U.S. patent application number 11/029964 was filed with the patent office on 2006-01-26 for performance additive for fuel cells.
Invention is credited to Hongli Dai, Joe Douglas Druliner, Gary A. Johansson, Mark A. Scialdone.
Application Number | 20060016122 11/029964 |
Document ID | / |
Family ID | 34885934 |
Filed Date | 2006-01-26 |
United States Patent
Application |
20060016122 |
Kind Code |
A1 |
Dai; Hongli ; et
al. |
January 26, 2006 |
Performance additive for fuel cells
Abstract
A fuel mixture comprising at least one performance additive,
wherein the performance additive comprises a cyclic tertiary amine
having a molecular weight of at least about 200. Alternately, the
performance additive may be present in the electrode composition.
The performance additive is capable of improving the
electrooxidation reaction rate of the fuel mixture, measured as
current density, by at least 2% compared to a fuel mixture not
containing the performance additive.
Inventors: |
Dai; Hongli; (Wilmington,
DE) ; Druliner; Joe Douglas; (Newark, DE) ;
Scialdone; Mark A.; (Oxford, PA) ; Johansson; Gary
A.; (Hockessin, DE) |
Correspondence
Address: |
E I DU PONT DE NEMOURS AND COMPANY;LEGAL PATENT RECORDS CENTER
BARLEY MILL PLAZA 25/1128
4417 LANCASTER PIKE
WILMINGTON
DE
19805
US
|
Family ID: |
34885934 |
Appl. No.: |
11/029964 |
Filed: |
January 5, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60535065 |
Jan 8, 2004 |
|
|
|
Current U.S.
Class: |
44/329 |
Current CPC
Class: |
H01M 8/1004 20130101;
C10L 1/2335 20130101; C10L 1/2383 20130101; H01M 8/1011 20130101;
H01M 4/921 20130101; H01M 8/0612 20130101; C10L 1/2368 20130101;
H01M 4/8663 20130101; H01M 8/04186 20130101; H01M 4/8668 20130101;
Y02E 60/523 20130101; H01M 4/8605 20130101; H01M 4/923 20130101;
Y02E 60/50 20130101; C10L 1/238 20130101; H01M 4/925 20130101; C10L
1/233 20130101 |
Class at
Publication: |
044/329 |
International
Class: |
C10L 1/22 20060101
C10L001/22 |
Claims
1. A fuel mixture comprising at least one performance additive,
wherein the performance additive comprises a cyclic tertiary amine
having a molecular weight of at least 200.
2. The fuel mixture of claim 1 wherein the performance additive is
capable of improving the electrooxidation reaction rate of the fuel
mixture, measured as current density, by at least 2% compared to a
fuel mixture not containing the performance additive.
3. The fuel mixture of claim 1 wherein the cyclic tertiary amine
has a molecular weight of about 200 to about 10,000.
4. The fuel mixture of claim 3 wherein the cyclic tertiary amine
has a molecular weight of about 400 to about 4,000.
5. The fuel mixture of claim 4 wherein the cyclic tertiary amine
has a molecular weight of about 400 to about 2000.
6. The fuel mixture of claim 1 wherein the cyclic tertiary amine
comprises at least one saturated monocyclic, bicyclic or polycyclic
ring containing at least one tertiary N atom, and wherein the
number of ring atoms ranges from 3 to 18.
7. The fuel mixture of claim 6 wherein the cyclic tertiary amine
further comprises at least one atom selected from the group of C,
N, O and Si.
8. The fuel mixture of claim 6 wherein the cyclic tertiary amine
comprises at least two saturated cyclic amine rings, and wherein
the rings are linked to one another by means of a linking group
selected from the group consisting of linear alkylidene, cyclic
alkylidene, oxyalkylidene, polyoxyalkylidene, alkoxylated
alkylidene and alkylaryl groups having 1 to 100 carbon atoms.
9. The fuel mixture of claim 8 wherein the cyclic tertiary amine is
selected from the group consisting of: ##STR5##
10. The fuel mixture of claim 6 wherein the cyclic tertiary amine
comprises at least two saturated cyclic amine rings, and wherein
the rings are attached to polymeric cores by means of side
chains.
11. The fuel mixture of claim 10 wherein the cyclic tertiary amine
has the following structure: ##STR6## wherein n=about 1 to about
50,000.
12. The fuel mixture of claim 11 wherein n=about 100 to about
50,000.
13. The fuel mixture of claim 12 wherein n=about 1000 to about
10,000.
14. The fuel mixture of claim 11 wherein n=1 to about 100.
15. The fuel mixture of claim 14 wherein n=1 to about 20.
16. The fuel mixture of claim 6 wherein the cyclic tertiary amine
is an end group of a dendrimer.
17. The fuel mixture of claim 16 wherein the dendrimer is a
dendrimeric polyether.
18. The fuel mixture of claim 16 wherein the dendrimer is based on
successive generations of polypropylenimines substituted onto a
central diamine.
19. The fuel mixture of claim 18 wherein the central diamine is
ethylenediamine, in which the terminal amine end groups are part of
saturated cyclic rings.
20. The fuel mixture of claim 1 wherein the concentration of the
performance additive in the methanol fuel is at least about 0.001
M.
21. The fuel mixture of claim 20 wherein the concentration of the
performance additive in the methanol fuel is at least about 0.05
M.
22. The fuel mixture of claim 21 wherein the concentration of the
performance additive in the methanol fuel is about 0.1 M to about
2.0 M
23. A fuel supply for supplying fuel to the anode of a fuel cell,
the fuel supply supplying fuel comprising at least one performance
additive, wherein the performance additive comprises a cyclic
tertiary amine having a molecular weight of at least about 200.
24. The fuel supply of claim 23 wherein the performance additive is
capable of improving the electrooxidation reaction rate of the fuel
mixture, measured as current density, by at least 2% compared to a
fuel mixture not containing the performance additive.
25. The fuel supply of claim 23 wherein the cyclic tertiary amine
has a molecular weight of about 200 to about 10,000.
26. The fuel supply of claim 23 wherein the cyclic tertiary amine
comprises at least one saturated monocyclic, bicyclic or polycyclic
ring containing at least one tertiary N atom, and wherein the
number of ring atoms ranges from 3 to 18.
27. The fuel supply of claim 26 wherein the cyclic tertiary amine
further comprises at least one atom selected from the group of C,
N, O and Si.
