NO177393B - Electrolytic cell for reduction of oxygen to hydrogen peroxide, agent for the construction of the cell and method for producing a gas diffusion cathode for the cell - Google Patents

Description

The present invention relates to an electrolysis cell for the reduction of oxygen to hydrogen peroxide at a cathode in the presence of an aqueous, alkaline electrolyte in a cell with an electrolyte inlet, an electrolyte outlet, devices for forcing the electrolyte from the electrolyte inlet to the electrolyte outlet, separation devices that divide the cell into an anode compartment containing an anode and a cathode compartment with a porous, gas-permeable cathode, and a means useful in the construction of said electrolytic cell, and a method of manufacturing a gas diffusion cathode for said electrolytic cell with a first cathode surface in contact with an electrolyte in the cell, and a second cathode surface forming an upper surface of the cell.

For over a hundred years it has been known that oxygen can be reduced at a cathode with the formation of hydrogen peroxide. Despite the very low voltage of the half-cell reaction, the method has never been commercialized.

U.S. patents 4,406,758 and 4,511,441 describe a method for operating an electrochemical cell using a gas cathode. The electrolyte is introduced into the cell in the anode chamber where a gas such as oxygen or chlorine is formed. The electrolyte then passes through a separator into a "dripped bed" or self-draining cathode. Oxygen gas is also introduced into the cathode and is reduced to form hydrogen peroxide. The hydrogen peroxide can optionally be split or collected and used as a bleaching solution. ■

Both of these patents describe that the desired electrolytic reaction with gas will only take place where there is a three-phase contact between a gas, an electrolyte solution and a solid electrical conductor. The patents teach that it is necessary to equalize the hydraulic pressure of the electrolyte on the anode side of the separator and on the cathode side of the separator in order to maintain a regulated flow of electrolyte into the cathode and to maintain oxygen gas throughout the cathode. Pores of sufficient size and in a sufficient number are provided in the cathode so that both gas and liquid can simultaneously flow through the cathode.

The presence of oxygen is necessary at an oxygen cathode not only to maintain high efficiency, but also to avoid a catastrophic explosion. In the presence of an alkali metal hydroxide, the total oxygen cathode reaction is the reaction between oxygen and water to form hydroxyl ions and perhydroxyl ions (anions of hydrogen peroxide, a very weak acid). The cathode reaction is

(1) 202 + 2HzO + 4e~ 2H02" + 20H", and the anode reaction is

(2) 40H" 02 + 2H20 + 4e" with the total reaction

(3) 02 + 20H" -> 2H02". In the absence of oxygen at the cathode, this half-cell reaction is (4) 2H20 + 4e'-» H2 + 20H".

Undesired side reactions can also take place at the cathode

(5) H02~ + H20 + 2e~ -<■ 30H" and at the anode

(6) H02" + OH" - > 02 + H20 + 2e~.

Consequently, it is important to avoid that a local high concentration of the perhydroxyl ion (H02_) accumulates in the catholyte.

Equation (4) can dominate if the cathode does not contain oxygen gas or hydrogen peroxide (equation 5) either because the cell is flooded with electrolyte or because the supply of oxygen is insufficient. In the absence of oxygen at the cathode, hydrogen gas will form. The hydrogen gas can form an explosive mixture with oxygen gas in the oxygen supply manifold. Alternatively, if insufficient oxygen was introduced into the cathode, hydrogen would form in the oxygen-poor portion which would mix with oxygen in the oxygen-rich zone to form an explosive mixture.

In the U.S. patents no. 3,454,477, 3,459,652, 3,462,351, 3,506,560, 3,507,769, 3,591,470 and 3,592,749 belonging to Grangaard, the cathode is a porous plate with the electrolyte and oxygen supplied from opposite sides for reaction on the cathode. The porous gas diffusion electrode requires a wax coating to fix the reaction zone and careful equalization of oxygen and electrolyte pressure to keep the reaction zone on the surface of the porous plate.

The electrolysis cells according to U.S. Pat. patents 4,406,758 and 4,511,441 have a problem in that the vertical dimension of the cell cannot be varied over a large area due to the need to equalize the hydraulic pressure differences across the separator and the need to avoid flooding the cathode with electrolyte; an unregulated flow of liquid through the separator is considered undesirable.

U.S. Patent No. 4,118,305 to Oloman attempts to overcome the problems of balancing the hydrostatic forces for maintaining a three-phase system of a solid electrode (cathode), a liquid electrolyte, and oxygen gas by having a mixture of oxygen gas and a liquid electrolyte continuously flows through a fluid-permeable cathode, such as a porous layer of graphite particles. A porous separator separates the packed-layer electrode from the neighboring electrode and is supported by the packed-layer electrode. The pores in the separator are sufficiently large that a regulated flow of electrolyte into the openings in the packed-layer electrode becomes possible. Electrochemical reactions take place inside the electrode at a gas-electrolyte-electrode interface.

The liquid products and unreacted electrolyte flow by gravity to the bottom of the packed-bed electrode. Mass transfer is a problem in such cells because the electrode is almost flooded with electrolyte. The reactions are slow and recycling of product is necessary for satisfactory product strength, and recycling of the excess oxygen gas is necessary for economical operation, and an above-atmospheric oxygen pressure is usually required.

Each of these electrolysis cells according to the state of the art has the disadvantage that a voltage is required which is mainly greater than the sum of the theoretical half-cell voltages due to the high ohmic resistance of the cells. A further disadvantage of these cells is that they lack devices for varying the cell's capacity during operation and the difficulty in establishing uniform electrolyte flow rates in the cell.

The characteristics of an ideal separation device are well known to those skilled in the art. It should be cheap, have a certain mechanical strength and stiffness, be resistant to cell reactants, products and operating conditions. Moreover, the ideal separator is described as permeable to ions but not to molecules, with high void fraction to minimize electrical resistance, with small mean pore size to prevent the passage of gas bubbles and minimize diffusion, homogeneous to ensure good current performance and uniform current distribution , and non-conductive to prevent functioning as an electrode.

