Title: Catalyst for use in fuel electrode of polymer solid electrolyte type fuel cell
Abstract: The present invention is a catalyst for use in a fuel electrode of a polymer solid electrotype fuel cell which is formed by making a carbon powder support platinum and ruthenium thereon, and is characterized in that the loading ratio between platinum and ruthenium is from 1:2.5 to 1:4 (in molar ratio). It is preferable that the loading density of the catalyst is 40 to 70%. Additionally, it is preferable that a carbon powder having a specific surface area of 600 to 1,200 m2/g is used as the carrier supporting the catalyst particles.
Patent Number: 7,001,865 Issued on 02/21/2006 to Tada, et al.
| Inventors:
|
Tada; Tomoyuki (Hiratsuka, JP);
Inoue; Masahiko (Hiratsuka, JP)
|
| Assignee:
|
Tanaka Kikinzoku Kogyo K.K. (Tokyo, JP)
|
| Appl. No.:
|
484962 |
| Filed:
|
April 11, 2003 |
| PCT Filed:
|
April 11, 2003
|
| PCT NO:
|
PCT/JP03/04613
|
| 371 Date:
|
January 28, 2004
|
| 102(e) Date:
|
January 28, 2004
|
| PCT PUB.NO.:
|
WO03/088387 |
| PCT PUB. Date:
|
October 23, 2003 |
Foreign Application Priority Data
| Apr 12, 2002[JP] | 2002-110211 |
| Current U.S. Class: |
502/185; 502/101; 502/182; 502/325; 429/40 |
| Current Intern'l Class: |
B01J 21/18 (20060101); B01J 23/40 (20060101); H01M 4/88 (20060101); H01M 4/86 (20060101) |
| Field of Search: |
502/101,185,182,325
429/40
|
References Cited [Referenced By]
U.S. Patent Documents
| 6284402 | Sep., 2001 | Mallouk et al.
| |
| 6326098 | Dec., 2001 | Itoh et al.
| |
| 6339038 | Jan., 2002 | Tada et al.
| |
| 6498121 | Dec., 2002 | Gorer.
| |
| 6551960 | Apr., 2003 | Laine et al.
| |
| 2003/0008198 | Jan., 2003 | Mukoyama et al.
| |
| 2003/0176277 | Sep., 2003 | Suh et al.
| |
| 2004/0126631 | Jul., 2004 | Uchida et al.
| |
| 2004/0184983 | Sep., 2004 | Paparatto et al.
| |
| Foreign Patent Documents |
| 10-334925 | Dec., 1998 | JP.
| |
| 11-250918 | Sep., 1999 | JP.
| |
| 2000-3712 | Jan., 2000 | JP.
| |
| 2000-12043 | Jan., 2000 | JP.
| |
| 2002/-358971 | Dec., 2002 | JP.
| |
| 2003-24798 | Jan., 2003 | JP.
| |
| 2003/-170725 | Jan., 2003 | JP.
| |
| 2003/-187851 | Jul., 2003 | JP.
| |
| WO 01/1524/7 | Mar., 2001 | WO.
| |
| WO 01/1525/4 | Mar., 2001 | WO.
| |
Primary Examiner: Lorengo; J. A.
Assistant Examiner: Hailey; Patricia L.
Attorney, Agent or Firm: Rothwell, Figg, Ernst & Manbeck, P.C.
Claims
The invention claimed is:
1. A catalyst for use in a fuel electrode of a polymer solid electrolyte type
fuel cell consisting essentially of a platinum and ruthenium alloy supported on
a carbon powder carrier, characterized in that a loading ratio between platinum
and ruthenium in said alloy ranges from 1:2.5 to 1:4 (in molar ratio) and a loading
density of the catalyst particle is about 60 to 70%.
2. The catalyst for use in a fuel electrode of a polymer solid electrolyte type
fuel cell according to claim 1, wherein the carrier is a carbon powder having a
specific surface area of 600 to 1,200 m
2/g.
