Boron-containing porous membranes and methods of using the same

Abstract

A proton exchange membrane, which comprises a porous structural framework and boron-based acid groups bonded to the porous structural framework. The porous structural framework can be formed of amorphous or crystalline inorganic materials and / or synthetic or natural polymers. The boron-based acid groups can be boric acid derivatives, such as cyclic boric acid derivatives, boronic acid or boronic acid derivatives. The boron-based acid groups can be reaction products of boric acid or boric acid derivatives with polyhydroxy compounds.

Description

[0001] Related Applications

[0002] This application claims priority to U.S. Provisional Patent Application No. 63 / 109,943, filed on November 5, 2020, the entire content of which is incorporated herein by reference. Background Art

[0003] A proton exchange membrane (PEM) is a semi-permeable membrane that is designed to transport protons (H + +) while being impermeable to gases such as hydrogen (H 2 2) and oxygen (O 2 2). PEMs can be used in hydrogen fuel cells and water electrolysis systems under acidic conditions. A PEM can consist of a porous framework that is mechanically and chemically resistant and has high acidic functional groups. For example, Nafion-based proton exchange membranes contain a polytetrafluoroethylene (PTFE) porous structure framework with sulfonic acid groups. The readily dissociable sulfonic acid groups act as proton transporters in the membrane. Summary of the Invention

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

[0005] In some exemplary embodiments, a proton exchange membrane comprises a porous structure framework and boron-based acid groups attached to the porous structure framework.

[0006] In some exemplary embodiments, the boron-based acid groups comprise cyclic boronic acid derivatives.

[0007] In some exemplary embodiments, the boron-based acid groups comprise borospiranic acid.

[0008] In some exemplary embodiments, the boron-based acid groups comprise catechol derivatives.

[0009] In some exemplary embodiments, the porous structure framework comprises solid carrier particles connected by the boron-based acid groups.

[0010] In some exemplary embodiments, the porous structure framework comprises a porous polymer network, and the boron-based acid groups are attached to the pore surfaces of the polymer network.

[0011] In some exemplary embodiments, the porous structure framework comprises an inorganic material.

[0012] In some exemplary embodiments, a method of preparing a proton exchange membrane comprises binding a boron-based acid group to the pore surface of a porous structure framework.

[0013] In some exemplary embodiments, the binding comprises reacting boric acid or a boric acid derivative with hydroxyl groups present on the pore surface.

[0014] In some exemplary embodiments, the binding comprises reacting a polyhydroxy compound with a boric acid derivative bound to the pore surface.

[0015] In some exemplary embodiments, the method further comprises coupling nanoparticles to the pore surface via the boron-based acid group.

[0016] In some exemplary embodiments, a membrane electrode assembly comprises a cathode, an anode, and a proton exchange membrane located between the cathode and the anode, the proton exchange membrane comprising a porous structure framework and boron-based acid groups bound to the pore surface of the porous structure framework.

[0017] In some exemplary embodiments, at least one of the anode or the cathode comprises a catalyst and an ionomer for binding the catalyst, and the ionomer comprises boron-based acid groups. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The drawings illustrate various embodiments and are part of the specification. The illustrated embodiments are merely examples and do not limit the scope of the invention. In all the drawings, the same or similar reference numerals denote the same or similar elements.

[0019] Figure 1 An exemplary proton exchange membrane comprising a porous structure framework and boron-based acid groups bound to the pore surface of the porous structure framework is shown.

[0020] Figure 2A Shows Figure 1 An exemplary configuration of the porous structure framework and boron-based acid groups within the PEM.

[0021] Figure 2B Shows Figure 1 Another exemplary configuration of the porous structure framework and boron-based acid groups within the PEM.

[0022] Figure 3A An exemplary reaction scheme for synthesizing a cyclic boric acid derivative is shown.

[0023] Figure 3B An exemplary reaction scheme for synthesizing boronic acid spirocycles is shown.

[0024] Figure 4A Shows an exemplary reaction scheme for synthesizing a solid support-bound monocyclic cyclic boronic acid derivative.

[0025] Figure 4B Shows an exemplary reaction scheme for synthesizing a solid support-bound boronic acid with a side group moiety.

[0026] Figure 5A Shows an exemplary reaction scheme for synthesizing a solid support-bound cyclic boronic acid derivative having a catechol derivative.

[0027] Figure 5B Shows an exemplary reaction scheme for synthesizing a solid support-bound boronic acid having a catechol derivative and a side group moiety.

[0028] Figure 6 Shows an exemplary reaction scheme 600 for synthesizing a crosslinked copolymer containing a boron-based acid group.

[0029] Figure 7 Shows an exemplary reaction scheme for functionalizing the pore surface of a porous structure framework with a boronic acid derivative.

[0030] Figure 8A Shows another exemplary reaction scheme for functionalizing the pore surface of a porous structure framework with a boronic acid derivative.

[0031] Figure 8B Shows another exemplary reaction scheme for functionalizing the pore surface of a porous structure framework with a boronic acid derivative (boronic acid).

[0032] Figure 9A Shows an exemplary reaction scheme for coupling particles (e.g., nanoparticles) to the pore surface of a porous structure framework with a boronic acid derivative (e.g., boronic acid).

