Boron-containing porous membrane and method of use thereof

Abstract

The proton exchange membrane comprises a porous framework and boron-based acidic groups bound to the porous framework. The porous framework may be formed of an amorphous or crystalline inorganic material and / or a synthetic or natural polymer. The boron-based acidic groups may be boric acid derivatives, such as cyclic boric acid derivatives, borospiranic acid, or borospiranic acid derivatives. The boron-based acidic groups may be the reaction product of boric acid or a boric acid derivative with a polyhydroxy compound.

Description

[Technical Field]

[0001] (Related Applications) This application claims priority to U.S. Provisional Patent Application No. 63 / 109,943, filed September 5, 2020, the contents of which are incorporated herein by reference in their entirety. [Background technology]

[0002] Proton exchange membranes (PEMs) are impermeable to gases such as hydrogen (H2) and oxygen (O2), but are permeable to protons (H + ) is a semipermeable membrane designed to transport protons. PEMs can be used in hydrogen fuel cells and water electrolysis systems under acidic conditions. PEMs can be composed of a mechanically and chemically resistant porous framework with strongly acidic functional groups. For example, Nafion-based proton exchange membranes contain a porous framework of polytetrafluoroethylene (PTFE) with sulfonic acid groups. The easily dissociated sulfonic acid groups function as proton transport agents in the membrane. Summary of the Invention

[0003] The following description presents a simplified summary of one or more aspects of the methods and systems described herein to provide a basic understanding of such aspects. This summary is not an extensive overview of all contemplated aspects, nor is it intended to identify key or critical elements of all aspects, nor is it intended 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 that is presented below.

[0004] In some exemplary embodiments, a proton exchange membrane comprises a porous structural framework and boron-based acidic groups attached to the porous structural framework.

[0005] In some exemplary embodiments, the boron-based acidic group comprises a cyclic boric acid derivative.

[0006] In some exemplary embodiments, the boron-based acidic group comprises a borospiranic acid.

[0007] In some exemplary embodiments, the boron-based acidic group comprises a catechol derivative.

[0008] In some exemplary embodiments, the framework of the porous structure comprises solid support particles linked by boron-based acidic groups.

[0009] In some exemplary embodiments, the porous structure framework comprises a porous polymer network, and the boron-based acidic groups are bonded to the pore surfaces of the polymer network.

[0010] In some exemplary embodiments, the framework of the porous structure comprises an inorganic material.

[0011] In some exemplary embodiments, a method of making a proton exchange membrane includes bonding boron-based acidic groups to the surfaces of pores contained in the framework of a porous structure.

[0012] In some exemplary embodiments, the binding comprises reacting boric acid or a boric acid derivative with hydroxyl groups present on the pore surface.

[0013] In some exemplary embodiments, the binding comprises reacting a polyhydroxy compound with a boric acid derivative that is bound to the pore surface.

[0014] In some exemplary embodiments, the method further comprises attaching nanoparticles to the pore surface via boron-based acidic groups.

[0015] In some exemplary embodiments, a membrane electrode assembly comprises a cathode, an anode, and a proton exchange membrane disposed between the cathode and the anode, the proton exchange membrane comprising a porous structural framework and boron-based acidic groups bound to the pore surfaces in the porous structural framework.

[0016] In some exemplary embodiments, at least one of the anode or cathode comprises a catalyst and an ionomer for binding the catalyst, the ionomer comprising boron-based acidic groups. [Brief explanation of the drawings]

[0017] The accompanying drawings illustrate various embodiments and are a part of the specification. The illustrated embodiments are merely examples and are not intended to limit the scope of the present disclosure. Throughout the drawings, the same or similar reference numerals refer to the same or similar elements. [Figure 1] FIG. 1 shows an exemplary proton exchange membrane that includes a porous framework and boron-based acidic groups attached to the pore surfaces of the porous framework. [Figure 2] Figure 2A shows an exemplary structure of the porous framework and boron-based acidic groups in the PEM of Figure 1. Figure 2B shows another exemplary structure of the porous framework and boron-based acidic groups in the PEM of Figure 1. [Figure 3] Figure 3A shows an exemplary reaction scheme for synthesizing a cyclic boronic acid derivative, and Figure 3B shows an exemplary reaction scheme for synthesizing a borospiranic acid. [Figure 4] Figure 4A shows an exemplary reaction scheme for synthesizing a solid support-bound monocyclic boronic acid derivative, and Figure 4B shows an exemplary reaction scheme for synthesizing a solid support-bound borospiranic acid with a pendant moiety. [Figure 5]Figure 5A shows an exemplary reaction scheme for synthesizing a solid support-bound cyclic boronic acid derivative incorporating a catechol derivative, and Figure 5B shows an exemplary reaction scheme for synthesizing a solid support-bound borospiranic acid incorporating a catechol derivative and a pendant moiety. [Figure 6] FIG. 6 shows an exemplary reaction scheme 600 for synthesizing a cross-linked copolymer containing boron-based acidic groups. [Figure 7] FIG. 7 shows an exemplary reaction scheme for functionalizing the pore surfaces of the framework of the porous structure with boric acid derivatives. [Figure 8] Figure 8A shows another exemplary reaction scheme for functionalizing the pore surface of the framework of a porous structure with a boric acid derivative, and Figure 8B shows another exemplary reaction scheme for functionalizing the pore surface of the framework of a porous structure with a boric acid derivative (borospiranic acid). [Figure 9] Figure 9A shows an exemplary reaction scheme for attaching particles (e.g., nanoparticles) to the pore surfaces of a porous framework using a boric acid derivative (e.g., borospiranic acid), and Figure 9B shows an exemplary reaction scheme for crosslinking multiple particles comprising multiple sheets within a polymer structure using a boric acid derivative. [Figure 10] FIG. 10 shows an exemplary proton exchange membrane water electrolysis system incorporating a boron-containing porous membrane. [Figure 11] FIG. 11 shows an exemplary proton exchange membrane fuel cell incorporating a boron-containing porous membrane. DETAILED DESCRIPTION OF THE INVENTION

