EP4665893A1 - Electrochemical cells for the reduction of carbon dioxide - Google Patents

Abstract

An electrochemical cell for the reduction of carbon dioxide to carbon monoxide. The electrochemical cell comprises a cathode, an anode, an ion-exchange membrane separating the anode and cathode, and a gas supply for providing carbon dioxide gas to the cathode; wherein the cathode is provided by a gas diffusion electrode comprising a gas diffusion layer and a molecular catalyst. The ion-exchange membrane comprises a cation-exchange layer and the ion-exchange membrane is arranged in direct contact with the cathode and the anode. The molecular catalyst comprises cobalt and an organic ligand. A method of electrochemical reduction of carbon dioxide in an electrochemical cell is also disclosed.

Description

Electrochemical Cells for the Reduction of Carbon Dioxide

Field

The present invention relates to an electrochemical cell for electrochemical reduction of carbon dioxide and to a method of electrochemical reduction of carbon dioxide. In particular, the present invention relates to an electrochemical cell comprising a cobalt molecular catalyst for the electrochemical reduction of carbon dioxide preferentially to carbon monoxide.

Background

The electrochemical reduction of carbon dioxide has the potential to provide a useful source of chemical feedstocks, such as carbon monoxide and hydrogen, from waste gas streams containing carbon dioxide. Carbon monoxide is a versatile chemical building block which can be used in the synthesis of a variety of bulk chemicals. Therefore if carbon monoxide could be produced efficiently and selectively from a waste gas stream, such a process has the potential to reduce the environmental impact of chemical feedstock production.

Electrochemical carbon dioxide reduction on metal electrodes has been studied extensively since the 1980s, when metal electrodes were identified which were able to produce three main classes of carbon-based products when used in water. Au, Ag, Zn, Pd are known to be selective towards carbon monoxide; Sn, In, Pd, Bi produce formate; and Cu a mixture CH4 and C²⁺ products depending on the nature of the Cu surface and the electrolyte. Since these early studies, advances have been made through the use of surface treatments, nanostructuring and alloying, but the identification of active sites is challenging, which makes it difficult to tune catalytic activity.

Researchers have recently been exploring the use of metal catalysts in gas diffusion electrodes for the electrochemical reduction of carbon dioxide. In water at room temperature, the dissolved CO2 concentration is limited to 34 mM (at room temperature and pressure) which means that CO2, the reaction substrate, is only available at low concentrations which limits current density in conventional electrolysis cells. Gas diffusion electrodes overcome this limitation by directly delivering a CO2 gas stream through the back of a porous electrode on which the catalyst is deposited, and which is also in contact with the electrolyte medium. For CO2 reduction to CO, studies have focused on Au and Ag catalysts. As the catalytic activity is related to pH, with a high pH being particularly desirable as it supresses H2 production, experiments are usually carried out around pH 13-14. This helps keep selectivity to CO2 reduction constant even if the load is fluctuated. However, operation at very high pH has a disadvantage as the majority of the CO2 delivered to the electrode enters the electrolyte forming carbonate salts. This means the majority of CO2 cannot be converted and a constant decrease in pH occurs. The drop in pH decreases selectivity and can lead to electrode failure due to carbonate deposits forming.

For carbon monoxide production, a further complication is that the most active metal surfaces (Au, Ag) show a strong dependence of activity on particle size. It is commonly reported that during CO2 electrolysis even on the <24 hr timescale that morphological changes occur with Au nanostructures and this leads to CO yields changing. This, coupled with local pH fluctuations as load is varied, leads to efficiencies varying significantly under operating conditions. Furthermore it is known that common impurities found in many flue gas streams from industry (e.g. H2S, SO_X, NO_X) can poison noble metal electrodes, as can common metal impurities in water feeds and support materials (e.g. from carbon supports), leading to a loss of activity. Technoeconomic analyses have suggested that CO2 electrolysers will need to have lifetimes of at least 4000 hours to be economical to run, a value which has not yet been reached by known systems.

Molecular electrocatalysts based on transition metal centres have been widely studied for carbon dioxide reduction, particularly to CO. A particular advantage of molecular electrocatalysts is that the use of a small, designed catalytic centre that is synthetically accessible facilitates the tuning of catalytic properties. Historically, molecular catalysts have been tested when dissolved in solution (homogenous) using an inert electrode (e.g. glassy carbon) and although remarkable turnover frequencies (up to 10⁶ s^-1) have been predicted to be achievable (through rate law analyses), they have been primarily viewed as of academic interest only. The need for many catalysts to operate in aprotic solvents coupled to the relatively low current densities and low turnover numbers have been barriers to application.

In a known configuration of an electrochemical cell for the reduction of carbon dioxide, flow cells with gas diffusion electrodes (GDE) using an alkaline catholyte and an anion-exchange membrane (AEM) can reach very high current densities and Faradaic efficiency for carbon products, but this cell configuration has an inherent problem in the parasitic reaction of feed CO2 with hydroxides, forming carbonates. The problem is threefold: a) carbonates formed in the catholyte needs to be regenerated to CO2, at an energetic penalty, for recycling of the CO2 feed, b) the generated carbonates act as the charge carrier through the AEM, resulting in CO2 crossover to the anode stream and increasing separation costs, and c) carbonates can precipitate out and cause blockages in the GDE.

Therefore there remains a need for improved methods and catalysts for the electrochemical reduction of carbon dioxide to useful products.

Summary of the Invention

It is one aim of the present invention, amongst others, to provide an electrochemical cell for electrochemical reduction of carbon dioxide and an associated method that addresses at least one disadvantage of the prior art, whether identified here or elsewhere, or to provide an alternative to existing gas diffusion electrodes. For instance, it may be an aim of the present invention to provide an electrochemical cell for the electrochemical reduction of carbon dioxide which has high carbon dioxide utilization and selectivity for carbon monoxide production and which may maintain performance over a longer time period than known electrochemical cells. The electrochemical cell of the present invention may suitably minimise undesired carbonate formation, which would adversely affect performance, compared to known electrochemical cells..

According to aspects of the present invention, there is provided an electrochemical cell, a method of electrochemical reduction of carbon dioxide and the use of a molecular catalyst for the electrochemical reduction of carbon dioxide, as set forth in the appended claims. Other features of the invention will be apparent from the dependent claims, and from the description which follows.

According to a first aspect of the present invention, there is provided an electrochemical cell comprising a cathode, an anode, an ion-exchange membrane separating the anode and cathode, and a gas supply for providing carbon dioxide gas to the cathode; wherein the cathode is provided by a gas diffusion electrode comprising a gas diffusion layer and a molecular catalyst; wherein the ion-exchange membrane comprises a cation-exchange layer and wherein the ionexchange membrane is arranged in direct contact with the cathode and the anode; and wherein the molecular catalyst comprises cobalt and an organic ligand.

