Self-humidifying fuel cell

A self-humidifying polymer electrolyte membrane (PEM) fuel cell assembly has an ion-exchange membrane interposed between hydrogen and oxygen diffusion layers to form a membrane electrode assembly (MEA). The MEA is in turn interposed between a pair of current collector plates having flow field channels for flowing the reactants adjacent the respective diffusion layers to produce corresponding anodic and cathodic electrochemical reactions. Various embodiments of the assembly incorporate one or more of the following features: interdigitated flow field channels, countercurrent reactant flows, opposing channel alignment, and uncatalyzed membrane hydration enhancement zones.

FIELD OF THE INVENTION
 This invention relates generally to electrochemical fuel cells, and more
 particularly to a self-humidifying polymer electrolyte membrane (PEM) fuel
 cell which, in operation, employs at least one dry reactant gas.
 BACKGROUND OF THE INVENTION
 Electrochemical fuel cells convert fuel and oxidant to electricity and
 reaction product. Solid polymer electrochemical fuel cells generally
 employ a membrane electrode assembly (MEA) interposed between gas
 diffusion backings. The MEA and gas diffusion backing arrangement is, in
 turn, interposed between anode and cathode flow field/current collector
 plates. The MEA includes a solid polymer electrolyte membrane (PEM)--also
 referred to as an ion exchange membrane or proton exchange
 membrane--having electrodes on either side thereof. The electrodes may be
 attached directly to the PEM membrane. Alternatively, the electrodes may
 be attached to or integrated into the gas diffusion backings, and the
 backings pressed against the PEM membrane. Gas diffusion backings are
 typically fabricated of porous, electrically conductive sheet materials,
 such as carbon/graphite fiber paper or carbon/graphite cloth. These gas
 diffusion layers often incorporate micro diffusion layers. A catalyst
 layer, typically in the form of platinum or platinum supported on carbon
 particles, is located at each membrane/electrode interface to induce the
 desired electrochemical reactions. The electrodes are typically coupled to
 one another to provide an electrical path for conducting electrons between
 the electrodes to an external load. Hydrophobic enhancement materials,
 such as Teflon.RTM., may be incorporated into the fuel cell to aid with
 product removal.
 At the anode, a fuel-reducing reagent reacts at the catalyst layer to form
 cations. These are thermodynamically driven through the membrane toward
 the cathode. At the cathode, oxidant reagent reacts at the catalyst layer
 to form anions. The anions formed at the cathode react with the cations to
 form reaction product. In electrochemical fuel cells employing hydrogen
 (or hydrogen-containing gas) as the fuel, and oxygen (or oxygen-containing
 gas) as the oxidant, the reaction at the anode produces hydrogen cations,
 or protons, from the fuel supply. The proton exchange membrane facilitates
 the migration of hydrogen protons from the anode to the cathode. In
 addition to conducting hydrogen protons, the membrane acts as a gas
 separator, generally isolating the hydrogen-containing fuel stream from
 the oxygen-containing oxidant stream (although nominal gas cross-over does
 occur). At the cathode, oxygen reacts at the catalyst layer to form
 anions. The anions formed at the cathode react with the hydrogen ions that
 have crossed the membrane, forming water as the reaction product. The
 anode and cathode reactions in hydrogen/oxygen fuel cells are as follows:
 Anode reaction:
EQU H.sub.2.fwdarw.2H.sup.+ +2e.sup.-
 Cathode reaction:
EQU 1/2+L O.sub.2 +2H.sup.+ +2e.sup.-.fwdarw.H.sub.2 O
 Typically, the MEA and gas diffusion backings are interposed between a pair
 of electrically conductive fluid flow field plates, or collector plates,
 each having at least one flow passage formed therein. These flow field
 plates are typically fabricated from graphite or metal. The flow passages,
 or channels, direct the fuel and oxidant to the respective electrodes;
 namely, fuel to the anode and oxidant to the cathode. The plates act as
 current collectors, provide structural support for the electrodes, provide
 access channels for transporting the fuel and oxidant to the anode and
 cathode, respectively, and provide channels for the removal of product
 water formed during operation of the cell.
 Hydrogen transport through the PEM requires the presence of water molecules
 within the membrane. Consequently, maintaining adequate membrane hydration
 is critical. In addition to maintaining adequate ionic conductivity and
 proton transport, uniform membrane hydration prevents localized drying, or
 hot spots, resulting from higher localized resistance. Overall,
 dehydration may impede performance, increase resistive power losses and
 degrade the structure of the membrane. In conventional fuel cells,
 membrane hydration is achieved by humidifying the fuel and oxidant gases
 prior to their introduction into the fuel cell.
