Functional modification of poly(chlorotrifluoroethylene) for ionomer and PEM applications

Abstract

An ion exchange-functionalized poly(chlorotrifluoroethylene) (PCTFE) polymer molecule includes a PCTFE main chain and a side group that includes an ion exchange group and a linker that links the ion exchange group to the PCTFE main chain.

Description

FUNCTIONAL MODIFICATION OF POLY(CHLOROTRIFLUOROETHYLENE) FOR IONOMER AND PEM APPLICATIONSRELATED APPLICATIONS

[0001] This application claims priority to U.S. Provisional Patent Application No. 63 / 532,262, filed August 11 , 2023, 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 (PFSA) 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 fromthe 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 PFSA PEMs, 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.

[0006] Moreover, synthesis of PFSA polymers has challenges. PFSA polymers are synthesized by co-polymerization of tetrafluoroethylene (TFE) and perfluorinated vinyl ether sulfonyl fluorides. Two types of perfluorinated vinyl ether sulfonyl fluorides are typically used: long side chain (LSC) and short side chain (SSC). LSCs generally have at least two ether linkages and four or more polyfluorinated carbon units (e.g., — CF2 — and / or — CF3). SSCs generally have one ether linkage and two polyfluorinated carbon units. SSC polymers offer better performance compared to LSC polymers. However, synthesis of the SSC monomer is cumbersome, low-yielding, and costly.

[0007] PCTFE polymers are homo-polymers of chlorotrifluoroethylene (CTFE) with the molecular formula (CF2CCIF)n. PCTFE is similar to PTFE 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.

[0008] Earlier attempts at chemical modification of PCTFE have produced structures that are also unsuitable for electrochemical applications. For example, some chemical modifications of PCTFE resulted in the formation of aliphatic carbon-carbon double bonds in the polymer backbone. See Beaver, L.G.; Danielson, N. D.; Polymer Bulletin, 1993, 30, 47 - 54. Dias, A. J.; McCarthy, T. J.; Macromolecules, 1985, 18, 1829 - 1833. However, these structures are incompatible with electrochemical applications because aliphatic C=C moieties are unstable and reactive under the electrochemical conditions of water electrolysis and hydrogen fuel cells. Other chemical modifications of PCTFE used sulfur nucleophiles, which produces alkyl- and aryl sulfides. See Taylor, R. T.; Pelter, M. W.; J. Polymer Science: part C: Polymer Letters, 1987, 25, 215 - 217. However, these structures containing bivalent sulfur atoms are also incompatible with electrochemical applications because alkyl- and aryl sulfides would undergooxidation under the redox conditions of water electrolysis and hydrogen fuel cells. Also, bivalent sulfur atoms react irreversibly with metal catalysts, stopping their catalytic activity.SUMMARY

[0009] In some illustrative examples, an ion exchange-functionalized poly(chlorotrifluoroethylene) (PCTFE) polymer molecule comprises: a PCTFE main chain; and a side group including an ion exchange group and a linker that links the ion exchange group to the PCTFE main chain.

[0010] In some illustrative examples, a method of making an ion exchange-functionalized PCTFE polymer comprises: substituting a chlorine atom of a PCTFE polymer molecule with a side group including an ion exchange group and a linker that links the ion exchange group to a main chain of the PCTFE polymer molecule.

[0011] In some illustrative examples, a membrane electrode assembly comprises: 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 an ion exchange-functionalized poly(chlorotrifluoroethylene) (PCTFE) polymer including: a PCTFE main chain; and a side group including an ion exchange group and a linker that links the ion exchange group to the PCTFE main chain.BRIEF DESCRIPTION OF THE DRAWINGS

[0012] FIG. 1 shows an illustrative reaction scheme for synthesis of an ion exchange- functionalized PCTFE polymer using PCTFE as a starting material, 4-hydroxybenzenesulfonic acid as a linking agent, and boron trifluoride as an ion exchange agent.

[0013] FIG. 2 shows another illustrative reaction scheme for synthesis of an ion exchange- functionalized PCTFE polymer using PCTFE as a starting material, 4-phenolic acid as a linking agent, and boron trifluoride as an ion exchange agent.

[0014] FIG. 3 shows another illustrative reaction scheme for synthesis of an ion exchange- functionalized PCTFE polymer using PCTFE as a starting material and 4-hydroxyphenylboronic acid as a linking agent and performing a fluoride treatment.

[0015] FIG. 4 shows another illustrative reaction scheme for synthesis of an ion exchange- functionalized PCTFE polymer using PCTFE as a starting material, sodium hydrosulfide acid as a linking agent, and boron trifluoride as an ion exchange agent.

[0016] FIG. 5 shows an illustrative reaction scheme for synthesis of an ion exchange- functionalized PCTFE polymer using PCTFE as a starting material, 4-mercaptobenzenesulfonic acid as a linking agent, and boron trifluoride as an ion exchange agent.

[0017] FIG. 6 shows another illustrative reaction scheme for synthesis of an ion exchange- functionalized PCTFE polymer using PCTFE as a starting material, 4-mercaptobenzoic acid as a linking agent, and boron trifluoride as an ion exchange agent.

[0018] FIG. 7 shows another illustrative reaction scheme for synthesis of an ion exchange- functionalized PCTFE polymer using PCTFE as a starting material and 4- mercaptophenylboronic acid as a linking agent and performing a fluoride treatment.

[0019] FIG. 8 shows an illustrative reaction scheme for synthesis of a functionalized PCTFE polymer molecule including a side group with an arylsulfonyl group.

[0020] FIG. 9 shows an illustrative proton exchange membrane water electrolysis system incorporating ion exchange-functionalized PCTFE polymer PEMs and / or ionomers.

[0021] FIG. 10 shows an illustrative proton exchange membrane fuel cell including ion exchange-functionalized PCTFE polymer PEMs and / or ionomers.DETAILED DESCRIPTION

[0022] Herein described are ion exchange-functionalized poly(chlorotrifluoroethylene) (PCTFE) polymers and methods of making ion exchange-functionalized PCTFE polymers. The ion exchange-functionalized PCTFE polymers are ion-conducting and may be used, for example, for ionomer and PEM applications in electrochemical cells, such as water electrolysis and / or fuel cell systems.

[0023] 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.

[0024] 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.

[0025] 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).

[0026] As used herein, “polymerization” refers to the process of converting a monomer, or a mixture of monomers, into a polymer.

[0027] As used herein, “oligomer” refers to a substance composed of oligomer molecules.

[0028] 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).

[0029] As used herein, “oligomerization” refers to the process of converting a monomer or a mixture of monomers into an oligomer.

[0030] 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.

[0031] As used herein, “ionomer” refers to a polymer composed of ionomer molecules.

[0032] 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.

[0033] As used herein, “monomer” refers to a substance composed of monomer molecules.

[0034] 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.

[0035] As used herein, “copolymer” refers to a polymer derived from more than one species of monomer.

[0036] 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).

[0037] As used herein, “repeating unit” refers to the constitutional unit the repetition of which constitutes a polymer molecule (or oligomer molecule, block, or chain).

[0038] 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.

[0039] 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.

[0040] 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.

[0041] 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).

[0042] As used herein, “side chain” refers to an oligomeric (short chain) or polymeric (long chain) offshoot from the main chain of a polymer molecule.

[0043] 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).

[0044] The principles, concepts, and features described herein with reference to side chains apply equally to side groups, and vice versa. Accordingly, any and all uses of the term side chain can be substituted by the term side group, and any and all uses of the term side group can be substituted by the term side chain, without departing from the scope of the disclosure herein.

[0045] 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.

[0046] 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.

[0047] 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.

[0048] 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.

[0049] 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.

[0050] 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).

[0051] 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.

[0052] As used herein, “metal” includes alkali metals, alkaline earth metals, transition metals, lanthanides, actinides, and post-transition metals.

[0053] As used herein, “transition metals” refers to elements of the d-block of the periodic table (Groups 3 to 12, inclusive).

[0054] As used herein, “post-transition metals” refers to aluminum, gallium, indium, tin, thallium, lead, bismuth, and polonium.

[0055] As used herein, “metalloids” refers to boron, silicon, germanium, arsenic, antimony, tellurium, and astatine.

[0056] As used herein, “platinum group metals” or “PGMs” refers to platinum, palladium, iridium, ruthenium, osmium, and rhodium

[0057] 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.

[0058] 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.

[0059] 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.

[0060] 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.

[0061] 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.

[0062] 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.

[0063] 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.

[0064] The term “aryl” refers to aromatic carbocyclic groups and heteroaryl groups, unsubstituted or wholly or partly substituted, having a single ring (e.g., phenyl), multiple rings (e.g., biphenyl), or multiple fused rings, wherein at least one ring of the aryl group is aromatic (e.g., 1,2,3,4-tetrahydronaphthyl, naphthyl, anthryl, or phenanthryl). That is, at least one ring of an aryl group has a conjugated Pi electron system, while other rings of the aryl group can be cycloalkyls, cycloalkenyls, cycloal kynyls, aryls, and / or heterocycyls. “Carbocyclic aryl groups” refer to aryl groups wherein the ring atoms on the aromatic ring are carbon atoms. Carbocyclic aryl groups include monocyclic carbocyclic aryl groups and polycyclic or fused compounds (e.g., two or more adjacent ring atoms are common to two adjoining rings) such as naphthyl group. Aryl groups are not limited to benzene and its derivatives but may have any suitable number of atoms in the ring. Examples of aryl groups include, without limitation, phenyl, naphthyl, tetrahydronaphthyl, anilyl, indanyl, and indenyl.

[0065] 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.

