Boronic acid-functionalized carbon materials

Abstract

A boronic acid-functionalized carbon material includes a carbon material and boronic acid groups linked to a surface of the carbon material. The carbon material may include carbon black, graphite, or graphene. The boronic acid groups comprise trivalent boronic acid groups and / or tetravalent boronic acid groups. The boronic acid-functionalized carbon material may be used as a catalyst support, a catalyst binder, and / or an ionomer for cation transport.

Description

RELATED APPLICATIONS

[0001] This application claims priority to U.S. Provisional Patent Application No. 63 / 604,311, filed November 30, 2023, which is hereby incorporated by reference in its entirety. BACKGROUND INFORMATION

[0002] In some electrochemical cells, such as hydrogen fuel cells and water electrolysis systems, proton exchange membranes (PEMs) are used to selectively transport protons. 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) based PEMs, such as Nafion™ (The Chemours Company, Wilmington, Delaware) and Aquivion® (Solvay SA Corporation, Brussels, Belgium)), contain a poly(tetrafluoroethylene) (PTFE) porous framework with 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 catalyst particles, such as metals, metal alloys, or metal oxides. The catalyst particles may be supported on a catalyst solid support, which generally includes an electrically conductive, high surface-area carbon material (e.g., carbon black, graphite, or graphene). The electrochemical catalysts reduce the activation energy needed to carry out electrochemical reactions at the electrodes, such as the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER) in water electrolysis applications and the hydrogen oxidation reaction (HOR) and the oxygen reduction reaction (ORR) in fuel cell applications.

[0004] In some applications, the catalyst layer includes a supported catalyst mixed with an ionomer, an ion-conducting polymer. The ionomer binds the catalysts within the electrode, binds the catalyst layer on the PEM, and provides a pathway for cations (e.g., protons), thereby improving cation conductivity. In some MEAs, the catalyst layers are formed separately from the PEM and layered on the PEM in the MEA stack. In other MEAs, the catalyst layers are coated on the PEM to form catalyst-coated membranes (CCMs).

[0005] Water electrolysis and fuel cell applications involve strong oxidation and reduction chemistries under ambient to high temperature and acidic conditions. Therefore, the materials used in electrochemical cells, such as PEMs, ionomers, catalysts, catalyst supports, electrodes, and gas diffusion layers, should remain robust under the harsh reaction conditions of redox stress. SUMMARY

[0006] The following description presents a simplified summary of one or more aspects of the apparatuses, compositions, and / or methods described herein in order to provide a basic understanding of such aspects. This summary is not an extensive overview of all contemplated aspects and is intended to neither identify key or critical elements of all aspects nor delineate the scope of any or all aspects. Its sole purpose is to present some concepts of one or more aspects of the apparatuses, compositions, and / or methods described herein in a simplified form as a prelude to the more detailed description that is presented below.

[0007] In some illustrative examples, a boronic acid-functionalized carbon material includes a carbon material; and a boronic acid group linked to a surface of the carbon material.

[0008] In some illustrative examples, a method of making a boronic acid-functionalized carbon material includes reacting an oxidized carbon material with a boronic compound to form a trivalent boronic acid-functionalized carbon material including trivalent boronic acid groups linked to a surface of the carbon material.

[0009] In some illustrative examples, the method further includes performing a fluoride treatment on the trivalent boronic acid-functionalized carbon material to convert a trivalent boronic acid group to a tetravalent boronic acid group.

[0010] In some illustrative examples, a catalyst support includes a carbon material; and a boronic acid group linked to a surface of the carbon material.

[0011] In some illustrative examples, a gas diffusion layer for an electrochemical cell includes a carbon material; and a boronic acid group linked to a surface of the carbon material. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] The accompanying drawings illustrate various embodiments and are a part of the specification. The illustrated embodiments are merely examples and do not limit the scope of the disclosure. Throughout the drawings, identical or similar reference numbers designate identical or similar elements.

[0013] FIG. 1 shows an illustrative reaction scheme for synthesis of an oxidized carbon material.

[0014] FIG. 2 shows an illustrative reaction scheme for synthesis of a boronic acid-functionalized carbon material having trivalent boronic acid groups at a surface of the carbon material.

[0015] FIG. 3 shows an illustrative reaction scheme for synthesis of a boronic acid-functionalized carbon material having both trivalent boronic acid groups and tetravalent boronic acid groups at a surface of the carbon material.

[0016] FIG. 4 shows another illustrative reaction scheme for the synthesis of a boronic acid-functionalized carbon material having tetravalent boronic acid groups in a one pot process.

[0017] FIG. 5 shows an illustrative proton exchange membrane water electrolysis system.

[0018] FIG. 6 shows an illustrative proton exchange membrane fuel cell. DETAILED DESCRIPTION

[0019] Described herein are boronic acid-functionalized carbon materials, including boronic acid-functionalized carbon black, boronic acid-functionalized graphite, and boronic acid-functionalized graphene. The boronic acid-functionalized carbon materials may be used in various applications, including in electrochemical cells, such as water electrolysis systems and / or fuel cell systems.

[0020] 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.

[0021] 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.

[0022] As used herein, “polymer molecule” or “macromolecule” refers to a molecule of high relative molecular mass, the structure of which comprises a relatively large repetition of units (e.g., about 60 or more monomer units) derived, actually or conceptually, from molecules of low relative molecular mass (e.g., monomer molecules).

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

[0024] As used herein, “oligomer molecule” refers to a molecule of intermediate relative molecular mass, the structure of which comprises a relatively small repetition of units (e.g., about 5 to about 60 monomer units) derived, actually or conceptually, from molecules of lower relative molecular mass (e.g., monomer molecules).

[0025] 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.

