FORMATION OF FORMIC ACID WITH THE HELP OF INDIUM-CONTAINING CATALYTIC ELECTRODE

Electrochemical conversion of CO2 to formic acid or a salt thereof, using an indium containing catalytic electrode, comprising (a) electrochemically converting CO2 to formic acid or a salt thereof by applying a voltage to an electrochemical cell comprising the catalytic electrode as cathode and an anode, wherein the electrochemical cell is fed with an electrolyte comprising CO2; and (b) regenerating the catalytic electrode by lowering the voltage and subsequently washing the catalytic electrode with an aqueous liquid and exposing the catalytic electrode to air without applying voltage; and (c) optionally repeating steps (a) and (b).

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Description
FIELD OF THE INVENTION

The present invention is in the field of electrochemistry, especially in the electrochemical conversion of carbon dioxide.

BACKGROUND ART

The electrochemical conversion of carbon dioxide into economically valuable materials such as fuels and industrial chemicals or intermediate products thereof is gaining interest in view of mitigating the emission of carbon dioxide into the atmosphere, which is responsible for damaging effects such as climate alterations, changes in pH of seawater, melting of polar ice and sea level rise. The electrochemical Indium-containing catalysts for the electrochemical reduction of CO2 to formate are known in the art, e.g. from WO 2019/141827, WO 2014/032000 and WO 2014/042781. Catalyst stability is one of the major bottlenecks in electroconversion of CO2 to valuable compounds such as formate. Solving this problem is therefore extremely important in order to enable the technological viability and scale-up for industrially relevant processes to be developed. The present invention provides in the need for an easy regeneration of indium-containing catalysts for the electrochemical reduction of CO2 to formate, which increased faradaic yields and electrode lifetime. US2013/105304 relates to a process for the electrochemical conversion of CO2 to formic acid or a salt thereof using a high surface area cathode which may include an indium coating and having a void volume of between 30 and 98%. The performance of this system may decrease with regard to formate yield which may result from catalyst loss or over-coating of the catalyst with impurities such as other metals that may be plated onto the cathode. The surfaces of the cathode may be renewed by the periodic addition of indium salts or a mix of indium/tin salts in situ during operation of the electrolyzer. Other or additional metal salts may be added in situ as well. During injection of the metal salts, the electrolyzer may be temporarily operated at a lower current density with or without carbon dioxide addition. US2015/218716 relates to reduction of carbon dioxide to products in a method wherein an indium cathode is oxidized. US2019/085477 relates to electrochemical conversion of carbon dioxide. It is taught that the catalyst can be refreshed by stopping the electrolytic voltage and reduction reaction, discharging the cathode solution and the anode solution and supplying a rinse solution while applying a refresh voltage. The refresh voltage may be cyclically applied so that the oxidation treatment of the ions and the impurities and the reduction treatment are alternately performed. Next, gas is supplied to dry the cathode and anode. When the rinse solution is supplied to the cathode solution flow path, the saturation degree of water in the gas diffusion layer increases and output reduction occurs due to the diffusibility of gas. By supplying the gas, the saturation degree of water lowers so that the cell performance is recovered and the refresh effect is increased. The exemplified process uses a membrane electro catalyst assembly (MEA).

SUMMARY OF THE INVENTION

The inventors have surprisingly found that an indium-containing catalytic electrode for the formation of formic acid could readily be regenerated by exposure to air. Even more surprisingly, the Faraday yields obtained at the electrode were increased and the lifetime of the electrode could be increased by this regeneration. The regeneration according to the invention can be employed in a process for the electrochemical conversion of CO2 to formic acid or a salt thereof, comprising: (a) electrochemically converting CO2 to formic acid or a salt thereof by applying a voltage to an electrochemical cell comprising the catalytic electrode as cathode and an anode, wherein the electrochemical cell is fed with an electrolyte comprising CO2; (b) regenerating the catalytic electrode by lowering the voltage and subsequently washing the catalytic electrode with an aqueous liquid and exposing the catalytic electrode to air without applying voltage; and (c) optionally repeating steps (a) and (b). The regeneration according to the invention is able to provide an improved faradaic yield of the electrochemical reaction and an improved lifetime of the electrode. The invention further concerns an electrochemical cell assembly for the continuous reduction of CO2 to formic acid for regeneration according to the invention. Voltage is not applied if there is a current of 0 mA/min in other words no current. Preferably, no voltage is applied during each the wash with aqueous liquid and exposure to air. It is preferred that step (b) comprises regenerating the catalytic electrode by lowering the voltage followed by washing the catalytic electrode with an aqueous liquid and subsequently exposing the catalytic electrode to air without applying voltage; and (c) optionally repeating steps (a) and (b). It is preferred to repeat steps (a) and (b) in step (c).

An aspect of the invention can be defined as a process for the electrochemical conversion of CO2 to formic acid or a salt thereof, using an indium containing catalytic electrode, comprising: (a) electrochemically converting CO2 to formic acid or a salt thereof by applying a voltage to an electrochemical cell comprising the catalytic electrode as cathode and an anode, wherein the electrochemical cell is fed with an electrolyte comprising CO2; (b) regenerating the catalytic electrode by lowering the voltage and subsequently washing the catalytic electrode with an aqueous liquid and exposing the catalytic electrode to air without applying voltage; and (c) optionally repeating steps (a) and (b).

