Note: Descriptions are shown in the official language in which they were submitted.
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FUEL CELL STACK ASSEMBLY
FIELD OF THE INVENTION
The present invention relates to a stack assembly primarily for an
electrochemical fuel cell of the proton exchange membrane (PEM) type, and
to a multi-function stratum for inclusion in a PEM-type fuel cell stack. The
multi-function stratum serves as (i) a heat dissipator; (ii) a compression
spring
;arrangement for maintaining the stack in compression; and (iii) an electrical
conductor between successive electrochemically active strata in the stack.
BACKGROUND
Electrochemical fuel cells convert fuel and oxidant to direct electric
current and reaction product. In electrochemical fuel cells employing hydrogen
as the fuel and oxygen as the oxidant, the reaction product is water. Solid
polymer fuel cells of the PEM type include a membrane electrode assembly
(MEA) layer comprising a solid polymer electrolyte serving as an ion exchange
membrane disposed between two electrode layers. The electrode layers
typically comprise porous, electrically conductive sheet material and an
electrocatalyst at each membrane-electrode interface to promote the desired
electrochemical reaction. At the anode, fuel from the fuel supply moves into
and through the porous electrode material and is oxidized with the aid of the
anode electrocatalyst. The reaction forms rations (typically protons obtained
from the hydrogen fuel supply), that migrate through the membrane to the
cathode, and electrons that move into an electric circuit that includes a load
and
is connected to the cathode. At the cathode, the oxidant moves into and
through the porous electrode material and reacts with the aid of the cathode
SUBSTITUTE SHEET (RULE 26)
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electrocatalvst to form anions. The anions so formed react with the cations
arriving through the PEM from the anode to produce a reaction product,
typically water. The cathode reaction requires the electric circuit to supply
the
electrons needed to provide the negative charge for the anions that neutralize
the arriving canons and form with the canons the electrically neutral reaction
product.
In conventional PEM fuel cells, the MEA is interposed between two
substantially fluid-impermeable, electrically conductive plates (sometimes
referred to as separator plates) to form a fuel cell unit. The plates serve as
current collectors and conductors, provide structural support for the
electrode
layers, typically include conduits for directing the fuel and oxidant to the
anode
and cathode layers, respectively, and typically provide means for removing
reaction products, typically water, formed during operation of the fuel cell.
When reactant channels for the fuel and oxidant are formed in the separator
plates, the plates are sometimes referred to as fluid flowfield plates. Since
in
stacked fuel cell arrangements a given separator plate typically has an anode
adjacent the obverse broad surface and a cathode adjacent the reverse broad
surface, such plates are sometimes referred to as bipolar plates.
Individual fuel cells are typically electrically interconnected in series
in a stacked array between stack end plates (to which terminals for connection
to the external electrical load are connected) in order to generate usable
electrical power. The dimension through the stack from one terminal end plate
to the other is referred to in this specification as the stack dimension. The
stack
must be retained between the terminal end plates under compression to
maintain sealing between MEA and plate subassemblies and to provide
adequate electrical conductivity between subassemblies. Conduits must be
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provided adjacent the electrode layers for the supply of fuel and oxidant
gases
respectively; these conduits are typically formed in the separator plates, as
mentioned. MEA layers alternate in the stack dimension with separator plates;
similarly fuel supply flowpaths alternate in the stack dimension with oxidant
supply flowpaths so that the fuel supply is isolated from the oxidant supply.
These flowpaths and associated exhaust conduits must also be supplied to
remove the spent reactant gases and the reaction product (typically water). In
most fuel cells, particularly those of larger dimensions, means such as
auxiliary
cooling passages for flow of cooling water through the stack must be provided
to remove heat from the stack, as the chemical reactions are exothermic.
As mentioned, in such stack arrangements, one broad surface of a given
fluid flow f eld plate, bipolar plate, or separator plate serves as the anode
conductor for one cell, and the other broad surface of the plate serves as the
cathode conductor for the adjacent cell. Fluid reactant streams are typically
supplied to channels in the flowfield plates via internal plena or manifolds
formed by aligning openings formed within the plates and MEA layers in the
stack. Similarly, fluid exhaust manifolds for spent gases and reaction
products
are also typically located internally within the stack; suitable apertures in
the
separator plates and MEA strata are frequently provided for this purpose.
Some types of fuel cell operate at relatively high temperatures. For
example, phosphoric acid fuel cells operate at about 200°C. Heat may be
readily removed from such cells by air cooling. By contrast, a conventional
PEM fuel cell operates at about 80°C. Unless a given PEM-type fuel
cell stack
is of sufficiently low power (say, lower than about 30 kW) and of sufficiently
small dimensions that heat can be dissipated into the environment without
special assistance (see I~letcher et al. tJ.S. Patent No. 5,470,671 granted to
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Ballard Power Systems Inc. on 28 November 1995 for an example of such
small low-power fuel cell stack), some heat dissipation means must be
designed into the fuel cell configuration so that the low-grade heat generated
from the fuel cell stack during its operation may be removed to maintain the
desired working temperature. In conventional PEM fuel cell stack design,
because the operating temperature does not differ by much from ambient (say,
room) temperature, air cooling has been found to be ineffective. Accordingly,
conventional stacks typically also have coolant passageways extending within
them for circulating a coolant liquid that absorbs the heat generated by the
exothermic fuel cell reaction. The coolant liquid typically flows out of the
stack to a heat exchanger and then the re-cooled liquid recycles into the
stack.
The requirement for such coolant flow necessitates the use of a circulating
pump as well as the heat exchanger and coolant flow conduits, thereby adding
to the cost, weight and bulk of conventional PEM fuel cell stack and
associated
assemblies. As PEM fuel cell stacks are otherwise suitable for use in mobile
applications such as powering vehicles. bulk and weight should be kept to a
minimum.
In some types of fuel cell stack. external manifolds connected to the
sides of the stack for providing side-located supply and exhaust plena for the
reactant gases may be provided instead of internal plena or manifolds. The use
of external manifolds with a fuel cell stack allows reactant and oxidant gases
to be fed directly to the sides of the stack and thence to the internal
reactant
fuel and oxidant gas conduits. Compared to a stack with internal manifolds,
a stack with external manifolds has simple manifold interfaces and assembly
requirements, which translate into lower manufacturing and assembly costs.
External manifolds have been used for molten carbonate fuel cells. However,
for a conventional PEM fuel cell stack, such external manifold architecture
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cannot maintain adequate sealing along the manifold interface, at least in the
case of a relatively large high-power stack. The reason for this problem is
that
during operation, a PEM fuel cell stack undergoes both thermal expansion
throughout and also hydro-expansion of its proton exchange membranes by
reason of absorption of water by those membraunes; further, compression of the
components tends over time to cause component shrinkage in the stack
dimension (i.e., as explained previously, along the dimension perpendicular to
the broad working surfaces of the stacked MEA layers and the separator plates)
of some of the fuel cell components, notably the porous electrode layers and
the seals. These expansions, contractions and shrinkages cause the stack as a
whole to expand and/or contract in the stack dimension, interfering with
conduit alignment, disrupting seals and presenting a serious impediment to the
use of external manifolds on such stacks. Sealing is critical in fuel cell
stacks;
especially important is the need to avoid any co-mingling of fuel and oxidant
gases, which at best would reduce the efficiency of the cell, and at worst
could
cause an explosion. For the foregoing reasons, virtually all commercially
manufactured PEM-type fuel cell stacks have used internal manifolding, as will
be discussed in further detail below.
Because of the movement over time of PEM fuel cell strata in the stack
dimension, in almost all high-power PEM fuel cell stacks currently
manufactured heretofore, stack expansion-compensating compression springs
are used at one or both ends of the stack to compensate for the expansion and
compression of the stack. The compressive force exerted by these springs
maintains internal seals between adjacent MEA/plate subassemblies and their
associated conduits for fuel, oxidant, and coolant supply, and reaction
product
disposal ,despite the tendency over time for the stack strata (subassemblies)
to
shrink in the stack dimension, and despite intermittent expansions and
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contractions of the stack strata. Such springs also facilitate adequate
electrical
conductivity between the serially connected strata. Unfortunately, as
mentioned above, PEM fuel cell stacks tend not to be stable in the stack
dimension; motion along the stack dimension is then propagated to all reactant
and oxidant gas delivery plates. Consequently, in accordance with
conventional PEM fuel cell stack design, manifolding and associated seals are
internally located, since external seal and manifold conduit alignment cannot
pursuant to conventional design be properly maintained in the stack dimension.
If external manifolding were attempted in such conventional PEM stacks, the
continual motion of the plates would tend to break the seals between the edges
of the plates and the fixed external manifolds coupled to the stack, thereby
rendering unworkable the otherwise desirable external manifolding for high-
power fuel cell stacks.
