WO1988003966A1

WO1988003966A1 - Cell for producing hydrogen peroxide

Info

Publication number: WO1988003966A1
Application number: PCT/US1987/002944
Authority: WO
Inventors: John Shu Chi Chiang; Charles John Nicholson
Original assignee: Fmc Corporation
Priority date: 1986-11-20
Filing date: 1987-11-09
Publication date: 1988-06-02
Also published as: FI892299A0; FI892299A; BR8707943A; NZ222596A; NO883207D0; SE8901803D0; FI92078C; MX172657B; NO883207L; SE8901803L; NO177393C; FI92078B; SE466500B; NO177393B

Abstract

An electrochemical cell which is useful to reduce oxygen to hydrogen peroxide at a cathode (106) and a process employing the cell. The cell not only avoids the safety hazards of a hydrogen explosion of the prior art cells but also avoids the need for a rigid separating means (112) and cathode (106).

Description

CELL FOR PRODUCING HYDROGEN PEROXIDE

The present invention is an electrochemical cell and a process suitable for safely reducing oxygen to hydrogen peroxide at a cathode in the presence of an alkaline electrolyte.

For over a hundred years it has been known that oxygen can be reduced at a cathode to form hydrogen peroxide. In spite of the very low voltage for the half-cell reaction the process has never been commercialized.

U. S. Patents 4,406,758 and 4,511,441 teach a method for operating an electrochemical cell employing a gas cathode. The electrolyte is introduced into the cell in the anode compartment where a gas such as oxygen or chlorine is formed. The electrolyte then passes through a separating means into a "trickle 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 decomposed or collected and employed as a bleach solution.

Both of these patents teach that the desired electrolytic reaction with gas will take place only 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 balance the hydraulic pressure of the electrolyte on the anode side of the separating means and on the cathode side of the separating means to maintain a controlled flow of electrolyte into the cathode and to maintain oxygen gas throughout the cathode. Pores of a sufficient size and number are provided in the cathode to allow both gas and liquid to flow simultaneously through the cathode. The presence of oxygen is required at an oxygen cathode not only to maintain a high efficiency, but also to avoid a disastrous explosion. In the pre sence of an alkali metal hydroxide the oxygen cathode overall reaction is the reaction of oxygen and water to form hydroxyl ions and perhydroxyl ions (anions of hydrogen peroxide, a very weak acid). The cathode reaction is

(1) 2O₂ + 2H₂O + 4e → 2HO₂-+2OH- and the anode reaction is

(2) 4OH- → O₂ + 2H₂O + 4e- with an overall reaction of (3) O₂ + 2OH- →2HO₂-.

In the absence of oxygen at the cathode that half cell reaction is

(4) 2H₂O + 4e- → H₂ + 2OH-.

Undesirable side reactions can also take place at the cathode

(5) HO₂- + H₂O + 2e- → 3OH- and at the anode

(6) HO₂- + OH- → O₂ + H₂O + 2e-

Consequently, it is important to avoid local high concentration of the perhydroxyl ion (HO₂-) from accumulating in the catholyte.

Equation (4) can predominate 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 inadequate. In the absence of oxygen at the cathode hydrogen gas will be formed. The hydrogen gas may form an explosive mixture with oxygen gas in the oxygen supply manifold. In the alternative, if insufficient oxygen were introduced into the cathode, hydrogen would be formed in the oxygen-depleted section which would mix with oxygen in the oxygen-rich zone to form an explosive mixture.

In U. S. Patents Nos. 3,454,477; 3,459,652; 3,462,351; 3,506,560; 3,507,769; 3, 591,470, and

3,592,749 to Grangaard the cathode is a porous plate with the electrolyte and oxygen delivered from oppo site sides for reaction on the cathode. The porous gas diffusion electrode requires a wax coating to fix the reaction zone and careful balancing of oxygen and electrolyte pressure to keep the reaction zone on the surface of the porous plate.

The electrolytic cells of U. S. Patents 4,406,758 and 4,511,441 have a problem in that vertical dimension of the cell cannot be varied over a large range because of the need to balance the hydraulic pressure differences across the separating means and the need to avoid flooding the cathode with electrolyte, an uncontrolled flow of liquid through the separator is considered to be undesirable.

U. S. Patent No. 4,118,305 to Oloman attempts to overcome the problems of balancing the hydrostatic forces to maintain a three-phase system of a solid electrode (cathode), a liquid electrolyte and oxygen gas by continuously flowing a mixture of oxygen gas and a liquid electrolyte through a fluid permeable cathode, such as, a porous bed of graphite particles. A porous separator separates the packed bed electrode from the adjoining electrode and is supported by the packed bed electrode. The pores of the separator are sufficiently large to allow a controlled flow of electrolyte into the openings of the packed bed electrode. Electrochemical reactions occur within 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. Reactions are slow and recycle of product is necessary for acceptable product strength, and recycle of the excess oxygen gas is essential for economic operation and a superatmospheric oxygen pressure is generally required.

Each of these prior art electrolytic cells have a disadvantage of requiring a voltage substantially greater than the sum of the theoretical half cell voltages because of the high ohmic resistance of the cells. A further drawback to these cells is that they lack the means to vary the capacity of the cell during operation and the difficulty in establishing uniform electrolyte flow rates in the cell.