[0018] Described herein are boron-containing porous membranes and methods for making and using the same. In some examples, the boron-containing porous membranes include a porous framework and boron-based acidic groups covalently bonded to the porous framework. The porous framework may be formed of an amorphous or crystalline inorganic material and / or a synthetic or natural polymer. The boron-based acidic groups may be boric acid derivatives, such as cyclic boric acid derivatives, borospiranic acid, or borospiranic acid derivatives. In some examples, the boron-based acidic groups are the reaction product of boric acid or a boric acid derivative with a polyhydroxy compound.

[0019] 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 achieved by protons ionically bonded to tetravalent negatively charged boron atoms. The presence of oxygen-boron bonds improves the hydrophilicity of the porous framework and stabilizes the negatively charged boron atoms. The boron-containing PEMs described herein also have high mechanical strength, high proton conductivity, low electronic conductivity, chemical stability under high pH gradients, durability, and low manufacturing costs. The boron-containing porous membranes can be fabricated from boric acid and its precursor (borax), which are naturally abundant and inexpensive. In some instances, the boron-containing porous membranes described herein are non-toxic.

[0020] The boron-containing porous membranes described herein can also be used to filter and / or neutralize pathogens such as bacteria, viruses, and fungal spores. For example, the boron-containing porous membranes may be incorporated into face masks, surgical masks, air filters, and air purifiers for enclosed spaces (e.g., homes, offices, hospitals, factories, vehicles, airplanes, etc.).

[0021] The devices, compositions, and methods described herein may provide one or more of the benefits described above and / or various additional and / or alternative benefits that may become apparent herein. Various embodiments will now be described in more detail with reference to the Figures. It will be understood that the following embodiments are merely illustrative and not limiting, as various variations may be made within the scope of this disclosure.

[0022] 1 illustrates an exemplary proton exchange membrane (PEM) 100. The PEM 100 includes a porous framework 102 and boron-based acidic groups 104 distributed throughout the porous framework 102 and attached to the pore surfaces of the porous framework 102.

[0023] The porous framework 102 can be formed of any suitable material or combination of materials, including inorganic and / or organic materials. Suitable inorganic materials include amorphous inorganic materials (e.g., glass, fused silica, or ceramic) and / or crystalline inorganic materials (e.g., quartz, single crystal silicon, or alumina). Suitable organic materials include, for example, synthetic and / or natural polymers (e.g., cellulose).

[0024] The PEM 100 may have a thickness d ranging from a few microns to hundreds of microns. With the structure described herein, the PEM 100 can withstand a pressure differential of up to 30 atmospheres across the membrane and an acidic pH gradient. The PEM 100 may be permeable to water and protons, which can travel through the PEM 100 as indicated by arrows 106, but is typically impermeable to gases, including hydrogen and oxygen.

[0025] The boron-based acidic groups 104 may be attached to the pore surfaces within the PEM 100 in two or more different configurations, as shown in Figures 2A and 2B, which illustrate exemplary configurations of the porous framework 102 and boron-based acidic groups 104 in the PEM 100. It will be appreciated that each of Figures 2A and 2B shows only a portion of the PEM 100 and is representative of the characteristics of the entire PEM 100.

[0026] In the first exemplary structure 200A shown in Figure 2A, the pore surface adjacent to the pore 204 is functionalized with boron-based acidic groups 104 such that the boron-based acidic groups 104 are bonded to the pore surface 202. Although Figure 2A shows only one boron-based acidic group 104 bonded to the pore surface 202, the framework 102 of the porous structure may have any number and concentration of boron-based acidic groups 104 bonded to the pore surface 202.

