Polymeric hydrogel-containing gas diffusion electrodes and the like and method of using the same in electrochemical systems

Abstract: This invention is concerned with catalytic gas-diffusion structures such as electrodes and barriers comprising, as an integral part of portion thereof, a non-ionic aqueous gel such as a polyvinyl alcohol, rendering said electrode impervious to gas percolation, while retaining electrolyte contact with the structures; and with methods of using said electrodes or barriers in a variety of electrochemical systems including electrolysis cells, fuel cells, batteries and others, as well as in metal-recovery systems involving no electricity.

Description: The present invention relates to catalytic gas-diffusion electrodes for use in an electrolytic cell containing an aqueous electrolyte, and a method of forming said electrode. The gas-diffusion electrode described in EP-A-0 106 621 has a porous electroconductive body of hydrophobic polymeric material and on its surface for contact with the electrolyte, a thin porous hydrophilic layer, e.g. of asbestos. Neither this known electrode nor the gas-diffusion electrode described in GB-A-1 398 843 use a carbon cloth. As is well known, catalytic gas-diffusion electrodes function usually in a three-phase environment consisting of a solid catalytic electrode, an aqueous electrolyte solution and an active gas. Though no theoretical interpretation is needed as explanation of this invention, it is plausible to accept the common view that the contact point between the aqueous electrolyte and the active gas on the catalytic sites of the solid electrode is the seat of the electrochemical reaction, which involves electron release or capture at practically acceptable rates. In practice, in the fuel cell area, gas-diffusion electrodes are made by catalyzing and wet-proofing gas-porous structures such as porous carbon papers, carbon cloth, metal screen and the like, with an appropriate mixture, for example, of polytetrafluorethylene (PTFE) and a platinum-on-carbon catalyst. Here the wet-proofing serves the purpose of preventing "flooding" of the pores of the electrode with an aqueous electrolyte solution, while retaining gaseous access to, and wetting of the catalyst sites. It is usually necessary to prevent at least partial flooding of electrodes, by supplying the gas at a sufficient pressure, thereby causing gas percolation through the otherwise porous electrodes. Excessively pressurized gases, on the other hand, cause often serious drops in electrode performance by preventing adequate wetting of the electrode. Generally, active gases such as hydrogen, oxygen (or even air, when it has to be freed from carbon dioxide for alkaline electrochemical systems), should not be allowed to percolate through the electrodes, as percolation at least causes a net loss of reagent. An object of the present invention, accordingly, is to provide a catalytic gas-diffusion electrode that is not subject to the above-described percolation and enables greater efficiency. In its broadest aspect, the present invention provides a gas-diffusion electrode as defined in claim 1. Further features are defined in claims 2-4. Methods of forming said gas-diffusion electrodes are defined in claims 5 and 6. The term non-ionic aqueous polymeric gel (also abbreviated to "hydrogel"), as used herein, refers to hydrated substantially water and/or liquid electrolyte-insoluble polymers, which are preferably, though not necessarily, cross-linked, said polymers being substantially free from ionizable groups chemically linked to said polymer. It is an essential characteristic of the non-ionic hydrogels of this invention that they are gas-impervious, while allowing electrolytes consisting of mobile ions of both signs, with and/or without associated water of hydration, to diffuse freely into the aqueous phase of the gel. The drawing shows the essential components of the gas-impervious, but solution-penetrable, gas-diffusion electrode. Figure 1 represents schematically a localized small area of an electrically conducting carbon cloth. A catalytic hydrophobic layer (2) is deposited on the electrically conducting fibers (1) of said cloth. The open pores (3) allow access of gas to the catalyst (2), provided they are not flooded with aqueous electrolyte. Such flooding must be minimized or prevented, for example, by supplying the gas under pressure. It is here that the detrimental percolation of the active gas occurs, in the absence of the non-ionic polymeric gel of this invention. Further, flooding can occur not only because the liquid pressure exceeds the gaseous pressure, but also as a result of the water moved, for example as water of hydration, with dissolved ions under D.C. current. Figure 2 is a cross-sectional view of an electrolytic cell. The hydrogen anode (A) of the cell used, for example, for the electrolytic recovery of zinc from an electrolyte solution (D) of zinc sulfate and sulfuric acid, said anode comprises the porous carbon cloth before described, and an aluminum (starting sheet) cathode (C). Whereas, as before described, a porous catalyzed hydrogen anode (A) would normally dissipate or percolate hydrogen gas into the electrolyte, and electrolyte would normally leak excessively into the hydrogen feed chamber (E), these deleterious effects have been overcome by coating the inner surface (A') of the catalyzed carbon cloth anode (A) with a layer of a hydrogel, e.g. polyvinyl alcohol (PVA) gel, cast as an intimate film (P) over the inner anode surface (A'), dried and then heat-treated to seal to the carbon cloth surface at (A'). The layer (P) is then permeated or filled with electrolyte, swelling to contain about fifty-percent