CATALYST MATERIAL FOR FUEL CELL
BACKGROUND
[0001] This disclosure relates to catalyst materials and, more particularly, to a
method for targeted deposition of a catalyst metal.
[0002] Fuel cells and other types of devices commonly utilize electroactive
materials. For instance, a typical fuel cell may include an anode catalyst layer, a cathode
catalyst layer and an electrolyte between the anode and cathode catalyst layers for generating
an electric current in a known electrochemical reaction between a fuel and an oxidant. The
catalyst layers typically include a catalytic material, such as platinum, that is supported on
carbon particles. However, the catalytic material is only active when it is accessible to
protons, electrons and the respective reactant fuel or oxidant. Regions in the catalyst layers
that are accessible to protons, electrons and the respective reactant are referred to as the threephase
boundary.
SUMMARY
[0003] Disclosed is a method of forming a catalyst material. The method includes
coating agglomerates of catalyst support particles with an ionomer material. After coating the
agglomerates of catalyst support particles, a catalyst metal precursor is deposited by chemical
infiltration onto peripheral surfaces of the agglomerates of catalyst support particles. The
catalyst metal precursor is then chemically reduced to form catalyst metal on the peripheral
surfaces of the agglomerates of catalyst support particles.
[0004] In another aspect, an example fuel cell apparatus includes an electrolyte
layer and first and second catalyst layers that are arranged on respective opposing sides of the
electrolyte layer. At least one of the first or second catalyst layers includes agglomerates of
catalyst support particles dispersed within an ionomer material and a catalyst metal deposited
substantially on peripheral surfaces of the agglomerates of catalyst support particles with
regard to non-peripheral surfaces within the agglomerates.
BRIEF DESCRIPTION OF THE DRAWINGS
[0005] The various features and advantages of the disclosed examples will
become apparent to those skilled in the art from the following detailed description. The
drawings that accompany the detailed description can be briefly described as follows.
[0006] Figure 1 illustrates an example fuel cell apparatus.
[0007] Figure 2 illustrates an example method of forming a catalyst material in a
targeted deposition process.
[0008] Figure 3 illustrates a prior art ionomer electrode material.
[0009] Figure 4 illustrates an example ionomer electrode material according to the
present disclosure.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0010] Figure 1 schematically illustrates selected portions of an example fuel cell
20. In this example, a single fuel cell unit 22 is shown. However, it is to be understood that
multiple fuel cell units 22 may be stacked in a known manner in the fuel cell 20 to generate a
desired amount of electric power. It is also to be understood that this disclosure is not limited
to the arrangement of the example fuel cell 20, and the concepts disclosed herein may apply
to other fuel cell arrangements or to other catalytic devices.
[0011] As shown, the fuel cell 20 includes an electrode assembly 24 arranged
between an anode flow field 26 and a cathode flow field 28. For instance, the anode flow
field 26 may deliver fuel, such as hydrogen gas, to the electrode assembly 24. Similarly, the
cathode flow field 28 may deliver oxygen gas, such as air, to the electrode assembly 24. In
this regard, the anode flow field 26 and the cathode flow field 28 are not limited to any
particular structure, but may include channels or the like for delivering the reactant gases to
the electrode assembly 24.
[0012] The electrode assembly 24 includes an anode catalyst layer 30 and a
cathode catalyst layer 32. An electrolyte layer 34 is arranged between the anode catalyst layer
30 and the cathode catalyst layer 32 for conducting ions there between in an electrochemical
reaction to generate an electric current. In some examples, the electrolyte layer 34 may be a
polymer electrolyte membrane, a solid oxide electrolyte or other type of electrolyte suitable
for sustaining the electrochemical reaction.
[0013] The hydrogen at the anode catalyst layer 30 dissociates into protons that
are conducted through the electrolyte layer 34 to the cathode catalyst layer 32 and electrons
that flow through an external circuit 36 to power a load 38, for example. The electrons from
the external circuit 36 combine with the protons and oxygen at the cathode catalyst layer 32
to form a water byproduct.
[0014] As will be described in further detail below, the anode catalyst layer 30,
the cathode catalyst layer 32 or both are ionomer electrodes. The ionomer electrodes include
agglomerates 40 of catalyst support particles that are dispersed within an ionomer material
42. A catalyst metal 44 is deposited substantially on peripheral surfaces 46 of the
agglomerates 40 with regard to non-peripheral surfaces 48 within the agglomerates 40 (see
Figure 4). As will also be described in further detail below, the ionomer electrodes disclosed
herein are fabricated using a targeted deposition technique that deposits the catalyst metal 44
substantially on the peripheral surfaces 46 of the agglomerates 40 to thereby increase the
amount of catalyst metal 44 at the three-phase boundary of the ionomer electrodes.
[0015] The anode catalyst layer 30, the cathode catalyst layer 32 or both may be
fabricated using a method 50 of forming a catalyst material, as shown in Figure 2. The
method 50 of targeted deposition of the catalyst metal 44 generally includes a coating step 52,
a deposition step 54 and a chemical reduction step 56. As shown, the coating step 52 includes
coating agglomerates 40 of catalyst support particles with the ionomer material 42. After the
coating in step 52, the deposition step 54 is used to deposit a catalyst metal precursor by
chemical infiltration onto the peripheral surfaces 46 of the agglomerates 40 of catalyst
support particles. In the reduction step 56, the catalyst metal precursor is chemically reduced
to form the catalyst metal 44.
