U.S. patent application number 12/177312 was filed with the patent office on 2009-03-26 for on-demand hydrogen gas generation device having gas management system.
This patent application is currently assigned to ROVCAL, INC.. Invention is credited to William C. Bushong, Gregory J. Davidson, Mario DeStephen, Zhihong Jin, Jon Luecke, Erik Mortensen, Jamie L. Ostroha, Joseph L. Passaniti, Karthik Ramaswami, Tony Rubsam, Juergen Scherer, Viet Vu.
Application Number | 20090078568 12/177312 |
Document ID | / |
Family ID | 40281756 |
Filed Date | 2009-03-26 |
United States Patent
Application |
20090078568 |
Kind Code |
A1 |
Ramaswami; Karthik ; et
al. |
March 26, 2009 |
ON-DEMAND HYDROGEN GAS GENERATION DEVICE HAVING GAS MANAGEMENT
SYSTEM
Abstract
The present disclosure generally relates to an on-demand
hydrogen gas generation device, suitable for use in a fuel cell,
which utilizes water electrolysis, and more particularly galvanic
cell corrosion, and/or a chemical hydride reaction, to produce
hydrogen gas. The present disclosure additionally relates to such a
device that comprises a switching mechanism that has an electrical
current passing therethrough and that repeatedly and reversibly
moves between a first position and a second position when exposed
to pressure differential resulting from hydrogen gas generation, in
order to (1) alter the rate at which hydrogen gas is generated,
such that hydrogen gas is generated on an as-needed basis for a
fuel cell connected thereto, and/or (2) ensure a substantially
constant flow of hydrogen gas is released therefrom. The present
disclosure additionally or alternatively relates to such an
on-demand hydrogen gas generation device that comprises a gas
management system designed to maximize the release or evolution of
hydrogen gas, and in particular dry hydrogen gas, therefrom once it
has been formed, thus maximizing hydrogen gas output. The present
disclosure is still further directed to a fuel cell comprising such
an on-demand hydrogen gas generation device, and in particular a
fuel cell designed for small-scale applications.
Inventors: |
Ramaswami; Karthik;
(Middleton, WI) ; Jin; Zhihong; (Peewaukee,
WI) ; Mortensen; Erik; (Waunakee, WI) ;
Davidson; Gregory J.; (Oregon, WI) ; DeStephen;
Mario; (Madison, WI) ; Passaniti; Joseph L.;
(Madison, WI) ; Luecke; Jon; (Denver, CO) ;
Ostroha; Jamie L.; (Edgerton, WI) ; Rubsam; Tony;
(Madison, WI) ; Scherer; Juergen; (Karlsdorf,
DE) ; Vu; Viet; (Joplin, MO) ; Bushong;
William C.; (Madison, WI) |
Correspondence
Address: |
Christopher M. Goff (27860);ARMSTRONG TEASDALE LLP
ONE METROPOLITAN SQUARE, SUITE 2600
ST. LOUIS
MO
63102
US
|
Assignee: |
ROVCAL, INC.
Madison
WI
|
Family ID: |
40281756 |
Appl. No.: |
12/177312 |
Filed: |
July 22, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60951618 |
Jul 24, 2007 |
|
|
|
Current U.S.
Class: |
204/266 |
Current CPC
Class: |
C25B 1/04 20130101; H01M
8/0606 20130101; H01M 8/04216 20130101; C25B 9/73 20210101; H01M
8/065 20130101; H01M 8/04201 20130101; Y02E 60/36 20130101; Y02E
60/50 20130101; C01B 3/065 20130101 |
Class at
Publication: |
204/266 |
International
Class: |
C25B 9/00 20060101
C25B009/00 |
Claims
1. An on-demand hydrogen gas generation device comprising: a
switching mechanism for regulating the generation of hydrogen gas
within the on-demand hydrogen gas generation device, the switching
mechanism being operable in a range of from about 1 psig to about
30 psig; and a gas management system for transporting hydrogen gas
out of the on-demand hydrogen gas generation device, the gas
management system comprising an anode, an electrolyte, a gas
management electrode comprising a conductive substrate and a
catalyst and having a first face and a second face, a gas
impermeable and liquid permeable hydrophilic layer, a gas permeable
and liquid impermeable hydrophobic layer and a gas exit region,
wherein the anode surrounds the gas impermeable and liquid
permeable hydrophilic layer, and further wherein the gas
impermeable and liquid permeable hydrophilic layer is disposed
between the anode and the first face of the gas management
electrode, the gas permeable and liquid impermeable hydrophobic
layer is disposed on the second face of the gas management
electrode and the gas exit region is interior of the gas permeable
and liquid impermeable hydrophobic layer.
2. The on-demand hydrogen gas generation device as set forth in
claim 1 wherein the device is capable of producing an average of at
least about 0.1 cubic centimeter/minute/cubic centimeter of fuel
volume for a period of time of at least about 1 hour.
3. The on-demand hydrogen gas generation device as set forth in
claim 2 wherein the device is capable of producing hydrogen having
a water content of less than about 1000 ppm water/gram of
hydrogen.
4. The on-demand hydrogen gas generation device as set forth in
claim 3 wherein the conductive substrate is comprised of a material
selected from the group consisting of carbon cloth, carbon paper, a
non-woven mat material, a metal screen, an expanded metal, and a
metal foam.
5. The on-demand hydrogen gas generation device as set forth in
claim 4 wherein the catalyst includes a metal or a Raney metal of
Group VIII.
6. The on-demand hydrogen gas generation device as set forth in
claim 5 wherein the catalyst includes a metal selected from the
group consisting of iron, nickel, nickel power, and Raney
nickel.
7. The on-demand hydrogen gas generation device as set forth in
claim 6 wherein the gas management electrode further includes a
material selected from the group consisting of carbon black,
graphite, polytetrafluoroethylene, activated carbon, and mixtures
thereof.
8. The on-demand hydrogen gas generation device as set forth in
claim 7 wherein the anode includes a metal selected from the group
consisting of zinc, aluminum, magnesium, and titanium.
9. The on-demand hydrogen gas generation device as set forth in
claim 8 wherein the anode further includes an alloying agent
selected from the group consisting of indium, bismuth, tin,
calcium, aluminum, lead, and combinations thereof.
10. The on-demand hydrogen gas generation device as set forth in
claim 1 wherein the device is orientation independent.
11. The on-demand hydrogen gas generation device as set forth in
claim 1 wherein the gas management electrode, gas impermeable and
liquid permeable layer, and gas permeable and liquid impermeable
layer are a single integral component.
12. The on-demand hydrogen gas generation device as set forth in
claim 1 wherein the device further includes a porous separator
disposed between the anode and the gas impermeable and liquid
permeable layer.
13. An on-demand hydrogen gas generation device comprising: a
switching mechanism for regulating the generation of hydrogen gas
within the on-demand hydrogen gas generation device, the switching
mechanism being operable in a range of from about 1 psig to about
30 psig; and a circular gas management system for transporting
hydrogen gas out of the on-demand hydrogen gas generation device,
the gas management system comprising an anode, an electrolyte, a
gas management electrode comprising a conductive substrate and a
catalyst and having a first face and a second face, a gas
impermeable and liquid permeable hydrophilic layer, a gas permeable
and liquid impermeable hydrophobic layer, wherein the gas
impermeable and liquid permeable layer substantially surrounds the
anode, the gas management electrode surrounds the gas impermeable
and liquid permeable hydrophobic layer, and wherein the gas
permeable and liquid impermeable layer surrounds the gas management
electrode.
14. The on-demand hydrogen gas generation device as set forth in
claim 13 wherein the device is capable of producing about an
average of at least about 0.1 cubic centimeter/minute/cubic
centimeter of fuel volume for a period of time of at least about 1
hour.
15. The on-demand hydrogen gas generation device as set forth in
claim 14 wherein the device is capable of producing hydrogen having
a water content of less than about 1000 ppm water/gram of
hydrogen.
16. The on-demand hydrogen gas generation device as set forth in
claim 15 wherein the conductive substrate is comprised of a
material selected from the group consisting of carbon cloth, carbon
paper, a non-woven mat material, a metal screen, an expanded metal,
and a metal foam.
17. The on-demand hydrogen gas generation device as set forth in
claim 16 wherein the catalyst includes a metal or Raney metal from
Group VIII.
18. The on-demand hydrogen gas generation device as set forth in
claim 17 wherein the catalyst includes a metal selected from the
group consisting of iron, nickel, nickel powder, and Raney
nickel.
19. The on-demand hydrogen gas generation device as set forth in
claim 13 wherein the device is orientation independent.
20. The on-demand hydrogen gas generation device as set forth in
claim 13 wherein the gas management electrode, gas impermeable and
liquid permeable layer, and gas permeable and liquid impermeable
layer are a single integrated component.
21. The on-demand hydrogen gas generation device as set forth in
claim 13 wherein the device additionally includes a porous
separator disposed between the anode and the gas impermeable and
liquid permeable layer.
22. An on-demand hydrogen gas generation device comprising: a
switching mechanism for regulating the generation of hydrogen gas
within the on-demand hydrogen gas generation device, the switching
mechanism being operable in a range of from about 1 psig to about
30 psig; and a gas management system for transporting hydrogen gas
out of the on-demand hydrogen gas generation device, the gas
management system comprising an anode, an electrolyte, a gas
management electrode comprising a conductive substrate and a
catalyst and having a first face and a second face, wherein the
first face comprises a gas impermeable and liquid permeable
hydrophilic layer and the second face comprises a gas permeable and
liquid impermeable hydrophobic layer, and a gas exit region,
wherein the first face is adjacent the anode and wherein the gas
exit region is positioned interior of the gas permeable and liquid
impermeable layer.
23. The on-demand hydrogen gas generation device as set forth in
claim 22 wherein the device is capable of producing an average flow
of hydrogen gas of at least about 0.1 cubic centimeter/minute/cubic
centimeter of fuel volume for a period of time of at least about 1
hour.
24. The on-demand hydrogen gas generation device as set forth in
claim 23 wherein the device is capable of producing hydrogen having
a water content of less than about 1000 ppm water/gram of
hydrogen.
25. The on-demand hydrogen gas generation device as set forth in
claim 23 wherein the conductive substrate is selected from a
material from the group consisting of carbon cloth, carbon paper, a
non-woven mat material, a metal screen, an expanded metal, and a
metal foam.
26. The on-demand hydrogen gas generation device as set forth in
claim 25 wherein the catalyst includes a metal or Raney metal of
Group VIII.
27. The on-demand hydrogen gas generation device as set forth in
claim 25 wherein the catalyst includes a metal selected from the
group consisting of iron, nickel, nickel powder, and Raney
nickel.
28. The on-demand hydrogen gas generation device as set forth in
claim 22 wherein the device is orientation independent.
29. The on-demand hydrogen gas generation device as set forth in
claim 22 wherein the gas management electrode, gas impermeable and
liquid permeable layer, and gas permeable and liquid impermeable
layer are a single integrated component.
30. The on-demand gas generation device as set forth in claim 22
wherein the device further includes a porous separator disposed
between the anode and the gas impermeable and liquid permeable
layer.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This patent application claims priority from U.S.
Provisional Patent Application 60/951,618 filed on Jul. 24, 2007,
the entire contents of which are incorporated herein by
reference.
FIELD OF THE DISCLOSURE
[0002] The present disclosure generally relates to an on-demand
hydrogen gas generation device, suitable for use with a fuel cell,
which utilizes electrolysis, and more particularly galvanic
corrosion of one or more metals or metal alloys, and/or a chemical
hydride reaction, to produce hydrogen gas. The present disclosure
additionally relates to such a device that comprises a switching
mechanism that has an electrical current passing therethrough, and
that rapidly and repeatedly (or reversibly) moves between a first
position and a second position when exposed to a pressure
differential resulting from hydrogen gas generation, in order to
alter the rate at which hydrogen gas is generated, such that
hydrogen gas is generated on an as needed basis for a fuel cell in
communication therewith. The present disclosure additionally or
alternatively relates to such an on-demand hydrogen gas generation
device that comprises a gas management system designed to maximize
or optimize the generation or release of hydrogen gas, and in
particular to minimize the loss of electrolyte from the gas
generation device and thereby release substantially dry hydrogen
gas, therefrom once it has been formed, thus maximizing hydrogen
gas output. The present disclosure is still further directed to a
fuel cell comprising such an on-demand hydrogen gas generation
device, and in particular a fuel cell designed for small-scale
applications.
BACKGROUND OF THE INVENTION
[0003] Attempts to generate and utilize hydrogen gas as an energy
source have been extensively pursued for a number of years, at
least in part because hydrogen gas is considered by many to be the
fuel of the future due to its potentially abundant supply, and the
often non-polluting by-products (e.g., water) resulting from its
consumption in a fuel cell. Fuel cells utilizing hydrogen as fuel
also operate at significantly higher efficiencies than fuel cells
utilizing other fossil fuels such as methanol. Hydrogen is
therefore considered by many to be the ideal fuel. The generation
and storage of hydrogen however, has significant challenges, in
that it must be economically viable and safe to store and
transport, in order to make it a useful alternative to traditional
fuels. The desire to utilize hydrogen as an energy source extends
not only to largescale applications (e.g., automobiles, homes,
commercial buildings, etc.), but also to smaller-scale
applications, such as consumer electronics (e.g., cell phones,
personal digital assistants, laptop computers, etc.). Although both
uses have challenges associated with them, small-scale applications
(e.g., applications requiring 30 watts of power or less) are
unique, due particularly to (1) size restrictions (i.e.,
limitations on the maximum weight and/or dimensions of the device),
(2) weight limitations for portable applications, and (3) safety,
particularly for hand-held portable devices. These size and weight
restrictions increase the need to maximize both energy output of
the fuel cell and the fuel supply in order to avoid having the user
replace the hydrogen source too often. Pressure limitations are
also relevant to size restrictions, given that fuel cell anodes are
generally porous electrodes in intimate contact with relatively
thin polymeric membranes and the pressure of the hydrogen in the
fuel cell has to be relatively low (e.g., less than 30 psig) in
order to avoid rupture of the thin polymeric membranes. If the
hydrogen source is operating at a pressure too great for the fuel
cell where the hydrogen is consumed, a pressure regulator(s) may be
needed to reduce the hydrogen gas pressure before it enters the
fuel cell. Additionally, thicker/bulkier components such as a
portable pressure tank, and high pressure plumbing leading to the
fuel cell, may be required to contain hydrogen that is much above
atmospheric pressure. These components take up space that could
otherwise be used for the fuel cell, and more particularly the
hydrogen fuel itself. Furthermore, stored high pressure hydrogen is
potentially unsafe (particularly for air transportation), while a
hydrogen generator that only produces hydrogen when the fuel cell
is in operation minimizes the amount of stored hydrogen, making it
intrinsically safer.
[0004] The size and weight restrictions for portable applications
tend to limit the design options to "passive" systems (both fuel
cell and fuel generator), which are systems that do not require
pumps, flow meters, pressure regulators, etc. that are generally
acceptable in stationary applications. In addition, smallscale,
portable consumer applications are unique in that the demand for
energy is typically not continuous; that is, such applications may
experience long periods of "down time" where they are shut off or
are in "sleep mode" (i.e., on but not actively being used), and
therefore are not consuming much hydrogen. A hydrogen generator
capable of producing hydrogen on demand is ideally suited for these
applications such that it only depletes the fuel supply when the
fuel cell is in use. It is therefore recognized that a fuel cell
hydrogen source viable for small-scale portable applications
ideally is small and portable, and desirably has a passive mode of
operation supplying low pressure (i.e., less than 30 psig) hydrogen
"on-demand" (where at any given time the amount of stored hydrogen
is minimized).
[0005] Hydrogen gas may be supplied to the fuel cell in a number of
ways. For example, the fuel cell may be designed to simply possess
a holding vessel or tank, which may be directly charged or
pressurized with hydrogen (in liquid or gas form) from an external
source. Such an approach may not be preferred in some applications
or uses, however, because it requires a large and heavy or bulky
external hydrogen source to be available for recharging the fuel
cell. Additionally, the carrying around or transport of a device
which includes a container of pressurized hydrogen gas creates
undesirable safety concerns.
[0006] Another alternative may be a fuel cell that is fed by a
hydrogen gas generator that uses methanol as the hydrogen gas
source. More specifically, the generator may possess a small vessel
or tank of methanol that is catalytically reformed as needed to
produce hydrogen gas. However, such an approach may also not be
preferred in some applications or uses for a number of reasons,
including the fact that it requires the user to carry or transport
a device that includes a container of a flammable liquid.
[0007] In view of the various safety and/or convenience concerns
surrounding the use of fuel cells in small-scale applications,
hydrogen production by (1) the direct electrolysis of water using
electrical energy, (2) the electrolysis of water, or more generally
an aqueous solution, in a galvanic corrosion cell, and/or (3) the
reaction of water, or more generally an aqueous solution, with a
solid hydrogen source, such as a hydride salt, may be desirable
alternatives (provide one or more of these can be achieved in a
compact, safe system). The direct electrolysis of water using
electrical energy generally requires expensive catalysts as well as
a source of electrical energy. For a truly portable system that is
disconnected from the electrical grid, electrolysis of water may
also be achieved by utilizing the spontaneous galvanic corrosion
reaction of a reactive metal in combination with a hydrogen
generator catalyst where the water is reduced to hydrogen. In one
particular arrangement, an anode, made of, for example, a metal
such as zinc, and a positive electrode, made of for example a
non-consumable metal such as iron, are immersed in some aqueous
electrolytic solution and connected by an external circuit of some
kind. As further illustrated by the equations presented below, when
the circuit is closed, the anode material is oxidized and electrons
flow through the circuit to the positive electrode, which acts as
the hydrogen generator catalyst and therefore is not consumed,
where water is reduced to produce hydrogen gas and hydroxide
ions:
(anode reaction) M.fwdarw.M.sup.+2+2e.sup.-
(positive electrode reaction)
2H.sub.2O+2e.sup.-.fwdarw.2OH.sup.-+H.sub.2 (gas)
(overall reaction) M+nH.sub.2O.fwdarw.M[OH].sub.n+n/.sub.2H.sub.2
(gas).
Similarly, the generation of hydrogen by means of a chemical
reaction with a solid hydrogen source may also be achieved using a
number of different techniques and reagents. For example, in one
approach, water or an aqueous solution can be reacted with sodium
borohydride resulting in the production of hydrogen gas and sodium
borate in accordance with the following reaction:
NaBH.sub.4+2H.sub.2O.fwdarw.NaBO.sub.2+4H.sub.2 (gas).
Such an approach may, under certain circumstances, be desirable
over water electrolysis due, for example, to the increased yield or
output of hydrogen gas and the increased volumetric and gravimetric
energy density that may be achieved.
[0008] In addition to the above-noted challenges, it is desirable
for fuel cells, and particularly small-scale fuel cell
applications, to maximize their "energy density," or their energy
output per unit of volume of the fuel cell. A number of factors are
to be considered in this context, including for example: (1)
selection of the means by which the hydrogen gas is to be generated
in order to maximize hydrogen generation; (2) once generated,
maximizing hydrogen gas evolution or output from the source thereof
and to the fuel cell itself, and/or (3) limiting the number of fuel
cell and hydrogen gas generator components, in order to maximize
energy density (that is, it is desirable for the fuel cell and
hydrogen gas generator to occupy as little space as possible, in
order to maximize space for the fuel from which the hydrogen is to
be generated). Accordingly, as previously indicated, it is
desirable for the hydrogen gas generator to be passive; that is, it
is desirable for the hydrogen gas generator to generate and
transport hydrogen gas to the fuel cell without the need of
commonly used pressure regulators, pumps, fans, etc. In addition,
desirably the hydrogen gas generator is an "on-demand" generator;
that is, it generates hydrogen gas on an as-needed basis, thus
eliminating unnecessary consumption of the fuel from which the
hydrogen gas is generated (and thus more frequent re-fuelings),
and/or the need for a tank to hold the hydrogen gas for later
consumption. Furthermore, in addition to proper selection of the
means by which hydrogen gas is generated (i.e., selection of a
source which provides maximum hydrogen generation per unit volume),
a challenge exists to efficiently maximize the transport or
evolution of the hydrogen gas, once generated, out of the generator
and into the fuel cell for consumption, in order to maximize energy
output. Generation of hydrogen gas from a liquid or solid/liquid
system would normally result in entrainment and "carry-over" of
some liquid with the gas, depending on the rate of gas generation.
In most cases, the liquid is a corrosive acidic or alkaline fluid.
Since the hydrogen generator device is in communication with the
fuel cell and the plumbing leading up to the fuel cell, this
carry-over of corrosive liquid is highly undesirable. Additionally,
any loss of liquid from the generator device results in less
available liquid for hydrogen generation, which is also highly
undesirable. Finally, in small-scale, and/or low pressure fuel cell
devices, steps are desirably taken to ensure the hydrogen gas that
is generated and transported out of the hydrogen gas generator has
relatively low moisture content. This is because the fuel cell
anode reaction itself generates water that must be ejected
passively out of the fuel cell through the membrane and ultimately
out through the porous cathode. Any additional incoming moisture
with the fuel (e.g. hydrogen gas) only increases the burden on the
system, which can become a significant problem with small systems
used in portable consumer devices.
SUMMARY OF THE DISCLOSURE
[0009] Briefly, therefore, the present disclosure is directed to an
on-demand hydrogen gas generation device that comprises: (a) a cell
comprising a means for generating an average flow of hydrogen gas
of at least about 0.1 cubic centimeter/minute/cubic centimeter of
fuel volume for a period of time of at least about 1 hour; and, (b)
a switching mechanism in communication with the cell comprising the
hydrogen gas generation means, the switching mechanism regulating
the generation of hydrogen gas therein, the switching mechanism
comprising a moveable member that is operable to repeatedly and
reversibly move between a first position and a second position in
response to a pressure differential created by said cell of less
than 30 psig, wherein (1) in the first position an electrical
current passes through the switching mechanism which enables the
generation of hydrogen gas from said cell, and (2) in the second
position resistance in the switching mechanism to the electrical
current passing therethrough increases to reduce the rate of
hydrogen gas generation from said cell.
[0010] The present disclosure is further directed to such an
on-demand hydrogen gas generation device, wherein said means for
hydrogen gas generation comprises galvanic cell corrosion, or is
achieved thereby.
[0011] The present disclosure is additionally or alternatively
directed to such an on-demand hydrogen gas generation device,
wherein said means for hydrogen gas generation comprises, or
additionally comprises, a chemical hydride reaction, or is achieved
thereby. In particular, the present disclosure is directed to such
an on-demand hydrogen gas generation device that utilizes both
galvanic cell corrosion and a chemical hydride reaction to generate
hydrogen, wherein hydrogen generated by galvanic cell corrosion
exerts pneumatic pressure on an aqueous solution (e.g., acidic
aqueous solution) to force the solution into contact with a
chemical hydride reagent, the resulting reaction between the
aqueous solution and the chemical hydride resulting in the
formation of hydrogen gas.
[0012] The present disclosure is still further directed to an
on-demand hydrogen gas generation device comprising: (a) a first
chamber comprising a gas-generating electrochemical cell in
communication with a switching mechanism for regulating the
generation of gas within the gas-generating electrochemical cell,
the switching mechanism being operable in a range of from about 1
to about 30 psig; (b) a second chamber containing an aqueous
solution; (c) a third chamber containing a chemical hydride; and
(d) a conduit for allowing the aqueous solution contained in the
second chamber to flow into the third chamber.
[0013] The present disclosure is still further directed to an
on-demand hydrogen gas generation device comprising: (a) a
gas-generating cell comprising a galvanic cell that contains a zinc
chloride electrolyte in communication with a switching mechanism
for regulating the generation of gas from the gas generating cell,
the switching mechanism being operable in a range of from about 1
to about 30 psig; (b) a first chamber containing an aqueous
solution having a pH of less than about 7; (c) a second chamber
containing a complex borohydride; and (d) a conduit for allowing
the aqueous solution contained in the first chamber to flow into
the second chamber.
[0014] The present disclosure is still further directed to a method
of producing a stream of hydrogen gas from a chemical hydride
(e.g., a complex chemical hydride). The method comprises: (a)
activating a gas-generating electrochemical cell to produce a
stream of gas sufficient to force an aqueous solution through a
conduit connecting a first chamber and a second chamber, the first
chamber including the gas-generating electrochemical cell and the
aqueous solution; (b) forcing the aqueous solution through the
conduit and into the second chamber, the second chamber including a
chemical hydride; and, (c) reacting the aqueous solution and the
chemical hydride to form a stream of hydrogen.