The catalytic electrode preferably is an indium-bismuth catalyst, indium-tin catalyst or an indium catalyst, more preferably an indium-bismuth catalyst. The process preferably is operated in cycles wherein step (c) is repeated at least 10 times, more preferably 1000-1000000 times. In each cycle the duration of step (b) preferably is 0.1-50%, more preferably 1-10%, most preferably 3-7% of the duration of step (a); and/or wherein the duration of step (a) in a single cycle is in the range of 1-100 h, preferably 20-30 h, and wherein the duration of step (b) in a single cycle is in the range of 0.1-2.5 h, preferably 0.5-1.5 h, most preferably wherein the duration of step (a) in a single cycle is 23 h and the duration of step (b) in a single cycle is in the range of 1 h. The exposure of the electrode to air in step (b) preferably is performed by feeding air to the electrochemical cell, more preferably wherein the electrochemical cell is equipped with air jets. The catalytic electrode preferably is a gas diffusion electrode, more preferably wherein air is led through the gas diffusion electrode during step (b). The process preferably is a continuous process, more preferably a process wherein a plurality of electrochemical cells are connected in parallel and wherein some of the cells are being subjected to the regeneration of step (b) while other cells are simultaneously used for the conversion of step (a). The aqueous liquid for use in the process preferably is deionized water or the electrolyte used during step (a). The process preferably further comprises a control system which determines the performance of the electrochemical cell, preferably by determining the faradaic yield, and wherein step (a) is interrupted and step (b) is initiated in case the performance drops below a predetermined threshold value. Step (b) of the process preferably involves (1) ramping down the current, preferably with a decrease of 0.1-10 mA/min, more preferably 1-4 A/min, and no longer applying an external voltage; (2) stopping the liquid and gas flows; (3) washing the catalytic electrode with the aqueous liquid; (4) feeding air, preferably at a rate of 0.01-10 L/min, more preferably 0.05-0.5 L/min; (5) starting the liquid and gas flows; (6) ramping up the current, preferably with an increase of 0.1-10 mA/min, more preferably 1-4 A/min. A further aspect of the invention is use of a regeneration step for improving the faradaic yield of an electrochemical process, wherein the regeneration involves lowering the voltage, washing the catalytic electrode with an aqueous liquid and exposing the catalytic electrode to air, and wherein the electrochemical process comprises the conversion of CO2 to formic acid or a salt thereof, using an indium-containing catalytic electrode.

Another aspect is use of a regeneration step for improving the lifetime of an indium-containing catalytic electrode, wherein the regeneration involves lowering the voltage, washing the catalytic electrode with an aqueous liquid and exposing the catalytic electrode to air, and wherein the electrode is used for an electrochemical process comprising the conversion of CO2 to formic acid or a salt thereof.

A further aspect is an electrochemical cell assembly for the continuous reduction of CO2 to formic acid, comprising a plurality of electrochemical cells, each cell comprising an anode and an indium-containing catalytic gas diffusion electrode as cathode, wherein the cathode is configured to receive either an electrolyte containing CO2 or air, and wherein each cell further contains an outlet for discharging formic acid or a salt thereof wherein the gas diffusion electrodes are equipped with an air jet stream to enable contacting of the electrode with air during regeneration. The electrochemical cell assembly preferably comprises a plurality of electrochemical cells arranged in blocks each containing an equal amount of electrochemical cells, preferably 1-25 electrochemical cells, most preferably 1 or 10 electrochemical cells, wherein each block alternates between a first position wherein it is used for conversion of CO2 to formic acid or a salt thereof and a second position wherein it is regenerated. Preferably, each electrochemical cell contains a cathode compartment and an anode compartment separated by at least one membrane and wherein the cathode compartment contains an inlet for receiving either the electrolyte containing CO2 or air and the anode compartment a separate inlet for receiving an anolyte.

DETAILED DESCRIPTION

Electrochemical cells are well-known in the art. They are equipped with an anode and a cathode and may comprise one or more semi-permeable membranes located in between the anode and cathode, as such forming an anode compartment and a cathode compartment. In operation, an oxidation reaction occurs at the anode and a reduction reaction occurs at the cathode. Indium-containing catalysts for the electrochemical reduction of CO2 to formate are known in the art. These catalytic electrodes are ideally suitable as cathode.

The present inventors have developed a process for the regeneration of such an indium-containing electrode. The regeneration process according to the invention not only regenerates the electrode, but even improves its performance, in particular the faradaic yield, and its lifetime. Hence, in one aspect the regeneration according to the present invention is used to improve the faradaic yield of the electrode. Alternatively, in one aspect the regeneration according to the present invention is used to improve the lifetime of the electrode. In another aspect, the present invention concerns a process for the electrochemical reduction of CO2 to formate, making use of the regeneration according to the present invention. The invention further concerns an electrochemical cell assembly for the continuous reduction of CO2 to formic acid, which is specifically designed to execute the regeneration according to the present invention. The definition of the electrode and the regeneration process according to the present invention are common to all aspects of the present invention.

Without being bound to a theory, the inventors believe that the catalytic layer is freed from deposits that block the catalyst during the washing step. The washing liberates the metal layer, which is activated by exposure to air. It is believed that the catalytic active sites of the catalyst may be rearranged during exposure to air. Since a colour change was observed after air regeneration, it is assumed the catalyst is at least partially oxidized during regeneration, leading to such structural and/or morphological rearrangement. The increase in faradaic yield after a regeneration step may only occur after several minutes or even an hour or so, indicating that the rearrangement may be triggered by air (oxidation), but occurs slowly.

