Copper catalysts for the electrochemical conversion of carbon dioxide or carbon monoxide to c2+ products

Abstract

Copper catalysts for the electrochemical conversion of carbon dioxide or carbon monoxide to C2+ products According to the invention there is provided a catalyst for the electrochemical conversion of carbon dioxide or carbon monoxide to a C2+ product. The catalyst comprises copper and a metal (M); wherein the molar ratio of Cu : M in the catalyst is from 100 : 0.1 to 100 : 10. The catalyst comprises a core and a modified surface layer, wherein the modified surface layer comprises a higher concentration of metal (M) than the core. The catalyst has particular use in a gas diffusion electrode, and a catalyst coated membrane of an electrolyser. [Figure 1a]

Description

[0001] Copper catalysts for the electrochemical conversion of carbon dioxide or carbon monoxide to C2+ products

[0002] Field of the Invention

[0003] The present invention relates to copper catalysts for the electrochemical conversion of carbon dioxide or carbon monoxide to C2+ products such as ethylene.

[0004] Declaration of funding

[0005] The project leading to this application has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 101006701.

[0006] Background

[0007] Energy storage is one of the greatest hurdles for the complete adoption of renewable electricity. One approach to sustainable fuels is to convert CO2 directly into C2+ products such as ethylene which in turn can be converted into fuels e.g. by thermocatalytic reactions. This approach is complementary to established methods producing fuels from synthesis gas (e.g. Fisher Tropsch synthesis) and may involve fewer process steps.

[0008] Direct CO2 conversion may be carried out in an electrochemical reactor called an electrolyser which uses electricity to drive chemical reactions by supplying electrons to the substrate directly, avoiding the need for oxidising or reducing agents. An electrolyser could use surplus electricity from intermittent renewable sources to convert CO2 into fuels and chemicals, thereby storing the renewable energy as chemical energy in fuel or chemical molecules. A simplified eguation for the half reactions occurring in direct CC to ethylene is shown below:

[0009] Cathode reaction: 2CO2 + 8H2O + 12 e- — > C2H4 + 12OH' Anode reaction: 12OH' — > 6H2O + 3O2 + 12e_Overall: 2H2O + 2CO2C2H4+ 3O2

[0010] A reaction which competes with the desired C2+ generation reaction is the hydrogen evolution reaction (HER): Hydrogen evolution reaction: 2H2O + 2 e- — > H2 + 2OH'

[0011] The cathode reaction is sometimes called the CO2 reduction reaction (CO2RR). Ideally a CO2RR catalyst needs to satisfy one or more of the following: (1) have a high selectively for the desired fuel or chemical (sometimes measured as Faradic efficiency (“FE”); (2) have low background activity for the competing hydrogen evolution reaction.

[0012] A variety of metals can be used as the CO2RR catalyst and the subject has been recently reviewed in the paper “A Comparison of Different Approaches to the Conversion of Carbon Dioxide into Useful Products: Part I” (Johnson Matthey Technol. Rev. 2021 , 65, (2), ISO- 196).

[0013] It is known that the choice of metal catalyst influences the mechanism of CO2 reduction and therefore the product(s) formed. Copper is an interesting metal for CO2RR because it offers a good balance between overpotential and strength of CO adsorption. Essentially, it allows the intermediate CO formed during the CO2RR to remain loosely adsorbed and mobile, meaning it is able to undergo C-C coupling reactions.

[0014] The article “Highly selective plasma-activated copper catalysts for carbon dioxide reduction to ethylene” (Nature Communications 7, 12123 (2016)) describes the preparation of plasma-activated Cu by treating polycrystalline Cu foils with 02 and H2 plasmas of varying power and duration.

[0015] The article “Subsurface Oxygen in Oxide-Derived Copper Electrocatalysts for Carbon Dioxide Reduction” (J. Phys. Chem. Lett. 2017, 8, 285-290) describes the treatment of a polycrystalline copper foil to electrochemical oxidation-reduction cycles which increased the overall CO2RR activity of the catalyst and improved the product yield toward more ethylene versus methane.

[0016] Oxide-derived copper catalysts have been shown to demonstrate higher activity and selectivity towards C2+ compounds compared to Cu metal. However, copper oxides are easily reduced to copper metal under the highly reducing conditions of CO2 reduction. To this end, efforts have been made to provide modified catalysts in which the copper is stabilised in a positive oxidation state by incorporating other metals into the structure. These are sometimes referred to as Cu5+materials. A doped or alloyed material has two advantages: first, the catalytic properties of another element may be utilised in conjunction with Cu and second, the doped material may have unique properties distinct from the elements of this it is composed.

[0017] The article “Turning the Selectivity of Carbon Dioxide Electroreduction toward Ethanol on Oxide-Derived CuxZn Catalysts” ACS Catal. 2016, 6, 8239-8247 describes an improvement in the FE towards Cn>2 products (C2+ products) by introducing Zn into the structure. Catalysts with stoichiometry Cu Zn, Cu4Zn and Cu2Zn were prepared from the corresponding bimetallic CuxZn oxides which were prepared by electrodeposition. These catalysts preferentially produced ethanol instead of ethylene.

