GB2598828A - Anion exchange lonomer with a poyarylene backbone and anion exchange membrane incorporating same - Google Patents

Abstract

An anion exchange ionomer contains a fluorinated, ether-free backbone, and a fluorinated ether based quaternary ammonium functional group: said backbone comprises a copolymer of a polyarylene and a fully fluorinated alkyl hydrocarbon, said hydrocarbon unit having a fluorinated ether side group. Such a polymer is produced by reaction of an arylene unit with 2-bromo-1,1,2,2-tetrafluoroethylene trifluorovinyl ether in the presence of a strong acid. The novel polymer has improved chemical and mechanical stability as compared to the state-of-the-art materials for incorporation in anion exchange membrane.

Description

Anion Exchange lonomer With A Poyarylene Backbone and Anion Exchange Membrane Incorporating Same

BACKGROUND OF THE INVENTION

Cross Reference To Related Applications

[0001] This application claims the benefit of priority to U.S. provisional patent application No. 63/047,808, filed on July 2, 2020; the entirety of which is hereby incorporated by reference.

Field of the Invention

[0002] This invention is directed to a novel anion exchange ionomer (AEI) incorporating a fluorinated ether functional group with a stable backbone for use as an anion exchange material, with enhanced mechanical and chemical stability compared to state of the art. Such materials are well suited for use in anion exchange membranes. Some of the structures provided can be optionally modified for use as cation exchange materials and bipolar membranes

Background

[0003] Anion exchange membranes (AEM) are more attractive than proton exchange membranes for applications such as fuel cells and electrolyzers because nonprecious metals can be used in place of precious metal catalysts. Unfortunately, most current anion exchange membranes suffer from poor stability in highly basic environments. This instability is primarily caused by two factors: degradation of the polymer backbone and degradation of the functional cationic groups.

[0004] Most AEM materials comprise of a backbone selected from the group of polysulfones, poly(phenylene oxide)s, poly(phenylene)s, poly(benzimidazolium)s, poly(arylene ether ketone)s, and poly(arylene ether sulfone)s. These polymers contain an arylene ether linkage in the mid-chain. These have been found to be chemically unstable and degrade easily under highly alkaline conditions. The hydroxyl ions and any radicals formed during the reaction cleave the ether bond causing the polymer to unzip. This unzipping leads to break down of the membrane structure and loss of mechanical strength and conductivity.

[0005] The main drawback of quaternary ammonium functional groups in AEMs is the poor chemical stability due to the ammonium group's susceptibility to OH-attack, leading to ammonium group degradation and reduction of IEC The OH-attack occurs via one of the following reaction pathways: e p-hydrogen(Hoffman elimination), nucleophile substitution (SN2) or ylide intermediate formation. Figure 1A, 1B and 1C highlight the respective degradation reaction pathways. Given that all these reactions can be initiated by nucleophiles such as OH-, the high-pH environment in AEMFC/AEMVVE makes it inevitable that the QA will be degraded over time. For many applications, long term stability is a critical issue.

[0006] Anion exchange membranes (AEMs) in fuel cells are solid polymer electrolyte membranes which transport anions (e.g. OH-, HCO3-) under an electrical potential. AEM is a critical component for direct ammonia fuel cells (DAFCs), where ammonia and oxygen are used to generate electricity together with nitrogen as a byproduct. Ammonia is an ideal carbon-free fuel for fuel cells and offers a reliable and clean source of energy, without many of the problems associated with the traditional hydrogen economy. Compared to proton exchange membrane fuel cells, DAFCs have attracted recent interest due to their potential to eliminate the requirement for expensive platinum-group catalysts, fluorinated ionomers, and acid-resistant metals in the system. The chemical reactions in DAFC are as follows: Cathode: 02 + 2H20 + 4e--> 40H- (1) Anode: 2NH3 + 60H--> N2 + 6H20 + 6e- (2) Overall: 4NH3 + 302 2N2 + 6H20 (3) [0007] To achieve high ionic conductivity and hydrophilic-domain phase separation, AEMs for DAFCs are designed to have high ion exchange capacity (IEC). High IEC also increases the water uptake and reduces the mechanical strength and dimensional stability in AEMs. To solve these issues, thicker membranes are traditionally used. However, thicker AEMs have higher ionic resistance, lowering the electrochemical performance in a device. To achieve a thin, dimensionally-stable AEM with low resistance, inert reinforcement must be explored.

