U.S. patent application number 10/667645 was filed with the patent office on 2004-03-25 for fluid diffusion layers for fuel cells.
Invention is credited to Beattie, Paul D., Chiem, Bien, Davis, Michael Todd, Erickson, Lynn C., Fong, Kelvin Keen-Ven, Gordon, John Robert, Kozak, Paul, Neumann, Sheilah, Wang, Haijiang, Wilkinson, David P., Wong-Cheung, Sonia Geillis.
Application Number | 20040058229 10/667645 |
Document ID | / |
Family ID | 27098525 |
Filed Date | 2004-03-25 |
United States Patent
Application |
20040058229 |
Kind Code |
A1 |
Beattie, Paul D. ; et
al. |
March 25, 2004 |
Fluid diffusion layers for fuel cells
Abstract
Fluid diffusion layers with favorable mechanical and electrical
properties are prepared for fuel cell electrodes by impregnating a
porous carbonaceous web with a carbonizable polymer having
pyrrolidone functionality and then carbonizing the pyrrolidone
polymer. The polymer having pyrrolidone functionality is stabilized
against vaporization by use of an oxidization step prior to
carbonization. The fluid diffusion layers are particularly suitable
for use as gas diffusion layers in solid polymer electrolyte fuel
cells.
Inventors: |
Beattie, Paul D.; (Port
Moody, CA) ; Wilkinson, David P.; (North Vancouver,
CA) ; Kozak, Paul; (Mission, CA) ; Wang,
Haijiang; (Burnaby, CA) ; Neumann, Sheilah;
(Vancouver, CA) ; Gordon, John Robert; (Vancouver,
CA) ; Fong, Kelvin Keen-Ven; (Burnaby, CA) ;
Wong-Cheung, Sonia Geillis; (Burnaby, CA) ; Davis,
Michael Todd; (Port Coquitlam, CA) ; Chiem, Bien;
(Burnaby, CA) ; Erickson, Lynn C.; (Vancouver,
CA) |
Correspondence
Address: |
Robert W. Fieseler
McAndrews, Held & Malloy, Ltd
34th Floor
500 West Madison Street
Chicago
IL
60661
US
|
Family ID: |
27098525 |
Appl. No.: |
10/667645 |
Filed: |
September 22, 2003 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
10667645 |
Sep 22, 2003 |
|
|
|
09951655 |
Sep 13, 2001 |
|
|
|
6667127 |
|
|
|
|
09951655 |
Sep 13, 2001 |
|
|
|
09662437 |
Sep 15, 2000 |
|
|
|
Current U.S.
Class: |
429/516 ;
429/532; 429/534 |
Current CPC
Class: |
H01M 8/0234 20130101;
H01M 8/0239 20130101; Y02E 60/50 20130101 |
Class at
Publication: |
429/044 ;
429/042 |
International
Class: |
H01M 004/96; H01M
004/94 |
Claims
What is claimed is:
1. A fluid diffusion layer suitable for use in a fuel cell
electrode, said fluid diffusion layer comprising a porous
carbonaceous web impregnated with a carbonization product of at
least one polymer having pyrrolidone functionality.
2. The fluid diffusion layer of claim 1 wherein said carbonization
product is carbonized polyvinylpyrrolidone.
3. The fluid diffusion layer of claim 1 wherein said fluid
diffusion layer is a gas diffusion layer.
4. An electrode suitable for use in a solid polymer electrolyte
fuel cell, said electrode comprising the fluid diffusion layer of
claim 1 and a catalyst.
5. The fluid diffusion layer of claim 1 wherein said porous
carbonaceous web is a non-woven carbon fiber mat comprising carbon
fibers and a binder.
6. The fluid diffusion layer of claim 5 wherein said binder is a
polymer having pyrrolidone functionality.
7. The fluid diffusion layer of claim 1 wherein said fluid
diffusion layer is greater than about 50% porous.
8. The fluid diffusion layer of claim 1 additionally comprising
carbon particles impregnated within said web.
9. The fluid diffusion layer of claim 8 wherein said carbon
particles comprise graphite.
10. The fluid diffusion layer of claim 1 wherein said fluid
diffusion layer has a Taber stiffness greater than 2 Taber
units.
11. The fluid diffusion layer of claim 1 wherein said fluid
diffusion layer has an electrical conductivity greater than about 1
(ohm-cm).sup.-1.
12. The fluid diffusion layer of claim 1 wherein said fluid
diffusion layer has a Gurley air permeability of less than about 20
seconds.
13. A fluid diffusion layer for use in a fuel cell electrode, the
fluid diffusion layer comprising a plurality of porous carbonaceous
webs impregnated with and bound together by a carbonization product
of a polymer having pyrrolidone functionality.
14. The fluid diffusion layer of claim 13 wherein the carbonization
product is uniformly distributed through the plurality of webs.
15. The fluid diffusion layer of claim 13 wherein the carbonization
product is mostly uniformly distributed throughout the plurality of
webs.
16. The fluid diffusion layer of claim 13 wherein the plurality of
webs form one or more interfaces between adjacent webs, and the
carbonization product is mostly disposed at the interfaces.
17. The fluid diffusion layer of claim 13 wherein the plurality of
webs comprises a first web and a second web, and the first web
comprises a different material than the second web.
18. The fluid diffusion layer of claim 13 wherein the plurality of
webs comprises a first web and a second web, and the first web has
a different structure than the second web.
19. The fluid diffusion layer of claim 1 wherein the fluid
diffusion layer further comprising a non-particulate carbon filler
bound to the porous carbonaceous web by the carbonization
product.
20. The fluid diffusion layer of claim 19 wherein the
non-particulate carbon filler comprises chopped carbon fibers.
21. A fluid diffusion layer for use in a fuel cell electrode, the
fluid diffusion layer comprising at least one porous carbonaceous
web impregnated with a carbonization product of at least one
polymer having pyrrolidone functionality, and the fluid diffusion
layer defines at least one fluid distribution channel.
