Catalyst and process

Abstract

An oxygen evolution catalyst material is provided, the catalyst material comprising iridium oxide (IrOx) and a potassium iridate crystalline phase. The potassium iridate crystalline phase provides a reflection in the x-ray diffraction (XRD) pattern of the catalyst material at 20 = 13° and offers high oxygen evolution catalyst activity.

Description

[0001] CATALYST AND PROCESS

[0002] Field of the Invention

[0003] The present invention relates to iridium-containing catalyst materials which are suitable for use as oxygen evolution reaction (OER) catalysts, for example in a water electrolyser or a fuel cell, and to improved processes for their manufacture.

[0004] Background of the Invention

[0005] The electrolysis of water to produce hydrogen and oxygen can be carried out in both alkaline and acidic electrolyte systems. Those electrolysers that employ a solid proton-conducting polymer membrane, or proton exchange membrane (PEM), are known as proton exchange membrane water electrolysers (PEMWEs). Those electrolysers that utilise a solid anion- conducting polymer membrane, or anion exchange membrane (AEM), are known as anion exchange membrane water electrolysers (AEMWEs).

[0006] Ion-conducting membranes, such as PEMs and AEMs, are also used in fuel cells. In a proton exchange membrane fuel cell (PEM FC) the membrane is proton conducting, and protons, produced at the anode, are transported across the membrane to the cathode, where they combine with oxygen to form water.

[0007] Catalyst-coated membranes (CCMs) may be employed within electrochemical devices, such as electrolysers and fuel cells. Such CCMs comprise an ion-conducting membrane, such as a PEM or AEM, with an anode catalyst layer and / or a cathode catalyst layer applied to a face of the membrane, the anode catalyst layer and cathode catalyst layer being applied to opposite faces of the membrane.

[0008] For water electrolyser applications, hydrogen evolution reaction (HER) catalysts are used in cathode catalyst layers, for example HER catalysts comprising platinum, such as platinum on a carbon support. Oxygen evolution reaction (OER) catalysts are utilised in electrolyser anode catalyst layers. Iridium-containing catalysts are well known for their properties as excellent OER catalysts and are preferred materials for the oxygen evolution reaction on the anode side of a water electrolyser.

[0009] For fuel cell applications, oxygen reduction reaction (ORR) catalysts are used in cathode catalyst layers and hydrogen oxidation reaction (HOR) catalysts are utilised in anode catalyst layers. For PEM FC applications, suitable cathode and anode catalyst materials comprise a platinum group metal or an alloy of a platinum group metal with one or more other metals, for example platinum or an alloy of platinum with one or more other metals. Iridium-based OER catalysts may also be incorporated into fuel cell anodes, to improve cell reversal tolerance.

[0010] In commercial applications iridium-containing OER catalysts are typically iridium oxide (I rOx) materials. Such iridium oxide (lrOx) materials may be amorphous, in which case the materials are typically a mixture of iridium oxides and oxyhydroxides, with both Ir (III) and Ir (IV) species present, or may be crystalline, such as iridium (IV) oxide with a rutile crystal structure.

[0011] JP2017141158A (UNIV SHINSHU) describes the formation of iridium oxide nanosheets with a thickness of 3 nm or less. The nanosheets are formed by conversion of iridium oxide nanosheets to a layered iridium acid salt, and then subsequent conversion of the layered iridium acid salt to layered iridium acid, from which the iridium oxide nanosheet is formed. The layered iridium acid salts are used as reaction intermediates and are not used or proposed as oxygen evolution reaction catalysts.

[0012] US2016376715A1 (UNIV SHINSHU) describes layered alkali iridates and layered iridic acids which are proposed for use as intermediates for producing iridium oxide nanosheets. Again, the layered alkali iridates are used as reaction intermediates and are not used or proposed as oxygen evolution reaction catalysts.

[0013] Due to the rising global demand for hydrogen and the scarcity of iridium, there is a need to develop iridium-containing OER catalyst materials with improved performance and / or equivalent performance using lower loadings of iridium. There is also a need to develop more efficient processes for the production of iridium-containing OER catalyst materials with high oxygen evolution catalytic activity.

