U.S. patent application number 10/633413 was filed with the patent office on 2004-02-05 for method and apparatus for reducing reactant crossover in a liquid feed electrochemical fuel cell.
Invention is credited to Campbell, Stephen A., Colbow, Kevin M., Johnson, Mark C., Wilkinson, David P..
Application Number | 20040023091 10/633413 |
Document ID | / |
Family ID | 24295360 |
Filed Date | 2004-02-05 |
United States Patent
Application |
20040023091 |
Kind Code |
A1 |
Wilkinson, David P. ; et
al. |
February 5, 2004 |
Method and apparatus for reducing reactant crossover in a liquid
feed electrochemical fuel cell
Abstract
In an electrochemical fuel cell, a sufficient quantity of
catalyst, effective for promoting the reaction of reactant supplied
to an electrode, is disposed within the volume of the electrode so
that a reactant introduced at a first major surface of the
electrode is substantially completely reacted upon contacting the
second major surface. Crossover of reactant from one electrode to
the other electrode through the electrolyte in an electrochemical
fuel cell is thereby reduced.
Inventors: |
Wilkinson, David P.; (North
Vancouver, CA) ; Johnson, Mark C.; (Phoenix, AZ)
; Colbow, Kevin M.; (North Vancouver, CA) ;
Campbell, Stephen A.; (Coquitlam, CA) |
Correspondence
Address: |
Robert W. Fieseler
McAndrews, Held & Malloy, Ltd.
34th Floor
500 West Madison Street
Chicago
IL
60661
US
|
Family ID: |
24295360 |
Appl. No.: |
10/633413 |
Filed: |
August 1, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10633413 |
Aug 1, 2003 |
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09255428 |
Feb 22, 1999 |
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6613464 |
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09255428 |
Feb 22, 1999 |
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08939673 |
Sep 29, 1997 |
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5874182 |
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08939673 |
Sep 29, 1997 |
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08574262 |
Dec 18, 1995 |
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5672439 |
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Current U.S.
Class: |
429/482 ;
429/450; 429/530; 429/532 |
Current CPC
Class: |
H01M 4/8605 20130101;
H01M 8/0213 20130101; Y02E 60/50 20130101; H01M 8/1004 20130101;
H01M 2300/0082 20130101; H01M 8/1009 20130101; H01M 4/926 20130101;
H01M 4/8626 20130101; H01M 8/1007 20160201; H01M 4/96 20130101;
H01M 4/86 20130101 |
Class at
Publication: |
429/30 ;
429/44 |
International
Class: |
H01M 008/10; H01M
004/86; H01M 004/96 |
Claims
What is claimed is:
1. A liquid feed electrochemical fuel cell comprising: (a) a first
electrode having first and second oppositely facing major surfaces,
said first electrode comprising a porous sheet material having a
thickness and a quantity of catalyst distributed through the
thickness of the porous sheet material between said major surfaces,
and said first major surface has a first major surface
hydrophilicity, and said second major surface has a second major
surface hydrophilicity; (b) a second electrode; (c) an ion-exchange
membrane interposed between said second major surface of said first
electrode and said second electrode; wherein said first electrode
is fluidly connected to a source of liquid reactant and the first
surface hydrophilicity is greater than the second major surface
hydrophilicity.
2. A liquid feed electrochemical fuel cell according to claim 1
wherein said second major surface of said first electrode comprises
a hydrophobic polymer.
3. A liquid feed electrochemical fuel cell according to claim 1
wherein said first major surface of said first electrode comprises
a hydrophilic polymer.
4. A liquid feed electrochemical fuel cell comprising: (a) a first
electrode having first and second oppositely facing major surfaces
and a porous volume between said major surfaces, said first
electrode comprising a sufficient quantity of catalyst concentrated
at said first major surface and disposed between said major
surfaces within the volume of said first electrode so that a
reactant introduced to said first major surface of said first
electrode is substantially completely reacted upon contacting said
second major surface of said first electrode; (b) a second
electrode; (c) an ion-exchange membrane interposed between said
second major surface of said first electrode and said second
electrode, and said second major surface of said first electrode is
adjacent the ion-exchange membrane.
5. A liquid feed electrochemical fuel cell according to claim 4
wherein said first electrode comprises a porous electrically
conductive sheet material which defines said volume, and said sheet
material comprises carbon fiber paper.
