A method of recycling a waste ionomer material

Abstract

A method of recycling a waste ionomer material contaminated with one or more metal cation contaminants, the method comprising: treating the waste ionomer material with a chelation reagent which forms soluble complexes with the one or more metal cation contaminants; and washing the soluble complexes from the waste ionomer material.

Description

[0001] A METHOD OF RECYCLING A WASTE IONOMER MATERIAL

[0002] Field

[0003] This specification relates to recycling methods for components of waste catalyst coated membranes such as those used in fuel cells and hydrogen producing water electrolysers.

[0004] Background

[0005] Fuel cell and hydrogen producing water electrolyser production is set for rapid growth as investment is placed into the global hydrogen economy. Catalyst coated membranes (CCMs) are a major functional component of both fuel cells and electrolysers. Such CCMs generally comprise a conductive polymer membrane coated on either side by a catalyst containing layer. The CCMs are configured to drive oxidation and reduction reactions and support proton and electron transport, these processes been required for the fuel cell and electrolyser technologies to function.

[0006] While variations in CCM component materials and configurations exist according to functional performance requirements in end use applications, they generally contain several components of value including one or more platinum group metal (PGM) catalysts and one or more proton conducting polymers.

[0007] Typically, the membrane is formed of one or more ionomers such as perfluorosulfonic-acid (PFSA) ionomers. Ionomer may also be provided in one or both of the catalyst layers. The ionomer in the catalyst layers may be the same or different to the ionomer in the main membrane component and / or in the other catalyst layer(s).

[0008] A CCM may comprise two different catalysts, one for driving an oxidation reaction on one side of the CCM and one for driving a reduction reaction on the other side of the CCM. A CCM may also comprise a recombination catalyst which is provided to catalyse the recombination of hydrogen and oxygen to form water, reducing the quantity of hydrogen crossing the membrane and mixing with oxygen to form a potentially explosive mixture. A CCM may also include a metal oxide (e.g., CeCh) as a peroxide scavenger.

[0009] CCM catalysts can be based on platinum group metals such as platinum, ruthenium, iridium, palladium, or mixtures thereof. The platinum group metals may be provided in elemental (metallic) form, in compound form (e.g., an oxide, such as an iridium oxide catalyst), or as a PGM-base metal alloy (e.g., PtCo). Furthermore, the PGM catalyst materials may be supported on a substrate material (e.g., carbon, such as a platinum-on-carbon catalyst comprising particles of carbon on which platinum is disposed or PtCo-on-carbon).

[0010] Catalyst coated membranes (CCMs) can also be provided in combination with additional functional layers to form multi-layer membrane electrode assemblies (MEAs). Such MEAs may have 3, 5, or 7 layers for example.

[0011] With the increase in CCM manufacture for fuel cells and electrolysers, there is an associated increase in CCM waste materials, including a significant volume of scrap material created during CCM manufacture (e.g., due to failure at quality control) and also an increase in end-of-life (EoL) CCMs. Since CCMs contain several components which are rare and / or valuable, including platinum group metals (notably Pt, Pd, Ir and Ru) and ionomer (both in the membrane and catalyst layers), there is a growing demand for methods of recycling such components from waste CCM materials.

[0012] One current method to recover PGMs from production scrap and end-of-life CCM material involves incineration. The incineration process yields a PGM rich (typically Pt and Ir) ash which is processed via 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 these gases have negative impacts as they pollute the atmosphere, increase the greenhouse effect, and / or have harmful effects in the human body. As such, there is a need for a cleaner process which reduces or eliminates the emission of these gases.

[0013] In addition to the above, the incineration method destroys the ionomer component which also has significant value. As such, it would also be desirable to provide a process which is capable of recovering both PGM and ionomer components as well as providing a process which is cleaner, safer, and more environmentally friendly. Processes for recovering perfluorosulphonic acid ionomer are known. See, for example, WO2016 / 156815 and US7255798. Furthermore, processes for recovering individual PGM catalyst components are known. See, for example, US7709135. However, to enable fuel cells and electrolysers to become more sustainable technologies, there is a need for commercially viable and environmentally friendly routes to recover, separate, and recycle both the PGMs and the ionomer components from waste CCM materials including production scrap and end-of-life material.

[0014] It is an aim of the present specification to address this problem.

[0015] Summary of Invention

[0016] The present specification is concerned with providing a method of recycling a waste ionomer material such as waste ionomer membrane, catalyst-coated ionomer membrane, or catalyst layer material containing ionomer. As described in the background section, such materials are key components of fuel cells and hydrogen producing water electrolysers and typically may contain platinum group metal catalyst material.