[0033] Figure 9B Shows an exemplary reaction scheme for crosslinking a plurality of particles including a plurality of sheets into a polymer structure using a boronic acid derivative.

[0034] Figure 10 Shows an exemplary proton exchange membrane water electrolysis system having a boron-containing porous membrane.

[0035] Figure 11 Shows an exemplary proton exchange membrane fuel cell having a boron-containing porous membrane. Detailed Description

[0036] This document describes boron-containing porous membranes and methods for preparing and using boron-containing porous membranes. In some examples, the boron-containing porous membrane includes a porous structural framework and boron-based acid groups covalently bonded to the porous structural framework. The porous structural framework can be formed from amorphous or crystalline inorganic materials and / or synthetic or natural polymers. The boron-based acid groups can be boric acid derivatives, such as cyclic boric acid derivatives, boronic acid or boronic acid derivatives. In some examples, the boron-based acid groups are reaction products of boric acid or boric acid derivatives with polyhydroxy compounds.

[0037] The boron-containing porous membranes described herein can be used as PEMs for water electrolysis and / or fuel cell applications operating under acidic conditions. In the boron-containing PEMs described herein, cation (e.g., proton) exchange is provided by protons ionically linked to negatively charged tetravalent boron atoms. The presence of the oxygen-boron bond increases the hydrophilicity of the porous structural framework and stabilizes the negatively charged boron atoms. The boron-containing PEMs described herein also have high mechanical strength, high proton conductivity, low electron conductivity, chemical stability under large pH gradients, durability, and low production costs. The boron-containing porous membranes can be prepared from boric acid and its precursors, such as borax, which are naturally abundant and inexpensive. In some examples, the boron-containing porous membranes described herein also do not contain toxic substances.

[0038] The boron-containing porous membranes described herein can also be used for filtering and / or neutralizing pathogens, such as bacteria, viruses, and fungal spores. For example, the boron-containing porous membranes can be implemented in face masks, surgical masks, air filters, and air purification systems for enclosed spaces (e.g., homes, offices, hospitals, factories, vehicles, airplanes, etc.).

[0039] The devices, compositions, and methods described herein can provide one or more of the above benefits and / or various additional and / or other benefits, which will become apparent herein. Various embodiments will now be described in more detail with reference to the accompanying drawings. It should be understood that the following embodiments are merely exemplary and not restrictive, as various modifications can be made within the scope of the present disclosure.

[0040] Figure 1 An exemplary proton exchange membrane 100 (PEM 100) is shown. The PEM 100 includes a porous structural framework 102 and boron-based acid groups 104 that are distributed throughout the porous structural framework 102 and bonded to the pore surfaces of the porous structural framework 102.

[0041] The porous structure framework 102 can be formed from any suitable material or combination of materials, including inorganic materials and / or organic materials. Suitable inorganic materials can include amorphous inorganic materials (e.g., glass, fused silica, or ceramics) and / or crystalline inorganic materials (e.g., quartz, single crystal silicon, or alumina). Suitable organic materials can include, for example, synthetic and / or natural polymers (e.g., cellulose).

[0042] The PEM 100 can have a thickness d ranging from a few micrometers to several hundred micrometers. With the configurations described herein, the PEM 100 can withstand a pressure differential of up to 30 atmospheres and an acidic pH gradient across the membrane. The PEM 100 is also permeable to water and protons, which can conduct through the PEM 100 as shown by the arrow 106, but the PEM 100 is generally impermeable to gases including hydrogen and oxygen.

[0043] The boronic acid groups 104 can be bound to the pore surfaces within the PEM 100 in at least two different configurations, as Figure 2A and 2B shown. Figure 2A and 2B show exemplary configurations of the porous structure framework 102 and the boronic acid groups 104 within the PEM 100. It should be recognized that Figure 2A and 2B each show only a portion of the PEM 100 and are representative of the characteristics of the entire PEM 100.

[0044] In Figure 2A the first exemplary configuration 200A shown, the pore surface 202 adjacent to the pore 204 is functionalized with the boronic acid groups 104 such that the boronic acid groups 104 are bound to the pore surface 202. Although Figure 2A only one boronic acid group 104 bound to the pore surface 202 is shown, the porous structure framework 102 can have any number and concentration of boronic acid groups 104 bound to the pore surface 202.

[0045] In the second exemplary configuration 200B shown in FIG. 2, the porous structure framework 102 comprises solid support particles 206 (e.g., solid support particles 206-1 to 206-4) connected together by the boronic acid groups 104. The connected solid support particles 206 form pores 208 in the spaces between the solid support particles 206. The boronic acid groups 104 are bound to the pore surfaces 210 (e.g., pore surfaces 210-1 to 210-4 of the solid support particles 206-1 to 206-4). While Figure 2BFour solid support particles 206 are shown connected by boronic acid groups 104, but any other number of solid support particles 206 can be connected by boronic acid groups 104. Additionally, each solid support particle 206 can be connected to one or more additional solid support particles by one or more additional boronic acid groups, thereby creating a porous structure framework 102, where the boronic acid groups 104 are bonded to the pore surfaces 210 within the porous structure framework 102.

