Catalyst system for electrochemical reduction of carbon dioxide

Abstract

Catalyst system for catalyzed electrochemical reactions comprising an electrically conductive support and a catalyst comprising indium and bismuth and zinc wherein the amount of zinc is 1- 50 wt.% based on the total amount of indium and bismuth and zinc, and gas-diffusion electrode comprising such catalyst system, electrochemical cell containing said gas-diffusion electrode and method for the electrochemical reduction of carbon dioxide with the help of such catalyst system.

Description

CATALYST SYSTEM FOR ELECTROCHEMICAL REDUCTION OF CARBON DIOXIDEField of the invention

[0001] The present invention relates to a catalyst system for catalyzed electrochemical reactions, in particular to a catalyst system for catalyzing the electrochemical reduction of carbon dioxide to prepare products or intermediates thereof like carboxylates and / or carboxylic acids.

[0002] The invention further relates to gas-diffusion electrodes and electrochemical cells comprising a catalyst system for catalyzed electrochemical reactions and to a method for electrochemically reducing carbon dioxide.Description of the background art

[0003] The conversion of carbon dioxide into economically valuable products and intermediates thereof is gaining interest in view of mitigating the emission of carbon dioxide into the atmosphere, resulting in environmental issues such as climate change, ocean acidification and global warming.

[0004] The electrochemical conversion of carbon dioxide is an attractive utilization technique, because of the mild operating conditions such as low temperature and pressure. Furthermore, when powered by renewable energy sources, it can solve the intermittency problem by storing energy in chemical bonds.

[0005] Formate or formic acid is an example of a valuable carbon dioxide reduction product, because the conversion of carbon dioxide into formate only requires two electrons. Furthermore, formate / formic acid has many applications in the pharmaceutical, textile, and food industry and shows promise as a reactant in liquid fuel cells or as a renewable hydrogen carrier molecule.

[0006] WO2019 / 141827 discloses a catalyst system comprising an indium bismuth catalyst for the electrochemical conversion of carbon dioxide into chemical products such as carboxylates and carboxylic acids.

[0007] WO2021 / 122323 teaches a method for regenerating a gas-diffusion electrode comprising an indium containing catalyst used in electrochemical conversion of carbon dioxide. In addition to indium, the catalyst can contain a plethora of other metals, preferably selected from the group consisting of Sn, Pb, Ga and Bi. Most preferably, the catalytic electrode is an indium-bismuth catalyst, an indium-tin catalyst or an indium catalyst. It now has been found that the Faradaic yields of InBi electrodes reduced even after regeneration.

[0008] EP3149228 discloses a method for reducing carbon dioxide in an electrochemical cell comprising receiving carbon dioxide and an alkali metal bicarbonate at the cathode and applying an electrical potential between the cathode and anode. In the cell of Fig. 15, the catalyst for reduction of carbon dioxide may include, for example, indium, tin, bismuth, lead, silver, gold, zinc, and cadmium including their binary and ternary alloys, intermetallics and combinations as single and multiple composition coatings deposited on various metals, carbon, or other conductive supports. An indium-tin alloy is considered most suitable for reducing carbon dioxide to formate.

[0009] There is an ongoing desire to develop catalyst systems with one or more improved catalytic properties such as activity and / or Faradaic efficiency towards the desired productespecially over time and / or after one or more regenerations.Brief summary of the invention

[0010] According to the present invention, a catalyst system for catalyzed electrochemical reactions has been developed which shows an improvement in one or more of said catalytic properties.

[0011] In particular, the invention aims to provide a catalyst system having a high Faradaic efficiency and / or an improved stability of the Faradaic efficiency over time for the electrochemical reduction carbon dioxide into valuable reduction products, in particular carboxylic acids or carboxylates.

[0012] Thus, the present invention relates to a catalyst system for catalyzed electrochemical reactions, the catalyst system comprising a catalyst and an electrically conductive support wherein the catalyst comprises indium and bismuth and zinc and wherein the catalyst comprises 1-50 percent by weight of zinc based on the total amount of indium and bismuth and zinc (wt.%). The metals can be present in metallic form or in the form of a compound containing the metal such as oxides, hydroxides and alloys. The amounts of metals mentioned herein are calculated as weight amounts of metal in metallic form independent of the actual form of each of the metals. The amount of metals can be determined with the help of inductively coupled plasma (ICP) analysis.

[0013] The catalyst system according to the invention comprises an electrically conductive support, preferably comprising a porous structure of carbon particles.

[0014] The catalyst preferably comprises 1-20 wt.% of zinc based on the total amount of indium, bismuth and zinc and preferably comprises 10-90 wt.% of bismuth, more preferably 40-60 wt.%, based on the total amount of indium and bismuth.

[0015] In another aspect, the invention relates to a gas-diffusion electrode comprising a gasdiffusion layer, wherein the gas-diffusion layer comprises the catalyst system according to the invention. The catalyst system is preferably present as a catalyst layer on the gas-diffusion layer. The catalyst system is preferably bound to the gas-diffusion layer with a binder material, preferably a binder that is stable under electrochemical conditions and / or alkaline conditions.

[0016] In another aspect, the invention relates to an electrochemical cell comprising at least one gas chamber and at least one liquid chamber, which chambers are separated by a gas-diffusion electrode according to the invention.