[0017] In order to recover ionomer from such materials, the waste ionomer material can be heated in a solvent to disperse the waste ionomer material forming a dispersion of ionomer which can be separated and recovered.

[0018] However, the present inventors have found that waste ionomer materials can be contaminated with metal cations, particularly transition metal cations such as Fe, Cr, Ni, Zn, Sn, Pb, Mo, Mn, and / or Co. The presence of such metal cation contaminants in the recycled ionomer stream is considered a barrier to reuse of the ionomer. As such a method of removing such metal cation contaminants during recycling of the ionomer material is required.

[0019] One possible method is ion exchange. However, the present inventors have found that such metal cations can be difficult to remove from sulfonic acid groups of PFSA ionomers in waste ionomer membranes via ion-exchange processes using strong acid ion exchange media. As such, the present specification provides an alternative approach to removing such metal cation contaminants.

[0020] Accordingly, the present specification provides a method of recycling a waste ionomer material contaminated with one or more metal cation contaminants, the method comprising: treating the waste ionomer material with a chelation reagent which forms soluble complexes with the one or more metal cation contaminants; and washing the soluble complexes from the waste ionomer material.

[0021] The specific chelation reagent which is utilized will depend on the target metal cation contaminant which is to be removed from the waste ionomer material during recycling. The chelation reagent may be selected, for example, from ethylenediaminetetraacetic acid (EDTA), diethylenetriaminepentaacetic acid (DTPA), or hydroxyethylethylenediaminetriacetic acid (HEDTA). Such reagents are useful for removing transition metal cation contaminants from waste ionomer materials.

[0022] In accordance with certain implementations, the waste ionomer material is a solid ionomer membrane waste material which is treated with the chelation reagent and, after washing the soluble complexes from the solid ionomer membrane waste material, the solid ionomer membrane waste material can be dispersed in a solvent for further processing and recovery of the ionomer for re-use.

[0023] In accordance with certain alternative implementations, the waste ionomer material is a solid ionomer membrane waste material which is first dispersed in a solvent to produce a dispersion of the waste ionomer material, and then the dispersion of the waste ionomer material is treated with the chelation reagent and washed to remove the soluble complexes. The ionomer dispersion can then be further processed to recover the ionomer for re-use.

[0024] In accordance with certain further alternative implementations, the waste ionomer material is a solid ionomer membrane waste material which is first dispersed in a solvent to produce a dispersion of the waste ionomer material. The waste ionomer material can then be recovered from the dispersion and the recovered waste ionomer material is then treated with the chelation reagent and washed to remove the soluble complexes.

[0025] As such, this methodology enables metal cation contaminants to be removed from solid ionomer membrane waste materials prior to, or after, dispersal of the waste ionomer material. The ionomer material can then be recovered for re-use. Since this recycling methodology removes metal cation contamination which is difficult to remove by other methods such as ion exchange, the recovered ionomer material can have better functional performance compared to other recycling methods.

[0026] Brief Description of the Drawings

[0027] For a better understanding of the present invention and to show how the same may be carried into effect, certain embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings.

[0028] Figure 1 shows a flow diagram of a method for recycling a solid ionomer membrane waste material including removal of metal contaminants from the solid ionomer membrane waste material by treating with a chelation reagent to form soluble chelate complexes which can then be washed from the solid ionomer membrane waste material prior to dispersing and recovering the ionomer material for re-use.

[0029] Figure 2 shows a flow diagram of a method for recycling a waste ionomer material where the waste ionomer material is first dispersed and then the ionomer dispersion is treated with a chelation reagent to form soluble chelate complexes which can then be washed from the solid ionomer membrane waste material.

[0030] Figure 3 shows a flow diagram of a method for recycling a solid catalyst coated membrane waste material including the methodology of Figure 1. Figure 4 shows a flow diagram of a method for recycling a solid catalyst coated membrane waste material including the methodology of Figure 2.

[0031] Detailed Description

[0032] As described in the summary section the present specification provides a method of recycling a waste ionomer material contaminated with one or more metal cation contaminants, the method comprising: treating the waste ionomer material with a chelation reagent which forms soluble complexes with the one or more metal cation contaminants; and washing the soluble complexes from the waste ionomer material.

[0033] Figure 1 illustrates an example of the methodology in which the waste ionomer material is a solid ionomer membrane waste material which is treated with the chelation reagent and, after washing the soluble complexes from the solid ionomer membrane waste material, the solid ionomer membrane waste material can then be dispersed in a solvent for further processing and recovery of the ionomer.

[0034] Figure 2 shows another example of the methodology in which the waste ionomer material is dispersed in a solvent to produce a dispersion of the waste ionomer material, and the dispersion of the waste ionomer material is treated with the chelation reagent and washed to remove the soluble complexes.

