Note: Descriptions are shown in the official language in which they were submitted.
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MICROCELL ELECTROCHEMICAL DEVICES AND ASSEMBLIES, AND METHOD OF
MAKING AND USING THE SAME
DESCRIPTION
Field of the Invention
This invention relates to microcell electrochemical devices and assemblies,
methods of making same
by various techniques, and use of such devices and assemblies.
Description of the Art
In the field of energy supplies and energy conversion devices, and
particularly in the development of
fuel cells and batteries, there has been continuing effort to develop devices
with significant power
outputs (high current and/or high voltage), high power density, and high
energy output per unit
volume.
Structurally, electrochemical cells such as batteries and fuel cells are
relatively simple, utilizing
respective positive and negative electrodes separated in such manner as to
avoid internal short
circuiting, and with the electrodes being arranged in contact with an
electrolyte medium. By chemical
reaction at the electrodes, the chemical energy of the reaction is converted
into electrical energy with
the flow of electrons providing power when the electrode circuit is coupled
with an external load.
Battery cells may use separator plates between respective electrodes so that
multiple sheet elements
are arranged in successive face-to-face assemblies, and/or such sheets may be
wound together in a
(spiral) roll configuration.
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The fuel cell is of significant current interest as a source of power for
electrically powered vehicles, as
well in distributed power generation applications.
In fuel cells, a fuel is introduced to contact with an electrode (a.node) and
oxidant is contacted with the
other electrode (cathode) to establish a flow of positive and negative ions
and generate a flow of
electrons when an external load is coupled to the cell. The current output is
controlled by a number of
factors, including the catalyst (e.g., platinum in the case of hydrogen fuel
cells) that is impregnated in
the electrodes, as well as the lcinetics of the particular fuel/oxidant
electrochemical reaction.
Currently, single cell voltages for most fuel cells are in the range of about
0.6-0.8 volts. The operating
voltage depends on the current; as current density increases, the voltage and
cell efficiency
correspondingly decline. At higher current densities, significant potential
energy is converted to heat,
thereby reducing the electrical energy of the cell.
Fuel cells also may be integrated with reformers, to provide an arrangement in
wllich the reformer
generates fuel such as hydrogen from natural gas, methanol or other feed
stocks. The resulting fuel
product from the reformer then is used in the fuel cell to generate electrical
energy.
Numerous types of fuel cells have been described in the art. These include:
polymer electrolyte fuel cells, in which the electrolyte is a fluorinated
sulfonic acid polymer or similar
polymeric material;
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alkaline fuel cells, using an electrolyte such as potassium hydroxide, in
which the KOH electrolyte is
retained in a matrix between electrodes including catalysts such as nickel,
silver, metal oxide, spinel or
noble metal;
phosphoric acid fuel cells using concentrated phosphoric acid as the
electrolyte in high temperature
operation;
molten salt fuel cells employing an electrolyte of alkali carbonates or
sodium/potassium, in a ceramic
matrix of lithium aluminate, operating at temperatures on the order of 600-700
degrees C, with the
alkali electrolyte forming a high conductive molten salt;
solid oxide fuel cells utilizing metal oxides such as yttria-stabilized
zirconia as the electrolyte and
operating at high temperature to facilitate ionic conduction of oxygen between
a cobalt-zirconia or
nickel-zirconia anode, and a strontium-doped lanthanum manganate cathode.
Fuel cells exhibit relatively high efficiency and produce only low levels of
gaseous/solid emissions.
As a result of these characteristics, there is great current interest in them
as energy conversion devices.
Conventional fuel cell plants have efficiencies typically in the range of 40-
55 percent based on the
lower heating value (LHV) of the fuel that is used.
In addition to low environmental emissions, fuel cells operate at constant
temperature, and heat from
the electrochemical reaction is available for cogeneration applications, to
increase overall efficiency.
The efficiency of a fuel cell is substantially size-independent, and fuel cell
designs thus are scalable
over a wide range of electrical outputs, ranging from watts to megawatts.
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A recent innovation in the electrochemical energy field is the development of
microcells - small-sized
electrochemical cells for battery, fuel cell and other electrochemical device
applications. The
microcell technology is described in U.S. Patent Nos. 5,916,514; 5,928,808;
5,989,300; and 6,004,691,
all to Ray R. Eshraghi. The microcell structure described in these patents
comprises hollow fiber
structures with which electrochemical cell components are associated.
The aforementioned Eshraghi patents describe an electrochemical cell structure
in which the single
cell is formed of a fiber containing an electrode or active material thereof,
a porous membrane
separator, electrolyte and a second electrode or active material thereof. Cell
designs are described in
the Eshraghi patents in which adjacent single fibers are utilized, one
containing an electrode or active
material thereof, the separator and electrolyte, with the second fiber
comprising a second electrode,
wliereby the adjacent fibers constitute positive and negative electrodes of a
cell.
The present invention embodies additional advances in the Eshraghi microcell
teclmology.
BRIEF DESCRIPTION OF THE DRAWINGS
Figures 1-4 are perspective views of fibrous element structures illustrating
the fabrication of a
microcell assembly.
Figure 5 is a perspective view of a connector for joining current collector or
electrode elements of a
-microcell fiber assembly.
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Figure 6 is a microcell asseinbly according to one embodiment of the
invention, with a terminal at one
end of the assembly.
Figure 7 is an exploded perspective view of a niicrocell asseinbly showing
series-comlected inicrocell
sheets.
Figure 8 is a schematic view of a layered arrangement of microcell sheets,
joined in series relationship.
Figure 9 is a 3-dimensional perspective view of a series-connected arrangement
of microcell layers.
Figure 10 shows a potted arrangement of microcell sheets.
Figure 11 is a perspective view of a duct that is perforated on the top
surface, and optionally on the
bottom surface for the fabrication of double stack bundles of electrochemical
cells.
Figure 12 is a cross-sectional elevation view of a microcell fiber bundle
potted in a vessel.
Figure 13 is a side elevation view of the vessel of Figure 12.
Figure 14 is an elevational cross-sectional view of a double stack of
microcell sheets.
Figure 15 is a side elevation view of a double stack of microcell devices
arranged in sheets,
comprising a stack on each side of a perforated duct.
Figure 16 is a perspective view of potted fibers on one side of a perforated
feed duct.
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Figure 17 shows a vessel with fibers laid on both sides of the perforated feed
duct.
Figure 18 is a side elevation view of an electrochemical cell device
comprising an assembly of
microcells.
Figure 19 shows a perforated feed tube used as a mandrel in forming microcell
structures.
Figure 20 shows fibrous microcell and shell side current collector sheets that
can be rolled or wound
around the perforated tube of Figure 19, with the sheets being shown during
rolling in Figure 21 and
as finally rolled -into shape in Figure 22.
Figure 23 shows sheets of fibrous microcell eleinents and shell side current
collectors, and an
insulating sheet (e.g., of fiberglass or porous plastic material).
Figure 24 is a perspective view of a sheet assembly including two sheets of
fibrous microcell elements
and shell side current collectors.
Figure 25 is a side elevation view of a microcell assembly with off-set fiber
layer sheets.
Figure 26 is a cross-sectional view of a microcell bundle.
Figure 27 is a side elevation view of series-connected rnicrocell sub-bundles
according to one
embodiment of the invention.
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Figure 28 is a perspective view of a connector that may be used to join
component microcell sub-
bundles in series.
Figure 29 is a cross-sectional elevation view of a multibundle assembly,
wherein each bundle has a
corresponding feed tube associated therewith.
Figure 30 is a cross-sectional elevation view of a inultibundle asseinbly,
wherein the respective
bundles are coimected in series.
Figure 31 is a cross-sectional view of a fuel cell module witli multiple sub-
bundles wllerein blank seal
elements provide closure members for the face sheet of the module enclosure.
Figure 32 is a side view of a fuel cell module with multiple sub-bundles of
microcell elements, with a
feed tube in a manifolded arrangement.
Figure 33 is a side elevation view in section, showing penetration of a feed
tube into the interior
volume of the housing of a module containing microcell sub-bundles according
to one embodiment of
the present invention.
Figure 34 is a cross-sectional view of a microcell assembly in which heat
exchange fibers or tubes are
provided in interspersed relationship to the microcell bundles.
Figure 35 is a cross-sectional elevation view of a fuel cell module, showing
air/fuel passages and heat
exchange passages, interspersed between the sub-bundles.
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Figure 36 is a cross-sectional view of a microcell bundle wlierein hollow
fibers function as outer
electrode elements and enable heat exchange.
Figure 37 is a side elevation in cross section of a fuel cell with heat
exchange/current collector hollow
fibers.
Figure 38 is a cross-sectional elevation view of a fuel cell module with heat
exchange from current
collectors by means of conduction.
Figure 39 is a schematic depiction of a fuel cell system, according to one
embodiment of the
invention.
Figure 40 is a cross-sectional view of a double membrane design with an
electrically conductive perm-
selective membrane on the anode or cathode element of the microcell.
Figure 41 is a cross-sectional view of a double separator design with perm-
selective membranes
protecting the anode or cathode elements of the microcell.
Figure 42 is a cross-sectional view of a double separator design with perm-
selective meinbranes
covering both anode and cathode elements of the microcell.
Figure 43 is a cross-sectional view of a double separator design with penn-
selective membranes
covering both anode and cathode elements of the microcell and with a porous,
electrically conductive
inner separator.
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Figure 44 is a cross-sectional view of a double separator design with perm-
selective inembranes
covering both anode and cathode elements of the niicrocell and with refonner
catalyst on the iimer
wall of the inner separator.
Figure 45 is a scheinatic flowsheet of a solution impregnation system for
iinpregnation of a membrane
fiber with Nafion or electrocatalyst.
Figure 46 is an elevation view of a metallic fiber having a polyineric
compound on its outer surface.
Figure 47 shows the corresponding fiber of Figure 46 after pyrolysis, with a
pyrolyzed carbon coating
on the outside surface thereof.
Figure 48 shows a fibrous carbon current collector laid along a coated
metallic fiber.
Figure 49 shows the fiber assembly of Figure 48 after a disconnection break of
the coated metallic
fiber.
Figure 50 shows a cross-section of a hollow fiber and microcell tube bundle,
in which the plane
hollow fiber elements are used for channeling water from the assembly.
Figure 51 shows a vertically upward extending bundle of microcells, arranged
so that water from the
module drains to a lower plenum space for removal.
