Method for preparing tetracoordinate boric acid functionalized polymer molecules

Abstract

A method for preparing a tetracoordinate boric acid-functionalized polymer molecule, comprising reacting pendant boric acid groups of the boric acid-functionalized polymer molecule with a fluoride reagent and / or a compound having the general formula HX, where HX is a Bronsted-Lowry acid. The tetracoordinate boric acid-functionalized polymer molecule comprises a backbone and tetracoordinate boric acid groups attached to the backbone. The tetracoordinate boric acid groups have the general formula -BF m X n (OH) (3-m-n) wherein B has four covalent bonds and is covalently attached to the polymer backbone, side chain, or side group; m and n are each independently 0, 1, 2, or 3; the sum of m+n is 1, 2, or 3; and X is an anion other than fluoride.

Description

[Technical Field]

[0001] This application claims priority to U.S. Provisional Patent Application No. 63 / 444,772 (filed February 10, 2023) and U.S. Provisional Patent Application No. 63 / 624,533 (filed January 24, 2024), which are incorporated by reference herein in their entireties. [Background technology]

[0002] In electrochemical cells, such as hydrogen fuel cells and water electrolysis systems, proton exchange membranes (PEMs) are used to selectively transport protons. + ) while blocking the permeation of gases. PEMs generally consist of a porous framework and strong acid functional groups. For example, polyfluorosulfonic acid-based PEMs, such as Nafion™ (The Chemours Company, Wilmington, Delaware) and Aquivion® (Solvay SA Corporation, Brussels, Belgium), contain a poly(tetrafluoroethylene) (PTFE) porous framework with pendant sulfonic acid side groups. The easily dissociated sulfonic acid groups function as proton transport agents within the membrane. In a hydrogen fuel cell, hydrogen gas (H2) is transported at the anode to form protons (H + ) and electrons. The protons pass through the PEM and combine with oxygen gas (O2) at the cathode to produce water, and the electrons flow through an external circuit to generate electricity. In a water electrolysis system, electricity is used to convert water into oxygen gas (O2) and protons (H + The protons pass through the PEM and combine with electrons at the cathode to produce hydrogen gas (H2).

[0003] A membrane electrode assembly (MEA) can include a PEM disposed between a first catalyst layer and a second catalyst layer. The catalyst layers are electrically conductive electrodes (anode and cathode) embedded with an electrochemical catalyst, such as a metal, metal alloy, or metal oxide. The catalyst can be supported on a catalytic solid support, typically an electrically conductive, high-surface-area carbon (e.g., graphite or graphene). The electrochemical catalyst reduces the activation energy of electrochemical reactions occurring at the electrodes, such as the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) in water electrolysis, and the hydrogen oxidation reaction (HOR) and oxygen reduction reaction (ORR) in fuel cells.

[0004] In some applications, the catalyst layer comprises a mixture of a supported catalyst and an ionomer (an ion-conducting polymer). The ionomer binds the catalyst within the electrode, anchors the catalyst layer on the PEM, and provides a pathway for cations (e.g., protons) to improve cation conductivity. In some MEAs, the catalyst layer is formed separately from the PEM and laminated onto the PEM in an MEA stack. In other MEAs, the catalyst layer is coated onto the PEM to form a catalyst-coated membrane (CCM).

[0005] Water electrolysis and fuel cell applications involve strong redox chemistry under acidic conditions ranging from ambient to elevated temperatures. Therefore, PEMs and ionomers, as well as the molecular functional groups they contain that are responsible for proton transport properties, must be robust even under harsh reaction conditions of redox stress. However, conventional polymers used in PEMs and ionomers, such as Nafion™ and Aquivion®, primarily contain sulfonic acid functional groups as proton transport agents. Due to the inherent physicochemical properties of sulfur, sulfonic acid functional groups have limited ability to withstand the redox stresses posed by electrochemical operations. Furthermore, sulfonic acid groups can participate in secondary redox reactions, poisoning platinum group metal catalysts and promoting depolymerization of the polymer matrix. [Brief explanation of the drawings]

[0006] The accompanying drawings illustrate various embodiments and constitute a part of this specification. The illustrated embodiments are merely examples and are not intended to limit the scope of the disclosure. Throughout the drawings, the same or similar reference numbers refer to the same or similar elements. [Figure 1] Figure 1A shows an example of a reaction scheme for the synthesis of a boric acid functionalized PBI polymer, and Figure 1B shows an example of a reaction scheme for the synthesis of a boric acid functionalized PBI polymer by boronation. [Figure 2] Figure 2A shows an example of a reaction scheme for synthesizing a boric acid-functionalized polystyrene polymer by boronation, and Figure 2B shows an example of a reaction scheme for synthesizing a boric acid-functionalized polymer using a vicinal diol. [Figure 3] Figure 3A shows an example of a reaction scheme for synthesizing a boric acid functionalized polymer by replacing sulfonic acid side groups with boric acid groups, and Figure 3B shows an example of a reaction scheme for synthesizing a boric acid functionalized polymer by replacing sulfonic acid side groups with boric acid groups. [Figure 4] 4A to 4C show an example of a reaction scheme for synthesizing a fluoroboric acid-functionalized polymer by subjecting a boric acid-functionalized polymer to a fluorination treatment. [Figure 5] 5A and 5B show an alternative reaction scheme for synthesizing a fluoroboric acid functionalized polymer by subjecting a boric acid functionalized polymer to a fluorination treatment. [Figure 6] FIG. 6 shows an example of a reaction scheme for synthesizing a fluoroboric acid-functionalized PBI polymer by subjecting a boric acid-functionalized PBI polymer to a fluorination treatment. [Figure 7] FIG. 7 shows an example of a reaction scheme for synthesizing a fluoroboric acid-functionalized PBI polymer by subjecting a boric acid-functionalized PBI polymer to a fluorination treatment. [Figure 8] FIG. 8 shows an alternative reaction scheme for synthesizing a fluoroboric acid-functionalized PBI polymer by subjecting a boric acid-functionalized PBI polymer to a fluorination treatment. [Figure 9]FIG. 9 shows an alternative reaction scheme for synthesizing a fluoroboric acid-functionalized PBI polymer by subjecting a boric acid-functionalized PBI polymer to a fluorination treatment. [Figure 10] 10A and 10B show an example of a reaction scheme for synthesizing a fluoroboric acid-functionalized polystyrene polymer by subjecting a boric acid-functionalized polystyrene polymer to a fluorination treatment. [Figure 11] 11A and 11B show an example of a reaction scheme for synthesizing a fluoroboric acid-functionalized polymer by subjecting a boric acid-functionalized polymer to a fluorination treatment. [Figure 12] FIG. 12 shows an alternative reaction scheme for synthesizing a fluoroboric acid functionalized polymer by subjecting a boric acid functionalized polymer to a fluorination treatment. [Figure 13] FIG. 13 shows an alternative reaction scheme for synthesizing a fluoroboric acid functionalized polymer by subjecting a boric acid functionalized polymer to a fluorination treatment. [Figure 14] FIG. 14 shows an example of another reaction scheme for synthesizing a fluoroboric acid functionalized polymer. [Figure 15] FIG. 15 shows an example of a proton exchange membrane water electrolysis system incorporating fluoroboric acid functionalized polymeric PEM and / or ionomer. [Figure 16] FIG. 16 shows an example of a proton exchange membrane fuel cell including a fluoroboric acid functionalized polymer PEM and / or ionomer. Summary of the Invention

[0007] <Detailed Description of the Invention> Tetracoordinate boric acid-functionalized polymer molecules comprise a backbone, side chains or side groups attached to the backbone, and pendant tetracoordinate boric acid groups present on the side chains or side groups. The pendant tetracoordinate boric acid groups comprise a tetracoordinate boron atom with a negative formal charge, which is neutralized by a cation (e.g., a proton). Tetracoordinate boric acid-functionalized polymers can be formed by post-polymerization functional modification of boric acid-functionalized polymers (e.g., boric acid-functionalized PBI polymers, polystyrene polymers, or PTFE polymers).

[0008] The tetracoordinate boric acid-functionalized polymers described herein can be used in electrochemical cell applications such as proton exchange membranes (PEMs), ionomers, catalyst layers, and membrane electrode assemblies (MEAs). Ionomers and PEMs based on tetracoordinate boric acid-functionalized polymers reduce or eliminate catalyst poisoning compared to sulfonic acid-functionalized polymers. In some examples, tetracoordinate boric acid-functionalized polymers, including tetracoordinate boric acid-functionalized PTFE polymers, are highly effective in PEM and ionomer applications, reducing or eliminating issues such as catalyst poisoning and depolymerization (polymer instability) while maintaining the functionality of Nafion™ and Aquivion®. Unlike conventional sulfonic acid-containing polymers, tetracoordinate boric acid-functionalized polymers are stable under the harsh electrochemical conditions of water electrolysis and hydrogen fuel cells without unzipping (i.e., depolymerization).

[0009] In order to understand the various aspects of this disclosure, several definitions are provided below. Each term or expression (e.g., alkyl, m, n, etc.) used herein, when used multiple times, is intended to be independent of its definition elsewhere in this specification. In the event of a conflict in a definition with a patent application or patent incorporated by reference herein, the present specification (including definitions) shall control.

[0010] As used herein, "polymer" refers to a material containing polymer molecules of the same or different polymeric species, including mixtures of polymer molecules of the same polymeric species, including variations in chain length and specific structural arrangements (e.g., irregularities in the orientation of monomer units, end groups, and / or the position and / or length of any side chains or side groups). "Polymer" includes homopolymers, copolymers, terpolymers, interpolymers, and the like.

[0011] As used herein, the term "polymer molecule" or "macromolecule" refers to a molecule of high relative molecular weight, the structure of which is a relatively large repeating structure (e.g., about 100 or more monomeric units) derived, actually or conceptually, from molecules of lower relative molecular weight (e.g., monomeric molecules).

[0012] As used herein, "polymerization" refers to the process of converting a monomer or mixture of monomers into a polymer.

[0013] As used herein, "oligomer" refers to a substance composed of oligomeric molecules.

[0014] As used herein, the term "oligomeric molecule" refers to a molecule of moderate relative molecular weight, whose structure is a relatively small repeating structure (e.g., about 5 to about 100 monomeric units) of units derived from molecules of lower relative molecular weight (e.g., monomeric molecules).

[0015] As used herein, "oligomerization" refers to the process of converting a monomer or mixture of monomers into an oligomer.

[0016] The principles, concepts, and features described herein relating to polymers, polymer molecules, and polymerizations apply equally to oligomers, oligomeric molecules, and oligomerizations, respectively. Thus, use of the terms polymer, polymer molecule, and polymerization herein can be substituted with the terms oligomer, oligomeric molecule, and oligomerization, respectively, without departing from the scope of the disclosure.

[0017] As used herein, "ionomer" refers to a polymer composed of ionomeric molecules.

[0018] As used herein, "ionomeric molecule" refers to a polymer molecule in which a small but significant proportion of its constitutional units have ionizable or ionic pendant groups (including ion-exchange groups as described herein), or both. Generally, the ionizable or ionic pendant groups do not exceed about 15 mole percent of the constitutional units.

[0019] As used herein, the term "monomer" refers to a substance made up of monomer molecules.

[0020] As used herein, a "monomer molecule" refers to a molecule that can undergo polymerization or oligomerization to form a polymer or oligomer molecule. Monomer molecules provide the building blocks that make up the basic structure of a polymer or oligomer molecule.

[0021] As used herein, the term "copolymer" refers to a polymer derived from more than one type of monomer.

[0022] As used herein, a "building block" refers to an atom or group of atoms (including pendant atoms or groups, if present) that constitutes part of the structure of a polymer molecule (or oligomeric molecule, block, chain).

[0023] As used herein, the term "repeating unit" refers to a structural unit that, when repeated, constitutes a polymer molecule (or oligomer molecule, block, or chain).

[0024] As used herein, the term "monomer unit" refers to the largest building block that a single monomer molecule contributes to the structure of a polymer or oligomer molecule.

[0025] As used herein, a "block" refers to a portion of a polymer molecule (or oligomer molecule) that is made up of multiple constitutional units and has at least one characteristic that is not present in adjacent portions.

[0026] As used herein, a "chain" refers to all or part of a polymer molecule (or oligomer molecule, block) having a linear or branched sequence of constitutional units between two boundary constitutional units, each of which may be an end group, a branch point, or other particular feature of a polymer molecule.

[0027] As used herein, "main chain" or "backbone" refers to the chain in a polymer molecule from which all other chains (long or short, or both) are considered pendant (e.g., side chains).

[0028] As used herein, "side chain" refers to an oligomeric chain (short chain) or polymeric chain (long chain) that branches off from the main chain of a polymer molecule.

