WO2002007241A2

WO2002007241A2 - Direct methanol fuel cell system

Info

Publication number: WO2002007241A2
Application number: PCT/US2001/022524
Authority: WO
Inventors: Chowdary Ramesh Koripella; William Jay Ooms; David L. Wilcox; Joseph W. Bostaph
Original assignee: Motorola, Inc., A Corporation Of The State Of Delaware
Priority date: 2000-07-18
Filing date: 2001-07-17
Publication date: 2002-01-24
Also published as: EP1358689A2; JP5143991B2; BR0112997A; RU2258277C2; KR20030076559A; ZA200300386B; CN1524311A; AU2001280596A1; CN1303715C; US6387559B1; TW508864B; KR100792361B1; JP2004504700A; WO2002007241A3

Abstract

A fuel cell system and method of forming the fuel cell system including a base portion (14), formed of a singular body, and having a major surface. At least one fuel cell membrane electrode assembly (16) is formed on the major surface of the base portion. A fluid supply channel(32) including a mixing chamber is defined in the base portion and communicating with the fuel cell membrane electrode assembly for supplying a fuel-bearing fluid (34) to the membrane electrode assembly. An exhaust channel (38) including a water recovery (56) and recirculation system is defined in the base portion and communicating with the membrane electrode assembly. The membrane electrode assembly and the cooperating fluid supply channel and cooperating exhaust channel forming a single fuel cell assembly.

Description

DIRECT METHANOL FUEL CELL SYSTEM

Field of Invention

The present invention pertains to fuel cells, and

more particularly to a direct methanol fuel cell system

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 the air.

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 the power

requirements .

Typically DMFCs designs are large stacks with forced

airflow at elevated temperatures. Smaller air breathing

DMFC designs are more complicated. In conventional PEM

fuel cells, stack connections are made between the fuels

cell assemblies with conductive plates, machined with

channels or grooves for gas distribution. A typical

conventional fuel cell is comprised of an anode (H₂ or

methanol side) current collector, anode backing, membrane electrode assembly (MEA) (anode/ion conducting

membrane/cathode) , cathode backing, and cathode current

collector. Each fuel cell is capable of approx. 1.0 V.

To obtain higher voltages, fuel cells are typically

stacked in series (bi-polar manor - positive to negative)

one on top another. Conventional fuel cells can also be

stacked in parallel (positive to positive) to obtain

higher power, but typically, larger fuel cells are simply

used.

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. 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 the system for portable

applications. 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.

Recirculation of the water fuel mixture after the fuel

cell reaction and recycling of the water generated in the

fuel cell reaction, in addition to the water diffused

across the membrane is also required for miniaturizing

the system.

Accordingly, it is a purpose of the present

invention to provide for a direct methanol fuel cell

system design in which at least one direct methanol fuel

cell is integrated into a miniaturized system.

It is a purpose of the present invention to provide

for a direct methanol fuel cell system^' including

microchannels and cavities and microfluidics technology

for fuel-bearing fluid mixing, pumping and recirculation.

It is a further purpose of the present invention to

provide for a direct methanol fuel cell system which is

orientation insensitive.

It is still a further purpose of the present

invention to provide for a direct methanol fuel cell system 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 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 array apparatus and method of forming the

fuel cell array apparatus including a base portion, formed of a singular body, and having a major surface. At least one membrane electrode assembly formed on the major

surface of the base portion. A fluid supply channel is

defined in the base portion and communicating with 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 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. There is additionally

included a top portion which includes a plurality of

electrical components for electrical integration of a

plurality of formed fuel cell assemblies.

Brief Description of the Drawings

Referring to the drawings :

FIG. 1 is a simplified sectional view of a plurality

of direct methanol fuel cell devices formed on a single

base portion including a plurality of microfluidic

channels, according to the present invention;

FIG. 2 is a simplified sectional view of a single

direct methanol fuel cell device formed on a single base

portion including a plurality of microfluidic channels,

according to the present invention; and FIG. 3 is a simplified schematic diagram

illustrating the system of the present invention.