[0066] 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.

[0067] The term “aryloxy” refers to an aryl group having an oxygen radical attached thereto. An example of an aryloxy group includes, without limitation, a phenoxy group.

[0068] Any of the above groups may be optionally substituted, in whole or in part. Examples of substituents include, without limitation, aliphatic, alicyclic, heteroaliphatic, heteroal icyclic, halogen, azide, alkyl, aralkyl, alkenyl, alkynyl, cycloalkyl, hydroxyl, alkoxyl, amino, nitro, sulfhydryl, imino, amido, phosphonate, phosphinate, carbonyl, carboxyl, alkyloxycarbonyl, silyl, ether, alkylthio, heteroalkylthio, heteroarylthio, sulfonyl, sulfonamido, ketone, aldehyde, ester, heterocyclyl, aromatic or heteroaromatic moieties, — CF3, — CN, aryl, aryloxy, perhaloalkoxy, aralkoxy, heteroaryl, heteroaryloxy, heteroarylalkyl, heteroaralkoxy, azido, amino, amine, halide, alkylthio, oxo, acylalkyl, carboxy esters, -carboxamido, acyloxy, aminoalkyl, alkylaminoaryl, alkylaryl, alkylaminoalkyl, alkoxyaryl, arylamino, aralkylamino, alkylsulfonyl, - carboxamidoalkylaryl, -carboxamidoaryl, hydroxyalkyl, haloalkyl, alkylaminoalkylcarboxy-,aminocarboxamidoalkyl-, cyano, alkoxyalkyl, perhaloalkyl, arylalkyloxyalkyl, (e.g., SO4(R')2), a phosphate (e.g., PO4(R')3), a silane (e.g., Si(R')4), a urethane (e.g., R'O(CO)NHR'), and the like. Additionally, the substituents may be selected from F, Cl, Br, I, — OH, — NO2, — CN, — NCO, — CF3, — CH2CF3,— CHCI2, — CH2ORx, — CH2CH2ORX, — CH2N(RX)2, — CH2SO2CH3, — C(O)RX, — O2(RX), — CON(RX)2, — OC(O)RX, — C(O)OC(O)RX, — OCO2RX, — OCON(RX)2, — N(RX)2, — S(O)2RX, — OCO2RX, — NRx(CO)Rx, — NRX(CO)N(RX)2, wherein each occurrence of Rxindependently includes, but is not limited to, hydrogen, aliphatic, alicyclic, heteroaliphatic, heteroal icyclic, aryl, heteroaryl, alkylaryl, or alkyl heteroaryl, wherein any of the aliphatic, alicyclic, heteroaliphatic, heteroal icyclic, alkylaryl, or alkyl heteroaryl substituents described above and herein may be substituted or unsubstituted, branched or unbranched, cyclic or acyclic, and wherein any of the aryl or heteroaryl substituents described above and herein may be substituted or unsubstituted.

[0069] Polytetrafluoroethylene (PTFE) polymers are a class of polymers composed of tetrafluoroethylene polymer molecules, and derivatives thereof, and are produced by the polymerization of tetrafluoroethylene (TFE). PTFE polymer molecules have a carbon main chain with two fluorine atoms bonded to each carbon, and derivatives thereof.

[0070] Poly(chlorotrifluoroethylene) (PCTFE) polymers are polymers of chlorotrifluoroethylene (CTFE) with the molecular formula (CF2CCIF)n, and derivatives thereof. PCTFE is similar to PTFE 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. Examples of PCTFE polymers include, without limitation, modified and functionalized PCTFE polymers, including the intermediate and ion exchange-functionalized PCTFE polymers described herein. A PCTFE polymer includes homopolymers as well as copolymers that include PCTFE repeating units and one or more additional repeating units other than a PCTFE repeating unit.

[0071] 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 positiveand 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.

[0072] 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.

[0073] In a PCTFE polymer molecule, the chlorine atoms of the main chain are leaving groups that can be substituted with a side group including a linker and an ion exchange group to thereby form an ion exchange-functionalized PCTFE polymer molecule. The ion exchange group is linked to the PCTFE main chain by way of the linker.

[0074] Illustrative ion exchange-functionalized PCTFE polymer molecules and oligomer molecules, and methods of making ion exchange-functionalized PCTFE polymer molecules and oligomer molecules, will now be described. In the description that follows, discussion of and reference to polymer molecules applies equally to oligomer molecules, and discussion of and reference to polymer molecules can be extended to polymers (and oligomers) composed of the polymer molecules described herein. Moreover, the principles and concepts described herein are not limited to PCTFE polymers and polymer molecules but may be applied to anyfluorocarbon polymers and polymer molecules that have one or more leaving groups in a main chain or repeating unit.

[0075] In some examples, an ion exchange-functionalized PCTFE polymer molecule is a PCTFE polymer molecule that has one or more ion exchange groups linked to the main chain. For example, an ion exchange PCTFE polymer molecule may include a PCTFE main chain and a side group including an ion exchange group (e.g., a tetravalent boron group or a metal fluoride group) and a linker that links the ion exchange group to the PCTFE main chain.

[0076] A PCTFE main chain is a main chain that is derived from a PCTFE polymer molecule wherein some or all of the chlorine atoms are substituted, such as by a side group having a linker and an ion exchange group. The ion exchange-functionalized PCTFE polymer molecule may have any suitable degree of side group loading on the PCTFE main chain (e.g., stoichiometric ratio of side group to residual chlorine in the PCTFE main chain). For example, the degree of side group loading may be 100% (e.g., all chlorine atoms of the PCTFE main chain are substituted with a side group), 90% (e.g., a 9:1 stoichiometric ratio of side group to residual chlorine), 80% (8:2 stoichiometric ratio), 75% (3:1 stoichiometric ratio), 67% (2:1 stoichiometric ratio), 50% (1:1 stoichiometric ratio), 40% (2:3 stoichiometric ratio), 25% (1:3 stoichiometric ratio), 10% (1:9 stoichiometric ratio), or any other suitable value. In some examples, the degree of side group loading ranges from about 95% to about 50%, from about 90% to about 60%, from about 85% to about 70%, or from about 80% to about 75%.

[0077] In some examples, the ion exchange group is a tetravalent boron group or a metal fluoride group comprising a multivalent metal atom. As used herein, “multivalent” means that an atom is not restricted to a specific number of valence bonds, but may have multiple different valence states each with a different number of valence bonds. Thus, a trivalent atom that is multivalent may expand its valence state, such as by one to three to form a tetravalent, pentavalent, or hexavalent structure with a negative one (-1), negative two (-2), or negative three (-3) formal charge.

[0078] For example, boron has three valence electrons and has a ground state electron configuration of 1s22s22p1. Boron forms trivalent, trigonal neutral compounds, such as boric acid (B(OH)3), boronic acid (RB(OH)2or R1R2B(OH)), and boronic acid groups (a boronic acid where R is a part of a main chain or side chain or side group of a polymer), in which boron has three covalent bonds through sp2hybridization. The sp2hybridized boron atom contains an empty p-orbital, which makes trivalent boron compounds strongly electron-deficient, two electrons short of a stable octet electronic configuration. Boron is multivalent due to the empty p-orbital, thus enabling boron to form negatively charged tetravalent compounds with four covalent bonds. Thus, trivalent boron compounds, such as boric acid and boronic acids, are Lewis acids and readily accept an electron pair at the boron atom. Addition of an anion, such as fluoride (e.g., by a fluoride treatment) or other anion, makes the octet electronic configuration,forming highly stable, negatively charged tetravalent, tetrahedral boron compounds with four covalent bonds. Tetravalent boron may also be synonymously called tetra-coordinated boron. PCTFE polymer molecules may be functionalized with functional groups having tetracoordinated boron, such as tetra-coordinated boronic acid groups (e.g., fluoroboric acid groups). Functional groups including tetra-coordinated boron have a negative formal charge and thus are intrinsically ionic and acidic and may serve as cation transport agents in electrochemical cell applications. Similarly to boron, multivalent metal atoms may expand their valence to form one or more additional covalent bonds and thereby gain a formal negative charge, which may be balanced by an appropriate number of cations.

[0079] A tetravalent boron group has the general formula — BX3 wherein each X is independently an alkyl group, an alkoxy group, an alkyloxycarbonyl group, an aryl group, an aryloxy group, a hydroxyl group, a fluoro group, a cyano group, or a pentafluorophenyl group. In embodiments in which each X is a fluoro group, the tetravalent boron group is a boron trifluoride group having the general formula — BF3.

[0080] The boron atom of the tetravalent boron group is covalently bonded to each X of the tetravalent boron group and to the linker. Thus, the boron atom has four covalent bonds, making the boron atom intrinsically ionic. As a result, the tetravalent boron group may serve as an ion exchange group (e.g., a proton transport agent).

[0081] A metal fluoride group has the general formula — MF3 or — MF4 where M is a multivalent metal atom. As used herein, a “metal atom” may be a transition metal atom, a metal atom, or a metalloid atom. In some examples, M comprises a Group 4 metal atom (e.g., zirconium (Zr)), a Group 13 metal atom (e.g., aluminum (Al), gallium (Ga), or indium (In)), or a Group 14 metal atom (e.g., silicon (Si), germanium (Ge), or tin (Sn)).