[0046] The solid support particles 206 can be formed from any suitable material, such as any of the materials used above for the porous structure framework 102, such as inorganic molecules (e.g., fused silica particles, ceramic particles, etc.) or organic molecules (e.g., polymers). The solid support particles 206 can have any suitable shape and size, ranging from tens of nanometers (nm) to 100 micrometers. The porosity of the PEM 100 can be controlled and defined by the size of the solid support particles 206. The solid support particles 206 can also be selected based on their mechanical strength, durability in a high pH gradient environment, and / or their affinity for water (e.g., they can be selected as hydrophilic or hydrophobic based on the desired hydrophilicity balance of the PEM 100).

[0047] In some instances, the boronic acid group 104 comprises a boronic acid derivative (e.g., a compound or group derived from boric acid). Boric acid has the molecular formula B(OH) 3 and the following structure given by formula (I):

[0048]

[0049] The boronic acid derivative can be any compound or group in which one, two, or all three hydroxyl (OH) groups of boric acid have reacted and bonded with one or more other compounds. Exemplary boronic acid derivatives will be described in more detail below.

[0050] The boronic acid derivative can be formed in any suitable manner. In some instances, the boronic acid derivative is formed by reacting boric acid or another boronic acid derivative with one or two hydroxyl groups of one or more other compounds. For example, boric acid or a boronic acid derivative can react with a polyhydroxy compound having at least two cis - vicinal hydroxyl groups. The boronic acid derivative can include any suitable boronic acid derivative, including but not limited to cyclic boronic acid derivatives, boronic acid spiro compounds, boronic acid spiro compound derivatives, and any other boronic acid derivatives described herein. The polyhydroxy compound can be any suitable polyhydroxy compound, such as a polyol, sugar, sugar alcohol (e.g., glycerol, mannitol, or sorbitol), catechol, or a derivative of any of the above. In some instances, the polyhydroxy compound has a structure represented by formula (IIa) or (IIb):

[0051]

[0052] wherein W, X, Y, and Z are side group moieties and can each independently be selected from the group consisting of: hydrogen (H), hydroxy (OH), fluoro (F), chloro (Cl), dialkylamino (NR 2 ), cyano (CN), carboxylic acid (COOH), carboxamide, carboxylic acid ester, alkyl, alkoxy, and aryl. In some instances, any one or more of the groups W, X, Y, and Z can represent a C 1 to C 30 alkyl chain, and the C 1 to C 30 alkyl chain can optionally contain one or more substituents such as oxygen (O), hydroxy (OH), fluoro (F), chloro (Cl), dialkylamino (NR 2 ), cyano (CN), carboxylic acid (COOH), carboxamide, carboxylic acid ester, alkyl, alkoxy, and aryl.

[0053] Reference will now be made to Figure 3A and 3B to show and describe exemplary reaction schemes for preparing boronic acid derivatives by the reaction of a boronic acid or a boronic acid derivative with a polyhydroxy compound. It should be recognized that the reaction schemes below are merely exemplary and not restrictive.

[0054] Figure 3A Exemplary reaction scheme 300A for synthesizing a cyclic boronic acid derivative is shown. As shown, boronic acid 302 combines with glycerol 304 to produce cyclic boronic acid derivative 306. Glycerol 304 is represented by formula (IIa), wherein X, Y, and Z are each hydrogen (H), and W is hydroxymethyl (CH 2 OH). While Figure 3A shows the reaction of boronic acid with glycerol 304, boronic acid 302 can react with any other suitable sugar (e.g., glucose, fructose, etc.), sugar alcohol (e.g., mannitol, sorbitol, etc.), or polyhydroxy compound. Additionally or alternatively, a boronic acid derivative can be used in place of boronic acid 302.

[0055] Figure 3B Exemplary reaction scheme 300B for synthesizing boronic acid spiro acid, which is also a boronic acid derivative, is shown. As shown, cyclic boronic acid derivative 306 prepared by the reaction scheme 300A shown by Figure 3A combines with another glycerol molecule 304 to form boronic acid spiro acid 308. Although boronic acid 302 is a weak acid, a strongly acidic boronic acid spiro acid 308 is formed in the presence of a polyhydroxy compound such as a sugar alcohol (e.g., glycerol 304). Although Figure 3BThe reaction of cyclic boronic acid derivative 306 with glycerol 304 is shown, but cyclic boronic acid derivative 306 can react with any other suitable sugar, sugar alcohol (e.g., mannitol, sorbitol, etc.) or polyhydroxy compound. Additionally or alternatively, cyclic boronic acid derivative 306 can be replaced with any other suitable cyclic boronic acid derivative.

[0056] Figure 3A and 3B The reaction schemes 300A and 300B shown in Figure 2A can be used to produce boronic acid groups 104 bound to the pore surface 202 (see Figure 2B ) and / or to produce boronic acid groups 104 bound to the pore surface 210 (see Figures 4A to 9B ). In some instances, the surface of the solid support material (e.g., pore surface 202 or particle surface 210) can be functionalized with a boronic acid derivative or a polyhydroxy compound. Exemplary reaction schemes for forming boronic acid groups 104 bound to the pore surface of the porous structure framework 102 will now be shown and described with reference to

[0057] Figure 4A An exemplary reaction scheme 400A for the synthesis of a solid support-bound monocyclic cyclic boronic acid derivative is shown. As shown, the solid support 402 is functionalized with cis-1,2-dihydroxy 404 having cis vicinal dihydroxyl groups. The solid support 402 can be formed from any inorganic or organic solid support material for the porous structure framework 102 described herein (e.g., glass, ceramic, synthetic polymer, natural polymer) and can be selected based on its mechanical strength, durability in a high pH gradient environment, and / or its affinity for water (e.g., it can be selected as hydrophilic or hydrophobic according to the hydrophilicity balance required for the PEM 100). The solid support 402 can be a particle similar to the solid support particle 206 described with reference to Figure 2B . Alternatively, the solid support 402 can be a part of the porous structure framework 102 described with reference to Figure 2A .

