U.S. patent application number 10/944455 was filed with the patent office on 2006-03-23 for carbon-polymer electrochemical systems and methods of fabricating them using layer-by-layer technology.
This patent application is currently assigned to Massachusetts Institute of Technology. Invention is credited to Tarek R. Farhat, Paula T. Hammond Cunningham.
Application Number | 20060062982 10/944455 |
Document ID | / |
Family ID | 36074391 |
Filed Date | 2006-03-23 |
United States Patent
Application |
20060062982 |
Kind Code |
A1 |
Hammond Cunningham; Paula T. ;
et al. |
March 23, 2006 |
Carbon-polymer electrochemical systems and methods of fabricating
them using layer-by-layer technology
Abstract
One aspect of the invention provides ion-exchange and
gas-diffusion membranes, fabricated by a layer-by-layer approach,
for use, e.g., in electrochemical cells; a process for making
membrane electrode assemblies fabricated using porous frameworks,
LBL composite membranes and LBL carbon-Polymer electrodes; and the
application of the membrane and electrode assemblies to a variety
of devices, both electrochemical and otherwise.
Inventors: |
Hammond Cunningham; Paula T.;
(Newton, MA) ; Farhat; Tarek R.; (Somerville,
MA) |
Correspondence
Address: |
FOLEY HOAG, LLP;PATENT GROUP, WORLD TRADE CENTER WEST
155 SEAPORT BLVD
BOSTON
MA
02110
US
|
Assignee: |
Massachusetts Institute of
Technology
Cambridge
MA
|
Family ID: |
36074391 |
Appl. No.: |
10/944455 |
Filed: |
September 17, 2004 |
Current U.S.
Class: |
428/220 ;
427/402; 428/411.1; 428/500 |
Current CPC
Class: |
Y02E 60/10 20130101;
H01M 50/46 20210101; B01D 71/28 20130101; H01M 8/1051 20130101;
Y10T 428/31855 20150401; H01M 8/1048 20130101; B01D 69/02 20130101;
B01D 69/141 20130101; B01D 71/60 20130101; C25B 13/04 20130101;
B01D 69/122 20130101; B01D 71/44 20130101; H01M 8/1004 20130101;
Y10T 428/31504 20150401; Y02E 60/50 20130101; B01D 69/12 20130101;
H01M 8/1039 20130101; H01M 50/411 20210101; H01M 4/92 20130101;
H01M 8/1053 20130101; H01M 4/926 20130101; H01M 8/1046 20130101;
H01M 8/103 20130101; B01D 71/82 20130101; Y02P 70/50 20151101; H01M
4/8605 20130101; B01D 71/40 20130101; H01M 8/1023 20130101 |
Class at
Publication: |
428/220 ;
428/500; 428/411.1; 427/402 |
International
Class: |
B32B 27/08 20060101
B32B027/08; B32B 27/32 20060101 B32B027/32 |
Goverment Interests
GOVERNMENT SUPPORT
[0001] This invention was made with support from the National
Science Foundation (grant number CTS-0136029); therefore, the
government has certain rights in the invention.
Claims
1. A method of forming a membrane, comprising sequentially
depositing, under pH controlled conditions, a plurality of polymer
layers on a surface; wherein each polymer layer is independently
selected from the group consisting of pH dependent cationic
polyelectrolytes, pH independent cationic polyelectrolytes, neutral
polymers, pH dependent anionic polyelectrolytes, and pH independent
anionic polyelectrolytes; wherein a polymer layer optionally
comprises at least one additional chemical entity selected from the
group consisting of hydrogels, polyions, colloids, latexes,
zeolites, platelets, proton sponges, organic molecules, organic
salts, inorganic salts, organic acids, inorganic acids, cationic
dendrimers, anionic dendrimers, metals and carbon; and wherein said
plurality of polymer layers comprises a first polymer layer and a
second polymer layer; thereby forming a membrane.
2. The method of claim 1, wherein said membrane further comprises
at least one additional chemical entity selected from the group
consisting of hydrogels, polyions, colloids, latexes, zeolites,
platelets, proton sponges, organic molecules, organic salts,
inorganic salts, organic acids, inorganic acids, cationic
dendrimers, anionic dendrimers, metals and carbon.
3. The method of claim 1, wherein said first polymer layer is a pH
dependent cationic polyelectrolyte, a pH independent cationic
polyelectrolyte or a neutral polymer; and wherein said second
polymer layer is a pH dependent anionic polyelectrolyte or pH
independent anionic polyelectrolyte; and wherein said membrane is
removed from said surface; thereby forming an ion-exchange membrane
or a LBL polyelectrolyte-carbon electrode.
4. The method of claim 1, wherein said first polymer layer is a pH
dependent cationic polyelectrolyte, a pH independent cationic
polyelectrolyte or a neutral polymer; and wherein said second
polymer layer is a pH dependent anionic polyelectrolyte or pH
independent anionic polyelectrolyte; and wherein said surface is
selected from the group consisting of organic hydrophilic porous
filter membranes and inorganic hydrophilic porous filter membranes;
thereby forming an ion-exchange membrane.
5. The method of claim 2, wherein said first polymer layer is a pH
dependent cationic polyelectrolyte, a pH independent cationic
polyelectrolyte or a neutral polymer; and wherein said second
polymer layer is a pH dependent anionic polyelectrolyte or pH
independent anionic polyelectrolyte; and wherein said membrane is
removed from said surface; thereby forming an ion-exchange membrane
or a LBL polyelectrolyte-carbon electrode.
6. The method of claim 2, wherein said first polymer layer is a pH
dependent cationic polyelectrolyte, a pH independent cationic
polyelectrolyte or a neutral polymer; and wherein said second
polymer layer is a pH dependent anionic polyelectrolyte or pH
independent anionic polyelectrolyte; and wherein said surface is
selected from the group consisting of organic hydrophilic porous
filter membranes and inorganic hydrophilic porous filter membranes;
thereby forming an ion-exchange membrane.
7. The method of claim 2, wherein said first polymer layer is a pH
dependent cationic polyelectrolyte, a pH independent cationic
polyelectrolyte or a neutral polymer; and wherein said second
polymer layer is a pH dependent anionic polyelectrolyte or pH
independent anionic polyelectrolyte; and wherein said surface is
organic, semi-metallic or metallic; and wherein said at least one
additional entity is carbon; thereby forming a LBL
polyelectrolyte-carbon electrode.
8. The method of claim 2, wherein said first polymer layer is a pH
dependent cationic polyelectrolyte, a pH independent cationic
polyelectrolyte or a neutral polymer; and wherein said second
polymer layer is a pH dependent anionic polyelectrolyte or pH
independent anionic polyelectrolyte; and wherein said at least one
chemical entity is selected from the group consisting of metals and
inorganic salts; and wherein said at least on additional entity is
carbon; thereby forming a LBL polyelectrolyte-carbon electrode.
9. The method of claim 1, wherein said first polymer layer is
selected from the group consisting of PAH, PDAC, PDME, PAAm, LPEI,
PEO, PVP, PVA, PEG and PANI.
10. The method of claim 1, wherein said first polymer layer is
selected from the group consisting of PDAC, PDME, PAAm, LPEI and
PEO.
11. The method of claim 1, wherein said second polymer layer is
selected from the group consisting of PAA, PMA, SPS, PAMPS, OEGDA,
PSSM3:1, and PAA-co-PAA.
12. The method of claim 1, wherein said second polymer layer is
selected from the group consisting of PAA, PAMPS, SPS, PSSM3:1,
PAA-co-PAAm.
13. The method of claim 1, wherein said at least one chemical
entity is selected from the group consisting of sulfonated latex,
sodium chloride, potassium chloride, lithium chloride, sulfonic
acid, nitric acid, hydrochloric acid, hydrobromic acid, phosphonic
acid, PEG, OEGDA, PAAm, PVA, PVP, polyphosphates, Nafions.RTM.,
ethylene glycol and glycerol.
14. The method of claim 1, wherein said at least on chemical entity
is selected from the group consisting of sodium chloride,
sulfonated latex, and Nafion 117.
15. The method of claim 1, wherein said first polymer layer is
selected from the group consisting of PAH, PDAC, PDME, PAAm, LPEI,
PEO, PVP, PVA, PEG and PANI; and wherein said at least one chemical
entity is selected from the group consisting of sulfonated latex,
sodium chloride, potassium chloride, lithium chloride, sulfonic
acid, nitric acid, hydrochloric acid, hydrobromic acid, phosphonic
acid, PEG, OEGDA, PAAm, PVA, PVP, polyphosphates, Nafions.RTM.,
ethylene glycol and glycerol.
16. The method of claim 1, wherein said second polymer layer is
selected from the group consisting of PAA, PMA, SPS, PAMPS, OEGDA,
PSSM3:1, and PAA-co-PAA; and wherein said at least one chemical
entity is selected from the group consisting of sulfonated latex,
sodium chloride, potassium chloride, lithium chloride, sulfonic
acid, nitric acid, hydrochloric acid, hydrobromic acid, phosphonic
acid, PEG, OEGDA, PAAm, PVA, PVP, polyphosphates, Nafions.RTM.,
ethylene glycol and glycerol.
17. The method of claim 1, wherein said first polymer layer is
selected from the group consisting of PAH, PDAC, PDME, PAAm, LPEI,
PEO, PVP, PVA, PEG and PANI; and wherein said second polymer layer
is selected from the group consisting of PAA, PMA, SPS, PAMPS,
OEGDA, PSSM3:1, and PAA-co-PAA; and wherein said at least one
chemical entity is selected from the group consisting of sulfonated
latex, sodium chloride, potassium chloride, lithium chloride,
sulfonic acid, nitric acid, hydrochloric acid, hydrobromic acid,
phosphonic acid, PEG, OEGDA, PAAm, PVA, PVP, polyphosphates,
Nafions.RTM., ethylene glycol and glycerol.
18. The method of claim 1, wherein said first polymer layer is
selected from the group consisting of PDAC, PDME, PAAm, LPEI and
PEO; and wherein said at least on chemical entity is selected from
the group consisting of sodium chloride, sulfonated latex, and
Nafion 117.
19. The method of claim 1, wherein said second polymer layer is
selected from the group consisting of PAA, PAMPS, SPS, PSSM3:1,
PAA-co-PAAm; and wherein said at least on chemical entity is
selected from the group consisting of sodium chloride, sulfonated
latex, and Nafion 117.
20. The method of claim 1, wherein said first polymer layer is
selected from the group consisting of PDAC, PDME, PAAm, LPEI and
PEO; and wherein said second polymer layer is selected from the
group consisting of PAA, PAMPS, SPS, PSSM3:1, PAA-co-PAAm; and
wherein said at least on chemical entity is selected from the group
consisting of sodium chloride, sulfonated latex, and Nafion
117.
21. The method of claim 1, wherein said first polymer layer is
LPEI.
22. The method of claim 21, wherein said second polymer layer is a
pH independent anionic polyelectrolyte.
23. The method of claim 1, wherein said first polymer layer is
PEO.
24. The method of claim 23, wherein said second polymer layer is a
pH dependent polyelectrolyte.
25. The method of claim 1, wherein said first polymer layer is
PAAm.
26. The method of claim 25, wherein said second polymer layer is a
pH dependent polyelectrolyte.
27. The method of claim 1, wherein said first polymer layer is
PDAC.
28. The method of claim 27, wherein said second polymer layer is a
PH dependent polyelectrolyte.
29. The method of claim 27, wherein said second polymer layer is a
pH independent polyelectrolyte.
30. The method of claim 1, wherein said first polymer layer is
PDME.
31. The method of claim 30, wherein said second polymer is a pH
independent polyelectrolyte.
32. The method of claim 1, wherein said second polymer layer is
PAMPS.
33. The method of claim 32, wherein said first polymer layer is a
neutral polymer.
34. The method of claim 32, wherein said first polymer layer is a
pH independent cationic polyelectrolyte.
35. The method of claim 1, wherein said second polymer layer is
SPS.
36. The method of claim 35, wherein said first polymer layer is a
neutral polymer.
37. The method of claim 1, wherein said second polymer layer is
PSSM3:1.
38. The method of claim 37, wherein said first polymer layer is a
neutral polymer.
39. The method of claim 1, wherein said second polymer layer is
PAA.
40. The method of claim 39, wherein said first polymer layer is a
neutral polymer.
41. The method of claim 1, wherein said second polymer layer is
PAA-co-PAAm.
42. The method of claim 41, wherein said first polymer layer is a
neutral polymer.
43. The method of claim 41, wherein said first polymer is a pH
independent cationic polymer.
44. The method of claim 1, wherein the first polymer layer is LPEI;
and wherein the second polymer layer is PAMPS.
45. The method of claim 1, wherein the first polymer layer is LPEI;
and the wherein the second polymer layer is SPS.
46. The method of claim 1, wherein the first polymer layer is LPEI;
and wherein the second polymer layer is PSSM.
47. The method of claim 1, wherein the first polymer layer is LPEI;
and wherein the second polymer layer is PAA.
48. The method of claim 1, wherein the first polymer layer is PEO;
and wherein the second polymer layer is PAA.
49. The method of claim 1, wherein the first polymer layer is PAAm;
and wherein the second polymer layer is PAA.
50. The method of claim 1, wherein the first polymer layer is PAAm;
and wherein the second polymer layer is PAA-coPAAm.
51. The method of claim 1, wherein the first polymer layer is PDAC;
and wherein the second polymer layer is PAA-co-PAAm.
52. The method of claim 1, wherein the first polymer layer is PDAC;
and wherein the second polymer layer is PAMPS.
53. The method of claim 1, wherein the first polymer layer is PDME;
and wherein the second polymer layer is PAMPS.
54. The method of claim 47, wherein the PAA is deposited at a pH of
about 4.0.
55. The method of claim 47, wherein the PAA is deposited at a pH
between about 3.5 and about 4.5.
56. The method of claim 48, wherein the PAA is deposited at a pH of
about 2.0.
57. The method of claim 48, wherein the PAA is deposited at a pH of
about 2.5.
58. The method of claim 48, wherein the PAA is deposited at a pH
between about 1.5 and about 3.0.
59. The method of claim 50, wherein the PAA-co-PAAm is deposited at
a pH of about 2.0.
60. The method of claim 50, wherein the PAA-co-PAAm is deposited at
a pH between about 1.5 and about 2.5.
61. The method of claim 51, wherein the PAA-co-PAAm is deposited at
a pH of about 5.
62. The method of claim 51, wherein the PAA-co-PAAm is deposited at
a pH between about 4.5 and about 5.5.
63. The method of claim 51, wherein the PAA-co-PAAm is deposited at
a pH between about 5.5 and about 6.5.
64. The method of claim 51, wherein the PAA-co-PAAm is deposited at
a pH between about 6.5 and about 7.5.
65. The method of claim 7, wherein said surface is selected from
the group consisting of carbon cloth, porous stainless steel,
porous silicon, porous titanium alloys and gold.
66. The method of claim 7 or 8, wherein said carbon is selected
from the group consisting of carbon powder, aqueous carbon paste,
and Hispec3000 powder.
67. The method of claim 8, wherein said metal is selected from the
group consisting of Pd, Pt, Au, Ru, Zn, Cu, Ag and Al.
68. The method of claim 8, wherein said first polymer layer is
PDAC; and wherein said second polymer layer is PAMPS; and wherein
said carbon is carbon powder; and wherein said metal is Pd.
69. The method of claim 8, wherein said first polymer layer is
PDAC; and wherein said second polymer layer is PAMPS; and wherein
said carbon is Hispec 3000; and wherein said metal is Pt.
70. The method of claim 8, wherein said first polymer layer is
LPEI; and wherein said second polymer layer is PAA deposited at a
pH of about 4.0; and wherein said carbon is aqueous carbon paste;
and wherein said metal is Pt.
71. A membrane, comprising a plurality of polymer layers held
together by electrostatic or hydrogen bonding interactions
deposited on a porous framework, wherein said membrane has a total
uniform thickness of less than about 10 .mu.m and a conductance of
less than about 1.0.times.10.sup.-3 S/cm and this conductance does
not degrade over time.
72. The membrane of claim 71, wherein said membrane has a total
thickness of less than about 1 .mu.m.
73. The membrane of claim 71, wherein said membrane has a total
thickness of less than about 0.75 .mu.m.
74. The membrane of claim 71, wherein said membrane has a total
thickness of less than about 0.5 .mu.m.
75. The membrane of claim 71, wherein said membrane has a total
thickness of less than about 0.25 .mu.m.
76. The membrane of claim 71, wherein said membrane has a total
thickness of less than about 0.1 .mu.m.
77. The membrane of claim 71, wherein said membrane has a
conductance of less than about 5.0.times.10.sup.-4 S/cm.