The electrochemical cell of this first aspect is configured with the ion-exchange membrane in direct contact with the cathode and the anode. Therefore the electrochemical cell is a “zero-gap” electrochemical cell with the ion-exchange membrane provided by or comprising a cation-exchange layer (CEL). For the purposes of the present invention, the term “membrane” can also be interpreted as “layer”.

In the electrochemical cell of this first aspect, the molecular catalyst of the cathode is suitably arranged in direct contact with the cation-exchange layer of the ion-exchange membrane.

In some embodiments, the ion-exchange membrane is a cation-exchange membrane. In such embodiments, the cation-exchange membrane provides the cation-exchange layer.

In some embodiments, the ion-exchange membrane is provided by a bipolar membrane (BPM) comprising a cation-exchange layer (CEL) and an anion-exchange layer (AEL). Therefore in such embodiments the bipolar membrane is arranged in direct contact with the cathode and the anode.

In such embodiments, the electrochemical cell may have a forward-bias configuration or a reversebias configuration. In the forward-bias BPM configuration, the AEL is towards the cathode, and carbonate and hydroxide ions are transported through the AEL. Whilst having the AEL towards the cathode can provide a local alkaline environment (thus enabling high Faradaic efficiency for carbon products), the generation of water and/or CO2 at the CEL/AEL interface can potentially lead to blistering and delamination of the BPM. In the reverse-bias BPM configuration, the CEL is towards the cathode, and water dissociation occurs at the CEL/AEL interface. Typical metal catalysts (e.g. Ag, Cu) in direct contact with an acidic CEL leads to significant H2 evolution and low selectivity towards carbonaceous products.

Suitably the anode is arranged in direct contact with the anion-exchange layer of the ion-exchange membrane, when present.

Suitably the electrochemical cell has a reverse-bias configuration, suitably wherein the molecular catalyst of the cathode is arranged in direct contact with the cation-exchange layer of the ionexchange membrane and the anode is arranged in direct contact with the anion-exchange layer of the ion-exchange membrane.

Suitably the molecular catalyst of the cathode is arranged in direct contact with the CEL of the BPM. The AEL is suitably arranged in direct contact with the anode of the electrochemical cell.

In such zero-gap electrochemical cells, no liquid electrolyte (catholyte) is present between the cathode and the ion-exchange membrane(s) and suitably no liquid electrolyte (anolyte) is present between the anode and the ion-exchange membrane(s). In such embodiments, the catholyte is a polymer catholyte, i.e. is provided by the cation-exchange membrane or the anion-exchange membrane, suitably by the cation-exchange membrane.

Suitably the zero-gap electrochemical cell is arranged to receive CO2 at the cathode (as described above) and to receive water at the anode. Suitably in operation, hydrated CO2 is flowed to the cathode and deionized H2O or an aqueous electrolyte is flowed to the anode.

The configuration of the zero-gap electrochemical cell described above creates a low pH (acidic) local environment at the cathode and a relatively high pH (alkaline) environment at the anode, which may be beneficial for the selectivity of the electrochemical reduction of CO2 to CO and the longevity of the electrochemical cell. Suitably the cation-exchange layer in contact with the cathode is therefore also at low pH (acidic).

In some embodiments, the anode is not in direct contact with the ion-exchange membrane of the electrochemical cell. In such embodiments, the electrochemical cell is zero-gap with respect to the cathode and the ion-exchange membrane, but not with respect to the anode and the ion-exchange membrane.

In such embodiments, the invention provides an electrochemical cell comprising a cathode, an anode, an ion-exchange membrane separating the anode and cathode, and a gas supply for providing carbon dioxide gas to the cathode; wherein the cathode is provided by a gas diffusion electrode comprising a gas diffusion layer and a molecular catalyst; wherein the ion-exchange membrane comprises a cation-exchange layer and wherein the ionexchange membrane is arranged in direct contact with the cathode; wherein an anolyte is arranged between the anode and the ion-exchange membrane; and wherein the molecular catalyst comprises cobalt and an organic ligand.

In such embodiments, the cathode, anode and molecular catalyst are as described herein.

The anolyte arranged between the anode and the ion-exchange membrane may be a liquid anolyte, as further described herein. For example, the anolyte may be water or may be an aqueous solution of a metal hydroxide salt, for example a Group I or Group II metal hydroxide, suitably a Group I metal hydroxide.

Therefore the present invention may provide an electrochemical cell comprising a cathode, an anode, an ion-exchange membrane separating the anode and cathode, and a gas supply for providing carbon dioxide gas to the cathode; wherein the cathode is provided by a gas diffusion electrode comprising a gas diffusion layer and a molecular catalyst; wherein the ion-exchange membrane comprises a cation-exchange layer and wherein the ionexchange membrane is arranged in direct contact with the cathode; and wherein the molecular catalyst comprises cobalt and an organic ligand; optionally wherein the anode is arranged in direct contact with the ion-exchange membrane.

Suitably the gas diffusion layer has an electro-active side comprising the molecular catalyst and a back side, wherein the gas supply is directed to the back side of the gas diffusion layer.

Suitably the electrochemical cell comprises a polymer catholyte at the electro-active side of the gas diffusion layer of the gas diffusion electrode.

The electrochemical cell of this first aspect is suitably adapted to be used for an electrochemical reduction of a gas, suitably carbon dioxide. Therefore the electrochemical cell comprises suitable connections between the anode and cathode, through suitable contacts. Such arrangements of contacts are known in the art.

The electrochemical cell of this first aspect configured as described above is suitably capable of carrying out an electrochemical reaction on the incoming gas stream, suitably an electrochemical reduction of carbon dioxide to selectively form carbon monoxide over hydrogen. Suitably the electrochemical cell operates by receiving gaseous carbon dioxide and reacting said gaseous carbon dioxide with the molecular catalyst in the presence of a polymer catholyte to produce gaseous electroreduction products, preferably a selective production of carbon monoxide.

The electrochemical cell of this first aspect is provided with a source of carbon dioxide gas, for example in a waste gas stream from an industrial process. Suitably the electrochemical cell is arranged such that the source of carbon dioxide gas is directed to the back side of the gas diffusion layer of the gas diffusion electrode.

The electrochemical cell is provided with an ion-exchange membrane which comprises a cationexchange layer (CEL). In embodiments wherein the ion-exchange membrane is a cation-exchange membrane, the cation-exchange membrane may be formed of a sulfonated tetrafluoroethylenebased fluoropolymer-copolymer, for example Nation™ 117.

In embodiments wherein the ion-exchange membrane is a bipolar membrane comprising a cationexchange layer (CEL) and an anion-exchange layer (AEL), the CEL may be provided by a sulfonated tetrafluoroethylene-based fluoropolymer-copolymer, for example Nation™ 117. The AEL may be provided by a copolymer of styrene and vinylbenzyl comprising imidazolium or pyridinium moieties, for example Sustainion X37-50. Alternatively, the BPM may be provided by Fumasep™.

