Strong polybenzimidazole (PBI) gel membranes with high conductivity

Abstract

Disclosed are polybenzimidazole gel membranes that exhibit both high proton conductivity and high tensile strength at break. The membranes are formed with a high percentage of tetraaminobiphenyl monomers and naphthalene dicarboxylic acid monomers.

Description

STRONG POLYBENZIMIDAZOLE (PBI) GEL MEMBRANES WITH HIGH CONDUCTIVITYCross Reference to Related Application

[0001] This application claims filing benefit of United States Provisional Patent Application Serial No. 63 / 491 ,301 having a filing date of March 21 , 2023, which is incorporated herein by reference for all purposes.Background

[0002] Polybenzimidazole (PBI) membranes have been considered for use in a variety of electrochemical applications. Traditionally, PBI membranes for use in electrochemical applications were prepared by solution casting in N,N’- dimethylacetamide (DMAc) to form a dense film followed by imbibing the formed film in the desired electrolyte, coined the “conventional imbibing process.” Traditional PBI membranes are most notably known for their performance as high temperature polymer electrolyte membranes as may be utilized in fuel cell applications. Traditional PBI membranes have also been considered for devices such as electrochemical hydrogen separators, SO2 depolarized electrolyzers, and redox flow batteries. Unfortunately, these conventional PBI membranes have been shown to exhibit extremely low ionic conductivities when imbibed in electrolyte solutions (less than 100 mS cm-1) and low power operations at current loads above about 100 mA cm-2. Moreover, the conventional imbibing process for traditional PBI membranes is a time consuming, environmentally unfriendly technique that adds cost to the membrane fabrication process.

[0003] More recently, a process to prepare PBI membranes has been developed that includes direct casting of a polymerization composition comprising the PBI polymer dissolved in polyphosphoric acid (PPA) solvent. Subsequent hydrolysis of the PPA solvent to phosphoric acid (PA, H3PO4), which is a poor solvent for PBI, induces gelation of the cast solution into a PBI membrane that is imbibed as formed with PA. Unfortunately , it has been found that these second generation polymer membranes are still in need of improvement as membranes formed to display high mechanical characteristics (e.g. , tensile characteristics) display low proton conductivities, while membranes that display high proton conductivities show undesirably low strength characteristics.

[0004] What is needed in the art is a PBI gel membrane that exhibits high mechanical characteristics in conjunction with high ionic conductivity.Summary

[0005] According to one embodiment, disclosed is a gel membrane that includes a PBI polymer and an imbibed solution comprising a PA. The gel membrane exhibits a high proton conductivity of about 180 mS / cm or greater at 160°C and also exhibits excellent mechanical characteristics, e.g., a tensile strength at break of about 5 MPa or greater as determined with a 10N load cell at a crosshead speed of 5 mm / min.The repeating units of the PBI of the gel membrane include reaction products of a tetraaminobiphenyl monomer and a naphthalene dicarboxylic acid monomer.

[0006] Also disclosed are methods for forming a gel membrane. A method can include forming a polymerization solution that includes PPA and PBI-forming monomers including a tetraaminobiphenyl monomer and a naphthalene dicarboxylic acid monomer. The tetraaminobiphenyl monomer and the naphthalene dicarboxylic acid monomer being about 70 mol% or more of the PBI-forming monomers of the solution. Following polymerization of the monomers, the resulting solution can be treated to hydrolyze at least a portion of the PPA and thereby induce gelation of the membrane.Brief Description of the Figures

[0007] A full and enabling disclosure of the present subject matter, including the best mode thereof to one of ordinary skill in the art, is set forth more particularly in the remainder of the specification, including reference to the accompanying figures in which:

[0008] FIG. 1 illustrates one embodiment of an electrochemical cell as may incorporate PBI membrane as described herein.

[0009] FIG. 2 illustrates a proton exchange PEM fuel cell as may incorporate a PBI membrane as described herein.

[0010] FIG. 3 illustrates a proton exchange PEM electrolyzer cell as may incorporate a PBI membrane as described herein.Detailed Description

[0011] Reference will now be made in detail to various embodiments of the disclosed subject matter, one or more examples of which are set forth below. Each embodiment is provided by way of explanation of the subject matter, not limitation thereof. In fact, it will be apparent to those skilled in the art that variousmodifications and variations may be made in the present disclosure without departing from the scope or spirit of the subject matter. For instance, features illustrated or described as part of one embodiment, may be used in another embodiment to yield a still further embodiment.

[0012] The present disclosure is generally directed to PBI gel membranes that exhibit both strong mechanical properties and high proton conductivities and to methods for forming the gel membranes. More specifically, a PBI gel membrane can exhibit high tensile strength at break while maintaining high ionic conductivity, allowing for formation of fuel cells, electrolyzers, and other products that can provide high power operation under high current load conditions.

[0013] The improved physical robustness combined with high proton conductivities of disclosed gel membranes can allow for longer lasting membranes and wider application of technologies incorporating the membranes. For instance, a fuel cell incorporating a PBI gel membrane as disclosed herein can be capable of withstanding multiple start-stop cycles without damage to the membrane, even at extremely low external temperatures. Such capabilities can prove highly beneficial in the automobile sector and residential appliance applications, among others. The higher mechanical durability combined with the high proton conductivities of disclosed gel membranes can also be advantageous in the production of membraneelectrode assemblies and fuel cell stacks. For instance, considerable forces act on the gel membrane in the lamination of a fuel cell stack, and the high tensile characteristics of disclosed gel membranes can be advantageous in such conditions. Forces on the gel membrane encountered during assembly of a device can be better withstood by disclosed membranes, leading to improved quality control and lower production costs.

[0014] Disclosed gel membranes can operate at high current density. For instance, disclosed gel membranes can exhibit proton conductivity at 160°C of about 180 millisiemens per centimeter (mS / cm) or higher, about 190 mS / cm, about 200 mS / cm, or about 220 mS / cm or greater e.g., from about 180 mS / cm to about 500 mS / cm in some embodiments.

