Recycling of membrane components for fuel cells and electrolytic cells

Abstract

1. A method for recycling fluoropolymer from waste membranes, wherein the fluoropolymer comprises a fluoropolymer backbone and a plurality of groups represented by the formula -SOZ, where Z is hydrogen or a cation, comprising: contacting the waste membrane with a liquid solvent to disperse the waste membrane in the solvent, thus forming a dispersion of fluoropolymer in the liquid solvent; and freeze-drying the dispersion to produce a dry, solid fluoropolymer material.

Description

Technical Field

[0001] This specification relates to a recycling method for components of membranes, such as those of catalyst coated membranes and membrane electrode assemblies used in fuel cells and hydrogen generating water electrolyzers.

Background Art

[0002] As investments are made in a global hydrogen economy, the production of fuel cells and hydrogen generating water electrolyzers is set to grow rapidly. Catalyst coated membranes (CCMs) are a major functional component of both fuel cells and electrolyzers. Such CCMs generally include a conductive polymer membrane coated on both sides by catalyst containing layers. The CCMs are configured to drive oxidation and reduction reactions and carry proton and electron transport, and these processes are required for fuel cell and electrolyzer technologies to function.

[0003] The materials and configurations of CCM components vary according to the functional performance requirements in the end use, but they generally contain several useful components including one or more platinum group metal (PGM) catalysts and one or more proton conductive polymers.

[0004] Typically, the membrane is formed from one or more ionomers such as perfluorosulfonic-acid (PFSA) ionomers. The ionomer may also be provided on one or both of the catalyst layers. The ionomer in the catalyst layer may be the same as or different from the ionomer in the main membrane component and / or other catalyst layers.

[0005] The CCM may contain two different catalysts, one for driving an oxidation reaction on one side of the CCM and the other for driving a reduction reaction on the other side of the CCM. The CCM may also contain a recombination catalyst provided to catalyze the recombination of hydrogen and oxygen to form water, reducing the amount of hydrogen passing through the membrane and mixing with oxygen to form a potentially explosive mixture. The CCM may also contain a metal oxide (e.g., CeO2) as a peroxide scavenger to slow the decomposition of the CCM and extend its lifespan.

[0006] CCM catalysts can be based on platinum group metals such as platinum, ruthenium, iridium, palladium, or mixtures thereof. Platinum group metals may be provided in elemental (metallic) form, compound form (e.g., oxides such as iridium oxide catalysts), or PGM-based metal alloys (e.g., PtCo). Furthermore, the PGM catalyst material may be supported on a substrate material (e.g., carbon such as a platinum-supported carbon catalyst or PtCo-supported carbon containing carbon particles on which platinum is arranged).

[0007] A catalyst coating (CCM) can also be provided in combination with additional functional layers to form a multi-layer membrane electrode assembly (MEA). Such an MEA may have, for example, 3, 5, or 7 layers.

[0008] With the increasing production of CCMs for fuel cells and electrolytic cells, there is also a corresponding increase in CCM waste materials, including large amounts of scrap material generated during CCM production (e.g., due to quality control failures), and an increase in end-of-life (EoL) CCMs. Since CCMs contain several rare and / or valuable components, including platinum group metals (particularly Pt, Pd, Ir, and Ru) and ionomers (in both membrane and catalyst layers), there is a growing demand for methods to recycle such components from scrap / waste CCM materials.

[0009] One current method for recovering PGMs from production scrap and end-of-life CCM materials involves incineration. The incineration process produces PGM-rich (typically Pt and Ir) ash that is processed through conventional PGM refining routes. However, the incineration process releases harmful and toxic gases such as CO2 and HF from the polymers that are part of the membrane. Both of these gases have adverse effects as they pollute the atmosphere, increase the greenhouse effect, and / or have harmful effects on human health. Therefore, cleaner processes that reduce or eliminate the release of these gases are needed.