[0026] As used herein, “ionomer” refers to a polymer composed of ionomer molecules.

[0027] 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 (e.g., a sulfonic acid group, a carboxylic acid group, a phosphoric acid group, a boronic acid group, etc.).

[0028] 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.

[0029] 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.

[0030] As used herein, a “catalyst support” refers to a substance, exclusive of a catalyst particle, that may be used to support catalyst particles (e.g., a substance or material to which catalyst particles may be bound or on which catalyst particles may be supported). Examples of catalyst supports include, without limitation, carbon-based materials (e.g., carbon black, graphite, carbon nanotubes, graphene, and / or boronic acid-functionalized carbon 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.

[0031] 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).

[0032] 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.

[0033] As used herein, “metal” includes alkali metals, alkaline earth metals, transition metals, lanthanides, actinides, and post-transition metals.

[0034] As used herein, “transition metals” refers to elements of the d-block of the periodic table (Groups 3 to 12, inclusive).

[0035] As used herein, “post-transition metals” refers to aluminum, gallium, indium, tin, thallium, lead, bismuth, and polonium.

[0036] As used herein, “metalloids” refers to boron, silicon, germanium, arsenic, antimony, tellurium, and astatine.

[0037] As used herein, “platinum group metals” or “PGMs” refers to platinum, palladium, iridium, ruthenium, osmium, and rhodium.

[0038] As used herein, a “composite” means a material having a combination of two or more distinct constituent materials, each of which retains its own distinctive properties, but which has properties that the constituent materials do not have acting alone.

[0039] 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.

[0040] 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.

[0041] 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.

[0042] 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.

[0043] 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.

[0044] 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.

[0045] 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.

[0046] 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, cycloalkynyls, 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.

[0047] 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.

[0048] 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.

[0049] 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.

[0050] Any of the above groups may be optionally substituted, in whole or in part. Examples of substituents include, without limitation, aliphatic, alicyclic, heteroaliphatic, heteroalicyclic, 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 Rx independently includes, but is not limited to, hydrogen, aliphatic, alicyclic, heteroaliphatic, heteroalicyclic, aryl, heteroaryl, alkylaryl, or alkylheteroaryl, wherein any of the aliphatic, alicyclic, heteroaliphatic, heteroalicyclic, alkylaryl, or alkylheteroaryl 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.

[0051] Boronic acid-functionalized carbon materials will now be described. A boronic acid-functionalized carbon material includes a carbon material that is functionalized with one or more boronic acid groups at a surface of the carbon material. The boronic acid groups may be linked to the carbon material by an ether linkage or an ester linkage. Any surface of the carbon material may be oxidized, including an outer surface of the carbon material and / or a pore surface (e.g., a surface of the carbon material at an interface with a pore within the carbon material).

[0052] Carbon materials are formed of hexagonal carbon rings with delocalized pi electrons, and include crystalline, semi-crystalline, paracrystalline, and amorphous carbon allotropes. Examples of carbon include, without limitation, carbon black, graphite, graphene, charcoal, activated charcoal, soot, and coal, as well as carbon molecules, such as fullerenes and carbon nanotubes, and materials and structures formed of carbon molecules. Carbon black is a paracrystalline carbon material containing randomly distributed graphite primary particles arranged as aggregates and / or agglomerates. Carbon black can exist in structures that are spheroidal, ellipsoidal, linear, or branched. Carbon black is highly porous and electrically conducting. The electrical conductivity of carbon black increases with smaller and more spherical structures. Graphite is a crystalline allotrope of carbon arranged in multiple carbon layers. Each carbon atom within a layer is covalently bonded to three other carbon atoms in hexagonal rings. Adjacent layers are bonded by weak van der Waals forces between layers. Graphite is a stable material under redox conditions and is highly electrically conducting. Graphene is a carbon material generally formed of one to ten carbon layers. Each carbon atom within a layer is covalently bonded to three other carbon atoms in hexagonal rings. Graphene is a highly durable material under redox conditions with very high electrical conductivity. Fullerenes are a class of molecules including polyhedral cages made up entirely of n three-coordinate carbon atoms and having 12 pentagonal and (n / 2 - 10) hexagonal faces, where n s 20. Examples of fullerenes include, without limitation, C20, C6o (buckminsterfullerene), C70, C72, C76, Cm, and Cioo- Carbon nanotubes include hexagonal rings and include single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs). Carbon materials may have any suitable shape, structure, and / or form, such as a film, a particle, a sphere, an ellipsoid, a carbon powder, a carbon cloth, a nanofilm, a nanotube, a nanobud, a nanoribbon, a scaffold, or a nanofoam. Carbon materials also include composites of carbon material and another material, such as a polymer or a ceramic material. For example, a carbon material may include a carbon-polymer composite, such as a polymer matrix reinforced with carbon fibers or graphite particles. In some examples a carbon material, such as graphite or a nanotube structure, is porous due to interconnected voids or pores within the bulk of the carbon material.

[0053] Carbon materials may be oxidized to form an oxidized carbon material, wherein surfaces of the carbon material are functionalized with oxide groups, such as but not limited to hydroxyl groups (OH), carbonyl groups (—C(=O)), carboxyl groups (—C(=O)OH), and epoxide groups (e.g., epoxide bridges between adjacent carbon atoms). Oxidation of carbon black generates oxide groups on the graphitic surfaces of carbon black to add new physical and chemical properties while retaining the electrical conductivity of carbon black. Oxidation of graphite forms a graphite oxide (GO) and adds oxide groups onto the graphite surfaces to add new physical and chemical properties while retaining its electrical conductivity. Graphene oxide (GrO) is a derivative of graphene containing oxide groups at the surfaces. Graphene oxide may be made from graphite oxide by exfoliating the multiple-layered sheets of graphite oxide into a single orfew-atom layer, such as by chemical exfoliation. In some examples, chemical exfoliation involves dispersion of graphite oxide in a polar solvent followed by ultra-sonication and / or mechanical agitation. Alternatively, graphene oxide may be made by oxidization of graphene. The degree of loading of oxide groups on carbon materials, and the types of oxide groups added to the carbon material surfaces, may be controlled through various oxidation processes to achieve desired physical, chemical, and / or electrical properties.

