U.S. patent application number 13/068651 was filed with the patent office on 2011-11-24 for novel catalyst for oxygen reduction reaction in fuel cells.
This patent application is currently assigned to Board of Trustees of Michigan State University. Invention is credited to Scott A. Calabrese Barton, Vijayadurga Nallathambi, Kothandaraman Ramanujam.
Application Number | 20110287174 13/068651 |
Document ID | / |
Family ID | 44972695 |
Filed Date | 2011-11-24 |
United States Patent
Application |
20110287174 |
Kind Code |
A1 |
Calabrese Barton; Scott A. ;
et al. |
November 24, 2011 |
Novel catalyst for oxygen reduction reaction in fuel cells
Abstract
A method for making a carbon-metal-nitrogen oxygen reducing
cathode catalyst, the method comprising mixing a carbon source with
a transition metal precursor to form a metal precursor loaded
carbon substrate; adding a nitrogen precursor compound to the metal
precursor loaded carbon substrate to form a carbon-metal-nitrogen
precursor; and pyrolyzing the carbon-metal-nitrogen precursor in a
closed vessel, thereby forming an oxygen reducing cathode catalyst.
The carbon-metal-nitrogen catalyst requires no precious metal such
as Pt, and also provides benefits such as controlled deposition of
catalytically active nitrogenous compounds that can increase the
catalytic activity of the catalyst when compared to gaseous
deposition of nitrogen to the surface of the carbon support.
Inventors: |
Calabrese Barton; Scott A.;
(East Lansing, MI) ; Ramanujam; Kothandaraman;
(Chennai, IN) ; Nallathambi; Vijayadurga; (East
Lansing, MI) |
Assignee: |
Board of Trustees of Michigan State
University
East Lansing
MI
|
Family ID: |
44972695 |
Appl. No.: |
13/068651 |
Filed: |
May 17, 2011 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
12583532 |
Aug 21, 2009 |
|
|
|
13068651 |
|
|
|
|
61090780 |
Aug 21, 2008 |
|
|
|
Current U.S.
Class: |
427/115 ;
502/174 |
Current CPC
Class: |
H01M 4/8652 20130101;
H01M 4/881 20130101; H01M 4/92 20130101; H01M 4/8807 20130101; Y02E
60/50 20130101; H01M 2008/1095 20130101; H01M 8/1004 20130101; Y02P
70/50 20151101; H01M 4/96 20130101; H01M 4/90 20130101; H01M 4/8825
20130101 |
Class at
Publication: |
427/115 ;
502/174 |
International
Class: |
B05D 5/12 20060101
B05D005/12; B01J 37/08 20060101 B01J037/08; B01J 27/20 20060101
B01J027/20 |
Claims
1. A method for making an oxygen reducing cathode catalyst, the
method comprising: (a) mixing a carbon source with a transition
metal precursor to form a metal precursor loaded carbon substrate,
wherein the substrate is substantially free of precious metals; (b)
adding a nitrogen precursor compound to the metal precursor loaded
carbon substrate to form a carbon-metal-nitrogen precursor; and (c)
pyrolyzing the carbon-metal-nitrogen precursor at an elevated
pressure ranging from about 2 bar to about 100 bar, thereby forming
the oxygen reducing cathode catalyst.
2. The method of claim 1, wherein the carbon source comprises one
or more of Norit.RTM. SX Ultra, Ketjenblack.RTM., pyrolyzed
perylene tetracarboxylic anhydride (PTCDA), polyacrylonitrile
(PAN), Black Pearls.RTM., Printex.RTM. XE2, pyrrole black,
graphitic powder, acetylene black, Vulcan.RTM. XC72, oxidized
carbon supports, and metal carbides.
3. The method of claim 1, wherein mixing the carbon source with a
transition metal precursor further comprises stirring the carbon
source with the transition metal precursor in a solvent for up to
12 hours and evaporating the solvent to form the carbon-metal
substrate.
4. The method of claim 1, wherein the transition metal precursor is
a transition metal macrocycle, a transition metal salt, or
combination thereof.
5. The method of claim 4, wherein the transition metal macrocycle
comprises cobalt pthalocyanine, iron pthalocyanine, cobalt
tetraazannulene, iron tetramethoxy phenyl porpyrin chloride,
tetracarboxylic cobalt, iron pthalocyanine, tetramethoxy phenyl
porpyrin chloride, cobalt salen-N,N' bissalicylidine,
ethylenediaminocobalt, cobalt-anten-O-amino, ferrocene,
benzaldehyde, ethylenediamino cobalt, iron phenanthroline, or
combinations thereof.
6. The method of claim 4, wherein the transition metal salt
comprises (1) a cation selected from the group consisting of iron,
cobalt, nickel, chromium, cerium, zinc, zirconium, molybdenum,
manganese, and mixtures thereof; and (2) an anion selected from the
group consisting of acetate, chloride, nitrate, sulfate, and
combinations thereof.
7. The method of claim 6, wherein the transition metal salt
comprises iron (II) acetate.
8. The method of claim 1, wherein the nominal amount of the metal
precursor added to the carbon source to form said metal precursor
loaded carbon substrate ranges from about 0.75% to about 10% by
weight of the substrate.
9. The method of claim 1, wherein the transition metal precursor is
a transition metal macrocycle, a transition metal salt, or
combination thereof.
10. The method of claim 1, wherein the nitrogen precursor compound
is selected from the group consisting of poly(quinoxaline),
nitroaniline, 1,10 phenanthroline, pthalocyanine, pyridine,
bipyridine, polyaniline, pyrrole, polyvinyl pyridine,
3-nitrophalimide, p-phenylazophenol, 6-quionoline carboxylic acid,
6-nitrobenzimidazole, 5-amino 6-nitro quinoline, 2,3
naphthalocyanine, 4,4'-azoxydibenzoic acid, 2 amino 5-nitro
pyrimidine, hematin, 4,4' azo-bis[cyanovaleric acid],
heamotoporpyrin dihydrochloride, 4,4' nitrophenyl azo catechol 4,6
dihydroxy pyrimidine, nitrophenyl, benzylamine, 1,6
phenylendiamine, tetracyanoquinodimethane, propylene di-amine,
ethylene di-amine, urea, selenourea, thiourea, dimethylformamide,
tetrahydrofuran, ammonia, acetonitrile and polymers, and
combinations thereof.
11. The method of claim 1, wherein the nitrogen precursor compound
comprises melamine.
12. The method of claim 10, wherein the nominal amount of nitrogen
in the carbon-metal-nitrogen precursor ranges from about 1.0% to
about 15% by weight of the carbon-metal-nitrogen precursor.
13. The method of claim 1, wherein the nitrogen precursor compound
is free of carbon.
14. The method of claim 1, wherein the nitrogen precursor compound
undergoes a decomposition reaction to form ammonia.
15. The method of claim 14, wherein the nitrogen precursor compound
comprises an ammonia generating precursor selected from the group
consisting of ammonium hydroxide, urea, ammonium carbamate, or
combinations thereof.
16. The method of claim 14, wherein the nitrogen precursor compound
comprises an ammonium salt.
17. The method of claim 1, wherein the pyrolyzing step comprises
pyrolyzing the carbon-metal-nitrogen precursor at a temperature
ranging from about 600.degree. C. to about 900.degree. C. in a
closed reaction vessel.
18. The method of claim 1, wherein the reaction vessel comprises
quartz.
19. The method of claim 1, wherein the pyrolyzing step further
comprises pyrolyzing the carbon-metal-nitrogen precursor using a
spray pyrolysis apparatus.
20. A low temperature fuel cell comprising the oxygen reducing
cathode catalyst of claim 1.
21. A method for making a membrane electrode assembly for a fuel
cell, comprising: (a) providing an ionomeric membrane, the membrane
having a first side and a second side; (b) applying an anode
catalyst on at least a portion of the first side of the ionomeric
membrane; and (c) applying a cathode catalyst on at least a portion
of the second side of the ionomeric membrane, wherein the cathode
catalyst is synthesized by: (i) mixing a carbon source with a
transition metal precursor to form a metal precursor loaded carbon
substrate, wherein the substrate is free of precious metals; (ii)
adding a nitrogen precursor compound to the metal precursor loaded
carbon substrate to form a carbon-metal-nitrogen precursor; (iii)
pyrolyzing the carbon-metal-nitrogen precursor at a pressure
ranging from about 2 bar to about 100 bar, thereby forming an
oxygen reducing cathode catalyst, and (iv) mixing the oxygen
reducing cathode catalyst with a recast ionomer.
22. The method according to claim 21, wherein the anode catalyst
comprises a catalyst ink having at least one transition metal
selected from the group consisting of platinum, ruthenium,
palladium, and combinations thereof.
23. The method according to claim 21, wherein the recast ionomer
comprises poly(perfluorosulphonic acid).
