Electrocatalyst layer

Abstract

An electrocatalyst layer for electrochemical reduction of carbon dioxide or carbon monoxide to form a C2+ product is described. The electrocatalyst layer comprises: an electrocatalytic component which

Description

Field of the Invention This invention relates to an electrocatalyst layer for electrochemical reduction of carbon dioxide or carbon monoxide to form a C2+ product. This invention also relates to associated catalyst coated membranes, gas diffusion electrodes, membrane electrode assemblies and electrolysers comprising said electrocatalyst layer. This invention also relates to associated methods of preparing said electrocatalyst layers. Declaration of funding 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. Background of the Invention 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. Direct CO2 conversion may be carried out in an electrochemical reactor called an electrolyser, 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 electrochemical conversion of CO2 to ethylene is shown below: Cathode reaction: 2CO2 + 8H2O + 12 e- C2H4 + 12OH' Anode reaction: 12OH' —► 6H2O + 3O2 + 12e_ Overall: 2H2O + 2CO2 —► C2H4 + 302 A reaction which competes with the desired C2+ generation reaction is the hydrogen evolution reaction (HER): Hydrogen evolution reaction: 2H2O + 2 e' H2 + 2OH' 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. A variety of metals can be used as the CO2RR catalyst and the subject has been 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), 180-196). It is known that the choice of metal catalyst influences the mechanism of CO2 reduction and therefore the product(s) formed. Copper is a commonly employed 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. There is a desire and a need for CO2 electrolysers which can convert CO2 into C2+ products, in particular ethylene, which improved selectivity, have low background activity for the hydrogen evolution reaction (HER), and which are simple to manufacture. The present invention provides an electrocatalyst layer which seeks to address this desire and need. Summary of the Invention While the choice of metal is known to influence the mechanism of CO2 reduction, which may affect the selectivity of forming C2+ products such as ethylene, the present inventors have now found that tuning certain properties of the electrocatalyst layer can surprisingly affect the observed selectivity of forming C2+ products, even if the same electrocatalyst or preelectrocatalyst is used. In particular, the present invention seeks to provide an electrocatalyst layer which provides favourable selectivity towards ethylene production with relatively lower contributions from the competing hydrogen evolution reaction. According to a first aspect of the invention there is provided an electrocatalyst layer for electrochemical reduction of carbon dioxide or carbon monoxide to form a C2+ product. As used herein, the term “C2+ products” means a product comprising at least two carbon atoms. The electrocatalyst layers are particularly suitable for the conversion of CO2 to ethylene. The electrocatalyst layer comprises an electrocatalytic component which is an electrocatalyst or a pre-electrocatalyst. Suitably, the electrocatalytic component comprises copper. The electrocatalyst layer also comprises an ion-conducting polymer comprising ionexchange groups. The electrocatalytic component can be present in the electrocatalyst layer at an areal loading in a range of up to 5.0 mg / cm2, suitably up to 4.0 mg / cm2, or suitably up to 3.0 mg / cm2, for example in a range of from 0.1 mg / cm2 to 5.0 mg / cm2. Preferably, the electrocatalytic component can be present in the electrocatalyst layer at an areal loading of 2.5 mg / cm2 or less, preferably 2.3 mg / cm2 or less, more preferably 2.0 mg / cm2 or less, suitably 1.8 mg / cm3 or less, more preferably 1.5mg / cm2 or less, and suitably 1.0mg / cm2 or less. The electrocatalytic component can be a pre-electrocatalyst (e.g. copper(ll) oxide, a mixed metal oxide comprising copper, or a mixture of a copper oxide and a metal oxide) or an electrocatalyst (e.g. Cu(0), an alloy of copper or copper doped with a dopant, such as a transition metal or lanthanide). A ratio (niOn / ncat) of the total number of moles of the ion exchange groups (n / on) to the total number of moles of the electrocatalytic component {ncat) can be at least 0.007, suitably at least 0.012, preferably at least 0.015, preferably at least 0.017, preferably at least 0.024. The nion / ncat ratio can be 0.035 or less, preferably 0.034 or less, preferably 0.033 or less. The nion / ncat ratio can be in a range comprising a combination of any of the aforementioned upper and lower limits. For example, the niOn / nCat ratio can be in a range of 0.007 to 0.035, preferably in a range of 0.012 to 0.035, and more preferably 0.017 to 0.033. The ion-conducting polymer can be a proton conducting polymer. The ion-exchange groups can be sulfonic acid groups. The ion-conducting polymer can be a partially- or fully-fluorinated sulfonic acid polymer. The ion-conducting polymer can have an equivalent weight (EW) in a range of 600 to 1200. The electrocatalyst layer can have a contact angle for 1 M KHCO3 of greater than 115°, preferably >117°. The electrocatalyst layer can have a cross-sectional thickness (in a through plane direction) in a range of 3 pm to 50 pm, preferably 3 pm to 30 pm, more preferably 5 pm to 15 pm. Suitably, the electrocatalyst layer is porous. That is, the electrocatalyst layer can comprise pores. The electrocatalyst layer can have a pore volume in a range of 0.10cm3 / g to 0.50 cm3 / g. The electrocatalyst layer can have a porosity in a range of 20 vol.% to 50 vol.%. The electrocatalyst layer can have a peak log™ differential intrusion volume (for pores having a pore diameter in a range of 10 nm to 1 pm) of <1 cm3 / g, suitably in a range of 0.3 cm3 / g to 0.8 cm3 / g, preferably in a range of 0.35 cm3 / g to 0.70 cm3 / g, preferably 0.4 cm3 / g to 0.60 cm3 / g. The pores of the electrocatalyst layer can have an entrapment factor in a range of 35 %v / v to 65 %v / v. The pores of the electrocatalyst layer can have a median pore diameter in a range of 80 nm to 110 nm, preferably 85 nm to 105, more preferably 90 m to 100 nm. The pores can have a bimodal pore size (diameter) distribution for pores having a pore diameter in a range of 10 nm to 1 pm, as measured by mercury intrusion porosimetry. In particular, the pore size distribution can comprise a first peak {Pi) positioned at a pore diameter in a range of 50-75 nm; and a second peak {P2) positioned at a pore diameter in a range of 80-110 nm. The ratio (Areap2 / Areapi) of the area under peak P2 (Areap2) to the area under peak Pi can be >0, suitably >0.10, suitably >0.15, more preferably >0.20. Suitably, the ratio Areap2 / Areap2 is <2.0, suitably <1.5, preferably <1.10, preferably <1.00, and more preferably <0.90. The ratio Areap2 / Areap2 can be in a range comprising any combination of the aforementioned upper and lower limits. For example, the ratio Areap2 / Areap2 can be in a range of >0 to 2.0, preferably 0.10 to 1.5, preferably 0.15 to 1.10, preferably 0.20 to 1.00, more preferably 0.30 to 0.90, suitably 0.35 to 0.85. The ratio of the peak logw differential intrusion volume of peak P2 to the peak logw differential intrusion volume of peak Pi can be >0, suitably >0.10, suitably >0.15, more preferably >0.20. Suitably, the ratio of the peak logw differential intrusion volume of peak P2 to the peak logw differential intrusion volume of peak Pi is <2.0, suitably <1.5, preferably <1.10, preferably <1.00, and more preferably <0.90. The ratio of the peak logw differential intrusion volume of peak P2 to the peak logw differential intrusion volume of peak Pi can be in a range comprising any combination of the aforementioned upper and lower limits. For example, the ratio of the peak logw differential intrusion volume of peak P2 to