[0017] In another aspect, the invention relates to a method of electrocatalytically reducing carbon dioxide, the method comprising introducing an anolyte to a first cell compartment of an electrochemical cell, the first cell compartment comprising an anode; introducing a catholyte and carbon dioxide to a second cell compartment of the electrochemical cell, the second cell compartment comprising a cathode; and applying an electrical potential between the anode and the cathode sufficient to reduce the carbon dioxide to a reduced reaction product; wherein the cathode comprises a catalyst system according to the invention. Preferably, the cathode is a gasdiffusion electrode according to the invention.

[0018] In the method according to the invention, the carbon dioxide is preferably reduced to a reaction product selected from carboxylates and carboxylic acids, preferably to formate or formicacid in an aqueous medium or to oxalate or oxalic acid in a non-aqueous medium.Detailed description of the invention

[0019] The following is a description of certain embodiments of the invention, given by way of example only and with reference to the drawings.

[0020] The invention relates to a catalyst system for catalyzed electrochemical reactions, the catalyst system comprising a catalyst, wherein the catalyst comprises indium (In) and bismuth (Bi) and zinc (Zn) and wherein the catalyst comprises 1-50 wt.% zinc based on the total amount of indium and bismuth and zinc. The catalyst will herein be referred to as a ZnlnBi catalyst.

[0021] Electrochemical reactions are chemical reduction or oxidation reactions which take place at a required potential and / or a required current density. Such reactions can be enhanced or influenced by catalysts.

[0022] Catalyst systems for catalyzed electrochemical reactions comprising indium-bismuth catalysts are known. Indium bismuth catalysts have been reported as catalysts for the electrochemical conversion of carbon dioxide into formate. WO2019 / 141827 discloses a catalyst system comprising a binary indium-bismuth catalyst.

[0023] A catalyst system typically comprises catalytically active material and support material suitable for supporting the catalytically active material.

[0024] Although zinc is generally regarded as a catalyst for the electrochemical conversion of carbon dioxide into carbon monoxide, it has surprisingly been found that a combination of bismuth, indium and zinc as catalyst for the electrochemical conversion of carbon dioxide shows good selectivity for the reduction of carbon dioxide into carboxylic acids and carboxylates, as well as a good Faradaic yield, in particular for the aqueous conversion of carbon dioxide into formate.

[0025] Compared to catalyst systems comprising an indium bismuth catalyst, the catalyst system comprising a ZnlnBi catalyst according to the invention, comprising 1-50 wt.% zinc based on the total amount of zinc, indium and bismuth can show improved Faradaic yield of CO2 to formate after 1 day at 100 mA / cm2.

[0026] Furthermore, it was found that catalyst systems comprising a ZnlnBi catalyst according to the invention can show improved stability of Faradaic yields over time. For example, a catalyst system comprising a catalyst according to the invention comprising 11.4 wt.% Zn (based on the total amount of ZnlnBi) resulted in high Faradaic yields of CO2 to formate during 1400 hours at 100 mA / cm2.

[0027] The catalyst system according to the invention comprises a ZnlnBi catalyst comprising 1- 50 wt.% zinc based on the total amount of zinc, indium, and bismuth. The ZnlnBi catalyst may comprise at least 1.5 wt.%, or at least 2 wt.%, or at least 3 wt.% of zinc based on the total amount of zinc and indium and bismuth.

[0028] In one embodiment, the catalyst comprises 1-20 wt.%, even more preferably 3-18 wt.% of zinc based on the total amount of indium, bismuth and zinc. Catalyst systems comprising these catalysts show improved stability over time in Faradaic yields in the CO2 to formate conversion.

[0029] In one embodiment, the catalyst system according to the invention preferably comprises a ZnlnBi catalyst comprising up to or equal to 40 wt.% zinc, or up to or equal to 30 wt.% zinc, or up to or equal to 20 wt.% zinc based on the total amount of indium, bismuth and zinc. It was found that catalyst systems comprising these particular amounts of zinc demonstrate improved stability in Faradaic yield of formate in the electrochemical reduction of carbon dioxide to formate.

[0030] The amount of bismuth in the ZnlnBi catalyst is preferably in the range of 10-90 wt.%, more preferably 30-90 wt.%, such as 35-90 wt.% based on the total amount of indium and bismuth. More preferably, the amount of bismuth is in the range of 40-60 wt.% based on the total amount of indium and bismuth, such as 45-55 wt.%. Most preferably, bismuth and indium are present in a weight ratio of 1:1. Catalysts comprising indium and bismuth within these weight ratios offer improved catalytic properties regarding the electrochemical conversion of carbon dioxide into formate.

[0031] The ZnlnBi catalyst in the catalyst system according to the invention may comprise a combination of zinc, indium, and bismuth in different thermodynamic phases. The zinc, indium, and bismuth may be present in the catalyst system as metals and may or may not form alloys. The zinc, indium, and bismuth may be partially oxidized. At the surface of the zinc, indium, and bismuth in the catalyst system, hydroxyl groups may be present. These hydroxyl groups may be linear or bridged hydroxyl groups. Preferably, the zinc, indium and bismuth are present in the catalyst system as an alloy which means a homogenous mixture. Determining whether an alloy is present is within the general knowledge of the skilled person. X-ray diffraction (XRD) may be used to determine whether crystalline alloy phases are present in the catalyst composition. Differential scanning calorimetry (DSC) is another way to determine the presence of alloy phases.

