Ozone generating electrolysis cell   

The present invention relates to an ozone generating electrolysis cell comprising a negative electrode, an ozone generating positive electrode comprising a mixture of lead(IV) oxide (referred to as lead dioxide) and polytetrafluoroethylene (PTFE), a membrane arranged between the negative electrode and the positive electrode, and an electrically conductive, liquid and gas permeable first electrode support in contact with a side of the positive electrode located opposite to the membrane, said side of the electrode support having a surface covered with a platinum-containing layer.

Several industrial processes are known for producing ozone from water by utilizing electrolysis cells. In these processes, electrolysis cells having a central part for effecting the electrolysis and thus for generating ozone are used, said central part being composed of an anode (positive electrode) usually in the form of a planar plate, a cathode (negative electrode) having the same construction, and a proton exchange membrane (e.g. Nafion™) constituting a solid electrolyte in the form of a planar plate arranged between the anode and the cathode, as it is described in e.g. U.S. Pat. No. 6,328,862.

According to the cited document, the anode itself is a porous member generally made of titanium and having liquid and gas permeable capabilities. On the (inner) surface of the anode facing the proton exchange membrane, a thin layer of platinum is applied, typically by an electroplating process. Between this layer of platinum and the proton exchange membrane, an electrode layer is arranged, said electrode layer comprising metallic or semimetallic conductors and/or oxides thereof (e.g. lead dioxide) characterized by high overvoltage with respect to the evolution potential of the oxygen. Adjacent to the outer surface of the anode, ie. at its surface opposite to the proton exchange membrane, an anode side collector plate (also referred to as an electrode support) is arranged in contact with the anode providing on the one hand an electrical connection between the anode and a power supply, and on the other hand, an outlet for the gas of oxygen/ozone produced. In addition, the anode side collector plate can also enable directing water required for the electrolysis to the anode itself.

The cathode also comprises of a member made of a porous material or a material having suitable channels in it, generally stainless steel or titanium, and having liquid and gas permeable capabilities as well. On the (inner) surface of the cathode facing the proton exchanging membrane, an electrode layer containing metal is applied for generating hydrogen. This metal containing electrode layer is generally a thin layer of platinum. Adjacent to the outer surface of the cathode, a cathode side collector plate (also referred to as an electrode support) is arranged in contact with the cathode, said cathode side collector plate providing an electrical connection between the cathode and the power supply, on the one hand, and directing the water required for the electrolysis to the proton exchange membrane and to the anode therethrough and, if necessary, providing an outlet for the produced gas of hydrogen, on the other hand.

The above described multilayer electrode structure is housed in a suitably formed cell housing. In order to make the mounting easier, the cell housing is usually formed of two halves, which are aligned and then fixed together in a sealed manner by means of e.g. through-bolts. The compressive force required for the perfect contact between the adjacent layers of the electrode structure is also provided by the mutual screwed fixation of the two halves of the housing.

During the operation of an ozone generating electrolysis cell having the above mentioned structure, water is fed to the cathode side of the cell and it reaches the proton exchange membrane and the anode therethrough via the porous cathode side collector plate and the porous cathode itself (or the channels formed therein). While applying a voltage for the cell at the same time, the electrolysis of the water is caused to start and hydrogen ions with positive charge move from the anode to the cathode through the proton exchange membrane. At the same time, oxygen and ozone are generated at the anode due to the electrolysis. The coefficient of efficiency of the ozone conversion, ie. the amount of ozone in the produced gas of oxygen, is determined by the quality of the anodic electrode layer and the operational parameters, therefore the ozone production capacity of the cell can be significantly affected through an appropriate manufacturing technology of the anodic electrode layer.

According to a common manufacturing process of the lead dioxide containing electrode layer, the lead dioxide film is formed on the anode by electroplating. The electrode layer thus obtained is rather uneven, which implies a change of the superficial electrical conductivity (resistance) of the electrode layer. In addition, the electrode layer produced by electroplating can be formed difficulty, it is rather rigid and it can easily break, therefore it is not suitable for mass production of ozone generating electrolysis cells containing solid electrolyte.

