Boronic acid-functionalized anion exchange polymers

Abstract

Boronic acid-functionalized anion exchange polymers have a main chain and tetra-coordinated boronic acid groups in side chains, side groups, or crosslinks. Tetra-coordinated boronic acid groups have a tetra-coordinated boron atom covalently bonded to two or three hydroxyl groups and to the polymer main chain, a side chain, or a side group. The tetra-coordinated boronic acid groups serve as hydroxide transport agents by a process known as reconstruction. Boronic acid-functionalized anion exchange polymers may be used as anion exchange membranes.

Description

BORONIC ACID-FUNCTIONALIZED ANION EXCHANGE POLYMERSRELATED APPLICATIONS

[0001] This application claims priority to U.S. Provisional Patent Application No. 63 / 624,533, filed January 24, 2024, which is hereby incorporated by reference in its entirety.BACKGROUND INFORMATION

[0002] Anion exchange membranes (AEMs) are semipermeable membranes that are engineered to selectively transport anions while being impermeable to gases. Alkaline anion exchange membranes (AAEMs) are AEMs containing alkaline anions such as hydroxide (OH-), carbonate (CO32-), and bicarbonate (HCO3-) ions. AEMs may be used in electrochemical applications, such as fuel cells and water electrolysis systems, among other applications.

[0003] Conventional AEMs are generally composed of a porous polymer framework with alkaline functional groups. However, conventional AEMs have a number of disadvantages. For example, tetraalkylammonium-based AEMs and electrolytes require exchange of chloride anions (Cl ) with hydroxyl anions (OH ) before use. Other conventional AEM technologies require strong alkaline conditions, which compromises the lifetime of electrolyzers due to unwanted reactions of hydroxyl anions under strong alkaline conditions, including carbonate formation with carbon dioxide (CO2).SUMMARY

[0004] The following description presents a simplified summary of one or more aspects of the apparatuses, compositions, and / or methods described herein in order to provide a basic understanding of such aspects. This summary is not an extensive overview of all contemplated aspects and is intended to neither identify key or critical elements of all aspects nor delineate the scope of any or all aspects. Its sole purpose is to present some concepts of one or more aspects of the apparatuses, compositions, and / or methods described herein in a simplified form as a prelude to the more detailed description that is presented below.

[0005] In some illustrative examples, a method comprises: conducting hydroxide ions from a first side of an anion exchange membrane to a second side of the anion exchange membrane by a reconstruction process, wherein the anion exchange membrane includes a boronic acid- functionalized anion exchange polymer.

[0006] In some illustrative examples, a method of making an anion exchange polymercomprises: hydroxylating a boronic acid-functionalized polymer including a trivalentboronic acid group to convert the trivalent boronic acid group to a tetra-coordinated boronic acid group.

[0007] In some illustrative examples, a method of making an anion exchange polymer comprises: crosslinking a polybenzimidazole (PBI) polymer with a second polymer using a boronic crosslinking agent, wherein the boronic crosslinking agent includes boric acid or a boronic acid having the general formula R-B(OH)2.

[0008] In some illustrative examples, an anion exchange polymer comprises: a main chain; and pendant tetra-coordinated boronic acid groups in at least one of side chains or side groups, wherein the pendant tetra-coordinated boronic acid groups have the general formula (I) or (II):

[0009] In some illustrative examples, a membrane electrode assembly comprises: a first catalyst layer; a second catalyst layer; and an anion exchange membrane positioned between the first catalyst layer and the second catalyst layer; wherein at least one of the first catalyst layer, the second catalyst layer, or the anion exchange membrane includes a boronic acid- functionalized anion exchange polymer including: a main chain; and a tetra-coordinated boronic acid group in at least one of a side chain, a side group, or a crosslink.BRIEF DESCRIPTION OF THE DRAWINGS

[0010] The accompanying drawings illustrate various embodiments and are a part of the specification. The illustrated embodiments are merely examples and do not limit the scope of the disclosure. Throughout the drawings, identical or similar reference numbers designate identical or similar elements.

[0011] FIG. 1 A shows an illustrative reaction scheme for synthesis of a boronic acid- functionalized PBI polymer.

[0012] FIG. 1 B shows an illustrative reaction scheme for synthesis of a boronic acid- functionalized PBI polymer by borylation.

[0013] FIG. 2A shows an illustrative reaction scheme for synthesis of a boronic acid- functionalized polystyrene polymer by borylation.

[0014] FIG. 2B shows an illustrative reaction scheme for synthesis of a boronic acid- functionalized polymer using a vicinal diol.

[0015] FIG. 3A shows an illustrative reaction scheme for synthesis of a boronic acid- functionalized polymer by coupling a boronic acid group with a pendant sulfonic acid group by a sulfonamide link.

[0016] FIG. 3B shows an illustrative reaction scheme for synthesis of a boronic acid- functionalized polymer by coupling a boronic acid group with a pendant sulfonic acid group by a sulfone link.

[0017] FIG. 4 shows an illustrative reaction scheme for synthesis of a boronic acid- functionalized PCTFE polymer using PCTFE as a starting material.

[0018] FIG. 5 shows another illustrative reaction scheme for synthesis of boronic acid- functionalized PCTFE polymer using PCTFE as a starting material.

[0019] FIG. 6A shows an illustrative reaction scheme for synthesis of a boronic acid- functionalized anion exchange polymer using a boronic acid-functionalized polymer as a starting material.

[0020] FIG. 6B shows an illustrative reaction scheme for hydroxide anion exchange using the boronic acid-functionalized anion exchange polymer produced by the reaction scheme of FIG. 6A.

[0021] FIG. 7A shows another illustrative reaction scheme for synthesis of a boronic acid- functionalized polymer by post-polymerization functional modification of a polymer having a vicinal diol in a side chain or side group.

[0022] FIG. 7B shows an illustrative reaction scheme for synthesis of a boronic acid- functionalized anion exchange polymer using the boronic acid-functionalized polymer produced by the reaction scheme of FIG. 7A.

[0023] FIG. 7C shows an illustrative reaction scheme for hydroxide anion exchange using the boronic acid-functionalized anion exchange polymer produced by the reaction scheme of FIG. 7B.

[0024] FIG. 8 shows an illustrative reaction scheme for intramolecular crosslinking of two PBI polymer molecules using boric acid as the crosslinking agent.

[0025] FIG. 9 shows an illustrative reaction scheme for intramolecular crosslinking of a PBI polymer molecule with a hydroxyl-functionalized PTFE polymer using boric acid as the crosslinking agent.

[0026] FIG. 10 shows an illustrative reaction scheme for crosslinking a PBI polymer molecule with a functionalized PTFE polymer molecule using an aminoboronic acid as the crosslinking agent.

[0027] FIG. 11 shows an illustrative reaction scheme for intramolecular crosslinking of a PBI polymer molecule with a PPA polymer molecule using boric acid as the crosslinking agent.

[0028] FIG. 12 shows an illustrative anion exchange membrane water electrolysis system.

[0029] FIG. 13 shows an illustrative anion exchange membrane fuel cell.DETAILED DESCRIPTION

[0030] Boronic acid-functionalized anion exchange polymers are described herein. Boronic acid-functionalized anion exchange polymers have a main chain and tetra-coordinated boronic acid groups in side chains, side groups, or crosslinks. A tetra-coordinated boronic acid group has a tetra-coordinated boron atom covalently bonded to two or three hydroxyl groups and to the polymer main chain, a side chain, or a side group.

[0031] Boronic acid-functionalized anion exchange polymers take advantage of the unique chemical bonding properties of boron. Boron has three valence electrons and has a ground state electron configuration of 1s22s22p1. Boron forms trivalent, trigonal neutral compounds, such as boric acid (B(OH)3), boronic acid (RB(OH)2or R1R2B(OH)), and boronic acid groups (a boronic acid where R is a part of a main chain, side chain, or side group of a polymer), in which boron has three covalent bonds through sp2hybridization. The sp2hybridized boron atom contains an empty p-orbital, which makes trivalent boron compounds strongly electrondeficient, two electrons short of a stable octet electronic configuration. Thus, trivalent boric acid, boronic acids, and boronic acid groups are Lewis acids and readily accept an electron pair at the boron atom. Addition of an anion, such as hydroxide (OH-) or other anion, completes the octet electronic configuration, forming highly stable, negatively charged tetravalent, tetrahedral boron compounds with four covalent bonds. (Tetra-coordinated boron may also be synonymously called tetravalent boron.)

[0032] Tetra-coordinated boronic acid groups have a negative formal charge and may serve as hydroxide transport agents by a process known as reconstruction (also referred to as reformation) in the presence of incoming hydroxide ions. Thus, an anion exchange membrane may be formed of boronic acid-functionalized anion exchange polymer as described herein. Reconstruction occurs when hydroxide ions are added to a boronic acid-functionalized anion exchange membrane at a first side of the anion exchange membrane. The added hydroxide ion causes a hydroxide ion of the tetra-coordinated boronic acid group to be released and bind with a neighboring tetra-coordinated boronic acid group deeper within the polymer membrane, which similarly causes another hydroxide ion of another tetra-coordinated boronic acid group to be released. Repetition of this dynamic reconstruction process proceeds through the anion exchange membrane until a hydroxide ion is released at an opposite side of the anion exchange membrane. Thus, this reconstruction process effectively results in the transport (or exchange) of hydroxide anions through the anion exchange membrane.

[0033] Boronic acid-functionalized anion exchange polymers can be used in electrochemical applications, such as in AEMs for hydroxide anion transport under near neutral to mildly alkaline conditions. The near neutral to mildly alkaline conditions offers stability of the AEMs in the presence of carbon dioxide and offers much milder alkaline conditions ascompared to conventional AEMs, thus extending the lifetime of the AEM and membrane electrode assembly. The boronic acid-functionalized anion exchange polymers described herein can also be produced from or based on a broad range of commercially available polymers, including synthetic polymers such as long side chain, mid side chain, and / or short side chain polytetrafluoroethylene (PTFE) polymers, polybenzimidazole (PBI) polymers, polystyrene polymers, polyfluoro sulfonic-acid PTFE polymers, polyvinyl alcohol (PVA) polymers, poly(phosphoric acid) (PPA)-doped polymers, and polychlorotetrafluoroethylene (PCTFE) polymers, polyphenylene sulfide (PPS) polymers, and natural polymers and / or biopolymers such as cellulose, lignin, chitin, and chitosan.

[0034] Various definitions will now be provided to aid in understanding various aspects of the present disclosure. As used herein, each term or expression, e.g. alkyl, m, n, etc., when used more than once, is intended to be independent of its definition elsewhere in this disclosure. In case of conflict with any patent application or patent incorporated herein by reference, the present specification, including definitions, will control.

[0035] As used herein, “polymer” refers to a substance comprising polymer molecules of the same or different polymer species, including a mixture of polymer molecules of the same polymer species which may differ from other polymer molecules within the same sample in chain length and / or particular structural arrangement (e.g., irregularities in the orientation of monomer units, end-groups, and / or in the locations and / or lengths of any side chains or side groups). “Polymer” includes homopolymers, copolymers, terpolymers, interpolymers, and so on.

[0036] As used herein, “polymer molecule” or “macromolecule” refers to a molecule of high relative molecular mass, the structure of which comprises a relatively large repetition of units (e.g., about 60 or more monomer units) derived, actually or conceptually, from molecules of low relative molecular mass (e.g., monomer molecules).

[0001] As used herein, “polymerization” refers to the process of converting a monomer, or a mixture of monomers, into a polymer.

[0037] As used herein, “oligomer” refers to a substance composed of oligomer molecules.

[0038] As used herein, “oligomer molecule” refers to a molecule of intermediate relative molecular mass, the structure of which comprises a relatively small repetition of units (e.g., about 5 to about 60 monomer units) derived, actually or conceptually, from molecules of lower relative molecular mass (e.g., monomer molecules).

[0002] As used herein, “oligomerization” refers to the process of converting a monomer or a mixture of monomers into an oligomer.

[0039] The principles, concepts, and features described herein with reference to polymers, polymer molecules, and polymerization apply equally to oligomers, oligomer molecules, and oligomerization, respectively. Accordingly, any and all uses of the terms polymer, polymer molecule, and polymerization herein can be substituted by the terms oligomer, oligomermolecule, and oligomerization, respectively, without departing from the scope of the disclosure herein.

[0040] As used herein, “ionomer” refers to a polymer composed of ionomer molecules.

[0041] 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 (such as the tetra-coordinated boronic acid groups described herein), or both.Generally, no more than approximately 15 mole percent of the constitutional units have ionizable or ionic pendant groups (e.g., a sulfonic acid group, a carboxylic acid group, a phosphoric acid group, a boronic acid group, etc.).