[0015] The present disclosure is still further directed to an
on-demand hydrogen gas generation device comprising: (a) a
switching mechanism for regulating the generation of hydrogen gas
within the on demand hydrogen gas generation device, the switching
mechanism being operable in a range of from about 1 to about 30
psig; and (b) a gas management system for transporting hydrogen gas
out of the on-demand hydrogen gas generation device, the gas
management system comprising an anode, an electrolyte, a gas
management electrode comprising a conductive substrate and a
catalyst and having a first face and a second face, a gas
impermeable and liquid permeable hydrophilic layer, a gas permeable
and liquid impermeable hydrophobic layer and a gas exit region,
wherein the anode surrounds the gas impermeable and liquid
permeable hydrophilic layer, and further wherein the gas
impermeable and liquid permeable hydrophilic layer is disposed
between the anode and the first face of the gas management
electrode, the gas permeable and liquid impermeable hydrophobic
layer is disposed on the second face of the gas management
electrode and the gas exit region is interior of the gas permeable
and liquid impermeable hydrophobic layer.
[0016] The present disclosure is still further directed to an
on-demand hydrogen gas generation device comprising: (a) a
switching mechanism for regulating the generation of hydrogen gas
within the on-demand hydrogen gas generation device, the switching
mechanism being operable in a range of from about 1 to about 30
psig; and (b) a circular gas management system for transporting
hydrogen gas out of the on-demand hydrogen gas generation device,
the gas management system comprising an anode, an electrolyte, a
gas management electrode comprising a conductive substrate and a
catalyst and having a first face and a second face, a gas
impermeable and liquid permeable hydrophilic layer, a gas permeable
and liquid impermeable hydrophobic layer, wherein the gas
impermeable and liquid permeable layer surrounds the anode, the gas
management electrode surrounds the gas impermeable and liquid
permeable hydrophobic layer, and wherein the gas permeable and
liquid impermeable layer surrounds the gas management
electrode.
[0017] The present disclosure is still further directed to an
on-demand hydrogen gas generation device comprising: (a) a
switching mechanism for regulating the generation of hydrogen gas
within the on-demand hydrogen gas generation device, the switching
mechanism being operable in a range of from about 1 to about 30
psig; and (b) a gas management system for transporting hydrogen gas
out of the on-demand hydrogen gas generation device, the gas
management system comprising an anode, an electrolyte, a gas
management electrode comprising a conductive substrate and a
catalyst and having a first face and a second face, wherein the
first face comprises a gas impermeable and liquid permeable
hydrophilic layer and the second face comprises a gas permeable and
liquid impermeable hydrophobic layer, and a gas exit region,
wherein the first face is adjacent the anode and wherein the gas
exit region is positioned interior of the gas permeable and liquid
impermeable layer.
[0018] The present disclosure is still further directed to an
on-demand hydrogen gas generation device comprising: (a) a cell
comprising a means for generating an average flow of hydrogen gas
of at least about 0.1 cubic centimeter/minute/cubic centimeter of
fuel volume for a period of time of at least about 1 hour; (b) a
switching mechanism in communication with the cell comprising the
hydrogen gas generation means for regulating the generation of
hydrogen gas therein, the switching mechanism comprising a moveable
member that is operable to repeatedly move between a first position
and a second position in response to a pressure differential
created by said cell of less than 30 psig, wherein (1) in the first
position an electrical current passes through the switching
mechanism which enables the generation of hydrogen gas from said
cell, and (2) in the second position resistance in the switching
mechanism to the electrical current passing therethrough increases
to reduce the rate of hydrogen gas generation from said cell; and,
(c) a gas management system for transporting hydrogen gas out of
the cell comprising the means for generating hydrogen gas. In
particular, the gas management system comprises an anode, an
electrolyte, a gas management electrode comprising a conductive
substrate and a catalyst and having a first face and a second face,
a gas impermeable and liquid permeable hydrophilic layer, a gas
permeable and liquid impermeable hydrophobic layer and a gas exit
region, wherein (1) the anode and the gas management electrode are
in serial, electrical communication with and through the switching
mechanism, (2) the anode surrounds the gas impermeable and liquid
permeable hydrophilic layer, and (3) the gas impermeable and liquid
permeable hydrophilic layer is disposed between the anode and the
first face of the gas management electrode, the gas permeable and
liquid impermeable hydrophobic layer is disposed on the second face
of the gas management electrode and the gas exit region is interior
of the gas permeable and liquid impermeable hydrophobic layer.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] FIG. 1A is a cross-sectional, schematic view of one
embodiment of the on-demand hydrogen gas generation device of the
present disclosure, generally illustrating a switching mechanism
therein.
[0020] FIG. 1B is an expanded, cross-sectional view of the hydrogen
gas generation means from FIG. 1A.
[0021] FIG. 1C is an expanded, cross-sectional view of the positive
electrode assembly (also referred to herein as a "gas management
electrode") generally illustrated in FIG. 1B.
[0022] FIG. 2A is a more detailed, cross-sectional, schematic view
of the switching mechanism generally illustrated in FIG. 1A,
wherein said switch is in the closed position.
[0023] FIG. 2B is a more detailed, cross-sectional, schematic view
of the switching mechanism generally illustrated in FIG. 1A,
wherein said switch is substantially open (as compared to FIG.
2A).
[0024] FIG. 3A is a cross-sectional, schematic view of an
alternative embodiment of the switching mechanism of the present
disclosure, wherein said switch is in the closed position.
[0025] FIG. 3B is a cross-sectional, schematic view of an
alternative embodiment of the switching mechanism of the present
disclosure, wherein said switch is in the substantially open
position (as compared to FIG. 3A).
[0026] FIG. 4 is a cross-sectional, schematic view of an
alternative embodiment of the on-demand hydrogen gas generation
device of the present disclosure, and in particular as illustrated
in FIG. 1A, wherein the alternative switching embodiment
illustrated in FIGS. 3A and 3B is incorporated therein (the
switching mechanism of 3A and 3B being inserted into the negative
end of the can of 1A, replacing the switching mechanism and end cap
illustrated therein).
[0027] FIG. 5A is a schematic view of another alternative
embodiment of the on-demand hydrogen gas generation device of the
present disclosure, the device having dimensions similar to a
conventional prismatic electrochemical cell.
[0028] FIG. 5B is a cross-sectional, schematic view of the
prismatic, on-demand hydrogen gas generation device of FIG. 5A, the
cross-sectional view being along line X therein.
[0029] FIG. 5C is an expanded, cross-sectional view of the
prismatic, on-demand hydrogen gas generation device of FIG. 5B.
[0030] FIG. 6 is a cross-sectional, schematic view of an
alternative embodiment of the switching mechanism of the present
disclosure.
[0031] FIGS. 7 and 8 are cross-sectional, schematic illustrations
of on-demand hydrogen gas generators utilizing chemical
hydrides.
[0032] FIG. 9 is an exploded, cross-sectional schematic view of a
gas management system of the present disclosure.
[0033] FIGS. 10, 11 and 12 are cross-sectional, schematic
illustrations of alternative embodiments of the gas management
system of the present disclosure.
[0034] FIG. 13 is a graph which illustrates the results of
measurements taken comparing the voltage versus the current
response for the unplated nickel foam with the averaged results of
10 nickel plated, nickel foam samples.
[0035] FIG. 14A is a cross-sectional, schematic view of an
alternative embodiment of the on-demand hydrogen gas generation
device of the present disclosure having a pouch cell or prismatic
cell design.
[0036] FIG. 14B is an expanded, cross-sectional view of a portion
of the embodiment illustrated in FIG. 14A.
[0037] FIG. 15 is a graph which illustrates the hydrogen
performance of the hydrogen generation devices prepared and
detailed in Example 2.
[0038] FIG. 16 is a graph which illustrates the comparison of the
theoretical (calculated) hydrogen gas generation rate versus the
measured hydrogen gas generation rate of a device detailed in
Example 2.
[0039] It is to be noted that corresponding reference characters
indicate corresponding parts throughout the several views of the
drawings.
[0040] It is to be further noted that the design or configuration
of the components presented in these figures are not scale, and/or
are intended for purposes of illustration only. Accordingly, the
design or configuration of the components may be other than herein
described without departing from the intended scope of the present
disclosure. These figures should therefore not be viewed in a
limiting sense.
DETAILED DESCRIPTION OF THE INVENTION
[0041] In accordance with the present disclosure, an on-demand
hydrogen gas generation device, particularly well-suited for use in
small-scale fuel cell applications, has been developed. As further
detailed herein below, the on-demand hydrogen gas generation device
advantageously utilizes water electrolysis and, more particularly,
galvanic cell corrosion, a chemical hydride reaction, or both, to
produce the hydrogen gas for fuel cell consumption. As a result,
many of the safety concerns associated with the use of flammable
fuels for the generator (e.g., methanol), or the need to carry or
transport a tank of hydrogen gas, are eliminated.
[0042] Also as further detailed herein below, the on-demand
hydrogen gas generation device desirably generates and releases a
flow of hydrogen gas on a substantially as-needed basis by the fuel
cell, and desirably at a substantially constant pressure. These
features are achieved by means of a switching mechanism that
repeatedly or reversibly moves between a first position and a
second position in response to particular low pressure conditions,
and more specifically in response to a pressure differential of
less than 30 psig (pounds per square inch gauge), in order to (1)
alter or regulate the hydrogen gas generation reaction, such that
hydrogen gas is generated on a substantially as-needed basis for
the fuel cell connected thereto, and/or (2) ensure that a
substantially constant flow of hydrogen gas is generated and
released from the hydrogen gas generation device.
[0043] As still further detailed herein below, the present
disclosure additionally relates to such an on-demand hydrogen gas
generation device that comprises a gas management system that is
designed to maximize the release or evolution of hydrogen gas, and
in particular dry hydrogen gas, from the device once it has been
formed, thus maximizing hydrogen gas output therefrom.
[0044] In one or more of the above-noted embodiments, the on-demand
hydrogen gas generation device is orientation independent.
Additionally, or alternatively, in one or more of the above-noted
embodiments, the on-demand hydrogen gas generator device is
passive.
[0045] In this regard it is to be noted that, as used herein, the
following terms or phrases, or variations thereof, generally have
the following meanings: "small-scale" fuel cell applications
generally refers to fuel cells that produce, or have an output of,
less than about 30 watts of power; "on-demand" or "substantially
as-needed basis" generally refers to a hydrogen gas generation
device that produces or generates hydrogen gas when needed by the
fuel cell of which it is a part (or to which it is in communication
with), such as for example when the device receiving power from the
fuel cell is turned on or in the active mode, and therefore does
not simply produce or generate hydrogen once activated until all of
the hydrogen-generating fuel is consumed, thus optionally
eliminating or limiting the need for (i) a tank to hold the
hydrogen that is generated (for later consumption or use), and/or
(ii) the need for the ability to safely vent hydrogen gas when the
device is not in use; a switching mechanism operable under "low
pressure conditions" generally refers to a switch that moves when
exposed to a pressure differential of less than 30 pounds per
square inch gauge (psig); "substantially constant flow" of hydrogen
gas generally refers to a hydrogen gas generator that, when active
(i.e., when the reaction that generates the hydrogen gas is
occurring), is capable of releasing a flow of hydrogen gas at a
particular target pressure (e.g., about 5 psi, about 10, psi, about
15 psi, about 20 psi, or even about 25 psi) that varies over a
period of time (e.g., at least about 1 hour) by less than about 25%
(e.g., less than about 10%, less than about 8%, less than about 6%,
less than about 4%, or even less than about 2%), until
substantially all of the fuel from which the hydrogen gas is
generated is consumed or gone; "orientation independent" means that
the on-demand hydrogen gas generation device does not have to be
maintained in a particular position or orientation (e.g., in an
upright position) in order to be operational (that is, the device
is operational regardless of its orientation); "passive" means the
on-demand hydrogen gas generation device generates and transports
hydrogen gas to the fuel cell without the need of commonly used
pressure regulators, pumps, fans, etc.; and, "high energy density"
generally refers to an on-demand hydrogen gas generation device
that is capable of generating an average flow of hydrogen gas of at
least about 0.1 cubic centimeter/minute/cubic centimeter of fuel
volume for a defined period of time (e.g., at least about 1
hour).
[0046] Additionally, a switch mechanism that moves "rapidly"
generally refers to a switch movement from the first position to
the second position, and conversely from the second position to the
first position, that occurs in a time period that is at least fast
enough to enable control of the hydrogen pressure within the
variation percentage described above. This time period may be
optimized for the specific hydrogen generation application. For
example, for low fuel flow applications, this time period may be
minutes, but for most applications it would be less than 1 minute
(e.g., less than about 1 minute, less than about 30 seconds, less
than about 1 second, less than about 100 milliseconds, or even less
than about 10 milliseconds).
I. Hydrogen Gas Generation/Hydrogen Gas Generation Device
[0047] The present disclosure is generally directed to an on-demand
hydrogen gas generation device that, in particular, comprises a
cell containing or comprising a means for generating a flow of
hydrogen gas of at an average rate of at least about 0.1 cubic
centimeter/minute/fuel volume of the cell, and in various
embodiments may generate a flow of hydrogen gas at an average rate
of at least about 0.2, at least about 0.4, at least about 0.6, at
least about 0.8, at least about 1 cubic centimeters/minute/fuel
volume of the cell, or more, for a defined period of time (e.g., at
least about 30 minutes, at least about 60 minutes, at least about
90 minutes, at least about 120 minutes, or more). As further
detailed herein below, hydrogen generation may advantageously
utilize electrolysis, and more particularly galvanic cell
corrosion, a chemical hydride reaction, or both, to produce the
hydrogen gas. In one particular embodiment, the means for
generating hydrogen is coupled to or in communication with a
pressure responsive switching mechanism to regulate the generation
or formation of hydrogen gas therein.
[0048] In this regard it is to be noted that, as used herein, "fuel
volume" in the phrase "cubic centimeter/minute/fuel volume" refers
to the total volume of fuel used to generate hydrogen gas for
consumption by a fuel cell. For example, when hydrogen gas is
produced by galvanic cell corrosion, this phrase generally refers
to the anode fuel (i.e., the total of the volume of the anode of
the device, including active material, plus the total volume of the
electrolyte, plus the total volume of any other additive in the
anode) in the anode compartment (e.g., the space defined by a
separator when present) at the time of assembling the anode. In
contrast, when a chemical hydride is used to produce the hydrogen
gas, this phrase generally refers to the combined volume of the
aqueous solution and the chemical hydride reagents, prior to the
initiation of the hydrogen gas-generating reaction.
[0049] With respect to suitable designs of the hydrogen gas
generation means, or more generally suitable designs for the cell
comprising the hydrogen gas generation means, it is to be noted
that, in general (and as further detailed elsewhere herein), the
design is desirably one that maximizes or optimizes the rate of
hydrogen gas evolution, at least in part because the accumulation
of hydrogen gas in the pores of the positive electrode material may
result in substantial polarization. Additionally, when a galvanic
cell corrosion reaction is employed for hydrogen gas generation, it
is desirable for the cell to be designed in a way that accounts for
the reduction in electrolyte volume during operation (the
electrolyte being consumed in the reaction). Furthermore, when a
galvanic cell corrosion reaction is employed, it is to be noted
that at high rates of hydrogen gas evolution a portion of the
electrolyte may be carried away by the hydrogen gas stream. It is
therefore desirable to design the cell in a way that minimizes this
loss of electrolyte, such as for example by including a membrane in
the cell that is permeable to hydrogen but impermeable to water
and/or the electrolyte (as further detailed elsewhere herein).
Ideally, it is desirable to design a gas management system wherein
there is increasing hydrophobicity in the direction away from the
reaction zone and toward the gas exit from the electrode
system.
[0050] In one particular embodiment, the on-demand hydrogen gas
generation device comprises a hydrogen gas generation means that is
in the form of a known alkaline cell "bobbin" design, wherein the
cell is generally cylindrical in shape and a cylindrical surface
separates the anode from the positive electrode therein. However,
because it is desirable to minimize positive electrode volume and
maximize anode volume, in order to maximize "fuel volume" (as
further detailed elsewhere herein), the positive electrode used in
accordance with the present disclosure in such a cell design is
generally much thinner than the positive electrode used in a
conventional alkaline cell design (or more generally a cell
utilizing galvanic cell corrosion), while the anode may be much
thicker. A thicker anode is not necessarily desirable, however,
because this may compromise discharge efficiency. Accordingly, as
further detailed elsewhere herein, in order to have a cell that
possess a high discharge rate, as well as a highly efficient
discharge, a design that maximizes the interfacial surface area
between the anode and the cathode is desirable. Since the anode gel
is highly flexible or pliable, and since the positive electrode is
also a flexible material (due, for example, to the use of high
surface area fiber, particles, cloth, etc. therein), cell designs
that possess high discharge rate and efficiency include, for
example, those wherein the anode and positive electrode are placed
in a separator bag, and then (i) wound (into a "jelly-roll" like
shape), (ii) folded into a "Z"-like shape, or (iii) folded into a
"S"-like shape, and then inserted into a cell cavity. Additionally,
electrolyte may be added to the cavity after insertion of this
separator bag. Other approaches to maximize hydrogen generation
rates and fuel consumption efficiency include thin prismatic cell
designs (see, e.g., the prismatic cell designs illustrated and
described in the working examples).
[0051] A. Switching Mechanism
[0052] In accordance with one or more of the various embodiments of
the present disclosure, a pressure-responsive switching mechanism
is employed in combination with a means for generating hydrogen gas
in order to regulate hydrogen gas production (i.e., to provide
hydrogen gas "on-demand"). More particularly, the switch mechanism
is in electrical and physical communication with the hydrogen gas
generation means; that is, the switching mechanism is in electrical
series within a galvanic cell circuit, from which the hydrogen gas
is generated. The switching mechanism is desirably activated (i.e.,
in a closed position) at the time of hydrogen gas generation device
manufacture, and is in a serial electrical relationship with an
external circuit (e.g., as described further elsewhere herein). At
the time of assembly, the external circuit is in an "open" position
and is desirably activated (or closed) at the time of use, as
described elsewhere, thereby allowing the galvanic corrosion
reaction to commence and current to flow in the external circuit
through the switching mechanism. When the external circuit is
activated, the switching mechanism repeatedly and reversibly moves
from a first (e.g., closed, or substantially closed) position to a
second (e.g., partially or fully open position) in response to a
pressure differential that is created as a result of one side of
the switching mechanism being exposed to atmospheric pressure (this
side, for example, being sealed or vented to the atmosphere), while
the other side is exposed to a pressure in excess of atmospheric
pressure resulting from hydrogen gas generation. Once this pressure
differential exceeds some predetermined threshold, the switching
mechanism moves (e.g., downstream, away from the hydrogen gas
source) in response thereto from a first position to a second
position. This movement results in an increase in the resistance
within the switching mechanism to the electrical current passing
therethrough, the same electrical current that controls the
hydrogen gas generation reaction rate. As a result, the rate of
hydrogen gas generation decreases as the resistance of the
switching mechanism to the electrical current increases. Once the
resistance is sufficiently high (e.g., the switching mechanism
becomes fully opened), the flow of electrical current, and thus
hydrogen gas generation, will stop. However, as the pressure
differential decreases (as a result of, for example, dissipation of
hydrogen gas by venting to a fuel cell where it is consumed), the
switching mechanism will move to or toward (e.g., upstream, toward
the hydrogen gas source) its original position (i.e., from the
second position to or toward the first position). Resistance will
therefore decrease and the flow of electrical current, and thus
hydrogen gas generation, will increase or begin once again.
[0053] The switching mechanism, in one or more embodiments (as
further detailed elsewhere herein), is constructed of or comprises
one or more components (e.g., a moveable (e.g., flexible) member
and one or more electrically conductive members, which may be
stand-alone or individual parts of the switching mechanism or may
be integrated with or part of the moveable member) that, in
combination, form electrical contact points for the flow of an
electrical current therethrough. Additionally, the switching
mechanism may optionally comprise one or more springs that contact
the moveable member, to add further elastic strength to the
switching mechanism (e.g., to ensure the switching mechanism
responds to the desired pressure differential). When present, the
springs may be comprised or made of essentially the same types of
materials as the moveable member (the spring and moveable member in
one or more embodiments having essentially the same composition,
while in alternative embodiments having different
compositions).
[0054] The switching mechanism, and more particularly one or more
components thereof (e.g., a moveable member, a first conductive
contact, a second conductive contact, a spring, etc.), may be
constructed of materials that enable an electrical current to flow
therethrough, and/or that is sufficiently moveable or flexible so
as to be moved or flexed from one position to another as the noted
pressure differential increases or decreases. For example, the
moveable member may be made of, or comprise, essentially any
material that will undergo a sufficient elastic deformation (e.g.,
moves or flexes a sufficient distance) in response to the load
created by the pressure differential, and more particularly will
sufficiently deform in response to a load created by the pressure
differential, and more particularly will sufficiently deform in
response to a predetermined pressure differential while remaining
within the elastic range of the material (so as, for example, to
return to its initial form, or substantially its initial form, once
the pressure differential is removed or substantially decreased).
Additionally, in one or more embodiments, the material desirably
(i) provides sufficient force (e.g., spring force) so as to
maintain electrical contacts within the switching mechanism when
there is essentially no pressure differential (i.e., the pressure
on both sides of the switching mechanism is substantially the same)
and/or when the pressure differential is less than the
predetermined value, but that (ii) sufficiently deforms or deflects
to allow the contacts to open or separate when the pressure
differential reaches a pre-determined value. Finally, the
components and materials of the switching mechanism are also
desirably selected to avoid excessive heating, as a result of the
electrical current flowing therethrough.
[0055] In view of the foregoing, the moveable member, and
optionally one or more of the other switching mechanism components
(such as the spring, when present), may be fabricated from various
known metals, plastics (e.g., conductive plastics), elastomers
(e.g., conductive elastomers), etc., that are sufficiently elastic
or flexible between the temperatures of about -30.degree. C. and
about 85.degree. C. or more. For example, in one exemplary
embodiment, the moveable member and/or spring is selected from a
plastic material, which may or may not contain standard additives
or fillers, that has a heat deflection temperature of at least
about 85.degree. C. at about 264 psi, and/or from a moldable,
castable or extrudable plastic. Exemplary plastic materials
suitable for use in the present disclosure include polypropylene,
polyethylene and nylon, as well as elastomers such as EPDM,
neoprene, nitrile, viton, natural rubber, and silicon. In an
alternative embodiment, the moveable member and/or spring is
selected from a metal or metal alloy such as steel, stainless
steel, nickel, cooper or aluminum, the metals optionally having a
metal plating, coating or surface treatment thereon. Additionally,
depending on the locations of these materials within the on-demand
hydrogen generation device, it is to be noted that they may be
exposed to the various chemicals used in or resulting from the
hydrogen gas generation reaction (e.g., located on the hydrogen gas
generation side of the switch, rather than the atmospheric side
thereof), and therefore are desirably selected to possess suitable
chemical compatibility (e.g., stability) therewith. Furthermore,
various known plating materials and methods may be used to enhance
conductivity of, for example, various contact surfaces or to
prevent corrosion.
[0056] In this regard it is to be noted that the moveable member,
the first conductive contact and/or the second conductive contact
may optionally have a resistive coating on a surface thereof (the
coating being comprised of a material generally known in the art,
and applied to the particular surface using means generally known
in the art). Such a coating may be used, for example, to design a
switch that can change resistance in controlled increments over
short distances. Such designs may be used to reduce variability in
the hydrogen pressure level maintained by the switch. Such designs
may also be used to aid in the overall design of thin or low
profile switching mechanisms.
[0057] The components of the switching mechanism, including for
example the moveable member and the spring (when present) may be
fabricated (e.g., cut, cast, molded, extruded, rolled, stamped,
punched or created) using essentially any mechanical process known
in the art. Additionally, the form or dimensions of the component
are generally dictated, at least in part, by the overall dimensions
of the on-demand hydrogen gas generation device and/or the hydrogen
gas output pressure requirements of the fuel cell with which it is
to be used. For example, as further illustrated elsewhere herein,
the moveable member may be in the form of a square, rectangular or
circular sheet, disk or washer (i.e., having a centrally located
hole therein). The thickness may also be used, in combination with
the composition thereof, to affect the pressure differential that
causes the moveable member to move or flex. For example, in one or
more embodiments of the present disclosure, the moveable member may
have a fixed thickness ranging from between about 0.01 mm and about
2.5 cm, about 0.025 mm and about 1.5 cm, about 0.1 mm and about 1
cm, or about 0.25 mm and about 0.5 cm.
