Oxygen-Consuming Zero-Gap Electrolysis Cells With Porous/Solid Plates

Abstract

An electrolysis stack (53) with oxygen-depolarized cathodes (31) employs solid-plate anodes (38) and porous-plate cathodes (42). The stack (53) of electrolysis cells (29) (e.g, hydrogen-chloride or chlor-allkali cells) each include an ion exchange membrane (32) sandwiched between an anode conductor (34) and a permeable cathode (35); an oxygen-consuming gas diffusion cathode (31) is adjacent the cathode conductor of each cell. Between the anode conductor of one cell and the gas diffusion cathode of an adjacent cell there is a composite bipolar plate (51) including a solid plate (38) having channels (39) for conducing salt solution and product of the process; the bipolar plates also include a porous plate (42) having channels (43) for conducting oxidant adjacent the gas diffusion cathode and channels (49) connected to a source of liquid (such as water or dilute sodium hydroxide).

Description

TECHNICAL FIELD

This invention relates to industrial electrolysis cells, such as may be used in the production of chlorine gas from hydrogen chloride or from sodium chloride solutions, employing an oxygen-consuming cathode, with a zero-gap structure, which employs solid plates adjacent the anode where chlorine is evolved and porous plates adjacent the cathode electrode having passageways for liquid solution and having passageways for oxygen-containing gas.

BACKGROUND ART

Traditional production of chlorine employs a process in which the electrolysis of brine (that is, sodium chloride and water) to form chlorine gas and sodium hydroxide (also known as caustic soda) utilizing hydrogen-evolving cathodes. In the year 2003, U.S. production of chlorine was about 13 million tons and production of caustic soda was about 15 million tons, the production of which consumed about 10 GW (about 317 trillion BTUs) of electrical energy; this corresponds to about two percent of the total electric power generated in the U.S.

In order to achieve greater energy savings, the hydrogen-evolving cathodes can be replaced by oxygen-consuming cathodes to produce an energy savings of as much as 30%, in what are sometimes called oxygen-depolarized cells.

Additionally, other industrial electrolysis processes that employ hydrogen-evolving cathodes can also realize an energy efficiency benefit by employing oxygen-consuming cathodes. Examples include the electrolysis of hydrogen chloride to recover chlorine or the electrolysis of hydrogen bromide solution to produce bromine.

The original oxygen-consuming chlor-alkali processes involves an anode chamber, through which brine is circulated and from which chlorine gas evolves, a cathode chamber through which oxygen is circulated, and a solution chamber between the other two chambers in which water is converted to sodium hydroxide. This is typically referred to as the finite-gap design. It is thought that the layer of sodium hydroxide between the oxygen electrode and the membrane unfavorably increases the cell resistance, and hydrostatic pressure of the sodium hydroxide solution leads to non-uniform gas/liquid interfaces within the oxygen electrode, which results in electrode flooding in some spots, and in leakage of the sodium hydroxide into the oxygen compartment in other spots.

To overcome these difficulties, a zero-gap oxygen-consuming chlor-alkali electrolysis cell was developed, as disclosed in U.S. Pat. No. 6,117,286 and described with respect to FIG. 1 herein. The cell 11 is portioned into an anode chamber 13 and a cathode chamber 14 by means of an ion-exchange membrane 12. The cell has a mesh-form insoluble anode 15, which may comprise a conventional insoluble titanium electrode known as a dimensionally-stable anode (DSA), in intimate contact with the ion-exchange membrane 12, on the side thereof adjacent the anode chamber 13. A sheet-form hydrophilic material 16 is in intimate contact with the ion-exchange membrane 12 on the side thereof adjacent the cathode chamber 14. The cell 11 also has a liquid-permeable oxygen gas diffusion cathode 17 in intimate contact with the hydrophilic material 16 on the side thereof adjacent the cathode chamber 14. A mesh-form cathode collector 18 is connected to the oxygen gas diffusion cathode 17 so that electricity is supplied through the anode 15 and the collector 18, as shown by the plus and minus signs.

