GB2178223A - Polymeric molded frames for an alkali electrochemical cell - Google Patents

Abstract

An alkali electrochemical cell having cell components contained within a cell housing comprising a plurality of polymeric molded stacked frame members 101, 131, 161 that provides more dependable cell performance. The frame members comprise a polyphenylene sulfide resin; polyether sulfone resin; or mixture thereof; and a filler. The frame members are preferably injected molded at a melt temperature of about 600 DEG F (316 DEG C), and a mold temperature of about 290 DEG F (143 DEG C) using pressure of about 10,000 psi (703 kg/cm<2>) and a mold cycle time of about 35 seconds. Following molding the resin is cured at about 425 DEG F (218 DEG C) for about 3 hours. Design features may be molded into the frame during processing by utilizing a sophisticated mold that incorporates the desired features or these features may be machined into a plain frame that has been molded. The molded frames are simpler in design and manufacture and weigh less than multilayer laminates. In addition, the materials utilized are stable in alkali environments and contribute less contaminates to a cell resulting in greater cell durability. <IMAGE>

Description

SPECIFICATION Polymeric molded frames for an alkali electrochemical cell Technical Field The field of art to which this invention pertains is alkali electrochemical cells, particulary container frames for alkali fuel cells.

Background Art Alkali fuel cells are used in a variety of applications including undersea and aerospace applications such as the orbital space shuttle. In space related applications, a malfunction-free long term life is of paramount importance.

Present alkali fuel cell structures are arranged in stacks of about 30 to about 150 cells depending on the required output voltage of the specific application. Conventional alkali fuel cells comprise multilayer component arrangements having an oxygen flow field, cathode, matrix, anode, electrolyte reservoir and hydrogen flow field. These fuel cell structures typically require a container device (cell housing) that holds together the cells and cell components in a unified rigid structure. In addition, the cell housing must allow for the exchange of reactants, products and coolants. The housing material should be compatible with the alkali fuel cell environment in order to contribute to the malfunction-free operation and long term life required for alkali fuel cell space applications. The housing should also be lightweight to facilitate a favourable weight to power ratio.

One solution for this problem currently used is a frame comprised of a multiply glass fiber cloth laminate. The glass fiber laminates can be laid up to the desired thickness, machine to the required configuration, and incorporated into the fuel cell stack. These laminates function well in containing the cell components and allowing for exchange of reactants, products and coolants.

However, given the importance of a malfunction-free long term life alkali fuel cell for space applications, there is always room for improvement in fuel cell structure components.

Accordingly, there is a constant search in this art for more dependable, long life alkali fuel cell structure components for aerospace applications.

Disclosure of Invention This invention is directed to a more dependable, long life alkali fuel cell. An electrochemical cell in which a cathode, matrix and anode cell components are contained within the interior of a cell housing. The cell housing comprises a plurality of molded stacked frame members. The frame members comprise a polyphenylene sulfide resin; polyether sulfone resin; or mixture thereof; and a filler.

This molded polymeric frame member provides the alkali fuel cell industry with a component that contributes to a more durable, long term life cell. Thus, it makes a significant advance in the alkali fuel cell field by providing durable components for aerospace applications.

Other features and advantages will be apparent from the specification and claims and from the accompany drawings which illustrate an embodiment of the invention.

Brief Description of the Drawings Figure 1 represents a top view of an exemplary set of molded frame members.

Figure 1A is a perspective view of one of the apertures (manifolds).

Figure 2 represents a cross-sectional view of a portion of a fuel cell which incorporates the exemplary set of molded frame members illustrated in Fig. 1.

Best Mode for Carrying Out the Invention It is preferred to use a polyphenylene sulfide polymer, polyether sulfone, or mixture thereof in the practice of this invention. These resins can be molded to a frame configuration (e.g. picture frame) of for example about 15.3 to about 38.1 cm on a side and about 0.64 cm thick. In addition the resin is capable of withstanding alkali fuel cell conditions, and does not contribute contaminates to the cathode, matrix, anode and electrolyte. It is especially preferred to use polyphenylene sulfide in the practice of this invention.