[0003] As used herein, “monomer” refers to a substance composed of monomer molecules.

[0004] As used herein, “monomer molecule” refers to a molecule that can undergo polymerization or oligomerization to form a polymer molecule or an oligomer molecule. A monomer molecule contributes constitutional units to the essential structure of a polymer molecule or an oligomer molecule.

[0005] As used herein, “copolymer” refers to a polymer derived from more than one species of monomer.

[0006] As used herein, “constitutional unit” refers to an atom or a group of atoms (with pendant atoms or groups, if any) comprising a part of the structure of a polymer molecule (or oligomer molecule, block, or chain).

[0007] As used herein, “repeating unit” refers to the constitutional unit the repetition of which constitutes a polymer molecule (or oligomer molecule, block, or chain).

[0008] As used herein, “monomer unit” refers to the largest constitutional unit contributed by a single monomer molecule to the structure of a polymer molecule or an oligomer molecule.

[0009] As used herein, “block” refers to a portion of a polymer molecule (or oligomer molecule) comprising many constitutional units and that has at least one feature which is not present in the adjacent portions.

[0010] As used herein, “chain” refers to the whole or part of a polymer molecule (or oligomer molecule or block), comprising a linear or branched sequence of constitutional units between two boundary constitutional units, each of which may be either an end-group, a branch point, or an otherwise-designated characteristic feature of the polymer molecule.

[0011] As used herein, “main chain” or “backbone” refers to the chain of a polymer molecule to which all other chains (long or short or both) may be regarded as being pendant (e.g., a side chain).

[0012] As used herein, “side chain” refers to an oligomeric (short chain) or polymeric (long chain) offshoot from the main chain of a polymer molecule.

[0013] As used herein, “side group” or “pendant group” refers to an offshoot, neither oligomeric nor polymeric, from a chain (e.g., from a main chain).

[0014] The principles, concepts, and features described herein with reference to side chains apply equally to side groups, and vice versa. Accordingly, any and all uses of the term side chain can be substituted by the term side group, and any and all uses of the term side group can be substituted by the term side chain, without departing from the scope of the disclosure herein.

[0015] As used herein, “crosslink” refers to a small region in a polymer molecule from which at least four chains emanate. A crosslink is generally formed by reactions involving sites or groups on existing polymer molecules or by interactions between existing polymer molecules.

[0016] The term “crosslinked” refers to the state in which polymer molecules that were earlier separate polymer molecules are linked to one another at points other than their ends.

[0042] As used herein, a “catalyst particle” refers to a particle in “black” or pure form (e.g., exclusive of any catalyst support to which the catalyst particle may be bound and exclusive of any catalyst additives) that increases the rate of a reaction without modifying the overall standard Gibbs free energy change in the reaction. A catalyst particle may be an individual molecule (including but not limited to a monomer molecule), a group of molecules, a crystal structure (e.g., as in a metal oxide), a polymer molecule, or an oligomer molecule. A catalyst particle may have any suitable size and shape, such as a microparticle, a nanoparticle, or a nanotube. A catalyst particle may include, for example, a metal, a metal alloy, a metal oxide, a metal halide (e.g., a metal chloride), or a composite including at least one of a metal, a metal alloy, a metal oxide, or a metal halide.

[0043] As used herein, an “electrocatalyst particle” or “electrochemical catalyst particle” refers to a catalyst particle that reduces the activation energy needed to carry out electrochemical reactions and / or increases the rate of electrochemical reactions, such as the OER, HER, HOR, and / or ORR. Suitable electrocatalyst particles may include, without limitation, metals such as platinum group metals (PGMs) (e.g., platinum, palladium, iridium, ruthenium, osmium, and rhodium), transition metals (e.g., silver, gold, cobalt, copper, iron, nickel, rhenium, and mercury), and post-transition metals (e.g., bismuth and tin), metal alloys (e.g., PGM- transition metal based alloys and platinum-ruthenium based alloys), metal oxides (e.g., PGM oxides, such as iridium(IV) oxide, ruthenium(IV) oxide, iridium ruthenium oxide, platinum(IV) oxide, magnesium oxide, and cerium(IV) oxide), metal halides (e.g., platinum(IV) chloride, iridium(lll) chloride, platinum(IV) bromide, iridium(lll) bromide), and / or composites of metals, metal alloys, metal oxides, and / or metal halides.

[0044] As used herein, a “catalyst support” refers to a substance, exclusive of a catalyst particle, that may be used to support catalyst particles (e.g., a substance or material to which catalyst particles may be bound or on which catalyst particles may be supported). Examples of catalyst supports include, without limitation, carbon-based materials (e.g., carbon black, graphite, carbon nanotubes, graphene, and / or boronic acid-functionalized carbon materialsdescribed herein), titanium dioxide, Sb-doped SnO2 nanoparticles, tin-doped indium oxide (ITO), and / or the ion exchange-modified catalyst supports described in International Patent Application No. PCT / US2022 / 046105, filed October 7, 2022, the contents of which are incorporated herein by reference in their entirety.

[0045] As used herein, a “catalyst” refers to a catalyst particle as well as a catalyst particle together with a catalyst support on which the catalyst particle is supported or to which the catalyst particle is bound. A catalyst may also include catalyst additives, such as promoters (such as, but not limited to, metalloids).

[0046] As used herein, an “electrocatalyst” or “electrochemical catalyst” refers to an electrocatalyst particle in “black” or pure form as well as an electrocatalyst particle together with a catalyst support on which the electrocatalyst particle is supported or to which the catalyst particle is bound. An electrocatalyst may also include catalyst additives, such as promoters.

[0047] As used herein, “metal” includes alkali metals, alkaline earth metals, transition metals, lanthanides, actinides, and post-transition metals.

[0048] As used herein, “transition metals” refers to elements of the d-block of the periodic table (Groups 3 to 12, inclusive).

[0049] As used herein, “post-transition metals” refers to aluminum, gallium, indium, tin, thallium, lead, bismuth, and polonium.

[0050] As used herein, “metalloids” refers to boron, silicon, germanium, arsenic, antimony, tellurium, and astatine.

[0051] As used herein, “platinum group metals” or “PGMs” refers to platinum, palladium, iridium, ruthenium, osmium, and rhodium.

[0052] As used herein, a “composite” material means a material having a combination of two or more distinct constituent materials, each of which retains its own distinctive properties, but which has properties that the constituent materials do not have acting alone.

[0053] As used herein, “aliphatic” compounds are hydrocarbons that are saturated or unsaturated, acyclic or cyclic, unbranched or branched, unsubstituted or wholly or partly substituted with one or more substituents or functional groups. As will be appreciated by one of ordinary skill in the art, “aliphatic” is intended herein to include, but is not limited to, alkyl, alkenyl, and alkynyl moieties. Illustrative 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.

[0054] As used herein, the term “alkyl” is given its ordinary meaning in the art and may include saturated aliphatic groups, including straight-chain alkyl groups, branched-chain alkyl groups, cycloalkyl (alicyclic) groups, alkyl substituted cycloalkyl groups, and cycloalkyl substituted alkyl groups. An analogous convention applies to other generic terms such as“alkenyl,” “alkynyl,” and the like. Furthermore, as used herein, the terms “alkyl,” “alkenyl,” “alkynyl,” and the like encompass both wholly or partly substituted and unsubstituted groups.

[0055] In some embodiments, a straight or branched alkyl chain may have 1 to 30 carbon atoms in its backbone, and, in some cases, 1 to 20 or fewer. In some embodiments, a straight or branched alkyl chain has 1 to 10 carbon atoms in its backbone (e.g., C1-C10 for straight chain, C3-C10 for branched chain), has 6 or fewer carbon atoms, or has 4 or fewer carbon atoms. Cycloalkyls may have from 3 to 10 carbon atoms in their ring structure or, in some case, from 3 to 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, and dodecyl. Examples of cyclic alkyl groups include, but are not limited to, cyclopropyl cyclobutyl, and cyclochexyl.

[0056] The terms “alkenyl” and “alkynyl” refer to unsaturated aliphatic groups analogous in length and possible substitution to the alkyls described above, but that contain at least one double or triple bond respectively. Alkenyl groups include, but are not limited to, for example, 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.

[0057] The term “heteroatom” refers to any atom other than carbon. Non-limiting examples of heteroatoms include B, N, O, Al, Si, P, S, Ge, As, Se, and Sb. In some examples, a heteroatom is an atom selected from the group consisting of B, N, O, P, and S.

[0058] 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 by a heteroatom. Examples of heteroalkyl groups include, without limitation, methoxy, ethoxy, propoxy, isopropoxy, methylthio, ethylthio, propylthio, isopropylthio, n-butylthio, methoxymethyl, and cyano groups.

[0059] 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.

[0060] The term “aryl” refers to aromatic carbocyclic groups and heteroaryl groups, unsubstituted or wholly or partly substituted, having a single ring (e.g., phenyl), multiple rings (e.g., biphenyl), or multiple fused rings, wherein at least one ring of the aryl group is aromatic (e.g., 1,2,3,4-tetrahydronaphthyl, naphthyl, anthryl, or phenanthryl). That is, at least one ring of an aryl group has a conjugated Pi electron system, while other rings of the aryl group can be cycloalkyls, cycloalkenyls, cycloal kynyls, aryls, and / or heterocycyls. “Carbocyclic aryl groups” refer to aryl groups wherein 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 to two adjoining rings) such as naphthyl group. Aryl groups are not limited to benzene and its derivatives but may have any suitablenumber of atoms in the ring. Examples of aryl groups include, without limitation, phenyl, naphthyl, tetrahydronaphthyl, anilyl, indanyl, and indenyl.

[0061] The term “heteroaryl” refers to aryl groups comprising at least one heteroatom as a ring atom (e.g., heteroaromatic groups), such as a heterocyclic group. Non-limiting examples of heteroaryl groups include, without limitation, pyridyl, pyrazinyl, pyrimidinyl, pyrrolyl, pyrazolyl, imidazolyl, thiazolyl, oxazolyl, isooxazolyl, thiadiazolyl, oxadiazolyl, thiophenyl, furanyl, quinolinyl, and isoquinolinyl. Heteroaryl groups may be referred to as A-heteroaryl where A is the element symbol of the heteroatom. For example, N-heteroaryl refers to an aryl group including at least one nitrogen atom as a ring atom.

[0062] The terms “alkoxyl” or “alkoxy” as used herein refers to an alkyl group having an oxygen radical attached thereto, and has the general formula R — O. Examples of alkoxyl groups include, without limitation, methoxy, ethoxy, propyloxy, and tert-butoxy groups.

[0063] The term “aryloxy” refers to an aryl group having an oxygen radical attached thereto. An example of an aryloxy group includes, without limitation, a phenoxy group.

[0064] Any of the above groups may be optionally substituted, in whole or in part. Examples of substituents include, without limitation, aliphatic, alicyclic, heteroaliphatic, heteroal icyclic, halogen, azide, alkyl, aralkyl, alkenyl, alkynyl, cycloalkyl, hydroxyl, alkoxyl, amino, nitro, sulfhydryl, imino, amido, phosphonate, phosphinate, carbonyl, carboxyl, alkyloxycarbonyl, silyl, ether, alkylthio, heteroalkylthio, heteroarylthio, sulfonyl, sulfonamido, ketone, aldehyde, ester, heterocyclyl, aromatic or heteroaromatic moieties, — CF3, — CN, aryl, aryloxy, perhaloalkoxy, aralkoxy, heteroaryl, heteroaryloxy, heteroarylalkyl, heteroaralkoxy, azido, amino, amine, halide, alkylthio, oxo, acylalkyl, carboxy esters, -carboxamido, acyloxy, aminoalkyl, alkylaminoaryl, alkylaryl, alkylaminoalkyl, alkoxyaryl, arylamino, aralkylamino, alkylsulfonyl, - carboxamidoalkylaryl, -carboxamidoaryl, hydroxyalkyl, haloalkyl, alkylaminoalkylcarboxy-, aminocarboxamidoalkyl-, cyano, alkoxyalkyl, perhaloalkyl, arylalkyloxyalkyl, (e.g., SO4(R')2), a phosphate (e.g., PO4(R')3), a silane (e.g., Si(R')4), a urethane (e.g., R'O(CO)NHR'), and the like. Additionally, the substituents may be selected from F, Cl, Br, I, — OH, — NO2, — CN, — NCO, — CF3, — CH2CF3J— CHCI2J— CH2ORX, — CH2CH2ORX, — CH2N(RX)2, — CH2SO2CH3J— 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, wherein each occurrence of Rxindependently includes, but is not limited to, hydrogen, aliphatic, alicyclic, heteroaliphatic, heteroal icyclic, aryl, heteroaryl, alkylaryl, or alkyl heteroaryl, wherein any of the aliphatic, alicyclic, heteroaliphatic, heteroal icyclic, alkylaryl, or alkyl heteroaryl substituents described above and herein may be substituted or unsubstituted, branched or unbranched, cyclic or acyclic, and wherein any of the aryl or heteroaryl substituents described above and herein may be substituted or unsubstituted.