78. The membrane of claim 71, wherein said membrane has a
conductance of less than about 2.0.times.10.sup.-4 S/cm.
79. The membrane of claim 71, wherein said membrane has a
conductance of less than about 5.0.times.10.sup.-5 S/cm.
80. The membrane of claim 71, wherein said membrane has a
conductance of less than about 2.0.times.10.sup.-5 S/cm.
81. The membrane of claim 71, wherein said membrane has a
conductance of less than about 5.0.times.10.sup.-6 S/cm.
82. The membrane of claim 71, wherein said membrane has a
conductance of less than about 1.0.times.10.sup.-6 S/cm.
83. The membrane of claim 71, wherein the first polymer layer is
LPEI; and wherein the second polymer layer is PAMPS.
84. The membrane of claim 71, wherein the first polymer layer is
LPEI; and the wherein the second polymer layer is SPS.
85. The membrane of claim 71, wherein the first polymer layer is
LPEI; and wherein the second polymer layer is PSSM.
86. The membrane of claim 71, wherein the first polymer layer is
LPEI; and wherein the second polymer layer is PAA.
87. The membrane of claim 71, wherein the first polymer layer is
PEO; and wherein the second polymer layer is PAA.
88. The membrane of claim 71, wherein the first polymer layer is
PAAm; and wherein the second polymer layer is PAA.
89. The membrane of claim 71, wherein the first polymer layer is
PAAm; and wherein the second polymer layer is PAA-coPAAm.
90. The membrane of claim 71, wherein the first polymer layer is
PDAC; and wherein the second polymer layer is PAA-co-PAAm.
91. The membrane of claim 71, wherein the first polymer layer is
PDAC; and wherein the second polymer layer is PAMPS.
92. The membrane of claim 71, wherein the first polymer layer is
PDME; and wherein the second polymer layer is PAMPS.
93. The membrane of claim 71, wherein said membrane can be used as
an ion-exchange membrane, a gas diffusion membrane or an
electrode.
94. The membrane of claim 71, wherein the said membrane can be used
in a membrane-electrode assembly, a battery, a galvanic cell, an
electrochemical cell, a micro-electrochemical cell, a catalytic
converter, a solid-state hydrogen pump, an electrolyzer, or an
electrochromic device.
Description
BACKGROUND OF THE INVENTION
[0002] Electrochemical cells in which a chemical reaction is forced
by adding electrical energy are called electrolytic cells. Central
to the operation of any cell is the occurrence of oxidation and
reduction reactions which produce or consume electrons. These
reactions take place at electrode/solution interfaces, where the
electrodes must be good electronic conductors. In operation, a cell
is connected to an external load or to an external voltage source,
and electric charge is transferred by electrons between the anode
and the cathode through the external circuit. To complete the
electric circuit through the cell an additional mechanism must
exist for internal charge transfer. This is provided by one or more
electrolytes, which support charge transfer by ionic conduction.
Electrolytes must be poor electronic conductors to prevent internal
short circuiting of the cell.
[0003] The simplest electrochemical cell consists of at least two
electrodes and one or more electrolytes. The electrode at which the
electron producing oxidation reaction occurs is the anode. The
electrode at which an electron consuming reduction reaction occurs
is called the cathode. The direction of the electron flow in the
external circuit is always from anode to cathode.
[0004] A typical electrochemical cell will have a positively
charged anode and a negatively charged cathode. The anode and
cathode are typically submerged in a liquid electrolytic solution
which may be comprised of water and certain salts, acids or base
materials. Generally speaking, the anode and cathode are made of
substrate materials such as titanium, graphite, or the likes coated
with a catalyst such as lead dioxide or other known materials. The
selection of a substrate and catalyst is determined by the
particular electrode reactions which are to be optimized in a given
situation.
[0005] The cathode and anode are positioned within the electrolytic
cell with electrical leads leading to the exterior. The cell may be
provided with appropriate plumbing and external structures to
permit circulation of the electrolyte to a separate heat exchanger.
Suitable inlet and outlet passages may also be provided in the cell
head space to permit the withdrawal of the gases evolved from the
cathode (if gases are to be evolved) and from the anode.
[0006] In order to maintain or reduce the temperature of the cell
electrodes, heat exchange passages may be provided within the
electrode structures. These coolant passages are connected to
external sources of coolant liquid which can be circulated through
the electrodes during the electrolysis process in order to maintain
or reduce their temperatures.
[0007] In order to drive the electrolysis reactions it is necessary
to apply electric power to the cell electrodes. The electrodes are
connected through the electrical leads to an external source of
electric power with the polarity being selected to induce the
electrolyte anion flow to the anode and the cation flow to the
cathode.
Layer-by-Layer (LBL) Technique
[0008] Organic thin films continue to attract great interest in the
materials science community due to their ease of processing, ease
of functionalization, light weight and flexibility. Significant
progress has been achieved in the past 10-20 years, presenting the
possibility of molecular-level control in molecular and
macromolecular composite films. The ionic, layer-by-layer assembly
technique, introduced by Decher in 1991, is among the most exciting
recent developments in this area. Makromol. Chem., Macromol. Symp.
1991, 46, 321; Ber. Bunsenges. Phys. Chem. 1991, 95, 1430; Thin
Solid Films 1992, 210/211, 831. This approach, which utilizes
electrostatic interactions between oppositely charged polyion
species to create alternating layers of sequentially adsorbed
polyions, provides a simple and elegant means of depositing
layer-by-layer sub-nanometer-thick polymer films onto a surface
using aqueous solutions. Crystallography Reports 1994, 39, 628;
Macromol. 1995, 28, 7107; Langmuir 1997, 13, 2171. This
layer-by-layer deposition process provides a means to create
polycation-polyanion polyelectrolyte multilayers one molecular
layer at a time, thereby allowing an unprecedented level of control
over the composition and surface functionality of these interesting
materials. Typically, alternate layers of positively and negatively
charged polymers are sequentially adsorbed onto a substrate from
dilute solution to build up interpenetrated multilayer structures.
Most studies have focused on polyelectrolytes in their fully
charged state, such as strong polyelectrolyte poly(styrene
sulfonate) (SPS). However, we have discovered unique properties
when at least one alternating layer in the polyelectrolyte
multilayer is a weak polyelectrolyte where the charge density along
the chain can be readily controlled by adjusting the pH values of
the polyelectrolyte solution. Thin Solid Films 1992, 210, 831.
[0009] More recently, applications have been extended to
electroluminescent LEDs, conducting polymer composites, and as the
assembly of proteins and metal-nanoparticle systems. Adv. Mater.
1995, 7, 395; Adv. Mat. 1998, 10, 1452; Thin Solid Films 1994, 244,
985; Thin Solid Films 1994, 244, 806; J. Am. Chem. Soc. 1995, 117,
6117. The electrostatic LBL technique has been extended to include
many charged systems other than polymers and even other
complexation mechanisms, such as hydrogen bonding. Chem. Lett.
1997, 125; Macromol. 1997, 30, 2717.
Solid Polymer Electrolytes (SPEs)
[0010] As mentioned above, all electrochemical systems consist of
electrodes separated by an electrolyte for ion conduction and a
load for electronic conduction, as electricity can be generated or
fed into the system. Early electrochemistry relied exclusively on
liquid electrolytes, but recent applications are more demanding.
Solid polymer electrolytes (SPEs) have replaced liquid electrolytes
in many high-performance applications, such as batteries, fuel
cells, sensors, and electrochromic devices. Compared to liquid
electrolytes, SPEs feature easier processing, enhanced chemical
compatibility, and better mechanical properties with only a modest
decrease in conductivity.
[0011] A major advantage gained from forming SPEs by the LBL
technique is the introduction of a large number of variables that
modify the electrolyte or the electrodes depending on the user's
application. Other advantages include the utilization of cheap
nontoxic polyelectrolyte materials, an economic and simple
fabrication process, and miniaturization of the electrochemical
components. For example, a composite membrane made by LBL
deposition of a poly(+)/poly(-) couple on a porous framework is
more than ten times cheaper than any common commercial
proton-exchange-membrane (PEM), yet it can deliver more than half
the power. In addition, a stainless steel composite electrode made
by LBL deposition of a colloid of platinum/carbon catalyst with a
poly(-)/poly(+) stabilizers acted similar to a pure platinum
electrode by furnishing the same open-circuit potential yet it is a
thousand times cheaper and, unlike solid platinum, allows the
conduction of ions.
Fabrication of Fuel Cells Via LBL
[0012] A fuel cell is a type of electrical energy generating
device. There are several types of fuel cells such as acid fuel
cells, molten carbonate fuel cells, solid polymer electrolyte fuel
cells and solid oxide fuel cells. A fuel cell is an apparatus for
continually producing electric current by electrochemical reaction
of a fuel with an oxidizing agent. More specifically, a fuel cell
is a galvanic energy conversion device that chemically converts a
fuel such as hydrogen or a hydrocarbon and an oxidant that
catalytically react at electrodes to produce a DC electrical
output. In one type of fuel cell, the cathode material defines
passageways for the oxidant and the anode material defines
passageways for fuel. An electrolyte separates the cathode material
from the anode material. The fuel and oxidant, typically as gases,
are continuously passed through the cell passageways for reaction.
The essential difference between a fuel cell and a battery is that
there is a continuous supply of fuel and oxidant from outside the
fuel cell. Fuel cells produce voltage outputs that are less than
ideal and decrease with increasing load (current density). Such
decreased output is in part due to the ohmic losses within the fuel
cell, including electronic impedances through the electrodes,
contacts and current collectors. A need therefore exists for fuel
cells which have reduced ohmic losses.
[0013] Recently, industrial nations have revived the usage of
alternative energy sources to address their energy problems. At the
forefront of alternative energy technologies are fuel cells which
consume hydrogen or methanol, rather than crude oil, to generate
electricity. Larminie, J.; Dicks, A. Wiley, New York 2000. Although
fuel cell technologies are relatively well known, there is a strong
need for more portable, lightweight and low-cost fuel cell devices
for portable devices, micropower applications, and new applications
requiring embedded power in textiles, paper, plastics and other
thin film geometries. Using the layer-by-layer (LBL) self-assembly
a new generation of fuel cells can be envisioned. Decher, G.; Hong,
J. D.; Schmitt, J. Thin Solid Films 1992, 210/211, 831-835; Arys,
X.; Jonas, A. M.; Laschewsky, A.; R., L. 2000, 505-564. Micro fuel
cells assembled using the LBL technology, are fundamentally
different from those described in the literature. Most authors use
expensive lithographic and sputtering techniques to fabricate a
large number of microelectrodes on a flat substrate and use
conventional PEMs as separators. However, micropatterned LBL fuel
cells would provide access to low cost, readily available, and
easily mass-produced micropower devices analogous to, but much
cheaper than, the traditional microelectronic processes. Such
systems might include the use of an ultrathin perm-selective
membrane on a porous, ionically transmissive support. A major
advantage of PEMs over classical membranes is that extremely thin
films can effectively reduce the flow of specific gases, while
maintaining a high flux of others. Krasemann, L.; Tieke, B. Journal
of Membrane Science 1998, 150, 23-30; Krasemann, L.; Tieke, B.
Material Science and Engineering 1999, 819, 513-519; Krasemann, L.;
Tieke, B. Mat. Sc. Eng. C-Bio S89 1999, 513-518; Krasemann, L.;
Tieke, B. Langmuir 2000, 16, 287; Krasemann, L.; Tieke, B. Chem.
Eng. Tech. 2000, 2, 211-213. With a typical thickness per layer
pair of about 1.0 to about 100 nm, it is possible to engineer a
broad range of systems which will act as effective components in
proton exchange membranes. Krasemann, L.; Tieke, B. Journal of
Membrane Science 1998, 150, 23-30; Levasalmi, J. M.; McCarthy, T.
J. Macromolecules 1997, 30, 1752.
[0014] The core of a fuel cell is the membrane-electrode assembly
(MEA). The MEA of a fuel cell is usually fabricated by sandwiching
a proton-exchange membrane (PEM) between two gas diffusion C/Pt
electrodes. Larminie, J.; Dicks, A. Wiley, New York 2000;
Gottesfield, S.; Zawodzinski, T. Adv. Electrochem. Sci. Eng. 1997,
5, 195-301. The most commonly used PEMs are the perfluorosulfonated
membranes (e.g., Nafion.RTM.) which are comprised of a PTFE
crosslinked hydrophobic backbone impregnated with hydrophilic
sulfonic acid sites needed for proton mobility. Larminie, J.;
Dicks, A. Wiley, New York 2000; Gottesfield, S.; Zawodzinski, T.
Adv. Electrochem. Sci. Eng. 1997, 5, 195-301; Mehta, V.; Cooper, J.
S. J. Power Sources 2003, 114, 32-53. Other types of membranes used
as PEMs are the hydrocarbon polymer, non-fluorinated, and
polymer-inorganic composite membranes that, in general, are less
expensive and recyclable. Glipa, X.; Hograth, M. Dept. of Trade and
Industry (UK) homepage 2001; Panero, S.; Ciuffa, F.; D'Epifano, A.;
Scrsati, B. Electrochim. Acta 2003, 48, 2009-2014; Rikukawa, M.;
Sanui, K. Prog. Polym. Sci. 2000, 25, 1463-1502. Some polymers such
as the polyphosphazenes, the polybenzimidazoles (PBI) and
zirconia-polymer gels exhibit an equal or better performance than
the conventional perfluorinated membranes, especially for water
retention at high temperature. Qunhui, G.; Pintauro, P. N.; Tang,
H.; O'Connor, S. J. Mem. Sci. 1999, 154, 175-181; Glipa, X.;
Bonnet, B.; Mula, B.; Jones, D. J.; Rozier, J. J. Mater. Chem.
1999, 9, 3045-3049; Alberti, G.; Casciola, M. Solid State Ionics
2003, 145, 3-16. However, it should be noted that the
polyphosphazenes and the zirconia-polymer gels are not commercially
available and the PBIs are relatively expensive. An emerging
membrane technology based on the layer-by-layer deposition of
polyelectrolytes multilayer films on solid substrates or detachable
films might be harnessed to perform like a classical PEM. Decher,
G.; Hong, J. D.; Schmitt, J. Thin Solid Films 1992, 210/211,
831-835; Dubas, S. T.; Farhat, T. R.; Schlenoff, J. B. J. Am. Chem.
Soc. 2001, 123, 5368-5369; Vazquez, E.; Dewitt, D. M.; Lynn, D. M.;
Hammond, P. T. J. Am. Chem. Soc. 2003, 125, 11452; Arys, X.; Jonas,
A. M.; Laschewsky, A.; R., L. 2000, 505-564.
[0015] Because the LBL films can be tailored to deposit any
polyelectrolyte (PE) couple to any desired thickness, ranging from
a few angstroms to a few microns, they are much less expensive
technology than conventional membranes. Ion permeability and ion
conductivity in LBL films have been extensively studied and
characterized. Krasemann, L.; Tieke, B. Langmuir 2000, 16, 287;
Farhat, T. R.; Schlenoff, J. B. Langmuir 2001, 17, 1184-1192;
Farhat, T. R.; Schlenoff, J. B. J. Am. Chem. Soc. 2003, 125,
4627-4636; Harris, J. J.; DeRose, P. M.; Bruening, M. L. J. Am.
Chem. Soc. 1999, 121, 1978; DeLongchamp, D. M.; Hammond, P. T.
Chem. Mater. 2003, 15, 1165-1173; DeLongchamp, D. M.; Hammond, P.
T. Abstr. Pap. Am. Chem. Soc. S22:136-PMSE, Part 2 2001. The
diffusion coefficient of ions of conventional polymer multilayers
is a few orders of magnitude lower than the classical ion exchanger
membranes hence their proton conduction is lower. However, a range
of multilayer systems which incorporate hydrophilic polymers using
electrostatic and hydrogen bonding mechanisms, and have shown
increases in ionic conductivity of 3 or 4 orders of magnitude.
DeLongchamp, D. M.; Hammond, P. T. Chem. Mater. 2003, 15,
1165-1173; DeLongchamp, D. M.; Hammond, P. T. Abstr. Pap. Am. Chem.
Soc. S22:136-PMSE, Part 2 2001; Tokuhisa, H.; Hammond, P. T. Adv.
Funct. Mater. 2003, 13, 831-839. These differences are further
enhanced by the fact that ultra thin films can be formed using the
LBL technique, making the final conductance closer to that required
for power applications. One can tune the thickness and
permeability, as well as the composition, of these films through
choice of polyelectrolytes and adsorption conditions. For example,
using strong polyelectrolytes with hydrocarbon backbones yields LBL
films that tend to be either strongly or moderately hydrophobic,
thus discouraging proton exchange. On the other hand, LBL films
assembled using weak electrostatic and secondary interactions (i.e.
long-range hydrogen bonding or dipole-dipole), particularly those
with hydrophilic backbones, support proton-exchange.
