ELECTROCHEMICAL CELL FOR CARBON DIOXIDE CONVERSION WITH IMPROVED CARBON DIOXIDE CONVERSION EFFICIENCY

Description

CROSS-REFERENCE TO RELATED APPLICATION

This application claims under 35 U.S.C. § 119(a) the benefit of priority to Korean Patent Application No. 10-2024-0103866 filed on Aug. 5, 2024, the entire contents of which are incorporated herein by reference.

BACKGROUND

(a) Technical Field

The present disclosure relates to an electrochemical cell for carbon dioxide conversion with improved carbon dioxide conversion efficiency that changes the type of fuel injected into an anode, the composition of a separator, the type of a cathode catalyst, etc., to improve the carbon dioxide conversion efficiency and reduce the driving voltage of the electrochemical cell.

(b) Background Art

As emissions of carbon dioxide, which is greenhouse gas, have increased worldwide over the past several decades and global worming has accelerated, causing climate change, solving these problems has become an important issue.

Accordingly, interest in technologies that convert flue gas emitted via flues of industrial manufacturing plants or carbon dioxide directly captured from the atmosphere into high value-added compounds has increased significantly. These technologies include a CO₂ conversion reaction, which converts carbon dioxide into other compounds through an electrochemical reduction reaction, or a CO₂ reduction reaction (CO₂RR).

Using these technologies have additional advantages of being able to consume and remove carbon dioxide as a reactant of the electrochemical reactions, while storing electricity based on renewable energy source, such as wind, sunlight, and water power, which are produced in excess and discarded, in the form of high value-added compounds.

The CO₂ conversion reaction or the CO₂ reduction reaction produces a wide variety of products depending on the cell structure of a device, the type of an electrolyte or an electrode-catalyst, the characteristics of components or materials that form the cell, and operating conditions.

Electrochemical cells for carbon dioxide conversion include a liquid batch cell called an H-type cell, a liquid flow cell in which an anolyte is located between a solid electrolyte and an anode and a catholyte is located between the solid electrolyte and a cathode, and an electrochemical cell having a zero-gap structure in which an anode, a cathode, and a separator interposed between the anode and the cathode are provided, and porous transport layers integrated with the anode and the cathode are located on the outer surfaces of the anode and the cathode so that the separator and electrodes, i.e., the anode and the cathode, are in direct contact.

Water is injected as fuel into the anode of this electrochemical cell, and oxygen is produced by oxidation reaction. In addition, carbon dioxide is injected as fuel into the cathode, and hydrogen and various types of products, such as carbon monoxide, ethylene, propylene, etc., are simultaneously produced by a reduction reaction. As such, if various types of products are simultaneously produced, there is a problem that an additional process for separating the products must be performed.

Further, an anion exchange membrane or a cation exchange membrane was conventionally used as the separator of the electrochemical cell for carbon dioxide conversion. In the case of the anion exchange membrane, a CO2 crossover problem may occur, and may thus reduce carbon dioxide conversion efficiency. In addition, a conventional perfluorinated cation exchange membrane is operated under very strong acidic conditions, and the carbon dioxide reduction reaction and hydrogen evolution reaction (HER) are in a competitive reaction relationship, and at a low pH, the hydrogen evolution reaction is dominant and may thus reduce CO2 conversion efficiency.

The above information disclosed in this Background section is only for enhancement of understanding of the background of the disclosure and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.

SUMMARY OF THE DISCLOSURE

The present disclosure has been made in an effort to solve the above-described problems associated with the prior art, and it is an object of the present disclosure to provide an electrochemical cell that increases carbon dioxide conversion efficiency by controlling a pKa at which the electrochemical cell is driven.

It is another object of the present disclosure to provide an electrochemical cell that produces a single product by a reduction reaction occurring at a cathode to omit a product separation process.

It is yet another object of the present disclosure to introduce a gaseous reactant, such as hydrogen, instead of water as fuel injected into an anode to reduce the driving voltage of an electrochemical cell.

It is a further object of the present disclosure to provide an electrochemical cell that has a low adsorption intensity with hydrogen and high selectivity for a carbon dioxide reduction reaction by controlling the dispersed form of a catalytic metal on a support in an electrode catalyst including the support and the catalytic metal.

The objects of the present disclosure are not limited to the above-mentioned objects. The objects of the present disclosure will become clearer from the following description, and may be realized by means stated in the claims and combinations thereof.

In one aspect, the present disclosure provides an electrochemical cell for carbon dioxide conversion including an anode including an anode catalyst, a cathode including a cathode catalyst, and a separator interposed between the anode and the cathode and including a buffer solution and a cation exchange material, wherein hydrogen (H₂) is supplied to the anode so that an oxidation reaction of the hydrogen occurs at the anode, and carbon dioxide (CO₂) is supplied to the cathode so that a reduction reaction of the carbon dioxide occurs at the cathode.

In a preferred embodiment, the electrochemical cell may further include at least one of an anode porous transport layer located on an outer surface of the anode, or a cathode porous transport layer located on an outer surface of the cathode.

In another preferred embodiment, the buffer solution may include phosphoric acid (H₃PO₄).

In still another preferred embodiment, a pKa of the buffer solution may satisfy 2≤pKa≤5.5.

In yet another preferred embodiment, the cation exchange material may include at least one of polybenzimidazole (PBI) and an ion-pair forming material.

In still yet another preferred embodiment, the ion-pair forming material may include at least one selected from the group consisting of quaternary ammonium coordinated polyphenylene, polynorbornene, polycarbazole, poly(aryl piperidinium)-based on terphenyl (PAP-TP), imidazolium-functionalized poly(pentafluorostyrene) and combinations thereof.