the peak logw differential intrusion volume of peak Pi can be in a range of >0 to 2.0, preferably 0.10 to 1.5, preferably 0.15 to 1.10, preferably 0.20 to 1.00, more preferably 0.30 to 0.90, suitably 0.35 to 0.85. The electrocatalyst layer can further comprise other additives. For example, the electrocatalyst layer can comprise a hydrophobic additive for modifying the hydrophobicity of the layer. Suitable hydrophobic additives can be hydrophobic polymers, such as fluoropolymers. Suitable hydrophobic additives can include polytetrafluoroethylene (PTFE), polyvinylidene difluoride (PVDF), and polystyrene. As a further example, the electrocatalyst layer can comprise an electrically conducting additive for modifying the electrical conductivity of the electrocatalyst layer. Suitable electrically conducting additives can include electrically conducting carbon-based materials, such as carbon black, graphite, carbon nanotubes, carbon fibres, graphene and the like. Preferably, the electrocatalyst layer is devoid of cracks having a smallest cross-sectional dimension (in a through-plane direction) of >1.5 pm. For example, in one embodiment, there is an electrocatalyst layer for electrochemical reduction of carbon dioxide or carbon monoxide to form a C2+ product, the electrocatalyst layer comprising: an electrocatalytic component which is an electrocatalyst or a pre-electrocatalyst; and an ion-conducting polymer comprising ion-exchange groups; wherein the electrocatalytic component is present in the electrocatalyst layer at an areal loading of 2.5 mg / cm2 or less, a ratio (niOn / nCai) of the total number of moles of the ion exchange groups ( / ?»«) to the total number of moles of the electrocatalytic component (ncat) is in a range of 0.007 to 0.035, and the electrocatalyst layer has a contact angle for 1 M KHCO3 of greater than 115°. In another embodiment, there is an electrocatalyst layer for electrochemical reduction of carbon dioxide or carbon monoxide to form a C2+ product, the electrocatalyst layer comprising: an electrocatalytic component comprising copper, wherein the electrocatalytic component is an electrocatalyst or a pre-electrocatalyst; an ion-conducting polymer; and pores; wherein the pores have a pore diameter distribution comprising a first peak (Pi) positioned at a pore diameter in a range of 50 nm to 75 nm, and a second peak (P2) positioned at a pore diameter in a range of 80 nm to 110 nm, wherein the maximum peak log™ differential intrusion volume for peak Pi and peak P2 is in a range of 0.3 cm3 / g to 0.8 cm3 / g, preferably 0.4 cm3 / g to 0.60 cm3 / g. In particular, the pore size distribution can comprise a first peak (Pi) positioned at a pore diameter in a range of 50-75 nm; and a second peak (P2) positioned at a pore diameter in a range of 80-110 nm. The ratio (Areap2 / Areapi) of the area under peak P2 (Areap2) to the area under peak Pi is can be >0, suitably >0.10, suitably >0.15, more preferably >0.20. Suitably, the ratio Areap2 / Areap2 is <2.0, suitably <1.5, preferably <1.10, preferably <1.00, and more preferably <0.90. The ratio Areap2 / Areap2 can be in a range comprising any combination of the aforementioned upper and lower limits. For example, the ratio Areap2 / Areap2 can be in a range of >0 to 2.0, preferably 0.10 to 1.5, preferably 0.15 to 1.10, preferably 0.20 to 1.00, more preferably 0.30 to 0.90, suitably 0.35 to 0.85. The ratio of the peak log™ differential intrusion volume of peak P2 to the peak log™ differential intrusion volume of peak Pi can be >0, suitably >0.10, suitably >0.15, more preferably >0.20. Suitably, the ratio Areap2 / Areap2 is <2.0, suitably <1.5, preferably <1.10, preferably <1.00, and more preferably <0.90. The ratio Areap2 / Areap2 can be in a range comprising any combination of the aforementioned upper and lower limits. For example, the ratio Areap2 / Areap2 can be in a range of >0 to 2.0, preferably 0.10 to 1.5, preferably 0.15 to 1.10, preferably 0.20 to 1.00, more preferably 0.30 to 0.90, suitably 0.35 to 0.85. In a second aspect, there is provided a gas diffusion electrode comprising the electrocatalyst layer of the first aspect and a gas diffusion layer. In a third aspect, there is provided a catalyst coated membrane comprising the electrocatalyst of the first aspect applied to at least one side of an ion-conducting membrane, such as a proton exchange membrane. In a fourth aspect, there is provided a membrane electrode assembly comprising the electrocatalyst layer of the first aspect, or the gas diffusion electrode of the second aspect, or the catalyst coated membrane of the third aspect. In a fifth aspect, there is provided an electrolyser for electrochemically converting CO2 into a C2+ product comprising the electrocatalyst layer of the first aspect, the gas diffusion electrode of the second aspect, the catalyst coated membrane of the third aspect, or the membrane electrode assembly of the fourth aspect. In a sixth aspect, there is a method of manufacturing an electrocatalyst layer, preferably an electrocatalyst layer according to the first aspect. The electrocatalyst layer is suitable for electrochemical reduction of carbon dioxide or carbon monoxide to form a C2+ product. The method comprises the steps of: (i) providing an electrocatalyst ink comprising an electrocatalyst component which is an electrocatalyst or a pre-electrocatalyst; an ion-conducting polymer comprising ion-exchange groups; and a solvent; (ii) depositing the electrocatalyst ink onto a substrate to provide a wet electrocatalyst layer; and (iii) drying the wet electrocatalyst layer. The electrocatalyst layer of the first aspect has particular utility when used in a cathode of an electrolyser for electrochemically reducing CO2 or CO into C2+ products. Any aspect of the invention may be combined with any other aspect of the invention, unless the context demands otherwise. Any of the preferred or optional features of any aspect or embodiment may be combined, singly or in combination, with any aspect or embodiment of the invention, unless the context demands otherwise. Brief Description of the Drawings Figures 1, 2 and 3 show possible arrangements for an electrolyser according to the disclosure; Figure 4 shows a plot of Log 10 differential intrusion volume (in cm3 / g) as a function of pore diameter (in Angstroms) using a log scale on the x-axis for Comparative Examples 3 and 5, and Examples 1, 2 and 5; Figure 5 shows the data fitting used to deconvolute the bimodal pore size distribution of Example 2; and Figure 6 shows the data fitting used to model the unimodal pore size distribution of Comparative Example 5. Detailed Description of the Invention Further preferred and / or optional features of the invention will now be described. Any aspect of the invention may be combined with any other aspect of the invention, unless the context requires otherwise. Unless the context requires otherwise, any of the preferred or optional features of any aspect may be combined with any other aspect of the invention, alone or in combination. Any sub-headings are included for convenience only, and are not to be construed as limiting the disclosure in any way. Electrocatalyst layer The electrocatalyst layer of the present disclosure comprises an electrocatalytic component and an ion-conducting polymer comprising ion exchange groups. The electrocatalytic component is suitably present in the electrocatalyst layer at an areal loading of 2.5 mg / cm2, preferably 2.3 mg / cm2 or less, more preferably 1.8 mg / cm2 or less, more preferably 1.5 mg / cm2 or less, and more preferably 1.0 mg / cm2 or less. The areal loading of the electrocatalytic component can be determined using X-ray fluorescence (XRF) and / or ICP-MS. The ratio (niOn / nCai) of the total number of moles of the ion exchange groups (nm) to the total number of moles of the electrocatalytic component (ncat) is suitably in a range of 0.007 to 0.035. Preferably, the niOn / ncat ratio is in a range of 