An inlet 20 receives saturated aqueous sodium chloride solution as well as discharging the chlorine gas which is produced, and an outlet 19 discharges the aqueous solution of unreacted sodium chloride. An inlet 21 receives humidified oxygen-containing gas and an outlet 22 allows discharge of excess oxygen-containing gas as well as sodium hydroxide formed in the process. Sodium hydroxide is generated on the surface of the ion-exchange membrane 12 which faces the cathode chamber 14 and descends in a dispersed fashion, especially due to gravity, within the hydrophilic material 16, which provides less flow resistance to the sodium hydroxide solution than would the cathode 17 itself. The sodium hydroxide solution drips from the lower edge of the hydrophilic material 16 and passes through the outlet 22. This avoids having the sodium hydroxide solution residing in the oxygen gas diffusion cathode and impeding the oxygen-containing feed gas from smoothly permeating through the cathode.

In patent application publication US 2005/0026005, instead of employing a hydrophilic sheet material 16 adjacent the oxygen gas diffusion cathode 17, the oxygen gas diffusion cathode is provided with a composite layer of carbon-supported platinum and polytetrafluorethylene on the side of the oxygen diffusion cathode facing the cathode chamber 14. The purpose is stated to avoid generation of peroxide, the precipitation of which as sodium peroxide would cause liquid flow maintenance problems and damage to the membrane 12 and/or the oxygen gas-diffusion cathode.

The aforementioned zero gap, oxygen consuming electrode chlor-alkali cells present challenging problematic conditions. For instance, the relatively high viscosity and strong corrosiveness of concentrated sodium hydroxide can impede the effective transport of reactants and products within the cathode and can damage the cathode. Although the structure of the aforementioned patent publication tends to avoid local dry out of the cathode which promotes the formation of harmful peroxide, it is extremely difficult to maintain the balance of having the cathode fully saturated but not flooded at practical reactant stoichiometries, especially under a wide variety of commercial operating conditions across extended periods of time.

Additionally, other industrial electrolysis processes, such as the conversion of hydrogen chloride to chlorine with an oxygen-depolarized cathode, face analogous problematic issues.

DISCLOSURE OF INVENTION

Objects of the invention include: maintaining proper liquid balance in any oxygen-consuming, electrolysis membrane cell; providing a gas diffusion electrode which will maintain three-phase boundaries of gas, liquid and solid, that include oxygen, water/other solutions (such as water/caustic-soda solutions), and the cathode catalyst/support particles; providing a gas diffusion electrode which effectively transports oxygen to the cathode catalyst layer while removing the liquid products away from the catalyst layer; provision of a gas diffusion electrode in an electrolysis cell which assures that the ion-exchange membrane remains well hydrated without local dryout regions, while at the same time preventing flooding of the cathode catalyst layer; a chlor-alkali cell which does not require a supply of air that is humidified; electrolysis cells that are not adversely affected by a gradient of liquid pressure from the top of the cell to the bottom of the cell; and improved electrolysis cells.

This invention is predicated in part on the discovery that the presence of a dilute solution of sodium hydroxide, in a relatively uniform concentration and pressure, across the entire gas diffusion cathode enhances operation of the oxygen-consuming chlor-alkali cell. The invention is also predicated on the discovery that the oxygen reactant gas can be provided directly to the oxygen-consuming cathode without interference from sodium hydroxide or other liquid flooding, by providing channels for a recirculating flow of sodium hydroxide, water, or other water-containing liquid separate from channels for a flow of oxygen-containing gas, such as air.

In accordance with the invention, oxygen-containing gas is flowed across the gas diffusion layer of an oxygen depolarized cathode of an electrolysis cell, and a liquid solution is flowed through channels separated from the diffusion layer by a porous plate to hydrate the membrane and remove excess water.

According to one form of the present invention, an electrochemical apparatus includes a composite, bipolar plate disposed between the gas diffusion cathode of one oxygen consuming electrolysis cell and the anode of an adjacent cell; the portion of the bipolar plate adjacent to the anode is solid and contains passageways which circulate salt solution (such as brine or halide acid solution) and recover the gaseous product (e.g., chlorine or bromine) that is produced. The other portion of the bipolar plate is highly porous and hydrophilic, having reactant air or oxygen channels in one surface which are disposed in intimate contact with the oxygen-consuming gas diffusion cathode, and having channels in the opposite surface of said porous plate, through which liquid (e.g., sodium hydroxide or water, in some embodiments) is circulated, the liquid solution entering those channels at a desired reduced concentration (which may typically be on the order of 32%). According to the invention, the pressure of the sodium hydroxide or other water-containing liquid circulating through the porous plate is lower than the pressure of the air or other oxygen-containing gas; this pressure differential provides a driving force for liquid removal from the gas-diffusion electrode, which prevents the gas diffusion electrode from being flooded, while at the same time the electrode and the adjacent membrane is kept well hydrated by the liquid solution circulating through the plate.