[0065] Boronic acid-functionalized anion exchange polymers will now be described. Boronic acid-functionalized anion exchange polymers have a main chain and tetra-coordinated boronic acid groups in at least one of side chains, side groups, or crosslinks. The main chain of a boronic acid-functionalized anion exchange polymer may be a main chain of any suitable polymer, including any polymer descirbed herein, such as a PBI polymer main chain, a sulfonic acid-functionalized polymer main chain, a polystyrene polymer main chain, a PCTFE polymer main chain, a PTFE polymer main chain, or a PPS polymer main chain, or a derivative of any of the foregoing (including substituted (e.g., fluorinated) and / or branched derivatives and boronic acid-functionalized derivatives). As described herein, a pendant tetra-coordinated boronic acid group is in a side chain or a side group and has the general formula (I) or (II) shown below, where tetra-coordinated boron is covalently bonded to three or two hydroxyl groups and is covalently bonded to the polymer main chain, a side chain, or a side group, with four total covalent bonds:(I) (H)A crosslinking tetra-coordinated boronic acid group crosslinks two polymer chains and has the general formula (III) shown below, where tetra-coordinated boron is covalently bonded to two hydroxyl groups, and is covalently bonded to a first polymer and a second polymer (e.g., a main chain, a side chain, or a side group of the first polymer and second polymer), with four total covalent bonds:Tetra-coordinated boronic acid groups have a negative formal charge and may serve as hydroxide transport agents by a process known as reconstruction in the presence of incoming hydroxide ions. Thus, boronic acid-functionalized anion exchange polymers may be used in electrochemical applications, such as in anion exchange membranes and membrane electrode assemblies. Boronic acid-functionalized anion exchange polymers encompass a broad range of polymers, including, without limitation, boronic acid-functionalized anion exchange PBI polymers, boronic acid-functionalized anion exchange PTFE polymers, boronic acid- functionalized anion exchange PCTFE polymers, boronic acid-functionalized anion exchange polystyrene polymers, boronic acid-functionalized anion exchange PPS polymers, and boronic acid-functionalized anion exchange cellulose polymers, among others.

[0066] In some examples, boronic acid-functionalized anion exchange polymers are synthesized by hydroxylation of a boronic acid-functionalized polymer to convert trivalent boronic acid groups of the boronic acid-functionalized polymer into tetra-coordinated boronic acid groups. A boronic acid-functionalized polymer molecule has a polymer main chain and pendant boronic acid groups in side chains and / or side groups. A boronic acid group has the general formula (IV) or (V), as shown below, where the boron atom is covalently bonded to two or one hydroxyl groups and by one or two covalent bonds to the main chain, a side chain, or a side group, with three total covalent bonds (i.e. , boron is trivalent):(IV) (V)Examples of boronic acid-functionalized polymers include, without limitation, boronic acid- functionalized PBI polymers, boronic acid derivatives of sulfonic acid-functionalized polymers (e.g., sulfonic acid-functionalized PTFE polymers and sulfonic acid-functionalized PCTFE polymers), boronic acid-functionalized polystyrene polymers, boronic acid-functionalized cellulose polymers, and others. Illustrative boronic acid-functionalized polymers and illustrative reaction schemes for synthesis of boronic acid-functionalized polymers will be described below in more detail.

[0067] In the hydroxylation reaction, the boronic acid-functionalized polymer is combined with a hydroxylating agent. In the hydroxylation reaction, a boron atom of the pendant trivalent boronic acid group (having the structure of general formula (IV) or (V)) accepts and covalently bonds with a hydroxide anion (OH-) from the hydroxylating agent. As a result, the boron atom becomes tetra-coordinated and has the structure of general formula (I) or (II). Any suitable hydroxylating agent may be used in the hydroxylation reaction, including but not limited to water or an aqueous alkali solution, such as a solution using ammonium hydroxide (NH4OH) or any water soluble metal hydroxide in which the metal cation remains electrochemically tolerant within the redox window of the electrochemical reaction, such as water electrolysis. Suitable examples of an alkali solution include, without limitation, sodium hydroxide (NaOH) and potassium hydroxide (KOH). Due to the unique electronic configuration and bonding properties of boron, the hydroxylation reaction is spontaneous in the presence of hydroxide anions. The hydroxylation reaction occurs at neutral pH when the hydroxylating agent is water, and occurs under mildly alkaline to alkaline pH for other hydroxylating agents. Thus, a range of pH may be used for successful reactions. The degree of hydroxide anion loading on boronic acid groups, and thus the pKa of the resulting boronic acid-functionalized anion exchange polymer, may be tuned as desired based on the stoichiometry of the reagents. The alkaline hydroxylating agent solution may be used in either a stoichiometric amount or in an excess amount relative to the quantity of boronic acid groups of the boronic acid-functionalized polymer. The hydroxylationreaction may be carried out at room temperature (e.g., about 25°C) or at a higher temperature, which would increase the rate of reaction.

[0068] In other examples, boronic acid-functionalized anion exchange polymers are synthesized by crosslinking a PBI polymer molecule and a second polymer molecule by using a boronic crosslinking agent, as described in more detail in International Patent Application No. PCT / US2023 / 011508, filed January 25, 2023, which is incorporated herein by reference in its entirety. A boronic crosslinking agent comprises boric acid (B(OH)3) or a boronic acid having the general formula R — B(OH)2. In some examples, the second polymer molecule is another PBI polymer molecule. For instance, PBI polymer sheets or layers may be crosslinked using a boronic crosslinking agent. When crosslinking two PBI polymer molecules, the boron atom of the boronic crosslinking agent covalently bonds with a secondary amine of each PBI polymer molecule main chain to form a tetravalent boronic acid group crosslink having the general formula (III). The tetra-coordinated boron atom has four covalent bonds and a negative formal charge. As a result, the tetra-coordinated boronic acid group crosslink may function as a hydroxide anion exchange agent by the reconstruction process described herein.

[0069] In other examples, the second polymer molecule is a hydroxyl-functionalized polymer, such as a hydroxyl-functionalized PTFE polymer, a poly(phosphoric acid) (PPA) polymer, a natural polymer (e.g., lignin, cellulose, chitin), or a PPA-doped polymer. In some examples, a hydroxyl-functionalized polymer is functionalized with an acid group represented by X-OH in which X is sulfur (S), carbon (C) or phosphorous (P) and is covalently bonded to the oxygen (O) atom of the hydroxyl group and to the polymer main chain, a side chain, or a side group. The acid group X-OH may be any suitable acid group such as, without limitation, a carboxylic acid group, a sulfonic acid group, a phosphoric acid group, a phosphonic acid group, or an alcohol. In some examples, X includes a Ci to C30 alkyl linker chain and optionally has one or more pendant moieties, which may be the same or different and may each be independently selected from the group consisting of hydrogen, a hydroxyl group, a fluoro group, a chloro group, a dialkylamino group, a cyano group, a carboxylic acid group, a carboxylic amide group, an ester group, an alkyl group, an alkoxy group, and an aryl group. In some examples, X may be omitted so that the polymer is functionalized with only the hydroxyl group. The polymer may have any degree of acid functionalization as may serve a particular implementation.

[0070] The boron atom of the crosslinking agent covalently bonds with the secondary amine of the PBI polymer molecule and with the acid group or hydroxyl group of the second polymer molecule. Specifically, the boron atom expands its valence to covalently bond with the acid group or hydroxyl group of the second polymer molecule through a substitution reaction. The resulting polymer compound has a tetra-coordinated boronic acid group crosslink having the general formula (III). The tetra-coordinated boron atom has four covalent bonds and anegative formal charge. As a result, the tetra-coord inated boronic acid group crosslink may function as a hydroxide anion exchange agent by the reconstruction process described herein.

[0071] Due to the tetra-coordinated boron atom, a tetra-coordinated boronic acid group in a boronic acid-functionalized anion exchange polymer molecule has a negative formal charge and may be counterbalanced by a cation (e.g., H+, Na+, K+, NF ). The boronic acid- functionalized anion exchange polymer may be used for anion exchange by reconstruction. As mentioned, reconstruction occurs when hydroxide ions are added, such as by addition of water or other hydroxide ion source, to the boronic acid-functionalized anion exchange polymer at a first side of an anion exchange membrane, causing a hydroxide ion of the tetra-coordinated boronic acid group to be released and bind with a neighboring tetra-coordinated boronic acid group deeper within the anion exchange membrane, which similarly causes another hydroxide ion of another tetra-coordinated boronic acid group to be released. Repetition of this dynamic reconstruction process proceeds through the anion exchange membrane until a hydroxide ion is released at an opposite side of the anion exchange membrane. Thus, this reconstruction process effectively results in the transport of hydroxide anions through the anion exchange membrane.

[0072] As explained above, boronic acid-functionalized anion exchange polymers may be synthesized by hydroxylation of a boronic acid-functionalized polymer to convert trivalent boronic acid groups of the boronic acid-functionalized polymer into tetra-coordinated boronic acid groups. A boronic acid-functionalized polymer may be synthesized in any suitable way. In some examples, boronic acid-functionalized polymers are synthesized by post-polymerization functional modification of polymers, such as polybenzimidazole (PBI) polymers, sulfonic acid- functionalized PTFE polymers, sulfonic acid-functionalized polymers, and polystyrene polymers.

[0073] Polybenzimidazole (PBI) polymers are a class of polymers composed of PBI polymer molecules. PBI polymer molecules have a repeating unit that includes a benzimidazole unit as at least part of a main chain. The benzimidazole unit comprises a benzimidazole moiety or a derivative thereof. Benzimidazole is a heterocyclic aromatic organic compound having a phenyl group and an imidazole group that share two carbon atoms in their ring structures. The general structure of benzimidazole is shown in the following Formula (VI):

[0074] An example of a PBI polymer with one benzimidazole unit per repeating unit in a main chain is poly(2,5-benzimidazole) (AB-PBI), shown below as Formula (VII), and examplesof PBI polymers with two benzimidazole units per repeating unit in a main chain are poly[2,2'- (n7-phenylene)-5,5'-bibenzimidazole] (m-PBI), shown below as Formula (VIII), and 4F-PBI (a fluorinated derivative of m-PBI), shown below as Formula (IX).(IX): 4F-PBI

[0075] Other examples of PBI polymers include, without limitation, poly{2,6-(2,6- naphtyliden)-1 ,7-dihydrobenzo[1 ,2-d;4,5-d]diimidazole}; poly 2,2'-(2,6-naphtyliden)-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,)-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'-(furylene-2",5")-5,5'-bibenzimidazole; poly-2.2-(naphthalene- r,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'-bibenzimidazole; poly-2, 2'-(m- phenylene)-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 (including substituted (e.g., fluorinated) and / or branched derivatives). In some examples, a PBIpolymer is a copolymer that comprises one or more additional repeating units, which may or may not include a benzimidazole unit in a main chain, in a side chain, or both.

[0076] Polytetrafluoroethylene (PTFE) polymers are a class of polymers composed of tetrafluoroethylene polymer molecules, and derivatives thereof, and are produced by the polymerization of tetrafluoroethylene. PTFE polymer molecules have a carbon main chain with two fluorine atoms bonded to each carbon, and derivatives thereof.

[0077] Polychlorotrifluoroethylene (PCTFE) polymers are derivatives of PTFE and are homo-polymers of chlorotrifluoroethylene (CTFE) with the molecular formula (CF2CCIF)n, and derivatives thereof. PCTFE is similar to PTFE (such as Teflon) except that PCTFE contains a chlorine atom in each repeating unit. The presence of this chlorine atom makes PCTFE a unique thermoplastic polymer with many applications. However, PCTFE has a hydrophobic main chain and is non-conducting for ions and thus is not suitable for electrochemical applications. Derivatives of PCTFE polymers include substituent groups (e.g., side chains or side groups) in place of chlorine atoms. Derivatives of PCTFE may be wholly or partly substituted. Derivatives of PCTFE polymers include, without limitation, modified and functionalized PCTFE polymers, including any of the modified or functionalized PCTFE polymers (e.g., acid-functionalized and ion-exchange functionalized PCTFE polymers) described in International Patent Application No. PCT / US2024 / 041922, filed August 12, 2024, which is hereby incorporated by reference in its entirety.