[0027] In the second exemplary structure 200B shown in Figure 2B, the porous structure framework 102 includes solid support particles 206 (e.g., solid support particles 206-1 to 206-4) linked together by boron-based acidic groups 104. The linked solid support particles 206 form pores 208 in the spaces between the solid support particles 206. The boron-based acidic groups 104 are bonded to pore surfaces 210 (e.g., pore surfaces 210-1 to 210-4 of solid support particles 206-1 to 206-4). Although Figure 2B shows four solid support particles 206 linked by boron-based acidic groups 104, any other number of solid support particles 206 may be linked by the boron-based acidic groups 104. Additionally, each solid support particle 206 may be linked to one or more additional solid support particles by one or more additional boron-based acidic groups, thereby forming a porous framework 102 having boron-based acidic groups 104 attached to the pore surfaces 210 of the porous framework 102.

[0028] The solid support particles 206 may be formed of any suitable material, such as any of the materials described above for the porous framework 102, for example, inorganic molecules (e.g., fused silica particles, ceramic particles, etc.) or organic molecules (e.g., polymers). The solid support particles 206 may have any suitable shape and size ranging from tens of nanometers (nm) to hundreds of microns. The porosity of the PEM 100 can be controlled and determined by the size of the solid support particles 206. The solid support particles 206 may also be selected for mechanical strength, durability in high pH gradient environments, and / or affinity for water (e.g., the solid support particles may be selected to be hydrophilic or hydrophobic depending on the balance of water affinity required for the PEM 100).

[0029] In some examples, the boron-based acidic group 104 comprises a boric 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 in formula (I): [ka] It has.

[0030] A boric acid derivative can be any compound or group in which one, two, or all of the hydroxyl (OH) groups of boric acid have reacted and become linked to one or more other compounds. Exemplary boric acid derivatives are described in detail below.

[0031] The boric acid derivative may be formed by any suitable method. In some examples, the boric acid derivative is formed by the reaction of boric acid or other boric acid derivatives with one or two hydroxyl groups of one or more other compounds. For example, boric acid or a boric acid derivative may be reacted with a polyhydroxy compound having two or more cis-vicinity hydroxyl groups. The boric acid derivative may include any suitable boric acid derivative, such as, but not limited to, cyclic boric acid derivatives, borospiranic acid, borospiranic acid derivatives, and any other boric acid derivatives described herein. The polyhydroxy compound may be any suitable polyhydroxy compound, such as a polyol, a sugar, a sugar alcohol (e.g., glycerol, mannitol, or sorbitol), a catechol, or a derivative of any of these. In some examples, the polyhydroxy compound has the following formula (IIa) or (IIb): [ka] wherein W, X, Y, and Z are pendant moieties and may each independently be selected from the group consisting of hydrogen (H), hydroxyl (OH), fluoro (F), chloro (Cl), dialkylamino (NR), cyano (CN), carboxylic acid (COOH), carboxylic acid amide, carboxylic acid ester, alkyl group, alkoxy group, and aryl group. In some examples, any one or more of the groups W, X, Y, and Z may be a C1-C aryl group that may optionally include one or more substituents, such as oxygen (O), hydroxyl (OH), fluoro (F), chloro (Cl), dialkylamino (NR), cyano (CN), carboxylic acid (COOH), carboxylic acid amide, carboxylic acid ester, alkyl group, alkoxy group, and aryl group. 30 It may represent an alkyl chain.

[0032] Exemplary reaction schemes for making boric acid derivatives by reaction of boric acid or a boric acid derivative with a polyhydroxy compound are shown and described with reference to Figures 3A and 3B. It will be appreciated that the following reaction schemes are illustrative only and not limiting.

[0033] FIG. 3A shows an exemplary reaction scheme 300A for synthesizing a cyclic boric acid derivative. As shown, boric acid 302 combines with glycerol 304 to produce cyclic boric acid derivative 306. Glycerol 304 is represented by formula (IIa), where X, Y, and Z are each hydrogen (H), and W is a hydroxymethyl group (CHOH). While FIG. 3A shows boric acid reacting with glycerol 304, boric acid 302 may 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 boric acid derivative may be used in place of boric acid 302.

[0034] FIG. 3B shows an exemplary reaction scheme 300B for synthesizing borospiranic acid, which is also a boric acid derivative. As shown, the cyclic boric acid derivative 306 produced by reaction scheme 300A shown in FIG. 3A combines with another glycerol molecule 304 to produce borospiranic acid 308. Boric acid 302 is a weak acid, but in the presence of a polyhydroxy compound, such as a sugar alcohol (e.g., glycerol 304), it forms borospiranic acid 308, which is a strong acid. While FIG. 3B shows cyclic boric acid derivative 306 reacting with glycerol 304, cyclic boric acid derivative 306 may react with any other suitable sugar, sugar alcohol (e.g., mannitol, sorbitol, etc.), or polyhydroxy compound. Additionally or alternatively, cyclic boric acid derivative 306 may be replaced with any other suitable cyclic boric acid derivative.