24. A method for making a cathode catalyst coated diffusion layer
for a fuel cell, comprising: (a) providing a gas diffusion layer;
and (b) applying a cathode catalyst on at least a portion of the
gas diffusion layer, wherein the cathode catalyst is synthesized
by: (i) mixing a carbon source with a transition metal precursor to
form a metal precursor loaded carbon substrate, wherein the
substrate is free of precious metals; (ii) adding a nitrogen
precursor compound to the metal precursor loaded carbon substrate
to form a carbon-metal-nitrogen precursor; and (iii) pyrolyzing the
carbon-metal-nitrogen precursor at a pressure ranging from about 2
bar to about 100 bar, thereby forming an oxygen reducing cathode
catalyst.
25. A method for making an oxygen reducing cathode catalyst, the
method comprising: (a) mixing a carbon source with a transition
metal precursor to form a metal precursor loaded carbon substrate
substantially free of precious metals; (b) adding a nitrogen
precursor compound having a N:C ratio of at least about 1:1 to the
metal precursor loaded carbon substrate to form a
carbon-metal-nitrogen precursor; and (c) pyrolyzing the
carbon-metal-nitrogen precursor at an elevated pressure ranging
from about 2 bar to about 100 bar, thereby forming the oxygen
reducing cathode catalyst.
26. The method according to claim 25, wherein the nitrogen
precursor compound has a N:C ratio of at least about 2:1.
27. The method according to claim 25, wherein the nitrogen
precursor compound comprises melamine.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation-in-part of U.S. patent
application Ser. No. 12/583,532, filed on Aug. 21, 2009, which
claims the benefit of U.S. Provisional Patent Application No.
61/090,780, filed Aug. 21, 2008. The entire disclosure of each of
the above applications is incorporated herein by reference.
INTRODUCTION
[0002] The present technology relates to methods for producing
improved metal, nitrogen, and carbon containing catalysts effective
for the reduction of oxygen in low temperature fuel cells and other
electrochemical reactions and cathode catalysts produced by these
methods.
[0003] There is an increasing interest to replace platinum (Pt)
based electro-catalysts with cost-effective non-noble catalysts for
the oxygen reduction reaction ("ORR") in low-temperature fuel
cells, such as Polymer Electrolyte Fuel Cells (PEFCs) and Direct
Methanol Fuel Cells ("DMFCs") etc. Non-noble metal catalysts based
on iron (Fe) and cobalt (Co) ions are among the possible candidates
for replacement of Pt based catalyst metals for ORR. These
catalysts are active towards ORR and exhibit selectivity towards
ORR in the presence of a fuel, thereby increasing the volumetric
energy density of a DMFC.
[0004] Others in the field have discovered the catalytic nature of
nitrogen-doped carbon materials, and subsequently various
non-precious metal catalysts were produced by pyrolyzing materials
such as metal-N.sub.4 macrocyles adsorbed on carbon black in an
inert atmosphere. Others have demonstrated an active catalyst for
ORR by pyrolyzing a metal precursor (cobalt acetate), carbon black
and a nitrogen precursor such as polyacrylonitrile in inert
atmosphere. Following this approach, many methods have been
developed to prepare non-noble metal catalysts, including these
steps: (a) heat-treating carbon-supported organometallic complexes
by pyrolyzing a metal source with carbon source in ammonia or
acetonitrile atmosphere, (b) cosputtering cobalt or iron and carbon
in a nitrogen atmosphere with or without subsequent heat-treatment,
and (c) mixing nitrogen-containing ligands with cobalt oxide
solution which are subsequently entrapped in polypyrrole matrix
supported on carbon.
[0005] Recently, investigators have ball-milled highly-ordered
synthetic graphite for use as a carbon support as it contains low
levels of iron as impurities and low surface area (3.5 m.sup.2/g),
Pyrolysis of the milled material with an iron source in ammonia
environment produced catalysts with nitrogen content as high as 4
atom %. These nitrogen containing catalysts demonstrate that the
catalytic activity increases as a result of decreasing metal
crystallite size, increasing degree of disorder, nitrogen content,
and microporous (<22 .ANG.) specific surface area. Others in the
field have also suggested that active sites containing pyridinic
nitrogen can be responsible for the catalytic activity for ORR and
reported low levels of H.sub.2O.sub.2 production while reducing
oxygen in an acidic medium.
[0006] U.S. Patent Application Publication No. 2007/0248752,
O'Brien et al., published Oct. 25, 2007, discloses making an
oxygen-reducing catalyst layer. The catalyst layer is prepared by
physical vapor depositing (PVD) a transition metal onto a carbon
support under a reduced pressure (e.g. about 1.times.10.sup.-5 Ton
or less). After a film of catalyst metal has been applied to a
substrate, the resulting coated substrate is thermally treated
either separately or as part of the PVD step. The thermal treatment
and/or PVD treatment can be performed under a nitrogen gas
environment to provide a source of nitrogen to the catalyst. The
thermal treatment can comprise heating the coated substrate for 15
minutes or so at temperatures of at least 600-900.degree. C.
However, the deposition of the nitrogen source is not readily
controllable. Moreover, it is believed that a greater amount of
nitrogen can be incorporated into a high-surface area support by
introducing nitrogen at higher activity (for example, higher
partial pressure) in the presence of the carbon support in contrast
to the use of gaseous nitrogen at reduced pressure.
[0007] As such there is a need for alternative methods for
producing alternative catalyst materials having improved catalytic
activity. There is also a need for methods to increase the
availability of nitrogen target sites on catalytic supports for
oxygen reduction reactions and provide enhanced stability of these
alternative catalysts when used in acidic fuel cell
environments.
SUMMARY
[0008] The present technology provides methods for making
non-precious metal electrochemical cathode catalysts for the
reduction of molecular oxygen, for example, in a fuel cell. In
addition, the present technology provides for a method to control
the anchoring of a nitrogen containing compound on a high surface
area carbon surface, which actively contributes to the catalytic
activity of the cathode catalyst over preexisting methods of
depositing nitrogen, thereby effectively increasing the catalytic
activity per unit mass of catalytic material on a substrate. The
cathode catalyst material produced in accordance with the present
technology lowers the cost for producing the catalyst material and
follows a simple synthesis method compared to platinum/carbon
catalysts and other non-precious metal catalysts conventionally
used in fuel cell designs.
[0009] In one aspect, the present technology provides a method for
making a carbon-metal-nitrogen oxygen reducing cathode catalyst,
the method comprising: [0010] (a) mixing a carbon source with a
transition metal precursor to form a metal precursor loaded carbon
substrate, preferably wherein the substrate is substantially free
of precious metals; [0011] (b) adding a nitrogen precursor compound
to the metal precursor loaded carbon substrate to form a carbon
supported metal-nitrogen complex precursor; and [0012] (c)
pyrolyzing the carbon-metal-nitrogen precursor at an elevated
pressure to form an oxygen reducing cathode catalyst.
[0013] In another aspect, the present technology provides for a
method for making a membrane electrode assembly for a fuel cell,
the membrane electrode assembly comprising: [0014] (a) an ionomeric
membrane; [0015] (b) an anode catalyst disposed on a first surface
of the ionomeric membrane; and [0016] (c) a cathode catalyst
disposed on a second surface of the ionomeric membrane wherein the
cathode catalyst is synthesized by: [0017] (i) mixing a carbon
source with a transition metal precursor to form a metal precursor
loaded carbon substrate, preferably wherein the substrate is free
of precious metals; [0018] (ii) adding a nitrogen precursor
compound to the metal precursor loaded carbon substrate to form a
carbon supported metal-nitrogen complex precursor; and [0019] (iii)
pyrolyzing the carbon supported metal-nitrogen complex precursor at
an elevated pressure to form an oxygen reducing cathode
catalyst.
[0020] Still further, the present technology provides a method for
making a cathode catalyst coated diffusion layer for a fuel cell,
the method comprising: [0021] (a) providing a gas diffusion layer;
and [0022] (b) applying a cathode catalyst on at least a portion of
the gas diffusion layer, wherein the cathode catalyst is
synthesized by: [0023] (i) mixing a carbon source with a transition
metal precursor to form a metal precursor loaded carbon substrate,
preferably wherein the substrate is free of precious metals; [0024]
(ii) adding a nitrogen precursor compound to the metal precursor
loaded carbon substrate to form a carbon-metal-nitrogen precursor;
and [0025] (iii) pyrolyzing the carbon-metal-nitrogen precursor at
a pressure ranging from about 2 bar to about 100 bar, thereby
forming an oxygen reducing cathode catalyst.
[0026] In yet another aspect, the present technology provides
method for making an oxygen reducing cathode catalyst, the method
comprising: [0027] (a) mixing a carbon source with a transition
metal precursor to form a metal precursor loaded carbon substrate
substantially free of precious metals; [0028] (b) adding a nitrogen
precursor compound having a N:C ratio of at least about 1:1 to the
metal precursor loaded carbon substrate to form a
carbon-metal-nitrogen precursor; and [0029] (c) pyrolyzing the
carbon-metal-nitrogen precursor at an elevated pressure ranging
from about 2 bar to about 100 bar, thereby forming the oxygen
reducing cathode catalyst.
DRAWINGS
[0030] The patent or application file contains at least one drawing
executed in color. Copies of this patent or patent application
publication with color drawings will be provided by the U.S. Patent
and Trademark Office upon request and payment of the necessary
fee.