6. A liquid feed electrochemical fuel cell comprising: (a) a first
electrode having first and second oppositely facing major surfaces,
said first electrode comprising a porous sheet material and a
quantity of catalyst concentrated at said first and second major
surfaces of said first electrode, said quantity of catalyst being
sufficient so that a reactant introduced to said first major
surface of said first electrode is substantially completely reacted
upon contacting said second major surface of said first electrode;
(b) a second electrode; (c) an ion-exchange membrane interposed
between said first electrode and said second electrode.
7. A liquid feed electrochemical fuel cell according to claim 6
wherein catalyst particles are impregnated into both major
surfaces.
8. A liquid feed electrochemical fuel cell comprising: (a) a first
electrode having first and second oppositely facing major surfaces,
said first electrode comprising at least one active layer, said at
least one active layer comprising a porous sheet material having a
volume and catalyst particles, and said first electrode further
comprising a plurality of inactive layers, and each of said at
least one active layer is disposed between two inactive layers; (b)
a second electrode; (c) an ion-exchange membrane interposed between
said first electrode and said second electrode.
9. The liquid feed fuel cell of claim 8, wherein said active layer
has a volume and the catalyst particles are disposed throughout the
volume of said active layer.
10. The liquid feed fuel cell of claim 8, wherein said first and
second major surfaces of said first electrode are defined by said
inactive layers.
11. A liquid feed electrochemical fuel cell comprising: (a) a first
electrode having first and second oppositely facing major surfaces,
said first electrode comprising a plurality of layers having
oppositely facing major planar surfaces, and each layer comprises
porous sheet material and catalyst particles disposed at both of
said major planar surfaces of each layer; (b) a second electrode;
(c) an ion-exchange membrane interposed between said first
electrode and said second electrode.
12. A liquid feed fuel cell according to claim 11, wherein said
first electrode comprises at least four of said layers.
13. A liquid feed electrochemical fuel cell comprising: (a) a first
electrode having first and second oppositely facing major surfaces,
said first electrode comprising a plurality of active layers, and
said active layers comprise catalyst particles and a porous sheet
material having a thickness, and said first electrode further
comprising at least one inactive layer, and said at least one
inactive layer is disposed between two active layers; (b) a second
electrode; (c) an ion-exchange membrane interposed between said
first electrode and said second electrode.
14. The liquid feed fuel cell of claim 13, wherein said first
electrode comprises at least three active layers.
15. The liquid feed fuel cell of claim 13, wherein said active
layers comprise carbon cloth filled with a matrix of said carbon
particles and polymeric binder.
16. The liquid feed fuel cell of claim 13, wherein said catalyst
particles are distributed throughout the thickness of said active
layers.
17. The liquid feed fuel cell of claim 13, wherein said first and
second major surfaces of said first electrode are defined by said
active layers.
18. The liquid feed fuel cell of claim 13, wherein said at least
one inactive layer has oppositely facing major planar surfaces and
has channels formed in said major planar surfaces.
19. The liquid feed fuel cell of claim 18, wherein said at least
one inactive layer comprises carbon fiber paper.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a division of U.S. patent application
Ser. No. 09/255,428 filed Feb. 22, 1999, entitled "Method and
Apparatus for Reducing Reactant Crossover in a Liquid Feed
Electrochemical Fuel Cell", which is a continuation of U.S. patent
application Ser. No. 08/939,673 filed Sep. 29, 1997, entitled
"Method and Apparatus for Reducing Reactant Crossover in a Liquid
Feed Electrochemical Fuel Cell", scheduled to issue as U.S. Pat.
No. 5,874,182 on Feb. 23, 1999, which is a continuation of U.S.
patent application Ser. No. 08/574,262 filed Dec. 18, 1995,
entitled "Method and Apparatus for Reducing Reactant Crossover in
an Electrochemical Fuel Cell", now U.S. Pat. No. 5,672,439 issued
Sep. 30, 1997.
FIELD OF THE INVENTION
[0002] The invention relates generally to electrochemical fuel
cells and, more particularly, to a fuel cell with an electrode
having catalyst disposed within the volume between its major
surfaces. A method and apparatus for reducing reactant crossover
from one electrode to the other in an electrochemical fuel cell is
provided.