Various specific fuel cell stack design approaches in the prior literature
have limited merit within the context of their objectives, but none has
resolved
the set of problems to which reference is made above. Because of the unique
physical and chemical properties of PEM fuel cell stacks, design approaches
directed to other types of fuel cell often are not transposable to PEM fuel
cell
stack design. In the literature relating to commercially manufactured PEM fuel
cell stack design, inevitably internal manifolding is utilized.
The following prior patent specifications are illustrative of the state of
fuel cell stack technology:
Bette, U.S. Patent No. 5,397,655, granted 14 March 1995, discloses a
PEM-type fuel cell stack with resilient spring plates between adjacent cells.
Bette does not teach any manifolding of the fuel cell stack. Bette apparently
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uses the spring plates to adjust voltage drop; the specification teaches that
the
voltage drop within the fuel cell stack is "adjustable within wide limits,
which
is achieved, for instance, by inserting additional, or differently shaped
spring
plates 10, or other intervening layers" (column 4, line 60ff). There is no
disclosure of the use of such spring plates for the purpose of maintaining
dimensional stability within the stack dimension, nor any teaching as to how
such spring plates could be adapted for use in multiple-function
configurations
nor in conjunction with an external manifold.
Mattejat, L~.S. Patent No. 5,472,801, granted 5 December 1995,
discloses a PEM-type fuel cell with intervening contact plates provided with
"stamped out contact tongues" (column 6, line 57) that might arguably function
as spring plates similar to those of Bette. Mattejat's teaching suffers from
the
same deficiencies as the Bette teaching mentioned above.
Kothmann, U.S. Patent No. 4,276,355, granted 30 June 1981, discloses
an externally manifolded air-cooled fuel cell stack. This stack is for use
with
a liquid electrolyte-type fuel cell employing phosphoric acid as the
electrolyte.
Such cells are not susceptible to the same sort of dimensional instability as
PEM fuel cell stacks. There is nothing in the Kothmann teaching to suggest
that compression occurs, and nothing to suggest the use of compensating
springs or the like. Kothmann discloses the use of a solid cooling block
between two bipolar plates. The cooling block cannot accommodate the
expansion and contraction of fuel cell components. Further, the cooling block
is bulky, adding to the weight and volume of the fuel cell stack, and tends to
be inefficient for heat dissipation. It would not be expected to be suitable
for
dissipating iow-grade heat generated by medium to high power PEM fuel cells.
Nothing in the Kothmann patent suggests that his phosphoric acid fuel cell
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design would be suitable for use in a PEM fuel cell stack.
Kumata, U.S. Patent No. 4,508,793, issued 2 April 1985, and Tajima,
U.S. Patent No. 5,541,015, granted 30 July 1996, each disclose an externally
manifolded phosphoric acid liquid electrolyte air-cooled fuel cell stack, and
each is subject to the same deficiencies in teaching as the Kothmann patent.
The specification of Siemens AG German Patent Application Serial
No. DE 44 42 285 Cl, published 8 February 1996, discloses a stamped metal
fuel cell configuration. An intermediate panel is disclosed whose functions
include improving mechanical support, transferring electric current and
providing a contact surface for air cooling of the stack. Although Siemens
teaches that interwoven or tangled wire or wire mesh may be used as the panel
material (none of which would have any appreciable springiness), nevertheless
the panel may have sufficient spring properties to generate a modest internal
thrust force against fuel cell plates. However, the rigid frames of the
component fuel cells must remain in contact. 'these frames determine the stack
dimension. Any thermal or hydro expansion has to be accommodated by
external springs. This fuel cell stack design uses internal manifolds; there
is
no discussion of the possibility of external manifolding nor of the need to
maintain dimensional stability in the stack dimension to accommodate such
possibility.
Baker, U.S. Patent No. 4,169,917, granted 2 October 1979, discloses
an air-cooled fuel cell incorporating layers configured as rectangular
corrugations and other geometries for use as corrugated layers. Baker does not
disclose any dimensional stability problems, nor any use of the corrugated
layer or any other layer as a compression spring, nor the use of internal
springs
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of any sort to maintain dimensional stability. 'there is no discussion of the
manifolding for the fuel cell.
Maru, U.S. Patent No. 4,444,851, granted 24 April 1984, discloses an
externally manifolded air-cooled fuel cell stack, but there is no disclosure
of
means for compensation of dimensional instability in the stack dimension;
further, Mare does not disclose that his configuration would be suitable for a
PEM-type fuel cell stack. The problem of dimensional alignment within an
externally manifolded fuel cell stack is not addressed.
Sasaki, U.S. Patent No. 5,378,247, granted 3 January 1995, discloses
a corrugated separator for a molten carbonate-type fuel cell. There is no
suggestion that this configuration would be suitable for use in a PEM-type
fuel
cell. There is no discussion of dimensional instability in the fuel cell stack
nor
means to maintain dimensional stability, nor of manifolding of this particular
type of fuel cell.
Leonida, U.S. Patent No. 5,446,354, granted 14 November 1995, is not
directed to fuel cell design at all, but rather to electrolysis cell design.
Leonida
discloses a metal compression pad interposed between pairs of stacked
electrolysis cells in order to compress the cells so as to compensate for
dimensional variations and thermal expansion, and to provide electrical
connection. Leonida does not teach the use of such pads for use in fuel cell
stacks, nor is Leonida concerned with manifolding or heat dissipation.
Other literature disclosing some fuel cell stack structures of interest
includes the following:
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I . Anahara, R., "Research, Development, and Demonstration of
Phosphoric Acid Fuel Cell Systems , (8.3.3.2 Air-Cooling System)" Fuel Cell
Systems, L. J. Blomen led.), Plenum, 1993, pp. 296-297.
2. Gibb, P., et al. "Electrochemical Fuel Cell Stack with
Compression Mechanism Extending Through Interior Manifold Headers," U.S.
Patent No. 5,484,666, Jan. 16, 1996.
3. Selman, J. R., "Research, Development, and Demonstration of
Molten Carbonate Fuel Cell Systems, (9.2.6 Stack Structure & 9.4.1. Stack
Configuration and Sealing)" Fuel C.'ell Systems, L. J. Blomen led.), Plenum,
1993, pp. 357-359 & pp. 398-399.
SUMMARY OF 'THE INVENTION
The invention comprises improvements in fuel cell stack design,
primarily for PEM-type fuel cell stacks, including a novel mufti-function
stratum (panel) for use in such stack, and an external manifolding arrangement
suitable for use with such stack.
A fuel cell stack assembly according to the invention (particularly
useful for a PEM-type fuel cell stack) in a preferred embodiment includes an
aligned series of uniform mufti-function strata or panels alternating with and
interposed between a mating series of uniform electrochemically active fuel
cell strata, which in a PEM-type fuel cell stack are either fuel cell units or
membrane electrode assembly (MEA) layers. (Obviously a terminating one of
such strata must be provided adjacent the terminal end plate at either end of
the
stack.) Each stratum is a discrete sub-assembly. Each fuel cell stratum may
be of conventional manufacture; such stratum includes the electrode layers,
electrocatalytic layers, and polymeric proton exchange membrane electrolytic
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layer in the case of an MEA layer-type fuel cell stratum and in the case of a
fuel cell unit-type fuel cell stratum includes separators. Each mufti-function
stratum comprises an interior open spring layer or equivalent sandwiched
between a pair of relatively rigid conductive layers. Such rigid conductive
layers in an orthogonal stack configuration (i.e., a stack having the overall
shape of a rectangular parallelepiped) are generally planar in the form of a
thin,
flat, parallelepiped, so that the mufti-function stratum has the overall shape
of
a generally flat panel, the interior open spring layer being sandwiched
between
the rigid conductive layers. The rigid conductive layers need not be rigid in
the
sense that a desktop, say, is rigid, but should impart sufficient rigidity to
the
sandwich structure that the mufti-function panel maintains its structural
integrity within the stack. As will be discussed below, it may in some
circumstances in which fuel cell units are used as fuel cell strata be
possible to
depart from strict alternation of fuel cell strata with mufti-function strata
and
to have two or possibiy more fuel cell strata in immediate succession in the
stack followed by a mufti-function stratum, but such is not normally
preferred.
The open spring layer of each mufti-function stratum may conveniently
be in the form of a corrugated or other undulate sheet. Alternatively, the
open
spring layer could be formed as a spaced array of fins of identical transverse
cross-section shaped to provide the function of a compression spring; that
cross-section could for example be S-shaped or undulate. As a further
alternative, the open spring layer could be an array of coil springs whose
axes
are parallel to the stack dimension. As a further alternative, the open spring
layer could be manufactured from a single sheet of elastic material subjected
to cutting and punching to create an inset array of integrally connected
springs.