[0018] There is a need for alternative copper-based electrocatalysts which can convert CO2 into C2+ products with high selectivity and which are simple to manufacture. The present invention addresses this need.

[0019] Description of the Figures

[0020] Figures 1a, 1 b and 1c show possible arrangements for an electrolyser according to the invention; and

[0021] Figure 2 show a chart of Faradaic Efficiency of each reaction product formed during electrochemical CO2 reduction for catalysts E4 and CE1.

[0022] Summary of Invention

[0023] It is known that during the initial operation of an electrolyser the pre-catalyst is converted into a reduced catalyst, i.e. by conversion of Cu(ll) or Cu(l) into Cu(0). The present inventors have now found that modifying the surface of the copper oxide pre-catalyst by including certain metals (M) improves the conversion efficiency of CO2 to ethylene. The following theory, which has been constructed in hindsight, explains why doping copper oxide with these particular metals improve the selectivity of the reduced catalyst towards ethylene formation.

[0024] A key step in the conversion of CO2 to ethylene is the coordination of -CO onto the catalyst surface followed by dimerization. A catalyst which is selective for ethylene production should therefore have the ability to bind CO, but not so strongly as to prevent dimerization. The standard reduction potentials of various metals M are reported in Table 1. Some of these metals have a standard reduction potential which is more negative than that of Cu(ll) and as a result it is thought that some or all of those modifying metals M remains in a positive oxidation state even after the majority of copper has been reduced to Cu(0).

[0025] Metal (M) Standard reduction potential

[0026] (V) vs SHE for the reaction Mn++ n e- M°

[0027] Ba -2.912

[0028] Y -2.372

[0029] La -2.38

[0030] Ce -2.336

[0031] Pr -2.353

[0032] Nd -2.323

[0033] Gd -2.279

[0034] Zr -1.45

[0035] Zn -0.7618

[0036] Ga -0.53

[0037] In -0.34

[0038] Ni -0.257

[0039] Pd +0.915

[0040] Pt +1.188

[0041] Table 1. While some of these metals have a lower standard reduction potential than Cu (Cu2++ 2 e- — > Cu° +0.337 V), not all metals having a lower standard reduction potential than Cu can be readily manufactured using wet chemical methods. For instance, Ga also has a lower standard reduction potential than Cu, but was not able to be co-precipitated with the CuO using wet chemical methods, as is explained in more detail in the comparative examples.

[0042] The presence of specific M ion modifiers within the Cu(0) provides a catalyst which binds CO more strongly than does Cu(0) alone, but not too strongly to prevent dimerization. It is not yet known whether the M ion coordinates CO directly, or whether CO is coordinated by Cu(l) which is stabilised in the surface modified catalyst.

[0043] Moreover, some metals having a higher standard reduction potential than Cu, such as Pd and Pt, can also be effective. In such cases, the surface modifying metal M may be present as M(0) (i.e. a metal in an oxidation state of 0), or as a metal M ion modifier. Without wishing to be bound by theory or conjecture, it is believed that the presence of Pt and Pd surface modifiers within the Cu(0) provides a catalyst which has a lower activation energy barrier for the dimerization process than Cu(0) alone, which is favourable for forming C2+ products.

[0044] In a first aspect the invention provides a catalyst for the electrochemical conversion of carbon dioxide or carbon monoxide to a C2+ product; wherein the molar ratio of Cu : M in the catalyst is from 100 : 0.1 to 100 : 10; and wherein the catalyst comprises a core and a modified surface layer, wherein concentration of metal (M) is higher in the surface layer than in the core.

[0045] As used herein, unless context requires otherwise, the term “catalyst” can refer to a precatalyst containing Cu(ll) and M ions, or may refer to the reduced catalyst produced following reduction of the Cu(ll) to Cu(l) and / or Cu(0). The Cu : M ratio is not changed when converting the pre-catalyst into the reduced catalyst.

[0046] As used herein, the term “modified” simply means that the catalyst contains M in addition to Cu. The term “surface modified” is used herein to mean that the catalyst contains M in a surface layer at a higher concentration than in a core (bulk) of the catalyst.

[0047] In a second aspect the invention relates to the use of a catalyst according to the first aspect of the invention for the electrochemical conversion of carbon dioxide (CO2) or carbon monoxide (CO) to C2+ products. As such, there is provided a method of electrochemically converting carbon dioxide or carbon monoxide to C2+ products using a catalyst according to the first aspect.