[0029] As used herein, the term "side group" or "pendant group" refers to a group that branches off from a chain (eg, a backbone) and is not an oligomeric or polymeric chain.

[0030] The principles, concepts, and features described herein with respect to side chains apply equally to side groups, and therefore, use of the term side chain can be substituted for the term side group without departing from the scope of the disclosure.

[0031] As used herein, a "crosslink" refers to a small region of at least four chains extending from within a polymer molecule. Crosslinks are generally formed by reaction with sites or groups on existing polymer molecules or by interactions between existing polymer molecules.

[0032] The term "crosslinked" refers to a state in which previously separate polymer molecules are joined to one another, usually by covalent bonds, at sites other than their ends.

[0033] As used herein, "catalyst particles" refer to particles in "black" or pure form (e.g., without catalyst support or catalyst additives to which the catalyst particles are bound) that increase the rate of a reaction without modifying the standard Gibbs free energy change of the reaction. Catalyst particles can be individual molecules (e.g., including but not limited to, monomer molecules), molecular groups, crystalline structures (e.g., metal oxides), polymeric molecules, or oligomeric molecules. Catalyst particles can have any suitable size and shape, such as microparticles, nanoparticles, or nanotubes. Catalyst particles include, by way of example, metals, metal alloys, metal oxides, metal halides (e.g., metal chlorides), or composites that include at least one of a metal, metal alloy, metal oxide, or metal halide.

[0034] As used herein, "electrocatalyst particle" or "electrochemical catalyst particle" refers to a catalyst particle that reduces the activation energy required to carry out an electrochemical reaction and / or increases the rate of an electrochemical reaction (e.g., OER, HER, HOR, and / or ORR). Applicable electrocatalyst particles may include, but are not limited to, platinum group metals (PGM) (e.g., platinum, palladium, iridium, ruthenium, osmium, and rhodium), transition metals (e.g., silver, gold, cobalt, copper, iron, nickel, rhenium, and mercury), 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 oxide, iridium(IV) oxide, ruthenium(IV) oxide, iridium ruthenium oxide, platinum(IV) oxide, magnesium oxide, cerium(IV) oxide), metal halides (e.g., platinum(IV) chloride, iridium(III) chloride, platinum(IV) bromide, iridium(III) bromide), and / or composites of metals, metal alloys, metal oxides, and metal halides.

[0035] As used herein, the term "catalyst support" refers to a substance, excluding catalyst particles, used to support catalyst particles (e.g., a substance or material to which catalyst particles are bonded or supported). Examples of catalyst supports include, but are not limited to, carbon-based materials (e.g., carbon black, graphite, carbon nanotubes, graphene, and the boron-functionalized carbon materials described herein), titanium dioxide, Sb-doped SnO nanoparticles, tin-doped indium oxide (ITO), and 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.

[0036] As used herein, "catalyst" refers to a catalyst particle or a catalyst particle together with a catalyst support on which the catalyst particle is supported or bound. The catalyst may also include catalytic additives such as promoters (e.g., but not limited to, metalloids).

[0037] As used herein, "electrocatalyst" or "electrochemical catalyst" refers to electrocatalyst particles in "black" or pure form, or together with a catalyst support on which the electrocatalyst particles are supported or bonded. The electrocatalyst may also include catalytic additives such as promoters.

[0038] As used herein, "metal" is intended to include alkali metals, alkaline earth metals, transition metals, lanthanides, actinides, and post-transition metals.

[0039] As used herein, the term "transition metal" refers to the d-block elements (groups 3 to 12) of the periodic table.

[0040] As used herein, "post-transition metals" refers to aluminum, gallium, indium, tin, thallium, lead, bismuth, and polonium.

[0041] As used herein, "metalloid" refers to boron, silicon, germanium, arsenic, antimony, tellurium, and astatine.

[0042] As used herein, "platinum group metals" or "PGM" refers to platinum, palladium, iridium, ruthenium, osmium, and rhodium.

[0043] As used herein, an "aliphatic" compound refers to a hydrocarbon that is saturated or unsaturated, acyclic or cyclic, straight-chain or branched, and unsubstituted or partially or fully substituted with one or more substituents or functional groups. As will be understood by those skilled in the art, the term "aliphatic" as used herein includes, but is not limited to, alkyl, alkenyl, and alkynyl moieties. Examples of aliphatic groups include, but are not limited to, 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.

[0044] As used herein, "alkyl" has the meaning commonly understood in the art and is intended to include saturated aliphatic groups, including straight-chain alkyl groups, branched-chain alkyl groups, cycloalkyl (alicyclic) groups, alkyl-substituted cycloalkyl groups, cycloalkyl-substituted alkyl groups, and the like. Similar rules apply to other general terms (e.g., alkenyl, alkynyl, and the like). Furthermore, as used herein, the terms "alkyl," "alkenyl," "alkynyl," and the like are intended to include both fully or partially substituted and unsubstituted groups.

[0045] In some embodiments, a straight chain or branched chain alkyl group can have 1-30 carbon atoms in their backbone, and in some cases, 1-20 or fewer carbon atoms. In some embodiments, a straight chain or branched chain alkyl group can have 1-10 carbon atoms in their backbone (e.g., C1-C10 for straight chain, C3-C10 for branched chain), 6 or fewer carbon atoms, or 4 or fewer carbon atoms. A cycloalkyl can have 3-10 carbon atoms in its ring structure, and in some cases, 3-5, 6, or 7 carbon atoms in the ring structure. Examples of acyclic alkyl groups include, but are not limited to, methyl, ethyl, propyl, isopropyl, butyl, isobutyl, n-butyl, t-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 cyclohexyl.

[0046] The terms "alkenyl" and "alkynyl" refer to unsaturated aliphatic groups of length and possible substitution analogous to the alkyls described above, but which contain at least one double or triple bond respectively. Examples of alkenyl groups include, but are not limited to, ethenyl, propenyl, butenyl, 1-methyl-2-buten-1-yl, and the like. Examples of alkynyl groups include, but are not limited to, ethynyl, 2-propynyl (propargyl), 1-propynyl, and the like.

[0047] The term "heteroalkyl" refers to an alkyl group in which one or more hydrogen atoms bonded to a carbon of the alkyl group or one or more carbon atoms is replaced with a heteroatom. A heteroatom refers to an atom other than carbon. In some examples, the heteroatom is selected from the group consisting of N, O, P, B, S, Si, Sb, Al, Sn, As, Se, and Ge. Examples of heteroalkyl groups include, but are not limited to, methoxy, ethoxy, propoxy, isopropoxy, methylthio, ethylthio, propylthio, isopropylthio, n-butylthio, methoxymethyl, cyano groups, and the like.

[0048] 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.

[0049] The term "aryl" refers to an aromatic carbocyclic group, unsubstituted or fully or partially substituted, including a single ring (e.g., phenyl), multiple rings (e.g., biphenyl), or fused rings in which at least one ring is aromatic (e.g., 1,2,3,4-tetrahydronaphthyl, naphthyl, anthryl, phenanthryl). That is, at least one ring has a conjugated π-electron system, and other adjacent rings can be cycloalkyl, cycloalkenyl, cycloalkynyl, aryl, and / or heterocyclic. A "carbocyclic aryl group" refers to an aryl group in which the ring atoms on the aromatic ring are carbon atoms. Hydrocarbon aryl groups include single-ring hydrocarbon aryl groups and polycyclic or fused compounds (e.g., adjacent ring atoms are common to two adjacent rings), such as naphthyl groups. Examples of aryl groups include, but are not limited to, phenyl, naphthyl, tetrahydronaphthyl, indanyl, and indenyl.

[0050] The term "heteroaryl" refers to an aryl group containing at least one heteroatom as a ring atom, i.e., a heterocycle. Examples of heteroaryl groups include, but are not limited to, pyridyl, pyrazinyl, pyrimidinyl, pyrrolyl, pyrazolyl, imidazolyl, thiazolyl, oxazolyl, isoxazolyl, thiadiazolyl, oxadiazolyl, thiophenyl, furanyl, quinolinyl, isoquinolinyl, and the like.

[0051] The terms "alkoxyl" or "alkoxy" refer to an alkyl group having an oxygen radical attached thereto and are represented by the general formula R O. Examples of alkoxyl groups include, but are not limited to, methoxy, ethoxy, propoxy, and t-butoxy groups.

[0052] The term "aryloxy" refers to an aryl group having an oxygen radical attached thereto. Examples of alkoxyl groups include, but are not limited to, phenoxy groups.

[0053] Any of the above groups may be optionally substituted in whole or in part. Examples of the substituents include aliphatic, alicyclic, heteroaliphatic, heteroalicyclic, halogen, azido, 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, heterocyclic, aromatic or heteroaromatic group, -CF3, -CN, aryl, allyloxy, perhaloalkoxy, aralkoxy, heteroaryl, heteroaryloxy, heteroarylalkyl, heteroaralkoxy, azido, amino, halide, These include, but are not limited to, alkylthio, oxo, acylalkyl, carboxyester, -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), phosphate (e.g., PO4(R')3), silane (e.g., Si(R')4), urethane (e.g., R'O(CO)NHR'), and the like. In addition, the substituents can be selected from F, Cl, Br, I, -OH, -NO2, -CN, -NCO, -CF3, -CH2CF3, -CHCl2, -CH2ORx, -CH2CH2ORx, -CH2N(Rx)2, -CH2SO2CH3, -C(O)Rx, -O2(Rx), -CON(Rx)2, -OC(O)Rx, -C(O)OC(O)Rx, -OCO2Rx, -OCON(Rx)2, -N(Rx)2, -S(O)2Rx, -OCO2Rx, -NRx(CO)Rx, -NRx(CO)N(Rx)2, etc.wherein each Rx independently includes, but is not limited to, hydrogen, aliphatic, alicyclic, heteroaliphatic, heteroalicyclic, aryl, heteroaryl, alkylaryl, or alkylheteroaryl, wherein the foregoing substituents may be wholly or partially substituted or unsubstituted, branched or unbranched, cyclic or linear, and the foregoing aryl or heteroaryl groups may also be wholly or partially substituted or unsubstituted.

[0054] The tetracoordinate boric acid-functionalized polymers (e.g., fluoroboric acid-functionalized polymers) described herein can be used in PEM and ionomer applications. In some examples, the tetracoordinate boric acid-functionalized polymers are synthesized by post-polymerization functionalization modification of PEM polymers, such as PBI polymers, sulfonic acid-functionalized PTFE polymers, sulfonic acid-functionalized polymers, styrene-based polymers, and / or boric acid-functionalized derivatives of the foregoing polymers.

[0055] Polybenzimidazole (PBI) polymers are a class of polymers composed of PBI polymer molecules. PBI polymer molecules have repeating units containing benzimidazole units as at least part of the backbone. The benzimidazole units contain a benzimidazole moiety or a derivative thereof. Benzimidazoles are aromatic heterocyclic organic compounds in which a phenyl group and an imidazole group share two carbon atoms in the ring structure. The general structure of benzimidazoles is shown in formula (I) below. [ka]

[0056] An example of a PBI polymer containing one benzimidazole unit in each repeat unit of the main chain is poly(2,5-benzimidazole) (AB-PBI), shown below in formula (II). An example of a PBI polymer containing two benzimidazole units in each repeat unit of the main chain is poly[2,2'-(m-phenylene)-5,5'-bibenzimidazole] (m-PBI), shown below in formula (III), and a fluorinated derivative of m-PBI, 4F-PBI, shown below in formula (IV). [ka]

[0057] Examples of other PBI polymers include, but are not limited to, poly{2,6-(2,6-naphthylidene)-1,7-dihydrobenzo[1,2-d;4,5-d']diimidazole}; poly2,2'-(2,6-naphthylidene)-5,5'-bibenzimidazole; poly-2,2'-(2,6-pyridine)-5,5'-bibenzimidazole; poly-2,2'-(2,5-pyridine)5,5'-bibenzimidazole; poly-2 ,2'-(2,2'-bipyridine-5,5')-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'-(naphtha Poly-2,2'-(biphenylene-1'',6'')-5,5'-bibenzimidazole;Poly-2,2'-(biphenylene-4'',4'')-5,5'-bibenzimidazole;Poly-2,2'-amylene-5,5'-bibenzimidazole;Poly-2,2'-octamethylene-5,5'-bibenzimidazole;Poly-2,6-(m-phenylene)-diimidazolebenzene;Poly-2,2'-cyclohexenyl-5,5'-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, the PBI polymer is a copolymer that includes one or more additional repeat units that may or may not include benzimidazole units in the backbone, side chains, or both.