0.015 to 0.035, more preferably in a range of 0.017 to 0.034, and more preferably in a range of 0.024 to 0.033. This combination of parameters provides improved ethylene selectivity in the electrochemical CO2 reduction reaction. However, the reasons for this are not yet fully understood. Without wishing to be bound by any theory or conjecture, it is believed that such a combination of nion / nCat ratio and catalyst layer loading provides favourable mass transport conditions to facilitate C-C coupling, while suppressing the undesirable hydrogen evolution reaction. Optionally, the electrocatalyst layer can further comprise an additive. The additive can be to control the hydrophobicity of the layer, for example the additive can be a hydrophobic additive. The additive can be to control the electronic conductivity of the electrocatalyst layer, for example, the additive can be an electrically conducting additive. Preferably, the additive does not react with any of the reactants or electrolyte in the electrode during operation. The additive is suitably a hydrophobic additive, for example a hydrophobic polymer, such as a fluoropolymer. Suitable hydrophobic additives can include polymers selected from polytetrafluoroethylene (PTFE), polystyrene, and polyvinylidene fluoride (PVDF), or a combination thereof, for example. Suitable electronically conducting additives can include electrically conducting carbon-based materials, such as carbon black, graphite, carbon nanotubes, carbon nanofibres, graphene and the like; and electrically conductive polymers, such as polyanilines, polypyrroles, and polythiophenes such as PEDOT. The additive is typically non-ion conducting. The additive can be in the form of particles, fibres or the like. The hydrophobic additive can be present in the electrocatalyst layer such that the weight ratio of the additive to the electrocatalytic component is in a range of and including 0 to 0.50, suitably 0 to 0.40, suitably 0 to 0.30, preferably 0 to 0.20, preferably 0 to 0.15, more preferably 0 to 0.13, and most preferably 0 to 0.11. The electrically conductive additive can be present in the electrocatalyst layer such that the weight ratio of the additive to the electrocatalytic component is in a range of and including 0 to 0.50, suitably 0 to 0.40, suitably 0 to 0.30, preferably 0 to 0.20, preferably 0 to 0.15, more preferably 0 to 0.13, and most preferably 0 to 0.11. The electrocatalyst layer can have a contact angle for aqueous 1 M KHCO3 of >115°, preferably >117°, and more preferably >120°. The electrocatalyst layer can have a contact angle for aqueous 1 M KHCO3 of <180°, preferably <160°, and more preferably <150°. The electrocatalyst can have a contact angle for aqueous 1 M KHCO3 in a range comprising any combination of the aforementioned limits, for example >115° to <180°, preferably >117° to <160°, and more preferably >120° to <150°. If the contact angle of the electrocatalyst layer is not sufficiently high, the electrocatalyst layer is more susceptible to flooding. Moreover, electrocatalyst layers having high contact angles (e.g. in a range 120° to 150°) in combination with a suitable areal loading of the electrocatalytic component and a suitable nm / ncat ratio, and / or suitable pore characteristics were associated with high ethylene selectivity while suitably mitigating electrode flooding. The contact angle can be measured using known techniques, such as using a contact angle meter at a temperature of 25 °C. The electrocatalyst layer can have a thickness of less than 50 pm, preferably less than 40 pm, suitably less than 30 pm, preferably less than 25 pm, more preferably less than 20 pm, more preferably less than 15 pm, more preferably less than 10 pm, and suitably less than 8 pm. The electrocatalyst can have a thickness of at least 3 pm, optionally at least 5 pm. The electrocatalyst layer can have a thickness in a range comprising any combination of the aforementioned upper and lower limits. For example, the electrocatalyst layer can have a thickness in a range of at least 3 pm and less than 30 pm, suitably at least 5 pm and less than 15 pm. The electrocatalyst layer suitably comprises pores. The electrocatalyst layer can have a pore volume in a range of 0.10cm3 / g to 0.50cm3 / g, suitably 0.25 cm3 / g to 0.40 cm3 / g. Preferably, the electrocatalyst layer has a pore volume of <0.35 cm3 / g, more preferably <0.310 cm3 / g, and more preferably <0.30cm3 / g. Preferably the electrocatalyst layer has a pore volume of >0.20 cm3 / g, preferably >0.26 cm3 / g, and more preferably >0.27 cm3 / g. The pore volume can be in a range comprising any combination of the aforementioned upper and lower limits. For example, the pore volume can be in a range of 0.20 cm3 / g to 0.40 cm3 / g, preferably 0.26cm3 / g to 0.35 cm3 / g, preferably 0.27 cm3 / g to 0.310 cm3 / g, and more preferably 0.280 cm3 / g to 0.30 cm3 / g. The pore volume can be determined using mercury intrusion porosimetry, for examples, using the methods described in the examples. The electrocatalyst layer can have a porosity in a range of 20 vol.% to 50 vol.%. Preferably the porosity of the electrocatalyst layer is at least 25 vol.%, more preferably at least 30 vol.%, preferably at least 32 vol.%. Suitably, the electrocatalyst layer has a porosity of <55 vol.%, preferably <50 vol.%, suitably <45 vol.%, preferably <40 vol.%, and preferably <37.0 vol.%. The electrocatalyst layer can have a porosity in a range comprising a combination of any of the aforementioned upper and lower limits. The porosity can be determined by mercury intrusion porosimetry, for example, as described in the examples. The electrocatalyst layer can have a peak logw differential intrusion volume (for pores having a pore diameter in a range of 10 nm to 1 pm) of <1 cm3 / g, suitably <0.8 cm3 / g, suitably <0.7 cm3 / g, preferably <0.6 cm3 / g, suitably <0.55 cm3 / g. The electrocatalyst layer can have a peak logw differential intrusion volume (for pores having a pore diameter in a range of 10 nm to 1 pm) of >0.3 cm3 / g, suitably >0.35 cm3 / g, preferably >0.40 cm3 / g, more preferably 0.45 cm3 / g. The peak logw differential intrusion volume (for pores having a pore diameter in a range of 10 nm to 1 pm) can be in a range comprising a combination of any of the aforementioned upper and lower limits. The logw differential intrusion volume can be determined using mercury intrusion porosimetry, for example, as described in the examples. The pores of the electrocatalyst layer can have an entrapment factor in a range of 35 vol.% to 65 vol.%. Suitably, the entrapment factor is less than 60 vol.%, preferably less than 55 vol.%. Suitably, the entrapment factor is at least 40 vol.%, preferably at least 45 vol.%. The entrapment factor can be in a range comprising any combination of the aforementioned upper and lower limits. For example, the entrapment factor can be in a range of 40 to 60 vol.%, preferably 45 to 55 vol.