The present invention overcomes the shortcomings of the previous cells. The invention is an electrolysis cell for the reduction of oxygen to hydrogen peroxide at a cathode in the presence of an aqueous, alkaline electrolyte of the type mentioned at the outset, characterized in that the cathode has a first surface in contact with the electrolyte in the cathode space and a second surface which forms an external surface of the cell in contact with an oxygen-containing gas, the cell containing means for directing oxygen gas away from the separation means, the separation means being substantially permeable to an ion in the electrolyte, and the anode compartment being provided with means for forcing the electrolyte to flow from the electrolyte inlet through the first surface of the cathode to the electrolyte outlet at a rate equal to the rate of electrolyte flow in the cathode space. Use of the cell for the production of hydrogen peroxide is considered to be within the scope of the invention.

In the first preferred embodiment, the separation device is permeable to electrolyte as well as to an ion in the electrolyte, this embodiment comprising a cell with an electrolyte inlet, a porous, self-draining cathode having a first surface in contact with the electrolyte and a second surface forming an outer surface of the cell, an electrolyte outlet arranged to receive electrolyte drained from the cathode, an anode, separation device between the cathode and the anode. The separator is substantially permeable to the electrolyte and defines an anode chamber containing the electrolyte intake and a cathode chamber. The second surface of the cathode is in contact with an oxygen-containing gas, and means are provided for controllably propelling the electrolyte from the electrolyte inlet through the separation device and into the self-draining cathode at a rate substantially equal to the rate of draining of the electrolyte from the cathode and in an amount which is sufficient to fill only part of the pores in the cathode and with means for blowing a gas in the anode chamber out of the electrolytic cell, the arrangement for conducting oxygen gas formed at the anode in the anode chamber electrolyte away from the anode is necessary to prevent increase of the ohmic resistance of the cell.

In a particularly desirable embodiment of the present invention, the cell is placed so that the cathode is held in a generally horizontal position. If the anode is placed in the cell in a position above the cathode, it may be desirable for the anode to provide holes or pores as means to direct the floating oxygen gas in the electrolyte in the anode chamber away from the separator. Desirable means for diverting oxygen gas may include not only shutter valves in the anode, but also channels in the anode which direct the bubbles up and to one or both sides, e.g. in an 11 herringbone" pattern. Equally effective are mechanical wipers or "paddle wheels" which can be operated by the rising bubbles both to push the other bubbles from the area and to push fresh solution into the space between the anode and the separation device.

In another particularly desirable embodiment of the present invention, the cathode is arranged in a generally horizontal position with an angle of 5 - 25°. The cathode is above (higher than) the anode and a porous felt material is arranged between the anode and the cathode, providing a means of directing oxygen gas away from the anode. The cathode consists of granular particles supported by the porous felt. The aid to driving the electrolyte from the electrolyte inlet into the self-draining cathode is the static pressure difference obtained by the electrolyte inlet being above the electrolyte outlet, and the wicking effect of the porous felt, the porous felt also serving as a separation device and a device for directing oxygen gas away from the anode .

The cathode is an electrically conductive porous mass through which a large number of pores and channels pass. It can be a layer of electrically conductive particles sintered to form a uniform mass or an agglomerate of loose particles. It must have pores of sufficient size and number to allow gas to flow through it. The channels must be of a size sufficient for non-volatile products to flow from the cathode by gravity, i.e. that the cathode should be "self-draining". Another way of expressing this is to describe the channels as large enough that gravity has a greater effect on the liquid in the electrode than the capillary pressure.

The devices for driving the electrolyte from the electrolyte intake through the separator and into the self-draining cathode and for controllably driving the electrolyte through the separator can be combined by tilting the cell so that the electrolyte intake is raised above the electrolyte outlet. Alternatively, the electrolyte may be driven by means of a pump or other device for providing a greater pressure at the electrolyte inlet, and the aid to controllable propulsion of the electrolyte through the separator and into the self-draining cathode may be a uniform reduction of the cell's cross-sectional area from its inlet end to its outlet end.

Any suitable separation device may be used in the cell; e.g. a ceramic diaphragm, an ion-selective membrane such as a cation membrane which is also porous to the aqueous electrolyte. Other separation devices such as microporous plastic, a mat of asbestos, woven or felted fibers or porous plastic may also be suitable. A support may be required as part of the separation device.

The initially mentioned agent consists of layers in the following order: a first non-conductive, porous device which is inert to an alkaline liquid, a separation device, a second, non-conductive, porous device inert to an alkaline liquid containing hydrogen peroxide, and a porous cathode, which separation means is substantially permeable to both ions and gases but is substantially impermeable to liquids, the first and second porous means being permeable to liquids, and attachment means for keeping each of the layers in contact with a surface of the adjacent see ict.

The method for producing the gas diffusion cathode is characterized by the fact that the cathode is produced by impregnating a graphite cloth with sufficient polytetrafluoroethylene resin to provide 40-70% by weight of the resin on the graphite cloth, coating the impregnated graphite cloth with an aqueous slurry of approximately equal parts of carbon black and polytetrafluoroethylene ethylene, and sintering of the coated impregnated cloth, the sex smoke being applied in a sufficient quantity to give 5-15 parts by weight of sex smoke per 100 parts graphite cloth was assumed, again applying a sufficient amount of a second coating of a slurry of a quantity of suspension of approx. 9 parts by weight of sex smoke to 1 part of polytetrafluoroethylene resin as a slurry for approx. 105 parts by weight of water and 15 parts by weight of a non-ionic surfactant, which gives 5-15 weight percent of the graphite cloth after sintering, and sintering the cloth in air at 360-370°C.

The following figures illustrate in detail three of the preferred embodiments of the invention. Fig. 1 is a cross-section of a cell where the cathode, separator and anode are arranged in a generally vertical position. Fig. 2 is a cross-section of a cell where the cathode, separator and anode are arranged in a generally horizontal position with the anode at the top. Fig. 3 is a cross-section of a cell where the cathode, separator and anode are arranged in a generally horizontal position with the cathode at the top. Fig. 1 illustrates an electrolysis cell. The cell has a shutter-ventilated anode 120 which is housed in an anolyte chamber 127. An electrolyte intake port 116 opens into the anolyte chamber. A gas product outlet opening 122 is provided in the anolyte chamber 127. The first surface of the cathode 106 is in contact with the electrolyte in the cathode chamber, and the second

surface constitutes an outer surface of the cell and is in contact with an oxygen-containing gas such as air. An electrolyte outlet port 108 collects liquid electrolyte from the cathode. Separation device 112 divides the cell into an anode chamber and a

cathode chamber.