In a further preferred embodiment, the separator may include the buffer solution at a doping level 3 to 15.

In another further preferred embodiment, the cathode catalyst may include a catalyst metal supported on a support.

In still another further preferred embodiment, the catalyst metal may include at least one of a single-atom catalyst and a nanoparticle catalyst.

In yet another further preferred embodiment, the single-atom catalyst may include at least one selected from the group consisting of nickel (Ni), iron (Fe), cobalt (Co) and combinations thereof.

In still yet another further preferred embodiment, the nanoparticle catalyst may include gold (Au).

In a still further preferred embodiment, the support may include a carbon-based support, and the carbon-based support may include at least one selected from the group consisting of carbon black, carbon nanotubes, graphite, graphene and combinations thereof.

In a yet still further preferred embodiment, the support may include a nitrogen-doped support.

In a further preferred embodiment, the cathode may include a binder configured to adjust cation conductivity of the cathode.

In another further preferred embodiment, the binder may include at least one selected from the group consisting of poly(2,3,5,6-tetrafluorostyrene-4-phosphonic acid) (PWN), perfluorinated ionomers, polyvinylpyrrolidone (PVP), polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), polybenzimidazole (PBI), polystyrene tetramethyl imidazolium chloride and combinations thereof.

In still another further preferred embodiment, a weight ratio of a support to the binder in the cathode catalyst may be 1:0.4 to 1:1.6.

In yet another further preferred embodiment, the electrochemical cell may further include a barrier layer interposed between the separator and the cathode to adjust a rate of a hydrogen evolution reaction (HER).

In still yet another further preferred embodiment, the barrier layer may include at least one selected from the group consisting of silicon dioxide (SiO₂), aluminum oxide (Al₂O₃), polyvinylpyrrolidone (PVP) and combinations thereof.

In a still further preferred embodiment, a single product including at least one selected from the group consisting of carbon monoxide (CO), formic acid (HCOOH), ethylene (C₂H₄), propylene (C₃H₆) and alcohol may be produced at the cathode.

In a yet still further preferred embodiment, the hydrogen supplied to the anode may include humidified hydrogen having a relative humidity of 4% or more.

Other aspects and preferred embodiments of the disclosure are discussed infra.

The above and other features of the disclosure are discussed infra.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features of the present disclosure will now be described in detail with reference to certain exemplary embodiments thereof illustrated in the accompanying drawings which are given hereinbelow by way of illustration only, and thus are not limitative of the present disclosure, and wherein:

FIG. 1 shows an electrochemical cell for carbon dioxide conversion according to one embodiment of the present disclosure;

FIG. 2 shows an electrochemical cell for carbon dioxide conversion according to another embodiment of the present disclosure;

FIG. 3 schematically shows the structure of a barrier layer;

FIGS. 4A and 4B are graphs showing carbon dioxide conversion efficiency while adjusting the pKa of an electrochemical cell according to Example 1;

FIGS. 5A and 5B are graphs showing carbon dioxide conversion efficiency while adjusting the pKa of an electrochemical cell according to Example 2;

FIGS. 6A and 6B are graphs showing carbon dioxide conversion efficiency while adjusting the pKa of an electrochemical cell according to Example 3;

FIGS. 7A and 7B are graphs showing carbon dioxide conversion efficiency while adjusting the pKa of an electrochemical cell according to Example 4;

FIGS. 8A and 8B are graphs showing carbon dioxide conversion efficiency while adjusting the pKa of an electrochemical cell according to Example 5;

FIGS. 9A and 9B are graphs showing carbon dioxide conversion efficiency while adjusting the pKa of an electrochemical cell according to Example 6;

FIG. 10 is a graph showing carbon dioxide conversion efficiency when the pKa of an electrochemical cell according to Comparative Example 1 is set to 1; and

FIG. 11 is a graph showing carbon dioxide conversion efficiency when the pKa of an electrochemical cell according to Comparative Example 2 is set to 5.

It should be understood that the appended drawings are not necessarily to scale, presenting a somewhat simplified representation of various preferred features illustrative of the basic principles of the disclosure. The specific design features of the present disclosure as disclosed herein, including, for example, specific dimensions, orientations, locations, and shapes, will be determined in part by the particular intended application and use environment.

In the figures, reference numbers refer to the same or equivalent parts of the present disclosure throughout the several figures of the drawing.

DETAILED DESCRIPTION

The above-described objects, other objects, advantages and features of the present disclosure will become apparent from the descriptions of embodiments given hereinbelow with reference to the accompanying drawings. However, the present disclosure is not limited to the embodiments disclosed herein and may be implemented in various different forms. The embodiments are provided to make the description of the present disclosure thorough and to fully convey the scope of the present disclosure to those skilled in the art.

In the drawings, the same or similar elements are denoted by the same reference numerals even though they are depicted in different drawings. In the accompanying drawings, the dimensions of structures may be exaggerated compared to the actual dimensions thereof, for clarity of description. In the following description of the embodiments, terms, such as “first” and “second”, may be used to describe various elements but do not limit the elements. These terms are used only to distinguish one element from other elements. For example, a first element may be named a second element, and similarly, a second element may be named a first element, without departing from the scope and spirit of the disclosure. Singular expressions may encompass plural expressions, unless they have clearly different contextual meanings.

