in direct contact with the electrolyte, for example where the anode is in the form of a hollow tube the conducting member may be provided within the lumen of _i I 15 WO 93/11281 PCT/GB92/02157 the tube, in order that the conducting member is protected from the electrolyte.

The cell is sealed prior to use, and the cell head space is provided with suitable inlet and outlet passages for water make-up, where necessary, and the withdrawal of the gases evolved from the cathode (where hydrogen is produced) and from the anode. Two discrete gas removal systems may, where necessary, be used to keep the cathode gases separate from the anode gases. Nitrogen and/or air may be added, for example pumped through, the gas handling system in order to entrain the evolved cathode and anode gases and carry them from the cell to the exterior where they may be stored or utilised in the desired application.

The anode and cathode structures are connected by the aforementioned electrical leads, optionally through a conducting member, to a source of power external to the cell. Typically the cell is operated at electrical potentials in the order of 3-7 volts. The current density at i the anode surface may be in the range from about a tenth of an ampere per square centimetre of effective anode surface up to about 1.0 ampere per square centimetre of effective anode surface.

The invention is illustrated with reference to the accompanying figures in which: Figure 1 is a diagrammatic partly cut away view of an electrolytic cell according to the invention, ii Figure 2 is a view in section along the line A-A in figure i, Figure 3 is a view in section through the cell of Figure 1 along the line B-B in figure 2, Figure 4 is a view in section through the cell of Figure 1 along the line C-C in figure 2, 16 WO 93/11281 PCT/GB92/02157 Figure 5 is a view in section along the line D-D in figure 3, and Figure 6 is a view in section along the line E-E in figure 3.

Figure 7 is a view in section along the line F-F in figure 3.

Referring to Figures 1 to 7, an electrolytic cell suitable for the generation of ozone shown generally as 1 in Figure 1 comprises a main cell body 2, having top and bottom portions 4, 6 and spaced columns 8 extending therebetween.

Although three such columns 8 are present in the illustrated embodiment, a pair of end columns 8A and an intermediate column 8B, there may be more or less according to the number of anode structures employed. A pair of air cathodes 10 with associated air chambers 12, and located between but spaced from the air cathodes 10, a pair of tubular glassy carbon anodes 14, are supported in the cell body. The columns 8, which support the anodes and cathodes in spaced relation with one another, constitute means for bounding, together with active surfaces 16 of the air cathodes 10 and parts only of the outer surfaces of the anodes 18 which are in confronting relation with the active surfaces of the air cathodes, electrolyte chambers 20. The electrolyte chambers extend longitudinally from the top portion 4 to the S;bottom portion 6 and at each end open into upper and lower cell spaces, 22, 24 respectively (see Figure 3) within the top and bottom portions 4 and 6, the arrangement being such that each cell space is in communication with a pair of electrolyte chambers 20 on opposite sides of an anode 14.

The columns have a wedge-like configuration and have inwardly directed surfaces 26, the surfaces of adjacent columns converging from the cathode to the anode at an angle of convergence so as to define a longitudinally extending active anode surface having an area half that of the cathode mkMMNW-W 17 WO 93/11281 17 PCT/GB92/02157 surface 16 which is bounded by the columns. The required differential in active anode and cathode surface area thereby achieved provides in use the required current density differential between the active anode and cathode surfaces but allows a mean distance between the active and cathode surfaces of less than 4mm.

The columns 8 mask the remainder of the anode surfaces 28 from the surfaces of the cathodes and are formed with grooves or channels which constitute re-circulation channels I0 30 of curved, e.g. semi-circular profile, adjacent at least a part 28a of the inactive surfaces of the anodes 14. Each channel 30 extends longitudinally from the top portion 4 to the bottom portion 6 and at each end opens into upper an lower cell spaces 22, 24 (see figure 4) within the tcr and bottom portions 4, 6, the arrangement being such that each cavity 22, 24 is in communication with a pair of channels associated with adjacent columns and a pair of electrolyte chambers 20 as described previously. Fluid tight seals are maintained between the surface of the anodes and the columns of the cell body by longitudinally extending resiliently deformable seals 32 which prevent circumferential flow of electrolyte about the .periphery of the anode from the electrolyte chambers 20 to the re-circulation channels The lumen 34 of each anode is provided with copper conductors 36 adjacent the active surfaces of the anodes 16 through which electrical connection is made between each electrolytically active surface of the anodes 18 and the source of electrical power. The anodes are constructed fromn glassy carbon which possesses poor electrical conductivity and which therefore serves to reduce the tendency for current to leak to the inactive anode surfaces adjacent the re-circulation channels 30. The lumen 34 also provides the flow path for circulation of a coolant in heat transfer relation with both the active and inactive surfaces of the anode structures.

