EP4695322A2 - Conjugate acid proton exchange polymers and methods of making and using conjugate acid proton exchange polymers - Google Patents

Abstract

A conjugate acid proton exchange polymer molecule includes an acidic aromatic unit in a main chain or a side chain, wherein the acidic aromatic unit is a conjugate acid of a basic aromatic unit, and a non-coordinating counter anion ionically linked with the acidic aromatic unit.

Description

CONJUGATE ACID PROTON EXCHANGE POLYMERS AND METHODS OF MAKING AND USING CONJUGATE ACID PROTON EXCHANGE POLYMERS

RELATED APPLICATIONS

[0001] This application claims priority to U.S. Provisional Patent Application No. 63/457,873, filed April 7, 2023, the contents of which is hereby incorporated by reference in its entirety.

BACKGROUND INFORMATION

[0002] In electrochemical cells, such as hydrogen fuel cells and water electrolysis systems, proton exchange membranes (PEMs) are used to selectively transport protons. Proton exchange membranes (PEMs) are semipermeable membranes that transport protons (H⁺) while being impermeable to gases. PEMs are generally composed of a porous framework with highly acidic functional groups. For example, polyfluorosulfonic acid-based PEMs, such as Nation™ (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 dissociable sulfonic acid groups serve as proton transport agents in the membrane. In hydrogen fuel cells, hydrogen gas (H2) separates at the anode into protons (H⁺) and electrons. The protons pass through a PEM and combine with oxygen gas (O2) at a cathode to produce water while the electrons flow through an external circuit to produce electricity. In water electrolysis systems, electricity splits water at the anode 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) may include a PEM positioned between a first catalyst layer and a second catalyst layer. The catalyst layers are electrically conductive electrodes (anode and cathode) with embedded electrochemical catalysts such as metals, metal alloys, or metal oxides. The catalysts may be bound to a catalyst solid support, which generally are an electrically conductive, high surface-area carbon (e.g., graphite or graphene). The electrochemical catalysts reduce the activation energy needed to carry out the electrochemical reactions at the electrodes, such as the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER) in water electrolysis applications and the hydrogen oxidation reaction (HOR) and the oxygen reduction reaction (ORR) in fuel cell applications. [0004] In some applications, the catalyst layer includes a supported catalyst mixed with an ionomer, an ion-conducting polymer. The ionomer binds the catalysts within the electrode, binds the catalyst layer on the PEM, and provides a pathway for cations (e.g., protons), thereby improving cation conductivity. In some MEAs, the catalyst layers are formed separately from the PEM and layered on the PEM in the MEA stack. In other MEAs, the catalyst layers are coated on the PEM to form catalyst-coated membranes (CCMs).

[0005] Water electrolysis and fuel cell applications involve strong oxidation and reduction chemistries under ambient to high temperature and acidic conditions. Therefore, PEMs and ionomers and the molecular functional groups therein responsible for proton transport properties should remain robust under the harsh reaction conditions of redox stress. However, conventional polymers used in PEMs and as ionomers, such as Nation™ and Aquivion®, mostly contain sulfonic acid functional groups as proton transport agents. The sulfonic acid functional groups have only limited ability to withstand the redox stress from electrochemical operations, mainly due to the intrinsic physicochemical properties of sulfur. Furthermore, the sulfonic acid groups participate in secondary redox reactions promoting platinum group metal catalyst poisoning and de-polymerization of the polymer matrices.

BRIEF DESCRIPTION OF THE DRAWINGS

[0006] The accompanying drawings illustrate various embodiments and are a part of the specification. The illustrated embodiments are merely examples and do not limit the scope of the disclosure. Throughout the drawings, identical or similar reference numbers designate identical or similar elements.

[0007] FIG. 1 shows an illustrative reaction scheme for producing a proton-conducting polybenzimidazolium (PBI⁺) polymer ionically linked with a tetrafluoroborate counter anion. [0008] FIG. 2 shows another illustrative reaction scheme for producing a PBI⁺ polymer ionically linked with a non-coordinating tetrafluoroborate counter anion.

[0009] FIG. 3 shows an illustrative reaction scheme for producing a PBI⁺ polymer ionically linked with any one of multiple different non-coordinating metal fluoride counter anions.

[0010] FIG. 4 shows another illustrative reaction scheme for producing a PBI⁺ polymer ionically linked with a non-coordinating tetrafluoroborate counter anion.

[0011] FIG. 5 shows an illustrative proton exchange membrane water electrolysis system incorporating conjugate acid proton exchange polymer PEMs and/or ionomers.

[0012] FIG. 6 shows an illustrative proton exchange membrane fuel cell including conjugate acid proton exchange polymer PEMs and/or ionomers.

DETAILED DESCRIPTION

[0013] Described herein are conjugate acid proton exchange polymers and methods of making and using conjugate acid proton exchange polymers by post-polymerization functional modification of basic aromatic polymers. Basic aromatic polymers include basic aromatic units (e.g., aryl groups and/or heteroaryl groups) within the main chain and/or side chains or side groups. Examples of basic aromatic polymers include, without limitation, polybenzimidazole (PBI) polymers and polyaniline (PANI) polymers, among many others. The basic aromatic units are weakly basic and react with a strong acid in an acid-base reaction to form an acidic aromatic unit that is ionically linked with a counter ion. The acidic aromatic unit is a conjugate acid of the basic aromatic unit. In some examples, the counter anion is the conjugate base of the strong acid, although the ion exchange group can be modified depending on the application and desired properties. The acidic aromatic unit easily dissociates in an aqueous environment to form an equilibrated mixture including hydrated proton (H₃O⁺). Thus, the easily dissociable acidic aromatic units may serve as proton transport agents.

[0014] The conjugate acid proton exchange polymers described herein may be used in electrochemical cell applications, such as in PEMs, ionomers, catalyst layers, and membrane electrode assemblies (MEAs), as well as other applications. The counter anion (conjugate base) is a non-coordinating anion that remains stable and passive under harsh electrochemical reaction conditions, does not poison electrochemical catalysts by reacting with the metal catalysts, and does not participate in secondary de-polymerization reactions. Thus, ionomers and PEMs based on conjugate acid proton exchange polymers, as described herein, reduce or eliminate catalyst poisoning as compared with conventional ionomers and polymers, including sulfonic acid-functionalized polymers, like Nation®.

[0015] Various definitions will now be provided to aid in understanding various aspects of the present disclosure. As used herein, each term or expression, e.g. alkyl, m, n, etc., when used more than once, is intended to be independent of its definition elsewhere in this disclosure. In case of conflict with any patent application or patent incorporated herein by reference, the present specification, including definitions, will control.

[0016] As used herein, “polymer” refers to a substance comprising polymer molecules of the same or different polymer species, including a mixture of polymer molecules of the same polymer species which may differ from other polymer molecules within the same sample in chain length and/or particular structural arrangement (e.g., irregularities in the orientation of monomer units, end-groups, and/or in the locations and/or lengths of any side chains or side groups). “Polymer” includes homopolymers, copolymers, terpolymers, interpolymers, and so on. [0017] As used herein, “polymer molecule” or “macromolecule” refers to a molecule of high relative molecular mass, the structure of which comprises a relatively large repetition of units (e.g., about 100 or more monomer units) derived, actually or conceptually, from molecules of low relative molecular mass (e.g., monomer molecules).

[0018] As used herein, “polymerization” refers to the process of converting a monomer, or a mixture of monomers, into a polymer. [0019] As used herein, “oligomer” refers to a substance composed of oligomer molecules.

[0020] As used herein, “oligomer molecule” refers to a molecule of intermediate relative molecular mass, the structure of which comprises a relatively small repetition of units (e.g., about 5 to about 100 monomer units) derived, actually or conceptually, from molecules of lower relative molecular mass (e.g., monomer molecules).

[0021] As used herein, “oligomerization” refers to the process of converting a monomer or a mixture of monomers into an oligomer.

[0022] The principles, concepts, and features described herein with reference to polymers, polymer molecules, and polymerization apply equally to oligomers, oligomer molecules, and oligomerization, respectively. Accordingly, any and all uses of the terms polymer, polymer molecule, and polymerization herein can be substituted by the terms oligomer, oligomer molecule, and oligomerization, respectively, without departing from the scope of the disclosure herein.

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

[0024] As used herein, “ionomer molecule” refers to a polymer molecule in which a small but relatively significant proportion of the constitutional units have ionizable or ionic pendant groups (including the ion exchange groups described herein), or both. Generally, no more than approximately 15 mole percent of the constitutional units have ionizable or ionic pendant groups.

[0025] As used herein, “monomer” refers to a substance composed of monomer molecules. [0026] As used herein, “monomer molecule” refers to a molecule that can undergo polymerization or oligomerization to form a polymer molecule or an oligomer molecule. A monomer molecule contributes constitutional units to the essential structure of a polymer molecule or an oligomer molecule.

[0027] As used herein, “copolymer” refers to a polymer derived from more than one species of monomer.

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

[0029] As used herein, “repeating unit” refers to the constitutional unit the repetition of which constitutes a polymer molecule (or oligomer molecule, block, or chain).

[0030] As used herein, “monomer unit” refers to the largest constitutional unit contributed by a single monomer molecule to the structure of a polymer molecule or an oligomer molecule. [0031] As used herein, “block” refers to a portion of a polymer molecule (or oligomer molecule) comprising many constitutional units and that has at least one feature which is not present in the adjacent portions. [0032] As used herein, “chain” refers to the whole or part of a polymer molecule (or oligomer molecule or block), comprising a linear or branched sequence of constitutional units between two boundary constitutional units, each of which may be either an end-group, a branch point, or an otherwise-designated characteristic feature of the polymer molecule.

[0033] As used herein, “main chain” or “backbone” refers to the chain of a polymer molecule to which all other chains (long or short or both) may be regarded as being pendant (e.g., a side chain).

[0034] As used herein, “side chain” refers to an oligomeric (short chain) or polymeric (long chain) offshoot from the main chain of a polymer molecule.

[0035] As used herein, “side group” or “pendant group” refers to an offshoot, neither oligomeric nor polymeric, from a chain (e.g., from a main chain).

[0036] The principles, concepts, and features described herein with reference to side chains apply equally to side groups. Accordingly, any and all uses of the term side chain can be substituted by the term side group without departing from the scope of the disclosure herein.

[0037] As used herein, “crosslink” refers to a small region in a polymer molecule from which at least four chains emanate. A crosslink is generally formed by reactions involving sites or groups on existing polymer molecules or by interactions between existing polymer molecules. [0038] The term “crosslinked” refers to the state in which polymer molecules that were earlier separate polymer molecules are linked to one another at points other than their ends. [0039] As used herein, a “catalyst particle” refers to a particle in “black” or pure form (e.g., exclusive of any catalyst support to which the catalyst particle may be bound and exclusive of any catalyst additives) that increases the rate of a reaction without modifying the overall standard Gibbs free energy change in the reaction. A catalyst particle may be an individual molecule (including but not limited to a monomer molecule), a group of molecules, a crystal structure (e.g., as in a metal oxide), a polymer molecule, or an oligomer molecule. A catalyst particle may have any suitable size and shape, such as a microparticle, a nanoparticle, or a nanotube. A catalyst particle may include, for example, a metal, a metal alloy, a metal oxide, a metal halide (e.g., a metal chloride), or a composite including at least one of a metal, a metal alloy, a metal oxide, or a metal halide.

