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

Abstract

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) selected from the group consisting of: cerium (Ce), praseodymium (Pr), neodymium (Nd), zirconium (Zr), barium (Ba), indium (In), nickel (Ni), palladium (Pd), platinum (Pt), and magnesium (Mg); wherein the molar ratio of Cu : M is from 100 : 0.1 to 100 : 10. The catalyst has particular use in a gas diffusion electrode, a catalyst coated membrane of an electrolyser.

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 equation for the half reactions occurring in direct CC o 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 CO2reduction 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 CO2reduction 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 CO2reduction. 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 is a plot showing Faradaic Efficiency of some reaction products formed during electrochemical CO2 reduction for catalysts of Examples 1 , 4, 7 and 8 at a current density of -250 mA / cm2.

[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 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] Ce -2.336

[0029] Pr -2.353

[0030] Nd -2 323

[0031] Zr -1.45

[0032] Ga -0.53

[0033] In -0.34

[0034] Ni -0.257

[0035] Cu +0.337

[0036] Pd +0.915

[0037] Pt +1.188

[0038] Table 1.

[0039] 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 are effective. For instance, Ga also has a lower standard reduction potential than Cu, but was not able to be co-precipitated with the CuO, as is explained in more detail in the examples. The presence of specific M ion modifiers within the Cu(0) material 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 modified catalyst, for example by the formation of a CUMO2 delaffosite phase.

[0040] Moreover, some metals having a higher standard reduction potential than Cu, such as Pd and Pt, can also be effective. In such cases, the 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 including Pt and Pd modifiers within the Cu(0) material 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.

[0041] 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 catalyst comprises copper and a metal (M) selected from the group consisting of: cerium (Ce), praseodymium (Pr), neodymium (Nd), zirconium (Zr), barium (Ba), indium (In), nickel (Ni), palladium (Pd), platinum (Pt), and magnesium (Mg); wherein the molar ratio of Cu : M is from 100 : 0.1 to 100 : 10.

[0042] 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, although it is expected that the distribution of Cu and M within the catalyst may differ. The catalyst is suitably an electrocatalyst, preferably a CO2 reduction reaction electrocatalyst.

[0043] As used herein, the term “modified” simply means that the catalyst contains M in addition to Cu. The term “modified” is not intended to imply anything about the distribution of M throughout the copper oxide (pre-catalyst) or copper (reduced catalyst).

[0044] 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. In a third aspect the invention relates to a method of manufacturing a pre-catalyst, comprising the steps of:

[0045] (i) providing an aqueous solution comprising a copper (II) salt and a metal M salt;

[0046] (ii) effecting a precipitation of the copper (II) salt and the metal M salt to form a precipitate;

[0047] (iii) isolating the precipitate;

[0048] (iv) drying the precipitate to provide the pre-catalyst;

[0049] (v) optionally heating the pre-catalyst to a temperature of 200-500°C; wherein M is selected from the group consisting of cerium (Ce), praseodymium (Pr), neodymium (Nd), zirconium (Zr), barium (Ba), indium (In), nickel (Ni), palladium (Pd), platinum (Pt), and magnesium (Mg); and wherein the molar ratio of Cu : M in the pre-catalyst is from 100 : 0.1 to 100 : 10.

[0050] 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.

[0051] In a fourth aspect the invention relates to an ink comprising a polymer and a pre-catalyst dispersed in a solvent or solvent mixture.

[0052] 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.

[0053] 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.

[0054] 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.

[0055] 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. Detailed Description

[0056] Any sub-headings are included for convenience only, and are not to be construed as limiting the disclosure in any way.

[0057] Catalyst

[0058] The catalyst comprises copper and a metal (M) selected from the group consisting of: cerium (Ce), praseodymium (Pr), neodymium (Nd), zirconium (Zr), barium (Ba), indium (In), nickel (Ni), palladium (Pd), platinum (Pt), and magnesium (Mg). Preferably, the metal (M) is selected from the group consisting of Zr, Mg, Ce, Pr, Nd, Pd and Pt. More preferably, the metal (M) is selected from Zr, Ce, Pr, and Nd. The catalyst does not comprise an alloy of copper and the metal (M).