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DETAILED DESCRIPTION OF THE INVENTION. AND PREFERRED
EMBODIMENTS THEREOF
The disclosures of Eshraghi U.S. Patent Nos. 5,916,514; 5,928,808; 5,989,300;
and 6,004,691
are noted as pertaining to the general field of the present invention.
As used herein, the term microcell refers to an electrochemical cell energy
generation or conversion
structure, including a porous membrane separator having electrolyte disposed
in porosity thereof The
porous membrane separator is in contact with electrically conductive fibers
that in turn are in contact
with or are coated with electrocatalyst forming positive and negative
electrodes for the electrochemical
cell.
'While the ensuing description herein is primarily directed to fuel cell
embodiments of the instant
iinvention, it will be appreciated that the description can be analogously
applied to corresponding
battery cells and to other forms of electrochemical cell devices, consistent
with the invention.
A battery cell of course differs from a fuel cell in that the (electrode)
active material in a battery is
present and stored in the cell, as opposed to being extemally furnished to the
structure when
electrochemieal activity is desired.
Accordingly, when used in a battery cell, the microcell does not require a
lumen at the center of the
fiber, thereby correspondingly simplifying the bundling of fibers in modular
assemblies for battery cell
applications. Microcells for battery cell applications thus have structural
and operational differences
from microcells used in fuel cells.
CA 02417682 2003-01-29
~
=a rrn
In a specific form, the microcell comprises an inner electrode, active
material, a microporous
membrane separator in contact with the inner electrode active element,
electrolyte in pores of the
microporous membrane separator, and an outer electrode active element, wherein
each of the inner and
outer electrode active elements comprises at least one of electrode, current
collector and
electrocatalyst components.
In another specific form, the microcell may include a fibrous, inner electrode
that is encapsulated by a
microporous membrane separator with an electrolyte disposed in porosity of the
microporous
membrane separator, and with electrocatalyst impregnated or coated on the bore
or shell side of the
fiber (to form an inner or outer electrode, respectively) along with
electrically conductive material.
In fuel cell applications, the bore of the microcell hollow fiber defines a
lumen for passage
therethrough of gaseous or liquid feed (e.g., fuel or oxidant) components.. A
wide variety of
electrolyte types can be used in the microcell fuel cell, depending on the
specific application involved.
In a preferred form, all components of the microcell are fabricated in a
single fiber assembly. The
microcells can be of any predetermined length, typically with a length to
diameter ratio significantly
~...,_~
greater than 1, and are readily formed into microcell assemblies, including
bundled forms as
hereinafter described in greater detail. Such microcell assemblies, or
collections of such assemblies,
may be aggregated to form a fuel cell module, similar in overall arrangernent
to a shell and tube heat
exchanger.
When the microcell elements are fabricated into bundled multi-cell modules in
a unitary overall
construction, the resulting compact unitary configuration provides high
density energy output and
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enables minimization of the volume (and "footprint") of the fuel cell or other
electrochemical cell
apparatus fabricated from such bundles.
The inicrocell apparatus of the invention in one embodiment is fabricated with
the inner electrode (or a
multiplicity of current collector fibers) being encapsulated by a microporous
membrane separator.
The electrocatalyst of the inner electrode in such embodiment is coated or
impregnated on the inner
wall of the membrane separator (or coated on the iimer current collector
fibers).
The electrocatalyst in one embodiment is impregnated onto the membrane
separator wall from a
catalyst solution. In another, alternative embodiment, a thin inlc formulation
of the catalyst is pumped
through the bore of the membrane separator during the membrane spinning
process.
One technique of forming porous separator membrane-electrode assemblies
involves coating current
collector fibers with an electrocatalyst formulation. Such coating in one
embodiment is carried out in
an extrusion process. In another embodiment, the current collector fibers are
coated from a plating
solution. In yet another embodiment, the current collector fibers are coated
by plasma deposition of a
metal catalyst.
In forming a fuel cell stack or module, the microcell fibers are bundled and
potted in order to isolate
and seal the bore side and the shell side of the cells. For large fuel cell
structures, microcells may be
bundled around a perforated mandrel, such that the mandrel becomes the gas
input structure for the
shell side of the cells.
With respect to the microporous membrane separator element as used in fuel
cell embodiments and in
other electrochemical cell embodiments of the present invention, any suitable
means and method for
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electrolyte impregnation or incorporation are usefully employed. An
illustrative and preferred
technique for impregnation of the electrolyte is solution impregnation.
The porous membrane separator element itself can be of widely varying type and
structure, and
formed for a specific type of fuel cell or other electrochemical cell
application. For polymer
electrolyte fuel cells, for example, an asymmetric channelized porous
structure is preferred to provide
a contiguous phase of the ion exchange polymer adjacent to the electrocatalyst
layer. For acid or
alkaline fuel cells, a foam-like structure of the porous membrane element is
desirable. The choice of
membrane separator conformation and morphology is readily determinable without
undue
experimentation, as will be appreciated by those skilled in the art.
Fuel cells formed from microcells in accordance with a preferred aspect of the
invention are
monopolar and do not require bipolar flow field plates. Since the cells and
current collectors are in
fiber form, a high level of electrode surface area can be compacted in very
small volumes. In parallel
connection of individual bundled cells, wherein current is additive, very high
current density per unit
volume is achievable, allowing the microcell assembly to operate at high
voltage and high efficiency.
hi one embodiment, inner electrodes of respective inicrocells are connected to
form a first terminal of
a microcell asseinbly, and current collectors on the outer shell of the fiber
elements or on the outer
shell of a bundle of such microcells, forms a second terminal. When such
assembly is constructed and
arranged for fuel cell usage, fuel and oxidant are passed over electrodes on
the corresponding
respective shell and bore sides of the bundle. In the individual microcell
elements of this fuel cell, the
microporous membrane is iinpregnated with an appropriate electrolyte and forms
a barrier or separator
element. Depending on the electrolyte type, the microporous matrix and
electrolyte can combine to
form a new structure in the form of a solid matrix or a liquid-solid matrix.
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In fuel cell applications utilizing microcell devices containing a single
fiber imier electrode element,
the size of the inner electrode element is selected to provide an
appropriately dimensioned lumen on
the bore side of the membrane separator containing the electrode. Multiple
fibers can also be
positioned in the bore of a hollow fiber membrane separator to provide
interstitial space forming a
lurrien in the hollow fiber. The formation of the lumen is important since the
lumen allows (liquid or
gaseous) fuel or oxidant to reach the inner electrode in the operation of the
fuel cell.
In a preferred form, an electrocatalyst and the electrically conductive
material of a second electrode is
coated, extruded or impregnated on the outer shell of the microporous membrane
separator and
electrolyte is disposed in the micropores of the membrane separator, to
coinplete the microcell
structure.
The microporous membrane separator may be formed of any suitable material of
construction. In one
embodiment, the microporous membrane separator is fabricated from a material
selected from the
group consisting of semi-permeable, ion-exchange membranes, and a porous
membrane coated on a
shell or bore side thereof with a penn-selective or an ion-exchange polymer.
In the microcell structure, the inner electrode or current collector is
retained in a tightly-held maimer in
the bore of the separator and is contiguous to the inner wall of the fiber,
for interfacial contact with the
electrolyte or electrolyte/electrocatalyst layer. The outer electrode or
current collector also makes
intimate contact with the shell side electrolyte, or with the
electrolyte/electrocatalyst layer of adjacent
cells, wlien the fibrous microcell structures are densely bundled with one
another.
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Accordingly, the lumen of the inicrocell structure in fuel cell applications
inust be sufficiently "open"
to allow passage of the gaseous feed (fuel or oxidant) througll the lumen
during noimal operation. For
such purpose, the fuel cell apparatus desirably includes a puinp, fan, blower,
compressor, eductor, or
the lilce. Since the flow rates required for fuel cell operation entail
relatively low pressure
differentials, pumping requirements (for gaseous feed flow through the lumen
of the microcell hollow
fiber) are readily accommodated by commercially available fluid driver devices
of the above-
mentioned types.
Series Connection of Microcell Structures and Assemblies
To achieve high current density at a single microcell voltage level, a number
of microcells are
connected together in parallel. Parallel connection of niicrocells for such
purpose is readily effected by
bundling the microcells in parallel relationship to one another and connecting
the end portions of the
current collectors at each extremity of the resulting microcell assembly.
In order to achieve high voltages, however, above the voltage afforded by a
single microcell, it is
necessary to comlect microcells in series with one another. As described more
fully hereinafter,
various methods may be employed to effect series connection, depending on the
geometry of the
microcell assembly that is desired. For example, a rectangular configuration
or a cylindrical
configuration may be desired.
In accordance with the invention, a sub-bundle of parallel fibrous microcells
is first constructed to
obtain the desired current. The sub-bundles then are connected in series to
achieve a desired voltage.
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One preferred approach to forining a sub-bundle microcell assembly is to form
a sheet arrangement of
generally parallelly aligned microcell elements, wherein the microcells are in
side-by-side relationship
to one another, with the current collectors extending from one end of the
generally planer sheet, in
side-by-side register with one another (i.e., so that the current collector
ends are generally arranged in
a single plane with respect to one another, or otherwise so that the current
collectors protruding from
the microcells are generally coextensive in length relative to the face of the
microcell sheet assembly
from which they protrude). Next, the first layer of microcell elements is
overlaid by a second layer
comprising outer current collector elements, arranged so that the outer
current collectors extend from
an opposite side of the superimposed sheet from that from which the inner
current collectors protrude.
The outer current collectors likewise extend outwardly to a generally same
lengtli, so that the ends of
the outer current collectors are in register with one another, residing
generally in a single vertical plane
relative to a flat, horizontal plane of the sheet assembly.
For purpose of fonning the above-described sheet assembly, the constituent
fibrous microcell elements
in the first layer may be secured to one another to provide a unitary web or
sheet form of such
elements. In like mamier, the outer current collectors overlaid on the fibrous
microcell elements may
be secured to one another to a sheet or web confirmation, such as by an inner
connecting mesh or
woven structure, transversely laid strips of adhesive tape, or other means by
which a parallel assembly
of current collector elements is provided.
It will be appreciated that any suitable means and methods may be employed to
form the respective
sheet-like layers of the microcell assembly just described. Such layers can be
pre-formed, for
example, by weaving the microcell or current collector fibers into sheets or
embedding them in a
resinous matrix, or in any other suitable manner.
