Patent Description:
Electrochemical carbon dioxide reduction (CO<NUM>R) offers an attractive route to upgrade greenhouse gases such as CO<NUM> to valuable fuels and feedstocks. However, today it is curtailed at least in part by the limits of having a high selectivity into specific products. Achieving a narrow product distribution with cheap CO<NUM>R catalysts is challenging and conventional material modifications offer limited control. In particular, in the case of formate (HCOO-) while a high selectivity has been reported by using a gold-based CO<NUM>R catalyst, it could be interesting to achieve such high selectivity with cheaper CO<NUM>R catalysts. Indeed, formate is a high energy density and cost-beneficial product and can be used as a hydrogen storage material and as an energy source in direct formate fuel cells. Formate also has wide applications in the textile, leather and pharmaceutical industries.

The study of <NPL>) has evaluated Au electrodes functionalized with thiol-tethered ligands for their ability to alter the selectivity of CO<NUM>R reaction. It was found that upon the use of <NUM>-pyridylethylmercaptan-modified Au CO<NUM>R catalyst, the Faradaic efficiency (FE) amounts to <NUM>% at -<NUM> V vs RHE, while on Au surfaces, the formate FE was only <NUM>% at -<NUM> V vs RHE. The partial current density for formate, which is an indication of the formate production rate, has been determined to be as high as -<NUM> mA/cm<NUM> when the pyridylethylmercaptan-modified Au CO<NUM>R catalyst has been used, while on the untreated gold surface, the partial current density was only -<NUM> mA/cm<NUM>. The good selectivity for formate is explained by the protonation of the pyridine moiety of the ligand which is anchored on the gold surface to effectively promote the activation and conversion of CO<NUM> to HCOOH - see for example the study of <NPL>) and to the adsorption of a hydrogen atom on the gold surface which favour the electrophilic attack of the CO<NUM> to yield activated HCO<NUM>*.

The study of <NPL>) describes the electrochemical CO<NUM>R to formate on a Cu/Au bimetallic system. The authors have shown that interactions with gold can turn copper, which by itself is neither selective nor active for the electrochemical CO<NUM> reduction to formate, into an improved catalyst for the same reaction. The Cu/Au bimetallic catalyst described allows for the reduction reaction to achieve a partial current density of <NUM> and a Faradaic efficiency of <NUM>% at -<NUM> V vs RHE.

The study of <NPL>) describes a square-wave electrochemical redox cycling treatment of ultrapure copper foil to produce submicron-thick films rich in (<NUM>)-oriented Cu<NUM>O nanoparticles anchored on Cu for the CO<NUM>R reaction in a high-pressure electrolyser under CO<NUM>-partial pressure of <NUM> to <NUM> MPa. It was observed that at <NUM> MPa, maximum Faradaic efficiency of <NUM>% at -<NUM> V vs RHE in formate was obtained while at high pressure of about <NUM> MPa, the Faradaic efficiency in formate increase up to <NUM>% at -<NUM> V vs RHE. However, at this potential, the partial current density attains only -<NUM> and for attaining a higher partial current density (of about -<NUM> mA/cm<NUM>), it is required to work at -<NUM> V vs RHE which as for consequence to decrease the Faradaic efficiency in formate to about <NUM>%. Such designs, although offering a good selectivity, are hampered due to the requirement of working under high pressure.

The design described in the study of <NPL>) further requires work with supercritical carbon dioxide to control the selectivity into formate.

In the study of <NPL>), it is shown that thiol ligands containing no nitrogen-based heterocycle and introduced by treating polycrystalline Au with an ethanolic solution of thiols, can alter the activity and selectivity of Au for the electrochemical carbon dioxide reduction, suppressing the formation of hydrogen or in other cases the formation of carbon monoxide.

In the study of <NPL>), it is revealed that on sulfur-, selenium- or tellurium-modified copper, the chalcogen adatoms are present on the surface and actively participate in the reaction, either by transferring a hydride or by tethering carbon dioxide thus suppressing the formation of carbon monoxide. Thus, a Faradaic efficiency for formate amounting to about <NUM>% at -<NUM> V vs RHE has been obtained. At this potential, the partial current density was about -<NUM> mA/cm<NUM>. Similar results have been described at -<NUM> V vs RHE in the study of <NPL>) or in the study of <NPL>).

<CIT> discloses copper electrocatalysts for carbon dioxide reduction. <NPL> discloses copper-cysteamine nanoparticles. <NPL> discloses <NUM>-mercaptopyridine adsorbed to gold, silver and copper oxide films. <NPL>; <NPL>; <NPL>; and <NPL> disclose electrochemical CO<NUM> reduction using gas diffusion electrodes.

Although the selectivity and production rate of formate could be considered as being important, there is still room for improvement.

There is a still need for an improvement of catalyst materials for efficient electrochemical carbon dioxide reduction (CO<NUM>R) reactions; in particular, there is still a need for a catalyst that shows improved selectivity for formate; for a system comprising such a catalyst and for process using such catalysts.

One or more of the above needs can be fulfilled by the gas diffusion electrode according to the present disclosure comprising copper nanoparticles functionalized with one or more pyridine-containing ligands being pyridine-containing ligands.

According to a first aspect, the disclosure provides a gas diffusion electrode suitable for carbon dioxide electrolysis, said gas diffusion electrode having a gas diffusion membrane, the gas diffusion electrode further comprising an ink deposited on the gas diffusion membrane; wherein the ink comprises an ion-conducting polymer, said gas diffusion electrode is remarkable in that the ink further comprises a catalyst comprising copper nanoparticles functionalized with one or more pyridine-containing ligands, wherein the one or more pyridine-containing ligands have an anchoring group comprising a sulphur atom tethered to the copper nanoparticles.

It has been found that copper nanoparticles functionalized with one or more pyridine-containing ligands show good results in the electrolysis of carbon dioxide when implemented into a gas-diffusion electrode of a gas-fed flow cell, notably in the selectivity and/or production rate in formate. Indeed, surprisingly, a selectivity into formate equivalent to a faradaic efficiency of at least <NUM>% at -<NUM> mA/cm-<NUM> can be reached.

With preference, the pyridine-containing ligands are selected from <NUM>-pyridylethylmercaptan, <NUM>-mercaptopyridine, <NUM>,<NUM>-dimethyl-<NUM>-mercaptopyridine, <NUM>-mercaptopyridine and any mixture thereof; more preferably, the one or more pyridine-containing ligands are or comprise <NUM>-mercaptopyridine.