[0040] As used herein, an “electrocatalyst particle” or “electrochemical catalyst particle” refers to a catalyst particle that reduces the activation energy needed to carry out electrochemical reactions and/or increases the rate of electrochemical reactions, such as the OER, HER, HOR, and/or ORR. Suitable electrocatalyst particles may include, without limitation, metals such as platinum group metals (PGMs) (e.g., platinum, palladium, iridium, ruthenium, osmium, and rhodium), transition metals (e.g., silver, gold, cobalt, copper, iron, nickel, rhenium, and mercury), and post-transition metals (e.g., bismuth and tin), metal alloys (e.g., PGM- transition metal based alloys and platinum-ruthenium based alloys), metal oxides (e.g., PGM oxides, such as iridium(IV) oxide, ruthenium(IV) oxide, iridium ruthenium oxide, platinum(IV) oxide, magnesium oxide, and cerium(IV) oxide), metal halides (e.g., platinum(IV) chloride, iridium(lll) chloride, platinum(IV) bromide, iridium(lll) bromide), and/or composites of metals, metal alloys, metal oxides, and/or metal halides.

[0041] As used herein, a “catalyst support” refers to a substance, exclusive of a catalyst particle, that may be used to support catalyst particles (e.g., a substance or material to which catalyst particles may be bound or on which catalyst particles may be supported). Examples of catalyst supports include, without limitation, carbon-based materials (e.g., carbon black, graphite, carbon nanotubes, graphene, and/or boron-functionalized carbon materials described herein), titanium dioxide, Sb-doped SnO2 nanoparticles, tin-doped indium oxide (ITO), and/or the ion exchange-modified catalyst supports described in International Patent Application No. PCT/US2022/046105, filed October 7, 2022, the contents of which are incorporated herein by reference in their entirety.

[0042] As used herein, a “catalyst” refers to a catalyst particle as well as a catalyst particle together with a catalyst support on which the catalyst particle is supported or to which the catalyst particle is bound. A catalyst may also include catalyst additives, such as promoters (such as, but not limited to, metalloids).

[0043] As used herein, an “electrocatalyst” or “electrochemical catalyst” refers to an electrocatalyst particle in “black” or pure form as well as an electrocatalyst particle together with a catalyst support on which the electrocatalyst particle is supported or to which the catalyst particle is bound. An electrocatalyst may also include catalyst additives, such as promoters.

[0044] As used herein, “metal” includes alkali metals, alkaline earth metals, transition metals, lanthanides, actinides, and post-transition metals.

[0045] As used herein, “transition metals” refers to elements of the d-block of the periodic table (Groups 3 to 12, inclusive).

[0046] As used herein, “post-transition metals” refers to aluminum, gallium, indium, tin, thallium, lead, bismuth, and polonium.

[0047] As used herein, “metalloids” refers to boron, silicon, germanium, arsenic, antimony, tellurium, and astatine.

[0048] As used herein, “platinum group metals” or “PGMs” refers to platinum, palladium, iridium, ruthenium, osmium, and rhodium.

[0049] As used herein, “aliphatic” compounds are hydrocarbons that are saturated or unsaturated, acyclic or cyclic, unbranched or branched, unsubstituted or wholly or partly substituted with one or more substituents or functional groups. As will be appreciated by one of ordinary skill in the art, “aliphatic” is intended herein to include, but is not limited to, alkyl, alkenyl, and alkynyl moieties. Illustrative aliphatic groups include, but are not limited to, for example, methyl, ethyl, n-propyl, isopropyl, allyl, n-butyl, sec-butyl, isobutyl, tert-butyl, n-pentyl, sec-pentyl, isopentyl, tert-pentyl, n-hexyl, and sec-hexyl moieties.

[0050] As used herein, the term “alkyl” is given its ordinary meaning in the art and may include saturated aliphatic groups, including straight-chain alkyl groups, branched-chain alkyl groups, cycloalkyl (alicyclic) groups, alkyl substituted cycloalkyl groups, and cycloalkyl substituted alkyl groups. An analogous convention applies to other generic terms such as “alkenyl,” “alkynyl,” and the like. Furthermore, as used herein, the terms “alkyl,” “alkenyl,” “alkynyl,” and the like encompass both wholly or partly substituted and unsubstituted groups. [0051] In some embodiments, a straight or branched alkyl chain may have 1 to 30 carbon atoms in its backbone, and, in some cases, 1 to 20 or fewer. In some embodiments, a straight or branched alkyl chain has 1 to 10 carbon atoms in its backbone (e.g., C1-C10 for straight chain, C3-C10 for branched chain), has 6 or fewer carbon atoms, or has 4 or fewer carbon atoms. Cycloalkyls may have from 3 to 10 carbon atoms in their ring structure or, in some case, from 3 to 5, 6 or 7 carbon atoms in the ring structure. Examples of non-cyclic alkyl groups include, but are not limited to, methyl, ethyl, propyl, isopropyl, butyl, isobutyl, n-butyl, tert-butyl, n-pentyl, neopentyl, n-hexyl, n-heptyl, n-octyl, n-decyl, n-undecyl, and dodecyl. Examples of cyclic alkyl groups include, but are not limited to, cyclopropyl cyclobutyl, and cyclochexyl.

[0052] The terms “alkenyl” and “alkynyl” refer to unsaturated aliphatic groups analogous in length and possible substitution to the alkyls described above, but that contain at least one double or triple bond respectively. Alkenyl groups include, but are not limited to, for example, ethenyl, propenyl, butenyl, 1-methyl-2-buten-1-yl, and the like. Non-limiting examples of alkynyl groups include ethynyl, 2-propynyl (propargyl), 1-propynyl, and the like.

[0053] The term “heteroatom” refers to any atom other than carbon. Non-limiting examples of heteroatoms include B, N, O, Al, Si, P, S, Ge, As, Se, and Sb. In some examples, a heteroatom is an atom selected from the group consisting of B, N, O, P, and S.

[0054] The term “heteroalkyl” refers to an alkyl group in which one or more hydrogen atoms bonded to any carbon of the alkyl group or one or more carbon atoms are replaced by a heteroatom. Examples of heteroalkyl groups include, without limitation, methoxy, ethoxy, propoxy, isopropoxy, methylthio, ethylthio, propylthio, isopropylthio, n-butylthio, methoxymethyl, and cyano groups.

[0055] 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.

[0056] The term “aryl” refers to aromatic carbocyclic groups, unsubstituted or partly or wholly substituted, having a single ring (e.g., phenyl), multiple rings (e.g., biphenyl), or multiple fused rings, wherein at least one ring of the aryl group is aromatic (e.g., 1 ,2,3,4- tetrahydronaphthyl, naphthyl, anthryl, or phenanthryl). That is, at least one ring of an aryl group has a conjugated Pi electron system, while other rings of the aryl group can be cycloalkyls, cycloalkenyls, cycloal kynyls, aryls, and/or heterocycyls. “Carbocyclic aryl groups” refer to aryl groups wherein the ring atoms on the aromatic ring are carbon atoms. Carbocyclic aryl groups include monocyclic carbocyclic aryl groups and polycyclic or fused compounds (e.g., two or more adjacent ring atoms are common to two adjoining rings), such as naphthyl group. Aryl groups are not limited to benzene and its derivatives but may have any suitable number of atoms in the ring. Examples of aryl groups include, without limitation, phenyl, naphthyl, tetrahydronaphthyl, anilyl, indanyl, and indenyl.

[0057] The term “heteroaryl” refers to aryl groups comprising at least one heteroatom as a ring atom (e.g., heteroaromatic groups), such as a heterocyclic group. Non-limiting examples of heteroaryl groups include, without limitation, pyridyl, pyrazinyl, pyrimidinyl, pyrrolyl, pyrazolyl, imidazolyl, thiazolyl, oxazolyl, isooxazolyl, thiadiazolyl, oxadiazolyl, thiophenyl, furanyl, quinolinyl, and isoquinolinyl. Heteroaryl groups may be referred to as A-heteroaryl where A is the element symbol of the heteroatom. For example, N-heteroaryl refers to an aryl group including at least one nitrogen atom as a ring atom.

[0058] The terms “alkoxyl” or “alkoxy” as used herein refers to an alkyl group having an oxygen radical attached thereto, and has the general formula R — O. Examples of alkoxyl groups include, without limitation, methoxy, ethoxy, propyloxy, and tert-butoxy groups.

[0059] The term “aryloxy” refers to an aryl group having an oxygen radical attached thereto. An example of an alkoxyl group includes, without limitation, a phenoxy group.

[0060] Any of the above groups may be optionally substituted, in whole or in part. Examples of substituents include, without limitation, aliphatic, alicyclic, heteroaliphatic, heteroal icyclic, halogen, azide, alkyl, aralkyl, alkenyl, alkynyl, cycloalkyl, hydroxyl, alkoxyl, amino, amine, nitro, sulfhydryl, imino, amido, phosphonate, phosphinate, carbonyl, carboxyl, alkyloxycarbonyl, silyl, ether, alkylthio, heteroalkylthio, heteroarylthio, sulfonyl, sulfonamido, ketone, aldehyde, ester, heterocyclyl, aromatic or heteroaromatic moieties, — CF3, — CN, aryl, aryloxy, perhaloalkoxy, aralkoxy, heteroaryl, heteroaryloxy, heteroarylalkyl, heteroaralkoxy, azido, amino, halide, alkylthio, oxo, acylalkyl, carboxy esters, -carboxamido, acyloxy, aminoalkyl, alkylaminoaryl, alkylaryl, alkylaminoalkyl, alkoxyaryl, arylamino, aralkylamino, alkylsulfonyl, - carboxamidoalkylaryl, -carboxamidoaryl, hydroxyalkyl, haloalkyl, alkylaminoalkylcarboxy-, aminocarboxamidoalkyl-, cyano, alkoxyalkyl, perhaloalkyl, arylalkyloxyalkyl, (e.g., SO4(R')2), a phosphate (e.g., PO4(R')₃), a silane (e.g., Si(R')4), a urethane (e.g., R'O(CO)NHR'), and the like. Additionally, the substituents may be selected from F, Cl, Br, I, — OH, — NO2, — CN, — NCO, — CF₃, — CH₂CF_3J — CHCI2, — CH₂ORx, — CH2CH2ORX, — CH₂N(RX)₂, — CH₂SO₂CH₃, — C(O)RX, — O₂( X), — CON(RX)₂, — OC(O)RX, — C(O)OC(O)RX, — OCO₂ X, — OCON(RX)₂, — N(RX)₂, — S(O)₂ X, — OCO2 X, — NRx(CO)Rx, — NR_X(CO)N(R_X)2, wherein each occurrence of R_x independently includes, but is not limited to, hydrogen, aliphatic, alicyclic, heteroaliphatic, heteroal icyclic, aryl, heteroaryl, alkylaryl, or alkyl heteroaryl, wherein any of the aliphatic, alicyclic, heteroaliphatic, heteroal icyclic, alkylaryl, or alkyl heteroaryl substituents described above and herein may be wholly or partly substituted or unsubstituted, branched or unbranched, cyclic or acyclic, and wherein any of the aryl or heteroaryl substituents described above and herein may be wholly or partly substituted or unsubstituted.