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Once the layer of microcell elements and the layer of outer current collector
elements is contacted in
superimposed relationsllip with one another, the composite structure then can
be rolled into a
cylindrical shape and potted at each of respective opposite ends, to forin a
sub-bundle assembly
comprising a multiplicity of microcells.
Potting of such assembly can be carried out in any suitable manner, using
methods conventionally
employed to pot hollow fiber membranes, e.g., in the fabrication of hollow
fiber filtration modules.
Each resultant potted sub-bundle thereby has a positive and negative terminal
at each end, with one
such terminal being formed by the iimer current collector elements protruding
from the microcell
elements of the first-described layer, and the other terminal being formed by
the outer current collector
elements protruding from the opposite end of the sub-bundle.
Sub-bundles then are connected in series by connecting the positive terminal
of a first sub-bundle to a
negative terminal of a next sub-bundle, and so on in consecutive fashion. The
resulting long strand of
connected sub-bundles then is re-bundled into a cylindrical shape, by folding
each bundle in an
alternating fashion at each end and at the connection between each succeeding
microcell. The
resulting assembly of sub-bundles folded into parallel arrangement with one
another then is potted
again at each end thereof to form a bundle as a composite structure comprising
a multiplicity of sub-
bundles.
The bundle in consequence contains fibrous microcells in both parallel and
series connection,
constituted in a unitary structure that may then be placed in a casing in the
manner of a shell and tube
heat exchange assembly, as hereinafter described in greater detail.
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It is evident from the foregoing discussion that the avoidance of short-
circuiting between sub-bundles
requires that each sub-bundle be covered or encased with a porous yet
electrically insulating material.
Accordingly, each sub-bundle inay be sheathed or sleeved in a fiberglass or
polymeric material
encasement member, in to which the sub-bundle may be inserted or about which
the encasement
material may be wrapped.
Sub-bundles alternatively may be formed and the packed into a bundle by
alternating each end so that
a positive terminal end of a sub-bundle is in proximity to a negative terminal
of another sub-bundle.
The sub-bundles in this altenlative teclulique can first be potted and then
connected in series, by
connection of the positive terminal of a first sub-bundle to a negative
terminal of a next adjacent sub-
bundle. The sub-bundles may be coiuiected simply be malcing an electrical
connection between each
microcell. Alternatively, an end plate having a mirror image of the location
of sub-bundle connection
nodes (where all the microcell fibers are connected in parallel in a sub-
bundle) on its face, and an
imprint of series connections of the terminals designed and built into the
plate may be employed, so
that electrical connection of the plate with each node of the bundle will
automatically yield a series
connection.
As yet another alternative to sub-bundle potting, eacll sub-bundle may be
fabricated with a sealed tube
sheet member at each end. Each sub-bundle then can be inserted into a casing
having openings at each
end tliereof that are the same size as the parameter (outer circumference) of
the sub-bundle. In such
fabrication, each sub-bundle may be sealed at each respective end of the
housing, e.g., with 0-ring
seals or other sealing means, without the requirement of having to pot the sub-
bundles again. In such
configuration, each sub-bundle can be removed from or introduced to the
housing in a simple and
readily affected manner, allowing for increase or reduction in power
generation capacity of the overall
microcell apparatus.
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Alternatively, a sub-bundle article can be fabricated in a rectangular
confiimation by placing layers of
microcells and outer current collectors over each other in alternating and
repeating sequence to
achieve a desired height and rectangular cross-section. The constituent layers
of microcell fibrous
elements and outer current collectors can be preformed in sheet-like form, as
previously described.
In forming a series connection of sub-layers of respective fibrous microcell
elements and outer current
collectors in the respective layers, the current collector elements -are
generally of similar length
characteristics to the fibrous microcell elements, such that respective
fibrous microcell element and
outer current collector layers are longitudinally off-set in relation to one
another. In such arrangement,
the outer current collector elements are longitudinally displaced beyond one
end of the fibrous
microcell element layer, and is correspondingly shorter at the opposite end so
that the first layer
(underlying layer) of fibrous microcell elements extends beyond the ends of
the layer of outer current
collectors.
Thus, at each end of the layered assembly, there is a line of "short ends" of
the upper or lower layer,
and it is at this short end that the potting member is formed at each of the
ends of the overall assembly.
On this sub-layer assembly a layer of porous, electrically insulating sheet
material is placed, and a
second sub-layer asseinbly then is formed on the porous, electrically
insulating sheet. Tii the second
sub-layer asseinbly, a bottom layer is placed directly on the porous,
electrically insulating sheet and
overlaid with a layer of outer current collectors, off-set from one another,
and arranged such that the
positive teiminal of the new sub-layer is on the same side of the overall
assembly as the negative
terminal of the first sub-layer. This pattern of fabrication is continued
until a desired sub-layer height
is reached and a desired voltage is achieved. The ends of the respective
positive and negative current
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collectors from each end are then connected to each other with, for example,
an electrically conductive
rod or strip member, as hereinafter described.
Alternatively, the layered assembly may be fabricated, with electrical
connection of the fiber sheets
with positive and negative ends from adjoining sub-layers initially, prior to
staclcing of the respective
sub-layers. A final stack of sub-layers is then potted at both ends of the
assembly, to isolate and seal
the bore of the sub-layer assembly from the shell side. The potted bundle of
the fiber stack then can
be placed on a perforated duct that will function as a feed inlet to the shell
side of the hollow fibers in
the assembly. The fibrous microcell elements and the outer current collectors
can alternatively be
potted as the fibers are being layered, e.g., by depositing a line or bead of
epoxy or other potting
compound at both ends as the respective layers are being laid. The viscosity
of the potting material is
suitably chosen so that complete wetting of the fibrous microcell elements
takes place, to ensure leak-
tightness of the resultant tube sheet.
Once potted, the bundle or stack of microcell layers is placed in a housing
such that the shell and bore
of the microcell elements are sealed and isolated when a feed is introduced on
either side (shell side or
bore side). The resulting unit has the confirmation of a rectangular shell and
tube heat exchanger, and
such unit is advantageously fabricated with at least one inlet to the housing
for introducing feed to the
bore side and at least one outlet in the housing for removing depleted feed
from the bore side.
When the inicrocell elements are provided in a stack of layers, such stack is
placed on a duct
perforated between the potting members at respective ends. A non-perforated
section of the duct
extends through one end of the housing, e.g., with the feed inlet or outlet on
the bore side of the
microcell elements, as described, and with the duct extending sealingly
tlirough the housing to provide
a feed inlet to the shell side of the microcell elements. The layered
microcell stacks inay be placed on
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both sides of a perforated feed duct to form a symmetric double stack, as
hereinafter described in
greater detail.
In accordance with one aspect of the present invention, small sub-bundles of
microcell assemblies can
be electrically connected in series in the same cell housing, or smaller fuel
cell modules can be
electrically connected in series to increase the overall cell voltage. One
approach for achieving high
voltage levels, in accordance with another embodiment of the present
invention, is to maiiifold fuel
cell stacks (each comprising a plurality of microcell devices) to gas feeds in
a parallel fashion, with the
stacks themselves being series-connected assemblies of microcell bundles.
In one einbodiment, electrically conductive fibers are bundled with microcell
devices, so that the
electrically conductive fibers fiinction as current collectors on the shell
side of the fibers. The shell
side current collectors, or alternatively the outer electrodes coated with
suitable electrocatalyst, are
connected to a conunon plate to constitute a first terminal for the bundled
assembly. Correspondingly,
inner electrodes extending through the bore of the microcell fibers are
coimected to a plate forming a
second terminal for the assembly.
In such fuel cell assembly, fuel or oxidant is passed over the electrodes on
the corresponding
respective bore or shell side of the fibers, and the electrolyte-incorporating
membrane separator
prevents migration of the fuel or oxidant to the other electrode.
In accordance with the invention, the microcell fiber structures are usefully
potted to form sub-bundles
of a larger ultimate bundled structure, with the sub-bundles being connected
in series or parallel (or, as
discussed hereinafter, some structures or sub-bundles can be parallel
connected, with the parallel-
connected assembly of microcell elements then being series-connected to other
sub-bundles; the
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converse arrangement, wherein series-connected microcell elements form sub-
bundles that are
parallel-connected to one another, also is usefully employed in some
applications).
In one preferred einbodiment, sub-bundles of the microcell fiber structures
are fabricated, and then the
sub-bundles are aggregated with other sub-bundles, and potted again to form
the fuel cell module. The
potting medium advantageously used for such structural fixation of the
microcell fiber structures or
sub-bundles is any suitable potting or encapsulant medium, such as epoxy,
urethane, silicone, EPDM
rubber, or other encapsulant media.
The sub-bundles can be made with tube sheets at each end with 0-ring seals,
similar to the process
employed in the final module assembly, and with the sub-bundles then inserted
in a metal or
polymeric sheet material having holes formed in it. The fuel cell casing then
will have two faces, one
at each end, with holes cut into it the size of the outer diameter of the sub-
bundled tube sheet.
By this arrangement, sub-bundles can be added to or removed from the overall
module to increase or
decrease power (e.g., in a power source for stationary application, or
alternatively for motive transport
applications such as electrical vehicles, to provide adjustable vehicle
power). The holes in the faces
can be sealed with blanlc sheets of the same size as the holes, if sub-
biuldles are removed from the
module. This feature also provides capability for servicing individual sub-
bundles, by removing
defective sub-bundles and replacing them with new sub-bundles. The sub-bundles
can themselves be
potted units comprising smaller sub-bundles.
Figures 1-4 are perspective views of fibrous element structures illustrating
the fabrication of a
microcell assembly.
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As shovcni in Figure 1, a fibrous microcell element sheet 10 is formed of a
plurality of fibrous
microcell elements 12, laid side by side one another in parallel aligmnent.
The respective fibrous
microcell elements 12 can be consolidated by a plurality of sewn seains 16 as
shown, or by use of
tape, adhesive bonding or other method of affixation to produce a unitary
fibrous microcell element
sheet.
The sheet 10 as illustrated is aligned with first ends 18 of the elements 12
being in transverse register
with one another, i.e., the ends are generally coextensive in axial extent
with one another, so that the
ends 18 lie in a common vertical plane extending across the face of the sheet
from wliich the intenlal
current collectors 14 protrude.
In like manner, the opposite ends 20 of the fibrous microcell elements 12 are
in transverse register
with one another, with the ends generally aligned with one another in a
transversely extending vertical
plane at the opposite face of the fibrous microcell elements 12.