Description of the Preferred Embodiment

Turning now to the drawings, FIG. 1 illustrates in

simplified sectional view a planar array direct methanol

fuel cell system fabricated according to the present

invention. More particularly, there is formed a planar

stack array 10, including two direct methanol fuel cells,

generally referenced 12. Fuel cells 12 are formed on a

base portion 14, each fuel cell 12 being spaced at least

lmm apart from an adjacent fuel cell 12. It should be

understood that dependent upon the required power output,

any number of fuel cells 12 can be fabricated to form a

planar array of fuel cells, from one fuel cell as

illustrated in FIG. 2 (discussed presently) , to numerous

fuel cells. The material of base portion 14 is designed

to be impermeable to the mixture of fuel and oxidizer

materials that is utilized to power fuel cells 12.

Typically a hydrogen-containing fuel/oxidizer mixture is

utilized to power fuel cells 12. Suitable fuels that are consumed by fuel cells 12 to produce electrical energy

are hydrogen-containing materials such as hydrogen,

methane and methanol. In this particular example,

methanol is used to fuel, cells 12. Base portion 14 is

typically formed of glass, plastic, silicon, ceramic, or

any other suitable material. In this particular

embodiment, planar stack array 10 is composed of the at

least two direct methanol fuel cells 12 each defined by a

fuel cell membrane electrode assembly (MEA) (discussed

presently FIG. 2) , accordingly, planar stack array 10

includes two fuel cell membrane electrode assemblies.

Base portion 14 has formed within a plurality of

micro-fluidic channels as illustrated. More

particularly, base portion 14 has formed a first fluid

inlet 30 and a second fluid inlet 31, in fluidic

communication with a fluid supply channel 32. Fluid

supply channel 32 is formed in base portion 14 utilizing

standard techniques, well known in the art, such as

multi-layer ceramic technology, micro-machining, or

injection molding. Fluid supply channel 32 supplies a

fuel-bearing fluid 34 to each of the at least two spaced

apart fuel cellsl2. In this particular example, fuel-

bearing fluid 34 is comprised of methanol and water being delivered directly from a methanol tank 35 and a water

tank 37. A mixing chamber 36 is formed in base portion

14 in micro-fluidic communication with fluid supply

channel 32 as illustrated. In a preferred embodiment,

fuel-bearing fluid 34 is 0.5%-4.0% methanol in water

(96%-99.5%) . The goal is to pump methanol into the

overall assembly 10 at a rate of 0.002ml/min and pump the

water into the assembly 10 at a rate of 0.098ml/min (2%

to 98%) . The fuel cell assembly 10 would also be able

to use other fuels, such as hydrogen or ethanol, but it

should be noted that ethanol is not as efficient, nor

does it produce as much power as does the use of

methanol. In this particular example a separate methanol

tank 35 and water tank 37 are utilized to supply the

fuel-bearing fluid 34. The methanol will be pumped in at

a given rate, and the water will be added as needed

determined by a methanol concentration sensor 39.

Methanol concentration sensor 39 helps maintain the

methanol ration in the mixture. The methanol and water

will be homogeneously mixed in mixing chamber 36 before

flowing to each individual fuel cell 12. It should be

understood that fluid supply channel 32 provides for an equal and simultaneous delivery of fuel-bearing fluid 34

to each individually formed fuel cell 12.

In addition, there is formed in base portion 14, an

exhaust channel 38 communicating with each of the at

least two spaced apart fuel cells 12. Exhaust channel 38

serves to remove exhaust products 42 from fuel cells 12, namely carbon dioxide and a water/methanol mixture .

During operation, exhaust products are separated in a carbon dioxide separation chamber 44 into the water/methanol mixture 46 and a carbon dioxide gas 48. Next, gas 48 is expelled through an exhaust outlet 52,

such as a gas permeable • membrane and water/methanol mixture 46 is recirculated through a recirculating channel 53 , having included as a part thereof a pump 54 ,

such as a MEMS pump, or check valve type assembly, back to mixing chamber 36. In addition, in microfluidic

communication is a gas permeable water recovery system

56, and a water recovery return channel 58. Water recovery system 56 serves to recapture water from the

cathode sides of fuel cells 12, and direct it toward

water recovery return channel 58, as illustrated. Water

recovery return channel 58 is in micro-fluidic communication with separation chamber 44 and ultimately

mixing chamber 36.

Recirculation of the water/methanol mixture,

subsequent to reaction in the fuel cell, and the

recycling of the water diffused across the cathode, is

required for miniaturizing the system. It is

anticipated that the fuel delivery system includes

methanol and water, in the form of methanol tank 35 and

water tank 37, which is to be carried in portable

disposable cartridge-like devices, connected through

tubing to the base portion 14.

