U.S. patent application number 10/213838 was filed with the patent office on 2003-02-27 for mems-based thin-film fuel cells.
This patent application is currently assigned to The Regents of the University of California. Invention is credited to Jankowski, Alan F., Morse, Jeffrey D..
Application Number | 20030039874 10/213838 |
Document ID | / |
Family ID | 22909493 |
Filed Date | 2003-02-27 |
United States Patent
Application |
20030039874 |
Kind Code |
A1 |
Jankowski, Alan F. ; et
al. |
February 27, 2003 |
MEMS-based thin-film fuel cells
Abstract
A micro-electro-mechanical systems (MEMS) based thin-film fuel
cells for electrical power applications. The MEMS-based fuel cell
may be of a solid oxide type (SOFC) having a thickness of 0.5-50
.mu.m at a temperature below 600.degree. C. and a thickness of
0.5-7.5 .mu.m at a temperature over 600.degree. C., or a solid
polymer type (SPFC), or a proton exchange membrane type (PEMFC),
each operating below 600.degree. C. and having a thickness of
0.5-50 .mu.m, and each fuel cell basically consists of an anode and
a cathode separated by an electrolyte layer. The electrolyte layer
can consist of either a solid oxide or solid polymer material, or
proton exchange membrane electrolyte materials may be used.
Additionally catalyst layers can also separate the electrodes
(cathode and anode) from the electrolyte. Gas manifolds are
utilized to transport the fuel and oxidant to each cell and provide
a path for exhaust gases. The electrical current generated from
each cell is drawn away with an interconnect and support structure
integrated with the gas manifold. The fuel cells utilize integrated
resistive heaters for efficient heating of the materials. By
combining MEMS technology with thin-film deposition technology,
thin-film fuel cells having microflow channels and full-integrated
circuitry can be produced that will lower the operating temperature
an will yield an order of magnitude greater power density than the
currently known fuel cells.
Inventors: |
Jankowski, Alan F.;
(Livermore, CA) ; Morse, Jeffrey D.; (Martinez,
CA) |
Correspondence
Address: |
Alan H. Thompson
Assistant Laboratory Counsel
Lawrence Livermore National Laboratory
P.O. Box 808, L-703
Livermore
CA
94551
US
|
Assignee: |
The Regents of the University of
California
|
Family ID: |
22909493 |
Appl. No.: |
10/213838 |
Filed: |
August 7, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10213838 |
Aug 7, 2002 |
|
|
|
09241159 |
Feb 1, 1999 |
|
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|
Current U.S.
Class: |
429/434 ;
429/458; 429/465; 429/482; 429/485; 429/492 |
Current CPC
Class: |
H01M 8/1213 20130101;
H01M 8/241 20130101; H01M 2300/0082 20130101; H01M 2008/1293
20130101; Y02E 60/50 20130101; H01M 8/1286 20130101; H01M 2300/0074
20130101; H01M 8/2485 20130101; H01M 8/2432 20160201; H01M 8/1097
20130101 |
Class at
Publication: |
429/26 ; 429/32;
429/38 |
International
Class: |
H01M 008/04; H01M
008/12; H01M 008/10; H01M 008/02 |
Goverment Interests
[0002] The United States Government has rights in this invention
pursuant to Contract No. W-7405-ENG-48 between the United States
Department of Energy and the University of California for the
operation of Lawrence Livermore National Laboratory.
Claims
What is claimed is:
1. A fuel cell, including: a fuel cell stack having an electrolyte
composed of material selected from the group consisting of solid
oxide, solid polymer, and proton exchange membrane materials, means
for supplying fuel and an oxidant to opposite sides of said fuel
cell stack, and means for heating said fuel cell stack, said means
for heating includes a resistive heater, said fuel cell stack
including at least a pair of electrodes, and an electrolyte
separating said electrodes, said electrolyte having a thickness of
about 0.5-50 .mu.m, said means for supplying fuel and oxidant
including at least one substrate having a manifold formed therein,
said manifold directing fuel to one side of said fuel cell
stack.
2. The fuel cell of claim 1, wherein said means for supplying fuel
and oxidant additionally includes a second substrate having a
manifold formed therein for directing an oxidant to an opposite
side of said fuel cell stack.
3. The fuel cell of claim 1, wherein said fuel cell stack includes
a catalyst adjacent the electrolyte.
4. The fuel cell of claim 1, wherein said resistive heater is
located in said fuel cell stack.
5. The fuel cell of claim 1, wherein said pair of electrodes of
said fuel cell stack includes a first electrode composed of
hydrogen catalyzing conducting material selected from nickel,
copper, carbon, platinum, and palladium, and a second electrode
composed of oxygen catalyzing conducting material selected from
silver, carbon, platinum, lanthanum strontium manganate (LSM).
6. The fuel cell of claim 1, wherein said pair of electrodes
comprises a first electrode composed of Ni, and a second electrode
composed of Ag and wherein said electrolyte is composed of YSZ.
7. The fuel cell of claim 1, wherein said at least one substrate is
composed of material selected from the group consisting of silicon,
glass, ceramic, and plastic.
8. The fuel cell of claim 1, wherein said means for supplying fuel
includes a second substrate bonded to said at least one substrate,
said second substrate having at least a fuel inlet extending
thereinto and in open communication with said manifold in said at
least one substrate.
9. The fuel cell of claim 8, wherein said second substrate
additionally includes a cutaway section in a surface located
adjacent said at least one substrate and which is in open
communication with said manifold and with said fuel inlet.
10. The fuel cell of claim 8, wherein said second substrate
additionally includes a plurality of channels in open communication
with said manifold and said fuel inlet.
11. The fuel cell of claim 1, additionally including a porous
member located intermediate said fuel cell stack and said manifold
in said at least one substrate.
12. The fuel cell of claim 1, wherein said manifold of said at
least one substrate is composed a plurality of channels.