For example, the one or more pyridine-containing ligands have a thiol group.

For example, the one or more pyridine-containing ligands are grafted onto the copper nanoparticles. For example, the one or more pyridine-containing ligands are present in a surface concentration ranging from <NUM> nmol cm-<NUM> to <NUM> nmol cm-<NUM> as determined by reductive desorption and UV-visible spectroscopy as set out in the description; preferably, ranging from <NUM> nmol cm-<NUM> to <NUM> nmol cm-<NUM>.

Advantageously, the one or more pyridine-containing ligands are grafted onto the copper nanoparticles and form a monolayer.

In an embodiment, the copper nanoparticles have an average diameter ranging from <NUM> to <NUM> as measured by transmission-electron microscopy, preferably from <NUM> to <NUM>, more preferably from <NUM> to <NUM>.

In an embodiment, the copper nanoparticles comprise facets selected from Cu(<NUM>) facets, Cu(<NUM>) facets and any mixture thereof.

Advantageously, the catalyst is suitable for electrochemical carbon dioxide reduction (CO<NUM>R) reactions to produce formate.

One or more of the following features advantageously define the gas diffusion membrane of the gas diffusion electrode of the disclosure:.

One or more of the following features advantageously define the ion-conducting polymer of the gas diffusion electrode of the disclosure:.

In an embodiment, the ink layer has a thickness ranging from <NUM> to <NUM> as measured by scanning electron microscopy, preferably from <NUM> to <NUM>, more preferably from <NUM> to <NUM>.

For example, the ink has a weight ratio of the copper nanoparticles functionalized with one or more pyridine-containing ligands over the ion-conducting polymer ranging from <NUM> to <NUM>, preferably from <NUM> to <NUM>, or from <NUM> to <NUM>. With preference, the ink has a weight ratio of the copper nanoparticles functionalized with thiol-tethered ligands over the ion-conducting polymer ranging from <NUM> to <NUM>, preferably from <NUM> to <NUM>, or from <NUM> to <NUM>.

For example, the gas diffusion electrode has a mass loading of the ink onto said gas diffusion membrane ranging from <NUM>/cm<NUM> to <NUM>/cm<NUM>, preferably from <NUM>/cm<NUM> to <NUM>/cm<NUM>.

For example, the copper nanoparticles have an average diameter ranging from <NUM> to <NUM> as measured by transmission electron microscopy, preferably from <NUM> to <NUM>, more preferably from <NUM> to <NUM>.

According to a second aspect, the disclosure provides a method for producing the gas diffusion electrode suitable for carbon dioxide electrolysis as defined according to the first aspect, said method is remarkable in that it comprises the following steps:.

In an embodiment, the solvent is water and step (b) of dispersing the copper nanoparticles to obtain a first dispersion in an aqueous solution.

With preference, the first organic solvent used in step (b) is a polar solvent selected from dichloromethane, ethyl acetate, acetone, dimethylformamide, acetonitrile, n-butanol, n-propanol, methanol, ethanol and any mixture thereof; with preference, the first organic solvent is or comprises dimethylformamide and/or methanol.

With preference, the second organic solvent used in step (d) is selected from dichloromethane, ethyl acetate, acetone, dimethylformamide, acetonitrile, n-butanol, n-propanol, methanol, ethanol and any mixture thereof; with preference, the second organic solvent is or comprises dimethylformamide and/or methanol; more preferably, the second organic solvent is or comprises methanol.

The first organic solvent can be the same or different from the second organic solvent.

For example, step (b) further comprises a sub-step of sonicating the first dispersion. With preference, the sub-step sonicating the first dispersion is performed at room temperature; for example, at a temperature ranging from <NUM> to <NUM>. With preference yet, the sub-step of sonicating the first dispersion is performed for at least <NUM> minutes, preferably for at least <NUM> minutes and/or for at most <NUM> minutes, preferably at most <NUM> minutes.

For example, step (b) and step (c) are performed simultaneously and step (c) of adding one or more pyridine-containing compounds comprises adding a solution of one or more pyridine-containing compounds into the organic solvent used in step (b). With preference, the concentration of the solution of one or more pyridine-containing ligands ranges from <NUM> nmol per mg of Cu to <NUM> nmol per mg of Cu; preferably from <NUM> to <NUM> nmol per mg of Cu; more preferably from <NUM> to <NUM> nmol per mg of Cu; even more preferably from <NUM> to <NUM> nmol per mg of Cu and most preferably from <NUM> to <NUM> nmol per mg of Cu.

For example, steps of washing and drying are performed after step (c) and/or before step (d). With preference, the step of washing is performed with an organic solvent and the step of drying lasts at least <NUM> hours, preferably at least <NUM> hours and/or lasts no more than <NUM> hours, preferably no more than <NUM> hours. The organic solvent used in the step of washing is selected from dichloromethane, ethyl acetate, acetone, dimethylformamide, acetonitrile, n-butanol, n-propanol, methanol, ethanol and any mixture thereof; with preference, the organic solvent used in the step of washing is or comprises dimethylformamide and/or methanol.

For example, the step of depositing the ink onto the gas-diffusion membrane is performed by spray-deposition.

For example, said method further comprises the step (g) of drying the gas diffusion electrode under reduced pressure, for example at a pressure ranging from <NUM> MPa to <NUM>-<NUM> MPa. According to a third aspect, the disclosure provides a gas diffusion electrode obtained by the method according to the second aspect. The third aspect is not claimed.

According to a fourth aspect, the disclosure provides a gas-fed flow cell suitable for carbon dioxide electrolysis, said gas-fed flow cell comprising a gas chamber, a catholyte chamber and an anolyte chamber, wherein said gas chamber is separated from the catholyte chamber by a gas diffusion electrode which is attached to the catholyte chamber by an electrically conductive connection, said gas diffusion electrode having a gas diffusion membrane being comprised within said gas chamber, wherein said catholyte chamber and said anolyte chamber are separated by an anion exchange membrane, and wherein said catholyte chamber and said anolyte chamber comprise respectively a cathode and an anode, said gas-fed flow cell is remarkable in that the gas diffusion electrode is as defined according to the first aspect and/or with the third aspect.

The cathode and the anode are different from the gas diffusion electrode.