The sectional views of figures 3 to 7 show more clearly the electrolyte flow around a single anode between a pair of i 1 18 WO 93/11281 PCT/GB92/02157 air cathodes. Figure 3 shows the flow of electrolyte from the cell head space 22 downwardly through the recirculati..

channels 30 which extend lengthwise of the anode structure to the lower cell space 24 and figure 4 shows the flow of electrolyte from the lower cell space 24 upwardly through the electrolyte chambers 20, which also extend lengthwise of the anode structure to the cell head space 22. Product gases are collected through gas outlet 38 provided from the cell head space. Figures 5 to 7 show clearly the provision within the cell of the cell head space 22 (Figure 5) and lower cel" space 24 (figure into and from which electrolyte from both the recirculation channels and the electrolyte chambers flows.

In operation of the cell, electrolyte is charged tc the cell and the electrodes 10 and 14 are connected to a source of electrical power (not shown). Air is pumped through tne air chambers 12 by an air pump (not shown), and a coolant fluid, for example a refrigerant, is caused to flow through the anode lumen 34 from a refrigeration system (not shown) externally of the cell. Gaseous products of electrolysis formed at the electrolytically active anode surface 18 cause the electrolyte to flow upwardly through the electrolyte chambers 20 to the cell head space 22 where the gaseous products are disentrained, the electrolyte thence flowing downwardly through the recirculation channels 30 adjacent the masked and electrolytically inactive anode surfaces at which no gaseous products of electrolysis are formed.

Product gases are collected via the gas outlet 38.

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Claims (29)

1. An electrolytic cell irnclutdin 'first and second electrode structures at least the first electrode structure having a surface area which in operation of the ceil is in heat transfer relation with the electrolyte to be electrolysed in the cell and a part of said surface area being electrolytically active and a part being electrolytically inactive in which the active surface area of the second electrode structure is greater than that of the first electrode structure and the electrolytic cell also comprises means for circulating a coolant in heat transfer relation with the first electrode structure in such a way that in operation heat exchange is secured between the coolant and the electrolyte.

2. An electrolytic cell as claimed in claim 1 in which the means for circulation a coolant in heat transfer relation with the first electrode structure in such a way that heat exchange is secured between the coolant and the electrolyte is such that in operation heat exchange is effected through the active and inactive surface area of the first electrode structure. S3. A cell electrolytic cell as claimed in Claim 1 or Claim 2 in which the electrolytically active surface areas of the electrode structures are of such an extent that, in operation, the current density established at said first electrode structure is at least 20% greater than that established at the second electrode structure.

K K I% 4

4. An electrolytic cell as claimed in anyone of claims 1 to 3 including first and second electrode structures, part only of the surface area of at least the first electrode structure being electrolytically active, and means masking the remaining part of the surface area of the first electrode structure in such a way as to render the same substantially electrolytically inactive.

An electrolytic cell as claimed in any one of claims 1 to 5 and including first and second electrode structures, at least part of the surface area of the first electrode structure being exposed to interaction with the electrolyte: and means for routing the electrolyte along a path in which it is in heat exchange relation with a substantially electrolytically inactive part of the surface areas of the first electrode structure. 1

6. A cell as claimed in any one of claims 1 to 5 in which the active and inactive surface areas of the first electrode structure are distributed around the periphery of the first electrode structure.

7. A cell as claimed in any one of claims 1 to 6 in which the first electrode structure is of elongated configuration and the active and inactive surface areas of the first electrode structure extend longitudinally of the first electrode structure. -i

8. A cell as claimed in any one of claims 5 to 7 in which the first electrode structure i0 is of elongated configuration and in which said routing means provides at least one flow path extending longitudinally of the first electrode structure.

