WO1996005336A1

WO1996005336A1 - Electrochemical device for removal and regeneration of oxygen and method

Info

Publication number: WO1996005336A1
Application number: PCT/US1994/009198
Authority: WO
Inventors: Melvyn I. Burk; Daniel A. Scherson
Original assignee: Burk Melvyn I; Scherson Daniel A
Priority date: 1992-04-27
Filing date: 1994-08-15
Publication date: 1996-02-22
Also published as: US5338412A

Abstract

An electrochemical device for concentrating oxygen is provided. The electrochemical device comprises a cathode (10) for reducing oxygen to peroxide. An anode (30) is provided for oxidizing the peroxide to form oxygen according to a two electron process. A solid polymer electrolyte (20) diffuses the peroxide from the cathode (10) to the anode (30).

Description

ELECTROCHEMICAL DEVICE FOR REMOVAL AMD REGENERATION OF OXYGEN AND METHOD

BACKGROUND OF THE INVENTION The present invention pertains to the art of oxygen concentration, and more particularly to the art of concentrating oxygen from a gaseous mixture, such as air, according to an electrochemical device and method. The invention finds usefulness in a variety of applications and situations such as those pertaining to the medical, industrial and space exploration fields, and will be described with particular reference thereto. It should be appreciated, however, that the invention can be applied to other areas not set forth herein, particularly where concentrated oxygen is required.

There is a need for providing substantially undiluted oxygen in a variety of medical, industrial, aeronautical and space exploration situations. Numerous electrochemical methods and devices are known in the art for concentrating oxygen, but these are not without several disadvantages. Such known electrochemical devices and methods for generating oxygen are based on electrolysis using solid polymer electrolyte (SPE) technology, static feed H₂0 electrolysis subsystems (SFWES) , or the less- developed electrolysis of carbon dioxide. Conventional electrochemical oxygen generators are based upon the electrolysis of water, a 4-electron process.

Existing solid polymer electrolyte (SPE) technology includes a thin (0.30 mm) perfluorinated sulfonic acid membrane (DuPont Nafion) which, when saturated with water, serves as an electrolyte having a resistivity of 15 ohm cm. The membrane also prevents mixing of 0₂ and H₂. Catalyzed electrodes are placed in intimate contact with both sides of the membrane. Deionized water is fed to the cathode (i.e., the H₂ producing side of the SPE) and acts as both a reactant and a coolant. A six person SPE electrolyzer, which operates at 1.72V per cell at a current density of 350mA/cm², calls for an input power of 2180 W and provides an 0₂ production rate of 6.82 kg/day. The specific power is roughly 320 W/kg 0₂ per day.

Existing static feed water electrolysis subsystem technology (SFWES) includes thin asbestos sheets. These serve as both the water feed and the cell matrices, and are saturated with an aqueous KOH solution. As a direct current power is supplied, water in the cell matrix is electrolyzed. As a result, the KOH electrolyte concentration increases and water vapor diffuses from the feed to the cell matrix. An SFWES module which operates at 1.52V per cell at a current density of 206mA/cm², calls for a power consumption of 174W, and provides a 0₂ production rate of 0.82 kg/day. The specific power for such an operation is roughly 212 W/kg 0₂ per day.

Another preexisting oxygen concentration method involves carbon dioxide electrolysis. C0₂ from a C0₂ concentrator is directly oxidized using a solid oxide electrolyte that is coated on both sides with a porous metal coating such as Pt. This operation takes place at high temperatures to produce 0₂. Only oxide ions (0²) migrate through the solid electrolyte driven by a DC voltage so that 0₂ separation is excellent. The processes are as follows:

At cathode: C0₂ + 2e → CO + O²" At anode: 20²" → 0₂ + 4e

In another reactor: 2CO → C + C0₂ Technological problems such as sealing exist with carbon dioxide electrolysis.

Other electrolysis technologies are known in the art. One example concerns the electrolysis of water. Electrolysis of water with 0₂-depolarization utilizes a fuel cell type cathode and operates at a theoretical cell voltage close to zero. In practice, overpotentials at the anode and cathode, as well as IR losses, raise the cell voltage to about 1.1 V, at current densities of 108 mA/cm². The specific power requirement is about 180-200 W/kg 0₂ per day. Another technology involves electroregeneration of an organometallic carrier compound capable of binding 0₂ (in a manner like hemoglobin) in its reduced state and releasing 0₂ according to a 2-electron process upon anodic oxidation is being studied. A major parasitic reaction with respect to power consumption arises from the oxidation of a carrier which has not bound to 0₂. The power consumption for such technology is estimated at about 30 W/kg 0₂ per day. A limitation is the low current density of practical operation, in the range of about 1 to 2 mA/cm². It should be noted that at higher current densities, the lifetime of the organometallic is severely limited.