Fuel cell array 10 typically has formed as a part

thereof, four individual fuel cells 12, having an overall

base portion 14 dimension of approximately 5.5cm x 5.5cm

x .5cm, and individual fuel cell 12 area of 4 x 1.5-2.0cm

squares. Each individual fuel cell 12 is capable of

2 generating approximately 0.5V and 22.5mA/cm of power.

Referring now to FIG. 2, illustrated is a fuel cell

system, generally referenced 10 " including a single fuel

cell assembly 12 ' . It should be noted that all components

of the first embodiment as illustrated in FIG. 1, that

are similar to components of this particular embodiment

as illustrated in FIG. 2, are designated with similar numbers, having a prime added to indicate the different

embodiment. Fuel cell 12 ' is comprised of a fuel cell

membrane electrode assembly 16 comprised of first

electrode 18, including a carbon cloth backing 19, a film

20, such as a porous protonically conducting electrolyte

membrane, and a second electrode 22, including a carbon

cloth backing 23. First and second electrodes 18 and 22

are comprised of a material selected from the group

consisting of platinum, palladium, gold, nickel, tungsten

carbide, ruthenium, molybdenum, and alloys of platinum,

palladium, gold, nickel, tungsten carbide, molybdenum,

and ruthenium. Film 20 is further described as formed of

a nafion type material that prevents the permeation of

fuel from the anode side (first electrode 18) to the

cathode side (second electrode 22) of fuel cell 12 ' .

Membrane electrode assembly 16 in this particular

example is positioned in a recess 24 formed in an

uppermost major surface 26 of base portion 14^*. It is

anticipated by this disclosure that membrane electrode

assembly 16 can be positioned on major surface 26 of base

portion 14 ' without the need for the formation of recess

24. In this instance, a spacer (not shown) would be utilized to avoid complete compression of membrane

electrode assembly 16.

Planar stack array 10 ' further includes a top

portion, more specifically, in this particular

embodiment, a current collector 28 positioned to overlay

membrane electrode assembly 16. Current collector 28 is

disclosed in a preferred embodiment as being formed

discretely over each individually formed fuel cell

membrane electrode assembly 16. Current collector 28 is

further described in a preferred embodiment as comprised

of a perforated corrugated gold coated stainless steel .

It should be understood that in addition it is

anticipated that current collector 28 can be formed of

any electrically conductive material.

During fabrication, individual fuel cell membrane

electrode assemblies 16 are formed using a direct

painting method or hot press method. More particularly,

the plurality of first electrodes 18 are formed or

positioned in contact with major surface 26 of base

portion 14 * . Various materials are suitable for the

formation of electrodes 18. Suitable materials include

platinum, palladium, gold, nickel, tungsten carbide, ruthenium, molybdenum and various alloys of these

materials.

In this specific embodiment, and for exemplary

purposes, each of the plurality of first electrodes 18

has a dimension of approximately 2.0cm x 2.0cm. When

planar stack 10 ' includes a plurality of fuel cells 12*,

such as that discussed previously with respect to FIG. 1,

there is included a separation of approximately 0.5mm to

lmm between adjacent fuel cells 12.

Film 20, formed of a protonically conducting

electrolyte, also referred to as a proton exchange

membrane (PEM), is comprised of a nafion type material.

Film 20 as previously stated serves to limit the

permeation of fuel from the anode 18 of fuel cell 12 to

the cathode 22 of fuel cell 12.

Next, during fabrication of membrane electrode

assembly 16, a plurality of second electrodes 22 are

formed to be correspondingly cooperating with the

plurality of first electrodes 18. Each second electrode

22 is formed having approximately the same dimension as

its corresponding first electrode 18. It should be

understood, that as described, fuel cell membrane electrode assemblies 16 are each comprised of first

electrode 18, film 20 and second electrode 22.

