U.S. patent application number 11/462739 was filed with the patent office on 2007-02-15 for electrocatalyst supports for fuel cells.
This patent application is currently assigned to GM GLOBAL TECHNOLOGY OPERATIONS, INC.. Invention is credited to Mei Cai, Lee Lizhong Feng, John T. Johnson, Yunfeng LU, Martin S. Ruthkosky, Frederick T. Wagner, Zhiwang WU.
Application Number | 20070037041 11/462739 |
Document ID | / |
Family ID | 37758098 |
Filed Date | 2007-02-15 |
United States Patent
Application |
20070037041 |
Kind Code |
A1 |
Cai; Mei ; et al. |
February 15, 2007 |
Electrocatalyst Supports for Fuel Cells
Abstract
Titanium oxide (usually titanium dioxide) catalyst support
particles are doped for electronic conductivity and formed with
surface area-enhancing pores for use, for example, in
electro-catalyzed electrodes on proton exchange membrane electrodes
in hydrogen/oxygen fuel cells. Suitable compounds of titanium and a
dopant are dispersed with pore-forming particles in a liquid
medium. The compounds are deposited as a precipitate or sol on the
pore-forming particles and heated to transform the deposit into
crystals of dopant-containing titanium dioxide. If the heating has
not decomposed the pore-forming particles, they are chemically
removed from the, now pore-enhanced, the titanium dioxide
particles.
Inventors: |
Cai; Mei; (Bloomfield Hills,
MI) ; LU; Yunfeng; (New Orleans, LA) ; WU;
Zhiwang; (New Orleans, LA) ; Feng; Lee Lizhong;
(Troy, MI) ; Ruthkosky; Martin S.; (Sterling
Heights, MI) ; Johnson; John T.; (Sterling Heights,
MI) ; Wagner; Frederick T.; (Fairport, NY) |
Correspondence
Address: |
GENERAL MOTORS CORPORATION;LEGAL STAFF
MAIL CODE 482-C23-B21
P O BOX 300
DETROIT
MI
48265-3000
US
|
Assignee: |
GM GLOBAL TECHNOLOGY OPERATIONS,
INC.
P.O. Box 300 Mail Code 482-C23-B21
Detroit
MI
|
Family ID: |
37758098 |
Appl. No.: |
11/462739 |
Filed: |
August 7, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60707937 |
Aug 12, 2005 |
|
|
|
Current U.S.
Class: |
429/524 ;
423/610; 429/532; 429/535; 502/350 |
Current CPC
Class: |
H01M 4/92 20130101; C01G
23/047 20130101; C01P 2006/12 20130101; C01G 23/053 20130101; H01M
2008/1095 20130101; H01M 4/881 20130101; Y02E 60/50 20130101; H01M
4/9075 20130101; C01G 33/00 20130101; H01M 4/925 20130101; H01M
4/8652 20130101 |
Class at
Publication: |
429/044 ;
423/610; 502/350 |
International
Class: |
H01M 4/86 20070101
H01M004/86; H01M 4/90 20070101 H01M004/90; C01G 23/047 20060101
C01G023/047; B01J 23/00 20060101 B01J023/00 |
Claims
1. A porous catalyst support comprising particles of titanium
oxide, the particles of titanium oxide containing a dopant element
for enhanced electron conductivity and the particles having surface
area-increasing pores resulting from vacated pore-forming
particles.
2. A catalyst support as recited in claim 1 in which the dopant
element comprises one or more elements selected from the group
consisting of lanthanum, manganese, molybdenum, niobium, tantalum,
tungsten, strontium, vanadium, and yttrium.
3. A catalyst support as recited in claim 1 in which the dopant
element comprises niobium.
4. An electrode for a fuel cell, the electrode comprising catalyst
particles dispersed on catalyst support particles of titanium
oxide, the titanium oxide containing a dopant element for enhanced
electronic conductivity and the particles having surface
area-increasing pores resulting from vacated pore-forming
particles.
5. An electrode for a fuel cell as recited in claim 4 in which the
electrode is formed on a surface of with a proton exchange membrane
and comprises noble metal catalyst particles.