The ion-exchange membrane may comprise a water dissociation catalyst, suitably provided at the junction of the CEL and the AEL. The water dissociation catalyst may be a suitable oxide material, for example selected from TiC>2, SnC>2, WO3, AI2O3, lrC>2, RuO2 and graphene oxide.

Other suitable ion-exchange membranes may be known in the art.

The electrochemical cell of this first aspect comprising the molecular catalyst of cobalt and an organic ligand is suitable for the electrochemical reduction of carbon dioxide. The inventors have found that such an electrochemical cell in the zero-gap configuration comprising the molecular catalyst of cobalt and an organic ligand, as described herein, can effectively and efficiently provide an electrochemical reduction of carbon dioxide to carbon monoxide which is highly selective for carbon monoxide production over hydrogen production. This selective production of carbon monoxide can be maintained for longer than comparable known electrochemical cells using alternative catalysts, such as Ag metal. For example, stable, selective production of carbon monoxide may be provided during operation over 4 h at 25 mA cm ².

Use of such a zero-gap cell configuration would be expected to provide relatively low selectivity due to the acidic CEL being in contact with the cathode and so presenting an acidic environment for the reduction reaction. Such acidic environments have been known to provide poor selectivity in carbon dioxide reduction. Also, the overall performance of such zero-gap cells comprising a BPM would be expected to be lower due to the additional voltage requirements to affect water dissociation within the ion-exchange membrane (for example a BPM as described above) compared to a configuration wherein the ion-exchange membrane is separated from the electrodes. Therefore the apparent improved performance of the electrochemical cell of the present invention comprising the catalysts of cobalt and an organic ligand was unexpected.

This high performance allows the electrochemical cell of the present invention to benefit from the low carbonate formation and/or low CO2 crossover made possible by the zero-gap configuration, which may further contribute to the longevity of effective operation of the electrochemical cell.

This favourable selectivity and stability of operation of the electrochemical cell of the present invention may make the process of electrochemical reduction of carbon dioxide economically feasible and beneficial. The electrochemical cell of the present invention may therefore allow the electrochemical reduction of carbon dioxide to carbon monoxide to be implemented on an industrial scale in order to efficiently provide this useful chemical feedstock and displace other more energy intensive and polluting methods of carbon monoxide production. The present invention may therefore provide a significant economic and environmental benefit.

In the electrochemical cell of this first aspect, the cathode is provided by a gas diffusion electrode comprising a gas diffusion layer and the molecular catalyst. Such arrangements of gas diffusion electrodes are known in the art. The gas diffusion electrode suitably has a porous structure which allows the molecular catalyst to contact a gas, for example carbon dioxide, and is electrically conductive to allow electrical current to flow through the electrode and the molecular catalyst. The gas diffusion layer may be any suitable porous material which can function as described above to allow a sufficient interaction of an incoming gas stream and the molecular catalyst. The gas diffusion layer may be a porous carbon material. For example, a suitable porous carbon material may be a fibrous carbon cloth or a carbon paper. Such a suitable carbon paper is a hydrophobic material which has a microporous carbon layer deposited on the catalyst side to increase the effective surface area. Suitably the gas diffusion layer is hydrophobic. Suitably the molecular catalyst is retained on the gas diffusion layer, suitably on one side of the gas diffusion layer, suitably the side which is to contact the ion-exchange layer in use. This side of the gas diffusion layer comprising the molecular catalyst may be referred to as the electro-active side and the other side of the gas diffusion layer may be referred to as the back side. Suitably the molecular catalyst is arranged on the gas diffusion layer on the electro-active side so that in use the incoming gas stream has to penetrate the pores of the gas diffusion layer from the back side to reach the molecular catalyst.

The anode may be provided by any suitable material known in the art, for example a ruthenium oxide or iridium oxide on a substrate, for example a carbon substrate such as carbon paper. In some embodiments, for example in larger scale implementations of the present invention, the substrate of the anode may be formed of stainless steel, nickel or titanium. The molecular catalyst is suitably retained on the gas diffusion layer, for example through non- covalent interactions between the molecular catalyst and the gas diffusion layer. Suitably the molecular catalyst is adhered to the gas diffusion layer, for example with a binder polymer. Suitably the molecular catalyst is adhered to the gas diffusion layer through such non-covalent interactions and through such a binder polymer. A suitable binder polymer may be a fluorocarbon polymer, for example PTFE and/or a sulfonated tetrafluoroethylene-based fluoropolymer-copolymer, for example Nation™. The molecular catalyst may be deposited and adhered to the gas diffusion layer using a catalyst ink comprising the molecular catalyst, a binder polymer and a solvent. The catalyst ink may be sprayed onto the gas diffusion electrode and dried, suitably in ambient conditions. The inventors have found that this arrangement can provide a robust gas diffusion electrode where the catalyst is retained on the gas diffusion layer throughout many operational cycles.

The back side of the gas diffusion layer may be treated with a hydrophobic polymer, for example a fluorocarbon polymer such as PTFE. This may help prevent flooding of the pores of the gas diffusion layer, which can adversely affect the performance of the gas diffusion electrode.

In some embodiments, the cathode may comprise a salt, for example a salt of a Group I or Group II metal, suitably a Group I metal, suitably a sodium, potassium and/or caesium salt. The salt may be a sulphate, perchlorate or halide salt, for example a sulphate, perchlorate or halide salt of a Group I or Group II metal. Suitably the salt is a halide salt such as a chloride salt. Suitable salts may be selected from NaCI, KOI and CsCI. Suitably such a salt is provided on the electro-active side of the cathode, in contact with the molecular catalyst. Therefore the molecular catalyst may be considered to have been treated with the salt.

The salt may be provided by treating the cathode, suitably on the electro-active side, with an aqueous solution of the salt. The concentration of the aqueous solution of the salt may be in the range of 0.1 to 5 M, suitably in the range 0.5 to 2 M, suitably approximately 1 M. Suitably the salt is present on the cathode in an amount of from 10 to 500 pmol cm^-2, suitably from 50 to 200 pmol cm'², for example approximately 100 pmol cm ². The presence of the salt may advantageously increase the activity of the cathode in CO production. For example, the salt may increase the faradic efficiency of the electrochemical cell for CO production. Furthermore, the salt may increase the stability of the cathode and therefore the electrochemical cell, to repeated usage cycles.

The molecular catalyst of the gas diffusion electrode comprises cobalt and an organic ligand. The term molecular catalyst is used to denote a catalyst species which is a molecule or complex and is therefore distinct from simple metal catalysts. A molecular catalyst may be generally described as a transition metal complex comprising one or more ligands and having a defined and finite molecular structure. Counterions may also be present in the molecular catalyst. The molecular catalyst may consist essentially of cobalt, the organic ligand and counterions, when present. The molecular catalyst may also be referred to as a complex of cobalt and the organic ligand (and any counterions present). The molecular catalyst may also contain solvent molecules functioning as ligands, for example through solvent exchange with a counterion of the molecular catalyst.