3. A production method of a catalyst for use in a fuel electrode of a polymer
solid electrolyte type fuel cell, comprising the steps of making the carrier support
platinum and ruthenium, and heating said carrier at 600° C. to 1,200°
C. to alloy substantially all platinum and ruthenium with each other, wherein a
loading ratio between platinum and ruthenium in said alloy ranges from 1:2.5 to
1:4 (in molar ratio) and a loading density of the catalyst particle is about 60
to 70%.
Description
CROSS REFERENCE TO RELATED APPLICATION
This application is a 35 USC § 371 National Phase Entry Application from
PCT/JP03/04613, filed Apr. 11, 2003, and designating the U.S.
TECHNICAL FIELD
The present invention relates to a catalyst for use in a polymer solid electrolyte
type fuel cell, particularly, to a catalyst used in a fuel electrode of a polymer
solid electrotype type fuel cell.
BACKGROUND ART
Fuel cells are highly expected as electricity generation systems, among which
polymer solid electrolyte type fuel cells, which use polymer electrolytes as the
electrolytes, have lower operation temperatures and are compact as compared to
phosphoric acid type fuel cells and the like, accordingly are regarded as promising
as power supplies for electric cars.
Now, a polymer solid electrolyte type fuel cell has a laminate structure comprising
two electrodes, namely, a fuel electrode and an air electrode, and a polymer solid
electrolyte film sandwiched between these electrodes; the fuel electrode is supplied
with a hydrogen containing fuel and the air electrode is supplied with oxygen or
air, and thus the electric power is generated by the oxidation and reduction reactions
occurring in the respective electrodes. To these two electrodes, a mixture containing
a catalyst for accelerating the chemical reactions and a solid electrolyte is generally
applied. As the catalysts constituting the electrodes, widely used are platinum
catalysts in which platinum, satisfactory in catalytic activity, is made to be supported.
Different characteristics are demanded for a catalyst for use in a polymer
solid electrolyte type fuel cell; the demanded characteristics for the fuel electrode
and those for the air electrode are probably different from each other. The catalyst
for use in the fuel electrode is demanded to have the resistance to catalyst poisoning
due to carbon monoxide, in addition to high catalytic activity. As the hydrogen
supplied to the fuel electrode, the reformed hydrogen obtained from methanol or
the like is regarded as promising; however, carbon monoxide is contained in the
reformed hydrogen as an impurity, and a problem occurs that the carbon monoxide
is adsorbed on the catalyst particles to deactivate the catalyst. This is the reason
why the resistance to catalyst poisoning due to carbon monoxide is demanded. Accordingly,
for the purpose of improving the resistance to catalyst poisoning due to carbon
monoxide, supported ruthenium catalysts are widely applied as the catalyst for
the fuel electrode, in addition to supported platinum catalysts.
In these years, however, the practical use of polymer solid electrolyte type
fuel
cells comes to be established, and in this context a new problem is confirmed to
be involved in the catalyst for use in the fuel electrode. This problem is the
one that the cell characteristics are degraded when the fuel becomes deficient
during the fuel cell operation. More specifically, when some abnormal condition
somehow occurs in fuel supply during the steady operation of a fuel cell, the activity
of the catalyst in the fuel electrode is degraded owing to the fuel deficiency
and the cell characteristics are degraded, causing trouble in steady supply of
electric power.
Additionally, when such catalyst activity degradation due to fuel deficiency
occurs, if the catalyst activity is recovered by the fuel supply that is once again
made normal, the cessation of the electric power supply is temporal and nonfatal.
However, according to the previous reports, the catalyst activity degradation caused
by the fuel deficiency is irreversible in nature, and it has been confirmed that
the catalyst activity is not fully recovered by once again supplying the fuel.
As a countermeasure against the irreversible deactivation of the catalyst caused
by the fuel deficiency, the establishment of a system free from cessation of fuel
supply can be said most important. However, even if such a peripheral system can
be improved, on the assumption of a worse case scenario, it is also preferable
to improve the catalyst of the fuel electrode and the fuel cell themselves so that
the characteristics thereof may not be degraded when the fuel becomes deficient.