[0058] It is to be noted that the various embodiments of the
switching mechanism, and more generally the on-demand hydrogen gas
generation device, illustrated herein are exemplary and therefore
should not be viewed in a limiting sense. For example, the
switching mechanism may comprise one or more electrical contact
points or poles. Furthermore, these contacts may be on the
atmospheric side or the hydrogen gas side of the switching
mechanism. More specifically, although the various embodiments
illustrated herein provide for the electrical contact in the
switching mechanism to be on the atmospheric side of the switch, it
may also be on the hydrogen gas side of the switching mechanism.
However, it is typically desirable for this contact to be on the
atmospheric side, at least in part because this acts to isolate the
contact from possible chemical attack and/or to isolate the switch
function from the hydrogen gas atmosphere.
[0059] It is to be further noted that, as detailed elsewhere
herein, the on-demand hydrogen gas generation device may comprise a
switching mechanism that is integrated with the hydrogen gas
generation means, or it may be a stand-alone or low-profile switch
that is part of a hydrogen gas passageway that is linked to one or
more hydrogen gas generation means. Additionally, or alternatively,
multiple switching mechanisms may be used in the on-demand hydrogen
gas generation device, for example in series to provide redundancy
for safety purposes. The on-demand hydrogen gas generation device
may also be constructed such that the hydrogen gas outlet vent
(which leads to the fuel cell to which the device is connected) is
independent of the switch, or alternatively the outlet vent may be
integrated with the switch.
[0060] It is to be still further noted that the switching mechanism
of the on-demand hydrogen gas generation device may be a first
switching mechanism that is in electrical communication with a
second switching mechanism (e.g., a switching mechanism of a
hydrogen gas consuming device, such as a fuel cell). Generation of
hydrogen gas by the on-demand hydrogen gas generation device (and
more particularly the hydrogen gas generation means thereof) is
initiated by allowing an electrical current to pass through the
second switching mechanism and to the first switching mechanism. In
this way, the on-demand hydrogen gas generation device may be
activated, to initiate hydrogen gas generation, by electrically
shorting the device through an external circuit (of, for example,
the fuel cell) that comprises the second switching mechanism. In
such an embodiment, the first conductive element or the second
conductive element of the first switching mechanism may be a part
of the second switching mechanism and therefore may be manufactured
and shipped in an inactive state where it contains substantially
little or no hydrogen.
[0061] Finally, it is to be noted that the resistance of the
hydrogen generation device series circuit (including the generator
cell, the first switching mechanism, the second switching mechanism
(if included), and all electrical interconnections) generally
dictates the current flow through the generator, since current is
substantially proportional to hydrogen generation, and hence
controls the rate of hydrogen generation. The sum of these
resistances is therefore carefully evaluated during the design of
the generator and adjusted to generate the desired current. In some
cases, additional resistance may be added to the external portion
of the series circuit in the form of a fixed resistance to reduce
the current and thus the rate of hydrogen gas generation. (For
example, a fixed resistance of about 0.3 ohms was added to the
external series circuits to control the rate of hydrogen generation
during testing of some the various designs discussed as examples
elsewhere herein. The external resistance was, in those instances,
applied using typical battery test equipment.) Optionally, the
current flow through the hydrogen generation device series circuit
may be initiated using external circuitry (e.g., a second switching
mechanism). In such an embodiment, the first conductive element or
the second conductive element of the first switching mechanism may
be a part of the second switching mechanism. Additionally, the
second switching mechanism may be incorporated in the gas outlet
interface between the hydrogen generating device and the fuel
cell.
[0062] In some situations the switch may be located in the fuel
cell housing or the interface plumbing area rather than in the
hydrogen generator itself. This is particularly true if the
generator is a low cost, disposable device, in which case the
relatively expensive switch is desirably located in a portion of
the system that is non-disposable.
[0063] B. Galvanic Cell Corrosion with Switching Mechanism
[0064] In one embodiment of the present disclosure, hydrogen gas is
formed in the on-demand hydrogen gas generation device by the
electrolysis of water, or more generally by a standard galvanic
cell corrosion reaction. In one particular arrangement, the device
comprises a cell that comprises a means for generating a flow of
hydrogen gas, and more particularly comprises an anode (or anode
material) and a positive electrode (or cathode) material, which are
in fluid communication with each other (and optionally separated by
an ionically conductive and electronically insulating separation
material). The positive electrode is in contact with an aqueous
electrolytic solution within the cell, and furthermore is
electrically connected (i.e., in an electrical series relationship)
to the anode by a circuit that passes through a switching mechanism
that is in electrical, as well as physical or mechanical,
communication with the cell. The switching mechanism comprises a
moveable (e.g., flexible) member that is operable to repeatedly or
reversibly move between a first and second position in response to
a pressure differential created by the hydrogen gas generation
means (or more generally the cell comprising the hydrogen gas
generation means) of less than 30 psig (e.g., less than about 25
psig, less than about 20 psig, less than about 15 psig, or even
less than about 10 psig) and typically greater than about 1 psig
(e.g., greater than about 2 psig, greater than about 3 psig,
greater than about 4 psig, greater than about 5 psig or more), the
pressure differential, for example, being in the range of from
about 1 psig to 30 psig, or from about 2 psig to about 20 psig, or
from about 3 psig to about 10 psig. In one or more embodiments, the
moveable member may be operable to move downstream (or away from
the high pressure side of the pressure differential) when the
pressure differential exceeds a predetermined value of less than 30
psig (e.g., less than about 25 psig, less than about 20 psig, less
than about 15 psig, less than about 10 psig, less than about 5
psig, or less) and to move upstream (or toward the high pressure
side) when the pressure differential falls below this same
predetermined value.
[0065] Once the generator is activated (as detailed elsewhere
herein), in the first position of the switching mechanism, the
circuit between the anode and positive electrode is essentially
fully closed, thus allowing an electrical current to pass through
the switching mechanism while experiencing as little resistance as
possible therein, thus enabling the maximum rate of reaction at the
positive electrode, and therefore hydrogen production, to occur. In
the second position, the moveable member acts to at least partially
open the circuit that connects the anode and positive electrode,
thus increasing the resistance in the switching mechanism to the
electrical current passing therethrough. As a result, the rate of
reaction at the positive electrode, and thus hydrogen production,
is decreased. If the pressure differential is sufficiently high,
the moveable member will be moved or displaced far enough from the
first position to effectively fully open the circuit that connects
the anode and positive electrode; as a result, the resistance to
the electrical current will be sufficiently great such that the
reaction at the positive electrode is effectively stopped, thus
stopping hydrogen gas production as well.
[0066] Generally speaking, the rate of hydrogen gas formation or
evolution in such a cell (i.e., a cell comprising a reactive metal
anode and a catalytic positive electrode for hydrogen gas
formation) is proportional to the current flowing between the anode
and positive electrode, which in turn is a function of the sum of
the internal and external resistance in the electrical circuit
which connects the anode and positive electrode. For example,
experience to-date suggests that in an AA sized cell a continuous
current of about 1.5 Amps needs to be sustained to reach a rate of
about 10 cc of hydrogen gas generation per minute. Design of the
cell desirably provides a high rate capability that is able to
sustain the high hydrogen gas generation rate. This generally
means: (i) the anode is designed or selected to provide a high
discharge efficiency at a high continuous current, the cell having
an optimum molar ratio of water to, for example, zinc (or, more
generally, an optimum (water)/(anode active) molar ratio); (ii) the
positive electrode has low hydrogen gas over-voltage and a high
surface area; (iii) the electrolyte is selected to provide high
ionic conductivity; (iv) a large anode-positive electrode
interfacial area is present, the (interfacial area)/(anode volume)
or (interfacial area)/(zinc volume) ratio is optimized; and, (v)
the separator is selected such that a high-rate discharge can be
sustained, utilizing its property of high water transport rate. The
interfacial area is defined as the geometric area of the positive
electrode (or cathode) that is directly against the anode surface.
Additionally, and as further detailed elsewhere herein, desirably
the hydrogen gas generation device is designed to provide proper
gas transport therein, in order for example to maximize output and
minimize the accumulation of hydrogen gas in the pores of the
positive electrode (which may lead for example to substantial
polarization); that is, the hydrogen gas generation device
desirably includes a gas management system, as further detailed
elsewhere herein. Finally, it is desirable to minimize positive
electrode volume, in order to maximize the volume of "fuel," and
more specifically in this embodiment the anode fuel (e.g., the
volume of the anode active material, such as zinc, plus the total
volume of the electrolyte, such as water) in the cell, which will
be consumed during operation of the cell, in order to form or
generate hydrogen gas.
[0067] Referring now to FIGS. 1A through 1C, one exemplary
embodiment of an on-demand hydrogen gas generation device 10 is
provided in accordance with the present disclosure. In particular,
the device 10, has an axially extending positive outer shell or can
12, a first or positive end (indicated generally at 14) that has
one or more holes 16 therein to allow the venting of hydrogen gas
from inside the can (which is connected by a passageway or conduit
of some kind (not illustrated) to the hydrogen gas-consuming device
or fuel cell (also not illustrated)), and a second or negative end
(indicated generally at 18) that is generally opposite and disposed
generally axially downstream of the positive, vented end 14, and a
cylindrical sidewall 19 between the positive and negative ends 14
and 18. The negative end 18 of the can 12 is closed by means of a
first negative end cap, 20, the first negative end cap and the
negative can end being adapted in size and shape such that the
first negative end cap and the negative can end are sealingly
connected (by, for example, crimping the negative end of the can
over the end cap and a flexible seal or gasket 21, which may or may
not be integrated with a switching mechanism as further detailed
elsewhere herein) during assembly, by means generally known in the
art.
[0068] Within the volume or space defined by the inside of the can
12 is a means for generating hydrogen (or a hydrogen gas generation
means), indicated generally at 22, that comprises an anode 24 and a
negative current collector 26 in contact therewith, and a
cylindrical positive electrode or electrode assembly 28 (which in
one or more embodiments may be referred to elsewhere herein as a
gas management electrode), which substantially surrounds the anode
and negative current collector. In general, and as detailed
elsewhere herein, the anode comprises or is selected from a
material with low thermodynamic nobility, such as for example zinc,
magnesium, aluminum, titanium, and combinations thereof. The amount
of anode active agent or material generally included in the anode
is less than about 80% by weight of the total weight of the anode
components. The total weight of the anode components includes the
weight of each component making up the anode such as, for example,
anode active agent, gelling agent, surfactant, alloying agent,
electrolyte, etc. Desirably, the amount of anode active agent is
from about 60% by weight to about 75% by weight, and more desirably
from about 67% by weight to about 71% by weight. The anode active
agent, such as zinc, can be present in the anode in the form of
particles, fines, or dust, for example. Also, combinations of these
forms may be utilized.
[0069] Conventional zinc powders contain particles having a wide
distribution of particle sizes ranging from a few microns to about
1000 microns, with most of the particle size distribution (PSD)
ranging between about 25 microns and about 500 microns. Therefore,
in order to achieve proper discharge of such conventional zinc
powders, a potassium hydroxide (KOH) concentration above about 34%
is conventionally used and necessary. While such concentrations of
electrolyte are still applicable and effective, in a hydrogen gas
generating device which involves consumption of the water from the
electrolyte, it is desirable to use more dilute electrolyte at
assembly, since the electrolyte will become more concentrated as
the water is consumed during hydrogen generation. Zinc powders with
conventional PSD are sensitive to low electrolyte concentrations
and can passivate easily. Therefore, use of the narrower PSDs
disclosed herein allows use of more dilute electrolytes without
premature passivation. A potassium hydroxide concentration of less
than about 35% (for example between about 20% and about 35%
potassium hydroxide concentration) may be desirable in various
embodiments.
[0070] With respect to the anode material, it is to be noted that
physical modifications thereof can also improve service life,
either alone or in combination with chemical modifications noted
elsewhere herein (or by means generally known in the art). For
example, in cells having a low concentration of hydroxide ions, the
hydrogen-generating reaction therein may be improved by reducing
diffusion resistance for the hydroxide ions. This can be
accomplished, for example, by adjusting the particle size
distribution to provide in the anode a narrow distribution of
similar zinc particle sizes, thereby enhancing porosity (diffusion
paths) for the hydroxide ion transport. In addition to improving
diffusion properties, controlling the particle size distribution
also provide the porosity sites for the precipitation of, for
example, ZnO, thereby delaying anode passivation. A narrow particle
size distribution (as described, for example in U.S. Patent
Application Publication No. 2005/0079415, the entire contents of
which are incorporated for all consistent and relevant purposes)
allows the use of electrolyte concentrations significantly lower
than in conventional alkaline cells. This approach is effective for
use in the anodes of different types of cells, including for
example zinc manganese dioxide and zinc-air alkaline cells, and can
be used alone or in combination with other improvements disclosed
herein (or generally known in the art). Similar advantages have
been observed when used in a zinc/Ni hydrogen generator cell as
detailed in this disclosure. One exemplary anode material (e.g.,
zinc) particle size distribution that may be suitable for one or
more embodiments of the present disclosure is one in which at least
about 70% of the particles have a standard mesh-sieved particle
size within about a 100 micron size range and in which the mode of
the distribution is between about 100 and about 300 microns. More
particularly, particle size distributions meeting the above-noted
tests and having a mode at about 100 microns, or at about 150
microns, or at about 200 microns, each plus or minus about 10%, may
be advantageously used in the present disclosure. It may be
particularly desirably that about 70% of the particles be
distributed in a size distribution range even more narrow than
about 100 microns (e.g., about 80 microns, about 60 microns, about
40 microns, or less).
[0071] In some embodiments of the present disclosure, the anode of
the on-demand hydrogen gas generation device may be prone to
various undesirable corrosion reactions when stored at or above
room temperature prior to use. It is to be noted that hydrogen
generation in the anode compartment is highly undesirable,
particularly during storage and transportation prior to use. Even
during use, the hydrogen generation is desirably confined to that
occurring on the cathode surface. The type of electrolyte in the
anode (e.g., an alkaline solution such as KOH) may corrode the zinc
(or other anode active agent) upon contact, forming oxidized zinc
products that decrease the availability of active zinc while
simultaneously also generating unwanted hydrogen gas in the anode
compartment. The rate of corrosion generally increases as the
storage temperature rises and can lead to a dramatic decrease in
anode active agent capacity. Gas generated in such reactions can
increase pressure in the anode, cause electrolyte leakage and
disrupt the device integrity. The rate at which the unwanted gas is
generated at the anode active surface accelerates when the anode
active material is partially discharged or consumed, thereby
decreasing the resistance of the anode to electrolyte corrosion.
The corrosion reactions that lead to gas evolution involve cathodic
and anodic sites on the anode active surface. Such sites can
include surface and bulk metal impurities, surface lattice
features, grain boundary features, lattice defects, point defects,
and inclusions.
[0072] To minimize undesirable corrosion and anode gassing during
storage, it is typical to employ corrosion-resistant zinc alloys
and to reduce the extent of impurities in the anode. A suitable
zinc powder (or other anode active) can be alloyed with one or more
of indium, bismuth, calcium, aluminum, lead, phosphorous, etc. A
particularly suitable alloying agent for minimizing gassing is
bismuth. Typically, alloy powders can include from about 0.01% to
about 0.5% by weight alloy agent alone, or in combination with,
from about 0.005% to about 0.2% by weight of a second alloying
agent, such as lithium, calcium, aluminum, and the like.
[0073] To further minimize undesirable corrosion and anode gassing
during storage as described above, it is typical to add organic
surfactants and inorganic corrosion-inhibiting agents to the anode.
Surfactants act at the anode-electrolyte interface by forming a
film that protects the anode active surface from the electrolyte
during storage. The inhibitive efficiency of surfactants to
increase the corrosion resistance of the anode active depends on
their chemical structure, concentration, and their stability in the
electrolyte. Among the surfactants known to be effective at
controlling gassing are organic phosphate esters such as the
ethylene oxide-adduct type disclosed by Rossler et al. (in U.S.
Pat. No. 4,195,120). Additionally, in U.S. Pat. No. 4,777,100,
Chalilpoyil et al. disclosed an anode containing single crystal
zinc particles with a surface-active heteropolar ethylene oxide
additive including organic phosphate esters. Specifically,
commercially available surfactants such as Rhodafac RM-510,
Rhodafac RA-600, Witconate 1840X, and Mafo 13 MOD1 are suitable
surfactants for use in the present disclosure, as described in for
example U.S. Pat. Nos. 6,872,489 and 7,226,696 (the entire contents
of which are incorporated herein by reference for all relevant and
consistent purposes).
[0074] The anode as described herein will generally include a
gelling agent to help suspend the anode active material throughout
the electrolyte to allow for the anode active to more fully react.
Essentially any gelling agent known in the art, which is suitably
or sufficiently compatible with the other components in the anode,
may be used in accordance with the present disclosure. Examples of
suitable gelling agents include polyacrylic acids, grafted starch
materials, salts of polyacrylic acids, polyacrylates,
carboxymethylcellulose, or combinations thereof. Examples of
suitable polyacrylic acids include Carbopol 940 and 934 (available
from Noveon) and Polygen 4P (available from 3V). An example of a
grafted starch material is Waterlock A221 (available from Grain
Processing Corporation). An example of a salt of a polyacrylic acid
is Alcosorb G1 (available from Ciba Specialties).
[0075] In this regard it is to be noted that, in order to optimize
or maximize the rate capability and/or discharge efficiency, the
anode is desirably designed to sustain a high discharge current
without passivation. As a result, the anode is desirably optimized
for continuous discharge with high discharge currents, or for
intermittent discharge with longer continuous discharge periods
than about one hour (which is typical for an alkaline cell).
Factors that influence the discharge performance or behavior
include, but are not limited to, the surface area of the anode
active material (e.g., zinc surface area), the particle size
thereof, the particle size distribution thereof, the loading
thereof, the electrolyte concentration, and/or the nature and
amount of the surfactant and/or gelling agent present therein.
(See, e.g., U.S. Pat. No. 7,226,696, the entire contents of which
are incorporated herein by reference for all relevant and
consistent purposes.) Additionally, as previously noted, the anode
is also desirably designed to contain the maximum amount of anode
active material (e.g., zinc) possible because, generally speaking,
this is fuel and therefore it directly affects how much hydrogen is
produced.
[0076] The proper choice of zinc powder PSD enables the use of
lower electrolyte concentrations without the premature passivation
that would otherwise occur with conventional zinc PSDs. In
particular, passivation generally occurs in electrochemical cells
when the anodic reaction produces zinc oxide, which covers the
remaining zinc in the anode, thereby preventing the KOH from
accessing and reacting with the remaining zinc. It is well known
that conventional MnO2 alkaline cell anodes having conventional PSD
prematurely passivate when lower electrolyte concentrations are
used. Conventional anode particle sizes are distributed between
45-500 microns, thus within a broad range of 455 microns, rather
than a narrow range of 100 to 150 microns that is envisaged by the
present inventors.
[0077] Alternatively, the zinc PSDs disclosed herein desirably can
be distributed within a narrow window of 200 microns and,
alternatively, 150 microns, meaning that between and including 90%
and 95%, and up to 100%, of the particle sizes, by weight, are
within the 150, or 200, micron window, and in particular are tight
distributions substantially centered around 100 .mu.m, 175 .mu.m,
and 250 .mu.m, and 300 .mu.m (meaning that between and including
about 90% and about 95%, and up to about 100% of the zinc particles
have particle sizes centered around the specified sizes). One
skilled in the art will recognize that mesh sizes corresponding to
these particle sizes can be identified using ASTM Designation:
B214-99.
[0078] In addition to the anode active material (and an alloying
agent, if present), the anode additionally includes an electrolyte
therein, which provides water for the galvanic corrosion reaction
and which facilitates ionic transfer between the anode and the
positive electrode. With respect to the electrolyte, it is to be
noted that essentially any electrolyte known to be suitable for use
in a galvanic cell corrosion reaction may be used in accordance
with the present disclosure. Typically, however, the electrolyte is
desirably selected from those materials that possess high ionic
conductivity, in order to provide the desired rate capability.
Additionally, the composition of the electrolyte is desirably
selected in view of the recognition that a substantial amount of
water is consumed during the hydrogen gas-generating reaction at
the positive electrode. As a result, the initial volume of the
electrolyte, like the anode active material, is desirably maximized
within the on-demand hydrogen gas generation device (because of the
water consumption, as well as because some of the electrolyte may
potentially be carried away with the hydrogen gas stream at high
rates of gas evolution). Common electrolytes suitable for use in
the present disclosure therefore include, for example, solutions
comprising hydroxide, chloride or acetate ions. Typically, the
electrolyte is a potassium hydroxide or sodium hydroxide solution.
However, in some cases, the electrolyte may contain dissolved
salts, oxides or hydroxides of bismuth, tin, indium, mercury, lead,
cadmium, or thallium. Additionally, the electrolyte may include a
dissolved cation or anion of the metal anode (e.g., an aluminum
oxide, sodium aluminate, potassium aluminate, a zinc oxide, a zinc
hydroxide, or calcium salts). In some embodiments, the electrolyte
may additionally contain a corrosion inhibitor such as a quaternary
ammonium salt, or a non-ionic, anionic, or cationic surfactant.
[0079] When potassium hydroxide is utilized as the electrolyte, the
concentration of potassium hydroxide may typically be from about
15% by weight to about 45% by weight, and desirably from about 20%
by weight to about 35% by weight. Generally, when zinc is the anode
active agent, the electrolyte may include a small amount of zinc
oxide to retard open circuit corrosion and stabilize the zinc
surface and reduce gassing. The amount of zinc oxide may be from
about 0.1% by weight of the anode to about 2% by weight of the
anode. The stoichiometric water/zinc molar ratio is about 2.
However, depending on the hydrogen generation rates needed and the
cell design used, it has been found that the water to zinc molar
ratio may typically range from about 1.4 to about 2.5, and
desirably from about 1.6 to about 2. One skilled in the art will
therefore recognize that similar ratios can be determined for other
fuel materials such as, for example, aluminum, magnesium, etc.
[0080] The Interfacial Area/Zinc Volume ratio has a strong
influence on the rate of generation of hydrogen gas and on fuel
(zinc) utilization efficiency. The cell design and electrode design
generally dictate this ratio, since a higher ratio generally means
that the zinc (anode) layer is thinner. A thinner anode layer is
generally capable of higher rate discharge and also higher
efficiency of utilization. The ratio can typically range from about
5 to about 60, and desirably from about 10 to about 50. The choice
of the appropriate ratio to use also depends upon other factors,
such as for example the hydrogen capacity (i.e., the total amount
of hydrogen needed from a certain volume of generator), which is
dictated by the amount of zinc. For a fixed volume of generator,
too high an interfacial area-to-zinc volume ratio will result in an
inadequate amount of zinc in the generator to supply the needed
hydrogen gas capacity. Generally, a higher ratio implies a lower
hydrogen capacity in a fixed volume. For low rates of gas
generation, a low ratio is adequate, providing high hydrogen
capacity.
[0081] Referring again to FIG. 1C, and as further detailed
elsewhere herein, the positive electrode or electrode assembly 28
may comprise a number of different layers of material. In general,
the positive electrode assembly comprises (i) a conductive
substrate 30 and (ii) a catalyst or catalyst layer 32 (which may
also be referred to generally as the positive electrode material)
disposed thereon or therein, and optionally also functions as a
significant component of the current collector, (iii) a gas
impermeable and liquid permeable hydrophilic layer, 34, which is
disposed adjacent the catalyst layer 32, between it and the anode
(as further detailed elsewhere herein), (iv) an optional separator
36, disposed between the anode and the gas impermeable and liquid
permeable hydrophilic layer 34, and (v) one or more layers (e.g.,
2, 3 or more) or wraps of a gas permeable and liquid impermeable
hydrophobic layer or material 38, also disposed adjacent the
catalyst and present on the outer surface thereof (as further
detailed elsewhere herein). The conductive substrate, catalyst
layer, gas impermeable and liquid permeable hydrophilic layer, and
gas permeable and liquid impermeable hydrophobic layer (and
optionally the separator) may be collectively referred to herein as
the "gas management electrode" or "gas management positive
electrode" (as further detailed elsewhere herein).