Various other polymers do not provide the properties required of the frame material such as are described above (e.g. they cannot be molded). For example, when polysulfone resins are molded to the sizes required of typical alkali fuel cells, i.e. frame length to width ratios of about 4 or larger, the resin cracks and crazes. When TeflonTH polymer (E. I. DuPont DeNemours, Wilmington, Delaware) is molded into the frame configuration, it tends to warp like a potato chip. Finally, the epoxy novalac family of polymers corrode under alkali fuel cell environmental conditions and contribute contaminates to the cathode, matrix, anode and electrolyte. For example, carbon dioxide may form and when this reacts with the cell electrolyte it forms potassium carbonate.Potassium carbonate is generally a poor electrolyte and specifically in creases the internal resistance of the cell thereby lowering its efficiency.

An exemplary polyphenylene sulfide can be empirically illustrated as

polyphenylene sulfide As reported in Volume 18 of the Encyclopedia of Chemical Technology by John Wiley and Sons which is hereby incorporated by reference, the molecular weight of polyphenylene sulfide before molding is typically about 18,000. Typical properties of a suitable resin (molded at 2750F (135 C) as described by the above reference) are detailed below.

Typical Properties of Polyphenylene Sulfide Glass-Filled (40 percent Unfilled by weight (%) Density, grams per cubic centimeter (g/cm3) 1.35 1.6 Tensile Strength, Mega Pascals (MPa) 65.5 121 Elongation, percent 1.6 1.25 Flexural Modulus, MPa 3800 12000 Flexural Strength, MPa 96 160 Compressive Strength, MPa 1100 145 Specific Heat, Joules per gram degree Kelvin (J/(g-K)) 1.09 1.05 Thermal Conductivity, watts per meter degree Kelvin (w/(m.K)) 0.288 0.288 Coefficient of Linear Thermal Expansion, X 1 0-5/degrees centigrade ( C) 4.9 4.0 Polyphenylene sulfide can be prepared by polymerizing p-dichlorobenzene and sodium sulfide at elevated temperatures in a polar solvent. Sodium sulfide can be prepared by the distillation of water from the reaction product of aqueous caustic and aqueous sodium hydrosulfide.During the purification of polyphenylene sulfide, sodium chloride, a reaction by-product, can be removed by washing. This process results in a linear polymer of modest molecular weight (ca. 18,000) and mechanical strength which is the feed stock for the production of various molding grade resins.

The molding grade resins of this disclosure may be produced by a curing process in which the virgin polymer contacts a small amount of air at elevated temperature in such a manner that a higher molecular weight resin is produced and low molecular weight polymers are removed by volatilization. The extent of curing is controlled by residence time and reaction temperature which can be followed by the measurement of melt viscosity. Polyphenylene sulfide is available commercially as RytonTH R-4, R-7, R-8 and R-10 polymers from Phillips Chemical Company, Bartlesville, Oklahoma.

An exemplary polyether sulfone can be empirically illustrated as

Typical properties of a suitable polyether sulfone as presented in the literature is shown below.

Typical Properties of Polyether Sulfone Glass-Filled Unfilled (40%) Density, g/cm3 1.37 1.6 Tensile Strength, MPa 84 140 Elongation, percent 60 3 Flexural Modulus, MPa 2900 8402 Flexural Strength, MPa 129 190 Coefficient of Linear 5.5 2.3 Thermal Expansion, X10-5/"C Polyether sulfone can be prepared from the reaction of bis-sulfonyl chloride with diaryl ethers.

As reported in the above reference, the condensation of diphenyl ether with the disulfonyl chloride of diphenyl ether yields polyether sulfone. The reaction can be carried out in the melt or in an inert solvent, such as acetonitrile. Ferric chloride is the preferred catalyst while certain other halides like antimony pentachloride and indium trichloride are acceptable. Examples of suitable polyether sulfones are PES-200P, PES-300P, PES-420P and PES-430P polymers available from ICI Americas, (Wilmington, Deleware).

Generally any organic or inorganic filler may be used in the practice of this invention that does not detract from the above-described properties of the molded frame. However, it is preferred to employ fibers like glass silica, Teflon polymer, potassium titanate, zirconium oxide or ceria. It is preferred to employ glass silica because this filler is widely used in the polymer compounding and molding industry and has been shown to produce dimensionally stable molded frames.