[0015] Disclosed gel membranes can also exhibit high tensile strength at break as determined with a 10N load cell at a crosshead speed of 5 mm / min in accord with ASTM D638. For instance, a PBI gel membrane can exhibit a tensile strength atbreak of about 5 megapascals (MPa) or greater, about 6 MPa or greater, or even higher in some embodiments, such as from about 5 MPa to about 10 MPa.

[0016] The combination of high proton conductivity and high tensile strength at break has not previously been attained, as methods for increasing one of these characteristics tends to decrease the other. For instance, Table 1 , below provides tensile strength at break and proton conductivities at 160°C for several representative PBI membranes previously described in the art. As indicated, none of these previously known membranes attain both a high proton conductivity and a high tensile strength at break as are exhibited by disclosed PBI membranes.TABLE 1

[0017] To form a PBI gel membrane, a polymerization composition can be formed that includes PPA and PBI-forming monomers. The monomer content of the polymerization composition can generally be low, for instance about 10 wt.% or less of the total weight of the monomers and PPA combined, such as about 8 wt.% or less, about 5 wt.% or less, about 4 wt.% or less, or about 3 wt.% or less, such as from about 1 wt.% to about 4 wt.%, or from about 2 wt.% to about 3 wt.% in some embodiments.

[0018] More specifically, the PBI forming monomers of the polymerization composition can include at least one tetraaminobiphenyl monomer and at least one naphthalene dicarboxylic acid monomer.

[0019] Examples of tetraaminobiphenyl monomers as may be utilized in forming a FBI gel membrane can include, without limitation 3,3',4,4'-tetraminobiphenyl; 3,3',4,4'-tetraminodiphenylsulfone; 3,3',4,4 -tetraminodiphenyl ether; 3, 3', 4,4'- tetraminodiphenylmethane; and 3,3',4,4'-tetraminodiphenyldimethyl-methane as well as the salts thereof, e.g., the mono-, di-, tri- and tetrahydrochloride salts, as well as any combination of tetraaminobiphenyl monomers.

[0020] Examples of naphthalene dicarboxylic acid monomers can include, without limitation, 1 ,4-naphthalenedicarboxylic acid; 1 ,5-naphthalenedicarboxylic acid; 2,6- naphthalenedicarboxylic acid; 2,7-naphthalenedicarboxylic acid; 1 ,8- dihydroxynaphthalene-3,6-dicarboxylic acid; or any combination thereof.

[0021] In one embodiment, the PBI-forming monomers of a polymerization composition can include exclusively tetraaminobiphenyl monomers and naphthalene dicarboxylic acid monomers. This is not a requirement, however, and in one embodiment, a polymerization composition can include one or more additional PBI- forming monomers in addition to at least one tetraaminobiphenyl monomer and at least one naphthalene dicarboxylic acid monomer. When present, additional PBI- forming monomers can be included in a polymerization composition in an amount of about 30 mol% or less of all PBI-forming monomers of the polymerization composition. Thus, the tetraaminobiphenyl and naphthalene dicarboxylic acid monomers of a polymerization composition (and thus also of the resulting PBI polymer) can together be about 70 mol% or more of the total amount of PBI-forming monomers of the polymerization composition, such as about 80 mol% or more, about 90 mol% or more, about 95 mol% or more, about 97 mol% or more, or about 98 mol% or more, in some embodiments. The amount of additional PBI-forming monomers can generally be about 30 mol % or less, for instance from about 0.1 mol% to about 20 mol %, or from about 0.5 mol% to about 10 mol % based on the total amount of PBI-forming monomers of the polymerization composition.

[0022] Additional PBI-forming monomers can include one or more aromatic and heteroaromatic tetraamino monomers. Examples of additional aromatic and heteroaromatic tetraamino monomers as may be utilized in forming the PBI membrane can include, without limitation, 2,3,5,6-tetraminopyridine; 1 , 2,4,5- tetraminobenzene; 3,3',4,4'-tetraminobenzophenone; and 3, 3', 4,4'- tetraminodiphenyldimethyl-methane and the salts thereof, e.g., the mono-, di-, tri-and tetrahydrochloride salts, as well as any combination of aromatic or heteroaromatic tetraamino monomers.

[0023] Additional PBI-forming monomers can include one or more aromatic or heteroaromatic polycarboxylic acids or ester, anhydride, or acid chloride thereof and / or one or more aromatic or heteroaromatic diaminocarboxylic acids. An ester of a polycarboxylic acid can be utilized such as C1-C20-alkyl esters or C5-C12-aryl esters of a polycarboxylic acid.

[0024] Examples of aromatic dicarboxylic acid monomers can include, without limitation, pyridine-2,5-dicarboxylic acid; pyridine-3,5-dicarboxylic acid; pyridine-2,6- dicarboxylic acid; pyridine-2,4-dicarboxylic acid; 4-phenyl-2, 5-pyridined icarboxylic acid; 3,5-pyrazoledicarboxylic acid; 2,6-pyrimidinedicarboxylic acid; 2,5- pyrazinedicarboxylic acid; 2,4,6-pyridinetricarboxylic acid; benzimidazole-5,6- dicarboxylic acid; 5-hydroxyisophthalic acid; 4-hydroxyisophthalic acid; 2- hydroxyterephthalic acid; 5-aminoisophthalic acid; 5-N,N-dimethylaminoisophthalic acid; 5-N,N-diethylaminoisophthalic acid; 2,5-dihydroxyterephthalic acid; 2,6- dihydroxyisophthalic acid; 4,6-dihydroxyisophthalic acid; 2 ,3-dihydroxyphthalic acid; 2,4-dihydroxyphthalic acid; 3,4-dihydroxyphthalic acid; diphenylsulfone-4,4'- dicarboxylic acid; isophthalic acid; terephthalic acid; phthalic acid; 3-fluorophthalic acid; 5-fl uoroisophthal ic acid; 2-fluoroterephthalic acid; tetrafluorophthalic acid; tetrafluoroisophthalic acid; tetrafluoroterephthalic acid; 3-sulfophthalic acid; 5- sulfoisophthalic acid; 2-sulfoterephthalic acid; tetrasulfophthalic acid; tetrasulfoisophthalic acid; tetrasulfoterephthalic acid; diphenic acid; diphenyl ether 4,4'-dicarboxy lie acid; benzophenone-4,4'-dicarboxylic acid; biphenyl-4,4'- dicarboxylic acid; 4-trif luoromethyl phthalic acid; 2 , 2-bis(4- carboxyphenyl)hexafluoropropane; 4,4'-stilbenedicarboxylic acid; and 4- carboxycinnamic acid or any combination thereof.