[0031] FIG. 1 is a flow diagram of a method for making an oxygen
reduction cathode catalyst in accordance with the methods of the
present technology.
[0032] FIG. 2 shows the polarization curves obtained for various
catalysts synthesized at 40.degree. C. in 1N aqueous sulfuric acid,
showing effect of heat-treatment temperature on the catalytic
activity towards ORR.
[0033] FIG. 3 shows the plot of current density as a function of
nominal nitrogen content observed at three different
potentials.
[0034] FIG. 4 shows a Koutecky-Levich analysis performed on
catalysts loaded with 10.3% nitrogen.
[0035] FIG. 5A shows a polarization curve obtained from rotating
ring disc electrode ("RRDE") measurements at 40.degree. C. in 1N
aqueous sulfuric acid; FIG. 5B shows a disk potential dependent
H.sub.2O.sub.2 production curve for the optimized catalyst of the
present technology having a nominal nitrogen of 10.3% coated on a
rotating ring disc electrode ("RRDE").
[0036] FIG. 6A shows the surface area distribution of various
catalysts with different nominal nitrogen % content obtained from
BJH desorption employing Halsey-Faas correction; FIG. 6B depicts a
calculated BET area for various catalysts with different nominal
nitrogen content (%).
[0037] FIGS. 7A-7D relate to oxygen reduction current density at
thin-film rotating disk electrodes with MNC catalysts of varying
precursor N:C ratios. FIG. 7A depicts pseudo-steady state
polarization; FIG. 7B depicts iR- and mass transfer corrected Tafel
curves; FIG. 7C depicts kinetic current density at 0.8 V/RHE as a
function of precursor N:C with the conditions: O2-saturated, 1N
aqueous sulfuric acid, 40.degree. C. Scan rate 0.5 mV s.sup.-1,
1200 rpm, nominal 6.3 wt % nitrogen loading; FIG. 7D depicts
observed BET surface area and bulk nitrogen obtained through CHN
analysis.
[0038] FIGS. 8A-8D relate to performance of fuel cell
membrane-electrode assemblies employing Fe-NC oxygen reduction
catalysts. FIGS. 8A and 8B depict polarization curves for
Fe-bipyridine and Fe-melamine based catalysts, in comparison to a
commercial Pt-catalyzed MEA (solid and dotted lines indicate
forward and reverse scans respectively); FIGS. 8C and 8D depict
durability of Fe-melamine and Fe-bipyridine based MEA fuel cells at
0.5 V/RHE; Conditions: H.sub.2--O.sub.2 feeds
(p.sub.O2=p.sub.H2=1.5 bar, 80.degree. C., 100% RH), MNC catalyst
loading 1.3 mg cm.sup.-2.
[0039] It should be noted that the figures set forth herein are
intended to exemplify the general characteristics of an apparatus,
materials and methods among those of the present technology, for
the purpose of the description of such embodiments herein. These
figures may not precisely reflect the characteristics of any given
embodiment, and are not necessarily intended to define or limit
specific embodiments within the scope of this technology.
DETAILED DESCRIPTION
[0040] The following description of technology is merely exemplary
in nature of the subject matter, manufacture and use of one or more
inventions, and is not intended to limit the scope, application, or
uses of any specific invention claimed in this application or in
such other applications as may be filed claiming priority to this
application, or patents issuing therefrom. A non-limiting
discussion of terms and phrases intended to aid understanding of
the present technology is provided at the end of this Detailed
Description.
[0041] In accordance with the various embodiments of the present
technology, it has been discovered that an effective way of
preparing a cathode-catalyst which, is used in low temperature fuel
cells and other electrochemical cell applications can be achieved
by pyrolyzing the catalyst components under high temperature in a
closed vessel. The vessel can be pressurized due to the
catalyst-components (autogenic pressure) to deliver a finite and
specific amount of nitrogen content to the catalyst and more
particularly, to control the ratio of nitrogen to carbon. In
various embodiments, methods produce cathode catalysts with higher
yields without gasifying carbon precursor. Having processes capable
of producing a more efficient and non-gasified catalysts directly
translates to a higher catalytic activity per unit of mass, volume
and a lower cost per unit catalytic activity.
[0042] Although the present technology is not limited to or
dependent on a particular theory, it is believed that the
transition metal/nitrogen component on the carbon support promotes
the reduction of molecular oxygen to water.
[0043] FIG. 1 represents a flow diagram of method 100 for making a
cathode catalyst for the reduction of oxygen typically found in a
fuel cell and includes steps 110-140. Method 100 initially involves
generating a carbon-metal substrate (step 110). Step 110 generally
involves mixing a carbon support and a transition metal precursor.
In some embodiments, the carbon support and transition metal
precursor are mixed in the presence of a chemically compatible
solvent, for example, a small chain alcohol and water. The alcohol
can be any C.sub.1-C.sub.6 alcohol, for example, ethanol, isopropyl
alcohol, n-propyl alcohol and butanol which are readily or
moderately miscible with water, the carbon support and the
transition metal. The dispersion can be stirred for one to six
hours to have the transition metal precursor deposited to the
carbon support.
[0044] In some embodiments, the amount of transition metal added to
the carbon support (as a percentage of dry weight of the two
components) can range from about 0.01% to about 30% by weight. In
various aspects, the amount of metal ranges from about 0.1% by
weight to about 10% by weight, or from about 0.5% to about 10% by
weight, or from about 0.75% to about 10% by weight, or from about
1% to about 10% by weight, or from about 2% to about 10% by weight,
or from about 5% to about 10% by weight, or from about 0.1% to
about 8% by weight, or from about 0.1% to about 6% by weight, or
from about 0.1% to about 5% by weight, or from about 0.1% to about
3% by weight, or from about 0.1% to about 2% by weight.
Carbon Support
[0045] The carbon support can include any activated or
non-activated carbon material, generally having a high surface
area. In some embodiments, the carbon support can include one or
more of the following illustrative examples of carbon supports
including: carbon powder, carbon black, acetylene black, activated
carbon, carbon fiber, fullerene, nano-carbon or combinations
thereof. Specific examples of carbon supports among those useful in
the present technology include Norit.RTM. SX Ultra (Marshall, Tex.,
USA), Ketjenblack.RTM. (600J and 300J, Akzo-Nobel Polymer
Chemicals, Chicago, Ill., USA), C55 carbon particles (Chevron
Phillips Chemical Company, Texas), Black Pearls.RTM. (Cabot
Corporation, Boston, Mass., USA), Printex.RTM. XE (Degussa
Engineered Carbons, Parsipanny, N.J., USA), pyrrole black,
activated charcoal, graphitic powder, Vulcan.RTM. XC72 (Cabot
Corporation, Boston, Mass., USA) and pyrolyzed form of perylene
tetracarboxylic anhydride (PTCDA), polyacrylonitrile (PAN), and
combinations thereof.
[0046] Ketjenblack.RTM. is an electroconductive carbon black, in
pellet form, having a pore volume of from about 300 ml/100 g to 520
ml/100 g (e.g., 310-345 ml/100 g, and 480-510 ml/100 g) with fines
(<125 micron) of less than about 7%, a pH of 8-10, and an
apparent bulk density of from about 100 to about 150 kg/m.sup.3
(e.g., 125-145 kg/m.sup.3, and 100-120 kg/m.sup.3). Black
Pearls.RTM. engineered pigment black has an OAN of 65 cc/100 g, a
325 mesh residue of less than 200 ppm, and a density of 430
kg/m.sup.3. C55 carbon black consists of acetylene black carbon
particles, 99.99% purity, having a surface area of 82 m.sup.2/g,
and is commercially available under the trade designation
"Shawinigan Black, Grade C55." Norit.RTM. SX Ultra is an acid
washed, steam activated carbon having a surface area (BET), of
about 1200 m.sup.2/g, an apparent density, tamped, of 0.32 g/mL, a
particle size distribution of d10 of 5 .mu.m, d50 of 25 .mu.m, d90
of 100 .mu.m, and a pH of about 7. Printex.RTM. carbon black has a
CTAB surface area of 600 m.sup.2/g, an OAN of 380 m/100 g, a COAN
of 370 ml/100 g, a sieve residue, 325 mesh of 20 ppm, and a pour
density of 130 g/dm.sup.3. Vulcan.RTM. is a conductive carbon black
pellet or powder having an OAN of about 174 cc/100 g, surface area
of 210 m.sup.2/g, 325 mesh residue of less than 25 ppm, and density
of about 264 kg/m.sup.3.
[0047] Oxidized carbon supports (oxidized for example by
HNO.sub.3/H.sub.2SO.sub.4) and other carbides, nitrides and
silicides of metals, for example, titanium carbide (TiC), tungsten
carbide (WC), titanium nickel carbide (TiN) and silicon carbide
(SiC) can all be used as a carbon support in the present methods.
The nano-carbon supports can include carbon nanotubes, carbon
nanofibers, carbon nanowires, carbon nanohorns and carbon
nanorings.
