U.S. patent application number 13/966556 was filed with the patent office on 2014-02-20 for ion conducting nanofiber fuel cell electrodes.
The applicant listed for this patent is Yossef A. Elabd, Francis W. Richey, Kevin H. Wujcik. Invention is credited to Yossef A. Elabd, Francis W. Richey, Kevin H. Wujcik.
Application Number | 20140051013 13/966556 |
Document ID | / |
Family ID | 50100272 |
Filed Date | 2014-02-20 |
United States Patent
Application |
20140051013 |
Kind Code |
A1 |
Elabd; Yossef A. ; et
al. |
February 20, 2014 |
ION CONDUCTING NANOFIBER FUEL CELL ELECTRODES
Abstract
The present invention is directed to methods of making a
nanofiber-nanoparticle network to be used as electrodes of fuel
cells. The method comprises electrospinning a polymer-containing
material on a substrate to form nanofibers and electrospraying a
catalyst-containing material on the nanofibers on the same
substrate. The nanofiber-nanoparticle network made by the methods
is suitable for use as electrodes in fuel cells.
Inventors: |
Elabd; Yossef A.;
(Wallingford, PA) ; Richey; Francis W.;
(Coatesville, PA) ; Wujcik; Kevin H.; (Mount
Laurel, NJ) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Elabd; Yossef A.
Richey; Francis W.
Wujcik; Kevin H. |
Wallingford
Coatesville
Mount Laurel |
PA
PA
NJ |
US
US
US |
|
|
Family ID: |
50100272 |
Appl. No.: |
13/966556 |
Filed: |
August 14, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61682820 |
Aug 14, 2012 |
|
|
|
Current U.S.
Class: |
429/530 ;
427/77 |
Current CPC
Class: |
D01D 5/0061 20130101;
Y02E 60/50 20130101; H01M 2008/1095 20130101; H01M 8/02 20130101;
H01M 4/8657 20130101; H01M 4/8853 20130101; H01M 4/926
20130101 |
Class at
Publication: |
429/530 ;
427/77 |
International
Class: |
H01M 4/86 20060101
H01M004/86; H01M 8/02 20060101 H01M008/02 |
Claims
1. A structure comprising: a polymer fiber structure comprising
fibers having a fiber diameter of less than 1 micron; and a
plurality of catalytic material particles supported on the polymer
fiber structure.
2. A structure as claimed in claim 1, where in the polymer fiber
structure comprises aligned fibers.
3. A structure as claimed in claim 1, wherein the catalytic
material has a particle size of less than 50 nm.
4. A structure as claimed in claim 1, wherein the catalytic
material has a particle size of less than 20 nm.
5. A structure as claimed in claim 1, wherein the catalyst loading
is less than 0.1 mg of catalyst per square centimeter.
6. A structure as claimed in claim 1, wherein the catalyst loading
is less than 0.05 mg of catalyst per square centimeter.
7. A structure as claimed in claim 1, wherein the catalyst loading
is less than 0.01 mg of catalyst per square centimeter.
8. An electrode comprising the structure of claim 1.
9. A fuel cell comprising the electrode of claim 8.
10. A method for making nanofiber-nanoparticle network to be used
as electrode of a fuel cell, the method comprises the step of:
electrospinning a polymer-containing material onto a substrate for
forming nanofibers on the substrate, and electrospraying a
catalyst-containing material onto the polymer-containing material
or nanofibers.
11. The method of claim 10, wherein said electrospinning and said
electrospraying are carried out simultaneously.
12. The method of claim 10, wherein the polymer comprises at least
one polymer selected from the group consisting of Nafion,
sulfonated poly(ether ether ketone), sulfonated
polyer(styrene-b-ethylene-r-butadiene-b-styrene), sulfonated
poly(styrene), sulfonated poly(arylene ether) copolymer, sulfonated
poly(styrene-b-isobutylene-b-styrene).
13. The method of claim 10, wherein said polymer is a sulfonated
tetrafluoroethylenes.
14. The method of claim 10, wherein said polymer-containing
material is a solution.
15. The method of claim 10, wherein said polymer-containing
material is molten polymer.
16. The method of claim 10, wherein the catalyst comprises at least
one material selected from the group consisting palladium,
platinum, gold, silver, nickel, rhodium, ruthenium, rhenium,
osmium, iridium, iron, chromium, cobalt, copper, manganese,
tungsten, niobium, titanium, tantalum, lead, indium, cadmium, tin,
bismuth and gallium, as well as compounds and alloys of these
metals.
17. The method of claim 16, wherein said at least one fuel cell
catalyst is synthetic platinum on carbide derived carbon
support.
18. The method of claim 16, wherein said catalyst-containing
material is a solution.
19. The method of claim 1, further comprising the step of heating
the materials.
20. A catalyst for a fuel cell selected from the group consisting
of platinum on carbide derived carbon support.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to electrodes for fuel cells.
In particular, it is directed to a process for manufacturing a
nanofiber-nanoparticle network that is useful for the manufacture
of electrodes for use in fuel cells, to electrodes made by this
process and to fuel cells incorporating such electrodes.
[0003] 2. Description of the Related Technology
[0004] Fuel cells provide several advantages over batteries, such
as high efficiency, high energy and power density, low weight,
low-temperature operation, rapid start-up time, and quick fuels
from renewable sources with no point-of-use greenhouse gas
emissions. However, one major factor that has limited the mass
commercialization of fuel cells, especially fuel cell vehicles, is
the high cost due to the catalysts requires for use in fuel cells.
The catalyst is usually a precious metal catalyst which currently
contributes to over 30% of the fuel cell engine cost.
[0005] Technologies that can reduce the amount of precious metal
catalyst needed (i.e. allow low catalyst loadings) while still
providing good performance (e.g. high power density) are critical
to the successful commercialization of fuel cells. Because the
catalyst functions on its surface where it is in contact with a
fuel, such as hydrogen or methane, the larger the surface area of
the catalyst per unit weight, the lower the required catalyst
loading. Extensive efforts have been invested towards increasing
the surface area of fuel cell catalysts.