[0025] Examples of aromatic tricarboxylic acids and esters, acid anhydrides, and acid chlorides thereof include, without limitation, 1 ,3,5-benzenetricarboxylic acid (trimesic acid); 1 ,2,4-benzenetricarboxylic acid (trimellitic acid); (2- carboxyphenyl)iminodiacetic acid; 3,5,3 -bipheny Itricarboxyl ic acid; and 3,5,4'- bi phenyltricarboxylic acid; or any combination thereof.

[0026] Examples of aromatic tetracarboxylic acids and esters, acid anhydrides, and acid chlorides thereof include, without limitation, 3,5,3',5'-biphenyltetracarboxylic acid; benzene-1 ,2,4,5-tetracarboxylic acid; benzophenonetetracarboxylic acid;3, 3',4,4'-biphenyltetracarboxyl ic acid; 2 , 2', 3, 3'-biphenyltetracarboxylic acid; 1 ,2,5,6- naphthalenetetracarboxylic acid; and 1 ,4,5,8-naphthalenetetracarboxylic acid; or any combination thereof.

[0027] Heteroaromatic carboxylic acids can include heteroaromatic dicarboxylic acids, heteroaromatic tricarboxylic acids and heteroaromatic tetracarboxylic acids, including their respective esters such as C1-C20-alkyl esters, C5-C12-aryl esters, or the acid anhydrides or the acid chlorides of the heteroaromatic carboxylic acids. Examples of heteroaromatic carboxylic acids include, without limitation, pyridine-2 , 5- dicarboxylic acid; pyridine-3, 5-dicarboxylic acid; pyridine-2, 6-dicarboxylic acid; pyridine-2, 4-dicarboxylic acid; 4-phenyl-2 , 5-pyridi nedicarboxylic acid; 3,5- pyrazoledicarboxylic acid; 2,6-py ri m idi nedicarboxyl ic acid; 2,5-pyrazinedicarboxylic acid; 2,4,6-py ridinetricarboxy lie acid; benzimidazole-5, 6-dicarboxylic acid; and also their C1-C20-alkyl esters or their C5-C12-aryl esters, or their acid anhydrides or their acid chlorides, or any combination thereof.

[0028] In one embodiment, the polymerization composition can include a diaminocarboxylic acid monomer, examples of which include, without limitation, diaminobenzoic acid and the mono- and dihydrochloride derivatives of said acid, as well as 1 ,2-diamino-3'-carboxy acid 4,4'-diphenyl ether, or any combination thereof.

[0029] The polymerization composition can also include PPA. PPA as can be utilized in the polymerization composition can include commercially available PPA as obtainable, for example, from Riedel-de Haen. PPA can include concentrated grades of PA above 100%. At high concentrations, the individual H3PO4 units are polymerized by dehydration and the PPA can be expressed by the formula Hn+2PnC>3n+1 (n>1).

[0030] The PPA [Hn+2PnO3n+i (n>1)] can have a P2O5 content as calculated by acidimetry of about 70 wt.% or more, for instance about 75 wt.% or more, or about 82 wt.% or more, for instance from about 70 wt.% to about 86 wt.% in some embodiments.

[0031] The polymerization composition can be in the form of a solution, a dispersion, or a suspension of the monomers in the PPA, generally depending upon the nature of the compounds to be polymerized and any additional components of the polymerization solution.

[0032] The polymerization can be carried out at a temperature and for a time until suitable polymerization of the monomers has taken place, which can generally bedetermined by an increase in viscosity of the composition. The increase in viscosity can be determined by visual inspection, through determination of the intrinsic viscosity, or by any other suitable means. For instance, the polymerization can continue until the polymerization composition exhibits an intrinsic viscosity of about 0.8 d L / g or greater, for instance about 1 .0 d L / g or greater, or about 1 .5 dL / g or greater, in some embodiments. The polymerization temperature can generally be about 220° C or less, for instance about 200° C or less, such as about 100° C to 195° C in some embodiments. The polymerization can be carried out over a time of from a few minutes (e.g., about 5 minutes) up to several hours (e.g., about 100 hours). In one embodiment, the polymerization composition can be heated in a stepwise fashion, for instance in three or more steps, each step lasting from about 10 minutes to about 5 hours and increasing the temperature by about 15° C or more for each step. Of course, the particular polymerization conditions can be varied, depending generally upon the reactivity and concentration of the particular monomers, as would be evident to one of skill in the art, and no particular polymerization conditions are required in formation of the membranes.

[0033] Following polymerization, PBI can be in solution in the PPA solvent, and the PBI polymer solution can be processed to form the membrane. In general, membrane formation can include initial formation of a membrane precursor having a desired thickness. In some embodiments, the viscosity of the PBI polymer solution can be adjusted in order to form the membrane precursor to a desired thickness and / or according to a desired formation methodology. By way of example, the solution can be combined with PA (most typically concentrated PA, e.g., 85% PA) to adjust the viscosity of the polymer solution to the desired value and facilitate formation of a membrane precursor.