13. The fuel cell of claim 12, wherein said means for supplying
fuel includes a second substrate bonded to said at least one
substrate, said second substrate having a fuel inlet and a
plurality of channels in open communication with said manifold in
said at least one substrate.
14. The fuel cell of claim 1, wherein said electrolyte is composed
of solid polymer or proton exchange membrane material, operating at
a temperature range of about 23-120.degree. C., and having a
thickness in the range of 0.5-50 .mu.m.
15. The fuel cell of claim 1, wherein said electrolyte is composed
of solid oxide materials operating at one of a temperature below
600.degree. C. with a thickness of 0.5-7.5 .mu.m or 0.5-50 .mu.m,
or at a temperature above 600.degree. C. with a thickness of
0.5-7.5 .mu.m.
16. A miniature thin-film fuel cell, comprising: a fuel cell stack
including a pair of electrodes and an electrolyte layer, selected
from the group consisting of solid oxides, solid polymers, and
proton exchange membrane materials, a resistive heater for heating
said fuel cell stack, a first substrate having an opening therein
located adjacent said fuel cell stack, a second substrate having at
least a fuel inlet therein secured to said first substrate and, a
resistive heating means located in said fuel cell stack for at
least said electrolyte, said electrolyte having a thickness of 0.5
to 50 .mu.m.
17. The fuel cell of claim 16, additionally including a third
substrate secured to said first substrate and having an oxidant
inlet and a manifold therein adjacent said fuel cell stack.
18. The fuel cell of claim 16, wherein said first substrate
additionally includes a plurality of channels forming said
opening.
19. The fuel cell of claim 18, wherein said second substrate
additionally includes a plurality of channels in open communication
with said fuel inlet and said plurality of channels in said first
substrate.
20. The fuel cell of claim 16, additionally including a porous
member intermediate said first substrate and said fuel cell
stack.
21. The fuel cell of claim 1, additionally including a porous
electrode formed by photolithographically patterning and etching a
continuous metal electrode layer.
22. The fuel cell of claim 16, additionally including a porous
electrode formed by photolithographically patterning and etching a
continuous metal electrode layer.
23. The fuel cell of claim 16, additionally including a porous
electrode formed by deposition onto a porous member.
24. The fuel cell of claim 21, additionally including a catalyst
and electrolyte layers deposited onto a porous electrode
structure.
25. The fuel cell of claim 22, additionally including a catalyst
and electrolyte layers deposited onto a porous electrode
structure.
26. The fuel cell of claim 1, in which a membrane electrode
assembly laminate structure is attached, bonded or mechanically
sealed to a micromachined manifold host structure.
27. The fuel cell of claim 16, in which a membrane electrode
assembly laminate structure is attached, bonded or mechanically
sealed to a micromachined manifold host structure.
28. The fuel cell of claim 17, in which the fuel cell modules can
be directly stacked on top of each other to scale voltage and
power.
29. The fuel cell of claim 28, additionally including a plurality
of fuel inlets to deliver fuel from a common reservoir.
30. The fuel cell of claim 16, wherein said electrolyte is composed
of solid polymer material or proton exchange membrane material
having a thickness of 0.5-50 .mu.m, and wherein said fuel cell
operates at a temperature below 600.degree. C.
31. The fuel cell of claim 30, wherein said operating temperature
is about 23.degree.-120.degree. C.
32. The fuel cell of claim 16, wherein said electrolyte is a solid
oxide material having a thickness of 0.5-50 .mu.m, and wherein said
fuel cell operates at a temperature above 600.degree. C. and below
600.degree. C.
33. The fuel cell of claim 32, wherein the operating temperature is
below 600.degree. C., and wherein said electrolyte has a thickness
of 0.5-50 .mu.m.
34. The fuel cell of claim 33, wherein the electrolyte has a
thickness of less than 10 .mu.m.
35. The fuel cell of claim 34, wherein the electrolye has a
thickness of 0.5-7.5 .mu.m.
36. The fuel cell of claim 32, wherein said electrolyte has a
thickness of about 0.5-7.5 .mu.m, and wherein the operating
temperature is above 600.degree. C.
37. The fuel cell of claim 36, wherein said operating temperature
is 675.degree. C.
Description
RELATED APPLICATION
[0001] This application is a Continuation-In-Part of U.S.
application Ser. No. 09/241,159 filed Feb. 1, 1999, and claims
priority thereto.
BACKGROUND OF THE INVENTION
[0003] The present invention relates to fuel cells, particularly to
small, compact fuel cells, and more particularly to a miniature
power source composed of a stack of fuel cells fabricated by
combining MEMS and thin film deposition technologies to produce
fuel cells with microflow channels, fully-integrated control
circuitry, and integrated resistive heaters.
[0004] Portable power sources of various types have been under
development for many years. A serious need exists for portable
power sources with significantly higher power density, longer
operating lifetime, and lower cost. Present rechargeable and
primary portable power sources have excessive weight, size, and
cost with limited mission duration. As an example, batteries
covering power range from 1-200 Watts have specific energies
ranging from 50-250 Whr/Kg, which represents two to three hours of
operation for a variety of commercial and military applications. An
alternative power source is the fuel cell which would potentially
provide higher performance power sources for portable power
applications if the stack structure, packaging, and cell operation
were made compatible with scaling down of size and weight.
[0005] Fuel cells typically consist of electrolyte materials based
on either polymer (proton exchange type) or solid oxide materials,
which are sandwiched between electrodes. The fuel cell operates
when fuel (usually hydrogen) is delivered to one electrode, and
oxygen to the other. By heating the electrode-electrolyte
structure, the fuel and oxidant diffuse to the
electrode-electrolyte interfaces where an electrochemical reaction
occurs, thereby releasing free electrons and ions which conduct
across the electrolyte. Typical fuel cells are made from bulk
electrode-electrolyte materials which are stacked and manifolded
using stainless steel or other packaging which is difficult to
miniaturize. These systems are bulky, requiring labor intensive
manual assembly, packaging and testing, and in the case of solid
oxide materials, typically operate at high temperatures
(>600.degree. C.). If the electrode-electrolyte stack can be
made very thin and deposited using thin film deposition techniques,
the temperature of operation will be significantly lower, and the
cost of integration, packaging and manufacturing can be
reduced.
