Polynorbornene-based ion exchange membranes derived from phosphonated polymer and methods of making same

Abstract

A proton exchange polymer comprises a polynorbornene copolymer with hydrophobic and hydrophilic blocks that can be phosphonated to produce phosphonic acid functional groups for proton exchange. Also, the polymer may be crosslinked to form quaternary ammonium groups on the side chains. The polynorbornene copolymer may be acid doped to ionically bond phosphonic acids to the quaternary ammonium groups that may for ion pairs for proton exchange. The proton exchange polymer has high temperature stability with the phosphonic acid functional group and can be mechanically durable with cross linking. Proton exchange membranes may utilize the proton exchange membrane in fuel cell and electrolyzer applications.

Claims

1. A proton exchange polymer comprising: a) a backbone comprising polynorbornene; b) a side chain extending from the backbone; and c) a functional group coupled to said side chain and comprising phosphonic acid covalently bonded to said side chain.

2. The proton exchange polymer of claim 1, wherein the functional group further comprises quaternary ammonium.

3. The proton exchange polymer of claim 2, further comprising ionically bonded phosphonic acid that forms an ion pair with the quaternary ammonium.

4. The proton exchange polymer of claim 3, further comprising a cross-linker; wherein the cross-linker is in a concentration of 0.1 mol % to 30 mol %.

5. The proton exchange polymer of claim 4, wherein the cross-linker is a branched crosslinker.

6. The proton exchange polymer of claim 5, wherein the branched cross-linker comprises triamine.

7. The proton exchange polymer of claim 4, wherein the cross-linker includes a diamine compound.

8. The proton exchange polymer of claim 7, wherein the cross-linker includes tetramethyl hexyldiamine (TMHDA).

9. The proton exchange polymer of claim 1, further comprising a secondary crosslinking agent that are tertiary diamine head groups which include DABCO (1,4-diazabicyclo[2,2,2]octane) and TMHDA (N,N,N,N-tetramethylhexane diammonium), 1,4-diiodobutane.

10. The proton exchange polymer of claim 1, wherein the side chain comprises aliphatic hydrocarbon side chains.

11. The proton exchange polymer of claim 10, wherein the aliphatic hydrocarbon side chains comprise four or more carbons.

12. The proton exchange polymer of claim 1, wherein the backbone is a co-polymer of hydrophobic blocks and hydrophilic blocks.

13. The proton exchange polymer of claim 12, wherein the hydrophilic block comprises bromopropyl norbornene.

14. The proton exchange polymer of claim 12, wherein the hydrophobic block comprises Butyl norbornene.

15. A proton exchange membrane comprising the proton exchange polymer of claim 1, wherein the proton exchange membrane is a thin sheet of material having a thickness of less than 200 m.

16. The proton exchange membrane of claim 15, further comprising a recombination catalyst selected from the group consisting of platinum nanoparticles, platinum nanoparticles supported on carbon, platinum nanoparticles supported on oxides, platinum alloy nanoparticles, platinum alloy nanoparticles supported on carbons, and platinum alloy nanoparticles supported on oxides.

17. The proton exchange membrane of claim 15, further comprising a radical scavenger that is an antioxidant selected from the group consisting of Cerium (Ce), Manganese (Mn), phenolic compounds, nitrogen-containing heterocyclic compounds, quinones, amine, phosphites, phosphonites, and thioesters.

18. The proton exchange membrane of claim 15, further comprising a free radical inhibitor selected from the group consisting of inorganic free radical scavenger such as metal organic frameworks (MOF) including cerium oxides and metal oxide.

19. The proton exchange membrane of claim 15, further comprising a plasticizer selected from the group consisting of nylon 6,6, Glycerol, ionic liquids.

20. The proton exchange membrane of claim 15, further comprising a filler selected from the group consisting of oxides of aluminum, silicon, titanium, zirconium and zirconium phosphate, cesium phosphate, zeolites, clays and carbon black, multiwall carbon nanotubes, reduced graphene oxide.

21. The proton exchange membrane of claim 15, further comprising: a) a support layer; wherein the proton exchange polymer extends through the support layer from a first side to a second side of the support layer to produce a composite proton exchange membrane.

22. The proton exchange membrane of claim 21, wherein the support layer has pores and wherein the proton exchange polymer extends on a first side of said support layer.