[0034] The membrane precursor can be formed according to any suitable formation process, such as, and without limitation to, casting, spray coating, knife coating, etc. Likewise, the membrane precursor can be formed to any suitable thickness. For instance, the membrane precursor can be formed to a thickness of from about 20 micrometers (pm) to about 4,000 pm in one embodiment, such as from about 30 pm to about 3,500 pm, or from about 50 pm to about 1 ,000 pm, in some embodiments.

[0035] To instigate gelation of the membrane precursor and form the RBI gel membrane, the PBI polymer solution can be treated in the presence of water and / or moisture so as to hydrolyze at least a portion of the PPA of the solution. Upon the hydrolysis, the PPA will form PA or PA and water mixtures, thereby causing a sol-gel transfer of the PBI polymer solution and gelation of the membrane, as the PBI polymer is less soluble in PA as compared to PPA.

[0036] The hydrolysis can be carried out at temperatures and for a time sufficient for the membrane to gel and thereby to be self-supporting and capable of being manipulated without destruction while incorporating high liquid content (e.g., about 60 wt.% or higher liquid content of the total solid and liquid content of the gel membrane). By way of example, the hydrolysis treatment can be carried out at a temperature of from about 0° C to about 150° C, for instance from about 10° C to about 120° C, or from about 20° C to about 90° C, e.g., at ambient temperature in some embodiments (e.g., at a relative humidity contacting environment of from about 35% to 100%).

[0037] The hydrolysis can be carried out by contact of the membrane precursor with H2O, for instance in the form of a liquid or vapor, and / or in the presence of other components. For instance, the membrane precursor can be contacted with water vapor and / or liquid water and / or steam and / or aqueous PA (e.g., a PA solution having a PA concentration of from about 10 wt.% to about 90 wt.%, e.g., about 30 wt.% to about 70 wt.% or about 45 wt.% to about 55 wt.%). The treatment can be carried out under standard pressure, but this is not a requirement of a gelation process, and in some embodiments, the hydrolysis treatment can be carried out under modified pressure.

[0038] In one embodiment, hydrolysis can be carried out in a climate-controlled environment in which the H2O content can be tightly controlled. For example, the moisture content of the local environment can be controlled through control of the temperature or saturation of the fluid contacting the precursor membrane. For example, carrier gases such as air, nitrogen, carbon dioxide or other suitable gases can carry H2O (e.g., steam) in a controlled amount for contact with the precursor membrane.

[0039] The hydrolysis treatment time can generally vary depending upon parameters such as, e.g., H2O content and form of the contact, precursor membrane thickness, contact temperature, etc. In general, the hydrolysis treatment can becarried out over a time period of between a few seconds to a few minutes, for instance when the hydrolysis treatment utilizes superheated steam, or alternatively over a period of several days, for example when the hydrolysis treatment is carried out at ambient temperature and low relative atmospheric moisture. In some embodiments, the hydrolysis treatment can be carried out over a period of time between about 10 seconds and about 300 hours, for instance from about 1 minute to about 200 hours. By way of example, in an embodiment in which the at least partial hydrolysis of the PPA of the PBI polymer solution is carried out at room temperature (e.g., about 20° C) with ambient air of relative atmospheric moisture (i.e. , relative humidity) content of from about 20% to 100%, for instance from about 40% to about 80%, the treatment time can generally be between about 5 hours and about 200 hours.

[0040] Upon hydrolysis of at least a portion of the PPA of the PBI polymer solution, the polymer can undergo gelation and form a PBI gel membrane. A PBI gel membrane can in one embodiment have a thickness of from about 15 pm to about 3000 pm, for instance from about 20 pm to about 2000 pm, or from about 20 pm to about 1500 pm, though any particular membrane thickness is not critical and can depend upon the thickness of the membrane precursor. In some embodiments, the PBI membrane can have a thickness that is less than that of the membrane precursor. Following hydrolysis, the PBI gel membrane can be self-supporting, even at high liquid content, which is believed to be due to the intra- and intermolecular polymer structures present in the gelled polymeric matrix.

[0041] The as-formed gel membrane can in one embodiment have a PBI solids content of from about 4 wt.% to about 40 wt.%, for instance from about 8 wt.% to about 30 wt.%, or from about 10 wt.% to about 25 wt.% of the total weight of the membrane including liquid content.

[0042] In one embodiment, a PBI gel membrane can be crosslinked, which can decrease the permeability of a gel membrane without strongly affecting the electrochemical characteristics of a gel membrane. The manner of crosslinking as well as the point in the formation process at which the gel membrane is crosslinked is not particularly limited.

[0043] In one embodiment, a PBI gel membrane can be crosslinked simply by heating in the presence of atmospheric oxygen. Crosslinking can also be affected bythe action of radiation, e.g., infrared (IR) radiation (having a wavelength of from about 700 nm to about 1 mm) including near IR (radiation having a wavelength of from about 700 to about 2000 nm or an energy in the range from about 0.6 to about 1.75 eV).

[0044] To crosslink a membrane, a PBI polymer can incorporate reactive functionality on the polymer chains so as to crosslink with itself or alternatively in conjunction with a crosslinking agent, i.e., a polyfunctional compound that can react with one or more functionalities of the PBI polymer (e.g., amines). Crosslinking agents can include any suitable functionality to effect crosslinking. Suitable crosslinking agents are not particularly limited, examples of which can include, without limitation, epichlorohydrin, diepoxides, diisocyanates, a,w-dihaloalkanes, diacrylates, and bisacrylamides, particular examples of which can include, without limitation, a,a'-dichloro-p-xylene, chloromethyl methyl ether, bis(chloromethyl) ether, terephthaloyl chloride, succinyl chloride, and dimethyl succinate, as well as combinations of crosslinking agents. In one embodiment, from 1 to 20 equivalents of crosslinking agent can be utilized per available aromatic ring, but crosslinked embodiments of the membranes are not limited to any particular crosslink density.