The separation device 112 can be a large number of layers or a single layer. However, the material should be essentially inert to the chemicals with which it will come into contact under normal operating conditions. The separation device is constructed so that it provides a somewhat limited opportunity for liquid to flow through it. The anode 120 is preferably provided with shutter valves and is connected by conductor 101 to a positive voltage source (not shown). Similarly, the cathode 106 is connected at conductor 102 to a negative voltage source.

During operation, electrolyte is introduced into the cell through intake opening 116 and driven through separator 112 into the cathode chamber and into cathode 106. The liquid drips down through the channels in the cathode by gravity and is collected and removed from the cell through electrolyte outlet opening 108. An electrical potential or voltage is applied between anode 120 and cathode 106; at the anode, oxygen gas is formed, which rises as bubbles in the electrolyte between anode 120 and separation device 112. The bubbles are led by means of the shutter valves to the other side of anode 120 and are then blown out through opening 122. At cathode 106, oxygen that diffuses from the air into in the cathode 106, forming hydrogen peroxide when it comes into contact with the electrolyte therein. The hydrogen peroxide-rich electrolyte drips down inside the channels in the cathode 106 and is collected at the electrolyte outlet opening 108.

For the purposes of this invention, the difference between the channels and pores is that in a channel the effect of gravity on the electrolyte is greater than the effect of capillary forces, and in a pore the effect of gravity is less on the electrolyte than the effect of capillary forces.

In the cell, the liquid flow through the separator 112 should be regulated at a level sufficient to fill only a portion of the pores in the cathode 106. If too much liquid passes through the separator and substantially all the pores in the cathode 106 are filled, the oxygen gas is displaced. This can result in the formation of explosive hydrogen gas. If, on the other hand, too little electrolyte passes through the separator 112, the electrochemical reactions will be minimized. The present invention prevents the almost total filling of the cathode pores while at the same time the almost total absence of electrolyte from the cathode is prevented.

Fig. 2 is similar to fig. l, except for the generally horizontal rather than vertical orientation of the cell. Each of the elements comprising the cell is numbered the same as the corresponding element in fig. 1, except that "200" is used instead of "100". One exception is that outlet opening 108 is replaced with a large number of small diameter outlet openings, represented by 238A, 238B - 238Z, which act as channels. The effect of gravity on the electrolyte in the outlet openings provides a slight suction inside the cathode 206, which draws the electrolyte into the outlet openings and thereby prevents the electrolyte from filling the pores containing oxygen gas.

For the purposes of this invention, the term "generally horizontal" may include angles of up to approx. 45°.

It is clear that the outlet openings 238A, 238B - 238Z need not be perpendicular to the cathode 206. E.g. the outlet openings may be inclined at an angle so that they are substantially vertical even though the cathode 206 is inclined from the absolute horizontal position.

A drawing of another embodiment according to the invention, cell 300, is shown in fig. 3.

Fig. 3. Anode 301, a nickel or stainless steel plate, is arranged in a generally horizontal position between electrolyte reservoir 302 and electrolyte surge tank 303. A piece of polyester felt cloth 304 is supported on anode 301 , with a first end in reservoir 302 constituting an electrolyte intake and the other end in equalization tank 303 constituting an electrolyte outlet. Electrolyte is driven through the cell by the wicking action of polyester felt 304 and by the static pressure difference between the level of electrolyte in reservoir 302 and electrolyte equalization tank 303. Reservoir 302 contains sufficient electrolyte 306 such that the upper surface of electrolyte 306 is higher than the other end of polyester felt. 304 at electrolyte equalization tank 303. An electrically conductive cathode 307 consisting of carbon black bonded to graphite shavings is arranged to provide a first surface in contact with and over polyester felt 304. The second surface of the cathode constitutes an outer surface of the cell 3 00. The cell 300 consists of the anode 301, the polyester felt part 304 which lies adjacent to the anode, and the cathode 307. The polyester felt 304 delimits the space between the anode 301 and the cathode 307 in an anode chamber (not shown) and a cathode chamber (not shown, but including a part of cathode 307). Conduit device 308 supplies the electrolyte reservoir 302 with electrolyte from a source (not shown). Conductors 310 and 311 provide voltage to anode 301 and cathode 307, respectively, from a source (not shown).

During operation, electrolyte is drawn from reservoir 302 by the wicking action of felt 304 into cell 300 where oxygen gas is formed. The oxygen is led from the anode chamber by the felt 304 into the cathode chamber and to the cathode 307 where it is reduced to hydrogen peroxide. Additional oxygen diffuses from the oxygen-containing gas at the electrolyte interface in the surface of the cathode 307 where it is also reduced to hydrogen peroxide. The electrolyte is driven from the electrolyte inlet to the electrolyte outlet by the static pressure difference between the level of electrolyte in reservoir 302 and electrolyte equalization tank 303 in combination with the wicking action of the separation device 304.

A person skilled in the art will understand that with the present invention, oxygen can always diffuse into the cathode because the cathode forms an outer surface of the cell and is always in contact with the atmosphere.

The cells exemplified in fig. 2 and 3, has a further advantage over a substantially vertical cell in that the hydrostatic pressures are uniform across the separator and the cathode so that the rate of oxygen diffusion into the cathode and the flow rate of electrolyte through the separator and into the cathode are also uniform throughout the cell .

There are two suitable methods for regulating the current through the separator into the electrode. One method is to vary the area of the separating device that is in contact with the liquid, and another method is to adjust the pressure drop across the separating device.

In a vertical cell, a suitable way to regulate the area of the separator in contact with the liquid is to increase or decrease the height of the liquid reservoir for the anode chamber adjacent to the separator. As the height is increased, the flow through the separator increases. On the other hand, the flow decreases as the height decreases. However, this area of cathode and anode in contact with the electrolyte and consequently the cell capacity varies.

Another method for regulating the flow through the separating device in a vertical cell is by regulating the pressure drop across the separating device. The pressure drop can be regulated in several ways.

One method for regulating the pressure drop across the separating device in the cell in fig. l is that the anode chamber is operated under gas or liquid pressure. In this method, the opposite chamber is sealed against the atmosphere, and gas pressure or liquid pressure is exerted on the electrolyte. Pumps may be used to force a fluid under pressure into the opposite chamber, or the pressure may be maintained by means of a valve associated with ports 122 or 222.