[0006] Previous efforts at Lawrence Livermore National Laboratory,
for example, have demonstrated the synthesis of a thin-film
solid-oxide based electrolyte fuel cell. See A. F. Jankowski et
al., Mat Res. Soc. Symp. Proc., Vol. 496, pp 155-158, 1998 Material
Research Society; and U.S. Pat. No. 5,753,385, issued May 19, 1998
to A. F. Jankowski. In one example, the thin film solid oxide fuel
cell (TFSOFC) stack was formed using physical vapor deposition
(PVD) techniques. The host substrate used was a silicon wafer
covered by a thin layer of silicon nitride. A layer of nickel was
first deposited, followed by a layer of yttria-stabilized zirconia
(YSZ). The conditions during the deposition were adjusted in order
to achieve smooth, dense, continuous films, thus, avoiding pinhole
formation which could result in direct mixing of the fuel and
oxidant as well as electrical shorting through the electrolyte
layer. This enables the electrolyte layer to be on the order of
about 0.5-10 .mu.m thick rather than typical thicknesses on the
order of >10 .mu.m for bulk solid oxide fuel cells. For solid
polymer and proton exchange membrane electrolytes, the thickness
may be in the range of 0.5-50 .mu.m. By thinning the electrolyte
layer, the diffusive path for the oxygen ion is shorter and the
solid oxide fuel cell operates at much lower temperatures. A silver
electrode layer is deposited on top of the YSZ layer. The
deposition conditions of this film are adjusted to create a porous
structure so that oxygen can readily diffuse to the electrolyte
interface.
[0007] The present invention combines an example of thin-film
deposition technology, referenced above, with
micro-electro-mechanical systems (MEMS) technology to produce a
thin-film miniature fuel cell with microflow channels and
full-integrated control circuitry, along with integrated resistive
heaters for effectively heating the fuel cell such that it will
yield and order of magnitude greater power density than any
currently known bulk fuel cell. Using this combined technology,
thin-film fuel cell stacks can be produced to provide a small,
compact miniature power source. The miniature fuel cells of this
invention may be either oxygen-ion based, solid oxide or proton
exchange membrane (e.g., solid polymer) electrolyte materials, and
may also utilize catalyst layers between the electrodes and the
electrolyte. The solid oxide electrolyte may have a thickness of
0.5-7.5 .mu.m and operate at temperatures above and below
600.degree. C.
SUMMARY OF THE INVENTION
[0008] It is an object of the present invention to provide a small,
compact fuel cell power source.
[0009] A further object of the invention is to provide thin-film
fuel cells for electrical power applications.
[0010] A further object of the invention is to provide MEMS-based
thin-film fuel cells.
[0011] Another object of the invention is to provide an MEMS-based
thin-film fuel cells having microflow channels and integrated
resistive heaters and control circuitry.
[0012] Another object of the invention is to provide a MEMS-based
thin-film fuel cell stack, manifold structure, and fuel reservoir
as a miniature portable power source.
[0013] Another object of the invention is to provide a MEMS-based
thin-film fuel cell capable of utilizing an electrolyte of either a
solid oxide, a solid polymer, or a proton exchange membrane
material.
[0014] Another object of the invention is to provide thin-film fuel
cell stack with microflow channels, full-integrated control
circuitry, and integrated resistive heaters capable of producing a
high power density.
[0015] Another object of the invention is to provide a MEMS-based
fuel cell which includes electrode catalyst/electrolyte materials
which enable the combination of a fuel and oxidant above room
temperature to produce continuous electric current.
[0016] Another object of the invention is to provide a MEMS-based
fuel cell having in the form of a module which can be directly
stacked as a means of scaling the power and voltage.
[0017] Another object of the invention is to provide a MEMS-based
fuel cell utilizing an electrolyte composed of solid oxide, solid
polymer and proton exchange membrane material, and wherein the
solid oxide electrolyte can operate below or above 600.degree.
C.
[0018] Other objects and advantages of the present invention will
become apparent from the following description and accompanying
drawings. Basically the invention involves combining MEMS
technology and thin-film deposition technology to produce a
miniature thin-film fuel cell or fuel cell stack as a portable
power source. The MEMS-based fuel cell of this invention may
utilize an electrolyte layer which may consist of either a solid
oxide or a proton exchange membrane material, and may utilize
catalyst layers between the electrodes and the electrolyte. The
fuel cell includes microflow channels and manifolding micromachined
into the host structure/substrate, and may contain a rechargeable
microbattery for startup control, and utilize integrated resistive
heaters for efficient heating of the stack materials, and may
utilize integrated microvalves, resistive heaters, or other means
to control the flow of fuel to the fuel cell stack. Furthermore,
the fuel cell may exploit the various fuel cell
electrode/electrolyte/catalyst materials systems as they are
developed such that either direct hydrogen fuel is used, or other
gaseous or liquid hydrocarbon fuels, including methanol or
propanol, can be used. Additionally, other structures or materials
may be readily integrated with a MEMS-based fuel cell which enable
reforming of fuel such that a variety of fuels having high
percentage concentrations of hydrogen may be used. The MEMS-based
fuel cell may incorporate a fuel reservoir as part of a package
approach, or as a modular cartridge which can be easily replaced or
refilled. Such a fuel reservoir may simply be a volume containing a
liquid if liquid fuels are used, or a volume containing a metal
hydride or other material which is capable of storing hydrogen
within it. In this case, some form of valve may be placed in the
micro-flow channels as a means of controlling the flow of fuel to
the stack. Additionally, heating elements may be used to control
the flow of hydrogen from the reservoir. The MEMS-based fuel cell
modules may be directly stacked on top of each other as a means of
scaling voltage and power, or connect fuel cell modules in series
on a planar level to do the same. Power transforming circuitry may
be utilized to provide the appropriate output voltage and power
requirements.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] The accompanying drawings, which are incorporated into and
form a part of the disclosure, illustrate embodiments of the
invention and, together with the description, serve to explain the
principles of the invention.