BACKGROUND OF THE INVENTION
[0003] Electrochemical fuel cells convert fuel and oxidant to
electricity and reaction product. Fluid reactants are supplied to a
pair of electrodes which are in contact with and separated by an
electrolyte. The electrolyte may be a solid or a liquid (supported
liquid matrix). Solid polymer electrochemical fuel cells generally
employ a membrane electrode assembly comprising a solid ionomer or
ion-exchange membrane disposed between two planar electrodes. The
electrodes typically comprise an electrode substrate and an
electrocatalyst layer disposed upon one major surface of the
electrode substrate. The electrode substrate typically comprises a
sheet of porous, electrically conductive material, such as carbon
fiber paper or carbon cloth. The layer of electrocatalyst is
typically in the form of finely comminuted metal, typically
platinum, and is disposed on the surface of the electrode substrate
at the interface with the membrane electrolyte in order to induce
the desired electrochemical reaction. In a single cell, the
electrodes are electrically coupled to provide a path for
conducting electrons between the electrodes through an external
load.
[0004] At the anode, the fuel moves through the porous anode
substrate and is oxidized at the anode electrocatalyst layer. At
the cathode, the-oxidant moves through the porous cathode substrate
and is reduced at the cathode electrocatalyst layer.
[0005] Electrochemical fuel cells most commonly employ gaseous
fuels and oxidants, for example, those operating on molecular
hydrogen as the fuel and oxygen in air or a carrier gas (or
substantially pure oxygen) as the oxidant. The anode and cathode
reactions in hydrogen/oxygen fuel cells are shown in the following
equations:
[0006] Anode reaction: H.sub.2.fwdarw.2H.sup.++2e.sup.-1
[0007] Cathode reaction:
1/2O.sub.2+2H.sup.++2e.sup.-.fwdarw.H.sub.2O
[0008] The catalyzed reaction at the anode produces hydrogen
cations (protons) from the fuel supply. The ion-exchange membrane
facilitates the migration of protons from the anode to the cathode.
In addition to conducting protons, the membrane isolates the
hydrogen-containing gaseous fuel stream from the oxygen-containing
gaseous oxidant stream. At the cathode electrocatalyst layer,
oxygen reacts with the protons that have crossed the membrane to
form water as the reaction product.
[0009] In liquid feed electrochemical fuel cells, one or more of
the reactants is introduced to the electrocatalyst in the liquid
form. Examples of electrochemical fuel cells which can be operated
with a liquid fuel feed are those employing a lower alcohol, most
commonly methanol, as the fuel supplied to the anode (so-called
"direct methanol" fuel cells) and oxygen to the cathode. In fuel
cells of this type the reaction at the anode produces protons, as
in the hydrogen/oxygen fuel cell described above, however the
protons (along with carbon dioxide) arise from the oxidation of
methanol. An electrocatalyst promotes the methanol oxidation at the
anode. The methanol may alternatively be supplied to the anode as
vapor, but it is generally advantageous to supply the methanol to
the anode as a liquid, preferably as an aqueous solution. In some
situations, an acidic aqueous methanol solution is the preferred
feed to the anode. The anode and cathode reactions in a direct
methanol fuel cell are shown in the following equations:
[0010] Anode reaction:
CH.sub.3OH+H.sub.2O.fwdarw.6H.sup.++CO.sub.2+6e.sup- .-
[0011] Cathode reaction:
3/2O.sub.2+6H.sup.++6e.sup.-.fwdarw.3H.sub.2O
[0012] Overall reaction:
CH.sub.3OH+3/2O.sub.2.fwdarw.CO.sub.2+2H.sub.2O
[0013] The protons formed at the anode electrocatalyst migrate
through the ion-exchange membrane from the anode to the cathode,
and at the cathode electrocatalyst layer, the oxidant reacts with
the protons to form water.
[0014] Other non-alcohol fuels may be used in liquid feed fuel
cells, for example formic acid. The oxidant may also be supplied as
a liquid, for example, as an organic fluid with a high oxygen
concentration (see U.S. Pat. No. 5,185,218), or as a hydrogen
peroxide solution.
[0015] In electrochemical fuel cells employing liquid or solid
electrolytes and gaseous or liquid reactant streams, crossover of a
reactant from one electrode to the other is generally undesirable.
Reactant crossover may occur if the electrolyte is permeable to the
reactant, that is, some of a reactant introduced at a first
electrode of the fuel cell may pass through the electrolyte to the
second electrode, instead of reacting at the first electrode.
Reactant crossover typically causes a decrease in both reactant
utilization efficiency and fuel cell performance. Fuel cell
performance is defined as the voltage output from the cell at a
given current density or vice versa; the higher the voltage at a
given current density or the higher the current density at a given
voltage, the better the performance.