The properties of an ideal separating means are well known to those skilled in the art. It should be cheap, of some mechanical strength and rigidity, resistant to cell reactants, products and operating conditions. Also, the ideal separating means is described as permeable to ions but not molecules, of high void fraction to minimize electrical resistance, of small mean pore size to prevent the passage of gas bubbles and minimize diffusion, homogeneous to ensure good current efficiency and even current distribution, and nonconducting to prevent action as an electrode. The present invention overcomes the deficiencies of the prior cells. The invention is an electrolytic cell for reducing oxygen to hydrogen peroxide at a cathode in the presence of an aqueous, alkaline electrolyte. The invention comprises a cell having an electrolyte inlet, an electrolyte outlet, a porous cathode permeable to a gas, the cathode having a first surface contacting the electrolyte and a second surface forming an exterior surface of the cell in contact with an oxygen-containing gas, an anode, means to divert oxygen gas away from the anode, separating means between the cathode and the anode, and means to urge the electrolyte from the electrolyte inlet to the electrolyte outlet. The separating means defines an anode compartment and a cathode compartment in the cell, the separating means being substantially permeable to an ion in the electrolyte. The cell, is disposed with the cathode and anode in a generally vertical, horizontal or inclined attitude , the anode compartment is provided with means to urge electrolyte to flow across the surface of the anode, and the cathode compartment being provided with means to urge the electrolyte from the electrolyte inlet across the first surface of the cathode. The process of employing the cell to manufacture hydrogen peroxide is considered to be within the scope of the invention. In the first preferred embodiment the separating means is permeable to the electrolyte as weli as to an ion in the electrolyte, this embodiment comprising a cell having an electrolyte inlet, a porous, selfdraining cathode with a first surface contacting electrolyte and a second surface forming an exterior surface of the cell, an electrolyte outlet disposed to receive electrolyte draining from the cathode, an anode, separating means between the cathode and the anode. The separating means is substantially permeable to the electrolyte and defines an anode compartment containing the electrolyte inlet and a cathode compartment. The second surface of the cathode is in contact with an oxygen-containing gas, and means are provided to controllably urge the electrolyte from the electrolyte inlet through the separating means and into the self-draining cathode at a rate substantially equal to the drainage rate of the electrolyte from the cathode and in a quantity sufficient to fill only a portion of the pores of the cathode and having means to exhaust a gas in the anode compartment out of the electrolytic cell.

The means to divert oxygen gas generated at the anode in the anode compartment electrolyte away from the anode is essential to prevent increasing the ohmic resistance of the cell.

In a particularly desirable embodiment of the present invention, the cell is disposed so that the cathode is maintained in a generally horizontal position . If the anode is disposed in the cell in a position superior to the cathode the anode may desirably provide holes or pores as means to divert the buoyant oxygen gas in the electrolyte in the anode compartment away from the separating means. Desirable means to divert oxygen gas can include not only louvres in the anode, but also channels in the anode leading the bubbles up and to either side or both sides, for example, in a "herring bone" pattern. Equally effective are mechanical wipers or "paddlewheels" which can be driven by the rising bubbles to both sweep the other bubbles from the area and sweep fresh solution into the space between the anode and the separating means.

In another particularly desirable embodiment of the present invention the anode and cathode are disposed in a generally horizontal position at an angle of about 5° to 25°. The cathode is above (superior to) the anode and a porous felt material is disposed between the anode and cathode providing means to direct oxygen gas away from the anode . The cathode is composed of granular particles supported by the porous felt. The means to urge the electrolyte from the electrolyte inlet and into the self-draining cathode is the static head of the electrolyte inlet above the electrolyte outlet and the wicking effect of the porous felt, the porous felt also serving as separating means and means to direct oxygen gas away from the anode.

The cathode is an electrically conductive porous mass having a plurality of pores and channels passing therethrough. It may be a bed of electroconductive particles sintered to form a unitary mass or an agglomeration of loose particles. It must have pores of sufficient size and number to allow gas to flow therethrough. The channels must be of a sufficient size such that nonvolatile products will flow by gravity from the cathode, that is, the cathode should be "self-draining". Another way of expressing this is to describe the channels as being large enough so that gravity has a greater effect on the liquid in the electrode than does capillary pressure.

The means to urge the electrolyte from the electrolyte inlet through the separating means and into the self-draining cathode and to controllably urge the electrolyte through the separating means may be combined by inclining the cell so that the electrolyte inlet is raised above the electrolyte outlet. Alternatively, the electrolyte may be urged by a pump or other means to provide a greater pressure at the electrolyte inlet, and the means to controllably urge the electrolyte through the separating means and into the self-draining cathode may be by uniformly reducing the cross sectional area of the cell from the inlet end to the outlet end of the cell. Any convenient separating means may be used in the cell. For example, a ceramic diaphragm, an ion selective membrane such as a cation membrane which is also porous to the aqueous electrolyte. Other separating means such as a microporous plastic, a mat of asbestos, woven or felted fibers or a porous plastic may also be suitable. Support may be required as part of the separating means.

The following figures illustrate three of the preferred embodiments of the invention in detail. Figure 1 is a cross sectional view of a cell in which the cathode, separating means and anode are disposed in a generally vertical position.

Figure 2 is a cross sectional view of a cell in which the cathode, separating means and anode are disposed in a generally horizontal position with the anode superior.

Figure 3 is a cross sectional view of a cell in which the cathode, separating means and anode are disposed in a generally horizontal position with the cathode superior.

Figure 1 illustrates an electrolytic cell. The cell has louvered anode 120 which is located in an anolyte compartment 127. An electrolyte inlet port 116 opens into the anolyte compartment. A gaseous product outlet port 122 is located in the anolyte compartment 127. The first surface of cathode 106 contacts the electrolyte in cathode compartment and the second surface forms, an exterior 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 cathode. Separating means 112 divides the cell into anode compartment and cathode compartment.

The separating means 112 may be a plurality of layers or a single layer. However, the material should be substantially inert to the chemicals that it will contact under ordinary operating conditions. The separating means is constructed so that it has a somewhat limited ability to allow liquid to flow therethrough. Anode 120 is preferably equipped with louvres and is connected by conductor 101 to a positive source of voltage (not shown). Similarly cathode 100 is connected by conductor 102 to a negative source of voltage.

