Patent Application
20090004550
The invention relates to fuel cells.
A fuel cell is a device whereby the energy produced by chemical reactions of a gaseous or liquid feed material is converted directly to electric energy. Some fuel cells consist of a membrane electrode assembly (MEA) formed by a polymer electrolyte membrane and a pair of electrodes with catalyst on them, located at each side of the membrane. The structural art for fuel cells is well developed, as illustrated by U.S. Pat. Nos. 6,893,767 (“Methods for Producing Fuel Cell Units and Fuel Cell Stacks) and 7,157,176 (Membrane-Electrode Assembly for Polymer Electrolyte Fuel Cell, and Process for its Production), the disclosures of which are incorporated herein by reference.
A useful membrane, among others, is Nafion®, a sulfonated tetrafluorethylene copolymer manufactured by DuPont de Nemours. One form can be referred to as tetrafluoroethylene-perfluoro-3,6-dioxa-4-methyl-7-octenesulfonic acid copolymer, represented more generally by the following structure where x, y, and z are dependent on processing conditions:
It is one of a class of synthetic polymers with ionic properties which are called ionomers. Nafion has excellent thermal and mechanical stability. Its ionic properties are a result of incorporating perfluorovinyl ether groups terminated with sulfonate groups onto a tetrafluoroethylene (Teflon) backbone. In presence of humidity the polymer membrane forms aqueous domains, hereby called channels, consisting of water, protons and SO3H (sulfonic acid) groups. The protons can move through the channel and if the channel traverses the membrane they will produce an ionic current when the membrane is under an electrical potential. This electric conductivity is essential for the function of the fuel cell.
Fuel cells and other electrochemical devices that make use of semipermeable or permeable membranes (such as Nafion) affixed to electrodes on which an electrocatalyst is deposited, have limitations. In existing fuel cells the catalysts are deposited first on the electrodes of the cell and then the membrane is placed between the electrodes. In general, any membrane will contain channels through which the protons have to travel. In the specific case of Nafion or other polymeric membranes proton migration takes place through hydrophilic domains consisting of water, protons and sulfonic groups. The catalyst consists of very small metal particles (e.g. Pt/Ru and Pt) at both anode and cathode. These are supported on small carbon particles which are, in turn, supported by a carbon cloth that serves as an electrode. When the membrane is pressed against the carbon cloth, to make the cell, there is no reason why the catalyst particles (e.g. Pt/Ru and the Pt) will be in contact with a hydrophilic domain or an ion conducting domain in a ceramic or other type of membrane. The electrocatalytic process, that makes the cell function, requires that protons are made at the anode and travel through the membrane to the cathode where they are reduced. The catalyst particles that are located in a hydrophobic domain (or under the ceramic wall in other membranes) cannot produce protons, hence electricity. The ones that are located in a hydrophilic domain (channel) that crosses the membrane produce electricity only if the domain has a catalyst particle at each end of the channel. All catalyst particles (e.g. Pt and Pt/Ru) that do not satisfy this condition are wasted. This poor utilization of the catalyst increases the cost of the cell. Our measurements estimate that about 40% of the Pt particle do not participate in the electrochemical current production.
Moreover, it is difficult to control the size of the conventional platinum catalyst currently used in fuel cells. The activity of the catalyst takes place on its surface and if the particles are too large the ratio of surface to volume is large and the Pt inside the particle is wasted (it does not contribute to the chemical activity but it adds to the cost).
In U.S. Pat. No. 4,959,132 (“Preparing In Situ Electrolyte Membranes, . . . ”), a polymer electrolyte membrane such as Nafion is impregnated with an ionic salt of a desired catalyst, such as Pt(NH3)4Cl2 is by saturating the membrane with a solution of the ionic salt, the metal ions moving into the membrane by ion exchange. This is followed by a reduction step by placing the membrane between a non-reactive medium such as nitrogen gas and a liquid containing a reductant to form an electrostatic film proximate to the surface of the membrane by diffusion of the ions. The resulting membrane is not useful for direct electrochemical reaction because of the electrical isolation of the particles. A bi-polar structure including a pair of electrocatalytic films proximate both surfaces are connected to anode and cathode simultaneous oxidation and reduction in a chloroalkai system. The method is not suggested for fuel cells and indeed would be similarly wasteful as described above with respect to catalysts deposited first on the electrodes of the cell and then contacted with a membrane, because, as described above, catalyst particles that are located in a hydrophobic domain cannot produce protons.