[0016] The advantages gained using polyelectrolytes to synthesize
the LBL PEM membrane should apply to the synthesis of LBL
electrodes. On top of fast ion conduction LBL electrodes demand
high electronic conduction, strongly hydrophobic to expel water,
stable to chemical and mechanical degradation, assessable to
control loading of catalysts, intimately adhere to the PEM and the
GDL to ensure proper passage of the ions, capable of producing
open-circuit-potentials similar to a pure metal. Larminie, J.;
Dicks, A. Wiley, New York 2000; Gottesfield, S.; Zawodzinski, T.
Adv. Electrochem. Sci. Eng. 1997, 5, 195-301; Glipa, X.; Hograth,
M. Dept. of Trade and Industry (UK) homepage 2001. Conducting
polymers were successfully used to assemble LBL electronically
conducting films. Rubner, M. F.; Stockton, W. B. Macromolecules
1997, 30, 2717-2725; Rubner, M. F.; Fou, A. C. Macromolecules 1995,
21, 7115.; Rubner, M. F.; Cheung, J. H.; Fou, A. F. Thin Solid
Films 1994, 244, 985; DeLongchamp, D. M.; Hammond, P. T. Abstr.
Pap. Am. Chem. Soc. S22:136-PMSE, Part 2 2001;. DeLongchamp, D. M.
PhD Thesis, Massachusetts Institute of Technology, MA 2003.
Unfortunately, LBL conducting polymer films are weak ionic
conductors, not stable and degrade in a sever electrochemical
environment. A more resilient combination is a
polyelectrolyte-colloid such that the colloid is electronically
conducting and ready to assemble. Many colloids can assemble with
polyelectrlytes but the LBL films are not conducting. Kotov, N. A.;
Dekany, I.; Fendler, J. H. J. Phys. Chem. 1995, 99, 13065.;
Mallouk, T. E.; Feldheim, D. L.; Crabar, K. C.; Natan, M. J. J. Am.
Chem. Soc. 1996, 1181, 7640-7641.; Grabar, K. C.; Natan, M. J.;
Freeman, R. G.; Hommer, M. B. Anal. Chem. 1995, 67, 735-743.;
Hammond, P. T.; Rubner, M. F.; Zheng, H. P.; Lee, I. Adv. Mater.
2002, 14, 569-572. Only one original approach used exfoliated
graphite oxide that is not conducting to make LBL films because
graphite cannot be dispersed in water and it forms micrometer-sized
irregular aggregates in organic solvents. The GO can be converted
to graphite under sever reduction conditions with H.sub.2 gas.
Fendler, J. H.; Cassagneau, T. Adv. Mater. 1998, 10, 877-881.;
Kotov, N. A.; Dekany, I.; Fendler, J. H. Adv. Mater. 1996, 8, 637.
Our method directly employs polyelectrolyte graphite mixtures to
assemble LBL electrodes without having to convert the graphite
powder to exfoliated GO and then back to graphite where in both
processes expensive and sever chemical and thermal conditions
applies. The LBL polyelectrolyte-Carbon electrodes [LPCE] achieved
most of the requirements stated above, thus providing a cheaper and
practical way of making electrochemical electrodes.
SUMMARY OF THE INVENTION
[0017] The invention provides improved ion exchange and gas
diffusion membranes for use in electrochemical cells, a process for
making porous frameworks, membrane and electrode assemblies
fabricated using porous frameworks, and the application of the
membrane and electrode assemblies to a variety of devices, both
electrochemical and otherwise.
BREIF DESCRIPTION OF THE FIGURES
[0018] FIG. 1 depicts SEM pictures of LBL films composed of (a)
PANI/PAMPS [10 kV, M=11000] covering the surface of a Nucleopore
membrane with the pores appearing in the crevice; and (b & c)
PEO/PAA [15 kV, M=8000, P=1.9 Torr] with the Nucleopore clearly
sandwiched between the LBL films (a cross sectional view). Also
shown a simplified sketch of the profile of the "composite
membrane".
[0019] FIG. 2 depicts (a) normalized drift velocity against the
fraction of sites in a porous (.diamond.) and a nonporous
(.box-solid.) matrix. A 2D coordinate space of 400.times.200
points, randomly distributed hopping sites [5000], a set of [50 or
a 100] conducting points making up a wave packet; (b) asymmetric
mode of hopping of a conducting point; and (c) normalized drift
velocity against the fraction of sites in a pore (i) Saturation of
sites representing aqueous phase (.diamond.), (ii) 60% population
of sites (.diamond-solid.), (iii) 20% population of sites
(.DELTA.), (iv) 10% population of sites (.tangle-solidup.). [%
population is out of 5000 hopping sites].
[0020] FIG. 3 depicts (a) AC impedance of a PDAC/PAMPS fuel cell
running under dry conditions (RH<30%, .quadrature.) while the
humidity was ramped up to RH=50 to 70% (.diamond-solid.), and then
to operating conditions Solid line (RH.about.90%); and (b) as the
fuel cell was further purged to saturation the high frequency
semi-circle referring to the pore's impedance disappeared and the
overall impedance Solid line sharply decreased. Frequency=1 to
2.times.10.sup.7 Hz, Applied potential=10 mV. T=22-24.degree. C.
Other PE couples exhibited similar behavior.
[0021] FIG. 4 depicts (a) A system composed of two Randel's cells
connected in series showing a Warberg constant-phase element (b) AC
impedance plots simulated using the circuit in part (a) at (i)
RH<30% (.quadrature.), R.sub.m=390.OMEGA., R.sub.p=590.OMEGA.,
C.sub.m=4.times.10.sup.-4F, C.sub.p=10.sup.-5F, C.sub.W=0.012F,
.phi.=0.8; (ii) RH=50 to 70% (.diamond-solid.), R.sub.m=220.OMEGA.,
R.sub.p=270.OMEGA., C.sub.m=2.times.10.sup.-4F,
C.sub.p=2.times.10.sup.-6F, C.sub.W=0.012F, .phi.=0.8; (iii)
RH.about.90% Solid line, R.sub.m=110.OMEGA., R.sub.p=141.OMEGA.,
C.sub.m=2.times.10.sup.-4F, C.sub.p=2.times.10.sup.-6F,
C.sub.W=0.012F, .phi.=0.8; (c) A fully purged "composite membrane"
exhibits a sharp drop in pore resistance and a decrease in the
overall impedance, Solid line, R.sub.m=22.OMEGA., R.sub.p=3.OMEGA.,
C.sub.m=2.times.10.sup.-4F, C.sub.p=2.times.10.sup.-6F,
C.sub.W=0.012F, .phi.=0.8.
[0022] FIG. 5 depicts ((a) Potential (V) vs Current density (mA
cm.sup.-2) for LPEI/PAMPS .box-solid., LPEI/PSS .quadrature.,
PEO/PMAA .tangle-solidup.; (b) Corresponding power density plots
for the same set of membranes. Conditions: LPEI/PAMPS,
P.sub.air=2.0 psi, P.sub.H2=0.5 psi, RH=50-65%. LPEI/PSS
P.sub.air=3.0 psi, P.sub.H2=0.5 psi, RH=65-75%. PEO/PMAA
P.sub.air=2.0 Psi, P.sub.H2=0.5 psi, RH=55-88%.
Temperature=22.5-24.degree. C. Area of the electrode is 0.5
cm.sup.-2. Solid lines are guide to the eye
[0023] FIG. 6 depicts (a) Potential (V) vs Current density (mA
cm.sup.-2) for LPEI/PAA .DELTA., PDAC/PAMPS .tangle-solidup.;
PEO/PAA .largecircle.; (b) Corresponding power density plot for the
same set of membranes. Conditions: LPEI/PAA P.sub.air=2.0 psi,
P.sub.H2=0.5 psi, RH=55-75%. PDAC/PAMPS P.sub.air=2.0 psi,
P.sub.H2=0.5 psi, RH=51-60%. PEO/PAA P.sub.air=2.0 psi,
P.sub.H2=0.5 psi, RH=50-60%. Temperature=22.5-24.degree. C. Area of
the electrode is 0.5 cm.sup.-2. Solid lines are guide to the
eye.
[0024] FIG. 7 depicts a soft fuel cell assembled on an insulating
customized porous support.
[0025] FIG. 8 depicts an electrode-catalyst layer atop of the
GDL.
[0026] FIG. 9 depicts an electrode-catalyst layer within the GDL
and the LBL membrane contains a colloidal porous framework.
[0027] FIG. 10 is a schematic of a Soft Carbon-Polymer Galvanic
cell.
[0028] FIG. 11 depicts a miniature micro fuel-cell.
[0029] FIG. 12 is a sketch of a small part of a catalytic
converters.
[0030] FIG. 13 depicts a thick, highly porous LPCE built on a
porous membrane.
DETAILED DESCRIPTION OF THE INVENTION
[0031] The invention will now be described more fully with
reference to the accompanying examples, in which certain preferred
embodiments of the invention are shown. This invention may,
however, be embodied in many different forms and should not be
construed as limited to the embodiments set forth herein; rather,
these embodiments are provided so that this disclosure will be
thorough and complete, and will fully convey the scope of the
invention to those skilled in the art.
I. Process for Making Ion-Exchange Membranes (IEMs)
[0032] Polyelctrolytes Used in IEM Fabrication. The
polyelectrolytes utilized were taken from the group consiting of
poly(styrene sulfonic acid, sodium salt) [PSS; MW=70,000];
poly(diallyldimethyl ammonium chloride) [PDAC; MW=240,000]; linear
poly(ethyleneimine) [LPEI; MW=25,000]; poly(acrylic acid) [PAA;
MW=90,000]; poly(styrenesulfonic acid-maleic acid, sodium salt)
[PSSM3:1; MW=20,000]; poly(ethylene oxide) [PEO; MW=4,000,000];
poly(methylacrylic acid) [PMA; MW=100,000];
poly(acrylic-co-acrylamide acid, sodium salt) [PAAcoAAm,
MW=10,000,000 40% carboxy]; poly(2-acrylamido-2-methyl-1-propane
sulfonic acid) [PAMPS=2,000,000];
poly(dimethylamine-co-epichlorohydrin) [PDME]; polyaniline
(Emarlidine base) [PANI, MW=100,000].
[0033] Choice of Polyelectrolytes. Ion exchange membranes (IEM),
particularly the proton-exchange membrane (PEM) of fuel cells, are
used in solid state electrochemical systems to replace the aqueous
electrolyte. Various types of IEMs, of acceptable ionic
conductivity, can be assembled using LBL technique. At least two
types of water-soluble macromolecules capable of electrostatic or
secondary interactions should be utilized, and one of them should
promote ionic conductivity. The concentrations of the
macromolecular solutions can range from about 1.0 mM to about 20.0
mM, regardless of the nature of the polyelectrolyte. The pH of
assembly is relevant only in the case of LBL films which
incorporate weak polyelectrolytes (i.e. polyelectrolytes where the
charge density along the chain can be readily controlled by
adjusting the pH values of the polyelectrolyte solution). For
example, weak polyelectrolytes such as LPEI/PAA at pH=4, PEO/PAA at
pH=2.5 or at pH=2.0, PAAm/PAA at pH=2.8, PAAm/PAA-co-PAAm at
pH=2.0, and PDAC/PAA-co-PAAm at a pH>5 form IEMs of acceptable
ionic conductivity. In contrast, the LBL assembly of strong
polyelectrolyte couples is pH independent; however, in these cases
the thickness of the film depends on the concentration of the
dosing salt (i.e. 10.0 mM PDAC (0.5M NaCl)/5.0 mM PAMPS (0.5M NaCl)
at any pH).
[0034] Water solubility of all the films components is not a
limitation when forming IEMs by the LBL technique. For example, ion
conducting colloids (e.g., cationic or anionic latexes, zeolites,
and zirconia salts) can also incorporated into LBL films to form
IEMs. Colloids that possess strong functional groups, such as
sulfonates, phosphates, and quaternary ammonium groups are pH
independent, while those with weak functional groups, such as
carboxylates, primary, and secondary amine groups, are pH dependent
and can only assemble LBL films of the instant invention at a pH
range of about 2 to about 4. In addition, cationic and anionic
dendrimers, which fall under the same category as the ion
conducting colloids, can also be assembled into films under similar
LBL conditions to form IEMs.
[0035] Choice of Substrates for LBL deposition. It will be
appreciated that materials with an inherently charged surface are
particularly attractive substrates for LBL assembly of an inventive
thin film. Alternatively, a range of methods are known in the art
that can be used to charge the surface of a substrate, including
but not limited to plasma processing, corona processing, flame
processing, and chemical processing (e.g., etching, micro-contact
printing, and chemical modification). For example, plastics can be
used as substrates, particularly if they have been chemically
modified to present polar or charged functional groups on the
surface.
[0036] In one approach to IEM fabrication, the LBL film can be
deposited directly onto a hydrophilic porous framework. Porous
polymer membranes include all types of organic and inorganic, nano
or micro-pore filter membranes which can be made hydrophilic, as
mentioned above, by plasma etching with an acidified dichromate
solution, a H.sub.2O.sub.2/H.sub.2SO.sub.4 solution or a
H.sub.2O.sub.2/NH.sub.3 solution. After etching, the porous
substrate can be coated on both sides by an LBL multilayer film to
form a composite membrane which can act as an IEM.
[0037] Interestingly, in some cases the LBL IEM can be directly
affixed to an electrode to form a membrane-electrode assembly
(MEA). It is important to note that depositing an LBL film on the
surface of an electrode requires a membrane thickness that is at
least two times thicker than the rough surface of the electrode;
the requirement is to prevent short-circuiting of the cell after
the electrodes are pressed together. A more detailed discussion of
MEAs is found in a subsequent section.
[0038] Certain LBL membranes themselves can be made hydrophilic
(i.e. converted to a porous structure suitable for LBL deposition)
simply by changing the pH or salt concentration (e.g., assemble
10.0 mM LPEI/PSSM at pH=4 then change the pH to <2.0, similarly
this can be done with LPEI/PAA, PAH/PAA) thus forming a framework
which can be coated on both sides by a second LBL multilayer film
to form a composite membrane which can act as an IEM.
[0039] Similarly, if an LBL film is made from polyelectrolytes and
micro or nano latex spheres, zeolites, platelets, or other
colloidal particulates (e.g., 10.0 mM PDAC/2% sulfonate latex
suspension), it can by fabricated in such a way that it posses
crevice structures, thereby generating a porous framework which can
be coated on both sides by a second LBL multilayer film to form a
composite membrane which can act as an IEM.
[0040] Methods of Assembly of LBL Films. In certain embodiments,
the LBL assembly of inventive films may involve a series of dip
coating steps in which the substrate is dipped in alternating
polycationic and polyanionic solutions. Additionally or
alternatively, it will be appreciated that deposition of
alternating polycationic and polyanionic layers may also be
achieved by spray coating, brush coating, roll coating, spin
casting, or combinations thereof.
[0041] Examples of Post Fabrication Modifications. The synthesis of
the IEM LBL multilayer film is a fully controlled process. For
example, an LBL film made from one type of macromolecules can be
capped with a thinner LBL film of another type of macromolecules
(e.g to repel water); or an LBL film can be chemically, thermally
or photochemically treated to induce cross-linkages, thus enhancing
its chemical and mechanical stability; or an LBL film can be
tailored to the desired thickness (i.e. up to several micrometers)
and to the desired size (i.e. from meters down to a few
microns).
[0042] Properties of Composite Membrane. Since at least two
water-soluble macromolecules are needed to make up an LBL film, it
follows that a composite membrane will possess different chemical
and physical properties depending on the chemical structure of the
constituent macromolecules. Some examples of this phenomenon
include: an LBL film containing zeolite clusters blocks methanol
permeation, while an LBL film containing only polyelectrolytes
cannot; an LBL film capped with inorganic colloids or latexes
resists peroxide degradation; an LBL film capped with a hydrophobic
polyelectrolyte repels water at the cathode; and most importantly,
in the case of the instant invention, the observation that
hydrophobic polyelectrolytes (e.g., PDAC/PSS) exhibit a lower ionic
conduction than hydrophilic polyelectrolytes (e.g., LPEI/PAA).
[0043] Interestingly, an LBL membrane of the present invention
behaves as a sponge which can absorbs ions allowing the enhancement
of the ionic conductivity of the film. Depending on the materials
used to form the composite membrane, either the matrix or the pores
can be embedded with chemicals to enhance the ionic conductivity of
the membrane. Chemicals which can be embeded can be selected from
the following: hydrogels (e.g., PEG, OEGDA, PAAm, PVA, PVP);
polyions (e.g., polyphosphates, Nafion.RTM.); micron or nano-size
colloids, platelets, and zeolites; all types of proton sponges;
organic molecules (e.g., ethylene glycol and glycerol) that are
known to increase proton conductivity; and all types of acids and
salts of an organic and inorganic nature.