[0014] Summary of the invention

[0015] The present inventors have surprisingly found that iridium oxide (IrOx) catalyst materials which comprise a potassium iridate crystalline phase show enhanced OER catalytic activity. The potassium iridate crystalline phase may be identified by a reflection in the x-ray diffraction (XRD) pattern at 20 = 13 °.

[0016] Therefore, in a first aspect of the invention there is provided an oxygen evolution catalyst material, the catalyst material comprising iridium oxide (IrOx) and a potassium iridate crystalline phase, wherein the potassium iridate crystalline phase provides a reflection in the x-ray diffraction (XRD) pattern of the catalyst material at 20 = 13 °.

[0017] The oxygen evolution catalyst material may be advantageously formulated with an ionconducting polymer to form an ink. Therefore, in a second aspect of the invention there is provided an ink comprising an oxygen evolution catalyst material according to the first aspect and an ion-conducting polymer.

[0018] Such inks may be suitably applied to a substrate to produce a catalyst layer. Therefore, in a third aspect of the invention there is provided a catalyst layer comprising an oxygen evolution catalyst material according to the first aspect and an ion-conducting polymer. Suitably, the catalyst layer is an anode catalyst layer for a water electrolyser. The oxygen evolution catalyst material may also be incorporated into a catalyst layer for a fuel cell, particularly in a fuel cell anode catalyst layer in combination with a hydrogen oxidation reaction (HOR) catalyst, such as a platinum catalyst, to improve cell reversal tolerance.

[0019] Such catalyst materials and catalyst layers may be used in the formation of catalyst-coated membranes. Therefore, in a fourth aspect of the invention there is provided a catalyst- coated membrane comprising an oxygen evolution catalyst material according to the first aspect or a catalyst layer according to the third aspect. Suitably, the catalyst-coated membrane comprises a proton exchange membrane (PEM) or an anion exchange membrane (AEM). The oxygen evolution catalyst material is preferably provided in an anode catalyst layer applied to a face of the membrane. Such CCMs are suitable for use in a water electrolyser or a fuel cell.

[0020] In a fifth aspect of the invention there is provided a water electrolyser or a fuel cell comprising a catalyst-coated membrane according to the fourth aspect.

[0021] In a sixth aspect of the invention there is provided the use of a catalyst material comprising a potassium iridate crystalline phase as an OER catalyst, such as in a water electrolyser, the crystalline phase providing a reflection in the x-ray diffraction (XRD) pattern of the catalyst material at 20 = 13. In a seventh aspect of the invention there is provided the use of a catalyst material comprising a KlrCh crystalline phase as an OER catalyst, such as in a water electrolyser.

[0022] It has also been found that the OER activity of iridium oxide catalyst materials may be enhanced by thermal treatment with a potassium salt, preferably potassium hydroxide. Therefore, in an eighth aspect of the invention there is provided a method of manufacturing an oxygen evolution catalyst material, the method comprising the step of the heat treatment of a mixture comprising iridium oxide and a potassium salt, such as potassium hydroxide. Suitably the formed oxygen evolution catalyst material is according to the first aspect of the invention. In a ninth aspect of the invention there is provided an oxygen evolution catalyst material obtained or obtainable by the method of the seventh aspect.

[0023] Description of the Figures

[0024] Figure 1 shows the XRD pattern of the material formed in Example 1.

[0025] Figure 2 shows the results of OER testing of the material formed in Example 1 and Comparative Example 1 .

[0026] Figure 3 shows the XRD pattern of the material formed in Example 2.

[0027] Figure 4 shows the results of OER testing of the material formed in Example 2 and the starting material used to form Example 2.

[0028] Figure 5 shows the XRD pattern of the material formed in Example 3.

[0029] Figure 6 shows the results of OER testing of the material formed in Example 3 and a Comparative Example.

[0030] Figure 7 shows the XRD pattern of the material formed in Example 4.

[0031] Detailed Description

[0032] Preferred and / or optional features of the invention will now be set out. Any of the preferred and / or optional features of any aspect may be combined, either singly or in combination, with any other preferred and / or optional features of any aspect of the invention unless the context demands otherwise.