In a particularly preferred embodiment of the present invention, the separation device is permeable to gas, but not to liquid. This embodiment comprises a cell with an electrolyte inlet, an electrolyte outlet, a porous cathode permeable to gas, the cathode having a first surface which is in contact with the electrolyte and a second surface which constitutes an outer surface of the cell in contact with an oxygen-containing gas, a anode, separating device between the cathode and the anode, and device for driving the electrolyte from the electrolyte inlet to the electrolyte outlet. The separator delimits an anode chamber and a cathode chamber in the cell, the separator being mainly permeable to an ion in the electrolyte and to a gas, but mainly impermeable to the flow of the electrolyte from the cathode chamber to the anode chamber. The cell is arranged with the cathode and anode in a generally horizontal position with the cathode above the anode, the anode chamber is provided with means for conducting oxygen gas formed at the anode to the separator and for driving electrolyte to flow over the surface of the anode, and the cathode chamber is provided with means for driving the electrolyte from the electrolyte inlet over the first surface of the cathode.

The means for directing the oxygen gas to the separator and driving the electrolyte to flow uniformly over the anode can be combined and can be any gas permeable porous material such as felt, woven cloth or a compound foam material. Other suitable devices include flow vanes in the anode chamber which direct the oxygen bubbles to the separator and which direct the electrolyte over the surface of the anode. A gas-permeable porous device is particularly desirable because its wicking effect which supports the propulsion of electrolyte from the electrolyte intake to the electrolyte outlet.

The aid for causing the electrolyte to flow uniformly over the surface of the cathode may be similar to the aid in the anode chamber. In both cases, the aid can be provided at a very small distance between the cathode and the separator so that the capillary effect of the first surface of the cathode and the adjacent surface of the separator on the electrolyte approaches the effect of gravity.

For the purposes of the present invention, the expression "mainly permeable both to an ion in the electrolyte and to a gas, but mainly impermeable to the flow of electrolyte from the cathode chamber to the anode chamber" is to be understood so that under normal operating conditions bubbles of oxygen gas formed at the anode can pass freely through the separation device from the anode chamber to the cathode chamber, but very little electrolyte is transferred from the cathode chamber to the anode chamber.

One commercially available separator suitable for the present invention is a hydrophilic laminate of polyester felt and an expanded polytetrafluoroethylene consisting of knots and interlocking fibrils marketed by W. L. Gore and

Associates. The separator is rated in a standard ASTM test F778 as 3.8 m<3>/S at 125 Pa. The polyester felt portion of the laminate is suitable both as a device for conducting oxygen gas from the anode to the separator and for driving the anolyte to flow smoothly across the anode, or as a device for directing the electrolyte to flow smoothly across the cathode.

Another suitable separation device is a microporous polypropylene film having a thickness of 2.5 x 10"<2> mm and having 38% porosity with an effective pore size of 0.02 micrometers, which is marketed by Celanese Corporation. The pores provide the desired electrical conductivity, but hinders electro-auditory flow. The film was perforated with openings without any material being removed. The openings function as non-return valves and are spaced about one centimeter apart in a row and column matrix. The openings, eg 0 .5 - 1 mm slits, function as small bunsen valves which are opened so that oxygen gas can flow from the anode chamber into the cathode chamber and which are closed so that the flow of electrolyte from the cathode chamber to the anode chamber is excluded.

An ion-conducting membrane, perforated in the same way, is also suitable for use as a separation device. A typical commercial membrane is marketed by RIA Research Corporation under the trade name Raipore BDM-10 membrane. It comprises a grafted low density polyethylene base film with a cationic weak base monomer as graft.

It is clear that the separator used in this embodiment of the present invention differs from the well-recognized "ideal separator" in that it not only has a small mean pore size which makes it permeable to ions and not to molecules, but also has openings of sufficient size for gas bubbles to pass (gas openings) without any significant diffusion or backmixing of hydrogen peroxide from the cathode chamber to the anode chamber being possible. The optimal size, shape and distribution of the gas openings can be determined without unnecessary trials. The shape of the openings can be straight slits, crosses, v's, or just point perforations. It is desirable that the openings are formed by piercing the separating device without any material being removed from the separating device. The separation device is usually arranged so that the oxygen bubbles pass through in the direction in which the holes were formed. In this way, the oxygen gas bubbles act as part of the "valve".

A particularly suitable product article for this embodiment of the cell comprises layers in sequence; a first non-conductive porous device which is inert to alkaline liquid, a separation device, a second non-conductive porous device which is inert to alkaline liquid containing hydrogen peroxide, and a porous cathode, the separation device being substantially permeable both to ions and to gases, but is substantially impermeable to liquids, the first and second porous means are permeable to fluids, and fasteners keep each of the layers in contact with a surface of the adjacent layer. The complete product article is referred to herein as a cellular mat.

An electrolysis cell where the cell mat is used is mounted by placing the cell mat on a generally horizontal conductive anode. The cell mat is arranged on top of the generally horizontal anode with the first porous device in contact with the anode. A current conducting device is placed on top of the cell mat in electrical contact with the cathode on the upper surface of the cell mat, and the electrically conducting device is provided with channels so that it is possible for a gas to come into contact with the anode.

Preferably, the first and second porous devices are formed of felted inert fibers, woven inert fibers, knitted inert fibers or an inert foam material with interconnected pores.

Any suitable porous inert conductive material known to be suitable as an oxygen electrode may be used as the cathode, such as a sheet of commercially available reticulated glassy carbon used in the U.S. Pat. Patent No. 4,430,176, porous graphite or a composite electrode consisting of carbon particles bonded to an electrically conductive, porous substrate as described in U.S. Pat. patent no. 3,459,652 where the binder is paraffin. An electrode of activated carbon bonded with PTFE and natural rubber on a nickel mesh described in U.S. Pat. patent no. 4,142,949 is also suitable. Other electrodes known to be suitable are described in U.S. Pat. patent no. 3,856,640 where carbon particles bound with polytetrafluoroethylene and porous carbon electrodes suitable for fuel cells are used. It is desirable that the cathode be flexible such as one where graphite felt or woven or knitted graphite cloth is used as a substrate for carbon particles such as any described in published French Patent No. 2,493,878. A cathode where a graphite substrate is used and carbon particles bound with polytetrafluoroethylene are used is particularly desirable.