%. The entrapment factor can be determined using the methods described in the examples. The pores of the electrocatalyst layer suitably have a median pore diameter in a range of 80 nm to 110 nm. Preferably, the pores have a median pore diameter of at least 85 nm, suitably at least 90 nm, and preferably at least 92 nm. Preferably, the pores have a median pore diameter of <100 nm. Without wishing to be bound by any theory or conjecture, it is believed that if the median pore diameter is too small, mass transport of reactants and products through the electrocatalyst layer is impeded, which can negatively affect ethylene selectivity. The median pore diameter can be calculated using mercury intrusion porosimetry, for example, as described in the examples. A plot of logw differential intrusion volume (in cm3 / g) as a function of pore diameter (in A) (using a log scale) for the electrocatalyst layer typically includes a major peak (Pcat) corresponding to pores having a pore diameter in a range of 10 nm to 1 pm, suitably 30 nm to 200 nm. The peak Pcat can have a substantially unimodal distribution, e.g. consisting of a single peak (Pi) in a range of about 50-75 nm. Preferably, the peak Pcat can comprise a substantially bimodal distribution, e.g. comprising a first peak (Pi) positioned at a pore diameter in a range of 50-75 nm; and a second peak (P2) positioned at a pore diameter in a range of 80-110 nm. Without wishing to be bound by any theory or conjecture, non-spherical electrocatalyst particles (e.g. plate-like shaped particles) can give rise to a bimodal pore size distribution due to their non-spherical shape. The peaks Pi and P2 can overlap with each other, whereby one of the peaks appears as a shoulder peak. Data fitting methods, as is known in the art and as described below, can be used to deconvolute the peaks Pi and P2. Electrocatalyst layers having a substantially bimodal pore size distribution (in a range of 10 nm to 300 nm) tended to exhibit an improved selectivity towards ethylene production in preference to hydrogen evolution (i.e. higher FEc2H4 / FEH2). Preferably, the ratio (Areap2 / Areap2) of the area under Peak P2 (Areap2) to the area under Peak Pi (Areapi) is >0, suitably >0.10, preferably >0.20. Suitably the ratio Areap2 / Areap2 is <1.10, preferably <1.00, and more preferably <0.90. The ratio Areap2 / Areap2 can be in a range comprising any combination of the aforementioned upper and lower limits. For example, the ratio Areap2 / Areap2 can be in a range of >0 to <1.10, preferably 0.20 to 1.00, more preferably 0.30 to 0.90, suitably 0.35 to 0.85. Electrocatalytic component It is known that during the initial operation of an electrolyser a pre-electrocatalyst (e.g. a Cu(ll)- or Cu(l)-containing pre-electrocatalyst) is converted into a reduced electrocatalyst (e.g. a Cu(0)-containing electrocatalyst). The term “electrocatalytic component” is used herein to encompass either the pre-electrocatalyst or the (reduced) electrocatalyst, for example, when referring to features that are applicable to both the pre-electrocatalyst and the (reduced) electrocatalyst. As such, the electrocatalytic component is a pre-electrocatalyst or an electrocatalyst. The electrocatalytic component can be supported or, preferably, unsupported. The choice of electrocatalytic component is not particularly limited provided that the electrocatalytic component is suitable for the electrochemical reduction of CO2 or CO to C2+ products. Preferably, the electrocatalytic component comprises copper. In some embodiments, the electrocatalytic component comprises copper and a metal (M). The metal (M) is suitably a lanthanide, a transition metal other than copper, or a Group 13 element (e.g. Al, Ga, In). 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, 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. In some embodiments, the electrocatalytic component comprises copper but is substantially devoid of metal (M). The pre-electrocatalyst suitably comprises copper (II) oxide. The pre-electrocatalyst can comprise a mixture of copper (II) oxide and a metal (M) or a metal (M) compound. Preferably, the pre-electrocatalyst comprises a mixed oxide of copper (II) oxide and a metal oxide. The term “mixed oxide” as used herein encompasses an oxide that contains cations of more than one chemical element, for example at least Cu and M in this case. The mixed oxide may be in a single phase, for example, as measured by XRD. The mixed oxide can be a solid solution. The term “solid solution” as used herein encompasses a mixture of two crystalline solids that coexist as a new crystalline solid, or crystal lattice, for example. In other words, the mixed oxide is typically substantially one phase, rather than a mixture of different phases. The pre-electrocatalyst is typically converted by reduction to a reduced electrocatalyst during initial operation of the electrolyser. For example, for a pre-electrocatalyst comprising copper (II) oxide, the majority of the copper (II) oxide is converted to copper (0). The term “majority” as used here means >50 at% of the copper is present as copper (0), typically >80 at%, such as >90 at%. The following preferred embodiments apply to both the pre-electrocatalyst and the reduced electrocatalyst. The electrocatalytic component typically comprises particles, i.e. particles of the preelectrocatalyst or particles of the electrocatalyst. The electrocatalytic component can comprise an aggregate or agglomerate of particles. The particles can be non-spherical. Preferably, the particles can be elongate. The particles can be plate-like. The particles can have a high aspect ratio, for example, the particles can have an aspect ratio in a range of 1:1 to 14:1, as determined by microscopy techniques, such as SEM. Suitably, the aspect ratio is greater than 3:1, suitably greater than 4:1. The particles can have a mode average largest cross-sectional dimension (e.g. length) in a range of 200 nm to 700 nm, preferably 300 nm to 600 nm, more preferably 400 nm to 500 nm, as measured by microscopy techniques such as SEM. The mode average largest cross-sectional dimension can be in a range comprising any combination of the aforementioned upper and lower limits. The particles can have a mode average cross-sectional width in a range of 50 nm to 200 nm, preferably 75 nm to 175 nm. and more preferably 80 nm to about 150 nm. The mode average cross-sectional width can be in a range comprising any combination of the aforementioned upper and lower limits. The electrocatalyst component can comprise a (porous) interconnected network of the particles, and preferably a (porous) interconnected network of plate-like particles. The pre-electrocatalyst can comprise CuO commercially available from Sigma Aldrich. Pre-electrocatalysts comprising copper and a metal (M) can be prepared according to methods disclosed in WO2023 / 156800, for example. Ion-conducting polymer The ion-conducting polymer can be a proton conducting polymer or an anion conducting polymer, such as a hydroxyl anion-conducting polymer. Preferably the ion-conducting polymer is a proton conducting polymer. The ion-conducting polymer comprises ion exchange groups, such as sulfonic acid groups. An “ion exchange group” is a group in which an ion (e.g. cation) contained in the group can be exchanged with another ion (e.g. cation). Preferably, the ion exchange group is a cation exchange group. Preferably the ion exchange groups are sulfonic acid groups. A “sulfonic acid group” is a general term for a salt-type sulfonic acid group (-SOs-Z+, wherein Z+ is H+, a monovalent metal ion, or an ammonium ion in which at least one hydrogen atom may be substituted by a hydrocarbon group) and an acid-type sulfonic acid group (-SO3H). Suitably, the ion-conducting polymer is a perfluorinated sulfonic acid ionomer, or a partially-fluorinated or non-fluorinated hydrocarbon sulfonic acid ionomer. Examples of suitable proton-conducting polymers include partially- or fully-fluorinated sulfonic acid polymers, such as perfluorosulfonic acid ionomers (e.g. Nation® (Chemours), Aciplex® (Asahi Kasei), Aquivion™ (Syensqo), Flemion® (Asahi Glass Co.), and from 3M; or ionomers based on a sulphonated hydrocarbon such as those available from FuMA-Tech GmbH as the fumapem® P, E or K series of products, and others. Examples of suitable anion-conducting polymers include A901 made by Tokuyama Corporation and Fumasep FAA from FuMA-Tech GmbH, Selemion™ (AGC Inc.), Sustainion™ (Dioxide Materials), PiperlON™ (Versogen™). Typically, the ion-conducting polymer has an equivalent weight of about 1200 or less, typically about 1100 or less. Typically, the ion-conducting polymer has an equivalent weight of at least about 450, suitably at least 600, and suitably at least 700. The ion-conducting polymer can have an equivalent weight a range comprising any combination of the aforementioned upper and lower limits. Electrocatalyst ink The electrocatalyst layer of the present disclosure is suitably formed from an electrocatalyst ink comprising the electrocatalytic component (e.g. the pre-electrocatalyst), the ion-conducting polymer, and optionally a hydrophobic additive, dispersed in a solvent or solvent mixture. The total solids content in the electrocatalyst ink is suitably less than 50 wt.