[0044] In an example of this process, a composite membrane (or the
soft MEA) can be soaked in about 40.0 mM of the dosing solution
(range about 10.0 to about 50.0 mM; 1% to 5% for some polymer
solutions) for about 30 minutes (range 5.0 minutes to 24 hour;
depending on the size of the molecule) and an increase in ionic
conductivity will be observed. For example, a soft MEA [PDAC/PAMPS
membrane-electrode] soaked in 2.5% Nafion117 solution showed a ten
fold increase in its ionic conductivity.
IIa. Membrane-Electrode Assemblies (MEAs) to Form LPCEs
[0045] Overview of the Fabrication of LBL polyelectrolyte-carbon
electrodes (LPCEs). LPCEs are formed by standard LBL techniques
using polyelectrolyte-carbon-catalyst colloidal suspensions. LPCEs,
along with IEM can be combined to form soft membrane-electrode
assemblies (MEAs). There are two conventional methods that
electrochemical and fuel cell technologists use to fabricate a
membrane-electrode assembly (MEA). One method is direct
application. Therein one deposits the LPCE directly on any
classical IEM (e.g., PSS gel, Nafion.RTM.) or an IEM consisting of
an LBL composite membrane (as described above in Section I). There
is also an indirect application approach. Therein one deposits the
LPCE directly on any gas diffusion substrate that is electronically
conductive (e.g., carbon cloth, porous stainless steel, porous
silicon, porous titanium alloys, etc.) to form the catalyst
layer.
[0046] Polyelctrolytes Used in LPCE Fabrication. The
polyelectrolytes utilized were taken from the group consisting of
poly(styrene sulfonic acid, sodium salt) [PSS; MW=70,000];
poly(diallyldimethyl ammonium chloride) [PDAC; MW=240,000]; linear
poly(ethyleneimine) [LPEI; MW=25,000]; poly(acrylic acid) [PAA;
MW=90,000]; poly(styrenesulfonic acid-maleic acid, sodium salt)
[PSSM3:1; MW=20,000]; poly(ethylene oxide) [PEO; MW=4,000,000];
poly(methylacrylic acid) [PMA; MW=100,000];
poly(acrylic-co-acrylamide acid, sodium salt) [PAAcoAAm,
MW=10,000,000 40% carboxy]; poly(2-acrylamido-2-methyl-1-propane
sulfonic acid) [PAMPS=2,000,000];
poly(dimethylamine-co-epichlorohydrin) [PDME]; polyaniline
(Emarlidine base) [PANI, MW=100,000] and Nafion 117 as a 5% resin
solution.
[0047] Synthesis of the Carbon-Catalyst Colloidal Suspensions. A
carbon colloidal suspension was considered reliable for LBL
assembly if the carbon particulates did not settle to the bottom
leaving a gray or clear layer above them. The settlement test was
done after stirring the mixture, followed by sonication for one
hour, then leaving the carbon colloidal suspension to rest (i.e. no
stirring or agitation of any kind) for one day. The carbon
colloidal suspension was prepared by dissolving 0.004 g of the
carbon powder in 1.0 mL of 10.0 mM polymer solution if the powder
was dry. For the aqueous carbon paste, that was not loaded with the
catalyst, a prior step was taken. The platinum powder (0.02 g) was
sonicated in 100.0 mL of 10.0 mM polymer solution for one or two
hours to ensure proper dispersal of the catalyst. Afterwards carbon
paste (1.0 g) was added to the 100.0 mL catalyst-polymer solution
followed by 1 hr sonication. The following properties indicate the
type of carbon and polymer products that can be used to prepare a
suitable carbon colloidal suspension. The properties of the carbon
needed for a suitable collidal suspension include a graphite type
that possess low electrical resistivity (i.e. <5.times.10.sup.-4
.OMEGA.cm). For example, 10% platinum on activated carbon was
successfully deposited on a PDAC/PAMPS LBL film.
[0048] For example, using scanning electron microscopy (SEM), an
LPCE synthesized by mixing carbon paste and Pt powder (size
.about.0.15-0.45 .mu.m) showed .about.10-20 .mu.m Pt agglomerates
dispersed between carbon particulates. In addition the carbon
should be treated or pretreated with a base such as ammonia
solution or its organic derivatives in order to disperse it
properly in aqueous solution. For example, the Hispec3000 powder
was processed at pH=10 to 11, the graphite paste is ammonia treated
according to manufacturer's specifications, while the untreated dry
graphite powder failed to form any colloids. Furthermore, the
carbon should have a small particle size (about 1.0 .mu.m or less)
to form good colloids as settlement depends on atomic mass
according to Stokes law, while sub micron size colloids are needed
to fabricate micron size power devises. Finally, the carbon should
be loaded with nanometer size catalyst colloids rather than having
the catalyst mixed with it. For example, using scanning electron
microscopy (SEM), an LPCE synthesized by mixing carbon paste and Pt
powder (size .about.0.15-0.45 .mu.m) showed .about.10-20 .mu.m Pt
agglomerates dispersed between carbon particulates. Unfortunately,
the bigger the platinum colloids the faster they settle down in
solution and their surface area becomes smaller hence less power is
generated.
[0049] It should be noted that all the carbon colloids used in this
study were not of the submicron size according to the
manufacturer's specifications and SEM. In colloidal suspensions
that were prepared but not used a colloidal layer formed on top of
the agglomerates indicating a segregation of the small particulates
that remained in solution while the larger ones settled down. Many
commercial electrodes use carbon colloids that are 100 nm or less
in size and therefore are expected to give positive tests with most
polymer solutions.
[0050] Properties of the catalyst. The catalyst could be inert type
(e.g., noble metals Pt, Au, Ru) or active type (e.g., any metal Zn,
Cu, Ag). For example, Pt and Al powders were mixed with carbon
paste to prepare colloidal suspensions for LPCEs according to the
procedure explained above. In addition, the catalyst should be
properly dispersed in solution using stabilizers such as PVP or
suitable polyelectrolytes in order to prevent agglomeration. For
example, Pt colloids were successfully prepared and dispersed in
PAA, PSS and PANI solutions. Finally, since Stokes law states the
size of the colloidal metallic catalyst used depends on its atomic
mass and since platinum colloids should have a maximum size of 60
nm to remain suspended for few days (vida supra), Pt-carbon-polymer
colloidal suspension are robust enough to deposit platinum
particulates of only a few microns in size.
[0051] Properties of the polymer solution. In general, all strong
polyelectrolytes such as poly(-) PSS, PAMPS, Nafion and poly(+)
PDAC, PAH successfully formed carbon colloidal suspensions. Best
results were obtained when Hispec3000 or 10% Pt on activated carbon
were treated with PDAC as poly(+) solution and PAMPS as a poly(-)
solution at pH=10 to 11. In general, weak polyelectrolytes, such as
poly(-) PAA, PAA-co-PAAM and poly(+) LPEI, yielded best results
with the aqueous carbon paste. The LPEI/carbon paste and PAA/carbon
paste colloidal suspensions were prepared and used in LBL
deposition at pH=4 without having to increase the pH to pH=11. In
fact, a PAA/carbon paste suspension lasted for months. The same
polyelectrolytes tended to produce agglomerates with the Hispec3000
or 10% Pt on activated carbon. At high pH PAA and PAA-co-PAAm yield
quality Hispec3000 or 10% Pt on activated carbon suspensions that
can be used in LBL deposition. Using a low pH<2 to render weak
polyelectrolytes, such as LPEI, strongly positive was not effective
because at low pH the carbon colloids tend to agglomerate.
[0052] LBL deposition to assemble the LPCE. To insure acceptable
electrical conductivity and good catalyst loading of both
polyelectolytes the poly(-) and the poly(+) solutions were loaded
with carbon and platinum (or catalyzed carbon). This method of
colloidal LBL deposition is unlike any other LBL method in the
literature as the colloids are placed in both the poly(-) and the
poly(+) solutions. All previous LBL methods have the colloid in one
beaker and the polyelectrolyte in another. Whether using
(+)LPEI(10.0 mM, 100 mL)/C(0.1 g)/Pt(0.02 g) with (-)PAA(10 mM, 100
mL)/C(0.1 g)/Pt(0.02 g) or (+)PDAC(10.0 mM, 100 mL)/Hispec(0.1 g)
with (-)PAMPS(5 mM, 100 mL)/Hispec(0.1 g) the C/Pt colloids were
deposited on the substrate in every dipping. Thus the graphite
particulates are held in intimate contact and an electronic
conductivity up to 2.0 Scm.sup.-1 was recorded. Unlike LBL
conducting films that use conducting polymers such as poly(aniline)
or poly(pyrrol), the LPCE electrical conductivity does not degrade
with time. Moreover, X-ray SEM analysis showed similar levels of
catalyst loading when a Hispec3000 powder sample was compared to an
LBL deposited sample. However, a 10 bilayer LPCE (Hispec3000 type)
showed nearly 2.5 times less in platinum loading compared to a
commercial E-TEK.RTM. electrode. This should not be a problem
because the catalyst layer in the E-TEK.RTM. is around 30 .mu.m
thick while the 10 bilayer LPCE was measured by profilometry to be
.about.6 .mu.m thick. The dipping time in the carbon colloidal
suspension was 20 minutes followed by 2.0 minutes drying and three
rinses with pure water where each rinse lasted for 2.0 minutes
without any agitation.
[0053] From an industrial and economic point of view the cost of
preparation and handling the carbon colloidal suspensions should
also be considered. We have tested the number of LBL depositions
and the total surface area of the assembled electrodes; a 100 mL
solution used over two weeks was capable of producing LPCEs of a
total surface area .about.50 cm.sup.2. The carbon colloidal
suspensions are also recoverable and recyclable. Recovery of the
carbon and platinum is achieved by evaportation of the solvent
(water). This reisolated material, plus an additional quantity of
material, added to account for the lost mass of solids, can be
stirred and sonicated in 100 mL poly(+) or poly(-) solution and the
process repeated.
[0054] Depositing and testing LPCEs on solid substrates. In order
to check on its effectiveness, the LPCE was first deposited on
solid substrates, such as platinum, gold, porous stainless steel,
and Silicon-100. Gold was first dipped in a poly(-) colloidal
suspension because it has a positively charged surface. After 10
bilayers, the gold surface was completely covered with the LPCE. On
a Si-100 substrate that was originally covered with 40 bilayers of
PDAC/PAMPS, a 10 bilayer Hispec3000 based LPCE showed an average
thickness of .about.6 .mu.m measured by profilometry.
[0055] Testing the open circuit potential (OCP) of the LPCE. To
check on the electrochemical activity of an LPCE-coated gold
electrode [Pt--C/Au] the OCP of bare gold and bare platinum
electrodes were tested against a standard calomel electrode (SCE)
in a 20 mM H.sub.2SO.sub.4/H.sub.2 (g) purged solution with the
following results: Pt/0.02M H.sub.2SO.sub.4/H.sub.2(g)//SCE the
OCP=+0.32 to +0.359 V; Au/0.02M H.sub.2SO.sub.4/H.sub.2(g)//SCE the
OCP=-0.005 to +0.02 V; and for a carbon coated gold electrode:
[C/Au]/0.02M H.sub.2SO.sub.4/H.sub.2(g)//SCE the OCP=+0.005 to
+0.03 V; while for a carbon/platinum coated gold electrode:
[Pt--C/Au]/0.02M H.sub.2SO.sub.4/H.sub.2(g)//SCE the OCP=+0.345 to
+0.347 V.
[0056] It is clear that the [Pt--C/Au] yielded an OCP in the same
range as an uncoated pure Pt electrode. Similarly, a porous
stainless steel SS316L filament was coated with the Hispec3000
catalyst to give an OCP=+0.338 V. In fact two LPCE-Hispec3000
coated SS316L filaments each purged with H.sub.2 and air
respectively yielded an OCP=+0.65 V. When a 10 bilayer
LPCE-Hispec3000 coated SS316L filaments were utilized in a galvanic
cell (acting as an aqueous fuel cell) where the cathode compartment
contained an acidified dichromate solution (10.0 mM
Cr.sub.2O.sub.7.sup.2- in 1.0 N H.sub.2SO.sub.4) as an oxidizer and
the anode compartment contained a basic borohydride solution (10.0
mM BH.sup.4- in 0.5M NaOH) as a reducer it generated an OCP=+1.63V,
while pure platinum electrodes yielded an OCP=+1.58V, and the
uncoated SS316L filament electrodes yielded an OCP=+0.89V. The
difference in the power generated is 2.0 mWcm.sup.-2 by the
LPCE-Hispec3000 coated SS316L, 0.088 mWcm.sup.-2 by the uncoated
SS316L filament electrodes, and 0.06 mWcm.sup.-2 by the pure
platinum electrodes. Results from the power generated signify
effective platinum loading in the matrix of the LPCE. Note that the
aqueous fuel cell (described below) was used to compare performance
because its salt bridge recorded several hundred ohms of internal
impedance.
[0057] Testing the ionic conductivity of the LPCE. The following
electrodes of the same area were placed in acidic solution at the
same distance from a bare platinum electrode and the impedance was
measured. For a bare gold electrode: Au/0.02M H.sub.2SO.sub.4/Pt
the Z=70.OMEGA.; and for a carbon coated gold electrode:
[Au--C]/0.02M H.sub.2SO.sub.4/Pt the Z=72.OMEGA.; and for a
carbon/platinum coated gold electrode: [Au--C/Pt]/0.02M
H.sub.2SO.sub.4/Pt the Z=72.OMEGA.. The results showed a 2.OMEGA.
increase in impedance, implying that the LPCE exhibits a high ionic
conductivity when fully wet and an indication of its highly porous
nature.
[0058] Depositing and testing the LPCEs on polymer membranes. The
LPCE was successfully deposited on membranes such as Nafion112.RTM.
and PDAC/PAMPS composite membrane. LPCEs based on carbon paste were
deposited directly on Nafion112.RTM. surface while LPCE based on
Hispec3000 failed. In this case, the Nafion can be functionalized
with a few bilayers of PDAC/PAMPS and the Hispec3000 can be
successfully deposited on Nafion112.RTM. membrane. LPCEs based on
the three carbon brands (that is, aqueous carbon paste, Hispec3000,
and 10% activated carbon) were successfully deposited on the
PDAC/PAMPS composite membrane to form soft "MEA" or catalytic
surfaces. The LPCE was found to adhere intimately to the ion
conducting membrane eliminating the need to apply hot pressing (at
a temperature of about 180-200.degree. C. and a pressure of about
70 to 80 atmospheres for about 2 minutes) to fabricate the MEA, as
is usually needed with conventional fabrication methods.
[0059] Conventional deposition techniques. In some cases, LBL
deposition to assemble an LPCE is not necessary. Analogous to
conventional methods of fabricating the catalyst layer, a
polyelectrolyte complex gel replaces a Nafion.RTM. ionomer solution
instead. The slurry of a polyelectrolyte complex solution plus
catalyst ink/PTFE emulsion/stabilizer can be poured directly onto
the composite membrane to form a uniform film which is then dried
and hot pressed. Uniformity and thickness depend on the method
used, such as spin coating, decaling, or spraying. These methods
are faster than LBL deposition but are incapable of thickness
control, uniformity, compactness, and performing microcontact
printing.
[0060] Properties of a Soft Membrane-Electrode-Assembly (MEA). When
properly mounted, a soft MEA, running on H.sub.2 and air, produced
a stable OCP up to +0.9V similar to any commercial MEAs. The
overall impedance Z.sub.T (i.e. contact impedance, electronic
impedance, and ionic impedance) hence the conductivity of a
PDAC/PAMPS soft MEA was measured and compared to both an
E-TEK.RTM./PDAC/PAMPS MEA and a commercial MEA under similar
conditions. A commercial MEA gave a Z.sub.T=2.OMEGA. at
RH.about.95%. An E-TEK.RTM./PDAC/PAMPS MEA gave Z.sub.T=85.OMEGA.
at RH.about.94% and Z.sub.T=70.OMEGA. during fuel cell operation.
In fact, around 60.OMEGA. were lost to contact resistance in our
E-TEK.RTM./LBL MEAs. With a PDAC/PAMPS soft MEA a Z.sub.T=10 to
20.OMEGA. was achieved at RH.about.95%. The reason for such a high
conductivity resides in the adaptability of the LBL technique. When
the PDAC/PAMPS soft MEA is installed dry without dosing with a
plastisizer solution its Z.sub.T=5000.OMEGA. at RH.about.55%. When
the same soft MEA is dosed with 0.02M H.sub.2SO.sub.4 solution its
Z.sub.T=250.OMEGA. at RH.about.60% and Z.sub.T=100.OMEGA. at
RH.about.95%. Dosing with a mixture of 0.02M H.sub.2SO.sub.4/2.5%
Nafion117.RTM. solution yielded a Z.sub.T=30.OMEGA. at RH.about.95%
and Z.sub.T=10.OMEGA. at RH.about.99% which is another indication
that a soft MEA is capable of high ionic conductivity.
