Method of recycling a catalyst coated membrane

Abstract

A method of recovering material from an ionomer membrane coated on both faces with a catalyst layer which comprises a platinum group metal (PGM) disposed on a support material (e.g. carbon), by removi

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 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. 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 catalyst coated membrane to recover both platinum group metal and ionomer components. In order to recover ionomer from such materials, the waste membrane material can be heated in a solvent to form a dispersion of ionomer. However, if such a dispersion process is applied to the catalyst coated membrane, then the resultant dispersion will comprise both ionomer material and catalyst material. The dispersion thus requires further processing to separate the ionomer and platinum group metal catalyst materials. One possibility to avoid the dispersion including the platinum group metal is to subject the catalyst coated membrane material to an acid leaching process to recover the platinum group metal prior to dispersing the ionomer. However, such acid leaching processes do not remove catalyst support material, such as carbon, from the ionomer membrane. Accordingly, after leaching the membrane of platinum group metal and then dispersing the ionomer membrane, the dispersion will still be contaminated with catalyst support material which requires removal to achieve a purified ionomer product. Another possibility to avoid the ionomer dispersion including the platinum group metal is to subject the catalyst coated membrane material to a delamination process in which that catalyst coatings are delaminated and separated from the ionomer membrane prior to dispersing the ionomer membrane. However, the delaminated material will then require further processing to recover the platinum group metal from support material and catalyst layer ionomer material. Additionally, ionomer membranes may comprise platinum group metal material in an interior of the membrane serving as a recombination catalyst. Delamination of surface catalyst layers will not remove such interior platinum group metal material. As such, if the catalyst layer coatings are first removed by delamination, both the remaining ionomer membrane and also the separated catalyst coating material will require further processing to recover the platinum group metal. As such, the present specification proposes another approach in which the catalyst coated membrane is first subjected to an acid leaching process to remove platinum group metal and then the remaining catalyst coated membrane, including catalyst support material, is subjected to a delamination process to remove the catalyst layer material which has already been leached of its platinum group metal content from the ionomer membrane. This approach enables an efficient three-way separation of platinum group metal material, catalyst support material, and ionomer material. In light of the above, the present specification provides a method of recycling a catalyst coated membrane, the catalyst coated membrane comprising an ionomer membrane coated with catalyst layers on either side thereof, at least one of the catalyst layers including a catalyst material composed of a platinum group metal disposed on a catalyst support material, the method comprising: subjecting the catalyst coated membrane to at least one acid leach to remove platinum group metal from the catalyst coated membrane, the remaining catalyst coated membrane comprising the ionomer membrane coated with catalyst layer material, including the catalyst support material, which has been leached of its platinum group metal content; subjecting the remaining catalyst coated membrane to a delamination step in which the catalyst layer material, including the catalyst support material which has been leached of its platinum group metal content, is delaminated from the ionomer membrane without dispersing the ionomer membrane; separating the delaminated catalyst layer material from the ionomer membrane; and heating the ionomer membrane in a solvent to form an ionomer dispersion. 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, in which: Figure 1 shows a flow diagram of a method for recycling a catalyst coated membrane according to the present specification; and Figure 2 shows a flow diagram of another method for recycling a catalyst coated membrane according to the present specification. Detailed Description As described in the summary section and shown in the flow diagram of Figure 1, the present specification provides method of recycling a catalyst coated membrane, the catalyst coated membrane comprising an ionomer membrane coated with catalyst layers on either side thereof, at least one of the catalyst layers including a catalyst material composed of a platinum group metal disposed on a catalyst support material. The method comprises subjecting the catalyst coated membrane to at least one acid leach to remove platinum group metal from the catalyst coated membrane, the remaining catalyst coated membrane comprising the ionomer membrane coated with catalyst layer material, including the catalyst support material, which has been leached of its platinum group metal content. The remaining catalyst coated membrane is then subjected to a delamination step in which the catalyst layer material, including the catalyst support material which has been leached of its platinum group metal content, is delaminated from the ionomer membrane without dispersing the ionomer membrane. The delaminated catalyst layer material can then be separated from the ionomer membrane and the ionomer membrane can then be heated in a solvent to form an ionomer dispersion. The acid leaching may comprise an oxidative acid leach which is suitable for leaching platinum, palladium and / or ruthenium. Furthermore, the acid leaching may comprise a reductive acid leach which is suitable for leaching iridium. Suitable leaching conditions are described in WO2023 / 247913. The catalyst support material may comprise particles of an inorganic support material, such as carbon, on which platinum group metal is disposed. The catalyst layer material may also comprise ionomer material. In that case, after delamination of the catalyst layer material, the catalyst support material may require further separation from the ionomer material. However, there will be much less ionomer material in this process stream compared with the approach in which the catalyst support material is dispersed with the bulk ionomer membrane. Various approaches to achieving delamination of the catalyst layer material from the bulk ionomer membrane may be envisaged. In one approach, the delamination step comprises contacting the leached catalyst coated membrane with a solvent to delaminate the catalyst layer material. The solvent can be agitated, for example by sonification, when contacting the leached catalyst coated membrane with the solvent to aid delamination of the catalyst layer material. The solvent can be a mixture of an alcohol and water, the alcohol optionally selected from n-butanol, n-propanol, i-propanol, or ethanol. The mixture of alcohol and water can have a volume ratio of alcohokwater which is: at least 50:50, 60:40, or 70:30; no more than 95:5, 90:10, or 85:15; or within a range defined by any combination of the aforementioned lower and upper limits. Furthermore, the solvent used to delaminate the catalyst layer material from the ionomer membrane may comprise a base, optionally a metal hydroxide or ammonium solution, to convert the ionomer to salt form prior to dispersion of the bulk ionomer membrane. Alternatively, the conversion of ionomer to salt form using a base can be performed in a separate step to the delamination process. Figure 2 shows another example 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. In accordance with the present specification, the membrane is then subjected to a delaminate process to separate the remaining catalyst layer material from the ionomer membrane. As an alternative, the delamination process can be done prior to conversion of the ionomer membrane to salt form. The ionomer membrane 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 which can be readily separated by filtration. The ionomer can then be subjected to an ion-exchange step to re-protonate the acid groups of the ionomer and freeze dried to yield a solid ionomer product ready for re-use. To demonstrate the method, a portion of catalyst coated membrane (0.0559 g) previously subject to a platinum leaching process was soaked in ethanol for 5 minutes. The ethanol-soaked membrane was transferred to a solution of 1-propanol and water (1:1) and subject to ultrasonication for 10 minutes. The sonicated membrane was washed with water to remove the delaminated, catalyst layer which was substantially free from platinum after the platinum leach. FTIR of the delaminated membrane exhibited signal characteristic of PFSA ionomer present in the membrane after delamination. This demonstrates bulk separation of ionomer used in the catalyst layers from that used in the membrane. The membrane could then be processed to disperse the membrane and recover the membrane ionomer. The method enables the membrane ionomer to be recovered separately from the ionomer used in the catalyst layers. 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