[0054] As explained in more detail herein, the oxide groups of oxidized carbon materials may be further used for covalent tethering of boronic moieties, such as boric acid or a boronic acid, to form boronic acid groups. For example, an oxidized carbon material can be functionalized with trivalent boronic acid groups and / or tetravalent boronic acid groups to form a boronic acid-functionalized carbon material. Partial covalent modification of the oxide groups of an oxidized carbon material using nitrogen, sulfur, and / or phosphorus ligands is also possible to thereby form nitrogen-, sulfur-, and / or phosphorus-containing functional groups, such as sulfonic acid and phosphonic acid functional groups, as may be suitable for their applications.

[0055] FIG. 1 shows an illustrative reaction scheme for synthesis of an oxidized carbon material (e.g., oxidized carbon black, graphite oxide, or graphene oxide). In FIG. 1, a portion of a single layer (e.g., an outer surface layer) of a carbon material is shown as a representative starting material. It will be understood that the carbon material may have any other structure, shape, and size not shown in FIG. 1, and any suitable carbon material may be used.

[0056] As shown in FIG. 1, the carbon material undergoes an oxidation process to functionalize the carbon material with oxide groups at a surface of the carbon material. Any suitable oxidation process for carbon materials may be used, including “top-down” methods involving the use of strong oxidizing agents, such as but not limited to Brodie’s method, Hummer’s method, and variations of the foregoing methods. Brodie’s method oxidizes carbon material by treating the carbon material with a mixture of potassium chlorate (KCIOs) and fuming nitric acid (HNO3). Hummer’s method treats the carbon material with a mixture of sulfuric acid (H2SO4), sodium nitrate (NaNOs), and potassium permanganate (KMnO4). Alternatively to “top-down” methods that oxidize a carbon material using an oxidizing agent, an oxidized carbon material may be synthesized by a “bottom-up” method, such as the Tang-Lau method and variations thereof. The Tang-Lau method uses glucose as the sole starting reagent in a self-assembly method and is described in Tang, L. et al. (2012). "Bottom-up synthesis of large-scale graphene oxide nanosheets," Journal of Materials Chemistry, 22 (12), 5676. Processes for synthesis of oxidized carbon materials, including Brodie’s method, Hummer’s method, the Tang-Lau method, and variations of the foregoing, are known to persons of ordinary skill in the art and thus will not be described further.

[0057] While the oxidized carbon material of FIG. 1 shows six oxygenated functional groups (two hydroxyl groups, two carboxyl groups, and two epoxide groups), the reaction chemistry may be controlled as desired to tune the degree of oxidation and to tune the types of oxide groups and / or to tune the carbon to oxygen ratio (C:O) of the carbon material. In some examples, the carbon to oxygen ratio of the carbon material ranges from 10:1 to 1:1, from 9:1 to 1:1, from 5:1 to 1:1, from 4:1 to 1:1, from 3:1 to 1:1, from 5:1 to 2:1, from 4:1 to 2:1, or from 3:1 to 2:1.

[0058] The oxide groups at the surface of the oxidized carbon material may be used for boron attachment while maintaining the layer structure of the carbon material. As mentioned, boronic acid-functionalized carbon materials include a carbon material and boronic acid groups linked to the carbon material at a surface of the carbon material. In some examples, the boronic acid groups are linked to the carbon material (e.g., to a carbon atom in a backbone of the carbon material) by way of a linker, such as an ether linkage, an ester linkage, a boronic acid linker (e.g., a derivative of a boronic acid), or a polymeric boric acid linker (e.g., a derivative of a polymeric boric acid). Boronic acid groups include trivalent boronic acid groups and tetravalent boronic acid groups.

[0059] Trivalent boronic acid groups have a terminal boronic acid group wherein the boron atom participates in three covalent bonds and has at least one pendant hydroxyl group. Examples of trivalent boronic acid groups include, without limitation, —B(OH)2, —B(OH)R, and —B(OH)(OR), wherein 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. Trivalent boronic acid groups may be linked to the carbon material (e.g., to a carbon atom of the carbon material backbone) by an ether linkage (—O—) or an ester linkage (—C(=O)O—).

[0060] Trivalent boronic acid-functionalized carbon materials may be used as catalyst supports. When used as catalyst supports in electrochemical applications, such as in catalyst layers / anodes / cathodes, the trivalent boronic acid-functionalized carbon materials provide enhanced catalyst performance as compared with traditional carbon-based catalyst supports. For example, the terminal trivalent boronic acid groups enhance electrical conductivity and improve catalyst binding.

[0061] Tetravalent boronic acid groups have a terminal fluoroboric acid group wherein the boron atom is tetra-coordinated and is covalently bonded to at least one fluorine atom. Boron generally has three valence electrons and a ground state electron configuration of 1s22s22p1. Boron forms trivalent, trigonal neutral compounds in which boron has three covalent bonds through sp2 hybridization, such as boric acid (B(OH)3) and trivalent boronic acid groups. The sp2 hybridized 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 is also synonymously referred to herein as tetra-coordinated boron. Examples of tetravalent boronic acid groups include, without limitation, —BF3, —BF2R, —BF2(OR), and —BF(OH)2, wherein 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. Tetravalent boronic acid groups may be linked to the carbon material (e.g., to a carbon atom of the carbon material) by an ether linkage (—O—) or an ester linkage (—C(=O)O—). Due to the tetra-coordinated nature of the boron atom, the tetravalent boronic acid group has a negative formal charge and may be counterbalanced by a cation, such as but not limited to H+, Li+, Na+, Al3+, Ni2+, or any other suitable cation, including cations used in battery applications.