[0082] As mentioned, the boron atom of the tetravalent boron group or the metal atom of the metal fluoride group is linked to the PCTFE main chain by way of a linker. In some examples, the linker is an aryloxy linker having the general formula — O — Ar — or — O — Ar — Y — , where O is oxygen and replaces chlorine (Cl) in the PCTFE main chain, Ar is an aryl group (e.g., a phenyl group), and Y is the conjugate base of an acid group such as a carboxylic acid group, a sulfonic acid group, a phosphonic acid group, a phosphoric acid group, a phosphinic acid group, or a hydroxyl group. In some examples, the aryloxy linker includes an alkyl chain of length 1 to 10 atoms between the oxygen atom and the aryl group and / or between the aryl group and Y. The aryl group may be variously substituted or unsubstituted. The aryloxy linker provides an ether linkage with the PCTFE main chain.

[0083] In other examples, the linker is a sulfonate group (the conjugate base of sulfonic acid group) and has the general formula — S(=O)2O — .

[0084] In further examples, the linker is an arylsulfonyl linker having the general formula — S(=O)2— Ar — or — S(=O)2 — Ar — Y- where — S(=O)2 — is a sulfonyl group and Ar and Y are asdescribed above. Thus, the arylsulfonyl linker provides a sulfonyl linkage with the PCTFE main chain.

[0085] The aryloxy linkers, sulfonate linkers, and arylsulfonyl linkers described herein include oxygen. Although the PCTFE main chain is hydrophobic, the oxygen-containing linkers provide hydrophilic domains and thus are compatible with electrochemical applications as ionomers, PEMs, and catalyst or anode coatings. Additionally, the electron-withdrawing sulfonate groups and sulfonyl groups in the sulfonate linkers and arylsulfonyl linkers further increase the acidity of the ion exchange-functionalized PCTFE polymers while remaining robust against the harsh chemical environment in electrochemical applications.

[0086] An ion exchange-functionalized PCTFE polymer molecule may be synthesized in any suitable way. In some examples, an ion exchange-functionalized PCTFE polymer molecule is synthesized by a post-polymerization process (e.g., after synthesis of a PCTFE polymer molecule) by substituting one or more chlorine atoms of the PCTFE main chain with a side group including an ion exchange group and a linker that links the ion exchange group to the PCTFE main chain.

[0087] An illustrative example of a post-polymerization process for converting a PCTFE polymer molecule to an ion exchange-functionalized PCTFE polymer molecule will now be described. In a first step, an intermediate linker-modified PCTFE polymer presenting a pendant acid group is formed. In some examples of the first step, a linking agent including a pendant acid group reacts with a PCTFE polymer molecule in a substitution reaction to substitute the linking agent for the chlorine atom in the PCTFE main chain. When bonded with the PCTFE main chain, the linking agent forms an intermediate linker presenting a pendant acid group or hydroxyl group. Thus, the resulting intermediate linker-modified PCTFE polymer molecule has a PCTFE main chain and a side group including a linker having a pendant acid group or hydroxyl group.

[0088] In some examples, the linking agent is an aromatic linking agent. An aromatic linking agent includes an aryl group, a linking group bonded to the aryl group, and an acid group bonded to the aryl group. For example, the aryl group may include the linking group and the acid group as substituents. The aryl group may be variously substituted or unsubstituted. Illustrative examples of suitable aryl groups include, without limitation, phenyl, biphenyl, naphthyl, and anthryl. The linking group reacts with chlorine atoms in the main chain of the PCTFE polymer molecule in a substitution reaction to form an intermediate linker-modified PCTFE polymer molecule with the pendant acid group. In some examples, the linking group is a hydroxyl group or a thiol group. When the linking agent is bonded to the PCTFE main chain, the acid group reacts with an ion exchange agent to form an ion exchange group. For examples, the acid group may be a Bronsted-Lowry acid which, upon deprotonation in solvent, becomes a Lewis base and donates an electron pair to the ion exchange agent to form acovalent bond with the ion exchange agent. Illustrative examples of the acid group include, without limitation, a sulfonic acid group, a carboxylic acid group, a phosphonic acid group, a phosphoric acid group, a phosphinic acid group, and a hydroxyl group. Illustrative examples of the linking agent include, without limitation, hydroxybenzenesulfonic (e.g., 4- hydroxybenzenesulfonic acid, 2-hydroxybenzenesulfonic acid), phenolic acid (e.g., 4-phenolic acid, 2-phenolic acid), hydroxybenzenephosphonic (e.g., 4-hydroxybenzenephosphonic acid, 2- hydroxybenzenephosphonic acid), and hydroxybenzenephosphonic (e.g., 4- hydroxybenzenephosphonic acid, 2-hydroxybenzenephosphonic acid).

[0089] In examples where the linking group of the linking agent is a thiol group, the intermediate linker-modified PCTFE polymer molecule includes an aryl group linked with the PCTFE main chain by a sulfide link (R — S — R'). In these examples, the first step may also include a sub-step of oxidizing the sulfide link between the PCTFE main chain and the aryl group to form a sulfonyl linkage ( — S(=O)2 — ). Any suitable oxidizing agent may be used, such as but not limited to aqueous hydrogen peroxide (H2O2). The intermediate product is a linker- modified PCTFE polymer molecule with a pendant acid group linked to the PCTFE main chain by an arylsulfonyl linker.

[0090] In a second step, an ion exchange agent reacts with the pendant acid group of the intermediate linker. The ion exchange agent readily reacts with the pendant acid group to form a pendant ion exchange group. In some examples, the ion exchange agent is a trivalent boron compound or a metal fluoride.

[0091] A trivalent boron compound has the general formula BX3 where X is as described above with reference to the tetravalent boron group. In the reaction of the second step, the boron atom of the trivalent boron compound expands its valence and covalently bonds with an oxygen atom of the pendant acid group or hydroxyl group, thus forming an ion exchange group in which the boron atom is linked with the PCTFE main chain by way of the linker. Thus, the boron atom has four covalent bonds and has a negative formal charge, making the boron atom intrinsically acidic and ionic. As a result, the tetravalent boron group may serve as an ion exchange group (e.g., a proton transport agent).

[0092] In some examples, the trivalent boron compound is boron trifluoride (BF3) (in the form of BF3etherate (Et20)), which reacts with the pendant acid group or pendant hydroxyl group of the intermediate linker-modified PCTFE polymer to form a pendant tetravalent boron group. The boron atom of the boron trifluoride covalently bonds with the oxygen atom of the pendant acid group or pendant hydroxyl group of the linker to form a tetravalent boron group. The boron atom of the tetravalent boron group is covalently bonded to each fluorine atom of the tetravalent boron group and to the oxygen atom of the linker.

[0093] A metal fluoride has the general formula MF3 or MF4 where M comprises a multivalent metal atom, as described above with reference to the metal fluoride group. In thereaction of the second step, the multivalent metal atom of the metal fluoride expands its valence and covalently bonds with an oxygen atom of the pendant acid group or hydroxyl group, thus forming an ion exchange group in which the metal atom is linked with the PCTFE main chain by way of the linker. Thus, the metal atom has a negative formal charge, making the metal atom intrinsically acidic and ionic. As a result, the metal fluoride group may serve as an ion exchange group (e.g., a proton transport agent).

[0094] Another illustrative example of a post-polymerization process for converting a PCTFE polymer molecule to an ion exchange-functionalized PCTFE polymer molecule will now be described. In a first step, a boronic acid-functionalized PCTFE polymer is formed. A boronic acid-functionalized PCTFE polymer may be formed in any suitable way, including in any way described in International Patent Application No. PCT / US2024 / 015409, filed February 12, 2024, which is incorporated herein by reference in its entirety. In some examples, a boronic acid- functionalized PCTFE polymer is formed by reacting an arylboronic acid linking agent with a PCTFE polymer molecule in a substitution reaction to substitute the arylboronic acid linking agent for the chlorine atom in the PCTFE main chain. When bonded with the PCTFE main chain, the linking agent forms an intermediate linker presenting a pendant boronic acid group. Thus, the resulting intermediate linker-modified PCTFE polymer molecule has a PCTFE main chain and a side group including a linker having a pendant boronic acid group.

[0095] The arylboronic acid linking agent is an aromatic linking agent, as described above, but wherein the pendant acid group is a boronic acid group having the general formula — B(OH)2. Illustrative examples of an arylboronic acid linking agent include, without limitation, 4- hydroxyphenylboronic acid and 2-hydroxyphenylboronic acid.

[0096] When bonded with the PCTFE main chain, the arylboronic linking agent forms an intermediate linker presenting a pendant boronic acid group. Thus, the resulting intermediate boronic acid-functionalized PCTFE polymer molecule has a PCTFE main chain and a side group including a linker having a pendant boronic acid group.

[0097] In examples where the linking group of the arylboronic linking agent is a thiol group, the intermediate linker-modified PCTFE polymer molecule includes an aryl group linked with the PCTFE main chain by a sulfide link (R — S — R'). In these examples, the first step may also include a sub-step of oxidizing the sulfide link between the PCTFE main chain and the aryl group to form a sulfonyl linkage ( — S(=O)2 — ). Any suitable oxidizing agent may be used, such as but not limited to aqueous hydrogen peroxide (H2O2). The intermediate product is a linker- modified PCTFE polymer molecule with a pendant boronic acid group linked to the PCTFE main chain by an arylsulfonyl linker.

[0098] In a second step, a fluoride treatment is performed to convert the pendant boronic acid group to a tetravalent boron group. The fluoride treatment covalently bonds a fluorine atom to the boronic acid group to form a fluoroboric acid group and, depending on stoichiometry, mayalso replace one or both hydroxyl groups of the pendant boronic acid groups with fluorine. The fluoride treatment may be performed in any suitable way.