[0061] Polytetrafluoroethylene (PTFE) polymers are a class of polymers composed of tetrafluoroethylene polymer molecules, and derivatives thereof, and are produced by the polymerization of tetrafluoroethylene. PTFE polymer molecules have a carbon main chain with two fluorine atoms bonded to each carbon, and derivatives thereof.

[0062] Polychlorotrifluoroethylene (PCTFE) polymers are derivatives of PTFE and are homo-polymers of chlorotrifluoroethylene (CTFE) with the molecular formula (CF2CCIF)_n, and derivatives thereof. PCTFE is similar to PTFE (such as Teflon) except that PCTFE contains a chlorine atom in each repeating unit. The presence of this chlorine atom makes PCTFE a unique thermoplastic polymer with many applications. However, PCTFE has a hydrophobic main chain and is non-conducting for ions and thus is not suitable for electrochemical applications. Derivatives of PCTFE polymers include substituent groups (e.g., side chains or side groups) in place of chlorine atoms. Derivatives of PCTFE may be wholly or partly substituted. Derivatives of PCTFE polymers include, without limitation, modified and functionalized PCTFE polymers, including any of the modified or functionalized PCTFE polymers (e.g., acid-functionalized and ion-exchange functionalized PCTFE polymers) described in U.S. Provisional Application No. 63/532,262, filed August 11, 2023, which is hereby incorporated by reference in its entirety.

[0063] Sulfonic acid-functionalized PTFE polymers are derivatives of PTFE polymers and have a PTFE main chain and a side chain or side group with one or more pendant sulfonic acid groups. In some examples, the side chain is a long side chain (LSC) having at least two ether linkages and four or more polyfluorinated carbon units (e.g., — CF2 — and/or — CF3). In other examples, the side chain is a short-side chain (SSC) having one ether linkage and two polyfluorinated carbon units. In further examples, the side chain is a mid-side chain (MSC) having one ether linkage and four polyfluorinated carbon units. In some examples, a sulfonic acid-functionalized PTFE polymer has the general formula [(CF₂CF2)m(CFACF2)_n]x, where A is a side chain comprising one or more pendant sulfonic acid groups, and m, n, and x are positive and are selected based on application, equivalent weight, molecular weight, etc. In some examples, m ranges from 4 to 7 and n is 1. In some examples, side chain A is a LSC, MSC, or SSC. Examples of LSC sulfonic acid-functionalized PTFE polymers include, without limitation, Nation™ series polymers (available from Chemours Company in various configurations and grades, including Nafion-H, Nation HP Nation 117, Nation 115, Nation 212, Nation 211, Nation NE1035, Nation XL, etc.) and any combination, derivative, grade, or configuration thereof. Examples of SSC sulfonic acid-functionalized PTFE polymers include, without limitation, Aquivion® series polymers (available from Solvay S.A. in different configurations and grades, including Aquivion® E98-05, Aquivion® PW98, Aquivion® PW87S, etc.), Gore-Select® (available from W.L. Gore & Associates, Inc.), Flemion™ (available from Asahi Glass Company), Pemion+™ (available from lonomr Innovations, Inc.), and any combination, derivative, grade, or configuration thereof. Examples of MSC sulfonic acid-functionalized PTFE polymers include, without limitation, polymers produced by 3M™ Company. In some examples, a PTFE polymer is a copolymer that comprises one or more other repeating units. In some examples, a PTFE polymer may be doped and/or may be crosslinked with itself and/or with another polymer.

[0064] Sulfonic acid-functionalized polymers include, without limitation, polyfluorosulfonic acid (PFSA) polymers and non-fluorinated sulfonic acid polymers. Examples of polyfluorosulfonic acid polymers include, without limitation, sulfonic acid-functionalized PTFE polymers and sulfonic acid-functionalized PCTFE polymers. Examples of non-fluorinated sulfonic acid polymers include, without limitation, poly(styrene sulfonic acid) polymers, sulfonated aromatic polymers (e.g., sulfonated poly(ether ketone) (SPEEK) polymers, sulfonated poly(aryl ether sulfone) (SPAES) polymers, sulfonated poly(arylene ether ketone) (SPAEK) polymers, sulfonated polysulfone (SPSF) polymers, sulfonated polyimide (SPI) polymers, and sulfonated polystyrene (SPS), sulfonated polyphenylene, and any other sulfonated polymer, including sulfonated derivatives of polymers described herein.

[0065] Polystyrene polymers are polymers composed of polystyrene polymer molecules. Polystyrene polymer molecules have a repeating unit that includes alternating carbon centers attached to a phenyl group. Examples of polystyrene polymers include, without limitation, polystyrene, poly(styrene sulfonic acid) (e.g., poly(4-styrene sulfonic acid)), polyhalostyrene, poly(3-trifluoromethyl styrene), poly(4-acetoxy styrene), poly(4-allyl styrene), poly(4- cyanostyrene), poly(4-dimethylsilyl styrene), poly(4-hydroxystyrene), poly(alpha-methyl styrene), poly(4-methyl styrene), poly(4-methoxystyrene), poly(4-[tert-butoxycarbonyl]oxy- styrene), poly(4-tert- butyl styrene), poly(4-[N,N-di(trimethylsilyl)aminomethyl]-styrene), poly(4- vinylbenzoic acid), poly(n-butyl 4-vinylbenzoate), poly(tert-butyl 4-vinylbenzoate), poly(2- ethylhexyl 4-vinylbenzoate), poly(methyl 4-vinylbenzoate), poly(vinylbenzyl chloride), poly(4- vinylbenzyl-N-methylphthalimide), poly(vinyl cyclohexane), and derivatives of any of the foregoing (including substituted (e.g., fluorinated) and/or branched derivatives). In some examples, a polystyrene polymer is a copolymer that comprises one or more additional repeating units, which may or may not include a carbon center attached to a phenyl group.

[0066] Aromatic polymers are polymers having one or more aromatic units in a main chain and/or a side chain. An aromatic unit includes one or more aryl groups and/or heteroaryl groups. Examples of aromatic polymers include, without limitation, polystyrene polymers, polycarbonate polymers (polyphenylene polymers (e.g., poly(1,4-phenylene), poly(1,4- phenylene-ethylene), poly(1,3-phenylene-methylene), poly(p-phenylene vinylidene), poly(p- phenylene vinylene), poly(1,4-phenylene oxide), poly(1,4-phenylene sulfide)), poly(ether sulfone), polyaryletherketone polymers, polysulfone polymers, poly(ethylene terephthalate), aromatic polyester polymers, poly(oxy-1,4-phenylenecarbonyl-1,4-phenylene), poly[(dimethylmethylene)bis(4,1 -phenylene) carbonate], phenolic resins, poly[(ethylazanediyl)ethyleneazanediyl-1,3-phenylene], poly-oxydiphenylene-pyromellitimide (Kapton®, manufactured by E. I. du Pont de Nemours and Company), poly(ester imide) polymers, aromatic polyimide polymers, lignin, and derivatives of any of the foregoing (including substituted (e.g., fluorinated) and/or branched derivatives).

[0067] Conjugate acid proton exchange polymers are synthesized by post-polymerization acid-base reactions between a basic aromatic polymer and an acid to form a conjugate acid proton exchange polymer. Basic aromatic polymers are polymers that have a basic aromatic unit in a main chain and/or a side chain of the polymer. A basic aromatic unit includes a basic aryl group and/or a basic heteroaryl group. The acid-base reaction converts the basic aromatic unit to an acidic aromatic unit by protonation of the basic aromatic unit. The acidic aromatic unit is the conjugate acid of the basic aromatic unit and has a positive formal charge by addition of the proton.

[0068] The resulting conjugate acid proton exchange polymer molecule includes an acidic aromatic unit in the main chain or the side chain of the polymer molecule and a counter anion ionically linked with the acidic aromatic unit. Conventional counter anions, such as sulfates (derived from sulfuric acid), phosphates (derived from phosphoric acid and polyphosphoric acids), and halides, have substantial coordinating properties. These coordinating anions participate in catalyst-poisoning reactions with metal catalysts and promote de-polymerization reactions, thereby reducing the throughput and lifetime of a membrane electrode assembly (MEA). The size of the counter anions, and more specifically the ionic potential (ratio of anion charge to anion radius) of the anions, also plays a role in modulating the hydrogen bond networks and water retention properties of proton exchange membranes.

[0069] To address these issues, the counter anion ionically linked with the acidic aromatic unit is a non-coordinating anion. Non-coordinating counter anions remain stable and passive under harsh electrochemical reaction conditions, do not poison catalysts by reacting with the metal catalysts, and do not participate in secondary de-polymerization reactions. Accordingly, in some examples the counter anion ionically linked with the acidic aromatic unit is the conjugate base of the acid reagent used in the acid-base reaction, wherein the acid reagent has a conjugate base that is a non-coordinating anion. In other examples, such as where the acid reagent has a coordinating conjugate base, a subsequent fluoride treatment step is performed after the acid-base reaction to convert the counter anion to a non-coordinating counter anion (e.g., tetrafluoroborate or a metal fluoride), or a subsequent anion exchange step is performed after the acid-base reaction to exchange the counter anion with a non-coordinating counter anion (e.g., tetrafluoroborate). The fluoride treatment step and the anion exchange step will be described below in more detail.

[0070] As mentioned, a basic aromatic unit includes a basic aryl group and/or a basic heteroaryl group. Basic aryl groups and basic heteroaryl groups have an amine with a lone pair of electrons available for accepting a proton from an acid, which makes the aryl group or heteroaryl group, and hence the aromatic unit, weakly basic. The amine may be in an aromatic ring of the aromatic unit, as in an N-heteroaryl group, or may be in a substituent of the aromatic ring, as in an arylamine (e.g., aniline). The amine may be a primary amine, a secondary amine, or a tertiary amine. As used herein, a primary amine refers to a nitrogen atom bonded to one carbon atom and two hydrogen atoms (C — NH2), a secondary amine refers to a nitrogen atom bonded to two carbon atoms and one hydrogen atom (C — NH — C), and a tertiary amine refers to a nitrogen atom bonded to two carbon atoms and no hydrogen atoms (C — N=C) or bonded to three carbon atoms (C — N( — C)( — C)). A tertiary amine of a basic aromatic unit is a stronger base than a secondary amine of a basic aromatic unit, and a secondary amine of a basic aromatic unit is a stronger base than a primary amine of a basic aromatic unit.