The Electrode

The electrode in the context of the present invention is an indium-containing catalytic electrode for the electrochemical reduction of CO2 to formate.

In addition to indium, the electrode may contain further elements, such as one or more elements selected from the group consisting of C, Pt, Pd, Rh, Mo, Zr, Nb, Os, Au, Ag, Ti, Cu, Ir, Ru, Re, Hg, Pb, Ni, Co, Zn, Cd, Sn, Fe, Cr, Mn, Ga, TI, Sb, Ga and Bi, preferably from the group consisting of Sn, Pb, Ga and Bi. The atoms are typically present in their metallic form, although oxides have also been known to reduce carbon dioxide. The cathode may contain further components, such as ligands to stabilize the metal atoms and/or to catalyse the reduction of CO2, e.g. hydrides, halides, phosphines and porphyrins. Single metal Indium-cathodes may be used as well as alloys. Indium-containing alloys have been found particularly effective in the reduction of 002. Preferred electrodes are selected from indium electrodes, indium-bismuth electrodes, indium-tin electrodes and indium-lead electrodes. In a preferred embodiment, the catalytic electrode is an indium-bismuth catalyst, indium-tin catalyst or an indium catalyst, most preferably an indium-bismuth catalyst.

In a preferred embodiment, the catalytic electrode is an indium-bismuth electrode, wherein the amount of bismuth is in the range of 5-94 wt. % based on the total amount of bismuth and indium, preferably in the range of 10-90 wt. %, more preferably 30-90 wt. %, such as 35-90 wt. %, most preferably in the range of 40-60 wt. %, such as 45-55 wt. %. Such ratios have shown to provide improved catalytic properties regarding carbon dioxide to formate conversion, see e.g. WO 2019/141827. The catalyst can comprise a combination of bismuth and indium in different thermodynamic phases.

The electrode may contain a porous support. The porous support allows gas and liquid to interact. The porous support may be structured as a foam, felt and/or mesh. The electrode can consist of the catalytic material, but the catalytic material may also be deposited on a support, such as a carbon support. Preferably, the catalyst is applied in combination with an electrically conductive support. As a conductive support a particulate material, in particular carbon particles, may be used. Preferably the conductive support comprises a porous structure of carbon particles bonded together. A preferred binding material is a hydrophobic binder, such as a fluorinated binder. The catalyst is deposited onto or adhered to the conductive material. The weight ratio of indium and bismuth to carbon can advantageously be in the range of 0.10-1.50, preferably 0.2-0.8.

In a preferred embodiment, the electrode is a gas diffusion electrode (GDE). Gas diffusion electrodes are highly suitable for the reduction of CO2, especially when CO2 in gaseous form is used as electrolyte. A gas-diffusion electrode provides a high surface area or interface for solid-liquid-gas contact. Such a gas-diffusion electrode typically comprises an electrically conductive substrate, which may serve as a supporting structure for 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 distract water. The gas diffusion layer may comprise the catalytically active material. By diffusion of gaseous CO2 through the pores of the cathode, the area that is available for reducing CO2 is maximized, as such increasing the overall efficacy of the process according to the invention. Additionally, the same gas inlet can be used to receiving air during the regeneration according to the present invention.

The gas diffusion electrode typically contains the indium-containing catalytic system embedded in the gas-diffusion layer or provided as one or more additional separate layers thereof. Examples of suitable substrates include metal structures like expanded or woven metals, metal foams, and carbon structures including wovens, cloth and paper. As explained above, the conductive support for the catalyst is preferably formed by particulate carbon. The catalyst system is preferably bonded to the electrically conductive substrate using a hydrophobic binder, such as PTFE.

Regeneration

The regeneration according to the present invention involves lowering the voltage. After lowering of the voltage, no external voltage is to be applied during the remainder of the regeneration. Subsequently, the catalytic electrode is washed with an aqueous liquid and exposed to air. The exposure of the electrode to air is typically performed by feeding air to the electrochemical cell, preferably wherein the electrochemical cell is equipped with air jets which are configured to feed the air to the electrochemical cell. Preferably, the feeding of air is directly to the electrode. In case the electrode is a gas diffusion electrode, exposure to air is conveniently performed by leading air through the gas diffusion electrode. In case the air originates from a compressor, it is preferred that the air is first led through a filter to remove any oil or other particles, before using the air for regeneration.

Before exposure to air, the catalytic electrode is washed with an aqueous liquid. Without being bound to a theory, the washing step was found to be essential for liberating the metal atoms (indium and possibly other metal(s)) before exposure to air. The water of the aqueous liquid may be pure (deionized) water or may contain other components, such as inert gases (e.g. N2, Ar), H2, bases such as bicarbonate and/or formate (e.g. potassium or sodium salts). Preferably, the aqueous liquid should be essentially free from divalent cations such as Ca2+ and Mg2+. In a preferred embodiment, the aqueous liquid is deionized water or the electrolyte used during step (a). Most preferably, deionized water is used.