The invention may be practiced as a mono-polar cell, if desired.

An oxygen-depolarized electrolysis cell according to the present invention provides the proper amount of hydration throughout the face of the entire gas diffusion cathode, removing excess liquid product where necessary and providing additional moisture where necessary, at various locations across the planform of the gas diffusion cathode. Similarly, since the oxygen, in accordance with the present invention, is presented to the gas diffusion cathode through separate air channels, at a pressure higher than that of the liquid solution, the presence of air at all areas of the gas diffusion electrode is ensured.

The invention may be practiced by providing solid and porous carbon plates in accordance with techniques which are customary in the production of solid plates and porous, hydrophilic plates for use in fuel cells, particularly for use in proton exchange membrane fuel cells.

Other objects, features and advantages of the present invention will become more apparent in the light of the following detailed description of exemplary embodiments thereof, as illustrated in the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional, side elevation view of a zero-gap, oxygen consuming chlor-alkali cell known to the prior art.

FIG. 2 is a sectioned side elevation view of a hydrogen-chloride electrolysis cell in accordance with the present invention, with stippling indicating the porous portion of the bipolar plate and section lines indicating the solid portion of the bipolar plate.

FIG. 3 is a sectioned side elevation view of a chlor-alkali cell in accordance with the present invention, with stippling indicating the porous portion of the bipolar plate and section lines indicating the solid portion of the bipolar plate.

FIG. 4 is a multiple representation of the chlor-alkali cell of the invention shown in FIG. 3, illustrating the simplicity of the repetitive structure of a stack of electrolysis cells in accordance with the invention.

MODE(S) FOR CARRYING OUT THE INVENTION

Referring to FIG. 2, a hydrogen-chloride electrolysis cell 29 employing an oxygen-consuming, gas-diffusion cathode 31 includes a conventional ion-exchange membrane 32 flanked by conductive anode and cathode screens 34, 35. The screens 34, 35 may contain catalysts that promote the respective electrochemical reactions, numbered 1-3 hereinafter, at respective electrodes. The anode may be a conventional DSA. On the anode side, a solid (non-porous) plate 38 includes passageways 39 for the circulation of hydrogen chloride solution and for the extraction of chlorine gas which is produced by the electrolysis process. On the cathode side, a micro-porous, hydrophilic plate 42 has passageways 43 for conducting non-hydrated, oxygen-containing gas, such as air, and passageways 44 for circulating a water-containing liquid such as sodium hydroxide or simply water.

The electrode conductors 34, 35 are respectively positive and negative terminals (as indicated by the plus and minus signs 46, 47) and these are connected to an appropriate source of direct current electrical (DC) power 48. With a hydrogen chloride solution, water, and air being provided to the cell 29, and with the anode and cathode conductors 34 and 35 connected across the DC power 48, the reactions at the anode are shown by equations 1 and 2, and the reaction at the cathode is shown by equation 3.

At anode

4HCl→4Cl⁻+4H⁺  1.

4Cl⁻→+2Cl₂+4e-  2.

At cathode

O₂+4H⁺+4e⁻→2H₂O   3.

A significant difference of the cell 29 compared to the prior art is that the oxygen-containing stream (e.g., air) need not be humidified prior to being fed to the cell 29. This is because the hydrophilic pores in the porous plate 42 are filled with water (or a dilute hydrogen chloride solution) and provide a means to fully hydrate (saturate with water) the gas in the cathode passageways 43. Keeping the cell well-hydrated is critical since the membrane is a poor ionic conductor if it dries out and the lifetime of the membrane is significantly reduced under dry conditions. Additionally, since the water in the passageways 44 is circulated at a pressure below that of the gas pressure in the cathode passageways 43, any excess liquid water in the cathode 31 is removed by this pressure gradient. The removal of excess liquid water from the cathode is necessary since liquid water is both produced at the cathode, via reaction 3, and is transported to the cathode from the anode with the flow of protons via electro-osmotic drag. If the cathode is flooded with water, even in local spots, this will prevent the access of oxygen to the cathode catalyst layer 35, and instead of reaction 3 the following reaction can occur in these flooded regions:

4H⁺+4e⁻→2H₂   4.

Obviously, this reaction is not desired from a safety perspective. Additionally, it decreases the efficiency of the oxygen-depolarized cathode.