In operation, electrolyte is introduced into the cell through inlet port 116 and is urged through separating meand 112 into the cathode compartment and into the cathode 106. The liquid trickles down through the channels of the cathode by gravity and is collected and removed from the cell through electrolyte outlet port 108. An electric potential or voltage is applied between anode 120 and cathode 106; at the anode oxygen gas is formed, and rises as bubbles in the electrolyte between anode 120 and separating means 112. The bubbles are diverted by the louvres to the other side of anode 120, and is then exhausted through port 122. At cathode 106 oxygen which diffuses from the air into the cathode 100 is reduced to form hydrogen peroxide when it contacts the electrolyte therein. The hydrogen peroxide rich electrolyte trickles down inside the channels of cathode 106 and is collected at electrolyte outlet port 108. For the purpose of this invention, the channels and pores are distinguished in that in a channel the effect of gravity is greater on the electrolyte 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, liquid flow through the separating means 112 should be controlled 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 of the pores of the cathode 106 are filled, oxygen gas is displaced. This can result in the formation of explosive hydrogen gas. Conversely, if too little electrolyte passes through the separating means 112, the electrochemical reactions will be minimized. The present invention prevents the almost total filling of the cathode pores while at the same time preventing the almost total absence of electrolyte from the cathode.

Figure 2 is similar to Figure 1 except for the generally horizontal rather than vertical orientation of the cell. Each of the elements comprising the cell is enumerated similarly to the corresponding element of Figure 1 except in the "200's", rather than "100's". One exception is that the outlet port 108 is replaced by a plurality of small diameter outlet ports represented by 238A, 238B to 238Z, which function as channels. Gravity acting on the electro lyte in the outlet ports provides a slight suction within cathode 206 drawing the electrolyte into the outlet ports and thereby prevents the electrolyte from filling the pores employed by oxygen gas.

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

It is clear that the outlet ports 238A, 238B to 238Z need not be perpendicular to cathode 206. For example, the outlet ports can be inclined at an angle to be essentially vertical even if cathode 206 is inclined from the absolute horizontal.

A view of another embodiment of the invention, cell 300 is shown in Figure 3.

Figure 3. Anode 301, a nickel or stainless steel plate, is disposed in a generally horizontal attitude between electrolyte reservoir 302 and electrolyte surge tank 303. A sheet of a polyester felt fabric 304 is supported on anode 301 with a first end in reservoir 302 forming an electrolyte inlet and the second end in surge tank 303 to form an electrolyte outlet. Electrolyte is urged through the cell by the wicking action of polyester felt 304 and by the static head between the level of electrolyte in reservoir 302 and the electrolyte surge tank 303. Reservoir 302 contains sufficient electrolyte 306 so that the upper surface of electrolyte 306 is higher than the second end of polyester felt 304 at electrolyte surge tank 303. An electroconductive cathode

307 composed of carbon black bonded to graphite chips is disposed to provide a first surface contacting and above polyester felt 304. The second surface of the cathode forms an exterior surface of cell 300. Cell 300 consists of anode 301, the portion of polyester felt 304 adjacent to the anode, and cathode 307. The polyester felt 304 defining the space between the anode 301 and cathode 307 into an anode compartment (not shown) and a cathode compartment (not shown but including part of cathode 307). Conduit means 308 provides electrolyte to electrolyte reservoir 302 from a source (not shown). Conductors 310 and 311 provide a voltage to anode 301 and cathode 307 respectively from a source (not shown).

In operation electrolyte from reservoir 302 is drawn by the wicking effect of felt 304 into cell 300 where oxygen gas is formed. The oxygen is directed from the anode compartment by the felt 304 into the cathode compartment and to 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 cathode 307 where it is also reduced to hydrogen peroxide. The electrolyte is urged from the electrolyte inlet to the electrolyte outlet by the static head between the level of electrolyte in reservoir 302 and the electrolyte surge tank 303 in combination with the wicking effect of separating means 304.

One skilled in the art will recognize that in the present invention oxygen is always able to diffuse into cathode because the cathode comprises an exterior surface of the cell and is always in contact with the atmosphere.

The cells exemplified in Figures 2 and 3 have an added advantage over a substantially vertical cell in that the hydrostatic pressures are uniform over the separating means and the cathode so that the rate of diffusion of oxygen into the cathode and the rate of flow of electrolyte through the separating means and into the cathode are also uniform throughout the cell. There are two convenient methods for controlling the flow through the separating means into the electrode. One method is by varying the area of the separating means contacted by the liquid and a second method is by adjusting the pressure drop across the separating means.

In a vertical cell a convenient way of controlling the area of the separating means exposed to the liquid is by increasing or decreasing the. height of the liquid reservoir of the anode compartment adjoining the separating means. As the height is increased, the flow through the separating means increases. Conversely, as the height is decreased, the flow decreases. However, this varies the area of cathode and anode in contact with the electrolyte and hence the cell capacity.

Another method of controlling the flow through the separating means of a vertical cell is by controlling the pressure drop across the separating means. The pressure drop may be controlled in several ways.

One method of controlling the pressure drop across the separating means of the cell of Figure 1 is by operating the anode compartment under gas or liquid pressure. In this method, the opposing compartment is sealed from the atmosphere and gas pressure or liquid pressure is exerted on the electrolyte. Pumps may be used to force a pressurized liquid into the opposing compartment or the pressure may be maintained by a valve attached to ports 122 or 222.