[0048] In a further aspect there is provided a method of manufacturing a pre-catalyst comprising the steps of:

[0049] (i) providing a mixture comprising particles of a copper compound and particles of a surface modifier precursor, the surface modifier precursor comprising a metal M or metal M compound;

[0050] (ii) mixing or milling the mixture so that the particles of the copper compound and the surface modifier precursor interact to form a mixed powder in which the surface modifier precursor decorates a surface layer of the particles of the copper compound; and

[0051] (iii) optionally, calcining the mixed powder; wherein the molar ratio of Cu : M in the pre-catalyst is from 100 : 0.1 to 100 : 10; and wherein the pre-catalyst comprises a core and a modified surface layer, wherein concentration of metal (M) is higher in the surface layer than in the core

[0052] In a third aspect the invention relates to a method of manufacturing a pre-catalyst comprising the steps of:

[0053] (i) providing a mixture comprising particles of a copper (II) compound and particles of a surface modifier precursor, the surface modifier precursor comprising a metal M or metal M compound;

[0054] (ii) mixing or milling the mixture so that the particles of the copper (II) compound and the surface modifier precursor interact mechanochemically to form a mechanochemically-mixed powder; and

[0055] (iii) optionally, calcining the mechanochemically-mixed powder; wherein the molar ratio of Cu : M in the pre-catalyst is from 100 : 0.1 to 100 : 10; and wherein the pre-catalyst comprises a core and a modified surface layer, wherein concentration of metal (M) is higher in the surface layer than in the core.

[0056] As used herein, the term “C2+ products” means a product comprising at least two carbon atoms. The catalysts are particularly suitable for the conversion of CO2 to ethylene.

[0057] As used herein, the term “mechanochemically” refers to a mechanical initiation of a chemical process. For example, the surface modifier precursor may mechanochemically interact with the copper (II) compound to form the modified surface layer. The modified surface layer suitably comprises particles of a surface modifier, which may be formed as a result of step (ii) or as a result of the optional calcination step (iii).

[0058] In a fourth aspect the invention relates to an ink comprising a polymer and a pre-catalyst dispersed in a solvent or solvent mixture.

[0059] In a fifth aspect the invention relates to a gas diffusion electrode comprising a gas diffusion layer and a catalyst layer on the gas diffusion layer, wherein the catalyst layer comprises a catalyst as defined in the first aspect.

[0060] In a sixth aspect the invention relates to a catalyst coated membrane comprising a membrane having an anode side and a cathode side, wherein a catalyst as defined herein is present at the cathode side.

[0061] In a seventh aspect the invention relates to a CO2 electrolyser comprising a gas diffusion electrode according to the fifth aspect or a catalyst coated membrane according to the sixth aspect.

[0062] In an eighth aspect the invention relates to a method for converting CO2 into C2+ products, comprising the step of providing a feed stream comprising CO2 to the cathode of a CO2 electrolyser according to the seventh aspect.

[0063] Detailed Description

[0064] Any sub-headings are included for convenience only, and are not to be construed as limiting the disclosure in any way.

[0065] Catalyst

[0066] The catalyst is a CO2 reduction reaction electrocatalyst. The catalyst is suitably in the form of particles. The catalyst comprises copper and a metal (M). The metal (M) is suitably a lanthanide, a transition metal other than copper, or a Group 13 element. Preferably, the metal can be a lanthanide or yttrium. The metal (M) is preferably selected from the group consisting of: yttrium (Y), lanthanum (La) and gadolinium (Gd); cerium (Ce), praseodymium (Pr), neodymium (Nd), zirconium (Zr), barium (Ba), indium (In), nickel (Ni), zinc (Zn), palladium (Pd) and platinum (Pt). More preferably, the metal (M) is selected from the group consisting of: yttrium (Y), lanthanum (La) and gadolinium (Gd); cerium (Ce), praseodymium (Pr), neodymium (Nd), zirconium (Zr), barium (Ba), indium (In), zinc (Zn), palladium (Pd). More preferably, the metal (M) is selected from the group consisting of: yttrium, lanthanum, gadolinium, cerium, praseodymium, neodymium, and palladium. More preferably still, the metal (M) is selected from the group consisting of: yttrium, lanthanum, gadolinium and cerium. In a preferred embodiment, the metal (M) is yttrium. In another preferred embodiment, the metal (M) is gadolinium.

[0067] The catalyst produced by the method of the invention is referred to herein as a pre-catalyst and suitably comprises a mixture of copper oxide (e.g. copper (II) oxide) and a metal M or metal M compound, such as a metal M oxide. Preferably, the pre-catalyst comprises a mixture of copper (II) oxide and a metal M oxide. The mixture comprises a copper (II) oxide phase and a metal M oxide phase, as distinct phases. This is in contrast to a “mixed metal oxide” which may be characterised by a single crystal structure comprising copper, the metal M, and oxygen in a lattice or as a solid solution. That is, the copper oxide and the metal M compound (e.g. metal M oxide) are suitably present as separate phases. For example, the copper oxide and the metal M oxide are preferably present as separate phases. The copper oxide and the metal M compound can have different crystal structures. Preferably, the copper oxide and the metal M compound (e.g. metal M oxide) in the pre- catalyst are not present as a mixed metal oxide (or solid solution) having a single crystal structure.