[0032] The catalyst system according to the invention may comprise additional metals or catalytic materials. In one embodiment, the ZnlnBi catalyst may comprise other metals, such as dopants. In a further embodiment, the catalyst system may comprise one or more other metals which may be chosen from the list of tin, antimony, tellurium, thallium and lead. In a preferred embodiment, the catalyst system comprises substantially no other metals than zinc, indium, and bismuth. That is, the zinc, indium, and bismuth present in the catalyst system according to the invention preferably make up at least 95 wt.% of all metals present in the catalyst system, more preferably at least 99 wt.%, even more preferably at least 99.9 wt.%. Preferably, the metals present in the catalyst system consist of indium, bismuth and zinc. In other words, the catalyst system contains as only metals indium, bismuth and zinc.

[0033] The catalyst system according to the invention comprises an electrically conductive support, for example a carbon-containing support. A support may help to form metallic particles with a small size during the synthesis of the catalyst, thereby increasing the overall metallic surface area. Without a support, there is an increased chance that the metals will aggregate forming larger particles. The support may also allow for the catalyst system to be more easily adhered to the cathode in the electrochemical cell. On an operation level, an electrically conductive support may enhance the conductivity during reaction.

[0034] As an electrically conductive support a particulate material, in particular carbon particles, may be used. Preferably, the electrically conductive support comprises a porous structure of carbon particles, such as for example carbon black, activated carbon, carbon nanotubes, Vulcan carbon, acetylene black, or graphite, bound together. The use of particles may allow for supports with high surface area. Compared to other known high surface area supports, carbon, while conductive, is not catalytically active in the reduction of carbon dioxide under electrochemical conditions. Therefore, a support comprising carbon particles has the advantage that the support material will not interfere with the desired reaction that is catalyzed by the metals ( / .e. zinc, indium, bismuth).

[0035] The ZnlnBi catalyst is deposited onto or adhered to the electrically conductive substrate. The weight ratio of the total metal loading, which means the weight amount of zinc, indium, andbismuth, based on the total weight of metal containing compounds and substrate, may advantageously be in the range of 10-90 wt.%. A too high total metal loading on the electrically conductive support may lead to aggregation of the metals resulting in a relative decrease in active metal surface area. A too low metal loading on the electrically conductive support leads to a low Faradaic yield during catalysis due to a low availability of active metal surface area.

[0036] The term "active metal surface area" herein refers to the surface of the zinc, indium, and bismuth that is accessible to the reagents and electrons during the reduction of carbon dioxide. The active surface area may be metallic or may include oxygen-containing species such as hydroxyl groups.

[0037] In a preferred embodiment, the weight ratio of the metal loading may be between 10-60 wt.%, more preferably between 25-50 wt.%, for example around 40 wt.% based on the total weight of the metals and the substrate.

[0038] Catalyst systems according to the invention may be prepared with any suitable method known to the skilled worker. As an example, catalyst systems according to the invention may be prepared by combining metal salts and a ligand in a liquid to obtain a precursor mixture. The precursor mixture may be combined with a reducing agent to obtain a suspension comprising the catalyst, from which the catalyst can be separated. A support material may be added to either the precursor mixture or to the reducing agent before combining it with the precursor mixture. The ligand may advantageously be an oxalate salt, which may allow for high and reproducible yields. The liquid may be water. Metal chlorides or nitrates are suitable metal salts.

[0039] The electrochemical reduction of carbon dioxide into chemical reduction products is generally performed in an electrochemical cell or photochemical cell having one or more cell compartments containing the respective electrodes. The electrodes are separated by a distance such that a current flows between them through an electrolytic solution. The potential that is reached between the two electrodes depends amongst others on the distance between the two electrodes and the composition of the electrolytic solution. The potential required for a reaction is characteristic for a certain chemical reaction. The direct current causes ions in the electrolyte to be attracted towards the respective oppositely charged electrode. Electrochemical reactions typically take place at the surface of the electrodes.

[0040] Typically, a divided cell having two cell compartments is used. One cell compartment contains the anode, and the other cell compartment contains a cathode, therefore generally referred to as the anodic and cathodic compartment respectively. The two cell compartments may be separated by a suitable membrane, e.g. made from porous glass frit, microporous material, ion exchanging membrane or ion conducting bridge, allowing (specific) ionic species to travel from one compartment to the other, such as protons generated at the anode to the cathode compartment.

[0041] Carbon dioxide is supplied to the cathode, where it can be reduced to a desired reduction product. Carbon dioxide may be supplied to the cathode in ways known to the skilled worker. For example, carbon dioxide can be introduced into the cathodic compartment by bubbling the carbon dioxide into a liquid medium in the cathodic compartment. The bubbling of the carbon dioxide into the liquid medium may also take place outside of the electrochemical cell after which the liquid medium comprising the carbon dioxide, either in the form of bubbles or (partially) dissolved in the liquid medium, is transported to the electrochemical cell. Carbon dioxide may also be introduced to the cathode / cathodic compartment by making use of a gas-diffusion electrode.

[0042] Preferably, the cathode is a gas-diffusion electrode via which the carbon dioxide is introduced to the electrochemical cell. Such a gas-diffusion electrode comprises a gas-diffusion layer. The gas-diffusion layer provides a thin porous structure or network e.g. made from carbon, for passing a gas like carbon dioxide from one side to the other. Typically the structure is hydrophobic to repel water. The gas-diffusion electrode may further comprise an electrically conductive substrate, which may serve as a supporting structure for a gas-diffusion layer, making the gas-diffusion electrode more structurally robust. Said conductive substrate may also allow for gas to diffuse through the substrate.