[0010] In addition to the above, incineration methods destroy ionomer components, which are of equally important value. Therefore, it is desirable to provide a process that can recover both PGMs and ionomer components, as well as a process that is cleaner, safer, and more environmentally friendly. Processes for recovering perfluorosulfonic acid ionomers are known; see, for example, International Publication No. 2016 / 156815 and U.S. Patent No. 7255798. Furthermore, processes for recovering individual PGM catalyst components are known; see, for example, U.S. Patent No. 7709135. However, in order to enable fuel cells and electrolytic cells to become more sustainable technologies, there is a need for commercially viable and environmentally friendly routes for recovering, separating, and recycling both PGMs and ionomer components from waste CCM materials, including production scrap and end-of-life materials.

[0011] This specification relates to a method for recycling ionomer material (perfluorosulfonic acid (PFSA) polymer) from ionomer membranes in fuel cells or electrolytic cells, and is also suitable for recovering catalyst material from catalyst coatings including ionomer membranes and catalyst coatings.

[0012] International Publication 2021 / 250576 discloses a process for recycling ionomer material from ionomer membranes in fuel cells or electrolytic cells. It is described that the solubility of fluorinated polymers used in such membranes decreases when the fluorinated polymers are heat-treated, as can occur during the production of membranes containing the fluorinated polymers. That is, fluorinated polymers having a fluorinated main chain and multiple groups represented by the formula -SO3H or a salt thereof readily dissolve in water and alcohol mixtures when newly prepared after being heated to a temperature of at least 100°C, whereas these polymers are typically insoluble in water and water / alcohol mixtures under standard conditions. International Publication 2021 / 250576 discloses that such heat-treated fluorinated polymers become soluble when heated in the presence of water and a base. Therefore, International Publication No. 2021 / 250576 discloses a method involving dissolving a fluorinated polymer film in water and a base to form a fluorinated polymer salt solution, and then converting the fluorinated polymer salt solution back into a fluorinated polymer solution by hydrogen cation exchange. The base has been shown to be typically an alkali metal hydroxide (e.g., lithium hydroxide, sodium hydroxide, or potassium hydroxide) or ammonium hydroxide. It has been further shown that the moles of the base used may be equivalent to the moles of the fluorinated polymer, or an excess of base (e.g., an excess of up to 100, 200, or 300 mole percent of the base relative to the fluorinated polymer) may be used.

[0013] The objective of this specification is to provide improved processes for recycling fluorinated polymer films, particularly processes that are more time and energy efficient, as well as processes for recovering ionomers in a form more suitable for reuse in the manufacture of new film materials. [Overview of the project]

[0014] A method for recycling fluorinated polymers from fluorinated polymer films involves placing the film in a suitable liquid solvent to disperse the film material in the solvent as either a fluorinated polymer dispersion or a fluorinated polymer salt dispersion. If a solid product is required, after dispersing the film material in the solvent, the dispersion can be optionally thermally dried under reduced pressure to remove the liquid solvent and obtain a dry solid particulate product of the fluorinated polymer or fluorinated polymer salt. Such a dry solid product may be desirable as an intermediate product before redispersing the solid polymer material in another solvent for purification, washing, storage, transport, and / or for film manufacturing methods.

[0015] One problem with such membrane recycling methods is that the dried fluorinated polymer (ionomer) material recovered from waste fluorinated polymer membranes can require significant time and energy to redisperse to achieve a stable dispersion suitable for use in further processing steps to produce new membranes for fuel cells or electrolytic cells. This is because, at least in part, the fluorinated polymer material recovered from waste membranes tends to have relatively high density, high particle size, low porosity, and / or low surface area.

[0016] This specification provides a method for recycling a fluorinated polymer from a waste film, wherein the fluorinated polymer comprises a fluorinated polymer main chain and a plurality of groups represented by the formula -SO3Z, where Z is a cation, and the method comprises contacting the waste film with a liquid solvent to disperse the waste film in the solvent, thereby forming a dispersion of the fluorinated polymer in the liquid solvent, and freeze-drying the dispersion to produce a dry solid fluorinated polymer material.