In an especially preferred embodiment of the present invention the separating means is permeable to a gas but not a liquid. This embodiment comprises a cell having an electrolyte inlet, an electrolyte outlet, a porous cathode permeable to a gas, the cathode having a first surface contacting the electrolyte and a second surface forming an exterior surface of the cell in contact with an oxygen- containing gas, an anode, separating means between the cathode and the anode, and means to urge the electrolyte from the electrolyte inlet to the electrolyte outlet. The separating means defines an anode compartment and a cathode compartment in the cell, the separating means being substantially permeable to an ion in the electrolyte and to a gas, but being substantially impermeable to the flow of the electrolyte from the cathode compartment to the anode compartment. The cell is disposed with the cathode and anode in a generally horizontal attitude with the cathode superior to the anode, the anode compartment is provided with means to direct oxygen gas generated at the anode to the separating means and to urge electrolyte to flow across the surface of the anode, and the cathode compartment being provided with means to urge the electrolyte from the electrolyte inlet across the first surface of the cathode.

The means to direct the oxygen gas to the separating means and to urge the electrolyte to flow uniformly across the anode may be combined, and can be any gas permeable porous material such as a felt, a woven fabric or an interconnecting foam material. Other suitable means include flow vanes in the anode compartment which direct the oxygen bubbles to the separating means, and which divert the electrolyte over the surface of the anode. A gas permeable porous means is particularly desirable because of its wicking action which aids in urging electrolyte from the electrolyte inlet to the electrolyte outlet. The means to urge the electrolyte to flow uniformly across the surface of the cathode can be similar to the means in the anode compartment. In both cases the means may be provided by very close spacing of the cathode and the separating means so that the capillary effect of the first surface of the cathode and adjacent surface of the separating means on the electrolyte approaches the effect of gravity. For the purpose of the present invention, the expression "substantially permeable both to an ion in the electrolyte and to a gas, but being substantially impermeable to the flow of the electrolyte from the cathode compartment to the anode compartment," shall be understood to mean that under normal operating conditions bubbles of oxygen gas generated at the anode can pass freely through the separating means from the anode compartment to the cathode compartment, but that very little electrolyte is transferred from the cathode compartment to the anode compartment.

One commercially-available separating means suitable for the present invention is a hydrophillic laminate of polyester felt and an expanded polytetra- fluoroethylene consisting of nodes and interconnecting fibrils marketed by W. L. Gore and Associates. The separating means is rated in a standard ASTM test F778 as 3.8 m³/S at 125 Pa. The polyester felt portion of the laminate is suitable both as a means to direct oxygen gas from the anode to the separating means and to urge the anolyte to flow uniformly across the anode, or as the means to direct the electrolyte to flow uniformly across the cathode. Another suitable separating means is a micro- porous polypropylene film 2.5 x 10^-2 mm thick having 38% porosity with an effective pore size of 0.02 micrometer which is marketed by Celanese Corporation. The pores provide the desired electrical conductivity but impede the flow of electrolyte. The film was perforated with openings without removing any material. The openings act as check valves and are spaced approximately every centimeter in a row and column matrix. The openings, for example, 0.5 mm to 1 mm slits, act as small bunsen valves which open to permit the flow of oxygen gas from the anode compartment into the cathode compartment and which close to exclude the flow of electrolyte from the cathode compartment to the anode compartment.

An ion conductive membrane, similarly punctured, is also suitable for use as a separating means. A typical commercial membrane is marketed by RIA

Research Corporation under the trade name of Raipore BDM-10 membrane. It comprises a grafted low density polyethylene base film having a weak base cationic monomer as the graft. It is clear that the separating means employed in this embodiment of the present invention differs from the well recognized "ideal separating means" in that it not only has a small mean pore size making it permeable to ions and not molecules, but also has openings of sufficient size to permit the passage of gas bubbles (gas openings) without permitting substantial diffusion or back mixing of hydrogen peroxide from the cathode compartment to the anode compartment. The optimum size, shape and distribution of the gas openings can be determined without undue experimentation. The shape of the openings may be straight slits, crosses, vees, or mere point punctures. The openings are formed, desirably, by puncturing the separating means, without removing any material from the separating means. The separating means is usually installed so that the oxygen bubbles pass through in the direction the punctures were formed. In this way the oxygen gas bubbles function as a part of the "valve". A particularly useful article of manufacture for this embodiment of the cell comprises layers in sequence; a first nonconductive porous means inert to an alkaline liquid, a separating means, a second nonconductive porous means inert to an alkaline liquid containing hydrogen peroxide, and a porous cathode, said separating means being substantially permeable both to ions and to gases but being sub stantially impermeable to liquids, said first and second porous means being permeable to fluids, fastening means holding each of said layers in contact with a surface of the adjacent layer. The complete article of manufacture is referred to herein as a cell quilt.

An electrolytic cell employing the cell quilt is assembled by placing the cell quilt on a generally horizontal conductive anode. The cell quilt is disposed on top of the generally horizontal anode with the first porous means in contact with the anode. A current conducting means is placed on top of the cell quilt in electrical contact with the cathode on the upper surface of the cell quilt, said electrical conducting means provided with channels to permit a gas to contact the anode.

Preferably the first and second porous means are formed from felted inert fibers, woven inert fibers, knit inert fibers or an inert foamed material having interconnected pores.