[0035] Reaction schemes 300A and 300B shown in Figures 3A and 3B may be used to generate boron-based acidic groups 104 attached to pore surfaces 202 (see Figure 2A) and / or to generate boron-based acidic groups 104 attached to pore surfaces 210 (see Figure 2B). In some examples, the surface of a solid support material (e.g., pore surfaces 202 or particle surfaces 210) may be functionalized with boric acid derivatives or polyhydroxy compounds. Exemplary reaction schemes for forming boron-based acidic groups 104 attached to pore surfaces of a porous structure framework 102 are shown and described with reference to Figures 4A-9B. It will be appreciated that these reaction schemes are merely illustrative and not limiting.

[0036] FIG. 4A shows an exemplary reaction scheme 400A for synthesizing a monocyclic boronic acid derivative bound to a solid support. As shown, solid support 402 is functionalized with a cis-1,2-dihydroxy group 404 having a cis-adjacent dihydroxy group. Solid support 402 may be formed from any inorganic or organic solid support material described herein for porous framework 102 (e.g., glass, ceramic, synthetic polymer, natural polymer) and may be selected for its mechanical strength, durability in high pH gradient environments, and / or affinity for water (e.g., hydrophilic or hydrophobic properties may be selected depending on the balance of water affinity required for PEM 100). Solid support 402 may be a particle similar to solid support 206 described with reference to FIG. 2B. Alternatively, solid support 402 may be part of porous framework 102 described with reference to FIG. 2A.

[0037] As shown in Figure 4A, cis-1,2-dihydroxy group 404 has a structure represented by formula (IIa) and includes a linker chain 406 (represented by pendant group W or X in formula (IIa)) attached to pendant moiety Y' and solid support 402. Linker chain 406 includes a C1-C 30is an alkyl chain, optionally having one or more pendant moieties X', which may be the same as or different from each atom of the linker chain 406. X' and Y' may each independently be selected from the group consisting of hydrogen (H), a hydroxyl group (OH), a fluoro group (F), a chloro group (Cl), a dialkylamino group (NR2), a cyano group (CN), a carboxylic acid (COOH), a carboxylic acid amide, a carboxylic acid ester, an alkyl group, an alkoxy group, and an aryl group.

[0038] The cis-1,2-dihydroxy group 404 reacts with boric acid 408 to produce a monocyclic boronic acid derivative 410 bound to the solid support 402. In this configuration, the solid support-bound cyclic boronic acid derivative 410 can provide boron-based acidic groups 104 bound to the pore surfaces of the framework 102 of the porous structure of the PEM 100. Alternatively, the solid support-bound cyclic boronic acid derivative 410 can be further reacted with polyhydroxy compounds to produce other boric acid derivatives (e.g., borospiranic acids), as described herein with reference to Figure 4B.

[0039] 4B shows an exemplary reaction scheme 400B for synthesizing a solid support-bound borospiranic acid having pendant moieties. As shown, a solid support-bound cyclic boronic acid derivative 410 (produced in reaction scheme 400A shown in FIG. 4A) is coupled to a polyhydroxy compound 412 having pendant moieties A', B', C', and D'. Polyhydroxy compound 412 has a structure represented by Formula (IIa). Thus, pendant moieties A', B', C', and D' can correspond to pendant moieties W, X, Y, and Z described above with respect to Formula (IIa).

[0040] The solid support-bound cyclic boric acid derivative 410 and polyhydroxy compound 412 react to produce a solid support-bound borospiranic acid 414 having pendant moieties A', B', C', and D'. With this structure, the solid support-bound borospiranic acid 414 can provide boron-based acidic groups 104 bound to the pore surfaces of the framework 102 of the porous structure of the PEM 100. The solid support-bound borospiranic acid 414 can exhibit strong proton exchange properties, depending on the electronic nature of the A', B', C', D', X', and Y' substitutions.

[0041] In examples where the solid support 402 comprises a polymer network, the solid support-bound boric acid derivative 410 and the solid support-bound borospiranic acid 414 may be used to form an ionomer for a catalyst bound to a catalyst layer in a membrane electrode assembly, as described below with reference to Figures 10 and 11, with controlled loadings of boric acid and borospiranic acid, respectively.

[0042] 5A shows an exemplary reaction scheme 500A for synthesizing a solid support-bound cyclic boronic acid derivative incorporating 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.

[0043] In reaction scheme 500A, solid support 502 is functionalized with catechol derivative 504. Solid support 502 can be the same as or similar to solid support 402. Catechol derivative 504 has a structure represented by formula (IIb) and includes pendant moieties W", Y", and Z", and linker chain 506 (represented by pendant group X in formula (IIb)) attached to solid support 502. Linker chain 506 can be any of C1 to C 30is an alkyl chain, optionally having one or more pendant moieties X″, which may be the same as or different from each atom of linker chain 506. The pendant moieties W″, X″, Y″, and Z″ may each independently be selected from the group consisting of hydrogen (H), hydroxyl (OH), fluoro (F), chloro (Cl), dialkylamino (NR), cyano (CN), carboxylic acid (COOH), carboxylic acid amide, carboxylic acid ester, alkyl group, alkoxy group, and aryl group.