[0020] FIG. 1 is a cross-sectioned view of an embodiment of a
miniature MEMS-based thin-film fuel cell of the invention.
[0021] FIG. 2 is an enlarged cross-section of an upper section of
the FIG. 1 embodiment.
[0022] FIG. 3 is an exploded view of another embodiment of the
miniature thin-film fuel cell of the invention.
[0023] FIG. 4 graphically illustrates results of initial testing of
the miniature fuel cell.
[0024] FIG. 5 is a cross-sectional view of another embodiment of
the thin-film fuel cell made in accordance with the present
invention.
[0025] FIG. 6 is a cross-sectional view of another embodiment of
the fuel cell using a multiple channel fuel manifold
arrangement.
[0026] FIG. 7 graphically illustrates cell potential and cell power
relative to cell current.
[0027] FIG. 8 is a view which illustrates the approach for stacking
the MEMS-based fuel cell modules for power and voltage scaling.
[0028] FIG. 9 is a cross-sectional view of another embodiment of
the fuel cell enabling the cells to be inverted and bonded in order
to deliver fuel to two cells from one common inlet.
DETAILED DESCRIPTION OF THE INVENTION
[0029] The present invention is a MEMS-based thin-film fuel cell or
stack of fuel cells of either a solid oxide fuel cell (SOFC)
(operating at temperatures above and below 600.degree. C.), a solid
polymer fuel cell (SPFC) or a proton exchange membrane fuel cell
(PEMFC), (each operating at temperature below 600.degree. C.)
utilizing electrode/catalyst/electro- lyte or electrode/electrolyte
materials which enable the combination of a fuel and oxidant at
elevated temperatures to produce continuous electric current. Fuel
manifolds and microflow channels are formed in the host
structure/substrate by MEMS-based technology and the
electrode/electrolyte/electrode, with or without catalyst layers
are formed along with resistive heaters and integrated control
circuitry by thin-film deposition technology and microfabrication
approaches in combination with MEMS fabrication techniques. Thus,
the invention provides a miniature power source composed of fuel
cells which can yield zero emissions (when operated on hydrogen and
air). The electrical current generated from each cell is drawn away
with an interconnect and support structure integrated with the gas
manifold. In the present invention, the strengths of
microelectronics, fabrication, micromachining approaches, and
microelectro-mechanical systems technology are combined with
thin-film deposition techniques for the purpose of producing a
MEMS-based, thin-film miniature fuel cell. The MEMS-based fuel cell
is materials flexible, therefore can utilize either solid oxide,
solid polymer or proton exchange membrane electrolyte materials
system.
[0030] A fuel cell stack has been fabricated which utilizes thin
film electrodes, catalysts, and ion conducting layers deposited by
physical vapor deposition techniques. The stack has been patterned
with electrical connections using standard microfabricaton
techniques and subsequently formed into free-standing or supported
membranes by micromachining away the silicon substrate. Manifold
structures have further been fabricated through silicon
micromachining techniques.
[0031] Stand-alone, single thin-film solid oxide fuel cells
(TFSOFCS) have been synthesized through multilayer and thin film
vacuum deposition technology, see A. F. Jankowski et al.,
referenced above. The present invention provides the ability to
design and build a single MEMS-based fuel cell module suitable for
assembly into a fuel cell stack. Development of a usable substrate
platform for cell stacking is seen in microelectronic processing
of, for example, silicon wafers. The Si wafers are patterned using
photolithography and etch processing to yield select areas of thin
nitride windows that served as the substrate for deposition of the
anode-electrolyte-cathode trilayer. Additional approaches can
utilize a host structure/substrate having controlled porosity on
which the fuel cell electrode/catalyst/electrolyte layers are
deposited. This host structure having controlled porosity is then
bonded on top of, mechanically sealed to, or formed within the
silicon (or other material) substrate through which the flow
channels have been etched. A solid oxide fuel cell (SOFC) trilayer
example, similar to that shown in FIGS. 1 and 2, has been
fabricated based on yttria-stabilized zirconia (YSZ) beginning with
deposition of a nickel (Ni) layer and concluding with a silver (Ag)
layer. Following deposition of the basic trilayer structure, the
nitride window, if present, can be removed by plasma etching or
other selective etching techniques. Creation of a multiple cell
stack may be pursued, for example, using the Ni--YSZ--Ag trilayers
or other solid oxide based trilayers. In this invention the
Si-based substrates are used to build the individual TFSOFCs, for
example, and use this platform to provide integration of the
circuitry, resistive heaters, and required fuel and oxidant
manifolding through microflow channel and MEMS fabrication
technology. The patterned circuitry provides for controlled power
output from each cell as well as a controlled heating of individual
fuel cell elements. The MEMS processes allows individual control of
gas flow to each cell through use of microvalves as well as control
and regulate gas pressure or fuel flow throughout the device.
Provision capabilities exist for incorporation of materials for the
electrode, catalyst and electrolyte interface that optimize
performance at lower temperature by reducing resistive losses along
with alternative materials that are suitable for high-temperature
internal fuel reforming. The substrate platform enables modular
cell assembly providing a capability for servicing and cell
upgrade, as well as scaling of voltage.
[0032] Referring now to the embodiment of FIGS. 1 and 2, FIG. 1
illustrates a fuel cell generally indicated at 10 a host structure
or substrate 11 having a nitride layer or other porous membrane 12
on which is deposited electrode-electrolyte-electrode layers (with
or without catalyst layers) 13, a substrate 14 having a fuel inlet
15, and a substrate 16 having an oxidant (oxygen) inlet 17.