 One commonly-used method for pre-humidifying fuel cell gas streams is to
 employ membrane-based humidifiers. Where membrane-based humidifiers are
 employed, reactant humidification is achieved by flowing the respective
 gases on one side of a water vapor exchange membrane while flowing
 deionized water on the opposite side of the membrane. In such
 arrangements, water is transported across the membrane to humidify the
 fuel and oxidant gases. Another known technique for pre-humidifying the
 reactant gas streams comprises exposing the gases directly to water in an
 evaporation chamber to permit the gas to absorb evaporated water. Yet
 another known pre-humidification technique comprises directly injecting or
 aspirating water into the respective gas streams before introducing them
 into the fuel cell.
 Generally, pre-humidification is undesirable because it requires auxiliary
 fuel cell components, increasing the relative complexity of fuel cell
 systems. For instance, pre-humidification generally requires dedicated
 components for storing and transporting water. Additional components may
 also present system reliability issues. For example, where fuel cells are
 operated in sub-freezing conditions, water solidification can result in
 the weakening of mechanical components. Auxiliary water storage and
 transport components also reduce operating efficiency and add to the
 overall cost of the system.
 For the foregoing reasons, the need exists for a PEM fuel cell assembly
 capable of maintaining hydration of the fuel cell membrane without
 requiring additional components for humidifying reactant streams prior to
 their introduction into the fuel cell stack.
 SUMMARY OF THE INVENTION
 It is an object of this invention to provide a polymer electrolyte membrane
 (PEM) fuel cell capable of maintaining membrane hydration upon the
 introduction of at least one dry reactant stream into the cell.
 It is another object of this invention to provide a PEM fuel cell in which
 membrane hydration is maintained using water provided as a byproduct of
 electrochemical reactions within the fuel cell.
 It is a further object of this invention to provide a PEM fuel cell which
 does not require components dedicated to pre-humidification of reactant
 gas streams.
 These and other objects of the invention are achieved by the PEM fuel cell
 assembly of the present invention. A self-humidifying polymer electrolyte
 membrane (PEM) fuel cell has a membrane electrode assembly (MEA),
 comprising an ion-exchange membrane interposed between catalyzed anode and
 cathode electrodes. The MEA is interposed between a pair of gas diffusion
 backings, and the resulting structure is interposed between fuel and
 oxidant flow field plates. The fuel flow field plate has a fuel stream
 inlet, a fuel stream outlet, and means for flowing the fuel stream
 therebetween. The oxidant flow field plate has an oxidant stream inlet, an
 oxidant stream outlet, and means for flowing the oxidant stream
 therebetween.
 In the preferred embodiment of the invention, fuel and oxidant flows are
 strategically directed to enhance the transfer of water byproduct, through
 the ion exchange membrane, between opposing gas streams. Dry fuel and
 oxidant streams are introduced into respective fuel and oxidant inlets.
 The corresponding flow field plate channels are oriented such that dry
 portions of reactant stream adjacent to one surface of the membrane
 generally oppose hydrated portions of reactant stream on the opposite
 surface of the membrane. In this embodiment, membrane hydration is
 maximized by providing countercurrent reactant flows. In other words, the
 reactant gases are directed in generally opposing directions through the
 fuel cell. Preferably, opposing channel lengths having disparate
 humidification levels are aligned to provide an increased humidification
 gradient therebetween, resulting in enhanced mass transfer of water across
 the membrane in the direction of the less humidified reactant stream.
 Channel alignment also minimizes the water transfer distance between
 opposing channels. At one extreme, portions of channels carrying dry
 reactant oppose portions of channels on the opposite side of the membrane
 carrying saturated reactant.
 Although contiguous channel designs (e.g., serpentine patterns) are
 effective, it is preferred that noncontiguous, interdigitated channels are
 employed to provide forced convection of the reactants through the
 respective fuel and oxidant diffusion layers. Forced convection propels
 the reactants closer to the respective catalyst layers, thereby enhancing
 the electrochemical reactions at the opposing cathode and anode sides of
 the membrane. As a result, the rate of production of water byproduct at
 the cathode is increased. Furthermore, although countercurrent flows are
 preferred, other orientations have proven effective. For example,
 perpendicular interdigitated flow fields, resulting in reactant cross
 flow, are effective for enhancing membrane hydration in fuel cells
 employing at least one dry reactant stream.