Surprisingly, it has been demonstrated that the gas-fed flow cell of the present disclosure, namely comprising the gas diffusion electrode of the first aspect and/or with the third aspect, allows for obtaining good results in the electrolysis of carbon dioxide, notably in the selectivity and/or in the production rate of formate. Thus, it is demonstrated in the present disclosure that selectivity into formate equivalent to a faradaic efficiency of at least <NUM>% at -<NUM> can be reached, preferably at least <NUM>%, and of at least <NUM>% at -<NUM> mA/cm<NUM>, preferably at least <NUM>%.

It is also demonstrated in the present disclosure that the formate production rate is reflected by the partial current density, is of at least <NUM> mA/cm<NUM>, preferably of at least <NUM> mA/cm<NUM>. These results, in the selectivity and/or in the production rate of formate have never been obtained before, especially the results in terms of formate production rate.

With preference, said reference electrode is an Ag/AgCl electrode filled with <NUM> of KCI. With preference, said anode is a Ni foam anode.

Advantageously, the electrically conductive connection from the gas diffusion electrode and the catholyte chamber is achieved by applying copper tape on said gas diffusion electrode, the copper tape being electrically connected to a metallic rod in contact with the catholyte chamber. For example, the metallic rod is a steel rod, preferably a stainless-steel rod.

According to a fifth aspect, the disclosure provides a process for electrolysing carbon dioxide, said process comprising the following steps:.

said process is remarkable in that the gas-fed flow cell provided at step (i) is as defined in the fourth aspect and in that in that said step (iii) is performed by injecting carbon dioxide at a potential gradient starting at -<NUM> V versus a reference electrode and ending at -<NUM> V versus said reference electrode at a sweep rate ranging from <NUM> mV s-<NUM> to <NUM> mV s-<NUM>, the reference electrode being preferably an Ag/AgCl electrode filled with <NUM> of KCI.

For example, the electrolyte flow provided in step (ii) has a flow rate that is comprised between <NUM> min-<NUM> to <NUM> min-<NUM>, preferably from <NUM> min-<NUM> to <NUM> min-<NUM>, more preferably from <NUM> min-<NUM> to <NUM> min-<NUM>.

For example, the electrolyte flow provided in step (ii) is a flow of an aqueous solution of one or more inorganic bases.

With preference, the aqueous solution of one or more inorganic bases has a concentration ranging from <NUM> to <NUM>; preferably from <NUM> to <NUM> or from <NUM> to <NUM>. For example, the aqueous solution of one or more inorganic bases has a concentration that is at least <NUM>.

With preference, the aqueous solution of one or more inorganic bases has a pH ranging from <NUM> to <NUM>.

With preference, the one or more inorganic bases are alkali selected from NaOH, KOH, Ca(OH)<NUM>, LiOH, Mg(OH)<NUM>, RbOH, CsOH and any mixture thereof. With preference, the one or more inorganic bases are or comprise KOH and/or NaOH.

Advantageously, said step (iii) is performed by injecting carbon dioxide at a potential gradient starting at -<NUM> V versus a reference electrode and ending at -<NUM> V versus said reference electrode at a sweep rate ranging from <NUM> mV s-<NUM> to <NUM> mV s-<NUM>, the reference electrode being preferably an Ag/AgCl electrode filled with <NUM> of KCI. With preference, the potential gradient starts at -<NUM> V versus a reference electrode and ends at -<NUM> V versus said reference electrode.

With preference, said step (iv) lasts at least <NUM> hour, more preferably at least <NUM> hours, even more preferably at least <NUM> hours, most preferably at least <NUM> hours, even most preferably at least <NUM> hours or at least <NUM> hours.

For example, the input flow of carbon dioxide provided in step (iv) has a flow rate that is ranging from <NUM> min-<NUM> to <NUM> min-<NUM>, preferably from <NUM> min-<NUM> to <NUM> min-<NUM>, more preferably from <NUM> min-<NUM> to <NUM> min-<NUM>, even more preferably from <NUM> min-<NUM> to <NUM> min-<NUM>.

For example, the input flow of carbon dioxide provided in step (iv) comprises at least <NUM> mol% of carbon dioxide based on the total molar content of the input flow; preferably at least <NUM> mol%.

Advantageously, said step (iv) is performed at room temperature, for example at a temperature ranging from <NUM> to <NUM>.

Advantageously, said step (iv) is performed at atmospheric pressure, for example at a pressure ranging from <NUM> MPa to <NUM> MPa.

With preference, the process is operated with a cathodic voltage no lower than -<NUM>. 5V vs. reversible hydrogen electrode (RHE).

According to a sixth aspect, the disclosure provides for the use of a catalyst for electrochemical carbon dioxide reduction (CO<NUM>R) reactions to produce formate, the use being remarkable in that the catalyst is according to the first aspect. Thus, the disclosure provides for the use of a catalyst for electrochemical carbon dioxide reduction (CO<NUM>R) reactions to produce formate, the use being remarkable in that the catalyst comprises copper nanoparticles functionalized with one or more pyridine-containing ligands, wherein the one or more pyridine-containing ligands have an anchoring group comprising a sulphur atom tethered to the copper nanoparticles.

For example, the one or more pyridine-containing ligands are grafted onto the copper nanoparticles. For example, the one or more pyridine-containing ligands are present in a surface concentration ranging from <NUM> nmol cm-<NUM> to <NUM> nmol cm-<NUM> as determined by reductive desorption and UV-visible spectroscopy; preferably, ranging from <NUM> nmol cm-<NUM> to <NUM> nmol cm-<NUM>.

The particular features, structures, characteristics or embodiments may be combined in any suitable manner, as would be apparent to a person skilled in the art from this disclosure, in one or more embodiments.

For the disclosure, the following definitions are given:
The terms "comprising", "comprises" and "comprised of" as used herein are synonymous with "including", "includes" or "containing", "contains", and are inclusive or open-ended and do not exclude additional, non-recited members, elements or method steps. The terms "comprising", "comprises" and "comprised of" also include the term "consisting of".

The recitation of numerical ranges by endpoints includes all integer numbers and, where appropriate, fractions subsumed within that range (e.g. <NUM> to <NUM> can include <NUM>, <NUM>, <NUM>, <NUM>, <NUM> when referring to, for example, a number of elements, and can also include <NUM>, <NUM>, <NUM> and <NUM>, when referring to, for example, measurements). The recitation of endpoints also includes the recited endpoint values themselves (e.g. from <NUM> to <NUM> includes both <NUM> and <NUM>).