[0082] In this regard it is to be noted that, as used herein, the
term "impermeable" means substantially impermeable; that is, the
term "impermeable" does not mean 100% impermeable in all situations
or conditions. Instead, it is recognized that although there may be
some pressure and/or temperature parameters wherein the material
becomes permeable to some extent, at standard operating conditions
for the device described herein, the material described as
"impermeable" is substantially impermeable to either gas or liquid
as described.
[0083] In this regard it is to be further noted that the catalyst,
or positive electrode active material, is generally a catalytic
redox material that is inert and non-consumable; that is, the
positive electrode material is present, in general, simply to
provide the electrons and does not otherwise participate in the
overall hydrogen-gas producing reaction or process. Accordingly,
generally any known material suitable for use as a positive
electrode material in a galvanic cell may be employed in accordance
with the present disclosure. However, the positive electrode
material is desirably selected from materials having as low a
hydrogen overvoltage as possible, and additionally is in a form
with optimized surface area, in order to provide a sufficient, and
desirably an optimum, rate capability. The stability of the
positive electrode toward corrosion or oxidation, under open
circuit conditions, may also be a consideration (under closed
circuit conditions positive electrode corrosion generally does not
occur because of cathodic protection) when selecting a suitable
positive electrode material. As previously noted, however, the
overall volume of the positive electrode material is desirably as
low as possible, in order to provide the maximum amount of space or
volume possible within the on-demand hydrogen generation device for
the fuel. Furthermore, it is desirable to limit the amount of
reducible metal oxides present in the positive electrode material,
given that these will reduce the amount of hydrogen that can be
generated due to the resultant need for a reduction of the oxides
present prior to hydrogen evolution. From a practical standpoint
however, as detailed in the examples section, some deliberate and
controlled oxidation or stabilization of the catalyst material may
be necessary in order to allow for processing of highly reactive or
oxygen sensitive catalysts, such as Raney nickel. Finally, the
migration of even small amounts of impurity metals (such as, for
example, iron or nickel, both of which are often candidates for use
in low-cost positive electrodes) to the anode can significantly
increase the self-discharge rate of the anode material (e.g., zinc
anode) and cause undesirable gassing in the anode compartment which
could lead to leakage of electrolyte or other undesirable
consequences including rupture in extreme cases. Accordingly, when
such metals are used, precautions are desirably taken to
substantially limit, or prevent, such migration. When properly
selected, an added benefit of the gas impermeable liquid permeable
hydrophilic layer (also referred to as the thin film separator) is
its ability to substantially reduce the migration of anode fouling
species into the anode compartment. Films comprising polyvinyl
alcohol (PVOH) and copolymers of PVOH have been previously
disclosed (see, e.g., U.S. Patent Application Publication No. US
2006/0257728A1 filed Feb. 15, 2006, which is incorporated herein by
reference) to possess such properties. A particularly suitable thin
film has as small a cross-sectional thickness as is practical,
while retaining manufacturing processibility (e.g., flexibility,
mechanical stability, integrity at processing temperatures,
integrity within the cell, and the like), adequate electrolyte
absorption, as well as the other advantageous properties noted
herein. Suitable films typically have a single layer, dry thickness
(i.e., a thickness prior to being contacted with an electrolyte and
after being equilibrated/stored for about 24 to about 48 hours, in
an environment where the relative humidity ranges from about 49%,
+/-about 6%, and the temperature is about 21.degree. C., +/-about
1.degree. C.) of typically less than about 250 microns, less than
about 200 microns, less than about 150 microns, or even less than
about 125 microns, the thickness ranging for example from about 5
microns to about 125 microns, from about 10 microns to about 100
microns, or from about 25 microns to about 75 microns. In this
regard it is to be noted that, depending on the difference between
the pH value of the bulk electrolyte and the pH value of the
electrolyte retained in the separator, the thickness of a film
separator may be selectively optimized, in one embodiment for
example, to effectively limit the migration of anode-fouling
soluble species.
[0084] As noted elsewhere herein, the thin film separator may, in
one specific embodiment, optionally be formed on the surface of the
anode or cathode, thereby forming a conformal separator thereon
(see, e.g., U.S. Patent Application Publication Nos. 2003/00446086
and 2004/0229116, the entire contents of which are incorporated
herein by reference.). Alternatively, however, in one particularly
desirable embodiment the separator may be laminated to the positive
electrode surface using a variety of means familiar to one skilled
in the art, including heat, pressure, adhesives, and combinations
thereof. A good interface between the gas impermeable and liquid
permeable layer and the cathode ensures good and transport of
liquid and ions, and also minimizes areas for accumulation of
hydrogen gas bubbles (on the positive electrode side) between the
catalyst layer and the gas impermeable layer.
[0085] The thin film may have an average pore size optimized for a
particular application or use. Typically, however, the average pore
size is less than about 0.5 microns (such as, for example, when a
multilayer separator structure, or multiple wraps of a single layer
separator, is used), less than about 0.1 microns, less than about
0.075 microns, less than about 0.05 microns, less than about 0.01
microns, or even less than about 0.001 microns (e.g., about 0.0005
microns, or about 0.0001 microns), the average pore size range
being within the range of, for example, about 0.001 microns and
about 0.5 microns, or about 0.01 microns and about 0.1 microns.
Alternatively, however, the average pore size may be within the
range of, for example, about 0.0001 to about 0.0005 microns.
[0086] In this regard it is to be noted that, as expressed herein,
the average pore size is for a dry film, being determined at an
ambient relative humidity of about 30% to about 80%, and at a
temperature of about 20 to about 30.degree. C. It is also to be
noted that, in order to effectively limit migration of dissolved
ionic species, the films or membranes must have extremely small
"pores," the size of which cannot easily be measured directly using
conventional methods. Instead, the pore sizes noted herein are
estimates, based on the physical and/or mathematical models that
incorporate the dimensions of the migrating ions or molecules, the
mechanism of transport and the rate of transport, etc. This is
known and understood by one skilled in the art of ultrafiltration,
reverse osmosis, etc.
[0087] The composition of the thin film separator may vary,
depending upon for example the particular properties that are
desired for the film (e.g., ionic resistance, water transport,
etc., as further detailed elsewhere herein), and/or the particular
conditions to which the film is to be subjected (e.g., process or
manufacturing conditions, and/or use conditions). In one particular
embodiment, however, the film is a PVA film, which may or may not
be modified in some way (e.g., copolymerized, and/or partially or
fully cross-linked, the degree of cross-linking therein being, for
example, at least about 10%, about 25%, about 50%, about 75% or
more, based on the total number of potential cross-linking sites
therein). Furthermore, the cross-linking may be performed such that
only one or other surface is cross-linked, leaving the interior
portion substantially un-cross-linked.
[0088] In general, the number of layers of the thin separator film
used in the electrochemical cell may be optimized for a given
application and/or to achieve a desired performance within the
cell. In addition, to preventing generated gas from being
transported to the anode compartment, it is also believed that the
thin film separator detailed herein acts to improve shorting
resistance, given that a film with small pore size provides
excellent internal shorting resistance. Rapid, preferential water
transport through the films described herein is a key factor in
providing superior high rate performance as described elsewhere.
Accordingly, in at least some embodiments, the separator will be as
thin as possible, in order to maximize the rate of discharge (i.e.,
achieve as high a current as possible).
[0089] It is to be noted that rapid water transport, or osmotic
transport, may act to enable or provide a high discharge rate.
Accordingly, the thin film separator may have a water osmosis rate
as detailed elsewhere herein (e.g., a rate of at least about
1.times.10.sup.-6 moles-cm/cm.sup.2/hr, at least about
1.times.10.sup.-5 moles-cm/cm.sup.2/hr, at least about
5.times.10.sup.-5 moles-cm/cm.sup.2/hr, or more.)
[0090] It is to be further noted that in the presence of
electrolyte, the thickness of the thin film separator may increase,
or it may swell, by greater than about 0% and less than about 100%,
less than about 75%, less than about 50%, or less than about 25%,
as compared to the initial or dry thickness of the thin film
separator. For example, in one particular embodiment, the thickness
of thin film separator, upon contact with an electrolyte, swells or
increases by about 0.1% to about 15%, by about 0.2% to about 10%,
by about 0.5% to about 5%, or by about 1% and less than about 2%.
Additionally, or alternatively, it is also to be noted that the
length and width dimensions (also defined as machine direction and
transverse direction, with respect to the orientation during
manufacture of the film) of the film also change with absorption of
electrolyte. The length and width dimensions, for example, may
increase by less than about 15%, less than by about 10%, less than
by about 5%, or less than about 2% each.
[0091] The thin film separator materials suitable for this
application desirably also minimize the transport of anode fouling
soluble species that may be present in the catalyst material. Such
anode fouling species can cause highly undesirable gassing in the
anode compartment. Such impurities include, for example ions of
metals such as molybdenum, antimony, iron, etc. that are often
found in various amounts in typical Raney Nickel catalysts. The
separator is thus associated with an "Exclusion Value" that refers
to a percentage of soluble species that is prevented from migrating
from the cathode through the separator to the anode. "Substantially
all" is intended to indicate that the separator has an Exclusion
Value of at least about 50%; alternatively at least about 60%;
alternatively at least about 70%, alternatively at least about 80%,
alternatively at least about 85%; alternatively at least about 90%;
alternatively at least about 95%; alternatively at least about 97%;
and finally alternatively at least about 99%, per the test method
developed and described herein.
[0092] It will be appreciated, however, that to the extent the
anode active material of a cell tolerates the soluble species, the
cell can tolerate some migration through the separator of
anode-fouling soluble species. Generally, therefore, a suitable
separator effectively limits the migration of anode-fouling soluble
species if the separator passes less of the species than the anode
active material can tolerate without becoming fouled. Substantially
lower amounts of the soluble species are desired, however.
[0093] A suitable hydrophilic gas impermeable and liquid permeable
thin film separator material also desirably transports water
preferentially over hydroxide ions, and hydroxide ions over soluble
species. This is an indication of "osmotic" transport. When such
materials are used as separators in electrochemical cells, this
property of the films can be advantageously leveraged to benefit
the discharge behavior of a cell by accelerating the rebalancing of
the OH-- and H2O concentrations in the electrolyte as the cell
discharges.
[0094] An osmotic transport test (such as disclosed in U.S. Patent
Application Publication No. 2005/0257728, the entire contents of
which are incorporated herein by reference for all relevant and
consistent purposes) may be used to measure water and KOH transport
through films and membranes. The test is performed using a glass
fixture. The film in question is placed between Side A and Side B
and sealed with an "O" ring such that fluid communication between
the two sides occurs only through the film. The tubes in this case
are graduated in steps of 0.1 ml, such that the volume changes on
each side may be monitored. After assembling the fixture, 14 ml of
45% KOH is placed in side A and 14 ml of 4.5% KOH solution in side
B. Prior to assembly, the separators are equilibrated with 29% KOH
for 12 hrs so as to minimize "wet-up" time.
[0095] The liquid level on the side containing 45% KOH rises, while
the level on the other side falls. This is because osmotic
transport causes the water to be transported through the film at a
faster rate than the OH.sup.- ions. The preferential water
transport to the side containing 45% KOH causes the liquid level to
increase. The rate of volume change is monitored, and after 4 hours
the solutions on both sides were sampled to analytically determine
KOH concentration. The volume change on the 45% KOH side can be
measured as a function of time for various films evaluated. The
rate of water transport can then be calculated. The exposed
cross-sectional surface area of the separators in this test is 1.4
cm.sup.2. Determination of the concentration of liquid on each side
provides an estimate of the average molar rate of transport through
the film. The calculations are based on the total moles of water
transported in a 4 hour period in the Osmotic Transport Test. Based
on this calculation method, the moles of water or OH.sup.- ions
transported in a 4 hour period can be estimated to determine the
rate. The flux of the species transported depends upon the
cross-sectional area of the film, as well as on the thickness of
the material. The thicker the film, the less the transport.
Similarly, the larger the cross-sectional surface area, the more
the flux. In order to incorporate both these parameters, the
transport rate here is reported as moles-cm/hr/cm.sup.2, or simply
moles/hr/cm.
[0096] Preferential water transport is believed to minimize
concentration gradients between the anode and cathode, and this
property is particularly suitable for a hydrogen generator as
described herein due to significant consumption of water in the
reaction. The reason for this is that rapid water transport will
help reduce concentration polarization and maintain a higher cell
operating voltage. While not being limited to specific discharge
rates, this property is particularly beneficial during high rate
discharges where concentration polarization tends to play a bigger
role.
[0097] In accordance with one aspect of this disclosure, therefore,
the electrochemical cell comprises film separators, the film having
a water osmosis rate greater than at least about 1.times.10.sup.-6
moles/cm/hr as determined using the Osmotic Transport Test
described herein. Further, the films possess the ability to
transport water at a rate greater than at least about
1.times.10.sup.-5 moles/cm/hr, and at least about 5.times.10.sup.-5
moles/cm/hr.
[0098] The ionic resistance of the hydrophilic thin film separator
is an important characteristic that influences the discharge
current and gas generation rate. Generally, the lower the
resistance, the better the performance; however, this is desirably
balanced against the need to provide adequate protection against
internal shorting. The ionic resistance of a separator material is
a function of the material characteristics according to the
following equation:
R=.rho.(l/A)
wherein "R" is the resistance in ohms; ".rho." is the resistivity
of the material with units of ohm-cm (derived from
(ohm*cm.sup.2/cm); "l" is the thickness of the material with units
of cm; and, "A"=cross-sectional area of the material perpendicular
to the axis of flow, cm.sup.2.
[0099] The resistivity of a material is a fundamental
characteristic of the material. The resistance "R" of a film in a
particular electrolyte can be directly measured in a special
fixture where a fixed gap is provided between two identical,
planar, solid graphite electrodes. The exposed cross-sectional area
for all measurements is fixed at 1 cm2, hence the measured
resistance is only a function of the material, its thickness, and
the electrolyte being used. The gap between the electrodes is
initially filled with a known electrolyte (e.g. 32% KOH in water)
and the resistance of the electrolyte alone (background) is first
measured across the gap by an AC impedance technique using a
Solartron Model SI 1255 Frequency Response Analyzer known to one
skilled in the art. A single high frequency (10 kHz), 10 mV
sinusoidal signal is applied to determine the ohmic resistance of
the gap between the two electrodes. The temperature is maintained
at 22.degree. C.+3.degree. C. This initial measurement provides the
background resistance between the two electrodes. Next, the
separator material in question (e.g. PVA film) is placed in the
holder and positioned directly between the two electrodes. The
entire apparatus is filled with electrolyte above the level of the
film and electrodes, air bubbles are removed, and the film is
allowed to soak for at least 24 hours prior to measurement, so as
to eliminate/minimize the effect of differences in the rate of
absorption of electrolyte by different films. It is also possible
to pre-soak the sample film in the electrolyte prior to fixture
assembly. With the separator in place, the impedance measurement is
again performed to determine the resistance between the two
electrodes to provide a second resistance measurement. The
difference between the second and the first measurement provides a
measure of the resistance of the separator material. This is
believed to be representative of the separator resistance in an
actual battery after it has equilibrated with the electrolyte in
the system. The method is used to screen various potential
separator films and film combinations, with or without a nonwoven
material backing.
[0100] Table A below shows the resistivity of various materials
evaluated in such a fixture using 32-0 KOH electrolyte. The
non-woven, being extremely open and porous, has very low
resistivity, but in a practical battery, a minimum of 3, desirably
4 wraps of the material are necessary. To determine the actual
resistance of a particular separator material in a battery, one
would need to know the thickness and the total surface area.
TABLE-US-00001 TABLE A Resistivity Material (ohm-cm) Viskase
Cellophane 23.6 M2000 PVA film 37.7 M1000 PVA film 38.9 M 1030 PVA
film 23.3
[0101] Suitably, the hydrophilic separator materials used in the
present disclosure have an ionic resistivity of less than about 100
ohm-cm. More suitably, the separator materials have an ionic
resistivity of less than about 50 ohm-cm, even more suitably, less
than about 40 ohm-cm, even more suitably, less than about 25
ohm-cm, even more suitably, less than about 10 ohm-cm, and even
more suitably less than about 5 ohm-cm.
[0102] The catalyst is ideally in a finely divided form and very
well distributed throughout the support, to ensure rapid and
effective reaction to ensure high operating voltage. Inexpensive,
highly active, well-distributed fine particle catalysts and
processes for making them are therefore important. The choice of
materials must take into account its ability to withstand a
corrosive environment that would be encountered in an
electrochemical cell. In view of the foregoing, the catalytic
positive electrode material is desirably selected from a material
having a BET surface area of at least about 0.1 m.sup.2/g, about 1
m.sup.2/g, about 10 m.sup.2/g, about 100 m.sup.2/g, about 250
m.sup.2/g, or even about 500 m.sup.2/g, the surface area for
example ranging from about 0.1 to about 500 m.sup.2/g, from about 1
to about 250 m.sup.2/g, or from about 10 to about 100 m.sup.2/g.
Other suitable surface area ranges, for various embodiments,
include for example: from about 0.1 to about 0.5 m.sup.2/g, from
about 0.5 to about 1 m.sup.2/g, from about 1 to about 10 m.sup.2/g,
from about 10 to about 50 m.sup.2/g, from about 50 to about 100
m.sup.2/g, or from about 100 to about 500 m.sup.2/g.
[0103] Suitable materials include, for example, metals or Raney
metals of Group VIII of the Periodic Table. Typically, however,
metals such as nickel (e.g., nickel foam, nickel mesh, Raney nickel
particles, nickel powder, etc.), platinum, platinum black,
palladium, or other metals such as silver, copper, iron, cobalt,
molybdenum and molybdenum compounds (e.g., molybdenum sulfide,
MoS2), as well as various combinations thereof (e.g.,
nickel-cobalt, iron-nickel, etc.), can be used. Materials such as
carbon in various forms can also be used as effective catalysts.
The quantity of catalyst in the catalyst layer
(catalyst+support+binder and any other additives) can range from
about 0.1% to about 90% by weight depending on the activity of the
catalyst and the cost. For example, platinum or platinum black
would typically fall near the lower end of the range whereas less
expensive catalysts such as Raney Nickel could be closer to the
upper range, depending on activity. In the examples described
herein, the de-activated Raney Nickel catalyst composition was
about 75%.
[0104] The positive electrode active material may be in a number of
different forms, including for example a powder or high surface
area particle (e.g., nickel particle) that are dispersed or
deposited (e.g., mixed, blended, coated, sprayed, sputtered, etc.,
using means generally known in the art) on a support of some kind.
Suitable supports include, for example, high surface area carbon or
ceramic particles, including for example carbon black (e.g., Vulcan
XC72, Shawinigan Black, Black Pearls 2000, Ketjen Black, etc.) and
activated carbon (e.g., PWA grade carbon from Calgon Corp.).
Exemplary catalyst/support combinations include, for example, Raney
nickel powder blended with PWA carbon, nickel plated on a nickel
foam or screen support, and nickel particles on a carbon support
(as further detailed in the Examples provided below). The quantity
of catalyst support in the catalyst layer (catalyst layer mass=mass
of catalyst+support+binder and any other additives) may range from
about 98% to about 20% by weight, depending upon the catalyst being
used. It is to be noted that if the catalyst/support combination
has the adequate electronic conductivity and structural stability
or strength then no additional current collector may be necessary.
The catalyst/support combination could then be directly coated or
laminated on to the gas impermeable but liquid permeable
hydrophilic layer. Conversely, the gas impermeable but liquid
permeable hydrophilic layer may be cast from a solution or
suspension on to the catalyst/support combination.
[0105] In one particular embodiment, Raney nickel is used as the
positive electrode material. In this regard however it is to be
noted that Raney nickel is typically very reactive in air (e.g., it
may spontaneous burn), and therefore is typically commercially
available as a fine powder in water. However, such material may be
rendered suitable for use in accordance with the present disclosure
by first de-activating the material temporarily (by forming a
protective oxide layer thereon, by means of controlled activation
of the surface thereof, using means generally known in the art,
such as those disclosed by M.A. Al-Saleh et al., Novel Methods of
Stabilization of Raney Nickel Catalysts for Fuel Cell Electrodes,
Journal of Power Sources, 72 (1998), pp. 159-64), in order to allow
the material to be processed in air as a dry powder. In use, once
hydrogen generation begins, the hydrogen is initially consumed to
reduce a protective oxide on the surface of the powder, which keeps
it from otherwise oxidizing in contact with air. Accordingly, when
such a material is used in the on-demand hydrogen generation device
of the present disclosure, it is to be noted that the activity of
the positive electrode, and therefore hydrogen gas production, may
generally improves with time (e.g., for the first several minutes
upon initial use).
[0106] As noted above, the positive electrode assembly includes a
conductive substrate, which may also be referred to in the art as a
current collector. The conductive substrate generally includes
thereon or therein a positive electrode, or catalyst, material for
interacting with the anode active agent and electrolyte to produce
hydrogen in the on-demand hydrogen gas generation device upon
activation. The conductive substrate may be in the form of a metal
screen, an expanded metal, a metal foam, carbon cloth, carbon
paper, or a non-woven mat material. In order to minimize inactive
materials (hence maximize energy density), it may be beneficial to
choose a conductive substrate material that is also a good catalyst
for hydrogen generation, thereby becoming a dual-purpose component.
The conductive substrate may, for example, be a nickel or iron
screen, a nickel or iron metal foam. The substrate may also
comprise a carbon material such as carbon cloth or fabric. The
conductive substrate may also provide integrity and strength to the
gas management electrode and, in some embodiments, may act as a
catalyst to facilitate the production of hydrogen (as detailed
elsewhere herein).
[0107] It has been determined that the physical structure of the
catalyst layer is an important determinant of the effectiveness of
the gas management electrode. As such, in addition to the
electrochemical activity and distribution of the catalyst itself,
the porosity and hydrophobicity of the structure are important
variables. In a desired embodiment, the catalyst layer contains a
mixture of a catalyst support, one or more hydrogen generation
catalysts, and a polymeric binder/water-proofing agent like
polytetrafluorethylene (PTFE).
[0108] The hydrogen generation reaction within the electrode
structure involves a three-dimensional solid/liquid/gas interface
requiring diffusion of liquid to the catalyst surface/site,
electrochemical reaction at the surface with generation of the gas
and transport of the gaseous product away from the reaction site.
At high gas generation rates (high current densities), mass
transport becomes dominant, where transport of electrolyte to the
reaction zone and gas transport away from the sites are critical
for sustained gas generation and supply. Poor mass transport, which
can be caused by low porosity or excessive hydrophobicity (too high
a PTFE binder content) can significantly increase polarization and
the electrode will no longer sustain the current, resulting in a
significant drop in hydrogen generation rates. Due to the need for
a good gas/liquid/solid interface for sustained gas generation, it
is important to have a good three-dimensional
hydrophobic/hydrophilic balance within the electrode.
Hydrophilicity is required to bring the water based electrolyte to
the reaction site and to wet the catalyst surface. Hydrophobicity
is required to facilitate efficient separation and transport of the
generated gas from the liquid with minimal foaming and entrainment
of liquid. Hence a porous, partially hydrophobic structure is
important for effective hydrogen generation and transport,
particularly at high current densities. Hydrophobicity may be
obtained partly from the inherent property of the catalyst support
(e.g. carbon which may have organic surface groups present), and
partly from hydrophobic additives such as PTFE (which also serves
as a binder to keep particles together), which must be well
distributed in the structure. This can be achieved by adjusting the
quantity of PTFE and by adjusting the processing conditions to
provide good distribution and fibrillation of the PTFE to develop a
porous, 3-dimensional partially hydrophobic structure as is well
known to one skilled in the art, particularly in the development of
air cathodes. The hydrophobic binder content can range from about
0.5% (of the total weight of the catalyst+support+binder) to about
20% by weight. Since effective transport of generated gas from the
reaction zone is an important property of the structure, it may be
beneficial in some embodiments to have a catalyst layer that has
increasing hydrophobicity in the direction away from the reaction
zone and in the direction of the gas exit. One means to achieve
this type of increasing hydrophobicity is to laminate multiple
layers together, with each successive layer having increasing
hydrophobic additive content. In such cases, depending on the layer
in question, the hydrophobic binder content can vary from about
0.5% to about 20% in the layer closest to the reaction zone and
from about 50% to about 100% in the layer farthest from the
reaction layer.