(In this last embodiment, the cut and punched sheet could function both as
one of the rigid layers and as the spring layer.) .Alternatively, the open
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layer could be a corrugated metal mesh. Further suitable structures may be
imagined. Not all will be equally convenient to manufacture nor equally
effective to provide spring compression, conduct electricity, or dissipate
heat.
The common characteristics of all of these alternative choices for the open
spring layer are elasticity (springiness), heat and electrical conductivity,
and an
open structure permitting cooling air to tlow through the multi-function
stratum
and absorb heat from the open spring material, which is conveniently a springy
(elastic) metal.
It is of course essential that continuity of the electric circuit in the stack
dimension through the strata of the stack be maintained, so choosing a
conductive material as that of which the spring layer is made meets that
requirement; further, good electrical conductors tend to be good heat
conductors and dissipators. Preferably the spring layer is electrically and
physically bonded to each of the rigid layers between which it is sandwiched;
if for example the spring layer is a continuous undulate sheet, then the
apices
of the undulations are bonded to the contiguous rigid layers.
The spring layer must be shaped to provide continuous open spaces
therethrough so that when the fuel cell stack is operational, heat from the
spring layer material (to which heat has been transferred from the fuel cell
strata adjacent) may be transferred efficiently to cooling fluid (typically
air)
passed through the open spaces in the sptiBg layer. For manufacturing
convenience, the spring layer is preferably uniformly constructed from one
airflow end to the other; it is desirably invariant in the airflow dimenstion,
subject to any need to generate some turbulence in the airflow to increase the
rate of heat dissipation.
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The open spring layer sandwiched between each pair of rigid
conductive sheets in a multi-function stratum constitutes a distributed spring
exerting a compressive force acting in the stack dimension on the rigid
conductive sheets between which it is sandwiched and consequently acting on
all of the strata in the stack. The set of~ such layers in the stack
constitutes a
distributed spring array that can eliminate the need for a separate
compression
spring at the end of the stack.
In this specification and the appended claims, the three mutually
perpendicular directions and dimensions of interest are for convenience of
description and definition named as follows:
(a) The stack dimension is the dimension extending from one extremity of
the fuel cell stack to the other, perpendicular to the layers of the stack and
perpendicular to the broad working surfaces of the fuel cell strata. In the
appended drawings showing the fuel cell stack, for convenience this dimension
is presented as the vertical dimension.
(b) The airflow dimension is the dimension parallel to the flow of cooling
air or other cooling fluid through the mufti-function panels.
(c) The transverse dimension is the dimension from one manifold cover
plate (see description below) to the other, perpendicular to the other two
dimensions. Where the open spring layer is formed as an undulate layer, this
dimension extends from the begimiing of the waveform to the end of the
waveforrn of the layer.
In embodiments of the invention in which the fuel cell strata are MEA
layers, the outer surfaces of the rigid conductive sheets (i.e., the surfaces
that
are in contact with adjacent MEA layers in the stack) of each mufti-function
stratum should be inert to reactant gases (i.e., the fuel and oxidant gases)
in the
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fuel cell stack. An inert coating or protective layer may be applied to the
outer
surfaces rigid sheets, or the rigid conductive sheets themselves may be made
of material inert to the reactant gases and non-poisonous to the fuel cell MEA
layers, such as graphite. The use in fuel cells of certain selected types of
fuel-
cell compatible stainless steel such as 316L stainless steel has also been
proposed in the literature. The rigid conductive layers also serve as
protective
layers for the spring layer, so that reactant gases do not reach such spring
layer
to react with it. Suitable protective layers include graphite foil and curable
graphite-containing inks or pastes.
The open spring layer, as mentioned, may preferably be an undulate or
corrugated sheet. The terms "undulate layer", "undulate sheet", "corrugated
sheet" or "corrugated layer" used herein mean any sheet or layer of a suitable
degree of springiness having undulations that provide suitable apices for
physical and electrical contact with the rigid conductive plates or layers,
and
which undulations also define air passageways to permit cooling air to flow
from one end of the corrugations or undulations to the other (each end being
exposed to the ambient air or coupled to a forced air circuit that forces
cooling
air over the undulations). Such undulate layers do not include, for example,
corrugated sheets having a rectangular waveform, because these lack the
requisite springiness, and would tend to buckle or crumple rather than flex
under load. The physical contact of the corrugated sheet with the protective
rigid plates or layers permits heat to flow from fuel cell strata through the
rigid
plates and thence to the corrugated sheet, which thus serves as a heat
exchanger
or dissipator to dissipate heat into the cooling air passing over the
corrugations.
This heat dissipation arrangement thus functions in a manner similar to the
operation of a conventional automobile radiator. The corrugated layer must be
made of metal or other suitable springry material so that the requisite degree
of
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compressive force can be applied by the corrugated sheet in the stack
dimension to the rigid plates, thereby maintaining an overall compressive
force
in the stack dimension within the stack. The undulate layer must be both
thermally and electrically conductive; thermal conductivity is necessary for
adequate heat dissipation, and electrical conductivity is necessary to enable
low-resistance current flow through the stack. While a technically acceptable
substitute for a conductive springy sheet having continuous corrugations or
undulations would be a layer provided with (i) fins or other suitable heat
transfer surfaces for heat dissipation (heat transfer to the cooling airflow),
(ii)
conductive contact surfaces for thermal, physical and electrical contact with
adjacent layers, and (iii) springs for applying compressive force in the stack
dimension, any such substitute layer is likely to be appreciably more
expensive
to manufacture and install than a continuous corrugated or undulating sheet or
layer, so the latter is preferred.
While the fuel cell stack of the invention has been and will be for the
most part described as including planar strata and layers (apart from
corrugations and the necessary flowpath structures), it is apparent that other
configurations are possible, for example generally parallel curved
subassemblies (strata) could be devised in lieu of planar strata. As a
practical
matter, the rigid sheets of the multi-function panel will usually be planar,
as
that choice lends design and manufacturing convenience to the implementation
of the present invention, in which case the stack dimension is perpendicular
to
the plane of the multi-function panel sheets, and the stack lends itself to an
orthogonal description. But it is possible to construct the fuel cell strata
and
multi-function panels as themselves undulate overall, or arcuate, or otherwise
departing from planar. The concepts described herein using the named
dimensions as applied to such planar and orthogonal stack configurations may
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be modified to accommodate non-orthogonal stacks or non-planar strata.
Further, while the invention has thus far been described as requiring the
use of conductive rigid sheets between which the corrugated sheet (or other
suitable equivalent open spring layer) is sandwiched, it is apparent that the
entirety of the sheets need not be conductive. What is required is adequate
electrical conductivity between adjacent fuel cell strata. As long as the
rigid
sheets include adequate provision for such electrical conductivity, they need
not be uniformly conductive throughout. It is also apparent that any
protective
coating applied to such sheets must not unacceptably interfere with the
electrical conductive path; accordingly, preferably any such protective
coating,
if applied at all, should be selected to be electrically conductive. If the
protective coating or layer is made of material that is conductive, non-
poisonous to the MEA layers, and inert to reactant gases (graphite being a
suitable choice), then no problem arises, as the entirety of the mufti-
function
stratum will then be both conductive and non-reactive. Otherwise, if the rigid
plate is not entirely conductive or a protective layer applied to it is not
conductive, some means such as an array of spaced chemically inert conductors
extending from or through the protective coating or rigid layer or both could
be provided.
There must also be in the fuel cell stack some suitable means for
providing a supply of reactant gases to the electrochemically active layers of
each fuel cell stratum. In a PEM-type fuel cell stack, fuel gas is supplied to
one active layer of a fuel cell stratum and oxidant gas is supplied to the
other
active layer of each fuel cell stratum. (An "active layer" is one to which
fuel
or oxidant, as the case may be, is delivered to enable the electrochemical
reaction to take place. Such active layer is typically a porous cathode layer
or
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a porous anode layer.) The gas delivery means may be conveniently integrally
formed on the outer surface of the conductive rigid layer of the multi-
function
panel as a pattern of flowpath channels if the fuel cell stratum is an MEA
layer,
or the gas delivery means may be formed integrally within the fuel cell
stratum
if the fuel cell stratum is a fuel cell unit.