23. A proton exchange system comprising: a) a proton exchange membrane comprising the proton exchange polymer claim 1; b) an anode; and c) a cathode; wherein protons are conducted through the proton exchange membrane.

24. The proton exchange system of claim 23, wherein the proton exchange membrane is heated to a temperature of 130 C. or more.

25. A method of making the proton exchange polymer of claim 1, comprising: a) providing said polynorbornene co-polymer comprising a backbone and side chains; and b) phosphonating the polynorbornene co-polymer to produce a phosphonated norbornene co-polymer comprising phosphonic acid groups covalently bonded to said side chains.

26. The method of claim 25, further comprising crosslinking the polynorbornene co-polymer with a crosslinker, prior to phosphonating.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The accompanying drawings are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention, and together with the description explaining the principles of the invention.

(2) FIG. 1 shows a flow diagram of a method of making an exemplary proton exchange polymer as described herein.

(3) FIG. 2 shows the polymer synthetic scheme.

(4) FIG. 3 shows the polymer synthetic scheme with an additional crosslinking step and functionalization steps.

(5) FIG. 4 shows the exemplary proton exchange polymer with dopped phosphoric acid that forms an ion pair with the quaternary ammonium.

(6) FIG. 5 shows cross-sectional view of an exemplary proton exchange membrane comprising a thin sheet, less than 200 m thick of proton exchange polymer.

(7) FIG. 6 shows cross-sectional view of an exemplary porous reinforced support having a porous structure and pores therein, wherein the proton exchange polymer substantially fills the pores of the scaffold support.

(8) FIG. 7 shows a cross-sectional view of an exemplary ultra-thin composite proton exchange polymer film having a layer of proton exchange polymer on either side of the porous reinforced support.

(9) FIG. 8 shows cross-sectional view of an exemplary ultra-thin composite proton exchange polymer film formed by imbibing proton exchange polymer copolymer into a porous reinforced support using solution casting process, wherein the proton exchange polymer substantially fills the pores of the reinforced support.

(10) FIG. 9 shows cross-sectional view of an exemplary membrane electrode assembly comprising a composite proton exchange membrane with a cathode on a first side and an anode on a second side.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

(11) Corresponding reference characters indicate corresponding parts throughout the several views of the figures. The figures represent an illustration of some of the embodiments of the present invention and are not to be construed as limiting the scope of the invention in any manner. Further, the figures are not necessarily to scale, some features may be exaggerated to show details of components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for teaching one skilled in the art to variously employ the present invention.

(12) As used herein, the terms comprises, comprising, includes, including, has, having or any other variation thereof, are intended to cover a non-exclusive inclusion. For example, a process, method, article, or apparatus that comprises a list of elements is not necessarily limited to only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. Also, use of a or an are employed to describe elements and components described herein. This is done merely for convenience and to give a general sense of the scope of the invention. This description should be read to include one or at least one and the singular also includes the plural unless it is obvious that it is meant otherwise.

(13) Certain exemplary embodiments of the present invention are described herein and are illustrated in the accompanying figures. The embodiments described are only for purposes of illustrating the present invention and should not be interpreted as limiting the scope of the invention. Other embodiments of the invention, and certain modifications, combinations, and improvements of the described embodiments, will occur to those skilled in the art and all such alternate embodiments, combinations, modifications, improvements are within the scope of the present invention.

(14) Also as used herein, and/or refers to and encompasses any and all possible combinations of one or more of the associated listed items, as well as the lack of combinations when interpreted in the alternative (or).

(15) A weight percent (wt. %) of a component, unless specifically stated to the contrary, is based on the total weight of the formulation or composition in which the component is included.

(16) A mole percent (mol %) of a component, unless specifically stated to the contrary, is based on the total number of moles of each unit of the formulation or composition in which the component is included.

(17) The term crosslinking agent or crosslinker as used herein refers to a molecule, ion or other chemical unit capable of forming a chemical unit linking two parts of the same polymer chain or two different polymer chains.

(18) The term alkyl as used herein is a branched or unbranched saturated hydrocarbon group of 1 to 20 carbon atoms, such as methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, t-butyl, pentyl, hexyl, heptyl, octyl, nonyl, decyl, dodecyl, tetradecyl, hexadecyl, eicosyl, tetracosyl, and the like. The alkyl group can also be substituted or unsubstituted.