<FIG> illustrates the gas-fed flow cell <NUM> of the present disclosure. Said gas-fed flow cell <NUM> comprises a gas diffusion electrode <NUM> suitable for carbon dioxide electrolysis. The following description first describes the gas diffusion electrode <NUM>.

The gas diffusion electrode <NUM> has a gas diffusion membrane <NUM>, the gas diffusion electrode <NUM> further comprising an ink <NUM> deposited on the gas diffusion membrane <NUM>; wherein the ink <NUM> comprises an ion-conducting polymer, said gas diffusion electrode <NUM> is remarkable in that the ink <NUM> further comprises a catalyst comprising copper nanoparticles functionalized with one or more pyridine-containing ligands, wherein the one or more pyridine-containing ligands have an anchoring group comprising one sulphur atom tethered to the copper nanoparticles. Advantageously, the one or more pyridine-containing ligands are grafted onto the copper nanoparticles and form a monolayer.

In an embodiment, the catalyst comprises copper nanoparticles wherein at least a part of the copper nanoparticles is functionalized with one or more pyridine-containing ligands, the one or more pyridine-containing ligands having an anchoring group comprising one sulphur atom tethered to the copper nanoparticles.

For example, the one or more pyridine-containing ligands have a thiol group before being grafted onto the copper nanoparticles, and thereby become thiol-tethered pyridine-containing ligands.

With preference, the one or more pyridine-containing ligands are selected from <NUM>-pyridylethylmercaptan, <NUM>-mercaptopyridine <NUM>,<NUM>-dimethyl-<NUM>-mercaptopyridine, <NUM>-mercaptopyridine and any mixture thereof. For example, the one or more pyridine-containing ligands are or comprise <NUM>-mercaptopyridine.

In an embodiment, the one or more pyridine-containing ligands are functionalized onto the copper nanoparticles and are present in a surface concentration ranging from <NUM> nmol cm-<NUM> to <NUM> nmol cm-<NUM> as determined by reductive desorption and UV-visible spectroscopy, preferably ranging from <NUM> nmol cm-<NUM> to <NUM> nmol cm-<NUM> or ranging from <NUM> nmol cm-<NUM> to <NUM> nmol cm-<NUM>; or ranging from <NUM> nmol cm-<NUM> to <NUM> nmol cm-<NUM>.

The gas diffusion membrane <NUM> allows for the diffusion of carbon dioxide as the main reactant of the electrolysis reaction into the electrochemical cell and is preferably a hydrophobic porous support. In a gas-fed flow cell <NUM>, the gas diffusion membrane <NUM> is comprised within the gas chamber <NUM> of said gas-fed flow cell <NUM>. With preference, said support shows a pore size ranging from <NUM> to <NUM> as determined by scanning electron microscopy, preferably from <NUM> to <NUM> or from <NUM> to <NUM>. The gas diffusion membrane is preferably selected from an ion-conducting polymer-based membrane, an ion-conducting inorganic material, a combination polymer/inorganic based membrane and the like.

It is preferred that the gas diffusion membrane is a hydrophobic, porous and chemically inert support; with preference, the gas diffusion membrane is not soluble in KOH. For example, the gas diffusion membrane <NUM> is or comprises polytetrafluoroethylene (PTFE). Examples of suitable membranes are commercially available from Fisher Scientific SAS under the commercial denomination Sartorius.

With preference, the gas diffusion membrane <NUM> has a circular shape and/or has a surface area of at least <NUM><NUM>, or of at least <NUM><NUM>. For example, the gas diffusion membrane <NUM> has a thickness ranging from <NUM> to <NUM> measured by scanning electron microscopy, preferably from <NUM> to <NUM>, more preferably from <NUM> to <NUM>.

An ink <NUM> is deposited on the gas diffusion membrane <NUM> and comprises an ion-conducting polymer. With preference, the ion-conducting polymer is or comprises an ionomer. For example, the ion-conducting polymer is or comprises an ionomer with a tetrafluoroethylene backbone group (-CF<NUM>-CF<NUM>-). Such ionomers are capable of creating strongly hydrophobic nanoporous networks. For example, said ion-conducting polymer is or comprises a perfluorinated sulfonic acid, such as Nafion® (tetrafluoroethylene-perfluoro-<NUM>,<NUM>-dioxa-<NUM>-methyl-<NUM>-octenesulfonic acid copolymer); and/or the ion-conducting polymer is or comprises tetrafluoroethylene-perfluoro(<NUM>-hydrophobioxa-<NUM>-pentenesulfonic acid) copolymer, such as Aquivion®. It can form a layer on the gas diffusion membrane, said layer having a thickness ranging from <NUM> and <NUM> measured by transmission electron microscopy, preferably from <NUM> and <NUM>, more preferably from <NUM> and <NUM>. For example, ink <NUM> has a ratio of the copper nanoparticles functionalized with thiol-tethered ligands over the ion-conducting polymer.

With preference, the ink has a weight ratio of the copper nanoparticles functionalized with thiol-tethered ligands over the ion-conducting polymer ranging from <NUM> to <NUM>, preferably from <NUM> to <NUM>, or from <NUM> to <NUM>.

For example, the gas diffusion electrode <NUM> has a mass loading of the ink <NUM> onto said gas diffusion membrane <NUM> ranging from <NUM>/cm<NUM> to <NUM>/cm<NUM>, preferably from <NUM>/cm<NUM> to <NUM>/cm<NUM>. The mass loading can be determined by weighing before and after deposition and drying.

The gas-fed flow cell <NUM> suitable for carbon dioxide electrolysis of the present disclosure will then be described.

The gas-fed flow cell <NUM> comprises a gas chamber <NUM>, a catholyte chamber <NUM> and an anolyte chamber <NUM>. For example, the gas chamber <NUM> has a gas channel <NUM>, through which a flow of CO<NUM> is circulating. The gas chamber <NUM> is separated from the catholyte chamber <NUM> by a gas diffusion electrode <NUM> which is attached to the catholyte chamber <NUM> by an electrically conductive connection. The catholyte chamber <NUM> and the anolyte chamber <NUM> are separated by an anion exchange membrane (AEM) <NUM>. The catholyte chamber <NUM> and the anolyte chamber <NUM> comprise respectively a cathode (not represented) and an anode <NUM>, for example a Ni foam anode. However, any oxygen evolution reaction (OER) catalyst and anode compartment design can be used. The gas-fed flow cell <NUM> of the present disclosure is remarkable in that the gas diffusion electrode <NUM> is as defined above and in that the gas diffusion membrane <NUM> of said gas diffusion electrode <NUM> is comprised within the gas chamber <NUM>. The ink <NUM> comprising the ion-conducting polymer and the copper nanoparticles functionalized with one or more pyridine-containing ligands as described above is comprised within the catholyte chamber <NUM>.