IIb. Membrane-Electrode Assemblies (MEAs) to Gas Diffusion Layers
(GDLs)
[0061] The synthesis mechanism is exactly the same as the LPCE
assembly described above with the omission of the catalyst in all
its forms. The size of the particle of carbon or any electronically
conducting colloid should be micron size to ensure good diffusion
of gases. GDLs made from aqueous carbon paste were found to adhere
intimately to the preceding catalyst (i.e. LPCE) layer and provided
high electrical conductivity. As with the LPCE, the GDL should also
be hydrophobic to expel water and allow the diffusion of gases.
IIIa. Devices--Fuel Cells: Polyelectrolyte Multilayer Thin Films as
PEMs
[0062] We have developed polyelectrolyte multilayer thin films with
relatively high ionic conductivity constructed with hydrophilic
weak polyelectrolytes, using either electrostatics or hydrogen
bonding as a means of assembly LPEI/PAA and PEO/PAA. In particular,
films constructed from linear poly-ethyleneimine (LPEI) and
poly(acrylic acid) (PAA), exhibited ionic conductivity as high as
10.sup.-5 Scm.sup.-1 at 100% relative humidity and room
temperature; these films also exhibited high ionic conductivity at
ambient conditions (50% RH, 25.degree. C.) when prepared and
post-treated with an oligoethyleneoxide/aqueous solution at low pH.
Polyethylene oxide (PEO) and PAA alternating thin films were
constructed through hydrogen bonding interactions, and stabilized
with light crosslinking, to obtain films with conductivities of
10.sup.-5 to as high as 10.sup.-4 Scm.sup.-1 at ambient conditions
when films were constructed at high salt concentrations. In both
cases, the ionic conductivity was optimized at film assembly
conditions which led to thick polyion bilayers, and loopy, coiled
polymer surface conformations. The use of LPEI, PEO and other
polymers with a relatively hydrophilic nature greatly enhanced ion
conduction in these films.
[0063] Herein, ultrathin LBL polymer films were utilized as the
top, continuous thin film layer of a composite polymer membrane. A
nonconducting porous substrate was chosen as the intermediate, or
core layer, in this construction. Here we use a commercially
available polycarbonate Nucleopore.RTM. membrane, which has a total
thickness of 6.0 .mu.m, and regular pores of 100 nm diameter, as
the base membrane. The pores in the Nucleopore membranes are
created via gamma irradiation, and are therefore cylindrical and
regular in geometry, with single pores traversing the entire
thickness of the film. This simple geometry provides a clear path
of ion transport, and thus the pores act as the conducting channels
between the two LBL film/electrode assemblies.
[0064] The Nucleopore base was then plasma treated and, in all
cases, a 40 bilayer thin film of the LBL polymer pair of interest
was constructed onto the surface of the membranes. By selecting the
molecular weight and ionic strength or solution pH, it was possible
to tune the hydrodynamic radius of the polyion of interest in
solution. If the hydrodynamic radius is relatively small compared
to the pore diameter, a multilayer thin film will form within the
pores; however, if the molecular weight and solution conditions are
such that the polymer chains are larger than or close to the pore
size of interest, assembly results in bridging of the pore
diameters, and the film builds up only on the outer surface of the
membrane. The molecular weight of the polymers used was such that
the pore diameters were bridged, and the multilayer film existed
only on the base membrane surface. This fact was confirmed by
scanning electron microscopy (SEM), as shown in FIG. 1(a-c). A top
view of the composite membrane, for which a part of the film was
removed with a stylus, indicates clearly the presence of open pores
underneath the top, uniform, pore free LBL film layer.
Cross-sectional images in FIG. 1(b-c) indicate that the LBL film is
present only on the top surface, and that a symmetric film forms on
both sides of the membrane.
[0065] Peak areas from transmission FTIR that were measured by
referring to the --COOH stretch in PEO/PAA, PEO/PMAA both at
(1878-1577 cm.sup.-1), and LPEI/PAA at (1762-1660 cm.sup.-1); or
the SO.sub.3H stretch in LPEI/PSS at (1058-1018 cm.sup.-1), and
finally the --CONH.sub.2 stretch in PAN/PAMPS at (1637-1536
cm.sup.-1) or PAN/PAAcoPAAm at (1632-1535 cm.sup.-1) were used to
determine the relative amount of the PE material deposited on both
substrates. Both the Si-100 and the IR transparent Nucleopore
substrates showed an average difference of only .+-.5% an
indication that the amount of PE material is similar on both
surfaces. Moreover, AC impedance and galvanostat measurements gave
much lower resistances than expected if the PE material were to
fill the pores. For example, the conductivity of the PEO/PAA LBL
film can reach a maximum of 10.sup.-4 Scm.sup.-1 at RH=100%.
Calculations show that a 20 .mu.m thick composite membrane (Table
1) is expected to have a resistance of about 40.OMEGA. if the pores
are filled completely with PEO/PAA film. Unexpectedly, the result
obtained experimentally was 5 to 13.OMEGA., indicating that the
pores and the film were fully soaked. The result of this composite
film geometry is that during fuel cell operation at high humidity,
the membrane allows the passage of water and ions, ultimately
resulting in water-filled pores between the LBL films; the
electrodes contacting the LBL films, which serve as a barrier to
gases across the membrane, and a gateway for ions generated at the
electrodes.
[0066] Because of differences in the nature of the multilayers
achieved with different polyion pairs, the thickness of the LBL
film covering the pores ranged between about 1 .mu.m to about 5
.mu.m on one side of the membrane. The thicknesses as determined by
profilometry, and the corresponding pH of assembly for all of the
polyion pairs studied here are shown in Table 1; in all cases, the
same adsorption conditions were used for both polyions in a given
polyion pair. Table 1 also contains the FC open circuit voltages
and a summary of the resistance across the composite membrane as
measured at 90 to 95% relative humidity and a temperature of
23.+-.1.degree. C. using AC impedance spectroscopy. The resistance
as determined from the galvanostat was assumed to be that at 100%
relative humidity and not at the humidity of the chamber (50 to
60%) because the fuel cell generates water that is soaked up by the
composite membrane. The corresponding conductivities were
determined using the equation: .sigma. = t R A ( 1 ) ##EQU1## where
.sigma. is the conductivity (Scm.sup.-1), t is the thickness (cm),
R is the resistance (.OMEGA.), and A is the area of the electrode
(cm.sup.-2). Each of the systems shown below were optimized to
achieve optimal ion conductivity via variations in the ionic
strength and pH of assembly. It is clear that these systems produce
unusually large bilayers, ranging in thickness from 26 to 120
nm/bilayer.
[0067] Fuel Cell Measurements and Ionic Conductivity. Beyond
providing a mechanical support and added stability to the LBL film,
the porous framework of the Nucleopore membrane provides unhindered
proton conductivity. One must consider the role of the multilayer
thin film and its ionic resistance with respect to the electrolyte
filled pores of the porous polycarbonate base. TABLE-US-00001 TABLE
1 Total thickness Resistance Resistance Fuel cell Open circuit
Membrane (.mu.m)/pH AC-Impd. Galvanostat conductivity potential
System assembly (.OMEGA.) (.OMEGA.) (S cm.sup.-1).sup.d OCP (V)
LPEI/PSSM .sup. 6.0/4.0 1400 2500 9.6E-07 0.92 LPEI/PSS
2.4.sup.a/2.7 114 136 1.23E-05 0.87 LPEI/PAMPS 2.1.sup.a/2.5 c 104
1.56E-05 0.82 PEO/PMAA 8.5.sup.a/2.0 91 145 2.0E-05 0.87 LPEI/PAA
6.4.sup.a/4.0 65 76.sup.e 3.26E-05 0.85 PDAC/PAMPS.sup.b
2.2.sup.a/6.0 28 45.sup.e 3.64E-05 0.89 PDME/PAMPS.sup.b
2.2.sup.a/6.0 24 30 5.46E-05 0.64 PEO/PAA 97.sup.a/2.0 .about.1.5-5
13.sup.e 0.00024 0.88 Nafion112 .RTM. 50 .about.0.5-1 6.7 0.0015
0.95 .sup.aOnly includes the thickness of the LBL film material on
both sides of the pore. .sup.bPolyelectrolyte solutions contained
0.5M NaCl. c delaminated. .sup.dArea = 0.5 cm.sup.-2; Thickness =
Nucleopore + LBL films; Resistance.fwdarw.Galvanostat.
.sup.edetermined using the fuel cell equation 3.11 in Larminie, J.;
Dicks, A. Fuel Cell Systems 2000, Wiley, New York.
[0068] Ionic Conductivity Analysis. The advantage of using a
track-etched polycarbonate membrane is the straight pores that
traverse the membrane from one side to the other, thus making
theoretical manipulation easier. What starts the conduction process
is a short burst of ions at the surface of the electrode that
disturbs the overall charge neutrality of the system. The protons
that propagate across the bulk of the solution by the Grotthuss
mechanism would do the same across the hydrophilic exchanger sites
of the LBL films. An exchanger site is an ion-pair site that is
surrounded by a cluster of water molecules that are in a dynamic
equilibrium with the surrounding water pools. Farhat, T.; Yassin,
G.; Dubas, S. T.; Schlenoff, J. B. Langmuir 1999, 15, 6621-6623;
Losche, M.; Schmitt, J.; Decher, G.; Bouwman, W. G.; Kjaer, K.
Macromolecules 1998, 31, 8893. The plane of scatter of ions drift
across these sites with a current density (A cm.sup.-2) such that:
j=vcF (2) where v (cms.sup.-1) is the drift velocity of the
scattering plane of protons, c (molcm.sup.-3) is the concentration
of ions, and F is the Faraday constant (about 96,500 Cmol.sup.-1).
In the presence of an electric field E (Vcm.sup.-1) the current
density is also defined as: j=.sigma.E (3) Combining equations (2)
& (3), the conductivity can be defined in terms of the drift
velocity of the plane of scatter of ions. .sigma. = v c F E ( 4 )
##EQU2## Using Monte Carlo algorithm the drift of the plane of
scatter of ions can be monitored across a fixed distance by
counting the number of cycles required for the plane to drift from
one electrode to the other. Lui, J. S. Monte Carlo Strategies in
Scientific computing, NY, Springer .COPYRGT. 2001; Madras, N. Monte
Carlo Methods, Providence R.I., American Mathematical Society
.COPYRGT. 2000. In a 2D coordinate space of 400.times.200 points a
random distribution of a population of hopping sites [5000,
representing either water molecules or "hydrated ion pair"
exchanger sites] would be used as a ground for a more dilute set of
conducting points [50 or a 100, representing the protons] to
propagate across the 2D system. A hopping point can hop in all
directions in an asymmetric way (FIG. 2b) with a tendency to move
in the direction dictated by the electric field. It should be noted
that interactions based on quantum mechanical treatments, which
affect the hopping time, were neglected and if a conducting point
strikes a site the hopping attempt is considered successful.
Normalized values of the drift velocity plotted against the
fraction of exchanger sites whether inside the bulk of the
electrolyte or within a pore surrounded by a hydrophobic matrix is
shown in FIG. 2a. In both cases the values are overlapping which
indicates that the conductivity won't be affected by porosity.
[0069] Since proton conduction can largely be related to the
concentration of the hopping sites (i.e. the degree of
hydrophobicity) then MCA shows that the drift velocity would
decrease linearly (plot not shown) as the concentration, expressed
here as the % population of sites, was decreased FIG. 2c.
Fortunately, MCA proves that by having aqueous pores traversing the
LBL composite membrane a maximum conductivity can be achieved (i.e.
.sigma..about.0.015 Scm.sup.-1 corresponding to a 20 mM
H.sub.2SO.sub.4 solution compared to a .sigma..about.10.sup.-4
Scm.sup.-1 of a PEO/PAA matrix).
[0070] AC Impedance Measurements of Polyion Pair Composite
Membranes. Results obtained by AC impedance on fuel cells running
under dry conditions (RH<40%) while the humidity was ramped up
to operating conditions are shown in FIGS. 3a, 3b. The complexity
of the Nyquist plots reflects the structure of the composite
membrane. Multiple semi-circles at low humidity indicated a system
that is composed of Randle's cells connected in series. As the
humidity was ramped up the impedance decreased because the pores of
the polycarbonate membrane filled with water. As the fuel cell was
further purged to operating conditions the high frequency
semi-circle referring to the polarization resistance of the porous
electrodes interface or pore's impedance, Z.sub.p, disappeared and
the polarization resistance of the LBL membrane-electrode interface
Z.sub.m impedance was the only feature left (FIG. 3b). The tail of
the Nyquist plot, which was assigned as a Warberg
Constant-Phase-Element (CPE), indicates that ion diffusion through
a microporous structure is indeed occurring in all measurements.
The circuit shown in FIG. 4a was used to simulate the behavior of
the fuel cell under different humidity conditions using the
following equation: Z T = Z p + Z m ( 5 ) Z p = 1 R p - j.omega.
.times. .times. C p ( 1 Rp ) 2 + ( .omega. .times. .times. C p ) 2
( 6 ) Z m = R m - X .phi. .times. cos .times. .times. .beta. - j
.function. ( .omega. .times. .times. C m .times. R - 2 .times.
.times. .omega. .times. .times. C m .times. R m .times. X .phi.
.times. cos .times. .times. .beta. + .omega. .times. .times. C m
.times. X 2 .times. .phi. + X .phi. .times. sin .times. .times.
.beta. ) ( .omega. .times. .times. C m .times. X .phi. .times. sin
.times. .times. .beta. + 1 ) 2 + ( .omega. .times. .times. C m
.times. R m - .omega. .times. .times. C m .times. X .phi. .times.
cos .times. .times. .beta. ) 2 .times. .times. .beta. = .pi. 2
.times. .phi. , X = 1 .omega. .times. .times. C W ( 7 ) ##EQU3##
where R.sub.p is the pore resistance, C.sub.p is the pore
capacitance, R.sub.m is the membrane resistance, C.sub.m is the
membrane capacitance, .phi. is the fractional exponent, C.sub.W the
CPE capacitance, and X.sup..phi. is the Warberg CPE reactance.
[0071] We have considered a contribution from two Randle's cells
connected in series. Both the Z.sub.m and Z.sub.p depend on the
concentration of H.sup.+ ions and on the water uptake, i.e. the
degree to which the channels and pools of water interconnect across
the composite membrane. The simulated Nyquist plots that utilized
the circuit in FIG. 4a showed two effects: (1) A drop in R.sub.p
and R.sub.m as humidity is increased leads to an overall drop in
the total impedance reflected by a shrinkage in the size of the
Nyquist plot; and (2) A sharp drop in R.sub.p (pores fully soaked)
diminished one semi-circle (i.e. the pores), leaving one Randle
cell that corresponds to Z.sub.m at maximum humidity.
[0072] Galvanostat Measurements of Polyion Pair Composite
Membranes. After each fuel cell operation the resistance of the MEA
was measured using AC impedance and compared to values of
resistance calculated from the open circuit potential [OCP] of the
fuel cell, and the junction potentials recorded for specified
applied currents, where: R j = OCP - V j I j .times. .times. j = 1
, 2 , 3 .times. .times. ( 8 ) ##EQU4## Equation (8) is a simplified
form of the fuel cell equation. Larminie, J.; Dicks, A. Wiley, New
York 2000. Simulations using the power density plots of LPEI/PAA
showed that results obtained by eq. (8) and the fuel cell equation
for R.gtoreq.10.OMEGA. are nearly similar. For all composite
membranes, we conclude that the system under study is a combination
of H.sup.+ ions propagating across a nonporous LBL film phase into
a porous aqueous phase thus creating a "nano salt bridge". Values
of the resistance showed that when the fuel cell is operating even
at low RH of .about.50% to 60% the composite membrane performs as
if it is wet or at a RH of about 100%. It is probable that dynamic
hydration equilibrium is occurring between the water produced at
the cathode, the LBL film, and the water within the pores.