[0078] Sulfonic acid-functionalized PTFE polymers are derivatives of PTFE polymers and have a PTFE main chain and a side chain or side group with one or more pendant sulfonic acid groups. In some examples, the side chain is a long side chain (LSC) having at least two ether linkages and four or more polyfluorinated carbon units (e.g., — CF2 — and / or — CF3). In other examples, the side chain is a short-side chain (SSC) having one ether linkage and two polyfluorinated carbon units. In further examples, the side chain is a mid-side chain (MSC) having one ether linkage and four polyfluorinated carbon units. In some examples, a sulfonic acid-functionalized PTFE polymer has the general formula [(CF2CF2)m(CFACF2)n]x, where A is a side chain comprising one or more pendant sulfonic acid groups, and m, n, and x are positive and are selected based on application, equivalent weight, molecular weight, etc. In some examples, m ranges from 4 to 7 and n is 1. In some examples, side chain A is a LSC, MSC, or SSC. Examples of LSC sulfonic acid-functionalized PTFE polymers include, without limitation, Nation™ series polymers (available from Chemours Company in various configurations and grades, including Nafion-H, Nation HP Nation 117, Nation 115, Nation 212, Nation 211, Nation NE1035, Nation XL, etc.) and any combination, derivative, grade, or configuration thereof. Examples of SSC sulfonic acid-functionalized PTFE polymers include, without limitation, Aquivion® series polymers (available from Solvay S.A. in different configurations and grades, including Aquivion® E98-05, Aquivion® PW98, Aquivion® PW87S, etc.), Gore-Select®(available from W.L. Gore & Associates, Inc.), Flemion™ (available from Asahi Glass Company), Pemion+™ (available from lonomr Innovations, Inc.), and any combination, derivative, grade, or configuration thereof. Examples of MSC sulfonic acid-functionalized PTFE polymers include, without limitation, polymers produced by 3M™ Company. In some examples, a PTFE polymer is a copolymer that comprises one or more other repeating units. In some examples, a PTFE polymer may be doped and / or may be crosslinked with itself and / or with another polymer.

[0079] Sulfonic acid-functionalized polymers include, without limitation, polyfluorosulfonic acid polymers and non-fluorinated sulfonic acid polymers. Examples of polyfluorosulfonic acid polymers include, without limitation, sulfonic acid-functionalized PTFE polymers and sulfonic acid-functionalized PCTFE polymers. Examples of non-fluorinated sulfonic acid polymers include, without limitation, poly(styrene sulfonic acid) polymers, sulfonated aromatic polymers (e.g., sulfonated poly(ether ketone) (SPEEK) polymers, sulfonated poly(aryl ether sulfone) (SPAES) polymers, sulfonated poly(arylene ether ketone) (SPAEK) polymers, sulfonated polysulfone (SPSF) polymers, sulfonated polyimide (SPI) polymers, and sulfonated polystyrene (SPS), sulfonated polyphenylene, and any other sulfonated polymer, including sulfonated derivatives of polymers described herein.

[0080] Polystyrene polymers are polymers composed of polystyrene polymer molecules. Polystyrene polymer molecules have a repeating unit that includes alternating carbon centers attached to a phenyl group. Examples of polystyrene polymers include, without limitation, polystyrene, poly(styrene sulfonic acid) (e.g., poly(4-styrene sulfonic acid)), polyhalostyrene, poly(3-trifluoromethyl styrene), poly(4-acetoxy styrene), poly(4-allyl styrene), poly(4- cyanostyrene), poly(4-dimethylsilyl styrene), poly(4-hydroxystyrene), poly(alpha-methyl styrene), poly(4-methyl styrene), poly(4-methoxystyrene), poly(4-[tert-butoxycarbonyl]oxy- styrene), poly(4-tert- butyl styrene), poly(4-[N,N-di(trimethylsilyl)aminomethyl]-styrene), poly(4- vinylbenzoic acid), poly(n-butyl 4-vinylbenzoate), poly(tert-butyl 4-vinylbenzoate), poly(2- ethylhexyl 4-vinylbenzoate), poly(methyl 4-vinylbenzoate), poly(vinylbenzyl chloride), poly(4- vinylbenzyl-N-methylphthalimide), poly(vinyl cyclohexane), and derivatives of any of the foregoing (including substituted (e.g., fluorinated) and / or branched derivatives). In some examples, a polystyrene polymer is a copolymer that comprises one or more additional repeating units, which may or may not include a carbon center attached to a phenyl group.

[0081] Aromatic polymers include any polymers having aromatic rings in a main chain and / or in side chains or side groups. Examples of aromatic polymers include, without limitation, polystyrene polymers, polycarbonate polymers (polyphenylene polymers (e.g., poly(1,4- phenylene), poly(1,4-phenylene-ethylene), poly(1,3-phenylene-methylene), poly(p-phenylene vinylidene), poly(p-phenylene vinylene), poly(1,4-phenylene oxide), poly(1,4-phenylene sulfide)), poly(ether sulfone), polyaryletherketone polymers, polysulfone polymers,polyethylene terephthalate), aromatic polyester polymers, poly(oxy-1,4-phenylenecarbonyl-1,4- phenylene), poly[(dimethylmethylene)bis(4,1 -phenylene) carbonate], phenolic resins, poly[(ethylazanediyl)ethyleneazanediyl-1,3-phenylene], poly-oxydiphenylene-pyromellitimide (Kapton®, manufactured by E. I. du Pont de Nemours and Company), poly(ester imide) polymers, aromatic polyimide polymers, polyphenylene sulfide (PPS) polymers, lignin, and derivatives of any of the foregoing (including substituted (e.g., fluorinated) and / or branched derivatives).

[0082] Natural polymers (also referred to as “biopolymers”) include, without limitation, cellulose, lignin, chitin, and derivatives of any of the foregoing, including any of the polymers described in U.S. Patent No. 11,331,631, issued May 17, 2022, and U.S. Patent No. 11,594,747, issued February 28, 2023, each of which is hereby incorporated by reference in its entirety.

[0083] Illustrative examples of synthesis of boronic acid-functionalized polymers by postpolymerization functional modification of polymers will now be described.

[0084] In some examples, a boronic acid-functionalized PBI polymer is synthesized by post-polymerization functional modification of a PBI polymer with a boronic acid group by coupling a boronic acid-functionalized linker with a secondary nitrogen atom in the benzimidazole moiety in the PBI polymer. The boronic acid-functionalized linker has a linking group X as a terminal group or as a side group and a boronic acid group as a terminal group and / or as a side group, where linking group X is a methyl group ( — CH3), a formyl group ( — C(=O)H), or a sulfonyl group ( — S(=O)2H). In some examples, the boronic acid-functionalized linker has the general formula XRB(OH)2, where R is an alkyl chain of length m, where m ranges from 0 to 30 (or from 0 to 20, or from 0 to 12, or 0 to 10, or 0 to 8, or 0 to 6), and has one or more side groups A, each of which may independently be hydrogen (H), a hydroxyl group (OH), a fluoro group (F), a chloro group (Cl), a boronic acid group, a dialkylamino group (NR'2, in which R' may represent hydrogen or an organic combining group, such as a methyl group (CH3)), a cyano group (CN), a carboxylic acid (COOH) group, a carboxylic amide group, an ester group, an alkyl group, an alkoxy group, or an aryl group. In some examples, the boronic acid-functionalized linker has one or more pendant boronic acid groups as a side group rather than, or in addition to, a terminal group.

[0085] In the reaction, linking group X of the boronic acid-functionalized linker bonds with the secondary nitrogen of the benzimidazole moiety, thus forming a side chain with a pendant boronic acid group. The loading of the boronic acid groups may be controlled by tuning the molar ratio of the boronic acid-functionalized linker to the benzimidazole moiety in the PBI polymer.

[0086] FIG. 1 A shows an illustrative reaction scheme for synthesis of a boronic acid- functionalized PBI polymer. As shown, a PBI polymer molecule is combined with a boronicacid-functionalized linker as described above, to produce a boronic acid-functionalized PBI polymer molecule. Any boronic acid-functionalized linker described herein may be used. It will be recognized that the PBI polymer molecule of FIG. 1A is merely representative and the reaction scheme of FIG. 1A can be carried out using any other suitable PBI polymer.

[0087] In other examples of post-polymerization functional modification, an aromatic polymer (e.g., a PBI polymer, a polystyrene polymer, a PPS polymer, or any other aromatic polymer described herein) is converted into a boronic acid-functionalized polymer by borylation of an aromatic ring in the main chain, a side chain, or a side group. In some examples, the aromatic ring is directly borylated by reaction with a borylating agent. Any suitable borylating agent may be used, including, but not limited to, a boronic acid, a borate ester having the general formula B(OR1)3 and / or a boronic ester having the general formula R2B(OR1)2, where each R1is independently an alkyl or aryl group having 1 to 20 carbon atoms, 1 to 10 carbon atoms, 1 to 6 carbon atoms, or 1 to 4 carbon atoms, wholly or partly substituted or unsubstituted, branched or unbranched, and R2is an alkyl, alkenyl, alkynyl, or aryl group having 1 to 20 carbon atoms, 1 to 10 carbon atoms, 1 to 6 carbon atoms, or 1 to 4 carbon atoms, wholly or partly substituted or unsubstituted, and branched or unbranched. Other suitable borylating agents may be used. Illustrative examples of borylating agents include, without limitation, trialkyl borates (e.g., trimethyl borate, triethyl borate), bis(pinacolato)diboron, bis(catecholato)diborane, pinacol borate, bis(2,4-dimethylpentane-2,4-glycolato)diboron, bis(hexylene glycolato)diboron, bis(neopentyl glycolato)diboron, vinyl boronic acid, and derivatives of the foregoing. Borylation reactions of the aromatic ring may also include metal catalyzed C-H borylation reactions, including but not limited to Suzuki-Miyaura metal-catalyzed coupling reactions, which use transition metals to directly convert a C-H bond into a C-B bond.

[0088] Borylation of the aromatic ring produces an intermediate protected boronic acid group (e.g., — B(OR1)2). In these examples, a hydrolysis step may be performed to remove the protecting groups R1, thus producing a pendant boronic acid group (e.g., having general formula (III)). In other examples, the hydrolysis step may be performed in situ during the borylation step (e.g., by combining the borylating agent and water in a one-pot process). The loading of the boronic acid groups on the polymer may be controlled by controlling the molar ratio of the borylating agent to the aromatic rings in the polymer. In other examples, the aromatic group (Ar) in the polymer molecule may be first converted to an active “Ar-X” intermediate for the subsequent borylation reaction, where X is a halo group (e.g., an iodo (I), bromo (Br), or chloro (Cl) group) or a metal (where Ar-X is formed by an aromatic ring metalation reaction). In some examples, X is lithium (Li).

[0089] FIG. 1 B shows an illustrative reaction scheme for synthesis of a boronic acid- functionalized PBI polymer by borylation. As shown, a PBI polymer molecule is combined with a borylating agent. The aromatic ring of the benzimidazole unit is borylated to produce anintermediate protected boronic acid group (not shown), which is then hydrolyzed to produce a pendant boronic acid group (e.g., having general formula (IV). Any suitable borylating agent can be used in place of boric acid, and the reaction scheme of FIG. 1B can be carried out using any other suitable PBI polymer.

[0090] FIG. 2A shows an illustrative reaction scheme for synthesis of a boronic acid- functionalized polystyrene polymer by borylation. As shown, a polystyrene polymer molecule is combined with a borylating agent. An aromatic ring of the polystyrene repeating unit is borylated to produce an intermediate protected boronic acid group (not shown), which is then hydrolyzed to produce a pendant boronic acid group (e.g., having general formula (IV)). Any suitable borylating agent can be used and the reaction scheme of FIG. 2A can be carried out using any other suitable polystyrene polymer.

[0091] In further examples of post-polymerization functional modification of a polymer, a polymer having a vicinal diol in a side group or a side chain is combined with boric acid (B(OH)3). The boric acid reacts with the vicinal hydroxyl groups to form a cyclic boronic acid group having only one hydroxyl group (e.g., having general formula (V)). Any suitable polymer having a vicinal diol may be used, including polysaccharides, cellulose, and 1 ,2- dihydroxyphenyl polymers.