In this manner, the fibers are laid flat adjacent to one another and
consolidated in a web structure, to
form a sheet of fibers.
A plurality of external current collectors 24 are lilcewise secured togetller
in parallelly aligned side by
side arrangement, by a sewn seam 26, or alternatively, a tape, glue strip, or
other consolidating means,
to form a sheet 22 as shown in Figure 2. In such sheet 22, the respective ends
28 and 30 of the
constituent current collectors 24 are in register with one another so that all
ends of the fibrous current
collectors at each extremity of the web lie in a transversely extending
vertical plane at such extremity.
23
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Next, the sheet 10 of fibrous inicrocell elements 12 and the sheet 22 of
fibrous current collector
elements 24 are staclced, with the current collector sheet 22 on top of the
fibrous microcell elements
sheet 10, to form a conjoint structure 32 as shown in Figure 3.
In such conjoint structure 32, the respective sheets 10 and 22 are
longitudinally off-set with respect to
one another, so that the internal current collector elements 14 of sheet 10
extend beyond the ends of
the external current collectors of sheet 22 as shown, and with the external
current collectors of sheet
22 correspondingly extending beyond the ends of the internal current
collectors 14 of sheet 10 at the
opposite end of the conjoint structure. The respective external current
collectors of the overlying sheet
22 thus are in contact with associated fibrous microcell elements in the
underlying sheet 10.
In Figure 4, the conjoint structure 32 of Figure 3 is a bottom layer of an
assembly that is formed by
overlying the bottom layer with a second layer 36 including a parallely
aligned arrangement of fibrous
microcell elements 38 forming a corresponding sheet, and overlaid in the
second layer by a sheet
including external current collectors 48 secured together by a sewn seam 40 as
shown.
In the second layer, the fibrous microcell elements 38 are in register with
one another at their
respective ends 42 and 44, and the sheet of external current collectors 48 is
longitudinally displaced
from the sheet of fibrous microcell elements 38. By such arrangeinent, the
extenlal current collectors
48 extend beyond the ends 42 of the fibrous microcell elements 38, and the
internal current collectors
46 of the fibrous microcell elements 38 extend beyond the ends of the external
current collectors 48.
Concurrently, the longitudinally protruding current collectors from the
respective first and second
layers at each of the ends of the assembly are coextensive in axial extent
with one another. A porous
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insulating layer of polymeric or fiberglass sheet 50 is placed between the
layers 32 and 36, as shown
in Figure 4.
Figure 5 is a perspective view of a comiector 52 for joining current collector
or electrode elements of a
inicrocell fiber assembly. The connector 52 has two leaves 54 and 56 that are
at a 90 angle in relation
to one another, with the leaves being crimpable toward one another. When a
group of current collector
or electrode elements is placed between the leaves of the connector and the
leaves are crimped
together, the current collector or electrode elements then are secured in
electrical contact with one
another.
Figure 6 shows the microcell assembly of Figure 4, with the current collector
elements at the right-
hand portion of the drawing shown as being secured to the comiector 52 so that
the current collector
elements are coupled in electrical contact with one another.
Figure 7 is an exploded perspective view of a microcell assembly 70 showing
series connected
microcell sheet layers 60, 62, 64 and 66. The bottom sheet layer 60 comprises
internal current
collector elements that are connected by connector 72, and the overlying sheet
of external current
collectors in such layer are in turn joined to connector 74.
The next upper layer in the assembly includes internal current collectors
connected by connector 78,
which is joined by interconnect 76 to connector 74, as well as external
current collectors joined to
connector 80.
Connector 80 is joined by interconnect 82 to connector 84 of the next upper
layer in the asseinbly.
Connector 84 connects the internal current collectors of such next upper
layer, and the connector 86 at
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the opposite end of the layer connects extenial current collectors of the
layer to the connector 90 of the
top layer in the assembly via interconnect 88.
Connector 90 comiects the internal current collectors of the top layer in the
assembly and the external
current collectors at the opposite end of the top layer of the asseinbly are
connected by connector 92.
Each of the constituent layers in the assembly is separated from an adjacent
layer by a corresponding
porous insulative sheet 94, 96 and 98, respectively.
By the foregoing arrangement, each of the constituent layers in the assembly
of Figure 7 is joined to a
next adjacent layer in head-to-tail series relationship, as is evident from
the indicated polarity of the
respective connectors in the drawing.
Figure 8 is a schematic view of an assembly 100 comprising a layered
arrangement of microcell layers
joined in series relationship, including layers 102 and 104, separated by
porous insulating sheet 110,
layers 104 and 106, separated by porous insulating sheet 112, and 106 and 108,
separated by porous
insulating sheet 114.
Figure 9 is a three-dimensional perspective view of a series-connected
arrangement 130 of microcell
layers. The lowermost layer is illustrative and comprises a sheet of fibrous
microcell elements 122
from which internal current collector elements 124 protrude at the left-hand
side of the layer, with
overlying sheet of extexnal current collector elements 126 completing the
microcell layer. The
lowermost layer is shown as being electrically segregated from the next upper
layer by a porous
insulating layer 128, as schematically illustrated. The other layers are
analogously constructed. The
uppermost layer 130 as shown comprises three fibrous microcell elements
arranged in side-by-side
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relationship, and the other sheets of fibrous microcell elements in the
assembly are correspondingly
constituted. In this manner, a bundled microcell structure is fonned.
Figure 10 shows a potted arrangement 136 of microcell sub-bundles 138, in
which component sub-
bundles are connected by series connection of their respective opposite
current collector elements 140
and 142, wherein adjacent sub-bundles are separated from electrical contact
and potential short-
circuiting by porous, insulative sheet 147. As shown, the sub-bundles 138 are
potted at their
respective ends by potting meiubers 144 and 146.
Figure 11 is a perspective view of a duct 150 that is perforated with openings
154 on the top surface
152, and optionally on the bottom surface (not shown in the view of Figure 11)
for double stack
bundles of the microcell layers. Two retaining walls 156, 158 are on each
side, to retain the fiber
sheets in position. Fibers are stacked on top of each other on the perforated
duct until the desired
voltage is achieved. A fluid ingress/egress conduit 160 is joined to the
interior plenum chamber of the
duct 150, as shown.
Fiber sheets can be potted with epoxy as they are laid. Alternatively, the
fiber sheets can be bundled
and potted in the vessel to finish the procedure. Fibers are potted at each
end such that the open end
remains open. The perforated duct will be the feed port to the shell side of
the fibers.
Figure 12 shows a cross-sectional elevation view of a fiber bundle 162 potted
in a vessel 150. The
fiber bundle comprises layers 164 and 166 of fibrous microcell elements, with
an interposed sheet 168
of external current collector elements and with a separator sheet 170 of
porous insulative material
between adjacent current collector and fibrous microcell element sheets. The
bundle is potted by
potting member 163.
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Figure 13 is a side elevation view of the vessel of Figure 12, showing the
retaining wall 156, and fluid
ingress/egress conduit 160 of the housing, as well as the terminal connections
at the respective faces
180 and 182 of the bundle.
Figure 14 shows a potted arrangement 186 of fibrous microcell element sheets,
in two sub-bundles 188
and 190 on opposite sides of feed duct 196 receiving feed gas via inlet 198.
The feed duct has
perforations on both top and bottom surfaces, and each of the constituent sub-
bundles is potted with
the top sub-bundle being potted by potting member 192 and the bottom sub-
bundle being potted by
potting member 194.
Figure 15 shows a side elevation view of a double stack arrangement 200,
comprising a stack of
microcell elements on either side of the perforated duct. The gas feed 198 is
shown in the drawing.
The arrangement shown in this drawing includes comiector/terminal elements
202, 204 and 206
connecting the corresponding current collector elements.
Figure 16 is a perspective view of an assembly 210 of potted fibrous microcell
elements on one side of
a perforated feed duct including gas inlet 224 and retaining wal1216. The
potted rectangular bundle of
microfibers is arranged with its respective ends potted by potting meinbers
218 and 220.
Figure 17 shows a corresponding vesse1230 when fibers are laid on both sides
of the perforated feed
duct 238. The vessel comprises a central section 232 with an outlet 242 for
discharging gas from the
shell side of the microcell assembly, end section 234 featuring outlet 248 for
exhausting bore-side
spent gas and end section 236 with inlet 246 for introducing bore-side gas
into the housing. The
28
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perforated feed duct is arranged to introduce feed gas into the central
section 232 of the housing for
flow on the shell side thereof.
Figure 18 is a sectional elevation view of system 250 including a microcell
bundle 280 potted at
respective ends tliereof by potting members 266 and 268, which are leak-
tightly secured to the inner
surface of the housing 252 by 0-ring elements 270 and 272.
The housing 252 has a flange element 256 joining the end section 258 of the
housing witli the central
section. The central section of the housing contains interior volume 252,
which is separated from end
volume 278 by potting member 268 and from end volume 282 by potting member
266. Feed inlet 276
communicates with end volume 278 and end volume 282 communicates with spent
gas outlet 284.
Spent gas outlet 264 communicates with the interior volume 262. Feed tube 260
extends into the
center of the microcell bundle 280 in the interior volume 262, and is
perforate along its length to
introduce feed gas to the shell side of the microcell bundle 280 in the
interior volume, with the spent
gas being discharged in outlet 264. Feed introduced into end volume 278 from
inlet 276 flows tlirough
the bore side of the inicrocell eleinents in the bundle 280, and flows out of
the bundle into end volume
282, following which it is discliarged from the housing 252 in outlet 284.
The current collectors are joined to terminal 292 in the end volume 282, with
the terminal structure
extending exteriorly of the housing 252. At the opposite end volume 278, the
other ones of the iimer
and outer current collectors are joined to termina1290, which extends
exteriorly of the housing.
Figure 19 shows a perforated feed tube 300 with open ends 302, having
perforations 308 along a
central part 306 of its length.
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Figure 20 shows fibrous inicrocell and sliell side current collector sheets
312, 314 that can be rolled or
wound around the perforated tube 300 of Figure 19, with the sheets being shown
during rolling in
Figure 21 and as finally rolled into shape in Figure 22.
The sheets will be placed on top of each other such that the ends of the
fibrous microcell sheet 312
extend farther than the shell side current collector sheet 314 on one side,
and the shell side current
collector sheet 314 extends farther on the other side. The sheets 312, 314
then are wrapped tightly
around the perforated tube 300 and then potted by potting members 322 and 324.