The duration of the exposure to air may be in the range of 1 min-24 h, preferably 0.1-10 h. optimal results have been obtained with regeneration for about 18 h but also with regeneration for about 1 h in total (air flow for about 30 min), indicating that the exact duration of this step is not crucial. In one embodiment, the duration of step (4) is 0.5-1.5 h, most preferably about 1 h. The air flow may have a flow rate of 0.1-100 mL/min per cm2 electrode surface area, more preferably 0.5-10 L/min per cm2 electrode surface area, most preferably 1-3 L/min per cm2 electrode surface area. Such flow rate and duration have been found particularly suitable to regenerate a gas diffusion electrode.

The regeneration according to the present invention preferably involves in the indicated sequence:

  • (1) ramping down the current;
  • (2) stopping the liquid and gas flows;
  • (3) washing the electrode;
  • (4) feeding air, preferably at a rate of 0.01-10 L/min, more preferably 0.05-0.5 L/min;
  • (5) starting the liquid and gas flows;
  • (6) ramping up the current.

The regeneration process according to this embodiment preferably starts with lowering the current such that the electrochemical conversion is halted. In step (1), the current is lowered, preferably with a decrease of 0.1-10 mA/min, more preferably 1-4 A/min. At the end of step (1), the current is typically reduced to 0 mA/min and no external voltage is applied. If the electrode is part of a gas diffusion electrode, the washing fluid of step (3) and air of step (4) preferably are fed to the outward facing surface of the indium containing electrode. The outward facing surface is the surface which is in first contact with the cathode electrolyte.

An electrochemical process typically involves the feeding of one or more electrolytes to the electrochemical cell. An electrolyte may be gaseous and/or liquid. In the context of reducing CO2, the electrolyte contains a source of CO2, in which case the electrolytes are often but not necessarily gaseous. In step (2), the flow of the electrolyte, in gaseous and/or liquid form, is stopped. If both liquid and gas flows are present, it is preferred that first the gas flow is stopped, and then the liquid flow. Step (2) is performed once step (1) is completed.

In step (3), the catalytic electrode is washed with an aqueous liquid without applying voltage. Such washing or rinsing is typically performed directly before step (4). The washing step is further defined above.

The regeneration may include a step of draining of the cell, or the catholyte compartment thereof, such that there are no substantial amounts of liquid present. If performed, this is typically done after step (3) and before the air is introduced. Even though using a drained cell for regeneration provides optimal results, in an alternative embodiment, air may be introduced when the aqueous liquid, as introduced in step (3), is still present. As such, step (4) is performed by bubbling air through the liquid.

During step (4), the actual regeneration by exposure to air takes places without applying voltage. The electrode is contacted with a stream of air, preferably with a flow rate of 0.01-10 L/min, more preferably 0.05-0.5 L/min. The duration of this step may be in the range of 1 min-24 h, preferably 0.1-10 h. Optimal results have been obtained with regeneration for about 18 h but also with regeneration for about 1 h in total (air flow for about 30 min), indicating that the exact duration of this step is not crucial. In one embodiment, the duration of step (4) is 0.5-1.5 h, most preferably about 1 h.

In step (5), the flow of electrolyte is started again, in order to restart normal operation after the current is reinstated. If both liquid and gas flows are present, it is preferred that first the liquid flow is started, and then the gas flow. In step (6), the current is ramped up again, which is preferably performed with an increase of 0.1-10 mA/min, more preferably 1-4 A/min. Step (6) may be started as soon as the electrolyte flow(s) has been started. At the end of step (6), the electrochemical cell is operative again.

The entire regeneration process is preferably performed at or near ambient pressure and temperature, although deviation from these conditions is possible without significantly affecting the regeneration. In one embodiment, the temperature during the regeneration is in the range of 10-50° C., preferably 15-40° C. Advantageously, the temperature and pressure during regeneration are the same as those during operation of the electrochemical cell.

Preferably, the regeneration is part of a cyclic process wherein operation and regeneration are alternated, such as in the process according to the invention. The regeneration according to the present invention can also be used for activating the catalytic electrode, such as at the start of an electrochemical conversion.

The Process

The process according to the invention utilizes the regeneration as defined above.

Herein, step (a) is the operation step of an electrochemical cell, wherein carbon dioxide is converted, and step (b) is the regeneration step. The process according to the invention can be used to prepare formic acid or formate, such as sodium formate or potassium formate. Whether formic acid or formate is formed, depends primarily on the pH within the electrochemical cell. When the pH is below the pKa of formic acid, formic acid will be formed, and when the pH is above the pKa of formic acid, formate will be formed. Typically, the pH will be too high for the formation of formic acid. The electrochemical conversion of CO2 to formic acid or formate is known to the skilled person. This conversion occurs at the cathode. This cathodic reaction can be coupled to any anodic reaction, such as oxygen or chlorine evolution. The formation of formic acid at the cathode may also be coupled to formation of formic acid at the anode, e.g. by oxidation of glycerol.

Step (a) involves feeding to the cathode an electrolyte comprising carbon dioxide. Typically, the carbon dioxide is comprised in a catholyte fed to the cathode, and the process may further comprise feeding an anolyte to the anode. Usually, the catholyte is fed to a first cell compartment of an electrochemical cell, comprising the cathode, while the anolyte is fed to a second cell compartment of the electrochemical cell, comprising the anode. The carbon dioxide conversion to formate/formic acid is typically performed in an aqueous medium, wherein the CO2 is bubbled through the aqueous medium or distributed through the gas-diffusion electrode, e.g. using perculator systems.