The fastening means which hold each of the layers of the cell mat in contact with a surface of the adjacent layer can be any non-conductive fastening means, such as an adhesive, or welding such as spot welding or line welding. Other suitable fastening devices include non-conductive fibers, rivets, needles, buckles, hooks and the like; fastening devices used for fastening clothes such as hem and end ("loop and pile") which engage each other, etc. A particularly desirable fastening device is to sew the layers together with an inert thread or yarn. Preferably, the sewing needle should pierce the layers from the first porous device, through the separating device and the second porous device and into the cathode. All the layers can be fixed with the same fixing device, or the layers of the cell mat can be fixed individually to an adjacent layer.

The cell mat is used to form an electrolysis cell in that the product article is placed on an anode such as a flat nickel sheet, and the power line device is placed over the cathode. The cathode and anode are led to an electrical current source and an electrolyte is introduced into and through the cell by the "wicking" action of the porous device.

The suitability of many different types of carbon for use in oxygen or air cells as a negative electrode was well documented in the fuel cell battery research.

For the fuel cell electrodes, paraffin, polyethylene, polypropylene and other binding materials were used so that the constructions would have the required hydrophobicity, as a three-phase reaction zone was produced where gas, electrolyte and conductive surface meet. Polytetrafluoroethylene (PTFE), available as an aqueous suspension under the trade name "Teflon" was marketed in the late 1950s and is suitable for binding the particles together. However, it is well known that the performance of such hydrophobic electrodes can vary significantly depending on how they are manufactured. Such electrodes eliminate the problems of a packed-bed electrode used in the U.S. Patent No. 4,118,305.

The fuel cell technology is suitable as a general guideline for the construction and operation of oxygen electrodes. However, there are several important differences between electrodes for fuel cells and electrodes for the production of hydrogen peroxide by reducing oxygen in an alkaline electrolyte. One important difference is that fuel cell electrodes use a catalyst to break down hydrogen peroxide as it is formed. This splitting provides part of the oxygen, so that the oxygen that needs to be supplied to the three-phase reaction zone is reduced. Another significant difference is that a fuel cell is intended to convert chemical compounds into electrical energy, while the present invention is a method for producing hydrogen peroxide in an electrolytic cell.

According to Maru et al., "Proceedings of the Symposium on Porous Electrodes: Theory and Practice", Volume 84.8, The Electrochemical Society, Pennington, N.J. (1984), the technological procedure for optimizing a fuel cell is well known, and each part must perform the work it is designed for. In terms of an electrode construction, this means that only a multi-layer composite electrode can be successful. Starting from the electrolyte side, there are, in order, three main layers to be considered. First, a platinum catalyzed carbon layer is required which should not be too hydrophobic; otherwise it will not achieve any good interface contact with the electrolyte. The second layer, the diffusion layer, has the purpose of transporting the gas with a minimal gas pressure drop (absolute or partial pressure drop) to the moist catalyzed carbon layer. This part of the electrode must be highly liquid-repellent, a barrier against the electrolyte's penetrative tendency. The third component is the pantograph. In thin composite electrodes for alkaline cells, it can be a porous nickel sheet impregnated with PTFE, or a nickel mesh.

Rusinko et al., Fuel Cell Materials, Proceedings I5th Annual Power Sources Conference page 9 describes that with regard to electrode pore size requirements, electrode pores larger than 1.0 µm in diameter are likely to fill with electrolyte and therefore do not contribute to the cell reaction. Moreover, the presence of a few large pores makes it impossible to operate an electrode without gas loss. The reference also describes that it has also been shown that gas flow drift characteristics can best be optimized with electrodes where the pores are completely homogeneous.

U.S. patent no. 4,118,305 belonging to Oloman describes that gas diffusion electrodes can provide sufficient electrode area for carrying out reactions that require low current densities such as the reduction of oxygen during the formation of hydrogen peroxide. Other disadvantages of the gas diffusion electrodes are that they are sensitive to contamination, deactivation by clogging and deactivation by flooding the pores with liquid.

A particularly desirable method for producing a gas diffusion cathode comprises piercing the cathode with a large number of perforations, each perforation having a sufficient open area so that the force of gravity on the electrolyte is greater than the capillary forces on the electrolyte.

Although the practice of this invention does not depend on any particular theory, it is convenient to explain the effect of the invention as a series of small openings which prevent a localized non-uniform flow of electrolyte in the cell from resulting in channeling in the cell.

It is clear that the optimum size, shape and distribution of the perforations will depend on the specific variable operating conditions of the electrolysis cell. Variables may include the specific gravity of the electrolyte, the rate of electrolyte flow in the cell, the dimensions of the cathode chamber, the surface tension, and other physical and electrical conditions of the electrolysis cell. However, a person skilled in the art can easily determine the optimal dimensions for a piercing for a particular cell without unnecessary trials.

It is desirable that the size of the perforation has an area that is equal to the area of a circle with a diameter of approx. 0.2 mm or larger, e.g. 0.1-1 mm, distributed over the surface of the cathode every 1-2 cm. Although there is no upper limit to the area of a via, an increase in the cumulative area of the vias in the cathode reduces the total area of the electrode available for electrolytic action.

The present invention is preferably carried out when the cathode is a gas diffusion cathode where a flexible conductive material such as graphite cloth is used as a substrate, which is made hydrophobic by the fact that the flexible graphite cloth is impregnated with approx. 40-70%, preferably approx. 45-65%, of a polytetrafluoroethylene resin, applying a sufficient amount of a first coating containing approximately equal parts by weight of carbon black and polytetrafluoroethylene resin, and then sintering the cloth in air at 360°-370°C while providing approx. 5-15 parts by weight of carbon black per 100 parts by weight of graphite cloth, and applying a sufficient amount of a second coating of a slurry with an amount in suspension of approx. 9 parts carbon black to 1 part polytetrafluoroethylene resin by weight as a slurry for approx.

105 parts by weight water and approx. 15 parts by weight non-ionic surfactant, and add approx. 5-15% carbon black to the graphite cloth after sintering and sintering the cloth in air at approx. 360-370°C.

The graphite cloth can be felted graphite fibres, a cloth of woven graphite fibers or a cloth of knitted graphite fibres. Carriers can also be produced from a metal substrate such as a nickel cloth impregnated with sintered nickel powder.

The best way to carry out the present invention will be apparent from the following, non-limiting examples.