(19) As used herein, copolymer is a polymer made by reaction of at least two different monomers, with units of more than one kind.

(20) The term monomer as used herein refers to one of the constituent units used to synthesize a polymer.

(21) The term quaternary ammonium as used herein is represented by the formula NA.sup.4+ where A can be hydrogen or hydrocarbons and N is nitrogen.

(22) An exemplary polynorbornene-based proton exchange polymer is prepared by vinyl addition polymerization of halogenated alkyl norbornene and alkyl norbornene. Note that the side chain segment can be modified to engineer the channel of polymer by adjusting the cation tether length, the molecular length of crosslinker, and cross-linker functionality.

(23) One aspect of the invention relates to a polynorbornene copolymer comprising one or more norbornene-based hydrophilic blocks and one or more hydrophobic blocks. The polynorbornene copolymer is synthesized via vinyl addition polymerization method. In some embodiments, the one or more hydrophobic blocks are norbornene-based hydrophobic blocks. Norbornene-based hydrophobic blocks comprise hydrophobic monomers having a norbornene structure substituted with a saturated C1-C20 alkyl chain (e.g., C1, C2, C3, C4, C5, C6, C7, C8, C9, C10, C11, C12, C13, C14, C15, C16, C17, C18, C19, or C20 alkyl chain) branched or unbranched. In some embodiments, the alkyl chain may be halogenated (i.e., comprising one or more halogens (e.g., Cl, Br, F, or I) located throughout the alkyl chain. For example, in some embodiments, the alkyl chain is halogenated with bromine, chlorine or fluorine. In some embodiments, the halogen is located at the terminal position of the halogenated alkyl chain of the hydrophobic monomer. In some embodiments, the alkyl chain is non-halogenated. In some embodiments, the alkyl chain is a C3-C6 alkyl chain. When one or more norbornene-based hydrophobic monomers are combined a norbornene-based hydrophobic block is formed, wherein the number (n) of norbornene-based hydrophobic monomers can vary. In some embodiments, the number (n) of norbornene-based hydrophobic monomers is from about 10 to about 1,000, from about 100 to about 1,000, or from about 500 to about 1,000. For example, a norbornene-based hydrophobic block present in a multiblock copolymer of the invention comprises a structure represented by structure below, where wherein Ri is a saturated C1-C20 alkyl chain or halogenated alkyl chain branched or unbranched; and n is an integer from 1 about 1500:

(24) ##STR00001##

(25) The phosphonated polymers of the invention are ion exchange polymers with phosphonated polynorbornene backbones. Note that the side chain segment can be modified to engineer the channel of polymer by adjusting the cation tether length, the molecular length of crosslinker, and cross-linker functionality.

(26) The ion exchange membranes of the invention are PEMs composed of all-hydrocarbon backbones with long alkyl tethered side chains which contain a fixed phosphonic acid head-group. The polymers exhibit suitable chemical stability and physical characteristics that are desirable for use in electronic and electrochemical devices.

Example 1

(27) FIG. 1 shows a method or process for making an exemplary proton exchange membrane as described herein. In one embodiment, a proton exchange membrane is prepared by dissolving the precursor polymer in toluene at a 12.5% weight ratio i.e. 12.5 grams of polymer to 87.5 g of solvent. The mixture was stirred until homogenous and translucent. The concentration of the precursor polymer solution can vary between 0.1 wt % to 60 wt. %, and may between about 1 wt. % and 30 wt %, and may be about 5 wt. % or more, 10 wt. % or more, 15 wt. % or more, 20 wt. % or more, 30 wt. % or more, 50 wt % or more, or from about 5 wt. % to about 60 wt. % and any other range between and including the values provided.

(28) The precursor polymer solution may then be applied or coated onto a support layer, such as a microporous support layer (e.g., expanded polytetrafluoroethylene) tensioned to maintain dimensions upon application of the precursor polymer solution, such as being restrained in a chemically resistant plastic frame. The precursor polymer solution may then be coated on to the microporous reinforcement substrate by a doctor blade or brush or by dipping. The membrane may then be dried at room temperature.

(29) The precursor polymer may be converted to a proton exchange membrane by exchanging with phosphoric acid. The bromide groups may be reacted with triethyl phosphite to produce dimethyl phosphite that can then be reactive with said an acid to configure phosphonic acid as functional groups on the side chains.