In the following description of the embodiments, terms, such as “including”, “comprising” and “having”, are to be interpreted as indicating the presence of characteristics, numbers, steps, operations, elements or parts stated in the description or combinations thereof, and do not exclude the presence of one or more other characteristics, numbers, steps, operations, elements, parts or combinations thereof, or possibility of adding the same. In addition, it will be understood that, when a part, such as a layer, a film, a region or a plate, is said to be “on” another part, the part may be located “directly on” the other part or other parts may be interposed between the two parts. In the same manner, it will be understood that, when a part, such as a layer, a film, a region or a plate, is said to be “under” another part, the part may be located “directly under” the other part or other parts may be interposed between the two parts.

All numbers, values and/or expressions representing amounts of components, reaction conditions, polymer compositions and blends used in the description are approximations in which various uncertainties in measurement generated when these values are obtained from essentially different things are reflected and thus it will be understood that they are modified by the term “about”, unless stated otherwise. In addition, it will be understood that, if a numerical range is disclosed in the description, such a range includes all continuous values from a minimum value to a maximum value of the range, unless stated otherwise. Further, if such a range refers to integers, the range includes all integers from a minimum integer to a maximum integer, unless stated otherwise.

In the following description of the embodiments, it will be understood that, when the range of a variable is stated, the variable includes all values within the stated range including stated end points of the range. For example, it will be understood that a range of “5 to 10” includes not only values of 5, 6, 7, 8, 9 and 10 but also arbitrary subranges, such as a subrange of 6 to 10, a subrange of 7 to 10, a subrange of 6 to 9, and a subrange of 7 to 9, and arbitrary values between integers which are valid within the scope of the stated range, such as 5.5, 6.5, 7.5, 5.5 to 8.5, and 6.5 to 9. Further, for example, it will be understood that a range of “10% to 30%” includes not only all integers including values of 10%, 11%, 12%, 13%, . . . 30% but also arbitrary subranges, such as a subrange of 10% to 15%, a subrange of 12% to 18%, and a subrange of 20% to 30%, and arbitrary values between integers which are valid within the scope of the stated range, such as 10.5%, 15.5%, and 25.5%.

FIG. 1 shows an electrochemical cell for carbon dioxide conversion according to one embodiment of the present disclosure. An electrochemical cell for carbon dioxide conversion according to the present disclosure may include an anode 100 including an anode catalyst, a cathode 200 including a cathode catalyst, and a separator 300 interposed between the anode 100 and the cathode 200 and including a buffer solution and a cation exchange material. In addition, the electrochemical cell may include at least one of an anode porous transport layer 400 located on the outer surface of the anode 100 or a cathode porous transport layer 500 located on the outer surface of the cathode 200.

Electrochemical reactions that occur in the electrochemical cell for carbon dioxide conversion are as follows. Hydrogen (H₂) may be supplied to the anode 100 through the anode porous transport layer 400 of the electrochemical cell so that an oxidation reaction of hydrogen may occur at the anode 100. Carbon dioxide may be supplied to the cathode 200 through the cathode porous transport layer 500 so that a reduction reaction of carbon dioxide may occur at the cathode 200.

Hydrogen, water, and a single product including one of formic acid, ethylene, propylene, and alcohol may be generated through the reduction reaction of carbon dioxide. This will be described later.

The anode 100 may include an anode catalyst. In addition, the anode 100 may include a binder which will be described later. The anode 100 receives hydrogen gas as fuel, and causes the oxidation reaction of hydrogen, as shown in Reaction Formula 1 below, to generate protons and electrons.

According to the present disclosure, by using hydrogen gas as the fuel injected into the anode 100, the hydrogen oxidation reaction may occur instead of an oxygen evolution reaction in which water is oxidized to generate oxygen, thereby reducing the driving voltage of the electrochemical cell. Further, as the hydrogen supplied to the anode 100, humidified hydrogen having a relative humidity of 4% or more may be used. If the humidified hydrogen having a relative humidity of 4% or more is injected into the anode 100, a material transfer problem in the anode 100, the separator 300, and the cathode 200 is improved, thereby reducing resistance at the anode 100.

As the anode catalyst, any material commonly used as an anode catalyst in the technical field to which the present disclosure pertains may be used without particular limitation. For example, the anode catalyst may include a precious metal catalyst, such as platinum (Pt), a non-precious metal catalyst, an alloy catalyst thereof, or the like. In addition, the anode catalyst may be supported on a carbon support, and preferably, may include a platinum catalyst supported on a carbon support (Pt/C).

The separator 300 according to the present disclosure may include the buffer solution that controls the pKa of the separator 300, and the cation exchange material that exchanges or transfers cations.

Ion exchange materials mainly used in electrochemical cells include anion exchange materials, cation exchange materials, and bipolar exchange materials. The anion exchange materials may exchange anions, such as hydroxide ions (OH⁻). The main chains of the anion exchange materials generally include non-fluorinated hydrocarbons. Representative types include non-limiting examples such as Sustainion™ based on polystyrene tetramethyl imidazolium chloride including a polystyrene (PS) main chain and an imidazolium group, Aemion™ based on benzimidazolium, and PiperION™ based on poly (aryl piperidinium) and the like.

Anion exchange materials have been widely used because it is easy to create conditions suitable for the carbon dioxide reduction reaction at the cathode 200 of a conventional electrochemical cell for carbon dioxide conversion. However, use of such materials can create problems given that carbon dioxide, which is a reactant at the cathode 200, reacts with hydroxide ions and is consumed and generates byproduct salts that can precipitate at the cathode 200, (salt formation and precipitation) and can degrade cell performance and durability. The salt byproducts can cross over from the cathode 200 to the anode 100 through the anion exchange material, thereby reducing carbon dioxide (CO₂) utilization. In addition, the anion exchange material has a problem of oxidative degradation when the oxygen evolution reaction (OER) occurs at the anode 100 due to the low durability of the anion exchange material. Furthermore, there is a problem of increased resistance when carbon dioxide gas comes into contact with the separator 300 including the anion exchange material.