During step (a), an electrical potential is applied between the anode and the cathode sufficient to reduce carbon dioxide to formic acid or a salt thereof. The anode is positively charged and the cathode negatively. In other words, an electrical potential to the electrochemical cell so that the anode is at a higher potential than the cathode. Cations, typically protons, will thus flow from the anode towards the cathode where they combine with an oxygen atom liberated from CO2 to form a water molecule. Electrons, liberated at the anode by the anodic reaction, are taken up by the anode, while they are generated at the cathode to be combined with the protons and oxygen atoms into water molecules and the product of the CO2 reduction (formic acid or formate). The electrical potential may be a DC voltage. In preferred embodiments, the applied electrical potential is generally between about −1.5 V vs. SCE and about −6 V vs. SCE, preferably from about −1.5 V vs. SCE to about −5 V vs. SCE, such as in the range of −3 V vs. SCE to −5 V vs SCE and more preferably from about −1.5 V vs. SCE to about −4 V vs. SCE.

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 (galvanostatic) or by setting a certain current (potentiostatic). 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. The process according to the invention operates with such a voltage difference and/or such a current that carbon dioxide is reduced at the cathode.

Preferably, the current density of the electrochemical cell during operation is at least 10 mA/cm2, such as in the range of 10 mA/cm2-5 A/cm2, more preferably at least 100 mA/cm2, such as in the range 100 mA/cm2-3 A/cm2. A certain minimal current of at least 10 mA/cm2, preferably at least 100 mA/cm2, is preferred in terms of process economics, as below these values too little product is formed for an economically viable process. The upper limit of the current at which the process can operate is solely determined by safety issues. For example, it the current is too high, the cell may heat up too much. Other than that, higher currents are preferred since it will result in more product formation. Excellent results have been obtained with a current density in the range of 100-200 mA/cm2. Herein, the currents are defined based on the projected area of the electrode. The optimal current for the process according to the invention may differ based on the exact conditions that are applicable in the electrochemical cell, and the skilled person is able to determine the optimal current in terms of product conversions.

The cathode is the electrode subject to the present invention and as further defined above. The anode may be any suitable anode known in the art. The material of the anode is preferably tailored to the desired anodic reaction, as will be understood by the skilled person.

The reduction of CO2 to formic acid is known in the art. The half reaction is typically as follows: CO2+2H++2 e−→HCO2H [E0=−0.20 V vs. RHE]. The carbon dioxide is supplied to the cathode and consumed there. CO2 can be fed in liquid or gaseous form. The solution of carbon dioxide may aqueous or non-aqueous and may include buffers such as bicarbonates and/or phosphates. Non-aqueous electrolytes have been found beneficial in the reduction of CO2 as the side-reaction at higher potentials wherein H2 is formed (due to reduction of protons in solution) is reduced. CO2 gas can also be fed to the cathode compartment through gas diffusion electrode (GDE). Neutral pH was found to give the best results in terms of CO2 reduction. In one preferred embodiment, the cathode is a GDE and is fed with a gaseous catholyte. In an alternative preferred embodiment, the catholyte is aqueous and liquid catholyte is present in the cathode compartment. It is well-known to the skilled person to select specific electrochemical conditions (e.g. the voltage applied and catholyte composition) in order to optimize the formation of formic acid or formate.

As protons may enter the cathode compartment during operation of the electrochemical cell, some base may be present to the catholyte. Thus, in one embodiment, the catholyte comprises a base, typically as contained in a buffer solution, in such an amount to keep the pH of the catholyte within the cathode compartment neutral. Herein, neutral pH refers to a pH in the range of 6-8, preferably 6.5-7.5, most preferably about 7. The type of buffer solution is not crucial for the operation of the electrochemical cell, and a suitable example is potassium bicarbonate. The skilled person knows how to determine the optimal amount of base, based on the desired pH in the cathode compartment. Since protons are also consumed by the reaction at the cathode, the base should not eliminate all protons. By maintaining the pH of the catholyte in the desired range, the amount of base will not be excessive.

The CO2 that is comprised in the catholyte may originate from any source. In a preferred embodiment, the CO2 originates from exhaust gases, flue gases or air. Typically, the CO2 originates from industrial flue gases, such as from power plants or the chemical industry. CO2 can be captured from exhaust gases, flue gases and air by methods known in the art. In case the CO2 is provided via a gas diffusion electrode as the electrode according to the present invention, it is preferred that the concentration of CO2 in the gas is as high as possible, such as above 90 wt %, preferably above 95 wt %, more preferably above 99% wt % or even above 99.9 wt %. In addition to CO2, some other gaseous species may be present, such as inert gases (N2, Ar) and/or H2. The presence of O2 in the gas fed to the electrode is preferably avoided.

The process according to the invention may contain a control system that determines when a regeneration step should be performed. This control system determines the performance of the electrochemical cell, preferably by determining the faradaic yield. Step (a) is interrupted and step (b) is initiated in case the performance drops below a predetermined threshold value. As such, regeneration is only initiated when needed, and the performance of the electrochemical cell is further optimized.