In an alternative method of manufacturing the lead dioxide containing electrode layer in the form of a separate plate, the pores of a thin porous PTFE sheet are filled with a mixture of lead dioxide and the material of the proton exchange membrane, as described in Japanese Patent No. 3,504,021 and U.S. Pat. No. 6,054,230. The proton exchange membrane is placed on the member thus obtained, and then its surface is covered with a platinum containing material. Subsequently, this multilayer structure is subject to a hot pressing at a temperature between 120° C. and 140° C. The pressed laminated member is inserted between the anode and the cathode, and then housed in a cell casing to obtain the electrolysis cell. The largest drawback of this method is that lead dioxide is an extremely unstable composition that easily decomposes due to heat. Hence, if at relatively high temperatures, regions with various electrical conductivities develop on certain parts of the surface of the anodic electrode layer produced by hot pressing due to decomposition of the lead dioxide, the operation of an electrolysis cell with such an electrode layer becomes unstable.

U.S. Pat. No. 6,328,862 discloses a method for producing an anodic electrode layer containing lead dioxide, wherein a dispersion of PTFE, pulverised lead dioxide and volatile dispersing agent (preferably ethanol or isopropyl alcohol) are mixed and the mixture thus obtained is shaped to a very thin sheet, preferably by calendering, and the dispersing agent is vaporised, for example, by heating. Each step of the production of the electrode layer is performed at a temperature of up to 100° C. in order to avoid possible thermal decomposition of the lead dioxide. The PTFE content of the mixture obtained by this method is about 5% by weight, the film itself is rigid, easily breaking and less ductile. Moreover, the cost of such an electrolysis cell used as an ozone generating electrode/electrode layer, where the cell contains lead dioxide/PTFE film produced from a liquid-phase raw material, is increased by the use of the corresponding solvent(s) and dispersing agent(s), and by the treatment performed after the evaporation thereof from the layers.

An object of the present inventions is therefore to provide an ozone generating electrolysis cell which allows to produce the ozone generating electrode from solid-phase raw materials at ambient temperature and without the use of dispersing agent. Another object of the invention is to provide a mixed material of lead dioxide and PTFE, for example for the anode of an ozone generating electrolysis cell, that is resilient and ductile due to its relatively high content of PTFE, that can be prepared at ambient temperature and that can be produced in less technological steps and with lower costs than the lead dioxide/PTFE films commonly used today. A further object of the present invention is to provide a negative electrode side (cathode side) electrode structure that in addition to its electrical conductivity and mechanical strength, due to its structure, inherently has liquid and gas permeable capabilities, too.

These and other objects of the present inventions are achieved by providing an ozone generating electrolysis cell, in which the positive electrode (ie. the anode) is made of a mixture prepared by high-pressure moulding of lead dioxide grains of colloid size and PTFE filaments having a dimension of at most 1 mm, and wherein the negative electrode (ie. the cathode) is adjoined to a side of the membrane located opposite to the positive electrode by a given compressing force and is formed on a surface of a porous second electrode support.

Preferred further embodiments of the ozone generating electrolysis cell according to the invention are specified by the dependant claims 2 to 10.

The invention will be now described in detail with references to the accompanying drawing, in which:

FIG. 1A is the cross-sectional view of a preferred embodiment of the electrode structure used for an ozone generating electrolysis cell according to the invention;

FIG. 1B is a schematic enlarged view of the material structure of a preferred embodiment of the second electrode support for supporting the negative electrode forming a part of the electrode structure shown in FIG. 1A; and

FIG. 2 is a longitudinal cross-sectional view of an assembled ozone generating electrolysis cell comprising the electrode structure schematically illustrated in FIG. 1A.

The electrode structure 10 of FIG. 1A used in the ozone generating electrolysis cell according to the invention primarily comprises a negative electrode (or cathode) 13, an ozone generating positive electrode (or anode) 16, a proton exchange membrane 15 arranged between the electrodes 15, 16, and a first (positive electrode or anode side) electrode support 17 arranged on a side of the positive electrode 16 located opposite to the membrane 15. The electrode support 17 is arranged on an (anode side) bearing member 18 provided with a through-hole 19 for an electrical contact. The electrode 13 is formed on a second (cathode side) electrode support 12 arranged in a (cathode side) bearing member 11.