28. The fuel supply of claim 23 wherein the cyclic tertiary amine
comprises at least two saturated cyclic amine rings, and wherein
the rings are linked to one another by means of a linking group
selected from the group consisting of linear alkylidene, cyclic
alkylidene, oxyalkylidene, polyoxyalkylidene, alkoxylated
alkylidene and alkylaryl groups having 1 to 100 carbon atoms.
29. The fuel supply of claim 23 wherein the cyclic tertiary amine
comprises at least two saturated cyclic amine rings, and wherein
the rings are attached to polymeric cores by means of side
chains.
30. The fuel supply of claim 23 wherein the cyclic tertiary amine
is an end group of a dendrimer.
31. The fuel supply of claim 30 wherein the dendrimer is a
dendrimeric polyether
32. The fuel supply of claim 30 wherein the dendrimer is based on
successive generations of polypropylenimines substituted onto a
central diamine.
33. The fuel supply of claim 23 wherein the concentration of the
performance additive in the methanol fuel is at least about 0.001
M.
34. The fuel supply of claim 33 wherein the concentration of the
performance additive in the methanol fuel is at least about 0.05
M.
35. The fuel supply of claim 34 wherein the concentration of the
performance additive in the methanol fuel is about 0.1 M to about
2.0 M
36. An electrode composition comprising an electrocatalyst, a
polymer binder and at least one performance additive, wherein the
performance additive comprises a cyclic tertiary amine having a
molecular weight of at least about 200.
37. The electrode composition of claim 36 wherein the performance
additive is capable of improving the electrooxidation reaction rate
of the electrode composition, measured as current density, by at
least 2% compared to an electrode composition not containing the
performance additive.
38. The electrode composition of claim 36 wherein the cyclic
tertiary amine has a molecular weight of about 200 to about
1,000,000.
39. The electrode composition of claim 38 wherein the cyclic
tertiary amine has a molecular weight of about 2000 to about
500,000.
40. The electrode composition of claim 39 wherein the cyclic
tertiary amine has a molecular weight of about 4000 to about
100,000.
41. The electrode composition of claim 36 wherein the cyclic
tertiary amine comprises at least one saturated monocyclic,
bicyclic or polycyclic ring containing at least one tertiary N
atom, and wherein the number of ring atoms ranges from 3 to 18.
42. The electrode composition of claim 41 wherein the cyclic
tertiary amine further comprises at least one atom selected from
the group of C, N, O and Si.
43. The electrode composition of claim 41 wherein the cyclic
tertiary amine comprises at least two saturated cyclic amine rings,
and wherein the rings are linked to one another by means of a
linking group selected from the group consisting of linear
alkylidene, cyclic alkylidene, oxyalkylidene, polyoxyalkylidene,
alkoxylated alkylidene and alkylaryl groups having 1 to 100 carbon
atoms.
44. The electrode composition of claim 43 wherein the cyclic
tertiary amine is selected from the group consisting of:
##STR7##
45. The electrode composition of claim 41 wherein the cyclic
tertiary amine comprises at least two saturated cyclic amine rings,
and wherein the rings are attached to polymeric cores by means of
side chains.
46. The electrode composition of claim 45 wherein the cyclic
tertiary amine has the following structure: ##STR8## wherein
n=about 1 to about 50,000.
47. The electrode composition of claim 46 wherein n=about 100 to
about 50,000.
48. The electrode composition of claim 46 wherein n=about 1000 to
about 10000.
49. The electrode composition of claim 46 wherein n=about 1 to
about 100.
50. The electrode composition of claim 46 wherein n=about 1 to
about 20.
51. The electrode composition of claim 41 wherein the cyclic
tertiary amine is an end group of a dendrimer.
52. The electrode composition of claim 51 wherein the dendrimer is
a dendrimeric polyether.
53. The electrode composition of claim 51 wherein the dendrimer is
based on successive generations of polypropylenimines substituted
onto a central diamine.
54. The electrode composition of claim 53 wherein the central
diamine is ethylenediamine, in which the terminal amine end groups
are part of saturated cyclic rings.
55. The electrode composition of claim 36 wherein the polymer
binder is a highly fluorinated ion-exchange polymer, optionally
containing sulfonate ion exchange groups
56. The electrode composition of claim 55 wherein the weight
percentage of the polymer binder is at least about 5%.
57. The electrode composition of claim 56 wherein the weight
percentage of the polymer binder is at least about 10% and less
than about 20%.
58. The electrode composition of claim 36 wherein the weight
percentage of the performance additive is at least about 0.1%.
59. The electrode composition of claim 36 wherein the weight
percentage of the performance additive is at least about 1% and
less than about 5%.
60. A membrane electrode assembly comprising an electrode
composition, wherein the electrode composition comprises an
electrocatalyst, a polymer binder and at least one performance
additive, wherein the performance additive comprises a cyclic
tertiary amine having a molecular weight of at least about 200.
61. The membrane electrode assembly of claim 60 further comprising
a solid polymer electrolyte membrane.
62. The membrane electrode assembly of claim 60 further comprising
at least one gas diffusion backing.
63. The membrane electrode assembly of claim 60 wherein the
performance additive is capable of improving the electrooxidation
reaction rate of the electrode composition, measured as current
density, by at least 2% compared to an electrode composition not
containing the performance additive.
64. The membrane electrode assembly of claim 60 wherein the cyclic
tertiary amine has a molecular weight of about 200 to about
1,000,000.
65. The membrane electrode assembly of claim 64 wherein the cyclic
tertiary amine has a molecular weight of about 4000 to about
100,000.
66. The membrane electrode assembly of claim 64 wherein the cyclic
tertiary amine comprises at least one saturated monocyclic,
bicyclic or polycyclic ring containing at least one tertiary N
atom, and wherein the number of ring atoms ranges from 3 to 18.
67. The membrane electrode assembly of claim 66 wherein the cyclic
tertiary amine further comprises at least one atom selected from
the group of C, N, O and Si.
68. The membrane electrode assembly of claim 66 wherein the cyclic
tertiary amine comprises at least two saturated cyclic amine rings,
and wherein the rings are linked to one another by means of a
linking group selected from the group consisting of linear
alkylidene, cyclic alkylidene, oxyalkylidene, polyoxyalkylidene,
alkoxylated alkylidene and alkylaryl groups having 1 to 100 carbon
atoms.
69. The membrane electrode assembly of claim 68 wherein the cyclic
tertiary amine is selected from the group consisting of:
##STR9##
70. The membrane electrode assembly of claim 66 wherein the cyclic
tertiary amine comprises at least two saturated cyclic amine rings,
and wherein the rings are attached to polymeric cores by means of
side chains.