[0092] FIG. 2B shows an illustrative reaction scheme for synthesis of a boronic acid- functionalized polymer using a vicinal diol. As shown, a polymer molecule having a pendant 1,2-di hydroxy phenyl group is combined with boric acid to produce a boronic acid-functionalized polymer having a pendant boronic acid group having general formula (V). In some examples, the polymer molecule includes a PTFE main chain and the linker is an LSC, MSC, or SSC. Other configurations are also contemplated by the linker. While FIG. 2B shows only one linker, the polymer molecule may have any other suitable number of side chains, linkers, and 1,2- dihydroxyphenyl groups. Additionally, the 1,2-dihydroxyphenyl group may be a part of a side chain or side group of any other polymer described herein, including but not limited to a PBI polymer, a sulfonic acid-functionalized polymer, a polystyrene polymer, or an aromatic polymer. In some examples, cellulose polymers and / or polysaccharides may be used to crosslink a boronic acid-functionalized polymer.

[0093] In yet further examples of post-polymerization functional modification of a polymer, a sulfonic acid-functionalized polymer molecule is converted into a boronic acid-functionalized polymer molecule by coupling a boronic acid group with a pendant sulfonic acid group (- S(=O)2(OH)) by a sulfonamide link. The reaction scheme includes multiple steps.

[0094] In a first step, the sulfonic acid group is activated to sulfonyl chloride (-S(=O)CI), sulfonyl fluoride (-S(=O)F), or a sulfonic ester. For example, the sulfonic acid-functionalized polymer molecule may be combined with hydrochloric acid (HCI) or hydrofluoric acid (HF), which performs a substitution reaction to replace the hydroxyl group of the sulfonic acid groupwith a chloride group, thus forming a sulfonyl chloride group or sulfonyl fluoride group. Other suitable chloride reagents and / or fluoride reagents may be used, including but not limited to thionyl chloride, sulfuryl chloride, oxalyl chloride, thionyl fluoride, and sulfuryl fluoride. A sulfonic ester has the general formula — S(=02)0R where R is hydrogen or a wholly or partly substituted or unsubstituted alkyl or aryl group having one to twenty, one to ten, one to eight, one to six, or one to four carbon atoms, such as but not limited to a methyl, ethyl, propyl, or butyl group. Examples of sulfonic ester reagents include, without limitation, dimethyl sulfate or a dialkyl sulfate.

[0095] In a second step, a bifunctional amino boronic acid linker is coupled with the sulfonyl chloride, sulfonyl fluoride, or sulfonic ester. The bifunctional amino boronic acid linker comprises an amino group as a terminal group or a side group, a boronic acid group as a terminal group or a side group, and a group R, where R is an alkyl chain of length m, where m ranges from 0 to 30 (or from 0 to 20, or from 0 to 12, or 0 to 10, or 0 to 8, or 0 to 6), and has one or more side groups A, each of which may independently be hydrogen (H), a hydroxyl group (OH), a fluoro group (F), a chloro group (Cl), a boronic acid group, a dialkylamino group (NR'2, in which R' may represent hydrogen or an organic combining group, such as a methyl group (CH3)), a cyano group (CN), a carboxylic acid (COOH) group, a carboxylic amide group, an ester group, an alkyl group, an alkoxy group, or an aryl group. In the second step, a primary or secondary amine in the bifunctional amino boronic acid linker orthogonally couples with the sulfonyl chloride, sulfonyl fluoride, or sulfonic ester, thus linking the amino boronic acid linker to the polymer main chain by a sulfonamide link. The resulting polymer molecule has a pendant boronic acid group (e.g., having general formula (IV)) linked to the polymer main chain (e.g., a PTFE main chain) by way of the linker (e.g., by way of the sulfonamide link).

[0096] In an alternative reaction scheme, in the second step an aromatic boronic acid having the general formula ArB(OR)2 reacts with the sulfonyl chloride, sulfonyl fluoride, or sulfonic ester by an aromatic electrophilic substitution reaction. In the aromatic boronic acid having the general formula ArB(OR)2, each R is independently hydrogen or a wholly or partly substituted or unsubstituted alkyl or aryl group having one to twenty, one to ten, one to eight, one to six, or one to four carbon atoms, such as but not limited to a methyl, ethyl, propyl, or butyl group. The aromatic boronic acid is a protected form of boronic acid. In this reaction scheme, the aryl group of the aromatic boronic acid couples directly with the sulfur atom of the sulfonyl chloride, sulfonyl fluoride, or sulfonic ester, forming an intermediate protected aromatic boronic acid group coupled with the polymer by a sulfone link. In a third step, the protecting groups R are removed to yield a free aromatic boronic acid group having the general formula ArB(OH)2. For example, a hydrolysis step may be performed to remove the protecting groups R, thus producing a pendant boronic acid group having general formula (IV) coupled with the main chain by way of an aromatic linker and a sulfone link. In other examples, the hydrolysisstep may be performed in situ during the borylation step (e.g., by combining the borylating agent and water in a one-pot process).

[0097] FIG. 3A shows an illustrative reaction scheme for synthesis of a boronic acid- functionalized polymer by coupling a boronic acid group with a pendant sulfonic acid group by a sulfonamide link. Any sulfonic acid-functionalized polymer molecule may be used as a starting reagent, including but not limited to a polyfluorosulfonic acid-functionalized PTFE polymer molecule or a sulfonic acid-functionalized PCTFE polymer molecule. In a first step, a pendant sulfonic acid group of the sulfonic acid-functionalized polymer molecule is activated to sulfonyl chloride, such as by combination with hydrogen chloride. However, the sulfonic acid group may be activated to sulfonyl chloride using any other chloride activation agent and / or any other suitable reaction. Alternatively, the sulfonic acid group may be activated to sulfonyl fluoride or a sulfonic ester. In a second step, the intermediate sulfonyl chloride-functionalized polymer molecule is combined with an amino boronic acid linker, which couples with the sulfonyl chloride by a sulfonamide link. In the example of FIG. 3A, the amino boronic acid linker has the formula H2N(CH)2B(OH)2. However, any other amino boronic acid linker may be used, including any amino boronic acid linker described herein. The reaction produces a boronic acid- functionalized polymer having a boronic acid group of general formula (IV) linked to the polymer main chain by a linker by way of a sulfonamide link.

[0098] FIG. 3B shows an illustrative reaction scheme for synthesis of a boronic acid- functionalized polymer by coupling a boronic acid group with a pendant sulfonic acid group by a sulfone link. In the second step, a sulfonyl chloride-functionalized polymer molecule (which may be produced as described with reference to FIG. 3A) is combined with phenylboronic acid having the general formula PhB(OR)2, where each R is independently hydrogen or a wholly or partly substituted or unsubstituted alkyl or aryl group having one to twenty, one to ten, one to eight, one to six, or one to four carbon atoms, such as but not limited to a methyl, ethyl, propyl, or butyl group. The phenylboronic acid couples with the sulfonyl chloride by a sulfone link to form an intermediate protected phenylboronic acid-functionalized polymer molecule. In a third step, the protecting groups R are removed from the intermediate phenylboronic acid- functionalized polymer molecule, as described above, to yield a polymer molecule having a pendant phenyl boronic acid group having the general formula PhB(OH)2 coupled with the polymer main chain by a sulfone link. The reaction produces a boronic acid-functionalized polymer having a boronic acid group of general formula (IV) linked to the polymer main chain by a linker by way of a sulfone link. In the example of FIG. 3B, any other aromatic boronic acid may be used in place of phenylboronic acid. Additionally, the aromatic boronic acid may be combined with sulfonyl fluoride or a sulfonic ester instead of sulfonyl chloride.

[0099] FIG. 4 shows an illustrative reaction scheme for synthesis of a boronic acid- functionalized PCTFE polymer using PCTFE as a starting material. In a first step, a PCTFEpolymer is combined with 4-hydroxyphenylboronic acid (the linking agent) in the presence of a base, such as a metal hydride (e.g., LiH, NaH, KH, K-tert-butoxide), in a 2:1 stoichiometric ratio (or any other suitable ratio) of the base to the linking agent. 4-hydroxyphenylboronic acid may be variously substituted or unsubstituted. 4-hydroxyphenylboronic acid reacts with chlorine atoms in the main chain of the PCTFE polymer in a substitution reaction and covalently bonds with the main chain. The reaction produces an intermediate phenoxyboronic acid-modified PCTFE polymer with a boronic acid group (B(OH)2) in a side group. The resulting boronic acid- functionalized PCTFE polymer includes a PCTFE main chain and a side group including a boronic acid group and a linker including a phenoxy linking group that links the boronic acid group to the main chain.

[0100] FIG. 5 shows another illustrative reaction scheme for synthesis of boronic acid- functionalized PCTFE polymer using PCTFE as a starting material. In a first step, a PCTFE polymer is combined with 4-mercaptophenylboronic acid (a linking agent) in the presence of a base, such as a metal hydride (e.g., LiH, NaH, KH, K-tert-butoxide), in a 2:1 stoichiometric ratio (or any other suitable ratio) of the base to the linking agent. 4-mercaptophenylboronic acid may be variously substituted or unsubstituted. It will be recognized that any other suitable aromatic linking agent may be used in place of 4-mercaptophenylboronic acid, such as 2- mercaptophenylboronic acid. 4-mercaptophenylboronic acid reacts with chlorine atoms in the main chain of the PCTFE polymer in a substitution reaction and covalently bonds with the main chain. The reaction produces an intermediate linker-modified PCTFE polymer with a pendant boronic acid group (B(OH)2) in a side group and a sulfide linkage. In a second step, the thiol group undergoes oxidation, such as by combination with aqueous hydrogen peroxide (H2O2), to produce a sulfonyl linkage. The boronic acid-functionalized PCTFE polymer molecule includes a pendant boronic acid group linked to the PCTFE main chain by an arylsulfonyl linker.

[0101] In the examples described above, a boronic acid-functionalized polymer is synthesized by post-polymerization functional modification of a polymer. In other examples, a boronic acid-functionalized polymer is synthesized by polymerization reactions using a boronic acid-functionalized monomer. For example, a monomer used to form a polymer, such as a tetrafluoroethylene (TFE) monomer, benzimidazole monomer, styrene polymer, or sulfonic acid- functionalized monomer, may be modified pre-polymerization with a boronic acid group, after which the functionalized monomer is polymerized to form a boronic acid-functionalized polymer. The monomer may be functionally modified with a boronic acid group in any way, including using any reaction scheme described herein for functional modification of a polymer with a boronic acid group.

[0102] Illustrative examples of boronic acid-functionalized anion exchange polymers, reaction schemes for synthesis of boronic acid-functionalized anion exchange polymers, andreaction schemes for hydroxide anion exchange using boronic acid-functionalized anion exchange polymers will now be described.

[0103] FIG. 6A shows an illustrative reaction scheme for synthesis of a boronic acid- functionalized anion exchange polymer using a boronic acid-functionalized polymer as a starting material. As shown in FIG. 6A, the boronic acid-functionalized polymer includes a polymer main chain (not shown), a linker, and a trivalent boronic acid group ( — B(OH)2) linked to the main chain by the linker. As shown, the linker is an alkyl chain of length m, where m ranges from 0 to 30 (or from 0 to 20, or from 0 to 12, or 0 to 10, or 0 to 8, or 0 to 6), and has one or more side groups A, each of which may independently be hydrogen (H), a hydroxyl group (OH), a fluoro group (F), a chloro group (Cl), a dialkylamino group (NR2, in which R may represent hydrogen or an organic combining group, such as a methyl group (CH3)), a cyano group (CN), a carboxylic acid (COOH) group, a carboxylic amide group, an ester group, an alkyl group, an alkoxy group, and an aryl group. In some examples, the linker is a long-side chain (LSC) having at least two ether linkages and four or more polyfluorinated carbon units (e.g., — CF2 — and / or — CF3), a short-side chain (SSC) having one ether linkage and two polyfluorinated carbon units, or a mid-side chain (MSC) having one ether linkage and four polyfluorinated carbon units. Other configurations are also contemplated by the linker. While FIG. 6A shows only one linker, the boronic acid-functionalized polymer may have any other suitable number of side chains, linkers, and trivalent boronic acid groups.

[0104] The boronic acid-functionalized polymer is generic and represents any suitable polymer or material, including any polymer or material described herein. In some examples, the boronic acid-functionalized polymer is hydrocarbon- based, such as a boronic acid- functionalized PTFE polymer, a boronic acid-functionalized polystyrene polymer, a boronic acid-functionalized PBI polymer, or a polyfluoro boronic acid-functionalized polymer (e.g., a Nation™ and / or Aquivion® polymer functionalized with boronic acid groups instead of sulfonic acid groups). In further examples, the boronic acid-functionalized polymer is a hydroxylated biopolymer, such as cellulose, functionalized with boric acid groups. The hydroxylated biopolymer may be functionalized with boric acid groups in any suitable way, including in a manner similar to that shown in FIG. 7 A. The boronic acid-functionalized polymer may be synthesized in any way described herein.

