Method for sealing acid-doped membranes

Abstract

A method for bonding two or more acid-doped polybenzimidazole films includes attaching a pair of first and second substrates to opposing surfaces of the first and second acid-doped polybenzimidazole films to form first and second film / substrate assemblies. A portion of each of the first and second acid-doped polybenzimidazole films is not covered by the respective first and second substrates. The method further includes immersing at least the uncovered portions of the first and second films in a solvent to remove acid from the uncovered portions; spraying at least one section of each of the uncovered portions of the first and second films with a fluoroelastomer coating; placing the second film / substrate assembly on top of the first film / substrate assembly to contact the spray-coated sections of the first and second films; and applying at least one of pressure and heat to the contacted sections of the first and second films.

Description

[Background technology]

[0001] The conversion of thermal or chemical energy to electrical energy, or vice versa, can be achieved in a variety of ways. For example, known electrochemical cells or batteries utilize a chemical reaction in which ions and electrons from an oxidized reactant are transferred via separate pathways to a reduced reactant. Specifically, the electrons are electrically transferred via wiring to an external load, where they perform work, while the ions are conducted through an electrolyte separator.

[0002] However, battery-type electrochemical cells can only produce a limited amount of energy because the dimensions of the battery enclosure limit the amount of available reactants that can be contained within. While such electrochemical cells can be designed to be recharged by applying a reverse polarity current / voltage between the electrodes, such recharging requires a separate power source. Also, during the recharging process, the electrochemical cell is typically unusable.

[0003] Fuel cells were developed as an attempt to overcome the problems associated with battery-type electrochemical cells. In conventional fuel cells, chemical reactants are continuously supplied to and removed from an electrochemical cell. In a manner similar to a battery, fuel cells operate by conducting ionized species through a selective electrolyte in a membrane electrode assembly (MEA) that normally blocks the passage of electrons and non-ionized species.

[0004] The most common type of fuel cell is the hydrogen-oxygen fuel cell, which passes hydrogen through one electrode and oxygen through the other. Porous electrodes, positioned on either side of an electrolyte separator membrane, are used to connect the electrons participating in the chemical reaction to an external load via an external circuit. Hydrogen ions are conducted through the electrolyte separator to the oxygen side of the cell under the potential of the hydrogen and oxygen chemical reaction. On the oxygen side, the electrons and hydrogen ions reconstitute hydrogen, completing the reaction with oxygen and producing water, which is released from the system. Continuous supply of hydrogen and oxygen to the cell results in a continuous current.

[0005] Mechanical heat engines have also been designed and used to generate electrical power. Such mechanical heat engines operate on a thermodynamic cycle, performing shaft work by compressing a working fluid using a piston or turbine. The compression process occurs at a low temperature, and after compression, the working fluid is raised to a higher temperature. At the high temperature, the working fluid can expand against a load, such as a piston or turbine, thereby producing shaft work. The key to the operation of all engines using a working fluid is that the amount of work required to compress the working fluid at a low temperature is less than the work produced by expanding the working fluid at a high temperature. This is true for all thermodynamic engines using a working fluid.

[0006] For example, steam engines operate on the Rankine thermodynamic cycle, where water is pumped to high pressure, then heated to steam and expanded by a piston or turbine to produce work. Internal combustion engines operate on the Otto cycle, where cold ambient air is compressed by a piston and then heated to very high temperatures by fuel combustion in a cylinder. As the cycle continues, the expansion of the heated air against the piston produces more work than is consumed during the cooler compression process.

[0007] Stirling engines were developed to operate on the Stirling cycle to provide an engine that was highly efficient and offered greater versatility in heat source selection. An ideal Stirling thermodynamic cycle would have an efficiency comparable to that of an ideal Carnot cycle, which defines the theoretical maximum efficiency of an engine operating with high-temperature heat input and low-temperature heat output. However, like all mechanical engines, Stirling engines suffer from reliability issues and efficiency losses associated with mechanical moving parts.

