Chemistry: electrical current producing apparatus – product – and – With pressure equalizing means for liquid immersion operation
Reexamination Certificate
2002-03-06
2004-10-26
Le, Hoa Van (Department: 1752)
Chemistry: electrical current producing apparatus, product, and
With pressure equalizing means for liquid immersion operation
C429S006000, C429S006000, C029S623000
Reexamination Certificate
active
06808837
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to direct oxidation fuel cell systems, and more particularly, to enclosures for such systems.
2. Background Information
Fuel cells are devices in which an electrochemical reaction is used to generate electricity. A variety of materials may be suited for use as a fuel depending upon the materials chosen for the components of the cell. Organic materials, such as methanol or natural gas, are attractive choices for fuel due to the their high specific energy.
Fuel cell systems may be divided into “reformer-based” systems (i.e., those in which the fuel is processed in some fashion to extract hydrogen from the fuel before it is introduced into the fuel cell system) or “direct oxidation” systems in which the fuel is fed directly into the cell without the need for separate internal or external processing. Most stationary fuel cells are reformer-based fuel cell systems. However, because fuel processing is expensive and requires significant volume, reformer-based systems are presently limited to comparatively high power applications.
Direct oxidation fuel cell systems may be better suited for applications in smaller mobile devices (e.g., mobile phones, handheld and laptop computers), as well as in some larger applications. Typically, in direct oxidation fuel cells, a carbonaceous liquid fuel in an aqueous solution (typically aqueous methanol) is applied to the anode face of a membrane electrode assembly (MEA). The MEA contains a protonically-conductive, but electronically non-conductive membrane (PCM). A catalyst or mixture of catalysts, which enable direct oxidation of the fuel on the anode, is disposed on the surface of the PCM (or is otherwise present in the anode chamber of the fuel cell). Upon the completion of a circuit, protons (from hydrogen in the fuel and water molecules involved in the anodic reaction) are separated from the electrons. Diffusion layers are typically in contact with each of the catalyzed anode and cathode faces of the PCM to facilitate the introduction of reactants and removal of products of the reaction from the PCM, and also serve to conduct electrons. The protons migrate through the PCM, which is impermeable to the electrons. The electrons thus seek a different path to reunite with the protons and oxygen molecules involved in the cathodic reaction and travel through a load, providing electrical power.
One example of a direct oxidation fuel cell system is a direct methanol fuel cell system or DMFC system. In a DMFC system, methanol or an aqueous methanol solution is used as fuel (the “fuel mixture”), and oxygen, preferably from ambient air, is used as the oxidizing agent. There are two half reactions that occur in a DMFC which allow a DMFC system to provide electricity to power consuming devices: the anodic disassociation of the methanol and water fuel mixture into CO
2
, protons, and electrons; and the cathodic combination of protons, electrons and oxygen into water. The overall reaction may be limited by the failure of either of these reactions to proceed to completion at an acceptable rate (more specifically, failure to oxidize the fuel mixture will limit the cathodic generation of water, and vice versa).
Fuel cells and fuel cell systems have been the subject of intensified recent development because of their ability to efficiently convert the energy in carbonaceous fuels into electric power while emitting comparatively low levels of environmentally harmful substances. The adaptation of fuel cell systems to mobile uses, however, is not straightforward because of the technical difficulties associated with reforming most carbonaceous fuels in a simple, cost effective manner, and within acceptable form factors and volume limits. Further, a safe and efficient storage means for substantially pure hydrogen (which is a gas under the relevant operating conditions) presents a challenge because hydrogen gas must be stored at high pressure and at cryogenic temperatures or in heavy adsorption matrices in order to achieve useful energy densities. It has been found, however, that a compact means for storing hydrogen is in a hydrogen rich compound with relatively weak chemical bonds, such as methanol or an aqueous methanol solution (and to a lesser extent, ethanol, propane, butane and other carbonaceous liquids or aqueous solutions thereof). In particular, DMFCs are being developed for commercial production for use in portable electronic devices. Thus, the DMFC system, including the fuel cell, and the balance of the system components are ideally fabricated using materials that optimize the electricity-generating reactions, and which are also cost effective. Furthermore, the manufacturing process associated with those materials should not be prohibitive in terms of labor intensity cost.
As noted, typical DMFC systems include a fuel source, fluid and effluent management systems, and a direct methanol fuel cell (“fuel cell”), and a means by which any electricity generated can be collected and delivered to a load. The fuel cell typically consists of a housing, and a membrane electrode assembly (“MEA”) disposed within the housing. A typical MEA includes a centrally disposed protonically conductive, electronically non-conductive membrane (“PCM”). By way of example, a commercially available PCM is Nafion® a registered trademark of E. I. Dupont de Nemours and Company, a cation exchange membrane based on perflouorocarbon polymers with side chain termini of perflourosulfonic acid groups, in a variety of thicknesses and equivalent weight. While the invention herein is described using one particular architecture of a representative fuel cell system, it is within the scope of the invention that the invention is equally applicable to fuel cell systems other than that described herein. For example, there are other electrolytes available that are well known in the art, including, but not limited to those with liquid (including encapsulated liquid) or “gel” electrolyte-based systems. The present invention is readily adaptable for use with a wide variety of fuel cells, with particular application to microfuel cells used for smaller devices due to its size and space-saving advantages, as discussed herein.
The PCM is typically coated on each face with an electrocatalyst such as platinum, or platinum/ruthenium mixtures or alloy particles. On either face of the catalyst coated PCM, a MEA typically includes a diffusion layer. The diffusion layer functions to evenly distribute the liquid fuel mixture across the anode in the case of the fuel, or the gaseous oxygen from air or other source across the cathode face of the PCM, and provides electron conductivity to allow the system to provide power to the power consuming application. In addition, flow field plates are often placed on the surface of the diffusion layers that are not in contact with the coated PCM. The flow field plates function to provide mass transport of the reactants and byproducts of the electrochemical reactions, and have a current collection functionality in that the flow field plates act to collect and conduct electrons through the load. Those skilled in the art will recognize that it is possible in some circumstances to use a metal screen or other conductive mesh as a current collector, rather than flow field plates, in order to minimize the volume consumed by these components.
As noted, one expected use for the direct oxidation fuel cell is to power small handheld and portable electronic devices. Such devices must conform to strict form factors, and consequently, a power supply unit for such devices must conform to those strict form factors. Thus, the power supply unit must be relatively small, and be capable of powering the device for sufficiently long periods of time without interruption. The fuel cell system is preferably in close mechanical contact with the application to which power is being supplied, to ensure that proper electrical and mechanical contact is established and maintained.
It is desirable to contain any leaks that may o
Cesari and McKenna LLP
Le Hoa Van
MTI Microfuel Cells Inc.
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