U.S. patent application number 11/533706 was filed with the patent office on 2007-04-12 for non-noble metal catalysts for the oxygen reduction reaction.
Invention is credited to Stephen A. Campbell.
Application Number | 20070082808 11/533706 |
Document ID | / |
Family ID | 31191353 |
Filed Date | 2007-04-12 |
United States Patent
Application |
20070082808 |
Kind Code |
A1 |
Campbell; Stephen A. |
April 12, 2007 |
NON-NOBLE METAL CATALYSTS FOR THE OXYGEN REDUCTION REACTION
Abstract
Non-noble metal transition metal catalysts can replace platinum
in the oxidation reduction reaction (ORR) used in electrochemical
fuel cells. A Ru.sub.xSe catalyst is prepared with comparable
catalytic activity to platinum. An environmentally friendly aqueous
synthetic pathway to this catalyst is also presented. Using the
same aqueous methodology, ORR catalysts can be prepared where Ru is
replaced by Mo, Fe, Co, Cr, Ni and/or W. Similarly Se can be
replaced by S.
Inventors: |
Campbell; Stephen A.; (Maple
Ridge, BC) |
Correspondence
Address: |
SEED INTELLECTUAL PROPERTY LAW GROUP PLLC
701 FIFTH AVE
SUITE 5400
SEATTLE
WA
98104
US
|
Family ID: |
31191353 |
Appl. No.: |
11/533706 |
Filed: |
September 20, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10630634 |
Jul 29, 2003 |
7125820 |
|
|
11533706 |
Sep 20, 2006 |
|
|
|
60400194 |
Jul 31, 2002 |
|
|
|
Current U.S.
Class: |
502/150 ;
502/305; 502/319; 502/321; 502/325; 502/337; 502/338 |
Current CPC
Class: |
Y02E 60/50 20130101;
H01M 4/92 20130101; B01J 37/03 20130101; B01J 27/02 20130101; B01J
27/0573 20130101; H01M 4/90 20130101; B01J 37/082 20130101; H01M
4/9075 20130101 |
Class at
Publication: |
502/150 ;
502/325; 502/321; 502/338; 502/319; 502/337; 502/305 |
International
Class: |
B01J 31/00 20060101
B01J031/00 |
Claims
1-6. (canceled)
7. A non-noble transition metal catalyst prepared by: dissolving
selenium and Ru.sub.3(CO).sub.12 in an organic solvent; refluxing
the organic solvent; obtaining a precipitate; and heating the
precipitate to a temperature greater than or equal to 600.degree.
C. under an inert atmosphere.
8. The catalyst of claim 7 wherein the catalyst is supported.
9. An electrochemical fuel cell comprising a non-noble transition
metal catalyst at the cathode wherein the catalyst is prepared by
the method of claim 7.
10-22. (canceled)
23. A non-noble transition metal catalyst prepared by: dissolving a
metal salt in an aqueous solution, the metal is ruthenium,
molybdenum, iron, cobalt, chromium, nickel or tungsten;
precipitating the metal; introducing a chalcogen, the chalcogen
being sulfur or selenium; and reacting the precipitated metal with
the chalcogen by heating under an inert atmosphere.
24. The catalyst of claim 23 wherein the catalyst is supported.
25. An electrochemical fuel cell comprising a non-noble transition
metal catalyst of claim 23 at the cathode.
26. An electrochemical fuel cell stack comprising at least one
electrochemical fuel cell of claim 24.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 10/630,634 filed Jul. 29, 2003, now allowed;
which claims the benefit of U.S. Provisional Patent Application No.
60/400,194 filed Jul. 31, 2002. Both of these applications are
incorporated herein by reference in their entireties.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The field of the invention relates to non-noble metal
catalysts for the oxygen reduction reaction including methods of
manufacture.
[0004] 2. Description of the Related Art
[0005] Electrochemical fuel cells convert fuel and oxidant to
electricity and reaction product. Solid polymer electrochemical
fuel cells generally employ a membrane electrode assembly ("MEA")
in which an electrolyte in the form of an ion-exchange membrane is
disposed between two electrode layers. The electrode layers are
made from porous, electrically conductive sheet material, such as
carbon fiber paper or carbon cloth. In a typical MEA, the electrode
layers provide structural support to the membrane, which is
typically thin and flexible.
[0006] The MEA contains an electrocatalyst, typically comprising
finely comminuted platinum particles disposed in a layer at each
membrane/electrode layer interface, to induce the desired
electrochemical reaction. The electrodes are electrically coupled
to provide a path for conducting electrons between the electrodes
through an external load.