Due to these problems, most anion exchange materials have low durability and are typically not used for large-scale manufacturing. For example, Sustainion™ has low handling ability because it must be stored in potassium hydroxide (KOH) before use, and has low efficiency in mass production because a sample treatment process is complicated.

In order to solve these problems, interest has increased in cation exchange materials, which have been used for decades in industries, such as chlor-alkali electrolysis, and have been verified in terms of mass production and durability.

In general, cation exchange materials have high durability and stability under various electrochemical operating conditions and suppress crossover of reactants or products at the cathode 200 to the anode 100, and can provide advantages in increasing the conversion rate of carbon dioxide. As non-limiting examples of these cation exchange materials, known cation exchange materials such as perfluorinated ionomers, for example, Nafion®, can be incorporated as part of the separator 300.

However, since a perfluorinated cation exchange material is operated under very strong acid conditions, there is a problem that the hydrogen evolution reaction, which is a competitive reaction of the carbon dioxide reduction reaction, dominantly occurs and the carbon dioxide conversion rate decreases. In addition, the separator 300 including the perfluorinated cation exchange material uses water as an electrolyte (or the buffer solution), and in this case, water acts as a main reactant of the hydrogen evolution reaction, which as the competitive reaction, may contribute to a decrease in the carbon dioxide conversion rate.

Furthermore, the electrochemical cell using the perfluorinated cation exchange material as the separator 300 is usable under low temperature and high humidity conditions; however, perfluorinated cation exchange material has a problem in that it is typically difficult to operate the electrochemical cell under high temperature conditions. Perfluorinated cation exchange material has other disadvantages, such as low carbon monoxide resistance, incorporation of large amounts of precious metal catalysts in the electrode, and is relatively expensive.

The separator 300 according to the present disclosure may include at least one of polybenzimidazole (PBI) and/or an ion-pair forming material as the cation exchange material. The above separator 300 comprising polybenzimidazole (PBI) and/or an ion-pair forming material as the cation exchange material may be operated at a higher pKa (or pH) than the conventional separator 300 using a perfluorinated ionomer, and suppresses the competitive hydrogen evolution reaction to promote the carbon dioxide reduction reaction.

In one embodiment, the ion-pair forming material is a material that may form ion pairs with the buffer solution within the separator 300, and as long as it satisfies such a definition, the type of the ion-pair forming material is not particularly limited. In some embodiments the ion-pair forming material may include at least one or more of the non-limiting examples selected from the group consisting of quaternary ammonium coordinated polyphenylene, polynorbornene, polycarbazole, poly(aryl piperidinium)-based on terphenyl (PAP-TP), imidazolium-functionalized poly(pentafluorostyrene), and combinations thereof.

Further, the electrochemical cell including the separator 300 of the present disclosure may be operated at a relatively high temperature of up to about 180° C. (as well as at lower temperatures), and thus has high resistance to impurities. Accordingly, low-purity hydrogen may be used, and carbon monoxide resistance may be increased. In some embodiments, wherein the the cation exchange material comprise if polybenzimidazole, the electrochemical cell may be operated at about 140° C. to 180° C. In addition, if the ion-pair forming material is used as the cation exchange material, the electrochemical cell may be operated at about 80° C. to 180° C.

The electrochemical cell according to the present disclosure uses hydrogen gas as fuel injected into (i.e., brought in contact with) the anode 100, and hydrogen gas is oxidized at the anode 100 to generate protons. The protons generated at the anode 100 migrate to the cathode 200 via the separator 300. The buffer solution may serve as an electrolyte configured or adapted to transfer the protons during the process in which the protons generated at the anode 100 migrate to the cathode 200 via the separator 300, and may prevent the pKa (or pH) of the separator 300 from decreasing to a very strong acidic environment (for example, pKa≤1).

In one embodiment, the pKa of the buffer solution may be in a range of about 2≤pKa≤5.5. If the pKa of the buffer solution drops to a value about less than 2, an exchange membrane becomes highly acidic, and thus the competitive hydrogen evolution reaction, may increase and the efficiency of the carbon dioxide reduction reaction may decrease. If the pKa of the buffer solution exceeds a value of about 5.5, the exchange membrane approaches neutrality or alkalinity, hydroxide ions generated by the carbon dioxide reduction reaction may react with carbon dioxide to generate carbonate ions, and these carbonate ions may cause a loss of carbon dioxide as fuel inserted into the cathode 200.

As the buffer solution, any material may be used without particular limitation as long as it functions as an electrolyte in the separator 300, a buffer to control the pKa of the separator 300, and can form ion pairs with the cation exchange material. In some embodiments the buffer solution may include phosphoric acid (H₃PO₄).

In one embodiment, the separator 300 may include the buffer solution at a doping level of 3 to 15. Preferably, the separator 300 may include the buffer solution at a doping level 7 to 12.

If the doping level of the buffer solution in the separator 300 is less than about 3, the buffer solution may not be in an amount sufficient to operate (e.g., may increase resistance and possibly cause short circuit, making it difficult to operate the electrochemical cell). If the doping level of the buffer solution exceeds about 15, the amount of the buffer solution may be in excess and degrade operation (e.g., phosphoric acid, if present in the separator 300, and may thus poison the catalytic surface of the electrode and deteriorate the performance thereof). In addition, if the doping level of the buffer solution in the separator 300 is 7 to 12, the electrochemical performance of the electrochemical cell may be desirably improved.