Now, as one of the remedies having hitherto been studied for the fuel cell catalyst,
for example, the addition of ruthenium oxide (RuO
2) or iridium oxide
(IrO
2) to the catalyst layer has been known. Additionally, it has been
claimed to be effective that, as additional remedies for improvement, the adopted
carrier is made to be a carrier stable in oxidation properties such as graphitized
carbon, titanium oxide (Ti
4O
7) and the like, and moreover,
the amount of the supported catalyst particles is increased (for details of these
remedies for improvement, see the international publication gazettes, WO 01/1527,
WO 01/152547).
However, according to the investigation conducted by the present inventors,
these remedies can be effective to a certain extent but not necessarily to a sufficient
extent, and yield non-negligible degradation in characteristics when the fuel is
deficient. Accordingly, as for the catalyst for use in the fuel electrode, it is
necessary to find additional improvements other than these remedies.
The present invention has been made with the background described above, and
takes as its object the provision of a catalyst hardly degradable in catalytic
activity even when the fuel deficiency occurs, as the catalyst for use in the fuel
electrode of a polymer solid electrolyte type fuel cell.
DISCLOSURE OF THE INVENTION
For the purpose of achieving the above described object, the present inventors
have studied at the beginning the factor contributing to the degradation of the
catalytic activity in the catalyst for use in the fuel electrode when the fuel
becomes deficient. Consequently, the present inventors have considered that the
factor involves the variation of the dominating reaction type in the fuel electrode
when the fuel becomes deficient.
In the usual condition where the fuel is being supplied, protons are supplied
to the fuel electrode through the electrolysis of hydrogen molecules, generating
the source for the electric power generation. In other words, usually the decomposition
reaction of hydrogen molecules is dominant in the fuel electrode, where the potential
of the fuel electrode is approximately taken as 0 V (vs. the standard hydrogen
electrode). On the other hand, when the fuel becomes deficient, the electrolysis
reaction of water occurs in the fuel electrode to supply protons coming short therein,
this reaction becoming dominant. The potential for the electrolysis of water amounts
to 1.23 V (vs. the standard hydrogen electrode), and hence it is conceivable that
the fuel electrode potential is elevated by the occurrence of the fuel deficiency.
The present inventors have interpreted that such environmental variation involving
the fuel electrode (the potential elevation) causes some transformation to occur
in the fuel electrode catalyst which degrades the activity. Investigation of the
transformation occurring in the catalyst has resulted in an interpretation that
the elevated potential results in generation of some type of coating film on the
surface of the catalyst, which degrades the activity of the catalyst. Additionally,
it is interpreted that the generation of the coating film is irreversible in such
a way that the coating film is neither decomposed nor faded away to remain on the
surface of the catalyst, inhibiting the reactivation of the catalyst.
Accordingly, the present inventors have come to an idea that it is preferable
to develop a technique to suppress the formation of the coating film for the purpose
of making the catalyst remain free from deactivation even when the potential of
the fuel electrode is elevated. Thus, as a result of the investigation concerned,
the present inventors have discovered that an intentional elevation of the loading
ratio of ruthenium relative to that in the conventional fuel electrode catalyst
makes it possible to suppress the coating film formation, and have thereby thought
up the present invention.
More specifically, the present invention is a catalyst for use in the fuel electrode
of the polymer solid electrolyte type fuel cell comprising platinum and ruthenium
supported on a carbon powder carrier, and is characterized in that the loading
ratio of platinum to ruthenium is from 1:2.5 to 1:4 (in molar ratio).
The loading ratio of ruthenium in the catalyst for use in the fuel electrode
involved in the present invention is made higher than the conventional loading
ratios (the loading ratio of platinum to ruthenium is of the order of 1:1 to 1:2).
The reason for such increase of the loading ratio of ruthenium to suppress the
coating film generation in the environment of the high potential is inferred such
that the crystalline structure of the catalyst particle is altered when the loading
ratio of ruthenium becomes high, and the coating film formation is suppressed in
such a condition. However, this inference is not based on any evidence at present,
but is merely based on the suppression effect confirmed by the test conducted by
the present inventors. Such a loading ratio range of ruthenium as specified above
comes from the following reasons: the ratio of the platinum to ruthenium of 1:2
or less overlaps with the corresponding ratios in the conventional catalysts for
use in the fuel electrode, and the ratio less than 1:2.5 leads to a certain but
insufficient suppression effect against the activity degradation due to fuel deficiency,
in particular, to degradation in characteristics when the fuel deficiency period
is extended; and on the other hand, when the ratio of platinum to ruthenium exceeds
1:4, the amount of the catalyst particles (involving both platinum particles and
ruthenium particles) becomes large and the dispersion condition of the particles
is degraded with an apprehension of degradation in characteristics.