[0006] U.S. Pat. No. 7,229,944 discloses a process of making fiber
structures based on interconnected carbon fibers for use with
catalytic material. The catalytic material may be in the form of
nanosize particles supported on the fibers. The structures are
produced by electrospinning a polymeric material fiber structure
that is subsequently converted to a carbon fiber structure in a
heat treatment step which also causes the catalyst particles to
nucleate on the carbon fibers and grow to a desired nanosize. The
catalyst may be uniformly distributed across the carbon fiber
structure before nucleation and the amount of catalyst may be
controlled. These factors may enhance catalytic performance and/or
enable use of less catalyst for equivalent catalytic performance
which can lead to cost savings, amongst other advantages.
[0007] U.S. Pat. No. 7,887,772 discloses an ultrafine graphitic
carbon fiber having a diameter of 1 to 3000 nm that is prepared by
electrospinning a halogenated polymer solution containing a metal
compound for inducing graphitization. An ultrafine porous graphitic
carbon fiber having a large specific surface area, micropores and
macropores is prepared by graphitization using a metal catalyst
generated from the metal compound. The ultrafine carbon fiber can
be used for storing hydrogen, an adsorbing material for
biochemically noxious substances, an electrode material for a
supercapacitor, a secondary cell material, a fuel cell material, or
a catalyst carrier material.
[0008] U.S. Patent Application Publication No. US 2012/0028170
discloses a fuel cell electrode made by synthesizing carbon
nanotubes grafted with poly(citric acid) and encapsulating a
platinum group metal nanoparticle. More specifically, carbon
nanotubes are oxidized, followed by mixing with monohydrated citric
acid, which results in carbon nanotubes grafted with poly(citric
acid). The carbon nanotubes grafted with poly(citric acid) are then
mixed with one or more sources of platinum group metal ions to
encapsulate the platinum group metal nanoparticles. Finally, the
carbon nanotubes encapsulated with platinum group metal
nanoparticles are electrosprayed onto an electrode of a fuel cell.
The present invention is aimed at achieving high power densities in
fuel cells with relatively low catalyst loadings.
SUMMARY OF THE INVENTION
[0009] In a first aspect, the present invention is directed to a
method for making a nanofiber-nanoparticle network useful in an
electrode for a fuel cell. The method includes the steps of
electrospraying a catalyst-containing material and electrospinning
a polymer-containing material to form a nanofiber-nanoparticle
network.
[0010] Another aspect of the present invention is an electrode for
fuel cells comprising the nanofiber-nanoparticle network
manufactured by the method of the present invention.
[0011] Yet another aspect of the present invention is a fuel cell
that uses an electrode comprising the nanofiber-nanoparticle
network manufactured by the method of the present invention.
[0012] Yet other aspects of the present invention relate to a
method for making a patterned nanofiber-nanoparticle network useful
in an electrode for a fuel cell, electrodes made with the patterned
nanofiber-nanoparticle network and fuel cells including such
electrodes.
[0013] In a still further aspect of the present, catalyst materials
for use in fuel cells are provided, as well as electrodes and fuel
cells including these catalyst materials. These catalysts may
include platinum/carbide derived carbon (Pt/CDC).
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] FIG. 1 shows a flow chart of a process for manufacturing a
nanofiber-nanoparticle network according to the present
invention.
[0015] FIG. 2 shows another embodiment of a process for
manufacturing a nanofiber-nanoparticle network according to the
present invention.
[0016] FIG. 3A shows a single Nafion nanofiber bridging two
electrodes.
[0017] FIG. 3B shows an enlarged image of FIG. 3A.
[0018] FIG. 3C is a plot of the proton conductivity of a Nafion
nanofiber vs. the nanofiber diameter.
[0019] FIG. 3D is a plot of the proton conductivity of a Nafion
nanofiber vs. relative humidity.
[0020] FIG. 4A is a scanning electron microscope (SEM) image of
ante mortem nanofiber-nanoparticle network manufactured according
to the present invention.
[0021] FIG. 4B is an SEM image of post mortem
nanofiber-nanoparticle network manufactured according to the
present invention.
[0022] FIG. 5A is an SEM image of a commercial Pt/C catalyst.
[0023] FIG. 5B is an SEM image of a platinum/carbide derived carbon
(Pt/CDC) supported catalyst according to the present invention.
[0024] FIG. 6 shows micro-patterned substrates that may be used in
the present invention.
[0025] FIG. 7 shows a fuel cell including an electrode made by the
process of the present invention.
[0026] FIG. 8 shows a diagram of the interface of three phases:
catalyst, polyelectrolyte, and pores.
[0027] FIG. 9A is an SEM image of an electrode with 0.022
mg/cm.sup.2 platinum (Pt) loading, fabricated according to the
method of Example 4 according to one embodiment of the present
invention.
[0028] FIG. 9B is an enlarged SEM image of the same electrode shown
in FIG. 9A.
[0029] FIG. 9C is plot of the distribution of the diameters of
nanofibers in the same electrode shown in FIGS. 9A-9B.
[0030] FIG. 9D is a plot of the distribution of the sizes of the
nanoparticles in the same electrode shown in FIGS. 9A-9B.
[0031] FIG. 9E is an SEM image of an electrode with 0.052
mg/cm.sup.2 Pt loading, fabricated by the method of Example 4
according to one embodiment of the present invention.
[0032] FIG. 9F is an enlarged SEM image of the electrode shown in
FIG. 9E.
[0033] FIG. 9G is plot showing the distribution of the diameters of
nanofibers in the electrode shown in FIG. 9E.
[0034] FIG. 9H is a plot showing the distribution of the sizes of
nanoparticles in the electrode shown in FIG. 9E.
[0035] FIG. 10A shows performance of fuel cells with the electrodes
fabricated in Example 4 and comparative hand-painted control
electrodes with operating conditions of H.sub.2/air at ambient
pressure.
[0036] FIG. 10B shows performance of the same fuel cells used in
FIG. 10A, at operating conditions using H.sub.2/air with 25 psi
back pressure.
[0037] FIG. 10C shows performance of the same fuel cells used in
FIG. 10A, at operating conditions using H.sub.2/O.sub.2 at ambient
pressure.