22. The fluid diffusion layer of claim 21 wherein the fluid
diffusion layer comprises a plurality of porous carbonaceous webs
impregnated with and bound together by the carbonization
product.
23. The fluid diffusion layer of claim 21 additionally comprising
non-particulate carbon filler bound to the web by the carbonization
product.
24. The fluid diffusion layer of claim 23 wherein the
non-particulate carbon filler comprises chopped carbon fibers.
Description
CROSS-REFERENCE TO RELATED APPLICATION(S)
[0001] This application is a divisional of U.S. patent application
Ser. No. 09/951,655 filed Sep. 13, 2001, now U.S. Pat. No. 6,______
issued ______ , entitled "Fluid Diffusion Layers For Fuel Cells".
The '655 application is, in turn, a continuation-in-part of U.S.
patent application Ser. No. 09/662,437 filed Sep. 15, 2000, now
abandoned, entitled "Fluid Diffusion Layers For Fuel Cells". Each
of the '655 and '437 applications is hereby incorporated by
reference herein in its entirety.
FIELD OF THE INVENTION
[0002] The present invention relates to fluid diffusion layers for
fuel cells and in particular to gas diffusion layers for solid
polymer electrolyte fuel cells. Further, it relates to the use of
carbonized polymers containing pyrrolidone groups in the
manufacture of fluid diffusion layers.
BACKGROUND OF THE INVENTION
[0003] Fuel cells convert reactants, namely fuel and oxidants, to
generate electric power and reaction products. Fuel cells generally
employ an electrolyte disposed between two electrodes, namely a
cathode and an anode. A catalyst typically induces the desired
electrochemical reactions at the electrodes. Preferred fuel cell
types include solid polymer electrolyte fuel cells that comprise a
solid polymer electrolyte and operate at relatively low
temperatures.
[0004] During normal operation of a solid polymer electrolyte fuel
cell, fuel is electrochemically oxidized at the anode catalyst,
typically resulting in the generation of protons, electrons, and
possibly other species depending on the fuel employed. The protons
are conducted from the reaction sites at which they are generated,
through the electrolyte, to electrochemically react with the
oxidant at the cathode catalyst. The catalysts are preferably
located at the interfaces between each electrode and the adjacent
electrolyte.
[0005] A broad range of fluid reactants can be used in solid
polymer electrolyte fuel cells and may be supplied in either
gaseous or liquid form. For example, the oxidant stream may be
substantially pure oxygen gas or a dilute oxygen stream such as
air. The fuel may be, for example, substantially pure hydrogen gas,
a gaseous hydrogen-containing reformate stream, or an aqueous
liquid methanol mixture in a direct methanol fuel cell. Reactants
are directed to the fuel cell electrodes and are distributed to
catalyst therein through fluid diffusion layers. In the case of
gaseous reactants, these layers are referred to as gas diffusion
layers.
[0006] Solid polymer electrolyte fuel cells employ a membrane
electrode assembly ("MEA") which comprises the solid polymer
electrolyte or ion-exchange membrane disposed between the two
electrodes. Each electrode comprises an appropriate catalyst,
preferably located next to the solid polymer electrolyte. The
catalyst may, for example, be a metal black, an alloy or a
supported metal catalyst such as platinum on carbon. The catalyst
may be disposed in a catalyst layer, and a catalyst layer typically
contains ionomer, which may be similar to that used for the solid
polymer electrolyte (for example, Nafion.RTM.). The catalyst layer
may also contain a binder, such as polytetrafluoroethylene. The
electrode may also contain a substrate (typically a porous,
electrically conductive sheet material) that may be employed for
purposes of mechanical support and/or reactant distribution, thus
serving as a fluid diffusion layer.
[0007] The MEA is typically disposed between two plates to form a
fuel cell assembly. The plates act as current collectors and
provide support for the adjacent electrodes. The fuel cell assembly
is typically compressed to ensure good electrical contact between
the plates and the electrodes, in addition to good sealing between
fuel cell components. A plurality of fuel cell assemblies may be
combined in series or in parallel to form a fuel cell stack. In a
fuel cell stack, a plate may be shared between two adjacent fuel
cell assemblies, in which case the plate also serves as a separator
to fluidly isolate the fluid streams of the two adjacent fuel cell
assemblies.
[0008] Flow fields are employed for the purpose of directing
reactants across the surfaces of the fluid diffusion electrodes or
electrode substrates. Flow fields are disposed on each side of the
MEA and comprise fluid distribution channels. The channels provide
passages for the distribution of reactants to the electrode
surfaces and also for the removal of reaction products and depleted
reactant streams. The flow fields may be incorporated in the
current collector/support plates on either side of the MEA (in
which case the plates are known as flow field plates) or,
alternatively, may be integrated into the fluid distribution layers
of the electrodes.
[0009] The fluid distribution layers in such fuel cells may
therefore have several functions, typically including: to provide
access of the fluid reactants to the catalyst, to provide a pathway
for removal of fluid reaction products, to serve as an electronic
conductor between the catalyst layer and an adjacent flow field
plate, to serve as a thermal conductor between the catalyst layer
and an adjacent flow field plate, to provide mechanical support for
the catalyst layer, and to provide mechanical support and
dimensional stability for the ion-exchange membrane.
[0010] Preferably, the fluid distribution layers are thin,
lightweight, inexpensive, and readily prepared using mass
production techniques (for example, reel-to-reel processing
techniques). Materials which have been employed in fluid
distribution layers for solid polymer electrolyte fuel cells
include commercially available carbon fiber paper and woven and/or
non-woven carbon fabrics. However, the mechanical and/or electrical
properties of these materials alone may not be adequate to meet all
the requirements for fuel cell applications.