[0008] To avoid the problems inherent in mechanical heat engines, alkali metal thermoelectrochemical conversion (AMTEC) cells have been designed as thermoelectrochemical heat engines. AMTEC heat engines use pressure to generate a voltage potential and current by forcing an ionizable working fluid, such as sodium, through an electrochemical cell (membrane electrode assembly, MEA) at high temperature. The electrodes connect the current to an external load. Electrical work is performed when molten sodium atoms are forced through the electrolyte by a pressure difference across an electrolyte separator. The sodium is ionized as it enters the electrolyte, thereby releasing electrons into the external circuit. On the other side of the electrolyte, the sodium ions recombine with electrons to reconstitute sodium as it leaves the electrolyte, much like the process that occurs in battery and fuel cell-type electrochemical cells. The reconstituted sodium leaves the electrochemical cell as an expanded gas at low pressure and high temperature. The gas is then cooled and condenses back into a liquid state. The resulting cold liquid is then repressurized. The operation of an AMTEC engine approximates the Rankine thermodynamic cycle.

[0009] Numerous publications are available on AMTEC technology. See, for example, Non-Patent Document 1. Another representative publication is Non-Patent Document 2. See also Non-Patent Document 3.

[0010] AMTEC heat engines suffer from reliability problems due to the highly corrosive nature of the alkali metal working fluid. AMTEC engines are also very limited in their practical utility. Specifically, they can only operate at extremely high temperatures because the ion-conducting solid electrolyte only reaches a practical conductivity level at high temperatures. In practice, even the low-temperature pressurization process must be carried out at relatively high temperatures because the alkali metal working fluid must always remain above its melting temperature as it moves through the cycle. Mechanical pumps, and even magnetohydrodynamic pumps, have been used to pressurize the low-temperature working fluid.

[0011] To overcome the aforementioned drawbacks of conventional mechanical and thermoelectrochemical heat engines, a Johnson Thermoelectrochemical Converter (JTEC) system (disclosed in U.S. Patent Application Publication No. 2003 / 0129999, filed April 28, 2003, the entire contents of which are incorporated herein by reference) was invented, which can approximate a Carnot equivalent cycle. A typical JTEC system is a heat engine including a first electrochemical cell (MEA) operating at one temperature, a second electrochemical cell (MEA) operating at a temperature different from that of the first electrochemical cell, a conduit system including a heat exchanger connecting the two cells, and a source of ionizable gas (such as hydrogen or oxygen) as a working fluid contained within the conduit system. Each MEA stack includes a non-porous membrane capable of conducting ions of the working fluid, flanked by porous electrodes.

[0012] In a JTEC, working fluid passes through each MEA stack by releasing electrons at the inlet electrode, allowing ions (protons) to be conducted through the membrane to the opposite electrode. As working fluid ions exit the membrane, the opposite electrode resupplies electrons that have passed through an external load or control device, reconstituting the working fluid within the opposite electrode. When a hydrogen pressure difference is applied across an MEA with an attached electrical load, the pressure difference powers the load as hydrogen passes from high pressure to low pressure. This process also works in the reverse direction: voltage and current can be applied to the MEA to pump hydrogen from low pressure to high pressure.

[0013] When operated under a pressure differential, the hot cell, according to the Nernst law, has a higher voltage than the cold cell. As in any engine, the working fluid (hydrogen in this case) is compressed at a low temperature and expanded at a high temperature to produce a net power output. A constant current through both MEAs maintains a constant pressure differential. Because the current (I) is the same through both cells, the voltage difference means that the amount of power produced by the expansion of hydrogen in the hot cell is higher than that of the cold cell.

[0014] Developing a JTEC suitable for widespread use faces several challenges, particularly those related to the use of hydrogen as the working fluid. For example, due to the small size of hydrogen molecules, small imperfections in the conduit system can lead to hydrogen leaks. In particular, hydrogen leaks can occur at the interconnections of the conduit couplers between the high-temperature and low-temperature cells. Such leaks are undesirable because they reduce the pressure differential of the working fluid across the membrane, thereby reducing electrical output and overall system efficiency.

[0015] Furthermore, unlike conventional fuel cells, whose open-circuit voltages can exceed 1 volt, the Nernst voltage due to the hydrogen pressure difference across the MEA stack is in the range of only about 0.2 volts. Therefore, to achieve a useful output voltage level, many cells must be connected in series. Additionally, each JTEC cell must have a large membrane / electrode surface area to achieve a useful level of output current and minimize voltage loss due to membrane resistance. That is, given the low operating voltage of each individual cell and the low conductivity of available membrane materials, a large membrane surface area is required to generate useful levels of power. The direct-bonded membrane structure alleviates the aforementioned challenges of hydrogen leakage by eliminating leaky conduit connectors.