[0068] The pre-catalyst is converted by reduction to a reduced catalyst (e.g. during initial operation of the electrolyser) in which the majority of the copper (II) oxide is converted to copper (0). By majority, we mean that >50 at% of the copper is present as copper (0), typically >80 at%, such as >90 at%. Without wishing to be bound by any theory or conjecture, it is thought that for metals M having a more negative reduction potential than the cathode potential during operation of the electrolyser, some or all of the metal M remains in a positive oxidation state , whereas for metals M having a less negative reduction potential than the cathode potential during operation of the electrolyser, some or all of the metal M is converted to M(0).

[0069] The following preferred embodiments apply to both the pre-catalyst and the reduced catalyst. In a preferred embodiment M is yttrium, lanthanum, gadolinium, cerium, praseodymium, neodymium. In other preferred embodiment, M is yttrium, lanthanum or gadolinium. In further preferred embodiment M is palladium or platinum.

[0070] The molar ratio of Cu : M in the catalyst is from 100 : 0.1 to 100 : 10 (i.e. 0.1 to 10 atom% M relative to Cu). A typical range is 100 : 1 to 10 : 1 , 100 : 2 to 100 : 8, such as 100 : 3 to 100 : 7, with 100 : 5 being typical.

[0071] The molar ratio of Cu : M in the modified surface layer is suitably less than 100 : 10 (i.e. at least 10 atom% M relative to Cu), and preferably less than 100 : 20. The molar ratio of Cu : M in the modified surface layer is preferably at least 100 : 100 (i.e. less than 100 atom% M relative to Cu), and preferably at least 100 : 80. The molar ratio of Cu : M in the modified surface layer can be a range comprising any combination of aforementioned upper and lower limits. The molar ratio of Cu : M in the modified surface layer can be determined using X-ray photoelectron spectroscopy (XPS).

[0072] The modified surface layer can have a thickness of 20 nm or less, preferably 10 nm or less, and more preferably 8 nm or less.

[0073] It is preferred that the content of metals other than Cu and M is <10 at.%, preferably < 5 at.%, preferably < 2 at.% or < 1 at.%. As an example, a catalyst containing the metals Cu, La and Al (where M = La) at a molar ratio of 94 : 5 : 1 has a content of metals other than Cu and M of 1 at.%.

[0074] Manufacture of the catalyst

[0075] The pre-catalysts described herein can be produced by a method comprising the steps of:

[0076] (i) providing a mixture comprising particles of a copper compound (e.g. a copper (II) compound) and particles of a surface modifier precursor, the surface modifier precursor comprising a metal M or metal M compound;

[0077] (ii) mixing or milling the mixture so that the particles of the copper compound and the surface modifier precursor interact (e.g. mechanochemically) to form a mixed powder (e.g. a mechanochemically-mixed powder); and

[0078] (iii) optionally, calcining the mixed powder. wherein the molar ratio of Cu : M in the pre-catalyst is from 100 : 0.1 to 100 : 10; and wherein the pre-catalyst comprises a core and a modified surface layer, wherein the modified surface layer comprises a higher concentration of metal (M) than the core.

[0079] In step (ii), the surface modifier precursor and the copper compound can suitably interact to form the modified surface layer. In some embodiments, step (ii) can mechanochemically convert the surface modifier precursor (e.g. a nitrate, acetate) to a surface modifier (e.g. an oxide). In some embodiments, the optional calcination step (step (iii)) can convert the surface modifier precursor to the surface modifier, and / or the copper compound to a copper oxide.

[0080] The pre-catalysts described herein can be produced by a mechanochemical process, such as resonant acoustic mixing, planetary ball milling or other high-energy milling process. Preferably the pre-catalysts described herein are produced using a resonant acoustic mixing process. The method can comprise the steps of:

[0081] (i) providing a mixture comprising particles of a copper (II) compound and particles of a surface modifier precursor, the surface modifier precursor comprising a metal (M) or metal (M) compound;

[0082] (ii) mixing or milling the mixture so that the particles of the copper (II) compound and the surface modifier precursor interact mechanochemically to form a mechanochemically-mixed powder; and

[0083] (iii) optionally, calcining the mechanochemically-mixed powder; wherein the metal (M) is suitably selected from the group consisting of: yttrium (Y), lanthanum (La) and gadolinium (Gd); cerium (Ce), praseodymium (Pr), neodymium (Nd), zirconium (Zr), barium (Ba), indium (In), nickel (Ni), zinc (Zn), palladium (Pd) and platinum (Pt); wherein the molar ratio of Cu : M in the pre-catalyst is from 100 : 0.1 to 100 : 10; and wherein the pre-catalyst comprises a core and a modified surface layer, wherein the modified surface layer comprises a higher concentration of metal (M) than the core.

