U.S. patent application number 14/042249 was filed with the patent office on 2016-12-01 for non-precious metal catalysts.
This patent application is currently assigned to Los Alamos National Security, LLC. The applicant listed for this patent is Los Alamos National Security, LLC. Invention is credited to Hoon Taek CHUNG, Piotr ZELENAY.
Application Number | 20160351915 14/042249 |
Document ID | / |
Family ID | 50066430 |
Filed Date | 2016-12-01 |
United States Patent
Application |
20160351915 |
Kind Code |
A9 |
CHUNG; Hoon Taek ; et
al. |
December 1, 2016 |
NON-PRECIOUS METAL CATALYSTS
Abstract
A catalyst for oxygen reduction reaction (ORR) for a fuel cell
was prepared by pyrolyzing a mixture of polyaniline, cyanamide,
carbon black, and a non-precious metal salt under an inert
atmosphere. The pyrolyzed product was treated to remove acid
soluble components and then pyrolyzed again. The resulting powder
was used to prepare a cathode for a membrane electrode assembly
that was used in a fuel cell. When iron(III) chloride was used as
the salt, the resulting catalyst was porous with a web-shaped
structure. It displayed a maximum power density of 0.79 W/cm at 0.4
V in H.sub.2/O.sub.2 at 1.0 bar back pressure.
Inventors: |
CHUNG; Hoon Taek; (Los
Alamos, NM) ; ZELENAY; Piotr; (Los Alamos,
NM) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Los Alamos National Security, LLC |
Los Alamos |
NM |
US |
|
|
Assignee: |
Los Alamos National Security,
LLC
Los Alamos
NM
|
Prior
Publication: |
|
Document Identifier |
Publication Date |
|
US 20140045098 A1 |
February 13, 2014 |
|
|
Family ID: |
50066430 |
Appl. No.: |
14/042249 |
Filed: |
September 30, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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13239109 |
Sep 21, 2011 |
9169140 |
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14042249 |
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61384883 |
Sep 21, 2010 |
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61408129 |
Oct 29, 2010 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01M 8/1004 20130101;
H01M 4/8668 20130101; H01M 4/881 20130101; H01M 4/9008 20130101;
H01M 4/8807 20130101; H01M 4/8828 20130101; Y02E 60/50
20130101 |
International
Class: |
H01M 4/90 20060101
H01M004/90 |
Goverment Interests
STATEMENT OF FEDERAL RIGHTS
[0002] The United States government has rights in this invention
pursuant to Contract No. DE-AC52-06NA25396 between the United
States Department of Energy and Los Alamos National Security, LLC
for the operation of Los Alamos National Laboratory.
Claims
1. A catalyst prepared by a process comprising: heating a mixture
of cyanamide, polyaniline, carbon black, and a non-precious metal
salt at a temperature in a range of from about 700.degree. C. to
about 1100.degree. C. under an inert atmosphere, wherein the
non-precious metal salt is selected from an iron salt, a cobalt
salt, a manganese salt, and combinations thereof, and removing acid
soluble components from the mixture.
2. The catalyst of claim 1, wherein the non-precious metal salt
comprises halide, acetate, sulfate, or phosphate.
3. The catalyst of claim 1, wherein the non-precious metal salt
comprises an iron halide salt.
4. A catalyst composition prepared by a process comprising: heating
a mixture of cyanamide, polyaniline, carbon black, and a
non-precious metal salt at a temperature in a range of from about
700.degree. C. to about 1100.degree. C. under an inert atmosphere,
wherein the non-precious metal salt is selected from an iron salt,
a cobalt salt, and a manganese salt, removing acid soluble
components from the mixture, and mixing the catalyst with water and
with an ionomer suspension.
5. The catalyst composition of claim 4, wherein the non-precious
metal salt comprises halide, acetate, sulfate, or phosphate.
6. The catalyst composition of claim 4, wherein the non-precious
metal salt comprises an iron halide salt.
7. The catalyst composition of claim 4, wherein the ionomer
suspension comprises an ionomer comprising copolymer of a
perfluorosulfonic acid and a polytetrafluoroethylene.