A need exists therefore to better utilize the electrocatalysts by placing it in the conducting channels, and to control the size of the catalyst particles to optimize current production per mass of platinum.
The present invention fulfills the foregoing needs and overcomes the foregoing drawbacks to greatly benefit both the cost and efficiency of such devices, by applying electrocatalyst to the electrode after a membrane containing ion conducting channels is attached. By depositing catalyst after the membrane is attached, the deposition of the electrocatalyst is such that it ensures that all catalyst particles in the cell are in contact with the conducting areas of the membrane. In addition the method of preparation allows a fine control of the size of the catalyst particles.
An electrochemical device is thus provided, such as a fuel cell, and a method of forming it, so that the catalyst particles are deposited only in those conducting channels of the membrane that cross the membrane and have good electrical contact with the carbon cloth membrane. We do this by depositing the catalyst on the carbon cloth electrochemically. This electrochemical deposition is done so that the ions that will form the catalyst deposits on the electrode, reach the electrode through the conducting channels of the membrane. In this way the catalyst particles can be formed only on those parts of the carbon cloth electrode that are in contact with the ion conducting channels in the membrane. Because of this all catalyst particle are able to participate in electricity production in the fuel cell. The catalyst is deposited successively first on one cell electrode then on the other.
In one embodiment a polymeric membrane fuel cell is prepared that has the carbon cloth electrodes and the membrane in place but it does not have the catalyst on the carbon cloth electrodes. The catalyst is then deposited on the carbon cloth electrochemically in a way that ensures that the catalyst particles are all in the ion conducting channels of the membrane. A second embodiment makes two half-cells that have the carbon cloth electrodes but no catalyst and then deposits the catalyst particle electrochemically through the membrane. After catalyst has been deposited in each half-cell, the half-cells are pressed together through a porous coupling layer. These procedures can be used for any electrodes (not necessarily carbon cloth) and for any membrane (not necessarily Nafion or polymeric).
In an alternative embodiment, what can be called “split-join” synthesis, catalyst particles are deposited from their respective ion solutions onto half membrane electrode assemblies, which are then coupled through a porous coupling layer to provide continuous pathways for ion transport across the membrane.
The invention comprises a method of making fuel cells and other electrochemical devices that require electrocatalysts in contact with a membrane as used in a variety of electrochemical devices including specifically fuel cells. The invention consists of a method of producing an electrochemical device with one or more electrodes which contain electrocatalysts prepared by the unique methodology of this invention.
For a more complete understanding of the present invention, reference is now made to the accompanying drawings, in which:
a-6c are schematic depictions of the assembly of a fuel cell in accordance with a second embodiment of the invention. Individual cathode and anode catalysts are first deposited separately using counter electrodes (65 and XX), then the two half cells are joined using a coupling layer composed of a material that is porous allowing free exchange of ions including but not limited to, porous silica, mesoporous alumina, or silica fibers, or, ion selective gels or other ionic exchange materials (67) in a split-join synthesis of the fuel cell;
The invention calls for depositing catalyst after an ion conducting membrane is attached to an electrode, so as to ensure that all catalyst particles in the cell are in contact with the conducting areas of the membrane. The catalyst particles are deposited only in those conducting channels of the membrane that cross the membrane and have good electrical contact with a carbon cloth electrode by depositing the catalyst on the carbon cloth electrochemically. The ions that will form the catalyst reach the electrode through the conducting channels of the membrane. In this way the catalyst particles can be formed only on those parts of the carbon cloth electrode that are in contact with the ion conducting channels in the membrane so that all catalyst particle are able to participate in electricity production in the fuel cell.