With preference, the cathode is a reference electrode. It is preferred that said reference electrode is an Ag/AgCl electrode filled with KCI at a concentration ranging from <NUM> to <NUM>; preferably from <NUM> to <NUM>; even more preferably with <NUM> of KCI. In other implementation of the invention, the reference electrode could also be a reversible hydrogen electrode (RHE).

Advantageously, the electrically conductive connection from the gas diffusion electrode <NUM> and the catholyte chamber <NUM> is achieved by applying copper tape on said gas diffusion electrode <NUM>, the copper tape being electrically connected to a metallic rod in contact with the catholyte chamber. For example, the metallic rod is a steel rod, preferably a stainless-steel rod.

According to the disclosure, at least a part of the copper nanoparticles is functionalized with one or more pyridine-containing ligands. For example, at least <NUM> wt. % of the copper nanoparticles are functionalized, based on the total weight of the copper nanoparticles; preferably, at least <NUM> wt. %; more preferably at least <NUM> wt. %, and even more preferably at least <NUM> wt. In a preferred embodiment, <NUM> wt. % of the copper nanoparticles are functionalized with one or more pyridine-containing ligands.

The copper nanoparticles are firstly functionalized (i.e., grafted) with one or more pyridine-containing ligands presenting an anchoring group. To achieve that, the copper nanoparticles, provided in step (a), are in a step (b) dispersed in an organic solvent is selected from dichloromethane, ethyl acetate, acetone, dimethylformamide, acetonitrile, n-butanol, n-propanol, methanol, ethanol and any mixture thereof; with preference, the organic solvent is or comprises dimethylformamide and/or methanol.

The first dispersion that is obtained can be then sonicated in a sub-step. With preference, the sonicating sub-step is performed at room temperature, for example at a temperature ranging from <NUM> to <NUM>. In an embodiment, the sonicating sub-step is performed for at least <NUM> minutes, preferably for at least <NUM> minutes and/or for no more than <NUM> minutes, preferably no more than <NUM> minutes. The sub-step of sonicating the first dispersion is preferably performed before step (c).

Thereafter, in step (c), the one or more pyridine-containing ligands, preferably in a solution of the organic solvent, is added to the first dispersion to obtain a suspension. For example, the solution of the one or more pyridine-containing ligands in the organic solvent has a concentration ranging from <NUM> nmol per mg of Cu to <NUM> nmol per mg of Cu; preferably from <NUM> to180 nmol per mg of Cu; more preferably from <NUM> to <NUM> nmol per mg of Cu; even more preferably from <NUM> to <NUM> nmol per mg of Cu and most preferably from <NUM> to <NUM> nmol per mg of Cu.

Said suspension can be sonicated to form copper nanoparticles functionalized with one or more pyridine-containing ligands. For example, the sonication is performed for at least <NUM> minutes, preferably at least <NUM> minutes and/or for no more than <NUM> minutes, preferably no more than <NUM> minutes.

For example, the sonication is achieved at room temperature, i.e., at a temperature ranging from <NUM> to <NUM>.

For example, the copper nanoparticles functionalized with the one or more pyridine-containing ligands described above are washed and dried after step (c) and/or before the steps required to prepare the gas diffusion electrode <NUM>. With preference, the step of washing is performed with an organic solvent and the step of drying lasts at least <NUM> hours, preferably at least <NUM> hours and/or last no more than <NUM> hours, preferably no more than <NUM> hours. With preference, the organic solvent is selected from dichloromethane, ethyl acetate, acetone, dimethylformamide, acetonitrile, n-butanol, n-propanol, methanol, ethanol and any mixture thereof; with preference, the organic solvent is or comprises dimethylformamide and/or methanol.

The copper nanoparticles are, in step (d), dispersed into methanol to obtain a second dispersion. It is preferred that the second dispersion is sonicated. With preference, said sonicating is achieved at room temperature, for example at a temperature ranging from <NUM> and <NUM>. With preference yet, said sonicating is achieved for at least <NUM> minutes, preferably for at least <NUM> minutes and/or for no more than <NUM> minutes, preferably no more than <NUM> minutes.

Then, in step (e), an ion-conducting polymer, such as Nafion®, is added to obtain the ink.

The ink is then deposited in step (f) on a gas-diffusion membrane, for example, a gas-diffusion membrane that is or comprises polytetrafluoroethylene. In a preferred embodiment, the ink is spray-deposited on the gas-diffusion membrane.

In a step (g), the gas diffusion electrode can be dried under reduced pressure, for example at a pressure ranging from <NUM> MPa and <NUM>-<NUM> MPa.

Finally, the present disclosure is about a process for electrolysing carbon dioxide, said process comprising the following steps:.

said process is remarkable in that the gas-fed flow cell <NUM> provided at step (i) is as defined above.

The process needs to have flowing catholyte but the anolyte can be flowing or static; with preference the anolyte is flowing as well.

For example, the current density is at least <NUM> mA, with preference, at least <NUM> mA.

The "electrolyte flow" hereafter refers to the catholyte flow but also apply to the anolyte flow in case of flowing anolyte.

For example, the electrolyte flow provided in step (ii) has a flow rate that is ranging from <NUM> min-<NUM> to <NUM> min-<NUM>, preferably from <NUM> min-<NUM> to <NUM> min-<NUM>, more preferably from <NUM> min-<NUM> to <NUM> min-<NUM>.

For example, the electrolyte flow provided in step (ii) is a flow of an aqueous solution of one or more inorganic bases. With preference, the one or more inorganic bases are alkali selected from NaOH, KOH, Ca(OH)<NUM>, LiOH, Mg(OH)<NUM>, RbOH, CsOH and any mixture thereof. With preference, the one or more inorganic bases are or comprise KOH and/or NaOH.

With preference, the aqueous solution of one or more inorganic bases has a concentration of at least <NUM>, of at least <NUM>, or at least <NUM>, or at least <NUM>. With preference, the aqueous solution of one or more inorganic bases has a concentration of at most <NUM>, of at most <NUM>, or at most <NUM>, or most <NUM>. For example, the concentration the aqueous solution of one or more inorganic bases is ranging from <NUM> to <NUM>; preferably from <NUM> to <NUM>, more preferably from <NUM> to <NUM>.