[0044] Catechol derivative 504 reacts with boric acid 508 to produce a cyclic boric acid derivative 510 attached to a solid support incorporating the catechol derivative. If solid support 502 is a polymer, the catechol structure may be introduced during polymerization (e.g., to form a catechol-formaldehyde resin), as described below with reference to Figure 6. Alternatively, the catechol structure may be introduced post-polymerization (e.g., by functional modification of a Merrifield-type resin).

[0045] With this structure, the cyclic boric acid derivatives 510 attached to the solid support incorporating the catechol derivatives can result in boron-based acidic groups 104 attached to the pore surfaces of the framework 102 of the porous structure of the PEM 100. Alternatively, the cyclic boric acid derivatives 510 attached to the solid support incorporating the catechol derivatives can be further reacted with polyhydroxy compounds to produce other boric acid derivatives (e.g., borospiranic acid derivatives), as described herein with reference to Figure 5B.

[0046] 5B shows an exemplary reaction scheme 500B for synthesizing a solid support-bound borospiranic acid derivative incorporating a catechol derivative and a pendant moiety. In reaction scheme 500B, a solid support-bound cyclic boronic acid derivative 510 incorporating a catechol derivative is coupled to a polyhydroxy compound 512 having pendant moieties A", B", C", and D". Polyhydroxy compound 512 has a structure represented by formula (IIa). Thus, pendant moieties A", B", C", and D" can correspond to pendant moieties W, X, Y, and Z described above with respect to formula (IIa).

[0047] The cyclic boric acid derivative 510, bound to a solid support incorporating a catechol derivative, combines with a polyhydroxy compound 512 to produce a borospiranic acid 514, bound to a solid support incorporating a catechol derivative and comprising pendant moieties A", B", C", and D". With this structure, the borospiranic acid 514, bound to a solid support incorporating a catechol derivative and comprising pendant moieties, can provide boron-based acidic groups 104 bound to the pore surfaces of the framework 102 of the porous structure in the PEM 100.

[0048] 6 shows an exemplary reaction scheme 600 for synthesizing a crosslinked copolymer containing boron-based acidic groups. In reaction scheme 600, catechol 602, formaldehyde 604, and boric acid 606 combine and polymerize to produce a catechol-formaldehyde-boric acid crosslinked copolymer 608 ("copolymer 608"). Copolymer 608 includes catechol structures 610 linked by borospiranic acid groups 612. Individual polymers of copolymer 608 are crosslinked by bridges 614 between the catechol structures 610. Copolymer 608 may also include other phenolic monomers, such as phenol, resorcinol, and trihydroxyphenol.

[0049] The copolymer 608 is suitable for PEM and ionomer applications. For example, the structure of the PEM 100 shown in Figure 2B is formed by bonding and linking borospiranic acid groups 612 (corresponding to the boron-based acid groups 104) with catechol structures 610 (corresponding to the solid support particles 206), thereby forming the porous framework 102. The mol% content of the borospiranic acid groups 612 may be controlled for optimal functional properties as a PEM 100 or as a catalytic ionomer for catalysts bound to catalyst layers in MEAs, as described below with reference to Figures 10 and 11.

[0050] As previously mentioned, in some examples, pore surfaces (e.g., pore surfaces 202) within the porous structure framework 102 are functionalized with boron-based acidic groups 104 (see FIG. 2A). Figures 7-9A show exemplary reaction schemes for functionalizing the pore surfaces of the porous structure framework with boron-based acidic groups. These reaction schemes are compatible with solid support materials that have one or more hydroxyl groups on the pore surfaces. Examples of such materials include, but are not limited to, silica, glass, alumina, clay, synthetic polymers, and cellulose.

[0051] FIG. 7 shows an exemplary reaction scheme 700 for functionalizing a pore surface with boron-based acidic groups. As shown in FIG. 7, a solid support 702 includes a surface 704 and hydroxyl groups 706 on the surface 704. The surface 704 of the solid support 702 can be a pore surface of the porous framework 102 (e.g., surface 202 shown in FIG. 2A) or a solid support particle (e.g., surface 210 of a solid support particle 206 shown in FIG. 2B). While FIG. 7 shows that the surface 704 has only one hydroxyl group 706, the surface 704 may have any other amount 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 groups 706 to form boric acid derivatives 710 attached to the surface 704. The boric acid derivative 710 bound to the solid support may result in boron-based acidic groups 104 attached to the pore surfaces of the framework 102 of the porous structure of the PEM 100. Alternatively, the boric acid derivative 710 bound to the solid support may be further reacted with boric acid, other boric acid derivatives, or polyhydroxy compounds to produce other boric acid derivatives.

[0052] FIG. 8A shows another exemplary reaction scheme 800A for functionalizing the pore surfaces of a porous structure framework with boron-based acidic groups. Reaction scheme 800A is similar to reaction scheme 700, except that boric acid 708 reacts with two hydroxyl groups 706 on surface 704 to produce cyclic boric acid derivative 802. In some examples, surface 704 may be pre-activated to increase the surface density of hydroxyl groups 706. Any suitable pre-activation process may be used. The cyclic boric acid derivative 802 may result in boron-based acidic groups 104 attached to the pore surfaces of the PEM porous structure framework 102. Alternatively, the cyclic boric acid derivative 802 may be further reacted to produce other boric acid derivatives, as described herein with reference to FIG. 8B.