[0011] Consequently, appropriate fillers and/or coatings have been
employed in the art to improve one or more of these properties. For
instance, the electrical conductivity of a carbonaceous web might
be increased by filling with an electrically conductive filler such
as graphite particles plus a binder. (Carbonaceous in this context
simply means containing carbon.) Alternatively, the stiffness of a
carbonaceous web might be increased by impregnating the web with a
suitable amount of curable polymer and then curing the polymer.
Further, both stiffness and conductivity might be increased by
impregnating the web with a carbonizable polymer, followed by
carbonization of the polymer-impregnated web in an inert
atmosphere. "Carbonization" is defined herein as increasing the
proportion of carbon by heating to temperatures of 600.degree. C.
or greater in a nonoxidizing environment. During carbonization,
carbon proportion increases as hydrogen, oxygen and nitrogen are
evolved. The carbonization product remaining after carbonization
can provide both mechanical support and additional electrical
pathways throughout the web. Carbonization product means the
reaction product after carbonization. Specific examples of webs
filled with a binder include the thin, highly porous, non-woven
carbon fiber web products of Technical Fibre Products Ltd., which
typically comprise non-woven carbon fiber webs bound with a
styrene-acrylic binder. While offering many desirable features,
such products may not be sufficiently stiff for handling purposes
nor of sufficient electrical conductivity for desired fuel cell
performance. The styrene-acrylic binder present in the web is
neither conductive nor carbonizable. However, both stiffness and
conductivity of such webs may be suitably improved by impregnating
with a carbon particle filler and a phenol-formaldehyde resin. The
resin is then cured and carbonized leaving behind a substantial
amount of carbonization product and resulting in a stiffer, more
conductive web.
[0012] Phenol-formaldehyde resins have been commonly used in many
impregnation applications. The phenol-formaldehyde resin is
typically diluted in a carrier solvent and then used as an
impregnant for fluid diffusion layers. However, a disadvantage of
phenol-formaldehyde resins is that the uncured resin and
by-products during curing are toxic as are certain preferred
carrier solvents. Thus appropriate precautions must be taken when
using these materials. Water can be used as a carrier solvent to
some extent. However, in situations where one wishes to also add a
conductive filler to the fluid diffusion layer, the preferable
method of application is by applying an ink to the web, where the
ink comprises the conductive filler, the resin, and the carrier
solvent. Such an ink usually requires a relatively large amount of
carrier solvent. If water is used as the carrier solvent in an ink,
the water is present in an amount large enough to cause the resin
to precipitate out of the ink. Thus, when using a water carrier
solvent for a phenol-formaldehyde resin, a two step impregnation
process is generally employed. An ink comprising the conductive
filler is used in a first impregnation step and then a water/resin
ink is added in a second step.
[0013] U.S. Pat. No. 6,037,073 employs a phenolic resin in its
preparation of a combination bipolar plate/diffuser component. The
combination component is prepared by making and screening an
aqueous slurry mixture of carbon fibers (such as chopped or milled
carbon fibers of various lengths) and about 20 wt % to about 50 wt
% phenolic resin powder binder to produce a wet monolithic, which
is subsequently dried at less than 80.degree. C. The dried monolith
is further densified and resin-cured via mechanical compression in
shaped graphite molds at about 120 to about 160.degree. C., and
carbonized at about 700 to about 1300.degree. C. in an inert
environment. A hermetic region on one side of the fluid diffusion
layer is then achieved via conventional masking and chemical vapor
infiltration (CVI) techniques. The porous region defines at least
portions of reactant channels and the hermetic region contains
coolant channels and prevents transport of fuel or oxidant to the
wrong electrode of the fuel cell. U.S. Pat. No. 6,037,073 is
incorporated by reference herein in its entirety.
[0014] Other carbonizable polymers may be considered for use in
preparing fluid diffusion layers in a like manner to
phenol-formaldehyde resins. For instance, polyacrylonitrile (PAN)
polymers may seem to be a suitable choice since carbonizing
processes are commonly used to make carbon products, such as carbon
fiber webs from PAN polymers. In these processes, PAN is
oxidatively stabilized prior to carbonization in order to obtain a
substantial yield from the carbonizing. However, the PAN residue is
extremely brittle and thus is not preferred for fluid diffusion
layer applications. Additionally, toxic organic solvents are
generally needed as carrier solvents.
[0015] Polymers having relatively low yields following conventional
carbonization would typically be considered unsuitable for purposes
of preparing fluid diffusion layers in this way. Pyrrolidone
polymers, for instance, have typically provided relatively low
yields following conventional carbonization.
SUMMARY OF THE INVENTION
[0016] An improved fluid diffusion layer for a fuel cell electrode
comprises a porous carbonaceous web impregnated with a
carbonization product of at least one polymer having pyrrolidone
functionality. An improved fluid diffusion layer may comprise a
plurality of porous carbonaceous webs impregnated with and bound
together by a carbonization product of at least one polymer having
pyrrolidone functionality. Pyrrolidone functionality is defined and
discussed below. In the present context, "impregnated" means
contained within, and the impregnated fluid diffusion layer does
not require that all pores or voids are completely filled; in fact,
it is specifically contemplated herein that the present fluid
diffusion layers are impregnated but may still have substantial
porosity. For example, the fluid diffusion layer preferably is at
least about 50% porous. In the present context, the web impregnated
with a carbonization product will preferably be made by
impregnating the web with a carbonizable polymer having pyrrolidone
functionality, followed by carbonization.
[0017] It has been found that polymers having pyrrolidone
functionality are preferred carbonizable impregnants in the
preparation of fluid diffusion layers for fuel cell electrodes.