[0099] In some examples, the fluoride treatment comprises reacting the boronic acid- functionalized PCTFE polymer molecule with a fluoride reagent. In some examples, the fluoride reagent comprises hydrogen fluoride (HF), a metal fluoride, ammonium fluoride (NH4F), ammonium bifluoride (NH4F2), or a tetraalkylammonium fluoride. The metal atom of the metal fluoride may be any suitable metal, such as an alkali metal (e.g., lithium (Li), sodium (Na), potassium (K)), an alkaline earth metal (e.g., magnesium (Mg), calcium (Ca)), a transition metal, or a post-transition metal. Illustrative examples of a metal fluoride include, without limitation, lithium fluoride (LiF), sodium fluoride (NaF), potassium fluoride (KF), potassium bifluoride (KHF2), and cesium fluoride (CsF). Tetraalkylammonium fluoride has the general formula [R4N+][F ] wherein each R is independently hydrogen or a wholly or partly substituted or unsubstituted alkyl or aryl group having one to twenty, one to ten, one to eight, one to six, or one to four carbon atoms, such as but not limited to a methyl, ethyl, propyl, or butyl group. In some examples, the fluoride treatment is performed with two or more different fluoride reagents (e.g., HF and NaF, HF and LiF, etc.).[000100] The fluoride reagent combines with a boronic acid group of the boronic acid- functionalized PCTFE polymer molecule to form a pendant fluoroboric acid group. One or more hydroxyl groups of the boronic acid group may also be replaced by fluoride from the fluoride compound and a fourth fluoride group is added. Thus, the boron atom becomes tetracoordinated so that the pendant fluoroboric acid group has a negative formal charge and is counterbalanced by a cation (e.g., H+, Li+, Na+, K+, Cs+, NH4+, or NR4+).[000101] In some examples in which the counter cation is not hydrogen (e.g., when the fluoride compound is one or more of LiF, NaF, KF, KHF2, CsF, NH4F, NH4F2, or NR4F), the fluoride treatment may be followed by a protonation step to replace the counter cation with a proton (H+). The protonation step may be performed in any suitable way. In some examples, the protonation step is performed by combining the fluoroboric acid-functionalized polymer molecule with a strong acid. Examples of suitable strong acids include, without limitation, hydrochloric acid (HCI), sulfuric acid (H2SO4), methanesulfonic acid (CH3SO3H), and trifluoracetic acid (CF3CO2H). After the protonation step, the pendant fluoroboric acid group is counterbalanced by a proton (H+).[000102] In other examples of the fluoride treatment, the hydrogen atom of one or more hydroxyl groups of the pendant boronic acid group is replaced with a fluoroboric acid group. In these examples, the fluoride treatment comprises combining the boronic acid-functionalized PCTFE polymer molecule with a fluoroboric compound of the formula BFm(OH)(3-m) where m is 1, 2, or 3 (e.g., fluorodihydroxy boric acid (BF(OH)2), difluorohydroxy boric acid (BF2(OH)), and / or boron trifluoride (BF3). In further examples, the fluoride treatment may also includecombining the boronic acid-functionalized polymer molecule with a boronic acid RB(OH)2 and fluoride reagent (e.g., HF) in situ in a one-pot process, with the fluoride reagent as the limiting reagent, where R is an alkyl or aryl group (e.g., 1 to 20, 1 to 10, 1 to 8, 1 to 6, 1 to 4, or 1 to 3 carbon atoms), branched or unbranched, wholly or partly substituted or unsubstituted. [000103] In examples where the reaction results in a tetra-coordinated boronic acid group having one or two remaining hydroxyl groups, the same reaction may be repeated or a different reaction may be performed to replace one or both of the remaining hydroxyl groups with a fluoride group or other anion. In other examples, conversion of a boronic acid group to a tetracoordinated boronic acid group may be performed using multiple reagents in the same step. [000104] Another illustrative example of a post-polymerization process for converting a PCTFE polymer molecule to an ion exchange-functionalized PCTFE polymer molecule will now be described. In a first step, a PCTFE polymer molecule is functionalized with one or more sulfonic acid groups. The PCTFE polymer molecule may be functionalized with one or more sulfonic acid groups in any suitable way.[000105] In some examples, a PCTFE polymer molecule is functionalized with one or more sulfonic acid groups in two sub-steps. In the first sub-step, a PCTFE polymer molecule is functionalized with one or more thiol groups ( — SH) by a substitution reaction with a linking agent having a thiol group. The thiol group of the linking agent reacts with the chlorine atoms in the main chain through a substitution reaction to form an intermediate thiol-modified PCTFE polymer having a pendant thiol group covalently bonded to the main chain. Any suitable linking agent having a thiol group may be used, such as but not limited to lithium hydrosulfide (LiSH), sodium hydrosulfide (NaSH), or potassium hydrosulfide (KSH). In the second sub-step, the thiol group undergoes oxidation with an oxidizing agent to form a pendant sulfonic acid group. Any suitable oxidizing agent may be used, such as but not limited to aqueous hydrogen peroxide (H2O2). The intermediate product is a sulfonic acid-functionalized PCTFE polymer molecule with a pendant sulfonic acid group. It will be recognized that a sulfonic acid-functionalized PCTFE polymer may be produced in other ways.[000106] In a second step, an ion exchange agent reacts with the pendant sulfonic acid group of the intermediate linker. The ion exchange agent readily reacts with the pendant sulfonic acid group to form a pendant ion exchange group. As described above, the ion exchange agent may be a trivalent boron compound (e.g., BX3 or BF3) or a metal fluoride. The reaction of the second step produces an ion exchange-functionalized PCTFE polymer molecule in which the ion exchange group has a negative formal charge and thus may serve as an ion transport agent. [000107] The ion exchange-functionalized PCTFE polymers are simple and advanced alternatives to current PFSA polymers. For example, the ion exchange-functionalized PCTFE polymers produced by the above-described processes are intrinsically ionic and strongly acidic and, therefore, highly proton-conducting. Under the aqueous conditions of water electrolysisand hydrogen fuel cells, the tetravalent boron groups may serve as ion exchange agents. As a result, the ion exchange-functionalized PCTFE polymers may be used for ionomer and proton exchange membrane (PEM) applications. The ion exchange-functionalized PCTFE polymers described herein are intrinsically ionic, resembling ionic liquids that are known to accelerate / facilitate electrochemical reactions. The ion exchange-functionalized PCTFE polymers are also highly cost-effective and have a robust polymer backbone. Moreover, residual chlorine atoms in the polymer main chain still provide the unique properties of PCTFE that may help control hydrogen gas crossover, a major issue in PEM water electrolysis. The oxygen-containing linkers also provide hydrophilic domains and are compatible with electrochemical applications as ionomers and PEM. Moreover, synthesis of the ion exchange- functionalized PCTFE polymers is simple and cost-effective. The ion exchange capacity (IEC) of the ion exchange-functionalized PCTFE polymers may also be controlled and tuned up for optimized performance.[000108] Illustrative examples of ion exchange-functionalized PCTFE polymers, and methods of making ion exchange-functionalized functionalized ion exchange-functionalized functionalized PCTFE polymers, will now be described.[000109] FIG. 1 shows an illustrative reaction scheme for synthesis of an ion exchange- functionalized PCTFE polymer using PCTFE as a starting material. In a first step, a PCTFE polymer is combined with 4-hydroxybenzenesulfonic acid (the linking agent) in the presence of a base, such as a metal hydride (e.g., LiH, NaH, KH, K-tert-butoxide), in a 2:1 stoichiometric ratio (or any other suitable ratio) of the base to the linking agent. 4-hydroxybenzenesulfonic acid may be variously substituted or unsubstituted. It will be recognized that any other suitable aromatic compound may be used in place of 4-hydroxybenzenesulfonic acid, such as 2- hydroxybenzenesulfonic acid. 4-hydroxybenzenesulfonic acid reacts with chlorine atoms in the main chain of the PCTFE polymer in a substitution reaction. The reaction produces an intermediate phenoxysulfonic acid-modified PCTFE polymer with a sulfonic acid group in a side group. In a second step, boron trifluoride (in the form of BF3etherate (Et20) in this example) reacts with the sulfonic acid group to form a tetravalent boron group. The resulting ion exchange-functionalized functionalized PCTFE polymer includes a PCTFE main chain and a side group including a tetravalent boron group ( — BF3) and a linker that links the tetravalent boron group to the main chain. The linker includes a phenoxy linking group ( — O-Ph — ) and a sulfonate linking group ( — S(=0)-0 — ). The boron atom has a negative formal charge and a proton is ionically linked with the boron atom.[000110] FIG. 2 shows another illustrative reaction scheme for synthesis of an ion exchange- functionalized PCTFE polymer using PCTFE as a starting material. The reaction scheme of FIG. 2 is similar to the reaction scheme of FIG. 1 except that, in the reaction scheme of FIG. 2,the linking agent is 4-phenolic acid (also known as p-hydroxybenzoic acid). However, any other suitable linking agent may be used, including any other phenolic acid.[000111] FIG. 3 shows another illustrative reaction scheme for synthesis of an ion exchange- functionalized PCTFE polymer using PCTFE as a starting material. In a first step, a PCTFE polymer is combined with 4-hydroxyphenylboronic acid (the linking agent) in the presence of a base, such as a metal hydride (e.g., LiH, NaH, KH, K-tert-butoxide), in a 2:1 stoichiometric ratio (or any other suitable ratio) of the base to the linking agent. 4-hydroxyphenylboronic acid may be variously substituted or unsubstituted. 4-hydroxyphenylboronic acid reacts with chlorine atoms in the main chain of the PCTFE polymer in a substitution reaction and covalently bonds with the main chain. The reaction produces an intermediate phenoxyboronic acid-modified PCTFE polymer with a boronic acid group (B(OH)2) in a side group. In a second step, a fluoride treatment is performed as described above to form a tetravalent boron group in the side group. In this example, the fluoride treatment replaces both hydroxyl groups of the boronic acid group with fluorine atoms and adds a third fluorine atom. Any suitable fluoride treatment may be performed, including any fluoride treatment described herein. The resulting ion exchange- functionalized PCTFE polymer includes a PCTFE main chain and a side group including a tetravalent boron group and a linker including a phenoxy linking group that links the tetravalent boron group to the main chain.[000112] FIG. 4 shows another illustrative reaction scheme for synthesis of an ion exchange- functionalized PCTFE polymer using PCTFE as a starting material. In this example, the first step (formation of an intermediate sulfonic acid-modified PCTFE polymer) occurs in two substeps. In the first sub-step, the PCTFE polymer is combined with sodium hydrosulfide (NaSH) as a first linking agent to form an intermediate thiol-mod ified PCTFE polymer having a thiol group ( — SH) covalently bonded to the main chain. In the second sub-step, the thiol group undergoes oxidation, such as by combination with aqueous hydrogen peroxide (H2O2), to form an intermediate linker-modified PCTFE polymer with a sulfonic acid group in a side group. In a second step of the reaction scheme, boron trifluoride (in the form of BF3 etherate (Et2O) in this example) reacts with the sulfonic acid group to form a tetravalent boron group. The resulting ion exchange-functionalized PCTFE polymer includes a PCTFE main chain and a side group including a tetravalent boron group and a linker including a sulfonic ester linking group ( — S(=0)-0 — ) that links the tetravalent boron group to the main chain.