[0053] FIG. 8B shows another exemplary reaction scheme 800B for functionalizing the pore surfaces of a porous framework with a boric acid derivative (e.g., borospiranic acid groups). In reaction scheme 800B, cyclic boric acid derivative 802 produced in reaction scheme 800A shown in FIG. 8A is coupled to polyhydroxy compound 804 having pendant moieties A′″, B′″, C′″, and D′″. Polyhydroxy compound 804 has a structure represented by Formula (IIa). Thus, pendant moieties A′″, B′″, C′″, and D′″ can correspond to pendant moieties W, X, Y, and Z described above with respect to Formula (IIa). Cyclic boric acid derivative 802 and polyhydroxy compound 804 react to produce borospiranic acid having pendant moieties 806, which are attached to pore surface 704. A borospiranic acid bound to a solid support having pendant moieties 806 can result in boron-based acidic groups 104 attached to the pore surfaces of the framework 102 of the porous structure of the PEM 100 .

[0054] 9A shows an exemplary reaction scheme 900A for attaching particles (e.g., nanoparticles or microparticles) to the pore surfaces of a porous framework using a boric acid derivative (e.g., borospiranic acid). The porosity of the PEM 100 can be controlled by the size of the particles 902. The particles 902 may be selected for mechanical strength, durability in high pH gradient environments, and / or affinity for water (e.g., solid support particles may be selected to be hydrophilic or hydrophobic depending on the balance of water affinity required for the PEM 100).

[0055] Scheme 900A is similar to Scheme 800A, except that particles 902 having hydroxyl groups 904 are bonded to boric acid 708. As a result, hydroxyl groups 706 on pore surfaces 704 are cross-linked with hydroxyl groups 904 on the surface of particles 902 using boric acid 708. This cross-coupling reaction results in the linking of particles 902 to pore surfaces 704 via borospiranic acid 906. Particles 902 may be formed of any suitable material (e.g., silica, glass, alumina, ceramic, clay, synthetic polymer, cellulose) and may be the same or different from the material of solid support 702.

[0056] Reaction Scheme 900A can be controlled and performed in any order. In some examples, the first step of Reaction Scheme 900A involves performing Reaction Scheme 800A to produce solid support-bound cyclic boronic acid derivative 802. In the second step, particle 902 can be exposed to solid support-bound cyclic boronic acid derivative 802 to produce solid support-bound borospiranic acid 906. In another method, in the first step, particle 902 can be combined with boric acid 708 to produce an intermediate boric acid derivative. In the second step, hydroxyl groups 706 on surface 704 of solid support 702 can be exposed to intermediate boric acid derivative and react to produce solid support-bound borospiranic acid 906, linking particle 902 to solid support 702. In yet another example, Reaction Scheme 900A can be performed by combining all reactants in a single step.

[0057] 9B shows an exemplary reaction scheme 900B for crosslinking multiple particles, including multiple sheets, within a polymer structure using a boric acid derivative. Reaction scheme 900B is similar to reaction scheme 900A, except that in reaction scheme 900B, particles 902 (first particles 902) are crosslinked with second particles 908 that present two hydroxyl groups 910 (instead of being crosslinked with hydroxyl groups 706 on pore surfaces 704). First particles 902 and second particles 908 are each microparticles or nanoparticles and can be of any size (e.g., ranging from a few nanometers to hundreds of microns) as may be suitable for a particular implementation. Reaction scheme 900B produces particles 912 linked with borospiranic acid groups 914. The first particle 902 and the second particle 908 may be made of the same material or using different materials, which may be selected from any solid support material described herein (e.g., silica, glass, alumina, ceramic, clay, synthetic polymer, cellulose, etc.).

[0058] Reaction scheme 900B can be used to efficiently use second structure 200B shown in Figure 2B to fabricate PEM 100. The porosity of PEM 100 can be determined by the size of first particles 902 and second particles 908.

[0059] The boron-containing porous membranes described herein may be used in water electrolysis and / or fuel cell applications, exemplary applications of which are now described with reference to Figures 10 and 11.

[0060] 10 illustrates an exemplary proton exchange membrane water electrolysis system 1000 (PEM water electrolysis system 1000) incorporating a boron-containing porous membrane. The PEM water electrolysis system 1000 uses electricity to split water into oxygen (O) and hydrogen (H) through an electrochemical reaction. The structure of the PEM water electrolysis system 1000 is exemplary only and not limiting, as other suitable structures, as well as other suitable water electrolysis systems, may incorporate a boron-containing porous membrane.

[0061] As shown in Figure 10, 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 supply 1008. PEM water electrolysis system 1000 may also include additional or alternative components not shown in Figure 10, as may be feasible for a particular implementation.