[0058] Polytetrafluoroethylene (PTFE) polymer is a class of polymers composed of tetrafluoroethylene polymer molecules and their derivatives, which are produced by the polymerization of tetrafluoroethylene. PTFE polymer molecules have a carbon backbone with two fluorine atoms attached to each carbon, including its derivatives.

[0059] Polychlorotrifluoroethylene (PCTFE) polymer is a derivative of PTFE and has the molecular formula (CF2CClF) n PCTFE is a homopolymer of chlorotrifluoroethylene (CTFE) and its derivatives. PCTFE is similar to PTFE (e.g., Teflon) but differs in that each repeat unit contains a chlorine atom. The presence of these chlorine atoms makes PCTFE a unique thermoplastic polymer suitable for many applications. However, PCTFE has a hydrophobic backbone and is non-conductive to ions, making it unsuitable for electrochemical applications. Derivatives of PCTFE polymers include substituents (e.g., side chains or groups) in place of the chlorine atoms. Derivatives of PCTFE may be fully or partially substituted. Derivatives of PCTFE polymers include, but are not limited to, modified and functionalized PCTFE polymers, including those described in U.S. Provisional Application No. 63 / 532,262, filed August 11, 2023, which is incorporated herein by reference in its entirety, including modified or functionalized PCTFE polymers (e.g., acid-functionalized and ion-exchange-functionalized PCTFE polymers).

[0060] Sulfonic acid functionalized PTFE polymers are derivatives of PTFE polymers, having a PTFE backbone and side chains or groups with one or more pendant sulfonic acid groups. In some examples, the side chains are long side chains (LSC) with at least two ether linkages and four or more polyfluorocarbon units (e.g., -CF2- and / or -CF3). In other examples, the side chains are short side chains (SSC) with one ether linkage and two polyfluorocarbon units. In yet other examples, the side chains are medium side chains (MSC) with one ether linkage and four polyfluorocarbon units. In some examples, the sulfonic acid functionalized PTFE polymers have the general formula [(CF2CF2) m (CFACF2) n ] xwhere A is a side chain containing one or more pendant sulfonic acid groups, and m, n, and x are selected based on the application, equivalent weight, molecular weight, etc. In some examples, m is 4-7 and n is 1. Side chain A is LSC, MSC, or SSC. Examples of LSC sulfonic acid functionalized PTFE polymers include, but are not limited to, the Nafion™ series of polymers (available from Chemours in various configurations and grades, including Nafion-H, Nafion HP, Nafion 117, Nafion 115, Nafion 212, Nafion 211, Nafion NE1035, Nafion XL, etc.), and combinations, derivatives, grades, and configurations thereof. Examples of SSC sulfonic acid-functionalized PTFE polymers include, but are not limited to, the Aquivion® series of polymers (available in different configurations and grades from Solvay SA, such as Aquivion® E98-05, Aquivion® PW98, and Aquivion® PW87S), Gore-Select® (WL Gore & Associates), Flemion™ (Asahi Glass), and Pemion+™ (Ionomr Innovations), as well as combinations, derivatives, grades, and configurations thereof. Examples of MSC sulfonic acid-functionalized PTFE polymers include, but are not limited to, polymers manufactured by 3M. In some instances, the PTFE polymer is a copolymer containing one or more other repeating units. Additionally, the PTFE polymer may be doped or crosslinked with itself or other polymers.

[0061] Sulfonic acid-functionalized polymers include, but are not limited to, polyfluorosulfonic acid polymers and non-fluorinated sulfonic acid polymers. Examples of polyfluorosulfonic acid polymers include, but are not limited to, sulfonic acid-functionalized PTFE polymers and sulfonic acid-functionalized PCTFE polymers. Examples of non-fluorinated sulfonic acid polymers include, but are not limited to, poly(styrene sulfonic acid) polymers, sulfonated aromatic polymers (e.g., sulfonated polyetherketone (SPEEK) polymers, sulfonated polyarylethersulfone (SPAES) polymers, sulfonated polyaryleneetherketone (SPAEK) polymers, sulfonated polysulfone (SPSF) polymers, sulfonated polyimide (SPI) polymers, sulfonated polystyrene (SPS), sulfonated polyphenylene), and other sulfonated polymers, including sulfonated derivatives of the polymers described herein.

[0062] A styrene polymer is a polymer composed of styrene polymer molecules. Styrene polymer molecules have repeating units containing alternating carbon centers bonded to phenyl groups. Examples of styrene polymers include, but are not limited to, polystyrene, poly(styrenesulfonic acid) (e.g., poly(4-styrenesulfonic acid)), polyhalostyrenes, poly(3-trifluoromethylstyrene), poly(4-acetoxystyrene), poly(4-allylstyrene), poly(4-cyanostyrene), poly(4-dimethylsilylstyrene), poly(4-hydroxystyrene), poly(α-methylstyrene), poly(4-methylstyrene), poly(4-methoxystyrene), poly(4-[tert-butoxycarbonyl]oxystyrene), poly(4-tert-butyl ... Examples of suitable repeating units include 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(vinylcyclohexane), and derivatives of any of the foregoing, including substituted (e.g., fluorinated) and / or branched derivatives. In some examples, the styrene polymer is a copolymer containing one or more other repeating units, which may or may not contain a carbon center attached to a phenyl group.

[0063] Aromatic polymers include any polymer that contains aromatic rings in the backbone and / or side chains or groups. Examples of aromatic polymers include, but are not limited to, styrene polymers, polycarbonate polymers, polyphenylene polymers (e.g., poly(1,4-phenylene), poly(1,4-phenylene-ethylene), poly(1,3-phenylene-methylene), poly(p-phenylenevinylidene), poly(p-phenylenevinylene), poly(1,4-phenylene oxide), poly(1,4-phenylene sulfide)), polyethersulfone, 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[(ethylazanidiyl)ethyleneazanidiyl-1,3-phenylene], polyoxydiphenylene-pyromellitimide (Kapton®, EI du Pont de Nemours), polyesterimide polymers, aromatic polyimide polymers, lignin, and derivatives of any of the foregoing, including substituted (e.g., fluorinated) and / or branched derivatives.

[0064] Natural polymers (also referred to as "biopolymers") include, but are not limited to, cellulose, lignin, chitin, and derivatives of any of the foregoing, including those 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), which are incorporated herein by reference in their entireties.

[0065] Boric acid-functionalized polymers can be used to synthesize tetracoordinate boric acid-functionalized polymers, as described below. Boric acid-functionalized polymer molecules contain pendant boric acid groups in the polymer backbone and either the backbone or side chains or side groups. Boric acid groups have the general formula -B(OH)2 or =B(OH), where the boron atom is attached to one or two hydroxyl groups and one or two covalent bonds to the backbone or side chains, forming a total of three covalent bonds. Examples of boric acid-functionalized polymers include, but are not limited to, boric acid-functionalized derivatives of PBI polymers, sulfonic acid-functionalized PTFE polymers, sulfonic acid-functionalized PCTFE polymers, sulfonic acid-functionalized polymers, styrene polymers, and cellulose polymers. A schematic reaction scheme for the synthesis of boric acid-functionalized polymers is shown below.

[0066] In some instances, the boric acid functionalized polymer is formed by post-polymerization functionalization of the polymer molecules. Various examples of post-polymerization functionalization are described below.

[0067] In some examples, boric acid-functionalized PBI polymers are synthesized by post-polymerization functionalization of PBI polymers by attaching a boric acid-functionalized linker to a secondary nitrogen atom of a benzimidazole moiety in the PBI polymer. The boric acid-functionalized linker has a linking group X as a terminal or side group and a boric acid group as a terminal and / or side group. The linking group X is a methyl group (-CH), a formyl group (-C(=O)H), or a sulfonyl group (-S(=O)H). In some examples, the boric acid-functionalized linker has the general formula XRB(OH), where R is an alkyl chain of length m, where m is 0-30 (or 0-20, or 0-12, or 0-10, or 0-8, or 0-6), and has one or more side groups A. Each A can independently be hydrogen (H), hydroxyl (OH), fluorine (F), chlorine (Cl), boric acid, dialkylamino (NR', where R' represents an organic linking group such as hydrogen or methyl (CH), cyano (CN), carboxyl (COOH), carboxylamido, ester, alkyl, alkoxy, or aryl. In some examples, the boric acid-functionalized linker has one or more pendant boric acid groups as side groups in addition to or instead of terminal groups.

[0068] In the reaction, the linking group X of the boric acid-functionalized linker bonds with the secondary nitrogen of the benzimidazole moiety, forming a side chain with pendant boric acid groups. The loading of the boric acid groups can be controlled by adjusting the molar ratio of the boric acid-functionalized linker to the benzimidazole moiety in the PBI polymer.

[0069] Figure 1A shows a schematic reaction scheme for the synthesis of a boric acid-functionalized PBI polymer. As shown, a PBI polymer molecule is reacted with a boric acid-functionalized linker to produce a boric acid-functionalized PBI polymer molecule. Any of the boric acid-functionalized linkers described herein can be used. The PBI polymer molecule in Figure 1A is merely exemplary, and the reaction scheme in Figure 1A can be carried out with other suitable PBI polymers.

[0070] As another example of post-polymerization functionalization of polymers, aromatic polymers can be converted to boric acid functionalized polymers by boronating the aromatic rings in the main chain or side chains / side groups. In some instances, the aromatic rings are directly boronated by reaction with a boronating agent. Any suitable boronating agent can be used, but examples include boric acid, compounds of the general formula B(OR 1 ) 3 borate esters and / or borate esters of the general formula R 2 B(OR 1 ) 2 borate esters, where each R 1 R can independently be a fully substituted, partially substituted, or unsubstituted, branched or straight-chain 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. 2 can be a fully substituted, partially substituted, or unsubstituted, branched or straight-chain 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. Other suitable boronating agents can also be used. Specific examples of boronating agents include trialkyl borates (e.g., trimethyl borate, triethyl borate), bis(pinacolato)diboron, bis(catecholato)diborane, pinacolboron, bis(2,4-dimethylpentane-2,4-diol)diboron, bis(hexylenediol)diboron, bis(neopentylglycol)diboron, vinylboric acid, and derivatives thereof. Aromatic ring boronation reactions include metal-catalyzed C-H boronation reactions and reactions that directly convert C-H bonds to C-B bonds using transition metals, such as the Suzuki-Miyaura coupling reaction.

[0071] Borylation of the aromatic ring provides a protective boronic acid group (e.g., -B(OR 1 )2). In these examples, the hydrolysis step results in the formation of the protecting group R 1can be removed to produce pendant boric acid groups of the general formula -B(OH). In other examples, the hydrolysis step can be performed in situ during the boronation step (e.g., by reacting the boronating agent and water together in a one-pot reaction). The loading of boric acid groups on the polymer can be adjusted by controlling the molar ratio of the boronating agent to the aromatic rings in the polymer. In other examples, aromatic groups (Ar) in the polymer molecule can be converted to activated "Ar-X" intermediates for subsequent boronation reactions, where X is a halogen (e.g., iodine (I), bromine (Br), chlorine (Cl)) or a metal (Ar-X is produced by an aromatic ring metallation reaction). In some examples, X is lithium (Li).

[0072] Figure 1B shows a schematic reaction scheme for the synthesis of boric acid-functionalized PBI polymers by boronation. As shown, a PBI polymer molecule is reacted with a boronating agent. The aromatic ring of the benzimidazole unit is boronated to generate an intermediate protected boric acid group, which is further hydrolyzed to generate the pendant boric acid group. A suitable boronating agent can be used in place of boric acid, and the reaction scheme in Figure 1B can be carried out with other suitable PBI polymers.

[0073] Figure 2A shows a schematic reaction scheme for the synthesis of boric acid-functionalized styrene polymers by boronation. As shown, a styrene polymer molecule is reacted with a boronating agent. The aromatic ring of the styrene polymerized unit is boronated to generate an intermediate protected boronic acid group, which is further hydrolyzed to generate the pendant boronic acid group. Suitable boronating agents can be used, and the reaction scheme in Figure 2A can also be carried out with other suitable styrene polymers.

[0074] As a further example of post-polymerization functionalization of polymers, polymers bearing vicinal diols in their side chains or groups are reacted with boric acid (B(OH)3). Boric acid reacts with the vicinal hydroxyl groups to form cyclic boric acid groups, leaving only one hydroxyl group. Any suitable polymer bearing vicinal diols can be used, including polysaccharides, cellulose, and 1,2-dihydroxyphenyl polymers.