[0071] Non-limiting examples of basic aromatic units include anilines (e.g., aniline, toluidines, xylidines, chloroanilines, aminobenzoic acids, nitroanilines, 2,5-diamino-1,4- phenylene), pyridines (e.g., 2,2'-bipyridine, 1,10-phenanthroline, 2,2';6'2"-terpyridine, 4-chloro- [3,3-bipyridine]- 5,5'-diyl; [2,3'-bipyridine]-4,5'-diyl; pyridine-4,2-diyl; pyridine-3,5-diyl; pyridine-

2.4-diyl; pyridine-2,6-diyl), pyrazines (e.g., alkyl pyrazines, such as 2,3-dimethylpyrazine, 2,5- dimethylpyrazine, 2,3,5-Trimethylpyrazine, 2,6-dimethylpyrazine, 2-and ethyl-3-methylpyrazine, methoxylated pyrazines, such as 3-isobutyl-2-methoxypyrazine), imidazoles (e.g., 1,5- dihydrobenzo[1,2-d:4,5-cf]= diimidazole-2,6-diyl; 4-phenyl-1H-imidazole-2,5-diyl; 4-phenyl-1H- imidazole-5,2-diyl; 5-phenyl-1H-imidazole-4,2-diyl), pyrazoles (e.g., 1H-pyrazole, 3H-pyrazole, 4H-pyrazole, aminopyraolzes (such as 3-aminopyrazoles, 4-aminopyrazoles, and 5- aminopyrazoles), diaminopyrazoles (e.g., 3,5-diaminopyrazoles), 1-(1-adamantyl)pyrazoles,

1.4-di(1-adamantyl)pyrazoles, aminophenazone, metamizole), pyrimidines (e.g., 2,4-dioxy pyrimidine, 2,4-dioxy-5-methyl pyrimidine, 2-oxy-4-amino pyrimidine, 2,4-dioxy-6-carboxy pyrimidine, 5-bromo-2,4-dichloropyrimidine, ), pyridazines, thiazoles (e.g., 6,6'-bi(1 ,3- benzothiazole)]-2,2'-diyl, [2,2'-bi(1 ,3-thiazole)]-4,4'-diyl), imidazolines, quinolines (e.g., quinoline-2,4-diyl; [3,3'-biquinoline]-6,6'-diyl), isoquinolines, acridines, quinoxilines (e.g., 3,3'- diphenyl= [6,6'-biquinoxaline]-2,2'-diyl; 1 H,TH-[5,5'-bibenzimidazole]-2,2'-diyl), benzimidazoles, purines, indazoles, quinazolines, cinnolines, tetrazines (e.g., 1 ,2,4,5-tetrazine-3,6-diyl), thiophenes (e.g., 3,4-dioctylthiophene-2,5-diyl; thiophene-2, 5-diyl; thiophene-2, 4-diyl), and derivatives of any of the foregoing. [0072] Basic aromatic polymers include polymers produced by polymerization of monomer units including any basic aromatic units, or otherwise based on or incorporating any basic aromatic units. Non-limiting examples of basic aromatic polymers include polyaniline (PANI) polymers (including leucoemeraldine, emeraldine, and pernigranilin forms), polypyrrole (PPy), polypyridine polymers (e.g., poly(2-isopropenyl pyridine), poly(2,5-pyridine), poly(3,5-pyridine), poly(2-vinyl pyridine), poly(4-vinyl pyridine)), polypyrazine (e.g., poly(2-vinyl pyrazine)), polyimidazole (e.g., poly(n-vinyl imidazole), poly(4-vinyl imidazole)), polypyrazole, polypyrimidine, polypyridazine (PPd), polythiazole, polyimidazolines, polyquinolines, polyisoquinolines, polyacridines, polyquinoxilines, polybenzimidazoles, polypurines, polyindazoles, polyquinazolines, polycinnolines, polytetrazines, and polymers in which a basic aromatic unit is included in side chains or side groups (e.g., poly(4-vinylpyridine) (PVP)).

[0073] The acid reagent donates a proton to the basic aromatic unit to produce a conjugate acid of the basic aromatic unit. Any suitable acid reagent may be used. In some examples, the acid reagent has a conjugate base that is non-coordinating, thus forming a counter anion that remains stable and passive under harsh electrochemical reaction conditions, does not poison the catalysts by reacting with the metal catalysts, and does not participate in secondary depolymerization reactions. Non-limiting examples of suitable acids having a non-coordinating conjugate base include: fluoroboric acid (HBF4) (also commonly known as tetrafluoroboric acid or hydrogen tetrafluoroborate ([H⁺] [BF₄-]), which has the general structure: phenyl trifluoroboric acid, which has the general structure: phenyl 1 ,4-di(trifluoro)boric acid, which has the general structure: hexafluorophosphoric acid (HPFe), which has the general structure:

^F^^F _H® fluoroantimonic acid (also referred to as hydrogen hexafluoroantimonate (HFeSb)), which has the general structure:

^F'_{S H}® F' . a sulfuric acid-boron trifluoride complex having the general structure: a benzenesulfonic acid-boron trifluoride complex having the general structure: a benzene disulfonic acid-boron trifluoride complex having the general structure: a phosphoric acid-boron trifluoride complex having the general structure:

[0074] It will be recognized that other acid reagents having one or more fluoroboric acid groups are contemplated by this disclosure and may be used in place of the acid reagents described above. In some examples, the non-coordinating conjugate base/anion produced by the acid reagent is tetrafluoroborate ([BF₄-]), phenyl trifluoroborate (PhBF₃ ^_), hexafluorophosphate ([PF₆-]), hexafluoroantimonate ([SbF₆-]), benzenesulfonato-trifluoroborate (PhSO₃(BF₃)^_), or sulfato-di(trifluoroborate) (SO₄(BF₃)₂ ^_).

[0075] In other examples, the acid reagent has a conjugate base/counter ion that is not non-coordinating. For example, sulfates, phosphates, and halides have substantial coordinating properties. As will be explained below, the coordinating counter ion may be converted into, or replaced with, a non-coordinating counter anion in a fluoride treatment step or an anion exchange step. The acid reagent in these examples may be an acid having the general formula HA where A is a halide (e.g., F, Cl, or Br), HSO4, SO4, H2PO4, HPO4, PO4, or any polyphosphate anion derived from any polyphosphoric acid. Suitable examples of polyphosphoric acids include, without limitation, diphosphoric acid, triphosphoric acid, tetraphosphoric acid, and trimetaphosphoric acid. Non-limiting examples of acid HA include, without limitation, hydrofluoric acid (HF), hydrochloric acid (HC), hydrobromic acid (HBr), sulfuric acid (H2SO4), fluorophosphoric acid (H2PO3F), a phosphoric acid (H3PO4), or a polyphosphoric acid.

[0076] The degree of protonation of the basic aromatic units may be tuned to the desired levels by adjusting the molar ratio of the acid reagent to the basic aromatic units of the polymer. For example, the molar ratio of the acid reagent to the basic aromatic units may be 1 : 1 , 1 :2, 1 :3, or any other suitable molar ratio. In some examples, the acid reagent is the limiting reagent so that the aromatic units are not fully protonated. In further examples, the molar ratio of the acid reagent to the basic aromatic unit is greater than 1 :1 (e.g., 1.5:1, 2:1, 3:1 , etc.) so that the basic aromatic units are fully protonated.

[0077] In the acid-base reaction, the basic aromatic unit is protonated by the acid and thus is converted to an acidic aromatic unit. The acidic aromatic unit is the conjugate acid of the basic aromatic unit. Thus, the polymer molecule produced by the acid-base reaction is referred to as a conjugate acid polymer molecule. The acid-base reaction is performed at standard conditions and proceeds without the need for a catalyst. In the reaction, the amine of the basic aromatic unit is protonated when the lone pair of the amine accepts the proton from the acid. Thus, the acidic aromatic unit is an N-protonated conjugate acid. When the basic aromatic unit is an imidazole unit or a pyridine unit, the acidic aromatic unit (N-protonated conjugate acid) may be referred to as an imidazolium unit or a pyridinium unit, respectively. When the basic aromatic unit includes aniline, as in PANI polymers, the acidic aromatic unit (N-protonated conjugate acid) is an ammonium derivative. The acidic aromatic unit has a positive charge at the protonated nitrogen and is counterbalanced by a counter anion ionically linked with the acidic aromatic unit. The counter ion is the conjugate base of the acid reagent.

[0078] When the acid reagent is HBF4, HPFe, or fluoroantimonic acid, the counter ion is a non-coordinating counter anion (e.g., tetrafluoroborate ([BF₄-]), hexafluorophosphate ([PF₆-]), or hexafluoroantimonate ([SbF₆-]), respectively). Accordingly, the conjugate acid proton exchange polymer may be used in electrochemical applications.

[0079] In some examples, the ion exchange capacity (IEC) of the conjugate acid proton exchange polymer is increased by using an acid reagent other than fluoroboric acid. In further examples, an acid reagent having multiple fluoroboric acid groups (e.g., phenyl trifluoroboric acid, phenyl 1 ,4-di(trifluoro)boric acid, a sulfonic acid-BF₃ complex, a phosphoric acid-BF₃ complex) also acts as a cross-linking agent wherein each fluoroboric acid group reacts with different basic aromatic units of the same or different polymer molecules. Cross-linking in this manner increases both polymer durability and IEC of the resulting conjugate acid proton exchange polymer.

[0080] When the acid reagent has a conjugate base that is coordinating, a fluoride treatment step or an anion exchange step is performed on the intermediate conjugate acid proton exchange polymer to convert the counter anion to a non-coordinating counter anion or to exchange the counter anion with a non-coordinating counter anion.

[0081] For example, when the acid reagent is hydrofluoric acid (HF), the acid-base reaction produces an intermediate conjugate acid proton exchange polymer in which the counter ion is a fluoride anion (F^_). A fluoride treatment step is then performed by combining the intermediate conjugate acid proton exchange polymer with boron trifluoride (e.g., in the form of BF3 etherate (Et20)), a difluoro(phenyl)borane having either of the general structures: or a metal fluoride having the general formula MF_n, wherein M is beryllium (Be), aluminum (Al), bismuth (Bi), antimony (Sb), phosphorous (P), tin (Sn), zirconium (Zr), or titanium (Ti) and n corresponds to a valence of M. Boron trifluoride reacts with the fluoride anion by an addition reaction to form a non-coordinating tetrafluoroborate counter anion wherein M increases its oxidation state to add the fluoride anion. Similarly, a difluoro(phenyl)borane as shown above reacts with the fluoride anion by an addition reaction to form a non-coordinating trifluoro(phenyl)borate counter anion. The metal fluoride reacts with the fluoride anion by an addition reaction to form a non-coordinating metal fluoride counter anion having the general formula [MF(_n+i)]^_, wherein M increases its oxidation state to add the fluoride anion. In some examples, the non-coordinating metal fluoride counter anion is beryllium fluoride (BF2), aluminum fluoride (AIF3), fluorinated bismuth (e.g., bismuth trifluoride (bismuth(lll) fluoride), fluorinated antimony (e.g., antimony(lll) trifluoride), fluorinated phosphorous (e.g., monofluorophosphate, difluorophosphate, or hexafluorophosphate), fluorinated tin (tin(ll) fluoride, also known as stannous fluoride), fluorinated zirconium (e.g., zirconium(IV) fluoride), or fluorinated titanium (e.g., titanium trifluoride (titanium(lll) fluoride) and titanium tetrafluoride (titanium(IV) fluoride)).