In a preferred embodiment, the means for providing fuel gas comprises
a configuration of one or (depending upon fuel cell dimensions) preferably two
or more meandering fuel channels formed in the outer surface of one of each
pair of the protective layers associated with each of said multiple-function
panels. A similar set of meandering oxidant channels is formed in the outer
surface of the other of each such pair of protective layers. The meandering
channels may be of conventional design. By "meandering" is meant that each
channel typically has a Length appreciably greater than either dimension of
the
major broad surface of the protective layer, and accordingly is provided with
a number of reverse curves to enable the channel to supply gas to a relatively
large portion of the MEA porous electrode layer adjacent. The pattern of
meandering channels on either surface of the multiple-function panel is
designed to be sufficient to provide gas to almost the entirety of the exposed
MEA porous electrode layer surface adjacent. The meandering fuel channel
pattern or configuration is exposed to an adjacent porous anode layer of an
adjacent MEA layer, while the meandering oxidant channel pattern or
configuration is exposed to an adjacent porous cathode layer of an adjacent
MEA layer. In current conventional fuel cell design, such meandering
channels are typically serpentine. The channel walls should be of uniform
height and should contact the adjacent membrane electrode assembly
throughout the length of the walls, but if all the fuel channels are formed on
one side of the multiple-function panel and all the oxidant channels are
formed
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on the other side, then minor imperfections or leaks in the channel walls will
be inconsequential.
Perforce each channel in the fuel supply channel configuration is
provided with an inlet fuel port and an outlet fuel port. The inlet fuel port
may
conveniently communicate with a fuel supply plenum and the outlet fuel port
may conveniently communicate with a fuel exhaust plenum. Similarly, each
channel in the oxidant supply channel configuration is provided with an inlet
oxidant port and an outlet oxidant port, the inlet oxidant port advantageously
communicating with an oxidant supply plenum and the outlet oxidant port
advantageously communicating with an oxidant exhaust plenum. Each plenum
is isolated from every other plenum (except via the reactant flowpaths) by
means of walls that may be formed in a manifold external to the various layers
within the fuel cell stack. The manifold is provided with conduits, at least
one
conduit per plenum, for connecting each plenum to a respective associated
source of fuel or oxidant gas, or sink of spent fuel or oxidant gas, as the
case
may be.
The invention comprises a fuel cell stack of the type heretofore
described, but it also comprises a mufti-function stratum (composite panel) of
the type described. Such mufti-function stratum is particularly suitable for
use
as a stratum or panel alternating with MEA layers in a PEM-type fuel cell
stack
assembly of the type defined above.
Such mufti-function strata or panels may each suitably comprise a
corrugated metal sheet sandwiched between two flat metal sheets. As
mentioned, such mufti-function strata serve as both the heat dissipating
elements of the stack and the springs to compensate for thermal and hydro
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expansion and age contraction of the fuel cells in the stack. The multi-
function
strata also provide structural support for the porous electrode layers
adjacent
and serve as current collectors to interconnect all fuel cells in the stack.
The
mufti-function panels form a distributed spring system that force the
"floating"
(otherwise free to move in the stack dimension) fuel cells and their reactant
gas
delivery plates into fixed positions. Undesired motion of the fuel cells and
their associated plates along the stack dimension is effectively eliminated,
making the sealing of external manifolds easy to accomplish, and thus making
feasible the use of external manifolds for a high-power PEM-type fuel cell
stack.
The associated structure of the fuel cell stack may advantageously
include pillars disposed peripherally about the stack and extending in the
stack
dimension. The terminal plates may be fixed to the pillars and may form with
the pillars a structurally sound "cage" or frame for the stack. The pillars
thereby provide both structural integrity and mounting surfaces for mounting
external manifold cover plates. 'Che fuel cell stack is thus seen to comprise,
within the framework of the pillars and terminal plates, an aligned series of
repeated combinations of elements, each of which combinations consists in one
preferred embodiment of the invention, of an MEA layer, a pair of reactant gas
delivery plates, and a mufti-function panel and, in a second preferred
embodiment of the invention, of an MEA layer and a mufti-function panel.
These repeated elements are flexible in the stack dimension and are
constrained
by the solid frame of the stack.
For obvious reasons, the fuel cell stack requires seals to prevent
reactant gases from commingling, to prevent such gases from reacting with
other materials in the stack (notably the metal in the corrugated layer), etc.
The.
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seals for the stack may include gaskets or sealant-filled channels around the
reactant and oxidant flowpath channels, gaskets at the manifold interface with
support pillars or other stack side surfaces, spacers inserted between the
supply
and exhaust plena and the sides of the mufti-function strata thereadjacent,
and
applied sealing cement where required.
It is also an aspect of the invention to use hydrogen as a fuel gas in a
fuel cell stack of the type heretofore described that includes a plurality of
mufti-function panels that are embodiments of the present invention stacked
together with the fuel cell units of the fuel cell stack, each mufti-function
panel
being interposed between a unique pair of said fuel cell units. Each multi-
function panel includes: (i) a pair of relatively rigid thermally and
electrically
conductive layers; and (ii) an electrically and thermally conductive elastic
open
spring layer sandwiched between and in mechanical, thermal and electrical
contact with the rigid layers, the open spring layer when the associated multi-
function panel is installed in a fuel cell stack providing electrical and
thermal
conductivity between the rigid layers, providing heat dissipation into the
surrounding fluid medium of heat transferred to the mufti-function panel from
the fuel cells in the stack, and serving as a compression spring exerting on
the
rigid layers a compressive force generally perpendicular to the panel for
providing compressive force within the stack in the stack dimension. Each fuel
cell unit in the fuel cell stack includes a membrane electrode assembly
(sometimes referred to herein as "MEA" or "MEA assembly" or "MEA
sandwich") formed as a "sandwich" having as the two outermost layers of the
sandwich an anode electrode comprising porous anode material, and a cathode
electrode comprising porous cathode material. The "filling" of the sandwich
comprises an electrolytic membrane layer disposed between the two electrodes,
an anode eiectro-catalyst layer disposed between the electrolytic membrane
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layer and the anode electrode, and a cathode electro-catalyst layer disposed
between the electrolytic membrane layer and the cathode eiectrode. A fuel cell
unit is formed when this MEA sandwich is installed between a first flow-field
plate, a selected side of which provides flow channels of a flow field for
hydrogen. and a second flow-field plate, a selected side of which provides
flow
channels of a flow field for a selected oxidant. The selected side of the
first
plate faces and is in contact with the anode electrode and the selected side
of
second plate faces and is in contact with the cathode electrode so that the
hydrogen flow channels arc closed to form a conduit for supplying hydrogen
to the membrane electrode assembly and the oxidant flow channels are closed
to form a conduit for supplying oxidant to the membrane electrode assembly.
According to another aspect of the invention, hydrogen may be used as
a fuel gas in a fuel cell stack that includes a plurality of mufti-function
panels
that are another embodiment of the present invention stacked together with the
MEA layers of the fuel cell stack, each mufti-function panel being interposed
between a unique pair of said MEA layers, the mufti-function panels acting as
a flow-field-defining structures. In this version of the fuel cell, the
membrane
electrode assembly, as in the previously described embodiment, is formed as
a sandwich as described above, but the two flow-field plates are omitted and
the flow fields, channels, and conduits are formed by mufti-function panels
that
have been coated or otherwise made inert to reactant gases present in the fuel
cell stack and non-poisonous to the membrane electrode assembly, but are
otherwise as described in the preceding paragraph. Flow channels are provided
on or in the surfaces of the rigid layers.
It can be seen that the principal difference beriveen the two fuel cell
stack embodiments just described is that in the first embodiment described
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above, the flow-fields are provided by the separator plates of the fuel cell
stratum itself, whereas in the second embodiment the flow-fields are provided
by the multi-function panels.
The use of hydrogen as a fuel in fuel cells of the described construction
constitutes an advantageous aspect of the invention. Both types of the fuel
cell
strata described in the preceding paragraphs operate when hydrogen is supplied
to the hydrogen conduit, oxygen is supplied to the oxidant conduit, and a
circuit that includes a load and is capable of receiving electrons from the
anode
electrode and supplying electrons to the cathode electrode is connected to the
anode electrode and the cathode electrode. In theory, the load could be
applied
across an individual fuel cell, but in practice a number of such fuel cells
are
stacked in a fuel cell stack, with the layers of all of the fuel cells being
substantially parallel in most conventional fuel cells. The stack is provided
with a terminal cathode at one end and a terminal anode at the other, across
which terminals the load is connected. When hydrogen and oxygen or other
suitable oxidant are supplied to the fuel cells in the stack, then in each
fuel cell,
hydrogen moves from the hydrogen flow field through the porous anode
electrode and is ionized at the anode electro-catalyst of the membrane
electrode
assembly to yield electrons and hydrogen ions. The electrolytic membrane
layer of the membrane electrode assembly is permeable to hydrogen ions
(protons), but is not conductive. Hence only the hydrogen ions may migrate
through the electrolytic membrane layer. The hydrogen ions after migrating
through the electrolytic membrane react with oxygen that has moved from the
oxidant flow field through the porous cathode electrode to the cathode electro-
catalyst and electrons supplied by the circuit. The reaction product is water.