71. The membrane electrode assembly of claim 70 wherein the cyclic
tertiary amine has the following structure: ##STR10## wherein
n=about 100 to about 50000.
72. The membrane electrode assembly of claim 64 wherein the cyclic
tertiary amine is an end group of a dendrimer.
73. An electrochemical cell comprising an anode and a cathode, a
membrane comprising ionomer having ion-exchange groups separating
the anode and cathode prepared from an electrode composition, and a
fuel supply for supplying fuel to the anode, the fuel supply
comprises at least one performance additive.
74. The electrochemical cell of claim 73 wherein the
electrochemical cell is a fuel cell.
75. The fuel cell of claim 74 wherein the fuel is methanol.
76. The electrochemical cell of claim 74 wherein the performance
additive is capable of improving the electrooxidation reaction rate
of the electrode composition, measured as current density, by at
least 2% compared to an electrode composition not containing the
performance additive.
77. The electrochemical cell of claim 74 wherein the performance
additive is a cyclic tertiary amine, and wherein the cyclic
tertiary amine has a molecular weight of about 200 to about
1,000,000.
78. The fuel cell of claim 74 wherein the performance additive is a
cyclic tertiary amine, and the cyclic tertiary amine comprises at
least one saturated monocyclic, bicyclic or polycyclic ring
containing at least one tertiary N atom, and wherein the number of
ring atoms ranges from 3 to 18.
79. An electrochemical cell comprising a membrane electrode
assembly, wherein the membrane electrode assembly comprises at
least one electrode composition, wherein the electrode composition
comprises an electrocatalyst, and wherein the electrochemical cell
comprises a fuel supply comprising a performance additive.
80. The electrochemical cell of claim 79 wherein the
electrochemical cell is a fuel cell.
81. The fuel cell of claim 80 wherein the membrane electrode
assembly further comprises a solid polymer electrolyte membrane
wherein the membrane comprises an ionomer having proton conductive
ion-exchange groups separating the anode and cathode.
82. The fuel cell of claim 80 wherein the membrane electrode
assembly further comprises at least one gas diffusion backing.
83. The fuel cell of claim 80 wherein the electrode composition
comprises a performance additive.
84. A process for operating a direct methanol fuel cell comprising
an anode and a cathode, a membrane comprising ion-exchange groups
separating the anode and cathode, and a fuel supply for supplying
liquid methanol fuel to the anode, the process comprising
contacting one or all of the membrane, anode or fuel with at least
one performance additive, wherein the performance additive
comprises a cyclic tertiary amine having a molecular weight of at
least about 200.
Description
FIELD OF THE INVENTION
[0001] This invention relates to additives for fuel cells, and in
particular performance additives that improve power density of
polymer electrolyte membrane (PEM) fuel cells.
BACKGROUND OF THE INVENTION
[0002] Electrocatalysts are known for oxidation of a variety of
fuels such as hydrogen, reformate, methanol, ethanol, propanol,
ethylene glycol, formaldehyde and formic acid etc. High power
density fuel cells require electrocatalysts with high
electrocatalytic activity. Various electrocatalyst compositions for
polymer electrolyte membrane (PEM) fuel cells are known in the art,
most of them contain expensive noble metal Pt. Therefore, there is
a strong economic driver towards improving activity of the
electrocatalyst. As an example, for methanol electro-oxidation,
bimetallic electrocatalysts Pt/Ru have been developed to replace
pure Pt electrocatalyst because Pt/Ru is more active. As seen in H.
Gasteiger, Journal of Physical Chemistry, 97, 12020-12029,1993 and
A. Crown, Surface Science, 506, L268-L274, 2002 various
optimizations of the atomic composition, structure and particle
morphology of Pt/Ru are being developed to further improve its
methanol electro-oxidation activity.
[0003] To reduce costs, electrocatalysts that do not contain noble
metals are also known in the art and might be applied to PEM fuel
cells. These include metal carbides, metal borides and
organometallic complexes such as iron and colbalt phthalocyanines
and porphyrins. However, these non-noble metal electrocatalysts
suffer low activity, usually orders of magnitude lower than that of
noble metal electrocatalysts.
[0004] The present invention deals with organic additives that can
promote the activity of electrocatalysts for PEM fuel cells. Many
organic compounds are known electrocatalyst inhibitors or even
poisons, notably sulfur containing compounds and most aromatic
compounds. The absorption of these organic compounds on metal
surfaces tends to block electrocatalyst active sites. In contrast,
organic additives that promote electrocatalytic reactions are rare.
H. Saffarian et. al. (Proceedings of Power Sources Conference, vol
39, page 116-119 (2000)) reported that some alkyl derivatives of
uracil may be able to promote the oxygen electrochemical reduction
reaction rate in acidic solutions. J. S. Bett et. al.
(Electrochimica Acta, Vol. 43, No. 24, pp 3645-3655, 1998) and R.
Venkataraman (Journal of Electrochemical Society, 151(5),A703-A709,
2004) reported that Ru and tetraaza-macrocycle complexes were able
to promote the activity of Pt towards methanol electrochemical
oxidation. However, the activity of the promoted Pt catalysts is
still lower than that of Pt/Ru bimetallic catalyst.
[0005] For fuel cell electrocatalysts and in particular for
electrocatalytic oxidation of small hydrocarbon molecules, there is
a need to make the electrocatalyst as active as possible.
SUMMARY OF THE INVENTION
[0006] In the first aspect, the invention provides a fuel mixture
comprising at least one said performance additive, wherein the
performance additive comprises a cyclic tertiary amine having a
molecular weight of at least about 200.
[0007] In the first aspect, the invention further provides a fuel
mixture comprising a performance additive capable of improving the
electrooxidation reaction rate of the fuel mixture, measured as
current density, by at least 2% compared to a fuel mixture not
containing the performance additive.
[0008] In the first aspect, the invention further provides a cyclic
tertiary amine comprising saturated monocyclic, bicyclic or
polycyclic rings containing at least one tertiary N atom; wherein
the cyclic tertiary amine contains one or more atoms selected from
C, N, O and Si, and wherein the number of ring atoms ranges from 3
to 18.
[0009] In the first aspect, the invention further provides a cyclic
tertiary amine comprising at least two saturated cyclic amine
rings, wherein the rings are linked to one another by means of
linking groups selected from the group of linear or cyclic
alkylidene, oxaalkylidene, polyoxaalkylidene, alkoxylated
alkylidene and alkylidenylaryl groups having 1 to 100 carbon
atoms.
[0010] In the first aspect, the invention further provides a cyclic
tertiary amine comprising at least two saturated cyclic amine rings
attached to polymeric cores by means of side chains.