[0059] The catalyst produced by the method of the invention is referred to herein as a pre-catalyst. The pre-catalyst suitably comprises (or consists essentially of or consists of) copper, the metal (M) and oxygen. Preferably, the pre-catalyst comprises elements other than copper, the metal (M) and oxygen in an amount of <10 at.%, preferably <5 at.%, and more preferably <1 at.%. Preferably, the pre-catalyst comprises (or consists essentially of) a mixture of copper (II) oxide and metal M oxide. The term “mixture of copper (II) oxide and metal M oxide” refers to a mixture comprising 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. The copper oxide and the metal M oxide are suitably present as separate phases in the pre-catalyst. The copper oxide and the metal M oxide can have different crystal structures. Preferably, the copper oxide and the metal M oxide in the pre-catalyst are not present as a mixed metal oxide (or solid solution) having a single crystal structure. The pre-catalyst does not comprise an alloy of copper and the metal M.

[0060] 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%. It is thought that for metals M having a more negative reduction potential than the cathode potential of the electrolyser during operation, 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 of the electrolyser during operation, some or all of the metal M is converted to M(0).

[0061] The following preferred embodiments apply to both the pre-catalyst and the reduced catalyst.

[0062] The molar ratio of Ou : M is from 100 : 0.1 to 100 : 10 (i.e. 0.1 to 10 atom% M relative to Cu). The preferred ratio of Cu : M differs depending on the choice of M. A typical range is 100 : 1 to 100 : 10, 100 : 2 to 100 : 8, such as 100 : 3 to 100 : 7, with 100 : 5 being typical.

[0063] In a preferred embodiment M is cerium (Ce), praseodymium (Pr), neodymium (Nd). In another preferred embodiment M is palladium (Pd) or platinum (Pt).

[0064] 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 Ba (M = Ba) at a molar ratio of 94 : 5 : 1 has a content of metals other than Cu and M of 5 at.%.

[0065] Manufacture of the catalyst

[0066] The pre-catalysts described herein can be produced by a simple co-precipitation procedure comprising the steps of:

[0067] (i) providing an aqueous solution comprising a copper (II) salt and a metal M salt;

[0068] (ii) effecting a precipitation of the copper (II) salt and the metal M salt to form a precipitate;

[0069] (iii) isolating the precipitate;

[0070] (iv) drying the precipitate to provide the pre-catalyst;

[0071] (v) optionally heating the pre-catalyst to a temperature of 200-500°C; wherein M is selected from the group consisting of cerium (Ce), praseodymium (Pr), neodymium (Nd), zirconium (Zr), barium (Ba), indium (In), nickel (Ni), palladium (Pd), platinum (Pt), and magnesium (Mg); and wherein the molar ratio of Cu : M in the pre-catalyst is from 100 : 0.1 to 100 : 10. It is preferred that in step (i) the only metal salts present are the copper (II) salt and the M salt. It is preferred that the counter anion of the copper (II) salt and M salt is the same. Nitrate salts are particularly suitable.

[0072] Optionally, between steps (i) and (ii), the method can comprise heating the solution from step (i) to a temperature of 60-80 °C. Heating an aqueous solution containing copper (II) above 60 °C can precipitate copper oxide. Regardless of whether step (ii) is carried out, the pH swing specified in step (iii) is carried out.