The electrogeneration of reducing agent 2,7- anthraquinone-disulfonate in solution reduces 0₂ to peroxide, which is then electrochemically oxidized to 0₂. This 2-electron process suffers from high solution IR and low energy efficiency.

Direct electroreduction of 0₂ can produce the superoxide ion (O²) , which may be followed by diffusion to the anode and one-electron oxidation to 0₂ at low power requirement. The key problem here is stabilization of the superoxide ion in water and avoiding coelectrolysis of water to give H₂ and 0₂. The present invention pertains to an electrochemical device and method for the selective removal and regeneration of oxygen from the ambient atmosphere. It is based on the use of a two gas-fed electrode system separated by a thin layer of liquid or solid electrolyte. An external potential difference is applied between the two electrodes to promote the reduction of dioxygen to either hydrogen peroxide or superoxide at the cathode. The hydrogen peroxide or superoxide is then reoxidized at the anode to yield gas-phase purified dioxygen. Hence, if the cathode is exposed to the atmosphere and the anode is exposed to an enclosed environment, the device will selectively enrich the enclosed environment with oxygen.

The subject development is well suited for applications in areas where oxygen is continuously consumed in or by the enclosed environment. A difference in chemical potential or partial pressure between the atmosphere and the enclosed environment will decrease the energy requirements needed for driving the process to reasonable rates, thus providing an energy efficient and economical source for purified oxygen. The subject new electrochemical filter and method for its application may lead to inexpensive sources or pure oxygen for medical uses and, on a larger scale, for industrial applications such as steel production or glass manufacturing. SUMMARY OF THE INVENTION The present invention is directed to an electrochemical device and method for concentrating and purifying oxygen. More specifically, an electrochemical device for concentrating oxygen comprises a cathode, an anode, and a solid polymer electrolyte. The cathode, a high-area Teflon-bonded carbon gas-fed electrode, is suited for reducing oxygen present in a gaseous feed to peroxide or superoxide. The anode, also a high area Teflon-bonded carbon gas-fed electrode, is suited for oxidizing the peroxide or superoxide to form oxygen according to a two- electron process. The solid polymer electrolyte located between the cathode and anode is an ultra-thin separator membrane suited for diffusing the peroxide or superoxide from the cathode to the anode.

A principal advantage of the subject development is that it provides for an oxygen concentrating unit that is more compact and of a lighter weight than oxygen concentration units of the prior art.

Another advantage of the present invention is that it provides for an oxygen concentration device having a lower power consumption at higher rates of oxygen production. Another advantage of the present invention is that it does not call for mixing in of additional chemicals, and it is fully self-contained.

Yet another advantage of the present invention is that it provides for a useful life which is greater than that of other electrochemical oxygen concentrating devices of the prior art. Moreover, the present invention concerns a device which calls for little maintenance.

Still other advantages and benefits of the invention will become apparent to those skilled in the art upon a reading and understanding of the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may take physical form in certain parts and arrangements of parts, a preferred embodiment of which will be described in detail in this specification and illustrated in the accompanying drawings which form a part hereof.

Figure 1 is a schematic representation of an electrochemical cell formed in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The oxygen concentrating device of the present invention takes advantage of thin film solid polymer electrolyte (SPE) technology and the use of a monolithic gas-fed fuel cell type cathode structure. A gas containing a low concentration of oxygen is fed to a cathodic electrode of the device. The gas can be comprised of ambient air, or it can be supplied by an environment incompatible with life support (such as, for example, an environment too high in carbon dioxide content for human habitation) . On a molar basis, air typically contains about 21% 0₂ and 79% N₂. The oxygen in the feed gas is selectively reduced at the cathode according to a two electron process to form peroxide. The peroxide is transferred through a thin separator and oxidized at the anode to form concentrated oxygen according to a two electron process. A careful search of the literature has shown that this specific approach to oxygen separation and concentration has not been previously described.