[0045] In one embodiment, a PBI gel membrane can be imbibed with an ionic transport agent, e.g., a supporting electrolyte, in conjunction with PA remaining in the membrane following gelation. For instance, a PBI membrane can be imbibed with a mineral acid (e.g., a strong inorganic acid) such as hydrochloric acid, nitric acid, fluorosulfonic acid, or sulfuric acid, or a mixture thereof, or a strong organic acid such as acetic acid, formic acid, p-toluene sulfonic acid, or trifluoromethane sulfonic acid or mixtures thereof, as well as mixtures of different types of acids, e.g., a combination of a mineral acid and an organic acid. Other examples of ionic transport agents that can be imbibed in the membrane can include, without limitation, sodium chloride, potassium chloride, sodium hydroxide, potassium hydroxide, sodium sulfide, potassium sulfide, and combinations thereof. By way of example, an ionic transport agent can include H2SO4, HBr, HBr / HCI mixtures, HCI, NaS2, NaS2 / NaBr mixtures, Br2 in HBr, Br2 in H2SO4, Br2 in HBr / H2SO4 mixtures, etc.Tetraalkylammonium supporting cations can be imbibed in the membranes in one embodiment, with tetraethyl ammonium (Et4N+) and tetrabutyl ammonium (BU4N+) being two non-limiting examples. A solution of a tetrafluoroborate (BF4-),perchlorate (CIO4-), or hexafluorophosphate (PF6-), or a combination thereof are additional examples of ionic transport agents that can be imbibed in the membranes.

[0046] The concentration of an ionic transport agent in a gel membrane is not particularly limited, and in general can be up to about 25 moles / liter (M), for instance from about 0.1 M to about 25 M, from about 0.5 M to about 10 M, or from about 1 M to about 5 M in some embodiments.

[0047] The PBI gel membrane can be imbibed with an ionic transport agent according to any suitable methodology. For example, a PBI gel membrane can be imbibed with an ionic transport agent in one embodiment by soaking in a solution of the ionic transport agent for a period of time from a few minutes up to hours or days, optionally in an environment of increased temperature.

[0048] A gel membrane can include one or more additives that can be incorporated in the membrane at the time of precursor membrane formation, of membrane gelation, or in conjunction with the addition of an ionic transport agent. By way of example, an organic small molecule, such as small C1-C4 alcohols (e.g., glycerol), small organic acids, urea, etc. can be incorporated in a PBI gel membrane in conjunction with or alternative to the imbibing of an ionic transport agent.

[0049] In one embodiment, a gel membrane can incorporate a particulate, e.g., a titanium dioxide or a PBI particulate, generally in an amount of about 2 wt.% or less by weight of the membrane, which can decrease the porosity of the membranes. For instance, nano-sized particulates of PBI can be incorporated into the polymeric matrix during gelation of the PBI gel membrane by addition of the particulate to the polymer solution during hydrolysis.

[0050] A gel membrane as described can be suitable for any use. Representative fields of use of the PBI gel membranes can include, without limitation, use in fuel cells, in electrolysis, in capacitors and in battery systems.

[0051] By way of example, in one embodiment, a PBI gel membrane as described can be combined with other electrochemical cell components as are known in the art. FIG. 1 illustrates one embodiment of an electrochemical cell 20 as may incorporate a PBI gel membrane as described herein. As illustrated, an electrochemical cell 20 can include a cathode 22 and an anode 24 separated by a PBI gel membrane 25, which can be imbibed with a suitable supporting electrolyte / ionic transport agent as a proton exchange membrane (PEM).

[0052] The electrochemical cell 20 can also include a porous layer 26, 27 adjacent to one or both of the electrodes 22, 24 that can improve contact of the chemical reactants and products with the electrodes 22, 24. For instance, when considering a gaseous reactant or product flow, a porous layer 26 can be a gas diffusion layer. A gas diffusion layer can be fibrous, particulate, or combinations thereof so as to provide uniform distribution of gases at the surface of the electrode and encourage electron transport between the electrode and an external electrical circuit. By way of example, a gas diffusion layer can be formed of carbon fibers, e.g. , in a woven or non-woven format. Porous layers 26, 27 can optionally include multiple sub-layers of different porosities, e.g., microporosity, mesoporosity, and / or microporosity, in any desired combination to further refine and define the flow field of the fluid at the electrode and encourage desired interactions between the electrode active materials and the reactants and products of the half-reaction.

[0053] An electrochemical cell 20 can also define flow fields 28, 29 at each side of the cell that can deliver and / or remove reactants and products from the cell. For instance, in the illustrated embodiment of FIG. 1 , the flow fields 28, 29, are defined by channels formed in bipolar plates 21 , 23, respectively. Bipolar plates 21 , 23, can be of any design and formation as is generally known in the art so as to provide the desired flow fields 28, 29 generally in conjunction with one or more of electrical connections, temperature control through heat removal, and prevention of leakage external to the cell 20.

[0054] While the representative electrochemical cell 20 illustrated in FIG. 1 includes flow into and out of each side of the cell 20, those skilled in the art will understand that in various embodiments, one side or the other of the cell will not require a flow to / from that side of the cell.

[0055] A single electrochemical cell 20 can be used alone or combined with other cells to provide a cell stack. A cell stack can include any number of individual electrochemical cell units, for instance 10 or more, 50 or more or hundreds of individual cells combined into a single cell stack.

[0056] In one embodiment, a PBI gel membrane can be incorporated into a fuel cell in which the electrochemical cell is designed to utilize electrical energy generated at a cell half-reaction. Referring to FIG. 2, one embodiment of a hydrogen fuel cell is shown that includes a cathode 34 and an anode 36 separated by a proton conducting PBI gel membrane 32. In operation of the fuel cell, hydrogen can be fed35 to the anode side of the cell and oxygen (e.g., air) 37 can be fed to the cathode side of the cell.