[000113] FIG. 5 shows another illustrative reaction scheme for synthesis of an ion exchange- functionalized PCTFE polymer using PCTFE as a starting material. A first step is performed with two sub-steps. In the first sub-step, a PCTFE polymer is combined with 4- mercaptobenzenesulfonic acid (the linking agent) in the presence of a base, such as a metal hydride (e.g., LiH, NaH, KH, K-tert-butoxide), in a 2:1 stoichiometric ratio (or any other suitable ratio) of the base to the linking agent. 4-mercaptobenzenesulfonic acid may be variouslysubstituted or unsubstituted. It will be recognized that any other suitable aromatic linking agent may be used in place of 4-mercaptobenzenesulfonic acid, such as 2-mercaptobenzenesulfonic acid. 4-mercaptobenzenesulfonic acid reacts with chlorine atoms in the main chain of the PCTFE polymer molecule in a substitution reaction to produce an intermediate linker-modified PCTFE polymer with a pendant sulfonic acid group in a side group. In the second sub-step, the thiol group undergoes oxidation, such as by combination with aqueous hydrogen peroxide (H2O2), to produce a sulfonyl linkage. The intermediate linker-modified PCTFE polymer molecule includes a pendant sulfonic acid group linked to the PCTFE main chain by an arylsulfonyl linker.[000114] In a second step, boron trifluoride (in the form of BF3 etherate (Et20) in this example) reacts with the sulfonic acid group to form a tetravalent boron group. The resulting ion exchange-functionalized functionalized PCTFE polymer includes a PCTFE main chain and a side group including a tetravalent boron group ( — BF3) and a linker that links the tetravalent boron group to the main chain. The linker includes a sulfonyl linking group, an aryl group, and a sulfonate group. The boron atom has four covalent bonds and, hence, a negative formal charge. A proton is ionically linked with the boron atom.[000115] FIG. 6 shows another illustrative reaction scheme for synthesis of an ion exchange- functionalized PCTFE polymer using PCTFE as a starting material. The reaction scheme of FIG. 6 is similar to the reaction scheme of FIG. 5 except that, in the reaction scheme of FIG. 6, the linking agent is 4-mercaptobenzoic acid. However, any other suitable linking agent may be used. The resulting ion exchange-functionalized functionalized PCTFE polymer includes a PCTFE main chain and a side group including a tetravalent boron group ( — BF3) and a linker that links the tetravalent boron group to the main chain. The linker includes a sulfonyl linking group, an aryl group, and a carboxylate group. The boron atom has four covalent bonds and, hence, a negative formal charge. A proton is ionically linked with the boron atom.[000116] FIG. 7 shows another illustrative reaction scheme for synthesis of an ion exchange- functionalized PCTFE polymer using PCTFE as a starting material. A first step is performed with two sub-steps. In the first sub-step, a PCTFE polymer is combined with 4- mercaptophenylboronic acid (the linking agent) in the presence of a base, such as a metal hydride (e.g., LiH, NaH, KH, K-tert-butoxide), in a 2:1 stoichiometric ratio (or any other suitable ratio) of the base to the linking agent. 4-mercaptophenylboronic acid may be variously substituted or unsubstituted. It will be recognized that any other suitable aromatic linking agent may be used in place of 4-mercaptophenylboronic acid, such as 2-mercaptophenylboronic acid. 4-mercaptophenylboronic acid reacts with chlorine atoms in the main chain of the PCTFE polymer in a substitution reaction and covalently bonds with the main chain. The reaction produces an intermediate linker-modified PCTFE polymer with a pendant boronic acid group (B(OH)2) in a side group and a sulfide linkage. In the second sub-step, the thiol groupundergoes oxidation, such as by combination with aqueous hydrogen peroxide (H2O2), to produce a sulfonyl linkage. The intermediate linker-modified PCTFE polymer molecule includes a pendant boronic acid group linked to the PCTFE main chain by an arylsulfonyl linker.[000117] In a second step, a fluoride treatment is performed as described above to form a tetravalent boron group in the side group. In this example, the fluoride treatment replaces both hydroxyl groups of the boronic acid group with fluorine atoms and adds a third fluorine atom. Any suitable fluoride treatment may be performed, including any fluoride treatment described herein. The resulting ion exchange-functionalized PCTFE polymer includes a PCTFE main chain and a side group including a tetravalent boron group (as the ion exchange group) and an arylsulfonyl group (as a linker) that links the tetravalent boron group to the PCTFE main chain. [000118] FIG. 8 shows an illustrative reaction scheme for synthesis of a functionalized PCTFE polymer including a side group with an arylsulfonyl group. In a first step, a PCTFE polymer is combined with benzenethiol in the presence of a base (e.g., in a 2:1 stoichiometric ratio). Benzenethiol may be variously substituted or unsubstituted. It will be recognized that any other suitable aromatic thiol may be used in place of benzenethiol. The thiol group of benzenethiol reacts with chlorine atoms in the main chain of the PCTFE polymer in a substitution reaction and covalently bonds benzenethiol with the PCTFE main chain. The reaction produces an intermediate aryl sulfide-mod ified PCTFE polymer with a pendant aryl group in a side group and a sulfide linkage. In a second step, the sulfide group undergoes oxidation, such as by combination with aqueous hydrogen peroxide (H2O2), to produce a sulfonyl linkage. The intermediate arylsulfonyl-modified PCTFE polymer molecule includes a pendant aryl group linked to the PCTFE main chain by sulfonyl linkage.[000119] In some examples, the pendant aryl group is functionalized with an acid of interest by an aromatic electrophilic substitution reaction or an aryl coupling reaction. Illustrative examples of acids that may be coupled with the aryl group include, without limitation, sulfuric acid, a sulfonic acid, a carboxylic acid, a phosphonic acid, phosphinic acid, boric acid, a boronic acid. In some examples, the aryl group is functionalized by aromatic sulfonation to produce a pendant sulfonic acid group. In some examples, the aryl group is functionalized by aromatic ring borylation to produce a pendant boronic acid group. Optionally, the pendant sulfonic acid group or pendant boronic acid group may be converted to a pendant tetravalent boron group. The aryl group may be sulfonated or borylated and / or the pendant boronic acid group may be converted to a tetravalent boron group in any suitable way, including in any way described in International Patent Application No. PCT / US2024 / 015409, incorporated herein by reference. [000120] As shown in the examples of FIGS. 1-4, the stoichiometric ratio of the side group (linking group and ion exchange group) to residual chlorine in the final product is 3:1, while in the examples of FIGS. 5-8 the stoichiometric ratio of the side group (linking group and ion exchange group) to residual chlorine in the final product is 1 :1. Residual chlorine atoms in thePCTFE main chain provide the unique properties of PCTFE that may help control hydrogen gas crossover. It will be recognized that the degree of modification of PCTFE with the side group may be tuned and modified to adjust the amount of residual chlorine and to optimize the ion exchange capacity (IEC) of the resulting PCTFE polymers. For example, the stoichiometric ratio of side groups to residual chlorine in the may be 1:1, 2:1, 3:1, 4:1, 5:1, 6:1, 7:1, 8:1, 9:1, 1 :2, 1:3, 2:3, 1:4, 2:4, 3:4, 1:5, 2:5, 3:5, 4:5, or any other suitable ratio. In some examples, the linking agent is rate limiting (the limiting reagent) in the reaction schemes of FIGS. 1-8. In other examples, all chlorine atoms in the PCTFE main chain are replaced with the side groups. [000121] While the main chains of the ion exchange-functionalized PCTFE polymers produced by the reaction schemes of FIGS. 1-7 are hydrophobic, the oxygen-containing linkers provide hydrophilic domains. As a result, the oxygen-containing linkers are compatible with electrochemical applications as ionomers and PEMs.[000122] In the reaction schemes of FIGS. 1, 2, 4, 5, and 6, the ion exchange agent is BF3, but any other suitable ion exchange agent described herein may be used, such as a trivalent boron compound BX3 or a metal fluoride having general formula MF3 or MF4. Moreover, the reaction schemes show that the ion exchange groups are counterbalanced by a proton, but any other suitable cation(s) may be used, such as, but not limited to, Li+, Na+, K+, Cs+, NH4+, or NFU+. In some examples, a cation exchange step is performed to replace the proton with a different cation.[000123] In the examples described above, an ion exchange-functionalized PCTFE polymer is synthesized by post-polymerization functional modification of a PCTFE polymer. In other examples, an ion exchange-functionalized PCTFE polymer is synthesized by polymerization reactions using an ion exchange-functionalized chlorotrifluoroethylene (CTFE) monomer. For example, a CTFE monomer may be modified pre-polymerization with an ion exchange group, after which the functionalized monomer is polymerized to form an ion exchange-functionalized PCTFE polymer molecule. The monomer may be functionally modified with an ion exchange group in any way, including using any reaction scheme described herein for functional modification of a PCTFE polymer molecule with an ion exchange group.[000124] The ion exchange-functionalized PCTFE polymers described herein may be formed as a porous polymer network and may be used as ionomers and / or PEMs. Additionally or alternatively, existing PCTFE membranes may be modified as described herein to form ion exchange-functionalized PCTFE polymer membranes. For example, in the reaction schemes describe herein, the PCTFE starting materials may be in the form of a membrane material. In some examples, PEMs may be formed via melt extrusion or gel casting of ion exchange- functionalized PCTFE polymers. PEMs produced in this way may be thinner but more mechanically robust than conventional PEMs.[000125] In some examples, an ion exchange-functionalized PCTFE polymer is blended with one or more additives, such as plasticizers, including other ionomers that function as plasticizers.[000126] An ion exchange-functionalized PCTFE polymer was synthesized as described in the following examples.Example 1[000127] 150 mmol of NaSH (sodium hydrosulfide), 15 mmol of tetramethylammonium chloride, and 100 ml of ultrapure water were combined in a 200 milliliter (mL) hydrothermal reactor and the resulting mixture was allowed to stir at room temperature to make a clear solution. PCTFE (10 grams (g)) was added to the solution, the hydrothermal reactor was sealed and allowed to heat in an oil bath at 170 °C for 48 hours with magnetic stirring. The reactor was cooled down to room temperature and the polymer was filtered off and washed with an excess of deionized water to remove the excess reagents. The crude polymer product, thiol- functionalized PCTFE (PCTFE — SH), was used directly for the next step.