[0073] Step (iii) can comprise adjusting the pH of the solution to a pH of between 6.5 and 10.5 to effect the precipitation. For example, in step (iii) the pH can be raised to effect the precipitation reaction. Any suitable base may be used, such as amines, alkali metal hydroxides or alkali metal carbonates e.g. NaOH or NazCCh. The pH aimed for in this step will differ depending on the choice of metal M. A typical range is 8.5 to 9.5. Alternatively, step (iii) can comprise effecting the precipitation at a substantially constant pH, for example, by addition of a suitable base, such as amines, alkali metal hydroxide or alkali metal carbonates e.g. NaOH or Na2CO3. The substantially constant pH is preferably between 6.5 and 10.5, and typically between 8.5 and 10.0. The constant pH chosen in this step will differ depending on the choice of metal M. Step (iii) can be performed at a temperature in the range of 60-80 °C.

[0074] In step (iv) the precipitate is isolated. Suitable techniques will be known to the skilled person, such as vacuum filtration.

[0075] It is preferred that between step (iv) and step (v) a washing step (iv-b) is carried out on the precipitate. The role of the washing step is to remove any entrained ions (e.g. Na+, NOs'). It is preferred that the material is washed with deionised water until the conductivity of the filtrate is < 20 pS.

[0076] In step (v) the precipitate is dried to remove excess water. Typical drying conditions are a temperature around 105 °C in air overnight. It will be appreciated that drying conditions may differ depending on scale. Typically step (v) is followed by a calcination step (vi). The calcination step (vi) may convert any residual hydroxide or carbonate of the metal or copper to the corresponding oxide. Typically calcination is carried out at 350 °C for 2 hours, but this may vary depending on the scale of material used. The calcination step may also remove some of the remaining anions that were not removed by the washing step (iv-b).

[0077] Ink

[0078] 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).

[0079] 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™.

[0080] Coated substrates

[0081] 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.

[0082] 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. In one embodiment the CCM is coated on the cathode side face with a cathode catalyst (cathode catalyst layer).

[0083] 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.

[0084] 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.

[0085] Figures 1a, 1b and 1c illustrate electrolysers containing a cation exchange membrane and using KHCO3 as the electrolyte. The skilled person will appreciate that other electrolytes (e.g. KOH) may be used and the membrane does not have to be a cation exchange membrane.

[0086] 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.

[0087] 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.

[0088] In one embodiment the catalyst layer on the gas diffusion layer comprises a polymer binder.

[0089] In one embodiment the gas diffusion electrode comprises a microporous layer on the catalyst layer.

[0090] 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.

[0091] 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.

[0092] 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.

[0093] 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.

[0094] 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.

[0095] Use of the catalyst

[0096] The catalyst may be used for the direct electrochemical conversion of CChto 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.

[0097] Examples

[0098] General procedure for the production of modified copper oxides

[0099] A copper(ll) salt (e.g. Cu(NO3)2-xH2O (x = 2 or 3)) and the respective M nitrate were dissolved in deionised water in the desired Cu : M ratio for the catalyst, herein called solution A. The metal concentration (Cu + M) was 0.4 M. Solution A was added dropwise to 100- 500 mL of deionised water heated to 60 °C. 1 M NaOH was simultaneously added to maintain a pH of 9-10 in the mixture. Once addition of solution A was complete, the solution was stirred at 60°C for 1-3 hours, before cooling to room temperature. The solid precipitate was collected by vacuum filtration and washed with deionised water until the conductivity of the filtrate was < 50 S. The precipitate was partially dried on the vacuum filtration bed before being completely dried in air at 80-110 °C in an oven overnight. The precipitate was ground and sieved to a powder with particle size < 500 pm. The as-prepared powders were then calcined at about 350 °C for 2 hours. The quantities used are shown in Table 2. This general method of preparation was used to prepare modified catalysts for all examples and comparative examples, except Example 7.

[0100] Procedure for the production of modified copper oxide in accordance with Example 7 (E7)

[0101] The modified copper material of Example 7 was made by an incipient wetness impregnation method involving a copper(ll) oxide precursor and Pd(NO3)2 aqueous solution. In Example 7, the copper(ll) oxide precursor was copper(ll) oxide. The respective M nitrate solution was weighed out and diluted with water to achieve the desired molar ratio of Cu:Pd. The Pd nitrate solution was poured directly onto the copper(ll) oxide precursor, and the material was mixed in a dual asymmetric centrifugal mixer at 3000 RPM for 1 minute (Speedmixer™ by Hauschild). The material was dried at 60 °C overnight, before calcination at about 350 °C for 2 hours.