Finally, current collector 28 is positioned relative

to second electrode 22. Current collector 28 is formed at

least 0. lmm thick and of a length dependent upon a point

of contact on fuel cell 12*. In the alternative, when

the device includes a plurality of fuel cells 12 ' , the

plurality of fuel cells 12 ' can be electrically

interfaced using silver conducting paint deposited by

evaporation or sputtering. Materials suitable for this

are gold (Au) , silver (Au) , copper (Cu) , or any other low

electrical resistant material. The bulk resistivity of

the electrode material and area of the electrode will

dictate the type of current collection scheme to minimize

ohmic losses. In addition, anticipated by this

disclosure to achieve electrical interface between the

plurality of direct methanol fuel cells 12 ', are

patterned conductive epoxy and pressing, wire bonding,

tab bonding, spring contacts, flex tape, or alligator

clips. It should be understood, that it is anticipated

that fuel cells 12 ' can be electrically interfaced

utilizing either a series connection or a parallel

connection, dependent upon the desired resultant voltage. To achieve electrical interfacing (not shown) of the

plurality of fuel cells 12 ' , each of the second

electrodes 22 is electrically connected to an adjacent

first electrode 18, thus connected in series electrical

interface, to increase the output voltage of the fuel

cell array apparatus 10 ' or each of the first electrodes

18 is electrically connected to an adjacent first

electrode 18, and each of the second electrodes 22 is

electrically connected to an adjacent second electrode

22, thus connected in parallel electrical interface, to

increase the output voltage of the fuel cell array

apparatus 10 ' .

Referring now to FIG. 3, illustrated is a simplified

schematic diagram detailing the system of the present

invention. Illustrated are methanol tank 35 and water

tank 37 in microfluidic communication with mixing chamber

36. Mixing chamber 36 as previously discussed serves to

achieve the proper ratio of methanol to water. Once

properly mixed, the fuel-bearing fluid flows through the

fluid supply channel toward the fuel cell 12. An

optional MEMs type pump 40 is utilized to assist with

this flow. Concentration sensors 39 are provided to

assist with monitoring the methanol concentration, and the temperature of the fuel-bearing fluid. The fuel-

bearing fluid next reaches fuel cell stack 12 and

generates power. The power is supplied to a DC-DC

converter 62 which converts the generated voltage to a

useable voltage for powering a portable electronic device, such as a cell phone 60 and included as a part

thereof a rechargeable battery 64. During operation

spent fluid is exhausted through the exhaust channel

toward a carbon dioxide separation chamber and carbon dioxide vent, generally referenced 44. In addition, water is recovered from the cathode side of the fuel cell 12, and from the separation chamber 44 and is recirculated through a recirculating channel back to the

mixing chamber 36. This recirculating of fluid provides for the consumption of less water from water tank 37 and thus less replenishment of water tank 37.

Accordingly, disclosed is a fuel cell system and

method of fabrication which provides for the fabrication

of the system, providing for inclusion of a single fuel

cell or a plurality of fuel cells to be formed on a

planar surface, thus allowing higher voltages and

currents to be gained on a single planar surface. More particularly, the design provides for a simplified system in which spent fuel is partially separated to recirculate

useable by-product, namely water, thereby providing for

less consumption and replenishment of a water supply. In

addition, it is disclosed that the system of the present

invention is a semi-self contained system, and is not

orientation sensitive, thus providing for ease in moving

the system, such as when providing power to a portable

electronic device .

While we have shown and described specific

embodiments of the present invention, further

modifications and improvements will occur to those

skilled in the art. We desire it to be understood,

therefore, that this invention is not limited to the

particular forms shown and we intend in the appended

claims to cover all modifications that do not depart from

the spirit and scope of this invention.

Claims

What is claimed is:

1. A fuel cell system comprising:

a base portion, formed of a singular body, and

having a major surface;

at least one fuel cell membrane electrode assembly

formed on the major surface of the base portion;

a fluid supply channel defined in the base portion

and communicating with the at least one fuel cell

membrane electrode assembly, the fluid supply channel

including a mixing chamber and at least two fuel-bearing

fluid inlets;

an exhaust channel defined in the base portion and

communicating with the at least one fuel cell membrane

electrode assembly, the exhaust channel including a water

recovery and recirculation system, the exhaust channel

spaced apart from the fluid supply channel for exhausting

fluid from the at least one fuel cell membrane electrode

assembly, the at least one fuel cell membrane electrode

assembly and the cooperating fluid supply channel and

cooperating exhaust channel forming a single fuel cell

assembly; a plurality of electrical components formed in the

base portion for electrical integration of the fuel cell

assembly.

2. A fuel cell system as claimed in claim 1 wherein

the base portion comprises a material selected from the

group consisting of ceramic, plastic, glass, and silicon.