[0082] In some examples, the ion exchange capacity (IEC) of the conjugate acid proton exchange polymer is increased by using a difluoro(phenyl)borane in the fluoride treatment as compared with boron trifluoride. In some examples, a difluoro(phenyl)borane having two difluoroboryl groups also acts as a cross-linking agent wherein each difluoroboryl group crosslinks different aromatic units of the same or different polymer molecules. Cross-linking in this manner increases both polymer durability and IEC of the resulting conjugate acid proton exchange polymer.

[0083] In other examples, when the acid reagent is an acid having the general formula HA described above, the acid-base reaction produces an intermediate conjugate acid proton exchange polymer in which the acidic aromatic unit is ionically linked with a non-coordinating counter anion A-. An anion exchange step is then performed by combining the intermediate conjugate acid proton exchange polymer with a metal tetrafluoroborate compound having the general formula MBF4 where BF4 is a tetrafluoroborate anion (BF4-) and M is a sodium ion (Na⁺), a potassium ion (K⁺), a cesium ion (Cs⁺), ammonium (NH4⁺), or tetraalkylammonium (R4N⁺ wherein each R is independently hydrogen or a wholly or partly substituted or unsubstituted alkyl group having one to twenty, one to ten, one to eight, one to six, or one to four carbon atoms, such as but not limited to a methyl, ethyl, propyl, or butyl group). The metal M combines with the counter anion A- to form a salt of general formula MA, and the tetrafluoroborate anion (BF4-) replaces A- as the counter ion linked with the acidic aromatic unit. The anion exchange step thus results in a conjugate acid proton exchange polymer ionically linked with a non-coordinating tetrafluoroborate counter anion.

[0084] In some examples, the conjugate acid proton exchange polymer is electrically conductive in addition to proton conductive. In these examples, the conjugate acid proton exchange polymer is produced from an electrically conductive polymer, such as a polyaniline or a polypyrrole polymer. In other examples, the conjugate acid proton exchange polymer is electrically non-conductive. In these examples, the conjugate acid proton exchange polymer is produced from an electrically non-conductive polymer.

[0085] Conversion of weakly basic polymers to proton-conducting conjugate acid polymers will now be described with reference to weakly basic PBI polymers, but the principles described herein may be applied to any other basic aromatic polymers.

[0086] PBI polymers are a class of polymers composed of PBI polymer molecules. PBI polymer molecules have a repeating unit that includes a benzimidazole unit as at least part of a main chain. The benzimidazole unit comprises a benzimidazole moiety or a derivative thereof. Benzimidazole is a heterocyclic aromatic organic compound having a phenyl group and an imidazole group that share two carbon atoms in their ring structures. The general structure of benzimidazole is shown in the following Formula (I): (I)

[0087] An example of a PBI polymer with one benzimidazole unit per repeating unit in a main chain is poly(2,5-benzimidazole) (AB-PBI), shown below as Formula (II), and examples of PBI polymers with two benzimidazole units per repeating unit in a main chain are poly[2,2'-(m- phenylene)-5,5'-bibenzimidazole] (m-PBI), shown below as Formula (III), and 4F-PBI (a fluorinated derivative of m-PBI), shown below as Formula (IV).

(IV): 4F-PBI

[0088] Other examples of PBI polymers include, without limitation, poly{2,6-(2,6- naphtyliden)-1 ,7-dihydrobenzo[1 ,2-d;4,5-d]diimidazole}; poly 2,2'-(2,6-naphtyliden)-5,5'- bibenzimidazole; poly-2, 2'-(2,6-pyridine)-5,5'-bibenzimidazole; poly-2, 2'-(2,5-pyridine) 5,5'- bibenzimidazole; poly-2, 2'-(2,2^,-bipyridine-5,5^,)-5,5'-bibenzimidazole); poly-2, 2'-(3,5-pyrazole)- 5,5-bibenzimidazole; poly-2, 2'-(m-phenylene)-5,5'-bibenzimidazole; poly-2, 2'-(pyridylene-3", 5")-5,5'-bibenzimidazole; poly-2, 2'-(furylene-2",5")-5,5'-bibenzimidazole; poly-2.2-(naphthalene- r,6")-5,5'-bibenzimidazole; poly-2, 2'-(biphenylene-4",4")-5,5'-bibenzimidazole; poly-2, 2'- amylene-5,5'-bibenzimidazole; poly-2, 2'-octamethylene-5,5'-bibenzimidazole; poly-2, 6-(m- phenylene)-diimidazolebenzene; poly-2, 2'-cyclohexenyl-5,5'-bibenzimidazole; poly-2, 2'-(m- phenylene)-5.5'di(benzimidazole)ether; poly-2, 2'-(m-phenylene)-5,5-di(benzimidazole)sulfide; poly-2, 2'-(m-phenylene)-5,5-di(benzimidazole)sulfone; poly-2, 2'-(m-phenylene)-5, 5- di(benzimidazole)methane; poly-2-2"-(m-phenylene)-5".5"-(di(benzimidazole)propane 2.2; poly- 2.2"-(m-phenylene)-5'5"-di(benzimidazole)ethylene-1,2; and derivatives of any of the foregoing (including substituted (e.g., fluorinated) and/or branched derivatives). In some examples, a PBI polymer is a copolymer that comprises one or more additional repeating units, which may or may not include a benzimidazole unit in a main chain, in a side chain, or both.

[0089] PBI polymers, which are weakly basic, may be converted to robust acidic polymers by treatments with strong acids in simple acid-base conjugation reactions. The resulting polybenzimidazolium polymers (referred to herein as “PBI⁺ polymers”) have acidic aromatic units that are conjugate acids of the weakly basic aromatic units of the original PBI polymers. The PBI⁺ polymers are ionic and strongly acidic and, therefore, have high conductivity for cations. Under the aqueous conditions of water electrolysis and hydrogen fuel cells, the polybenzimidazolium unit easily dissociates to form an equilibrium mixture of neutral PBI and a hydrated proton (H₃O⁺). Thus, the easily dissociable polybenzimidazolium units may serve as proton transport agents. As a result, the PBI⁺ polymers may be used for ionomer and proton exchange membrane (PEM) applications. The acidic PBI⁺ polymers are simple and advanced alternatives to current versions of a poly(phosphoric acid) (PPA)-doped polymer (e.g., PBI- PPA).

[0090] In a PBI polymer molecule, a tertiary amine of a benzimidazole unit (e.g., N of the N=C group) reacts with a strong acid by an acid-base reaction to produce a benzimidazolium unit (a conjugate acid of the weakly basic benzimidazole unit) and a counter anion (a conjugate base of the strong acid) ionically linked with the benzimidazolium unit.

[0091] The molecular structures of the PBI⁺ polymers suggest that the non-coordinating counter anions do not have a direct role in the electrochemical processes because the main function of the counter anions is to neutralize the cationic charges in the PBI⁺ polymer structures. As a result, the non-coordinating counter anions do not participate, directly or indirectly, in the electrochemical processes and do not inhibit the catalyst cycle by binding with the metal catalysts.

[0092] In some examples, the counter anion comprises tetrafluoroborate ([BF₄-]), a tetravalent boron-containing non-coordinating anion that remains stable and passive during electrochemical processes. Other suitable counter anions may include, for example, hexafluorophosphate ([PF₆-]), hexafluoroantimonate ([SbF₆-]), BF₂, AIF₃, fluorinated bismuth, fluorinated antimony, fluorinated phosphorous, fluorinated tin, fluorinated zirconium, and/or fluorinated titanium.

[0093] The PBI⁺ polymers described herein are all intrinsically ionic, resembling ionic liquids that are known to accelerate/facilitate electrochemical reactions. Illustrative proton-conducting PBI⁺ polymers, and methods of making proton-conducting PBI⁺ polymers, will now be described. [0094] FIG. 1 shows an illustrative reaction scheme for producing a proton-conducting PBI⁺ polymer ionically linked with a tetrafluoroborate counter anion. As shown in FIG. 1, an m-PBI polymer is combined with tetrafluoroboric acid (HBF4). Tetrafluoroboric acid is a very strong acid and reacts with the tertiary nitrogen of a benzimidazole unit in an acid-base reaction. The reaction produces a benzimidazolium unit and a tetrafluoroborate counter anion (BF4-) ionically linked with the benzimidazolium unit. The benzimidazolium unit is a strong conjugate acid of the weakly basic aromatic benzimidazole unit. The tetrafluoroborate counter anion is the conjugate base of tetrafluoroboric acid. The tetrafluoroborate counter anion contains a tetravalent anionic boron atom and thus is non-coordinating and non-oxidizing, making the tetrafluoroborate counter anion highly stable under the harsh electrochemical conditions of electrochemical cell applications, such as water electrolysis and fuel cells. The tetrafluoroborate counter anion is also incapable of catalyst poisoning due to its lack of binding with metal catalysts.

[0095] In the example of FIG. 1, only one benzimidazole unit is converted to a benzimidazolium unit in each repeating unit. In other examples, both benzimidazole units of each repeating unit are converted to a benzimidazolium unit. In yet further examples, the degree of proton loading may be controlled as desired by controlling the molar ratio of HBF4 to benzimidazole units.

[0096] FIG. 2 shows another illustrative reaction scheme for producing a PBI⁺ polymer ionically linked with a tetrafluoroborate counter anion. In a first step, m-PBI is combined with hydrofluoric acid (HF). Hydrofluoric acid is a very strong acid and reacts with the tertiary nitrogen of a benzimidazole unit in an acid-base reaction. The first step produces an intermediate PBI⁺ polymer having a benzimidazolium unit ionically linked with a fluoride counter anion. In a second step (a fluoride treatment step), the intermediate PBI⁺ polymer is combined with boron trifluoride (e.g., in the form of BF3 etherate (Et20)). Boron trifluoride combines with the fluoride counter anion to form a tetrafluoroborate counter anion, resulting in a PBI⁺ polymer including a benzimidazolium unit ionically linked with a tetrafluoroborate counter ion.