The speed of the electrolyte flow through the cell can be varied during operation by increasing or decreasing the angle of the cell from a horizontal position and by varying the hydrostatic pressure difference at the cell inlet or outlet. The generally horizontal cell position provides an advantage to the present cell over all previous cells in that it is not necessary to provide a support for any part of the cell or to fabricate any part of the cell from a rigid material. This enables the use of a very thin separation device and enables very little space between adjacent elements in the cell. As a result, the ohmic resistance of the cell can be reduced far below the resistance of previous cells.

This particularly preferred embodiment of the present invention is better explained when looking at fig. 4 and 5. Fig. 4 is a cross-section of a cell where a commercial PTFE felt cloth is used bonded to a microporous polyfluoroethylene membrane which can "breathe" in air. Fig. 5 is an expanded drawing showing an alternative embodiment of the cell 400 according to fig. 4. Fig. 4. Anode 401, a nickel or stainless steel plate, is arranged in a generally horizontal position between electrolyte reservoir 402 containing electrolyte 406, and electrolyte equalization tank 403. A piece of a polyester felt cloth 405 bonded to a microporous PTFE membrane 404 is supported by anode 401 with a first end in reservoir 402 which constitutes an electrolyte intake, and the other end in equalization tank 403 constitutes an electrolyte outlet. Electrolyte is driven to flow into and through polyester felt 405 into equalization tank 403 by the static pressure difference between the level of electrolyte 406 in reservoir 402 and electrolyte 413 in equalization tank 403. Reservoir 402 contains sufficient electrolyte 406 such that the upper surface of electrolyte 406 is higher than electrolyte 413 or the other end of polyester felt 405. A porous, electrically conductive cathode 407 is provided to provide a first surface higher than or above and close to polyester felt 405 and the second surface of the cathode forms an outer surface of cell 400 which consists of anode 401, the part of polyester felt 405 which is adjacent to the cathode, PTFE membrane 404 and cathode 407. The PTFE membrane 404 delimits the space between the anode 401 and the cathode 407 in an anode chamber, the liquid film between the anode 401 and the separator 404 (not shown) and a cathode chamber filled with polyester felt 405. Conduit assembly 408 supplies electrolyte to electr olyte reservoir 402 from a source (not shown). Optionally, conduit device 409 supplies additional electrolyte to the cathode chamber. Conductors 410 and 411 provide a voltage to respectively anode 401 and cathode 407 from a source (not shown).

In operation, electrolyte is drawn from reservoir 402 by the wicking action of polyester felt 405 into the cathode chamber of cell 400. Sufficient electrolyte wets the lower surface of PTFE membrane 404 prior to its contact with anode 401 to supply electrolyte to the anode chamber. In the presence of electrical energy, oxygen gas is formed in the anode chamber. The oxygen is led to separation device 4 04 and into the cathode chamber of cathode 407 where it is reduced to hydrogen peroxide. Additional oxygen diffuses from the oxygen-containing gas at the second surface of cathode 407 to the first surface where it is also reduced to hydrogen peroxide. The electrolyte in the anode chamber and the cathode chamber can either be driven from the electrolyte inlet to the electrolyte outlet by the wicking action of the polyester felt 405 or by the static pressure difference between the level of electrolyte in reservoir 402 and electrolyte equalization tank 403.

Fig. 5 is an expanded drawing of the elements in another preferred second embodiment of a cell. The elements, which are normally in contact with each other, comprise a nickel or stainless steel anode 501 which forms the bottom of the cell where a first porous device 502, separating device 503, a second porous device 504 and a porous cathode 505 are placed on top of each other in order which forms the upper surface of the cell and which is exposed to a gas containing oxygen. Nickel mesh 506 and anode 501 are connected to a negative and a positive voltage source (not shown).

During operation, electrolyte 511 enters the cell from electrolyte reservoir 510 through the extension of porous devices 502 and 504, which extensions constitute electrolyte intake 520. Porous devices 502 and 504 each function as a wick and distribute the electrolyte evenly over the surface of cathode 505 and anode 501. Anode 501 and nickel mesh 506 are connected to an electricity source (not shown). At anode 501, oxygen gas is formed which rises through the porous anode chamber device 502 and is led to the sub-surface of separating device 503.

Bubbles of oxygen gas pass through gas openings in separation device 503 into the porous cathode chamber device 504 and come into contact with cathode 505. Further oxygen gas also diffuses through cathode 505 to the surface of the electrolyte in the porous cathode chamber device 504. There oxygen is reduced from both sources forming a dissolving hydrogen peroxide in the electrolyte in the porous cathode chamber device 504. The electrolyte is driven from the electrolyte inlet 520 over the surface of the cathode 505 and anode 501 by the difference in static pressure between the surface of the electrolyte 511 in the electrolyte reservoir 510 and the lower levels of the anolyte equalization tank 512 and the catholyte equalization tank 513. The electrolyte flows from the porous catholyte device 504 and the porous anolyte device 502 into the respective electrolyte

equalization tanks 512 and 513.

It is not necessary that the inlet or outlet end of the porous devices 502 and 504 be immersed in the electrolyte as illustrated in the figures. E.g. a funnel can be used for collecting electrolyte from porous devices 502 and 504 at the cell outlet. In a similar way, electrolyte at the cell intake can be applied directly to the porous device.

The porous devices 502 and 504 may include any inert porous devices, preferably felted inert fibers, woven inert fibers, knitted inert fibers, or an inert material with interconnected pores. The inert porous device may comprise polyester, wool, glass foam or fibre, mineral wool, asbestos, polyvinylidene and the like.

Elements 502, 503, 504 and 505 in fig. 5 can be combined to form mat 530, a particularly suitable article of manufacture for the particularly preferred embodiment of this invention as illustrated in FIG. 5.

The following examples illustrate the invention:

Example 1

An electrolysis cell was constructed according to fig. 3. The cathodes were manufactured in a manner similar to the U.S. patents 4,457,953 and 4,481,303 and consisted of carbon black bonded to graphite shavings (-10 and +20 mesh) with colloidal polytetrafluoroethylene (PTFE). The separator was a commercially available 38 cm x 17 cm polyester felt with a thickness of 1.15 mm, and the anode was a 27 cm x 19 cm nickel plate. A 12 x 12 mesh nickel mesh was used as current collector. A 3.7% solution of sodium hydroxide containing 0.05% disodium EDTA was used as the electrolyte. The cell was tilted at an angle of approx. 12°, and oxygen gas was in contact with the other surface of the cathode. The average electrolyte flow rate was 8.3 g/min. The electrolyte contained 0.7% H 2 O 2 and the current yield after 5 hours was calculated to be 72.3%. The current density was 0.02 A/cm<2> at a voltage of 1.3V.