[0011] In the first aspect, the invention further provides a cyclic
tertiary amine, wherein the cyclic tertiary amine is an end group
of a dendrimer. U.S. Pat. No. 4,507,466 defines dendrimers as a
novel class of branched polymers containing dendritic branches
having functional groups uniformly distributed on the periphery of
such branches. Some suitable dendrimers include those disclosed in
U.S. Pat. No. 4,507,466, which is incorporated herein by
reference.
[0012] In the second aspect, the invention provides a fuel supply
for supplying fuel to the anode of a fuel cell, the fuel supply
supplying fuel comprising at least one performance additive.
[0013] In the third aspect, the invention provides an electrode
composition comprising an electrocatalyst and at least one
performance additive, wherein the performance additive comprises a
cyclic tertiary amine having a molecular weight of at least about
200.
[0014] In a fourth aspect, the invention provides a membrane
electrode assembly comprising an electrode composition, wherein the
electrode composition comprises an electrocatalyst and at least one
performance additive, wherein the performance additive comprises a
cyclic tertiary amine having a molecular weight of at least about
200.
[0015] The membrane electrode assembly further comprises a solid
polymer electrolyte membrane and at least one gas diffusion
backing.
[0016] In a fifth aspect, the invention provides an electrochemical
cell, such as a direct methanol fuel cell, comprising an anode and
a cathode, a membrane comprising ionomer having ion-exchange groups
separating the anode and cathode, and a fuel supply for supplying
fuel, such as liquid methanol fuel, to the anode, the fuel supply
comprises at least one performance additive.
[0017] In a sixth aspect, the invention provides an electrochemical
cell, such as a fuel cell, comprising a membrane electrode
assembly, wherein the membrane electrode assembly comprises at
least one electrode composition, wherein the electrode composition
comprises an electrocatalyst and at least one performance additive.
The membrane electrode assembly further comprises a solid polymer
electrolyte membrane wherein the membrane comprises an ionomer
having proton conductive ion-exchange groups separating the anode
and cathode. The membrane electrode assembly further comprises at
least one gas diffusion backing. The fuel cell further comprises a
fuel supply for supplying fuel, such as liquid methanol fuel, to
the anode.
[0018] In a seventh aspect, the invention provides a process for
operating a direct methanol fuel cell comprising an anode and a
cathode, a membrane comprising ion-exchange groups separating the
anode and cathode, and a fuel supply for supplying liquid methanol
fuel to the anode, the process comprising adding at least one
performance additive to the methanol fuel during the operation of
the fuel cell, wherein the performance additive comprises a cyclic
tertiary amine having a molecular weight of at least about 200.
BRIEF DESCRIPTION OF THE DRAWING
[0019] FIG. 1 is a schematic illustration of a single cell
assembly.
[0020] FIG. 2 shows the chronoamperometry of MeOH oxidation at 0.4V
vs. Normal Hydrogen Electrode (NHE) at 40.degree. C. The starting
electrolyte is 2M methanol with 0.05M H2SO4, wherein at 600 sec.,
0.15M N-methylmorpholine was injected into the solution, as
described in Example 1.
[0021] FIG. 3 shows the cyclic voltammogram of 2M MeOH/0.05M
H.sub.2SO.sub.4 before and after 0.15M N-methylmorpholine was
added, as described in Example 1.
[0022] FIG. 4 shows the chronoamperometry of MeOH oxidation at 0.4V
vs. NHE at 40.degree. C. wherein at 600 sec., 0.15M
4,4'-(Oxydi-2,1-ethanediyl)bismorpholine was injected into the
solution, as described in Example 2.
[0023] FIG. 5 shows the cyclic voltammogram of 2M MeOH/0.05M
H.sub.2SO.sub.4 before and after 0.15M
4,4'-(Oxydi-2,1-ethanediyl)bismorpholine was added, as described in
Example 2
[0024] FIG. 6 shows the current vs. time for a direct methanol fuel
cell at a constant voltage of 0.45 V, as described in Example 3,
wherein at about the 1800th second,
4,4'-(Oxydi-2,1-ethanediyl)bismorpholine is injected into the fuel
compartment of the cell to result in 0.175M
4,4'-(Oxydi-2,1-ethanediyl)bismorpholine in the 2M methanol fuel.
Current is seen to increase upon the injection.
[0025] FIG. 7 shows the current vs. time for a direct methanol fuel
cell at a constant voltage of 0.45 V, as described in Example 3,
wherein at about the 1800th second,
4,4'-(Oxydi-2,1-ethanediyl)bismorpholine is injected into the fuel
compartment of the cell to result in 0.125M
4,4'(Oxydi-2,1-ethanediyl)bismorpholine in the 2M methanol fuel.
Current is seen to increase upon the injection.
[0026] FIG. 8 shows the current vs. time for a direct methanol fuel
cell at a constant voltage of 0.45 V, as described in Example 3,
wherein at about the 1800th second, N-methylmorpholine is injected
into the fuel compartment of the cell to result in 0.15M N-methyl
morpholine in the 2M methanol fuel. Current is seen to decrease
upon the injection.
DETAILED DESCRIPTION OF THE INVENTION
[0027] Fuel cells electrochemically oxidize hydrogen or hydrocarbon
fuels such as alcohols, aldehydes, formic acid, and methoxy-methane
to generate electricity. Among these, H.sub.2 is the most reactive
fuel and there is little kinetic overpotential when Pt is used as
the electrocatalyst. On the other hand, electrooxidation of CO
containing reformate and hydrocarbon fuels, such as methanol,
suffers from low reaction rate and high reaction overpotential
compared to that of H.sub.2. Thus, fuel cells employ hydrocarbon or
CO containing reformate have lower power density than H.sub.2 fuel
cells.
[0028] The present invention provides a novel approach that has
several surprising and important benefits over the present state of
the art. In the present invention, a performance additive herein
described when added to the fuel supply or to the electrode
structure provides surprising enhancement of fuel cell power
density while the performance additive is not substantially
decomposed or consumed in the process. While there are many known
catalytic and electrocatalyst poisons, there are few known organic
additives that promote catalytic reaction. The fact that such an
organic performance additive exists for fuel cell oxidation
reactions is surprising. When used as a fuel or electrode additive,
the performance additive of the present invention is capable of
promoting the electrocatalytic reaction therefore providing higher
fuel cell power density than that without the additive. It is also
surprising that the performance additive of this invention may
simultaneously provide lower fuel crossover from the anode side to
the cathode side. Given the fact that these improvements come
without any increase of noble metal loading and without any complex
chemical or physical modification to the electrocatalyst, this is a
novel and comparatively low cost way to improve fuel cell
performance.