Although the glass fibers may contribute contaminates when utilized in laminate structures, the molded frames of this disclosure incorporate fibers that do not contribute contaminates as they are encapsulated by the resin. In addition the frames of this disclosure utilize about 30% to about 80% less glass fiber filler than laminate structures. This results in less potential contamination of the cell. Glass silica is available from Uniglass Industries, (Statesville, North Carolina).

Alternatively, it is preferred to utilize Teflon polymer, potassium titanate or zirconium oxide, as fillers because of their high temperature corrosion resistance in an alkali fuel cell environment.

Teflon polymer is available from E. I. DuPont DeNemours, (Wilmington, Delaware). Potassium titanate is available from Otsuka Chemical Company, (Osaka, Japan) and zirconium oxide fibers are available from the Transelco Division of Ferro Corporation, (New York, New York).

Typically, about 10% to about 40% filler is used in the final molding mixture. It is preferred that about 25% to about 35% filler is used and especially preferred that about 30% filler is used in the final molding mixture. Generally any proportion of polyphenylene sulfide resin to polyether sulfone resin can be used.

Any method of molding the frame may be utilized that results in a polymeric molded frame having the above-described properties. These conventional molding processes include injection, compression, extrusion and blow molding techniques. It is preferred to utilize an injection molding process at melt temperatures of about 5750F (302 C) to about 6750F (357 C). In addition, a mold temperature of about 2750F (135 C) to about 3000F (149 C) is preferred and it is especially preferred that the mold temperature is about 2900F (143 C). Molding pressures of about 7,000 pounds per square inch (psi) (492 kg/cm2) to about 12,000 psi (844 kg/cm2) are preferred when using a mold cycle time of about 25 to about 45 seconds. After molding the resin is cured and annealed at about 4000F (204 C) to about 4500F (232 C) with a preferred temperature of about 4250F (218 C) for about 2 hours to about 4 hours with 3 hours being preferred.The particular features of the preferred frame described below may be molded into the frame during processing by utilizing a sophisticated mold that incorporates the desired features or these features, may be machined into a basic frame that has been molded.

These polymers are molded into frame members that are capable of housing (containing) the various electrochemical cell components (e.g. anode, cathode, matrix) in a unified rigid structure.

The number and type of cell components would vary with application. The range of frame designs and thus cell (also stack designs) are varied and depend on the particular application.

For example, in one of the simplest designs a conventional nickel-cadmium battery cell would utilize a frame having a picture frame shape. The frame would essentially be a flat plate that would have an opening therein. The opening could be square, round, retangular etc. The cell components would be stacked within and between at least two (a plurality) of the frames and a gasket material placed between the frames. The cell(s) are then placed in a containment vessel of a size sufficient to hold the cell(s) and reactant. In addition, the vessel should be able to withstand the pressures, conventional of electrochemical cells. When used for fuel cell applica tions, these container frames (holders) must additionally be capable of allowing for the exchange of reactants, products and coolants.However, once these basic parameters are fulfilled, the range of frame member and thus cell and stack designs are numerous. The following paragraphs describe an exemplary design which uses three different frames and the Example illustrates an alternative stack design which uses two different frames.

A clear understanding of this frame may be had by reference to Fig. 1 which details an oxygen frame, a hydrogen/oxygen (H2/O2) frame and a hydrogen frame. The preferred frames are flat squares which resemble picture frames that have two annular rings resembling handles on each of 3 sides, said annular rings (manifolds) coexistent within the plane of the frame. The inside dimensions of these frames are typically about 8.5 inches (21.6 cm) by about 8.5 inches (21.6 cm) by about 0.25 inches (0.64 cm). This frame design facilitates the housing of a cell configuration that provides a lightweight stack that is simple in design and yet still provides a highly efficient alkaline fuel cell. Specifically, the oxygen frame 101 has a recess (e.g. step, indentation) 102- which bounds the inside periphery (edge) of the frame 101.This recess is typically about 127 #m to about 508 #m deep and about 0.2 cm to about 1.2 cm wide. The oxygen frame 101 contains hydrogen inlet 103 and hydrogen exit 104 manifolds (apertures), oxygen inlet 105 and oxygen exit 106 manifolds, and coolant inlet 107 and coolant exit 188 manifolds. When the frames are stacked into a housing the apertures should be aligned to form longitudinal fluid conduits which allow for the exchange of reactants, products and coolants. The manifolds are typically about 8 cm by about 4 cm and have an inside dimension of about 5 cm by about 2.5 cm. Fig. 1A illustrates a typical manifold. Although this cell design uses frames having six apertures, typically for the fuel applications at least four apertures are necessary.