[0016] In solid polymer electrochemical fuel cells the ion-exchange
membrane may be permeable to one or more of the reactants. For
example, ion-exchange membranes typically employed in solid polymer
electrochemical fuel cells are permeable to methanol, thus methanol
which contacts the membrane prior to participating in the oxidation
reaction can cross over to the cathode side. Diffusion of methanol
fuel from the anode to the cathode leads to a reduction in fuel
utilization efficiency and to performance losses (see, for example,
S. Surampudi et al., Journal of Power Sources, vol. 47, 377-385
(1994) and C. Pu et al., Journal of the Electrochemical Society,
vol. 142, L119-120 (1995)).
[0017] Fuel utilization efficiency losses arise from methanol
diffusion away from the anode because some of the methanol which
would otherwise participate in the oxidation reaction at the anode
and supply electrons to do work through the external circuit is
lost. Methanol arriving at the cathode may be lost through
vaporization into the oxidant stream, or may be oxidized at the
cathode electrocatalyst, consuming oxidant, as follows:
CH.sub.3OH+3/2O.sub.2.fwdarw.CO.sub.2+2H.sub.2O
[0018] Methanol diffusion to the cathode may lead to a decrease in
fuel cell performance. The oxidation of methanol at the cathode
reduces the concentration of oxygen at the electrocatalyst and may
affect access of the oxidant to the electrocatalyst (mass transport
issues). Further, depending upon the nature of the cathode
electrocatalyst and the oxidant supply, the electrocatalyst may be
poisoned by methanol oxidation products, or sintered by the
methanol oxidation reaction.
[0019] The electrode structures presently used in direct methanol
solid polymer fuel cells were originally developed for
hydrogen/oxygen fuel cells. The anode electrocatalyst which
promotes the oxidation of methanol to produce protons is typically
provided as a thin layer adjacent to the ion-exchange membrane (see
U.S. Pat. Nos. 5,132,193 and 5,409,785 and European Patent
Publication No. 0090358). The anode electrocatalyst layer is
typically applied as a coating to one major surface of a sheet of
porous, electrically conductive sheet material or to one surface of
the ion-exchange membrane. This provides a limited reaction zone in
which the methanol can be oxidized before contacting the membrane
electrolyte. Thus, with this type of electrode, the methanol
concentration at the anode-electrolyte interface will typically be
high.
[0020] Reactant crossover may be substantially eliminated if a
reactant introduced to a first major surface of a fuel cell
electrode is substantially completely reacted on contacting the
second major surface of the electrode. In this case essentially no
unreacted reactant would be available to pass from the second
surface through the electrolyte to the other electrode. As
described herein, this may be accomplished by ensuring that the
reactant contacts sufficient catalyst so that it is substantially
completely reacted before it contacts the second surface of the
electrode.
[0021] It is therefore an object of the invention to provide an
electrochemical fuel cell in which crossover of a reactant from one
electrode to the other is reduced.
[0022] It is a further object of the invention to provide a solid
polymer electrochemical fuel cell in which a reactant is
substantially completely reacted before it contacts the membrane
electrolyte.
[0023] It is a still further object of the invention to provide a
direct methanol solid polymer fuel cell in which methanol crossover
from the anode to the cathode is reduced.
[0024] Another object of the invention is to provide a method for
reducing reactant crossover in an electrochemical fuel cell.
SUMMARY OF THE INVENTION
[0025] The above and other objects are achieved by an
electrochemical fuel cell in which an electrode has catalyst
disposed within the volume thereof. The electrochemical fuel cell
comprises:
[0026] (a) a first electrode, the first electrode having first and
second oppositely facing major surfaces, the first electrode
comprising at least one layer of porous material and a sufficient
quantity of catalyst disposed within the volume of the electrode
between the major surfaces so that a reactant in a fluid introduced
to the first major surface of the first electrode is substantially
completely reacted upon contacting the second major surface of the
first electrode;
[0027] (b) a second electrode;
[0028] (c) an electrolyte interposed between the second major
surface of the first electrode and the second electrode.
[0029] In a preferred aspect of an electrochemical fuel cell the
first and second oppositely facing major surfaces of the first
electrode are planar.
[0030] In one embodiment the catalyst is distributed substantially
uniformly within the volume between the first and second oppositely
facing major surfaces of the first electrode. In an alternative
embodiment the catalyst is distributed nonuniformly, such as, for
example, in discrete layers or regions. The at least one layer of
porous material may optionally comprise a plurality of stacked
layers, and may optionally further comprise carbon particles.