Here, the “doping level” may be understood as a meaning commonly used in the technical field to which the present disclosure pertains, and may mean, for example, the number of molecules of the buffer solution molecules doped per repeating unit (monomer) of the cation exchange material in the separator 300.

The cathode 200 may receive carbon dioxide as fuel and generate hydrogen gas through a reduction reaction of hydrogen, as shown in Reaction Formula 2 below. In addition, a reduction reaction of carbon dioxide shown in any one of Reaction Formulas 3 to 8 below may be performed to generate a single product.

However, one of skill will appreciate that the reduction reaction of carbon dioxide is not limited to the above Reaction Formulas 3 to 8, and other reactions may occur depending on the type of the cathode catalyst, operating conditions, etc. In some embodiments, a single product may be generated by the reduction reaction of carbon dioxide.

In embodiments wherein a single product is generated by the reduction reaction of carbon dioxide at the cathode 200, there is an advantage in that the overall process is simplified by omitting a subsequent process of separating products.

In one embodiment, the cathode 200 may include the cathode catalyst. Further, the cathode catalyst may include a catalytic metal on a support. The catalytic metal of the cathode catalyst may include at least one of a single-atom catalyst and a nanoparticle catalyst.

In general, the catalytic metal is present in the form particles having a predetermined diameter which are supported on a support material. The “single-atom catalyst” according to the present disclosure may mean a catalyst in which each of catalytic metal atoms are directly bonded to a support material. That is, it differs from a general catalyst wherein the catalytic metal atoms agglomerated or clumped together to form particles having a predetermined diameter and attach to the support material. Since the single-atom catalyst causes unsaturated coordination of the catalytic metal it facilitates catalyst active sites. In embodiments, the single-atomic catalyst may reduce a particle size to increase the ratio of unsaturated coordinated surface atoms, thereby being capable of promoting increased catalyst activity per metal atom.

Non-limiting examples, the single-atom catalyst can include any non-precious metal catalyst as long as it can be combined with a support in the form of single atoms. In some non-limiting embodiments, the single-atom catalyst may include at least one metal selected from the group consisting of nickel (Ni), iron (Fe), cobalt (Co), and combinations thereof.

Further, the cathode catalyst according to the present disclosure may include a catalytic metal in the form of nanoparticles in addition to the single-atom catalyst. The nanoparticle catalyst may use a catalytic metal having a particle size of about 1 nm to 50 nm. For example, the nanoparticle catalyst may include gold (Au). The cathode catalyst according to the present disclosure may use a catalytic metal in the form of nanoparticles to increase a reaction area, thereby being capable of increasing the reduction reaction of carbon dioxide.

The cathode support material can comprise a carbon-based support, and the carbon-based support. In some embodiments, the support can comprise one or more of carbon black, carbon nanotubes, graphite, graphene, and combinations thereof. Further, in embodiments comprising a single-atom catalyst as the cathode catalyst, a nitrogen-doped support may be used to more easily combine or support a single-atom type catalytic metal with or on the support.

In one embodiment, the cathode 200 may include a binder configured to control cation conductivity of the cathode 200. The binder can be incorporated in the cathode 200 in the form of a mixture with the cathode catalyst, and may control the cation conductivity in the cathode 200 to suppress the competitive hydrogen reduction reaction and may control the amount of an acid present on the surface of the cathode catalyst to reduce the poisoning phenomenon by phosphoric acid.

In embodiments, the binder comprises a hydrophobic ionomer or a cation exchange ionomer. In some non-limiting embodiments, the binder may include at least one of poly(2,3,5,6-tetrafluorostyrene-4-phosphonic acid) (PWN), perfluorinated ionomers, polyvinylpyrrolidone (PVP), polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), polybenzimidazole (PBI), polystyrene tetramethyl imidazolium chloride and combinations thereof. Examples of the perfluorinated ionomers include Nafion®, and examples of polystyrene tetramethyl imidazolium chloride include Sustainion®.

In one embodiment, a weight ratio of the support to the binder in the cathode catalyst may range from about 1:0.4 to 1:1.6. If the weight ratio of the support to the binder is less than about 1:0.4, it may increase the difficultly to fully achieve the effects of suppressing the hydrogen reduction reaction and suppressing poisoning of the cathode catalyst by phosphoric acid. If the weight ratio of the support to the binder exceeds about 1:1.6, the content of the cathode catalyst may be too small, which may reduce the reduction reaction of carbon dioxide.

The electrochemical cell may include porous transport layers located on the outer surfaces of the electrodes, such as the anode 100 and the cathode 200. That is, with reference to the Figures, the electrochemical cell may include at least one of the anode porous transport layer 400 located on the outer surface of the anode 100 or the cathode porous transport layer 500 located on the outer surface of the cathode 200.

The porous transport layer is typically formed on the surface of the electrode to diffuse and pass fuel (for example, hydrogen gas or carbon dioxide gas) flowing through a bipolar plate to the separator 300. The porous transport layers may include a microporous layer including carbon particles and a substrate layer including carbon fibers. In some embodiments, the microporous layer may be disposed to face toward the electrode direction and the substrate layer may be disposed to face toward the bipolar plate but the microporous layer and the substrate layer are not limited thereto.

In some embodiments, the microporous layer includes particulate porous carbon particles, such as, for example, carbon powder of carbon black, such as acetylene black or Black Pearls. In some embodiments, the microporous layer may include a mixture of a polytetrafluoroethylene (PTFE)-based hydrophobic material with the carbon powder.