[0073] Fuel cell performance. In order to compare the performance
of our composite membranes to the classical PEM (Nafion112.RTM.,
both types of membrane were moderately pressed between two
commercial Pt/C (ELAT.RTM.) electrodes to fabricate the MEA. The
MEA was then sandwiched between two homemade bipolar plates that
allowed gas diffusion and acted as current collectors. The whole
assembly was enclosed inside a two-chamber manifold that contains
the fuel gases, and through which the pressure, temperature, and
humidity can be controlled at ambient conditions. From the brief
description of the FC design it should be noted that all the fuel
cell systems studied were run under a regime of non-optimized
performance. This design was chosen to minimize any interferences
from outside factors such as ionomer wicking inside the composite
membrane and thermal or mechanical degradation if press baked or
operated at high temperature (> about 60.degree. C.) and
pressure (> about 10 psi).
[0074] Two control experiments were made to analyze the performance
of composite membrane. First, a Nucleopore.RTM. membrane sandwiched
between two ELAT electrodes should show the highest conductance,
hence the maximum power delivery, because the conducting protons
are not impeded by the LBL film sealing the pores. A fuel cell
running on an uncoated Nucleopore.RTM. membrane delivered a maximum
power density of 9.6 mWcm.sup.-2 and its OCP was 0.8V, which
decreased with operation to 0.56V due to uncontrolled fuel cross
over, thus illustrating the important role of the LBL film in
regulating fuel cross over, and the need for this intermediate
layer in the MEA. Second, a home-built E-TEK.RTM./Nafion112.RTM.
fuel cell system was capable of delivering a maximum power of 43 mW
cm.sup.-2 at a RH.gtoreq.80% and a temperature of 25.degree. C. A
strong flow of hydrogen was maintained to obtain high humidity with
P.sub.air=4.0 psi, and the operation time was up to two hours. If
the humidity was decreased to a range of 60% to 50%, the power
dropped to .about.23 mW cm.sup.-2 while the voltage continuously
decreased. When the RH was dropped below 50% the
E-TEK.RTM./Nafion112.RTM. fuel cell system simply collapsed.
[0075] We have organized the LBL fuel cell systems into two
categories where the low-power systems are shown in FIG. 5, and the
high-power systems in FIG. 6. At ambient conditions, the low-power
couples LPEI/PAMPS, LPEI/PSSM, PEO/PMAA, and LPEI/PSS did not
deliver an electrical power higher than 4 mWcm.sup.-2, while the
high power couples PDME/PAMPS, PDAC/PAMPS, LPEI/PAA, and PEO/PAA
delivered greater than 5 mWcm.sup.-2. By comparing the systems in
FIG. 5 & FIG. 6 the effects of film structure and composition
on the performance of the fuel cell can be discussed. Results in
Table 1 show that MEAs containing different PE couples show
different internal resistances, and in general, those that are less
hydrophobic are more conductive to H.sup.+. However, the
hydrophilic-hydrophobic factor is not the only important factor,
but the stability of the LBL film and the size of the polymer
molecule may also play a role in power delivery. For example, wet
LBL films of LPEI/PAA deposited on gold-coated substrates have a
much higher conductivity than LPEI/PSS films (5.times.10.sup.-6
Scm.sup.-1>>1.2.times.10.sup.-8 Scm.sup.-1). The difference
is attributed to the higher hydrophilic character of PAA compared
to PSS. One might assume that the conductivity of PSSM would
therefore have an intermediate value of 0.9.times.10.sup.-7
Scm.sup.-1) as PSSM contains a mix of a 3:1 ratio of sulfonate to
carboxylate groups. By referring to the power plots in FIG. 5 and
FIG. 6, it is clear that the power delivered by the LPEI/PAA fuel
cell (.about.5.5 mWcm.sup.-2) is higher than that of LPEI/PSS
(.about.3.0 mWcm.sup.-2) for the reasons stated. Unexpectedly, the
power delivered by the LPEI/PSSM system (0.16 mWcm.sup.-2 at I=0.4
mA cm.sup.-2) is not intermediate, but lower than that of the more
hydrophobic LPEI/PSS. Although the LPEI/PSSM film is 2.5 times
thicker than the LPEI/PSS film, this difference does not explain
the 18.8 times difference in power density (i.e. 3.0:0.16
mWcm.sup.-2). Both the LPEI and the PSSM polymer molecules possess
a low average molecular weight that allows the PE couple to
penetrate through the pores and form a few bilayers on the inner
walls of the pore, and thus leading to greater resistance to proton
mobility, and hence much lower power density.
[0076] Comparing LPEI/PAMPS (3.6 mWcm.sup.-2), FIG. 5, and
PDAC/PAMPS (.about.11.5 mWcm.sup.-2), FIG. 6, the former couple
might have been expected to produce more power because LPEI is more
hydrophilic than PDAC. Under operating conditions PDAC/PAMPS showed
a better performance probably because the LPEI/PAMPS film was
viscous and unstable, allowing fuel cross over, and under the
severe environment of the fuel cell, the film delaminated and
leaked gases. The PDME/PAMPS (6.0 mWcm.sup.-2, plot not shown)
system was expected to perform better than PDAC/PAMPS because PDME
is less hydrophobic than PDAC due to the presence of an OH group on
the polymer backbone. However, it yielded a lower OCP (0.81
degrading to 0.64V) and a maximum power density of 6.0 mWcm.sup.-2,
similar to the LPEI/PAA system. In general, films that were
suspected of fuel crossover yielded a low OCP. A PDME/PAMPS fuel
cell operating at 0.46V and a current of 12.0 mAcm.sup.-2 did not
show any degradation in its voltage even after 30 minutes. The most
stable couple among the low power LBL fuel cells was the LPEI/PSS
system which after a few hours of operation kept delivering the
initial power or even higher, up to 3.7 mWcm.sup.-2.
[0077] Among the PE couples studied, the PEO/PAA system delivered
the highest power (16.6 mWcm.sup.-2), which was nearly 50% the
performance of the E-TEK/Nafion112 fuel cell operated at RH>80%
in our laboratory, and close to many commercial monocells at 20
mWcm.sup.-2 operated under the same conditions. The PEO/PAA system
is followed by the PDAC/PAMPS system at 11.6 mWcm.sup.-2 and the
LPEI/PAA system at .about.5.5 mWcm.sup.-2. The PDAC/PAMPS film had
higher conductance than LPEI/PAA film because its thickness is
nearly 2.5 times lower at an equivalent number of layers. All three
systems were stable especially PEO/PAA that was tested, after a two
hour experiment, by operating at half power for 10 minutes with
hardly any change in potential. Additionally, the capability of the
PEO/PAA system, as well as the other LBL fuel cell systems, to
operate normally at RH=50% is a major advantage over the
Nafion112.RTM. membrane used. For example, separate tests on the
LPEI/PAA (5.5 mWcm.sup.-2) and LPEI/PSS (3.7 mWcm.sup.-2) fuel cell
systems operated using a dry flow of H.sub.2 and air at RH.about.5%
were successfully performed. The significance of having the LBL
film deposited on the Nucleopore platform and not directly on the
ELAT electrodes was demonstrated when we tested an LBL PEO/PAA film
directly deposited onto the ELAT electrodes, where the latter were
separated by a composite membrane made up of only 10 bilayers of
PEO/PAA. The power delivered by this system (1.84 mWcm.sup.-2) was
extremely low, and was attributed to the permeation and probably
precipitation of the polyelectrolytes within the pores of the
electrode during assembly, thus creating resistances within the
electrodes to proton conductivity as well as a hydrophilic medium
that absorbs water into the electrodes, blocking the passage of
fuel. These early results indicate that the PEO/PAA system is a
promising one for actual fuel cell applications.
[0078] Therefore, there are three important factors that determine
the power performance of a PE-LBL fuel cell. To simplify, consider
the three PE couples PDAC/PAMPS, PDAC/PSS, and LPEI/PSS. The
PDAC/PSS couple (.sigma.<10.sup.-9 S cm.sup.-1) is characterized
by a strong hydrophobic electrostatic interaction between the
tertiary ammonium group [--N--.sup.+] of PDAC and the sulfonate
group [--SO.sup.3-] of PSS leaving no stations for proton
conduction across the PE segments. Farhat, T.; Yassin, G.; Dubas,
S. T.; Schlenoff, J. B. Langmuir 1999, 15, 6621-6623; Dubas, S. T.;
Schlenoff, J. B. Macromolecules 1999, 32, 8153-8160. As a result,
no power was delivered by this system. Replacing the PDAC by LPEI
with a polycation backbone that constitute secondary ammonium
groups [--NH--.sup.+] that are pH dependent and hydrophilic
resulting in weaker interactions with the [--SO.sup.3-] of PSS.
Yoo, D.; Shiratori, S. S.; Rubner, M. F. 1998, 31, 4309-4318;
Clark, S. L.; Hammond, P. T. Langmuir 2000, 16, 10206-10214.
Consequently, proton exchange can progress moderately across the
functional groups. When PDAC/PAMPS is compared to LPEI/PSS, the
former delivers about 4 times the power even though both LBL films
have the same thickness. Although, in PDAC/PAMPS there is a strong
hydrophobic electrostatic interaction between the tertiary ammonium
group [--N--.sup.+] of PDAC and the sulfonate group [--SO.sup.3-]
of PAMPS; however, PAMPS differs from PSS in possessing the amide
group [--CO--NH.sub.2--] that is strongly hydrophilic. DeLongchamp,
D. M.; Hammond, P. T. Chem. Mater. 2003, 15, 1165-1173. There is a
possibility that the amide group acts as a sponge for the water
generated at the cathode thus enriching the LBL film with water
pools and channels across which the hydronium ions can
propagate.
[0079] The impact of hydrophobicity clearly appears when comparing
the PEO/PAA (16.6 mWcm.sup.-2) and PEO/PMAA (2.64 mWcm.sup.-2) fuel
cell systems. The PEO/PAA film is nearly the same thickness as the
PEO/PMAA film, yet the PEO/PAA system was capable of delivering a
maximum electrical power around 16.6 mWcm.sup.-2, which is 6 times
higher than that of PEO/PMAA. The PMAA is more hydrophobic than PAA
because it has an additional methyl group in its repeat unit. Among
the PE couples studied, the PEO/PAA system delivered the highest
power, which was nearly 50% the performance of the E-Tek/Nafion112
fuel cell operated in our laboratory, and close to many commercial
monocells at 20 mWcm.sup.-2 operated under the same conditions.
However, the capability of the PEO/PAA system, as well as the other
LBL fuel cell systems, to operate normally at RH=50% is a major
advantage over the Nafion112 membrane. For example, separate tests
on the LPEI/PAA (5.5 mWcm.sup.-2) and LPEI/PSS (3.7 mWcm.sup.-2)
fuel cell systems operated using dry flow of H.sub.2 and air at
RH.about.5% were successfully performed. The significance of having
the LBL film deposited on the Nucleopore and not directly on the
ELAT electrodes was demonstrated when we tested an LBL PEO/PAA film
directly deposited onto the ELAT electrodes, where the latter were
separated by a composite membrane made up of only 10 bilayers of
PEO/PAA in order to maintain continuity. The power delivered by
this system (1.84 mWcm.sup.-2) was low, and was attributed to the
permeation of the polyelectrolytes within the pores of the
electrode during assembly, thus creating large resistances within
the electrodes to proton conductivity.
[0080] In terms of power delivery, the PEO/PAA system is followed
by the PDAC/PAMPS system at 11.6 mWcm.sup.-2 and the LPEI/PAA
system at .about.5.5 mWcm.sup.-2. The PDAC/PAMPS film had higher
conductance than LPEI/PAA film because its thickness is nearly 2.5
times less, although FTIR showed that both films have similar water
content by referring to the OH stretch at 3500 cm.sup.-1. Most of
the fuel cell systems were operated for at least two hours before a
complete shut down. Repetitive isolation of the FC from its load
did not affect its performance. The average time taken for the FC
to regain its OCP or an operating voltage when switched on was one
minute. Our simple design did not use serpentine bipolar plates
that allow intimate flow of gases close to the electrodes to
increase efficiency and remove water; therefore, after prolonged
operation, a disassembled MEA was found flooded with water between
the composite membrane and the cathode. Moreover, the Nucleopore
membrane had performed its role in providing support to the LBL
film, but the thermal and mechanical stability of these
polycarbonate membranes are less than optimal. In the fuel cell
module, the Nucleopore membrane is subjected to some compression
and excessive heat that in a number of fuel cell samples caused
cracks in the "composite membrane." Different support membranes
with more optimal mechanical and thermal properties and different
pore structures are currently being investigated in this work.
IIIb. Devices--Soft Polymer Electrolyte Fuel Cells (SPEFC)
[0081] Unlike conventional PEFCs, where the MEA is fabricated by
hot pressing, the MEA of a "soft-PEFC" is synthesized in one
process that does not require hot pressing and gasketing (if the
membrane and the electrodes are separate). Using an automated
dipper, the process starts by alternately dipping a customized
porous-insulating support, FIG. 7, in poly(+)/poly(-) solutions
until the required IEM thickness is assembled. The solutions that
make up the IEM can then be replaced by the electrode colloidal
suspensions to deposit the cathode and the anode. The electrode
colloidal suspensions are in turn replaced by the "gas diffusion
layer" colloidal suspensions to deposit the GDL. If a metallic
film, acting as a current collector, can now be applied on both
sides by sputtering or spraying then a complete soft fuel cell is
assembled. Apart from the metallic contacts, we have discovered the
above experimental procedure for soft fuel cell assembly.
[0082] There were no limitations on the soft fuel cell assembly
when strong polyelectrolytes (i.e. pH independent such as PDAC,
PAMPS) were used through out the process. However, if weak
polyelectrolytes (i.e. pH dependent such as LPEI, PAA, PAA-co-PAAm)
were used to assemble the IEM, then the pH of the carbon colloidal
suspension should not exceed 4. For example, carbon colloidal
suspensions of PDAC/PAMPS maintained at pH=11 cannot be used to
deposit LPCEs on an LPEI/PAA or PEO/PAA membranes that break down
at pH>4. In this case, carbon colloidal suspensions of either
PDAC/PAA-co-PAAm or LPEI/PAA maintained at pH=4 can be successfully
used.
[0083] Soft fuel-cells can be stacked according to conventional
gasketing methods when the soft fuel cells are assembled on
insulating-porous supports. Alternatively, soft MEAs can be stacked
on multiple insulated-porous-metallic supports that act as GDLs and
current collectors. In this case, stacking does not involve any
gasketing or mechanical parts and the whole stack of MEAs is
assembled in one process. The LBL process starts with the support
executing multiple dipping in the poly(-)/poly(+) slurry of the
electrodes until the desired thickness is achieved. The thickness
of the electrodes is determined by the power delivered, the size of
the colloids, and the separation between the supports. After
building the electrodes, the gap between the electrodes can be
filled by an LBL film (with or without a colloidal layer that mimic
the porous membrane). The whole stack can then be immersed in an
ionomer and dried (e.g., Nafion117.RTM.) to fill any lose gaps for
proton conduction. Finally, to insure tight seal against gas
leakages the whole stack is dipped in a resin solution and dried.
FIGS. 8 & 9 show a schematic of the soft stack design. In a
third embodiment, soft MEAs can be stacked on top of each other by
following the same procedure described above, but starting with a
metallic support rather than an insulating porous support to form
one complete fuel cell. Keeping the whole setup in position but
reversing the dipping procedure (i.e. GDL, Electrodes, Membrane)
two new soft fuel cells self assemble on the original. The process
can be repeated to form a "soft fuel-cell stack". The whole stack
can then be immersed in an ionomer and dried (e.g., Nafion117.RTM.)
to fill any lose gaps for proton conduction. Finally, to insure
tight seal against gas leakages the whole stack is dipped in a
resin solution and dried.
IIIc. Devices--Soft Carbon-Polymer Galvanic Cells and Batteries
(SCPGC)
[0084] The design of a soft fuel-cell can be adapted to design
carbon-polymer based solid-state galvanic cells and batteries, FIG.
10. Unlike conventional types the SCPGC can be tailored to be ultra
thin and extremely small (i.e. be applicable to micro-power
systems). Also unlike conventional solid-electrolytes, in which
only one type of ion (either cation or anion) is predominantly
modile and conducts electivity, the membranes of the instant
invention can be fabricated to conduct protons, cations and anions.
In addition, since the LPCE possesses a large surface area, the MEA
can be a potential candidate for state-of-art super-capacitors.
[0085] As with soft fuel-cells, the same type of composite
membrane, part of the electrolyte phase, can be used as a separator
or a salt bridge. The composite membrane is best assembled from
water-soluble macromolecules (i.e. all types of polyelectrolytes
e.g., LPEI/PAA, PEO/PAA, PAAm/PAA, PAAm/PAA-co-PAAm, PDAC/PAMPS,
PDAC/PAA-co-PAAm) that possess high ionic conductivity. Since LBL
films can be capped and stratified, a half-cell can be assembled by
depositing a suitable polyelectrolyte couple (e.g., LPEI/PSS)
directly onto the electrode then dosing the film with the required
salt ion such as CuSO.sub.4, NH.sub.4Cl, and ZnCl.sub.2.