[0109] The entire electrode structure must also have high
electronic conductivity to ensure effective collection of the
current, without which the ohmic resistance of the electrode will
be too high, resulting in an undesirable voltage drop. The catalyst
layer can be adhered or laminated to the current collector using a
variety of methods known in the art such as pressure, adhesives,
heat, or combinations thereof. To effectively separate the gas from
the liquid a gas permeable, liquid impermeable hydrophobic layer
may be laminated or adhered to one surface of the catalyst layer
using similar means. The thickness of the catalyst layer excluding
the current collecting substrate can range from about 0.01 mm to
about 2 mm, from about 0.1 mm to about 1 mm, from about 0.2 mm to
about 0.5 mm. The current collector thickness may range from about
0.02 mm to about 1 mm, or from about 0.05 mm to about 0.5 mm, or
desirably from about 0.1 mm to about 0.25 mm.
[0110] In addition to the catalyst, the positive electrode assembly
may include thereon one or more additive agents to improve the
functionality and efficiency of the positive electrode. For
example, the positive electrode assembly may include thereon carbon
black, graphite, polyteterfluoroethylene, and mixtures thereof.
[0111] Porosity of the various gas permeable layers is important,
as discussed elsewhere. One measure of permeability of a material
is through the use of a Gurley Air permeability instrument (as
disclosed for example in US Patent Application Publication No.
2006/0257728 A1, the entire of contents of which is incorporated
herein by reference for all relevant and consistent purpose). Air
permeability can be measured in Gurley seconds, as appreciated by
one having ordinary skill in the art. Because the Gurley test
measures the length of time necessary to pass a predetermined
volume of air through a material, a longer time measurement is an
indication of low air permeability. One skilled in the art will
recognize that hydrogen permeability of the same material is
expected to be higher, due to the smaller size of the hydrogen
molecule. The lower the Gurley Number, the higher the permeability.
The Air Permeability of the combination of catalyst layer, current
collector and gas permeable (liquid impermeable) hydrophobic
layers, of less than about 500 Gurley seconds, desirably less than
about 200 seconds, most desirably less than about 100 seconds has
been found suitable for use in the Gas Management electrode. Table
B lists the Gurley Seconds (SG) for the various layers used to
produce the Raney Nickel Gas Management electrode used in some of
the examples discussed elsewhere. The Gurley measurement was taken
using Model No. 4150N, commercially available from Gurley Precision
Instruments (located in Troy, N.Y.), at a pressure drop of 12.2
inches of water to displace 10 cc air through a 0.1 sq. inch
area.
TABLE-US-00002 TABLE B Air Permeability Component (SG), sec
Description Catalyst Layer w/ Current 16 .+-. 1 75% Ni (Raney),
Collector 5% PTFE, w/ C painted screen Single PTFE Layer 2 .+-. 1
Grade 600A, Plastomer Catalyst Layer laminated 47 .+-. 3 Laminator
rolls set to with 2 layers of PTFE 0.027'' Gap
[0112] It is to be noted that optionally a conventional porous
separator material may be disposed between the anode and the
positive electrode or electrode assembly. This separator may be
utilized to hold electrolyte and enhance the transport of water and
ions between the anode and cathode and assist in the preventing
inadvertent shorting of the device by particles of anode coming
into contact with the positive electrode (or cathode) surface.
Generally, this separator may be a hydrophilic separator made from
a nonwoven material such that it is both gas and liquid permeable.
It is desirable that this separator is capable of swelling and
stretching in order to accommodate changes in the device during
hydrogen generation.
[0113] Referring again to FIGS. 1A and 1B, one end of the hydrogen
gas generation means 22, generally denoted 40, is closed (i.e.,
sealed) by a second negative end cap 42 and a negative grommet seal
44. More specifically, this end (i.e., the negative end 40) of the
hydrogen gas generation means is closed by sealingly connecting the
second end cap 42 and grommet 44 to one end 40 of the cylindrical
electrode assembly (by, for example, crimping the second end cap
over the grommet and end of the positive electrode assembly).
Similarly, the end generally opposite and disposed axially upstream
of the negative end (i.e., the positive end) of the hydrogen gas
generation means 22, generally denoted 46, is closed (i.e., sealed)
by a positive end cap 48 and a positive grommet seal 50, this end
being closed by sealingly connecting the positive end cap and
grommet to the end of the positive electrode assembly (by, for
example, crimping the positive end cap over the grommet and end of
the positive electrode assembly). Additionally, present between the
positive grommet and the anode is a bottom separator cup 52, which
acts to prevent contact between anode particles and the positive
electrode end-cap which can result in an electrical short circuit
of the cell.
[0114] The hydrogen gas generation means, 22, present within the
can 12 is held in place therein, at least in part by means of a
positive connection tab 54, which is present between the positive
end cap 48 of the hydrogen gas generation means and the positive,
or vented, end 14 of the can 12 and portion of the sidewall 19 of
the can proximate thereto. Additionally, the positive connective
tab 54 acts to electrically connect the positive electrode (or more
generally the positive electrode assembly 28) to the positive outer
can 12. The connective tab 54 generally conforms to the shape and
dimensions of the positive end cap 48 of the hydrogen gas
generation means 22, as well as the positive end 14 and the portion
of the sidewall 19 of the can 12 that is proximate thereto, in
order to form a physical and electrical contact therebetween (i.e.,
between the positive end cap 48 and the positive end 14 and
sidewall 19 of can 12). Accordingly, the connective tab 54 is
prepared from a material that is (i) sufficiently conductive, in
order to create a sufficient electrical contact between the can 12
and the positive end cap 48 of the hydrogen gas generation means
22, thus allowing a low resistance circuit to be completed between
the anode 24 and positive electrode assembly 28 of the hydrogen gas
generation means (as further detailed elsewhere herein), and (ii)
sufficiently flexible or elastic, in order to conform to the
positive end cap 48 of the hydrogen gas generation means, as well
as the positive end 14 of the can 12 and the portion of sidewall 19
proximate thereto. The connective tab 54 is desirably a strip of
conductor (e.g., a strip of nickel 200 which is about 0.125 inch
wide by 0.007 inch thick) and is either physically offset from the
vent hole or sufficiently perforated to allow gas, particularly
hydrogen gas to pass therethrough and out of the hydrogen gas vent
16 (for consumption by a fuel cell connected thereto). This
positive tab connection 54 does not necessarily cover the entire
circumference of the positive end. Exemplary materials suitable for
use for the connective tab 54 include, for example, a nickel foam,
copper, brass, plated steel (plated for example with copper, tin,
silver, nickel), or a nickel 200 strip (e.g., a 0.125 inch wide by
0.007 inch thick strip of nickel 200, such as for example when the
hydrogen gas generation means is configured to have dimensions
similar to a AAA cell and/or the on-demand hydrogen gas generation
device is configured to have dimensions similar to a AA cell).
[0115] Additionally, the hydrogen gas generation means 22 is held
in place within the can 12 by means of an insulating wrap 55, which
is present between the second negative end cap 42 (of the hydrogen
gas generation means 22), and the portion of the sidewall 19 of the
can proximate the negative end 18 thereof. More particularly, the
insulating wrap 55 generally conforms to the shape and dimensions
of the second negative end cap 42 of the hydrogen gas generation
means 22, as well as the portion of the sidewall 19 of the can 12
that is proximate the negative end thereof, in order to form an
insulated, physical contact therebetween (i.e., an insulated
physical contact between the second negative end cap 42 and the
portion of the sidewall 19 near the negative end of the can 12).
Accordingly, the insulating wrap 55 is prepared from a material
that is (i) sufficiently insulating, in order to create a physical
but nonconductive contact between the sidewall 19 of the can 12 and
the second negative end cap 42 of the hydrogen gas generation means
22, and (ii) sufficiently flexible or elastic, in order to conform
to the second negative end cap 42, as well as the negative end 18
of the can 12 and the portion of sidewall 19 proximate thereto.
Additionally, the insulating wrap 55 is sufficiently porous or
prepared from a material that is sufficiently gas permeable, and
more particularly permeable to hydrogen, or is configured to allow
hydrogen gas, formed by the hydrogen gas generation means, to pass
therethrough and exert pressure on a pressure responsive switching
mechanism (which may also be referred to herein as a hydrogen gas
generation control), generally indicated at 56, which is further
detailed elsewhere herein.
[0116] Alternatively, the hydrogen gas generation means 22 is
insulated from the can 12 by means of an insulating wrap 55, which
is present between the second negative end cap 42 (of the hydrogen
gas generation means 22), and the portion of the sidewall 19 of the
can proximate the negative end 18 thereof. More particularly, the
insulating wrap 55 generally conforms to the shape and dimensions
of the second negative end cap 42 of the hydrogen gas generation
means 22, and is spaced from the portion of the sidewall 19 of the
can 12 that is proximate the negative end thereof, in order to form
an insulated, loose physical contact therebetween (i.e., an
insulated physical contact between the second negative end cap 42
and the portion of the sidewall 19 near the negative end of the can
12 that is loose enough to allow hydrogen gas to pass
therethrough). Accordingly, the insulating wrap 55 is prepared from
a material that is (i) sufficiently insulating, in order to create
a nonconductive contact between the sidewall 19 of the can 12 and
the second negative end cap 42 of the hydrogen gas generation means
22, and (ii) sufficiently flexible or elastic, in order to conform
to the second negative end cap 42. Additionally, the insulating
wrap 55 is configured to allow hydrogen gas, formed by the hydrogen
gas generation means, to pass therethrough and exert pressure on a
pressure responsive switching mechanism (which may also be referred
to herein as a hydrogen gas generation control), generally
indicated at 56, which is further detailed elsewhere herein.
Exemplary materials suitable for use for the insulating wrap 55
therefore include, for example, adhesive-backed PVC shrink film,
shrink tubing, electrical tape, etc.
[0117] Referring now to FIGS. 2A and 2B, and again to FIG. 1A, the
pressure responsive switching mechanism, generally indicated at 56,
is present within the can 12 and is physically or mechanically, as
well as electrically, connected to both the second negative end cap
42 of the hydrogen gas generation means 22 (by means of for example
an electrically conductive connection tab 58) and the first
negative end cap 20 of the can. The pressure responsive switching
mechanism 56 is adapted in size and shape such that it fits
securely in the negative end 18 of can 12, and is sealingly
connected thereto as well as to the first negative end cap 20 (by,
for example, crimping the negative end of the can over the end cap,
the switching mechanism and a flexible, electrically insulating
seal or gasket 21, which may or may not be integrated with the
switching mechanism itself) during assembly, by means generally
known in the art. More particularly, the switching mechanism
comprises a moveable (e.g., flexible) member or diaphragm 60, which
extends inward (e.g., radially inward) from the can 12, or more
particularly the gasket 21. The moveable member has an opening
(e.g., centrally located opening, the moveable member having a
circular washer-like design in the illustrated embodiment) that has
a conductive connector 62 therein that is spool-like or rivet-like
in shape. The movable member 60 in this particular embodiment is
electrically non-conductive (an additional electrical insulation
component may therefore be required to enable use of a conductive
movable member in this embodiment). The connector has a pair of
oppositely disposed radially extending outer flanges 64 and 66, the
flange which faces the second negative end cap 42 being
mechanically and electrically connected thereto by means of the
connective tab 58. The connector 62 is sized to fit securely in the
opening of the moveable member 60, such that the connector moves in
concert with the moveable member.
[0118] The switching mechanism 56 additionally comprises a first
conductive contact 68, which is annular in shape and which also has
an opening (e.g., centrally located opening, which like the
moveable member gives the first conductive contact a washer-like
design, and in the illustrated embodiment may be a metal washer,
for example). The upper surface of the first conductive contact 68
is in mechanical and electrical contact with the upper flange 64 of
the connector 62, while the lower surface is in mechanical (and
optionally electrical) contact with the upper surface of the
moveable member. In the illustrated embodiment, the first
conductive contact 68 surrounds the upper flange 64 of the
connector 62 and is annular in shape, the contact extending
outwardly from the connector 68 toward the sidewall 19 of the can
12, and/or the first negative end cap 20. Additionally, the first
conductive contact is sized and shaped, and/or positioned within
the switching mechanism, so as to move in concert with the moveable
member and/or the conductive connector 62.
[0119] The switching mechanism 56 additionally comprises a second
conductive contact 70, which in the illustrated embodiment is also
annular in shape and which also has an opening (e.g., centrally
located opening, which like the moveable member gives and the first
conductive contact gives the second conductive contact a
washer-like design, and in the illustrated embodiment may be a
metal washer, for example). The second conductive contact 70
surrounds the upper flange 64 of the connector 62, but is not in
direct contact therewith. Rather, the upper surface of the second
conductive contact 70 is in mechanical and electrical contact with
the lower surface of the first negative end cap 20, and in
removable electrical and mechanical contact with the lower surface
of the first conductive contact 68 (as further detailed elsewhere
herein). Additionally, the second conductive contact is sized and
shaped, and/or positioned within the switching mechanism, so as to
be immovable (or have a fixed position) relative to the first
conductive contact and/or the moveable member.
[0120] Finally, the switching mechanism 56 may optionally comprise
a spring or springs 72 that act, in concert with the flexible
member 60, to determine the pressure differential at which the
switching mechanism operates (i.e., move downstream, to fully or
partially open the switching mechanism, or upstream, to fully or
partially close the switching mechanism, as further detailed
elsewhere herein). In the illustrated embodiment, an annular spring
72, which surrounds the upper flange 64 of the connector 58, is in
contact with upper surface of the first conductive element and the
lower surface of the first negative end cap 20.
[0121] The switching mechanism 56 of the on-demand hydrogen gas
generation device of the present disclosure may be initially
constructed to be in a substantially closed position. As a result,
an electrical and physical contact is present between the first
negative end cap 20 and the second negative end cap 42, and thus
the anode 24, of the hydrogen gas generation means 22, through the
switching mechanism. However, there is essentially no hydrogen gas
generation by the hydrogen gas generation means 22 until an
electrical circuit is completed between the cylindrical positive
electrode assembly 28 and the anode 24 of the hydrogen gas
generation means, by means of an external circuit (not shown) which
links or connects the first negative end cap 20 with the positive
end 14 of the can 12, thereby allowing current to flow through the
galvanic cell (between the anode 24 and the positive electrode
assembly 28), in order to initiate the galvanic cell corrosion
reaction there. Once the external circuit has been closed (by, for
example, inserting the on-demand hydrogen gas generation device
into a fuel cell, which contains a means for completing the
external circuit between the negative end cap 20 and the positive
end 14 of can 12), hydrogen gas will begin to form in, and be
evolved from, the hydrogen gas generation means 22. More
specifically, with respect to FIG. 1A, it is to be noted that
hydrogen gas formed at the positive electrode 32 passes through the
gas permeable membrane 38 into a space 39, which is present between
the positive electrode assembly 28 and the can 12. As this space is
filled, a pressure differential is eventually created proximate the
switching mechanism 56, while at the opposite end of the device
hydrogen gas is allowed to exit space 39 for consumption by a fuel
cell (not shown).
[0122] As further illustrated in FIGS. 2A and 2B, in operation or
use, the switching mechanism 56 responds to a pressure differential
that results from, in the illustrated embodiment, the pressure on
the side of the switching mechanism facing the hydrogen gas
generation means 22 increasing (as a result of hydrogen gas
generation) relative to the pressure on the opposite side of
thereof (i.e., the side of the switching mechanism facing the first
end cap 20), which is essentially designed to remain at about
atmospheric pressure (due, for example, to the vent hole 74 present
in the first negative end cap). More particularly, as further
detailed elsewhere herein, when this pressure differential is below
a predetermined value, the moveable member 60 is essentially at
rest; that is, there is insufficient force being applied thereto by
the pressure on the hydrogen gas generation means side thereof,
relative to the opposite side thereof, and therefore it has
essentially not moved (e.g., flexed) downstream from its initial
(i.e., fully closed) position. However, as the hydrogen gas
generation means continues to form or evolve hydrogen gas, pressure
on one side (i.e., the upstream side) of the switching mechanism,
and more particularly the moveable member 60 thereof, increases and
therefore the pressure differential increases, as well. Once this
pressure differential is sufficiently high (i.e., reaches a
predetermined threshold value of less than 30 psig), the moveable
member deforms and moves downstream, thus moving from a first
(e.g., closed) position to a second (e.g., partially or fully open)
position.
[0123] As the switching mechanism opens, or more particularly the
moveable member deforms or moves downstream, the spring 72 (when
present) is compressed, and the connector 58 and the first
conductive contact 68 are also moved downstream in concert with the
moveable member. This movement acts to increase the resistance in
the switching mechanism to the electrical current passing
therethrough, by means of lessening the physical and electrical
contact between the first conductive contact 68 and the second
conductive contact 70. As a result, the rate at which hydrogen gas
is formed and evolved from the hydrogen gas generation means
decreases, which in turn decreases the rate at which the pressure
differential is increasing. If the pressure differential becomes
sufficiently high, the physical and electrical contact between the
first and second conductive contacts is essentially eliminated,
thus halting the flow of electrical current therethrough (i.e., the
resistance is sufficiently high to stop the flow of electrical
current therethrough). The switching mechanism is therefore
effectively in the fully open position, thus halting the flow of
electrical current therethrough and the formation or evolution of
hydrogen gas associated therewith.
[0124] Although the rate of hydrogen gas formation is decreased, or
stopped, hydrogen gas will continue to be consumed as long as the
fuel cell, to which the on-demand hydrogen gas generation device is
attached or connected, is active or in operation. As a result, the
pressure differential will naturally decrease over time. Once the
pressure differential is sufficiently low, the switching mechanism,
and more particularly the moveable member, will move upstream (due
to the natural flexibility of the moveable member alone, or in
combination the natural flexibility of the spring 72 which is in
contact with, and exerting upstream or downward pressure on the
first conductive element 68, or more generally the moveable member
60) and reestablish, or more fully establish, contact between the
first and second conductive contacts. Hydrogen gas formation will
then begin once again, or the rate of hydrogen gas formation will
increase. This downstream and upstream movement of the moveable
member will continue as long as (i) the on-demand hydrogen gas
generation device remains activated (i.e., in use), and (ii) there
is fuel for consumption in the reaction that forms the hydrogen
gas. As a result, the hydrogen gas generation device of the present
disclosure, and more particularly the switching mechanism thereof,
is designed to be operable to provide an on-demand flow of hydrogen
gas that is substantially constant when the device is activated.
Stated another way, the switching mechanism, and more particularly
the moveable member thereof, is designed to repeated or reversibly
move downstream or upstream as the pressure differential increases
above or decreases below, respectively, a predetermined value. As a
result, as detailed elsewhere herein, the on-demand hydrogen gas
generation device may produce or emit a flow of hydrogen gas at an
average rate of at least about 0.1 cubic centimeters/minute/cubic
centimeters of fuel volume, or more for a period of time of, for
example, at least about 1 hour (and which may optionally remain
substantially constant for this period of time).
[0125] It is to be noted that the repeated or reversible movement
of the switching mechanism, or more particularly the moveable
member, will continue in response to the pressure differential
resulting from hydrogen gas generation until, for example, (i) the
fuel, from which the hydrogen gas is formed, is essentially
completely consumed, and/or (ii) the switching mechanism becomes
fully opened, thus essentially stopping all flow of electrical
current therethrough, and remains in this position (e.g., the
on-demand hydrogen gas generation device is turned off, or
deactivated, in some way).
[0126] It is to be further noted that the design or configuration
of the can which contains the hydrogen gas generation means, the
hydrogen gas generation means, the switching mechanism, and any or
all of the various components thereof, may be other than herein
described without departing from the intended scope of the present
disclosure. Accordingly, it is to be still further noted that the
various embodiments provided in the figures presented herein are
for purposes of illustration, and therefore should not be viewed in
a limiting sense. For example, the moveable member may be
constructed (by means generally known in the art) to itself serve
as or comprise the means through which an electrical current passes
between the anode and positive electrode, and thus serve as or
comprise the means by which the anode and positive electrode of the
hydrogen gas generation means are placed in removable electrical
contact (i.e., the moveable member may be constructed, using means
generally known in the art, to comprise, or serve as, the first
conductive contact or the second conductive contact of the
switching mechanism).
[0127] Referring now to FIGS. 3A and 3B, an alternative embodiment,
and more particularly a low profile (e.g., low thickness) or
stand-alone embodiment, of the switching mechanism (generally
indicated at 76) of the present disclosure is provided, the
switching mechanism for example being constructed to have
dimensions (e.g., thickness) similar to or smaller than that of a
button cell (e.g., a cell having, for example, a diameter of about
8 mm and a thickness of about 1.5 mm). In this embodiment, the
moveable member 78 comprises a conductive material or a conductive
coating (as detailed elsewhere herein), and therefore the
electrical current may pass through the switching mechanism by
means of the flexible member, rather than by the first and second
conductive contacts (as illustrated in FIGS. 2A and 2B). In this
particular embodiment, the moveable member 78 further comprises a
point or nub 80 that is in removable mechanical and electrical
contact with a metal outer can 82, which in turn is in electrical
contact with the anode (not shown) of the hydrogen gas generation
means through a connector (not shown); that is, the connector is in
physical and electrical contact with both the anode and the outer
can 82. As previously noted, the outer can 82 in the illustrated
embodiment is button-like in shape, the open end thereof being
closed by means of a negative end cap 84. The negative end cap is
sized and shaped such that it and the end of the can are sealingly
connected (by, for example, crimping the end of the can over the
end cap, a flexible electrically insulating seal or gasket 86, and
optionally the ends of the moveable member 60, by means generally
known in the art).
[0128] The switching mechanism illustrated in FIGS. 3A and 3B is in
communication with the hydrogen gas generation means, and operates
in a manner similar to the switching mechanism detailed elsewhere
herein. More specifically, the upstream side of the moveable member
78 is in communication with the hydrogen gas generation means by
one or more vents 88, which are sealingly connected to said means.
In operation, hydrogen gas, once formed, enters through vent 88 and
exerts increasing pressure on the switching mechanism, or more
specifically the moveable member thereof, a pressure differential
being created as a result of the fact that the pressure on the
opposite side of the moveable member is at about atmospheric
pressure (a vent (not shown) in the end cap being optional for this
purpose, as detailed in other embodiments illustrated elsewhere
herein). Eventually, the pressure reaches a predetermined threshold
(as detailed elsewhere herein), causing the moveable member to move
or flex downstream and ultimately separate or break the contact
interface 90 that exists between the point or nub 80 of the
moveable member and the surface 92 of the can 82 facing the nub.
This separation acts to increase the resistance to the electrical
current passing through the switching mechanism, or more
specifically the moveable member, which in turn acts to slow or
stop the hydrogen gas formation. Once the pressure differential
falls below this predetermined value, the moveable member moves or
flexes upstream, eventually resulting in the re-establishment of
the contact interface.
[0129] It is to be noted that the switching mechanism may be placed
in communication with the hydrogen gas generation means using
essentially any means generally known in the art. For example, as
illustrated in FIG. 4, in one particular embodiment the entire
switching mechanism 76 of FIGS. 3A and 3B may be used to replace
the end cap 20 and switching mechanism 56 in the on-demand hydrogen
gas generation device illustrated in FIG. 1A. More specifically, as
illustrated in FIG. 4, the stand-alone switching mechanism
illustrated in FIGS. 3A and 3B may be used with the hydrogen gas
generation device illustrated in FIG. 1A, the switching mechanism
76 and/or can 12 being sized and shaped such that the switching
mechanism is essentially inserted into the negative end 18 of the
can and then the two are sealingly connected (by, for example,
crimping the negative end 18 of the can 12 over the switching
mechanism 76, a flexible electrically insulating seal or gasket
(not shown) being optionally present therebetween, by means
generally known in the art). In this embodiment, the electrically
conductive connection tab 58 is physically and electrically
connected to the side of the switching mechanism can 82 facing
it.