[0017] It has been found that freeze-drying can be used to recover solid ionomer materials from waste fluorinated polymer films with a larger surface area and higher porosity. The recovered ionomer material can be dispersed more effectively within a shorter timeframe than high-density, heat-dried ionomer materials previously recovered from films. Freeze-drying is a known process in which a dispersion is frozen and then the solid freeze solvent is removed by sublimation. Freeze-drying has also been mentioned in connection with the manufacture of ionomer materials (see, for example, International Publication 2021 / 111342). However, while freeze-drying is known as a general technique, the inventors are unaware that this technique has been previously proposed as a way to provide an improved process for recycling fluorinated polymers from waste films. The use of freeze-drying allows for improved energy efficiency in the recycling process and recovers ionomers in a form more suitable for reuse in the manufacture of new film materials.

[0018] In addition to the above, it has been noted that one problem with the methodology described in International Publication No. 2021 / 250576 is that the use of hydroxide bases to form dispersions of fluorinated polymer salts can cause several problems in further processing of ionomer materials. Excess hydroxide bases are corrosive to metal and glass-lined containers that may be used in subsequent dispersion processes at high temperatures and pressures. Furthermore, excess hydroxide bases can cause problems in the speciation and extraction of other components, such as platinum group metal catalysts present in catalyst coatings of fuel cells or electrolytic cells. Moreover, any excess hydroxide bases must be recovered during the ion exchange process to convert the fluorinated polymer salts back into protonated acid forms, which can adversely affect the overall balance of the material. While this freeze-drying methodology can be used to remove water and bases, it also ensures that the resulting solid ionomer (salt) material can be easily redispersed in a different, less corrosive solvent for further processing. Advantageously, before contacting the waste film with a liquid solvent and dispersing it in the solvent, the waste film is treated with a reagent providing a cation source (e.g., an aqueous basic solution) to form a fluorinated polymer salt, and the reagent is maintained at a sufficiently low temperature so that the waste film remains in a solid, non-dispersible form. Excess reagent can then be removed from the solid fluorinated polymer salt before dispersing the waste film in the liquid solvent. This ensures that excess reagent (e.g., excess base) is removed before lyophilization. The dispersion can be lyophilized in ionomer salt form, or the ionomer salt can be converted back to a protonated form by ion exchange before lyophilization. This methodology is particularly advantageous because the conversion of the weight ionomer material to salt form allows for better dispersion of the ionomer before lyophilization of the ionomer dispersion. This then results in a more dispersible lyophilized ionomer product. While freeze-drying has been shown to be useful for producing dispersible products, if the waste ionomer material is not sufficiently dispersed before the freeze-drying process, the resulting freeze-dried product will not have good dispersion properties.Therefore, to achieve a good dispersible product free of contaminants, a favorable process flow is to (i) convert the waste ionomer material into a salt form using a reagent that provides cations without dispersing the ionomer (while maintaining a temperature below the temperature at which the ionomer disperses, e.g., below 150°C, 100°C, or 80°C); (ii) remove excess reagent; (iii) disperse the salt form of the ionomer material by heating in a solvent (e.g., above 180°C, 200°C, or 240°C); (iv) optionally convert the salt form of the ionomer back into a protonated form by ion exchange; and (v) freeze-dry the ionomer material. This process flow leads to a better ionomer product material in terms of both dispersibility and purity when starting from waste ionomer material.

[0019] Therefore, the process of the present invention provides an improved method for recycling scrap or used membranes from fuel cells or electrolytic cells. [Brief explanation of the drawing]

[0020] For a better understanding of the present invention and to illustrate how it can be implemented, certain embodiments of the present invention are described hereby by reference only to the accompanying drawings.