[0016] A comparative technique of forming ionomer electrode material results in
a substantial amount of the catalyst metal 44 being deposited on non-peripheral surfaces 48 of
the agglomerates 40 of catalyst support particles, as seen in Figure 3. In the comparative
technique, the catalyst metal 44 is predeposited on the individual catalyst support particles
and then mixed with the ionomer material 42. During the mixing, the catalyst support
particles form the agglomerates 40 such that a substantial amount of the catalyst metal 44 is
located between adjacent catalyst support particles. The adjacent surfaces of the support
particles constitute the non-peripheral surfaces 48 and such surfaces are not at the three-phase
boundary as desired. As a result, much of the catalyst metal 44 that resides at the nonperipheral
surfaces 48 is inactive within the device.
[0017] As will now be described, the method 50 of targeted deposition of the
catalyst metal 44 deposits the catalyst metal 44 substantially on the peripheral surfaces 46 of
the agglomerates 40, as shown in Figure 4 . In this case, at least a majority of the catalyst
metal 44 resides on the peripheral surfaces 46. In a further example, the non-peripheral
surfaces 48 of the agglomerates 40 are substantially free of the catalyst metal 44.
[0018] In one example, the coating step 52 includes mixing the catalyst support
particles with an ionomer and a solvent. As an example, the ionomer may be a fluoropolymer,
such as Nafion by E.I. Dupont USA. The solvent may be isopropyl alcohol, water, or a
mixture thereof. The mixing coats the catalyst support particles with the ionomer material 42
and forms the agglomerates 40. The solvent is then removed and the resulting agglomerates
40 and ionomer material 42 that is coated on the agglomerates 40 is mixed with or suspended
in sodium bicarbonate (NaHCCb). The amount of sodium bicarbonate is controlled to
establish a target pH level. For instance, the target pH level is between 8.5 and 9.0. The target
pH level facilitates the coating of the catalyst support particles with the ionomer. That is, the
ionomer adheres to the catalyst support particles at the targeted pH level.
[0019] Additionally, the sodium bicarbonate dopes the ionomer material 42, such
as the disclosed fluoropolymer, with sodium ions. The sodium that is doped into the ionomer
material 42 thermally stabilizes the ionomer material 42, as will be described below.
[0020] Next, in the deposition step 54, a catalyst metal precursor is deposited by
chemical infiltration onto the peripheral surfaces 46 of the agglomerates 40. In one example,
the catalyst metal precursor is deposited by mixing chloroplatinic acid H2PtCl6-(H20)6 with
the mixture of the catalyst support particles and sodium bicarbonate. A buffer, such as
ammonium hydroxide, may be added to the mixture to control the pH to establish a target pH
level. As an example, the target pH level during the deposition step 54 is 5.75 - 6.25.
[0021] The target pH level permits the chemical infiltration of the chloroplatinic
acid (i.e., the precursor catalyst metal) through the ionomer material 42 that is coated onto the
agglomerates 40 of catalyst support particles such that the catalyst metal precursor deposits
onto the peripheral surfaces 46 of the agglomerates 40. Without being bound by any
particular theory, it is hypothesized that the target pH range during the deposition step 54
opens channels within the ionomer material 42 that allows the infiltration of the catalyst
metal precursor through the ionomer material 42 to the peripheral surfaces 46 of the
agglomerates 40. That is, the target pH range opens up such channels to allow the deposition
while pH ranges outside of the target pH range close the channels and preclude or
substantially hinder deposition of the precursor catalyst metal.
[0022] Optionally, carbon monoxide gas may be bubbled through the mixture
during the deposition step 54 to "poison" the catalyst metal precursor and thereby prevent
concentrated depositions that might otherwise result in large agglomerates of the catalyst
metal 44.
[0023] The mixture of the chloroplatinic acid, buffer, and the coated agglomerates
40 of catalyst support particles may be stirred for a predetermined amount of time. In one
example, the mixture is magnetically stirred for approximately 30-60 minutes.
[0024] In a further example, the target pH level during the coating step 52 and the
target pH level during the deposition step 54 are controlled to establish a target ratio. As an
example, a ratio of the target pH level during the coating step 52 to the target pH level during
the deposition step 54 (i.e., pHdeposition divided by pHcoating) is between 0.5 and 3. In a further
example based on the disclosed target pH levels above, the ratio is between 1.3 and 1.6.
[0025] After the deposition step 54, the catalyst metal precursor is chemically
reduced in the reduction step 56. As an example, formaldehyde is added to the mixture of the
chloroplatinic acid, buffer, and the coated catalyst support particles to reduce the catalyst
metal precursor. The pH level of the mixture during the reduction step 56 is controlled to
establish the pH of the mixture to be in a range of 5.5 - 6.0.
