Functionalized Polybenzimidazole Polymers for Ionomer and Proton Exchange Membrane Applications

Abstract

The ion exchange functionalized polymer molecules include repeat units having benzimidazole units as at least a portion of the backbone, side chains, or both. The ion exchange functionalized polymer molecules also include ion exchange groups linked to the repeat units. The ion exchange groups may be tetravalent boron groups or metal fluorides, and the metal fluorides may be polyvalent metal atoms.

Description

[Technical field]

[0001] [Related Applications] This application claims priority to U.S. Provisional Patent Application No. 63 / 302,755, filed January 25, 2022, which is incorporated by reference in its entirety herein. The present disclosure relates to functionalized polybenzimidazole polymers for ionomer and proton exchange membrane applications. [Background technology]

[0002] Proton exchange membranes (PEMs) are used in electrochemical cells, such as hydrogen fuel cells and water electrolysis systems, to selectively transport protons. + ) and is impermeable to gases. PEMs are generally composed of a porous scaffold with strong acid functional groups. For example, polyfluorosulfonic acid-based electrolyte membranes contain a poly(tetrafluoroethylene) (PTFE) porous scaffold with sulfonic acid groups. The easily dissociated sulfonic acid groups act as proton transport agents in the membrane. In a hydrogen fuel cell, hydrogen gas (H2) is pumped to the anode and converted to protons (H + ) and electrons. The protons pass through the electrolyte membrane and combine with oxygen gas (O2) at the cathode to produce water, and the electrons flow through an external circuit to produce electricity. In a water electrolysis system, electricity converts water at the anode into oxygen gas (O2) and protons (H + The protons pass through the electrolyte membrane and combine with electrons at the cathode to produce hydrogen gas (H2).

[0003] A membrane electrode assembly (MEA) can include an electrolyte membrane disposed between a first catalyst layer and a second catalyst layer. The catalyst layers are conductive electrodes (anode and cathode) with embedded electrochemical catalysts, such as metals, metal alloys, or metal oxides. The catalysts may be bound to catalytic solid supports, which are typically conductive, high surface area carbons (such as graphite or graphene). The electrochemical catalysts reduce the activation energy required to carry out electrochemical reactions at the electrodes, such as the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) in water electrolysis applications, and the hydrogen oxidation reaction (HOR) and oxygen reduction reaction (ORR) in fuel cell applications.

[0004] In some applications, the catalyst layer comprises a supported catalyst mixed with an ion-conducting polymer, an ionomer. The ionomer binds the catalyst in the electrode and the catalyst layer on the PEM, providing passage for cations (e.g., protons) and improving cation conductivity. In some MEAs, the catalyst layer is formed separately from the PEM and laminated onto the PEM in an MEA stack. In other MEAs, the catalyst layer is coated onto the PEM to form catalyst-coated membranes (CCMs). Summary of the Invention [Problem to be solved by the invention]

[0005] PEMs and ionomers, and the molecular functional groups therein that carry the proton transport properties, should be robust under harsh reaction conditions of redox stress. Most of the conventional polymers used as PEMs and ionomers in water electrolysis and fuel cell applications contain sulfonic acid functional groups as proton transport agents. The sulfonic acid groups, which are easy to dissociate, function as proton transport agents in PEMs. However, water electrolysis and fuel cell applications involve strong redox chemical reactions at ambient to elevated temperatures and acidic conditions. The sulfonic acid functional groups have limited ability to withstand the redox stresses of electrochemical operations, mainly due to the intrinsic physicochemical properties of sulfur. [Means for solving the problem]

[0006] The following description presents a simplified summary of one or more embodiments of the devices, compositions, and / or methods described herein, intended to provide a basic understanding of such embodiments. This summary is not an extensive overview of all contemplated embodiments, and is not intended to identify key or critical elements of all embodiments, nor to delineate the scope of any or all embodiments. Its sole purpose is to present some concepts of one or more aspects of the devices, compositions, and / or methods described herein in a simplified form as a prelude to the more detailed description that is presented below.

[0007] In some illustrative examples, the ion exchange functionalized polymer molecules include repeat units that include benzimidazole groups as at least a portion of the backbone, side chains, or both, and ion exchange groups linked to the repeat units, the ion exchange groups being comprised of tetravalent boron groups or metal fluorides, the metal fluorides comprising polyvalent metal atoms.

[0008] In some illustrative examples, a method for preparing an ion exchange functionalized polymer molecule includes linking an ion exchange agent to a repeat unit of a polymer molecule, the polymer molecule including benzimidazole units as at least a portion of a main chain or a side chain of the polymer molecule, the ion exchange agent including a trivalent boron compound or a metal fluoride, the metal fluoride including a polyvalent metal atom.

[0009] In some exemplary embodiments, a polybenzimidazole (PBI) polymer comprises a backbone comprising benzimidazole units and ion exchange groups linked to the benzimidazole units, the ion exchange groups comprising tetravalent boron groups or metal fluoride groups, each metal fluoride group comprising a polyvalent metal atom.

[0010] In some illustrative examples, the polymer composition includes a first polymer crosslinked with a second polymer by a tetravalent boron bridge, the first polymer including a first polybenzimidazole (PBI) polymer.

[0011] In some illustrative examples, a method for crosslinking a first polymer and a second polymer with a tetravalent boron bridge comprises reacting a first polymer, a second polymer, and a crosslinker, where the first polymer comprises a first polybenzimidazole (PBI) polymer including benzimidazole units as at least a portion of the backbone, and the crosslinker comprises boric acid, boronic acid, or a derivative of boronic acid.

[0012] In order to better understand the concepts described herein, various embodiments will now be described, by way of example only, with reference to the drawings, which illustrate various embodiments and which form a part of this specification. The illustrated embodiments are merely exemplary and are not intended to limit the scope of the present disclosure. Throughout the drawings, the same or similar reference numerals refer to the same or similar elements. [Brief description of the drawings]

[0013] [Figure 1]1 shows various exemplary reaction schemes for the synthesis of ion-exchange functionalized PBI polymer molecules. [Diagram 2] 1 shows various exemplary reaction schemes for the synthesis of ion-exchange functionalized PBI polymer molecules. [Diagram 3] 1 shows various exemplary reaction schemes for the synthesis of ion-exchange functionalized PBI polymer molecules. [Figure 4A] 1 shows various exemplary reaction schemes for the synthesis of ion-exchange functionalized PBI polymer molecules. [Figure 4B] 1 shows various exemplary reaction schemes for the synthesis of ion-exchange functionalized PBI polymer molecules. [Diagram 5] 1 shows an exemplary reaction scheme for crosslinking two PBI polymer molecules using boric acid as a crosslinking agent. [Figure 6] 1 shows an exemplary reaction scheme for crosslinking PBI polymer molecules with a hydroxyl-functionalized PTFE polymer using boric acid as a crosslinking agent. [Figure 7] 1 shows an exemplary reaction scheme for crosslinking PBI polymer molecules with hydroxyl-functionalized PTFE polymer molecules using aminoboronic acid as a crosslinker. [Figure 8] 1 shows an exemplary reaction scheme for crosslinking PBI polymer molecules with PPA polymer molecules using boric acid as a crosslinking agent. [Figure 9] 1 shows an exemplary proton exchange membrane water electrolysis system incorporating an ion exchange functionalized PBI polymer. [Figure 10] 1 shows an exemplary proton exchange membrane fuel cell incorporating an ion exchange functionalized PBI polymer. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] Described herein are ion-exchange functionalized polybenzimidazole (PBI) polymers, methods for making ion-exchange functionalized PBI polymers, and methods and devices for using ion-exchange functionalized PBI polymers. As described herein, an ion-exchange functionalized polymer molecule includes a repeat unit and an ion-exchange group. The repeat unit includes a benzimidazole unit as at least a portion of the backbone, side chain, or both. The ion-exchange group is linked to the repeat unit. The ion-exchange group can be, for example, a tetravalent boron group or a metal fluoride group having a multivalent metal atom. In some examples, the ion-exchange group is directly linked to the secondary amine of the benzimidazole unit, such as by a covalent bond between the boron atom or metal atom of the ion-exchange group and the nitrogen atom of the secondary amine. In other examples, the ion-exchange group is indirectly linked to the secondary amine via a linker. In a further example, the ion-exchange group is indirectly linked to the phenyl unit of the repeat unit via a linker. The boron atoms of the tetravalent boron groups or the multivalent atoms of the metal fluoride groups have extended valence and are therefore ionic and acidic in nature and can function as ion (e.g., proton) transport agents. Thus, ion-exchange functionalized PBI polymers can be used in ionomer and PEM applications.