[0007] A significant emphasis has been placed to reduce MEA costs
by reducing the platinum loading while maintaining or even
improving performance and reliability. One approach is to eliminate
platinum entirely and replace it with a cheaper alternative
catalytic material. In particular, a significant amount of work has
been done working on replacing platinum for the oxygen reduction
reaction at the cathode.
[0008] Aside from cost, platinum catalysts have a further
disadvantage when used in direct methanol fuel cells (DMFCs) in
which methanol is used as the fuel. Namely, platinum at the cathode
oxidizes methanol that crosses over from the anode leading to
depolarisation and hence serious power losses in the cell.
[0009] Bron et al. (Journal of Electroanalytical Chemistry 500,
2001, 510-517) discloses a ruthenium-based catalyst for oxygen
reduction. The catalysts were prepared by reacting
Ru.sub.3(CO).sub.12 with selenium for 20 hours in deaerated xylene
under reluxing conditions. The product was filtered, washed with
diethylether and dried in an oven at 90.degree. C. to produce a
black powder. Bron et al. studied the effect of selenium and found
a maximum benefit at about 15 mol % Se though catalytic activity
was still observed in a selenium free catalyst. Bron concluded that
the catalytic center in the selenium-containing catalyst is
different from the catalytic center in the selenium-free catalyst.
Selenium was also found to protect the catalyst against
electrochemical oxidation and therefore led to enhanced
stability.
[0010] In a second publication produced by the same group,
Tributsch et al. (Journal of Applied Electrochemistry 31, 2001,
739-748), found that heating of this product resulted in the loss
of carbon species in well defined steps. The first step involved
the loss of CO and CO.sub.2 between 250 and 350.degree. C. and a
second step was observed at temperatures above 600.degree. C.
Further, Tributsch et al. found a loss of catalytic activity
resulting from the release of carbon species at elevated
temperatures. This led Tributsch et al. to propose a complicated
catalytic structure comprising a cubane-like organometallic
ruthenium-complex on the surface of a ruthenium nanoparticle doped
with a chalcogen (selenium or sulfur). Inspiration for this model
appears to be an iron hydrogenase from the Clostridium pasteurianum
bacterium.
[0011] In a prior study on a related system, namely a MoRuS and
MoRuSe system, Trapp et al. (J. Chem. Soc, Faraday Trans. 92(21),
1996, 4311-4319) arrived at significantly different conclusions. In
the synthesis carried out by Trapp et al., Ru.sub.3(CO).sub.12 and
Mo(CO).sub.6 were refluxed in xylene with sulfur or selenium for 20
hours. The catalyst powder was then filtered and dried at room
temperature before being introduced into a tubular furnace at
350.degree. C. for one hour. Though Trapp et al. also performed a
heating step, instead of reduced catalytic activity as reported by
Tributsch et al. supra, Trapp et al. observed improved activity. In
fact, such heating step was referred to as "catalyst activation."
In addition, Trapp et al. concluded that the Ru species is the
active center of the catalyst with some synergistic effects
observed between the ruthenium and the molybdenum sites in the
mass-transport region. Trapp et al. also found that catalytic
activity of the MoRuS was not affected by methanol. Under
conditions of simulated methanol cross-over, the activated MoRuS
catalyst had a similar activity to platinum. However, similar
activity was only observed with methanol present. In the absence of
methanol, the activity of activated MoRuS catalyst was
significantly worse than platinum.
[0012] Despite considerable efforts, a non-noble metal-based
catalyst with activity similar to platinum has yet to be developed.
In addition, existing synthetic methodologies are directed to
experimental scale and, as such, are not necessarily amenable to
commercial scale production. For example, metal carbonyls, which
are typically used as starting materials, are relatively expensive
and typical solvent systems used, namely xylene, are toxic and
environmentally damaging. Thus, even if the catalysts were suitable
for use in fuel cells, an environmentally friendly synthetic method
would be needed.
[0013] The present invention fulfills these and other needs and
provides further related advantages.
BRIEF SUMMARY OF THE INVENTION
[0014] In a first aspect of the present invention, a novel
non-noble transition metal catalyst for the oxidation reduction
reaction is prepared by:
[0015] dissolving selenium and Ru.sub.3(CO).sub.12 in an organic
solvent;
[0016] refluxing the organic solvent;
[0017] obtaining a precipitate; and
[0018] heating the precipitate to a temperature greater than or
equal to 600.degree. C. under an inert atmosphere.
[0019] In one embodiment, the organic solvent may be xylene.