[0016] Therefore, membranes, such as those used in thermoelectric engines, must have sufficiently high ionic conductivity to maximize output voltage and sufficiently high diffusion barrier properties to minimize pressure-induced diffusion of the working fluid, e.g., hydrogen gas, and any accompanying gases (e.g., oxygen), across the membrane and prevent the associated degradation of electrical output and efficiency. However, available proton-conducting membrane materials with useful ionic conductivities, such as DuPont Corp.'s polymer Nafion, generally have very low molecular diffusion barrier properties, resulting in losses in the pressure differential required for operation. Conversely, available membrane materials, such as ceramic ionic conductors with high molecular diffusion barrier properties, generally have relatively low ionic conductivities, especially at low to moderate temperatures, resulting in high system impedance and high polarization losses when used with such materials. Therefore, a practical method is needed to use available high-barrier, high-ionic- or proton-conducting materials as thin, large-surface-area membranes to provide a thermoelectrochemical heat engine that can approximate the Carnot equivalent cycle and eliminates the reliability and inefficiency problems associated with conventional mechanical engines.

[0017] For this reason, there has been significant interest in the use of solid polymer electrolytes. Unlike conventional alternative membrane materials, such as Nafion™, whose conductivity depends on water availability and therefore requires external humidification for optimal operation, the proton conductivity of solid polymer electrolytes is independent of water availability, and thus solid polymer electrolytes can operate at high temperatures without external humidification. To this end, one membrane of particular interest for solid polymer electrolytes is based on polybenzimidazole (PBI) polymers. PBI polymers are a family of polymers known for their excellent thermal and chemical stability. More specifically, PBIs possess inherently high thermal and chemical stability due to the strong and rigid nature of their aromatic structures and the bonds between aromatic structures.

[0018] A method has been developed to prepare PBI solutions that can be cast into thin films. Specifically, PBI films can be used as solid polymer electrolytes by casting a membrane from the solution and then doping the membrane with phosphoric acid (PA) to make the polymer proton conductive. See, for example, Non-Patent Document 4. [Prior art documents] [Patent documents]

[0019] [Patent Document 1] U.S. Patent No. 7,160,639 [Non-patent literature]

[0020] [Non-Patent Document 1] Conceptual design of AMTEC demonstrative system for 100 t / d garbage disposal power generation facility,Qiuya Ni et al.(Chinese Academy of Sciences,Inst.of Electrical Engineering,Beijing,China) [Non-patent document 2] Intersociety Energy Conversion Engineering Conference and Exhibit(IECEC),35th,Las Vegas,NV(July 24-28,2000),Collection of Technical Papers.Vol.2(A00-37701 10-44) [Non-patent document 3] American Institute of Aeronautics and Astronautics,190,p.1295-1299.REPORT NUMBER(S)-AIAA Paper 2000-3032 [Non-patent document 4] JSWainright et al., “Acid-doped polybenzimidazoles: a new polymer electrolyte,” Journal of the Electroochemical Society, 142(7) (1995) Summary of the Invention [Problem to be solved by the invention]

[0021] Xiao et al. developed a sol-gel process called the "PPA" (polyphosphoric acid) method that can synthesize PA-doped PBI membranes (see, for example, L. Xiao et al., "High-temperature polybenzimidazole fuel cell membranes via a sol-gel process," Chemistry of Materials, 17(21), 5328-333 (2005)). Acid-doped gel membranes synthesized by the polyphosphoric acid (PPA) method have a high acid content per polymer repeat unit, which results in high proton conductivity and membranes that retain sufficient mechanical properties to allow the polymer to be used in fuel cell applications. Therefore, acid-doped PBI membranes are particularly desirable for use in fuel cell applications. However, one limitation of acid-doped PBI-based membranes is that they do not readily bond with themselves or with other materials. This makes it difficult to bond PBI membranes to form subassemblies used to fabricate electrochemical cell stacks. Therefore, it is desirable to provide a method for efficiently bonding acid-doped PBI membranes. [Means for solving the problem]