Similar to phenol-formaldehyde resins, carbonizable impregnants
having pyrrolidone functionality can significantly improve the
mechanical and electrical properties of certain porous carbonaceous
webs for fuel cell applications. However, carbonizable impregnants
having pyrrolidone functionality offer advantages in that a
sufficient amount can be impregnated using water as a carrier
solvent in a single impregnation step. The yield of carbonization
product obtained from the impregnant having pyrrolidone
functionality may be significantly increased by oxidizing the
impregnant having pyrrolidone functionality prior to carbonization.
Further, neither of the impregnant having pyrrolidone functionality
or the water carrier solvent pose toxicity problems during
impregnation.
[0018] The present method includes making a fluid diffusion layer
for a fuel cell electrode that includes impregnating a porous
carbonaceous web with a polymer and carbonizing the polymer wherein
the polymer has pyrrolidone functionality.
[0019] An improved fluid diffusion layer can comprise a porous
carbonaceous web, a non-particulate carbon filler, and a
carbonization product of at least one polymer having pyrrolidone
functionality, wherein the carbonization product binds the
non-particulate carbon filler to the web. An improved fluid
diffusion layer can comprise at least one porous carbonaceous web
impregnated with a carbonization product of at least one polymer
having pyrrolidone functionality, and the fluid diffusion layer
defines at least one fluid distribution channel.
[0020] The present method also includes making a fluid diffusion
layer by (a) coating a porous carbonaceous web with a mixture
comprising a carbonizable polymer having pyrrolidone functionality;
(b) disposing a non-particulate carbon filler in the coating of
carbonizable polymer; and (c) carbonizing the carbonizable polymer
so that the carbonization product binds the non-particulate carbon
filler to the web.
[0021] As yet another aspect, a method of making a fluid diffusion
layer defining a flow field is provided. The method comprises the
steps of (a) impregnating at least one porous carbonaceous web with
a polymer having pyrrolidone functionality, (b) carbonizing the
polymer having pyrrolidone functionality to form a carbonization
product, thereby forming a fluid diffusion layer having first and
second major surfaces; and (c) forming at least one fluid
distribution channel on the first major surface of the fluid
diffusion layer.
[0022] Polymers having pyrrolidone functionality are derived from
monomers containing a pyrrolidone functional group. Such monomers
are represented by the following general chemical formula: 1
[0023] where X represents chemical groups suitable for
polymerization, such as for example, alkenyl groups. For example, X
is a vinyl group in N-vinyl-2-pyrrolidone monomer.
[0024] In principle, the web may be impregnated with a monomer
having pyrrolidone functionality that is polymerized in the web
thereafter. A preferred polymer is polyvinylpyrrolidone, although
other polymers (or suitable monomers) with pyrrolidone functional
groups may be employed.
[0025] Polymers having pyrrolidone functionality vary widely in
structure and include homopolymers of N-vinyl-2-pyrrolidone, and
copolymers formed by polymerizing two or more polymerizable
monomers, at least one of which provides pyrrolidone functionality.
The term "pyrrolidone functionality" means the presence of one or
more pendant pyrrolidone rings.
[0026] The polymer having pyrrolidone functionality can be a
homopolymer of N-vinyl-2-pyrrolidone or a copolymer of
N-vinyl-2-pyrrolidone with one or more ethylenically-unsaturated
copolymerizable monomers. The pendant pyrrolidone rings of the
polymer can be substituted or unsubstituted. For example, it may be
advisable to employ pyrrolidone rings substituted with alkyl,
alkenyl, or other groups.
[0027] In the method, the polymer may be dissolved in a solvent and
then applied in solution to the carbonaceous web. For environmental
and other reasons, a preferred solvent is water.
[0028] The impregnated polymer is preferably stabilized before
carbonizing. The stabilization step may increase the carbon yield
after carbonization. This stabilization may be accomplished by
heating the polymer in an oxidizing atmosphere at a temperature
below about 420.degree. C. before carbonizing. Then, the
carbonizing may be performed in an inert atmosphere at a
temperature above about 600.degree. C.
[0029] The method is suitable for preparing fluid diffusion layers
for various fuel cell embodiments operating either on gaseous or
liquid reactants. However, the method is particularly suitable for
preparing gas diffusion layers for use in solid polymer electrolyte
fuel cell electrodes.
[0030] In the preparation of a fuel cell electrode from the fluid
diffusion layer, an appropriate catalyst may be applied to the
fluid diffusion layer either before or after the carbonizing step.
Further, catalyst may be applied in a mixture which also comprises
the carbonizable polymer having pyrrolidone functionality.
[0031] The carbonaceous webs employed in the present method are
preferably non-woven carbon fiber mats comprising carbon fibers and
a binder. The binder may be any suitable material including
styrene-acrylic or even the carbonizable polymer having pyrrolidone
functionality.
[0032] The method is particularly suitable for preparing acceptable
fuel cell fluid diffusion layers using lightweight carbonaceous
webs that are greater than about 80% porous, less than about 250
micrometers thick, and have a weight per unit area less than about
100 g/m.sup.2 (although materials with higher weights per unit area
may also be suitable).
[0033] It may be advantageous to effect additional improvements
(for example, improved electrical conductivity) by additionally
incorporating carbon particles or non-particulate carbon filler
(such as chopped carbon fibers) into the impregnated web. Suitable
carbon particles include graphite particles, and they may be
incorporated in a fill mixture comprising carbon particles and the
carbonizable polymer having pyrrolidone functionality.
Alternatively, it may be advantageous to incorporate
non-particulate carbon filler as part of the fluid diffusion layer.
Such a filler need not "fill" the porous carbonaceous web but may
be disposed on a surface of such a web. The non-particulate carbon
filler can be chopped carbon fibers. A fill mixture may comprise
non-particulate carbon filler and/or carbon particles and a
carbonizable polymer having pyrrolidone functionality. The fill
mixture may also comprise a suitable pore former, such as methyl
cellulose.