(30) FIG. 2 shows the synthetic scheme for the phosphonation of a precursor polymer to produce a phosphonated polymer. In one embodiment, the precursor polymer 60 was added drop wise to triethyl phosphite 80 slowly under high temperature (e.g., 180 degree c.) and magnetically stirred overnight. This step is called addition of phosphonate esters. After the reaction was complete the solution with the intermediate polymer 62 was cooled. The intermediate polymer may have dimethyl phosphite on the side chains. There are two (2) options to deprotect of phosponate esters and form phosphonated polymer. First option: a) dissolve the intermediate polymer 62 in chloroform (CHCI.sub.3) and then add bromotrimethylsilane (BrMe.sub.3Si) 70 at room temp for 24 hours while stirring followed by aqueous work up. The resulting powder was filtered at least three times. Second option: b) Reflux the phosphonate esters in the hydrochloric acid 72 for 12 h. The resulting powder was filtered at least three times. A proton exchange polymer 66 having covalently bonded phosphonic acid 68 functional groups, covalently bonded to the side chain is produced. The precursor polymer may have bromide 61 groups as terminal group of a side chain on a hydrophilic block 63 of the co-polymer and a hydrophobic block 65.

(31) FIG. 3 shows an optional step to cross link the precursor polymer 60. The precursor polymer 60 solution can contain a crosslinker 75, such as tetramethyl hexyldiamine (TMHDA), to cross-link some of the haloalkyl moieties to produce a cross-linked precursor polymer 67 having a crosslinker 80. The cross-linking is preferably carried out before the phosphonation step as shown and described in FIG. 2, however, cross-linking may also be carried out after phosphonation. As shown in FIG. 3 a phosphonated crosslinked proton exchange polymer 95 may be functionalized to produce phosphonic acid functional groups as shown and described in FIG. 2. The crosslinker 80 extends between side chains which may be along the same backbone of the proton exchange polymer or with a second co-polymer, as shown. Other crosslinkers may be used to produce quaternary ammonium functional groups that are cross linked and that may be subsequently dopped with phosphoric acid to produce ionically bonded phosphonic acid 69 functional groups, ionically bonded to the quaternary ammonium. The proton exchange polymer 90 is crosslinked and has both ionically bonded phosphonic acid 69 and covalently bonded phosphonic acid 68 functional groups to provided increased proton conductivity. Also, the quaternary ammonium is a functional group. Cross-linking may greatly improve the mechanical properties of the proton exchange polymer by increasing modulus, toughness, and preventing swelling and degradation. The cross-linked proton exchange polymer may be much more dimensionally stable and thermally stable, having less thermal expansion.

(32) As shown in FIG. 4, a crosslink precursor polymer 67 as shown and described for FIG. 3, is dopped with phosphoric acid and subsequently dopped with phosphoric acid to produce a proton exchange polymer 92 with only ionically bonded phosphonic acid 69 functional groups. The precursor polymer did not undergo phosphonation however, so this proton exchange polymer 92 has only ionically bonded phosphonic acid 69 functional groups and quaternary ammonium functional groups. The ionically bonded phosphonic acid 69 functional groups are ionically bonded to the quaternary ammonium.

Example 2: Proton Exchange Membrane

(33) A precursor polymer was coated onto a support structure to produce a precursor proton exchange membrane. A crosslinker 75 such as (N,N,N,N-Tetramethyl-1,6-hexanediamine) was added to crosslink the precursor polymer. Note this step could happen before or after the amination process. After the crosslinking, the crosslinked polymer was aminated in the trimethylamine to form quaternary ammonium groups. Then, the halide ions were exchanged with hydroxide ions. The proton exchange membrane was converted to hydroxide exchange membrane by immersed in the sodium hydroxide solution. Then, the hydroxide exchange membrane was converted to proton exchange polymer with ion pair by immersing the polymer in the phosphoric acid solution. Dihydrogen phosphate anion (H.sub.2PO.sup.4) was formed after the first molecule of phosphoric acid (PA) was deprotonated by hydroxide anion. The resulting ion exchange membrane was left alone for 72 h to let additional free phosphoric acid molecules leach out, during which the membrane is gently blotted by Kimwipe every 12 h. In some embodiments, there are some phosphoric acids remaining in the proton exchange polymer. The number of phosphoric acid molecules per phosphonic acid group was about between 1 to 15.