[0015] Also described herein is the use of tetravalent boron bridges to crosslink PBI polymers with other polymers, such as another PBI polymer, a functionalized poly(tetrafluoroethylene) (PTFE) molecule, or a poly(phosphoric acid) (PPA) polymer. In tetravalent boron bridges, the boron atom has a negative formal charge and is therefore ionic and acidic in nature and can function as an ion (e.g., proton) transport agent. Thus, crosslinked polymers with tetravalent boron bridges can be used in ionomer and PEM applications.

[0016] In the following, various definitions are provided to facilitate understanding of various aspects of the present disclosure.As used herein, each term or expression, such as alkyl, m, n, etc., when used multiple times, is intended to be independent of its definition in other parts of this disclosure.In the event of any conflict with the patent application or patent incorporated herein by reference, the present specification, including definitions, shall prevail.

[0017] As used herein, "polymer" refers to a material composed of polymer molecules of the same or different polymeric species, including mixtures of polymer molecules of the same polymeric species that may differ in chain length and / or specific structural arrangements (e.g., orientation of monomer units, irregularities in end groups, and / or position and / or length of any side chains or side groups) from other polymer molecules within the same sample. "Polymer" includes homopolymers, copolymers, terpolymers, interpolymers, and the like.

[0018] As used herein, a "polymer molecule" or "macromolecule" refers to a molecule of high relative molecular weight, the structure of which consists of a relatively large number of repeating units (e.g., about 100 or more monomeric units) that are actually or conceptually derived from molecules of lower relative molecular weight (e.g., monomeric molecules).

[0019] As used herein, "polymerization" refers to the process of converting a monomer or mixture of monomers into a polymer.

[0020] As used herein, "oligomer" refers to a substance composed of oligomeric molecules.

[0021] As used herein, "oligomer molecule" refers to a molecule of intermediate relative molecular weight, the structure of which consists of repeating relatively small units (e.g., about 10 to about 100 monomeric units) that are actually or conceptually derived from a molecule of lower relative molecular weight (e.g., a monomeric molecule).

[0022] As used herein, "oligomerization" refers to the process of converting a monomer or mixture of monomers into an oligomer.

[0023] As used herein, "ionomer" refers to a polymer composed of ionomeric molecules.

[0024] As used herein, "ionomer molecule" refers to a polymer molecule in which a small but relatively significant proportion of the constitutional units have ionizable or ionic pendant groups (including ion-exchange groups as described herein), or both. Generally, the constitutional units having ionizable or ionic pendant groups are about 15 mole percent or less.

[0025] As used herein, "monomer" refers to a substance that is made up of monomer molecules.

[0026] As used herein, a "monomer molecule" refers to a molecule that can undergo polymerization or oligomerization to form a polymeric or oligomeric molecule. A monomer molecule contributes a building block to the essential structure of a polymeric or oligomeric molecule.

[0027] As used herein, "copolymer" refers to a polymer derived from two or more types of monomers.

[0028] As used herein, "constitutional unit" refers to an atom or group of atoms (with pendant atoms or groups, if any) that constitutes part of the structure of a polymer molecule (or oligomeric molecule, block, or chain).

[0029] As used herein, "repeating unit" refers to the repeating constitutional units that make up a polymer molecule (or oligomeric molecule, block, or chain).

[0030] As used herein, "monomer unit" refers to the largest building block that a single monomer molecule contributes to the structure of a polymeric or oligomeric molecule.

[0031] As used herein, "block" refers to a portion of a polymer molecule (or oligomer molecule) that is composed of multiple constitutional units and has at least one characteristic that is not present in adjacent portions.

[0032] As used herein, "chain" refers to all or a portion of a polymer molecule (or oligomer molecule or block) consisting of a linear or branched sequence of constitutional units between two boundary constitutional units, each of which is either an end group, a branch point, or other designated feature of the polymer molecule.

[0033] As used herein, "main chain" or "backbone" refers to the chain of a polymer molecule from which all other chains (long or short or both) can be considered to be pendant (e.g., side chains).

[0034] As used herein, "side chain" refers to an oligomer (short chain) or polymer (long chain) branch off from the main chain of a polymer molecule.

[0035] As used herein, a "side group" or "pendant group" refers to a non-oligomeric or non-polymeric branch from a chain (e.g., from the main chain).

[0036] As used herein, "crosslink" refers to a small region in a polymer molecule from which at least four chains emanate. Crosslinks are generally formed by reactions involving sites or groups on existing polymer molecules or by interactions between existing polymer molecules.

[0037] The term "crosslinked" refers to a state in which previously separate polymer molecules are joined to one another, usually by covalent bonds, at points other than the terminal ends.

[0038] As used herein, an "aliphatic" compound is a hydrocarbon that is saturated or unsaturated, acyclic or cyclic, unbranched or branched, unsubstituted or substituted with one or more substituents or functional groups. As will be appreciated by those skilled in the art, "aliphatic" is intended herein to include, but is not limited to, alkyl, alkynyl, and alkynyl moieties. Exemplary aliphatic groups include, but are not limited to, for example, methyl, ethyl, n-propyl, isopropyl, allyl, n-butyl, sec-butyl, isobutyl, tert-butyl, n-pentyl, sec-pentyl, isopentyl, tert-pentyl, n-hexyl, and sec-hexyl moieties.

[0039] As used herein, the term "alkyl" is given its ordinary meaning in the art and may include straight-chain alkyl groups, branched-chain alkyl groups, cycloalkyl (alicyclic) groups, alkyl-substituted cycloalkyl groups, and saturated aliphatic groups, including cycloalkyl-substituted alkyl groups. A similar convention applies to other common terms, such as "alkenyl," "alkynyl," and the like. Furthermore, as used herein, the terms "alkyl," "alkenyl," "alkynyl," and the like, include both substituted and unsubstituted groups.

[0040] In some embodiments, a straight or branched alkyl chain may have 1-30 carbon atoms in its backbone, and in some cases, 1-20 or less. In some embodiments, a straight or branched alkyl chain may have 1-10 carbon atoms in its backbone (e.g., C1-C10 for straight chain, C3-C10 for branched chain), 6 or less carbon atoms, or 4 or less carbon atoms. A cycloalkyl may have 3-10 carbon atoms in its ring structure, and in some cases, 3-5, 6, or 7 carbon atoms in the ring structure. Examples of non-cyclic alkyl groups include, but are not limited to, methyl, ethyl, propyl, isopropyl, butyl, isobutyl, n-butyl, tert-butyl, n-pentyl, neopentyl, n-hexyl, n-heptyl, n-octyl, n-decyl, n-undecyl, dodecyl, and the like. Examples of cyclic alkyl groups include, but are not limited to, cyclopropyl, cyclobutyl, and cyclohexyl.

[0041] The terms "alkenyl" and "alkynyl" refer to unsaturated aliphatic groups analogous in length and possible substitution to the alkyls described above, but which contain at least one double or triple bond, respectively. Alkenyl groups include, but are not limited to, ethenyl, propenyl, butenyl, 1-methyl-2-buten-1-yl, and the like. Non-limiting examples of alkynyl groups include ethynyl, 2-propynyl (propargyl), 1-propynyl, and the like.

[0042] The term "heteroalkyl" refers to an alkyl group in which one or more hydrogen atoms bonded to any carbon of the alkyl group or one or more carbon atoms are replaced with a heteroatom. A heteroatom is an atom other than carbon. In some examples, the heteroatom is an atom selected from the group consisting of N, O, P, B, S, Si, Sb, Al, Sn, As, Se, and Ge. Examples of heteroalkyl groups include, but are not limited to, methoxy, ethoxy, propoxy, isopropoxy, methylthio, ethylthio, propylthio, isopropylthio, n-butylthio, methoxymethyl, and cyano.

[0043] The terms "heteroalkenyl" and "heteroalkynyl" refer to unsaturated aliphatic groups analogous in length and possible substitution to the heteroalkyls described above, but that contain at least one double or triple bond respectively.

[0044] The term "aryl" refers to unsubstituted or substituted aromatic carbocyclic groups having a single ring (e.g., phenyl), multiple rings (e.g., biphenyl), or multiple fused rings, in which at least one ring is aromatic (e.g., 1,2,3,4-tetrahydronaphthyl, naphthyl, anthryl, or phenanthryl). That is, at least one ring may have a conjugated Pi electron system, and other adjacent rings may be cycloalkyl, cycloalkenyl, cycloalkynyl, aryl, and / or heterocyclyl. "Carbocyclic aryl groups" refer to aryl groups in which the ring atoms on the aromatic ring are carbon atoms. Carbocyclic aryl groups include monocyclic carbocyclic aryl groups, and polycyclic or fused compounds (e.g., two or more adjacent ring atoms are common), such as naphthyl groups. Examples of aryl groups include, but are not limited to, phenyl, naphthyl, tetrahydronaphthyl, indanyl, and indenyl.