Suitably the organic ligand is a nitrogen-containing heterocyclic ligand. Suitably the ligand is a bidentate ligand or a poly-dentate ligand. The organic ligand may be an azamacrocycle.

Suitably the organic ligand is an aromatic nitrogen-containing heterocyclic ligand, for example a phthalocyanine or a porphyrin.

Suitably the molecular catalyst has a formula which comprises MLaXb, wherein M is cobalt, L is the organic ligand and X is an anion, wherein a is an integer from 1 to 3 and b is an integer from 0 to 3. Suitable anions may be selected from halogens, perchlorates and PFe’. Suitably a is 1. Suitably b is an integer from 0 to 2.

The organic ligand may be an optionally substituted phthalocyanine or porphyrin. In some embodiments, the organic ligand is an optionally substituted phthalocyanine. The phthalocyanine or porphyrin may be optionally substituted with C1-C20 alkyl, alkenyl, aryl or alkaryl groups, halogens, -NH₂, -NHR, -NR₂, -NR₃ ⁺, -OH, -OR, -SH, -SR, -CO₂H, -CO₂R, -C(O)NH₂, -C(O)NHR, C(O)NR₂, -SO3H, -SO₂NH₂, -SO₂NHR, -SO₂NR₂, wherein R is an optionally substituted C1-C20 alkyl, alkenyl, aryl or alkaryl group. Suitably R is a C1-C20 alkyl group.

The term optionally substituted suitably refers to optional substitution of the group with one or more halogen, amino, hydroxy, thiol or C1-C5 alkyl or alkenyl group.

In some embodiments wherein the organic ligand is a phthalocyanine, the organic ligand suitably has the formula (I): wherein R¹, R², R³, and R⁴ are each independently selected from C1-C20 alkyl, alkenyl, aryl or alkaryl groups, a heteroaryl group, halogens, CF3, CHF₂, CH₂F, -NH₂, -NHR, -NR₂, -NR₃ ⁺, -OH, -OR, -SH, -SR, -CO2H, -CO2R, -C(O)NH₂, -C(O)NHR, -C(O)NR₂, -SO3H, -SO2NH2, -SO2NHR, -SO2NR2 or a C1-C5 alkyl phosphonate wherein R is an optionally substituted C1-C20 alkyl, alkenyl, aryl or alkaryl group. Suitably R is a C1-C20 alkyl group.

Suitably R¹ , R², R³, and R⁴ are each independently selected from C1 -C10 alkyl, alkenyl, aryl or alkaryl groups, a heteroaryl group, halogens, CF3, CHF2, CH2F, -NH2, -NHR, -NR2, -NR3⁺, -OH, -OR, -SH,

-SR, -CO2H, -CO2R, -C(O)NH₂, -C(O)NHR, -C(O)NR₂, -SO3H, -SO2NH2, -SO2NHR, -SO2NR2 or a C1-C5 alkyl phosphonate wherein R is an optionally substituted C1-C10 alkyl, alkenyl, aryl or alkaryl group.

Suitably R¹ , R², R³, and R⁴ are each independently selected from C1 -C10 alkyl, alkenyl, aryl or alkaryl groups, -NH2, -NHR, -NR2, -NR3⁺, -OH, -OR, -CO2H, -CO2R, wherein R is an optionally substituted C1-C10 alkyl, alkenyl, aryl or alkaryl group.

In such embodiments wherein the organic ligand is a phthalocyanine, the molecular catalyst suitably has the formula (II): wherein R¹, R², R³, and R⁴ are as defined above.

In embodiments wherein the organic ligand is a porphyrin, the organic ligand suitably has the formula (HI):

wherein R¹, R², R³, and R⁴ are as defined above and R⁵, R⁶, R⁷, and R⁸ are as defined above for R¹, R², R³, and R⁴. In such embodiment, the organic ligand suitably has the formula (IV): wherein R¹, R², R³, and R⁴ are as defined above.

Suitably the organic ligand has the formula (I) or (IV), as defined above. It will be understood by the skilled person that two hydrogens will be removed from the nitrogen atoms of structures (I) and (IV) when forming the molecular catalyst (complex) with cobalt.

In embodiments where the organic ligand is a porphyrin, the molecular catalyst suitably has the formula (V): wherein R¹, R², R³, and R⁴ are as defined above.

Suitably the molecular catalyst has the formula (II) or (V), as defined above.

Suitably the complexes described herein may be neutral and may comprise counterions, such as a halogen ion, e.g. chloride, to balance the charge of the metal and/or ligand. The molecular catalyst may be formally neutral before incorporation into the electrochemical cell, for example by applying the catalyst to the gas diffusion electrode. After incorporation of the molecular catalyst into the electrochemical cell, the molecular catalyst may be charged, for example having a positive charge.

In the molecular catalyst, the cobalt may have any oxidation state accessible to cobalt, suitably an oxidation state of III, II, I or 0, or the catalyst may contain a mixture of catalyst species having different cobalt oxidation states, selected from the above. The oxidation state of the cobalt in the molecular catalyst prior to incorporation into the electrochemical cell may be different to the oxidation state of the cobalt in the molecular catalyst in use. The oxidation state of the cobalt in the molecular catalyst prior to incorporation into the electrochemical cell may be II and therefore the cobalt suitably has a charge of 2+. The cobalt molecular catalyst therefore suitably comprises ligands and/or counterions with an overall 2- charge, such as a phthalocyanine or a porphyrin ligand.

According to a second aspect of the present invention, there is provided a method of electrochemical reduction of carbon dioxide, the method comprising: a) providing an electrochemical cell according to the first aspect of the present invention comprising a cathode, an anode, and an ion-exchange membrane and a molecular catalyst comprising cobalt and an organic ligand; b) contacting the carbon dioxide in gaseous form with the molecular catalyst, optionally in the presence of water; and c) applying an electrical potential between the cathode and the anode, thereby electrochemically reducing the carbon dioxide to gaseous products including carbon monoxide. Suitably the steps of the method are carried out in the order step a) followed by step b) followed by step c).

The electrochemical cell provided in step a) may have any of the suitable features and advantages described in relation to the first aspect.

Step b) involves contacting the carbon dioxide in gaseous form with the molecular catalyst, optionally in the presence of water. Therefore in some embodiments, step b) does not involve a source of carbon dioxide which is dissolved in a solution or electrolyte. The method of this second aspect is a method of gas diffusion electrolysis which suitably involves directly delivering carbon dioxide gas through the back side of the gas diffusion electrode on which the molecular catalyst is deposited.

Step b) suitably involves delivering carbon dioxide, for example as part of a waste gas stream from an industrial process, to the back side of the gas diffusion layer in the cathode, suitably penetrating into the gas diffusion layer. Suitably a polymer catholyte is simultaneously provided to the electroactive side of the gas diffusion layer in the cathode. Suitably step b) also involves providing an anolyte to the anode. The anolyte may be water and have a neutral pH. In some embodiments, the anolyte may be an aqueous solution with an alkaline pH, for example a potassium hydroxide solution. In such embodiments the pH of the anolyte is suitably at least 10, suitably at least 12, suitably at least 14.