[000128] The thiol-modified PCTFE polymer (11.5 g) was transferred into a round bottomed flask, to which 50 mL of deionized water was added with stirring, followed by the addition of a 35% aqueous solution of hydrogen peroxide. The resulting mixture was allowed to stir at room temperature for 72 hours. The polymer was filtered and washed successively with 1 M sulfuric acid followed by deionized water and then dried under vacuum.[000129] The resulting polymer (9.55 g), sulfonic acid-functionalized PCTFE (PCTFE — SO3H), was transferred into a flask with a magnetic stirrer and treated with a solution of BF3 etherate (50 mmol) in a 1:1 mixture of tetrahydrofuran (THF) and dichloromethane (DCM) for 18 hours. The polymer was then filtered, washed with THF, and dried under vacuum to obtain the final product, PCTFE — S(=O)2 — OBF3H as a black solid.Example 2[000130] 100 mL dry DMF and 4 g (100 mmol) of NaH (60% in mineral oil) were added to a 200 mL hydrothermal reactor with stirring by a clean Teflon-coated magnet bar, followed by addition of 11.3 mL of thiophenol. When the evolution of gas stopped, PCTFE powder (10 g) was added. The reactor was sealed and the mixture was allowed to heat at 170 °C for 96 hours. The reactor was then cooled to room temperature. The product polymer, PCTFE — S — Ph, was isolated by filtration followed by washing with acetone and methanol and drying under vacuum.[000131] The product polymer, PCTFE — S — Ph (9.8 g), was transferred into a 200 mL hydrothermal vessel, to which 100 mL dichloroethane and 10 mL of chlorosulfonic acid were added. The reactor was sealed and allowed to heat at 170 °C with magnetic mixing for 15 hours. The reaction was allowed to cool down to room temperature. The reaction contents were slowly transferred into an Erlenmeyer flask containing 150 mL of ice-cold deionized water withmixing. The product polymer, PCTFE — S — C6H4SO3H, was isolated by filtration followed by washing with acetone and methanol and then dried under vacuum.[000132] The product polymer (8.9 g) was transferred into a round bottomed flask, to which was added 40 mL of deionized water with magnetic stirring. To this mixture a 35% aqueous solution of hydrogen peroxide (40 mL) was slowly added with stirring. The resulting mixture was stirred for 15 hours at room temperature. The pH of the reaction became ~1.0. The product polymer, PCTFE — S(=O)2 — C6H4SO3H, was isolated by filtration followed by washing with acetone and methanol and then dried under vacuum.[000133] The polymer product (7.5 g) was transferred into a flask, to which was added 100 mL diethyl ether with magnetic stirring, followed by addition of 10 mL BF3 etherate. The resulting mixture was stirred at room temperature for 15 hours. The product polymer, PCTFE — S(=O)2— C6H4— OBF3H, was isolated by filtration followed by washing with acetone and methanol and then dried under vacuum.Example 3[000134] 21 mmol of potassium 4-hydroxy phenyl trifluoroborate was added to a dry round bottomed flask, followed by the addition of 50 mL dry DMF under nitrogen with magnetic stirring. To this mixture was added 20 mmol NaH with stirring, followed by addition of 5.0 g of PCTFE powder. The mixture was heated under nitrogen for 96 hours. The product polymer was isolated by filtration followed by acidification with aqueous H2SO4 and washing with acetone and methanol and then dried under vacuum to a constant mass (5.09 g).[000135] The ionomers and PEMs described herein may be used in electrochemical systems, such as water electrolysis systems and fuel cell systems.[000136] FIG. 9 shows an illustrative proton exchange membrane water electrolysis system 900 (PEM water electrolysis system 900) incorporating ion exchange-functionalized PCTFE polymer PEMs and / or ionomers. PEM water electrolysis system 900 uses electricity to split water into oxygen (O2) and hydrogen (H2) via an electrochemical reaction. The configuration of PEM water electrolysis system 900 is merely illustrative and not limiting, as other suitable configurations as well as other suitable water electrolysis systems may incorporate ion exchange-functionalized PCTFE polymers.[000137] As shown in FIG. 9, PEM water electrolysis system 900 includes a membrane electrode assembly 902 (MEA 902), porous transport layers 904-1 and 904-2 (e.g., gas diffusion layers), bipolar plates 906-1 and 906-2, and an electrical power supply 908. PEM water electrolysis system 900 may also include additional or alternative components not shown in FIG. 9 as may serve a particular implementation.[000138] MEA 902 includes a PEM 910 positioned between a first catalyst layer 912-1 and a second catalyst layer 912-2. PEM 910 electrically isolates first catalyst layer 912-1 from second catalyst layer 912-2 while providing selective conductivity of cations, such as protons (H+), andwhile being impermeable to gases such as hydrogen and oxygen. PEM 910 may be implemented by an ion exchange-functionalized PCTFE polymer as described herein, or by any other suitable polymer.[000139] First catalyst layer 912-1 and second catalyst layer 912-2 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 912-1 and second catalyst layer 912-2 may also include one or more ionomers mixed with the catalyst solid supports and electrochemical catalyst particles. The ionomers may be implemented by an ion exchange-functionalized PCTFE polymer as described herein, or by any other suitable ionomer.[000140] MEA 902 is placed between porous transport layers 904-1 and 904-2, which are in turn placed between bipolar plates 906-1 and 906-2 with flow channels 914-1 and 914-2 located in between bipolar plates 906 and porous transport layers 904.[000141] In MEA 902, first catalyst layer 912-1 functions as an anode and second catalyst layer 912-2 functions as a cathode. When PEM water electrolysis system 900 is powered by power supply 908, an oxygen evolution reaction (OER) occurs at first catalyst layer / anode 912- 1, facilitated by the electrocatalysts bound to the catalyst solid supports in first catalyst layer / anode 912-1. The OER is represented by the following electrochemical half-reaction:2 H2O -> O2+ 4 H++ 4 e[000142] Protons are conducted from first catalyst layer / anode 912-1 to second catalyst layer / cathode 912-2 through PEM 910, and electrons are conducted from first catalyst layer / anode 912-1 to second catalyst layer / cathode 912-2 by conductive path around PEM 910. PEM 910 allows for the transport of protons (H+) and water from the first catalyst layer / anode 912-1 to the second catalyst layer / cathode 912-2 but is impermeable to oxygen and hydrogen. At second catalyst layer / cathode 912-2, 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 / cathode 912-2. The HER is represented by the following electrochemical half-reaction:4 H++ 4 e -> 2 H2[000143] 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[000144] In some examples, PEM 910 is an ion exchange-functionalized PCTFE polymer. In water electrolysis, the first step (OER) involves splitting water to generate oxygen and protons (H+) at the anode side (first catalyst layer 912-1 side). The protons are transported across PEM 910 to the cathode side (second catalyst layer 912-2 side) by the ion exchange groups of the PEM 910. Proton transport is a facile process for ion exchange-functionalized PCTFE polymersand ionomers as they have generous proton transporting groups (e.g., tetravalent boron groups and / or metal fluoride groups). At the cathode side, the protons undergo reduction in the HER, thereby producing molecular hydrogen (H2).[000145] In further examples, the anode (first catalyst layer 912-1) is coated with an ion exchange-functionalized PCTFE polymer. A polymer matrix of PCTFE is non-permeable to both molecular oxygen (O2) and hydrogen (H2) and most other gases except helium. See Yavari et al, The rote of halogens in polychlorotriflooroethytene (PCTFE) in membrane1gas separations, J. Membr. Sci, 548, 380-389, 2018. Due to the intrinsic non-permeability of PCTFE to molecular oxygen, first catalyst layer 912-1, when coated with an ion exchange-functionalized PCTFE polymer, allows only minimal oxygen absorption. Thus, the active local concentration of oxygen at the surfaces of the anode (first catalyst layer 912-1) remain low and the OER becomes much faster, shifting the water-splitting reaction equilibrium to the right, thereby offering a more energy-efficient water electrolysis process.[000146] In additional or alternative examples, the cathode (second catalyst layer 912-2) is coated with an ion exchange-functionalized PCTFE polymer. Due to the intrinsic nonpermeability of PCTFE to molecular hydrogen, second catalyst layer 912-2, when coated with an ion exchange-functionalized PCTFE polymer, only minimally allows hydrogen absorption and permeation. Thus, the HER becomes faster with faster ejection of hydrogen. As a result, the active concentration of hydrogen at cathode surfaces remains low, thereby minimizing or stopping altogether any hydrogen crossover to the anode side. Minimizing or stopping hydrogen crossover also increases safety of PEM water electrolysis system 900.[000147] FIG. 10 shows an illustrative proton exchange membrane fuel cell 1000 (PEM fuel cell 1000) including ion exchange-functionalized PCTFE polymer PEMs and / or ionomers. PEM fuel cell 1000 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 1000 is merely illustrative and not limiting.[000148] As shown in FIG. 10, PEM fuel cell 1000 includes a membrane electrode assembly 1002 (MEA 1002), porous transport layers 1004-1 and 1004-2 (e.g., gas diffusion layers), bipolar plates 1006-1 and 1006-2. An electrical load 1008 may be electrically connected to MEA 1002 and driven by PEM fuel cell 1000. PEM fuel cell 1000 may also include additional or alternative components not shown in FIG. 10 as may serve a particular implementation.[000149] MEA 1002 includes a PEM 1010 positioned between a first catalyst layer 1012-1 and a second catalyst layer 1012-2. PEM 1010 electrically isolates first catalyst layer 1012-1 from second catalyst layer 1012-2 while providing selective conductivity of cations, such as protons (H+), and while being impermeable to gases such as hydrogen and oxygen. PEM 1010may be implemented by any suitable PEM described herein, including any ion exchange- functionalized PCTFE polymer described or contemplated herein.[000150] First catalyst layer 1012-1 and second catalyst layer 1012-2 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 1012-1 and second catalyst layer 1012-2 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 ion exchange-functionalized PCTFE polymers described herein, or by any other suitable ionomers. [000151] MEA 1002 is placed between porous transport layers 1004-1 and 1004-2, which are in turn placed between bipolar plates 1006-1 and 1006-2 with flow channels 1014 located in between. In MEA 1002, first catalyst layer 1012-1 functions as a cathode and second catalyst layer 1012-2 functions as an anode. First catalyst layer / cathode 1012-1 and second catalyst layer / anode 1012-2 are electrically connected to load 1008, and electricity generated by PEM fuel cell 1000 drives load 1008.