[0043] The use of a gas-diffusion electrode to introduce carbon dioxide into the cathodic compartment is advantageous, because it allows for an enhanced mass transport of carbon dioxide to the active sites of the catalyst. In addition, it provides a high surface area / interface for liquid-gas contact, which allows for increased dissolution speed of the carbon dioxide into the liquid medium. This is advantageous, because gaseous bubbles hamper the electrical conductivity in the liquid medium, whereas dissolved carbon dioxide improves the conductivity.

[0044] A gas-diffusion layer may comprise a catalytically active material. Therefore a further aspect of the invention relates to a gas-diffusion electrode, comprising a gas-diffusion layer, wherein the gas-diffusion layer comprises the catalyst system according to the invention as outlined above. The catalyst system comprising zinc, indium, and bismuth according to the invention may be embedded in the gas-diffusion layer structure or provided as one or more additional separate layers thereof, i.e. one or more catalyst layers. The advantage of applying the catalyst system as a catalyst layer is that it maximizes contact between the three phase boundary (gas-liquid-catalyst). Applying the catalyst system as a catalyst layer on the gas-diffusion layer allows for a more effective use of the catalyst system: less material is necessary and the interaction with the liquid in the cathode compartment and the catalyst surface is increased.

[0045] The catalyst system is preferably bound to the gas-diffusion electrode using a binder. The binder is preferably stable under alkaline conditions and / or under reducing conditions, such as a fluorinated binder. The skilled person knows how to select a binder based on the requirements. In one embodiment, the binder is a hydrophobic binder. This is in particular useful when the catalyst system is applied to catalyze electrochemical reactions that take place in an aqueous environment. A hydrophobic binder will less likely disintegrate and / or dissolve in an aqueous medium, allowing for the electrically conductive support and the metals in the catalyst system to stay bound during said electrochemical reactions. The use of a binder to bind or adhere the catalyst system to the gas-diffusion electrode improves the mechanical stability. It may further help to prevent leaching of the metals during reaction. Additionally, it allows for a more efficient way of preparing the gas-diffusion electrode. Suitable binders include but are not limited to polytetrafluoroethylene (PTFE), polyvinylidene fluroride (PVDF), imidazolium functionalized styrene polymers such as those commercially available under the trade name Sustainion, polybenzimidazole (PBI), sulfonated tetrafluoroethylene based fluoropolymer-copolymers such as those commercially available under the trade name Nation and perfluorosulfonic acid such as those commercially available under the trade name Aquivion.

[0046] As explained above, a particulate carbon is a preferred example of the conductive support for the catalyst in the catalyst system according to the invention. The use of a catalyst system comprising the conductive support allows for improved adhering of the catalyst system to the gas diffusion layer of the gas-diffusion electrode.

[0047] Examples of suitable materials for the gas-diffusion electrode include metal structures like expanded or woven metals, metal foams, and carbon structures including wovens, cloth and paper. Suitable metals for the gas-diffusion electrodes are metals that are not active under electrochemical conditions and that are corrosion-resistant. An example of a suitable metal is titanium. Carbon structures are preferred, because they are lighter and more pliable than metal.

[0048] Afurther aspect of the invention concerns a method of preparing a gas-diffusion electrode as defined above, comprising the tertiary metal electrocatalyst system according to the invention. This manufacturing method comprises a step of providing a gas-diffusion electrode comprising a gas-diffusion layer and a step of applying the catalyst system according to the invention, comprising zinc and indium and bismuth and an electrically conductive support, in particular a particulate carbon support material, and a binder to the gas-diffusion layer of the gas-diffusion electrode.

[0049] The gas-diffusion electrode comprising the catalyst system according to the invention may be manufactured in various ways including spraying, casting and sintering, which may comprise using one or more suitable binders.

[0050] The catalyst system and a suitable binder are applied to the gas-diffusion layer to achieve a dispersion of the catalyst system over the gas-diffusion layer. This may be achieved by spraying a formulation containing the catalyst system and a suitable binder onto the gas-diffusion layer, thereby forming a layer, the so-called catalyst layer.

[0051] In another aspect the invention relates to an electrochemical cell comprising at least one gas chamber and at least one liquid chamber, which chambers are separated by a gas-diffusion electrode according to the invention. The gas chamber is arranged to receive carbon dioxide gas which may diffuse through the gas-diffusion electrode towards the liquid chamber. This can be established by, for example, building up a pressure of carbon dioxide in the gas chamber which results in a pressure difference with the liquid chamber, allowing gaseous carbon dioxide molecules to diffuse through the gas-diffusion cathode to the liquid chamber. The liquid chamber on the other side of the gas-diffusion cathode may also comprise an anode at a suitable distance from the gas-diffusion cathode. The liquid chamber may also be divided into a cathodic compartment and an anodic compartment (comprising the anode) separated by a suitable membrane. The liquid chamber is arranged to receive an electrolytic solution which may flow through the liquid chamber via an inlet and outlet. In case the liquid chamber comprises two compartments separated by a membrane, the two compartments may receive the same electrolytic solution or may be arranged to each receive a separate electrolytic solution flow.

[0052] In another aspect, the invention further relates to a method of electrocatalytically converting carbon dioxide into valuable products or product intermediates. This method comprises: introducing an anolyte to a first cell compartment of an electrochemical cell, the first cell compartment comprising an anode; introducing a catholyte and carbon dioxide to a second cell compartment of the electrochemical cell, the second cell compartment comprising a cathode, and applying an electrical potential between the anode and the cathode sufficient to reduce carbon dioxide to a reduced reaction product, wherein the cathode comprises a catalyst system according to the invention. Preferably, the cathode is a gas-diffusion electrode according to the invention.