[0097] FIG. 3 shows an illustrative reaction scheme for producing a PBI⁺ polymer ionically linked with any one of multiple different counter anions. In a first step, m-PBI is combined with hydrofluoric acid (HF). Hydrofluoric acid is a very strong acid and reacts with the tertiary nitrogen of a benzimidazole unit in an acid-base reaction. The first step produces an intermediate PBI⁺ polymer having a benzimidazolium unit ionically linked with a fluoride counter anion. In a second step (a fluoride treatment step), the intermediate PBI⁺ polymer is combined with a metal fluoride having the general formula MF_n where M is beryllium (Be), aluminum (Al), bismuth (Bi), antimony (Sb), phosphorous (P), tin (Sn), zirconium (Zr), or titanium (Ti) and n corresponds to a valence of M. The metal fluoride combines with the fluoride counter anion to form a counter anion having the general formula [MF(_n+i)]^_ and thus produce a strongly acidic PBI⁺ polymer ionically linked with a counter anion. [0098] FIG. 4 shows another illustrative reaction scheme for producing a PBI⁺ polymer ionically linked with a tetrafluoroborate counter anion. In a first step, m-PBI is combined with an acid having the general formula HA where A is fluorine (F), chlorine (Cl), bromine (Br), HSO4, SO4, H2PO4, HPO4, PO4, or any type of polyphosphate anion derived from any type of polyphosphoric acid. Suitable examples of polyphosphoric acids include, without limitation, diphosphoric acid, triphosphoric acid, tetraphosphoric acid, and trimetaphosphoric acid. The acid HA reacts with the tertiary nitrogen of a benzimidazole unit in an acid-base reaction. The first step produces an intermediate PBI⁺ polymer having a benzimidazolium unit ionically linked with a counter anion A-. In a second step (an anion exchange step), the counter ion A- is exchanged with a tetrafluoroborate counter anion ([BF4-]), which is a non-coordinating counter anion. The second step is performed by combining the intermediate PBI⁺ polymer with a metal tetrafluoroborate compound having the general formula MBF4 where BF4 is a tetrafluoroborate anion (BF4-) and M is a sodium ion (Na⁺), a potassium ion (K⁺), a cesium ion (Cs⁺), ammonium (NH₄ ⁺), or tetraalkylammonium (R4N⁺ wherein each R is independently hydrogen or a wholly or partly substituted or unsubstituted alkyl group, which may have one to twenty, one to ten, one to eight, one to six, or one to four carbon atoms, such as but not limited to a methyl, ethyl, propyl, or butyl group). The metal tetrafluoroborate compound combines with the fluoride counter anion to form a tetraflouroborate counter anion, thus resulting in a strongly acidic PBI⁺ polymer ionically linked with a non-coordinating tetrafluoroborate counter anion.

[0099] The examples and reaction schemes described above may be extended to other applications, including cross-linking of fluoroboric acid-functionalized polymers with basic aromatic polymers to form a conjugate acid proton exchange polymer. In some examples, the fluoroboric acid-functionalized polymer is a fluoroboric acid-functionalized sulfonic acid polymer, which includes sulfonic acid-functionalized polymers (e.g., Nation™ and Aquivion® polymers) in which one or more pendant sulfonic acid groups have reacted with BF3, resulting in the following structure having a pendant fluoroboric acid group ( — BF3H) (e.g., in one or more side chains or side groups):

[0100] The fluoroboric acid-functionalized polymers are combined with a basic aromatic polymer, such as PBI, PANI, or any other basic aromatic polymer described herein. The pendant fluoroboric acid groups react with the basic aromatic units of the basic aromatic polymer in an acid-base reaction, in a manner similar to the acid-base reaction of FIG. 1, to thereby cross-link the fluoroboric acid-functionalized polymer with the basic aromatic polymer. A cross-linked conjugate acid proton exchange polymer molecule includes a first polymer molecule comprising an acidic aromatic unit, wherein the acidic aromatic unit is a conjugate acid of a basic aromatic unit; and a second polymer molecule comprising a trifluoroborate group, wherein the trifluoroborate group is a conjugate base of a fluoroboric acid group. The first polymer molecule and the second polymer molecule are cross-linked by an ionic linkage between the acidic aromatic unit and the trifluoroborate group. Examples of cross-linked conjugate acid proton exchange polymers include, without limitation, PBI⁺-PFSA (polyfluorosulfonic acid) polymers (e.g., PBI⁺-Nafion™ polymers and PBI⁺-Aquivion® polymers), PBI⁺-SPSF (sulfonated polysulfone) polymers, PBI⁺-SPEEK (sulfonated poly(ether ketone)) polymers, PANI-PFSA polymers (e.g., PANI-Nafion™ polymers and PANI-Aquivion® polymers), PANI-SPSF polymers, PANI-SPEEK polymers, PPy-PFSA polymers (e.g., PPy-Nafion™ polymers and PPy-Aquivion® polymers), PPy-SPSF polymers, and PPy-SPEEK polymers. In some examples, more than two polymers may be cross-linked, such as three or four polymers. [0101] It will be recognized that other fluoroboric acid-functionalized polymers may be cross-linked with a basic aromatic polymer besides fluoroboric acid-functionalized sulfonic acid polymers. Fluoroboric acid-functionalized polymers include any polymers having pendant fluoroboric acid groups, such as, without limitation, any fluoroboric acid-functionalized polymers described in International Patent Application No. PCT/US2024/015409 filed February 12, 2024, which is incorporated herein by reference in its entirety. Cross-linking a fluoroboric acid- functionalized polymer with a basic aromatic polymer in this way enhances the dimensional tolerance of the cross-linked conjugate acid proton exchange polymer.

[0102] FIG. 5 shows an illustrative proton exchange membrane water electrolysis system 500 (PEM water electrolysis system 500) incorporating conjugate acid proton exchange polymer PEMs and/or ionomers (e.g., PBI⁺ polymer PEMs and/or ionomers). PEM water electrolysis system 500 uses electricity to split water into oxygen (O2) and hydrogen (H₂) via an electrochemical reaction. The configuration of PEM water electrolysis system 500 is merely illustrative and not limiting, as other suitable configurations as well as other suitable water electrolysis systems may incorporate conjugate acid proton exchange polymers.

[0103] As shown in FIG. 5, PEM water electrolysis system 500 includes a membrane electrode assembly 502 (MEA 502), porous transport layers 504-1 and 504-2 (e.g., gas diffusion layers), bipolar plates 506-1 and 506-2, and an electrical power supply 508. PEM water electrolysis system 500 may also include additional or alternative components not shown in FIG. 5 as may serve a particular implementation.

[0104] MEA 502 includes a PEM 510 positioned between a first catalyst layer 512-1 and a second catalyst layer 512-2. PEM 510 electrically isolates first catalyst layer 512-1 from second catalyst layer 512-2 while providing selective conductivity of cations, such as protons (H⁺), and while being impermeable to gases such as hydrogen and oxygen. PEM 510 may be implemented by a conjugate acid proton exchange polymer (e.g., a PBI⁺ polymer) as described herein, or by any other suitable polymer. [0105] First catalyst layer 512-1 and second catalyst layer 512-2 are electrically conductive electrodes that include catalyst solid supports bound with electrocatalyst particles (not shown), such as platinum group metals, metal alloys, and/or metal oxides. First catalyst layer 512-1 and second catalyst layer 512-2 may also include one or more ionomers mixed with the catalyst solid supports and electrochemical catalyst particles. The ionomers may be implemented by a conjugate acid proton exchange polymer (e.g., a PBI⁺ polymer) as described herein, or by any other suitable ionomer. In some examples, the ionomers are implemented by an electrically conductive conjugate acid proton exchange polymer (e.g., a conjugate acid proton exchange polyaniline polymer).

[0106] MEA 502 is placed between porous transport layers 504-1 and 504-2, which are in turn placed between bipolar plates 506-1 and 506-2 with flow channels 514-1 and 514-2 located in between bipolar plates 506 and porous transport layers 504.

[0107] In MEA 502, first catalyst layer 512-1 functions as an anode and second catalyst layer 512-2 functions as a cathode. When PEM water electrolysis system 500 is powered by power supply 508, an oxygen evolution reaction (OER) occurs at first catalyst layer/anode 512- 1, facilitated by the electrocatalysts bound to the catalyst solid supports in first catalyst layer/anode 512-1. The OER is represented by the following electrochemical half-reaction:

2 H₂O -> O₂ + 4 H⁺ + 4 e

[0108] Protons are conducted from first catalyst layer/anode 512-1 to second catalyst layer/cathode 512-2 through PEM 510, and electrons are conducted from first catalyst layer/anode 512-1 to second catalyst layer/cathode 512-2 by conductive path around PEM 510. PEM 510 allows for the transport of protons (H⁺) and water from the first catalyst layer/anode 512-1 to the second catalyst layer/cathode 512-2 but is impermeable to oxygen and hydrogen. At second catalyst layer/cathode 512-2, the protons combine with the electrons in a hydrogen evolution reaction (HER), facilitated by the electrocatalysts bound to the catalyst solid supports in second catalyst layer/anode 512-2. The HER is represented by the following electrochemical half-reaction:

4 H⁺ + 4 e -> 2 H₂

[0109] The OER and HER are two complementary electrochemical reactions for splitting water by electrolysis, represented by the following overall water electrolysis reaction:

2 H₂O 2 H₂ + O₂

[0110] FIG. 6 shows an illustrative proton exchange membrane fuel cell 600 (PEM fuel cell 600) including conjugate acid proton exchange polymer PEMs and/or ionomers (e.g., PBI⁺ polymer PEMs and/or ionomers). PEM fuel cell 600 produces electricity as a result of electrochemical reactions. In this example, the electrochemical reactions involve reacting hydrogen gas (H₂) and oxygen gas (O₂) to produce water and electricity. The configuration of PEM fuel cell 600 is merely illustrative and not limiting. [0111] As shown in FIG. 6, PEM fuel cell 600 includes a membrane electrode assembly 602 (MEA 602), porous transport layers 604-1 and 604-2 (e.g., gas diffusion layers), bipolar plates 606-1 and 606-2. An electrical load 608 may be electrically connected to MEA 602 and driven by PEM fuel cell 600. PEM fuel cell 600 may also include additional or alternative components not shown in FIG. 6 as may serve a particular implementation.

[0112] MEA 602 includes a PEM 610 positioned between a first catalyst layer 612-1 and a second catalyst layer 612-2. PEM 610 electrically isolates first catalyst layer 612-1 from second catalyst layer 612-2 while providing selective conductivity of cations, such as protons (H⁺), and while being impermeable to gases such as hydrogen and oxygen. PEM 610 may be implemented by any suitable PEM described herein, including any conjugate acid proton exchange polymer described or contemplated herein.

[0113] First catalyst layer 612-1 and second catalyst layer 612-2 are electrically conductive electrodes that include catalyst solid supports that bind electrocatalyst particles (not shown), such as platinum metals, metal alloys, and/or metal oxides. First catalyst layer 612-1 and second catalyst layer 612-2 may also include one or more ionomers mixed with the catalyst solid supports and electrochemical catalyst particles. The ionomers may be implemented by any suitable polymers or ionomers described herein, including any conjugate acid proton exchange polymers described herein, or by any other suitable ionomers. In some examples, the ionomers are implemented by an electrically conductive conjugate acid proton exchange polymer (e.g., a conjugate acid proton exchange polyaniline polymer).