[0034] Through such methods, lightweight carbonaceous webs may be
stiffened such that their Taber stiffness is greater than about 2
Taber units in the processing direction, or machine direction, of
the porous carbonaceous web. Further, the electrical conductivity
of such webs may be increased to be greater than about 1
(ohm-cm).sup.-1. The Gurley air permeability of the web however may
be kept below about 20 seconds.
[0035] The present methods are suitable for preparing fluid
diffusion layers for various fuel cell embodiments operating either
on gaseous or liquid reactants. However, the methods are
particularly suitable for preparing gas diffusion layers for use in
solid polymer electrolyte fuel cell electrodes.
[0036] A method is also provided for preparing a fluid diffusion
layer that defines one or more fluid distribution channels. For
instance, a fluid diffusion layer can be made in accordance with
the present techniques to be suitably thick to facilitate the
formation of fluid distribution channels by embossing or other
suitable means.
BRIEF DESCRIPTION OF THE DRAWINGS
[0037] FIG. 1 is a schematic view of an electrode comprising a
fluid diffusion layer for a solid polymer electrolyte fuel
cell.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENT(S)
[0038] FIG. 1 illustrates an electrode 1 for a typical solid
polymer electrolyte fuel cell that has been prepared using the
present method. Electrode 1 comprises catalyst layer 2 and fluid
diffusion layer 3. While FIG. 1 shows the catalyst layer 2 and the
fluid diffusion layer 3 as distinct layers, they may also overlap
to various extents. In this embodiment, catalyst layer 2 comprises
carbon-supported catalyst particles 4 along with ionomer 5 and
polytetrafluoroethylene (PTFE) binder 6, both of which are
dispersed around catalyst particles 4. The use of binder 6 and/or
ionomer 5 is optional. Furthermore, an ionomer 5 and a binder 6
need not be used simultaneously. Fluid diffusion layer 3 comprises
non-woven carbon fiber mat 7, graphite particle filler 8, and
carbonized polyvinylpyrrolidone (PVP) 9, which is dispersed around
fiber mat 7 and graphite filler 8. Fluid diffusion layer 3 may
optionally include ionomer (not shown) and/or PTFE binder (not
shown). Further, electrode 1 may optionally include a carbon-based
sublayer (not shown) between catalyst layer 2 and fluid diffusion
layer 3. Such a sublayer may also contain a carbon (for example,
graphite or carbon black), ionomer, and/or PTFE. The techniques for
applying such sublayers are known in the art and may involve the
use of pore forming agents (for example, methyl cellulose).
[0039] The fluid diffusion layer is highly porous so that fluid
reactant can readily pass through the fluid diffusion layer and
access the catalyst layer and so that by-products can be readily
removed. Preferably, the fluid diffusion layer is greater than
about 50% porous and shows a Gurley air permeability of about 20
seconds or less (as measured by a Gurley densitometer in which 300
cm.sup.2 of air is passed through a layer 2 area of 0.1 in (0.65
cm.sup.2) using a 5 ounce (1.39 Joules/meter) force).
[0040] To keep ohmic losses within the fuel cell at an acceptable
level, the through-plane resistance of the fluid diffusion layer is
preferably less than about 20 m.OMEGA. per cm.sup.2 under the
mechanical load within a solid polymer electrolyte fuel cell stack
(approximately 85 psi (586 kPa)). For layers of the order of 200
micrometers in thickness, this corresponds to layer conductivities
preferably more than about 1 (ohm-cm).sup.-1.
[0041] The fluid diffusion layer is also preferably fabricated
using a continuous process. The fluid diffusion layer preferably
has a stiffness of more than 2 Taber units, and preferably more
than 8 Taber units, for dimensional and mechanical stability.
(Taber measurements are commonly employed in the paper industry and
are indicative of a web's resistance to bending. Herein, the Taber
units are determined according to standard test method ASTM
D5342-95.)
[0042] Porous non-woven carbon fiber mat 7 can be conventionally
obtained in rolls of continuous web (for example, the
aforementioned Technical Fibre Products Ltd. carbon mats) and may
be used as a porous carbonaceous web. Such mats may have the
desired dimensions, porosity, and chemical resistance needed for
use in fuel cell electrodes, but typically do not have sufficient
stiffness or electrical conductivity. Several of these
characteristics can however be improved by adding conductive filler
(particulate or non-particulate carbon) and by coating the mat(s)
with a carbonizable polymer having pyrrolidone functionality
followed by carbonization, without unduly affecting air
permeability.
[0043] In FIG. 1, filler 8 and PVP are applied to mat 7 using a
conventional coating method (for example, a knife coater) and using
an aqueous mixture that includes filler 8 and an aqueous solution
of PVP. The mixture may optionally contain a pore former (for
example, methyl cellulose) that vaporizes when heated for purposes
of controlling the pore structure to some extent during the
subsequent heat treatments. The amounts of filler 8 and PVP in the
mixture and the amount of coating applied to mat 7 are adjusted in
accordance with the desired characteristics of mat 7 and with the
desired mixture properties for the given coating method. Typically,
a dry fill weight in the range of approximately 1-15 mg/cm.sup.2 of
graphite filler and PVP is added to mat 7. In general, a high
molecular weight PVP is used in order to increase the concentration
of carbon in the mixture and thereby reduce the loss of material
during oxidation and carbonization. Typically, a sufficient amount
of PVP can be solvated in the mixture such that both the desired
amount of PVP and the desired amount of filler can be incorporated
into mat 7 at the same time. Graphite is a preferred choice of
filler 8 since graphitic carbon has a relatively high conductivity.
An appropriate graphite particle size is selected such that pores
in mat 7 are not unduly blocked yet such that the particles remain
supported to some extent in mat 7.