[0038] FIG. 10D shows performance of the same fuel cells used in
FIG. 10A, at operating conditions using H.sub.2/O.sub.2 with 25 psi
back pressure.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)
[0039] For illustrative purposes, the principles of the present
invention are described by referencing various exemplary
embodiments. Although certain embodiments of the invention are
specifically described herein, one of ordinary skill in the art
will readily recognize that the same principles are equally
applicable to, and can be employed in other systems and methods.
Before explaining the disclosed embodiments of the present
invention in detail, it is to be understood that the invention is
not limited in its application to the details of any particular
embodiment shown. Additionally, the terminology used herein is for
the purpose of description and not of limitation. Furthermore,
although certain methods are described with reference to steps that
are presented herein in a certain order, in many instances, these
steps may be performed in any order as may be appreciated by one
skilled in the art; the novel method is therefore not limited to
the particular arrangement of steps disclosed herein.
[0040] It must be noted that as used herein and in the appended
claims, the singular forms "a", "an", and "the" include plural
references unless the context clearly dictates otherwise.
Furthermore, the terms "a" (or "an"), "one or more" and "at least
one" can be used interchangeably herein. The terms "comprising",
"including", "having" and "constructed from" can also be used
interchangeably.
[0041] In a first aspect, the present invention relates to a novel
process for manufacturing a nanofiber-nanoparticle network useful
for fuel cell electrodes. The process comprises two steps:
electrospinning of a polymer-containing material; and
electrospraying of a catalyst-containing material onto the
electrospun polymer. The electrospinning and electrospraying may be
carried out in any suitable manner. Some examples include
electrospinning and electrospraying simultaneously, or first
electrospinning and then electrospraying.
[0042] Electrodes made in this manner have more catalyst surface
area available for an oxygen reduction reaction because a large
percentage of catalyst in such electrodes can directly contact the
fuel in the fuel cell. Such electrodes therefore require less
catalyst than electrodes using conventional supported catalysts,
thereby permitting low or ultralow catalyst loadings. In addition,
these catalysts are more efficient for oxidization of the fuel,
i.e. provide a higher power density than conventional fuel cell
electrodes
[0043] Referring to FIG. 1, step 1 of the process may involve
preparation of a polymer-containing material. The
polymer-containing material contains at least one polymer,
preferably Nafion. However, any suitable polymer may be used to
make the nanofiber material. In general, the polymer needs to have
properties that make it possible to fabricate a nanofiber support
using electrospinning. In addition, the polymer may have a proton
conductivity of from about 0.001 mS/cm to about 10 S/cm, or from
about 0.1 mS/cm to about 1 S/cm, or from about 0.1 S/cm to about 1
S/cm. Suitable polymer materials include, but are not limited to,
Nafion, sulfonated poly(ether ether ketone), sulfonated
polyer(styrene-b-ethylene-r-butadiene-b-styrene), sulfonated
poly(styrene), sulfonated poly(arylene ether) copolymer, sulfonated
poly(styrene-b-isobutylene-b-styrene).
[0044] In one embodiment, the polymer may be dissolved in a
suitable solvent to provide a polymer solution for electrospinning
Suitable solvents are known to those of skill in the art and their
suitability depends, at least in part, on the characteristics of
the polymer. Suitable solvents may include, for example,
N,N-dimethylformamide (DMF), ethanol, methanol, acetone, water,
tetrahydrofuran (THF), methylene chloride (MC or dichloromethane)
and combinations thereof. It should be understood that a skilled
person in the art may also choose other solvents that are suitable
for a particular process.
[0045] The concentration of the polymer in the polymer solution
used for electrospinning can be determined by a skilled person
based on, for example, the desired viscosity of the polymer
solution. Typical polymer concentrations may be between about 8%
and about 20% by weight of the solution, and more preferably
between about 8% and about 15% by weight of the solution. Such
concentrations generally result in the solution having a suitable
viscosity for electrospinning. It should be understood that
concentrations outside the above ranges may also be used if the
resultant polymer solution is suitable for electrospinning.
[0046] In another embodiment, the polymer may be melted, preferably
by heating, to make a liquid polymer having a suitable viscosity
for electrospinning. Depending on the characteristics of the
polymer used, it may be preferred to melt the polymer, instead of
using a solvent to provide a polymer solution. When a melted
polymer is used as the electrospinning material, the melted polymer
may need to be maintained at elevated temperature for
electrospinning. Thus, some modification of the electrospinning
apparatus may be needed to accommodate this heating requirement,
such as providing a heated electrospinning needle/nozzle.
[0047] In step 2 of FIG. 1, the polymer-containing materials
supplied to at least one electrospinning needle/nozzle (hereinafter
needle/nozzle is referred to as "needle"). In a preferred
embodiment, the polymer-containing material is pumped to the
electrospinning needle. Any other suitable means for supplying the
polymer-containing material to the electrospinning needle may be
used in the present invention.
[0048] In step 3 of FIG. 1, the electrospinning needle electrospins
the polymer-containing material onto a grounded substrate as a fine
stream with its diameter in the nanometer range. In a typical
apparatus for electrospinning, the electrospinning needle is
connected to a high voltage power supply, while the conductive
substrate is grounded. Thus an electromagnetic field is formed
between the electrospinning needle and the grounded substrate. The
polymer-containing material, aided by the force of the
electromagnetic field, travels to the substrate by electrostatic
attraction.
[0049] The voltage applied to the electrospinning needle may depend
on the polymer and the viscosity of the polymer-containing material
prepared from the polymer. In an exemplary embodiment, the voltage
applied to the electrospinning needle is between about 3 kV and
about 50 kV, or between about 10 kV and about 40 kV.
[0050] The distance between the tip of the electrospinning needle
and the substrate may depend on the diameter of the
polymer-containing material stream, as well as the viscosity of the
polymer-containing material. In general, a finer stream and lower
viscosity may require a shorter distance between the
electrospinning needle and the substrate. In some exemplary
embodiments, the distance between the tip of the electrospinning
needle and the substrate may be between about 1 cm and about 50 cm,
more preferably between about 5 cm and 35 cm.
[0051] The speed of electrospinning may depend on the polymer.
Generally speaking, the faster the polymer-containing material
solidifies or dries, the higher the speed of electrospinning that
may be used. The polymer material may be dispensed for example at
about 0.1 to about 10 mL/hour) through the electrospinning
needle.