[0084] The copper compound is preferably a copper (II) compound. The copper (II) compound can be copper (II) oxide or preferably can be a copper (II) compound that is thermally decomposable to form copper (II) oxide, such as copper (II) carbonate. Preferably, the copper (II) compound is copper (II) oxide. Preferably, the copper (II) oxide provided in step (i) is crystalline. For example, the copper (II) oxide provided in step (i) can have been subject to a prior calcination treatment.

[0085] The surface modifier precursor is preferably a metal M nitrate, metal M acetate, metal M oxide, metal M hydroxide, metal M oxyhydroxide, or metal M carbonate. More preferably, the surface modifier precursor is a nitrate, acetate or oxide of the metal M. For example, the metal M compound can be a nitrate, acetate, oxide, hydroxide, oxyhydroxide or carbonate of a lanthanide, transition metal (other than copper) or a Group 13 element. Preferably, the metal M compound can be a nitrate, acetate, oxide, hydroxide, oxyhydroxide or carbonate of yttrium or a lanthanide. Preferably, the metal M compound can be a nitrate, acetate or oxide of: yttrium (Y), lanthanum (La) and gadolinium (Gd); cerium (Ce), praseodymium (Pr), neodymium (Nd), zirconium (Zr), barium (Ba), indium (In), nickel (Ni), zinc (Zn), palladium (Pd) or platinum (Pt). Preferably, the surface modifier precursor is a nitrate or acetate of yttrium, lanthanum, gadolinium, cerium, praseodymium, neodymium, zirconium, barium, indium, nickel, or zinc. More preferably, surface modifier precursor is a nitrate or acetate of yttrium, lanthanum, gadolinium, cerium, praseodymium, neodymium, zirconium or barium. Preferably, the surface modifier precursor is in a hydrated form. Typically, hydrated surface modifier precursors can more readily break-up during step (ii).

[0086] The surface modifier precursor can have a D50 particle size that is smaller than the D50 particle size of the copper (II) compound. In particular, when the surface modifier precursor comprises a metal M oxide, the D50 particle size of the metal M oxide can be smaller than the D50 particle size of the copper (II) oxide.

[0087] Preferably step (ii) is performed by resonant acoustic mixing (RAM).

[0088] Step (ii) is followed by an optional calcination step (iii). The role of step (iii) is to remove residual counter ions that were present in the mixture used in step (i), for example, to fully decompose the thermally decomposable copper (II) compound to form copper (II) oxide, and to convert the surface modifier precursor to the surface modifier, as required. Typically calcination is carried out at a temperature of at least 200 °C, preferably at least 250 °C, more preferably at least 300 °C. The temperature should be sufficiently high to remove the counter ions present. The calcination temperature is preferably does not exceed temperatures that would cause the copper (II) species to alloy or form a mixed phase with the surface modifier. For example, the calcination step is preferably carried out at a temperature of 600 °C or less, preferably 500 °C or less, preferably 450 °C or less, and preferably 400 °C or less. A calcination temperature in this range can provide a more even distribution of surface modifier in the surface layer and avoid aggregation of the surface modifier. The calcination temperature can be in a range comprising any combination of the aforementioned lower and upper limits. The calcination treatment can last for about 2 hours, but this may vary depending on the scale of material used.

[0089] Ink

[0090] The pre-catalyst may be formulated as an ink for application to a substrate. The substrate may be any substrate on which it is desirable to carry out CO2 electrolysis. Preferred substrates include: an ion exchange membrane (e.g. an ion exchange membrane such as Nation™, FumaSep, Pemion™, Aemion™, Sustainion™) or a gas diffusion layer (e.g. Freudenberg or Sigracet carbon paper or a porous PTFE sheet).

[0091] The ink comprises of a pre-catalyst (as defined above), a polymer, and a solvent or solvent mixture. The polymer is typically a binder or a filler. Suitable polymers will be known to those skilled in the art, and an exemplary polymer is Nation™.

[0092] Coated substrates

[0093] The person skilled in the art will be familiar with the design of a CO2 electrolyser. A typical CO2 electrolyser includes a gas diffusion electrode (GDE) and / or a catalyst coated membrane (CCM). Various arrangements of catalyst coated membranes are possible, all of which may benefit from using the catalysts defined herein on the cathode side.

[0094] In one aspect the invention relates to a catalyst coated membrane comprising a membrane having an anode side and a cathode side, wherein a pre-catalyst or a reduced catalyst as defined herein is present at the cathode side. As used herein, the term “catalyst coated membrane” refers to a membrane in which at least one of the faces of the membrane is coated with a catalyst. The term “anode side” refers to the side at which the anode reaction (e.g. OER) occurs. The term “cathode side” refers to the side at which the CO2RR occurs. Various arrangements are possible, and for the avoidance of doubt it is not required that both the anode and / or cathode are applied on the membrane; there may be a gap between the membrane and the anode, or between the membrane and the cathode.