In embodiments, the substrate layer can include irregularly arranged carbon fibers, and may have an irregular fibrous porous structure. In some embodiments, the substrate layer may further include a polytetrafluoroethylene (PTFE)-based hydrophobic agent. The substrate layer may include, for example, carbon fiber cloth, carbon fiber felt, carbon fiber paper, or the like.

In some embodiments, the porous transport layer may be attached to the electrode to be integrated therewith. As used herein, “integrated” or “integration” indicates that respective components are in direct contact, e.g., by directly applying raw materials of the anode 100 to the anode porous transport layer 400, transferring the anode 100 to the anode porous transport layer 400, or attaching the anode 100 to the anode porous transport layer 400, and then applying high pressure to form integrated components. The same meaning as above applies when the cathode porous transport layer 500 and the cathode 200 are integrated.

FIG. 2 shows an electrochemical cell for carbon dioxide conversion according to another embodiment of the present disclosure. Further, FIG. 3 schematically depicts a structure of a barrier layer 600. The electrochemical cell embodiments, as shown in FIGS. 2 and 3, may further include the barrier layer 600 located between the separator 300 and the cathode 200 to control the rate of the hydrogen evolution reaction.

The barrier layer 600 may include at least one material selected from the group consisting of silicon dioxide (SiO₂), aluminum oxide (Al₂O₃), polyvinylpyrrolidone (PVP), and combinations thereof. In embodiments wherein the barrier layer 600 comprises a combination of silicon dioxide and polyvinylpyrrolidone or a combination of aluminum oxide and polyvinylpyrrolidone, the content of polyvinylpyrrolidone in the barrier layer 600 may be from about 0 wt % to 0.2 wt %.

In some embodiments wherein the barrier layer 600 includes only a hydrophobic ionomer or a cation exchange ionomer described herein as a binder it may be difficult to achieve the effect of the barrier layer 600.

In some embodiments, the barrier layer 600 may include at least one of silicon dioxide (SiO₂), aluminum oxide (Al₂O₃), polyvinylpyrrolidone (PVP), and combinations thereof in an amount content to the content of the cathode catalyst in the cathode 200. For example, when coating electrode porous transport layer with a catalyst to form electrode, assuming that the amount of catalyst coated is uniformly applied at 0.1 mg/cm², it can be understood to mean that the material acting as a barrier layer on top of it is also coated at 0.1 mg/cm².

In some embodiments, the barrier layer 600 may further include an organic filler. As the organic filler, any organic filler may be used without particular limitation as long as it is commonly used in the relevant technical field.

In some embodiments, the barrier layer 600 comprises a separate layer from the cathode 200 (i.e., the barrier layer 600 does not include any cathode catalyst).

In such embodiments, a barrier layer 600 may minimize the amount of the acid contacting or present on the surface of the cathode catalyst and further suppress poisoning by buffer (e.g., phosphoric acid). In such embodiments, the barrier layer 600 may promote the carbon dioxide reduction reaction of the cathode catalyst, and improve the conversion efficiency of carbon dioxide.

Hereinafter, the present disclosure will be described in more detail through the following Examples and Comparative Examples, which merely serve as illustrative embodiments of the disclosure and should not be considered as limiting to the scope of disclosure or appended claims.

EXAMPLE 1—SEPARATOR: ION-PAIR FORMING MATERIAL; CATHODE CATALYST: NI—N—C

An electrochemical cell for carbon dioxide conversion having the structure shown in FIG. 1 was manufactured.

A Pt/C catalyst in which a platinum catalyst is supported on carbon black, which is a carbon-based support, was prepared as an anode catalyst. An anode integrated with an anode porous transport layer by mixing the anode catalyst with a PVP binder and then coating carbon paper prepared as an anode porous transport layer with an obtained mixture.

Nitrogen-doped carbon black was prepared as a support.

An Ni—N—C cathode catalyst was manufactured by supporting nickel on the nitrogen-doped carbon black in the form of a single-atom catalyst. A cathode integrated with a cathode porous transport layer was manufactured by mixing the cathode catalyst with a PVP binder and then coating carbon paper prepared as a cathode porous transport layer with an obtained mixture.

Phosphoric acid (H₃PO₄) having a pKa of 2 to 3 was prepared as a buffer solution. A separator was manufactured by adding quaternary ammonium coordinated polyphenylene, which is an ion-pair forming material, to the phosphoric acid and mixing the same.

The integrated anode and the integrated cathode were stacked on one surface and the other surface of the separator. At this time, the anode and the cathode faced the separator, and the anode porous transport layer and the cathode porous transport layer faced the outside. Separators, each of which has a fuel inlet, a product outlet, and a flow field, are stacked on the anode porous transport layer and the cathode porous transport layer, respectively. An electrochemical cell was manufactured by applying a predetermined pressure to an obtained stack.

EXAMPLE 2—SEPARATOR: PBI; CATHODE CATALYST: NI—N—C

An electrochemical cell was manufactured through the same process as in Example 1, except that a separator was manufactured using polybenzimidazole (PBI) as a cation exchange material, and use phosphoric acid having a pKa of 4 to 5 as a buffer solution.

EXAMPLE 3—SEPARATOR: ION-PAIR FORMING MATERIAL; CATHODE CATALYST: FE—N—C

An electrochemical cell was manufactured through the same process as in Example 1, except that Fe—N—C, in which iron (Fe) is supported on nitrogen-doped carbon black in the form of a single-atom catalyst, was used as a cathode catalyst.

EXAMPLE 4—SEPARATOR: PBI; CATHODE CATALYST: FE—N—C

An electrochemical cell was manufactured through the same process as in Example 2, except that Fe—N—C, in which iron (Fe) is supported on nitrogen-doped carbon black in the form of a single-atom catalyst, was used as a cathode catalyst.