Metal Precursor Molecules
[0048] The transition metal plays an important role in the
catalytic activity of the present catalysts. The transition metal,
and hence the resulting substrate and catalyst containing the
transition metal, is preferably substantially free of all precious
metals, such as ruthenium, rhodium, palladium, osmium, iridium,
platinum, gold, and silver. As used herein, the term substantially
free of precious metals means that precious metals are not intended
to be included with the transition metal, and the transition metal
precursor is either free of precious metals, or the presence of
precious metal is a negligible amount, for example, less than 0.1%
by weight. Precious materials have high material costs, and are
required in large amounts to achieve desirable operation voltages
and currents. In comparison, examples of suitable transition metals
for the transition metal target include iron, cobalt, nickel,
chromium, cerium, zinc, zirconium, molybdenum and manganese. These
suitable transition materials are less expensive than precious
metals, thereby reducing material costs during manufacturing. In
addition to the transition metals in ionic form, the present
methods also contemplate the use of these transition metals in the
form of transition metal macrocycles, transition metal salts and
combinations thereof.
[0049] The transition metal macrocycle comprises a large, generally
ring or crown-like molecule such as a phthalocyanine, having a
metal atom retained in its central portion, generally by
co-ordinatively bonding with one or more of nitrogen, oxygen and/or
other atoms having an unshared pair of electrons, or delocalized
electrons, as for example in a bond. Other examples of macrocycles
include metallocenes, porphyrins, chlorophyll derivatives of
imidazoles or pyrroles and the like. While a variety of transition
metals may be employed in the practice of the present technology,
some particularly preferred transition metals include iron, cobalt,
nickel, chromium, cerium, zinc, zirconium, molybdenum and
manganese.
[0050] Dispersion of the transition metal containing macrocycles
may be accomplished by dissolving the macrocyclic compound in a
solvent, dispersing the carbon support material into the solvent,
and evaporating the solvent to provide a support material having
the transition metal macrocyclic compound adsorbed onto the carbon
support. In other embodiments, the adsorption may be accomplished
by ball milling the materials together or by evaporating the
macrocyclic compound onto the support substrate provided that the
macrocyclic compound has sufficient volatility.
[0051] In some embodiments, the transition metal macrocycles can
include one or more of transition metal organometallic derivative
complexes. Such organometallic macrocycles can include for example
cobalt pthalocyanine, iron pthalocyanine, iron and cobalt
naphthalocyanine, cobalt tetraazannulene, iron tetramethoxy phenyl
porpyrin chloride, tetracarboxylic cobalt, iron pthalocyanine,
tetramethoxy phenyl porpyrin chloride, cobalt
salen-N,N'-bissalicylidine, ethylenediaminocobalt,
cobalt-anten-O-amino, ferrocene, benzaldehyde, dimethylglyoxime,
ethylenediamino cobalt and iron phenanthroline.
[0052] In some embodiments, the transition metal precursor may be a
transition metal salt. In various embodiments, the transition metal
salt is a combination of a transition metal cation, for example:
Fe.sup.2+, Fe.sup.3+, Co.sup.2+, Co.sup.3+, Cr.sup.2+, Cr.sup.3+,
Cr.sup.6+, Mn.sup.2+, Mn.sup.3+, Mn.sup.4+, Mn.sup.7+, Zn.sup.2+,
Ni.sup.2+ and Ni.sup.3+ paired with a common anion species, for
example, acetate, formate, nitrate, chloride, sulfate, oxy-chloride
and phosphate. In some embodiments, preferred transition metal
salts can include, for example, ferrous and ferric salts with one
or more of acetate, formate, chloride, sulfate, oxy-chloride,
phosphate anions; cobaltous and cobaltic salts with one or more of
chloride, acetate, nitrate, sulfate anions; chromium acetate;
cerium acetate; zinc chloride and zirconium acetate.
[0053] As shown in FIG. 1, step 120 provides for the addition or
blending of a nitrogen precursor compound with the metal precursor
loaded carbon substrate to form a carbon-metal-nitrogen precursor.
The amount of the nitrogen precursor added to the carbon-metal
substrate prior to pyrolysis can vary depending on the specific
application requiring the cathode catalysts of the present
technology. In some embodiments, the metal precursor loaded
carbon-metal substrate in the form of powder or granular particles
can be admixed with the nitrogen precursor in any suitable mixing
vessel. In some embodiments, the carbon-metal substrate and the
nitrogen precursor can be mixed together in a mortar with a
pestle.
[0054] Typically, for fuel cell cathode catalysis (both hydrogen
and methanol fuel cells), a mixture of metal precursor loaded
carbon substrate and a nitrogen precursor in a range of from about
0.1 to about 40 nominal weight % can be used to prepare the cathode
catalysts of the present technology. In some embodiments, the
carbon-metal-nitrogen precursor can contain an amount of nitrogen
precursor (weight % nominal nitrogen) ranging from 0.1% to about
18%, or from about 1% to about 15%, or from about 1% to about 12%,
or from about 1% to about 9%, or from about 1.5% to about 15%, or
from about 6% to about 15%, or from about 9% to about 15%, or from
about 10% to about 15%, or from about 12% to about 15%. In some
embodiments the carbon-metal-nitrogen precursor can contain an
amount of nitrogen precursor (weight % nominal nitrogen content)
ranging from 1% to about 15%, more preferably from 1.5 to about 12%
by weight.
Nitrogen Precursor Molecules
[0055] In various embodiments of the present technology, a nitrogen
containing precursor compound is added to the metal precursor
loaded carbon substrate as shown in method step 120. Without
wishing to be bound to any specific theory, it is believed that the
after pyrolysis metal-N.sub.xC.sub.y type of catalytic sites are
formed possessing high catalytic activity for oxygen reduction as
well as enhanced resistance to methanol poisoning while reducing
oxygen. (Gupta, S. et al., J. Appl. Electrochem. (1998),
28:673-682).
[0056] In some embodiments, the nitrogen precursor is one or more
heterocyclic nitrogen containing organic aromatic compounds and
polymers comprising heterocyclic nitrogen containing organic
aromatic compounds, including, for example, porphryins, pyridines,
pyrimidines, quinolines, aromatic amines and polymers of pyrrole
and aniline.
[0057] In some embodiments, the nitrogen precursor can include one
or more nitrogen containing precursor molecules, for example,
porphryins, pyridines, pyrimidines, aromatic amines, amines,
melamine, urea and urea derivatives, poly(quinoxaline),
nitroaniline, 1,10 phenanthroline, pthalocyanine, pyridine,
bipyridine, polyaniline, pyrrole, polyvinyl pyridine, Pyridine
based ligands-1,6 bis(4'-pyridine)-2,5-diazahexane (BPDH),
Bipyridine based ligands e.g. 4,4' bipyridine, terpyridine ligands:
4' phenyl 2,2'-6',2'-terpyridine, 2-2'' bipyrimidine, 4-7
phenanthroline dipyrido[3,2,2'3' phenazine], 3-nitrophalimide,
p-phenylazophenol, 6-quionoline carboxylic acid,
6-nitrobenzimidazole, 5-amino 6-nitro quinoline, 2,3
naphthalocyanine, 4,4'-azoxydibenzoic acid, 2 amino 5-nitro
pyrimidine, hematin, 4,4' azo-bis[cyanovaleric acid],
heamotoporpyrin dihydrochloride, 4,4' nitrophenyl azo catechol 4,6
dihydroxy pyrimidine, nitrophenyl, benzylamine, 1,6
phenylendiamine, tetracyanoquinodimethane, propylene di-amine,
ethylene diamine, urea, selenourea, thiourea, dimethylformamide,
ammonia and acetonitrile.
[0058] It has been found that the nitrogen/carbon (N:C) ratio may
demonstrate an important property of nitrogen precursors for
metal-nitrogen-carbon catalysts. As described in more detail in
Example 3, below, increasing the N:C ratio of the nitrogen
precursor may increase the accessible active site density by
reducing carbon deposition in the pores of the carbon support
during pyrolysis. For example, carbon deposition from various
organic precursors post pyrolysis may lead to pore blockages and
decreased oxygen reduction activity. As such, in certain aspects of
the present technology, the nitrogen precursor compound used with
the present disclosure may have a N:C ratio of 1:1, or greater. In
other aspects, the nitrogen precursor compound may have a N:C ratio
of at least about 2:1, or greater.
[0059] In still other aspects, the nitrogen precursor compound may
be provided such that it is free of carbon, or its decomposition
components are substantially free of carbon. As used herein, the
term substantially free of carbon means that once the nitrogen
precursor is subject to pyrolysis, the transformed product does not
end up containing anything more than a negligible amount of carbon
containing components that could potentially form deposits or lead
to pore blockage. In certain embodiments, the pyrolyzed nitrogen
precursor may contain less than 3 percent by weight of carbon
containing components based on the total weight of the nitrogen
precursor; for example, less than about 1 percent, less than about
0.5 percent, or less than about 0.1 percent by weight. For example,
the nitrogen precursor compound itself may be free of carbon, in
the case of ammonium hydroxide, or the precursor compound may
undergo a decomposition reaction during pyrolysis that forms
ammonia as a nitrogen precursor, along with water or carbon dioxide
as a by-product, such that the nitrogen precursor does not provide
any additional carbon material. In certain instances, it should be
understood that the decomposition reaction may not provide for 100%
conversion of all of the carbon containing compounds, thus there
may be trace amounts of organic carbon containing compounds in the
nitrogen precursor component. Table 1, below, provides
decomposition reactions for non-limiting exemplary nitrogen
precursor compounds that are either initially free of carbon, or
that decompose and release the carbon as gaseous carbon dioxide,
leaving nitrogen containing compounds free of carbon, such as
ammonia. Numerous ammonia generating nitrogen precursors are known
to those skilled in the art, and it has been found that these
precursors may assist in minimizing any unwanted carbon deposits.