[0035] In accordance with certain further alternative implementations, the waste ionomer material is dispersed in a solvent to produce a dispersion of the waste ionomer material, the waste ionomer material is then recovered from the dispersion, and then the recovered waste ionomer material is treated with the chelation reagent and washed to remove the soluble complexes.

[0036] The methodology may further comprise adjusting pH to achieve the formation of the soluble complexes. Target metal contaminants may be optimally removed in this manner at the optimal pH for complex formation. The specific pH value will depend on the target metal contaminant and the specific chelation reagent which is utilized and can readily be found by testing for a specific chelation reagent / target metal combination.

[0037] After washing the soluble complexes from the waste ionomer material to form a solution of the complexes in which the waste ionomer material is disposed, the waste ionomer material can be separated from the solution via a filtration process or centrifugation. For example, the waste ionomer material can be separated from the solution comprising the soluble complexes via a filtration process which is selected from vacuum filtration or cross-flow membrane filtration. Cross-flow membrane filtration is particularly useful when the chelation reagent treatment is applied to a dispersion of the ionomer.

[0038] The metal cation contaminants may comprise one or more transition metal cations such as one or more cations of Fe, Cr, Ni, Zn, Sn, Pb, Mo, Mn, and / or Co. The waste ionomer material may comprise sulfonic acid groups to which the metal cation contaminants are bound. The present inventors have found that such metal cations can be difficult to remove from sulfonic acid groups of PFSA ionomers in waste ionomer membranes via ion-exchange processes using strong acid ion exchange media. As such, the present specification provides an alternative approach to removing such metal cation contaminants. The present methodology is particularly useful for recycling of manufacturing waste material or end-of life material from fuel cell or electrolyser applications. The chelation reagent may be selected, for example, from ethylenediaminetetraacetic acid (EDTA), diethylenetriaminepentaacetic acid (DTPA), or hydroxyethylethylenediaminetriacetic acid (HEDTA). Such chelation reagents form soluble complexes with the metal cation contaminants such that the soluble complexes can be readily washed from the solid ionomer membrane waste material or from an ionomer dispersion using, for example, an appropriate separation technology and a suitable solvent which will dissolve the metal complex. Appropriate separation processes to separate the ionomer material from solution include membrane filtration processes combined with the use of a compatible solvent in which the metal complexes are soluble with the ionomer remaining dispersed rather than dissolved as a true solution. Examples of suitable solvents include water, aqueous mixtures with organic solvents including alcohols, alcohols, or other suitable organic solvent. As previously noted, the pH of the metal ion chelation process can be adjusted according to the nature of the chelation reagent and target metal contaminant using an appropriate acid, base, or buffer.

[0039] The soluble complexes are washed in a solvent in which the metal complexes are soluble but the ionomer is insoluble. Solvent systems such as those mentioned about may be utilized, i.e., an aqueous solvent, an organic solvent, an alcohol, a mixture of water and an organic solvent, or a mixture of water and an alcohol.

[0040] The solid ionomer membrane waste material can be dispersed in a solvent to recover the ionomer material for re-use. As previously described, in certain methods the dispersion is performed after treating the membrane with a chelation reagent and washing to remove metal contaminants. Preferably, after washing the soluble complexes from the solid ionomer membrane waste material and prior to dispersing the solid ionomer membrane, the solid ionomer membrane waste material is subjected to an ion-exchange process (e.g., using an alkali or alkali earth metal base such as NaOH or LiOH) to convert the solid ionomer membrane waste material to salt form. Figure 3 shows a flow diagram of a method for recycling a catalyst coated membrane using this approach. The catalyst coated membrane is first subjected to acid leaching as described, for example, in WO2023 / 247913 to remove and recover platinum group metal components. The acid leaching may, for example, include an oxidative acid leach to recover platinum and a reductive acid leach to recover iridium. At this stage, the remaining solid waste ionomer membrane material can be treated with a chelation reagent which forms soluble complexes with one or more metal cation contaminants which can then be washed from the solid ionomer membrane waste material. The solid ionomer membrane material is then treated with a base, such as NaOH or LiOH, to convert the ionomer to salt form without dispersing the solid ionomer membrane material. The ionomer salt is more readily dispersed when in such salt form. Furthermore, as the saltation is performed without undue heating to disperse the ionomer, excess base can readily be removed via solid-liquid separation to avoid contaminating further process steps.