Furthermore, the temperature for the heating step may be, for
example, between 600 and 700.degree. C. Similarly, the heating step
may be, for example, for more than 10 hours or it may be for 12
hours. The inert gas may be, for example, nitrogen or argon.
[0020] The Ru.sub.xSe catalyst thus prepared has an activity to the
oxidation reduction reaction comparable to platinum such that it
can be used at the cathode in a polymer electrolyte membrane fuel
cell. The Ru.sub.xSe catalyst may be supported on, for example,
carbon or unsupported.
[0021] In a second aspect of the present invention, the catalyst is
prepared using aqueous chemistry by:
[0022] dissolving a metal salt in an aqueous solution;
[0023] precipitating the metal;
[0024] introducing a chalcogen such as sulfur or selenium;
[0025] reacting the precipitated metal with the chalcogen by
heating under an inert atmosphere.
[0026] If the metal salt is a ruthenium salt such as ruthenium
(III) chloride, ruthenium (III) nitrate or ruthenium (III) acetate
and the chalcogen is selenium, then a Ru.sub.xSe catalyst as above
will be synthesized. However, the aqueous methodology allows new
non-noble transition metal catalysts to be synthesized where the
metal could be molybdenum, iron, cobalt, chromium, nickel and/or
tungsten.
[0027] Precipitation of the metal may be done by reducing the metal
with a reducing agent such as sodium borohydride or formaldehyde.
Alternatively, the metal may be precipitated with alkali, for
example NaOH or NaHCO.sub.3 to precipitate the corresponding metal
hydroxide or metal carbonate.
[0028] One method of introducing selenium is by dissolving selenium
dioxide in the aqueous solution and co-precipitating elemental
selenium with the same reducing agent as used for the precipitation
of the metal.
[0029] Similarly, the catalyst thus prepared has an activity to the
oxidation reduction reaction such that it can be used at the
cathode in an electrochemical fuel cell.
[0030] These and other aspects of the invention will be evident
upon reference to the attached drawings and following detailed
description.
BRIEF DESCRIPTION OF THE DRAWINGS
[0031] FIG. 1 is a plot illustrating the oxygen reduction current
as a function of the applied potential for an unsupported
ruthenium-selenium catalyst.
[0032] FIG. 2 is a corrected Tafel plot for mass activity of the
ruthenium-selenium catalyst in FIG. 1 and a supported platinum
catalyst.
[0033] FIG. 3 is a corrected Tafel plot for oxygen reduction
catalysts supported on carbon in 0.5M sulfuric acid at room
temperature comparing the activities of platinum, ruthenium and
ruthenium-selenium catalysts.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0034] A novel ruthenium-selenium (Ru.sub.xSe) catalyst with
catalytic activity for the oxygen reduction reaction was
synthesized (1) using an organic solvent, namely xylene, and (2)
using water as solvent. These catalysts were then tested for their
catalytic activity for the oxygen reduction reaction useful in
electrochemical fuel cells. The aqueous methodology is also easily
amenable to synthesizing a series of novel non-noble transition
metal chalcogen catalysts.
Organic Methodologies
[0035] Ru.sub.3(CO).sub.12 may be used as the starting material for
the Ru.sub.xSe catalyst and dissolved with selenium in an organic
solvent such as xylene. While other suitable organic solvents are
well known, the subsequent discussion will only refer to xylene. As
selenium may not be readily soluble in xylene, a selenium-xylene
mixture may be refluxed for a period of time to effect dissolution
before adding any Ru.sub.3(CO).sub.12. After the
Ru.sub.3(CO).sub.12 is added to the reaction mixture, the reaction
mixture may be refluxed again for a period of time to effect
reaction between ruthenium and selenium. A black precipitate may
then be filtered and collected. Complete reaction is not necessary
nor required and unreacted selenium may be observed in the black
precipitate.
[0036] The precipitate is then heated in a furnace under an inert
atmosphere, such as, for example, nitrogen. Complete reaction
between the ruthenium the selenium occurs as well as activation to
produce the Ru.sub.xSe catalyst. In a specific embodiment, heating
may be done at a temperature greater than or equal to 600.degree.
C., for example, between 600 and 700.degree. C.
Aqueous Methodologies
[0037] As mentioned above, heating of the RuxSe catalyst completes
reaction between the ruthenium and selenium and causes
decarbonylation. However, it is unnecessary to begin with the
relatively expensive starting material of Ru.sub.3(CO).sub.12 if
the final step causes decarbonylation. An alternate synthesis
involves the same heating of a ruthenium compound in the presence
of elemental selenium through a more environmentally friendly
aqueous methodology.