Additional non-limiting exemplary nitrogen precursor compounds may
include amides; carbamates; carbodimides; and thiocarbamides; as
well as various ammonium salts, including those of acetate,
carbonate, bicarbonate, sulfate, chloride, bisulfate, iodide, and
the like.
TABLE-US-00001 TABLE 1 Exemplary Carbon-free Nitrogen Precursors.
Precursor Decomposition reactions Ammonium hydroxide NH.sub.4OH
.fwdarw. NH.sub.3 + H.sub.2O Urea (+ water) NH.sub.2CONH.sub.2
.fwdarw. NH.sub.3 + HCNO HCNO + H.sub.2O .fwdarw. NH.sub.3 +
CO.sub.2 Ammonium carbamate NH.sub.4COONH.sub.2 .fwdarw. 2NH.sub.3
+CO.sub.2
[0060] Once the carbon-metal-nitrogen precursor has been prepared,
the next step in the synthesis of the present cathode catalysts is
to pyrolyze the carbon-metal-nitrogen precursor in a closed vessel,
in which reactions occur at its autogenic pressure. The autogenic
pressure may be based on the nitrogen precursor evaporation, as the
decomposition reactions typically increase nitrogen activity and
mobility.
[0061] As shown in FIG. 1, method step 130, the
carbon-metal-nitrogen precursor is placed in a pressure resistant
vessel capable of sustaining the interior of the vessel with a
reducing or neutral (inert) gaseous environment. Placing
carbon-metal-nitrogen precursor in a vessel capable of withstanding
both elevated temperatures and internal pressures, sealing of the
vessel (with the precursor compounds inside) and heating of that
vessel to elevated temperature, where the elevated internal
pressure results from the existence of a gaseous phase for some or
all of the resulting chemical constituents. While the inner vessel
wall can in principle react to some extent (catalytically or
non-catalytically) with the confined chemical species, and permit
some degree of diffusion of atoms or molecules into the vessel from
the interior of the vessel, the vessel must limit such processes to
the extent of maintaining a substantial portion of the initial
atoms in the vessel (as opposed to permitting substantial diffusion
of atoms and/or molecules into and/or through the containment
vessel or forming compounds with the inner vessel wall material and
thus not being further available for reaction). The vessel
materials, apart from the above general requirements, can in
principle vary widely. A thick-walled quartz vessel was found to
possess the necessary mechanical strength at high temperature and
pressure, minimize chemical reactions with the reactants and
minimize diffusion of the reactants into the vessel wall. However,
other containment materials could be used for this purpose. In some
embodiments, the vessel may be made from any industrial metal, for
example heat and pressure resistant stainless steel. In some
embodiments, the vessel can be made from quartz commonly used to
digest or pyrolyze organic matter. Alternatively, any industrial
vessel capable of passing a reducing or neutral gas into a chamber
and capable of operating at an internal pressure of at least 2 bar
can be used.
[0062] In method step 130, the carbon support is pyrolyzed (i.e.,
heated) at a temperature preferably in the range of from about
500.degree. C. to about 1,200.degree. C., and more preferably from
about 600.degree. C. to about 1,000.degree. C. The pyrolysis step
may be accomplished, for example, using a rotary kiln, a fluidized
bed reactor, or a conventional furnace. The contents of the vessel
can be then thermally treated by placing the vessel in a furnace or
other heating apparatus capable of thermally treating the contents
of the pressurized vessel to at least 1,000.degree. C. This is
accomplished by thermally treating the precursor material under
elevated pressure, for example, pyrolizing the
carbon-metal-nitrogen precursor in a vessel with an internal
pressure of about 2 bar to about 100 bar.
[0063] A typical pyrolysis process 130 can employ a thermal
treatment schedule, for example, the carbon-metal-nitrogen
precursor material can be heated from a starting temperature of
5.degree. C. over a period of 15 minutes to a temperature of
150.degree. C. and held at that temperature for 20 minutes.
Thereafter, the temperature can be raised over a period of 30
minutes to a pyrolysis temperature in the range of 600-900.degree.
C., and held at that temperature for approximately 30-360
minutes.
[0064] Thereafter, the pyrolyzed material is rapidly cooled to room
temperature. The cooling can be facilitated by opening the furnace
or microwave device while maintaining the flow of reducing gas over
the material. The contemplated pyrolysis vessel enables the
pyrolysis of the carbon-metal-nitrogen precursor to yield a carbon
nano-structure, for example, porous carbon nanotubes containing
disordered surfaces and coated with nitrogen precursor and the
transition metal.
[0065] Other embodiments may require or preferentially use a more
automated form of substrate pyrolysis under elevated pressure. For
example, a continuous flow spray pyrolizer (SP) injects a spray of
carbon-metal-nitrogen precursor into a connected furnace under
elevated pressure. The droplets are atomized from the starting
precursor solution with an atomizer and the droplets are then
placed in a furnace. A variety of activities may occur inside the
furnace during formation of the final product including evaporation
of the solvent, diffusion of solutes, drying, precipitation,
reaction between the precursor and surrounding gas, pyrolysis and
sintering.
[0066] Once the carbon-metal-nitrogen precursor has been pyrolized
in accordance with step 130, a carbon-metal-nitrogen catalyst,
which can effectively reduce oxygen, is obtained. Although, the
present catalyst will find primary use in low temperature fuel
cells, as the cathode catalyst in membrane electrode assemblies for
oxygen reduction reactions, the present cathode catalyst also finds
utility in batteries and in electrochemical sensors.
Devices Employing the Cathode Catalyst
[0067] Several electrochemical catalytic applications can be
envisioned for the present cathode carbon-metal-nitrogen catalysts
of the present technology. Returning back to FIG. 1, step 140, the
catalyst powder or carbon-metal-nitrogen cathode catalyst is taken
from the reaction vessel and can be admixed with an ionomeric
substrate, for example, Nafion.RTM. (E. I. du Pont de Nemours,
Wilmington, Del., USA) to form a catalytic ink. The catalytic ink
can be applied to any variety of solid supports, including, for
example, any well known cathode material used in fuel cell
manufacture. In some embodiments, the catalytic ink comprising the
catalyst powder or carbon-metal-nitrogen cathode catalyst is
deposited on an electrolyte membrane to form a membrane electrode
assembly for use in a hydrogen or methanol fuel cell.
[0068] As described above, the catalysts produced using the methods
described herein have particular efficacy in polymer electrolyte
fuel cells requiring oxygen reduction reactions to generate
electric current. As such, the present methods can be employed to
produce cathode catalysts that can be used in direct methanol fuel
cells, conventional hydrogen fuel cells and other electrochemical
applications requiring a oxygen reducing cathode catalyst. In some
embodiments, these carbon-metal-nitrogen cathode catalysts find
particular utility in membrane electrode assemblies that can be
used in the aforementioned methanol and hydrogen fuel cells.
Essentially, a membrane electrode fuel cell comprises an
electrolyte membrane disposed between a pair catalyst layers, i.e.
an anode and cathode catalyst layer. The respective sides of the
electrolyte membrane are referred to as the anode surface and the
cathode surface. In a typical proton exchange membrane fuel cell,
("PEM fuel cell") hydrogen fuel is introduced into the anode
portion where the hydrogen reacts and separates into protons and
electrons. The electrolyte membrane transports the protons to the
cathode portion, while allowing a current of electrons to flow
through an external circuit to the cathode portion to provide
power. Oxygen is introduced into the cathode portion and reacts
with the protons and electrons to form water and heat. The
reduction of the oxygen at the cathode is catalyzed by the
catalysts produced by the methods described herein.