To make up for the loss of electrons provided to the cathode electrode, the
circuit receives electrons from the anode electrode. A useful current of
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WO 99/57781 PCT/GB99/01 lfi9
electrons through the load is thereby provided. A fuel cell stack may be
formed from two or more of the fuels cells described above in a conventional
manner known to those familiar with fuel cell technology and therefore need
not be described in detail here.
While the advantages of all of the described aspects of the invention
have particular application to PEM-type fuel cell stack design, some of there
can be utilized to good effect in other types of fuel cell stack. For example,
the
mufti-function panels could be used in other types of fuel cell stack as heat
dissipators, even if they were not required for dimensional stability of the
stack
to compensate for thermal and hydro expansion or fuel cell shrinkage with age.
The efficiency of the overall stack architecture could be advantageous for
other types of fuel cell. ~-lowever, the full benefit of the fuel cell stack
variants
and mufti-function panels discussed herein is expected to be achieved only i:f
the fuel cells are PEM-type fuel cells.
SUMMARY OF THE DRAWINGS
Figure 1 is a schematic isometric view of an embodiment of a PEM-
type fuel cell stack constructed in accordance with the present invention.
Figure 2 is a schematic exploded isometric view of the fuel cell stack
of Figure 1.
Figure 3 is a schematic isometric exploded view of a side portion of the
fuel cell stack of Figure 2 showing also gaskets suitable for insertion in the
space between the cover plate and side posts of the fuel cell stack of Figure
2.
Figure 4 is a schematic front elevation view of the fuel cell stack of
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WO 99/57781 PCT/GB99/01169
Figure 1.
Figure 5 is a schematic section view of a representative corrugated
sheet layer of the fuel cell stack of Figure. 1 taken along the line V-V of
Figure
4.
Figure 6 is a schematic section view of an embodiment of a fuel
flowpath configuration of a representative fuel cell in the stack of Figure 1
taken along the line VI-VI of Figure 4.
Figure 7 is a schematic section view of an embodiment of an oxidant
flowpath configuration of a representative fuel cell in the stack of Figure 1
taken along the line VII-VII of Figure 4.
Figure 8 is a schematic section view of an alternative fuel flowpath
configuration that may be substituted for that of Figure 6.
Figure 9 is a schematic section view of an alternative oxidant flowpath
configuration that may be substituted for that of Figure 7.
Figure 10 is a schematic fragmentary exploded view of a mufti-function
stratum of the fuel cell stack of Figure 1, showing in greater detail the
configuration of the corrugated sheet layer.
Figure 11 is a schematic front elevation view of two strata of the fuel
cell stack of Figure I showing in greater detail the configuration of the
multi-
function stratum.
Figure 12 is a schematic exploded fragmentary front elevation view of
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WO 99/57781 PCT/GB99/01169
a portion of Figure 11, showing the spacer and mufti-function stratum
arrangement of the fuel cell stack of Figure 1.
Figure 13 (prior art) is a schematic diagram representing the
conventional spring-loading of fuel cells in a PEM-type fuel cell stack, as
practised prior to the present invention.
Figure 14 is a schematic diagram illustrating the distributed-spring
arrangement of corrugated sheets and fuel cells in a fuel cell stack in
accordance with the principles of the present invention.
DETAILED DESCRIPTION
A fuel cell stack 10 constructed in accordance with the principles of the
present invention is schematically illustrated in Figure 1 f~ By way of
example only, eight fuel cell strata 12 are shown in Figure 1 in stacked
arrangement between an upper terminal plate 14 and a lower (base) terminal
plate 16 for the fuel cell stack assembly 10. (An actual relatively high-power
fuel cell stack might typically have several dozen fuel cells in the stack.)
The
terminal plates 14, 16 are fixed to comer support pillars or posts 18 and
intermediate support pillars or posts 20, all of which pillars 18, 20 rest on
the
base terminal plate 16. The fixing of the pillars 18 and 20 to the base plate
16
may be conveniently effected by screwing bolts 22 through mating holes (not.
shown) in base terminal plate 16 and thence into mating threaded holes (not
shown) in the bases of pillars 18 and 20. The upper teminal plate 14 may be
fixed to the pillars 18 and 20 by means of bolts 24 that penetrate through
mating holes 26 in the upper terminal plate 14 and thread into mating threaded
receptacle holes 28 in the tops of pillars 18 and 20.
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A complete repeating unit of the stack 10 comprises a fuel cell stratum
I2 and an adjacent mufti-function stratum or panel 30. The structure of the
mufti-function stratum 30 is more clearly illustrated in Figure 1 l; see also
the
exploded view of Figure 10.
Figure 11 illustrates, in schematic vertical sequence, an exemplary pair
of consecutive repeating units of the stack, each repeating unit comprising a
fuel cell stratum 12 and a mufti-function stratum 30. Each fuel call stratum
12
may be a fuel cell unit of conventional manufacture comprising an upper
separator/flow-field plate 13 for fuel gas, an intermediate MEA layer 1 S, and
a lower separator/flow field plate 17 for oxidant gas. Alternatively, each
fuel
cell stratum 12 may include a discrete MEA layer without separator/flow field
plates. Fuel cell strata that are fuel cell units are referred to herein as
fuel cell
units I2 and fuel cell strata that are MEA layers without separator/flow field
plates are referred to herein as MEA layers 12, where it is necessary to
distinguish between the two forms of fuel cell strata. If it is not necessary
to
distinguish between the two forms of fuel cell strata, then they are both
referred
to as fuel cell strata 12. Both forms of fuel cell strata are conventional.
The mufti-function strata 30 each comprise an upper rigid layer or plate
32, an intermediate open spring layer in the form of corrugated layer 34, and
a lower rigid layer or plate 36. The three layers 32, 34, 36 may conveniently
be made of metal, provided that care is taken to prevent metal poisoning of
the
MEA layers if the mufti-function strata 30 are to be used with MEA layers 12.
While some choices of metal are relatively inert, others, such as copper,
would
be suitable for electrical and thermal conductivity, but would contaminate the
MEA layers 12. So when such fuel cell poisoning materials as copper are
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WO 99/57781 PCT/GB99/01169
chosen for the material of which the rigid layers 32, 36 are made, it is
necessary to interpose an inert medium between the metal and the neighbouring
MEA layers 12. Such inert medium can be a graphite coating, for example,
applied to the metal, preferably after roughening the metal surface by
sandblasting or the like to improve the adhesion of the coating to the rigid
layer.
A protective coating found suitable for use in fuel cells is a castable
graphite tooling material that is similar to monolithic graphite in a
manufacturing environment, but has increased strength. One such material is
sold as Hyper CastT"'' Graphite by Hyper Industries of Bonita, California.
Such
graphite is sold in a paste form for ease of application. It will cure through
a
catalyzing process. The curing can take place in several steps at both ambient
temperature or somewhat elevated temperatures (40°C - 110°C). If
the material
is sintered at a high temperature, around 700°C, few if any residual
organic or
inorganic contaminants will be left (leading to a final coating composition of
close to 99% carbon). Alternatively, the metal (say) sheets forming rigid
layers 32, 36 could be coated with a graphite paint to forni a solid graphite
coating over the metal. Alternatively, a separate protective layer such as a
graphite foil sheet could be interposed between each rigid layer 32. 36 and
the
neighbouring MEA layer 12.
If the fuel cell strata are MEA layers 12, then it is possible to form the
requisite reactant gas flowfield on the outer surface of each rigid layer 32,
3~
so that such layer does double duty, both bearing the walls defining the
flowfield structure, and forming a portion of the multi-function sandwich. The
castable ~aphite mentioned in the preceding paragraph can be used to form the
flowfield walls on planar layers 32, 36, or the walls may be molded in the
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WO 99/57781 PCT/GB99/Ollti9
layers 32, 36 prior to coating. Other manufacturing alternatives are possible.
The corrugated sheet layer 34 functions as an electrical conductor, as
a heat dissipator, and as a spring. The corrugated layer 34 is most easily
manufactured if the corrugations are uniform in both the airflow and
transverse
dimensions, but there may be particular reasons to provide non-uniform
corrugations in either or both dimensions, e.g. for generation of air
turbulence
in the interest of enhancing heat transfer to the cooling air flow. The apices
38
of the corrugated layer 34 make electrical and physical contact with the
respective rigid plates 32, 36 adjacent the apices of the corrugated layer 34.
The air spaces formed by the corrugations of layer 34 constitute cooling
passages 35 for permitting cooling air to flow over the corrugations of the
layer
34, thereby resulting in heat transfer from the stack 10 to the cooling air.
The
cooling air flow can be from the front of the stack 10 to the back (as seen in
Figure 1 ) or vice versa, and may be augmented by fans or the like. Spacer
bars
64 made of rubber or similar resilient sealing material seal the multi-
function
panels 30 from the reactant gas plena of the fuel stack, to be described
below..