[0033] The present invention provides an oxygen evolution catalyst material. The oxygen evolution catalyst material comprises iridium oxide (IrOx). It will be understood by the skilled person that iridium oxide (IrOx) may comprise iridium (IV) oxide (I rC>2), iridium oxyhydroxides, or a mixture thereof. Iridium oxyhydroxides are iridium compounds having both oxo (lr=O) and hydroxo (Ir-OH) functionalities and may have a composition which can be represented, for example, by the following formula: lrOa(OH)b wherein 1 < a < 2 and 0 < b < 2, and 3 < 2a+b < 4. The presence of an iridium oxyhydroxide may be determined by methods known to those skilled in the art, such as infra-red spectroscopy or NMR spectroscopy following a hydrogen-deuterium exchange. The iridium oxide (IrOx) may be hydrated.

[0034] It may be preferred that the iridium oxide (IrOx) in the oxygen evolution catalyst material is substantially amorphous. In such cases the XRD pattern is essentially free of well-defined diffraction peaks associated with crystalline iridium oxide. It may be preferred that the XRD pattern of the oxygen evolution catalyst material does not have any reflections associated with the crystalline iridium (IV) oxide (rutile) phase, for example reflections associated with crystalline iridium (IV) oxide at 20 = 28° and 54°. It may be preferred that the iridium oxide (IrOx) material comprises or consists of iridium oxyhydroxide.

[0035] The oxygen evolution catalyst material also comprises a potassium iridate crystalline phase. Such a crystalline phase may be identified by XRD of the catalyst material which includes a reflection at 20 = 13 °, such as a reflection in the 20 range of and including 12.6 to 12.8 °. Such materials have been shown to have high oxygen evolution catalytic activity and may be obtained using the methods described herein. The reflection at 13 ° is distinct from and distanced from peaks associated with crystalline iridium oxide. The XRD pattern may include a refection at 20 = 13 ° and a reflection at 20 = 25 °, such as a reflection in the 20 range 12.6 to 12.8 ° and a reflection in the 20 range 25.4 to 25.6 °. The potassium iridate may be hydrated. The potassium iridate crystalline phase may have the space group Im 3. The potassium iridate crystalline phase may comprise, or be isostructural with, KlrCh. Suitably, the oxygen evolution catalyst material does not comprise potassium iridate with a hollandite crystalline structure.

[0036] It may be preferred that the oxygen evolution catalyst material has a surface which is predominantly iridium in oxidation state (IV). It may be preferred that iridium (III) is substantially absent from the surface of the oxygen evolution catalyst material as assessed using cyclic voltammetry (CV). In such cases no iridium (III) to iridium (IV) transition is observed by CV. CV is suitably carried out using a potentiostat, for example using a glass three electrode electrochemical cell and a catalyst loading of 0.2 mg / cm-2.

[0037] Typically, the potassium content of the oxygen evolution catalyst material is in the range of and including 1 to 20 wt %. The potassium content of the oxygen evolution catalyst material may be determined by inductively coupled plasma mass spectrometry (ICP-MS). It may be preferred that the potassium content of the oxygen evolution catalyst material is in the range of and including 5 to 15 wt%.

[0038] It may be preferred that the oxygen evolution catalyst material is provided on a support. Suitable support materials are known to the person skilled in the art. For example, the support material may be an oxide of a transition metal, such as a titanium, zirconium, niobium or cerium, and oxide of a main group metal, such as tin or aluminium, a carbon material, or a mixture of two or more thereof.

[0039] The oxygen evolution catalyst material may be formulated as an ink, typically by dissolving or dispersing the oxygen evolution catalyst material in a mixture of an ion-conducting polymer and water, or a mixture of ion-conducting polymer, water and an organic solvent, such as ethanol or propan-1 -ol. Suitable ion-conducting polymers are known to those skilled in the art and include sulphonic acid-containing polymers, such as a fully fluorinated sulphonic acid polymer, or a partially- or non-fluorinated hydrocarbon sulphonic acid polymer.

[0040] The inks may be used to form a catalyst layer. Such layers suitably comprise the oxygen evolution catalyst material and an ion-conducting polymer. The catalyst layers may also comprise additional components, such as additional catalysts, radical scavengers, etc., as will be known to those skilled in the art.