[0008] However, a critical issue with thin AEMs is fuel On the case of a DAFC, i.e. ammonia) crossover across the membrane during operation. Crossover results in both degradation of the membrane and reduced cell performance in terms of open circuit voltage and efficiency. Crossover in an electrochemical cell is often characterized by electrochemical methods and is described as a crossover current density. For low temperature DAFC based on polymeric membrane electrolytes, the real challenge is how to minimize the crossover of ammonia. Novel chemical and coating structure design for anion exchange polymer and AEM become the key for its application in DAFCs.

[0009] In the case of anion exchange polymers, various cationic functional groups grafted to the same polymer backbone can exhibit various degrees of fuel crossover and/or ionic conductivity. For example, an anion exchange polymer may contain a tetramethylammonium functional group to provide certain anion exchange capacity. Substitution of this functional group with a pyridinium or piperidinium functional group may result in varying ammonia crossover and/or ionic conductivity. Besides, various polymer backbone and molecular weight can also exhibit various degrees of ammonia crossover. In the case of AEM, multilayer structure design will also result in reducing ammonia crossover.

[0010] The anion conducting membrane may be made much thinner and durable with the use of a microporous support scaffold, wherein the anion polymer is coated onto or at least partially into the microporous support scaffold. A microporous support scaffold may be microporous polyethylene, polypropylene, and polytetrafluoroethylene, such as expanded polytetrafluoroethylene (ePTFE) membrane, for example. An ion conducting membrane and the microporous support scaffold may be any suitable thickness including about 20microns or less, and preferably 10 microns or less, and even more preferably about 5microns or less.

[0011] There is therefore an urgent need for inexpensive, chemically stable AEM to enable the performance and durability for developing fuel cells including DAFCs.

SUMMARY OF THE INVENTION

[0012] The present invention provides an anion exchange ionomer and an anion exchange membrane and incorporation said ionomer with enhanced mechanical and chemical stability compared to the state of the art.

[0013] An exemplary anion exchange ionomer has a partially fluorinated or fully fluorinated backbone to enhance the chemical stability of the backbone. The backbone of the fluorinated polymer should contain no ether linkages, as ether linkage free backbones were developed for anion exchange membranes to obtain improved alkaline stability. This backbone is very stable because there are no C-0 bonds.

[0014] An exemplary anion exchange ionomer has a polyarylene structure, such as a phenyl group. An exemplary anion exchange ionomer is comprised of a fluorinated carbon chain, preferably containing sp2 carbons. An exemplary anion exchange ionomer may comprise Polyphenylenes, fluorinated hydrocarbon or carbon-based polymers.

[0015] One of the degradation mechanisms for the functional group is the Hoffman elimination mechanism, where a nucleophile (OH-) attacks the p-hydrogen, which causes loss of the functional group and reduction in IEC. One of the solutions to reduce or eliminate the Hoffman elimination mechanism is to synthesize the fluorine based quaternary ammonium cations in place of p-hydrogen quaternary ammoniums. The multiple carbon-fluorine bonds in the fluorine based quaternary ammonium cations also strengthen the "skeletal" carbon-carbon bonds from the inductive effect which would eliminate the degradation due to the SN2 mechanism.

[0016] There are two mechanistic models for how an alkyl halide can undergo nucleophilic substitution. In a first model, the reaction takes place in a single step, and bond-forming and bond-breaking occur simultaneously,SN2 mechanism. The term SN2, S stands for 'substitution', the subscript N stands for 'nucleophilic', and the number 2 refers to the fact that this is a bimolecular reaction.

[0017] An exemplary side chain contains a functional group that is either partially or fully fluorinated; this will eliminate all the degradation mechanisms of nucleophilic attack on the functional group. Preferably, the functional groups are fully fluorinated as they are more stable chemically and thermally. Fluorine forms the strongest single bond to carbon. Fluorocarbons are more chemically and thermally stable than their corresponding hydrocarbon counterparts, and indeed any other organic compound.

[0018] Exemplary anion exchange polymers may have functional groups selected from the group of quaternary ammonium, tertiary diamines, phosphonium, benz(imidazolium), sulphonium, guanidinium, metal cations, pyridinium. Preferably the functional group is quaternary ammonium.