[0022] Briefly, one embodiment includes a method for bonding two or more acid-doped polybenzimidazole films. The method includes attaching a pair of first substrates to opposing surfaces of a first acid-doped polybenzimidazole film to form a first film / substrate assembly. A portion of the first acid-doped polybenzimidazole film is not covered by the first substrate. The method further includes attaching a pair of second substrates to opposing surfaces of a second acid-doped polybenzimidazole film to form a second film / substrate assembly. A portion of the second acid-doped polybenzimidazole film is not covered by the second substrate. The method further includes the steps of immersing at least the uncovered portions of the first and second acid-doped polybenzimidazole films in a solvent to remove acid from at least the uncovered portions of the first and second acid-doped polybenzimidazole films, spraying a fluoroelastomer coating onto at least one section of the uncovered portions of each of the first and second acid-doped polybenzimidazole films, placing a second film / substrate assembly over the first film / substrate assembly to contact the spray-coated sections of the uncovered portions of the first and second acid-doped polybenzimidazole films with each other, and applying at least one of pressure or heat to the contacted sections of the uncovered portions of the first and second acid-doped polybenzimidazole films.

[0023] In one embodiment, the solvent is deionized water. In a further embodiment, the deionized water is at room temperature and the immersion step is carried out for at least about 40 seconds.

[0024] In another embodiment, the step of applying at least one of pressure or heat comprises applying pressure to the contacted sections for about 24 to 48 hours.

[0025] In yet another embodiment, the step of applying at least one of pressure or heat comprises applying a hot press at a temperature of about 60° C. for about 3 minutes.

[0026] In yet another embodiment, the method further comprises heating the first and second film / substrate assemblies at a temperature of about 180° C. for about 45 minutes after the applying step and before the immersion step.

[0027] In yet another embodiment, the method further comprises drying the first and second film / substrate assemblies in a dryer after the spraying step.

[0028] In yet another embodiment, the method further includes, after the dipping step, placing the first and second film / substrate assemblies on a vacuum plate and imprinting first and second meshes onto the uncovered portions of the first and second acid-doped polybenzimidazole films, respectively.

[0029] Another embodiment includes a method for making a first film / substrate assembly for bonding to another film / substrate assembly. The method includes attaching a pair of substrates to opposing surfaces of a first acid-doped polybenzimidazole film to form a first film / substrate assembly. A portion of the first acid-doped polybenzimidazole film is not covered by the first substrate. The method further includes immersing at least the uncovered portion of the first acid-doped polybenzimidazole film in a solvent to remove acid from at least the uncovered portion of the first acid-doped polybenzimidazole film, and spraying a fluoroelastomer coating onto at least one section of the uncovered portion of the first acid-doped polybenzimidazole film.

[0030] In one embodiment, the solvent is deionized water. In another embodiment, the deionized water is at room temperature and the immersion step is carried out for at least about 40 seconds.

[0031] In another embodiment, the method further comprises heating the first film / substrate assembly at a temperature of about 180° C. for about 45 minutes after the applying step but before the immersing step.

[0032] In another embodiment, the method further comprises placing the first film / substrate assembly in a dryer after the spraying step.

[0033] In other embodiments, the method further comprises, after the dipping step, placing the first film / substrate assembly on a vacuum plate and imprinting a first mesh onto the uncovered portion of the first acid-doped polybenzimidazole film.

[0034] The following detailed description of the preferred embodiments will be more clearly understood when read in conjunction with the accompanying drawings, in which there is shown, for purposes of illustration, embodiments which are presently preferred, it being understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown. [Brief explanation of the drawings]

[0035] [Figure 1] FIG. 2 is a flow diagram of an example of a method according to a first exemplary embodiment of the present invention. [Figure 2A] FIG. 2 is a schematic diagram of some of the components and materials used in the various steps of the method shown in FIG. 1. [Figure 2B] FIG. 2 is a schematic diagram of some of the components and materials used in the various steps of the method shown in FIG. 1. [Figure 2C] FIG. 2 is a schematic diagram of some of the components and materials used in the various steps of the method shown in FIG. 1. DETAILED DESCRIPTION OF THE INVENTION

[0036] Certain terminology is used in the following description for convenience only and is not limiting. The terms "right," "left," "lower," and "upper" designate directions in the drawings to which reference is made. The terms "inward" and "outward" refer to directions toward and away from the geometric center of the device and designated parts thereof, respectively. The terminology includes the above terms, their derivatives, and synonyms. Additionally, the terms "a" and "an," as used in the claims and corresponding parts of the specification, mean "at least one."