8. An electrode prepared by a process comprising: preparing a
catalyst by heating a mixture of cyanamide, polyaniline, carbon
black, and an iron salt at a temperature of from about 700.degree.
C. to about 1100.degree. C. under an inert atmosphere and
thereafter removing acid soluble components from the mixture and
thereafter heating the mixture from about 700.degree. C. to about
1100.degree. C. under an inert atmosphere to form the catalyst,
mixing the catalyst with water and with an ionomer suspension to
form a catalyst composition; applying the catalyst composition onto
a first side of a membrane; applying the catalyst composition onto
a first side of a gas diffusion layer; placing the first side of
the membrane in direct contact with the first side of the gas
diffusion layer; and applying heat and pressure to the membrane
electrode assembly.
9. The electrode of claim 8, wherein the iron salt is an iron
halide salt.
10. The electrode of claim 8, wherein the iron salt is
FeCl.sub.3.
11. The electrode of claim 8, wherein the ionomer suspension
comprises an ionomer comprising copolymer of a perfluorosulfonic
acid and a polytetrafluoroethylene
Description
RELATED APPLICATIONS
[0001] This application is a continuation-in-part of U.S.
application Ser. No. 13/239,109 entitled "Non-Precious Metal
Catalysts," filed Sep. 21, 2011, which claimed the benefit of U.S.
Provisional Application No. 61/384,883 entitled "Non-Precious Metal
Catalysts" filed Sep. 21, 2010 and U.S. Provisional Application No.
61/408,129 entitled "Non-Precious Metal Catalysts" filed Oct. 29,
2010, all incorporated by reference herein.
FIELD OF THE INVENTION
[0003] The present invention relates to non-precious metal
catalysts suitable for use in fuel cells and in particular to
non-precious metal catalysts comprising a metal, a nitrogen source
combination of cyanamide (CM) and polyaniline (PANI), and a carbon
source.
BACKGROUND OF THE INVENTION
[0004] Fuel cells are suited for transportation vehicles because of
their fast startup time, low sensitivity to orientation, and
favorable power-to-weight ratio. Relatively low temperature
operation at around 80.degree. C. makes fast startup possible.
Scarce, expensive platinum-based catalysts are needed for the
oxygen reduction reaction (ORR) at the cathode of these fuel
cells.
[0005] A need exists for less expensive catalysts that exhibit a
catalytic activity similar to that for platinum-based metal
catalysts.
[0006] Metal-nitrogen-carbon (M-N--C) type catalysts having
non-precious metals have been studied for almost 50 years since the
discovery of their ORR activity in macrocycles bound with
non-precious metals. These M-N--C type catalysts are currently
considered to be promising alternatives to platinum-based catalysts
in fuel cells. M-N--C type catalysts that are iron-based, for
example, are currently being investigated as possible alternatives
to platinum-based cathode catalysts. For example, Lefevre et al. in
"Iron-Based Catalysts with Improved Oxygen Reduction Activity in
Polymer Electrolyte Fuel Cells, Science, April 2009, vol. 324, pp.
71-74, incorporated by reference, reported the preparation of
various iron-based M-N--C type catalysts for ORR. Lefevre et al.
identified the following factors for producing active Fe-based
catalysts for ORR: (1) disordered carbon content in the catalyst
precursor, (2) iron, (3) surface nitrogen, and (4) micropores in
the catalyst. Lefevre et al., noted targets set by the U.S.
Department of Energy of 130 A/cm.sup.3 by 2010 and 300 A/cm.sup.3
by 2015 for volumetric activity as measured in a fuel cell at 0.8 V
iR-free cell voltage (i.e. after correction for ohmic loss R) at
80.degree. C., and at O.sub.2 and H.sub.2 absolute pressures of 1
bar and 100% relative humidity. According to Lefevre et al.,
volumetric activity is a meaningful measure of activity because the
product of volumetric activity with electrode thickness predicts
the kinetic current density (in A/cm.sup.2) of the cathode. FIG. 1,
taken from Lefevre et al. is a plot of iR-free cell voltage vs.
volumetric current density, including volumetric current density of
their best (solid circle, 99 A/cm.sup.3) non-precious metal
catalyst (NPMC). The original polarization curves of Lefevre et al.
were obtained from H.sub.2--O.sub.2 fuel cell tests at 80.degree.