[0053] An anolyte is an electrolytic solution, in other words an electrolyte, that flows through the anodic compartment comprising the anode. A catholyte is an electrolytic solution, i.e. electrolyte, that flows through the cathodic compartment comprising the cathode. Electrolytes are typically liquid mediums comprising free ions that allow for the conduction of the electric current that is applied to the electrodes of the electrolysis cell. The composition of the anolyte and catholyte may be the same or different depending on the electrochemical reactions that take place in the respective compartments. The electrolyte may include one or more suitable salts, such as KCI, NaNOs, NajSC , NaCI, NaF, NaCIO4, KCIO4, KjSiOa or CaCL, e.g. at a concentration of about 0.5 M. Other additives may include Group I cations (H, li, Na, K, Rb and Cs except Fr), divalent cations (e.g., Ca2+, Mg2+, Zn2+) ammonium, alkylammonium cations and alkyl amines. Examples of anions comprise halides, carbonates, bicarbonates, nitrates, nitrites, perchlorates, phosphates, polyphosphates, silicates and sulfates. Bicarbonate is a preferred anion, because it is present as a buffer (pKa = 6.4). At low pH values, carbonic acid is liberated as gaseous carbon dioxide. It is within the common knowledge of the skilled worker to select a solute and determine the composition of the electrolytes.

[0054] The method according to the invention allows to reduce carbon dioxide to carboxylic acid and / or intermediates, including carboxylates such as formate, glycolate, glyoxylate, oxalate and lactate, carboxylic acids, and glycols. The production of a carboxylic acid or carboxylic acid intermediate, such as carboxylate, may be dependent on the pH of the electrolyte solution in the cell, with lower pH ranges favoring carboxylic acid production.

[0055] The pH of the cathode compartment may be adjusted to favor production of one of a carboxylic acid or carboxylic acid intermediate over production of the other, such as by introducing an acid (e.g., HCI or HzSO4) to the cathode compartment. The pH of the catholyte is preferably between about 1 and 8. A pH range of 1-4 is preferable for production of carboxylic acids from carbon dioxide. A pH range of 4-8 is preferable for production of carboxylic acid intermediates, such as carboxylates, from carbon dioxide. A pH between 6-8, preferably around pH 7 is preferred however. At a too low pH the Faradaic yield towards the desired carbon dioxide reduction products decreases. This is due to the fact that a competing side reaction becomes thermodynamically favored, namely the reduction of protons (H+) to hydrogen. At a too high pH, hydroxyl ions (OH ) react with carbon dioxide to form bicarbonate, which is also an undesired side reaction. The use of a pH around 7, that is between 6.5 and 7.5, preferably between 6.8 and 7.2 has demonstrated good Faradaic yields for the electrochemical reduction of carbon dioxide to formate.

[0056] It is noted that applying an electrical potential is considered synonymous with creating a voltage difference between the cathode and the anode, so that the anode is at a higher potential than the cathode. The process may be controlled by setting a certain voltage (potentiostatic) or by setting a certain current (galvanostatic). If the voltage is set, the current will automatically follow from the reactions that occur in the cell. If the current is set, the voltage will automatically follow from the reactions that occur in the cell. The process according to the invention is equally workable in both operation modes. Typically, the current is controlled in the start-up phase of an electrochemical cell, in order to find the optimal voltage for the desired reaction, while during standard operation of the electrochemical cell, the voltage will be controlled.

[0057] The applied current density may be in the range of 0.05 and 10 kA / m2. In preferred embodiments, the applied current density is at least 0.1 kA / m2, more preferably at least 0.5 kA / m2. Herein, the currents are defined based on the projected area of the electrode. The optimal currentfor the process according to the invention may differ based on the exact conditions that are applicable in the electrolysis cell, and the skilled person is able to determine the optimal current in terms of product conversions.

[0058] The applied electrical potential may be a DC voltage. In preferred embodiments, the applied electrical potential is generally between about -1.5V vs. SHE (Standard Hydrogen Electrode) and about -6V vs. SHE, preferably from about -1.5V vs. SHE to about -5V vs. SHE, such as in the range of -3V vs. SHE to -5V vs SHE and more preferably from about -1.5V vs. SHE to about -4V vs. SHE. The thermodynamic potential of the electrochemical reduction of carbon dioxide to formate is around -1.3 V. At a too low voltage, little current will pass resulting in a low carbon dioxide turnover. At a too high voltage, a very large current will pass, resulting in a decreased Faradaic efficiency. The selection of an applied voltage depends not only the desired reaction's thermodynamic potential, but also on the resistance of the used electrolyte. It is within the knowledge of the skilled worker to select a suitable voltage for the desired electrochemical reaction.

[0059] The skilled person is capable of selecting an anodic reaction to take place in the anodic compartment while carbon dioxide is reduced in the cathodic compartment. High Faradaic yield and selectivity of the catalyst system according to the invention for conversion of carbon dioxide into formate / formic acid have been shown at the cathode according to the reaction CO2 + 2H++ 2e -> HCOOH, while at the anode water may be oxidized into oxygen and hydrogen ions according 4H++ O2 + 4e_. In this embodiment, the hydrogen ions (i.e. protons) pass through an ion exchange membrane from the anodic compartment to the cathodic compartment in the electrochemical cell.

[0060] The carbon dioxide conversion to formate / formic acid is typically performed in an aqueous medium, wherein the carbon dioxide is bubbled through the aqueous medium using for example percolator systems and / or wherein the carbon dioxide in introduced to the aqueous medium via a gas-diffusion electrode making use of a gas chamber which is separated from the liquid medium via the gas-diffusion electrode. In a preferred method according to the invention, a gas-diffusion electrode, in particular a gas-diffusion cathode, is used.