[0130] Yet another alternative embodiment of the on-demand hydrogen
gas generation device is illustrated in FIGS. 5A, 5B and 5C. In
this embodiment, the device, generally indicated at 96 has
dimensions similar to those of a standard prismatic electrochemical
cell, used for example in a cellular telephone. In this embodiment,
the switching mechanism, generally indicated at 98, is constructed
in a manner consistent with the embodiments detailed elsewhere
herein, and in particular is constructed like the low-profile or
stand-alone switching mechanism detailed above, with the exception
that in this particular embodiment a connector (not shown)
electrically connects the switching mechanism, or more particularly
the moveable member thereof (in the case of the stand-alone or
low-profile switching mechanism), to the positive electrode rather
than the anode. As illustrated in greater detail in FIGS. 5B and
5C, the device comprises a hydrogen gas generation means, which in
turn comprises an anode 100, an anode current collector 102 in
physical and electrical contact therewith, a positive electrode 104
(or more generally a positive electrode assembly, as detailed
elsewhere herein and referred to as a gas management electrode),
and an optional separator 106 disposed between the anode and
positive electrode. Additionally, the hydrogen gas generation
means, or more generally the on-demand hydrogen gas generation
device, comprises a space (e.g., a channel, passageway or chamber)
108, present between the positive electrode and the outer can 109
to which the hydrogen gas migrates or evolves after being formed at
the positive electrode. This space is in sealing communication with
the switching mechanism, the hydrogen gas present therein exerting
pressure on the moveable member thereof as a pressure differential
is created. As previously noted, when this pressure differential is
sufficiently high to exceed a predetermined value, the moveable
member moves downstream (in conformance with the direction of the
pressure differential), resulting in the disruption in the flow of
electrical current through the switching mechanism (and potentially
completing stopping the flow of current, if the pressure
differential becomes sufficiently great) and the slowing of the
rate of hydrogen gas generation (if not essentially stopping
hydrogen gas generation). As this pressure differential decreases
(such as by the escape of the hydrogen gas that has been formed
through a hydrogen gas vent or exit port, 110, which is in sealing
communication with the fuel cell (not shown) by a passageway or
conduit (not shown)), the moveable member moves upstream toward its
original position, resulting in the reestablishment (or further
establishment) of the flow of electrical current through the
switching mechanism.
[0131] In this regard it is to be noted that, as indicated by the
embodiment illustrated in FIGS. 5A through 5C, the switching
mechanism may be in physical and electrical contact with either the
anode or the positive electrode through, for example, a connective
member. Accordingly, the switching mechanism may operate to disrupt
the flow of electrical current between the anode and the positive
electrode in either way, and therefore the illustrations provided
herein which show the switching mechanism in physical and
electrical communication with one particular electrode should not
be viewed in a limiting sense.
[0132] Yet another alternative embodiment of the on-demand hydrogen
gas generation device is illustrated in FIG. 6. In this embodiment,
the device, generally indicated at 112, comprises a switching
mechanism, generally indicated at 114. The switching mechanism
comprises a moveable member, 116, that is metal and that is in
physical and electrical contact with the hydrogen gas generation
means (either the anode or positive electrode thereof, not shown)
through a conductive tab 120 (which is in physical and electrical
contact with the hydrogen gas generation means). The switching
mechanism further comprises a first conductive contact 118, which
is in physical and electrical contact with the moveable member 116.
The first conductive contact 118 is in removable physical and
electrical contact with a second conductive contact 122. One side
(i.e., the downstream side) of the second conductive contact 122 is
in physical and electrical contact with the first negative end cap
124, while the other side (i.e., the upstream side) thereof is in
physical contact with an insulator 126 (e.g., a plastic insulator)
that physically and electrically isolates the second conductive
contact from the moveable member 116. The switching mechanism
(i.e., the moveable member, the first and second conductive
contacts, and the insulator), as well as the end cap, are sized and
shaped such that they, and the end of the outer can 128 are
sealingly connected (by, for example, crimping the end of the can
over the end cap, the moveable member, the insulator and the second
conductive contact, as well as a flexible electrically insulating
seal or gasket 130, by means generally known in the art).
[0133] It is to be noted that the devices detailed herein above, as
well as the devices detailed elsewhere herein, may be directionally
independent. Therefore, it is to be understood that upper and lower
surfaces, the terms "downstream" and "upstream", etc., are provided
for purposes of illustration only.
[0134] It is to be further noted that the various gaskets, seals,
insulating material, etc. present in the on-demand hydrogen gas
generation device of the present disclosure may be made from
essentially any material that possesses the desired properties
(e.g., chemical stability, flexibility or elasticity, insulating or
non-conductive properties, etc.).
[0135] C. Chemical Hydride
[0136] As noted and discussed above, galvanic corrosion is one
chemical reaction method that can be utilized in the on-demand
hydrogen gas generation device to produce hydrogen gas. In an
alternative embodiment of the present disclosure, hydrogen gas can
be produced in the on-demand hydrogen gas generation device by
reacting water, or more generally an aqueous solution, and a
chemical hydride (e.g., a complex chemical hydride). Utilizing a
chemical hydride may be advantageous in certain embodiments where
the production of hydrogen is required over an extended period of
time as the chemical hydride has a high energy and hydrogen density
as compared to water, which is the hydrogen source in galvanic
corrosion. The mixing of water, or an aqueous solution, and the
complex chemical hydride sodium borohydride results in the
production of hydrogen according to the following reaction:
NaBH.sub.4+4H.sub.2O--->NaB(OH).sub.4+4H.sub.2 (gas).
[0137] As such, each mole of sodium borohydride produces 4 moles of
hydrogen. This is an efficient method of producing a large volume
of hydrogen from a compact energy source. As would be recognized by
one skilled in the art based on the disclosure herein, to control
the hydrogen generation, the chemical hydride and water (or an
aqueous solution) are kept separate and only mixed together
on-demand and only in the predetermined amounts.
[0138] One embodiment for producing hydrogen utilizing a chemical
hydride in the on-demand hydrogen gas generation device is
illustrated in FIG. 7. FIG. 7 illustrates a first container 150 and
a second container 152 that are connected via conduit 154 having
check valve 156 to allow for the flow of a liquid in one direction
only from container 150 into container 152. The first container 150
includes a gas-generating electrochemical cell 158 in electrical
communication via wire 176 with a switching mechanism 160 and a
first chamber 162 including an aqueous solution 164. The second
container includes the switching mechanism 160, a chemical hydride
166 and reaction zone 168. A gas 170 is produced in gaseous
production area 172 by the gas-generating electrochemical cell 158
and hydrogen gas 174 is produced in the reaction zone 168 by the
mixing of the complex chemical hydride 166 and aqueous solution
164.
[0139] When the electrical circuit is closed and the switching
mechanism 160 is in electrical communication with the
gas-generating electrochemical cell 158, the latter produces a gas
170, such as hydrogen, that pressurizes the first chamber 162 which
contains the aqueous solution 164. The pressurization forces the
aqueous solution 164 through the conduit 154 and check valve 156
and into the reaction zone 168 where the aqueous solution 164 can
react with the chemical hydride 166. Upon this reaction occurring,
hydrogen gas 174 is formed and can exit the second container 152
through the switching mechanism 160. So long as the pressure in the
second container 152 remains below a specified value, the switching
mechanism 160 maintains the electrical connection with the
gas-generating electrochemical cell 158 so that further gas 170 is
produced which continues to force aqueous solution 164 through
check valve 156 into reaction zone 168. In that way, hydrogen is
production by reaction of the aqueous solution 164 with the complex
chemical hydride 166 is maintained. Once the pressure in the second
container 152 reaches a specified value, the switch closes and no
further gas 170 is produced as the electrical communication between
the switching mechanism 160 and the gas-generating electrochemical
cell 158 is interrupted. This reduces the pressure in the first
chamber 162. Consequently, aqueous solution 164 is no longer forced
through check valve 156 into reaction zone 168. In that way,
hydrogen production by reaction of the aqueous solution 164 with
the complex chemical hydride 166 in reaction zone 168 ceases (one
the aqueous solution already present has reacted).
[0140] In an alternative embodiment of the present disclosure, a
chemical hydride can be utilized to produce hydrogen and all of the
required components and chambers can be incorporated into a single
unit. Referring now to FIG. 8, there is illustrated a single unit
on-demand hydrogen gas generation device 200 including a switching
mechanism 202 in electrical communication with gas-generating
electrochemical cell 204. Device 200 further includes a first
chamber 206 that contains aqueous solution 208 and check valve 214,
a chemical hydride 210, and reaction zone 212.
[0141] When the electrical circuit is closed and the switching
mechanism 202 is in electrical communication with the
gas-generating electrochemical cell 204, the gas-generating
electrochemical cell 204 produces a gas (not shown), such as
hydrogen, that creates pressure against the aqueous solution 208.
The pressurization forces the aqueous solution 208 through the
check valve 214 and into the reaction zone 212 where the aqueous
solution 208 can react with the chemical hydride 210. Upon this
reaction occurring, hydrogen gas 216 is formed and can exit the
switching mechanism 202. So long as electrical connection between
the gas-generating electrochemical cell 204 and the switching
mechanism 202 is maintained, further gas is produced which
continues to force aqueous solution 208 through check valve 214
into the reaction zone 212. In that way, hydrogen 216 production by
reaction of the aqueous solution 208 with the complex chemical
hydride 210 is maintained. Once the electrical communication
between the switching mechanism 202 and the gas-generating
electrochemical cell 204 is interrupted, gas production is stopped.
This reduces the pressure in chamber 206. Consequently, aqueous
solution 208 is no longer forced through check valve 214 into
reaction zone 212. In that way, hydrogen production by reaction of
the aqueous solution 208 with the complex chemical hydride 210 in
reaction zone 212 ceases (once the aqueous solution already present
has reacted).
[0142] The gas-generating electrochemical cell is utilized to
produce a gas upon-demand (that is, when the gas-generating
electrochemical cell is in electrical communication with the
switching mechanism and the switch is closed allowing for
electrical contact between the gas-generating electrochemical cell
and the switching mechanism) to force an aqueous solution from one
area of the on-demand hydrogen gas generation device to a second
area of the on-demand hydrogen gas generation device where a
chemical hydride is located. This gas-generating electrochemical
cell can be any type of electrochemical cell that is capable of
producing a gas upon-demand. For example, the gas-generating
electrochemical cell (e.g., a zinc chloride or nickel-zinc cell)
may utilize galvanic corrosion to produce a hydrogen gas to create
pressure to force the movement of the aqueous solution. Several
cell types capable of producing a gas may be utilized in accordance
with the present disclosure. As an example, the gas-generating
electrochemical cell may be a galvanic cell capable of producing
hydrogen gas to force the movement of the aqueous solution where
the electrolyte in the galvanic cell and the aqueous solution
forced into the hydride gas-generating chamber comprises zinc
chloride. The gas generating electrochemical cell may be self
contained or it may be integrated with the first chamber and
optionally utilize the aqueous solution as its electrolyte.
[0143] The gas-generating electrochemical cell may optionally
include a gas management system as described herein to facilitate
the movement or transport of gas from the gas-generating
electrochemical cell to improve the efficiency of the
pressurization required to pump the aqueous solution such that it
can be brought in to contact with the chemical hydride.
[0144] The aqueous solution utilized to react with the chemical
hydride can be at a neutral pH, acidic pH, or at a basic pH. In
most embodiments, it is generally desirable to utilize an aqueous
solution having a pH of less than 7, and more desirably less than
about 5. By utilizing an aqueous solution having an acidic pH, any
passivation present on the surface of the chemical hydride may be
disrupted by the acid in the aqueous solution to allow for a more
complete reaction of the chemical hydride with the aqueous
solution.
[0145] Additionally, the aqueous solution utilized to react with
the chemical hydride may be characterized by the water to chemical
hydride (e.g., complex chemical hydride) molar ratio. In most
embodiments, it is generally desirable to utilize an aqueous
solution with a low water to chemical hydride ratio, and more
desirably close to the stoichiometric molar ratio. By utilizing an
aqueous solution with a low water to chemical hydride ratio, higher
energy and hydrogen density as compared to an aqueous with a high
water to complex chemical hydride can be achieved. For example, in
order to achieve a high energy and hydrogen density with a zinc
chloride electrolyte, in most embodiments it is desirable to use a
water to chemical hydride molar ratio of less than about 20:1
(e.g., less than about 15:1, less than about 10:1 or even less than
about 5:1). Utilizing an aqueous solution with a low pH (e.g., less
than about 6, about 5 or even about 4), allows high hydrogen yields
to be obtained at low water to chemical hydride ratios.
Consequently, it may be desirable to use a water to chemical
hydride ratio of less than about 10:1, for example, with an aqueous
solution having a low pH.
[0146] As noted above, a chemical hydride is utilized to react with
an aqueous solution to produce hydrogen for use in a fuel cell
application. In general, many chemical hydrides known in the art
are satisfactory for use in the on-demand hydrogen gas generation
device. Some specific examples of suitable chemical hydrides
include Al(BH.sub.4).sub.3, LiBH.sub.4,
LiAlH.sub.2(BH.sub.4).sub.2, Mg(BH.sub.4).sub.2,
Ti(BH.sub.4).sub.3, Fe(BH.sub.4).sub.3, Ca(BH.sub.4).sub.2,
NaBH.sub.4, KBH.sub.4, LiAlH.sub.4, Mg(AlH.sub.4).sub.2,
Ti(AlH.sub.4).sub.4, Zr(BH.sub.4).sub.3, Mg(AlH.sub.4).sub.2,
NaAlH.sub.4, LiH, CaH.sub.2, H.sub.3BNH.sub.3 and combinations
thereof. In one particular embodiment, however, the chemical
hydride is a complex chemical hydride (such as, for example, a
hydride selected from Al(BH.sub.4).sub.3, LiBH.sub.4,
LiAlH.sub.2(BH.sub.4).sub.2, Mg(BH.sub.4).sub.2,
Ti(BH.sub.4).sub.3, Fe(BH.sub.4).sub.3, Ca(BH.sub.4).sub.2,
NaBH.sub.4, KBH.sub.4, LiAlH.sub.4, Mg(AlH.sub.4).sub.2,
Ti(AlH.sub.4).sub.4, Zr(BH.sub.4).sub.3, Mg(AlH.sub.4).sub.2,
NaAlH.sub.4, H.sub.3BNH.sub.3 and combinations thereof). The
chemical hydride can be present in the on-demand hydrogen gas
generation device as a solid, slurry, or stabilized solution (e.g.,
in combination with a stabilized hydrate of an appropriate metal
borate). It is generally desirable to have the chemical hydride
present as solid or slurry.
[0147] The on-demand hydrogen gas generation device utilizing a
chemical hydride hydrogen source may be a compact device that is
capable of providing a large stream of hydrogen gas upon activation
over a prolonged period of time. In some embodiments of the present
disclosure, the on-demand hydrogen gas generation device utilizing
a chemical hydride is capable of providing an average of at least
about 0.1 cubic centimeters/minute/cubic centimeter of fuel volume.
In some embodiments of the present disclosure, the on-demand
hydrogen gas generation device utilizing a chemical hydride is
capable of providing an average of from about 0.1 cubic
centimeters/minute/cubic centimeter of fuel volume to about 1.5
cubic centimeter/minute/cubic centimeter of fuel volume, and
desirably an average of about 1.0 cubic centimeter/minute/cubic
centimeter of fuel volume (over a defined period of time, as
detailed elsewhere herein).
[0148] In one embodiment of the present disclosure, the on-demand
hydrogen gas generation device utilizing a chemical hydride is
capable of producing hydrogen having a water content of less than
about 1000 ppm. Desirably, the device is capable of producing
hydrogen having a water content of less than about 500 ppm,
desirably less than about 200 ppm, desirably less than about 100
ppm, still more desirably less than about 50 ppm and still more
desirably less than about 10 ppm.
[0149] The on-demand hydrogen generation device utilizing a
chemical hydride is substantially orientation independent. The
device can produce substantially the same amount of hydrogen upon
activation regardless of the orientation of the device. This is
significant as it enables the device to be used in almost any
application regardless of orientation.
[0150] D. Gas Management System
[0151] The present disclosure is further directed to an on-demand
hydrogen gas generation device, such as for example one of the
cells detailed above or elsewhere herein, that includes a gas
management system for producing hydrogen and efficiently
transporting the substantially dry hydrogen gas out of the
on-demand hydrogen device. The gas management system is generally
desirable when hydrogen is being produced by the on-demand hydrogen
gas generation device by galvanic corrosion as described herein.
The gas management system facilitates the efficient movement or
transport of hydrogen out of the region wherein the hydrogen is
produced and into a region wherein it can be used as fuel in a fuel
cell. The gas management system also allows the hydrogen being
produced in the on-demand hydrogen gas generation device to be
produced with relatively low humidity or water content, which keeps
the device from prematurely drying out due to water loss.
Additionally, this can be advantageous in fuel cell systems that
benefit from the use of dry hydrogen. As discussed in more detail
herein, the gas management system renders the on-demand hydrogen
gas generation device substantially orientation independent; that
is, the on-demand hydrogen generation device produces hydrogen at
substantially the same rate regardless of its orientation. This is
a significant advantage as many of the current means for supplying
fuel to fuel cell systems are not orientation independent.
[0152] The gas management system may include a number of components
and/or layers to facilitate the efficient transporting of hydrogen
gas out of the on-demand hydrogen gas generation device upon
production. Referring now to FIG. 9, wherein various components or
layers are not drawn to scale with respect to other components or
layers, there is illustrated an exploded view of one embodiment of
a gas management system in accordance with the present disclosure
that includes anode 230, optional separator 232 disposed between
anode 230 and a gas impermeable and liquid permeable hydrophilic
layer 234, a conductive substrate 236 disposed between the gas
impermeable and liquid permeable hydrophilic layer 234 and a
catalyst layer 238, and a gas permeable liquid impermeable
hydrophobic layer 240 disposed adjacent the catalyst layer 238. The
conductive substrate, catalyst layer, gas impermeable and liquid
permeable hydrophilic layer and gas permeable and liquid
impermeable hydrophobic layer may be collectively referred to
herein as the "gas management electrode" or "gas management
positive electrode." As used herein, the term "impermeable" means
substantially impermeable; that is, the term "impermeable" does not
mean 100% impermeable in all situations or conditions. Instead, it
is recognized that although there may be some pressure and/or
temperature parameters wherein the material becomes permeable to
some extent, at standard operating conditions for the device
described herein, the material described as "impermeable" is
substantially impermeable to either gas or liquid as described.
[0153] The gas management electrode may be comprised of multiple
layers or components as illustrated in FIG. 9 that are discrete
layers in intimate contact with each other; that is, layers that
although touching and in intimate contact, can be physically
separated into individual layers without substantial damage to
neighboring layers. Alternatively, the gas management electrode may
be comprised of a single integrated component; that is, the gas
management layer may be comprised of a single component that is
substantially indistinguishable into individual layers, even though
each layer is physically present. In one embodiment, the gas
management electrode has each individual layer laminated together
to produce a laminated gas management electrode wherein the layers
are physically indistinguishable. Additionally, the gas management
electrode may be produced as a single integral component utilizing
heat, pressure, adhesives, or a combination thereof. Utilizing a
single integral component as the gas management electrode may
simplify the manufacturing process for the on-demand hydrogen gas
generation device, and may improve overall efficiency as a result.
In some embodiments the conductive substrate 236 may be embedded in
the catalyst layer or may be adhered to one surface of the catalyst
layer. In some embodiments some of the layers can be coated or cast
or sprayed on an adjacent layer. For example, the gas impermeable
and liquid permeable hydrophilic layer 234 may be cast on the
current collector/catalyst layer so as to produce a thin conformal
layer. In still other embodiments, the catalyst layer with or
without a hydrophobic binder, may be sprayed on to the conductive
substrate. The gas permeable liquid impermeable hydrophobic layer
240 may be sprayed on to the surface of the catalyst layer in some
embodiments. Additionally and alternately, a thin layer of tacky
PTFE suspension (e.g., T 30 grade PTFE suspension from DuPont) may
be sprayed on to the catalyst layer and a porous PTFE film is then
adhered on to the tacky surface using means familiar to one skilled
in the art, such as heat, pressure or a combination thereof.
[0154] As previously noted, the anode component of the gas
management system provides the fuel (anode active agent) for the
on-demand hydrogen gas generation device to produce hydrogen. In
order to provide the necessary hydrogen production rate capability
and discharge efficiency, the anode will generally be capable of
sustaining a high discharge current without significant passivation
of the anode active agent. As such, the anode is generally
optimized for continuous discharge with high discharge currents, or
for intermittent discharge with longer than one hour continuous
discharge periods. Factors that influence the optimization of the
anode for the specific purposes include, for example, anode active
agent surface area, particle size, particle size distribution,
amount of anode active agent, electrolyte concentration, and the
nature and amount of gelling agents and surfactants. Generally, the
anode is designed to contain the maximum amount of anode active
agent that can be efficiently utilized since this fuel dictates how
much hydrogen can be produced by the device.
[0155] The anode will generally include an anode active agent such
as a metal or metals as the fuel for producing hydrogen. The anode
active agent is desirably a metal having low thermodynamic
nobility. Such metals include, for example, zinc, aluminum,
magnesium, titanium, and combinations thereof. The amount of anode
active agent generally included in the anode is less than about 80%
by weight of the total weight of the anode components. The total
weight of the anode components includes the weight of each
component making up the anode such as, for example, anode active
agent, gelling agent, surfactant alloying agent, electrolyte, etc.
Desirably, the amount of anode active agent is from about 60% by
weight to about 75% by weight, and more desirably from about 67% by
weight to about 71% by weight. The anode active agent, such as
zinc, can be present in the anode in the form of particles, flakes,
fines, or dust, for example. Also, combinations of these forms may
be utilized.
[0156] In some embodiments of the present disclosure, the anode of
the on-demand hydrogen gas generation device may be prone to
various undesirable corrosion reactions when stored at or above
room temperature prior to use. The electrolyte in the anode may
corrode the zinc (or other anode active agent) upon contact,
forming oxidized zinc products that decrease the availability of
active zinc while simultaneously generating unwanted gas. The rate
of corrosion increases as the storage temperature rises and can
lead to a dramatic decrease in anode active agent capacity. Gas
generated in such reactions can increase pressure in the anode,
cause electrolyte leakage and disrupt the device integrity. The
rate at which the unwanted gas is generated at the anode active
surface may accelerate when the anode active is partially utilized
thereby decreasing the resistance of the anode to electrolyte
leakage. The corrosion reactions that lead to gas evolution involve
cathodic and anodic sites on the anode active surface. Such sites
can include surface and bulk metal impurities, surface lattice
features, grain boundary features, lattice defects, point defects,
and inclusions.
[0157] To minimize undesirable corrosion and gassing during
storage, it is typical to employ corrosion-resistant zinc alloys
and to reduce the extent of impurities in the anode. For example, a
suitable zinc powder (or other anode active) can be alloyed with
one or more metals selected from indium, bismuth, calcium,
aluminum, lead and phosphorous. A particularly desired alloying
agent for minimizing gassing is bismuth. Typically, alloy powders
can include from about 0.01% to about 0.5% by weight alloy agent
alone, or in combination with, from about 0.005% to about 0.2% by
weight of a second alloying agent such as lithium, calcium,
aluminum, and the like.
[0158] The anode is typically and preferably a "gelled anode".
Along with the anode active and alloying agent (if any), the anode
includes an electrolyte therein to provide water for the galvanic
corrosion reaction near the gas management electrode and to
facilitate ionic transfer between the anode and the gas management
electrode. The electrolyte desirably has high ionic conductivity.
Generally, the electrolyte is a potassium hydroxide or sodium
hydroxide solution, but can also include other electrolytes known
to those of ordinary skill in the art. In some cases, the
electrolyte may contain dissolved salts, oxides or hydroxides of
bismuth, tin, indium, mercury, lead, cadmium, or thallium.
Additionally, the electrolyte may include a dissolved cation or
anion of the metal anode (e.g., an aluminum oxide, sodium
aluminate, potassium aluminate, a zinc oxide, a zinc hydroxide, or
calcium salts.) In some embodiments, the electrolyte may
additionally contain a corrosion inhibitor such as a quaternary
ammonium salt, or a non-ionic, anionic, or cationic surfactant.
[0159] When potassium hydroxide is utilized as the electrolyte, the
concentration of potassium hydroxide may be from about 15% by
weight to about 45% by weight, and desirably from about 20% by
weight to about 35% by weight. Generally, when zinc is the anode
active agent, the electrolyte may include a small amount of zinc
oxide to retard open circuit oxidation and stabilize the zinc
surface and reduce gassing. The amount of zinc oxide may be from
about 0.1% by weight of the anode to about 2% by weight of the
anode.