[0075] Figure 2B shows a schematic reaction scheme for the synthesis of boric acid-functionalized polymers using vicinal diols. As shown, polymer molecules with pendant 1,2-dihydroxyphenyl groups are reacted with boric acid to produce boric acid-functionalized polymers with pendant boric acid groups. In some examples, the polymer molecules have a PTFE backbone and the linker is LSC, MSC, or SSC. Other configurations are also envisioned depending on the linker. While Figure 2B shows only one linker, the polymer molecules can have any number of side chains, linkers, and 1,2-dihydroxyphenyl groups. Furthermore, the 1,2-dihydroxyphenyl groups can be part of a side chain or side group in other polymers described herein (e.g., PBI polymers, sulfonic acid-functionalized polymers, styrene polymers, aromatic polymers, etc.). In some examples, cellulose polymers and / or polysaccharides can be used to crosslink the boric acid-functionalized polymers.

[0076] As yet another example of post-polymerization functionalization of polymers, sulfonic acid-functionalized polymer molecules can be converted to boric acid-functionalized polymer molecules by coupling pendant sulfonic acid groups with boric acid groups through sulfonamide bonds. This reaction scheme involves several steps.

[0077] In the first step, the sulfonic acid group is activated to a sulfonyl chloride (-S(=O)Cl), sulfonyl fluoride (-S(=O)F), or sulfonate ester. For example, reaction of a sulfonic acid-functionalized polymer molecule with hydrochloric acid (HCl) or hydrogen fluoride (HF) replaces the hydroxy group of the sulfonic acid group with a chlorine group to form a sulfonyl chloride or sulfonyl fluoride group. Other suitable chloride and / or fluoride reagents can also be used, including, but not limited to, thionyl chloride, sulfuryl chloride, oxalyl chloride, thionyl fluoride, sulfuryl fluoride, and the like. Sulfonate esters have the general formula -S(=O2)OR, where R is hydrogen or a fully, partially, or unsubstituted alkyl or aryl group having 1 to 20, 1 to 10, 1 to 8, 1 to 6, or 1 to 4 carbon atoms, such as methyl, ethyl, propyl, or butyl. Sulfonate ester reagents include, but are not limited to, dimethyl sulfate, dialkyl sulfate, and the like.

[0078] In the second step, the bifunctional aminoboric acid linker is coupled with a sulfonyl chloride, sulfonyl fluoride, or sulfonate ester. The bifunctional aminoboric acid linker has an amino group as a terminal or side group, a boric acid group as a terminal or side group, and an alkyl chain R of length m, where m ranges from 0 to 30 (or 0 to 20, or 0 to 12, or 0 to 10, or 0 to 8, or 0 to 6). R further has one or more side groups A, each of which independently can be hydrogen (H), hydroxyl (OH), fluoro (F), chloro (Cl), boric acid, dialkylamino (NR', where R' is hydrogen or an organic linking group such as methyl), cyano (CN), carboxyl (COOH), carboxylamido, ester, alkyl, alkoxy, or aryl. In the second step, the primary or secondary amine of the bifunctional aminoboric acid linker is orthogonally coupled with a sulfonyl chloride, sulfonyl fluoride, or sulfonate ester, attaching the aminoboric acid linker to the polymer backbone via a sulfonamide bond. The resulting polymer molecule has pendant boric acid groups attached to the polymer backbone (e.g., a PTFE backbone) via the linker (e.g., via a sulfonamide bond).

[0079] In another reaction scheme, in the second step, an aromatic boric acid of the general formula ArB(OR)2 reacts with a sulfonyl chloride, sulfonyl fluoride, or sulfonate ester via an aromatic electrophilic substitution reaction. In the general formula of aromatic boric acid, ArB(OR)2, each R is independently hydrogen or a fully, partially, or unsubstituted alkyl or aryl group having 1 to 20, 1 to 10, 1 to 8, 1 to 6, or 1 to 4 carbon atoms. The aromatic boric acid is a protected form of boric acid. In this reaction scheme, the aryl group of the aromatic boric acid bonds directly with the sulfonyl chloride, sulfonyl fluoride, or sulfonate ester sulfonate to form an intermediate protected aromatic boric acid group that is coupled to the polymer via a sulfone bond. In the third step, the protecting group R is removed to yield the free aromatic boric acid group of the general formula ArB(OH)2. For example, a hydrolysis step can be performed to remove the protecting group R, yielding a pendant boronic acid group (general formula -B(OH)2) coupled to the backbone via an aromatic linker and a sulfone bond. In other examples, the hydrolysis step can be performed in situ during the boronation step (by combining a boronating agent with water in a one-pot process).

[0080] Figure 3A shows a schematic reaction scheme for synthesizing boric acid-functionalized polymers by coupling pendant sulfonic acid groups to boric acid groups via a sulfonamide bond. Any sulfonic acid-functionalized polymer molecule can be used as the starting reagent, including, but not limited to, polyfluorosulfonic acid-functionalized PTFE polymer molecules and sulfonic acid-functionalized PCTFE polymer molecules. In the first step, the pendant sulfonic acid groups of the sulfonic acid-functionalized polymer molecule are activated to sulfonyl chlorides. For example, activation can be achieved by reaction with hydrogen chloride. However, other chloride-activating reagents can also be used to activate the sulfonic acid groups to sulfonyl chlorides. Furthermore, the sulfonic acid groups can alternatively be activated to sulfonyl fluorides or sulfonate esters. In the second step, the intermediate sulfonyl chloride-functionalized polymer molecule is reacted with an aminoboric acid linker, coupling via a sulfonamide bond. In the example shown in Figure 3A, the aminoboric acid linker has the general formula H2N(CH)2B(OH)2. However, any aminoboronic acid linker described herein can be used.

[0081] Figure 3B shows a schematic reaction scheme for synthesizing boric acid-functionalized polymers by coupling sulfonic acid and boric acid groups through sulfone bonds. In the second step, the sulfonyl chloride-functionalized polymer molecule is reacted with phenylboronic acid, which has the general formula PhB(OR)2, where each R is independently hydrogen or a fully, partially, or unsubstituted alkyl or aryl group having 1 to 20, 1 to 10, 1 to 8, 1 to 6, or 1 to 4 carbon atoms, such as methyl, ethyl, propyl, or butyl. The phenylboronic acid forms a sulfone bond with the sulfonyl chloride to produce an intermediate protected phenylboronic acid-functionalized polymer molecule. In the third step, the protecting group R of the intermediate is removed as described above, yielding a polymer molecule with pendant phenylboronic acid groups (general formula PhB(OH)2) coupled to the polymer backbone through sulfone bonds. Other aromatic boronic acids can also be used in place of phenylboronic acid in the example shown in Figure 3B. It is also possible to react aromatic boronic acids with sulfonyl fluorides or sulfonate esters instead of sulfonyl chlorides.

[0082] In the above example, the boric acid-functionalized polymer is synthesized by post-polymerization functionalization of the polymer. In other examples, the boric acid-functionalized polymer can also be synthesized by polymerization using a boric acid-functionalized monomer. For example, the monomer to be used for polymerization, such as tetrafluoroethylene (TFE) monomer, benzimidazole monomer, styrene polymer, or sulfonic acid-functionalized monomer, can be modified with a boric acid group before polymerization, and the modified monomer can be polymerized to obtain the boric acid-functionalized polymer. The monomer can be functionalized with a boric acid group using any of the reaction schemes described herein for boric acid functionalization of polymers.

[0083] In some examples, boric acid functionalized polymers are used to synthesize tetracoordinate boric acid functionalized polymers (including fluoroboric acid functionalized polymers). The tetracoordinate boric acid functionalized polymers described herein take advantage of the unique chemical bonding properties of boron. Boron has three valence electrons and its ground state electron configuration is 1s. 2 2s 2 2p1 Boron is found in trivalent and neutral compounds such as boric acid (B(OH)3), boric acid (RB(OH)2 or R 1 R 2 B(OH)), and boric acid groups (boric acid where R is part of the polymer backbone or side chain), where the boron is sp 2 It has three covalent bonds due to hybridization. sp 2 Hybridized boron atoms have empty p-orbitals, making trivalent boron compounds highly electron-deficient, two electrons short of a stable octet electron configuration. Therefore, boric acid and its derivatives are Lewis acids, readily accepting electron pairs at the boron atom. Addition of anions such as fluoride (e.g., by fluorination) or other anions completes the octet electron configuration, forming stable, negatively charged tetravalent boron tetrahedral structures with four covalent bonds. Tetravalent boron is also known as tetracoordinated boron. Polymers can be functionalized with functional groups containing tetracoordinated boron, such as tetracoordinated boron acid groups (e.g., fluoroboric acid groups). Because functional groups containing tetracoordinated boron have a formal negative charge, they are inherently ionic and acidic and can function as cation transport agents in electrochemical cell applications.

[0084] The tetracoordinate boric acid functionalized polymer molecule has a main chain and tetracoordinate boric acid groups in side chains and / or side groups. The tetracoordinate boric acid groups have the general formula -BF m X n (OH) (3-m-n)where B has four covalent bonds and is covalently attached to the polymer backbone, side chain, or side group. m and n are each independently 0, 1, 2, or 3, and the sum of m and n is 1, 2, or 3. X is an anion other than fluoride. In some instances, X is a conjugate base derived from the general formula HX (Brønsted-Lowry acid), such as alkylsulfonic acids with the general formula RSO2(OH) (where R is an alkyl group, e.g., methanesulfonic acid (CH3SO3H), vinylsulfonic acid, perfluorooctanesulfonic acid, and taurine), arylsulfonic acids with the general formula ArSO2(OH) (where Ar is an aryl group, e.g., benzenesulfonic acid (CH5SO3H) and p-toluenesulfonic acid (CH7H7SO3H)), alkylarylsulfonic acids (e.g., alkylbenzenesulfonic acids), sulfuric acid (H2SO4) (e.g., ammonium bisulfate ((NH4)HSO4), pyridinium bisulfate, nitrogen-containing heterocyclic bisulfates, alkylammonium bisulfates, and sulfuramides), phosphoric acid, phosphates, carboxylic acids, phenols, and their derivatives. X can be derived from both small and polymeric Brønsted-Lowry acids. In the case of polymeric acids, crosslinking can be introduced via chemical reactions, broadening the scope of applications. Tetracoordinate boric acid-functionalized polymers include a wide range of polymers, such as tetracoordinate boric acid-functionalized PTFE polymers, tetracoordinate boric acid-functionalized PCTFE polymers, tetracoordinate boric acid-functionalized polystyrene polymers, tetracoordinate boric acid-functionalized cellulose polymers, and tetracoordinate boric acid-functionalized PBI polymers.

[0085] In some examples, tetracoordinate boric acid functionalized polymer molecules are synthesized by nucleophilic substitution reaction of the pendant boric acid groups of the boric acid functionalized polymer molecule with a fluoride reagent (described below) and / or a compound HX (described above). The boron atom is converted from the fluoride reagent to a fluoride anion (F - ) and / or HX to X - Accepts an anion and, depending on the stoichiometry of the reaction, converts one or more hydroxy groups to F by nucleophilic substitution. - and / or X -Due to the tetracoordinated boron atoms, the tetracoordinated boric acid groups in the tetracoordinated boric acid functionalized polymer molecules have a negative formal charge, which is neutralized by a proton.

[0086] The degree of anion incorporation into the boric acid groups, and therefore the pKa of the resulting tetracoordinate boric acid functionalized polymer, can be adjusted based on the stoichiometry of the reagents. For example, when the molar ratio of HX to boric acid groups is 3:1 or greater, the resulting tetracoordinate boric acid groups have the general formula -B3. When HX is the limiting reagent, the pKa of the resulting tetracoordinate boric acid functionalized polymer will be higher than when HX is not limiting. For example, when the molar ratio of HX to boric acid groups is 2:1, the resulting tetracoordinate boric acid groups have the formula -BX2(OH). When the molar ratio is 1:1, the resulting tetracoordinate boric acid groups have the formula -BX(OH)2.

[0087] In some examples, the tetracoordinate boron-functionalized polymer is a fluoroboric acid-functionalized polymer. The fluoroboric acid-functionalized polymer molecule has a polymer backbone and fluoroboric acid groups in side chains and / or side groups. The fluoroboric acid groups contain a tetracoordinate boron atom covalently bonded to at least one fluorine atom and have the general formula -BF m X n (OH) (3-m-n) where m is 1, 2, or 3, n is 0, 1, or 2, and the sum of m and n is 1, 2, or 3. In some examples, the fluoroborate group has the formula -BF, -BF(OH), or -BF(OH), and optionally one or two of the hydroxy groups are replaced by X. - Due to the four-coordinated boron atom, the fluoroborate groups in the fluoroborate-functionalized polymer molecule have a negative formal charge, H + , Li + , Na + , Al 3+ , Ni 2+, or neutralized with a suitable cation such as that used in battery applications. Fluoroborate-functionalized polymers include fluoroborate-functionalized PTFE polymers, fluoroborate-functionalized PCTFE polymers, fluoroborate-functionalized polystyrene polymers, fluoroborate-functionalized cellulose polymers, fluoroborate-functionalized PBI polymers, and the like.