6. An electrode for a fuel cell as recited in claim 4 in which the
electrode is a cathode for reduction of oxygen in a stream of
air.
7. An electrode for a fuel cell as recited in claim 5 in which the
electrode is a cathode for reduction of oxygen in a stream of
air.
8. An electrode for a fuel cell as recited in claims 4 in which the
catalyst comprises platinum and the dopant element is niobium.
9. An electrode for a fuel cell as recited in claims 5 in which the
catalyst comprises platinum and the dopant element is niobium.
10. A method of making titanium dioxide particles for supporting
particles of a catalyst comprising: co-dispersing compounds of
titanium and a dopant element as solutes or an sol in a liquid
medium; dispersing insoluble particles for pore-forming in the
liquid medium, the particles being no larger than about twenty
nanometers in largest dimension; precipitating the dispersed
compounds of titanium and dopant on the pore-forming particles;
separating the titanium compound and dopant compound coated
particles from the liquid medium; heating the coated particles in
an atmosphere to form crystalline, dopant element-containing
titanium oxide; and if necessary after the heating, removing the
embedded template particles from the crystalline, dopant
element-containing titanium oxide particles to leave template
particle-vacated pores in the titanium oxide particles.
11. A method as recited in claim 10 in which the titanium compounds
are titanium (IV) alkoxide compounds and the liquid medium
comprises an alcohol and/ or water.
12. A method as recited in claim 10 in which the template particles
are silica particles.
13. A method as recited in claim 10 in which the dopant element
comprises one or more elements selected from the group consisting
of lanthanum, manganese, molybdenum, niobium, tantalum, tungsten,
strontium, vanadium, and yttrium.
14. A method as recited in claim 10 in which the dopant element
comprises niobium.
Description
[0001] This application claims priority based on provisional
application Ser. No. 60/707,937, filed Aug. 12, 2005 and titled
"Electrocatalyst Supports for Fuel Cells", which is incorporated
herein by reference.
TECHNICAL FIELD
[0002] This invention pertains to electrode catalysts for fuel
cells. More specifically, this invention pertains to corrosion
resistant catalyst supports for fuel cells, especially for cells
having a cathode at which oxygen is reduced in air.
BACKGROUND OF THE INVENTION
[0003] Fuel cells are electrochemical cells that are being
developed for mobile and stationary electric power generation. One
fuel cell design uses a solid polymer electrolyte (SPE) membrane or
proton exchange membrane (PEM), to provide ion transport between
the anode and cathode. Gaseous and liquid fuels capable of
providing protons are used. Examples include hydrogen and methanol,
with hydrogen being favored. Hydrogen is supplied to the fuel cell
anode. Oxygen (as air) is the cell oxidant and is supplied to the
cell's cathode. The fuel cell electrodes are formed of porous
conductive materials, such as woven graphite, graphitized sheets,
or carbon paper to enable the fuel to disperse over the surface of
the membrane facing the fuel supply electrode. Each electrode
comprises finely divided catalyst particles (for example, platinum
particles), supported on carbon particles, to promote ionization of
hydrogen at the anode and reduction of oxygen at the cathode.
Protons flow from the anode through the ionically conductive
polymer membrane to the cathode where they combine with oxygen to
form water, which is discharged from the cell. Conductor plates
carry away the electrons formed at the anode.
[0004] Currently, state of the art PEM fuel cells utilize a
membrane made of perfluorinated ionomers such as Dupont
Nafion.sup.TM. The ionomer carries pendant ionizable groups (e.g.
sulfonate groups) for transport of protons through the membrane
from the anode to the cathode.
[0005] Currently, platinum (Pt) supported on a high surface area
carbon is the most effective electrocatalyst for PEM fuel cell
systems. However, a significant problem hindering large-scale
implementation of proton exchange membrane (PEM) fuel cell
technology is the loss of performance during extended operation and
automotive cycling. Recent investigations of the deterioration of
cell performance have revealed that a considerable part of the
performance loss is due to the degradation of the electrocatalyst.