C. and 100% relative humidity (smaller open circles represent
PO.sub.2=PH.sub.2=1.5 bar). The smaller open diamonds are for data
obtained for a catalyst reported by Wood et al., "Non-precious
metal oxygen reduction catalyst for PEM fuel cells based on
nitroaniline precursor," J. Power Sources, 2008, vol. 178, pp.
510-516, incorporate by reference. FIG. 1 also shows corrected
polarization curves (larger circles and larger diamonds) that are
based on the DOE fuel cell test reference conditions (vide
supra).
SUMMARY OF THE INVENTION
[0007] Expensive platinum-based catalysts are currently used for
both anode catalysts and cathode catalysts in fuel cells. Platinum
is used for oxygen reduction in the cathode side due to the high
overpotential. Replacing an expensive platinum-based catalyst with
a less expensive material would have a tremendous impact on one of
the main obstacles to commercializing PEMFCs, namely the high cost
of precious metals. Embodiments described herein include iron-based
catalysts that are suitable for ORR in fuel cells.
[0008] An embodiment catalyst was prepared by a process that
includes heating a mixture of an iron salt, cyanamide (CM),
polyaniline (PANI), and carbon black at a temperature in a range of
from about 700.degree. C. to about 1100.degree. C. under an inert
atmosphere, and removing acid soluble components from the
mixture.
[0009] An embodiment electrode includes a catalyst prepared by
heating a mixture of an iron salt, CM, PANI, and carbon black at a
temperature of from about 700.degree. C. to about 1100.degree. C.
under an inert atmosphere and thereafter removing acid soluble
components from the mixture and thereafter heating the mixture from
about 700.degree. C. to about 1100.degree. C. under an inert
atmosphere to form the catalyst; mixing the catalyst with water and
with an ionomer suspension to form a catalyst composition; applying
the catalyst composition onto a first side of a membrane; applying
the catalyst composition onto a first side of a gas diffusion
layer; forming a membrane electrode assembly by placing the first
side of the membrane in direct contact with the first side of the
gas diffusion layer; and applying heat and pressure.
[0010] An embodiment fuel cell includes a catalyst prepared by a
process that includes heating a mixture of an iron salt, CM, PANI,
and carbon black at a temperature in a range of from about
700.degree. C. to about 1100.degree. C. under an inert atmosphere,
and removing acid soluble components from the mixture.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] FIG. 1 shows a plot taken from Lefevre et al. of iR-free
cell voltage vs. volumetric current density, including volumetric
current density of their best (solid circle, 99 A/cm.sup.3)
non-precious metal catalyst (NPMC).
[0012] FIG. 2 shows a schematic diagram of pyridinic, pyrrolic,
graphitic, and authentic pyrrole nitrogen incorporated into the
graphene carbon layer. Pyridinic nitrogen contributes 1 electron to
the .pi. band of carbon, while pyrrolic, graphitic, and authentic
pyrrole nitrogen contributes two electrons. Total nitrogen content
was obtained from the sum of pyridinic, pyrrolic, and graphitic
nitrogen content.
[0013] FIG. 3a shows fuel cell performance as plots of fuel cell
polarization at different backpressures for a
FeCl.sub.3-PANI-Ketjenblack ORR catalyst, and FIG. 3b shows plots
comparing measured (filled triangles) vs. calculated (filled
circles) volumetric activities of the catalyst. The name
FeCl.sub.3-PANI-Ketjenblack refers to the materials iron(III)
chloride, polyaniline, and a carbon black, which were used to
prepare the catalyst, and ORR refers to Oxygen Reduction
Reaction.
[0014] FIG. 4a compares fuel cell performance of a
FeCl.sub.3-CM-PANI-Ketjenblack ORR catalyst cathode with a
FeCl.sub.3-PANI-Ketjenblack ORR catalyst cathode, and FIG. 4b shows
volumetric activity of the FeCl.sub.3-CM-PANI-Ketjenblack
cathode.