[0058] As Figure 4A shown, cis-1,2-dihydroxy 404 has a structure represented by formula (IIa) and includes a side group portion Y' and a linking chain 406 bound to the solid support 402 (represented by side groups W or X in formula (IIa)). The linking chain 406 is a C 1 to C 30 alkyl chain and optionally has one or more side group portions X', which can be the same or different for each atom in the linking chain 406. X' and Y' can each independently be selected from the group consisting of hydrogen (H), hydroxyl (OH), fluoro (F), chloro (Cl), dialkylamino (NR2 ) cyano (CN), carboxylic acid (COOH), carboxamide, carboxylic acid ester, alkyl, alkoxy and aryl.

[0059] Cis-1,2-dihydroxy 404 reacts with boric acid 408 to obtain a monocyclic cyclic boric acid derivative 410 bound to a solid support 402. With this configuration, the solid support-bound cyclic boric acid derivative 410 can achieve binding to the boron-based acid group 104 on the pore surface of the porous structural framework 102 in the PEM 100. Alternatively, the solid support-bound cyclic boric acid derivative 410 can further react with a polyhydroxy compound to generate another boric acid derivative (e.g., boronic acid spiro compound), which will now be described with reference to Figure 4B as follows.

[0060] Figure 4B An exemplary reaction scheme 400B for synthesizing a solid support-bound boronic acid spiro compound with side chain moieties is shown. As shown, the solid support-bound cyclic boric acid derivative 410 (prepared by Figure 4A the reaction scheme 400A shown) binds to a polyhydroxy compound 412 having side chain moieties A', B', C' and D'. The polyhydroxy compound 412 has the structure shown in formula (IIa). Thus, the side chain moieties A', B', C' and D' can correspond to the side chain moieties W, X, Y and Z described with reference to formula (IIa).

[0061] The reaction of the solid support-bound cyclic boric acid derivative 410 and the polyhydroxy compound 412 produces a solid support-bound boronic acid spiro compound 414 having side chain moieties A', B', C' and D'. With this configuration, the solid support-bound boronic acid spiro compound 414 can achieve binding to the boron-based acid group 104 on the pore surface of the porous structural framework 102 in the PEM 100. The solid support-bound boronic acid spiro compound 414 can exhibit strong proton exchange properties, which depend on the electronic properties of the A', B', C', D', X' and Y' substitutions.

[0062] In embodiments where the solid support 402 comprises a polymer network, the solid support-bound cyclic boric acid derivative 410 and the solid support-bound boronic acid spiro compound 414, each having a controlled boric acid and boronic acid spiro compound loading, can also be used to form an ionomer for binding a catalyst in the catalyst layer of a membrane electrode assembly, which will be described below with reference to Figure 10 and 11 as follows.

[0063] Figure 5AShows an exemplary reaction scheme 500A for synthesizing a solid support - bound cyclic boronic acid derivative having a catechol derivative. Reaction scheme 500A is similar to reaction scheme 400A, except that in reaction scheme 500A the polyhydroxy compound includes a catechol derivative.

[0064] In reaction scheme 500A, the solid support 502 is functionalized with a catechol derivative 504. The solid support 502 can be the same as or similar to the solid support 402. The catechol derivative 504 has a structure represented by formula (IIb) and includes side - group portions W", Y", and Z", and a linking chain 506 (represented by the side - group X in formula (IIb)) that binds to the solid support 502. The linking chain 506 is a C 1 to C 30 alkyl chain and optionally has one or more side - group portions X", which can be the same or different for each atom in the linking chain 506. The side - group portions W", X", Y", and Z" can each independently be selected from the group consisting of: hydrogen (H), hydroxy (OH), fluoro (F), chloro (Cl), dialkylamino (NR 2 ), cyano (CN), carboxylic acid (COOH), carboxamide, carboxylic acid ester, alkyl, alkoxy, and aryl.

[0065] The catechol derivative 504 reacts with boric acid 508 to produce a solid - support - bound cyclic boronic acid derivative 510 having a catechol derivative. When the solid support 502 is a polymer, the catechol structure can be introduced during the polymerization process (e.g., to form a catechol - formaldehyde resin), as described below with reference to Figure 6 Or, the catechol structure can be introduced after polymerization (e.g., by functional modification of a Merrifield - type resin).

[0066] With this configuration, the solid - support - bound cyclic boronic acid derivative 510 having a catechol derivative can achieve binding to the pore surface of the porous - structure framework 102 in the PEM 100 of the boron - based acid group 104. Alternatively, the solid - support - bound cyclic boronic acid derivative 510 having a catechol derivative can further react with a polyhydroxy compound to produce another boronic acid derivative (e.g., a boronic acid spiro - derivative), as will now be described with reference to Figure 5B described.