[000152] During operation of PEM fuel cell 1000, hydrogen gas (H2) flows into the anode side of PEM fuel cell 1000 and oxygen gas (O2) flows into the cathode side of PEM fuel cell 1000. At second catalyst layer / anode 1012-2, 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 1012-2:2 H2-> 4 H++ 4 e[000153] The protons are conducted from anode 1012-2 to first catalyst layer / cathode 1012-1 through PEM 1000, and the electrons are conducted from second catalyst layer / anode 1012-2 to first catalyst layer / cathode 1012-1 around PEM 1010 through a conductive path and load 1008. At first catalyst layer / cathode 1012-1, 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 1012-1:O2+ 4 H++ 4 e -> 2 H2O[000154] Thus, the overall electrochemical reaction for the PEM fuel cell 1000 is: 2 H2+ O2-> 2 H2O[000155] In the overall reaction, PEM fuel cell 1000 produces water at first catalyst layer / cathode 1012-1. Water may flow from first catalyst layer / cathode 1012-1 to second catalyst layer / anode 1012-2 through PEM 1010 and may be removed through outlets at thecathode side and / or anode side of PEM fuel cell 1000. The overall reaction generates electrons at the anode that drive load 1008.[000156] In the examples of FIGS. 9 and 10, MEA 902 and MEA 1002 include catalyst layers 912 / 1012 formed on PEM 910 / 1010. In alternative configurations, catalyst layers 912 / 1012 may be coated on PEM 910 / 1010 to thereby form a catalyst coated membrane (COM). For example, catalyst layers 912 / 1012 may be formed in a one-pot process or in stages and sprayed onto PEM 910 / 1010.[000157] The ion exchange-functionalized PCTFE polymers have been described herein for use in electrochemical cell applications, such as water electrolysis and hydrogen fuel cell applications. However, ion exchange-functionalized PCTFE polymers may also be used in other applications. For example, the pKa of the ion exchange-functionalized PCTFE polymers can be controlled to a desired level for many applications, including electrochemical processes for ammonia production. For example, the pKa of the ion exchange-functionalized PCTFE polymers can be controlled to be higher than the pKa of conventional PEMs currently used in ammonia synthesis processes.[000158] Short chain, low molecular weight ion exchange-functionalized PCTFE oligomers (also called PCTFE oils) functionalized as described herein may also be used as cross-linking reinforcement agents with various polymers, such as polybenzimidazole (PBI) polymers. Ion exchange-functionalized PCTFE oligomers may also be used as coating agents or surfacemodification agents for various perfluorinated membranes and ionomers for enhanced functional applications.[000159] Other ion exchange-functionalized PCTFE polymers may also be synthesized in a manner similar to the reaction schemes described herein (e.g., using PCTFE as a starting material) and used for alkaline anion exchange membranes (AEMs). However, the ion exchange-functionalized PCTFE polymers for AEMs contain different functional groups than the acidic functional groups described above.[000160] In some examples, an ionomer for use in PEM and / or AEM applications is a blend of unfunctionalized PCTFE and / or PCTFE ionomer with another ionomer, natural or synthetic, including but not limited to a PFSA ionomer (e.g., Nation™, Aquivion®, etc.) and / or an ion exchange-functionalized PCTFE polymer as described herein. Blending PCTFE, whether in unfunctionalized or ionomer form as described herein, with one or more other ionomers helps enhance performance, tailor casting and mechanical properties, and minimize or prevent hydrogen crossover. In some examples, an ion exchange-functionalized PCTFE polymer may be used to reinforce ionomers and membranes formed of other ionomers. For example, an ion exchange-functionalized PCTFE polymer may be used to cast (stretched or not stretched) reinforced layers to reinforce PEMs and ionomer layers formed of other ionomers.[000161] In some embodiments, the polymers, ionomers, and membranes described herein may be used in other applications, such as separation membranes in batteries (e.g., lithium ion batteries), in flow batteries, and / or in the synthesis of ammonia.[000162] It will be recognized that any of the intermediate acid-functionalized PCTFE polymers described herein can be used as an ionomer in electrochemical and other applications without converting the pendant acid groups to ion exchange groups. Additionally, it will be understood that the reaction steps described herein can be performed independently and separately and are not mutually dependent. For example, any of the reaction schemes described herein can be modified to be performed using an acid-functionalized PCTFE polymer as a starting material.[000163] In the preceding description, various exemplary embodiments have been described with reference to the accompanying drawings. It will, however, be evident that various modifications and changes may be made thereto, and additional embodiments may be implemented, without departing from the scope of the claims that follow. For example, certain features of one embodiment described herein may be combined with or substituted for features of another embodiment described herein. The description and drawings are accordingly to be regarded in an illustrative rather than a restrictive sense.[000164] Advantages and features of the present disclosure can be further described by the following examples:[000165] Example 1. An ion exchange-functionalized poly(chlorotrifluoroethylene) (PCTFE) polymer molecule comprising: a PCTFE main chain; and a side group comprising an ion exchange group and a linker that links the ion exchange group to the PCTFE main chain. [000166] Example 2. The ion exchange-functionalized PCTFE polymer molecule of example1, wherein the ion exchange group comprises a tetravalent boron group.[000167] Example 3. The ion exchange-functionalized PCTFE polymer molecule of example2, wherein the tetravalent boron group has the general formula --BX3 wherein each X is independently an alkyl group, an alkoxy group, an alkyloxycarbonyl group, an aryl group, an aryloxy group, a hydroxyl group, a fluoro group, a cyano group, or a pentafluorophenyl group. [000168] Example 4. The ion exchange-functionalized PCTFE polymer molecule of example 2, wherein the tetravalent boron group is a boron trifluoride group having the general formula - BF3.[000169] Example 5. The ion exchange-functionalized PCTFE polymer molecule of example 1, wherein the ion exchange group comprises a metal fluoride having the general formula -MF3 or -MF4 where M is a multivalent metal atom.[000170] Example 6. The ion exchange-functionalized PCTFE polymer molecule of any of the preceding examples, wherein the side group comprises an oxygen atom bonded to a carbon atom of the PCTFE main.[000171] Example 7. The ion exchange-functionalized PCTFE polymer molecule of any of the preceding examples, wherein the linker comprises an aryloxy group or an arylsulfonyl group.[000172] Example 8. The ion exchange-functionalized PCTFE polymer molecule of any of the preceding examples, wherein the linker comprises a sulfonate group or a sulfonyl group.[000173] Example 9. The ion exchange-functionalized PCTFE polymer molecule of any of the preceding examples, wherein the ion exchange group is counterbalanced by a cation.[000174] Example 10. The ion exchange-functionalized PCTFE polymer molecule of any of the preceding examples, wherein the PCTFE main chain comprises one or more residual chlorine atoms.[000175] Example 11. The ion exchange-functionalized PCTFE polymer molecule of any of the preceding examples, further comprising a plurality of the side groups, wherein a degree of loading of side groups on the PCTFE main chain ranges from about 95% to about 90%.[000176] Example 12. A method of making an ion exchange-functionalized PCTFE polymer, comprising: substituting a chlorine atom of a PCTFE polymer molecule with a side group comprising an ion exchange group and a linker that links the ion exchange group to a main chain of the PCTFE polymer molecule.[000177] Example 13. The method of example 12, wherein the substituting comprises: in a first step, substituting the chlorine atom of the PCTFE polymer with a linking agent to form an intermediate linker presenting a pendant acid group or a pendant hydroxyl group.[000178] Example 14. The method of example 13, wherein the substituting further comprises: in a second step, reacting an ion exchange agent with the pendant acid group or the pendant hydroxyl group to form the ion exchange group.[000179] Example 15. The method of example 14, wherein the ion exchange agent comprises a trivalent boron compound having general formula BX3 wherein each X is independently an alkyl group, an alkoxy group, an alkyloxycarbonyl group, an aryl group, an aryloxy group, a hydroxyl group, a fluoro group, a cyano group, or a pentafluorophenyl group.[000180] Example 16. The method of example 14, wherein the ion exchange agent comprises boron trifluoride.[000181] Example 17. The method of example 14, wherein the ion exchange agent comprises a metal fluoride having the general formula MF3 or MF4 where M is a multivalent metal atom.[000182] Example 18. The method of example 14, wherein the linking agent comprises an aryl group, a linking group, and the pendant acid group.[000183] Example 19. The method of any of examples 13 to 18, wherein the pendant acid group comprises a sulfonic acid group, a carboxylic acid group, or a phosphinic acid group.[000184] Example 20. The method of any of examples 13 to 18, wherein the linking agent comprises hydroxybenzenesulfonic acid, phenolic acid, mercaptobenzenesulfonic acid, mercaptobenzoic acid, hydroxyphenylboronic acid, or mercaptophenylboronic acid.[000185] Example 21. The method of any of examples 13 to 20, wherein, in the first step, the linking agent is rate limiting.[000186] Example 22. The method of any of examples 13 to 18, 20, and 21, wherein the linking agent comprises an arylboronic acid comprising an aryl group and a pendant boronic acid group, wherein the pendant acid group is the boronic acid group.[000187] Example 23. The method of example 22, wherein the substituting further comprises: in a second step, performing a fluoride treatment to convert the boronic acid group to a tetravalent boron group.[000188] Example 24. The method of example 23, wherein: the fluoride treatment comprises reacting the boronic acid group with a fluoride reagent; and the fluoride reagent comprises at least one of hydrogen fluoride (HF), a metal fluoride, ammonium fluoride (NH4F), ammonium bifluoride (NH4F2), or a tetraalkylammonium fluoride.[000189] Example 25. The method of example 24, wherein the fluoride reagent comprises a metal fluoride comprising lithium fluoride (LiF), sodium fluoride (NaF), potassium fluoride (KF), potassium bifluoride (KHF2), or cesium fluoride (CsF).[000190] Example 26. The method of example 25, further comprising: protonating the ion exchange group to replace a counter cation derived from the fluoride reagent with a proton (H+).[000191] Example 27. The method of example 13, wherein the first step comprises: a first sub-step comprising substituting the chlorine atom with a thiol group; and a second sub-step comprising oxidizing the thiol group to form a pendant sulfonic acid group, wherein the intermediate linker comprises the pendant sulfonic acid group.[000192] Example 28. 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 comprises an ion exchange-functionalized poly(chlorotrifluoroethylene) (PCTFE) polymer comprising: a PCTFE main chain; and a side group comprising an ion exchange group and a linker that links the ion exchange group to the PCTFE main chain.[000193] Example 29. The membrane electrode assembly of example 28, wherein the ion exchange group comprises a tetravalent boron group.