 In an alternate embodiment of the invention, liquid-to-gas humidification
 is accomplished by directing dry fuel and oxidant inlet flow streams over
 liquid deionized water zones prior to their introduction into catalyzed
 regions of the cell. In particular, a stream of deionized water is
 directed over one side of an uncatalyzed portion of membrane, while dry
 reactant gas is directed on the opposite side of the membrane. The water
 is transported through the bare membrane and diffuses into the inlet gas
 stream as water vapor.
 In a further embodiment of the invention, membrane hydration enhancement
 zones are incorporated to provide additional membrane hydration/gas stream
 humidification. In particular, the zones comprise uncatalyzed areas of the
 PEM. These regions may consist of bare membrane interposed between gas
 diffusion layers or, alternatively, bare membrane directly interposed
 between opposing flow field plates. Uncatalyzed membrane is less
 hydrophobic than an catalyzed membrane. Consequently, liquid water is more
 apt to be absorbed into uncatalyzed areas of the membrane. Absorption of
 liquid water into uncatalyzed regions of the membrane is further aided by
 the elimination of exothermic electrochemical reactions which normally
 occur at catalyzed membrane/diffusion layer interfaces. By eliminating
 this reaction heat, the membrane within the zone is maintained at a
 reduced temperature and is less prone to drying.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
 Referring now to FIGS. 1 and 2, a polymer electrolyte membrane (PEM) fuel
 cell includes a membrane electrode assembly (MEA) 10, gas diffusion
 backings 14,16, and current collector/reactant flow field plates 20, 22.
 Membrane electrode assembly 10 consists of a polymer electrolyte membrane
 12 interposed between a pair of electrodes 13, 15. The electrodes include
 a catalyst, such as platinum or platinum supported on carbon, for inducing
 desired electrochemical reactions. The catalyst can be attached to the
 diffusion backings 14, 16, and the backings subsequently pressed against
 the membrane 12. However, it is preferred that the catalyst is directly
 adhered to the membrane. Membrane electrode assembly 10 is interposed
 between porous, electrically-conductive gas diffusion backings 14 and 16.
 Examples of gas diffusion backings include carbon/graphite fiber paper and
 carbon/graphite cloth sheets. Fuel and oxidant flow field plates 20 and 22
 are positioned on opposing anode and cathode sides of the assembly,
 respectively. In operation, multiple fuel cell assemblies are positioned
 side by side to form a fuel cell stack (not shown).
 Fuel flow field plate 20 has fuel inlet channels 34 extending from fuel
 inlet port 30, and fuel outlet channels 36 extending from fuel outlet port
 32. Similarly, oxidant flow field plate 22 has oxidant inlet channels 46
 extending from oxidant inlet port 42, and oxidant outlet channels 44
 extending from oxidant outlet port 40. The respective inlet and outlet
 channels formed in each plate are noncontiguous, or dead-ended, and
 interdigitated. By employing dead-ended, interdigitated inlet and outlet
 channels, the reactants are forced to flow through the respective porous
 diffusion layers 14, 16. For instance, referring to fuel flow field plate
 20 on the anode side of the fuel cell, hydrogen gas entering through fuel
 inlet port 30 and filling inlet channels 34 is forced through porous
 diffusion layer 14 before continuing to flow through fuel outlet channels
 36 toward fuel outlet 32. Conventional contiguous channel designs, such as
 those employing serpentine patterns, rely primarily on diffusion for fluid
 transport through the diffusion layer. In contrast, the interdigitated
 channels of the present invention employ forced convection of the reactant
 through the diffusion layer, urging the reactant closer to the catalyst
 layer. Forced convection results in more rapid reactant flow through the
 diffusion layer vis-a-vis diffusion. As a result, the electrochemical
 reaction rates at the respective catalyst sites are increased. The
 interdigitated flow channels on the cathode (oxidant) side operate in a
 similar manner. As a result of the enhanced electrochemical reactions, the
 production rate of water byproduct at the cathode is increased. A portion
 of the additional water produced at the cathode back-diffuses through the
 membrane toward the anode. Consequently, the availability of water at the
 anode is also increased when interdigitated channels are used.
 Referring briefly to FIG. 3, a front view of flow field plate 20 is
 depicted to illustrate a unique interdigitated flow field pattern in
 accordance with the preferred embodiment of the present invention.
 Conventional interdigitated designs typically employ a plurality of flow
 channels branching off from a main flow channel, e.g., see U.S. Pat. No.