[0095] In one embodiment the CCM is coated on the cathode side face with a cathode catalyst (cathode catalyst layer).

[0096] In one embodiment the CCM is coated on the anode side face with an anode catalyst (anode catalyst layer) and a gas diffusion electrode according to the sixth aspect is on the cathode side; this arrangement is shown in Figure 1 b.

[0097] In one embodiment the CCM is coated on the cathode side face with a cathode catalyst (cathode catalyst layer) and on the anode side face with an anode catalyst (anode catalyst layer); this arrangement is shown in Figure 1c.

[0098] Figures 1a, 1 b and 1c illustrate electrolysers containing a cation exchange membrane and using KHCO3 as the electrolyte. The skilled person will appreciate that other electrolytes may be used and the membrane does not have to be a cation exchange membrane.

[0099] The cathode catalyst layer and anode catalyst layer may be applied to the membrane by any techniques known to those skilled in the art, such as by using an ink or a decal.

[0100] In one aspect the invention relates to a gas diffusion electrode comprising a gas diffusion layer and a catalyst layer on the gas diffusion layer, wherein the catalyst layer comprises a pre-catalyst or a reduced catalyst as defined herein.

[0101] In one embodiment the catalyst layer on the gas diffusion layer comprises a polymer binder.

[0102] In one embodiment the gas diffusion electrode comprises a microporous layer on the catalyst layer.

[0103] In one aspect the invention relates to an electrolyser comprising a gas diffusion electrode as defined herein or a catalyst coated membrane as defined herein. In a first embodiment the electrolyser comprises a gas diffusion electrode, an ion exchange membrane and an anode catalyst layer. The gas diffusion electrode includes a catalyst layer comprising pre-catalyst or reduced catalyst as defined herein. The anode catalyst layer is separated from the ion exchange membrane by an electrode gap. An exemplary embodiment is shown in Figure 1a.

[0104] In a second embodiment the electrolyser comprises a CCM which comprises a gas diffusion electrode, an ion exchange membrane and an anode catalyst layer. The gas diffusion layer includes a catalyst layer comprising a pre-catalyst or reduced catalyst as defined herein. The anode catalyst layer is present on one side of the ion exchange membrane. An exemplary embodiment is shown in Figure 1b, in which a porous transport layer (PTL) contacts the anode catalyst layer.

[0105] In a third embodiment the electrolyser comprises a CCM which comprises an ion exchange membrane, an anode catalyst layer and a cathode catalyst layer. The anode catalyst layer is present on one side of the ion exchange membrane and the cathode catalyst layer is present on the other side. The cathode catalyst is pre-catalyst or reduced catalyst as defined herein. An exemplary embodiment is shown in Figure 1c.

[0106] It will be understood that the cathode catalyst may be present on the cathode side (gas diffusion layer on the cathode side or cathode catalyst layer on the ion exchange membrane) either as a pre-catalyst or a reduced catalyst. The pre-catalyst may be reduced to the reduced catalyst before operating the electrolyser for the first time, or may be reduced in situ during start up.

[0107] Use of the catalyst

[0108] The catalyst may be used for the direct electrochemical conversion of CC to C2+ products, such as ethylene. It is known that CO2 electroreduction involves the conversion of adsorbed CO2 to adsorbed CO and it is therefore expected that the catalysts could be used for the direct conversion of CO to C2+ products.

[0109] Examples General procedure for the production of surface modified copper oxides using mechanochemical process

[0110] A powder of a copper (II) compound was combined with a metal M compound powder to form a mixture. The mixture was added to a resonant acoustic mixing chamber together with YSZ 5 mm diameter milling beads. The mixture was subjected to resonant acoustic mixing (RAM) using a Resodyn™ Acoustic Mixer at 80 G for a total of 20 minutes to produce a RAM-mixed powder. Except for Example E3, the RAM-mixed powder was calcined at a temperature of 350 °C to 500 °C for about 2 hours.

[0111] Table 2:

[0112] General procedure for the production of modified copper oxides using wet chemistry (comparative) Cu(NO3)2-xH2O and the respective M(lll) nitrate were dissolved in deionised water in the desired Cu : M ratio for the catalyst. The metal concentration (Cu + M) was 15 g / L.. The solution was heated to 60 °C with stirring and then 1M NaOH solution was added dropwise until a stable pH of 9 was reached. The temperature was then raised to 70 °C and pH maintained with stirring overnight. The reaction mixture was cooled to room temperature and the solid precipitate was collected by vacuum filtration. The precipitate was washed with deionised water until the filtrate conductivity was < 20 pS. The precipitate was dried in vacuum and then dried in air at 105 °C in an oven overnight. The solid was ground and sieved to a powder with particle size < 500 pm. The quantities used are shown in Table 3. The as prepared powders were calcined at 350 °C for 2 hours. The same procedure was used to produce an unmodified precipitated CuO.