With preference, the aqueous solution of one or more inorganic bases is an aqueous solution of KOH at a concentration of at least <NUM>, of at least <NUM>, or at least <NUM>, or at least <NUM>. With preference, said at least one alkaline compound is an aqueous solution of KOH at a concentration of at most <NUM>, of at most <NUM>, or at most <NUM>, or most <NUM>. For example, the concentration of KOH in a solution of water is ranging from <NUM> to <NUM>; preferably from <NUM> to <NUM>, more preferably from <NUM> to <NUM>.

Said step (iii) is performed by injecting carbon dioxide at a potential gradient starting at -<NUM> V versus a reference electrode and ending at -<NUM> V versus said reference electrode at a sweep rate ranging from <NUM> mV s-<NUM> to <NUM> mV s-<NUM> or from <NUM> mV s-<NUM> to <NUM> mV s-<NUM>. With preference, the potential gradient starts at -<NUM> V versus a reference electrode and ends at -<NUM> V versus said reference electrode. For example, the reference electrode is an Ag/AgCl electrode filled with <NUM> of KCI.

The output flow <NUM> of the liquid component, in addition to comprising formate, can also comprise one or more selected from acetate, ethanol and propanol.

In step (iv), it is possible to produce an additional output flow <NUM> of gaseous components which exits through the gas chamber <NUM> via the gas channel <NUM>. The additional output flow <NUM> can comprise, for example, one or more ligands selected from hydrogen, carbon monoxide and ethylene. With preference, the additional output flow <NUM> is devoid of ethylene and but comprises hydrogen and/or carbon monoxide.

For example, the input flow of carbon dioxide provided in step (iv) has a flow rate that is ranging from <NUM> min-<NUM> to <NUM> min-<NUM>, preferably from <NUM> min-<NUM> to <NUM> min-<NUM>, more preferably from <NUM> min-<NUM> to <NUM> min-<NUM>, even more preferably from <NUM> min-<NUM> to <NUM> min-<NUM>. In case, the flow rate of carbon dioxide is too low, for example, in case the flow rate is below10 mL min-<NUM>, the selectivity to formate can be lost.

Mass loading of the ink onto the gas diffusion membrane: The membrane was weighed using an analytical balance before deposition and after drying overnight in a vacuum desiccator.

Fourier-Transform Infrared (FT-IR) spectra were recorded using a Shimadzu Prestige <NUM> Spectrometer on solid powder samples in transmission mode.

TEM analysis was conducted using a Jeol 2100F microscope equipped with Schottky Field Emission electron gun and an ultra-high resolution polar piece.

SEM images were obtained using a SU-<NUM> Hitachi FEG-SEM.

UV-vis absorption spectra were recorded on liquid samples using an Agilent Cary <NUM> spectrometer.

Bruker Advance III <NUM> spectrometer at <NUM> has been used. D<NUM>O was used as the lock solvent and an aqueous solution of terephthalic acid (TPA) was used as an internal standard for quantification.

Gas products were detected online using SRI instruments <NUM> GC with Ar as the carrier gas. The GC was fitted with a thermal conductivity detector for H<NUM> quantification, where the gas was separated using a HaySepD precolumn with a <NUM> molecular sieve column. Carbon products were separated using either a <NUM> molecular sieve column (CH<NUM>) or a <NUM> HaySepD column (CO, C<NUM>H<NUM>, C<NUM>H<NUM>) and detected using a flame-ionization detector fitted with a methanizer. Calibration was performed using a custom standard gas mixture in CO<NUM>.

The FE for gas products was calculated using equation (<NUM>): <MAT>.

Where nproduct is the amount of product (mol), nelectrons is the number of electrons used to make the product, F is the Faraday constant (C mol-<NUM>), Qt=<NUM> is the charge at the time of the injection, and Qt=x is the charge at time x seconds before the injection, representing the time taken to fill the sample loop, with x depending on the combined flow rate of Ar and CO<NUM> as well as the loop size.

The full cell energy efficiency EEfull for formate was calculated using equation (<NUM>): <MAT>.

Where Eproduct is the thermodynamic potential for formate (-<NUM> V), Ecell is the measured cell potential, and FEproduct is the faradaic efficiency (%). For EE<NUM>/<NUM> values, Ecell = E<NUM>/<NUM> + EH<NUM>O/O<NUM>, where E<NUM>/<NUM> is the iR-corrected potential measured in the cell (V vs. RHE) and EH<NUM>O/O<NUM> is <NUM> V.

Liquid products were analysed using <NUM>H NMR with a Pre-SAT180 water suppression method. Formate values were confirmed with a standard calibration using sodium formate solutions (in <NUM> KOH) to ensure the accuracy of the internal standard method. The crossover of formate through the anion exchange membrane was accounted for by also liquid sampling from the anode compartment.

Calculations were based on the volumetric flow entering and leaving the cell as well as the consumed flow rates involved in both product generation and reactions with OH-. The ideal gas law - see equation (<NUM>) - was used to relate the volumetric flow rate to the molar flow: <MAT> wherein P is pressure, Qf the volumetric flow, Nf the molar flow, R the gas constant, and T the temperature. The molar flow can be calculated using the molar values of each gas in the loop as long as the time is taken to fill the loop is known. In the present, the loop has a known size, so the time can be calculated given that the flow rate into the GC is known - see equation (<NUM>): <MAT>.

From here, the volumetric flow of products is calculated which represents the additional flow for gasses generated in the CO<NUM>R reaction - see equation (<NUM>): <MAT>.

The difference between the outlet flow and the sum of the product volumetric flow rates is then simply the flow rate of unreacted CO<NUM> in the system. This contributes to the flow but is not going into the generation of products - see equation (<NUM>): <MAT>.

From here, the amount of CO<NUM> (in terms of volumetric flow) consumed by generating gas and liquid products can be calculated using equations (<NUM>) and (<NUM>): <MAT> <MAT>.

This allows a total flow rate for CO<NUM> converted into carbonate to be determined using the inlet flow and the calculated product-based flow rates - see equation (<NUM>): <MAT>.

A product conversion percentage can also be calculated - as in equation (<NUM>) - which represents the amount of consumed CO<NUM> that goes into product generation instead of consumption through reaction with OH-: <MAT>.

The single-pass conversion efficiency - see equation (<NUM>) - is the best representation of the full system efficiency as it takes into account the CO<NUM> consumed and utilised as well as the products generated.