[0088] A fluoroboric acid-functionalized PTFE polymer molecule has a PTFE backbone, side groups or chains attached to the PTFE backbone, and fluoroboric acid groups in or attached to the side groups or chains. In some examples, the fluoroboric acid-functionalized PTFE polymer molecule is a derivative of a sulfonic acid-functionalized PTFE polymer molecule (such as a polyfluorosulfonic acid-functionalized PTFE polymer, Nafion™, or Aquivion® polymer) in which one or more pendant sulfonic acid groups have been replaced or appended with one or more fluoroboric acid groups.

[0089] The fluoroboric acid-functionalized PCTFE polymer molecule comprises a PCTFE main chain, side groups or chains (e.g., substituted at the chlorine atom) attached to the PTFE main chain, and fluoroboric acid groups in or attached to the side groups or chains. Thus, the PCTFE polymer molecule includes polymers in which one or more chlorine atoms in the PCTFE main chain are replaced with side chains or groups containing one or more fluoroboric acid groups.

[0090] Fluoroborate-functionalized polystyrene polymer molecules comprise a polystyrene backbone and fluoroborate groups attached directly to the polystyrene backbone (e.g., attached to a phenyl group of the polystyrene backbone) or indirectly via a linker. In some examples, the fluoroborate-functionalized polystyrene polymer molecules are sulfonic acid-functionalized polystyrene polymer molecules (e.g., polystyrene sulfonic acid polymer (CH2CHC6H4SO3H) n ) in which one or more pendant sulfonic acid groups are replaced or appended with one or more fluoroborate groups.

[0091] Fluoroborate-functionalized PBI polymer molecules comprise a polybenzimidazole (PBI) backbone and fluoroborate groups attached to the PBI backbone either directly (e.g., attached to aromatic groups) or via linkers (attached to the PBI backbone via side groups or side chains). In some examples, the fluoroborate-functionalized PBI polymer is crosslinked with other polymers (e.g., another PBI polymer, a PTFE polymer, or a poly(phosphoric acid) (PPA) polymer). In some examples, the fluoroborate-functionalized PBI polymer comprises a PPA-doped PBI polymer (PPA-PBI).

[0092] The fluoroboric acid-functionalized cellulose polymer molecule comprises a cellulose backbone, a side group or chain attached to the backbone, and a fluoroboric acid group in or attached to the side group or chain. In some examples, the fluoroboric acid-functionalized cellulose polymer molecule is a derivative of a sulfonic acid-functionalized cellulose polymer molecule or a boric acid-functionalized cellulose polymer molecule, in which one or more pendant sulfonic acid or boric acid groups are replaced with a side chain or group containing one or more fluoroboric acid groups.

[0093] In some examples, fluoroboric acid-functionalized polymer molecules are synthesized by subjecting boric acid-functionalized polymer molecules to a fluoride treatment. Any of the boric acid-functionalized polymer molecules described herein can be used. The fluoride treatment functionalizes the boric acid groups of the boric acid-functionalized polymer molecules with fluoroboric acid groups having four-coordinate boron atoms. The fluoride treatment can be carried out by any suitable method.

[0094] In some examples, the fluoride treatment involves reacting the boric acid functionalized polymer molecule with a fluoride reagent. The reagent compound can be hydrogen fluoride (HF), lithium fluoride (LiF), sodium fluoride (NaF), potassium fluoride (KF), potassium difluoride (KHF), cesium fluoride (CsF), ammonium fluoride (NHF), ammonium difluoride (NHF), or tetraalkylammonium fluorides (general formula NRF, where each R is independently hydrogen or a substituted or unsubstituted alkyl or aryl group having 1 to 20, 1 to 10, 1 to 8, 1 to 6, or 1 to 4 carbon atoms). The fluoride groups may be selected from the group consisting of methyl, ethyl, propyl, butyl, and the like, but are not limited to these. In some examples, the fluoride treatment is carried out using two or more different fluoride reagents (e.g., HF and NaF, HF and LiF, etc.). The fluoride treatment introduces fluoride groups to the pendant boric acid groups, forming fluoroborate groups in which the boron atom is covalently bonded to the fluoride group in a tetracoordinated arrangement. Depending on the stoichiometry of the reaction, one or more hydroxy groups of the pendant boric acid groups may be replaced with fluoride groups.

[0095] The fluoride compound bonds with the boric acid groups of the boric acid-functionalized polymer molecule to form pendant fluoroborate groups. One or more hydroxy groups of the boric acid group are replaced with fluoride from the fluoride compound, and a fourth fluoride group is introduced. This results in the boron atom being tetracoordinated, and the pendant fluoroborate group has a formal negative charge and can be attached to a cation (e.g., H + , Li + , Na + , K. + , Cs + , NH4 + , or NR4 + ) is neutralized.

[0096] If the countercation is not hydrogen (e.g., if the fluoride compound is LiF, NaF, KF, KHF2, CsF, NHF, NHF2, or NRF), a protonation step is performed after the fluoride treatment to convert the countercation to a proton (H +) The protonation step can be carried out by any applicable method. In some examples, protonation is achieved by reacting the fluoroboric acid functionalized polymer molecule with a strong acid. Examples of applicable strong acids include hydrochloric acid (HCl), sulfuric acid (H2SO4), methanesulfonic acid (CH3SO3H), and trifluoroacetic acid (CF3CO2H). After the protonation step, the pendant fluoroboric acid groups are substituted with protons (H + ) is neutralized.

[0097] In another example of a fluoride treatment, the hydrogen atom of one or more hydroxy groups of the pendant boric acid groups is replaced with a fluoroborate group. In this case, the fluoride treatment is carried out by combining a boric acid functionalized polymer molecule with a compound of the formula BF m (OH) (3-m) The fluoride treatment can involve reacting a borate-functionalized polymer molecule with boric acid (RB(OH)2) and a fluoride reagent (e.g., HF) in situ, where R is an alkyl or aryl group (e.g., 1-20, 1-10, 1-8, 1-6, 1-4, or 1-3 carbon atoms), branched or linear, fully or partially substituted, or unsubstituted.

[0098] If the reaction results in one or two remaining hydroxy groups in the tetracoordinated boric acid group, the same reaction can be repeated or a different reaction can be carried out to convert one or both of the remaining hydroxy groups to a fluoride group or an anion group X. - As previously mentioned, in another example, the conversion of the boric acid group to a tetracoordinate boric acid group can be carried out using multiple reagents in the same step (e.g., at least two of compound XH, a fluoride reagent, a fluoroborate compound, or boric acid).

[0099] An illustrative reaction scheme for synthesizing a fluoroboric acid functionalized polymer starting from a boric acid functionalized polymer is shown below.

[0100] 4A-4C show illustrative reaction schemes for synthesizing fluoroboric acid-functionalized polymers by subjecting boric acid-functionalized polymers to fluoride treatment. Any boric acid-functionalized polymer described herein can be used, including, but not limited to, boric acid-functionalized PBI polymers, boric acid-functionalized PTFE polymers, boric acid-functionalized polystyrene polymers, etc.

[0101] In the example shown in Figure 4A, the fluoride treatment involves reacting a boric acid-functionalized polymer molecule with hydrogen fluoride or sodium fluoride. The fluoride groups of the fluoride reagent replace the hydroxyl groups of the boric acid groups. The product is a fluoroboric acid-functionalized polymer molecule with pendant fluoroboric acid groups. The fluoroboric acid groups have a tetracoordinated boron atom covalently bonded to the main chain, or side chain, or side group, and are further covalently bonded to three fluorine atoms. The fluoroboric acid groups therefore have a negative formal charge, are ionic and acidic in nature, and are neutralized by the cations derived from the fluoride reagent. If the fluoride reagent is HF, the cations are protons. If the fluoride reagent is NaF (or other fluoride reagent), a protonation step is performed after the fluoride treatment to replace the countercations with protons. However, in some examples, the protonation step is omitted, and the fluoroboric acid groups are neutralized by the cations derived from the fluoride reagent.

[0102] The degree of fluorine loading on the boric acid groups and boric acid-functionalized polymer molecules, and therefore the pKa of the resulting fluoroboric acid-functionalized polymer, can be tailored based on the stoichiometry of the reagents. Fluorine is the most electronegative element, and increasing the number of fluorine atoms covalently bonded to the boron atom increases the acidity of the fluoroboric acid. As shown in the example in Figure 4A, a molar ratio of fluoride reagent to boric acid groups of approximately 3:1 or greater (≥3:1) results in trifluoroboric acid groups. The presence of three fluorine atoms covalently bonded to the tetracoordinated boron atom in the resulting polymer structure makes trifluoroboric acid the most acidic fluoroboric acid (lowest pKa). Therefore, using the reaction scheme in Figure 4A, a boric acid-functionalized polymer can be converted into a fluoroboric acid-functionalized polymer with tetracoordinated anionic boron atoms, which is inherently ionic, superacidic, and has strong proton conductivity.

[0103] The versatile chemistry of fluoride treatment allows the synthesis of fluoroborate groups with lower acidity than the trifluoroborate group shown in Figure 4A. For example, when the molar ratio of fluoride reagent (e.g., HF) to borate groups is less than 3:1, the fluoride reagent becomes the limiting reagent. In this case, the pKa of the resulting fluoroborate is higher (less acidic) than the superacidic trifluoroborate but lower (more acidic) than the weakly acidic borate groups in the starting borate-functionalized polymer molecule. The pKa can be controlled to a desired level for many applications, including electrochemical processes such as ammonia production. For example, when the molar ratio of fluoride reagent to borate groups is approximately 1:1, one fluorine atom is covalently bonded to the boron atom (as shown in the reaction scheme in Figure 4B). When the molar ratio of fluoride reagent to borate groups is approximately 2:2, two fluorine atoms are covalently bonded to the boron atom (as shown in the reaction scheme in Figure 4C). Any molar ratio of fluoride reagent to boric acid groups can be used, such as 3:1 or greater, 2.5:1, 2:1, 1.5:1, 1:1, 0.5:1, or other applicable ratios.

[0104] 5A and 5B show another illustrative reaction scheme for synthesizing a fluoroboric acid-functionalized polymer by subjecting a boric acid-functionalized polymer to a fluoride treatment. Any of the boric acid-functionalized polymers described herein can be used, including, but not limited to, boric acid-functionalized PBI polymers, boric acid-functionalized PTFE polymers, boric acid-functionalized polystyrene polymers, etc.

[0105] In the example shown in Figure 5A, the fluoride treatment involves reacting boric acid-functionalized polymer molecules with boron trifluoride (BF3) to obtain superproton-conducting divalent fluoroboric acid-functionalized polymer molecules. Boron trifluoride is available as a diethyl ether and / or tetrahydrofuran complex. The fluoroboric acid groups replace the hydrogen atoms of both hydroxyl groups of the boric acid groups. The three fluorine atoms covalently bonded to the tetracoordinated boron atom in the resulting polymer structure result in a superacidic fluoroboric acid. Therefore, the boric acid groups are converted into superproton-conducting divalent acids using boron trifluoride. Divalent acids have twice the ion exchange capacity (IEC) compared to monovalent acids, and the increased IEC improves proton conductivity and enhances the efficiency of ionomers and PEMs. Furthermore, the additional fluorine atoms contribute to adjusting the hydrophobic-hydrophilic balance and hydrogen-bonding network in the polymer, further facilitating proton transport.

[0106] The degree of fluorine loading on the boric acid groups and the boric acid-functionalized polymer, i.e., the pKa of the resulting fluoroboric acid-functionalized polymer, can be adjusted based on the stoichiometry of the reagents. As shown in the example in Figure 5A, the molar ratio of boron trifluoride to boric acid groups is ≥ 2:1. The three fluorine atoms covalently bonded to the tetracoordinated boron atom in the resulting polymer structure result in the most acidic fluoroboric acid (lowest pKa). Therefore, using the reaction scheme in Figure 5A, a boric acid-functionalized polymer can be converted into a fluoroboric acid-functionalized polymer with tetracoordinated anionic boron atoms, which is inherently ionic, superacidic, and has strong proton conductivity.

[0107] The versatile chemistry of fluoride treatment allows for the synthesis of fluoroborate-functionalized polymers with less acidity than the trifluoroborate-functionalized polymer shown in Figure 5A. For example, if the molar ratio of boron trifluoride to boric acid groups is less than 2:1, boron trifluoride becomes the limiting reagent, resulting in products with intermediate pKas. When the molar ratio of boron trifluoride to boric acid groups is approximately 1:1, only one of the hydrogen atoms of the boric acid group is replaced by a fluoroborate group (as shown in Figure 5B). The pKa can be controlled to a desired level by using any applicable molar ratio of boron trifluoride to boric acid groups (e.g., 2:1 or greater, 1.5:1, 1:1, 0.5:1, or others).