[0102] Table 2.

[0103] The as-prepared powders of CE1 and E1-E6 were calcined at about 350 °C for 2 hours and then were analysed by X-ray diffraction (XRD) and transmission electron microscopy (TEM). XRD can be used to determine the crystalline phases present in the sample, lattice parameters, unit cell volumes and crystallite sizes. TEM can be used to determine particle size, shape, dispersion of location of modifiers and Cu within each sample.

[0104] General procedure for ink formulation and gas diffusion electrode fabrication

[0105] The modified copper oxide material (prepared using the methods above) was added to a glass vial, followed by 12 wt% aqueous Nation™ 1100 EW dispersion (~20 wt.% with respect to the modified copper oxide material), n-propanol and water were then 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.

[0106] General procedure for electrochemical testing of modified copper oxides

[0107] 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 250 mL and 500 mL of 1 M KHCO3, respectively, with flow rates of 50 ml / min and 100 ml / min, respectively. The catholyte was purged with CO2 at a flow-rate of 45-50 ml / min. Galvanostatic measurements were then performed at a current density from -50 mA / cm2to -300 mA / cm2with an increment of 50 mA / cm2. Experiments were performed at each current density for 20 mins. 2 gas chromatography (GC) runs were collected during this 20 mins and averaged out for FE calculation The Faradaic Efficiency (FE) for producing the reaction products was determined. Faradaic Efficiency for gaseous products (FEgas), e.g. H2, was determined using Equation 1 : Equation 1 where C is the concentration of gaseous product as measured by gas chromatography (volproduct / vohotai product), n is the number of transferred electrons per mole, F is the Faraday constant (96485 C mol1), is the volumetric flow rate (mL min-1), is the molar volume of gas (mL mol'1), and jtotai is the total current density (A cm'2).

[0108] Results of electrochemical testing of modified copper oxides

[0109] Figure 2 shows the Faradaic Efficiency of some different reaction products formed during electrochemical CO2 reduction (i.e. H2, carbon monoxide and ethylene) for catalysts E1 , E4, E7 and E8. Each of these catalysts were active in the electrochemical conversion of carbon dioxide to 02+ products, such as ethylene. Moreover, each of these catalysts exhibited favourable ethylene selectivity compared to other products (e.g. H2 and CO) at a current density of -250 mA / cm2.

[0110] A non-exhaustive list of aspects of the invention is provided in the following numbered clauses:

[0111] 1. 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) selected from the group consisting of: cerium (Ce), praseodymium (Pr), neodymium (Nd), zirconium (Zr), barium (Ba), indium (In), nickel (Ni), palladium (Pd) and platinum (Pt); wherein the molar ratio of Cu : M is from 100 : 0.1 to 100 : 10.

[0112] 2. A catalyst according to clause 1, wherein the catalyst is a pre-catalyst comprising a mixture of copper (II) oxide and M oxide. 3. A pre-catalyst according to clause 2, wherein M is cerium (Ce), praseodymium (Pr) , neodymium (Nd).

[0113] 4. A pre-catalyst according to clause 2, wherein M is palladium or platinum.

[0114] 5. A pre-catalyst according to any one of clauses 2 to 4, wherein the molar ratio of Cu : M is from 100 : 1 to 100 : 10, and preferably 100 : 2 to 100 : 8.

[0115] 6. A pre-catalyst according to any one of clauses 2 to 5, wherein the content of metals other than Cu and M in the catalyst is < 5 at% based on the total amount of metals in the catalyst.

[0116] 7. A pre-catalyst according to any one of clauses 2 to 5, wherein the content of metals other than Cu and M in the catalyst is < 1 at% based on the total amount of metals in the catalyst.