[0044] After coating, the carrier solvent is dried off. The coated
mat 7 is then preferably subjected to an oxidation treatment to
stabilize the coated mat containing PVP and thus maintain a higher
carbon yield following carbonization. A typical oxidation treatment
involves baking the PVP for about an hour in air in the range of
200.degree. C. to 300.degree. C. Alternatively, other equivalent
oxidation treatments may be performed instead while still achieving
effective stabilization of the PVP polymer. For instance,
significantly shorter treatment times might be used with higher
temperatures (for example, above 420.degree. C.), or other means of
oxidation might be used. Finally, the stabilized PVP in mat 7 is
carbonized by heating to temperatures of about 600.degree. C. or
greater, preferably about 1000.degree. C. or greater, for up to
approximately 1 hour, thereby completing the preparation of fluid
diffusion layer 3. Generally, fluid diffusion layer 3 has an area
weight of less than 100 g/m.sup.2. During oxidation and subsequent
carbonization, a certain portion of the impregnated material is
removed. Without a suitable oxidizing step to stabilize the PVP,
generally most of the impregnated polymer would be volatilized
during carbonization.
[0045] Those skilled in the art will appreciate that various
properties of fluid diffusion layer 3 (including pore structure,
wettability, and other mechanical or electrical properties) can be
controlled to a certain extent by varying the type of carbon mat 7
and/or by varying the amounts of filler 8 and polymer having
pyrrolidone functionality. Additionally, the properties of fluid
diffusion layer 3 may be varied somewhat in the thickness direction
by applying multiple applications of filler and PVP onto mat 7
using mixtures of varied composition.
[0046] Those skilled in art will also appreciate that the various
properties of fluid diffusion layer 3 (including pore structure,
wettability, and other mechanical or electrical properties) can be
controlled by varying the type of porous carbonaceous webs
employed.
[0047] Under some circumstances, it may be preferable to employ a
fluid diffusion layer comprising a plurality of carbonaceous webs.
As shown herein, a thicker and more porous fluid diffusion layer
with better mechanical properties can be obtained using at least
two carbon fiber mats impregnated with and bound together by the
carbonization product of a polymer containing pyrrolidone
functionality. For instance, fluid diffusion layers for use in a
fuel cell electrode can be made which include at least two carbon
fiber mats impregnated with and bound together by the carbonization
product of a pyrrolidone polymer.
[0048] One embodiment of the process involves applying a coating of
a carbonizable polymer having pyrrolidone functionality, with or
without electrically conductive fillers or other additives present
in the coating, between two carbon fiber mats, compacting the
carbon fiber mats together, removing any release films (before or
after drying), and carbonizing the polymer impregnated in the mats
to produce a fluid diffusion layer for a fuel cell electrode. For
example, one carbon fiber mat is coated with an ink coating
comprising PVP, electrically conductive filler, and a pore former,
and another carbon fiber mat is placed on top of the ink coating,
then the two mats are compacted to "sandwich" the ink coating
therebetween. Thereafter, the "sandwich" is subjected to a
temperature above about 600.degree. C. in an inert atmosphere so
that the PVP is carbonized. Preferably, the PVP is stabilized
before carbonization.
[0049] In the resultant fluid diffusion layer, the two carbon fiber
mats are bound together by the carbonization product. The
carbonization product may be uniformly distributed through both
mats or may be mostly uniformly distributed in the mats. One
skilled in the art will also appreciate that the carbonization
product may be distributed in an uneven manner, so long as it binds
the carbon fiber mats together. For example, the carbonization
product may be mostly disposed at the interface of the webs. One
skilled in the art will also appreciate that a sub-layer may be
added to a side of the carbon fiber mat before or after heat
treatment. The porous carbonaceous webs may be two different
materials and/or have different structures.
[0050] The carbonizable polymer having pyrrolidone functionality
can be applied to the porous carbonaceous webs by methods other
than the sandwich method. Other coating and mat combinations are
facilitated by employing compaction to distribute the polymer and
any other additives throughout the webs. Although alternative
methods may be employed, the sandwich method is preferred.
[0051] Another embodiment of the present fluid diffusion layers
comprises a porous carbonaceous web, a non-particulate carbon
filler, and a carbonization product of a polymer having pyrrolidone
functionality that binds the filler to the web. The fluid diffusion
layer may be made by coating at least one porous carbonaceous web
with a carbonizable polymer having pyrrolidone functionality and
non-particulate carbon filler, such as chopped carbon fibers.
Chopped carbon fibers can be applied to a carbon fiber mat as part
of an ink coating containing the carbonizable polymer having
pyrrolidone functionality. Alternatively, chopped carbon fibers can
be applied directly to the surface of the carbon fiber mat and then
covered with the ink coating. Thereafter, the carbonizable polymer
having pyrrolidone functionality is carbonized, preferably after
being stabilized.
[0052] The carbonization product may be uniformly, mostly uniformly
distributed, or non-uniformly distributed. If the chopped carbon
fibers and ink coating are added together, a compaction step is
optional. If the chopped carbon fibers are added separately from
the ink coating, a compaction step is especially desirable.
Compaction tends to distribute the ink coating through the web.
Both embodiments work well because following the heat treatment,
the polymer having pyrrolidone functionality is carbonized to
mechanically hold the materials together in a porous and conductive
matrix.
[0053] Non-particulate carbon filler of several diameters, lengths
and materials can be used. For instance, more than one type of
fiber length can be used to substantially vary pore structure in
the x-y direction. Alternatively, a substantial amount of carbon
fibers may be oriented in the fluid diffusion layer so as to vary
structure in the z-direction or to obtain desired properties (for
example, improved through-plane conductivity).
[0054] Before the carbon fiber mats are bound together, each can
have a unique coating to incorporate a z-directional structure of
particle fill or carbonized binder. Further, any or each carbon
fiber mat or chopped carbon fibers used may have a previous coating
of binder, graphite and/or carbon, and additives before being
combined and heat-treated to form the fluid diffusion layer.