The electrode support 12 serves for providing electrical contact between an external DC power supply (not shown) and the negative electrode 13, on the one hand, and for directing the water required for the electrolysis to the electrode 13 during the operation of the cell, and diverting the produced gas of hydrogen from the electrode 13, on the other hand. Accordingly, the electrode support 12 is in the form of a member with high electrical conductivity and porous structure, as well as with high mechanical strength in order to tolerate the high pressures of up to 20 bars that may develop inside the cell. In particular, the electrode support 12 is a thin and porous titanium frit arranged in the bearing member 11 and produced by high-pressure cold moulding of a titan granulate. In the description, the term “frit” is referred to as a material produced from pulverised grains by cold moulding. The technological parameters of the moulding process are adjusted in such a manner that the obtained titanium frit have the desired mechanical strength, while reaching substantial porosity. In a preferred embodiment, as shown in FIG. 1B, the titanium granulate preferably comprises three different sizes of titanium grains in a layered structure, in which the layers are arranged in the order of the grain size in such a way that before moulding, a relatively coarse-grained titanium powder 12a (preferably comprising grains having a dimension of 600-1200 μm) is put into the bearing member 11, then a titanium powder 12b of medium sized grains (preferably comprising grains having a dimension of 350-600 μm) is applied thereon, and finally, a fine-grained titanium powder 12c (preferably comprising grains having a dimension of 150-350 μm) is applied thereon. Accordingly, the titanium frit produced by moulding and the cathode side electrode support 12 made therefrom will issue a grain size gradient in the direction of depth.

The cathode side bearing member 11 is made of a special, chemically resistant plastic shaped to e.g. an annular member. It is obvious, however, that the bearing member 11 may be made of any other material and may have any other shape as well.

An essential condition for the efficient cell operation is the good electrical contact between the electrodes 13, 16 and the membrane 15. Therefore, formation of the electrode 13 on the electrode support 12 made from the titanium frit has key importance. In the electrode structure 10 according to the invention, it is preferred that for the negative electrode 13, extra fine-grained platinum powder (so called platinum black) is used.

The platinum black is applied to the electrode support 12 at ambient temperature and pressure, without the use of a protective gas (ie. at ambient air) and in the form of a suspension. The suspension is made from the aqueous solution of 40 mg platinum black and sodium dodecyl sulphate (SDS) of 1 ml with a concentration of 0.001 mol/l. For homogenisation of the suspension, an ultrasonic bath is used for a period of 5 minutes. The suspension remains stable till its application, that is, no deposition can be detected. The electrode support 12 is placed onto an absorbent paper, and then the suspension is applied onto the surface of the electrode support 12 comprising, the finest grains, in small quantities by means of an automated pipette. While the solution is leaking through the electrode support 12 made of the porous titan frit, the electrode 13 in the form of a continuous platinum layer is caused to develop on the surface of the electrode support 12. Smoothness of the surface thus obtained may be improved, if necessary, for example by pressing. In an alternative method of applying the electrode 13, water may be used instead of the SDS solution to produce the suspension, which reduces the production costs.

The proton exchange (or proton conducting) membrane 15 is preferably in the form of a sulphonylated, perfluorinated polymeric resin membrane, most preferably the polymeric membrane Nafion® of DuPont de Nemours, Co. The membrane 15 constitutes the solid electrolyte of the ozone generating electrolysis cell according to the invention. In addition, the membrane 15 also provides the separation of the gases produced on the cathode side and the anode side. The water required for the electrolysis is introduced at one side of the membrane 15, through the second electrode support 12 provided with the electrode 13, whereas the gaseous mixture of oxygen and ozone to be processed is produced on the other side of the membrane 15, that is, at the ozone generating electrode 16. It should be noted that harmful deformation/straining of the membrane 15 resulted from the pressure effected through the electrode structure 10 may be reduced to the lowest possible extent by providing an extremely smooth surface on the electrode 13 (formed by the membrane 15) produced in the above mentioned manner. This contributes to the elongation of life time of the ozone generating cell according to the invention.