Claims

CLAIMSWhat is claimed is:

1. An ion exchange-functionalized poly(chlorotrifluoroethylene) (PCTFE) polymer molecule comprising: a PCTFE main chain; and a side group comprising an ion exchange group and a linker that links the ion exchange group to the PCTFE main chain.

2. The ion exchange-functionalized PCTFE polymer molecule of claim 1 , wherein the ion exchange group comprises a tetravalent boron group.

3. The ion exchange-functionalized PCTFE polymer molecule of claim 2, wherein the tetravalent boron group has the general formula — BX3 wherein each X is independently an alkyl group, an alkoxy group, an alkyloxycarbonyl group, an aryl group, an aryloxy group, a hydroxyl group, a fluoro group, a cyano group, or a pentafluorophenyl group.

4. The ion exchange-functionalized PCTFE polymer molecule of claim 2, wherein the tetravalent boron group is a boron trifluoride group having the general formula — BF3.

5. The ion exchange-functionalized PCTFE polymer molecule of claim 1 , wherein the ion exchange group comprises a metal fluoride having the general formula — MF3 or — MF4 where M is a multivalent metal atom.

6. The ion exchange-functionalized PCTFE polymer molecule of claim 1 , wherein the side group comprises an oxygen atom bonded to a carbon atom of the PCTFE main.

7. The ion exchange-functionalized PCTFE polymer molecule of claim 1 , wherein the linker comprises an aryloxy group or an arylsulfonyl group.

8. The ion exchange-functionalized PCTFE polymer molecule of claim 1 , wherein the linker comprises a sulfonate group or a sulfonyl group.

9. The ion exchange-functionalized PCTFE polymer molecule of claim 1 , wherein the ion exchange group is counterbalanced by a cation.

10. The ion exchange-functionalized PCTFE polymer molecule of claim 1 , wherein the PCTFE main chain comprises one or more residual chlorine atoms.

11. The ion exchange-functionalized PCTFE polymer molecule of claim 1 , further comprising a plurality of the side groups, wherein a degree of loading of side groups on the PCTFE main chain ranges from about 95% to about 90%.

12. A method of making an ion exchange-functionalized PCTFE polymer, comprising: substituting a chlorine atom of a PCTFE polymer molecule with a side group comprising an ion exchange group and a linker that links the ion exchange group to a main chain of the PCTFE polymer molecule.

13. The method of claim 12, wherein the substituting comprises: in a first step, substituting the chlorine atom of the PCTFE polymer with a linking agent to form an intermediate linker presenting a pendant acid group.

14. The method of claim 13, wherein the substituting further comprises: in a second step, reacting an ion exchange agent with the pendant acid group to form the ion exchange group.

15. The method of claim 14, wherein the ion exchange agent comprises a trivalent boron compound having general formula BX3 wherein each X is independently an alkyl group, an alkoxy group, an alkyloxycarbonyl group, an aryl group, an aryloxy group, a hydroxyl group, a fluoro group, a cyano group, or a pentafluorophenyl group.

16. The method of claim 14, wherein the ion exchange agent comprises boron trifluoride.

17. The method of claim 14, wherein the ion exchange agent comprises a metal fluoride having the general formula MF3 or MF4 where M is a multivalent metal atom.

18. The method of claim 13, wherein the linking agent comprises an aryl group, a linking group, and the pendant acid group.

19. The method of claim 13, wherein the pendant acid group comprises a sulfonic acid group, a carboxylic acid group, or a phosphinic acid group.

20. The method of claim 13, wherein the linking agent comprises hydroxybenzenesulfonic acid, phenolic acid, mercaptobenzenesulfonic acid, mercaptobenzoic acid, hydroxyphenylboronic acid, or mercaptophenylboronic acid.

21. The method of claim 13, wherein, in the first step, the linking agent is rate limiting.

22. The method of claim 13, wherein the linking agent comprises an arylboronic acid comprising an aryl group and a pendant boronic acid group, wherein the pendant acid group is the boronic acid group.

23. The method of claim 22, wherein the substituting further comprises: in a second step, performing a fluoride treatment to convert the boronic acid group to a tetravalent boron group.

24. The method of claim 23, wherein: the fluoride treatment comprises reacting the boronic acid group with a fluoride reagent; and the fluoride reagent comprises at least one of hydrogen fluoride (HF), a metal fluoride, ammonium fluoride (NH4F), ammonium bifluoride (NH4F2), or a tetraalkylammonium fluoride.

25. The method of claim 24, wherein the fluoride reagent comprises a metal fluoride comprising lithium fluoride (LiF), sodium fluoride (NaF), potassium fluoride (KF), potassium bifluoride (KHF2), or cesium fluoride (CsF).

26. The method of claim 25, further comprising: protonating the ion exchange group to replace a counter cation derived from the fluoride reagent with a proton (H+).

27. The method of claim 13, wherein the first step comprises: a first sub-step comprising substituting the chlorine atom with a thiol group; and a second sub-step comprising oxidizing the thiol group to form a pendant sulfonic acid group, wherein the intermediate linker comprises the pendant sulfonic acid group.

28. 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 comprises an ion exchange-functionalized poly(chlorotrifluoroethylene) (PCTFE) polymer comprising: a PCTFE main chain; and a side group comprising an ion exchange group and a linker that links the ion exchange group to the PCTFE main chain.

29. The membrane electrode assembly of claim 28, wherein the ion exchange group comprises a tetravalent boron group.

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