[0041] The catalyst layer may form a component of a catalyst coated membrane (CCM) which comprises a membrane with the catalyst layer on a first face thereof and, optionally, a second catalyst layer on a second face thereof. Suitably, the membrane is a proton exchange membrane (PEM) or an anion exchange membrane (AEM). The membrane in the CCM may include additional components (e.g. recombination catalysts, radical scavengers, reinforcements, multiple layers) as will be known to those skilled in the art.

[0042] Typically, the catalyst layer is the anode layer of a CCM for a water electrolyser. In such cases the CCM typically comprises: (i) an ion-conducting membrane with a first face and a second face; (ii) an anode layer comprising the oxygen evolution catalyst material and an ion-conducting polymer which is applied to the first face of the membrane; and (iii) a cathode catalyst layer comprising a hydrogen evolution reaction (HER) catalyst, such as a HER catalyst comprising platinum (for example platinum on carbon), which is applied to the second face of the membrane. The oxygen evolution catalyst material may also be used in a fuel cell, especially in a fuel cell anode for the purposes of cell reversal tolerance. For a discussion of the use of I rC>2 materials in fuel cells for the purpose of cell reversal tolerance see WO2012 / 107738 (Johnson Matthey PLC).

[0043] The present invention also provides a method of manufacturing an oxygen evolution catalyst material comprising the step of the heat treatment of a mixture comprising iridium oxide (IrOx) and a potassium salt.

[0044] The process involves heat treatment of iridium oxide (IrOx). Suitably, the iridium oxide is in the form of a solid, such as a particulate solid. Preferably, the iridium oxide (IrOx) is amorphous. The use of amorphous iridium oxide provides materials with higher OER activity after the method as set out herein. By ‘amorphous’ it is meant herein that the XRD pattern of the iridium oxide is substantially free of well-defined diffraction peaks. It may be preferred that the iridium oxide (IrOx) starting material comprises, or consists of, amorphous iridium oxyhydroxide.

[0045] The iridium oxide (IrOx) is mixed with a potassium salt. Preferably, the potassium salt is an inorganic potassium salt, such as potassium hydroxide or potassium carbonate. It may be further preferred that the potassium salt is potassium hydroxide.

[0046] Suitably, the mixture comprises the potassium salt in an amount such that the molar ratio of potassium to iridium is at least 1 :1 , such as at least 2:1. It may be preferred that the mixture comprises the potassium salt in an amount such that the molar ratio of potassium to iridium is in the range of and including 1 : 1 to 20:1.

[0047] Typically, the potassium salt and the iridium oxide (IrOx) are dry mixed. By ‘dry mixing’ it is meant herein that the components are mixed in the absence of an added liquid, such as water. It still be understood by the skilled person that this does not preclude the presence of minor amounts water within the starting materials themselves, for example the use of a hydrate. Dry mixing may be carried out using techniques known to those skilled in the art, for example by using a pestle and mortar, shaking or by milling.

[0048] The potassium salt and the iridium oxide (IrOx) may also be mixed in the presence of a liquid, such as water. For example, an aqueous solution of potassium salt, such as potassium hydroxide, may be added to the iridium oxide (IrOx), and the components then mixed, for example by, stirring shaking or by milling. Suitably, the mixture is then dried prior to heat treatment.

[0049] The mixture is then heat treated. Suitably the heat treatment is carried out at a temperature of at least 400 °C. It may be preferred that the heat treatment is carried out at a temperature of at least 450 °C, 475°C, 500 °C, 525 °C, or 550 °C. Suitably, the heat treatment is carried out at a temperature less than or equal to 800 °C. It may be preferred that the heat treatment is carried out at a temperature less than or equal to 700 °C, or less than or equal to 650 °C. Typically, the heat treatment is carried out at a temperature in the range of and including 400 to 800 °C, such as 400 to 700 °C, 500 to 700 °C, or 550 to 650 °C.

[0050] Suitably the heat treatment is carried out for a period in the range of and including 30 mins to 4 hours. Longer heat treatment times are not thought to provide any benefit and have the downside of higher energy costs. It may be preferred that the heat treatment is carried out at a temperature in the range of and including 400 to 800 °C for a period in the range of and including 30 mins to 4 hours. It may be further preferred that the heat treatment is carried out at a temperature in the range of and including 400 to 700 °C for a period in the range of and including 30 mins to 4 hours.