[0062] Tetravalent boronic acid-functionalized carbon materials may be used as an electrically conducting ionomer. The oxygen-containing linkers offer hydrophilic domains near the hydrophobic carbon material backbone, and the degree of functional modification can be used to tune the hydrophilic-lipophilic balance (HLB) of the carbon material. Moreover, the oxygen-containing linkers are compatible with electrochemical applications as ionomers and PEMs. Moreover, the ion exchange capacity of the modified carbon materials may be controlled and tuned as desired.

[0063] In some examples, boronic acid-functionalized carbon materials include both trivalent boronic acid groups and tetravalent boronic acid groups. In these examples, a modified carbon material serves dual functions as an electrically-conducting catalyst support with enhanced catalyst binding and as a proton-conducting ionomer. Accordingly, such modified carbon materials may be used in electrochemical cells, such as in catalyst and electrode layers.

[0064] Boronic acid-functionalized carbon materials may be synthesized in any suitable way. In some examples, a trivalent boronic acid-functionalized carbon material is synthesized by combining a boronic compound with an oxidized carbon material (e.g., oxidized carbon black, graphite oxide, or graphene oxide) having oxide groups at a surface of the carbon material. The boronic compound reacts with the pendant hydroxyl groups of the oxide groups in a condensation reaction to form a trivalent boronic acid group that is linked to the carbon material by an ether linkage or an ester linkage.

[0065] A boronic compound includes, without limitation, boric acid (B(OH)3), a boronic acid, a boronic ester, or a polymeric boric acid. A boronic acid has the general formula B(OH)(R1)(R2) where R1 is a hydroxyl group, an alkyl group, and / or an aryl group (such as phenyl) and R2 is an alkyl group and / or an aryl group (such as phenyl). In some examples, R1 and / or R2 have one to twenty carbon atoms. In other examples, R1 and / or R2 have one to ten carbon atoms. In further examples, R1 and / or R2 have one to five carbon atoms. A boronic ester has the general formula B(OH)(R3)(R4) where R3 is a hydroxyl group, an alkoxy group, or an aryloxy group and R4 is an alkoxy group or an aryloxy group. In some examples, R3 and / or R4 have one to ten carbon atoms. In other examples, R3 and / or R4 have one to five carbon atoms. In further examples, R3 and / or R4 have one to three carbon atoms (e.g., a methyl group, an ethyl group, a propyl group, or an isopropyl group). A polymeric boric acid is formed of a boric acid monomer. For example, metaboric acid (formula I) is a cyclic trimer of boric acid and tetraboric acid (formula II) is a bicyclic tetramer of boric acid: OH O'%                  O \0 B-OH B B                       xBs LO HO' "O' "OH (I), and H0 O-B           (II). A polymeric metaboric acid is formed by further linear polymerization of the metaboric acid unit and has the general formula HO[-B(BOH)2O3O-]nH. A polymeric tetraboric acid is also formed by further linear polymerization of the tetraboric acid unit. Other polymeric boric acid structures are contemplated herein and may be used as the boronic compound.

[0066] In some examples, a tetravalent boronic acid-functionalized carbon material is synthesized by performing a fluoride treatment on a trivalent boronic acid-functionalized carbon material to convert trivalent boronic acid groups to tetravalent boronic acid groups. The fluoride treatment functionalizes the boronic acid-functionalized carbon material with a fluoroboric acid group having a tetra-coordinated boron atom. The fluoride treatment may be performed in any suitable way.

[0067] In some examples, the fluoride treatment comprises combining a trivalent boronic acid-functionalized carbon material with a fluoride reagent. In some examples, the fluoride reagent is hydrogen fluoride (HF). In other examples, the fluoride reagent is a metal fluoride. The metal 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, ora post-transition metal. Examples of a metal fluoride include, without limitation, lithium fluoride (LiF), sodium fluoride (NaF), potassium fluoride (KF), potassium bifluoride (KHF2), and cesium fluoride (CsF). In further examples, the fluoride reagent is ammonium fluoride (NH4F), ammonium bifluoride (NH4F2), or a tetraalkylammonium fluoride having the general formula NR4F 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 using two or more different fluoride reagents (e.g., HF and NaF, HF and LiF, etc.).

[0068] The fluoride treatment adds a fluoride group to a pendant boronic acid group to form a fluoroboric acid group in which the boron atom is tetra-coordinated and covalently bonded to the fluoride group. Depending on the stoichiometry of the reaction, one or more hydroxyl groups of the boronic acid group may also be replaced by fluoride from the fluoride reagent. Thus, 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+).

[0069] 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 carbon material 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+).

[0070] In other examples of the fluoride treatment, the hydrogen atom of one or more hydroxyl groups of the pendant trivalent boronic acid group is replaced with a fluoroboric acid group. In these examples, the fluoride treatment comprises combining the trivalent boronic acid-functionalized carbon material with a fluoroboric compound of the formula BFm(OH)(3^) 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 also includes combining the trivalent boronic acid-functionalized carbon material with a boronic acid 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.

[0071] 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 (e.g., a halide).

[0072] It will be recognized that other fluoride treatments are possible and may be used. The fluoride treatment results in a carbon material functionalized with one or more tetravalent boronic acid groups linked to a surface of the carbon material. Illustrative examples of boronic acid-functionalized carbon materials and methods of making boronic acid-functionalized carbon materials will now be described.