of water, by weight of film; the electrolyte in the layer (P) now contacts the catalyst of the carbon cloth electrode at or near the surface (A'), that is the electrode area which has been merged with the hydrogel. This provides an interface section within the electrode (A) for gas entering the left-hand surface (B) to cause the effective catalytic conversion of hydrogen gas to hydrogen ion within the carbon cloth electrode adjacent the layer (P), as fresh dissolved ions continually enter the hydrogel layer (P) supplying the adjacent section (A') of the anode therewith, thereby completing the three-phase environment required for efficient anodic hydrogen oxidation. The H₂ gas is stopped at the gas-impervious layer (P) and does not pass through into the electrolyte (D). When the open pores of the cloth are blocked by anchoring the hydrogel, on the electrolyte-facing surface of the electrode, while retaining the remainder of the electrode accessible to the gas, diffusion and/or electrochemical transport of said ions from the liquid into the hydrogel occurs. In this fashion the necessary contact between the catalytic layer and the ions of the electrolyte is maintained by the solid gel coating while the gel-free gas side of the electrode insures gaseous access thereto. Alternately, the same result is attained when the hydrogel is anchored within the open pores and the gas is fed to the electrode by "edge feed", as shown in U.S. Patent No. 4,478,696. Preferred and best embodiments hereinafter set forth illustrate the invention using polyvinyl alcohol and polyethylene oxide as typical examples of the non-ionic hydrogel. Suitable for the purpose of this invention in general, however, is the broad class of non-ionic hydrogels which includes gels comprising at least one polymer or co-polymer of a hydrophilic substantially non-ionic monomer. Typical monomers include glycol and glycerol, monoacrylates and monomethacrylates, propylene oxide, acrylamide and methacrylamide, in addition to the above named polymers from vinyl alcohol and ethylene oxide. Such gels of the prior art are generally described in U.S. Patents 4,331,783, 4,337,327 and 4,379,874. It is understood that the selection of a suitable hydrogel is made on the basis of combining an adequate water content of the gel with an adequate dimensional stability, which combination is readily determined experimentally in any particular electrolyte solution, by simply varying cross-linking agents, cross-linking levels and/or molecular weights of the polymer. The carbon cloth type electrode comprises for example the platinum-on-carbon PTFE layer and a particle platinum size range of 15-25Å, as described in U.S. Patents 4,044,193, 4,293,396 and 4,248,682, such as are specifically described in Example 1 of said U.S. Patent No. 4,293,396. In practice, the problem of gas percolation as well as excessive loss of liquid electrolyte is particularly serious, when large size electrodes are required which are extending into sizable depths of the order of several feet, into an electrolyte bath. Here the standard porous gas-diffusion electrode cannot prevent serious gas percolation at the upper surface of the electrode, because excessive flooding occurs at the lower part due to the liquid electrolyte. This becomes especially serious when the electrode is a hydrogen anode in an electrolytic cell, where the highly diffusible hydrogen percolates through the upper portion, constituting a significant economic loss, and where excessive flooding of electrolyte at the bottom causes unacceptable performance losses. EXAMPLE 1 As an example of coating one surface of a gas-diffusion electrode with a hydrogel, 50 grams of a high molecule weight PVA made and sold by the Dupont Company under the trade name ELVANOL 72-60 (described, for example, in Dupont Company's brochure entitled elvanol, Third Edition, 1961) was dissolved in one liter of distilled water containing also 1.0 gram of ammonium chloride, by boiling for several hours until substantially complete dissolution of the PVA. The solution was cooled to ambient temperature and a clear solution was obtained upon decantation from a small insoluble residue. The resulting approximately 5% PVA solution was stable, when stored in a location protected from light. The solution was then applied, by brushing, to one surface of the above-referred to electrode, of Example 1, U.S. Patent No. 4,293,396, and the resulting film was dried at room temperature. Three coats were applied, each being dried for about one-half hour between applications. The coated electrode was then heated for about one-half hour at 125°C to increase the film's insolubility, and cooled to ambient temperature. The coated electrode was then ready for use in an electrochemical system comprising an aqueous electrolyte solution, the PVA-coated surface to be contacted by the solution and the uncoated surface to be contacted by an active gas. The PVA-coated electrode tolerates continuous, for example, hydrogen pressure of up to 0.3 bar (12 inch of water) above the liquid electrolyte pressure without separation of the coating from the structure. EXAMPLE 2 Stronger adherence of the PVA gel was attained, when the aqueous 5% PVA solution of Example 1 was diluted with isopropanol to the extent of 33% by volume of isopropanol contained solution, and applied to the electrode, as described in Example 1. The isopropanol causes the PVA to penetrate the electrode structure, resulting in strengthening the bond between the electrode and the PVA gel, allowing gas pressures of up to 1 bar (40 inch of water) to be used with no percolation and no PVA gel separation from the electrode. Here the hydrogel appears to have been anchored