(34) A precursor polymer or intermediate polymer of proton exchange polymer may be coated onto, imbibed, or otherwise combined with a support structure, such as a porous or microporous support membrane to produce a proton exchange membrane. Multiple coatings can be applied to increase the membrane thickness or to facilitate filling of the porous reinforcement substrate.

(35) The precursor polymer membrane can be soaked in trimethylamine solution in water or ethanol to convert the haloalkyl moieties within the precursor polymer to a trialkyl ammonium head-group enabling anion conduction within the membrane (i.e., proton exchange membrane).

(36) Optionally, the precursor polymer solution can contain diamine or triamine, such as tetramethyl hexyldiamine (TMHDA), to cross-link some or all of the haloalkyl moieties. The cross-linking is preferably carried out before the coating process and before the amination reaction in trimethylamine; however, cross-linking may also be carried out after amination.

(37) A proton exchange membrane may be soaked in the base solution (e.g., 1M NaOH) to exchange the mobile halogen counter ion (e.g. bromide, chloride or iodide) with hydroxide ions (OH.sup.). The resulting membrane is washed with DI water to remove residual NaOH and then immersed in phosphoric acid solution for forming the quaternary ammonium-phosphonate ion-pair membrane, which demonstrates both anion and cation exchange capabilities.

(38) As shown in FIG. 5, a cation exchange membrane 31 is a planar thin layer of proton exchange polymer 32 having a planar first side 34 and second side 36, wherein the first side and second side extend in parallel to produce a substantially uniform thickness of the proton exchange membrane, with variations in thickness of no more than about 35% and preferably no more than 25% or even 10%.

(39) As shown in FIG. 6, the proton exchange polymer 32 may be configured in a composite proton exchange membrane 30 has a support layer 33 and with proton exchange polymer 32 extending through the pores 50 from a first side 34 or anode side, to a second side 36 or cathode side. The thickness 35 of the composite proton exchange membrane 30 may be about 200 m or less, about 150 m or less, about 100 m or less, about 50 m or less, about 30 m or less, about 25 m or less, about 15 m or less, about 10 m or less, or even 5 m or less.

(40) As shown in FIG. 7, a composite proton exchange membrane 30 has a support layer 33 with proton exchange polymer 32 extending through the pores 50 of the support layer from a first side to a second side 36 of the support layer. Also, there is a layer of proton exchange polymer extending on the anode and cathode side, or first 34 and opposing second side of the composite proton exchange membrane 30. The thickness 35 of the composite proton exchange membrane 30 may be 50 m or less, about 30 m or less, about 25 m or less, about 15 m or less, about 10 m or less, or even 5 m or less. The proton exchange polymer 32 may be configured on just one side of a support layer 33, or may be only with the support layer or on one side and within at least partially the support layer.

(41) As shown in FIG. 8, a composite proton exchange membrane 30 has a plurality of support layers 33 and 33 and with proton exchange polymer 32 extending through the pores 50, 50 of each layer from a first side to a second side 36 of each support layer. Also, there is a layer of proton exchange polymer extending on the anode and cathode side, or first 34 and opposing second side of the composite proton exchange membrane 30. The thickness 35 of the composite proton exchange membrane 30 may be 50 m or less, about 30 m or less, about 25 m or less, about 15 m or less, about 10 m or less, or even 5 m or less.

(42) As shown in FIG. 9, the composite proton exchange membrane 30 may be incorporated into a membrane electrode assembly 55, having an anode 40 and cathode 60 on an opposing side of the composite proton exchange membrane 30. The anode may comprise an anode catalyst 42 and may also include an anode proton exchange polymer 44, which may be the same or different from the proton exchange polymer in the composite proton exchange membrane 30 or proton exchange membrane. Likewise, the cathode may comprise a cathode catalyst 62 and may also include an anode proton exchange polymer 64, which may be the same or different from the proton exchange polymer in the composite proton exchange membrane 30 or proton exchange membrane.

(43) It will be apparent to those skilled in the art that the mentioned embodiment can be scaled up to a roll-to-roll, continuous process.

(44) In the case of either embodiment, multiple coatings can be applied to increase the membrane thickness or to facilitate filling of the porous reinforcement substrate. Multiple reinforcement can also be applied to increase the membrane mechanical strength.