Performance Additive:
[0029] The performance additive of this invention comprises a
cyclic tertiary amine having a molecular weight of at least about
200. These performance additives are capable of increased
electrooxidation reaction rate, measured as current density, by at
least 2%, and more typically by 5 to 300% than that achieved
without the performance additive. Suitable cyclic tertiary amines
comprise saturated monocyclic, bicyclic or polycyclic rings
containing at least one tertiary N atom; wherein the cyclic
tertiary amine contains one or more atoms selected from C, N, O and
Si, and wherein the number of ring atoms ranges from 3 to 18.
[0030] In one embodiment, the cyclic tertiary amine may comprise at
least two saturated cyclic amine rings, wherein the rings are
linked to one another by means of linking groups selected from the
group of linear or cyclic alkylidene, oxaalkylidene,
polyoxaalkylidene, alkoxylated alkylidene and alkylidenylaryl
groups having 1 to 100 carbon atoms.
[0031] A representative structure for these cyclic tertiary amines
would be: ##STR1## where X1 and X2 are independently selected from
C, N, O, and Si.
[0032] Some specific examples include: ##STR2##
[0033] In another embodiment, a cyclic tertiary amine comprises at
least two saturated cyclic amine rings attached to polymeric cores
by means of side chains. A representative structure corresponding
to this embodiment would be: ##STR3## where n=1 to 50000.
[0034] In a further embodiment, the cyclic tertiary amine is a part
of a dendrimer. U.S. Pat. No. 4,507,466 defines dendrimers as a
novel class of branched polymers containing dendritic branches
having functional groups uniformly distributed on the periphery of
such branches. Some suitable dendrimers include those disclosed in
U.S. Pat. No. 4,507,466, which is incorporated herein by
reference.
[0035] A representative structure corresponding to this embodiment
would be a dendrimeric polyether ending with saturated cyclic
tertiary amines (Dendrimer I) and a dendrimer based on successive
generations of polypropylenimines substituted onto a central
diamine, such as ethylenediamine, in which the terminal amine end
groups are part of saturated cyclic rings (Dendrimer II).
##STR4##
[0036] Typically, the cyclic tertiary amine should not diffuse into
the membrane of the fuel cell. To achieve this, the amines may be
made with large enough molecular size so as to eliminate or
minimize their diffusion into the membrane. A molecular weight of
at least 200 is typical. Said amines may be immobilized by means
known in the art, including but not limited to grafting onto
insoluble substrates such as polymers, metal catalysts, inorganic
oxides, and amorphous carbon, carbon nanotubes and carbon
nanohorns, etc. Other immobilization means such as chelating and
coordination may also be used.
[0037] In one aspect of this invention, the performance additive is
used as a fuel additive, in which case, the molecular weight of the
additive is typically about 200 to about 10,000 and still more
typically about 400 to about 5,000, and most typically about 400 to
about 2000. The fuel cell utilizes a fuel source that may be in the
gas or liquid phase, and may comprise hydrogen or a hydrocarbon. In
the case of gaseous fuel, the performance additive in its gases
form is mixed with the gaseous fuel. In the case of liquid fuel,
the performance additive is dissolved in and mixed with the liquid
fuel.
[0038] Typically, the performance additive is used as liquid
methanol fuel additive. In the present application, "methanol fuel"
refers to the fuel in contact with the anode and the membrane.
"Fuel mixture" is methanol and the performance additive with or
without water. "Fuel supply" is the apparatus for supplying
methanol fuel to the anode.
[0039] It is desirable that the performance additive selected be
soluble in the methanol fuel under the conditions of temperature
and concentration at which the methanol fuel is used in the fuel
cell. Similarly, for any fuel mixtures to be supplied to the fuel
cell which incorporate the performance additive, the performance
additive should be soluble in the fuel mixture at the desired
concentration.
[0040] The concentration of the performance additive in the
methanol fuel is at least about 0.001 molar, typically at least
about 0.05 molar, and more typically at least about 0.1 molar; no
more than about 2.0 molar, typically no more than about 1.0 molar,
and more typically no more than about 0.3 molar. The performance
additive may advantageously be premixed with the fuel mixture
supplied to the fuel cell, in which case concentrations can be in
the same preferred ranges as the methanol fuel, or may be added to
the fuel mixture during operation of the fuel cell.
[0041] The methanol fuel for the fuel cell can be supplied by a
fuel supply, which can be a single container, or it may be supplied
by a plurality of containers from which feeds are mixed. For
example, a three-container system could have separate containers of
methanol, water, and performance additive. A two-container system
could have separate containers, one for up to 100% aqueous
methanol, and the other for pure performance additive.
Alternatively, a two-container system could have one for fuel
mixture, typically performance additive in pure methanol, the other
for methanol fuel, typically aqueous mixture of methanol and
performance additive. If water is present, the fuel mixture
advantageously has the same percentages of methanol and water
discussed above for the methanol fuel. Water generated by the fuel
cell can be a source of water for the methanol fuel. Therefore, the
fuel supply can be fed from a fuel concentrate, which may contain
up to 100% methanol or only methanol and the performance additive.
The concentrate is added to the operating fuel cell to keep the
methanol fuel in desired concentration range for both methanol and
the performance additive.
[0042] The performance additive is not consumed at all or is not
consumed at the rate at which methanol is consumed in the fuel
cells of this invention and therefore, depending upon operating
conditions such as temperature and current density, the
concentration of the performance additive in the methanol fuel
during operation may increase in the anode compartment and/or the
fuel reservoir. In this event, the performance additive
concentration can be maintained by stop adding the performance
additive or reducing the amount of performance additive being added
to the methanol fuel. The performance additive may be added to the
methanol fuel intermittently when operating conditions indicate a
drop in performance. This process can be automated, for example, by
monitoring the amount of methanol and water being consumed and
using the monitor signal to control a metering system to add
performance additive to the methanol fuel as necessary to maintain
performance. Alternatively, the performance additive concentration
may be adjusted in real time so as to meet the power demand on the
fuel cell.
[0043] Because the amount of performance additive in the fuel
mixture needed for "make up" or to maintain the desired
steady-state concentration of performance additive in the methanol
fuel may be less than needed at the start of operation, the
concentration of performance additive in the fuel mixture of a fuel
cell that is in operation may be lower than that at the start of
operation.