[0045] The term "heteroaryl" refers to an aryl group that contains at least one heteroatom as a ring atom, such as a heterocycle. Non-limiting examples of heteroaryl groups include, but are not limited to, pyridyl, pyrazinyl, pyrimidinyl, pyrrolyl, pyrazolyl, imidazolyl, thiazolyl, oxazolyl, isoxazolyl, thiadiazolyl, oxadiazolyl, thiophenyl, furanyl, quinolinyl, and isoquinolinyl.

[0046] As used herein, the term "alkoxyl" or "alkoxy group" refers to an alkyl group having an oxygen radical attached thereto and has the general formula RO. Examples of alkoxyl groups include, but are not limited to, methoxy, ethoxy, propyloxy, and tert-butoxy groups.

[0047] The term "aryloxy" refers to an aryl group having an oxygen radical attached thereto. Examples of alkoxyl groups include, but are not limited to, phenoxy groups.

[0048] Any of the above groups may be optionally substituted. Examples of substituents include, but are not limited to, aliphatic, alicyclic, heteroaliphatic, heteroalicyclic, halogen, azido, alkyl, aralkyl, alkenyl, alkynyl, cycloalkyl, hydroxyl, alkoxyl, amino, nitro, sulfhydryl, imino, amide, phosphonate, phosphinate, carbonyl, carboxyl, alkyloxycarbonyl, silyl, ether, alkylthio, heteroalkylthio, heteroarylthio, sulfonyl, sulfonamide, ketone, aldehyde, ester, heterocyclyl, aromatic or heteroaromatic moiety, -CF3, -CN, aryl, aryloxy, perhaloalkoxy, aralkoxy, heteroaryl, heteroaryloxy, heteroarylalkyl, heteroaryl. Aralkoxy, azido, amino, halide, alkylthio, oxo, acylalkyl, carboxyester, -carboxamido, acyloxy, aminoalkyl, alkylaminoaryl, alkylaryl, alkylaminoalkyl, alkoxyaryl, arylamino, aralkylamino, alkylsulfonyl, -carboxamidoalkylaryl, hydroxyalkyl, haloalkyl, alkylaminoalkylcarboxy-, aminocarboxamidoalkyl-, cyano, alkoxyalkyl, perhaloalkyl, arylalkyloxyalkyl (e.g., SO4(R')2), phosphate (e.g., PO4(R')3), silane (e.g., Si(R')4), urethane (e.g., R'O(CO)NH RFurther, the substituents are F, Cl, Br, I, -OH, -NO2, -CN, -NCO, -CF3, -CH2CF3, -CHCl2, -CH2ORx, -CH2CH2ORx, -CH2N(Rx)2, -CH2SO2CH3, -C(O)Rx, -O2(Rx), -CON(Rx)2, -OC(O)Rx, -C(O)OC(O)Rx, -OCO2Rx, -OCON(Rx)2, -N(Rx)2, -S(O)2Rx, -OCO2Rx, -NRx(CO)Rx, -NRx(CO)N(Rx)2, where each occurrence of Rx is independently hydrogen. The substituents include, but are not limited to, aliphatic, alicyclic, heteroaliphatic, heteroalicyclic, aryl, heteroaryl, alkylaryl, or alkylheteroaryl, where any of the aliphatic, alicyclic, heteroaliphatic, heteroalicyclic, alkylaryl, or alkylheteroaryl substituents described above and herein can be substituted or unsubstituted, branched or unbranched, cyclic or acyclic, and any of the aryl or heteroaryl substituents described above and herein can be substituted or unsubstituted.

[0049] Polybenzimidazole (PBI) polymers are a type of polymer made up of PBI polymer molecules. PBI polymer molecules have repeating units that contain benzimidazole units as at least a portion of the backbone. The benzimidazole units consist of benzimidazole moieties or derivatives thereof. Benzimidazoles are heteroalicyclic aromatic organic compounds with phenyl and imidazole groups sharing two carbon atoms in a ring structure. The general structure of benzimidazole is shown in formula (I) below. [ka]

[0050] An example of a PBI polymer having one benzimidazole unit per main chain repeat unit is poly(2,5-benzimidazole) (AB-PBI) shown as formula (II) below, and an example of a PBI polymer having two benzimidazole units per main chain repeat unit is poly[2,2'-(m-phenylene)-5,5'-bibenzimidazole] (m-PBI) shown as formula (III) below, and 4F-PBI (a fluorinated derivative of m-PBI) shown as formula (IV) below. [ka] [ka] [ka]

[0051] Other examples of PBI polymers include, but are not limited to, poly{2,6-(2,6-naphthylidene)-1,7-dihydrobenzo[1,2-d;4,5-d']diimidazole}, poly2,2'-(2,6-naphthylidene)-5,5'-bibenzimidazole, poly-2,2'-(2,6-pyridine)-5,5'-bibenzimidazole, poly-2,2'-(2,5-pyridine)-5,5'-bibenzimidazole, poly-2,2'-(2,2'-bipyridine ... ′)-5,5′-bibenzimidazole), poly-2,2′-(3,5-pyrazole)-5,5′-bibenzimidazole, poly-2,2′-(m-phenylene)-5,5′-bibenzimidazole, poly-2,2′-(pyridylene-3″,5″)-5,5′-bibenzimidazole, poly-2,2′-(phthalene-2″,5″)-5,5′-bibenzimidazole, poly-2,2-(naphthalene-1″,6″)-5,5′-bibenzimidazole, poly-2, 2'-(biphenylene-4",4")-5,5'-bibenzimidazole, poly-2,2'-amylene-5,5'-bibenzimidazole, poly-2,2'-octamethylene-5,5'-bibenzimidazole, poly-2,6-(m-phenylene)-diimidazolebenzene, poly-2,2'-cyclohexenyl-5,5'-di(benzimidazole)ether, poly-2,2'-(m-phenylene)-5,5-di(benzimidazole)sulfide, poly-2 ,2′-(m-phenylene)-5,5-di(benzimidazole)sulfone, poly-2,2′-(m-phenylene)-5,5-di(benzimidazole)methane, poly-2-2″-(m-phenylene)-5″.5″-(di(benzimidazole)propane)2.2, poly-2.2″-(m-phenylene)-5′5″-di(benzimidazole)ethylene-1,2, and derivatives of any of the foregoing (e.g., substituted (fluorinated) and / or branched derivatives).

[0052] In some instances, the PBI polymer is a copolymer that includes one or more additional repeat units that may or may not include benzimidazole units in the backbone, side chains, or both.

[0053] In the PBI polymer molecule, the secondary amine group of the benzimidazole unit (e.g., N of the C-NH-C group) is available for direct or indirect linking with an ion exchanger, thereby forming an ion exchange functionalized PBI polymer molecule in which the PBI polymer molecule is functionalized with an ion exchange group. In a direct link, the boron or metal atom of the ion exchange group is covalently bonded to the nitrogen atom of the secondary amine group. In some examples of indirect linking, the ion exchange group is bonded to the secondary amine through a linker (e.g., an alkyl chain). In another example of indirect linking, the ion exchange group is linked to the phenyl group of the benzimidazole unit by a sulfonic acid linker.

[0054] Next, exemplary ion-exchange functionalized PBI polymer and oligomer molecules and methods for making the ion-exchange functionalized PBI polymer and oligomer molecules are described. In the following description, discussions and references to polymer molecules apply equally to oligomer molecules, and discussions and references to polymer molecules can be extended to polymers (and oligomers) composed of the polymer molecules described herein.

[0055] Ion-exchange functionalized PBI polymers and polymer molecules are described below, however, the principles and concepts described herein are not limited to PBI polymers and polymer molecules, but may be applied to any polymer and polymer molecule having benzimidazole, imidazole, or phenyl units (in the case of indirect linkages) as at least a portion of the backbone, side chains, or both.

[0056] In some embodiments, the ion exchange functionalized PBI polymer molecule (or oligomer molecule) comprises (i) a repeat unit comprising a benzimidazole unit and, optionally, a phenyl unit, and (ii) at least one ion exchange group linked to a secondary amine of the benzimidazole unit and / or the phenyl unit.

[0057] The repeat units of the ion-exchange functionalized PBI polymer molecules contain benzimidazole units as at least a portion of the main chain, as at least a portion of the side chain, or both. The benzimidazole units consist of benzimidazole moieties or derivatives thereof, as described above with reference to formula (I). The repeat units may contain any additional moieties and / or groups that may be useful in a particular implementation. In some embodiments, the repeat units consist of the repeat units of the PBI polymer molecules described above, or derivatives thereof. In some examples, such as m-PBI (see formula (III)) and its derivatives, the repeat units also contain separate phenyl groups as at least a portion of the main chain, as at least a portion of the side chain, or both.

