Patent Application
20050089746
1. Field of the Invention
The present invention relates to sealing techniques for electrochemical fuel cells and more particularly to preventing degradation of electrochemical fuel cell seals and contamination of other fuel cell parts such as the ion-exchange membrane.
2. Description of the Related Art
Electrochemical fuel cells convert reactants, namely fuel and oxidant fluid streams, to generate electric power and reaction products. Electrochemical fuel cells employ an electrolyte disposed between two electrodes, namely a cathode and an anode. The electrodes each comprise an electrocatalyst disposed at the interface between the electrolyte and the electrodes to induce the desired electrochemical reactions. The location of the electrocatalyst generally defines the electrochemically active area.
Polymer electrolyte membrane (PEM) fuel cells generally employ a membrane electrode assembly (MEA) consisting of an ion-exchange membrane disposed between two electrode layers comprising porous, electrically conductive sheet material as fluid diffusion layers, such as carbon fiber paper or carbon cloth. In a typical MEA, the electrode layers provide structural support to the ion-exchange membrane, which is typically thin and flexible. The membrane is ion conductive (typically proton conductive), and also acts as a barrier for isolating the reactant streams from each other. Another function of the membrane is to act as an electrical insulator between the two electrode layers. The electrodes should be electrically insulated from each other to prevent short-circuiting. A typical commercial PEM is a sulfonated perfluorocarbon membrane sold by E.I. Du Pont de Nemours and Company under the trade designation NAFION®.
The MEA contains an electrocatalyst, typically comprising finely comminuted platinum particles disposed in a layer at each membrane/electrode layer interface, to induce the desired electrochemical reaction. The electrodes are electrically coupled to provide a path for conducting electrons between the electrodes through an external load.
In a fuel cell stack, the MEA is typically interposed between two separator plates that are substantially impermeable to the reactant fluid streams. The plates act as current collectors and provide support for the electrodes. To control the distribution of the reactant fluid streams to the electrochemically active area, the surfaces of the plates that face the MEA may have open-faced channels formed therein. Such channels define a flow field area that generally corresponds to the adjacent electrochemically active area. Such separator plates, which have reactant channels formed therein are commonly known as flow field plates. In a fuel cell stack a plurality of fuel cells are connected together, typically in series, to increase the overall output power of the assembly. In such an arrangement, one side of a given plate may serve as an anode plate for one cell and the other side of the plate may serve as the cathode plate for the adjacent cell. In this arrangement, the plates may be referred to as bipolar plates.
The fuel fluid stream that is supplied to the anode typically comprises hydrogen. For example, the fuel fluid stream may be a gas such as substantially pure hydrogen or a reformate stream containing hydrogen. Alternatively, a liquid fuel stream such as aqueous methanol may be used. The oxidant fluid stream, which is supplied to the cathode, typically comprises oxygen, such as substantially pure oxygen, or a dilute oxygen stream such as air. In a fuel cell stack, the reactant streams are typically supplied and exhausted by respective supply and exhaust manifolds. Manifold ports are provided to fluidly connect the manifolds to the flow field area and electrodes. Manifolds and corresponding ports may also be provided for circulating a coolant fluid through interior passages within the stack to absorb heat generated by the exothermic fuel cell reactions.
It is desirable to seal reactant fluid stream passages to prevent leaks or inter-mixing of the fuel and oxidant fluid streams. U.S. Pat. No. 6,057,054, incorporated herein by reference in its entirety, discloses a sealant material impregnating into the peripheral region of the MEA and extending laterally beyond the edges of the electrode layers and membrane (i.e., the sealant material envelopes the membrane edge).
For a PEM fuel cell to be used commercially in either stationary or transportation applications, a sufficient lifetime is necessary. For example, 5,000 hour operations may be routinely required. In practice, there are significant difficulties in consistently obtaining sufficient lifetimes as many of the degradation mechanisms and effects remains unknown. Accordingly, there remains a need in the art to understand degradation of fuel cell components and to develop design improvements to mitigate or eliminate such degradation. The present invention fulfills this need and provides further related advantages.
Membrane contamination represents a serious problem that can significantly reduce the lifetime of the PEM fuel cell. Specifically, it has been found that sealant impregnated into the edge of the electrode layers as in the 054' patent may degrade such that contaminants from the sealant then migrates to the membrane.
Sealant degradation and/or membrane contamination can be reduced or eliminated if the sealant in the electrode is separated from the electrochemical reactions taking place at the catalyst layer. This can be accomplished in many different ways as disclosed below. For example, in an embodiment, a membrane electrode assembly for an electrochemical fuel cell comprises:
More particularly, the barrier film may be, for example, located between the electrocatalyst layer and the fluid diffusion layer, between the ion-exchange membrane and the electrocatalyst layer, or even impregnated into the fluid diffusion layer.