[0086] LPCE electrodes can be of the inert (insoluble) type where
the carbon electrodes are pure or embedded with noble metals, such
as platinum, palladium, and gold. Both types of electrodes were
assembled. The LPCE electrodes can also be of the active (soluble)
type embedded with metal colloids such as copper, zinc, and silver
or salt colloids such as PbO.sub.2, and MnO.sub.2 which allows it
to generate electricity.
IIId. Devices--Soft Carbon-Polymer Electrochemical Cell (SCPEC)
[0087] The design of a soft fuel-cell can be adapted to design
carbon-polymer based solid-state electrochemical cells. An
electrochemical cell is used to monitor electrochemical processes
by allowing currents to pass through rather than generating them.
The major difference from the SCPFC is to have a passage for
electrolytic solutions through the porous support in order to dose
the analytical cell. Unlike conventional types, the SCPEC can be
tailored to be ultra thin and extremely small (i.e. application to
micro-electro-analytical systems) thus a potential candidate for
state-of-art detectors. As with the fuel cell, the composite
membrane is best assembled from water-soluble macromolecules (i.e.
all types of polyelectrolytes e.g., LPEI/PAA, PEO/PAA, PAAm/PAA,
PAAm/PAA-co-PAAm, PDAC/PAMPS, PDAC/PAA-co-PAAm) that possess high
ionic conductivity. One of the LPCE electrodes must be of the inert
(insoluble) type to represent the counter electrode. The LPCE
counter electrode can be pure graphite colloid or graphite colloid
embedded with noble metals such as platinum, palladium, and gold.
For the working electrode the LPCE electrodes can be either inert
or active (soluble) type where the graphite colloid is embedded
with metal colloids, such as copper, zinc, or silver.
IIIe. Devices--Micro-Electrochemical Systems
[0088] Unlike conventional electrochemical systems, such as
electrochemical cells, fuel cells, and galvanic cells the LBL
electrochemical systems can be miniaturized using methods such as
micro-contact-printing. Conventional microfuel cells made using
expensive lithographic techniques still need a relatively large PEM
to separate the microelectrodes. Holladay, J. D.; Jones, E. O.;
Phelps, M.; Hu, J. J. Power Sources 2002, 108, 21-27.; Lee, S. J.;
Chang-Chien, A.; Cha, S. W.; O'Hayre, R.; Park, Y. I.; Saito, Y.;
Prinz, F. B. J. Power Sources 2002, 112, 410-418.; Maynard, H. L.;
Meyers, J. P. J. Power Sources 2002, 109, 76-88. Therefore, it
would be difficult to assemble a single microfuel cell using
lithography, a job that is made easy using the LBL technology.
[0089] Polyelectrolyte carbon colloidal suspensions that make up
the LPCE do not assemble well on hydrophobic or weakly bonding
surfaces that prevent electrostatic interactions or hydrogen
bonding. For example, a surface functionalized with long-chain
alkyl thiols, weakly bonding polyelectrolyte, or hydrophobic
polyelectrolytes prevents the deposition of carbon colloids at
these sites. If part of the surface were treated with a suitable
LBL film the LPCE would assemble over the LBL film only. On a
micro-fluidic substrate a micron size electrochemical system can be
assembled with its XY coordinates specified at a particular point
on the surface provided that a channel allows the delivery of the
electrolyte or the fuel, FIG. 11. For example, in the aqueous
fuel-cell system described above, the liquid fuel can be easily
delivered by capillary action through microfluidic channels.
IIIf. Devices--Catalytic Converters
[0090] The ability to choose the required particle size of the
catalyst (i.e. platinum or any suitable metal plus its carbon
support) as well as the size of the polymer (i.e. the appropriate
molecular weight) can be utilized to fabricate catalytic
converters. The polymer/carbon LBL films can either be deposited on
a porous framework with longitudinal tubes, FIG. 12. Or the LPCE
can be built thick and porous enough in order to allow the porosity
of the particles to mimic the porous support usually used in
catalytic converters, FIG. 13. For example, a 40 bilayer LPCE of
PDAC/PAMPS 10% platinum on activated carbon was successfully
deposited on a porous Nucleopore.RTM. support with a thickness up
to 30 .mu.m.
IIIg. Devices--Other Applications
[0091] A solid state hydrogen pump has all of the same problems as
a fuel cell, without the presence of a water forming reaction at
the cathode. As with a fuel cells, eliminating the need for a
humidifier will lead to a smaller, simpler, and lighter system.
[0092] An electrolyzer, especially one designed to produce hydrogen
and oxygen from water offers a different set of problems. A water
electrolyzer contains essentially the same parts as a fuel cells
but the polarity is reversed, as are all of the electrochemical
reactions. Instead of generating electricity and water from
hydrogen and oxygen, it produces hydrogen and oxygen from water and
electricity.
[0093] In an electrolyzer there is always water present to keep the
membrane hydrated. The problems arise in the electrodes and on the
gas outlet side. Because liquid water is present in the same
compartment that gas is being generated in, the gas outlet flow
will nearly always be two phase with a large quantity of water
being carried out with the gas.
[0094] A more fundamental problem arises in the electrodes. Since
maximum current efficiency requires that liquid water be in contact
with the membrane, at least one of the electrodes must be
hydrophilic. While a hydrophilic electrode is best for the
membrane, it tends to impede gas bubble formation and gas removal.
If the water is supplied directly to the membranes fully
hydrophobic electrodes could be used, to maintain efficient gas
evolution.
[0095] These problems are further exacerbated in a regenerative
fuel cell. Since a regenerative fuel cell by definition must
operate in turn as both an electrolyzer and a fuel cell, using
hydrophilic electrodes that produce effective operation in a liquid
water environment for electrolyzer operation virtually guarantees
electrode flooding during fuel cell operation. If operation with
liquid water present in the electrode compartment can be avoided,
then hydrophobic electrodes can function well in both modes.
[0096] One method that has previously been proposed for directly
humidifying a proton exchange membrane is the inclusion of water
conducting wicks as part of the membrane structure. While this
method has some effectiveness, the amount of flow that can be
achieved through the membrane is limited. A further drawback to the
wicks is that they rely on wetting to promote flow. This precludes
their use to introduce non-aqueous streams into the proton exchange
membrane. In addition, the wicks act as filtering elements to
remove any particles in the stream. This limits their use to
systems with pure water, or where care is taken to prevent the
solution from becoming saturated and beginning to precipitate.
[0097] Electrochemical water desalination or clean-up systems based
on the electroosmosis occurring in a hydrogen pump has some
additional difficulties other than those noted above for a simple
hydrogen pump. This type of system uses the fact that every proton
passing through the membrane carries water with it, typically about
two water molecules per proton. In devices described previously,
the hydrogen and water to be purified are fed into the cell
together as a solution saturated with hydrogen. Since the
solubility of hydrogen in water is low, the current density is
limited to a relatively low value. A low current density produces a
low water purification rate.
DEFINITIONS
[0098] For convenience, certain terms employed in the
specification, examples, and appended claims are collected
here.
[0099] The term "electrolyte" as used herein means any chemical
compound that ionizes when dissolved.
[0100] The term "polyelectrolyte" as used herein means a polymeric
electrolyte, such as polyacrylic acid.
[0101] The term "pH" as used herein means a measure of the acidity
or alkalinity of a solution, equal to 7, for neutral solutions and
increasing to 14 with increasing alkalinity and decreasing to 0
with increasing acidity.
[0102] The term "pH dependent" as used herein means a weak
electrolyte or polyelectrolyte, such as polyacrylic acid, in which
the charge density can be adjusted by adjusting the pH.
[0103] The term "pH independent" as used herein means a strong
electrolyte or polyelectrolyte, such as polystyrene sulfonate, in
which the ionization is complete or very nearly complete and does
not change appreciably with pH.
[0104] The term "K.sub..alpha." as used herein means the
equilibrium constant describing the ionization of a weak acid.
[0105] The term "pK.sub..alpha." as used herein means a shorthand
designation for an ionization constant and is defined as
pK.sub..alpha.=-log K.sub..alpha.. pK.sub..alpha. values are useful
when comparing the relative strength of acids.
[0106] The term "multilayer" as used herein means a structure
comprised of two or more layers.
[0107] For purposes of this invention, the chemical elements are
identified in accordance with the Periodic Table of the Elements,
CAS version, Handbook of Chemistry and Physics, 67th Ed., 1986-87,
inside cover.
[0108] The following are selected polymers used in the multilayer
depositions of the present invention: TABLE-US-00002 Charge/pH
dependent Polymer Name Polymer Abbreviation Polymer Structure or
independent Polyacrylic acid PAA ##STR1## Anionic/pH dependent
Polyallylamine hydrochloride PAH ##STR2## Cationic/pH dependent
Polyacrylamide PAAm ##STR3## Neutral Polymethacrylic acid PMA
##STR4## Anionic/pH dependent Polystyrene sulfonate SPS ##STR5##
Anionic/pH independent Polydiallyldimethyl-ammonium chloride PDAC
##STR6## Cationic/pH independent Linear Poly(ethyleneimine) LPEI
##STR7## Neutral Poly(ethyleneoxide) PEO ##STR8## Neutral
Poly(2-acrylamido-2-methyl-1-propane sulfonic acid) PAMPS ##STR9##
Anionic/pH independent Poly(vinylpyrrolidone) PVP ##STR10## Neutral
Poly(vinylalcohol) PVA ##STR11## Neutral Poly(ethylene glycol) PEG
##STR12## Neutral Poly(aniline) PANI ##STR13## Neutral Poly(styrene
sulfonic acid-maleic acid, sodium salt) PSSM3:1 ##STR14##
Anionic/pH dependent Poly(acryl-co-acrylamide acid, sodium salt)
PAA-co-AAm ##STR15## Anionic/pH dependent
Poly(dimethylamine-co-epichlorohydrin) PDME ##STR16## Cationic/pH
independent
Methods of the Invention
[0109] The present invention also relates to a method of forming a
membrane, comprising sequentially depositing, under pH controlled
conditions, a plurality of polymer layers on a surface; wherein
each polymer layer is independently selected from the group
consisting of pH dependent cationic polyelectrolytes, pH
independent cationic polyelectrolytes, neutral polymers, pH
dependent anionic polyelectrolytes, and pH independent anionic
polyelectrolytes; wherein a polymer layer optionally comprises at
least one additional chemical entity selected from the group
consisting of hydrogels, polyions, colloids, latexes, zeolites,
platelets, proton sponges, organic molecules, organic salts,
inorganic salts, organic acids, inorganic acids, cationic
dendrimers, anionic dendrimers, metals and carbon; and wherein said
plurality of polymer layers comprises a first polymer layer and
second polymer layer; thereby forming a membrane.
[0110] The present invention also relates to the aforementioned
method, wherein said membrane further comprises at least one
additional chemical entity selected from the group consisting of
hydrogels, polyions, colloids, latexes, zeolites, platelets, proton
sponges, organic molecules, organic salts, inorganic salts, organic
acids, inorganic acids, cationic dendrimers, anionic dendrimers,
metals and carbon.
[0111] The present invention also relates to the aforementioned
methods, wherein said first polymer layer is a pH dependent
cationic polyelectrolyte, a pH independent cationic polyelectrolyte
or a neutral polymer; and wherein said second polymer layer is a pH
dependent anionic polyelectrolyte or pH independent anionic
polyelectrolyte; and wherein said membrane is removed from said
surface; thereby forming an ion-exchange membrane or a LBL
polyelectrolyte-carbon electrode.
[0112] The present invention also relates to the aforementioned
methods, wherein said first polymer layer is a pH dependent
cationic polyelectrolyte, a pH independent cationic polyelectrolyte
or a neutral polymer; and wherein said second polymer layer is a pH
dependent anionic polyelectrolyte or pH independent anionic
polyelectrolyte; and wherein said surface is selected from the
group consisting of organic hydrophilic porous filter membranes and
inorganic hydrophilic porous filter membranes; thereby forming an
ion-exchange membrane.
[0113] The present invention also relates to the aforementioned
methods, wherein said first polymer layer is a pH dependent
cationic polyelectrolyte, a pH independent cationic polyelectrolyte
or a neutral polymer; and wherein said second polymer layer is a pH
dependent anionic polyelectrolyte or pH independent anionic
polyelectrolyte; and wherein said membrane is removed from said
surface; thereby forming an ion-exchange membrane or a LBL
polyelectrolyte-carbon electrode.
[0114] The present invention also relates to the aforementioned
methods, wherein said first polymer layer is a pH dependent
cationic polyelectrolyte, a pH independent cationic polyelectrolyte
or a neutral polymer; and wherein said second polymer layer is a pH
dependent anionic polyelectrolyte or pH independent anionic
polyelectrolyte; and wherein said surface is selected from the
group consisting of organic hydrophilic porous filter membranes and
inorganic hydrophilic porous filter membranes; thereby forming an
ion-exchange membrane.
[0115] The present invention also relates to the aforementioned
methods, wherein said first polymer layer is a pH dependent
cationic polyelectrolyte, a pH independent cationic polyelectrolyte
or a neutral polymer; and wherein said second polymer layer is a pH
dependent anionic polyelectrolyte or pH independent anionic
polyelectrolyte; and wherein said surface is organic, semi-metallic
or metallic; and wherein said at least one additional entity is
carbon; thereby forming a LBL polyelectrolyte-carbon electrode.
[0116] The present invention also relates to the aforementioned
methods, wherein said first polymer layer is a pH dependent
cationic polyelectrolyte, a pH independent cationic polyelectrolyte
or a neutral polymer; and wherein said second polymer layer is a pH
dependent anionic polyelectrolyte or pH independent anionic
polyelectrolyte; and wherein said at least one chemical entity is
selected from the group consisting of metals and inorganic salts;
and wherein said at least on additional entity is carbon; thereby
forming a LBL polyelectrolyte-carbon electrode.
[0117] The present invention also relates to the aforementioned
methods, wherein said first polymer layer is selected from the
group consisting of PAH, PDAC, PDME, PAAm, LPEI, PEO, PVP, PVA, PEG
and PANI.
[0118] The present invention also relates to the aforementioned
methods, wherein said first polymer layer is selected from the
group consisting of PDAC, PDME, PAAm, LPEI and PEO.
[0119] The present invention also relates to the aforementioned
methods, wherein said second polymer layer is selected from the
group consisting of PAA, PMA, SPS, PAMPS, OEGDA, PSSM3:1, and
PAA-co-PAA.
[0120] The present invention also relates to the aforementioned
methods, wherein said second polymer layer is selected from the
group consisting of PAA, PAMPS, SPS, PSSM3:1, PAA-co-PAAm.
[0121] The present invention also relates to the aforementioned
methods, wherein said at least one chemical entity is selected from
the group consisting of sulfonated latex, sodium chloride,
potassium chloride, lithium chloride, sulfonic acid, nitric acid,
hydrochloric acid, hydrobromic acid, phosphonic acid, PEG, OEGDA,
PAAm, PVA, PVP, polyphosphates, Nafions.RTM., ethylene glycol and
glycerol.
[0122] The present invention also relates to the aforementioned
methods, wherein said at least on chemical entity is selected from
the group consisting of sodium chloride, sulfonated latex, and
Nafion 117.
[0123] The present invention also relates to the aforementioned
methods, wherein said first polymer layer is selected from the
group consisting of PAH, PDAC, PDME, PAAm, LPEI, PEO, PVP, PVA, PEG
and PANI; and wherein said at least one chemical entity is selected
from the group consisting of sulfonated latex, sodium chloride,
potassium chloride, lithium chloride, sulfonic acid, nitric acid,
hydrochloric acid, hydrobromic acid, phosphonic acid, PEG, OEGDA,
PAAm, PVA, PVP, polyphosphates, Nafions.RTM., ethylene glycol and
glycerol.
[0124] The present invention also relates to the aforementioned
methods, wherein said second polymer layer is selected from the
group consisting of PAA, PMA, SPS, PAMPS, OEGDA, PSSM3:1, and
PAA-co-PAA; and wherein said at least one chemical entity is
selected from the group consisting of sulfonated latex, sodium
chloride, potassium chloride, lithium chloride, sulfonic acid,
nitric acid, hydrochloric acid, hydrobromic acid, phosphonic acid,
PEG, OEGDA, PAAm, PVA, PVP, polyphosphates, Nafions.RTM., ethylene
glycol and glycerol.