% based on the total weight of the electrocatalyst ink, suitably less than 30 wt.%, and preferably less than 15 wt.%. The solvent can comprise water and / or an organic solvent, such as an alcohol. The solvent mixture can comprise water and an organic solvent, such as an alcohol. Preferably, the organic solvent is miscible with water. Preferably, the solvent mixture can comprise water or can comprise water and at least one alcohol selected from methanol, ethanol, n-propanol and iso-propanol. In some embodiments, the solvent can consist essentially of or consist only of water. The choice and relative amounts of solvent can be determined by the skilled person depending on the desired coating method and the ink properties (e.g. viscosity) required. Methods of preparing an electrocatalyst layer A method of preparing an electrocatalyst layer typically comprises the steps of: depositing an electrocatalyst ink (as described above) onto a substrate to form a wet electrocatalyst layer; and drying the wet electrocatalyst layer to form the electrocatalyst layer. The step of depositing the electrocatalyst ink onto the substrate can comprise coating the substrate with the electrocatalyst ink. Suitable methods of depositing an electrocatalyst ink onto a substrate can include spray coating, slot die (slot, extrusion) coating (whereby the electrocatalyst ink is squeezed out under pressure via a slot onto the substrate), screen printing, rotary screen printing, inkjet printing, painting, bar coating, pad coating, gap coating techniques such as knife or doctor blade over roll (whereby the coating is applied to the substrate then passed through a split between the knife and a support roller), and metering rod application such as with a Meyer bar. The substrate can be an ion-conducting membrane, such as an electrolyte membrane. The substrate can be a gas diffusion layer. The substrate can be a decal transfer substrate, such as a PTFE sheet. The step of drying typically comprises heating the wet electrocatalyst layer at an elevated temperature (e.g. 60 °C) until substantially all (e.g. >99 wt.%, and preferably 100 wt.%) of the solvent is removed. CO2 electrolyser The electrocatalyst layer of the disclosure can be used as a cathode electrocatalyst layer of a CO2 electrolyser for converting CO2 to C2+ products, such as ethylene. A typical CO2 electrolyser includes a gas diffusion electrode (GDE) and / or a catalyst coated membrane (CCM). Various arrangements are possible, all of which may benefit from using the electrocatalyst layer of the disclosure on the cathode side. Figures 1-3 show schematic diagrams of electrolysers comprising 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, for example, the membrane may be an anion exchange membrane. Figure 1 shows an electrolyser comprising an electrocatalyst layer according to an embodiment of the disclosure. The electrolyser comprises a (cathode) gas diffusion electrode, an ion exchange membrane and an anode catalyst layer. The gas diffusion electrode comprises an electrocatalyst layer according to this disclosure. The anode catalyst layer is separated from the ion exchange membrane by an electrode gap. Figure 2 shows a further embodiment of an electrolyser comprising an electrocatalyst layer according to the disclosure. The electrolyser comprises a (cathode) gas diffusion electrode, a catalyst coated membrane (CCM). The CCM comprises an ion-exchange membrane an anode catalyst layer applied to one side of the ion-exchange membrane. The gas diffusion electrode comprises an electrocatalyst layer according to this disclosure. In this embodiment, a porous transport layer (PTL) contacts the anode electrocatalyst layer (on the side facing away from the ion-exchange membrane). Figure 3 shows a further embodiment of an electrolyser comprising an electrocatalyst layer according to the disclosure. The electrolyser comprises a CCM which comprises an ionexchange membrane, an anode electrocatalyst layer applied to a first face of the ion-exchange membrane, and a cathode electrocatalyst layer applied to a second face of the ion-exchange membrane. In the embodiment shown in Figure 3, a porous transport layer (PTL) contacts each of the anode and cathode electrocatalyst layers (on the side facing away from the ionexchange membrane). Accordingly, a catalyst coated membrane can comprise an ion-exchange membrane having an anode and a cathode side, and an electrocatalyst layer as defined herein applied to 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 an electrocatalyst layer. The term “anode side” refers to the side at which the anode reaction (e.g. OER) occurs. The anode may also be referred to as an “oxygen electrode”. The term “cathode side” refers to the side at which the C02RR 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 electrocatalyst layer (according to the disclosure) and on the anode side face with an anode electrocatalyst layer. This arrangement is shown in Figure 3. In another embodiment, CCM is coated only on the cathode face with a (cathode) electrocatalyst layer according to the disclosure. The cathode electrocatalyst layer and anode electrocatalyst layer may be applied to the ion-exchange membrane by any techniques known to those skilled in the art, such as by using an ink or a decal. In one aspect the invention relates to a gas diffusion electrode comprising a gas diffusion layer and an electrocatalyst layer (according to the disclosure) on the gas diffusion layer. In one embodiment the gas diffusion electrode comprises the electrocatalyst layer on a microporous layer. 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. It will be understood that the cathode electrocatalyst may be present in the cathode electrocatalyst layer on the cathode side (e.g. gas diffusion layer on the cathode side or cathode catalyst layer on the ion exchange membrane) either as a pre-electrocatalyst or a reduced electrocatalyst. The pre-electrocatalyst may be reduced to the reduced electrocatalyst before operating the electrolyser for the first time, or may be reduced in situ during start up. Examples General method for preparing an electrocatalyst layer An electrocatalytic component for CO2 reduction, such as copper(ll) oxide particles, an aqueous solution of an ion-conducting polymer (e.g. Nation®, EW 1100) and optionally a hydrophobic additive (e.g. polytetrafluoroethylene (PTFE)) were dispersed in a water-alcohol solvent mixture and probe-sonicated for 10 mins to form an electrocatalyst ink (total solids content 8-10 wt.