METHOD FOR REDUCING HALOGEN ION CONTAMINANT IN SOLID POLYMER ELECTROLYTE FUEL CELL

Abstract

A method is disclosed for improving the durability of membrane electrode assemblies in solid polymer electrolyte fuel cells by reducing the amount of halogen ion contaminants present. Silver carbonate is dissolved in an ionomer dispersion and incorporated into an appropriate component in the fuel cell where it reacts with halogen ion present to form silver halides. The component is then exposed to a suitable light source that decomposes the halide into halogen gas which is then removed prior to final assembly of the fuel cell.

Claims

1. A method for reducing the amount of halogen contaminant in a solid polymer electrolyte fuel cell, the fuel cell comprising an electrolyte comprising electrolyte ionomer, a cathode comprising a cathode catalyst and cathode ionomer, and an anode comprising an anode catalyst and anode ionomer, the method comprising: preparing a dispersion comprising a dispersion ionomer, silver carbonate, and an aqueous solvent whereby the dispersion ionomer and the silver carbonate react to form a silver containing ionomer, carbon dioxide and water; incorporating the dispersion into a component selected from the electrolyte, the cathode, and the anode whereby the silver cations in the silver containing ionomer react with halogen ion contaminant in the component to form a silver halide; removing the solvent; exposing the component to light capable of decomposing the silver halide into halogen and silver metal; removing the halogen; and assembling the component into the fuel cell.

2. The method of claim 1 wherein the halogen is chlorine or bromine.

3. The method of claim 1 wherein the incorporating step comprises applying the dispersion to the component.

4. The method of claim 1 wherein the incorporating step comprises making the component using the dispersion.

5. The method of claim 4 wherein the component is the electrolyte and the dispersion ionomer is the electrolyte ionomer.

6. The method of claim 4 wherein the component is the cathode and the dispersion ionomer is the cathode ionomer.

7. The method of claim 4 wherein the component is the anode and the dispersion ionomer is the anode ionomer.

8. The method of claim 1 wherein the dispersion ionomer, the electrolyte ionomer, the cathode ionomer, and the anode ionomer are the same type of ionomer.

9. The method of claim 1 wherein the dispersion ionomer is perfluorosulfonic acid ionomer or hydrocarbon ionomer.

10. The method of claim 1 wherein the light used in the exposing step is visible light or ultraviolet light.

11. A solid polymer electrolyte fuel cell comprising an electrolyte comprising electrolyte ionomer, a cathode comprising a cathode catalyst and cathode ionomer, and an anode comprising an anode catalyst and anode ionomer wherein the fuel cell comprises silver metal in a component selected from the electrolyte, the cathode, and the anode.

12. The fuel cell of claim 11 wherein the component comprises essentially no silver halide.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] FIG. 1 illustrates the reactions taking place when silver carbonate is added to the ionomer dispersion used in the method of the invention.

DETAILED DESCRIPTION

[0020] The present invention provides for improved durability of membrane electrode assemblies in solid polymer electrolyte fuel cells and stacks by reducing the amount of halogen ion contaminants present therein. Halogen ions, and especially Cl.sup., can cause dissolution of the typical Pt catalysts used in the fuel cell electrodes and thereby form PtCl.sub.4. Significant dissolution can take place even with very low levels of halogen present. Such dissolution significantly contributes to the degradation of the electrode catalysts. For instance, in certain developmental anode catalyst material, amounts of Cl as high as several thousand ppm have been found. These amounts are significant and lead to significant dissolution of the fuel cell catalysts.

[0021] In the present invention, a source of halogen ion scavenger, specifically silver ions (Ag.sup.+) from silver carbonate (Ag.sub.2CO.sub.3), is dispersed in a water based ionomer dispersion which is further incorporated into the electrolyte and/or one or both electrodes of the fuel cell as is desired for removing contaminants. In the ionomer dispersion, the silver carbonate can desirably react with the ionomer (a strong acid), with dissociated protons (from the ionomer) and carbonate ions reacting to form carbon dioxide gas and water, and exchanging dissolved silver ions for proton in the ionomer. The carbon dioxide formed simply vents to atmosphere. This series of reactions is illustrated in FIG. 1.

[0022] In the presence of halogen ions, the silver anions in the dispersion react to form insoluble silver halide precipitates. For chloride and bromide contaminants, silver chloride and silver bromide are formed respectively. Advantageously though, in the present invention, the precipitates are exposed to a suitable light source which is capable of decomposing the halide precipitate into halogen and silver metal. For chloride contaminant, chlorine gas and silver metal are produced when AgCl is exposed to a suitable visible light source (as per the below equation).

##STR00001##

[0023] The chlorine gas is readily removed under ambient conditions. For bromide contaminant, under ambient conditions, bromine liquid and silver metal are produced when AgBr is exposed to a suitable light source (e.g. ultraviolet light, as per the below equation).

##STR00002##

[0024] Bromine however has a relatively low boiling point of about 59 C. and thus it can also be readily removed as a gas with moderate heating, and without damaging the ionomer or other cell components.

[0025] In this way, the halogen ions present in the electrolyte and/or one or both electrodes can be removed prior to final assembly of the fuel cell. This is accomplished by incorporating a dispersion comprising silver carbonate into each desired component. The halogen ions react to form silver halide precipitate which is then decomposed to produce halogen that can readily be removed in gaseous form.