[0069] In one embodiment, ionomeric membrane is any commercially
available electrolyte membrane, for example, Nafion.RTM. (poly
(perfluorosulphonic acid), also commercially available as
Aciplex.RTM. or Flemion.RTM.). Other ionomeric membrane materials
known in the art, such as sulfonated
styrene-ethylene-butylene-styrene; polystyrene-graft-poly(styrene
sulfonic acid); poly(vinylidene fluoride)-graft-poly(styrene
sulfonic acid); poly(arylene ether), such as poly(arylene ether
ether ketone) and poly(arylene ether sulfone); polybenzimidazole;
polyphosphazene, such as poly [(3-methylphenooxy) (phenoxy)
phosphazene] and poly [bis(3-methylphenoxy) phosphazene]; and
combinations thereof, may also be used. An anode catalyst comprises
at least one metal. The at least one metal can include platinum,
ruthenium, palladium, and combinations thereof, that are known and
used in the art as fuel cell anode materials. The anode catalyst is
typically deposited on ionomeric membrane by preparing a catalyst
ink containing the at least one metal and applying the ink to one
side of the ionomeric membrane. The anode catalyst can comprise a
mixture of platinum and ruthenium, such as, for example,
platinum-ruthenium black. The cathode catalyst of the present
technology can similarly be applied to the other side of the
ionomeric membrane. In some embodiments the cathode catalyst to be
applied to the other side can include the oxygen reducing cathode
catalyst of the present technology mixed with one or more recast
ionomers. The recast ionomer can be an ionic conductor including,
for example, poly(perfluorosulphonic acid), such as Nafion.RTM.,
Aciplex.RTM., or Flemion.RTM.; sulfonated
styrene-ethylene-butylene-styrene; polystyrene-graft-poly(styrene
sulfonic acid); poly(vinylidene fluoride)-graft-poly(styrene
sulfonic acid); poly(arylene ether), such as poly(arylene ether
ether ketone) and poly(arylene ether sulfone); polybenzimidazole;
polyphosphazene, such as poly [(3-methylphenooxy) (phenoxy)
phosphazene] and poly [bis(3-methylphenoxy) phosphazene]; and
combinations thereof. In a preferred embodiment, the recast ionomer
is Nafion.RTM..
[0070] Yet, in other aspects of the present technology, the cathode
catalyst may be coated on a gas diffusion media, or gas diffusion
layer for use in an electrochemical cell. Thus, the present
technology also relates to methods for making a cathode catalyst
coated diffusion layer for a fuel cell. In various aspects, the
method includes providing a gas diffusion layer, which may comprise
a typical carbon fiber or carbon paper substrate as is generally
known in the art to allow for gas and water transport. The method
includes applying a cathode catalyst on at least a portion of the
gas diffusion layer. As described in detail above, the cathode
catalyst may be synthesized by: (i) mixing a carbon source with a
transition metal precursor to form a metal precursor loaded carbon
substrate, wherein the substrate is free of precious metals; (ii)
adding a nitrogen precursor compound to the metal precursor loaded
carbon substrate to form a carbon-metal-nitrogen precursor; and
(iii) pyrolyzing the carbon-metal-nitrogen precursor at a pressure
ranging from about 2 bar to about 100 bar, thereby forming an
oxygen reducing cathode catalyst. The cathode catalyst may also be
applied to the gas diffusion layer using a catalyst ink, as
detailed in Example 3, below.
[0071] The following examples illustrate the various features and
advantages of the technology and are not intended to limit the
technology thereto. While the examples refer to Ketjenblack.RTM.
600JD, iron (II) acetate and 2,2' bipyridine, etc., it is
understood that these materials represent one embodiment and that
other embodiments describing different carbon supports, transition
metals and nitrogen precursors described herein can be used.
EXAMPLES
Example 1
Production of Carbon-Fe-Pyridine Oxygen Reduction Cathode
Catalyst
[0072] Nafion.RTM. solution (1100 EW, 5 wt. %) were purchased from
Alfa Aesar, (Ward Hill, Mass., USA). A 5 mm glassy carbon rotating
disk electrode ("RDE") was purchased from Pine Instruments (Grove
City, Pa., USA). Ketjenblack.RTM. 600JD (Akzo-Nobel Polymer
Chemicals, Chicago, Ill., USA) (CAS No. 1333-86-4) is used as
carbon support, which is dispersed in 95% ethanol. To this
solution, Iron (II) acetate corresponding to 1 wt. % of iron on
carbon is added and the slurry is kept stirring for about 6 hr.
After the solvent is evaporated and a dry composite powder is
obtained, 55 mg of the composite material thus obtained is ground
with varying amounts of 2,2' bipyridine ranging from 35 to 85 mg
and the powder is subsequently charged into a stainless steel bomb
that has a volume of 1.7 ml. The pyrolysis vessel (bomb) can be a
closed vessel made from steel, ceramics or quartz. The material was
charged into the bomb in an inert atmosphere. At around 273.degree.
C., bipyridine decomposes and increases the pressure inside the
bomb and this in turn fixes nitrogen in the catalyst. Final
chemical analyses of the resulting carbon-metal-nitrogen cathode
catalysts having different quantities of pyridine nitrogen were
determined using CHN analysis and provided in Table 2.
TABLE-US-00002 TABLE 2 Chemical data on the finalized
carbon-metal-nitrogen cathode catalyst. Nominal Carbon, Hydrogen
and Nitrogen Nitrogen content data content Final Weight (mg)
derived from CHN analysis (%) Sample 1 Sample 2 C H N 1.5 58 57
95.68 0.18 0.71 6.9 70 68 93.88 0.21 1.75 8.9 64 69 93.56 0.19 1.86
9.7 65 65 94.23 0.10 1.49 10.3 65 68 94.15 0.09 1.66 10.9 68 93.75
0.20 1.53 11.3 123 93.4 0.30 2.93 12.5 138
Example 2
Electrochemical Evaluation of the Cathode Catalyst Performance
[0073] The catalysts thus obtained were tested in a rotating disc
electrode set up, using 1N H.sub.2SO.sub.4 as the electrolyte at
40.degree. C. As will be appreciated by those of ordinary skill in
the art, the rotating disk electrode (RDE) consists of a disk on
the end of an insulated shaft that is rotated at a controlled
angular velocity. Providing the flow is laminar over the entire
disk, the mathematical description of the flow is surprisingly
simple, with the solution velocity towards the disk being a
function of the distance from the surface, but independent of the
radial position. The rotating disk electrode is used for studying
electrochemical kinetics under conditions, such as those of testing
the present technology, when the electrochemical electron transfer
process is a limiting step rather than the diffusion process.
Hg/Hg.sub.2SO.sub.4 was used as the reference electrode for all the
studies and a platinum wire serves as the counter electrode. A
glassy carbon rotating disk electrode (4 mm diameter and 0.2
cm.sup.2 area) was used as the substrate for the supported
catalysts. The catalyst ink was prepared by dispersing 4 mg of the
carbon-metal-nitrogen oxygen reducing cathode catalyst powder
ultrasonically in a solution mixture containing 150 .mu.l isopropyl
alcohol and 50 .mu.l Nafion.RTM. (5 wt. % solution, E. I. du Pont
de Nemours, Wilmington, Del., USA). 5 .mu.l of the cathode catalyst
suspension was deposited onto the glassy carbon RDE, which was
subsequently air-dried. The electrode thus obtained was immersed in
a solution of 1N H.sub.2SO.sub.4 saturated with O.sub.2. The flow
rate of oxygen was maintained at 100 ml/min using a Rotameter.
[0074] Steady state voltammograms were recorded on the RDE using a
scan rate of 0.5 mV/s and the potential was scanned between 0.85V
to 0.2V vs. SHE (Standard Hydrogen Electrode) at a rotation rate of
1500 rpm. The foregoing describes the results of testing and
experiments utilizing an embodiment of the catalyst of the present
technology while employing the well-known rotating disk electrode
system. In order to gain insight into the nature of the catalytic
active sites, the surface area of the catalysts was measured using
adsorption and desorption techniques using the BET theory. The BET
theory is a well-known rule for the physical adsorption of gas
molecules on a solid surface, and is widely used for the
calculation of surface areas of solids. (Brunauer, S., Emmett, P.
H. and Teller, E., J. Am. Chem. Soc., (1938), 60:309). The surface
nitrogen content of the pyrolyzed samples were evaluated using
X-ray photoelectron spectroscopy ("XPS") technique and CHN and
elemental analyses on the samples quantified the bulk amounts of C,
H, N and Fe present in the catalysts. The morphology of the
catalysts and nano-structural formation in these were observed
using transmission electron microscopy ("TEM").
[0075] As a first step towards characterizing the catalyst, the
composite (carbon+Fe(II)-acetate+bipyridine) containing 9.7 wt %
nominal nitrogen content was subjected to pyrolysis at temperatures
between 700-900.degree. C. Polarization curves shown in FIG. 2 were
obtained at 40.degree. C., while keeping RDE rotation at 900 rpm.
The catalyst obtained at 700.degree. C. and 800.degree. C. shows
similar onset potential, but improved kinetics appears for
catalysts pyrolyzed at 800.degree. C., at 2 mA/cm.sup.2 current
density about 20 mV more anodic potential was observed for
800.degree. C. pyrolyzed catalysts in relation to 700.degree. C.
pyrolyzed catalyst. Loss of activity at 900.degree. C. could be due
to due to poor fixation of nitrogen in the carbon support, which
could be confirmed from the CHN analysis. At 700.degree. C., the
decrease in surface area of the catalyst in relation to its
800.degree. C. counterpart could be the reason for decreased
activity.
[0076] As shown in FIG. 3, the polarization curves recorded at 1200
rpm and 40.degree. C. for a series of catalysts that were
synthesized varying the amounts of nitrogen from 1.5% to 11.3% in
the sample. An anodic shift in the onset potential can be observed
until a nominal nitrogen content of about 10.3%. Beyond this, a
further increase in nominal nitrogen content results in a very high
overpotential towards ORR, primarily because of the excess amount
of bipyridine that is not effectively decomposed during pyrolysis.