In another embodiment, the barrier film may be placed by a barrier plug. Specifically, in this embodiment, a membrane electrode assembly for an electrochemical fuel cell comprises:
If the integral seal is a silicone based seal, contaminants may include mobile siloxanes that migrate into the membrane. The barrier film or barrier plug, may be, for example a thermoset or a thermoplastic.
Degradation of the sealant material may be greater on the cathode as compared to the anode as greater oxidative degradation would be expected at the cathode. While reduced membrane contamination would be expected when barrier films and barrier plugs are located at both the anode and cathode, benefits may be seen if such a barrier film is only located at one electrode, particularly if that electrode is the cathode.
Similarly, electrochemical degradation may be expected to be increased at certain locations within the fuel cell, for example, near the reactant inlets and/or outlets. Thus the barrier seal or barrier plug may either circumscribe the central, electrochemically active area or be located in only specific areas of increased sealant degradation.
A physical barrier may not be necessary to separate the sealant from electrochemical reactions. In another embodiment, a membrane electrode assembly for an electrochemical fuel cell comprises:
The main difference in this last embodiment is that at least a portion of the sealing region of at least one of the fluid diffusion layers is substantially free of active electrocatalyst particles. For example, the electrocatalyst layer may not extend into the sealing region such that the portion of the sealing region is substantially free of any electrocatalyst particles. In an alternate further embodiment, electrocatalyst particles in the sealing region are poisoned such that the portion of the sealing region becomes substantially free of active electrocatalyst particles even though inactive particles remain.
Similarly, the region substantially free of active electrocatalyst particles may be on only one of the cathode or anode, more particularly only the cathode, or on both electrodes. Further, this region may circumscribe the central, electrochemically active area or be located only at areas of expected increased degradation.
These and other aspects of the invention will be evident upon reference to the attached figures and following detailed description.
a-d are partial cross-sectional views of a membrane electrode assembly comprising further embodiments of the present invention.
In the above figures, similar references are used in different figures to refer to similar elements.
A cross-sectional representation of a perimeter edge of a sealed membrane electrode assembly (MEA) 10 as disclosed in U.S. Pat. No. 6,057,054, is illustrated in
As disclosed in the '054 patent, sealant material 40 may be a flow processable elastomer, such as, for example, a thermosetting liquid injection moldable compound (e.g., silicones, fluoroelastomers, fluorosilicones, ethylene propylene diene monomer (EPDM), and natural rubber). However, it has been discovered that sealant material 40 may not be chemically stable within the acidic, oxidative and reductive environment found in a fuel cell, particularly over the fuel cell lifetime.
Specifically, when silicones are used as sealant material 40, mobile siloxanes may migrate into membrane 20 where they may then be chemically oxidized to form silicon dioxide derivatives. This contamination may subsequently lead to internal fractures within membrane 20 and ultimate failure of the fuel cell. Without being bound by theory, the source of the mobile siloxanes may include leachable oligomers, volatile low molecular weight siloxanes and/or degradation products from the hydrolysis of silicone.
In particular, degradation appears to be localized within the region of MEA 10 where sealant material 40 is in close proximity to the active area of MEA 10. Thus sealant degradation can be reduced by physically separating sealant material 40 from the active area of MEA 10.
Even though access to reactants is limited in sealing region 45 in a typical MEA 10, the presence of electrocatalyst in such sealing region 45 may still lead to oxidative and reductive degradation of sealing material 40. In comparison, the embodiment as illustrated in
In an alternative embodiment not shown, the electrocatalyst is specifically poisoned in the sealing region 45 of MEA 10. Though catalyst layer 50 may extend to the edge of MEA 10, active catalyst is thus only present in the central area of MEA 10 and sealant material 40 is physically separated from active catalyst as in the embodiment illustrated in
In another embodiment illustrated in
For example, in
d shows an alternative embodiment wherein barrier films 60 impregnate sheet material 35. A typical fluid diffusion layer 30, comprises a porous, electrically conductive sheet material 35, such as carbon fiber paper or carbon cloth with a carbon sub-layer 70 applied thereto. While carbon sub-layer 70 is not explicitly shown in the above embodiments, it would likely be present in a typical fluid diffusion layer 30. However, in the embodiment illustrated in
It is also understood that in an MEA, particularly after bonding at elevated temperatures, individual layers may not remain as discrete layers as shown in
Barrier film 60 may comprise a material more stable to acid hydrolysis as compared to sealant material 40. For example, if sealant material is silicone, then barrier film 60 may be a thermoplastic or a thermoset that is processable up to 500° C. and forms a physical barrier between the sealant material 40 and membrane 20 (see for example Handbook of Plastics, Elastomers and Composites, 3rd edition, C. A. Harper ed., 1996, McGraw-Hill incorporated herein by reference in its entirety). Representative thermoplastics include polyvinylidene fluoride, polypropylene, polyethylene, polyolefins, PTFE and aromatic thermoplastics such as polyaryl ethers, PEEK, polysulfone etc. Representative thermosets include polyimide, epoxy, polyurethane, nitrile, butyl, TPEs, etc. Barrier film 60 may also comprise additives such as carbon black which may improve adhesion with catalyst layer 50.