For purposes of optimizing the spring performance of the multi-
functional strata 30, each of the apices of the corrugated layer 34 is
preferably
welded or soldered to the plate 32 or 36 respectively with which such apex of
the corrugated layer 34 comes into contact. This can be accomplished by
prefabrication of each multi-function stratum 30, for example by providing a
layer of solder on the inner surfaces of sheets 32, 36, placing the corrugated
layer 34 therebetween, and placing the assembly thus formed into an oven,
permitting the solder to melt and form the connections, and then permitting
the
assembly to cool so that the liquid solder connections solidify, leaving the
corrugated layer 34 firmly electrically and conductively bonded between the
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upper and lower rigid plates 32, 36.
In the embodiment illustrated in Figure l, there is a mufti-function
stratum 30 between each consecutive pair of fuel cell strata 12. However, it
is
conceivable that in some fuel cell stack designs in which fuel cell strata are
fuel
cell units 12, the fuel cell units 12 would be thin enough and the fuel cells
sufficiently dimensionally stable that two or more fuel cell units 12 could be
placed in the stack in immediate contact with one another to form an fuel cell
unit subset, between which subsets the mufti-function strata 30 might be
interposed. This optional arrangement would be expected to work successfully
only if the expansion and contraction over time ol'the fuel cell units are
slight
and only if the seals provided in the assembly are flexible. Experiments with
two fuel cell units per repeated fuel cell unit subset have been successful,
but
the safer design for most PEM-type fuel cell stacks is expected to be one
providing a mufti-function stratum between each consecutive pair of fuel cell
units.
It is necessary for the reactant gases in the stack assembly of Figure 1
to be independently fed to the respective porous electrode layers of the fuel
cell
strata 12. Accordingly, each fuel cell stratum 12 is designed so that inlet
and
outlet ports 90, 94, 104, 106 (Figures 6 and 7) for the reactant flowpaths
provided in the fuel cell strata 12 communcate with associated discrete plenum
chambers 76, 78, 80 and 82. Each plenum chamber 76, 78, 80 and 82 has a
substantially continuous inner boundary surface formed by the side edges 60
of the fuel cell strata 12 and side surfaces 66 of spacer bars 64 (Figures 2,
5, 11
and 12), the side surfaces 66 being aligned with the side edges 60 of the fuel
cell strata 12. It will be noted that the side edges 60 of the fuel cell
strata 12
are inset from the outer side surfaces 62 of the pillars 18 and 20. The spacer
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WO 99/57781 PCT/GB99/01169
bars 64 interposed between successive fuel cell strata 12 occupy the entire
space between any two adjacent corner and intermediate pillars 18, 20 and
between any two consecutive fuel cell strata 12, so as to completely seal off
the
mufti-function panels 30 from the plenum chambers 76, 78, 80 and 82.
As a result of the foregoing configuration, when side cover plates 68,
70 are assembled onto the sides of the fuel stack assembly 10, a gap is left
between the inset outer side surfaces 6f> of spacer bars 64 and side edges 60
fuel cell strata 12, on the one hand, and the interior surfaces 72, 74
respectively
of the side cover plates 68, 70, on the other hand, thereby forming reactant
gas
plenum chambers 76, 78, 80, 82 respectively. Plenum chambers 76, 78 can be
seen to lie within a left-hand manifold arrangement on the left side of the
fuel
cell stack 10 (as seen in Figures 2, 5, 6, 7) and plenum chambers 80, 82 can
be
seen to lie within a right-hand manifold arrangement on the right side of the
fuel cell stack 10.
Bearing in mind that one of the reactant gases is typically hydrogen,
which has a tendency to permeate materials and to escape easily if there is
any
leak, it is important that adequate sealing be provided for the fuel cell
stack 10.
It is thus important that the plenum chambers 76, 78, 80, 82 be completely
sealed off from one another (apart from the flowpaths interconnecting supply
and exhaust plenum chambers for a given reactant gas) and from the ambient
environment of the fuel cell stack. Typically fuel cell strata are
manufactured
as discrete composites provided with peripheral margin portions that are made
of material that facilitates sealing. The somewhat reslient sealed margins 29
(Figure 11 ) of each fuel cell stratum 12 would be expected to provide an
adequate seal around the contacting portions of the corner and intermediate
posts 18, 20. The spacer bars 64 are preferably formed of resilient material
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WO 99/57781 PCT/GB99/01169
such as rubber inert to reactant gases, and are preferably made slightly
oversized relative to the dimensions of the rigid units of the fuel cell stack
10,
so that they are under compression between adjacent intermediate and corner
pillars 18, 20 and between consecutive fuel cell strata 12, thereby
maintaining
a tight seal between the side edges of the mufti-function panels 30 and the
plenum chambers 76 through 82.
Figures 11 and 12 illustrate the spacer bars 64 inserted into cavities 27
(Figure 12) at the ends of the mufti-function strata 30. For convenience, the
spacer bars 64 may be provided with oversize outer end caps 25. The interior
dimensions of cavity 27 are preferably slightly smaller than the dimensions of
the mating spacer bars 64 so as to facilitate a tight seal, but note that
there is
normally a positive pressure drop between the outer and inner surfaces of
spacer bars 64 (the outer surfaces of spacer bars 64 being under pressure from
the reactant gases), so that this pressure drop tends also to maintain the
spacer
bars 64 in place and to force the inner surfaces of the cap portion 25 of the
spacer bars into close contact with the adjacent surfaces of the fuel cell
strata
12.
If desired, spaces 23 outside the boundaries of open spring panels 34
may be filled with a resilient latex sealing compound deposited as a viscous
paste. Preferably, the spacer bars 64 are glued in place within cavities 27 by
means of an epoxy glue, or the like.
To complete the sealing of the plenum chambers 76 through 82, gaskets
such as the gaskets 71, 73 illustrated in Figure 3 are preferably provided
between the side cover plates 68, 70 and the neighbouring pillars I 8, 20
against
which they are mounted. Mating recesses 7~, 77 to receive gaskets 71, 73 may,
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WO 99157781 PCT/GB99/01169
if desired, be provided in the inner surface 74 of side plate 70 (and a
similar
arrangement would, of course, be devised for cover plate 68). Sealing
compound may be applied along the gasket surfaces as required in order to
augment the seal. <;askets 71, 73 may be made of an inert material such as
rubber, resistant to reactant gases, or they may simply take the form of seals
such as silicone sealing strips applied as a viscous liquid to the inner
surfaces
of the cover plate 68 and 70 and then permitted to solidify. Note that ports
96,
98 are within the boundary of the recesses 75, 77 and mating gaskets 71, 73;
this is, of course, essential in order that the ports 96, 98 communicate
properly
with the plena 80, 82 within the respective sealed-off areas thus bounded.
The mounting arrangement for the cover plates 68, 70 is not critical; no
specific mounting means is illustrated in the schematic drawings, but, for
example, threaded receptacles (not shown) could be provided along the side
edges 62 of pillars 18 and 20, mating holes (not shown) could be provided in
the cover plates 68 and 70, and bolts (not shown) could be passed through the
mating holes in the cover plate 68 and 70 and threaded into the threaded
receptacles (not shown) in the associated pillars 18, 20.
Four plenum chambers 76, 78, 80, 82 having been provided by the fuel
cell stack assembly thus far described, it remains to describe the use of
these
plenum chambers to provide supply and exit passages for the reactant gases.
The specific flowpath embodiments illustrated in Figures 6 and 7 will
now be described. First referring to Figure 6, the fuel inlet plenum is the
plenum chamber 76 communicating with fuel inlet ports 90 of the flow field
92 in flow field panel 89 illustrated in Figure 6. Flow field 92 comprises a
pair
of discrete fuel flowpaths 93, 95 running from the inlet ports 90 to
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corresponding outlet ports 94. The meandering flowpaths 93, 95 of flow field
92 are serpentine in character so as to provide fuel gas to the active working
surface of the underlying (or overlying) porous electrode layer of the
associated
fuel cell stratum 12. For the embodiment of Figure 6, the plenum chamber 78
constitutes the exhaust plenum for exhaustion of the spent fuel gas. Since the
plenum chambers 76 and 78 cannot function without connection to an external
source and sink respectively of fuel gas, a fuel inlet port I08 and a fuel
outlet
port 110 are provided in end cover plate 68 for coupling to an external source
(not shown) and sink (not shown) respectively of fuel gas. For this purpose,
any suitable coupling element may be provided at each of the ports 108, 110
for coupling to externally mounted tubing (not shown) for connection to the
source and sink respectively of the fuel gas.