[0037] It should also be understood that the terms "about," "approximately," "generally," "substantially," and similar terms used herein when referring to dimensions or characteristics of components indicate that the described dimensions / characteristics are not precise boundaries or parameters, but do not exclude minor variations from functionally similar ones. At a minimum, such references involving numerical parameters will include variations that do not alter the least significant digit using mathematical and industrial principles recognized in the art (e.g., rounding, measurement or other systematic errors, manufacturing tolerances, etc.).

[0038] It should also be understood that terms such as "first," "second," etc. are provided for clarity purposes only, and elements or components, and operations thereof, identified by these terms may be readily interchanged.

[0039] Certain embodiments provided herein relate to methods for bonding two or more acid-doped PBI films or membranes. More specifically, the methods relate to methods for bonding two or more acid-doped polymer films or membranes, preferably prepared by a sol-gel process, and more preferably synthesized by a PPA process, preferably using a PPA. Even more specifically, the methods relate to methods for bonding two or more PA-doped polymer films or membranes synthesized by a PPA process. While the following discussion relates to a scheme for bonding two such films or membranes, it should be understood that the method can be repeated or replicated as needed to bond additional films or membranes. Also, while the following discussion primarily refers to polymer membranes, it should be understood that the method is fully applicable to any polymer film.

[0040] In the PPA method, PPA is used as both a polycondensation reagent and a casting solvent in the production of highly acid-doped PBI membranes. Acid-doped PBI membranes are highly hygroscopic due to the presence of acid. Therefore, when acid-doped PBI membranes are exposed to ambient air, a layer of water forms on the exposed surface of the membrane, making it extremely difficult to bond the PBI membrane to anything, including another PBI membrane.

[0041] 1 and 2A-2C, first and second acid-doped PBI films or membranes 10, 12 may be formed. In one embodiment, the polymer of membranes 10, 12 is poly-2,2″-(m-phenylene)-5,5-bibenzimidazole (m-PBI) or poly[2,2′-(p-phenylene)-5,5′-bibenzimidazole] (p-PBI). Preferably, each film or membrane 10, 12 may be an acid-doped p-PBI film or membrane. In one embodiment, first and second acid-doped PBI films or membranes 10, 12 are formed by a PPA method.

[0042] In some embodiments, in step 102, the first PBI film 10 can be attached to a pair of first substrates 14a, b to form a first film / substrate assembly. To form an MEA, the substrates 14a, b can be carbon-based electrode substrates, although other types of substrates, including multilayer substrates, can be used as well. To attach the first PBI film 10 to the carbon-based electrode substrates, the substrates 14a, b can be hot-pressed to the first PBI film 10 at a temperature of about 160° C. and a compression of about 30% or less.

[0043] Upon attachment, portions 11 of the first PBI film 10 may remain uncovered by the first substrates 14 a, b. For example, each of the first substrates 14 a, b may have a surface area smaller than that of the first PBI film 10, such that when the first substrates 14 a, b are attached (e.g., centered), the peripheral edges of the first PBI film 10 may extend beyond the edges of the first substrates 14 a, b. For example, the uncovered portions 11 of the first PBI film 10 may extend approximately 1 inch beyond the edges of the first substrates 14 a, b, although this amount may vary depending on various factors, including the thicknesses of the various layers. In some embodiments, the uncovered portions 11 of the first PBI film may extend in all directions from the first substrates 14 a, b, although the uncovered portions 11 may be shaped and positioned as needed to enable bonding. For example, in the case of rectangular shaped substrates 14a,b, the uncovered portions 11 may be present on only two opposing sides of the rectangle, rather than on all four sides, if desired.

[0044] After the first film / substrate assembly is formed, in step 104, the assembly can then be heated, for example, by placing it in an oven at a temperature of about 180° C. for about 45 minutes, although other temperatures and durations can be used as well. This process pre-shrinks the first film / substrate assembly.

[0045] In step 106, at least the uncovered portion 11 of the first PBI film 10 may be immersed in a solvent to leach or otherwise remove the acid (e.g., PA) from at least the uncovered portion 11. This can be done by immersing the entire first film / substrate assembly in the solvent, or simply soaking the uncovered portion 11 in the solvent may be sufficient. Alternatively, various ends of the uncovered portion 11 of the first PBI film 10 may be sequentially immersed in the solvent to avoid excessive contact of the first substrates 14a,b with the solvent. The solvent may be deionized water at room temperature, although other solvents and / or temperatures may be used as well, depending on the nature of the acid desired to be removed from the PBI film and the ends. The purpose of the solvent is typically to remove all of the acid, not to densify or otherwise harden the uncovered portion 11 of the first PBI film 10. Thus, when the edges of the first film / substrate assembly are immersed in deionized water, each edge may be immersed for at least about 40 seconds or even a few minutes, unlike other processes where the PBI film is immersed for hours to completely remove the acid.