[0067] Figure 5BShows an exemplary reaction scheme 500B for synthesizing a solid support-bound boronic acid derivative (which has a catechol derivative and a side group portion). In reaction scheme 500B, a solid support-bound cyclic boronic acid derivative 510 having a catechol derivative is combined with a polyhydroxy compound 512 having side group portions A", B", C", and D". The polyhydroxy compound 512 has a structure represented by formula (IIa). Thus, the side group portions A", B", C", and D" may correspond to the side group portions W, X, Y, and Z described with reference to formula (IIa).

[0068] The solid support-bound cyclic boronic acid derivative 510 having a catechol derivative is combined with the polyhydroxy compound 512 to produce a solid support-bound boronic acid 514 having a catechol derivative and including side group portions A", B", C", and D". With this configuration, the solid support-bound boronic acid 514 having a catechol derivative and including side group portions can achieve a boronic acid group 104 that binds to the pore surface of the porous structural framework 102 in the PEM 100.

[0069] Figure 6 Shows an exemplary reaction scheme 600 for synthesizing a crosslinked copolymer containing a boronic acid group. In reaction scheme 600, catechol 602, formaldehyde 604, and boric acid 606 are combined and polymerized to produce a catechol-formaldehyde-boric acid crosslinked copolymer 608 ("copolymer 608"). The copolymer 608 includes catechol structures 610 connected by boronic acid groups 612. The individual polymers of the copolymer 608 are crosslinked by crosslinking chains 614 between the catechol structures 610. The copolymer 608 may also contain other phenolic monomers such as phenol, resorcinol, and trihydroxyphenol.

[0070] The copolymer 608 is suitable for PEM and ionomer applications. For example, Figure 2B the configuration of the PEM 100 shown can be achieved by the copolymer 608, where the boronic acid groups 612 (corresponding to the boronic acid groups 104) bond and connect the catechol structures 610 (corresponding to the solid support particles 206) to form the porous structural framework 102. The molar percentage content of the boronic acid groups 612 can be controlled to obtain the optimal functional performance as the PEM 100 or as an ionomer for binding a catalyst in the catalyst layer in the MEA, which will be described below with reference to Figure 10 and 11 be described.

[0071] As previously described, in some instances, the pore surface (e.g., pore surface 202) within the porous structural framework 102 is functionalized with boronic acid groups 104 (see Figure 2A ). Figures 7 to 9AShows exemplary reaction schemes for functionalizing the pore surface of a porous structural framework with boronic acid groups. These reaction schemes are compatible with solid support materials having one or more hydroxyl groups present on the pore surface. Examples of such materials include, but are not limited to, silica, glass, alumina, clay, synthetic polymers, and cellulose.

[0072] Figure 7 Shows an exemplary reaction scheme 700 for functionalizing the pore surface with boronic acid groups. As Figure 7 shown, the solid support 702 includes a surface 704 and hydroxyl groups 706 at the surface 704. The surface 704 of the solid support 702 can be the pore surface of the porous structural framework 102 (e.g., Figure 2A the surface 202 shown) or the surface of a solid support particle (e.g., Figure 2B the surface 210 of the solid support particle 206 shown). Although Figure 7 shown with the surface 704 having only one hydroxyl group 706, the surface 704 can have any other number and concentration of hydroxyl groups 706. In reaction scheme 700, the surface 704 of the solid support 702 is exposed to boric acid 708, which reacts with the hydroxyl group 706 to form a borate derivative 710 bound to the surface 704. The solid support-bound borate derivative 710 can effect the binding of boronic acid groups 104 to the pore surface of the porous structural framework 102 incorporated into the PEM 100. Alternatively, the solid support-bound borate derivative 710 can further react with boric acid, another borate derivative, or a polyhydroxy compound to produce another boron derivative.

[0073] Figure 8A Shows another exemplary reaction scheme 800A for functionalizing the pore surface of a porous structural framework with boronic acid groups. Reaction scheme 800A is similar to reaction scheme 700, except that boric acid 708 reacts with two hydroxyl groups 706 on the surface 704 to produce a cyclic borate derivative 802. In some instances, the surface 704 can be pre-activated to increase the surface density of hydroxyl groups 706. Any suitable pre-activation process can be used. The cyclic borate derivative 802 can effect the binding of boronic acid groups 104 to the pore surface of the porous structural framework 102 incorporated into the PEM. Alternatively, the cyclic borate derivative 802 can further react to produce another borate derivative, as will now be described with reference to Figure 8B as follows.

[0074] Figure 8B Shows another exemplary reaction scheme 800B for functionalizing the pore surface of a porous structural framework with a borate derivative (e.g., a boronic acid group). In reaction scheme 800B, in Figure 8AThe cyclic boronic acid derivative 802 produced in the reaction scheme 800A shown binds to a polyhydroxy compound 804 having side group moieties A''', B''', C''', and D'''. The polyhydroxy compound 804 has the structure shown in formula (IIa). Thus, the side group moieties A''', B''', C''', and D''' can correspond to the side group moieties W, X, Y, and Z with reference to formula (IIa) above. The cyclic boronic acid derivative 802 and the polyhydroxy compound 804 react to produce a boronic acid spiro compound having side group moiety 806, which binds to the pore surface 704. The boronic acid spiro compound bound to the solid support having side group moiety 806 can effectuate a boronic acid group 104 that binds to the pore surface of the porous structural framework 102 incorporated in the PEM 100.