[0021] [Figure 1] This diagram shows a flow chart of a method for recycling waste fluorinated polymer films, including a waste film dispersion step and a freeze-drying step. [Figure 2] A flowchart of the process for recycling waste fluorinated polymer films is shown, including a waste film dispersion step, a freeze-drying step, an ionomer redispersion step, and the use of a dispersion to produce a new fluorinated polymer film. [Figure 3]Another example of a method step for recycling a waste fluorinated polymer membrane is provided, including a fluorinated polymer salt formation step, a membrane dispersion step, a freeze-drying step, a further ionomer dispersion step, and a cation exchange step prior to using the fluorinated polymer to produce a new fluorinated polymer membrane. [Figure 4] An example of a method step for recycling a catalyst-coated waste fluorinated polymer membrane is provided, including separating a catalyst material from the waste fluorinated polymer membrane prior to dispersion of the membrane and freeze-drying the dispersion. [Figure 5] An example of a method step for recycling a catalyst-coated waste fluorinated polymer membrane is provided, including dispersing the membrane, then separating the catalyst material from the dispersion, and then freeze-drying the dispersion.

Mode for Carrying Out the Invention

[0022] As illustrated in FIG. 1, the present specification provides a method for recycling a fluorinated polymer from a waste membrane, such as scrap or end-of-life fluorinated polymer membrane material from fuel cells and electrolyzers. Such a fluorinated polymer membrane is formed from a fluorinated polymer comprising a fluorinated polymer backbone and a plurality of groups represented by the formula -SO3Z, wherein Z is a cation, such as hydrogen, a metal cation, or an ammonium cation. The method includes contacting the waste membrane with a liquid solvent to disperse the waste membrane in the solvent, thus forming a dispersion of the fluorinated polymer in the liquid solvent. The fluorinated polymer dispersion is then freeze-dried to yield a dry solid fluorinated polymer material.

[0023] The lyophilization process includes cooling the dispersion to a temperature at which the liquid solvent freezes to form a frozen solid or below that temperature, and reducing the pressure above the frozen solid to a pressure at which the frozen solid solvent sublimates to yield a dried solid fluorinated polymer material or below that pressure. For example, the dispersion can be cooled to a temperature of less than -20°C, -30°C, -40°C, -50°C, -60°C, or -70°C, optionally up to -200°C or -100°C, to freeze the liquid solvent. Further, the dispersion is maintained at a temperature of less than -10°C, -20°C, -30°C, -40°C, or -50°C, optionally at -200°C, -100°C, -70°C, or -60°C or above during sublimation of the solvent. Still further, the pressure is maintained below 1 mbar, 0.1 mbar, 0.01 mbar, or 0.005 mbar during sublimation of the solvent. The exact temperature and pressure values utilized depend on the nature of the solvent used to form the dispersion and the operating capabilities of the lyophilization equipment.

[0024] The liquid solvent can be an organic solvent such as an alcohol or a diol, optionally mixed with water. Alternatively, the liquid solvent can be an aqueous solvent.

[0025] [[ID=⑧]]The dried solid fluorinated polymer material optionally has a BET surface area of less than 1 g / m 2 Subsequently, the dried solid fluorinated polymer material can be dispersed in a solvent, which can be the same as or different from the solvent used to disperse the waste film, and the dispersion is used to produce a new fluorinated polymer membrane. The dispersion optionally has a viscosity of less than 1000 cps, less than 500 cps, or less than 300 cps at 1 s -1 The lyophilized solid fluorinated polymer material recovered from the waste film is more easily dispersed than the previous form of the solid fluorinated polymer material recovered from the waste film. Thus, this requires less energy and time to form a suitable dispersion and is thus more suitable for reuse in the manufacture of new membranes.