[0061] Non-aqueous media may also be used, e.g. in the direct conversion of carbon dioxide to oxalic acid or oxalate.

[0062] A homogeneous heterocyclic catalyst may be added to the cathode compartment of the cell containing the cathode. The homogeneous heterocyclic catalyst may include, for example, one or more of 4-hydroxy pyridine, adenine, a heterocyclic amine containing sulfur, a heterocyclic amine containing oxygen, an azole, a benzimidazole, a bipyridine, furan, an imidazole, an imidazole related species with at least one five-member ring, an indole, a lutidine, methylimidazole, an oxazole, phenanthroline, pterin, pteridine, a pyridine, a pyridine related species with at least one six-member ring, pyrrole, quinoline, or a thiazole, and mixtures thereof. A homogeneous heterocyclic catalyst allows to steer the activity and / or selectivity towards desired end products, such as formate, and / or steer the activity / selectivity away from undesired side reactions such as hydrogen evolution. If present, the homogeneous heterocyclic catalyst is preferably present at a concentration of between about 0.001M and about IM, and more preferably between about 0.01M and 0.5M. A too low concentration does not lead towards the desired effect of steering the reaction activity / selectivity, while a too high concentration might lead to a change in viscosity or a change in conductivity.

[0063] The chemicals derived as reaction products from the direct electrochemical conversion according to the invention can be processed further into industrial products. An example is oxalic acid which can be used as a starting material for the production of ethylene glycol and / or glycine as described in US2016 / 0017503. A further example is formic acid which is used in the pharmaceutical, textile, and food industry. Hydrogen may be introduced to the carboxylic acid or carboxylic acid intermediate to produce a glycol or a carboxylic acid, respectively. Hydrogen may be derived from natural gas or water.Description of the drawings

[0064] The features and advantages of the invention will be appreciated upon reference to the following drawings, in which:

[0065] Figure I shows an embodiment of an electrochemical cell comprising a gas-diffusion electrode comprising a catalyst system according to the invention.

[0066] Figure II shows the Faraday yield of formate in the catalyzed electrochemical carbon dioxide reduction after one day using different ZnlnBi catalysts according to the invention.

[0067] Figure III shows the stability over time of the Faraday yield of formate in the catalyzed electrochemical carbon dioxide reduction using different ZnlnBi catalysts according to the invention.

[0068] Figure IV compares a ZnlnBi catalyst according to the invention with an InBi catalyst with and without refreshment cycles.

[0069] The drawings are intended for illustrative purposes only, and do not serve as a restriction of the scope or the protection as specified in the claims.Detailed description of the drawings

[0070] In Figure I, an electrochemical cell comprising a gas-diffusion electrode according to the invention is schematically depicted. The electrochemical cell (1) comprises a gas chamber (2) which is arranged to allow the introduction of gaseous carbon dioxide (3). The electrochemical cell(1) further comprises a liquid chamber (17) which contains a cathodic compartment (8) and an anodic compartment (10), which are separated with a membrane (9). The membrane (9) may be an ion exchange membrane (9). The cathodic compartment comprises a cathode (4) and the anodic compartment comprises anode (11).

[0071] The cathode (4) of this embodiment of an electrochemical cell according to the invention is a gas-diffusion electrode (4). The gas-diffusion electrode (4) separates the gas chamber (2) from the liquid chamber (17) and allows for the diffusion of carbon dioxide (3) from the gas chamber(2) towards the cathodic compartment (8). The side of the gas-diffusion electrode near the gas chamber is typically referred to as the back-side of the gas-diffusion electrode. The gas-diffusion electrode (4) comprises a gas-diffusion layer (6) which allows for the diffusion of carbon dioxide from the gas chamber (2) into the liquid chamber (17), in particular into the cathodic compartment (8). The gas-diffusion layer may comprise a catalyst system according to the invention. In the embodiment depicted in Figure I, the catalyst system is depicted as a catalyst layer (7) that is sprayed onto the gas-diffusion layer (6). The gas-diffusion electrode (4) of this embodiment further comprises a conductive substrate (5) which allows for the diffusion of the gaseous carbon dioxide (3).

[0072] The two electrodes (4, 11) may be electrically connected via electrical power source (12), such that the electrical power source (12) allows for applying a potential sufficient to reduce carbon dioxide to reduction products such as formate, and obtain a current density, e.g. in the range of 0.05 and 10 kA / m2at the cathode (4). In preferred embodiments, the applied current density is at least 0.1 kA / m2, more preferably at least 0.5 kA / m2.

[0073] The cathodic compartment (8) may be arranged such that a catholyte solution may be introduced into the cathodic compartment (8) via a first bottom entry (13) of the cathodic compartment and may be discharged via first exit (14) at the top of the cathodic compartment (8). Alternatively, the flow of catholyte solution may be reversed such that the flow is in the downward vertical direction. The catholyte solution may be fed at specific rates, such as in the range of 0.001 to 10 liters per minute or more depending on the electrochemical cell dimensions, so that the cathode gas diffusion electrode (4) may not be flooded with the catholyte solution due to excessive pressure, and so as to maintain good ionic contact with the cathode gas diffusion electrode (4) for the transfer of electrons into the solution in the reduction of carbon dioxide.