Although carbon has been considered as the most favorable catalyst
support because of its low cost, good electron conductivity, high
surface area, and chemical stability, corrosion of carbon supports
on the cathode side of PEM fuel cells is emerging as a challenging
issue for long-term stability of PEM fuel cells.
[0006] It is an object of this invention to provide a porous
titanium oxide electrocatalyst support having suitable properties
for a PEM fuel cell environment including suitable surface area,
electrical conductivity and chemical stability.
SUMMARY OF THE INVENTION
[0007] This invention uses a porous form of titanium dioxide
(sometimes called "titania") as a high surface area support for
platinum, or other suitable catalyst. Preferably, the titanium
dioxide is mixed or doped with an element such as niobium to
enhance the electrical conductivity of the support material. The
titanium oxide is formed around removable filler particles
(particulate templates), such as silica particles, that are
chemically dissolved (etched) from the titanium dioxide particles
to yield highly porous catalyst particle carriers. Particles of
noble metal or other catalyst material are then deposited on the
porous carrier material. Such a titanium dioxide carrier material
is particularly useful in a catalytic electrode material in
association with a proton exchange membrane in a fuel cell in which
oxygen is electrochemically reduced.
[0008] In accordance with a preferred embodiment of the invention,
a titanium alkoxide compound is formed as a solution or sol in an
alcohol or aqueous/alcohol medium. For example, a solution or sol
of titanium (IV) isopropoxide or titanium (IV) 2-ethylhexyloxide
may be formed. A salt or alkoxide of a suitable dopant element may
also be dissolved or dispersed in the medium. Examples of suitable
dopant elements include lanthanum, manganese, molybdenum, niobium,
tantalum, tungsten, strontium, vanadium, and yttrium. Also
dispersed in the liquid medium are suitably sized particles (e.g.
less than twenty nanometers in greatest dimension) of silica,
polymer beads, or the like (preferably with the aid of ultrasonic
energy). The titanium and dopant element compounds are then
precipitated or gelled on the dispersed particles.
[0009] The gelled or precipitated composite material is separated
from the liquid medium and dried as necessary. The composite
material is heated to a suitable temperature in a controlled
atmosphere, for example of hydrogen or ammonia, to form very small
particles (nanometer size) of titanium dioxide doped with a
suitable quantity of niobium, or the like. When the template
particles consist of an organic polymer they may be removed by
heating to leave pores in the agglomerated particles of titania.
When the template particles are inorganic, like silica, they may be
chemically dissolved from the titanium dioxide particles leaving
internal and external surface pores for receiving and dispersing
fine particles of catalyst metal.
[0010] The porous and doped titanium dioxide particles provide
ample surface for the effective dispersion of platinum particles
for use as cathodic electrode material on a Nafion.sup.TM proton
exchange membrane in a hydrogen/ oxygen fuel cell environment. The
titania carrier resists oxidative weight loss in a high temperature
air environment and displays electrical conductivity.
[0011] Other objects and advantages of the invention will be
apparent from a detailed description of illustrative preferred
embodiments.
DESCRIPTION OF PREFERRED EMBODIMENTS
[0012] The titanium dioxide catalyst support materials of this
invention have general utility in catalyst applications. Their
utility includes applications as catalyst supports for catalyst
particles in fuel cell electrodes. For example, these durable
catalyst supports may be useful in an electrochemical fuel cell
assembly including a solid polymer electrolyte membrane and a
cathode that is exposed to oxygen or air. Many United States
patents assigned to the assignee of this invention describe
electrochemical fuel cell assemblies having an assembly of a solid
polymer electrolyte membrane and electrode assembly. For example,
FIGS. 1-4 of U.S. Pat. No. 6,277,513 include such a description,
and the specification and drawings of that patent are incorporated
into this specification by reference. In the '513 patent, carbon
particles are used to carry or support catalyst particles for
electrode (anode or cathode) operation. In this invention, porous
and doped titanium dioxide particles are used to carry the catalyst
for the electrode function.