All of the embodiments illustrated above in
A further embodiment is illustrated in
Barrier plug 80 may impregnate the entire thickness of fluid diffusion layer 30 as illustrated though beneficial effects may still be seen if only a portion of the thickness is impregnated.
Specific regions of MEA 10 may be more susceptible to degradation of integral seal 40 than others. For example, degradation may be higher near the inlet/outlet reactant ports. As such, it may not be necessary to circumscribe the entire active area and significant improvements in sealant degradation may be observed if only portions of sealant material 40 are separated from the active area of MEA 10. Similarly, degradation may be higher on one electrode as compared to the other, for example on the cathode as compared to the anode. Thus less sealant degradation may be seen if only one side of MEA 10 has sealant material 40 physically separated from the active area of MEA 10. Alternatively, different methods could be used to physically separate the sealant material on the anode fluid diffusion layer from that used on the cathode fluid diffusion layer. For example, a barrier plug could be used with the anode fluid diffusion layer while a barrier film is used with the cathode fluid diffusion layer. Further, various embodiments as described above may be combined in one MEA. For example, a barrier plug as illustrated in
Seven membrane electrode assemblies (MEAs) were prepared incorporating the various embodiments of the present invention as described above. MEA design 1 is a conventional MEA. The MEAs were then run for a period of time and then the membrane was analyzed by SEM EDX line scans to determine the ratio of Si to S. The larger the ratio of Si in the membrane indicates greater contamination which would likely lead to earlier failure of the fuel cell.
Conventional MEA Design 1
MEA Design 1 was a conventional MEA. Carbon fiber paper was impregnated with PTFE (TGP-090 grade from Toray) and then screen printed with a 0.6 mg/cm2 carbon base. The cathodes employed a conventional loading of carbon supported platinum catalyst and the anodes had a conventional loading of carbon supported platinum-ruthenium catalyst. The membrane electrolyte employed was Nafion® 1112. The MEA was then bonded at 160° C., 325 psi for 3 min followed by cooling at ambient conditions. The MEA was then cut to the desired size and a flow processable silicone elastomer (supplied by Wacker Chemie GmbH) was then injection molded into the edge of the MEA.
The same general procedure was then followed in preparing the remaining MEAs except as specifically noted below.
MEA Design 2
In MEA Design 2, the catalyst layers were selectively printed as shown in
MEA Design 3
In MEA Design 3, a barrier layer was introduced to the MEA as shown in
MEA Designs 4a and 4b
In MEA Designs 4a and 4b, a barrier layer was introduced to the MEA as shown in
Differences between anode and cathode fluid diffusion layers are due to the conventional fluid diffusion layer used and not to a different application of the Technoflon® barrier layer. Specifically, it is believed that increased impregnation of the anode fluid diffusion layer with PTFE as compared to the cathode fluid diffusion layer results in the decreased air permeability. Differences in air permeability between the conventional anode FDL and cathode FDL are believed to be hidden by the large air permeability measured, specifically 2400 cc/min.
MEA Design 5
In MEA Design 5, a barrier layer was introduced to the MEA as shown in
MEA Design 6
In MEA Design 6, a barrier plug was introduced to the MEA as shown in
Analysis
Each MEA was then subjected to dynamic cycling testing. After a period of time, the respective membrane was then tested to determine the silicon to sulfur ratio present in the membrane. The results of this measurement are shown in Table 2. A larger ratio of silicon to sulfur indicates greater contamination of the membrane. The exact chemical species of silicon present in the membrane was not tested but is not believed to be important. Silicon in any form represents a contaminant within the membrane which could lead to premature failure of the fuel cell.
Limit means that the amount of silicon present in the membrane was below the detection limit of the measurement.
As can be seen from table 1, all of MEA Designs 2 to 6 reduced the amount of silicon contamination in the membrane as compared to the conventional MEA Design 1. With MEA Designs 2, 3, 5 and 6, the amount of silicon contamination was reduced to such a great extent that the detection limit of silicon in the membrane was reached. Even for design 4a with a 5 μm Technoflon® barrier film, the amount of silicon contamination was reduced. Increasing the thickness of the barrier film to 10 μm and consequently reducing the air permeability (as shown in table 1 above) resulted in yet further improvements in reduced silicon contamination. Increasing the thickness further would be expected to result in additional reductions in silicon contamination, perhaps approaching the detection limit as seen with the other MEA Designs discussed above. Thus the barrier film does not need to be gas impermeable and even a gas permeability of 1300 cc/min of a coated fluid diffusion layer is acceptable to provide reduced silicon contamination.
From the foregoing, it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention. Accordingly, the invention is not limited except as by the appended claims.