[0044] For portable devices powered by fuel cells designed
according to this invention, containers of fuel will be convenient
for refueling the cell. The containers can be made from polymer or
metal materials suitable for the fuel, i.e. having low permeability
to the fuel components and being resistant to interaction with the
fuel components. It is preferred that the container be
substantially nonvitreous, that is, not be made of glass or other
vitreous material, though such material may comprise no more than
about 10% of the total mass of the container, typically no more
than about 5%. Such containers will have at least one dispensing
port, sealed by a cap or plug, or other sealing means, such as by a
foil membrane, or typically a septum of elastomeric material. The
contents of the container may be used to fill the anode compartment
of the fuel cell when fuel replenishment is necessary.
Alternatively, the fuel cell can be designed to accept such
containers, so they may be joined to the cell, replacing empty
containers that have been removed. In either case, the container
may hold a concentrated fuel mixture to which water is added to
achieve the desired methanol fuel composition. The water may be in
a separate compartment and may be water that is generated during
operation of the fuel cell. In this respect, the containers may be
used as disposable batteries are now used in devices such as flash
lights and portable radios and may be used to provide an instant
"recharge" for devices such as cell phones, portable computers, and
portable digital assistants which currently employ rechargeable
batteries.
Electrode Compositions:
[0045] In another embodiment, the invention provides an
electrochemical cell, such as a fuel cell, comprising a membrane
electrode assembly comprising at least one electrode composition,
wherein the electrode composition comprises an electrocatalyst, a
polymer binder, typically a highly fluorinated ion-exchange
polymers (as described below) and at least one performance
additive. The weight percentage of the polymer binder in the
electrode composition is at least about 5%, and typically at least
about 10%; no more than about 30%, and typically no more than about
20%. The weight percentage of the performance additive in the
electrode composition is at least about 0.1%, and typically at
least about 1%; no more than 15% and typically no more than 5%.
[0046] The molecular weight of the performance additive in the
electrode composition is typically about 200 to about 1,000,000 and
still more typically about 2000 to about 500,000, and most
typically about 4000 to about 100,000. The electrode compositions
that form the anode and cathode in a membrane electrode assembly
may be made from well-known electrically conductive, catalytically
active particles or materials and may be made by methods well known
in the art. The electrode compositions may be formed as a film of a
polymer that serves as a binder for the catalyst particles.
Electrocatalyst:
[0047] Electrocatalysts in the composition are selected based on
the particular intended application for the membrane electrode
assembly. Electrocatalysts suitable for use in the present
invention include one or more noble group metals such as platinum,
ruthenium, rhodium, and iridium and electroconductive oxides
thereof, and electroconductive reduced oxides thereof. The catalyst
may be supported or unsupported. For direct methanol fuel cells, a
(Pt--Ru)O.sub.X electocatalyst has been found to be useful.
Typically used electorcatalysts for hydrogen fuel cells are
platinum on carbon, for example, 60 wt % carbon, 40 wt % platinum
such as the material with this composition obtainable from E-Tek
Corporation Natick, Mass., and 60% platinum, 40% carbon obtainable
from John Matthey as FC-60.
[0048] Suitable electrocatalysts for this invention also include
electrocatalysts that do not contain noble metal but have
electrocatalytic activity for fuel cell reactions. These include
Raney nickel, metal carbides, metal borides and organometallic
complexes such as iron, copper, or zinc metalloporphyrins.
Binder:
[0049] Since the ion exchange polymer employed in the electrode
composition serves not only as binder for the electrocatalyst
particles but also assists in securing the electrode to the
substrate, e.g. membrane, it is typical for the ion exchange
polymers in the composition to be compatible with the ion exchange
polymer in the membrane. Most typically, exchange polymers in the
composition are the same type as the ion exchange polymer in the
membrane.
[0050] Ion exchange polymers for use in accordance with the present
invention are typically highly fluorinated ion-exchange polymers.
"Highly fluorinated" means that at least 90% of the total number of
univalent atoms in the polymer are fluorine atoms. Most typically,
the polymer is perfluorinated. It is also preferred for use in fuel
cells for the polymers to have sulfonate ion exchange groups. The
term "sulfonate ion exchange groups" is intended to refer to either
sulfonic acid groups or salts of sulfonic acid groups, typically
alkali metal or ammonium salts. For applications where the polymer
is to be used for proton exchange as in fuel cells, the sulfonic
acid form of the polymer is preferred. If the polymer in the
electrode composition is not in sulfonic acid form when used, a
post treatment acid exchange step will be required to convert the
polymer to acid form prior to use. Suitable highly fluorinated
ion-exchange polymers include Nafion.RTM. polymers, available from
E. I. DuPont de Nemours, Wilmington, Del.
[0051] The electrode compositions formed on the membrane should be
porous so that they are readily permeable to the gases/liquids that
are consumed and produced in cell. The average pore diameter is
typically in the range of 0.01 to 50 .mu.m, most typically 0.1 to
30 .mu.m. The porosity is generally in a range of 10 to 99%,
typically 10 to 60%.
Membrane:
[0052] The membranes may typically be made by known extrusion or
casting techniques and typically have a thickness of about 5 .mu.m
to about 250 .mu.m , more typically about 10 .mu.m to about 200
.mu.m, most typically about 20 .mu.m to about 125 .mu.m. While the
polymer may be in alkali metal or ammonium salt form, it is typical
for the polymer in the membrane to be in acid form to avoid post
treatment acid exchange steps.
[0053] Ionomers for the membranes used in accordance with this
invention may be any number of ion exchange polymers including
polymers with cation exchange groups in the acid or proton form,
hereinafter referred to as acid groups. Such acid groups include
sulfonic acid groups, carboxylic acid groups, phosphonic acid
groups, and boronic acid groups. Typically, the ionomer has
sulfonic acid and/or carboxylic acid groups.
[0054] Polymers for use in accordance with the present invention
are typically fluorinated, more typically highly fluorinated
ion-exchange polymers having sulfonic acid and/or carboxylic acid
groups. "Fluorinated" means that at least 10% of the total number
of univalent atoms in the polymer are fluorine atoms. "Highly
fluorinated" means that at least 90% of the total number of
univalent atoms in the polymer are fluorine atoms. Most typically,
the polymer is perfluorinated.
[0055] Typically, the polymer comprises a polymer backbone with
recurring side chains attached to the backbone with the side chains
carrying the acid groups. Possible polymers include homopolymers or
copolymers of two or more monomers. Copolymers are typically formed
from at least one monomer which is a nonfunctional monomer and
which provides carbon atoms for the polymer backbone. A second
monomer provides both carbon atoms for the polymer backbone and
also contributes the side chain carrying the acid group or its
precursor, e.g., a sulfonyl halide group such as sulfonyl fluoride
(--SO.sub.2F), which can be subsequently hydrolyzed and converted
to a sulfonic acid group; or a carbomethoxy group (--COOCH.sub.3)
which can be subsequently hydrolyzed to a carboxylic acid group.