[0160] The anode as described herein will generally include at
least one gelling agent to help suspend the anode active material
throughout the electrolyte to allow for the anode active to more
fully react. Essentially any gelling agent known in the art, which
is suitably or sufficiently compatible with the other components in
the anode may be used in accordance with the present disclosure.
Examples of suitable gelling agents include polyacrylic acids,
grafted starch materials, salts of polyacrylic acids,
polyacrylates, carboxymethylcellulose, or combinations thereof.
Examples of suitable polyacrylic acids include Carbopol 940 and 934
(available from B.F. Goodrich) and Polygen 4P (available from 3V).
An example of a grafted starch material is Waterlock A221
(available from Grain Processing Corporation). An example of a salt
of a polyacrylic acid is Alcosorb G1 (available from Ciba
Specialties). The gelling agent amount can range from about 0.1% to
about 1% of the total mass of the anode.
[0161] To further minimize undesirable corrosion and gassing during
storage as described above, it is typical to add organic
surfactants and inorganic corrosion-inhibiting agents to the anode.
Surfactants act at the anode-electrolyte interface by forming a
hydrophobic film that protects the anode active surface during
storage. The inhibitive efficiency of surfactants to increase the
corrosion resistance of the anode active depends on their chemical
structure, concentration, and their stability in the electrolyte.
Total amount of surfactant can typically range from about 0.1% to
about 1% by weight of the anode.
[0162] Among the surfactants known to be effective at controlling
gassing are organic phosphate esters such as the ethylene
oxide-adduct type disclosed by Rossler et al. in U.S. Pat. No.
4,195,120, incorporated herein by reference. In U.S. Pat. No.
4,777,100, Chalilpoyil et al. disclosed an anode containing single
crystal zinc particles with a surface-active heteropolar ethylene
oxide additive including organic phosphate esters. Specifically,
commercially available surfactants such as Rhodafac RM-510,
Rhodafac RA-600, Witconate 1840X, and Mafo 13 MOD1 are suitable
surfactants for use in the present disclosure, as described in for
example U.S. Pat. Nos. 6,872,489 and 7,226,696 (the entire contents
of which are incorporated herein by reference for all relevant and
consistent purposes).
[0163] As noted above, the gas management electrode includes a
conductive substrate, which may also be referred to in the art as a
current collector. The conductive substrate generally includes
thereon or therein a catalyst material for interacting with the
anode active agent and electrolyte to produce hydrogen in the
on-demand hydrogen gas generation device upon activation. The
conductive substrate may be in the form of a metal screen, an
expanded metal, a metal foam, carbon cloth, carbon paper, or a
non-woven mat material. The conductive substrate may, for example,
be a nickel or iron screen, a nickel or iron metal foam, a carbon
material such as carbon black or activated carbon, a ceramic
material or combinations thereof. The conductive substrate provides
integrity and strength to the gas management electrode and, in some
embodiments, may act as a catalyst to facilitate the production of
hydrogen.
[0164] In some embodiments of the present disclosure, the
conductive substrate will include a catalyst (which is inert and
non-consumable in the hydrogen-producing reactions as it simply
provides electrons) therein and/or thereon to further the chemical
reactions to produce hydrogen. The catalyst is generally a powder
or combination of powders that is dispersed onto the conductive
substrate through mixing, coating, spraying, sputtering, and the
like. In order to ensure the desired rate of hydrogen production,
the catalyst desirably has a hydrogen over-voltage as small as
possible and sufficient surface area as discussed below.
Additionally, the stability of the catalyst (and gas management
electrode generally) towards corrosion/oxidation during periods of
open circuit (i.e., no hydrogen being produced) is important and
should be contemplated during design.
[0165] It is generally preferred that the catalyst be a high
surface area material to further the desired reactions. For
example, in one embodiment, the catalyst has a BET surface area in
the range of between about 0.1 m2/g and about 500 m2/g, and
desirably between about 0.1 m2/g and about 100 m2/g. It is to be
noted that it is generally desirable to experimentally
determine/confirm the activity of a catalyst layer in a flooded
galvanic corrosion cell prior to actual use in a generator device.
It is to be further noted that the BET surface area of the Raney
Nickel catalysts disclosed herein appears to decrease
significantly, following the de-activation procedure described
elsewhere.
[0166] Suitable catalyst materials include, for example, metals or
Raney metals of Group VIII of the Periodic Table. Some specific
suitable catalysts may include iron, nickel, nickel powder, Raney
nickel, platinum, platinum black, palladium, cobalt, mixtures of
nickel and cobalt, mixtures of iron and nickel, molybdenum sulfide,
and mixtures thereof. In one particularly desirable embodiment, the
catalyst is a Raney nickel catalyst. Because Raney nickel is
generally highly reactive with air, and can burn spontaneously, it
may be desirable to de-activate this (and other highly reactive
catalysts as described herein) catalyst temporarily during the
manufacturing process to allow for processing in an ambient
environment. Such deactivation may be accomplished by methods known
to those of ordinary skill in the art and may include, for example,
providing an oxide layer on the surface of the catalyst. This
protective oxide layer is then consumed by hydrogen produced in the
device during use. As such, when a protective layer, such as an
oxide, is utilized on the catalyst, the activity of the catalyst,
and hence the performance of the device, may generally improve with
time over an initial time period as the oxide is being chemically
removed by the hydrogen.
[0167] As noted above and illustrated in FIG. 9, disposed between
the anode and the conductive substrate is the gas impermeable and
liquid permeable hydrophilic layer. This layer facilitates ion and
water transport between the anode and the conductive substrate
including the catalyst and blocks the passage of hydrogen produced
at the surface of the conductive substrate and catalyst from
passing into the anode and causing unwanted foaming and bubbling in
the anode. Such foaming and/or bubbling in the anode are highly
undesirable as they can significantly reduce the efficiency of
hydrogen generation and ultimately render the device unusable and
unstable. Stated another way, this gas impermeable and liquid
permeable hydrophilic layer, while allowing for ion and water
transport between the anode and the gas management electrode, helps
to produce a gradient within the device to transport the produced
hydrogen gas in the desired direction (i.e., away from the anode)
in the on-demand hydrogen generation device by providing a gas
impermeable backstop to block gaseous access to the anode.
[0168] The gas impermeable and liquid permeable hydrophilic layer
desirably has characteristics that allow for a sustained high rate
of discharge of the device, while being somewhat stretchable in
order to accommodate changes within the device during hydrogen
generation. Additionally, this layer will desirably block soluble
impurities that can migrate to the anode and cause spontaneous
gassing during periods of nonuse of the device.
[0169] The gas impermeable and liquid permeable hydrophilic layer
can be any suitable material that provides the desired liquid and
ion transport and gas blockage. For example, the layer may be
comprised of a polyvinyl alcohol (PVA) film such as Monosol 2000
(Monosoll LLC). Cellulosic materials such as cellophane may also be
utilized as the material for this layer. The gas impermeable and
liquid permeable hydrophilic layer can be made of single layer or
can be a multiple layer film.
[0170] In this regard it is to be noted that one version of a
suitable gas impermeable and liquid permeable hydrophilic layer has
a polymer backbone formed from a straight chain, a branched chain,
or variants thereof. Examples of materials having such a backbone
that have been found to provide a suitable gas impermeable and
liquid permeable hydrophilic layer include not only PVA, but also
polymers of PVA (e.g., copolymers of PVA), and possibly
poly(ethylene-co-vinyl alcohol, or "EVOH"), copolymers of
polystyrene, blends or co-extrusions of these and like materials
with materials such as polyethylene, polypropylene, polystyrene,
and variants of the foregoing.
[0171] As further illustrated in FIG. 9, adjacent the catalyst
layer is the gas permeable and liquid impermeable hydrophobic
layer. This hydrophobic layer is generally a porous hydrophobic
structure that allows gas to easily pass therethrough while
stopping the flow of liquids. Because the layer is a hydrophobic
layer, it retards water passage while allowing gas to pass
therethrough. This property provides at least four benefits to the
device: (i) it keeps the device from leaking electrolyte, (ii) it
helps separate liquid from the hydrogen gas passing therethrough
toward the exit, (iii) it keeps the device from "drying out" due to
water loss and, as such, makes the device capable of producing a
higher level of drier hydrogen for a longer period of time as water
is required for the galvanic corrosion to produce hydrogen, and
(iv) multiple wraps of it can help provide structural integrity to
the cell and to keep the form and shape without electrolyte
leakage.
[0172] Generally, this layer is produced from a suitable
fluorinated polymer. In one specific embodiment, this layer is a
porous polyteterfluoroethylene. The thickness may range from about
0.02 mm to about 0.25 mm.
[0173] In addition to the layers and components described above,
the cell may include an optional conventional separator material
disposed between the anode and the gas impermeable and liquid
permeable hydrophilic layer. This separator may be utilized to hold
electrolyte and further the transfer of water and ions from the
anode to the surface of the conductive substrate having the
catalyst and assist in the prevention of shorting of the device.
Generally, this separator may be a hydrophilic separator made from
a non-woven material such that it is both gas and liquid permeable.
It is desirable that this separator be capable of swelling and
stretching in order to accommodate changes in the device during
hydrogen generation.
[0174] The on-demand hydrogen gas generation device as described
herein may be a compact device that is capable of providing a large
stream of hydrogen gas upon activation over a prolonged period of
time. In some embodiments of present disclosure, the on-demand
hydrogen gas generation device is capable of providing an average
of at least about 0.1 cubic centimeters/minute/cubic centimeter of
fuel volume. As used herein, the term "fuel volume" means the total
anode volume of the device including the anode active, electrolyte,
and any other additive in the anode at assembly of the device
(excluding the current collector). In some embodiments of the
present disclosure, the on-demand hydrogen gas generation device is
capable of providing an average of from about 0.1 cubic
centimeters/minute/cubic centimeter of fuel volume to about 1.5
cubic centimeter/minute/cubic centimeter of fuel volume, and
desirably an average of about 1.0 cubic centimeter/minute/cubic
centimeter of fuel volume (for a defined period of time, as
detailed elsewhere herein).
[0175] As described above, the gas management system is designed
such that the produced hydrogen is efficiently transported out of
the device through the gas permeable and liquid impermeable
hydrophobic layer. Because this layer is designed to be
hydrophobic, it repels water contained in the gas management system
and keeps the water internal to the system where it can be used in
the galvanic corrosion reactions to produce hydrogen. Additionally,
by keeping the water from the electrolyte internal of the gas
management system, the hydrogen that is produced is substantially
dry; that is, the hydrogen exiting from the generator may contain
very small amounts of water vapor and may generally be referred to
as "dry hydrogen." As noted herein, this may be important for some
fuel cell applications that require substantially dry hydrogen to
operate properly.
[0176] In one embodiment of the present disclosure, the on-demand
hydrogen gas generation device is capable of producing hydrogen
having a water content of less than about 50,000 ppm by volume, or
less than about 30,000 ppm by volume. Desirably, the device is
capable of producing hydrogen having a water content of less than
about 20,000 ppm, desirably less than about 10,000 ppm, desirably
less than about 5,000 ppm, and still more desirably less than about
1,000 ppm.
[0177] As noted above, the on-demand hydrogen generation device
including the gas management system is substantially orientation
independent. Because of the design characteristics of the gas
management system, the device can produce substantially the same
amount of hydrogen upon activation regardless of the orientation of
the device. This is significant as the device is capable of being
used in almost any application regardless of orientation.
[0178] Referring now to FIG. 10, wherein various components or
layers are not drawn to scale with respect to other components or
layers, there is illustrated one specific embodiment of the gas
management system of the present disclosure. This embodiment shows
anode 302, gas impermeable and liquid permeable hydrophilic layer
304 disposed between anode 302 and conductive substrate 306 (which
contains the catalyst that is not illustrated), gas permeable and
liquid impermeable hydrophobic layer 308 disposed adjacent
conductive substrate 306, and gas exit region 310 being interior of
the gas permeable and liquid impermeable hydrophobic layer 308.
This specific embodiment shows the anode being on the outside of
the gas management system and surrounding the various
components/layers; that is, this embodiment shows a design that is
the reverse of a typical alkaline battery where the cathode
constitutes an outer ring adjacent an outer can, and the anode
fills the middle of the annular ring with a separator between the
two). In this embodiment, when hydrogen is produced at the surface
of the conductive substrate (which contains or is in contact with
the catalyst layer), it is pushed or transported away from the
anode and into the center of the system through the gas permeable
and liquid impermeable hydrophobic layer and into the gas exit
region where it can exit the gas management system.
[0179] Referring now to FIG. 11, wherein various components or
layers are not drawn to scale with respect to other components or
layers, there is illustrated another specific embodiment of the gas
management system of the present disclosure. FIG. 11 illustrates
anode 312 being surrounded by gas impermeable and liquid permeable
hydrophilic layer 314 which is surrounded by conductive substrate
316 (which contains the catalyst that is not illustrated) and gas
permeable and liquid impermeable hydrophobic layer 318 which
surrounds the conductive substrate 316. This Figure illustrates a
gas management system design wherein the anode is on the interior
of the system and is surrounded by the other layers/components.
When hydrogen is produced at the surface of the conductive
substrate, it is transported away from the anode and exits the gas
management system through the gas permeable and liquid impermeable
hydrophobic layer 318.
[0180] Referring now to FIG. 12, wherein various components or
layers are not drawn to scale with respect to other components or
layers, there is illustrated another specific embodiment of the gas
management system of the present disclosure. FIG. 12 illustrates
anode 320, gas impermeable and liquid permeable hydrophilic layer
322 disposed between the anode 320 and the conductive substrate 324
(which contains the catalyst that is not illustrated) and gas
permeable and liquid impermeable hydrophobic layer 326 adjacent the
conductive substrate 324. This Figure illustrates a gas management
system which first is formed into a "V` by folding upon itself,
followed by folding the double layer gas management electrode
ribbon into an "s" ribbon (or "z") folded shape with the hydrogen
produced exiting through a gas exit region interior of the gas
permeable and liquid impermeable hydrophobic layer 326. In the
embodiment illustrated in FIG. 12, as well as in one or more other
embodiments described herein, the anode, including the anode active
agent, electrolyte, gelling agent (if any), surfactant (if any)
etc. may be enclosed in a pouch-type enclosure that acts as the gas
impermeable and liquid permeable hydrophilic layer. This embodiment
may allow for substantial bending and/or rolling of the anode into
a ribbon, z, s, prismatic, or other desired conformation including
a spiral wound configuration. In other embodiments, the "s" or "z"
shaped cathode ribbon is surrounded by the anode gel.
[0181] E. Configurations
[0182] It is to be noted that the size or dimensions of the
on-demand hydrogen gas generation device may be other than herein
described without departing from the intended scope of the present
disclosure. In one particular embodiment, however, the on-demand
hydrogen gas generation device may be configured to have a
cylindrical shape (the device, for example, having dimensions or a
shape substantially similar to a standard AA or AAA electrochemical
cell), or alternatively a flat prismatic or rounded (i.e.,
"race-track") prismatic shape (the device, for example, having
dimensions or a shape substantially similar to a standard prismatic
or elliptical electrochemical cell used in cellular telephones, for
example). In yet another alternative embodiment, the device may
have a button-like shape. Such embodiments are particularly
advantageous because, when employed with other features
conventionally used in electrochemical cells (e.g., standard anode,
positive electrode, electrolyte, etc., such as those detailed
herein above), the on-demand hydrogen gas generation device may be
mass produced using existing manufacturing techniques and
equipment.
II. Fuel Cell with On-Demand Hydrogen Gas Generation Drive
[0183] The on-demand hydrogen gas generation device may be used in
combination with a hydrogen gas consumption device (e.g., a fuel
cell) in order to generate power or electricity for a number of
applications, particularly small-scale applications or uses. The
on-demand hydrogen gas generation device may be initially
constructed with the switching mechanism in the closed position,
but with an open external circuit such that there is no completed
circuit between the anode and positive electrode therein and thus
no hydrogen gas is generated. In such an embodiment, the hydrogen
gas generation device is desirably designed so the act of
connecting the device to the fuel cell, or more generally a
hydrogen consuming device, such as for example by means of an
interface connector (in one embodiment) that connects the hydrogen
gas outlet or vent of the on-demand hydrogen gas generation device
with the hydrogen gas inlet of the fuel cell, causes the external
circuit to close and hydrogen gas generation to commence.
[0184] The fuel cell typically includes a hydrogen gas plenum
(i.e., a compartment adjacent to or leading to the anode of the
fuel cell), which begins to fill as hydrogen gas is generated by
and evolved from the hydrogen gas generation device. As the plenum
fills, pressure builds in the internal volume of the hydrogen gas
generation device and the hydrogen gas plenum. If the consumption
rate of hydrogen is greater than the production rate, hydrogen will
continue to be produced by the hydrogen gas generation device at
the maximum rate possible, which in general is a function of the
resistance of the electrical circuit therein (when galvanic cell
corrosion is the means by which hydrogen gas is generated for fuel
cell consumption). In contrast, if the consumption rate of hydrogen
is less than the production rate, the pressure in the hydrogen gas
generator will begin to increase until it reaches a threshold or
predetermined value (as detailed elsewhere herein). The switching
mechanism will then begin to open, increasing the resistance
therein to the electrical current passing therethrough. This will
slow the rate of hydrogen gas production, and may ultimately
terminate it for a period of time (if the pressure is sufficient to
fully open the switch). Once this pressure dissipates (i.e., falls
below the threshold pressure), by for example further hydrogen gas
consumption by the fuel cell, the switching mechanism will close
once again (or more fully close), thus resulting in increased
hydrogen gas production (or resulting in the re-commencing of
hydrogen gas production).
[0185] It is to be noted that the on-demand hydrogen gas generation
device of the present disclosure is particularly well-suited for
use in small-scale applications (optionally being configured to fit
within a fuel cell having dimensions or a shape substantially
similar to a standard AA or AAA electrochemical cell, for example);
that is, the device is well-suited for use in hydrogen
gas-consuming devices (e.g., fuel cells) that produce, or have an
output of, less than about 30 watts, less than about 25 watts, or
even less than about 20 watts, of power (the output, for example,
being in the range of about 1 watt to about 5 watts, about 5 watts
to about 30 watts, about 7.5 watts to about 25 watts, or about 10
watts to about 20 watts).
[0186] The following Examples describe various embodiments of the
present invention. Other embodiments within the scope of the
appended claims will be apparent to a skilled artisan considering
the specification or practice of the invention as described herein.
It is intended that the specification, together with the Examples,
be considered exemplary only, with the scope and spirit of the
invention being indicated by the claims, which follow the
Examples.
EXAMPLES
Example 1
Positive Electrode Preparation
[0187] This is an example that illustrates the preparation of a
suitable positive electrode material, and more specifically
illustrates the deactivation of Raney Nickel 3202 and the resulting
use thereof to prepare a catalyst layer and ultimately a gas
management electrode for use in accordance with the present
disclosure:
[0188] A. Deactivation of Raney Nickel
[0189] Raney Nickel is spontaneously combustible in air when it is
dry. While it may be possible to process it safely in a wet form,
for the purposes of this example, a procedure to deactivate it was
used based on literature information (see, e.g., "Novel Methods of
stabilization of Raney-Nickel catalyst for fuel cell electrodes",
M. A. Al-Saleh, et. al., Journal of Power Sources 72 (1998) pp
159-164). The procedure is described in greater detail below.
[0190] Material/Equipment List: Raney 3202 Nickel, slurry in water
(Sigma Aldrich product #510068); 5% hydrogen peroxide solution,
diluted from 50% solution (Sigma Aldrich product #516813); ice
water bath; mixing motor (Arrow Engineering model JR4000) with
three blade teflon mix shaft; fume hood; scoop; vacuum filtration
apparatus with Whatman 90 mm ashless filter paper (Cat #1440-090);
and, vacuum drying oven.
[0191] Procedure (the steps were performed in a fume hood to avoid
exposure to potentially hazardous gases products): (1) Hydrogen
peroxide and Raney Nickel solutions were chilled to a temperature
in the range of 0-10.degree. C. before starting the reaction. (2)
An ice water bath was prepared, and the batch and the mixing
apparatus were set up inside the fume hood. (3) The chilled 15%
hydrogen peroxide solution (50 ml) was added to a 250 ml beaker and
placed in the ice water bath under the mixer. (4) Wet Raney Nickel
(10-14 g) was added to the hydrogen peroxide solution at one time
using a small scoop (because the reaction begins immediately upon
contact). Accordingly, the wet Raney Nickel solution was dense
enough so that the entire weight used could be picked up with one
scoop. (5) The mixer was activated (low setting--setting no. 1)
within approximately three seconds of adding the Raney Nickel. (6)
After agitating for approximately ten seconds, bubbling in the
solution stopped, indicating the reaction was essentially complete.
(7) The solution was allowed to continue mixing for a total of
three minutes (timed from the addition of Raney Nickel), to allow
time to cool down before collecting the reaction product. (8) The
reaction product was collected by filtration (using a vacuum
filtration apparatus), and rinsed using two liters of deionized
water. (9) The product was thoroughly dried at 45.degree. C. in a
vacuum oven (vacuum pressure of at least 25 in Hg), which took at
least 12 hours. (10) The product was then removed from the oven and
stored under argon until being subjected to further processing.
[0192] B. Positive Electrode Formulation and Construction
[0193] Table 1, below, provides a summary of the composition of the
positive electrode material prepared using the above-described
deactivated Raney nickel, as well as commercially available carbon
and PTFE (the positive electrode material being subsequently used
in most of the examples described elsewhere herein, except the one
using Ni foam).
TABLE-US-00003 TABLE 1 Component Wt % Supplier Grade Lot # Carbon
20 Calgon PWA N04B29GB Ni Catalyst 75 Aldrich Raney Ni 3202 10928AD
deactivated PTFE 5 DuPont 6C 01302(12-7-05)
[0194] These three dry components were combined in a coffee grinder
(Mr. Coffee model IDS77) and mixed for 40 seconds. The grinder was
hand-shaken during its operation in order to more effectively
disperse and homogenize the dry mixture. This mixture was then
compressed into a flexible sheet by a set of pinch rollers, with a
gap set to achieve a sheet thickness of between 0.012 and 0.016
inches at a linear speed of 10.5 inches/minute. This sheet was then
reground in the coffee grinder for 20 seconds, again using hand
agitation during the grinding operation. This powder was then fed
back into the same pinch rollers to make another sheet, this time
with a gap adjusted to achieve a flexible sheet thickness of
approximately 0.0085 inches (0.21 mm). This process helped to
fibrillate the PTFE better, and provide a more flexible, continuous
sheet.
[0195] The positive electrode utilizes a Nickel wire screen as the
integrated current collector (although expanded metal may also be
used). The screen may be, as in this case, coated with a conductive
carbon primer to aide in adhesion of the active flexible sheet
thickness, 40 mesh double cold bonded, GDC Corporation) was coated
with Electrodag 109 (Acheson Colloids Co, Port Huron, Mich.).
Alternate solvent/binder systems can be used to enhance adhesion.
The flexible positive electrode sheet and screen were then passed
back through a set of pinch rollers to achieve an overall thickness
of between 0.009 and 0.010 inches (0.254 mm). This process produced
an electrode with an overall coating density of 0.039 g/cm.sup.2
(0.252 g/in.sup.2) and a nickel catalyst loading of 0.0292
g/cm.sup.2.
[0196] The positive electrode was cut into rectangles 1.86 inches
in height by 1.65 inches in width, for use in the cylindrical cell
configuration.
[0197] On the non-coated side (i.e., screen side) of the positive
electrode, a thin layer of PVOH (i.e., polyvinyl alcohol) and CMC
(i.e., carboxymethyl cellulose) solution was applied, to serve as a
glue to laminate a layer of the gas impermeable film of PVOH. The
aforementioned solution can be comprised of a variety of grades of
similar water soluble polymers. The following table (Table 2)
provides an example of one composition used herein.