[0052] In an exemplary embodiment, one electrospinning needle is
used to spin the polymer-containing material onto the substrate.
However, in some embodiments, it may be desirable to use multiple
electrospinning needles. A general description of an
electrospinning process is provided, for example, in "Polymer
Nanofibers Assembled by Electrospinning", Frenot et. al, Current
Opinion in Colloid and Interface Science, vol. 8, pages 64-75,
2003, which is incorporated herein by reference in its
entirety.
[0053] In some embodiments, the polymer-containing material may
form a continuous nanofiber, or the nanofiber may break and a
plurality of separate nanofibers may form on the substrate.
[0054] In step 4 of FIG. 1, a catalyst-containing material suitable
for electrospraying is prepared. In some embodiments, the
catalyst-containing material is a solution that contains at least
one catalyst that can function as an oxidation/reduction catalyst
in a fuel cell. The catalyst may be selected from, but is not
limited to, palladium, platinum, gold, silver, nickel, rhodium,
ruthenium, rhenium, osmium, iridium, iron, chromium, cobalt,
copper, manganese, tungsten, niobium, titanium, tantalum, lead,
indium, cadmium, tin, bismuth, gallium, as well as mixtures,
compounds and alloys of these metals. In some embodiments,
palladium and platinum may be the preferred catalysts.
[0055] The present invention may also employ new catalysts for fuel
cells. These catalysts include platinum/carbide derived carbon
(Pt/CDC). The platinum on CDC supports (FIG. 5B) has a higher
nanoporosity than a comparable platinum on amorphous carbon
catalyst (FIG. 5A). Therefore, higher platinum surface area to
volume ratios can be achieved with the Pt/CDC catalyst. Examples of
such catalysts can be found, for example, in US 2010/0285392
A1.
[0056] In step 5 of FIG. 1, the catalyst-containing material is
supplied to the electrospraying nozzle or needle (hereinafter
collectively referred to as a "needle"). In one embodiment, the
catalyst-containing material is pumped to the electrospraying
needle. Any other means known to persons skilled in the art that
are capable of supplying the catalyst-containing material to the
needle may be used in the present invention.
[0057] In step 6 of FIG. 1, the electrospraying needle sprays the
catalyst-containing material as fine droplets onto the electrospun
material or the formed nanofibers. The sprayed fine droplets
containing catalyst material are electrostatically attracted to the
substrate as discussed below by electrostatic attraction. For the
present invention, the same high voltage power supply may be
connected to both the electrospray needle and the electrospinning
needle, or they may be connected to different high voltage power
supplies. In one exemplary embodiment, the voltage applied to the
electrospraying needle is between about 3 kV and about 45 kV.
[0058] The distance between the tip of the electrospray needle and
the substrate may depend on the size of the sprayed droplets, as
well as the viscosity of the sprayed catalyst-containing material.
In general, the smaller the size of the sprayed material droplets
and lower viscosity will require shorter distance between the
electrospray needle and the substrate. In some exemplary
embodiments, the distance may be between about 1 cm and about 50
cm, more preferably between about 3 cm and 30 cm.
[0059] The speed of electrospraying of the catalyst-containing
material may depend on the speed at which the nanofibers are formed
on the substrate and the desired nanoparticle density on the
nanofibers. For example, the catalyst material may be sprayed at
about 0.01 to 50 mL/hour.
[0060] In an exemplary embodiment, one electrospraying needle is
used to spray the catalyst-containing material onto the nanofibers
on the substrate. However, in some embodiments, it may be desirable
to use multiple electrospray needles.
[0061] The substrate onto which the catalyst-containing droplets
are electrosprayed and polymer-containing fine streams are
electrospun is conductive such that, when grounded, there is an
electromagnetic field formed between the substrate and
electrospinning needle/electrospray needle. The substrate can be
made from any suitable conductive material. In preferred
embodiments, the substrate is made of a metal, such as, for
example, aluminum (Al).
[0062] In some exemplary embodiments, the substrate may be
patterned or micro-patterned as shown in FIG. 6 to provide control
over the the nanofiber morphology and provide an improved fuel cell
performance even at low catalyst loadings. Micro-patterned
substrates are disclosed in Zhang and Chang, "Electrospinning of
three-dimensional nanofibrous tubes with controllable
architectures," Nano Letters, vol. 8, no. 10, pages 3283-3287,
2008), which is incorporated herein in its entirety by reference.
The use of a patterned substrate allows at least some or all of the
nanofibers in the nanofiber/nanoparticle network to be aligned in a
substantially parallel relationship relative to one another.
[0063] In step 7 of FIG. 1, the nanofiber-nanoparticle network is
formed. If the polymer-containing material is a solution, the
solvent evaporates to solidify the polymer and form nanofibers on
the substrate. If molten polymer is employed, loss of heat from the
polymer-containing material will form nanofibers on the
substrate.
[0064] The nanofibers generally have circular-shaped
cross-sections, though other cross-sections may also be suitable.
In some embodiments, the nanofibers are preferably solid (i.e., not
hollow). However, it should be understood that in other
embodiments, the fibers may be hollow at least at some sections of
the nanofibers (e.g., nanotubes).
[0065] The nanofibers can have any suitable dimension. In some
embodiments, the average nanofiber diameter is greater than about
10 nm; in some embodiments, greater than about 50 nm; and, in some
embodiments, greater than about 100 nm. Nanofiber diameters less
than these ranges may cause the structure to have insufficient
mechanical integrity for some polymers used. The nanofiber
diameters may be less than about 1 micron, more preferably less
than about 500 nm. In some embodiments, the nanofiber diameters may
be less than about 300 nm.
[0066] The length of the nanofibers may vary. In some embodiments,
one or more nanofibers may have a length of at least about 500
microns; and, in some embodiments, at least one nanofiber has a
length greater than about 1 mm. In some embodiments, it may be
possible to achieve nanofiber lengths of greater than about 1 cm,
or even significantly greater.
[0067] Forming of the nanofiber-nanoparticle network of step 7 also
includes forming or depositing of catalyst nanoparticles on the
nanofibers. The fine droplets containing catalyst material will dry
by loss of solvent. The loss of solvent results in the formation of
catalyst nanoparticles on the nanofibers. In some embodiments heat
treatment may be needed to facilitate or accelerate the formation
of nanoparticles.