[0026] The amount of silver carbonate to be used in the inventive method depends on how much halogen ion content may be present and how low an amount of halogen ion can be tolerated in the fuel cell. In principle though, the halogen ion content is as low as possible to avoid any impact on MEA durability. The following discussion provides guidance for determining suitable amounts of silver carbonate to use in order to achieve a desired level of halogen ion in an electrode component. For instance, based on the known solubility constant of AgCl, the concentration of Cl.sup. in a saturated AgCl solution at 25 C. is 0.443 ppm. Thus, with sufficient Ag.sup.+ added to match the amount of CL present, a level of 0.443 ppm of free Cl ion is obtained in the relevant component, with the remaining Cl ion tied up as solid AgCl. After removing the dispersion solvent and exposing the component to light in accordance with the inventive method, the AgCl decomposes, releasing the chloride as chlorine gas which escapes to atmosphere. However, a component level even lower than 0.443 ppm can be obtained by using an excess of Ag.sup.+. For instance, to achieve a level of less than 10 ppb of chloride ion in 50 g of the dry solid component (assuming the volume of dispersion is 250 mL before removing all solvent), the Cl ion concentration in the dispersion should be less than 5.6410.sup.8 mol/L and thus the Ag ion concentration should be more than 2.7710.sup.3 mol/L in the dispersion. In a like manner, based on the known solubility constant of AgBr, the concentration of Br.sup. in a saturated AgBr solution at 25 C. is 0.070 ppm. Thus, with sufficient Ag.sup.+ added to match the amount of Br.sup. present, a level of 0.070 ppm of free Br ion can be obtained in the relevant component. And further, even lower levels can be obtained using an excess of Ag.sup.+ ion.

[0027] The dispersion comprising the silver carbonate can be incorporated into the desired component using various conventional methods. For instance, the dispersion can be applied (e.g. by spray or roll coating) to the component after the usual fabrication of the component. Alternatively, the dispersion might instead be suitably incorporated during the usual fabrication of the component. Because the electrolyte contains electrolyte ionomer and the cathode and anode electrodes usually contain ionomer (cathode ionomer and anode ionomer respectively), silver carbonate might simply be incorporated appropriately into the dispersions used to form the ionomers appearing in these components. Thus, the dispersion ionomers used in the invention may include any or all of the electrolyte, cathode, and anode ionomer types. These types potentially include perfluorosulfonic acid ionomers, hydrocarbon ionomers, and any other ionomers suitable for use in solid polymer electrolyte fuel cells. Although different types of ionomer may be used in each of these components, often the same type of ionomer is used in a given fuel cell construction.

[0028] In the general method of the invention, once the incorporated silver carbonate can react with halogen ion contaminants, it is then possible to remove halogen via exposure to a suitable light source. Thus, some variation in the order of certain steps in the method is possible and certain steps may be done concurrently. For instance, consideration may be given to exposing the component to the light source before all the solvent has been removed. And further, halogen may thus be removed (e.g. as a gas) also before all the solvent is removed. Further still, the exposing step may be done on individual components or for instance continuously on moving webs comprising the components.

[0029] During various stages of the fabrication process, silver carbonate and silver halides may be found in the relevant fuel cell components (electrolyte and electrodes). However, if the relevant reactions are allowed to go to completionincluding the decomposition reactions resulting from the exposure to light, once assembly of the fuel cell is complete, there may be essentially no silver halide remaining in the fuel cell. In such embodiments, this can be a distinguishing feature of the present invention. (Note however that, as discussed below, AgCl may be created again in very small amounts via reaction with PtCl.sub.4.)

[0030] By incorporating silver carbonate into halogen contaminated fuel cell components in this way, the present invention advantageously eliminates halogen ions, and particularly chloride ions, and thereby slows down the degradation of the catalysts and membrane electrolyte in the cell. In turn, this improves cell performance and durability. The method is relatively simple and introduces little in the way of additional steps in fuel cell fabrication. The carbonate counter ion in the incorporated silver carbonate is benign to the cell and further, as explained above, can be eliminated as carbon dioxide anyway during fabrication.

[0031] The metallic silver that is ultimately formed in the cell components provides additional benefits for the inventive method. Silver metal is a free radical scavenger which desirably decomposes any hydrogen peroxide which may be present in the fuel cell. In turn, decomposing any hydrogen peroxide present further slows down catalyst degradation and improves durability of the membrane electrode assembly in the fuel cell. Further still, silver metal is the only metal which can readily reduce PtCl.sub.4 to Pt. Undesirably, PtCl.sub.4 may be formed by inadvertent dissolution of Pt catalysts in the fuel cell. The small amounts of silver present however can react with any formed PtCl.sub.4 to form AgCl and Pt, thereby regenerating the catalyst material. This reaction is favored at a temperature of 27 C.

[0032] All of the above U.S. patents, U.S. patent application publications, U.S. patent applications, foreign patents, foreign patent applications and non-patent publications referred to in this specification, are incorporated herein by reference in their entirety.

[0033] While particular elements, embodiments and applications of the present invention have been shown and described, it will be understood, of course, that the invention is not limited thereto since modifications may be made by those skilled in the art without departing from the spirit and scope of the present disclosure, particularly in light of the foregoing teachings. Such modifications are to be considered within the purview and scope of the claims appended hereto.