[0105] As shown in FIG. 6A, the trivalent boronic acid group of the boronic acid- functionalized polymer is hydroxylated by addition of water (the hydroxylating agent) to form a tetra-coord inated boronic acid group having a negative formal charge counterbalanced by a cation (e.g., a proton). However, any other suitable hydroxylating agent may be used in addition to, or in place of, water. By converting the trivalent boronic acid group to a tetra-coordinated boronic acid group, the hydroxylation reaction converts the boronic acid-functionalized polymer to a boronic acid-functionalized anion exchange polymer.

[0106] FIG. 6B shows an illustrative reaction scheme for hydroxide anion exchange using the boronic acid-functionalized anion exchange polymer produced by the reaction scheme of FIG. 6A. As shown in FIG. 6B, the tetra-coordinated boronic acid group undergoes reconstruction in the presence of an incoming hydroxide ion, as explained above, releasing a hydroxide anion from the tetra-coordinate boronic acid group. The hydroxide anion that is released is available for anion exchange with a neighboring hydroxide group of the same or a different tetra-coordinated boronic acid group. This process repeats throughout the polymer material, thus effectively transporting hydroxide anions from a first side of a polymer material to an opposite side of the polymer material.

[0107] FIG. 7A shows another illustrative reaction scheme for synthesis of a boronic acid- functionalized polymer by post-polymerization functional modification of a polymer having a vicinal diol in a side chain or side group. In a first step, a boronic acid-functionalized polymer is produced by anchoring a boronic acid group onto a 1,2-diphenolic polymer (e.g., a 1,2- di hydroxy phenyl polymer). The polymer is generic and represents any suitable polymer. In some examples, the polymer is hydrocarbon-based, such as a PTFE polymer, a polystyrene polymer, a PBI polymer, or a polyfluoro sulfonic acid-functionalized polymer (e.g., Nation® and / or Aquivion® polymer). The polymer includes a 1,2-diphenolic group linked to the polymer main chain by a linker. The linker may be any linker described above with reference to FIG. 6A. The reaction produces a boronic acid-functionalized polymer having a pendant trivalent boronic acid group having general formula (V) in a side chain or side group.

[0108] FIG. 7B shows an illustrative reaction scheme for synthesis of a boronic acid- functionalized anion exchange polymer using the boronic acid-functionalized polymer produced by the reaction scheme of FIG. 7A. As shown in FIG. 7B, the trivalent boronic acid group of the boronic acid-functionalized polymer is hydroxylated by addition of water (the hydroxylating agent) to form a tetra-coordinated boronic acid group having a negative formal charge counterbalanced by a cation (e.g., a proton). However, any other suitable hydroxylating agent may be used in addition to, or in place of, water. By converting the trivalent boronic acid group to a tetra-coordinated boronic acid group, the hydroxylation reaction converts the boronic acid- functionalized polymer to a boronic acid-functionalized anion exchange polymer.

[0109] FIG. 7C shows an illustrative reaction scheme for hydroxide anion exchange using the boronic acid-functionalized anion exchange polymer produced by the reaction scheme of FIG. 7B. As shown in FIG. 7C, the tetra-coordinated boronic acid group undergoes reconstruction in the presence of an incoming hydroxide ion, as explained above, releasing a hydroxide anion from the tetra-coordinate boronic acid group. The hydroxide anion that is released is available for anion exchange with a neighboring hydroxide group of the same or a different tetra-coordinated boronic acid group. This process repeats throughout the polymermaterial, thus effectively transporting hydroxide anions from a first side of a polymer material to an opposite side of the polymer material.

[0110] Illustrative examples of crosslinked PBI polymer molecules, and reaction schemes for crosslinking PBI polymer molecules with another polymer molecule, will now be shown and described with reference to FIGS. 8-10. The following examples are merely illustrative and are not limiting. In the drawings, only a benzimidazole unit of the repeating unit of the PBI polymer molecules is shown, but the repeating unit may have any other units and structure as may serve a particular implementation.

[0111] FIG. 8 shows an illustrative reaction scheme for intramolecular crosslinking of two PBI polymer molecules using boric acid (B(OH)3) as the crosslinking agent. In some examples, the reaction scheme of FIG. 8 is carried out to crosslink PBI polymer layers or sheets. PBI polymer molecules are combined with boric acid. Boric acid reacts with the secondary amines of the PBI polymer molecules to form a tetravalent boron crosslink having the general formula — B(OH)2 — . The boron atom has four covalent bonds and thus gains a negative formal charge, thereby making the tetravalent boron crosslink intrinsically ionic and acidic and capable of functioning as a hydroxide anion exchange agent by the reconstruction process described herein. In some examples, boric acid is a limiting reagent to control the extent of crosslinking as well as the anion exchange capacity of the resulting boronic acid-functionalized anion exchange PBI polymer composition.

[0112] FIG. 9 shows an illustrative reaction scheme for intramolecular crosslinking of a PBI polymer molecule with a hydroxyl-functionalized PTFE polymer using boric acid as the crosslinking agent. As shown, PTFE (represented by the open circle) is functionalized with an acid group represented by X-OH in which X is sulfur (S), carbon (C), or phosphorous (P) and is covalently bonded to the oxygen (O) atom of the hydroxyl group. The acid group X-OH may be any suitable acid group such as, without limitation, a carboxylic acid group, a sulfonic acid group, a phosphoric acid group, a phosphonic acid group, or an alcohol. In some examples, X is a Ci to C30 alkyl linker chain and optionally has one or more pendant moieties, which may be the same or different and may each be independently selected from the group consisting of hydrogen, a hydroxyl group, a fluoro group, a chloro group, a dialkylamino group, a cyano group, a carboxylic acid group, a carboxylic amide group, an ester group, an alkyl group, an alkoxy group, and an aryl group. In other examples, X is omitted so that PTFE is functionalized with only the hydroxyl group. While FIG. 9 shows that PTFE is functionalized with only one acid group X-OH, PTFE may have any degree of functionalization as may serve a particular implementation.

[0113] In the reaction scheme of FIG. 9, the PBI polymer molecule and the acid- functionalized PTFE polymer molecule are combined with boric acid. Boric acid reacts with the secondary amine of the first PBI polymer molecule and with the hydroxyl group of thefunctionalized PTFE to form a tetra-coordinated boronic acid group crosslink having the general formula — B(OH)2 — . The boron atom has four covalent bonds and a negative formal charge, thereby making the tetravalent boron crosslink intrinsically ionic and acidic and capable of functioning as a hydroxide anion exchange agent through the reconstruction process described herein. In some examples, boric acid is a limiting reagent to control the extent of crosslinking as well as the anion exchange capacity of the resulting boronic acid-functionalized anion exchange PBI / PTFE polymer composition.

[0114] While FIG. 9 shows that a PBI polymer molecule is crosslinked with a functionalized PTFE polymer molecule, the PBI polymer molecule may be crosslinked in a similar manner with any other suitable acid- or hydroxyl-functionalized polymer, such as a synthetic polymer (e.g., poly(phosphoric acid) (PPA)) or a natural polymer (e.g., lignin, cellulose, chitin, etc.). For example, a PBI polymer may be crosslinked with a PPA-doped polymer. The crosslinked PBI polymer prevents or reduces leaching out of the PPA dopant from the PPA-doped polymer.

[0115] In the examples of FIGS. 8 and 9, two polymer molecules are crosslinked using boric acid. In modifications of these reaction schemes, two polymer molecules may be crosslinked using a boronic acid (e.g., (R — B(OH)2.

[0116] FIG. 10 shows an illustrative reaction scheme for crosslinking a PBI polymer molecule with a functionalized PTFE polymer molecule using an aminoboronic acid as the crosslinking agent. In a first step 1002, a sulfonyl fluoride-functionalized PTFE (the PTFE backbone is represented by an open circle) is combined with 4-aminophenylboronic acid. Sulfonyl fluoride-functionalized PTFE forms a strong covalent bond with aminoboronic acid to form an intermediate aminoboronic acid-functionalized PTFE polymer molecule. A non-aqueous base may optionally be added to capture hydrogen fluoride (HF), thus helping to shift the reaction equilibrium to the right (to the product). However, any other suitable aminoboronic acid or derivative thereof may be used, including, without limitation, aminophenyl, aminoaryl, and aminoalkyl boronic acids. In some examples, an aminoboronic acid having a trivalent boronic acid group may be hydroxylated as described herein to convert the trivalent boronic acid group of the aminoboronic acid to a tetra-coordinated boronic acid group.

[0117] In a second step 1004, the aminoboronic acid-functionalized PTFE polymer molecule is combined with a PBI polymer molecule. The tetra-coordinated boronic acid group of the aminoboronic acid-functionalized PTFE polymer molecule reacts with the secondary amine of the PBI polymer molecule, thereby crosslinking the PBI polymer molecule with the functionalized PTFE polymer molecule. The boron atom of the tetra-coordinated boronic acid group crosslink has four covalent bonds and a negative formal charge, thereby making the tetra-coordinated boronic acid group crosslink intrinsically ionic and acidic and capable of functioning as a hydroxide anion exchange agent.

[0118] While FIG. 10 shows that a PBI polymer molecule is crosslinked with a functionalized PTFE polymer molecule, a PBI polymer molecule may be crosslinked in a similar manner with any other suitable activated polymer molecule, including, without limitation, synthetic or natural polymer molecules.

[0119] As shown in FIG. 10, the reaction scheme begins with a sulfonyl fluoride- functionalized PTFE polymer molecule. In other examples (not shown), the reaction scheme may begin with a sulfonic acid-functionalized PTFE polymer molecule (or other polymer molecule). However, since a sulfonic acid group generally does not directly covalently bond with the primary amine of an aminoboronic acid, the sulfonic acid group is activated to a sulfonyl fluoride group (SO2F) for reaction with the primary amine of the aminoboronic acid. Alternatively, the sulfonic acid group may be activated to a sulfonyl chloride group (SO2CI) for reaction with the primary amine of the aminoboronic acid.

[0120] FIG. 11 shows an illustrative reaction scheme for intramolecular crosslinking of a PBI polymer molecule with a PPA polymer molecule using boric acid as the crosslinking agent. PPA has the general structure shown below as Formula (X):(X): poly(phosphoric acid) (PPA)In some examples, PPA is used as a dopant to improve ionic conductivity of PBI polymers, such as 4F-PBL However, the direct linking of PPA with PBI happens through weak, equilibrating acid-base interactions only between the acidic phosphates of PPA and basic imidazole nitrogen atoms of PBI. As a result, acidic phosphate residues leach out of the PEMs and ionomers, thus degrading the ion exchange performance of the PBI polymer. To improve PBI / PPA polymer performance, a PBI polymer may be crosslinked with a PPA polymer using a tetra-coord inated boronic acid group crosslink, as shown in the example of FIG. 9. Note that, in the example of FIG. 9, only a single repeating unit of a PPA polymer molecule is shown. However, the degree of crosslinking may be controlled as desired.

[0121] As shown in FIG. 11, the PBI polymer molecule and the PPA polymer molecule are combined with boric acid. Boric acid reacts with the secondary amine of the PBI polymer molecule and with the hydroxyl group of the PPA polymer molecule to form a tetra-coordinated boronic acid group crosslink having the general formula — B(OH)2 — . The boron atom bonds with the nitrogen of the secondary amine and the oxygen atom of the PPA molecule in addition to the two hydroxyl groups. Thus, the boron atom has four covalent bonds and a negative formal charge. As a result, the tetra-coordinated boronic acid group crosslink is intrinsicallyionic and acidic and capable of functioning as a hydroxide anion exchange agent. Moreover, crosslinking through the tetra-coordinated boron atom occurs through multiple boron-oxygen bonds, thus strongly binding PPA and adding robustness and minimizing leaching of PPA. In some examples, boric acid is a limiting reagent to control the extent of crosslinking as well as the ion exchange capacity of the resulting boronic acid-functionalized anion exchange PBI / PPA polymer composition.

[0122] In the example of FIG. 11 , PBI and PPA polymer molecules are crosslinked using boric acid. In a modification of this reaction scheme, PBI and PPA polymer molecules may be crosslinked using a boronic acid (e.g., (R — B(OH)2.

[0123] The boronic acid-functionalized anion exchange polymers and polymer compositions described herein may be formed as a porous polymer network and / or may be used as ionomers, membranes (e.g., AEMs), support structures, among other uses, in electrochemical systems, such as water electrolysis systems and fuel cell systems, as well as in 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 generation of electricity used for separation of nitrogen gas from air and / or during the Haber-Bosch process).