The catalyst is deposited successively first on one cell electrode then on the other. The size of the catalyst particles is varied and controlled by adjusting the pulse sequence in the electrochemical deposition, the concentration of the salt used to provide the catalyst cations during deposition, and the duration of the electrolysis. In the first embodiment of this concept we prepare a polymeric membrane fuel cell that has the carbon cloth electrodes and the membrane in place but it does not have the catalyst on the carbon cloth electrodes. The catalyst is then deposited on the carbon cloth electrochemically in a way that ensures that the catalyst particles are all in the ion conducting channels of the membrane. A second embodiment makes two half-cells that have the carbon cloth electrodes but no catalyst and then deposits the catalyst particle electrochemically through the membrane. After catalyst has been deposited in each half-cell, the half-cells are pressed together through a porous coupling layer. These procedures can be used for any electrodes (not necessarily carbon cloth) and for any membrane (not necessarily Nafion or polymeric).
More specifically, the catalytic particles are prepared by electrolysis. The membrane is first placed between the two electrodes (e.g. carbon cloth, with or without carbon particles on them—or any other electrode used in making a fuel cell), which is then impregnated with a solution containing the salt of the catalyst metal to be deposited. Such a solution can be, for example, PtCl6H2 if Pt is the catalyst. In general one can use any salt of any metal that one wants to deposit. However acids similar to PtCl6H2 are preferable because the positive ions are protons and the negative ion PtCl62− does not harm the membrane because the —SO3− ions in the membrane are chemically tied to the walls of the conducting channels and cannot be pushed out of the membrane by PtCl62−. Had we used another salt which contained a cation we would run the risk that the cation expels the protons from the conducting channels of the membrane. If that were to happen the protons have to be put back in the membrane by ion exchange performed after catalyst deposition. The salt is then electrolyzed to deposit the metal catalyst on one of the electrodes (e.g. carbon cloth). In this way the catalytic particles are formed only inside those channels that cross the membrane, i.e., in this case, the hydrophilic domains. As a result, no catalytic particles are wasted. Moreover, one has very good control of the size of the formed particles by applying a sequence of voltage pulses of adjustable magnitude and duration.
In an alternative embodiment, what can be called “split-join” synthesis, catalyst particles are deposited from their respective ion solutions onto half membrane electrode assemblies, which are then coupled through a porous coupling layer to provide continuous pathways for ion transport across the membrane.
The invention is applicable to any ion conducting membrane and any catalyst, be it unary, binary or ternary. In those cases when spontaneous metal deposition takes place from a salt to a metal (for example a Ru salt in contact with a Pt particle will deposit metallic Ru on the Pt particle) one can deposit a second metal on top of the catalyst (deposited first by the method explained above) by exposing the MEA to the salt.
Thus in broader embodiments, the membrane can be any ion conducting membrane, semipermeable or fully permeable. A semipermeable membrane, also termed a selectively-permeable membrane, a partially-permeable membrane or a differentially-permeable membrane, is a membrane that will allow only certain molecules or ions to pass through it by diffusion, exemplified herein by Nafion. Examples of fully permeable membranes that may be used herein are porous oxides and polymers including but not limited to fritted glass, alumina (including mesoporous), glass fibers, and porous polyacrylonitrile. Moreover, while the ion conducting membrane will be illustrated herein by an ionomer, specifically, a sulfonated tetrafluoroethylene copolymer such as Nafion, the concepts of the invention are broadly applicable to any ion conducting membrane, for example functionalized mesoporous films such as mesoporous alumina or silica functionalized with per-fluorinated ligands (including Nafion-like species), hydroxylated polyols, poly(methyl methacrylate), and amine's. Other examples include commercial cation selective gels and resins.
The present invention has several distinct advantages over existing technology:
Although the present invention has been described in connection with the preferred embodiments, it is to be understood that modifications and variations may be utilized without departing from the principles and scope of the invention, as those skilled in the art will readily understand. Accordingly, such modifications may be practiced within the scope of the following claims.
This application claims the benefit of U.S. Provisional Patent Application No. 60/904,604 filed Mar. 1, 2007, which application is herein incorporated by reference.
This invention was made with Government support under Grant No. DAAD 19-03-1-0121 from the United States Army Research Office. The Government has certain rights in the invention.
| Number | Date | Country | |
|---|---|---|---|
| 60904604 | Mar 2007 | US |