Suitable carbon particles include acetylene blacks, furnace blacks
and graphite particles.
[0031] The electrolyte may be a liquid or a solid. With a liquid
electrolyte a porous, electrically non-conductive separator is
typically employed between the two electrodes. In a preferred
embodiment the electrochemical fuel cell is a solid polymer fuel
cell and the electrolyte comprises an ion-exchange membrane. The at
least one layer of porous material is preferably electrically
conductive and in a further embodiment comprises a proton
conductor. Preferred porous materials comprise electrically
conductive sheet material such as carbon fiber paper or carbon
cloth. In an alternative aspect the at least one layer of porous
material comprises carbon particles and a polymeric binder.
[0032] The fluid in which the reactant is introduced may be a
liquid or a gas.
[0033] In a preferred embodiment of a liquid feed electrochemical
fuel cell, the first electrode is an anode and the reactant
comprises an alcohol, preferably methanol. In this case the
catalyst promotes the oxidation of methanol. The fluid in which the
methanol is introduced preferably comprises water and may
optionally further comprise acid. In a preferred embodiment of a
liquid feed direct methanol fuel cell the at least one layer of
porous material comprises a plurality of carbon fiber paper layers
each of the layers having two oppositely facing major surfaces,
wherein catalyst is disposed on at least one major surface of each
of the layers.
[0034] In any of the above embodiments of a solid polymer
electrochemical fuel cell, catalyst may also be applied to the
surface of the ion-exchange membrane.
[0035] The electrode has first and second oppositely facing major
surfaces and comprises:
[0036] (a) at least one layer of porous material;
[0037] (b) a sufficient quantity of catalyst disposed within the
volume of the electrode between the major surfaces so that a
reactant in a fluid introduced to the first major surface of the
electrode is substantially completely reacted upon contacting the
second major surface of the electrode.
[0038] In a preferred aspect of an electrode the first and second
oppositely facing major surfaces of the electrode are planar.
[0039] In the electrode, the porous material acts as a carrier for
the catalyst, and is preferably liquid and gas permeable, to allow
gas or liquid feed reactant to penetrate it and to allow gaseous
products to escape. The porous material may be electrically
non-conductive or preferably electrically conductive. The
electrode, as a whole, is electrically conductive, however
nonconductive porous material incorporating sufficient catalyst
and/or other electrically conductive material to render the
electrode electrically conductive may be used. The porous material
may, for example, be one or more layers of electrically conductive
particles, such as carbon particles, and a polymeric binder. The
preferred porous material includes a sheet material which is
self-supporting and has structural integrity, thus providing
structural support for the adjacent ion-exchange membrane in solid
polymer fuel cells. Suitable non-conductive sheet materials include
expanded polytetrafluoroethylene and glass fiber matting, which
preferably have electrically conductive particles, such as, for
example, carbon particles associated therewith. Suitable
electrically conductive sheet materials include carbon aerogel,
carbon foam, carbon sponge, expanded metals and reticulated metals.
Preferred sheet materials include carbon fiber paper and carbon
cloth. The electrode may optionally incorporate structures to
facilitate movement of gaseous products away from the electrode,
for example, channels, grooves and layers or regions of different
porosities.
[0040] In general, in electrochemical fuel cells, higher catalyst
loadings lead to improved performance. The electrode structures
described herein facilitate use of higher catalyst loadings.
[0041] In a method of substantially completely reacting a reactant
in a fluid within a first electrode of an electrochemical fuel
cell, the first electrode having first and second oppositely facing
major surfaces, the reactant introduced at the first major surface,
the first electrode comprising at least one layer of porous
material and a catalyst, and the fuel cell further comprising a
second electrode and an electrolyte interposed between the second
major surface of the first electrode and the second electrode, the
method comprises:
[0042] disposing a sufficient quantity of the catalyst within the
volume of the first electrode between the major surfaces thereof
such that the reactant is substantially completely reacted upon
contacting the second major surface.
BRIEF DESCRIPTION OF THE DRAWINGS
[0043] FIG. 1A is an exploded side view of a typical solid polymer
electrochemical fuel cell showing a conventional membrane electrode
assembly interposed between two separator plates, the separator
plates having reactant flow channels formed in the surfaces for
directing the reactants to the electrodes.
[0044] FIG. 1B is a side sectional view of a conventional (prior
art) solid polymer electrochemical fuel cell electrode having the
catalyst disposed in a single, discrete layer at the interface with
the membrane electrolyte.