Any suitable porous inert conductive material known to be useful as an oxygen electrode may be employed as a cathode, such as, a sheet of commercially available reticulated vitreous carbon employed in U. S. Patent No. 4,430,176, porous graphite, or a composite electrode consisting of carbon particles bonded to an electrically concuctive, porous base as taught by U. S. Patent No. 3,459,652 in which the bonding agent is paraffin. Also suitable is an electrode of activated carbon bonded with PTFE and natural rubber onto a nickel screen taught by U. S. Patent No. 4,142,949. Other electrodes known to be useful are taught by U. S. Patent No. 3,856,640 employing carbon particles bonded with polytetra- fluoroethylene and porous carbon electrodes suitable for fuel cells. It is desirable for the cathode to be flexible such as one employing graphite felt or woven or knit graphite fabric as a base for carbon particles such as any taught in French Patent Publication 2,493,878. Particularly desirable is a cathode employing a graphite base and employing carbon particles bonded with polytetrafluoroethylene. The fastening means holding each of the layers of the cell quilt in contact with a surface of the adjacent layer may be any nonconductive fastening means, such as an adhesive, or a weld such as a spot weld or a linear weld. Other suitable fastening means include nonconductive staples, rivets, pins, snaps, hooks and the like. Fastening means employed fastening textiles such as, interlocking loop and pile and the like. A particularly desirable fastening means is by sewing the layers together with an inert thread or yarn. Preferably the sewing needle, should puncture the layers from the first porous means, through the separating means and second porous means and into the cathode. All of the layers may be fastened by the same fastening means or the layers of the cell quilt may be fastened individually to an adjacent layer.

The cell quilt is employed to form an electrolytic cell by placing the article of manufacture on an anode, such as a planar nickel sheet and the current conductor means applied over the cathode. The cathode and anode are conducted to a source of electrical power and electrolyte introduced into and through the cell by the "wicking" action of the porous means.

The suitability of many different carbon types for their use in oxygen or air cells as negative electrode was well established by the fuel cell battery studies. Fuel cell electrodes used paraffin, polyethylene, polypropylene and other binding materials for providing the necessary hydrophobicity to the structures, creating a three-phase reaction zone where gas, electrolyte and conductive surface meet. Polytetrafluoroethylene (PTFE), available as aqueous suspension under the trade name "Teflon" was marketed in the later 1950's, and is useful 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 prepared. Such electrodes eliminate the problems of a packed bed electrode employed in U.S. Patent No. 4,118,305.

The fuel cell technology is useful as a general guide to the construction and operation of oxygen electrodes. However, there are several important distinctions between electrodes for fuel cells and electrodes to produce hydrogen peroxide by reducing oxygen in an alkaline electrolyte. One important difference is that fuel cell electrodes employ a catalyst to decompose hydrogen peroxide as it is formed. This decomposition provides part of the oxygen, decreasing oxygen needed to be supplied to the three phase reaction zone. Another significant distinction is that a fuel cell is intended to convert chemical compounds into electrical energy, while the present invention is a process to product 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 approach to optimizing a fuel cell is well known and each part should do the job it is designed for. In respect to an electrode structure it means that only a raulti-layered, composite electrode can be successful. In sequence, beginning from the electrolyte side there are three principal layers to consider. First, a platinum catalyzed carbon layer is required which should not be too hydrophobic, otherwise it will not achieve a good interfacial contact with the electrolyte. The second layer, the diffusion layer, has the purpose of transporting the gas with a minimum of gas-pressure drop (absolute or partial pressure drop) to the wet catalyzed carbon layer. This part of the electrode must be highly liquid repellent, a barrier against the penetration tendency of the electrolyte. The third component is the current collector. In thin composite electrodes for alkaline cells it can be a porous nickel sheet impregnated with PTFE, or a nickel screen.

Rusinko et al., Fuel Cell Materials, Proceedings 15th Annual Power Sources Conference page 9 discloses that with regard to electrode pore size requirements, electrode pores greater than 1.0 μm in diameter are probably filled with electrolyte and, therefore, do not contribute to the cell reaction. In addition, the presence of a few large pores makes it impossible to operate an electrode without gas losses. The reference also discloses it has also been shown that gas flow operating characteristics can best be optimized with electrodes whose pores are completely homogeneous. U. S. Patent No. 4,118,305 to Oloman discloses that although gas diffusion electrodes can provide sufficient electrode area to carry out reactions requiring low current densities such as the reduction of oxygen to form hydrogen peroxide. Other disadvantages of the gas diffusion electrodes are that they are susceptible to contamination, to deactivation by plugging and to deactivation by flooding of the pores with liquid.

A particularly desirable method for fabricating a gas diffusion cathode comprises perforating the cathode with a plurality of perforations, each perforation having a sufficient open area 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 vents which prevent a localized nonuniform 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 electrolytic cell. Variables may include the specific gravity of the electrolyte, the rate of flow of the electrolyte in the cell, the dimension of the cathode compartment, the surface tension and other physical and electrical conditions of the electrolytic cell. However, one skilled in the art can easily determine the optimum dimensions of a perforation for a particular cell without undue experimentation. It is desirable for the size of the perforation to have the area equivalent to that of a circle about 0.2 mm in diameter or larger, for example 0.1 to 1 mm, distributed over the surface of the cathode every 1-2 cm. Although there is no upper limit to the area of a perforation, increasing the cumulative area of the perforations in the cathode reduces the total area of the electrode available for the electrolytic action.

The present invention is preferably employed when the cathode is a gas diffusion cathode employing as a base a flexible conductive material such as graphite fabric which is made hydrophobia by impregnating the flexible graphite fabric with about 40% to 70%, desirably about 45% to 65% of a polytetrafluoro- ethylene resin, applying a sufficient quantity of a first coating, containing about equal parts by weight of carbon black and polytetrafluoroethylene resin. and subsequently sintering the fabric in air at 360° to 370°C to provide about 5 to 15 parts by weight of carbon black per 100 parts by weight of graphite fabric, and applying a sufficient quantity of a second coating of a slurry of quantity of a suspension of about 9 parts of carbon black to one part of a polytetrafluoroethylene resin by weight as a slurry in about 105 parts by weight water and about 15 parts by weight of a nonionic surfactant to add about 5% to 15% carbon black by weight to the graphite fabric after sintering, and sintering the fabric in air at about 360ºC to 370°C.