[0055] The advantages of the disclosed methods include simple
processing steps to incorporate a high and varied degree of
z-direction asymmetric properties in a fluid diffusion layer
substrate. This method allows two or more different carbon fiber
mats and/or chopped carbon fiber filler to be utilized to form a
thicker, more porous fluid diffusion layer which is structurally
stable. Further, these methods permit better penetration and
uniform distribution of filler and other additives into the fluid
diffusion layer substrate.
[0056] Prior to carbonization the coated carbon fiber mats of any
of these embodiments can be allowed to dry with one or more release
materials. The release material may be any substance that is
capable of forming a backing for a carbonaceous web during
application, compaction and/or heating, yet remains easily
removable such as by peeling. The release material can be used to
form a release material/impregnant/web sandwich before the sandwich
is subjected to other processing steps. Suitable release materials
include Mylar, Channeled Resources Blue R/L 41113 release film, and
polyethylene coated paper. The release material is removed prior to
carbonizing.
[0057] The fabrication of electrode 1 may be completed by applying
catalyst in a conventional manner to fluid diffusion layer 3 to
form catalyst layer 2. Preferably, a catalyst mixture would be
applied, such as a catalyst mixture comprising catalyst particles 4
along with ionomer 5 and PTFE binder 6.
[0058] An alternative sequence may be employed however in making
electrode 1. For instance, a catalyst may be applied to a mat that
has been coated and filled, but prior to the carbonization step.
This may offer certain advantages in that the presence of
conventional reduced catalyst can promote carbonization and thus
lower the temperature (and hence energy) for carbonization of the
polymer having pyrrolidone functionality. Alternatively, catalyst
particles may be applied in unreduced form to a coated, filled mat
prior to the carbonization step. The reduction of the catalyst
particles could then take place concurrently with the carbonization
of the polymer having pyrrolidone functionality, thereby obviating
the usual reduction step performed in the preparation of catalyst
particles. An ionomer and/or a binder, if desirable, can be added
after the carbonization step.
[0059] The present methods may be used to produce a fluid diffusion
layer having fluid distribution channels. A fluid diffusion layer
having a sufficient thickness can be embossed to form fluid
distribution channels. A thicker fluid diffusion layer can be
prepared in several ways. For instance, a thicker fluid diffusion
layer can be prepared by using a thicker carbon fiber mat. More
preferably, a thicker fluid diffusion layer comprises at least two
carbon fiber mats bound together with a carbonization product of a
polymer having pyrrolidone functionality, which may be prepared by
the foregoing methods described herein. A thicker fluid diffusion
layer can also comprise non-particulate carbon filler such as
chopped carbon fibers bound to a porous carbonaceous web with a
carbonization product of a polymer having pyrrolidone
functionality. A fluid diffusion layer made using these thicker
mats is then embossed to form fluid distribution channels. The
fluid diffusion layer having a flow field is then further processed
and incorporated into an MEA as previously described.
[0060] Alternatively, a fluid diffusion layer may be prepared by a
process which is similar to the method disclosed in U.S. Pat. No.
6,037,073 except that the aqueous pyrrolidone solutions
contemplated herein replace the phenolic resin used therein.
[0061] The following examples have been included to illustrate
different embodiments and aspects of the invention but these should
not be construed as limiting in any way.
[0062] Product number 20352 non-woven carbon fiber mat from
Technical Fibre Products Limited is composed of non-woven RK-30
carbon fibers that are held together by means of a styrene-acrylic
binder (approximately 15% by weight). The mat has a weight per unit
area of 17 g/m.sup.2 and is approximately 200 micrometers thick.
The Gurley air permeability was determined to be about 2 seconds
(using 300 cm.sup.2 of air and 5 ounces (1.39 Joules/meter) of
force over an area of 0.1 in.sup.2 (0.65 cm.sup.2). The
through-plane resistance was determined to be about 410 m.OMEGA.
per cm.sup.2 area (using a fixture applying 85 psi (586 kPa) to the
mat) which corresponds to a conductivity of about 0.05
(ohm-cm).sup.-1. The Taber stiffness in the machine (processing)
direction of the mat was about 2 Taber units.
EXAMPLES
Comparative Example 1
[0063] As is, this carbon fiber mat is sufficiently thin,
lightweight, and permeable for use in solid polymer electrode fluid
diffusion layers. However, it is preferable that the electrical
conductivity and stiffness are increased.
Comparative Example 2
[0064] The non-woven carbon fiber mat of Comparative Example 1 was
then impregnated with a conductive filler and carbonized
phenol-formaldehyde impregnant using conventional methods.
Initially, the mat was impregnated with an aqueous mixture of a
conductive filler (A99 grade synthetic graphite from Asbury with a
particle size in the range from 20-44 micrometers in diameter) and
a pore former (86000 MW methyl cellulose). The mixture was 15%
solids by weight (89% graphite, 11% methyl cellulose) and was
prepared by mixing appropriate amounts of aqueous methyl cellulose
solution (2% by weight), graphite, and water followed by shear
mixing with a Silverson mixer at 5500 rpm for 15 minutes. The
mixture was then applied to one side of the carbon fiber mat using
a knife coater and air dried at ambient temperature. The other side
of the carbon fiber mat was then coated with the same mixture in a
like manner. The resulting filled mat contained about 4 mg/cm.sup.2
of graphite/methylcellulose solids. The filled mat was then
immersed in an aqueous solution containing about 21% by weight
phenol-formaldehyde resin (Durite AL-5801A grade of Borden
Chemicals), drained of excess solution, and then dried in air at
100.degree. C. for about a minute. The resin was then cured by
heating the resin impregnated, filled mat in an oven at 300.degree.
C. for about an hour. The resin loading in the mat was determined
to be 28% by weight. The resin impregnated, filled mat was then
carbonized by heating at 1000.degree. C. under nitrogen for an
hour.