[0015] FIG. 5a shows a scanning electron micrograph (SEM) of a
FeCl.sub.3-PANI-CM-Ketjenblack catalyst, and FIG. 5b shows a SEM of
a FeCl.sub.3-CM-PANI-Ketjenblack catalyst which has a webbed
structure.
[0016] FIGS. 6a and 6b show plots related to fuel cell testing of
FeCl.sub.3-CM-PANI-Ketjenblack catalysts with different ratios of
(CM+PANI)/Ketjenblack. FIG. 6a shows plots of fuel cell
polarization with power density appearing on the right axis, and
FIG. 6b shows volumetric activities.
[0017] FIGS. 7a and 7b show plots related to fuel cell testing of
FeCl.sub.3-CM-PANI-Ketchenblack catalysts with different ratios of
CM/FeCl.sub.3. FIG. 7a shows a volumetric activity of 64 A/cm.sup.3
for a catalyst having a CM/FeCl.sub.3 ratio of 14.0/6.8. For a
catalyst having CM/FeCl.sub.3 ratio of 10.5/5.0, a Tafel slope
shown in FIG. 7b was 64 A/cm.sup.3. An embodiment catalyst having a
CM/FeCl.sub.3 ratio of 10.5/5.0 gave a power density of 0.79
W/cm.sup.2 at 0.4 volts (V); this value exceeds the highest
reported value thus far, to our knowledge, of power density for a
non-precious metal catalyst (0.56 W/cm.sup.2 at approximately 1.5
bar).
DETAILED DESCRIPTION
[0018] Embodiment ORR catalyst compositions were synthesized using
a nitrogen source combination of polyaniline and cyanamide.
Typically, an oxidant was combined with solution that included both
aniline and cyanamide. The oxidant caused polymerization of the
aniline to polyaniline. A typical synthesis of an embodiment
catalyst involves combining a non-precious metal salt (an iron
salt, a cobalt salt, a manganese sale, or a combination of two of
these salts, or a combination of all three salts) such as an iron
salt (an iron halide salt such as FeBr.sub.3 or FeCl.sub.3, for
example), aniline, and cyanamide with an aqueous solution of
hydrochloric acid. Other types of salts that fall within the scope
of this invention include metal acetates, metal sulfates, and metal
phosphates. The combination of the non-precious metal salt,
aniline, cyanamide, and aqueous hydrochloric acid produces a
solution. An oxidant is added (ammonium persulfate, for example) to
the solution. The oxidant causes polymerization of the aniline to
polyaniline. A pretreated carbon black is then added. The resulting
mixture is heated until a powder is produced. The powder is ground
and then pyrolyzed at a temperature in a range from 700.degree. C.
and 1100.degree. C. The pyrolyzed powder is treated with sulfuric
acid solution to remove acid soluble components, and afterward,
washed, dried, and pyrolyzed again. One or more of these steps is
performed with stirring and/or under a nitrogen atmosphere. The
resulting composition is a non-precious metal based, M-N--C type
catalyst that is suitable for use in fuel cells. A non-limiting
example of an embodiment catalyst is referred to herein as a
FeCl.sub.3-CM-PANI-Ketjenblack catalyst. CM is the acronym for
cyanamide and PANI is the acronym for polyaniline. The nomenclature
for the embodiment FeCl.sub.3-CM-PANI-Ketjenblack catalyst is used
to represent the materials used to prepare the embodiment
catalyst.