% of total weight of the electrocatalyst ink). The electrocatalyst ink was then spray-coated onto a carbon gas diffusion layer (Freudenberg H23C8) placed in a hot plate at 120 °C, to produce a gas diffusion electrode with a desired catalyst areal loading. The ratio of the total number of moles of ion exchange groups in the ion-conducting polymer to the total number of moles of the electrocatalytic component was varied by varying the relative amount of the electrocatalytic component and the ion-conducting component in the electrocatalyst ink. The weight ratio of the hydrophobic additive to the electrocatalytic component was varied by varying the relative amounts of the hydrophobic additive and the electrocatalytic component in the electrocatalyst ink. For Examples 1-9 and Comparative Examples 1-9, the electrocatalytic component was copper(ll) oxide, the ion-conducting polymer was Nation® with an equivalent weight of 1100, and the hydrophobic additive (where present) was PTFE. Contact angle measurement Contact angles were measured at room temperature by dropping 1 M KHCO3 solution onto the electrocatalyst layer. Contact angle measurements were measured using a Drop Shape Analyser DSA30, which is commercially available from Kruss Scientific. Mercury intrusion porosimetry measurement The pore volume and pore size distribution can be determined using mercury intrusion porosimetry. The determination of the pore volume and pore size by mercury intrusion porosimetry can be carried out by the following process. The gas diffusion electrode (i.e. comprising gas diffusion layer and an electrocatalyst layer) to be measured was cut into strips which were stacked and rolled prior to loading into a specialised sample holder known as a penetrometer. The penetrometer containing the strips was mounted into a Micromeritics Autopore IV 9520 mercury porosimeter and the mercury pressure increased from ~3.0 to 60,000 psia in small steps, with accompanying measurements of the volume of mercury intruded into the sample, derived from capacitance changes measured along the stem of the penetrometer. The pore size distribution was then calculated from the Washburn equation, assuming a contact angle for Hg of 130°, which relates the applied pressure to the diameter of the pores into which mercury is intruded, thereby giving the amount of porosity in pores from ~60 pm to 3 nm in diameter. The intrusion curves were corrected for the gas diffusion layer by measuring samples of the bare gas diffusion layer in the penetrometer over the same pressure range (~3.0 to 60,000 psia). The resulting apparent volume of intrusion, due to the gas diffusion layer, was subtracted from the data for the gas diffusion electrode to ensure that no apparent pore volume due to the gas diffusion layer was assigned to the electrocatalyst layer. The porosity at <1 pm and >3 nm pore diameter is selected as the appropriate pore size range for calculating porosity characteristics in the layers of this disclosure. This avoids misleading information from large, inhomogeneous features such as cracks or voids. To provide an indication of the structure and tortuosity of the pores of the electrocatalyst layer, an “entrapment factor” was calculated as follows: Volume of Hg retained in pores after extrusion Entrapment factor (vol. %) = -------------------------------—--------- Total volume of Hg intruded The entrapment factor is correlated to the tortuosity of the pores. A high entrapment factor is indicative of high tortuosity and associated with species having a high residence time in the electrocatalyst layer. A low entrapment factor is indicative of low tortuosity and associated with species having a low residence time in the electrocatalyst layer. Data fitting of mercury intrusion porosimetry measurements A plot of logw differential intrusion volume as a function of pore diameter (on a log scale) typically includes a major peak (Pcaf) associated with pores having a pore diameter in a range of 10 nm to 1 pm, suitably 30 nm to 200 nm. The peak Pcat can have a substantially unimodal distribution, e.g. consisting of a single peak (Pi) in a range of about 50-75 nm. In some embodiments, the peak Peat comprises a substantially bimodal distribution, e.g. comprising a first peak (Pi) positioned at a pore diameter in a range of 50-75 nm; and a second peak (Pf) positioned at a pore diameter in a range of 80-110 nm. Without wishing to be bound by any theory or conjecture, non-spherical electrocatalyst particles (e.g. plate-like shaped particles) can give rise to a bimodal pore size distribution. The peaks Pi and P2 can overlap with each other, whereby one of the peaks appears as a shoulder peak. Data fitting methods, as is known in the art, can be used to deconvolute the peaks Pi and P2. In the examples disclosed herein, data fitting was performed using Fityk™ software (version 1.3.1, available as open-source software for nonlinear curve fitting and data analysis). First, the plot of logw differential intrusion volume as a function of pore diameter (on a log scale) was baseline corrected (0 to 1600 A) using a spline function (using at least 10, preferably at least 15 data points) as is known in the art. For substantially unimodal pore size distributions (i.e. where Pcat consists of Pi), the baseline-corrected data was fitted with a Pseudo-Voigt function to determine the peak position (dif full width half maximum (FWHM), baseline-corrected area under the curve, and a shape factor by using an iterative approach to minimise the residuals, which is a measure of closeness of fit between the experimental data and the theoretical model. For substantially bimodal pore size distributions, the baseline-corrected data was fitted with two Pseudo-Voigt functions to deconvolute the overlapped peaks. The Pseudo-Voigt function is an approximation for the Voigt function, which is a convolution of Gaussian and Lorentzian (Cauchy) function, as is known in the art. The first Pseudo-Voigt function had a peak position (di) centred in a range of about 650-750 A. The second Pseudo-Voigt function had a peak position (dz) centred in a range of about 800-1100 A, typically 900-1050 A. An iterative fitting approach was used to minimise the residuals, thereby determining the peak position, FWHM, baseline-corrected area under the curve, and a shape factor for each peak Pi and P2. General procedure for electrochemical testing 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 1. 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 cm2 for 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 / cm2 to -300 mA / cm2 with an increment of 50 mA / cm2. Experiments were performed at each current density for 20 mins. 2 GC runs were collected during this 20 mins and averaged out for FE calculation. Liquid products were collected at the end of each galvanostatic 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), e.g. H2, was determined using Equation 1: FE^as (%) =----——— x 100% Equation 1 9 Vm x Jtotal 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), Qfiow is the volumetric flow rate (mL min-1), Vm is the molar volume of gas (mL mol’1), and jtotai is the total current density (A cm’2). Faradaic Efficiency for liquid products {FEnquid), e.g. ethylene, was determined using Equation 2: FEuqUid (%) = —----X 100% Equation 2 Qtotal where Qtota / 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. The ratio (FEc2h<i / FEh2) of Faradaic efficiency for ethylene (FEc2H4) to Faradaic efficiency for hydrogen (FEw) is used as a measure of the selectivity of the electrocatalyst layer towards ethylene in preference to the hydrogen evolution reaction. Electrode flooding is a significant cause of degradation of the gas diffusion electrode (GDE). In these experiments, the electrode was considered to be flooded when at least one of the following three conditions were satisfied: (i) presence of liquid flowing from gas chamber to liquid trap upon visual inspection during the aforementioned galvanostatic measurements; (ii) presence of liquid or salt deposits on the back side of the gas diffusion electrode (i.e. side facing gas chamber) after the aforementioned galvanostatic measurements; (iii) an increase in FEh2 as a function of time or current density during the aforementioned galvanostatic measurements. Results and discussion Table 1 shows a summary of the electrocatalyst layers for Comparative Examples 1-9 and Examples 1-9, including the ratio (nim / ncat) of the total number of moles of the ion exchange groups on the ion-conducting polymer (n,on) to the total number of moles of the electrocatalytic component (ncar); the electrocatalytic loading in mg / cm2, the ratio of the weight of PTFE to weight of CuO in the electrocatalyst layer; the contact angle for 1 M KHCO3 of the electrocatalyst layer; the ratio (FEc2H4 / FEh2) of Faradaic Efficiency of ethylene (FEc2H4) to the Faradaic Efficiency of hydrogen evolution (FEhz); and whether the electrode flooded during the electrochemical cell testing. Table 1: nion / ricat ratio CuO loading (mg / cm2) PTFE:CuO wt. ratio Contact Angle (°) FEc2H4 / FEh2 Electrode