[0075] Figure 9A An exemplary reaction scheme 900A is shown for coupling particles (e.g., nanoparticles or microparticles) to the pore surface of a porous structural framework via a boronic acid derivative (e.g., boronic acid spiro compound). The porosity of the PEM 100 can be controlled and defined by the size of the particles 902. The particles 902 can also be selected based on their mechanical strength, durability in a high pH gradient environment, and / or their affinity for water (e.g., they can be selected as hydrophilic or hydrophobic depending on the desired hydrophilicity balance of the PEM 100).

[0076] Scheme 900A is similar to scheme 800A, except that particles 902 having hydroxyl groups 904 bind to boric acid 708. Thus, using boric acid 708, the hydroxyl groups 706 on the pore surface 704 are crosslinked with the hydroxyl groups 904 on the surface of the particles 902. This cross-coupling reaction results in the particles 902 being linked to the pore surface 704 via a boronic acid spiro compound 906. The particles 902 can be formed from any suitable material (e.g., silica, glass, alumina, ceramic, clay, synthetic polymer, cellulose) and can be the same as or different from the material of the solid support 702.

[0077] The reaction scheme 900A can be controlled to proceed in any order. In some instances, the first step of the reaction scheme 900A includes conducting the reaction scheme 800A to produce a solid support-bound cyclic boronic acid derivative 802. In the second step, the particles 902 can be exposed to the solid support-bound cyclic boronic acid derivative 802 to produce a solid support-bound boronic acid spiro compound 906. Alternatively, the particles 902 can be bound to boric acid 708 in the first step to produce an intermediate boronic acid derivative. In the second step, the hydroxyl groups 706 on the surface 704 of the solid support 702 are exposed to the intermediate boronic acid derivative, which reacts to produce a solid support-bound boronic acid spiro compound 906 and link the particles 902 to the solid support 702. In additional instances, the reaction scheme 900A can be conducted by combining all the reactants in a single step.

[0078] Figure 9BShows exemplary reaction scheme 900B for crosslinking multiple particles including multiple sheets within a polymer structure using a borate derivative. Reaction scheme 900B is similar to reaction scheme 900A, except that in reaction scheme 900B, particle 902 (first particle 902) is crosslinked with a second particle 908 having two hydroxyl groups 910 (instead of crosslinking with the hydroxyl group 706 on the pore surface 704). The first particle 902 and the second particle 908 can each be a micron particle or a nano particle and can have any size suitable for a particular implementation (e.g., the size range can be from a few nanometers to several hundred microns). Reaction scheme 900B produces a linked particle 912 having a borospiro group 914. The first particle 902 and the second particle 908 can be made of the same material or different materials, which can be selected from any of the solid support materials described herein (e.g., silica, glass, alumina, ceramics, clay, synthetic polymers, cellulose, etc.).

[0079] Reaction scheme 900B can be used to produce PEM 100, effectively using Figure 2B the second configuration 200B shown. The porosity of PEM100 can be defined by the sizes of the first particle 902 and the second particle 908.

[0080] The boron-containing porous membranes described herein can be used in water electrolysis and / or fuel cell applications. Exemplary applications will now be described with reference to Figure 10 and 11 .

[0081] Figure 10 Shows an exemplary proton exchange membrane water electrolysis system 1000 (PEM water electrolysis system 1000) having a boron-containing porous membrane. PEM water electrolysis system 1000 uses electricity to decompose water into oxygen (O 2 ) and hydrogen (H 2 ) through an electrochemical reaction. The configuration of PEM water electrolysis system 1000 is merely exemplary and not restrictive, as other suitable configurations as well as other suitable water electrolysis systems can incorporate the boron-containing porous membrane.

[0082] As Figure 10 shown, PEM water electrolysis system 1000 includes a membrane electrode assembly 1002 (MEA 1002), porous transport layers 1004-1 and 1004-2, bipolar plates 1006-1 and 1006-2, and a power source 1008. PEM water electrolysis system 1000 can also include Figure 10 additional or other elements not shown in

[0083] MEA 1002 includes a PEM 1010 located between a first catalyst layer 1012-1 and a second catalyst layer 1012-2. The PEM 1010 electrically isolates the first catalyst layer 1012-1 from the second catalyst layer 1012-2 while providing selective conduction of cations (such as protons (H + )) and being impermeable to gases (such as hydrogen and oxygen) at the same time. The PEM 1010 can be implemented by any suitable PEM. For example, the PEM 1010 can be implemented by a boron-containing porous membrane (e.g., PEM 100) including a porous structural framework having boron-based acid groups bonded to the pore surfaces within the porous structural framework.