[0041] The solid ionomer membrane material is then dispersed in a solvent such as water, an alcohol, or a mixture of water and an alcohol. This is typically done in an autoclave at elevated pressure and temperature. The resultant ionomer dispersion will typically include membrane reinforcement materials such as ePTFE and catalyst support materials such as carbon. These can be removed from the ionomer dispersion by, for example, filtration. The ionomer can then be subjected to an ionexchange step to re-protonate the acid groups of the ionomer. Unlike the transition metal cation contaminants which bind strongly with the sulfonic groups of the ionomer and are difficult to remove, cations such as Na+or Li+used in the saltation step prior to dispersion can be readily ion exchanged to re-protonate the ionomer. The ionomer can then be freeze dried to yield a solid ionomer product ready for re-use.

[0042] Figure 4 shows an alternative process flow to that illustrated in Figure 3. In the process flow of Figure 4, the solid waste ionomer membrane material is dispersed and then the dispersion is treated with a chelation reagent to form soluble metal complexes which can be washed and separated from the ionomer material. The process steps are as follows: acid leach the waste catalyst coated ionomer membrane material to remove and recover platinum group metal; treat the remaining ionomer membrane material with a base to convert the solid ionomer membrane waste material to salt form; disperse the solid ionomer membrane waste material in a solvent; remove solid components such as membrane reinforcement material and catalyst support material, e.g., via filtration; add chelation reagent to the ionomer dispersion and adjust solution pH to form soluble metal complexes; wash the soluble complexes from the ionomer dispersion (e.g., using a cross-flow membrane filtration technique); ion-exchange the ionomer to re-protonate the ionomer; and freeze dry the ionomer to produce a solid ionomer product ready for re-use, e.g., to manufacture new ionomer membranes for fuel cells and / or electrolysers.

[0043] Further details of the steps involved in the recycling methodology of the present specification are described below.

[0044] Ionomer salt formation and dispersion

[0045] As previously described, in certain preferred approaches the ionomer material from a leached catalyst coated membrane is converted to a salt form prior to dispersion. In this regard, the leached catalyst coated membrane is subjected to excess base (at least 1.5 molar equivalents) compared to sulfonic acid groups in the ionomer. The base is added as an aqueous solution and can be lithium hydroxide, sodium hydroxide, or potassium hydroxide. Typically, the membrane is subjected to the basic solution and agitated on a roller mixer for at least 2 hours at room temperature. The excess base is then typically removed by filtration and the membrane solids washed with deionised water. The membrane solids can be washed repeatedly with water until the wash liquor is neutral (pH 7).

[0046] Following the saltation step, the collected solids are typically suspended in an aqueous or organic solvent system to afford a concentration of at least 5 wt.% solids. To disperse the salted ionomer from the leached catalyst coated membrane, the suspension is typically heated to 240°C and stirred for 3 hours in a sealed autoclave vessel (pressure ca. 40-60 bar). The incubation period may be under three hours and the pressure of the vessel depends on the nature of the solvent medium. Typically, the resulting dispersion is neutral (pH 7) to slightly basic (pH 8-9).

[0047] Filtrations - separation of ionomer from membrane reinforcement material (e.g., ePTFE) and catalyst support materials

[0048] Following the ionomer dispersion step, separation of the dispersed ionomer from ePTFE solids is typically achieved by vacuum filtration. The filtrate liquor typically consists of ionomer and residual catalyst support and may consist of other small molecule contaminants such as metal ions.

[0049] The residual solid catalyst support materials can be extracted from the ionomer dispersion via centrifugation. It may be preferable that the centrifuge rotor speed is at least 10,000 rpm and the separation process is conducted below room temperature.

[0050] In some embodiments, it is preferable to separate the dispersed ionomer from catalyst support materials using cross-flow membrane filtration. Membranes with a pore size of equal to or less than 200 nm is preferable for this separation to obtain a filtrate with no residual catalyst support.

[0051] Ion Exchange- re-protonation of ionomer and removal of trace metal contaminants

[0052] Typically, a dispersion of salted ionomer in water or mixed water-alcohol solvent systems can be reprotonated back to acid form using an ion-exchange process. Additionally, some trace metal ion contaminants can be removed. It is preferable to use an acidic ion-exchange resin to capture the cations used in the prior saltation step (such as Na+and Li+) and re-protonate the sulfonic acid groups in the ionomer.