As evident from Table 2, a huge increase in the mass obtained after
pyrolysis can be observed and this reaffirms that the bi-pyridine
has not decomposed completely. The high pressure pyrolysis was
carried out in a stainless steel bomb and in order to evaluate the
leaching effects of iron from the bomb, a sample was synthesized
without the presence of any metal and after pyrolysis, this sample
was tested. The electrochemical activity of this catalyst is shown
in FIG. 3 and as observed there is a 100 mV cathodic shift in the
potential when compared with the optimized 10.3% nitrogen content.
This reinstates the significance of the metal precursor in the
generation of active sites.
[0077] Koutecky-Levich analysis for the first order oxygen
reduction reaction on the catalysts having a nominal nitrogen
content of 10.3% and the chosen potential window for the same was
between 0.65V to 0.5V. The kinetic current can be obtained using
the following reaction.
1 i = 1 i k + 1 i d = 1 i k + 1 B C 0 .omega. 1 2 ##EQU00001##
[0078] Where B is the Levich slope given as
B=0.62nFScD.sup.2/3v.sup.-1/6
[0079] FIG. 4 shows the some typical plots of 1/I versus
.omega.-1/2 for various potentials in the range of 0.65V to 0.5V
vs. SHE. Linearity in the fit can be observed at all potentials
with less than 2% error in standard deviation. The near parallelism
of the slopes obtained from these plots indicates that there is no
change in the reaction mechanism and the number of electrons
exchanged does not vary substantially in these potential ranges.
The number electron is estimated through this analysis is close be
4.39, and the values of the constants used in the calculations are
D.sub.O2=2.1*10-5 cm.sup.2/s, C.sub.O2=1.03*10.sup.-6 mol/cm.sup.3,
kinematic viscosity v=0.01 cm.sup.2/s.
[0080] In order to evaluate the amount of hydrogen peroxide
produced and hence the selectivity of the reduction of oxygen to
water, a rotating ring disc electrode (RRDE) was used instead of an
RDE. The disc potential was scanned at a rate of 0.5 mV/s, while
the ring electrode was held at 1.2V vs. SHE, a potential
sufficiently high to oxidize any peroxide if generated. FIG. 5A
shows the disk and ring currents obtained using the optimized
catalysts. The polarization curve obtained on ring is similar in
shape to disk current, and shows well-defined plateau where
disk-current shows plateau. The number of electrons involved in the
reaction and the amount of hydrogen peroxide evolved can be
obtained employing following equations.
n = 4 I D I D + ( I R N ) ##EQU00002## % H 2 O 2 = 100 * ( 4 - n )
2 ##EQU00002.2##
[0081] Where n is the number of electrons transferred and N is the
collection efficiency, which is 0.39 for our system. FIG. 5B shows
a maximum H.sub.2O.sub.2 generation of about 7% while scanning the
disk potential from 0.8-0.4V vs. SHE. In order to correlate the
catalytic activity with respect to the surface area of the
catalysts, BET surface area measurements on the catalyst surface
were performed and area/pore volume distribution extracted from BJH
desorption is shown in FIG. 6A. Initially a decrease in BET surface
area from 1440 m.sup.2/g to 711 m.sup.2/g can be observed, which
could be due to the reduction in the area of pores less than 4 nm.
However, when the N content is increased beyond 6.9%, the surface
area begins to raise up to 914 m.sup.2/g, which could be due to the
formation of new pores of that are greater than 3 nm in size. As
shown in FIG. 6B when the nitrogen content is increased beyond
10.9%, there is a sudden decrease in the surface area, which is due
to the increased mass of catalyst due to the partial decomposition
of bipyridine.
Example 3
The Nitrogen/Carbon (N:C) Ratio Affect on Cathode Catalyst
Performance
[0082] As mentioned above, the Nitrogen/Carbon (N:C) ratio
demonstrates an important property of nitrogen precursors for
metal-nitrogen-carbon (MNC) catalysts. Increasing the N:C ratio of
the nitrogen precursor increases the accessible active site density
by reducing carbon deposition in the pores of the carbon support
during pyrolysis.
[0083] In one example, Ketjenblack.RTM. 600JD is dispersed in a 95%
ethanol solution, to which iron (II) acetate corresponding to 0.75
wt % Fe is added. This slurry is stirred for 6 hr followed by
solvent evaporation to yield a dry powder. Powder samples of 55 mg
are ground with varying amounts of pyridinic nitrogen rich
precursors, such as bipyridine (having a N:C ratio of 0.2),
pyrazine (N:C=0.5), purine (N:C=0.8), and melamine (N:C=2.0) to
achieve nominal 6.3 wt % nitrogen loading. The powder is
subsequently charged into a 1.7 mL quartz ampule. The ampule is
flame-sealed under vacuum and subjected to heat treatment at
800.degree. C. in a tube furnace. To remove excess iron, the
resulting catalysts are exposed to aqueous 1N sulfuric acid at
80.degree. C. for about 5 hr and rinsed with deionized water.
[0084] Electrochemical characterization is conducted using a glassy
carbon rotating disk electrode (RDE, 0.2 cm.sup.2 area) and a
rotating ring-disk electrode (RRDE, Pine Research Instrumentation,
Raleigh, N.C.) having a glassy carbon disk (0.25 cm.sup.2 disk
area) and a platinum ring (6.25 mm inner diameter, 7.92 outer
diameter) in aqueous 1N sulfuric acid at 40.degree. C.
[0085] Single fuel cell tests are carried out in a fuel cell test
stand (Fuel Cell Technologies Inc.). A cathode catalyst ink is
prepared by ultrasonically blending the MNC catalyst powder with 5
wt % Nafion.RTM. at a 2:1 ratio by weight in a solution of ethanol
and water. The catalyst ink is then sprayed onto a 5 cm.sup.2 gas
diffusion layer (ELAT LT 1200W, BASF) using a Paasche airbrush to a
loading of 2 mg cm.sup.2. A 5 cm.sup.2 commercial anode (BASF) with
a gas diffusion layer catalyzed by 0.4 mg cm.sup.-2 of 20% Pt/C is
used. A thin layer of Nafion.RTM. (0.5 mg cm.sup.2) is applied to
both catalyst layers to improve adhesion to the Nafion.RTM.
membrane. The electrodes are hot-pressed onto a Nafion.RTM. 112
membrane (ion power) at 140.degree. C. for 3 min at 10 atm.
Impedance measurements are carried out at open circuit (.about.0.9
V cell potential) to measure ionic membrane resistance, with values
of 60.+-.7 m.OMEGA. cm.sup.2 obtained at 10 kHz. All MEAs are
broken in at 0.5 V cell potential for at least 10 h, followed by
polarization at a cell temperature of 80.degree. C., with
humidification temperatures at 105-110.degree. C., and pure H.sub.2
and O.sub.2 feeds (backpressure 1 bar) at 100 and 50 sccm,
respectively. Durability of the catalyst layer was then measured by
holding the cell potential at 0.5 V for 100 hours while recording
steady-state current.
[0086] CHN analysis is performed using a combustion analyzer
(PerkinElmer series II 2400). A sample of 4 mg is subjected to
combustion in excess oxygen and the mass of collected products
(carbon dioxide, water and nitric-acid) are used to calculate
sample composition.
[0087] Thin-film rotating disc electrode (RDE) and rotating ring
disc electrode (RRDE) studies are used to assess oxygen reduction
activity and stability. FIG. 7A shows a series of polarization
curves measured at 1200 rpm and 40.degree. C. in 1N H.sub.2SO.sub.4
for catalysts synthesized using the four nitrogen precursors. A
positive shift in the potential is observed with increasing N:C
ratio of the nitrogen precursors. The polarization curves are
corrected for mass transfer resistance and the resulting kinetic
current density (ik) is shown in FIG. 7B. All of these catalysts
exhibit a very high onset potential (.about.0.88-0.93V vs. RHE, at
currents 0.1 mA/mg) and the kinetic current at 0.8 V vs. RHE can be
read directly from the Tafel plot. As the precursor N:C ratio
increased from 0.2 to 2.0, the kinetic current density increased by
a factor of 4, from 0.6 A cm.sup.-3 to 2.4 A cm.sup.-3. This may be
attributed to increased nitrogen retention and surface area of
these catalysts after pyrolysis. Barret-Joyner-Halenda (BJH)
analyses of nitrogen desorption of the four catalysts (figure not
shown) indicate that mesoporous pore volume appears in the 20-40
.ANG. range. A 33% increase in BET surface area is observed for the
melamine catalysts as compared to bipyridine (FIG. 7D), indicating
that increased surface accessibility contributes to improved
activity. Similarly, nitrogen retention of these catalysts, as
measured by CHN combustion analysis, increases with an increasing
N:C ratio as shown in FIG. 7D. A 50% increase in nitrogen retention
was observed for melamine catalysts as compared to bipyridine. RRDE
experiments indicate very low levels of H.sub.2O.sub.2 as shown in
Table 3, below. These levels are comparable to conventional
platinum based electrocatalysts, and much lower than similar iron
based MNC catalysts.