[0058] At least one ion exchange group is comprised of a tetravalent boron group or a metal fluoride group of a polyvalent metal atom. As used herein, "multivalent" means that an atom is not limited to a specific number of valence bonds, but may have multiple different valence states, each with a different number of valence bonds. Thus, a trivalent atom that is multivalent can "expand its valence state" from monovalent to trivalent to form a tetravalent, pentavalent, or hexavalent structure with a formal charge of negative monovalent (-1), negative divalent (-2), or negative trivalent (-3). For example, boron has three valence electrons and the ground state electron configuration is 1s. 2 2s 2 2p 1 Boron generally forms trivalent neutral compounds in which boron has three covalent bonds. Therefore, the boron atom has an empty p orbital and an sp 2 The trivalent boron compounds are hybridized, making them electron deficient. However, boron is polyvalent due to an empty p orbital, and can form negatively charged tetravalent compounds with four covalent bonds. Similarly, polyvalent metal atoms can expand their valence to form one or more additional covalent bonds, thereby gaining a formal negative charge.

[0059] The tetravalent boron group has the general formula -BX3, where each X is independently an alkyl group, an alkoxy group, an alkyloxycarbonyl group, an aryl group, an aryloxy group, a hydroxyl group, a fluoro group, a cyano group, or a pentafluorophenyl group. In embodiments where each X is a fluoro group, the tetravalent boron group is a boron trifluoride group having the general formula -BF3.

[0060] The boron atom of the tetravalent boron group is covalently bonded to each X of the tetravalent boron group and to the secondary amine group of the benzimidazole unit or to a linker chain that connects the tetravalent boron group to the secondary amine group. Therefore, the boron atom has four covalent bonds, and the boron atom is essentially ionic. As a result, the tetravalent boron group can function as an ion exchange group (e.g., a proton transport agent).

[0061] Metal fluoride groups have the general formula -MF3 or -MF4, where M is a polyvalent metal atom. As used herein, a "metal atom" can be a transition metal atom, a metal atom, or a metalloid atom. In some examples, M comprises a Group 4 metal atom (e.g., zirconium (Zr)), a Group 13 metal atom (e.g., aluminum (Al), gallium (Ga), or indium (In)), or a Group 14 metal atom (e.g., silicon (Si), germanium (Ge), or tin (Sn)).

[0062] As mentioned above, in some examples, the boron atom of the tetravalent boron group or the metal atom of the metal fluoride group is linked to the secondary amine group via a linker, such as an alkyl linker. In some examples, the linker is an alkyl group having 1 to 10 atoms, optionally substituted with one or more side groups or side chains, each of which may independently be hydrogen, a hydroxyl group, a fluoro group, a chloro group, a dialkylamino group (NR2, where R may represent hydrogen or an organic linking group, such as a methyl group (CH3)), a cyano group, a carboxylic acid group, a carboxylic acid amide group, an ester group, an alkyl group, an alkoxy group, or an aryl group.

[0063] In a further example, the boron atom of a tetravalent boron group or the metal atom of a metal fluoride group is indirectly linked to a phenyl group of a repeat unit via a linker, such as a sulfonate ester linker (e.g., a derivative of sulfonic acid). As described below, the phenyl group can be activated by sulfonation with a pendant sulfonic acid group and bonded to an ion exchange agent to form an ion exchange group.

[0064] The ion-exchange functionalized PBI polymer molecules can be synthesized by any suitable method. In some examples, the ion-exchange functionalized PBI polymer molecules are synthesized by a post-polymerization process (e.g., after synthesis of the PBI polymer molecules).

[0065] In some instances, the post-polymerization process can be carried out in a single step by reacting an ion exchange agent with the PBI polymer molecule. The ion exchange agent consists of a trivalent boron compound or a metal fluoride, which forms an ion exchange group when linked to the PBI polymer molecule.

[0066] The trivalent boron compound has the general formula BX3, where X is as defined above. In this reaction, the boron atom of the trivalent boron compound expands its valence and covalently bonds with the nitrogen atom of the secondary amine of the benzimidazole unit, thereby forming an ion exchange group in which the boron atom is directly bonded to the secondary amine of the benzimidazole unit.

[0067] Metal fluorides have the general formula MF3 or MF4, where M consists of a polyvalent metal atom as described above. In the reaction, the polyvalent metal atom of the metal fluoride expands its valence and covalently bonds with the nitrogen atom of the secondary amine of the benzimidazole unit, thus forming an ion exchange group in which the metal atom is directly linked to the secondary amine of the benzimidazole unit.

[0068] In another example, the post-polymerization step for indirect linking with secondary amines can be carried out in a multi-step process. In the first step, a linking agent is reacted with the PBI polymer molecule. The linking agent combines with the secondary amine of the benzimidazole unit and, upon linking with the PBI polymer molecule, forms an intermediate linker with a chain length in the range of 2-10 carbon atoms and exhibiting pendant acid or hydroxyl groups.

[0069] In some examples, the linking agent comprises a sulfonated compound. The sulfonated compound is an ester of sulfonic acid and has the general formula -S(=O)2OR, where R is an electron pair, hydrogen, an alkyl group, or an aryl group. The sulfonated compound is bonded to the secondary amine of the benzimidazole to form a linker chain having a terminal sulfonic acid group. Examples of sulfonated compounds include, but are not limited to, mesylates (methanesulfonate salts), triflates (trifluoromethanesulfonate salts), ethanesulfonates (esylates, esylate), tosylates (p-toluenesulfonate), benzenesulfonates (besylates), clodylates (clodylates, chlorobenzenesulfonates), camphorsulfonates (camsylates, camsylates), pipsylates (p-iodobenzenesulfonate derivatives), and the like.

[0070] In some instances, the sulfonated linking agent comprises a sultone. Sultones are cyclic sulfonic acid esters, many of which have 4-6 ring members (2-4 carbon atoms), but some have 7 or more ring members (5 or more carbon atoms). Sultones are reactive with primary and secondary amines, including the secondary amines of benzimidazoles. Examples of sultones include, but are not limited to, β-sultone, 1,3-propane sultone, 1,4-butane sultone, prop-1-ene-1,3-sultone, 3,4,4-trifluoro-3-(trifluoromethyl)oxathiethane 2,2-dioxide, 3,3,4,4-tetrafluoro-1,2-oxathiethane 2,2-dioxide, naphtho[1,8-cd]-1,2-oxathiol, 2,2-dioxide, 5-methyl-1,2-oxathiolane, 2,2-dioxide, 3-methyl-1,2-oxathiolane, 2,2-dioxide, 1,2-oxathiane, 6-dodecyl-, 2,2-dioxide, 4,6-dimethyl-1,2-oxathiine, 2,2-dioxide, acetyl fluoride, difluoro(fluorosulfonyl). Reaction of the sultone with the secondary amine of the imazole unit opens the alkyl linker chain with a terminal sulfonic acid group.

[0071] In other examples, the linking agent comprises an alkyl linking agent that reacts with a secondary amine to form a linker with a pendant acid group (e.g., a terminal carboxylic acid group, a phosphate group, a phosphonic acid group, a sulfonic acid group, a sulfate group, etc.) or a hydroxyl group (e.g., an alcohol linker). In some examples, the alkyl linking agent has a carbon chain length of 2 to 10. Exemplary alkyl linking agents include, but are not limited to, halogenated acids, alcohols, aldehydes, and ketones with pendant or terminal acid groups. In some examples, the alkyl linking agent comprises a halogenated carboxylic acid where the halogen is selected from the group consisting of chlorine, bromine, and iodine. Examples of halogenated carboxylic acids include, but are not limited to, bromoacetic acid and chloroacetic acid. The linking agent may be unsubstituted or substituted in whole or in part, as described above for linkers. In some examples, the linking agent is substituted with fluorine (e.g., tetrafluoro β-sultone).

[0072] In the second step, an ion exchanger is reacted with the pendant acid or hydroxyl groups of the intermediate linker. The ion exchanger reacts readily with the pendant acid or hydroxyl groups. The ion exchanger is as described above, and when linked to the PBI polymer molecule via the linker chain, it forms an ion exchange group.

[0073] In some examples, the alkyl linker comprises a halogenated alkylsulfonyl fluoride, such as a chloroalkylsulfonyl fluoride or a bromoalkylsulfonyl fluoride, to form a first intermediate sulfonyl fluoride-terminated alkyl linker. Prior to carrying out the second step, the sulfonyl fluoride-terminated alkyl linker is activated (by any suitable method) to a sulfonic acid, and then the second step is carried out to react the ion exchange agent with the sulfonic acid group.

[0074] In other examples, the post-polymerization process for indirectly linking ion exchange groups to the phenyl groups of the PBI polymer (e.g., m-PBI) can be carried out in a multi-step process. In the first step, the PBI polymer is sulfonated (e.g., activated with sulfonic acid) at the phenyl units to introduce intermediate pendant sulfonic acid groups. The PBI polymer can be sulfonated by any suitable method. In some examples, the PBI polymer is sulfonated by direct sulfonation by combining with a sulfonating agent such as sulfur trioxide (SO3) and / or sulfuric acid (H2SO4).