Copper nanopowder (Sigma-Aldrich, <NUM>), <NUM>-mercaptopyridine (ACROS organics, <NUM>%), thiophenol (Sigma-Aldrich, ≥<NUM>%), N,N-dimethylformamide (Carlo Erba, <NUM>%), and methanol (Carlo Erba, <NUM>%) were used to form thiol-modified Cu nanoparticles. Polytetrafluoroethylene (PTFE) membranes (SartoriusTM, <NUM> pore size) and NafionTM (Sigma-Aldrich, <NUM> wt. % in lower aliphatic alcohols and water) were used for electrode preparation. Milli-Q H2O and KOH (Sigma-Aldrich, <NUM>%) were used for electrochemical experiments. Terephthalic acid (Sigma-Aldrich, <NUM>%), D<NUM>O (<NUM>% D), and sodium formate (Sigma-Aldrich, ≥<NUM>%) were used for NMR experiments and calibration.

<NUM>,<NUM>-dimethyl-<NUM>-mercaptopyridine (DMSPy) was synthesised according to a previously reported procedure: Under inert conditions, <NUM>,<NUM>-dimethyl-<NUM>-chloropyridine (<NUM>, <NUM> mmol) was dissolved in DMF and NaHS (<NUM>, <NUM> mmol) was added. The mixture was heated to <NUM> for <NUM> then concentrated under vacuum. The product was purified using column chromatography (silica, DCM : MeOH = <NUM> : <NUM>) and dried under vacuum. <NUM>H NMR (thione tautomer, d<NUM>-DMSO): δ (ppm) = <NUM> (s, <NUM>), <NUM> (s, <NUM>), <NUM> (s, <NUM>).

Cu nanopowder (particle size of <NUM> as measured by transmission electron microscopy) (commercially available from Sigma-Aldrich, <NPL>) featuring a native oxide layer formed from ambient exposure were dispersed in N,N-dimethylformamide (DMF) (<NUM>) and sonicated for <NUM> at <NUM>. A solution containing <NUM>-mercaptopyridine (SPy, <NUM>, DMF) was added under inert conditions to obtain a mixture of <NUM> nmol mgCu-<NUM>. The suspension was sonicated for <NUM> at <NUM> then the particles were washed three times with DMF, twice with MeOH, and dried in vacuo for <NUM> to form SPy-modified nanoparticles.

Thiophenol (SPh) modification was conducted using the same method with the same molar ratio of ligand to Cu nanoparticles.

Cu-SPy nanoparticles (<NUM>) (i.e., Cu25-SPy nanoparticles) were formed in the same way but all treatments were carried out in a glovebox to avoid exposure of the particles to oxygen.

Successful anchoring of <NUM>-mercaptopyridine (SPy) on Cu nanoparticles (NPs) was confirmed using Fourier transform infrared (FTIR) spectroscopy. <FIG> shows indeed FTIR spectra of the bare Cu nanoparticles, the SPy compound and Cu-SPy nanoparticles.

X-ray photoelectron spectroscopy (XPS) has demonstrated that a mixture of thiol and thiolate environments were present.

No differences in morphology or particle size were observed between Cu and Cu-SPy particles. This can be observed in <FIG>, <FIG> and <FIG> that are transmission electron microscopy images showing images of initial copper nanoparticles, the as-prepared Cu-SPy nanoparticles, and Cu-SPy nanoparticles after <NUM> hour of electrolysis at -<NUM> mA cm-<NUM>.

Gas diffusion electrodes (GDEs) were prepared by airbrushing a methanolic solution of ionomer and copper nanoparticles onto PTFE membranes (<NUM> pore size) to form a porous network (approximately <NUM> thick) (see <FIG>).

An ink containing a weight ratio of <NUM>:<NUM>, Cu-SPy:Nafion (<NUM>%) was prepared in methanol and sonicated for <NUM> at <NUM>. The ink was spray deposited onto a PTFE membrane (Sartorius, <NUM> pore size) confined to a circular diameter of <NUM><NUM> to obtain a total mass loading of approximately <NUM> cm-<NUM> after drying under vacuum. The same mass loading was used for Cu, Cu-SPy, Cu-SPh, electrodes.

The GDEs of example <NUM> were electrically connected in a gas-fed flow cell for electrochemical testing.

All electrochemical experiments were conducted with a BioLogic VSP300 or VMP3 potentiostat with a <NUM> A current booster. Ohmic drop (iR) correction was conducted manually using resistance values obtained using electrochemical impedance spectroscopy.

Electrocatalysis was conducted in a custom-made gas-fed flow cell (Sphere Ltd). An anion exchange membrane (Sustanion, pre-treated in KOH), a Ni-foam anode, and a leak-free Ag/AgCl electrode filled with <NUM> of KCI (reference electrode from Innovative Instruments Ltd. ) were used. The PTFE-based GDEs were electrically contacted using Cu tape and confined to a geometric area of <NUM><NUM>. Pre-activation was required, which involves consecutive linear sweep voltammograms (LSVs) under CO<NUM> flow with a sweep rate of <NUM> mV s-<NUM> from -<NUM> to -<NUM> V vs. the reference electrode (Ag/AgCl electrode filled with <NUM> of KCI) until stabilisation of the current response. A CO<NUM> inlet flow rate was maintained at <NUM> min-<NUM> using a mass flow controller (Bronkhurst) for initial studies and the electrolyte solution (<NUM> KOH) was circulated at a rate of <NUM> min-<NUM> using a peristaltic pump. The catholyte was constantly purged with Ar at a fixed flow rate of <NUM> min-<NUM> and the outlet was connected to the CO<NUM> outlet gas trap to carry any liquid saturated gas products to the GC. Additionally, calibrated flow meters (MesaLabs Defender <NUM>+ and Ellutia <NUM>) were used to verify flow rates before and after the GC inlet to ensure the correct flow value was recorded and to establish the portion of CO<NUM> utilised to account for mass balance. The catholyte and anolyte volumes were <NUM> and the electrolysis time was <NUM> for all experiments apart from the <NUM> electrolysis, where the volumes were increased to <NUM>.

Gas products for CCE experiments were recorded at <NUM> and <NUM> minutes to ensure consistent selectivity and liquid products were taken after <NUM>. For the <NUM> electrolysis experiment, liquid samples were extracted using a syringe every <NUM> and the gas products recorded every <NUM> minutes.