[0013] Compounds of titanium (IV) alkoxides, such as titanium
(isopropoxide).sub.4 or titanium (2-ethylhexyloxide).sub.4, are
readily available and are, therefore, suitable and even preferred
for use in the practice of this invention. These compounds have
suitable solubility in alcohol (ethanol) for use in this method. As
summarized above, suitable dopant elements include lanthanum,
manganese, molybdenum, niobium, tantalum, tungsten, strontium,
vanadium, and yttrium. Atoms of the dopant element(s) may be added
to promote electronic conductivity by introducing defects in the
crystalline titanium oxide support material. The dopant(s) is
suitably added in an amount up to about half of the atoms of
titanium in the support material. Alkoxide compounds or salts of
these dopant elements are available and may be used for introducing
one or more dopant element (s) into the titanium oxide catalyst
support particles.
[0014] For example, titanium (IV) isopropoxide and niobium (V)
chloride, or niobium (V) ethoxide, are dissolved in ethanol in
proportions of two atomic parts titanium per atom of niobium.
Silica particles (10-15 nm in largest dimension) are dispersed in
the alcohol solution or sol of titanium and niobium compounds.
Silica is suitably added to the sol in an amount to provide about
1.2 parts by weight of silicon per part of titanium. As an
alternative nanometer size particles of nylon or vinyl chloride may
be used as pore-forming templates in the dispersion. The uniformity
of mixing of the constituents of the dispersion may be enhanced by
sonic vibration of the dispersion.
[0015] The solution (sol) is then acidified with aqueous
hydrochloric acid to hydrolyze the titanium and niobium compounds
and form a gel or precipitate of titanium-containing and
niobium-containing material entraining the silica particles. The
titanium containing material contains sufficient oxygen for the
formation of titanium dioxide.
[0016] The precipitate or gel is separated from the liquid medium
and dried. The solid material is then heated to about 1000.degree.
C. in an atmosphere of hydrogen (or suitably, ammonia) so as to
form crystalline titanium dioxide doped with elemental niobium. The
particles of titanium dioxide are very small, nanometer size, and
the particles of silica are dispersed in the doped titanium
dioxide.
[0017] The niobium doped oxide particles are chemically etched with
aqueous sodium hydroxide or hydrogen fluoride to remove the
pore-forming silica particles. The residue of the chemical etching
is a mass of very small, pore containing, Nb-doped, TiO.sub.2
particles where the pores are formed principally by the removal of
the silica particles.
[0018] In a specific experimental example, the resulting porous
TiO.sub.2 was crystalline, contained Ti/Nb in an atomic ratio of 2,
and had a BET surface area of 125 m.sup.2/g.
[0019] In a continuation of the experimental illustration, Pt was
deposited on this Nb-doped TiO.sub.2 using an aqueous solution of
diamineplatinum (II) nitrite, Pt (NO.sub.2).sub.2 (NH.sub.3).sub.2,
as a precursor. The Nb-doped TiO.sub.2 was dispersed in water at
80.degree. C. using ultrasonic energy. The platinum precursor was
also separately dissolved in 70-80.degree. C. water with stirring.
The TiO.sub.2 dispersion and the platinum precursor solution were
mixed. The pH of the resulting platinum deposition medium was
adjusted to 3.0 using acetic acid and carbon monoxide gas was
diffused through the medium at a rate of two liters per minute. The
reaction medium was stirred at 90.degree. C.
[0020] Hydrazine hydrate was used for reduction of the platinum and
its deposition as very small particles on the niobium-doped
TiO.sub.2 particles. Hydrazine hydrate was added drop wise with
stirring to the platinum deposition medium (at 90.degree. C., pH 3,
and with CO diffusion) over a period of one hour. Then the
TiO.sub.2-containing medium with deposited platinum was cooled to
room temperature. The reaction product of platinum deposited on
niobium-doped titanium dioxide particles was filtered through a
0.45 micrometer pore-size cellulose nitrate membrane, washed with
distilled water, and dried overnight in a vacuum oven at 50.degree.
C.