9. A cell as claimed in claim 7 or claim 8 when dependent on claim 7 in which the active and inactive surface areas extend alongside one another longitudinally of the first electrode structure.

A cell as claimed in claim 9 in which said active and inactive surface areas are i generally co-extensive with one another longitudinally of the first electrode structure.

11. A cell as claimed in any one of claims 1 to 10 including means for preventing electrolyte flow peripherally of the first electrode means from a region where the Selectrolyte in communication with said active surface area to a region where the inactive surface area is located. S 25

12. A cell as claimed in any one of claims 1 to 11 including means for applying electrical potential to said first electrode structure by contact with part only of a peripheral surface of the latter corresponding to the active surface area of the first electrode structure.

13. A cell as claimed in claim 12 in which the first electrode structure is fabricated from a material which possesses poor electrical conductivity. i I -21

14. A cell as claimed in any one of claims 1 to 13 in which said first electrode structure is of tubular configuration.

A cell as claimed in any one of claims 1 to 14 in which said second electrode structure is of planar configuration.

16. A cell as claimed in claim 15 in which two of said second electrode structures are present and in which the first electrode structure is located between, and in spaced relation with, said second electrode structures and has separate active surface areas in confronting relation with the second electrode structures.

17. A cell as claimed in claim 16 in which said first electrode structure has inactive surface areas located between said separate surface areas.

18. A cell as claimed in any one of the preceding claims including a plurality of said first electrode structures, each having active and inactive surface areas as aforesaid.

19. A cell as claimed in claim 17 or 18 when dependent on claim 16 in which said first electrode structures are arranged in spaced relation in a direction parallel to said second electrode structures.

A cell as claimed in any one of the preceding claims comprising means for supplying electrical potential to said first and second electrode structures, said means including a contact which engages said first electrode structure at a limited region thereof corresponding to said active surface area, the first electrode structure being composed of a material having poor conductivity whereby the electrical potential prevailing at the inactive surface area is reduced.

21. An electrolytic cell as claimed in claim 1 comprising: an anode structure of elongate configuration; a cathode structure; Smeans supporting the anode and cathode structures in spaced relation with one -22- first boundary means for bounding together with said cathode structure and part only of the surface area of said anode structure a chamber for enclosure of an electrolyte whereby said part constitutes an electrolytically active surface area of the anode structure; second boundary means for bounding together with a further part or parts of the surface area of said anode structure at least one channel in fluid communication with said chamber to provide for flow of electrolyte over said further part(s) of the surface area of the anode structure, said fi.rther part(s) constituting a substantially electrolytically inactive surface area(s) of the anode structure; and means for providing at least one path for flow of coolant through the anode structure in such a way that heat transfer between the coolant and the electrolyte can take place through said active and substantially inactive surface areas.

22. A cell as claimed in claim 21 in which the anode structure is of tubular configuration and in which the hollow interior of the anode structure constitutes said S path for coolant flow.

23. A cell as claimed in claim 21 or 22 in which said chamber and said channel(s) extend lengthwise of the anode structure. Si

24. A cell as claimed in any one of claims 21 to 23, said first and/or said second S boundary means being formed integrally with said supporting means. A cell as claimed in any one of claims 21 to 24 in which the cathode structure is S 25 constituted by a planar air cathode.

J I J

26. A cell as claimed in any one of the preceding claims containing an electrolyte suitable for the electrochemical generation of ozone.

27. An electrolytic cell as claimed in claim 1 for the production of ozone comprising at least one tubular electrode, the outer surface of which functions as an anode, and at lepst one planar cathode structure. %V C:I R\NiENDYNIPINGU95i3TlDOC 'Vr 0 23-

28. A cell as claimed in claim 27 when modified in accordance with any one of claims 1 to 26.

29. A cell as claimed in claim 1 substantially as hereinbefore described with reference to any one of the embodiments illustrated in the accompanying drawings. DATED: 17 October, 1995 PHILLIPS ORMONDE FITZPATRICK Attorneys for: S 10 IMPERIAL CHEMICAL INDUSTRIES PLC i r .1. I I