The process according to the invention is performed in an electrochemical cell, preferably the electrochemical cell according to the invention and further defined below. The process according to the invention may be a continuous process, preferably wherein a plurality of electrochemical cells are connected in parallel and wherein some of the cells are being subjected to the regeneration of step (b) while other cells are simultaneously used for the conversion of step (a). Preferred embodiments for the catalytic electrode as cathode and the regeneration step are further defined above.

The Electrochemical Cell

The inventors have further designed an electrochemical cell assembly for operating the process according to the invention. The electrochemical cell assembly according to the invention is for the continuous reduction of CO2 to formic acid, comprising a plurality of electrochemical cells, each cell comprising an anode and an indium-containing catalytic gas diffusion electrode as cathode, wherein the cathode is configured to receive either an electrolyte containing CO2 or air, and wherein each cell further contains an outlet for discharging formic acid or a salt thereof and the assembly is equipped with an air jet stream to enable contacting of the electrode with air during regeneration.

Each electrochemical cell may contain a cathode compartment and an anode compartment separated by at least one membrane and wherein the cathode compartments contains the inlet for receiving either an electrolyte containing CO2 or air to the indium-containing catalytic gas diffusion electrode, and the anode compartment a separate inlet for receiving an anolyte. The membrane may be made from porous glass frit, microporous material, ion exchanging membrane or ion conducting bridge, and allows ionic species to travel from one compartment to the other, such as protons generated at the anode to the cathode compartment.

The electrochemical cell assembly may contain the plurality of electrochemical cells arranged in blocks, wherein each block typically contains an equal amount of electrochemical cells, preferably 1-25 electrochemical cells, most preferably 1 or 10 electrochemical cells. During operation, each block alternates between a first position wherein it is used for conversion of CO2 to formic acid or a salt thereof, i.e. step (a) of the process according to the present invention, and a second position wherein it is regenerated, i.e. step (b) of the process according to the present invention.

EXAMPLES Example 1

CO2 was converted into potassium formate using an electrochemical cell equipped with a gas diffusion electrode consisting of a high-hydrostatic head gas diffusion layer and a highly active catalytic layer made of indium/bismuth (50/50 w/w) particles. The catalyst was prepared as described in WO 2019/141827. The gas diffusion electrode was prepared as described in US 2014/0227634 A1. 308 mg of the In-Bi particles were dispersed in 90 mL of isopropanol and stirred for at least 1 hour at room temperature. 77 mg PVDF (flex binder mass from Kynar) was dissolved in 90 mL of acetone. The catalyst ink and the PVDF solution were sprayed on the GDL in a layer-by-layer fashion. Care was taken not to flood the GDL during the spraying process. After the spraying, the electrode was allowed to dry overnight (˜15 h) at room temperature before being used for electrochemical testing. The catalyst loading was 1.83 mg/cm2 over a total electrode surface of 168 cm2. At the anode, H2O was oxidized to O2 using an anolyte containing 0.5 M H2SO4.

The electrode was tested in three 6-hour runs for a cumulative runtime of 18 hours, performed at a current density of 150 mA/cm2. After each run, the electrode was taken out from the cell, rinsed thoroughly with deionized water and exposed to air overnight at room temperature. After the first run, the colour of the catalytic layer on the electrode turned darker compared to the as-synthesized electrode (see FIG. 1).

The Faradaic yield during each of the three runs is depicted in FIG. 2. Average values for pH, cell voltage and Faradaic yields per run are given in the table below:

Average cell Average Faradaic Run Average pH voltage (V) yield (%) 1 6.90 +/− 0.34 4.49 +/− 0.10 63.4 +/− 3.9 2 6.51 +/− 0.40 4.33 +/− 0.05  79.5 +/− 13.6 3 6.51 +/− 0.45 4.34 +/− 0.06 87.9 +/− 4.4

The faradaic yield increased from 63% in the first run to 80% in the second run and 88% in the third run. Interestingly, the faradaic yield in the second run started off at around 60% and then steadily increased to 80%, indicative of activation of the catalyst after the regeneration cycle. Visual inspection of the electrode revealed no salt accumulation in its structure.

Example 2

CO2 was converted into potassium formate using an electrochemical cell equipped with a gas diffusion electrode consisting of a high-hydrostatic head gas diffusion layer and a highly active catalytic layer made of indium/bismuth (50/50 w/w) particles. The catalyst was prepared as described in WO 2019/141827. The gas diffusion electrode was prepared as described in US 2014/0227634 A1. 308 mg of the In-Bi particles were dispersed in 90 mL of isopropanol and stirred for at least 1 hour at room temperature. 77 mg PVDF (flex binder mass from Kynar) was dissolved in 90 mL of acetone. The catalyst ink and the PVDF solution were sprayed on the GDL in a layer-by-layer fashion. Care was taken not to flood the GDL during the spraying process. After the spraying, the electrode was allowed to dry overnight at room temperature before being used for electrochemical testing. The catalyst loading was 0.386 mg/cm2 over a total electrode surface of 233 cm2.

The electrode was tested over 7 days with refresh cycles every 20-24 hours for a cumulative runtime of 101 hours, performed at a current density of 100 mA/cm2 and an active area of 100 cm2. At each 20-24 hours increment the cell was ramped down at 2 A/min, flow of gas was stopped [depressurized], anolyte and catholyte flows were stopped, water was introduced to the front of the cathode for the washing cycle. After 2 washes, the air was introduced to the front of the cathode for 20-30 minutes. After the each regeneration cycle, the faradic yield was maintained. A control run was performed, wherein operation and catalyst was the same as above, except no regeneration cycles were performed. In the control run, the faradaic yield gradually declined.