With attention now to Figure 1, a depiction of the structure and operation of the composite electrode of the subject new development is shown. A feed gas including oxygen is fed to a cathode 10 and converted to peroxide. The peroxide diffuses or migrates through a solid polymer electrolyte (SPE) separator material 20 to the anode 30. At the anode, the peroxide is oxidized to form purified oxygen. The reaction taking place at the cathode is as follows:

0₂ + 2H⁺ + 2e → H₂0₂ E° = 0.695V or 0₂ + H₂ + 2e → H0₂ + OH E° = 0.649V At the anode, the following reactions take place:

H₂0₂ → 0₂ + 2H⁺ + 2e or H0₂ → 0₂ H⁺ + 2e

Both the cathode and anode of the electrochemical oxygen concentrating device of the present invention are comprised of adherent coatings formed from fuel cell type gas diffusion electrode components on the SPE separator material.

The separator material itself may be microporous, wetted with an aqueous electrolyte, or it may be an ion- exchange membrane wetted with water. The separator material is thin.

The electrochemical reduction of oxygen to peroxide under acidic or alkaline conditions is a highly reversible process occurring at a high exchange current density. Isotopic studies of the cathodic reduction of 0₂ to H0₂ and the reverse anodic process on high area porous carbons and alkaline solutions demonstrate that the oxygen- oxygen bond is not ruptured during these reactions. For instance, an E° value on carbon for the 0₂/H0₂ couple in alkaline solutions has been determined to be -0.048V. Polarographic measurements at Hg provide E° = -0.045V.

In contrast, the electrochemical reduction of 0₂ to H₂0 or OH, and the electrolysis of water under acidic or alkaline conditions, are irreversible processes. Such processes are highly dependent on the electrode materials' catalytic capability in facilitating oxygen-oxygen bond breaking (reduction) or bond making (oxidation) .

Cell voltages are dependent upon the activation overpotentials at the electrodes and on the IR contributions of the separator, electrode components, solution and electrical connections. Mixed electrochemical processes, caused by inefficiencies resulting fro catalytic decomposition of peroxide and 0₂ formation from electrolysis of water instead of peroxide, will raise the cell voltage if uncontrolled. The overall cell voltage will be close to zero volts.

Since the electrolyte pumping costs are eliminated by the device discussed herein, the energy efficiency of the present development may be greater than 90%. An estimated specific power requirement assuming a cell voltage of 0.4V at 100 mA/cm² is about 40-50 W/kg 0₂ per day. Like conventional SPE electrolysis, the subject new device would deliver oxygen at high pressure, if needed, by back pressuring both sides of the composite. The oxygen concentrator of the subject new development is operable either continuously or cyclically.

The SPE composites as set forth in Figure 1 can be prepared from various carbon, gold and other known separator materials. It is important that the materials and solutions used in preparing the SPE composites are of the highest purity to avoid traces of substances such as Fe, Cu, Mn, Ni, Cr and Pt. These and other substances could catalytically decompose peroxide and result in upsetting the charge balance required for efficient operation of the composites.

The thin SPE composites can be prepared according to a number of different approaches. Ion-exchange resins, microporous separator materials and even high purity paper can be used as substrates.

In preparing supported SPE composites, the ion- exchange membranes or relatively high-strength microporous separator materials such as microporous polycarbonate (e.g., Nucleopore) or polypropylene (e.g., Celgard) polymer films, or even high purity paper, can be used as substrates. These microporous materials will be well- coated or soaked in the ion exchange resin solution and dried. Ion exchange resins include materials such as Tosoh and Tosflex anionic resins. The microporous separator materials are perfluorinated materials available in thin membrane films or as solutions. After the microporous materials have been well- coated or soaked in ion exchange resin solution and dried, a purified conductive carbon black such as Shawinigan Black is mixed with polytetrofluoroethylene (Teflon) emulsion and coated or air-brushed onto both sides of the substrate and dried in air or in a vacuum oven supported between polytetrafluoproethylene (Teflon) pressure plates. In some instances, the composites are heated to higher temperatures but below temperatures that would promote degradation of the polymeric substrates. These preparation methods correspond to procedures followed in preparing diffusion electrodes for use in fuel cells.

In unsupported SPE layers, the anionic exchange resin solution is sprayed or coated onto one surface of two uncatalyzed Teflon-bonded gas diffusion electrodes. While still wet, the resin sides are sandwiched together to form the composite which will be dried under pressure. Uncatalyzed Teflon-bonded gas diffusion electrodes are available commercially. It is within the scope of the invention to use thin carbon paper or carbon felt electrodes as an alternative to gas diffusion electrodes in a sandwich structure.