[0084] The first catalyst layer 1012-1 and the second catalyst layer 1012-2 are conductive electrodes having embedded electrocatalysts (not shown), such as platinum, ruthenium, and / or cerium(IV) oxide. In some instances, the first catalyst layer 1012-1 and the second catalyst layer 1012-2 are formed using an ionomer to bind catalyst nanoparticles. As previously described, the ionomer used to form the first catalyst layer 1012-1 and the second catalyst layer 1012-2 can include the boron-based acid groups described herein, such as copolymer 608 (see Figure 6 ).

[0085] The 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, and flow channels 1014-1 and 1014-2 are located between the bipolar plates 1006 and the porous transport layers 1004.

[0086] In the MEA 1002, the first catalyst layer 1012-1 serves as the anode and the second catalyst layer 1012-2 serves as the cathode. When the PEM water electrolysis system 1000 is powered by a power source 1008, an oxygen evolution reaction (OER) occurs at the anode 1012-1, which is represented by the following electrochemical half-reaction:

[0087] 2H 2 O → O 2 + 4H + + 4e -

[0088] Protons are conducted from the anode 1012-1 to the cathode 1012-2 through the PEM 1010, and electrons are conducted from the anode 1012-1 to the cathode 1012-2 through a conduction path around the PEM 1010. The PEM 1010 allows protons (H +) and water are transported from the anode 1012-1 to the cathode 1012-2, but oxygen and hydrogen cannot permeate. At the cathode 1012-2, protons and electrons combine in the hydrogen evolution reaction (HER), which is represented by the following electrochemical half-reaction:

[0089] 4H + +4e - →2H 2

[0090] OER and HER are two complementary electrochemical reactions used to decompose water by electrolysis, which is represented by the following overall water electrolysis reaction:

[0091] 2H 2 O → 2H 2 +O 2

[0092] Figure 11 shows an exemplary proton exchange membrane fuel cell 1100 (PEM fuel cell 1100) with a boron-containing porous membrane. The PEM fuel cell 1100 generates electricity due to an electrochemical reaction. In this example, the electrochemical reaction includes reacting hydrogen (H 2 ) and oxygen (O 2 ) to produce water and electricity. The construction of the PEM fuel cell 1100 is merely exemplary and not restrictive, as other suitable constructions and other suitable proton exchange membrane fuel cells can incorporate the boron-containing porous membrane.

[0093] As Figure 11 shown, the PEM fuel cell 1100 includes a membrane electrode assembly 1102 (MEA 1102), porous transport layers 1104-1 and 1104-2, and bipolar plates 1106-1 and 1106-2. An electrical load 1108 can be electrically connected to the MEA 1102 and driven by the PEM fuel cell 1100. The PEM fuel cell 1100 may also include Figure 11 additional or other elements not shown in

[0094] The MEA 1102 includes a PEM 1110 located between a first catalyst layer 1112-1 and a second catalyst layer 1112-2. The PEM 1110 electrically isolates the first catalyst layer 1112-1 from the second catalyst layer 1112-2 while providing selective conduction of cations such as protons (H + ) and at the same time not permeating gases such as hydrogen and oxygen. The PEM 1110 can be implemented by any suitable PEM. For example, the PEM 1110 can be implemented by a boron-containing porous membrane (e.g., PEM 100) that includes a porous structural framework having boron-based acid groups bonded to the pore surfaces within the porous structural framework.

[0095] The first catalyst layer 1112-1 and the second catalyst layer 1112-2 are conductive electrodes having an embedded electrocatalyst (not shown). In some instances, the first catalyst layer 1112-1 and the second catalyst layer 1112-2 are formed using an ionomer to bind catalyst nanoparticles. In some instances, the ionomer used to form the first catalyst layer 1112-1 and the second catalyst layer 1104-2 includes a boric acid derivative as described herein, such as copolymer 608 (see Figure 6 ).

[0096] The MEA 1102 is placed between the porous transport layers 1104-1 and 1104-2, which are in turn placed between the bipolar plates 1106-1 and 1106-2, with flow channels 1114 therebetween. In the MEA→1102, the first catalyst layer 1112-1 serves as the cathode and the second catalyst layer 1112-2 serves as the anode. The cathode 1112-1 and the anode 1112-2 are electrically connected to the load 1108, and the load 1108 is driven by the electricity generated by the PEM fuel cell 1100.

[0097] During operation of the PEM fuel cell 1100, hydrogen (H 2 ) flows into the anode side of the PEM fuel cell 1100, and oxygen (O 2 ) flows into the cathode side of the PEM fuel cell 1100. At the anode 1112-2, hydrogen molecules are catalytically decomposed into protons (H + ) and electrons (e - ) according to the following hydrogen oxidation reaction (HOR):

[0098] 2H 2 →4H + +4e -

[0099] Protons are conducted from the anode 1112-2 to the cathode 1112-1 through the PEM 1100, while electrons are conducted from the anode 1112-2 to the cathode 1112-1 around the PEM 1110 through the conduction path and the load 1108. At the cathode 1112-1, protons and electrons combine with oxygen according to the following oxygen reduction reaction (ORR):

[0100] O 2 +4H + +4e - →2H 2 O

[0101] Thus, the overall electrochemical reaction of the PEM fuel cell 1100 is:

[0102] 2H 2 +O 2→2H 2 O

[0103] During the entire reaction, the PEM fuel cell 1100 generates water at the cathode 1112-1. The water can flow from the cathode 1112-1 to the anode 1112-2 through the PEM 1110 and can be removed through the outlets on the cathode side and / or the anode side of the PEM fuel cell 1100. Electrons are generated at the anode during the entire reaction, and these electrons drive the load 1108.