[0019] An embodiment FeCl.sub.3-CM-PANI-Ketjenblack catalyst was
prepared as follows: 3.0 ml aniline and 7.0 ml cyanamide (CM) were
added into 500 ml of 1.0M HCl with stirring by a magnetic bar. 5.0
g FeCl.sub.3 were dissolved in the aniline solution. 5.0 g of the
oxidant (NH.sub.4).sub.2S.sub.2O.sub.8 (ammonium persulfate APS)
were added and the resulting solution was stirred vigorously at
room temperature for 4 hours, which allowed the aniline to
polymerize to form PANI. Separately, and in advance of the above
polymerization, 0.4 g of carbon (Ketjenblack EC-300J) was
pretreated with 70% nitric acid at 80.degree. C. for 8 hours. The
pretreated Ketjenblack was ultrasonically dispersed for 1 hour in
40 ml of 1.0 M HCl solution. The resulting Ketjenblack-containing
dispersion was mixed into the polymerization reaction. The
resulting mixture was stirred for 48 hours, and then heated at
90.degree. C. on a hot plate with stirring by magnetic bar. As the
mixture heated, the liquid evaporated. The resulting dry powder was
ground using a mortar and pestle. The ground powder was pyrolyzed
at 900.degree. C. in a nitrogen atmosphere for 1 hour. The
pyrolyzed powder was ground by a mortar and pestle and
approximately 1 gram of the powder was subsequently pre-leached in
150 ml of 0.5 M H.sub.2SO.sub.4 at 80-90.degree. C. for 8 hours,
and washed thoroughly with 1 liter of deionized water. After drying
at 100.degree. C. in a vacuum overnight, the dried powder was
pyrolyzed at 900.degree. C. in a nitrogen atmosphere for 3 hours.
The result was a FeCl.sub.3-CM-PANI-Ketjenblack catalyst.
[0020] Another M-N--C type ORR catalyst was prepared using the
above process but without adding the CM. An example of this
catalyst is referred to herein as FeCl.sub.3-PANI-Ketjenblack. An
example of FeCl.sub.3-PANI-Ketjenblack was prepared as follows: 3.0
ml aniline was added into 500 ml of 1.0M HCl solution with stirring
by a magnetic bar. 5.0 g FeCl.sub.3 was dissolved in the aniline
solution, and then 5.0 g (NH.sub.4).sub.2S.sub.2O.sub.8 (ammonium
persulfate, APS) as oxidant was added into the solution to
polymerize the aniline. The solution was stirred vigorously at room
temperature for 4 hours to allow the aniline to polymerize.
Separately, and in advance of the above polymerization, 0.4 g of
carbon (Ketjenblack EC-300J) pretreated with 70% nitric acid at
80.degree. C. for 8 hours was ultrasonically dispersed for 1 hour
in 40 ml of 1.0 M HCl solution in advance, and mixed with the above
polymerized solution. After 48 hours of stirring, the mixture was
dried at 90.degree. C. on a hot plate with stirring by magnetic
bar. As the mixture was heating, the liquid evaporated. The
resulting dried powder was ground by a mortar and pestle and then
pyrolyzed at 900.degree. C. in nitrogen atmosphere for 1 hour. The
pyrolyzed powder was ground up by a mortar and pestle.
Approximately 1 gram of the ground up powder was pre-leached in 150
ml of 0.5 M H.sub.2SO.sub.4 at 80-90.degree. C. for 8 hours, and
washed thoroughly with 1 liter of deionized water. After drying at
100.degree. C. in a vacuum overnight, the dried powder was
pyrolyzed at 900.degree. C. in a nitrogen atmosphere for 3 hours.
The result is a FeCl.sub.3-PANI-Ketjenblack catalyst.
[0021] Inks of the catalysts for fuel cells were prepared. An
embodiment ink was prepared by combining a small amount of
catalyst, a small amount of ionomer solution (e.g. 5% solution of
NAFION.RTM.), isopropanol, and deionized water. The relative
amounts by weight for a non-limiting embodiment ink were
catalyst:isopropanol:deionized water: 5% NAFION.RTM.
solution=1:12:12:11. These ingredients were mixed ultrasonically
for 1 hour. NAFION.RTM. is a commercially available material that
is a copolymer of a perflurorsulfonic acid and a
polytetrafluoroethylene.
[0022] An embodiment ink for a rotation disk electrode (RDE) was
prepared by ultrasonically blending 10 milligrams (mg) of catalyst,
approximately 30 mg of 5% NAFION.RTM. in alcohol (SOLUTION
TECHNOLOGY, INC), and 2.5 ml of aqueous isopropanol (by volume: 4
parts isopropanol to 1 part deionized water). The catalyst to
ionomer ratio of the ink was approximately 0.15. Pipetting 30
microliters of the ink onto a 0.196 cm.sup.2-disk gave a loading of
0.6 mg/cm.sup.2. For 0.247 cm.sup.2-disk, 30 microliters of the ink
were pipetted.