[0073] FIG. 2 shows an illustrative reaction scheme for synthesis of a boronic acid-functionalized carbon material having trivalent boronic acid groups at a surface of the carbon material. Boric acid molecules react with hydroxyl groups to produce trivalent boronic acid groups having the general formula —OB(OH)2. Boric acid molecules also react with carboxyl groups to produce trivalent boronic acid groups having the general formula —C(=O)OB(OH)2. Boric acid molecules also react with epoxide groups to produce trivalent boronic acid groups having the general formula —OB(OH)2. The reaction with epoxide groups also produces another hydroxyl group at the surface of the carbon material, which hydroxyl group may also react with a boric acid molecule, as described herein, depending on the stoichiometry of the reaction and the desired loading of boronic acid groups. The reaction scheme of FIG. 2 may be controlled to achieve a desired loading of trivalent boronic acid groups. The boronic acid-functionalized carbon material produced by the reaction scheme of FIG. 2 may be used as an electrically conducting catalyst solid-support with enhanced catalyst binding.

[0074] FIG. 3 shows another illustrative reaction scheme for synthesis of a boronic acid-functionalized carbon material having both trivalent boronic acid groups and tetravalent boronic acid groups. In a first reaction step, the oxidized carbon material produced in the method of FIG. 1 is combined with boric acid (B(OH)3) to produce a boronic acid-functionalized carbon material having trivalent boronic acid groups, such as by performing the reaction scheme of FIG 2. In a second reaction step, a fluoride treatment is performed to convert at least some of the trivalent boronic acid groups to tetravalent boronic acid groups. In the example of FIG. 3, the fluoride treatment includes combining the trivalent boronic acid-functionalized carbon material with potassium hydrogen difluoride (KHF2) and / or hydrogen fluoride (HF) to form tetravalent boronic acid groups (e.g., boron trifluoride groups (—BF3)). The boron atoms of the tetravalent boronic acid groups are covalently bonded to three fluorine atoms and to a carbon atom of the carbon material backbone by an ester linkage. The tetravalent boronic acid groups have a negative formal charge, making these boronic acid groups atoms intrinsically acidic and ionic. As a result, the tetravalent boronic acid groups may serve as cation exchange groups (e.g., proton transport agents).

[0075] In the example shown in FIG. 3, the boronic acid-functionalized carbon material produced in the second step includes both trivalent boronic acid groups and tetravalent boronic acid groups. The degree of trivalent boronic acid group loading and tetravalent boronic acid group loading may be tuned as desired. In some examples, all or substantially all (e.g., greater than 90%, 95%, 98%, or 99%) trivalent boronic acid groups are converted to tetravalent boronic acid groups.

[0076] The boronic acid-functionalized carbon material produced by the reaction scheme of FIG. 3 includes both trivalent and tetravalent boronic acid groups and thus may be used as an all-in-one material: an electrically-conducting solid-support, a catalyst binder through boron for enhanced performance, and a cation-conducting ionomer. The ion exchange capacity (IEC) of the boronic acid-functionalized carbon material may be optimized or tuned as desired using the fluoride reagent (e.g., KHF2 or HF) as the limiting reagent.

[0077] FIG. 4 shows another illustrative reaction scheme for the synthesis of a boronic acid-functionalized carbon material having tetravalent boronic acid groups in a one pot process. In the reaction scheme of FIG. 4, the oxidized carbon material produced in the method of FIG. 1 is combined with a boronic compound (e.g., boric acid) and a fluoride reagent (e.g., KHF2) and HCI, which react with the oxide groups of the oxygenated carbon material to form tetravalent boronic acid groups (e.g., tetravalent boron trifluoride groups). The tetravalent boron atoms have a negative formal charge, making these boron atoms intrinsically acidic and ionic. As a result, the tetravalent boronic acid groups may serve as ion exchange groups (e.g., proton transport agents).

[0078] The boronic acid-functionalized carbon material produced by the reaction scheme of FIG. 4 may be used as a dual purpose electrically-conducting solid-support and protonconducting ionomer. The ion exchange capacity (IEC) of the boronic acid-functionalized carbon material may be tuned or optimized by controlled boron loading.

[0079] Various modifications may be made to the materials and reaction schemes described above. As mentioned, a boronic acid, a boronic ester, and / or a polymeric boric acid may be used in place of boric acid. A polymeric boric acid can modify surface properties of the carbon material through boron cross-linking. In some examples, the boronic acid groups are linked to the carbon backbone of the carbon material further by way of a linker. The linker may be a Ci to C30 alkyl linker chain and optionally has one or more pendant moieties, which may be the same or different and may each be independently selected from the group consisting of hydrogen, a hydroxyl group, a fluoro group, a chloro group, a dialkylamino group, a cyano group, a carboxylic acid group, a carboxylic amide group, an ester group, an alkyl group, an alkoxy group, and an aryl group. In some examples, the oxidizing agent used in the reaction scheme of FIG. 1 includes the linker. In further examples, a boronic compound that reacts with the oxide group includes the linker. For example, the linker may be a boronic acid linker (e.g., a derivative of a boronic acid) or a polymeric boric acid linker (e.g., a derivative of a polymeric boric acid).

[0080] In further examples, the tetravalent boronic acid groups of boronic acid-functionalized carbon materials may be ionically linked with any other cations other than protons, such as, but not limited to, H+, Li+, Na+, K+, Cs+, NH4+, or NR4+).