Figure 23 shows sheets 332, 334 of fibrous microcells and shell side current
collectors, and an
insulating sheet 330 (e.g., of fiberglass or porous plastic material). Figure
24 is a perspective view of
a sheet assembly 338, 340, 342, 344 and 346, including two sheets of fibrous
microcells and shell side
current collectors.
Figure 25 is a side elevation view of a microcell assembly 338, 340, 342, 344
and 346 with off-set
fiber layers. The electrically insulating sheet is placed between two layers
of fibers forming a cell. If
the sheets on either side of the insulator are extended beyond the edge of the
insulator as shown in
Figure 25, then the fiber layers can be connected to one another in series.
Figure 26 is a cross-sectional view of a microcell bundle 350 comprising an
assembly of positive
electrodes 354 interspersed with negative electrodes 352 in a bundled
conformation.
Figure 27 is a side elevation view of series-connected microcell sub-bundles
360 including sub-
bundles 362, 366, 370 and 374 interconnected by connectors 364, 368 and 372,
respectively. The
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coiuiectors are desirably highly flexible and most preferably
omnidirectionally flexible to
accommodate accordion folding of the chain of sub-bundles, so that when folded
back against a
preceding sub-bundle or folded forwardly against the succeeding sub-bundle.
Figure 28 is a perspective view of a connector 376 that may be used to join
coinponent microcell sub-
bundles in series. The connector 376 coinprises a spaced-apart pair of
crimpable leaves 378, 380, each
of which is crimpable by means of a pliers or similar tool, to coinpressively
grip a protruberant group
of current collectors of a sub-bundle. The leaves are electrically conductive,
and are themselves
interconnected by a flexible yolce eleinent 382, which may coinprise wire or
metal filament, etc. that
serves to electrically intercomlect the respective sub-bundles witll which
leaves 378 and 380 are
coupled.
Figure 29 is a cross-sectional elevation view of a inultibundle assembly 390,
wherein each bundle 391
has a corresponding feed tube 394 associated therewith, and is mounted in a
tubesheet 393 and leak-
tightly sealed therein with an 0-ring sealant element 392.
Figure 30 is a cross-sectional elevation view of the multibundle assembly of
Figure 29, wherein the
respective bundles are connected in series and are numbered correspondingly to
Figure 29. The
respective adjacent bundles are interconnected by terminal elements 396 and
400 joined to one another
by coupling wire 398 in series arrangement.
Figure 31 is a cross-sectional view of a fuel cell module with multiple sub-
bundles, numbered
correspondingly to Figure 29, and wherein blank seal elements 402 and 404
provide closure members
for the tubesheet 393 of the module enclosure, when sub-bundles are removed.
31
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Figure 32 is a side view of a fuel cell module 410 with lnultiple sub-bundles
460, 462 and 464 of
microcell elements, with a feed tube 450 in a manifolded anangement. The
module includes a
housing 422 enclosing a central interior volume 424 bounded by the housing
wall of the module and
by tubesheets 472, to which the sub-bundles are lealc-tightly secured by means
of 0-ring elements 438,
and 474, to wliich the sub-bundles are leak-tightly secured by means of 0-ring
elements 434.
The end sections of the housing enclose respective end volumes 426 and 428.
The end volume 426
contains a manifold to which the feed tube 450 is joined in gas flow
communication, for introduction
of feed gas to each of the three sub-bundles 460, 462 and 464 by means of the
manifold line 452 in
communication with branch lines 454, 456 and 458 coupled to the respective sub-
bundles.
The sub-bundles are joined in series relationship to one another in sequence,
by connection line 440
interconnecting sub-bundles 460 and 462 and connection line 442
interconnecting sub-bundles 462
and 464. The exterior sub-bundles in such series are in turn joined
respectively with terminals 444 and
446, as shown.
The right-hand end section of the housing is flangedly comiected to the main
central section of the
housing by flange 430, witli wliich mechanical fastener means may be coupled
to lealc-tightly secure
the component sections of the housing to one another.
The housing is provided with a feed inlet 466 for introducing one of the fuel
and oxidant streams into
the end voluine 426 for flow through the sub-bundles on the bore side thereof.
An outlet 468 is joined to the housing 422 at the left-hand section as shown,
for discharge of spent
feed gas from the end volume of the housing.
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The spent gas outlet 470 is provided in the main central section of the
housing, for discharge of spent
feed from the shell side of the sub-bundle in the interior volume 424 of the
housing.
Figure 33 is a side elevation view in section, showing penetration of a feed
tube 514 into the interior
volume 506 of the housing 515 of a module 480 containing microcell sub-bundles
494, 496, 489 and
498. hZ this arrangement, the sub-bundles are mounted in correspondingly sized
receiving openings in
tubesheets 500 and 502, leak-tightly secured in the housing by means of 0-ring
sealing elements 492.
In this way, the internal volume of the housing is divided into a central
volume 506 and end volumes
526 and 528.
The housing is provided with feed gas inlet 510, spent gas outlet 508 and
spent gas outlet 512. Spent
gas on the shell side of the sub-bundle is discharged from the housing in
outlet 508, and feed gas
introduced in inlet 510 is flowed through the bore side of the sub-bundle and
discharged into end
volume 528. From end volume 528 bore side spent gas is discharged from the
housing in outlet 512.
The sub-bundles in the interior volume of housing 515 are joined in series
relationship to one another
by means of series connector lines 516, 518 and 520, and the outside sub-
bundles in the series
arrangement are in turn joined to terminals 522 and 524.
The housing 515 is openable at flange 443 to remove the right-hand end
section, following which the
respective sub-bundles can be accessed for repair or replacement.
Thus, microcell articles in accordance with the present invention may be
readily connected in series
with one another, with successive adjacent articles (fibrous microcell sheet
layers, sub-bundles) being
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insulated from eacli otller by sheets or sheathing of porous insulating
electrically non-conductive
material, or in other manner ensuring the absence of electrical interference
between such adjacent
microcell articles. It will be appreciated by those skilled in the art that
the nuinbers of sub-bundles
shown in Figures 32 and 33, are illustrative only, and that the nuinber of sub-
bundles in a given
application of the invention may be widely varied depending on the energy
generation requirements
and other structural and operational parameters of the system in specific
embodiments.
In the fabrication of high voltage electrochemical cells utilizing microcell
articles of the invention, a
bundle or sheet-form assembly of microcells is fabricated. For example, if a
design current of 200
amps is required, a number of fibrous microcell articles are connected in
parallel to generate the
necessary current. The resultant inicrocell structure then is either bundled
in a cylindrical shape or
used to form a multi-layered assembly. In a bundle, the positive and negative
fibrous elements must
be electrically insulated yet in intimate contact with each other. To achieve
higher voltages, the sheets
or bundles are connected in series, i.e., the positive of one cell is
connected to the negative of the next
adjacent cell. The cells, bundles or sheets connected in series with one
another are then potted and
sealed in the same housing to provide the desired high voltage electrochemical
cell module.
Thermal Management
When microcell elements are bundled or otherwise aggregated in a compact
structural configuration to
form modular electrochemical cell assemblies, the resulting electrocheniical
energy generation or
energy conversion device generates significant heat in its operation.
Various methods can be utilized in accordance with the present invention to
remove heat from the
microcell assembly.
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In one aspect of the invention, heat exchange tubes are distributed in the
microcell bundles, sub-
bundles, or other aggregated microcell assembly. In a preferred embodiment,
such heat exchange
tubes are aligned parallel with the fibrous microcell elements in the
microcell asseinbly.
In another embodiment heat exchange tubes are placed between sub-bundles in
the assembly, so that
the heat exchange tubes extend at least from one end of a tubesheet face (in
wliich the extremeties or
outer portions of the sub-bundles are mounted) to the opposite end. The
number, size, and material of
the heat exchange tubes are readily determined based on the amount of heat
that must be recovered,
the fuel cell operating temperature, the type of heat exchange fluid used, and
the pumping requirement
or flow rate of the fluid, as will be appreciated by those skilled in the art.
In order to maintain separation of the heat exchange fluid from the feed that
is flowed to the bore side
of the microcell fibers in the fuel cell module, the length of the heat
exchange tubes can be selected
such that the heat exchange tubes extend beyond the tube sheet that seals the
bore side of the microcell
hollow fibers from the shell side. The extended heat exchange tubes then are
potted again to form a
barrier between the bore of the heat exchange tubes and the bore of the
microcell hollow fibers.
The final assembly of the fuel cell module with the heat exchange tubes
preferably includes the
formation of a first housing with an inlet for the introduction of heat
exchange fluid in one end, a
second housing between the two potted sections, i.e., the potted heat exchange
tubes and the potted
microcell elements, with an inlet for introduction of feed to the bore side of
the niicrocell, and with the
structure of the housing being correspondingly constructed at the opposite
end, to provide
corresponding respective outlets for discharge of the heat exchange fluid and
the spent feed.
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An alternative thermal inanagement design for microcell electrochemical cell
inodules according to
the present invention employs hollow, nonporous, electrically and thennally
conductive tubes, as
current collectors for either the bore side or the shell side or both the
shell and bore side of the
inicrocell structures. Since the current collectors terminate at opposite ends
of each tube sheet, the
heat exchange current collector tube will be potted as described hereinabove,
to separate the heat
exchange fluid housing from the bore side/feed only at one end. At the
opposite end the heat
exchange tube is terminated at the tube sheet.
This arrangement allows the heat exchange fluid and feed to the bore to be
mixed at the outlet. In this
system design the heat exchange fluid does not enter the bore of the microcell
to contact the catalyst or
the electrolyte. For exainple, the feed to the bore and the heat exchange
fluid can be supplied to the
module in the same direction, such that the heat exchange fluid and the feed
to the bore can only mix
at the feed outlet from the microcells.
The heat exchange fluid then is recovered in a separate unit, or a plenum in
the housing can be
provided to collect the heat exchange fluid for recycle. The separation of
heat exchange fluid from the
feed can be readily achieved, e.g., in the case where the feed is air or
hydrogen gas.
In a specific embodiment, where the heat exchange fluid and the feed to the
bore are the same (for
example, air), the heat exchange fluid and the feed can be allowed to mix
without further separation
requirement.
In a further embodiment, heat is removed from the microcell module by
conduction of heat from the
current collectors on the shell side or bore side of the microcell elements.