[0074] In the embodiment of Figure I, carbon dioxide may diffuse through the gas-diffusion electrode (4) to be reduced at the catalyst layer (7) comprising a catalyst system according to the invention, into preferably formate. The anodic reaction may be the oxygen evolution reaction (OER), wherein water is oxidized to form oxygen gas. The anodic compartment (10) may be arranged such that an anolyte solution may be introduced into the anodic compartment (10) via a second bottom entry (15) and flow through the anodic compartment (10) to a second exit (17). Alternatively, the flow of anolyte solution may be reversed such that the flow is in the downward vertical direction. The electrical source (12) applying a potential to the two electrodes (4, 11) allows for water to be oxidized to oxygen gas.ExamplesMaterials

[0075] Bismuth(lll) chloride (BiCU, 99.999%, Thermo Scientific), Indium(lll) chloride (InCU, 99.999%, Thermo Scientific), Zinc(ll) chloride (ZnCL, 99.999%, Thermo Scientific), Milli-Q. grade H2O (deionized, 18.2 MQ-cm), carbon black (Vulcan XC72R, FuelCellStore), sodium oxalate (NazCzC , 99.5+%, Thermo Scientific), and sodium borohydride (NaBH4, 98+%, ACROS) were used for the catalyst synthesis.

[0076] Potassium bicarbonate (KHCO3, 99%, Thermo Scientific) was used in the catholyte, sulfuric acid (H2SO4, 95-98 wt.% / 17.8-18.4 M, Thermo Scientific) was used in the anolyte.

[0077] Hydrochloric acid (HCI, 37 wt.% / 12.1 M, Thermo Scientific) was used to dilute a sample when determining the Faradaic yield towards formate.Characterization techniques

[0078] The catalyst composition was determined with Inductively Coupled Plasma (ICP) analysis using a Perkin Elmer Avio200 system equipped with an Optical Emission Spectrometer (OES). Yttrium served as the internal standard. Prior to analysis, 30 mg catalyst was dissolved in 5 mL concentrated HNO3 and an Anton Paar Microwave digestion system was employed for acid digestion.

[0079] Thermalgravimetric analysis (TGA) was performed on a Mettler Toledo TGA / DSC 3+ STARe System to determine the total metal loading of the catalysts. The samples (~ 6 mg, 40 pL alumina crucible) were heated to 600 °C / min with a 50 °C / min temperature slope in air (50 mL / min). Thistemperature was maintained for 2 h. Any loss of mass during the measurement was assumed to solely be due to the combustion of the support.Electrode preparation

[0080] Gas diffusion electrodes (GDE's) were prepared by airbrushing catalyst ink onto a commercial gas diffusion electrode (GDL) by FuelCellStore (CeTech Carbon Cloth with MPL, W1S1011). The ink was formulated by combining the catalyst and a fluorinated polymer (binder) in a water / isopropanol mixture. To homogenize the ink, the resulting combination was sonicated for 1 h at room temperature. Next, the ink was airbrushed onto the GDL using a Paasche VL varnish gun until a theoretical metal loading of 0.5 mg / cm2was reached. The metal loading was determined by weighing the GDL before and after spraying.Catalyst system synthesis

[0081] All catalyst systems were prepared using a wet chemical reduction method based on the article by Pavesi et al. in the article "CO2 electroreduction on bimetallic Pd-ln nanoparticles" in Catalysis, Science & Technology 10.13 (2020), 4264-4270. In each synthesis, the amounts used are to prepare 630-960 mg catalyst with a 40 wt.% total metal loading. First, BiCU, InCU, and ZnCL, 2120 mg sodium oxalate, and 547 mL water were combined in a 1 L glass jar. The resulting dispersion was sonicated in a 60 °C water bath for 1 h to dissolve the precursors. The catalyst composition was varied by keeping the total metal concentration constant (5.0 mM), but adjusting the (relative) amounts of In, Bi, and Zn. Thereafter, carbon black was added and the sonication step was repeated. To reduce the metals, 246 mL of a 100 mM sodium borohydrate solution was added with a constant flow rate of 10 mL / min. This step was performed in a 60 °C water bath and under vigorous magnetic stirring (1000 rpm). After 2 h, the sample was washed with approximately 2 L water (50 °C) using a pressure filter (0.22 pm Durapore membrane filter paper). Finally, the catalyst system was dried in a 50 °C oven overnight and grinded into a fine powder. The catalyst systems were analyzed with thermogravimetric analysis (TGA) and inductively coupled plasma (ICP) and the results are shown in Table 1.

[0082] Comparative example 1 is a catalyst system comprising a catalyst containing approximately 50:50 wt% of indium and bismuth. It is known from WO2019 / 141827 that this ratio of indium : bismuth is effective for the catalyzed electrochemical conversion of carbon dioxide into formate. The same weight ratio was aimed for in the examples according to the invention to demonstrate the effect of zinc at different weight ratios while keeping the ln:Bi ratio approximately constant.