These four apertures would allow for the inflow and outflow of the two reactants. Such a system would utilize a separate cooling system.

Typically at least two of said apertures are in fluid communication with the interior of the housing. This allows for the exchange of fluids from the apertures (manifolds) to the cell components, e.g. anode, cathode. The fluid communication is through fluid passages (metering ports) which are generally oriented perpendicular to the longitudinal fluid conduits. These fluid passages are preferably channels that extend across the frame and are typically about 1.27 mm to about 5.08 mm in depth and about 1.27 mm to about 5.08 mm wide and are located on the same side of the frame as the step 102. Although other preferable frames could have the passages located on the bottom because of numerous variations in alkali fuel cell configuration and cooling approaches.Through patterns impressed into the nickel field flow barrier 201, the oxygen is directed through the oxygen flow field 203 of the barrier 201 and the coolant is directed through the coolant flow field 205 of the barrier 201. After passage through the oxygen and coolant flow fields, the oxygen and coolant are directed toward their respective exit ports.

Specifically, the preferred embodiment of this frame contains metering ports 109 which connect the oxygen inlet manifold 105 to an oxygen flow field 203 and metering ports 110 which connect the oxygen outlet manifold 106 to an oxygen flow field 203. This frame also contains metering ports 111 which connect the coolant inlet manifold 107 to a coolant flow field 205 and metering ports 112 which connect the coolant exit manifold 108 to a coolant flow field 205.

The H2/02 frame 131 has a similar recess 132 to that described above which bounds the inside edge of the frame 131. The K2/O2 frame 131 contains hydrogen inlet 133 and hydrogen exit 134 manifolds, oxygen inlet 135 and oxygen exit 136 manifolds, and coolant inlet 137 and coolant exit 138 manifolds. These manifolds are similar to those described above. This frame also contains metering ports 139 which connect the oxygen inlet manifold 135 to an oxygenflow field 221 and metering ports 140 which connect the oxygen exit manifold 136 to the oxygen flow field 221. The frame also contains metering ports 141 which connect the hydrogen inlet manifold 133 to a hydrogen flow field 219 and metering ports 142 which connect the hydrogen exit manifold 134 to a hydrogen flow field 219. These metering ports are similar to those described above.As above through patterns impressed into the nickel flow field barrier 217 the two fluids, this time hydrogen and oxygen are directed across the surfaces of the anode and cathode respectively, toward their respective exit ports.

The hydrogen frame 161 also has a recess (e.g. step, indentation which is similar to 102) 162 which bounds the inside periphery of the frame 161. The hydrogen frame 161 contains hydrogen inlet 163 and hydrogen exit 164 manifolds, oxygen inlet 165 and oxygen exit 166 manifolds, and coolant inlet 167 and coolant exit 168 manifolds. These manifolds are similar to those described above. This frame also contains fluid communication metering ports (passages, channels) 169 which connect the hydrogen inlet manifold 163 to a hydrogen flow field 235 and a metering ports 170 which connect the hydrogen exit manifold 164 to a hydrogen flow field 235. Through a pattern impressed into nickel flow field barrier 233 the hydrogen is directed across the surfaces of the anode toward the hydrogen exit ports. This edge frame also contains metering ports 171 which connect the coolant inlet manifold 167 to a coolant flow field 237 and metering ports 172 which connect the coolant exit manifold 168 to a coolant flow field 237. Again through a pattern impressed into the nickel flow field barrier 233 the coolant is directed through the coolant flow field 237 of the barrier 233. After passage through the coolant flow fields 237, the hydrogen is directed toward its exit port. These passages are similar to those described above.

The molded frame of this disclosure can be used to contain stacks of conventional alkali fuel cells such as are illustrated in Fig. 2. Fig. 2 details a side view of a alternate alkali fuel cell configuration. In this scheme successive fuel cell components are placed in layers to form one fuel cell and then successive fuel cells are layered to form a stack. In Fig. 2, an oxygen frame 101 contains a nickel flow field barrier 201 which provides oxygen flow fields 203 and coolant flow fields 205. The nickel flow field barrier 201 is disposed below a conventional cathode 207, and a conventional fuel cell matrix 209 is disposed above the cathode 207. In the step 102 of the oxygen frame 101, a layer of butyl rubber surrounds the cathode and matrix edges and provides a positive reactant seal that prevents cross-mixing of hydrogen and oxygen.A polymeric gasket 211 is disposed adjacent to the matrix 211 and between the oxygen frame 101 and the H2/O2 frame 131. The polymeric gasket above the oxygen, hydrogen and coolant passages can be cut back to prevent intrusion of the gasket into the passage (metering point).