Additionally, if the fuel inlet 15 in substrate 14 and the oxidant
inlet 17 in substrate 16 enter from the sides, then the fuel cell
modules 10 can be directly stacked on top of each other with fuel
and oxidant being supplied through the respective inlets for each
module from a common reservoir. In this stacking embodiment, the
electrodes for each module are connected in series so as to add the
voltage produced by each individual module. The substrates 14 and
16 are bonded or mechanically sealed to host structure 11 as
indicated at 18 and 19. As pointed out above, if a nitride layer is
used, the central area of the nitride layer 12 is removed following
deposition of the electrode-electrolyte-electrode layers 13. The
host structure or substrate 11 is provided with a plurality of
openings, channels, pores or windows having tapered or straight
sides 20 and 21 which approximately align with tapered or straight
sides 22 of a cutaway section 23 of substrate 14 which includes a
section 24 which communicates with fuel inlet 15. The
electrode-electrolyte-electrode layers 13, as seen in FIG. 2, is
composed of an electrode (anode or cathode depending on type of
electrolyte) 25, an electrolyte layer 26 and an electrode (anode or
cathode depending on type of electrolyte) 27, with the bottom
electrode providing a continuous coverage of the plurality of open
channels, or pores 20 and the electrolyte 26 providing a continuous
electrically insulating barrier between the electrodes 25 and 27.
The top electrode 27 may step down over the edge of the bottom
electrode 25 in order to draw the electrical current off of the
substrate 11 to an external circuit. The
electrode-electrolyte-electrode layers 13 may be composed of a Ni
anode, a YSZ electrolyte and a Ag cathode, or other materials which
are typical of the solid oxide and solid polymer or proton exchange
membrane, fuel cell construction, such as Carbon bottom electrode,
Pt catalyst, Nafion electrolyte, and Pt catalyst/electrode. The
substrate 16 is provided with a cutaway section 28 which is in
communication with oxidant inlet 17. Thus the opening 20 in
substrate 11 and the cutaway section 23 of substrate 14 form a fuel
channel 29 in contact with electrode 25, while the cutaway section
28 of substrate 16 forms an oxidant (oxygen) channel 30 in contact
with electrode 27. As described above, if the fuel inlet 15 and
oxidant inlet 17 enter from the sides of the fuel cell module 10,
or in a manner which does not block the inlets, the individual
cells 10 can be directly stacked on one another. Furthermore, if
the location of the fuel and oxidant inlet channels are offset such
that they are easily acessible then fuel can be delivered to the
plurality of inlets from a common fuel or oxidant reservoir. Not
illustrated in the FIGS. 1-2 embodiment is a heater for the
electrolyte layer 26, but, for example, a resistive heater as shown
in FIG. 3 may be incorporated into the trilayer 13. The substrates
11, 14 and 16 may be composed of silicon, glass, ceramic, plastic,
or any material which has sufficient mechanical strength, can
withstand the temperature of operation, and can be micromachined or
cast to form manifold channels to deliver fuel and oxidant from an
external reservoir. The micromachining methods include those known
in the general art and include etching, laser drilling, diamond
turning, injection molding, and hot embossing.
[0033] As has been shown in FIG. 1, the incorporation of manifold
structure within the host substrate through micromachining
techniques enables a complete fuel cell device to be realized which
can be readily attached to fuel and oxidant sources. FIG. 3
illustrates an exploded view a complete fuel cell stack including
the resistive heater, not shown in the FIGS. 1-2 embodiment. In the
approach of FIG. 3, the fuel cell generally indicated at 40,
includes a membrane-electrode assembly created by thin-film
deposition techniques, or a combination of laminate, thin film and
thick film assembly techniques and generally indicated at 41, and a
micromachined substrate-manifold assembly or system generally
indicated at 42. Integrated circuit type microfabrication processes
are used to pattern the electrode contacts, as well as to form a
resistive heater element within the fuel cell stack structure. The
components of the membrane-electrode assembly 41 is subsequently
formed into a free standing membrane by selective etching of
openings or windows 43 in a substrate 44 on which the components
are deposited as in the FIG. 2 embodiment, the substrate 44 being
an upper component of the manifold assembly 42. In an alternative
embodiment, the components of the membrane-electrode assembly 41 is
subsequently formed on a porous thick film membrane or host
structure which is positioned over the openings or windows 43 in a
substrate 44 on which the components are formed, the substrate 44
being an upper component of the manifold assembly 42. Manifold
channels are micromachined in another substrate 45, the lower
components of the manifold assembly 42, which is subsequently
bonded to the substrate 44. The membrane electrode assembly 41
includes an electrode (anode) 46 having a contact pad 47, a heater
isolation member 48 having openings or window 49 which align with
openings or windows 43 of substrate 44, a resistive heater 50
having contact pads 51 and 52 and constructed so as not to cover
the openings 49 in member 48, an electrolyte 53, and an electrode
(cathode) 54 having a contact pad 55. For example, the
electrode/electrolyte/electrode (components 46, 53 and 54) may be
composed of nickel (Ni)-yttria stabilized zirconia (YSZ)-silver
(Ag), the resistive heater 50 composed of platinum, and the heater
isolation member 48 composed of silicon dioxide or other material
which provides electrical isolation from the electrodes with
openings 49 being 2 mm.times.2 mm, and with the substrates 44 and
45 composed preferably of silicon but may be composed of glass,
ceramic, plastic, or other material having the qualities described
above, with windows 43 being 2 mm.times.2 mm.
[0034] This approach provides an effective means to efficiently
form fuel cell stacks and electrode structures monolithically, and
distribute fuel to the entire stack without the need for bulky
complex manifolding. Furthermore, since the stack is now only a
small percentage of the mass of the entire structure, appropriate
thermal design of the fuel cell device, package, and resistive
heating elements will allow efficient, low power heating of the
stacks.