In some embodiments, step b) may be carried out at an alkaline pH. In such embodiments the pH of the cathode is suitably at least 10, suitably at least 12, suitably at least 14 or approximately 14.

In some embodiments, step b) may be carried out at an acidic pH. In such embodiments the pH of the cathode is suitably up to 6, suitably up to 4, suitably up to 3, for example up to 2 or approximately 2. Suitably steps b) and c) take place at a pH of less than 6.

Suitably, the electrochemical cell has an acidic environment at the cathode, as discussed above, for carrying out step b). Suitably the electrochemical cell has a neutral or alkaline environment at the anode, as discussed above.

The electrical potential applied in step c) depends on the scale of the apparatus on which the method is carried out and can be arrived at by using common knowledge in this field in light of the teachings herein.

The method suitably involves a step e) of collecting the gaseous products. Suitable methods for collecting and storing the gaseous products are known in the art. For example, the gaseous products may be dried, optionally purified and bottled. Suitably steps b) and c) of the method provide a conversion efficiency of carbon dioxide to carbon monoxide of at least 20%, at least 40%, suitably at least 60%.

In some embodiments the gaseous products including carbon monoxide produced in step c) are recirculated to the gas diffusion electrode comprising the molecular catalyst to undergo further electrochemical reduction of the carbon dioxide remaining in the gaseous products of step c). Therefore the method may be considered to comprise a step d) of recirculating the gaseous products including carbon monoxide produced in step c) to the gas diffusion electrode. Such recirculation may be continued until the concentration of the carbon monoxide in the gaseous products mixture rises to a desired threshold, for example at least 20%, suitably at least 40%, at least 60% or at least 80%, by volume.

Suitably the method of this second aspect selectively produces carbon monoxide over other products of electrochemical reduction of carbon dioxide, over hydrogen in particular. Therefore the method of this second aspect may be considered to be a method of selective electrochemical reduction of carbon dioxide to carbon monoxide, suitably a method of electrochemical reduction of carbon dioxide selectively to carbon monoxide over hydrogen. The gaseous products produced in step c) and collected in step e) may therefore be enriched in carbon monoxide, suitably compared to hydrogen and unreacted carbon dioxide.

Suitably the method of this second aspect provides stable selectivity for the production of carbon monoxide over other products of electrochemical reduction of carbon dioxide, over hydrogen in particular. Therefore the method of this second aspect may achieve high Faradaic efficiencies while carried out at high current densities for extended durations. The method may achieve Faradaic efficiencies of over 20%, over 30%, over 40%, preferably over 50%. The method may be carried out using current densities of more than 400 mA cm'², for example more than 200 mA cm^-2, suitably more than 150 mA cm^-2, or more than 100 mA cm^-2, more than 50 mA cm^-2, suitably more than 30 mA cm^-2, or more than 10 mA cm ². The method may be carried out for durations over 0.5 h, for example over 1 h, suitably over 2 h, or over 4 h.

In some preferred embodiments the method of this second aspect may achieve Faradaic efficiencies of over 30% at a total current density of over 100 mA cm ².

Suitably the method of this second aspect produces carbon monoxide with a single pass conversion yield of at least 20%, at least 30%, preferably at least 40%.

This second aspect therefore suitably provides a stable, selective method of producing carbon monoxide from the electrochemical reduction of carbon dioxide for extended durations at high Faradaic efficiencies and/or with high single pass conversion yields. Advantageously, the method can be carried out at a relatively high or a relatively low pHs and may therefore provide an efficient industrial-scale process for obtaining this useful chemical building block from different industrial waste gas streams.

According to a third aspect of the present invention, there is provided the use of a molecular catalyst comprising cobalt and an organic ligand for the electrochemical reduction of carbon dioxide in an electrochemical cell, the electrochemical cell comprising a cathode, an anode, an ion-exchange membrane separating the anode and cathode, and a gas supply for providing carbon dioxide gas to the cathode; wherein the cathode is provided by a gas diffusion electrode comprising a gas diffusion layer and a molecular catalyst; wherein the ion-exchange membrane comprises a cation-exchange layer and wherein the ionexchange membrane is arranged in direct contact with the cathode and the anode.

The molecular catalyst used in this third aspect may have any of the suitable features and advantages described above in relation to the first aspect.

The use of this third aspect may have any of the suitable features and advantages described above in relation to the method of the second aspect.

Suitably the use according to this third aspect provides a conversion efficiency of carbon dioxide to carbon monoxide of at least 20%, at least 40%, suitably at least 60%.

Suitably the use of this third aspect provides Faradaic efficiencies of over 30% at a total current density of over 100 mA cm ².

Suitably the use of this third aspect produces carbon monoxide with a single pass conversion yield of at least 20%, at least 30%, preferably at least 40%.

Examples

Materials

Co(ll) phthalocyanine (Sigma Aldrich, 97%), Ensaco 350G carbon powder (Imerys), TiC>2 (anatase, <25 nm, Sigma Aldrich, 99.7%), isopropanol (Sigma Aldrich, 99.5%), carbon paper (Sigracet 39BB), RuO₂ (FuelCellStore, nanoparticles ~5-10 nm), Fumasep FBM bipolar membrane (stored in 1 M NaCI). Nation solution (Sigma Aldrich, 5% in a mixture of a lower aliphatic alcohols and water), CO₂ (BOC, CP grade), Nation 117 membrane (FuelCellStore), Sustainion membrane (X37-50, Grade RT). Pure water was Milli-Q grade (18.2 MQ). Electrode and membrane fabrication

For the cathode, first the Ensaco carbon powder was dispersed in isopropanol by sonication for 1 h, then Co(ll) phthalocyanine (CoPc) was added then sonicated for a further 30 min. The CoPc/carbon suspension was left to stir overnight (~16 h), Nation solution was added then stirred for 3 h, then sprayed (Harder & Steenbeck Evolution with a N2 stream) onto carbon paper over a hot plate set to 40 °C (17 wt% CoPc, total loading 1 .2 mg/cm²). For the anode, the RuO2 powder was dispersed in isopropanol by sonication for 1 h, then sprayed onto carbon paper over a hot plate set to 80 °C. The electrodes were left to dry in ambient air.

For the custom BPM, we adapted a procedure from the literature [L. Chen, Q. Xu, S. Z. Oener, K. Fabrizio, S. W. Boettcher, Nature Communications 2022, 13, 3846], TiC>2 (anatase) was suspended in a mixture of isopropanol and water then sonicated for 1 h. This suspension was sprayed onto a sheet of Nation 117 (at a loading of 30 pg cm'²), held on a glass holder on a hot plate set at 90 °C. After spraying, the TiC>2-loaded Nation was returned to and stored in H2O, then it was interfaced with Sustainion (with the TiC>2-loaded side in the middle) directly before assembly of the electrochemical cell.