 5,641,586 to Wilson. In contrast, the present invention employs
 non-overlapping flow channels 34, 36 each extending from a reactant stream
 port 30, 32. The interdigitated flow pattern of the present invention
 results in more uniform fluid flow distribution, reduced channel moisture
 retention, and decreased channel pressure drop. Consequently, more uniform
 electrical current density is achieved across the fuel cell.
 Referring now to FIG. 4, the fuel and oxidant flow field plate channels are
 shown superimposed for the purpose of discussion. In the preferred
 embodiment of the present invention, the fuel and oxidant flow fields are
 oriented to achieve countercurrent fuel and oxidant flow streams. In other
 words, the fuel and oxidant stream flow in generally opposed directions
 across the fuel cell assembly. The water content of both the fuel and
 oxidant streams increases in the flow direction of the respective reactant
 through the fuel cell. Therefore, by employing a countercurrent flow
 arrangement, the region of the anode layer with the highest water content
 substantially coincides with the region of the cathode layer having the
 lowest water content, and vice versa. As a result, countercurrent reactant
 flow streams result in more uniform hydration of the ion-exchange
 membrane.
 Fuel and oxidant inlet ports are denoted by reference numerals 30 and 42,
 respectively. Fuel and oxidant outlet ports are denoted by reference
 numerals 32 and 40, respectively. The horizontal lengths of the
 corresponding fuel and oxidant channels are shown slightly offset from one
 another in a vertical direction for illustrative purposes; although such
 an arrangement is possible, it is not preferred. Preferably, the
 horizontal lengths of fuel inlet channels 34 and oxidant outlet channels
 44, and the horizontal lengths of oxidant inlet channels 46 and fuel
 outlet channels 36, overlie one another when viewed perpendicular to the
 reactant flow planes. As used herein, the term "superposed" is used to
 describe this overlapping relationship of opposing collector plate
 channels. There are a number of advantages to providing superposed
 channels. For instance, channel alignment results in corresponding
 alignment of lands, or non-channel areas, of opposing flow field plate
 surfaces. Maximizing land-to-land alignment between flow field plates
 results in optimized electrical conductivity between opposing current
 collector plates upon compression of the individual cells making up the
 fuel cell stack. In other words, aligning the opposing collector plate
 lands minimizes increases in electrical resistance across the cell
 associated with land-to-channel alignment. Furthermore, channel alignment
 increases the mass transfer rate of water across the membrane, especially
 in regions where a well-hydrated gas stream is positioned opposite a dry,
 or non-hydrated, gas stream. Where this is the case, the water transfer
 rate across the membrane is increased due to the gradient created by the
 hydration level differential on opposite sides of the membrane. Channel
 alignment is especially beneficial for hydrating peripheral regions of the
 membrane, where dry gases are typically introduced into the cell.
 Employing the preferred countercurrent reactant flow arrangement, the
 ingress of dry reactant gas on one side of the membrane is aligned with
 the egress of well-hydrated reactant gas on the opposite side of the
 membrane. In conventional fuel cell assemblies, inlet gas streams are
 pre-humidified to prevent dehydration of the membrane at gas inlet areas.
 In the present invention, humidified reactant streams may be provided
 initially when membranes are completely dry, to initiate the required
 electrochemical reactions at the respective electrodes. However, upon
 achieving an adequate fuel cell operating level, adequate membrane
 hydration can be maintained running only dry reactant flows.
 Referring now to FIG. 5, an alternate embodiment of the present invention
 incorporates at least one membrane hydration enhancement zone.
 Specifically, portions 82, 84 of the ion-exchange membrane are left
 uncatalyzed to enhance self-humidification of the fuel cell assembly,
 especially in regions of the membrane prone to drying. Uncatalyzed
 membrane areas are less hydrophobic than catalyzed membrane areas.
 Consequently, uncatalyzed areas are more apt to attract and retain liquid
 water. Accordingly, as a reactant stream containing liquid water flows
 through an uncatalyzed region of the membrane, the water is more likely to
 pool atop, and subsequently absorb into, the membrane. This results in
 both local membrane hydration and increased humidification of the reactant
 stream flowing on the opposite side of the membrane. Lateral absorption of
 water through the membrane results in increased hydration of catalyzed
 regions of the membrane as well.
 Membrane hydration is further enhanced by the lack of electrochemical
 reactions in these hydration enhancement zones. As previously described,
 the fuel cell electrochemical reactions at the respective electrodes are
 initiated by catalyst layers at the membrane/diffusion layer interfaces.
 The heat produced by these electrochemical reactions tends to dry the
 membrane locally. However, absent a catalyst, the gas reactants flow
 without producing these exothermic electrochemical reactions. The reduced
 membrane temperatures decrease local membrane dehydration.