In this approach, the ends of
the current collectors are extended and immersed in a heat exchange fluid in a
plenuin inside the
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housing containing the inicrocell module or in a heat exchange passage located
within the housing, at
the feed inlet to or outlet from the fiber bores. In the latter case, the
inlet and outlet of the heat
exchaige passage are lealc-tightly segregated from the interior volume of the
microcell module.
Referring to the drawings, Figure 34 is a cross-sectional view of a microcell
assembly 530 in which
heat exchange fibers or tubes 538 are provided in interspersed (distributed)
relationship to the
microcell bundles 532, as shown.
In the illustrated microcell assembly, each microcell bundle is mounted in a
correspondingly sized
opening in a tubesheet 536, with the microcell bundle being lealc-tightly
sealed in such opening by
means of an 0-ring sealing element. Alternatively, the microcell bundles 532
and heat exchange tubes
538 are potted to form tube sheet 536.
Figure 35 is a sectional elevation view of a fuel cell module, showing
air/fuel passages and heat
exchange passages thereof.
The fuel cell module 540 comprises a housing 541 in which a microcell assembly
550 is mounted, by
means of potting members 552 and 554, which are circumferentially sealingly
engaged with the inner
wall of the housing by means of 0-ring sealing elements 556 and 558. In this
manner, there is formed
an interior volume 560 in the housing, bounded by the potting members 552 and
554. A gas discharge
outlet 586 is provided in the main central portion of the housing, in gas flow
communication with the
shell side of the microcell elements in the assembly 550.
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The fuel cell module of Figure 35 also features respective tubesheets 562,
sealingly engaged with the
inner wall of the housing 541 by means of 0-ring sealing element, and
tubesheet 578, sealingly
engaged with the inner wall of the housing by means of 0-ring sealing element
580.
By such arrangement, an intermediate volume 576 is provided between the
potting 552 and tubesheet
578, and an end volume is provided at the extremity of the housing, in the
left-hand portion in the
view shown.
Correspondingly, an intermediate volume 568 is fonned between the potting
member 554 and the
tubesheet 562, as well as an end volume at the right-hand end portion of the
housing in the view
shown in Figure 35.
Coolant inlet 582 is provided at the right-hand end volume portion of the fuel
cell module housing,
and a coolant outlet 590 is provided at the left-hand end portion of such
housing.
A feed inlet 584 is provided in communication with the intermediate volume 568
of the module and a
spent feed outlet 588 is provided in flow communication with the intermediate
volume 576 at the
opposite end of the module.
Distributed across (transverse to the longitudinal axis) cross-section of the
microcell assembly 550 is a
plurality of hollow fiber heat exchange passages 604, which extend through the
entire length of the
microcell assembly and intermediate volumes through the tubesheets 562 and 578
into the end
volumes 566 and 565, respectively.
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A central feed tube 592 enters the vessel from the right-hand side thereof and
extends centrally into the
microcell asseinbly 550. Within the microcell assembly, the feed tube is of a
perforate character, to
provide feed to the shell side of the fibrous microcell elements in the
microcell asseinbly.
Current collector elements in the respective intermediate volumes 568 and 576
engage respective
tenninals 600 and 602, which extend exteriorly of the housing 541.
The housing 541 is provided with a flange 570 coiuiection, secured by suitable
mechanical fasteners,
whereby the right-hand intermediate volume and end volume portion of the
housing is removable to
access the interior elements of the fuel cell module.
In operation, the coolant inedium (from an external source, not shown in
Figure 35) is flowed into the
end volume 566 and passes through the open-ended heat exchange tubes 604 and
flows axially
through such tubes to the opposite end volume 565, from which the coolant is
discharged through
outlet 590, and may for example be subjected to heat recovery for re-
circulation of coolant to the inlet
582 in a continuous loop fashion. Concurrently, feed (oxidant and fuel) are
introduced to respective
shell side and bore side of microcell elements in the microcell assembly 550
to effect electrochemical
reaction generating power transmitted to an external load through the
respective terminals 600 and
602, which are joined to appropriate circuitry and external load componentry,
for such purpose.
Figure 36 is a cross-sectional view of a microcell bundle 610 incorporating
hollow fibers 614
interspersed with fibrous microcell elements 612. In such bundle, the hollow
fibers function as outer
electrode elements, as well as enabling heat exchange. Accordingly, the hollow
fibers may be coated,
impregnated or extruded with electrocatalyst material or otherwise configured
for functional use as
electrode elements, in addition to providing a throughbore passage in the
lumen thereof, for flow of a
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heat transfer medium, e.g., air, there through, to effect heat removal from
the bundle, incident to
electrocheinical reaction heat generation in the operation of the microcell
assembly.
Figure 37 is a side elevation in section of a fuel cell module utilizing
hollow fiber heat exchange
elements.
The fuel cell module 620 of Figure 37 comprises a housing 625, which is
flanged with flange structure
624, to allow separation of the right-hand portion of the housing to be
removed from the main central
portion, to access internal structures of the module. The housing 625 contains
a microcell bundle 626
which is potted by potting numbers 628 and 630, and leak-tightly sealed
against the interior wall
surface of the housing 625, by 0-ring sealing elements 632 and 634, to define
an interior volume 636
within the housing bounded by the interior walls and respective potting
members 628 and 630.
In axially spaced relationship to the potting number 630 is a tubesheet 640,
thereby defining an
intermediate volume 660, which is sealed by 0-ring element 642 against the
interior wall of the
housing.
The heat exchange tubes constituting current collectors, terminate at tube
sheet 628, with heat
exchange/current collector tubes communicating with end volume 662 of housing
622.
Exterior of the tubesheet 640 within the housing is an end volume 658.
A central feed tube 641 extends through the end-wall 622 of the housing and is
centrally extended in
to the microcell assembly 626. Such central feed tube is perforate within the
microcell assembly, to
provide fuel to the shell side of the assembly.
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The riglit-hand portion of the housing is removable at flange 624 to provide
access to the interior
elements of the module.
The intermediate volume 660 is provided with an inlet 646 for introduction of
fuel thereto for flow
through assembly 626 to volume 662, the latter being provided with outlet 648
for discharge of spent
fuel therefrom.
The intermediate volume 636 of the housing is provided with outlet 638 for
discharge of shell side
spent feed.
The end volume 658 of the module is provided with inlet 644 for introduction
of coolant for flow
through hollow fiber elements extending in to sucli volume, for axial flow
through the hollow fiber
electrode elements to the opposite end volume 662.
The hollow fiber heat exclzange passages in this embodiment are formed by
hollow fiber electrodes,
and such electrodes are coupled in the respective end volumes to the
corresponding terminals 652 and
656, as illustrated.
Figure 38 is a sectional elevation view of a fuel cell module with heat
exchange from current
collectors by means of conduction. The module 700 includes a housing 702
containing microcell
assembly 704, potted by respective potting numbers 706, sealed by 0-ring
sealing element 710, and
potting number 708, sealed by 0-ring sealing element 712. An interior volume
720 is thereby defined,
communicating with the outlet 740 for discharge of spent feed from the
interior volume 720.
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A central feed tube 714 extends centrally in to the microcell assembly 704 and
is perforate over its
length within the microcell assembly, to provide feed to the shell side of the
assembly.
The end volume 724 of the housing 702 is provided with an inlet 742 for
introduction of feed for flow
through bore passages of the microcell assembly 704 to the end voluine 722
from wliich spent feed
can be discharged from outlets 732.
In this module, a heat exchanger 746 is contained in end volume 724 and joined
in heat exchange
contact with current collector elements of the microcell assembly. A heat
exchange fluid (from a
source not shown in Figure 38) is introduced to heat exchange or inlet 748 and
circulated there
through for discharge from outlet 750.
In like manner, the opposite end volume 722 contains a heat exchanger 780 with
an inlet 728 receiving
heat exchange fluid for flow there through and discharge from the second heat
exchanger 780 through
outlet 730.
The current collector elements at respective ends are joined in electrically
conductive relationship to
terminals 738 and 736. The left-hand portion of the housing 702 is flanged by
flange 726, whereby
the housing can be readily opened to access internal elements of the housing.
Figure 39 is a schematic representation of a fuel cell system, according to
one embodiment of the
invention.
The fuel cell system 780 includes a microcell module 782, which includes a
housing 784 having
joined thereto a coolant medium inlet 810, a coolant mediuin outlet 792, a
fuel inlet 794, an oxidant
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inlet 799, a spent fuel outlet 786 and a spent oxidant outlet 804. The feed
outlet 786 is joined to a
discharge line containing back pressure regulating valve 788 therein. In lilce
manner, the spent oxidant
outlet 804 is joined to discharge line 806 containing back pressure regulating
valve 808 therein. The
respective back pressure regulating valve 788 and 808 may be modulated to
control the rate and extent
of electrochemical reaction involving the fuel and oxidant species.
The system includes fuel supply tank 798 joined by fuel feed line 796 to the
feed inlet tube 794.
Correspondingly, an oxidant tank 802 is provided, joined to oxidant feed line
800 coupled to oxidant
inlet 799.
The system involves a coolant recirculation arrangement, including
recirculation line 816 joined to
coolant outlet 792 and having dispose therein a pump 818 and heat exchanger
820. Heat exchanger
820 effects heat removal from the warmed coolant medium, so that same is
recycled to the surge tank
814 for return in feed line 812 to coolant inlet 810.
Accordingly, an operation of the system shown in Figure 39, the coolant medium
is flowed through
hollow fiber heat exchange tubes in the housing and is continuously
recirculated to the surge tank to
provide a hold-up inventory of coolant for high rate electrochemical
oxidation.
Double Membrane Microcell Structures and Assemblies
Microcell structures are usefully employed in specific applications of the
invention in a double
membrane configuration.
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In one embodiment, microcell structures of such type are readily formed using
an iimer hollow fiber
separator having an inner current collector and electrocatalyst of the inner
electrode on its shell side.
Such imier hollow fiber separator is encapsulated by an outer hollow fiber
meinbrane. The pores of
the outer hollow fiber ineinbrane are impregnated with an electrolyte and the
electrocatalyst of the
outer electrode is coated on the shell side of the outer hollow fiber
membrane, to forin a double
membrane microcell structure.
This double membrane microcell structure is advantageous to enable the imier
hollow fiber separator
to be used as a meinbrane to selectively allow permeation of feed (e.g.,
hydrogen or oxygen), as
desired. This may be effected, for example, by coating the inner wall or the
outer shell of the inner
separator with a penn-selective material that preferentially allows the
desired gas to permeate to the
electrode. This double membrane design thus is advantageous in reducing or
eliminating the exposure
of the electrocatalyst or the electrolyte to potential poisonous impurities in
the feed. Materials that
may be used in the perm-selective membrane include cellulose esters,
polyimides, polysulfones and
palladium.