[0068] The catalyst-containing nanoparticles are preferably
substantially evenly distributed on the surface of nanofibers,
though some aggregation of nanoparticles to aggregates of up to
about 0.5 microns is generally acceptable. The nanoparticle size is
generally in the nanometer range. For example, the nanoparticles
may have an average particle size of less than about 50 nm; and, in
some cases, less than about 20 nm. Small nanoparticle sizes may
advantageously lead to the relatively uniform distribution of
catalytic material throughout the nanofibers, as well as larger
surface areas for oxidation, amongst other positive effects.
However, the nanoparticles typically (though not always) have an
average particle size of greater than 20 nm. It should be
understood that nanoparticle sizes outside the above ranges may be
used in certain embodiments of the present invention.
[0069] Average nanoparticle sizes may be determined by averaging
the nanoparticle sizes of a representative number of nanoparticles
using, for example, scanning electron microscope (SEM) techniques.
As used herein, the average nanoparticle size includes sizes of
primary nanoparticles and sizes of nanoparticle agglomerates. It
may be preferred for the nanoparticle size distribution to be
relatively narrow, and/or relatively homogenously distributed.
Narrow nanoparticle size distributions promote the uniform
distribution of catalytic material throughout the
nanofiber-nanoparticle network.
[0070] During step 7 of FIG. 1, for the purpose of forming the
nanofiber-nanoparticle network, heat may be applied to the
materials deposited on the surface of the substrate. Heat may be
introduced to the deposited nanofiber material and nanoparticle
material by heating the substrate itself, or by some other suitable
means such as heated air or infra-red radiation. The heat may
accelerate the evaporation of solvent. Some further desirable
transformations and/or reactions may also be initiated or
accelerated by heating.
[0071] In a preferred embodiment of the present invention, the
speed of electrospinning of nanofiber material and the speed of
electrospraying of the nanoparticle material are controlled
relative to one another. The optimal speed of electrospinning and
optimal speed of electrospraying may be determined by a skilled
person.
[0072] The movement of the electrospinning needle and/or the
movement of the substrate are also preferably controlled to ensure
that the nanofibers are substantially evenly distributed over at
least portion of the surface of the substrate. In addition, the
motion of the electrospinning needle and the electrospray needle
are also preferably controlled to ensure that catalyst
nanoparticles are evenly deposited on substantially all of the
nanofibers.
[0073] In some exemplary embodiments, the substrate may be kept
stationary, while both the electrospinning needle and electrospray
needle move in the space over the substrate to spread the
nanofibers and nanoparticles on the surface of the substrate. In
one embodiment, the stationary substrate may be cylindrical. The
electrospinning needle and electrospray needle may both rotate
around the cylindrical substrate. In yet another embodiment, the
stationary substrate may be flat. The electrospinning needle and
electrospraying needle may both travel from one end of the flat
substrate to the other end.
[0074] Referring to FIG. 2, in an exemplary embodiment, the process
of the present invention is carried out in a closed chamber 10. The
chamber 10 has an air inlet 1 and an air outlet 8. A tube 2
supplies a catalyst-containing material to an electrospraying
needle 5, which is connected to a high voltage power supply by a
wire 3. The substrate, a rotating collector 6 is grounded through a
wire 9. A tube 4 supplies the polymer-containing material to
electrospinning needle 7, which is connected to a high voltage
power supply through a wire 3.
[0075] In this embodiment, electrospinning and electrospraying are
carried out in any suitable manner. Because the collector 6
rotates, the nanofibers are substantially evenly distributed on the
surface of the collector 6, and the nanoparticles are substantially
evenly distributed on the nanofibers.
[0076] In certain embodiments, the process of the present invention
may further comprise a treatment step. The treatment may be
initiated either during or after the formation step 7 in FIG. 1.
One such treatment step is the fusion of at least portion of the
nanofibers at points where the nanofibers intersect. The nanofibers
may be fused together during the beginning stage of heat treatment.
This fusion may lead to increased mechanical integrity and/or
increased conductivity. Some of the nanofibers may be merely in
physical contact with one another at intersection points without
being fused together.
[0077] The nanofiber-nanoparticle network made by the method
depicted in FIG. 1 is suitable for use as an electrode in fuel
cells. The conductive substrate used for
electrospinning/electrospraying may be removed from the
nanofiber-nanoparticle network before the nanofiber-nanoparticle
network is used as an electrode. FIG. 7 shows an exemplary
embodiment of a hydrogen fuel cell using electrodes with a
nanofiber-nanoparticle network made by the process described in the
present invention. In this exemplary fuel cell, the electrolyte is
made from Nafion.
[0078] One advantage of the nanofiber-nanoparticle network made by
the method depicted in FIG. 1 is the availability of a larger
surface area per unit of catalyst, where the reaction of the fuel
cells us carried out, as shown in FIG. 8. For example, the
electrodes embodying the nanofiber-nanoparticle network may be
porous electrodes consisting of, for example, platinum (Pt) as
catalyst in the form of nanoparticles, Nafion polymer, and pores.
Hydrogen and oxygen react on the Pt surface and protons are
transported through the Nafion polymer connected the network.
Nafion acts as both a binder and a proton transporter and the pores
serve as channels for gas diffusion to the catalyst surface.
[0079] The electrodes of the present invention may have catalyst
loading of, for example, less than 0.1 mgPt/cm.sup.2, more
preferably, less than 0.05 mgPt/cm.sup.2, and most preferably, less
than 0.01 mgPt/cm.sup.2. It has been found that these relatively
low catalyst loadings for electrodes of present invention provided
power densities comparable to electrodes having a standard loading
of 0.4 mgPt/cm.sup.2, (0.77 gPt/kW, 0.522 W/cm.sup.2), which
electrodes were made by a standard electrode fabrication technique.
Electrodes made by the present invention may have a Pt
loading/power of at least 0.1 gPt/kW, more preferably, at least
0.06 gPt/kW. This is much lower than the 2015 Department of Energy
target of 0.15 gPt/kW.