FIG. 3 is described using an HCl solution as the source of chlorine product; however, other halide acids, such as HBr, may be used.

Referring to FIG. 3, a chlor-alkali cell 29 employing an oxygen-consuming gas diffusion cathode 31 includes a conventional ion exchange membrane 30 flanked by conductive anode and cathode screens 34, 35. The screens may have catalysts to promote reactions 5-8, hereinafter, on respective electrodes. On the anode side, a solid (non-porous) plate 38 includes passageways 39 for the circulation of brine (sodium chloride solution) and for the extraction of chlorine gas which is produced by the electrolysis process. On the cathode side, a porous, hydrophilic plate 42, passageways 43 for conducting oxygen-containing gas, such as air, and passageways 44 for circulating sodium hydroxide solution, which enters the passageways 44 in a desired concentration, such as on the order of 32%, and leaves the passageways in a more concentrated solution, due to the formation of sodium hydroxide by the electrolysis process.

The electrode conductors 34, 35 are respectively positive and negative terminals (as indicated by the plus and minus signs 46, 47) and these are connected to an appropriate source of DC power 48. With brine, sodium hydroxide, and air being provided to the cell 29, and with the anode and cathode conductors 34 and 35 connected across the source of power 48, the reactions at the anode are shown by equations 5 and 6, and the reactions on the cathode are shown by equations 7 and 8.

At anode

4NaCl→4Cl⁻+4Na⁺  5.

4Cl⁻→2Cl₂+4e⁻  6.

At cathode

2H₂O+O₂+4e⁻→4OH⁻  7.

4Na⁺+4OH⁻→4NaOH   8.

A significant difference of the cell 29 compared to the prior art is that sodium hydroxide in an appropriate solution strength is provided to the cell, the solution thereby providing the desired water (reaction 7) to eventually provide sodium chloride (reaction 8). By providing a sodium hydroxide solution, not only is water provided so that air need not be moisturized as in the prior art, but the concentration of sodium hydroxide solution, and therefore water, will be substantially uniform across the planform of the gas diffusion cathode 31. This is achieved by the porous plate 42 which is hydrophilic and allows passage of sodium hydroxide solution through the porous plate 42 and air channels 43 to reach the surface of the gas diffusion cathode 31. Additionally, the pressure gradient provided across the porous plate 42 will remove any excess liquid water in the cathode, which will enable adequate oxygen access to support reaction 7 and prevent hydrogen evolution from occurring at the cathode via:

4Na⁺+4H₂O→4NaOH+4H⁺  9.

followed by reaction 4, hereinbefore.

Even in a low concentration of alkali solution, carbon dioxide will result in the formation of carbonate salts that will precipitate out of solution; therefore, the oxidant-containing source (e.g., air) should first be scrubbed to remove CO₂, as is conventional. However, the air stream need not be pre-humidified, or saturated with water, as described hereinbefore. The invention may be practiced in a mono-polar cell design (a single cell), or it may be practiced in a stack of cells. The cell 29 of FIG. 3 is shown in FIG. 4 with an additional cell 29 to illustrate the ease of repeatability so as to form a stack 53 of chlor-alkali cells 29. As easily seen in FIG. 4, the solid plates 38 and porous plate 42 together comprise a composite bipolar plate 51. If desired, mono-polar cells could also be readily constructed by those skilled in the art using the porous-plate cathode concept taught herein.

The configuration of the porous plate 42 may be very similar to similar porous, hydrophilic plates which are known in fuel cells, and typically comprise woven carbon sheets which are rendered hydrophilic by treating with tin, or by other known processes. The solid plate 38 may comprise solid carbon, solid metal, or any other suitable material, such as a plastic with carbon or glass fibers, metal or the like. The porous plate has to be conducting in a bipolar plate configuration, and preferably should be conducting in a mono-polar configuration, since it is the thickest part and current must flow parallel to the plate direction. The porous plate 42 can be constructed of a variety of materials that have been used as porous reactant gas flow field plates in fuel cell applications or it may be a porous metallic plate made from powdered metal.