[0051] Typically the heat treatment is carried out in an oxygen-containing atmosphere, such as air, mixtures of nitrogen and oxygen, or oxygen.

[0052] Optionally, following heat treatment, the oxygen evolution catalyst material is washed. Suitably, the oxygen evolution catalyst material is sequentially washed with water (such as deionised water) and an aqueous acid (such as nitric acid). The use of an aqueous acid for washing reduces the undesirable dissolution of iridium from the oxygen evolution catalyst during the washing step. The material is then typically dried, for example by heating to a temperature of 105 °C for a period of 2 to 6 hours.

[0053] The product may be ground to remove large agglomerates. For example, using a pestle and mortar or by milling.

[0054] The present invention will now be described with reference to the following examples, which are provided to assist with understanding the present invention and are not intended to limit its scope.

[0055] Examples Analysis by XRD

[0056] XRD analysis was carried out using a Bruker D8 Advance Davinci diffractometer with a Lynxeye-XE PSD detector with 0.0125° Ni Filter. The analysis was carried out an ambient temperature using Cu Ka (A = 1.5406 + 1.54439 A) radiation. Phase identification was carried out using Bruker AXS Diffrac Eva V5 (2010-2018) software and the PDF-4+ database (2020 release).

[0057] Assessment of PER catalytic activity

[0058] The iridium oxide catalysts formed were tested for PER catalytic activity using a potentiostat measuring an electrode with a 0.2 mg / cm-2catalyst loading (prepared using a catalyst ink made with an acidic Nation binder, spray coated onto carbon) as part of a glass three electrode electrochemical cell. The electrochemical activity was measured using an initial activity test (cyclic voltammetry (CV): 300, 300, 100, 50 and 10 mV s’1; linear sweep voltammetry (LSV): 1-1.55 V; electrochemical impedance spectroscopy (EIS): 1.45 V), a degradation cycling test (0.6-1.35V x 1000 cycles (4h 10 min), and a final activity test (CV: 300, 100, 50 and 10 mV s’1; LSV: 1-1.55 V; EIS: 1.45 V).

[0059] Comparative Example 1 (CE1) Preparation of an iridium oxide (IrPx) material via hydrolysis of dihydrogen hexachloroiridate (IV)

[0060] An aqueous solution of ^IrCle (22.1g) was adjusted with de-ionised water to a volume of 200mL. The pH was adjusted to pH 13 through the addition of 5M NaOH. The mixture was heated at 90 °C for seven hours whilst maintaining the pH of the mixture at 13. The mixture was allowed to cool to ambient temperature. The pH was adjusted to 3 with nitric acid and the mixture stirred at ambient temperature for 1 h. The mixture was filtered and the filtered solid washed with demineralised water until the conductivity of the water washings was measured by a conductivity meter to be less than 20 microcoulombs. The material was dried at 105 °C in a drying furnace to yield an iridium oxide material.

[0061] XRD analysis of the formed iridium oxide material showed it to be amorphous.

[0062] Example 1 : Formation of an oxygen evolution catalyst material with a potassium iridate phase

[0063] 2.5 g of KPH was dissolved in 5 ml water. 2.5 g of the lrPxmaterial formed in CE1 was added and mixed together. The mixture was heated in a static furnace in an air atmosphere to 600 °C at a ramp rate of 5 °C / min and then held at 600 °C for 1 hour and then allowed to cool. The mixture was filtered and the filtered solid washed until the conductivity of the water washings was measured by a conductivity meter to be less than 20 microcoulombs)

[0064] The solid was dried overnight at 105 °C.

[0065] The formed material was analysed by XRD (Figure 1). This showed a substantially amorphous iridium oxide material and potassium iridate crystalline phase which has a reflection at 2 theta 12.7 ° and 25.6°.

[0066] The results of OER testing are shown in Figure 2. This shows that the material formed in Example 1 has a significantly higher OER activity that the iridium oxide starting material (CE1).