[0124] Boronic acid-functionalized anion exchange materials also include non-polymeric materials (e.g., inorganic materials, ceramics, composites, etc.), such as catalysts, catalyst particles, and catalyst supports. For example, a metal oxide catalyst or catalyst particle (e.g., an electrocatalyst particle) may be functionalized with a trivalent boronic acid group, as described in International Patent Application No. PCT / US2023 / 026114, filed February 24, 2023, which is incorporated herein by reference in its entirety. The boronic acid group-fu notional ized catalyst or catalyst particle may then be hydroxylated, as described herein, to produce a tetracoordinated boronic acid-functionalized anion exchange catalyst or catalyst particle. A tetracoordinated boronic acid-functionalized anion exchange catalyst or catalyst particle may be used, for example, in catalyst layers, catalyst coated membranes, and membrane electrode assemblies.

[0125] A carbon material, such as carbon black, graphite, graphene, charcoal, activated charcoal, soot, coal, fullerenes, and carbon nanotubes, may also be functionalized with a trivalent boronic acid group, as described in International Patent Application No. PCT / US2024 / 058113, filed December 2, 2024, which is incorporated herein by reference in its entirety. The boronic acid group-functionalized carbon material may then be hydroxylated, as described herein, to produce a tetra-coordinated boronic acid-functionalized anion exchange carbon material. A tetra-coordinated boronic acid-functionalized anion exchange carbon material may be used, for example, as a catalyst support in catalyst layers, catalyst coated membranes, and membrane electrode assemblies.

[0126] FIG. 12 shows an illustrative anion exchange membrane water electrolysis system 1200 (AEM water electrolysis system 1200). AEM water electrolysis system 1200 uses electricity to split water into oxygen (O2) and hydrogen (H2) via an electrochemical reaction. The configuration of AEM water electrolysis system 1200 is merely illustrative and not limiting, as other suitable configurations as well as other suitable water electrolysis systems may incorporate boronic acid-functionalized anion exchange polymers.

[0127] As shown in FIG. 12, AEM water electrolysis system 1200 includes a membrane electrode assembly 1202 (MEA 1202), porous transport layers 1204-1 and 1204-2 (e.g., gas diffusion layers), bipolar plates 1206-1 and 1206-2, and an electrical power supply 1208. AEM water electrolysis system 1200 may also include additional or alternative components not shown in FIG. 12 as may serve a particular implementation.

[0128] MEA 1202 includes an AEM 1210 positioned between a first catalyst layer 1212-1 and a second catalyst layer 1212-2. AEM 1210 electrically isolates first catalyst layer 1212-1 from second catalyst layer 1212-2 while providing selective conductivity of anions, such as hydroxide anions (OH-), and while being impermeable to gases such as hydrogen and oxygen. AEM 1210 may be implemented by any suitable AEM, including any boronic acid-functionalized anion exchange polymer membrane described herein. First catalyst layer 1212-1 and second catalyst layer 1212-2 are electrically conductive electrodes that include catalyst solid supports bound with electrocatalyst particles (not shown), such as platinum group metals, metal alloys, and / or metal oxides.

[0129] In some examples, first catalyst layer 1212-1 and / or second catalyst layer 1212-2 include a supported catalyst mixed with an ionomer (an ion-conducting polymer). The ionomer binds the catalysts within the electrode, binds the catalyst layer on the AEM, and provides a pathway for anions, thereby improving anion conductivity. The ionomer used in first catalyst layer 1212-1 and / or second catalyst layer 1212-2 may include any suitable ionomer, including any boronic acid-functionalized anion exchange polymer described herein. In some examples, first catalyst layer 1212-1 and / or second catalyst layer 1212-2 is coated with a boronic acid- functionalized anion exchange polymer, as described herein.

[0130] MEA 1202 is placed between porous transport layers 1204-1 and 1204-2, which are in turn placed between bipolar plates 1206-1 and 1206-2 with flow channels 1214-1 and 1214-2 located between porous transport layers 1204 and bipolar plates 1206.

[0131] In MEA 1202, first catalyst layer 1212-1 functions as an anode and second catalyst layer 1212-2 functions as a cathode. When AEM water electrolysis system 1200 is powered by power supply 1208, electrons are conducted from first catalyst layer 1212-1 to second catalyst layer 1212-2 by a conductive path around AEM 1210, a hydrogen evolution reaction (HER) occurs at second catalyst layer 1212-2, and an oxygen evolution reaction (OER) occurs at first catalyst layer 1212-1. The OER and HER are two complementary electrochemical reactions forsplitting water by electrolysis, represented by the following overall water electrolysis reaction (1):2H2O - > 2H2+ O2(1)

[0132] The HER is represented by the following electrochemical half-reaction (2): catalyst4H2O + 4e“ - > 2H2+ 40H“ (2)The HER is facilitated by the electrocatalysts bound to the catalyst solid supports in second catalyst layer 1212-2. At second catalyst layer 1212-2, the electrons split water to form hydrogen gas and hydroxide anions, and the hydroxide anions are conducted from second catalyst layer 1212-2 to first catalyst layer 1212-1 through AEM 1210. AEM 1210 allows for the transport of hydroxide anions from second catalyst layer 1212-2 to first catalyst layer 1212-1 but is impermeable to oxygen and hydrogen gases. The hydroxide anions are driven through AEM 1210 by an electric field established by a potential difference between first catalyst layer 1212-1 and second catalyst layer 1212-2 and / or by an anion concentration gradient between the two sides of AEM water electrolysis system 1200. The hydroxide anions are transported across AEM 1210 to the anode side (first catalyst layer 1212-1 side) by the tetra-coordinated boronic acid groups of AEM 1 10 through the process of reconstruction, as described above.Anion transport is a facile process for boronic acid-functionalized anion exchange polymers and ionomers because the tetra-coordinated boronic acid groups function as anion exchange groups.

[0133] The OER is represented by the following electrochemical half-reaction (3): catalyst40H- - > O2+ 2H2O + 4e“ (3)The OER is facilitated by the electrocatalysts bound to the catalyst solid supports in first catalyst layer 1212-1. At first catalyst layer 1212-1, the hydroxide anions combine to form water and oxygen gas and lose electrons in the process.

[0134] FIG. 13 shows an illustrative anion exchange membrane fuel cell 1300 (AEM fuel cell 1300). AEM fuel cell 1300 produces electricity as a result of electrochemical reactions. In this example, the electrochemical reactions involve reacting hydrogen gas (H2) and oxygen gas (O2) to produce water and electricity. The configuration of AEM fuel cell 1300 is merely illustrative and not limiting.

[0135] As shown in FIG. 13, AEM fuel cell 1300 includes a membrane electrode assembly 1302 (MEA 1302), porous transport layers 1304-1 and 1304-2 (e.g., gas diffusion layers), bipolar plates 1306-1 and 1306-2. An electrical load 1308 may be electrically connected to MEA 1302 and driven by AEM fuel cell 1300. AEM fuel cell 1300 may also include additional or alternative components not shown in FIG. 13 as may serve a particular implementation.

[0136] MEA 1302 includes an AEM 1310 positioned between a first catalyst layer 1312-1 and a second catalyst layer 1312-2. AEM 1310 electrically isolates first catalyst layer 1312-1from second catalyst layer 1312-2 while providing selective conductivity of anions, such as hydroxide anions, and while being impermeable to gases such as hydrogen and oxygen. AEM 1310 may be implemented by any suitable AEM, including any boronic acid-functionalized anion exchange polymer membrane described herein. First catalyst layer 1312-1 and second catalyst layer 1312-2 are electrically conductive electrodes that include catalyst solid supports that bind electrocatalyst particles (not shown), such as platinum metals, metal alloys, and / or metal oxides.

[0137] In some examples, first catalyst layer 1312-1 and / or second catalyst layer 1312-2 include a supported catalyst mixed with an ionomer. The ionomer binds the catalysts within the electrode, binds the catalyst layer on the AEM, and provides a pathway for anions, thereby improving anion conductivity. The ionomer used in first catalyst layer 1312-1 and second catalyst layer 1312-2 may include any suitable ionomer, including any boronic acid- functionalized anion exchange polymer described herein. In some examples, first catalyst layer 1312-1 and / or second catalyst layer 1312-2 is coated with a boronic acid-functionalized anion exchange polymer, as described herein.

[0138] MEA 1302 is placed between porous transport layers 1304-1 and 1304-2, which are in turn placed between bipolar plates 1306-1 and 1306-2 with flow channels 1314 located between porous transport layers 1304 and bipolar plates 1306.

[0139] In MEA 1302, first catalyst layer 1312-1 functions as an anode and second catalyst layer 1312-2 functions as a cathode. First catalyst layer 1312-1 and second catalyst layer 1312- 2 are electrically connected to load 1308, and electricity generated by AEM fuel cell 1300 drives load 1308.

[0140] During operation of AEM fuel cell 1300, hydrogen gas (H2) flows into the anode side of AEM fuel cell 1300 and oxygen gas (O2) flows into the cathode side of AEM fuel cell 1300. At first catalyst layer 1312-1, the hydrogen gas combines with hydroxide anions according to the following hydrogen oxidation reaction (HOR), resulting in the production of water and electrons:40H" + 2H2- > 4H2O + 4e“ (4)The HOR is facilitated by the electrocatalysts particles bound to the catalyst solid supports in first catalyst layer 1312-1. The electrons produced at first catalyst layer 1312-1 are conducted from first catalyst layer 1312-1 to second catalyst layer 1312-2 around AEM 1310 through a conductive path and load 1308.

[0141] At second catalyst layer 1312-2, oxygen molecules combine with water and the electrons according to the oxygen reduction reaction (ORR) to produce hydroxide anions: catalyst O2+ 2H2O + 4e“ - > 40H" (5)The ORR is facilitated by the electrochemical catalysts particles bound to the catalyst solid supports in first catalyst layer 1312-1. The hydroxide anions are conducted from second catalyst layer 1312-2 to first catalyst layer 1312-1 through AEM 1310. AEM 1310 allows for thetransport of hydroxide anions from second catalyst layer 1312-2 to first catalyst layer 1312-1 but is impermeable to oxygen and hydrogen gases. The hydroxide anions are driven through AEM 1310 by an electric field established by a potential difference between second catalyst layer 1312-2 and first catalyst layer 1312-1 and / or by an anion concentration gradient between the two sides of AEM full cell 1300. The hydroxide anions are transported across AEM 1310 to the anode side (first catalyst layer 1312-1 side) by the tetra-coordinated boronic acid groups of AEM 1310 through the process of reconstruction, as described above. Anion transport is a facile process for boronic acid-functionalized anion exchange polymers and ionomers because the tetra-coordinated boronic acid groups function as anion exchange groups.

[0142] Thus, the overall electrochemical reaction for AEM fuel cell 1300 is:2H2+ O2- 2H2O (6)

[0143] In the overall reaction, AEM fuel cell 1300 produces water at first catalyst layer 1312-1. Water may flow from first catalyst layer 1312-1 to second catalyst layer 1312-2 through AEM 1310 and may be removed through outlets at the cathode side and / or anode side of AEM fuel cell 1300. The overall reaction generates electrons at the anode that drive load 1308.

[0144] In the examples of FIGS. 12 and 13, MEA 1202 and MEA 1302 include catalyst layers 1212 and 1312 formed on AEM 1210 and 1310. In alternative configurations, catalyst layers 1212 and 1312 may be coated on AEM 1210 and 1310 to thereby form a catalyst coated membrane (COM). For example, catalyst layers 1212 and 1312 may be formed in a one-pot process or in stages and sprayed onto AEM 1210 and 1310.

[0145] The boronic acid-functionalized anion exchange polymers have been described herein for use in electrochemical cell applications, such as water electrolysis and hydrogen fuel cell applications. However, boronic acid-functionalized anion exchange polymers may also be used in other applications. In some examples, the boronic acid-functionalized anion exchange polymers may be used in batteries or flow batteries.

[0146] In the preceding description, various exemplary embodiments have been described with reference to the accompanying drawings. It will, however, be evident that various modifications and changes may be made thereto, and additional embodiments may be implemented, without departing from the scope of the claims that follow. For example, certain features of one embodiment described herein may be combined with or substituted for features of another embodiment described herein. The description and drawings are accordingly to be regarded in an illustrative rather than a restrictive sense.

[0147] Advantages and features of the present disclosure can be further described by the following examples:

[0148] Example 1. A method comprising: conducting hydroxide ions from a first side of an anion exchange membrane to a second side of the anion exchange membrane by areconstruction process, wherein the anion exchange membrane comprises a boronic acid- functionalized anion exchange polymer.