The positive electrode 16 serves for supporting the anode side electrochemical reaction. For the positive electrode 16, electrically conducting metals, semimetals and/or oxides thereof are used in general. The use of the oxides of transition metals is advantageous because those are commonly available and inexpensive. However, the mechanical strength of these oxides is low, thus they have to be placed on a substrate with high mechanical strength and chemical resistance against the highly corrosive gaseous mixture of oxygen and ozone so that said oxides could tolerate the high pressures arising in the cell during operation without being mechanically damaged.

For the electrode support 17 used to support the positive electrode 16, noble metals (e.g. platinum) with good electrical conductivity or the alloys and/or mixture thereof can be used. In the cell according to the invention, a suitably perforated platinum sheet provided with through-holes preferably having a diameter of at least 0.8 mm is used as the electrode support 17.

The anode side bearing member 18 serves for removing the gaseous mixture of oxygen and ozone produced at the electrode 16 during operation of the cell away from the electrode 16. The bearing member 18 is additionally used to fasten the electrode support 17 to the electrode 16 and the latter to the membrane 15 in order to provide a perfect electrical contact, as well as to provide a homogenous transition surface therebetween. In a preferred embodiment of the electrode structure 10 shown in FIG. 1A, the bearing member 18 is made of a resilient, porous, chemically resistive material, preferably PTFE frit produced from grained PTFE by high-pressure moulding. The bearing member 18 is provided with a through-hole 19. In the assembled ozone generating electrolysis cell according to the invention, the through-hole 19 is adapted to receive an anode side conducting member used for electrically connecting the anode side electrode support 17 to an external DC power supply (see FIG. 2).

In the electrolysis cell according to the invention, the ozone generating electrode 16 is made of a material with good electrical conductivity, plasticity, high overvoltage with respect to the evolution potential and chemical resistance against the highly corrosive gaseous mixture of oxygen and ozone, preferably a mixture of lead dioxide and PTFE comprising PTFE in an amount of at least 10% by weight. The mixture of lead dioxide and PTFE is produced from solid-phase raw materials at ambient temperature by a process described below, with no use of further additives.

On the lead dioxide, which constitutes a component of the mixture of lead dioxide and PTFE, the evolution potential of the oxygen is very high and thus, the desired ozone can be produced thereon with a high conversional efficiency. Said component is advantageous because it is inexpensive, commonly available, chemically inert (due to not having a higher oxidation state) and insoluble in the majority of solvents and it has better electrical conductivity than certain metals. It is well known that during the ozone generation, the crystal modification β of the two possible crystal modifications α and β of lead dioxide can be used to perform the desired oxygen-to-ozone conversion, wherein during the conversion, as proved by X-ray diffraction measurements, a β-type interfacial recrystallization takes place. It means that an alteration of the applied lead dioxide (ie. recrystallization in the course of the reaction) is needed, which shows a constant value after a period of 2 to 12 days. Before producing the positive electrode 16, the lead dioxide is subject to continuous grinding, which results in the production of lead dioxide grains of colloid size, ie. with an average grain size of 0.5-100 μm, from the initial macroscopic sized lead dioxide pieces.

For the other component of the material of the electrode 16, PTFE elementary filaments having a fibrous (cotton wool-type) structure, a thickness of 50-100 μm and a length of up to 1 mm are used. PTFE filaments with such dimensions can be produced by abrasive machining or abrasion of a PTFE block. The dimension of the initial PTFE elementary filaments has a definite effect on the plasticity and resiliency of the final mixture of lead dioxide and PTFE.

In order to produce the material of the positive electrode 16, lead dioxide ground into grains of colloid size in an amount of, for example, about 1600 mg and PTFE in the form of fine elementary filaments in an amount of, for example, about 300 mg are put into a mixing jar. The apolar materials can easily mix with one another. After some agitation, preferably for a period of 10 minutes, the mixture thus obtained is poured into a frit moulding tool especially formed for this purpose and then pressed therein by applying a pressure of at least 50 MPa, preferably 250 MPa, to shape a sheet with a thickness of 0.25 mm. During the moulding process, the PTFE filaments get tangled and fused, causing the lead dioxide grains to be joined at the same time. According to a microscopic examination of the resulted lead dioxide/PTFE sheet, it has been established that the material thus obtained has compact dimensions and a continuous surface, it can be easily formed mechanically, and in addition, it is resilient and ductile. Finally, the electrode 16 is produced by cutting to the desired size and shaping the resulted lead dioxide/PTFE sheet.