The graphite fabric may be felted graphite fibers, a fabric of woven graphite fibers or a fabric of knit graphite fibers. Supports may also be made of a metal base such as a nickel fabric impregnated with sintered nickel powder.

The best mode of practicing the present invention will become evident from the following, nonlimiting examples.

The rate of flow of electrolyte through the cell can be varied during operation by increasing or decreasing the angle of the cell from horizontal and by varying the hydrostatic pressure difference at the cell inlet or outlet. The generally horizontal attitude of the cell provides an advantage of the present cell over all prior cells in that it is not necessary to provide a support for any part of the cell or to make any part of the cell of a rigid material. This permits employing a very thin separating means and permits very close spacing of adjacent elements of the cell. As a result, the ohmic resistance of the cell can be reduced far below that of prior cells. This especially preferred embodiment of the present invention is better explained in view of Figures 4 and 5.

Figure 4 is a cross-section of a cell employing a commercial PTFE felt fabric bonded to an air breathable microporous polyfluoroethylene membrane.

Figure 5 is an exploded view showing an alternative embodiment to the cell 400 of Figure 4. Figure 4. Anode 401, a nickel or stainless steel plate, is disposed in a generally horizontal attitude between electrolyte reservoir 402 containing electrolyte 406 and electrolyte surge tank 403. A sheet of a polyester felt fabric 405 bonded to a microporous PTFE membrane 404 is supported on anode 401 with a first end in reservoir 402 forming an electrolyte inlet and the second end in surge tank 403 to form an electrolyte outlet. Electrolyte is urged to flow into and through polyester felt 405 into surge tank 403 by the static head between the level of electrolyte 406 in reservoir 402 and electrolyte 413 in surge tank 403. Reservoir 402 contains sufficient electrolyte 406 so that the upper surface of electrolyte 406 is higher than electrolyte 413 or the second end of polyester felt 405. A porous, electroconductive cathode 407 is disposed to provide a first surface superior to or above and closely adjacent to polyester felt 405 and the second surface of the cathode forms an exterior surface of cell 400 which consists of anode 401, the portion of polyester felt 405 adjacent to the cathode, PTFE membrane 404 and cathode 407. The PTFE membrane 404 defining the space between the anode 401 and cathode 407 into an anode compartment, the liquid film between the anode 401 and separating means 404 (not shown) and a cathode compartment occupied by polyester felt 405. Conduit means 408 provides electrolyte to electrolyte reservoir 402 from a source (not shown). Optionally, conduit means 409 provides additional electrolyte for the cathode compartment. Conductors 410 and 411 provide a voltage to anode 401 and cathode 407 respectively from a source (not shown). In operation electrolyte from reservoir 402 is drawn by the wicking effect of polyester felt 405 into the cathode compartment 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 compartment. In the presence of electrical energy Oxygen gas is formed in the anode compartment. The oxygen is directed to separating means 404 and into the cathode compartment to cathode 400 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 compartment and the cathode compartment may either be urged from the electrolyte inlet to the electrolyte outlet by the wicking effect of the polyester felt 405 or by static head between the level of electrolyte in reservoir 402 and the electrolyte surge tank 400.

Figure 5 is an exploded view of the elements of another preferred second embodiment of a cell. The elements, normally in contact with each other, comprise a nickel or stainless steel anode 501 forming the bottom of the cell surmounted sequentially by a first porous means 502, separating means 503, a second porous means 504, and porous cathode, 505 forming the upper surface of the cell exposed to a gas containing oxygen. Nickel screen 506 and anode 501 are connected to a negative and positive source of voltage (not shown).

In operation electrolyte 511 enters the cell from electrolyte reservoir 510 through the extension of porous means 502 and 504 which extensions form electrolyte inlet 520. Porous means 502 and 504 each act as a wick and distribute the electrolyte uniformly over the surface of cathode 505 and anode 501. Anode 501 and nickel screen 506 are connected to a source of electricity (not shown). At anode 501, oxygen gas is formed which rises through anode compartment porous means 502 and is directed to the lower surface of separating means 503.

Bubbles of oxygen gas pass through gas openings of separating means 503 into the cathode compartment porous means 504 and contact cathode 505. Additional oxygen gas also diffuses through cathode 505 to the surface of the electrolyte in cathode compartment porous means 504. There oxygen from both sources is reduced to form a solution of hydrogen peroxide in the electrolyte in the cathode compartment porous means 504. The electrolyte is urged from electrolyte inlet 520 across the surface of cathode 505 and anode

001 by the difference of static head of the surface of electrolyte 511 in electrolyte reservoir 510 and the lower levels of anolyte surge tank 512 and catholyte surge tank 513. The electrolyte flows from catholyte porous means 504 and anolyte porous means

502 into electrolyte surge tanks 512 and 513 respectively.

It is not necessary for the inlet or the outlet end of porous means 502 and 504 to be immersed in electrolyte as illustrated in the figures. For example, a funnel can be employed to collect electrolyte from porous means 502 and 504 at the cell outlet. Similarly at the cell inlet electrolyte can be applied directly to the porous means. The porous means 502 and 504 may include any inert porous means, preferably felted inert fibers, woven inert fibers, knit inert fibers or an inert material having interconnected pores. The inert porous means may comprise polyester, wool, glass foam or fiber, mineral wood, asbestos, polyvinylidene, and the like.