[0117] 8. A catalyst according to clause 1 , wherein the catalyst is a reduced catalyst in which the majority of the copper is present as copper (0).

[0118] 9. A reduced catalyst according to clause 8 wherein M is cerium (Ce), praseodymium (Pr), neodymium (Nd).

[0119] 10. A reduced catalyst according to clause 8, wherein M is palladium or platinum.

[0120] 11. A reduced catalyst according to any one of clauses 8 to 10, wherein the molar ratio of Cu : M is from 100 : 1 to 100: 10, and preferably 100 : 2 to 100 : 8.

[0121] 12. A reduced catalyst according to any one of clauses 8 to 11 , wherein the content of metals other than Cu and M in the catalyst is < 5 at% based on the total amount of metals in the catalyst.

[0122] 13. Use of a catalyst according to any one of clauses 1 to 12 for the electrochemical conversion of carbon dioxide or carbon monoxide to a C2+ product. 14. A method of manufacturing a pre-catalyst comprising the steps of:

[0123] (i) providing an aqueous solution comprising a copper (II) salt and a metal M salt;

[0124] (ii) effecting a precipitation of the copper (II) salt and the metal M salt to form a precipitate;

[0125] (iii) isolating the precipitate;

[0126] (iv) drying the precipitate to provide the pre-catalyst;

[0127] (v) optionally heating the pre-catalyst to a temperature of 200-500°C; wherein M is selected from the group consisting of cerium (Ce), praseodymium (Pr), neodymium (Nd), zirconium (Zr), barium (Ba), indium (In), nickel (Ni), palladium (Pd) and platinum (Pt); and wherein the molar ratio of Cu : M in the pre-catalyst is from 100 : 0.1 to 100 : 10.

[0128] 15. A method according to clause 14, wherein the pre-catalyst is as defined in any of clauses 2 to 7.

[0129] 16. An ink comprising a polymer and a pre-catalyst dispersed a solvent or solvent mixture, wherein the pre-catalyst is a catalyst as defined in any of clauses 2 to 7.

[0130] 17. 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 clauses 1 to 12.

[0131] 18. A gas diffusion electrode according to clause 17, wherein the catalyst layer comprises a polymer binder.

[0132] 19. A gas diffusion electrode as defined in clause 17 or clause 18, wherein the gas diffusion electrode comprises a microporous layer on the catalyst layer.

[0133] 20. A catalyst coated membrane comprising a membrane having an anode side and a cathode side, wherein a catalyst as defined in any of clauses 1 to 12 is present at the cathode side. 21. A catalyst coated membrane according to clause 20, 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 clauses 1 to 12.

[0134] 22. A catalyst coated membrane according to clause 20, comprising a gas diffusion electrode as defined in any of clauses 17 to 19 on the cathode side of the membrane, and an anode catalyst layer on the anode side face of the membrane.

[0135] 23. A catalyst coated membrane according to clause 20, comprising an anode catalyst layer on the anode side of the membrane and a cathode catalyst layer on the cathode side of the membrane, wherein the cathode catalyst layer comprises a catalyst as defined in any of clauses 1 to 12.

[0136] 24. An electrolyser comprising a gas diffusion electrode according to any of clauses 17 to 19 or a catalyst coated membrane according to any of clauses 20 to 23.

[0137] 25. A method for converting CO2 into C2+ products, comprising the step of providing a feed stream comprising CO2 to an electrolyser as defined in clause 24.

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) selected from the group consisting of: cerium (Ce), praseodymium (Pr), neodymium (Nd), zirconium (Zr), barium (Ba), indium (In), nickel (Ni), palladium (Pd), platinum (Pt), and magnesium (Mg); wherein the molar ratio of Cu : M is from 100 : 0.1 to 100 : 10.

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

3. A catalyst as claimed in claim 1 or 2, wherein the catalyst is a pre-catalyst comprising a mixture of copper (II) oxide and M oxide.