As described above, the present invention provides a catalyst free from degradation
in characteristics through suppressing the coating film formation on the catalyst
surface even in the environment of high potential where the fuel is deficient with
the aid of the elevated loading ratio of ruthenium. Incidentally, ruthenium is
an element also serving to improve the resistance to catalyst poisoning due to
carbon monoxide. Now, it is preferable that platinum and ruthenium are in an alloyed
condition as a form of solid solution, for the purpose of more enhancing the suppression
effect against the activity degradation occurring when the fuel is deficient and
more enhancing the resistance to catalyst poisoning due to carbon monoxide.
Additionally, it is preferable that the catalyst involved in the present
invention has a loading density of 40 to 70% for the catalyst particle comprising
platinum and ruthenium in consideration of the electrode characteristics. The loading
density as referred to here signifies the ratio of the mass of the catalyst particle
to be supported by the carrier (in the present invention, the total weight of the
mass of the platinum and the mass of the ruthenium) to the mass of the carrier
in the whole catalyst. Such specification of the loading density as described above
reflects the consideration both of the electrode characteristics and of the acceleration
of alloying of platinum with ruthenium. More specifically, the specification of
the range made to be 40% or more comes from the fact that the loading density of
at least 40% or more is needed to meet the purpose of making the total amount of
the catalyst as small as possible while securing a desired catalyst particle amount,
in the fuel cell electrode design conducted with reference to the catalyst particle
amount. Additionally, this is because a decreased loading density results in a
large mutual separation between the two types of metal particles, making the alloying
difficult when alloying platinum with ruthenium. On the other hand, when the loading
density becomes as high as exceeding 70%, the alloy particles obtained when alloying
of platinum with ruthenium become coarse and large, resulting in degradation of
the cell characteristics.
Furthermore, as for the carrier made to support the catalyst, carbon
powders having the specific surface areas of 600 to 1,200 m
2/g are particularly
preferable. This is because making the specific surface area be 600 m
2/g
or more permits increasing the area for the catalyst adhesion so that the catalyst
particles can be dispersed widely and the effective surface area can be made high.
Additionally, this is because a carrier with a specific surface area exceeding
1,200 m
2/g is high in the abundance ratio of the ultramicropores (micropores
smaller than about 20 Å) which reject the intrusion of the ion-exchange
resin when forming the electrode so that such a carrier lowers the exploitation
efficiency of the catalyst particles. Thus, the specification of such a specific
surface area as described above permits dispersing the noble metal particles widely
so that the activity per unit mass of the catalyst is improved, and ensures the
exploitation efficiency of the catalyst.
A production method of the catalyst involved in the present invention comprises
a process for making the carrier support platinum and ruthenium comprising the
catalyst particle, and a process for alloying the supported platinum and ruthenium
with each other. As for these processes, there is no particular restriction to
the process for making the carrier support platinum and ruthenium. More specifically,
the carrier can be made to support platinum and ruthenium by impregnating the carrier
with a platinum salt solution and a ruthenium salt solution as conventionally performed.
Incidentally, as for the order of supporting platinum and ruthenium, no particular
effect is provided either by whether any one of platinum and ruthenium is supported
in advance of the other or by concurrently supporting platinum and ruthenium. As
for alloying the supported platinum with a supported auxiliary metal, it is appropriate
to heat the carrier at 600° C. to 1,200° C. in the hydrogen reduction
atmosphere in order to realize a sufficiently alloyed state. Here, it is preferable
that the hydrogen concentration in the reaction atmosphere is made to be nearly 100%.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a graph showing the time-course variations of the potentials of the
anodes in the fuel deficiency tests of the embodiment of the present invention.