It should be here noted that an amount of about 16% by weight of PTFE in the above mentioned mixture of lead dioxide and PTFE is advantageous in respect of both plasticity/resiliency and electrical conductivity. In case of utilisation of larger amount of PTFE, the mixture will be more plastic but electrically less conductive. In case of addition of smaller amount of PTFE, however, the mixture will be less plastic but electrically more conductive.

It is important that by means of grinding or another treatment exerting great shearing forces, the PTFE is subject to a structural conversion that, according to our experiences, results in a stabilizing effect on the β-type crystal modification of lead dioxide. As the method according to the invention, unlike prior art methods, does not include a step of heat treatment, no harmful crystal modification changes occur due to that. It has been experienced that the conductivity of the fibrous electrode is significantly higher than that of a material having a grained structure.

In the embodiment of the electrode structure 10 described above, the positive electrode 16 and the anode side electrode support 17 are formed as separate members. It should be noted, however, that the ozone generating electrode 16 and the first electrode support 17 may be formed as a combined electrode in such a manner that a thin platinum layer is applied on a (external) surface of the electrode 16 made from the mixture of lead dioxide and PTFE.

When designing the construction of the cell 100 manufactured of the electrode structure 10, as shown in FIG. 2 in its assembled state, and when selecting the materials for the cell 100, the chemical resistance against the gaseous mixture of oxygen and ozone and the mechanical strength coming from the pressure of the gas produced by the electrolysis of water are kept in view. The cell 100 in its assembled state is composed of a cathode side half cell 110 and an anode side half cell 115 that are fixed together in a form-fitting and thereby sealed manner. The electrode structure 10 is arranged in a seat 140 formed in the half cell 110 and defined by a bottom wall and a side wall, wherein the bearing member 11 of said electrode structure 10 abuts on the bottom wall of the seat 140 (see FIG. 1A). The form-fitting abutment is established between the outer surface of a compressive flange 145 of the half cell 115 and the side wall of the seat 140. The half cell 115 is provided with a depression 148 for receiving the anode side of the electrode structure 10, wherein said depression 148 is laterally defined by the compressive flange 145. In the assembled cell 100, the bearing member 18 of the electrode structure 10 (shown in FIG. 1A) is in close contact with the half cell 115 in the depression 148, whereas compressive flange 145 pushes the electrode structure 10 to the bottom wall of the seat 140 with firmly fixing it thereby.

The cathode side half cell 110 is provided with through-holes (without reference numbers in the drawing) for sealingly receiving a water feeding connector 160, a hydrogen and water discharging connector 162 and a cathode side electrical connector casing 130. The anode side half cell 115 is provided with through-holes (not marked in the drawings) for sealingly receiving an ozone/oxygen gas discharging connector 165 and an anode side electrical connector casing 135. The half cells 110, 115 are made of a chemically resistant, gas-proof material, preferably some kind of plastic, and formed preferably by injection moulding, machining or another shaping process.

In the electric connector casing 130, there is at least one current conducting member 150 (see FIG. 1A) arranged for providing electrical connection between the external power supply and the negative electrode 13. The current conductive member 150 is in the form of a member with the capability of reversible deformation along its longitudinal axis and thereby the exertion of a compressing force, said member 150 preferably being in the form of a cylindrical spring. It is also preferred that the electrical conductive member 150 is made of titanium.

In the electrical connector casing 135, there is at least one current conducting member 155 (see FIG. 1A) arranged for providing electrical connection between the external power supply and the electrode support 17. The current conductive member 155 is in the form of a member with the capability of reversible deformation along its longitudinal axis and thereby the exertion of a compressing force, said member 150 preferably being in the form of a cylindrical spring. It is also preferred that the electrical conductive member 155 is made of platinum. The use of electrical conductive members 150, 155 in the form of resilient parts allows to eliminate the changes in dimension due to size deviations and temperature fluctuations.