[0108] Figures 6 and 7 show illustrative reaction schemes for synthesizing fluoroboric acid-functionalized PBI polymers by subjecting boric acid-functionalized PBI polymers to a fluoride treatment. In the example shown in Figure 6, the fluoride treatment involves reacting a boric acid-functionalized polymer molecule with hydrogen fluoride or sodium fluoride. The fluoride group of the fluoride reagent replaces the hydroxyl group of the boric acid group. The product is a fluoroboric acid-functionalized polymer molecule with pendant trifluoroboric acid groups. The trifluoroboric acid group has a tetracoordinate boron atom covalently bonded to a side chain and three fluorine atoms. Therefore, the fluoroboric acid group has a formal negative charge, is ionic and acidic in nature, and is charge-neutralized by cations.

[0109] The degree of fluorine loading on the borate groups and borate-functionalized PBI polymers, i.e., the pKa of the resulting fluoroborate-functionalized PBI polymers, can be adjusted based on the stoichiometry of the reagents. As shown in the example in Figure 6, the molar ratio of HF or NaF to borate groups is about 6:1 or higher, and a trifluoroborate group can be obtained for each borate group in a single or multiple steps.

[0110] In another example, because HF is the limiting reagent, the resulting fluoroboric acid has a higher (less acidic) pKa than the superacidic trifluoroboric acid in Figure 6, but a lower (more acidic) pKa than the weakly acidic boric acid groups in the starting boric acid-functionalized PBI polymer. For example, when the molar ratio of fluoride reagent to boric acid groups is about 2:1, one fluorine atom is covalently bonded to the boron atom of the boric acid group attached to each PBI repeat unit (as shown in the reaction scheme in Figure 7). When the molar ratio of fluoride reagent to boric acid groups is about 4:2, or when additional steps of fluoride treatment are performed, another fluorine atom is covalently bonded to the boron atom of each boric acid group. Any molar ratio of fluoride reagent to boric acid groups can be used, such as 6:1 or greater, 5:1, 4:1, 3:1, 2:1, 1:1, or other applicable ratios.

[0111] Figures 8 and 9 show another illustrative reaction scheme for synthesizing fluoroboric acid-functionalized PBI polymers by fluoride treatment of boric acid-functionalized PBI polymers. The reaction scheme in Figure 8 is similar to that in Figures 6 and 7, except that in Figure 8, the boric acid group is directly attached to the aromatic group of the PBI repeat unit. In the example in Figure 8, the fluoride treatment involves reacting the boric acid-functionalized polymer molecule with hydrogen fluoride or sodium fluoride (followed by protonation with hydrochloric acid), and the fluoride group of the fluoride reagent replaces the hydroxy group of the boric acid group. The product is a fluoroboric acid-functionalized polymer molecule with pendant trifluoroboric acid groups. The trifluoroboric acid group has a tetracoordinate boron atom covalently bonded to a side chain and three fluorine atoms. Therefore, the fluoroboric acid group has a formal negative charge and is ionic and acidic in nature, with the charge neutralized by cations.

[0112] The degree of fluorine loading on the boric acid groups and the boric acid-functionalized PBI polymer, i.e., the pKa of the resulting fluoroborate-functionalized PBI polymer, can be adjusted based on the stoichiometry of the reagents. As shown in the example in Figure 8, a molar ratio of HF or NaF to boric acid groups of about 3:1 or higher results in trifluoroborate groups.

[0113] In another example, because the fluoride reagent is the limiting reagent, the resulting fluoroboric acid has a higher (less acidic) pKa than the superacidic trifluoroboric acid in Figure 8 and a lower (more acidic) pKa than the weakly acidic boric acid groups in the starting boric acid-functionalized PBI polymer. For example, when the molar ratio of fluoride reagent to boric acid groups is about 1:1, one fluorine atom is covalently bonded to the boron atom of the boric acid group (as shown in the reaction scheme in Figure 9). When the molar ratio of fluoride reagent to boric acid groups is about 2:2, or when an additional step of fluoride treatment is performed, another fluorine atom is covalently bonded to the boron atom of the boric acid group (as shown in Figure 9). Any molar ratio of fluoride reagent to boric acid groups can be used, such as 3:1 or greater, 2.5:1, 2:1, 1.5:1, 1:1, 0.5:1, or other applicable ratios.

[0114] Figures 10A and 10B show illustrative reaction schemes for synthesizing fluoroboric acid-functionalized polystyrene polymers by fluoride treatment of boric acid-functionalized polystyrene polymers. In the example of Figure 10A, the fluoride treatment involves reacting boric acid-functionalized polystyrene polymer molecules with hydrogen fluoride or sodium fluoride. The fluoride groups of the fluoride reagent replace the hydroxyl groups of the boric acid groups. The product is a fluoroboric acid-functionalized polymer molecule with pendant trifluoroboric acid groups. The trifluoroboric acid group has a tetracoordinate boron atom covalently bonded to a side chain and three fluorine atoms. Therefore, the fluoroboric acid group has a formal negative charge, is ionic and acidic in nature, and is charge-neutralized by cations.

[0115] The degree of fluorine loading on the boric acid groups and the boric acid-functionalized polystyrene polymer, i.e., the pKa of the resulting fluoroboric acid-functionalized polystyrene polymer, can be adjusted arbitrarily based on the stoichiometry of the reagents. As shown in the example in Figure 10A, a molar ratio of HF or NaF to boric acid groups of about 3:1 or higher results in trifluoroboric acid groups.

[0116] In another example, the fluoride reagent is the limiting reagent, resulting in a fluoroboric acid with a higher (less acidic) pKa than the superacidic trifluoroboric acid of Figure 10A, but lower (more acidic) than the weakly acidic boric acid groups of the starting boric acid-functionalized polystyrene polymer. For example, when the molar ratio of fluoride reagent to boric acid groups is about 1:1, one fluorine atom of the trifluoroboric acid group is covalently bonded to the boron atom of the boric acid group (as shown in the reaction scheme in Figure 10B). When the molar ratio of fluoride reagent to boric acid groups is about 2:2, or when a separate step of fluoride treatment is performed, another fluorine atom is covalently bonded to the boron atom of the boric acid group. Any molar ratio of fluoride reagent to boric acid groups can be used, such as 3:1 or greater, 2.5:1, 2:1, 1.5:1, 1:1, 0.5:1, or other applicable ratios.

[0117] Figures 11A and 11B show illustrative reaction schemes for synthesizing fluoroboric acid-functionalized polymers by subjecting boric acid-functionalized polymers to a fluoride treatment. In the example of Figure 11A, the fluoride treatment involves reacting a boric acid-functionalized polymer molecule with hydrogen fluoride or sodium fluoride. The boric acid-functionalized polymer molecule is a derivative of a sulfonic acid-functionalized polymer molecule, with a boric acid group in place of the sulfonic acid group. The boric acid group is attached to the backbone via a linker through a sulfonamide bond. The fluoride group of the fluoride reagent replaces the hydroxy group of the boric acid group. The product is a fluoroboric acid-functionalized polymer molecule with a pendant trifluoroboric acid group attached to the backbone via a linker through a sulfonamide bond. The trifluoroboric acid group has a tetracoordinate boron atom covalently bonded to a side chain and three fluorine atoms. Thus, the fluoroborate group has a formal negative charge and is ionic and acidic in nature, with the charge being neutralized by the cation.

[0118] The degree of fluorine loading on the boric acid groups and the boric acid-functionalized polymer, i.e., the pKa of the resulting fluoroborate-functionalized polymer, can be adjusted based on the stoichiometry of the reagents. As shown in the example in Figure 11A, a molar ratio of HF or NaF to boric acid groups of about 3:1 or higher results in trifluoroborate groups.

[0119] In another example, the fluoride reagent is the limiting reagent, resulting in a fluoroboric acid with a higher (less acidic) pKa than the superacidic trifluoroboric acid of Figure 11A, but lower (more acidic) than the weakly acidic boric acid groups of the starting boric acid-functionalized polystyrene polymer. For example, when the molar ratio of fluoride reagent to boric acid groups is about 1:1, one fluorine atom is covalently bonded to the boron atom of the boric acid group (as shown in the reaction scheme in Figure 11B). When the molar ratio of fluoride reagent to boric acid groups is about 2:2, or when an additional step of fluoride treatment is performed, another fluorine atom is covalently bonded to the boron atom of the boric acid group (as shown in Figure 11B). Any molar ratio of fluoride reagent to boric acid groups can be used, such as 3:1 or greater, 2.5:1, 2:1, 1.5:1, 1:1, 0.5:1, or other applicable ratios.

[0120] 12 and 13 show alternative illustrative reaction schemes for synthesizing fluoroboric acid-functionalized polymers by fluoride treatment of boric acid-functionalized polymers. In the example of FIG. 12, the boric acid-functionalized polymer is a derivative of a sulfonic acid-functionalized polymer, with the boric acid groups linked to the backbone via sulfonamide bonds. However, other boric acid-functionalized polymers can also be used, including, but not limited to, boric acid-functionalized PBI polymers, boric acid-functionalized PTFE polymers, boric acid-functionalized PCTFE polymers, boric acid-functionalized polystyrene polymers, and the like.

[0121] In the example of Figure 12, the fluoride treatment involves reacting boric acid-functionalized polymer molecules with boron trifluoride (BF3) to obtain superproton-conducting dibasic fluoroboric acid-functionalized polymer molecules. Boron trifluoride is available as a diethyl ether and / or tetrahydrofuran complex. The trifluoroboric acid groups replace the hydrogen atoms of both hydroxyl groups of the boric acid groups. In the resulting polymer structure, the presence of three fluorine atoms covalently bonded to the tetracoordinated boron atom results in a superacidic fluoroboric acid. Thus, the boric acid groups are converted to superproton-conducting dibasic acids using boron trifluoride. Dibasic acids have twice the ion exchange capacity compared to monobasic acids.

[0122] The degree of fluorine loading on the boric acid groups and the boric acid-functionalized polymer, i.e., the pKa of the resulting fluoroborate-functionalized polymer, can be adjusted based on the stoichiometry of the reagents. As shown in the example in Figure 12, the molar ratio of BF3 to boric acid groups is about 2:1 or higher, resulting in two trifluoroborate groups.

[0123] In another example, because BF3 is the limiting reagent, the resulting fluoroboric acid has a higher (less acidic) pKa than the superacidic trifluoroboric acid of Figure 12, but a lower (more acidic) pKa than the weakly acidic boric acid groups of the starting boric acid-functionalized polystyrene polymer. For example, when the molar ratio of BF3 to boric acid groups is about 1:1, one of the trifluoroboric acid groups is covalently bonded to the oxygen atom of the boric acid group, as shown in reaction scheme Figure 13. When the molar ratio of BF3 to boric acid groups is about 2:2, or when a separate fluoride treatment step is performed as shown in Figure 13, another trifluoroboric acid group is covalently bonded to the other oxygen atom of the boric acid group. Any molar ratio of BF3 to boric acid groups can be used, such as 2:1 or greater, 1.5:1, 1:1, 0.5:1, or other applicable ratios.

[0124] Figure 14 shows another illustrative reaction scheme for synthesizing a fluoroboric acid-functionalized polymer. As shown, the boric acid-functionalized polymer is a derivative of a sulfonic acid-functionalized polymer. However, other boric acid-functionalized polymers can also be used, including, but not limited to, boric acid-functionalized PBI polymer, boric acid-functionalized PTFE polymer, boric acid-functionalized polystyrene polymer, and the like.

[0125] In the example of Figure 14, boric acid-functionalized polymer molecules are reacted with fluoroboric acid (BF(OH)2) to yield fluoroboric acid-functionalized polymer molecules. The fluoroboric acid groups replace the hydrogen atoms of one or both hydroxyl groups of the boric acid groups, depending on the stoichiometry of the reaction. The presence of fluorine atoms covalently bonded to the tetracoordinated boron atoms in the resulting polymer structure results in acidic fluoroboric acid groups. Therefore, the boric acid groups of the starting polymer molecules are converted to proton-conducting fluoroboric acid groups using a fluoroboric acid reagent. In other examples, the fluoroboric acid reagent has the formula BF2(OH) or RBF(OH), where R is an alkyl or aryl group.

[0126] The degree of fluorine loading on the boric acid functionalized polymer, i.e., the pKa of the resulting fluoroboric acid functionalized polymer, can be adjusted arbitrarily based on the type and stoichiometry of the reagents.

[0127] The tetracoordinate boric acid functionalized polymers described herein, including fluoroboric acid functionalized polymers, can be used in proton exchange membranes and ionomers in water electrolysis and fuel cell applications.