[0057] At the anode 36, hydrogen can be reacted according to the half reaction: 2 H2— > 4 H++ 4 e-

[0058] The RBI gel membrane 32 allows transport of the protons formed at the anode 36 to the cathode 34, and the generated electrons e~ can be utilized as they pass through the circuit from the anode 36 to the cathode 34.

[0059] At the cathode 34, the protons, oxygen, and electrons can react according to the half reaction:O2+ 4 H++ 4 e“ 2 H2O

[0060] The water and any unreacted gas of the input flow 37 is then discharged 39 from the cathode side of the cell and unreacted hydrogen is discharged 31 from the anode side of the cell.

[0061] Disclosed PBI gel membranes can also be useful in electrolyzers, in which electricity is utilized to encourage a chemical reaction. Referring to FIG. 3, for instance, one embodiment of a proton exchange PBI gel membrane PEM water electrolyzer is shown that contains a cathode 44 and an anode 46 separated by a PBI gel membrane 42. In the illustrated embodiment, water can be fed 45 to the anode side of the electrolyzer cell. In some embodiments, water may be fed to both sides of the cell, in order to improve hydration of the PBI gel membrane 42.

[0062] At the anode 46, water is reacted according to the half reaction:2 H2O — ► 4 H++ O2 + 4 e-

[0063] The PBI gel membrane 42 allows transport of the protons formed at the anode 46 to the cathode 44.

[0064] At the cathode 44, the protons and electrons react according to the half reaction:4 H++ 4 e“ 2 H2

[0065] Oxygen and unreacted water is then discharged 41 from the anode side of the cell and hydrogen is discharged 49 from the cell at the cathode side of the cell. In general, the products can be discharged with water so far as water has been supplied in an amount great enough to purge them from the cell. Thereafter, the oxygen and hydrogen products can be separated from the water, as desired.

[0066] While the above illustrate representative products as may incorporate disclosed PBI gel membranes, one of skill in the art will readily understand that disclosed gel membraned can provide use in a variety of applications.

[0067] The present disclosure may be better understood with reference to the Examples set forth below.ExamplesMaterials and Methods

[0068] 3,3’,4,4’-Tetraaminobiphenyl (TAB, polymer grade, -97.5%) was donated by BASF Fuel Cell, Inc. and used as received. Naphthalene-1 ,4-dicarboxylic acid (>98.00% HPLC grade purity) was purchased from AK Scientific and used as received. PPA (115%) was supplied from FMC Corporation and used as received.

[0069] A typical polymerization consisted of a molar equivalent mixture of tetraaminobiphenyl (TAB) and a dicarboxylic acid species, which were added to PPA, mixed together with an overhead stirrer, and purged with dry nitrogen. The contents were heated in a high-temperature silicone oil bath, and the temperature was controlled by a programmable temperature controller with ramp and soak features. The reaction temperature was increased stepwise from room temperature to 120°C, 150°C, 170°C, and 190°C. In a typical polymerization, the final reaction temperature was approximately 190°C and held for 12 hours. Once the reaction was completed, determined by visual inspection of viscosity, the polymer solution was cast onto clear glass plates using a doctor blade with a controlled gate thickness of 15 mils. The cast solution was hydrolyzed into gel membranes in a humidity chamber regulated to 55% R.H. at 25°C.Membrane Composition

[0070] The composition of each PBI gel membrane was determined by measuring the relative amounts of polymer solids, water, and acid in the each membrane. The PA content of a gel membrane was determined by titrating a membrane sample with a standardized sodium hydroxide solution (0.10 M) using a Metrohm 716 DMS Titrino auto titrator. Once titrated, the sample was thoroughly washed with DI water and dried at reduced pressures at 120 °C overnight. The dried sample was then weighed to determine the polymer solids content of the membrane.

[0071] The polymer weight percentage and PA weight percentage was determined via eq. 1 and eq. 2:polymer 100 (1)acid(2 )where WsamPie is the weight of the sample before titration, Wdry is the weight of the final dried sample after titration, acid is the molecular weight of PA, and VNaoH and CNaoH are the volume and concentration of the sodium hydroxide solution required to neutralize the PA to the first equivalence point. The number of moles of PA per mole of PBI repeat unit (or the PA doping levels, X, PA / RU in the below tables) was calculated from the eq. (3):X=VNaOHCNaOHx 100 (3 )Wdry / MpOiymerwhere VNaoH and CNaoH are the volume and concentration of the sodium hydroxide solution required to neutralize the PA to the first equivalence point, Wdry is the final weight of the dried sample after titration, and Mpoiymer is the molecular weight of the polymer repeat unit.

[0072] The monomer concentration was determined as the weight ratio of the total amount (g) of monomers included in the polymerization mixture to the total amount (g) of monomers and PPA included in the polymerization mixture x 100. Tensile Properties.

[0073] The tensile properties of the gel membranes were tested at room temperature using an Instron Model 5543A system with a 10 N load cell and a crosshead speed of 5 mm / min. Dog-bone-shaped specimens were cut according to ASTM standard D638 (Type V specimens) and preloaded to 0.1 N prior to testing. Proton Conductivity.

[0074] Proton conductivities of the membranes were measured by a four-probe electrochemical impedance spectroscopy method using a Zahner IM6e electrochemical workstation over the frequency range from 1 Hz to 100 kHz with an amplitude of 5 mV. A two-component model with an ohmic resistance in parallel with a capacitor was employed to fit the experimental data. The conductivities of the membrane at different temperatures were calculated from the membrane resistance obtained from the model simulation with the following eq. (4):where d is the distance between the two inner probes, I is the thickness of the membrane, w is the width of the membrane, and Rmis the ohmic resistance determined by the model fitting. Membrane samples underwent two heating ramps to 180 °C. Conductivity data reported were recorded on the second heat ramp after water was removed from the membrane during the first heating cycle.Example 1

[0075] 1 ,4 naphthalene-PBI (n-PBI) membrane was polymerized and hydrolyzed as described above. 2.74 g tetraaminobiphenyl (TAB, 12.8 mmol), and 2.76 g naphthalene-1 ,4-dicarboxylic acid (NDA, 12.8 mmol) were added to 245 g PPA and reacted under a nitrogen atmosphere for 36 hours. The solution was applied by means of a doctor blade with a 15-mil gate thickness to a glass plate and subsequently hydrolyzed to produce a gel membrane.