TABLE-US-00003 TABLE 3 Catalytic Activity of Carbon-Metal-Nitrogen
Catalysts RDE.sup..dagger. MEA.sup..dagger. i.sub.k/ ik ik ik
Nitrogen mA mg.sup.-1 (A cm.sup.-3) (A cm.sup.-2) (A cm.sup.-3) %
H.sub.2O.sub.2 Bulk N Precursor (RDE) (RDE) (MEA) (MEA) @ 0.6 V (wt
%) Bipyridine 1.6 0.64 0.0035 2.0 4.8 1.3 Pyrazine 2.2 0.88 -- --
-- 1.0 Purine 3.9 1.7 -- -- -- 1.5 Melamine 5.9 2.4 0.021 12.4 5.2
4.1 .sup..dagger.at 0.8 V/RHE, iR-corrected
[0088] Polarization curves for a single fuel cell
membrane-electrode assemblies (MEAs) fabricated using these MNC
cathode catalysts and commercial PtJC anodes are recorded in a fuel
cell test stand. Polarization curves obtained using catalysts based
on bipyridine and melamine precursors are shown in FIGS. 8A and 8B.
Current densities around 210 mA/cm.sup.2 are obtained using
melamine based catalysts in comparison with 50-430 mA/cm.sup.2 at
0.6 VIR-free/RHE reported for similar Metal-Nitrogen-Carbon
catalysts. The volumetric current density (A cm.sup.-3, corrected
for ohmic losses) is given as a Tafel plot in FIG. 8B. The cathode
catalyst loading is maintained at less than 1.3 mg cm.sup.-2 to
avoid mass transport limitations. To obtain volumetric current
density from mass current density, an electrode density of 0.4 g
cm.sup.-3 is assumed, a typical value for porous carbon materials.
The volumetric current density of the melamine based catalysts is
close to 12.4 A cm.sup.-3, comparable to other literature reports.
For the four nitrogen precursors considered, Table 3 summarizes
kinetic current density measured via rotating disk electrode (RDE)
experiments, and bulk nitrogen content measured via CHN combustion
experiments. For the baseline material, bipyridine, and the more
promising melamine, fuel cell membrane electrode assembly (MEA)
current density measurements and hydrogen peroxide levels measured
via rotating ring disk electrode (RRDE) experiments are included as
well.
[0089] FIG. 8C presents the current density decay for melamine and
bipyridine-based MEAs poised at 0.5 V over a period of 100 hr under
pure H.sub.2 and O.sub.2. The performance loss for both the
catalysts is less than 10% over this period. The degradation rate
is similar for both precursors, though the current density of the
bipyridine is much lower, suggesting that the carbon support,
comprising 95% of the catalyst material, may play a crucial role in
determining catalyst layer durability. Other issues, such as
electrode structure and water management may also contribute to
performance losses. FIG. 8D shows polarization curves obtained from
melamine-based MEAs before and after durability measurements,
showing a similar degree of degradation.
[0090] A class of MNC catalysts are synthesized for oxygen
reduction in fuel cells, using a nitrogen precursor with a high N:C
ratio that resulted in high nitrogen content, surface area, and
catalytic kinetic activity as measured using a single fuel cell. In
moving from a low N:C precursor (bipyridine) to one of high N:C
(melamine), a four-fold (RDE) and six-fold (MEA) increase in
kinetic current density is observed. This may be attributed to the
three-fold increase in bulk nitrogen content for the improved
catalyst and hence increased density of active catalytic sites,
without significant pore blockage via carbon deposition. Stable
activity for over 100 hr is demonstrated using the melamine based
catalysts in a single fuel cell.
[0091] The embodiments described herein are exemplary and not
intended to be limiting in describing the full scope of
compositions and methods of the present technology. Equivalent
changes, modifications and variations of embodiments, materials,
compositions and methods can be made within the scope of the
present technology, with substantially similar results.
Non-limiting Discussion of Terminology
[0092] The headings (such as "Introduction" and "Summary") and
sub-headings used herein are intended only for general organization
of topics within the present disclosure, and are not intended to
limit the disclosure of the technology or any aspect thereof. In
particular, subject matter disclosed in the "Introduction" may
include novel technology and may not constitute a recitation of
prior art. Subject matter disclosed in the "Summary" is not an
exhaustive or complete disclosure of the entire scope of the
technology or any embodiments thereof. Classification or discussion
of a material within a section of this specification as having a
particular utility is made for convenience, and no inference should
be drawn that the material must necessarily or solely function in
accordance with its classification herein when it is used in any
given composition or method.
[0093] The citation of references herein does not constitute an
admission that those references are prior art or have any relevance
to the patentability of the technology disclosed herein. Any
discussion of the content of references cited in the Introduction
is intended merely to provide a general summary of assertions made
by the authors of the references, and does not constitute an
admission as to the accuracy of the content of such references. All
references cited in the "Description" section of this specification
are hereby incorporated by reference in their entirety.
[0094] The description and specific examples, while indicating
embodiments of the technology, are intended for purposes of
illustration only and are not intended to limit the scope of the
technology. Moreover, recitation of multiple embodiments having
stated features is not intended to exclude other embodiments having
additional features, or other embodiments incorporating different
combinations of the stated features. Specific examples are provided
for illustrative purposes of how to make and use the compositions
and methods of this technology and, unless explicitly stated
otherwise, are not intended to be a representation that given
embodiments of this technology have, or have not, been made or
tested. Equivalent changes, modifications and variations of some
embodiments, materials, compositions and methods can be made within
the scope of the present technology, with substantially similar
results.
[0095] As used herein, the words "preferred" and "preferably" refer
to embodiments of the technology that afford certain benefits,
under certain circumstances. However, other embodiments may also be
preferred, under the same or other circumstances. Furthermore, the
recitation of one or more preferred embodiments does not imply that
other embodiments are not useful, and is not intended to exclude
other embodiments from the scope of the technology.
[0096] As used herein, the word "include," and its variants, is
intended to be non-limiting, such that recitation of items in a
list is not to the exclusion of other like items that may also be
useful in the materials, compositions, devices, and methods of this
technology. Similarly, the terms "can" and "may" and their variants
are intended to be non-limiting, such that recitation that an
embodiment can or may comprise certain elements or features does
not exclude other embodiments of the present technology that do not
contain those elements or features.
[0097] Although the open-ended term "comprising," as a synonym of
non-restrictive terms such as including, containing, or having, is
used herein to describe and claim embodiments of the present
technology, embodiments may alternatively be described using more
limiting terms such as "consisting of" or "consisting essentially
of." Thus, for any given embodiment reciting materials, components
or process steps, the present technology also specifically includes
embodiments consisting of, or consisting essentially of, such
materials, components or processes excluding additional materials,
components or processes (for consisting of) and excluding
additional materials, components or processes affecting the
significant properties of the embodiment (for consisting
essentially of), even though such additional materials, components
or processes are not explicitly recited in this application. For
example, recitation of a composition or process reciting elements
A, B and C specifically envisions embodiments consisting of, and
consisting essentially of, A, B and C, excluding an element D that
may be recited in the art, even though element D is not explicitly
described as being excluded herein.
[0098] As referred to herein, all compositional percentages are by
weight of the total composition, unless otherwise specified.
Disclosures of ranges are, unless specified otherwise, inclusive of
endpoints and include disclosure of all distinct values and further
divided ranges within the entire range. Thus, for example, a range
of "from A to B" or "from about A to about B" is inclusive of A and
of B. Disclosure of values and ranges of values for specific
parameters (such as temperatures, molecular weights, weight
percentages, etc.) are not exclusive of other values and ranges of
values useful herein. It is envisioned that two or more specific
exemplified values for a given parameter may define endpoints for a
range of values that may be claimed for the parameter. For example,
if Parameter X is exemplified herein to have value A and also
exemplified to have value Z, it is envisioned that Parameter X may
have a range of values from about A to about Z. Similarly, it is
envisioned that disclosure of two or more ranges of values for a
parameter (whether such ranges are nested, overlapping or distinct)
subsume all possible combination of ranges for the value that might
be claimed using endpoints of the disclosed ranges. For example, if
Parameter X is exemplified herein to have values in the range of
1-10, or 2-9, or 3-8, it is also envisioned that Parameter X may
have other ranges of values including 1-9, 1-8, 1-3, 1-2, 2-10,
2-8, 2-3, 3-10, and 3-9.
[0099] When an element or layer is referred to as being "on",
"engaged to", "connected to" or "coupled to" another element or
layer, it may be directly on, engaged, connected or coupled to the
other element or layer, or intervening elements or layers may be
present. In contrast, when an element is referred to as being
"directly on", "directly engaged to", "directly connected to" or
"directly coupled to" another element or layer, there may be no
intervening elements or layers present. Other words used to
describe the relationship between elements should be interpreted in
a like fashion (e.g., "between" versus "directly between,"
"adjacent" versus "directly adjacent," etc.). As used herein, the
term "and/or" includes any and all combinations of one or more of
the associated listed items.
* * * * *