Electrolyzer cell configuration

The CoPc/carbon cathode prepared as described above was cold-pressed in the electrolyzer cell, together with the BPM and a RuO2 anode, with the CEL of the BPM towards the cathode (reversebias configuration). The anode was arranged in direct contact with the BPM. The configuration of the electrolyzer cell is shown in Figure 1 .

Electrochemical measurement

Electrochemical measurements were carried out using an Ivium Vertex potentiostat. The membrane electrode assembly was assembled in the electrolyzer cell (Dioxide Materials, cathode area 5 cm²) at ambient conditions (‘cold pressing’). The Fumasep membrane was soaked in H2O for 1 h before use. The cation-exchange layer of the BPM was towards the cathode (‘reverse bias’). The cell was tightened to 3 Nm using a torque wrench. The CO2 inlet stream was passed through a water saturator at room temperature, at a flow rate of 20 seem unless specified. The anolyte was pure H2O, typically 100 mL, recirculated at a rate of 15 mL min^-1. All measurements were taken at room temperature (20 - 22 °C). After assembly, the cell was preconditioned at open circuit, with CO2 and H2O flowing, for 1 h, before starting electrochemical measurements. The CO2 inlet flow rate was controlled by a mass flow controller (Alicat), and the outlet stream flow rate was measured using a digital flow meter (Agilent).

Measurement of electroactive coverage was conducted in a one-compartment glass cell with 0.1 M KHCO3 electrolyte, Ag/AgCI reference electrode, Pt wire counter electrode, and Ar purging. The turnover frequency (TOF) was calculated from the maximum CO partial current density, J_Co, and the electroactive coverage, /", as follows:

J CO TOF = nFV where n is 2, and F is Faraday’s constant.

Electrochemical impedance spectroscopy (EIS) was conducted galvanostatically, with a 20 mA amplitude, with frequencies from 100 kHz to 1 Hz. The cell was held at the operating current for 3 min for equilibration prior to starting the measurement. The equivalent circuit fitting was conducted using the IviumSoft potentiostat control program.

The equilibrium (thermodynamic) voltage was calculated from cathode: -0.11 V vs RHE anode: 1.23 V vs RHE yielding Eceii.eq = 1 .34 V, assuming the same pH at both electrodes.

The voltage requirements for cell resistance and water dissociation resistance were calculated by summing the

Product detection

The outlet stream of the electrolyzer was connected to a gas chromatograph (Varian CP-4900 MicroGC) with a Molsieve 5A column. The Ar carrier gas for H2 and CO detection by a thermal conductivity detector

Faradaic efficiency calculation: where F is Faraday’s constant, J is current density, A is electrode area, v_out is the total volumetric outlet flow rate, x is the outlet molar fraction of CO, P is the pressure, R is the gas constant, and T is the temperature.

CO yield calculation: 100 where x is the outlet molar fraction of CO, v_out is the total volumetric outlet flow rate, and Vin is the inlet volumetric CO2 flow rate.

Characterization

Scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) measurements were carried out with a Hitachi SEM S4800 at 20 kV. X-ray Photoelectron Spectroscopy (XPS) examinations were carried out using Thermo Scientific K-Alpha X-ray photoelectron spectrometer fitted with Al Ka X-ray source (1486.7 eV). The samples were analysed without further surface cleaning on a spot of 400 x 400 pm² area. The survey measurements were recorded in 0-1350 eV range at 200 eV pass energy and the high-resolution scans for elements of interest were obtained in the appropriate range at 50 eV pass energy. The spectra were calibrated by using C 1s peak at 285.0 eV as reference.

Results

Figure 2 shows the performance of CoPc/carbon in a reverse-bias zero-gap BPM electrolyzer. Conditions: Cathode area 5 cm², cathode feed: CO2 saturated by a water bubbler, 20 seem, anode RUO₂ 9 cm², anolyte pure water, recirculated at 15 mL min^-1, Membrane Fumasep FBM, room temperature, Error bars correspond to 1 standard deviation, from 3 independent samples.

Figure 2A shows the initial faradaic efficiencies (FE) to H2 and CO, obtained during 2-electrode chronopotentiometric measurements. The FE for CO reached 69±4 % at a total current density of 25 mA cm^-2, decreasing to 50±2 % at 100 mA cm^-2 and 34±2 % at 200 mA cm ². The selectivity reached here is a large improvement over previously reported results with Ni cyclam catalysts and derivatives in the same cell configuration (~20-30 % at 100 mA cm ²). It is believed that this may be due to the CoPc catalyst not showing noticeable product inhibition by CO and subsequent reductive deactivation. The CoPc also significantly outperforms the known Ag nanoparticle benchmark catalyst in this cell configuration (20±2 % at 100 mA cm ²).

Figure 2B shows the full cell voltages and the CO partial current densities. The CO partial current density increased towards a plateau with total current density, reached 68±3 mA cm^-2 at 200 mA cm^-2 total current density.

As a minor product, CH4 was also observed, but at the levels of <0.1 % Faradaic efficiency. A small amount of CH4 was previously reported on a Co-protoporphyrin complex in acidic electrolyte (~0.1 % FE at ambient conditions, rising to 2% with 10 atm CO2). The CH4 is generated from further protonation and reduction of the adsorbed CO, and while this is promoted under acidic conditions, the contribution of this pathway is still small relative to CO generation and desorption. Longer measurements were conducted at 25 and 100 mA cm^-2 (see Figures 3A-D). Figures 3A-D show the stability of the product selectivity and cell voltage of CoPc/carbon in a reverse-bias zerogap BPM electrolyzer at A,B) 25 mA cm^-2 and C,D) 100 mA cm ². Conditions: Cathode area 5 cm², cathode feed: CO2 saturated by a water bubbler, 20 seem at inlet, anode RuO2 9 cm², anolyte pure water, recirculated at 15 mL min^-1, Membrane Fumasep FBM.

At 25 mA cm'², there was an initial decline in CO FE but it stabilised at ~40 % which was sustained for the duration of the experiment with no further decreases (up to 4 h tested). For the run at 100 mA cm'², the selectivity reached ~30% after 3 h. As for cell voltage, we note that this commercial BPM (Fumasep) is not designed for operation at high current density at reverse bias (> 100 mA cm'²) for long periods, and the voltage increase with time at 100 mA cm ² is partly attributable to membrane degradation. Pausing the applied current (while keeping the CO2 and H2O feed on) had no effect in restoring the selectivity, indicating that the partial loss in selectivity is not due to product inhibition or catalyst desorption, since in both cases we would expect some recovery in CO selectivity, either due to CO being flushed out, or catalyst re-adsorption, during the pause period.