The invention will now be more fully described by way of the following Example: Example - Cell Fabrication

Very thin gas-fed/gas-generating-electrode/solid polymer electrolyte structures are fabricated in order to implement the oxygen purification and concentration features described above. The overall process involves a series of steps which call for preparing a thin-film solid polymer electrolyte phase and fabricating an in situ electrode and cell. Once fabricated, the cell is tested.

A. Preparation of the Thin-Film Solid Polymer Electrolyte Phase

Two different approaches may be employed to produce very thin SPE films. A first involves a supported

SPE. Here, a relatively high strength, highly porous material such as a Nucleopore membrane or a high purity paper, is placed in contact with a solution containing the SPE, such as Nafion or other ionically conducting polymers. After a certain period of time has elapsed, the membrane is removed from the solution, and the excess liquid is allowed to drain. The entire structure is left to air dry. A second approach involves unsupported SPE layers. In this instance, the SPE solution is either sprayed or painted onto the surface of a self-supported porous Teflon-bonded high area carbon electrode. B. In situ Electrode and Cell Fabrication

A mixture of emulsified Teflon and high are carbon dispersed under ultrasonic agitation is sprayed ont each side of the supported SPE membrane, either with a atomizer or by other means, so as to form a thin layer o material. A soft felt type carbon current collector i then placed onto each of the electrodes and the entir structure pressed in a roller.

Alternatively, the entire cell is formed directl onto the carbon felt current collector by spraying i sequence (1) the high area carbon/Teflon; (2) the SP (which could also be painted) ; and, (3) the othe electrode. Subsequently, the second felt collector i placed on the layered structure which is then rolled b conventional means to form the complete cell.

If necessary, the cells are dried in air or unde vacuum above room temperature in order to remove th emulsifier.

C. Cell Testing Circular cells about 1 inch in diameter ar pressed between two annular connectors forming behind eac electrode a cavity isolated from the atmosphere. Durin the actual measurements, air is flowed past the cathode an an inert gas, such as argon, is flowed past the anode ga compartments. For testing purposes, air of known chemica composition is obtained from a commercial vendor while the amount of dioxygen produced at the anode is assayed using a commercial oxygen-gas sensor. Faradaic efficiencies are determined as a function of current density and temperature. Optimized conditions for cell performance are found by determining the effects associated with:

(i) the partial pressure of dioxygen in the (cathode) feed gas by, for example, mixing air with pure oxygen; and, (ii) the decrease in the total pressure in the anode compartment.

The invention has been described with reference to the preferred embodiment. Obviously, modifications and alterations will occur to others upon a reading and understanding of this specification. It is intended to include all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.

Claims

I Claim:

1. An electrochemical device for concentrating oxygen from a feed gas, including oxygen, the electrochemical device comprising: a cathode for reducing oxygen in the feed gas to peroxide; a solid polymer electrolyte communicating with the cathode for diffusing the peroxide therethrough; and, an anode communicating with the solid polymer electrolyte for oxidizing the peroxide to produce a high concentration of oxygen.

2. An electrochemical device for concentrating oxygen, according to claim 1, wherein the feed gas is air.

3. An electrochemical device for concentrating oxygen, according to claim 1, wherein the solid polymer electrolyte comprises a polymer film.

4. An electrochemical device for concentrating oxygen, according to claim 3, wherein the polymer film may be selected from the group consisting of microporous polycarbonate and polypropylene.

5. A method for concentrating oxygen, comprising the steps of: bringing a gas comprising oxygen into contact with a gas-permeable cathode; converting the oxygen to peroxide; diffusing the peroxide through a wetted solid polymer electrolyte from the gas-permeable cathode to a gas-permeable anode; and, converting the peroxide to oxygen at the anode.

6. A method for concentrating oxygen, according to claim 5, wherein the gas comprising oxygen is air.

7. A method for concentrating oxygen, according to claim 5, wherein the gas permeable cathode is a high area carbon electrode.

8. A method for concentrating oxygen, according to claim 5, wherein the anode is a high area carbon electrode.

9. A method for concentrating oxygen, according to claim 5, wherein the solid polymer electrolyte comprises a polymer film selected from the group consisting of microporous polycarbonate and polypropylene.