[0053] Addition of chelating agents

[0054] Metal contamination during the recycling process may occur prior to leaching the catalyst coated membrane or during the ionomer dispersion stage. Certain metal contaminants bind strongly to the ionomer and are difficult to remove through filtration processes alone. Addition of chelating agents, such as EDTA, DTPA, or HEDTA to these contaminated ionomer materials (either in solid form or in ionomer dispersion form) results in a mixture whereby the metal complexes can be washed away from the ionomer. The wash solvent may be acidic (pH 2-6), neutral (pH 7), or slightly basic (pH 8-9). Aqueous or mixed aqueous / alcohol solvent systems can be utilized. It can be preferable to add the chelating agents to the leached catalyst coated membrane or the ionomer dispersion before the ion exchange process. A vacuum filtration or cross-flow filtration step can be employed to separate the soluble metal complexes from the ionomer membrane solid or ionomer dispersion.

[0055] By way of example, a molar excess of EDTA (28 mg) was added to a 10 mL sample of ionomer dispersion containing trace metal contaminants (Fe). The sample was agitated on a roller mixer for 16 hours at room temperature. The resulting dispersion was diluted to 30 mL and processed with a hollow fibre membrane filter (pore size 35 nm). The sample was subjected to diafiltration whereby the feed / retentate vessel and membrane filter was washed with 5 x 30 g of deionised water (5 diafiltration volumes). ICP-OES analysis after membrane filtration with and without EDTA showed that a significant reduction in metal contaminant is achieved with the addition of the chelating agent.

[0056] The table below shows ICP-OES analysis results of the ionomer dispersion after addition of EDTA (which forms a soluble complex with Fe) and subsequent filtering to yield a retentate depleted in Fe as compared to a process without EDTA addition.

[0057] As such, it will be evident that the since this recycling methodology removes metal cation contamination which is difficult to remove by other methods such as ion exchange, the recovered ionomer material will have better functional performance compared to other recycling methods which yield contaminated ionomer material.

[0058] While this invention has been particularly shown and described with reference to certain examples, it will be understood to those skilled in the art that various changes in form and detail may be made without departing from the scope of the invention as defined by the appended claims.

Claims

Claims1. A method of recycling a waste ionomer material contaminated with one or more metal cation contaminants, the method comprising: treating the waste ionomer material with a chelation reagent which forms soluble complexes with the one or more metal cation contaminants; and washing the soluble complexes from the waste ionomer material.

2. A method according to claim 1, wherein the waste ionomer material is a solid ionomer membrane waste material which is treated with the chelation reagent and, after washing the soluble complexes from the solid ionomer membrane waste material, the solid ionomer membrane waste material is dispersed in a solvent.

3. A method according to claim 1, wherein the waste ionomer material is a solid ionomer membrane waste material which is first dispersed in a solvent to produce a dispersion of the waste ionomer material, and then the dispersion of the waste ionomer material is treated with the chelation reagent and washed to remove the soluble complexes.

4. A method according to claim 1, wherein the waste ionomer material is a solid ionomer membrane waste material which is first dispersed in a solvent to produce a dispersion of the waste ionomer material, the waste ionomer material is recovered from the dispersion, and then the waste ionomer material is treated with the chelation reagent and washed to remove the soluble complexes.

5. A method according to any preceding claim, further comprising adjusting pH to achieve the formation of the soluble complexes.

6. A method according to any preceding claim, wherein, after washing the soluble complexes from the waste ionomer material to form a solution of the complexes in which the waste ionomer material is disposed, the waste ionomer material is separated from the solution via a filtration process or centrifugation.

7. A method according to claim 6, wherein the filtration process is selected from vacuum filtration or cross-flow membrane filtration.

8. A method according to any preceding claim, wherein the metal cation contaminants comprise one or more transition metal cations.

9. A method according to any preceding claim, wherein the metal cation contaminants comprise one or more cations of Fe, Cr, Ni, Zn, Sn, Pb, Mo, Mn, and / or Co.

10. A method according to any preceding claim, wherein the waste ionomer material comprises sulfonic acid groups to which the metal cation contaminants are bound.

11. A method according to any preceding claim, wherein the waste ionomer material is manufacturing waste material or end-of life material from fuels cell or electrolyser applications.

12. A method according to any preceding claim, wherein the chelation reagent is selected from ethylenediaminetetraacetic acid (EDTA), diethylenetriaminepentaacetic acid (DTPA), or hydroxyethylethylenediaminetriacetic acid (HEDTA).

13. A method according to any preceding claim, wherein the soluble complexes are washed from the waste ionomer material using one of: an aqueous solvent; an organic solvent; an alcohol; a mixture of water and an organic solvent; a mixture of water and an alcohol.

14. A method according to any preceding claim, wherein the waste ionomer material is subjected to an ion-exchange process to convert the waste ionomer material to salt form.

15. A method according to claim 14, wherein the waste ionomer material is converted to salt form before or after treating the waste ionomer material with the chelation reagent and washing the soluble complexes from the waste ionomer material.

Images