[0128] 15 shows an illustrative proton exchange membrane water electrolysis system 1500 (PEM water electrolysis system 1500) incorporating a tetracoordinate boric acid-functionalized polymer PEM and / or ionomer. The PEM water electrolysis system 1500 uses electricity to electrochemically split water into oxygen (O) and hydrogen (H). The configuration of the PEM water electrolysis system 1500 is illustrative and not limiting; other applicable configurations and other applicable water electrolysis systems can also incorporate a tetracoordinate boric acid-functionalized polymer.

[0129] 15, PEM water electrolysis system 1500 includes a membrane electrode assembly 1502 (MEA 1502), porous transport layers 1504-1 and 1504-2 (e.g., gas diffusion layers), bipolar plates 1506-1 and 1506-2, and a power supply 1508. PEM water electrolysis system 1500 may include additional or alternative components not shown in FIG. 15 and may be subject to specific examples.

[0130] The MEA 1502 includes a PEM 1510 disposed between a first catalyst layer 1512-1 and a second catalyst layer 1512-2. The PEM 1510 electrically insulates the first catalyst layer 1512-1 from the second catalyst layer 1512-2 and also functions to transport cations (e.g., protons (H + )) and is impermeable to gases such as hydrogen and oxygen. PEM1510 can be implemented by a tetracoordinate boric acid functionalized polymer (e.g., a fluoroboric acid functionalized polymer) described herein or by other applicable polymers.

[0131] First catalyst layer 1512-1 and second catalyst layer 1512-2 are electrically conductive electrodes and include a catalytic solid support bonded to electrocatalyst particles (not shown), such as platinum-group metals, metal alloys, and / or metal oxides. First catalyst layer 1512-1 and second catalyst layer 1512-2 may further include one or more ionomers mixed with the catalytic solid support and electrocatalyst particles. The ionomers can be tetracoordinate borate-functionalized polymers (e.g., fluoroborate-functionalized polymers) described herein or other applicable ionomers.

[0132] MEA 1502 is disposed between porous transport layers 1504-1 and 1504-2, which are further disposed between bipolar plates 1506-1 and 1506-2 having flow channels 1514-1 and 1514-2.

[0133] In the MEA 1502, the first catalyst layer 1512-1 functions as the anode, and the second catalyst layer 1512-2 functions as the cathode. When the PEM water electrolysis system 1500 is energized by the power source 1508, the oxygen evolution reaction (OER) occurs at the first catalyst layer / anode 1512-1 and is facilitated by an electrocatalyst bound to the catalytic solid support of the first catalyst layer / anode 1512-1. The OER is represented by the following electrochemical half-reaction: 2H2O→O2+4H + +4e -

[0134] Protons are conducted from the first catalyst layer / anode 1512-1 through the PEM 1510 to the second catalyst layer / cathode 1512-2, and electrons are conducted from the first catalyst layer / anode 1512-1 to the second catalyst layer / cathode 1512-2 through a conductive path around the PEM 1510. The PEM 1510 converts protons (H + ) and water from the first catalyst layer / anode 1512-1 to the second catalyst layer / cathode 1512-2, but is impermeable to oxygen and hydrogen. In the second catalyst layer / cathode 1512-2, protons combine with electrons to undergo the hydrogen evolution reaction (HER), which is facilitated by an electrocatalyst attached to the catalytic solid support of the second catalyst layer / cathode 1512-2. The HER is represented by the following electrochemical half-reaction: 4H + +4e - →2H2

[0135] OER and HER are two complementary electrochemical reactions for the splitting of water by water electrolysis, and the overall water electrolysis reaction is expressed as follows: 2H2O → 2H2 + O2

[0136] FIG. 16 shows an illustrative proton exchange membrane fuel cell 1600 (PEM fuel cell 1600) including a tetracoordinate boric acid functionalized polymeric PEM and / or ionomer (e.g., a fluoroboric acid functionalized polymeric PEM and / or ionomer). The PEM fuel cell 1600 produces electricity through an electrochemical reaction. In this example, the electrochemical reaction reacts hydrogen gas (H) and oxygen gas (O) to produce water and electricity. The configuration of the PEM fuel cell 1600 is illustrative and not limiting.

[0137] 16, PEM fuel cell 1600 includes a membrane electrode assembly 1602 (MEA 1602), porous transport layers 1604-1 and 1604-2 (e.g., gas diffusion layers), and bipolar plates 1606-1 and 1606-2. An electrical load 1608 may be electrically connected to MEA 1602 and driven by PEM fuel cell 1600. PEM fuel cell 1600 may include additional or alternative components not shown in FIG. 16, depending on the particular embodiment.

[0138] The MEA 1602 includes a PEM 1610 disposed between a first catalyst layer 1612-1 and a second catalyst layer 1612-2. The PEM 1610 electrically insulates the first catalyst layer 1612-1 from the second catalyst layer 1612-2 and also functions to transport cations (e.g., protons (H + )) and is impermeable to gases such as hydrogen and oxygen. PEM 1610 can be implemented by any applicable PEM described herein.

[0139] First catalyst layer 1612-1 and second catalyst layer 1612-2 are electrically conductive electrodes and include a catalytic solid support to which are attached electrocatalyst particles (not shown), such as platinum-based metals, metal alloys, and / or metal oxides. First catalyst layer 1612-1 and second catalyst layer 1612-2 may further include one or more ionomers mixed with the catalytic solid support and electrochemical catalyst particles. The ionomers can be any applicable polymer or ionomer described herein, including any of the tetracoordinate borate-functionalized polymers described herein, or other applicable ionomers.

[0140] The MEA 1602 is disposed between porous transport layers 1604-1 and 1604-2, which are further disposed between bipolar plates 1606-1 and 1606-2 having flow channels 1614. In the MEA 1602, the first catalyst layer 1612-1 functions as the cathode and the second catalyst layer 1612-2 functions as the anode. The first catalyst layer / cathode 1612-1 and the anode 1612-2 are electrically connected to a load 1608, and electricity generated by the PEM fuel cell 1600 powers the load 1608.

[0141] During operation of the PEM fuel cell 1600, hydrogen gas (H) flows into the anode side of the PEM fuel cell 1600, and oxygen gas (O) flows into the cathode side. In the second catalyst layer / anode 1612-2, hydrogen molecules are converted, facilitated by electrocatalyst particles attached to the catalytic solid support of the second catalyst layer / anode 1612-2, to protons (H) and electrons (e) according to the following hydrogen oxidation reaction (HOR): - ) is catalytically decomposed into 2H2→4H + +4e -

[0142] Protons are conducted from the anode 1612-2 through the PEM 1610 to the first catalyst layer / cathode 1612-1, and electrons are conducted from the second catalyst layer / anode 1612-2 through a conductive path around the PEM 1610 and the load 1608 to the first catalyst layer / cathode 1612-1. At the first catalyst layer / cathode 1612-1, the protons and electrons combine with oxygen gas, facilitated by electrochemical catalyst particles attached to the catalytic solid support of the first catalyst layer / cathode 1612-1, according to the following oxygen reduction reaction (ORR): O2+4H + +4e - →2H2O

[0143] Therefore, the overall electrochemical reaction of the PEM fuel cell 1600 is as follows: 2H2+O2→2H2O

[0144] In an overall reaction, the PEM fuel cell 1600 produces water at the first catalyst layer / cathode 1612-1. The water flows from the first catalyst layer / cathode 1612-1 through the PEM 1610 to the second catalyst layer / anode 1612-2 and may be removed from an outlet on the cathode and / or anode side of the PEM fuel cell 1600. The overall reaction produces electrons at the anode, which power the load 1608.

[0145] 14 and 15, the MEA 1502 and MEA 1602 include a catalyst layer 1512 / 412 formed on a PEM 1510 / 410. In an alternative configuration, the catalyst layer 1512 / 412 can be coated onto the PEM 110 / 410 to form a catalyst-coated membrane (CCM). For example, the catalyst layer 1512 / 212 can be formed in a one-pot or stepwise manner and sprayed onto the PEM 1510 / 410.

[0146] Various examples and embodiments have been described and illustrated herein. However, it will be apparent that various modifications and variations thereto may be made, and that additional embodiments may be implemented. Specific features of an embodiment described herein may be combined with or substituted for features of other embodiments. Accordingly, the descriptions and drawings herein are to be understood in an illustrative rather than a restrictive sense.

[0147] The advantages and features of the present disclosure are further illustrated by the following examples. [Example]

[0148] Example 1 A method for preparing a tetracoordinate boric acid functionalized polymer molecule, the method comprising reacting pendant boric acid groups of the boric acid functionalized polymer molecule with a fluoride reagent and / or a compound having the general formula HX, where HX is a Bronsted-Lowry acid.

[0149] Example 2 The method of Example 1, wherein the boric acid functionalized polymer molecules comprise boric acid functionalized polybenzimidazole (PBI) polymer molecules.

[0150] Example 3 The method of Example 1, wherein the boric acid functionalized polymer molecules comprise boric acid functionalized aromatic polymer molecules.

[0151] Example 4 The method of Example 1, wherein the boric acid functionalized polymer molecules comprise boric acid functionalized PTFE polymer molecules.

[0152] Example 5 The method of Example 1, wherein the boric acid functionalized polymer molecules comprise boric acid functionalized PCTFE polymer molecules.

[0153] Example 6 The method of Example 1, wherein the boric acid functionalized polymer molecules comprise boric acid functionalized cellulose polymer molecules.

[0154] Example 7 The method of any of Examples 1-6, wherein the method comprises reacting pendant boric acid groups with a fluoride reagent, and the fluoride reagent comprises hydrogen fluoride (HF), lithium fluoride (LiF), sodium fluoride (NaF), potassium fluoride (KF), potassium difluoride (KHF), cesium fluoride (CsF), ammonium fluoride (NHF), ammonium difluoride (NHF), or a tetraalkylammonium fluoride having the general formula NRF, where each R is independently hydrogen, a substituted or unsubstituted alkyl group, or an aryl group.

[0155] Example 8 The method of Example 7, wherein the method comprises reacting the pendant boronic acid groups with a fluoride reagent, and the fluoride reagent comprises boron trifluoride.

[0156] Example 9 The method of any one of Examples 1-8, wherein the method comprises reacting pendant boronic acid groups with a Bronsted-Lowry acid, and the Bronsted-Lowry acid comprises an alkyl sulfonic acid, an aryl sulfonic acid, or an alkylaryl sulfonic acid.

[0157] Example 10 The method of any one of Examples 1-9, wherein the method comprises reacting pendant boronic acid groups with a Bronsted-Lowry acid, and the Bronsted-Lowry acid comprises sulfuric acid or a derivative thereof.

[0158] Example 11 The method of any one of Examples 1-10, wherein the method comprises reacting pendant boronic acid groups with a Bronsted-Lowry acid, and the Bronsted-Lowry acid comprises a phosphoric acid, a phosphinic acid, or a derivative thereof.

[0159] Example 12 The method of any one of Examples 1-11, wherein the method comprises reacting pendant boronic acid groups with a Bronsted-Lowry acid, and the Bronsted-Lowry acid comprises a carboxylic acid, a phenol, or a derivative thereof.

[0160] Example 13 The method of any one of Examples 1-12, further comprising preparing a boric acid functionalized polymer molecule.

[0161] Example 14 The method of example 13, wherein preparing the boric acid-functionalized polymer molecules comprises functionalizing polymer molecules with boric acid groups.

[0162] Example 15 The method of Example 14, wherein the polymer molecules comprise a PBI polymer.

[0163] Example 16 The method of example 14, wherein the polymer molecules comprise aromatic polymer molecules.

[0164] Example 17 The method of example 14, wherein the polymer molecules comprise PTFE polymer molecules.

[0165] Example 18 The method of example 14, wherein the polymer molecules comprise PCTFE polymer molecules.

[0166] Example 19 The method of Example 14, wherein the polymer molecules comprise cellulose polymer molecules.

[0167] Example 20 The method of Example 14, wherein the polymer molecules comprise sulfonic acid-functionalized polymer molecules.

[0168] Example 21 The method of Example 14, wherein functionalizing the polymer molecules with boronic acid groups comprises boronating an aromatic ring in a repeat unit of the backbone.

[0169] Example 22 The method of Example 21, wherein the polymer molecules comprise PBI polymer molecules or polystyrene polymer molecules.

[0170] Example 23 The method of Example 14, wherein functionalizing the polymer molecules with boronic acid groups comprises attaching a boronic acid-functionalized linker to a secondary nitrogen in a repeat unit of the backbone.

[0171] Example 24 The method of Example 23, wherein the secondary nitrogen is contained in a benzimidazole unit of the backbone.