[0113] With the exception of the Ga-modified catalyst (CE2), the amount of modifier measured closely matched that expected. The Ga-modified catalyst (CE2) had a significantly lower amount of modifier than expected. It is thought that the low wt% of Ga incorporated into the catalyst is because the conditions of the co-precipitation were not harsh enough for the Ga to be incorporated into the CuO structure.

[0114] TEM analysis of catalysts E1-E6 before the calcination step showed a regular distribution of the surface modifier on the surface of the CuO, which is suggestive of a surface-doped structure. TEM analysis of catalysts E1-E6 after the calcination step did not show any noticeable change in structure or distribution of the surface modifier on the surface of the CuO particle. Without wishing to be bound by any theory or conjecture, it is believed that the surface modifier particles, which are typically softer and smaller than the copper (II) species, decorate the surface of the copper (II) species, thereby forming a surface layer having a higher concentration of metal M compared to the bulk material. TEM analysis of catalysts CE1 (Y-modified) and CE2 (Ga-modified) showed an even distribution of Y or Ga throughout the CuO, which is suggestive of a doped structure throughout the particle.

[0115] General procedure for ink formulation and gas diffusion electrode fabrication

[0116] 200 mg of (surface) modified copper oxide material (prepared using the methods above) was added to a 10 mL glass vial, followed by 333.3 mg of 12wt% aqueous Nation™ 1100 EW dispersion (20 wt.% with respect to the modified copper oxide material). 3400 mg of ethanol and 1100 mg of water were added to the vial and the mixture was sonicated for 1 hour to produce the catalyst ink. The ink was then spray coated onto a carbon gas diffusion layer (Freudenberg H23C8) to produce a gas diffusion electrode with a catalyst loading of 1 mg / cm2.

[0117] General procedure for electrochemical testing of modified copper oxides

[0118] Electrochemical CO2 reduction was performed using a MicroFlowCell electrochemical reactor (electrolyser) commercially available from ElectroCell Europe A / S, which had an arrangement as shown in Figure 1a. A gas diffusion electrode (made using the procedure described above) was used on the cathode side of the electrolyser for electrochemical CO2 reduction. The anode comprised an iridium mixed metal oxide (Ir-MMO) plate (commercially available from ElectroCell Europe A / S). The exposed electrode area was 10 cm2for both the cathode and the anode. Catholyte and anolyte chambers were filled with 120 mL and 500 mL of 1 M KHCO3, respectively. During electrochemical testing, catholyte and anolyte were flowed through the electrolyser at a flow rate of 50 mL / min and 100 mL / min respectively. The electrolyser was pre-activated by performing cyclic voltammetry (0 V to - 0.6 V vs. Ag / AgCI, 20 cycles, scan rate: 50 mV / s) whilst the electrolyser was simultaneously purged with CO2 at a flow rate of 20 mL / min.

[0119] Potentiostatic measurements were then performed at a potential of -2.25 V vs. Ag / AgCI held for 30 minutes. Gaseous products were directly analysed using gas chromatography (GC). Liquid products were collected at the end of each potentiostatic test and analysed using high-performance liquid chromatography (HPLC), equipped with a refractive index detector (RID), or proton-NMR spectroscopy. The Faradaic Efficiency (FE) for producing the reaction products was determined. Faradaic Efficiency for gaseous products (FEgas) was determined using Equation 1 : , . n x c x Qfi„„x F

[0120] FE9qas(%) = - X Vmx]total100% Equation 1 where C is the concentration of gaseous product as measured by gas chromatography (volproduct / voltotai product), n is the number of transferred electrons per mole, F is the Faraday constant (96485 C mol-1), Qfiowis the volumetric flow rate (mL min-1), Vmis the molar volume of gas (mL mol'1), and jtotai is the total current density (A cm'2). Faradaic Efficiency for liquid products (FEnquid) was determined using Equation 2:

[0121] FEliquid(%) =m x n x Fx 100% Equation 2

[0122] Qtotal where Qtotai is the total charge consumed during the potentiostatic measurement, m is the number of moles of liquid products formed as determined by HPLC analysis or NMR, n is the number of transferred electrons per mole, and F is the Faraday constant.

[0123] Results of electrochemical testing of modified copper oxides

[0124] Figure 2 shows the Faradaic Efficiency of different reaction products formed during electrochemical CO2 reduction for catalysts E4 and CE1 . Each catalyst tested was active in the electrochemical conversion of carbon dioxide to C2+ products, such as ethylene, ethanol, and 1 -propanol. E4 showed improved selectivity towards ethylene and ethanol production under the conditions tested compared to CE1. Furthermore, E4 exhibited a slight suppression of hydrogen generation compared to CE1 .

Claims

Claims1 . A catalyst for the electrochemical conversion of carbon dioxide or carbon monoxide to a C2+ product, wherein the catalyst comprises copper and a metal (M); wherein the molar ratio of Cu : M in the catalyst is from 100 : 0.1 to 100 : 10; and wherein the catalyst comprises a core and a modified surface layer, wherein the modified surface layer comprises a higher concentration of metal (M) than the core.