[0075] In the second step, an ion exchange agent is reacted with the pendant sulfonic acid groups, as described above. The ion exchange agent reacts readily with the pendant sulfonic acid groups, as described above, and when bonded to the PBI polymer molecule via the pendant sulfonic acid groups forms ion exchange groups.

[0076] The degree to which the PBI polymer molecules are functionalized with ion exchange groups can be controlled as desired. In some examples, the PBI polymer molecules are composed of ionomer molecules in which about 15 mole percent of the repeating units (or benzimidazole units or phenyl units) are functionalized with ion exchange groups. In a further example, about 10 mole percent of the repeating units (or benzimidazole units or phenyl units) are functionalized with ion exchange groups. In yet another example, about 5 mole percent of the repeating units (or benzimidazole units or phenyl units) are functionalized with ion exchange groups. In other examples, 15 mole percent or more of the repeating units (or benzimidazole units or phenyl units) are functionalized with ion exchange groups, such as 20 mole percent or more, 30 mole percent or more, 50 mole percent or more, or 75 mole percent or more.

[0077] In the above example, the ion-exchange functionalized PBI polymer is formed by post-polymerization functional group modification of the PBI polymer. In another example, the ion-exchange functionalized PBI polymer may be formed by pre-polymerization functional group modification of a monomer molecule consisting of a benzimidazole moiety and optionally a phenyl moiety.

[0078] Exemplary ion-exchange functionalized PBI polymers and reaction schemes for synthesizing ion-exchange functionalized PBI polymers are now shown and described with reference to Figures 1-4B. The following examples are merely illustrative and not limiting. Although the figures show only the benzimidazole units of the repeat units of the PBI polymer molecules, the repeat units may have any other units and structures as may be useful in a particular embodiment.

[0079] FIG. 1 shows an exemplary reaction scheme for the synthesis of ion-exchange functionalized PBI polymer molecules. In the first step, a tetrafluoro β-sultone linking agent reacts with the secondary amine of a benzimidazole unit to form an intermediate linker chain. The intermediate linker chain is a perfluoro 2-carbon chain with a sulfonic acid group at its end. In the second step, BF3 (in the form of BF3 etherate (Et2O)) is used to convert the sulfonic acid group to a tetravalent boron group to produce an ion-exchange functionalized PBI polymer molecule. The boron atom of the tetravalent boron group has a negative formal charge, so the ion-exchange group is ionic in nature, thereby enabling ion (e.g., proton) exchange. Although FIG. 1 shows the use of a perfluorinated sultone, in an alternative example, the sultone is not fluorinated.

[0080] FIG. 2 shows another exemplary reaction scheme for the synthesis of ion-exchange functionalized PBI polymer molecules. FIG. 2 is similar to FIG. 1, except that in FIG. 2, the linking agent is 1,3-propane sultone. In the first step, 1,3-propane sultone reacts with the secondary amine of the benzimidazole unit to form an intermediate linker chain. The intermediate linker chain is a 3-carbon chain with a sulfonic acid group at the end. In the second step, the sulfonic acid group is converted to a tetravalent boron group using BF3 (in the form of BF3 etherate). Because the boron atom of the tetravalent boron group has a negative formal charge, the ion-exchange group is ionic in nature, thereby enabling ion (e.g., proton) exchange. Although FIG. 2 shows the use of an unsubstituted sultone, in alternative examples, the sultone may be substituted, such as with one or more fluorine atoms.

[0081] FIG. 3 shows another exemplary reaction scheme for the synthesis of ion-exchange functionalized PBI polymer molecules. FIG. 3 is similar to FIG. 1, except that in FIG. 3 the linking agent is 1,4-butane sultone. In the first step, 1,4-butane sultone reacts with the secondary amine of the benzimidazole unit to form an intermediate linker chain. The intermediate linker chain is a four carbon chain with a sulfonic acid group at the end. In the second step, BF3 etherate is used to convert the sulfonic acid group to a tetravalent boron group. The boron atom of the tetravalent boron group has a negative formal charge, so the ion-exchange group is ionic in nature, thereby enabling ion (e.g., proton) exchange. Although FIG. 3 shows the use of an unsubstituted sultone, in alternative examples, the sultone may be substituted, such as with one or more fluorine atoms.

[0082] Figure 4A shows another exemplary reaction scheme for the synthesis of ion-exchange functionalized PBI polymer molecules. Figure 4 is similar to Figure 1, except that in Figure 4, the linking agent is boron trifluoride (in the form of BF3 etherate salt), which reacts directly with the secondary amines of the benzimidazole units to form ion-exchange groups (tetravalent boron groups with the general formula -BF3). The boron atoms of the ion-exchange groups carry a negative formal charge, making them ionic and acidic in nature, which allows for the exchange of ions (e.g., protons).

[0083] FIG. 4B shows another exemplary reaction scheme for the synthesis of ion-exchange functionalized PBI polymer molecules. FIG. 4B is similar to FIG. 4A, except that in FIG. 4B, the ion-exchange group is a trivalent boron compound having the general formula BXYZ, where X, Y, and Z are the same or different and each independently represent an alkyl group, an alkoxy group, an alkyloxycarbonyl group, an aryl group, an aryloxy group, a hydroxyl group, a fluoro group, a cyano group, or a pentafluorophenyl group. Boron trifluoride reacts directly with the secondary amine of the benzimidazole group to form the ion-exchange group (a tetravalent boron group having the general formula -BXYZ). The boron atom of the ion-exchange group has a negative formal charge, so that the ion-exchange group is inherently ionic and acidic, allowing for ion (e.g., proton) exchange. The acidic character of the tetravalent boron group can be adjusted by substituting some or all of the X, Y, and Z groups with highly electronegative fluorine atoms.

[0084] In other examples, the reaction schemes of Examples 1-4B can be modified by using a metal fluoride (e.g., MF3 or MF4) as the ion exchange agent instead of boron trifluoride or a trivalent boron compound. The metal fluoride can be any of the metal fluorides described above.

[0085] In some embodiments, the first polymer molecule may be crosslinked with the second polymer molecule using a tetravalent boron bridge to form a crosslinked polymer composition. The first polymer molecule may be any PBI polymer molecule, including any PBI polymer molecule described herein. In some examples, the second polymer molecule is a second PBI polymer molecule, which may be the same or different from the first PBI polymer molecule. In further examples, the second polymer molecule is a polymer molecule functionalized with an acid group (e.g., a sulfonic acid group, a carboxylic acid group, a phosphoric acid group, a phosphonic acid group) or a hydroxyl group. Examples of acid-functionalized and hydroxyl-functionalized polymer molecules include, but are not limited to, functionalized poly(tetrafluoroethylene) (PTFE) molecules, poly(phosphoric acid) (PPA) molecules, cellulose, lignin, etc.

[0086] Tetravalent boron bridges have the general formula -BX2-, where each X is different or the same and is independently a hydroxyl group or a halogen, such as fluorine or chlorine. The boron atom is covalently bonded to a secondary amine of a first PBI polymer molecule and to a secondary amine, acid group, or hydroxyl group of a second polymer molecule. In this configuration, the boron atom is tetravalent and formally negatively charged, making the tetravalent boron bridge ionic and acidic in nature. Thus, the tetravalent boron bridge can also function as an ion (e.g., cation) exchange group.

[0087] In some instances, a method of crosslinking a PBI polymer molecule with a second polymer molecule includes combining a first PBI polymer molecule, a second polymer molecule, and a crosslinker comprising a trivalent boron compound.

[0088] Any suitable crosslinking agent that produces a tetravalent boron bridge can be used. In some examples, the crosslinking agent is boric acid having the general formula B(OH)3, or a boronic acid having the general formula RB(OH)2, or a derivative of boronic acid. Exemplary crosslinking agents include, but are not limited to, trimethyl borate, triethyl borate, tributyl borate, n-octyl borate, tridecyl borate, tritetradecyl borate, triisopropyl borate, tris(hexafluoroisopropyl) borate, trimethoxycyclotriboroxane, triphenyl borate, tri-o-tolylborate, tris(trimethylsilyl)borate, tetraacetyl diborate, tris(2,2,2-trifluoroethyl)borate, bispinacol diboronate, pinacol boronate, allylboronic acid pinacol ester, diisopropoxymethylborane, and the like. The boron atom of the crosslinker covalently bonds to a secondary amine of the first PBI polymer molecule and to a secondary amine, acid group, or hydroxyl group of the second polymer molecule. Specifically, the boron atom expands its valence to covalently bond to a nitrogen of a secondary amine of the first PBI polymer molecule. The boron atom also reacts with a nitrogen atom of a secondary amine, acid group, or hydroxyl group of the second polymer molecule by a substitution reaction.