[0062] The MEA 1002 includes a PEM 1010 disposed between a first catalyst layer 1012-1 and a second catalyst layer 1012-2. The PEM 1010 converts protons (H + The PEM 1010 electrically separates the first catalyst layer 1012-1 from the second catalyst layer 1012-2 while providing selective conductivity for cations such as cations (e.g., cations of cations), and being impermeable to gases such as hydrogen and oxygen. The PEM 1010 can be any suitable PEM. For example, the PEM 1010 can be a boron-containing porous membrane (e.g., PEM 100) that includes a porous framework having boron-based acidic groups attached to the pore surfaces within the porous framework.

[0063] First catalyst layer 1012-1 and second catalyst layer 1012-2 are conductive electrodes with embedded electrochemical catalysts (not shown), such as platinum, ruthenium, and / or cerium (IV) oxide. In some examples, first catalyst layer 1012-1 and second catalyst layer 1012-2 are formed using ionomers that bind catalytic nanoparticles. As previously mentioned, the ionomers used to form first catalyst layer 1012-1 and second catalyst layer 1012-2 may include boron-based acidic groups, such as copolymer 608 (see FIG. 6), as described herein.

[0064] MEA 1002 is disposed between porous transport layers 1004-1 and 1004-2, which in turn is disposed between bipolar plates 1006-1 and 1006-2, with channels 1014-1 and 1014-2 located between bipolar plate 1006 and porous transport layer 1004.

[0065] In the MEA 1002, the first catalyst layer 1012-1 functions as the anode and the second catalyst layer 1012-2 functions as the cathode. When the PEM water electrolysis system 1000 is powered by a power supply 1008, the oxygen evolution reaction (OER) occurs at the anode 1012-1, as represented by the following electrochemical half-reaction: 2H2O→O2+4H + +4e - 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 by a conduction path around the PEM 1010. The PEM 1010 transports protons (H + At cathode 1012-2, protons combine with electrons in the hydrogen evolution reaction (HER), which is represented by the following electrochemical half-reaction: 4H + +4e - →2H2

[0066] OER and HER are two complementary electrochemical reactions that split water using electricity and are represented by the overall water electrolysis reaction: 2H2O → 2H2 + O2

[0067] FIG. 11 illustrates an exemplary proton exchange membrane fuel cell 1100 (PEM fuel cell 1100) including a boron-containing porous membrane. The PEM fuel cell 1100 produces electricity as a result of an electrochemical reaction. In this example, the electrochemical reaction involves reacting hydrogen gas (H) and oxygen gas (O) to produce water and electricity. The structure of the PEM fuel cell 1100 is illustrative only and not limiting, as other suitable structures may incorporate a boron-containing porous membrane, as well as other suitable proton exchange membrane fuel cells.

[0068] As shown in Figure 11, 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 device 1108 is electrically connected to MEA 1102 and is powered by PEM fuel cell 1100. PEM fuel cell 1100 may also include additional or alternative components not shown in Figure 11 to effect a particular implementation.

[0069] The MEA 1102 includes a PEM 1110 disposed between a first catalyst layer 1112-1 and a second catalyst layer 1112-2. The PEM 1110 converts protons (H + The first catalyst layer 1112-1 electrically separates the second catalyst layer 1112-2 from the second catalyst layer 1112-2 while providing selective conduction of cations such as cations (e.g., cations of ...

[0070] The first catalyst layer 1112-1 and the second catalyst layer 1112-2 are conductive electrodes with an embedded electrochemical catalyst (not shown). In some examples, the first catalyst layer 1112-1 and the second catalyst layer 1112-2 are formed using an ionomer that binds catalyst nanoparticles. In some examples, the ionomer used to form the first catalyst layer 1112-1 and the second catalyst layer 1104-2 includes an ionomer incorporating a boric acid derivative as described herein, such as copolymer 608 (see FIG. 6).

[0071] The MEA 1102 is disposed between porous transport layers 1104-1 and 1104-2, and consequently between bipolar plates 1106-1 and 1106-2, with a flow channel 1114 located therebetween. In the MEA 1102, the first catalyst layer 1112-1 functions as the cathode, and the second catalyst layer 1112-2 functions as the anode. The cathode 1112-1 and the anode 1112-2 are electrically connected to a device 1108, and electricity is produced by the PEM fuel cell 1100 that powers the device 1108.

[0072] During operation of the PEM fuel cell 1100, hydrogen gas (H) flows into the anode side of the PEM fuel cell 1100, and oxygen gas (O 2 ) flows into the cathode side of the PEM fuel cell 1100. At the anode 1112-2, the hydrogen molecules are catalytically converted to protons (H + ) and electrons (e - ) 2H2→4H + +4e - Protons travel through PEM 1100 from anode 1112-2 to cathode 1112-1, and electrons are conducted from anode 1112-2 to cathode 1112-1 around PEM 1110 through conduction paths and device 1108. At cathode 1112-1, the protons and electrons combine with oxygen gas according to the oxygen reduction reaction (ORR): O2+4H + +4e - →2H2O Therefore, the overall electrochemical reaction in the PEM fuel cell 1100 is: 2H2+O2→2H2O is.

