A method of recycling a waste ionomer material

Abstract

Waste ionomer material, which is preferably a persulfonic acid ionomer, and preferably is obtained from waste proton exchange membranes from fuel cells or electolysers, is heated in a solvemt to form

Description

Field 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. Background 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. 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. 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). 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., CeOj) as a peroxide scavenger. 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). 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. 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. 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. 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 perfluorosulfonic 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. It is an aim of the present specification to address this problem. Summary of Invention 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. 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. Such ionomer dispersions can be contaminated with catalyst particles, catalyst support particles, membrane reinforcement material, and / or metal cations. A significant amount of such contamination can be removed via centrifugation and / or filtration. However, the present inventors have found that even after such treatment, trace contaminants such as trace catalyst particles, trace catalyst support particles and / or metal cations can remain in the ionomer dispersion. It has further been found that such trace contamination can be reduced or removed by treating the ionomer dispersion with a membrane filtration process (e.g., a crossflow filtration process such as one which utilizes a hollow fibre membrane filtration configuration). Such a processes can be used to wash the ionomer. Such a process can also be used to concentrate the ionomer dispersion and, if desired, change the solvent in which the ionomer is dispersed. In light of the above, the present specification provides a method of recycling a waste ionomer material, the method comprising: heating the waste ionomer material in a solvent to disperse the waste ionomer material forming an ionomer dispersion in the solvent; and treating the ionomer dispersion with a membrane filtration process to remove contaminants. Brief Description of the Drawings 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. Figure 1 shows a flow diagram of a method for recycling a solid ionomer membrane waste material according to the present specification. Figure 2 shows a flow diagram of a method for recycling a catalyst coated membrane according to the present specification. Figure 3 shows a photo of aliquots taken from an ionomer dispersion processed with a hollow fibre membrane filter. The original feed (left) contains ionomer and residual catalyst support. The retentate (middle) contains the catalyst and ionomer retained by the filter. The permeate (right) contains the isolated ionomer free of catalyst support. Figure 4 shows 19 F NMR of an ionomer dispersion sample before membrane filtration (bottom trace), purified ionomer dispersion after membrane filtration (middle trace), and the recovered permeate stream containing small molecule contaminants (top trace). Figure 5 shows 19 F NMR of an ionomer dispersion sample before membrane filtration (bottom trace), purified ionomer dispersion after membrane filtration (middle trace), and the recovered permeate stream containing small molecule contaminants (top trace). Detailed Description As described in the summary section and shown in the flow diagram of Figure 1, the present specification provides a method of recycling a waste ionomer material, the method comprising: heating the waste ionomer material in a solvent to disperse the waste ionomer material forming an ionomer dispersion in the solvent; and treating the ionomer dispersion with a membrane filtration process to remove contaminants. The membrane filtration process can be a crossflow filtration process such as one which utilizes a hollow fibre membrane filtration configuration. Crossflow filtration (also known as tangential flow filtration) differs from more conventional "dead-end" filtration in that the majority of the feed flow travels tangentially across the surface of the filter rather than directly into the filter. One type of crossflow filtration uses hollow fibre membranes. Commercial hollow fibre membrane units typically comprise a plurality of permeable (porous) hollow fibres packed into a cartridge which can be used for a variety of liquid and gaseous separations. Such units are used in water treatment, desalination, cell culture, medicine, and tissue engineering. In accordance with the present specification membrane filtration units of this type are used in a waste ionomer recycling process. Typically, membrane filtration falls into two categories: ultrafiltration (nano-scale, with pores typically below 30 nm in size) or microfiltration (micron scale, with pores typically of the order of 1 to 10 micrometres). However, it has been found that for the present ionomer recycling method, the membrane filtration process advantageously utilizes membranes with a pore size in a range 30 nm to 500 nm which lies between the typical pore sizes using in ultrafiltration or microfiltration. The pore size of the membranes for the present application can be: at least 30 nm, 35 nm, 50 nm, 75 nm, 100 nm, or 150 nm; no more than 500 nm, 400 nm, 300 nm, or 200 nm; or within a range defined by any combination of the aforementioned lower and upper limits. For example, the pore size may lie in a range 100 to 200 nm. The membrane filtration process may comprise at least one concentration step in which the concentration of ionomer is increased. Furthermore, the membrane filtration process may comprise at least one diafiltration step in which the ionomer is washed. Advantageously, the membrane