An alternate approach to prevent gasket intrusion would be to place a thin nickel foil above the passage between the passage and the gasket.

In place of the polymeric gasket, the edge frames, the H2/02 edge frame can be bonded, glued, or sealed together by a seal formulated from a fiberglass cloth or polyphenylene sulfide resin substrate impregnated with butyl rubber. One suitable material is butyl latex BL-100 rubber available from Burke-Palmason Chemical Company, (Pompano Beach, Florida).

A conventional substrate 213 backed alkali fuel cell anode 215 is placed adjacent to the matrix. Preferably a porous carbon substrate having a layer of catalyst bonded thereon is used.

A second nickel flow field barrier 217 provides hydrogen flow field channels 219 and oxygen flow field channels 221. A conventional alkali fuel cell cathode 223 is disposed above the nickel flow field 217 and a conventional fuel cell matrix 225 is disposed above the cathode 223. As with the oxygen frame, in the step 132 of the H2/O2 frame 131, a layer of butyl rubber surrounds the cathode and matrix edges and provides a positive reactant seal that prevents cross-mixing of hydrogen and oxygen. Again a polymeric gasket 227 is disposed adjacent to the matrix 225 and between the H2/O2 frame 131 and the hydrogen frame 161. A conventional substrate 229 backed alkali fuel cell anode 231 is disposed above the matrix 225. As before, a porous carbon substrate having a layer of catalyst bonded thereon is preferred.A third nickel flow field 233 provides hydrogen flow field channels 235 and coolant flow field channels 237.

An alternative fuel cell configuration which incorporates different frame designs is described in the Example below.

Example Polyphenylene sulfide frames with 40% glass fiber filler, about 1.78 mm thick were molded by Plastic Tooling Aids Laboratory, (Oxford, Connecticut) from Ryton polyphenylene R-4 resin obtained from Phillips Chemical Company. The molding procedure followed the recommendations provided by the resin supplier for injection molding. The compounded resin was air dried at about 3000F (149 C) for about 6 hours before molding. Resin pellets were melted at about 6000F (316 C) and molded at about 2900F (143 C) with a molding pressure of 10,000 psi (703 kg/cm2) and a mold cycle time of approximately 25 seconds to approximately 35 seconds.

Following molding of the frame, the part was annealed at a temperature of about 4250F (218 C) for approximately 3 hours.

The frames were contoured machined to the desired alkali fuel cell configuration. The molded frame was machined to form hydrogen, oxygen and coolant inlet and exit manifolds. A hydrogen frame was fabricated by machining three metering ports about 0. 1 cm wide by about 0.1 cm deep at the hydrogen inlet and exit manifolds. An oxygen frame was fabricated by machining three metering ports of about 0.1 cm wide by about 0.1 cm deep at the oxygen inlet and exit manifolds. Into the hydrogen and oxygen frame was machined a step about 0.25 mm deep and about 0.6 cm wide around the interior perimeter of each frame on the side opposite to the metering ports.

A cell was assembled by containing a cathode, matrix and anode in the center of the oxygen and hydrogen frame; the step of each frame facing inward. A thin layer of butyl rubber was applied to the frame step on each frame prior to assembling the cell. The oxygen and hydrogen frames together with the cell assembly in between, were bonded into a unit. The bond or seal gasket consisted of a fiberglass cloth substrate of about 0.25 mm thick which had been impregnated with butyl rubber. The cell assembly comprised a porous nickel plate (electrolyte reservoir plate) having machined oxygen channels (flow fields) adjacent to the oxygen frame. On top of the nickel plate and frame sat, in order, a conventional cathode, matrix, and anode. On top of the anode and adjacent to the hydrogen edge frame was disposed another conventional porous nickel plate having machined hydrogen channels.