[0038] The first step involves precipitation of a ruthenium salt in
aqueous solution. The ruthenium salt may be any ruthenium salt such
as, for example, ruthenium (III) chloride, ruthenium (III) nitrate
or ruthenium (III) acetate. In a specific embodiment, the ruthenium
salt is ruthenium (III) chloride, as it is the least expensive and
the most readily available ruthenium salt. Precipitation may be
carried out with a suitable reducing agent such as, for example,
formaldehyde or sodium borohydrides to produce a metal precipitate.
Alternatively, an alkali solution, for example NaOH or NaCO.sub.3,
may be used to precipitate ruthenium hydroxide or ruthenium
carbonate, respectively.
[0039] In one embodiment, elemental selenium is produced by the
addition of selenium dioxide to the aqueous solution prior to
reduction of the ruthenium salt. In water, selenium dioxide
dissolves to produce selenous acid, which in turn precipitates to
the elemental selenium when reduced by, for example,
NaBH.sub.4.
[0040] After precipitation, the reaction mixture is filtered and
heated in a furnace under an inert atmosphere, such as, for
example, nitrogen. The ruthenium deposit decomposes and reacts with
the elemental selenium to produce the Ru.sub.xSe catalyst. In a
specific embodiment, heating may be done at a temperature greater
than or equal to 600.degree. C., for example, between 600 and
700.degree. C.
[0041] Not only does the above aqueous synthesis avoid both the use
of the costly starting material Ru.sub.3(CO).sub.12, but it also
avoids the use of a toxic and dangerous solvent system, namely
refluxing xylene. As such, the above aqueous synthesis of a
Ru.sub.xSe catalyst is not only environmentally friendly but also
amenable to large-scale commercial production.
[0042] Sulfur may also be used instead of or in addition to
selenium as ruthenium is known to react preferably with sulfur as
compared to selenium (see, e.g., Trapp et al., supra).
Precipitation of elemental sulfur in aqueous solution is likely to
be impractical and, as such, a more specific method involves
directly adding colloidal sulfur to the ruthenium solution prior to
precipitation of the metal so that the ruthenium and sulfur are
combined within a single powder. The colloidal sulfur can be
produced from, for example, polysulfide alkaline solutions. If
sulfur is used as the chalcogen, hydrogen should be avoided in the
heating step as sulfur reacts with hydrogen to produce hydrogen
sulfide. In contrast, selenium would not be expected to appreciably
react with hydrogen.
[0043] In addition to substitution of the chalcogen, novel
catalysts can be synthesized in which ruthenium is replaced by
other non-noble transition metals such as molybdenum, iron, cobalt,
chromium, nickel and/or tungsten. Without being limited thereto,
examples of suitable salts would include: ammonium molybdate,
ammonium iron (III) citrate, ammonium cobalt (II) sulfate
hexahydrate, ammonium tungstate, and cobalt (II) nitrate
hexahydrate. As with ruthenium, either one of selenium or sulfur or
both may be used. Further, mixed catalytic systems wherein the
catalyst contains more than one non-noble transition metal may also
be synthesized by dissolving and precipitating a mixture of at
least two different metal salts in the aqueous solution. While not
being bound by theory, the chalcogen appears to stabilize the
transition metal such that it does not dissolve within the acidic
environment of an electrochemical fuel cell. This allows a greater
variety of non-noble transition metals to be used as catalysts for
the oxygen reduction reaction.
EXAMPLE 1
Synthesis of Unsupported Ru.sub.xSe with an Organic Solvent
[0044] 0.15 g Se was added to 100 ml xylene and refluxed under
bubbling nitrogen overnight before being allowed to cool to room
temperature. 2.85 g Ru.sub.3(CO).sub.12 was then added to the
reaction mixture and refluxed under nitrogen for a further 20
hours. A black precipitate was then washed and dried. On grinding,
the black precipitate was found to contain reddish brown streaks
that was presumed to be unreacted elemental selenium. The material
was then heated under nitrogen to 600.degree. C. in a quartz tube
furnace for 12 hours. After heating, the Ru.sub.xSe powder was
completely black without any reddish brown streaks thereby
indicating complete reaction.