[0021] In this example platinum was deposited at 72 weight percent
on porous niobium doped titanium dioxide and the resulting catalyst
was tested with a gas phase accelerated thermal sintering method
intended to induce oxidative corrosion of the catalyst. The test
was conducted at 250.degree. C. for 30 hours under an atmosphere,
by volume, of 0.7% O.sub.2, 8% H.sub.2O, and the balance helium.
Two commercial platinum-on-carbon catalysts were subjected to the
same corrosion testing for comparison. Table 1 records the mass
loss resulting from the platinum-on-titanium dioxide catalyst
produced in accordance with this invention and the two comparison
carbon supported platinum catalysts. TABLE-US-00001 TABLE 1 Mass
Loss Comparison Catalysts Pt loading Mass Loss Pt/TiO.sub.2 (no Nb)
42% -1.1% Pt/TiO.sub.2 (Nb/Ti = 1/2) 72% -4.4% Pt on carbon (1)
46.6% -55.8% Pt on carbon (2) 45.9% -76.2%
[0022] It is seen that the titanium oxide supported catalysts
survives an oxidizing environment better than the carbon supported
catalyst.
[0023] The above porous, niobium-doped titanium oxide supported
platinum catalyst was further tested for its oxygen reduction
activity. The catalyst sample was prepared for electrochemical
measurement by a special method (mixing and sonication in a
suspension) to form an ink for application to a rotating disk
electrode (RDE). The suspension contained the platinum on
doped-titanium dioxide support (designated 41305 TJ) and a
commercial electrically conductive particulate carbon dispersed in
isopropanol and water. The dispersion also contained a 5% solution
of Nafion.sup.TM ionomer in water.
[0024] The supported platinum and carbon containing mixture was put
into a sealed 60 ml glass bottle. The content was subsequently
mixed by shaking and sonicated for 2-4 hours. Once a homogeneous
ink suspension was formed, 10-20 micro liters of the suspension
were dispensed on a glassy carbon electrode surface. After drying
at room temperature, the electrode was put on the Rotating Disk
Electrode (RDE) device for activity measurement (in micro-amperes
per square centimeter of platinum at 0.9V). The resulting dried
catalyst on the electrode contained 52.6 wt. % Pt.
[0025] A sample of platinum on non-doped TiO.sub.2 was prepared for
comparison testing. The platinum on non-doped TiO.sub.2 (sample
0131005TJ) was applied as in ink to a RCE for comparative electrode
activity measurement by the technique described above. Also, a
second platinum on niobium-doped TiO.sub.2 catalyst was prepared
(sample 061705KV). This sample contained niobium in an amount of 5%
of the titanium and the platinum loading on the electrode was lower
(33.4%) than sample 131005TJ.
[0026] In the electrode activity tests the electrode was rotated at
1600 RPM in the 0.1 M HClO.sub.4 electrolyte at 60.degree. C. with
a flowing, saturated oxygen atmosphere at one atmosphere. The
electrode voltage scan rate was 5mV/s over a voltage range of
0-1V.
[0027] Table 2 summarizes the specific oxygen reduction activities
of two illustrative platinum-on-doped titanium dioxide support
catalysts and like data obtained using the non-doped TiO.sub.2
sample and two commercial platinum-on-carbon comparison catalysts.
TABLE-US-00002 TABLE 2 Specific activity (uA/cm.sup.2 Pt at
Catalyst Pt (wt %) Type 0.90 V) 0131005TJ 27.8 Pt/TiO.sub.2 (no Nb)
153 041305TJ 52.6 Pt/Nb--TiO.sub.2 (1:2) 548 061705KV 33.4
Pt/Nb--TiO.sub.2 (5%) 494 Pt/C (3) 46.4 Pt Co/C 298 Pt/C (4) 46.5
Pt/HSC 172
[0028] It is seen that the niobium-doped titanium support particles
with platinum catalyst provided highly suitable specific electrode
activity in the tests. The specific activities of the tow samples
in uA/cm.sup.2 Pt at 0.90V were higher than either of the platinum
on carbon electrocatalysts or the platinum on non-doped TiO.sub.2
electrode material.
[0029] While the invention has been illustrated by certain
preferred embodiment, these illustrations are intended to be
non-limiting.
* * * * *