4. A pre-catalyst as claimed in claim 3, wherein M is cerium (Ce), praseodymium (Pr), neodymium (Nd).

5. A pre-catalyst as claimed in claim 3, wherein M is palladium or platinum.

6. A pre-catalyst as claimed in any of claims 3 to 5, wherein the molar ratio of Cu : M is from 100 : 1 to 100 : 10, and preferably 100 : 2 to 100 : 8.

7. A pre-catalyst as claimed in any of claims 3 to 6, wherein the content of metals other than Cu and M in the catalyst is < 5 at% based on the total amount of metals in the catalyst.

8. A pre-catalyst as claimed in any of claims 3 to 6, wherein the content of metals other than Cu and M in the catalyst is < 1 at% based on the total amount of metals in the catalyst.

9. A catalyst as claimed in claim 1 or 2, wherein the catalyst is a reduced catalyst in which the majority of the copper is present as copper (0).

10. A reduced catalyst as claimed in claim 9 wherein M is cerium (Ce), praseodymium (Pr), neodymium (Nd).

11. A reduced catalyst as claimed in claim 9, wherein M is palladium or platinum.

12. A reduced catalyst as claimed in any of claims 9 to 11 , wherein the molar ratio of Cu : M is from 100 : 1 to 100: 10, and preferably 100 : 2 to 100 : 8.

13. A reduced catalyst as claimed in any of claims 9 to 12, wherein the content of metals other than Cu and M in the catalyst is < 5 at% based on the total amount of metals in the catalyst.

14. Use of a catalyst as claimed in any of claims 1 to 13 for the electrochemical conversion of carbon dioxide or carbon monoxide to a C2+ product.

15. A method of manufacturing a pre-catalyst comprising the steps of:(i) providing an aqueous solution comprising a copper (II) salt and a metal M salt;(ii) effecting a precipitation of the copper (II) salt and the metal M salt to form a precipitate;(iii) isolating the precipitate;(iv) drying the precipitate to provide the pre-catalyst;(v) optionally heating the pre-catalyst to a temperature of 200-500°C; wherein M is selected from the group consisting of cerium (Ce), praseodymium (Pr), neodymium (Nd), zirconium (Zr), barium (Ba), indium (In), nickel (Ni), palladium (Pd), platinum (Pt), and magnesium (Mg); and wherein the molar ratio of Cu : M in the pre-catalyst is from 100 : 0.1 to 100 : 10.

16. A method as claimed in claim 15, wherein M is selected from the group consisting of cerium (Ce), praseodymium (Pr), neodymium (Nd), zirconium (Zr), barium (Ba), indium (In), nickel (Ni), palladium (Pd), and platinum (Pt).

17. A method as claimed in claim 15 or 16, wherein the pre-catalyst is as claimed in any of claims 2 to 7.

18. An ink comprising a polymer and a pre-catalyst dispersed a solvent or solvent mixture, wherein the pre-catalyst is a catalyst as defined in any of claims 3 to 8.

19. 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 13.

20. A gas diffusion electrode according to claim 19, wherein the catalyst layer comprises a polymer binder.

21. A gas diffusion electrode as claimed in claim 19 or claim 20, wherein the gas diffusion electrode comprises a microporous layer on the catalyst layer.

22. 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 13 is present at the cathode side.

23. A catalyst coated membrane according to claim 22, 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 13.

24. A catalyst coated membrane according to claim 22, comprising a gas diffusion electrode as defined in any of claims 19 to 21 on the cathode side of the membrane, and an anode catalyst layer on the anode side face of the membrane.

25. A catalyst coated membrane according to claim 22, comprising an anode catalyst layer on the anode side of the membrane and 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 13.

26. An electrolyser comprising a gas diffusion electrode according to any of claims 19 to 21 or a catalyst coated membrane according to any of claims 22 to 25.

27. 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 26.

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