Patent Application
20080038612
Exemplary embodiments of the present invention will become more fully understood from the detailed description and the accompanying drawings.
The following description of embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses.
Referring to
Solid polymer electrolyte membranes 30 useful in the present invention are ion-conductive materials. Suitable membranes useful in the present invention are described in U.S. Pat. Nos. 4,272,353 and 3,134,697, and in the Journal of Power Sources, Volume 29 (1990), pages 367-387. Such membranes are also referred to as ion exchange resin membranes. The resins include ionic groups in their polymeric structure; one ionic component for which is fixed or retained by the polymeric matrix and at least one other ionic component being a mobile replaceable ion electrostatically associated with the fixed component. The ability of the mobile ion to be replaced under appropriate conditions with other ions imparts ion exchange characteristics to these materials.
The ion exchange resins can be prepared by polymerizing a mixture of ingredients, one of which contains an ionic constituent. One broad class of cation exchange, proton conductive resins is the so-called sulfonic acid cation exchange resin. In the sulfonic acid membranes, the cation exchange groups are sulfonic acid groups which are attached to the polymer backbone.
The formation of these ion exchange resins into membranes or sheets is well known to those skilled in the art. The preferred type is perfluorinated sulfonic acid polymer electrolyte in which the entire membrane structure has ionic exchange characteristics. These membranes are commercially available, and a typical example of a commercial sulfonic perfluorocarbon, proton conductive membrane is sold by E.I. DuPont de Nemours & Company under the trade designation NAFION. Other such membranes are available from Asahi Glass and Asahi Chemical Company. The use of other types of membrane such as, but not limited to, perfluorinated cation-exchange membranes, hydrocarbon based cation-exchange membranes as well as anion-exchange membranes are also within the scope of the invention. In other alternative embodiments, the membrane may include sulfonated hydrocarbon polymers, including sulfonated polysulfones, polyetheretherketone, polybenzimidazoles, polyimides, polyphenylenes or polyphenylenesulfide.
The deactivated edge region 30b of the membrane body 31 is formed typically by adding a material, such as barium or another element or molecule, in the membrane body 31 along the deactivated edge region 30b. In another embodiment of the invention, the deactivated region 30b is formed by introducing ions that form insoluble sulfate and sulfonate salts, including ions of strontium, lead, calcium, silver, and mercury (I). These metal ions form low solubility products with the sulfonic acid groups of the membrane.
One embodiment of the invention includes preparing a membrane deactivating solution, such as a 0.01 M solution of barium hydroxide (Ba(OH)2), and immersing each edge of the membrane 30 in the membrane deactivating solution, for example, for about 10 minutes. After each immersion, the membrane 30 is removed from the solution and excess solution is removed. In one embodiment of the invention, excess barium ions are washed away from the membrane with water to make certain that electrode catalyst and ionomer contamination does not result. Alternatively, the deactivating solution may be applied to edge regions or other portion or portions of the membrane 30 using a screen printing or other process known by those skilled in the art. In one embodiment of the invention, the ionic conductivity of the deactivated edge region 30b is zero while the ionic conductivity of the active region 30a remains unaffected.
Referring next to
A cathode side bipolar plate 32 may abut the gas diffusion medium 10, and an anode side bipolar plate 32a may abut the gas diffusion medium 10a. The cathode side bipolar plate 32 has flow field channels 34 which distribute oxygen 38 to the cathode catalyst layer 26 through the gas diffusion medium 10. The channels 34 also receive product water 40 from the cathode catalyst layer 26 through the gas diffusion medium 10. The anode side bipolar plate 32a has flow field channels 34a which distribute hydrogen gas 36 to the anode catalyst layer 28 through the gas diffusion medium 10a.
During operation of the fuel cell 22, hydrogen gas 36 flows through the channels 34a of the bipolar plate 32a and diffuses through the substrate 10a to the anode catalyst layer 28. In like manner, oxygen 38 flows through the channels 34 of the bipolar plate 32 and diffuses through the gas diffusion medium 10 to the cathode catalyst layer 26. The anode catalyst layer 28 disassociates the hydrogen gas 36 into electrons and protons. The electrons are distributed as electric current from the anode catalyst layer 28, through a load, such as a drive motor (not shown), and then to the cathode catalyst layer 26.
Protons from the hydrogen gas 36 migrate from the anode catalyst layer 28, through the membrane 30 to the cathode catalyst layer 26. At the cathode catalyst layer 26, the protons are combined with electrons returning from the drive motor and oxygen 38 to form water 40. The water 40 diffuses from the cathode catalyst layer 26, through the gas diffusion medium 10 into the channels 34 of the bipolar plate 32 and is discharged from the fuel cell 22. Due to the deactivated edge region 30b of the membrane 30, the protons are conducted or are capable of passing from the anode catalyst layer 28 to the cathode catalyst layer 26 only through the active region 30a and not through the deactivated edge region 30b of the membrane 30.
Referring next to the flow diagram of
In another embodiment of the invention, the membrane deactivating solution may alternatively be applied to the membrane by any of a variety of methods including, but not limited to, brushing, spraying, pouring, dropping, or screen printing or other method known by those skilled in the art rather than by immersing the membrane portions to be deactivated into the solution. Alternatively, in another embodiment, the portion of the membrane to be deactivated is immersed in water, and barium hydroxide solution is added dropwise until the barium ions are exchanged for the protons of the sulfonic acid groups on the membrane wherein an endpoint is reached where the pH is 7. Other hydroxides of elements other than barium (II) can be used and these include those ions which form insoluble sulfate and sulfonate salts including strontium, lead, calcium, silver, and mercury (I). These metals form low solubility products with the sulfonic acid groups of the membrane.
The above description of embodiments of the invention is merely exemplary in nature and, thus, variations thereof are not to be regarded as a departure from the spirit and scope of the invention.