[0026] In one example, the formaldehyde is added at a controlled rate, such as 5
milliliters per minute, to promote uniform chemical reduction of the platinum of the catalyst
metal precursor. Additionally, the amount of formaldehyde may be precalculated to
correspond to the amount of chloroplatinic acid that was used. The reduction may be
conducted over a predetermined amount of time, such as 1.5 hours.
[0027] After the chemical reduction, the synthesis is essentially complete and the
mixture may be filtered to remove the catalyst material. The catalyst material may then be
washed in water and/or ammonium bicarbonate and subsequently dried to form the catalyst
material into the respective anode catalyst layer 30 and/or cathode catalyst layer 32 in a
known manner.
[0028] After the washing and drying, the catalyst material may be heat treated at a
temperature above 150°C/302°F. The heat treatment removes impurities from the catalyst
metal 44 and thereby improves catalytic performance. The thermal stabilization of the
ionomer material 42 from the sodium doping enables the heat treatment at the temperature
above 150°C/302°F and as high as 220°C. Without the sodium dopant, the ionomer material
tends to degrade and render the catalyst material unsuitable for use in the fuel cell 20 or other
catalytic device. After the heat treatment, the sodium dopant can be removed and replaced
with hydrogen in an acid washing step. As an example, sulfuric acid may be used.
[0029] Although a combination of features is shown in the illustrated examples,
not all of them need to be combined to realize the benefits of various embodiments of this
disclosure. In other words, a system designed according to an embodiment of this disclosure
will not necessarily include all of the features shown in any one of the Figures or all of the
portions schematically shown in the Figures. Moreover, selected features of one example
embodiment may be combined with selected features of other example embodiments.
[0030] The preceding description is exemplary rather than limiting in nature.
Variations and modifications to the disclosed examples may become apparent to those skilled
in the art that do not necessarily depart from the essence of this disclosure. The scope of legal
protection given to this disclosure can only be determined by studying the following claims.
15255WO; 67124-178PCT

CLAIMS
What is claimed is:
1. A method of forming a catalyst material, the method comprising:
coating agglomerates of catalyst support particles with an ionomer material;
after the coating of the agglomerates of catalyst support particles with the ionomer
material, depositing by chemical infiltration a catalyst metal precursor onto peripheral
surfaces of the agglomerates of catalyst support particles; and
chemically reducing the catalyst metal precursor to form catalyst metal on the
peripheral surfaces of the agglomerates of catalyst support particles, the catalyst material
comprising the ionomer material, the agglomerates of catalyst support particles and the
catalyst metal.
2. The method as recited in claim 1, wherein the coating of the agglomerates of catalyst
support particles with the ionomer material includes mixing the catalyst support particles and
ionomer material with sodium bicarbonate.
3. The method as recited in claim 2, including controlling a pH of the mixture to be
within a predetermined range of 8.5 - 9.0.
4. The method as recited in claim 1, wherein the depositing includes mixing the coated
agglomerates of catalyst support particles with chloroplatinic acid and establishing a pH of
5.75-6.25.
5. The method as recited in claim 4, further comprising bubbling carbon monoxide gas
through the mixture.
6. The method as recited in claim 1, wherein the depositing includes mixing the coated
agglomerates of catalyst support particles with chloroplatinic acid and establishing a pH level
of the mixture to be within a predetermined range.
15255WO; 67124-178PCT
7. The method as recited in claim 1, wherein the coating of the agglomerates of catalyst
support particles includes mixing the catalyst support particles, the ionomer material and
sodium bicarbonate and establishing a first predetermined pH level of the mixture, and then
the depositing includes mixing the coated agglomerates of catalyst support particles with
chloroplatinic acid and establishing a second predetermined pH level such that a ratio of the
first predetermined pH level to the second predetermined pH level is between 0.5 and 3.
8. The method as recited claim 7, wherein the ratio is between 1.3 and 1.6.
9. The method as recited in claim 1, wherein the chemically reducing includes using
formaldehyde and establishing a pH level during the reducing to be 5.5 - 6.0.
10. The method as recited in claim 1, wherein the coating of the agglomerates of catalyst
support particles includes mixing the catalyst support particles and ionomer material with
sodium bicarbonate to dope the ionomer with sodium.
11. The method as recited in claim 10, further comprising, after the chemically reducing,
drying the catalyst material and heating the catalyst material at a temperature that is greater
than 150°C/302°F.
12. The method as recited in claim 11, further comprising washing the catalyst material in
acid to remove the sodium.
15255WO; 67124-178PCT
13. A fuel cell apparatus comprising:
an electrolyte layer; and
a first catalyst layer and a second catalyst layer arranged on respective opposing sides
of the electrolyte layer, at least one of the first catalyst layer or the second catalyst layer
including agglomerates of catalyst support particles dispersed within an ionomer material and
a catalyst metal deposited substantially on peripheral surfaces of the agglomerates of catalyst
support particles with regard to non-peripheral surfaces within the agglomerates.
14. The fuel cell apparatus as recited in claim 13, wherein the catalyst metal comprises
platinum and the catalyst support particles are carbon.
15. The fuel cell apparatus as recited in claim 14, wherein the ionomer material is a
fluoropolymer.