2. A catalyst as claimed in claim 1 , wherein M is selected from the group consisting of: yttrium (Y), lanthanum (La) and gadolinium (Gd); cerium (Ce), praseodymium (Pr), neodymium (Nd), zirconium (Zr), barium (Ba), indium (In), nickel (Ni), zinc (Zn), palladium (Pd) and platinum (Pt).

3. A catalyst as claimed in claim 1 or 2, wherein M is selected from the group consisting of: yttrium, lanthanum, gadolinium, cerium, praseodymium, neodymium.

4. A catalyst as claimed in any one of claims 1 to 3, wherein M is yttrium, lanthanum, gadolinium, or cerium.

5. A catalyst as claimed in any previous claim, wherein the molar ratio of Cu : M in the catalyst is from 100 : 1 to 100 : 10, and preferably 100 : 2 to 100 : 8.

6. A catalyst as claimed in any previous claim, wherein the molar ratio of Cu : M in the modified surface layer is more than 100 : 10, preferably more than 100 : 20.

7. A catalyst as claimed in any of claims 2 to 6, wherein the content of metals other than Cu and M in the catalyst is < 5 at%, and preferably < 1 at%, based on the total amount of metals in the catalyst.

8. A catalyst as claimed in any previous claim, wherein the catalyst is a pre-catalyst comprising a mixture of copper (II) oxide and metal M or metal M compound.

9. A catalyst as claimed in any of claims 1 to 7, wherein the catalyst is a reduced catalyst in which the majority of the copper is present as copper (0).

10. Use of a catalyst as claimed in any of claims 1 to 9 for the electrochemical conversion of carbon dioxide or carbon monoxide to a C2+ product.

11. A method of manufacturing a pre-catalyst comprising the steps of:(i) providing a mixture comprising particles of a copper (II) compound and particles of a surface modifier precursor, the surface modifier precursor comprising a metal M or metal M compound;(ii) mixing or milling the mixture so that the particles of the copper (II) compound and the surface modifier precursor interact mechanochemically to form a mechanochemically-mixed powder; and(iii) optionally, calcining the mechanochemically-mixed powder; wherein the molar ratio of Cu : M in the pre-catalyst is from 100 : 0.1 to 100 : 10; and wherein the pre-catalyst comprises a core and a modified surface layer, wherein the modified surface layer comprises a higher concentration of metal (M) than the core.

12. A method as claimed in claim 11 , wherein step (ii) is performed by resonant acoustic mixing (RAM).

13. A method as claimed in claim 11 or 12, wherein the surface modifier precursor has a D50 particle size that is smaller than the D50 particle size of the copper (II) compound.

14. A method as claimed in any of claims 11 to 13, wherein the copper (II) compound is decomposable to form copper (II) oxide or is copper (II) oxide.

15. A method as claimed in any of claims 11 to 14, wherein the surface modifier precursor is a metal M nitrate, a metal M acetate, or a metal M oxide.

16. A method as claimed in any of claims 11 to 15, wherein the pre-catalyst is as claimed in claim 8.

17. An ink comprising a polymer and a pre-catalyst dispersed in a solvent or solvent mixture, wherein the pre-catalyst is a catalyst as defined in claim 8.

18. A gas diffusion electrode comprising a gas diffusion layer and a catalyst layer on the gas diffusion layer, wherein the catalyst layer comprises a catalyst as defined in any of claims 1 to 9.

19. A catalyst coated membrane comprising a membrane having an anode side and a cathode side, wherein a catalyst as defined in any of claims 1 to 9 is present at the cathode side.

20. A catalyst coated membrane according to claim 19, comprising a cathode catalyst layer on the cathode side of the membrane, wherein the cathode catalyst layer comprises a catalyst as defined in any of claims 1 to 9.

21. A catalyst coated membrane according to claim 19, comprising a gas diffusion electrode as defined in claim 20 on the cathode side of the membrane.

22. An electrolyser comprising a gas diffusion electrode according to claim 18 or a catalyst coated membrane according to any of claims 19 to 21 .

23. A method for converting CO2 into C2+ products, comprising the step of providing a feed stream comprising CO2 to an electrolyser as defined in claim 22.

24. A method of manufacturing a pre-catalyst comprising the steps of:(i) providing a mixture comprising particles of a copper compound and particles of a surface modifier precursor, the surface modifier precursor comprising a metal M or metal M compound;(ii) mixing or milling the mixture so that the particles of the copper compound and the surface modifier precursor interact to form a mixed powder in which the surface modifier precursor decorates a surface layer of the particles of the copper compound; and(iii) optionally, calcining the mixed powder. wherein the molar ratio of Cu : M in the pre-catalyst is from 100 : 0.1 to 100 : 10; and wherein the pre-catalyst comprises a core and a modified surface layer, wherein the modified surface layer comprises a higher concentration of metal (M) than the core.

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