A similar such arrangement is made for the oxidant gases. Figure 7
illustrates a representative set of serpentine oxidant flowpaths 103, I OS,
107,
109, lll, 113 constituting a meandering flowpath arrangment generally
indicated as 100 in the associated oxidant flowpath panel 102. An exemplary
six flowpaths are illustrated that are provided with a set of inlet ports 104
communicating with supply plenum chamber 80 and a set of outlet ports I06
communicating with exhaust plenunu chamber 82. Again, the plenum
chambers 80, 82 are provided with a supply port 96 and an exhaust port 98
respectively (Figure 2) in end cover plate 70 for attachment via suitable
couplings (not shown) to tubing (not :shown) providing a supply and a sink
respectively of oxidant gas to the fuel cell stack.
In the flowplates 89, I02, auxiliary sealing cement or the like may be
applied in more critical areas if need be. For example, the flowpath
configurations of Figures 6 and 7 include disconnected spaces 91, 101
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WO 99/57781 PCT/GB99/01169
respectively into which epoxy cement or other suitable sealant may be
injected.
Various tlowpath arrangements can be devised by the designer to suit
individual fuel stack designs. The choice of flowpaths will depend in part
upon
the intended power output, the choice of fuel and oxidant gases, the supply
pressure and port size for each gas, the area of the substrate to be covered,
the
height of the boundary walls for the flowpath, the degree of porosity of the
porous electrode layer, the surface texture of the flow channels, and other
parameters influcing the pressure drop, flow velocity and Reynold number of
the reactant and oxidant gases. Generally speaking, having regard to the
respective molecular weights and densities, each fuel flowpath will usually be
appreciably longer than each oxidant flowpath; this implies that the number of
discrete flowpaths per layer will be higher for the oxidant than for the
fuel..
'Three principal flowpath layouts are possible, as follows:
1 ) Each fuel flowpath may both enter and exit on the same side of the stack
and
each oxidant flowpath may enter and exit on the opposite side of the stack.
Or,
2) the fuel flowpath may enter on one side of the stack and exit on the other
side, the oxidant flowpath entering on the fuel path exit side and exiting on
the
fuel path entrance side. Or
3) as the final option, both flowpaths may begin on one side of the stack and
both may exit on the opposite side of the stack.
'there are various advantages and disadvantages associated with each
of the foregoing flowpath design options. For example, if the stack is placed
on its side (relative to the orientation shown in Figure 1) such that the exit
ports
are as a consequence located along the lowermost fuel cell stack surface, then
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WO 99/57781 PCT/GB99/01169
the third option mentioned above would be optimal for drainage of any surplus
water out of the fuel cell stack. (If the stack is designed properly and
operating
normally, all water should evaporate and be carried off with the effluent
spent
gases, but sometimes there can be accumulations of liquid water that have to
be dealt with.) If the fuel enters and leaves on one side of the stack and the
oxidant enters and leaves on the other side of the stack, then it is easier to
isolate fuel and oxidant gases from one another so that leaks permitting the
two
gases to combine outside the fuel cell will be less likely; for this safety
reason,
the first option mentioned above would be preferred (that is the option
embodied in Figures 6 and 7). Finally, the second option, in which the fuel
and
oxidant flowpaths tend to be oriented overall in roughly opposite directions,
has been found to be the most chemically efficient arrangement. So depending
upon the priority of importance of the factors mentioned above and possibly
others, the designer will select a flowpath design that best meets design
criteria
and priorities.
By way of further example, Figures 8 and 9 illustrate optional fuel and
oxidant flowpath configurations respectively that implement the second design
option mentioned above. In Figure 8, fuel flow field panel 51 is provided with
two meandering fuel f7owpaths 47, 4J running from fuel flowpath entry pons
53, 55 to fuel flowpath exit ports 57, 59 Again a serpentine flowpath
arrangement is maintained so that the adjacent porous electrode layer (not
shown in Figure 8) next to the flowpaths 47, 49 may be maximally exposed to
fuel gas. By arranging the location of fuel flowpath exit ports 57, 59 on the
opposite side of the flow field of Figure 8 from the side on which fuel path
entry ports 53, 55 are located, it is possible to select (as discussed below)
the
oxidant fuel path to flow in roughly the opposite direction, thereby achieving
the objective of the second design option mentioned above. Note that with the
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configuration of Figure 8, it is necessary to re-assign the plenum chamber
functions relative to the assignment of Figure 6; accordingly, plenum chamber
76 serves as a fuel supply plenum chamber communicating with the inlet ports
S3, SS of the fuel flowpaths, and plenum chamber 82 serves as a spent fuel
exhaust plenum chamber communicating with the exit ports S7, S9 for the fuel
flowpaths.
'turning to Figure 9, six serpentine oxidant flowpaths (unlabelled) are
illustrated proceeding from oxidant flowpath entry ports 63 through respective
individual flowpaths thence to oxidant flowpath exit ports 65. Note that the
inlet ports 63 for the oxidant flowpaths are on the opposite side of the fuel
stack 10 from the inlet ports 53, SS for the fuel flowpaths, and equally the
outlet ports for fuel (57, S9) and oxidant (6S) flowpaths respectively again
are
on opposite sides of the fuel stack 10, thereby enabling the objective of the
second design option mentioned above to be achieved. For this purpose,
plenum chamber 80 now serves as the oxidant supply plenum, and plenum
chamber 78 serves as the spent oxidant exhaust plenum. The appropriate
revised connections of the plenum chambers 76 through 82 will be made to
external sources and sinks of fuel and oxidant gases.
The choice of inlet and outlet ports for the flowpaths of Figures 8 and
9 are arbitrary and could be reversed. So ports S7, S9 (Figure 8) could serve
as fuel inlet ports and pons 53, SS as fuel exhaust ports; similarly inlet and
outlet ports 63, 6S could be reversed in Figure q. If such reversals were
made,
of course the plenum chamber assignments for pienum chambers 76 through
82 would have to be reallocated. Note that such reversal of inlet and outlet
port
selection is feasible only if the flow channels are of uniform cross-section
from
end to end. If the flow channels are designed to be tapered, then inlet and
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outlet ports cannot be interchanged.
Kecali that in conventional PEM-type fuel cell stacks, MEA layers and
separator plates are stacked together in a stack that does not include any
constituent elements resembling the corrugated layers 34 interposed between
successive fuel cell strata 12 in stack 10. In conventional stack assemblies,
a
compression spring is mounted between one terminal end plate of the stack and
the nearest separator plate (or other rigid plate such as a special bearing
plate)
at one end of the stack, so that expansion or contraction of the fuel cell
stack
layers can be accommodated by corresponding variations in the extent of
compression of the compression spring. The ideal. internal pressures within
the fuel cell stack can be of the order of 3 bars or higher; the spring
stiffness
chosen must provide adequate compression over the range of operating
conditions of the stack. The spring stiffness chosen should not be so high as
to raise the internal pressure above the ideal when the spring is under the
maximum compression provided by the operating conditions of the stack. In
other words, the spring stiffness must provide a suitable compressive force to
maintain the internal pressure within the ideal internal pressure range
regardless of whether the spring is relatively compressed, due to, say hydro
expansion, or is relatively uncompressed, due to, say,-expected MEA layer
shrinkage over tlrne. Thls previously known spring arrangement is illustrated
schematically in Figure 13, which shows a stack of fuel cells 40 of a
representative PEM-type fuel cell stack. The fuel cells 40 are spaced from
upper end terminal plate 44 of the fuel cell stack 42 to accommodate a spring
4f that maintains the stack in compression between upper end terminal plate
44 and lower end terminal plate 43 notwithstanding continual compression and
expansion of the fuel cells 40 in the stack 42. (In actual practice, a
plurality of
springs 46 spaced from one another over the area of upper terminal plate 44
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would typically be provided, rather than the single spring 46 shown in the
interest of simplifying the schematic diagram.)
By contrast, in accordance with the fuel stack assembly of the present
invention, the multi-function strata 30 schematically illustrated in Figure 14
together provide a distributed-spring arrangement throughout the entire fuel
stack 10, so that compression and expansion of fuel cell strata 12 are
accommodated locally by variations in the extent of compression. 'The strata
30 must always be under some compression in order to assure alignment and
help avoid leakage. The result is that there is no tendency of any individual
fuel cell stratum 12 to move along the stack dimension within the fuel cell
stack I 0. If a given fuel cell stratum (say) I 2 expands or contracts, the
extent
of compression of the associated adjacent mufti-function strata 30 changes to
compensate for such expansion or contraction of the fuel cell strata 12. The
entire assembly of fuel cell strata I2 and multi-function strata 30 are
mounted
under compression between terminal plates 14 and 16. The initial compression
is chosen to be sufficient to compensate for expected stack shrinkage over the
lifetime of the stack 10.
In the normal state of affairs, spring 46 (Figure 13) in the conventional
design would be under a compression load tending to force together the fuel
cells 40 in the stack dimension. Equally, each individual spring layer 56 in
the
distributed-spring arrangement according to the invention would always be
under slight compression, such that the overall compressive load provided by
the entirety of the springs 56 of Figure 14 would be roughly equal to the
single
compressive load exerted by spring 46 of Figure 13 (assuming comparable age,
materials, dimensions, power capacity and operating conditions of the fuel
cell
stacks being compared).