For the process according to the invention, including regeneration cycles, the lapsed time and average pH, cell voltage and Faradaic yields per cycle are given in the table below:

Time Average cell Average Faradaic (h) Average pH voltage (V) yield (%) 20 6.8 +/− 0.50 4.3 +/− 0.10  96.3 +/− 3 44 6.8 +/− 0.50 4.3 +/− 0.10 86.87 +/− 3 67 6.8 +/− 0.50 4.3 +/− 0.10 93.33 +/− 3 74 6.8 +/− 0.50 4.3 +/− 0.10 91.91 +/− 3 80 6.8 +/− 0.50 4.124 +/− 0.10  95.11 +/− 3 97 6.79 +/− 0.50  4.164 +/− 0.10  95.76 +/− 3 101 6.74 +/− 0.50  4.153 +/− 0.10  95.60 +/− 3

For the control process, without regeneration cycles, the lapsed time and average pH, cell voltage and Faradaic yields per cycle are given in the table below:

Time Average cell Average Faradaic (h) Average pH voltage (V) yield (%) 17  7.7 +/− 0.50 4.1 +/− 0.10 76.5 +/− 3 24 7.54 +/− 0.50 4.2 +/− 0.10 78.1 +/− 3 89 5.22 +/− 0.50 4.1 +/− 0.10 80.4 +/− 3 113  7.4 +/− 0.50 4.3 +/− 0.10 50.88 +/− 3 

The faradaic yield achieved during the control was consistently below 80%, until the end where the yield dropped to 50%. In the regeneration run, the faradaic yield was consistently above 90%, indicative of activation of the catalyst after the regeneration cycle.

Example 3

CO2 was converted into potassium formate using an electrochemical cell equipped with a gas diffusion electrode consisting of a high-hydrostatic head gas diffusion layer and a highly active catalytic layer made of indium/bismuth (50/50 w/w) particles. The catalyst was prepared as described in WO 2019/141827. The gas diffusion electrode was prepared as described in US 2014/0227634 A1. 308 mg of the In-Bi particles were dispersed in 90 mL of isopropanol and stirred for at least 1 hour at room temperature. 38.5 mg PVDF (flex binder mass from Kynar) was dissolved in 90 mL of acetone. The catalyst ink and the PVDF solution were sprayed on the GDL in a layer-by-layer fashion. Care was taken not to flood the GDL during the spraying process. After the spraying, the electrode was allowed to dry overnight at room temperature before being used for electrochemical testing. The catalyst loading was 1.62 mg/cm2 over a total electrode surface of 253 cm2.

The electrode was tested over 7 days with regeneration cycles every 20-24 hours for a cumulative runtime of 110 hours, performed at a current density of 200 mA/cm2 and an active area of 10 cm2. At each 20-24 hours increment the cell was ramped down at 0.5 A/min, the flow of gas was stopped [depressurized], the anolyte and catholyte flows were stopped, water was introduced to the front of the cathode for the washing step. After 2 washes, the air was introduced to the front of the cathode for 20-30 minutes. After the each regeneration cycle, the faradic yield was maintained at about the same level. A control run was performed, wherein operation and catalyst was the same as above, except no regeneration cycles were performed (See Example 2). In the control run, the faradaic yield gradually declined.

For the process according to the invention, including regeneration cycles, the lapsed time and average values for pH and Faradaic yields per cycle are given in the table below:

Time Average Faradaic (h) Average pH yield (%) 17 7.2 +/− 0.50  107 +/− 10 41 7.2 +/− 0.50 102 +/− 3 47 7.2 +/− 0.50 100 +/− 3 64 7.2 +/− 0.50 100 +/− 3 69 7.2 +/− 0.50 101 +/− 3 90 7.2 +/− 0.50 100 +/− 3 110 7.2 +/− 0.50  96 +/− 3

The faradaic yield increased from the control was consistently below 80%, until the end where the yield dropped to 50%. In the regeneration run all faradaic yields were consistently 100%, indicative of activation of the catalyst after the regeneration cycle.

Example 4

CO2 was converted into potassium formate using an electrochemical cell equipped with a gas diffusion electrode consisting of a high-hydrostatic head gas diffusion layer and a highly active catalytic layer made of indium/bismuth (50/50 w/w) particles. The catalyst was prepared as described in WO 2019/141827. The gas diffusion electrode was prepared as described in US 2014/0227634 A1. 308 mg of the In-Bi particles supported on carbon in a ratio of 50% metal: 50% carbon were dispersed in 90 mL of isopropanol and stirred for at least 1 hour at room temperature. 41.3 mg PVDF (flex binder mass from Kynar) was dissolved in 90 mL of acetone. The catalyst ink and the PVDF solution were sprayed on the GDL in a layer-by-layer fashion. Care was taken not to flood the GDL during the spraying process. After the spraying, the electrode was allowed to dry overnight at room temperature before being used for electrochemical testing. The catalyst loading was 0.315 mg/cm2 over a total electrode surface of 250 cm2.