Elements 502, 503, 504 and 505 of Figure 5 may be combined to form quilt 530, a particularly useful article of manufacture for the especially preferred embodiment of this invention as illustrated in Figure 5. The best mode of practicing the present invention is exemplified by the following nonlimiting examples:

Example 1 An electrolytic cell was constructed in accordance with Figure 3. The cathodes were prepared in a manner similar to U. S. Patents 4,457,953 and

4,481,303 and consisted of carbon black bonded to graphite chips (-10 and +20 mesh) with colloidal polytetrafluoroethylene (PTFE). The separating means was a commercial 38 cm x 17 cm polyester felt 1.15 mm thick, and the anode was a 27 cm x 19 cm nickel plate. A 12 x 12 mesh nickel screen was employed as a current collector. A 3.7% solution of sodium hydroxide containing 0.05% disodium EDTA was employed as the electrolyte. The cell was inclined at an angle of about 12° and oxygen gas contacted the second surface of the cathode. The average electrolyte flow rate was 8.3 g/min. The electrolyte contained 0.7% H₂O₂ and current efficiency after 5 hours was calculated to be 72.3%. The current density was 0.02 A/cm² at a voltage of 1.3v.

Examples 2 to 5 A cell was set up in the configuration of Figure 4. The cathode was a 24 cm x 15 cm x 0.6 cm foam reticulated vitreous carbon (RVC) used for fuel cell electrodes having a pore volume of 97%. Oxygen gas contacted the second surface of the cathode at atmospheric pressure. A 38 cm x 17 cm x 1.3 mm Gortex brand fabric which provided the separating means and porous means rested on a 27 cm x 19 cm 316 ss plate. The combination of wicking action of the felt and the static head urged 4% NaOH electrolyte through the cell. The static head is indicated by the tilt of the cell from horizontal. The results are compared in Table I. The cell was operated for 6 hours.

Example 2 The cathode was commercial untreated RVC employed in U. S. Patent 4,430,176 and the electrolyte contained no stabilizer.

Example 3 Example 2 was repeated except anode was nickel and the RVC was impregnated carbon black bonded to the RVC with colloidal polytetrafluoroethylene (PTFE) to make. it hydrophobic. The electrolyte was 4% NaOH containing 0.05% disodium ethylenediaminetetraacetic acid (EDTA) as a stabilizer.

Example 4 Example 3 was repeated except the cathode was carbon black supported on a porous graphite cloth. The cloth was impregnated with colloidal PTFE and carbon black applied to the second surface.

Example 5 Example 4 was repeated using the carbon black - a graphite felt cathode of Example 4.

The above examples have a relatively poor current efficiency. The electrolyte was provided to the anode compartment by seepage of electrolyte from the catholyte compartment prior to contact with the anode. In the cell oxygen bubbles acted as part of the valve to prevtnt the electrolyte in the cathode compartment from diffusing into the anode compartment. However, the examples are useful in showing that a separating means can be effective even if it permits electrolyte to transfer from the anode compartment to the cathode compartment.

Examples 6 to 8 The cell from Examples 2 to 5 was set up in a manner similar to Figure 5 except the electrolytes from both the anode compartment and the cathode compartment were collected in a single electrolyte surge tank. The cell employed a 51 cm x 15 cm cathode. A 0.025 mm thick water-wettable microporous polypropylene film was employed as a separating means having 38% porosity with an effective pore size of 0.02 μmeter. Slits were punctured through the film approximately 0.7 mm in length in a 1 cm x 1 cm matrix. The first porous means for the anode compartment was a 64 cm x 17 cm polyester felt 0.1 mm thick, while the second porous means for the cathode compartment was a 64 cm x 17 cm polyester felt about 1 mm thick. Unless specified otherwise, 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 presented as Table II.

Example 6 The cathode was carbon black deposited on 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 air was employed 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 employed air as the gas containing oxygen. The carbon dioxide was removed from the air by contacting it with sodium hydroxide.

In comparing Examples 2 to 5 and 6 to 8, it is clear that Examples 6 to 8 are superior in terms of current efficiency and hydrogen peroxide concentration, although Examples 2 to 5 are operative examples. The superiority of Examples 6 to 8 appears to be that the bunsen valve slits punctured through the separating means were more effective than the air valves of the expanded PTFE which relied on gas bubbles for their operation.

Comparative Example 9 A cell was set up similar to Figure 5 employing separate, unfastened layers, the electrolyte was 3.6% sodium hydroxide, and air scrubbed free of carbon dioxide was directed over the exterior surface of the cathode. The cell was operated for 5 hours at a current density of 0.025 A/cm². The current efficiency for an average of two runs was 96% producing an electrolyte containing an average of 0.93% H₂O₂.

Inventive Example 9 The comparative example 9 was repeated but the assembly was stitched with nylon thread. Each stitch was about 10 cm apart. The cell was operated for 5 hours with a current efficiency of 96.4% and produced an electrolyte containing 0.95% H₂O₂.

Example 10 A cathode was prepared of carbon black supported on a duPont Teflon 30B polytetrafluoroethylene (PTFE) impregnated graphite fabric (25 cm x 15 cm x 0.12 cm). The fabric which weighed 11.14g was first cleaned to remove H₂O₂ decomposition catalysts by 4% NaOH, 10% nitric acid and thoroughly rinsed. The graphite fabric was made hydrophobic or water repellent by impregnating with an aqueous suspension of duPont brand Teflon 30B PTFE to provide 7.1 mg/cm² of PTFE on the graphite fabric. A first coating of equal parts by weight of carbon black and PTFE (3.6 mg/cm² each) was applied to one surface and the coated graphite fabric was dried and sintered at 360°C to 370°C for about an hour. The increase in weight was 2.75 g.

A second coating was applied consisting of a suspension of 9 parts by weight carbon black to 1 part PTFE. The suspension was prepared by mixing 150 g water, 22 g Triton X-100 brand nonionic surfactant. 0.13 g of a 1 M NaOH solution, 2.1 g Teflon 30B, and 12.8 g 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 to 370°C for one hour in air. The amount of carbon black added on the cloth in the layer was calculated to be about 3.4 mg/cm². The increase in weight was 1.48 g.