[0046] In step 108, the first film / substrate assembly can be placed on a vacuum plate (not shown) to which a mesh is attached (e.g., welded, etc.). In step 110, the mesh can be imprinted onto the uncovered portion 11 of the first PBI film 10, and the first film / substrate assembly can be placed on the vacuum plate for at least about 7 minutes. The mesh serves to etch or otherwise roughen the surface of the uncovered portion 11 to aid in the bonding process. Other etching / roughening processes can be used in addition to or instead of the mesh. The vacuum plate can be used to ensure that the PBI film remains flat during processing, and other similar restraint methods can be used instead if necessary.

[0047] In step 112, at least one section of the uncovered portion 11 of the first PBI film 10 can be spray-coated with a fluoroelastomer. An example of a coating layer 20 surrounding the uncovered portion 11 of the first PBI film 10 can be seen in FIG. 2B. While the coating is shown only on the first PBI film 10, the coating 20 can be applied to portions of the first substrate 14a,b as well. The fluoroelastomer can be a VITON™ spray, commercially available from Chemours Company, although other types of similar fluoroelastomer sprays can be used as well. When spray-coating, it is preferable to position the first film / substrate assembly under a fume hood. While FIG. 2B shows the entire uncovered portion 11 of the first PBI film 10 covered with the coating layer 20, the coating layer 20 can alternatively be selectively applied to various sections of the uncovered portion 11. For example, depending on the structure, materials, and / or bonding scheme, only the upwardly (or downwardly) facing surface of the uncovered portion 11 of the first PBI film 10 may be coated, the coating layer 20 may be provided only at selected intervals along the uncovered portion, or other coating patterns may be used as well.

[0048] In step 114, after the spray coating 20 is applied, the first film / substrate assembly may be dried, for example, in an oven at about 100°C for at least about 10 minutes, although other times and / or temperatures may be used as well.

[0049] Steps 102-114 constitute one example of a method for preparing a first film / substrate assembly for bonding with another assembly. Steps 116-128 shown in FIG. 1 are similar to steps 102-114 and may be performed on a second or subsequent film / substrate assembly, such as that shown in FIGS. 2A-2B, in which a second PBI film 12 is sandwiched between second substrates 16a,b, resulting in an uncovered portion 13 that is dipped and coated. Steps 116-128 may be performed in parallel with steps 102-114, after steps 102-114, alternating with steps 102-114, combinations thereof, etc. Additional film / substrate assemblies may be similarly fabricated and processed.

[0050] In step 130, the first and second substrate assemblies can be joined. In particular, as seen in FIG. 2C, the second film / substrate assembly can be positioned on top of the first film / substrate assembly. At least the spray-coated sections of the uncovered portions 11, 13 of the first and second PBI films 10, 12 can be brought into contact with each other, for example, by bending or otherwise manipulating the uncovered portions 11, 13 of the first and second PBI films 10, 12. It should be noted that in some embodiments, when two uncovered portions 11, 13 are in contact, such as when only one of the PBI films 10, 12 is spray coated, or when the coating layer 20 is patterned (e.g., a section of the uncovered portion 11 of the first PBI film 10 having the coating layer 20 contacts a section of the uncovered portion 13 of the second PBI film 12 without the coating layer 20, and vice versa), the coating layer 20 need only be applied to one of the PBI films 10, 12.

[0051] In step 132, at least one of pressure and heat can be applied to the contacted sections of the uncovered portions 11, 13 of the first and second PBI films 10, 12 to form a bond. In one embodiment, the process involves applying pressure to the contacted sections for about 24-48 hours, or until the materials bond. In another embodiment, the process involves applying a hot press (not shown) at a temperature of about 60°C for about 3 minutes, although other times and / or temperatures may be used as well, depending on the materials and conditions. The intervening fluoroelastomer coating can further enhance the bond between the first and second (and any additional) PBI films.

[0052] Although the present invention has been described above and illustrated in the drawings as being used with a PBI film sandwiched between a pair of substrates, the methods disclosed herein, or minor modifications thereof, can be used with a PBI film attached to a single substrate or to two or more substrates. The methods disclosed herein, or minor modifications thereof, can also be used with a PBI film that is not attached to any substrate.