BEST MODE FOR CARRYING OUT THE INVENTION
Description will be made below on the preferred embodiment of the present
invention with reference to the drawing.
In the present embodiment, platinum/ruthenium alloy catalysts having various
loading
ratios and loading densities were produced, and the characteristics thereof were
to be confirmed. The production of the catalysts was performed by preliminarily
producing platinum catalysts in which platinum was supported on the above described
carbon powder; making these catalysts support ruthenium thereon through impregnating
these catalysts with a ruthenium compound solution; and alloying the supported
metals with each other through an additional heat treatment.
[Selection of Carriers]
The carrier used in the present embodiment was a commercially available carbon
fine powder (brand name: Ketjenblack EC). The specific surface area of the carrier
was measured by the BET one-point method to be 800 m
2/g.
[Preparation of a Platinum Catalyst]
30 g of the above described carbon powder was soaked in 900 g (the platinum content:
20 g) of a dinitrodiammine platinum solution having a platinum concentration of
2.2 wt % as a platinum solution, the solution was stirred, and then 100 ml of 100%
ethanol was added as a reducing agent to the solution. The solution was stirred
and mixed at the boiling point (about 95° C.) for 6 hours to make the carbon
powder support platinum. Then the solution was filtered, and thus the filtered-out
solid content was dried to yield a platinum catalyst.
[Supporting of Ruthenium]
Then, 10 g of the above described platinum catalyst (the platinum content:
4 g) was mixed in 100 g of a ruthenium chloride solution containing 5.2 wt % of
ruthenium (the ruthenium content: 5.2 g), and the mixture was stirred, then filtered,
and the filtered out solid content was dried to yield a catalyst in which platinum
and ruthenium were supported.
[Heat Treatment]
The heat treatment for alloying of platinum with ruthenium was conducted for
the carrier made to support platinum and ruthenium in the above described processes.
The heat treatment for alloying was conducted by maintaining the carrier in a 50%
hydrogen gas (the balance was nitrogen) at 900° C. for one hour.
The ratio between the supported metals in the platinum-ruthenium alloy catalyst
produced by the above described processes is 1:2.5. Additionally, the loading density
for platinum and ruthenium is 60%. The values of the loading ratio and loading
density can be easily controlled by varying the platinum content in the platinum
solution impregnated into the carrier and the ruthenium content in the ruthenium
solution impregnated into the platinum catalyst.
In the present embodiment, in a manner similar to that described above, catalysts
were produced in which the loading ratio between platinum and ruthenium was varied,
and fuel electrodes were produced therefrom and the characteristics of the fuel
electrodes were examined. The production procedures of an electrode are as follows:
1 g of the catalyst (as the weight of the carbon powder) was checkweighed and mixed
in 1.2 g of a resin powder prepared through spray drying of 5% solution of an ion-exchange
resin (brand name: Nafion, manufactured by DuPont, Inc.); the mixture obtained
was placed in 25 mL of an aqueous 1-propanol and the mixture obtained was mixed
for 100 minutes using a ball mill to yield a catalyst paste; the above described
catalyst paste was applied, so as for the platinum amount to be 0.56 g/cm
2,
by printing onto a sheet of a carbon paper impregnated with PTFE, to be used as
the gas diffusion layer, having the surface layer coated with carbon and an ion-exchange
resin; and furthermore, the sheet of carbon paper was dried at 100° C. and
subsequently subjected to hot press at 130° C. and 20 kg/cm
2 for
one minute to yield an electrode.
The examination of the electrode characteristics conducted in the present invention
was as follows. The fuel was supplied to the fuel electrode (a half-cell), when
the polarization value at the current density of 500 mA/cm
2 was measured;
and subsequently the fuel supply was halted and in that condition the time variation
of the anode potential was measured as a function of time for 5 minutes while the
current was made to flow at 200 mA/cm
2 (hereinafter, the anode potential
measurement when the fuel is deficient will be referred to as a fuel deficiency
test). Then, the fuel was supplied to the electrode after being subjected to the
fuel deficiency test, when the polarization value was measured at the current density
of 500 mA/cm
2 to examine whether the electrode characteristics were
degraded or not (hereinafter, the polarization value measurement conducted when
the fuel was supplied to the electrode after the fuel deficiency test will be referred
to as a performance test). Additionally, after the performance test, the fuel supply
was once again halted and the fuel deficiency test was conducted for 5 minutes
(accumulated to be 10 minutes); after the second fuel deficiency test, the fuel
was once again supplied to the electrode and the performance test was conducted.