[0035] A completely fabricated fuel cell module with an integrated
heating element, with the fuel cell stack free standing membranes
has been tested. The silicon substrate was selectively etched with
potassium hydroxide using patterned silicon nitride as the mask to
form the window therein. Manifold channels were etched in a silicon
substrate utilizing similar techniques. These components were
ultimately bonded together, such as illustrated in FIG. 1, to form
a fuel cell module having inlet and outlet channels with
approximately 50 .mu.m by 200 .mu.m openings for fuel delivery.
[0036] Initial testing of fuel cell structures consisted of a
manifolded tube that can be sealed to the backside of the fuel cell
stack to deliver hydrogen gas diluted to 4% or less concentration.
The oxident source was a 20% oxygen gas air mixture delivered to
the top side of the structure. Contact wires were attached to the
anode and cathode electrode pads, from a parameter analyzer that
was utilized to monitor the potential voltage drop across the stack
and subsequent current output as a function of temperature. Initial
results of this testing are illustrated in FIG. 4. While not yet
optimal, these results exhibit the expected overpotential for this
electrolyte materials system with no output current, along with
increasing current output as temperature increases.
[0037] Alternate solid oxide electrolyte materials and proton
exchange membrane materials are being tested with improvements as
described in greater detail with respect to FIGS. 5 and 6. As an
example, test results for a PEMFC are shown.
[0038] FIGS. 5 and 6 illustrate embodiments of a non-stacked
MEMS-based miniature fuel cells, described as being either a solid
oxide fuel cell (SOFC), a solid polymer fuel cell (SPFC) or a
proton exchange membrane fuel cell (PEMFC), and where air is the
oxidant thereby eliminating the oxygen channel and upper substrate
16 of the FIG. 1 embodiment unless stacking of cells is utilized,
in which case the embodiment of FIG. 1 including the upper
substrate 16 have pores or channel inlets formed to allow the flow
of air to the top electrode would be required. The MEMS-based
miniature fuel cell of FIG. 5 utilizes electrode/catalyst/elec-
trolyte materials which enable the combination of a fuel and
oxidant at elevated temperatures to produce continuous electric
current. As pointed out above, the basis of each fuel cell is an
anode and cathode separated by an electrolyte layer which can
consist, in these embodiments, of either a solid oxide or a solid
polymer material, or proton exchange membrane electrolyte
materials. Additional layers of catalyst materials can also
separate the electrode (cathode or anode) from the electrolyte. In
these embodiments micromachined manifolds can be utilized to
transport the fuel and oxidant, to each cell and a provide a path
for exhaust gases. The electrical current generated from each cell
is drawn away with an interconnect and support structure integrated
with the gas manifold.
[0039] The MEMS-based fuel cell of FIG. 5 consists of three main
features. The first two are the substrate host structure on which
the fuel cell stack (electrode/electrolyte/electrode) is formed,
and in which the manifold structures are formed to deliver the fuel
and oxidant to the stack. The substrate can be made of silicon,
glass, ceramic, plastic, or any material having the above-described
qualities. By utilizing micromachining techniques, the substrate
host structure and manifolding for the fuel cell can be made light
weight. Furthermore, the manifold structures can be manufactured in
a variety of methods using MEMS techniques, including
micromachining, injection molding, laser drilling, hot embossing,
or diamond machining. As illustrated in FIG. 5, the substrate host
structure has two parts, one being a substrate host structure 61
with a plurality of channels or pores through it 73 which transport
the gas fuel to the fuel cell stack 65. As described previously,
the substrate host structure 61 can be a micromachined substrate
formed using the aforementioned techniques, or it can be a porous
substrate with pores suitable to allow gas to pass through, as well
as complete which can have the fuel cell stack materials deposited
on the top side. The second part is a manifold substrate 62 with
channels formed in it to transport fuel from one common reservoir,
distributing the fuel over the entire area of the channels or pores
73 which deliver fuel to the stack layers. These two parts can be
fabricated separately using the techniques cited above, then bonded
or mechanically sealed together to form the structure of FIG. 5. A
variety of bonding techniques, as used in the art are available,
including use of adhesive films or eutectics, or mechanical sealing
and packaging techniques so long as they are robust enough through
the temperature range of operation, which is in the 50-500.degree.
C. range, for example, depending on the electrolyte material
used.
[0040] Referring now to FIG. 5, the fuel cell stack generally
indicated at 60 comprises a micromachined host structure or
substrate 61, a micromachined substrate 62 bonded to substrate 61
at 63, a porous thick-film 64 located on the top surface of
substrate 61, and an electrode/electrolyte/electrode assembly or
fuel cell stack 65 deposited on the porous thick film or member 64.
The substrate 61 has a single or plurality of openings, pores or
windows defined by tapered or straight surfaces 66 and 67 which
approximately align with tapered or straight surfaces 68 and 69 of
a cutaway section 70 in substrate 62 which is connected by a
cutaway 71 to a fuel inlet 72, with the single or plurality of
openings, pores or windows formed by tapered or straight surfaces
66 and 67 and cutaway section 70 forming a micromachined manifold
or pore 73. The electrode/electrolyte/electrode assembly or fuel
cell stack 65 consists of a thin-film electrode 74, a thin or thick
film electrolyte/catalyst 75, and a thin-film electrode 76. The
porous film 64 includes numerous pores 77. While not shown the fuel
cell stack 65 would include a resistive heater as in FIG. 3
integrated within the porous thick film 64 or fuel cell stack
65.