[0053] Those skilled in the art will recognize that the boundaries of the operations described above are merely exemplary. Operations may be combined into a single operation, a single operation may be divided into additional operations, and operations may be performed with at least partial overlap in time. Furthermore, alternative embodiments may include multiple instances of a particular operation, and the order of operations may be changed in various other embodiments.

[0054] While the drawings show specific different embodiments, various individual elements or combinations of elements from different embodiments can be combined with each other while maintaining the spirit and scope of the invention. Thus, individual features described herein are described with respect to only one embodiment and should not be construed as incompatible with other embodiments described herein or otherwise encompassed by the invention.

[0055] Those skilled in the art will appreciate that changes could be made to the above-described embodiments without departing from the broad inventive concept thereof. It is understood, therefore, that the invention is not limited to the particular embodiments disclosed, but that it covers modifications within the spirit and scope of the invention as defined by the appended claims.

Claims

1. 1. A method for bonding two or more acid-doped polybenzimidazole films, comprising: attaching a pair of first substrates to opposing surfaces of a first acid-doped polybenzimidazole film to form a first film / substrate assembly, a portion of the first acid-doped polybenzimidazole film not covered by the first substrate; and attaching a pair of second substrates to opposing surfaces of a second acid-doped polybenzimidazole film to form a second film / substrate assembly, a portion of the second acid-doped polybenzimidazole film not covered by the second substrate; immersing at least the uncovered portions of the first and second acid-doped polybenzimidazole films in a solvent to remove acid from at least the uncovered portions of the first and second acid-doped polybenzimidazole films; spraying a fluoroelastomer coating onto at least one section of each of the uncovered portions of the first and second acid-doped polybenzimidazole films; placing a second film / substrate assembly over the first film / substrate assembly so that the uncovered spray-coated sections of the first and second acid-doped polybenzimidazole films contact each other; applying at least one of pressure or heat to the contacted sections of the uncovered portions of the first and second acid-doped polybenzimidazole films; A method comprising:

2. The method of claim 1 wherein the solvent is deionized water.

3. 3. The method of claim 2, wherein the deionized water is at room temperature and the immersion step is performed for at least about 40 seconds.

4. The method of claim 1, wherein the step of applying at least one of pressure or heat comprises applying pressure to the contacted sections for about 24 to 48 hours.

5. 10. The method of claim 1, wherein the step of applying at least one of pressure or heat comprises applying a hot press at a temperature of about 60°C for about 3 minutes.

6. 10. The method of claim 1, further comprising heating the first and second film / substrate assemblies at a temperature of about 180°C for about 45 minutes after the applying step and before the immersing step.

7. The method of claim 1 , further comprising drying the first and second film / substrate assemblies in a dryer after the spraying step.

8. 10. The method of claim 1, further comprising, after the dipping step, placing the first and second film / substrate assemblies on a vacuum plate and imprinting the first and second meshes onto the uncovered portions of the first and second acid-doped polybenzimidazole films, respectively.

9. 1. A method of making a first film / substrate assembly for bonding to another film / substrate assembly, comprising: attaching a pair of substrates to opposing surfaces of a first acid-doped polybenzimidazole film to form a first film / substrate assembly, wherein a portion of the first acid-doped polybenzimidazole film is not covered by the first substrate; immersing at least the uncovered portion of the first acid-doped polybenzimidazole film in a solvent to remove acid from at least the uncovered portion of the first acid-doped polybenzimidazole film; spraying a fluoroelastomer coating onto at least one section of the uncovered portion of the first acid-doped polybenzimidazole film; A method comprising:

10. 10. The method of claim 9, wherein the solvent is deionized water.

11. 11. The method of claim 10, wherein the deionized water is at room temperature and the soaking step is performed for at least about 40 seconds.

12. 10. The method of claim 9, further comprising heating the first film / substrate assembly at a temperature of about 180°C for about 45 minutes after the applying step and before the immersing step.

13. 10. The method of claim 9, further comprising placing the first film / substrate assembly in a dryer after the spraying step.

14. 10. The method of claim 9, further comprising, after the dipping step, placing the first film / substrate assembly on a vacuum plate and imprinting the first mesh into the uncovered portions of the first acid-doped polybenzimidazole film.

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