Chemistry: electrical current producing apparatus – product – and – With pressure equalizing means for liquid immersion operation
Reexamination Certificate
2000-12-15
2002-12-24
Ryan, Patrick (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
With pressure equalizing means for liquid immersion operation
C429S006000, C429S006000, C429S006000, C429S047000, C427S115000
Reexamination Certificate
active
06497975
ABSTRACT:
FIELD OF INVENTION
The present invention pertains to fuel cells, and more particularly to a direct methanol fuel cell including an integrated fuel flow field and a method of fabricating the device, in which even distribution of the fuel into the fuel cell is achieved during the process of generating electrical energy.
BACKGROUND OF THE INVENTION
Fuel cells in general, are “battery replacements”, and like batteries, produce electricity through an electrochemical process without combustion. The electrochemical process utilized provides for the combining of protons with oxygen from air or as a pure gas. The process is accomplished utilizing a proton exchange membrane (PEM) sandwiched between two electrodes, namely an anode and a cathode. Fuel cells, as known, are a perpetual provider of electricity. Hydrogen is typically used as the fuel for producing the electricity and can be processed from methanol, natural gas, petroleum, or stored as pure hydrogen. Direct methanol fuel cells (DMFCs) utilize methanol, in a gaseous or liquid form as fuel, thus eliminating the need for expensive reforming operations. DMFCs provide for a simpler PEM cell system, lower weight, streamlined production, and thus lower costs.
In a standard DMFC, a dilute aqueous solution of methanol is fed as the fuel on the anode side (first electrode) and the cathode side (second electrode) is exposed to forced or ambient air (or 02). A Nafion® type proton conducting membrane typically separates the anode and the cathode sides. Several of these fuel cells can be connected in series or parallel depending on power requirements.
Typically, DMFC designs are large stacks with forced airflow operating at elevated temperatures of approximately 60-80° C. Smaller air breathing DMFC designs require the miniaturization of all the system components and are thus more complicated. In conventional PEM fuel cells, stack connections are made between the fuel cell assemblies with conductive plates, having channels or grooves for gas distribution formed therein. A typical conventional fuel cell is comprised of an anode (H
2
or methanol side) current collector, anode backing, membrane electrode assembly (MEA) (anode/ion conducting membrane/cathode), cathode backing, and cathode current collector. Typical open circuit voltage under load for a direct methanol fuel cell is approximately in the range of 0.3-0.5 V To obtain higher voltages, fuel cells are typically stacked in series (bi-polar manner—positive to negative) one on top of another, or by connecting different cells in series in a planar arrangement. Conventional fuel cells can also be stacked in parallel (positive to positive) to obtain higher current, but generally, larger active areas are simply used instead.
During operation of a direct methanol fuel cell, a dilute aqueous methanol (usually 3-4% methanol) solution is used as the fuel on the anode side. If the methanol concentration is too high, then there is a methanol crossover problem that will reduce the efficiency of the fuel cell. If the methanol concentration is too low then there will not be enough fuel on the anode side for the fuel cell reaction to take place. Current DMFC designs are for larger stacks with forced airflow. The smaller air breathing DMFC designs are difficult to accomplish because of the complexity in miniaturizing all the required system components and integrating them in a small unit required for portable applications. Carrying the fuel in the form of a very dilute methanol mixture would require carrying a large quantity of fuel which is not practical for the design of a miniature power source for portable applications. Miniaturizing the DMFC system requires having on hand separate sources of methanol and water and mixing them in-situ for the fuel cell reaction. In addition, even distribution of the fuel onto the anode of the fuel cell is critical for optimum performance.
In the instance where a designated fuel flow is not present, the fuel flow will follow the path of least resistance to the fuel cell. This path of least resistance results in uneven distribution of the fuel to the anode. In addition, if an inefficient flow field is present, carbon dioxide by-products can accumulate in areas and prevent fuel from accessing the anode, or electrocatalyst. This results in back pressure which is formed due to the lack of means for exhausting of the carbon dioxide. To aid in supplying fuel, and more specifically methanol and water to the anode, it would be beneficial to form a fuel flow field that would provide for the even distribution of the fuel onto the anode, and more specifically onto the anode backing, and thus into the membrane electrode assembly (MEA). This provision for the equal distribution of the fuel would provide for optimum performance of the fuel cell device.
Accordingly, it is a purpose of the present invention to provide for a direct methanol fuel cell system design in which a fuel flow field is integrated into a miniaturized device.
It is a purpose of the present invention to provide for a direct methanol fuel cell including an integrated fuel flow field, comprised of microchannels, cavities, and microfluidics technology for the equal distribution of a fuel-bearing fluid to the anode of a fuel cell device.
It is still a further purpose of the present invention to provide for a direct methanol fuel cell including an integrated fuel flow field in which all of the system components are embedded inside a base portion, such as a ceramic base portion.
It is yet a further purpose of the present invention to provide for method of fabricating a direct methanol fuel cell including an integrated fuel flow field, comprised of microchannels, cavities, and microfluidics technology for the equal distribution of a fuel-bearing fluid to the anode of a fuel cell device.
SUMMARY OF THE INVENTION
The above problems and others are at least partially solved and the above purposes and others are realized in a fuel cell device and method of forming the fuel cell device including a base portion, formed of a singular body, and having a major surface. At least one membrane electrode assembly is formed on the major surface of the base portion. The base portion includes an integrated fuel flow field for the equal distribution of fuel to the membrane electrode assembly. A fluid supply channel is defined in the base portion and communicating with the fuel flow field and the at least one membrane electrode assembly for supplying a fuel-bearing fluid to the at least one membrane electrode assembly. An exhaust channel is defined in the base portion and communicating with the at least one membrane electrode assembly. The exhaust channel is spaced apart from the fluid supply channel for exhausting by-product fluid, including water, from the at least one membrane electrode assembly. The membrane electrode assembly and the cooperating fluid supply channel and cooperating exhaust channel forming a single fuel cell assembly.
REFERENCES:
patent: 6387559 (2002-05-01), Koripella et al.
Bostaph Joseph W.
Fisher Allison M.
Koripella Chowdary R.
Neutzler Jay K.
Crepeau Jonathan
Koch William E.
Motorola Inc.
Ryan Patrick
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