3. A fuel cell system as claimed in claim 2 further

wherein the at least one fuel cell membrane electrode

assembly formed on the major surface of the base portion

includes a plurality of fuel cell membrane electrode

assemblies formed on the major surface of the base

joortion wherein each of the plurality of fuel cell

membrane electrode assemblies is spaced at least lmm from

an adjacent fuel cell membrane electrode assembly.

4. A fuel cell system as claimed in claim 1 wherein

the fuel cell membrane electrode assembly includes a

first electrode, a film adjacent the first electrode,

formed of a protonically conductive electrolyte, and a

second electrode in contact with the film.

5. A fuel cell system as claimed in claim 4

wherein the first and second electrodes comprise a

material selected from the group consisting of platinum,

palladium, gold, nickel, tungsten carbide, ruthenium,

molybdenum, and alloys of platinum, palladium, gold,

nickel, tungsten carbide, molybdenum, and ruthenium.

6. A fuel cell system as claimed in claim 5

wherein the film overlying the first electrode comprises

of a proton exchange type material .

7. A fuel cell system as claimed in claim 1 wherein

the fuel-bearing fluid is comprised of mixture of

methanol and water, supplied by a separate methanol

source and a separate water source, and mixed in-situ.

8. A fuel cell system as claimed in claim 1 wherein

the fluid supply channel defined in the base portion and

communicating with the fuel cell membrane electrode

assembly includes a methanol concentration sensor.

9. A fuel cell system as claimed in claim 1 wherein

the exhaust channel defined in the base portion and communicating with the fuel cell membrane electrode

assembly further includes a carbon dioxide separation

chamber and a carbon dioxide exhaust vent .

10. A fuel cell system as claimed in claim 1

wherein the water recovery and recirculation system

provides for the recovery and recirculation of a spent

water and ethanol mixture from the fuel cell back to the

mixing chamber.

11. A fuel cell array apparatus comprising:

a base portion, formed of a singular body, and

having a major surface, the base portion formed of a

material selected from the group consisting of ceramic,

plastic, glass, and silicon;

at least one fuel cell membrane electrode assembly

formed on the major surface of the base portion, the at

least one fuel cell membrane electrode assembly including

a first electrode, a film in contact with the first

electrode and formed of a protonically conductive

electrolyte, and a second electrode in contact with the

film; a fluid supply channel defined in the base portion

and communicating with the at least one fuel cell

membrane electrode assembly for supplying a fuel-bearing

fluid to the at least one fuel cell membrane electrode

assembly, the fluid supply channel further including a

first fuel-bearing fluid inlet, and a second fuel-bearing

fluid inlet, and a mixing chamber;

an exhaust channel defined in the base portion and

communicating with the at least one fuel cell membrane

electrode assembly, the exhaust channel spaced apart from

the fluid supply channel for exhausting fluid from the at

least one spaced apart fuel cell membrane electrode

assembly, the exhaust channel further including a water

recovery and recirculation assembly in fluidic

communication with the second electrode of the at least

one fuel cell membrane electrode assembly, in combination

the at least one fuel cell membrane electrode assembly

and the cooperating fluid supply channel and cooperating

exhaust channel forming a single fuel cell assembly; and

a top portion including a plurality of electrical

components for electrical integration of the plurality of

formed fuel cell assemblies.

12. A method of fabricating a fuel cell array

apparatus comprising the steps of :

providing a base portion formed of a material

selected from the group consisting of ceramic, plastic, glass, and silicon;

forming a fluid supply channel in the base portion for supplying a fuel-bearing fluid to at least one fuel

cell membrane electrode assembly, the fluid supply channel further including a mixing chamber and a methanol

concentration sensor; forming an exhaust channel in the base portion, the

exhaust channel spaced apart from the fluid supply channel for exhausting fluid from the at least one spaced apart fuel cell membrane electrode assembly, the exhaust channel further including a water recovery and

recirculation system for the recover and recirculation of

a spent fuel-bearing fluid;

forming the at least one fuel cell membrane

electrode assembly on the major surface of the base

portion, the step of forming the at least one spaced

apart fuel cell membrane electrode assembly including the

steps of providing for a first electrode on a major surface of the base portion, providing for a film in contact with the first electrode and formed of a

protonically conductive electrolyte, and providing for a

second electrode in contact with the film, the at least

one spaced apart fuel cell membrane electrode assembly

and the cooperating fluid supply channel and cooperating

exhaust channel forming a single fuel cell assembly; and

forming a top portion including a plurality of

electrical components for electrical integration of the

formed fuel cell assembly.