Potentials were converted to the reversible hydrogen electrode scale (RHE) using the Nernst equation: ERHE = EAg/AgCl + <NUM> + <NUM> × pH and were iR-corrected to account for the solution resistance, which was obtained from electrochemical impedance spectroscopy scans. Note that this does not account for any local pH changes at the electrode/solution interface, however, only small changes are expected for such highly alkaline systems.

A geometrical molecular loading of ≈<NUM> nmol cm-<NUM> was determined by UV-Vis spectroscopy after reductive desorption of the thiol at highly cathodic potentials. The electrodes were exposed to potentials more negative than -<NUM> V vs. Ag/AgCl/KCl3. <NUM> under Ar flow for <NUM> to ensure that all of the molecule was removed. Under Ar flow (see <FIG>), the desorption of SPy forms <NUM>,<NUM>'-dithiodipyridine with an absorption peak at <NUM> in KOH. The absorption peak for the desorbed molecule was correlated with a calibration curve for the complex to obtain a molar loading of SPy based on the geometrical area (<NUM><NUM>). The SPy loading value obtained was <NUM> ± <NUM> nmol cm-<NUM>. Under CO<NUM> flow, the desorbed thiolate reacts to form a new species with an altered UV-Vis spectrum with a much higher absorption coefficient (peak at <NUM> used for analysis) - this likely corresponds to a thiocarbonate derivative. The low concentrations excluded conventional molecular characterisation, however through ligand stripping under CO<NUM> flow for a blank electrode, and comparison with the absorption peak at <NUM> for the experiments conducted under CO<NUM> flow, an approximate percentage of desorbed species could be obtained.

As shown by <FIG>, it has been observed that in alkaline conditions (<NUM> KOH) these GDEs could reach -<NUM> mA cm-<NUM> at applied potentials of approximately -<NUM> V vs. RHE.

There were no significant differences in the current-voltage responses for Cu and Cu-SPy samples confirming the negligible influence of the molecule on the physical properties of the catalyst (<FIG>). Also, no mass transport-limited current was observed.

<FIG> shows the FEHCOO- values obtained from controlled current electrolysis (CCE) over <NUM> hour with varying current densities for Cu and Cu-SPy electrodes. It was revealed that Cu-SPy GDEs displayed a high selectivity for HCOO- whereas unmodified Cu showed a wide product distribution. Tables <NUM> and <NUM> display the faradaic efficiencies for the unmodified copper nanoparticles and the copper nanoparticles as prepared in the present disclosure respectively. FEHCOO- values of <NUM> ± <NUM>% at -<NUM> mA cm-<NUM> and <NUM> ± <NUM>% at -<NUM> mA cm-<NUM> were obtained for Cu-SPy GDEs with a maximal partial current density for formate (jHCOO-) of <NUM> ± <NUM> mA cm-<NUM>.

Table <NUM> displays Cu-SPh, Cu25-SPy, and Cu-DMSPy faradaic efficiencies of main products from CO<NUM> reduction (<NUM>) at different current densities. The results highlight the advantage of pyridine-containing ligands over thiophenol ligands.

At -<NUM> mA cm-<NUM>, the selectivity was lower (FEHCOO- = <NUM> ± <NUM>%) suggesting that ligand loss occurs at these potentials (≈ -<NUM> V vs. RHE). This was verified using solution-phase UV-Vis spectroscopy of the electrolyte solutions following electrolysis.

<FIG> displays the faradaic efficiency for all the products obtained following the present disclosure.

<FIG> displays the partial current densities for all the products obtained following the present disclosure.

It is also highlighted that at -<NUM> mA cm-<NUM> (≈ -<NUM> V vs. RHE) about <NUM>% desorption was observed whereas at -<NUM> mA cm-<NUM> about <NUM>% of the total ligand loading was lost after <NUM>.

The loss of formate selectivity was correlated with an increase in ligand desorption using UV-Vis analysis throughout longer-term electrolysis at -<NUM> mA cm-<NUM>, as shown in <FIG> in which the correlation between FEHCOO- and the percentage of SPy ligand lost to solution as determined by UV-Vis from an electrode held at -<NUM> mA cm-<NUM> over the course of <NUM> hours is illustrated. The FEHCOO- of a bare Cu GDE was recovered after <NUM> hours showing that the permanent effects of molecule desorption are minimal and highlighting the key role of the SPy compound in directing selectivity towards formate.

As shown in <FIG> and <FIG> obtained by scanning electron microscopy, no clear morphological changes which could be responsible for ligand desorption were observed over the electrolysis.

The cathodic energy efficiency for HCOO- (EE½HCOO-) was <NUM> ± <NUM>% at -<NUM> mA cm-<NUM> with an optimal single-pass efficiency of <NUM>% attained through alteration of the CO<NUM> flow rate as illustrated in <FIG>. The optimum flow rate that does not affect FEHCOO- is <NUM> min-<NUM>. Each point was determined from <NUM>-minute electrolysis at -<NUM> mA cm-<NUM> with a fresh electrolyte solution.

The stability of the gas-fed flow cell at a potential required to drive CO<NUM> reduction coupled with oxygen evolution using a Ni foam anode, by conducting CCE at -<NUM> mA cm-<NUM> for <NUM> (see <FIG>) was studied. A stable potential of -<NUM> V was observed and an average FEHCOO-value of approximately <NUM>% was maintained giving a full-cell energy efficiency for HCOO- of <NUM>% and single-pass conversion efficiency of <NUM>%. After <NUM> hours, a fresh solution of electrolyte was added, which is depicted in <FIG> by the vertical line at <NUM> hours. Retention of the molecule after electrolysis was confirmed with UV-Vis spectroscopy, which showed no additional signal from desorbed species after <NUM>. The molecular stability in a gas-fed flow cell, therefore, permits the use of a broad range of organic thiols for CO<NUM>R.

Claim 1:
Gas diffusion electrode (<NUM>) suitable for carbon dioxide electrolysis, said gas diffusion electrode (<NUM>) having a gas diffusion membrane (<NUM>), the gas diffusion electrode (<NUM>) further comprising an ink (<NUM>) deposited on the gas diffusion membrane (<NUM>); wherein the ink (<NUM>) comprises an ion-conducting polymer, said gas diffusion electrode (<NUM>) is characterized in that the ink (<NUM>) further comprises a catalyst comprising copper nanoparticles functionalized with one or more pyridine -containing ligands, wherein the one or more pyridine-containing ligands have an anchoring group comprising a sulphur atom tethered to the copper nanoparticles.