[0067] Example 2 - Formation of an oxygen evolution catalyst material with a potassium iridate phase from a crystalline iridium oxide

[0068] A sample of a commercially available amorphous iridium oxide (J&J Materials) was heated to 600 °C for 2 hours to form a crystalline iridium oxide precursor. The formed crystalline iridium oxide precursor was then heat treated with KOH using the method described in Example 1.

[0069] The material formed in Example 2 was analysed by XRD (Figure 3). This showed a crystalline iridium oxide rutile phase and a crystalline potassium iridate phase which has reflections at 2 theta 12.7 ° and 25.6°.

[0070] The results of OER testing of the crystalline precursor and the material formed after heat treatment with KOH are shown in Figure 4. The results show that the material formed in Example 2 has a significantly higher OER activity that the iridium oxide starting material.

[0071] Example 3: Formation of an oxygen evolution catalyst material with a potassium iridate phase from an amorphous iridium oxide

[0072] A sample of a commercially available amorphous iridium oxide material (J&J Materials) was heat treated with KOH using the method described in Example 1. The formed material was analysed by XRD (Figure 5). This showed an amorphous iridium oxide and a crystalline potassium iridate phase which has reflections at 2 theta 12.7 ° and 25.5.

[0073] The results of OER testing are shown in Figure 6. The results show that the material formed in Example 3 has a significantly higher OER activity that the amorphous iridium oxide starting material.

[0074] Example 4 - Formation of an oxygen evolution catalyst material with a potassium iridate phase from iridium oxide with a 2-hour heat treatment

[0075] A sample of amorphous iridium oxide was heat treated with KOH using the method described in Example 1 but with a heat treatment at 600 °C for 2 hours.

[0076] The formed material was analysed by XRD (Figure 7). This showed a crystalline iridium oxide rutile phase and crystalline potassium iridate phase which has reflections at 2 theta 12.65 and 25.4 °.

Claims

Claims1 . An oxygen evolution catalyst material, the catalyst material comprising:(i) iridium oxide (IrOx); and(ii) a potassium iridate crystalline phase; and wherein the potassium iridate crystalline phase provides a reflection in the x-ray diffraction (XRD) pattern of the catalyst material at 20 = 13 °.

2. A catalyst material according to claim 1 , wherein iridium (III) is substantially absent from the surface of the catalyst material as assessed by cyclic voltammetry.

3. A catalyst material according to any one of the preceding claims, wherein the XRD pattern of the catalyst material does not have any reflections associated with the lr(IV)C>2 (rutile) phase.

4. A catalyst material according to any one of the preceding claims, wherein the catalyst material comprises iridium oxyhydroxide.

5. A catalyst material according to any one of the preceding claims, wherein the potassium content is in the range of and including 1 to 20 wt%.

6. An ink comprising an oxygen evolution catalyst material according to any one of claims 1 to 5 and an ion-conducting polymer.

7. A catalyst layer comprising an oxygen evolution catalyst material according to any one of claims 1 to 5 and an ion-conducting polymer.

8. A catalyst-coated membrane comprising an oxygen evolution catalyst material according to any one of claims 1 to 5, or a catalyst layer according to claim 7.

9. A water electrolyser or a fuel cell comprising a catalyst-coated membrane according to claim 8.

10. Use of a catalyst material comprising a potassium iridate crystalline phase as an OER catalyst, such as in a water electrolyser, the crystalline phase providing a reflection in the x-ray diffraction (XRD) pattern of the catalyst material at 20 = 13.11 . A method of manufacturing an oxygen evolution catalyst material according to any one of claims 1 to 5, the method comprising the step of heat treatment of a mixture comprising iridium oxide (IrOx) and a potassium salt.

12. A method according to claim 11 , wherein the potassium salt is potassium hydroxide.

13. A method according to claim 11 or claim 12, wherein the iridium oxide starting material is amorphous.

14. A method according to any one of claims 11 to 13, wherein the heat treatment is carried out at a temperature in the range of and including 500 to 700 °C.

15. A method according to any one of claims 11 to 14, wherein the heat treatment is carried out for a period in the range of and including 30 mins to 4 hours.

16. A method according to any one of claims 11 to 15, wherein the mixture comprises the potassium salt in an amount such that the molar ratio of potassium to iridium is is in the range of and including 1 :1 to 20: 1.

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