[0023] Another embodiment ink for a RDE was prepared by
ultrasonically blending 20 mg of catalyst, and approximately 60 mg
of 5% NAFION.RTM. in alcohol (SOLUTION TECHNOLOGY, INC) in 2.0 ml
isopropanol. Pipetting 12 microliters onto a 0.196 cm.sup.2-disk
(15 microliters onto a 0.247 cm.sup.2-disk) gave a loading of 0.6
mg/cm.sup.2.
[0024] Electrodes (e.g. cathodes) were prepared from the inks. An
embodiment cathode was prepared by applying (by brushing, for
example) ink to a side of a membrane, and applying ink to a side of
a gas diffusion layer (GDL). The ink was applied until a catalyst
loading of approximately 3.5 mg/cm.sup.2 was obtained. The inked
sides of the membrane and GDL were pressed together and the
resulting assembly was heated. This procedure resulted in an
electrode that may be used as a cathode of a membrane electrode
assembly.
[0025] The anodes of embodiment membrane electrode assemblies were
commercially available Pt-based anodes. An embodiment membrane
electrode assembly included an anode of Pt-catalyzed cloth
gas-diffusion layer having a loading of 0.5 milligrams of Pt per
square centimeter, available from E-TEK.
[0026] Embodiment membrane electrode assemblies were prepared by
hot pressing a cathode and an anode onto a membrane that was a
copolymer of a perfluorosulfonic acid and a
polytetrafluoroethylene. Such membranes are commercially available
under the name NAFION.RTM.. An embodiment membrane assembly was
prepared by, for example, hot-pressing a cathode and anode onto a
NAFION 212 membrane at 125.degree. C. for 3 minutes. The geometric
area of the membrane electrode assembly was 5.0 cm.sup.2.
[0027] Fuel cells were assembled using the embodiment membrane
electrode assemblies. The activities of the embodiment ORR
catalysts in these fuel cells were measured. Rotating disk
electrode (RDE) and rotating disk-ring electrode (RRDE)
measurements were performed using a CHI Electrochemical Station
(Model 750b) in a standard three electrode cell. For RDE, 5 mm
diameter glassy carbon disks (geometric area: 0.196 cm.sup.2) were
used. For RRDE, 5.61 mm diameter glassy carbon disks (geometric
area 0.247 cm.sup.2) with a platinum ring were used. To avoid any
potential contamination of the N-M-C catalyst by platinum, all
experiments for the N-M--C catalysts were carried out with a
graphite rod as the counterelectrode. The reference electrode was
(Ag/AgCl (in 3M NaCl)). The reference electrode was calibrated
against a reversible hydrogen electrode (RHE).
[0028] Fuel cell testing was carried out in a single cell with
single serpentine flow channels. Pure hydrogen and air/oxygen,
humidified at 80.degree. C., were supplied to the anode and cathode
at a flow rate of 200 and 600/200 mL/min, respectively. The
backpressures at both electrodes were changed from 0 to 30 psig.
Fuel cell polarization plots were recorded using standard fuel cell
test stations (Fuel Cell Technologies, Inc.) at current control
mode.
[0029] Catalyst morphology was characterized by scanning electron
microscopy (SEM) using a Hitachi S-5400 instrument. High-resolution
transmission electron microscopy (HR-TEM) images were taken on a
JEOL 2010 microscope operating at 200 kV. X-ray photoelectron
spectroscopy (XPS) measurements were performed on a Physical
Electronics VersaProbe XPS System using monochromatic A1 K.alpha.
line (1486.6 eV) as an X-ray source at UTRC. The background was
subtracted from all the experimental data presented herein. Surface
area of the samples was measured by Quantachrome Autosorb-iQ using
N.sub.2.
[0030] According to previous reports, the nitrogen content and type
present in M-N--C catalysts is important for ORR activity. As
depicted in FIG. 2, there are several types of nitrogen species
that can be largely classified as "two p electrons donor" (to the
pi-band of carbon) and "one p electron donor" (to the pi-band of
carbon). The two p electrons donor species (especially graphitic
and pyrrolic-N) are expected to lower the carbon band gap energy
and possibly promote catalytic activity. The one p electron donor
(pyridinic-N) specie also has a lone pair of electrons available
for binding with metal atoms; indeed, this pyridinic nitrogen
content has been the most closely correlated to the activities of
M-N--C catalysts.