[0089] In some instances, fluorine treatment may be performed after crosslinking to substitute fluorine groups in place of hydroxyl groups.

[0090] Illustrative examples of crosslinked PBI polymer molecules and reaction schemes for crosslinking a PBI polymer molecule with another polymer molecule are now shown and described with reference to Figures 5-7. The following examples are merely illustrative and not limiting. Although only the benzimidazole units of the repeat units of the PBI polymer molecules are shown in the figures, the repeat units may have any other units and structures as may be useful in a particular embodiment.

[0091] FIG. 5 shows an exemplary reaction scheme for the intramolecular crosslinking of two PBI polymer molecules using boric acid (B(OH)3) as a crosslinking agent. The reaction scheme of FIG. 5 can be carried out to crosslink a PBI polymer layer or sheet. As a first step, a PBI polymer molecule is bonded with boric acid. The boric acid reacts with a secondary amine of the PBI polymer molecule to form a tetravalent boron bridge having the general formula -B(OH)2-. Because the boron atom has four covalent bonds, it acquires a negative formal charge, which makes the tetravalent boron bridge ionic and acidic in nature and capable of functioning as an ion (e.g., cation) transport agent. In some instances, boric acid is the limiting reagent for controlling the degree of crosslinking and the proton exchange capacity of the resulting ion exchange functionalized PBI polymer composition.

[0092] In an optional second step, the acidity of the tetravalent boron crosslinks is increased by fluoride treatment, which can be done in any suitable manner, such as by combining the crosslinked PBI polymer molecules with sodium fluoride (NaF).

[0093] 6 shows an exemplary reaction scheme for intramolecular crosslinking of PBI polymer molecules with hydroxy-functionalized PTFE polymer using boric acid as a crosslinking agent. As shown, PTFE (represented by an open circle) is functionalized with acid groups represented by X-OH, where X is a substituent that includes a sulfur (S), carbon (C), or phosphorus (P) atom covalently bonded to the oxygen (O) atom of the hydroxyl group. The acid group X-OH can be any suitable acid group, such as, but not limited to, a carboxylic acid group, a sulfonic acid group, a phosphoric acid group, a phosphonic acid group, or an alcohol. In some examples, the substituent group X is a C1-C 30It includes an alkyl linker chain, and optionally has one or more pendant moieties, which may be the same or different, and each may be independently selected from the group consisting of hydrogen, hydroxyl, fluoro, chloro, dialkylamino, cyano, carboxylic acid, carboxylic acid amide, ester, alkyl, alkoxy, and aryl. In some instances, the substituent X can be omitted, such that the PTFE is functionalized with only hydroxyl groups. Although FIG. 6 shows the PTFE functionalized with only one acid group, X-OH, the PTFE may have any degree of functionalization as is useful in a particular implementation.

[0094] In the first step, the PBI polymer molecules and the acid-functionalized PTFE polymer molecules are combined with boric acid. The boric acid reacts with the secondary amines of the first PBI polymer molecule and the hydroxyl groups of the functionalized PTFE to form tetravalent boron crosslinks having the general formula -B(OH)2-. Because the boron atom has four covalent bonds, it acquires a negative formal charge, which makes the tetravalent boron crosslinks ionic and acidic in nature and capable of functioning as ion (e.g., cation) transport agents. In some instances, boric acid is the limiting reagent for controlling the degree of crosslinking and the proton exchange capacity of the resulting ion exchange functionalized PBI / PTFE polymer composition.

[0095] In an optional second step, the acidity of the tetravalent boron crosslinks is increased by fluoride treatment, such as by combining the crosslinked PBI / PTFE polymer molecules with sodium fluoride (NaF).

[0096] Although FIG. 6 shows the PBI polymer molecules crosslinked with functionalized PTFE polymer molecules, the PBI polymer molecules may be crosslinked in a similar manner with any other suitable acid or hydroxy functionalized polymer, such as synthetic polymers (e.g., polyphosphoric acid (PPA)) or natural polymers (e.g., lignin, cellulose, chitin, etc.). For example, a PBI polymer may be crosslinked with a PPA-loaded polymer. The crosslinked PBI polymer prevents or reduces leaching of the PPA dopant from the PPA-loaded polymer.

[0097] In the examples of Figures 5 and 6, boric acid is used to crosslink two polymer molecules. In a modification of these reaction schemes, the two polymer molecules are crosslinked with a boric acid derivative, such as a boronic acid (e.g., (RB(OH)2 or (R 1 -B(OH)-R 2 )) or may be crosslinked with trivalent boron having the general formula BXYZ as described above with respect to FIG. 4B.

[0098] FIG. 7 shows an exemplary reaction scheme for crosslinking PBI polymer molecules with functionalized PTFE polymer molecules using aminoboronic acid as the crosslinking agent.

[0099] In the first step 702, sulfonyl fluoride functionalized PTFE 704 (representing the PTFE backbone as an open circle) is coupled with 4-aminophenylboronic acid 706. The sulfonyl fluoride functionalized PTFE 704 forms a strong covalent bond with the aminoboronic acid to form an intermediate aminoboronic acid functionalized PTFE polymer molecule 708. A non-aqueous base can optionally be added to scavenge the hydrogen fluoride (HF), which helps shift the reaction equilibrium to the right (product side). It will be appreciated that any other suitable aminoboronic acid or derivatives thereof may be used, including but not limited to aminophenyl, aminoaryl, and aminoalkylboronic acids, or boronic acid surrogates such as boronic acid pinacolates.

[0100] In a second step 710, the aminoboronic acid functionalized PTFE polymer molecule 708 is coupled with a PBI polymer molecule 712. The boronic acid groups of the aminoboronic acid functionalized PTFE polymer molecule 708 react with the secondary amines of the PBI polymer molecule 712, thereby crosslinking the PBI polymer molecule 708 with the functionalized PTFE polymer molecule 704. The boron atom of the tetravalent boron bridge has four covalent bonds and therefore acquires a negative formal charge, thereby making the tetravalent boron bridge ionic and acidic in nature and capable of functioning as an ion (e.g., cation) transport agent.

[0101] In an optional third step 714, the acidity of the tetravalent boron crosslinkers can be increased by fluoride treatment, such as by combining the crosslinked polymer molecules with sodium fluoride (NaF).

[0102] Although FIG. 7 shows the PBI polymer molecules 712 being crosslinked with functionalized PTFE polymer molecules 704, the PBI polymer molecules 712 may be crosslinked in a similar manner with any other suitable activated polymer molecules, including but not limited to, synthetic or natural polymer molecules.

[0103] As shown in Figure 7, the reaction scheme begins with a sulfonyl fluoride functionalized PTFE polymer molecule 704. In other examples (not shown), the reaction scheme may begin with a sulfonic acid functionalized PTFE polymer molecule (or other polymer molecule). However, because sulfonic acid groups generally do not directly covalently bond with the primary amine of an aminoboronic acid, the sulfonic acid groups are activated to sulfonyl fluoride groups (SO2F) for reaction with the primary amine of an aminoboronic acid. Alternatively, the sulfonic acid groups can be activated to sulfonyl chloride groups (SO2Cl) for reaction with the primary amine of an aminoboronic acid.

[0104] 8 shows an exemplary reaction scheme for intramolecular crosslinking of PBI and PPA polymer molecules using boric acid as a crosslinking agent. PPA has the general structure shown below as formula (V). [ka] In some examples, PPA is used as a dopant to improve the cationic conductivity of PBI polymers, such as 4F-PBI. However, direct binding of PPA to PBI occurs only through a weak equilibrium acid-base interaction between the acidic phosphate of PPA and the basic imidazole nitrogen atoms of PBI. As a result, the acidic phosphate residues are leached out of the PEM and ionomer, decreasing the cation exchange performance of the PBI polymer. To improve the performance of PBI / PPA polymers, the PBI polymer may be crosslinked with the PPA polymer using a tetravalent boron bridge, as shown in the example in Figure 8. Note that in the example in Figure 8, only the repeat units of the PPA polymer molecule are shown. However, the degree of crosslinking can be controlled as desired.

[0105] As shown in FIG. 8, in the first step, the PBI and PPA polymer molecules are bonded with boric acid. The boric acid reacts with the secondary amines of the PBI polymer molecules and the hydroxyl groups of the PPA polymer molecules to form tetravalent boron crosslinks having the general formula -B(OH)2-. In addition to the two hydroxyl groups, the boron atom also bonds with the nitrogen of the secondary amine and the oxygen atom of the PPA molecule. Thus, the boron atom has four covalent bonds and acquires a negative formal charge. As a result, the tetravalent boron crosslinks are ionic and acidic in nature and can function as ion (e.g., cation) transport agents. Furthermore, crosslinking through the tetravalent boron atoms occurs through multiple boron-oxygen bonds, thus strongly binding the PPA, imparting robustness, and minimizing leaching of the PPA. In some instances, boric acid is the limiting reagent for controlling the degree of crosslinking as well as the proton exchange capacity of the resulting ion exchange functionalized PBI / PPA composition.