EXAMPLE 2
Synthesis of Carbon Supported Ru.sub.xSe with an Aqueous
Solvent
[0045] 1.0361 g Vulcan XC72R carbon was added to 1l water in a 4 l
beaker. 0.4034 g RuCl.sub.3 and 0.1071 g SeO.sub.2 were dissolved
in 500 ml water and subsequently added to the 4 l beaker. Wetting
was assured by adding 100 ml propan-1-ol and then stirred at
80.degree. C. for 1 hour. The mixture was then allowed to cool to
room temperature. A 1 l solution of 0.1M NaBH.sub.4 in 0.2M NaOH
was added to the beaker and allowed to react slowly. Excess
NaBH.sub.4 was removed by heating to 80.degree. C. for 5 minutes
and cooling. The powder was then filtered and washed in water and
dried at 80.degree. C. overnight. The powder was then placed in a
quartz lined tube furnace under nitrogen and heated at 15.degree.
C. min.sup.-1 to 600.degree. C. and held at 600.degree. C. for 2
hours. The catalyst was then removed from the furnace, cooled and
ground to a fine powder.
EXAMPLE 3
Synthesis of Carbon Supported Ru Catalyst
[0046] The same methodology for synthesizing the RuxSe catalyst of
Example 2 was employed except that no SeO.sub.2 was added to the
reaction mixture.
EXAMPLE 4
Preparation of Electrode for Testing Oxygen Reduction Reaction
Activity
[0047] The catalyst powders were tested for oxygen reduction
reaction activity. Catalyst powder was dispersed in glacial
ethanoic acid and a portion thereof deposited onto a clean gold
mesh electrode. After drying with a stream of warm air, the
electrode was then placed in a standard three electrode cell
containing 0.5M H.sub.2SO.sub.4 as electrolyte, a gold wire counter
electrode and a reversible hydrogen reference electrode. After
bubbling with oxygen gas to saturate the acid, the potential was
swept at 5 mVs.sup.-1 from 1.0V to 0.1V vs RHE to give the oxygen
reduction current as a function of the applied potential.
[0048] FIG. 1 illustrates the oxygen reduction current as a
function of the applied potential for the unsupported Ru.sub.xSe
catalyst. To compare the activity of the unsupported Ru.sub.xSe
catalyst with platinum, a Tafel plot was prepared as in FIG. 2. In
both of FIGS. 1 and 2, .lamda. is used to indicate the unsupported
Ru.sub.xSe catalyst and .sigma. indicates a baseline measurement of
a platinum catalyst supported on XC72R carbon supplied by Johnson
Matthey Inc. As the platinum was supported by carbon and the
Ru.sub.xSe catalyst was unsupported, the results as illustrated in
FIG. 2 are normalized by mass of actual metal present. FIG. 2
indicates that on a mass basis, the Ru.sub.xSe catalyst is
comparable in activity as platinum for the oxygen reduction
reaction. Further observations of the Ru.sub.xSe catalyst indicated
that the particle size was very small, i.e., less than 50 nm in
diameter. Further, no dissolution of the Ru.sub.xSe catalyst was
observed during electrochemical experiments as may be expected with
elemental ruthenium. Without being bound by theory, this indicates
an increased stability of the catalyst as compared to elemental
ruthenium. From the amounts of starting material used, the
unsupported Ru.sub.xSe catalyst would have a Ru:Se ratio of
approximately 7:1.
[0049] FIG. 3 illustrates a corrected Tafel plot for oxygen
reduction catalysts supported on carbon in 0.5M sulfuric acid at
room temperature comparing the activities of supported Ru.sub.xSe
catalyst with platinum and elemental ruthenium catalysts. Plot A
illustrates the results obtained for 40% Pt on XC72R carbon
(supplied by Johnson Matthey) whereas plot B is for 40% Ru on XC72R
carbon and plot C is for 40% Ru.sub.xSe on XC72R carbon. FIG. 3
also illustrates two key features in addition to the points raised
above with respect to the unsupported Ru.sub.xSe catalyst. First,
FIG. 3 is a more accurate comparison between the activities of the
catalysts as compared to FIG. 2 as all catalysts are supported. The
activity of the supported Ru.sub.xSe catalyst is shown to approach
the activity of the supported platinum catalyst and represents an
improvement in activity and stability as compared to the supported
Ru catalyst. Second, the aqueous methodology used to make the
Ru.sub.xSe produces an active catalyst in a relatively
environmentally friendly and cost effective manner.
[0050] While particular steps, elements, embodiments and
applications of the present invention have been shown and
described, it will be understood, of course, that the invention is
not limited thereto since modifications may be made by persons
skilled in the art, particularly in light of the foregoing
teachings. It is therefore contemplated by the appended claims to
cover such modifications as incorporate those steps or elements
that come within the spirit and scope of the invention.
* * * * *