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The expected thermal expansion of fuel cell components can be
calculated given expected temperature variations, once the coefficients of
thermal expansion of the critical materials are ascertained. (The coefficients
of thermal expansion are known for virtually all critical materials that may
be
used in the fuel cell stack; the critical materials are those such as any
metals
that do expand and contract appreciably with temperature variations. Materials
that do not appreciably expand and contract with temperature variations, such
as the polymeric electrolyses in PEM fuel cells, can be igmored.) Note that
the
fuel cell stack frame will expand and contract with temperature changes, and
its dimensional variation has to be taken into account also. The expected
range
of hydro expansion of the proton exchange membrane layer is also known from
previous work in fuel cell design and testing, and in any case is best
ascertained
empirically. Given the calculated total expected expansion of the stack and
the
ideal internal pressure to be applied to the stack (which will vary with stack
dimensions, materials chosen, etc.), the ideal stiffness of the compression
k = PA
L+01,
spring, k, can thus be calculated by
where P is the ideal internal pressure of the stack, .4 is the cross-
sectional area of the stack perpendicular to the stack dimension, L is the
minimum compression of the compression spring, and dL is the
expected change in stack length (in the stack dimension) due to
expected net thermal and hydro expansion and age-induced shrinkage.
Generally, 41. is small relative to L.
Note that in each multi-function stratum, the spring could be either only
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the corrugated conductive sheet 34 or equivalent, or instead (and preferably)
it is the welded assembly 30 of the corrugated conductive sheet (or
equivalent)
34 and the two rigid conductive sheets 32, 36 bonded to the corrugated layer
34. The latter is a preferred structure because of its superior stiffness and
strength.
The collective effect of all multi-function strata 30 in the stack 10 is
equivalent to the effect of the single spring or spring combination 4~ that is
normally used at the end of a conventional stack 42 (Figure 13). The multi-
function strata 30 collectively have a i;ombined spring stiffness equal to the
ideal stiffness K given by the above equation, which would be the stiffness of
the single spring 46 at the end of an equivalent conventional stack 42. Having
calculated or determined total required spring stiffness K for the stack 10 as
a
whole, the next step is to calculate the required stiffness k; for each
stratum 30
1 I 1
-_ __+....+-
K k~ kn
based upon the series spring formula:
where n is the number of multi-function strata in the stack, and k" is the
stiffness of the nth multi-function stratum.
Since the strata are all identical, k; =k~=k" . For n identical strata, the
required
stiffness k of any given mufti-function stratum can accordingly be calculated
by:
k=nK
This required stiffness could be accomplished by suitably selecting the
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WO 99/57781 PCT/GB99/01169
properties of the multi-function panels 30. Selectable parameters include the
thickness of the corrugated conductive sheet 34 or equivalent, the thickness
(or
height) of the stratum 30, the material of which layers 32, 34, 36 are made,
and
the shape and period of the waveforro of layer 34. Given a set of panel
parameters for mufti-function panel 30, the deformation of the panel 30 under
various loads can be calculated using a finite element analysis computer
model.
The calculated result may be used to guide the designer to fine-tune the
foregoing parameters to make the stratum stiffer or softer until the target
stiffness is achieved. Since design criteria and objectives will vary
considerably from one fuel cell stack design to another, an empirical approach
is recommended.
Note that the fuel stack assembly described herein embodies not only
an electrochemically effective design that provides cooling efficiency and
mechanical stability, but it also embodies a design that can be realized with
a
relatively small number of constituent elements, many of which perform more
than one function. The several functions of the mufti-function stratum has
already been discussed above - each such stratum contributes to the
distributed
spring design and maintains dimensional stability in the stack direction. The
mufti-function stratum also provides electrical contact between fuel cell
units
and provides heat-dissipating surfaces and cooling passages through the stack.
Other constituent elements of the stack equally contribute to economy of
manufacture. For example, the pillar'spacer/cover plate design described
permits the pillars to serve as strong rigid structural elements extending
longitudinally throughout the stack; these pillars also serve to define, with
the
inset spacer bars and marginal edges of the MEA layer, the spaces inset from
the manifold cover that constitute the supply and exhaust plena for fuel and
oxidant gases. The space bars function not only as fillers and to the
structural
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WO 99/57781 PCT/GB99/01 l69
integrity of the stack but also serve as sealing units, since they are
selected to
be of a resilient material that can expand or contract to fill completely the
space
between adjacent pillars and alongside adjacent multi-function strata.
Note that the described embodiment also provides for easy maintenance
in the event of a failure of an MEA layer or other unit of~ the stack. It not
infrequently happens that in a fuel cell stack, some electrochemical problem
arises - for example, an MEA layer fails to function properly. In such cases,
with conventional PEM-type fuel cells, it is often necessary to disassemble
the
entire fuel cell stack and remove and replace the defective unit in order to
cure
the problem. By contrast, with the design of the present invention, it is
possible simply to remove the manifold cover plates, block the fuel and
oxidant
inlet and outlet passages of the defective unit, and provide a conductive
bridge
to pass current from the sound unit on one side of the defective unit to the
sound unit Iying on the other side of the defective unit, thereby providing
electrical continuity throughout the fuel stack assembly. The result is that
if,
for example, a fuel stack assembly includes 120 fuel cells arranged in series,
and one defective unit is bypassed according to the foregoing procedure, there
will be a loss of 1 fuel cell, representing a power capacity decline of the
120-
unit stack of something less than 1 %, which is tolerable for many fuel cell
applications.
Note also that the fuel stack design described herein lends itself to
modular construction; modular stacks of, say, twenty fuel cells each could be
manufactured and placed end to end in a series configuration to bring the
total
number of cells in a composite stack to a desired target number (say, 120 fuel
cells). The terminal plates 14, 1G of fuel stack 10 illustrated would, of
course,
have to be suitably modified for convenient end-to-end physical and electrical
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WO 99/57781 PCT/GB99/01169
contact for use in such modular arrangement.
While temperature differentials between interior stack temperatures and
ambient temperature will normally promote a sufficient flow of air through the
stack 10 to dissipate the low-grade heat produced during stack operation,
nevertheless, for high power applications, it may be desirable to provide one
or more fans to blow cooling air through the stack 10, or devise a
turbocharger-
type arrangement that would force air through the stack 10 in a closed conduit
communicating with the cooling passages 35 provided by the corrugated spring
layers 34. Further, the structural characteristics of the open spring layers
34
may be varied to increase heat dissipation. For example, the layers 34 may be
provided with punched openings and/or a rough finish for improved heat
dissipation capability.
Hydrogen may be used as a fuel gas in the fuel cell stack 10 illustrated
in Figure 1 and described above. If the fuel cell strata are fuel cell units
12,
then conduits for supplying hydrogen to the anode electrode layers and oxidant
to the cathode electrode layers are provided by the fuel cell units 12.
Alternatively, if the fuel cell strata are MEA layers 12, then when the fuel
cell
stack 10 is assembled, the hydrogen and oxidant flow channels provided by the
rigid plates 32, 36 are thereby closed to form conduits for supplying hydrogen
to the anode electrode layers and oxidant to the cathode electrode layers,
respectively, of the MEA layers. If we focus on a single fuel cell stratum 12
and consider the rest of the fuel cell stack 10 to be part of an external
circuit
(the rest of which is not shown) that includes a load and is capable of
receiving
electrons from the anode electrode layer of that fuel cell stratum 12 and
supplying electrons to the cathode electrode layer of that fuel cell stratum
12,
then when hydrogen is supplied to the hydrogen conduit adjacent the anode
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WO 99/57781 PCT/GB99/01169
electrode layer of that fuel cell stratum 12 and oxygen is supplied to the
oxidant conduit adjacent the cathode electrode layer of that fuel cell stratum
12,
hydrogen moves from the hydrogen tlow field through the anode electrode
layer and is ionized at the electro-catalyst between the anode electrode layer
and the electrolytic membrane layer to yield electrons and hydrogen ions. The
hydrogen ions migrate through the electrolytic membrane layer to react with
oxygen that has moved from oxidant flow field through the cathode electrode
layer to the electro-catalyst layer between the cathode electrode layer and
the
electrolytic membrane layer and electrons supplied by the external circuit to
form water. To make up for the electrons provided to the cathode electrode
layer the external circuit receives electrons from the anode electrode layer.
A
useful current of electrons through the load is thereby provided.
Further equivalents, variants, modifications, and improvements of the
fuel cell stack assembly and multi-function stratum described herein will
readily occur to those skilled in the technology. The scope of the invention
is
as defined in the appended claims.
44