Flooded? Comparative Examples 1 0.000 3.5 0.30 0.0 0.09 Yes 2 0.004 2.0 0.24 72.6 0.26 Yes 3 0.007 3.0 0.20 102.9 0.34 Yes 4 0.007 2.7 0.00 122.2 0.64 No 5 0.010 3.8 0.16 117.3 0.63 No 6 0.005 0.9 0.06 122.8 0.80 No 7 0.015 2.7 0.09 122.8 0.84 No 8 0.036 2.0 0.00 144.6 1.00 No 9 0.019 3.6 0.03 114.2 1.19 Yes Examples 1 0.012 1.5 0.13 117.9 1.19 No 2 0.022 1.8 0.00 124.0 1.20 No 3 0.007 0.9 0.02 129.1 1.21 No 4 0.022 2.3 0.00 123.9 1.50 No 5 0.017 0.9 0.06 132.1 1.54 No 6 0.018 0.7 0.11 121.4 2.22 No 7 0.024 0.8 0.00 142.6 2.23 No 8 0.033 0.7 0.00 137.3 2.90 No 9 0.029 0.7 0.00 134.6 2.91 No Despite using the same CuO electrocatalytic component in all examples and comparative examples, the inventors have surprisingly found that a sufficiently low loading of the 5 electrocatalytic component in combination with a ratio (n / bn / ncat) of the total number of moles of the ion exchange groups on the ion-conducting polymer (n»n) to the total number of moles of the electrocatalytic component (ncat) in a range of 0.0007 to 0.035 provides an electrocatalyst layer exhibiting improved selectivity towards ethylene production in an electrochemical CO2 reduction reaction. For example, Comparative Example 1 has a loading 10 of the electrocatalytic component of more than 2.0 mg / cm2 and a nion / nCat ratio outside the range of 0.007 to 0.035 and exhibits a very low FEc2H4 / FEh2. As a further example, Comparative Examples 2, 6 and 8 have a loading of the electrocatalytic component of 2.0 mg / cm2 or less, however, the nion / ncai ratio in these comparative examples falls outside the range of 0.007 to 0.035 and these comparative examples exhibit unsatisfactory selectivity towards ethylene production. Comparative Examples 3 to 5, and 7 have a njOn / ncat ratio within the range of 0.010 to 0.019, but have an electrocatalytic component loading of greater than 2.5 mg / cm2, and these comparative examples exhibit unsatisfactory selectivity towards ethylene production. On the other hand, Examples 1 to 9 demonstrate that when both the loading of the electrocatalytic layer is sufficiently low and the niOn / nCat ratio is sufficiently high (up to a value of 0.035) the selectivity towards ethylene production (in preference to the hydrogen evolution reaction) is increased. Particularly high selectivity towards ethylene production was observed when the loading of the electrocatalytic component was less than 1.0 mg / cm2 and the nion / ncat ratio was in a range of 0.017 to 0.033. Appropriate combination of loading of the electrocatalytic component and niOn / nCat ratio provides improved ethylene selectivity of the electrochemical CO2 reduction reaction. However, the reasons for this are not fully understood. Without being bound by any theory or conjecture it is believed that suitable selection of these parameters enables the reactants and intermediates of the electrochemical CO2 reduction reaction to reside in the cathode for a suitable period to facilitate C-C coupling, while also favouring easy removal of reaction products. The ion-exchange groups present on the ion-conducting polymer interact with the electrocatalytic surface of the electrocatalytic component. Having a suitably high amount of ion-exchange groups, relative to the amount of electrocatalytic component, provides an optimum configuration to facilitate ionic and mass transport, e.g. to allow reactants and products to reach and leave the catalytic surface, while also optimising ionic conductivity. The hydrophobicity of the electrocatalyst layer is associated with the amount of ionconducting polymer (which typically comprises a hydrophobic fluorinated main chain) and any hydrophobic (non-ion conducting) additive, such as PTFE, present in the layer. Comparative Examples 1-3 and 9 had a contact angle of 114.2° or less. In each case, the electrode flooded during the electrochemical testing. However, when the contact angle was above this value (i.e. more hydrophobic), the electrode did not flood in the electrochemical testing. Without wishing to be bound by any theory or conjecture, it is believed that a suitably high contact angle can help to prevent electrolyte from permeating into the pores of the cathode, and can therefore increase the tolerance of the cathode to flooding. Suitably, the contact angle is in a range of 115° to 145°, and preferably in a range of 117° to 143°. Mercury intrusion porosimetry was performed on Comparative Examples 1,2,3, 5, 7, and 9 and Examples 1, 2, 5 and 6. The results are summarised in Table 2 below. The corrected intrusion volume corresponds to the pore volume of the electrocatalyst layer, and has an uncertainty of ±1 %. The median pore diameter has an uncertainty of ±0.2 %. Entrapment factor is calculated as described above. The “peak logw differential intrusion volume” shown in Table 2 is the maximum logw differential intrusion volume for pores having a pore diameter in a range of from 10 nm to 1 pm. Figure 4 shows a plot of logw differential intrusion volume (cm3 / g) as a function of pore diameter (log scale), for pores having a pore diameter in a range of about 10 nm to about 500 nm for Comparative Examples 3 and 5, and Examples 1, 2 and 5. Comparative Examples 1,2,3, and 5 had a unimodal pore size distribution. Comparative Examples 7 and 9, and Examples 1,2,5 and 6 had a bimodal pore size distribution. The peak positions, FWHM, shape factor and area under the curve were determined using the data fitting methods described above. The fitting parameters are also included in Table 2 below. Electrocatalyst layers having a lower peak logw differential intrusion volume tended to exhibit improved selectivity towards ethylene production in preference to hydrogen evolution (i.e. higher FEc2H4 / FEh2). Preferably, the peak logw differential intrusion volume is less than 0.60, and more preferably in a range of 0.49 to 0.60. Electrocatalyst layers having a substantially bimodal pore size distribution (in a range of 10 nm to 300 nm) tended to exhibit and improved selectivity towards ethylene production in preference to hydrogen evolution (i.e. higher FEc2hj / FEh2). Preferably, the ratio (Areap2 / Areap2) of the area under Peak 2 (Areap2) to the area under Peak 1 (Areapi) is >0, preferably >0.20. Suitably the ratio Areap2 / Areap2 is <1.10, preferably <1.00, and more preferably <0.90. Favourable ethylene selectivity was observed for electrocatalyst layers exhibiting an entrapment factor in a range of about 35-51 vol.%, suitably 46-51 vol.%. Without wishing to be bound by any theory of conjecture, it is believed that if the entrapment factor is too low, the residence time of the reactive species in the electrocatalyst layer is not sufficient for favourable C-C coupling. If the entrapment factor is too high, the mass transport of reactant to the electrocatalyst and reaction product away from the electrocatalyst is poor, which can increase contribution of the hydrogen evolution reaction (i.e. higher FEh2). Table 2: Corrected intrusion volume (cm3 / g) Entrap, factor (%v / v) Median pore diameter (A) Porosity (vol.%) Peak log io differential intrusion volume (cm3 / g) Areapi (A.cm3 / g) Comp. Ex. 1 0.342 44 905 39.8 0.65 121 Comp. Ex. 2 0.312 42 841 35.9 0.62 155 Comp. Ex. 3 0.314 39 851 37.4 0.67 239 Comp. Ex. 5 0.315 59 875 38.9 0.60 281 Comp. Ex. 7 0.283 54 948 33.9 0.48 105 Comp. Ex. 9 0.286 56 932 35.9 0.49 146 Example 1 0.284 51 965 33.0 0.49 160 Example 2 0.297 49 927 36.3 0.52 159 Example 5 0.296 46 945 33.3 0.53 140 Example 6 0.313 46 848 35.3 0.52 104 Peak 1 (Pi) Peak 2 (P2) Peak Position w FWHM (A) Shape factor Areap2 (A.cm3 / g) Peak Position (½) FWHM (A) Shape factor Areapi / Areapz 676 199 0.165 Unimodal 648 135 0.334 Unimodal 748 196 0.117 Unimodal 802 220 0.200 Unimodal 689 156 0.059 113 983 171 0.316 1.07 686 177 0.440 135 981 191 0.200 0.93 703 185 0.104 82 1023 164 0.361 0.51 699 172 0.255 60 999 151 0.205 0.38 692 173 0.179 116 999 177 0.248 0.83 575 126 0.339 68 810 276 0.200 0.65