filtration process comprises a sequence of concentration and diafiltration steps. For example, the ionomer dispersion can be concentrated, then washed via diafiltration, then concentrated again using the membrane filtration methodology. A diafiltration step can also be used to change the solvent in which the ionomer is dispersed using the membrane filtration methodology. Furthermore, a diafiltration step can be used to wash out soluble species such as small molecules, inorganic species and metal complexes. For example, the waste ionomer material can be treated with a chelation reagent (e.g., ethylenediaminetetraacetic acid (EDTA), diethylenetriaminepentaacetic acid (DTPA), or hydroxyethylethylenediaminetriacetic acid (HEDTA)) which forms soluble complexes with one or more metal cation contaminants and the membrane filtration process comprises a diafiltration step which washes the soluble complexes from the ionomer. The present membrane filtration process can be combined with more conventional centrifugation and / or dead-end filtration processes. That is, after heating the waste ionomer material to form the ionomer dispersion, the ionomer dispersion can be subjected to centrifugation or dead-end filtration to remove one or more of catalyst particles, catalyst support particles, and membrane reinforcement material and then the ionomer dispersion can be further treated with the membrane filtration process to remove trace or small molecule contaminants, concentrate the ionomer dispersion, and / or change the solvent of the dispersion. Further still, the waste ionomer material can be treated with a base to convert the ionomer to salt form either before, during or after dispersing the ionomer in the solvent and prior to subjecting the ionomer dispersion to the membrane filtration process. Following the above-described methodology, after dispersion of an ionomer membrane including reinforcement, catalyst particles and support, the catalyst and support particles can be recovered via centrifugation or conventional "dead-end" filtration. Remaining catalyst particles and support particles can then be separated by membrane filtration wherein the ionomer permeates, and catalyst and support are retained. All of the ionomer content can be washed through via diafiltration and dilution with water (Example 1). In the next stage of the process, the ionomer can be concentrated using a membrane filtration process. Furthermore, in a subsequent step, a membrane filter can be used in diafiltration mode to wash the ionomer particles free of small molecule contaminants such as ionomer breakdown products and soluble metal species including inorganic salts and chelated metal complexes (e.g., transition metal complexes such as Fe, Co, Cr, Mn, Mo, Ni, Zn) formed with chelation reagents such as EDTA (Example 2). A subsequent concentration can then be carried out using a membrane filtration process. Additionally, the solvent in which the ionomer is dispersed can be changed using a diafiltration process, for example from water to water / solvent or solvent, or vice versa. It should also be noted that the same membrane filtration rig / unit can be used to both concentrate an ionomer dispersion (concentration mode) and wash the ionomer free of small molecule contaminants such as ionomer breakdown products and soluble metal species (diafiltration mode). That is, the membrane filtration rig / unit can be operated in different modes for the different processing steps. Such, programmable membrane filtration units are commercially available. Alternatively, separate membrane filtration units can be provided for the different membrane filtration steps. Figure 2 shows a flow diagram of a method for recycling a catalyst coated membrane according to the present specification. The catalyst coated membrane is subjected to acid leaching as described 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. 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. 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. 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, centrifugation or filtration. The ionomer dispersion can then be treated with the membrane filtration processes as described herein to remove trace contaminants (Examples 3 and 4) and to concentrate the ionomer dispersion in the desired solvent (Example 5). The ionomer can then be subjected to an ion-exchange step to re-protonate the acid groups of the ionomer (Example 6). The ionomer can then be freeze dried to yield a solid ionomer product or concentrated further using the membrane filtration process ready for re-use Examples Removal of suspended catalysts and supports Example 1 - Approximately, 800 mL of ionomer dispersion (6.32 wt.% in deionised water and lithium hydroxide) containing residual catalyst support was processed with a hollow fibre membrane filter (pore size 0.1 pm). An initial concentration step reduced the feed sample weight by a factor of 2.73 (ca. 820 g to 300 g). The sample was then subjected to diafiltration whereby the feed / retentate vessel and membrane filter was washed with 2 x 300 g of deionised water (two diafiltration volumes). The final filtrate was colourless, containing 1.56 wt.% ionomer, free of residual catalyst support (black particulates). The final retentate was a viscous black suspension (containing residual catalyst support and ionomer). Figure 3 shows a photo of aliquots taken from an ionomer dispersion processed with a hollow fibre membrane filter. The original feed (left) contains ionomer and residual catalyst support. The retentate (middle) contains the catalyst and ionomer retained by the filter. The permeate (right) contains the isolated ionomer free of catalyst support. Removal of soluble metal salts and complexes Example 2 - To a 10 sample of ionomer dispersion containing trace metal contaminants (453 ppm of Fe, Ce, Cr and Zn) a molar excess of EDTA (28 mg) was added. 