[0080] The invention will now be illustrated by the following
examples which are not to be construed as limiting of the
invention.
EXAMPLES
Example 1
[0081] A nanofiber-nanoparticle network made by the process
according to the present invention was used in electrodes for a
fuel cell. These electrodes used platinum as the catalyst. It was
observed that these electrodes had a high fuel cell power density
at Pt loadings that are 40 times lower than typical Pt loadings
used in state-of-the-art fuel cells.
[0082] More specifically, a 0.01 mgPt/cm.sup.2 (0.158 W/cm.sup.2,
0.06 gPt/kW) loading for electrodes of present invention provided
power densities comparable to the standard catalyst loading of 0.4
mgPt/cm.sup.2, (0.77 gPt/kW, 0.522 W/cm.sup.2) of electrodes made
by a standard electrode fabrication technique. It was also observed
that the Pt loading/power was 0.06 gPt/kW for the electrode of the
present invention, which was much lower than the 2015 Department of
Energy target of 0.15 gPt/kW.
Example 2
[0083] Nanofibers made from Nafion were found to have extremely
high proton conductivity. FIG. 3A, is an SEM image of a single
Nafion nanofiber bridging the gap between two metal electrodes.
FIG. 3B is an enlarged image of FIG. 3A. The Nafion nanofiber's
proton conductivities at 30.degree. C. and 90% relative humidity as
a function of nanofiber diameter are shown in FIG. 3C. The proton
conductivities of nanofibers with diameters >2 .mu.m was similar
to that of bulk Nafion film (.about.0.1 S/cm). However, when the
nanofiber diameter was <1 .mu.m, proton conductivity increased
sharply with decreasing nanofiber diameter and reached a value as
high as 1.5 S/cm for the 400 nm nanofiber, which was an order of
magnitude higher than the bulk Nafion film. The relative humidity
also affected the proton conductivity of the Nafion nanofiber, as
shown in FIG. 3D. See also Dong, B.; Gwee, L.; Salas-de la Cruz,
D.; Winey, K. I. Elabd, Y. A. Super Proton Conductive High Purity
Nafion Nanofibers. Nano Letters 2010, 10, 3785-3790.
Example 3
[0084] The ion-conducting nanofiber-nanoparticle network
manufactured according to one embodiment of the present invention
is shown in FIGS. 4A and 4B. This network is ideal for use as a
fuel cell electrode. SEM images of the electrode ante mortem (FIG.
4A) and post mortem (FIG. 4B) demonstrate the nanofiber morphology,
where catalyst nanoparticles are substantially evenly distributed
on the surface of the nanofibers create an electrode with high
surface area. Such electrodes have a low catalyst loading and a
high power density.
Example 4
[0085] A custom-designed apparatus for producing electrodes
according to the electrospinning/electrospraying (E/E) process of
the present invention was used to fabricate electrodes in this
example. The apparatus consisted of two high-voltage power supplies
(Model PS/EL50R00.8 of Glassman High Voltage, Inc. and Model
ES40P-10W/DAM of Gama High Voltage Research), two syringe pumps
(Model NE-1000, New Era Pump Systems), two syringe needles
(i.d.=0.024 in.) (Hamilton), tubing (Pt No. 30600-65, Cole-Parmer),
and a grounded collector (aluminum foil coated drum, o.d.=4.85 cm).
The collector drum was connected to a motor (Model 4IK25GN-SW2,
Oriental Motor) to allow for rotation during the E/E process. A gas
diffusion layer was adhered to the collector drum, where
nanofibers/nanoparticles were directly collected via the E/E
process. The needle tip to collector distances, applied voltages,
and solution flow rates were 15 cm and 9 cm, 10.5 kV and 12.5 kV,
and 0.3 ml/h and 3 ml/h for the E/E process, respectively. As a
comparison, a hand-painting process was used to make hand-painted
electrodes as a control.
[0086] The materials used to fabricate the electrodes included
de-ionized (DI) water, isopropanol (IPA, Sigma-Aldrich, 99.5%),
ethanol (Decon Labs, Inc., 99.5%), and 5 wt % Nafion in a
water/isopropanol solution (1000 EW, Ion Power), poly(acrylic acid)
(PAA) (M.sub.v=450,000 g/mol, Aldrich), 20 wt % Pt on carbon
catalyst (Vulcan XC-72, Premetek Co.), and a SGL-25BC gas diffusion
layer (Fuel Cells Etc.). These materials were used to fabricate the
electrodes in both the hand-painting process (to fabricate
hand-painted electrodes) and the E/E process of the present
invention (to fabricate E/E electrodes).
[0087] Catalyst ink used in the electrospraying portion of the E/E
process according to the present invention consisted of 20 mg
platinum (Pt) catalyst, 0.248 ml DI water, 0.043 ml 5 wt % Nafion
solution, 0.171 ml IPA/H.sub.2O (3/1 v/v), and 1.970 ml ethanol.
This mixture was sonicated for 3 min (Model CL-18, Qsonica
Sonicator) prior to electrospraying. This mixture corresponds to
10/1 wt/wt (Pt/C)/Nafion, which was five times greater than the
same ratio in the catalyst ink prepared for the hand-painted
electrodes. Significantly less Nafion was required for
electrospraying, because Nafion was also supplied from the
electrospinning to produce the E/E electrode. The final
(Pt/C)/Nafion ratio in the E/E electrode was similar to that of the
hand-painted electrode, since Nafion was supply to electrodes from
both electrospraying and electrospinning
[0088] The polymer solution used in the electrospinning portion of
the E/E process consisted of a mixture of 4/1 wt/wt Nafion/PAA. A
solution of 5 wt % PAA and Nafion was prepared by combining 131.1
mg PAA, 10494 mg of 5 wt % Nafion solution, and 2491.7 mg IPA/water
(3/1 v/v). This solution was stirred at 70-80.degree. C. for
.about.12 h to ensure complete dissolution. The solution was cooled
down to ambient temperature before electrospinning The electrodes
were annealed at 135.degree. C. for 5 min to stabilize the
nanoparticles and nanofibers right after the E/E process. For the
E/E electrodes, the Pt loading was varied by changing the E/E
process time for making a particular electrode.