[0081] The boronic acid-functionalized carbon materials and methods of making the boronic acid-functionalized carbon materials have various benefits and advantages over conventional materials and methods of making conventional materials. The reaction schemes described herein are applicable to any standard carbon materials, including commercially available carbon materials, such as carbon powders, carbon cloths, Vulcan XC 72, and carbon-polymer composite membranes. The reagents in the reaction schemes are easy to handle and the reaction schemes are easily scalable. Moreover, the reagents are generally common and inexpensive materials, thus offering a cost reduction as compared with conventional materials used in electrochemical cells.

[0082] Additionally, the oxygen-containing linkers that link the boronic acid groups of the boronic acid-functionalized carbon materials to the carbon backbone offer hydrophilic domains in the hydrophobic carbon backbone and enable tuning of the hydrophilic-lipophilic balance (HLB). Additionally, the oxygen-containing linkers are compatible with electrochemical applications as ionomers and PEMs, such as polyfluorosulfonic acid (PFSA) PEMs (e.g., Nafion and Aquivion). The ion exchange capacity (IEC) of the boronic acid-functionalized carbon materials may be controlled and tuned up for desired or optimized performance and intended functional applications.

[0083] The oxygenated trivalent boronic acid groups are ideal catalyst supports that enhance catalyst performances. Additionally, the tetravalent boronic acid groups are intrinsically ionic and acidic and thus may serve as cation exchange agents.

[0084] The boronic acid-functionalized carbon materials described herein may be used in electrochemical systems, such as water electrolysis systems and fuel cell systems. For example, the boronic acid-functionalized carbon materials described herein may be used as carbon-based solid supports for catalysts. Additionally or alternatively, the boronic acid-functionalized carbon materials may be used in porous transport layers / gas diffusion layers (GDLs) (e.g., as gas diffusion electrodes) in water electrolysis systems and fuel cell systems. The boronic acid-functionalized carbon materials described herein are believed to reduce dissociation of carbon material under strong redox conditions, thus enabling use of a carbonbased GDL at the anode of an electrochemical cell to replace titanium-based porous transport layers. In some examples, gas diffusion electrodes may be directly modified following any of the above reaction schemes.

[0085] FIG. 5 shows an illustrative proton exchange membrane water electrolysis system 500 (PEM water electrolysis system 500). PEM water electrolysis system 500 uses electricity to split water into oxygen (O2) and hydrogen (H2) via an electrochemical reaction. The configuration of PEM water electrolysis system 500 is merely illustrative and not limiting.

[0086] As shown in FIG. 5, PEM water electrolysis system 500 includes a membrane electrode assembly 502 (MEA 502), porous transport layers 504-1 and 504-2 (e.g., gas diffusion layers), bipolar plates 506-1 and 506-2, and an electrical power supply 508. PEM water electrolysis system 500 may also include additional or alternative components not shown in FIG. 5 as may serve a particular implementation.

[0087] MEA 502 includes a PEM 510 positioned between a first catalyst layer 512-1 and a second catalyst layer 512-2. PEM 510 electrically isolates first catalyst layer 512-1 from second catalyst layer 512-2 while providing selective conductivity of cations, such as protons (H+), and while being impermeable to gases such as hydrogen and oxygen. PEM 510 may be implemented by any suitable organic or inorganic PEM. Illustrative examples of organic PEMs include, without limitation, synthetic polymers and natural polymers. Examples of synthetic polymers include sulfonic acid-functionalized polymers such as Nation® (available from E.L Dupont de Nemours and Company in various configurations and grades, including Nafion-H, Nafion HP Nation 117, Nation 115, Nation 212, Nation 211, Nation NE1035, Nation XL, etc.), Aquivion® (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+TM (available from lonomr Innovations, Inc.), and any combination, derivative, grade, or configuration thereof. Examples of natural polymers include, without limitation, lignin, cellulose, or chitin. Examples of inorganic PEMs include, without limitation, amorphous inorganic materials (e.g., glass, fused silica, or ceramics) and / or crystalline inorganic materials (e.g., quartz, single crystal silicon, or alumina).

[0088] First catalyst layer 512-1 and second catalyst layer 512-2 are electrically conductive electrodes that include catalyst solid supports bound with electrocatalyst particles (not shown), such as platinum group metals, metal alloys, and / or metal oxides. In some examples, the catalyst particles are supported on a catalyst support formed of a boronic acid-functionalized carbon material, as described herein.

[0089] In some examples, first catalyst layer 712-1 and / or second catalyst layer 712-2 include a supported catalyst mixed with an ionomer (an ion-conducting polymer). The ionomer binds the catalysts within the electrode, binds the catalyst layer on the PEM, and provides a pathway for cations (e.g., protons), thereby improving cation conductivity. The ionomer used in first catalyst layer 712-1 and second catalyst layer 712-2 may include any suitable ionomer, including any ionomer described herein.

[0090] In some examples, first catalyst layer 512-1 and / or second catalyst layer 512-2 includes an electrically-conductive boronic acid-functionalized carbon material comprising trivalent boronic acid groups and tetravalent boronic acid groups, as described herein. Thus, the boronic acid-functionalized carbon material functions as the catalyst support, catalyst binder, and proton conducting ionomer.

[0091] MEA 502 is placed between porous transport layers 504-1 and 504-2, which are in turn placed between bipolar plates 506-1 and 506-2 with flow channels 514-1 and 514-2 located in between bipolar plates 506 and porous transport layers 504. In some examples, porous transport layers 504-1 and / or 504-2 include an electrically-conductive boronic acid-functionalized carbon material comprising trivalent boronic acid groups and / or tetravalent boronic acid groups, as described herein.