[0083] Comparative example 2 is a catalyst system comprising a catalyst containing zinc and no indium nor bismuth to demonstrate the advantage of the combination of zinc, indium, and bismuth comprised in a catalyst in a catalyst system.Tabel 1: Catalyst systems prepared[b]as determined with ICP - based on the total amount of indium, bismuth and zinc.Electrolysis

[0084] The catalyst systems, applied on the GDE's, were tested for their performance in the electrochemical conversion of carbon dioxide into formate. All experiments were performed in a three-compartment Micro Flow Cell by Electrocell. The cells were connected to an electrochemical workstation and galvanostatically operated at 100 mA / cm2. The working electrode was the prepared GDE inside a Ti picture frame current collector. The anode was a Ti current collector coated with IrOz and with the OER (oxygen evolution reaction) as the anodic reaction. Both electrodes had an exposed area of 10 cm2. The catholyte (0.5 M KHCO3) and anolyte (0.5 M H2SO4) were circulated in a closed-loop at 50 mL / min (STP) using a peristaltic pump (Masterflex L / S). A Nation N324 Cation Exchange Membrane (CEM) separated the electrolyte compartments. To maintain a pH of 7.0 at the GDE, the catholyte was dosed with 3 M KHCO3 using an Iwaki metering pump when the pH fell below this value. Carbon dioxide gas was fed to the back-side of the GDE with a constant flow rate of 50 mL / min (ambient temperature and pressure) at the gas inlet. This was regulated by a mass flow controller obtained from Alicat Scientific.

[0085] To monitor the Faraday yield of formate over time, catholyte samples were taken every 24 hours and the formate yield was determined with UV-Vis spectroscopy (using a GENESYS 150 UV-Vis spectrophotometer using a single wavelength (220 nm) quantification method).Results

[0086] Figure II is a bar graph showing the Faraday yield of formate after 1 day. All catalyst systems according to the invention (comprising indium, bismuth and zinc) demonstrate an improved yield compared to a catalyst system comprising only zinc or only indium and bismuth.

[0087] Without being bound to theory, it is believed that the presence of zinc may stabilize the indium hydroxide bonds at the surface of the catalyst, which are believed to be active species in the electrochemical reduction of carbon dioxide into formate.

[0088] Without being bound to theory, it is believed that catalyst systems comprising zinc, indium and bismuth, comprise a relative high amount of surface hydroxyl groups, which are believed to be active species in the electrochemical reduction of carbon dioxide into formate.

[0089] Figure III shows the stability of the Faraday yield of formate in the electrochemical reduction of carbon dioxide into formate using some of the catalyst systems of Table 1. While a catalyst system comprising indium and bismuth (comparative example 1) demonstrates reasonably good initial Faraday yields towards formate, as can also be seen in Figure II, the stability of the Faraday yield decreases rapidly over time. In comparison, all catalyst systems according tothe invention, comprising indium, bismuth and zinc, demonstrate improved stability of the Faraday yield towards formate. The increased stability was particularly improved for catalyst systems comprising relatively little zinc. Examples 2 and 4, for example, show Faradaic yields over 90% after 1200 h on stream.

[0090] Comparative example 1 comprises an I n Bi catalyst and shows low stability in Faradaic yield towards formate. In WO2021 / 122323, a regeneration method is disclosed for regenerating the gas-diffusion electrode comprising an InBi catalyst in the electrochemical conversion of carbon dioxide. Figure IV shows the Faradaic yield of comparative example 1 (comprising an InBi catalyst) without and with a regeneration treatment every 24 hours as described in WO2021 / 122323 and compares this to example 6 according to the invention. Catalyst systems according to the invention show improved Faradaic yield with improved stability over longer times on stream without the need for a regeneration.

Claims

CLAIMS1. Catalyst system for catalyzed electrochemical reactions, the catalyst system comprising a catalyst and an electrically conductive support, wherein the catalyst comprises indium and bismuth and zinc and wherein the catalyst comprises 1-50 wt.% zinc based on the total amount of indium and bismuth and zinc.

2. Catalyst system according to claim 1, wherein the electrically conductive support comprises a porous structure of carbon particles.

3. Catalyst system according to any of the preceding claims, wherein the catalyst comprises 1-20 wt.% zinc based on the total amount of indium and bismuth and zinc.

4. Catalyst system according to any of the preceding claims, wherein the catalyst comprises 10- 90 wt.% bismuth based on the total amount of indium and bismuth.

5. Catalyst system according to any of the preceding claims, wherein the catalyst comprises 40- 60 wt.% bismuth based on the total amount of indium and bismuth.

6. Gas-diffusion electrode comprising a gas-diffusion layer, wherein the gas-diffusion layer comprises the catalyst system according to any one of the preceding claims.

7. Gas-diffusion electrode according to claim 6, wherein the catalyst system is bound to the gasdiffusion layer with a binder material, wherein the binder material is preferably a hydrophobic binder material.

8. Gas-diffusion electrode according to any of claims 6 or 7, wherein the catalyst system is applied on the gas-diffusion layer as a catalyst layer.

9. Electrochemical cell comprising at least one gas chamber and at least one liquid chamber, which chambers are separated by a gas-diffusion electrode according to any of claims 6 to 8.

10. Method of electrocatalytically reducing carbon dioxide, comprising: introducing an anolyte to a first cell compartment of an electrochemical cell, the first cell compartment comprising an anode; introducing a catholyte and carbon dioxide to a second cell compartment of the electrochemical cell, the second cell compartment comprising a cathode; and applying an electrical potential between the anode and the cathode sufficient to reduce the carbon dioxide to a reduced reaction product; wherein the cathode comprises a catalyst system according to any one of claims 1 to 5.

11. Method according to claim 10, wherein the cathode is a gas-diffusion electrode according to any one of claims 6 to 8.

12. Method according to claim 10 or 11, wherein carbon dioxide is reduced to a reaction product selected from carboxylates and carboxylic acids.

13. Method according to any one of claims 10 to 12, wherein carbon dioxide is reduced to formate or formic acid in an aqueous medium.

14. Method according to any one of claims 10 to 12, wherein carbon dioxide is reduced to oxalate or oxalic acid in a non-aqueous medium.

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