Between cells (below the oxygen frame and above the hydrogen frame) were cooler assemblies. These cooler assemblies comprised thin nickel sheets having folded nickel cooler fields (e.g. flow channels). These nickel sheets were contained within their own cooler seal frame consisting of a metal frame into which was molded a rubber gasket. The entire cooler assembly was attached to the molded frame assembly with a butyl rubber impregnated fiberglass cloth gasket identical to that employed to bond the two cell frame members together.

The above alkali fuel cells constructed with glass fiber filled polyphenylene sulfide housing frames formed a six-cell alkali fuel cell stack which was tested at 1 800F (82 C) for over 6000 hours without incident.

These polymeric frames can be used to house single alkali fuel cells or a stack of alkali fuel cells. They can be used to contain alternate cell cooling structures or every cell cooling structures. Although this disclosure has been directed to frames for use with alkali fuel cells, these frames may be used to advantage with other electrochemical devices such as alkali electrolysis cells, nickel-hydrogen and nickel-cadmium batteries. The alkali fuel cells of this disclosure operate using conventional alkali fuel cell processes.

This invention provides a molded frame for alkali fuel cells which has a variety of desirable advantages over the previous glass fiber laminate frames. For example, a molded polymeric frame can reduce the amount of silica filler which is required by the glass fiber laminate frame.

This is important as silica contaminates the fuel cell and reduces cell life. In addition, the hydrogen and oxygen flows tended to erode the epoxy from the port area of glass fiber laminates resulting in the plugging of ports and eventually a reactant starved cell. Also, the glass fiber epoxy laminate is less stable in the potassium hydroxide enviornment of the alkali fuel cell than the polymeric frames of this disclosure. This glass fiber frame instability can lead to carbonation of the electrolyte which is the major cause of cell failure. Also, there are a myriad of advantages of a single molded part over a multilayer laminate. These range from the fact that it is simpler to make one part than to fabricate various layers of lamination to the fact that a multilayered part is difficult to reproduce.In addition, an inherent design defect of the glass fiber laminate frames was that the frame was bonded to one cell requiring the disposal of the whole cell if the frame was defective, e.g. voids in the epoxy. However, since the molded frame is independent of the cell, it can be discarded separately from a cell if the frame is found to be defective. Also, the high temperature processing required of the laminate frame fabrication, which could result in cell damage, has been eliminated. Finally, the polymeric molded frames of this disclosure contribute to a lighter weight cell and a thinner cell package as the thicker cell components (glass fiber container frames) are eliminated.

This invention provides lightweight frames for containing alkali fuel cells. These frames result in longer life alkali fuel cells that have fewer defects. This is a result of the superior compatibility of these molded frames in the alkali environment than the previous frame containers. In addition, these molded frames require less fabrication complexity and result in a simpler overall fuel cell structure design. Thus, these alkali fuel cell molded frames make a significant advance in the fuel cell technology by providing durable long life cell structures, particularly for use in aerospace and undersea applications which require a high degree of reliability.

It should be understood that the invention is not limited to the particular embodiment shown and described herein, but that various changes and modifications may be made without departing from the spirit or scope of this concept as defined by the following claims.

Claims (5)

1. An electrochemical alkali fuel cell including cathode, matrix, and anode cell components, said components contained within a cell housing having an interior which accommodates said components wherein the improvement is characterized by said cell housing comprising a plurality of molded stacked frame members, said frame members comprising a polyphenylene sulfide resin; polyether sulfone resin; or mixture thereof; and a filler about 10% to about 40% encapsulated filler selected from the group consisting of ceria, glass, potassium titanate or zirconium oxide.

2. The alkali fuel cell as recited in claim 1 characterized in that said housing includes at least four longitudinal fluid conduits, each fluid conduit comprising aligned apertures disposed in said frame members and at least two of said apertures being in fluid communication with said interior through fluid passages oriented generally perpendicularly to said longitudinal fluid conduits and said frame members having a recess bounding said interior.

3. The alkali fuel cell according to any of claims 1 or 2 characterized in that said frame member comprises a polyether sulfone resin.

4. The alkali fuel cell according to any of claims 1 to 3, characterized in that said filler is glass fiber.

5. The alkali fuel cell according to any of claims 1 to 4, characterized in that said frame members are about 0.18 cm to about 0.64 cm thick and have sides about 15.3 cm to about 38.1 cm in length.