Examples 2-5

A cell was constructed with a design as in fig. 4. The cathode was a 24 cm x 15 cm x 0.6 cm mesh foamed glass carbon (RVC) cathode used for fuel cell electrodes with a pore volume of 97%. Oxygen gas was in contact with the other surface of the cathode at atmospheric pressure. A 38 cm x 17 cm x 1.3 mm cloth of the trademark "Gortex" which provided the separation device and the porous device rested on a 27 cm x 19 cm plate 316 pp. The combination of the wicking action of the felt and the static pressure drove 4% NaOH electrolyte through the cell. The static pressure is indicated by the inclined position of the cell in relation to the horizontal position. The results are compared in Table I. The cell was operated for 6 hours.

Example 2

The cathode is commercially available untreated RVC used in the U.S. patent no. 4,430,176 and the electrolyte contained no stabilizer.

Example 3

Example 2 was repeated, except that the anode was nickel and the RVC cathode was impregnated carbon black bonded to the RVC with colloidal polytetrafluoroethylene (PTFE) to render it hydrophobic. The electrolyte was 4% NaOH containing 0.05% disodium ethylenediaminetetraacetic acid (EDTA) as stabilizer.

Example 4

Example 3 was repeated, except that the cathode was carbon black supported on a porous graphite cloth. The cloth was impregnated with colloidal PTFE and carbon black applied to the other surface.

Example 5

Example 4 was repeated using carbon black - a graphite felt cathode according to Example 4.

The above examples have a relatively poor current yield. The anode chamber was supplied with electrolyte by seeping electrolyte from the catholyte chamber before contact with the anode. In the cell, oxygen bubbles functioned as part of the valve to prevent the electrolyte in the cathode chamber from diffusing into the anode chamber. However, the examples are suitable to show that a separation device can be effective even if it allows electrolyte to transfer from the anode chamber to the cathode chamber.

Examples 6-8

The cell from examples 2-5 was constructed in a manner similar to fig. 5, except that the electrolytes from both the anode chamber and the cathode chamber were collected in a single electrolyte equalization tank. A 51 cm x 15 cm cathode was used in the cell. A 0/025 mm thick water-wettable microporous polypropylene film was used as the separator with a porosity of 38% and an effective pore size of 0.02 µm. Slits were punched through the film with a length of approximately 0.7 mm in a 1 cm x 1 cm matrix. The first porous device for the anode chamber was a 64 cm x 17 cm polyester felt with a thickness of 0.1 mm, while the second porous device for the cathode chamber was a 64 cm x 17 cm polyester felt with a thickness of approx. 1 mm. Unless otherwise stated, the electrolyte in the reservoir was 4% NaOH containing 0.05% EDTA. The cells were operated for 5 hours with oxygen gas at atmospheric pressure in contact with the second surface of the cathode. The results are shown in Table II.

Example 6

The cathode was carbon black deposited on a 1.25 mm thick graphite cloth impregnated with PTFE and a mixture of carbon black and PTFE.

Example 7

Example 7 was similar to Example 6, except that air was used as the gas containing oxygen instead of pure oxygen.

Example 8

Example 6 was repeated using a cationic membrane perforated with slits as above, and air was used as the gas containing oxygen. The carbon dioxide was removed from the air by bringing it into contact with sodium hydroxide.

When comparing examples 2-5 and 6-8, it is clear that examples 6-8 are superior in terms of current yield and hydrogen peroxide concentration, although examples 2-5 are effective examples. The superiority of Examples 6-8 appears to be that the bunsen valve slots passing through the separator were more efficient than the air valves in the expanded PTFE, which depended on gas bubbles for their operation.

Comparative example 9

A cell was constructed similar to fig. 5 using separate, unattached layers, the electrolyte was 3.6% sodium hydroxide, and carbon dioxide-purified air was passed over the outer surface of the cathode. The cell was operated for 5 hours at a current density of 0.025 A/cm<2.> The current yield for an average of two runs of 96%, producing an electrolyte containing an average of 0.93% H 2 O 2 .

Example 9 according to the invention Comparative example 9 was repeated, but the assembly was sewn with nylon thread. Between each stitch there was a distance of approx. 10 cm. The cell was operated for 5 hours with a current yield of 96.4% and produced an electrolyte containing 0.95% H 2 O 2 .

Example 10

A cathode was prepared from carbon black supported on a duPont Teflon 30B type polytetrafluoroethylene (PTFE)-impregnated graphite cloth (25 cm x 15 cm x 0.12 cm). The cloth weighing 11.14 g was first cleaned to remove H 2 O 2 cleavage catalysts using 4% NaOH, 10% nitric acid, and rinsed

thorough.

The graphite cloth was made hydrophobic or water repellent by impregnation with an aqueous suspension of duPont trademark Teflon 30B PTFE providing 7.1 mg of PTFE per cm<2> of the graphite cloth. A first coating of equal parts carbon black and PTFE (3.6 mg/cm<2> each) was applied to one surface, and the coated graphite cloth was dried and sintered at 360°C - 370°C for about one hour. The weight gain was 2.75 g.

A second coating was applied which consisted of a suspension of 9 parts by weight carbon black and 1 part by weight PTFE. The suspension was prepared by mixing 150 g of water, 22 g of nonionic surfactant of the trademark "Triton X-100", 0.13 g of a 1 M NaOH solution, 2.1 g of Teflon 30B and 12.8 g of carbon black ("Vulcan XC-72R"). The mixture was applied to the cloth with a brush. The resulting cloth was then dried and sintered at 360°C - 370°C for one hour in air. The amount of carbon black added to the cloth in the layer was calculated to be approx. 3.4 mg/cm<2>. The weight gain was 1.48 g.