TABLE-US-00004 TABLE 2 Component Wt % Supplier Grade Lot #
H.sub.2O(DI) 93.14 n/a n/a n/a PVOH 5.65 Celvol 125 Celanese
34080042 CMC-Na 1.21 CAS 9004-32-4 Acros A0166091
[0198] Within 15 seconds of the solution application, a 0.012 mm
thick film of biaxially oriented PVOH film (Bovlon, Nippon Gohsei,
Osaka, Japan, lot #6525A2) was applied and pressed onto the
solutioncoated side of the electrode. The pressure lamination was
carried out with a 1 inch diameter stainless steel rod, wherein
hand pressure was used to ensure a smooth film coating while
removing the excess PVOH/CMC solution from between the electrode
and PVOH film. The PVOH was permeable to water and KOH, so adequate
electrolyte could be transported to the positive electrode. The
PVOH was laminated to ensure that gas generated in the positive
electrode does not enter the anode compartment, nor accumulate in
any space between the positive electrode surface and the
hydrophilic PVOH layer. The path of least resistance is through the
cell wall exterior.
[0199] Additionally, in the case of a cylindrical cell
configuration, the laminated electrode was allowed to dry, at room
temperature, for between 10 and 15 minutes, after which time a 2 mm
strip of PVOH was removed from the shorter side of the electrode.
This material removal was done to expose part of the electrode and
enable electrical contact between the current collector (screen)
and a bottom cap (that will be introduced later). After the PVOH
strip was removed, the electrode was placed in a 45.degree. C. oven
for between 30 and 45 minutes to fully dry the laminated positive
electrode.
Cell Configurations:
[0200] For a galvanic hydrogen generator, four basic embodiments
were developed for purposes of illustration and evaluation: (1) a
spiral wound design for which an electrode of the configuration
illustrated in FIG. 12 was used; (2) anode surrounded by the
positive electrode in a cylindrical configuration (such as the
design illustrated in FIGS. 1A-1C herein and in the configuration
illustrated in FIG. 11); (3) positive electrode "immersed" in an
anode mass using an "s" shaped ribbon of electrode and the
configuration illustrated in FIG. 12; and, (4) anode surrounded by
the positive electrode in flat prismatic configuration as
illustrated in FIGS. 14A and 14B. With respect to embodiment (1),
plated Nickel foam was used for the positive electrode. For the
other embodiments, a Raney nickel catalyst (as detailed above), in
combination with the substrate that was part of a gas management
electrode system (as detailed elsewhere herein), was used for the
positive electrode (with embodiment (3) utilizing such an electrode
folded on itself, or having a "pouch" configuration, as detailed
elsewhere herein).
[0201] It is to be noted that, prior to testing, the generator
cells of (2), (3) and (4) were placed in an O-ring sealed Delrin
container with electrical feed-throughs and a gas burp valve set to
about 2 to 3 psi, to periodically release the hydrogen generated
therein. Such a container was necessary to test the cells because
the Raney nickel catalyst is reactive with oxygen, and exposure to
ambient air is undesirable. Ambient air can penetrate and diffuse
into the gas permeable hydrophobic outer layers of the gas
management electrode. During each cycle, the generated hydrogen
accumulates in the outer container and builds up pressure in the
container. At the threshold pressure of about 2 to 3 psig, the burp
valve opens instantaneously and releases the pressure, but keeps
oxygen (i.e., air) out of the chamber.
Example 2
Correlation Between Hydrogen Gas Generation Rate and Current
[0202] The hydrogen gas generation rate was observed to correlate
well with the current measured during generation of gas. Once this
correlation was established for different currents, in the interest
of simplicity and speed all the reported results were based on
current measurements.
[0203] A cylindrical cell design of Example 4 (i.e., the
Bobbin-type cell design), further detailed below, was used to make
the noted correlation. Individual generator cells were discharged
at a fixed current of 350 mA and 650 mA respectively, and the
hydrogen generation rates were measured by the displacement of a
column of water in a graduated burette. An additional experiment
was performed that involved a single cell that was tested at
various constant current levels in a stepped manner. A comparison
of the theoretical (calculated) vs. the actual measured gas
generation rate is shown in the graph presented in FIG. 16, which
indicates this correlation is excellent.
Example 3
Spiral-Wound Cell Design
[0204] For this example, a D-size alkaline cell can was used. A
spiral wound design is generally known to one skilled in the art to
enable higher discharge rates (i.e., discharge currents, and hence
gas generation rates in this application) than a typical bobbin
cell design. The positive electrode comprised a compressed nickel
foam material which was post-plated with nickel to produce a more
active nickel surface than the as-received Ni foam (as further
detailed herein below). The spiral wound D cell design consisted of
a layered arrangement of two positive electrode sheets and three
anode pouches made up of the hydrophilic gas impermeable, liquid
permeable layer material. The anode pouches comprised 1.5 wraps of
1 mil M2000 and were filled with a gelled Zn anode. The filled
anode pouches were about 2 mm thick, to enable a higher anode rate
and efficiency than a "bobbin" design used in typical alkaline
batteries.
[0205] As noted, the positive electrode for H2 evolution was
Ni-plated Ni foam, to one surface of which was laminated a layer of
porous PTFE. One challenge of a wound design is to provide a dry
path (i.e., a "chimney") for H2 diffusion. This was achieved by
putting a piece of woven Teflon fabric between two layers of PTFE
film to provide a three-dimensional diffusion path. The two layers
of PTFE were carefully sealed at the bottom and the sides to
maintain a dry interface. The dry vent was made taller than the
electrodes to avoid electrolyte spill over at the top of the cell.
After inserting the wound assembly into the cell can, electrolyte
was added until the pores of the positive electrode and interface
were completely filled (this involved use of with 16 g of 15% KOH).
The anode comprised 61 g of a gelled Zn anode with 66% loading and
28% KOH-2% ZnO anode electrolyte. The initial equilibrium KOH
concentration was calculated to be 22.6%. The KOH concentration
increased during discharge (due to consumption of water) and was
calculated to be 26.3% at 50% and 30.7% at 90% anode discharge. The
cell was discharged using a constant resistance intermittent test
(1 hr cycles 4.times./day). The external resistance was set so that
the initial current was about 4 amps.
[0206] The procedure used for electrolytic nickel plating of nickel
metal foam, for use as a hydrogen generating positive electrode
(including equipment and materials), and the subsequent testing
method used, was as follows:
[0207] Equipment/Material List: (1) A nickel substrate, such as
nickel foam sheet (commercially available from INCO Advanced
Technology Materials Co, Ltd.), having a density of 420 g/m2, a
cell size of 590 um, and a thickness of 1.7 mm; (2) Electrochemical
interface, such as Solartron SI1287, computer and Scribner
Associates Corrware software; (3) Electrochemical cells (such as
glass beakers); (4) Nickel plating solution, such as Buehler
Edgemet Nickel Plating Kit 20-8192; (5) Nickel counter electrode,
such as 40 mesh nickel wire screen, or GDC/Keystone Wire Cloth
(0.005 inch Nickel 200 nominal diameter wire, double cold bonded);
(6) Nickel tabbing material, such as Nickel 200, 0.007 by 0.125
inches, with a length as appropriate for its use; (7) Non-woven
separator material, such as PA160VS30E (commercially available from
Papeteries de Mauduit, France); and, (8) A reference electrode,
such as Zinc metal wire, Alfa Aesar (1.0 mm, 99.95% metal
basis).
[0208] Plating Procedure: (1) The nickel foam sheet was cut into a
rectangle, approximately 2.5 inches by 11 inches (approximately 1.0
cm by 27.9 cm or 27.5 cm2). (2) A nickel tab was attached (by spot
welding) to the nickel foam sheet to provide electrical contact
during the plating process. (3) The nickel foam was placed along
the inside wall of a 2000 ml Pyrex beaker with the tab up. (4) A
strip of separator material approximately 3.8 inches by 14 inches
was cut and placed against the inner wall of the nickel foam. (5) A
length of nickel screen counter electrode was cut (approximately 4
inches by 14 inches). A "flag" was cut on the short end of the
rectangle by cutting the screen about half an inch from the short
edge, cutting from bottom and stopping about a half an inch short
of the top (the flag being folded up to form a tab for electrical
connection), and then placing the nickel screen against the inner
wall of the separator material with the flag up. (6) Equal
quantities of the two Edgemet Nickel Plating solutions were mixed
and then a sufficient quantity thereof was poured into the beaker
to cover the electrodes (a weighted 600 ml beaker may be placed in
the center to allow, by displacement, the use of less Edgemet
solution), followed by mixing or agitating as needed to remove
trapped air around the electrodes. (7) Two electrodes on the
electrochemical interface, such that the nickel foam was the
working electrode and the nickel screen was the counter electrode,
were connected to the electrochemical interface. (8) Plating was
conducted at ambient temperature (approximately 22.degree. C.);
accordingly, at this temperature an anodic or positive 2 amps was
applied to the nickel foam for 5 minutes to clean the surface
through electrochemical etching. (9) Following the etching step, a
cathodic or negative 2 amps was applied to the nickel foam for 60
minutes to plate fresh nickel onto its surface. (10) Finally, the
plated nickel foam was removed from the electrochemical cell and
washed several times in flowing deionized water. The plated nickel
foam was stored under deionized water until use.
[0209] Testing: The resulting plated nickel foam was tested using
the following procedure: (1) A 2 cm2 coupon of the sample plated
nickel foam material was cut, and then an appropriate length of a
nickel tab was attached thereto (to provide an electrical contact)
for testing. (2) The counter electrode was a rectangle of a nickel
screen, approximately 4 inches in size, folded in half and then in
half again to form a rectangle about 1 inch by 4 inches. (3) The
reference electrode was a piece of zinc wire. (4) The electrolyte
was a 31% solution of potassium hydroxide dissolved in water. (5)
The test cell was a 100 ml beaker with the electrode to be tested
on one side, the counter electrode on the other side, and the zinc
wire reference placed near the test electrode. A sufficient
quantity of the electrolyte was added in order to completely
immerse the 2-cm2 test coupon. (6) The two electrodes were
connected to an electrochemical interface, such that the nickel
foam was the working electrode, the nickel screen was the counter
electrode, and the zinc wire was the reference electrode. (7) The
test electrode was evaluated for rate of hydrogen evolution by
applying a voltage range and recording the delivered current (the
voltage was decreased from an initial 0.5 volts to 0.1 volts (test
electrode verses the zinc reference) at a rate of 1 mV per second).
(8) The determined current values were divided by two and reported
as amps per square centimeter (A/cm2), wherein hydrogen production
at a given voltage was directly proportional to the current
delivered at that voltage (the higher the current value, the
greater the rate of hydrogen production).
[0210] The graph presented in FIG. 13 compares the voltage/current
response for the unplated nickel foam with the averaged results of
10 nickel plated, nickel foam samples. These results clearly
indicated that plated foam was much more active for hydrogen
generation.
[0211] Samples of the unplated and plated nickel foam materials
were also subjected to Scanning Electron Microscopic (SEM) and
Energy Dispersive X-ray (EDAX) analyses. The plating of the nickel
foam resulted in a slight observed increase in irregularity of the
surface structure. The EDS data are shown in Table 3, below. This
analysis also found the additional presence of magnesium (Mg),
phosphorous (P) and oxygen (O) on the surface of the plated nickel.
It is possible that, in addition to the higher roughness, the
incorporation of the Mg and P may have increased the activity of
the foam.
TABLE-US-00005 TABLE 3 Energy Dispersive Spectroscopy Analysis of
the Nickel Surfaces Determined A/cm.sup.2 Ni Foam at 0.1 V vs. Zn O
Mg P Ni Plated 070410A-2 2.04 2.9 0.7 0.5 95.9 Unplated Foam 0.06
-- -- -- 100
Example 4
Anode Surrounded By Positive Electrode (AA Size Bobbin-Type)
Design, With AA Positive Electrode Tube Formation Process
[0212] This example utilized the configuration depicted in FIG. 11,
where the entire outer wall of the cylindrical cell is a porous
surface through which the generated hydrogen can easily pass. This
design approach provides a simple means to achieve orientation
independence for the generator since the hydrogen escapes out of
the walls of the sealed cell with hydrophobic porous walls. Using
laminating/winding equipment, a sheet of PTFE (Low density 600A,
Plastomer Technologies, Newtown, Pa.) was applied to the coated
side of the positive electrode with a lamination pressure of 21
psig (+/-1 psig). The PTFE sheet width is about 1 to 2 mm wider
than that of the electrode. The PTFE lamination was performed
simultaneously with forming the electrode into a cylinder, where
the cylindrical electrode contained an overlap of about 5.0 mm. The
PTFE sheet was wound externally around the cylindrical positive
electrode a total of 3 times (one wrap covering the coated surface
and 2 full additional overwraps).
[0213] Once the cylindrical shell was formed by means of the PTFE
wrap, the inner electrode seam was sealed to prevent any loss of
electrolyte. The PVOH/CMC solution discussed above was applied at
the interior axial joint of the electrode overlap, using a syringe
with a 22 gage, 1.5 inch needle. This viscous solution was allowed
to dry for 30 minutes at room temperature with the axial bead of
sealant parallel to the floor to contain the solution in the
overlap area while a skin formed. After 30 minutes, the cylinder
was placed in a 45.degree. C. oven and dried for an additional 30
to 45 minutes (until the liquid from the solution evaporated).
[0214] A grommet and cap were placed on the end of the cylindrical
electrode where the PVOH film strip was removed. Once radially
crimped, electrical contact exists between this bottom cap and the
current collector. A piece of separator paper, in this case PA 160
VS 30E PDM (commercially available from Papeteries de Mauduit,
29393 Quimperle Cedex, France) was inserted into the interior of
the cylindrical electrode in such a way that the entirety inner
surface of the positive electrode was covered by the separator.
Desirably, an overlap of at least about 10% (by circumference) was
used to ensure that zinc particles from the anode did not directly
contact the positive electrode surface. Additionally, the separator
was used (as is commonly practiced by those skilled in this art) as
a means of maintaining a reservoir of electrolyte and a wetted
interface between the anode and positive electrode. An insulating
bottom disk or cup was used to electrically insulate the metallic
bottom cup from the anode. This was inserted after the separator
layer was placed within the cylinder and acts as an anchor at the
bottom of the cell to ensure maximum interior volume is attained as
well as minimizing the gap in the separator overlap to prevent
accidental particulate zinc transport to the positive electrode
surface that can cause shorting.
[0215] This cell was filled using 10.6 g of 66% zinc 25-2
(KOH--ZnO) anode, 0.8 g of a 20-0 gelled KOH, and 0.3 g of 15-0
liquid electrolyte. The liquid electrolyte was used for initial
wetting of the separator, and some gelled electrolyte was added to
the top of the anode after the anode was filled into the
cavity.
[0216] The generator cell was tested at 0.5.OMEGA. external load
under intermittent conditions to a capacity of 0.35 Ah for each
step, with a 3 hour rest period between cycles. The results of the
test for this cell are provided in the graph of FIG. 15.
Example 5
Positive Electrode-In-Anode Design--AA Size
[0217] This cell design is considered an "inside out" cell design
(compared to a conventional alkaline battery). The objective was to
develop a higher rate design than Example 3 to increase rate of
hydrogen production and to increase efficiency of zinc utilization.
The inside walls of a AA alkaline cell can were plated with copper
(see procedure below) to prevent contact between the zinc anode and
the nickel can (which would spontaneously produce hydrogen gas if
not protected). A double layer of positive electrode (5 cm by 4 cm)
was formed into an approximate "S" shape and inserted into the can.
In addition to reducing the thickness of the anode layer, this
design allowed two surfaces of the positive electrode to be
utilized (since the gas management electrode here was folded on
itself as depicted in FIG. 12). The positive electrode used was
essentially the same composition as the one used in Example 3 above
(i.e., a composite positive electrode comprising deactivated Raney
Nickel catalyst). The anode consisted of 10.2 to 10.4 g of 66% 25-2
anode and 2.0 g of 15-2 gelled electrolyte. The gelled zinc anode
was filled into all the cavities between the positive electrode and
the can. The cells were discharged using a constant resistance
intermittent test (i.e., 1 hr cycles 4.times./day). The resistance
was set so that the initial current was about 500 mA.
[0218] Copper plating of the inside of the AA can was performed
using electroless plating, via the reduction of copper sulfate
solution (CuSO4.5H2O) with formaldehyde (HCHO). The electroless
plating solution consisted of 0.1M CuSO4.5H2O, 0.05 M
ethylenediamine tetraacetic acid (EDTA) and formaldehyde (35%). The
pH of the plating solution was adjusted to between 11 and 12 by
potassium hydroxide, and the plating temperature was
50.+-.5.degree. C. Prior to electroless plating, the inside of
positive electrode can was first thoroughly washed with water and
then dried at room temperature. Finally, the dried can was filled
with the plating solution and held at 50.+-.5.degree. C. in a water
bath for about 1 hour. The copper plated can was thoroughly washed
with distilled water and then dried at room temperature.
Example 6
Prismatic Design (Pouch Cell)
[0219] The prismatic cell design (or pouch cell design), as further
illustrated in FIGS. 14A and 14B, has the advantage of not only
having a thin anode layer (optimized for high rate), but also a
uniform anode thickness, so as the discharge progresses, the
interfacial anode area remains essentially constant. In a
cylindrical bobbin type design, the interfacial anode area shrinks
as discharge progresses, making it more difficult for high rate
discharge as more of the anode is consumed. This cell design builds
on the orientation independent approach disclosed in Example 3 but
is a significant improvement over Example 4 due to the higher rate
capability achieved by the thinner and substantially even thickness
of the anode layer. The cell design comprises an anode current
collector 400 and anode 405, around which is wrapped or folded (in
a generally "U" shape) a gas management electrode (i.e., an
optional separator (e.g., nonwoven material) 410, a gas
impermeable, hydrophilic layer 415, a positive electrode and
support (collectively 420), which is in electrical communication
with a current collector (425), and a gas permeable, hydrophobic
layer (430), the outer edges 440 and 445 thereof being sealingly
connected to each other and around the anode current collector post
and the positive electrode post (i.e., the outer edges of one or
more of, for example, the gas impermeable, hydrophilic layer and/or
the gas permeable, hydrophobic layer being sealed to substantially
prevent leakage).
[0220] The cell structure is provided by the positive electrode,
which is 75% Raney nickel, 20% carbon and 5% PTFE coated onto a
carbon coated nickel screen or support. The positive electrode is
cut to dimensions of about 100 mm by 50 mm and then a nickel tab is
welded to the positive electrode current collector. The positive
electrode is then laminated to 2 layers of Plastomer Technologies
A600 Low Density PTFE (polytetrafluoroethylene) at a gap of 22
mils. The PTFE is trimmed so that approximately 8 mm of overhang
remains. The positive electrode and PTFE are then laminated to 1.5
mil M2000 PVA (polyvinyl alcohol), obtained from Monosol LLC
(Portage, Ind.). An adhesive gel comprising PVA/CMC (carboxymethyl
cellulose) is used for the lamination, at a pressure of about 60
psi. The PVA layer overhangs the positive electrode on each side by
about 4 mm. With the positive electrode, PTFE and PVA layer as a
single unit, it is folded in half the long way and the side seams
are heat sealed. These side seams include both the overhanging PVA
and PTFE layers to prevent leakage between the anode and positive
electrode compartments as well as to the outside of the cell. As an
extra precaution, each seam has two heat seals. The first heat seal
bonds the PVA and PTFE layers, while the second heat seal is on the
outer edge where the PTFE extends beyond the PVA layers and bonds
just the PTFE layers. An optional separator layer of nonwoven F3T23
(from Kuraray Corporation, Japan) is inserted into the cell pouch
to hold additional electrolyte near the PVA layer interface and as
an extra shorting protection. The anode compartment contains 17 g
of 28-2 66% anode and 2 g of 26-2 electrolyte. The anode current
collector is a 36.9 mm by 31.3 mm by 0.076 mm perforated brass
foil. The base of the anode current collector tab is wrapped in a
hydrophilic F3T23 grade nonwoven material (Kuraray Corporation,
Japan) that is first soaked in a 7% PVA/water solution, to improve
the bonding with PVA in the top seam area. Once this was dry, the
top seam was then heat sealed in the same manner as the sides, with
the two tabs extending outward. With the top seam completed, the
tabs were then further sealed to the top of the cell by heat
sealing them between two layers of an aluminum polymer composite
(GLAM-085SL, Pliant) to prevent KOH electrolyte leakage.
Example 7
Performance Evaluation
[0221] The cells prepared above were placed in a O-ring sealed
Delrin container with electrical feed-throughs and a gas burp valve
set to between about 2 and 3 psi to periodically release the
generated hydrogen. Such a container was used to test the cells
because the Raney nickel catalyst is reactive with oxygen, and
exposure to ambient air is undesirable. Since there is no hydrogen
consuming device connected to the outer container, the hydrogen
generated during each cycle accumulates in the outer container and
builds up pressure in the container. At the threshold pressure of
between about 2 and 3 psi, the burp valve opens instantaneously and
releases the pressure but keeps oxygen (air) out of the
chamber.
[0222] The generator cells were tested at 0.3.OMEGA. under
intermittent conditions to a capacity of 0.35 Ah for each step,
with a 3 hour rest period between cycles. The results of the test
are presented in the graph of FIG. 15. The results indicate that
the cell design and configuration can be modified to obtain the
desired generation rate and fuel utilization efficiency. For high
gas generation rates, it is desirable to utilize a flat prismatic
type of design.
[0223] Table 4 shows some of the key design parameters, including
the interfacial area-to-anode volume ratio.
TABLE-US-00006 TABLE 4 Cell Design Parameters Interfacial Anode
Interfacial Cell Design Area, cm.sup.2 Volume, cc Area/Zinc Cath
Surrounding 16.86 0.92 18.3 Anode - Example 4 Cathode in Anode - 24
0.98 24.5 Example 5 Prismatic Pouch 43.68 1.42 30.8 Cell - Example
6
Example 8
Determination of Water Content in Hydrogen From Galvanic Cell
[0224] The anode of a fuel cell produces water which is transported
through the membrane to the positive electrode, from where it is
expelled out of the cell as vapor. The water content in the
incoming hydrogen stream is desirably controlled to avoid a
flooding situation in the cell. Hence, the water content of the
hydrogen stream is important for optimum functioning. To analyze
this parameter, a gas chromatograph fitted with on column gas
injection and thermal conductivity detector was used. (Instrument
Details: Thermo Finnigan Trace GC 2000 equipped with TCD detector
and on column gas injection loop; Supelco 100/120 10'.times.1/8''
Hayesep D column; 120.degree. C. oven temperature; 5 minute run
time; 0.25 mL sample loop size; 25 mL/min Helium carrier gas.
Desiccant Materials: Anhydrous Calcium Sulfate from Drierite (Stock
#26930); 30/40 mesh Molecular Sieve 5A from Supelco.)
[0225] The procedure used in this example was as follows: A 30 mL
hydrogen sample was collected from a hydrogen gas generator with a
gas tight syringe. The sample was injected into the instrument
within five minutes of its collection to preserve the integrity of
the sample. For concentrations of water vapor above 2 g/m.sup.3,
the result was obtained by comparing the water peak area against a
standard curve that was generated. For concentrations below 1000
ppm, the result was compared against results obtained from samples
taken from a dry room environment, which has less than 35 ppm water
concentration, and then an approximate concentration was
determined. Using this method, the moisture content of the hydrogen
gas from cells of example 4 and example 6 are in the range of 8-12
g/m.sup.3, or about 1% to about 1.5%.
[0226] When introducing elements of the present invention or the
various versions, embodiment(s) or aspects thereof, the articles
"a", "an", "the" and "said" are intended to mean that there are one
or more of the elements. The terms "comprising", "including" and
"having" are intended to be inclusive and mean that there may be
additional elements other than the listed elements. The use of
terms indicating a particular orientation (e.g., "top", "bottom",
"side", etc.) is for convenience of description and does not
require any particular orientation of the item described.
[0227] In view of the above, it will be seen that the several
advantages of the invention are achieved and other advantageous
results attained. As various changes could be made in the above
processes and composites without departing from the scope of the
invention, it is intended that all matter contained in the above
description and shown in the accompanying drawings shall be
interpreted as illustrative and not in a limiting sense.
[0228] This written description uses examples to disclose the
invention, including the best mode, and also to enable any person
skilled in the art to practice the invention, including making and
using any devices or systems and performing any incorporated
methods. The patentable scope of the invention is defined by the
claims, and may include other examples that occur to those skilled
in the art. Such other examples are intended to be within the scope
of the claims if they have structural elements that do not differ
from the literal language of the claims, or if they include
equivalent structural elements with insubstantial differences from
the literal languages of the claims.
* * * * *