[0041] By way of example, the substrates 61 and 62 are composed of
silicon; the porous thick-film 64 is composed of alumina, plastic
or porous silicon with a thickness of 10 microns to 200 microns and
with pores 77 having a diameter of 200 .ANG. to 5 microns and the
film 64 having mechanical strength to support the fuel cell stack
65; the electrode 74 being composed of copper, nickel, carbon,
palladium, or platinum with a thickness of 1000 .ANG. to 2 microns;
and the electrolyte/catalyst 75 being composed Ni/YSZ, Pt/Nafion,
or Pt/C/Nafion with a thickness of 1 micron to 50 microns and the
electrode 76 being composed of oxygen catalyzing conducting
material, such as Ag, Pt, lanthanum strontium manganate (LSM), or
carbon with a thickness of 1000 .ANG. to 2 microns. The
micromachined pore or manifold 73 may have a cross-sectional area
of 10 .mu.m by 10 .mu.m to 10 mm by 10 mm, with the fuel inlet 72
having a diameter of 25 .mu.m to 1 mm. While the above description
specifies examples of fuel cell stack materials known to those
familiar with the art, additional combinations can readily be
substituted as dictated by performance and compatibility with a
MEMS fabrication approach.
[0042] The host substrate or structure on which the fuel cell stack
is formed has the requirement of having many channels or pores
which will enable the fuel, whether it be in gas or liquid form, to
transport to the electrode structure. This host structure can be
created by selective etching of channels in a silicon substrate,
for example, which would result in a free standing fuel cell
membrane. This host structure could also be created by etching or
molding of the channel features in a variety of other materials as
described previously, and can be ceramic, glass, or plastic. This
host structure could also be made from a thick film porous material
which is bonded on top of the manifold substrate or the channel
host structure, as illustrated in FIG. 6. The requirements of the
thick film porous structure are that it have enough mechanical
strength that it will support the fuel cell stack that can be
created on it when positioned over manifold channels or pores of
suitable size (0.1-10 mm diameter). Additionally, the pore size
must be large enough to provide flow of fuel from the bottom
manifolds to the fuel cell stack, but the pores must be small
enough that the thin-films which are deposited on top to form the
electrode will not deposit down into the pores too deep, so as to
completely seal the pores. This issue is less important if a fuel
cell membrane electrode assembly in laminate form were bonded,
attached or mechanically sealed over the host structure rather than
deposited directly onto it. Several materials exist in sheet form
which already have the desired porosity characteristics. These
include glass, plastics, ceramics, or anodized alumina. In the case
of the anodized alumina, sheets can be formed which are
approximately 50 .mu.m thick which have pore sizes on the order of
0.2 .mu.m, with a small fraction of the structure having pores
which are only 0.2 .mu.m diameter. This would allow thin-film
deposition on the top side without depositing down into and sealing
into the pores, and can be accomplished using a variety of
deposition techniques, including sol-gel, or spin cast approaches.
Similar structures can be achieved using two porous ceramic
materials, or two porous plastic thick films, each having different
pore sizes. Again, these materials are readily available from
commercial suppliers, or fabricated using techniques reported in
the open literature.
[0043] As shown in FIG. 6, the fuel cell assembly or device,
generally indicated at 80 basically comprises a substrate/host
structure 81, a manifold substrate 82 bonded to substrate 81 at 83,
a porous thick-film layer 84 located on or within substrate 81, and
a thin-film fuel cell stack generally indicated at 85 located on
layer 84. The fuel cell stack 85 includes an electrode 86, an
electrolyte 87, an electrode 88, and a heater element, not shown,
but which may be constructed as in the FIG. 3 embodiment.
Alternatively, the heater element may be formed on top of the
substrate/host structure 81 or within the porous thick film layer
84, so long as it is electrically isolated from the electrodes 86
or 88. The substrate/host structure 81 includes a number of
vertical channels or pores 89 which are in open communication with
manifold channels 90 in substrate 82. The porous thick-film layer
84 includes numerous pores 91 which are in open communication with
channels 89 of substrate 81. If the porous thick film layer 84 is
mechanically strong enough and allows sufficient flow of fuel
through to the fuel cell stack 80, it can be directly bonded or
mechanically sealed to the manifold substrate 82 with pores 91 in
open communication with manifold channels 90. The substrate 82
includes an opening or passageway forming a fuel inlet not shown in
the FIG. 6, but similar to the fuel inlet 72 of FIG. 5, in open
communication with the plurality of manifold channels 90, which is
adapted to be connected to a fuel source 94 via connector 95. Note
that the substrate/host structure 81 is etched such that the porous
thick film 84 aligns with the pores 89 of the host structure 81
enabling fuel to flow to the fuel cell stack 80 and preventing fuel
from flow elsewhere. Additionally shown in FIG. 6 is a package 96
and seals 97 which enable the manifold substrate 82, host
structure/substrate 81 and porous thick film layer 84 supporting
fuel cell stack 80 to be assembled and mechanically sealed. If the
substrate 82 is positioned such that the pores 90 are in open
communication with the bottom portion of the package 96, the
package 96 may include an opening or passageway forming a fuel
inlet 92 which is adapted to be connected to a fuel source 94 via
connector 95.
[0044] The third main feature of the MEMS-based miniature fuel cell
is the thin film fuel cell stack itself. The series of thin film
materials, as illustrated in FIG. 5, for example, are an electrode,
a catalyst, an electrolyte, a catalyst, and an electrode. Depending
on the particular electrolyte the catalyst layers may not be
necessary. The thin film fuel cell stack can be formed by a variety
of thin-film deposition techniques including sputter or evaporative
techniques, sol-gel, spin cast, dip coating approaches. The
individual layer thicknesses may vary from 0.5-50 .mu.m. The
operating temperatures may be from room temperature to about
600.degree. C. or higher for solid oxide electrolytes. The stack
structure may also be formed using a sheet of the electrolyte
material which is then sandwiched between the electrode/catalyst
layers forming a membrane electrode assembly component or laminate
and attached, bonded or mechanically sealed on a MEMS manifold host
structures. The requirements of the electrodes are that they have
low resistance, continuous conductivity, and are porous enough for
the fuel and oxidant to diffuse through to the catalyst/electrolyte
interface where the electrochemical reaction occurs, and allow the
byproduct of the reaction to diffuse out of the electrode
structure. Electrode materials can be made porous through the
techniques, including rf-sputtering, electron beam deposition, or
sol-gel processes. The electrodes can also be made porous using
photolithographic techniques to etch pores into the metal film.