A cell was assembled using a 27 cm x 19 cm nickel plate anode as a substrate, and in successive layers a 38 cm x 16 cm x 0.1 mm polyester felt, an acrylic/polyester membrane 25 cm x 15 cm x 0 .1 mm with an average pore size of 0.45 micrometers by 0.75 mm slits through the hole therein for every centimeter. A second polyester felt 38 cm x 15 cm x 1.1 mm was placed on the membrane and then the cathode. A nickel mesh was in contact with the upper surface of the cathode as a current collector. The two pieces of polyester felt hung beyond the two ends of the anode with one end immersed in a 3.6% NaOH solution and functioned as the electrolyte intake for the cell. The cell was inclined downwards from the electrolyte inlet at an angle of 12° and the solution was drawn through the cell by the wicking action of the polyester felt pieces. The cell was operated at a current density of 0.025 A/cm<2> and air purged with 4% NaOH blown over the cathode. After two 5-hour runs, the cathode was pierced with 0.5 mm vent holes spaced 1 cm apart, and the electrolysis was continued with two additional 5-hour runs. The results are listed in Table III. It is clear that the cell performance increased significantly with the ventilation holes in runs 3 and 4 according to the invention.

Claims (12)

1. Electrolysis cell for the reduction of oxygen to hydrogen peroxide at a cathode in the presence of an aqueous alkaline electrolyte in a cell with an electrolyte inlet (116), an electrolyte outlet (108), means for forcing the electrolyte from the electrolyte inlet to the electrolyte outlet, separation devices (112) which divides the cell into an anode compartment (127) containing an anode (101) and a cathode compartment (128) with a porous, gas-permeable cathode (106), characterized in that the cathode (106) has a first surface in contact with the electrolyte in the cathode compartment (128 ) and a second surface which forms an external surface of the cell in contact with an oxygen-containing gas, the cell containing devices for directing oxygen gas away from the separation device (112), the separation device (112) being substantially permeable to an ion in the electrolyte, and the anode compartment (128) is provided with means to force the electrolyte to flow from the electrolyte inlet (116) through the first surface of the cathode (106) t il the electrolyte outlet (108) at a speed equal to the speed of the electrolyte flow in the cathode space (128). 2. Electrolysis cell according to claim 1, characterized in that the cathode (106) is a porous, self-draining cathode (106.) with a first surface in contact with electrolyte and a second surface containing an electrolyte outlet (108) positioned to receive electrolyte that is drained from the cathode (106), the separation device (112) being substantially permeable to the electrolyte, to force the electrolyte from the electrolyte inlet (116), through the separation device (112) and into the self-draining cathode (106) at a rate approximately equal to the discharge rate of the electrolyte from the cathode (106), and in a sufficient amount to fill only a part of the pores of the cathode (106) and the cell has means to lead oxygen gas away from the separation device (112) and to discharge the gas in the anode space (127) out of the electrolytic cell . 3. Electrolysis cell according to claim 2, characterized in that the cathode (106) is mainly vertical and the cell contains devices to lead oxygen gas in the anode compartment electrolyte away from the separation device (112) comprising shutter valves in the anode. 4. Electrolysis cell according to claim 2, characterized in that the cathode (106) and the anode (120) are placed in an essentially horizontal position with the cathode (106) above the anode (120), and the separation device (112) is essentially permeable to the electrolyte and to oxygen gas to force oxygen gas out of the anode compartment (127) to the cathode compartment (128). 5. Electrolysis cell according to claim 2, 3 or 4, characterized in that the cathode (106) comprises carbon black bound to graphite tiles with polytetrafluoroethylene. 6. Electrolysis cell according to claim 1, characterized in that the separation device (503) is mainly permeable both to an ion in the electrolyte and to bubbles of oxygen gas that are formed at the anode (501), but is mainly impermeable to the flow of electrolyte from the cathode space (504) to the anode compartment (502), the cathode (505), and the anode (501) in the cell are placed in a substantially horizontal position with the cathode (505) above the anode (501), the anode compartment (502) being equipped with devices for distributing bubbles of oxygen gas formed at the anode (501) of the separation device (503) and to force electrolyte to flow through the surface of the anode (501), and the cathode space (504) is provided with means to force the electrolyte from the electrolyte inlet (116) through the first surface of the cathode (505). 7. Electrolysis cell according to claim 6, characterized in that the separation device (503) is a microporous film pierced with openings to enable the flow of oxygen gas from the anode compartment to the cathode compartment. 8. Electrolysis cell according to claim 6, characterized in that the separation device (503) is an ion-conducting membrane pierced with openings that allow the flow of oxygen gas from the anode compartment to the cathode compartment. 9. Electrolysis cell according to claim 6, 7 or 8, characterized in that the first and second porous devices (502) and (504) are selected from the group consisting of a felt of inert fibers, a woven fabric of inert fibers, a tightly knitted fabric of inert fibers and an inert material with pores in between. 10. Electrolysis cell according to claim 6, 7, 8 or 9, characterized in that the cathode is inclined at an angle from 5° to 25°, the electrolyte inlet being above the electrolyte outlet to provide a static column, and the porous separation device (503) and the cathode (505) provides means for passing oxygen gas out of the electrolysis cell. 11. Agent (530) usable in the construction of the electrolysis cell according to claim 6, 7, 8, 9 or 10 suitable for the production of hydrogen peroxide by reducing oxygen at a cathode, which agent (530) is characterized by layers in order: a first non-conductive, porous device (502) which is inert to an alkaline liquid, a separation device (503), a second, non-conductive, porous device (504) inert to an alkaline liquid containing hydrogen peroxide , and a porous cathode (505), which separation means is substantially permeable to both ions and gases, but is substantially impermeable to liquids, the first and second porous means being permeable to liquids, and attachment means for keeping each of the layers in contact with a face of the adjacent layer. 12. Method for producing a gas diffusion cathode for an electrolysis cell according to claim 6, 7, 8, 9 or 10 with a first cathode surface in contact with an electrolyte in the cell, and a second cathode surface that forms an upper surface of the cell, characterized in that the cathode is produced by impregnating a graphite cloth with sufficient polytetrafluoroethylene resin to provide 40-70% by weight of the resin on the graphite cloth, coating the impregnated graphite cloth with an aqueous slurry of approximately equal parts carbon black and polytetrafluoroethylene, and sintering the coated impregnated cloth, the carbon black applied in a sufficient amount to give 5-15 parts by weight of sex smoke per 100 parts graphite cloth was assumed, again applying a sufficient amount of a second coating of a slurry of a quantity of suspension of approx. 9 parts by weight of sex smoke to 1 part of polytetrafluoroethylene resin as a slurry for approx. 105 parts by weight of water and 15 parts by weight of a non-ionic surfactant, which gives 5-15 weight percent of the graphite cloth after sintering, and sintering the cloth in air at 360-370°C.