The electrodes were tested over 3 days with regeneration cycles every 20-24 hours (except control run 1) for a cumulative runtime of 72 hours, performed at a current density of 100 mA/cm2 and an active area of 10 cm2. At each 20-24 hours increment the cell of runs 2-4 were ramped down at 0.5 A/min, the flow of gas was stopped [depressurized], and anolyte and catholyte flows were stopped. In runs 2 and 4, water was introduced to the front of the cathode for the washing cycle. In runs 3 and 4 (for run 4: after the washing step), air was introduced to the front of the cathode for 20-30 minutes.

The average values for the faradaic yields (%) per cycle, for each of the runs, are given in the table below:

Time Run 1 Run 2 Run 3 Run 4 (h) [control] [water only] [air only] [water and air] 24 — 56 +/− 5 77 +/− 5 103 +/− 5  48 — 51 +/− 5 77 +/− 5 88 +/− 5 72 50 +/− 5 42 +/− 5 74 +/− 5 80 +/− 5

The faradaic yield for the control run was at 50% at the end of the experiment. In the regeneration run with water only and air only, the faradaic yields were consistently below 80%, a while for the regeneration run with water only and air the faradaic yield was consistently above 80%, indicative of activation of the catalyst after the regeneration cycle of both water and air being necessary to complete the regeneration.

Claims

1. A process for the electrochemical conversion of carbon dioxide to formic acid or a salt thereof, using an indium-containing catalytic electrode, comprising:

(a) electrochemically converting carbon dioxide to formic acid or a salt thereof by applying a voltage to an electrochemical cell comprising the catalytic electrode as cathode and an anode, wherein the electrochemical cell is fed with an electrolyte comprising carbon dioxide; and
(b) regenerating the catalytic electrode by lowering the voltage and subsequently washing the catalytic electrode with an aqueous liquid and exposing the catalytic electrode to air without applying voltage; and
(c) optionally repeating steps (a) and (b).

2. The process according to claim 1, wherein the catalytic electrode is an indium-bismuth catalyst, indium-tin catalyst or an indium catalyst.

3. The process according to claim 1, wherein the process is operated in cycles wherein step (c) is repeated at least 10 times.

4. The process according to claim 1, wherein in each cycle the duration of step (b) is 0.1 50% of the duration of step (a); wherein the duration of step (a) in a single cycle is in the range of 1-100 h, and wherein the duration of step (b) in a single cycle is in the range of 0.1-2.5 h.

5. The process according to claim 1, wherein the exposure of the electrode to air in step (b) is performed by feeding air to the electrochemical cell, wherein the electrochemical cell is equipped with air jets.

6. The process according to claim 1, wherein the catalytic electrode is a gas diffusion electrode, wherein air is led through the gas diffusion electrode during step (b).

7. The process according to claim 1, wherein a plurality of electrochemical cells are connected in parallel and wherein some of the cells are being subjected to the regeneration of step (b) while other cells are simultaneously used for the conversion of step (a).

8. The process according to claim 1, wherein the aqueous liquid is deionized water or the electrolyte used during step (a).

9. The process according to claim 1, wherein a control system is in place which determines the performance of the electrochemical cell, by determining the faradaic yield, and wherein step (a) is interrupted and step (b) is initiated in case the performance drops below a predetermined threshold value.

10. The process according to claim 1, wherein feeding of the electrolyte comprising carbon dioxide of step (a) involves feeding a liquid and gas flow and step (b) involves:

(1) ramping down the current, preferably with a decrease of 0.1-10 mA/min;
(2) stopping the liquid and gas flows;
(3) washing the catalytic electrode with the aqueous liquid;
(4) feeding air at a rate of 0.01 10 L/min;
(5) starting the liquid and gas flows;
(6) ramping up the current with an increase of 0.1 10 mA/min.

11. (canceled)

12. (canceled)

13. An electrochemical cell assembly for reduction of carbon dioxide to formic acid, comprising a plurality of electrochemical cells, each cell comprising an anode and an indium-containing catalytic gas diffusion electrode as cathode, wherein the cathode is configured to receive either an electrolyte containing carbon dioxide or washing liquid or air, and wherein each cell further contains an outlet for discharging formic acid or a salt thereof, wherein the gas diffusion electrodes are equipped with an air jet stream to enable contacting of the electrode with air during regeneration.

14. The electrochemical cell assembly according to claim 13, wherein the plurality of electrochemical cells are arranged in blocks each containing an equal amount of electrochemical cells, wherein each block alternates between a first position wherein it is used for conversion of carbon dioxide to formic acid or a salt thereof and a second position wherein it is regenerated.

15. The electrochemical cell assembly according to claim 13, wherein each electrochemical cell contains a cathode compartment and an anode compartment separated by at least one membrane and wherein the cathode compartment contains an inlet for receiving either the electrolyte containing carbon dioxide or air and the anode compartment a separate inlet for receiving an anolyte.

Patent History
Publication number: 20220411942
Type: Application
Filed: Dec 10, 2020
Publication Date: Dec 29, 2022
Inventors: Davide Ansovini (Amsterdam), Matthew Francis Philips (Amsterdam), Julia L. Krasovic (Plainsboro, NJ), Marta Catarina Costa Figueiredo (Amsterdam), Klaas Jan Pieter Schouten (Amsterdam)
Application Number: 17/781,600
Classifications
International Classification: C25B 3/07 (20060101); C25B 3/26 (20060101); C25B 15/02 (20060101); C25B 11/032 (20060101); C25B 9/70 (20060101); C25B 11/089 (20060101);