[0172] Example 25 The method of any of Examples 14-24, wherein functionalizing the polymer molecule with a boronic acid group comprises activating a sulfonic acid group of the sulfonic acid-functionalized polymer molecule to a sulfonyl chloride, sulfonyl fluoride, or sulfonyl ester, and coupling an amino group of an aminoboronic acid linker with the sulfonyl chloride, sulfonyl fluoride, or sulfonyl ester.

[0173] Example 26 The method of any of Examples 14-25, wherein functionalizing the polymer molecules with boronic acid groups comprises activating the sulfonic acid groups of the sulfonic acid-functionalized polymer molecules to sulfonyl chloride, sulfonyl fluoride, or sulfonyl ester, combining an aromatic boronic acid with the sulfonyl chloride, sulfonyl fluoride, or sulfonyl ester, and protonating the aromatic boronic acid.

[0174] Example 27 A tetracoordinate boric acid functionalized polymer molecule comprising a backbone and tetracoordinate boric acid groups attached to the backbone, the tetracoordinate boric acid groups having the general formula -BF m X n (OH) (3-m-n) wherein the boron atom (B) has four covalent bonds and is covalently attached to the polymer backbone, side chain, or side group; m and n are each independently 0, 1, 2, or 3; the sum of m+n is 1, 2, or 3; and X is an anion other than fluoride.

[0175] Example 28 The tetracoordinate boric acid-functionalized polymer molecule of Example 27, wherein the tetracoordinate boric acid groups comprise fluoroborate groups, wherein m is 1, 2, or 3, n is 0, 1, or 2, and the sum of m+n is 1, 2, or 3.

[0176] Example 29 The tetracoordinate boric acid functionalized polymer molecule of Example 28, wherein the fluoroborate groups comprise trifluoroborate groups having the general formula -BF3.

[0177] Example 30 The tetracoordinate boric acid functionalized polymer molecule of Example 28, wherein the fluoroboric acid group has the formula -BF2OH.

[0178] Example 31 The tetracoordinate boric acid functionalized polymer molecule of Example 28, wherein the fluoroboric acid groups have the formula -BF(OH)2.

[0179] Example 32 The tetracoordinate boric acid functionalized polymer molecule of any of Examples 27-31, wherein the backbone comprises repeat units comprising benzimidazole units.

[0180] Example 33 A tetracoordinate boric acid functionalized polymer molecule as described in Example 32, wherein the boron atom is attached to the secondary amine of the benzimidazole unit via a linker.

[0181] Example 34 In the tetracoordinate boric acid functionalized polymer molecule described in any one of Examples 27 to 31, A tetracoordinate boric acid functionalized polymer molecule, wherein the backbone comprises repeat units that include an aromatic ring, and the boron atom is covalently bonded to the aromatic ring.

[0182] Example 35 The tetracoordinate boric acid-functionalized polymer molecule of Example 34, wherein the backbone comprises polystyrene.

[0183] Example 36 The tetracoordinate boric acid-functionalized polymer molecule of any of Examples 27-35, wherein the tetracoordinate boric acid groups are attached to the backbone directly or indirectly via sulfonamide or sulfone bonds.

[0184] Example 37 In the tetracoordinate boric acid functionalized polymer molecule described in any of Examples 27 to 36, A tetracoordinate boric acid functionalized polymer molecule, wherein the backbone comprises PTFE having side chains, the side chains being long, medium, or short, and the boron atom is covalently bonded to the side chains.

[0185] Example 38 The tetracoordinate boric acid functionalized polymer molecule of any of Examples 27-37, wherein X is a conjugate base of a Bronsted-Lowry acid.

[0186] Example 39 The tetracoordinate boric acid functionalized polymer molecule of any of Examples 27-38, wherein the Bronsted-Lowry acid comprises an alkyl sulfonic acid, an aryl sulfonic acid, an alkylaryl sulfonic acid, a sulfuric acid, a phosphoric acid, a phosphinic acid, a carboxylic acid, a phenol, or a derivative thereof.

[0187] Example 40 A membrane electrode assembly (MEA) comprising a first catalyst layer, a second catalyst layer, and a proton exchange membrane (PEM) disposed between the first catalyst layer and the second catalyst layer, wherein at least one of the first catalyst layer, the second catalyst layer, and the proton exchange membrane is formed from polymer molecules comprising a main chain and a tetracoordinated boric acid group attached to the main chain, the tetracoordinated boric acid group having a general formula -BFmXn(OH) (3-m-n) wherein the boron atom (B) has four covalent bonds and is covalently attached to a polymer backbone, side chain, or side group; m and n are each independently 0, 1, 2, or 3; the sum of m+n is 1, 2, or 3; and X is an anion other than fluoride.

Claims

1. 1. A method for preparing a tetracoordinate boric acid functionalized polymer molecule, comprising: reacting the pendant boric acid groups of the boric acid functionalized polymer molecule with a fluoride reagent and / or a compound having the general formula HX; wherein HX is a Bronsted-Lowry acid.

2. The method of claim 1 , wherein the boric acid functionalized polymer molecules comprise boric acid functionalized polybenzimidazole (PBI) polymer molecules.

3. The method of claim 1 , wherein the boric acid-functionalized polymer molecules comprise boric acid-functionalized aromatic polymer molecules.

4. The method of claim 1 , wherein the boric acid functionalized polymer molecules comprise boric acid functionalized PTFE polymer molecules.

5. The method of claim 1 , wherein the boric acid functionalized polymer molecules comprise boric acid functionalized PCTFE polymer molecules.

6. The method of claim 1 , wherein the boric acid functionalized polymer molecules comprise boric acid functionalized cellulose polymer molecules.

7. reacting the pendant boronic acid groups with the fluoride reagent; The fluoride reagent may be hydrogen fluoride (HF), lithium fluoride (LiF), sodium fluoride (NaF), potassium fluoride (KF), potassium difluoride (KHF ２ ), cesium fluoride (CsF), ammonium fluoride (NH ４ F), ammonium difluoride (NH ４ F ２ ), or the general formula NR ４ F, and each R is independently hydrogen, or a substituted or unsubstituted alkyl or aryl group.

8. reacting the pendant boronic acid groups with the fluoride reagent; 8. The method of claim 7, wherein the fluoride reagent is boron trifluoride.

9. 2. The method of claim 1, comprising reacting the pendant boric acid groups with the Bronsted-Lowry acid, wherein the Bronsted-Lowry acid comprises an alkylsulfonic acid, an arylsulfonic acid, or an alkylarylsulfonic acid.

10. 10. The method of claim 1, comprising reacting the pendant boric acid groups with the Bronsted-Lowry acid, wherein the Bronsted-Lowry acid comprises sulfuric acid or a derivative thereof.

11. 10. The method of claim 1, comprising reacting the pendant boric acid groups with the Bronsted-Lowry acid, wherein the Bronsted-Lowry acid comprises a phosphoric acid, a phosphinic acid, or a derivative thereof.

12. 10. The method of claim 1, comprising reacting the pendant boric acid groups with the Bronsted-Lowry acid, wherein the Bronsted-Lowry acid comprises a carboxylic acid, a phenol, or a derivative thereof.

13. The method of claim 1 further comprising the step of preparing said boric acid functionalized polymer molecules.

14. 14. The method of claim 13, wherein the preparation of the boric acid functionalized polymer molecules comprises functionalizing polymer molecules with boric acid groups.

15. The method of claim 14 , wherein the polymer molecules comprise PBI polymer molecules.

16. The method of claim 14 , wherein the polymer molecules comprise aromatic polymer molecules.

17. The method of claim 14 , wherein the polymer molecules comprise PTFE polymer molecules.

18. The method of claim 14 , wherein the polymer molecules comprise sulfonic acid-functionalized polymer molecules.

19. 15. The method of claim 14, wherein the polymer molecules comprise boric acid functionalized PCTFE polymer molecules.

20. The method of claim 1 , wherein the boric acid functionalized polymer molecules comprise boric acid functionalized cellulose polymer molecules.

21. 15. The method of claim 14, wherein the step of functionalizing the polymer molecules with the boronic acid groups comprises boronating aromatic rings in repeat units of the backbone of the polymer molecules.

22. 20. The method of claim 19, wherein the polymer molecules comprise PBI polymer molecules or polystyrene polymer molecules.

23. 15. The method of claim 14, wherein the step of functionalizing the polymer molecules with the boronic acid groups comprises attaching a boronic acid-functionalized linker to a secondary nitrogen in a repeat unit of the backbone of the polymer molecules.

24. 24. The method of claim 23, wherein the secondary nitrogen is contained in a backbone benzimidazole unit.

25. functionalizing said polymer molecules with said boronic acid groups, activating the sulfonic acid groups of the sulfonic acid-functionalized polymer molecule to sulfonyl chloride, sulfonyl fluoride, or sulfonyl ester; and 15. The method of claim 14, comprising coupling an amino group of an aminoboronic acid linker with the sulfonyl chloride, sulfonyl fluoride, or sulfonyl ester.

26. functionalizing said polymer molecules with said boronic acid groups, Activating the sulfonic acid groups of the sulfonic acid-functionalized polymer molecules to sulfonyl chlorides, sulfonyl fluorides, or sulfonyl esters; combining an aromatic boronic acid with said sulfonyl chloride, sulfonyl fluoride, or sulfonyl ester; and 15. The method of claim 14, comprising protonating the aromatic boric acid.

27. The main chain and and a tetracoordinate boric acid group bonded to the main chain, the tetracoordinate boric acid group having the general formula -BF ｍ X ｎ (OH) （３－ｍ－ｎ） wherein the boron atom (B) has four covalent bonds and is covalently attached to the polymer backbone, side chain, or side group; m and n are each independently 0, 1, 2, or 3; the sum of m+n is 1, 2, or 3; and X is an anion other than fluoride.

28. 28. The tetracoordinate boric acid functionalized polymer molecule of claim 27, wherein the tetracoordinate boric acid comprises a fluoroboric acid group, where m is 1, 2, or 3, n is 0, 1, or 2, and the sum of m+n is 1, 2, or 3.

29. The fluoroboric acid group is represented by the general formula -BF ３ 30. The tetracoordinate boric acid functionalized polymer molecule of claim 28, comprising a trifluoroborate group having the formula:

30. The fluoroboric acid group is represented by the formula -BF ２ 30. The tetracoordinate boronic acid functionalized polymer molecule of claim 28 having the formula: OH.

31. The fluoroboric acid group has the formula -BF(OH) ２ 30. The coordinated boric acid functionalized polymer molecule of claim 28 having the formula:

32. 28. The tetracoordinate boric acid-functionalized polymer molecule of claim 27, wherein the backbone has repeating units comprising benzimidazole units.

33. 33. The tetracoordinate boronic acid-functionalized polymer molecule of claim 32, wherein the boron atom is attached to the secondary amine of the benzimidazole unit by a linker.

34. the main chain has a repeating unit containing an aromatic ring, 28. The tetracoordinate boronic acid-functionalized polymer molecule of claim 27, wherein said boron atom is covalently bonded to said aromatic unit.

35. 35. The tetracoordinate boric acid-functionalized polymer molecule of claim 34, wherein the backbone comprises polystyrene.

36. 28. The tetracoordinate boric acid functionalized polymer molecule of claim 27, wherein the tetracoordinate boric acid groups are attached directly or indirectly to the backbone or side chains by sulfonamide or sulfone bonds.

37. The main chain comprises PTFE having side chains, the side chain is a long side chain, a medium side chain, or a short side chain; 28. The coordinated boric acid-functionalized polymer molecule of claim 27, wherein said boron atom is covalently bonded to said side chain.

38. 28. The tetracoordinate boric acid-functionalized polymer molecule of claim 27, wherein X is the conjugate base of a Bronsted-Lowry acid.

39. 40. The tetracoordinate boric acid-functionalized polymer molecule of claim 38, wherein the Bronsted-Lowry acid comprises an alkylsulfonic acid, an arylsulfonic acid, an alkylarylsulfonic acid, a sulfuric acid, a phosphoric acid, a phosphinic acid, a carboxylic acid, a phenol, or a derivative of any of these.

40. a first catalyst layer; a second catalyst layer, and a proton exchange membrane disposed between the first catalyst layer and the second catalyst layer; At least one of the first catalyst layer, the second catalyst layer, and the proton exchange membrane is formed from a polymer molecule having a main chain and a tetracoordinated boric acid group bonded to the main chain, and the tetracoordinated boric acid group has the general formula -BF ｍ X ｎ (OH) （３－ｍ－ｎ） wherein B has four covalent bonds and is covalently attached to the polymer backbone, side chain, or side group; m and n are each independently 0, 1, 2, or 3; the sum of m+n is 1, 2, or 3; and X is an anion other than fluoride.

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