[0125] The present invention also relates to the aforementioned
methods, wherein said first polymer layer is selected from the
group consisting of PAH, PDAC, PDME, PAAm, LPEI, PEO, PVP, PVA, PEG
and PANI; and wherein said second polymer layer is selected from
the group consisting of PAA, PMA, SPS, PAMPS, OEGDA, PSSM3:1, and
PAA-co-PAA; and wherein said at least one chemical entity is
selected from the group consisting of sulfonated latex, sodium
chloride, potassium chloride, lithium chloride, sulfonic acid,
nitric acid, hydrochloric acid, hydrobromic acid, phosphonic acid,
PEG, OEGDA, PAAm, PVA, PVP, polyphosphates, Nafions.RTM., ethylene
glycol and glycerol.
[0126] The present invention also relates to the aforementioned
methods, wherein said first polymer layer is selected from the
group consisting of PDAC, PDME, PAAm, LPEI and PEO; and wherein
said at least on chemical entity is selected from the group
consisting of sodium chloride, sulfonated latex, and Nafion
117.
[0127] The present invention also relates to the aforementioned
methods, wherein said second polymer layer is selected from the
group consisting of PAA, PAMPS, SPS, PSSM3:1, PAA-co-PAAm; and
wherein said at least on chemical entity is selected from the group
consisting of sodium chloride, sulfonated latex, and Nafion
117.
[0128] The present invention also relates to the aforementioned
methods, wherein said first polymer layer is selected from the
group consisting of PDAC, PDME, PAAm, LPEI and PEO; and wherein
said second polymer layer is selected from the group consisting of
PAA, PAMPS, SPS, PSSM3:1, PAA-co-PAAm; and wherein said at least on
chemical entity is selected from the group consisting of sodium
chloride, sulfonated latex, and Nafion 117.
[0129] The present invention also relates to the aforementioned
methods, wherein said first polymer layer is LPEI.
[0130] The present invention also relates to the aforementioned
methods, wherein said second polymer layer is a pH independent
anionic polyelectrolyte.
[0131] The present invention also relates to the aforementioned
methods, wherein said first polymer layer is PEO.
[0132] The present invention also relates to the aforementioned
methods, wherein said second polymer layer is a pH dependent
polyelectrolyte.
[0133] The present invention also relates to the aforementioned
methods, wherein said first polymer layer is PAAm.
[0134] The present invention also relates to the aforementioned
methods, wherein said second polymer layer is a pH dependent
polyelectrolyte.
[0135] The present invention also relates to the aforementioned
methods, wherein said first polymer layer is PDAC.
[0136] The present invention also relates to the aforementioned
methods, wherein said second polymer layer is a PH dependent
polyelectrolyte.
[0137] The present invention also relates to the aforementioned
methods, wherein said second polymer layer is a pH independent
polyelectrolyte.
[0138] The present invention also relates to the aforementioned
methods, wherein said first polymer layer is PDME.
[0139] The present invention also relates to the aforementioned
methods, wherein said second polymer is a pH independent
polyelectrolyte.
[0140] The present invention also relates to the aforementioned
methods, wherein said second polymer layer is PAMPS.
[0141] The present invention also relates to the aforementioned
methods, wherein said first polymer layer is a neutral polymer.
[0142] The present invention also relates to the aforementioned
methods, wherein said first polymer layer is a pH independent
cationic polyelectrolyte.
[0143] The present invention also relates to the aforementioned
methods, wherein said second polymer layer is SPS.
[0144] The present invention also relates to the aforementioned
methods, wherein said first polymer layer is a neutral polymer.
[0145] The present invention also relates to the aforementioned
methods, wherein said second polymer layer is PSSM3:1.
[0146] The present invention also relates to the aforementioned
methods, wherein said first polymer layer is a neutral polymer.
[0147] The present invention also relates to the aforementioned
methods, wherein said second polymer layer is PAA.
[0148] The present invention also relates to the aforementioned
methods, wherein said first polymer layer is a neutral polymer.
[0149] The present invention also relates to the aforementioned
methods, wherein said second polymer layer is PAA-co-PAAm.
[0150] The present invention also relates to the aforementioned
methods, wherein said first polymer layer is a neutral polymer.
[0151] The present invention also relates to the aforementioned
methods, wherein said first polymer is a pH independent cationic
polymer.
[0152] The present invention also relates to the aforementioned
methods, wherein the first polymer layer is LPEI; and wherein the
second polymer layer is PAMPS.
[0153] The present invention also relates to the aforementioned
methods, wherein the first polymer layer is LPEI; and the wherein
the second polymer layer is SPS.
[0154] The present invention also relates to the aforementioned
methods, wherein the first polymer layer is LPEI; and wherein the
second polymer layer is PSSM.
[0155] The present invention also relates to the aforementioned
methods, wherein the first polymer layer is LPEI; and wherein the
second polymer layer is PAA.
[0156] The present invention also relates to the aforementioned
methods, wherein the first polymer layer is PEO; and wherein the
second polymer layer is PAA.
[0157] The present invention also relates to the aforementioned
methods, wherein the first polymer layer is PAAm; and wherein the
second polymer layer is PAA.
[0158] The present invention also relates to the aforementioned
methods, wherein the first polymer layer is PAAm; and wherein the
second polymer layer is PAA-coPAAm.
[0159] The present invention also relates to the aforementioned
methods, wherein the first polymer layer is PDAC; and wherein the
second polymer layer is PAA-co-PAAm.
[0160] The present invention also relates to the aforementioned
methods, wherein the first polymer layer is PDAC; and wherein the
second polymer layer is PAMPS.
[0161] The present invention also relates to the aforementioned
methods, wherein the first polymer layer is PDME; and wherein the
second polymer layer is PAMPS.
[0162] The present invention also relates to the aforementioned
methods, wherein the PAA is deposited at a pH of about 4.0.
[0163] The present invention also relates to the aforementioned
methods, wherein the PAA is deposited at a pH between about 3.5 and
about 4.5.
[0164] The present invention also relates to the aforementioned
methods, wherein the PAA is deposited at a pH of about 2.0.
[0165] The present invention also relates to the aforementioned
methods, wherein the PAA is deposited at a pH of about 2.5.
[0166] The present invention also relates to the aforementioned
methods, wherein the PAA is deposited at a pH between about 1.5 and
about 3.0.
[0167] The present invention also relates to the aforementioned
methods, wherein the PAA-co-PAAm is deposited at a pH of about
2.0.
[0168] The present invention also relates to the aforementioned
methods, wherein the PAA-co-PAAm is deposited at a pH between about
1.5 and about 2.5.
[0169] The present invention also relates to the aforementioned
methods, wherein the PAA-co-PAAm is deposited at a pH of about
5.
[0170] The present invention also relates to the aforementioned
methods, wherein the PAA-co-PAAm is deposited at a pH between about
4.5 and about 5.5.
[0171] The present invention also relates to the aforementioned
methods, wherein the PAA-co-PAAm is deposited at a pH between about
5.5 and about 6.5.
[0172] The present invention also relates to the aforementioned
methods, wherein the PAA-co-PAAm is deposited at a pH between about
6.5 and about 7.5.
[0173] The present invention also relates to the aforementioned
methods, wherein said surface is selected from the group consisting
of carbon cloth, porous stainless steel, porous silicon, porous
titanium alloys and gold.
[0174] The present invention also relates to the aforementioned
methods, wherein said carbon is selected from the group consisting
of carbon powder, aqueous carbon paste, and Hispec3000 powder.
[0175] The present invention also relates to the aforementioned
methods, wherein said metal is selected from the group consisting
of Pd, Pt, Au, Ru, Zn, Cu, Ag and Al.
[0176] The present invention also relates to the aforementioned
methods, wherein said first polymer layer is PDAC; and wherein said
second polymer layer is PAMPS; and wherein said carbon is carbon
powder; and wherein said metal is Pd.
[0177] The present invention also relates to the aforementioned
methods, wherein said first polymer layer is PDAC; and wherein said
second polymer layer is PAMPS; and wherein said carbon is Hispec
3000; and wherein said metal is Pt.
[0178] The present invention also relates to the aforementioned
methods, wherein said first polymer layer is LPEI; and wherein said
second polymer layer is PAA deposited at a pH of about 4.0; and
wherein said carbon is aqueous carbon paste; and wherein said metal
is Pt.
[0179] The present invention also relates to a membrane, comprising
a plurality of polymer layers held together by electrostatic or
hydrogen bonding interactions, wherein said membrane has a total
uniform thickness of less than about 10 .mu.m and a conductance of
less than about 1.0.times.10.sup.-3 S/cm and this conductance does
not degrade over time.
[0180] The present invention also relates to the aforementioned
membrane, wherein said membrane has a total thickness of less than
about 1 .mu.m.
[0181] The present invention also relates to the aforementioned
membranes, wherein said membrane has a total thickness of less than
about 0.75 .mu.m.
[0182] The present invention also relates to the aforementioned
membranes, wherein said membrane has a total thickness of less than
about 0.5 .mu.m.
[0183] The present invention also relates to the aforementioned
membranes, wherein said membrane has a total thickness of less than
about 0.25 .mu.m.
[0184] The present invention also relates to the aforementioned
membranes, wherein said membrane has a total thickness of less than
about 0.1 .mu.m.
[0185] The present invention also relates to the aforementioned
membranes, wherein said membrane has a conductance of less than
about 5.0.times.10.sup.-4 S/cm.
[0186] The present invention also relates to the aforementioned
membranes, wherein said membrane has a conductance of less than
about 2.0.times.10.sup.-4 S/cm.
[0187] The present invention also relates to the aforementioned
membranes, wherein said membrane has a conductance of less than
about 5.0.times.10.sup.-5 S/cm.
[0188] The present invention also relates to the aforementioned
membranes, wherein said membrane has a conductance of less than
about 2.0.times.10.sup.-5 S/cm.
[0189] The present invention also relates to the aforementioned
membranes, wherein said membrane has a conductance of less than
about 5.0.times.10.sup.-6 S/cm.
[0190] The present invention also relates to the aforementioned
membranes, wherein said membrane has a conductance of less than
about 1.0.times.10.sup.-6 S/cm.
[0191] The present invention also relates to the aforementioned
membranes, wherein the first polymer layer is LPEI; and wherein the
second polymer layer is PAMPS.
[0192] The present invention also relates to the aforementioned
membranes, wherein the first polymer layer is LPEI; and the wherein
the second polymer layer is SPS.
[0193] The present invention also relates to the aforementioned
membranes, wherein the first polymer layer is LPEI; and wherein the
second polymer layer is PSSM.
[0194] The present invention also relates to the aforementioned
membranes, wherein the first polymer layer is LPEI; and wherein the
second polymer layer is PAA.
[0195] The present invention also relates to the aforementioned
membranes, wherein the first polymer layer is PEO; and wherein the
second polymer layer is PAA.
[0196] The present invention also relates to the aforementioned
membranes, wherein the first polymer layer is PAAm; and wherein the
second polymer layer is PAA.
[0197] The present invention also relates to the aforementioned
membranes, wherein the first polymer layer is PAAm; and wherein the
second polymer layer is PAA-coPAAm.
[0198] The present invention also relates to the aforementioned
membranes, wherein the first polymer layer is PDAC; and wherein the
second polymer layer is PAA-co-PAAm.
[0199] The present invention also relates to the aforementioned
membranes, wherein the first polymer layer is PDAC; and wherein the
second polymer layer is PAMPS.
[0200] The present invention also relates to the aforementioned
membranes, wherein the first polymer layer is PDME; and wherein the
second polymer layer is PAMPS.
[0201] The present invention also relates to the aforementioned
membranes, wherein said membrane can be used as an ion-exchange
membrane, a gas diffusion membrane or an electrode.
[0202] The present invention also relates to the aforementioned
membranes, wherein the said membrane can be used in a
membrane-electrode assembly, a battery, a galvanic cell, an
electrochemical cell, a micro-electrochemical cell, a catalytic
converter, a solid-state hydrogen pump, an electrolyzer, or an
electrochromic device.
Exemplification
[0203] The invention now being generally described, it will be more
readily understood by reference to the following examples, which
are included merely for purposes of illustration of certain aspects
and embodiments of the present invention, and are not intended to
limit the invention.
EXAMPLE 1
Membrane Preparation
[0204] All polyelectrolytes used to fabricate the LBL films were
used as received. Poly(styrene sulfonic acid, sodium salt) [PSS;
MW=70,000], Poly(diallyldimethyl ammonium chloride) [PDAC;
MW=240,000], Linear Poly(ethyleneimine) [LPEI; MW=25,000],
Poly(acrylic acid) [PAA; MW=90,000], Poly(styrenesulfonic
acid-maleic acid, sodium salt) [PSSM3:1; MW=20,000], Poly(ethylene
oxide) [PEO; MW=4,000,000], Poly(methylacrylic acid) [PMAA;
MW=100,000], Poly(acrylic-co-acrylamide acid, sodium salt)
[PAAcoAAm, MW=10,000,000 40% carboxy], all from Polysciences Inc.
Poly(2-acrylamido-2-methyl-1-propane sulfonic acid) [PAMPS;
MW=2,000,000], Poly(dimethylamine-co-epichlorohydrin) [PDME], and
Polyaniline(Emarlidine base) [PAN, MW=100,000], from Aldrich.
Nafion117 as a 5% resin solution from Fluka. Sodium chloride from
Mallinckrodt C.P., Sulfuric acid 98%, Nitric acid 96.6%, 30%
hydrogen peroxide from Fischer.
[0205] Nucleopore.RTM. membranes (25mm diameter) were from SPI
supplies. Nafion112.RTM. from DuPont, and ELAT.RTM. C/Pt electrodes
(.about.0.5mg platinum loading) from E-Tek Inc.; Profilometer from
Tencore Ins. Type P-10, Scanning electron microscope (SEM) JOEL
5910 and an environmental SEM (XL30 FEG-ESEM) for non-conducting
polymer samples, EG&G Princeton Applied Research potentiostat
model 276, AC Impedance Solartron Inc. type SI 1260, plasma
cleaner/sterilizer PDC-32G, Nicolet 550 SeriesII FTIR.
[0206] The track etched Nucleopore.RTM. membrane with 0.1 .mu.m
pores and 3.times.10.sup.8 pore density was plasma etched for 90
seconds. A positively charged layer of PDAC was first deposited to
prepare the porous membrane for multilayer deposition. For the
entire PE couples used 40 bilayers were deposited using an
automated ZEISS DS50 dipper. Whether the LBL film was deposited on
a Nucleopore or a Si-100 wafer the thickness of the composite
membrane or the LBL film was determined using a profilometer and
characterized by FTIR. The Nucleopore membrane is fairly
transparent to IR (around 75% transmission peak-to-peak on the
interferogram window).
EXAMPLE 2
Fuel Cell Assembly
[0207] The "composite membranes" were soaked in 20.0 mM sulfuric
acid solution for 20 minutes then placed on permanox slides ready
to be folded between the ELAT C/Pt electrodes. All the "composite
membranes", including the Nafion112, were moderately hand pressed
to the electrodes at room temperature. The untreated electrodes
were twice soaked in 2.5% Nafion117 solution for 15 minutes and
oven dried at 90.degree. C. for 3 minutes.
[0208] The Nafion112.RTM. was pretreated by heating at 80.degree.
C. with 2% H.sub.2O.sub.2 solution for 2 hours followed by rinsing
with Milli-Q pure water. The Nafion membrane was then soaked in a
0.5M sulfuric acid solution for two days and then boiled in 20 mM
sulfuric acid solution for 1 hour before storing in pH=1.7
H.sub.2SO.sub.4 solution. Slade, S.; Campbell, S. A.; Ralph, T. R.;
Walsh, F. C. J. Electrochem. Soc. 2002, 149, A1556-A1564.
[0209] The MEA was fitted inside a fuel cell module that acts as
bipolar plates but with no serpentine channels that provide
intimate fuel flow. The module was enclosed in a homemade
two-chamber manifold where the pressures of air and hydrogen (BOC
grade) were monitored and adjusted using digital pressure gauges
ACSI from McMaster-Carr Supply Company. The temperature and
humidity in the H.sub.2 chamber were monitored using a MASTECH
MS6503 humidity-temperature meter. Pressures were never taken above
10 psi and glass humidifiers in a water bath thermostat were
used.
[0210] The power delivered by the cell had its cathode connected to
the working electrode terminal while the anode was connected to
both the reference and the counter electrodes. Measurements were
made using the EG&G 276 software that dials a particular
current in the galvanostat mode in order to measure the
corresponding potential as a function of time. Measurements were
checked against resistive loads using an ammeter and a
voltmeter.
EQUIVALENTS & INCORPORATION BY REFERENCE
[0211] It is understood that the examples and embodiments described
herein are for illustrative purposes only and that various
modifications or changes in light thereof will be suggested to
persons skilled in the art and are to be included within the spirit
and purview of this application and scope of the appended claims.
All publications, patents, and patent applications cited herein are
hereby incorporated by reference in their entirety for all
purposes.
* * * * *