[0019] An exemplary anion exchange membrane may be a composite anion exchange membrane comprising a porous scaffold support. An anion exchange polymer may be coupled to the porous scaffold, such as by being imbibed into the pores of the porous scaffold. The porous scaffold may enable the composite anion exchange membrane to be ultra-thin and therefore reduce water management issues. An ultra-thin anion exchange membrane may have a thickness of about 50pm or less, about 30pm or less, about 25pm or less, about lOpm or less, and may even be as thin as about 5pm or less. An exemplary porous scaffold support is selected from the group comprising expanded polytetrafluoroethylene (ePTFE) or polyolefins. Exemplary polyolefins include polypropylene, polyethylene, and polyether ether ketone.

[0020] Anion exchange membranes as described herein may be use in anion exchange membrane fuel cells, electrolyzers, sensors, compressors, batteries, super capacitors.

Examples

[0021] A polyarylene, such as triphenyl or biphenyl, is reacted with 2-Bromo1,1,2,2-tetrafluoroethyl trifluorovinyl ether in the presence of a strong acid to form a bromoalkylated precursor polymer; The bromoalkylated precursor polymer is reacted with a trialkylamine and sodium hydroxide to form a polyarylene having a backbone free of ether linkages. The strong acid may have to have a pH of about 2 or less, about 1 or less, about 0.5 or less and any range between and including the pH listed. An exemplary strong acid is triflic acid.

[0022] The present invention provides a mechanically reinforced anion exchange membrane comprising one or more layers of functional polymer based on a terphenyl backbone with quaternary ammonium functional groups and an inert porous scaffold material for reinforcement. Typically, the present invention describes an anion exchange membrane comprising multilayers of anion exchange polymers which each contain varying types of backbones, varying degrees of functionalization, or varying functional groups to reduce ammonia crossover through the membrane. Typically, the membrane consists of at least two layers with varying properties. Typically, the thickness of anion exchange membrane is 30 microns or less, more typically 15 microns or less, and in some embodiments 5 microns or less [0023] A fluorinated carbon chain, such as TFE, is reacted with 2-Bromo-1,1,2,2-tetrafluoroethyl trifluorovinyl ether in the presence of an initiator and/or a strong acid to form a bromoalkylated precursor polymer; The bromoalkylated precursor polymer is reacted with a trialkylamine and sodium hydroxide to form a perfluorinated anion exchange ionomer having a backbone free of ether linkages.

[0024] 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.

[0025] Anion exchange polymers described in this invention is composed of an aromatic/polyaromatic ring in polymer backbone (e.g. biphenyl, terphenyl, fluorenyl) and a tethered alkyl halide (e.g. bromide) side chain which can be converted to quaternary ammonium hydroxide groups.

[0026] In one embodiment, a membrane is prepared by dissolving p-TPN1-Pyr in DMF at a 10 % weight ratio i.e. 0.3 grams of polymer to 9.7 g of solvent. The mixture was stirred until homogenous and translucent.

[0027] The anion exchange polymer solution was then applied to a microporous polyethylene material tensioned around a chemically-resistant plastic frame. The polymer solution was then poured on to the microporous scaffold. The frame was covered with a lid to slow down the solvent evaporation process. The membrane was dried at 60-degree C. The final thickness of the anion exchange membrane was 15 microns.

[0028] In another embodiment, a membrane is prepared by dissolving p-TPN1-TMA in DMF at an 8% weight ratio i.e. 0.8 grams of polymer to 9.2 g of solvent. The mixture was stirred until homogenous and translucent.

[0029] The anion exchange polymer was then applied to a microporous poly(tetrafluoroethylene) material with a doctor blade. The membrane was dried at room temperature. The final thickness of the membrane was 6 microns.

[0030] In another embodiment, a membrane is prepared by providing two distinct solutions of TPN polymers of various functional groups in DMF (e.g. m-TPN1-TMA and m-TPN1-Pip) along with a porous reinforcement material. The porous reinforcement material is coated on one side with the first solution with a doctor blade and then dried. Then, the porous reinforcement material is coated on the second side with the second solution and then dried, filling the remainder of the pores in the reinforcement material and creating a multilayer reinforced anion exchange membrane. The final thickness of the membrane was 10 microns.

[0031] It will be apparent to those skilled in the art that all the embodiment discussed above can be scaled up to a roll-to-roll continuous coating process.

BRIEF DESCRIPTION OF THE DRAWINGS

[0032] 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 serve to explain the principles of the invention.