[0026] Fluorinated polymers in dispersions and / or dried solid fluorinated polymer materials can be in salt form or protonated acid form. Figure 3 shows a method for recycling waste fluorinated polymer films in which the fluorinated polymer is converted to salt form, the film is dispersed, and the dispersion is freeze-dried to produce a dried solid fluorinated polymer material in salt form. The fluorinated polymer in the film can be converted to salt form before the film is dispersed. Alternatively, the fluorinated polymer in the film can be converted to salt form during the film dispersion. Salt conversion can be achieved, for example, using a solvent containing water and a base, where the base reacts with the sulfonic acid groups of the fluorinated polymer to form a fluorinated polymer salt. The base can be a metal hydroxide or a hydroxide such as ammonium hydroxide. At low temperatures (e.g., room temperature or gentle heating only), conversion to salt form can be achieved without dispersing the film. When heated (e.g., above 180°C under pressure), the film can be dispersed. Converting the ionomer to salt form before dispersion is useful because it can remove excess base, and then the salt form of the base forms a better dispersion. Next, this better dispersion, free from contamination with excess base, can be freeze-dried to form a highly dispersible solid fluorinated polymer salt material.

[0027] Freeze-dried solid fluorinated polymer salt materials can then be used to produce new film materials. In this regard, fluorinated polymer salt materials can be dispersed in a solvent, converted to protonated acid form via cation exchange, and then used to produce new films.

[0028] This method is similar in some respects to the method described in International Publication No. 2021 / 250576, in that it can form fluorinated polymer salts using water and bases such as hydroxides. However, excess hydroxide bases are corrosive to metal and glass-lined containers that may be used in subsequent dispersion processes at high temperatures and pressures. Furthermore, excess hydroxide bases can cause problems in the speciation and extraction of other components, such as platinum group metal catalysts present in catalyst coating films of fuel cells or electrolytic cells. Moreover, any excess hydroxide bases must be recovered during the ion exchange process to convert the fluorinated polymer salts back into protonated acid forms, which can adversely affect the overall balance of the material. Therefore, it is preferable that the fluorinated polymer film be converted to a salt without dispersing the film. The bases can then be removed, and the film can then be dispersed in a non-basic solution (e.g., water) before freeze-drying. The resulting solid ionomer (salt) material can be easily redispersed in another solvent for further processing. It is worth noting that, as an alternative to using bases such as hydroxides as cation sources for forming polymer salts, other reagents such as carbonates and halide salts can be used.

[0029] The solid fluorinated polymer salt formed by freeze-drying can be stored as an intermediate product until needed for use in the production of a new fluorinated polymer, or it can be immediately converted into a fluorinated polymer. In this regard, the fluorinated polymer salt can then be dispersed before the step of converting the fluorinated polymer salt to a fluorinated polymer by cation exchange. After the conversion of the fluorinated polymer salt to a fluorinated polymer by cation exchange, the fluorinated polymer can be reused to produce a new film.

[0030] The membrane may be a catalyst coating membrane for a fuel cell or electrolytic cell. In this case, it is desirable to recycle the catalyst components and polymer materials of the membrane. Therefore, at least one catalyst material can be separated from the membrane before dispersion (e.g., by delamination or leaching), and / or at least one catalyst material can be separated after dispersion (e.g., via solid-liquid separation such as filtration). Such a process flow is illustrated in Figures 4 and 5.

[0031] experiment A dispersion of solid PFSA ionomer in water (89.67 g) was formed by autoclaving a fuel cell ionomer membrane in water. The dispersion was frozen on dry ice (approximately -78°C) before sublimating the water at approximately 0.002 mbar for 43 hours (using a Christ Alpha 1-2 LDPLUS -55°C freeze-dryer) to produce a fuzzy, off-white PFSA ionomer solid material (8.72 g). The solid ionomer product material has low viscosity, high porosity, and a large surface area, and is readily dispersible for use in the manufacture of ionomer membranes or for use in the manufacture of catalyst layers, for example, for incorporation into catalyst ink formulations for catalyst coating films of fuel cells or hydrogen production electrolytic cells.