[0114] MEA 602 is placed between porous transport layers 604-1 and 604-2, which are in turn placed between bipolar plates 606-1 and 606-2 with flow channels 614 located in between. In MEA 602, first catalyst layer 612-1 functions as a cathode and second catalyst layer 612-2 functions as an anode. First catalyst layer/cathode 612-1 and anode 612-2 are electrically connected to load 608, and electricity generated by PEM fuel cell 600 drives load 608.

[0115] During operation of PEM fuel cell 600, hydrogen gas (H2) flows into the anode side of PEM fuel cell 600 and oxygen gas (O2) flows into the cathode side of PEM fuel cell 600. At second catalyst layer/anode 612-2, hydrogen molecules are catalytically split into protons (H⁺) and electrons (e^_) according to the following hydrogen oxidation reaction (HOR), which is facilitated by the electrocatalysts particles bound to the catalyst solid supports in second catalyst layer/anode 612-2:

2 H₂ -> 4 H⁺ + 4 e

[0116] The protons are conducted from anode 612-2 to first catalyst layer/cathode 612-1 through PEM 600, and the electrons are conducted from second catalyst layer/anode 612-2 to first catalyst layer/cathode 612-1 around PEM 610 through a conductive path and load 608. At first catalyst layer/cathode 612-1, the protons and electrons combine with the oxygen gas according to the following oxygen reduction reaction (ORR), which is facilitated by the electrochemical catalysts particles bound to the catalyst solid supports in first catalyst layer/anode 612-1 :

O₂ + 4 H⁺ + 4 e -> 2 H₂O

[0117] Thus, the overall electrochemical reaction for the PEM fuel cell 600 is:

2 H₂ + O₂ -> 2 H₂O

[0118] In the overall reaction, PEM fuel cell 600 produces water at first catalyst layer/cathode 612-1. Water may flow from first catalyst layer/cathode 612-1 to second catalyst layer/anode 612-2 through PEM 610 and may be removed through outlets at the cathode side and/or anode side of PEM fuel cell 600. The overall reaction generates electrons at the anode that drive load 608.

[0119] In the examples of FIGS. 14 and 5, MEA 502 and MEA 602 include catalyst layers 512/412 formed on PEM 510/410. In alternative configurations, catalyst layers 512/412 may be coated on PEM 110/410 to thereby form a catalyst coated membrane (COM). For example, catalyst layers 512/212 may be formed in a one-pot process or in stages and sprayed onto PEM 510/410.

[0120] The conjugate acid proton exchange polymers have been described herein for use in electrochemical cell applications, such as water electrolysis and hydrogen fuel cell applications. However, conjugate acid proton exchange polymers may also be used in other applications. For example, the pKa of the conjugate acid proton exchange polymers can be controlled to a desired level for many applications, including electrochemical processes for ammonia production. For example, the pKa of the conjugate acid proton exchange polymers can be controlled to be higher than the pKa of conventional PEMs currently used in ammonia synthesis processes.

[0121] Various examples and embodiments have been described and illustrated herein. It will, however, be evident that various modifications and changes may be made thereto, and additional embodiments may be implemented, without departing from the scope of the claims that follow. For example, certain features of one embodiment described herein may be combined with or substituted for features of another embodiment described herein. The description and drawings are accordingly to be regarded in an illustrative rather than a restrictive sense.

[0122] Advantages and features of the present disclosure can be further described by the following examples:

[0123] Example 1. A conjugate acid proton exchange polymer molecule comprising: an acidic aromatic unit in a main chain or a side chain, wherein the acidic aromatic unit is a conjugate acid of a basic aromatic unit; and a non-coordinating counter anion ionically linked with the acidic aromatic unit. [0124] Example 2. The conjugate acid proton exchange polymer molecule of example 1, wherein the non-coordinating counter anion is a conjugate base of a strong acid.

[0125] Example 3. The conjugate acid proton exchange polymer molecule of example 1, wherein the non-coordinating counter anion comprises tetrafluoroborate.

[0126] Example 4. The conjugate acid proton exchange polymer molecule of example 1, wherein the non-coordinating counter anion comprises hexafluorophosphate.

[0127] Example 5. The conjugate acid proton exchange polymer molecule of example 1, wherein the non-coordinating counter anion comprises hexafluoroantimonate.

[0128] Example 6. The conjugate acid proton exchange polymer molecule of example 1, wherein the non-coordinating counter anion comprises benzenesulfonato-trifluoroborate (PhSO₃(BF₃)-).

[0129] Example 7. The conjugate acid proton exchange polymer molecule of example 1, wherein the non-coordinating counter anion comprises sulfato-di(trifluoroborate) (SO4(BF₃)₂ ^2-). [0130] Example 8. The conjugate acid proton exchange polymer molecule of example 1, wherein the non-coordinating counter anion comprises beryllium fluoride, aluminum fluoride, fluorinated bismuth, fluorinated antimony, fluorinated phosphorous, fluorinated tin, fluorinated zirconium, or fluorinated titanium.

[0131] Example 9. The conjugate acid proton exchange polymer molecule of example 1, wherein the non-coordinating counter anion comprises a metal fluoroborate having the general formula [MF(_n+i)]^_ wherein M is beryllium (Be), aluminum (Al), bismuth (Bi), antimony (Sb), phosphorous (P), tin (Sn), zirconium (Zr), or titanium (Ti) and n corresponds to a valence of M. [0132] Example 10. The conjugate acid proton exchange polymer molecule of example 1, wherein the basic aromatic unit comprises aniline or a derivative of aniline.

[0133] Example 11. The conjugate acid proton exchange polymer molecule of example 1 , wherein the basic aromatic unit comprises an imidazole group or a benzimidazole group.

[0134] Example 12. The conjugate acid proton exchange polymer molecule of example 1, wherein the conjugate acid proton exchange polymer molecule is electrically conductive.

[0135] Example 13. A method of making a conjugate acid proton exchange polymer molecule, comprising: protonating a basic aromatic unit of a basic aromatic polymer molecule to form an acidic aromatic unit, wherein the acidic aromatic unit is a conjugate acid of the basic aromatic unit; and ionically linking a non-coordinating counter anion with the acidic aromatic unit.

[0136] Example 14. The method of example 13, wherein protonating the basic aromatic unit and ionically linking the non-coordinating counter anion with the acidic aromatic unit comprise reacting the basic aromatic unit with an acid in an acid-base reaction, wherein a conjugate base of the acid comprises the non-coordinating counter anion. [0137] Example 15. The method of example 14, wherein the acid comprises fluoroboric acid.

[0138] Example 16. The method of example 14, wherein the acid comprises hexafluorophosphoric acid.

[0139] Example 17. The method of example 14, wherein the acid comprises fluoroantimonic acid.

[0140] Example 18. The method of example 14, wherein the acid comprises a sulfuric acid- boron trifluoride complex having the general structure:

[0141] Example 19. The method of example 14, wherein the acid comprises a benzenesulfonic acid-boron trifluoride complex having the general structure:

[0142] Example 20. The method of example 14, wherein the acid comprises a benzene disulfonic acid-boron trifluoride) complex having the general structure:

[0143] Example 21. The method of example 14, wherein the acid comprises phenyl trifluoroboric acid having the general structure: or phenyl 1 ,4-di(trifluoro)boric acid having the general structure:

[0144] Example 22. The method of example 14, wherein the acid comprises a phosphoric acid-boron trifluoride complex having the general structure:

[0145] Example 23. The method of example 13, wherein: protonating the basic aromatic unit comprises reacting the basic aromatic unit with hydrofluoric acid in an acid-base reaction, wherein a conjugate base of the hydrofluoric acid comprises a fluoride counter anion ionically linked with the acidic aromatic unit; and ionically linking the non-coordinating counter anion with the acidic aromatic unit comprises reacting the fluoride counter anion with boron trifluoride.

[0146] Example 24. The method of example 13, wherein: protonating the basic aromatic unit comprises reacting the basic aromatic unit with hydrofluoric acid in an acid-base reaction, wherein a conjugate base of the hydrofluoric acid comprises a fluoride counter anion ionically linked with the acidic aromatic unit; and ionically linking the non-coordinating counter anion with the acidic aromatic unit comprises reacting the fluoride counter anion with a metal fluoride having the general formula MF_n, wherein M is beryllium (Be), aluminum (Al), bismuth (Bi), antimony (Sb), phosphorous (P), tin (Sn), zirconium (Zr), or titanium (Ti) and n corresponds to a valence of M.

[0147] Example 25. The method of example 13, wherein: protonating the basic aromatic unit comprises reacting the basic aromatic unit with an acid in an acid-base reaction, wherein a conjugate base of the acid comprises a coordinating counter anion ionically linked with the acidic aromatic unit; and ionically linking the non-coordinating counter anion with the acidic aromatic unit comprises performing an anion exchange reaction to exchange the coordinating counter anion with the non-coordinating counter anion.

[0148] Example 26. The method of example 25, wherein the acid has the general formula HA where A is F, Cl, Br, HSO4, SO4, H2PO4, HPO4, PO4, or a polyphosphate anion.

[0149] Example 27. The method of example 25, wherein the anion exchange reaction comprises reacting a metal tetrafluoroborate compound having the general formula MBF4 where M is a sodium ion (Na⁺), a potassium ion (K⁺), a cesium ion (Cs⁺), ammonium (NH4⁺), or tetraalkylammonium having the general formula R4N⁺ wherein each R is independently hydrogen or a wholly or partly substituted or unsubstituted alkyl group.

[0150] Example 28. The method of example 13, wherein the basic aromatic polymer molecule comprises a polyaniline polymer.

[0151] Example 29. The method of example 13, wherein the basic aromatic polymer molecule comprises a polybenzimidazole polymer. [0152] Example 30. The method of example 13, wherein the basic aromatic polymer molecule is electrically conductive.

[0153] Example 31. A membrane electrode assembly comprising: a first catalyst layer; a second catalyst layer; and a proton exchange membrane positioned between the first catalyst layer and the second catalyst layer; wherein at least one of the first catalyst layer, the second catalyst layer, and the proton exchange membrane is formed of a conjugate acid proton exchange polymer comprising: acidic aromatic units in a main chain or a side chain, wherein the acidic aromatic units are a conjugate acid of a basic aromatic unit; and non-coordinating counter anions ionically linked with the acidic aromatic units.

[0154] Example 32. The membrane electrode assembly of example 31 , wherein: the conjugate acid proton exchange polymer is electrically conductive; and at least one of the first catalyst layer or the second catalyst layer is formed of the conjugate acid proton exchange polymer.

[0155] Example 33. The membrane electrode assembly of example 32, wherein the proton exchange membrane is formed of an additional conjugate acid proton exchange polymer that is electrically non-conductive.

[0156] Example 34. The membrane electrode assembly of example 31 , wherein the conjugate acid proton exchange polymer comprises a polybenzimidazolium polymer.

[0157] Example 35. The membrane electrode assembly of example 31 , wherein the conjugate acid proton exchange polymer comprises a polyaniline polymer.