In another microcell structure including a double membrane separator, the
inner wall of the inner
separator may be impregnated or coated with a CO-H20 shift low temperature
refonning catalyst. In
such design, the shell side of the inner separator is coated with an anode or
cathode feed-selective
material.
Another double membrane design involves coating both anode and cathode with a
hydrogen- or
oxygen-selective material. In such instance, the protective perm-selective
material on the shell side of
the outer hollow fiber membrane must be electrically conductive to allow
electrical contact between
the current collector of the outer electrode and the electrocatalyst on the
shell side. A perm-selective
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material such as palladium can be used for such purpose. Alternatively, an
electrically conductive
perm-selective material can be applied only to one of the cathode and anode
components, if desired.
Yet another design utilizing double membrane fabrication employs an
electrically conductive inner
hollow fiber separator. Such electrically conductive hollow fiber separator
may be formed of sintered
metal, carbon or graphite. In some embodiments of such double membrane design,
an inner current
collector may not be needed depending on the electrical conductivity of the
imler hollow fiber.
The inner and outer hollow fiber membrane can be of any suitable commercially
available membrane
material, including, for example, polypropylene, polysulfones,
polyacrylonitrile, etc. In one
embodiment, the membrane is treated to impart penn-selective characteristics,
e.g., to selectively
allow permeation of the feed gases (fuel, oxidant) wliile remaining
impermeable to other gases and
components (such as fuel impurities) that may be present. By way of specific
example, a protective
hydrogen-permeable barrier layer can be deposited by solution deposition,
electrolytic coating, etc., to
provide a film of palladium on the membrane surface that allows passage of
hydrogen therethrough,
but occludes nitrogen and oxygen. See, for example, Gryaznov et al.,
"Selectivity in Catalysis by
Hydrogen-Porous Membranes," Discussions of the Faraday Society, No. 72 (1982),
pp. 73-78;
Gryaznov, "Hydrogen Permeable Palladium Membrane Catalysts," Platinuin Metals
Review, 1986, 30
(2), pp. 68-72; and Armor, "Catalysis with Permselective Inorganic Membranes,"
Applied Catalysis,
49 (1989), pp. 1-25.
Figure 40 is a cross-sectional view of a double membrane design of a microcell
900 with an
electrically conductive penn-selective membrane on the anode or cathode
element of the microcell.
The microcell 900 comprises an outer electrocatalyst layer 912, the
microporous membrane/electrolyte
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matrix 910, electrocatalyst 908, an inner hydrogen- or oxygen-selective
membrane 906, and current
collector or electrode elements 902 in the iiuier bore 904.
Figure 41 is a cross-sectional view of a double separator design of a
microce11914 with perm-selective
membranes protecting the anode or cathode elements of the microcell. The
microcell 914 comprises
an outer electrocatalyst layer 930, the microporous membrane/electrolyte
matrix 928, electrocatalyst
926, current collector or electrode elements 922, inner porous separator 920,
an inner hydrogen- or
oxygen-selective meinbrane 918 and an imler bore 916.
Figure 42 is a cross-sectional view of a double separator design of a
microce11932 with perm-selective
membranes covering both anode and cathode elements of the microcell. The
microce11932 comprises
an outer hydrogen- or oxygen-selective electrically conductive membrane 948,
electrocatalyst layer
946, the microporous membrane/electrolyte matrix 944 electrocatalyst 942,
current collector or
electrode element 940, iimer porous separator 938, an iiuier hydrogen- or
oxygen-selective membrane
936 and an inner bore 934.
Figure 43 is a cross-sectional view of a double separator design of a
microce11950 with perm-selective
membranes covering both anode and cathode elements of the microcell and with
an electrically
conductive inner separator. The microcell 950 comprises an outer hydrogen- or
oxygen-selective
electrically conductive membrane 966, electrocatalyst layer 964, the
microporous
membrane/electrolyte matrix 962, electrocatalyst 960, electrically conductive
porous current collector
or electrode element 958, an inner hydrogen- or oxygen-selective membrane 956
and an inner bore
952.
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Figure 44 is a cross-sectional view of a double separator design of a
microce11970 with penn-selective
membranes covering both anode and cathode elements of the microcell and with
reformer catalyst on
the inner wall of the inner separator. The microcell 970 comprises an outer
electrocatalyst layer 986,
the microporous membrane/electrolyte matrix 984, electrocatalyst 982, current
collector or electrode
elements 980, an inner hydrogen- or oxygen-selective meinbrane 978, inner
porous separator 976, CO
water shift/reforming catalyst 974, and an inner bore 972.
Manufacture of Microcell Structures and Assemblies Comprising Same
For commercial high-volume production, the microcell device with most of its
components desirably
is fabricated in a single extrusion step, at high rate. A critical aspect of
the high-volume fabrication
process is encapsulating the inner electrode with the microporous ineinbrane
separator.
For such purpose, a strand or tow of electrically conductive fibers can be
passed through the center of
the bore foimer tube of an extrusion mold (spimierette). The material that
will fonn the baclcbone of
the microporous membrane separator, referred to as a "dope," is extruded
around the bore former tube
in continuous fashion onto the strand or tow of electrically conductive
fiber(s). An intemal coagulant
fluid, e.g., a gas such as nitrogen or a liquid such as water, is passed
through the bore former tube
along with the inner electrode fiber(s) or fibrous current collector(s).
In the above-described operation, the size of the microcell fiber is
determined by the size of the orifice
of the extrusion mold. Such orifice can be widely varied in size, e.g., from
as small as 100 niicrons or
smaller, with the meinbrane correspondingly being as thin as a few microns in
thickness.
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An electrocatalyst paste is simultaneously extruded through the bore if the
metliod of microcell
fabrication utilizes an ink paste. Extruded fiber is innnersed in a quenching
bath or an extemal
coagulant medium, such as water. As the extruded fiber passes tllrough the
coagulation/quench
operation, the microporous membrane structure is instantaneously formed around
the inner electrode
as the water-soluble pore former compound is leached out in the
coagulant/quenching medium.
Pore structure, porosity and pore size of the membrane separator thereby are
accurately controlled by
selection and corresponding control of parameters such as the membrane dope
formulation, type of
coagulant used, temperature of the spinning operation, etc. Specific
conditions are readily
determinable for such process by simple experiinent without undue effort, by
those skilled in the art.
A wide variety of materials is useful to form the microporous membrane
separator, including, without
limitation, polysulfone, polyacrylonitrile, other high temperature polymers,
glass and ceramic
materials.
By the above-described spinning process, microcell articles can be fabricated
at high rate on a
continuous basis.
After formation of the microporous membrane separator-encapsulated inner
electrode structure, such
encapsulated structure is coated or impregnated on the outside (shell side)
with an ion exchange
polymer in the case of polymer electrolyte fuel cells, and/or electrocatalyst
of the outer electrode.
Such exterior coating can be advantageously performed by a similar extrusion
process.
Figure 45 is a schematic flowsheet of a. solution in7pregnation system 988 for
impregnation of a
membrane fiber 992 with Nafion or electrocatalyst. The membrane fiber 992 is
dispensed from a fiber
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spool 990 and passes, by action of the roller 994, through a solution bath 996
in which the fiber is
impregnated. The impregnated fiber then passes over guide roll 998 and
througli the bank of heating
elements 999 for final collection on talce up winder 1000.
Additional applications for electrochemical cells of the invention include
production of chemicals.
Chemical synthesis applications are advantageously effected utilizing
microcells fabricated in
accordance with the invention, which provide: high current density per unit
volume, as necessary for
chemical synthesis; low internal resistance due to minimal electrode membrane
distance (thiclrness);
and high efficiency due to low mass transfer resistance.
In addition, microcells fabricated in accordance with the present invention
may be utilized to generate
hydrogen and oxygen where other forms of electric power are available. In such
applications,
hydrogen (or other fuel gas) generated by the cell can be stored and used for
generation of electricity.
For example, after the porous polymeric membrane has been formed around
current collector(s) of a
inicrocell fiber structure, the structure can be directly passed through a
solution of aqueous polymeric
electrolyte, such as a solution of Nafion (5% solids in water and alcohol)
polymeric electrolyte, to
impregnate the pores of the porous polymeric membrane with the polymeric
electrolyte. The amount
of the impregnated polymeric electrolyte may be selectively varied depending
on the residence time of
the porous polymeric membrane in the electrolyte impregnant composition, and
the number of times
that the structure is repetitively exposed to the composition (i.e., in single-
pass or multi-pass fashion)
during processing.
On the same process line in which the electrolyte is impregnated, or
alternatively in a subsequent
phase of the fabrication operation, the microcell fibers in one process
embodiment are dried and
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impregnated with platinuin as the electrocatalyst material, using a plating
solution containing
Hz[PtC16], following which the fibers are passed througli a bath of reducing
agent, such as sodium
borohydride (NaBH4), to reduce the platinum composition to elemental platinum
metal.
This continuous technique according to one embodiment of the invention is used
to impregnate only
the outer shell of the membrane with platinum. The inner wall of the membrane
is impregnated after
the fibers have been potted in the vessel by pumping the platinum plating
solution through the bore of
the fibers.
In another einbodiment, both the shell and the bore side of the fibers are
impregnated after the fibers
have been potted.
After the ion exchange Nafion electrolyte solution is impregnated in the pores
of the membrane, the
electrocatalyst is coated according to another aspect of the invention by
using platinum loaded on
activated carbon of suitable particle size. The platinum loading on the
activated carbon particles
typically is in the range of from about 5 to about 10 percent by weight. A
paste is prepared consisting
essentially of platinum loaded activated carbon, Nafion ionomer as the binder,
and a Teflon
polytetrafluoroethylene emulsion. The paste then is coated, or alternatively
extruded, on the shell side
of the fibers.
Coating of the paste inside the fiber wall may be accomplished in various
ways. In one approach, the
paste is coextruded while the porous membrane separator element is being spun
around the current
collector. A second approach is to pre-extrude the paste around the current
collector before inserting
the current collector into the membrane fiber. As a third approach, a thin
paste can be puinped into the
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bore of the porous meinbrane separator element after the cell assembly and
potting has been
completed.