[0092] In MEA 502, first catalyst layer 512-1 functions as an anode and second catalyst layer 512-2 functions as a cathode. When PEM water electrolysis system 500 is powered by power supply 508, an oxygen evolution reaction (OER) occurs at first catalyst layer / anode 5121, facilitated by the electrocatalysts bound to the catalyst solid supports in first catalyst layer / anode 512-1. The OER is represented by the following electrochemical half-reaction: catalyst             , 2H2O ----► O2 + 4H+ + 4e“ (1)

[0093] Protons are conducted from first catalyst layer / anode 512-1 to second catalyst layer / cathode 512-2 through PEM 510, and electrons are conducted from first catalyst layer / anode 512-1 to second catalyst layer / cathode 512-2 by conductive path around PEM 510. PEM 510 allows for the transport of protons (H+) and water from the first catalyst layer / anode 512-1 to the second catalyst layer / cathode 512-2 but is impermeable to oxygen and hydrogen. At second catalyst layer / cathode 512-2, the protons combine with the electrons in a hydrogen evolution reaction (HER), facilitated by the electrocatalysts bound to the catalyst solid supports in second catalyst layer / cathode 512-2. The HER is represented by the following electrochemical half-reaction:

[0094] The OER and HER are two complementary electrochemical reactions for splitting water by electrolysis, represented by the following overall water electrolysis reaction: 2H2O ----► 2H2 + 02 (3)

[0095] FIG. 6 shows an illustrative proton exchange membrane fuel cell 600 (PEM fuel cell 600). PEM fuel cell 600 produces electricity as a result of electrochemical reactions. In this example, the electrochemical reactions involve reacting hydrogen gas (H2) and oxygen gas (O2) to produce water and electricity. The configuration of PEM fuel cell 600 is merely illustrative and not limiting.

[0096] As shown in FIG. 6, PEM fuel cell 600 includes a membrane electrode assembly 602 (MEA 602), porous transport layers 604-1 and 604-2 (e.g., gas diffusion layers), bipolar plates 606-1 and 606-2. An electrical load 608 may be electrically connected to MEA 602 and driven by PEM fuel cell 600. PEM fuel cell 600 may also include additional or alternative components not shown in FIG. 6 as may serve a particular implementation.

[0097] MEA 602 includes a PEM 610 positioned between a first catalyst layer 612-1 and a second catalyst layer 612-2. PEM 610 electrically isolates first catalyst layer 612-1 from second catalyst layer 612-2 while providing selective conductivity of cations, such as protons (H+), and while being impermeable to gases such as hydrogen and oxygen. PEM 610 may be implemented by any suitable PEM, including any PEM described herein.

[0098] First catalyst layer 612-1 and second catalyst layer 612-2 are electrically conductive electrodes that include catalyst solid supports that bind electrocatalyst particles (not shown), such as platinum metals, metal alloys, and / or metal oxides. In some examples, the catalyst particles are supported on a catalyst support formed of a boronic acid-functionalized carbon material, as described herein.

[0099] In some examples, first catalyst layer 612-1 and / or second catalyst layer 612-2 include a supported catalyst mixed with an ionomer (an ion-conducting polymer). The ionomer binds the catalysts within the electrode, binds the catalyst layer on the PEM, and provides a pathway for cations (e.g., protons), thereby improving cation conductivity. The ionomer used in first catalyst layer 612-1 and second catalyst layer 612-2 may include any suitable ionomer, including any ionomer described herein.

[0100] In some examples, first catalyst layer 612-1 and / or second catalyst layer 612-2 includes an electrically-conductive boronic acid-functionalized carbon material comprising trivalent boronic acid groups and tetravalent boronic acid groups, as described herein. Thus, the boronic acid-functionalized carbon material functions as the catalyst support, catalyst binder, and proton conducting ionomer.

[0101] MEA 602 is placed between porous transport layers 604-1 and 604-2, which are in turn placed between bipolar plates 606-1 and 606-2 with flow channels 614 located in between. In some examples, porous transport layers 564-1 and / or 604-2 include an electrically-conductive boronic acid-functionalized carbon material comprising trivalent boronic acid groups and / or tetravalent boronic acid groups, as described herein.

[0102] In MEA 602, first catalyst layer 612-1 functions as a cathode and second catalyst layer 612-2 functions as an anode. First catalyst layer / cathode 612-1 and second catalyst layer / anode 612-2 are electrically connected to load 608, and electricity generated by PEM fuel cell 600 drives load 608.

[0103] During operation of PEM fuel cell 600, hydrogen gas (H2) flows into the anode side of PEM fuel cell 600 and oxygen gas (02) flows into the cathode side of PEM fuel cell 600. At second catalyst layer / anode 612-2, hydrogen molecules are catalytically split into protons (H+) and electrons (e-) according to the following hydrogen oxidation reaction (HOR), which is facilitated by the electrocatalysts particles bound to the catalyst solid supports in second catalyst layer / anode 612-2: 2H2 ----> 4H+ + 4e“                       (4)

[0104] The protons are conducted from anode 612-2 to first catalyst layer / cathode 612-1 through PEM 610, and the electrons are conducted from second catalyst layer / anode 612-2 to first catalyst layer / cathode 612-1 around PEM 610 through a conductive path and load 608. At first catalyst layer / cathode 612-1, the protons and electrons combine with the oxygen gas according to the following oxygen reduction reaction (ORR), which is facilitated by the electrochemical catalysts particles bound to the catalyst solid supports in first catalyst layer / anode 612-1: ,           catalyst O2 + 4H+ + 4e“----> 2H2O                (5)

[0105] Thus, the overall electrochemical reaction for the PEM fuel cell 600 is: 2H2 + O2 ----> 2H2O                       (6)

[0106] In the overall reaction, PEM fuel cell 600 produces water at first catalyst layer / cathode 612-1. Water may flow from first catalyst layer / cathode 612-1 to second catalyst layer / anode 612-2 through PEM 610 and may be removed through outlets at the cathode side and / or anode side of PEM fuel cell 600. The overall reaction generates electrons at the anode that drive load 608.