[0031] FIG. 3a shows fuel cell polarization plots of cell voltage
in volts versus current density in amperes per square centimeter
for a FeCl.sub.3-PANI-Ketjenblack cathode catalyst with various
applied backpressures. The bottom curve which includes filled
circles shows data plotted for a backpressure of 14.5 psig, the
curve above it which includes empty circles is iR-corrected for
this backpressure. The next curve which includes filled squares
shows data plotted for a backpressure of 30 psig, and the topmost
curve which includes empty squares is iR-corrected for this
backpressure. The curves show that the activity of the embodiment
catalyst decreases with decreasing back pressure. FIG. 3b plots
measured and calculated volume activity of the same catalyst. There
was little difference between the measured and calculated values.
The measured volumetric activity for the catalyst at 0.8 V
(iR-corrected) 1.0 bar (applied) was 3 A/cm.sup.3.
[0032] FIG. 4a provides fuel cell polarization curves of cell
voltage in volts versus current density for a
FeCl.sub.3-CM-PANI-Ketjenblack catalyst. The volumetric activity at
0.8 V (iR-corrected) of the embodiment catalyst measured at 1.0 bar
(applied) was 10 A/cm.sup.3 (see FIG. 3b).
[0033] XPS results show a higher pyridinic nitrogen content (39.2%)
for FeCl.sub.3-CM-PANI-Ketjenblack than for
FeCl.sub.3-PANI-Ketjenblack (31.4%).
[0034] BET surface area measurements indicate a higher BET surface
area (607 m.sup.2/g) for the FeCl.sub.3-CM-PANI-Ketjenblack
catalyst than for the FeCl.sub.3-PANI-Ketjenblack catalyst (264
m.sup.2/g).
[0035] FIG. 5a provides a scanning electron micrograph (SEM) of the
FeCl.sub.3-PANI-Ketjenblack catalyst, and FIG. 5b provides an SEM
of the FeCl.sub.3-CM-PANI-Ketjenblack catalyst. The SEM of the
CM-containing catalyst of FIG. 5b reveals a webbed structure for
this catalyst. This webbed structure may contribute to the observed
high activity in the high current region, which would facilitate
mass transportation. The CM might act as a forming agent, resulting
in the porous structure of the CM-containing catalyst.
[0036] The effects of varying the amount and type of carbon black
were examined in a series of embodiment
FeCl.sub.3-CM-PANI-Ketchenblack catalysts prepared with the same
amounts of FeCl.sub.3, CM, and PANI, but with varying amounts of
Ketchenblack (KB). Several embodiments were prepared using Black
Pearl (BP) carbon black instead of Ketchenblack carbon black. Table
2 provides a listing of the amounts of starting materials for
several embodiment catalysts that were prepared according to the
embodiment procedure (vide supra).
TABLE-US-00001 TABLE 2 Sample cyanamide aniline Carbon support
10/0.4 (KB) 7.0 grams 3.0 milliliters 0.4 grams (KB) 10/0.8 (KB)
7.0 grams 3.0 milliliters 0.8 grams (KB) 10/1.2 (KB) 7.0 grams 3.0
milliliters 1.2 grams (KB) 10/1.2 (BB) 7.0 grams 3.0 milliliters
1.2 grams (BP) 10/2.0 (BB) 7.0 grams 3.0 milliliters 2.0 grams (BP)
10/2.8 (BP) 7.0 grams 3.0 milliliters 2.8 grams (BP)
Results of cell performance for the first three catalysts listed in
Table 2, all of which included the KB carbon black, are plotted in
FIG. 6a and FIG. 6b. As the plots show, volumetric activity
increased with increasing amounts of KB (up to 1.2 g) at the
expense of decreases in current density at lower voltages.