[0104] The boron-containing porous membranes described herein (e.g., PEM 100) can also be used as porous membranes for neutralizing pathogens. For example, the porous structure framework 102 can have pores small enough to prevent pathogens such as bacteria, fungal spores, and viruses from passing through. The boron-based acid groups 104 can also have anti-pathogenic activity against bacteria, fungi, and viruses (including SARS-CoV-2). For example, the basic protein sites of pathogens (including SARS-CoV-2) can ionically bond to the acidic boron sites of the proton exchange membrane, thereby preventing the pathogens from passing through the proton exchange membrane. Therefore, the proton exchange membrane can be implemented in face masks, surgical masks, and air filters and air purification systems for enclosed spaces (e.g., homes, offices, hospitals, factories, vehicles, airplanes, etc.).

[0105] In the foregoing description, various exemplary embodiments have been described with reference to the accompanying drawings. However, it is apparent that various modifications and changes can be made thereto and other implementations can be achieved without departing from the scope of the appended claims. For example, certain features of one embodiment described herein can be combined with or substituted for features of another embodiment described herein. Accordingly, the specification and drawings are to be regarded as illustrative rather than restrictive.

[0106] In the foregoing description, various exemplary embodiments have been described with reference to the accompanying drawings. However, it is apparent that various modifications and changes can be made thereto and additional embodiments can be achieved without departing from the scope of the invention as set forth in the appended claims. For example, certain features of one embodiment described herein can be combined with or substituted for features of another embodiment described herein. Accordingly, the specification and drawings are to be regarded as illustrative rather than restrictive.

Claims

1. A proton exchange membrane, which comprises a porous structural framework; and tetravalent boron-based acid groups covalently bonded to the porous structural framework, wherein the boron atoms in the tetravalent boron-based acid groups are covalently bonded to the porous structural framework through oxygen; the boron atoms in the tetravalent boron-based acid groups are negatively charged, and cations are ionically connected to the boron atoms.

2. The proton exchange membrane according to claim 1, wherein the tetravalent boron-based acid groups comprise cyclic boric acid derivatives.

3. The proton exchange membrane according to claim 1, wherein the tetravalent boron-based acid groups comprise boronic acid.

4. The proton exchange membrane according to claim 1, wherein the tetravalent boron-based acid groups comprise catechol derivatives.

5. The proton exchange membrane according to claim 1, wherein the porous structural framework comprises solid support particles connected by the tetravalent boron-based acid groups.

6. The proton exchange membrane according to claim 1, wherein: the porous structural framework comprises a porous polymer network, and the tetravalent boron-based acid groups are covalently bonded to the pore surface of the porous polymer network.

7. The proton exchange membrane according to claim 1, wherein the porous structural framework comprises an inorganic material.

8. A method for preparing a proton exchange membrane, which comprises: obtaining the proton exchange membrane by functionalizing the pore surface contained in a porous structural framework with tetravalent boron-based acid groups through covalent bonding; the boron atoms in the tetravalent boron-based acid groups are covalently bonded to the porous structural framework through oxygen; the boron atoms in the tetravalent boron-based acid groups are negatively charged, and cations are ionically connected to the boron atoms.

9. The method according to claim 8, wherein the tetravalent boron-based acid groups comprise cyclic boric acid derivatives.

10. The method according to claim 8, wherein the tetravalent boron-based acid groups comprise boronic acid.

11. The method according to claim 8, wherein the tetravalent boron-based acid groups comprise catechol derivatives.

12. The method according to claim 8, wherein the functionalization comprises reacting boric acid or a boric acid derivative with hydroxyl groups present on the pore surface.

13. The method according to claim 8, wherein the functionalization comprises reacting a polyhydroxy compound with a boric acid derivative bound to the pore surface.

14. The method according to claim 8, further comprising coupling nanoparticles to the pore surface through the tetravalent boron-based acid groups.

15. A membrane electrode assembly, which comprises a cathode; an anode ； and a proton exchange membrane located between the cathode and the anode, the proton exchange membrane comprising a porous structural framework and tetravalent boron-based acid groups covalently bonded to the pore surface in the porous structural framework; the boron atoms in the tetravalent boron-based acid groups are covalently bonded to the porous structural framework through oxygen; the boron atoms in the tetravalent boron-based acid groups are negatively charged, and cations are ionically connected to the boron atoms.

16. The membrane electrode assembly according to claim 15, wherein the tetravalent boron-based acid groups comprise cyclic boric acid derivatives.

17. The membrane electrode assembly according to claim 15, wherein the tetravalent boron-based acid groups comprise boronic acid.

18. The membrane electrode assembly according to claim 15, wherein, the tetravalent boron-based acid group contains a catechol derivative.

19. The membrane electrode assembly according to claim 15, wherein the porous structure framework contains an inorganic material.

20. The membrane electrode assembly according to claim 15, wherein: at least one of the anode or the cathode contains a catalyst and an ionomer for binding the catalyst; and the ionomer contains a tetravalent boron-based acid group.

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