1. A method of recycling a catalyst coated membrane, the catalyst coated membrane comprising an ionomer membrane coated with catalyst layers on either side thereof, at least one of the catalyst layers including a catalyst material composed of a platinum group metal disposed on a catalyst support material, the method comprising:subjecting the catalyst coated membrane to at least one acid leach to remove platinum group metal from the catalyst coated membrane, the remaining catalyst coated membrane comprising the ionomer membrane coated with catalyst layer material, including the catalyst support material, which has been leached of its platinum group metal content;subjecting the remaining catalyst coated membrane to a delamination step in which the catalyst layer material, including the catalyst support material which has been leached of its platinum group metal content, is delaminated from the ionomer membrane without dispersing the ionomer membrane;separating the delaminated catalyst layer material from the ionomer membrane; andheating the ionomer membrane in a solvent to form an ionomer dispersion.

2. A method according to claim 1,wherein the at least one acid leach comprises an oxidative acid leach.

3. A method according to claim 1 or 2,wherein the at least one acid leach comprises a reductive acid leach.

4. A method according to any preceding claim,wherein the catalyst support material comprises particles of an inorganic support material on which platinum group metal is disposed.

5. A method according to claim 4,wherein the inorganic support material is carbon.

6. A method according to any preceding claim,wherein the catalyst layer material comprises an ionomer.

7. A method according to any preceding claim,wherein the delamination step comprises contacting the leached catalyst coated membrane with a solvent to delaminate the catalyst layer material.

8. A method according to claim 7,wherein the solvent is agitated when contacting the leached catalyst coated membrane with the solvent to delaminate the catalyst layer material.

9. A method according to claim 8,wherein the solvent is agitated by sonification.

10. A method according to any one of claims 7 to 9,wherein the solvent used to delaminate the catalyst layer material from the ionomer membrane is a mixture of an alcohol and water.

11. A method according to claim 10,wherein the alcohol in the mixture of the alcohol and water is selected from n-butanol, n-propanol, i-propanol, or ethanol.

12. A method according to claim 10 or 11,wherein the mixture of the alcohol and water has a volume ratio of alcohokwater which is: at least 50:50, 60:40, or 70:30; no more than 95:5, 90:10, or 85:15; or within a range defined by any combination of the aforementioned lower and upper limits.

13. A method according to any preceding claim,wherein the solvent used to delaminate the catalyst layer material from the ionomer membrane comprises a base, optionally a metal hydroxide or ammonium solution, to convert the ionomer to salt form.T +44(0)30 0300 2000A

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