[0045] FIG. 2 is a side sectional view of an electrode having
catalyst substantially uniformly distributed throughout the volume
of the electrode.
[0046] FIG. 3 is a side sectional view of an electrode having a
catalyst layer impregnated into, and disposed in the volume
underlying, the surface of the electrode facing away from the
membrane electrolyte.
[0047] FIG. 4 is a side sectional view of an electrode having a
catalyst layer impregnated into both surfaces of the electrode.
[0048] FIG. 5 is a side sectional view of an electrode comprising
two layers of porous electrically conductive material and an active
layer comprising catalyst interposed between them.
[0049] FIG. 6 is a side sectional view of a multilayer electrode
comprising four layers of porous electrically conductive material
and catalyst disposed at both surfaces of each layer.
[0050] FIG. 7 is a side sectional view of a multilayer electrode
comprising five layers of porous electrically conductive material
and catalyst disposed within alternate layers.
[0051] FIG. 8 is a side sectional view of a multilayer electrode
structured to facilitate escape of gaseous products, comprising
three layers of porous electrically conductive material.
[0052] FIG. 9 is a plot of cell voltage versus methanol utilization
for a fuel cell with the conventional anode (plot A) shown in FIG.
1B and with a multi-layer anode (plot B) shown in FIG. 6.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0053] FIG. 1A illustrates a typical solid polymer fuel cell 10.
Fuel cell 10 includes a membrane electrode assembly 12 consisting
of an ion-exchange membrane 14 interposed between two electrodes,
namely an anode 16 and a cathode 17. In conventional solid polymer
fuel cells, anode 16 and cathode 17 comprise a substrate of porous
electrically conductive sheet material, 18 and 19, respectively.
Each substrate has a thin layer, 20, 21, of electrocatalyst
disposed on one of the major surfaces at the interface with the
membrane 14. The membrane electrode assembly 12 is typically
interposed between anode flow field or separator plate 22 and
cathode flow field or separator plate 24. Anode separator plate 22
has at least one fuel flow channel 23 engraved, milled or molded in
its surface facing anode. Similarly, cathode separator plate 24 has
at least one oxidant flow channel 25 engraved, milled or molded in
its surface facing the cathode. When assembled against the
cooperating surfaces of electrodes 16 and 17, channels 23 and 25
form the reactant flow field passages for the fuel and oxidant
respectively.
[0054] FIG. 1B shows a conventional (prior art) electrode 30 of the
type typically used in solid polymer fuel cells. Electrode 30
comprises a sheet of porous, electrically conductive material 32,
typically carbon fiber paper or carbon cloth. The electrode 30 has
oppositely facing major planar surfaces 30a, 30b. In a conventional
solid polymer fuel cell, electrode surface 30b is adjacent to the
membrane electrolyte. A thin layer comprising electrocatalyst
particles 36 is disposed at electrode surface 30b.
[0055] FIG. 2 shows an electrode 40 comprising porous material 42.
The electrode 40 has oppositely facing major planar surfaces 40a,
40b. In a direct methanol fuel cell, electrode surface 40b is
adjacent to the membrane electrolyte. Catalyst particles 46,
effective for promoting the oxidation of methanol, are distributed
between the electrode surfaces 40a, 40b. The catalyst particles 46
may be distributed substantially uniformly throughout the volume
between the electrode surfaces 40a, 40b, as shown in FIG. 2, or may
be distributed nonuniformly, for example, in discrete layers or
regions. Sufficient catalyst is provided so that substantially all
of the methanol, which is introduced in a fluid to the electrode 40
at surface 40a is oxidized upon contacting surface 40b. The
thickness of the electrode 40 and the quantity of catalyst required
will depend for example on the rate of methanol supply to the
electrode 40, and the rate of fluid transport through the electrode
40. In one example of an electrode 40, porous material 42 is one or
more layers of carbon particles mixed with a polymeric binder, and
catalyst particles 46 are distributed throughout porous material
42. In another example of an electrode 40, porous material 42 is
glass fiber mat or expanded (porous) polytetrafluoroethylene and a
matrix of carbon particles and a polymeric binder which, along with
catalyst particles 46, is distributed throughout the thickness of
the mat. In preferred example of an electrode 40, porous material
42 is carbon cloth and a matrix of carbon particles and a polymeric
binder which, along with catalyst particles 46, is distributed
throughout the thickness of the carbon cloth.