EXAMPLE 5—SEPARATOR: ION-PAIR FORMING MATERIAL; CATHODE CATALYST: AU/C

An electrochemical cell was manufactured through the same process as in Example 1, except that Au/C, in which gold (Au) is supported on carbon black, which is not doped with nitrogen, in the form of a nanoparticle catalyst, was used as a cathode catalyst.

EXAMPLE 6—SEPARATOR: PBI: CATHODE CATALYST: AU/C

An electrochemical cell was manufactured through the same process as in Example 2, except that Au/C, in which gold (Au) is supported on carbon black, which is not doped with nitrogen, in the form of a nanoparticle catalyst, was used as a cathode catalyst.

EXAMPLE 7—EXAMPLE 2+BARRIER LAYER: PVP, SIO₂

A slurry to form a barrier layer was prepared by mixing silicon dioxide (SiO₂) and PVP in a weight ratio of 99.8 wt %: 0.2 wt %. A cathode integrated with a cathode porous transport layer was manufactured through the same process as in Example 1, and then the barrier layer was formed by applying the slurry to the integrated cathode and drying the slurry.

The integrated cathode and the barrier layer were stacked on the separator. Here, the barrier layer was configured to face the separator.

An electrochemical cell was manufactured through the same process as in Example 2, except that the barrier layer located between the separator and the cathode was formed through the above process, as shown in FIG. 2.

COMPARATIVE EXAMPLE 1—SEPARATOR: NAFION; CATHODE CATALYST: NI—N—C

An electrochemical cell was manufactured through the same process as in Example 1, except that Nafion (DUPONT), which is a conventional cation exchange material, was used as a separator and phosphoric acid (H₃PO₄) having a pKa of 1 was used as an electrolyte.

COMPARATIVE EXAMPLE 2—SEPARATOR: FBI; CATHODE CATALYST: SN—N—C

An electrochemical cell was manufactured through the same process as in Example 2, except that Sn—N—C, in which tin (Sn) is supported on nitrogen-doped carbon black in the form of a single-atom catalyst, was used as a cathode catalyst.

TEST EXAMPLE 1—COMPARISON OF CO₂ CONVERSION EFFICIENCY DEPENDING ON PKA CHANGE

In order to check the conversion efficiency of carbon dioxide depending on pKa change based on a phosphoric acid solution as a buffer solution, after the electrochemical cells according to Example 1, Example 2, and Comparative Example 1 were operated, the compositions of products were analyzed. Specifically, hydrogen gas was used as fuel supplied to the anode, and carbon dioxide gas was used as fuel supplied to the cathode. At this time, the electrochemical cells according to Example 1 and Example 2 were operated at a voltage of 1.5 V, and the electrochemical cell according to Comparative Example 1 was operated at a voltage of 3.0 V.

The electrochemical cell according to Example 1 was operated at pKa 2 and pKa 3, and the results thereof are shown in FIGS. 4A and 4B. The electrochemical cell according to Example 2 was operated at pKa 4 and pKa 5, and the results thereof are shown in FIGS. 5A and 5B. The electrochemical cell according to Comparative Example 1 was operated at pKa 1, and the results thereof are shown in FIG. 10.

Referring to FIGS. 4A to 5B and FIG. 10, it may be confirmed that the carbon dioxide conversion efficiency increased as the pKa value increased, and the carbon dioxide conversion efficiency of 94.3% to maximum was exhibited at pKa 5. Without being limited by any possible theory, this result is thought to be due to the fact that the hydrogen evolution reaction, which is the competitive reaction, decreases as the pKa increases under acidic conditions, and thus the carbon dioxide conversion efficiency increases. In addition, in the case of Comparative Example 1 using the conventional Nafion separator, it was confirmed that the electrochemical cell was operated under very strong acidic conditions, and thus the hydrogen evolution reaction, which is the competitive reaction, dominantly occurred.

Further, if an anion exchange material is used as a separator, although no separate test was conducted, the ratio of CO₂, which is a reactant in a reaction phase, to CO, which is converted from the reactant, becomes 2:1, and therefore, even if 100% conversion is assumed, the efficiency may only reach a maximum of 50%. Therefore, it is calculated that the carbon dioxide conversion efficiency will be much lower than the case in which a cation exchange material is used as a separator.

TEST EXAMPLE 2—COMPARISON OF CO₂ CONVERSION EFFICIENCY DEPENDING ON TYPE OF CATALYST

In order to check the carbon dioxide conversion efficiency depending on the type of the cathode catalyst, the electrochemical cells according to Examples 1 to 6 and Comparative Example 2 were operated, and the compositions of products were analyzed. Specifically, hydrogen gas was used as fuel supplied to the anode, and carbon dioxide gas was used as fuel supplied to the cathode.

The electrochemical cell according to Example 1 was operated at pKa 2 and pKa 3, and the results thereof are shown in FIGS. 4A and 4B. The electrochemical cell according to Example 2 was operated at pKa 4 and pKa 5, and the results thereof are shown in FIGS. 5A and 5B.

The electrochemical cell according to Example 3 was operated at pKa 2 and pKa 3, and the results thereof are shown in FIGS. 6A and 6B. The electrochemical cell according to Example 4 was operated at pKa 4 and pKa 5, and the results thereof are shown in FIGS. 7A and 7B.

The electrochemical cell according to Example 5 was operated at pKa 2 and pKa 3, and the results thereof are shown in FIGS. 8A and 8B. The electrochemical cell according to Example 6 was operated at pKa 4 and pKa 5, and the results thereof are shown in FIGS. 9A and 9B.