The test conditions involved in these tests were as follows:
Electrode area: 7 cm2
Temperature: 60° C.
Pressure: Atmospheric pressure
Fuel used for the fuel supply: 100% Hydrogen
In the first place, Table 1 shows the results of the fuel deficiency tests, among
the examination results. In the fuel deficiency test, the elevation of the anode
potential signifies the degradation in the electrode characteristics, and an anode
potential falling within a certain low level range indicates the successful maintenance
of the characteristics. Additionally, the results (polarization values) of the
performance tests conducted after the fuel deficiency periods of 5 minutes and
10 minutes are shown in Table 1.
| |
TABLE 1 |
| |
|
| |
Before |
After fuel deficiency test |
| |
deficiency |
Deficiency period: |
Deficiency period: |
| Loading ratio |
(mv) |
5 minutes (mv) |
10 minutes (mv) |
|
| Pt:Ru = 1:0.5 |
4 |
110 |
231 |
| Pt:Ru = 1:1 |
2 |
4 |
241 |
| Pt:Ru = 1:1.5 |
5 |
3 |
138 |
| Pt:Ru = 1:2 |
3 |
4 |
214 |
| Pt:Ru = 1:2.5 |
3 |
5 |
7 |
| Pt:Ru = 1:3 |
6 |
5 |
7 |
| Pt:Ru = 13.5 |
2 |
3 |
5 |
| Pt:Ru = 1:4 |
6 |
4 |
4 |
|
Among the results thus obtained, according to FIG. 1, the electrode produced
with the catalyst having the platinum/ruthenium loading ratio of 1:0.5 showed the
anode potential rise within 5 minutes in the fuel deficiency test, revealing the
degradation in characteristics. Additionally, the electrode produced with the catalyst
having the platinum/ruthenium loading ratio of 1:0.5 showed a high polarization
value in the performance test, in which the fuel was once again supplied, conducted
after the fuel deficiency period of 5 minutes, indicating that the irreversible
degradation occurred in the electrode characteristics. The electrode also exhibited
an elevation of the anode potential immediately after the resumption of the fuel
deficiency test after the performance test.
According to FIG. 1 and Table 1, the electrodes produced with the catalysts
having the loading ratios of 1:1 to 1:2 exhibited no degradation in characteristics
in the first fuel deficiency period of 5 minutes. However, the elevation of the
anode potentials was observed after the elapsed time of 6 to 8 minutes (accumulated
time) in the second fuel deficiency test conducted after the performance test,
and accordingly it was confirmed that these electrodes were unable to maintain
the characteristics against the long period of fuel deficiency. Additionally, remarkable
potential elevation was confirmed in the performance test conducted after the fuel
deficiency period of 10 minutes, indicating that the electrode characteristics
were degraded.
On the other hand, as FIG. 1 shows, the electrodes produced with the catalysts
having the loading ratios of 1:2.5 to 1:4 exhibited no anode potential elevation
even after the fuel deficiency periods of 5 and 10 minutes. In the performance
tests after the fuel deficiency periods of 5 and 10 minutes, the polarization values
as observed when the fuel was supplied were approximately the same as those observed
at the beginning. In other words, it was confirmed that these electrodes were able
to maintain the electrode characteristics even after the accumulated fuel deficiency
period of 10 minutes.
INDUSTRIAL APPLICABILITY
As described above, according to the catalyst for use in the fuel electrode involved
in the present invention, the irreversible activity degradation due to the fuel
supply halting as observed in conventional catalysts can be suppressed. According
to the present invention, the reliability of a polymer solid electrolyte type fuel
cell can be improved, and the present invention can contribute to the acceleration
of the practical application thereof.
*