[0106] In an optional second step, the acidity of the tetravalent boron crosslinks is increased by fluoride treatment, such as by combining the crosslinked PBI / PPA polymer molecules with sodium fluoride (NaF).

[0107] In the example of FIG. 8, the PBI and PPA polymer molecules are crosslinked using boric acid. In a modification of this reaction scheme, the PBI and PPA polymer molecules are crosslinked using a boric acid derivative, such as a boronic acid (e.g., (RB(OH)2 or (R 1 -B(OH))-R 2 ) or may be crosslinked with trivalent boron having the general formula BXYZ as described above with respect to FIG. 4B.

[0108] The ion exchange functionalized PBI polymers and polymer compositions described herein, including tetravalent boron crosslinked polymer compositions, may be formed as porous polymer networks and / or used as ionomers, membranes, and / or PEMs. The polymers, ionomers, and membranes (e.g., PEMs) described herein may be used in water electrolysis systems and fuel cells, as well as batteries (e.g., as separation membranes), and in the production of ammonia (e.g., for the production of H2 as a precursor to the Haber-Bosch process, and / or for the separation of nitrogen gas from air, and / or for the generation of electricity used during the Haber-Bosch process). Exemplary applications of the described ion exchange functionalized PBI polymers are now described with reference to Figures 9 and 10.

[0109] 9 illustrates an exemplary proton exchange membrane water electrolysis system 900 (PEM water electrolysis system 900). The PEM water electrolysis system 900 uses electricity to split water into oxygen (O2) and hydrogen (H2) via an electrochemical reaction. The configuration of the PEM water electrolysis system 900 is exemplary only and not limiting, as other suitable configurations and other suitable water electrolysis systems may incorporate a boron-containing porous membrane.

[0110] As shown in Figure 9, PEM water electrolysis system 900 includes a membrane electrode assembly 902 (MEA 902), porous transport layers 904-1 and 904-2, bipolar plates 906-1 and 906-2, and a power source 908. PEM water electrolysis system 900 may also include additional or alternative components not shown in Figure 9, as may be useful in a particular implementation.

[0111] The MEA 902 includes a PEM 910 disposed between a first catalyst layer 912-1 and a second catalyst layer 912-2. The PEM 910 converts protons (H + ), and is impermeable to gases such as hydrogen and oxygen, while electrically isolating the first catalyst layer 912-1 from the second catalyst layer 912-2. The PEM 910 may be implemented by any suitable PEM, including any PEM described herein. For example, the PEM 910 may be implemented by an ion-exchange functionalized PBI polymer described herein.

[0112] The first catalyst layer 912-1 and the second catalyst layer 912-2 are conductive electrodes embedded with an electrochemical catalyst (not shown), such as platinum, ruthenium, and / or cerium (IV) oxide. In some examples, the first catalyst layer 912-1 and the second catalyst layer 912-2 are formed using an ionomer to bind the catalyst nanoparticles. The ionomer used to form the first catalyst layer 912-1 and the second catalyst layer 912-2 may include an ion-exchange functionalized PBI polymer as described herein.

[0113] The MEA 902 is disposed between porous transport layers 904-1 and 904-2, which are disposed between bipolar plates 906-1 and 906-2, with channels 914-1 and 914-2 disposed between the bipolar plates 906 and the porous transport layer 904.

[0114] In the MEA 902, the first catalyst layer 912-1 functions as the anode and the second catalyst layer 912-2 functions as the cathode. When power 908 is applied to the PEM water electrolysis system 900, the oxygen evolution reaction (OER) occurs at the anode 912-1 and is represented by the following electrochemical half-reaction: 2H2O→O2+4H + +4e -

[0115] Protons are conducted from the anode 912-1 to the cathode 912-2 through the PEM 910, and electrons are conducted from the anode 912-1 to the cathode 912-2 by a conductive path around the PEM 910. The PEM 910 transports protons (H + ) and water transport, but is impermeable to oxygen and hydrogen. At the cathode 912-2, protons combine with electrons in the hydrogen evolution reaction (HER), which is represented by the following electrochemical half-reaction: 4H + +4e - →2H2

[0116] The OER and HER are two complementary electrochemical reactions for splitting water by electrolysis and are represented by the overall water electrolysis reaction: 2H2O → 2H2+O2

[0117] FIG. 10 illustrates an exemplary proton exchange membrane fuel cell 1000 (PEM fuel cell 1000) that includes a boron-containing porous membrane. The PEM fuel cell 1000 produces electricity as a result of an electrochemical reaction. In this example, the electrochemical reaction involves reacting hydrogen gas (H2) with oxygen gas (O2) to produce water and electricity. The configuration of the PEM fuel cell 1000 is exemplary only and not limiting, as other suitable configurations and other suitable proton exchange membrane fuel cells may incorporate a boron-containing porous membrane.

[0118] As shown in Figure 10, the PEM fuel cell 1000 includes a membrane electrode assembly 1002 (MEA 1002), porous transport layers 1004-1 and 1004-2, and bipolar plates 1006-1 and 1006-2. An electrical load 1008 may be electrically connected to the MEA 1002 and driven by the PEM fuel cell 1000. The PEM fuel cell 1000 may also include additional or alternative components not shown in Figure 10 as may be conducive to a particular implementation.

[0119] The MEA 1002 includes a PEM 1010 disposed between a first catalyst layer 1012-1 and a second catalyst layer 1012-2. The PEM 1010 converts protons (H + ), and is impermeable to gases such as hydrogen and oxygen, while electrically insulating the first catalyst layer 1012-1 from the second catalyst layer 1012-2. The PEM 1010 may be implemented by any suitable PEM, including any PEM described herein. For example, the PEM 1010 may be implemented by an ion-exchange functionalized PBI polymer described herein.

[0120] The first catalyst layer 1012-1 and the second catalyst layer 1012-2 are conductive electrodes with embedded electrochemical catalysts (not shown). In some examples, the first catalyst layer 1012-1 and the second catalyst layer 1012-2 are formed using an ionomer to bond the catalyst nanoparticles. In some examples, the ionomer used to form the first catalyst layer 1012-1 and the second catalyst layer 1004-2 comprises an ion-exchange functionalized PBI polymer as described herein.

[0121] The MEA 1002 is disposed between porous transport layers 1004-1 and 1004-2, which are disposed between bipolar plates 1006-1 and 1006-2, with flow channels 1014 disposed between the porous transport layers and the bipolar plates. In the MEA 1002, the first catalyst layer 1012-1 functions as a cathode and the second catalyst layer 1012-2 functions as an anode. The cathode 1012-1 and anode 1012-2 are electrically connected to a load 1008, such that electricity generated by the PEM fuel cell 1000 powers the load 1008.

[0122] During operation of the PEM fuel cell 1000, hydrogen gas (H2) flows into the anode side of the PEM fuel cell 1000 and oxygen gas (O2) flows into the cathode side of the PEM fuel cell 1000. At the anode 1012-2, hydrogen molecules are catalytically converted into protons (H + ) and electrons (e - ) 2H2→4H + +4e - Protons are conducted from the anode 1012-2 through the PEM 1010 to the cathode 1012-1, and electrons are conducted from the anode 1012-2 through the conductive path and the load 1008 to the surroundings of the PEM 1010 to the cathode 1012-1. At the cathode 1012-1, the protons and electrons combine with oxygen gas according to the oxygen reduction reaction (ORR): O2+4H + +4e - →2H2O The overall electrochemical reaction in the PEM fuel cell 1000 is therefore: 2H2+O2→2H2O

[0123] In an overall reaction, the PEM fuel cell 1000 produces water at the cathode 1012-1. The water flows from the cathode 1012-1 through the PEM 1010 to the anode 1012-2 and may be removed through an outlet on the cathode and / or anode side of the PEM fuel cell 1000. The overall reaction produces electrons at the anode that power a load 1008.

[0124] In the preceding description, various exemplary embodiments have been described with reference to the accompanying drawings. However, it will be apparent that various modifications and changes may be made thereto, and additional embodiments may be implemented, without departing from the scope of the following claims. For example, certain features of one embodiment described herein may be combined with or substituted for features of another embodiment described herein. The specification and drawings are therefore to be regarded in an illustrative rather than restrictive sense.