[0065] The physical characteristics of the mat were then
remeasured. The Gurley air permeability was about 6 seconds, the
conductivity was about 2.7 (ohm-cm).sup.-1, and the Taber stiffness
in the machine direction was greater than about 10 Taber units.
[0066] The carbon fiber mat impregnated with conductive filler and
carbonized phenol-formaldehyde resin polymer in a conventional
manner exhibits desirable electrical conductivity and stiffness for
use in fluid diffusion layers. The air permeability has been
reduced, but is still acceptable.
Inventive Example 1
[0067] Several samples of non-woven carbon fiber mat were
impregnated with a conductive filler and carbonized polymer
impregnant as in Comparative Example 2, except that
polyvinylpyrrolidone (PVP) polymer was used and the polymer and
filler were impregnated in a single step.
[0068] In this example, an aqueous mixture comprising PVP in
solution and graphite in suspension was knife coated onto each side
of the carbon fiber mat samples and air dried as before. The
mixture contained 33% by weight of graphite and PVP, of which 75%
was graphite and 25% was PVP. The weight of the solids in the
coating was about 12 mg/cm.sup.2. The PVP in the samples was then
stabilized to different degrees by oxidizing in air at various
temperatures in a convection oven for an hour. The graphite-filled,
PVP impregnated mats were then carbonized at 1000.degree. C. under
nitrogen for an hour. The physical characteristics of the samples
were determined and are summarized in Table 1 below. However, one
sample was prepared without any stabilizing step prior to
carbonization. The PVP in this sample vaporized, leaving the filler
effectively unbound to the mat. The characteristics of this sample
were not determined.
1 TABLE 1 Taber units Oxidation Through-plane Gurley (average in
temperature Conductivity permeability machine (.degree. C.)
(ohm-cm).sup.-1 (seconds) direction) 200 4.1 11.4 4.2 250 3.2 6.6
8.1 300 3.3 10.0 1.5 350 3.3 8.6 2.1
[0069] By impregnating with conductive filler and carbonized PVP,
the carbon fiber mats can have the characteristics desired for use
in fluid diffusion layers. However, the results obtained are
influenced by the oxidation process employed. Advantageously, both
filler and PVP polymer can be impregnated into the mat in a single
step. Further, the aqueous PVP/graphite mixture employed contains
no toxic uncured resin.
Inventive Example 2
[0070] Several samples of non-woven carbon fiber mat were
impregnated in a like manner to those of Inventive Example 1 except
that methyl cellulose was also used in the mixture and shorter
oxidation periods were used. The methyl cellulose acted to
stabilize the graphite suspension and improve coating quality and
acted as a pore former. Shorter oxidation periods are preferred in
mass production.
[0071] In this example, an aqueous mixture containing 10% PVP, 11%
graphite, and 1% methyl cellulose by weight in water was employed.
The applied weight of solids mixture (after air drying) to the
samples was about 6 mg/cm.sup.2. Samples were again oxidized at
various temperatures but this time by passing the samples through a
convection oven over a period of 10 minutes. The convection oven in
this case simulated a conveyor oven suitable for a mass production
process. The samples were then carbonized at 1000.degree. C. under
nitrogen for an hour.
[0072] The first sample was prepared by oxidizing at 255.degree. C.
prior to carbonization. The PVP in This sample appeared to be lost
after the carbonizing step, so higher oxidation temperatures were
used subsequently. The physical characteristics of these subsequent
samples after carbonization were determined and are summarized in
Table 2 below. (This example was primarily for purpose of
determining mechanical stability and electrical conductivity.
Gurley air permeability values were not measured.)
2 TABLE 2 Oxidation Through-plane Taber units temperature
conductivity (average in machine (.degree. C.) (ohm-cm).sup.-1
direction) 350 2.3 5.4 (cracked) 375 2.4 8.8 400 2.2 5.3 415 2.7
4.5
[0073] By impregnating with conductive filler and carbonized PVP,
the carbon fiber mats can have the characteristics desired for use
in fluid diffusion layers. Again, the results obtained are
influenced by the oxidation process employed. Advantageously, some
or all desired components can be impregnated into the mat in a
single step. Further, the process appears amenable to mass
production techniques.
[0074] Subsequent to these tests, an additional sample was oxidized
at 375.degree. C. for ten minutes and then carbonized more rapidly
in a conveyor oven using nitrogen gas blankets on either side of
the heat treatment zone. This sample was carbonized at 1000.degree.
C. but only for approximately 6 minutes. The through-plane
conductivity, Taber stiffness, and Gurley air permeability for this
sample were 2.1 (ohm-cm).sup.-1, 8.0 Taber units, and 12.8 seconds
respectively. This data suggests that the fluid diffusion layer
properties are less sensitive to the carbonization process than the
oxidation process.
Inventive Example 3
[0075] A fluid diffusion layer was prepared by laminating two 17
g/m.sup.2 TFP carbon fiber mats together using the carbonization
product of an ink containing polyvinlypyrrolidone. An aqueous ink
comprising 8.9% polyvinylpyrrolidone resin, 11.1% graphite
particles, and 0.89% methyl cellulose by weight in water was
employed. The ink was knife coated onto one of the carbon fiber
mats to form a coating. The second carbon fiber mat was placed on
top of the ink coating to form a "sandwich," which was then
compacted at a pressure of 80 psi. The PVP was then stabilized by
oxidizing in air at a temperature of 250.degree. C. for 1 hour. The
sandwich was then carbonized at 1000.degree. C. under nitrogen for
an hour. No loose fibers were observed on the surfaces of the fluid
diffusion layers (unlike from single, thicker carbon fiber mats).
Porosity (calculated basis weight/thickness) was greater than
substrates of the same thickness obtained from a single carbon
fiber mat with a greater amount of carbonization product
impregnated therein.
[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 without
departing from the scope of the present disclosure, particularly in
light of the foregoing teachings.
* * * * *