[0076] The characteristics of the membrane are described in Table 2, below.Table 2

[0077] As indicated, the gel membrane had a proton conductivity of 240 mS / cm and the average tensile strength was 6.5 MPa.Example 2

[0078] 3.11 g tetraaminobiphenyl (TAB, 14.5 mmol), and 3.14 g naphthalene-1 ,4- dicarboxylic acid (1 ,4-NDA, 14.5 mmol) were added to 243 g PPA, and polymerized as described above according to the following reaction scheme to form n-PBI. The polymerization was conducted for 36 hours in a nitrogen atmosphere at 190 °C. The solution was applied by means of a doctor blade with a 15-mil gate thickness to a glass plate and subsequently hydrolyzed to produce a gel membrane.

[0079] The characteristics of the membrane are described in Table 3, below.Table 3

[0080] As indicated, the gel membrane had a higher PBI solids content than the membrane of Example 1 , and exhibited a proton conductivity of 260 mS / cm and average tensile strength of 9.2 MPa.Comparative Example 3

[0081] A para-PBI (p-PBI) membrane was polymerized as described above utilizing a terephthalic acid rather than a naphthalene dicarboxylic acid.

[0082] 4.23 g tetraaminobiphenyl (TAB, 19.7 mmol), and 3.27 g terephthalic acid(TPA, 19.7 mmol) were added to 243 g PPA and reacted under a nitrogen atmosphere for 36 hours. The solution was applied by means of a doctor blade with a 15-mil gate thickness to a glass plate and subsequently hydrolyzed to produce a gel membrane.

[0083] The characteristics of the membrane are described in Table 4, below.Table 4

[0084] As indicated, the gel membrane had a proton conductivity of 270 mS / cm but displayed an average tensile strength of only 2.0 MPa.Comparative Example 4

[0085] A sulfonated-PBI (s-PBI) membrane was polymerized as described above utilizing a sulfonated terephthalic acid rather than a naphthalene dicarboxylic acid.

[0086] 3.89 g tetraaminobiphenyl (TAB, 18.1 mmol), and 4.86 g 2- sulfoterephthalic acid monosodium salt (s-TPA, 18.1 mmol) were added to 241 g PPA and reacted under a nitrogen atmosphere for 36 hours. The solution wasapplied by means of a doctor blade with a 15-mil gate thickness to a glass plate and subsequently hydrolyzed to produce a gel membrane.

[0087] The characteristics of the membrane are described in Table 5, below.Table 5

[0088] As indicated, the gel membrane had a conductivity of 220 mS / cm but only exhibited an average tensile strength of 2.4 MPa.Comparative Example 5

[0089] A dihydroxy-PBI (diOH-PBI) membrane was polymerized as described above utilizing a dihydroxyterephthalic acid rather than a naphthalene dicarboxylic acid.

[0090] 3.64 g tetraaminobiphenyl (TAB, 17.0 mmol), and 3.36 g 2,5- dihydroxyterephthalic acid (DiOH-TPA, 17.0 mmol) were added to 243 g PPA and reacted under a nitrogen atmosphere for 36 hours. The solution was applied by means of a doctor blade with a 15-mil gate thickness to a glass plate and subsequently hydrolyzed to produce a gel membrane.

[0091] The characteristics of the membrane are described in Table 6, below.Table 6

[0092] As indicated, the gel membrane had a conductivity of 275 mS / cm but only exhibited an average tensile strength of 1 .8 MPa.Comparative Example 6

[0093] A meta / para-PBI (m / p-PBI) membrane was polymerized as described above utilizing a 7:1 wt. ratio isophthalic acid:terephthalic acid rather than a naphthalene dicarboxylic acid.

[0094] 11 .27 g tetraaminobiphenyl (TAB, 52.3 mmol), 7.64 g isophthalic acid(I PA, 46.0 mmol), and 1 .09 g terephthalic acid (TPA, 6.6 mmol) were added to 230 g PPA and reacted under a nitrogen atmosphere for 36 hours. The solution was applied by means of a doctor blade with a 15-mil gate thickness to a glass plate and subsequently hydrolyzed to produce a gel membrane.

[0095] The characteristics of the membrane are described in Table 7, below.Table 7

[0096] As indicated, this gel membrane with much higher polymer solids content as compared to Examples 1 and 2 exhibited high average tensile strength of 7.0 MPa, but only showed a proton conductivity of 170 mS / cm.Example 7

[0097] A copolymer (1 :1 n / p-PBI membrane) membrane was polymerized as described above utilizing a 1 :1 ratio of 1 ,4-naphthalene dicarboxylic acid and terephthalic acid as the diacid components. Thus, 3.9638 g tetraaminobiphenyl (TAB, 18.5 mmol), 1.9996 g naphthalene-1 ,4-dicarboxylic acid (NDA, 9.25 mmol), and terephthalic acid (9.25 mmol) (for a total of TAB+NDA = 27.75 mmol, or 75mol% of the total monomer content) were added to 243 g polyphosphoric acid and reacted under a nitrogen atmosphere for 36 hours. The solution was applied by means of a doctor blade with a 15-mil gate thickness to a glass plate and subsequently hydrolyzed to produce a gel membrane.