 Preferably, uncatalyzed areas 82 and 84 are strategically located proximate
 to reactant flow inlet and outlet areas on opposite sides of catalyzed
 area 80. Since reactant humidification typically increases in the reactant
 flow direction, the lowest reactant humidification levels typically occur
 near the reactant channel inlets, and the highest reactant humidification
 levels typically occur near the reactant channel outlets. At least a
 portion of the humidified outlet channel lengths are routed through the
 humidification zones 82, 84, hydrating the membrane locally through
 absorption of reactant stream water. Inlet channels carrying dry reactant
 gas are routed through the humidification zones 82, 84 on the opposite
 side of the membrane. The humidification gradient across the membrane,
 caused by the opposing dry and humidified reactant streams, results in the
 transfer of water absorbed by the membrane to the respective dry inlet gas
 streams.
 Humidification zones may be created by utilizing an ion-exchange membrane
 with integrally attached electrodes that maintain uncatalyzed regions.
 Alternatively, humidification zones may be created by providing a
 diffusion layer having both catalyzed and uncatalyzed areas. In some
 instances, it may be desirable to further treat the diffusion layer such
 that the catalyzed areas are hydrophobic, and the uncatalyzed areas,
 corresponding to humidification zones, are hydrophilic. This is easily
 accomplished by selectively coating the diffusion layer with a hydrophobic
 material such as Teflon.RTM.. Alternatively, humidification zones can
 comprise an ion-exchange membrane directly interposed between opposing
 flow field plates, where one or both sides of the membrane are not
 supported by a diffusion layer. In this embodiment, it is preferable to
 use an integrally reinforced PEM membrane, such as GORE PRIMEA.RTM.,
 manufactured by W.L. Gore & Associates, Inc. of Elkton, Md. Elimination of
 the diffusion layer reduces gas flow and permits localized "pooling" of
 water. The elimination of the diffusion layer also permits gas-to-water
 sealing along the periphery of the zone. These water-to-membrane contact
 zones may directly humidify a gas flowing on the opposite side of the
 membrane, or extend sealed deionized water zones via lateral water
 absorption/hydration through the membrane. Hydration enhancement zones 86
 may be distributed throughout the fuel cell assembly for further membrane
 hydration. Although specific humidification zone quantities and geometries
 are illustrated, the invention is not intended to be so limited. For
 instance, the membrane can incorporate hundreds, or even thousands, of
 uncatalyzed areas (e.g., microdots), having dimensions on the order of
 microns, distributed throughout the assembly to enhance membrane
 hydration.
 Referring now to FIG. 6, in a further embodiment of the invention,
 liquid-to-gas membrane hydration/reactant humidification is enhanced by
 directing dry fuel and oxidant inlet flow streams over liquid deionized
 water zones prior to their introduction into catalyzed regions of the
 cell. In particular, a stream of deionized water introduced into the fuel
 cell assembly is directed over one side of an uncatalyzed portion of
 membrane, while dry reactant gas is directed over the opposite side of the
 membrane. Water absorbed by, and transported through, the bare membrane
 diffuses into the dry inlet gas stream as water vapor.
 Referring now to FIG. 7, yet a further embodiment of the invention
 incorporates a double-interdigitated flow field channel design. The
 cathode and anode flow field channels are shown superimposed and slightly
 offset. The vertical offset is for illustrative purposes only and is not
 preferred. The fuel cell assembly has at least one uncatalyzed
 humidification zone and at least one catalyzed reaction zone. In
 operation, two reactants are communicated through the assembly. However,
 for simplicity, the following description only refers to one of the
 reactants. A first reactant is introduced through inlet 50 into inlet
 channels 52. Inlet channels 52 extend across the humidification zone, and
 function solely to humidify the first reactant. Preferably, the first
 reactant is humidified to the point of saturation. The saturated first
 reactant is subsequently transferred, by forced convection, from inlet
 channels 52, through intermediate channels 54, to outlet channels 56. As a
 result of the double convection, both reactant and water vapor are more
 evenly distributed during transport across the polymer electrolyte
 membrane. This embodiment may further incorporate features of previously
 described embodiments, e.g., countercurrent reactant flow, superposed
 channels, etc.
 While the preferred embodiments of the invention have been illustrated and
 described, it will be clear that the invention is not so limited. Numerous
 modifications, changes, variations, substitutions and equivalents will
 occur to those skilled in the art without departing from the spirit and
 scope of the present invention as described in the claims.