The external walls of the half cells 110, 115, ie. the walls not contacting with the electrode structure 10, are provided with a cathode side confining plate 120 and an anode side confining plate 125, respectively. The confining plates 120, 125 serve for protecting the half cells 110, 115 against the external mechanical influences. Accordingly, the confining plates 120, 125 are made of a material with high mechanical strength, preferably stainless steel. The water feeding connector 160, the hydrogen and water discharging connector 162 and the cathode side electrical connector casing 130 are firmly (but releasably) fixed into through-holes (not marked in the drawings) formed in the confining plate 120. Similarly, the ozone/oxygen gas discharging connector 165 and the anode side electrical connector casing 135 are firmly (but releasably) fixed into through-holes (without reference numbers in the drawing) formed in the confining plate 125. Finally, in order to hold the cell 100 in one piece, to seal the electrode structure 10 constituting the central part of the cell 100 and to provide the required electrical and mechanical contacts between the parts of the cell 100 (shown in detail in FIG. 1A) in the half cells 110, 115, through-bolts 185 are arranged in through-holes formed in the half cells 110, 115 and in the confining plates 120, 125, said through-bolts 185 being fastened by screw nuts 190.

After producing the above mentioned structural elements, the cell 100 according to the invention is assembled in the steps described below. First, the through-bolts 185 are inserted into the through-holes formed in the cathode side confining plate 120, then the cathode side half cell 110 is arranged on the confining plate 120 with its seat 140 facing upwards. Next, the cathode side electrode support 12 and the negative electrode 13 accommodated in the bearing member 11 are arranged in the seat 140 in a position of contacting with the half cell 110. The electrode support is then wetted and the proton conductive membrane 15, which has been cut to size and shaped, is placed thereon, followed by wetting said membrane 15 as well. Subsequently, the electrode 16 already cut to size and shaped and the anode side electrode support 17 are arranged on the membrane 15. Then the anode side bearing member 18 is placed onto the electrode support 17 and the anode side half cell 115 is pushed onto the assembly thus obtained, causing thereby the various parts of the electrode structure 10 to be securely fixed. Next, the bearing member 18 is wetted, the confining plate 125 is placed onto the half cell 115 and the structural elements of the cell 100 are forced to each other by screwing the screw nuts 190 to the through-bolts 185, thereby providing the electrical and mechanical contacts between the structural elements, as well as the sealed joints. Finally, the connectors 160, 162, 165 and the connector casings 130, 135 with the current conducting members 150, 155 are mounted into the cell 100.

During operation of the ozone generating electrolysis cell 100 according to the invention, water is fed into the side of the cell 100 adjacent to the negative electrode 13, and through the porous cathode side electrode support 12 and the porous cathode, the water flows to the proton conductive membrane 15 and further to the positive electrode 16 through the membrane 15. While applying a DC voltage with a proper polarity for the cell 100 at the same time, the electrolysis of the water is caused to start at the electrodes 13, 16, and hydrogen ions with positive charge move from the positive electrode 16 to the negative electrode 13 through the proton conductive membrane 15. The hydrogen ions transform into hydrogen of neutral charge by accepting an electron from the negative electrode 13. At the same time, oxygen and ozone is generated at the positive electrode 16 as a result of the electrolysis. The efficiency of the ozone conversion, ie. the amount of ozone in the produced gaseous mixture of oxygen and ozone, is determined by the quality of the electrode 16 and the operational parameters. The amount of the generated gaseous mixture of oxygen and ozone and thereby its pressure under particular conditions may be adjusted by changing the electrolysing current. The amount of ozone in the gaseous mixture of oxygen and ozone generated by the cell 100 according to the invention is preferably at most 12% by volume.

Cooling the cell is provided by means of a water flow introduced through the connector 160 and diverted partly through the connector 162. It should be noted that on the anode side of the cell 100 according to the invention, there is no need to divert water since water not taking part in the electrolysis moves away from the anode side with exhausting from the cell 100 through the connector 165, together with the gaseous mixture of oxygen and ozone, in the form of steam.