One possible cause of the selectivity decrease is the partial flooding of the gas diffusion layer (GDL) of the cathode, which could result in lower CO2 supply to the catalyst. Flooding can occur irrespective of carbonate formation, due to water transport through the BPM as well as water formation as part of the reaction of CO2 conversion to CO. Periodic drying has been shown to help sustain the activity of CoPc on GDEs at higher pH’s. In a separate experiment, the double layer capacitance of the cathode was measured (as a quantification of water penetration into the GDE) by cyclic voltammetry. Initially, after preconditioning, the cathode capacitance was 51 pF cm'², which increased to 514 pF cm^-2 after chronopotentiometry at 100 mA cm^-2 for 2 h. During this period, the CO Faradaic efficiency decreased from 42 % to 18 %. The cell was then disassembled and the cathode taken out to dry in ambient air overnight, and after reassembly the capacitance decreased to 306 pF cm ². When the chronopotentiometry was restarted at 100 mA cm'², there was a recovery of CO FE to 41 %, which then declined again during operation, thus confirming that at least part of the selectivity decline is due to flooding.

A key performance metric is the CO2 utilization efficiency, or single-pass yield of carbon products (defined as the proportion of inlet CO2 that was converted into the desired carbon products, CO in this case) (this has also been referred to as single-pass conversion in the literature). At constant current (100 mA cm^-2), the inlet CO2 flow rate was varied, and the CO Faradaic efficiency and the CO single pass yield were measured (Figure 4A). Figure 4A shows dependence of CO Faradaic efficiency (left axis) and CO single-pass yield (right axis) on the CO2 inlet flow rate. This was conducted on the same sample, measured after the initial drop in selectivity (after ~1 h) such that the performance is relatively stable. Conditions: Cathode CoPc/carbon area 5 cm², cathode feed: CO2 saturated by a water bubbler, anode RuO2 9 cm², anolyte pure water, recirculated at 15 mL min^-1, Membrane Fumasep FBM. The CO Faradaic efficiency only decreased slightly with lower inlet flow rates, resulting in a maximum of 51 % CO yield at 3 seem inlet flow. There is a trade-off between CO productivity (measured in amount of CO produced per unit time) and the CO yield. Higher inlet flow results in higher productivity (defined as moles or mass of CO produced per unit time) but lower percentage yield. This trade-off is illustrated in Figure 4B, for 25 and 100 mA cm'², together with trendlines obtained from leastsquares fitting to the relationship.

The commercially available Fumasep BPM used in the experiments described above is not suited for long-term operation at >100 mA cm'², according to the supplier. This has resulted in variation in cell voltage after long operation, as well as batch-to-batch differences. The voltage requirement of a BPM electrolyzer depends on the ion transport characteristics of the CEL and AEL, and the rate of water dissociation (WD) at the CEL/AEL junction. The addition of catalysts (e.g. metal oxides, graphene oxide) has been shown to increase the WD rate at the junction, leading to higher currents during reverse-bias operation.

A custom BPM was constructed composed of a Nation 117 membrane as the CEL, a Sustainion X37-50 membrane as the AEL, and TiC>2 (anatase) particles as WD catalysts at the CEL/AEL junction (at a loading of 30 pg cm^-2 by airbrushing). Figure 5A compares the CO FE obtained from this custom BPM to the commercial Fumasep BPM (from Figure 2A). Conditions: Cathode CoPc/carbon area 5 cm², cathode feed: CO2 saturated by a water bubbler, 20 seem, anode RuO2 9 cm², anolyte pure water, recirculated at 15 mL min^-1. The cathode and anode remained the same CoPc/carbon and RuO2, respectively, as used in the experiments described above. The CO selectivity using the custom BPM was significantly lower than the Fumasep BPM throughout the current density range measured here, because the Nation membrane as the CEL presented a more acidic local environment at the cathode compared to the Fumasep BPM. Future improvements to the selectivity can be expected, for example by modifying the acidity of the CEL, or by inserting a thin intervening layer between the CEL and the cathode. The comparison of cell voltages is shown in Figure 5B. A custom BPM with no WD catalyst showed very high cell voltages (>7 V at 30 mA cm^-2), demonstrating that TiC>2 was effective in promoting water dissociation. Overall, the cell voltage from the custom BPM with a TiO2 WD catalyst was ~0.5 - 0.8 V lower than that of the commercial Fumasep membrane.

The effect of changing the anolyte from pure H2O to a highly alkaline (1 M KOH) electrolyte was investigated, and the results are shown in Figures 6A and B, using the Fumasep BPM for comparison with pure H2O anolyte results (from Figure 2A, left bars). 1 M KOH are shown in the right bars of Figure 6A. Conditions: Cathode CoPc/carbon area 5 cm², cathode feed: CO2 saturated by a water bubbler, anode RuO2 9 cm², recirculated at 15 mL min^-1. Error bars correspond to 1 standard deviation, from 2 independent samples for KOH run. As expected, the cell voltage decreased by ~0.7 - 1.0 V, due to the improved anode kinetics as well as lower solution resistance. Although the cathode is separated from the anolyte, we found that CO selectivity was increased at high current density compared to using pure water anolyte (62 % vs 34 % at 200 mA cm ²).

The results described above show that relatively high Faradic efficiencies for CO production can be achieved by using the electrochemical cell of the present invention comprising a cobalt molecular catalyst. The electrochemical cell showed a stable selective production of CO over 4 hours. Switching the anolyte from pure water to 1 M KOH improved both CO selectivity and cell voltage, due to cation crossover however further studies are required to assess the mechanism of how K⁺ enhances the selectivity of this molecular catalyst cathode. These results show that the electrochemical cell of the present invention can provide significant improvements over know devices for the electrochemical reduction of carbon dioxide to carbon monoxide which may lead to a more economic and sustainable production of this useful chemical feedstock. Although a few preferred embodiments have been shown and described, it will be appreciated by those skilled in the art that various changes and modifications might be made without departing from the scope of the invention, as defined in the appended claims.

Effect of salt additives

The effect of additions of salts to the cathode was investigated as follows. CoPc/C cathodes were prepared as described above and then treated, on the side of the cathode comprising the catalyst, with an aliquot of 1 M solution of either NaCI, KCI, or CsCI, to give 100 pmol cm^-2 loading of the salt. The cathodes were left to dry at 40°C in ambient air, before assembly into electrolyzer cells as described above with an lrO_x anode and a Fumasep FBM bipolar membrane. The cells were then tested as described to provide the faradaic efficiency for CO production at various total current densities, as shown in Figure 7. Figure 7 shows the results for the CoPc/C cathode with no additional salts, with NaCI treatment, with KCI and with CsCI treatment (of 100 pmol cm^-2). The anode was lrO_x, and the anolyte was pure H2O. The cathode feed was CO2 saturated with H2O vapor, at 80 seem, and the experiments were carried out at room temperature.

The results show an improvement in faradaic efficiency for CO production at current densities of up to 100 mA cm'², compared to the untreated cathode, for the cathodes treated with KCI, CsCI and NaCI, and an improvement at current densities of up to 200 mA cm^-2 for the cathodes treated with KCI and CsCI.