For example, copolymers of a first fluorinated vinyl monomer
together with a second fluorinated vinyl monomer having a sulfonyl
fluoride group (--SO.sub.2F) can be used. Possible first monomers
include tetrafluoroethylene (TFE), hexafluoropropylene, vinyl
fluoride, vinylidine fluoride, trifluorethylene,
chlorotrifluoroethylene, perfluoro(alkyl vinyl ether),
hexafluoroisobutylene ((CH.sub.2.dbd.C(CF.sub.3).sub.2), ethylene,
and mixtures thereof. Possible second monomers include a variety of
fluorinated vinyl ethers with sulfonic acid groups or precursor
groups which can provide the desired side chain in the polymer.
Additional monomers can also be incorporated into these polymers if
desired.
[0056] Other sulfonic acid ionomers are known and have been
proposed for fuel cell applications. Polymers of trifluorostyrene
bearing sulfonic acid groups on the aromatic rings are an example
(U.S. Pat. No. 5,773,480). The trifluorostyrene monomer may be
grafted to a base polymer to make the ion-exchange polymer (U.S.
Pat. No. 6,359,019).
[0057] Suitable perfluorinated sulfonic acid polymer membranes in
acid form are available under the trademark Nafion.RTM. by E. I. du
Pont de Nemours and Company.
[0058] Reinforced perfluorinated ion exchange polymer membranes can
also be utilized in CCM manufacture. Reinforced membranes may be
made by impregnating porous, expanded PTFE (ePTFE) with ion
exchange polymer. ePTFE is available under the tradename "Goretex"
from W. L. Gore and Associates, Inc., Elkton Md., and under the
tradename "Tetratex" from Tetratec, Feasterville Pa. Impregnation
of ePTFE with perfluorinated sulfonic acid polymer is disclosed in
U.S. Pat. Nos. 5,547,551 and 6,110,333.
[0059] Alternately, the ion exchange membrane may be a porous
support for the purposes of improving mechanical properties, for
decreasing cost and/or other reasons. The porous support may be
made from a wide range of components, for e.g., hydrocarbons such
as a polyolefin, e.g., polyethylene, polypropylene, polybutylene,
copolymers of those materials, and the like. Perhalogenated
polymers such as polychlorotrifluoroethylene may also be used. The
membrane may also be made from a polybenzimadazole polymer. This
membrane may be made by casting a solution of polybenzimadazole in
phosphoric acid (H.sub.3PO.sub.4) doped with trifluoroacetic acid
(TFA) as described in U.S. Pat. Nos. 5,525,436; 5,716,727,
6,025,085 and 6,099,988.
Electrochemical Cell:
[0060] As shown in FIG. 1, an electrochemical cell, such as a fuel
cell comprises a catalyst coated membrane (CCM) (10) in combination
with at least one gas diffusion backing (GDB) (13) to form an
unconsolidated membrane electrode assembly (MEA). The catalyst
coated membrane (10) comprises an polymer electrolyte membrane (11)
discussed above and catalyst layers or electrodes (12) formed from
an electrocatalyst coating composition. The fuel cell is further
provided with an inlet (14) for fuel, such as hydrogen; liquid or
gaseous alcohols, e.g. methanol and ethanol; or ethers, e.g.
diethyl ether, etc., an anode outlet (15), a cathode gas inlet
(16), a cathode gas outlet (17), aluminum end blocks (18) tied
together with tie rods (not shown), a gasket for sealing (19), an
electrically insulating layer (20), graphite current collector
blocks with flow fields for gas distribution (21), and gold plated
current collectors (22).
[0061] Alternately, gas diffusion electrodes comprising a gas
diffusion backing having a layer of an electrocatalyst coating
composition thereon may be brought into contact with a solid
polymer electrolyte membrane to form the MEA.
Catalyst Coated Membrane (CCM) and Membrane Electrode Assembly
(MEA):
[0062] A variety of techniques are known for CCM manufacture which
apply an electrocatalyst coating composition similar to that
described above onto the solid fluorinated polymer electrolyte
membrane. Some known methods include spraying, painting, patch
coating and screen, decal, pad or flexographic printing.
[0063] In one embodiment of the invention, the MEA (30) may be
prepared by thermally consolidating the gas diffusion backing (GDB)
with a CCM at a temperature of under 200.degree. C., preferably
140-160.degree. C. The CCM may be made of any type known in the
art. In this embodiment, an MEA comprises a solid polymer
electrolyte (SPE) membrane with a thin catalyst-binder-performance
additive layer disposed thereon. The catalyst may be supported
(typically on carbon) or unsupported. In one method of preparation,
a catalyst film is prepared as a decal by spreading the catalyst
ink on a flat release substrate such as Kapton.RTM. polyimide film
(available from the DuPont Company). After the ink dries, the decal
is transferred to the surface of the SPE membrane by the
application of pressure and heat, followed by removal of the
release substrate to form a catalyst coated membrane (CCM) with a
catalyst layer having a controlled thickness and catalyst
distribution. Alternatively, the catalyst layer is applied directly
to the membrane, such as by printing, and then the catalyst film is
dried at a temperature not greater than 200.degree. C.
[0064] The CCM, thus formed, is then combined with a GDB to form
the MEA of the present invention. The MEA is formed, by layering
the CCM and the GDB, followed by consolidating the entire structure
in a single step by heating to a temperature no greater than
200.degree. C., preferably in the range of 140-160.degree. C., and
applying pressure. Both sides of the MEA can be formed in the same
manner and simultaneously. Also, the composition of the catalyst
layer and GDB could be different on opposite sides of the
membrane.
[0065] The following examples illustrate but do not limit the
invention.
EXAMPLES
Sample Preparation and Test Methods Chronoamperometry and Cyclic
Voltammetry:
[0066] Chronoamperometry and cyclic voltammetry were conducted in a
modified three-electrode cell commercially available from Princeton
Applied research (microcell kit K0264). The cell temperature was
precisely controlled within .+-.0.25.degree. C. by supplying
temperature controlled water to a glass jacket of the cell. The
counter electrode was Pt (K0266 Princeton Applied Research) and the
reference electrode was Ag/AgCl electrode commercially available
from Bioanalytical Systems, Inc. (BAS) (RE-5B). The working
electrode was made as described below.