[0056] FIG. 3 shows an electrode 50 comprising porous electrically
conductive material 52. The electrode 50 has oppositely facing
major planar surfaces 50a, 50b. In a direct methanol solid polymer
fuel cell, electrode surface 50b is adjacent the membrane
electrolyte. Catalyst particles 56, effective for promoting the
oxidation of methanol, are concentrated at electrode surface 50a.
Sufficient catalyst is provided so that substantially all of the
methanol, which is introduced in a fluid to the electrode 50 at
surface 50a, is oxidized upon contacting surface 50b. In an example
of an electrode 50, catalyst particles 56 are applied to and
impregnated into surface 50a of a porous electrically conductive
material sheet material such as carbon fiber paper.
[0057] FIG. 4 shows an electrode 60 comprising porous electrically
conductive material 62. The electrode 60 has oppositely facing
major planar surfaces 60a, 60b. In a direct methanol solid polymer
fuel cell electrode, surface 60b is adjacent to the membrane
electrolyte. Catalyst particles 66, effective for promoting the
oxidation of methanol, are concentrated at electrode surfaces 60a,
60b. Sufficient catalyst is provided so that substantially all of
the methanol, which is introduced in a fluid to electrode 60 at
surface 60a is oxidized upon contacting surface 60b. In an example
of an electrode 60, catalyst particles 66 are applied to and
impregnated into both surfaces 60a, 60b of porous electrically
conductive material 62.
[0058] FIG. 5 shows a multi-layer electrode 70 comprising two
layers of porous electrically conductive sheet material 72, 74. In
a direct methanol solid polymer fuel cell layer 74 is adjacent to
the membrane electrolyte. An active layer 78, comprising catalyst
particles 76 is interposed between layers 72, 74. The layers 72, 74
provide structural support for the catalyst-containing layer 78.
Sufficient catalyst is provided so that substantially all of the
methanol, introduced in a fluid at layer 72 is oxidized upon
contacting layer 74.
[0059] FIG. 6 shows a multi-layer electrode 80 comprising four
layers of porous electrically conductive sheet material 82, 83, 84,
85. The electrode 80 has oppositely facing major planar surfaces
80a, 80b. In a direct methanol solid polymer fuel cell electrode,
surface 80b is disposed adjacent the membrane electrolyte. Catalyst
particles 86 are disposed at both major planar surfaces of each
layer. Sufficient catalyst is provided so that substantially all of
the methanol, which is introduced in a liquid to the electrode 80
at surface 80a is oxidized upon contacting surface 80b.
[0060] FIG. 7 shows a multi-layer electrode 90 comprising five
stacked layers of porous electrically conductive sheet material
92a, 92b, 92c, 93a, 93b. The electrode 90 has oppositely facing
major planar surfaces 90a, 90b. In a direct methanol solid polymer
fuel cell electrode, surface 90b is disposed adjacent the membrane
electrolyte. Catalyst particles 96 are disposed in layers 92a, 92b
and 92c. Sufficient catalyst is provided so that substantially all
of the methanol, which is introduced in a fluid to the electrode 90
at surface 90a is oxidized upon contacting surface 90b. In an
example of an electrode 90 structured to facilitate escape of
gaseous carbon dioxide product, porous layers 92a, 92b, 92c are
carbon cloth filled with a matrix of carbon particles and a
polymeric binder, and catalyst particles 96 are distributed
throughout the thickness of the layers 92a, 92b, 92c. Porous layers
93a and 93b are carbon cloth which is not filled with a matrix and
catalyst particles, and are therefore more porous.
[0061] FIG. 8 shows a multi-layer electrode 100 structured to
facilitate escape of gaseous carbon dioxide product. Electrode 100
comprises three stacked layers of porous electrically conductive
sheet material 102, 103, 104, and has oppositely facing major
planar surfaces 10a, 10b. In a direct methanol solid polymer fuel
cell electrode, surface 100b is disposed adjacent the membrane
electrolyte. Catalyst particles 106 are disposed in layers 102 and
104. Sufficient catalyst is provided so that substantially all of
the methanol, which is introduced in a fluid to electrode 100 at
surface 100a is oxidized upon contacting surface 100b. Layer 103
has channels 103a formed in its major planar surfaces to facilitate
gas transport. In an example of an electrode 100, porous material
102 and 104 is carbon cloth filled with a matrix of carbon
particles and a polymeric binder, and catalyst particles 106 are
distributed throughout the thickness of layers 102, 104. Layer 103
is carbon fiber paper with channels 103a formed in the surfaces
thereof.