Increases beyond 1.2 grams had little or no effect on catalyst
activity. FIG. 6b provides a plot of iR-free cell voltage versus
volumetric current density. According to FIG. 6b, the iR-corrected
volumetric activity at 0.8 V and 1.0 bar for the 10/1.2 sample
(i.e. the third sample from Table 2) was 38 A/cm.sup.3. The first
entry gave the highest value for power density (0.75 W/cm.sup.2).
Surprisingly, no increase in activity was observed for the samples
prepared with Black Pearl 2000 (BP) instead of KB as the carbon
support, even though the BP carbon support had a higher surface
area than did the KB. Transmission electron micrograph (TEM) images
of these materials revealed a complete encapsulation of the carbon
black particles, which may explain the observed lack of dependence
of activity on surface area of carbon black.
[0037] Embodiment FeCl.sub.3-CM-PANI-Ketchenblack catalysts were
prepared according to the embodiment procedure (vide supra) but
with the amounts of CM, FeCl.sub.3, aniline, and Ketjenblack carbon
support shown in Table 3. Embodiment cathodes and membrane
electrode assemblies were prepared using these catalysts, and their
fuel cell performance was evaluated. Fuel cell performance is
plotted in FIGS. 7a and 7b.
TABLE-US-00002 TABLE 3 Cyanamide FeCl.sub.3 carbon support Sample
(CM/FC) (grams) Aniline (ml) (grams) (grams) 10.5/5.0 10.5 3.0 5.0
1.2 10.5/5.9 10.5 3.0 5.9 1.2 14.0/5.9 14.0 3.0 5.9 1.2 14.0/6.8
14.0 3.0 6.8 1.2
FIG. 7a provides polarization plots for various embodiment
catalysts with varying CM/FC ratios (see column 1 of Table 3). The
polarization plot of cell voltage (V) vs. current density
(A/cm.sup.2) reveals little change in fuel cell performance between
the first entry (10.5/5.0) and the second entry (10.5/5.9). A
slight increase in current density for cell voltage above 0.8 V was
observed, but current densities decreased for voltages less than
0.7 volts. The volumetric activity for the fourth entry (14.0/6.8)
was 64 A/cm.sup.3, and the Tafel slope shown in FIG. 7b, which was
calculated from linear regression method using three points, was
approximately 56 mV/dec. The third entry (10.5/5.0) provided a
value for power density of 0.79 W/cm.sup.2 at 0.4 volts (V), which
to our knowledge is the highest value for power density for a
non-precious metal catalyst.
[0038] The effects of backpressure on activity were also examined
An embodiment FeCl.sub.3-CM-PANI-Ketjenblack catalyst showed
relatively high performance even without an applied
backpressure.
[0039] In summary, embodiment catalysts suitable for cathodes for
fuel cells were prepared using a iron salt, cyanamide, polyaniline,
and carbon black. An embodiment FeCl.sub.3-CM-PANI-Ketjenblack
catalyst gave a volumetric activity of 64 A/cm.sup.3 at 0.8 V
(iR-corrected) in a membrane electrode assembly operated on
H.sub.2/O.sub.2 at 1.0 bar applied backpressure. This catalyst
shows a maximum power density of 0.79 W/cm.sup.2 at 0.4 V in
H.sub.2/O.sub.2 at 1 bar applied backpressure. This performance
might be due at least in part to a porous webbed catalyst structure
and increased nitrogen content and surface area.
[0040] In all embodiments of the present invention, all percentages
are by weight of the total composition, unless specifically stated
otherwise. All ratios are weight ratios, unless specifically stated
otherwise. All ranges are inclusive and combinable. All documents
cited in the Detailed Description of the Invention are, in relevant
part, incorporated herein by reference; the citation of any
document is not to be construed as an admission that it is prior
art. To the extent that any meaning or definition of a term in this
document conflicts with any meaning or definition of the same term
in a document incorporated by reference, the meaning or definition
assigned to that term in this document shall govern.
[0041] Whereas particular embodiments of the present invention have
been illustrated and described, it would be obvious to those
skilled in the art that various other changes and modifications can
be made without departing from the spirit and scope of the
invention. It is therefore intended to cover in the appended claims
all such changes and modifications that are within the scope of
this invention.
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