[0033] Figure 1 shows representative examples of anion exchange polymers disclosed in this invention [0034] Figure 2 shows representative examples of anion exchange polymers disclosed in this invention [0035] Figure 3 shows an exemplary porous scaffold reinforcement material employed in the present invention.

[0036] Figure 4 shows an exemplary anion exchange membrane formed from imbibing an anion exchange polymer into a porous scaffold reinforcement material.

[0037] Figure 5 shows an exemplary anion exchange membrane formed from imbibing two various anion exchange polymers into a porous scaffold reinforcement material.

[0038] FIGS. 6A, 6B, and 6C show degradation mechanisms of quartnerary ammonium functional groups.

[0039] FIG. 7 shows the reaction mechanism for forming a polyarylene-based anion exchange ionomer.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

[0040] 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 particular 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.

[0041] 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.

[0042] Figures 1 and 2 shows the chemical structure of exemplary anion exchange polymers of the present invention.

[0043] Figure 3 shows an exemplary cross-sectional diagram of porous scaffold 9 has a thickness 40 from a first side 10 and an opposite second side 20. The porous scaffold has pores 30 and an open structure from the first side 10 to the second side 20, allowing for an appropriate fluid to flow from the first to the second side. The porous scaffold is air permeable.

[0044] Figure 4 shows a cross-sectional diagram of an anion exchange membrane 90 comprising a porous scaffold 9 imbibed with an anion exchange polymer 70 which contributes ionic conductivity. The anion exchange polymer forms surface layers 50 and 60 on the two faces of the imbibed porous scaffold.

[0045] Figure 5 shows a multilayered anion exchange membrane 900 comprise a first layer 100 comprising an anion exchange polymer with a functional group 200 and a second layer 110 comprising another anion exchange polymer with a second functional group 210, both imbibed into the porous scaffold 9.

[0046] Figures 6A, 6B and 6C highlight the respective degradation reaction pathways. Given that all these reactions can be initiated by nucleophiles such as OH-, the high-pH environment in AEMFC/AEMWE makes it inevitable that the OA will be degraded over time.

[0047] As shown in FIG. 7, an aromatic compound reacts in the presence of a strong acid with 2-Bromo-1,1,2,2-tetrafluoroethyl trifluorovinyl ether to form bromoalkylated precursor polymer. The bromoalkylated precursor polymer is reacted with a trialkylamine (TMA) and sodium hydroxide to form a perfluorinated anion exchange ionomer having a backbone free of ether linkages.

[0048] 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.

Claims (14)

1. What is claimed is: 1 An anion exchange ionomer comprising: a) a polyarylene backbone bonded with a fully fluorinated alkyl hydrocarbon, and b) a fluorinated ether side group.
2. 2. The anion exchange ionomer of claim 1, wherein the backbone comprises a tetrafluoroethylene compound.
3. 3. The anion exchange ionomer of claim 1, wherein the backbone comprises a polyphenylene compound.
4. 4. The anion exchange ionomer of claim 1, comprising a fluorinated ether based quaternary ammonium functional group.
5. An anion exchange membrane comprising the polymer of claim 1 and a porous scaffold.
6. 6 The anion exchange membrane of claim 5, wherein the porous scaffold is comprises poly(tetrafluoroethylene).
7. 7 The anion exchange membrane of claim 6, wherein the porous polytetrafluoroethylene is expanded polytetrafluoroethylene.
8. 8 The anion exchange membrane of claim 5, wherein the porous scaffold is selected from the group consisting of: polyethylene, polypropylene, polyetherether-ketone (PEEK), and poly(tetrafluoroethylene).
9. 9. The ion exchange membrane of claim 8, wherein a thickness of the exchange membrane is no more than 50pm.
10. 10. The ion exchange membrane of claim 8, wherein a thickness of the exchange membrane is no more than 25pm.
11. 11.A method of making the anion exchange ionomer of claim 1, comprising: a) reacting polyarylene with fluoroalkyl ketone 2-Bromo-1,1,2,2-tetrafluoroethyl trifluorovinyl ether in the presence of a strong acid having a pH of no more than 2.0.
12. 12 The method of making the anion exchange ionomer of claim 11, wherein the strong acid has a pH of no more than 1.0.
13. 13. The method of making the anion exchange ionomer of claim 11, wherein the strong acid has a pH of no more than 0.5.
14. 14. The method of making the anion exchange ionomer of claim 11, wherein the strong acid triflic acid.