[0107] In the examples of FIGS. 5 and 6, MEA 502 and MEA 602 include catalyst layers 512 / 612 formed on PEM 510 / 610. In alternative configurations, catalyst layers 512 / 612 may be coated on PEM 510 / 610 to thereby form a catalyst coated membrane (CCM). For example, catalyst layers 512 / 612 may be formed in a one-pot process or in stages and sprayed onto PEM 510 / 610.

[0108] The boronic acid-functionalized carbon materials have been described herein for use in electrochemical cell applications, such as water electrolysis and hydrogen fuel cell applications. However, boronic acid-functionalized carbon materials may also be used in other applications. For example, the pKa of the boronic acid-functionalized carbon materials can be controlled to a desired level for many applications, including electrochemical processes for ammonia production.

[0109] In some examples, the boronic acid-functionalized carbon materials may be used in batteries, flow batteries, and / or in the synthesis of ammonia. For example, boronic acid-functionalized carbon materials having tetravalent boronic acid groups may be used in graphite electrodes for lithium ion batteries.

[0110] 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 5 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.

Claims

1. A boronic acid-functionalized carbon material comprising:a carbon material; anda boronic acid group linked to a surface of the carbon material.

2. The boronic acid-functionalized carbon material of claim 1, wherein the carbon material comprises carbon black, graphite, or graphene.

3. The boronic acid-functionalized carbon material of claim 1, wherein the carbon material comprises charcoal, activated charcoal, soot, or coal.

4. The boronic acid-functionalized carbon material of claim 1, wherein the carbon material comprises a fullerene or a carbon nanotube.

5. The boronic acid-functionalized carbon material of claim 1, wherein the boronic acid groups comprise trivalent boronic acid groups.

6. The boronic acid-functionalized carbon material of claim 1, wherein the boronic acid groups have the general formula —B(OH)2.

7. The boronic acid-functionalized carbon material of claim 1, wherein the boronic acid groups comprise tetravalent boronic acid groups.

8. The boronic acid-functionalized carbon material of claim 1, wherein the boronic acid groups comprise a fluoroboric acid group.

9. The boronic acid-functionalized carbon material of claim 1, wherein the boronic acid groups comprise a combination of trivalent boronic acid groups and tetravalent boronic acid groups.

10. The boronic acid-functionalized carbon material of claim 1, wherein the boronic acid groups are linked to the surface of the carbon material by an ether linkage.

11. The boronic acid-functionalized carbon material of claim 1, wherein the boronic acid groups are linked to the surface of the carbon material by an ester linkage.

12. The boronic acid-functionalized carbon material of claim 1, wherein the boronic acid groups are linked to the surface of the carbon material by a boronic acid linker or a polymeric boric acid linker.

13. The boronic acid-functionalized carbon material of claim 1, wherein the surface of the carbon material comprises a surface of the carbon material at an interface with a pore within the carbon material.

14. The boronic acid-functionalized carbon material of claim 1, further comprising a cation ionically linked with the boronic acid group.

15. A method of making a boronic acid-functionalized carbon material, comprising: reacting an oxidized carbon material with a boronic compound to form a trivalent boronic acid-functionalized carbon material comprising trivalent boronic acid groups linked to a surface of the carbon material.

16. The method of claim 15, wherein the oxidized carbon material comprises oxidized carbon black.

17. The method of claim 15, wherein the oxidized carbon material comprises graphite oxide or graphene oxide.

18. The method of claim 15, wherein the boronic compound comprises boric acid.

19. The method of claim 15, wherein the boronic compound comprises a boronic acid having the general formula B(OH)(R1)(R2) where R1 is a hydroxyl group, an alkyl group, and / or an aryl group and R2 is an alkyl group and / or an aryl group.

20. The method of claim 15, wherein the boronic compound comprises a boronic ester having the general formula B(OH)(R3)(R4) where R3 is a hydroxyl group, an alkoxy group, or an aryloxy group and R4 is an alkoxy group or an aryloxy group.

21. The method of claim 15, wherein the boronic compound comprises a polymeric boric acid.

22. The method of claim 21, wherein the polymeric boric acid comprises a polymeric metaboric acid.

23. The method of claim 21, wherein the polymeric boric acid comprises a polymeric tetraboric acid.

24. The method of claim 15, further comprising performing a fluoride treatment on the trivalent boronic acid-functionalized carbon material to convert a trivalent boronic acid group to a tetravalent boronic acid group.

25. The method of claim 24, wherein the fluoride treatment comprises:reacting a trivalent boronic acid group of the boronic acid-functionalized carbon material with a fluoride reagent.

26. The method of claim 25, wherein the fluoride reagent comprises hydrogen fluoride (HF), lithium fluoride (LiF), sodium fluoride (NaF), potassium fluoride (KF), potassium bifluoride (KHF2), cesium fluoride (CsF), ammonium fluoride (NH4F), ammonium bifluoride (NH4F2), or a tetraalkylammonium fluoride having the general formula NR4F, wherein each R is independently hydrogen or a substituted or unsubstituted alkyl or aryl group.

27. The method of claim 25, wherein the fluoride reagent comprises boron trifluoride.

28. The method of claim 24, wherein the reacting the oxidized carbon material withthe boronic compound and the performing the fluoride treatment are performed in a one-pot process.

29. A catalyst support comprising:a carbon material; anda boronic acid group linked to a surface of the carbon material.

30. A gas diffusion layer for an electrochemical cell, comprising:a carbon material; anda boronic acid group linked to a surface of the carbon material.

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