A cell was assembled employing a nickel plate anode 27 cm x 19 cm as a base, 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 having an average pore size of 0.45 micrometer with 0.75 mm slits punctured therein 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 screen contacted the upper surface of the cathode as a current collector. The two polyester felts overhung the two ends of the anode with the one end immersed in a 3.6% NaOH solution and acted as an electrolyte inlet for the cell. The cell was inclined downward from the electrolyte inlet at an angle of 12° and the solution was drawn through the cell by the wicking effect of the polyester felts. The cell was operated at a current density of 0.025 A/cm² and air scrubbed with 4% NaOH was blown over the cathode. After two 5 hour runs, the cathode was perforated with 0.5 mm vent holes 1 cm apart and electrolysis was continued for two additional 5 hour runs. The results are presented as Table III. It is clear that the cell performance increased substantially with the vent holes of inventive Runs 3 and 4.

Claims

Claims :

1. An electrolytic cell characterized by an electrolyte inlet, an electrolyte outlet, a porous cathode permeable to a gas, the cathode having a first surface contacting the electrolyte and a second surface forming an exterior surface of the cell in contact with an oxygen-containing gas, an anode, means to divert oxygen gas away from the anode, separating means between the cathode and the anode, and means to urge the electrolyte from the electrolyte inlet to the electrolyte outlet, the separating means defining an anode compartment and a cathode compartment in the cell, the separating means being substantially permeable to an ion in the electrolyte, the cell being disposed with the cathode and anode in a generally vertical, horizontal or inclined attitude, the anode compartment being provided with means to urge electrolyte to flow across the surface of the anode, and the cathode compartment being provided with means to urge the electrolyte from the electrolyte inlet across the first surface of the cathode.

2. The cell of claim 1 characterized in that porous cathode is self-draining and the separating means is permeable to an ion of the electrolyte and is sufficiently permeable to the electrolyte to controllably urge the electrolyte from the electrolyte inlet through the separating means into the self- draining cathode at a rate substantially equal to the drainage rate of the cathode into the electrolyte outlet and in a quantity sufficient to fill only a portion of the cathode, the means in the anode compartment which urge the electrolyte to flow across the surface of the anode divert the oxygen gas in the anode compartment away from the separating means and out of the electrolytic cell.

3. The cell of claim 2 characterized in that the anode and cathode are disposed in a generally horizontal attitude with a difference of static head between the electrolyte inlet and the electrolyte outlet urging the electrolyte from the electrolyte inlet to the electrolyte outlet and the separating means has a wicking action sufficient to urge the electrolyte across the surface of the anode and cathode and controllably urge electrolyte into the cathode and is also sufficiently gas permeable to urge the oxygen gas out of the anode compartment.

4. The cell of claim 1 characterized in that the cathode and anode are disposed in a generally horizontal attitude with the cathode superior to the anode and the separating means being substantially permeable to an ion in the electrolyte and to oxygen gas to urge oxygen gas out of the anode compartment into the cathode compartment, but being substantially impermeable to the electrolyte.

5. The cell of claim 4 characterized in that the separating means is a microporous film punctured with openings to permit the flow of oxygen gas from the anode compartment into the cathode compartment.

6. The cell of claims 4 or 5 characterized in that the anode compartment is provided with a first porous means to distribute oxygen gas generated at the anode to the separating means and to urge the electrolyte to flow across the surface of the anode and the cathode compartment is provided with a second porous means to urge the electrolyte from the electrolyte inlet across the surface of the cathode.

7. The process of manufacturing hydrogen peroxide by introducing an aqueous alkaline electrolyte into the electrolyte inlet of the cells of claims 1 to 6 and reducing oxygen at the cathode to hydrogen peroxide and withdrawing hydrogen peroxide from the cell.

8. An article of manufacture useful to construct the electrolytic cell of claim 6 suitable for the manufacture of hydrogen peroxide by the reduction of oxygen at a cathode, the article of manufacture characterized by layers in sequence; a first nonconductive porous means inert to an alkaline liquid, separating means, a second nonconductive porous means inert to an alkaline liquid containing hydrogen peroxide, and a porous cathode, said separating means being substantially permeable both to ions and to gases but being substantially impermeable to liquids, said first and second porous means being permeable to fluids, fastening means holding each of said layers in contact with a surface of the adjacent layer.

9. A method for fabricating a gas diffusion cathode for an electrolytic cell of claims 4 to 6 having a first cathode surface contacting electrolyte in the cell and a second cathode surface forming an upper surface of the cell characterized by perforating the cathode with a plurality of perforations, each perforation having sufficient open area that the force of gravity on the electrolyte is greater than the capillary forces on the electrolyte and sufficient to prevent a localized nonuniform flow of electrolyte in the cell.

10. The method of claim 9 characterized in that the cathode is prepared by a. impregnating a graphite fabric with sufficient polytetrafluoroethylene resin to provide 40% to 70% by weight of the resin on the graphite fabric, coating the impregnated graphite fabric with an aqueous slurry of about equal parts of carbon black and polytetrafluoroethylene and sintering the coated impregnated fabric, the carbon black being applied in an amount sufficient to provide about 5 to 15 parts by weight of carbon black per 100 aarts of graphite fabric originally employed, and applying a sufficient quantity of a second coating of a slurry of quan tity of a suspension of about 9 parts of carbon black to one part of a polytetrafluoroethylene resin by weight as a slurry in about 105 parts by weight water and about 15 parts by weight of a nonionic surfactant to provide about 5% to 15% carbon by weight of the graphite fabric after sintering, and sintering the fabric in air at about 360°C to 370ºC.