Chemistry: electrical current producing apparatus – product – and – With pressure equalizing means for liquid immersion operation
Reexamination Certificate
2000-12-15
2004-02-24
Gulakowsi, Randy (Department: 1746)
Chemistry: electrical current producing apparatus, product, and
With pressure equalizing means for liquid immersion operation
C429S006000, C429S006000, C429S006000, C429S047000, C429S047000, C427S115000
Reexamination Certificate
active
06696189
ABSTRACT:
FIELD OF INVENTION
The present invention pertains to fuel cells, and more particularly to a direct methanol fuel cell system including an integrated methanol concentration sensor and a method of fabricating the system, in which electrical energy is produced through the consumption of gaseous or liquid fuels.
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 hydrogen 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 O
2
). 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 thus more complicated. In conventional PEM fuel cells, stack connections are made between the fuel cell assemblies with conductive plates, machined with channels or grooves for gas distribution. A typical conventional fuel cell is comprised of an anode (H
2
or methanol side) current collector, anode backing, membrane electrode assembly (MEA) (anode electrocatalyst/ion conducting membrane/cathode electrocatalyst), 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 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 fuel cells are simply used instead.
During operation of a direct methanol fuel cell, a dilute aqueous methanol (usually 3-4 vol % methanol) solution is used as the fuel on the anode side. 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 carrying methanol and water separately and mixing them in-situ for the fuel cell reaction to take place. 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.
Accordingly, the integration of a chemical sensor, such as a methanol sensor, into the DMFC system to monitor the concentration of the fuel consisting of a mixture of methanol in deionized water, would prove beneficial. A chemical sensor can be defined as a measurement device that utilizes chemical reactions to detect and quantify a specific analyte or event. In a DMFC system, the analyte is methanol in deionized water. There are a variety of chemical sensors that have been developed such as electrochemical, photometric, calorimetric, acoustical or mechanical. Of these, electrochemical sensors operating on the potentiometric or amperometric principle would prove beneficial if integrated into a DMFC system for the purpose of monitoring the concentration of the fuel.
Accordingly, it is a purpose of the present invention to provide for a direct methanol fuel cell system design that has included an integrated sensor for the monitoring of the fuel supply.
It is a purpose of the present invention to provide for a direct methanol fuel cell system and integrated sensor that includes microchannels and cavities and microfluidics technology for fuel-bearing fluid mixing, pumping and recirculation.
It is still a further purpose of the present invention to provide for a direct methanol fuel cell system and integrated sensor 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 system and integrated sensor which includes the steps of providing for microchannels and cavities in which microfluidic technology is a basis for the mixing, pumping and recirculation of a fuel-bearing fluid.
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 system and method of forming the fuel cell system 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. A fluid supply channel is defined in the base portion and communicates with the at least one membrane electrode assembly for supplying a fuel-bearing fluid to the at least one membrane electrode assembly. An integrated methanol concentration sensor is provided in fluidic communication with the fluid supply channel and the membrane electrode assembly for regulating the fuel supply to the 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 fluid 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.
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Barton et al., “A Methanol Sensor for Portable Direct Methanol Fuel Cells,” J. Electrochem Soc. 145 (11), pp. 3783-3788, Nov. 1998.*
Kelley et al., “A miniature methanol/air polymer electrolyte fuel cell,” Electromechanical and Solid-State Letters, 3, (2000), pp. 407-409.
Morse et al., “A novel thin film solid oxide fuel cell for microscale energy conversion,” SPIE conference on micromachined devices and components, Sep. 1999, pp. 223-226.
Bostaph Joseph W.
Fisher Allison M.
Koripella Chowdary R.
Crepean Jonathan
Gulakowsi Randy
Koch William E.
Motorola Inc.
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