The gas diffusion electrode 31 may be constructed of conventional materials used in fuel-cell cathodes, such as porous carbon papers, cloths, or non-woven materials. Alternatively, the gas-diffusion electrode could be constructed of a metal screen or a combination of metal and carbon. The cathode catalyst layer 35 may be constructed in a manner similar to state-of-the-art proton exchange membrane (PEM) fuel cell cathodes, which typically are a combination of catalyst and ion-exchange ionomer. In addition to the ionomers in the catalyst layer, other polymers (e.g., PTFE) may be used as binders and/or to control the desired hydrophobicity and porosity of this layer. The catalyst should be one that promotes the oxygen-reduction reaction, which typically requires a noble metal such as platinum or some platinum-based alloy. Preferably, this catalyst should not be adversely affected by the presence of chlorine, which may be present in small amounts at the cathode. The catalyst layer is typically formed by mixing the catalyst with the polymers in solution and carefully casting the resultant ink onto the membrane, or some other suitable substrate, to obtain the desired porous structure after the solvent(s) are removed by evaporation. The ion-exchange membrane 12 may be the same as those used in conventional chlor-alkali membrane cells or in PEM fuel cells. These membranes are typically fluorinated polymers with sulfonate groups to provide the ionic sites, such as Du Pont NAFION®. These membranes are formed into thin films by extrusion or casting and they can be reinforced with other materials (e.g., fibers of expanded PTFE) to improve their mechanical properties. The anodes can be constructed with conventional materials used in chlor-alkali and hydrogen-chloride electrolysis cells.

Claims

1. A method of operating an oxygen-depolarized electrolysis cell having an anode and having a cathode including a water permeable gas diffusion electrode, said method comprising:

feeding a solution to the anode of the cell selected from (a) a salt solution and (b) a solution of halide acid;

applying DC power between the anode and the cathode of the cell to drive electrochemical reactions in the cell to produce desired product; and

recovering desired product from the cell;

characterized by:

flowing oxygen-containing gas through passageways on a side of a porous hydrophilic plate adjacent to the gas diffusion electrode of the cathode; and

circulating a water-containing liquid solution through passageways in the porous hydrophilic plate which are separated from the flow of oxygen-containing gas.

2. A method according to claim 1 wherein said step of flowing is further characterized by:

flowing non-hydrated oxygen-containing gas.

3. A method according to claim 1 wherein said step of flowing is further characterized by:

flowing oxygen-containing gas which is not saturated with water.

4. A method according to claim 1 wherein said step of flowing is further characterized by:

flowing oxygen-containing gas at a pressure which is lower than the pressure of the water-containing liquid.

5. A method according to claim 1 further characterized by:

removing substantially all carbon dioxide from the oxygen-containing gas before flowing the oxygen-containing gas through said cell.

6. A method according to claim 1 further characterized by:

said step of feeding comprising feeding a halide acid solution in water; and

said step of circulating comprises circulating water.

7. A method according to claim 6 further characterized by:

said step of feeding comprises feeding hydrochloric acid.

8. A method according to claim 6 further characterized by:

said step of feeding comprises feeding brine; and

said step of circulating comprises circulating a dilute solution of sodium hydroxide.

9. An electrolysis cell (29) with an oxygen-depolarized cathode (31), comprising:

a permeable anode conductor (34), a permeable cathode conductor (35), an ion exchange membrane (32) disposed between and contacting said conductors, a solid plate (38) having salt/product channels (39) adjacent to said anode conductor, configured to receive salt solution and configured to conduct product of said salt/product channels, and an oxygen consuming, gas diffusion cathode (31) contacting said cathode conductor;

characterized by the improvement comprising:

a porous, hydrophilic plate (42) having oxidant channels (43), extending from a first surface thereof contacting said gas diffusion cathode, configured to receive an oxygen-containing gas, said porous hydrophilic plate also having liquid channels (44), extending from a second surface thereof opposite to said first surface, configured to receive a water-containing liquid.

10. An electrolysis cell (29) according to claim 9 wherein:

a noble metal or noble metal alloy catalyst is disposed in said cathode conductor (35) adjacent to said membrane (32).

11. A stack (53) of electrolysis cells (29) according to claim 9.

12. An electrolysis cell (29) according to claim 9 wherein:

said salt/product channels (39) are configured to receive brine;

said liquid channels (43) are configured to receive a dilute solution of water and sodium hydroxide; and

said salt/product channels are configured to provide chlorine as product.

13. An electrolysis cell (29) according to claim 9 wherein:

said salt/product channels (39) are configured to receive halide acid solution;

said liquid channels (43) are configured to receive water; and

said salt/product channels are configured to provide chlorine as product.

14. An electrolysis cell (29) according to claim 13 wherein:

said salt/product channels (39) are configured to receive hydrogen chloride solution.