Throughout this specification, the term “comprising” or “comprises” means including the component(s) specified but not to the exclusion of the presence of other components. The term “consisting essentially of’ or “consists essentially of’ means including the components specified but excluding other components except for materials present as impurities, unavoidable materials present as a result of processes used to provide the components, and components added for a purpose other than achieving the technical effect of the invention. Typically, when referring to compositions, a composition consisting essentially of a set of components will comprise less than 5% by weight, typically less than 3% by weight, more typically less than 1 % by weight of non-specified components.

The term “consisting of’ or “consists of’ means including the components specified but excluding addition of other components.

Whenever appropriate, depending upon the context, the use of the term “comprises” or “comprising” may also be taken to encompass or include the meaning “consists essentially of’ or “consisting essentially of’, and may also be taken to include the meaning “consists of’ or “consisting of’.

For the avoidance of doubt, wherein amounts of components in a composition are described in wt%, this means the weight percentage of the specified component in relation to the whole composition referred to.

The optional features set out herein may be used either individually or in combination with each other where appropriate and particularly in the combinations as set out in the accompanying claims. The optional features for each aspect or exemplary embodiment of the invention as set out herein are also to be read as applicable to any other aspect or exemplary embodiments of the invention, where appropriate. In other words, the skilled person reading this specification should consider the optional features for each exemplary embodiment of the invention as interchangeable and combinable between different exemplary embodiments.

Attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

All of the features disclosed in this specification (including any accompanying claims, and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive.

Each feature disclosed in this specification (including any accompanying claims, and drawings) may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

The invention is not restricted to the details of the foregoing embodiment(s). The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

Claims

Claims

1. An electrochemical cell comprising a cathode, an anode, an ion-exchange membrane separating the anode and cathode, and a gas supply for providing carbon dioxide gas to the cathode; wherein the cathode is provided by a gas diffusion electrode comprising a gas diffusion layer and a molecular catalyst; wherein the ion-exchange membrane comprises a cation-exchange layer and wherein the ionexchange membrane is arranged in direct contact with the cathode and the anode; and wherein the molecular catalyst comprises cobalt and an organic ligand.

2. The electrochemical cell according to claim 1 or claim 2, wherein the organic ligand is a nitrogen-containing heterocyclic ligand.

3. The electrochemical cell according to any one of the preceding claims, wherein the molecular catalyst has a formula which comprises MLaXb, wherein M is cobalt, L is the organic ligand and X is an anion, wherein a is an integer from 1 to 3 and b is an integer from 0 to 3.

4. The electrochemical cell according to claim 3, wherein the nitrogen-containing heterocyclic ligand is an optionally substituted phthalocyanine.

5. The electrochemical cell according to claim 4, wherein the molecular catalyst has the formula (II) or (V): wherein R¹, R², R³, and R⁴ are each independently selected from C1-C20 alkyl, alkenyl, aryl or alkaryl groups, a heteroaryl group, halogens, CF3, CHF2, CH2F, -NH2, -NHR, -NR2, -NR3⁺, -OH, -OR,

-SH, -SR, -CO₂H, -CO2R, -C(O)NH₂, -C(O)NHR, -C(O)NR₂, -SO3H, -SO2NH2, -SO2NHR, -SO2NR2 or a C1-C5 alkyl phosphonate wherein R is an optionally substituted C1-C20 alkyl, alkenyl, aryl or alkaryl group.

6. The electrochemical cell according to any one of the preceding claims, wherein the molecular catalyst of the cathode is arranged in direct contact with the cation-exchange layer of the ionexchange membrane.

7. The electrochemical cell according to any one of the preceding claims, wherein the ionexchange membrane is a bipolar membrane comprising the cation-exchange layer and an anion- exchange layer.

8. The electrochemical cell according to claim 7, wherein the anode is arranged in direct contact with the anion-exchange layer of the ion-exchange membrane.

9. The electrochemical cell according to any one of the preceding claims, wherein the cationexchange layer of the ion-exchange membrane is acidic.

10. The electrochemical cell according to any one of the preceding claims, wherein the ionexchange membrane comprises a water dissociation catalyst.

11 . The electrochemical cell according to any one of the preceding claims, wherein the gas diffusion layer is a porous carbon material.

12. The electrochemical cell according to any one of the preceding claims, wherein the molecular catalyst is adhered to the gas diffusion layer by a composition comprising a fluorocarbon polymer.

13. The electrochemical cell according to any one of the preceding claims, wherein the gas diffusion layer has an electro-active side comprising the molecular catalyst and a back side, wherein the gas supply is directed to the back side of the gas diffusion layer.

14. The electrochemical cell according to any one of the preceding claims, wherein the cathode comprises a salt, preferably in contact with the molecular catalyst.

15. An electrochemical cell comprising a cathode, an anode, an ion-exchange membrane separating the anode and cathode, and a gas supply for providing carbon dioxide gas to the cathode; wherein the cathode is provided by a gas diffusion electrode comprising a gas diffusion layer and a molecular catalyst; wherein the ion-exchange membrane comprises a cation-exchange layer and wherein the ionexchange membrane is arranged in direct contact with the cathode; wherein an anolyte is arranged between the anode and the ion-exchange membrane; and wherein the molecular catalyst comprises cobalt and an organic ligand.

16. A method of electrochemical reduction of carbon dioxide in an electrochemical cell, the method comprising: a) providing an electrochemical cell according to any one of claims 1 to 15, comprising a cathode, an anode, and an ion-exchange membrane and a molecular catalyst comprising cobalt and an organic ligand; b) contacting carbon dioxide in gaseous form with the molecular catalyst, optionally in the presence of water; and c) applying an electrical potential between the cathode and the anode, thereby electrochemically reducing the carbon dioxide to gaseous products including carbon monoxide.

17. The method according to claim 16, wherein step b) takes place at a pH of less than 7.

18. The method according to claim 16 or claim 17, wherein step b) involves providing an anolyte to the anode, wherein the anolyte is an aqueous solution with an alkaline pH.

19. The method according to any one of claims 16 to 18, wherein the carbon monoxide is produced with a Faradic Efficiency of at least 30% at a total current density of 100 mA cm ².

20. The method according to any one of claims 16 to 19, wherein the carbon monoxide is produced with a single pass conversion yield of at least 30%.

21 . Use of a molecular catalyst comprising cobalt and an organic ligand for the electrochemical reduction of carbon dioxide in an electrochemical cell, the electrochemical cell comprising a cathode, an anode, an ion-exchange membrane separating the anode and cathode, and a gas supply for providing carbon dioxide gas to the cathode; wherein the cathode is provided by a gas diffusion electrode comprising a gas diffusion layer and a molecular catalyst; wherein the ion-exchange membrane comprises a cation-exchange layer and wherein the ionexchange membrane is arranged in direct contact with the cathode and the anode.