[0067] After assembly of the cell, the cell electrolyte solution
was purged with ultrahigh purity Argon for several minutes to
remove air. The solution was blanketed by Argon during the
experiments.
[0068] The cell was electronically connected to a potentiostat
(Solatron 1287) which was controlled by the computer software
CorrWare.RTM. (Scribner Associates Inc.) via a standard GPIB
(General Purpose Input Board) interface.
[0069] The chronoamperometry was typically performed at 0.4V vs.
NHE. The methanol oxidation current was recorded as a function of
time at least twice per second. The performance additive of
interest was typically injected into the cell in the middle of the
chronoamperometry run using a digital pipette through an injection
port. Cyclic voltammetry in the range of -0.5 to 1.1 V vs. NHE was
performed before and after chronoamperometry. The scan rate was
typically 20 mV/sec.
Preparation of the Working Electrode:
[0070] A working electrode was prepared by coating the tip (1 cm
wide 1.5 cm long) of a 1 cm wide 6 cm long carbon paper strip
(Toray) with 0.07 to 0.09 gram of catalyst ink. The ink formulation
is 0.1 g Pt/Ru black, 2 g of Nafion.RTM. solution (proton form, 1%
solids in water) and 2 g of nanopure water. The ink was stirred and
sonicated to a uniform dispersion. The coated carbon paper was
dried in the chemical hood before use.
Example 1
N-methvlmorpholine
[0071] N-methylmorpholine (>99.5%) was obtained from Aldrich and
used as received. A carbon strip with Pt/Ru black (1:1 atomic
ratio) catalyst was used as the working electrode. The starting
electrolyte solution is 0.05M H.sub.2SO.sub.4 with 2M MeOH in
water. Cell temperature was at 40.degree. C. A constant potential
of 0.4 V vs. normal hydrogen electrode (NHE) was applied. At about
600 sec, N-methylmorpholine was injected into the solution to
result in 0.15M N-methylmorpholine. FIG. 2 shows the result of this
experiment: the methanol oxidation current increased by about 50%
after injection.
[0072] It is known in the art, and shown here in FIG. 3 curve 1,
methanol oxidation on Pt/Ru catalyst commences at about 0.25V vs.
NHE, below which there is no appreciable oxidation current.
However, in the presence of N-methylmorpholine, a significant
oxidation current was obtained at voltages below 0.25V. In fact,
below 0.42V, curve 2 has a higher oxidation current than curve 1
indicating increased methanol electrooxidation activity as a result
of the additive.
Example 2
4,4'-(Oxydi-2,1-ethanedivl)bismorpholine
[0073] Example 1 was repeated with the following exception: the
injected chemical was 4,4'-(Oxydi-2,1-ethanediyl)bismorpholine
(Aldrich, GC, >95%) and its concentration in the cell solution
was 0.15 M. FIG. 4 shows that the methanol oxidation current
increased after injection. FIG. 5 shows cyclic voltammetry before
(curve 1) and after (curve 2) the
4,4'-(Oxydi-2,1-ethanediyl)bismorpholine injection. In the presence
of 4,4'-(Oxydi-2,1-ethanediyl)bismorpholine, the methanol oxidation
current is higher than without it at potentials below 0.5V vs. NHE.
This shows that 4,4'-(Oxydi-2,1-ethanediyl)bismorpholine enhances
methanol oxidation rate.
Example 3
[0074] A direct methanol fuel cell was operated with a Nafion.RTM.
117 membrane, 1050 equivalent weight (EW), and 7 mils (178 .mu.m)
thick. The cell active area was 5 cm.sup.2. The anode catalyst
layer was comprised of 4 mg/cm.sup.2 Pt/Ru (1:1 atom ratio) black
and 0.5 mg/cm.sup.2 Nafion.RTM. perflurosulfonic acid. The cathode
catalyst was comprised of 4 mg/cm.sup.2 Pt black and 0.5
mg/cm.sup.2 Nafion.RTM. perflurosulfonic acid. The catalyst layers
were applied to the Nafion.RTM. membrane by a screen printing
process. Porous carbon paper obtained from SGL Inc. (GDL31BC) was
used in both anode and cathode as diffusion backing. Stainless
steel mesh was used in both anode and cathode as current collector.
The cell body was made of polytetrafluroethylene and was comprised
of an anode compartment that measured 4 cm.times.3 cm.times.2.5 cm.
Methanol fuel in this compartment diffused onto the surface of the
catalyst layer through the stainless mesh and porous carbon paper.
There was no forced methanol fuel circulation. The cathode
compartment was supplied with house air at a constant flow rate of
200 sccm. Operating temperature was 40.degree. C. A Solartron 1287
potentiostat (Solartron Analytical Hampshire, England) was used to
control the fuel cell.
[0075] Initially, 27 g of 2 molar methanol fuel was loaded in the
fuel compartment. The cell potential was set at 0.45V constant and
the current output was monitored as a function of time. At the 30
minutes mark, 1.15 cc of the additive
4,4'-(Oxydi-2,1-ethanediyl)bismorpholine were injected via a
syringe into the fuel compartment to result in about 0.175 molar of
the additive. As seen in the FIG. 6, the cell current increased
from 134 mA to 142 mA. Methanol crossover in the cell with and
without additive was measured voltammetrically according to the
method of X. Ren et al. (J. Electrochemical Society, 147 (1), 92-98
(2000)) When measuring the crossover current, 200 sccm dry N.sub.2
was supplied to the cathode side of the fuel cell and a voltage of
0.8V was applied to the cell (cathode side positive). The steady
state current was taken as the methanol crossover current. When the
fuel was 2 molar MeOH without any additive, the crossover current
density was 110 mA/cm.sup.2; when the fuel was 2 molar MeOH with
0.175 molar additive, the crossover current density was 7
mA/cm.sup.2.
Example 4
[0076] Example 3 was repeated with the following exception:
(Oxydi-2,1-ethanediyl)bismorpholine was injected, resulting in
0.125 molar of the additive. As shown in FIG. 7, the cell current
increased from 196 mA to 206 mA upon injection of the additive. The
crossover current reduced from 130 mA/cm.sup.2 to 126 mA/cm.sup.2
after additive injection.
Comparative Example A
[0077] Example 3 was repeated with the following exception: a
cyclic tertiary amine, N-methylmorpholine having a molecular weight
of 101 was injected resulting in 0.15 molar of the additive. As
shown in FIG. 8, the cell current quickly decreased. The results
show that the cyclic tertiary amine having a molecular weight of
less than 200 failed the fuel cell test.
* * * * *