Claims

1. An electrocatalyst layer for electrochemical reduction of carbon dioxide or carbon monoxide to form a C2+ product, the electrocatalyst layer comprising:an electrocatalytic component which is an electrocatalyst or a pre-electrocatalyst; andan ion-conducting polymer comprising ion-exchange groups;wherein the electrocatalytic component is present in the electrocatalyst layer at an areal loading of 2.5 mg / cm2 or less, a ratio (niOn / nCat) of the total number of moles of the ion exchange groups (nkm) to the total number of moles of the electrocatalytic component (ncai) is in a range of 0.007 to 0.035, and the electrocatalyst layer has a contact angle for 1 M KHCO3 of greater than 115°.

2. An electrocatalyst layer according to claim 1, wherein the electrocatalytic component is present in the electrocatalyst layer at an areal loading of 2.3 mg / cm2 or less.

3. An electrocatalyst layer according to claim 2, wherein the electrocatalytic component is present in the electrocatalyst layer at an areal loading of 1.0 mg / cm2 or less.

4. An electrocatalyst layer according to any of claims 1 to 3, wherein the ratio of the total number of moles of the ion exchange groups to the total number of moles of the electrocatalytic component is in a range of 0.012 to 0.035, preferably 0.017 to 0.033.

5. An electrocatalyst layer according to any previous claim, wherein the electrocatalyst layer has a contact angle of >117°, preferably >120°.

6. An electrocatalyst layer according to any previous claim, wherein the electrocatalytic component comprises particles of the electrocatalyst or the pre-electrocatalyst.

7. An electrocatalyst layer according to any previous claim wherein the particles of the electrocatalyst component comprise plate-like particles.

8. An electrocatalyst according to any previous claim, wherein the electrocatalytic component comprises an interconnected network of the particles, preferably an interconnected network of the plate-like particles.

9. An electrocatalyst according to any previous claim, wherein the electrocatalytic component comprises copper.

10. An electrocatalyst layer according to any previous claim, wherein the preelectrocatalyst comprises an oxide of copper, a mixed metal oxide comprising copper, or a mixture of a copper oxide and a metal oxide.

11. An electrocatalyst layer according to any previous claim, wherein the electrocatalyst comprises Cu(0), a copper alloy, or copper doped with a dopant.

12. An electrocatalyst layer according to any previous claim, wherein the ion-conducting polymer is a proton conducting polymer.

13. An electrocatalyst layer according to any previous claim, wherein the ion-exchange groups are sulfonic acid groups.

14. An electrocatalyst layer according to any previous claim, wherein the ion-conducting polymer is a partially- or fully-fluorinated sulfonic acid polymer.

15. An electrocatalyst layer according to any previous claim, wherein the electrocatalyst layer further comprises a fluoropolymer, such as polytetrafluoroethylene (PTFE).

16. An electrocatalyst layer according to any previous claim wherein the electrocatalyst layer comprises pores.

17. An electrocatalyst layer according to claim 16, wherein the electrocatalyst layer has a pore volume in a range of 0.10 cm3 / g to 0.50 cm3 / g.

18. An electrocatalyst layer according to claim 16 or 17, wherein the electrocatalyst layer has a peak logw differential intrusion volume for pores having a pore diameter in a range of 10 nm to 1 pm of less than 1.0 cm3 / g.

19. An electrocatalyst layer according to any of claims 16 to 18, wherein the pores have an entrapment factor in a range of 35 to 55 vol.%.

20. An electrocatalyst layer according to any of claims 16 to 19, wherein the pores have a bimodal pore diameter distribution.

21. An electrocatalyst layer according to any previous claim, wherein the electrocatalyst layer is substantially devoid of cracks having a smallest cross-sectional dimension of >1.5 pm.

22. A gas diffusion electrode comprising a gas diffusion layer and the electrocatalyst layer according to any of claims 1 to 21.

23. A catalyst coated membrane comprising the electrocatalyst layer according to any of claims 1 to 21.

24. A membrane electrode assembly comprising the electrocatalyst layer according to any of claims 1 to 21, the gas diffusion electrode according to claim 22, or a catalyst coated membrane according to claim 23.

25. An electrolyser for electrochemically converting CO2 into a C2+ product comprising the electrocatalyst layer according to any of claims 1 to 21, the gas diffusion electrode according to claim 22, a catalyst coated membrane according to claim 23, or the membrane electrode assembly according to claim 24.

26. A method of manufacturing an electrocatalyst layer suitable for electrochemical reduction of carbon dioxide or carbon monoxide to form a C2+ product, the method comprising the steps of:providing an electrocatalyst ink comprising an electrocatalytic component which is an electrocatalyst or a pre-electrocatalyst; an ion-conducting polymer comprising ion-exchange groups; and a solvent; wherein a ratio (nim / ncat) of the total number of moles of the ionexchange groups (nkm) to the total number of moles of the electrocatalytic component (neat) is in a range of 0.007 to 0.035; anddepositing the electrocatalyst ink onto a substrate to provide a wet electrocatalystlayer in which the electrocatalytic component is present at an areal loading of 2.5 mg / cm2 or5 less; anddrying the wet electrocatalyst layer.27

Images