Claims

1. 1. An ion-exchange functionalized polymer molecule comprising: repeat units comprising benzimidazole units as at least a portion of the main chain, side chain, or both; and an ion exchange group linked to the repeating unit, the ion exchange group being composed of a tetravalent boron group or a metal fluoride, the metal fluoride being composed of a polyvalent metal atom; 1. An ion-exchange functionalized polymer molecule comprising:

2. 2. The ion-exchange functionalized polymer molecule of claim 1, wherein the repeating repeat unit comprises 4F-PBI.

3. 2. The ion-exchange functionalized polymer molecule of claim 1, wherein the ion-exchange groups comprise tetravalent boron groups.

4. 4. The ion-exchange functionalized polymer molecule of claim 3, wherein the tetravalent boron group comprises a boron trifluoride group.

5. 4. The ion exchange functionalized polymer molecule of claim 3, wherein the tetravalent boron group has the general formula -BX ３ where each X independently comprises an alkyl group, an alkoxy group, an alkyloxycarbonyl group, an aryl group, an aryloxy group, a hydroxyl group, a fluoro group, a cyano group, or a pentafluorophenyl group.

6. 4. An ion-exchange functionalized polymer molecule according to claim 3, wherein the boron atom of the tetravalent boron group is covalently bonded to a nitrogen atom of the secondary amine of the benzimidazole unit.

7. 2. The ion-exchange-functionalized polymer molecule of claim 1, wherein the ion-exchange groups comprise a metal fluoride.

8. 8. The ion-exchange functionalized polymer molecule of claim 7, wherein the polyvalent metal atom comprises a Group 4 metal, a Group 13 metal, or a Group 14 metal.

9. 8. The ion exchange functionalized polymer molecule of claim 7, wherein the polyvalent metal atom comprises zirconium (Zr), aluminum (Al), gallium (Ga), indium (In), silicon (Si), germanium (Ge), or tin (Sn).

10. 8. The ion-exchange functionalized polymer molecule of claim 7, wherein the polyvalent metal atom is covalently bonded to a nitrogen atom of a secondary amine.

11. 2. The ion-exchange functionalized polymer molecule of claim 1, wherein the boron atom of the tetravalent boron group or the polyvalent metal atom is covalently bonded to a nitrogen atom of a secondary amine of the benzimidazole unit.

12. 2. The ion-exchange functionalized polymer molecule of claim 1, wherein the ion-exchange group is attached to a phenyl unit of a repeat unit.

13. 2. The ion-exchange functionalized polymer molecule of claim 1, wherein the ion-exchange group is linked to the secondary amine of the benzimidazole unit via a linker.

14. 14. The ion-exchange functionalized polymer molecule of claim 13, wherein the linker comprises an alkyl chain having from 2 to 10 atoms in the backbone of the alkyl chain.

15. 15. The ion-exchange-functionalized polymer molecule of claim 14, wherein the linker is fluorinated.

16. 1. A method for producing ion-exchange functionalized polymer molecules comprising the steps of: linking the ion exchange agent to a repeat unit of a polymer molecule; the polymer molecule comprises a benzimidazole unit as at least a portion of the backbone or side chain of the polymer molecule; and The ion exchange agent includes a trivalent boron compound or a metal fluoride, and the metal fluoride includes a polyvalent metal atom. A method comprising the step of linking said ion exchange agent with a repeating unit.

17. 17. The method of claim 16, wherein the polymer molecules comprise polybenzimidazole (PBI) polymer molecules.

18. The method of claim 17, wherein the PBI polymer molecules comprise 4F-PBI.

19. 17. The method of claim 16, wherein the ion exchange agent comprises a trivalent boron compound.

20. 20. The method of claim 19, wherein the trivalent boron compound comprises boron trifluoride.

21. 20. The method of claim 19, wherein the trivalent boron compound has the general formula -BX ３ wherein each X independently comprises an alkyl group, an alkoxy group, an alkyloxycarbonyl group, an aryl group, an aryloxy group, a hydroxyl group, a fluoro group, a cyano group, or a pentafluorophenyl group.

22. 20. The method of claim 19, wherein the step of linking the ion exchange agent to the repeat unit comprises covalently bonding a boron atom of the trivalent boron compound to a nitrogen atom of a secondary amine of the benzimidazole unit.

23. 17. The method of claim 16, wherein the ion exchange agent comprises the metal fluoride.

24. 24. The method of claim 23, wherein the polyvalent metal atom comprises a Group 4 metal, a Group 13 metal, or a Group 14 metal.

25. 24. The method of claim 23, wherein the polyvalent metal atom comprises zirconium (Zr), aluminum (Al), gallium (Ga), indium (In), silicon (Si), germanium (Ge), or tin (Sn).

26. 24. The method of claim 23, wherein linking the ion exchange agent to the repeat unit comprises covalently bonding the polyvalent metal atom of the metal fluoride to a nitrogen atom of a secondary amine of the benzimidazole unit.

27. 17. The method of claim 16, wherein the step of linking the ion exchanger to the repeat unit comprises: forming an intermediate linker linked to the secondary amine of said benzimidazole unit, said intermediate linker comprising a pendant acid group or a pendant hydroxyl group; forming an intermediate linker; and reacting the ion exchange agent with the pendant acid groups or the pendant hydroxyl groups; A method comprising:

28. 28. The method of claim 27, wherein the pendant acid groups comprise sulfonic acid groups.

29. 28. The method of claim 27, wherein the intermediate linker comprises an alkyl chain having from 2 to 10 atoms in the backbone of the alkyl chain.

30. 30. The method of claim 27, wherein forming the intermediate linker comprises coupling a linking agent to the secondary amine.

31. 31. The method of claim 30, wherein the linking agent comprises a sulfonated compound.

32. 31. The method of claim 30, wherein the linking agent comprises a sultone.

33. 17. The method of claim 16, wherein the step of linking the ion exchanger to the repeating unit comprises: forming an intermediate linker comprising a pendant sulfonic acid group linked to a phenyl unit of said repeat unit; and reacting the ion exchange agent with the pendant sulfonic acid groups of the intermediate linker; A method comprising:

34. A polybenzimidazole (PBI) polymer, a backbone consisting of benzimidazole units, and an ion exchange group linked to the benzimidazole unit; Equipped with The ion exchange group contains a tetravalent boron group or a metal fluoride group, and each metal fluoride group is composed of a polyvalent metal atom; PBI polymer.

35. 35. The PBI polymer of claim 34, further comprising a side chain attached to the secondary amine of the benzimidazole unit; The ion exchange group is contained in a side chain. PBI polymer.

36. 35. A PBI polymer according to claim 34, wherein the PBI polymer ion exchange groups comprise the tetravalent boron groups bonded to the secondary amines of the benzimidazole units.

37. 35. The PBI polymer of claim 34, wherein the ion exchange groups comprise the metal fluoride groups linked to secondary amines of the benzimidazole units.

38. 1. A polymer composition comprising a first polymer crosslinked with a second polymer by a tetravalent boron bridge, the first polymer comprising a first polybenzimidazole (PBI) polymer.

39. 40. The polymeric composition of claim 38, wherein the first PBI polymer comprises 4F-PBI.

40. 40. The polymeric composition of claim 38, wherein the second polymer comprises a second PBI polymer.

41. 40. The polymer composition of claim 38, wherein the second polymer comprises an acid-functionalized or hydroxyl-functionalized polymer.

42. 42. The polymer composition of claim 41, wherein the second polymer comprises a poly(phosphoric acid) (PPA) polymer.

43. 40. The polymer composition of claim 38, wherein the tetravalent boron crosslinks comprise boron atoms covalently bonded to nitrogen atoms of benzimidazole units of the first PBI polymer.

44. 40. The polymeric composition of claim 38, wherein the tetravalent boron bridge is fluorinated.

45. 1. A method for crosslinking a first polymer and a second polymer with a tetravalent boron bridge, comprising: reacting the first polymer, the second polymer, and a crosslinker; the first polymer comprises a first polybenzimidazole (PBI) polymer comprising benzimidazole units as at least a portion of the backbone; and The crosslinker comprises boric acid, boronic acid, or a derivative of boronic acid. The reacting step comprises: method.

46. 46. ​​The method of claim 45, wherein the first PBI polymer comprises 4F-PBI.

47. 46. ​​The method of claim 45, wherein the second polymer comprises a second PBI polymer.

48. 46. ​​The method of claim 45, wherein the second polymer comprises an acid-functionalized or hydroxyl-functionalized polymer.

49. 50. The method of claim 48, wherein the second polymer comprises a poly(phosphoric acid) (PPA) polymer.

50. 46. ​​The method of claim 45, wherein the tetravalent boron crosslinks comprise boron atoms covalently bonded to nitrogen atoms of benzimidazole units of the first PBI polymer.

51. 46. ​​The method of claim 45 further comprising: The method comprises a step of subjecting the tetravalent boron-bridged product to a fluoride treatment.

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