[0089] The catalyst ink used to make hand-painted electrodes was
prepared by combining 100 mg solid Pt catalyst, 550 mg DI water,
1000 mg 5 wt % Nafion solution, and 1350 mg IPA and mixing via
sonication for 3 min. This mixture corresponds to 2/1 wt/wt
(Pt/C)/Nafion and 3/1 v/v IPA/water.
[0090] Membrane electrode assemblies (MEA's) were made by
sandwiching together the hand-painted electrodes or E/E electrodes
and NR-212 Nafion membrane. The anode catalyst layers of all of the
membrane electrode assemblies were hand-painted with a Pt loading
of 0.15 mg/cm.sup.2. All of the membrane electrode assemblies were
hot-pressed at 135.degree. C., 33 psi for 5 minutes.
Example 5
[0091] The catalyst loading of the electrodes fabricated in Example
4 was determined by thermogravimeteric analysis (hereinafter "TGA")
(TGA 7, Perkin Elmer). A small portion of the E/E electrode
(.about.5-7 mg) was heated during the TGA from ambient temperature
to 900.degree. C. at 5.degree. C./min in air flowing at 20 ml/min.
Since all components in the E/E electrode volatilize above
900.degree. C., with the exception of Pt, the Pt loading was
determined by comparing the weight of the E/E electrode before and
after exposure to a temperature of 900.degree. C. in the TGA.
[0092] Morphological characterization of the E/E electrodes
fabricated in Example 4 was carried out with scanning electron
microscopy (SEM, Model FEI/Philips XL-30, 10 kV). SEM images of the
E/E electrode were obtained after the E/E process (on a gas
diffusion layer) and before MEA fabrication. All samples were
sputter-coated (Denton Desk II Sputtering System) with platinum at
40 mA for 30 seconds before the SEM images were taken.
[0093] SEM images of the E/E electrode with 0.022 mg/cm.sup.2 Pt
loading are shown in FIGS. 9A and 9B. FIG. 9A shows that the E/E
electrode was porous and uniform over large length scales
(.about.10 .mu.m). The porous structure allows gas transport from
the gas flow channel to the catalyst layer. The Nafion/PAA
nanofibers serve as pathways for proton transport, while the
platinum/carbon (Pt/C) nanoparticles are the active sites for
electrochemical reactions. FIG. 9B shows a magnified view of the
E/E electrode of FIG. 9A, where several large agglomerates
(.about.1-2 .mu.m) were observed, while the majority of catalyst
particles ranged in size from .about.50-300 nm. The larger
agglomerates were rough and porous, which still allow for gas/fuel
to penetrate and diffusion for electrochemical reaction. When the
E/E electrode was compressed, the nanoparticles and nanofibers
formed a tightly packed network for both electron and proton
transport.
[0094] Using SEM, the size distributions of the nanoparticles and
nanofibers in the E/E electrodes were also studied. Thirty of the
nanofibers shown in FIG. 9B were randomly selected using ImageJ
software for determination of nanofiber diameter by SEM. FIG. 9C
presents the distribution of the diameters of the thirty randomly
selected nanofibers. It was observed that the majority of the
nanofibers had diameters of .about.200 nm.
[0095] The sizes of the same nanoparticles randomly selected from
FIG. 9B was also determined by SEM. FIG. 9D shows the distribution
of nanoparticle size (diameter) in the E/E electrode. The size
distribution of the nanoparticles was broader than that of the
nanofibers, with .about.80% of the nanoparticles being less than
450 nm in diameter.
[0096] A similar morphological study was also conducted on an E/E
electrode with a higher Pt loading (0.052 mg/cm.sup.2), which was
also fabricated in Example 4 (see FIGS. 9E-9H). Surprisingly, more
catalyst agglomerates were seen in FIG. 9E than for the E/E
electrode with the lower Pt loading shown in FIG. 9A. This may be
due to a slight change in the ambient temperature and relative
humidity that may have affected the viscosity and evaporation rate
of the solvent in the catalyst ink. The distributions of nanofiber
diameters and nanoparticle sizes in FIGS. 9G and 9H show that that
the nanofiber sizes were similar to those shown in FIG. 9C, but
that the nanoparticle sizes were on average larger than those in
FIG. 9D. This suggests that, though a change in ambient conditions
may affect nanoparticle size, it may not have a significant effect
on nanofiber diameters.
[0097] Additionally, it was observed that the electrosprayed
nanoparticles were more sensitive to slight changes in operating
conditions. The flow rate of the electrospraying process (3 ml/h)
was higher than the flow rate of the electrospinning process (0.3
ml/h). The higher flow rate of the electrospraying relative to the
electrospinning was selected to achieve a desired ratio of Pt/C to
Nafion (2:1) in the catalyst layer. The higher flow rate of the
electrospraying solution required a longer time to fully evaporate
the solvent inside the nanoparticles. As a result, a slight change
in operating conditions such as relative humidity and temperature
could significantly affect the morphology and size of the
nanoparticles in the E/E electrodes, but appears to have a lesser
effect on the diameter of the nanofibers.
Example 6
[0098] The membrane electrode assembly (1.21 cm.sup.2, made in
Example 4) was put into fuel cells (as cathode) with 100 lb force
torque. The membrane electrode assembly was placed between two
serpentine flow field graphite plates separated by two 0.160 mm
thick Teflon.RTM. coated gaskets (Pt No. 381-6, Saint Gobian). A
hand-painted cathode with a 0.42 mg/cm.sup.2 Pt loading was used as
a control. For the E/E cathode electrodes, the Pt loading had two
levels: 0.052 and 0.022 mg/cm.sup.2. Hand-painted electrodes with
0.15 mg/cm.sup.2 Pt loading were used as the anode in all fuel
cells in this Example.
[0099] The fuel cell performances were evaluated with a Compact
Fuel Cell Test System (850C, Scribner Associates, Inc.). Fuel cell
tests were conducted at ambient temperature and 25 psi back
pressure with anode and cathode flow rates of 0.42 L/min hydrogen
and 1.0 L/min air, respectively. The cathode, anode, and fuel cells
were kept at 80.degree. C. When H.sub.2 and O.sub.2 were supplied
to the anode and cathode, the flow rates were 0.42 L/min and 0.50
L/min, respectively. Polarization curves were collected from open
circuit to 0.2 V at increments of 0.05 V/min.