[0158] Example 36. A method of making a conjugate acid proton exchange polymer, the method comprising: cross-linking a fluoroboric acid-functionalized polymer molecule with a basic aromatic polymer molecule.

[0159] Example 37. The method of example 36, wherein the fluoroboric acid-functionalized polymer molecule comprises a polyfluorosulfonic acid polymer molecule having one or more sulfonic acid groups functionalized with a fluoroboric acid group.

[0160] Example 38. The method of example 36, wherein the basic aromatic polymer molecule comprises a polybenzimidazole (PBI) polymer molecule.

[0161] Example 39. The method of example 36, wherein the basic aromatic polymer molecule comprises a polyaniline (PANI) polymer molecule.

[0162] Example 40. The method of example 36, wherein cross-linking the fluoroboric acid- functionalized polymer molecule with the basic aromatic polymer molecule comprises: reacting a fluoroboric acid group of the fluoroboric acid-functionalized polymer molecule with a basic aromatic unit of the basic aromatic polymer molecule in an acid-base reaction.

[0163] Example 41. A conjugate acid proton exchange polymer comprising: a first polymer molecule comprising an acidic aromatic unit, wherein the acidic aromatic unit is a conjugate acid of a basic aromatic unit; and a second polymer molecule comprising a trifluoroborate group, wherein the trifluoroborate group is a conjugate base of a fluoroboric acid group; wherein the first polymer molecule and the second polymer molecule are cross-linked by an ionic linkage between the acidic aromatic unit and the trifluoroborate group.

[0164] Example 42. The conjugate acid proton exchange polymer of example 40, wherein the first polymer molecule comprises a polybenzimidazole (FBI) polymer.

[0165] Example 43. The conjugate acid proton exchange polymer of example 40, wherein the first polymer molecule comprises a polyaniline (PANI) polymer.

[0166] Example 44. The conjugate acid proton exchange polymer of example 40, wherein the second polymer molecule comprises a fluoroboric acid-functionalized sulfonic acid polymer.

Claims

CLAIMS What is claimed is:

1. A conjugate acid proton exchange polymer molecule comprising: an acidic aromatic unit in a main chain or a side chain, wherein the acidic aromatic unit is a conjugate acid of a basic aromatic unit; and a non-coordinating counter anion ionically linked with the acidic aromatic unit.

2. The conjugate acid proton exchange polymer molecule of claim 1, wherein the non-coordinating counter anion is a conjugate base of a strong acid.

3. The conjugate acid proton exchange polymer molecule of claim 1 , wherein the non-coordinating counter anion comprises tetrafluoroborate.

4. The conjugate acid proton exchange polymer molecule of claim 1 , wherein the non-coordinating counter anion comprises hexafluorophosphate.

5. The conjugate acid proton exchange polymer molecule of claim 1 , wherein the non-coordinating counter anion comprises hexafluoroantimonate.

6. The conjugate acid proton exchange polymer molecule of claim 1 , wherein the non-coordinating counter anion comprises benzenesulfonato-trifluoroborate (PhSOsCBFs)-).

7. The conjugate acid proton exchange polymer molecule of claim 1 , wherein the non-coordinating counter anion comprises sulfato-di(trifluoroborate) (SO4(BF₃)₂ ^2-).

8. The conjugate acid proton exchange polymer molecule of claim 1 , wherein the non-coordinating counter anion comprises beryllium fluoride, aluminum fluoride, fluorinated bismuth, fluorinated antimony, fluorinated phosphorous, fluorinated tin, fluorinated zirconium, or fluorinated titanium.

9. The conjugate acid proton exchange polymer molecule of claim 1 , wherein the non-coordinating counter anion comprises a metal fluoroborate having the general formula [MF(n+i)]^_ wherein M is beryllium (Be), aluminum (Al), bismuth (Bi), antimony (Sb), phosphorous (P), tin (Sn), zirconium (Zr), or titanium (Ti) and n corresponds to a valence of M.

10. The conjugate acid proton exchange polymer molecule of claim 1, wherein the basic aromatic unit comprises aniline or a derivative of aniline.

11. The conjugate acid proton exchange polymer molecule of claim 1 , wherein the basic aromatic unit comprises an imidazole group or a benzimidazole group.

12. The conjugate acid proton exchange polymer molecule of claim 1, wherein the conjugate acid proton exchange polymer molecule is electrically conductive.

13. A method of making a conjugate acid proton exchange polymer molecule, comprising: protonating a basic aromatic unit of a basic aromatic polymer molecule to form an acidic aromatic unit, wherein the acidic aromatic unit is a conjugate acid of the basic aromatic unit; and ionically linking a non-coordinating counter anion with the acidic aromatic unit.

14. The method of claim 13, wherein protonating the basic aromatic unit and ionically linking the non-coordinating counter anion with the acidic aromatic unit comprise reacting the basic aromatic unit with an acid in an acid-base reaction, wherein a conjugate base of the acid comprises the non-coordinating counter anion.

15. The method of claim 14, wherein the acid comprises fluoroboric acid.

16. The method of claim 14, wherein the acid comprises hexafluorophosphoric acid.

17. The method of claim 14, wherein the acid comprises fluoroantimonic acid.

18. The method of claim 14, wherein the acid comprises a sulfuric acid-boron trifluoride complex having the general structure:

19. The method of claim 14, wherein the acid comprises a benzenesulfonic acid- boron trifluoride complex having the general structure:

20. The method of claim 14, wherein the acid comprises a benzene disulfonic acid- boron trifluoride) complex having the general structure:

21. The method of claim 14, wherein the acid comprises phenyl trifluoroboric acid having the general structure: or phenyl 1 ,4-di(trifluoro)boric acid having the general structure:

22. The method of claim 14, wherein the acid comprises a phosphoric acid-boron trifluoride complex having the general structure:

23. The method of claim 13, wherein: protonating the basic aromatic unit comprises reacting the basic aromatic unit with hydrofluoric acid in an acid-base reaction, wherein a conjugate base of the hydrofluoric acid comprises a fluoride counter anion ionically linked with the acidic aromatic unit; and ionically linking the non-coordinating counter anion with the acidic aromatic unit comprises reacting the fluoride counter anion with boron trifluoride.

24. The method of claim 13, wherein: protonating the basic aromatic unit comprises reacting the basic aromatic unit with hydrofluoric acid in an acid-base reaction, wherein a conjugate base of the hydrofluoric acid comprises a fluoride counter anion ionically linked with the acidic aromatic unit; and ionically linking the non-coordinating counter anion with the acidic aromatic unit comprises reacting the fluoride counter anion with a metal fluoride having the general formula MF_n, wherein M is beryllium (Be), aluminum (Al), bismuth (Bi), antimony (Sb), phosphorous (P), tin (Sn), zirconium (Zr), or titanium (Ti) and n corresponds to a valence of M.

25. The method of claim 13, wherein: protonating the basic aromatic unit comprises reacting the basic aromatic unit with an acid in an acid-base reaction, wherein a conjugate base of the acid comprises a coordinating counter anion ionically linked with the acidic aromatic unit; and ionically linking the non-coordinating counter anion with the acidic aromatic unit comprises performing an anion exchange reaction to exchange the coordinating counter anion with the non-coordinating counter anion.

26. The method of claim 25, wherein the acid has the general formula HA where A is F, Cl, Br, HSO4, SO4, H2PO4, HPO4, PO4, or a polyphosphate anion.

27. The method of claim 25, wherein the anion exchange reaction comprises reacting a metal tetrafluoroborate compound having the general formula MBF4 where M is a sodium ion (Na⁺), a potassium ion (K⁺), a cesium ion (Cs⁺), ammonium (NH4⁺), or tetraalkylammonium having the general formula R4N⁺ wherein each R is independently hydrogen or a wholly or partly substituted or unsubstituted alkyl group.

28. The method of claim 13, wherein the basic aromatic polymer molecule comprises a polyaniline polymer.

29. The method of claim 13, wherein the basic aromatic polymer molecule comprises a polybenzimidazole polymer.

30. The method of claim 13, wherein the basic aromatic polymer molecule is electrically conductive.

31. A membrane electrode assembly comprising: a first catalyst layer; a second catalyst layer; and a proton exchange membrane positioned between the first catalyst layer and the second catalyst layer; wherein at least one of the first catalyst layer, the second catalyst layer, and the proton exchange membrane is formed of a conjugate acid proton exchange polymer comprising: acidic aromatic units in a main chain or a side chain, wherein the acidic aromatic units are a conjugate acid of a basic aromatic unit; and non-coordinating counter anions ionically linked with the acidic aromatic units.

32. The membrane electrode assembly of claim 31 , wherein: the conjugate acid proton exchange polymer is electrically conductive; and at least one of the first catalyst layer or the second catalyst layer is formed of the conjugate acid proton exchange polymer.

33. The membrane electrode assembly of claim 32, wherein the proton exchange membrane is formed of an additional conjugate acid proton exchange polymer that is electrically non-conductive.

34. The membrane electrode assembly of claim 31 , wherein the conjugate acid proton exchange polymer comprises a polybenzimidazolium polymer.

35. The membrane electrode assembly of claim 31 , wherein the conjugate acid proton exchange polymer comprises a polyaniline polymer.

36. A method of making a conjugate acid proton exchange polymer, the method comprising: cross-linking a fluoroboric acid-functionalized polymer molecule with a basic aromatic polymer molecule.

37. The method of claim 36, wherein the fluoroboric acid-functionalized polymer molecule comprises a polyfluorosulfonic acid polymer molecule having one or more sulfonic acid groups functionalized with a fluoroboric acid group.

38. The method of claim 36, wherein the basic aromatic polymer molecule comprises a polybenzimidazole (PBI) polymer molecule.

39. The method of claim 36, wherein the basic aromatic polymer molecule comprises a polyaniline (PANI) polymer molecule.

40. The method of claim 36, wherein cross-linking the fluoroboric acid-functionalized polymer molecule with the basic aromatic polymer molecule comprises: reacting a fluoroboric acid group of the fluoroboric acid-functionalized polymer molecule with a basic aromatic unit of the basic aromatic polymer molecule in an acid-base reaction.

41. A conjugate acid proton exchange polymer comprising: a first polymer molecule comprising an acidic aromatic unit, wherein the acidic aromatic unit is a conjugate acid of a basic aromatic unit; and a second polymer molecule comprising a trifluoroborate group, wherein the trifluoroborate group is a conjugate base of a fluoroboric acid group; wherein the first polymer molecule and the second polymer molecule are cross-linked by an ionic linkage between the acidic aromatic unit and the trifluoroborate group.

42. The conjugate acid proton exchange polymer of claim 40, wherein the first polymer molecule comprises a polybenzimidazole (PBI) polymer.

43. The conjugate acid proton exchange polymer of claim 40, wherein the first polymer molecule comprises a polyaniline (PANI) polymer.

44. The conjugate acid proton exchange polymer of claim 40, wherein the second polymer molecule comprises a fluoroboric acid-functionalized sulfonic acid polymer.