in its entirety within the electrode, with, however, exposed area available on the electrode's "backside" for gaseous access. There is no surface PVA film present on the "front side", as determined visually and by the fact that no PVA film can be lifted from the electrode under pressures in excess of 1 bar (40 inch of water) (which can cause some percolation). Combined partial embedding with film coating was also obtained by using a 10% isopropanol solution, in lieu of the above 33.3% solution. Finally, by using the above referred to "edge feed" technique such as described in Example 1 of the above referred to U.S. Patent No. 4,478,696, a PVA-gel containing electrode was made covering both surfaces of the electrode, but allowing the PVA-free edge portion accessible to, for example, hydrogen gas. EXAMPLE 3 As a first example of a successful test of the invention applied to the hydrogen anode-involving recovery of zinc from a before-mentioned ZnSO₄ aqueous electrolyte (100 grams H₂SO₄, 60 grams ZnSO₄), an anode 15 x 15 cm (6 x 6 inches) constructed of the before-described carbon cloth (on Stackpole Company's cloth labeled PWB 6), catalyzed by platinum particles in the 15-25Å range distributed evenly over both surfaces of the cloth and in the interstices thereof in a PTFE binder, and a 15 x 15 cm (6 x 6 inch) aluminum starting sheet cathode were employed. The thickness of the anode was about 0.38 mm (15 mils) and its platinum loading was 0.3 mg/cm². The same cell without the use of the invention, operated with the electrodes at 92 cm (3 foot depths), was found to cause flooding of electrolyte in the bottom third thereof and into the hydrogen feed chamber, as well as substantial H₂ gas percolation into the upper third of the electrolyte chamber, requiring a cell voltage of about 2.56 volts at 0.1 amperes/cm² (90 amperes/square foot), and necessitating hydrogen back pressure of 0.9 bar (36 inches) to prevent substantially total flooding of the anode. With two coats of PVA, applied as a 4 mil thick film on the anode, as described in Example 1 above, the cell was operable at a H₂ back pressure of 0.95 bar (38 inches) without H₂-percolation, requiring the substantially smaller cell voltage of 1.88 volts, at 0.1 A/cm² (90 amperes/square foot), resulting in a thirty percent energy savings over operation without the invention (the cell voltage, under a given current, being directly proportional to the energy consumption of the cell). In addition, the cathode zinc recovered at the aluminum cathode was remarkably uniform (0.75-0.78 mm (30-33 mils)near the top and 0.85-0.9 mm (34-36 mils) near the bottom after several hours of operation), as contrasted with the same-dimensioned cell without the PVA coating (0.35-0.4 mm (14-16 mils) near the bottom, and 0.7 mm (28 mils) near the top). The following example illustrates the advantage of the hydrogel-containing structure, when used in the metal-recovery process of U.S. Patent 4,331,520 with the gaseous supply means of U.S. Patent 4,478,696. EXAMPLE 4 As described in Example 1, a 0.1 mm (4 mil) thick layer of the PVA solution was applied, by a doctor's blade, to one surface of the carbon cloth electrode. While the applied PVA film was still wet, the coated electrode was exposed to two doses, of 10 Megarads each, of γ-radiation from Cobalt-60 source. The resulting crosslinked polyvinyl alcohol gel film contained about 80% water, by weight of film, and was, preferably, kept wet to avoid the danger of embrittlement during drying. The coated anode was used as a hydrogen anode in the zinc-plating electrolysis cell as in Example 3 above, the anode having an active area of 50.8 x 50.8 mm (2 x 2 inch). At 0.1 A/cm² (90 amperes per square foot) a cell voltage of 1.67 was obtained. This example illustrates the use of a crosslinked non-ionic hydrogel. The value of cross-linked film, versus, for example, the non-cross-linked film of Example 1, resides in the much lower water-solubility of the former. Evidently, other known techniques of cross-linking, utilizing, for example, known multifunctional chemical agents (such as di-aldehydes) may also be used.

Claims: A catalytic gas-diffusion electrode for use in an electrolytic cell containing an aqueous electrolyte, said electrode comprises an electrically conducting porous carbon cloth with a catalytic hydrophobic layer on the fibers of said cloth and at the electrolyte-facing surface of the electrode there is a layer of a non-ionic aqueous polymeric gel (hydrogel) bonded to said cloth, said layer of hydrogel is gas-impervious. An electrode as claimed in claim 1, wherein said catalytic layer contains platinum particles. An electrode as claimed in claim 1 or 2, wherein said hydrogel is selected from the group composed of polyvinyl alcohol, polyethylene oxide, and a polymer or copolymer of a hydrophobic substantially non-ionic monomer. An electrode as claimed in claim 3, wherein the polymer is cross-linked. A method of forming a catalytic gas-diffusion electrode for use in an electrolytic cell containing an aqueous electrolyte, comprising providing an electrically conducting porous carbon cloth with a hydrophobic layer on the fibers of said cloth, said layer has catalytic particles, and said cloth is coated at its electrolyte-facing surface with a gas-impervious layer of a non-ionic aqueous polymeric gel (hydrogel). A method as claimed in claim 5, wherein said cloth is coated with an aqueous solution of polyvinyl alcohol and isopropyl alcohol to form said gas-impervious layer of hydrogel.

Inventors: ILAN AMIRAM BAR, JUDA WALTER, ILAN, AMIRAM BAR, JUDA, WALTER
Applicants: METALLGESELLSCHAFT AG
Application year: 1985
Main classification: H01M4
IPC codes: C25B11, G01N27, H01M4