[0032] Similar freeze-drying experiments were also performed on a dispersion formed from an autoclave process on an eluted fuel cell catalyst coating film. Water (60 mL) was added to the eluted catalyst coating film (2.71 g). This mixture was heated at 250 °C for 3 hours to produce an ionomer dispersion, as well as a residual solid containing reinforcing material, residual catalyst, and residual ionomer. The dispersion was filtered, frozen with dry ice, and freeze-dried for 48 hours using a Christ Alpha 1-2 LDplus freeze-dryer to produce a cottony, off-white solid (0.89 g).

[0033] Although the present invention has been specifically illustrated and described with reference to certain embodiments, it will be understood by those skilled in the art that various modifications of form and detail can be made without departing from the scope of the invention as defined by the appended claims.

Claims

1. A method for recycling fluorinated polymers from waste films, wherein the fluorinated polymer comprises a fluorinated polymer main chain and formula -SO ３ It contains multiple groups represented by Z, where Z is hydrogen or a cation. The waste film is brought into contact with a liquid solvent to disperse the waste film in the liquid solvent, thereby forming a dispersion of the fluorinated polymer in the liquid solvent. A method comprising freeze-drying the dispersion to produce a dry solid fluorinated polymer material.

2. The method according to claim 1, wherein the fluorinated polymer in the dispersion is in salt form.

3. Before contacting the waste film with the liquid solvent and dispersing the waste film in the liquid solvent, the waste film is treated with a reagent that provides a cation source to form a fluorinated polymer salt, and the reagent is maintained at a sufficiently low temperature so that the waste film remains in a solid non-dispersible state. The method according to claim 2, wherein excess reagent is removed from the fluorinated polymer salt before the waste film is dispersed in the liquid solvent.

4. The method according to claim 2, further comprising converting the fluorinated polymer in the dispersion in the salt form to a fluorinated polymer in which Z is hydrogen by cation exchange before freeze-drying.

5. The method according to claim 2, wherein the fluorinated polymer in salt form is freeze-dried.

6. The method according to claim 1, wherein the fluorinated polymer in the dispersion and the dried solid fluorinated polymer material is in the form of a protonated acid.

7. The method according to claim 1, wherein the liquid solvent is an organic solvent or an aqueous solvent.

8. The aforementioned dry solid fluorinated polymer material is 1 g / m ２ The method according to claim 1, having a BET surface area of ​​less than 1.

9. The method according to claim 1, further comprising dispersing the dry solid fluorinated polymer material in another solvent to produce a dispersion for further processing.

10. The aforementioned dispersion, －１ The method according to claim 9, wherein the viscosity is less than 1000 cps, less than 500 cps, or less than 300 cps.

11. The method according to claim 9, wherein the dry solid fluorinated polymer material is in salt form, and after dispersing the dry solid fluorinated polymer material in another solvent, the method further comprises converting the fluorinated polymer in salt form to a fluorinated polymer in which Z is hydrogen by cation exchange.

12. The method according to claim 1, wherein the freeze-drying step includes cooling the dispersion to a temperature at or below which the liquid solvent freezes to form a frozen solid solvent, and reducing the pressure above the frozen solid to a pressure at or below which the frozen solid solvent sublimes to produce the dried solid fluorinated polymer material.

13. The method according to claim 12, wherein the dispersion is cooled to a temperature below -20°C, -30°C, -40°C, -50°C, -60°C, or -70°C in order to freeze the liquid solvent.

14. The method according to claim 12, wherein the dispersion is maintained at a temperature below -10°C, -20°C, -30°C, -40°C, or -50°C during the sublimation of the frozen solid solvent.

15. The method according to claim 12, wherein the pressure is maintained below 1 mbar, 0.1 mbar, 0.01 mbar, or 0.005 mbar during the sublimation of the frozen solid solvent.

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