[0098] The characteristics of the membrane are described in Table 8 below.Table 8

[0099] As indicated, the gel membrane had a conductivity of 250 mS / cm and an average tensile strength of 6.2 MPa.Example 8

[0100] 2.39 g tetraaminobiphenyl (TAB, 11.1 mmol), and 2.41 g naphthalene-1 ,4- dicarboxylic acid (1 ,4-NDA, 11.1 mmol) were added to 170.2 g PPA, and polymerized as described above according to the following reaction scheme to form n-PBI. The polymerization was conducted for 25 hours in a nitrogen atmosphere at 190 °C. The solution was applied by means of a doctor blade with a 15-mil gate thickness to a glass plate and subsequently hydrolyzed to produce a gel membrane.

[0101] The characteristics of the membrane are described in Table 9, below.Table 9

[0102] As indicated, the gel membrane exhibited a proton conductivity of 219 mS / cm and average tensile strength of 5.08 MPa.Example 9

[0103] 2.61 g tetraaminobiphenyl (TAB, 12.1 mmol), and 2.63 g naphthalene-1 ,4- dicarboxylic acid (1 ,4-NDA, 12.1 mmol) were added to 169.76 g PPA, and polymerized as described above according to the following reaction scheme to form n-PBI. The polymerization was conducted for 25 hours in a nitrogen atmosphere at 190 °C. The solution was applied by means of a doctor blade with a 15-mil gate thickness to a glass plate and subsequently hydrolyzed to produce a gel membrane.

[0104] The characteristics of the membrane are described in Table x, below.Table 10

[0105] As indicated, the gel membrane exhibited a proton conductivity of 250 mS / cm and average tensile strength of 9.54 MPa.

[0106] While certain embodiments of the disclosed subject matter have been described using specific terms, such description is for illustrative purposes only, and it is to be understood that changes and variations may be made without departing from the spirit or scope of the subject matter.

Claims

WHAT IS CLAIMED IS:

1. A gel membrane comprising a polybenzimidazole and an imbibed solution comprising a phosphoric acid, the polybenzimidazole comprising a reaction product of a tetraaminobiphenyl monomer and a reaction product of a naphthalene dicarboxylic acid monomer, the polybenzimidazole membrane exhibiting a proton conductivity of about 180 mS / cm or greater at 160°C and exhibiting a tensile strength at break of about 5 MPa or greater as determined with a 10N load cell at a crosshead speed of 5 mm / min.

2. The gel membrane of claim 1 , wherein the tetraaminobiphenyl monomer is selected from the group consisting of 3,3',4,4'-tetraminobiphenyl; 3, 3', 4,4'- tetraminodiphenylsulfone; 3,3',4,4'-tetraminodiphenyl ether; 3, 3', 4,4'- tetraminodiphenylmethane; 3,3',4,4'-tetraminodiphenyldimethyl-methane; mono-, di-, tri- or tetrahydrochloride salts thereof, and any combination thereof.

3. The gel membrane of claim 1 or claim 2, wherein the naphthalene dicarboxylic acid is selected from the group consisting of 1 ,4-naphthalenedicarboxylic acid; 1 ,5- naphthalenedicarboxylic acid; 2,6-naphthalenedicarboxylic acid; 2,7- naphthalenedicarboxylic acid; 1 ,8-dihydroxynaphthalene-3,6-dicarboxylic acid; and any combination thereof.

4. The gel membrane of any of the preceding claims, wherein the tetraaminobiphenyl monomer and the naphthalene dicarboxylic acid monomer together comprise about 70 mol% or more of all monomers of the polybenzimidazole.

5. The gel membrane of any of the preceding claims, further comprising a reaction product of one or more additional monomers, the one or more additional monomers comprising an aromatic tetraamino monomer; a heteroaromatic tetraamino monomer; an aromatic polycarboxylic acid or an ester, anhydride, or acid chloride thereof; a heteroaromatic polycarboxylic acid or an ester, anhydride, or acid chloride thereof; an aromatic diaminocarboxylic acid; a heteroaromatic diaminocarboxylic acid; or any combination thereof.

6. The gel membrane of any of the preceding claims, wherein the gel membrane has a solids content of from about 4 wt.% to about 40 wt.% of the gel membrane.

7. The gel membrane of any of the preceding claims, wherein the polybenzimidazole is crosslinked.

8. The gel membrane of any of the preceding claims, wherein the imbibed solution further comprises a mineral acid, an organic acid, or a mixture thereof, such as hydrochloric acid, nitric acid, fluorosulfonic acid, sulfuric acid, acetic acid, formic acid, p-toluene, sulfonic acid, trfluoromethane sulfonic acid, sodium chloride, potassium chloride, sodium hydroxide, potassium hydroxide, sodium sulfide, potassium sulfide, tetraethyl ammonium, tetrabutyl ammonium, tetrafluoroborate, perchlorate, hexafluorophosphate, or any combination thereof.

9. The gel membrane of any of the preceding claims, further comprising an organic small molecule or a particulate.

10. A fuel cell comprising the gel membrane of any of the preceding claims.

11. A method for forming a gel membrane, comprising forming a polymerization solution, the polymerization solution comprising a polyphosphoric acid and polybenzimidazole-forming monomers, the polybenzimidazole-forming monomers including a tetraaminobiphenyl monomer and a naphthalene dicarboxylic acid monomer, the tetraaminobiphenyl monomer and the naphthalene dicarboxylic acid monomer together comprising about 70 mol% or more of the polybenzimidazole-forming monomers; establishing a polymerization condition upon which the polybenzimidazole- forming monomers polymerize in the polymerization solution; and hydrolyzing at least a portion of the polyphosphoric acid and thereby inducing gelation of the gel membrane.

12. The method of claim 11 , wherein the monomer content of the polymerization solution is about 10 wt.% or less of the total weight of the polybenzimidazole-forming monomers and the polyphosphoric acid combined.

13. The method of claim 11 or claim 12, further comprising shaping a gel membrane precursor prior to the step of establishing the polymerization condition.

14. The method of any of claims 11 through 13, further comprising crosslinking the gel membrane.

15. The method of any of claims 11 through 14, further comprising imbibing the gel membrane with an ionic transport agent.

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