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). The final filtrate was yellow and contained no ionomer. ICP-OES analysis of the sample prior to membrane filtration revealed the presence of Ce (135 ppm) and Fe (295 ppm). After chelation and membrane filtration the ionomer dispersion contained <25 ppm of Fe. The table below shows ICP-OES analysis results of an ionomer dispersion sample before and after treatment with a chelation agent (EDTA) and purification with a hollow fibre membrane filter, showing effective chelation of Fe by the EDTA and reduction in the concentration of this soluble Fe-EDTA complex by permeation through the filter membrane. Sample S(ppm) Fe (ppm) Ce (ppm) Feed Dispersion 2868 295 139 Processed Dispersion 2188 24 134 Concentration of ionomer and diafiltration to remove dissolved small molecules and metal salts Example 3 -1 L of ionomer dispersion (6.52 wt.% in deionised water) containing trace small molecule contaminants was processed with a hollow fibre membrane filter (pore size 35 nm). An initial concentration step reduced the feed sample weight by a factor of 2.15 (ca. 1000 g to 465 g). The sample was then subjected to diafiltration whereby the feed / retentate vessel and membrane filter was washed with 2 x 465 g of deionised water (two diafiltration volumes). The final retentate contained ionomer dispersed (14.01 wt.%) in water free of small molecule contaminants. The table below shows the concentration of free fluoride species present in an ionomer dispersion sample before (feed) and after processing using a hollow fibre membrane filter (retentates) showing free fluoride is washed from the ionomer during concentration and diafiltration processes. Sample F(ppm) Solids Content (wt.%) Feed 186 6.52 Retentate after Concentration 113 14.01 Retentate after Diafiltration 0 14.01 Trace free fluoride species from the feed sample (186 ppm F ) was also removed in the final retentate (0 ppm F ). After concentration 113 ppm F- was present in the retentate and after one diafiltration volume the F- concentration was reduced to 12 ppm. The 19F NMR of permeate contains sharp peaks, indicative of LiF (-120 ppm) and a small perfluorinated molecule (-124 ppm) which were present in the feed but are now absent from the retentate. Figure 4 shows19 F NMR of an ionomer dispersion sample before membrane filtration (bottom trace), purified ionomer dispersion after membrane filtration (middle trace), and the recovered permeate stream containing small molecule contaminants (top trace). Example 4-1.9 Lof ionomer dispersion (5.86 wt.% in deionised water) containing trace small molecule contaminants was processed with a hollow fibre membrane filter (pore size 35 nm). An initial concentration step reduced the feed sample weight by a factor of 2.12 (ca. 1900 g to 894 g). The sample was then subjected to diafiltration whereby the feed / retentate vessel and membrane filter was washed with 2 x 894 g of deionised water (two diafiltration volumes). The final retentate contained ionomer dispersed (13.80 wt.%) in water free of small molecule contaminants. The table below shows concentration of free fluoride species present in an ionomer dispersion sample before (feed) and after processing using a hollow fibre membrane filter (retentates). Sample F(ppm) Solids Content (wt.%) Feed 80 5.86 Retentate after Concentration 51 13.80 Retentate after Diafiltration 0 13.80 Trace free fluoride species from the feed sample (80 ppm F ) was also removed in the final retentate (0 ppm F ). After concentration, 113 ppm F- was present in the retentate and after diafiltration volume the F- concentration was reduced to 0 ppm. The 19F NMR of permeate contains sharp peaks, indicative of LiF (-120 ppm) and a small perfluorinated molecule (-124 ppm) which were present in the feed but are now absent from the retentate. Figure 5 shows19 F NMR of the ionomer dispersion sample before membrane filtration (bottom trace), purified ionomer dispersion after membrane filtration (middle trace), and the recovered permeate stream containing small molecule contaminants (top trace). Example 5 - Approximately 1 L of ionomer dispersion (6.52 wt.% in deionised water) was processed with a hollow fibre membrane filter (35 nm pore size). The first concentration step reduced the feed sample weight by a factor of 2.3 (ca. 1023 g to 445 g) resulting in an ionomer concentration of 15.94 wt.%. The second concentration step reduced the sample weight by a factor of 1.6 (ca. 445 g to 279 g) resulting in a final ionomer concentration of 24.34 wt.%. The table below shows the masses and solids content (wt.%) of an ionomer dispersion concentrated using a hollow fibre membrane filter. The initial ionomer dispersion is the feed, the concentrated ionomer dispersions are the retentates, and the permeates contain the solvent removed from the ionomer dispersion. Sample Concentration Factor Feed / Retentate Mass (g) Permeate Mass (g) Feed / Retentate Solids Content (wt.%) Permeate Solids Content (wt.%) Feed 0 1023 0 6.52 0.00 After 1st Concentration 2.3 445 577 15.94 0.00 After 2nd Concentration 1.6 279 746 24.34 0.00 Ion exchange of membrane filtered ionomer dispersions to convert sulfonate salts to sulfonic acid Example 6 - A batch of Amberlyst 15 (H) resin was added to an omnifit glass column and treated as follows: three bed volumes (BVs) of deionised water, three BVs of acid rinse (e.g. 3 M HCI), and three BVs of deionised water. Then 1 L of ionomer dispersion (6 wt.% in deionised water) was injected into the column at a flow rate of one bed volume per hour (1 BV h1) for 5 h. Amberlyst 15 (H) is commercially available from Thermo Scientific Chemicals, the CAS number is 39389-20-3. ICP-OES analysis before and after ion-exchange show reduction in metal contaminants. The table below shows ICP-OES analysis results of the ionomer dispersion sample before and after processing with an ionexchange resin. Sample Cr(ppm) Fe (ppm) Li (ppm) Ni (ppm) Before Ion-Exchange 36 47 19 278 After Ion-Exchange 3 4 5 9 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.

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