[0073] In an overall reaction, the PEM fuel cell 1100 produces water at the cathode 1112-1. The water can flow through the PEM 1110 from the cathode 1112-1 to the anode 1112-2 and can be removed through outlets on the cathode and / or anode sides of the PEM fuel cell 1100. The overall reaction generates electrons at the anode that power the device 1108.

[0074] The boron-containing porous membranes described herein (e.g., PEM100) may be used as pathogen-neutralizing porous membranes. For example, the porous structural framework 102 may have pores small enough to prevent the migration of pathogens, such as bacteria, fungal spores, and viruses. The boron-based acidic groups 104 may also have anti-pathogenic activity against bacteria, fungi, and viruses, including SARS-CoV-2. For example, basic protein sites on pathogens, including SARS-CoV-2, may ionically bind with the acidic boron sites of the proton exchange membrane, thereby preventing the migration of the pathogens through the proton exchange membrane. As a result, the proton exchange membranes may be implemented in face masks, surgical masks, and air filters and air purifiers for enclosed spaces (e.g., homes, offices, hospitals, factories, vehicles, aircraft, etc.).

[0075] In the foregoing description, various exemplary embodiments have been described with reference to the accompanying figures. However, it will be apparent that various modifications and variations may be made and further embodiments may be implemented without departing from the scope of the following claims. For example, certain features of one embodiment described herein may be combined with or substituted for features of other embodiments described herein. Accordingly, the specification and drawings should be regarded in an illustrative, rather than a restrictive, sense.

[0076] In the foregoing description, various exemplary embodiments have been described with reference to the accompanying figures. However, it will be apparent that various modifications and variations may be made and further embodiments may be implemented without departing from the scope of the invention as set forth in the following claims. For example, certain features of one embodiment described herein may be combined with or substituted for features of other embodiments described herein. Accordingly, the specification and drawings should be regarded in an illustrative, rather than a restrictive, sense.

Claims

1. A porous framework; a plurality of tetravalent boron-based acidic groups attached to the framework of the porous structure by linkers; A proton exchange membrane, wherein the plurality of tetravalent boron-based acidic groups comprises borospiranic acid, and the proton exchange membrane provides selective conduction of protons through the porous structural framework.

2. 2. The proton exchange membrane of claim 1, wherein the linker comprises a C1-C30 alkyl chain and optionally has one or more pendant moieties X', each of which may be independently selected from the group consisting of hydrogen (H), hydroxyl (OH), fluoro (F), chloro (Cl), dialkylamino (NR2), cyano (CN), carboxylic acid (COOH), carboxylic acid amide, carboxylic acid ester, alkyl group, alkoxy group, and aryl group.

3. The proton exchange membrane of claim 1 , wherein the plurality of tetravalent boron-based acidic groups comprises a catechol derivative.

4. 10. The proton exchange membrane of claim 1, wherein the porous structural framework comprises solid support particles linked by the plurality of tetravalent boron-based acidic groups.

5. the porous structural framework comprises a porous polymer network; 10. The proton exchange membrane of claim 1, wherein the plurality of tetravalent boron-based acidic groups are attached to the pore surfaces of the polymer network.

6. The proton exchange membrane of claim 1 , wherein the porous structural framework comprises an inorganic material.

7. functionalizing a plurality of pore surfaces contained in the framework of the porous structure by a linker with a plurality of tetravalent boron-based acidic groups; A method of making a proton exchange membrane, wherein the plurality of tetravalent boron-based acidic groups comprises borospiranic acid and provides selective conduction of protons through the porous structural framework.

8. 8. The method of claim 7, wherein the linker comprises a C1-C30 alkyl chain and optionally has one or more pendant moieties X', each of which may be independently selected from the group consisting of hydrogen (H), hydroxyl (OH), fluoro (F), chloro (Cl), dialkylamino (NR2), cyano (CN), carboxylic acid (COOH), carboxylic acid amide, carboxylic acid ester, alkyl group, alkoxy group, and aryl group.

9. The method of claim 7 , wherein the plurality of tetravalent boron-based acidic groups comprises a catechol derivative.

10. 8. The method of claim 7, wherein said functionalizing comprises reacting a polyhydroxy compound with a boric acid derivative attached to the surface of said plurality of pores.

11. The method of claim 7, further comprising binding nanoparticles to the pore surfaces by the tetravalent boron-based acidic groups.

12. a cathode; an anode; a proton exchange membrane disposed between the cathode and the anode; A membrane electrode assembly, wherein the proton exchange membrane comprises a porous framework and tetravalent boron-based acidic groups attached to the surfaces of pores in the porous framework.

13. at least one of the anode or the cathode includes a catalyst and an ionomer for binding the catalyst; 13. The membrane electrode assembly of claim 12, wherein the ionomer comprises tetravalent boron-based acidic groups.

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