[0062] Additional materials, such as hydrophobic or hydrophilic
polymers, and particulate fillers, may optionally be incorporated
into the electrode, for example, to control gas and liquid
transport in the electrode.
[0063] In the direct methanol solid polymer fuel cells described
herein, protons are generated by oxidation of methanol at catalyst
sites which are remote from the anode-membrane interface. A
mechanism is provided for transporting protons from the catalyst
sites to the membrane electrolyte. In preferred embodiments, the
anode further comprises proton conductive material which provides a
path for transport of protons from the catalyst sites to the
membrane electrolyte. For example, proton conductive material may
be impregnated into the at least one porous layer of the anode or
may be applied in a mixture with the catalyst. The proton
conductive material may, for example, be an ionomer such as a
sulfonated fluoroionomer, for example Nafion.RTM.. Alternatively,
or in addition, the methanol may be supplied to the anode in
aqueous acidic solution, the aqueous acid thereby providing a path
for transport of protons from the catalyst sites to the membrane
electrolyte. Suitable acids include sulfuric acid and perchloric
acid.
[0064] Any catalyst which is effective for the oxidation of
methanol may be employed in the anode of a direct methanol fuel
cell. For example, the catalyst can be a metal black such as
platinum, a mixture of metals, an alloy, a catalyst mixed with
additives to promote electrocatalytic activity and/or inhibit
catalyst poisoning, or a supported catalyst such as, for example, a
noble metal on a carbon support.
EXAMPLE 1
Comparison of Methanol Utilization
[0065] (a) Preparation of a Conventional Anode
[0066] A single layer of carbon supported platinum ruthenium
catalyst (Pt/Ru/C: 20/10/70%) was applied to one surface of a sheet
of carbon fiber paper (14.times.14 cm, thickness 0.27 mm, Grade
CFP090) to give a platinum loading of 1.80 mg/cm.sup.2.
[0067] (b) Preparation of a Multi-layer Anode
[0068] Carbon supported platinum ruthenium catalyst (Pt/Ru/C:
20/10/70%) was applied to both surfaces of three sheets of carbon
fiber paper (14.times.14 cm, thickness 0.10 mm, Grade CFP030), and
the sheets were stacked together to give a multi-layer anode with
the same total platinum loading as in Example 1(a) above, that is
1.80 mg/cm.sup.2.
[0069] Each of the above anodes was tested in a liquid feed direct
methanol fuel cell employing Nafion.RTM. 117 as the ion-exchange
membrane (electrolyte) and a platinum black cathode (4 mg/cm.sup.2
platinum loading). In both cases the fuel cell was supplied with a
fixed amount of fuel which was recirculated past the anode. The
fixed amount of fuel was 250 mL of aqueous 2M methanol solution
with a 0.5M sulfuric acid concentration, and the operating
conditions were as follows:
[0070] current density--constant at 200 MA/cm.sup.2
[0071] temperature=115.degree. C.
[0072] air inlet pressure=35 psig
[0073] air stoichiometry=3
[0074] FIG. 9 is a plot of cell voltage versus methanol utilization
for a fuel cell with a conventional (plot A) and multi-layer anode
(plot B) prepared as described in Example 1. The quantity of
electricity which would be produced if all of the methanol were
used (with complete oxidation) can be calculated. The ratio of the
observed output (current.times.time) to the theoretical output of
the fuel cell is expressed as a percentage methanol utilization in
FIG. 9. For the conventional anode, the cell voltage drops sharply
when the utilization approaches 60%. This indicates an inadequate
concentration of methanol at the electrode. For the multi-layer
anode the voltage drop occurs closer to 80% utilization. The
primary reason for fuel utilization loss (that is, the fuel cell
producing less current than is theoretically possible) is methanol
diffusion to the cathode. Additional contributions to the loss may
be due to incomplete oxidation of methanol less than its
electrochemical equivalent of six electrons per methanol molecule.
The results indicate that the multi-layer anode improves fuel
utilization and reduces methanol diffusion to the cathode.
[0075] As used herein the term "substantially completely reacted"
indicates that the amount of reactant remaining unreacted is
insufficient to detrimentally affect fuel cell performance.
[0076] While particular elements, embodiments and applications of
the present invention have been shown and described, it will be
understood, of course, that the invention is not limited thereto
since modifications may be made by those skilled in the art,
particularly in light of the foregoing teachings. It is therefore
contemplated by the appended claims to cover such modifications as
incorporate those features which come within the spirit and scope
of the invention.
* * * * *