The electrochemical cell according to Comparative Example 2 was operated at pKa 5, and the results thereof are shown in FIG. 11.

Referring to FIGS. 4A to 5B, when evaluating the carbon dioxide conversion efficiency in the range of pKa 2-5, it was confirmed that the hydrogen evolution reaction, which is the competitive reaction, decreased as the pKa value increased under acidic conditions, and thus the CO₂ conversion efficiency increased. At this time, it was confirmed that the carbon dioxide conversion efficiency of 94.3% to maximum was exhibited at pKa 5.

Referring to FIGS. 6A to 7B, it was confirmed that the electrochemical cells using Fe—N—C as the cathode catalyst according to Examples 3 and 4 also showed excellent carbon dioxide conversion efficiency and hydrogen evolution reaction suppression ability, although lower than those of the electrochemical cells using Ni—N—C as the cathode catalyst according to Examples 1 and 2.

Referring to FIGS. 8A to 9B, it was confirmed that the electrochemical cells using Ag/C, which is the nanoparticle catalyst, as the cathode catalyst according to Examples 5 and 6 also showed excellent carbon dioxide conversion efficiency and hydrogen evolution reaction suppression ability, although lower than those of the electrochemical cells using Ni—N—C or Fe—N—C, which is the single-atom catalyst, as the cathode catalyst according to Examples 1 to 4. It was confirmed that the carbon dioxide conversion efficiency of 40% to maximum was exhibited at pKa 5.

Referring to FIG. 11, it was confirmed that the carbon dioxide conversion efficiency of the electrochemical cell using Sn—N—C as the cathode catalyst according to Comparative Example 2 was lower than those of the electrochemical cells according to Examples 1 to 6 of the present disclosure. Further, it was confirmed that only carbon monoxide (CO), which is a single product, was generated as a reduction reaction product at the cathode, as shown in FIGS. 4A to 9B according to Examples 1 to 6 of the present disclosure, but in the case of Comparative Example 2, in addition to carbon monoxide (CO), a small amount of propanol was also generated.

In the case of Comparative Example 2, hydrogen and carbon monoxide in a gaseous phase and propanol in a liquid phase were generated simultaneously. In order to separate them from each other, an additional process, such as gas-liquid separation, is required, and thus it may be said that the product selectivity of the electrochemical cell according to Comparative Example 2 is lower than those of the electrochemical cells according to Examples. In addition, a low carbon dioxide conversion efficiency of 40% or less was exhibited even at pKa 5, and this value is lower than those of the electrochemical cells according to Example 2 (FIG. 5B), Example 4 (FIG. 7B), and Example 6 (FIG. 9B).

Therethrough, it was confirmed that, in the case of Comparative Example 2, the hydrogen evolution reaction was not properly suppressed even under a pKa condition where carbon dioxide conversion generally occurs well.

TEST EXAMPLE 3—COMPARISON OF EFFECTS DEPENDING ON PRESENCE OR ABSENCE OF BARRIER LAYER

In order to compare the carbon dioxide conversion efficiency depending on presence or absence of the barrier layer, the electrochemical cells according to Examples 2 and 7 were operated at pKa 5, and then the carbon dioxide conversion efficiencies thereof were measured. The results thereof are set forth in Table 1 below.

TABLE 1
	Cathode		Barrier	Fraction of
Category	catalyst	Binder	layer	product (CO)

Example 2	Ni—N—C	PVP	x	2.56%
Example 7	Ni—N—C	PVP	PVP + SiO2	2.78%

As set forth in Table 1, it may be seen that the carbon dioxide conversion efficiency of Example 7, in which the barrier layer was formed under the same catalyst, binder, and separator conditions, was improved. Therethrough, it was confirmed that the rate of the hydrogen evolution reaction, which is the competitive reaction, may be adjusted by applying the barrier layer between the separator and the cathode of the electrochemical cell, and thus the carbon dioxide conversion efficiency may be improved.

As such, according to the present disclosure, as hydrogen is used as fuel injected into the anode, the driving voltage of the electrochemical cell may be reduced.

In addition, by using polybenzimidazole or an ion-pair forming material as the cation exchange material in the separator, the pKa at which the electrochemical cell is operated may be adjusted to an appropriate acidic range to increase the carbon dioxide conversion efficiency.

Further, the electrochemical cell according to the present disclosure may lower an adsorption intensity with hydrogen and increase selectivity for the carbon dioxide reduction reaction because the catalyst metal is supported on the support in the form of a single-atom catalyst.

Moreover, a single product is generated by the reduction reaction occurring at the cathode, and thus a process of separating products may be omitted.

As is apparent from the above description, according to the present disclosure, as hydrogen is used as fuel injected into an anode, the driving voltage of an electrochemical cell may be reduced.

In addition, by using polybenzimidazole or an ion-pair forming material as a cation exchange material in a separator, a pKa at which the electrochemical cell is operated may be adjusted to an appropriate acidic range to increase carbon dioxide conversion efficiency.

Further, the electrochemical cell according to the present disclosure may lower an adsorption intensity with hydrogen and increase selectivity for the carbon dioxide reduction reaction because a catalyst metal is supported on a support in the form of a single-atom catalyst.

Moreover, a single product is generated by a reduction reaction occurring at a cathode, and thus a process of separating products may be omitted.

The effects of the present disclosure are not limited to the above-mentioned effects. The effects of the present disclosure should be understood to include all effects that may be inferred from the above description.

The disclosure has been described in detail with reference to preferred embodiments thereof. However, it will be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the disclosure, the scope of which is defined in the appended claims and their equivalents.