In another embodiment, the electrolyte is deposited inside the porous membrane
separator element,
and the catalyst is applied by electrodeposition from a solution coiitaining
platinum ions, by an
electrolytic plating solution process, or by an electroless plating solution
process.
Corrosion Management in the Microcell Assembly
In applications of conventional fuel cell tecluiology, current collectors
generally have been limited to
the use of graphite type materials. Current collectors formed of aluminum or
titanium can be coated
with corrosion-resistant coatings such as gold, but such coatings tend to peel
and delaminate from the
current collector element under the severe corrosive conditions and thermal
cycles that characterize the
fuel cell operation.
The use of microcell elements permits current collector materials of
construction other than graphitic
materials to be employed. Metal fibers utilized in microcell structures in the
electrochemical cell
module can be coated by variety of techniques to achieve durable corrosion
resistance. Useful coating
techniques for such purpose include, witliout limitation, electrocheniical
deposition, electroless
coating, dipcoating, extrusion, etc., using corrosion resistant metal
compositions or polymeric
materials such as polyanaline.
A preferred approach for coating metal substrates for use as current collector
involves use of
amorphous metal compositions deposited by plasma coating techniques. In
general, better corrosion
resistance is attributable to the amorphous nature of the coating structure.
Further, various amorphous
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metal compositions generate extremely high surface areas. Examples of such
high surface area metal
compositions include niclcel inetal hydride electrocatalyst materials. The use
of such high surface
metal compositions coupled with the inherently high surface area of the
fibrous geometry of the
microcells enables such amorphous metal coatings to be effectively utilized
for hydrogen storage
capability in the fuel cell, a potentially significant structural and
operational advantage.
As another approach to increase the corrosion resistance of metallic fiber
substrates, the metal fibers
can be coated with a polymeric precursor or other organic coating, and the
coating then iscarbonized.
Carbonization of the polymer to form graphitic material on metallic fibers
yields a coating that is
corrosion resistant, yet possesses electrical conductivity that is higher than
that of carbon or graphite
alone.
The presence of pinholes in any coating application can cause corrosion and
electrical disconnection
of one section of the microcell from others, which in turn reduces the useful
power density of the cell.
In anotl7er approach, such electrical disconnection deficiencies are avoided
by a fabrication method
involving co-placement of a carbon fiber in the bore or on the shell side of
the microcell, so that the
carbon fiber is in intimate contact with the associated current collector of
the microcell. With such
arrangement, if the current collector is corrosively attacked in the operation
of the electrochemical
cell, the carbon or graphitic fiber then continues to maintain a flow of
current therethrough, thereby
providing electrical continuity despite even gross corrosion-mediated breakage
or deterioration of the
current collector element.
To enhance the service life of metallic current collector fibers in the
corrosive environment of a fuel
cell, the metal fiber is advantageously coated with a compound such as a
polymeric material,
following which the coated fiber is subjected to pyrolysis conditions for the
polymeric material. The
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fiber coating material is pyrolyzed and converted to carbon using techniques
that are conventionally
employed to form carbon fibers per se.
Formation of a continuous layer of carbon on a metallic current collector
fiber (of any size) produces a
fiber that is electrically conductive radially and longitudinally and at the
same time is corrosion-
resistant due to the surface layer protecting the underlying metal from
corrosive attack.
Figure 46 is an elevation view of a conductor element 1002 including a
metallic fiber 1004 having a
polymeric compound coating 1006 on its outer surface. The fiber is coated in
any suitable manner,
e.g., by spraying, dip-coating, roller coating, etc.
Figure 47 shows the corresponding fiber 1002 of Figure 46 after the pyrolysis
step, as comprising a
pyrolyzed carbon coating 1008 on the outside surface thereof.
Concerning current collector and electrode preparation, the electrically
conductive metal fibers of the
microcell in one embodiment of the invention comprise copper, aluminum or
titanium fibers, having a
diameter in the range of from about 100 to about 10,000 microns, coated with a
suitable thickness of
corrosion-resistant material such as gold or platinum.
Alternatively, carbon/graphite fibers having diameter in the range of from
about 100 to about 10,000
microns and having good electrical conductivity characteristics can be
employed, and metallized with
the electrocatalyst, e.g., platinum. Such platinum metallization is
advantageously effected by
contacting the fibers with a plating solution containing HZ[PtC16], followed
by reduction of the
platinum compound to elemental platinum metal via contact with sodium
borohydride (NaBH4).
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With respect to current collectors, the presence of pinholes or coating
defects causes accelerated
corrosion of metallic current collectors. Iu consequence of such corrosion,
the fiber cell can
disconnect (as a result of the continuity of the conductor being iinpaired)
and become inoperable. To
avoid this disconnection of part of the microcell voltage and current, fibrous
carbon current collectors
advantageously are laid along the coated metallic fibers. Figure 48 shows a
conductor 1010 including
a fibrous carbon current collector 1014 laid along a coated metallic fiber
1012. The carbon fiber 1014
will be in intimate contact with the coated fiber 1012, as shown in Figure 48.
Figure 49 shows the fiber assembly of Figure 48 after a disconnection break of
the coated metallic
fiber 1012. In the event of a corrosion point break in continuity of the
coated metallic fiber, the carbon
fiber 1014 in contact with both sections of the corroded metal fiber 1012
provides continuity enabling
the current to pass from one side to the other along the length of the carbon
fiber/metallic fiber
arrangement.
Water Management in Microcell Assemblies
In microcell electrochemical reactions wherein water is a reaction by-product,
a feed may be
humidified to prevent drying of the membrane, the microcell assembly desirably
includes a water
management system for addition and removal of excess water from the microcell
assembly.
In general, the high surface area of microcell structures, and lower mass
transfer resistance, mean that
the removal of water from the microcell module is less problematic than in
conventional planar fuel
cell structures.
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Various alternatives can be employed to further enhance the water manageinent
capacity of the
microcell fuel cell module. For example, if heat exchange tubes are einployed
in the fuel cell
assembly, comprising hollow fiber membranes coated with Nafion or other ion-
exchange polymer or
material that will selectively allow water permeation, and if the heat
exchange liquid is water, the heat
exchange tubes can be used for water supply to the fuel cell and removal of
heat from the fuel cell.
One approach for water removal from the fuel cell is to provide porous plane
hollow fiber membranes
in the microcell bundle, in distributed fashion therein. In this structural
arrangement, water will
permeate through the membrane wall by a wicking action during operation of the
fuel cell and will be
channeled down the bore of the hollow fiber and away from the active surfaces.
The resultingly
channeled water then can be collected in a plenuin provided in the housing
containing the module, for
discharge from the system.
Concerning the removal of water from fuel cells, various approaches are
contemplated by the present
invention. To remove water produced in the fuel cell made from fiber cells or
microcells, hollow fiber
membranes treated witli a hydrophilic compound can be paclced intermittently
with fiber cells
containing an electrode or current collector. Since these hollow fiber
membranes are in intimate
contact with the shell side of the cells and are open on the bore side, water
produced in the fuel cell is
absorbed by a wiclcing action and channeled down the bore of the membrane
hollow fiber membrane
away from the cells containing the electrode, thereby eliminating the water
flooding in the cell.
If the module is mounted vertically, then water may be collected by gravity
collection at the bottom of
the cell a.nd discharged therefrom.
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Figure 50 shows a cross-section of a hollow fiber and microcell tube bundle
1020, in which the plane
hollow fiber elements 1026 are interspersed witli the microcell fiber elements
1022 and shell side
electrodes 1024, and such hollow fiber elements are used for channeling water
from the assembly.
Figure 51 is a sectional elevation view of a inicrocell fuel cell module 1030,
including a housing 1032
containing a microcell assembly 1036 arranged vertically as shown. The housing
1032 has a flange
1034 by means of which the upper end of the housing can be removed to access
the microcell
assembly and other internal components of the module.
The microcell assembly 1036 is potted at its upper end by potting member 1040
leak-tightly sealed to
the inner wall of the housing by 0-ring sealing element 1042. In like manner,
the microcell assembly
1036 is potted at its lower end by potting member 1044 leak-tightly sealed to
the inner wall of the
housing by 0-ring sealing element 1046.
The microcell assembly 1036 engages a central feed tube 1080, which is
perforate within the interior
volume of the microcell assembly. Additionally, feed inlet 1060 provides feed
to the bore side of the
microcell elements in the assembly, from upper eind volume 1048. Feed
discharged at the lower end
from the hollow fiber elements enters the lower end volume 1050 and is
discharged from the housing
from outlet 1072 or outlet 1070.
Outlet 1078 is provided for interior volume 1038 of the housing, for discharge
of spent feed from the
interior volume (shell side).
The lower end of housing 1032 constitutes a plenuin chamber 1076 which
receives access water
(condensate) gravitationally flowed to such lower end of the housing, and
discharged by overflow
through outlet 1072 or outlet 1070.
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The current collector elements at respective ends of the microcell assembly
are joined to respective
terzninals 1082 and 1084, as illustrated.
Accordingly, the hollow fiber tubular elements employed in the microcell
assembly allow permeation
of excess water in to the bore passages of such hollow fibers and drainage
thereof to the plenum
chainber, to readily remove excess water from the electrochemical fuel cell
module.
Any other suitable means or methods can be used to channel water from the
microcell assembly,
including elements or structures that utilize surface tension or capillarity
effects to induce channelized
flow of water from the microcell bundle to a collection vessel or locus. By
way of example, the
enhancement structure for film condensation apparatus that is described in
U.S. Patent 4,253,519
issued March 3, 1981 to Leslie C. Kun and Elias G. Ragi is usefully employed
as an overlay structure
on the microcell fibers or bundles or sub-bundles comprising same, to effect
channelized flow of
liquid for recovery and discharge tliereof from the fuel cell module.
In each of the foregoing approaches, the electrolyte/catalyst-impregnated
coated fiber can be
optionally coated with a Teflon polytetrafluoroethylene emulsion, to impart
hydrophobicity to the
membrane/electrode assembly. By sucli expedient, water introduced or formed in
the cell will be
repelled from the catalyst surface, to enhance the availability of the
catalyst site to the fuel or the
oxidant (e.g., hydrogen, or oxygen).
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While the invention has been described herein wit11 reference to specific
embodiments, features and
aspects, it will be recognized that the inveiition is not thus limited, but
rather extends in utility to other
modifications, variations, applications, and embodiments, and accordingly all
such other
modifications, variations, applications, and embodiments are to be regarded as
being witliin the spirit
and scope of the invention.
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