Additionally, the porous electrodes could be created using a porous
metallic sheet which could also be used as the porous manifold host
structure. The catalyst and electrolyte layers are dense continuous
films deposited by the techniques described previously. The
electrolyte can be either solid oxide, solid polymer, or proton
exchange type materials.
[0045] FIG. 7 graphically illustrates cell potential (V) and cell
power (mW) relative to cell current (mA).
[0046] FIG. 8 illustrates an embodiment wherein individual cells
can be directly stacked on one another. In FIG. 1, the fuel inlet
15 and the oxidant inlet 17 enter from the sides of the fuel cell
module 10, or in a manner in which the inlets 15 and 17 remain in
open communication with respective flow channels 29 and 30, the
individual cells 10 can be directly stacked on one another, as
shown in FIG. 8, with the cells indicated a 110, the fuel inlets
indicated at 115, and the oxidant inlets indicated at 117. Further,
if the location of the fuel and oxidant inlet channels of the cells
are offset, as shown in FIG. 8, such that they are easily
accessible then fuel can be delivered to the plurality of inlets
from a common fuel or oxidant reservoir. Additionally, substrates
14 and 16 of FIG. 1 can be the same substrate, as indicated at 114
in FIG. 8, having appropriate inlets 115 and 117 for fuel and
oxidant formed in top and bottom areas of the substrate 114 which
have open communication to the appropriate flow channels in order
to deliver the fuel to the required electrodes 25 and oxidant to
the electrode 27, as seen in FIG. 1. In the case of the oxidant
inlet 117, the oxidant or air flow is enabled by forming a channel
or pore using micromachining techniques as previously described, or
using a material having adequate porosity and mechanical strength
which enable ambient flow of air to the electrode. In this stacking
embodiment, the electrodes for each module are connected in series
so as to add the voltage produced by each individual module. This
stacking approach is relevant to all embodiments as described in
FIGS. 2, 3, 5 and 6.
[0047] An additional embodiment for stacking of MEMS-based fuel
cells is illustrated in FIG. 9. In this embodiment, the
substrate/host structure 211' having fuel cell
electrode/electrolyte stack 213 and single or plurality of
openings, channels or pores 220 is now bonded or mechanically
sealed as indicated at 219 to a similar substrate/host structure
211 which is positioned upside down as illustrated in FIG. 9 such
that a symmetrical structure is formed and a single fuel inlet 215
can now deliver fuel to double the effective fuel cell stack area.
The fuel inlet 215 is formed using a cutaway section in
communication with the flow channel openings, channels or pores
220. The flow channels and cutaway sections are formed using one or
more of the various micromachining techniques as previously
described. If additional stacking or packaging is required, another
substrate structure 216, similar to the substrate 16 of FIG. 1, is
positioned over each fuel cell electrode/electrolyte stack 213 and
bonded or mechanically sealed as indicated at 219' to the
substrate/host structure 211 and 211' similar to that shown in FIG.
1. This embodiment, as illustrated in FIG. 8, enables each of the
fuel cell stacks to have a cavity and oxygen inlet structure 217
formed in said substrate 216 as shown using micromachining
techniques. This additionally enables direct stacking of fuel cell
modules as previously described and illustrated in FIG. 8.
[0048] An integrated resistive heating element can be utilized to
efficiently heat the fuel cell stack without heating the entire
host structure and package. The resistive heating element is
attached to a rechargeable microbattery located within the package,
and is used to turn-on the fuel cell by uniformly heating the fuel
cell stack. Once the fuel cell is powered on, the resistive heater
draws its power from the operating fuel cell. The resistive heater
can be located within the fuel cell stack but will require
electrical isolation. The heating element can also be located
within the porous host structure and be designed such that the heat
is conducted to the fuel cell stack structure, and insulated from
the manifold substrate and exterior packaging.
[0049] It has thus been shown that the present invention advances
the state of the fuel cell art by miniature MEMS-based thin-film
fuel cells that will yield an order of magnitude greater power
density than any fuel cell currently available. Further the fuel
cells of this invention are small and compact and thus offers
improved performance over rechargeable batteries as a miniature
power source. Since the fuel cell electrolyte layer can be made
thinner, e.g. 0.5-10 .mu.m thick as compared to 1 mil, then the
operating temperature of room temperature to about 600.degree. C.
(for example, .about.25.degree.-550.degree. C.) can be reduced by
hundreds of degrees from the conventional operation temperatures of
>600.degree. C. for the case of solid oxide electrolytes which
can operate at about 675.degree. C., for example. The electrolyte
material may be a solid oxide, such as YSZ, ceria and ceria-based
compounds, or a solid polymer, or a proton exchange membrane. For
example, solid polymer or proton exchange membrane electrolytes
operate at a temperature range of 23-120.degree. C., and with a
thickness ranging from 0.5-50 .mu.m. Solid oxide electrolytes
operate at a temperature below 600.degree. C. with a preferred
thickness of about 0.5-7.5 .mu.m, but can have a thickness of
0.5-50 .mu.m. Also, the solid oxide electrolyte may operate at a
temperature above 600.degree. C. (675.degree. C.) with a thickness
of 0.5-7.5 .mu.m. Also, the MEMS-based fuel cells have microflow
channels and manifolding micromachined into the host
structure/substrate, utilize an integrated thin-film resistive
heater for efficient heating of the fuel cell stack, and contain a
rechargeable microbattery connected to the heater for startup
control. The fuel may be hydrogen (liquid or gaseous) or
hydrocarbon fuels, including methanol and propanol.
[0050] While particular embodiments have been illustrated and
described along with specific materials and parameters, to
exemplify and explain the principles of the invention, such are not
intended to be limiting. Modifications may become apparent to those
skilled in the art, and it is intended that the invention be
limited only be the scope of the appended claims.
* * * * *