[0100] The fuel cell performance was recorded after a new membrane
electrode assembly was fully activated. The activation process for
the fuel cells included operating a membrane electrode assembly at
0.7 V for 1-2 hours followed by scanning the voltage from open
circuit potential to 0.2 V several times. This activation process
lasted .about.4-6 h and may be repeated until the membrane
electrode assembly reached steady state. A steady state was reached
when no further increase in the current was observed and the fuel
cell was held at constant voltage. All fuel cell performances were
measured using fully saturated anode and cathode feeding gases
(Relative Humidity=100% for anode and cathode).
[0101] FIGS. 10A-10D show the performance of fuel cells with the
E/E cathodes at both Pt loading levels, as well as fuel cells with
the hand-painted cathodes as control. FIG. 10A shows that the fuel
cell performance increased with the Pt loading. The maximum power
density of the control experiment (hand-painted cathode with 0.42
mg/cm.sup.2 Pt loading) was 0.59 W/cm.sup.2, while the E/E cathodes
with 0.052 and 0.022 mg/cm.sup.2 Pt loadings resulted in maximum
power densities of 0.436 W/cm.sup.2 and 0.376 W/cm.sup.2,
respectively. When the Pt loading of the E/E electrode (0.052
mg/cm.sup.2 Pt loading) was 8-fold lower than the hand-painted
electrode (0.42 mg/cm.sup.2 Pt loading), the output power was only
reduced by 37%. The output power of the E/E electrode with 0.022
mg/cm.sup.2 Pt loading was 20-fold lower than the output power
hand-painted control electrode, but this only resulted in a 46%
reduction in the maximum output.
[0102] The effect of the catalyst loading on the fuel cell
performance when back pressure was applied to the fuel cells was
also investigated. Under back pressure, the fuel cell performance
was less affected by the catalyst loading on the electrode (FIG.
10B), probably because back pressure led to a higher concentration
of the reactants at the catalyst surface. When 25 psi back pressure
(39.7 psi absolute pressure) was applied to both the cathode and
anode in the fuel cells, the maximum output power for the control
experiment (0.42 mg/cm.sup.2 Pt loading on the hand-painted
cathode) was 0.839 W/cm.sup.2. For the E/E cathodes with 0.022
mg/cm.sup.2 and 0.052 mg/cm.sup.2 Pt loading, the maximum output
power was 0.625 W/cm.sup.2 and 0.656 W/cm.sup.2, respectively. This
corresponded to a reduction of only 26% and 22% in the maximum
output power for the E/E electrodes compared to the hand-painted
control electrode. Thus, the fuel cells with E/E electrodes were
capable of generating high power densities at ultra-low Pt loadings
(significantly lower cost than control).
[0103] FIG. 10C shows fuel cell performance with oxygen (O.sub.2),
instead of air, as the cathode fuel. It was observed that the fuel
cell performance in the high current region (mass
transport-limiting region) improved significantly in comparison
with when air was used as the cathode fuel. This might be due to
the fact that pure oxygen drives a more efficient electrochemical
reaction at the anode. An output power density of 0.962 W/cm.sup.2
was achieved when pure oxygen was supplied to the hand-painted
cathode in the control experiment. For the E/E cathodes with
significantly lower Pt loadings than the control hand-painted
cathode, the maximum power output was still high (0.724 W/cm.sup.2
and 0.614 W/cm.sup.2 for the 0.052 mg/cm.sup.2 and 0.022
mg.sup.2/cm Pt loadings, respectively). This corresponded to only a
25-36% lower maximum output power density compared to the control
experiment with an 8-20-fold reduction in Pt loading. When 25 psi
back pressure was applied to both the cathode and anode sides, a
further improvement in fuel cell performance was observed (FIG.
10D), with back pressure reducing the difference in performance
between the hand-painted cathode and E/E cathodes.
Example 7
[0104] Electrochemical properties of the electrodes fabricated in
Example 4 were studied by cyclic voltammetry (CV), which was
conducted on a two-electrode membrane electrode assembly (made in
Example 4) with a potentiostat (Solartron SI 1287, Corrware
Software) at 20 mV/s over a range of 0 to 1.2 V. The anode worked
as both the counter and reference electrodes. The electrochemical
surface area (ECSA) of the electrodes was determined from the
hydrogen adsorption area from 0.1 to 0.4 V. The fuel cell anode and
cathode were supplied with H.sub.2 at 40 sccm (standard cubic
centimeters per minute) and N.sub.2 at 18 sccm, respectively.
Temperatures of the cathode, anode and fuel cell were held at
30.degree. C. The Pt catalyst was assumed to have an average site
density of 210 .mu.C/cm.sup.2. Using the Pt loading and maximum
power density (H.sub.2/air at ambient pressure), the platinum
utilization (g of Pt in cathode/kW of fuel cell max power) was
calculated.
[0105] Several electrochemical properties of the electrodes are
summarized in Table 1. The E/E electrodes had much higher ECSA than
the hand-painted electrodes. The platinum utilization value for the
E/E electrodes was lower than the target set by the U.S. Department
of Energy (DOE 2012 target) indicating that these electrodes meet
DOE specifications. The superior performance of fuel cells with E/E
electrodes even with less Pt loading may be at least partially due
to the reason that the E/E electrodes had ECSAs significantly
higher than the hand-painted control electrode.
TABLE-US-00001 TABLE 1 Electrochemical properties of catalyst
layers in electrodes Pt utilization at Loading, maximum power,
Cathode mg/cm.sup.2 ECSA, m.sup.2/g g.sub.Pt/kW Hand-painted 0.42
53.2 0.71 E/E 0.022 121.3 0.059 E/E 0.052 101.7 0.121 DOE 2017
target -- -- 0.125
[0106] It is to be understood, however, that even though numerous
characteristics and advantages of the present invention have been
set forth in the foregoing description, together with details of
the structure and function of the invention, the disclosure is
illustrative only, and changes may be made in detail, especially in
matters of shape, size and arrangement of parts within the
principles of the invention to the full extent indicated by the
broad general meanings of the terms in which the appended claims
are expressed.
* * * * *