[0149] Example 2. The method of example 2, wherein the boronic acid-functionalized anion exchange polymer comprises a main chain and tetra-coordinated boronic acid groups in at least one of side chains, side groups, or crosslinks.

[0150] Example 3. The method of example 2, wherein the tetra-coordinated boronic acid groups have the general formula (I):

[0151] Example 4. The method of example 2, wherein the tetra-coordinated boronic acid groups have the general formula (II):

[0152] Example 5. The method of example 2, wherein the tetra-coordinated boronic acid groups have the general formula (III):

[0153] Example 6. The method of any of examples 1 to 5, wherein the boronic acid- functionalized anion exchange polymer comprises a boronic acid-functionalized anion exchange polybenzimidazole (PBI) polymer.

[0154] Example 7. The method of any of examples 1 to 5, wherein the boronic acid- functionalized anion exchange polymer comprises a boronic acid-functionalized anion exchange polystyrene polymer.

[0155] Example 8. The method of any of examples 1 to 5, wherein the boronic acid- functionalized anion exchange polymer comprises a boronic acid-functionalized anion exchange polychlorotrifluoroethylene (PCTFE) polymer.

[0156] Example 9. The method of any of examples 1 to 5, wherein the main chain comprises a main chain of a sulfonic acid-functionalized polytetrafluoroethylene (PTFE) polymer.

[0157] Example 10. The method of any of examples 1 to 5, wherein the boronic acid- functionalized anion exchange polymer comprises a boronic acid-functionalized polyphenylene sulfide (PPS) polymer.

[0158] Example 11. The method of any of the preceding examples, wherein the reconstruction process comprises adding water to the first side of the anion exchange membrane.

[0159] Example 12. A method of making an anion exchange polymer, comprising: hydroxylating a boronic acid-functionalized polymer comprising a trivalent boronic acid group to convert the trivalent boronic acid group to a tetra-coordinated boronic acid group.

[0160] Example 13. The method of example 12, wherein the hydroxylating the boronic acid- functionalized polymer comprises combining the boronic acid-functionalized polymer with a hydroxylating agent.

[0161] Example 14. The method of example 13, wherein the hydroxylating agent comprises water.

[0162] Example 15. The method of example 13, wherein the hydroxylating agent comprises an aqueous alkali solution.

[0163] Example 16. The method of example 15, wherein the aqueous alkali solution comprises ammonium hydroxide (NH4OH).

[0164] Example 17. The method of example 15, wherein the aqueous alkali solution comprises sodium hydroxide (NaOH) or potassium hydroxide (KOH).

[0165] Example 18. The method of any of examples 12 to 17, wherein the boronic acid- functionalized polymer comprises a boronic acid-functionalized polybenzimidazole (PBI) polymer.

[0166] Example 19. The method of any of examples 12 to 17, wherein the boronic acid- functionalized polymer comprises a boronic acid-functionalized polystyrene polymer.

[0167] Example 20. The method of any of examples 12 to 17, wherein the boronic acid- functionalized polymer comprises a boronic acid-functionalized polychlorotrifluoroethylene (PCTFE) polymer.

[0168] Example 21. The method of any of examples 12 to 17, wherein the boronic acid- functionalized polymer comprises a boronic acid derivative of a sulfonic acid-functionalized polytetrafluoroethylene (PTFE) polymer.

[0169] Example 22. The method of any of examples 12 to 17, wherein the boronic acid- functionalized anion exchange polymer comprises a boronic acid-functionalized polyphenylene sulfide (PPS) polymer.

[0170] Example 23. The method of any of examples 12 to 17, further comprising: making the boronic acid-functionalized polymer.

[0171] Example 24. The method of example 23, wherein the synthesizing the boronic acid- functionalized polymer comprises functionalizing a polymer with a pendant trivalent boronic acid group.

[0172] Example 25. The method of example 24, wherein the polymer comprises a polybenzimidazole (PBI) polymer.

[0173] Example 26. The method of example 24, wherein the polymer comprises a polystyrene polymer.

[0174] Example 27. The method of example 24, wherein the polymer comprises a polychlorotrifluoroethylene (PCTFE) polymer.

[0175] Example 28. The method of example 24, wherein the polymer comprises a sulfonic acid-functionalized polytetrafluoroethylene (PTFE) polymer.

[0176] Example 29. A method of making an anion exchange polymer, comprising: crosslinking a polybenzimidazole (PBI) polymer with a second polymer using a boronic crosslinking agent, wherein the boronic crosslinking agent comprises boric acid or a boronic acid having the general formula R — B(OH)2.

[0177] Example 30. The method of example 29, wherein the second polymer molecule comprises a PBI polymer molecule.

[0178] Example 31. The method of example 29, wherein the second polymer molecule comprises a hydroxyl-functionalized PTFE polymer, a poly(phosphoric acid) (PPA) polymer, or a PPA-doped polymer.

[0179] Example 32. An anion exchange polymer comprising: a main chain; and pendant tetra-coord inated boronic acid groups in at least one of side chains or side groups, wherein the pendant tetra-coordinated boronic acid groups have the general formula (I) or (II):

[0180] Example 33. The anion exchange polymer of example 34, wherein the main chain comprises a polybenzimidazole (PBI) polymer main chain or a derivative thereof.

[0181] Example 34. The anion exchange polymer of claim 32, wherein the main chain comprises a polystyrene polymer main chain or a derivative thereof.

[0182] Example 35. The anion exchange polymer of example 32, wherein the main chain comprises a polychlorotrifluoroethylene (PCTFE) polymer main chain or a derivative thereof.

[0183] Example 36. The anion exchange polymer of example 32, wherein the main chain comprises a polytetrafluoroethylene (PTFE) polymer main chain or a derivative thereof.

[0184] Example 37. The anion exchange polymer of example 32, wherein the main chain comprises a polyphenylene sulfide (PPS) polymer main chain or a derivative thereof.

[0185] Example 38. A membrane electrode assembly comprising: a first catalyst layer; a second catalyst layer; and an anion exchange membrane positioned between the first catalyst layer and the second catalyst layer; wherein at least one of the first catalyst layer, the secondcatalyst layer, or the anion exchange membrane comprises a boronic acid-functionalized anion exchange polymer comprising: a main chain; and a tetra-coordinated boronic acid group in at least one of a side chain, a side group, or a crosslink.

[0186] Example 39. The membrane electrode assembly of example 38, wherein the boronic acid-functionalized anion exchange polymer comprises a boronic acid-functionalized anion exchange polymer made by the method of any of examples 12 to 31.

[0187] Example 40. The membrane electrode assembly of example 38, wherein the boronic acid-functionalized anion exchange polymer comprises the anion exchange polymer of any of examples 32 to 37.

[0188] Example 41. The method of any of claims 1 to 11 , wherein the method is performed using the membrane electrode assembly of any of examples 38 to 40.

Claims

CLAIMSWhat is claimed is:

1. A method comprising: conducting hydroxide ions from a first side of an anion exchange membrane to a second side of the anion exchange membrane by a reconstruction process, wherein the anion exchange membrane comprises a boronic acid-functionalized anion exchange polymer.

2. The method of claim 1, wherein the boronic acid-functionalized anion exchange polymer comprises a main chain and tetra-coord inated boronic acid groups in at least one of side chains, side groups, or crosslinks.

3. The method of claim 2, wherein the tetra-coordinated boronic acid groups have the general formula (I):

4. The method of claim 2, wherein the tetra-coordinated boronic acid groups have the general formula (II):

5. The method of claim 2, wherein the tetra-coordinated boronic acid groups have the general formula (III):

6. The method of claim 1, wherein the boronic acid-functionalized anion exchange polymer comprises a boronic acid-functionalized anion exchange polybenzimidazole (PBI) polymer.

7. The method of claim 1, wherein the boronic acid-functionalized anion exchange polymer comprises a boronic acid-functionalized anion exchange polystyrene polymer.

8. The method of claim 1, wherein the boronic acid-functionalized anion exchange polymer comprises a boronic acid-functionalized anion exchange polychlorotrifluoroethylene (PCTFE) polymer.

9. The method of claim 1, wherein a main chain of the boronic acid-functionalized anion exchange polymer comprises a main chain of a sulfonic acid-functionalized polytetrafluoroethylene (PTFE) polymer.

10. The method of claim 1, wherein the boronic acid-functionalized anion exchange polymer comprises a boronic acid-functionalized polyphenylene sulfide (PPS) polymer.

11. The method of claim 1 , wherein the reconstruction process comprises adding water to the first side of the anion exchange membrane.

12. A method of making an anion exchange polymer, comprising: hydroxylating a boronic acid-functionalized polymer comprising a trivalent boronic acid group to convert the trivalent boronic acid group to a tetra-coordinated boronic acid group.

13. The method of claim 12, wherein the hydroxylating the boronic acid- functionalized polymer comprises combining the boronic acid-functionalized polymer with a hydroxylating agent.

14. The method of claim 13, wherein the hydroxylating agent comprises water.

15. The method of claim 13, wherein the hydroxylating agent comprises an aqueous alkali solution.

16. The method of claim 15, wherein the aqueous alkali solution comprises ammonium hydroxide (NH4OH).

17. The method of claim 15, wherein the aqueous alkali solution comprises sodium hydroxide (NaOH) or potassium hydroxide (KOH).

18. The method of claim 12, wherein the boronic acid-functionalized polymer comprises a boronic acid-functionalized polybenzimidazole (PBI) polymer.

19. The method of claim 12, wherein the boronic acid-functionalized polymer comprises a boronic acid-functionalized polystyrene polymer.

20. The method of claim 12, wherein the boronic acid-functionalized polymer comprises a boronic acid-functionalized polychlorotrifluoroethylene (PCTFE) polymer.

21. The method of claim 12, wherein the boronic acid-functionalized polymer comprises a boronic acid derivative of a sulfonic acid-functionalized polytetrafluoroethylene (PTFE) polymer.

22. The method of claim 12, wherein the boronic acid-functionalized anion exchange polymer comprises a boronic acid-functionalized polyphenylene sulfide (PPS) polymer.

23. The method of claim 12, further comprising: making the boronic acid-functionalized polymer.

24. The method of claim 23, wherein making the boronic acid-functionalized polymer comprises functionalizing a polymer with a pendant trivalent boronic acid group.

25. The method of claim 24, wherein the polymer comprises a polybenzimidazole (PBI) polymer.

26. The method of claim 24, wherein the polymer comprises a polystyrene polymer.

27. The method of claim 24, wherein the polymer comprises a polychlorotrifluoroethylene (PCTFE) polymer.

28. The method of claim 24, wherein the polymer comprises a sulfonic acid- functionalized polytetrafluoroethylene (PTFE) polymer.

29. A method of making an anion exchange polymer, comprising: crosslinking a polybenzimidazole (PBI) polymer molecule with a second polymer molecule using a boronic crosslinking agent, wherein the boronic crosslinking agent comprises boric acid or a boronic acid having the general formula R — B(OH)2.

30. The method of claim 29, wherein the second polymer molecule comprises a PBI polymer molecule.

31. The method of claim 29, wherein the second polymer molecule comprises a hydroxyl-functionalized PTFE polymer molecule, a poly(phosphoric acid) (PPA) polymer molecule, or a PPA-doped polymer molecule.

32. An anion exchange polymer comprising: a main chain; and pendant tetra-coordinated boronic acid groups in at least one of side chains or side groups, wherein the pendant tetra-coordinated boronic acid groups have the general formula (I) or (II):

33. The anion exchange polymer of claim 32, wherein the main chain comprises a polybenzimidazole (PBI) polymer main chain or a derivative thereof.

34. The anion exchange polymer of claim 32, wherein the main chain comprises a polystyrene polymer main chain or a derivative thereof.

35. The anion exchange polymer of claim 32, wherein the main chain comprises a polychlorotrifluoroethylene (PCTFE) polymer main chain or a derivative thereof.

36. The anion exchange polymer of claim 32, wherein the main chain comprises a polytetrafluoroethylene (PTFE) polymer main chain or a derivative thereof.

37. The anion exchange polymer of claim 32, wherein the main chain comprises a polyphenylene sulfide (PPS) polymer main chain or a derivative thereof.

38. A membrane electrode assembly comprising: a first catalyst layer; a second catalyst layer; and an anion exchange membrane positioned between the first catalyst layer and the second catalyst layer; wherein at least one of the first catalyst layer, the second catalyst layer, or the anion exchange membrane comprises a boronic acid-functionalized anion exchange polymer comprising:a main chain; and a tetra-coordinated boronic acid group in at least one of a side chain, a side group, or a crosslink.

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